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1 Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, United States; Biochemistry, Wake Forest University Health Sciences, Winston-Salem, North Carolina, United States
* To whom correspondence should be addressed. E-mail: clynch{at}psu.edu.
In vitro, leptin secretion is regulated at the level of mRNA translation by the rapamycin-sensitive mammalian target of rapamycin (mTOR) and its agonist leucine (Leu). Studies were conducted on meal-trained rats to evaluate the potential physiological relevance of these in vitro observations and the role of leucine in affecting rises in plasma leptin observed after a meal. In the first study we correlated changes in plasma insulin and Leu, to mTOR signaling pathway activation and plasma leptin at different times during meal feeding. Rapid rises in plasma insulin and Leu, along with mTOR signaling (phosphorylation of eIF-4G, S6K1, ribosomal protein S6 and 4E-BP1) in adipose tissue were observed during the 3h meal and declined thereafter. Plasma leptin rose more slowly, peaking at 3 h, and was inhibited by rapamycin (0.75 mg/kg) pretreatment. In another experiment, oral Leu or norleucine was provided instead of a meal. Leu and norleucine stimulated a rise in plasma leptin, however the magnitude was less than the response to a complete meal. In a third study, rats were provided a meal that lacked either Leu, branched chain amino acids or all amino acids. Stimulation of leptin secretion was reduced approximately 40% in animals provided the Leu deficient meal. Further reductions where not observed by removing the other amino acids. Thus Leu appears to regulate most of the effects of dietary amino acids upon the postprandial rise in plasma leptin, but is only responsible for part of the leptin response to meal feeding.
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