Investigating the Mechanisms of Cellular Senescence

Senescent cells are those that have left the cell cycle without being destroyed, either by the immune system or by one of the processes of programmed cell death. They remain active, however, exhibiting what is termed a senescence-associated secretory phenotype (SASP): these cells secrete all sorts of chemical signals that prove harmful to surrounding tissues and the body as a whole - through promotion of chronic inflammation, for example.

The number of senescent cells in tissue grows with age, and this increase in numbers is one of the root causes of aging. Researchers have demonstrated benefits in mice through destroying senescent cells without harming other cells. Regular targeted destruction of senescent cells could be the basis for therapies that remove this contribution to degenerative aging.

Any other approach would require understanding more about SASP and how to control or reverse the unpleasant effects of senescence - and here is an example of this sort of research, aimed at identifying controlling mechanisms with an eye to building therapies to reduce SASP:

With advancing age, senescent cells accumulate in tissues and the SASP-elicited proinflammatory state is believed to have a complex influence on age-related conditions. For example, two major SASP factors, IL-6 and IL-8, together with other SASP factors, attract immune cells to the tissue in which senescent cells reside; depending on the tissue context, this immune surveillance can promote processes such as wound healing, the resolution of fibrosis, and tumor regression. At the same time, SASP factors can compromise the integrity of the ECM, thus facilitating cancer cell migration. In addition, the systemic proinflammatory phenotype seen in the elderly is believed to affect a broad range of age-related pathologies, including diabetes, cancer, neurodegeneration and cardiovascular disease and contributes to an age-related decline of the adaptive immune system (immunosenescence).

Despite the great potential impact of the SASP on the biology of senescence and aging, the mechanisms that regulate SASP are poorly understood. ... Here, we report the identification of NF90 as an RNA-binding protein that binds to numerous mRNAs encoding SASP factors (collectively named SASP mRNAs) and coordinately influences their post-transcriptional fate in a senescence-dependent manner.

In young, early-passage, proliferating fibroblasts, high NF90 levels contributed to the repression of SASP factor production. This repression was elicited mainly via reduction in SASP factor translation ... By contrast, in senescent cells NF90 levels were markedly reduced, which allowed increased expression of numerous SASP factors. Our results are consistent with a role for NF90 as a coordinator of the inhibition of SASP factor production in early-passage, proliferating fibroblasts; in senescent cells, the lower levels of NF90 lead to SASP de-repression, permitting higher expression of SASP factors