INVITED REVIEW
The adipoinsular axis: effects of leptin on pancreatic -cells

¹ Departments of Medicine and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 2S2; and ² Laboratory of Molecular Endocrinology, Massachusetts General Hospital, Howard Hughes Medical Institute and Harvard Medical School, Boston, Massachusetts 02114

	ABSTRACT

TOP ABSTRACT OVERVIEW REGULATION OF LEPTIN PRODUCTION... RATIONALE FOR PROPOSING AN... DETECTION OF LEPTIN RECEPTORS... ACTIONS OF LEPTIN ON... MECHANISM OF LEPTIN ACTION... CONCLUDING REMARKS REFERENCES

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic -cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on -cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.

insulin secretion; diabetes

	OVERVIEW

TOP ABSTRACT OVERVIEW REGULATION OF LEPTIN PRODUCTION... RATIONALE FOR PROPOSING AN... DETECTION OF LEPTIN RECEPTORS... ACTIONS OF LEPTIN ON... MECHANISM OF LEPTIN ACTION... CONCLUDING REMARKS REFERENCES

The ob/ob and db/db mouse strains are both characterized by the development of obesity, hyperinsulinemia, and hyperglycemia, a syndrome resembling type 2 diabetes mellitus (28, 29, 43, 62, 63, 79). By a series of parabiosis experiments thirty years ago, in which the circulations of two animals became partially fused (ob/ob with db/db; ob/ob with +/+; db/db with +/+), Coleman (29) predicted that the ob/ob mice could respond to an appetite-suppressing hormone that it could not produce itself, whereas the db/db mice could produce this satiety factor but could not respond to it. The ob gene was identified in 1994 by use of positional cloning techniques; it encodes a 167-amino acid product termed leptin (188), which is comprised of a four-helical bundle similar to that of the long-chain helical cytokine family (97, 187). It was discovered that mutations in the ob gene of ob/ob mice prevent the synthesis of a functional leptin protein (188).

Although leptin is expressed at low levels in gastric epithelium (6), placenta (120), muscle (178), and possibly the central nervous system (46, 113), it is predominantly produced by and secreted from adipose tissue (188). As a consequence of this circumstance, plasma leptin levels are directly proportional to fat mass (33, 36, 42, 77, 104, 118, 121). It now appears that leptin is the long-sought-after hormone that communicates information about adipocyte metabolism and body weight to the appetite centers in the hypothalamic regions of the brain. Treatment of leptin-deficient ob/ob mice with recombinant leptin markedly increases energy expenditure and decreases food intake, body weight, and adiposity (17, 69, 135, 166, 182). The exogenous administration of leptin to mice also attenuates the neuroendocrine responses to food restriction, indicating that leptin may also play an important role in signaling nutritional status during periods of food deprivation (1).

The leptin receptor was first isolated from the mouse by the expression cloning of complementary DNA prepared from the choroid plexus (171). Positional cloning of the leptin receptor gene (db) revealed that it encodes five alternatively spliced forms of mRNAs (110). As had been predicted earlier from the parabiosis experiments, abnormal splicing of the mRNA transcribed from the leptin receptor gene in db/db mice results in a truncated version of the otherwise longer signaling isoform (ObRb) (110). Therefore ObRb, the so-called long form of the leptin receptor, is essential for the weight-reducing effects of leptin. The functions of the other isoforms of the leptin receptor remain unclear. Notably, however, it is now recognized that leptin receptor isoforms are widely distributed throughout peripheral organs (52, 66, 80, 115). These observations support the concept that leptin has peripheral actions independent of its actions on the central nervous system. In particular, the ObRb leptin receptor is expressed in the pancreatic -cells that produce insulin, raising the possibility that leptin directly regulates insulin release (94). Because the adipogenic actions of insulin are well known, the idea was postulated that leptin may suppress insulin release as part of a bidirectional adipoinsular axis (94).

The reader is referred to several comprehensive reviews of leptin biology (5, 19, 32, 56, 61, 170). In this review we present the evidence that supports the existence of an adipoinsular axis in which leptin suppresses insulin production and secretion. The other limb of the proposed adipoinsular axis, the stimulation of leptin production and secretion in adipose tissue by insulin, seems to be well established and will only be reviewed briefly. To place the concept of the adipoinsular axis into a comprehensible body of understanding, it is helpful to describe briefly the biology of the endocrine pancreas and adipose tissue.

The Endocrine Pancreas

The endocrine pancreas consists of circumscribed clusters of endocrine tissue (islets of Langerhans) imbedded in the exocrine tissue of the pancreas. The islets are composed of at least four major phenotypically specific hormone-producing cells: the -, -, -, and PP-cells that produce glucagon, insulin, somatostatin, and pancreatic polypeptide, respectively. The islets are highly vascularized, with a rate of blood flow similar to that of the brain, and thus are designed to respond rapidly to changes in nutrients in the bloodstream that signal the islet cells to secrete, or not to secrete, hormones. The hormones insulin, glucagon, and somatostatin clearly have important physiological actions in the maintenance of blood glucose levels and the metabolism of glucose and other nutrients, such as amino acids and lipids. Insulin and glucagon exert opposing actions on nutrient metabolism and are secreted in a reciprocal fashion. Insulin is an anabolic hormone whose functions are manifested during the fed state. Insulin is secreted in response to meals and acts on its key target tissues, skeletal muscle, liver, and adipose tissue, to stimulate the uptake of glucose and to store calories predominantly in the form of glycogen and fat. Glucagon is primarily a catabolic hormone that is important in maintaining blood glucose levels during fasting by promoting glycogenolysis in muscle and liver and lipolysis in adipose tissues. An absolute or relative lack of insulin production and secretion is responsible for type 1 and type 2 diabetes mellitus, respectively. Excessive secretion of glucagon that characteristically occurs in diabetes worsens hyperglycemia by increasing hepatic glucose output.

It is important to appreciate that the factors involved in the regulation of the secretion of insulin and glucagon in response to meals are complex. The temporal aspects and the relative amounts of insulin secreted in response to a meal involve precise timing. These temporal and quantitative aspects of insulin secretion are orchestrated not only by absorbed nutrients, but also by complex neural and hormonal factors that are activated by feeding. Changes in sympathetic and parasympathetic tone in the autonomic nervous system, which enervates the islets, are of critical importance. Likewise, the secretion of intestinally derived hormones, so-called incretins, such as glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are of paramount importance in the modulation of meal-induced insulin secretion. The intestinal incretin hormones augment glucose-dependent insulin secretion in response to the ingestion of nutrients. Therefore, there are multiple regulatory systems to ensure appropriate secretion of insulin and glucagon so as to maintain plasma glucose levels within a narrow range.

Adipose Tissue

There are two distinct types of adipose tissue, white and brown, which have entirely separate physiological functions. Brown adipose tissue resides in the central axis of the body and functions to dissipate energy and provide heat, i.e., brown adipose is a thermogenic organ. Brown adipose tissue plays a major role in the regulation of body temperature in small animals such as mice and rats, which have a high surface area to body volume and require hormonal regulation (adrenergic) of thermogenesis to maintain body temperature. In human infants, brown adipose tissue may provide a similar role. However, with aging, brown adipose atrophies, and thus the importance of brown adipose tissue in humans remains to be identified.

White adipose tissue is widely distributed throughout the body and serves as a major depot of stored energy in the form of triglycerides. The triglycerides are stored during periods of excess energy availability and are mobilized during periods of energy deprivation. Thus white adipose tissue is unique in that it has enormous potential for changes in volume (and hence mass). Over the long term, adipose mass reflects the net balance between energy expenditure and energy intake. Remarkably, total adipose mass remains relatively stable in most individuals. It is now evident that the hormone leptin, produced and secreted predominantly by white adipose tissue, plays a key role in signaling to the hypothalamic centers controlling food intake and energy expenditure to maintain this balance.

	REGULATION OF LEPTIN PRODUCTION BY INSULIN

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Because leptin is produced and secreted from adipocytes, and the relative body mass of most individuals is determined to a large extent by the fat mass, circulating leptin levels correlate directly with body mass index (BMI) (33, 36, 42, 77, 104, 118, 121). Those individuals with higher BMIs typically have a higher adipose tissue mass and therefore higher levels of circulating leptin. This direct relationship between BMI and plasma leptin levels has been verified by correlating circulating leptin to body fat content (42, 104, 118). It is worth noting that the regression between body weight or body fat content and circulating leptin is not complete, in the range of 0.7-0.8, with a correlation, or r², of 0.5-0.6 (3), indicating that other factors must also be involved in leptin regulation. Furthermore, plasma leptin levels and leptin gene expression increase at night and decrease acutely during fasting (33, 98, 118, 150, 160). These observations cannot be adequately accounted for by corresponding changes in body fat content. In this regard a role for nutrients, hormones, and neurotransmitters in the regulation of leptin expression and secretion is emerging. For example, leptin production by white adipose tissue is stimulated by insulin and cortisol and attenuated by -adrenergic agonists, cAMP, and thiazolidinediones.

Insulin has been considered to be a potential regulator of leptin, because plasma insulin concentrations decrease during fasting and increase after refeeding in parallel with plasma leptin levels. Current evidence suggests that insulin plays a chronic role in the regulation of leptin gene expression and production by white adipose tissue. Hyperinsulinemia increases plasma leptin levels and gene expression in white adipose tissue after many hours in rodents and humans (34, 99, 100, 119, 149, 150, 174, 176). Plasma leptin levels and leptin mRNA are also increased in subjects with insulinoma, and plasma leptin levels return to normal after removal of the insulinoma (35, 138). Insulin deficiency induced by streptozotocin results in diminished plasma leptin levels and leptin mRNA, and this suppression is rapidly reversed by treatment with insulin (11, 78, 86, 117, 134, 162). This action of insulin appears to be mediated, at least in part, directly at the level of adipocytes, because insulin also increases leptin secretion and mRNA levels in vitro (8, 64, 70, 99, 111, 129, 143, 148, 164, 177, 185), perhaps due to increased glucose transport and metabolism (126). A candidate transcription factor, adipocyte determination differentiation dependent factor 1/sterol regulatory element binding protein 1 (ADD1/SREBP1), linking changes in insulin levels to ob gene expression in mice, was recently reported (96). ADD1/SREBP1 expression increases upon treatment of adipocytes with insulin, and the increased ADD1/SREBP1 transactivates the leptin gene (96). Undoubtedly other transcription factors, including members of the C/EBP and PPAR families, are involved in the regulation of leptin gene expression by hormones such as insulin (47, 85). For additional reviews on the regulation of leptin production, see the following references (3, 31, 83, 85).

	RATIONALE FOR PROPOSING AN ADIPOINSULAR AXIS

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The hypothesis for the presence of an adipoinsular axis, or feedback loop (see Fig. 1) between adipose tissue and the endocrine pancreas (94), was generated on the basis of existing literature on the leptin-deficient or -resistant ob/ob or db/db mouse, respectively. Studies spanning three decades have reported that in both of these obese, diabetic mouse models, hyperinsulinemia precedes the development of both obesity and insulin resistance (22, 25, 30, 43, 55, 62, 63, 68, 114). It seemed reasonable that, if in the absence of a functional leptin signal, the first metabolic abnormality observed is hyperinsulinemia, then perhaps in normal circumstances leptin acts to curtail insulin release. Physiologically such a pathway would be important in the regulation of fat deposition. Insulin is a major adipogenic hormone. Therefore, as fat stores increase, rising plasma leptin concentrations would reduce insulin levels, thereby directing less energy to the formation of adipose tissue. In the event that adipose stores diminish, falling plasma leptin levels would permit increased insulin production, thereby resulting in the deposition of additional fat. Therefore, it was postulated that leptin might be considered as a thermostat or set point to appropriately regulate insulin levels according to the extent of adiposity (94). Such a putative feedback loop must be superimposed on the well-established enteroinsular axis, whereby insulin secretion is acutely regulated during meals by way of the secretion of intestinal incretin hormones.

View larger version (35K): [in this window] [in a new window]   Fig. 1.   The adipoinsular axis. It is well established that the fat-derived hormone leptin acts at the level of the hypothalamus to reduce appetite. There is growing evidence for a bidirectional feedback loop, or adipoinsular axis between adipose tissue and pancreatic -cells via the hormones leptin and insulin, respectively. Insulin is adipogenic and increases the production of leptin by adipose tissue. Leptin feeds back to reduce both insulin secretion and insulin gene expression. The suppressive effect of leptin on insulin production is mediated both by the autonomic nervous system (ANS) and by direct actions via leptin receptors on -cells.

In support of the hypothesis that leptin may directly suppress insulin release from pancreatic -cells are the observations that the administration of leptin for 5-30 days, either via injection, osmotic mini-pump, or gene therapy, lowers plasma insulin levels in ob/ob mice (112, 128, 135, 151, 166, 182). In a recent study, a single injection of leptin reduced plasma insulin levels in ob/ob mice by ~75% after only 24 h (152). Furthermore, a significant reduction in plasma insulin levels occurs within 10 min after the administration of leptin to ob/ob mice (103). This acute fall in plasma insulin levels is accompanied by a concomitant increase in plasma glucose levels. By 2 h after a leptin injection, the plasma glucose excursion after an oral glucose challenge was significantly increased relative to saline-treated animals (71). Therefore, the acute reduction in plasma insulin levels after leptin treatment does not appear to be secondary to either diminished body weight or a fall in plasma glucose concentrations. Notably, however, leptin treatment for longer periods of time appears to normalize plasma glucose levels in hyperglycemic rodents (69, 72, 112, 128, 135, 151, 152, 166, 182). This effect of leptin appears to be mediated by an increase in insulin sensitivity at the levels of the liver and peripheral tissues (9, 13, 26, 92, 147, 154, 163, 179). Thus reductions in circulating insulin levels after several hours or days of leptin treatment likely result from both increased insulin sensitivity and a direct leptin-mediated reduction in insulin secretion.

The autonomic nervous system likely contributes to the inhibitory effect of leptin on insulin secretion. The administration of leptin in vivo inhibits neuropeptide Y (NPY) gene expression (151, 166) and secretion (166), which in turn could result in reduced insulin secretion by either inhibiting the parasympathetic or activating the sympathetic nervous system. In support of this notion, insulin levels are reduced by one-half in ob/ob mice deficient in NPY (45). Direct evidence has been obtained for the regulation by leptin of sympathetic inputs to pancreatic -cells (123). In vagotomized rats, leptin significantly suppresses the increase in plasma insulin levels after an intravenous glucose injection. However, the effect of leptin in vagotomized rats is abolished after chemical sympathicectomy (123). Therefore, it appears that leptin can regulate insulin secretion through the autonomic nervous system. However, there is now abundant evidence to support the hypothesis that leptin also directly modulates insulin secretion by actions on leptin receptors on pancreatic -cells.

	DETECTION OF LEPTIN RECEPTORS IN ISLETS

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There are multiple isoforms of leptin receptors (60, 110, 170). Although the short isoform of the receptor (ObRa) is capable of signaling (15, 127), the long isoform (ObRb) is currently believed to convey most of the physiological actions of leptin. The db/db strain of mice has a genetic defect that results specifically in the absence of expression of ObRb, whereas other isoforms of the receptor are not altered (21, 27, 110). Given that the resultant phenotype closely resembles that of the leptin-deficient ob/ob mice, including the presence of hyperinsulinemia, it seems probable that the actions of leptin on pancreatic islets are mediated largely by ObRb. This concept is substantiated by the observation that leptin has no insulin-lowering action either in vivo or in vitro in db/db mice, whereas leptin lowers plasma insulin levels in ob/ob mice studied in parallel (44, 103, 151).

To support the hypothesis of direct actions of leptin on pancreatic -cells, evidence must be provided for the presence of leptin receptors on these cells. On the basis of results of tissue surveys, it is often argued that the only tissue where ObRb is found in high abundance is the hypothalamus, raising doubts as to the possibility of peripheral actions of leptin (60, 170). It is important to consider, however, that pancreatic islets comprise only 1-2% of the entire pancreas; the majority of the pancreas consists of an exocrine organ in which large quantities of digestive enzymes are synthesized. Therefore, pancreatic islet RNA represents a minor fraction of total pancreatic RNA. Yet of 21 tissues examined by sensitive RNase protection assay, total pancreas appears to have the most abundant proportion of ObRb mRNA relative to ObRa mRNA of all the peripheral tissues examined, and it is only second in amount to the brain and hypothalamus overall (11% ObRb vs. 18 and 36%, respectively) (66). Thus, just as the proportion of ObRb mRNA is considerably enriched in hypothalamic preparations compared with total brain, the same is true of islet ObRb mRNA compared with total pancreas. Messenger RNA for ObRb has been found in relatively high abundance in murine pancreatic islets but was not detected in whole pancreas by RNA blot hybridization because of the limited sensitivity of the technique used and the dilution of islet-specific RNA by exocrine pancreas RNA (44).

By use of a probe common to all leptin receptor isoforms, leptin receptor mRNA was detected in rat islets in greater abundance than was found in total brain (94). Leptin receptor mRNA was also observed by RT/PCR in rat islets and the tumor-derived -cell line TC3 (94). Expression of a functional protein was confirmed by the observation that TC3 cells bind ¹²⁵I-labeled leptin (94). These initial observations support the hypothesis of the existence of an adipoinsular axis. The presence of leptin receptors in pancreatic islets has since been confirmed by a variety of techniques. Although shorter isoforms of the leptin receptor may predominate, clearly ObRb is expressed in pancreatic -cells (44, 51, 89, 95, 101, 103, 107, 133, 136, 157, 169, 189). The distribution of leptin receptors within rat pancreatic islets was examined using antisera generated against an extracellular epitope of the leptin receptor. Leptin receptor immunoreactivity was observed on the majority of - and -cells, but not on glucagon-producing -cells (95). Similar results were obtained using leptin tagged with the fluorescent compound iodocarbocyanine (Cy3) (95). The function of leptin receptors expressed on somatostatin-secreting -cells remains to be determined.

	ACTIONS OF LEPTIN ON PANCREATIC ISLETS

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Insulin Secretion

The effects of leptin on insulin secretion have been studied in several different experimental settings, with variable and sometimes apparently conflicting results. The pancreatic islets (-cells) of ob/ob mice have been particularly useful in investigating the effect of leptin on insulin secretion, probably as a consequence of upregulation of the leptin receptor signaling in these leptin-deficient mice. In contrast, the -cells of normal rodents are chronically exposed to leptin and thus may be relatively less responsive under acute experimental conditions. The variable ways in which experiments have been performed often make the findings difficult to compare, and many uncertainties remain regarding optimal models and conditions. Other problematic issues remain, such as the considerable time delay for the action of leptin on -cells (76, 95) and uncertainties regarding the stability/bioactivity of various leptin solutions. Leptin circulates bound to one or more proteins, which may alter its bioavailability (40, 84, 161). In one study (103), it was suggested that the presence of bovine serum albumin or fetal calf serum may diminish or even obliterate the effects of leptin. Furthermore, it remains unclear why relatively high doses of leptin are required to induce weight loss (17, 69, 135, 166). In -cells, leptin appears to exhibit a biphasic dose-response curve. In rat islets ~2 nM leptin maximally suppresses insulin release, whereas high concentrations are ineffective (133, 189). Until these issues have been resolved, it will be difficult to reconcile all of the observations discussed below.

The application of leptin (6.25-100 nM) for 1-2 h to pancreatic islets isolated from ob/ob mice reduces insulin secretion from ~13 to almost 80% (44, 95). Leptin also significantly reduces insulin release from the perfused pancreas of ob/ob mice (44), a standard, reliable model to assess islet function. Notably, in ob/ob islets, the suppressive effect of leptin may be reduced at higher glucose levels and is absent in the presence of the incretin hormone GLP-1 (24, 95). One explanation for this observation is that leptin acts primarily during fasting to dampen insulin secretion and that this tonic inhibition is overcome by the nutrient and incretin signals (e.g., glucose and GLP-1) that accompany feeding (95). Leptin attenuates insulin release from ob/ob islets stimulated by either acetylcholine or the phorbol ester phorbol-12-myristate 13-acetate (PMA), both activators of protein kinase C (PKC) (24). At concentrations as low as 2.5 nM, leptin partially constrained acetylcholine-induced insulin secretion from islets of ob/ob mice, with an IC₅₀ (effective concentration) of ~5 nM. The finding that leptin curtails acetylcholine-induced insulin secretion may in part explain why islets from leptin-deficient ob/ob mice have enhanced sensitivity to PKC-stimulated insulin secretion (22, 23).

In contrast to -cells of leptin-deficient ob/ob mice, the -cells in normal rodents are continually exposed to leptin and therefore may be expected to be less sensitive. Overall, the reports regarding the effect of leptin on insulin secretion in normal rodents are conflicting. There were no significant effects of leptin (0.625-100 nM) on insulin release from isolated rat or mouse islets at glucose concentrations ranging from 1.7 to 16.7 mM (93, 136) or the perfused rat pancreas with 8.3 mM glucose and leptin at either 1.0 nM (107) or 10.0 nM (108, 109). In one study, leptin (1 nM) stimulated insulin release from isolated rat islets incubated in 5 mM glucose (169). However, in the majority of studies, leptin (1-20 nM) significantly reduced insulin release from the perfused rat pancreas and isolated rat or mouse islets in the presence of either low (2.8-3.3 mM) (51, 123), normal (5.5-8.0 mM) (87, 103), or high (10.0-20.0 mM) glucose (44, 51, 123, 132, 133, 144). There are no clear answers for these disparate observations.

Insulin secretion stimulated by agents that elevate cellular concentrations of cAMP, such as glucose + IBMX, a phosphodiesterase inhibitor, or forskolin and GLP-1, is inhibited by leptin during both static incubations and perifusions of mouse or rat islets (49, 136, 189). These observations suggest that in normal rodents, leptin antagonizes cAMP signaling. Leptin also suppresses the elevations of cellular cAMP levels that occur in response to exposure of -cells to GLP-1 or to forskolin (189). A mechanism proposed to explain this action of leptin on cAMP levels is a phosphoinositide 3-kinase (PI 3-kinase)-dependent activation of phosphodiesterase 3B (PDE3B) and subsequent reduction of the intracellular cAMP (189). Notably, leptin restrains insulin secretion from both mouse ob/ob islets (24) and rat islets (132) induced by PMA, a PKC activator. Therefore, it appears that leptin may antagonize insulin secretion from -cells by interacting with both cAMP-dependent protein kinase A (PKA) and PKC signaling pathways.

Studies of cultured human islets in vitro indicate that they respond to leptin in a manner similar to that of mouse and rat islets. Leptin suppresses insulin secretion from human islets at concentrations as low as 0.01 nM (48, 103, 153). Thus it appears from these findings in studies of human islets that humans are likely to have a functional adipoinsular axis.

The effects of leptin on insulin secretion have also been investigated in various -cell lines derived from islet cell tumors (insulinomas). The results obtained in different insulinoma cell lines have been contradictory and less consistent than those of studies in cultured islets and the perfused pancreas. In mouse MIN6 cells and hamster HIT-T15 cells, murine leptin increased insulin secretion (157, 169). In contrast, in other studies, leptin (2 nM) inhibited both glucose and GLP-1-stimulated insulin secretion from HIT-T15 (189), rat RIN5AH, rat RINm5F, and mouse TC6 cells (103). The inhibition of insulin secretion by leptin in TC6 cells was observed at 10 min and achieved a maximum by 60 min. In the rat insulinoma cell line INS-1, leptin significantly reduced insulin secretion stimulated by agents that increase intracellular content of cAMP (i.e., GLP-1), but not insulin secretion stimulated by activators of PKC (i.e., carbachol) (2, 49). These latter observations suggest that, in the INS-1 cell line, the cAMP-PKA signal transduction pathway is a target by which leptin may inhibit insulin secretion. A proliferative response to leptin was observed in RINm5F cells and mouse MIN6 cells, as measured by [³H]thymidine incorporation (89, 168). Notably, however, incubation of mouse islets with 100 nM murine leptin for 48 h had no effect on islet cell replication, as assessed by the same [³H]thymidine incorporation assay (93). Thus leptin responses in transformed cell lines may not necessarily reflect the response of native -cells.

Insulin Gene Expression

In a few studies, leptin had no effect on insulin biosynthesis (93, 144), and in one other study (157) leptin (unreported source) treatment increased preproinsulin mRNA levels in HIT-T15 cells. However, leptin has been shown to suppress preproinsulin mRNA expression in isolated rat islets (103, 133), in mouse TC6 cells (103), in ob/ob mouse islets, in the rat pancreatic -cell line INS-1 (152), and in human islets (153). When ob/ob mice are injected intraperitoneally with murine leptin (1 µg/g body weight), steady-state levels of preproinsulin mRNA are reduced by 40% after 24 h compared with vehicle-treated animals (152). Preproinsulin mRNA levels are also reduced by 50% in isolated ob/ob islets incubated with leptin (6.25 nM) for 24 h, suggesting that this action of leptin is mediated at least in part by a direct interaction with receptors on -cells (152). In studies of humans islets, human leptin (6.25 nM) evokes a time-dependent decrease in proinsulin mRNA levels in the presence of 11.1 mM glucose, but not 5.6 mM glucose (153); by 24 h, the level of proinsulin mRNA declines to 25% of the initial value. GLP-1 stimulates transcription of the rat insulin gene and increases proinsulin mRNA levels in -cells (41, 50). Remarkably, leptin (6.25 nM) inhibits GLP-1-stimulated expression of preproinsulin mRNA in human islets incubated in conditions of both 5.6 and 11.1 mM glucose (153). At 16 h, GLP-1-stimulated preproinsulin mRNA levels are inhibited by 80%.

Incubation of INS-1 cells with leptin (0.625 nM or 6.25 nM) for 16 h significantly reduces preproinsulin mRNA expression when incubated at 25 mM glucose, but not 5.6 or 11.1 mM glucose (152). These findings indicate a dependence of leptin-mediated reductions of preproinsulin mRNA expression on glucose augmentation. However, as with human islets, the stimulation of preproinsulin mRNA expression by GLP-1 (10 nM) is also inhibited by leptin (6.25 nM) at both 5.6 and 11.1 mM glucose (152). Of note, there was no effect of leptin on steady-state levels of preproinsulin mRNA after a 6-h incubation.

The effect of leptin on the transcriptional activity of the insulin gene promoter has been examined. A reporter vector expressing the luciferase gene under control of 410 bp of the rat insulin I gene promoter was transfected into INS-1 cells. Leptin (6.25 nM) inhibited reporter gene expression at 25 mM glucose by 50%, but no such inhibition was observed at 5.6 or 11.1 mM glucose (152). However, the induction of transcriptional activity by GLP-1 (10 nM) in the presence of 11.1 mM glucose was nearly completely inhibited by leptin. These observations indicate that activation of transcription by either GLP-1 or high glucose (25 mM) restores the inhibitory actions of leptin on rat insulin I promoter activity at a glucose concentration (11.1 mM) in which leptin alone fails to reduce insulin promoter activity (152).

	MECHANISM OF LEPTIN ACTION ON -CELLS

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The leptin receptor gene encodes multiple splice variants of the leptin receptor (ObRa-ObRf), which closely resemble the class I cytokine receptors (110, 170, 171). The longest form of the leptin receptor, ObRb, contains intracellular consensus motifs for interaction with cytoplasmic protein kinases, known as JAKs (Janus kinase), and protein transcription factor kinase substrates of the signal transducers and activators of transcription (STAT) family. Leptin appears to interact with homo-oligomerized ObRb (10, 183, 184), resulting in activation of JAK2 (65), which phosphorylates a number of substrates, including ObRb. When phosphorylated, ObRb serves as a scaffold for recruitment of STATS (10, 15, 66, 175). STATS themselves are also substrates for JAK, and when activated, either homo- or heterodimerize and translocate to the nucleus, where they modulate the transcription of target genes (37). STATS 1, 3, 5, and 6 are all reportedly activated by leptin (10, 66, 125, 146, 158, 175). Signaling capabilities have also been reported for short (ObRa) isoforms of the leptin receptor (15, 127). However, because leptin has no effect on islets isolated from db/db mice, which harbor mutations resulting in aberrant mRNA that encodes only a shortened form of ObRb (21, 110), the direct effects of leptin on -cells are likely mediated via ObRb.

A brief review of intracellular signaling pathways within pancreatic -cells is required to interpret possible pathways involved in leptin signaling. Insulin secretion from pancreatic -cells is controlled in part by the activity of K_ATP channels. Closure (inactivation) of K_ATP channels in response to glucose or other insulin secretagogues depolarizes -cells, resulting in the activation of voltage-dependent Ca²⁺ channels, a rise in cytosolic calcium concentration ([Ca²⁺]_i), and insulin secretion (4). Glucose metabolism within -cells also results in an elevation of fatty acyl-CoA esters, which synergize with rises in [Ca²⁺]_i to cause induction of insulin release (139). Glucose-induced insulin secretion is further potentiated by hormone-mediated elevation of the intracellular second messengers cAMP/PKA (81) and phospholipase C/PKC (186). Thus there are multiple potential sites for leptin signaling to influence insulin secretion.

Clues to the cellular mechanisms by which leptin suppresses insulin release come from earlier studies utilizing ob/ob and db/db mouse islets. Notably, over two decades ago it was reported that islets from db/db mice are partially depolarized, even in the absence of glucose (122). This circumstance is associated with elevated basal insulin release and unresponsiveness to further elevation of the glucose concentration (122). The K⁺ permeability of both ob/ob and db/db islets is relatively insensitive to changes in glucose (12, 59, 145), yet this insensitivity does not appear because of a failure of the cells to express functional K_ATP channels (58, 102). Because these phenotypes both result from the absence of leptin signaling, it was postulated that leptin may normally open (activate) K_ATP channels on -cells and thereby hyperpolarize the membrane. Such a mechanism would be consistent with the observations that leptin reduces insulin secretion (95).

Patch-clamp electrophysiological analyses on both normal and ob/ob mouse -cells show that the application of leptin (1-10 nM) to single mouse -cells (95) or the CRI-G1 cells (75, 76) consistently results in hyperpolarization, consistent with an inhibitory action of leptin on insulin secretion. The effect of leptin is delayed by 10-15 min, similar to that observed previously for potassium channel activation by janus kinase-associated prolactin receptors in Chinese hamster ovary cells (140). Leptin also increases membrane conductance, consistent with the opening (activation) of ATP-sensitive K channels (K_ATP channels). The activation of K_ATP channels by leptin is further supported by the observations that the leptin response is sensitive to the application of the sulphonylureas tolbutamide and glibenclamide, selective blockers of K_ATP channels. Leptin-induced increases in membrane conductance and the resulting hyperpolarization are immediately and completely reversed by the application to -cells of tolbutamide (100 µM) or glibenclamide (0.5 µM) (73, 75, 76, 95). Notably, in single ob/ob mouse -cells, the increase in conductance is biphasic in nature (95). The reversal potential associated with the increase in cell conductance in response to leptin is characteristic of the activation of a potassium current (75, 76, 95).

A confirmation of the K_ATP channel as the molecular target for action of leptin in -cells was obtained from single-channel recordings. Application of leptin (1-10 nM) induces an increase in the activity of single potassium channels that are sensitive to ATP and sulphonylureas (76, 95). Leptin increases the K_ATP open channel probability with no significant effect on open times (95). Notably, the activation of K_ATP channels in CRI-G1 cells by leptin is reversed by nanomolar concentrations of insulin (74). This effect is specific for leptin, because insulin fails to prevent opening of K_ATP channels by diazoxide (74). These observations suggest that leptin suppresses insulin secretion during fasting, when ambient insulin levels are low (74). However, during feeding, the resultant increases in insulin levels inhibit the opening of K_ATP channels by leptin and prevent the inhibition of insulin secretion. Such a model could explain in part the apparent resistance to leptin actions observed in obese individuals who are typically hyperinsulinemic, even during fasting.

Given the important role of K_ATP channels in determining [Ca²⁺]_i in -cells, it is anticipated that application of leptin to -cells results in a decrease in [Ca²⁺]_i. A fall in [Ca²⁺]_i occurs in both mouse and human single -cells after application of leptin (6.25 nM) (95, 153). Notably, this fall in [Ca²⁺]_i is reversed by co-incubation of the -cells with 20 mM glucose and GLP-1 (10 nM) (95). A 15-min preincubation of INS-1 cells with leptin reduces the subsequent rise in [Ca²⁺]_i in response to either forskolin (1 µM) or GLP-1 (10 nM) (49). Leptin (10 nM) also attenuated glucose-induced elevations in [Ca²⁺]_i in mouse islets (51). Of note, leptin also hyperpolarizes glucose-receptive hypothalamic neurons via opening (activation) of K_ATP channels (165). Therefore, the K_ATP channel may function as the molecular end point of the leptin signaling pathway in both hypothalamic neurons and pancreatic -cells.

By what mechanisms might leptin open K_ATP channels and thereby lower [Ca²⁺]_i and inhibit insulin secretion? Dialysis with AMP-PNP, a nonhydrolyzable analog of ATP that does not serve as a substrate for protein kinases, prevents leptin activation of K_ATP channels, indicating that phosphorylation or dephosphorylation is involved (75). The serine/threonine-specific protein phosphatase inhibitors okadaic acid (50 nM) and cyclosporin A (1 µM) fail to block activation of K_ATP channels by leptin (75). In contrast, whole cell dialysis with the tyrosine phosphatase inhibitor orthovanadate (500 µM) prevents the actions of leptin, suggesting that protein phosphorylation of tyrosine, but not serine/threonine, residues underlies actions of leptin in CRI-G1 cells. Furthermore, inhibition of tyrosine kinase activity by application of genistein (10 µM), tyrphostin B42 (10 µM), or herbimycin A (500 nM) mimics the leptin-induced hyperpolarization and increase in K⁺ conductance (75). Therefore, the mechanism underlying the opening of K_ATP channels by leptin may involve inhibition of tyrosine kinases or activation of tyrosine phosphatases and subsequent tyrosine dephosphorylation. Because the opening of K_ATP channels by leptin is blocked by wortmannin (1-10 nM) and LY-294002 (10 µM), inhibitors of PI 3-kinase, the mechanism underlying leptin activation of K_ATP channels appears to involve dephosphorylation of cytosolic proteins before activation of PI 3-kinase (74).

Further support for the concept that leptin activates PI 3-kinase is provided by the findings that treatment of insulinoma HIT-T15 cells with murine leptin (2 nM) for 30 min results in a threefold activation of PI 3-kinase (189). PDE3B, which reduces the cellular content of cAMP, is also activated by leptin. Thus, in HIT-T15 cells, leptin (1-5 nM) suppresses the elevation of cAMP induced by GLP-1 (5 nM) (189). This suppression is a PI 3-kinase-dependent process, because the addition of the PI 3-kinase inhibitors wortmannin (20 nM) or LY-294002 (2 µM) abolishes the activation of PDE3B by leptin and the inhibitory effect of leptin on glucose- and GLP-1-stimulated insulin secretion from the HIT cells (189). Furthermore, treatment of HIT cells with either milrinone (1 µM) or enoximone (1 µM), selective inhibitors of type 3 PDE, completely blocked the inhibitory effect of leptin on glucose- or GLP-1-potentiated insulin secretion. These findings suggest that the inhibitory actions of leptin on insulin secretion are primarily mediated through the PI 3-kinase-dependent activation of PDE3B and a subsequent reduction of the intracellular cAMP (189). Because cAMP potentiates insulin secretion in part by closing (inactivating) K_ATP channels (81, 82), a reduction of cAMP by leptin is consistent with the activation of K_ATP channels by leptin.

Another mechanism by which leptin may open K_ATP channels is through lipid metabolism in -cells. It has been proposed that leptin increases the storage of triglycerides in adipocytes to maintain a constant level of intracellular triglycerides in nonadipocytes, such as -cells (173). Culture of isolated islets with leptin depletes the content of triglycerides and increases the oxidation of free fatty acids (101, 156). This lipopenic action of leptin is not observed in islets isolated from leptin-resistant fa/fa Zucker diabetic fatty rats but is reestablished upon the restoration of expression of the leptin receptor (ObRb) in the islets (155, 180, 181). This action of leptin appears to be dependent on formation of fatty acyl-CoA. Remarkably, long-chain acyl-CoA esters (LC-CoA), the metabolically active form of free fatty acids, bind to and open K_ATP channels in pancreatic -cells (16, 105). Therefore, it is possible that elevations in LC-CoA within -cells after exposure to leptin result in activation of K_ATP channels and thereby inhibition of insulin secretion.

The inhibitory effects of leptin on preproinsulin gene expression appear to be independent of the activation of K_ATP channels. Diazoxide, which also opens K_ATP channels, has no effect itself on preproinsulin mRNA levels in INS-1 cells and does not alter the suppressive effect of leptin (152). Furthermore, inhibition of insulin promoter activity in INS-1 cells at 25 mM glucose was detected in both the presence and absence of K_ATP channel activation by diazoxide (152). These findings indicate that inhibition of preproinsulin gene expression by leptin is independent of the opening of K_ATP channels and suggests that the molecular mechanisms underlying inhibition of insulin secretion and preproinsulin mRNA by leptin are mediated by different intracellular signaling pathways.

Activation of STAT proteins in -cells after leptin treatment is clearly a candidate pathway for the mechanism of action of leptin on insulin gene expression. Treatment of RINm5F cells and isolated rat islets with leptin induces the binding of STATs to a labeled oligonucleotide containing a high-affinity binding element for STAT1 and STAT3 (124). This binding is not altered by treatment with plasminogen activator (PMA, a PKC activator; 100 nM) but is reduced by treatment with acetylcholine (10 µM), ionomycin (calcium ionophore; 1 µg/ml), forskolin (adenylyl cyclase activator; 10 µM), and IBMX (cAMP phosphodiesterase inhibitor; 50 µM). The activation of STAT3 is suggested by the observation that treatment of rat pancreatic islets with leptin produces an increase in the tyrosine phosphorylation of STAT3 (124). Similarly, in MIN6 cells, leptin (5 nM) increases tyrosine phosphorylation of STAT1 and STAT3, but not STAT5 (168). Leptin also increases the transcriptional activation of reporter plasmids containing STAT3 binding elements, but not the STAT5 consensus element, in RINm5F cells (124). These observations implicate STAT3 as a potential mechanism by which leptin regulates gene expression in -cells. Furthermore, the STAT3 pathway appears to be antagonized by elevations of [Ca²⁺]_i and cAMP.

That STAT proteins may mediate the inhibition of insulin gene expression after leptin treatment was shown by DNA-binding assays in which sequences between 307 and 410 bp of the rat insulin I gene promoter bind multiple protein complexes contained in nuclear extracts from leptin-treated ob/ob islets. One of the complexes contained STAT5b, which formed on a previously described consensus STAT binding site (152). Although STAT proteins are generally considered to be activators of gene transcription, they may also be inhibitory, depending on the promoter context and cell type. For example, STAT5b stimulated by prolactin induces the -casein promoter but inhibits the interferon regulatory factor-1 promoter (116).

In addition to activating a STAT pathway within pancreatic -cells, the leptin receptor appears to be coupled to a mitogen-activated protein kinase (MAPK) pathway in MIN6 and RINm5F cells (124, 168). The phosphorylation of MAPK in response to leptin was observed with 0.3 nM leptin and was maximal with 5 nM leptin (168). However, the activation of MAPK by leptin may be limited to tumor-derived -cell lines, because treatment of rat islets with concentrations of leptin up to 100 nM did not result in activation of MAPK (124). Please see Fig. 2 for a summary of leptin signaling pathways within pancreatic -cells.

View larger version (25K): [in this window] [in a new window]   Fig. 2.   Leptin signaling in pancreatic -cells. Glucose enters the -cell via the GLUT-2 transporter and is metabolized, resulting in generation of ATP. Change in the ADP/ATP ratio closes the ATP-sensitive K+ channel (KATP), and the resultant decrease in efflux of K+ leads to depolarization of the -cell with the consequent opening of voltage-dependent calcium channels (VDCC) and influx of Ca2+ down its concentration gradient. The Ca2+ binds calmodulin and activates Ca2+/calmodulin-dependent protein kinase (CaMK). Subsequent protein phosphorylation ultimately results in insulin secretion. Glucose-induced insulin secretion is potentiated by other insulin secretagogues. Hormones such as glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) bind to cell surface receptors that are coupled via the heterotrimeric G protein Gs to adenylyl cyclase (AC). Receptor occupancy activates AC, resulting in the generation of cAMP from ATP. The cAMP activates protein kinase A (PKA), which phosphorylates protein substrates, leading to insulin secretion. Other cell surface receptors (e.g., acetylcholine, Ach; cholecystokinin, CCK) are coupled via the heterotrimeric G protein Gq to phospholipase C (PLC). Receptor occupancy activates PLC and generates inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) from the hydrolysis of phosphatidylinositol 4,5-biphosphate (PIP2). DAG activates protein kinase C (PKC), while IP3 mobilizes Ca2+ release from the endoplasmic reticulum. There is now evidence to suggest that leptin may interact at multiple points within the -cell signaling pathways (dashed lines). Binding of leptin to its receptor may result in activation of the janus kinase/signal transducer activation of transcription (JAK/STAT) pathway and a mitogen-activated protein kinase (MAPK) pathway, which act to inhibit preproinsulin gene transcription within the nucleus. Leptin also appears to act via a phosphoinositide 3-kinase (PI 3-kinase) pathway to activate phosphodiesterase 3B (PDE3B), which can reduce intracellular levels of cAMP, generated by, for example, GIP or GLP-1. Leptin may also alter the phosphorylation of proteins resulting from activation of either CaMK, PKC, or PKA. Finally, the KATP channel, which is closed in response to glucose, appears to be opened by leptin, with the net result of inhibition of insulin secretion.

	CONCLUDING REMARKS

TOP ABSTRACT OVERVIEW REGULATION OF LEPTIN PRODUCTION... RATIONALE FOR PROPOSING AN... DETECTION OF LEPTIN RECEPTORS... ACTIONS OF LEPTIN ON... MECHANISM OF LEPTIN ACTION... CONCLUDING REMARKS REFERENCES

Leptin is an important controller of food intake and energy expenditure by its actions on receptors located in regions of the hypothalamus that regulate feeding behavior. There is now growing evidence that leptin also acts to suppress insulin production from pancreatic -cells. Because insulin is adipogenic and increases the expression of leptin, there is a bidirectional feedback loop between adipose tissue and pancreatic islets, termed the adipoinsular axis. The majority of evidence in support of an adipoinsular axis has come from studies in rodents or cell lines, but there is also evidence to suggest that a similar pathway exists in humans. Given the well-known direct relationship between increased adiposity (leptin) and increased insulin levels (7, 88, 137, 167), it has been questioned whether leptin inhibits insulin release in humans (18). If it did, why would obese hyperleptinemic individuals be hyperinsulinemic?

It is generally believed that hyperinsulinemia is simply a compensatory response to insulin resistance (20, 38, 39, 53, 67, 90, 91, 131, 141, 142, 172). Hyperglycemia is believed to occur when -cells can no longer compensate adequately for insulin resistance, resulting in the development of diabetes. However, many recent studies challenge this dogma because of the observations that hyperinsulinemia appears to precede decreases in insulin sensitivity and perhaps even the development of obesity in humans, thus arguing against the belief that obesity-induced insulin resistance fully explains the development of type 2 diabetes (54, 106, 130, 159). Hyperinsulinemia promotes adipogenesis and may initiate a cycle of increasing insulin resistance, compensatory hyperinsulinemia, and a progression to the diabetic state.

In obese individuals, sustained elevated levels of plasma leptin are proposed to uncouple leptin actions on its receptors in the hypothalamus, thereby attenuating signal transduction pathways that exert the effects of the hormone on satiety and energy expenditure (14, 57). Given that leptin suppresses insulin secretion by autonomic outputs originating in hypothalamic circuitry, breakdown of the central leptin signal may contribute to hyperinsulinemia. By analogy to leptin actions on the hypothalamus, in conditions of increasing adiposity and prolonged elevated plasma leptin levels, the leptin signaling system in pancreatic -cells may become unresponsive. Such a putative loss of leptin reception by -cells could result in dysregulation of the adipoinsular axis and a corresponding failure to suppress insulin secretion, resulting in chronic hyperinsulinemia. Additionally, the hyperinsulinemia exacerbates the obesity by increasing adipogenesis and increasing leptin production. This positive feedback loop of leptin desensitization at both the hypothalamus and pancreatic -cells may result in hyperphagia and hyperinsulinemia, respectively, and thus may be an important factor in the pathogenesis of obesity-associated diabetes mellitus. Furthermore, it has recently been proposed that such dysregulation of the adipoinsular axis may occur independently of obesity in subjects who are susceptible to the development of diabetes (88). Thus investigation of the adipoinsular axis dysregulation hypothesis may lead to the identification of determinants of both human obesity and diabetes.

	ACKNOWLEDGEMENTS

We are grateful to Townley Budde for assistance in the preparation of this manuscript. T. J. Kieffer acknowledges support from the Alberta Heritage Foundation for Medical Research, the Medical Research Council of Canada, and the Canadian Diabetes Association. J. F. Habener is an Investigator with the Howard Hughes Medical Institute.

	FOOTNOTES

Please address correspondence and reprint requests to either T. J. Kieffer, Div. of Endocrinology and Metabolism, 370 Heritage Medical Research Centre, Univ. of Alberta, Edmonton, AB, Canada T6G 2S2 (E-mail: tim.kieffer{at}ualberta.ca); or J. F. Habener, Laboratory of Molecular Endocrinology, Massachusetts General Hospital, 55 Fruit St., WEL320, Boston, MA 02114 (E-mail: jhabener{at}partners.org).

	REFERENCES

TOP ABSTRACT OVERVIEW REGULATION OF LEPTIN PRODUCTION... RATIONALE FOR PROPOSING AN... DETECTION OF LEPTIN RECEPTORS... ACTIONS OF LEPTIN ON... MECHANISM OF LEPTIN ACTION... CONCLUDING REMARKS REFERENCES

1.   Ahima, R. S., D. Prabakaran, C. Mantzoros, D. Qu, B. Lowell, E. Maratos-Flier, and J. S. Flier. Role of leptin in the neuroendocrine response to fasting. Nature 382: 250-252, 1996[Medline].

2.   Ahrén, B., and P. J. Havel. Leptin inhibits insulin secretion induced by cellular cAMP in INS-1 cells. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 277: R959-R966, 1999[Abstract/Free Full Text].

3.   Ahren, B., H. Larsson, C. Wilhelmsson, B. Nasman, and T. Olsson. Regulation of circulating leptin in humans. Endocrine 7: 1-8, 1997[Medline].

4.   Ashcroft, F. M., and P. Rorsman. Electrophysiology of the pancreatic -cell. Prog. Biophys. Molec. Biol. 54: 87-143, 1989[Medline].

5.   Auwerx, J., and B. Staels. Leptin. Lancet 351: 737-742, 1998[Web of Science][Medline].

6.   Bado, A., S. Levasseur, S. Attoub, S. Kermorgant, J. P. Laigneau, M. N. Bortoluzzi, L. Moizo, T. Lehy, M. Guerre-Millo, Y. Le Marchand-Brustel, and M. J. Lewin. The stomach is a source of leptin. Nature 394: 790-793, 1998[Medline].

7.   Bagdade, J. D. Basal insulin and obesity. Lancet 2: 630-631, 1968[Web of Science][Medline].

8.   Barr, V. A., D. Malide, M. J. Zarnowski, S. I. Taylor, and S. W. Cushman. Insulin stimulates both leptin secretion and production by rat white adipose tissue. Endocrinology 138: 4463-4472, 1997[Abstract/Free Full Text].

9.   Barzilai, N., J. Wang, D. Massilon, P. Vuguin, M. Hawkins, and L. Rossetti. Leptin selectively decreases visceral adiposity and enhances insulin action. J. Clin. Invest. 100: 3105-3110, 1997[Web of Science][Medline].

10.   Baumann, H., K. K. Morella, D. W. White, M. Dembski, P. S. Bailon, H. Kim, C. F. Lai, and L. A. Tartaglia. The full-length leptin receptor has signaling capabilities of interleukin 6-type cytokine receptors. Proc. Natl. Acad. Sci. USA 93: 8374-8378, 1996[Abstract/Free Full Text].

11.   Becker, D. J., L. N. Ongemba, V. Brichard, J. C. Henquin, and S. M. Brichard. Diet- and diabetes-induced changes of ob gene expression in rat adipose tissue. FEBS Lett. 371: 324-328, 1995[Web of Science][Medline].

12.   Berglund, O., J. Sehlin, and I.-B. Täljedal. ⁸⁶Rb⁺ fluxes and K⁺-stimulated nitro-phenyl phosphatase activity in the pancreatic islets of genetically diabetic mice (C57BL/KsJ-db/db). Diabetologia 15: 191-195, 1978[Web of Science][Medline].

13.   Berti, L., M. Kellerer, E. Capp, and H. U. Haring. Leptin stimulates glucose transport and glycogen synthesis in C2C12 myotubes: evidence for a PI3-kinase mediated effect. Diabetologia 40: 606-609, 1997[Web of Science][Medline].

14.   Bjorbaek, C., J. K. Elmquist, J. D. Frantz, S. E. Shoelson, and J. S. Flier. Identification of SOCS-3 as a potential mediator of central leptin resistance. Mol. Cell 1: 619-625, 1998[Web of Science][Medline].

15.   Bjorbaek, C., S. Uotani, B. da Silva, and J. S. Flier. Divergent signaling capacities of the long and short isoforms of the leptin receptor. J. Biol. Chem. 272: 32686-32695, 1997[Abstract/Free Full Text].

16.   Branstrom, R., B. E. Corkey, P. O. Berggren, and O. Larsson. Evidence for a unique long chain acyl-CoA ester binding site on the ATP-regulated potassium channel in mouse pancreatic beta cells. J. Biol. Chem. 272: 17390-17394, 1997[Abstract/Free Full Text].

17.   Campfield, L. A., F. J. Smith, Y. Guisez, R. Devos, and P. Burn. Recombinant mouse OB protein: evidence for a peripheral signal linking adiposity and central neural networks. Science 269: 546-549, 1995[Abstract/Free Full Text].

18.   Carantoni, M., F. Abbasi, S. Azhar, Y. D. Chen, M. Klebanov, P. W. Wang, F. Warmerdam, and G. M. Reaven. Plasma leptin concentrations do not appear to decrease insulin-mediated glucose disposal or glucose-stimulated insulin secretion in women with normal glucose tolerance. Diabetes 47: 244-247, 1998[Abstract].

19.   Caro, J. F., M. K. Sinha, J. W. Kolaczynski, P. L. Zhang, and R. V. Considine. Leptin: the tale of an obesity gene. Diabetes 45: 1455-1462, 1996[Web of Science][Medline].

20.   Cerasi, E. Insulin deficiency and insulin resistance in the pathogenesis of NIDDM: is a divorce possible? Diabetologia 38: 992-997, 1995[Web of Science][Medline].

21.   Chen, H., O. Charlat, L. A. Tartaglia, E. A. Woolf, X. Weng, S. J. Ellis, N. D. Lakey, J. Culpepper, K. J. Moore, R. E. Breitbart, G. M. Duyk, R. I. Tepper, and J. P. Morgenstern. Evidence that the diabetes gene encodes the leptin receptor: identification of a mutation in the leptin receptor gene in db/db mice. Cell 84: 491-495, 1996[Web of Science][Medline].

22.   Chen, N.-G., and D. R. Romsos. Enhanced sensitivity of pancreatic islets from preobese 2-wk-old ob/ob mice to neurohormonal stimulation of insulin secretion. Endocrinology 136: 505-511, 1995[Abstract].

23.   Chen, N.-G., and D. R. Romsos. Persistently enhanced sensitivity of pancreatic islets from ob/ob mice to PKC-stimulated insulin secretion. Am. J. Physiol. Endocrinol. Metab. 272: E304-E311, 1997[Abstract/Free Full Text].

24.   Chen, N. G., A. G. Swick, and D. R. Romsos. Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice. J. Clin. Invest. 100: 1174-1179, 1997[Web of Science][Medline].

25.   Chick, W. L., R. L. Lavine, and A. A. Like. Studies in the diabetic mutant mouse: V. Glucose tolerance in mice homozygous and heterozygous for the diabetes (db) gene. Diabetologia 6: 257-262, 1970[Web of Science][Medline].

26.   Chinookoswong, N., J. L. Wang, and Z. Q. Shi. Leptin restores euglycemia and normalizes glucose turnover in insulin-deficient diabetes in the rat. Diabetes 48: 1487-1492, 1999[Abstract].

27.   Chua, S. C., Jr., W. K. Chung, X. S. Wu-Peng, Y. Zhang, S. M. Liu, L. Tartaglia, and R. L. Leibel. Phenotypes of mouse diabetes and rat fatty due to mutations in the OB (leptin) receptor. Science 271: 994-996, 1996[Abstract].

28.   Coleman, D. L. Obese and diabetes: two mutant genes causing diabetes-obesity syndromes in mice. Diabetologia 14: 141-148, 1978[Web of Science][Medline].

29.   Coleman, D. L. Diabetes-obesity syndromes in mice. Diabetes 31, Suppl.1: 1-6, 1982[Abstract].

30.   Coleman, D. L., and K. P. Hummel. Hyperinsulinemia in pre-weaning diabetes (db) mice. Diabetologia 10: 607-610, 1974.

31.   Coleman, R. A., and T. S. Herrmann. Nutritional regulation of leptin in humans. Diabetologia 42: 639-646, 1999[Web of Science][Medline].

32.   Considine, R. V., and J. F. Caro. Leptin and the regulation of body weight. Int. J. Biochem. Cell Biol. 29: 1255-1272, 1997[Web of Science][Medline].

33.   Considine, R. V., M. K. Sinha, M. L. Heiman, A. Kriauciunas, T. W. Stephens, M. R. Nyce, J. P. Ohannesian, C. C. Marco, L. J. McKee, T. L. Bauer, and J. F. Caro. Serum immunoreactive-leptin concentrations in normal-weight and obese humans. N. Engl. J. Med. 334: 292-295, 1996[Abstract/Free Full Text].

34.   Cusin, I., A. Sainsbury, P. Doyle, F. Rohner-Jeanrenaud, and B. Jeanrenaud. The ob gene and insulin. A relationship leading to clues to the understanding of obesity. Diabetes 44: 1467-1470, 1995[Abstract].

35.   D'Adamo, M., A. Buongiorno, E. Maroccia, F. Leonetti, F. Barbetti, A. Giaccari, D. Zorretta, G. Tamburrano, and P. Sbraccia. Increased OB gene expression leads to elevated plasma leptin concentrations in patients with chronic primary hyperinsulinemia. Diabetes 47: 1625-1629, 1998[Abstract].

36.   Dagogo-Jack, S., C. Fanelli, D. Paramore, J. Brothers, and M. Landt. Plasma leptin and insulin relationships in obese and nonobese humans. Diabetes 45: 695-698, 1996[Abstract].

37.   Darnell, J. E., Jr. Reflections on STAT3, STAT5, and STAT6 as fat STATs. Proc. Natl. Acad. Sci. USA 93: 6221-6224, 1996[Abstract/Free Full Text].

38.   DeFronzo, R. A. Pathogenesis of type 2 (non-insulin dependent) diabetes mellitus: a balanced overview. Diabetologia 35: 389-397, 1992[Web of Science][Medline].

39.   DeFronzo, R. A., and E. Ferrannini. Insulin resistance. A multifaceted syndrome responsible for NIDDM, obesity, hypertension, dyslipidemia, and atherosclerotic cardiovascular disease. Diabetes Care 14: 173-194, 1991[Abstract].

40.   Diamond, F. B., Jr., D. C. Eichler, G. Duckett, E. V. Jorgensen, D. Shulman, and A. W. Root. Demonstration of a leptin binding factor in human serum. Biochem. Biophys. Res. Commun. 233: 818-822, 1997[Web of Science][Medline].

41.   Drucker, D. J., J. Philippe, S. Mojsov, W. L. Chick, and J. F. Habener. Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line. Proc. Natl. Acad. Sci. USA 84: 3434-3438, 1987[Abstract/Free Full Text].

42.   Dua, A., M. I. Hennes, R. G. Hoffmann, D. L. Maas, G. R. Krakower, G. E. Sonnenberg, and A. H. Kissebah. Leptin: a significant indicator of total body fat but not of visceral fat and insulin insensitivity in African-American women. Diabetes 45: 1635-1637, 1996[Abstract].

43.   Dubuc, P. U. The development of obesity, hyperinsulinemia, and hyperglycemia in ob/ob mice. Metabolism 25: 1567-1574, 1976[Web of Science][Medline].

44.   Emilsson, V., Y. L. Liu, M. A. Cawthorne, N. M. Morton, and M. Davenport. Expression of the functional leptin receptor mRNA in pancreatic islets and direct inhibitory action of leptin on insulin secretion. Diabetes 46: 313-316, 1997[Abstract].

45.   Erickson, J. C., G. Hollopeter, and R. D. Palmiter. Attenuation of the obesity syndrome of ob/ob mice by the loss of neuropeptide Y. Science 274: 1704-1707, 1996[Abstract/Free Full Text].

46.   Esler, M., M. Vaz, G. Collier, P. Nestel, G. Jennings, D. Kaye, D. Seals, and G. Lambert. Leptin in human plasma is derived in part from the brain, and cleared by the kidneys. Lancet 351: 879, 1998[Web of Science][Medline].

47.   Fajas, L., J.-C. Fruchart, and J. Auwerx. Transcriptional control of adipogenesis. Curr. Opin. Cell. Biol. 10: 165-173, 1998[Web of Science][Medline].

48.   Fehmann, H. C., P. Berghofer, D. Brandhorst, H. Brandhorst, B. Hering, R. G. Bretzel, and B. Goke. Leptin inhibition of insulin secretion from isolated human islets. Acta Diabetol. 34: 249-252, 1997[Web of Science][Medline].

49.   Fehmann, H. C., H. P. Bode, T. Ebert, A. Karl, and B. Goke. Interaction of GLP-I and leptin at rat pancreatic B-cells: effects on insulin secretion and signal transduction. Horm. Metab. Res. 29: 572-576, 1997[Web of Science][Medline].

50.   Fehmann, H. C., and J. F. Habener. Insulinotropic hormone glucagon-like peptide-I(737) stimulation of proinsulin gene expression and proinsulin biosynthesis in TC-1 insulinoma cells. Endocrinology 130: 159-166, 1992[Abstract/Free Full Text].

51.   Fehmann, H. C., C. Peiser, H. P. Bode, M. Stamm, P. Staats, C. Hedetoft, R. E. Lang, and B. Goke. Leptin: a potent inhibitor of insulin secretion. Peptides 18: 1267-1273, 1997[Web of Science][Medline].

52.   Fei, H., H. J. Okano, C. Li, G. H. Lee, C. Zhao, R. Darnell, and J. M. Friedman. Anatomic localization of alternatively spliced leptin receptors (Ob-R) in mouse brain and other tissues. Proc. Natl. Acad. Sci. USA 94: 7001-7005, 1997[Abstract/Free Full Text].

53.   Ferrannini, E. Insulin resistance versus insulin deficiency in non-insulin-dependent diabetes mellitus: problems and prospects. Endocr. Rev. 19: 477-490, 1998[Abstract/Free Full Text].

54.   Ferrannini, E., A. Natali, P. Bell, P. Cavallo-Perin, N. Lalic, G. Mingrone, and the European Group for the Study of Insulin Resistance (EGIR). Insulin resistance and hypersecretion in obesity. J. Clin. Invest. 100: 1166-1173, 1997[Web of Science][Medline].

55.   Flatt, P. R., C. J. Bailey, A. M. Cameron, and B. J. Gould. Age effects on glycosylated blood proteins in lean and obese hyperglycaemic (ob/ob) mice. Diabetes Res. 3: 241-243, 1986[Web of Science][Medline].

56.   Flier, J. S. Leptin expression and action: new experimental paradigms. Proc. Natl. Acad. Sci. USA 94: 4242-4245, 1997[Free Full Text].

57.   Flier, J. S. Clinical review 94: What's in a name? In search of leptin's physiologic role. J. Clin. Endocrinol. Metab. 83: 1407-1413, 1998[Free Full Text].

58.   Fournier, L. A., N. Bégin-Heick, J. F. Whitfield, and J.-L. Schwartz. Comparison of the properties of the ATP-sensitive K⁺ channels of pancreatic -cells of lean and obese (ob/ob) C57BL/6J mice. J. Membrane Biol. 129: 267-276, 1992[Web of Science][Medline].

59.   Fournier, L. A., H. M. C. Heick, and N. Bégin-Heick. The influence of K⁺-induced membrane depolarization on insulin secretion in islets of lean and obese (ob/ob) mice. Biochem. Cell. Biol. 68: 243-248, 1990[Web of Science][Medline].

60.  Friedman, J. M. Leptin, leptin receptors, and the control of body weight. Nutr. Rev. 56: 38-46; 54-75, 1998.

61.   Friedman, J. M., and J. L. Halaas. Leptin and the regulation of body weight in mammals. Nature 395: 763-770, 1998[Medline].

62.   Genuth, S. M. Hyperinsulinism in mice with genetically determined obesity. Endocrinology 84: 386-391, 1969[Abstract/Free Full Text].

63.   Genuth, S. M., R. J. Przybylski, and D. M. Rosenberg. Insulin resistance in genetically obese, hyperglycemic mice. Endocrinology 88: 1230-1238, 1971[Abstract/Free Full Text].

64.   Gettys, T. W., P. J. Harkness, and P. M. Watson. The beta 3-adrenergic receptor inhibits insulin-stimulated leptin secretion from isolated rat adipocytes. Endocrinology 137: 4054-4057, 1996[Abstract].

65.   Ghilardi, N., and R. C. Skoda. The leptin receptor activates janus kinase 2 and signals for proliferation in a factor-dependent cell line. Mol. Endocrinol. 11: 393-399, 1997[Abstract/Free Full Text].

66.   Ghilardi, N., S. Ziegler, A. Wiestner, R. Stoffel, M. H. Heim, and R. C. Skoda. Defective STAT signaling by the leptin receptor in diabetic mice. Proc. Natl. Acad. Sci. USA 93: 6231-6235, 1996[Abstract/Free Full Text].

67.   Groop, L. C., E. Widen, and E. Ferrannini. Insulin resistance and insulin deficiency in the pathogenesis of type 2 (non-insulin-dependent) diabetes mellitus: errors of metabolism or of methods? Diabetologia 36: 1326-1331, 1993[Web of Science][Medline].

68.   Grundleger, M. L., V. Y. Godbole, and S. W. Thenen. Age-dependent development of insulin resistance of soleus muscle in genetically obese (ob/ob) mice. Am. J. Physiol. Endocrinol. Metab. 239: E363-E371, 1980[Abstract/Free Full Text].

69.   Halaas, J. L., K. S. Gajiwala, M. Maffei, S. L. Cohen, B. T. Chait, D. Rabinowitz, R. L. Lallone, S. K. Burley, and J. M. Friedman. Weight-reducing effects of the plasma protein encoded by the obese gene. Science 269: 543-546, 1995[Abstract/Free Full Text].

70.   Hardie, L. J., N. Guilhot, and P. Trayhurn. Regulation of leptin production in cultured mature white adipocytes. Horm. Metab. Res. 28: 685-689, 1996[Web of Science][Medline].

71.   Harris, R. B. Acute and chronic effects of leptin on glucose utilization in lean mice. Biochem. Biophys. Res. Commun. 245: 502-509, 1998[Web of Science][Medline].

72.   Harris, R. B., J. Zhou, S. M. Redmann, Jr., G. N. Smagin, S. R. Smith, E. Rodgers, and J. J. Zachwieja. A leptin dose-response study in obese (ob/ob) and lean (+/?) mice. Endocrinology 139: 8-19, 1998[Abstract/Free Full Text].

73.   Harvey, J., and M. L. Ashford. Diazoxide- and leptin-activated K(ATP) currents exhibit differential sensitivity to englitazone and ciclazindol in the rat CRI-G1 insulin-secreting cell line. Br. J. Pharmacol. 124: 1557-1565, 1998[Web of Science][Medline].

74.   Harvey, J., and M. L. Ashford. Insulin occludes leptin activation of ATP-sensitive K+ channels in rat CRI-G1 insulin secreting cells. J. Physiol. (Lond.) 511: 695-706, 1998[Abstract/Free Full Text].

75.   Harvey, J., and M. L. Ashford. Role of tyrosine phosphorylation in leptin activation of ATP-sensitive K+ channels in the rat insulinoma cell line CRI-G1. J. Physiol. (Lond.) 510: 47-61, 1998[Abstract/Free Full Text].

76.   Harvey, J., F. McKenna, P. S. Herson, D. Spanswick, and M. L. Ashford. Leptin activates ATP-sensitive potassium channels in the rat insulin-secreting cell line, CRI-G1. J. Physiol. (Lond.) 504: 527-535, 1997[Abstract/Free Full Text].

77.   Havel, P. J., S. Kasim-Karakas, W. Mueller, P. R. Johnson, R. L. Gingerich, and J. S. Stern. Relationship of plasma leptin to plasma insulin and adiposity in normal weight and overweight women: effects of dietary fat content and sustained weight loss. J. Clin. Endocrinol. Metab. 81: 4406-4413, 1996[Abstract].

78.   Havel, P. J., J. Y. Uriu-Hare, T. Liu, K. L. Stanhope, J. S. Stern, C. L. Keen, and B. Ahren. Marked and rapid decreases of circulating leptin in streptozotocin diabetic rats: reversal by insulin. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 274: R1482-R1491, 1998[Abstract/Free Full Text].

79.   Herberg, L., and D. L. Coleman. Laboratory animals exhibiting obesity and diabetes syndromes. Metabolism 26: 59-99, 1977[Web of Science][Medline].

80.   Hoggard, N., J. G. Mercer, D. V. Rayner, K. Moar, P. Trayhurn, and L. M. Williams. Localization of leptin receptor mRNA splice variants in murine peripheral tissues by RT-PCR and in situ hybridization. Biochem. Biophys. Res. Commun. 232: 383-387, 1997[Web of Science][Medline].

81.   Holz, G. G., and J. F. Habener. Signal transduction crosstalk in the endocrine system: pancreatic beta-cells and the glucose competence concept. Trends Biochem. Sci. 17: 388-393, 1992[Web of Science][Medline].

82.   Holz, G. G., IV, W. M. Kühtreiber, and J. F. Habener. Pancreatic beta-cells are rendered glucose-competent by the insulinotropic hormone glucagon-like peptide-1(737). Nature 361: 362-365, 1993[Medline].

83.   Houseknecht, K. L., C. A. Baile, R. L. Matteri, and M. E. Spurlock. The biology of leptin: a review. J. Anim. Sci. 76: 1405-1420, 1998[Abstract/Free Full Text].

84.   Houseknecht, K. L., C. S. Mantzoros, R. Kuliawat, E. Hadro, J. S. Flier, and B. B. Kahn. Evidence for leptin binding to proteins in serum of rodents and humans: modulation with obesity. Diabetes 45: 1638-1643, 1996[Abstract].

85.   Hwang, C. S., T. M. Loftus, S. Mandrup, and M. D. Lane. Adipocyte differentiation and leptin expression. Annu. Rev. Cell. Dev. Biol. 13: 231-259, 1997[Web of Science][Medline].

86.   Igel, M., H. Kainulainen, A. Brauers, W. Becker, L. Herberg, and H. G. Joost. Long-term and rapid regulation of ob mRNA levels in adipose tissue from normal (Sprague Dawley rats) and obese (db/db mice, fa/fa rats) rodents. Diabetologia 39: 758-765, 1996[Web of Science][Medline].

87.   Ishida, K., T. Murakami, A. Mizuno, M. Iida, M. Kuwajima, and K. Shima. Leptin suppresses basal insulin secretion from rat pancreatic islets. Regul. Pept. 70: 179-182, 1997[Web of Science][Medline].

88.   Ishikawa, M., M. L. Pruneda, B. Adams-Huet, and P. Raskin. Obesity-independent hyperinsulinemia in nondiabetic first-degree relatives of individuals with type 2 diabetes. Diabetes 47: 788-792, 1998[Abstract].

89.   Islam, M. S., N. M. Morton, A. Hansson, and V. Emilsson. Rat insulinoma-derived pancreatic beta-cells express a functional leptin receptor that mediates a proliferative response. Biochem. Biophys. Res. Commun. 238: 851-855, 1997[Web of Science][Medline].

90.   Kahn, B. B. Type 2 diabetes: when insulin secretion fails to compensate for insulin resistance. Cell 92: 593-596, 1998[Web of Science][Medline].

91.   Kahn, C. R. Insulin action, diabetogenes, and the cause of type II diabetes. Diabetes 43: 1066-1084, 1994[Web of Science][Medline].

92.   Kamohara, S., R. Burcelin, J. L. Halaas, J. M. Friedman, and M. J. Charron. Acute stimulation of glucose metabolism in mice by leptin treatment. Nature 389: 374-377, 1997[Medline].

93.   Karlsson, E., M. Stridsberg, and S. Sandler. Leptin regulation of islet amyloid polypeptide secretion from mouse pancreatic islets. Biochem. Pharmacol. 56: 1339-1346, 1998[Web of Science][Medline].

94.   Kieffer, T. J., R. S. Heller, and J. F. Habener. Leptin receptors expressed on pancreatic beta-cells. Biochem. Biophys. Res. Commun. 224: 522-527, 1996[Web of Science][Medline].

95.   Kieffer, T. J., R. S. Heller, C. A. Leech, G. G. Holz, and J. F. Habener. Leptin suppression of insulin secretion by the activation of ATP-sensitive K+ channels in pancreatic beta-cells. Diabetes 46: 1087-1093, 1997[Abstract].

96.   Kim, J. B., P. Sarraf, M. Wright, K. M. Yao, E. Mueller, G. Solanes, B. B. Lowell, and B. M. Spiegelman. Nutritional and insulin regulation of fatty acid synthetase and leptin gene expression through ADD1/SREBP1. J. Clin. Invest. 101: 1-9, 1998[Web of Science][Medline].

97.   Kline, A. D., G. W. Becker, L. M. Churgay, B. E. Landen, D. K. Martin, W. L. Muth, R. Rathnachalam, J. M. Richardson, B. Schoner, M. Ulmer, and J. E. Hale. Leptin is a four-helix bundle: secondary structure by NMR. FEBS Lett. 407: 239-242, 1997[Web of Science][Medline].

98.   Kolaczynski, J. W., R. V. Considine, J. Ohannesian, C. Marco, I. Opentanova, M. R. Nyce, M. Myint, and J. F. Caro. Responses of leptin to short-term fasting and refeeding in humans: a link with ketogenesis but not ketones themselves. Diabetes 45: 1511-1515, 1996[Abstract].

99.   Kolaczynski, J. W., M. R. Nyce, R. V. Considine, G. Boden, J. J. Nolan, R. Henry, S. R. Mudaliar, J. Olefsky, and J. F. Caro. Acute and chronic effects of insulin on leptin production in humans: studies in vivo and in vitro. Diabetes 45: 699-701, 1996[Abstract].

100.   Koopmans, S. J., M. Frolich, E. H. Gribnau, R. G. Westendorp, and R. A. DeFronzo. Effect of hyperinsulinemia on plasma leptin concentrations and food intake in rats. Am. J. Physiol. Endocrinol. Metab. 274: E998-E1001, 1998[Abstract/Free Full Text].

101.   Koyama, K., G. Chen, M. Y. Wang, Y. Lee, M. Shimabukuro, C. B. Newgard, and R. H. Unger. -Cell function in normal rats made chronically hyperleptinemic by adenovirus-leptin gene therapy. Diabetes 46: 1276-1280, 1997[Abstract].

102.   Kukuljan, M., M. Y. Li, and I. Atwater. Characterization of potassium channels in pancreatic cells from ob/ob mice. FEBS Lett. 266: 105-108, 1990[Web of Science][Medline].

103.   Kulkarni, R. N., Z. L. Wang, R. M. Wang, J. D. Hurley, D. M. Smith, M. A. Ghatei, D. J. Withers, J. V. Gardiner, C. J. Bailey, and S. R. Bloom. Leptin rapidly suppresses insulin release from insulinoma cells, rat and human islets and, in vivo, in mice. J. Clin. Invest. 100: 2729-2736, 1997[Web of Science][Medline].

104.   Larsson, H., S. Elmstahl, and B. Ahren. Plasma leptin levels correlate to islet function independently of body fat in postmenopausal women. Diabetes 45: 1580-1584, 1996[Abstract].

105.   Larsson, O., J. T. Deeney, R. Branstrom, P. O. Berggren, and B. E. Corkey. Activation of the ATP-sensitive K+ channel by long chain acyl-CoA. A role in modulation of pancreatic beta-cell glucose sensitivity. J. Biol. Chem. 271: 10623-10626, 1996[Abstract/Free Full Text].

106.   Le Stunff, C., and P. Bougnères. Early changes in postprandial insulin secretion, not in insulin sensitivity, characterize juvenile obesity. Diabetes 43: 696-702, 1994[Abstract].

107.   Leclercq-Meyer, V., R. V. Considine, A. Sener, and W. J. Malaisse. Do leptin receptors play a functional role in the endocrine pancreas? Biochem. Biophys. Res. Commun. 229: 794-798, 1996[Web of Science][Medline].

108.   Leclercq-Meyer, V., and W. J. Malaisse. Failure of leptin to counteract the effects of glucose on insulin and glucagon release by the perfused rat pancreas. Med. Sci. Res. 25: 257-259, 1997[Web of Science].

109.   Leclercq-Meyer, V., and W. J. Malaisse. Failure of human and mouse leptin to affect insulin, glucagon and somatostatin secretion by the perfused rat pancreas at physiological glucose concentration. Mol. Cell. Endocrinol. 141: 111-118, 1998[Web of Science][Medline].

110.   Lee, G. H., R. Proenca, J. M. Montez, K. M. Carroll, J. G. Darvishzadeh, J. I. Lee, and J. M. Friedman. Abnormal splicing of the leptin receptor in diabetic mice. Nature 379: 632-635, 1996[Medline].

111.   Leroy, P., S. Dessolin, P. Villageois, B. C. Moon, J. M. Friedman, G. Ailhaud, and C. Dani. Expression of ob gene in adipose cells. Regulation by insulin. J. Biol. Chem. 271: 2365-2368, 1996[Abstract/Free Full Text].

112.   Levin, N., C. Nelson, A. Gurney, R. Vandlen, and F. de Sauvage. Decreased food intake does not completely account for adiposity reduction after ob protein infusion. Proc. Natl. Acad. Sci. USA 93: 1726-1730, 1996[Abstract/Free Full Text].

113.   Li, H. Y., L. L. Wang, and R. S. Yeh. Leptin immunoreactivity in the central nervous system in normal and diabetic rats. Neuroreport 10: 437-442, 1999[Web of Science][Medline].

114.   Lin, P. Y., D. R. Romsos, and G. A. Leveille. Food intake, body weight gain, and body composition of the young obese (ob/ob) mouse. J. Nutr. 107: 1715-1723, 1977.

115.   Lollmann, B., S. Gruninger, A. Stricker-Krongrad, and M. Chiesi. Detection and quantification of the leptin receptor splice variants Ob- Ra, b, and, e in different mouse tissues. Biochem. Biophys. Res. Commun. 238: 648-652, 1997[Web of Science][Medline].

116.   Luo, G., and L. Yu-Lee. Transcriptional inhibition by Stat5. Differential activities at growth-related versus differentiation-specific promoters. J. Biol. Chem. 272: 26841-26849, 1997[Abstract/Free Full Text].

117.   MacDougald, O. A., C. S. Hwang, H. Fan, and M. D. Lane. Regulated expression of the obese gene product (leptin) in white adipose tissue and 3T3-L1 adipocytes. Proc. Natl. Acad. Sci. USA 92: 9034-9037, 1995[Abstract/Free Full Text].

118.   Maffei, M., J. Halaas, E. Ravussin, R. E. Pratley, G. H. Lee, Y. Zhang, H. Fei, S. Kim, R. Lallone, S. Ranganathan, Leptin levels in human and rodent: measurement of plasma leptin and ob RNA in obese and weight-reduced subjects. Nat. Med. 1: 1155-1161, 1995[Web of Science][Medline].

119.   Malmstrom, R., M. R. Taskinen, S. L. Karonen, and H. Yki-Jarvinen. Insulin increases plasma leptin concentrations in normal subjects and patients with NIDDM. Diabetologia 39: 993-996, 1996[Medline].

120.   Masuzaki, H., Y. Ogawa, N. Sagawa, K. Hosoda, T. Matsumoto, H. Mise, H. Nishimura, Y. Yoshimasa, I. Tanaka, T. Mori, and K. Nakao. Nonadipose tissue production of leptin: leptin as a novel placenta-derived hormone in humans. Nat. Med. 3: 1029-1033, 1997[Web of Science][Medline].

121.   McGregor, G. P., J. F. Desaga, K. Ehlenz, A. Fischer, F. Heese, A. Hegele, C. Lammer, C. Peiser, and R. E. Lang. Radiommunological measurement of leptin in plasma of obese and diabetic human subjects. Endocrinology 137: 1501-1504, 1996[Abstract].

122.   Meissner, H. P., and H. Schmidt. The electrical activity of pancreatic beta-cells of diabetic mice. FEBS Lett. 67: 371-374, 1976[Web of Science][Medline].

123.   Mizuno, A., T. Murakami, S. Otani, M. Kuwajima, and K. Shima. Leptin affects pancreatic endocrine functions through the sympathetic nervous system. Endocrinology 139: 3863-3870, 1998[Abstract/Free Full Text].

124.   Morton, N. M., V. Emilsson, R. P. de Groot, A. L. Pallet, and M. A. Cawthorne. Leptin signalling in pancreatic islets and clonal insulin-secreting cells. J. Mol. Endocrinol. 22: 173-184, 1999[Abstract].

125.   Morton, N. M., V. Emilsson, Y. L. Liu, and M. A. Cawthorne. Leptin action in intestinal cells. J. Biol. Chem. 273: 26194-26201, 1998[Abstract/Free Full Text].

126.   Mueller, W. M., F. M. Gregoire, K. L. Stanhope, C. V. Mobbs, T. M. Mizuno, C. H. Warden, J. S. Stern, and P. J. Havel. Evidence that glucose metabolism regulates leptin secretion from cultured rat adipocytes. Endocrinology 139: 551-558, 1998[Abstract/Free Full Text].

127.   Murakami, T., T. Yamashita, M. Iida, M. Kuwajima, and K. Shima. A short form of leptin receptor performs signal transduction. Biochem. Biophys. Res. Commun. 231: 26-29, 1997[Web of Science][Medline].

128.   Muzzin, P., R. C. Eisensmith, K. C. Copeland, and S. L. Woo. Correction of obesity and diabetes in genetically obese mice by leptin gene therapy. Proc. Natl. Acad. Sci. USA 93: 14804-14808, 1996[Abstract/Free Full Text].

129.   Nolan, J. J., J. M. Olefsky, M. R. Nyce, R. V. Considine, and J. F. Caro. Effect of troglitazone on leptin production. Studies in vitro and in human subjects. Diabetes 45: 1276-1278, 1996[Abstract].

130.   Odeleye, O. E., M. de Courten, D. J. Pettitt, and E. Ravussin. Fasting hyperinsulinemia is a predictor of increased body weight gain and obesity in Pima Indian children. Diabetes 46: 1341-1345, 1997[Abstract].

131.   Olefsky, J. M., and J. J. Nolan. Insulin resistance and non-insulin-dependent diabetes mellitus: cellular and molecular mechanisms. Am. J. Clin. Nutr. 61: 980S-986S, 1995[Abstract/Free Full Text].

132.   Ookuma, M., K. Ookuma, and D. A. York. Effects of leptin on insulin secretion from isolated rat pancreatic islets. Diabetes 47: 219-223, 1998[Abstract].

133.   Pallett, A. L., N. M. Morton, M. A. Cawthorne, and V. Emilsson. Leptin inhibits insulin secretion and reduces insulin mRNA levels in rat isolated pancreatic islets. Biochem. Biophys. Res. Commun. 238: 267-270, 1997[Web of Science][Medline].

134.   Patel, B. K., J. I. Koenig, L. M. Kaplan, and S. C. Hooi. Increase in plasma leptin and Lep mRNA concentrations by food intake is dependent on insulin. Metabolism 47: 603-607, 1998[Web of Science][Medline].

135.   Pelleymounter, M. A., M. J. Cullen, M. B. Baker, R. Hecht, D. Winters, T. Boone, and F. Collins. Effects of the obese gene product on body weight regulation in ob/ob mice. Science 269: 540-543, 1995[Abstract/Free Full Text].

136.   Poitout, V., C. Rouault, M. Guerre-Millo, I. Briaud, and G. Reach. Inhibition of insulin secretion by leptin in normal rodent islets of Langerhans. Endocrinology 139: 822-826, 1998[Abstract/Free Full Text].

137.   Polonsky, K. S., B. D. Given, L. Hirsch, E. T. Shapiro, H. Tillil, C. Beebe, J. A. Galloway, B. H. Frank, T. Karrison, and E. Van Cauter. Quantitative study of insulin secretion and clearance in normal and obese subjects. J. Clin. Invest. 81: 435-441, 1988.

138.   Popovic, V., D. Micic, S. Danjanovic, S. Zoric, M. Djurovic, S. Obradovic, M. Petakov, C. Dieguez, and F. F. Casanueva. Serum leptin and insulin concentrations in patients with insulinoma before and after surgery. Eur. J. Endocrinol. 138: 86-88, 1998[Abstract].

139.   Prentki, M., K. Tornheim, and B. E. Corkey. Signal transduction mechanisms in nutrient-induced insulin secretion. Diabetologia 40: S32-S41, 1997.

140.   Prevarskaya, N. B., R. N. Skryma, P. Vacher, N. Daniel, J. Djiane, and B. Dufy. Role of tyrosine phosphorlation in potassium channel activation: functional association with prolactin receptor and JAK2 tyrosine kinase. J. Biol. Chem. 270: 24292-24299, 1995[Abstract/Free Full Text].

141.   Reaven, G. M. Banting lecture 1988. Role of insulin resistance in human disease. Diabetes 37: 1595-1607, 1988[Abstract].

142.   Reaven, G. M., and A. Laws. Insulin resistance, compensatory hyperinsulinaemia, and coronary heart disease. Diabetologia 37: 948-952, 1994[Web of Science][Medline].

143.   Rentsch, J., and M. Chiesi. Regulation of ob gene mRNA levels in cultured adipocytes. FEBS Lett. 379: 55-59, 1996[Web of Science][Medline].

144.   Roduit, R., and B. Thorens. Inhibition of glucose-induced insulin secretion by long-term preexposure of pancreatic islets to leptin. FEBS Lett. 415: 179-182, 1997[Web of Science][Medline].

145.   Rosario, L. M., I. Atwater, and E. Rojas. Membrane potential measurements in islets of Langerhans from ob/ob obese mice suggest an alteration in [Ca²⁺]_i-activated K⁺ permeability. Q. J. Exp. Physiol. 70: 137-150, 1985[Abstract/Free Full Text].

146.   Rosenblum, C. I., M. Tota, D. Cully, T. Smith, R. Collum, S. Qureshi, J. F. Hess, M. S. Phillips, P. J. Hey, A. Vongs, T. M. Fong, L. Xu, H. Y. Chen, R. G. Smith, C. Schindler, and L. H. Van der Ploeg. Functional STAT 1 and 3 signaling by the leptin receptor (OB-R); reduced expression of the rat fatty leptin receptor in transfected cells. Endocrinology 137: 5178-5181, 1996[Abstract].

147.   Rossetti, L., D. Massillon, N. Barzilai, P. Vuguin, W. Chen, M. Hawkins, J. Wu, and J. Wang. Short term effects of leptin on hepatic gluconeogenesis and in vivo insulin action. J. Biol. Chem. 272: 27758-27763, 1997[Abstract/Free Full Text].

148.   Russell, C. D., R. N. Petersen, S. P. Rao, M. R. Ricci, A. Prasad, Y. Zhang, R. E. Brolin, and S. K. Fried. Leptin expression in adipose tissue from obese humans: depot-specific regulation by insulin and dexamethasone. Am. J. Physiol. Endocrinol. Metab. 275: E507-E515, 1998[Abstract/Free Full Text].

149.   Saad, M. F., A. Khan, A. Sharma, R. Michael, M. G. Riad-Gabriel, R. Boyadjian, S. D. Jinagouda, G. M. Steil, and V. Kamdar. Physiological insulinemia acutely modulates plasma leptin. Diabetes 47: 544-549, 1998[Abstract].

150.   Saladin, R., P. De Vos, M. Guerre-Millo, A. Leturque, J. Girard, B. Staels, and J. Auwerx. Transient increase in obese gene expression after food intake or insulin administration. Nature 377: 527-529, 1995[Medline].

151.   Schwartz, M. W., D. G. Baskin, T. R. Bukowski, J. L. Kuijper, D. Foster, G. Lasser, D. E. Prunkard, D. Porte, Jr., S. C. Woods, R. J. Seeley, and D. S. Weigle. Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice. Diabetes 45: 531-535, 1996[Abstract].

152.   Seufert, J., T. J. Kieffer, and J. F. Habener. Leptin inhibits insulin gene transcription and reverses hyperinsulinemia in leptin-deficient ob/ob mice. Proc. Natl. Acad. Sci. USA 96: 674-679, 1999[Abstract/Free Full Text].

153.   Seufert, J., T. J. Kieffer, C. A. Leech, G. G. Holz, W. Moritz, C. Ricordi, and J. F. Habener. Leptin suppression of insulin secretion and gene expression in human pancreatic islets: implications for the development of adipogenic diabetes mellitus. J. Clin. Endocrinol. Metab. 84: 670-676, 1999[Abstract/Free Full Text].

154.   Shi, Z. Q., A. Nelson, L. Whitcomb, J. Wang, and A. M. Cohen. Intracerebroventricular administration of leptin markedly enhances insulin sensitivity and systemic glucose utilization in conscious rats. Metabolism 47: 1274-1280, 1998[Web of Science][Medline].

155.   Shimabukuro, M., M. Higa, Y. T. Zhou, M. Y. Wang, C. B. Newgard, and R. H. Unger. Lipoapoptosis in beta-cells of obese prediabetic fa/fa rats. Role of serine palmitoyltransferase overexpression. J. Biol. Chem. 273: 32487-32490, 1998[Abstract/Free Full Text].

156.   Shimabukuro, M., K. Koyama, G. Chen, M. Y. Wang, F. Trieu, Y. Lee, C. B. Newgard, and R. H. Unger. Direct antidiabetic effect of leptin through triglyceride depletion of tissues. Proc. Natl. Acad. Sci. USA 94: 4637-4641, 1997[Abstract/Free Full Text].

157.   Shimizu, H., K. Ohtani, T. Tsuchiya, H. Takahashi, Y. Uehara, N. Sato, and M. Mori. Leptin stimulates insulin secretion and synthesis in HIT-T 15 cells. Peptides 18: 1263-1266, 1997[Web of Science][Medline].

158.   Siegrist-Kaiser, C. A., V. Pauli, C. E. Juge-Aubry, O. Boss, A. Pernin, W. W. Chin, I. Cusin, F. Rohner-Jeanrenaud, A. G. Burger, J. Zapf, and C. A. Meier. Direct effects of leptin on brown and white adipose tissue. J. Clin. Invest. 100: 2858-2864, 1997[Web of Science][Medline].

159.   Sigal, R. J., M. El-Hashimy, B. C. Martin, S. Soeldner, A. S. Krolewski, and J. H. Warram. Acute postchallenge hyperinsulinemia predicts weight gain. A prospective study. Diabetes 46: 1025-1029, 1997[Abstract].

160.   Sinha, M. K., J. P. Ohannesian, M. L. Heiman, A. Kriauciunas, T. W. Stephens, S. Magosin, C. Marco, and J. F. Caro. Nocturnal rise of leptin in lean, obese, and non-insulin-dependent diabetes mellitus subjects. J. Clin. Invest. 97: 1344-1347, 1996[Web of Science][Medline].

161.   Sinha, M. K., I. Opentanova, J. P. Ohannesian, J. W. Kolaczynski, M. L. Heiman, J. Hale, G. W. Becker, R. R. Bowsher, T. W. Stephens, and J. F. Caro. Evidence of free and bound leptin in human circulation. Studies in lean and obese subjects and during short-term fasting. J. Clin. Invest. 98: 1277-1282, 1996[Web of Science][Medline].

162.   Sivitz, W. I., S. Walsh, D. Morgan, P. Donohoue, W. Haynes, and R. L. Leibel. Plasma leptin in diabetic and insulin-treated diabetic and normal rats. Metabolism 47: 584-591, 1998[Web of Science][Medline].

163.   Sivitz, W. I., S. A. Walsh, D. A. Morgan, M. J. Thomas, and W. G. Haynes. Effects of leptin on insulin sensitivity in normal rats. Endocrinology 138: 3395-3401, 1997[Abstract/Free Full Text].

164.   Slieker, L. J., K. W. Sloop, P. L. Surface, A. Kriauciunas, F. LaQuier, J. Manetta, J. Bue-Valleskey, and T. W. Stephens. Regulation of expression of ob mRNA and protein by glucocorticoids and cAMP. J. Biol. Chem. 271: 5301-5304, 1996[Abstract/Free Full Text].

165.   Spanswick, D., M. A. Smith, V. E. Groppi, S. D. Logan, and M. L. Ashford. Leptin inhibits hypothalamic neurons by activation of ATP-sensitive potassium channels. Nature 390: 521-525, 1997[Medline].

166.   Stephens, T. W., M. Basinski, P. K. Bristow, J. M. Bue-Valleskey, S. G. Burgett, L. Craft, J. Hale, J. Hoffmann, H. M. Hsiung, A. Kriauciunas, W. MacKellar, P. R. J. Rosteck, B. Schoner, D. Smith, F. C. Tinsley, X.-Y. Zhang, and M. Heiman. The role of neuropeptide Y in the antiobesity action of the obese gene product. Nature 377: 530-532, 1995[Medline].

167.   Stout, R. W. Insulin and atheroma: 20-years perspective. Diabetes Care 13: 631-654, 1990[Abstract].

168.   Tanabe, K., S. Okuya, Y. Tanizawa, A. Matsutani, and Y. Oka. Leptin induces proliferation of pancreatic beta cell line MIN6 through activation of mitogen-activated protein kinase. Biochem. Biophys. Res. Commun. 241: 765-768, 1997[Web of Science][Medline].

169.   Tanizawa, Y., S. Okuya, H. Ishihara, T. Asano, T. Yada, and Y. Oka. Direct stimulation of basal insulin secretion by physiological concentrations of leptin in pancreatic beta cells. Endocrinology 138: 4513-4516, 1997[Abstract/Free Full Text].

170.   Tartaglia, L. A. The leptin receptor. J. Biol. Chem. 272: 6093-6096, 1997[Free Full Text].

171.   Tartaglia, L. A., M. Dembski, X. Weng, N. Deng, J. Culpepper, R. Devos, G. J. Richards, L. A. Campfield, F. T. Clark, J. Deeds, Identification and expression cloning of a leptin receptor, OB-R. Cell 83: 1263-1271, 1995[Web of Science][Medline].

172.   Taylor, S. I., D. Accili, and Y. Imai. Insulin resistance or insulin deficiency. Which is the primary cause of NIDDM? Diabetes 43: 735-740, 1994[Web of Science][Medline].

173.   Unger, R. H., Y.-T. Zhou, and L. Orci. Regulation of fatty acid homeostasis in cells: novel role of leptin. Proc. Natl. Acad. Sci. USA 96: 2327-2332, 1999[Abstract/Free Full Text].

174.   Utriainen, T., R. Malmstrom, S. Makimattila, and H. Yki-Jarvinen. Supraphysiological hyperinsulinemia increases plasma leptin concentrations after 4 h in normal subjects. Diabetes 45: 1364-1366, 1996[Abstract].

175.   Vaisse, C., J. L. Halaas, C. M. Horvath, J. E. Darnell, Jr., M. Stoffel, and J. M. Friedman. Leptin activation of Stat3 in the hypothalamus of wild-type and ob/ob mice but not db/db mice. Nat. Genet. 14: 95-97, 1996[Web of Science][Medline].

176.   Vidal, H., D. Auboeuf, P. De Vos, B. Staels, J. P. Riou, J. Auwerx, and M. Laville. The expression of ob gene is not acutely regulated by insulin and fasting in human abdominal subcutaneous adipose tissue. J. Clin. Invest. 98: 251-255, 1996[Web of Science][Medline].

177.   Wabitsch, M., P. B. Jensen, W. F. Blum, C. T. Christoffersen, P. Englaro, E. Heinze, W. Rascher, W. Teller, H. Tornqvist, and H. Hauner. Insulin and cortisol promote leptin production in cultured human fat cells. Diabetes 45: 1435-1438, 1996[Abstract].

178.   Wang, J., R. Liu, M. Hawkins, N. Barzilai, and L. Rossetti. A nutrient-sensing pathway regulates leptin gene expression in muscle and fat. Nature 393: 684-688, 1998[Medline].

179.   Wang, J. L., N. Chinookoswong, S. Scully, M. Qi, and Z. Q. Shi. Differential effects of leptin in regulation of tissue glucose utilization in vivo. Endocrinology 140: 2117-2124, 1999[Abstract/Free Full Text].

180.   Wang, M. Y., K. Koyama, M. Shimabukuro, D. Mangelsdorf, C. B. Newgard, and R. H. Unger. Overexpression of leptin receptors in pancreatic islets of Zucker diabetic fatty rats restores GLUT-2, glucokinase, and glucose-stimulated insulin secretion. Proc. Natl. Acad. Sci. USA 95: 11921-11926, 1998[Abstract/Free Full Text].

181.   Wang, M. Y., K. Koyama, M. Shimabukuro, C. B. Newgard, and R. H. Unger. OB-Rb gene transfer to leptin-resistant islets reverses diabetogenic phenotype. Proc. Natl. Acad. Sci. USA 95: 714-718, 1998[Abstract/Free Full Text].

182.   Weigle, D. S., T. R. Bukowski, D. C. Foster, S. Holderman, J. M. Kramer, G. Lasser, C. E. Lofton-Day, D. E. Prunkard, C. Raymond, and J. L. Kuijper. Recombinant ob protein reduces feeding and body weight in the ob/ob mouse. J. Clin. Invest. 96: 2065-2070, 1995.

183.   White, D. W., K. K. Kuropatwinski, R. Devos, H. Baumann, and L. A. Tartaglia. Leptin receptor (OB-R) signaling. Cytoplasmic domain mutational analysis and evidence for receptor homo-oligomerization. J. Biol. Chem. 272: 4065-4071, 1997[Abstract/Free Full Text].

184.   White, D. W., and L. A. Tartaglia. Evidence for ligand-independent homo-oligomerization of leptin receptor (OB-R) isoforms: a proposed mechanism permitting productive long-form signaling in the presence of excess short-form expression. J. Cell. Biochem. 73: 278-288, 1999[Web of Science][Medline].

185.   Yoshida, T., M. Hayashi, T. Monkawa, and T. Saruta. Regulation of obese mRNA expression by hormonal factors in primary cultures of rat adipocytes. Eur. J. Endocrinol. 135: 619-625, 1996[Abstract/Free Full Text].

186.   Zawalich, W. S., M. Bonnet-Eymard, and K. C. Zawalich. Signal transduction in pancreatic beta-cells: regulation of insulin secretion by information flow in the phospholipase C/protein kinase C pathway. Front. Biosci. 2: d160-d172, 1997[Medline].

187.   Zhang, F., M. B. Basinski, J. M. Beals, S. L. Briggs, L. M. Churgay, D. K. Clawson, R. D. DiMarchi, T. C. Furman, J. E. Hale, H. M. Hsiung, B. E. Schoner, D. P. Smith, X. Y. Zhang, J. P. Wery, and R. W. Schevitz. Crystal structure of the obese protein leptin-E100. Nature 387: 206-209, 1997[Medline].

188.   Zhang, Y., R. Proenca, M. Maffei, M. Barone, L. Leopold, and J. Friedman. Positional cloning of the mouse obese gene and its human homolog. Nature 372: 425-432, 1994[Medline].

189.   Zhao, A. Z., K. E. Bornfeldt, and J. A. Beavo. Leptin inhibits insulin secretion by activation of phosphodiesterase 3B. J. Clin. Invest. 102: 869-873, 1998[Web of Science][Medline].