Progression to androgen independence is the lethal end stage of prostate cancer where the tumors are no longer dependent on circulating androgens for growth regulation. However, it has been shown that the androgen receptor (AR) still plays a central role in regulating downstream androgen-responsive genes in these cancer cells. One of the mechanisms that contribute to the continual activation of the AR is the effect of non-steroidal stimulating agents such as cytokines, growth factors and PKA activators exerting their effects on AR in the absence of ligands. We used a short hairpin RNA (shRNA) directed towards the AR to examine if AR is required for ligand-independent activation of androgen-regulated genes. LNCaP human prostate cancer cells were transiently transfected with either the AR shRNA or a scrambled control together with the PSA-luciferase reporter gene and then treated with forskolin (FSK), interleukin-6 (IL-6) and epidermal growth factor (EGF) as stimulatory agents. While AR knockdown by shRNA efficiently abrogated PSA reporter activation by the androgen R1881 (96%), its effect on FSK and IL-6 activation was less complete (75%). Moreover, the antiandrogen bicalutamide could not further reduce the residual activity to background level, suggesting that there exists an alternative pathway that is AR-independent. A LNCaP cell line, stably expressing AR shRNA that was inducible by doxycycline (DOX), was grown as a xenograft in immunocompromised mice. Induction of AR shRNA alone or in combination with castration resulted in a ∼3-fold retardation of tumor growth and >10-fold decrease in serum PSA. Furthermore, induction of AR shRNA prevented progression to androgen independence over a 40-day treatment period. These results suggest that knockdown of AR may provide additional therapeutic benefit in delaying or preventing progression of prostate cancers to androgen independence. (Supported by grants from the Terry Fox Foundation and Health Canada).
[Proc Amer Assoc Cancer Res, Volume 47, 2006]