Steroid-independent activation of ER by GnRH in gonadotrope pituitary cells

In the rat pituitary gland the mechanism responsible for ER alpha regulatio n has not been fully elucidated. Using transient transfection assays in alp ha T3-1 cells, a cell line of gonadotrope origin, we show that GnRH stimula tes estrogen response element-containing promoters in an estrogen-independe nt manner. This effect was strictly ER and GnRH receptor dependent, as no a ctivation of the reporter gene was observed in presence of the anti-estroge n ICI 182,780 or a GnRH antagonist. These data suggest that the GnRH-trigge red signaling pathway results in 17 beta -estradiol-independent trans-activ ation of the ER alpha in aT3-1 cells. Furthermore, an additive activation w as achieved when cells were treated with both GnRH and 17 beta -estradiol. In primary pituitary cells, GnRH alone (100 nM) did not cause a significant stimulation of reporter gene activity, presumingly due to the low amount o f gonadotropes. Interestingly, the combination of 17 beta -estradiol and Gn RH resulted in a significant increase in ER alpha transactivation compared with that in cells treated with 17 beta -estradiol alone. This enhancement was prevented by ICI 182,780, showing an ER alpha requirement. Moreover, we show that the effects of GnRH on ER alpha transcriptional activity in gona dotrope cell lines are mediated by the PKC/MAPK pathway. In conclusion, our data demonstrate that GnRH is an important signal in the regulation of ER alpha trans-activation in gonadotrope cells.