Cellular senescence suppresses cancer by arresting cell proliferation, essentially permanently, in response to oncogenic stimuli, including genotoxic stress. We modified the use of antibody arrays to provide a quantitative assessment of factors secreted by senescent cells. We show that human cells induced to senesce by genotoxic stress secrete myriad factors associated with inflammation and malignancy. This senescence-associated secretory phenotype (SASP) developed slowly over several days and only after DNA damage of sufficient magnitude to induce senescence. Remarkably similar SASPs developed in normal fibroblasts, normal epithelial cells, and epithelial tumor cells after genotoxic stress in culture, and in epithelial tumor cells in vivo after treatment of prostate cancer patients with DNA-damaging chemotherapy. In cultured premalignant epithelial cells, SASPs induced an epithelial–mesenchyme transition and invasiveness, hallmarks of malignancy, by a paracrine mechanism that depended largely on the SASP factors interleukin (IL)-6 and IL-8. Strikingly, two manipulations markedly amplified, and accelerated development of, the SASPs: oncogenic RAS expression, which causes genotoxic stress and senescence in normal cells, and functional loss of the p53 tumor suppressor protein. Both loss of p53 and gain of oncogenic RAS also exacerbated the promalignant paracrine activities of the SASPs. Our findings define a central feature of genotoxic stress-induced senescence. Moreover, they suggest a cell-nonautonomous mechanism by which p53 can restrain, and oncogenic RAS can promote, the development of age-related cancer by altering the tissue microenvironment.

The unfolded protein response (UPR) regulates gene expression in response to stress in the endoplasmic reticulum (ER). We determined the transcriptional scope of the UPR using DNA microarrays. Rather than regulating only ER-resident chaperones and phospholipid biosynthesis, as anticipated from earlier work, the UPR affects multiple ER and secretory pathway functions. Studies of UPR targets engaged in ER-associated protein degradation (ERAD) reveal an intimate coordination between these responses: efficient ERAD requires an intact UPR, and UPR induction increases ERAD capacity. Conversely, loss of ERAD leads to constitutive UPR induction. Finally, simultaneous loss of ERAD and the UPR greatly decreases cell viability. Thus, the UPR and ERAD are dynamic responses required for the coordinated disposal of misfolded proteins even in the absence of acute stress.

Cellular senescence suppresses cancer by stably arresting the proliferation of damaged cells1. Paradoxically, senescent cells also secrete factors that alter tissue microenvironments2. The pathways regulating this secretion are unknown. We show that damaged human cells develop persistent chromatin lesions bearing hallmarks of DNA double-strand breaks (DSBs), which initiate increased secretion of inflammatory cytokines such as interleukin-6 (IL-6). Cytokine secretion occurred only after establishment of persistent DNA damage signaling, usually associated with senescence, not after transient DNA damage responses (DDR). Initiation and maintenance of this cytokine response required the DDR proteins ATM, NBS1 and CHK2, but not the cell cycle arrest enforcers p53 and pRb. ATM was also essential for IL-6 secretion during oncogene-induced senescence and by damaged cells that bypass senescence. Further, DDR activity and IL-6 were elevated in human cancers, and ATM-depletion suppressed the ability of senescent cells to stimulate IL-6-dependent cancer cell invasiveness. Thus, in addition to orchestrating cell cycle checkpoints and DNA repair, a novel and important role of the DDR is to allow damaged cells to communicate their compromised state to the surrounding tissue.

Cellular senescence suppresses cancer by forcing potentially oncogenic cells into a permanent cell cycle arrest. Senescent cells also secrete growth factors, proteases, and inflammatory cytokines, termed the senescence-associated secretory phenotype (SASP). Much is known about pathways that regulate the senescence growth arrest, but far less is known about pathways that regulate the SASP. We previously showed that DNA damage response (DDR) signalling is essential, but not sufficient, for the SASP, which is restrained by p53. Here, we delineate another crucial SASP regulatory pathway and its relationship to the DDR and p53. We show that diverse senescenceinducing stimuli activate the stress-inducible kinase p38MAPK in normal human fibroblasts. p38MAPK inhibition markedly reduced the secretion of most SASP factors, constitutive p38MAPK activation was sufficient to induce an SASP, and p53 restrained p38MAPK activation. Further, p38MAPK regulated the SASP independently of the canonical DDR. Mechanistically, p38MAPK induced the SASP largely by increasing NF-jB transcriptional activity. These findings assign p38MAPK a novel role in SASP regulation-one that is necessary, sufficient, and independent of previously described pathways.