Synergistic interactions between transcription factors control expression of the apolipoprotein AI gene in liver cells

NKX2.2 and KLF4 cooperate to regulate α-cell identity

Transcription factors (TFs) are indispensable for maintaining cell identity through regulating cell-specific gene expression. Distinct cell identities derived from a common progenitor are frequently perpetuated by shared TFs, yet the mechanisms that enable these TFs to regulate cell-specific targets are poorly characterized. We report that the TF NKX2.2 is critical for the identity of pancreatic islet α cells by directly activating α-cell genes and repressing alternate islet cell fate genes. When compared with the known role of NKX2.2 in islet β cells, we demonstrate that NKX2.2 regulates α-cell genes, facilitated in part by α-cell-specific DNA binding at gene promoters. Furthermore, we have identified the reprogramming factor KLF4 as having enriched expression in α cells, where it co-occupies NKX2.2-bound α-cell promoters, is necessary for NKX2.2 promoter occupancy in α cells, and coregulates many NKX2.2 α-cell transcriptional targets. Overexpression of Klf4 in β cells is sufficient to manipulate chromatin accessibility, increase binding of NKX2.2 at α-cell-specific promoter sites, and alter expression of NKX2.2-regulated cell-specific targets. This study identifies KLF4 as a novel α-cell factor that cooperates with NKX2.2 to regulate α-cell identity.

Regulation of Apolipoprotein A-I Gene Expression in Human Macrophages by Oxidized Low-Density Lipoprotein

Apolipoprotein A-I (ApoA-I) is a key component of reverse cholesterol transport in humans. In the previous studies, we demonstrated expression of the apoA-I gene in human monocytes and macrophages; however, little is known on the regulation of the apoA-I expression in macrophages during the uptake of modified low-density lipoprotein (LDL), which is one of the key processes in the early stages of atherogenesis leading to formation of foam cells. Here, we demonstrate a complex nature of the apoA-I regulation in human macrophages during the uptake of oxidized LDL (oxLDL). Incubation of macrophages with oxLDL induced expression of the apoA-I gene within the first 24 hours, but suppressed it after 48 h. Both effects depended on the interaction of oxLDL with the TLR4 receptor, rather than on the oxLDL uptake by the macrophages. The oxLDL-mediated downregulation of the apoA-I gene depended on the ERK1/2 and JNK cascades, as well as on the NF-κB cascade.

FOXO1 and LXRα downregulate the apolipoprotein A-I gene expression during hydrogen peroxide-induced oxidative stress in HepG2 cells

Reactive oxygen species damage various cell components including DNA, proteins, and lipids, and these impairments could be a reason for severe human diseases including atherosclerosis. Forkhead box O1 (FOXO1), an important metabolic transcription factor, upregulates antioxidant and proapoptotic genes during oxidative stress. Apolipoprotein A-I (ApoA-I) forms high density lipoprotein (HDL) particles that are responsible for cholesterol transfer from peripheral tissues to liver for removal in bile in vertebrates. The main sources for plasma ApoA-I in mammals are liver and jejunum. Hepatic apoA-I transcription depends on a multitude of metabolic transcription factors. We demonstrate that ApoA-I synthesis and secretion are decreased during H2O2-induced oxidative stress in human hepatoma cell line HepG2. Here, we first show that FOXO1 binds to site B of apoA-I hepatic enhancer and downregulates apoA-I gene activity in HepG2 cells. Moreover, FOXO1 and LXRα transcription factors participate in H2O2-triggered downregulation of apoA-I gene together with Src, JNK, p38, and AMPK kinase cascades. Mutations of sites B or C as well as the administration of siRNAs against FOXO1 or LXRα to HepG2 cells abolished the hydrogen peroxide-mediated suppression of apoA-I gene.

Insulin-Mediated Downregulation of Apolipoprotein A-I Gene in Human Hepatoma Cell Line HepG2: The Role of Interaction Between FOXO1 and LXRβ Transcription Factors

Apolipoprotein A-I (ApoA-I) is a key component of high density lipoproteins which possess anti-atherosclerotic and anti-inflammatory properties. Insulin is a crucial mediator of the glucose and lipid metabolism that has been implicated in atherosclerotic and inflammatory processes. Important mediators of insulin signaling such as Liver X Receptors (LXRs) and Forkhead Box A2 (FOXA2) are known to regulate apoA-I expression in liver. Forkhead Box O1 (FOXO1) is a well-known target of insulin signaling and a key mediator of oxidative stress response. Low doses of insulin were shown to activate apoA-I expression in human hepatoma HepG2 cells. However, the detailed mechanisms for these processes are still unknown. We studied the possible involvement of FOXO1, FOXA2, LXRα, and LXRβ transcription factors in the insulin-mediated regulation of apoA-I expression. Treatment of HepG2 cells with high doses of insulin (48 h, 100 nM) suppresses apoA-I gene expression. siRNAs against FOXO1, FOXA2, LXRβ, or LXRα abrogated this effect. FOXO1 forms a complex with LXRβ and insulin treatment impairs FOXO1/LXRβ complex binding to hepatic enhancer and triggers its nuclear export. Insulin as well as LXR ligand TO901317 enhance the interaction between FOXA2, LXRα, and hepatic enhancer. These data suggest that high doses of insulin downregulate apoA-I gene expression in HepG2 cells through redistribution of FOXO1/LXRβ complex, FOXA2, and LXRα on hepatic enhancer of apoA-I gene. J. Cell. Biochem. 118: 382-396, 2017. © 2016 Wiley Periodicals, Inc.

The nuclear receptors COUP-TF: a long-lasting experience in forebrain assembly

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are nuclear receptors belonging to the superfamily of the steroid/thyroid hormone receptors. Members of this family are internalized to the nucleus both in a ligand-dependent or -independent manner and act as strong transcriptional regulators by binding to the DNA of their target genes. COUP-TFs are defined as orphan receptors, since ligands regulating their activity have not so far been identified. From the very beginning of metazoan evolution, these molecules have been involved in various key events during embryonic development and organogenesis. In this review, we will mainly focus on their function during development and maturation of the central nervous system, which has been well characterized in various animal classes ranging from ctenophores to mammals. We will start by introducing the current knowledge on COUP-TF mechanisms of action and then focus our discussion on the crucial processes underlying forebrain ontogenesis, with special emphasis on mammalian development. Finally, the conserved roles of COUP-TFs along phylogenesis will be highlighted, and some hypotheses, worth exploring in future years to gain more insight into the mechanisms controlled by these factors, will be proposed.

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.

Genistein and daidzein induced apoA-1 transactivation in hepG2 cells expressing oestrogen receptor-α

Studies have shown that soya consumption has been associated with low incidence of CVD. Because the chemical structures of soya isoflavones are similar to oestrogen, the beneficial outcome may be attributed to the oestrogenicity of these compounds. In this study, effect of the soya isoflavone genistein on the mRNA expression of apoA-1 in the human hepatoma HepG2 cell was investigated. Without oestrogen receptor (ER) α transfection, soya isoflavones in the physiological range had no effect on the apoA-1 transcription. Once ERα was ectopically expressed in these cells, soya isoflavone dramatically increased the apoA-1 mRNA abundance quantified by real-time PCR.ApoA-1-reporter assays with plasmid constructed from the 5′-flanking segment upstream to the coding region revealed that the transactivation of theapoA-1promoter was induced by the soya isoflavone in HepG2 cells expressing ERα. This induction was reduced by the anti-oestrogen ICI 182780, but not the inhibitors of protein kinase (PK) C, PKA, or mitogen-activated PK. Based on the previously identified response elements on the promoter, a series of truncated promoter reporter plasmids were then constructed. An induction profile of genistein was built and insulin response core element at − 411 to − 404 appeared to be a potential site of interaction. This study illustrated that soya isoflavones at physiological concentrations could up regulate apoA-1 mRNA expression in ERα-transfected HepG2 cells.

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.

Mechanisms by which dietary fatty acids modulate plasma lipids

Dietary fatty acids have a considerable effect on plasma LDL cholesterol (LDL-C) concentrations and therefore on the risk for coronary heart disease. Numerous studies have been conducted in animal models to elucidate the mechanisms by which different types of fatty acids modulate plasma cholesterol concentrations. In addition, multiple clinical trials and epidemiological data have demonstrated the effects of fatty acids in determining the concentrations of circulating LDL. SFAs and trans fatty acids have a detrimental effect on plasma lipids, whereas PUFAs of the (n-6) family and monounsaturated fatty acids decrease plasma LDL-C concentrations. Among the SFAs, stearic acid (18:0) appears to have a neutral effect on LDL-C, while lauric (12:0), myristic (14:0), and palmitic (16:0) acids are considered to be hypercholesterolemic. SFAs increase plasma LDL-C by increasing the formation of LDL in the plasma compartment and by decreasing LDL turnover. Although unsaturated fatty acids increase cholesterol synthesis, they also increase hepatic LDL receptor number and LDL turnover in vivo. Fatty acids are also ligands of important regulatory elements, which can play a role in determining plasma cholesterol. This article presents a summary of the major effects of various types of fatty acids on plasma lipid concentrations and the mechanisms involved.

The APOA1/C3/A4/A5 gene cluster, lipid metabolism and cardiovascular disease risk

PURPOSE OF REVIEW

APOA1/C3/A4/A5 are key components modulating lipoprotein metabolism and cardiovascular disease risk. This review examines the evidence regarding linkage disequilibrium and haplotype structure within the A1/C3/A4/A5 cluster, and assesses its association with plasma lipids and cardiovascular disease risk. In addition, we use genomic information from several species to draw inferences about the location of functional variants within this cluster.

RECENT FINDINGS

The close physical distance of these genes and the interrelated functions of these apolipoproteins have encumbered attempts to determine the role of individual variants on lipid metabolism. Therefore, current research aims to define linkage disequilibrium and haplotype structure within this cluster. Functional variants in regulatory regions are most interesting as they are potentially amenable to therapy. Comparative genomics can contribute to the identification of such functional variants.

SUMMARY

Genetic variability at the APOA1/C3/A4/A5 cluster has been examined in relation to lipid metabolism and cardiovascular disease risk. However, the findings are inconsistent. This is partly due to the classic approach of studying single and mostly nonfunctional polymorphisms. Moreover, allelic expression may depend on the concurrent presence of environmental factors. Association studies using haplotypes should increase the power to detect true associations and interactions. We hypothesize that phenotypes observed in association with transcriptional regulatory variants can be readily modified by environmental factors. Therefore, studies focusing on regulatory variants may be more fruitful to locate/define future therapeutic targets.

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.

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.

Isolation and characterization of unsaturated fatty acids as natural ligands for the retinoid-X receptor

The retinoid-X receptor (RXR) is a ligand activated nuclear receptor that is the heterodimer partner for many class II nuclear receptors. Previously identified natural ligands for this receptor include 9-cis retinoic acid (9cRA), docosahexaenoic acid, and phytanic acid. Our studies were performed to determine if there are any unidentified, physiologically important RXR ligands. Agonists for RXR were purified from rat heart and testes lipid extracts with the use of a cell-based reporter assay to monitor RXR activation. Purified active fractions contained a variety of unsaturated fatty acids and components were quantified by gas-liquid chromatography of derivatized samples. The corresponding fatty acid standards elicited a similar response in the reporter cell assay. Competition binding analysis revealed that the active fatty acids compete with [3H]9cRA for binding to RXR. Non-esterified fatty acids were analyzed from lipid extracts of isolated heart and testes nuclei and endogenous concentrations were found to be within the range of their determined binding affinities. Our studies reveal tissue dependent profiles of RXR agonists and support the idea of unsaturated fatty acids as physiological ligands of RXR.

Regulation of apoA-I gene expression: mechanism of action of estrogen and genistein

We have previously shown that 17-beta-estradiol (E2) and genistein increase the expression of apolipoprotein A-I (apoA-I), the major protein component of HDL, in Hep G2 cells. To elucidate the mechanism mediating the increase in apoA-I gene expression by these compounds, plasmid constructs containing serial deletions of the apoA-I promoter region were generated. The smallest region maintaining response to E2 and genistein spanned the -220 to -148 sequence, and the estrogen antagonist ICI182,780 completely inhibited the E2 and genistein effect. Nuclear extracts from cells treated with E2 and genistein showed increased binding to site B oligonucleotide (-169 to -146), and nuclear extracts from genistein-treated cells showed increased binding to an early growth response factor 1 (Egr-1) oligonucleotide compared to control cells. An increase in the concentrations of Egr-1 and hepatocyte nuclear factor-3beta was observed in nuclear extracts of cells treated with both compounds compared to control cells. Treatment with a specific inhibitor of mitogen-activated protein (MAP) kinase, but not with other inhibitors, abolished the stimulation of apoA-I gene expression by E2 and genistein. These results indicate that the MAP kinase pathway is involved in the regulation of apoA-I gene expression by genistein and E2, possibly through downstream regulation of transcription factors binding to the promoter region.

Zinc depletion reduced Egr-1 and HNF-3beta expression and apolipoprotein A-I promoter activity in Hep G2 cells

We examined the influence of zinc status on expression of certain transcription factors involved in regulation of apolipoprotein A-I (apoAI) expression in human hepatoblastoma Hep G2 cells. A low zinc basal medium (zinc deficient, ZD) consisting of DMEM and 10% Chelex100-treated fetal bovine serum was used to deplete cellular zinc over one passage. Cells were also cultured for one passage in medium supplemented with 0.4 (ZD0.4), 4.0 (zinc normal, ZN), 16.0 (zinc adequate, ZA), or 32.0 microM zinc (zinc supplemented, ZS). Compared with ZN cells, cellular zinc levels were 43 and 31% lower in ZD and ZD0.4 cells but 70 and 146% higher in ZA and ZS cells, respectively. Supplementation of 0.4 microM zinc significantly increased DNA contents per plate, from 65% in ZD cells to 83% in ZD0.4 cells compared with ZN cells. Addition of >4 microM zinc in medium did not further increase DNA contents. The proportion of cells in G(1)/S and S phase was about fourfold higher and threefold lower, respectively, in ZD cells compared with ZN and other groups. Nuclear Egr-1 protein was markedly decreased in ZD and ZD0.4 cells. Moreover, hepatocyte nuclear factor (HNF)-3beta was severely degraded in ZD and ZD0.4 cells. In contrast, HNF-4alpha remained stable in all groups and was not significantly lower in ZD and ZD0.4 cells. Furthermore, downregulation of trans-acting factor Egr-1 and cleavage of HNF-3beta were associated with reduction of apoAI promoter activity in zinc-deficient Hep G2 cells. Thus zinc is critical in transcriptional regulation of apoAI gene expression in hepatocytes.

Simvastatin, an HMG-CoA reductase inhibitor, induces the synthesis and secretion of apolipoprotein AI in HepG2 cells and primary hamster hepatocytes

Clinical studies have recently suggested that statin treatment may beneficially elevate plasma concentrations of high density lipoprotein (HDL)-cholesterol in patients with hyperlipidemia. Here, we have investigated the effect of a potent inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase on the synthesis and secretion of apolipoprotein AI (apoAI) in two model systems, HepG2 cells and primary hamster hepatocytes. Cultured cells were incubated with different doses of simvastatin (0.1-10 microM) for a period of 18 h. A dose-dependent increase in synthesis and secretion of apoAI was observed in both cell types. There was a significant increase in the synthesis of apoAI in HepG2 cells (44.3+/-12.1%), and hamster hepatocytes (212+/-2%) after treatment with 10 microM of the statin. The increase in apoAI synthesis appeared to result in a higher level of apoAI secreted into the culture media in both cell types (49.2+/-7.8% in HepG2, 197+/-0.2% in hamster hepatocytes). ApoAI mRNA levels were also significantly increased in both cell types in response to statin treatment. Control experiments with transferrin confirmed specificity of the effect on apoAI secretion. Analysis of a density fraction containing HDL particles in culture media revealed an increase in HDL-associated apoAI of 94.3+/-2.1% in HepG2 cells and 27.0+/-0.03% in hamster hepatocytes following 10 microM simvastatin-treatment. Comparative studies of simvastatin and lovastatin indicated a differential ability to induce apoAI synthesis and secretion, with simvastatin having a more significant effect. Thus, acute statin treatment of cultured hepatocytes (transformed as well as primary) resulted in a significant upregulation of apoAI mRNA and apoAI synthesis, causing oversecretion of apoAI and HDL extracellularly. The stimulatory effect on apoAI synthesis and secretion may thus explain the clinical observation of an elevated plasma HDL-cholesterol level in hyperlipidemic patients treated with certain statins.

Expression of the human beta(3)-adrenergic receptor gene in SK-N-MC cells is under the control of a distal enhancer

Mechanisms of transcriptional regulation of the human beta(3)-adrenergic receptor were studied using SK-N-MC cells, a human neuroblastoma cell line that expresses beta(3)- and beta(1)-adrenergic receptors endogenously. Deletions spanning different portions of a 7-kb 5'-flanking region of the human beta(3)-adrenergic receptor gene were linked to a luciferase reporter and transfected in SK-N-MC, CV-1, and HeLa cells. Maximal luciferase activity was observed when a 200-bp region located between -6.5 and -6.3 kb from the translation start site was present. This region functioned only in SK-N-MC cells. Electrophoretic mobility shift assays of nuclear extracts from SK-N-MC, CV-1, and HeLa cells using double stranded oligonucleotides spanning different portions of the 200-bp region as probes and transient transfection studies revealed the existence of three cis-acting regulatory elements: A) -6.468 kb-AGGTGGACT--6.458 kb, B) -6.448 kb-GCCTCTCTGGGGAGCAGCTTCTCC-6.428 kb, and C) -6.405 kb-20 repeats of CCTT-6.385 kb. These elements act together to achieve full transcriptional activity. Mutational analysis, antibody supershift, and electrophoretic mobility shift assay competition experiments indicated that element A binds the transcription factor Sp1, element B binds protein(s) present only in nuclear extracts from SK-N-MC cells and brown adipose tissue, and element C binds protein(s) present in both SK-N-MC and HeLa cells. In addition, element C exhibits characteristics of an S1 nuclease-hypersensitive site. These data indicate that cell-specific positive cis-regulatory elements located 6.5 kb upstream from the translation start site may play an important role in transcriptional regulation of the human beta(3)-adrenergic receptor. These data also suggest that brown adipose tissue-specific transcription factor(s) may be involved in the tissue-specific expression of the beta(3)-adrenergic receptor gene.

Genistein activates apolipoprotein A-I gene expression in the human hepatoma cell line Hep G2

Soy phytoestrogens have been shown to increase plasma levels of HDL cholesterol and apolipoprotein (apo) A-I, its major protein component, in animal studies and in some human studies. The human hepatoma cell line Hep G2 was used to study the effect of the phytoestrogens genistein and daidzein on apo A-I secretion and gene expression in liver cells. Both genistein and daidzein increased apo A-I secretion in a dose-dependent fashion. Apo A-I concentration in the media of treated cells was increased approximately fivefold by 10 micromol/L genistein (P: < 0.001) and approximately onefold by 10 micromol/L daidzein (P: < 0.001) compared with control cells. The effect of genistein on apo A-I secretion was similar to that observed with 17-beta-estradiol. Treatment of cells with genistein for 24 h increased the transcriptional activity of the apo A-I gene as measured by nuclear run-on assay. Transfection experiments with plasmids containing regulatory regions of the apo A-I gene cloned in front of the luciferase reporter gene indicated that the 5' region of the apo A-I gene contained between nucleotides -256 and -41 is responsible for the increased expression of this gene by genistein.

Serum-free, long-term cultures of human hepatocytes: maintenance of cell morphology, transcription factors, and liver-specific functions

Since human hepatocytes are available only in limited number, the development of a serum-free culture system for long-term cultivation of differentiated and functional hepatocytes is of great importance. Here we describe the culture of human hepatocytes in a chemically defined serum-free medium for up to 5 weeks. Cell morphology was assayed by light and electron microscopy and revealed a well-preserved cellular morphology. Marker proteins for epithelial and bile duct cells, cytokeratin (CK) 18 and 19, and liver-specific proteins, like phosphoenolpyruvate carboxykinase-2 (PCK2) and serum proteins, were expressed. Liver-enriched transcription factors CCAAT/enhancer binding protein alpha (C/EBPalpha) and hepatocyte nuclear factor-4 (HNF-4), cytokine and mitogen activated factors (nuclear factor kappa B) NFkappaB, and activator protein-1 (AP-1) were maintained and active for several weeks in our cultures. In summary, our serum-free culture system allows the culture of differentiated human hepatocytes for several weeks. It may serve as a model system for metabolic, pharmacologic-toxicologic studies, and studies on human pathogens under defined chemical conditions.

Mechanism of action of niacin on lipoprotein metabolism

It is generally accepted that the increased concentrations of apolipoprotein (apo) B containing very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL), and decreased levels of apo AI containing high-density lipoproteins (HDL) are correlated to atherosclerotic cardiovascular disease. Current evidence indicates that the post-translational apo-B degradative processes regulate the hepatic assembly and secretion of VLDL and the subsequent generation of LDL particles. The availability of triglycerides (TG) for the addition to apo B during intracellular processing appears to play a central role in targeting apo B for either intracellular degradation or assembly and secretion as VLDL particles. Based on the availability of TG, the liver secretes either dense TG-poor VLDL2 or large TG-rich VLDL1 particles, and these particles serve as precursors for the formation of more buoyant or small, dense LDL particles by lipid transfer protein- and hepatic lipase-mediated processes. HDLs are a heterogenous class of lipoproteins, and apo AI (the major protein of HDL) participates in reverse cholesterol transport, a process by which excess cholesterol is eliminated. Recent studies indicate that HDL particles containing only apo A-I (LPA-I) are more effective in reverse cholesterol transport and more anti-atherogenic than HDL particles containing both apo A-I and apo A-II (LPA-I + A-II).

Estrogen increases apolipoprotein (apo) A-I secretion in hep G2 cells by modulating transcription of the apo A-I gene promoter

Estrogen administration to postmenopausal women has been shown to increase plasma levels of apolipoprotein (apo) A-I. A human hepatoma cell line, Hep G2, was used to test the hypothesis that estrogen increases the hepatic production of apo A-I by modulating gene expression. When Hep G2 cells were treated for 24 hours with E(2), the apo A-I content in the medium increased 4.3+/-1.0-fold at 10 micromol/L E(2) and 1.8+/-0.4-fold at 1 micromol/L E(2) compared with untreated cells. A time-course experiment indicated that there was no E(2)-dependent (10 micromol/L) increase in apo A-I medium content at 1 hour and 2 hours and that apo A-I was 165% of controls at 6 hours and 440% at 24 hours. Hep G2 cells were transfected, by the cationic lipid method, with constructs containing serial deletions of the 5' region of the apo A-I gene (-41/+397, -256/+397, and -2500/+397) cloned in front of the luciferase gene and with or without a 7-kb region spanning the apo C-III/A-IV intergenic region, which has been shown to contain regulatory elements for the expression of the apo A-I gene. With the exception of the construct containing only the basal promoter (-41/+397), the expression of all constructs was 2- to 3-fold greater in the presence of E(2). The smallest construct that maintained E(2) responsiveness, the -256/+397 construct, does not contain a typical estrogen-responsive element. In the same transfection experiments, the 4-fold increase in apo A-I in the culture medium was preserved. However, when the same set of transfections was performed by the calcium phosphate precipitation method, the E(2) effect on the apo A-I content in the culture medium and on transcription activation was nearly abolished. This effect was probably mediated by Ca(2+), because incubation of cells with 20 mmol/L CaCl(2) abolished the E(2) response. In conclusion, E(2) increases apo A-I production in hepatic cells by increasing the transcription of the apo A-I gene.

A bifunctional regulatory element of the human ApoA-I gene responsive to a distal enhancer

Promoter elements located up to 2 kb upstream of the apolipoprotein A-I (apoA-I) gene are necessary for apoA-I expression in liver and intestine cells in tissue culture. In transgenic mice, a distal enhancer located between the apoA-IV and apoC-III genes is additionally necessary for tissue-specific expression of apoA-I in liver and intestine. We have identified a previously uncharacterized regulatory element between 746 and 856 nucleotides 5' of the apoA-I transcription start site that differentially affects the expression of apoA-I reporter plasmids in intestine cells dependent on the presence of the distal apolipoprotein enhancer. Deletion of the -856/-746 sequence strongly repressed transcription in the presence of the apolipoprotein enhancer, but in the absence of the enhancer, deletion of the -856/-746 element increased transcription. By contrast, in liver cells, deletion of the -856/-746 element strongly repressed transcription in the presence of the distal enhancer but had no detectable effect on transcription in the absence of the distal enhancer. Electrophoretic mobility shift analysis revealed tissue-specific and sequence-specific protein-DNA complexes formed by the -856/-746 element in intestine, liver, and HeLa cell nuclear extracts. The complexes formed by extracts of intestinal cells differed from those of liver and HeLa cells by their sensitivity to DNase digestion and their pattern of protein footprints. Collectively, the data suggest that the -856/-746 sequence is a composite regulatory element that interacts with multiple proteins and the apolipoprotein distal enhancer to achieve tissue-specific expression of apoA-I.

The effect on transcription efficiency of the apolipoprotein AI gene of DNA variants at the 5' untranslated region

Elevated circulating levels of high-density lipoprotein and apolipoprotein AI are associated with reduced coronary artery disease risk. We have shown that a C to T substitution at +83 bp and a G to A substitution at -75 bp of the apolipoprotein AI gene are both related to increased high-density lipoprotein levels in a healthy population but not in a coronary population, among whom the same mutations are associated with increased disease severity. In the present study, we explored the effects of these base changes on transcriptional efficiency in vitro. We directionally cloned (using polymerase chain reaction) the 5' region of the apolipoprotein AI gene (-281 to +330 bp) with GC, GT, and AC haplotypes into a pGL3-luciferase reporter gene basic vector, and transfected the constructed vectors into HepG2 cells. The cells carrying the T allele at the +83 bp site (GT 112.3 +/- 12.4) had the same transcriptional efficiency as those bearing the C allele (GC 126.3 +/- 9.6). However, for cells with the A allele at -75 bp there was a twofold decrease in transcription (AC 63.1 +/- 9.3) accompanied by similar changes in Luc+ mRNA levels; this reduced transcription was only present if the apolipoprotein AI leader sequence was included in the insert. While the findings are inconsistent with the T or A allele being associated with higher high-density lipoprotein levels, they are consistent with the finding that the alleles are associated with an increased coronary artery disease risk, and demonstrate that the 5' leader region of the apolipoprotein AI gene participates in regulating apolipoprotein AI transcription. They also suggest that other regions of the apolipoprotein AI gene may have an active role in such regulation, and that environmental effects may influence allele-specific expression.

The peroxisome proliferator-activated receptor alpha (PPARalpha) ligand WY 14,643 does not interfere with leukotriene B4 induced adhesion of neutrophils to endothelial cells

Peroxisome proliferator-activated receptors (PPAR) control discrete genes involved in fatty acid and lipid metabolism. Recently, it was suggested that activation of the alpha isoform of PPAR by the potent proinflammatory mediator leukotriene B4 (LTB4) enhanced degradation of this eicosanoid, offersuggesting a new aspect of down-regulation of inflammation. Here, we studied whether PPARalpha activation (by means of the selective agonist WY 14,643) of endothelial cells, pivotal in the regulation of inflammatory responses, interfered with LTB4 induced adhesion of PMN neutrophil granulocytes in vitro. When endothelial cells were treated with WY 14,643 prior to activation with LTB4 (or fMLP, IL-1beta or TNFalpha, as controls) we could not document any effect on the number of adhering PMN or duration of the response. Thus, this study provides no evidence indicating a regulatory function of PPARalpha in LTB4 induced adhesive interactions between endothelial cells and neutrophils.

Estrogen regulation of the apolipoprotein AI gene promoter through transcription cofactor sharing

Estrogen replacement therapy increases plasma concentrations of high density lipoprotein and its major protein constituent, apolipoprotein AI (apoAI). Studies with animal model systems, however, suggest opposite effects. In HepG2 cells stably expressing estrogen receptor alpha (ERalpha), 17beta-estradiol (E2) potently inhibited apoAI mRNA steady state levels. ApoAI promoter deletion mapping experiments indicated that ERalpha plus E2 inhibited apoAI activity through the liver-specific enhancer. Although the ERalpha DNA binding domain was essential but not sufficient for apoAI enhancer inhibition, ERalpha binding to the apoAI enhancer could not be detected by electrophoretic mobility shift assays. Western blotting and cotransfection assays showed that ERalpha plus E2 did not influence the abundance or the activity of the hepatocyte-enriched factors HNF-3beta and HNF-4, two transcription factors essential for apoAI enhancer function. Expression of the ERalpha coactivator RIP140 dramatically repressed apoAI enhancer function in cotransfection experiments, suggesting that RIP140 may also function as a coactivator on the apoAI enhancer. Moreover, estrogen regulation of apoAI enhancer activity was dependent upon the balance between ERalpha and RIP140 levels. At low ratios of RIP140 to ERalpha, E2 repressed apoAI enhancer activity, whereas at high ratios this repression was reversed. Regulation of the apoAI gene by estrogen may thus vary in direction and magnitude depending not only on the presence of ERalpha and E2 but also upon the intracellular balance of ERalpha and coactivators utilized by ERalpha and the apoAI enhancer.

E1A represses apolipoprotein AI enhancer activity in liver cells through a pRb- and CBP-independent pathway

The apolipoprotein AI (apoAI) promoter/enhancer contains multiple cis -acting elements on which a variety of hepatocyte-enriched and ubiquitous transcription factors function synergistically to regulate liver-specific transcription. Adenovirus E1A proteins repress tissue-specific gene expression and disrupt the differentiated state in a variety of cell types. In this study expression of E1A 12Sor 13S in hepatoblastoma HepG2 cells repressed apoAI enhancer activity 8-fold. Deletion mapping analysis showed that inhibition by E1A was mediated by the apoAI promoter site B. E1A selectively inhibited the ability of HNF3beta and HNF3alpha to transactivate reporter genes controlled by the apoAI site B and the HNF3 binding site from the transthyretin promoter. The E1A-mediated repression of HNF3 activity was not reversed by overexpression of HNF3beta nor did E1A alter nuclear HNF3beta protein levels or inhibit HNF3 binding to DNA in mobility shift assays. Overexpression of two cofactors known to interact with E1A, pRb and CBP failed to overcome inhibition of HNF3 activity. Similarly, mutations in E1A that disrupt its interaction with pRb or CBP did not compromise its ability to repress HNF3beta transcriptional activity. These data suggest that E1A inhibits HNF3 activity by inactivating a limiting cofactor(s) distinct from pRb or CBP.

Engineering novel specificities for ligand-activated transcription in the nuclear hormone receptor RXR

BACKGROUND

The retinoid X receptor (RXR) activates transcription of target genes in response to its natural ligand, 9-cis retinoic acid (9cRA), and a number of RXR-specific synthetic ligands. To discover the potential for engineering nuclear receptors for activation of transcription by novel ligands, we used structure-based mutagenesis to change the ligand specificity of RXR.

RESULTS

By making substitutions at only two positions (Phe313 and Leu436) we engineered two new classes of RXR proteins that had altered ligand specificities. The first class exhibits decreased activation by 9cRA and increased activation by synthetic ligands. The second class continues to be activated by 9cRA but no longer responds to synthetic ligands. The magnitude of the change in specificity that can be accomplished is greater than 280-fold.

CONCLUSIONS

These results confirm that Phe313 and Leu436 are crucial determinants of ligand specificity for RXR and demonstrate that nuclear receptors are exceptionally promising protein scaffolds for the introduction of novel ligand specificities through structure-based protein engineering.

Regulation of apolipoprotein A-I gene expression in Hep G2 cells depleted of Cu by cupruretic tetramine

Studies were designed to examine the regulation of apolipoprotein (apo) A-I gene expression in Cu-depleted Hep G2 cells. The cupruretic chelator N,N'-bis(2-aminoethyl)-1,3-propanediamine 4 HCl (2,3,2-tetramine or TETA) was used to maintain a 77% reduction in cellular Cu in Hep G2 cells. After two passages of TETA treatment, the relative abundance of apoA-I mRNA was elevated 52%. In TETA-treated cells, the rate of apoA-I mRNA decay measured by an actinomycin D chase study was accelerated 108%, and the synthesis of apoA-I mRNA determined by a nuclear runoff assay was enhanced 2.5-fold in TETA-treated cells. All of those changes could be reverted toward the control values with Cu supplementation for only 2 days. In transient transfection assays, a 26.7% increase in chloramphenicol O-acetyltransferase (CAT) activity for the reporter construct -256AI-CAT was observed in the treated cells. However, the ability of apoA-I regulatory protein 1 (ARP-1) to repress the CAT activity was not affected by the depressed Cu status. In addition, gel retardation experiments demonstrated that Cu depletion enhanced the binding of hepatocyte nuclear factor 4 (HNF-4) and other undefined nuclear factors to oligonucleotides containing site A, one of three regulatory sites of the apoA-I gene promoter. Moreover, the relative abundance of HNF-4 mRNA was increased 58% in the Cu-depleted cells. Thus the observed increase in apoA-I gene transcription may be mediated mostly by an elevated level of the regulatory factor, HNF-4. In summary, the present findings established the mechanism by which a depressed cellular Cu status can enhance apoA-I mRNA production and subsequently increase apoA-I synthesis.

Cloning and characterization of hcKrox, a transcriptional regulator of extracellular matrix gene expression

cKrox is a novel zinc finger-containing transcription factor that binds to the alpha1(I) and alpha2(I) collagen gene promoters. The gene coding for cKrox is a new member of a family of early growth response genes, that play important roles in development. In the mouse, cKrox is expressed, beginning at 9.5 days of gestation and at 10.5 days in regions destined to become skin. In adult animals, expression is predominantly in skin, one of the two major organs where type I collagen is expressed. We have isolated cDNA clones for human cKrox. Theoretical translation of the nucleic acid sequence reveals 90% conservation of amino acids between the mouse and human proteins; however, the human gene product contains a 117-amino-acid N-terminal extension. The amino acid sequences of the zinc-finger DNA binding domains of mouse and human cKrox are identical. RT-PCR analysis of human fibroblasts indicates constitutive low-level expression of cKrox which can be transiently elevated by treatment with retinoic acid. Transient transfection assays indicate that hcKrox represses transcription of the alpha1(I) procollagen promoter, and electrophoretic mobility shift assays demonstrate that hcKrox binds to both the human and murine promoter DNA. Deletion derivatives of hcKrox demonstrate transcription-activating potential that is promoter-dependent. NIH3T3 cells permanently expressing hcKrox demonstrate a threefold and 10-fold decrease in alpha1(I) procollagen and fibronectin mRNA levels, respectively, compared to control cells. Consistent with this finding, a fibronectin promoter reporter construct is repressed more than 80% by hcKrox. These data suggest that hcKrox represses collagen transcription directly, and it may function as a repressor of fibronectin and possibly other matrix genes.

Identification, characterization, and tissue distribution of human peroxisome proliferator-activated receptor (PPAR) isoforms PPARgamma2 versus PPARgamma1 and activation with retinoid X receptor agonists and antagonists

We describe the cloning, characterization, and tissue distribution of the two human peroxisome proliferator activated receptor isoforms hPPARgamma2 and hPPARgamma1. In cotransfection assays the two isoforms were activated to approximately the same extent by known PPARgamma activators. Human PPARgamma binds to DNA as a heterodimer with the retinoid X receptor (RXR). This heterodimer was activated by both RXR agonists and antagonists and the addition of PPARgamma ligands with retinoids resulted in greater than additive activation. Such heterodimer-selective modulators may have a role in the treatment of PPARgamma/RXR-modulated diseases like diabetes. Northern blot analysis indicated the presence of PPARgamma in skeletal muscle, and a sensitive RNase protection assay confirmed the presence of only PPARgamma1 in muscle that was not solely due to fat contamination. However, both PPARgamma1 and PPARgamma2 RNA were detected in fat, and the ratio of PPARgamma1 to PPARgamma2 RNA varied in different individuals. The presence of tissue-specific distribution of isoforms and the variable ratio of PPARgamma1 to PPARgamma2 raised the possibility that isoform expression may be modulated in disease states like non-insulin-dependent diabetes mellitus. Interestingly, a third protected band was detected with fat RNA indicating the possible existence of a third human PPARgamma isoform.

Distal apolipoprotein C-III regulatory elements F to J act as a general modular enhancer for proximal promoters that contain hormone response elements. Synergism between hepatic nuclear factor-4 molecules bound to the proximal promoter and distal enhancer sites

Transient transfection assays have shown that the distal apoC-III promoter segments that contain the regulatory elements F to J enhance the strength of the tandemly linked proximal apoA-I promoter 5- to 13-fold in hepatic (HepG2) cells. Activation in intestinal (CaCo-2) cells to levels comparable to those obtained in HepG2 cells requires a larger apoA-I promoter sequence that extends to nucleotide -1500 as well as the presence of hepatic nuclear factor-4 (HNF-4). The distal apoC-III regulatory elements can also enhance 4- to 8-fold the strength of the heterologous apoB promoter in HepG2 and CaCo-2 cells. Finally, these elements in the presence of HNF-4 enhance 14.5- to 18.5-fold the strength of the minimal adenovirus major late promoter linked to two copies of the hormone response element (HRE) AID of apoA-I in both HepG2 and CaCo-2 cells. In vitro mutagenesis of the promoter/enhancer cluster established that the enhancer activity is lost by a mutation in the HRE present in the 3' end of the regulatory element I (-736 to -714) and is reduced significantly by point mutations or deletions in one or more of the regulatory elements F to J of the apoC-III enhancer. The enhancer activity also requires the HREs of the proximal apoA-I promoter. The apoC-III enhancer can also restore the activity of the proximal apoA-I and apoB promoters that have been inactivated by mutations in CCAAT/enhancers binding protein binding sites, indicating that C/EBP may not participate in the synergistic activation of the promoter/enhancer cluster. The findings suggest that the regulatory elements F to J of the apoC-III promoter act as a general modular enhancer that can potentiate the strength of proximal promoters that contain HREs. Such potentiation in the HepG2 cells can be accounted for by synergistic interactions between HNF-4 or other nuclear hormone receptors bound to the proximal and distal HREs and SP1 or other factors bound to the apoC-III enhancer. Additional factors may be required for optimal activity in CaCo-2 cells as well as for the function of this region as an intestinal enhancer.

Transgenic rabbits expressing human apolipoprotein A-I in the liver

Human apolipoprotein A-I (apo A-I) transgenic rabbits were created by use of an 11-kb genomic human apo A-I construct containing a liver-specific promoter. Five independent transgenic lines were obtained in which human apo A-I gene had integrated and was expressed. Plasma levels of human apo A-I ranged from 8 to 100 mg/dL for the founder and up to 175 mg/dL for the progeny. Rabbit apo A-I levels were substantially decreased in the transgenic rabbits. HDL cholesterol (HDL-C) levels were higher in two of the five transgenic rabbit lines than in controls (line 20 versus nontransgenic littermate, HDL-C = 80 +/- 7 versus 37 +/- 6 mg/dL; line 8 versus nontransgenic littermate, HDL-C = 54 +/- 16 versus 35 +/- 6 mg/dL). This resulted in less atherogenic lipoprotein profiles, with very low (VLDL + LDL-C)/HDL-C ratios. HDL size and protein and lipid compositions were similar between transgenic and littermate nontransgenic rabbits. However, a large amount of pre-beta apo A-I-containing lipoproteins was observed in the plasma of the highest human apo A-I expressor. Cell cholesterol efflux was evaluated with the incubation of whole serum from transgenic and control rabbits. Cell cholesterol efflux was highly correlated with HDL cholesterol, with apo A-I, and with the presence of pre-beta apo A-I-containing lipoproteins. These rabbits will be an extremely useful model for the evaluation of the effect of increased hepatic apo A-I expression on atherosclerosis.

Transcription of the human hepatic lipase gene is modulated by multiple negative elements in HepG2 cells

The expression of the hepatic lipase (HL) gene is highly tissue specific. In order to identify cis-acting elements which regulate the expression of this gene in the liver, multiple deletion mutants of the 5'-flanking region of the HL gene fused to the human growth hormone gene were transfected in HepG2 cells, which normally produce HL. Transient expression assays indicated the presence of negative (at nucleotides (nt) -1576(/)-1342 and -623(/)-407) and positive (at nt -1862(/)-1576 and -50(/)-9) regulatory elements. Transfection of HeLa cells, which do not produce HL, with the same deletion constructs resulted in a similar pattern of promoter activities. However, additional negative (nt -138/-50) and positive (nt -407(/)-138) elements were found. DNase I footprint analysis of the proximal and distal HLpromoter sequences with HepG2 and HeLa cell nuclear extracts identified seven protected regions: A, nt -1540(/)-1527; B, -1505(/)-1473; C, -1467(/)-1460; D, -592(/)-577; E, -565(/)-545; F, -234(/)-220; and G, -70(/) -48. Sites A, B, C, D and E were located within regions containing negative regulatory elements. In order to determine which nuclear factor interacts with the negative elements, sites B, D and E were mutated and the effects of mutation on competition in a gel retardation assay and on promoter activity were studied. When the binding motif for AP1 in sites B, D and E was mutated, the specific DNA-protein complexes were not competed with the mutant oligonucleotides and promoter activity increased twofold. The magnitude of the increase is less than expected from the deletion analysis, and simultaneous mutations did not cause further increase in promoter activity, which suggests that other sites are involved in this negative modulation. These results suggest that the transcription of the HLgene in HepG2 cells is negatively modulated by multiple cis-acting negative elements and AP1-like nuclear factor may play some role in this modulation.

The peroxisome proliferator activated receptors (PPARS) and their effects on lipid metabolism and adipocyte differentiation

The three types of peroxisome proliferator activated receptor (PPAR), alpha, beta (or delta), and gamma, each with a specific tissue distribution, compose a subfamily of the nuclear hormone receptor gene family. Although peroxisome proliferators, including fibrates and fatty acids, activate the transcriptional activity of these receptors, only prostaglandin J2 derivatives have been identified as natural ligands of the PPAR gamma subtype, which also binds thiazolidinedione antidiabetic agents with high affinity. Activated PPARs heterodimerize with RXR and alter the transcription of target genes after binding to specific response elements or PPREs, consisting of a direct repeat of the nuclear receptor hexameric DNA core recognition motif spaced by one nucleotide. The different PPARs can be considered key messengers responsible for the translation of nutritional, pharmacological and metabolic stimuli into changes in the expression of genes, more specifically those genes involved in lipid metabolism. PPAR alpha is involved in stimulating beta-oxidation of fatty acids. In rodents, a PPAR alpha-mediated change in the expression of genes involved in fatty acid metabolism lies at the basis of the phenomenon of peroxisome proliferation, a pleiotropic cellular response, mainly limited to liver and kidney and which can lead to hepatocarcinogenesis. In addition to their role in peroxisome proliferation in rodents, PPAR is also involved in the control of HDL cholesterol levels by fibrates and fatty acids in rodents and humans. This effect is, at least partially, based on a PPAR-mediated transcriptional regulation of the major HDL apolipoproteins, apo A-I and apo A-II. The hypotriglyceridemic action of fibrates and fatty acids also involves PPARs and can be summarized as follows: (1) an increased lipolysis and clearance of remnant particles, due to changes in LPL and apo C-III levels, (2) a stimulation of cellular fatty acid uptake and their conversion to acyl-CoA derivatives by the induction of FAT, FATP and ACS activity, (3) an induction of fatty acid beta-oxidation pathways, (4) a reduction in fatty acid and triglyceride synthesis, and finally (5) a decrease in VLDL production. Hence, both enhanced catabolism of triglyceride-rich particles as well as reduced secretion of VLDL particles are mechanisms that contribute to the hypolipidemic effect of fibrates and FFAs. Whereas for PPAR beta no function so far has been identified, PPAR gamma triggers adipocyte differentiation by inducing the expression of several genes critical for adipogenesis.

Transcriptional induction of rat liver apolipoprotein A-I gene expression by glucocorticoids requires the glucocorticoid receptor and a labile cell-specific protein

Treatment with glucocorticoids increases the concentration of plasma high-density lipoprotein (HDL), which is inversely correlated to the development of atherosclerosis. Previously, we demonstrated that repeated administration of glucocorticoids increases apolipoprotein (apo) A-I gene expression and decreases apoA-II gene expression in rat liver. In the present study, the mechanism of glucocorticoid action on hepatic apoA-I and apoA-II expression was studied. A single injection of rats with dexamethasone increased hepatic apoA-I mRNA levels within 6 h and further increases were observed after 12 h and 24 h. In contrast, liver apoA-II mRNA levels gradually decreased after dexamethasone treatment to less than 25% control levels after 24 h. In rat primary hepatocytes and McARH8994 hepatoma cells, addition of dexamethasone increased apoA-I mRNA levels in a time-dependent and dose-dependent manner, whereas apoA-II mRNA levels were unchanged. Simultaneous addition of the glucocorticoid antagonist RU486 prevented the increase in apoA-I mRNA levels after dexamethasone treatment, which suggests that the effects of dexamethasone are mediated through the glucocorticoid receptor. Inhibition of transcription by actinomycin D and nuclear-run-on experiments in McARH8994 cells and primary hepatocytes showed that dexamethasone induced apoA-I, but not apoA-II, gene transcription. Transient-transfection assays in McARH8994 cells with a chloramphenicol acetyl transferase vector driven by the rat-apoA-I-gene promoter demonstrated that the proximal apoA-I promoter could be induced by dexamethasone, and this effect could be abolished by simultaneous treatment with RU486. However, in COS-1 cells, apoA-I promoter transcription was not induced by dexamethasone or cotransfected glucocorticoid receptor. In addition, the induction of apoA-I gene transcription by dexamethasone was blocked by the protein-synthesis inhibitor cycloheximide, which suggests the presence of a labile protein involved in apoA-I gene activation by dexamethasone. In conclusion, our results demonstrate that dexamethasone regulates rat apoA-I, but not apoA-II, gene expression through direct action on the hepatocyte. The induction of apoA-I gene transcription by dexamethasone requires the glucocorticoid receptor and a labile cell-specific protein.

Control of apolipoprotein AI gene expression through synergistic interactions between hepatocyte nuclear factors 3 and 4

Apolipoprotein AI (apoAI) gene expression in liver depends on synergistic interactions between transcription factors bound to three distinct sites (A, B, and C) within a hepatocyte-specific enhancer in the 5'-flanking region of the gene. In this study, we showed that a segment spanning sites A and B retains substantial levels of enhancer activity in hepatoblastoma HepG2 cells and that sites A and B are occupied by the liver-enriched hepatocyte nuclear factors (HNFs) 4 and 3, respectively, in these cells. In non-hepatic CV-1 cells, HNF-4 and HNF-3beta activated this minimal enhancer synergistically. This synergy was dependent upon simultaneous binding of these factors to their cognate sites, but it was not due to cooperativity in DNA binding. Separation of these sites by varying helical turns of DNA did not affect simultaneous binding of HNF-3beta and HNF-4 nor did it influence their functional synergy. The synergy was, however, dependent upon the cell type used for functional analysis. In addition, this synergy was further potentiated by estrogen treatment of cells cotransfected with the estrogen receptor. These data indicate that a cell type-restricted intermediary factor jointly recruited by HNF-4 and HNF-3 participates in activation of the apoAI enhancer in liver cells and suggest that the activity of this factor is regulated by estrogen.

Binding specificity and modulation of the ApoA-I promoter activity by homo- and heterodimers of nuclear receptors

Three proximal regulatory elements, AIB, AIC, and AID, of the apoA-I gene are necessary and sufficient for its hepatic expression in vivo and in vitro. DNA binding and competition assays showed that elements AIB and AID contain hormone response elements composed of imperfect direct repeats that support the binding of the hepatic nuclear factor-4, other nuclear orphan receptors, and the ligand-dependent nuclear receptors retinoic X receptor (RXRalpha), RXRalpha/RARalpha, and RXRalpha/T3Rbeta. Substitution mutations on repeats 1 and 2 in the hormone response sites of elements AIB and AID, respectively, abolished the binding of all nuclear receptors and reduced promoter activity to background levels, indicating the importance of both hormone response elements for the hepatic expression of the apoA-I gene. Cotransfection experiments in HepG2 cells with normal and mutated promoter constructs and plasmids expressing nuclear hormone receptors showed that RXRalpha homodimers transactivated the wild type promoter 150% of control, in the presence of 9-cis-retinoic acid (RA), whereas RXR alpha/T3R beta heterodimers repressed transcription to 60% of control, in the presence of T3. RXR alpha/RAR alpha and hepatic nuclear factor-4 did not affect the transcription, driven by the proximal apoA-I promoter. Potassium permanganate and dimethyl sulfate interference experiments showed that RXRalpha homodimers, RXRalpha/RARalpha, and RXRalpha/T3Rbeta heterodimers participate in protein-DNA interactions with 12, 13, and 11 out of the 14 nucleotides, respectively, that span repeats 1 and 2 and the spacer region separating them on the hormone response element of element AID. The binding of RXRalpha homodimers and RXRalpha/T3Rbeta heterodimers is associated with ligand-dependent activation by 9-cis-RA or repression by T3. Upon deletion or mutation of repeat 1, homodimeric binding of RXRalpha is lost whereas heterodimeric binding is retained. This heterodimeric binding to the mutated element AID is mediated solely by interactions with repeat 2 and one adjacent nucleotide and is confined to a heptameric core recognition motif. The interactions of the RXRalpha heterodimers with repeat 2 are associated with low levels of ligand-independent transcriptional activity. The findings suggest that the specific types of homo- and heterodimers of nuclear hormone receptors occupying the hormone response elements of apoA-I and the availability of the ligand may play an important role in the transcriptional regulation of the human apoA-I gene.

Transcriptional regulation of the cholesteryl ester transfer protein gene by the orphan nuclear hormone receptor apolipoprotein AI regulatory protein-1

We have defined a 105-base pair tissue-restricted promoter for the cholesteryl ester transfer protein (CETP) gene that contains a nuclear hormone receptor response element essential for transcriptional activity. DNaseI protection and electrophoretic mobility shift assays showed specific binding of nuclear extracts from HepG2 (hepatic) and Caco-2 (intestinal) cells (expressing cell types) to 3 sites (designated A (-26 to -57), B (-59 to -87), and C (-93 to -118)) within the 105-base pair minimal promoter element between -138 and -33. Mutagenesis studies indicated that the function of the promoter was dependent upon synergistic interactions between transcription factors bound to these sites. Mutation of site C reduced transcription by 50 and 80%, respectively, in HepG2 and Caco-2 cells, and electrophoretic mobility shift assays showed that nuclear hormone receptors, including ARP-1 and its homologue Ear-3/COUP-TF, were occupants of site C in both of these cell types. Overexpression of ARP-1 or Ear-3/COUP-TF with CETP promoter/chloramphenicol acetyltransferase gene reporter plasmids repressed transcriptional activity of the CETP promoter containing sequences up to -300, but activated transcription in the context of larger constructs containing sequences up to -636. Thus ARP-1 may assume a dichotomous role as both a transcriptional repressor and a transcriptional activator dependent on the promoter context. In addition, the architecture of the CETP gene promoter suggests that its expression is under the control of multiple transcriptional signaling pathways mediated by inducible transcription factors as well as nuclear hormone receptors.