Cellular senescence has been theorized to oppose neoplastic transformation triggered by activation of oncogenic pathways in vitro 1-3 , but the relevance of senescence in vivo has not been established. The PTEN and p53 tumour suppressors are among the most commonly inactivated or mutated genes in human cancer including prostate cancer 4,5 . Although they are functionally distinct, reciprocal cooperation has been proposed, as PTEN is thought to regulate p53 stability, and p53 to enhance PTEN transcription 6-10 . Here we show that conditional inactivation of Trp53 in the mouse prostate fails to produce a tumour phenotype, whereas complete Pten inactivation in the prostate triggers nonlethal invasive prostate cancer after long latency. Strikingly, combined inactivation of Pten and Trp53 elicits invasive prostate cancer as early as 2 weeks after puberty and is invariably lethal by 7 months of age. Importantly, acute Pten inactivation induces growth arrest through the p53-dependent cellular senescence pathway both in vitro and in vivo, which can be fully rescued by combined loss of Trp53. Furthermore, we detected evidence of cellular senescence in specimens from early-stage human prostate cancer. Our results demonstrate the relevance of cellular senescence in restricting tumorigenesis in vivo and support a model for cooperative tumour suppression in which p53 is an essential failsafe protein of Pten-deficient tumours.'Cellular senescence' describes a permanent form of cell cycle arrest in primary cultured cells, which can be triggered by DNA damage or activated oncogenes 1-3 . Although it has been implicated in mediating the response to anti-tumour treatments 11 , there is still no evidence that senescence opposes tumorigenesis.Up to 70% of primary prostate tumours lose one PTEN allele and retain the other copy 12-15 . Similarly, p53 is found completely lost or mutated almost exclusively in advanced prostate cancer 16,17 . Because complete loss of Pten in the mouse seems to be crucial for the development of invasive prostate tumours 18,19 , why human invasive prostate cancer wouldCorrespondence and requests for materials should be addressed to P.P.P. (p-pandolfi@ski.mskcc.org). Author Information Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.Supplementary Information is linked to the online version of the paper at www.nature.com/nature. (Supplementary Fig. S1). As expected, in the presence of Pb-Cre4, recombination of Pten and Trp53 was restricted to the three prostatic lobes, namely the anterior prostate (AP), ventral prostate (VP) and dorsolateral prostate (DLP), with minor recombination occurring in seminal vesicles ( Supplementary Fig. S2a). NIH Public AccessTo study early effects of Pten and/or Trp53 inactivation in the prostate, mice were killed at 9 weeks of age and histopathological analysis was performed. Wild-type (WT) mice displayed normal prostate histology, whereas age-matched Pten pc−/− littermate...

Summary Prostate cancer is characterized by its dependence on androgen receptor and frequent activation of PI3K signaling. We find that AR transcriptional output is decreased in human and murine tumors with PTEN deletion and that PI3K pathway inhibition activates AR signaling by relieving feedback inhibition of HER kinases. Similarly, AR inhibition activates AKT signaling by reducing levels of the AKT phosphatase PHLPP. Thus, these two oncogenic pathways cross-regulate each other by reciprocal feedback. Inhibition of one activates the other, thereby maintaining tumor cell survival. However, combined pharmacologic inhibition of PI3K and AR signaling caused near complete prostate cancer regressions in a Pten-deficient murine prostate cancer model and in human prostate cancer xenografts, indicating that both pathways coordinately support survival. Significance The two most frequently activated signaling pathways in prostate cancer are driven by AR and PI3K. Inhibitors of the PI3K pathway are in early clinical trials and AR inhibitors confer clinical responses in most patients. However, these inhibitors rarely induce tumor regression in preclinical models. Here we show that these pathways regulate each other by reciprocal negative feedback, such that inhibition of one activates the other. Therefore, tumor cells can adapt and survive when either single pathway is inhibited pharmacologically. Our demonstration of profound tumor regressions with combined pathway inhibition in preclinical prostate tumor models provides rationale for combination therapy in patients.

Complete inactivation of the PTEN tumor suppressor gene is extremely common in advanced cancer, including prostate cancer (CaP). However, one PTEN allele is already lost in the vast majority of CaPs at presentation. To determine the consequence of PTEN dose variations on cancer progression, we have generated by homologous recombination a hypomorphic Pten mouse mutant series with decreasing Pten activity: Ptenhy/+ > Pten+/− > Ptenhy/− (mutants in which we have rescued the embryonic lethality due to complete Pten inactivation) > Pten prostate conditional knockout (Ptenpc) mutants. In addition, we have generated and comparatively analyzed two distinct Ptenpc mutants in which Pten is inactivated focally or throughout the entire prostatic epithelium. We find that the extent of Pten inactivation dictate in an exquisite dose-dependent fashion CaP progression, its incidence, latency, and biology. The dose of Pten affects key downstream targets such as Akt, p27Kip1, mTOR, and FOXO3. Our results provide conclusive genetic support for the notion that PTEN is haploinsufficient in tumor suppression and that its dose is a key determinant in cancer progression.

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