We have identified the VgEcR-RXR gene expression system as a tool for studying the expression of SEPP1. Our results indicate that once activated by ponasterone A, VgEcR-RXR is capable of inducing transcription of SEPP1 through a GRE located at position −87 or a RRE at position −73 of the promoter. In the EcR-GR cells, treatment with the GR agonist dexamethasone resulted in an attenuation of the ponasterone A-induced transcription of SEPP1 compared to ponasterone A treatment alone. This suggests that once activated by dexamethasone, the GR can travel to the nucleus and alter VgEcR-RXR binding at the site identified as GRE #1. While the EMSA failed to demonstrate GR binding by supershifting the protein:DNA complex, nuclear extract from the EcR-GR cells does display dexamethasone-dependent modulation of the protein:DNA complex that was consistent with the heterologous reporter expression assays. When a functional GR was stably integrated to make the EcR-GR cells, a generalized repression of SEPP1 was observed compared to the 293-EcR cells. This data supports the idea that the GR may indirectly regulate expression of this gene, and this effect
