28 March 2007
Gonadotropin-releasing hormone, Prostatic cancer, Calcium-dependent tyrosine kinase, Pyk2, Non-receptor tyrosine kinase, c-Src, Hydrogen peroxide-inducible clone 5 protein, Hic-5, Testosterone, Androgen receptor
Gonadotropin-releasing hormone (GnRH) analogs constitute the most widely employed medical treatment for prostatic cancer. The predominant mechanism of action is presumed to be via the inhibition of gonadotropins and resultant decrease in androgen. However, GnRH analogs have also been shown to directly inhibit prostate cancer cells both in vitro and in vivo through antiproliferative cell cycle arrest and stimulation of apoptosis. Since the GnRH receptor has been shown to affect sex steroid hormone receptor function, we considered that part of GnRH analog actions on prostate cells may be mediated through modulation of the human androgen receptor. Using a model HEK293 cell line expressing the GnRH receptor, we demonstrated a novel signalling pathway of the GnRH receptor that induces nuclear translocation of the androgen receptor that renders it transcriptionally inactive. This mechanism involves the calcium-dependent tyrosine kinase Pyk2, the non-receptor tyrosine kinase c-Src and the focal adhesion protein/steroid receptor co-factor, Hic-5. In this setting there is a GnRH-induced association and nuclear translocation of the androgen receptor with Hic-5. GnRH-induced Pyk2 activation opposed the association of Hic-5 with androgen receptor as overexpression of a dominant negative Pyk2 enhanced the GnRH-induced nuclear translocation of a green fluorescent protein-tagged human androgen receptor. GnRH-induced c-Src activation resulted in the phosphorylation of expressed Hic-5 and promoted its association with the human androgen receptor. In contrast to testosterone, GnRH-induced nuclear translocation did not transcriptionally activate the androgen receptor. We then demonstrated that GnRH can also stimulate androgen receptor mobilization in human prostate PC3, BPH-1 and LNCaP cells, and in cultured rat ventral prostate cells through the same mechanism. To determine if GnRH could antagonize androgen effects in normal tissue, we examined the effect of GnRH on rat ventral prostate organ cultures and demonstrated that GnRH can functionally antagonize the actions of testosterone on prostate cell proliferation and tissue growth. This antagonism of testosterone action by GnRH may underlie in part the capacity of GnRH receptor activation to inhibit prostate tumor growth.