Won Dong Lee^1,2,7, Anna Chiara Pirona^1,3,4,7, Boris Sarvin^1,7, Alon Stern⁵, Keren Nevo-Dinur¹, Elazar Besser¹, Nikita Sarvin¹, Shoval Lagziel⁵, Dzmitry Mukha¹, Shachar Raz¹, Elina Aizenshtein⁶, Tomer Shlomi^1,5,6,8,*

¹Faculty of Biology, Technion, 32000 Haifa, Israel
²Department of Chemistry and Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544, USA
³Division of Functional Genome Analysis, German Cancer Research Center (DKFZ), Heidelberg 69120, Germany
⁴Faculty of Bioscience, University of Heidelberg, Heidelberg 69120, Germany
⁵Faculty of Computer Science, Technion, 32000 Haifa, Israel
⁶Lokey Center for Life Science and Engineering, Technion, 32000 Haifa, Israel
⁷These authors contributed equally
⁸Lead Contact

^*Corresponding author

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.