Serotonin (5-HT), a conserved monoamine derived from tryptophan metabolism, has emerged as a multifaceted regulator in cancer biology. Beyond receptor signaling, serotonylation allows transglutaminase 2 (TGM2) to covalently incorporate serotonin into target proteins, linking intracellular serotonin availability to regulation of tumor metabolism, chromatin state, and immune function. Despite recent progress, the determinants of substrate selection and the physiological scope of serotonylation remain incompletely defined. In this review, we integrate advances in serotonin metabolism and receptor signaling with emerging insights into serotonylation, emphasizing its roles in tumors and the tumor microenvironment, and outline priorities for future investigation.
Many cancer patients remain at risk of metastatic relapse for the remainder of their lives, despite initial disease remission, due to disseminated tumour cells (DTCs) persisting at secondary sites in a dormant, therapy-resistant state. Dormant DTCs can reawaken and develop into clinical metastasis after a prolonged latency period. Interactions between DTCs and their niche are key to metastatic dormancy and subsequent reawakening and outgrowth. Ageing has profound effects on tissues and the immune system and, as such, the field is evolving to delineate the impact of an aged microenvironment on metastatic dormancy. We summarise the latest insights in this review, focussing on recent advances in immune- and stroma-mediated regulation of dormancy and reawakening in the context of age-related microenvironmental perturbations.

