Cellular and molecular mechanisms of fatty liver disease

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Project Summary Cytosolic accumulation of triglyceride (TG)-rich lipid droplets (LDs) is a hallmark of nonalcoholic fatty liver disease (NAFLD). Many LD-associated proteins are implicated in the pathogenesis of human and rodent NAFLD, including CGI-58 (Comparative Gene Identification-58). Mutations in human CGI-58 gene cause ichthyosis (thickened dry scaly skin) and TG-rich LD accumulation in most cell types. Patients display NAFLD ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), fibrosis and cirrhosis. CGI-58 was shown to promote TG hydrolysis in vitro by activating Adipose Triglyceride Lipase (ATGL), but ATGL mutations in humans cause no ichthyosis. While liver-specific CGI-58 knockout (LivKO) mice develop severe hepatic steatosis, NASH and fibrosis even on a regular chow diet, liver-specific ATGL KO mice display only mild hepatic steatosis without NASH and fibrosis. These phenotypic differences between ATGL and CGI-58 mutations provide us a unique opportunity to exploit molecular mechanisms of NAFLD progression. Two major pathways are implicated in intracellular LD breakdown: 1) Cytosolic/Neutral Lipolysis mainly mediated by ATGL, and 2) Lysosomal/Acidic Lipolysis mediated by a lipid-specific macroautophagy (lipophagy). Lipophagy brings cytosolic LDs to lysosomes for degradation by acidic lipases, but its molecular details are unknown. Our preliminary data suggest that hepatic autophagy-related proteins are reduced, the major upstream autophagy inhibitory signaling pathway (mTORC1) is activated, and there is a defect in association between LDs and acidic organelles in CGI-58-deficient liver and/or hepatocytes. It was recently shown that perilipin 2, a major LD coat protein known to interact with CGI-58, also binds the heat shock cognate protein of 70 kDa (hsc70) for degradation via chaperone-mediated autophagy (CMA), and inhibition of CMA reduces both Neutral and Acidic Lipolysis, leading to severe hepatic steatosis and liver damage. Based on these observations and our preliminary data, we hypothesize that CGI-58 may coordinate with hsc70 to stimulate LD uncoating by promoting perilipin 2 degradation via CMA, thereby activating both ATGL lipolysis and lipophagy. Lipophagy failure may in turn inhibit autophagy by altering cellular energy balance. This hypothesis may explain why ATGL and CGI-58 mutations cause overlapping yet distinct phenotypes. We will test this central hypothesis by comparing liver-specific CGI-58 and ATGL KO mice to determine whether liver CGI-58 and ATGL differentially, regulates lipophagy and autophagy. We will then perform detailed protein-protein interaction studies to examine whether CGI-58 and hsc70 coordinate CMA-mediated LD uncoating (degradation of perilipin 2). Finally we will determine if mTORC1 inhibition and autophagy induction protect against NAFLD progression in CGI-58 LivKO mice. Given that CGI-58 resides at the critical crossroad of cellular fat breakdown, this project holds promise of revealing general mechanisms for progression of NAFLD, a major public health problem.