Is Senescence Reversible?

Senescence was originally identified by the finite lifespan of normal cells that is a consequence of telomere shortening with each cycle of DNA replication. Cells undergoing replicative senescence display pronounced morphological and biochemical changes such as flattening and/or enlargement, increases in p21(WAF1) and/or p16(INK4A), a senescence-associated secretory phenotype, and often senescence-associated heterochromatic foci. Senescence also occurs in tumor cells in response to various forms of chemotherapy or radiation (therapy-induced senescence), which could be the basis for prolonged or (ideally) permanent growth arrest. Alternatively, therapy-induced senescence could represent a means whereby tumor cells evade the potential toxicity of chemotherapy and radiation, allowing for the eventual re-emergence or escape from senescence that could lead to disease recurrence. This review discusses the experimental data in the literature that support the premise that senescence is potentially reversible through the inactivation of p53, p16(INK4A) and/or Rb, over-expression of Cdc2/cdk1 and survivin, the development of polyploidy, the survival of cancer stem cells and/or restoration of the nuclear landscape. If senescence is truly reversible, then the re-emergence of tumor cells from senescent arrest induced by chemotherapy or radiation could represent a barrier to the development of effective and curative cancer therapies.

Yet another function of p53--the switch that determines whether radiation-induced autophagy will be cytoprotective or nonprotective: implications for autophagy inhibition as a therapeutic strategy.

The influence of autophagy inhibition on radiation sensitivity was studied in human breast, head and neck, and non-small cell lung cancer cell lines, in cell lines that were either wild type or mutant/null in p53, and in cells where p53 was inducible or silenced. Whereas ionizing radiation promoted autophagy in all tumor cell lines studied, pharmacological inhibition of autophagy and/or genetic silencing of autophagy genes failed to influence sensitivity to radiation in p53 mutant Hs578t breast tumor cells, HN6 head and neck tumor cells, and H358 non-small cell lung cancer cells. The requirement for functional p53 in the promotion of cytoprotective autophagy by radiation was confirmed by the observation that radiation-induced autophagy was nonprotective in p53 null H1299 cells but was converted to the cytoprotective form with induction of p53. Conversely, whereas p53 wild-type HN30 head and neck cancer cells did show sensitization to radiation upon autophagy inhibition, HN30 cells in which p53 was knocked down using small hairpin RNA failed to be sensitized by pharmacological autophagy inhibition. Taken together, these findings indicate that radiation-induced autophagy can be either cytoprotective or nonprotective, a functional difference related to the presence or absence of function p53. Alternatively, these findings could be interpreted to suggest that whereas radiation can induce autophagy independent of p53 status, inhibition of autophagy promotes enhanced radiation sensitivity through a mechanism that requires functional p53. These observations are likely to have direct implications with respect to clinical efforts to modulate the response of malignancies to radiation through autophagy inhibition.

Autophagy inhibition for chemosensitization and radiosensitization in cancer: do the preclinical data support this therapeutic strategy?

Recognition of the cytoprotective functions of autophagy that occur in tumor cells exposed to various forms of chemotherapy or radiation has generated intense interest in the possibility that pharmacological interference with autophagy could provide a clinical strategy for overcoming therapeutic resistance. Multiple clinical trials are currently in progress to evaluate the antimalarial agent chloroquine (generally in its clinical formulation as hydroxychloroquine) and its impact on various forms of cancer therapy. In this commentary/review, we focus on the relatively limited number of studies in the literature where chloroquine has been tested in combination with chemotherapy or radiation in experimental tumor-bearing animal models. We also present recent data from our own laboratories, in cell culture experiments as well as in vivo studies, which demonstrate that neither chloroquine nor silencing of an autophagy regulatory gene was effective in conferring radiation sensitivity in an experimental model of breast cancer. The capacity for sensitization by chloroquine appears to be quite wide-ranging, with dramatic effects for some drugs/tumor models and modest or minimal effects in others. One possible caveat is that, with only a few exceptions, experiments have generally been performed in xenograft models, thereby eliminating the involvement of the immune system, which might ultimately be proven to play a central role in determining the effectiveness of autophagy inhibition in chemosensitization or radiosensitization. Nevertheless, a careful review of the current literature suggests that caution is likely to be warranted in translating preclinical findings relating to autophagy inhibition as an adjunctive therapeutic strategy.