Enhancer of zeste homolog 2 depletion induces cellular senescence via histone demethylation along the INK4/ARF locus

Abstract

Polycomb group proteins are epigenetic transcriptional repressors that function through recognition and modification of histone methylation and chromatin structure. As a member of PcG proteins, enhancer of zeste homolog 2 (EZH2) targets cell cycle regulatory proteins which govern cell cycle progression and cellular senescence. In previous work, we reported that EZH2 depletion functionally induced cellular senescence in human gastric cancer cells with mutant p53. However, whether EZH2 expression contributes to the change of key cell cycle regulators and the mechanism involved are still unclear. To address this issue, we investigated the effects of EZH2 depletion on alteration of histone methylation pattern. In gastric cancer cells, INK4/ARF locus was activated to certain extent in consequence of a decrease of H3K27me3 along it caused by EZH2 silence, which contributed substantially to an increase in the expression of p15INK4b, p14ARF and p16INK4a and resulted in cellular senescence ultimately. Furthermore, MKN28 cells, which did not express p16INK4a and p21cip, could be induced to senescence via p15INK4b activation and suppression of p15INK4b reversed senescence progression induced by EZH2 downregulated. These data unravel a crucial role of EZH2 in the regulation of INK4/ARF expression and senescence procedure in gastric cancer cells, and show that the cellular senescence could just depend on the activation of p15INK4b/Rb pathway, suggesting the cell-type and species specificity involved in the mechanisms of senescence inducement.