Aryl hydrocarbon receptor as a molecular mediator in the induction of human aldehyde oxidase gene expression

Relatively little is known about receptor-mediated regulation of aldehyde oxidase (AOX). 2,3,7,8-Tetrachlorodibenzo-p-dioxin, an aryl hydrocarbon receptor (AHR) agonist, increases mouse Aox expression. However, given the species differences in ligand-dependent AHR activation and AOX regulation, it remains to be investigated whether human AHR regulates the expression of AOX1 (the human AOX isoform). Therefore, the present study was designed to determine the effect of modulating the functionality of human AHR on AOX1 gene expression in cell culture models (HepG2 and MCF-7) known to express AHR and AOX1. Treatment of HepG2 or MCF-7 cells with a noncytotoxic concentration of a classical AHR agonist (β-naphthoflavone, benzo[a]pyrene, or 3-methylcholanthrene [3-MC]), an endogenous AHR agonist (indoxyl sulfate or 6-formylindolo[3,2-b]carbazole), or a selective AHR modulator (quercetin, 3,3'-diindolylmethane, or omeprazole) increased AOX1 and AHR-regulated CYP1A1 mRNA expression. The increases in AOX1 and CYP1A1 mRNA levels were not correlated among our panel of AHR activators. The induction of AOX1 mRNA by 3-MC was time- and concentration-dependent. It was accompanied by an increase in AOX1 protein expression and AOX1-mediated enzyme activity (O⁶-benzylguanine 8-oxidation). An AHR antagonist (3',4'-dimethoxyflavone) completely abolished the increase in AOX1 and CYP1A1 expression by an AHR agonist (3-MC). Gene knockdown by an AHR small interfering RNA substantially decreased AHR mRNA expression and attenuated AOX1 and CYP1A1 mRNA induction by 3-MC. Overall, based on the experimental approaches involving AHR activation, AHR antagonism, and AHR gene knockdown by small interfering RNA, our novel data indicate that AHR plays a role in mediating the induction of human AOX1 gene expression. SIGNIFICANCE STATEMENT: The deployment of multiple experimental approaches involving receptor activation, receptor antagonism, and gene knockdown by small interfering RNA led to the identification of the human aryl hydrocarbon receptor as a molecular mediator of aldehyde oxidase gene induction. This novel discovery expands our limited knowledge of the identity of pharmacological receptors contributing to the functionality of human aldehyde oxidase.