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Transciptional regulation of human gonadotropin-releasing hormone (GnRH) and GnRH receptor genes Cheng, Chi Keung

Abstract

In addition to its pivotal role in controlling gonadotropin synthesis and secretion, mammalian gonadotropin-releasing hormone (termed GnRH-I) functions as an autocrine and/or paracrine factor in certain extrapituitary tissues, where expression of its receptor (GnRH-I receptor) has been demonstrated. Although our laboratory has previously addressed a number of issues regarding the transcriptional regulation of the human GnRH-I receptor gene, some aspects including tissue-specific and hormonal regulation of the gene expression remain unclear. In the present study, we have identified by stepwise deletion analysis a new upstream GnRH-I receptor promoter that is primarily used by ovarian granulosa-luteal (GL) cells. Site-directed mutagenesis indicated that the activity of this promoter was governed by a cooperative action among two CCAAT/enhancerbinding protein (C/EBP) and one GATA motifs. On the one hand, these data strengthen the notion that tissue-specific expression of the human GnRH-I receptor gene is mediated by differential promoter usage in various cell types. On the other hand, we have identified an evolutionarily conserved octamer sequence, which constitutively and ubiquitously suppresses the GnRH-I receptor promoter. Electrophoretic mobility shift assays (EMSAs) showed that the repressor binding to this element was the POU homeodomain transcription factor Oct-1. These findings thus uncover a novel role of Oct-1 in silencing GnRH-I receptor gene transcription. Furthermore, we have demonstrated that an activator protein-1 (AP-l)-like motif mediates 17β-estradiol (E2) repression of the GnRH-I receptor promoter via an estrogen receptor (ER)ct-dependent mechanism that may involve competition for the transcriptional coactivator cAMP responsive element (CRE)-binding protein (CREB)-binding protein (CBP) between the steroid receptor and c-Jun/c-Fos heterodimer. The identification of a second form of GnRH from chicken hypothalamus (termed GnRH-II) revealed that it is the most widely expressed form of GnRH and that its structure is conserved over all vertebrate classes from primitive fishes to humans. However, the molecular mechanism governing human GnRH-II gene transcription is poorly understood. Here we have shown that two E-box binding sites (EBSs) and one Ets-like element (ELE) in the untranslated first exon function cooperatively to stimulate the basal transcription of the GnRH-II gene and demonstrated that the basic helix-loophelix (bHLH) transcription factor AP-4 is an enhancer protein for the GnRH-II promoter. The results of this research provide novel and significant insights into the molecular mechanisms underlying the multi-faceted actions of GnRH in humans.

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