The biological functions of the aryl hydrocarbon receptor (AHR) can be delineated into dioxin response element (DRE)-dependent or independent activities. Ligands exhibiting either full or partial agonist activity, e.g. TCDD and α-naphthoflavone, have been demonstrated to potentiate both DRE-dependent and independent AHR function. In contrast, the recently identified selective AHR modulators (SAhRMs), e.g. SGA360, bias AHR towards DRE-independent functionality while displaying antagonism with regard to ligand-induced DRE-dependent transcription. Recent studies have expanded the physiological role of AHR to include modulation of hematopoietic progenitor expansion and immunoregulation. It remains to be established whether such physiological roles are mediated through DRE-dependent or independent pathways. Here, we present evidence for a third class of AHR ligand, 'pure' or complete antagonists with the capacity to suppress both DRE-dependent and independent AHR functions, which may facilitate dissection of physiological AHR function with regard to DRE or non-DRE-mediated signaling. Competitive ligand binding assays together with in silico modeling identify GNF351 (N-[2-(3H-indol-3-yl)ethyl]-9-isopropyl-2-(5-methyl-3-pyridyl)purin-6-amine) as a high-affinity AHR ligand. DRE-dependent reporter assays, in conjunction with quantitative PCR analysis of AHR targets, reveal GNF351 as a potent AHR antagonist that demonstrates efficacy in the nM range. Furthermore, unlike many currently utilized AHR antagonists, e.g. α-naphthoflavone, GNF351 is devoid of partial agonist potential. Interestingly, in a model of AHR-mediated DRE-independent function, i.e. suppression of cytokine-induced acute phase gene expression, GNF351 has the capacity to antagonize agonist and SAhRM-mediated suppression of SAA1. Such data indicate that GNF351 is a pure antagonist with the capacity to inhibit both DRE-dependent and independent activity.
- Received December 18, 2010.
- Revision received March 16, 2011.
- Accepted March 16, 2011.
- The American Society for Pharmacology and Experimental Therapeutics