Proatherogenic and antiatherogenic effects of probucol and phytosterols in apolipoprotein E-deficient mice: possible mechanisms of action

Cholesterol absorption inhibitors for the treatment of hypercholesterolaemia

The benefits of lipid lowering therapy on coronary heart disease have been clearly established in many clinical trials on primary and secondary prevention. Despite the availability of potent lipid lowering drugs, many patients do not reach the current treatment goals. This paper reviews new therapeutic approaches in lipid lowering drugs focusing on compounds which lower cholesterol absorption. The role of plant sterols and stanols, new acyl-CoA:cholesterol O-acyl transferase (ACAT) inhibitors, microsomal triglyceride transfer protein (MTP) inhibitors, and ezetimibe are summarised. Although the lipid lowering effect of plant sterols and plant stanols is only moderate, their use as functional foods is beneficial for patients with mild hypercholesterolaemia and is able to enhance the lipid lowering effect of HMG-CoA reductase inhibitors (statins). The role of ACAT inhibitors that might also inhibit cholesterol absorption remains unclear. Avasimibe, the first oral bioavailable ACAT inhibitor, has entered phase III trials. However, the presently available data in humans do not indicate a clear clinical benefit. The role of MTP inhibitors, which exhibit remarkable effects on all plasma lipids, also remains unclear, as safety concerns must first be addressed. Ezetimibe, the first available 2-azetidinone, succeeded in phase III trials showing remarkable effects in inhibition of cholesterol absorption as well as cholesterol lowering. The synergistic effect of co-administration of ezetimibe with statins seemingly offers a new approach in reaching the therapeutic goals.

Pathophysiology of apolipoprotein E deficiency in mice: relevance to apo E-related disorders in humans

Apolipoprotein E (apo E) deficiency (or its abnormalities in humans) is associated with a series of pathological conditions including dyslipidemia, atherosclerosis, Alzheimer's disease, and shorter life span. The purpose of this study was to characterize these conditions in apo E-deficient C57BL/6J mice and relate them to human disorders. Deletion of apo E gene in mice is associated with changes in lipoprotein metabolism [plasma total cholesterol (TC) (>+400%), HDL cholesterol (-80%), HDL/TC, and HDL/LDL ratios (-93% and -96%, respectively), esterification rate in apo B-depleted plasma (+100%), plasma triglyceride (+200%), hepatic HMG-CoA reductase activity (-50%), hepatic cholesterol content (+30%)], decreased plasma homocyst(e)ine and glucose levels, and severe atherosclerosis and cutaneous xanthomatosis. Hepatic and lipoprotein lipase activities, hepatic LDL receptor function, and organ antioxidant capacity remain unchanged. Several histological/immunohistological stainings failed to detect potential markers for neurodegenerative disease in the brain of 37-wk-old male apo E-KO mice. Apo E-KO mice may have normal growth and development, but advanced atherosclerosis and xanthomatosis may indirectly reduce their life span. Apo E plays a crucial role in regulation of lipid metabolism and atherogenesis without affecting lipase activities, endogenous antioxidant capacity, or appearance of neurodegenerative markers in 37-wk-old male mice.

Effects of a novel hydrophilic phytostanol analog on plasma lipid concentrations in gerbils

This study was designed to determine the effects of a novel hydrophilic phytostanol analog, FM-VP4, on total plasma cholesterol, total plasma triglyceride, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol concentrations after acute oral administration to gerbils. Gerbils were administered a standard gerbil diet for 4 continuous weeks, and daily water and food intake was monitored and replaced. The diet contained either no FM-VP4 (control) or FM-VP4 at the following concentrations: 0.25, 0.50, 1.0, or 2.0% w/w; six gerbils were fed each diet formulation. After 4 weeks of receiving a single diet formulation, blood was obtained from each gerbil by cardiac puncture and the animals were sacrificed humanely. Plasma obtained from this blood was analyzed for total cholesterol, total triglyceride, and HDL cholesterol levels by standard enzymatic and precipitation techniques. LDL cholesterol levels were calculated using the Friedewald equation. Administration of dietary FM-VP4 resulted in significant decreases in total plasma cholesterol and LDL cholesterol concentrations compared with controls. Dietary FM-VP4 at concentrations of 1% and 2% (w/w) decreased total plasma cholesterol by 3.4 mmol/L compared with controls. This decrease was entirely due to the loss of cholesterol from the LDL pool because LDL cholesterol was decreased by 3.3 and 3.2 mmol/L after 1% and 2% (w/w) FM-VP4, respectively. There were no significant changes in plasma triglyceride or HDL cholesterol concentrations after the administration of FM-VP4. Animals administered 1% or 2% (w/w) FM-VP4 also had significantly lower body weight after 4 weeks of treatment compared with the other groups. However, no unusual behavior was observed in these animals. No major differences in daily water or food intake were observed throughout the study. These findings indicate that FM-VP4 decreases total and LDL cholesterol concentrations.

Taurine inhibits development of atherosclerotic lesions in apolipoprotein E-deficient mice

1. The effects of taurine on the development of atherosclerotic lesions were investigated in apolipoprotein (apo) E-deficient mice, an animal model with severe hypercholesterolaemia and extensive atherosclerosis. These mice were fed a normal laboratory chow containing 2% taurine for 12 weeks. 2. Serum total cholesterol was significantly elevated after 12 weeks treatment with taurine. This elevation was due to increases in very low-density lipoprotein- and low-density lipoprotein-cholesterol. 3. Despite such effects on serum lipoproteins, analysis using en face oil red O staining revealed that taurine reduced the area of arterial lipid accumulation by 28%, as measured quantitatively as an index of atherosclerosis. Histological examination also demonstrated a decrease in the size of aortic lesions in taurine-treated mice. 4. Serum levels of thiobarbituric acid reactive substances (TBARS) in apoE-deficient mice were higher than in normolipidaemic C57BL/6J mice. Serum TBARS levels were significantly decreased by 12 weeks treatment of apoE-deficient mice with taurine. 5. Thus, taurine prevents the formation of atherosclerotic lesions, independently of serum cholesterol levels, and the results suggest that the anti-oxidative effects of taurine are related to its anti-atherosclerotic actions.

Advances in experimental dyslipidemia and atherosclerosis

Among the models of dyslipidemia and atherosclerosis, a number of wild-type, naturally defective, and genetically modified animals (rabbits, mice, pigeons, dogs, pigs, and monkeys) have been characterized. In particular, their similarities to and differences from humans in respect to relevant biochemical, physiologic, and pathologic conditions have been evaluated. Features of atherosclerotic lesions and their specific relationship to plasma lipoprotein particles have been critically reviewed and summarized. All animal models studied have limitations: the most significant advantages and disadvantages of using a specific animal species are outlined here. New insights in lipid metabolism and genetic background with regard to variations in pathogenesis of dyslipidemia-associated atherogenesis have also been reviewed. Evidence suggests that among wild-type species, strains of White Carneau pigeons and Watanabe Heritable Hyperlipidemic and St. Thomas's Hospital rabbits are preferable to the cholesterol-fed wild-type animal species in dyslipidemia and atherosclerosis research. Evidence for the usefulness of both wild-type and transgenic animals in studying the involvement of inflammatory pathways and Chlamydia pneumoniae infection in pathogenesis of atherosclerosis has also been summarized. Transgenic mice and rabbits are excellent tools for studying specific gene-related disorders. However, despite these significant achievements in animal experimentation, there are no suitable animal models for several rare types of fatal dyslipidemia-associated disorders such as phytosterolemia and cerebrotendinous xanthomatosis. An excellent model of diabetic atherosclerosis is unavailable. The question of reversibility of atherosclerosis still remains unanswered. Further work is needed to overcome these deficiencies.

Assessing plasma pharmacokinetics of cholesterol following oral coadministration with a novel vegetable stanol mixture to fasting rats

The purpose of this project was to assess the plasma pharmacokinetics of [(3)H]cholesterol following coadministration of a novel vegetable stanol mixture composed of sitostanol and campestanol (FCP-3P4) to fasting rats. Following an overnight fast (12-16 h) and 48 h post-surgery, adult male Sprague Dawley rats were divided into six treatment groups and received a single-dose oral gavage at 0700 h of either: [(3)H]cholesterol (25 microCi/mL), FCP-3P4 (5 mg/kg) + [(3)H]cholesterol (25 microCi/mL), FCP-3P4 (12.5 mg/kg) + [(3)H]cholesterol (25 microCi/mL), FCP-3P4 (25 mg/kg) + [(3)H]cholesterol (25 microCi/mL), FCP-3P4 (50 mg/kg) + [(3)H]cholesterol (25 microCi/mL), or FCP-3P4 (100 mg/kg) + [(3)H]cholesterol (25 microCi/mL). Intralipid (10%) was the vehicle used to solubilize and coadminister [(3)H]cholesterol and FCP-3P4. Liquid chromatography-mass spectrometry analysis confirmed minimal cholesterol and vegetable stanol content within 10% Intralipid. Analysis of plasma pharmacokinetics was initiated by sampling 0.5 mL of blood prior to and 0.25, 0.5 1.0, 2.0, 4.0, 6.0, 8.0, 10, 24, 28, 32, and 48 h post-oral gavage. Plasma samples were obtained by centrifugation of the blood samples and analyzed for [(3)H]cholesterol radioactivity. Pharmacokinetics analysis was performed by standard noncompartmental methods using statistical moment theory. Thin-layer chromatography was used to confirm that the majority of radioactivity measured in plasma was cholesterol (in the form of esterified or unesterified cholesterol). Greater than 90% of the radioactivity measured in all plasma samples was cholesterol-associated (in the form of either esterified or unesterified cholesterol). The coadministration of FCP-3P4 significantly decreased the area under the curve of [(3)H]cholesterol concentration versus time from 0 to 48 h (AUC(0-48h)) and maximum concentration (C(max)) in a dose-dependent manner. However, coadministration of FCP-3P4 at 25, 50, and 100 mg/kg resulted in a significant increase in apparent total body clearance (CL/F, where F is the bioavailability constant), apparent volume of distribution (V(d)/F), and oral absorption rate constant (k(a)) of [(3)H]cholesterol compared with controls. These findings suggest that the novel vegetable stanol mixture, FCP-3P4, modifies the plasma pharmacokinetics of [(3)H]cholesterol in fasting rats on oral coadministration.

Therapeutic potential of plant sterols and stanols

Products enriched with plant sterol and stanol esters selectively lower LDL cholesterol. Consumption appears to be safe, and these functional foods thus have great potential for cardiovascular risk management. Although values remain within the normal range, one possible concern is that they lower lipid-standardized concentrations of the plasma carotenoids. Whether this affects health in the longer-term or in selected patient groups is not known. Therefore, especially in view of the increasing number of functional foods that will be on the market in the very near future, there is a clear need to establish an effective post-marketing safety net.

The oxidative modification hypothesis of atherogenesis: an overview

The literature relating lipid and lipoprotein oxidation to atherosclerosis has expanded enormously in recent years. Papers on the "oxidative modification hypothesis" of atherogenesis have ranged from the most basic studies of the chemistry and enzymology of LDL oxidation, through studies of the biological effects of oxidized LDL on cultured cells, and on to in vivo studies of the effects of antioxidants on atherosclerosis in animals and humans. The data in support of this theory are mounting but many key questions remain unanswered. For example, while it is generally agreed that LDL undergoes oxidation and that oxidized LDL is present in arterial lesions, it is still not known how and where LDL gets oxidized in vivo nor which of its many biological effects demonstrable in vitro are relevant to atherogenesis in vivo. This brief review is not intended to be comprehensive but rather to offer a perspective and a context for this Forum. We discuss the strengths and weaknesses of each line of evidence, try to identify areas in which further research is needed, assess the relevance of the hypothesis to the human disease, and point to some of the potential targets for therapy.

The value of apolipoprotein E knockout mice for studying the effects of dietary fat and cholesterol on atherogenesis

The ability of the apolipoprotein E-deficient mouse to develop spontaneous atherosclerosis, which resembles the human process, is an excellent model in which to assess the impact of dietary factors. This review discusses the role of several nutrients in the development of atherosclerosis and the mechanisms through which they act.

Dietary and pharmacological antioxidants in atherosclerosis

Antioxidants that inhibit LDL oxidation are thought to be potential anti-atherogenic compounds. The results of major human randomized trials with antioxidants have, however, been disappointing, except for probucol, which consistently inhibits restenosis. Similarly, animal intervention studies show that antioxidants do not generally inhibit atherosclerosis, although some compounds provide protection. Direct evidence for the oxidation of LDL causing atherosclerosis is needed. This article summarizes results from antioxidant intervention studies, and highlights some of the key issues that need to be addressed to link biochemical changes in the arterial wall more directly to the oxidation theory of atherosclerosis.

Effects of dietary phytosterols on cholesterol metabolism and atherosclerosis: clinical and experimental evidence

Although plant sterols (phytosterols) and cholesterol have similar chemical structures, they differ markedly in their synthesis, intestinal absorption, and metabolic fate. Phytosterols inhibit intestinal cholesterol absorption, thereby lowering plasma total and low-density lipoprotein (LDL) cholesterol levels. In 16 recently published human studies that used phytosterols to reduce plasma cholesterol levels in a total of 590 subjects, phytosterol therapy was accompanied by an average 10% reduction in total cholesterol and 13% reduction in LDL cholesterol levels. Phytosterols may also affect other aspects of cholesterol metabolism that contribute to their antiatherogenic properties, and may interfere with steroid hormone synthesis. The clinical and biochemical features of hereditary sitosterolemia, as well as its treatment, are reviewed, and the effects of cholestyramine treatment in 12 sitosterolemic subjects are summarized. Finally, new ideas for future research into the role of phytosterols in health and disease are discussed.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

Major advances have been made in our understanding of the role of apolipoprotein E (apoE) in the onset and development of atherosclerosis. Increasing evidence from both animal and human studies suggests that apoE is able to protect against atherosclerosis by: a) promoting efficient uptake of triglyceride-rich lipoproteins from the circulation; b) maintaining normal macrophage lipid homeostasis; c) playing a role in cellular cholesterol efflux and reverse cholesterol transport; d) acting as an antioxidant; e) inhibiting platelet aggregation; and f) modulating immune function. In humans, apoE is polymorphic, and this genetic variation has a strong effect on its antiatherogenic characteristics. Thus, compared to the epsilon3 allele, the epsilon4 allele promotes atherosclerosis, whereas the epsilon2 allele is either pro- or anti-atherogenic, depending on the influence of both environmental and genetic factors. ApoE and its gene are prime targets for therapeutic intervention aimed at preventing or treating atherosclerotic vascular disease.

Dissociation of atherogenesis from aortic accumulation of lipid hydro(pero)xides in Watanabe heritable hyperlipidemic rabbits

Antioxidants can inhibit atherosclerosis, but it is unclear how inhibition of intimal lipid oxidation relates to atherogenesis. Here we tested the effect of probucol and its metabolite bisphenol on aortic lipid (per)oxidation and atherogenesis in Watanabe heritable hyperlipidemic (WHHL) rabbits. LDL and aortas from rabbits fed probucol contained bisphenol at concentrations comparable to those in bisphenol-treated animals. Bisphenol treatment increased plasma cholesterol slightly, and plasma and aortic alpha-tocopherol more substantially; these parameters were unaffected by probucol. Bisphenol and probucol treatment both enhanced the resistance of circulating LDL to peroxyl radical-induced lipid peroxidation; this was due to bisphenol, not probucol. Only probucol enhanced LDL's resistance to Cu(2+)-induced oxidation. Both bisphenol and probucol treatment strongly inhibited aortic accumulation of hydroperoxides and hydroxides of cholesteryl esters and triglycerides [LO(O)H]. Despite this, however, probucol had a modestly significant effect on the extent of lesion formation; bisphenol had no inhibitory effect. In addition, the extent of atherosclerosis did not correlate with amounts of aortic LO(O)H present, but, as expected, it did correlate with aortic alpha-tocopherol and cholesterol. Together, these results suggest that aortic accumulation of LO(O)H is not required for, nor is alpha-tocopherol depleted during, the initiation and progression of atherogenesis in WHHL rabbits.