Hypoxia Rewires Histone Methylation in Glioblastoma Cells via Enzyme Reprogramming Despite Disruption of One-Carbon Metabolism

Hypoxia is a hallmark of the tumor microenvironment that profoundly alters the cellular metabolism and epigenetic regulation. In this study, we investigated how oxygen limitation reprograms histone methylation in glioblastoma cells by integrating stable isotope tracing with high-resolution proteomics and epigenomics. Using deuterium-labeled serine and the RQMID-MS platform, we demonstrated that hypoxia impairs methyl group transfer from serine to histones due to the downregulation of the vitamin B₁₂ transporter TCN2, which is critical for homocysteine remethylation and SAM synthesis. Despite this blockade in one-carbon metabolism, global histone methylation patterns were not uniformly suppressed. Instead, we observed site-specific changes driven by altered expression of methyltransferases and demethylases, particularly decreased KMT1F (H3K9 methylation) and KMT2B (H3K4 methylation) and increased KDM2A (H3K36 demethylation), KDM3A (H3K9 demethylation), and KMT5A/SETD8 (H4K20 monomethylation). These findings reveal that the histone methylation landscape under hypoxia is governed by a compensatory interplay between one-carbon metabolism and chromatin-modifying enzyme regulation.