For several years cellular senescence has been considered the result of replicative exhaustion of primary cells in vitro or the artifactual overexpression of oncogenes, with little evidence to prove the functional relevance of senescence in vivo. Recently, we and others have demonstrated that cellular senescence occurs in vivo and can oppose tumorigenesis in mouse models (2
). Specifically, our prostate mouse model has shown how potent the senescence response elicited by Pten
loss is in restricting tumor initiation and progression (3
). This finding has straightforward therapeutic implications for prostate cancer prevention and treatment and is especially relevant given that prostate cancer is still the most commonly diagnosed malignancy in men and the second most common cause of male cancer deaths (40
). The recent discovery that senescence can drive tumor clearance trough the stimulation of a peritumoral immune response gives credence to the therapeutic application of pro-senescence compounds that can target senescence pathway for cancer therapy (16
). Conventional chemotherapy and radiotherapy has been previously shown to induce cellular senescence in tumors samples of cancer patients but at the cost of cytotoxic effects for normal and cancer cells (41
). Thus, it remained to be established whether the targeting of specific genes involved in senescence pathway (e.g., p53 or PTEN) could be used to induce senescence in vivo and whether this approach would efficiently oppose tumorigenesis. In this sense, targeted therapy oriented to modulate pro-senescence mediators could avoid the unselective genotoxicity delivered by conventional chemotherapeutics, thereby representing an alternative therapeutic strategy. Moreover, it was tempting to hypothesize that the genetic defects harbored by cancer cells (e.g., Pten
heterozygous or homozygous loss) would render them differentially vulnerable to pro-senescence mediators. While our previous data regarding PICS clearly established an important role for the senescence response in opposing prostate tumorigenesis, it did not address the molecular and mechanistic details of the process. The new knowledge obtained in this study regarding the molecular and mechanistic requirements for PICS in turn offered important therapeutic opportunities that we could test both in vivo and in vitro.
Taken together, our findings lead to a number of relevant conclusions. First, we have characterized PICS as a novel form of cellular senescence, that unlike OIS does not trigger hyperproliferation and a classic DDR. These findings integrate the current notion whereby cellular senescence in response to some oncogenic stimuli would invariably represent, a checkpoint triggered by hyperproliferation, aberrant DNA replication, and DNA damage. These findings have therapeutic implications, since they suggest that compounds that enhance PICS by targeting PTEN or one of its downstream effectors will drive a beneficial senescence response without inducing DNA damage.
Second, we show that PICS can also be triggered in arrested cells and hence, in principle, be used to target for therapy even nonproliferating cells, including the quiescent cancer-initiating cell (24
). While the therapeutic enhancement of OIS would by definition spare the quiescent cell, in turn promoting the accumulation of arrested, albeit, damaged cells, therapeutic enhancement of PICS can be used to trigger the irreversible arrest of both proliferating and nonproliferating cells in the absence of DNA damage. Importantly, this also suggests that therapeutic delivery of DNA-damaging agents to cells undergoing PICS in the form, for instance, of ionizing radiation, could be used to further potentiate PICS or to morph this fail-safe response into full-blown apoptosis.
Third, we have further shown that a transient inhibition of PTEN can drive senescence in vivo in tumors with decreased PTEN protein levels. Given that current efforts in the fight against cancer aim at inhibiting PI3K-mTOR signalling, it is provocative to think that such PTEN inhibitors may represent a powerful class of molecules for pro-senescence therapy in PTEN
heterozygous tumors and cells, keeping in mind that PTEN
heterozygosity is commonly observed in human cancer (12
Fourth, we have found that compounds that block mTOR can blunt senescence in tumors with activated senescence programs. Our unpublished observations also suggest that rapamycin and its analogues can antagonize the p53 response, upon treatment with ionizing radiation and chemotherapeutic agents. Thus, in addition to stabilization of p53, there can be a significant translational component to p53 upregulation in these conditions as well. Treatment with rapamycin specifically abrogates this upregulation and in doing so can blunt the p53 response so critical for opposing tumor progression. These undesired effects may limit the efficacy of rapamycin as a single agent in some instances.
Fifth, we have shown that the Ptenpc–/– mouse model provides a unique setting to study the pro-senescence efficacy of novel and previously identify compounds alone or in combination. This in vivo approach has also allowed us to analyze the biological and genetic determinants of drug action in a manner that would be impossible using cell lines or in patients. Thus, this in vivo model has great potential for evaluating drug action and predicting treatment responses to pro-senescence therapeutic modalities.
Sixth, the elucidation of the molecular requirements underlying PICS has allowed us to identify a number of critical entry points for the rationale development of a pro-senescence therapy for cancer chemoprevention and treatment (Figure I). This will allow for an effective evaluation in the clinic of drugs that modulate PICS-inducing and PICS-suppressive pathways and for the development and fine tuning of protocols to improve efficacy and response to current treatments. This is exemplified by the case in hand, whereby inhibitors of p53 degradation, like Nutlin-3, can enhance senescence and overcome the negative effect of rapamycin and convert a rapamycin-mediated transitory growth arrest into terminal senescence. This approach could also prove very beneficial, as mTOR inhibition along with senescence induction could blunt at the same time the HIF-1α–mediated pro-angiogenic response, so critical for tumor progression (31