Transcriptional activation of gonadotropin-releasing hormone (GnRH) receptor gene by GnRH and cyclic adenosine monophosphate.

Abstract

GnRH appears to regulate messenger RNA levels and synthesis of its own receptor (GnRHR). In this study, we examined the regulation of GnRHR gene transcription by GnRH and cAMP in the GGH3 cell line (GH3 cells stably expressing GnRHR). Transient transfection of GGH3 cells with luciferase reporter gene vector (GnRHR-pXP2) containing a 1226-bp promoter fragment (-1164 to +62, relative to the major transcription start site) of mouse GnRHR gene resulted in an increase in reporter gene (GnRHR-Luc) activity (11- to 22-fold) compared with the promoterless vector. GnRH or a GnRH agonist (Buserelin) significantly stimulated the GnRHR-Luc activity in a dose-dependent manner. Time-course studies using 10(-7) M Buserelin revealed that GnRHR-Luc activity increased progressively from 1.5-6 h, with a peak at 6 h. The increase in GnRHR-Luc activity was lower at 12 and 24 h. Both cholera toxin and dBcAMP significantly stimulated GnRHR-Luc activity. Pretreatment with dBcAMP also enhanced the extent of stimulation of GnRHR-Luc activity in response to Buserelin. Pertussis toxin did not induce basal or Buserelin-stimulated GnRHR-Luc activity. Treatment of GGH3 cells with 10(-9) or 10(-7) M Buserelin for 6 h was sufficient to stimulate a significant increase in cAMP release. An adenylate cyclase inhibitor SQ 22536 did not affect the basal GnRHR-Luc activity but significantly reduced Buserelin-activated GnRHR-Luc activity. These results suggest that GnRH and cAMP activate transcriptional activity of the GnRHR gene and that GnRH activates GnRHR transcriptional activity, in part, through the cAMP pathway. Progressive 5'-deletion analysis revealed that basal and Buserelin- or dBcAMP-stimulated GnRHR-Luc activity were consistently retained after 5'-deletion at position -456, -381, or -331 relative to the major transcription start site but were significantly decreased after subsequent truncation of the promoter from -331 to -255 relative to the major transcription start site. However, the -255 construct still retained responsiveness to Buserelin and dBcAMP, and the relative activity remained similar under both stimulation conditions. These results suggest that elements located between -331 and -255 necessary for transcriptional activity of the GnRHR gene in GGH3 cells, and that the response elements on the mouse GnRHR gene for both GnRH and cAMP reside at two different sites: between -331 and -255 and between -255 and +62.