Department of Cell Stress Biology, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
Proc Natl Acad Sci U S A. 2012 Aug 14;109(33):13314-8. doi: 10.1073/pnas.1205690109. Epub 2012 Jul 30.
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.
与可逆性静止不同，细胞衰老的特征是细胞形态大而扁平、β-半乳糖苷酶染色阳性和再生（即复制）潜能的不可逆丧失。从增殖停滞向不可逆衰老的转化，即所谓的衰老转化，部分由促进生长的途径驱动，如哺乳动物雷帕霉素靶蛋白（mTOR）。在细胞周期停滞期间，mTOR 将可逆性停滞转化为衰老。mTOR 的抑制剂可以抑制衰老转化，维持静止状态。已经表明缺氧抑制 mTOR。因此，我们认为缺氧可能抑制衰老转化。在这里，我们测试了这一假设。在 HT-p21-9 细胞中，诱导型 p21 的表达导致细胞周期停滞而不抑制 mTOR，导致衰老。缺氧并没有阻止 p21 的诱导和增殖停滞，而是抑制了 mTOR 通路和衰老转化。在 p21 诱导期间暴露于缺氧会阻止衰老形态的出现和再生潜能的丧失，从而维持可逆性静止，使细胞在关闭 p21 后能够重新开始增殖。衰老转化的抑制与 p53 和 HIF-1 无关，因为缺氧也抑制了缺乏功能性 p53 和 HIF-1α 的细胞中的衰老转化。此外，在正常成纤维细胞和视网膜细胞中，缺氧抑制 mTOR 通路，并抑制依托泊苷引起的衰老，而不影响 DNA 损伤反应、p53/p21 诱导和细胞周期停滞。此外，缺氧还抑制了用 nutlin-3a 处理的细胞中的衰老转化，nutlin-3a 是一种非遗传毒性的 p53 诱导剂，在易受 nutlin-3a 诱导衰老的细胞系（MEL-10、A172 和 NKE）中。因此，在正常和癌细胞系中，缺氧抑制了由多种刺激引起的衰老转化。讨论了本研究结果的生理和临床意义。
