Nutritional and hormonal regulation of leptin: Importance of posttranscriptional mechanisms

Leptin is a hormone made by adipocytes in proportion to body fat stores and functions to regulate food intake, energy expenditure, endocrine systems and immunity. Chronic as well as acute nutritional status influences serum leptin levels. Counter-regulation of leptin expression by insulin (stimulation) and beta-adrenergic receptor (inhibition) is thought to mediate leptin responses to feeding/starvation but the mechanisms involved are not well understood. Studies using metabolic labeling and immunoprecipitation showed that relative rates of leptin biosynthesis in human adipose tissue (hAT) were highly correlated with tissue content and release. Culture with insulin increased leptin biosynthesis in hAT of obese without affecting mRNA levels. However, hAT from nonobese was unresponsive to insulin, suggesting that both chronic hyperinsulinemia and enhanced responsiveness to insulin contribute to higher leptin biosynthesis in obesity. Cellular mechanisms by which obesity and starvation influence leptin production were studied using rat AT. Starvation decreased leptin biosynthesis with little or no changes in leptin mRNA levels. Insulin increased leptin biosynthesis by 2∼3 fold without affecting leptin mRNA levels. Analysis of polysome profiles and reporter constructs containing various regions of leptin UTRs supported the hypothesis that leptin translation is regulated. Whereas most leptin mRNA was associated with monosomes in starved state, it shifted to the polysome fractions in fed rats and after in vitro insulin treatment. The 5'UTR of leptin increased translational efficiency by 2--3 fold while the 3'UTR inhibited it by 65%. Insulin increased translational efficiency of the inhibitory constructs. Pulse-chase studies showed that insulin also stimulated leptin secretion and prevented intracellular degradation in AT from fed but not in starved rats. Although isoproterenol (a beta-AR agonist) alone did not affect basal but decreased insulin-stimulated leptin secretion by blocking the insulin-induced leptin biosynthesis in rat AT. In contrast, isoproterenol decreased leptin release without affecting leptin mRNA levels or rates of leptin biosynthesis in hAT. In conclusion, nutritional status and insulin regulate leptin production by modulating leptin translation, storage, degradation and secretion. This multi-level modulation ensures accurate adjustments in circulating leptin in response to changes in adiposity and food intake that are critical to maintenance of energy homeostasis.