Role of PAT proteins in ectopic fat deposition

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Ectopic fat deposition, which is the accumulation of lipid droplets outside the adipose tissue, is well recognized as a strong prognostic factor for the development of insulin resistance and "metabolic syndrome" in obesity and people with lipodystrophies. Therefore a key question has been raised as to whether ectopic fat is not simply a marker but in fact a mediator of disease. We now have preliminary results in a liver cell culture system where siRNA gene inhibitors of lipid droplet surface proteins induce insulin resistance, providing some of the first evidence suggesting that ectopic fat is indeed a mediator of disease. To continue this promising line of investigation to better understand the pathophysiology associated with ectopic fat deposition, we propose to investigate the function and mechanism of lipid droplet surface protein to control lipolysis in non-adipogenic cells and tissues as well as potential changes in lipid droplet composition and activity as the underlying cause of these pathophysiological consequences of obesity, and potentially as a new biochemical pathway for therapeutic intervention. In the healthy state, lipid droplets maintain cellular non-esterifedfatty acid (NEFA) homeostasis by storing any excess NEFA as triglyceride (TAG) and releasing it when needed. This ability to minimize excess NEFA is thought to provide protection from "lipotoxicity", characterized by insulin resistance leading to inflammation, and in extreme cases cellular apoptosis. In addition, both of the pathways for storage and release of NEFA involve the intermediate production of diacylglyceride (DAG), which also has a signaling function. It is possible that an increase in DAG production by disregulated lipolysis may have a role in the adverse effects associated with ectopic fat. Therefore, regulation of lipid droplet storage AND release of NEFA is a strong candidate for a mechanism by which ectopic fat contributes to disease. Although lipid droplet regulation remains poorly understood, numerous studies have revealed that one or more membersof the PAT protein family always seem to be associated with the lipid droplet surface and one, Perilipin, has been shown to help regulate adipocyte lipolysis. The PAT protein family is composed of lipid droplet coat proteins sharing high primary sequences, and similarity with apolipoproteins, and some are tissue specific. Therefore, the overall goal of this proposal is to understand the pathophysiological importance of three functionally uncharacterized PAT proteins, ADRP,TIP47 and PAT-1 (or MLDP) known to be expressedin non-adipogenic tissues. Given our preliminary results, these proteins are strong candidates for regulating lipid droplet lipolysis in non-adipogenic tissues and their disfunction is a likely cause of ectopic fat mediated insulin resistance. These proposed studies will test this hypothesis, and if correct provide an approach for prevention of disease or therapeutic intervention.