Human endometrial stromal cells express novel isoforms of the transcriptional modulator CREM and up-regulate ICER in the course of decidualization

Decidualization of human endometrial stromal (ES) cells in culture can be triggered by a sustained elevation of intracellular cAMP for several days and is characterized by activation of the cAMP-responsive decidual PRL (dPRL) gene promoter. We investigated the expression of the cAMP response element (CRE) binding protein CREB, and the modulators CREM (cAMP response element modulator) and ICER (inducible cAMP early repressor), in relation to decidualization of ES cells. We isolated all four known ICER isoforms from ES cells, which differ by the presence or absence of the small exon gamma and the presence of either DNA-binding domain (DBD) I or II. Of the various CREM isoforms, we cloned six transcript species, all containing DBD I. These were the known repressor CREM-alpha, the potential activator CREM-tau 2 alpha, and four novel forms whose reading frames were blocked upstream of the DBD. Two of these forms contained a novel exon psi, which is 100 bp in length, resides downstream of the first protein-coding exon of the CREM gene, and introduces an early in-frame stop codon. Surprisingly, in cotransfection assays, all four novel CREM isoforms were potent inhibitors of protein kinase A-stimulated transcription of a reporter gene construct driven by a CRE. By in vitro transcription/translation of all six CREM cDNAs, we demonstrated internal translation initiation at three different methionine residues, giving rise to novel short and very short C-terminal proteins comprising DBD I. These proteins bound to a cAMP response element as homodimers or as heterodimers with each other or with CREB. Immunofluorescence showed nuclear localization of C-terminal CREM proteins expressed from all six CREM cDNAs. Comparison of undifferentiated and decidualized ES cells showed no difference in the level of expression of any of the CREM transcript species. Likewise, CREB was evenly expressed between the two populations. In contrast, ICER transcripts were strongly up-regulated in decidualized ES cells in parallel with the induction of dPRL expression. It appears paradoxical that in vivo, in response to a permanent cAMP stimulus, ICER is up-regulated without displaying negative autoregulation of its own gene or suppression of the dPRL promoter. Elevated ICER levels in decidualized ES cells may be indicative of the presence of overriding amounts of transcriptional activators such as full length CREM-tau or CREB which, in turn, upon cAMP-induced phosphorylation, contribute to the induction of the dPRL gene.