[Cell Signaling Technology] Cellular Senescence 연구는 Cell Signaling Technology와 함께하세요!

Only cells with stable cell cycle arrest are considered senescent. Unlike a quiescent cell, a senescent cell will not reenter the cell cycle in response to any known physiological stimuli. Cell cycle arrest is mediated by the p53/p21^CIP1 and p16^INK4A/pRb tumor suppressor pathways, described in more detail below. Expression of p16^INK4A is frequently observed in senescent cells, serving as a useful biomarker. However, p16^INK4A is also highly expressed in pRb-negative tumors and cell lines.

Senescent cells typically have an enlarged size and flattened shape compared to their dividing cell counterparts. Senescent cells display extensive vacuolization and are sometimes multi-nucleated. In addition, disrupted nuclear envelope integrity is observed due to a loss of lamin B1 expression. Senescent cells accumulate dysfunctional mitochondria and display increased levels of reactive oxygen species (ROS). Increased lysosomal content and altered lysosomal activity is also observed, which is reflected by increased levels of β-galactosidase activity at pH 6.0, leading this to be widely adopted as a biomarker of cellular senescence.

Loss of Lamin B1 (shown here in green) is a marker of cellular senescence.

A hallmark feature of senescent cells is extensive chromatin reorganization, most notably the formation of senescence-associated heterochromatin foci (SAHF). These sites of facultative heterochromatin play a role in silencing genes that promote proliferation including E2F target genes like cyclin A. Senescent cells typically contain 30-50 SAHF which are characterized by bright DAPI staining and macroH2A, heterochromatin protein 1 (HP1), and lysine 9 di-or-tri-methylated histone H3 (H3K9Me2/3) immunoreactivity. Although SAHF is frequently observed during senescence, some cells undergo senescence without forming SAHF.

HP1 is a commonly used marker of cellular senescence. Immunohistochemical analysis of HP1 expression in paraffin-embedded human breast carcinoma using HP1 alpha antibody.

DNA damage, such as DNA double strand breaks, is a prominent feature of senescence. Senescent cells display a persistent DNA damage response (DDR) which ultimately triggers cell cycle arrest. Senescent cells contain nuclear foci called DNA segments with chromatin alterations reinforcing senescence (DNA-SCARS), which associate with PML nuclear bodies and accumulate DDR proteins such as activated p53, ATR, and ATM. DNA-SCARS that occur at uncapped telomeres are called telomere dysfunction-induced foci (TIF). Another indicator of DNA damage is γ-H2A.X, which is the phosphorylated form of H2A.X, a variant histone required for checkpoint-mediated cell cycle arrest and DNA repair following double-stranded DNA breaks. DNA damage, caused by ionizing radiation, UV-light, or radiomimetic agents, results in rapid phosphorylation of H2A.X at Ser139.

H2A.X is a commonly used marker of cell senescence. Confocal immunofluorescent analysis of HeLa cells, untreated (left), treated with UV (100 mJ/cm²) using Phospho-Histone H2A.X (Ser139) Mouse mAb (green). Actin filaments were labeled with DyLight™ 554 Phalloidin (red).

Many senescent cells acquire a pro-inflammatory senescence-associated secretory phenotype (SASP) that mediates non-cell autonomous effects of senescence, both beneficial and deleterious. 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). Additionally, chronic senescence-induced inflammation can induce systemic immunosuppression, potentially leading to the onset of diseases including cancer. This chronic inflammation may also drive tissue damage and degeneration associated with aging.