Lipid transfer inhibitor protein defines the participation of high density lipoprotein subfractions in lipid transfer reactions mediated by cholesterol ester transfer protein (CETP)

Cholesterol Ester Transfer Protein Taq1B Polymorphism and Its Association with Cardiovascular Risk Factors in Patients Undergoing Angiography in Yazd, Eastern Iran: A Cross-Sectional Study

Background

Several studies assessed the relationship between the cholesterol ester transfer protein (CETP) Taq1B gene polymorphism (rs708272) with risk factors of cardiovascular diseases (CVDs). However, their findings were inconsistent. The present study investigated the relationship between CVD risk factors and the Taq1B variant in patients undergoing coronary angiography.

Methods

This cross-sectional study was conducted on 476 patients aged 30-76 years old of both sexes from 2020-2021, in Yazd (Iran). The Taq1B polymorphism genotypes were evaluated using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) on DNA extracted from whole blood. Standard protocols were used to measure cardio-metabolic markers. To determine the association between CVDs risk factors and the rs708272 variant, binary logistic regression was used in crude and adjusted models.

Results

Taq1B polymorphism genotype frequencies were 10.7% for B1B1, 72.3% for B1B2, and 17% for B2B2. There was no significant association between abnormal levels of CVDs risk factors and different genotypes of the Taq1B variant, Gensini score (P=0.64), Syntax score (P=0.79), systolic blood pressure (P=0.55), diastolic blood pressure (P=0.58), and waist circumference (P=0.79). There was no significant association between genotypes of the rs708272 variant and any abnormal serum lipid levels. After adjusting for confounders, the results remained non-significant.

Conclusion

There was no significant association between CVDs risk factors and CETP rs708272 polymorphism. The relationship between CETP gene variants and CVD occurrences varied across groups, implying that more research in different regions is required.A preprint version of this manuscript is available at https://www.researchsquare.com/article/rs-2575215/v1 with doi: 10.21203/rs.3.rs-2575215/v1.

Interaction between CETP Taq1B polymorphism and dietary patterns on lipid profile and severity of coronary arteries stenosis in patients under coronary angiography: a cross-sectional study

AIM

Evidence indicates there are still conflicts regarding CETP Taq1B polymorphism and coronary artery disease risk factors. Current findings about whether dietary patterns can change the relationship of the Taq1B on lipid profile and the severity of coronary arteries stenosis appears to be limited. The present research made an attempt to investigate this possible relationship.

METHODS

This cross-sectional study involved 453 male and female participants with a mean age of 57 years. A validated 178-item food frequency questionnaire (FFQ) was used to assess dietary usual intake. Dietary patterns were extracted through principal component analysis (PCA). Taq1B variant was genotyped by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Two-way ANOVA was used to test the interaction between Taq1B polymorphism and dietary patterns.

RESULTS

Two dietary patterns were detected: the western dietary pattern (WDP) and the traditional dietary pattern (TDP). The frequency of Taq1B genotypes turned out to be 10.4, 72.4, and 17.2% for B1B1, B1B2, and B2B2, respectively. A significant difference was observed in TG and TG/HDL-C levels among TaqIB genotypes in higher adherence to TDP (P = 0.01 and P = 0.03, respectively). Taq1B showed a significant interaction with TDP for modulating TG levels and TG/HDL-C ratio (P = 0.02 and P = 0.04, respectively). Greater compliance to WDP demonstrated a significant difference in TG and TG/HDL-C levels across rs708272 genotypes (P = 0.03) after adjusting for confounding factors. Other lipid components and coronary arteries stenosis scores failed to show any relationship or significant difference across Taq1B genotypes or dietary patterns.

CONCLUSION

Adherence to TDP may adjust the association between the Taq1B variant and TG and TG/HDL-C levels in patients undergoing coronary angiography. To better understand the relationships, we suggest prospective studies in different race groups with multivariate approaches.

High-density lipoproteins (HDL): Novel function and therapeutic applications

The failure of high-density lipoprotein (HDL)-raising agents to reduce cardiovascular disease (CVD) together with recent findings of increased cardiovascular mortality in subjects with extremely high HDL-cholesterol levels provide new opportunities to revisit our view of HDL. The concept of HDL function developed to explain these contradictory findings has recently been expanded by a role played by HDL in the lipolysis of triglyceride-rich lipoproteins (TGRLs) by lipoprotein lipase. According to the reverse remnant-cholesterol transport (RRT) hypothesis, HDL critically contributes to TGRL lipolysis via acquirement of surface lipids, including free cholesterol, released from TGRL. Ensuing cholesterol transport to the liver with excretion into the bile may reduce cholesterol influx in the arterial wall by accelerating removal from circulation of atherogenic, cholesterol-rich TGRL remnants. Such novel function of HDL opens wide therapeutic applications to reduce CVD in statin-treated patients, which primarily involve activation of cholesterol flux upon lipolysis.

The combined effects of cholesteryl ester transfer protein (CETP) TaqIB gene polymorphism and canola, sesame and sesame-canola oils consumption on metabolic response in patients with diabetes and healthy people

Introduction: Cholesteryl ester transfer protein (CETP) is a key regulating enzyme in the lipid metabolism pathway, and its gene polymorphism may be a candidate for modulating the metabolic responses to dietary intervention. We thus examined whether the effects of the CETP TaqIB polymorphism on metabolic profiles were modified by dietary plant oils.

Methods: This is a retrospective analysis of data collected during a randomized triple-blind cross over trial. A total of 95 patients with type 2 diabetes and 73 non-diabetes individuals completed a 9-weekof the intake of sesame, canola and sesame-canola oils. Blood samples were collected at the beginning and at the end of each intervention period for biochemical analysis. Genotyping was done using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.

Results: In diabetes patients, B1B1 homozygotes of the CETP TaqIB polymorphism compared with B2 carriers (B1B2 + B2B2) had significantly lower diastolic blood pressure, apoB and apoB: apoA-1,and higher Lp(a) after the intake of sesame-canola oil, as well as lower insulin and HOMA-IR after the intake of sesame oil. There was also a significant effect of genotype on adjusted changes of apoB, apoB: apoA-1, insulin, HOMA-IR and QUICKI. A significant genotype-dietary oils combined effects were observed for diastolic blood pressure, and LDL: HDL, TC: HDL and TG: HDL ratios in diabetes patients. No independent or combined effects of dietary oils and genotypes on outcomes were found in healthy people.

Conclusion: There was a modulatory effect of the CETP TaqIB polymorphism on some metabolic traits in response to plant oils in patients with diabetes. Taken together, the intake of sesame-canola and canola oils showed more favorable effects in diabetes patients with B1B1 genotype. Future investigations are needed to confirm these results.

Free cholesterol transfer to high-density lipoprotein (HDL) upon triglyceride lipolysis underlies the U-shape relationship between HDL-cholesterol and cardiovascular disease

BACKGROUND

Low concentrations of high-density lipoprotein cholesterol (HDL-C) represent a well-established cardiovascular risk factor. Paradoxically, extremely high HDL-C levels are equally associated with elevated cardiovascular risk, resulting in the U-shape relationship of HDL-C with cardiovascular disease. Mechanisms underlying this association are presently unknown. We hypothesised that the capacity of high-density lipoprotein (HDL) to acquire free cholesterol upon triglyceride-rich lipoprotein (TGRL) lipolysis by lipoprotein lipase underlies the non-linear relationship between HDL-C and cardiovascular risk.

METHODS

To assess our hypothesis, we developed a novel assay to evaluate the capacity of HDL to acquire free cholesterol (as fluorescent TopFluor® cholesterol) from TGRL upon in vitro lipolysis by lipoprotein lipase.

RESULTS

When the assay was applied to several populations markedly differing in plasma HDL-C levels, transfer of free cholesterol was significantly decreased in low HDL-C patients with acute myocardial infarction (-45%) and type 2 diabetes (-25%), and in subjects with extremely high HDL-C of >2.59 mmol/L (>100 mg/dL) (-20%) versus healthy normolipidaemic controls. When these data were combined and plotted against HDL-C concentrations, an inverse U-shape relationship was observed. Consistent with these findings, animal studies revealed that the capacity of HDL to acquire cholesterol upon lipolysis was reduced in low HDL-C apolipoprotein A-I knock-out mice and was negatively correlated with aortic accumulation of [³H]-cholesterol after oral gavage, attesting this functional characteristic as a negative metric of postprandial atherosclerosis.

CONCLUSIONS

Free cholesterol transfer to HDL upon TGRL lipolysis may underlie the U-shape relationship between HDL-C and cardiovascular disease, linking HDL-C to triglyceride metabolism and atherosclerosis.

ApoF knockdown increases cholesteryl ester transfer to LDL and impairs cholesterol clearance in fat-fed hamsters

Cholesteryl ester transfer protein (CETP) regulates intravascular lipoprotein metabolism. In vitro studies indicate that ApoF alters CETP function by inhibiting its activity with LDL. To explore in vivo the complexities driving ApoF's effects on CETP, we developed a siRNA-based hamster model of ApoF knockdown. In both male and female hamsters on chow- or fat-fed diets, we measured lipoprotein levels and composition, determined CETP-mediated transfer of cholesteryl esters (CEs) between lipoproteins, and quantified reverse cholesterol transport (RCT). We found that apoF knockdown in chow-fed hamsters had no effect on lipoprotein levels or composition, but these ApoF-deficient lipoproteins supported 50-100% higher LDL CETP activity in vitro. ApoF knockdown in fat-fed male hamsters created a phenotype in which endogenous CETP-mediated CE transfer from HDL to LDL increased up to 2-fold, LDL cholesterol increased 40%, HDL declined 25%, LDL and HDL lipid compositions were altered, and hepatic LDLR gene expression was decreased. Diet-induced hypercholesterolemia obscured this phenotype on occasion. In fat-fed female hamsters, ApoF knockdown caused similar but smaller changes in plasma CETP activity and LDL cholesterol. Notably, ApoF knockdown impaired HDL RCT in fat-fed hamsters but increased sterol excretion in chow-fed animals. These in vivo data validate in vitro findings that ApoF regulates lipid transfer to LDL. The consequences of ApoF knockdown on lipoproteins and sterol excretion depend on the underlying lipid status. By minimizing the transfer of HDL-derived CE to LDL, ApoF helps control LDL cholesterol levels when LDL clearance mechanisms are limiting.

Serious Adverse Effects of Extended-release Niacin/Laropiprant: Results From the Heart Protection Study 2-Treatment of HDL to Reduce the Incidence of Vascular Events (HPS2-THRIVE) Trial

PURPOSE

The Heart Protection Study 2-Treatment of HDL to Reduce the Incidence of Vascular Events (HPS2-THRIVE) trial of patients at high risk of vascular disease found that adding extended-release niacin-laropiprant to intensive statin-based LDL-lowering therapy had no benefit on cardiovascular outcomes. However, the trial also identified previously unrecognized serious adverse effects (including new-onset diabetes, bleeding, and infection). Our objective was to explore the safety profile of niacin-laropiprant and examine whether any patients were at lower (or higher) risk of its adverse effects.

METHODS

HPS2-THRIVE was a randomized, double-blind trial of niacin-laropiprant (2000/40 mg/d) versus placebo among 25,673 patients at high risk of vascular disease. Information on all serious adverse events was collected during a median of 3.9 years of study treatment. Effects of niacin-laropiprant on new-onset diabetes, disturbances of diabetes control, bleeding, infection, and gastrointestinal upset were estimated by (1) time after randomization, (2) severity, (3) baseline characteristics, (4) baseline risk of the adverse event of interest, and (5) risk of major vascular event.

FINDINGS

The hazard ratio (HR) for new-onset diabetes with niacin/laropiprant was 1.32 (95% CI, 1.16-1.51; P < .001), which corresponded to an absolute excess of 4 people (95% CI, 2-6) developing diabetes per 1000 person-years in the study population as a whole. Among the 8299 participants with diabetes at baseline, the HR for serious disturbances in diabetes control was 1.56 (95% CI, 1.35-1.80), corresponding to an absolute excess of 12 (95% CI, 8-16) per 1000 person-years. The HR was 1.38 (95% CI, 1.17-1.63; P < .001) for serious bleeding, corresponding to an absolute excess of 2 (95% CI, 1-3) per 1000 person-years and 1.22 (95% CI, 1.11-1.34; P < .001) for serious infection, corresponding to an absolute excess of 4 (95% CI, 2-6) per 1000 person-years. The excess risks of these serious adverse events were larger in the first year after starting niacin-laropiprant therapy than in later years (except for the excess of infection, which did not appear to attenuate with time), and the risks of nonfatal and fatal events were similarly increased. The absolute excesses of each of these adverse effects were similar regardless of the baseline risk of the outcome.

IMPLICATIONS

Practitioners or patients considering the use of niacin (in addition to, or instead of, a statin) despite the lack of evidence of cardiovascular benefits (at least when added to effective statin therapy) should take account of the significant risks of these serious adverse effects when making such decisions. ClinicalTrials.gov identifier: NCT00461630.

Establishing the role of PLVAP in protein-losing enteropathy: a homozygous missense variant leads to an attenuated phenotype

BACKGROUND

Intestinal integrity is essential for proper nutrient absorption and tissue homeostasis, with damage leading to enteric protein loss, that is, protein-losing enteropathy (PLE). Recently, homozygous nonsense variants in the plasmalemma vesicle-associated protein gene (PLVAP) were reported in two patients with severe congenital PLE. PLVAP is the building block of endothelial cell (EC) fenestral diaphragms; its importance in barrier function is supported by mouse models of Plvap deficiency.

OBJECTIVE

To genetically diagnose two first-degree cousins once removed, who presented with PLE at ages 22 and 2.5 years.

METHODS

Family-based whole exome sequencing was performed based on an autosomal recessive inheritance model. In silico analyses were used to predict variant impact on protein structure and function.

RESULTS

We identified a rare homozygous variant (NM_031310.2:c.101T>C;p.Leu34Pro) in PLVAP, which co-segregated with the disease. Leu34 is predicted to be located in a highly conserved, hydrophobic, α-helical region within the protein's transmembrane domain, suggesting Leu34Pro is likely to disrupt protein function and/or structure. Electron microscopy and PLVAP immunohistochemistry demonstrated apparently normal diaphragm morphology, predicted to be functionally affected.

CONCLUSIONS

Biallelic missense variants in PLVAP can cause an attenuated form of the PLE and hypertriglyceridaemia syndrome. Our findings support the role of PLVAP in the pathophysiology of PLE, expand the phenotypic and mutation spectrums and underscore PLVAP's importance in EC barrier function in the gut.

Present therapeutic role of cholesteryl ester transfer protein inhibitors

Therapeutic interventions aimed at increasing high-density lipoprotein (HDL) levels in order to reduce the residual cardiovascular (CV) risk of optimally drug treated patients have not provided convincing results, so far. Transfer of cholesterol from extrahepatic tissues to the liver appears to be the major atheroprotective function of HDL, and an elevation of HDL levels could represent an effective strategy. Inhibition of the cholesteryl ester transfer protein (CETP), raising HDL-cholesterol (HDL-C) and apolipoprotein A-I (apoA-I) levels, reduces low-density lipoprotein-cholesterol (LDL-C) and apoB levels, thus offering a promising approach. Despite the beneficial influence on cholesterol metabolism, off-target effects and lack of reduction in CV events and mortality (with torcetrapib, dalcetrapib and evacetrapib) highlighted the complex mechanism of CETP inhibition. After the failure of the above mentioned inhibitors in phase III clinical development, possibly due to the short duration of the trials masking benefit, the secondary prevention REVEAL trial has recently shown that the inhibitor anacetrapib significantly raised HDL-C (+104%), reduced LDL-C (-18%), with a protective effect on major coronary events (RR, 0.91; 95%CI, 0.85-0.97; p = 0.004). Whether LDL-C lowering fully accounts for the CV benefit or if HDL-C-rise is a crucial factor still needs to be determined, although the reduction of non-HDL (-18%) and Lp(a) (-25%), should be also taken into account. In spite of the positive results of the REVEAL Study, Merck decided not to proceed in asking regulatory approval for anacetrapib. Dalcetrapib (Dal-GenE study) and CKD-519 remain the two molecules within this area still in clinical development.

Inhibition of cholesteryl ester transfer protein increases cholesteryl ester content of large HDL independently of HDL-to-HDL homotypic transfer: in vitro vs in vivo comparison using anacetrapib and dalcetrapib

The increase in high density lipoprotein (HDL)-cholesterol observed with cholesteryl ester transfer protein (CETP) inhibition is commonly attributed to blockade of cholesteryl ester (CE) transfer from HDL to low density lipoprotein particles. In vitro, it has been observed that CETP can mediate transfer of CE between HDL particles ("homotypic transfer"), and it is postulated that this contributes to HDL remodeling and generation of anti-atherogenic pre-beta HDL. Inhibition of CETP could limit this beneficial remodeling and reduce pre-beta HDL levels. We observed that anacetrapib does not reduce pre-beta HDL in vivo, but the role of HDL homotypic transfer was not examined. This study evaluated the effects of anacetrapib on homotypic transfer from HDL3 to HDL2 in vivo using deuterium-labeled HDL3, and compared this to in vitro settings, where homotypic transfer was previously described. In vitro, both anacetrapib and dalcetrapib inhibited transfer of CE from HDL3 to HDL2 particles. In CETP transgenic mice, anacetrapib did not inhibit the appearance of labeled CE derived from HDL3 in HDL2 particles, but rather promoted the appearance of labeled CE in HDL2. We concluded that inhibition of CETP by anacetrapib promoted HDL particle remodeling, and does not impair the flux of cholesterol ester into larger HDL particles when studied in vivo, which is not consistent with in vitro observations. We further conclude, therefore, that the in vitro conditions used to examine HDL-to-HDL homotypic transfer may not recapitulate the in vivo condition, where multiple mechanisms contribute to cholesteryl ester flux into and out of the HDL pool.

Transcriptional regulation of the apolipoprotein F (ApoF) gene by ETS and C/EBPα in hepatoma cells

Apolipoprotein F (ApoF) inhibits cholesteryl ester transfer protein (CETP) activity and plays an important role in lipid metabolism. In the present study, the full-length human ApoF promoter was cloned, and the molecular mechanism of the regulation of ApoF was investigated. The ApoF promoter displayed higher activities in hepatoma cell lines, and the -198 nt to +79 nt promoter region contained the maximum promoter activity. In the promoter region of -198 nt to -2 nt there were four putative binding sites for transcription factors ETS-1/ETS-2 (named EBS-1 to EBS-4) and one for C/EBP. Mutation of EBS-2, EBS4 and the C/EBP binding site abolished the promoter activity, and ETS-1/ETS-2 and C/EBPα could interact with corresponding binding sites. In addition, overexpression of ETS-1/2 or C/EBPα enhanced, while dominant-negative mutants of ETS-1/2 and knockdown of C/EBPα decreased, ApoF promoter activities. ETS-1 and C/EBPα associated physically, and acted synergistically to activate ApoF transcription. These results demonstrated combined activation of the ApoF promoter by liver-enriched and ubiquitous transcription factors. Direct interactions between C/EBPα and ETS-1 were important for high liver-specific expression of ApoF.

Anacetrapib and dalcetrapib differentially alters HDL metabolism and macrophage-to-feces reverse cholesterol transport at similar levels of CETP inhibition in hamsters

Cholesteryl ester transfer protein (CETP) inhibitors dalcetrapib and anacetrapib differentially alter LDL- and HDL-cholesterol levels, which might be related to the potency of each drug to inhibit CETP activity. We evaluated the effects of both drugs at similar levels of CETP inhibition on macrophage-to-feces reverse cholesterol transport (RCT) in hamsters. In normolipidemic hamsters, both anacetrapib 30 mg/kg QD and dalcetrapib 200 mg/kg BID inhibited CETP activity by ~60%. After injection of 3H-cholesteryl oleate labeled HDL, anacetrapib and dalcetrapib reduced HDL-cholesteryl esters fractional catabolic rate (FCR) by 30% and 26% (both P<0.001 vs. vehicle) respectively, but only dalcetrapib increased HDL-derived 3H-tracer fecal excretion by 30% (P<0.05 vs. vehicle). After 3H-cholesterol labeled macrophage intraperitoneal injection, anacetrapib stimulated 3H-tracer appearance in HDL, but both drugs did not promote macrophage-derived 3H-tracer fecal excretion. In dyslipidemic hamsters, both anacetrapib 1 mg/kg QD and dalcetrapib 200 mg/kg BID inhibited CETP activity by ~65% and reduced HDL-cholesteryl ester FCR by 36% (both P<0.001 vs. vehicle), but only anacetrapib increased HDL-derived 3H-tracer fecal excretion significantly by 39%. After 3H-cholesterol labeled macrophage injection, only anacetrapib 1 mg/kg QD stimulated macrophage-derived 3H-tracer appearance in HDL. These effects remained weaker than those observed with anacetrapib 60 mg/kg QD, which induced a maximal inhibition of CETP and stimulation of macrophage-derived 3H-tracer fecal excretion. In contrast, dalcetrapib 200 mg/kg BID reduced macrophage-derived 3H-tracer fecal excretion by 23% (P<0.05 vs. vehicle). In conclusion, anacetrapib and dalcetrapib differentially alter HDL metabolism and RCT in hamsters. A stronger inhibition of CETP may be required to promote macrophage-to-feces reverse cholesterol transport in dyslipidemic hamsters.

Altered relationship of plasma triglycerides to HDL cholesterol in patients with HIV/HAART-associated dyslipidemia: further evidence for a unique form of metabolic syndrome in HIV patients

INTRODUCTION

Plasma triglycerides (TG) and HDL-C are inversely related in Metabolic Syndrome (MetS), due to exchange of VLDL-TG for HDL-cholesteryl esters catalyzed by cholesteryl ester transfer protein (CETP). We investigated the relationship of TG to HDL-C in highly-active antiretroviral drug (HAART)-treated HIV patients.

METHODS

Fasting plasma TG and HDL-C levels were compared in 179 hypertriglyceridemic HIV/HAART patients and 71 HIV-negative persons (31 normotriglyceridemic (NL) and 40 hypertriglyceridemic due to type IV hyperlipidemia (HTG)). CETP mass and activity were compared in 19 NL and 87 HIV/HAART subjects.

RESULTS

Among the three groups, a plot of HDL-C vs. TG gave similar slopes but significantly different y-intercepts (9.24±0.45, 8.16±0.54, 6.70±0.65, sqrt(HDL-C) for NL, HIV and HTG respectively; P<0.001); this difference persisted after adjusting HDL-C for TG, age, BMI, gender, glucose, CD4 count, viral load and HAART strata (7.18±0.20, 6.20±0.05 and 4.55±0.15 sqrt(HDL-C) for NL, HIV and HTG, respectively, P<0.001). CETP activity was not different between NL and HIV, but CETP mass was significantly higher in HIV (1.47±0.53 compared to 0.93±0.27μg/mL, P<0.0001), hence CETP specific activity was lower in HIV (22.67±13.46 compared to 28.46±8.24nmol/μg/h, P=0.001).

CONCLUSIONS

Dyslipidemic HIV/HAART patients have a distinctive HDL-C plasma concentration adjusted for TG. The weak inverse relationship between HDL-C and TG is not explained by altered total CETP activity; it could result from a non-CETP-dependent mechanism or a decrease in CETP function due to inhibitors of CETP activity in HIV patients' plasma.

Identification of candidate genes encoding an LDL-C QTL in baboons

Cardiovascular disease (CVD) is the leading cause of death in developed countries, and dyslipidemia is a major risk factor for CVD. We previously identified a cluster of quantitative trait loci (QTL) on baboon chromosome 11 for multiple, related quantitative traits for serum LDL-cholesterol (LDL-C). Here we report differentially regulated hepatic genes encoding an LDL-C QTL that influences LDL-C levels in baboons. We performed hepatic whole-genome expression profiling for LDL-C-discordant baboons fed a high-cholesterol, high-fat (HCHF) diet for seven weeks. We detected expression of 117 genes within the QTL 2-LOD support interval. Three genes were differentially expressed in low LDL-C responders and 8 in high LDL-C responders in response to a HCHF diet. Seven genes (ACVR1B, CALCOCO1, DGKA, ERBB3, KRT73, MYL6B, TENC1) showed discordant expression between low and high LDL-C responders. To prioritize candidate genes, we integrated miRNA and mRNA expression profiles using network tools and found that four candidates (ACVR1B, DGKA, ERBB3, TENC1) were miRNA targets and that the miRNAs were inversely expressed to the target genes. Candidate gene expression was validated using QRT-PCR and Western blotting. This study reveals candidate genes that influence variation in LDL-C in baboons and potential genetic mechanisms for further investigation.

Cholesteryl ester transfer-protein modulator and inhibitors and their potential for the treatment of cardiovascular diseases

Elevated low-density lipoprotein (LDL) cholesterol and lowered high-density lipoprotein (HDL) cholesterol are important risk factors for cardiovascular disease. Accordingly, raising HDL cholesterol induced by cholesteryl ester transfer protein (CETP) inhibition is an attractive approach for reducing the residual risk of cardiovascular events that persist in many patients receiving low-density LDL cholesterol-lowering therapy with statins. The development of torcetrapib, a CETP inhibitor, was terminated due to its adverse cardiovascular effects. These adverse effects did not influence the mechanism of CETP inhibition, but affected the molecule itself. Therefore a CETP modulator, dalcetrapib, and a CETP inhibitor, anacetrapib, are in Phase III of clinical trials to evaluate their effects on cardiovascular outcomes. In the dal-VESSEL (dalcetrapib Phase IIb endothelial function study) and the dal-PLAQUE (safety and efficacy of dalcetrapib on atherosclerotic disease using novel non-invasive multimodality imaging) clinical studies, dalcetrapib reduced CETP activity by 50% and increased HDL cholesterol levels by 31% without changing LDL cholesterol levels. Moreover, dalcetrapib was associated with a reduction in carotid vessel-wall inflammation at 6 months, as well as a reduced vessel-wall area at 24 months compared with the placebo. In the DEFINE (determining the efficacy and tolerability of CETP inhibition with anacetrapib) clinical study, anacetrapib increased HDL cholesterol levels by 138% and decreased LDL cholesterol levels by 36%. In contrast with torcetrapib, anacetrapib had no adverse cardiovascular effects. The potential of dalcetrapib and anacetrapib in the treatment of cardiovascular diseases will be revealed by two large-scale clinical trials, the dal-OUTCOMES (efficacy and safety of dalcetrapib in patients with recent acute coronary syndrome) study and the REVEAL (randomized evaluation of the effects of anacetrapib through lipid modification, a large-scale, randomized placebo-controlled trial of the clinical effects of anacetrapib among people with established vascular disease) study. The dal-OUTCOMES study is testing whether dalcetrapib can reduce cardiovascular events and the REVEAL study is testing whether anacetrapib can reduce cardiovascular events. These reports are expected to be released by 2013 and 2017, respectively.

The effects of apolipoprotein F deficiency on high density lipoprotein cholesterol metabolism in mice

Apolipoprotein F (apoF) is 29 kilodalton secreted sialoglycoprotein that resides on the HDL and LDL fractions of human plasma. Human ApoF is also known as Lipid Transfer Inhibitor protein (LTIP) based on its ability to inhibit cholesteryl ester transfer protein (CETP)-mediated transfer events between lipoproteins. In contrast to other apolipoproteins, ApoF is predicted to lack strong amphipathic alpha helices and its true physiological function remains unknown. We previously showed that overexpression of Apolipoprotein F in mice reduced HDL cholesterol levels by 20–25% by accelerating clearance from the circulation. In order to investigate the effect of physiological levels of ApoF expression on HDL cholesterol metabolism, we generated ApoF deficient mice. Unexpectedly, deletion of ApoF had no substantial impact on plasma lipid concentrations, HDL size, lipid or protein composition. Sex-specific differences were observed in hepatic cholesterol content as well as serum cholesterol efflux capacity. Female ApoF KO mice had increased liver cholesteryl ester content relative to wild type controls on a chow diet (KO: 3.4+/−0.9 mg/dl vs. WT: 1.2+/−0.3 mg/dl, p<0.05). No differences were observed in ABCG1-mediated cholesterol efflux capacity in either sex. Interestingly, ApoB-depleted serum from male KO mice was less effective at promoting ABCA1-mediated cholesterol efflux from J774 macrophages relative to WT controls.

Modulating cholesteryl ester transfer protein activity maintains efficient pre-β-HDL formation and increases reverse cholesterol transport

The mechanism by which cholesteryl ester transfer protein (CETP) activity affects HDL metabolism was investigated using agents that selectively target CETP (dalcetrapib, torcetrapib, anacetrapib). In contrast with torcetrapib and anacetrapib, dalcetrapib requires cysteine 13 to decrease CETP activity, measured as transfer of cholesteryl ester (CE) from HDL to LDL, and does not affect transfer of CE from HDL3 to HDL2. Only dalcetrapib induced a conformational change in CETP, when added to human plasma in vitro, also observed in vivo and correlated with CETP activity. CETP-induced pre-β-HDL formation in vitro in human plasma was unchanged by dalcetrapib ≤3 µM and increased at 10 µM. A dose-dependent inhibition of pre-β-HDL formation by torcetrapib and anacetrapib (0.1 to 10 µM) suggested that dalcetrapib modulates CETP activity. In hamsters injected with [³H]cholesterol-labeled autologous macrophages, and given dalcetrapib (100 mg twice daily), torcetrapib [30 mg once daily (QD)], or anacetrapib (30 mg QD), only dalcetrapib significantly increased fecal elimination of both [³H]neutral sterols and [³H]bile acids, whereas all compounds increased plasma HDL-[³H]cholesterol. These data suggest that modulation of CETP activity by dalcetrapib does not inhibit CETP-induced pre-β-HDL formation, which may be required to increase reverse cholesterol transport.

Modification of high density lipoprotein by myeloperoxidase generates a pro-inflammatory particle

High density lipoprotein (HDL) is the major atheroprotective particle in plasma. Recent studies demonstrate that myeloperoxidase (MPO) binds to HDL in vivo, selectively targeting apolipoprotein A1 (apoA1) of HDL for oxidative modification and concurrent loss in cholesterol efflux and lecithin cholesterol acyl transferase activating activities, generating a "dysfunctional HDL" particle. We now show that (patho)physiologically relevant levels of MPO-catalyzed oxidation result in loss of non-cholesterol efflux activities of HDL including anti-apoptotic and anti-inflammatory functions. One mechanism responsible is shown to involve the loss of modified HDL binding to the HDL receptor, scavenger receptor B1, and concurrent acquisition of saturable and specific binding to a novel unknown receptor independent of scavenger receptors CD36 and SR-A1. HDL modification by MPO is further shown to confer pro-inflammatory gain of function activities as monitored by NF-kappaB activation and surface vascular cell adhesion molecule levels on aortic endothelial cells exposed to MPO-oxidized HDL. The loss of non-cholesterol efflux activities and the gain of pro-inflammatory functions requires modification of the entire particle and can be recapitulated by oxidation of reconstituted HDL particles comprised of apoA1 and nonoxidizable phosphatidylcholine species. Multiple site-directed mutagenesis studies of apoA1 suggest that the pro-inflammatory activity of MPO-modified HDL does not involve methionine, tyrosine, or tryptophan, oxidant-sensitive residues previously mapped as sites of apoA1 oxidation within human atheroma. Thus, MPO-catalyzed oxidation of HDL results not only in the loss of classic atheroprotective reverse cholesterol transport activities of the lipoprotein but also both the loss of non-cholesterol efflux related activities and the gain of pro-inflammatory functions.

Localization of multiple pleiotropic genes for lipoprotein metabolism in baboons

We employed a novel approach to identify the key loci that harbor genes influencing lipoprotein metabolism in approximately 2,000 pedigreed baboons fed various diets differing in levels of fat and cholesterol. In this study, 126 overlapping traits related to both LDL and HDL metabolism were normalized and subjected to genome-wide linkage screening. As was expected, the traits were highly, but not completely, correlated. We exploited the information in these correlated traits by focusing on those genomic regions harboring quantitative trait loci (QTL) for multiple traits, reasoning that the more influential genes would impact a larger number of traits. This study identified five major QTL clusters (each with at least two significant logarithm of the odds scores >4.7), two of which had not been previously reported in baboons. One of these mapped to the baboon ortholog of human chromosome 1p32-p34 and influenced concentrations of LDL-cholesterol on Basal and high-fat, low-cholesterol diets. The other novel QTL cluster mapped to the baboon ortholog of human chromosome 12q13.13-q14.1 and influenced LDL size properties on high-fat, low-cholesterol and high-fat, high-cholesterol, but not Basal, diets. Confirming the value of this approach, three of the QTL clusters replicated published linkage findings for the same or similar traits.

Molecular cloning of hamster lipid transfer inhibitor protein (apolipoprotein F) and regulation of its expression by hyperlipidemia

Lipid transfer inhibitor protein (LTIP) is a regulator of cholesteryl ester transfer protein (CETP) function. Factors affecting plasma LTIP levels are poorly understood. In humans, plasma LTIP is elevated in hypercholesterolemia. To define possible mechanisms by which hyperlipidemia modifies LTIP, we investigated the effects of hypercholesterolemic diets on plasma LTIP and mRNA levels in experimental animals. The hamster, which naturally expresses CETP, was shown to express LTIP. Hamster LTIP mRNA, exclusively detected in the liver, defined a predicted LTIP protein that is 69% homologous to human, with an isoelectric point of 4.15 and Mr = approximately 16.4 kDa. Hyperlipidemia induced by feeding hydrogenated coconut oil, cholesterol, or both lipids increased plasma LTIP mass up to 2.5-fold, with LTIP mass correlating strongly with plasma cholesterol levels. CETP mass was similarly affected by these diets. In contrast, these diets reduced LTIP hepatic mRNA levels by >50%, whereas CETP mRNA was increased. Similar results for both CETP and LTIP were also observed in cholesterol-fed rabbits. In conclusion, we report in hamster and rabbit that dietary lipids regulate LTIP. Diet-induced hypercholesterolemia markedly increased plasma LTIP mass while concomitantly depressing LTIP gene expression. CETP and LTIP have distinct responses to dietary lipids.

Combined statin and niacin therapy remodels the high-density lipoprotein proteome

BACKGROUND

Boosting low high-density lipoprotein (HDL) levels is a current strategy for preventing clinical events that result from cardiovascular disease. We previously showed that HDL(3) of subjects with coronary artery disease is enriched in apolipoprotein E and that the lipoprotein carries a distinct protein cargo. This observation suggests that altered protein composition might affect the antiatherogenic and antiinflammatory properties of HDL. We hypothesized that an intervention that increases HDL levels-combined statin and niacin therapy-might reverse these changes.

METHODS AND RESULTS

HDL(3) isolated from 6 coronary artery disease subjects before and 1 year after combination therapy was analyzed by liquid chromatography-Fourier transform-mass spectrometry. Alterations in protein composition were detected by spectral counting and confirmed with extracted ion chromatograms. We found that combination therapy decreased the abundance of apolipoprotein E in HDL(3) while increasing the abundance of other macrophage proteins implicated in reverse cholesterol transport. Treatment-induced decreases in apolipoprotein E levels of HDL(3) were validated biochemically in a second group of 18 coronary artery disease subjects. Interestingly, the changes in HDL(3) proteome with niacin/statin treatment resulted in a protein composition that more closely resembled that of HDL(3) in healthy control subjects.

CONCLUSIONS

Combined statin and niacin therapy partially reverses the changes in the protein composition seen in HDL(3) in coronary artery disease subjects. Our observations raise the possibility that quantifying the HDL proteome could provide insights into the therapeutic efficacy of antiatherosclerotic interventions.

Control of cholesteryl ester transfer protein activity by sequestration of lipid transfer inhibitor protein in an inactive complex

Lipid transfer inhibitor protein (LTIP) is a physiologic regulator of cholesteryl ester transfer protein (CETP) function. We previously reported that LTIP activity is localized to LDL, consistent with its greater inhibitory activity on this lipoprotein. With a recently described immunoassay for LTIP, we investigated whether LTIP mass is similarly distributed. Plasma fractionated by gel filtration chromatography revealed two LTIP protein peaks, one coeluting with LDL, and another of approximately 470 kDa. The 470 kDa LTIP complex had a density of 1.134 g/ml, indicating approximately 50% lipid content, and contained apolipoprotein A-I. By mass spectrometry, partially purified 470 kDa LTIP also contains apolipoproteins C-II, D, E, J, and paraoxonase 1. Unlike LDL-associated LTIP, the 470 kDa LTIP complex does not inhibit CETP activity. In normolipidemic subjects, approximately 25% of LTIP is in the LDL-associated, active form. In hypercholesterolemia,this increases to 50%, suggesting that lipoprotein composition may influence the status of LTIP activity. Incubation (37 degrees C) of normolipidemic plasma increased active, LDL-associated LTIP up to 3-fold at the expense of the inactive pool. Paraoxon inhibited this shift by 50%. Overall, these studies show that LTIP activity is controlled by its reversible incorporation into an inactive complex. This may provide for short-term fine-tuning of lipoprotein remodeling mediated by CETP.

Lipid transfer inhibitor protein (apolipoprotein F) concentration in normolipidemic and hyperlipidemic subjects

Lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein function. We report the development of an immunoassay for LTIP and its use to quantify LTIP in plasma of varying lipid contents. A rabbit antibody against bacterially produced recombinant LTIP detected two LTIP isoforms in plasma differing in carbohydrate content. This antibody was used in a competitive, enzyme-linked immunoassay that uses partially purified LTIP bound to microtiter plates. To optimize LTIP immunoreactivity, plasma samples required preincubation in 1% Tween-20 and 0.5% Nonidet P-40. In normolipidemic plasma, LTIP averaged 83.5 microg/ml. LTIP was 31% higher in males than in females. LTIP was positively associated with HDL cholesterol in normolipidemic males but not in females. In hypertriglyceridemic males, LTIP was only 56% of control values, whereas in hypertriglyceridemic females, LTIP tended to increase. Additionally, in males with normal cholesterol and triglyceride (TG) < or = 200 mg/dl, LTIP varied inversely with plasma TG. Overall, we have confirmed the negative association between plasma TG levels and LTIP previously suggested by a small data set, but now we demonstrate that this effect is seen only in males. The mechanisms underlying this gender-specific response to TG, and why LTIP and HDL levels correlate in males but not in females, remain to be determined.

Interaction between dietary fat intake and the cholesterol ester transfer protein TaqIB polymorphism in relation to HDL-cholesterol concentrations among US diabetic men

BACKGROUND

A low plasma HDL-cholesterol concentration is a major characteristic of diabetic dyslipidemia. HDL concentrations are determined by both environmental factors and genetic factors. Cholesterol ester transfer protein (CETP) plays an important role in the regulation of HDL metabolism, and the TaqIB polymorphism of the CETP gene has been associated with elevated HDL concentrations.

OBJECTIVE

We examined the association between the CETP TaqIB polymorphism and plasma HDL concentrations and evaluated whether this association was modified by dietary fat intake.

DESIGN

We followed 780 diabetic men aged 40-75 y who participated in the Health Professionals Follow-Up Study since its initiation in 1986. The participants had confirmed type 2 diabetes and were free of cardiovascular disease at the time blood was drawn.

RESULTS

After adjustment for age, smoking, alcohol consumption, fasting status, hemoglobin A(1c), physical activity, total energy intake, and body mass index, HDL concentrations were significantly higher in men with the B2B2 or B1B2 genotype than in those with the B1B1 genotype (adjusted x +/- SE: 37.9 +/- 0.02, 40.3 +/- 0.01, and 42.6 +/- 0.02 mg/dL for B1B1, B1B2, and B2B2, respectively; P for trend = 0.0004). This inverse association of the B1 allele with plasma HDL concentrations existed for those with a high consumption of animal fat (P for interaction = 0.02), saturated fat (P for interaction = 0.02), and monounsaturated fat (P for interaction = 0.04).

CONCLUSION

These data confirmed a significant effect of the CETP Taq1 gene on HDL concentrations and suggested a potential interaction between the CETP TaqIB polymorphism and intake of dietary fat on plasma HDL concentration.

Chemistory of Fibrates

Since the description of the synthetic chemical clofibrate in 1962, various derivatives of fibrates with a diversity of chemical structures have been developed. Several of these are used clinically to treat dyslipidemia because they are generally effective in lowering elevated plasma triglycerides and cholesterol. Studies suggest that several biochemical mechanisms underlie fibrate-mediated modulation of lipoprotein and related metabolites. These mechanisms are: 1) induced lipoprotein lipolysis; 2) induced hepatic fatty acid uptake and reduced hepatic triglyceride formation; 3) amplified removal of low density lipoprotein (LDL) particles; 4) reduced neutral lipid (cholesteryl ester and triglyceride) exchange between very low density lipoprotein (VLDL) and high density lipoprotein (HDL) resulting from decreased plasma levels of triglyceride-rich lipoprotein (TRL); and 5) increased HDL production and stimulation of reverse cholesterol transport. Recent studies of structure-based inhibitor design strategy revealed that an independent enzyme, aldose reductase (AR), is a target of fibrate activity, an additional biochemical mechanism. AR has been implicated as a major player in the development of diabetes and diabetic complications because of its ability to catalyze the conversion of glucose to sorbitol. This article discusses various targets of fibrate action, biochemical pathways and commonalities in potential molecular interactions.

No association of CETP genotype with cognitive function or age-related cognitive change

A cholesteryl ester transfer protein (CETP) genotype (V/V homozygosity for I405V, NCBI dbSNP rs5882) has been associated with preservation of cognitive function in old age, in addition to its associations with exceptional longevity and cardiovascular disease. We tested the hypotheses that this polymorphism was associated with either level of cognitive function or lifetime cognitive change in 525 participants who took part in the Scottish Mental Survey of 1932. Participants took the same well-validated mental ability test at ages 11 and 79. Scores were also available for several other mental ability tests at age 79, and history of cardiovascular disease was available. Frequency of the V/I genotype was slightly greater than expected in the sample, possibly consistent with some role for the V polymorphism in longevity, and the V/V genotype was associated with a lower rate of cardiovascular disease history. There were, however, no significant associations of CETP genotype with either childhood IQ or current cognitive function in old age, or with lifetime change in cognitive function.

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.

Partial suppression of CETP activity beneficially modifies the lipid transfer profile of plasma

Cholesteryl ester transfer protein (CETP) regulates human lipoprotein metabolism. Because reducing CETP increases plasma HDL, CETP inhibitors are currently being investigated for their pharmacologic value. However, complete CETP deficiency may have undesirable consequences. In contrast, based on previous studies with purified components, we hypothesized that partial CETP inhibition, which will still elevate HDL, may induce beneficial changes in plasma lipid metabolism. To address this, CETP activity in human plasma was variably inhibited with monoclonal antibody. In control plasma, VLDL to LDL lipid transfer was >2-fold higher than to HDL(3) with lipid transfer to HDL(2) intermediate. However, individual lipid transfer events were uniquely sensitive to CETP suppression such that when CETP activity was inhibited by 60%, lipid transfer from VLDL to LDL, HDL(2) and HDL(3) were equal. The ratio of lipid transfers to LDL versus HDL declined linearly with CETP inhibition. In mass lipid transfer experiments, 25-50% inhibition of CETP significantly reduced lipid flux between VLDL and LDL but minimally affected cholesteryl ester (CE) loss from HDL. Complete CETP inhibition did not reduce cholesterol esterification rates but completely blocked the delivery of new CE to VLDL, whereas, 50% inhibition of CETP reduced this CE flux to VLDL by <20%. Thus, inhibition of CETP by <or=50% preferentially blocks lipid transfers involving LDL while largely maintaining lipid flux through HDL. These results suggest that a more beneficial therapeutic outcome may be achieved with partial, rather than extensive, CETP suppression.

Dual role of peroxiredoxin I in macrophage-derived foam cells

We and others have shown that foam cell formation initiated by exposing macrophages to oxidized low density lipoprotein (oxLDL) triggers the differential expression of a number of proteins. Specifically, our experiments have identified peroxiredoxin I (Prx I) as one of these up-regulated proteins. The peroxiredoxins, a family of peroxidases initially described for their antioxidant capability, have generated recent interest for their potential to regulate signaling pathways. Those studies, however, have not examined peroxiredoxin for a potential dual functionality as both cytoprotective antioxidant and signal modulator in a single, oxidant-stressed system. In this report, we examine the up-regulation of Prx I in macrophages in response to oxLDL exposure and its ability to function as both antioxidant enzyme and regulator of p38 MAPK activation. As an antioxidant, induction of Prx I expression led to improved cell survival following treatment with oxLDL or tert-butyl hydroperoxide. The improved survival coincided with a decrease in measurable reactive oxygen species (ROS), and both the increased survival and reduced ROS were reversed by Prx I small interfering RNA transfection. Additionally, our data show that activation of p38 MAPK in oxLDL-treated macrophages was dependent on the up-regulation of Prx I. Reduction of Prx I expression by small interfering RNA transfection resulted in a significant decrease in p38 MAPK activation, whereas the up-regulation of Prx I expression with either oxLDL or ethoxyquin led to increased p38 MAPK activation. These results are consistent with multiple roles for Prx I in macrophage-derived foam cells that include functionality as both an antioxidant and a regulator of oxidant-sensitive signal transduction.

Plasma lipid transfer proteins

PURPOSE OF REVIEW

Plasma cholesteryl ester transfer protein and phospholipid transfer protein are involved in lipoprotein metabolism. Conceivably, manipulation of either transfer protein could impact atherosclerosis and other lipid-driven diseases.

RECENT FINDINGS

Cholesteryl ester transfer protein mediates direct HDL cholesteryl ester delivery to the liver cells; adipose tissue-specific overexpression of cholesteryl ester transfer protein in mice reduces the plasma HDL cholesterol concentration and adipocyte size; cholesteryl ester transfer protein TaqIB polymorphism is associated with HDL cholesterol plasma levels and the risk of coronary heart disease. In apolipoprotein B transgenic mice, phospholipid transfer protein deficiency enhances reactive oxygen species-dependent degradation of newly synthesized apolipoprotein B via a post-endoplasmic reticulum process, as well as improving the antiinflammatory properties of HDL in mice. Activity of this transfer protein in cerebrospinal fluid of patients with Alzheimer's disease is profoundly decreased and exogenous phospholipid transfer protein induces apolipoprotein E secretion by primary human astrocytes in vitro.

SUMMARY

Understanding the relationship between lipid transfer proteins and lipoprotein metabolism is expected to be an important frontier in the search for a therapy for atherosclerosis.

Current knowledge of chromosome 12 susceptibility genes for late-onset Alzheimer's disease

In the last decade, it has become more apparent the important role genes play in the development of late-onset Alzheimer's disease (AD). Great efforts, involving human genome scans and candidate gene studies, have been given towards identifying susceptibility genes for AD. A number of regions on different chromosomes have been reported to demonstrate linkage for AD. Of these, findings on chromosome 12 are some of the most compelling. Worldwide genetic association studies pre-dating and subsequent to recent linkage studies have identified and focused upon a number of genes that map to the areas of reported linkage on chromosome 12, however, analyses of those genes studied to date, on the whole, remain inconclusive and ambiguous. This paper reviews studies that have provided evidence of linkage for AD on chromosome 12 and in turn discusses the work conducted to date on candidate genes that have been identified and map to the chromosome 12 regions of interest.

Proteomic and transcriptomic analyses of macrophages with an increased resistance to oxidized low density lipoprotein (oxLDL)-induced cytotoxicity generated by chronic exposure to oxLDL

The uptake of oxidized low density lipoprotein (oxLDL) by macrophages leads to foam cell formation and fatty streaks, which represent early sites of potential atheroma development. We developed a cell culture model of chronic oxLDL exposure to determine whether hallmark parameters of oxLDL uptake and cytotoxicity are altered during foam cell formation and to determine changes in protein and mRNA expression that distinguish acute and chronic oxLDL exposure. Although the extent of oxLDL uptake did not change, a resistance to oxLDL-induced cytotoxicity was observed in the chronically exposed cells. Macrophages that have been chronically exposed to oxLDL required a 40% higher concentration of oxLDL to achieve 50% survival in a 48-h treatment relative to macrophages subjected to a single oxLDL exposure. A main feature of the differentially expressed proteome was a series of significantly overexpressed antioxidant and antioxidant-related proteins in the oxLDL-exposed cells. A large proportion of these proteins (45%) was overexpressed in the chronically exposed cells prior to the oxLDL treatment, indicative of the unique phenotype produced by the chronic treatment. Analysis of the transcriptome also revealed a broad increase in the expression of antioxidant and antioxidant-related proteins. In addition, the transcriptome experiments found an increased inflammatory response under conditions of both acute and chronic oxLDL exposure. Overall the combined functional, proteomic, and transcriptomic experiments show that macrophages respond to oxLDL by developing an oxidative stress resistance that increases and stabilizes with chronic exposure. Furthermore this protective response and the increased foam cell survival that it supports amplifies their proatherogenic role by promoting a continued inflammatory state.

New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.

High-density lipoprotein metabolism and progression of atherosclerosis: new insights from the HDL Atherosclerosis Treatment Study

PURPOSE OF REVIEW

The purpose of this review is to summarize the current understanding of the potentially antiatherogenic properties of high-density lipoprotein related to its different components.

RECENT FINDINGS

Recent findings on the role of the different high-density lipoprotein subspecies in reverse cholesterol transport, inflammation, endothelial dysfunction, and low-density lipoprotein oxidation are covered. Special emphasis is put on the heterogeneity of high-density lipoprotein and functional changes related to specific high-density lipoprotein particles with the potential therapeutic alterations of high-density lipoprotein metabolism.

SUMMARY

The diverse action of high-density lipoprotein observed could be explained by the heterogeneity of high-density lipoprotein particles with completely different composition and properties. The modification of specific high-density lipoprotein subpopulations to reach the maximum atheroprotective effects under various pathologic conditions bears great potential in lipid research.

CETP and lipid transfer inhibitor protein are uniquely affected by the negative charge density of the lipid and protein domains of LDL

Lipoprotein surface charge influences cholesteryl ester transfer protein (CETP) activity and its association with lipoproteins; however, the relationship between these events is not clear. Additionally, although CETP and its regulator, lipid transfer inhibitor protein (LTIP), bind to lipoproteins, it is not known how the charge density of lipoprotein protein and lipid domains influences these factors. Here, the electronegativity of the protein (by acetylation) and surface lipid (oleate addition) domains of LDL were modified. LDL-only lipid transfer assays measured changes in CETP and LTIP activities. CETP activity was stimulated by <10 microM oleate but completely suppressed by >20 microM. The same electronegative potential induced by acetylation mildly stimulated CETP. Modification-induced enhanced binding of CETP did not correlate with CETP activity. LTIP activity was completely blocked by approximately 10 microM oleate but only mildly suppressed by acetylation. LTIP binding to LDL was not decreased by oleate. Thus, the negative charge of LDL surface lipids, but not protein, is an important regulator of CETP and LTIP activity. Altered binding could not explain changes in CETP activity, suggesting that the extent of CETP binding is not normally rate limiting to its activity. Physiologic and pathophysiologic conditions that modify the negative charge of lipoprotein surface lipids will suppress LTIP activity first, followed by CETP.