Targeting tryptophan hydroxylase 1 restricts growth and suppresses plasticity in neuroendocrine prostate cancer.

Advanced prostate cancer has increasingly developed a lethal neuroendocrine form, small cell/neuroendocrine prostate cancer (NEPC), as a consequence of the widespread use of highly potent androgen receptor signaling inhibitors in castration-resistant disease. The molecular mechanisms remain unclear and no effective therapies currently exist. We report that tryptophan hydroxylase 1 (TPH1), the enzyme responsible for peripheral serotonin biosynthesis - a neurotransmitter enriched in neuroendocrine tumors and a classical neuroendocrine biomarker - was upregulated in both de novo and therapy-induced human NEPC. TPH1 upregulation was necessary and sufficient for neuroendocrine differentiation and the NEPC phenotype through its enzymatic activity. Silencing TPH1 suppressed neuroendocrine plasticity and various aggressive behaviors of NEPC cells, including proliferation, invasion, sphere formation, and NEPC tumor xenograft growth. Mechanistically, TPH1 activated mTOR via intracellular serotonin-dependent serotonylation of mTOR at glutamine 2453, which triggered the induction of FOXM1 and E2F1 to drive NEPC differentiation and growth. Importantly, pharmacological inhibition of TPH1 using the clinically available inhibitor LX1606 effectively restricted growth and neuroendocrine marker expression in multiple NEPC cell lines and patient-derived xenografts. Collectively, these findings characterize TPH1's contribution to NEPC and suggest TPH1 as a potential therapeutic target.