| Research Abstract |
In the present study, we investigated the mechanism for the decrease in levels of serum thyroid hormones, especially thyroxine (T_4), by polychlorinated biphenyls (PCBs) such as Kanechlor-500 (KC500),2,2',4',5,5'-pentachlorobiphenyl (PentaCB), and 2,2',3',4',5,6-hexachlorobiphenyl (HexaCB), and studied species difference among mice, hamsters, rats, and guinea pigs in the PCB effect. Significant decrease in serum total T_4 level by KC500 was observed in all four species. On the other hand, there were differences in the level of decrease of serum total T_4 level by PentaCB and HexaCB. Differences in the level of hepatic methylsulfonyl-PCB and OH-PCB metabolites of KC500, PentaCB and HexaCB, which were thought to be associated with the PCB-toxic effects, did not necessarily correlate with the magnitude of decrease in serum total T_4 level. Likewise, the induction of CYP1A, CYP2B, CYP3A, iodothyronine deiodinase and UDP-glucuronosyltransferases (T_4-UDP-GT) toward T_4 by PCB did not necess
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arily correlate with the decrease in serum T_4 level in the animals used. In addition, the PCB-mediated decrease in serum total T_4 level in UGT1A-deficient Wistar rats (Gunn rats) occurred without an increase in hepatic T_4-UDP-GT activity. Further studies on transthyretin (TTR) and serum T_4-transpoter suggested that decrease in serum total T_4 level by PCB occurred not only by induction of T_4-UDP-GT but also by the alteration of levels of T_4-TTR binding and hepatic T_4-transporter. In addition, species difference in the decrease of serum total T_4 level was associated with various PCB-induced total effects, including induction of T_4-UDP-GT, decrease in T_4-TTR binding level, the increase of hepatic thyroid hormone transporter, and other thyroid function correlates. In conclusion, the present findings suggest that in the animals used, the PCB-induced decrease in serum T_4 level dose not occur only through increase in hepatic T_4-UDP-GT, and further suggest that in all four species, the PCB-mediated decrease occurs through the increased transportation of T_4 to the liver. Furthermore, the decrease in the binding of T_4 to serum TTR and hepatic hyperplasia might be attributed to the increase in the level of T_4 in the liver. Further studies are necessary for understanding the susceptibility toward a PCB-mediated decrease in serum T_4 level in animals including humans. Less
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