Investigating the role of tumor-intrinsic BCAT1 in HCC growth and anti-tumor immunity - PROJECT SUMMARY Hepatocellular carcinoma (HCC) is the predominant subtype of liver cancer and arises almost exclusively in the context of chronic inflammation. The healthy liver is a modulator of systemic immune tolerance, recognizing harmless dietary and environmental antigens from the gut and degrading them without inducing inflammation. Dysregulation of this tightly controlled response can lead to chronic inflammation, compensatory hepatocyte regeneration, and eventually fibrosis and hepatocarcinogenesis. While viral hepatitis is the predominant risk factor for HCC, metabolic disorders such as metabolic-associated fatty liver disease (MAFLD) are expected to increase the global incidence of HCC in the coming decades. With treatment options limited due to the mutational heterogeneity of HCC and poor immunotherapeutic responses in patients with MAFLD-associated HCC, there is a clinical need to identify the metabolic vulnerabilities of cancer and immune cells in the tumor micro- environment to exploit specific dependencies of HCC and improve cancer therapy. To this end, we have identified one of the most downregulated metabolic pathways in HCC to be catabolism of the branched chain amino acids (BCAAs). The BCAAs are leucine, isoleucine, and valine, all of which are essential amino acids that contribute to protein synthesis or undergo catabolism to support anaplerotic processes. The majority of BCAA catabolic enzymes are often downregulated in HCC, apart from two enzymes responsible for the first step in the pathway: BCAT1 and BCAT2. Both enzymes catalyze the reversible transamination of BCAAs by transferring the amino group to α-ketoglutarate to generate glutamate and the respective branched chain ketoacids. Loss of BCAA catabolism downstream of the BCATs may represent a unique vulnerability of HCC and confer dependence on BCAT1 or BCAT2 expression for tumor growth. Here, I demonstrate that high BCAT1 expression, but not BCAT2, correlates with worse overall survival in patients according to data from The Cancer Genome Atlas. Moreover, BCAT1 protein expression is high in many well-characterized human cell lines of HCC, and BCAT1 knockdown inhibits growth in a high BCAT1-expressing line, which can be rescued by BCAT1 re-expression. This suggests that tumors with high BCAT1 still utilize the BCAA pathway to support their growth, and this could impact BCAA availability within the tumor microenvironment as a result. Thus, I hypothesize that BCAT1 promotes HCC cell growth through a tumor-intrinsic mechanism, and this suppresses anti-tumor immunity by reducing the amount of BCAAs available to immune cells in the tumor microenvironment. For my first aim, I will determine how BCAT1 knockdown leads to growth inhibition with metabolite supplementation and will confirm the relevance of BCAT1 to HCC growth in vivo. In my second aim, I will investigate whether BCAT1 expression regulates BCAA uptake and how changes in BCAA levels impact immune cell populations in the liver. Collectively, these findings will provide insight into BCAA utilization in the context of BCAT1 and establish a critical crosstalk between tumor and immune cell metabolism in HCC.
