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Institut für Ernährungswissenschaften, Martin-Luther-Universität, Halle-Wittenberg, Germany
(Received June 7, 2006; accepted August 4, 2006)
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Abstract |
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1 (P < 0.05) than control pigs. In conclusion, this study shows that clofibrate treatment induces a strong activation of T3- and T4-UGT in pigs, leading to increased glucuronidation and markedly reduced plasma concentrations of these hormones, accompanied by a moderate stimulation of thyroid function.
(PPAR), a nuclear receptor also activated by natural ligands like free fatty acids or some eicosanoids. Activation of PPAR leads to up-regulation of transcription of several genes involved mainly in mitochondrial and peroxisomal ß-oxidation, ketogenesis, and gluconeogenesis (Mandard et al., 2004
). Fibrates have been in clinical use as hypolipidemic agents for several decades. Several studies in rodents and cell culture systems have shown that fibrates, like many other drugs (e.g., phenobarbital, 3-methylcholantrene, polychlorinated biphenyl, tetrachlorobiphenyl, pregnenolone-16-carbonitrile, or dexamethasone), induce UDP glucuronosyltransferases (UGT) (Beetstra et al., 1991; Saito et al., 1991; Barter and Klaassen, 1992a,b, 1994; Visser et al., 1993a,b; Jemnitz et al., 2000; Viollon-Abadie et al., 2000; Vansell and Klaassen, 2002). UGT, consisting of UGT1 and UGT2 isoforms, are localized in the endoplasmatic reticulum of hepatocytes and extrahepatic tissue and belong to the enzymes of phase II metabolism. With broad and overlapping substrate specificities, the UGT isoenzymes catalyze the glucuronidation of differential functional groups, using UDP-glucuronic acid as the cofactor (Miners and Mackenzie, 1991; Mackenzie et al., 1997). Thyroid hormones thyroxine (T4) and triiodothyronine (T3) are substrates of hepatic UGT, and glucuronidation of these hormones is the main metabolic pathway for deactivating them (Jemnitz et al., 2000). In rats, several of the drugs acting as inducers of microsomal enzymes have been shown to produce hypertrophy and hyperplasia of thyroid follicular cells, most probably through increased deactivation of thyroid hormones by UGT, leading to a reduction of serum T4 and possibly T3 (Beetstra et al., 1991; Saito et al., 1991; Barter and Klaassen 1992a, 1994). In mice, in contrast to rats, clofibrate treatment did not alter T3- and T4-UGT activities and plasma concentrations of thyroid hormones (Viollon-Abadie et al., 1999). These studies show species-specific differences in the effects of clofibrate on hepatic thyroid hormone metabolism (i.e., glucuronidation of thyroid hormones).
In rodents, PPAR agonists not only induce many genes involved in various metabolic pathways such as ß-oxidation, ketogenesis, and gluconeogenesis but also cause severe peroxisome proliferation in the liver, hepatomegaly, and hepatocarcinogenesis (Peters et al., 2005). In contrast to rodents, PPAR agonists do not induce peroxisome proliferation or tumor in the liver of many other species, such as guinea pigs, swine, monkeys, and humans, although they retain a hypotriglyceridemic effect in these species (Holden and Tugwood, 1999). In nonproliferating species, expression of PPAR in the liver is much lower, and the response of many genes to PPAR activation is weaker than in proliferating species (Cheon et al., 2005). It is known that PPAR activation can modulate metabolizing enzymes of phase I and II biotransformation (Rushmore and Kong, 2002; Zhou et al., 2005). Moreover, it has been shown that some UGT isoforms (UGT1A9, UGT2B4) are PPAR target genes (Barbier et al., 2003a,b). Therefore, nonproliferating species could respond differently from proliferating species to clofibrate with respect to induction of UGT (i.e., UGT involved in glucuronidation of thyroid hormones). To our knowledge, the effect of clofibrate on the hepatic thyroid hormone metabolism has not yet been investigated in vivo in a nonproliferating species.
The aim of our study was to investigate the effects of clofibrate treatment on hepatic thyroid hormone metabolism (i.e., activities of T3- and T4-UGT in pigs, representing a nonproliferating species). Therefore, as well as determining hepatic activities of T3- and T4-UGT, we also measured plasma concentrations of thyroid hormones, thyroid weights, thyroidal epithelial cell height, and follicle lumen diameter and gene expression levels of several thyroidal genes involved in thyroid hormone biosynthesis [thyroid-stimulating hormone (TSH) receptor, sodium iodide symporter, thyroid peroxidase, dual oxidase 2, thyroglobulin, cathepsin B, and type II iodothyronine deiodinase]. We also investigated the effect of clofibrate on mRNA expression of genes involved in thyroid hormone transport (transthyretin, thyroxine-binding globulin, and albumin), peripheral conversion of thyroid hormones (type I iodothyronine deiodinase), and thyroid hormone signaling (thyroid hormone receptor 1) in the liver.
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Materials and Methods |
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Animals and Treatments. Eighteen male 8-week-old crossbred [(German Landrace x Large White) x Pietrain] pigs, bred in the local animal facility, were used. They weighed between 11.0 and 13.5 kg. They were individually housed in a room maintained at 23°C and 50 to 60% relative humidity with light from 6:00 AM to 6:00 PM. On the day before the start of the experimental feeding period, all the pigs were weighed and assigned to two groups with body weights of 12.0 ± 1.1 (S.D.) kg (control group) and 11.9 ± 0.6 (S.D.) kg (treatment group). Both groups of pigs received a nutritionally adequate diet (National Research Council, 1998) for growing pigs containing wheat (400 g/kg), soybean meal (230 g/kg), wheat bran (150 g/kg), barley (100 g/kg), sunflower oil (90 g/kg), and mineral premix including L-lysine, DL-methionine, and L-threonine (30). This diet contained 14.4 MJ metabolizable energy and 185 g of crude protein/kg. The diet of the treatment group was supplemented with 5 g of clofibrate/kg. To standardize feed intake, each pig within the experiment received 700 g of the diet daily, which was completely consumed by all the animals in the experiment. The clofibrate dosage in the treated pigs was 220 mg/kg b.wt./day. The pigs had free access to water via nipple drinking systems. The experimental diets were administered for 28 days. All the experimental procedures described followed established guidelines for the care and use of laboratory animals and were approved by the local veterinary office.
Sample Collection. After completion of the feeding period, the animals were killed under a light anesthesia. Blood was collected into heparinized polyethylene tubes. Liver and thyroid gland were dissected and weighed. Plasma was obtained by centrifugation of the blood (1100g; 10 min). All the samples were stored at –80°C pending analysis.
Total RNA Preparation and cDNA Synthesis. Total RNA from liver and thyroid tissue was isolated by TRIzol reagent (Invitrogen, Karlsruhe, Germany) following the manufacturer's protocol, resuspended in diethyl pyrocarbonate-treated water, and stored at –80°C until use. The concentration of total RNA was determined by ultraviolet absorbance at 260 nm. The quality of all the RNA samples was assessed by agarose gel electrophoresis. cDNA was prepared from total RNA (1.2 µg) by reverse transcription using M-MuLV reverse transcriptase (MBI Fermentas, St. Leon-Rot, Germany) and oligo(dT)18 primers (Operon, Cologne, Germany).
Semiquantitative Polymerase Chain Reaction. Expression analysis for semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was normalized using glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as an internal standard. cDNA templates (2 µl) were used in a final volume of 20 µl containing 0.2 µM concentration of the corresponding primers (Roth, Karlsruhe, Germany) (see Table 1), 1.5 mM magnesium chloride, 1x PCR buffer, 1 U Taq polymerase (Gene Craft, Luedinghausen, Germany), and 0.2 mM deoxyribonucleoside triphosphates (Roth). Each PCR cycle comprised denaturation for 30 s at 94°C, annealing for 30 s at 60 to 64°C (see Table 1), and elongation for 1 min at 72°C, followed by a final extension period for 10 min at 72°C. Number of cycles for each primer pair was tested previously. Cycle number was as follows: genes of thyroid gland: GAPDH, 23; sodium iodide symporter, 25; dual oxidase 2, 42; cathepsin B, 35; TSH receptor, 28; type II iodothyronine deiodinase, 40; and thyroglobulin, 33; hepatic genes: GAPDH, 32; acyl CoA oxidase (ACO), 32; carnitine palmitoyl transferase 1 (CPT-1), 32; albumin, 20; thyroid hormone receptor 1, 35; thyroxine-binding globulin, 42; transthyretin, 30; and type I iodothyronine deiodinase, 40. A water control was included in all the PCRs for detection of contamination, and dilutions of the isolated total RNA corresponding to the cDNA synthesis were used as template to exclude impurities caused by genomic DNA. A volume of 10 µl per PCR was submitted to agarose gel electrophoresis (1.5%). Ethidium bromide-stained gels were digitized for quantification (apparatus and software from Syngene, Cambridge, UK).
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Preparation of Hepatic Microsomes. One gram of liver was homogenized in a medium (10 ml) containing 0.25 M sucrose and 0.1 M phosphate buffer (pH 7.4) using a Potter-Elvehjem homogenizer. Homogenates were centrifuged at 1000g for 10 min at 4°C, and the supernatant was centrifuged at 15,000g for 15 min. The microsomal pellet was obtained by centrifugation of the 15,000g supernatant at 105,000g for 60 min. Microsomal pellets were suspended in the homogenization medium, and the protein concentration was determined with the bicinchoninic acid reagent according to the supplier's protocol using bovine serum albumin as standard.
Enzyme Assays. The activity of pNP-UGT in hepatic microsomes was assayed by the method of Thurman et al. (1981). The assay mixture consisted of 50 mM potassium phosphate buffer (pH 7.4), 0.2 mg of bovine serum albumin/ml, 1 mM magnesium chloride, 0.5 mM dithiothreitol, 0.5 mg of Triton X-100/ml, 0.2 mM pNP, 0.2 mM UDPGA, and 0.25 mg of microsomal protein/ml. The final volume of the assay was 200 µl. A blank was incubated without UDPGA. The reaction was stopped after 30 min at 37°C by addition of 1 ml of 0.1 M sodium hydroxide solution. The pNP glucuronidation was quantified by measuring the decrease of absorbance at 400 nm. The concentration of pNP glucuronide was calculated using a molar extinction coefficient of 18,300/cm.
Activity of bilirubin-UGT in hepatic microsomes was measured in an assay mixture containing 0.1 M Tris-hydrochloride (pH 7.8), 0.1 mM bilirubin, and 5 mM UDPGA in a final volume of 200 µl. Suspended microsomes were preincubated with 20 mg/ml sodium cholate (1:1, v/v) for 10 min at 4°C. The reaction was started by addition of 1 mg of microsomal protein/ml for 60 min at 37°C. The amount of bilirubin glucuronide formed during the incubation was quantified with a commercial kit (DiaSys Diagnostic Systems, Holzheim, Germany).
T3- and T4-UGT activities were determined in separate assays using a modified version of the method of Beetstra et al. (1991) by incubating 1 µM T3 or T4, respectively, and 0.1 µCi of 125I-labeled T3 or T4 in reaction mixture containing 75 mM Tris-hydrochloride (pH 7.8), 7.5 mM magnesium chloride, 0.25 mg of Brij 56/ml, 5 mM UDPGA, and 1 mM 6-propyl-2-thiouracil. The final volume of the assay was 200 µl. Reactions were started by adding 0.5 mg of microsomal protein/ml at 37°C. Blanks were performed in the absence of UDPGA. After 30 min, reactions were terminated by addition of 200 µl of ice-cold methanol, and the mixtures were centrifuged at 3500g for 8 min. Fifty microliters of the supernatants was injected into a high-performance liquid chromatograph for separation of T3 or T4 glucuronides formed during the incubation by a modified version of the method of Jemnitz et al. (2000). The high-performance liquid chromatography equipment consisted of a 1100 series pump (isocratic), an autosampler, a LiChrospher 100 RP 18e column (125 x 4 mm, 5-µm particle size) with matching guard column (4 x 4 mm; Agilent Technologies, Waldbronn, Germany). The mobile phase consisted of 50 mM potassium dihydrogen phosphate and methanol (43:57, v/v, pH 7.0). For separation of T3-glucuronide, the flow rate was 0.8 ml/min. For separation of T4-glucuronide, the flow rate was 1.25 ml/min. Fractions containing T3- or T4-glucuronide, respectively, were collected with a fraction collector 203 (Gilson International, Bad Camberg, Germany). The radioactivity of the fractions was counted to calculate T3- and T4-UGT activities.
Histology of Thyroid Gland. Samples of thyroid glands were fixed by immersion in 10% neutral buffered formalin, processed for embedding into paraffin wax, and cut into 4-µm sections. For light microscopy, the sections were stained with hemalum and eosin. The epithelial cell height was measured using 4 cells per follicle in 100 follicles of each thyroid. The lumen diameter was measured in 10 sections for 10 follicles per section of each thyroid. All the pictures were digitized, and the parameters were measured using the Lucia G (Nikon, Düsseldorf, Germany) software (release 4.81).
Analysis of Plasma Hormones. The plasma concentrations of free and total T4 and total T3 were measured with radioimmunoassay kits (MP Biomedicals, Eschwege, Germany).
Statistics. The results were analyzed using Minitab (State College, PA) statistical software (release 13). Statistical significance of differences between control group and treatment group was evaluated using Student's t test. Mean values were considered significantly different for P < 0.05.
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Results |
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target genes ACO and CPT-1 was higher (P < 0.05) in pigs treated with clofibrate than in control pigs (ACO: 1.39 ± 0.27 versus 1.00 ± 0.35; CPT-1: 1.60 ± 0.13 versus 1.00 ± 0.12; mean ± S.D., n = 9 for each group). Moreover, concentrations of total and free T4 and total T3 in plasma were markedly lower in pigs treated with clofibrate than in control pigs (P < 0.05), whereas the T4/T3 ratio did not differ between both groups of pigs (Table 2).
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Pigs treated with clofibrate had a higher activity of bilirubin-UGT in the liver than control pigs (1.08 ± 0.05 versus 0.44 ± 0.02 nmol/min/mg; mean ± S.D., n = 9 for each group; P < 0.05). The activity of hepatic pNP-UGT was lower in pigs treated with clofibrate than in control pigs (44 ± 5 versus 70 ± 8 nmol/min/mg; mean ± S.D., n = 9 for each group; P < 0.05). Activities of hepatic T3- and T4-UGT were higher in pigs treated with clofibrate than in control pigs (P < 0.05) (Fig. 1).
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Pigs treated with clofibrate had lower hepatic mRNA concentrations of transthyretin, albumin, and thyroid hormone receptor 1 than control pigs (P < 0.05) (Table 4). Hepatic mRNA concentration of thyroxine-binding globulin tended to be lower in pigs treated with clofibrate compared with control pigs (P < 0.10), whereas hepatic mRNA concentration of type I iodothyronine deiodinase did not differ between both groups of pigs.
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Discussion |
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agonist causes no or only weak peroxisome proliferation in the liver. Interestingly, in this study clofibrate treatment caused a significant increase in liver weights of pigs, by about 15%. This is in disagreement with a recent study in which pigs did not show significantly increased liver weights after a 1-week treatment with a dose of clofibrate similar to those used in our study (Cheon et al., 2005). The difference in these results could be because of the longer treatment period in our study compared with that in the study of Cheon et al. (2005). Moreover, we observed a moderate up-regulation of the PPAR target genes ACO and CPT-1 in the liver of pigs treated with clofibrate, which indicates that clofibrate treatment caused PPAR activation in these pigs. Nevertheless, increases in liver weights and hepatic ACO and CPT-1 mRNA concentration were much lower than those observed in rodents treated with clofibrate. In rats and mice, feeding PPAR agonists increases liver weights by 50% or more and mRNA concentrations of ACO 5- to 10-fold compared with untreated controls (Kawashima et al., 1990; He et al., 2002; Frederiksen et al., 2004; Li et al., 2004).
Several families of UGT enzymes are expressed in the liver. To study the effect of clofibrate treatment on the induction of microsomal enzymes, we determined the activities of bilirubin- and pNP-UGT. The finding that clofibrate treatment strongly increases bilirubin-UGT is in accordance with studies in rats and mice (Visser et al., 1993a; Viollon-Abadie et al., 1999). The finding that clofibrate reduces the activity of pNP-UGT is also in accordance with a study in which clofibrate significantly reduced the activity of pNP-UGT in the liver of Wistar rats (Visser et al., 1993a). These observations suggest that clofibrate stimulated the microsomal enzyme system in pigs in a similar way as in rats. This study also shows that clofibrate treatment strongly increases the activity of T3- and T4-UGT in the liver, which in turn leads to a dramatic reduction of plasma T3 and T4 concentrations. Increased activity of T4-UGT had previously been observed in Wistar rats but not in mice treated with clofibrate (Visser et al., 1993a; Viollon-Abadie et al., 1999). The increased activity of T3-UGT in pigs treated with clofibrate, however, is in strong contrast to rats, in which clofibrate treatment did not increase T3-UGT activity (Visser et al., 1993a).
In rats, T4 is accepted as a substrate by hepatic bilirubin-UGT (UGT1A1) and phenol-UGT (UGT1A6), and it was shown that increased activities of these enzymes were associated with increased glucuronidation of T4 in the liver (Beetstra et al., 1991; Magdalou et al., 1993; Visser et al., 1993a,b; Viollon-Abadie et al., 2000; Vansell and Klaassen, 2002). The enzymes involved in glucuronidation of thyroid hormones in pigs have not yet been identified. The fact that activities of both bilirubin- and T4-UGT were increased suggests that in pig liver T4 was also glucuronidated by bilirubin-UGT, as happens in rats. It is probable that enzymes other than UGT1A1 and UGT1A6 can also be induced by clofibrate in rats and are involved in T4 conjugation (Jemnitz et al., 2000). In rat liver, glucuronidation of T3, unlike glucuronidation of T4, is catalyzed by androsterone-UGT (Beetstra et al., 1991; Visser et al., 1993b). The increased T3-UGT activity in pigs treated with clofibrate could therefore also have been caused by an increased activity of androsterone-UGT, although this was not assayed in this study. The UGT in pig liver have been less extensively investigated and have not yet been phenotyped. Therefore, it remains unknown which specific UGT were responsive for the increased glucuronidation of T3 and T4 in pigs treated with clofibrate.
It has been shown that activation of PPAR leads to transcriptional up-regulation of the CYP4A genes, which are also constituents of the microsomal biotransformation system in both proliferating and nonproliferating species (Lawrence et al., 2001; Cheon et al., 2005). It has further been shown that some UGT isoforms (UGT1A9, UGT2B4) are PPAR target genes (Barbier et al., 2003a,b). PPAR is naturally activated during fasting, and Visser et al. (1996) showed that food restriction resulted in increased bilirubin and thyroid hormone UGT activities in rats. These findings suggest that UGT catalyzing the glucuronidation of thyroid hormones may be transcriptionally up-regulated by activation of PPAR. It is well known that expression of PPAR in the liver is much lower in nonproliferating species and that the response of many genes to PPAR activation is weaker than in proliferating species. This is also true for up-regulation of microsomal CYP4A genes by treatment with PPAR agonists (Lawrence et al., 2001; Cheon et al., 2005). If PPAR plays a crucial role in the activation of UGT catalyzing thyroid hormone glucuronidation, the effect of clofibrate on up-regulation of these enzymes in pigs would be expected to be much lower than in proliferating species such as rats or mice. But activation of T3- and T4-UGT by clofibrate was even stronger in pigs than reported for rats or mice (Visser et al., 1993a; Viollon-Abadie et al., 1999). These findings suggest that activation of PPAR does not play a key role in clofibrate-induced up-regulation of thyroid hormone UGT. Nevertheless, the role of PPAR in the regulation of thyroid hormone glucuronidation should be further investigated.
The increased activities of T3- and T4-UGT make it highly probable that the markedly reduced plasma concentrations of T3 and T4 in pigs treated with clofibrate are mainly caused by increased glucuronidation of these hormones in the liver. Because most T3 is generated in peripheral tissues, mainly the liver, by deiodination of T4, a reduced T3 concentration could potentially be caused by an inhibition of type I iodothyronine deiodinase. Indeed, in the study of Visser et al. (1993a), clofibrate treatment of rats reduced the activity of that enzyme, which might be responsible for the reduced concentration of T3 observed in their study. We did not determine the activity but only the mRNA concentration of that enzyme in the liver, which was not influenced by clofibrate treatment. Interestingly, in contrast to hepatic type I iodothyronine deiodinase, type II deiodinase in the thyroid showed a reduced mRNA concentration in pigs treated with clofibrate compared with control pigs. A reduced activity of type II deiodinase, which converts T4 to T3 in the thyroid, may play some role in the reduced T3 concentration in plasma. However, because the thyroid produces less than 20% of total T3 (Findlay et al., 2000), a reduced activity of type II deiodinase most probably plays a minor role in the reduced plasma concentration of T3. The reduction of plasma concentrations of total T3 (by 47% versus control), free T4 (by 32% versus control), and total T4 (by 35%) concentrations by clofibrate are also stronger than those observed in Wistar rats. In Wistar rats, a dose of 800 mg of clofibrate/kg b.wt./day reduced plasma T3 concentration by 27% but did not reduce plasma total and free T4 (Visser et al., 1993a). In mice, a dose of 300 mg of clofibrate/kg b.wt./day reduced plasma free T4 concentration by 13% but did not significantly reduce plasma concentration of free T3 (Viollon-Abadie et al., 1999). It is clear that different studies cannot be directly compared with each other, but these data nevertheless suggest that pigs could be even more sensitive to disruptions of thyroid hormone metabolism by clofibrate than rodents.
Reduced plasma concentrations of T3 and T4 are expected to increase the release of TSH from the pituitary gland. It has indeed been shown that microsomal enzyme inducers elevate TSH plasma concentrations in rodents, which in turn stimulates proliferation of epithelial cells in thyroid tissue as a result of increased glucuronidation of thyroid hormones (e.g., Curran and DeGroot, 1991; De Sandro et al., 1991; Saito et al., 1991; Liu et al., 1995). As no assay was available for measuring TSH concentration in plasma of the pigs, we determined mRNA concentrations of various genes in the thyroid that are responsive to TSH treatment. The finding that mRNA concentrations of TSH receptor, sodium iodide symporter, thyroid peroxidase, and cathepsin B, all genes responsive to TSH, were moderately increased by 40 to 70% suggests that the thyroid was stimulated by the increased plasma concentration of TSH. This suggestion is confirmed by a study that showed that TSH plasma concentrations are increased by microsomal enzyme inducers, which stimulate the glucuronidation of T3 (Klaassen and Hood, 2001). Our study further shows that expression levels of dual oxidase 2, a hydrogen peroxide-generating system that constitutes the rate-limiting step of thyroid hormone synthesis, and of thyroglobulin, a protein involved in thyroid hormone synthesis and storage, are not altered by clofibrate treatment.
The finding that thyroid weights, epithelial cells, and follicle lumen diameter were not increased by clofibrate was unexpected and suggests that stimulation of the thyroid was moderate, only increasing gene expression of TSH-responsive genes in the thyroid, whereas histological alterations (i.e., increased epithelial cell height) may take longer than 4 weeks to become evident.
The action of thyroid hormones like T3 is mediated by thyroid hormone receptors that belong to the family of nuclear hormone receptors. The present study shows that clofibrate treatment reduces gene expression of thyroid hormone receptor 1 in the liver of rats. This finding agrees with a recent study in which bezafibrate down-regulated thyroid hormone receptors in rat liver (Bonilla et al., 2001). That study further showed that down-regulation of thyroid hormone receptors was caused by activation of PPAR. Therefore, it is likely that in our study down-regulation of thyroid hormone receptor 1 in pigs treated with PPAR was also caused by PPAR activation by clofibrate. Down-regulation of thyroid hormone receptor implies that the biological activity of T3 may have been reduced in pigs treated with clofibrate.
Thyroxine-binding globulin, transthyretin, and albumin are the major plasma transport proteins in pigs (Janssen et al., 2002). These proteins are synthesized in the liver. We found in our study that gene expression of these proteins in the liver was reduced by clofibrate treatment of pigs. In studies by Motojima et al. (1992, 1997), the same effect of clofibrate on expression of transthyretin was observed in rats and several mouse strains, whereas there was no effect in PPAR-null mice. This suggests that down-regulation of transthyretin expression was induced by PPAR activation. Consequently, down-regulation of transthyretin and possibly also of thyroxine-binding globulin and albumin could be the result of PPAR activation by clofibrate. Our data suggest, although we did not measure concentrations of these proteins in blood, that clofibrate treatment lowers not only concentrations of thyroid hormones in plasma but also could reduce the transport capacity for thyroid hormones.
In conclusion, this study shows for the first time that clofibrate treatment induces a strong activation of T3- and T4-UGT in pigs, leading to increased glucuronidation and markedly reduced plasma concentrations of these hormones. These alterations were accompanied by moderately increased mRNA concentrations of various TSH-responsive enzymes in the thyroid gland, reduced hepatic mRNA concentrations of proteins involved in thyroid hormone transport, and thyroid hormone receptors. Because the pig represents a species that does not respond with peroxisome proliferation to treatment with PPAR agonists, the study shows that clofibrate treatment also disrupts the metabolism of thyroid hormones in nonproliferating species.
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ABBREVIATIONS: PPAR, peroxisome proliferator-activated receptor ; UGT, UDP glucuronosyltransferase(s); T4, thyroxine; T3, 3,3',5-triiodothyronine; TSH, thyroid-stimulating hormone; pNP, p-nitrophenol; UDPGA, UDP-glucuronic acid; RT-PCR, reverse transcriptase polymerase chain reaction; GAPDH, glycerinaldehyde-3-phosphate dehydrogenase; ACO, acyl CoA oxidase; CPT-1, carnitine palmitoyl transferase 1.
Address correspondence to: Klaus Eder, Institut für Ernährungswissenschaften, Martin-Luther-Universität Halle-Wittenberg, Emil-Abderhalden-Str. 26, D-06108 Halle/S., Germany. E-mail: klaus.eder{at}landw.uni-halle.de
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. J Biol Chem 34: 31521–31527.. FASEB J 18: A863. target genes. Cell Mol Life Sci 61: 393–416.[CrossRef][Medline]-carbonitrile do not affect hepatic thyroid hormone UDP-glucuronosyl transferase activity, and thyroid gland function in mice. Toxicol Appl Pharmacol 155: 1–12.[CrossRef][Medline]
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