Therapy-induced stromal senescence promotes prostate cancer progression and aggressiveness

Although chemotherapy still represents the gold-standard treatment option for cancer cure, it often results in several long-term negative side effects, including cancer relapse and acquired resistance to the therapy. Docetaxel is the preferred anticancer drug for several tumor types, including metastatic castrate-resistant prostate cancer. However, it exhibits only modest efficacy and many patients who received Docetaxel treatment experience tumor progression and chemotherapy insensitivity. Therefore, identifying the molecular mechanisms that associate anticancer therapy with its negative bystander effects is an urgent need in order to develop novel strategies targeting chemotherapy vulnerabilities. Interestingly, a possible link is provided by therapy-induced cellular senescence. Indeed, a recent body of evidence highlights that several antineoplastic agents promote the senescent phenotype in both stromal and tumor cells. Despite it has been originally identified as a tumor-suppressive mechanism, many findings underline that cellular senescence sustains tumor growth and dissemination. The tumor-promoting effects of cellular senescence are mainly mediated by secreted factors, that exert profound paracrine effects through the generation of a pro-inflammatory and immunosuppressive microenvironment. For instance, it is well known that cancer progression and aggressiveness are strongly supported by the bi-directional crosstalk between cancer cells and the surrounding tumor microenvironment, that favors cancer cell migration and invasion, neo-angiogenesis and homing of distant organs. In this scenario, stromal cells exert a promoting role in cancer progression providing tumor with energy sources, growth factors and cytokines. In this study, we found that Docetaxel treatment strongly promotes the senescent phenotype in stromal prostate fibroblasts, as revealed by a sharp increase in numerous senescence markers, including SA-β-Galactosidase activity, γ-H2AX nuclear foci, and p53 expression. We also provided evidence that Docetaxel treatment induces the senescence-associated secretory phenotype (SASP) in prostate stromal fibroblasts, by increasing the levels of pro-inflammatory cytokines (IL-6, IL-8), growth factors (VEGF-A) and matrix-metalloproteinases (MMP-2 and MMP-3). The clinical relevance of the senescence-inducing effects of Docetaxel in the stromal compartment of prostate cancer was determined analyzing the accumulation of lipofuscin aggregates in tissue specimens from 20 patients with prostate cancer, 10 of whom received neo-adjuvant Docetaxel chemotherapy before radical prostatectomy. Remarkably, we found that lipofuscin staining is significantly higher in patients treated with Docetaxel therapy and preferentially accumulates in the stromal compartment of prostate cancer tissues. Besides, this study underlined that Docetaxel-induced senescent stromal cells exhibit a strong mitochondrial dysfunction, characterized by increased mitochondrial mass and oxidative stress, reduced mitochondrial membrane potential, and morphological changes of the mitochondrial structure. Moreover, we observed that the anticancer drug promotes metabolic alterations in senescent prostate fibroblasts, including increased extracellular acidification rate and lactate secretion, suggesting that senescent cells may shift towards a more glycolytic metabolism to meet their energetic demands, as a consequence of impaired mitochondria. We then investigated the role of the paracrine factors and metabolites secreted by senescent fibroblasts on prostate cancer aggressiveness by incubating PC3 tumor cells with conditioned media from control or senescent fibroblasts. We highlighted that therapy-induced stromal senescence supports the increase in the invasive and migratory abilities, and clonogenic and stemness potential of prostate cancer cells. Interestingly, these effects are directly correlated to stromal senescence. Indeed, clearance of senescent fibroblasts through administration of ABT263, a senolytic drug, reverts the malignant phenotype of prostate cancer cells. The results obtained in this study highlight that the long-term adverse effects of Docetaxel therapy could be correlated to its ability to induce the senescent phenotype in the stromal compartment, thus generating a supportive tumor microenvironment, that further promotes prostate cancer progression and aggressiveness. In addition, this study shed new light on the use of senolytic drugs to improve Docetaxel efficacy and overcome its detrimental bystander effects in prostate cancer.