Role of osteoblasts in castration resistant prostate cancer (CRPC) progression

Prostate cancer has the propensity to metastasize to the bone forming mainly osteoblastic lesions. Although androgen ablation remains the standard of care for advanced metastatic disease an eventual tumour progression occurs and androgen-independent disease develops. Prostate cancer can be considered a microenvironment-driven disease, indeed bone and tumour cells interaction is crucial for tumour progression and bone metastasis onset. Since this complex interplay is currently not completely elucidated, this research project was focused on osteoblast (OB) role in the progression of castration resistant prostate cancer (CRPC). We found that in direct co-cultures, OBs strongly inhibited AR receptor of CRPC cell line (C4-2B cells). Since AR activation is the driving signal for prostate cancer cell growth, we analysed the impact of AR inhibition on tumour cell proliferation. Contrary to expectations, C4-2B cells proliferated significantly more when cultured with OBs, supporting the hypothesis that OBs stimulate an androgen-independent cancer growth. Moreover, we demonstrated that OB effects were mediated by the release of osteoblastic soluble factors, since AR inhibition persists when C4-2B cells were treated with osteoblastic conditional media (OCM). Our results showed also that OB-mediated AR inhibition is only partially restored adding an exogenous androgen, demonstrating that its repression could be in part androgen independent. In addition, the effect of Enzalutamide, an AR inhibitor was significantly lower in C4-2B treated with OCM. The lower effect of AR agonist (R1881) and inhibitor (Enzalutamide) could be a consequence of the reduced expression of AR mRNA and protein levels found in C4-2B cells after OCM treatment. Once again, together with the repression of the receptor, we observed that OBs promoted tumour cell growth also in indirect co-culture. In order to identify the molecules involved in this process, we focused on IL-6, IL-8, WNT16, DKK-1 and CXCL12 expression, finding that OBs presented higher mRNA levels of all these factors compared with C4-2B cells. According to literature data, IL-8 and IL-6 represent the best candidates to mediate OB effects, since their protein levels were highly express in OCM. However, further analyses are necessary to fully elucidate their role in the androgen-independent cancer cell growth. The identification of the targets responsible of OBs pro-tumour effects could represent a strong rationale for the development of new target therapies that could support the existing strategies of CRPC treatment.