TPH2 and colorectal carcinoma: The CRC-associated microbiota metabolite isovalerate (IVA) initiates transcription of the rate-limiting enzyme tryptophan hydroxylase 2 (Tph2) to synthesize gut 5-hydroxytryptamine (5-HT), and then 5-HT triggers Wnt/β-catenin signaling by engagement with 5-HT receptor (HTR) 1B / 1D / 1 F on colorectal CSCs, promoting the self-renewal and metastasis of colorectal CSCs [59].