MOLECULAR CONTROL OF LEPTIN SECRETION FROM WHITE ADIPOCYTES

Abstract

Leptin plays a critical role in energy homeostasis by increasing energy expenditure and reducing food intake. Little is known about how leptin secretion is regulated, which is important, as dysregulation of leptin secretion leads to the disturbance of the homeostasis in different systems. In the present study, the leptin secretory pathway and the potential role of calcium, a common trigger for regulated secretion, in leptin secretion was investigated. Leptin was found to be stored in membrane-bound vesicles localizing predominantly in the endoplasmic reticulum (ER) and the plasma membrane (PM) of both primary and 3T3-L1 adipocytes. Insulin increased leptin secretion as fast as 15 minutes without affecting the leptin mRNA level within 2 hours (h). Both extra- and intracellular calcium was found to be required for basal and insulin-stimulated leptin secretion, but calcium influx alone is not sufficient for leptin secretion. Protein synthesis inhibitor cycloheximide (CHX) and ER-Golgi trafficking blocker Brefeldin A (BFA) treatment inhibited both basal and insulin-stimulated leptin secretion, suggesting that leptin-containing vesicles go through the classic ER-Golgi route and insulin stimulates leptin secretion by up regulating the constitutive leptin secretion. To further explore the potential role of calcium, the signaling pathways involved in insulin-stimulated leptin secretion were investigated. The phosphoinositide 3-kinase (PI3K)-Akt but not MAPK pathway was demonstrated to be involved in insulin-stimulated leptin secretion in vitro and in vivo. Akt phosphorylation was significantly attenuated when extra- and intracellular calcium was removed, suggesting that calcium is required for insulin-stimulated Akt activation.