A mitotic cell that rests in permanent cell cycle arrest without the ability to divide is considered as a senescent cell. Cellular senescence is essential to limit the function of cells with heavy DNA damages. The lack of senescence is in favour of tumorigenesis, whereas the accumulation of senescent cells in tissues is likely to induce ageing and age-related pathologies on the organismal level. Understanding of cellular senescence is thus critical to both cancer and ageing studies.
Senescence, essentially permanent cell cycle arrest, is one of the results of DNA damage response, such as the ataxia telangiectasia mutated and the ataxia telangiectasia and Rad3-related signaling pathways. In other cases, mild DNA damages can usually be repaired after DNA damage response, while the cells with heavy damages on DNA end in apoptosis. The damage to the special structure of telomere, however, prone to result in permanent cell cycle arrest after activation of DNA damage response. In fact, a few previous pieces of research on ageing have largely focused on telomere and considered it a primary contributor to different types of senescence. For instance, its reduction in length after each replication turns on a timer for replicative senescence, and its tandem repeats specific to binding proteins makes it susceptible to DNA damage from oxidative stress, and thus stress-induced premature senescence. In most of the senescent cells, the accumulation of biomarkers is found around the telomere which has either its tail structure disassembled or damage foci exposed on the tandem repeats.
In this review, among several types of senescence, I will investigate two of the most common and widely discussed types in eukaryotic cells -replicative senescence and stress-induced premature senescence - in terms of their mechanism, relationship with telomere, and implication to organismal ageing.
DNA damage response activation in mouse embryonic fibroblasts undergoing replicative senescence and following spontaneous immortalization
