Adrenergic regulation of ICER (inducible cyclic AMP early repressor) and β₁-adrenergic receptor gene expression in C6 glioma cells

ICER (inducible cyclic AMP early repressor), a member of the cyclic AMP response element (CRE) modulator (CREM) family of transcription factors, is a powerful repressor of cyclic AMP-mediated transactivation. Our studies characterize the regulation of ICER in C6 glioma cells and investigate its role in repressing transcription of the β₁-adrenergic receptor (β₁AR) gene. Incubation with isoproterenol (100 nM) results in a rapid induction in levels of mRNA for ICER and its splice variant ICERγ, with maximal induction occurring after 2 h of treatment. Incubation with isoproterenol also increased levels of CREM isoforms within 1 h; this was unexpected given previous reports that these isoforms are not rapidly induced. Increased expression of ICER and CREM was accompanied by induction of two CRE-binding complexes. The presence of ICER in these two CRE-binding complexes is demonstrated by their disruption with CREM antibody and by their comigration with recombinant ICER. Because the time course for isoproterenol induction of ICER mRNA and CRE binding corresponds to that for down-regulation of β₁AR mRNA levels in C6 glioma cells, the influence of ICER on β₁AR transcription was directly examined. Coexpression of ICER significantly decreased transcriptional activity of a rat β₁AR promoter-luciferase reporter construct that contains a CRE. In contrast, coexpression of ICER did not influence two truncated rat β₁AR promoter constructs that lack the CRE site. These data demonstrate that ICER can interact at the β₁AR promoter to repress transcription.