Mechanisms linking the Branched-Chain alpha-Keto Acid regulatory network to the pathogenesis of NASH

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Non-alcoholic steatohepatitis (NASH) is a major global health concern that continues to rise at an alarming rate driven by the tide of the obesity pandemic. It is well appreciated that NASH significantly raises risk for development of hepatocellular carcinoma, cirrhosis, and acute liver failure as well as type 2 diabetes and cardiovascular disease. However, there are currently no approved therapies for the treatment or reversal of NASH. Our foundational work defining the molecular pathways linking disturbances in branched-chain amino acid (BCAA) metabolism to the etiology of metabolic disease recently identified a novel regulatory node that exerts a powerful influence on hepatic lipid deposition in obese and lean animals. We discovered that the branched-chain α-keto acid dehydrogenase (BCKDH) kinase, BDK that inhibits branched-chain α-keto acid (BCKA) oxidation robustly stimulates de novo lipogenesis (DNL), by phosphorylating the lipogenic enzyme ATP citrate lyase (ACLY) on its activating serine. Likewise, we found that the BCKDH phosphatase, protein phosphatase M1K (PPM1K), that promotes BCKA oxidation, dephosphorylates ACLY on its activating serine. Accordingly, adenoviral mediated overexpression of BDK in liver of lean healthy Wistar rats was found to be sufficient to raise hepatic DNL by 2.5 fold. Whereas, treatment of genetically obese Zucker Fatty rats with the BDK inhibitor, BT-2, or adenovirus expressing recombinant PPM1K lowered circulating BCKA, reduced phosphorylation of ACLY, and remarkably prompted a 40% reduction in liver triglyceride content in these severely obese animals without altering food intake, body weight, adiposity, or physical activity. Subsequent, studies in our lab have identified an additional effect of BT2 to lower expression of the fatty acid transporter, CD36, in liver. Thus, our current working model is that modulation of the hepatic BCKA regulatory network exerts robust effects on lipid content due to its dual effects on CD36-mediated lipid uptake and ACLY-mediated DNL. Beyond these mechanisms, it remains unclear whether the BCKA themselves exert any direct or synergistic effects on hepatic lipid metabolism. Importantly, our recent medRxiv preprint demonstrates that circulating BCKA and liver BDK expression are strongly associated with NASH status in a cohort of 288 bariatric surgery patients with severe obesity that are discordant for NAFLD and NASH. In the current proposal, we will leverage our newly developed mouse models, established molecular/pharmacologic armamentarium, and novel insight to resolve the molecular mechanisms connecting the BCKA regulatory network to the pathogenesis of NASH by completing three specific aims: 1) Characterize the relative contribution of hepatic BDK, PPM1K, and BCKA to NASH progression. 2) Evaluate the therapeutic potential of small molecule inhibitors of BDK for reversing NASH. 3) Define the mechanisms connecting the BCKA regulatory network to hepatic lipid content. The successful completion of the studies outlined in specific aims 1-3 will define the BCKA regulatory network as an important modulator of NASH progression with strong translation relevance for the treatment of NASH in humans.