Increased Biliary Excretion of Thyroxine by Microsomal Enzyme Inducers

doi:10.1006/taap.2001.9278

Toxicology and Applied Pharmacology

Volume 176, Issue 3, 1 November 2001, Pages 187-194

https://doi.org/10.1006/taap.2001.9278 Get rights and content

Abstract

The extrathyroidal mechanism by which endocrine disruptors promote thyroid tumors has been proposed to be increased glucuronidation and biliary elimination of thyroxine (T₄), followed by disruption of the hypothalamic—pituitary–thyroid axis. The ability of a chemical to increase T₄ glucuronidation in vitro correlates with the ability to reduce serum T₄ concentrations. Pregnenolone-16α-carbonitrile (PCN), 3-methylchloranthrene (3-MC), and Aroclor 1254 (PCB) each increase T₄ glucuronidation in rat liver microsomes and reduce serum T₄. However, whether reductions in serum T₄ result directly from increases in T₄ glucuronidation and biliary excretion in vivo has not been thoroughly examined. It is also unclear whether reduced serum T₄ concentrations following microsomal enzyme inducer treatment elicit increases in serum thyrotropin (TSH), the primary stimulus for thyroid cell proliferation, because only PCN treatment increases serum TSH. This study sought to determine whether increases in T₄-glucuronide biliary excretion in vivo are responsible for reductions in serum T₄ and increases in serum TSH. Male rats were fed control diet or diet containing either 1000 ppm PCN, 250 ppm 3-MC, or 100 ppm PCB for 7 days. Animals were then given [¹²⁵I]T₄ iv, and bile was collected for 2 h. Radiolabeled metabolites in bile were analyzed by reverse-phase HPLC with γ-detection. PCN, 3-MC, and PCB treatments reduced serum T₄ concentrations by 42, 45, and 73%, respectively, while TSH was only increased by PCN (180%). The biliary excretion of [¹²⁵I]thyronines was increased 103% by PCN, 157% by 3-MC, and 193% by PCB. T₄-glucuronide was the primary metabolite in bile, accounting for up to 86% of the radiolabeled metabolites in controls. The amount of T₄-glucuronide excreted in bile was increased 161% and 226% by 3-MC and PCB, respectively, but was only increased 55% by PCN. None of the treatments had any effect on the urinary excretion of [¹²⁵I]T₄. Thus, increased glucuronidation and biliary excretion of T₄ appears likely to be responsible for reductions in serum T₄ produced by microsomal enzyme inducers. Furthermore, increases in T₄ biliary excretion produced by microsomal enzyme inducer treatment are not consistent with changes in TSH. Thus, it can be concluded that differential changes in serum TSH do not stem from differential increases in T₄ biliary excretion.

References (48)

R.A. Barter et al.
UDP-glucuronosyltransferase inducers reduce thyroid hormone levels in rats by an extrathyroidal mechanism
Toxicol. Appl. Pharmacol.
(1992)
R.A. Barter et al.
Rat liver microsomal UDP-glucuronosyltransferase activity toward thyroxine: Characterization, induction, and form specificity
Toxicol. Appl. Pharmacol.
(1992)
K.R. Chauhan et al.
Assessing the role of ortho-substitution on polychlorinated biphenyl binding to transthyretin, a thyroxine transport protein
Toxicol. Appl. Pharmacol.
(2000)
C.P. Comer et al.
Changes in thyroidal function and liver UDPglucuronosyltransferase activity in rats following administration of a novel imidazole (SC-37211)
Toxicol. Appl. Pharmacol.
(1985)
E.C. Henry et al.
Changes in thyroid hormones and thyroxine glucuronidation in hamsters compared with rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin
Toxicol. Appl. Pharmacol.
(1987)
A. Hood et al.
Effects of microsomal enzyme inducers on thyroid-follicular cell proliferation, hyperplasia, and hypertrophy
Toxicol. Appl. Pharmacol.
(1999)
A. Hood et al.
Effects of microsomal enzyme inducers on outer-ring deiodinase activity toward thyroid hormones in various rat tissues
Toxicol. Appl. Pharmacol.
(2000)
N. Masubuchi et al.
Effects of proton pump inhibitors on thyroid hormone metabolism in rats: A comparison of UDP-glucuronyltransferase induction
Biochem. Pharmacol.
(1997)
R.M. McClain et al.
The effect of phenobarbital on the metabolism and excretion of thyroxine in rats
Toxicol. Appl. Pharmacol.
(1989)
D.E. Semler et al.
The effects of chronic ingestion of spironolactone on serum thyrotropin and thyroid hormones in the male rat
Toxicol. Appl. Pharmacol.
(1989)

A.J. Sonderfan et al.

Inhibition of steroid 5 alpha-reductase and its effects on testosterone hydroxylation by rat liver microsomal cytochrome P-450

Arch. Biochem. Biophys.

(1988)

J.A. vanRaaij et al.

Increased glucuronidation of thyroid hormone in hexachlorobenzene-treated rats

Biochem. Pharmacol.

(1993)

T.J. Visser et al.

Glucuronidation of thyroid hormone by human bilirubin and phenol UDP-glucuronyltransferase isoenzymes

FEBS Lett.

(1993)

T.J. Visser et al.

Multiple UDP-glucuronyltransferases for the glucuronidation of thyroid hormone with preference for 3,3′,5′-triiodothyronine (reverse T3)

FEBS Lett.

(1993)

D.J. Waxman

P450 gene induction by structurally diverse xenochemicals: Central role of nuclear receptors CAR, PXR, and PPAR

Arch. Biochem. Biophys.

(1999)

A.G.E. Wilson et al.

Mode of action of thyroid tumor formation in the male Long-Evans rat administered high doses of alachlor

Fundam. Appl. Toxicol.

(1996)

M. Abe et al.

Cell-associated activation of latent transforming growth factor-beta by calpain

J. Cell. Physiol.

(1998)

C.H. Bastomsky

Effects of a polychlorinated biphenyl mixture (Aroclor 1254) and DDT on biliary thyroxine excretion in rats

Endocrinology

(1974)

C.H. Bastomsky

Enhanced thyroxine metabolism and high uptake goiters in rats after a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Endocrinology

(1977)

C.H. Bastomsky et al.

The effect of methylcholanthrene on biliary thyroxine excretion in normal and Gunn rats

J. Endocrinol.

(1973)

J.B. Beetstra et al.

Thyroxine and 3,3′,5-triiodothyronine are glucuronidated in rat liver by different uridine diphosphate-glucuronyltransferases

Endocrinology

(1991)

A. Brouwer

Inhibition of thyroid hormone transport in plasma of rats by polychlorinated biphenyls

Arch. Toxicol. Suppl.

(1989)

C.C. Capen

Mechanistic data and risk assessment of selected toxic end points of the thyroid gland

Toxicol. Pathol.

(1997)

Cited by (45)

Practical considerations for developmental thyroid toxicity assessments: What's working, what's not, and how can we do better?
2019, Regulatory Toxicology and Pharmacology
Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.
Human relevance of follicular thyroid tumors in rodents caused by non-genotoxic substances
2018, Regulatory Toxicology and Pharmacology
Citation Excerpt :
The hydrophilic conjugates are readily excreted via the urine and bile, resp. ( Vansell and Klaassen, 2001; Visser et al., 1998). Deiodination of the thyroid hormones is qualitatively similar in animals and humans (see Fig. 1).
Chronic stimulation of the thyroid gland of rodents by TSH leads to thyroid follicular hyperplasia and subsequently to thyroid follicular adenomas and carcinomas. However, the interpretations of rodent thyroid tumors are contradictory. The U.S. Food and Drug Administration (FDA) concluded that findings with drugs that lead to increased levels of thyroid-stimulating hormone (TSH) in rats are not relevant to humans, whereas the U.S. Environmental Protection Agency (US EPA) concluded that chemicals that produce rodent thyroid tumors may pose a carcinogenic hazard for humans although the thyroid of rodents appears to be more sensitive to a carcinogenic stimulus than that of humans. Meanwhile, based on the CLP Criteria of the European Chemicals Agency (ECHA), rodent thyroid tumors caused by the induction of uridine-diphosphate-glucuronosyl transferases (UDGT) were assessed as not relevant to humans.
To clarify these discrepant positions, the function and regulation of the thyroid gland are described and the types of thyroid tumors and the causes of their development in humans and animals are examined. Based on these data and the evidence that so far, except radiation, no chemical is known to increase the incidence of thyroid tumors in humans, it is concluded that rodent thyroid tumors resulting from continuous stimulation of the thyroid gland by increased TSH levels are not relevant to humans. Consequently, compounds that induce such tumors do not warrant classification as carcinogenic.
Intracellular toxicity exerted by PCBs and role of VBNC bacterial strains in biodegradation
2018, Ecotoxicology and Environmental Safety
Polychlorinated biphenyls (PCBs) are xenobiotic compounds that persists in the environment for long-term, though its productivity is banned. Abatement of the pollutants have become laborious due to it's recalcitrant nature in the environment leading to toxic effects in humans and other living beings. Biphenyl degrading bacteria co-metabolically degrade low chlorinated PCBs using the active metabolic pathway. bph operon possess different genetic arrangements in gram positive and gram negative bacteria. The binding ability of the genes and the active sites were determined by PCB docking studies. The active site of bphA gene with conserved amino acid residues determines the substrate specificity and biodegradability. Accumulation of toxic intermediates alters cellular behaviour, biomass production and downturn the metabolic activity. Several bacteria in the environment attain unculturable state which is viable and metabolically active but not cultivable (VBNC). Resuscitation-promoting factor (Rpf) and Rpf homologous protein retrieve the culturability of the so far uncultured bacteria. Recovery of this adaptive mechanism against various physical and chemical stressors make a headway in understanding the functionality of both environmental and medically important unculturable bacteria. Thus, this paper review about the general aspects of PCBs, cellular toxicity exerted by PCBs, role of unculturable bacterial strains in biodegradation, genes involved and degradation pathways. It is suggested to extrapolate the research findings on extracellular organic matters produced in culture supernatant of VBNC thus transforming VBNC to culturable state.
Image analysis for TSH mRNA in situ hybridization in pituitary glands from rats with thyroid follicular cell hypertrophy after treatment with three different test compounds
2017, Research in Veterinary Science
The goal of this in situ hybridization and image analysis technique is to study the effects of new pharmacological/chemical entities on the thyroid and pituitary gland in rats, reveal the pathogenesis of thyroid follicular cell hypertrophy and to retrospectively exclude the risk of thyroid tumor development in humans.
In the present study, we describe the increase of thyroid-stimulating hormone- (TSH-) beta subunit mRNA in the pars distalis of the pituitary gland and the quantitative measurement of TSH mRNA positive cells from rats of three 4-week toxicity studies treated with three different test compounds inducing thyroid follicular cell and hepatocellular hypertrophy in rats. Compared to immunohistochemistry (IHC), in situ hybridization (ISH) for TSH was found to be more sensitive.
With this technique we are able to exclude a direct effect of the test compound on the thyroid gland by showing the activation of thyrotrope cells from the pituitary gland and therefore this technique retrospectively enables us to exclude a possible risk for humans at an early stage of drug development. Also in case blood serum samples for evaluation of TSH are not available anymore or hepatocellular hypertrophy is not present (close metabolic relationship between thyroid gland and liver in rodents), the described method allows retrospective investigations on thyroid follicular cell hypertrophy or hyperplasia. This can be of high relevance in human safety assessment for certain drugs in order to exclude a primary effect on the thyroid gland especially when it comes to thyroid neoplasia in rodents as previously described.
Evidence for triclosan-induced activation of human and rodent xenobiotic nuclear receptors
2013, Toxicology in Vitro
Citation Excerpt :
Specifically, hepatic mRNA expression levels of Cyp2b2, Cyp3a1/23, Ugt1a1, and Sult1c1, as well as hepatic microsomal uridine diphosphate glucuronyltransferase (UGT) activity and pentoxyresorufin-O-deethylase (PROD) activity, were induced by TCS exposure (Paul et al., 2010b). These effects are consistent with other microsomal enzyme inducers that are thought to decrease thyroid hormones at least partially via up-regulation of hepatic catabolism (Barter and Klaassen, 1994; Chen et al., 2003; Hood et al., 2003; Vansell and Klaassen, 2002, 2001). In aggregate, the data suggest that TCS may activate CAR and/or PXR in the rat.
The bacteriostat triclosan (2,4,4′-trichloro-2′-hydroxydiphenylether) (TCS) decreases rat serum thyroxine via putative nuclear receptor (NR) interaction(s) and subsequent transcriptional up-regulation of hepatic catabolism and clearance. However, due to the evolutionary divergence of the constitutive androstane and pregnane-X receptors (CAR, PXR), TCS-mediated downstream effects may be species-dependent. To test the hypothesis that TCS activates xenobiotic NRs across species, cell-based NR reporter assays were employed to assess potential activation of rat, mouse, and human PXR, and rat, mouse, and three splice variants of human CAR. TCS activated hPXR, acted as an inverse agonist of hCAR1, and as a weak agonist of hCAR3. TCS failed to activate rPXR in full-length receptor reporter assays, and instead acted as a modest inverse agonist of rCAR. Consistent with the rat data, TCS also failed to activate mPXR and was a modest inverse agonist of mCAR. These data suggest that TCS may interact with multiple NRs, including hPXR, hCAR1, hCAR3, and rCAR in order to potentially affect hepatic catabolism. Overall these data support the conclusion that TCS may interact with NRs to regulate hepatic catabolism and downstream thyroid hormone homeostasis in both rat and human models, though perhaps by divergent mechanisms.
Developmental triclosan exposure decreases maternal, fetal, and early neonatal thyroxine: A dynamic and kinetic evaluation of a putative mode-of-action
2012, Toxicology
Citation Excerpt :
In weanling female rats, TCS exposure for 4 days decreased serum T4 and up-regulated markers of hepatic catabolism, including uridine diphosphate glucuronyltransferase (UGT) activity, and Ugt1a1 and Sult1c1 expression (Paul et al., 2010b). The UGT 1A subfamily, specifically Ugt1a1 and Ugt1a6, and SULTs 1b1 and 1c, conjugate T4 in rat liver (Kester et al., 2003; Vansell and Klaassen, 2002), and increased activity of these enzymes previously has been demonstrated to correlate with decreased systemic T4 concentrations in rodents (Barter and Klaassen, 1994; Buckley and Klaassen, 2009; Cheng et al., 2005; Guo et al., 2002; Johnson and Klaassen, 2002; Kohn, 2000; Lecureux et al., 2009; Schuur et al., 1997; Vansell and Klaassen, 2001, 2002; Visser et al., 1993; Wong et al., 2005). In addition, TCS increased hepatic Cyp2b1 and Cyp3a1 expression, suggestive of a hepatic nuclear receptor-mediated increase in T4 catabolism (Kretschmer and Baldwin, 2005).
This work tests the mode-of-action (MOA) hypothesis that maternal and developmental triclosan (TCS) exposure decreases circulating thyroxine (T4) concentrations via up-regulation of hepatic catabolism and elimination of T4. Time-pregnant Long-Evans rats received TCS po (0–300 mg/kg/day) from gestational day (GD) 6 through postnatal day (PND) 21. Serum and liver were collected from dams (GD20, PND22) and offspring (GD20, PND4, PND14, PND21). Serum T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) concentrations were measured by radioimmunoassay. Ethoxy-O-deethylase (EROD), pentoxyresorufin-O-depentylase (PROD) and uridine diphosphate glucuronyltransferase (UGT) enzyme activities were measured in liver microsomes. Custom Taqman^® qPCR arrays were employed to measure hepatic mRNA expression of select cytochrome P450s, UGTs, sulfotransferases, transporters, and thyroid hormone-responsive genes. TCS was quantified by LC/MS/MS in serum and liver. Serum T4 decreased approximately 30% in GD20 dams and fetuses, PND4 pups and PND22 dams (300 mg/kg/day). Hepatic PROD activity increased 2–3 fold in PND4 pups and PND22 dams, and UGT activity was 1.5 fold higher in PND22 dams only (300 mg/kg/day). Minor up-regulation of Cyp2b and Cyp3a expression in dams was consistent with hypothesized activation of the constitutive androstane and/or pregnane X receptor. T4 reductions of 30% for dams and GD20 and PND4 offspring with concomitant increases in PROD (PND4 neonates and PND22 dams) and UGT activity (PND22 dams) suggest that up-regulated hepatic catabolism may contribute to TCS-induced hypothyroxinemia during development. Serum and liver TCS concentrations demonstrated greater fetal than postnatal internal exposure, consistent with the lack of T4 changes in PND14 and PND21 offspring. These data support the MOA hypothesis that TCS exposure leads to hypothyroxinemia via increased hepatic catabolism; however, the minor effects on thyroid hormone metabolism may reflect the low efficacy of TCS as thyroid hormone disruptor or highlight the possibility that other MOAs may also contribute to the observed maternal and early neonatal hypothyroxinemia.

View all citing articles on Scopus

^☆: Supported by NIH Grant ES-008156.

²: Supported by NIH Training Grant ES-07079.

View full text

Regular ArticleIncreased Biliary Excretion of Thyroxine by Microsomal Enzyme Inducers☆

Abstract

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Biochem. Pharmacol.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Arch. Biochem. Biophys.

Biochem. Pharmacol.

FEBS Lett.

FEBS Lett.

Arch. Biochem. Biophys.

Fundam. Appl. Toxicol.

Cell-associated activation of latent transforming growth factor-beta by calpain

J. Cell. Physiol.

Effects of a polychlorinated biphenyl mixture (Aroclor 1254) and DDT on biliary thyroxine excretion in rats

Endocrinology

Enhanced thyroxine metabolism and high uptake goiters in rats after a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Endocrinology

The effect of methylcholanthrene on biliary thyroxine excretion in normal and Gunn rats

J. Endocrinol.

Thyroxine and 3,3′,5-triiodothyronine are glucuronidated in rat liver by different uridine diphosphate-glucuronyltransferases

Endocrinology

Inhibition of thyroid hormone transport in plasma of rats by polychlorinated biphenyls

Arch. Toxicol. Suppl.

Mechanistic data and risk assessment of selected toxic end points of the thyroid gland

Toxicol. Pathol.

Regular Article
Increased Biliary Excretion of Thyroxine by Microsomal Enzyme Inducers☆