Triiodothyronine increases rat apolipoprotein A-I synthesis and alters high-density lipoprotein composition in vivo

Role of thyroid receptor β in lipid metabolism

Thyroid hormones (THs) exert their actions by binding to thyroid hormone receptors (TRs) and thereby affect tissue differentiation, development, and metabolism in most tissues. TH-deficiency creates a less favorable lipid profile (e.g. increased plasma cholesterol levels), whereas TH-excess is associated with both positive (e.g. reduced plasma cholesterol levels) and negative (e.g. increased heart rate) effects. TRs are encoded by two genes, THRA and THRB, which, by alternative splicing, generate several isoforms (e.g. TRα1, TRα2, TRβ1, and TRβ2). TRα, the major TR in the heart, is crucial for heart rate and for cardiac contractility and relaxation, whereas TRβ1, the major TR in the liver, is important for lipid metabolism. Selective modulation of TRβ1 is thus considered as a potential therapeutic target to treat dyslipidemia without cardiac side effects. Several selective TH analogs have been tested in preclinical studies with promising results, but only a few of these compounds have so far been tested in clinical studies. This review focuses on the role of THs, TRs, and selective and non-selective TH analogs in lipid metabolism. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Piperine, an active principle from Piper nigrum, modulates hormonal and apo lipoprotein profiles in hyperlipidemic rats

PURPOSE

To study the effect of piperine, an alkaloid, on thyroid hormones and apolipoproteins in high-fat-diet (HFD) and antithyroid drug-induced hyperlipidemic rats.

EXPERIMENTAL

Male Wistar rats were first divided into two groups, control diet and high-fat diet (HFD) and then subdivided into four subgroups of ten animals each. The animals were treated with the following regimens for 10 weeks: 1% carboxymethyl cellulose; 10 mg carbimazole (CM)/kg body weight; 10 mg CM + 40 mg piperine/kg body weight, and 10 mg CM + 2 mg atorvastatin /ATV//kg body weight. Lipid profiles, hormone levels, and apolipoprotein levels were studied in all groups.

RESULTS

HFD and/or CM administration significantly elevated the plasma levels of total cholesterol, VLDL, LDL, triglycerides, free fatty acids, and phospholipids, but significantly reduced the HDL levels. Moreover, CM administration significantly reduced apo A-I levels and T3, T4 and testosterone levels while significantly elevating plasma apo B, thyroid stimulating hormone (TSH) and insulin levels. The simultaneous administration of piperine and HFD significantly reduced plasma lipids and lipoproteins levels, except for HDL, which was significantly elevated. Piperine supplementation also improved the plasma levels of apo A-I, T3, T4, testosterone, and I and significantly reduced apo B, TSH, and insulin to near normal levels.

CONCLUSIONS

The data presented here provide evidence that piperine possesses thyrogenic activity, thus modulating apolipoprotein levels and insulin resistance in HFD-fed rats, opening a new view in the management of dyslipidemia by dietary supplementation with nutrients.

Changes in serum apolipoprotein concentrations after L-thyroxine therapy in infants with congenital hypothyroidism

To further characterize the impact of thyroid hormones on the serum lipid profile, we studied serum apolipoproteins in infants with congenital hypothyroidism before and after L-thyroxine (L-T4) replacement therapy. Serum high-density lipoprotein cholesterol (HDLC) decreased after L-T4 therapy. Total cholesterol (TC) and low-density lipoprotein cholesterol (LDLC) levels did not change significantly after therapy. Two months after L-T4 replacement therapy, serum apolipoprotein A-I (apo A-I), C-III, and E declined and apo B increased significantly. No significant changes were observed for serum concentrations of apo A-II and C-II after L-T4 substitution. We conclude that in infants, thyroid hormone reduces serum levels of apo A-I, the principal protein component of HDLC, and this may contribute to the decline of serum HDLC concentrations after L-T4 replacement therapy.

Effects of a major gene for apolipoprotein A-I concentration are thyroid hormone dependent in Mexican Americans

Apolipoprotein A-I (apoA-I) is the principal protein component of HDL cholesterol. The thyroid hormone triiodothryonine (T3) is known to be a potent mediator of expression of the apoA-I structural gene (APOA1). Using complex segregation analysis, we detected a major gene influencing plasma concentration of apoA-I and examined its interaction with T3 serum level in Mexican Americans participating in the San Antonio Family Heart Study. Strong evidence for a major locus with two alleles (A and a) determining apoA-I level was obtained when interaction with T3 was allowed. The major gene appears not to be linked to the APOA1 structural locus. Genotypes differed significantly in their relationships to T3 level. The AA and Aa genotypes showed a positive relationship with T3 level, while the rarer aa homozygote showed a strong negative relationship with T3. The relative variance in apoA-I concentration due to this major gene varied from 56% to 18%, depending on T3 level. On average, the major gene accounts for 30% of apoA-I variation, and shared-household effects account for an additional 11%. These findings suggest that thyroid hormone has an important role in the genetic control of lipoprotein metabolism.

The rat, a useful animal model for pharmacological studies on apolipoprotein E

Apolipoprotein E is a major protein component of lipoproteins and plays an important role in cholesterol transport. The structure of the gene and the polymorphism of apolipoprotein E have been studied in human and rat, which show similar structures of apolipoprotein E. The wide tissue distribution of this apolipoprotein suggests diverse functions like cholesterol distribution between cells, intracellular cholesterol trafficking and tissue reparation. Nevertheless, the presence of apolipoprotein E in atherosclerotic plaques and amyloid deposits in brains of Alzheimer's disease patients also indicate pathologic functions staying misunderstood. The aim of this paper is to review the present knowledge on the distribution of apolipoprotein E between the different organs with the related functions and to make an overview of the implications of this apolipoprotein is physiological events and pathological states in the rat. The rat is widely used for drug metabolism studies. Its serum levels are 5-10 times higher than in human and thus this animal provides an useful pharmacological model to elucidate the functions of apo E.

Thyroid hormone modulates apolipoprotein-AI gene expression at the post-transcriptional level in Hep G2 cells

Hyperthyroidism is associated with elevated plasma levels of apolipoprotein AI (apo AI). We have examined the effects of 3,3',-5-triiodothyronine on apo AI mRNA, transcription run-on activity, apo AI mRNA half-life, and the rate of protein synthesis in Hep G2 cells, to understand the molecular mechanism by which thyroid hormone regulates apo AI gene expression. Incubation with thyroid hormone increased the apo AI and apo AII mRNA concentrations twofold. Cycloheximide alone caused a significant increase in apo AI mRNA. Nuclear run-on assays indicate that thyroid hormone did not change the rate of the apo AI gene transcription at 6, 12 or 24 h, showing that thyroid hormone did not modulate apo AI gene transcription. Kinetic studies performed in the presence of actinomycin D showed that the half-life of apo AI mRNA was increased 2-3-fold by thyroid hormone over control cells. Thyroid hormone did not change the incorporation of [35S]methionine into immunoprecipitable apo AI. Pulse-chase experiments demonstrated that there was no change in the secretion and degradation rates of labeled apo AI in response to T3. This suggests that thyroid hormone does not affect the catabolism of apo AI (degradation or/and uptake) and that translation control strongly influences the regulation of apo AI gene expression. The stabilization of apo AI mRNA by thyroid hormone and its role in translation remain to be elucidated.

The modulation of apolipoprotein E gene expression by 3,3'-5-triiodothyronine in HepG2 cells occurs at transcriptional and post-transcriptional levels

The regulation of the synthesis and secretion of apolipoprotein E (apoE) is incompletely understood. This study examines the mechanisms responsible for regulating apoE gene expression in HepG2 cells by thyroid hormone (3,3'-5-triiodothyronine). The secretion rate of apoE was by thyroid hormone increased (1.5-1.8-fold) in pulse/chase experiments. Thyroid hormone doubled apoE mRNA concentration as determined by Northern-blot analysis. Inhibition of protein synthesis by cycloheximide increased the thyroid-hormone-induced stimulation of apoE mRNA. This suggests that the synthesis of new protein is not required for thyroid hormone to stimulate apoE mRNA. Actinomycin D was used to inhibit new transcription; there was a more rapid degradation of mature apoE mRNA in thyroid hormone-treated HepG2 cells than in control cells, suggesting that thyroid hormone acts post-transcriptionally to regulate apoE gene expression. Cycloheximide blocked the action of thyroid hormone, suggesting that thyroid hormone regulates the turnover of apoE mRNA via the synthesis of de novo protein. Nuclear run-on transcription assays demonstrated that thyroid hormone stimulated apoE gene transcription threefold in 24 h. These findings indicate that the expression of the apoE gene is controlled at both transcriptional and post-transcriptional loci by the thyroid hormone.

Human apolipoprotein A-I forms small high density lipoprotein particles in rats in vivo

The effects of injection of purified human or rat apolipoprotein (apo) A-I (1.7 mg/100 g body weight) on the size and composition of rat high density lipoprotein (HDL) particles have been investigated. The injection of human apo A-I results in the formation (over a period of 3 to 6 h) of a population of smaller HDL particles resembling human HDL3. This population of smaller particles contains human apo A-I and rat apo A-IV but lacks rat apo A-I and rat apo E. Small HDL3-like particles are not detected in rat plasma following the injection of rat apo A-I. Associated with the injection of either human or rat apo A-I is a gradual increase of plasma cholesterol levels of 20 to 50% (over 24 h) and the appearance of larger HDL particles. The results suggest that the smaller HDL particles in human plasma compared to rat plasma are not simply due to the action of lipid modifying enzymes or lipid transfer proteins but a specific property of human apo A-I.