Effect of dioxins on regulation of tyrosine hydroxylase gene expression by aryl hydrocarbon receptor: a neurotoxicology study

N2a-Rβ cells were established by transfecting murine neuroblastoma Neuro2a with the rat AhR cDNA. TH expression induced by TCDD was assessed by RT-PCR and Western blotting. Participation of AhR in TCDD-induced TH gene expression was confirmed by suppressing AhR expression using the siRNA method. Catecholamines including dopamine were measured by high-performance liquid chromatography. A reporter gene assay was used to identify regulatory motifs in the promoter region of TH gene. Binding of AhR with the regulatory motif was confirmed by an electrophoretic mobility shift assay (EMSA).Induction of TH by TCDD through AhR activation was detected at mRNA and protein levels. Induced TH protein was functional and its expression increased dopamine synthesis. The reporter gene assay and EMSA indicated that AhR directly regulated TH gene expression. Regulatory sequence called aryl hydrocarbon receptor responsive element III (AHRE-III) was identified upstream of the TH gene from -285 bp to -167 bp. Under TCDD exposure, an AhR complex was bound to AHRE-III as well as the xenobiotic response element (XRE), though AHRE-III was not identical to XRE, the conventional AhR-binding motif.Our results suggest TCDD directly regulate the dopamine system by TH gene transactivation via an AhR-AHRE-III-mediated pathway. The AhR- mediated pathway could have a particular AhR-mediated genomic control pathway transmitting the effects of TCDD action to target cells in the development of dopaminergic disabilities.Halogenated aromatic hydrocarbons (HAHs) such as polychlorinated biphenyls (PCBs) and poly-chlorodibenzo-p-dioxins (PCDDs) affect human health when they are absorbed by the body. Their effects are predominantly negative, such as oncogenesis, reproductive toxicity, immunosuppression, and neurological dysfunction [1-4]. One of the dioxins, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), leads to neurobehavioral abnormalities associated with both cognitive and locomotor systems [5,6]. While th