Tryptophan metabolism: From physiological functions to key roles and therapeutic targets in cancer (Review).

Tryptophan (Trp) metabolism is a complex and important biochemical process in humans. It is vital in protein synthesis and is a precursor of various bioactive molecules. Trp is metabolized through the kynurenine, serotonin and indole pathways, mediating diverse physiological functions, including neurotransmitter synthesis, immune regulation, antioxidant effects, and biosynthesis of niacin and melatonin. These metabolic pathways maintain essential functions under normal physiological conditions. However, they are significantly affected by various types of cancers. Trp metabolites regulate tumor angiogenesis, affect the self‑renewal of cancer stem cells, and participate in immune evasion and cell death through complex mechanisms. As the mechanisms underlying Trp metabolism in diseases are increasingly being elucidated, targeting Trp metabolic pathways has emerged as a promising therapeutic strategy. Further investigation of the molecular mechanisms underlying Trp metabolism and its role in diseases may provide new perspectives and approaches for diagnosing and treating diseases.