CARF (CDKN2AIP) Regulates Hepatic Lipid Metabolism and Protects Against Development of Non-Alcoholic Fatty Liver Diseases

Non-alcoholic fatty liver diseases (NAFLD) is the most common form of liver diseases in the USA with 30–40% of American being affected and about 12% with nonalcoholic steatohepatitis (NASH), a leading cause of end-stage liver diseases. NAFLD has been linked with insulin resistance, type2 diabetes, obesity, and cardiovascular diseases but molecular mechanisms underlying the development of NAFLD and its association with metabolic syndromes are poorly understood. In this study, we explored the role of CARF (collaborator of ARF) also known as CDKN2AIP, a novel gene of ARF-MDM2-p53 pathway in the development of NAFLD. It has been shown that, p53, beyond its tumor suppressor functions, can regulate the cellular glucose and lipid metabolism and its activation has been reported to induce hepatic steatosis in mice. However, as a regulator of p53 pathway, the role of CARF in the lipid metabolism and associated metabolic diseases has not been studied yet. Using high-fat diet (HFD) fed obesity mouse model of NAFLD we found that the expression of CARF along with Sirt1, pAMPK, and pACC was significantly decreased in the HFD induced fatty livers compared to control. Similarly, CARF expression was also down-regulated in palmitate (PA)-treated HepG2 cells, an in vitro model of steatosis. We also observed that shRNA mediated knockdown or lentiviral vector mediated overexpression of CARF induced or reduced the endogenous fat accumulation, respectively, in HepG2 cells, suggesting that CARF expression is negatively regulated in NAFLD. Additionally, we performed RNA seq analysis after CARF silencing in HepG2 cells and demonstrated that silencing of CARF altered the expression of genes regulating hepatic de novo lipogenesis, beta-oxidation, and lipid secretion all of which favor the accumulation of fat in the hepatocytes. Furthermore, genes associated with mitochondrial functions such as the TCA cycle and oxidative phosphorylation were also altered which could play a role in the development of NAFLD. Finally, we demonstrated that AAV mediated hepatic overexpression of CARF in HFD fed mouse model significantly reduced the fat accumulation in the liver as evident by H&E staining of liver sections and intrahepatic triglyceride level. Altogether we conclude that CARF plays a vital role in hepatic lipid metabolism and its downregulation perturbs lipid homeostasis leading to hepatic steatosis and the development of NAFLD.