Gonadotropins regulate inducible cyclic adenosine 3',5'-Monophosphate early repressor in the rat ovary: Implications for inhibin α subunit gene expression

Many hormones that stimulate intracellular signaling pathways utilizing the second messenger cAMP affect gene expression in target cells through the activation of cAMP-responsive transcriptional regulatory proteins. Two of the best characterized of these are the cAMP-response element (CRE)-binding protein (CREB) and the CRE-modulatory protein (CREM). CREB and CREM are expressed as a family of proteins that have diverse activities in either stimulating or repressing gene transcription. In this study we examined the expression and regulation of the CREM gene in the rat ovary and in granulosa cells, to determine whether repressor isoforms of CREM might have a role in the LH-mediated suppression of inhibin α-subunit gene expression that occurs just before ovulation. We found that the predominant CREM mRNAs in the ovary correspond to previously described internal transcripts of the CREM gene that encode the inducible cAMP early repressor (ICER). ICER mRNAs are strongly induced in the ovary by exogenous gonadotropins in immature rats and are transiently expressed in the ovary immediately after the preovulatory LH surge in adult cycling rats. Although ICER is expressed in multiple ovarian cell types, expression in granulosa cells is observed only in response to LH stimulation. ICER mRNAs are also induced by the activation of cAMP-signaling pathways in cultured primary granulosa cells. To determine whether ICER can act as a functional repressor to modulate potential target genes such as the inhibin α-subunit gene, an ICER expression construct was transiently cotransfected into a granulosa cell line along with an inhibin α-subunit promoter-luciferase reporter gene. Both basal and cAMP-induced expression of the inhibin α-subunit promoter were suppressed by ICER. These studies reveal that CREM, a tissue-specific factor, is expressed and regulated by gonadotropins in the ovary, that the predominant CREM transcripts encode the repressor protein ICER, and that ICER is capable of inhibiting cAMP-induced expression of the inhibin α-subunit gene. Our findings are consistent with a role for repressors such as ICER in mediating the suppression of inhibin α-subunit gene expression that occurs in the ovary at the time of the preovulatory LH surge.