Absence of Vitamin D Signaling Increases Lipolysis and Fatty acid Synthesis, Leading to NAFLD

Nonalcoholic fatty liver disease (NAFLD) afflicts 20-30% of the population in Western countries. Type II diabetes patients demonstrate NAFLD that worsens to hepatic fibrosis. In healthy individuals, adipose tissue is the primary source of free fatty acids (~70%) for liver triglyceride synthesis. In insulin resistance, insulin fails to inhibit lipolysis in adipocytes, causing excess free fatty acid supply to the liver, resulting in excess hepatic triglyceride synthesis and NAFLD. Therefore, molecular interventions to inhibit lipolysis could prevent or treat NAFLD. However, because the molecular mechanisms underlying adipose lipolysis in NAFLD remain poorly defined, no targeted therapies exist. Low vitamin D levels have been reported in obesity, Type II diabetes, prediabetes, and the metabolic syndrome. Vitamin D exerts its physiological effects by binding to the vitamin D receptor (VDR), a nuclear hormone receptor. While direct VDR actions in liver have been shown to suppress hepatic fibrosis, the role of impaired vitamin D signaling in adipocytes as a cause of NAFLD and hepatic fibrosis remains unknown. In this regard, our preliminary data in global VDR null mice show a fatty liver phenotype as well as increased PPARγ mRNA expression and increased expression of genes involved in fatty acid synthesis and lipolysis in gonadal white adipose tissue (gWAT). Therefore, we hypothesize that absence of vitamin D signaling increases adipose lipolysis and fatty acid synthesis, leading to NAFLD. Herein, we will use Adiponectin-Cre to conditionally knockout VDR expression in adipocytes. We will analyze lipolysis and fatty acid synthesis in gWAT; serum lipid profiles; and inflammation, steatosis, and fibrosis in the liver. mRNA expression of genes involved in lipolysis (LPL and its transcriptional regulator PPARγ and fatty acid synthesis (FASN) will be measured in gWAT. VDR and PPARγ co-recruitment to regulatory sites on the PPARγ and LPL genes will be evaluated by sequential ChIP-qPCR analysis in primary adipocytes. Interaction of VDR and PPARγ proteins will be examined by co-immunoprecipitation analysis in primary adipocytes. Our studies will define a role for the VDR in suppression of lipolysis and fatty acid synthesis and demonstrate that impaired vitamin D signaling in adipose tissue leads to NAFLD. Importantly, the results of the proposed studies are expected to identify the VDR as a therapeutic target in the prevention and treatment of NAFLD in Type II diabetes.