Methionine synthase is essential for cancer cell proliferation in physiological folate environments

Abstract

Targeting folate metabolism can be an effective way to treat cancer. The enzyme methionine synthase catalyzes a key reaction in both folate and methionine metabolism. Early work suggested that inhibiting methionine synthase might restrain tumor growth, though the mechanism remains unclear. We find that due to its specific role in processing folates, methionine synthase is required for cancer proliferation. However, widely used cell culture conditions obscure the proliferative and metabolic consequences of methionine synthase inhibition. Complete dependence on methionine synthase only arises when 5-methyl tetrahydrofolate, the major folate found in circulation, is the predominant folate source provided to cells. In these physiological folate conditions, methionine synthase activity is necessary to maintain intracellular levels of nucleotides, but not methionine. These data reveal that the extracellular environment can alter the essentiality of methionine synthase and suggest that this enzyme plays a crucial cell-autonomous role in supporting nucleotide synthesis and cell proliferation in physiological contexts. Fellowship. A.M.D. was supported by a Jane Coffin Childs Postdoctoral Fellowship. M.G.V.H. acknowledges support from R35CA242379, R01CA201276, the Ludwig Center at MIT, the MIT Center for Precision Cancer Medicine, SU2C, the Emerald Foundation, the Lustgarten Foundation, and a Faculty Scholars Grant from HHMI. AUTHOR CONTRIBUTIONS Conceptualization, M.R.S., M.G.V.H.; Methodology, M.R.S., M.F.R., C.A.L.; Formal Analysis, M.R.S., A.M.D.; Investigation, M.R.S., A.M.D., M.F.R.; Resources, C.A.L.; Visualization, M.R.S., A.M.D.; Writing -Original Draft, M.R.S.; Writing -Review & Editing, M.R.S., A.M.D., M.G.V.H.; Funding Acquisition, M.R.S., A.M.D., M.G.V.H.

S-adenosylmethionine synthases specify distinct H3K4me3 populations and gene expression patterns during heat stress

“…Reduction in SAM levels has major phenotypic consequences in animals, altering lipid levels in murine liver and C. elegans (Lu et al, 2001; Walker et al, 2011), altering differentiation potential in iPS cells (Shyh-Chang et al, 2013) and changing stress resistance (Ding et al, 2018). In addition, 1CC has been identified as a causal regulator of aging (Annibal et al, 2021) and is important in cancer development (Gao et al, 2019; Sullivan et al, 2021). However, the abundance of SAM and its targets have made it difficult to connect changes in methylation to these physiological effects.…”

“…However, the abundance of SAM and its targets have made it difficult to connect changes in methylation to these physiological effects. In addition, studying effects of SAM is difficult in culture because SAM itself is labile (Sun and Locasale, 2021) and tissue culture media is replete with 1CC metabolites (Sullivan et al, 2021). Important insights have been made into the impact of SAM on the breadth of H3K4me3 peaks using methionine depletion (Dai et al, 2018; Mentch et al, 2015; Tang et al, 2017), however, this approach could affect other pathways.…”

“…However, the abundance of SAM and its targets have made it difficult to connect changes in methylation to molecular pathways regulating these physiological effects. In addition, studying effects of SAM is difficult in culture because SAM itself is labile 56 and tissue culture media is replete with 1CC metabolites 19 .…”

“…Studies exploring specificity of metazoan SAM synthase function have been difficult, as MAT1A expression decreases ex vivo and MAT2A is essential for cell viability (Mato et al, 2002). Finally, the high methionine content of traditional cell culture media has limited functional studies (Sullivan et al, 2021).…”

Screening in serum-derived medium reveals differential response to compounds targeting metabolism

“…Two inhibitors of one-carbon metabolism, methotrexate and pyrimethamine, were both more effective in slowing proliferation of cells cultured in ftABS (Figure S3D). This was surprising, given that cells are more resistant to methotrexate when cultured in media containing the physiological folate source 5-methyl tetrahydrofolate (5-MeTHF), rather than the non-physiological folate source present in RPMI, folic acid (FA) 34,35 . We quantified the levels of FA and 5-MeTHF in ftABS to be 2 nM and 10 nM, respectively, which are both lower than the concentration of FA present in RPMI (Figure S3E).…”