Hypoxia suppresses conversion from proliferative arrest to cellular senescence

Abstract

Unlike reversible quiescence, cellular senescence is characterized by a large flat cell morphology, β-gal staining and irreversible loss of regenerative (i.e., replicative) potential. Conversion from proliferative arrest to irreversible senescence, a process named geroconversion, is driven in part by growth-promoting pathways such as mammalian target of rapamycin (mTOR). During cell cycle arrest, mTOR converts reversible arrest into senescence. Inhibitors of mTOR can suppress geroconversion, maintaining quiescence instead. It was shown that hypoxia inhibits mTOR. Therefore, we suggest that hypoxia may suppress geroconversion. Here we tested this hypothesis. In HT-p21-9 cells, expression of inducible p21 caused cell cycle arrest without inhibiting mTOR, leading to senescence. Hypoxia did not prevent p21 induction and proliferative arrest, but instead inhibited the mTOR pathway and geroconversion. Exposure to hypoxia during p21 induction prevented senescent morphology and loss of regenerative potential, thus maintaining reversible quiescence so cells could restart proliferation after switching p21 off. Suppression of geroconversion was p53-and HIF-1-independent, as hypoxia also suppressed geroconversion in cells lacking functional p53 and HIF-1α. Also, in normal fibroblasts and retinal cells, hypoxia inhibited the mTOR pathway and suppressed senescence caused by etoposide without affecting DNA damage response, p53/p21 induction and cell cycle arrest. Also hypoxia suppressed geroconversion in cells treated with nutlin-3a, a nongenotoxic inducer of p53, in cell lines susceptible to nutlin-3a-induced senescence (MEL-10, A172, and NKE). Thus, in normal and cancer cell lines, hypoxia suppresses geroconversion caused by diverse stimuli. Physiological and clinical implications of the present findings are discussed.oncology | gerontology | biology R ecent evidence emerges that the mammalian target of rapamycin (mTOR) pathway is involved in cellular aging (1, 2). Nutrients, cytokines, growth factors, and hormones activate the mTOR pathway, which drives cellular mass growth (3, 4). In proliferating cells, cellular growth in size is balanced by cell division. When the cell cycle is arrested and cells thus do not divide, inappropriate activation of growth-promoting pathways such as mTOR converts cell cycle arrest into senescence (1, 2). Senescence is characterized by a large flat cell morphology, β-gal staining, and a hypersecretory phenotype (5, 6). In a widely used cellular model, induction of ectopic p21 by isopropyl-thio-galactosidase (IPTG) arrests HT-p21-9 cells (7,8). Initially (during 2-3 d), this condition is reversible: when p21 is switched off, cells resume proliferation (7,8). While inhibiting the cell cycle, p21 does not inhibit mTOR, which in turn converts arrest into irreversible senescence (1). By day 3, cells become large, flat, and β-gal-positive, and lose regenerative potential (RP): cells cannot resume proliferation when p21 is switched off. The conversion from reversible arrest to senescence, a process na...

MEK drives cyclin D1 hyperelevation during geroconversion

“…[16][17][18] We emphasize that our investigations are different from previous work on senescence. It has been known for over a decade that strong mitogenic/oncogenic signaling can cause senescence.…”

Cyclin-dependent kinases regulate lysosomal degradation of hypoxia-inducible factor 1α to promote cell-cycle progression

“…Hypoxia induces reversible cell-cycle arrest in a HIF-dependent manner (31-36), but suppresses senescence, which is the irreversible loss of replicative potential, in a HIFindependent manner (46). In the present study, we have delineated a molecular mechanism by which Cdks regulate HIF-1.…”

Vascular ageing and endothelial cell senescence: Molecular mechanisms of physiology and diseases

“…This suggests that physiological reduced level of oxygen supply in most of the normal tissues and in particular the cancer stem cell niche might contribute to deceleration of premature senescence promoted by different stressors. Indeed in vitro cell based experiments have proven that similarly to rapamycin hypoxia suppresses geroconversion after cell cycle arrest induced by DNA damageing agents (etoposide treatment), activation of p53/p21 axis (nutlin3a treatment) or ectopic expression of p21 (Leontieva et al, 2012). Hypoxia therefore appears to directly counteract the p53-mediated senescence and ageing, however whether HIFs play a direct role in this process is currently unknown.…”