Figure 2. Hallmarks of SNCs.

Based predominately on in vitro experimentation, senescent cells (SNCs) possess several key features, namely engagement of a permanent cell cycle arrest, resistance to cell death signalling and production of a bioactive secretome, known as the senescence-associated secretory phenotype (SASP). a | In response to pro-senescence stresses, including reactive oxygen species (ROS), DNA damage and others, the p53–p21 and p16^INK4A cell cycle arrest pathways are activated, which inhibits cyclin-dependent kinase 2 (CDK2), CDK4 and CDK6, respectively. Consequently, retinoblastoma protein (RB) is maintained in a hypophosphorylated state that suppresses expression of S-phase genes by binding to and sequestering the transcription factors E2F, DP1 and DP2 as well as recruiting histone deacetylases (HDACs) that act on heterochromatin. b | SNCs resist mitochondria-mediated apoptosis, in part by upregulating B cell lymphoma 2 (BCL-2) family members (BCL-2, BCL-X_L and BCL-W), which bind to and sequester BAX and BCL-2-associated agonist of cell death (BAD). This sequestration blocks pore formation by BAX–BAD, which inhibits mitochondrial outer membrane permeabilization (MOMP) and release of pro-apoptotic cytochrome c, second mitochondria-derived activator of caspase (SMAC; also known as DIABLO) and OMI (also known as HTRA2). SNCs also resist extrinsic apoptosis by overexpressing decoy receptor 2 (DCR2), which intercepts FAS ligand expressed on cytotoxic immune cells. c | SNCs produce a dynamic, bioactive secretome. SASP production is initiated and sustained by a chronic DNA damage response (DDR) called ‘DNA-SCARS’ (DNA segments with chromatin alterations reinforcing senescence). Initially, Notch signalling drives transforming growth factor-β (TGFβ) secretion (‘early SASP’), which acts in a cell-autonomous manner to promote cell cycle arrest. A subsequent decrease in Notch signalling promotes a shift to a DDR-dependent ‘transitional SASP’ that is enhanced by mechanistic target of rapamycin (mTOR), in which cell surface-associated interleukin-1α (IL-1α) binds to interleukin-1 receptor (IL-1R). Either this cell-autonomous IL-1α signal or p38 mitogen-activated protein kinase (p38 MAPK) activity is transmitted through nuclear factor-κB (NF-κB), resulting in the secretion of a ‘late SASP’, which contains metalloproteinases (MMPs), IL-6, IL-8 and numerous other factors. At this stage, IL-6 and IL-8 can also reinforce the cell cycle arrest. Crucially, many of the specific details of SNC growth arrest, death resistance and SASP have not been confirmed in vivo and are likely to show substantial differences. ATM, ataxia telangiectasia mutated; ATR, ataxia telangiectasia and RAD3-related protein; CHK2, checkpoint kinase 2; NBS1, Nijmegen breakage syndrome protein 1.