The role of MDM2 in the pathogenesis of obesity-induced non-alcoholic fatty liver disease

Abstract

Background and objective: Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent form of the chronic liver disease that affects about a quarter of the population in the world, and its prevalence is continuously increasing with the growing of obesity. NAFLD comprises a wide spectrum of liver dysfunction, ranging from simple steatosis, steatohepatitis to fibrosis and subsequent cirrhosis. Recent research demonstrated that dysregulation of lipid metabolism plays a crucial role in NAFLD progression. In particular, reduced hepatic very-low-density-lipoprotein (VLDL) triglyceride secretion is observed in human subjects with non-alcoholic steatohepatitis but not in those with simple steatosis. Abrogation of VLDL-triglyceride secretion by inhibiting apolipoprotein B (ApoB) or microsomal triglyceride transfer protein (MTP) (two key molecules responsible for assembly and secretion of VLDL-triglyceride) induces liver injury in a rodent model. The E3 ubiquitin ligase MDM2 is a well-known negative regulator of p53. In addition, our research team and others recently demonstrated that this oncogene also plays a vital role in metabolic diseases including type 2 diabetes and adipose tissue dysfunction. In this study, I further explored the potential role of MDM2 in the regulation of liver metabolism and dysfunction in obesity.

Key findings: 1. Protein expression of MDM2 was significantly increased in dietary-induced and genetically-inherited NAFLD mouse models. 2. By using Cre-LoxP tissue-specific knockout approach, I generated a hepatocyte-specific MDM2 knockout mice (HMDM2KO) and fed the mice with a high-fat-high-cholesterol (HFHC) diet to induce obesity and NALFD. This knockout mouse model exhibited normal energy homeostasis and glucose metabolism under lean or obese condition. 3. HFHC diet-induced hepatic steatosis and inflammation in wild-type (WT) mice, but this detrimental effect of HFHC diet was alleviated by hepatic deletion of MDM2. 4. HMDM2KO mice fed with HFHC diet displayed an increase of the postprandial circulating level of triglyceride, and the increase was due to upregulation of VLDL-triglyceride secretion. On the other hand, genetic deletion of MDM2 had no effect on cholesterol metabolism. 5. Protein expression of ApoB in the liver and circulation was markedly increased in HFHC diet-fed HMDM2KO mice when compared to their WT controls. In addition, siRNA-mediated knockdown of MDM2 increased VLDL-triglyceride secretion and ApoB expression in HepG2 cells, whereas overexpression of MDM2 exerted opposite effects. 6. At the molecular level, MDM2 interacted with ApoB, which in turn induced lysine 48-linked polyubiquitination of ApoB, leading to proteasomal degradation ApoB in HepG2 cells. On the other hand, blocking the interaction between ApoB and MDM2 by the chemical compound nutlin-3a increased ApoB expression and VLDL-triglyceride secretion in HepG2 cells.

Conclusion: In this study, I uncovered a new role of MDM2 in the regulation of hepatic VLDL metabolism in obesity. My findings raise a possibility that blocking MDM2 activity is able to alleviate obesity-induced NALFD by promoting VLDL-triglyceride secretion, without any effect on glucose or energy metabolism.