Overexpression of the mitochondrial folate and glycine-serine pathway: a new determinant of methotrexate selectivity in tumors

Abstract

Previous studies have documented the roles of transport via the reduced folate carrier, retention via polyglutamylation, and increased levels of the target enzyme, dihydrofolate reductase in sensitivity to methotrexate. Recent studies have shown that the mitochondrial enzymes in the cellular metabolism of serine, folate, and glycine are overexpressed in a subset of human cancers and that their expression is required for tumor maintenance. In this Perspective article, we propose that the expression of mitochondrial enzymes in the metabolism of serine and glycine, in addition to those involved in folate metabolism, are determinants of the response to methotrexate. Furthermore, we show that myc activation in tumors is associated with upregulation of these enzymes. We propose that patients whose tumors show this phenotype will be sensitive to folate antagonists targeting thymidylate or purine biosynthesis.

Figure 1

The role of the mitochondria in the generation of purines and thymidylate for DNA synthesis. Reactions 1, 3, 4, and 5 occur in both the mitochondria and the cytoplasm, with reaction 2 limited only to the mitochondria. Reaction 1 and 2 are catalyzed by SHMT2 and the glycine cleavage system, respectively. In the cytoplasm, reactions 3, 4, and 5 are carried out by the trifunctional enzyme (MTHFD1); in the mitochondria, 2 enzymes are required (MTHFD2 catalyzes reactions 3 and 4 and MTHFD1L catalyzes reaction 5). The enzyme 10-formyl tetrahydrofolate dehydrogenase (not shown) has been reported to be absent in most cancer cells (10). Serine hydroxymethytransferase (1); glycine oxidase complex (2); 5–10 methylene tetrahydrofolate dehydrogenase (3); methenyltetrahydrofolate cyclohydrolase (4); and formyltetrahydrofolate synthetase (5).

Figure 2

Heatmap showing the expression of genes coding for enzymes in folate metabolism (blue, underexpressed; red, overexpressed). A, a collection of 515 tumor-derived cell lines sorted in decreasing order of their sensitivity for methotrexate (MTX), based on data from ref. (9). The samples were divided into 2 groups: sensitive (1/3 of samples with lowest IC₅₀) and other (remaining samples). The right column shows a color-coded quantification of the P value for enrichment of samples with high expression (2-fold or above) in the sensitive group, whereas red indicates significantly enriched, the brighter the more significant, and blue not significantly enriched. B, a collection of 161 ALL sorted in decreasing order of their sensitivity to MTX, based on data from ref. (11). The samples were divided into 2 groups: sensitive (1/3 of samples with highest fold decrease in the leukemia cell count) and other (remaining samples). The right column shows a color-coded quantification of the P value as in A. C, a collection of 221 lymphomas grouped by subtype and sorted in decreasing order of c-myc gene signature, based on data from ref. (16). The right column shows a color-coded quantification of the P value for enrichment of samples with high expression (2-fold or above) among samples with a significant upregulation of the c-myc signature. D, samples from 508 breast cancers, based on data from ref. (18). The samples were divided in 3 groups with significant upregulation (top), nonsignificant (middle), and significant downregulation (Bottom) of the folate metabolism gene signature. The right column shows a color-coded quantification of the P value for enrichment of samples with high expression in the top group.