MST3 and MST4: Critical Regulators of Liver Lipid Partitioning and Hepatocarcinogenesis

Abstract: Non-alcoholic fatty liver disease (NAFLD) encompasses a disease spectrum ranging from hepatic steatosis to non-alcoholic steatohepatitis (NASH), fibrosis, and finally cirrhosis. With no approved pharmacological treatment, NAFLD is currently the most common etiology of chronic liver disease in Western countries and its prevalence continues to grow. Recently, NASH, characterized by liver steatosis, inflammation, cell injury (hepatocyte ballooning), and different stages of fibrosis, has emerged as a major catalyst for hepatocellular carcinoma (HCC), which is one of the most fatal and fastest-growing malignancies. The mechanistic underpinnings of the etiology of NAFLD, as well as disease progression from simple steatosis to NASH and NASH-induced HCC, remain inadequately elucidated. Our studies described in this thesis have identified STE20-type kinases, MST3 and MST4, localized to intracellular lipid droplets (LDs), as critical regulators of ectopic fat accumulation in human hepatocytes. In addition, we found that MST3 and MST4 expression in human liver biopsies was positively correlated with the key features of NAFLD (i.e., hepatic steatosis, lobular inflammation, and hepatocellular ballooning). Our results further demonstrate that Mst3-targeting antisense oligonucleotides can effectively ameliorate the full spectrum of high-fat diet-induced NAFLD in mice including liver steatosis, inflammation, fibrosis, and hepatocellular damage. Mechanistically, our studies revealed that the silencing of MST4 stimulated LD catabolism through enhanced β-oxidation and triacylglycerol (TAG) secretion and inhibited LD anabolism through suppressed free fatty acid influx and TAG synthesis. Furthermore, the knockdown of MST4 protected hepatocytes from oxidative and ER stress. Notably, the opposite effects on lipid storage and metabolic stress were observed when MST4 was overexpressed. Our studies based on analyses of public datasets and in-house cohorts also showed that the expression of hepatic MST3 and MST4 was positively correlated with the incidence and severity of HCC. In addition, the depletion of both MST3 and MST4, but also either of them individually, markedly suppressed the tumorigenesis of human HCC cells including attenuated proliferation, migration, invasion, and epithelial-mesenchymal transition. Mechanistic investigations revealed lower activation of STAT3 signaling in MST3/MST4-deficient hepatocytes and identified GOLGA2 and STRIPAK complex as the binding partners of both MST3 and MST4 in HCC cells. Together, our results suggest that hepatic LD-binding proteins MST3 and MST4 are critical regulatory nodes governing susceptibility to NAFLD and warrant future investigations to address the therapeutic potential of MST3 and MST4 antagonism as a strategy to prevent or mitigate the development and aggravation of NAFLD/NASH as well as NASH-induced HCC.