Rat Thyroid Hormone Depression Effect Of Perfluorooctane Sulfonate(PFOS) And Its Mechanism Of Action

Perfluorooctane sulfonate (PFOS) is an environmentally persistent organic pollutant (POPs), which was widely used in industrial and consumer applications. Related animal experiments suggest that PFOS is associated with hepatotoxicity, carcinogenicity, reproductive and developmental effects, immunotoxicity neuroendocrine and neurobehavioural effects as well as hormone disruption. In May 2009, PFOS was added to the Stockholm Convention on POPs.Thyroid hormone is essential for the normal physiological function of virtually all tissues in mammals, playing important roles in regulating metabolism, growth and development, especially for the development of central nervous system and brain function. The present paper investigated that whether prenatal or postnatal exposure to PFOS alone can disturb the thyroid function in rat pups, and if so, which kind of exposure is a major cause of thyroid disruption. Besides, mechanisms of action by which PFOS interferes with thyroid systems in rats were also investigated. The present study comprise of several parts as follows:(1) A cross-foster animal model was established, yield the following four groups: unexposed control (CC), prenatal exposure (TC), postnatal exposure (CT), and prenatal+ postnatal exposure (TT). Serum and liver PFOS concentrations as well as serum TH levels in rat pups at the age of various postnatal days 0,7,14,21 or 35 under different conditions of PFOS exposure were evaluated. The occurrence of PFOS transfer from dams to pups, both via placenta and milk, was observed. Neither total T3 (TT3) nor rT3 in pups was affected by PFOS exposure. Gestational exposure to PFOS alone (TC) significantly (p< 0.05) decreased total T4 (TT4) level in pups on PNDs 21 and 35,20.3 and 19.4% lower than the control on the same PND, respectively. Having postnatal accumulation of PFOS, TT4 levels were 11.9,28.6, and 35.9% lower than controls on PNDs 14,21 and 35, respectively. No significant difference in TT4 level (p> 0.05) was observed between TC and CT on PND 21.(2) Thyroid gland colloid histomorphology paremeters and hepatic expression of genes involved in TH transport, metabolism and receptors were also investigated in rat pups at the age of various postnatal days under different conditions of PFOS exposure. The results demonstrate that none of the thyroid gland colloid histomorphology paremeters, including colloid area, longest axis, shortest axis, roundness and aspect ratio, was affected by PFOS. None of the selected TH related transcripts was affected by PFOS in pups on PND 0. Only transcript level of transthyretin, TH binding protein, in group TT significantly increased to 150% of the control on PND 21.(3) A subchronic PFOS exposure animal model was established. Male rats were exposed to 1.7,5.0 and 15.0 mg/L of PFOS in drinking water for 91 consecutive days. Thyroid and liver were removed for the measurement of the endpoints closely related to TH biosynthesis and metabolism following PFOS exposure. The results showed that serum TT4 level decreased significantly at all applied dosages. Hepatic UGT1A1 mRNA but not UGT1A6 was up-regulated at 5.0 and 15.0 mg/L of PFOS. Correlation coefficients revealed that hepatic UGT1A1 mRNA expression correlated with TT4 level (r=-0.754; p= 0.002). Treatment with PFOS lowered hepatic DIO1 mRNA at 15.0 mg/L but increased thyroidal DIO1 mRNA dose-dependently. The activity of TPO was not affected by PFOS. NIS and TSHR mRNA in thyroid increased slightly but not significantly. These results indicate that increased hepatic T4 glucuronidation via UGT1A1 and increased thyroidal conversion of T4 to T3 via DIO1 were in part responsible for PFOS-induced hypothyroxinemia in rats.(4) A 5 day PFOS exposure was to further identify the major factors contributing to PFOS-induced TH decrease in rats.The consequence of exposure to PTU was also analyzed as a positive control of TH inhibition. The animals were given either (1) vehicle; (2) PFOS; (3) PTU; or (4) PTU+PFOS once a day by gavage for 5 consecutive days. Parameters including contents of TT4 and TT3 in both serum and bile, serum concentrations of transthyretin and thyroglobulin, as well as transcripts of transporters involved in hepatic uptake and efflux of T4 were determined in control and PFOS exposed groups. TT4 and TT3 were also analyzed in PTU and PTU+PFOS groups to reflect the different hormone effects between PFOS, PTU, and PFOS+PTU. Results showed that serum TT4 and TT3 decreased, while bile TT4 and TT3 remained stable following PFOS exposure. Exposure to 3.0 mg/kg of PFOS enhanced hepatic organic anions transporter OATP2 mRNA expression (1.43 times of control). Treatment with PFOS increased hepatic expression of multidrug resistance-associated protein MRP2, approximately 1.80 and 1.69 times of control in 1.0 and 3.0 mg/kg groups, respectively. Spearman's correlation coefficients revealed that MRP2 mRNA expression correlated well with serum TT4 level (r=-0.528,p= 0.012). Serum thyroglobulin and transthyretin levels remained stable. In addition to serum TT4, PFOS and PTU groups demonstrated significantly difference with each other with respect to serum TT3, bile TT4, and bile TT3. No significant differences of TT4 and TT3 in both serum and bile were observed between PTU and PTU+PFOS (p> 0.05).In conclusion, gestational and lactational exposure to PFOS depresses TT4 in weaned rat to a similar extent. PFOS has little effects on the synthesis of TH. PFOS-induced TT4 deficiency is mainly due to increased hepatic uptake of T4 and enhanced hepatic metabolism and excretion of T4, which is probably different from the classic goitrogen, PTU.