Nuclear Envelope, Lipoprotein Metabolism, and Hepatic Steatosis

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The liver is a major site of lipid metabolism. Abnormalities of hepatic lipid metabolism cause nonalcoholic fatty liver disease (NAFLD), a burgeoning public health epidemic estimated to affect approximately 25% of the U.S. population. NAFLD encompasses a spectrum of liver pathologies, beginning with lipid droplet accumulation in hepatocytes, called simple steatosis, which may progress to nonalcoholic steatohepatitis (NASH). Individuals with NASH are at increased risk for subsequent fibrosis, cirrhosis and hepatocellular carcinoma. Although there is appropriate focus on the progression of simple steatosis to NASH and fibrosis, there is also an urgent need to advance understanding of the mechanisms governing the development of simple steatosis. This proposal addresses this need, identifying novel molecular components that are critical to the normal regulation of hepatic lipid metabolism and, when impaired, cause profound steatosis. Our extensive preliminary studies in vivo and in vitro newly identify the torsinA/lamina-associated polypeptide 1 (LAP1) complex of the inner membrane of the nuclear envelope as a novel regulator of intrahepatic lipid metabolism, and a potent driver of steatosis. Conditional hepatocyte-specific depletion of either torsinA (A-CKO) or LAP1 (L-CKO) causes significant steatosis, which can progress to NASH in mice fed a regular chow diet. These mice demonstrate significant reductions in the assembly and secretion of very low density lipoprotein (VLDL) triglycerides (TG) and apolipoprotein B (apoB). The hepatic steatosis in these mice is far more severe, however, than observed in existing murine models of reduced VLDL secretion or in humans with mutations in genes that affect VLDL secretion, indicating additional effects of loss of the torsinA/LAP1 complex. These effects are cell autonomous as the genetic deletion is hepatocyte-specific, and these mice exhibit normal body mass and no evidence of insulin resistance. Based on these and additional preliminary results, we hypothesize that disruption of the torsinA/LAP1 complex at inner nuclear membrane causes steatosis by impairing the transfer of newly synthesized lipids to downstream processes. In Aim 1, we will conduct in vivo, ex vivo, and in vitro studies to test the hypothesis that loss of the torsinA/LAP1 complex impairs VLDL assembly and secretion by preventing the transfer of TG to nascent apoB. In Aim 2, we will examine hepatic lipid metabolism in detail, including studies of nuclear lipid droplets (LD) present in L-CKO mice and lipid accumulated in the ER in A-CKO mice. Studies of fatty acid and TG synthesis, fatty acid oxidation, phospholipid synthesis, and lipid droplet formation and turnover, as well as examination of key proteins in LD biogenesis, will allow for identification of critical pathways involved in hepatic lipid accumulation. In Aim 3, we will define the epistatic relationship between torsinA and LAP1 in steatosis by testing if we can prevent it in L-CKO mice by overexpressing torsinA. Successful completion of these Aims will implicate new intrahepatic targets to control VLDL secretion and prevent hepatic steatosis. It will also advance understanding of the role of the nuclear envelope as a critical node of liver lipid metabolism.