Dioxin increases C/EBPbeta transcription by activating cAMP/protein kinase A

Area of Science:

• Toxicology
• Molecular Biology
• Gene Regulation

Background:

• 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an environmental pollutant known to affect gene expression.
• TCDD exposure is linked to toxic effects, including wasting syndrome, diabetes, and impaired adipocyte differentiation.
• Modulation of CCAAT/enhancer-binding protein beta (C/EBPbeta) expression is implicated in TCDD's toxic responses.

Purpose of the Study:

• To elucidate the regulatory mechanism behind TCDD-mediated transcriptional activation of the C/EBPbeta gene.
• To investigate the role of specific promoter regions and transcription factors in TCDD's effect on C/EBPbeta.

Main Methods:

• Gene expression analysis (mRNA and protein levels) in mouse embryonic and hepatoma cell lines.
• Reporter gene assays using deletion and mutation constructs of the C/EBPbeta promoter.
• Inhibition studies with protein kinase A (PKA) inhibitors and analysis of cAMP levels and PKA activity.
• Gel shift assays and cotransfection experiments to assess transcription factor binding and activation.

Main Results:

• TCDD treatment elevated C/EBPbeta mRNA and protein levels, correlating with increased protein binding affinity.
• A specific promoter region (-60 to -120 bp upstream) was identified as essential for TCDD-induced C/EBPbeta activation.
• Activation was mediated through incomplete cAMP-response element-binding protein (CREB) sites, dependent on a cAMP/PKA pathway.
• TCDD increased intracellular cAMP levels and PKA activity; PKA inhibition blocked TCDD's effect on C/EBPbeta promoter activity.
• CREB directly bound to the identified motif, confirming its role in mediating the TCDD response.

Conclusions:

• TCDD activates C/EBPbeta transcription via a cAMP/PKA-dependent pathway involving CREB activation.
• This molecular mechanism contributes to the understanding of dioxin's toxicological effects.