Alcoholic liver disease (ALD) remains the most common chronic liver diseases worldwide. During last several decades, extensive studies have shown that ALD progression involves multiple events such as hepatic lipid accumulation, intestinal barrier dysfunction, and activation of an inflammatory response. Currently, no FDA approved therapy is available for any stage of ALD. Therefore, the unmet need to identify novel targets for the development of effective therapeutics against ALD is urgent. It has been shown that alcohol alters bile acid metabolism and disrupts intestinal barrier function, which results in leaky gut and bacterial translocation and activation of systemic and hepatic inflammation. Bile acids are important signaling molecules involved in regulating lipid metabolism. Our lab first discovered a link between conjugated bile acids and sphingosine-1 phosphate (S1P) signaling in regulating hepatic lipid and glucose metabolism. The conjugated bile acids activate S1P receptor 2 (S1PR2), which further activate sphingosine kinase 2 (SphK2). S1P is a membrane-derived lipid mediator, which is synthesized from sphingosine by either SphK1 or SphK2. It has been identified that S1P can regulate various fundamental cellular responses either as an intracellular signaling molecule or a ligand for five cell membrane G-protein coupled receptors (GPCRs), S1PR1-5. SphK1 is primarily located in the cytoplasm of mammalian cells, whereas SphK2 is located in the nucleus and mitochondria. SphK2-generated nuclear S1P is a powerful natural inhibitor of histone deacetylases (HDAC1/2). Increased histone acetylation is often associated with an increase in the transcriptional activity of genes involved in cell proliferation, migration and angiogenesis. We recent reported that both S1PR2 and SphK2 knock out mice (S1PR2-/- and SphK2-/-) are highly susceptible to high fat diet-induced fatty liver and hepatic lipotoxicity. Activation of SphK2 in response to ER stress ameliorates hepatic steatosis. The preliminary results further showed that SphK2-/-mice developed more severe fatty liver and hepatic injury as indicated by increased hepatic lipid accumulation and inflammation after 10-day?s feeding with 5% ethanol liquid diet (Lieber-DeCarli) followed by a binge via 31.5% ethanol gavage or two-month chronic feeding with 5% alcohol liquid diet. In addition, alcohol-feeding significantly increased intestinal permeability and bacterial translocation in SphK2-/- mice. Furthermore, both LPS and thapsigargin (an ER stress inducer) inhibited intestinal organoid growth. Based on these key findings, we HYPOTHESIZE that disruption of SphK2/S1P-mediated signaling pathways plays a critical role in alcohol-induced liver injury.
Two specific aims are proposed to test our central hypothesis:1) To investigate the role of SphK2 in alcohol induced-liver injury. 2) To identify the potential mechanisms by which lack of SphK2 promotes hepatic injury in response to chronic alcohol consumption. Results from these studies will provide novel information for the development of effective therapeutics not only for ALD but also for other related metabolic diseases. Therefore, the subject matter of this proposal is timely, important, and has direct clinical application.
Alcoholic hepatitis, the clinically most devastating presentation of alcoholic liver disease (ALD), is a significant global health problem with a lack effective therapeutics due to the limited understanding of the mechanisms of disease progression, and liver transplantation remains the only curative option. The focus of this application will be on the roles of sphingosine kinase 2 (SphK2)/sphingosine-1 phosphate (S1P)-mediated signaling pathways in alcohol-induced liver injury. Completion of this study will not only identify the potential cellular/molecular mechanisms involved in the initiation and progression of ALD, but will also make significant advances conceptually, linking bile acids and S1PR2/SphK2 with alcoholic liver injury and creating a fundamental basis for developing novel therapies for ALD to reduce morbidity and mortality in patients with alcoholic hepatitis.
