To the Editor:

I read with interest the article by Kato and colleagues (2007) titled “A novel mechanism for polychlorinated biphenyls-induced decrease in serum thyroxine level in rats” in which the authors investigated several mechanisms by which Kaneclor-500 (KC500), a commercial polychlorinated biphenyl (PCB) mixture, decreased thyroxine (T4) serum levels in rats. The enhancement of thyroid hormone metabolism by PCB via induction of UDP- glucuronosyltransferases (UGTs) involved in T4 elimination was examined using Wistar and UGT1A deficient Gunn rats. KC500 treatment (10 mg/kg ip for 10 days) resulted in an increase in UGT1A expression in Wistar rats, but not Gunn rats. Despite having non-existent levels of UGT1A (as measured by Western blot), T4 serum levels decreased and T4 total body clearance increased in Gunn rats to a similar magnitude as Wistar rats. The involvement of hepatic type-I iodothyonine deiodinase activity in this observed increase in T4 total body clearance was unlikely since KC500 was shown to decreased hepatic type-I iodothyonine deiodinase activity in a previous study by the authors (Kato et al., 2004). In addition, 7- benzyloxyresorufin O-dealkylase, 7-pentoxyresorufin O-dealkylase, and 7- ethoxyresorufin O-dealkylase activities following KC500 treatment were significantly increased in both strains of rat. The magnitude of increase in the activities of these P450 substrates was greater in Wistar than in Gunn rats.

The impact of KC500 on T4 distribution was also investigated in serum protein binding and tissue distribution studies using [125I] T4. KC500 treatment caused significant decreases in T4 binding to serum transthyretin (TTR). In contrast, T4 binding to serum albumin increased. Overall, these alterations in serum protein binding resulted in a significant increase in T4 unbound fraction for Wistar rats, but not for Gunn rats. In tissue distribution studies, KC500 treatment caused an increased uptake/ accumulation of [125I] T4 particularly in the liver of both strains of rat. Decreases in serum T4 levels were attributed mainly to this enhanced liver uptake/ accumulation of T4 rather than an induction of hepatic T4 UGTs. The authors concluded by speculating that the enhanced uptake/ accumulation of T4 in liver was due to changes in T4 distribution related to a PCB- and its metabolite(s)-mediated inhibition of T4 binding to TTR.

In addition to inhibition of T4 binding to TTR, I would like to provide an additional point of discussion on the mechanism of the enhanced hepatic uptake of T4 and perhaps provide an additional avenue of investigation that may be considered by the authors. Alterations in T4 serum levels in rats due to the accumulation/ enhanced uptake of T4 in the liver has been observed following treatment of compounds such as SK&F 93479 (lupitidine), SK&F 93944 (temelastine) (Poole et al. 1989, 1990), and DMP 904 (Wong et. al., 2004). DMP 904 treatment resulted in an increase in T4 total body clearance accompanied by an increase in volume of distribution. Small alterations in T4 serum unbound fraction due to DMP 904 were observed, but could not account for the magnitude of the change in T4 total body clearance in rats. A substantial portion of the increase in total body clearance was due to an increase in biliary clearance of unconjugated T4. Consistent with the observed increase in biliary clearance, a whole body autoradiography study showed that DMP 904 treatment resulted in an increase in accumulation of T4 in both the liver and small intestine. In addition, DMP 904 appeared to increase expression of Oatp2, a hepatic transporter involved in T4 transport. Overall, the effects of DMP 904 on T4 homeostasis appeared to be in large part related to activation of CAR and PXR followed by up-regulation of hepatic transporters involved in T4 transport (Wong et al., 2004).

Although differing in magnitude, similarities exist in the effects of KC500 and DMP 904 on T4 disposition in rats. Both compounds caused a concurrent increase in T4 total body clearance and volume of distribution, and enhanced T4 liver accumulation. Similar to DMP 904, KC500 treatment resulted in an increased concentration of [125I] T4 in the small intestines of Wistar and Gunn rats consistent with increases in biliary clearance. In particular, for UGT1A deficient Gunn rats, the T4 in the small intestine is very likely to be in the form of unconjugated T4 rather than its glucuronide metabolite. Higher levels of 7-benzyloxyresorufin O- dealkylase (CYP2B1/2 and CYP3A1/2) and 7-pentoxyresorufin O-dealkylase (CYP2B1/2) activities in Wistar and Gunn rats along with an increase in expression of UGT1A in Wistar rats suggest activation of CAR and PXR by KC500 treatment. Thus, KC500 mediated activation of CAR and PXR and subsequent up-regulation of hepatic T4 transporters may be an additional mode of action by which KC500 decreases serum T4 levels and serves as additional avenue of investigation that may be considered by the authors. In follow up experiments, measurement of mRNA and/or protein levels of hepatic T4 transporters following KC500 pretreatment in Wistar and Gunn rats may aid in clarifying the situation. I will continue to read with interest on further investigations examining the mechanism by which PCBs such as KC500 reduce T4 serum levels in rats.

REFERENCES

Kato Y, Ikushiro S, Takiguchi R, Haraguchi K, Koga N, Uchida S, Sakaki T, Yamada S, Kanno J and Degawa M (2007) A novel mechanism for polychlorinated biphenyls-induced decrease in serum thyroxine level in rats. Drug Metab and Disp. Doi:10.1124/dmd.107.017327.

Kato Y, Ikushiro S, Haraguchi K, Yamazaki T, Ito Y, Suzuki H, Kimura R, Yamada S, Inoue T and Degawa M (2004) A possible mechanism for decrease in serum thyroxine level polychlorinated biphenyl in Wistar and Gunn rats. Tox Sci. 81: 309-315

Poole, A., Jones, R.B., Pritchard, D., Catto, L., and Leonard, T. (1989). In vitro accumulation of thyroid hormones by cultured heepatocytes and the biliary excretion of iodothyronines in rats treated with a novel histamine H2-receptor antagonist. Toxicology 59, 23-36.

Poole, A., Pritchard, D., Jones, R.B., Catto, L., and Leonard, T. (1990). In vivo biliary excretion and in vitro cellular accumulation of thyroxine by rats or cultured rat hepatocytes treated with a novel histamine H1-receptor antagonist. Arch. Toxicol. 64, 474-481.

Wong H, Lehman-McKeeman LD, Grubb MF, Grossman SJ, Bhaskaran VM, Solon EG, Shen HSL, Gerson RJ, Car BD, Zhao B, and Gemzik B (2004) Increased hepatobiliary clearance of unconjugated thyroxine determines DMP904-induced alterations in thyroid hormone homeostasis in rats. Tox Sci. 84: 232-242.