Regulation of fatty acid metabolism in adipocytes

The regulation of energy storage and utilization in adipocytes is a dynamic process that influences overall energy homeostasis. Adipocytes store nutrients in lipid droplets as triglycerides (TG), and mobilize them as needed. While these cells respond to sympathetic signals by increasing TG lipolysis to release free fatty acids (FFA) and glycerol, they also reabsorb FFA for re-esterification as triglycerides or alternatively for oxidation. Activation of b-adrenergic receptors and downstream cyclic AMP signaling not only increases lipolysis, but also promotes fatty acid oxidation at the expense of re-esterification, although the underlying mechanisms remain poorly understood. We hypothesize that catecholamines direct fatty acids for oxidation through regulation of signal transducer and activator of transcription 3 (STAT3) and suppression of glycerol-3-phosphate acyltransferase 3 (GPAT3). Our preliminary data demonstrate that STAT3 specifically undergoes Ser727 phosphorylation at the lipid droplet in response to stimulation of b-3 adrenergic receptors and activation of lipolysis in adipocytes. The pool of lipid droplet-associated STAT3 binds to and inhibits GPAT3, effectively suppressing GPAT3-catalyzed re-esterification, to promote fatty acid oxidation. Adipocyte-specific Stat3 KO mice exhibit normal rates of lipolysis, but manifest a specific defect in lipolysis-driven oxidative metabolism, resulting in reduced energy expenditure and increased adiposity on high fat diet. The experiments outlined in this proposal are designed to expand insights into this novel function of STAT3, determining its metabolic consequences and delineating the mechanism of action. Aim 1 will focus on the stimulation of STAT3 phosphorylation by catecholamines in vivo, delineating the signaling pathway and specific kinase(s) responsible for the critical STAT3 Ser727 phosphorylation event. The interaction of STAT3 with GPAT3 will be investigated in aim 2 using co-immunoprecipitation and in vitro binding assays. Additionally, the mechanism by which STAT3 interaction results in suppression of GPAT3 activity will be investigated using in vitro GPAT activity assays and proteomic approaches. Finally, in aim 3, the physiological relevance of this novel regulatory pathway in the development of obesity in males and females will be examined. Additionally, non-phosphorylatable STAT3 S727A mutant adipocytes and mice will be employed to determine the in vivo role of this phosphorylation site. These studies will provide a more complete understanding of the regulation of lipolysis-driven oxidative metabolism, and will improve our understanding of this energy expending pathway in white adipose tissue. This may lead to the development of new therapeutic approaches to curtail obesity and the devastating metabolic diseases with which obesity is associated.