Role of thyroid state on induction by ciprofibrate of laurate hydroxylase and peroxisomal enzymes in rat liver microsomes

Thyroid hormone contributes to the hypolipidemic effect of polyunsaturated fatty acids from fish oil: in vivo evidence for cross talking mechanisms

n-3 polyunsaturated fatty acids (n-3 PUFA) from fish oil (FO) exert important lipid-lowering effects, an effect also ascribed to thyroid hormones (TH) and TH receptor β1 (TRβ1)-specific agonists. n-3 PUFA effects are mediated by nuclear receptors, such as peroxisome proliferator-activated receptors (PPAR) and others. In this study, we investigated a role for TH signaling in n-3 PUFA effects. Euthyroid and hypothyroid adult rats (methimazole-treated for 5 weeks) received FO or soybean oil (control) by oral administration for 3 weeks. In euthyroid rats, FO treatment reduced serum triglycerides and cholesterol, diminished body fat, and increased protein content of the animals. In addition, FO-treated rats exhibited higher liver expression of TRβ1 and mitochondrial α-glycerophosphate dehydrogenase (mGPD), at protein and mRNA levels, but no alteration of glutathione S-transferase or type 1 deiodinase. In hypothyroid condition, FO induced reduction in serum cholesterol and increase in body protein content, but lost the ability to reduce triglycerides and body fat, and to induce TRβ1 and mGDP expression. FO did not change PPARα liver abundance regardless of thyroid state; however, hypothyroidism led to a marked increase in PPARα liver content but did not alter TRβ1 or TRα expression. The data suggest that part of the effect of n-3 PUFA from FO on lipid metabolism is dependent on TH signaling in specific steps and together with the marked upregulation of PPARα in liver of hypothyroid rats suggest important in vivo consequences of the cross-talking between those fatty acids and TH pathways in liver metabolism.

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.

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.

Comparative quantification of two hepatic UDP-glucuronosyltransferase bilirubin isoforms mRNAs in various thyroid states in rat

The study was designed to compare the effects of 3,5,3' triiodo-L-thyronine (L-T3) on the levels of hepatic mRNAs encoding two UDP-glucuronosyltransferase bilirubin isoforms (UGT1*1 and UGT1*0) in rats, by reverse transcription and quantitative polymerase chain reaction (RT-PCR). The administration of L-T3 decreased the UGT1*O mRNA by 2.2-fold and that of UGT1*1 by only 1.4-fold. In contrast, thyroidectomy increased the UGT1*O mRNA level by twofold but did not change that of the UGT1*1 isoform significantly. Interestingly, treatment with a known inducer of UGT bilirubin, ciprofibrate, induced the hepatic mRNA levels encoding for the UGT1*0 isoform by 3.5-fold and for the UGT1*1 isoform by only twofold. The results indicate for the first time that, although UGT1*1 mRNA is indeed a major transcript, its level is weakly affected by these compounds. In contrast, the minor UGT1*0 form is much more sensitive both to the action of this drug and to changes in thyroid status. The data support the notion that the various members of exon1 of the UGT1 locus have their own individual regulatory region.

Suppression of clofibrate-induction of peroxisomal and microsomal fatty acid-oxidizing enzymes by growth hormone and thyroid hormone in primary cultures of rat hepatocytes

Using primary cultures of rat hepatocytes on a matri-gel, effects of peroxisome proliferator and omega-hydroxydodecanoic acid on cellular levels of acyl-CoA oxidase and CYP4A have been studied to determine the hormonal influence in serum-free media. Peroxisomal acyl-CoA oxidation, microsomal CYP4A content and laurate omega-hydroxylation were increased in rat hepatocytes by the addition of 100 microM clofibrate or Wy14,643 for two days. omega-Hydroxydodecanoic acid (100 microM) also increased peroxisomal acyl-CoA oxidation, but had no clear effect on microsomal CYP4A level and laurate omega-hydroxylation. CYP4A-mediated laurate omega-hydroxylation in hepatocytes was suppressed by the addition of pituitary growth hormone (0.05 mU/ml), but was not altered by the addition of triiodothyronine (30 nM). In contrast, clofibrate-mediated induction of acyl-CoA oxidase activity was decreased by the addition of either one of the hormones in hepatocytes. Suppression by those hormones was also observed with omega-hydroxydodecanoic acid-mediated induction of acyl-CoA oxidase activity. These results indicate the possibility that GH and T3 exert the suppressive effects on peroxisomal acyl-CoA oxidation through plural mechanisms with and without the alteration of CYP4A levels in livers.

Biochemical properties of liver peroxisomes from rat, guinea pig and human species and the influence of hormonal status on rat liver acyl-CoA oxidase mRNA content

Liver peroxisomes from three different species, rat, guinea pig and man, have been purified by ultracentrifugation on a discontinuous Nycodenz gradient. Several biochemical parameters were tested in order to compare the basic peroxisomal properties of liver from rat, a species strongly responsive to peroxisome proliferators, and guinea pig and man, two weakly responsive species. Polypeptide patterns were compared and the bands in guinea pig and man comigrating with the two major bands in rat, catalase at 66 kDa and urate oxidase at 35 kDa, appeared in low amounts. However, other polypeptides are similar throughout these species especially in guinea pig as revealed by cross-immunoreactivity using an anti-rat peroxisomal protein rabbit immune serum. Specific activities of peroxisome acyl-CoA oxidase and microsome omega-lauryl hydroxylase have comparable rates in rat and guinea pig liver, but in human liver the activities are much lower. There is a cross-hybridization between acyl-CoA oxidase mRNA probed by rat liver acyl-CoA oxidase cDNA among the three species at a medium stringency. But interestingly, acyl-CoA oxidase mRNA from guinea pig and man appear to be larger in size. On the other hand, the hormonal status does not seem to have a significant effect on the rat liver acyl-CoA oxidase mRNA level suggesting at most that insulin, corticosterone and estradiol have no direct effect on acyl-CoA oxidase gene expression, which contrasts with the well-known effect of peroxisome proliferators.