Lack of ligand-selective binding of the aryl hydrocarbon receptor to putative DNA binding sites regulating expression of Bax and paraoxonase 1 genes

•Ligand-specific differences in AhR:ARNT DNA binding and transcription were studied.•DRE-like sequences in the promoter regions of Bax and PON1 genes were analyzed.•The AhR:ARNT complex did not bind to either DRE-like sequence regardless of ligand.•AhR-dependent luciferase gene expression was not observed from either sequence.•Our results support the established role of the DRE in AhR:ARNT DNA binding.

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxicological effects of structurally diverse chemicals through its ability to bind specific DNA recognition sites (dioxin responsive elements (DREs)), and activate transcription of adjacent genes. While the DRE has a highly conserved consensus sequence, it has been suggested that the nucleotide specificity of AhR DNA binding may be ligand-dependent. The upstream regulatory regions of the murine Bax and human paraoxonase 1 (PON1) genes reportedly contain unique DRE-like sequences that respond to AhRs activated by some ligands but not others. Given the significant implications of this observation to understanding the diversity in AhR responses and that of other ligand-dependent nuclear receptors, a combination of DNA binding, nuclear translocation and gene expression analysis was used to investigate the molecular mechanisms underlying these ligand-selective responses. Although known AhR agonists stimulated AhR nuclear translocation, DRE binding and gene expression, the ligand-selective DRE-like DNA elements identified in the Bax and PON1 upstream regulatory regions failed to bind ligand-activated AhR or confer AhR-responsiveness upon a reporter gene. These results argue against the reported ligand-selectivity of AhR DNA binding and suggest DNA binding by ligand activated AhR involves DRE-containing DNA.