9-cis-Retinoic acid represses transcription of the gonadotropin-releasing hormone (GnRH) gene via proximal promoter region that is distinct from all-trans-retinoic acid response element.
We previously reported an enhancing effect of all-trans-retinoic acid (all-trans-RA) on gonadotropin-releasing hormone (GnRH) gene transcription via distal promoter elements of the rat GnRH gene. The present study examined the effects of another biologically active retinoid, 9-cis-retinoic acid (9-cis-RA), on GnRH transcription in GT1-1 cells. Similar to the action of all-trans-RA, 9-cis-RA significantly induced the luciferase activity of the strong retinoic acid response element (RARE) reporter construct, 3X beta RARE-Luc, by about 60-fold, indicating that GT1-1 cells are also responsive to 9-cis-RA. In contrast to the stimulatory effect of all-trans-RA on GnRH transcription, 9-cis-RA inhibited the GnRH promoter activity in a dose- and time-dependent manner. Significant inhibition by 9-cis-RA required at least an 18 h treatment and a further decrease of GnRH promoter-driven luciferase activity was observed up to 48 h of incubation. Accordingly, GnRH mRNA levels were decreased by 9-cis-RA treatment in a similar dose- and time-related manner, indicating that mouse GnRH expression is also negatively regulated by 9-cis-RA. Transient transfections of serial deletion constructs of the rat GnRH promoter revealed that the --230/--110 sequence of the rat GnRH promoter is responsible for 9-cis-RA-induced inhibition of GnRH transcription. Within this region, however, no consensus retinoid X receptor response element was found. To gain insights into the role of retinoid X receptors (RXRs) in GnRH expression, we examined the effects of RXR overexpression on GnRH transcriptional activity. Interestingly, co-transfection of RXR overexpression vectors significantly increased the GnRH promoter-driven luciferase activity, while treatment with 9-cis-RA not only nullified the enhancing effect of RXR overexpression but also decreased the basal GnRH promoter-driven luciferase activity by 50% compared to vehicle-treated controls. This implies that RXRs in the absence of its cognate ligand 9-cis-RA contribute to the maintenance of basal GnRH gene transcription. Northern blot analysis revealed that 9-cis-RA, but not all-trans-RA, down-regulated RXR beta expression in GT1-1 cells, suggesting that one possible mechanism of 9-cis-RA-induced repression involves down-regulation of RXR expression. In conclusion, the present study clearly demonstrates that 9-cis-RA is a negative regulator of GnRH gene expression in immortalized GnRH neurons.
Cho
S
S
School of Biological Sciences and Research Center for Cell Differentiation, Seoul National University, Seoul, 151-742, South Korea.
Chung
J
J
Han
J
J
Ju Lee
B
B
Han Kim
D
D
Rhee
K
K
Kim
K
K
D000077556:Alitretinoin; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D007987:Gonadotropin-Releasing Hormone / Q000235:genetics* / Q000378:metabolism; D008024:Ligands; D011401:Promoter Regions, Genetic / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D018168:Receptors, Retinoic Acid / Q000378:metabolism; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology; D014162:Transfection; D014212:Tretinoin / Q000494:pharmacology*
D000970:Antineoplastic Agents; D008024:Ligands; D012333:RNA, Messenger; D018168:Receptors, Retinoic Acid; D047488:Retinoid X Receptors; D014157:Transcription Factors; D000077556:Alitretinoin; D007987:Gonadotropin-Releasing Hormone; D014212:Tretinoin