Excess energy is stored as triglyceride (TG) in cytosolic lipid droplets (LDs). In the liver, mobilization of LD associated TG for fatty acid oxidation (FAO) and lipoprotein-mediated secretion requires TG hydrolysis by hydrolase(s) and other factors. A protein implicated in TG hydrolysis is Comparative Gene ldenfificafion-58 (CGI-58), a member of the alpha/beta-hydrolase fold protein family. Mutations in CGI-58 are associated with Chanarin-Dorfman Syndrome, an autosomal recessive disease characterized by the accumulation of TGrich LDs in most cells, including hepatocytes. CGI-58 appears to be a coactivator of adipose triglyceride lipase (ATGL). Recently, CGI-58 was also shown to function as a lysophosphafidic acid acyltransferase (LPAAT) to generate phosphafidic acid (PA), an important lipid second messenger in many cell signaling pathways including inflammatory responses. In preliminary studies, we found that silencing of CGI-58 expression in hepatoma cells increases TG stores and reduces FAO and lipoprotein-TG secretion. When we overexpressed CGI-58, the opposite resulted. Interestingly, inhibifion of protein tyrosine kinases completely abolished CGI-58-induced decreases in cellular TG stores. We also found that transgenic mice overexpressing CGI-58 in the liver showed a much lower hepatic TG content only after infiammation was stimulated by a low dose of lipopolysaccharide, but not on a basal low fat diet. These data suggest that CGI-58 somehow links inflammafion to TG metabolism. A low-grade inflammation is associated with high fat diet (HFD) feeding that can cause fatty liver. Thus, the major goal of this project will be to define how CGI-58 limits hepatic TG stores in response to metabolic overioad and inflammation. We hypothesize that CGI-58 protects against TG stores by generating a lipid second messgenger (PA) in inflammatory signaling to facilitate TG hydrolysis.
In Specific Aim 1, we will examine the effects of hepafic overexpression of CGI-58 on hepatic steatosis, FAO and lipoprotein-TG production in mice challenged with a HFD.
In Specific Aim 2, we will define how CGI-58's LPAAT acfivity regulates cellular TG stores, inflammatory signaling, and PA production in response to fatty acid loading and inflammatory stimulation. Together, these studies will provide novel insight into molecular mechanisms responsible for the partitioning of TG within organisms during overnutrition and inflammation, and potenflally reveal new approaches for the treatment of fatty liver and associated metabolic risk factors, which substantially contribute to disease morbidity and mortality.
