Bile Acids and Sphingosine 1-Phosphate in Non-Alcoholic Steatohepatitis (NASH)

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Non-alcoholic fatty liver disease (NAFLD) is becoming a globally prevalent disease, especially among US veterans. NAFLD and non-alcoholic steatohepatitis (NASH) are recognized as hepatic manifestations of metabolic syndrome. The disease progression from simple steatosis to NASH, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC) is promoted by multiple-hit instead of two-hit, including disruption of intrahepatic bile acid (BA) homeostasis, aberrant activation of the innate immune response, insulin resistance, dyslipidemia, and dysbiosis. Although much new information on the pathogenesis of NAFLD/NASH has been gained during the last decade, no effective therapy has been developed due to the incomplete understanding of the pathogenesis of this complex disease. Considering the rapid increase in the incidence and prevalence worldwide of fatty liver diseases, the development of new therapeutic interventions for NAFLD is especially urgent and is the long-term objective of this application. BAs are important signaling molecules involved in regulating lipid, glucose, and energy metabolism by activating nuclear receptors and G protein-coupled receptors (GPCRs), such as farnesoid X receptor (FXR), TGR5, and sphingosine-1 phosphate receptor 2 (S1PR2). We have reported that conjugated primary BAs activate the AKT and ERK1/2 pathways via Gαi protein-coupled S1PR2. It also has been reported that phosphorylation of sphingosine kinase 2 (SphK2) by ERK1/2 increases nuclear sphingosine- 1 phosphate (S1P) levels. Nuclear S1P is a potent endogenous inhibitor of specific histone deacetylase 1 &2 (HDAC1 and 2). Inhibition of HDAC1 has been reported to reduce hepatic lipid accumulation. BAs are also important modulators of innate immunity. Our previous studies reported that deletion of S1PR2 or SphK2 in hepatocytes significantly increased lipid accumulation. Both S1PR2-/- and SphK2-/- mice are more prone to western diet (WD)-induced hepatic steatosis and inflammation. In the liver, both S1PR2 and SphK2 are highly expressed in hepatocytes, macrophages, and cholangiocytes. A recent study reported that SphK2-generated S1P activated an anti-inflammatory response by suppressing the stimulator of type 1 interferon gene (STING) signaling in macrophages. It has also been reported that hepatic STING expression is upregulated in NASH patients and activation of STING in macrophages contributes to NASH disease progression. Our preliminary data further indicate that serum BA composition and levels were significantly changed in a diet-induced mouse NASH model, which was similar to the finding in human NASH patients. Furthermore, the expression of SphK2 was significantly down-regulated, but the expression of STING was upregulated in the livers of human NASH patients and the NASH mouse model. Therefore we HYPOTHESIZE that BA-mediated activation of S1PR2/SphK2 is pivotal in the maintenance of hepatic lipid homeostasis and modulation of innate immune response under metabolic stress. Two specific aims are proposed to test this hypothesis. Aim 1: To investigate the role and mechanisms of BA-mediated activation of S1PR2/SphK2 in regulating hepatic lipid metabolism during NASH disease progression. Aim 2: To define the role of BA-mediated activation of S1PR2/SphK2 in modulating the innate immune response under metabolic stress and further identify the underlying cellular/molecular mechanisms. The hepatic cell-specific knockout mice of S1PR2 and SphK2 and a well-characterized western diet plus sugar water (WDSW)-induced NASH mouse model will be used. Completion of this study will not only identify the potential novel cellular/molecular mechanisms in the initiation and progression of NAFLD/NASH but will also make a significant conceptual advance linking BAs and sphingolipids with metabolic diseases which will create a fundamental base for developing novel therapies for NAFLD/NASH. Therefore, the subject matter of this proposal is timely, clinically significant, and directly fits into the mission of the US Veteran's Administration.