Dioxin Increases C/EBPβ Transcription by Activating cAMP/Protein Kinase A

Abstract

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD = dioxin) has been shown to increase the expression of C/EBPβ. The modulated expression of C/EBPβ has been suggested to be associated with toxic responses of TCDD such as wasting syndrome, diabetes, and inhibition of adipocyte differentiation. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of C/EBPβ. Elevated C/EBPβ mRNA and protein levels in mouse embryonic fibroblasts (C3H10T1/2) and in mouse hepatoma cells (Hepa1c1c7) were correlated with increased binding affinity of the C/EBPβ protein. Transfection studies with different deletion constructs of the CCAAT/enhancer-binding protein promoter indicated that a small region located 60-120 bp upstream of the start site of transcription is required for activation of the C/EBPβ gene by TCDD in both cell lines tested. Further analysis using mutation constructs of the C/EBPβ promoter demonstrated that activation of the C/EBPβ promoter is mediated through incomplete cAMP-response element-binding protein (CREB) sites located close to the TATA box of the C/EBPβ gene. The protein kinase A (PKA) inhibitor H89 completely blocks the TCDD-dependent effect on C/EBPβ promoter activity, indicating that TCDD activates CREB binding via a cAMP/PKA pathway, which is supported by the increased cAMP level and PKA activity observed after TCDD treatment. Gel shift analyses demonstrated that CREB itself binds to the putative CREB motif that mediates the TCDD-dependent effect on C/EBPβ gene transcription. Cotransfection experiments with CREB and PKA expression plasmids further supported our conclusions that the TCDD-dependent effect on C/EBPβ transcription is mediated via PKA-dependent CREB activation.