SIGNALING AND TRANSACTING FACTORS IN THE TRANSCRIPTIONAL INHIBITION OF GONADOTROPIN-RELEASING-HORMONE GENE BY HUMAN CHORIONIC-GONADOTROPIN IN IMMORTALIZED HYPOTHALAMIC GT1-7 NEURONS

We recently demonstrated that immortalized GT1-7 neurons co-express lu teinizing hormone (LH)/human chorionic gonadotropin (hCG) receptor and gonadotropin releasing hormone (GnRH) genes. Treatment of GT1-7 neuro ns with LH/hCG resulted in a transcriptional inhibition of GnRH gene. In the present study, we investigated the signaling and transacting fa ctors involved in the action of hCG. Eight-bromo-cyclic AMP can mimic the down-regulating action of hCG on GnRH mRNA levels. H-89, a protein kinase (PK) A inhibitor, but not bisindolylmaleimide, a PKC inhibitor , blocked the down-regulating actions of hCG as well as of 8-bromocycl ic AMP. Treatment with the PKA inhibitor alone modestly decreased GnRH mRNA levels suggesting that PKA signaling also controls the basal exp ression of the GnRH gene. The direct measurement of PK activities reve aled that hCG treatment of GT1-7 neurons increased the PKA but not the PKC activity. New protein synthesis is required for the down-regulati ng action of hCG on GnRH mRNA levels. Since some of the new proteins c ould be nuclear transcription or transacting factors, we investigated the effects of hCG on cyclic AMP response element binding protein (CRE B), c-Fos and c-Jun protein levels. Treatment of GT1-7 neurons with hC G resulted in an increase of 43 kDa phosphorylated CREB, 50 kDa c-Fos and 40 kDa c-Jun proteins compared to the corresponding controls. The kinetics of increases were different and in all cases the increases of the proteins preceded the decrease of GnRH mRNA levels. In summary, P KA signaling and transacting factors such as CREB, Fos and Jun are pro bably involved in transcriptional inhibition of GnRH gene by hCG in GT 1-7 neurons.