UPP1 in Cancer: Context-Dependent Roles in Metabolic Adaptation and Treatment Response.

Uridine phosphorylase 1 (UPP1) catalyzes the reversible phosphorolysis of uridine to uracil and ribose-1-phosphate, thereby linking pyrimidine salvage to central carbon metabolism. In cancer, UPP1 is functionally relevant when UPP1-linked pathway engagement and a perturbation-dependent phenotype converge within a defined cellular, nutrient/substrate, molecular, or treatment setting. The direction of effect is determined by the dominant biochemical or signaling output, whereas the strength of UPP1 dependence may be attenuated by compensation through related nucleoside-metabolic pathways. This review examines the roles of UPP1 in metabolic adaptation under nutrient stress, remodeling of the tumor microenvironment, and treatment response across cancer types. The strongest evidence concerns uridine-derived ribose-carbon utilization under glucose limitation and modulation of fluoropyrimidine response by UPP1. Mechanistic support for immune and microenvironmental effects is strongest in lung adenocarcinoma and selected metabolite-centered models, whereas most patient-outcome and pan-cancer findings remain cohort- or signature-based. This heterogeneity argues against treating UPP1 as a universal oncogenic driver or a stand-alone pan-cancer biomarker. Its biological and translational value lies in identifying tumor states in which UPP1-linked metabolism becomes functionally important or therapeutically informative.