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Am J Physiol Endocrinol Metab 286: E560-E567, 2004. First published November 18, 2003; doi:10.1152/ajpendo.00561.2002
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Rosiglitazone prevents the impairment of human islet function induced by fatty acids: evidence for a role of PPAR{gamma}2 in the modulation of insulin secretion

R. Lupi,1 S. Del Guerra,1 L. Marselli,1 M. Bugliani,1 U. Boggi,2 F. Mosca,2 P. Marchetti,1 and S. Del Prato1

1Department of Endocrinology and Metabolism, Metabolic Unit, and 2Department of Oncology, Surgical Unit, University of Pisa, 56100 Pisa, Italy

Submitted 23 December 2002 ; accepted in final form 12 November 2003


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Peroxisome proliferator-activated receptors (PPARs) are a subgroup of the superfamily of nuclear receptors, with three distinct main types: {alpha}, {beta} and {gamma} (subdivided into {gamma}1 and {gamma}2). Recently, the presence of PPAR{gamma} has been reported in human islets. Whether other PPAR types can be found in human islets, how islet PPAR{gamma} mRNA expression is regulated by the metabolic milieu, their role in insulin secretion, and the effects of a PPAR{gamma} agonist are not known. In this study, human pancreatic islets were prepared by collagenase digestion and density gradient purification from nonobese adult donors. The presence of PPAR mRNAs was assessed by RT-PCR, and the effect was evaluated of exposure for up to 24 h to either 22.2 mmol/l glucose and/or 0.25, 0.5, or 1.0 mmol/l long-chain fatty acid mixture (oleate to palmitate, 2:1). PPAR{beta} and, to a greater extent, total PPAR{gamma} and PPAR{gamma}2 mRNAs were expressed in human islets, whereas PPAR{alpha} mRNA was not detected. Compared with human adipose tissue, PPAR{gamma} mRNA was expressed at lower levels in the islets, and PPAR{beta} at similar levels. The expression of PPAR{gamma}2 mRNA was not affected by exposure to 22.2 mmol/l glucose, whereas it decreased markedly and time dependently after exposure to progressively higher free fatty acids (FFA). This latter effect was not affected by the concomitant presence of high glucose. Exposure to FFA caused inhibition of insulin mRNA expression, glucose-stimulated insulin release, and reduction of islet insulin content. The PPAR{gamma} agonists rosiglitazone and 15-deoxy-{Delta}-12,14prostaglandin J2 prevented the cytostatic effect of FFA as well as the FFA-induced changes of PPAR and insulin mRNA expression. In conclusion, this study shows that PPAR{gamma} mRNA is expressed in human pancreatic islets, with predominance of PPAR{gamma}2; exposure to FFA downregulates PPAR{gamma}2 and insulin mRNA expression and inhibits glucose-stimulated insulin secretion; exposure to PPAR{gamma} agonists can prevent these effects.

pancreatic islets; peroxisome proliferator-activated receptors; polymerase chain reaction; messenger ribonucleic acid; lipotoxicity; glucotoxicity; prostaglandin; free fatty acids

SINCE THE APPRECIATION that chronic exposure to glucose impairs glucose-stimulated insulin secretion (43), lipotoxicity has been described to account for the alteration of {beta}-cell function associated with elevation of plasma free fatty acids (FFA) (23). Incubation of isolated rodent (3, 17, 50, 51, 53) or human islets (27, 52) in the presence of high FFA concentration has been consistently shown to impair glucose-mediated insulin release and cause apoptosis. The deleterious effect of FFA on insulin release has been confirmed in perfused-pancreas studies as well (36). In vivo studies have led to more controversial results. Paolisso et al. (31) reported increased insulin secretion after short elevation of plasma FFA but an inhibitory effect after more prolonged Intralipid infusion. Similarly, Carpentier et al. (11) observed impaired glucose-stimulated insulin secretion in the presence of 1 mM plasma FFA. On the other hand, other authors were not able to confirm a deleterious effect of high plasma FFA on insulin secretion (5, 10, 19). More recently, Jensen et al. (22) have reported that short-term (24-h), low-grade Intralipid infusion was associated with reduced peripheral insulin action but increased insulin secretion, although the possibility that over a longer period {beta}-cell exhaustion could ensue was not ruled out. Not only does controversy exist about the impact of increased plasma FFA concentration on insulin secretion in vivo, but also uncertainties remain concerning the intimate mechanism(s) responsible for the FFA effects on the human islet. Moreover, different fatty acids (i.e., saturated vs. unsaturated) are known to exert divergent effects on the {beta}-cell (6, 20, 28).

Expression of peroxisome proliferator-activated receptors (PPARs) has been demonstrated in rat pancreatic islets (8). Interestingly, in the islets of normal rats, PPAR{alpha} is expressed to a higher extent than PPAR{gamma} (54), whereas this proportion is inverted in islets from obese Zucker diabetic fatty (ZDF) rats. The balance between PPAR{alpha} and PPAR{gamma} activities plays a major role in determining cell fat storage (2). More recently, PPAR{gamma} expression has been proven in normal human pancreatic islet cells as well (15). Because PPAR{gamma} is thought to stimulate fatty acid uptake and their intracellular binding and esterification (24, 37), a role of these nuclear receptors in mediating the FFA effects on insulin secretion was postulated. Therefore, an experimental design was developed 1) to evaluate the expression of PPAR isoforms in isolated human islets compared with adipose tissue and pancreatic acinar cells, 2) to assess the effect of 24-h culture in the presence of high glucose and/or FFA concentrations on PPAR expression and insulin release, and 3) to determine the effects of concomitant incubation with the PPAR{gamma} agonists rosiglitazone and 15-deoxy-{Delta}-12,14prostaglandin J2 (PGJ2) on PPAR expression and insulin secretion.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Human islet preparation. Pancreatic islets were prepared by collagenase digestion and density gradient purification, as previously reported in detail (29, 35). For the purpose of this study, 13 human pancreases (from donors aged 25-68 yr) were used with the approval of the local ethics committee. After isolation, islets were cultured in M-199 culture medium as previously described (29, 35) for 3-5 days before experiments were carried out. Within this period, aliquots of ~500 hand-picked islets were cultured free floating for 24 h in M-199 culture medium under the following conditions: 1) control M-199, 2) M-199 containing 22 mmol/l glucose and/or 0.25, 0.5, or 1.0 mmol/l long-chain fatty acid mixture (oleate to palmitate 2:1; Sigma Chemicals, St. Louis, MO), 3) M-199 containing 1.0 mmol/l long-chain fatty acid mixture plus 1.5 µg/ml rosiglitazone, or 10 µM PGJ2, and 4) M-199 containing 1.5 µg/ml rosiglitazone or 10 µM PGJ2. The FFA mixtures were prepared as described in detail previously (4, 26, 39). The final pH of all media was buffered at pH 7.4.

Intracellular triglyceride content. Triglyceride content in human islets was determined in three different experiments before and after 24-h incubation with 1.0 mM FFA mixture, rosiglitazone (1.5 µg/ml), and FFA plus rosiglitazone as well as in control islets. Aliquots of 500 cultured islets were selected under microscope from each pool of human islets. Very large and very small islets were excluded in an effort to minimize intergroup size differences. Islets were washed twice with Hanks' balanced salt solution and suspended in 50 µl of 2 mol/1 NaCl, 2 mmol/1 EDTA, and 50 mmol/1 sodium phosphate buffer (pH 7.4). After 1-2 min of sonication, 10 µl of the homogenate were mixed with 10 µl of tert-butyl alcohol and 5 µl of Triton X-100-methyl alcohol mixture (1:1 by volume) for lipid extraction. Measurements were made in duplicate using the Sigma Triglyceride (GPO-Trinder) kit.

Glucose-stimulated insulin release. After a 24-h incubation, islet glucose-stimulated insulin secretion was determined as previously described. Briefly, following a 45-min preincubation period at 3.3 mmol/l glucose, batches of ~30 islets of comparable size were kept at 37°C for 45 min in Krebs-Ringer bicarbonate solution (KRB) and 0.5% albumin, pH 7.4, containing 3.3 mmol/l glucose. At the end of this period, the medium was completely removed and replaced with KRB containing 16.7 mmol/l glucose. After an additional 45-min incubation, the medium was removed. Samples (500 µl) from the different media were stored at -20°C until insulin concentrations were measured by immunoradiometric assay (Pantec Forniture Biomediche, Turin, Italy). Total insulin content was measured after acid-alcohol extraction, as previously reported (29, 35).

Islet mRNA expressions. mRNA expression of the different PPARs and insulin was measured by reverse transcription reaction followed by polymerase chain reaction (RT-competitive PCR) or real-time quantitative RT-PCR. Total RNA was extracted from human pancreatic islets, adipose tissue, or acinar cells by use of the SV Total RNA Isolation System (Promega). Total RNA was quantified by absorbance at A260/A280 nm (ratio >1.65) in a Perkin-Elmer spectrophotometer, and its integrity was assessed after electrophoresis in 1.0% agarose gels by ethidium bromide staining.

For RT-PCR, each mRNA first-strand cDNA synthesis was performed from 1 µg of total RNA in the presence of dNTP mix (final concentration 200 µM) and 200 µU of a thermostable reverse transcriptase (Superscript II, GIBCO-BRL). After the reaction, the RT fluid was added to a PCR mix (5 µl buffer 10x, 0.2 µM of Deoxynucleotide Mix, 500 pmol of specific sense and antisense primers, and 2 U of Taq polymerase) in a final volume of 25 µl. The tubings were subjected to 35 cycles of amplification, including denaturation for 60 s at 95°C, hybridization for 60 s at 52-60°C for the varying PPARs, and elongation for 90 s at 72°C.

For real-time quantitative RT-PCR, the genes of interest were obtained from GenBank (National Center of Biotechnology Information), the cDNA synthesis was performed from 2 µg of total RNA, and the primer and probe sequences were designed using Primers Express Software (PE Applied Biosystems). To avoid amplification of genomic DNA, the primer-probe sets were designed to span exonexon borders. The probes were labeled with FAM at the 5' end and TAMRA at the 3' end. PCR amplifications were performed in a total volume of 25 µl containing 2 µl of cDNA sample, 200 nmol/l of each primer, 100 nmol/l of corresponding probe, and 12.5 µl of TaqMan Universal PCR Master Mix. For each reaction, the polymerase was activated by preincubation at 95°C for 10 min. Amplification was then performed by 40 cycles of switching between 95°C for 15 s and 60°C for 60 s. The PPAR{gamma}2 and human insulin cDNA quantity of each cDNA sample was normalized to the housekeeping gene for {beta}-actin.

According to Refs. 1, 16, 42, and 47, the primers (sense and antisense) for RT-PCR described below were used to generate human PPAR{alpha}, -{beta}, total {gamma}, -{gamma}2, human insulin, and {beta}-actin cDNAs. For PPAR{alpha}: 5'-CCA GTA TTT AGG ACG CTG TCC-3' and 5'-AAG TTC TTC AAG TAG GCC AGC-3' (492 bp); for PPAR{beta}: 5'-AAC TGC AGA TGG GCT GTG AC-3', and 5'-GTC TCG ATG TCG TGG ATC AC-3' (484 bp); for total PPAR{gamma}: 5'-TCT CTC CGT AAT GGA AGA CC-3' and 5'-GCA TTA TGA GAC ATC CCC AC-3' (474 bp); for PPAR{gamma}2: 5'-GCG ATT CCT TCA CTG ATA C-3' and 5'-GCA TTA TGA GAC ATC CCC AC AC-3' (580 bp); for human insulin: 5'-GCC TTT GTG AAC CAA CAC CTG-3' and 5'-GTT GCA GTA GTT CTC CAG CTG-3' (261 bp); for {beta}-actin: 5'-ACC AAC TGG GAG GAG ATG GAG-3' and 5'-CGT GAG GAT CTT CAT GAG GTA AGT C-3' (354 bp). For real-time RT-PCR, the primers (sense and antisense) and probes described below were used to generate human PPAR{gamma}2 and human cDNAs. For PPAR{gamma}2: 5'-GAA CCA CCG CAC GAT GTT G-3' (sense), 5'-TGA ACG ACT GGA AGT CGA ACA C-3' (antisense) and 5'-TGT CGG CCA CAC AGT GCT TCA GC-3' (probes) (100 bp); for human insulin: 5'-TGG CAT TGT GGA ACA ATG CT-3' (sense), 5'-GGC TGG TTC AAG GGC TTT ATT-3' (antisense) and 5'-CCA GCT GGA GAA CTA CTG CAA CTA GAC GCA-3' (probes) (100 bp). The probe and primer sequences for human {beta}-actin were obtained from PE Applied Biosystems (Pre-Developed TaqMan Assay Reagents Control Kit). The final PCR products were analyzed on a 1-1.5% nondenaturing agarose gel.

To determine whether islet incubation in the presence of PPAR{gamma} agonists can induce fibroblast differentiation into adipocytes, expression of fatty acid-binding protein (FABP4) mRNA was determined in human islets as well as in human adipocytes by the use of the primers 5'-TCC AGT GAA AAC TTT GAT GAT TAT-3' (sense) and 5'-ACG CAT TCC ACC ACC AGT TTA TCA-3' (antisense).

Statistical analysis. Data are presented as means ± SD. Statistical analysis was performed by the ANOVA test, followed by Bonferroni correction as appropriate.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
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 REFERENCES
 
PPAR mRNAs are expressed in isolated human islets. As shown in Fig. 1, mRNA expression for PPAR{beta}, total PPAR{gamma}, and PPAR{gamma}2, but not PPAR{alpha}, was detected during 10 experiments performed in isolated human islets. Compared with human adipose tissue, total PPAR{gamma} and PPAR{gamma}2 mRNAs were expressed at lower levels in the islets (Fig. 1). Only PPAR{beta} mRNA was found in pancreatic acinar tissue (not shown). Because PPAR{gamma}2 mRNA accounted for most of total PPAR{gamma} mRNA in islets, we decided to determine whether high glucose or fatty acid concentration in the culture medium could simultaneously alter the expression of this specific isoform and glucose-mediated insulin release.



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  		Fig. 1. mRNA expression of peroxisome proliferator-activated receptors (PPAR){alpha}, -{beta}, total {gamma}, and -{gamma}2 in adipose tissue (A.T.) and human islets (H.I.). BA, {beta}-actin. The figure is representative of 10 experiments.

 

FFA, but not high glucose concentration, modulate mRNA expression of PPAR{gamma}2. Human islets from five pancreases were used to assess the effect of high FFA or glucose concentration on the mRNA expression of PPAR{gamma}2. Twenty-four-hour incubation of human islets in the presence of high glucose concentration did not cause any major change in the expression of PPAR{gamma}2 mRNA (Fig. 2). On the other hand, inhibition of PPAR{gamma}2 mRNA expression occurred after exposure to FFA (Fig. 2). Addition of glucose to FFA did not prevent the decline in PPAR{gamma}2 mRNA expression noted with FFA alone (not shown).



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  		Fig. 2. A: effect of 24-h exposure to glucose (22.2 mM) and free fatty acids (FFA; 0.25, 0.5, and 1.0 mmol/l) on PPAR{gamma}2 expression. The figure is representative of 5 experiments. B: effect of 2-, 8-, and 24-h incubation with 0.5 or 1.0 mmol/l FFA on the expression of PPAR{gamma}2 mRNA. The figure is representative of 5 experiments.

 

The effects of 0.5 and 1.0 mmol/l FFA on PPAR{gamma}2 mRNA expression were evaluated as a function of time as well, and results are shown in Fig. 2. With 0.5 mmol/l FFA, inhibition of PPAR{gamma}2 mRNA expression was complete after the eighth hour. When the FFA concentration was doubled, a quicker inhibition occurred (Fig. 2).

PPAR{gamma}2 agonists, rosiglitazone and PGJ2, prevent lipotoxicity in isolated human islets. To explore the role of PPARs in the impairment of human islet function, human islets from four different pancreases were used to perform experiments with rosiglitazone and PGJ2. As illustrated in Figs. 3 and 4, incubation of human islets in the presence of the two agonists was associated with increased PPAR{gamma}2 mRNA expression. Moreover, both rosiglitazone and PGJ2 were able to completely prevent the suppression of PPAR{gamma}2 mRNA expression induced by 1.0 mM FFA. The changes in PPAR{gamma}2 mRNA expression under the different experimental conditions were paralleled by similar variations in the insulin mRNA expression: reduced by FFA and recovered by concomitant human islet incubation with 1.0 mM FFA and rosiglitazone or PGJ2 (Figs. 3 and 4).



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  		Fig. 3. Expression of PPAR{gamma}2 mRNA in control human islets and in islets incubated for 24 h with rosiglitazone (1.5 µg/ml), 15-deoxy-{Delta}-12,14prostaglandin J2 (PGJ2; 10 µM), FFA (1.0 mM), and rosiglitazone + FFA (A) or PGJ2 + FFA (B). The figure is representative of 4 experiments.

 


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  		Fig. 4. Real-time quantitative RT-PCR PPAR{gamma}2 (A) and insulin mRNA expression in human islets (B) after 24-h incubation with rosiglitazone (1.5 µg/ml), PGJ2 (10 µM), FFA (1.0 mM), and FFA + rosiglitazone or PGJ2. *P < 0.05 vs. all conditions. Results represent the means of 4 experiment series from 4 different pancreases.

 

In control islets, acute elevation of glucose concentration in the medium was associated with an increase in insulin release from 2.2 ± 1.5 to 4.7 ± 1.5% of insulin content. In contrast, elevation of fatty acids in the medium resulted in a lower basal insulin release (data not shown). Table 1 summarizes the results of insulin release as well as total insulin content in the different experimental condition explored. The addition to the medium of the two PPAR{gamma}2 agonists rosiglitazone and PGJ2 did not alter glucose-mediated insulin release nor total insulin content (Table 1 and Fig. 5). As already observed, incubation of human islets in the presence of 1.0 mM fatty acid was associated with a marked impairment of insulin release (0.6 ± 0.05 and 0.9 ± 0.6% of islet content for rosiglitazone or PGJ2 experiments, respectively; P < 0.01 vs. control) and significant impoverishment of islet insulin content (Table 1).


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  		Table 1. Effects of 24-h incubation of human islets with FFA (1.0 mM rosiglitazone (1.5 µg/ml), PGJ2 (10 µM), rosiglitazone plus FFA, or PGJ2 plus FFA on glucose-stimulated insulin release and insulin content

 


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  		Fig. 5. Glucose-mediated insulin release (%total insulin content) from human islets after 24-h incubation with rosiglitazone (1.5 µg/ml), PGJ2 (10 µM), FFA (1.0 mM), and FFA + rosiglitazone or PGJ2. Data are presented as percent change from control islets. *P < 0.05 vs. all conditions. Results represent the means of 4 experimental series from 4 different pancreases.

 

When the two PPAR{gamma}2 agonits were coincubated with 1.0 mM fatty acids, insulin release at low glucose concentration was no more different compared with control islets (data not shown). In addition, insulin release in response to high glucose and insulin content increased significantly with no difference compared with control islets (Fig. 5 and Table 1).

Rosiglitazone reduces triglyceride content in human islets. Triglyceride content was determined in human islets exposed for 24 h to 1.0 mM fatty acids, rosiglitazone (1.5 µg/m) and the combination of the two. Compared with control islets (1.1 ± 0.4 µg/ml), fatty acids cause an increase in triglyceride content (2.1 ± 0.8 µg/ml, P < 0.05), whereas a decrease (0.3 ± 0.1 µg/ml, P < 0.05) followed incubation in the presence of rosiglitazone. When coincubated, rosiglitazone completely prevented triglyceride accumulation in the isolated islets (0.8 ± 0.4 µg/ml).

To assess whether changes in lipid metabolism could be due to differentiation of fibroblasts present within the islet into adipocytes FABP4 mRNA expression was determined under the aforementioned experimental conditions (Fig. 6). The level of expression normalized by {beta}-actin mRNA was not modified by fatty acids (0.059 ± 0.004), rosiglitazone (0.061 ± 0.002), or the combination of the two (0.024 ± 0.006) compared with control islets (0.077 ± 0.001). In all conditions, moreover, the level of expression was at least 10-fold lower compared with adipose tissue (0.743 ± 0.085).



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  		Fig. 6. Expression of fatty acid-binding protein (FABP4) mRNA in control human islets and in islets incubated for 24 h with rosiglitazone (1.5 µg/ml), FFA (1.0 mM), rosiglitazone + FFA, and in human adipose tissue. The figure is representative of 3 experiments.

 


   DISCUSSION
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The present study strongly supports a role for PPAR{gamma}2 in the modulation of insulin secretion from isolated human islets. In particular, our results suggest that PPAR{gamma}2 may prevent the lipotoxic effect on the {beta}-cell exerted by increasing the fatty acid concentration in culture medium.

Our finding confirms that the mRNA for PPAR{gamma} is expressed in human pancreatic islets (21) and provides evidence for expression of PPAR{beta} mRNA as well. In contrast, by using specific primers previously employed with other human tissues (1, 16, 42, 47), we could not detect PPAR{alpha} mRNA expression, suggesting a tissue-specific distribution of the PPAR subtypes. No specific assay was performed in the present study to determine the contribution of endocrine cells compared with the nonendocrine component. Nonetheless, contamination by adipose cells is unlikely, given the different pattern of expression of the PPAR subtypes in the islets compared with adipose tissue (Fig. 1). The same was true by comparison with pancreatic acinar cells (data not shown). Therefore, as indicated by Dubois et al. (15), PPAR{gamma} appears to be preferentially localized in the endocrine cells of pancreatic islets.

Interestingly, we observed that FFA, but not glucose, induced a dose- and time-dependent decrease of PPAR{gamma}2 mRNA expression. PPARs are fine modulators of intracellular lipid metabolism. PPAR{gamma} (and in particular the {gamma}2-isoform) plays a major role in stimulating the uptake of fatty acids, their intracellular binding, and their esterification, whereas PPAR{alpha} promotes {beta}-oxidation (23, 24, 37). Therefore, the ratio between these two PPAR subtypes is likely to affect the metabolic status of the cell, in particular lipid accretion vs. lipid depletion (14, 23, 24, 37). In turn, this is likely to affect insulin-secretory capacity. Triglycerides are of importance for the stimulus-secretion coupling in the {beta}-cells, and a lipid-derived signal, possibly provided by hormone-sensitive lipase, may participate in the modulation of insulin secretion (30, 32). On the other hand, chronic elevation of the fatty acids leads to islet triglyceride accumulation and increased production of fatty acyl-CoA derivatives, such as ceramide, causing adverse effects on insulin secretion and {beta}-cell survival (lipotoxicity) (17, 23, 27, 36, 40, 50, 51, 52).

Our finding that human islets express PPAR{gamma}2 mRNA to a greater extent than PPAR{alpha} mRNA could be interpreted as a condition favoring lipid accumulation and, hence, lipid-induced damage. From this point of view, the inhibition of PPAR{gamma}2 mRNA expression in response to FFA but not glucose elevation represents an intriguing finding, as it may reflect a defensive adaptation to lipid accretion inside the cell. A downregulation of PPAR{gamma}2 mRNA after exposure to its agonists has also been observed in brown adipocytes (46). These findings suggest the presence of feedback between the mRNA expression of PPAR{gamma}2 and at least some PPAR ligands.

With such a scenario in mind, we explored the possible link between PPAR{gamma}2 mRNA expression and glucose-mediated insulin release. To this purpose both a natural (PGJ2) and a synthetic (rosiglitazone) PPAR{gamma} ligand were employed. Twenty-four-hour incubation with the two agonists did not affect insulin release, total insulin content, and insulin mRNA expression. This finding is in agreement with previous reports obtained both in human islets (15) and in lean wild-type (+/+) ZDF rats (39), although more recent studies have reported a dose-dependent effect of rosiglitazone on insulin release from rat pancreatic islets (48). On the other hand, a marked reduction in both basal and glucose-stimulated insulin release was observed after a 24-h incubation of human islets in the presence of 1 mM fatty acids. In agreement with previous findings (7), elevation of fatty acids not only affected glucose-mediated insulin release but also led to a significant reduction in total insulin content. The inhibition of insulin release was well expected on the basis of the lipotoxicity concept (17, 23, 27, 36, 40, 50, 51, 52).

Although the increased availability of fatty acids may be associated with specific metabolic alteration at the {beta}-cell level that might account for defective insulin secretion, other mechanisms could be invoked. This also was suggested by the suppression of PPAR{gamma}2 mRNA in the presence of fatty acid-enriched medium, whereas concomitant incubation with rosiglitazone and PGJ2 was associated with prevention of the suppression of PPAR{gamma}2 mRNA expression, insulin release, and islet insulin content. These results are very much in line with those obtained in islets from obese homozygous (fa/fa) ZDF rats, where troglitazone restored {beta}-cell insulin secretion pattern by lowering islet fat content (38).

A possible limitation of our study could be the difficulty in separating a toxic vs. a functional effect of increased FFA availability due to the low albumin-to-FFA ratio we have employed. In a separate set of experiments, islets (n = 3) were incubated in the presence of medium enriched with more physiological albumin-to-FFA mixture (6% human albumin:1 mM FFA). For the sake of discussion, data are presented in Table 2. At variance with the original set of data, no suppression of insulin release was found at 3.3 mM glucose. Nonetheless, glucose-stimulated insulin release was suppressed by 53% in the presence of high FFA concentration, similarly to what observed in the original set of studies. It is of note that, even assuming a toxic effect, the effects of PPAR{gamma} agonists are retained, probably due to prevention of triglyceride accumulation.


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  		Table 2. Insulin release (µU/ml) in 3 different experiments performed in the absence or presence of 6% human albumin: 1 mM fatty acid mixture

 

In summary, although PPAR agonists may not have a direct effect in modulating insulin secretion, they may preserve the {beta}-cell under condition of an excess of fatty acid availability, and therefore preserve the {beta}-cell from lipotoxicity. This conclusion is supported by recent studies by Finegood et al. (18), showing that rosiglitazone treatment hampers the loss of {beta}-cell mass in obese ZDF rats by maintaining {beta}-cell proliferation and preventing net {beta}-cell death.

The mechanism(s) triggered by PPAR{gamma}2 were not investigated in this study. In fat-laden pancreatic islets from obese ZDF rats, troglitazone has been shown to exert a lipopenic action that was accompanied by a >30-fold increase in glucose-mediated insulin release and a doubling of the response to arginine (39). Such a lipopenic effect is confirmed in the present study. In fact, rosiglitazone alone did reduce the triglyceride content in isolated human islets. Moreover, rosiglitazone completely prevented the triglyceride accretion associated with islet incubation in the presence of high fatty acids. In vivo, this effect is likely to be amplified because of the ability of glitazones to prevent the spillover of lipids from adipose tissues into nonadipose tissues. The changes occurring in islet triglyceride content cannot be taken as direct evidence for {beta}-cell triglyceride accumulation. One could argue that fibroblasts within the islet may differentiate into adipocytes in response to PPAR{gamma} activation leading to accumulation of triglyceride and diversion of fatty acids from {beta}-cell metabolism. According to this view, a lipogenic rather than a lipopenic effect would be responsible for the rescue of {beta}-cell function in the presence of fatty acid abundance. Our results, however, dismiss this hypothesis on the basis of the absence of any transcription of FABP4 mRNA (Fig. 6), a marker for adipocyte differentiation.

The modulation of insulin secretion by FFA excess is unlikely to be mediated exclusively by suppression of PPAR{gamma}, since other compounds, not necessarily affecting these nuclear receptors, also have been shown to protect human islet function from FFA insult. In a recent study (25), addition of 2.4 µg/ml metformin to a high-FFA medium also resulted in a preservation of insulin release. Although metformin does not interact with PPARs, a common pathway of lipid metabolism may be activated by both metformin and rosiglitazone. The work by Unger and colleagues (43-45) has clearly shown a lipopenic effect of troglitazone in the {beta}-cell. In our hands (25), metformin prevented the accumulation of triglycerides. More recently, Cnop et al. (13) have suggested that FFA can cause the death of normal rat islet cells through a nitric oxide-independent mechanism, suggesting that accumulation of triglyceride may serve as a cytoprotective mechanism against FFA-induced apoptosis by preventing a cellular rise of toxic FFA-derived moieties. Therefore, we cannot exclude the possibitlity that the final mechanism involved in prevention of islet function (i.e., insulin release) may be mediated through the activation of metabolic pathways that reduce the intracellular formation of toxic compounds.

Our results may lead to a paradox because PPAR{gamma} activation appears to exert a lipogenic effect in adipocytes but a lipopenic action in human islets. A possibility to reconcile this paradox may rely on different molecular mediators. For instance, white adipose tissue does not express PPAR{gamma} coactivator-1{alpha} (PGC-1{alpha}) (33), whereas the {beta}-cell and brown adipose tissue do so. Forced expression of PGC-1{alpha} in white adipose tissue is associated with mitochondrial biogenesis (34). Recent data suggest that this coactivator is responsible for the activation of thermogenesis and oxidative metabolism in both brown fat and muscle (41). The PGC-1-dependent program includes both mitochondrial biogenesis and tissue-specific expression of un-coupling proteins. Recent work from Spiegelman et al. (49) has shown that the transcriptional coactivator PGC-1{alpha} mRNA and protein levels are significantly increased in islets from multiple animal models of diabetes and that adenovirus-mediated expression of PGC-1{alpha} also produces inhibition of glucose-stimulated insulin secretion. Thus one may postulate that, by suppressing PPAR{gamma}, fatty acid reduces {beta}-oxidation in the {beta}-cell, whereas treatment with the agonists restores the protection via PGC-1{alpha}.

In summary, our unique data, obtained with studies performed in isolated human islets, indicate that PPAR agonists such as rosiglitazone, initially proposed as insulin sensitizers, may exert a more comprehensive action, including a direct effect at the level of the {beta}-cell. This effect is of particular interest because it seems to be operative mainly under conditions of islet fat accretion or increased fatty acid availability. A disturbance of fatty acid metabolism characterizes type 2 diabetes (21) and exerts a lipotoxic effect both in peripheral tissues and on the pancreatic {beta}-cell. It is this simultaneous effect on both sites that may account for improvement of insulin sensitivity and insulin secretion that has been documented with glitazone treatment in impaired glucose tolerance patients (12) and in women with previous gestational diabetes (9).


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 GRANTS
REFERENCES
 
This work was supported by grants from the Italian Ministero Università e Ricerca Scientifica e Tecnologica (Grant no. MM06AQ2414).


   FOOTNOTES
 

Address for reprint requests and other correspondence: P. Marchetti, Dept. of Endocrinology and Metabolism, Section of Diabetes, Ospedale Cisanello, via Paradisa 2, 56100, Pisa, Italy (E-mail: marchant{at}immr.med.unipi.it).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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