Regulation of human dioxin receptor function by indolocarbazoles, receptor ligands of dietary origin

The intracellular basic helix-loop-helix (bHLH) dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). In analogy to nuclear receptors that are members of the steroid hormone receptor superfamily the dioxin receptor is a ligand-inducible transcriptional regulator that directly binds to response elements within regulated genes. The most commonly studied dioxin receptor ligands are dioxin itself and structurally related environmental contaminants. A physiological ligand has not yet been identified. Interestingly, however, indolo[3,2-b]carbazole, a compound formed from precursors in the diet, has been shown to bind the murine dioxin receptor with high affinity in vitro. In the present study we show that this compound and its methylated derivative 5,11-dimethylindolo[3,2-b]carbazole very potently activated transcription from a dioxin or xenobiotic response element (XRE)-driven reporter gene in both murine and human hepatoma cells. This effect was not observed in mutant, dioxin-resistant hepatoma cells which are either deficient in expression of dioxin receptor or the bHLH receptor partner factor Arnt. In vitro indolocarbazoles induced XRE binding activity by the human dioxin receptor-Arnt complex in a dose-dependent manner. Thus, both dioxin- and indolocarbazole-activated forms of dioxin receptor regulate target gene expression by the same mechanism involving recruitment of the bHLH factor Arnt and recognition of the XRE element. Finally, the indolo[3,2-b]carbazole-activated human dioxin receptor appeared to generate more stable complexes with the XRE target sequence relative to those produced by the dioxin-activated receptor form, indicating interesting mechanistic differences between different classes of dioxin receptor ligands in their abilities to modulate human dioxin receptor function.