Cellular senescence is defined by permanent cell cycle arrest. Senescent cells accumulate with age and contribute to the normal aging process as well as age-related disorders. The link between senescence, aging, and age-related pathologies, including cancer, neurodegeneration, and metabolic and cardiovascular diseases have largely fueled the senescence research field.
Cellular senescence was first described by Hayflick and Moorhead as a permanent cell cycle arrest occurring when cultured cells reach their replicative limit (replicative senescence). This is defined by cells undergoing sufficient divisions to cause telomere shortening, which elicit a DNA damage response (DDR) and consequentially cell cycle arrest.
Senescence can be induced by various cell stressors such as oncogenes, oxidative stress, or chemotherapy. Cellular senescence is now characterized as a stable stress-induced cell cycle arrest accompanied by an enhanced secretory phenotype known as the Senescence Associated Secretory Phenotype (SASP).
Senescence-associated cell cycle arrest typically involves cyclin dependent kinase (CDK) inhibition through the p53/p21 Waf1/Cip1 and/or Rb/p16 INK4A pathways as a result of persistent DNA damage. Oncogene-induced senescence (OIS) is a sustained antiproliferative response brought about by oncogene activation or tumor suppressor inactivation. Withdrawal of pre-neoplastic cells from the cell cycle plays a protective anti-tumorigenic role. Chemotherapeutic agents can also cause cells to undergo senescence by inducing persistent DDR. Chemotherapy-induced fatigue may be a result of senescent cell accumulation, as it can be alleviated in mice by through clearance of senescent cells.
The SASP is comprised of a highly complex mixture of secreted cytokines, chemokines, growth factors, and proteases, with the precise composition varying markedly by cell and tissue context and the senescence-inducing stimulus. These secreted factors facilitate communication with neighboring cells and the immune system, which ultimately influences the fate of the senescent cell. For example, the SASP recruits immune cells to senescent cells, thereby facilitating their elimination, which serves a tumor suppressor function. Paradoxically, however, the SASP has been shown to promote tumor cell progression through secretion of factors that promote angiogenesis, extracellular matrix remodeling, or epithelial-mesenchymal transition (EMT).
It is important to note that there is currently no universal marker of senescence. Investigators must assess several senescence-associated markers in aggregate to provide evidence of a senescent phenotype as the markers that are expressed in senescent cells vary depending on the senescence stimulus, cell type, and timing.
Some commonly used markers of senescence are related to senescence-associated cell cycle arrest. Examples are cyclin dependent kinase inhibitors such as p16 INK4A or p21 Cip1, persistent DNA damage foci as seen by staining for DNA repair proteins such as 53BP1 or gammaH2A.X, and lack of the proliferation marker Ki67.
SASP factors such as TNFalpha, IL-1alpha, IL-1beta, IL-6, IL-8, matrix metalloproteinases (MMPs) and loss of nuclear localization of HMGB1 are also commonly used senescence markers.
Additional markers include senescence-associated beta-galactosidase activity due to increased lysosomal content and activity, loss of lamin B1 due to changes in the nuclear envelope in senescent cells, increased lipofuscin staining by Sudan Black B (SBB) and morphological changes such as enlarged, flattened cell bodies.
Senescent cells are also categorized by extensive epigenetic remodeling. HIRA and ASF1A chromatin remodelers establish senescence-associated heterochromatin foci (SAHF), which serve as another marker of senescence along with an increase in trimethylated histone H3 lysine 9 (H3K9me3).
Aging and Disease
While senescence plays beneficial roles in tumor prevention, remodeling during development, and in wound healing, accumulation of senescent cells over the course of a lifetime contributes to aging and age-related disease. Senescent cells accumulate as organisms age and, if not cleared by the immune system, can contribute to many age-related pathologies including cancer, cardiovascular disease, atherosclerosis and type 2 diabetes.
The detrimental effects of senescent cells on aged organisms are thought to be carried out primarily by SASP factors. In addition to creating a pro-inflammatory environment, SASP factors can induce paracrine senescence, hindering regenerative capacity of the surrounding tissue.
In mouse models, clearance of senescent cells increases lifespan and improves healthspan. Senolytics, drugs that eliminate senescent cells, are currently being tested in clinical trials to treat age-related disease in humans. Some senolytic drugs target anti-apoptotic pathways, which, when active in senescent cells, allow resistance to cell cycle checkpoint-induced cell death.