Silencing of the CKIIα and CKIIα' genes during cellular senescence is mediated by DNA methylation

Previously we reported that down-regulation of CKII activity is tightly associated with cellular senescence and that the mRNA and protein levels of CKIIα decrease during senescence. The present study demonstrates that the mRNA and protein levels of CKIIα' also decrease during senescence. Knockdown of CKIIα' in IMR-90 cells by RNA interference induced the senescent phenotype. Treatment of senescent IMR-90 cells with a demethylating agent 5-aza-2′-deoxycytidine induced CKIIα and CKIIα' expression, suggesting that DNA hypermethylation might be involved in the silencing of CKIIα and CKIIα' genes in senescent cells. However, bisulfite sequencing analysis revealed that the methylation status of the CpG islands within the reported CKIIα and CKIIα' promoters was not associated with senescence. Instead, senescence-dependent hypermethylation was observed in the region ranging from position + 1112 to + 1128 of the CKIIα gene and at positions - 527 and + 829 of the CKIIα' gene. In addition, this study indicates that DNA methylation-dependent down-regulation of transcription factors Sp1, Ets1 and NF-κB might be involved in silencing of the CKIIα and CKIIα' genes during cellular senescence.