Cellular senescence is a stable cell-cycle arrest triggered by genotoxic stress (replicative telomere erosion, oxidative damage, oncogene activation — OIS) — initially tumor-suppressive but accumulates with age and drives "inflammaging" + age-related disease via the senescence-associated secretory phenotype (SASP). Two main routes: (1) p16^INK4a^/Rb axis — irreversible cell-cycle arrest in G1; (2) p21^CIP1^/p53 axis — initial arrest, more reversible. Hallmarks: SA-β-galactosidase activity, enlarged flattened morphology, SAHF (senescence-associated heterochromatin foci), LADs (lamin-associated domains) reorganization, lysosomal expansion. SASP: senescent cells secrete a pro-inflammatory cocktail — IL-6, IL-8, MCP-1, MMPs, GROα, TIMPs, GDF15 — driven by NF-κB + C/EBPβ + cGAS-STING (cross-link). SASP paracrine: spreads senescence to neighbors; recruits immune clearance (when functional); promotes tumor microenvironment + fibrosis when persistent. Tissue burden of p16-positive cells ↑ with age; targeted clearance in mice extends healthspan + reverses age-related dysfunction (Baker 2016 — INK-ATTAC). Senolytics: drugs that selectively kill senescent cells (which depend on anti-apoptotic SCAPs — BCL-2, BCL-xL, PI3K-AKT). Established: dasatinib + quercetin (D+Q — Kirkland canonical combo; first-in-human trials in idiopathic pulmonary fibrosis, diabetic kidney disease); fisetin (mono-senolytic, gentler); navitoclax + venetoclax (BCL-2/BCL-xL inhibitors — too toxic systemically, focal use); rapamycin/mTOR inhibitors (gerosuppression but not senolytic per se); metformin (gerosuppressive, ↓SASP); resveratrol/curcumin (modest SASP suppression). Cross-links: cell cycle cdk (p16/p21 arrest), nfkb signaling (SASP transcription), cgas sting type1 ifn (DNA-damage trigger), mtor signaling (gerosuppression), autophagy lc3 axis (lysosomal-senescence).
Organ Systems
immune-hematologic
musculoskeletal
cardiovascular
integumentary
Pathway Steps
genotoxic stress (telomere erosion, ROS, oncogene) → DDR + p53-p21 / p16-Rb activation — via CHK1/CHK2 + ATM/ATR; commitment over 7–10 days. Senescence is triggered by diverse genotoxic stresses — telomere erosion (replicative senescence), oxidative damage, and oncogene activation (OIS) — which converge on the DNA-damage response and engage the two tumor-suppressor arms, p53-p21 and p16-Rb. It evolved as an anti-cancer brake but accumulates with age.
p16^INK4a^ ↑ → Rb hypophosphorylation → stable G1 arrest — via irreversible vs p21 (transient). p16 (the INK4a CDK4/6 inhibitor) rises and keeps Rb hypophosphorylated, so Rb stays bound to E2F and blocks cell-cycle entry — locking in a stable G1 arrest that, unlike quiescence, resists mitogens. p16 is the most widely used biomarker of senescent-cell burden in aging tissue.
persistent DNA damage / cytosolic DNA → cGAS-STING + NF-κB activation — via cross-link to cgas sting type1 ifn — drives SASP transcription. Senescence becomes inflammatory partly via nucleic-acid sensing: persistent DNA damage and cytoplasmic chromatin fragments (and leaked mitochondrial DNA) activate cGAS-STING, which with NF-κB launches the secretory response. This links the genome-surveillance machinery to the senescent secretome.
NF-κB + C/EBPβ + GATA4 → SASP transcription (IL-6, IL-8, MCP-1, MMPs, GDF15) — via paracrine senescence spread + immune chemotaxis. The SASP is driven transcriptionally by NF-κB with C/EBPβ and GATA4, producing IL-6, IL-8, MCP-1, matrix metalloproteinases, and GDF15. The SASP reinforces arrest and recruits immune clearance, but when chronic it spreads senescence to neighbors and fuels age-related “inflammaging”.
BCL-2 / BCL-xL upregulation (SCAP) → apoptosis resistance — via senescent cells survive via SCAPs — pharmacological vulnerability for senolytics. Senescent cells resist their own apoptosis by upregulating BCL-2-family survival proteins (BCL-2, BCL-xL) through senescent-cell anti-apoptotic pathways (SCAPs). This survival dependence is the therapeutic Achilles’ heel — precisely what senolytics exploit.
senolytic drug (D+Q, fisetin, navitoclax) → selective apoptosis of senescent cells — via SCAP inhibition removes survival signal. Senolytics selectively kill senescent cells by disabling those survival pathways: dasatinib+quercetin (D+Q), the flavonoid fisetin, and the BCL-2/BCL-xL inhibitor navitoclax. Clearing senescent cells improves healthspan in animal models, and translational trials are now testing this in age-related disease.