Differential effects of gonadotropin-releasing hormone (GnRH)-I and GnRH-II on prostate cancer cell signaling and death

Context: GnRH is known to directly regulate prostate cancer cell proliferation, but the precise mechanism of action of the peptide is still under investigation. Objective: This study demonstrates differential effects of GnRH-I and GnRH-II on androgen-independent human prostate cancer cells. Results: Both GnRH-I and GnRH-II increased the intracellular Ca²⁺ concentration ([Ca²⁺]_i) either through Ca²⁺ influx from external Ca²⁺ source or via mobilization of Ca ²⁺ from internal Ca²⁺ stores. Interestingly, the [Ca ²⁺]_i increase was mediated by activation of the ryanodine receptor but not the inositol trisphosphate receptor. Trptorelix-1, a novel GnRH-II antagonist but not cetrorelix, a classical GnRH-I antagonist, completely inhibited the GnRH-II-induced [Ca²⁺]_i increase. Concurrently at high concentrations, trptorelix-1 and cetrorelix inhibited GnRH-I-induced [Ca²⁺]_i increase, whereas at low concentrations they exerted an agonistic action, inducing Ca²⁺ influx. High concentrations of trptorelix-1 but not cetrorelix-induced prostate cancer cell death, probably through an apoptotic process. Using photoaffinity labeling with ¹²⁵I-[azidobenzoyl-D-Lys⁶]GnRH-II, we observed that an 80-kDa protein specifically bound to GnRH-II. Conclusions: This study suggests the existence of a novel GnRH-II binding protein, in addition to a conventional GnRH-I receptor, in prostate cancer cells. These data may facilitate the development of innovatory therapeutic drugs for the treatment of prostate cancer.