Bcat1 is controlled by Tsc2/mTORC1 pathway at expression levels and its deficiency together with Bcat2 inactivation suppresses the growth of a Tsc2^-/- tumor cell line

The tuberous sclerosis complex (TSC) gene products (TSC1/TSC2) negatively regulate mTORC1. Although mTORC1 inhibitors are used for the treatment of TSC, incomplete tumor elimination and the adverse effects from long-term administration are problems that need to be solved. Branched-chain amino acid (BCAA) metabolism is involved in the growth of many tumor cells via the mTORC1 pathway. However, it remains unclear how BCAA metabolism affects the growth of mTORC1-dysregulated tumors. We show here that the expression of branched-chain amino transferase1 (Bcat1) was suppressed in Tsc2-deficient murine renal tumor cells either by treatment with rapamycin or restoration of Tsc2 expression suggesting that Bcat1 is located downstream of Tsc2-mTORC1 pathway. We also found that gabapentin, a Bcat1 inhibitor suppressed the growth of Tsc2-deficient tumor cells and increased efficacy when combined with rapamycin. We investigate the functional importance of Bcat1 and the mitochondrial isoform Bcat2 by inhibiting each enzyme separately or both together by genome editing and shRNA in Tsc2-deficient cells. We found that deficiency of both enzymes, but not either alone, inhibited cell growth, indicating that BCAA-metabolic reactions support Tsc2-deficient cell proliferation. Our results indicate that inhibition of Bcat1 and Bcat2 by specific drugs should be a useful method for TSC treatment.