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Abstract

Human placental GnRHR cDNA isolated from human choriocarcinoma JEG-3 cells, immortalized human extravillous trophoblasts (IEVT) and primary culture of cytotrophoblasts was identical to the pituitary counterpart. In addition, placental GnRHR wasshown coupling to both the protein kinase C (PKC) and protein kinase A (PKA) signaling transduction pathways. Interestingly, homologous down-regulation of GnRHR mRNA level was not observed in placental cells as in pituitary cells, suggesting that a different regulatory mechanism may exist in controlling the expression of this gene in these two tissues. UsingJEG-3 and IEVT cells as models, an upstream promoter was shown to confer the placental cell-specific expression of hGnRHR gene both in vitro and in vivo. Four putative transcription factor binding sites, namely hGR-Oct-1, hGR-CRE, hGR-GATA and hGR-AP-1, were located and confirmed to be essential for the placental expression of this gene. Importantly, hGR-CRE and hGR-GATA motifs were subsequently found to be placenta specific. A differential regulation of human GnRHR promoter activity by progesterone (P) in the pituitary and placenta was observed. P treatment decreased the promoter activity at the level of pituitary. In contrast, P stimulated the expression of this gene in the placenta. A progesterone response element, namely hGR-PRE, mediated the P-action. Interestingly, human progesterone receptor (PR)-B exhibits a cell-dependent transcriptional activity, such that it functions as a transcription activator in the placenta but a transcription repressor in the pituitary. In contrast, human PR-A acts as a transcription repressor in both tissues. The increase in hGnRHR promoter activity after cAMP/PKA pathway activation by either pharmacological agents or by PACAP and hCG in the pituitary and placenta, respectively, implies that any hormones, which activate cAMP/PKA pathway, may increase the hGnRHR gene transcription. Two elements, namely hGR-AP/CRE-1 and -2, were subsequently demonstrated to be responsible for mediating this stimulatory effect. The comparison studies on the transcriptional regulation of hGnRHR gene by P and cAMP/PKA pathway at the level of the pituitary and placenta implicate that the regulation of hGnRHR gene transcription is constantly under fine-tuning by a complex regulatory mechanism through the availability of different transcription factors and the activation of multiple signal transduction pathways.