Gonadotropin-Releasing Hormone (GnRH) Antagonists Promote Proapoptotic Signaling in Peripheral Reproductive Tumor Cells by Activating a Gαi-Coupling State of the Type I GnRH Receptor

Abstract

Gonadotropin-releasing hormone (GnRH) receptor agonists are extensively used in the treatment of sex hormone-dependent cancers via the desensitization of pituitary gonadotropes and consequent decrease in steroid sex hormone secretion. However, evidence now points to a direct inhibitory effect of GnRH analogs on cancer cells. These effects appear to be mediated via the G␣ i -type G protein, in contrast to the predominant G␣ q coupling in gonadotropes. Unlike G␣ q coupling, G␣ i coupling of the GnRH receptor can be activated by both agonists and antagonists. This unusual pharmacology suggested that the receptor involved in the cancer cells may not be the classical gonadotrope type I GnRH receptor. However, we have previously shown that a functional type II GnRH receptor is not present in man. In the present study, we show that GnRH agonists and selective GnRH antagonists exert potent antiproliferative effects on JEG-3 choriocarcinoma, benign prostate hyperplasia (BPH-1), and HEK293 cells stably expressing the type I GnRH receptor. This antiproliferative action occurs through a G␣ i -mediated activation of stress-activated protein kinase pathways, resulting in caspase activation and transmembrane transfer of phosphatidlyserine to the outer membrane envelope. Structurally related antagonistic GnRH analogs displayed divergent antiproliferative efficacies but demonstrated equal efficacies in inhibiting GnRH-induced G␣ q -based signaling. Therefore the ability of GnRH receptor antagonists to exert an antiproliferative effect on reproductive tumors may be dependent on ligand-selective activation of the G␣ i -coupled form of the type I GnRH receptor.

Diversity of actions of GnRHs mediated by ligand-induced selective signaling

“…For both of these the pharmacology of the effects of GnRH agonists and antagonists is distinctly different from their effects on pituitary gonadotropin secretion. The third arises from this phenomenon and encompasses the concept of ligand-induced selectivesignaling (LiSS) by GnRH analogs which we developed to explain the differing pharmacology in different cellular environments [81,89,95]. In support of this we present evidence of the existence of different conformations of the GnRH receptor which give rise to selective binding of GnRH analogs and differential intracellular signaling.…”

“…Antiproliferative effects of GnRH analogs and the expression of GnRH receptors have now been demonstrated in a number of cell lines of reproductive tract tumors, including prostate, uterine and ovarian cancers, and also in nonreproductive tract tumors [21,43,47,70,73,79]. In contrast to GnRH actions at the gonadotrope, which are mediated through the G αq protein, these antiproliferative and apoptotic effects on tumor cells are thought to be mediated via the G αi protein [42,54,81], focal adhesion complexes involving c-Src, and the JNK and P38 stress-activated kinases [42,54,70,73,81]. Other mechanisms which have been implicated in the antiproliferative effects, include the downregulation of growth factor actions by decreased expression of growth factors and their receptors and activation of phosphotyrosine phosphatase [42], and the inhibition of Akt and the 60S acidic ribosomal phosphoproteins which modulate cell survival and protein synthesis [18,62,63,69,74,136].…”

“…Other mechanisms which have been implicated in the antiproliferative effects, include the downregulation of growth factor actions by decreased expression of growth factors and their receptors and activation of phosphotyrosine phosphatase [42], and the inhibition of Akt and the 60S acidic ribosomal phosphoproteins which modulate cell survival and protein synthesis [18,62,63,69,74,136]. We have demonstrated that the effects of a series of GnRH analogs on antiproliferation and apoptosis in cancer cell lines is very different from their effects on gonadotropin secretion [81]. We have dubbed the phenomenon "ligand-induced selectivesignaling" (LiSS) [81,89,95] (Fig.…”

“…We have demonstrated that the effects of a series of GnRH analogs on antiproliferation and apoptosis in cancer cell lines is very different from their effects on gonadotropin secretion [81]. We have dubbed the phenomenon "ligand-induced selectivesignaling" (LiSS) [81,89,95] (Fig. 14).…”

“…15). A number of antagonists of gonadotrope GnRH receptors act as GnRH agonists in the inhibition of tumor cell lines [32,34,44,81,128].…”