AhR-Agonist-Induced Transcriptional Changes of Genes Involved in Thyroid Function in Primary Porcine Thyrocytes

Abstract

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.

Comparison of the in vitro effects of TCDD, PCB 126 and PCB 153 on thyroid-restricted gene expression and thyroid hormone secretion by the chicken thyroid gland

“…It couples the inward translocation of Na + down its electrochemical gradient to the simultaneous inward translocation of I − against its electrochemical gradient (Spitzweg and Morris, 2002), using the energy source of Na + /K + ATPase (Ajjan et al, 1998). Our findings are consistent with studies of Pocar et al (2006) who using a primary porcine thyrocyte culture as the experimental model found reduced NIS gene expression in their thyrocytes following PCB 126 exposure. Nevertheless, we did not observe the negative effect of TCDD on this gene expression found by those authors.…”

“…Our in vitro experiment revealed that TCDD and PCB 126 (but not PCB 153) up-regulated TG and TPO gene expression in chicken thyroid explants. Pocar et al (2006) did not find effects of these two chemicals on mRNA expression of TG and TPO in porcine thyrocytes. These discrepancies might be due to the varying doses of TCDD and PCB or to different species used in the experiment.…”

“…increased amounts of lysosomal bodies with strong acid phosphatase activity which prevented T 4 disconnection from thyroglobulin (Collins et al, 1977). Moreover, both in vivo (Pocar et al, 2006) and in vitro experiments (Kędzior et al, 2015) revealed that dioxin and PCBs downregulate the mRNA expression and activity of cathepsin B, the enzyme responsible for solubilization of thyroglobulin and subsequent thyroid hormone liberation (Friedrichs et al, 2003). These assumptions, however, need further investigation.…”

An in vitro approach to assess the toxicity of certain food contaminants: Methylmercury and polychlorinated biphenyls

“…TT cells were sensitive to the toxicity of PCB 126, but not to that of PCB 153. Dioxinlike PCBs are known to affect thyroid function in vivo much more than non-dioxin-like PCBs, as they decrease circulating levels of T3 and T4 (Fisher et al, 2006;Pocar et al, 2006). Suggested mechanisms include induction of T4 metabolism by upragulation of uridine diphosphoglucuronosyl transferase (Capen, 1997), displacement of T4 from transthyretin (Brouwer et al, 1998), or alteration of thyroid hormone iodination (Morse et al, 1993).…”

Toxicological effects of clofibric acid and diclofenac on plasma thyroid hormones of an Indian major carp, Cirrhinus mrigala during short and long-term exposures

“…The alterations of THs in fish collected from contaminated sites may be due to reduced thyroid levels resulting from induced hepatic biotransformation enzymes (UDPGT and sulfatase) and excretion (Kohn et al, 1996;Crofton, 2008), and/or changes in expression of genes involved in TH production (Pocar et al, 2006;Brar et al, 2010). Moreover, thyroid disruption due to chemical toxicity may also result in the alterations of T 4 and T 3 (Li et al, 2014).…”