Identification of functional positive and negative thyroid hormone-responsive elements in the rat apolipoprotein AI promoter

Prospective Analysis of Lipid Variations in Hyperthyroid Subjects from Lahore, Pakistan

Perturbations in the actions of T3 and T4 influence the normal metabolic pathways. Responsiveness of lipid biomarkers like LDL-C, HDL-C, TC, TG, Apo-A, and Apo-B after rehabilitation of thyroid profile attaining euthyroid state was determined. A total of 179 age-matched subjects of both genders were recruited for this research. Sixty healthy controls, thirty-four subclinical, fifty overt hyperthyroid, and thirty-five follow-up subjects having 3 months of Carbimazole therapy were enrolled. Biochemical analysis was performed by chemistry analyzer, RIA, and ELISA. One-way ANOVA was applied for the statistical analysis, while significance (P < 0.05) of means was compared by the Student-Newman-Keuls (SNK) test. Pronounced reduction (P < 0.001) of cholesterol in overt as compared to control and subclinical was noticed, whereas marked improvement (P < 0.001) was evidenced in follow-up. Prominent elevation (P < 0.05) of TG in follow-up was evidenced as compared to control. Overt presented marked reduction of HDL-C as compared to subclinical and control (P < 0.01 and P < 0.001), respectively. Pronounced elevation (P < 0.001) of HDL-C was evidenced after treatment. Overt presented reduction of LDL-C as compared to subclinical and control (P < 0.01 and P < 0.05, respectively). The follow-up group demonstrated considerable (P < 0.001) improvement of LDL-C after treatment and elevation (P < 0.05) as compared to control. Overt presented reduction of Apo-B as compared to subclinical and control (P < 0.05 and P < 0.001, respectively). Improvement (P < 0.05) of Apo-B was evidenced in follow-up. Reduction (P < 0.05) of Apo-A in overt as compared to control and elevation (P < 0.05) in follow-up as compared to overt was evidenced. Conclusively, improvement after treatment was evidenced in lipid profile.

Type 1 Deiodinase Regulates ApoA-I Gene Expression and ApoA-I Synthesis Independent of Thyroid Hormone Signaling

OBJECTIVE

Plasma levels of high-density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (ApoA-I) are reduced in individuals with defective insulin signaling. Initial studies using liver-specific insulin receptor (InsR) knockout mice identified reduced expression of type 1 deiodinase (Dio1) as a potentially novel link between defective hepatic insulin signaling and reduced expression of the ApoA-I gene. Our objective was to examine the regulation of ApoA-I expression by Dio1.

APPROACH AND RESULTS

Acute inactivation of InsR by adenoviral delivery of Cre recombinase to InsR floxed mice reduced HDL-C and expression of both ApoA-I and Dio1. Overexpression of Dio1 in InsR knockout mice restored HDL-C and ApoA-I levels and increased the expression of ApoA-I. Dio1 knockout mice had low expression of ApoA-I and reduced serum levels of HDL-C and ApoA-I. Treatment of C57BL/6J mice with antisense to Dio1 reduced ApoA-I mRNA, HDL-C, and serum ApoA-I. Hepatic 3,5,3'-triiodothyronine content was normal or elevated in InsR knockout mice or Dio1 knockout mice. Knockdown of either InsR or Dio1 by siRNA in HepG2 cells decreased the expression of ApoA-I and ApoA-I synthesis and secretion. siRNA knockdown of InsR or Dio1 decreased activity of a region of the ApoA-I promoter lacking thyroid hormone response elements (region B). Electrophoretic mobility shift assay demonstrated that reduced Dio1 expression decreased the binding of nuclear proteins to region B.

CONCLUSIONS

Reductions in Dio1 expression reduce the expression of ApoA-I in a 3,5,3'-triiodothyronine-/thyroid hormone response element-independent manner.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Inhibition of apolipoprotein A-I expression by TNF-alpha in HepG2 cells: requirement for c-jun

Tumor necrosis factor alpha (TNF α) signals in part through the mitogen activated protein (MAP) kinase c-jun-N-terminal kinase (JNK). Activation of JNK has been shown to promote insulin resistance and dyslipidemia, including reductions in plasma high-density lipoprotein (HDL) and apolipoprotein A-I (apo A-I). To examine how TNF α-mediated JNK activation inhibits hepatic apo A-I production, the effects of c-jun activation on apo A-I gene expression were examined in HepG2 cells. Apo A-I gene expression and promoter activity were measured by Northern and Western blotting and transient transfection. Transient transfection and siRNA were used to specifically over-express or knockout c-jun, c-jun-N-terminal kinase-1 and -2 (JNK1 and JNK2, respectively) and mitogen-activated protein kinase-4 (MKK4). TNF α-treatment of HepG2 cells induced rapid phosphorylation of c-jun on serine 63. In cells treated with phorbol-12-myristate-13-acetate (PMA), apo A-I gene promoter activity was inhibited and apo A-I mRNA content and apo A-I protein secretion decreased. Likewise, over-expression of JNK1 and JNK2 inhibited apo A-I promoter activity. Over-expression of constitutively active MKK4, an upstream protein kinase that directly activates JNK, also inhibited apo A-I promoter activity, while over-expression of a dominant-negative MKK4 de-repressed apo A-I promoter activity in TNF α-treated cells. Inhibition of c-jun synthesis using siRNA but not a control siRNA prevented TNF α-mediated inhibition of apo A-I. These results suggest that the MKK4/JNK/c-jun signaling pathway mediates TNF α-dependent inhibition of apo A-I synthesis.

Induction of apolipoprotein A-I gene expression by glucagon-like peptide-1 and exendin-4 in hepatocytes but not intestinal cells

OBJECTIVE

Diabetic dyslipidemia is an important risk factor for the development of macrovascular complications. Recent clinical trials suggest that diabetics treated with glucagon-like peptide-1 (GLP-1) have normalized lipid levels, including an increase in plasma high-density lipoprotein cholesterol (HDLc) levels.

METHODS

To determine if GLP-1 (7-36 amide) and the GLP-1-like insulinotropic peptide exendin-4 regulate expression of apolipoprotein A-I (apo A-I), the primary anti-atherogenic component of high-density lipoprotein (HDL), HepG2 hepatocytes and Caco-2 intestinal cells, representative of tissues that express the majority of apo A-I, were treated with increasing amounts of each peptide and apo A-I gene expression was measured in the conditioned medium.

RESULTS

Apo A-I secretion increased in both GLP-1 and exendin-4-treated HepG2, but not Caco-2 cells, and this was accompanied by similar changes in apo A-I mRNA levels and apo A-I promoter activity. Induction of apo A-I promoter activity by GLP-1 and exendin-4 required an SP1-responsive element. Hepatic ATP binding cassette protein A1 (ABCA1) expression, but not scavenger receptor class B type1 receptor expression was also induced by GLP-1 and exendin-4.

CONCLUSIONS

These results suggest that GLP-1- and exendin-4-mediated changes in HDLc are likely due to changes in hepatic expression of apo A-I and ABCA1.

Inhibition of apolipoprotein A-I gene expression by obesity-associated endocannabinoids

Obesity is associated with increased serum endocannabinoid (EC) levels and decreased high-density lipoprotein cholesterol (HDLc). Apolipoprotein A-I (apo A-I), the primary protein component of HDL is expressed primarily in the liver and small intestine. To determine whether ECs regulate apo A-I gene expression directly, the effect of the obesity-associated ECs anandamide and 2-arachidonylglycerol on apo A-I gene expression was examined in the hepatocyte cell line HepG2 and the intestinal cell line Caco-2. Apo A-I protein secretion was suppressed nearly 50% by anandamide and 2-arachidonoylglycerol in a dose-dependent manner in both cell lines. Anandamide treatment suppressed both apo A-I mRNA and apo A-I gene promoter activity in both cell lines. Studies using apo A-I promoter deletion constructs indicated that repression of apo A-I promoter activity by anandamide requires a previously identified nuclear receptor binding site designated as site A. Furthermore, anandamide-treatment inhibited protein-DNA complex formation with the site A probe. Exogenous over expression of cannabinoid receptor 1 (CBR1) in HepG2 cells suppressed apo A-I promoter activity, while in Caco-2 cells, exogenous expression of both CBR1 and CBR2 could repress apo A-I promoter activity. The suppressive effect of anandamide on apo A-I promoter activity in Hep G2 cells could be inhibited by CBR1 antagonist AM251 but not by AM630, a selective and potent CBR2 inhibitor. These results indicate that ECs directly suppress apo A-I gene expression in both hepatocytes and intestinal cells, contributing to the decrease in serum HDLc in obese individuals.

Effect of retinoic acid on cell proliferation and differentiation as well as on lipid synthesis, lipoprotein secretion, and apolipoprotein biogenesis

Dietary vitamin A and its active metabolites are essential nutrients for many functions as well as potent regulators of gene transcription and growth. Although the epithelium of the small intestine is characterized by rapid and constant renewal and enterocytes play a central role in the absorption and metabolism of alimentary retinol, very little is known about the function of retinoids in the human gastrointestinal epithelium and mechanisms by which programs engage the cell cycle are poorly understood. We have assessed the effects of 10 microM 9- and 13-cis-retinoic acid (RA) on proliferation and differentiation processes, lipid esterification, apolipoprotein (apo) biogenesis and lipoprotein secretion along with nuclear factor gene transcription. Treatment of Caco-2 cells with RA at different concentrations and incubation periods revealed the reduction of thymidine incorporation in 60% preconfluent or 100% confluent cells. Concomitantly, RA 1) modulated D-type cyclins by reducing the mitogen-sensitive cyclin D1 and upregulating cyclin D3 expressions and 2) caused a trend of increase in p38 MAPK, which triggers CDX2, a central protein in cell differentiation. RA remained without effect on lipoprotein output and apo synthesis, even for apo A-I that possesses RARE in its promoter. RA, in combination with 22-hydroxycholesterol, could induce apo A-I gene expression without any impact on apo A-I mass. Only the gene expression of peroxisome proliferator-activated receptor (PPAR)beta, retinoic receptor (RAR)beta, and RARgamma was augmented and no alteration was noted in PPARalpha, PPARgamma, liver X receptor (LXR)alpha, LXRbeta, and retinoid X receptors. Taken together, these data highlight RA-induced cell differentiation via specific signaling without a significant impact on apo A-I synthesis.

Differential regulation of apolipoprotein A-I gene expression by vitamin D receptor modulators

We have found that 1,25-dihydroxy-cholecalciferol (1,25-(OH)(2)D(3)) represses the expression of the apolipoprotein A-I (apo A-I) gene in hepatocytes. In this manuscript we examined the effects of the vitamin D receptor (VDR) modulators EB1089 (EB) and ZK191784 (ZK) on expression of the apo A-I gene in liver (HepG2) and in intestinal (Caco-2) cells. In HepG2 cells, EB and ZK induced apo A-I secretion and gene promoter activity in a dose-dependent manner. This induction did not require the VDR since antisense-mediated inhibition of VDR had no appreciable effect on apo A-I promoter activity in cells treated with EB or ZK. Although repression of apo A-I gene expression by 1,25-(OH)(2)D(3) in hepatocytes required nuclear receptor binding to site A in the promoter, this cis-element was insufficient for induction of apo-AI by EB and ZK. In Caco-2 cells, treatment with 1,25-(OH)(2)D(3) had no effect on apo A-I protein secretion or promoter activity while EB induced and ZK inhibited apo A-I gene expression. Gel shift assays showed that none of the treatments resulted in a change in site A binding activity. These results indicate that VDR modulators in hepatocytes and intestinal cells differentially regulate expression of the apo A-I gene.

A phosphorimager-based filter binding thyroid hormone receptor competition assay for chemical screening

INTRODUCTION

A phosphorimager-based filter binding thyroid hormone receptor (THR) competition assay has been developed for use in verifying hits from compound library screens.

METHODS

This method employs in vitro translated ligand binding domains (LBDs) of THRalpha and THRbeta, separation through nitrocellulose via a 96-well vacuum manifold, and analysis of receptor-bound radioactivity by phosphorimaging.

RESULTS

A standard curve of [I(125)]T3 showed a linear response over the dynamic range of a competition assay, and a comparison of Sephadex G-25 column separation and gamma counting with en masse filtration and phosphorimaging revealed similar IC(50) and K(i) values when using unlabeled T3 as competitor. In addition, this method produced IC(50) and K(i) values for the known T3 competitors [3,5-Dimethyl-4-(4'-hydoxy-3'-isopropylbenzyl) phenoxy] acetic acid (GC-1) and 3,5-diiodothyropropionic acid (DITPA) similar to those reported elsewhere.

DISCUSSION

These data suggest that filtration and phosphorimaging adequately and properly reproduces binding values associated with THR competition. Further, this method gave a 3-fold reduction in time and a 40-fold reduction in radioactive waste over the column-based method. These reductions allow for a substantial increase in assay throughput. Taken together, these data suggest that en masse filtration and phosphorimaging is an efficient and tractable method for verifying large groups of putative T3 competitors in vitro.

The effect of select nutrients on serum high-density lipoprotein cholesterol and apolipoprotein A-I levels

One of the factors contributing to the increased risk of developing premature atherosclerosis is low plasma concentrations of high-density lipoprotein (HDL) cholesterol (HDLc). Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein A-I (apo A-I). The low plasma concentrations of HDL could be the result of increased fractional clearance and reduced expression of apo A-I. To this end, nutrients play an important role in modulating the fractional clearance rate, as well as the rate of apo A-I gene expression. Because medical nutrition therapy constitutes the cornerstone of management of dyslipidemias, it is essential to understand the mechanisms underlying the changes in HDL level in response to alterations in dietary intake. In this review, we will discuss the effect of select nutrients on serum HDLc and apo A-I levels. Specifically, we will review the literature on the effect of carbohydrates, fatty acids, and ketones, as well as some of the nutrient-related metabolites, such as glucosamine and the prostanoids, on apo A-I gene expression. Because there are multiple mechanisms involved in the regulation of serum HDLc levels, changes in gene transcription do not necessarily correlate with clinical observations on serum levels of HDLc.

Ascorbic acid and alpha-tocopherol down-regulate apolipoprotein A-I gene expression in HepG2 and Caco-2 cell lines

HepG2 cells and Caco-2 cells were treated with various concentrations of select antioxidants to study some of the molecular pathways underlying antioxidant-related changes in apolipoprotein A-I (apoA-I) expression. Both alpha-tocopherol and ascorbate treatment over a time course of 72 hours caused a significant reduction in apoA-I messenger RNA and protein levels in a dose-dependent fashion. Albumin levels did not change with any treatment, suggesting that the effect is protein-specific. Similar changes were seen in Caco-2 cells. In contrast, apoA-I messenger RNA and protein levels significantly increased after 28 and 280 micromol/L dimethyl sulfoxide (DMSO) treatment. Cells were transfected with chloramphenicol acetyltransferase (CAT) reporter gene plasmid containing the full-length apoA-I promoter to determine if these changes occur at a transcriptional level, and after 24 hours, the HepG2 or Caco-2 cells were treated with varying concentrations of ascorbate or alpha-tocopherol. At the highest concentration of ascorbate and alpha-tocopherol used (5 mmol/L), there was a significant reduction in apoA-I promoter activity. DMSO treatment up-regulated apoA-I promoter activity, whereas increasing oxidative load with 50, 100, and 200 micromol/L hydrogen peroxide treatment did not significantly alter apoA-I promoter activity. Studies with deletional constructs of the promoter containing or lacking the antioxidant response element (ARE) indicated that the effect of ascorbate and alpha-tocopherol, unlike that of DMSO, was independent of this ARE. Using a series of apoA-I deletion constructs, it was shown that site A-containing segment of the promoter has a critical regulatory element. However, electromobility shift assays indicated that there was no significant change in nuclear factor binding activity to site A as a result of treatment with ascorbate or alpha-tocopherol. As expected, treatment with DMSO increased factor binding to the previously described ARE. It is concluded that the apoA-I promoter-stimulating effect of DMSO may be independent of its antioxidant activity and that some antioxidants at very high concentrations may have suppressive effect on the apoA-I gene expression. It appears that the inhibitory effect of ascorbate or alpha-tocopherol on the apoA-I promoter is either indirect or is the result of posttranslational modifications of the nuclear binding factors. The previously described ARE is not a response element for the ascorbate or alpha-tocopherol.

Inhibition of apolipoprotein AI gene expression by 1, 25-dihydroxyvitamin D3

Members of the steroid receptor superfamily are known to alter the transcription of apolipoprotein AI (apo AI), the major apoprotein of high-density lipoprotein (HDL). To assess the role of vitamin D receptor (VDR) in apo AI gene expression, we investigated the effect of 1alpha, 25-dihydroxycholecalciferol (1, 25-(OH)2 D3) as well as the vitamin D antagonist ZK-191784 (ZK), on apo AI gene expression and promoter activity in the human hepatoma cell line HepG2. Apo AI secretion and mRNA levels were both suppressed in a dose-dependent manner in HepG2 cells treated 1, 25-(OH)2 D3. This was accompanied by a similar decrease in apo AI promoter activity. Mapping of the vitamin D response element showed that suppression required a region of the apo AI gene promoter identified previously to contain site A. However, vitamin D treatment had no effect on nuclear factor binding to site A of the apo AI promoter. Treatment with vitamin D receptor antagonist ZK inhibited the ability of 1, 25-(OH)2 D3 to repress apo AI promoter activity, while higher doses of ZK increased apo AI promoter activity. ZK did not alter estradiol stimulated apo AI promoter activity. The VDR antisense ODN had no effect on apo AI promoter activity in control cells, however, it reversed the repression normally seen in cells treated with 1, 25-(OH)2D3. It is concluded that 1, 25-(OH)2 D3 suppresses apo A1 gene expression at the transcriptional level, possibly by altering coactivators or corepressors. This effect requires the VDR as well as a vitamin D response element in the apo AI promoter.

Thyroid hormone action in the heart

The heart is a major target organ for thyroid hormone action, and marked changes occur in cardiac function in patients with hypo- or hyperthyroidism. T(3)-induced changes in cardiac function can result from direct or indirect T(3) effects. Direct effects result from T(3) action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T(3) effects, which occur independent of nuclear T(3) receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T(3) effects are mediated by the binding of T(3) to specific nuclear receptor proteins, which results in increased transcription of T(3)-responsive cardiac genes. The T(3) receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. T(3) also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum. This T(3) effect results from T(3)-induced increases in the level of the mRNA coding for the sarcoplasmic reticulum calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the sarcoplasmic reticulum.

Induction of the apolipoprotein AI promoter by Sp1 is repressed by saturated fatty acids

Insulin induces transcription of the hepatic apolipoprotein AI (apo AI) gene by increasing Sp1 binding to the promoter. To determine the effect of fatty acids on this process, HepG2 cells cotransfected with the plasmid pAI.474.CAT containing the full-length apo AI promoter and the Sp1-expressing plasmid, pCMV-Sp1, were studied. Chloramphenicol acetyl transferase (CAT) activity (% acetylation) increased 1.98-fold in cells receiving the Sp1 expression construct relative to control cells (46.4% +/- 0.6% v 23.4% +/- 1.3%, P < .05). Treatment of cells with 3 saturated fatty acids, stearic, myristic, and palmitic acid, repressed the ability of exogenous Sp1 to induce apo AI reporter gene expression (15.2% +/- 1.7%, 22.5% +/- 0.3%, 22.9% +/- 0.1%, 23.5% +/- 0.8%, respectively, P < .05). Unsaturated fatty acids, oleic, linoleic, or linolenic acid had no effect on Sp1-mediated induction of the apo AI promoter. In the presence of the trans fatty acids, CAT activity in the Sp1-transfected cells was similar to control cells (16.7% +/- 3.3%, 19.3% +/- 0.5%, and 21.0% +/- 2.1% acetylation in cells exposed to elaidic acid, linolelaidic, or linolenelaidic acid, respectively). In cells treated with an equimolar mixture of oleic acid and stearic acid, apo AI promoter activity was suppressed in a manner similar to that observed in stearic acid-treated cells. Insulin (100 microU/mL) induced apo AI promoter activity 2.9-fold (22.4% +/- 1.7% v 7.8% +/- 2.4%, P < .05). However, in the presence of stearic acid, insulin was unable to induce apo AI promoter (6.3% +/- 1.6%). Stearic acid treatment did not alter Sp1-DNA binding as measured by gel shift analysis. Therefore, saturated fatty acids blunt Sp1 induction of apo AI promoter probably at a step beyond DNA binding.

Thyroid hormone receptor-dependent transcriptional regulation of fibrinogen and coagulation proteins

Thyroid hormone (T(3)) regulates growth, development, and differentiation. These activities are mediated by the nuclear thyroid hormone receptors (TRs), which belong to the steroid/TR superfamily of ligand-dependent transcription factors. The effect of T(3) treatment on target gene regulation was investigated in a TRalpha-overexpressing hepatoma cell line (HepG2-TRalpha), by performing cDNA microarrays. We demonstrate that 148 of the 7597 genes represented were up-regulated by T(3), including fibrinogen and several other components of the coagulation factor system. To confirm the microarray results, fibrinogen and a small number of the blood clotting components were further investigated using quantitative RT-PCR. The T(3)-induction ratios observed with quantitative RT-PCR for factors such as thrombin (8-fold), coagulation factor X (4.9-fold), and hepatoglobin (30-fold) were similar to those observed by the cDNA microarray analysis. Further investigation, using HepG2-TRalpha (cell lines, revealed a 2- to 3-fold induction of fibrinogen transcription after 24 h of T(3) treatment. In addition, T(3) treatment increased the level of fibrinogen protein expression 2.5- to 6-fold at 48 h. The protein synthesis inhibitor, cycloheximide, did not inhibit the induction of fibrinogen by T(3), indicating that this regulation was direct. Furthermore, transcription run-on experiments indicate that the induction of fibrinogen by T(3) is regulated largely at the level of transcription. Similar observations were made on the regulation of fibrinogen by T(3) using rats that received surgical thyroidectomy (TX) as an in vivo model. These results suggest that T(3) plays an important role in the process of blood coagulation and inflammation and may contribute to the understanding of the association between thyroid diseases and the misregulation of the inflammatory and clotting profile evident in the circulatory system of these patients.

Effect of glucosamine on apolipoprotein AI mRNA stabilization and expression in HepG2 cells

Previously published studies suggest that an alteration in hexosamine flux induces a state of insulin resistance in muscle, liver, and other cell types. Glucosamine also alters the expression of several genes through an effect on transcription factors such as Sp1. Since the anti-atherogenic protein apolipoprotein AI (apoAI) is positively regulated by insulin, at least partly through its effect on Sp1, we investigated the effect of glucosamine on apoAI gene expression in the hepatocyte cell line, HepG2. By 24 hours of treatment with 0.1, 1, or 3 mmol/L glucosamine, the amount of apoAI protein secreted into the culture media increased 1.8-fold, 5.5-fold, and 2.3-fold, respectively. The decline in apoAI secretion at the highest glucosamine levels may be due to toxicity since the percentage of cells able to exclude trypan blue was lower in this group than in control cells (98.5% +/- 1.5% in control cells v 89.2% +/- 2.1% in cells treated with 3 mmol/L glucosamine, P <.01). ApoAI mRNA levels increased 2.4-fold in hepatocytes treated with 1 mmol/L glucosamine for 24 hours (1,158.1 +/- 78.8 v 482.2 +/- 24.3 arbitrary integrator units [AIU], P <.02), suggesting that the increase in apoAI protein secretion was due, at least partly, to an increase in apoAI mRNA levels. However, glucosamine had no effect on apoAI gene transcription rate as measured by nuclear runoff analysis (3,155 +/- 46.0 in control cells v 3,181 +/- 30.0 AIU in glucosamine-treated cells). Similarly, apoAI promoter activity measured in HepG2 cell transfected with an apoAI reporter plasmid containing the full-length apoAI promoter including an insulin-responsive Sp1 binding site did not change with glucosamine addition. In this assay, the chloramphenicol acetyltransferase (CAT) activity was 12.4% +/- 3.1%, 10.1% +/- 2.4%, 9.8% +/- 2.0%, 9.7% +/- 2.2%, and 11.9% +/- 2.9% in cells treated with 0, 0.03, 0.1, 0.3, and 1 mmol/L glucosamine, respectively. The apoAI mRNA turnover studies showed that 1 mmol/L glucosamine treatment of HepG2 cells was associated with increased apoAI mRNA half-life, from 7.6 to 16.6 hours. These findings suggest that increases in apoAI gene expression by glucosamine occur primarily through stabilizing apoAI mRNA.

Transcriptional control of apolipoprotein A-I gene expression in diabetes

Cardiovascular disease continues to be the leading cause of mortality in diabetes. One of the factors contributing to the increased risk is the high prevalence rate of low plasma concentrations of HDL cholesterol. Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein (apo) A-I. The reduced plasma concentrations of HDL could be the result of increased fractional clearance of HDL and reduced expression of apo A-I. In animal models of diabetes and in cell cultures treated with high concentrations of glucose, apo A-I expression is reduced. In this review we will discuss the alterations in transcriptional control of apo A-I in diabetes. The role of select nutritional and hormonal alterations commonly found in diabetes will be reviewed. Specifically, we will review the literature on the effect of hyperglycemia, hypoinsulinemia, and ketoacidosis, as well as the role of various mediators of insulin resistance, such as fatty acids, cytokines, and prostanoids, on apo A-I promoter activity. Identifying the mechanisms that modulate apo A-I gene expression will aid in the new development of therapeutic agents that increase plasma apo A-I and HDL concentrations.

Cyclooxygenase inhibition is associated with downregulation of apolipoprotein AI promoter activity in cultured hepatoma cell line HepG2

Prostanoids have been implicated in the transcriptional control of several genes. Since prostanoid synthesis inhibitors are commonly used in subjects with coronary heart disease we studied the effect of cyclooxygenase (COX) inhibition on apolipoprotein AI (apoAI) expression in a human hepatoma cell line (HepG2) transfected with full-length apoAI promoter attached to the chloramphenicol acetyl transferase (CAT) reporter gene. To control for transfection efficiency, the cells were cotransfected with the plasmid pCMV.SPORT-beta-gal containing the beta-galactosidase gene driven by the cytomegalovirus promoter. Treatment of these cells with varying concentrations of indomethacin (INDO, 0, 50, 100, and 300 micromol/L) resulted in a dose-dependent decrease in apoAI promoter activity (% acetylation corrected for beta-galactosidase activity: were 46.1 +/- 2.6, 29.9 +/- 1.2, 25.2 +/- 2.9, and 17.2 +/- 2.8, respectively, P <.001). INDO treatment did not cause significant changes in beta-galactosidase activity. A similar reduction in apoAI promoter activity was found after treating the cells with 50 micromol/L acetylsalicylic acid (ASA) (31.8 +/- 1.8%, P <.001), suggesting that the effect of INDO is related to COX inhibition rather than a peculiar effect of INDO. Nuclear run-off assays indicated that treatment of cells with 50 micromol/L INDO resulted in 31.4% reduction in apo A1 transcription rate (P <.0002). Northern blot analysis of RNA from HepG2 cells treated with 50 micromol/L of INDO for 72 hours showed that the apoAI mRNA concentration relative to G3PDH mRNA was 4,043.0 +/- 84.6 and 3,064.0 +/- 49.8 in control and INDO-treated cells, respectively (P <.0006). Kinetic studies of apoAI mRNA in HepG2 cells indicated that the half-life of apoAI mRNA was not significantly altered with 50 micromol/L INDO treatment. Apo AI mRNA half-life was 25.3 hours in control cells and 26.9 hours in INDO-treated cells. Western blot analysis of culture media of HepG2 cells treated with 50 micromol/L of INDO for 72 hours showed a significant reduction in apoAI protein (6,760.0 +/- 318.1 v 4,773.0 +/- 112.0 arbitrary units, P <.004). Treatment of cells with either arachidonic acid (COX substrate) or various prostanoids including prostaglandin I(2), thromboxane B(2), (+/-)5-HETE, or (+/-)12-HETE did not significantly alter apoAI promoter activity. However, prostaglandin E(1) and E(2) at the highest concentration tested (50 nmol/L) significantly repressed apoAI promoter activity. COX activity measurements in HepG2 cells verified the efficacy of COX inhibition by INDO. It is concluded that COX inhibition with INDO or ASA downregulates apoAI expression at the transcriptional level. This effect could not be attributed to either arachidonic acid excess or to a deficiency in various prostanoids tested.

Suppression of apolipoprotein AI gene expression in HepG2 cells by TNF alpha and IL-1beta

Plasma inflammatory cytokines are elevated in obese subjects as well as in those with type 2 diabetes. This presumably results in systemic insulin resistance, characterized by a pro-atherogenic plasma lipid profile and reduced apolipoprotein AI (apoAI) protein levels. To determine how cytokine-mediated insulin resistance suppresses apoAI gene expression, we investigated the effect of tumor necrosis factor alpha (TNF alpha) and interleukin-1beta (IL-1beta) on apoAI protein, mRNA, and transcriptional activity in the human hepatoma cell line HepG2. ApoAI secretion was suppressed in a dose-dependent manner in HepG2 cells treated with both cytokines. ApoAI protein levels were 2892+/-22.0, 2263+/-117, 2458+/-25.0, 3401+/-152, 2333+/-248, 1520+/-41.5 and 956.0+/-11.0 arbitrary units (AU) in cells treated with 0, 0.3, 1.0, 3.0, 10, 30, and 100 ng/ml TNF alpha, achieving statistical significance in the 30 and 100 ng/ml range (P<0.0009). ApoAI protein levels were 4055+/-360, 3697+/-101, 3347+/-327, 1561+/-33.0, 1581+/-182, 810.0+/-59.5, and 1766+/-717 AU in cells treated with similar doses of IL-1beta, achieving statistical significance within the range of 3-100 ng/ml (P<0.02). ApoAI mRNA levels were suppressed 50.8% in HepG2 cells treated with 30 ng/ml TNF alpha for 24 h (P<0.05), and remained suppressed for up to 96 h. Similarly, treatment of cells with 30 ng/ml IL-1beta for 24 h, resulted in 42.9% reduction in apoAI mRNA levels (P<0.05) and remained suppressed for up to 96 h. In order to determine if the effect of TNF alpha and IL-1beta occurs at the transcriptional level, HepG2 cells were transfected with a chloramphenicol acetyltransferase (CAT) reporter gene plasmid containing the full-length apoAI promoter, and after 24 h, treated with TNF alpha (30 ng/ml), IL-1beta (30 ng/ml), or both cytokines. CAT activity was suppressed by both cytokines (24.0+/-1.9% acetylation in control cells vs. 5.6+/-1.2% (P<0.0004), 10.2+/-1.5% (P<0.0006), and 3.9+/-0.9% acetylation (P<0.0002) in cells treated with TNF alpha, IL-1beta, and the combination of both cytokines, respectively) suggesting that cytokine-mediated suppression occurs at the transcriptional level. Using a series of apoAI deletion constructs, the cytokine response element was mapped between nucleotides -325 and -186 (relative to the transcriptional start site). This region contains a previously identified and characterized cis-element, site A, which binds several different transcription factors. Finally, electrophoretic mobility shift assays (EMSA) showed that TNF alpha treatment of HepG2 cells is associated with reduced nuclear factor binding to site A. These studies suggest that inflammatory cytokines down-regulate apoAI expression at least partly through inhibition of binding of the nuclear factors to site A of the apoAI promoter.

Effect of chromium on apolipoprotein A-I expression in HepG2 cells

OBJECTIVE

Chromium is a key micronutrient required for lipid and carbohydrate metabolism. Some but not all clinical trials have associated use of chromium supplements with improved insulin sensitivity and lipid profile including increased high-density lipoprotein cholesterol levels.

METHODS

Because apolipoprotein A-I (apoA-I) is the principal protein of high-density lipoprotein, the molecular pathways underlying chromium-related changes in apoA-I expression were studied in a human hepatoma cell line (HepG2) transfected with full-length apoA-I promoter attached to the reporter chloramphenicol acetyl transferase gene.

RESULTS

Exposure of these cells to different concentrations of chromium chloride (0, 0.5, 1.0, and 3.0 mM) resulted in a dose-dependent decrease in apoA-I promoter activity (chloramphenicol acetyl transferase activity expressed as a percentage of an internal control was 99.4 +/- 7.2% in control cells versus 87.6 +/- 5.0%, 73.4 +/- 2.3%, and 36.6 +/- 3.9%, respectively, P < 0.01). Chromium chloride at 10 mM concentration was toxic and caused death in a large number of cells. Treating HepG2 cells with other minerals known to have insulin-sensitizing effects such as magnesium (1 mM), zinc (0.2 mM), and vanadyl sulfate (0.1 mM) significantly reduced apoA-I promoter activity in the presence and absence of 100 microU/mL of insulin. Northern blot analyses showed that the apoA-I mRNA content of cells treated with 0.2 mM of chromium chloride relative to G3PDH mRNA was not significantly increased compared with controls (0.652 +/- 0.122 versus 0.745 +/- 0.143, the ratio of apoA-I to glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA in control and chromium-treated cells, respectively). Western blot analyses of proteins secreted in culture media indicated that neither chromium treatment of the HepG2 cells (858.0 +/- 151.4 arbitrary units) nor treatment with magnesium (1323.3 +/- 175.7) or vanadium (1102 +/- 78.7) significantly altered apoA-I concentrations compared with controls (1061.7 +/- 114.7). However treatment of HepG2 cells with 0.2 mM of zinc significantly reduced apoA-I concentrations (291.0 +/- 29.2 versus 1061.7 +/- 114.7; P < 0.001).

CONCLUSIONS

Supraphysiologic concentrations of chromium and other minerals with known insulin-sensitizing activity may reduce apoA-I promoter activity in cultured cells. Whether similar changes may occur in vivo remains to be shown. However, these observations do not support the use of pharmacologic amounts of chromium supplementation to enhance the cardioprotective lipid profile.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

Apolipoprotein A-I expression in rats is not altered by troglitazone

Insulin is known to upregulate apolipoprotein A-I (apoA-I) promoter activity and to increase apoA1 gene expression in vivo. To determine if enhancement of insulin action with insulin sensitizers can also increase the apoA-I expression, we studied the in vivo effect of troglitazone, a potent insulin sensitizer, on the expression of rat hepatic and intestinal apoA-I mRNA using Northern blot analysis. The plasma, hepatic, and intestinal apoA-I content was also measured with immunoblot analysis using a specific anti-rat apoA-I antiserum. Troglitazone, given mixed with rat chow (0.2%) for 18 days, did not increase either plasma or tissue apoA-I mRNA or protein content. Intestinal apoA-I mRNA content relative to glyceraldehyde-3 phosphate dehydrogenase (G(3)PDH) mRNA was significantly lower compared with hepatic tissue content in both control and troglitazone-treated rats. The effect of troglitazone on the rat apoA-I promoter was examined using transient transfection analysis in HepG2 cells transfected with the apoA-I-chloramphenicol acetyl transferase (CAT) reporter plasmid (pAI.474.CAT). CAT activity (percentage acetylation of chloramphenicol as means +/- SEM) was not significantly different in ethanol (vehicle)-treated cells compared with cells treated with troglitazone (50.5% +/- 2.5% in control cells vs 57.7% +/- 8.2% and 53.5% +/- 4.2% in cells treated with 10 and 100 mM troglitazone, respectively). It is concluded that troglitazone doses known to achieve insulin sensitization did not enhance rat apoA-I promoter activity sufficiently to result in an increased apoA-I mRNA or protein expression in the intact rat. However, peroxisome proliferator activator receptor (PPAR) agonists that have significant PPAR alpha activity in addition to their PPAR gamma effects, may well be able to induce apoA-I expression.

A distinct thyroid hormone response element mediates repression of the human cholesterol 7alpha-hydroxylase (CYP7A1) gene promoter

We examined the molecular basis by which T3 regulates the human cholesterol 7alpha-hydroxylase gene (CYP7A1) promoter. L-T3 decreased chloramphenicol acetyltransferase activity in hepatoma cells cotransfected with a plasmid encoding the T3 receptor (TR) alpha [NR1a1] and a chimeric gene containing nucleotides -372 to +61 of the human CYP7A1 gene fused to the chloramphenicol acetyltransferase structural gene. Deoxyribonuclease I footprinting revealed that recombinant TRalpha protected two regions in this segment of the human CYP7A1 gene promoter. In EMSAs, TRalpha bound to both regions. The binding was competed by oligonucleotides bearing an idealized TRalpha binding motif and abolished by mutation of these elements. In assays of promoter function, mutation of only one of the TRalpha binding sites blocked repression by T3. The results indicate that T3-dependent repression of human CYP7A1 gene expression is mediated via a novel site in the human CYP7A1 gene promoter.

Two thyroid hormone-mediated gene expression patterns in vivo identified by cDNA expression arrays in rat

Thyroid hormone (T3) is essential for normal development, differentiation and metabolic balance. Only a limited number of T3-target genes have been identified so far and their complex regulation pattern is poorly understood. We performed cDNA expression array hybridisation to identify T3-regulated genes and to investigate their expression pattern after various time points in vivo. Radioactively labelled cDNA was prepared from hepatic RNA of hypothyroid and hyperthyroid rats 6, 24 and 48 h after the administration of T3. Labelled cDNA probes were hybridised to rat Atlas Arrays. Twenty-three of 588 genes were shown to be differentially regulated, 18 of which were previously not known to be regulated by T3. The expression of 19 genes was verified by independent northern blot hybridisation. Two different expression time courses of T3 expression were observed. In a first expression profile ('early' expression) the transcription level of the target genes rises within 6 h, drops by 24 h and increases again within 48 h after the administration of T3. In a second expression profile ('late' expression) the mRNA level rose in the first 6 h and rose further by 48 h, indicating an additional regulation mechanism. Nuclear respiratory factor (NRF)-1 and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), but not NRF-2, were up-regulated within 6 h after T3 administration, suggesting NRF-1 and/or PGC-1 as key regulators for mediating the 'late' expression pattern.

Negative response elements in keratin genes mediate transcriptional repression and the cross-talk among nuclear receptors

Very little is known about the mechanisms responsible for the findings that binding of nuclear receptors (NR) to some promoter elements leads to transcriptional activation, whereas binding to others leads to repression. Case in point is the group of epidermal keratin genes and their DNA sequences responsible for repression by NR. Keratin response elements (KREs) interact with receptors for retinoic acid, thyroid hormone, and glucocorticoids. KREs, by their structure and sequence, direct the binding of retinoic acid and thyroid hormone as homodimers and glucocorticoids as monomers. Such specific DNA-receptor interactions are crucial for the repression signal of transcription. In this paper we have analyzed the interactions between the KREs and NR that lead to such repression. We have found that KREs are promoter-independent. They not only provide a docking platform for the receptors, but also play a key role in directing the receptors to bind into particular configurations and coordinating the interactions among different receptors. Both an intact KRE and an intact receptor DNA-binding domain are necessary for the regulation to occur, which emphasizes the importance of interaction between the DNA and NR for proper signaling. Furthermore, KREs allow simultaneous binding of multiple receptors, thus providing fine-tuning of transcriptional regulation. The DNA/DNA-binding domain interactions in keratin promoters exemplify tissue and gene specificity of hormone action.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Epidermal growth factor induction of apolipoprotein A-I is mediated by the Ras-MAP kinase cascade and Sp1

Insulin induces apolipoprotein A-I, apoA-I gene transcription via a membrane receptor with intrinsic tyrosine kinase activity. This finding prompted us to ask whether the gene is stimulated by epidermal growth factor (EGF), EGF a peptide hormone that binds to another member of the receptor superfamily with tyrosine kinase activity. Our data showed that like insulin, EGF increased abundance of apoA-I protein and transcription of the gene in human hepatoma, Hep G2 cells. The effects of both hormones appeared direct because their induction of apoA-I gene transcription was not affected by the protein synthesis inhibitor, cycloheximide. Although both insulin and EGF stimulate apoA-I expression, each hormone binds to a distinct membrane receptor thus suggesting differential intracellular signaling. Therefore, we used a panel of inhibitors to define the pathway(s) that mediate the actions of these hormones. Whereas, the actions of EGF required only the Ras-mitogen-activated protein, MAP kinase, those of insulin were mediated by equal participation of both the Ras-MAP kinase and protein kinase C, PKC cascades. Despite differences in signaling pathways triggered by each hormone receptor, the activation of apoA-I transcription required the participation of a single transcription factor, Sp1. Furthermore, EGF induction of transcription was attenuated by mutating the MAP kinase site at amino acid, Thr(266) rendering Sp1 phosphorylation deficient. In summary, EGF stimulation of apoA-I expression is mediated solely by the Ras-MAP kinase cascade and enhanced activity of this pathway requires Sp1 with an intact phosphorylation site at Thr(266). However, insulin induction of this gene is different and requires both Ras-MAP kinase and PKC pathways but their actions are also mediated by Sp1.

Thyroid hormone stimulates acetyl-coA carboxylase-alpha transcription in hepatocytes by modulating the composition of nuclear receptor complexes bound to a thyroid hormone response element

Triiodothyronine (T3) stimulates a 7-fold increase in transcription of the acetyl-CoA carboxylase-alpha (ACCalpha) gene in chick embryo hepatocytes. Here, we characterized an ACCalpha T3 response element (ACCalpha-T3RE) with unique functional and protein binding properties. ACCalpha-T3RE activated transcription both in the absence and presence of T3, with a greater activation observed in the presence of T3. In nuclear extracts from hepatocytes incubated in the absence of T3, ACCalpha-T3RE bound protein complexes (complexes 1 and 2) containing the liver X receptor (LXR) and the retinoid X receptor (RXR). In nuclear extracts from hepatocytes incubated in the presence of T3 for 24 h, ACCalpha-T3RE bound a different set of complexes. One complex contained LXR and RXR (complex 3) and another contained the nuclear T3 receptor (TR) and RXR (complex 4). Mutations of ACCalpha-T3RE that inhibited the binding of complexes 1 and 2 decreased transcriptional activation in the absence of T3, and mutations of ACCalpha-T3RE that inhibited the binding of complexes 3 and 4 decreased transcriptional activation in the presence of T3. The stimulation of ACCalpha transcription caused by T3 was closely associated with changes in the binding of complexes 1-4 to ACCalpha-T3RE. These data suggest that T3 regulates ACCalpha transcription by a novel mechanism involving changes in the composition of nuclear receptor complexes bound to ACCalpha-T3RE. We propose that complexes containing LXR/RXR ensure a basal level of ACCalpha expression for the synthesis of structural lipids in cell membranes and that complexes containing LXR/RXR and TR/RXR mediate the stimulation of ACCalpha expression caused by T3.

Negative regulation of the antimetastatic gene Nm23-H1 by thyroid hormone receptors

Metastasis of various malignant cells is inversely related to the abundance of the Nm23-H1 protein. The possible role of thyroid hormones in tumor metastasis has now been investigated by examining the effect of T3 on the expression of the Nm23-H1 gene. Human hepatoma HepG2 cells, in which endogenous thyroid hormone receptor subtype alpha1 (TRalpha1) is expressed at a low level, were stably transfected, either with expression plasmids encoding wild-type TRalpha1 or a dominant negative mutant of TRalpha1, or with the empty vector (yielding HepG2-Wt, HepG2-Mt, and HepG2-Neo cells, respectively). Immunoblot analysis revealed that exposure of HepG2-Wt and HepG2-Neo cells, but not HepG2-Mt cells, to T3-induced time-dependent decreases in the abundance of Nm23-H1 messenger RNA and protein, with the extent of these effects correlating with the level of expression of TRalpha1. An in vitro assay also revealed that T3 induced a marked increase in the invasive activity of HepG2-Wt cells; it induced a smaller increase in that of HepG2-Neo cells but had no effect on that of HepG2-Mt cells. Finally, the promoter region of Nm23-H1 spanning nucleotides -471 to -437 (relative to the transcriptional initiation site) inhibited the expression of a downstream reporter gene, in a T3-dependent manner, in COS-1 cells also transfected with an expression plasmid encoding TRalpha1 or TRbeta1. The DNA binding domain of TRbeta1 was required for this inhibitory effect. These results indicate that T3, acting through TRs, inhibits transcription of Nm23-H1, and that this effect is mediated by a negative regulatory element in the promoter region of the gene. Thus, it is possible that T3 promotes tumor metastasis by inducing down-regulation of Nm23-H1 expression.

Increased high density lipoprotein (HDL), defective hepatic catabolism of ApoA-I and ApoA-II, and decreased ApoA-I mRNA in ob/ob mice. Possible role of leptin in stimulation of HDL turnover

Abnormalities of plasma high density lipoprotein (HDL) levels commonly reflect altered metabolism of the major HDL apolipoproteins, apoA-I and apoA-II, but the regulation of apolipoprotein metabolism is poorly understood. Two mouse models of obesity, ob/ob and db/db, have markedly increased plasma HDL cholesterol levels. The purpose of this study was to evaluate mechanisms responsible for increased HDL in ob/ob mice and to assess potential reversibility by leptin administration. ob/ob mice were found to have increased HDL cholesterol (2-fold), apoA-I (1.3-fold), and apoA-II (4-fold). ApoA-I mRNA was markedly decreased (to 25% of wild-type) and apoA-II mRNA was unchanged, suggesting a defect in HDL catabolism. HDL apoprotein turnover studies using nondegradable radiolabels confirmed a decrease in catabolism of apoA-I and apoA-II and a 4-fold decrease in hepatic uptake in ob/ob mice compared with wild-type, but similar renal uptake. Low dose leptin treatment markedly lowered HDL cholesterol and apoA-II levels in both ob/ob mice and in lean wild-type mice, and it restored apoA-I mRNA to normal levels in ob/ob mice. These changes occurred without significant alteration in body weight. Moreover, ob/ob neuropeptide Y-/- mice, despite marked attenuation of diabetes and obesity phenotypes, showed no change in HDL cholesterol levels relative to ob/ob mice. Thus, increased HDL levels in ob/ob mice reflect a marked hepatic catabolic defect for apoA-I and apoA-II. In the case of apoA-I, this is offset by decreased apoA-I mRNA, resulting in apoA-II-rich HDL particles. The studies reveal a specific HDL particle catabolic pathway that is down-regulated in ob/ob mice and suggest that HDL apolipoprotein turnover may be regulated by obesity and/or leptin signaling.

Compound heterozygosity for an apolipoprotein A1 gene promoter mutation and a structural nonsense mutation with apolipoprotein A1 deficiency

Apolipoprotein (apo) A1 plays a central role in the metabolism of HDL. We describe a novel genetic variant of the apoA1 gene identified in a patient with low concentrations of plasma HDL cholesterol. The proband, a 12-year-old Japanese boy, exhibited markedly low levels of both plasma apoA1 and HDL cholesterol. Genomic DNA sequencing of apoA1 genes of the patient showed a compound heterozygosity for an A to C substitution at 27 bp upstream of the transcription start site of 1 apoA1 allele, and a C to T substitution in another allele at residue 84 resulting in aberrant termination. The point mutation at nucleotide position -27 changed ATAAATA of the putative TATA box signal sequence to ATACATA. In addition to this mutation, the patient was heterozygous for a G to A substitution at position -75. Immunoblotting of an isoelectric focusing electrophoresis gel of the proband's plasma showed a trace amount of normal apoA1. No measurable plasma apoA1 and HDL cholesterol in a patient with homozygosity for nonsense mutation at residue 84 has been reported previously. To determine the effects of substitution either at position -27 or -75, plasmids containing the 5'-flanking region of the human apoA1 promoter fused to the CAT reporter gene were constructed and transfected in HepG2 cells. A construct with the A to C substitution at position -27 showed 41. 8+/-4.2%, and G to A substitution at position -75 showed 72.8+/-15. 2% (means+/-SD, n=3) of CAT activities, compared with the wild-type promoter sequence. A construct with the double substitutions at positions -27 and -75 showed only 22.8+/-1.3% (mean+/-SD, n=3) activity relative to the wild type. Our patient is the first case with a TATA box mutation etiologically related to lipoprotein disorders.

Transcription of the juvenile hormone esterase gene under the control of both an initiator and AT-rich motif

The binding of transcription factors to the core promoter of the juvenile hormone esterase gene was functionally characterized using both a cell-free in vitro transcription functional assay and a cell transfection assay. A core JHE promoter (-61 to +28 bp relative to transcription start site) supported faithful transcription from the in vivo transcription start site. The nuclear extracts from the Sf9 insect cell line that provided transcription from that template also bound to that template as a probe in gel-mobility shift assays. Deletion or transversion of the initiator-binding motif (-1 to +4 bp) abolished detectable transcription either in vitro or in transfected cells. An AT-rich motif (ATATAT; -28 to -23 bp) serves another transcription factor-binding site. Mutation of the AT-rich motif to a canonical TATA-box preserved transcription, while either its deletion or complete transversion abolished or significantly reduced detectable transcriptional activity. These results indicate that, under these conditions, the functional operation of this core promoter approaches that of a composite promoter in which both the TATA- and initiator-binding protein complexes are necessary, even for basal transcription. On the other hand, these debilitating mutations to either the TATA box or initiator motif did not prevent the ability of the corresponding gel-shift competitive probes to compete with the wild-type promoter for binding by the transcription factors. Even a double transversion of both the AT-rich motif and the initiator-binding motif was able to competitively displace the protein complex that bound to the labelled wild-type probe. These data strongly indicate the presence of (an) additional core-promoter-associated transcription factor(s) (that is not the 'downstream element') that contact(s) the AT-binding complex and/or initiator-binding factor with sufficient avidity to remove them from binding to the competing wild-type promoter sequence.

Beneficial effects of a novel thyromimetic on lipoprotein metabolism

Although L-triiodothyronine (L-T3) lowers cholesterol, this hormone is not used to treat hypercholesterolemia because of its cardiotoxic effects. Thyromimetics, such as the novel compound CGS 23425, that mimic the beneficial but lack the detrimental effects of T3, may be useful in the treatment of hypercholesterolemia. To show that CGS 23425 has no cardiotoxicity, atrial contractility and force were both measured and found to be unchanged in rats treated with up to 10 mg/kg drug. The lipid lowering actions of this drug resulted in a 44% decrease in low-density lipoprotein (LDL) cholesterol in hypercholesterolemic rats treated with 10 microg/kg of the compound. Normal rats required a higher dose of 1000 microg/kg to elicit a similar 50% reduction in LDL cholesterol. Both CGS 23425 or T3 (10 nM) increased the specific binding of 125I-labeled LDL to Hep G2 cells and increased LDL receptor number by 44 and 49%, respectively. These data indicate that CGS 23425 enhances hepatic clearance of serum LDL cholesterol. Normal and fat-fed animals treated with the drug showed a dose-dependent increase in apolipoprotein AI, a protein that promotes the efflux of cholesterol from peripheral tissues. Transient transfection of a rat apolipoprotein AI promoter-chloramphenicol acetyltransferase construct, in human hepatoma cells, showed a dose-dependent increase in chloramphenicol acetyltransferase activity with EC50 values of 2 x 10(-12) M and 10(-10) M for thyroid hormone receptors beta1 and alpha1, respectively, with maximal responses at 10(-7) M. These data indicate that CGS 23425 is a thyromimetic that increases apolipoprotein AI expression via thyroid hormone receptor. In summary, CGS 23425 ameliorates hypercholesterolemia by increasing apolipoprotein A1 and the clearance of LDL cholesterol. Therefore, a compound like CGS 23425 may be useful for the prevention and reversal of atherosclerosis.

Localization of a negative thyroid hormone-response region in hepatic stearoyl-CoA desaturase gene 1

The effect of thyroid hormone on stearoyl-CoA desaturase gene 1 (SCD1) expression was investigated in mouse liver. Daily injections of 15 micrograms triiodothyronine (T3)/100 g body weight to hypothyroid mice resulted in repression of SCD1 mRNA levels by more than 50% in 48 hours and up to 65% in 6 days. Transient co-transfections were performed with an expression vector for T3 receptor alpha (T3R alpha) in HepG2 cells using chimeric reporter gene constructs of the SCD1 5'-flanking region. Transcriptional repression of the SCD1 putative promoter was observed upon treatment with 100 nM T3 when cotransfected with T3R alpha, but not without cotransfection of receptor. Transient gene expression studies localized a T3 response region to a 70-bp sequence in the SCD1 putative promoter. Eliminating the TATA box and an AP-2 binding site, DNA mobility shift analysis demonstrated specific binding of in vivo nuclear protein from mouse liver nuclear extract to a 43-bp sequence. DNA mobility shift with purified T3R alpha confirmed the presence of a T3 receptor binding site in this thyroid hormone-responsive region. These data indicate that SCD1 contains a negative T3 response region in its proximal promoter.