RESUMO
The gonadotrope-specific and regulated expression of the GnRH receptor (GnRH-R) gene is dependent on multiple transcription factors that interact with the noncanonical GnRH-R activating sequence (GRAS), the activator protein-1 (AP-1) element, and the steroidogenic factor-1 (SF-1) binding site. However, these three elements are not sufficient to mediate the complete cell-specific expression of the rat GnRH-R gene. In the present study, we demonstrate, by transient transfection in gonadotrope-derived alphaT3-1 and LssT2 cell lines, the existence of a distal enhancer [GnRH-R- specific enhancer (GnSE)] that is highly active in the context of the GnRH-R gene promoter. We show that the GnSE activity (-1,135/-753) is mediated through a functional interaction with a proximal region (-275/-226) that includes the SF-1 response element. Regions of similar length containing either the AP-1 or GRAS elements are less active or inactive. Transfection assays using an artificial promoter containing two SF-1 elements fused to a minimal PRL promoter indicate that SF-1 is crucial in this interaction. In addition, by altering the promoter with deletion and block- replacement mutations, we have identified the active elements of GnSE within two distinct sequences at positions -983/-962 and -871/-862. Sequence analysis and electrophoretic mobility shift experiments suggest that GnSE response elements interact, in these two regions, with GATA- and LIM-related factors, respectively. Altogether, these data establish the importance of the GnSE in the GnRH-R gene expression and reveal a novel role for SF-1 as a mediator of enhancer activity, a mechanism that might regulate other SF-1 target genes.
