AhR-agonist-induced transcriptional changes of genes involved in thyroid function in primary porcine thyrocytes.

The Ah receptor (AhR) is a ligand transcription factor mediating toxic effects of chemicals such as dioxins. The 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) and the coplanar polychlorinated biphenyl 126 (PCB 126) are member of the polyhalogenated aromatic hydrocarbons family exerting a variety of toxic effects in a tissue-specific and species-specific manner including thyroid function. In the present study, we aimed to investigate the effects of TCDD (1 and 10 nM) and dioxin-like PCB 126 (306 nM) on the AhR signaling pathway and on the gene expression profiles of key factors involved in thyroid function, including thyroglobulin (TG), thyroid peroxidase (TPO), the sodium iodide symporter (NIS), TSH receptor (TSHR), and cathepsins (Cat B and L), using a primary porcine thyrocyte culture as the experimental model. AhR and ARNT expression was detected both as mRNA and on the protein level. Expression did not vary upon treatment with either TCDD or PCB 126. However, treatment with TCDD and PCB 126 induced an AhR signaling response, as indicated by the expression of the AhR-target gene cytochrome P-450 1A1 (CYP1A1). Both 10 nM TCDD and PCB 126 treatment induced a significant downregulation in the expression of NIS and cathepsin B without affecting any of the other parameters investigated. In conclusion, these data indicate that (a) thyrocytes are targets of TCDD and TCDD-like compounds and (b) there is evidence for two independent most likely AhR-mediated molecular mechanisms, by which these compounds negatively interfere with thyroid function.