Tumor Reliance on Cytosolic versus Mitochondrial One-Carbon Flux Depends on Folate Availability

Folate metabolism supplies one-carbon (1C) units for biosynthesis and methylation and has long been a target for cancer chemotherapy. Mitochondrial serine catabolism is considered the sole contributor of folate-mediated 1C units in proliferating cancer cells. Here, we show that under physiological folate levels in the cell environment, cytosolic serine-hydroxymethyltransferase (SHMT1) is the predominant source of 1C units in a variety of cancers, while mitochondrial 1C flux is overly repressed. Tumor-specific reliance on cytosolic 1C flux is associated with poor capacity to retain intracellular folates, which is determined by the expression of SLC19A1, which encodes the reduced folate carrier (RFC). We show that silencing SHMT1 in cells with low RFC expression impairs pyrimidine biosynthesis and tumor growth in vivo. Overall, our findings reveal major diversity in cancer cell utilization of the cytosolic versus mitochondrial folate cycle across tumors and SLC19A1 expression as a marker for increased reliance on SHMT1. [Display omitted] •A variety of cancer cells rely on cytosolic 1C flux in physiological folate medium•Intracellular folate levels determine the cytosolic versus mitochondrial 1C flux•SLC19A1 regulates compartmentalized 1C flux by modulating intracellular folate levels•SHMT1 is essential for growth of low RFC-expressing tumors in vitro and in vivo Lee et al. reveal that tumor reliance on cytosolic versus mitochondrial 1C flux depends on intracellular folate availability, which is regulated by SLC19A1 expression. Their findings suggest SHMT1 as a promising anti-cancer drug target, and SLC19A1 expression as a biomarker for identifying tumors susceptible to SHMT1 inhibition.