Non-alcoholic fatty liver disorders (NAFLD) are the most common cause of abnormal serum liver chemistry tests in the United States and are the leading causes of cryptogenic cirrhosis. Although NAFLD is associated with the metabolic syndrome, obesity, hyperlipidemia and insulin resistance, the pathophysiology of fatty liver disorders remains poorly understood. There is increasing evidence that endoplasmic reticulum (ER) stress may be involved in the progression of the hepatic injury that occurs with NAFLD. The ER is an intracellular organelle involved in protein folding, packaging and secretion. Pathophysiologic conditions such as obesity, fatty liver and hyperinsulinemia result in excess misfolding of proteins and ER stress. The cellular response to ER stress is activation of the Unfolded Protein Response (UPR);a cellular signaling mechanism that reduces protein translation, increases ER associated protein degradation and increases production of chaperone molecules to enhance protein trafficking. Prolonged UPR activation can then induce inflammatory and apoptotic pathways. SREBP-lc is a hepatic protein that regulates hepatic lipid and glucose metabolism. The high-fat, high calorie (HFHC) diet is a well accepted nutritional model for causing non-alcoholic fatty liver disorders in mice. When mice are fed a HFHC diet, hepatic triglycerides increase, producing ER stress and activation of SREBP-lc pathway, lnsig-1 and lnsig-2 are regulatory proteins that bind to SREBP-lc and prevents its translocation to the nucleus. I hypothesize that when mice are fed a HFHC diet, lnsig-1 and lnsig-2 levels decline thereby allowing SREBP-lc to translocate to nucleus and increase hepatic triglyceride synthesis and hepatic steatosis. Thus, I will examine the role of Insig in the pathogenesis of murine fatty liver by feeding C57BL/6J mice a high fat, high calorie (HFHC) diet and determining the role of Insig signaling and hepatic steatosis (Specific Aim #1A). To demonstrate a direct role of Insig in hepatic steatosis, I will feed Insig 2-null and wild-type mice a HFHC diet and study the resulting effects on hepatic steatosis (Specific Aim #1B). To further investigate the importance of lnsig-1 and lnsig-2 in averting hepatic steatosis in mice fed a HFHC, I will upregulate insig expression in vivo by activating PPAR-gamma pathway which has been shown to increase insig expression. This will be done via gene-mediated therapy and phamacologically with Rosiglitazone, a PPAR-g agonist (Specific Aim #2).
These studies will help delineate the role of the Insig pathway in non-alcoholic fatty liver disorders and enhance our understanding of these common hepatic diseases, especially with the increasing incidence of obesity and metabolic syndrome in the United States.