Hypoxia increases the methylated histones to prevent histone clipping and redistribution of heterochromatin during Raf-induced senescence.
Hypoxia increases histone methylation by inhibiting O2- and a-ketoglutarate-dependent histone lysine demethylases (KDMs). This study is the first to demonstrate how the hypoxic increment of methylated histones cross-talks with other epigenetic changes, such as histone clipping, and heterochromatin redistribution (senescence-associated heterochromatin foci, SAHF) found during oncogene-induced senescence (OIS). Raf activation in primary human fibroblasts IMR90 increased cathepsin L (CTSL)-mediated clipping of histone 3 (H3), H2B and H4 at H3 A21/T22, H2B T19/K20, and H4 G11/K12, respectively. Hypoxia protected H3 from CTSL by increasing histone methylation, especially at H3K23me3 without reducing the activity of CTSL. The maintenance of methylated histones is sufficient for protecting histones from CTSL, not sufficient but necessary for inhibiting SAHFs. Expression of cleaved H3 induces senescence even under hypoxia, suggesting that hypoxia disrupts this positive feedback loop of OIS by increasing histone methylation. Thus, hypoxia protects histones and chromatin from dramatic epigenetic changes by increasing histone methylation. These observations suggest that the OIS process increases the accessibility of chromatin, which hypoxic conditions inhibited.