Cholesteryl ester transfer protein and atherosclerosis

Plasma cholesteryl ester transfer protein facilitates the transfer of cholesteryl ester from HDL to apolipoprotein B-containing lipoproteins. Its significance in atherosclerosis has been debated in studies of human population genetics and transgenic mice. The current review will focus on human plasma cholesteryl ester transfer protein research, including TaqIB, 1405V, and D442G polymorphisms. Plasma cholesteryl ester transfer protein has a dual effect on atherosclerosis, depending on the metabolic background. In hypercholesterolaemia or combined hyperlipidaemia, plasma cholesteryl ester transfer protein may be pro-atherogenic and could be a therapeutic target.     

Cholesteryl ester transfer protein in metabolic syndrome

Objective: Low high‐density lipoprotein cholesterol (HDL‐C), hypertriglyceridemia, and small dense‐low density lipoprotein (LDL) are key components of metabolic syndrome (MS). Cholesteryl ester transfer protein (CETP) mediates the transfer of triglycerides (TGs) from TG‐rich lipoproteins to HDL and LDL particles in exchange for cholesteryl esters, leading to low HDL‐C and small dense‐LDL. The aim of this study was to investigate the role of CETP in subjects with MS. Research Methods and Procedures: In a cross‐sectional cohort of 234 middle‐aged men and 252 women randomly selected from the Salzburg Atherosclerosis Prevention Program in Subjects at High Individual Risk (SAPHIR) study, MS was diagnosed according to the National Cholesterol Education Program guidelines. CETP mass was determined by enzyme‐linked immunosorbent assay and LDL size‐by‐gradient polyacrylamide gel electrophoresis. Results: Men and women with MS had lower HDL‐C (45 ± 7 vs. 58 ± 13 and 48 ± 10 vs. 71 ± 14 mg/dL for men and women, respectively; p < 0.001 for all) and higher TG levels (222 ± 71 vs. 98 ± 54 and 167 ± 67 vs. 90 ± 35 mg/dL for men and women, respectively; p < 0.001 for all) than healthy subjects. LDL size was lower in subjects with MS (256 ± 11 Å vs. 267 ± 11 Å and 262 ± 10 Å vs. 273 ± 8 Å for men and women, respectively; p < 0.001 for all). CETP mass was higher in men with MS (1.87 ± 0.78 vs. 1.40 ± 0.65 μg/mL; p < 0.001) but not in women (1.74 ± 0.79 vs. 1.62 ± 0.62 μg/mL). CETP mass correlated inversely with LDL size in both men and women (r = ?0.19, p < 0.01 and r = ?0.13, p < 0.05 in men and women, respectively). Discussion: MS is associated with increased CETP mass in men. Increased CETP mass may be responsible for reduced HDL‐C and reduced LDL particle diameter in MS.     

Cholesteryl ester transfer protein and hyperalphalipoproteinemia in Caucasians

It is unclear whether cholesteryl ester transfer protein (CETP) contributes to high density lipoprotein cholesterol (HDL-C) levels in hyperalphalipoproteinemia (HALP) in Caucasians. Moreover, even less is known about the effects of hereditary CETP deficiency in non-Japanese. We studied 95 unrelated Caucasian individuals with HALP. No correlations between CETP concentration or activity and HDL-C were identified. Screening for CETP gene defects led to the identification of heterozygosity for a novel splice site mutation in one individual. Twenty-five heterozygotes for this mutation showed reduced CETP concentration (-40%) and activity (-50%) and a 35% increase of HDL-C compared with family controls. The heterozygotes presented with an isolated high HDL-C, whereas the remaining subjects exhibited a typical high HDL-C/low-triglyceride phenotype. The increase of HDL-C in the CETP-deficient heterozygotes was primarily attributable to increased high density lipoprotein containing apolipoprotein A-I and A-II (LpAI:AII) levels, contrasting with an increase in both high density lipoprotein containing apolipoprotein A-I only and LpAI:AII in the other group. This study suggests the absence of a relationship between CETP and HDL-C levels in Caucasians with HALP. The data furthermore indicate that genetic CETP deficiency is rare among Caucasians and that this disorder presents with a phenotype that is different from that of subjects with HALP who have no mutation in the CETP gene.     

Cholesteryl ester transfer protein expression attenuates atherosclerosis in ovariectomized mice

Reduced estrogen levels result in loss of protection from coronary heart disease in postmenopausal women. Enhanced and diminished atherosclerosis have been associated with plasma levels of cholesteryl ester transfer protein (CETP); however, little is known about the role of CETP-ovarian hormone interactions in atherogenesis. We assessed the severity of diet-induced atherosclerosis in ovariectomized (OV) CETP transgenic mice crossbred with LDL receptor knockout mice. Compared with OV CETP expressing ((+)), OV CETP non-expressing ((-)) mice had higher plasma levels of total, VLDL-, LDL-, and HDL-cholesterol, as well as higher antibodies titers against oxidized LDL. The mean aortic lesion area was 2-fold larger in OV CETP(-) than in OV CETP(+) mice (147 +/- 90 vs. 73 +/- 42 x 10(3) micro m(2), respectively). Estrogen therapy in OV mice blunted the CETP dependent differences in plasma lipoproteins, oxLDL antibodies, and atherosclerosis severity. Macrophages from OV CETP(+) mice took up less labeled cholesteryl ether (CEt) from acetyl-LDL than macrophages from OV CETP(-) mice. Estrogen replacement induced a further reduction in CEt uptake and an elevation in HDL mediated cholesterol efflux from pre-loaded OV CETP(+) as compared with OV CETP(-) macrophages. These findings support the proposed anti-atherogenic role of CETP in specific metabolic settings.     

Cholesteryl ester transfer protein and its plasma regulator: lipid transfer inhibitor protein.

The interconnections between cholesteryl ester transfer protein (CETP) expression and lipid metabolism, and the possible roles of CETP in atherogenesis are examined. The importance of lipid transfer inhibitor protein in modulating CETP activity is detailed, and the consequences of this inhibitory activity on CETP-mediated events are proposed.     

Cholesteryl ester transfer proteins from different species do not have equivalent activities

Site-specific changes in the amino acid composition of human cholesteryl ester transfer protein (CETP) modify its preference for triglyceride (TG) versus cholesteryl ester (CE) as substrate. CETP homologs are found in many species but little is known about their activity. Here, we examined the lipid transfer properties of CETP species with 80–96% amino acid identity to human CETP. TG/CE transfer ratios for recombinant rabbit, monkey, and hamster CETPs were 1.40-, 1.44-, and 6.08-fold higher than human CETP, respectively. In transfer assays between VLDL and HDL, net transfers of CE into VLDL by human and monkey CETPs were offset by equimolar net transfers of TG toward HDL. For hamster CETP this process was not equimolar but resulted in a net flow of lipid (TG) into HDL. When assayed for the ability to transfer lipid to an acceptor particle lacking CE and TG, monkey and hamster CETPs were most effective, although all CETP species were able to promote this one-way movement of neutral lipid. We conclude that CETPs from human, monkey, rabbit, and hamster are not functionally equivalent. Most unique was hamster CETP, which strongly prefers TG as a substrate and promotes the net flow of lipid from VLDL to HDL.     

Cholesteryl ester transfer protein expression is down-regulated in hyperinsulinemic transgenic mice

Cholesteryl ester transfer protein (CETP) mediates cholesteryl ester (CE) and triglyceride redistribution among plasma lipoproteins. In this work, we investigated whether varying levels of insulin regulate the CETP expression in vivo. Insulin deficiency [streptozotocin (STZ) injection], and hyperinsulinemia (insulin injections, 14 days) were induced in transgenic mice expressing a human CETP minigene flanked by its natural regulatory sequences. Glucose supplementation was provided to the hyperinsulinemic group (INS+GLUC) and to an extra group of mice (GLUC). In the STZ group, endogenous CE transfer rate, plasma CETP, and hepatic CETP mRNA levels were enhanced 3.0-, 1.5-, and 2.5-fold, respectively, as compared with controls. Insulin replacement in STZ mice normalized their glycemia and liver mRNA levels. Higher plasma CETP levels were observed in GLUC mice, which were decreased in INS+GLUC mice. Hepatic CETP mRNA was not altered in GLUC mice and was reduced by one-third in INS+GLUC mice. These results show that: 1) STZ treatment increases CETP plasma levels and liver mRNA expression; 2) diet glucose supplementation increases plasma CETP levels but does not change liver mRNA abundance; and 3) daily insulin injections blunt the glucose-stimulated CETP expression by reducing its liver mRNA levels. These data suggest that insulin down-regulates CETP gene expression.     

Cholesteryl ester transfer protein, high density lipoprotein and arterial disease

The reported relationships between cholesteryl ester transfer protein, high density lipoproteins and arterial disease are confusing and conflicting. Several papers published during the review period add substantially to the evidence base regarding the atherogenicity (or anti-atherogenicity) of cholesteryl ester transfer protein, although none clearly resolves the continuing conflict. These new papers are presented against the backdrop of the previous state of knowledge.     

Cholesteryl ester transfer protein biosynthesis and cellular cholesterol homeostasis are tightly interconnected

Cholesteryl ester transfer protein (CETP) mediates triglyceride and cholesteryl ester (CE) transfer between lipoproteins, and its activity is strongly modulated by dietary cholesterol. To better understand the regulation of CETP synthesis and the relationship between CETP levels and cellular lipid metabolism, we selected the SW872 adipocytic cell line as a model. These cells secrete CETP in a time-dependent manner at levels exceeding those observed for Caco-2 or HepG2 cells. The addition of LDL, 25OH-cholesterol, oleic acid, or acetylated LDL to SW872 cells increased CETP secretion (activity and mass) up to 6-fold. In contrast, CETP production was decreased by almost 60% after treatment with lipoprotein-deficient serum or beta-cyclodextrin. These effects, which were paralleled by changes in CETP mRNA, show that CETP biosynthesis in SW872 cells directly correlates with cellular lipid status. To investigate a possible, reciprocal relationship between CETP expression and cellular lipid homeostasis, CETP biosynthesis in SW872 cells was suppressed with CETP antisense oligonucleotides. Antisense oligonucleotides reduced CETP secretion (activity and mass) by 60% compared with sense-treated cells. When CETP synthesis was suppressed for 24 h, triglyceride synthesis was unchanged, but cholesterol biosynthesis was reduced by 20%, and acetate incorporation into CE increased 31%. After 3 days of suppressed CETP synthesis, acetate incorporation into the CE pool increased 3-fold over control. This mirrored a similar increase in CE mass. The efflux of free cholesterol to HDL was the same in sense and antisense-treated cells; however, HDL-induced CE hydrolysis in antisense-treated cells was diminished 2-fold even though neutral CE hydrolase activity was unchanged. Thus, CETP-compromised SW872 cells display a phenotype characterized by inefficient mobilization of CE stores leading to CE accumulation. These results strongly suggest that CETP expression levels contribute to normal cholesterol homeostasis in adipocytic cells. Overall, these studies demonstrate that lipid homeostasis and CETP expression are tightly coupled.     

Cholesteryl ester transfer protein inhibition as a strategy to reduce cardiovascular risk

Human and rabbit plasma contain a cholesteryl ester transfer protein (CETP) that promotes net mass transfers of cholesteryl esters from high density lipoproteins (HDL) to other plasma lipoprotein fractions. As predicted, inhibition of CETP in both humans and rabbits increases the concentration of cholesterol in the potentially protective HDL fraction, while decreasing it in potentially proatherogenic non-HDL fractions. Inhibition of CETP in rabbits also inhibits the development of diet-induced atherosclerosis. However, use of the CETP inhibitor torcetrapib in humans did not reduce atheroma in three imaging trials and caused an excess of deaths and cardiovascular events in a large clinical outcome trial. The precise explanation for the harm caused by torcetrapib is unknown but may relate to documented, potentially harmful effects unrelated to inhibition of CETP. More recently, a trial using the weak CETP inhibitor dalcetrapib, which raises HDL levels less effectively than torcetrapib and does not lower non-HDL lipoprotein levels, was terminated early for reasons of futility. There was no evidence that dalcetrapib caused harm in that trial. Despite these setbacks, the hypothesis that CETP inhibitors will be antiatherogenic in humans is still being tested in studies with anacetrapib and evacetrapib, two CETP inhibitors that are much more potent than dalcetrapib and that do not share the off-target adverse effects of torcetrapib.     

Cholesteryl ester transfer protein deficiency causes slow egg embryonation of Schistosoma japonicum

It was investigated whether proteasome activity was implicated in susceptibility of human vascular smooth muscle cells (VSMCs) to Fas-mediated death. Human fetal aorta smooth muscle cells were treated with agonistic anti-Fas antibody (CH11) and proteasome inhibitors (MG115 or MG132) and then cell death was determined by morphology, viability, and DNA fragmentation. The present study reports that: (a) crosslinking of Fas receptor with anti-Fas antibody in the presence of proteasome inhibitor-induced death and DNA degradation in human VSMCs that were blocked by caspases inhibitor z-DEVD.fmk; (b) cotreatment with anti-Fas antibody and proteasome inhibitor activated caspase-3; (c) proteasome inhibitors did not influence expression of procaspase-8, procaspase-3, c-FLIP, and Bcl-2; and (d) proteasome inhibitors up-regulated Fas and FADD. The data indicate that proteasome activity is important in survival of VSMCs and provide the first evidence that proteasome is involved in Fas signal transduction. The present study proposes novel mechanism(s) by which VSMCs become susceptible to FasL.     

Cholesteryl ester transfer protein inhibition, high-density lipoprotein metabolism and heart disease risk reduction

PURPOSE OF REVIEW: Cholesteryl ester transfer protein (CETP) inhibitors (JTT-705 and torcetrapib) are currently in clinical testing, and significantly raise high-density lipoprotein (HDL) cholesterol levels. Low HDL cholesterol is a significant independent predictor of coronary heart disease (CHD) and HDL raising has been associated with coronary heart disease risk reduction, but there is debate about whether CETP inhibition will reduce coronary heart disease risk. RECENT FINDINGS: It has been documented in transgenic mouse models that apolipoprotein (apo) C-I inhibits CETP, and that high mono-unsaturated fat diets prevent the normal stimulation of CETP activity by dietary cholesterol. In rabbits, torcetrapib markedly decreases clearance of HDL cholesteryl ester via an indirect pathway, but has no effect on total plasma cholesteryl ester clearance. In humans, torcetrapib raises HDL apoA-I by modestly decreasing its fractional catabolic rate, while having a very profound effect on raising HDL cholesterol and large alpha-1 migrating HDL particles by more than 50%, with no effect on fecal cholesterol excretion. When JTT-705 at 600 mg/day was given to hypercholesterolemic patients already on pravastatin 40 mg/day, the combination was well tolerated and increases in HDL cholesterol of 28% were noted. SUMMARY: In our view, CETP inhibitors in combination with statins will be profoundly beneficial in reducing human atherosclerosis, primarily because they normalize HDL particles and prevent the transfer of cholesteryl ester from HDL to atherogenic lipoproteins.     

Cholesteryl ester transfer protein activity and atherogenic parameters in rabbits supplemented with cholesterol and garlic powder

The current study was conducted to examine the effect of garlic supplementation on CETP activity, along with its anti-atherosclerotic effect in cholesterol-fed rabbits. Rabbits were fed a 1% cholesterol diet for 12 weeks, including a 1% garlic powder supplement. The garlic-supplemented group exhibited significantly lower CETP activity than the control group during the experimental period (P < 0.05). Among the atherogenic parameters, the total cholesterol, TG, LDL-C, VLDL-C, and atherogenic index were all significantly lower in the garlic group than in the control group during the experimental period (P < 0.05), whereas the HDL-C concentration was significantly higher in the garlic group than in the control group after 12 weeks (P < 0.05). Atherosclerotic lesion area in the aorta arch was also significantly lower in the garlic group (P < 0.05). In the morphological examination, the garlic-supplemented group exhibited far fewer fat droplet deposits than the control group. Furthermore, the garlic supplement also lowered the aortic and hepatic cholesterol, and triglyceride. Accordingly, the current results suggest that garlic exerts hypocholesterolemic and/or antiatherogenic and that plasma CETP activity might be a risk marker related with atherogenesis. As such, the inhibition of CETP activity may delay the progression of atherosclerosis, thereby supporting the atherogenicity of CETP and the inhibitory activity of garlic supplementation against CETP.     

Cholesteryl ester transfer protein: the controversial relation to atherosclerosis and emerging new biological roles

Cholesteryl ester transfer protein (CETP) exerts a profound impact on high-density lipoprotein (HDL) metabolism and, consequently, on the risk of atherosclerosis development and cardiovascular mortality. Here, we review the complex relationship between CETP and atherosclerosis based upon the experimental, clinical, and epidemiological studies. In addition, we discuss the recent findings that expand the functions of CETP to new areas of interest such as Alzheimer's disease, inflammation, and obesity.     

Cholesteryl ester transfer protein. Size of the functional unit determined by radiation inactivation

Radiation inactivation was used to determine the functional Mr of cholesteryl ester transfer protein (CETP) in rabbit plasma from control and irradiated animals. This technique reveals the size of the functional unit required to carry out the transfer function. The functional Mr was calculated to be 70 000 +/- 3000 (mean +/- SD) for both control and irradiated rabbits. This result is in accordance with the Mr obtained by a completely different method, namely SDS-polyacrylamide gel electrophoresis of a partially purified (110-fold) rabbit CETP. The pI of this CETP was found by isoelectric focusing to be equal to 5.95. The results suggest that the functional unit of this enzyme is the monomer.     

Cholesteryl ester transfer protein: gathering momentum as a genetic marker and as drug target

PURPOSE OF REVIEW: Cholesteryl ester transfer protein facilitates the exchange of neutral lipids between HDL and apolipoprotein B containing lipoproteins, which hold powerful opposing roles as risk factors for coronary artery disease. The question as to whether cholesteryl ester transfer protein promotes or protects from atherosclerosis, however, has not been answered. RECENT FINDINGS: This review considers studies dealing with cholesteryl ester transfer protein variants and their effect on blood lipids in various metabolic and clinical settings. Other studies discussed deal with the association between the transfer protein and cardiovascular disease. Research on the biological activity of the cholesteryl ester transfer protein molecule is described including a first clinical study where pharmacological inhibition of the protein proved to be effective in raising HDL cholesterol. SUMMARY: Data concerning the potential marker role of cholesteryl ester transfer protein, although accumulating, are still inconclusive and, at present, not useful for clinical decision making. Inhibition of the protein was demonstrated to be feasible and appears to be promising.     

Concerted actions of cholesteryl ester transfer protein and phospholipid transfer protein in type 2 diabetes: effects of apolipoproteins

PURPOSE OF REVIEW: Type 2 diabetes frequently coincides with dyslipidemia, characterized by elevated plasma triglycerides, low high-density lipoprotein cholesterol levels and the presence of small dense low-density lipoprotein particles. Plasma lipid transfer proteins play an essential role in lipoprotein metabolism. It is thus vital to understand their pathophysiology and determine which factors influence their functioning in type 2 diabetes. RECENT FINDINGS: Cholesteryl ester transfer protein-mediated transfer is increased in diabetic patients and contributes to low plasma high-density lipoprotein cholesterol levels. Apolipoproteins A-I, A-II and E are components of the donor lipoprotein particles that participate in the transfer of cholesteryl esters from high-density lipoprotein to apolipoprotein B-containing lipoproteins. Current evidence for functional roles of apolipoproteins C-I, F and A-IV as modulators of cholesteryl ester transfer is discussed. Phospholipid transfer protein activity is increased in diabetic patients and may contribute to hepatic very low-density lipoprotein synthesis and secretion and vitamin E transfer. Apolipoprotein E could stimulate the phospholipid transfer protein-mediated transfer of surface fragments of triglyceride-rich lipoproteins to high-density lipoprotein, and promote high-density lipoprotein remodelling. SUMMARY: Both phospholipid and cholesteryl ester transfer proteins are important in very low and high-density lipoprotein metabolism and display concerted actions in patients with type 2 diabetes.