Tumor dependence on cytosolic one-carbon metabolism is determined by cellular capacity to retain folates

Folate metabolism supplies one-carbon (1C) units for nucleotide biosynthesis. Mitochondrial serine catabolism is considered the sole contributor of 1C units in proliferating cancer cells, facilitated via the shuttling of mitochondria produced formate to cytosol. Here, we show that the concentration of folate in cell culture media determines the relative contribution of the cytosolic versus mitochondrial folate pathway to 1C unit production. We find that under physiological folate levels, the cytosolic pathway is the predominant source for 1C units in a variety of cancers. Tumor specific reliance on cytosolic 1C flux is determined by the capacity of cancer cells to retain high intracellular folate pools, which is determined by the expression of the Reduced Folate Carrier (RFC). We show that inhibition of the cytosolic serine hydroxymethyltransferase (SHMT1) in cells with low RFC expression impairs pyrimidine biosynthesis and tumor growth. Our findings reveal major diversity in the utilization of the cytosolic versus the mitochondrial folate cycle across cancers and RFC as a marker for induced dependence on SHMT1.