Opposite regulation of human versus mouse apolipoprotein A-I by fibrates in human apolipoprotein A-I transgenic mice

Fenofibrate Reduces Atherogenesis in ApoE*3Leiden Mice

OBJECTIVE

To demonstrate, quantify, and mechanistically dissect antiatherosclerotic effects of fenofibrate besides lowering plasma cholesterol per se.

METHODS AND RESULTS

ApoE*3Leiden transgenic mice received either a high-cholesterol diet (HC) or HC containing fenofibrate (HC+FF) resulting in 52% plasma cholesterol-lowering. In a separate low-cholesterol diet (LC) control group, plasma cholesterol was adjusted to the level achieved in the HC+FF group. Low plasma cholesterol alone (assessed in LC) resulted in reduced atherosclerosis (lesion area, number and severity) and moderately decreased plasma serum amyloid-A (SAA) concentrations. Compared with LC, fenofibrate additively reduced lesion area, number and severity, and the total aortic plaque load. This additional effect in HC+FF was paralleled by an extra reduction of aortic inflammation (macrophage content; monocyte adhesion; intercellular adhesion molecule-1 [ICAM-1], soluble vascular cell adhesion molecule-1, granulocyte-macrophage colony-stimulating factor (GM-CSF), MCP-1, and NF-kappaB expression), systemic inflammation (plasma SAA and fibrinogen levels), and by an upregulation of plasma apoE levels. Also, enhanced expression of ABC-A1 and SR-B1 in aortic macrophages may contribute to the antiatherosclerotic effect of fenofibrate by promoting cholesterol efflux.

CONCLUSIONS

Fenofibrate reduces atherosclerosis more than can be explained by lowering total plasma cholesterol per se. Impaired recruitment of monocytes/macrophages, reduced vascular and systemic inflammation, and stimulation of cholesterol efflux may all contribute to these beneficial effect of fenofibrate.

Activation of the rat scavenger receptor class B type I gene by PPARalpha

Peroxisomal proliferator activated receptor alpha (PPARalpha) is activated by fibrate drugs which are known to protect against atherosclerosis. The present study examines the effects of PPARalpha on SR-BI expression. For this study, a rat SR-BI promoter-luciferase reporter gene construct was co-transfected into different cell lines with expression vectors that encode for PPARalpha+/-retinoic X receptor alpha (RXRalpha). PPARalpha/RXR increased the activity of the SR-BI promoter, an effect that was enhanced by clofibrate. Sequence analysis of the rat SR-BI promoter revealed the presence of a putative peroxisomal proliferator response element (PPRE) at bp -1,622. Electrophoretic mobility shift assays demonstrated that PPARalpha and RXRalpha are able to bind to the SR-BI PPRE motif. In addition, mutational analysis studies confirmed that this PPRE motif is responsible for the PPARalpha/RXRalpha-dependent activation of the rat SR-BI promoter in the cell lines examined.

Reciprocal and coordinate regulation of serum amyloid A versus apolipoprotein A-I and paraoxonase-1 by inflammation in murine hepatocytes

OBJECTIVE

During inflammation, the serum amyloid A (SAA) content of HDL increases, whereas apolipoprotein A-I (apoA-I) and paraoxonase-1 (PON-1) decrease. It remains unclear whether SAA physically displaces apoA-I or if these changes derive from coordinated but inverse transcriptional regulation of the HDL apolipoprotein genes. Because cytokines stimulate the hepatic expression of inflammatory markers, we investigated their role in regulating SAA, apoA-I, and PON-1 expression.

METHODS AND RESULTS

A cytokine mixture (tumor necrosis factor [TNF]-alpha, interleukin [IL]-1beta, and IL-6) simultaneously induced SAA and repressed apoA-I and PON-1 expression levels. These effects were partially inhibited in cells pretreated with either nuclear factor kappaB (NF-kappaB) inhibitors (pyrrolidine dithiocarbamate, SN50, and overexpression of super-repressor inhibitor kappaB) or after exposure to the peroxisome proliferator-activated receptor-alpha (PPARalpha) ligands (WY-14643 and fenofibrate). Consistent with these findings, the basal level of SAA was increased, whereas apoA-I and PON-1 decreased in primary hepatocytes from PPARalpha-deficient mice as compared with wild-type mice. Moreover, neither WY-14643 nor fenofibrate had any effect on SAA, apoA-I, or PON-1 expression in the absence of PPARalpha.

CONCLUSIONS

These results suggest that cytokines increase the expression of SAA through NF-kappaB transactivation, while simultaneously decreasing the expression of apoA-I and PON-1 by inhibiting PPARalpha activation. Inflammation may convert HDL de novo into a more proatherogenic form by coordinate but inverse transcriptional regulation in the liver, rather than by physical displacement of apoA-I by SAA.

PEROXISOME PROLIFERATOR-ACTIVATED RECEPTORS: How Their Effects on Macrophages Can Lead to the Development of a New Drug Therapy Against Atherosclerosis

Peroxisome proliferator-activated receptors (PPARs) alpha (alpha), beta/delta (beta/delta), and gamma (gamma) are members of the nuclear receptor superfamily, which also includes the estrogen, androgen, and glucocorticoid receptors. Recent evidence suggests that PPARs regulate genes involved in lipid metabolism, glucose homeostasis, and inflammation in various tissues; however, the mechanisms involved are not completely understood. Anti-diabetic drugs, called glitazones, can selectively activate PPARgamma, and hypolipidemic drugs, called fibrates, can weakly activate PPARalpha. Both classes of drugs can decrease insulin resistance and dyslipidemias, which also makes them attractive for treating the metabolic syndrome. The metabolic syndrome exhibits a constellation of risk factors for atherosclerosis that include obesity, insulin resistance, dyslipidemias, and hypertension. Interestingly, all three PPARs are present in macrophages and can therefore have a profound effect on several disease processes, including atherosclerosis. Macrophages are key players in atherosclerotic lesion development. Currently, the first line of defense in reducing the risk of atherosclerosis is aimed at lowering low-density lipoproteins (LDL) and raising high-density lipoproteins (HDL), but a large percentage of patients on statins still succumb to coronary artery disease. However, with the development of drugs selectively activating PPARs, a new arsenal of drugs specifically targeting to the macrophage/foam cell may potentially have a profound impact on how we treat cardiovascular disease.

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.

9-cis beta-carotene-rich powder of the alga Dunaliella bardawil increases plasma HDL-cholesterol in fibrate-treated patients

The effect of fibrates on high density lipoprotein (HDL)-cholesterol levels is suggested to be mediated by its binding to peroxisome proliferator-activated receptor-alpha (PPARalpha). Upon ligand binding, PPARalpha heterodimerizes with the 9-cis retinoic acid receptor (RXR) and it is this heterodimer which regulates gene expression. We assessed the hypothesis that a combined treatment with fibrate plus 9-cis beta-carotene-rich powder of the alga Dunaliella bardawil, as a source of 9-cis retinoic acid, would improve the drug's effect on HDL-cholesterol levels. In an open-labeled first trial, 20 fibrate-treated men with plasma HDL-cholesterol levels below 40 mg/dl were given Dunaliella capsules, providing 60 mg beta-carotene/day, containing all-trans and 9-cis beta-carotene (1:1 ratio, w/w). Twenty-two fibrate-treated patients participated in a double-blind placebo-controlled second trial. Eleven patients were treated with Dunaliella capsules, and 11 patients were treated with beta-carotene-deficient Dunaliella capsules. Following 6 weeks of the dual treatment plasma HDL-cholesterol increased by 24.5 and 12.7% in the first and second trials, respectively (P=0.002 and 0.012). The dual treatment also increased HDL-cholesterol levels in human apolipoprotein A-I transgenic mice by 87.5% (P=0.021). The results show that a combination treatment of fibrate plus 9-cis beta-carotene-rich Dunaliella powder amplifies the effect of the drug on HDL-cholesterol levels.

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARalpha activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid beta-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARalpha target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARalpha-signaling.

Fibrates inhibit aldose reductase activity in the forward and reverse reactions

Fibrates such as bezafibrate, gemfibrozil, clofibric acid, ciprofibrate and fenofibrate, are ligands for peroxisome proliferator-activated receptor alpha (PPARalpha), and are used as therapeutic agents in the treatment of hyperlipidemia. Synthesis and accumulation of sorbitol in cells due to aldose reductase (AR) activity is implicated in secondary diabetic complications. In pursuit of finding a lead compound identification to design an effective AR inhibitor employing fragment-based design-like approach, we found that this class of compounds and their nearest neighbors could inhibit AR. Bezafibrate and gemfibrozil displayed a mixed non-competitive inhibition pattern in the glyceraldehyde reduction activity and pure non-competitive inhibition pattern in the benzyl alcohol oxidation activity of AR. Clofibric acid, ciprofibrate and fenofibrate showed pure non-competitive inhibition patterns in the forward reaction. In the reverse reaction, clofibric acid displayed a non-competitive inhibition pattern while ciprofibrate and fenofibrate displayed competitive inhibition patterns. This finding reveals for the first time a novel attribute of the fibrates in the regulation of AR activity and may be useful as lead compounds to control the function of AR in the progression and treatment of secondary diabetic complications in addition to other clinical conditions. Alternatively, these findings demonstrate that AR plays a significant role in the fibrate metabolism under various scenarios.

Expression of the pregnane X receptor in mice antagonizes the cholic acid-mediated changes in plasma lipoprotein profile

OBJECTIVE

Modification of lipoprotein metabolism by bile acids has been mainly explained by activation of the farnesyl X receptor (FXR). The aim of the present study was to determine the relative contribution of the pregnane X receptor (PXR), another bile acid-activated nuclear receptor to changes in plasma lipoprotein profile.

METHODS AND RESULTS

Wild-type mice, Pxr-deficient mice, and Pxr-null mice expressing human PXR (Pxr-null SXR-Tg mice) were fed a cholic acid-containing diet, and consequences on plasma lipoprotein profiles and target gene expression were assessed. Cholic acid produced significant decreases in high-density lipoprotein (HDL) cholesterol, plasma apolipoprotein (apo)A-I and hepatic apoA-I mRNA in wild-type mice. Interestingly, the effect of cholic acid was significantly more pronounced in Pxr-deficient mice, indicating that PXR contributes to the weakening of the effect of bile acids on lipoprotein metabolism. Reciprocally, changes in HDL/apoA-I profiles were abolished in Pxr-null SXR-Tg mice in which PXR-responsive genes, particularly those involved in bile acid detoxification were readily activated after cholic acid treatment.

CONCLUSIONS

PXR expression in mice antagonizes the cholic acid-mediated downregulation of plasma HDL cholesterol and apoA-I, and magnification of PXR/SXR-mediated changes may constitute a new mean to counteract the effects of bile acids on plasma lipoproteins.

Alteration of 20:5n-3 and 22:6n-3 fat contents and liver peroxisomal activities in fenofibrate-treated rainbow trout

Fish easily accumulate n-3 PUFA of exogenous origin, but the underlying mechanisms are not well established in the whole animal. This study was undertaken to investigate whether this feature was physiologically associated with mitochondrial and peroxisomal capacities that differentially affect FA oxidation. For this purpose, peroxisomal FA oxidation was increased by treating rainbow trout with fenofibrate, which strongly stimulates the peroxisome proliferator-activated receptor-a in rodents. Diets containing EPA and DHA, with or without fenofibrate added, were administered to male trout for 12 d. After treatment, neither liver hypertrophy nor accumulation of fat was apparent within the liver and muscle cells. However, fenofibrate treatment decreased the contents of EPA and DHA in the liver, white muscle, and intraperitoneal fat tissue, which represented (per whole body) at least 280 mg less than in controls. Carnitine-dependent palmitate oxidation rates, expressed per gram of liver, were slightly increased by fenofibrate when measured from tissue homogenates and were unchanged when calculated from isolated mitochondria, relative to control fish. The treatment altered neither carnitine palmitoyltransferase I activity rates, expressed per gram of liver, nor the sensitivity of the enzyme to malonyl-CoA inhibition, but did increase the malonyl-CoA content (+45%). Meanwhile, fenofibrate increased (by about 30%) the peroxisome-related activities, i.e., catalase, carnitine-independent palmitate oxidation, acyl-CoA oxidase, and the peroxisomal FA-oxidizing system, relative to the control group. The data strongly suggest that the induction of peroxisomal activities, some of which being able to oxidize very long chain FA, was responsible for the lower contents of EPA and DHA in the body lipids of fenofibrate-treated trout.

HDL: the 'new' target of cardiovascular medicine

Clinical, experimental and epidemiological research has shown the undeniable causal relationship between low HDL plasma concentrations and cardiovascular disease. Low HDL levels are present in about 10% of the general population and represent the most frequent form of dyslipidemia in patients with coronary disease. Reduced HDL concentrations seem to be unable to eliminate efficiently the cholesterol excess at vascular wall level, contributing to the onset of the inflammatory response that typically occurs in the pathogenesis of atherosclerosis right from its earliest stages. The results of numerous studies quite convincingly suggest that HDL is capable of exerting anti-inflammatory activity either directly or by modulating the expression of a number of acute phase proteins. Although the therapeutic options currently available for raising HDL levels still show modest efficacy, both in experimental and pre-clinical fields, genetic investigation and specifically aimed pharmacological treatment have produced more encouraging results, shedding some light on the concrete possibility of being able to treat this disease in the very near future.

Identification of a novel selective peroxisome proliferator-activated receptor alpha agonist, 2-methyl-2-(4-{3-[1-(4-methylbenzyl)-5-oxo-4,5-dihydro-1H-1,2,4-triazol-3-yl]propyl}phenoxy)propanoic acid (LY518674), that produces marked changes in serum lipids and apolipoprotein A-1 expression

Low high-density lipoprotein-cholesterol (HDL-c) is an important risk factor of coronary artery disease (CAD). Optimum therapy for raising HDL-c is still not available. Identification of novel HDL-raising agents would produce a major impact on CAD. In this study, we have identified a potent (IC50 approximately 24 nM) and selective peroxisome proliferator-activated receptor alpha (PPARalpha) agonist, 2-methyl-2-(4-{3-[1-(4-methylbenzyl)-5-oxo-4,5-dihydro-1H-1,2,4-triazol-3-yl]propyl}phenoxy)propanoic acid (LY518674). In human apolipoprotein A-1 (apoA-1) transgenic mice, LY518674 produced a dose-dependent increase in serum HDL-c, resulting in 208 +/- 15% elevation at optimum dose. A new synthesis of apoA-1 contributed to the increase in HDL-c. LY518674 increased apoA-1 mRNA levels in liver. Moreover, liver slices from animals treated with LY518674 secreted 3- to 6-fold more apoA-1 than control liver slices. In cultured hepatocytes, LY518674 produced 50% higher apoA-1 secretion, which was associated with increase in radiolabeled methionine incorporation in apoA-1. Thus, LY518674 is a potent and selective PPARalpha agonist that produced a much greater increase in serum HDL-c than the known fibrate drugs. The increase in HDL-c was associated with de novo synthesis of apoA-1.

Fenofibrate Induces a Novel Degradation Pathway for Scavenger Receptor B-I Independent of PDZK1

Fibrate drugs improve cardiovascular health by lowering plasma triglycerides, normalize low density lipoprotein levels, and raise high density lipoprotein (HDL) levels in patients with dyslipidemias. The HDL-raising effect of fibrates has been shown to be due in part to an increase in human apolipoprotein AI gene expression. However, it has recently been shown that fibrates can affect HDL metabolism in mouse by significantly decreasing hepatic levels of the HDL receptor scavenger receptor B-I (SR-BI) and the PDZ domain containing protein PDZK1. PDZK1 is essential for maintaining hepatic SR-BI levels. Therefore, decreased SR-BI might be secondary to decreased PDZK1, but the mechanism by which fibrates lower SR-BI has not been elucidated. Here we show that feeding PDZK1-deficient mice fenofibrate resulted in the near absence of SR-BI in liver, definitively demonstrating that the effect of fenofibrate on SR-BI is PDZK1-independent. Metabolic labeling experiments in primary hepatocytes from fenofibrate-fed mice demonstrated that fenofibrate enhanced the degradation of SR-BI in a post-endoplasmic reticulum compartment. Moreover, fenofibrate-induced degradation of SR-BI was independent of the proteasome, calpain protease, or the lysosome, and antioxidants did not inhibit fenofibrate-induced degradation of SR-BI. Using metabolic labeling coupled with cell surface biotinylation assays, fenofibrate did not inhibit SR-BI trafficking to the plasma membrane. Together, the data support a model in which fenofibrate enhances the degradation of SR-BI in a post-ER, post-plasma membrane compartment. The further elucidation of this novel degradation pathway may provide new insights into the physiological and pathophysiological regulation of hepatic SR-BI.

A Peroxisome Proliferator-Activated Receptor α/γ Dual Agonist with a Unique in Vitro Profile and Potent Glucose and Lipid Effects in Rodent Models of Type 2 Diabetes and Dyslipidemia

LSN862 is a novel peroxisome proliferator-activated receptor (PPAR)α/γ dual agonist with a unique in vitro profile that shows improvements on glucose and lipid levels in rodent models of type 2 diabetes and dyslipidemia. Data from in vitro binding, cotransfection, and cofactor recruitment assays characterize LSN862 as a high-affinity PPARγ partial agonist with relatively less but significant PPARα agonist activity. Using these same assays, rosiglitazone was characterized as a high-affinity PPARγ full agonist with no PPARα activity. When administered to Zucker diabetic fatty rats, LSN862 displayed significant glucose and triglyceride lowering and a significantly greater increase in adiponectin levels compared with rosiglitazone. Expression of genes involved in metabolic pathways in the liver and in two fat depots from compound-treated Zucker diabetic fatty rats was evaluated. Only LSN862 significantly elevated mRNA levels of pyruvate dehydrogenase kinase isozyme 4 and bifunctional enzyme in the liver and lipoprotein lipase in both fat depots. In contrast, both LSN862 and rosiglitazone decreased phosphoenol pyruvate carboxykinase in the liver and increased malic enzyme mRNA levels in the fat. In addition, LSN862 was examined in a second rodent model of type 2 diabetes, db/db mice. In this study, LSN862 demonstrated statistically better antidiabetic efficacy compared with rosiglitazone with an equivalent side effect profile. LSN862, rosiglitazone, and fenofibrate were each evaluated in the humanized apoA1 transgenic mouse. At the highest dose administered, LSN862 and fenofibrate reduced very low-density lipoprotein cholesterol, whereas, rosiglitazone increased very low-density lipoprotein cholesterol. LSN862, fenofibrate, and rosiglitazone produced maximal increases in high-density lipoprotein cholesterol of 65, 54, and 30%, respectively. These findings show that PPARγ full agonist activity is not necessary to achieve potent and efficacious insulin-sensitizing benefits and demonstrate the therapeutic advantages of a PPARα/γ dual agonist.

Fenofibric acid, an active form of fenofibrate, increases apolipoprotein A-I-mediated high-density lipoprotein biogenesis by enhancing transcription of ATP-binding cassette transporter A1 gene in a liver X receptor-dependent manner

OBJECTIVE

Fibrates are widely used drugs to reduce plasma triglyceride and increase high-density lipoprotein. Their active forms, fibric acids, are peroxisome proliferator-activated receptor-alpha activators, but no direct evidence has been demonstrated for their activation of ATP-binding cassette transporter A1 (ABCA1) in relation to clinically used fibrates. We investigated the reaction of fenofibric acid in this regard.

METHODS AND RESULTS

Fenofibric acid was examined for the effect of increase of ABCA1 activity. It enhanced ABCA1 gene transcription and its protein level in macrophage cell line cells and fibroblasts and increased apolipoprotein A-I-mediated cellular lipid release, all in a dose-dependent manner. Enhancement of the gene transcription was examined by using a reporter assay system for liver X receptor responsive element (LXRE) and its inactive mutant. The results demonstrated that the effect of fenofibric acid is dependent on active LXRE.

CONCLUSIONS

Fenofibric acid increased transcription of ABCA1 gene in a liver X receptor-dependent manner.

HDL as a target in the treatment of atherosclerotic cardiovascular disease

Lipid abnormalities are among the key risk factors for cardiovascular disease. Indeed, lipid-modifying drugs - in particular, the statins, which primarily lower plasma levels of low-density lipoprotein (LDL) cholesterol - considerably reduce the risk of cardiovascular events, leading to their widespread use. Nevertheless, it seems that there might be limits to the degree of benefit that can be achieved by lowering LDL-cholesterol levels alone, which has led to increased interest in targeting other lipid-related risk factors for cardiovascular disease, such as low levels of high-density lipoprotein (HDL) cholesterol. In this article, we first consider the mechanisms that underlie the protective effect of HDL cholesterol, and then discuss several strategies that have recently emerged to increase levels of HDL cholesterol to treat cardiovascular disease, including nuclear receptor modulation, inhibition of cholesteryl ester transfer protein and infusion of apolipoprotein/phospholipid complexes.

Nuclear receptors in macrophage biology: at the crossroads of lipid metabolism and inflammation

Macrophages are essential modulators of lipid metabolism and the innate immune system. Lipid and inflammatory pathways induced in activated macrophages are central to the pathogenesis of human diseases including atherosclerosis. Recent work has shown that expression of genes involved in lipid uptake and cholesterol efflux in macrophages is controlled by peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs). Other studies have implicated these same receptors in the modulation of macrophage inflammatory gene expression. Together, these observations position PPARs and LXRs at the crossroads of lipid metabolism and inflammation and suggest that these receptors may serve to integrate these pathways in the control of macrophage gene expression. In this review, we summarize recent work that has advanced our understanding of the roles of PPARs and LXRs in macrophage biology and discuss the implication of these results for cardiovascular physiology and disease.

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Regulation of human apoA-I by gemfibrozil and fenofibrate through selective peroxisome proliferator-activated receptor alpha modulation

OBJECTIVE

The objective of this trial was to study the effects of fenofibrate (FF) and gemfibrozil (GF), the most commonly used fibrates, on high-density lipoprotein (HDL) and apolipoprotein (apo) A-I.

METHODS AND RESULTS

In a head-to-head double-blind clinical trial, both FF and GF decreased triglycerides and increased HDL cholesterol levels to a similar extent, whereas plasma apoA-I only increased after FF but not GF. Results in human (h) apoA-Itransgenic (hA-ITg) peroxisome proliferator-activated receptor (PPAR) alpha-/- mice demonstrated that PPARalpha mediates the effects of FF and GF on HDL in vivo. Although plasma and hepatic mRNA levels of hapoA-I increased more pronouncedly after FF than GF in hA-ITgPPARalpha+/+ mice, both fibrates induced acylCoAoxidase mRNA similarly. FF and GF transactivated PPARalpha with similar activity and affinity on a DR-1 PPAR response element, but maximal activation on the hapoA-I DR-2 PPAR response element was significantly lower for GF than for FF. Moreover, GF induced recruitment of the coactivator DRIP205 on the DR-2 site less efficiently than did FF.

CONCLUSIONS

Both GF and FF exert their effects on HDL through PPARalpha. Whereas FF behaves as a full agonist, GF appears to act as a partial agonist due to a differential recruitment of coactivators to the promoter. These observations provide an explanation for the differences in the activity of these fibrates on apoA-I.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

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.

Guinea pigs as models to study the hypocholesterolemic effects of drugs

Guinea pigs are useful models to investigate the mechanisms of the hypocholesterolemic effects of drugs. Like humans, guinea pigs are one of the few species that carry the majority of cholesterol in LDL. This animal model has also been shown to develop atherosclerosis when challenged with hypercholesterolemic diets. In addition, plasma lipid profiles in males, females and ovariectomized guinea pigs, a model for menopause, follow similar patterns to those observed in humans. In this report, drugs aimed at lowering plasma cholesterol and triglycerides in hyperlipidemic individuals are reviewed. Studies analyzing the hypolipidemic effect of HMG-CoA reductase inhibitors, acyl CoA cholesterol acyltransferase inhibitors, fibrates, bile acid resins, apical sodium bile acid transporter inhibitors, and others show that guinea pigs and humans have comparable responses to drug therapy. In addition, results from the limited clinical reports addressing specific effects of drugs on LDL catabolism or VLDL synthesis are in agreement with observations in guinea pigs. From the review of these studies, it is apparent that the guinea pig is a useful animal model to further explore the mechanisms of action of lipid lowering drugs including effects on specific receptors and regulatory enzymes involved in cholesterol metabolism and on early atherosclerosis development.

ABBREVIATIONS

ACAT, acyl-CoA:cholesterol acyltransferase; ASBT, apical sodium co-dependent bile acid transporter; ApoB, apolipoprotein B; CHD, coronary heart disease; CYP7, cholesterol 7alpha-hydroxylase; HDL, high density lipoprotein; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; FCR, free catabolic rate; LDL, low density lipoprotein; PPAR, peroxisome proliferators-activated receptor; TC, total cholesterol; TG, triglycerides; VLDL, very low density lipoprotein.

A novel oxyiminoalkanoic acid derivative, TAK-559, activates human peroxisome proliferator-activated receptor subtypes

A novel oxyiminoalkanoic acid derivative, TAK-559, (E)-4-[4-[(5-methyl-2-phenyl-1, 3-oxazol-4-yl)methoxy]benzyloxyimino]-4-phenylbutyric acid, was synthesized as a candidate of a new type of insulin-sensitizing agent. We report here activation of human peroxisome proliferator-activated receptor (hPPAR) subtypes by TAK-559. In a transient transactivation assay, TAK-559 was a potent hPPARgamma1 and hPPARalpha agonist with EC50 values of 31 and 67 nM, respectively. Furthermore, TAK-559 was a partial agonist for hPPARgamma1 with about 68% of maximal activation obtained with rosiglitazone (5-(4-(2-(methyl(2-pyridinyl)amino)ethoxy) benzyl)-1,3-thiazolidine-2,4-dione), a thiazolidinedione derivative, which is known as a PPARgamma agonist. PPARdelta was significantly activated at a high concentration (10 microM) of TAK-559. Competition-binding assays using radiolabeled ligand indicated that the transactivation of all hPPAR subtypes by TAK-559 was due to direct binding of TAK-559 to each subtype. We also demonstrated that TAK-559 acts to recruit the coactivator SRC-1 to each of hPPARgamma1 and hPPARalpha, and to dissociate the corepressor NCoR from each of hPPARgamma1 and hPPARalpha. Taken together, we conclude that TAK-559 is a dual agonist for hPPARgamma1 and hPPARalpha with nearly equal EC50 values, a partial agonist for hPPARgamma1, and has a rather slight agonist activity for hPPARdelta.

Pharmacological characterization of a human-specific peroxisome proliferater-activated receptor α (PPARα) agonist in dogs

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a key regulator in lipid metabolism and a potential therapeutic target for lipid-related metabolic diseases. It has been shown that there are species differences between human and mouse in response to several PPARalpha agonists in a transactivation assay. In the present study, we cloned a full length of dog PPARalpha and investigated the effects of a novel and potent agonist (KCL) for human PPARalpha. In a transactivation assay using the full length of PPARalpha, agonistic activity of KCL for dog PPARalpha (EC(50): 0.007 microM) was comparable to that for human PPARalpha (EC(50): 0.003 microM), but not that for rat PPARalpha (EC(50): 11.49 microM). Similar results were obtained from a transactivation assay using a GAL4/PPARalpha ligand-binding domain (LBD) chimera. A point-mutation study showed that I272 on PPARalphaLBD is a major contributor to species differences in response to KCL between human, dog, and rat PPARalpha. KCL also induced mRNA levels of HMG-CoA synthase in dog hepatocytes. When administered orally to dogs and rats, KCL significantly decreased plasma triglyceride levels in a dose-dependent manner. The triglyceride-lowering effects of KCL in dogs were >100-fold more potent than those in rats. These results suggest that KCL may induce activation of highly potent PPARalpha in humans as well as dogs, and that dog is a suitable animal model for studying and predicting the biological actions of potent agonists for human PPARalpha.

Phenytoin treatment reduces atherosclerosis in mice through mechanisms independent of plasma HDL-cholesterol concentration

Phenytoin (PHT) increases high density lipoprotein cholesterol (HDL-C) and reduces coronary artery disease mortality in humans. We report the results of PHT treatment on atherosclerosis susceptibility and lipid profile in four different types of mouse: control C57BL/6 mice and cholesteryl ester transfer protein transgenic mice as models of fatty streak, and LDL receptor-deficient mice and apolipoprotein E-deficient mice as models of mature atherosclerosis. Each mouse type was fed an appropriate diet to induce atherosclerosis and prevent liver toxicity. PHT treatment demonstrated a protective effect in all models. Reduction in aortic atherosclerotic area by PHT treatment was more evident in early atherosclerosis (2.3-fold) than in mature atherosclerosis (decreases of 40 and 23%, respectively, but only in mice in the upper 50% percentile of plasma PHT concentration). Atherosclerosis prevention was not concomitant with a consistent increase in HDL-C or any other protective change in the lipid profile. Different analyses of potential antiatherogenic HDL functions did not provide additional information. Microarray liver gene expression analyses identified a potential atheroprotective mechanism characterized by decreased expression of syndecan-4, RhoA2, double LIM protein-1, zeta-chain-associated protein kinase-70 and interleukin 6 receptor-alpha. However, to demonstrate that these changes are part of a PHT-antiatherogenic effect, they will need to be found also in arteries, maintained at protein level and proved to be causal rather than reactive.

Expression and secretion of human apolipoprotein A-I in the heart

Various studies have correlated apolipoprotein (apo) A-I, the major component high-density lipoprotein, with protection against development of cardiovascular disease. Although apoA-I expression has been previously detected in the liver and intestine, we have discovered that the human apoA-I gene is also expressed in the heart. Using transgenic (Tg) mice generated with the human apoA-I/C-III/A-IV gene cluster and Tg mice produced with just the 2.2 kb human apoA-I gene, we have detected significant levels of apoA-I expression in the heart. Furthermore, the detection of apoA-I expression in the hearts of human apoA-I Tg mice indicates that the minimal regulatory elements necessary for cardiac expression of the gene are located near its coding sequence. To determine if the apoA-I gene is also expressed in the human heart, similar analyses were performed, where apoA-I expression was found in both adult and fetal hearts. Furthermore in-depth investigation of the various regions of human and Tg mouse hearts revealed that the apoA-I mRNA was present in the ventricles and atria, but not in the aorta. In situ hybridization of Tg mouse hearts revealed that apoA-I expression was restricted to the cardiac myocyte cells. Finally, heart explants and cardiac primary culture experiments with Tg mice showed secretion of particles containing the human apoA-I protein, and metabolic labeling experiments have also detected a 28 kDa human apoA-I protein secreted from the heart. From these novel findings, new insights into the role and function of apoA-I can be extrapolated.

A Novel Selective Peroxisome Proliferator-Activated Receptor α Agonist, 2-Methyl-c-5-[4-[5-methyl-2-(4-methylphenyl)-4-oxazolyl]butyl]-1,3-dioxane-r-2-carboxylic acid (NS-220), Potently Decreases Plasma Triglyceride and Glucose Levels and Modifies Lipoprotein Profiles in KK-Ay Mice

2-Methyl-c-5-[4-[5-methyl-2-(4-methylphenyl)-4-oxazolyl]butyl]-1,3-dioxane-r-2-carboxylic acid (NS-220) was newly synthesized and demonstrated to be a novel potent peroxisome proliferator-activated receptor alpha (PPARalpha) agonist with high subtype selectivity. In cell-based reporter gene assays, the EC(50) values of NS-220 for human PPARalpha, PPARgamma, and PPARdelta were 1.9 x 10(-8), 9.6 x 10(-6), and >10(-4) M, respectively, and for mouse PPARalpha, PPARgamma, and PPARdelta were 5.5 x 10(-8), 3.3 x 10(-5), and >10(-4) M, respectively. In addition, [(3)H]NS-220 bound to the ligand-binding domain of human PPARalpha with a K(D) value of 1.85 x 10(-7) M. Fenofibric acid and bezafibrate showed weak agonist activity for PPARalpha (EC(50), 2-8 x 10(-5) M), with poor subtype selectivity. NS-220 (0.1-3 mg/kg p.o.) decreased plasma triglyceride levels in ddY mice in a dose-dependent manner, but its hypolipidemic activity was abolished in PPARalpha-deficient mice. In KK-A(y) mice, an animal model of type-2 diabetes, NS-220 (0.3-1 mg/kg p.o.; 4 days) and fenofibrate (100-300 mg/kg p.o.; 4 days) decreased plasma triglyceride and glucose levels in a dose-dependent manner. In a 2-week repeated administration test, NS-220 (0.3-1 mg/kg p.o.) decreased plasma glucose levels markedly without increasing in plasma insulin levels. Furthermore, NS-220 increased high-density lipoprotein levels and decreased triglyceride-rich lipoprotein levels. In conclusion, a newly synthesized dioxanecarboxylic acid derivative, NS-220, is a potent and highly selective PPARalpha agonist that ameliorates metabolic disorders in diabetic mice. These results strongly suggest that it will be a promising drug for the treatment of hyperlipidemia or metabolic disorders in type-2 diabetes.

Coadministration of a liver X receptor agonist and a peroxisome proliferator activator receptor-alpha agonist in Mice: effects of nuclear receptor interplay on high-density lipoprotein and triglyceride metabolism in vivo

Liver X receptors (LXRs) are master transcription factors regulating cholesterol and fatty acid metabolism. Treatment of C57B6 mice with a specific synthetic LXR agonist, N-(2,2,2-trifluoroethyl)-N-[4-[2,2,2-trifluoro-1-hydroxy-1(trifluoromethyl)-ethyl]phenyl]-benzenesulfonamide (T0901317), resulted in elevated high-density lipoprotein (HDL) cholesterol as well as plasma and liver triglycerides. Peroxisome proliferator-activated receptor-alpha (PPARalpha) agonists are known to induce peroxisomal fatty acid beta-oxidation and also mediate HDL cholesterol metabolism. We have explored the hypothesis that simultaneous activation of PPARalpha and LXR may lead to additive effects on HDL cholesterol elevation as well as attenuation of triglyceride accumulation. Coadministration of T0901317 and the specific PPARalpha agonist [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (Wy14643)] in mice led to synergistic elevation of HDL cholesterol that was primarily associated with enlarged HDL particles enriched with apoE and apoAI. Liver phospholipid transfer protein (PLTP) mRNA and plasma PLTP activity were additively elevated, suggesting a role of PLTP in the observed HDL cholesterol elevation. Moderate increases in plasma triglyceride levels induced by LXR activation was reduced, whereas the accumulation of triglyceride in the liver was not altered upon coadministration of the PPARalpha agonist. Peroxisomal fatty acid beta-oxidation in the liver was dramatically elevated upon PPARalpha activation as expected. Interestingly, activation of LXRs via T0901317 also led to a significant increase in peroxisomal fatty acid beta-oxidation. Sterol regulatory element binding protein 1c expression was dramatically up-regulated by the LXR agonist but was not changed with PPARalpha agonist treatment. Liver lipoprotein lipase expression was additively increased upon LXR agonist and PPARalpha agonist coadministration. Our studies mark the first exploration of nuclear receptor interplay on lipid homeostasis in vivo.

Les anticholestérolémiants, nouvelles approches thérapeutiques

Statins and fibrates constitute the two major families of lipid-lowering agents. Statins are widely used for the treatment of pure hypercholesterolaemia while fibrates are used for the treatment of hypertriglyceridemia. Both drugs are also used for the treatment of mixed dyslipidemia. Some fibrates efficiently lower serum LDL-cholesterol. Statins inhibit HMG-CoA reductase and decrease cellular cholesterol synthesis. The resulting lower intracellular cholesterol concentration induces the activation of SREBP thus inducing the over expression and transcription of the LDL receptor gene. This over expression of the LDL receptor in the liver increases the clearance of circulating LDL thus decreasing the LDL-cholesterol plasma levels. The effects of fibrates on lipid metabolism are entirely due to their capacity to activate PPAR-alpha and to induce the over expression of genes containing a PPRE in their promoter. Fibrates decrease triglyceride concentrations by increasing the beta-oxidation of fatty acids in the liver and by decreasing triglyceride-VLDL synthesis. Fibrates also decrease triglycerides by increasing the hydolysys of triglycerides in chylomicron and VLDL through their capacity to increase and to decrease the lipoprotein lipase and the apo C-III transcription, respectively. Fibrates could decrease triglycerides partly by inducing apo A-V over-expression. These molecules increase HDL-cholesterol by increasing apo A-I and apo A-II transcription. Therefore the mechanisms of action of statins and fibrates depend on their capacity to modulate the expression of genes controlling lipoprotein metabolism.

HIV protease inhibitor ritonavir induces lipoatrophy in male mice

We investigated the effects of the HIV protease inhibitor ritonavir on body composition, serum lipids, and gene expression in C57BL/6 mice. Dual-energy X-ray absorptiometry measurements in ritonavir-treated male mice revealed whole-body lipoatrophy. In female mice fat reduction was restricted to the gonadal depot. A histopathological analysis showed no visible abnormalities in liver or adipose tissue from ritonavir-treated mice, although adipocytes were significantly smaller in diameter. Serum triglyceride levels were increased in ritonavir-treated male mice. Ritonavir was coadministered with the peroxisome proliferator-activated receptor alpha (PPARalpha) agonist gemfibrozil and the PPARgamma agonist rosiglitazone for 8 weeks. Neither drug alleviated the hypertriglyceridemia or lipoatrophy in ritonavir-treated male mice. Rather, gemfibrozil exacerbated the lipoatrophy. Ritonavir reduced basal expression of two PPARalpha target genes in liver, as well as the PPARgamma target gene phosphoenolpyruvate carboxykinase (PEPCK) in adipose tissues. Ritonavir partially inhibited induction of PPAR target genes by gemfibrozil and rosiglitazone. Gemfibrozil induced expression of fatty acid oxidation genes in liver, and this induction was less substantial when ritonavir was coadministered. Similarly, rosiglitazone induced expression of uncoupling protein-1, uncoupling protein-2, and PEPCK in adipose tissues, and this effect was partially inhibited by ritonavir. Thus, the effects of ritonavir on serum triglycerides and body composition may be due, at least in part, to an inhibition of PPAR function.

Design and Synthesis of a Potent and Selective Triazolone-Based Peroxisome Proliferator-Activated Receptor α Agonist

A new series of hPPARalpha agonists containing a 2,4-dihydro-3H-1,2,4-triazol-3-one (triazolone) core is described leading to the discovery of 5 (LY518674), a highly potent and selective PPARalpha agonist.

NFkappaB regulates plasma apolipoprotein A-I and high density lipoprotein cholesterol through inhibition of peroxisome proliferator-activated receptor alpha

The levels of plasma HDL cholesterol and apoA-I in NFkappaB p50 subunit-deficient mice were significantly higher than those in wild-type mice under regular and high fat diets, without any significant difference in the level of total cholesterol. To examine the role of NFkappaBin lipid metabolism, we studied its effect on the regulation of apoA-I secretion from human hepatoma HepG2 cells. Lipopolysaccharide-induced activation of NFkappaB reduced the expression of apoA-I mRNA and protein, whereas adenovirus-mediated expression of IkappaBalpha super-repressor ameliorated the reduction. This IkappaBalpha-induced apoA-I increase was blocked by preincubation with MK886, a selective inhibitor of peroxisome proliferator-activated receptor alpha (PPARalpha), suggesting that NFkappaB inactivation induces apoA-I through activation of PPARalpha. To further support this idea, the expression of IkappaBalpha increased apoA-I promoter activity, and this increase was blocked by preincubation with MK886. Mutations in the putative PPARalpha-binding site in the apoA-I promoter or lack of the site abrogated these changes. Taking these results together, inhibition of NFkappaB increases apoA-I and HDL cholesterol through activation of PPARalpha in vivo and in vitro. Our data suggest a new aspect of lipid metabolism and may lead to a new paradigm for prevention and treatment of atherosclerotic disease.

The potential role of peroxisome proliferator-activated receptors on inflammation in type 2 diabetes mellitus and atherosclerosis

Increasing attention has focused on the role of inflammation in various chronic diseases, including atherosclerosis. Recent compelling data have begun to unite work from various arenas, such as epidemiology and vascular biology, and even clinical trials to provide evidence for inflammation as a mechanism underlying cardiovascular disease. Inflammation has been implicated in the pathogenesis, progression, and complications of both atherosclerosis and diabetes mellitus-2 complex disorders often found intertwined in patients. Although this story continues to evolve, peroxisome proliferator-activated receptors (PPARs) have been implicated as a molecular pathway involved in both these disease processes. In vitro data, animal work, and some human studies suggest that synthetic PPAR agonists in clinical use, such as thiazolidinediones, may not only regulate metabolic processes but may also limit inflammatory responses, including some involved in atherosclerosis.

Action of ciprofibrate in type IIb hyperlipoproteinemia: modulation of the atherogenic lipoprotein phenotype and stimulation of high-density lipoprotein-mediated cellular cholesterol efflux

The effects of ciprofibrate (100 mg/d) on apolipoprotein (apo)B- and apoAI-containing lipoprotein subclasses, cholesteryl ester (CE) transfer protein activity, and plasma high-density lipoprotein (HDL)-mediated cellular cholesterol efflux were evaluated in 10 patients displaying type IIB hyperlipidemia. Plasma concentrations of large very low-density lipoprotein (VLDL)-1 (Sf 60-400) and of small VLDL-2 (Sf 20-60) were markedly diminished after fibrate treatment (-40%, P = 0.001; and -25%, P = 0.003, respectively). We observed a reduction (-17%; P = 0.005) in plasma low-density lipoprotein (LDL) levels resulting from significant reductions in concentrations of dense LDL particles (-46%; P < 0.0001). Ciprofibrate induced elevation in plasma total HDL (+13%; P = 0.005) levels; such elevation occurred preferentially in HDL-3 (+22%; P = 0.009). Marked reduction in numbers of atherogenic apoB100-containing particle acceptors was associated with a 25% decrease (P < 0.02) in CE transfer protein-mediated CE transfer from HDL. Finally, a significant fibrate-mediated elevation (+13%; P = 0.01 compared with baseline) in the capacity of plasma from type IIB subjects to mediate free cholesterol efflux from scavenger receptor class B, type I-expressing Fu5AH hepatoma cells was observed. In conclusion, the action of ciprofibrate in type IIB dyslipidemia leads to preferential reduction in particle numbers of atherogenic VLDL-1, VLDL-2, and dense LDL and, concomitantly, to elevation in HDL-3 levels that are associated with stimulation of HDL-mediated cellular free cholesterol efflux through the scavenger receptor class B, type I receptor pathway.

Adipocyte low density lipoprotein receptor-related protein gene expression and function is regulated by peroxisome proliferator-activated receptor gamma

The alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor that interacts with a variety of molecules. It is implicated in biologically important processes such as lipoprotein metabolism, neurological function, tissue remodeling, protease complex clearance, and cell signal transduction. However, the regulation of LRP gene expression remains largely unknown. In this study, we have analyzed 2 kb of the 5'-flanking region of the LRP gene and identified a predicted peroxisome proliferator response element (PPRE) from -1185 to -1173. Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands such as fatty acids and rosiglitazone increased functional cell surface LRP by 1.5-2.0-fold in primary human adipocytes and in the SW872 human liposarcoma cell line as assessed by activated alpha(2)-macroglobulin binding and degradation. These agents were found to increase LRP transcription. Gel shift analysis of the putative PPRE demonstrated direct binding of PPARgamma/retinoid X receptor alpha heterodimers to the PPRE in the LRP gene. Furthermore, these heterodimers could no longer interact with a mutated PPRE probe. The isolated promoter was functional in SW872 cells, and its activity was increased by 1.5-fold with the addition of rosiglitazone. Furthermore, the isolated response element was similarly responsive to rosiglitazone when placed upstream of an ideal promoter. Mutagenesis of the predicted PPRE abolished the ability of this construct to respond to rosiglitazone. These data demonstrate that fatty acids and rosiglitazone directly stimulate transcription of the LRP gene through activation of PPARgamma and increase functional LRP expression.

Fibrates down-regulate hepatic scavenger receptor class B type I protein expression in mice

Fibrates are normolipidemic drugs used in atherogenic dyslipidemia because of their ability to raise high density lipoprotein (HDL) and decrease triglyceride levels. They exert multiple effects on lipid metabolism by activating the peroxisome proliferator-activated receptor-alpha (PPAR-alpha), which controls the transcriptional regulation of genes involved in hepatic fatty acid, cholesterol, and lipoprotein metabolism. The hepatic expression of the scavenger receptor class B type I (SR-BI) plays a critical role in lipoprotein metabolism, mainly due to its ability to mediate selective cholesterol uptake. Because fibrates and PPAR-alpha agonists up-regulate SR-BI expression in human and murine macrophages, we tested whether fibrates raised a similar regulatory response on hepatic SR-BI expression in mice. Surprisingly, fibrate treatment suppressed SR-BI protein expression in the liver without changing steady state SR-BI mRNA levels. Decreased hepatic SR-BI protein expression correlated with enlarged HDL particle size. This effect was concomitant with down-regulation of CLAMP, a putative SR-BI-stabilizing protein found in the hepatic plasma membrane, which was also not associated to changes in CLAMP mRNA levels. The post-transcriptional regulatory effect of fibrates over hepatic SR-BI protein levels was dependent on PPAR-alpha expression, because it was absent in PPAR-alpha-deficient mice. Restoring hepatic SR-BI expression in fibrate-treated mice by recombinant adenoviral gene transfer abolished fibrate-mediated HDL particle size enlargement. This study describes a novel effect of fibrates on hepatic SR-BI expression providing an alternative mechanism by which this drug family modulates HDL metabolism in vivo.

[Mechanisms of actions of statins and fibrates]

Statins and fibrates constitute the two major families of hypolipidaemic drugs. Statins are widely used in the treatment of patients with pure hypercholesterolaemias and mixed dyslipidaemias while fibrates are used to treat hypertriglyceridaemias and mixed hyperlipidaemias. Some fibrates efficiently reduce low density-lipoprotein (LDL)-cholesterol. Statins inhibit HMG-CoA reductase and decrease cellular cholesterol synthesis. The resulting lower intracellular cholesterol concentrations suppress the capacity of Insing-1 and Insing-2 to inhibit the interaction of SCAP with SREBP-2 in the membrane of the endoplasmic reticulum and the formation of the SCAP: SREBP-2:SP-1 complex. When formed, this complex migrates towards the Golgi where activated SP-1 and SP-2 protease cleave SREBP-2 to give a free NH2-terminal-SREBP-2 peptide which migrates towards the nucleus. In the nucleus, this free NH2-terminal-SREBP-2 peptide binds to the SRE contained in the promoter of the gene of the LDL(B/E)-receptor and induces the transcription of this gene, and the over-expression of the LDL(B/E)-receptor in the cytoplasmic plasma membrane of hepatocytes. The over-expression of the LDL-receptor in the liver increases the clearance of circulating LDL, decreasing the LDL-cholesterol plasma levels. Fibrates decrease plasma triglycerides by decreasing their hepatic synthesis and increasing their catabolism. They decrease the triglyceride-very low density-lipoprotein (VLDL) synthesis through their capacity to increase the beta-oxidation of fatty acids in the liver. They increase the plasma triglyceride catabolism by inducing the lipoprotein lipase gene transcription and decreasing the apoC-III gene transcription. Fibrates increase high density-lipoprotein (HDL)-cholesterol by increasing apoA-I and apoA-II gene transcription. These bio-molecular effects of fibrates are entirely due to their capacity to activate PPAR alpha and to induce the over expression of genes containing a PPRE in their promoter. Therefore, the mechanism of action of the statins and fibrates depends on their capacity to modulate the expression of genes controlling the lipoprotein metabolism.

Reduction of atherosclerosis by the peroxisome proliferator-activated receptor alpha agonist fenofibrate in mice

Several clinical and angiographic intervention trials have shown that fibrate treatment leads to a reduction of the coronary events associated to atherosclerosis. Fibrates are ligands for peroxisome proliferator-activated receptor alpha (PPARalpha) that modulate risk factors related to atherosclerosis by acting at both systemic and vascular levels. Here, we investigated the effect of treatment with the PPARalpha agonist fenofibrate (FF) on the development of atherosclerotic lesions in apolipoprotein (apo) E-deficient mice and human apoA-I transgenic apoE-deficient (hapoA-I Tg x apoE-deficient) mice fed a Western diet. In apoE-deficient mice, plasma lipid levels were increased by FF treatment with no alteration in the cholesterol distribution profile. FF treatment did not reduce atherosclerotic lesion surface area in the aortic sinus of 5-month-old apoE-deficient mice. By contrast, FF treatment decreased total cholesterol and esterified cholesterol contents in descending aortas of these mice, an effect that was more pronounced in older mice exhibiting more advanced lesions. Furthermore, FF treatment reduced MCP-1 mRNA levels in the descending aortas of apoE-deficient mice, whereas ABCA-1 expression levels were maintained despite a significant reduction of aortic cholesterol content. In apoE-deficient mice expressing a human apoA-I transgene, FF increased human apoA-I plasma and hepatic mRNA levels without affecting plasma lipid levels. This increase in human apoA-I expression was accompanied by a significant reduction in the lesion surface area in the aortic sinus. These data indicate that the PPARalpha agonist fenofibrate reduces atherosclerosis in these animal models of atherosclerosis.

The effects of peroxisome proliferators on global lipid homeostasis and the possible significance of these effects to other responses to these xenobiotics: an hypothesis

Peroxisome proliferators (PPs) have been shown to regulate hepatic lipid metabolism via activation of the peroxisome proliferator-activated receptor alpha (PPAR-alpha). Recent studies have revealed that PPs also exert considerable influence on certain extrahepatic tissues, including adipose tissue and lymphoid organs, in an indirect fashion. Inhibition of the proliferation of thymocytes and splenocytes and alteration of fatty acid uptake into and release from adipose tissue might be consequences of the hypolipidemic effect of PPs involving both PPARalpha-dependent and -independent pathways. Exposure to PPs reduces the cholesterol content of circulating low-density lipoprotein (LDL), which is the major supply of this steroid to most peripheral tissues. In addition, PPs increase serum levels of high-density lipoprotein (HDL), which extracts cholesterol from peripheral tissues and returns it to the liver, thereby further decreasing the cholesterol content of peripheral tissues. This net flux of cholesterol from extrahepatic tissues to the liver represents a change in global lipid homeostasis. In normal healthy young mice, this hypolipidemic effect could result in loss of cholesterol and other lipids from peripheral tissues (e.g., adipose tissue, thymus, and spleen), especially from plasma membrane caveolae, which might perturb normal cellular signaling and result in tissue atrophy. On the other hand, the increased hepatic cholesterol content in the hepatocyte plasma membrane might actually enhance signaling, playing a role in the liver hypertrophy and hepatocarcinogenecally associated with long-term PP treatment. In conclusion, it is important to consider the systemic effects of PPs, rather than to focus on the liver alone.

Micronised fenofibrate: an updated review of its clinical efficacy in the management of dyslipidaemia

Micronised fenofibrate is a synthetic phenoxy-isobutyric acid derivative (fibric acid derivative) indicated for the treatment of dyslipidaemia. Recently, a new tablet formulation of micronised fenofibrate has become available with greater bioavailability than the older capsule formulation. The micronised fenofibrate 160mg tablet is bioequivalent to the 200mg capsule. The lipid-modifying profile of micronised fenofibrate 160mg (tablet) or 200mg (capsule) once daily is characterised by a decrease in low-density lipoprotein cholesterol (LDL-C) and total cholesterol (TC) levels, a marked reduction in plasma triglyceride (TG) levels and an increase in high-density lipoprotein cholesterol (HDL-C) levels. Micronised fenofibrate 200mg (capsule) once daily produced greater improvements in TG and, generally, in HDL-C levels than the hydroxymethylglutaryl coenzyme A reductase inhibitors simvastatin 10 or 20 mg/day, pravastatin 20 mg/day or atorvastatin 10 or 40 mg/day. Combination therapy with micronised fenofibrate 200mg (capsule) once daily plus fluvastatin 20 or 40 mg/day or atorvastatin 40 mg/day was associated with greater reductions from baseline than micronised fenofibrate alone in TC and LDL-C levels. Similar or greater changes in HDL-C and TG levels were seen in combination therapy, compared with monotherapy, recipients. Micronised fenofibrate 200mg (capsule) once daily was associated with significantly greater improvements from baseline in TC, LDL-C, HDL-C and TG levels than placebo in patients with type 2 diabetes mellitus enrolled in the double-blind, randomised Diabetes Atherosclerosis Intervention Study (DAIS) [> or =3 years follow-up]. Moreover, angiography showed micronised fenofibrate was associated with significantly less progression of coronary atherosclerosis than placebo. Micronised fenofibrate has also shown efficacy in patients with metabolic syndrome, patients with HIV infection and protease inhibitor-induced hypertriglyceridaemia and patients with dyslipidaemia secondary to heart transplantation. Micronised fenofibrate was generally well tolerated in clinical trials. The results of a large (n = 9884) 12-week study indicated that gastrointestinal disorders are the most frequent adverse events associated with micronised fenofibrate therapy. Elevations in serum transaminase and creatine phosphokinase levels have been reported rarely with micronised fenofibrate. In conclusion, micronised fenofibrate improves lipid levels in patients with primary dyslipidaemia; the drug has particular efficacy with regards to reducing TG levels and raising HDL-C levels. Micronised fenofibrate is also effective in diabetic dyslipidaemia; as well as improving lipid levels, the drug reduced progression of coronary atherosclerosis in patients with type 2 diabetes mellitus. The results of large ongoing studies (e.g. FIELD with approximately 10 000 patients) will clarify whether the beneficial lipid-modifying effects of micronised fenofibrate result in a reduction in cardiovascular morbidity and mortality.

Effects of fenofibrate on high-density lipoprotein particle size in patients with hyperlipidemia: a randomized, double-blind, placebo-controlled, multicenter, crossover study

BACKGROUND

Fenofibrate lowers serum total cholesterol and triglyceride levels while it elevates serum high-density lipoprotein cholesterol (HDL-C) level.

OBJECTIVE

The aim of this study was to investigate the effects of fenofibrate on the particle size of high-density lipoprotein (HDL).

METHODS

Patients with hyperlipidemia (as defined by serum triglyceride level > or = 150 mg/dL in the fasting state) were enrolled in this randomized, double-blind, placebo-controlled, multicenter, crossover study. Fenofibrate 300 mg (corresponding to 200 mg of micronized fenofibrate) or placebo was administered orally once daily after dinner for 8 weeks, followed by crossover of the 2 drugs for an additional 8 weeks.

RESULTS

Fifty hyperlipidemic patients (31 men, 19 women; mean [SD] age, 54.6 [12.7] years) were enrolled. Serum total cholesterol and triglyceride levels were significantly reduced with fenofibrate treatment compared with placebo (9.4% [P = 0.007] and 34.4% [P < 0.001], respectively), whereas HDL-C levels were significantly elevated (by 25.8% [P < 0.001]). Lipoprotein lipase (LPL) activity, LPL protein level, and hepatic triglyceride lipase activity increased by 10.5%, 13.4%, and 11.4%, respectively, with fenofibrate compared with placebo. HDL was classified into 3 groups by particle size: HDL3 <88 A; HDL2a > or = 88 A but <98 A; and HDL2b > or = 98 A. The amount of HDL3 increased significantly with fenofibrate compared with placebo (P < 0.001). Fenofibrate was well tolerated during the study. Abnormal clinical laboratory values were noted in 20 of 48 patients (41.7%), but these events were mild and not clinically significant.

CONCLUSION

Taken together, these findings indicate that fenofibrate therapy increased the HDL subfraction with the smallest diameter (HDL3), which is largely responsible for withdrawing cholesterol from peripheral cells.

Advances in lipid-lowering therapy in atherosclerosis

The accrued evidence that lipid-lowering therapy limits the progression of atherosclerosis and reduces CAD events is overwhelming. The focus has been on LDL-C reduction with statins, but recent evidence also stresses the importance of raising HDL-C and reducing triglyceride-rich lipoproteins (TRL). Treatment should take into account the type of dyslipidemia, combination therapy, drug interactions and pleiotropic effects of drugs (multiple effects in different systems). Statins and fibrates are the most widely prescribed. Fibrates have a major impact on plasma TRL and HDL-C levels. They enhance lipoprotein lipase, apoAI and apoAII transcription and reduce that of apoCIII. The discovery that their multiple actions are in large part mediated by the PPAR alpha pathway is a breakthrough. Fibrates also lower plasma fibrinogen and plasma viscosity but their ability to inhibit smooth muscle cell activation is one of their most promising pleiotropic effects. Statins are safe and potent LDL-C-lowering agents but also lower TRL and raise HDL. Their pleiotropic effects are numerous, and include vasodilatory, anti-thrombotic, antioxidant, anti-proliferative, anti-inflammatory and plaque stabilizing properties. Many findings make a case for their early use in CAD to improve myocardial perfusion after a myocardial infarction, and they are indicated in heart transplant recipients to improve survival and reduce graft rejection. Fibrates and statins have complementary lipid modifying and pleiotropic effects so that their combination, carried out with caution to avoid potential untoward effects, should provide the highest cardiovascular benefit. This hypothesis is currently being tested in the Lipid in Diabetes Study (LDS), an outcome trial comparing monotherapy with fenofibrate and cerivastatin to combination therapy conducted in England.

Influences of the PPAR alpha-L162V polymorphism on plasma HDL(2)-cholesterol response of abdominally obese men treated with gemfibrozil

PURPOSE

The effect of gemfibrozil is mediated by the activation of peroxisome proliferator-activated receptor alpha (PPAR alpha). The objective of this study was to determine whether the lipid response to gemfibrozil therapy is influenced by the PPAR alpha-L162V polymorphism.

METHODS

Sixty-three abdominally obese men were randomly assigned to a 6-month-intervention program with either receiving a placebo (N = 31) or gemfibrozil (N = 32).

RESULTS

In response to gemfibrozil therapy, L162-homozygotes exhibited a 5.5% increase in high-density lipoprotein 2 cholesterol (HDL(2)-C) levels compared with a 50.0% increase among carriers of the V162 allele (P = 0.03).

CONCLUSION

These results suggest that the HDL(2)-C response to gemfibrozil is modulated by the PPAR alpha-L162V polymorphism.

Peroxisome proliferator-activated receptor agonists, hyperlipidaemia, and atherosclerosis

Dyslipidaemia is a major risk factor in the development of atherosclerosis, and lipid lowering is achieved clinically using fibrate drugs and statins. Fibrate drugs are ligands for the fatty acid receptor peroxisome proliferator-activated receptor (PPAR)alpha, and the lipid-lowering effects of this class of drugs are mediated by the control of lipid metabolism, as directed by PPARalpha. PPARalpha ligands also mediate potentially protective changes in the expression of several proteins that are not involved in lipid metabolism, but are implicated in the pathogenesis of heart disease. Clinical studies with bezafibrate and gemfibrozil support the hypothesis that these drugs may have a significant protective effect against cardiovascular disease. The thiazolidinedione group of insulin-sensitising drugs are PPARgamma ligands, and these have beneficial effects on serum lipids in diabetic patients and have also been shown to inhibit the progression of atherosclerosis in animal models. However, their efficacy in the prevention of cardiovascular-associated mortality has yet to be determined. Recent studies have found that PPARdelta is also a regulator of serum lipids. However, there are currently no drugs in clinical use that selectively activate this receptor. It is clear that all three forms of PPARs have mechanistically different modes of lipid lowering and that drugs currently available have not been optimised on the basis of PPAR biology. A new generation of rationally designed PPAR ligands may provide substantially improved drugs for the prevention of cardiovascular disease.

Genetic determinants of the response to bezafibrate treatment in the lower extremity arterial disease event reduction (LEADER) trial

Genetic determinants of baseline levels and the fall in plasma triglyceride and fibrinogen levels in response to bezafibrate treatment were examined in 853 men taking part in the lower extremity arterial disease event reduction (LEADER) trial. Three polymorphisms in the peroxisome proliferator activated receptor alpha (PPARalpha) gene were investigated (L162V, G>A in intron 2 and G>C in intron 7), two in the apolipoprotein CIII (APOC3) gene (-482C>T and -455T>C) and one in the beta-fibrinogen (FIBB) gene (-455G>A). The presence of diabetes (n=158) was associated with 15% higher triglyceride levels at baseline compared to non-diabetics (n=654) (P<0.05). Among the diabetic group, carriers of the PPARalpha intron 7 C allele had 20% lower triglyceride levels compared to homozygotes for the common G allele (P<0.05), with a similar (non-significant) trend for the L162V polymorphism, which is in linkage disequilibrium with the intron 7 polymorphism. For the APOC3 gene, carriers of the -482T allele had 13% lower baseline triglyceride levels compared to -482C homozygotes (P<0.02), but no effect was observed with the -455T>C substitution. In the non-diabetic patients, the PPARalpha V162 allele was significantly associated with 9% higher baseline triglyceride levels (P<0.03) and a similar, but non-significant trend was seen for the intron 7 polymorphism. Overall, triglyceride levels fell by 26% with 3 months of bezafibrate treatment, and current smokers showed a poorer response compared to ex/non-smokers (23% fall compared to 28% P=0.03), but none of the genotypes examined had a significant influence on the magnitude of response. Carriers of the -455A polymorphism of the FIBB gene had, as expected, marginally higher baseline fibrinogen levels, 3.43 versus 3.36 g/l (P=0.055), but this polymorphism did not affect response to treatment. Overall, fibrinogen levels fell by 12%, with patients with the highest baseline fibrinogen levels showing the greatest decrease in response to bezafibrate. For both the intron 2 and the L162V polymorphisms of the PPARalpha gene there was a significant interaction (both P<0.01) between genotype and baseline levels of fibrinogen on the response of fibrinogen levels to bezafibrate, such that individuals carrying the rare alleles in the lowest tertile showed essentially no overall decrease compared to a 0.18 g/l fall in homozygotes for the common allele. Thus while these genotypes are a minor determinant of baseline triglyceride and fibrinogen levels, there is little evidence from this study that the magnitude of response to bezafibrate treatment in men with peripheral vascular disease is determined by variation at these loci.