Treatment with luteinizing-hormone-releasing hormone antagonist sb-75 decreases levels of epidermal growth-factor receptor and its messenger-RNA in ov-1063 human epithelial ovarian-cancer xenografts in nude-mice

Role of Gonadotropin-Releasing Hormone (GnRH) in Ovarian Cancer

The hypothalamus–pituitary–gonadal (HPG) axis is the endocrine regulation system that controls the woman’s cycle. The gonadotropin-releasing hormone (GnRH) plays the central role. In addition to the gonadotrophic cells of the pituitary, GnRH receptors are expressed in other reproductive organs, such as the ovary and in tumors originating from the ovary. In ovarian cancer, GnRH is involved in the regulation of proliferation and metastasis. The effects on ovarian tumors can be indirect or direct. GnRH acts indirectly via the HPG axis and directly via GnRH receptors on the surface of ovarian cancer cells. In this systematic review, we will give an overview of the role of GnRH in ovarian cancer development, progression and therapy.

The Role of Gonadotropin-Releasing Hormone in Cancer Cell Proliferation and Metastasis

In several human malignant tumors of the urogenital tract, including cancers of the endometrium, ovary, urinary bladder, and prostate, it has been possible to identify expression of gonadotropin-releasing hormone (GnRH) and its receptor as part of an autocrine system, which regulates cell proliferation. The expression of GnRH receptor has also been identified in breast cancers and non-reproductive cancers such as pancreatic cancers and glioblastoma. Various investigators have observed dose- and time-dependent growth inhibitory effects of GnRH agonists in cell lines derived from these cancers. GnRH antagonists have also shown marked growth inhibitory effects on most cancer cell lines. This indicates that in the GnRH system in cancer cells, there may not be a dichotomy between GnRH agonists and antagonists. The well-known signaling mechanisms of the GnRH receptor, which are present in pituitary gonadotrophs, are not involved in forwarding the antiproliferative effects of GnRH analogs in cancer cells. Instead, the GnRH receptor activates a phosphotyrosine phosphatase (PTP) and counteracts with the mitogenic signal transduction of growth factor receptors, which results in a reduction of cancer cell proliferation. The PTP activation, which is induced by GnRH, also inhibits G-protein-coupled estrogen receptor 1 (GPER), which is a membrane-bound receptor for estrogens. GPER plays an important role in breast cancers, which do not express the estrogen receptor α (ERα). In metastatic breast, ovarian, and endometrial cancer cells, GnRH reduces cell invasion in vitro, metastasis in vivo, and the increased expression of S100A4 and CYR61. All of these factors play important roles in epithelial-mesenchymal transition. This review will summarize the present state of knowledge about the GnRH receptor and its signaling in human cancers.

Role of gonadotropin-releasing hormone (GnRH) in ovarian cancer

The expression of GnRH (GnRH-I, LHRH) and its receptor as a part of an autocrine regulatory system of cell proliferation has been demonstrated in a number of human malignant tumors, including cancers of the ovary. The proliferation of human ovarian cancer cell lines is time- and dose-dependently reduced by GnRH and its superagonistic analogs. The classical GnRH receptor signal-transduction mechanisms, known to operate in the pituitary, are not involved in the mediation of antiproliferative effects of GnRH analogs in these cancer cells. The GnRH receptor rather interacts with the mitogenic signal transduction of growth-factor receptors and related oncogene products associated with tyrosine kinase activity via activation of a phosphotyrosine phosphatase resulting in downregulation of cancer cell proliferation. In addition GnRH activates nucleus factor kappaB (NFkappaB) and protects the cancer cells from apoptosis. Furthermore GnRH induces activation of the c-Jun N-terminal kinase/activator protein-1 (JNK/AP-1) pathway independent of the known AP-1 activators, protein kinase (PKC) or mitogen activated protein kinase (MAPK/ERK). Recently it was shown that human ovarian cancer cells express a putative second GnRH receptor specific for GnRH type II (GnRH-II). The proliferation of these cells is dose- and time-dependently reduced by GnRH-II in a greater extent than by GnRH-I (GnRH, LHRH) superagonists. In previous studies we have demonstrated that in ovarian cancer cell lines except for the EFO-27 cell line GnRH-I antagonist Cetrorelix has comparable antiproliferative effects as GnRH-I agonists indicating that the dichotomy of GnRH-I agonists and antagonists might not apply to the GnRH-I system in cancer cells. After GnRH-I receptor knock down the antiproliferative effects of GnRH-I agonist Triptorelin were abrogated while the effects of GnRH-I antagonist Cetrorelix and GnRH-II were still existing. In addition, in the ovarian cancer cell line EFO-27 GnRH-I receptor but not putative GnRH-II receptor expression was found. These data suggest that in ovarian cancer cells the antiproliferative effects of GnRH-I antagonist Cetrorelix and GnRH-II are not mediated through the GnRH-I receptor.