Viewpoint Cite This: Biochemistry 2018, 57, 889−890
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Formaldehyde Detoxification Creates a New Wheel for the FolateDriven One-Carbon “Bi”-cycle Zachary T. Schug* The Wistar Institute, 3601 Spruce Street, Philadelphia, Pennsylvania 19104, United States
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ome of the primary functions of metabolism in nonproliferating cells are to convert the nutrients we eat into energy in the form of ATP, to store excess nutrients, and to detoxify and excrete any potentially harmful agents or metabolic byproducts. In proliferating cells, metabolism is further taxed by the generation of sufficient biomass to support the metabolic demands of cell growth and division. Many normal, healthy cellular processes, such as hematopoiesis and the expansion of lymphocytes in response to infection, require cell proliferation. However, there are also instances in which cell proliferation can be pathogenic. For example, cancer, atherosclerosis, and psoriasis are all associated with uncontrolled proliferation. For many of these diseases, targeting metabolic pathways associated with growth and proliferation has proven to be an effective treatment option. Indeed, the very first cancer chemotherapy, methotrexate, functions by blocking folate metabolism and therefore the synthesis of DNA and RNA and has more recently been successfully applied to other proliferative diseases, namely rheumatoid arthritis. Therefore, the potential of exploiting metabolism as a therapeutic modality in proliferative diseases cannot be overstated. Formaldehyde is a known carcinogen and teratogen that forms protein and DNA adducts that can lead to severe genotoxic stress. Formaldehyde toxicity can occur through environmental exposure, but given the ubiquitous presence of formaldehyde (∼100 μM) in our bloodstream, scientists have long speculated that an endogenous source of formaldehyde must exist in our bodies. However, the provenance of this endogenously formed formaldehyde has remained elusive. Prior studies suggest that oxidative demethylation of RNA, DNA, and proteins can generate formaldehyde, but this mechanism is insufficient to account for the total formaldehyde pool.1 In a recent issue of Nature, Patel and co-workers discovered that the main source of endogenous formaldehyde is the oxidative decomposition of certain folate derivatives (i.e. dihydrofolate, tetrahydrofolate, and 5,10-methylene-tetrahydrofolate).2 They further show that the majority of cell types possess a “two-tier” system that guards against the accumulation of toxic levels of formaldehyde. In the first tier, the detoxification of formaldehyde is initiated by the natural cellular antioxidant defense afforded by glutathione, which spontaneously reacts with formaldehyde to form S-hydroxymethylglutathione (Figure 1). Next, the NADP+-dependent oxidation of S-hydroxymethylglutathione to S-formylglutathione is catalyzed by the enzyme alcohol dehydrogenase 5 (ADH5). S-Formylglutathione is subsequently converted by S-formylglutathione hydrolase (FGH) to formate, which is then free to enter the one-carbon (1C) cycle. Using Adh5 knockout mice and CRISPR-cas9 ADH5 knockout human cell lines, Patel and co-workers clearly demonstrate that ADH5 activity is necessary © 2018 American Chemical Society
Figure 1. New folate−formaldehyde−formate cycle of 1C metabolism. The recycling of the formaldehyde resupplies the one-carbon cycle. Folate derivatives dihydrofolate (DHF), tetrahydrofolate (THF), and 5,10-methylene-tetrahydrofolate (5,10-me-THF) undergo spontaneous oxidative degradation to formaldehyde. Formaldehyde subsequently reacts with glutathione (GSH) to form S-hydroxymethylglutathione (HM-GSH). Alcohol dehydrogenase 5 (ADH5) catalyzes the conversion of HM-GSH to S-formylglutathione (F-GSH). F-GSH is hydrolyzed to formate and GSH by formylglutathione hydrolase (FGH). Formate is then free to enter the canonical 1C metabolic pathway. It is possible that certain cells can take advantage of formaldehyde detoxification as a nutrient source for purine and pyrimidine biosynthesis as well as NADPH production. Overall, the former 1C cycle has now added another wheel involving formaldehyde recycling to formate to create a new folate−formaldehyde−formate “bi”-cycle. It will be interesting to see how active this new arm of the 1C cycle is in proliferative diseases such as cancer. Dotted lines represent spontaneous chemical reactions, while solid lines indicate enzymatic catalysis.
to recycle (or recapture) the formaldehyde formed from the oxidative decomposition of folate derivatives and thereby keep formaldehyde levels in check. In essence, the constant spontaneous formation of folate-derived formaldehyde must be matched by an ADH5-dependent neutralization of formaldehyde. The second tier of formaldehyde defense requires the activity of the Fanconi anemia (FA) protein complex, which performs recombination repair of DNA damage caused by DNA− formaldehyde cross-links. CRISPR-cas9-mediated knockout of ADH5 in combination with a FA complex protein (i.e., FANCD2 or FANCC) rendered many cell types hypersensitive to oxidative degradation of exogenously added tetrahydrofolate. The authors additionally point out that breast cancer cell lines harboring mutations in BRCA1 or BRCA2, two genes that are also FA family members and tumor suppressors, are also sensitive to tetrahydrofolate treatment. Received: December 15, 2017 Published: January 25, 2018 889
DOI: 10.1021/acs.biochem.7b01261 Biochemistry 2018, 57, 889−890
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Armed with the knowledge of this new formaldehyde cycle, Patel and co-workers next sought to understand the extent to which the recovery of formate from formaldehyde contributes to metabolic flux through the 1C cycle. To this end, they used metabolomic profiling of cells treated with 13C-labeled formaldehyde. The results of their stable isotope tracing experiments show that recapture of formaldehyde is a major contributor of carbon to the 1C cycle. Indeed, in cell lines defective in mitochondrial formate synthesis, physiological levels of formaldehyde are able to compensate and support the 1C cycle through an ADH5-dependent conversion of formaldehyde to formate. A more in-depth metabolomics analysis revealed that formaldehyde-derived formate is also readily incorporated into DNA deoxyadenosine and deoxythymidine, as well as the ATP pool, suggesting that formaldehyde recapture may be an important nutrient source for nucleotide synthesis, particularly in proliferating cells that require biomass precursors, such as cancer cells. Altogether, these findings will compel biochemists to redraw their maps of folate metabolism. The 1C cycle now includes a second cycle (or wheel) in which formaldehyde spontaneously formed from folate derivatives is recycled back into the canonical 1C cycle through the combined activity of glutathione, ADH5, and FGH (Figure 1). With an eye on the future, there have a been number of studies in recent years that have suggested that targeting 1C catabolism and, by extension, serine metabolism may be an effective therapy in cancer, especially in breast cancers that harbor DNA copy number amplification of phosphoglycerate dehydrogenase (PHGDH).3,4 One current hypothesis is that increased flux through the serine catabolism pathway supports 1C metabolism and helps to maintain cellular redox. Serine and 1C metabolism are key regulators of the NADP+/NADPH ratio, and knockdown of PHGDH or other serine metabolism enzymes [i.e., serine hydroxymethyltransferase 2 (SHMT2)] inhibits 1C metabolism flux, induces ROS formation, and significantly retards tumor growth.4,5 Despite this, many tumors continue to slowly grow after shRNA-mediated knockdown of PHGDH or SHMT2, suggesting the existence of a compensatory mechanism(s) for cellular NADPH regeneration. It is tempting to speculate that the NADPH-generating conversion of S-hydroxymethylglutathione to S-formylglutathione by ADH5 might be this mechanism and that combining 1C (or serine) metabolism inhibitors with ADH5 inhibitors might prove to be a more effective treatment option in cancer and other diseases. Additionally, some patient tumors are refractory to anti-folate drugs, such as methotrexate. Could ADH5-dependent recovery of formate help explain (at least in some cases) anti-folate drug resistance? The findings of Patel and co-workers are likely to have significant ramifications on our understanding of targeting folate metabolism in disease. It will be interesting in the years to come to see the extent to which diseases “ride the bi-cycle” to exploit formaldehyde as an alternative formate source but also how researchers will use the knowledge of this new folate−formaldehyde−formate cycle to predict and manage drug resistance in patients.
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Research in the Schug lab is supported by the Pew Charitable Trust, the Commonwealth of Pennsylvania (Tobacco Settlement), the Louis C. Washburn Fund for Cancer Research (Wistar Institute), and the National Institutes of Health (CA010815). Notes
The author declares no competing financial interest.
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REFERENCES
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[email protected]. ORCID
Zachary T. Schug: 0000-0003-4197-8227 890
DOI: 10.1021/acs.biochem.7b01261 Biochemistry 2018, 57, 889−890
