Final Report: Endocrine Disruptors: Effects on the Thyroid

EPA Grant Number: R826297
Title: Endocrine Disruptors: Effects on the Thyroid
Investigators: Klaassen, Curtis D.
Institution: University of Kansas Medical Center
EPA Project Officer: Hahn, Intaek
Project Period: December 15, 1997 through December 14, 2000
Project Amount: $594,660
RFA: Endocrine Disruptors (1997) RFA Text |  Recipients Lists
Research Category: Endocrine Disruptors , Economics and Decision Sciences , Health , Safer Chemicals


The ultimate goal of this project was to assess the significance of endocrine disruptors that increase thyroxine (T4) glucuronidation on thyroid carcinogenesis because many endocrine disruptors are suspected to be thyroid tumor promoters. The mechanism by which endocrine disruptors promote thyroid tumors has been proposed to result from alterations in the thyroid-pituitary-hypothalamus axis. Endocrine disruptors alter the thyroid-pituitary-hypothalamus axis by increasing T4 glucuronidation and elimination, which reduces serum T4. As a compensatory feedback mechanism, thyroid stimulating hormone (TSH) will be released from the pituitary, which will stimulate the thyroid, and result in thyroid follicular cell proliferation and ultimately neoplasia. However, the preliminary studies suggest that a number of endocrine disruptors interfere with the normal hypothalamus-pituitary-thyroid axis because these endocrine disruptors do not increase serum TSH. Therefore, the central hypothesis of this application is that endocrine disruptors that increase T4 glucuronidation are thyroid tumor promoters only when they increase serum TSH.

Summary/Accomplishments (Outputs/Outcomes):

Polychlorinated biphenyl (PCB) mixtures and congeners effectively reduce circulating concentrations of thyroxine (T4). This is thought to occur because of their ability to induce the UDP-glucuronosyl transferases that conjugate T4 and the subsequent excretion of the glucuronide into bile. To determine whether there is a good correlation between the ability of PCBs to reduce T4 and increase the biliary excretion of T4 glucuronide, PCB congeners 95 (16 mg/kg), 99 (16 mg/kg), 118 (16 mg/kg), and 126 (40 µg/kg); Aroclors 1242 (32 mg/kg) and 1254 (32 mg/kg); and TCDD (3.9 µg/kg) were administered via gavage to male Sprague-Dawley rats for 7 days. Twenty-four hours after the last dose, the femoral artery and vein and the common bile duct were cannulated. Following administration of [125I] T4, bile was collected at 30-minute intervals for 2 hours. Blood was collected at the midpoint of each bile collection period. Urine also was collected at 2 hours. The total excretion of T4 and its metabolites was quantified by gamma spectrometry, followed by high performance liquid chromatography analysis. All seven treatments decreased the plasma concentration of T4. Of the pure congeners, PCB 99 and 118 produced the largest decreases in plasma T4 concentration, whereas PCB 95, PCB 126, and TCDD had the least effect. None of the seven treatments had a marked effect on the urinary excretion of T4 and its metabolites. In contrast, biliary excretion of T4 glucuronide after administration of TCDD, PCB 118, or Aroclor 1254 was increased six to eight-fold, PCB 126 produced a three to five-fold increase, whereas PCB 95 and PCB 99 produced less than a doubling. TCDD, one of the treatments that had the least effect on decreasing the concentration of T4 in blood, increased the biliary excretion the most. In contrast, PCB 99, one of the congeners that decreased the plasma concentration of T4 the most, had the least effect on its biliary excretion. Therefore, there does not appear to be a good correlation between the ability of PCBs to decrease plasma T4 concentration and increase its biliary excretion.

Circulating levels of thyroid hormones can be reduced by a number of compounds, including PCBs. Numerous studies have been conducted using mixtures of PCB congeners known as Aroclors, but little is known about the effects of the individual congeners. The objective of this study was to determine the effects of PCB congeners on serum levels of thyroxine (T4) and triiodothyronine (T3). PCB congeners are commonly divided into three general categories. The "2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) type" are those congeners that have affinity for the aryl hydrocarbon receptor (AhR) and induce cytochrome P450 1A1 (CYP1A1); the "Phenobarbital type" are those congeners that induce CYP2B1, and the "mixed type" are those congeners capable of induction of both CYP1A1 and CYP2B1. Four PCB congeners were selected for use in this study: PCB 126, a "TCDD type"; PCBs 95 and 99, which are "PB types"; and PCB 118, a "mixed type." Aroclors 1242 and 1254 and TCDD also were evaluated. Each compound was administered at a minimum of four dose levels via gavage to male Sprague-Dawley rats for 7 days. Blood was obtained via the retro orbital sinus on days –3, 3, and at necropsy on day 7. On day 7, serum T4 levels were reduced to 5 to 10 percent of control values in rats administered Aroclor 1254 and PCBs 99 and 118. Reductions up to 50 percent of control values occurred for rats administered each of the remaining compounds. Serum T3 levels were reduced to 30 percent of the control values in rats administered Aroclor 1254. The remaining compounds caused only slight or no reductions in serum T3. Results of this study suggest that PCB congeners may induce the majority of their effects on thyroid functions via pathways that are not mediated by the AhR.

PCB mixtures and congeners have been shown to markedly reduce serum concentrations of T4. This is thought to occur, in part, by induction of hepatic UDP-glucuronosyl-transferases that conjugate T4, and subsequent excretion of T4-glucuronide into bile. However, in an attempt to determine other mechanisms by which PCB treatment decreases serum T4 in rats, we examined the disappearance of [125I]-T4 from the serum of rats that received PCBs daily for 7 days. Male Sprague-Dawley rats were administered PCB mixtures Aroclor 1254 (32 mg/kg/day), Aroclor 1242 (32 mg/kg/day) or individual congeners PCB 95 or PCB 99 (16 mg/kg/day, phenobarbital-like congeners), PCB 118 (16 mg/kg/day, a mixed-type congener), PCB 126 (40 µg/kg/day, a dioxin-like congener), or TCDD (3.9 µg/kd/day) in corn oil via oral gavage. Control rats received corn oil. Twenty-four hours after the last dose, [125I]-T4 was administered intravenously and blood was sampled at 15, 45, 75, and 105 minutes postinjection. The presence of [125I]-T4 in serum was quantified by gamma spectrometry. Within 15 minutes after injection of [125I]-T4, serum T4 levels were decreased approximately 75 percent in rats administered PCB 99, PCB 118, or Aroclor 1254, and 50 percent in rats administered Aroclor 1242, PCB 95, or PCB 126, with respect to control rats. There was no significant decrease in serum T4 in TCDD-treated rats. Serum concentrations of T4 continued to be significantly reduced in all of the PCB-treated rats at all subsequent time points. The disappearance of T4 from the serum of PCB-treated rats appears to be too rapid to be explained solely by increased glucuronidation and biliary excretion. This suggests that other mechanism(s), in addition to glucuronidation and biliary excretion, may be involved in the decreased serum T4 elicited by PCB exposure.

Transfer of thyroxine (T4) across plasma membranes is accomplished, in part, via transport proteins. Two proteins possibly involved in this process are organic anion transporting polypeptides Oatp2 and Oatp3. Additionally, Mrp2, a member of the superfamily of ATP binding cassette transporters, is implicated in the excretion of T4-glucuronide (T4-G) into bile. The purpose of this study was to determine whether PCBs, which markedly reduce serum T4, might do so by enhancing the hepatic uptake of T4, and subsequent excretion into bile, via increases in the expression of these proteins. Male rats were administered Aroclor 1254 or Aroclor 1242 for 7 consecutive days and 24 hours after the last dose, [125I]-T4 was administered intravenously. At 1, 3, 5, 10, 15, and 30 minutes postinjection, blood and liver were sampled. The [125I]-T4 present in serum and liver was quantified by gamma spectrometry, and transporter protein levels in liver were determined by Western blot analysis. Within 1 minute postinjection, Aroclor 1254 produced a 56 percent decrease in serum [125I]-T4 and Aroclor 1242 a 30 percent reduction within 3 minutes, as compared to controls. At the 1 minute timepoint, Aroclor 1254 and Aroclor 1242 produced a 105 percent and a 54 percent increase, respectively, in the uptake of [125I]-T4 into liver, as compared to the control value. Oatp2 protein levels were suppressed 87 percent by Aroclor 1254, compared to the control, but Aroclor 1242 had no significant effect on Oatp2. No significant changes in Oatp3 protein levels were observed. In contrast, Aroclor 1254 and Aroclor 1242 produced significant increases in Mrp2 protein levels of 108 percent and 92 percent, respectively, relative to control. These data suggest that the increase in hepatic uptake of T4 in rats exposed to PCBs cannot be explained by upregulation of Oatp2 and Oatp3. However, upregulation of Mrp2 may contribute to increased biliary excretion of T4-G.

Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other project views: All 5 publications 2 publications in selected types All 2 journal articles
Type Citation Project Document Sources
Journal Article Martin LA, Gallo MA, Ruehl KR, Klassen CD. Rapid dissappearance of intravenously administered [125]I-thyroxine (T4) from serum following repeated administration of polychlorinated biphenyls (PCBs). Toxicologist 2001;60:380-381 R826297 (Final)
not available
Journal Article Martin LA. Differential effects of polychlorinated biphenyl (PCB) mixtures and congeners on the disposition of thyroxine(T4). Toxicol.Sci. 2002;60:Abstr 1314 R826297 (Final)
not available

Supplemental Keywords:

thyroid, polycyclic aromatic hydrocarbons, polychlorinated byphenyls, PCBs, endocrine disruptors., RFA, Health, Scientific Discipline, Toxics, Environmental Chemistry, Health Risk Assessment, Chemistry, Endocrine Disruptors - Environmental Exposure & Risk, HAPS, endocrine disruptors, Biochemistry, Children's Health, Biology, Endocrine Disruptors - Human Health, adverse outcomes, toxicokinetic, DHPN, PCBs, Polychlorinated biphenyls (Aroclors), pharmacokinetics, animal models, developmental processes, mice, polychlorinated biphenyls, tumors, carcinogens, models, thyroid function, pituitary, biological effects

Progress and Final Reports:

Original Abstract
  • 1998
  • 1999 Progress Report