Differential expression and ciprofibrate induction of hepatic UDP-glucuronyltransferases for thyroxine and triiodothyronine in Fischer rats

Xanthohumol, a prenylated chalcone from hops, modulates hepatic expression of genes involved in thyroid hormone distribution and metabolism

In the present study, we analyzed the influence of xanthohumol (XN) on thyroid hormone (TH) distribution and metabolism in rats. A potent and selective competition of XN for thyroxine (T4) binding to transthyretin (IC(50)=1 microM at 1.7 nM [(125)I]T4) was found in human and rat sera in vitro. Female rats treated orally with XN showed increased hepatic expression of T4-binding globulin and decreased transthyretin and albumin. Thyrotropin levels and hepatic type 1 deiodinase activity were moderately increased. Northern blot analysis revealed diminished expression of liver sulfotransferase (Sult1a1) and uridine-diphosphate glucuronosyltransferase (Ugt1a1) after XN treatment. The transcript levels of constitutive androstane receptor (CAR), known to be involved in regulation of enzymes metabolizing hormones, drugs and xenobiotics, was lower in rats treated with >10 mg XN/kg body weight per day. Immunoblot analysis indicates reduced amounts of CAR protein. The phenobarbital-inducible cytochrome P450 mRNA level was decreased in rats treated with >10 mg XN/kg/day, in agreement with reduced CAR protein. Although only moderate changes in TH serum levels were observed, the XN-dependent altered expression of components involved in TH homeostasis might be important not only for hormone metabolism, but also for hepatic phase I and II elimination of drug metabolites and xenobiotics.

Role of UDP-glucuronosyltransferase (UGT) 2B2 in metabolism of triiodothyronine: effect of microsomal enzyme inducers in Sprague Dawley and UGT2B2-deficient Fischer 344 rats

Microsomal enzyme inducers (MEI) that increase UDP-glucuronosyltransferases (UGTs) can impact thyroid hormone homeostasis in rodents. Increased glucuronidation can result in reduction of serum thyroid hormone and a concomitant increase in thyroid-stimulating hormone (TSH). UGT2B2 is thought to glucuronidate triiodothyronine (T(3)). The purposes of this study were to determine the role of UGT2B2 in T(3) glucuronidation and whether increased T(3) glucuronidation mediates the increased TSH observed after MEI treatment. Sprague Dawley (SD) and UGT2B2-deficient Fischer 344 (F344) rats were fed a control diet or diet containing pregnenolone-16alpha-carbonitrile (PCN; 800 ppm), 3-methylcholanthrene (3-MC; 200 ppm), or Aroclor 1254 (PCB; 100 ppm) for 7 days. Serum thyroxine (T(4)), T(3), and TSH concentrations, hepatic androsterone/T(4)/T(3) glucuronidation, and thyroid follicular cell proliferation were determined. In both SD and F344 rats, MEI treatments decreased serum T(4), whereas serum T(3) was maintained (except with PCB treatment). Hepatic T(4) glucuronidation increased significantly after MEI in both rat strains. Compared with the other MEI, only PCN treatment significantly increased T(3) glucuronidation (281 and 497%) in both SD and UGT2B2-deficient F344 rats, respectively, and increased both serum TSH and thyroid follicular cell proliferation. These data demonstrate an association among increases in T(3) glucuronidation, TSH, and follicular cell proliferation after PCN treatment, suggesting that T(3) is glucuronidated by other PCN-inducible UGTs in addition to UGT2B2. These data also suggest that PCN (rather than 3-MC or PCB) promotes thyroid tumors through excessive TSH stimulation of the thyroid gland.

Vitamin A modulates the effects of thyroid hormone on UDP-glucuronosyltransferase expression and activity in rat liver

We studied the influence of thyroid hormones and vitamin A status on the regulation of UDP-glucuronosyltransferase (UGT) expression and the glucuronidation of thyroid hormones by UGTs. For this, we used an original model of rats fed with different vitamin A diets and implanted subcutaneously by osmotic minipumps delivering vehicle or thyroid hormones, which permitted the control of plasma thyroid hormone concentrations. The activity and expression of family 1 UGTs are correlated and were significantly modified by both thyroid status and amounts of retinol in the diet. Dietary vitamin A did not perturbe the UGT1A expression in thyroidectomized animals. Thyroid hormones and dietary vitamin A did not affect the activity and expression of family 2 UGTs. We conclude that thyroid hormones and vitamin A are co-regulator of the UGT1 family expression, without affecting the UGT2 family; by modifying activity and expression of the bilirubin UOT isoform, a member of UGT1 family, thyroid hormone reduced the glucuronidation of T4 and rT3.

Role of thyroid hormones in hepatic effects of peroxisome proliferators

Peroxisome proliferators are endocrine disrupting chemicals that cause liver tumors in rodents but not humans. Although the receptor that mediates key hepatic effects, the peroxisome proliferator-activated receptor alpha (PPAR-alpha), and its endogenous ligands have been identified, the mechanism whereby these commonly used chemicals cause liver tumors in rodents has yet to be elucidated. Species differences in PPAR-alpha and DNA response elements may explain some of the variability in response upon exposure to peroxisome proliferators. The possibility that thyroid-modulating effects of peroxisome proliferators may contribute to the hepatic effects of peroxisome proliferators has yet to be fully explored. When the potent peroxisome proliferator, WY-14,643, was given to hypothyroid rats, there was a blunting of the hepatomegaly and hepatocyte proliferative responses seen in thyroid-intact animals. Acyl-CoA oxidase activity was unaltered by changes in thyroid hormone status. In addition, preliminary evidence indicates that peroxisome proliferators increased hepatic thyroid receptor (TRalpha1) expression, but TRalpha1 levels in liver tumors were similar to those in unexposed animals. Significant differences between humans and rodents with respect to thyroid hormone physiology and metabolism, in conjunction with the results of these studies, may be indicative of yet another mechanism to explain differential sensitivity to hepatic effects of peroxisome proliferators.

Extraperoxisomal targets of peroxisome proliferators: mitochondrial, microsomal, and cytosolic effects. Implications for health and disease

Peroxisome proliferators are a structurally diverse group of compounds that include the fibrate hypolipidemic drugs, the phthalate ester industrial plasticizers, the phenoxy acid herbicides, and the anti-wetting corrosion inhibitors perfluorinated straight-chain monocarboxylic fatty acids. Administration of these chemicals to rodents results in a number of effects, the most prominent being hepatomegaly and induction of peroxisomal enzyme activities. Several of these compounds have also been associated with the production of liver tumors in rodents and are classified as nongenotoxic hepatocarcinogens. Experimental evidence suggests that humans are not susceptible to these effects following exposure to peroxisome-proliferating compounds. This has led to the proposal that an "actual threat to humans" from exposure to one of these compounds seems "rather unlikely". Indeed, recent reports suggest that peroxisome proliferators may prove valuable as antitumor agents in humans. However, this assessment is preliminary given that peroxisome proliferators also produce a myriad of extraperoxisomal effects in livers and other tissues of experimental animals. Such effects include both stimulation and inhibition of mitochondrial and microsomal metabolism and alteration of the activities of various cytosolic enzymes. These responses may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether the extraperoxisomal effects of these compounds occur in humans is not known and their potential impact on human health remains to be investigated.

Effects of proton pump inhibitors on thyroid hormone metabolism in rats: a comparison of UDP-glucuronyltransferase induction

The effects of proton pump inhibitors on thyroid hormone metabolism in rats were examined. Pantoprazole, omeprazole, and lansoprazole were administered repeatedly to female SD rats at doses of 5, 50, and 300 mg/kg/day for 1 week, and changes in UDP-glucuronyltransferase activities were examined. Increases in o-aminophenol UDP-glucuronyltransferase activity, which was measured as that responsible for the glucuronidation of thyroxine, were evident following 7-day high-dose administration of all the proton pump inhibitors tested. Of the three proton pump inhibitors investigated, o-aminophenol UDP-glucuronyltransferase activity was greatest following the high-dose administration of omeprazole. Androsterone UDP-glucuronyltransferase activity in rats treated with the proton pump inhibitors increased significantly, but these increases were smaller than those of o-aminophenol UDP-glucuronyltransferase. Pantoprazole and omeprazole treatment did not affect plasma T4 or T3 significantly, whereas lansoprazole treatment produced marked reductions in plasma T4 but did not affect plasma T3 significantly. After administration of 125I-labeled thyroid hormone to rats treated with the proton pump inhibitors, biliary excretion of radioactivity increased significantly in omeprazole- and lansoprazole-treated rats; these increases were attributed to induction of liver thyroxine UDP-glucuronyltransferase activities. The order of biliary excretion of radioactivity, as well as the o-aminophenol UDP-glucuronyltransferase activity, in the treated animals was: omeprazole > lansoprazole > pantoprazole. Therefore, repeated administration of the proton pump inhibitors increased thyroxine-metabolizing activity via induction of UDP-glucuronyltransferase, and this induction by pantoprazole was less pronounced than that by omeprazole or lansoprazole.

Transcriptional regulation by triiodothyronine of the UDP-glucuronosyltransferase family 1 gene complex in rat liver. Comparison with induction by 3-methylcholanthrene

This study demonstrates that the expression of the phenol UDP-glucuronosyltransferase 1 gene (UGT1A1) is regulated at the transcriptional level by thyroid hormone in rat liver. Following 3,5, 3'-triiodo-L-thyronine (T3) stimulation in vivo, there is a gradual increase in the amount of UGT1A1 mRNA with maximum levels reached 24 h after treatment. In comparison, induction with the specific inducer, 3-methylcholanthrene (3-MC), results in maximal levels of UGT1A1 mRNA after 8 h of treatment. In primary hepatocyte cultures, the stimulatory effect of both T3 and 3-MC is also observed. This induction is suppressed by the RNA synthesis inhibitor actinomycin D, indicating that neither inducer acts at the level of mRNA stabilization. Indeed, nuclear run-on assays show a 3-fold increase in UGT1A1 transcription after T3 treatment and a 6-fold increase after 3-MC stimulation. This transcriptional induction by T3 is prevented by cycloheximide in primary hepatocyte cultures, while 3-MC stimulation is only partially affected after prolonged treatment with the protein synthesis inhibitor. Together, these data provide evidence for a transcriptional control of UGT1A1 synthesis and indicate that T3 and 3-MC use different activation mechanisms. Stimulation of the UGT1A1 gene by T3 requires de novo protein synthesis, while 3-MC-dependent activation is the result of a direct action of the compound, most likely via the aromatic hydrocarbon receptor complex.

Ion-pair high-performance liquid chromatographic separation of two thyroxine glucuronides formed by rat liver microsomes

A simple reversed-phase ion-pair high-performance liquid chromatographic separation method has been developed for thyroxine (T4) and its glucuronide metabolites formed by liver microsomes of untreated and 3-methylcholanthrene-treated rats. Besides the phenol-T4-glucuronide, another, probably acyl-T4-glucuronide, formation has been detected. The effect of pH and temperature on the stability of the acyl-T4-glucuronide was also investigated. The lowering of pH to 2 and cooling the samples to 5 degrees C is necessary to prevent the hydrolysis of acylglucuronide, while both pH and temperature do not affect the stability of the phenol-T4-glucuronide. The retention times of T4 and phenol-T4-glucuronide are highly influenced by the pH of the mobile phase, but not that of acyl-T4-glucuronide.

Effects of thyroid status and thyrostatic drugs on hepatic glucuronidation of lodothyronines and other substrates in rats : Induction of phenol UDP-glucuronyltransferase by methimazole

Glucuronidation of iodothyronines in rat liver is catalyzed by at least three UDP-glucuronyltransferases (UGTs): bilirubin UGT, phenol UGT, and androsterone UGT. Bilirubin and phenol UGT activities are regulated by thyroid hormone, but the effect of thyroid status on hepatic glucuronidation of iodothyronines is unknown. We examined the effects of hypothyroidism induced by treatment of rats with propylthiouracil (PTU) or methimazole (MMI) or by thyroidectomy as well as the effects of T4-induced hyperthyroidism on the hepatic UGT activities for T4, T3, bilirubin,p-nitrophenol (PNP), and androsterone. Bilirubin UGT activity was increased in MMI- or PTU-induced hypothyroid and thyroidectomized rats, and decreased in hyperthyroid animals. T4 and, to a lesser extent, T3 UGT activities were increased in MMI- or PTU-induced hypothyroid rats, and T4 but not T3 glucuronidation also showed a significant increase in thyroidectomized rats. T4 but not T3 UGT activity was slightly decreased in hyperthyroid rats. While PNP UGT activity was decreased in thyroidectomized rats and increased in hyperthyroid animals, it was also markedly increased by MMI and slightly increased by PTU-induced hypothyroidism. In T4-substituted rats, MMI did not affect T4, T3, bilirubin and androsterone UGT activities but again strongly induced PNP UGT activity, indicating that this represented a direct induction of PNP UGT by the drug independent of its thyrostatic action. Androsterone UGT activity was hardly affected by thyroid status. Our results suggest a modest, negative control of the hepatic glucuronidation of thyroid hormone by thyroid status, which may be mediated by changes in bilirubin UGT activity. To our knowledge, this is the first report of the marked induction of a hepatic enzyme by MMI, which is not mediated by its thyroid hormone-lowering effect.

The relative hypolipidaemic activity and hepatic effects of ciprofibrate enantiomers in the rat

The aim of this study was to establish whether the individual enantiomers of racemic ciprofibrate, a potent hypolipidaemic agent and peroxisome proliferator, differ significantly in either pharmacological potency or toxic potential. After a single oral dose to male Fischer F344 rats at dosages below 10 mg/kg, S(-) ciprofibrate produced slightly, but statistically significantly, greater reductions in plasma concentrations of cholesterol than R(+) ciprofibrate. Similarly, at low concentrations in F344 rat hepatocyte cultures, S(-) ciprofibrate produced slightly, but statistically significantly, greater inductions of peroxisomal beta-oxidation activity than R(+) ciprofibrate. However, after seven daily doses, the differences in pharmacological effects of the two enantiomers were no longer apparent. Furthermore, in contrast to its effects in vitro, R(+) ciprofibrate produced slightly, but statistically significantly, greater inductions of peroxisomal beta-oxidation activity in vivo than S(-) ciprofibrate. These observations may be possibly explained on the basis that following multiple dosing, plasma concentrations of R(+) ciprofibrate 24 hr post-dose were greater than those of its optical antipode. Thus the slightly greater potency of the S(-) enantiomer after a single dose may have been overcome by the greater plasma concentrations of the less potent enantiomer. Both enantiomers produced similar reductions in plasma concentrations of thyroxine. The data indicate that at low dosages S(-) ciprofibrate is a slightly more potent hypolipidaemic agent after a single dose in rats and a slightly more potent peroxisome proliferator at low concentrations in vitro. However, following multiple dosing, both enantiomers produced changes in plasma concentrations of lipids, hepatic enzyme activities and plasma concentrations of thyroxine which were of comparable magnitude to those produced by the racemate. Since these early changes have been linked mechanistically to the chronic toxicity of the racemate in the rat, it could be predicted that the individual enantiomers of ciprofibrate under conditions employed in chronic safety studies, would produce the same spectrum of rodent toxicity as the racemate.

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

The glucuronidation of thyroid hormone by UDP-glucuronyltransferases (UGTs) stably transfected in Chinese hamster V79 lung fibroblasts was investigated. Human bilirubin UGT (HP3) and phenol UGT (HP4) both catalysed the glucuronidation of T4 and rT3, whereas glucuronidation of T3 was not significant, rT3 was the preferred substrate for both isoenzymes, glucuronidation rates being 1.6- and 6.4-times higher than conjugation of T4 by HP3 and HP4 clones, respectively. This is the first identification of thyroid hormone as potential alternative endogenous substrate for bilirubin UGT.

Increased glucuronidation of thyroid hormone in hexachlorobenzene-treated rats

Metabolism of thyroid hormones was investigated in WAG/MBL rats that had been exposed to hexachlorobenzene (HCB). Serum thyroxine (T4) levels were lowered by 35.5%, whereas triiodothyronine (T3) levels were not changed. Bile flow, as well as T4 excretion in bile were increased by HCB treatment. Analysis of bile by HPLC revealed a more than 3-fold increase of T4 glucuronide (T4G) and a concomitant reduction of non-conjugated T4. T4 UDP-glucuronyltransferase activity (T4 UDPGT) activity in hepatic microsomes was increased more than 4.5-fold in animals exposed to HCB. p-Nitrophenol (PNP) UDPGT showed a comparable increase by HCB. Both T3 and androsterone UDPGT activities were low in WAG/MBL rats compared with normal Wistar rats. T3 UDPGT activity was increased 2.5-fold by HCB, but androsterone UDPGT activity was unchanged. These results suggest that T4 is a substrate for HCB-inducible PNP UDPGT and T3 for androsterone UDPGT. In the absence of the latter, T3 is also glucuronidated to some extent by PNP UDPGT. Type 1 iodothyronine deiodinase activity was decreased by HCB treatment. It is concluded that decreased T4 levels in serum of animals after exposure to HCB may be due to a combined effect of displacement of T4 from carriers, an increased glucuronidation of T4 and enhanced bile flow.

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

We have studied the glucuronidation of the thyroid hormones T4, T3 and rT3 by liver microsomes of Wistar, Gunn and WAG rats. Gunn rats have a defect in the gene coding for bilirubin and phenol UDP-glucuronyltransferase (UGT) isoenzymes; WAG rats have a genetic defect in androsterone UGT. In normal Wistar rats UGT activity was approximately 5-fold higher for rT3 than for T4 or T3. UGT activities for T4 and rT3, but not for T3, were impaired in Gunn rats. Conversely, UGT activity for T3, but not for T4 or rT3, was impaired in WAG rats. Thus, in rat liver rT3 is glucuronidated much more rapidly than T4 and T3. Our results support the view that T4 and rT3 are glucuronidated by bilirubin and phenol UGTs and T3 by androsterone UGT.

Rat liver microsomal UDP-glucuronosyltransferase activity toward thyroxine: characterization, induction, and form specificity

Glucuronidation of thyroxine (T4) by liver microsomal UDP-glucuronosyltransferase (UDP-GT) is a predominant pathway by which T4 is deactivated. This study was conducted to characterize in vitro T4 UDP-GT activity in rat liver microsomal preparations, to determine if T4 glucuronidation is mediated by a particular form of UDP-GT, and to determine if T4 glucuronidation can be increased by microsomal enzyme inducers. Characterization of microsomal T4 UDP-GT activity led to the establishment of optimal assay conditions. UDP-GT activity toward T4 was determined in hepatic microsomal preparations from Wistar and Gunn rats, a mutant strain of Wistar rats deficient in several forms of UDP-GT. Hepatic microsomal preparations from Gunn rats glucuronidated T4 at one-third the rate catalyzed by microsomal preparations from Wistar rats. To determine the effect of four inducers that each increase a separate class of UDP-GT, phenobarbital (PB), 3-methylcholanthrene (3MC), pregnenolone-16 alpha-carbonitrile (PCN), clofibrate (CLO), saline, or corn oil was administered to male Sprague-Dawley rats ip for 4 days. T4 UDP-GT activity was increased by PB, 3MC, PCN, and CLO 88, 150, 100, and 160%, respectively on a per-milligram-microsomal-protein basis and 138, 125, 100, and 145% on a per-kilogram-body-weight basis, respectively. Therefore, all four classes of UDP-GT inducers increase T4 glucuronidation, suggesting that T4 is not a selective substrate for a particular form of UDP-GT.