Mechanistic studies on protein factors dependent formation of the aryl hydrocarbon receptor -DNA complex

Dioxins and several halogenated polycyclic aromatic hydrocarbons belong to a class of toxic environmental pollutants that give rise to a myriad of teratogenic and carcinogenic responses and are of major concern from a human health perspective due to their widespread distribution. Apart from an array of toxic endpoints, they affect the expression of a variety of xenobiotic metabolizing enzymes including CYP1A1 and 1A2. Data generated by rodent studies have shown that most, if not all, of their biological and toxic effects are mediated through binding to the aryl hydrocarbon receptor (AhR). Upon ligand binding, AhR translocates into the nucleus and heterodimerizes with AhR-nuclear translocator (Arnt); the heterodimer binds to the dioxin response element (DRE) located upstream to the promoter region of target genes, leading to their transcription. The AhR/Arnt/DRE complex has been well characterized and can be observed readily by the gel shift assay. However, the mechanism for this AhR complex formation is unclear. Baculovirus expressed, metal resin-purified human AhR and Arnt are unable to bind the DRE in a ligand-dependent manner unless crude extracts, such as the rabbit reticulocyte lysate (RRL), are reconstituted with these proteins. Proteins in the RRL are responsible for this restoration of the gel shift complex because the activity is sensitive to both heat and proteolytic treatments. Fractionation of the RRL using centricon devices gave the enriched activity in the C10 retentate fraction (C10R). Screening gel shift assays and immunodepletion studies showed that p23 and CyP40, but not hsp90 and hsp70, could be the protein factors. Purified bacterial expressed p23 restored the gel shift complex; and the mechanism is mediated at the heterodimerization step and is hsp90-dependent. However, p23 is not the major factor since the same amount of C10R as that of purified p23 produced a much more pronounced gel shift activity and was insensitive to geldanamycin and apyrase. CyP40 is unable to restore the complex formation directly; however, our data suggested that some of the CyP40-interacting proteins restore the AhR/Arnt/DRE complex formation