Expression und Wirkungsmechanismen von Gonadotropin-Releasing Hormon Typ II (GnRH-II) und seines Rezeptors in humanen Ovarial- und Endometriumkarzinomen

Abstract

The majority of human endometrial- (>80%) and ovarian (>80%) cancer cell lines express gonadotropin releasing hormone (GnRH-I) and its receptor as a part of a negative autocrine/paracrine mechanism of cell proliferation. The classical GnRH-receptor signaltransduction mechanisms of the pituitary are not involved in the mediation of antiproliferative effects of GnRH analogs in cancer cells. The GnRH-receptor instead interacts with the mitogenic signaltransduction of growth factor receptors. The mediation of the signaltransduction in human ovarian and endometrial cancers works through coupling to G protein αi. This binding activates a phosphotyrosine phosphatase (PTP) counteracting the EGF-induced tyrosine phosphorylation of EGF-receptor. As a result of, cell proliferation is reduced.Recently, the expression of a second GnRH (GnRH-II, chicken GnRH) was detected in mammals. The antiproliferative effects in human endometrial and ovarian cancer cell lines are significantly greater then those of the GnRH-I agonist. It was shown that these antiproliferative effects of GnRH-II are not regulated by the GnRH-I receptor. The GnRH-II receptor system is well known. In 2001, Neill et al. and Miller et al. cloned and characterised the GnRH-II receptor in mammals and primates. By using the reverse transcriptase (RT) -PCR technique, the expression of the receptor could also detect in human ovarian and endometrial cancers. A functional receptor for human GnRH-II has not yet been identified because of a stop codon in the second exon.This study assessed, that GnRH-II agonist [D-Lys6]GnRH-II reduces the EGF-induced autotyrosine phosphorylation of EGF-receptor through the activation of a PTP. The activation of mitogen activated proteinkinase (MAPK) is also decreased in this context. As a result of, the EGF-induced c-fos expression is reduced and that is the reason for the reduction of cell proliferation. It can assume that the signaltransduction of GnRH-II and its agonist [D-Lys6]GnRH-II is comparable to that of GnRH-I analogs.To identify the GnRH-II receptor protein it was generated a polyclonal antiserum to the putative human GnRH-II receptor using a peptide (YSPTMLTEVPPC) corresponding to the highly conserved third extra cellular domain of the Callithrix jacchus. Data base search showed no similar peptide sequences in any other human gene. The expression of GnRH-II receptor protein could also shown in immunhistological sections and western blot analysis of human placenta and human gynecological tumors. To identify the GnRH-II receptor-like antigene we used the photo-affinity-labeling technique. In summary, these investigations give the first clear evidence for the existence of a GnRH-II receptor-like protein with GnRH-II binding affinity in human placenta and in cancers of human reproductive organs. To complete the identification and characterisation of the receptor the sequencing of the protein and/or gene has to follow.