Abstract
Hypercholesterolemia is found in patients with hypothyroidism and resistance to thyroid hormone. In this study, we examined cholesterol metabolism in a thyroid hormone receptor beta (TR-beta) mutant mouse model of resistance to thyroid hormone. Whereas studies of cholesterol metabolism have been reported in TR-beta knock-out mice, generalized expression of a non-ligand binding TR-beta protein in this knock-in model more fully recapitulates the hypothyroid state, because the hypothyroid effect of TRs is mediated by the unliganded receptor. In the hypothyroid state, a high cholesterol diet increased serum cholesterol levels in wild-type animals (WT) but either did not change or reduced levels in mutant (MUT) mice relative to hypothyroidism alone. 7alpha-Hydroxylase (CYP7A1) is the rate-limiting enzyme in cholesterol metabolism and mRNA levels were undetectable in the hypothyroid state in all animals. triiodothyronine replacement restored CYP7A1 mRNA levels in WT mice but had minimal effect in MUT mice. In contrast, a high cholesterol diet markedly induced CYP7A1 levels in MUT but not WT mice in the hypothyroid state. Elevation of CYP7A1 mRNA levels and reduced hepatic cholesterol content in MUT animals are likely because of cross-talk between TR-beta and liver X receptor alpha (LXR-alpha), which both bind to a direct repeat + 4 (DR+4) element in the CYP7A1 promoter. In transfection studies, WT but not MUT TR-beta antagonized induction of this promoter by LXR-alpha. Electromobility shift analysis revealed that LXR/RXR heterodimers bound to the DR+4 element in the presence of MUT but not WT TR-beta. A mechanism for cross-talk, and potential antagonism, between TR-beta and LXR-alpha is proposed.
Highlights
Thyroid hormone is known to regulate each of these pathways at the transcriptional level, and the predominate thyroid hormone receptor (TR) isoform in the liver is thyroid hormone receptor  (TR-)1 (9 –12)
We characterize an interaction between liver X receptor (LXR) and TR, which bind to a similar DNA-response element, and show a unique role for the unliganded mutant TR- in cholesterol metabolism
Because TR- mutant (MUT) animals have significantly higher TH levels, we considered that excess TH (Table 1) may overcome some of the dominant negative effects of the mutant TR-
Summary
Animals—TR- knock-in mice have a mutation that deletes three nucleotides in exon 6 of the TR- gene, resulting in loss of a threonine residue in the receptor, which prevents ligand binding [17,18,19,20]. Lipid Determination—Serum total cholesterol levels were measured with a Determiner TC555 kit (Kyowa Medex, Tokyo, Japan). The liver samples were homogenized in a Polytron in chloroform/methanol (2:1, v/v) and lipid was extracted as previously described [24] and analyzed for cholesterol content with a Determiner TC555 kit (Kyowa Medex). Pool size was determined as the total bile acid content of small intestine, gallbladder, and the liver, combined as described [25] These organs were homogenized in 20 ml of ethanol (at 60 °C), the extract was filtered and a 1-ml aliquot dried with a centrifuge concentrator at room temperature. For CYP7A1, HMG-CoA reductase, and LDL receptor, Northern blot analysis was performed with the use of [␣-32P]UTP-labeled antisense riboprobes. Free thyroxine (ng/dl) at baseline (regular chow) and after induction of hypothyroidism (MMI/PTU chow)
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