Regulation of human apolipoprotein A-I gene expression by gramoxone

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.

Proteomic analysis of changes in protein expression in serum from animals exposed to paraquat

Paraquat (PQ) poisoning remains a major public health concern in many countries. Extensive research has focused on finding early diagnostic biomarkers of acute PQ poisoning. In order to investigate the characterization of diagnostic biomarkers in PQ poisoning, we utilized proteomic analysis using serum from rats exposed to PQ, and we identified 8 differentially expressed proteins from over 500 protein spots. The expression of apolipoprotein E (ApoE), preprohaptoglobin (Pphg), a precursor of haptoglobin (Hp), and complement component 3 (C3) proteins was greatly induced by PQ exposure while the expression of fibrinogen γ-chain (FGG) and Ac-158 was dramatically reduced. To further investigate the possibility of ApoE, Pphg and FGG as useful diagnostic biomarkers of PQ poisoning, western blot and qRT-PCR analyses were conducted using cell lines as well as rat and human sera. The expression levels of ApoE, Hp and FGG were significantly altered in the presence of PQ in both rat and human serum suggesting that these proteins may be appropriate candidate molecular biomarkers for the early diagnosis of acute PQ intoxication.

Comparison of chemical and cell-based antioxidant methods for evaluation of foods and natural products: generating multifaceted data by parallel testing using erythrocytes and polymorphonuclear cells

The objective of this study was to compare three tests frequently used for evaluation of antioxidant potential in natural products: (1) oxygen radical absorbance assay (ORAC), (2) cell-based antioxidant protection in an erythrocyte model (CAP-e), and (3) reactive oxygen species formation in polymorphonuclear cells (ROS PMN). The methods were applied to four natural products, all containing antioxidants capable of entering and protecting cells in the CAP-e assay. The magnitude of this effect was not directly correlated to the ORAC value of each product. Furthermore, the products showed different effects in the ROS PMN assay. Acai provided strong inhibition of ROS formation, indicating anti-inflammatory properties. In contrast, Immunel and EpiCor mildly enhanced ROS formation, suggesting activation of the innate immune response. HA Joint Formula showed a complex, nonlinear dose-response in the ROS PMN assay. This illustrates that complex natural products may have similar antioxidant properties but different effects on human cells. Cell-based antioxidant protection is addressed best in the CAP-e assay, since some natural products contain compounds that may provoke cellular signaling in other cell types. The PMN cell type is a useful model for assessment of overall anti-inflammatory versus immune supportive properties of a product. The sequential use of the three methods serves to bridge analytical and biological testing methods.

Increasing apoA-I production as a target for CHD risk reduction

Dyslipidemia leading to coronary heart diseases (CHD) enables venues to prevent or treat CHD by other strategies than only lowering serum LDL cholesterol (LDL-C) concentrations, which is currently the most frequently targeted change. Unlike LDL-C, elevated high-density lipoprotein cholesterol (HDL-C) concentrations may protect against the development of CHD as demonstrated in numerous large-scale epidemiological studies. In this review we describe that besides elevating serum HDL-C concentrations by increasing alpha-HDL particles, approaches to elevate HDL-C concentrations by increasing pre-beta HDL particle concentrations seems more attractive. Besides infusion of apoA-I(Milano), using apoA-I mimetics, or delipidation of alpha-HDL particles, elevating de novo apoA-I production may be a suitable target to functionally increase pre-beta HDL particle concentrations. Therefore, a detailed description of the molecular pathways underlying apoA-I synthesis and secretion, completed with an overview of known effects of pharmacological and nutritional compounds on apoA-I synthesis will be presented. This knowledge may ultimately be applied in developing dietary intervention strategies to elevate apoA-I production and serum HDL-C concentrations and consequently lower CHD risk.

Proteomic analysis of liver tissues subjected to early ischemia/reperfusion injury during human orthotopic liver transplantation

Knowledge of early molecular events occurring upon ischemia/reperfusion (I/R) during liver transplantation (LT) is of great importance to improve the therapeutic intervention of surgical treatment. However, nowadays, few data are available on early protein targets of I/R injury. To identify these proteins, we used a differential proteomics approach in the characterization human liver biopsies during I/R upon LT. Analyses were performed on nine donor livers during LT. By using 2-DE and MALDI-TOF MS, we identified 36 proteins which resulted significantly altered upon I/R injury. The majority of these proteins are functionally involved in lipid and energy metabolism, in different metabolic pathways, in redox signalling and in oxidative-stress response. Our data represent the first global approach in the study of I/R injury in liver.

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.

Antioxidant vitamins and lipid therapy: end of a long romance?

During the past decade, the perception flourished that lipid and antioxidant therapy were 2 independent avenues for cardiovascular protection. However, studies have shown that commonly used antioxidant vitamin regimens do not prevent cardiovascular events. We found that the addition of antioxidant vitamins to simvastatin-niacin therapy substantially blunts the expected rise in the protective high density lipoprotein (HDL)2 cholesterol and lipoprotein(A-I) subfractions of HDL, with apparent adverse effects on the progression of coronary artery disease. To better understand this effect, 12 apolipoproteins, receptors, or enzymes that contribute to reverse cholesterol transport have been examined in terms of their relationship to HDL2 and lipoprotein(A-I) levels and the potential for antioxidant modulation of their gene expression. Three plausible candidate mechanisms are identified: (1) antioxidant stimulation of cholesteryl ester transfer protein expression/activity, (2) antioxidant suppression of macrophage ATP binding cassette transmembrane transporter A1 expression, and/or (3) antioxidant suppression of hepatic or intestinal apolipoprotein A-I synthesis or increase in apolipoprotein A-I catabolism. In summary, antioxidant vitamins E and C and beta-carotene, alone or in combination, do not protect against cardiovascular disease. Their use for this purpose may create a diversion away from proven therapies. Because these vitamins blunt the protective HDL2 cholesterol response to HDL cholesterol-targeted therapy, they are potentially harmful in this setting. We conclude that they should rarely, if ever, be recommended for cardiovascular protection.

Transcriptional regulatory mechanisms of the human apolipoprotein genes in vitro and in vivo

The present review summarizes recent advances in the transcriptional regulation of the human apolipoprotein genes, focusing mostly, but not exclusively, on in-vivo studies and signaling mechanisms that affect apolipoprotein gene transcription. An attempt is made to explain how interactions of transcription factors that bind to proximal promoters and distal enhancers may bring about gene transcription. The experimental approaches used and the transcriptional regulatory mechanisms that emerge from these studies may also be applicable in other gene systems that are associated with human disease. Understanding extracellular stimuli and the specific mechanisms that underlie apolipoprotein gene transcription may in the long run allow us to selectively switch on antiatherogenic genes, and switch off proatherogenic genes. This may have beneficial effects and may confer protection from atherosclerosis to humans.

Apolipoprotein A-I gene expression is upregulated by polychlorinated biphenyls in rat liver

Xenobiotics such as polychlorinated biphenyls (PCB) increase serum cholesterol level (especially high density lipoprotein cholesterol) and apolipoprotein A-I (apo A-I) level in rats. The effect of PCB on serum apo A-I and hepatic apo A-I gene expression and the relationship between apo A-I and drug-metabolizing enzymes in rats were investigated. Serum levels of cholesterol and apo A-I were increased by dietary addition of PCB in a dose-dependent manner (0-500 mg/kg diet). Hepatic apo A-I mRNA level was also elevated by PCB in a similar fashion. Serum level of cholesterol gradually increased during feeding period of PCB (200 mg/kg diet, 105 days) and reached a two-fold higher level in PCB group than in controls. The levels of serum apo A-I and hepatic apo A-I mRNA linearly elevated during feeding period of PCB and were increased 3- or 4-fold, respectively, compared to controls. Although acute administration (16 hr) of PCB, 3-methylcholanthrene, and phenobarbital induced cytochrome P-450 gene expression in the liver, hepatic apo A-I gene expression was not increased by these xenobiotics. These results indicated that the serum levels of cholesterol and apo A-I had positive correlation with hepatic level of apo A-I mRNA in rats fed PCB, and that hepatic apo A-I gene expression was dependent upon intake of PCB but was not directly related to the induction of drug-metabolizing enzymes. This study demonstrated that xenobiotic-induced hyper-alpha-cholesterolemia would be caused by the increased apo A-I gene expression and cholesterol synthesis in the liver, coordinately.

High density lipoprotein, apolipoprotein A-I, and coronary artery disease

High density lipoproteins (HDL), one of the main lipoprotein particles circulating in plasma, is involved in the reverse cholesterol transport. Several lines of evidence suggest that elevated levels of HDL is protective against coronary heart disease. The role of HDL in the removal of body cholesterol and in the regression of atherosclerosis add to the importance of understanding the molecular-cellular processes that determine plasma levels of HDL. Factors modulating plasma levels of HDL may have influence on the predisposition of an individual to premature coronary artery disease. Apolipoprotein (apo) A-I is the main apolipoprotein component of HDL and, to a large extent, sets the plasma levels of HDL. Thus, understanding the regulation of apoA-I gene expression may provide clues to raise plasma levels of HDL. This review discusses the various pathways that alter plasma levels of HDL. Since apoA-I is the main protein component of HDL and determines the plasma levels of HDL, this review also covers the regulation of apoA-I gene expression.

Serum malondialdehyde and prevalent cardiovascular disease in hemodialysis

BACKGROUND

Oxidative stress has been proposed as a mechanism by which the accelerated rate of cardiovascular disease (CVD) observed in maintenance hemodialysis (HD) patients may be explained. This study examined the effects of HD and CVD on serum malondialdehyde (MDA) levels as a marker of oxidative stress in HD patients with and without prevalent CVD. Serum MDA levels and CVD prevalence in HD were modeled.

METHODS

Serum MDA was determined using spectrophotometry in HD patients (N = 76, 53 men and 23 women, mean age 63.8 years) immediately prior to and at the conclusion of one midweek HD treatment. Traditional CVD risk factors, including serum lipids, lipoproteins, apolipoproteins, and fibrinogen, were also measured, as were serum chemistry and dialysis adequacy.

RESULTS

Mean serum MDA levels were significantly elevated in HD patients with prevalent CVD compared with those without, whereas serum lipoprotein and plasma fibrinogen levels did not differ between the two groups. Patients in the highest compared with the lowest tertile of postdialysis MDA were nearly four times as likely to have prevalent CVD, and serum MDA was the single strongest predictor of prevalent CVD in this patient population.

CONCLUSIONS

These findings indicate the presence of oxidative stress in HD patients, and are consistent with the theory of oxidative stress as a factor in accelerated CVD in this population.