Important role for bone marrow-derived cholesteryl ester transfer protein in lipoprotein cholesterol redistribution and atherosclerotic lesion development in LDL receptor knockout mice

CETP Expression in Bone-Marrow-Derived Cells Reduces the Inflammatory Features of Atherosclerosis in Hypercholesterolemic Mice

CETP activity reduces plasma HDL-cholesterol concentrations, a correlate of an increased risk of atherosclerotic events. However, our recent findings suggest that CETP expression in macrophages promotes an intracellular antioxidant state, reduces free cholesterol accumulation and phagocytosis, and attenuates pro-inflammatory gene expression. To determine whether CETP expression in macrophages affects atherosclerosis development, we transplanted bone marrow from transgenic mice expressing simian CETP or non-expressing littermates into hypercholesterolemic LDL-receptor-deficient mice. The CETP expression did not change the lipid-stained lesion areas but decreased the macrophage content (CD68), neutrophil accumulation (LY6G), and TNF-α aorta content of young male transplanted mice and decreased LY6G, TNF-α, iNOS, and nitrotyrosine (3-NT) in aged female transplanted mice. These findings suggest that CETP expression in bone-marrow-derived cells reduces the inflammatory features of atherosclerosis. These novel mechanistic observations may help to explain the failure of CETP inhibitors in reducing atherosclerotic events in humans.

High-density lipoproteins and non-alcoholic fatty liver disease

Background and aims

Non-alcoholic fatty liver disease (NAFLD), a high incidence liver pathology, is associated with a ∼1.5-fold higher cardiovascular disease risk. This phenomenon is generally attributed to the NAFLD-associated increase in circulating levels of pro-atherogenic apolipoprotein B100-containing small dense low-density lipoprotein and plasma hypertriglyceridemia. However, also a significant reduction in cholesterol transported by anti-atherogenic high-density lipoproteins (HDL) is frequently observed in subjects suffering from NAFLD as compared to unaffected people. In this review, we summarize data regarding the relationship between NAFLD and plasma HDL-cholesterol levels, with a special focus on highlighting potential causality between the NAFLD pathology and changes in HDL metabolism.

Methods and results

Publications in PUBMED describing the relationship between HDL levels and NAFLD susceptibility and/or disease severity, either in human clinical settings or genetically-modified mouse models, were critically reviewed for subsequent inclusion in this manuscript. Furthermore, relevant literature describing effects on lipid loading in cultured hepatocytes of models with genetic alterations related to HDL metabolism have been summarized.

Conclusions

Although in vitro observations suggest causality between HDL formation by hepatocytes and protection against NAFLD-like lipid accumulation, current literature remains inconclusive on whether relative HDL deficiency is actually driving the development of fatty liver disease in humans. In light of the current obesity pandemic and the associated marked rise in NAFLD incidence, it is of clear scientific and societal interest to gain further insight into the relationship between HDL-cholesterol levels and fatty liver development to potentially uncover the therapeutic potential of pharmacological HDL level and/or function modulation.

Cholesterol-Ester Transfer Protein Alters M1 and M2 Macrophage Polarization and Worsens Experimental Elastase-Induced Pulmonary Emphysema

Cholesterol-ester transfer protein (CETP) plays a role in atherosclerosis, the inflammatory response to endotoxemia and in experimental and human sepsis. Functional alterations in lipoprotein (LP) metabolism and immune cell populations, including macrophages, occur during sepsis and may be related to comorbidities such as chronic obstructive pulmonary disease (COPD). Macrophages are significantly associated with pulmonary emphysema, and depending on the microenvironment, might exhibit an M1 or M2 phenotype. Macrophages derived from the peritoneum and bone marrow reveal CETP that contributes to its plasma concentration. Here, we evaluated the role of CETP in macrophage polarization and elastase-induced pulmonary emphysema (ELA) in human CETP-expressing transgenic (huCETP) (line 5203, C57BL6/J background) male mice and compared it to their wild type littermates. We showed that bone marrow-derived macrophages from huCETP mice reduce polarization toward the M1 phenotype, but with increased IL-10. Compared to WT, huCETP mice exposed to elastase showed worsened lung function with an increased mean linear intercept (Lm), reflecting airspace enlargement resulting from parenchymal destruction with increased expression of arginase-1 and IL-10, which are M2 markers. The cytokine profile revealed increased IL-6 in plasma and TNF, and IL-10 in bronchoalveolar lavage (BAL), corroborating with the lung immunohistochemistry in the huCETP-ELA group compared to WT-ELA. Elastase treatment in the huCETP group increased VLDL-C and reduced HDL-C. Elastase-induced pulmonary emphysema in huCETP mice promotes lung M2-like phenotype with a deleterious effect in experimental COPD, corroborating the in vitro result in which CETP promoted M2 macrophage polarization. Our results suggest that CETP is associated with inflammatory response and influences the role of macrophages in COPD.

The haemochromatosis gene Hfe and Kupffer cells control LDL cholesterol homeostasis and impact on atherosclerosis development

AIMS

Imbalances of iron metabolism have been linked to the development of atherosclerosis. However, subjects with hereditary haemochromatosis have a lower prevalence of cardiovascular disease. The aim of our study was to understand the underlying mechanisms by combining data from genome-wide association study analyses in humans, CRISPR/Cas9 genome editing, and loss-of-function studies in mice.

METHODS AND RESULTS

Our analysis of the Global Lipids Genetics Consortium (GLGC) dataset revealed that single nucleotide polymorphisms (SNPs) in the haemochromatosis gene HFE associate with reduced low-density lipoprotein cholesterol (LDL-C) in human plasma. The LDL-C lowering effect could be phenocopied in dyslipidaemic ApoE-/- mice lacking Hfe, which translated into reduced atherosclerosis burden. Mechanistically, we identified HFE as a negative regulator of LDL receptor expression in hepatocytes. Moreover, we uncovered liver-resident Kupffer cells (KCs) as central players in cholesterol homeostasis as they were found to acquire and transfer LDL-derived cholesterol to hepatocytes in an Abca1-dependent fashion, which is controlled by iron availability.

CONCLUSION

Our results disentangle novel regulatory interactions between iron metabolism, KC biology and cholesterol homeostasis which are promising targets for treating dyslipidaemia but also provide a mechanistic explanation for reduced cardiovascular morbidity in subjects with haemochromatosis.

Cholesteryl Ester Transfer Protein and Lipid Metabolism and Cardiovascular Diseases

In this chapter, we present the major advances in CETP research since the detection, isolation, and characterization of its activity in the plasma of humans and several species. Since CETP is a major modulator of HDL plasma levels, the clinical importance of CETP activity was recognized very early. We describe the participation of CETP in reverse cholesterol transport, conflicting results in animal and human genetic studies, possible new functions of CETP, and the results of the main clinical trials on CETP inhibition. Despite major setbacks in clinical trials, the hypothesis that CETP inhibitors are anti-atherogenic in humans is still being tested.

Plasma cholesteryl ester transfer protein is predominantly derived from Kupffer cells

The role of Kupffer cells (KCs) in the pathophysiology of the liver has been firmly established. Nevertheless, KCs have been underexplored as a target for diagnosis and treatment of liver diseases owing to the lack of noninvasive diagnostic tests. We addressed the hypothesis that cholesteryl ester transfer protein (CETP) is mainly derived from KCs and may predict KC content. Microarray analysis of liver and adipose tissue biopsies, obtained from 93 obese subjects who underwent elective bariatric surgery, showed that expression of CETP is markedly higher in liver than adipose tissue. Hepatic expression of CETP correlated strongly with that of KC markers, and CETP messenger RNA and protein colocalized specifically with KCs in human liver sections. Hepatic KC content as well as hepatic CETP expression correlated strongly with plasma CETP concentration. Mechanistic and intervention studies on the role of KCs in determining the plasma CETP concentration were performed in a transgenic (Tg) mouse model expressing human CETP. Selective elimination of KCs from the liver in CETP Tg mice virtually abolished hepatic CETP expression and largely reduced plasma CETP concentration, consequently improving the lipoprotein profile. Conversely, augmentation of KCs after Bacille-Calemette-Guérin vaccination largely increased hepatic CETP expression and plasma CETP. Also, lipid-lowering drugs fenofibrate and niacin reduced liver KC content, accompanied by reduced plasma CETP concentration.

CONCLUSIONS

Plasma CETP is predominantly derived from KCs, and plasma CETP level predicts hepatic KC content in humans.

Mouse models of disturbed HDL metabolism

High-density lipoprotein (HDL) is considered to be an anti-atherogenic lipoprotein moiety. Generation of genetically modified (total body and tissue-specific knockout) mouse models has significantly contributed to our understanding of HDL function. Here we will review data from knockout mouse studies on the importance of HDL's major alipoprotein apoA-I, the ABC transporters A1 and G1, lecithin:cholesterol acyltransferase, phospholipid transfer protein, and scavenger receptor BI for HDL's metabolism and its protection against atherosclerosis in mice. The initial generation and maturation of HDL particles as well as the selective delivery of its cholesterol to the liver are essential parameters in the life cycle of HDL. Detrimental atherosclerosis effects observed in response to HDL deficiency in mice cannot be solely attributed to the low HDL levels per se, as the low HDL levels are in most models paralleled by changes in non-HDL-cholesterol levels. However, the cholesterol efflux function of HDL is of critical importance to overcome foam cell formation and the development of atherosclerotic lesions in mice. Although HDL is predominantly studied for its atheroprotective action, the mouse data also suggest an essential role for HDL as cholesterol donor for steroidogenic tissues, including the adrenals and ovaries. Furthermore, it appears that a relevant interaction exists between HDL-mediated cellular cholesterol efflux and the susceptibility to inflammation, which (1) provides strong support for the novel concept that inflammation and metabolism are intertwining biological processes and (2) identifies the efflux function of HDL as putative therapeutic target also in other inflammatory diseases than atherosclerosis.

Association of cholesteryl ester transfer protein (CETP) gene polymorphism, high density lipoprotein cholesterol and risk of coronary artery disease: a meta-analysis using a Mendelian randomization approach

BACKGROUND

Recent randomized controlled trials have challenged the concept that increased high density lipoprotein cholesterol (HDL-C) levels are associated with coronary artery disease (CAD) risk reduction. The causal role of HDL-C in the development of atherosclerosis remains unclear. To increase precision and to minimize residual confounding, we exploited the cholesteryl ester transfer protein (CETP)-TaqIB polymorphism as an instrument based on Mendelian randomization.

METHODS

The Mendelian randomization analysis was performed by two steps. First, we conducted a meta-analysis of 47 studies, including 23,928 cases and 27,068 controls, to quantify the relationship between the TaqIB polymorphism and the CAD risk. Next, the association between the TaqIB polymorphism and HDL-C was assessed among 5,929 Caucasians. We further employed Mendelian randomization to evaluate the causal effect of HDL-C on CAD based on the findings from the meta-analysis.

RESULTS

The overall comparison of the B2 allele with the B1 allele yielded a significant risk reduction of CAD (P < 0.0001; OR = 0.88; 95% CI: 0.84-0.92) with substantial between-study heterogeneity (I² = 55.2%; P(heterogeneity) <0.0001). The result was not materially changed after excluding the Hardy-Weinberg Equilibrium (HWE)-violation studies. Compared with B1B1 homozygotes, Caucasian carriers of the B2 allele had a 0.25 mmol/L increase in HDL-C level (95% CI: 0.20-0.31; P <0.0001; I² = 0; P(heterogeneity) =0.87). However, a 1 standard deviation (SD) elevation in HDL-C levels due to the TaqIB polymorphism, was marginal associated with CAD risk (OR =0.79; 95% CI: 0.54-1.03; P =0.08).

CONCLUSIONS

Taken together, our results lend support to the concept that increased HDL-C cannot be translated into a reduction in CAD risk.

Prolactin receptor antagonism uncouples lipids from atherosclerosis susceptibility

The pituitary-derived hormone prolactin has been suggested to stimulate the development of atherosclerosis and cardiovascular disease through its effects on metabolism and inflammation. In this study, we aimed to challenge the hypothesis that inhibition of prolactin function may beneficially affect atherosclerosis burden. Hereto, atherosclerosis-susceptible LDL receptor (Ldlr) knockout mice were transplanted with bone marrow from transgenic mice expressing the pure prolactin receptor antagonist Del1-9-G129R-hPRL or their non-transgenic littermates as control. Recipient mice expressing Del1-9-G129R-hPRL exhibited a decrease in plasma cholesterol levels (-29%; P<0.05) upon feeding a Western-type diet (WTD), which could be attributed to a marked decrease (-47%; P<0.01) in the amount of cholesterol esters associated with pro-atherogenic lipoproteins VLDL/LDL. By contrast, Del1-9-G129R-hPRL-expressing mice did not display any change in the susceptibility for atherosclerosis after 12 weeks of WTD feeding. Both the absolute atherosclerotic lesion size (223 ± 33 × 10(3) μm(2) for Del1-9-G129R-hPRL vs 259 ± 32 × 10(3) μm(2) for controls) and the lesional macrophage and collagen contents were not different between the two groups of bone marrow recipients. Importantly, Del1-9-G129R-hPRL exposure increased levels of circulating neutrophils (+91%; P<0.05), lymphocytes (+55%; P<0.05), and monocytes (+43%; P<0.05), resulting in a 49% higher (P<0.01) total blood leukocyte count. In conclusion, we have shown that prolactin receptor signaling inhibition uncouples the plasma atherogenic index from atherosclerosis susceptibility in Ldlr knockout mice. Despite an associated decrease in VLDL/LDL cholesterol levels, application of the prolactin receptor antagonist Del1-9-G129R-hPRL does not alter the susceptibility for initial development of atherosclerotic lesions probably due to the parallel increase in circulating leukocyte concentrations.

Bone marrow-derived HL mitigates bone marrow-derived CETP-mediated decreases in HDL in mice globally deficient in HL and the LDLr

The objective of this study was to determine the combined effects of HL and cholesteryl ester transfer protein (CETP), derived exclusively from bone marrow (BM), on plasma lipids and atherosclerosis in high-fat-fed, atherosclerosis-prone mice. We transferred BM expressing these proteins into male and female double-knockout HL-deficient, LDL receptor-deficient mice (HL(-/-)LDLr(-/-)). Four BM chimeras were generated, where BM-derived cells expressed 1) HL but not CETP, 2) CETP and HL, 3) CETP but not HL, or 4) neither CETP nor HL. After high-fat feeding, plasma HDL-cholesterol (HDL-C) was decreased in mice with BM expressing CETP but not HL (17 ± 4 and 19 ± 3 mg/dl, female and male mice, respectively) compared with mice with BM expressing neither CETP nor HL (87 ± 3 and 95 ± 4 mg/dl, female and male mice, respectively, P < 0.001 for both sexes). In female mice, the presence of BM-derived HL mitigated this CETP-mediated decrease in HDL-C. BM-derived CETP decreased the cholesterol component of HDL particles and increased plasma cholesterol. BM-derived HL mitigated these effects of CETP. Atherosclerosis was not significantly different between BM chimeras. These results suggest that BM-derived HL mitigates the HDL-lowering, HDL-modulating, and cholesterol-raising effects of BM-derived CETP and warrant further studies to characterize the functional properties of these protein interactions.

Dusty punch cards and an eternal enigma: high-density lipoproteins and atherosclerosis

Epidemiological, clinical, and experimental evidence has accumulated during the last decades suggesting that high-density lipoproteins (HDLs) may protect from atherosclerosis and its clinical consequences. However, more than 55 years after the first description of the link between HDL and heart attacks, many facets of the biochemistry, function, and clinical significance of HDL remain enigmatic. This applies particularly to the completely unexpected results that became available from some recent clinical trials of nicotinic acid and of inhibitors of cholesteryl ester transfer protein (CETP). The concept that raising HDL cholesterol by pharmacological means would decrease the risk of vascular disease has therefore been challenged.

Farnesoid X receptor activation increases cholesteryl ester transfer protein expression in humans and transgenic mice

Cholesteryl ester transfer protein (CETP) activity results in a proatherogenic lipoprotein profile. In cholestatic conditions, farnesoid X receptor (FXR) signaling by bile acids (BA) is activated and plasma HDL cholesterol (HDL-C) levels are low. This study tested the hypothesis that FXR-mediated induction of CETP contributes to this phenotype. Patients with cholestasis and high plasma BA had lower HDL-C levels and higher plasma CETP activity and mass compared with matched controls with low plasma BA (each P < 0.01). BA feeding in APOE3*Leiden transgenic mice expressing the human CETP transgene controlled by its endogenous promoter increased cholesterol within apoB-containing lipoproteins and decreased HDL-C (each P < 0.01), while hepatic CETP mRNA expression and plasma CETP activity and mass increased (each P < 0.01). In vitro studies confirmed that FXR agonists substantially augmented CETP mRNA expression in hepatocytes and macrophages dependent on functional FXR expression (each P < 0.001). These transcriptional effects are likely mediated by an ER8 FXR response element (FXRE) in the first intron. In conclusion, using a translational approach, this study identifies CETP as novel FXR target gene. By increasing CETP expression, FXR activation leads to a proatherogenic lipoprotein profile. These results have clinical relevance, especially when considering FXR agonists as emerging treatment strategy for metabolic disease and atherosclerosis.

HDL inhibits endoplasmic reticulum stress by stimulating apoE and CETP secretion from lipid-loaded macrophages

The role of HDL in the modulation of endoplasmic reticulum (ER) stress in macrophage-derived foam cells is not completely understood. Therefore, we aimed to investigate whether HDL may inhibit ER stress in correlation with the secretion of apoE and CETP from lipid-loaded macrophages. To this purpose, THP-1 macrophages were loaded with lipids by incubation with human oxidized LDL (oxLDL) and then exposed to human HDL3. ER stress signaling markers, protein kinase/Jun-amino-terminal kinase (SAPK/JNK p54/p46) and eukaryotic initiation factor-2α (eIF2α), as well as the secreted apoE and CETP, were evaluated by immunoblot analysis. Out of the many different bioactive lipids of oxLDL, we tested the effect of 9-hydroxy-octadecadienoic acid (9-HODE) and 4-hydroxynonenal (4-HNE) on ER stress. Tunicamycin was used as positive control for ER stress induction. Results showed that oxLDL, 9-HODE and 4-HNE induce ER stress in human macrophages by activation of eIF-2α and SAPK/JNK (p54/p46) signaling pathways. OxLDL stimulated apoE and CETP secretion, while tunicamycin determined a reduction of the secreted apoE and CETP, both in control and lipid-loaded macrophages. The addition of HDL3 to the culture medium of tunicamycin-treated cells induced: (i) the reduction of ER stress, expressed as decreased levels of eIF-2α and SAPK/JNK, and (ii) a partial recovery of the secreted apoE and CETP levels in lipid-loaded macrophages. These data suggest a new mechanism by which HDL3 diminish ER stress and stimulate cholesterol efflux from lipid-loaded macrophages.

Niacin reduces plasma CETP levels by diminishing liver macrophage content in CETP transgenic mice

The anti-dyslipidemic drug niacin has recently been shown to reduce the hepatic expression and plasma levels of CETP. Since liver macrophages contribute to hepatic CETP expression, we investigated the role of macrophages in the CETP-lowering effect of niacin in mice. In vitro studies showed that niacin does not directly attenuate CETP expression in macrophages. Treatment of normolipidemic human CETP transgenic mice, fed a Western-type diet with niacin for 4 weeks, significantly reduced the hepatic cholesterol concentration (-20%), hepatic CETP gene expression (-20%), and plasma CETP mass (-30%). Concomitantly, niacin decreased the hepatic expression of CD68 (-44%) and ABCG1 (-32%), both of which are specific markers for the hepatic macrophage content. The decrease in hepatic CETP expression was significantly correlated with the reduction of hepatic macrophage markers. Furthermore, niacin attenuated atherogenic diet-induced inflammation in liver, as evident from decreased expression of TNF-alpha (-43%). Niacin similarly decreased the macrophage markers and absolute macrophage content in hyperlipidemic APOE*3-Leiden.CETP transgenic mice on a Western-type diet. In conclusion, niacin decreases hepatic CETP expression and plasma CETP mass by attenuating liver inflammation and macrophage content in response to its primary lipid-lowering effect, rather than by attenuating the macrophage CETP expression level.

Plasma lipid profiling across species for the identification of optimal animal models of human dyslipidemia

In an attempt to understand the applicability of various animal models to dyslipidemia in humans and to identify improved preclinical models for target discovery and validation for dyslipidemia, we measured comprehensive plasma lipid profiles in 24 models. These included five mouse strains, six other nonprimate species, and four nonhuman primate (NHP) species, and both healthy animals and animals with metabolic disorders. Dyslipidemic humans were assessed by the same measures. Plasma lipoprotein profiles, eight major plasma lipid fractions, and FA compositions within these lipid fractions were compared both qualitatively and quantitatively across the species. Given the importance of statins in decreasing plasma low-density lipoprotein cholesterol for treatment of dyslipidemia in humans, the responses of these measures to simvastatin treatment were also assessed for each species and compared with dyslipidemic humans. NHPs, followed by dog, were the models that demonstrated closest overall match to dyslipidemic humans. For the subset of the dyslipidemic population with high plasma triglyceride levels, the data also pointed to hamster and db/db mouse as representative models for practical use in target validation. Most traditional models, including rabbit, Zucker diabetic fatty rat, and the majority of mouse models, did not demonstrate overall similarity to dyslipidemic humans in this study.

Bone marrow transplantation in mice as a tool for studying the role of hematopoietic cells in metabolic and cardiovascular diseases

Hematopoietic cells have been established as major players in cardiovascular disease, with an important role in the etiology of atherosclerotic plaque. In addition, hematopoietic cells, and in particular the cells of monocyte and macrophage lineages, have recently been unmasked as one of the main causes of metabolic abnormalities leading to insulin resistance and type 2 diabetes. With the availability of transgenic mouse models that reproduce many aspects of these diseases, research in these areas has been able to make exceptional progress. Much of the work exploring the role of hematopoietic cells has been carried out on chimeric mice made by the recipient disease model mice being given donor bone marrow cells from transgenic mice harboring a genetic alteration in a relevant pathway. Here, we will describe the potential of the bone marrow transplantation approach and discuss recent developments, including the use of virally transduced cells. We will explain some of the caveats, their effect on the experimental outcomes, and some possible solutions. Taken as a whole, this technology offers great advantages in efficiency and cost-effectiveness, and it is expected to continue to be a crucial technique in cardiovascular research work.

Restoration of high-density lipoprotein levels by cholesteryl ester transfer protein expression in scavenger receptor class B type I (SR-BI) knockout mice does not normalize pathologies associated with SR-BI deficiency

OBJECTIVE

Disruption of scavenger receptor class B type I (SR-BI) in mice impairs high-density lipoprotein (HDL)-cholesterol (HDL-C) delivery to the liver and induces susceptibility to atherosclerosis. In this study, it was investigated whether introduction of cholesteryl ester transfer protein (CETP) can normalize HDL-C transport to the liver and reduce atherosclerosis in SR-BI knockout (KO) mice.

METHODS AND RESULTS

Expression of human CETP in SR-BI(KO) mice resulted in decreased plasma HDL-C levels, both on chow diet (1.8-fold, P<0.001) and on challenge with Western-type diet (1.6-fold, P<0.01). Furthermore, the presence of CETP partially normalized the abnormally large HDL particles observed in SR-BI(KO) mice. Unexpectedly, expression of CETP in SR-BI(KO) mice did not reduce atherosclerotic lesion development, probably because of consequences of SR-BI deficiency, including the persistence of higher VLDL-cholesterol (VLDL-C) levels, unchanged elevated free cholesterol/total cholesterol ratio, and the increased oxidative status of the animals. In addition, CETP expression did not normalize other characteristics of SR-BI deficiency, including female infertility, reticulocytosis, thrombocytopenia, and impaired platelet aggregation.

CONCLUSIONS

CETP restores HDL-C levels in SR-BI(KO) mice, but it does not change the susceptibility to atherosclerosis and other typical characteristics that are associated with SR-BI disruption. This may indicate that the pathophysiology of SR-BI deficiency is not a direct consequence of changes in the HDL pool.

The liver-selective thyromimetic T-0681 influences reverse cholesterol transport and atherosclerosis development in mice

BACKGROUND

Liver-selective thyromimetics have been reported to efficiently reduce plasma cholesterol through the hepatic induction of both, the low-density lipoprotein receptor (LDLr) and the high-density lipoprotein (HDL) receptor; the scavenger receptor class B type I (SR-BI). Here, we investigated the effect of the thyromimetic T-0681 on reverse cholesterol transport (RCT) and atherosclerosis, and studied the underlying mechanisms using different mouse models, including mice lacking LDLr, SR-BI, and apoE, as well as CETP transgenic mice.

METHODOLOGY/PRINCIPAL FINDINGS

T-0681 treatment promoted bile acid production and biliary sterol secretion consistently in the majority of the studied mouse models, which was associated with a marked reduction of plasma cholesterol. Using an assay of macrophage RCT in mice, we found T-0681 to significantly increase fecal excretion of macrophage-derived neutral and acidic sterols. No positive effect on RCT was found in CETP transgenic mice, most likely due to the observed decrease in plasma CETP mass. Studies in SR-BI KO and LDLr KO mice suggested hepatic LDLr to be necessary for the action of T-0681 on lipid metabolism, as the compound did not have any influence on plasma cholesterol levels in mice lacking this receptor. Finally, prolonged treatment with T-0681 reduced the development of atherosclerosis by 60% in apoE KOs on Western type diet. In contrast, at an earlier time-point T-0681 slightly increased small fatty streak lesions, in part due to an impaired macrophage cholesterol efflux capacity, when compared to controls.

CONCLUSIONS/SIGNIFICANCE

The present results show that liver-selective thyromimetics can promote RCT and that such compounds may protect from atherosclerosis partly through induction of bile acid metabolism and biliary sterol secretion. On-going clinical trials will show whether selective thyromimetics do prevent atherosclerosis also in humans.

Liver X receptor-mediated induction of cholesteryl ester transfer protein expression is selectively impaired in inflammatory macrophages

OBJECTIVE

Cholesteryl ester transfer protein (CETP) is a target gene for the liver X receptor (LXR). The aim of this study was to further explore this regulation in the monocyte-macrophage lineage and its modulation by lipid loading and inflammation, which are key steps in the process of atherogenesis.

METHODS AND RESULTS

Exposure of bone marrow-derived macrophages from human CETP transgenic mice to the T0901317 LXR agonist increased CETP, PLTP, and ABCA1 mRNA levels. T0901317 also markedly increased CETP mRNA levels and CETP production in human differentiated macrophages, whereas it had no effect on CETP expression in human peripheral blood monocytes. In inflammatory mouse and human macrophages, LXR-mediated CETP gene upregulation was inhibited, even though ABCA1, ABCG1, and SREBP1c inductions were maintained. The inhibition of CETP gene response to LXR agonists in inflammatory cells was independent of lipid loading (ie, oxidized LDL increased CETP production in noninflammatory macrophages with a synergistic effect of synthetic LXR agonists).

CONCLUSIONS

LXR-mediated induction of human CETP expression is switched on during monocyte-to-macrophage differentiation, is magnified by lipid loading, and is selectively lost in inflammatory macrophages, which suggests that inflammatory cells may not increase the circulating CETP pool on LXR agonist treatment.

A Translational Medicine perspective of the development of torcetrapib: Does the failure of torcetrapib development cast a shadow on future development of lipid modifying agents, HDL elevation strategies or CETP as a viable molecular target for atherosclerosis? A case study of the use of biomarkers and Translational Medicine in atherosclerosis drug discovery and development

Although the relationship between HDL (high density lipoprotein) function and cardiovascular (CV) risk has been extensively explored, the premise that HDL elevation is linked to reduced CV risks and that high HDL cholesterol (HDL-C) might be a potential surrogate biomarker for reduced CV risk remains controversial. Substantial genetic, molecular, biochemical and preclinical evidence have raised the hope that HDL-C elevation via CETP inhibition might generate clinical benefits. However, four large-scale clinical trials with the CETP inhibitor torcetrapib failed to demonstrate benefits on CV clinical outcomes. Likewise, biomarkers that were supposed to predict vascular risk reduction provided disappointing results. The sad tale of torcetrapib development emphasizes the need for a paradigm shift from the conventional drug development mode to a biomarker-based Translational Medicine (TMed) strategy. Emergence of further CETP inhibitors encourage continued development of such compounds for cardiovascular risk management. However, there is a need to adopt biomarker-driven TMed strategies in target validation, target-compound interaction, pharmacodynamic activities, disease modification and patient selection to guide future drug development efforts. This commentary analyzes the issues surrounding the demise of torcetrapib and proposes a TMed-based road map towards successful development of new CETP inhibitors.

Scavenger receptor class B type I-mediated uptake of serum cholesterol is essential for optimal adrenal glucocorticoid production

Impaired scavenger receptor class B type I (SR-BI)-mediated uptake of HDL-cholesterol esters (HDL-CE) induces adrenal insufficiency in mice. Humans contain an alternative route of HDL-CE clearance, namely through the transfer by cholesteryl ester transfer protein (CETP) to apolipoprotein B lipoproteins for subsequent uptake via the LDL receptor. In this study, we determined whether CETP can compensate for loss of adrenal SR-BI. Transgenic expression of human CETP (CETP Tg) in SR-BI knockout (KO) mice increased adrenal HDL-CE clearance from 33-58% of the control value. SR-BI KO/CETP Tg and SR-BI KO mice displayed adrenal hypertrophy due to equally high plasma adrenocorticotropic hormone levels. Adrenal cholesterol levels and plasma corticosterone levels were 38-52% decreased in SR-BI KO mice with and without CETP expression. SR-BI KO/CETP Tg mice also failed to increase their corticosterone level after lipopolysaccharide challenge, leading to an identical >4-fold increased tumor necrosis factor-alpha response compared with controls. These data indicate that uptake of CE via other routes than SR-BI is not sufficient to generate the cholesterol pool needed for optimal adrenal steroidogenesis. In conclusion, we have shown that CETP-mediated transfer of HDL-CE is not able to reverse adrenal insufficiency in SR-BI knockout mice. Thus, SR-BI-mediated uptake of serum cholesterol is essential for optimal adrenal function.

Reduced leucocyte cholesteryl ester transfer protein expression in acute coronary syndromes

OBJECTIVE

Cholesterol ester transfer protein (CETP) plays an important role in HDL cholesterol metabolism. Leucocytes, including monocyte-derived macrophages in the arterial wall synthesize and secrete CETP, but its role in atherosclerosis is unclear. The aim of the current study was to investigate the effect of acute coronary syndromes (ACS) on leucocyte CETP expression.

RESEARCH DESIGN

Peripheral blood mononuclear cells (PBMCs) were freshly isolated from hospitalized ACS patients displaying Braunwald class IIIB unstable angina pectoris (UAP) on admission (t = 0) and at 180 days post inclusion (t = 180) for analysis of CETP expression. In addition, to prove the potential correlation between leucocyte CETP and ACS the effect of acute myocardial infarction on leucocyte CETP expression was studied in CETP transgenic mice.

RESULTS

Upon admission, UAP patients displayed approximately 3-6 fold (P < 0.01) lower CETP mRNA and nearly absent CETP protein expression in PBMCs, as compared to healthy age-/sex-matched controls. Interestingly, CETP mRNA and protein levels were significantly elevated in PBMCs isolated from UAP patients (both stabilized and refractory) at t = 180 as compared to t = 0 (P < 0.01), which was correlated with a reduced inflammatory status after medical treatment. In agreement with the data obtained in UAP patients, markedly down-regulated leucocyte CETP mRNA expression was observed after coronary artery ligation in CETP transgenic mice, which also correlated with increased serum amyloid A levels.

CONCLUSIONS

We are the first to report that episodes of UAP in humans and myocardial infarction in CETP transgenic mice are associated with reduced leucocyte CETP expression. We propose that the impairment in leucocyte CETP production is associated with an enhanced inflammatory status, which could be clinically relevant for the pathogenesis of ACS.

The role of plasma lipid transfer proteins in lipoprotein metabolism and atherogenesis

The plasma lipid transfer proteins promote the exchange of neutral lipids and phospholipids between the plasma lipoproteins. Cholesteryl ester transfer protein (CETP) facilitates the removal of cholesteryl esters from HDL and thus reduces HDL levels, while phospholipid transfer protein (PLTP) promotes the transfer of phospholipids from triglyceride-rich lipoproteins into HDL and increases HDL levels. Studies in transgenic mouse models and in humans with rare genetic deficiencies (CETP) or common genetic variants (CETP and PLTP) highlight the central role of these molecules in regulating HDL levels. Human CETP deficiency is associated with dramatic elevations of HDL cholesterol and apolipoprotein A-I levels, while PLTP variants with increased expression are associated with higher HDL levels. A recent meta-analysis suggests that common CETP alleles causing reduced CETP and increased HDL levels are associated with reduced coronary heart disease. The failure of a clinical trial with the CETP inhibitor torcetrapib may have been related in part to off-target toxicity. Ongoing phase 3 clinical trials with other CETP inhibitors may help to clarify if this strategy can ultimately be successful in the treatment of atherosclerosis.

Leukocyte-derived hepatic lipase increases HDL and decreases en face aortic atherosclerosis in LDLr-/- mice expressing CETP

In addition to hepatic expression, cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) are expressed by human macrophages. The combined actions of these proteins have profound effects on HDL structure and function. It is not known how these HDL changes influence atherosclerosis. To elucidate the role of leukocyte-derived HL on atherosclerosis in a background of CETP expression, we studied low density lipoprotein receptor-deficient mice expressing human CETP (CETPtgLDLr -/-) with a leukocyte-derived HL deficiency (HL -/- BM). HL(-/-) bone marrow (BM), CETPtgLDLr(-/-) mice were generated via bone marrow transplantation. Wild-type bone marrow was transplanted into CETPtgLDLr(-/-) mice to generate HL +/+ BM, CETPtgLDLr(-/-) controls. The chimeras were fed a high-fat, high-cholesterol diet for 14 weeks to promote atherosclerosis. In female HL(-/-) BM, CETPtgLDLr(-/-) mice plasma HDL-cholesterol concentration during high-fat feeding was decreased 27% when compared with HL +/+ BM, CETPtgLDLr(-/-) mice (P < 0.05), and this was associated with a 96% increase in en face aortic atherosclerosis (P < 0.05). In male CETPtgLDLr(-/-) mice, leukocyte-derived HL deficiency was associated with a 16% decrease in plasma HDL-cholesterol concentration and a 25% increase in aortic atherosclerosis. Thus, leukocyte-derived HL in CETPtgLDLr(-/-) mice has an atheroprotective role that may involve increased HDL levels.

Is raising HDL a futile strategy for atheroprotection?

The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein inhibitor torcetrapib has led to considerable doubt about the value of raising high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These results have underscored the intricacy of HDL metabolism, with functional quality perhaps being a more important consideration than the circulating quantity of HDL. As a result, HDL-based therapeutics that maintain or enhance HDL functionality warrant closer investigation. In this article, we review the complexity of HDL metabolism, discuss clinical-trial data for HDL-raising agents, including possible reasons for the failure of torcetrapib, and consider the potential for future HDL-based therapies.

Sex-Dependent Attenuation of Plaque Growth After Treatment With Bone Marrow Mononuclear Cells

There are clinically relevant differences in symptomatology, risk stratification, and efficacy of therapies between men and women with coronary artery disease. Sex-based differences in plaque attenuation after administration of bone marrow mononuclear cells (BMNCs) are unknown. Forty-five male and 57 female apolipoprotein-E knockout (apoE −/− ) mice were fed a high-fat diet. At 14 weeks of age, animals received 4 biweekly intravenous sex-matched (males, n=11; females, n=13) or -mismatched (males, n=12; females, n=14) BMNCs obtained from C57BL6/J mice. The rest of the apoE −/− mice were vehicle treated (males, n=13; females, n=20) or were age-matched untreated controls (males, n=9; females, n=10). Aortic plaque burden, progenitor cell profiles in bone marrow (BM) and 22 circulating cytokines/chemokines were examined 1 week following the final injection. Only female BMNCs infused into male apoE −/− recipients significantly decreased plaque formation ( P <0.001). This reparative response univariately correlated with increased CD34 + ( P =0.02), CD45 + ( P =0.0001), and AC133 + /CD34 + ( P =0.001) cell percentages in the BM of recipients but not with total serum cholesterol or percentage of BM-CD31 + /CD45 low cells. In a multivariate analysis, BM-AC133 + /CD34 + and BM-CD45 + percentage counts correlated with a lower plaque burden ( P <0.05). Increased granulocyte colony-stimulating factor levels highly correlated with plaque attenuation ( r =−0.86, P =0.0004). In untreated apoE −/− mice of either sex, BM-AC133 + /CD34 + cells rose initially and then fell as plaque accumulated; however, BM-AC133 + /CD34 + percentages were higher in females at all times ( P ≤0.01). We have demonstrated an atheroprotective effect of female-derived BMNCs administered to male atherosclerotic apoE −/− mice; this reparative response correlated with the upregulation of BM-AC133 + /CD34 + and CD45 + cells and of circulating granulocyte colony-stimulating factor. Atherosclerotic female apoE −/− mice did not exhibit atheroprotection after BMNCs of either sex. Our findings may have implications for clinical cell therapy trials for coronary artery disease. Further exploration of sex-based differences in atheroprotection and vascular repair is warranted.

Expression of cholesteryl ester transfer protein in mice promotes macrophage reverse cholesterol transport

BACKGROUND

Cholesteryl ester transfer protein (CETP) transfers cholesteryl esters from high-density lipoproteins to apolipoprotein (apo) B-containing lipoproteins and in humans plays an important role in lipoprotein metabolism. However, the role that CETP plays in mediation of reverse cholesterol transport (RCT) remains unclear. We used a validated in vivo assay of macrophage RCT to test the effect of CETP expression in mice (which naturally lack CETP) on macrophage RCT, including in mice that lack the low-density lipoprotein receptor or the scavenger receptor class B, type I.

METHOD AND RESULTS

A vector based on adeno-associated virus serotype 8 (AAV8) with a liver-specific thyroglobulin promoter was used to stably express human CETP in livers of mice and was compared with an AAV8-lacZ control vector. The RCT assay was performed 4 weeks after vector injection and involved the intraperitoneal injection of acetylated low-density lipoprotein cholesterol-loaded and 3H-cholesterol-labeled J774 macrophages in mice with plasma sampling at several time points, liver and bile sampling at 48 hours, and continuous fecal collection to measure 3H-sterol as an integrated readout of macrophage RCT. In apobec-1-null mice, CETP expression reduced plasma high-density lipoprotein cholesterol levels but significantly increased fecal 3H-sterol excretion. In low-density lipoprotein receptor/apobec-1 double-null mice, CETP expression reduced high-density lipoprotein cholesterol levels and had no effect on fecal 3H-sterol excretion. Finally, in scavenger receptor class B, type I-null mice, CETP expression reduced high-density lipoprotein cholesterol levels and significantly increased fecal 3H-sterol excretion.

CONCLUSION

The present results demonstrate that CETP expression promotes macrophage RCT in mice, that this effect is dependent on the low-density lipoprotein receptor, and that CETP expression restores to normal the impaired RCT in mice deficient in scavenger receptor class B, type I.

CETP activity variation in mice does not affect two major HDL antiatherogenic properties: macrophage-specific reverse cholesterol transport and LDL antioxidant protection

CETP inhibition increases HDL cholesterol levels and presumably could contribute to human atheroprotection via increasing macrophage-specific reverse cholesterol transport (RCT) and antioxidant properties of HDL. However, the impact of CETP activity variation on these two antiatherogenic functions of HDL remain unknown. In this study, we assessed the effects of overexpressing CETP in transgenic (Tg) mice on macrophage-specific RCT and HDL ability to protect against LDL oxidative modification. [(3)H]cholesterol-labeled macrophages were injected intraperitoneally into mice maintained on a chow diet or an atherogenic diet, after which the appearance of [(3)H]cholesterol in plasma, liver and feces over 48 h was determined. The degree of protection of oxidative modification of LDL coincubated with HDL was evaluated by measuring relative electrophoretic mobility and dichlorofluorescein fluorescence. CETP-Tg mice presented decreased radiolabeled HDL-bound [(3)H]cholesterol 24 and 48 h after the label injection. However, the magnitude of macrophage-derived [(3)H]cholesterol in liver and feces did not differ between CETP-Tg and control mice on either diet. Similar results were found when [(3)H]cholesterol-labeled endogenous peritoneal macrophages were injected into the CETP-Tg and control mice. Further, the injection of endogenous macrophages from CETP-Tg mice did not alter macrophage RCT in control mice. HDL from CETP-Tg and control mice protected LDL from oxidative modification similarly, and paraoxonase 1, platelet activated factor acetyl-hydrolase and lecithin-cholesterol acyl transferase activities of transgenic mice did not differ from those of control mice. In conclusion, CETP overexpression in transgenic mice does not affect RCT from macrophages to feces in vivo or the protection conferred by HDL against LDL oxidative modification.