One of the master regulators of placental cell fusion in mammals leading to multi-nucleated syncytiotrophoblasts is the transcription factor GCMa. Recently, we proved that the cAMP-driven protein kinase A signaling pathway is fundamental for up-regulation of GCMa transcript levels and protein stability. Here, we show that Transducer of Regulated CREB activity (TORC1), the human co-activator of cAMP response element-binding protein (CREB), but not a dominant-negative CREB mutant, significantly up-regulates the GCMa promoter. We identified potential cAMP response element (CRE)-binding sites within the GCMa promoter upstream of the transcriptional start site. Only the CRE site at -1337 interacted strongly with CREB in promoter mapping experiments. The characterization of GCMa promoter mutants and additional bZIP-type family members demonstrated that also old astrocyte specifically-induced substance (OASIS) is able to stimulate GCMa transcription. Knockdown of endogenous CREB or OASIS in BeWo cells decreased endogenous GCMa mRNA level and activity. Overexpression of TORC1 or OASIS in choriocarcinoma cells led to placental cell fusion, accompanied by placental expression of gap junction forming protein connexin-43. Further, we show that CREB expression is replaced by OASIS expression around E12.5 suggesting that a sequential order of bZIP-type family members ensures a high rate of GCMa transcription throughout placentation.

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