Senescence-associated exosome release from human prostate cancer cells.

Males of advanced age represent a rapidly growing population at risk for prostate cancer. In the contemporary setting of earlier detection, a majority of prostate carcinomas are still clinically localized and often treated using radiation therapy. Our recent studies have shown that premature cellular senescence, rather than apoptosis, accounts for most of the clonogenic death induced by clinically relevant doses of irradiation in prostate cancer cells. We show here that this treatment-induced senescence was associated with a significantly increased release of exosome-like microvesicles. In premature senescence, this novel secretory phenotype was dependent on the activation of p53. In addition, the release of exosome-like microvesicles also increased during proliferative senescence in normal human diploid fibroblasts. These data support the hypothesis that senescence, initiated either by telomere attrition (e.g., aging) or DNA damage (e.g., radiotherapy), may induce a p53-dependent increase in the biogenesis of exosome-like vesicles. Ultrastructural analysis and RNA interference-mediated knockdown of Tsg101 provided significant evidence that the additional exosomes released by prematurely senescent prostate cancer cells were principally derived from multivesicular endosomes. Moreover, these exosomes were enriched in B7-H3 protein, a recently identified diagnostic marker for prostate cancer, and an abundance of what has recently been termed "exosomal shuttle RNA." Our findings are consistent with the proposal that exosomes can transfer cargos, with both immunoregulatory potential and genetic information, between cells through a novel mechanism that may be recruited to increase exosome release during accelerated and replicative cellular senescence.

Distinct roles for p107 and p130 in Rb-independent cellular senescence.

Telomere attrition, DNA damage and constitutive mitogenic signaling can all trigger cellular senescence in normal cells and serve as a defense against tumor progression. Cancer cells may circumvent this cellular defense by acquiring genetic mutations in checkpoint proteins responsible for regulating permanent cell cycle arrest. A small family of tumor suppressor genes encoding the retinoblastoma susceptibility protein family (Rb, p107, p130) exerts a partially redundant control of entry into S phase of DNA replication and cellular proliferation. Here we report that activation of the p53-dependent DNA damage response has been found to accelerate senescence in human prostate cancer cells lacking a functional Rb protein. This novel form of irradiation-induced premature cellular senescence reinforces the notion that other Rb family members may compensate for loss of Rb protein in the DNA damage response pathway. Consistent with this hypothesis, depletion of p107 potently inhibits the irradiation-induced senescence observed in DU145 cells. In contrast, p130 depletion triggers a robust and unexpected form of premature senescence in unirradiated cells. The dominant effect of depleting both p107 and p130, in the absence of Rb, was a complete blockade of irradiation-induced cellular senescence. Onset of the p107-dependent senescence was temporally associated with p53-mediated stabilization of the cyclin-dependent kinase inhibitor p27 and decreases in c-myc and cks1 expression. These results indicate that p107 is required for initiation of accelerated cellular senescence in the absence of Rb and introduces the concept that p130 may be required to prevent the onset of terminal growth arrest in unstimulated prostate cancer cells lacking a functional Rb allele.