Also available in: Rat

INDIGO’s Aryl Hydrocarbon Receptor (AhR) Assay is a cell-based genetic reporter assay that utilizes the luciferase reporter gene Aryl Hdrocarbon Receptor Agonist and Antagonist Assays functionally linked to an AhR-responsive promoter. Thus, quantifying changes in luciferase expression in the treated reporter cells provides a sensitive surrogate measure of the changes in AhR activity. The principle application of this reporter assay system is in the screening of test samples to quantify any functional activity, either agonist or antagonist, that they may exert against human AhR.

Although technically not a member of the Nuclear Receptor (NR) superfamily, the AhR shares many of the same attributes. The AhR is a member of the basic helix-loop-helix, Per-Arnt-Sim (bHLH-PAS) family of transcription factors and is responsible for the toxicologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, often referred to as simply “dioxin”) and several other related polycyclic aromatic hydrocarbons. The basic mechanism of action of dioxin and related compounds has been extensively studied, in particular as it relates to regulation of cytochrome P450 1A1 (CYP1A1).

AhR is present in most cell types and in the unactivated state is cytosolic and exists in a complex with chaperone proteins such as heat shock protein 90 (Hsp90). Binding of TCDD and related molecules to AhR leads to nuclear translocation and heterodimerization with its partner protein ARNT, another bHLH-PAS family member. The AhR-ARNT heterodimer binds to specific cognate DNA sequence elements known as dioxin/xenobiotic response elements (DRE/XRE) present in the regulatory region of specific genes such as CYP1A1. Binding of the AhR:ARNT heterodimer to these elements, and subsequent recruitment of transcription coactivator complexes, leads to increased transcription of the specific gene, known as “target genes.” There is a battery of genes affected in this manner and targets include certain xenobiotic-metabolizing enzymes, such as CYP1A1,CYP1A2, CYP2B1, and UGT1A6. In addition, genes affected directly and indirectly by the TCDD/AhR-complex code for both inhibitory and stimulatory growth factors and their gene products affect cellular growth and differentiation leading to tumor promotion and carcinogenicity as well as other forms of toxicity.

For more information on AhR, visit the Nuclear Receptor Resource.