Submitted on February 8, 2006
Revised on May 3, 2006
Accepted on May 31, 2006
Multilevel regulation of leptin storage, turnover and secretion by feeding and insulin in rat adipose tissue
Mi-Jeong Lee and Susan K. Fried
Division of Endocrinology, Dept of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201
Corresponding Author: sfried{at}medicine.umaryland.edu
The mechanisms the increased serum leptin in response to feeding are poorly understood. We therefore used metabolic labeling to directly assess leptin biosynthesis, secretion and turnover in adipose tissue from 14h starved compared to fed 12-14 week old rats. Starvation decreased serum leptin (-477%), adipose tissue (AT) leptin content (-325%), and leptin secretion during 3 h incubation (-6512%). Starvation did not affect leptin mRNA levels, but decreased rates of leptin biosynthesis by tissue fragments as determined by 35S-methionine/cysteine incorporation into immunoprecipitable leptin. Insulin in vitro did not acutely increase leptin biosynthesis or rates of 125I-leptin degradation. Pulse-chase studies showed that in AT from fed but not starved rats, insulin accelerated the secretion of 35S-leptin by ~2-fold after 30 and 60 minutes of chase. Degradation of newly-synthesized leptin was slower in AT of starved than fed rats (half lives of 50 and 150 minutes respectively). Inhibitor experiments showed that both lysosomes and proteosomes contributed to leptin degradation. In conclusion, feeding as compared to starvation influences leptin production at multiple post-transcriptional levels: synthesis, tissue storage, turnover and secretion. The insulin-stimulated release of leptin from a preformed intracellular leptin pool may contribute to increases in serum leptin levels after meals.
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