Mitochondrial MTHFD isozymes display distinct expression, regulation, and association with cancer.

Mitochondrial folate metabolism is central to the generation of nucleotides, fuelling methylation reactions, and redox homeostasis. Uniquely among the reactions of the mitochondrial folate pathway, the key step of the oxidation of 5,10-methylene-tetrahydrofolate (CH2-THF) can be catalysed by two isozymes, MTHFD2 and MTHFD2L. The MTHFD2 enzyme has recently received considerable attention as an oncogenic enzyme upregulated in several tumour types, which is additionally required by cancer cells in vitro and in vivo. However, much less is currently known about MTHFD2L and its expression in cancer. In this study, we examine and compare the expression and regulation of the two mitochondrial MTHFD isozymes in normal human and cancer cells. We found that normal and cancer cells express both enzymes, although MTHFD2 has a much higher baseline expression. Unlike MTHFD2, the MTHFD2L isozyme does not show an association with proliferation and growth factor stimulation. In addition, we did not find evidence of a compensatory increase of MTHFD2L following suppression of its isozyme. This study supports that MTHFD2L is unlikely to have an important function in increased proliferation or cancer. Furthermore, therapeutic strategies aiming to block the mitochondrial folate pathway in cancer should focus on MTHFD2, with MTHFD2L being unlikely to be involved in the development of chemoresistance to targeting of its mitochondrial isozyme.