The mammalian type II gonadotropin-releasing hormone receptor : cloning, distribution and role in gonadotropin gene expression

[摘要] ENGLISH ABSTRACT: Gonadotropin-releasing hormone (GnRH) is well known as the central regulator of thereproductive system through its stimulation of gonadotropin synthesis and release fromthe pituitary via binding to its specific receptor, known as the gonadotropin-releasinghormone receptor type I (GnRHR-I). The gonadotropins, luteinising hormone (LH) andfollicle-stimulating hormone (FSH), bind to receptors in the gonads, leading to effects onsteroidogenesis and gametogenesis. The recent finding of a second form of the GnRHreceptor, known as the type II GnRHR or GnRHR-II, in non-mammalian vertebratestriggered the interest into the possible existence and function of a GnRHR-II in humans.The current study addressed this issue by investigating the presence of transcripts for aGnRHR-II in various human tissues and cells. While it was demonstrated that antisensetranscripts for this receptor, containing sequence of only two of the three coding exons,are ubiquitously and abundantly expressed in all tissues examined, potentially full-length(containing all three exons), sense transcripts for a GnRHR-II were detected only inhuman ejaculate. Further analysis revealed that the subset of cells in the ejaculateexpressing these transcripts is mature sperm. These findings, together with the reportedrole for GnRH in spermatogenesis and reproduction led to the further analysis of thepresence of a local GnRH/GnRHR network in human and vervet monkey ejaculate orsperm. Indeed, such a network seems to be present in humans since transcripts forboth forms of GnRH present in mammals, as well as transcripts for the GnRHR-I, areexpressed in human ejaculate. Furthermore, transcripts for the GnRHR-II are expressedin both human and vervet monkey ejaculate. Thus, it would appear that locally producedGnRH-1 and/or GnRH-2 in the human male reproductive tract might mediate their effectson fertility via a local GnRHR-I, and possibly via GnRHR-II.Remarkably, in the pituitary, LH and FSH are present in the same gonadotropes, yetthey are differentially regulated by GnRH under various physiological conditions. Whileit is well established that post-transcriptional regulatory mechanisms occur, thecontribution of transcriptional regulation to the differential expression of the LHβ- andFSHβ-subunit genes is unclear. In this study, the role of GnRH-1 and GnRH-2 via theGnRHR-I and the GnRHR-II in transcriptional regulation of mammalian LHβ- and FSHβgenes was determined in the LβT2 mouse pituitary gonadotrope cell-line. It isdemonstrated for the first time that GnRH-1 may affect gonadotropin subunit gene expression via GnRHR-II in addition to GnRHR-I, and that GnRH-2 also has the ability toregulate gonadotropin subunit gene expression via both receptors. Similar to otherreports, it is shown that the transcriptional response to GnRH-1 of LHβ and FSHβ is low(about 1.4-fold for bLHβLuc and 1.2-fold for oFSHβLuc). In addition, evidence issupplied for the first time that GnRH-2 transcriptional regulation of the gonadotropin βsubunits is also low (about 1.5-fold for bLHβLuc and 1.1-fold for oFSHβLuc). It isdemonstrated that GnRH-1 is a more potent stimulator of bLHβ promoter activity ascompared to GnRH-2 via the GnRHR-I, yet both hormones result in a similar maximuminduction of bLHβ. However, GnRH-2 is a more efficacious stimulator of bLHβtranscription via the GnRHR-II than GnRH-1. No discriminatory effect of GnRH-1 vs.GnRH-2 was observed for oFSHβ promoter activity via GnRHR-I or GnRHR-II. Bycomparison of the ratio of expression of transfected oFSHβ- and bLHβ promoterreportersvia GnRH-1 with that of GnRH-2, it is shown that GnRH-2 is a selectiveregulator of FSHβ gene transcription. This discriminatory effect of GnRH-2 is specific forGnRHR-I, as it is not observed for GnRHR-II, where GnRH-1 results in a greater oFSHβ-to-bLHβ ratio. These opposite selectivities for GnRHR-I and GnRHR-II on the ratios ofoFSHβ:bLHβ promoter activity for GnRH-1 vs. GnRH-2 suggest a mechanism for finecontrol of gonadotropin regulation in the pituitary by variation of relative GnRHR-I vs.GnRHR-II levels. In addition, a concentration-dependent modulatory role for PACAP onGnRH-1- and GnRH-2-mediated regulation of bLHβ promoter activity, via both GnRHR-Iand GnRHR-II, and of oFSHβ promoter activity, via GnRHR-I, is indicated. Theconcentration-dependent effects suggest the involvement of two different signallingpathways for the PACAP response. Together these findings suggest that transcription ofthe gonadotropin genes in vivo is under extensive hormonal control that can be finetunedin response to varying physiological conditions, which include changing levels ofGnRH-1, GnRH-2, GnRHR-I and GnRHR-II as well as PACAP.

[发布机构] Stellenbosch University