Role of Sphingosine Kinases in Liver Metabolism and Cancer

Hepatocellular carcinoma (HCC), a primary liver cancer, ranks the sixth morbidity of all cancers and the fourth cause among all cancer deaths. Non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), and associated lipid metabolic conditions are emerging as chief risk factors for HCC. Sphingolipids are a family of essential lipids in cells. Of them, ceramide, sphingosine, and sphingosine 1-phosphate (S1P) are well-known signalling molecules regulating cellular metabolism and functions. Sphingolipid homeostasis is determined by sphingosine kinases (SphKs), the enzymes that convert ceramide/sphingosine into S1P. My host laboratory and others have implicated both SphK isoforms in chronic liver diseases. SphK1 has been defined as a tumour-promoting factor. It is highly expressed in multiple cancers, including HCC. My host laboratory and collaborators have found that knockout of SphK1 suppresses the Diethylnitrosamine (DEN)-induced HCC in mice. Meanwhile, I and my colleagues have identified SphK2 as a metabolically protective factor against hepatic insulin resistance in diet-induced obese mice. Mechanistically, SphK2 deficiency results in an inhibition of hepatic insulin signalling. Extended from previous findings, this Master of Philosophy project aims to 1) test if PF-543, a novel, highly selective SphK1 inhibitor, can suppress DEN-induced HCC in vivo; 2) examine if the liver-specific knockout of Sphk2 can predispose NAFLD progression to NASH in mice. PF-543 has never been tested in any primary cancer models in vivo, although it holds promise to be a new anti-HCC therapy. In addition, liver-specific Sphk2 knockout mouse is a long-coveted tool to achieve a definitive, in-depth understanding of the pathophysiological roles of SphK2. In this project, I found that intraperitoneal injection of PF-543 for 12 weeks significantly reduced tumour number and size and mitigated hepatic inflammation in DEN-induced HCC. Notably, the treatment didn’t result in cytotoxicity in liver cancer cells. Instead, it profoundly suppressed tumour neo-vasculature. In addition, I demonstrated that liver-specific ablation of SphK2 promoted the development of glucose intolerance, insulin resistance, hepatic steatosis, and liver fibrosis in mice on a high-fat, high-cholesterol diet for 12 weeks, indicative of NAFLD progression to NASH. In conclusion, my study provides the first experimental evidence to demonstrate the anti-HCC effects on PF-543 in vivo, offering a potential therapeutic option for difficult-to-treat HCC. Furthermore, my findings of the metabolically protective roles of SphK2 in NAFLD/NASH generate new knowledge of lipid metabolic causes of chronic liver diseases and provide a novel target for fundamental research and drug development