Differential maintenance and de novo methylating activity by three DNA methyltransferases in aging and immortalized fibroblasts

Abstract

Genomic methylation, which influences many cellular processes such as gene expression and chromatin organization, generally declines with cellular senescence although some genes undergo paradoxical hypermethylation during cellular aging and immortalization. To explore potential mechanisms for this process, we analyzed the methylating activity of three DNA methyltransferases (Dnmts) in aging and immortalized WI-38 fibroblasts. Overall maintenance methylating activity by the Dnmts greatly decreased during cellular senescence. In immortalized WI-38 cells, maintenance methylating activity was similar to that of normal young cells. Combined de novo methylation activity of the Dnmts initially decreased but later increased as WI-38 cells aged and was strikingly elevated in immortalized cells. To further elucidate the mechanisms for changes in DNA methylation in aging and immortalized cells, the individual Dnmts were separated and individually assessed for maintenance and de novo methylating activity. We resolved three Dnmt fractions, one of which was the major maintenance methyltransferase, Dnmt1, which declined steadily in activity with cellular senescence and immortalization. However, a more basic Dnmt, which has significant de novo methylating activity, increased markedly in activity in aging and immortalized cells. We have identified this methyltransferase as Dnmt3b which has an important role in neoplastic transformation but its role in cellular senescence and immortalization has not previously been reported. An acidic Dnmt we isolated also had increased de novo methylating activity in senescent and immortalized WI-38 cells. These studies indicate that reduced genome-wide methylation in aging cells may be attributed to attenuated Dnmt1 activity but that regional or gene-localized hypermethylation in aging and immortalized cells may be linked to increased de novo methylation by Dnmts other than the maintenance methyltransferase.

Age-related loss of CpG methylation in the tumour necrosis factor promoter

“…The potential mechanism of age-related demethylation of a single locus is not clear; the cellular activity of DNA methyltranserase 1 decreases with age but this is likely to lead to a global loss in methylation which we did not observe [33]. Another potential mechanism is related to the effects of chronic age-related low grade inflammation leading to the production of reactive oxidative molecules with resultant conversion of 5-methylcytosine to 5-hydroxymethylcytosine [34].…”

Epigenetics, aging, and autoimmunity

“…A recently published longitudinal study of 718 elderly individuals between 55 and 92 years of age demonstrated that repetitive element methylation, particularly in Alu sequences, decreases throughout aging (31). It has been postulated that the reduction of Dnmt1 activity, the predominant maintenance DNA methyltransferase, with age contributes to the decrease in global DNA methylation (32,33). However, although genome-wide levels of methylation decrease with age in mammals, methylation levels can be either increased, decreased or unchanged depending on the specific strain of mice, tissue or gene examined (recently reviewed in (34)).…”

Biological Effects of Polyamines on the Prevention of Aging-associated Diseases and on Lifespan Extension

“…However, aging-associated increases in the methylation of some genes are also reported (Issa et al, 1994;Issa et al, 1996;Wallace et al, 2010). Generally, ODC (Minois et al, 2011) and Dnmt (Lopatina et al, 2002;Oliveira et al, 2012;Romanenko et al, 1998) et al, 2004;Li et al, 2010;Morgan et al, 2005). Because there is a close relationship between Dnmt activities and the methylation status of LFA-1 promoter regions, and since aging is associated with decreased Dnmt activities as well as increased demethylation of the LFA-1 promoter region (Zhang et al, 2002), the agingassociated enhancement of LFA-1 expression seems to be due to the age-dependent decreases in Dnmt activities.…”