Inhibition of BCAT1-mediated cytosolic leucine metabolism regulates Th17 responses <i>via</i> the mTORC1-HIF1α pathway

초록

Branched-chain amino acids (BCAAs), particularly leucine, are indispensable AAs for immune regulation through metabolic rewiring. However, the molecular mechanism underlying this phenomenon remains unclear. Our investigation revealed that T-cell receptor (TCR)-activated human CD4(+) T cells increase the expression of BCAT1, a cytosolic enzyme responsible for BCAA catabolism, and SLC7A5, a major BCAA transporter. This upregulation facilitates increased leucine influx and catabolism, which are particularly crucial for Th17 responses. Activated CD4(+) T cells induce an alternative pathway of cytosolic leucine catabolism, generating a pivotal metabolite, beta-hydroxy beta-methylbutyric acid (HMB), by acting on BCAT1 and 4-hydroxyphenylpyruvate dioxygenase (HPD)/HPD-like protein (HPDL). Inhibition of BCAT1-mediated cytosolic leucine metabolism, either with BCAT1 inhibitor 2 (Bi2) or through BCAT1, HPD, or HPDL silencing using shRNA, attenuates IL-17 production, whereas HMB supplementation abrogates this effect. Mechanistically, HMB contributes to the regulation of the mTORC1-HIF1 alpha pathway, a major signaling pathway for IL-17 production, by increasing the mRNA expression of HIF1 alpha. This finding was corroborated by the observation that treatment with L-beta-homoleucine (L beta hL), a leucine analog and competitive inhibitor of BCAT1, decreased IL-17 production by TCR-activated CD4(+) T cells. In an in vivo experimental autoimmune encephalomyelitis (EAE) model, blockade of BCAT1-mediated leucine catabolism, either through a BCAT1 inhibitor or L beta hL treatment, mitigated EAE severity by decreasing HIF1 alpha expression and IL-17 production in spinal cord mononuclear cells. Our findings elucidate the role of BCAT1-mediated cytoplasmic leucine catabolism in modulating IL-17 production via HMB-mediated regulation of mTORC1-HIF1 alpha, providing insights into its relevance to inflammatory conditions.

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