Serum cholesterol efflux potential is an independent predictor of coronary artery atherosclerosis

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

Elevated Lipoprotein(a) Levels Lower ABCA1 Cholesterol Efflux Capacity

CONTEXT

Elevated serum lipoprotein(a) [Lp(a)] levels are associated with increased cardiovascular disease risk. ABCA1-mediated cholesterol efflux from macrophages may be an antiatherogenic process. Plasminogen (PLG) is a driver of ABCA1-mediated cholesterol efflux, and its action is inhibited by purified human Lp(a).

OBJECTIVE

To determine the effects of Lp(a) in human serum on ABCA1 cholesterol efflux.

METHODS

Cholesterol efflux capacity (CEC) was measured with two different cell-culture models using serum from 76 patients with either low (<50 mg/dL) or high (>50 mg/dL) Lp(a) levels.

RESULTS

Using cAMP-stimulated J774 macrophages or baby hamster kidney fibroblasts overexpressing human ABCA1, we show that CEC was lower in patients with high Lp(a) levels compared with patients with low levels (-30.6%, P = 0.002 vs -24.1%, P < 0.001, respectively). Total-serum CEC negatively correlated with Lp(a) levels (r = -0.433, P = 0.0007 vs r = -0.505, P = 0.0011, respectively). These negative associations persisted after adjusting for serum cholesterol, age, sex, and statin use in a multiple linear regression model (adjusted R2 = 0.413 or 0.405, respectively) and were strengthened when further adjusting for the interaction between Lp(a) and PLG levels (adjusted R2 = 0.465 and 0.409, respectively). Total-serum and isolated Lp(a) from patients with high Lp(a) inhibited PLG-mediated ABCA1 cholesterol efflux.

CONCLUSION

Total-serum CEC is reduced in patients with high Lp(a) levels. This is in part due to the inhibition of PLG-mediated ABCA1 cholesterol efflux by Lp(a). Our findings suggest an atherogenic role for Lp(a) through its ability to inhibit CEC.

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

Pharmacodynamic effects of the fetal estrogen estetrol in postmenopausal women: results from a multiple-rising-dose study

OBJECTIVE

Estetrol (E4) is an estrogen produced exclusively by the human fetal liver during pregnancy. In this study the pharmacodynamic effects of escalating doses of E4 in postmenopausal women were investigated.

METHODS

This was a partly randomized, open-label, multiple-rising-dose study in 49 postmenopausal women. Participants were randomized to receive either 2 mg E4 or 2 mg estradiol-valerate (E2 V) for 28 days. Subsequent dose-escalation groups were (non-randomized): 10, 20 and 40 mg E4. Blood samples were collected regularly for measuring endocrine and hemostasis variables, lipids and lipoproteins, fasting glucose and bone turnover markers.

RESULTS

Estetrol treatment resulted in a decrease of follicle-stimulating hormone and luteinizing hormone and an increase of sex-hormone binding globulin. Changes in hemostasis variables were small. A lowering effect on low-density lipoprotein cholesterol was accompanied with an increase in high-density lipoprotein cholesterol and no or minimal changes in triglycerides. The considerable decrease in osteocalcin levels in the three highest E4 dose groups and the small decrease in C-telopeptide levels were comparable to the E2 V control group and suggest a preventive effect on bone loss. All changes observed were dose-dependent.

CONCLUSIONS

In this study, estetrol treatment showed dose-dependent estrogenic effects on endocrine parameters, bone turnover markers, and lipids and lipoproteins. The effect on triglycerides was small as were the effects on hemostatic variables. These results support the further investigation of estetrol as a candidate for hormone therapy. Quantitatively, the effects of 10 mg estetrol were similar to the study comparator 2 mg estradiol valerate.

Communication-Microelectrode Detection of Cholesterol Efflux from the Human Buccel Mucosa

It has previously demonstrated that cholesterol efflux from the cell plasma membrane is increased in a mouse model of cystic fibrosis (CF) compared to a wild-type control. A noninvasive means of characterizing plasma membrane cholesterol efflux at the surface of airway tissue of CF patients is needed to extend the trends found in animal models of CF to the human disease state. Microelectrode-induced cholesterol efflux from the plasma membrane of cells at the surface of tissue is proposed as a strategy to demonstrate increased cholesterol efflux for CF in human subjects. Data demonstrating detection of cholesterol efflux from the human buccal mucosa is reported as proof-of-concept for an in vivo diagnostic assay.

Endothelial lipase and reverse cholesterol transport in type 2 diabetes mellitus

Aims/Introduction:  Endothelial lipase (EL) plays an important role in high‐density lipoprotein (HDL) metabolism and experimental data suggest that EL might be proatherogenic. We have investigated whether serum EL concentration is associated with changes in serum capacity to induce cholesterol efflux and arterial stiffness in type 2 diabetes.

Materials and Methods:  Serum EL was assayed by ELISA in 172 diabetic patients and 175 controls. The ability of serum to induce cholesterol efflux was measured using a cell culture system and arterial stiffness was determined by measuring pulse wave velocity (PWV) between carotid and femoral arteries.

Results:  Diabetic patients had significantly higher C‐reactive protein (CRP) and EL (27.7 ± 16.6 ng/mL vs 24.0 ± 11.3, P < 0.05). Cholesterol efflux to serum mediated through scavenger receptor class B type I was impaired (15.1 ± 2.5%vs 16.7 ± 3.1, respectively, P < 0.01). In controls, serum EL correlated with cholesterol efflux to serum (r = −0.16, P = 0.025), but only a trend was seen in the diabetic patients. Linear regression showed that in controls, HDL, serum EL and waist circumference were major independent determinants of cholesterol efflux; whereas in the diabetic cohort, the major independent determinants of cholesterol efflux were HDL, CRP and age. PWV was increased in the diabetic patients (P < 0.01), but no association between serum EL and PWV was seen in either groups.

Conclusions:  Serum EL was increased in diabetic patients, but impaired serum capacity to induce cholesterol efflux in these patients was mainly related to low HDL and subclinical inflammation. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00016.x, 2010)

Future of cholesteryl ester transfer protein (CETP) inhibitors: a pharmacological perspective

In almost 30 years since the introduction of HMG-CoA reductase inhibitors (statins), no other class of lipid modulators has entered the market. Elevation of high-density lipoprotein-cholesterol (HDL-C) via inhibiting cholesteryl ester transfer protein (CETP) is an attractive strategy for reducing the risk of cardiovascular events in high-risk patients. Transfer of triglyceride and cholesteryl ester (CE) between lipoproteins is mediated by CETP; thus inhibition of this pathway can increase the concentration of HDL-C. Torcetrapib was the first CETP inhibitor evaluated in phase III clinical trials. Because of off-target effects, torcetrapib raised blood pressure and increased the concentration of serum aldosterone, leading to higher cardiovascular events and mortality. Torcetrapib showed positive effects on cardiovascular risk especially in patients with a greater increase in HDL-C and apolipoprotein A-1 (apoA-1) levels. The phase III clinical trial of dalcetrapib, the second CETP inhibitor that has entered clinical development, was terminated because of ineffectiveness. Dalcetrapib is a CETP modulator that elevated HDL-C levels but did not reduce the concentration of low-density lipoprotein cholesterol (LDL-C). Both heterotypic and homotypic CE transfer between lipoproteins are mediated by some CETP inhibitors, including torcetrapib, anacetrapib, and evacetrapib, while dalcetrapib only affects the heterotypic CE transfer. Dalcetrapib has a chemical structure that is distinct from other CETP inhibitors, with a smaller molecular weight and a lack of trifluoride moieties. Moreover, dalcetrapib is a pro-drug that must be hydrolyzed to a pharmacologically active thiol form. Two other CETP inhibitors, anacetrapib and evacetrapib, are currently undergoing evaluation in phase III clinical trials. Both molecules have shown beneficial effects by increasing HDL-C and decreasing LDL-C concentration. The success of anacetrapib and evacetrapib remains to be confirmed upon the completion of phase III clinical trials in 2017 and 2015, respectively. Generally, the concentration of HDL-C has been considered a biomarker for the activity of CETP inhibitors. However, it is not clear whether a fundamental relationship exists between HDL-C levels and the risk of coronary artery diseases. The most crucial role for HDL is cholesterol efflux capacity in which HDL can reverse transport cholesterol from foam cells in atherosclerotic plaques. In view of the heterogeneity in HDL particle size, charge, and composition, the mere concentration of HDL-C may not be a good surrogate marker for HDL functionality. Recent clinical studies have reported that increased HDL functionality inversely correlates with the development of atherosclerotic plaque. Future development of CETP inhibitors may therefore benefit from the use of biomarkers of HDL functionality.

HDL-apoA-I exchange: rapid detection and association with atherosclerosis

High density lipoprotein (HDL) cholesterol levels are associated with decreased risk of cardiovascular disease, but not all HDL are functionally equivalent. A primary determinant of HDL functional status is the conformational adaptability of its main protein component, apoA-I, an exchangeable apolipoprotein. Chemical modification of apoA-I, as may occur under conditions of inflammation or diabetes, can severely impair HDL function and is associated with the presence of cardiovascular disease. Chemical modification of apoA-I also impairs its ability to exchange on and off HDL, a critical process in reverse cholesterol transport. In this study, we developed a method using electron paramagnetic resonance spectroscopy (EPR) to quantify HDL-apoA-I exchange. Using this approach, we measured the degree of HDL-apoA-I exchange for HDL isolated from rabbits fed a high fat, high cholesterol diet, as well as human subjects with acute coronary syndrome and metabolic syndrome. We observed that HDL-apoA-I exchange was markedly reduced when atherosclerosis was present, or when the subject carries at least one risk factor of cardiovascular disease. These results show that HDL-apoA-I exchange is a clinically relevant measure of HDL function pertinent to cardiovascular disease.

Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis

BACKGROUND

High-density lipoprotein (HDL) may provide cardiovascular protection by promoting reverse cholesterol transport from macrophages. We hypothesized that the capacity of HDL to accept cholesterol from macrophages would serve as a predictor of atherosclerotic burden.

METHODS

We measured cholesterol efflux capacity in 203 healthy volunteers who underwent assessment of carotid artery intima-media thickness, 442 patients with angiographically confirmed coronary artery disease, and 351 patients without such angiographically confirmed disease. We quantified efflux capacity by using a validated ex vivo system that involved incubation of macrophages with apolipoprotein B-depleted serum from the study participants.

RESULTS

The levels of HDL cholesterol and apolipoprotein A-I were significant determinants of cholesterol efflux capacity but accounted for less than 40% of the observed variation. An inverse relationship was noted between efflux capacity and carotid intima-media thickness both before and after adjustment for the HDL cholesterol level. Furthermore, efflux capacity was a strong inverse predictor of coronary disease status (adjusted odds ratio for coronary disease per 1-SD increase in efflux capacity, 0.70; 95% confidence interval [CI], 0.59 to 0.83; P<0.001). This relationship was attenuated, but remained significant, after additional adjustment for the HDL cholesterol level (odds ratio per 1-SD increase, 0.75; 95% CI, 0.63 to 0.90; P=0.002) or apolipoprotein A-I level (odds ratio per 1-SD increase, 0.74; 95% CI, 0.61 to 0.89; P=0.002). Additional studies showed enhanced efflux capacity in patients with the metabolic syndrome and low HDL cholesterol levels who were treated with pioglitazone, but not in patients with hypercholesterolemia who were treated with statins.

CONCLUSIONS

Cholesterol efflux capacity from macrophages, a metric of HDL function, has a strong inverse association with both carotid intima-media thickness and the likelihood of angiographic coronary artery disease, independently of the HDL cholesterol level. (Funded by the National Heart, Lung, and Blood Institute and others.).

Impaired serum capacity to induce cholesterol efflux is associated with endothelial dysfunction in type 2 diabetes mellitus

OBJECTIVE

Reverse cholesterol transport (RCT) plays a protective role against atherosclerosis and cholesterol efflux from cells is an early step in the RCT pathway. We investigated whether the capacity of serum to induce cholesterol efflux was associated with endothelial dysfunction in type 2 diabetes.

METHODS

Endothelium-dependent and -independent vasodilation of the brachial artery was measured by high-resolution vascular ultrasound and serum cholesterol efflux capacity was determined by measuring the transfer of [3H]cholesterol from Fu5AH cells to serum in 137 patients with type 2 diabetes and 75 controls.

RESULTS

Serum cholesterol efflux capacity was lower in diabetic patients than in the controls (13.6+/-2.5% vs. 14.6+/-3.4%, respectively, p=0.02), and both endothelium-dependent vasodilation (4.9+/-2.2% vs. 8.8+/-4.1%, respectively, p<0.01) and endothelium-independent vasodilation were impaired (13.4+/-4.3% vs. 16.3+/-5.5%. respectively, p<0.01). Endothelium-dependent vasodilation correlated with serum cholesterol efflux capacity (r=0.26, p=0.003) in diabetic patients and controls (r=0.24, p=0.037). On general linear model analysis, the presence of diabetes, brachial artery diameter and serum cholesterol efflux capacity were significant independent determinants of endothelium-dependent vasodilation.

CONCLUSION

Impaired serum cholesterol efflux capacity was associated with endothelial dysfunction independent of other cardiovascular risk factors.

Age-associated decrease of high-density lipoprotein-mediated reverse cholesterol transport activity

High-density lipoproteins (HDL) are considered atheroprotective in contrast to low-density lipoproteins (LDL), which are atherogenic in their oxidized form. A growing body of evidence suggests that HDL exert part of their antiatherogenic effect by counteracting LDL oxidation as well as their proinflammatory effect. However, a number of studies, carried over the past 30 years, have shown that cholesterol efflux plays a major role in the atheroprotective effects of HDL and cholesterol homeostasis. These studies have further identified the scavenger receptor type B-I (SR-BI), the adenosine triphosphate (ATP)-binding cassette transporters ATP-binding cassette subfamily A1 (ABCA1), ATP-binding cassette subfamily G1 (ABCG1) and ABCG4, the liver X receptor/retinoid X receptor (LXR/RXR) and peroxisome proliferator-activated receptorgamma(PPAR gamma) transcription factors, the HDL components apolipoprotein A-I (apoA-I), lecithin-cholesterol acyltransferase (LCAT), and phospholipids as additional mediators of cholesterol transport. Cholesterol efflux occurs via three independent pathways: (1) aqueous diffusion, (2) nonspecific efflux via SR-BI receptors, and (3) specific efflux via cholesterol-responsive members of the ABC superfamily. Whereas aqueous diffusion and scavenger receptor class B, type I (SR-BI)-mediated efflux transport free cholesterol to a wide variety of cholesterol acceptors (particles containing phospholipids, HDL, and lipidated apo-lipoproteins; LDL, etc), the ABCA1 pathway mediates the transport of cholesterol in a unidirectional manner, mainly to lipid-poor apoA-I. In contrast, the ABCG1 pathway is responsible for the transport of cholesterol to all the subfamily members of HDL. Although HDL-mediated cholesterol efflux is apoA-I-dependent, recent studies have suggested an involvement of the enzyme paraoxonase 1 (PON1). Cholesterol efflux is carried on by a number of factors such as genetic mutations, smoking, stress, and high-fat diets. It is attenuated with aging due to changes in the composition and structure of HDL, especially the phosphatidylcholine/sphingomyelin ratio, the fluidity of the phospholipidic layer, the concentration of apoA-I, and the activity of PON1. This review summarizes the findings that cholesterol homeostasis is disrupted with aging as a consequence of dysfunctional cholesterol efflux and the impairment of physiological functions.

Cellular cholesterol efflux to serum is impaired in diabetic nephropathy

BACKGROUND

Cholesterol efflux from cells is an early step of reverse cholesterol transport (RCT) and the capacity of serum to induce cellular cholesterol efflux has recently been shown to be an independent predictor of coronary artery atherosclerosis. Our aim is to evaluate the capacity of serum to induce ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-BI) mediated cholesterol efflux in type 2 diabetic patients with nephropathy.

METHODS

Diabetic patients were recruited according to their urinary albumin excretion rate (normoalbuminuria, microalbuminuria and proteinuria) with 20 subjects in each group and compared with 20 age-matched controls. The ability of the serum to induce cholesterol efflux was measured using a cell culture system.

RESULTS

Serum capacity to induce ABCA1-mediated cholesterol efflux was decreased in patients with microalbuminuria or proteinuria (p < 0.05) whereas SR-BI-mediated cholesterol efflux was impaired in all three groups of diabetic patients (p < 0.05). Plasma high-density lipoprotein (HDL) cholesterol and apoAI were reduced in all groups of diabetic patients, but pre-beta-HDL was only significantly decreased in those with microalbuminuria or proteinuria. Serum advanced glycation end products (AGEs) were significantly increased in diabetic patients with microalbuminuria or proteinuria. Serum AGEs and pre-beta-HDL were the significant independent determinants of ABCA1-mediated cholesterol efflux, whereas plasma HDL and log (creatinine) were the significant determinants of SR-BI-mediated cholesterol efflux.

CONCLUSION

The capacity of serum to induce ABCA1- and SR-BI-mediated cholesterol efflux was impaired in diabetic patients with incipient or overt nephropathy. These abnormalities may contribute to the accelerated development of atherosclerotic vascular disease in these patients.

In vitro effects of tibolone and its metabolites on human vascular coronary cells

OBJECTIVES

Tibolone is a tissue selective compound with estrogenic, androgenic and progestogenic properties in classical bioassays. It is used for alleviation of menopausal symptoms and for osteoporosis prophylaxis in postmenopausal women. Only few data are available regarding the effects of tibolone on the cardiovascular system. We investigated therefore the in vitro effects of tibolone and its metabolites on the vasculature under special controlled conditions, using human female coronary endothelial and smooth muscle cells.

METHODS

The effect on the production of the following markers in endothelial cells from human female coronary arteries was evaluated: nitric oxide synthase, prostacyclin, endothelin, plasminogen-activator-inhibitor-1 (PAI-1), E-Selectin, Intercellular adhesion molecule (ICAM-1), monocyte attracting protein-1 (MCP-1) and the precursor of matrix metalloproteinase-1 (pro-MMP-1). Tibolone, its metabolites, estradiol (E2), E2/norethisterone (NET) and E2/medroxyprogesterone acetate (MPA) were tested at 0.1 microM and 1 microM. The markers were determined by enzyme immunoassays in the cell supernatant. Cell proliferation of smooth muscle cells from female coronary artery was measured by an adenosine triphosphate-assay.

RESULTS

Tibolone, its 3-hydroxy metabolites, E2/NET, E2/MPA and estradiol alone had significant effects on the synthesis of all markers tested. The magnitude of the tibolone effects, however, was mostly smaller than that of E2/NET and E2/MPA. Concerning smooth muscle cells tibolone and its 3-hydroxy metabolites also elicited an inhibition of the proliferation compared to control values. The strongest effect here was found for E2/NET and E2 alone, whereas E2/MPA had no effect.

CONCLUSION

The results of this in vitro study conducted with cells of the most important vascular bed with respect to the problem of cardiovascular risk suggest that tibolone can positively influence the vasculature. However, these tibolone effects may depend on intact vascular cells and may vary due to the different atherosclerotic stages of the vessels. Thus, experimental studies are useful to explore mechanisms, but clearly cannot replace clinical studies.

Indices of reverse cholesterol transport in subjects with metabolic syndrome after treatment with rosuvastatin

OBJECTIVE

The effects of the statin, rosuvastatin on indices of reverse cholesterol transport were studied in a randomized, placebo-controlled, cross-over trial in 25 overweight subjects with defined metabolic syndrome.

RESULT

Four weeks' treatment with 40 mg/day rosuvastatin significantly reduced levels of plasma cholesterol (44%), LDL cholesterol (60%) and triglyceride (38%). HDL cholesterol (mean [S.D.]) rose (0.97[0.17] to 1.05[0.17]mmol/L; P<0.05) and the LpA-I component of HDL from 39[7] to 45[9]mg/dL (P<0.05). LCAT activity fell (0.55[0.13] to 0.35[0.07]nmol/mL/h; P<0.05); CETP activity and mass fell from 89[13] to 80[11]nmol//L/h and from 1.66[0.57] to 1.28[0.41]mug/mL respectively, (P<0.05). Cholesterol efflux in vitro (to plasmas from THP-1 activated cells) fell from 7.1[1.8]% (placebo) to 6.2[1.7]% (rosuvastatin); P<0.05, but when plasmas depleted of apoB lipoproteins were studied, the difference in efflux was no longer statistically significant. During placebo efflux was paradoxically inversely correlated with HDL-C (P=0.016) and LpA-I (P=0.035) concentrations but these correlations were absent after rosuvastatin.

CONCLUSIONS

The data suggest possible HDL dysfunctionality in subjects with metabolic syndrome. The reduced capacity of plasmas following statin treatment to stimulate cholesterol efflux in vitro occurred in association with reduction in apoB lipoproteins and reduced activities of CETP and LCAT, and despite increased levels of HDL cholesterol.

Measurement of cholesterol bidirectional flux between cells and lipoproteins

We developed an assay that quantitates bidirectional cholesterol flux between cells and lipoproteins. Incubating Fu5AH cells with increasing concentrations of human serum resulted in increased influx and efflux; however, influx was 2- to 3-fold greater at all serum concentrations. With apolipoprotein B (apoB)-depleted serum, the ratio of influx to efflux (I/E) was close to 1, indicating cholesterol exchange. The apoB fraction of serum induced influx and little efflux, with I/E > 1. Using block lipid transport-1 to block scavenger receptor class B type I (SR-BI)-mediated flux with different acceptors, we determined that 50% to 70% of efflux was via SR-BI. With HDL, 90% of influx was via SR-BI, whereas with LDL or serum, 20% of influx was SR-BI-mediated. Cholesterol-enriched hepatoma cells produced increased efflux without a change in influx, resulting in reduced I/E. The assay was applied to cholesterol-normal and -enriched mouse peritoneal macrophages exposed to serum or LDL. The enrichment enhanced efflux without shifts in influx. With cholesterol-enriched macrophages, HDL efflux was enhanced and influx was greatly reduced. With all lipoproteins, cholesterol enrichment of murine peritoneal macrophages led to a reduced I/E. We conclude that this assay can simultaneously and accurately quantitate cholesterol bidirectional flux and can be applied to a variety of cells exposed to isolated lipoproteins or serum.

Elevated HDL Cholesterol is Functionally Ineffective in Cardiac Transplant Recipients: Evidence for Impaired Reverse Cholesterol Transport

BACKGROUND

Cardiac transplant recipients frequently have high plasma HDL levels but it is unclear whether these promote a cardioprotective profile.

METHODS

Parameters of reverse cholesterol transport and endothelial function were compared in 25 cardiac transplant recipients with low (<1.4 mmol/L; n=11) or high (>1.4 mmol/L; n=14) plasma levels of HDL and in a reference healthy group.

RESULTS

Patients with high HDL had lower levels of triglyceride and prebeta1-HDL and a higher proportion of large HDL particles. When normalized to apoA-I content, non-ABCA1-dependent cholesterol efflux from RAW 264.7 macrophage cells to plasma from high HDL patients was 33% lower when compared to plasma from patients with low HDL, whereas ABCA1-dependent cholesterol efflux was not impaired. Forearm vascular responses to acetylcholine and sodium nitroprusside were not influenced by HDL levels in these patients. Compared to a reference healthy group (n=26), cardiac transplant recipients had higher levels of triglyceride, lower levels of prebeta1-HDL and LCAT, and lower activities of cholesteryl ester transfer protein and phospholipid transfer protein.

CONCLUSIONS

Hyperalphalipoproteinaemia in cardiac transplant recipients is associated with the formation of partially dysfunctional HDL. We conclude that high levels of HDL may not confer cardioprotection in this group of patients.

Serum cholesterol efflux potential in postmenopausal women treated with isolated isoflavones

OBJECTIVE

Based on the low cardiovascular risk in Asian populations, phytoestrogens are believed to provide vascular benefits. To elucidate the mechanisms behind the possible cardiovascular effects of phytoestrogens, we evaluated reverse cholesterol transport by assessing the capacity of serum to promote cholesterol efflux in postmenopausal women treated with isolated isoflavones.

DESIGN

Thirty postmenopausal women were treated in a randomized, placebo-controlled, crossover trial with isoflavones or placebo for 3 months interrupted by a 2-month washout period. Serum samples were collected before and after each treatment period, and the cholesterol efflux potential was investigated by using H-cholesterol--labeled Fu5AH cells in culture.

RESULTS

Serum promoted 20.2% +/- 3.0% and 19.9% +/- 3.4% (mean +/- SD) cholesterol efflux after isoflavonoid treatment and after placebo treatment, respectively. Thus, the isoflavone treatment did not affect serum cholesterol efflux. We also studied separately women who produced high concentrations of the isoflavone metabolite equol into serum because some studies suggest that equol could exert favorable vascular effects. However, there was no difference in serum cholesterol efflux capacity between the equol producers (n = 15) and non-equol producers (n = 15).

CONCLUSIONS

In conclusion, isoflavone treatment did not affect serum cholesterol efflux potential in postmenopausal women. Based on our findings, isolated isoflavones do not provide vascular benefits by improving cholesterol efflux.

Increased cellular cholesterol efflux in glycogen storage disease type Ia mice: A potential mechanism that protects against premature atherosclerosis

Glycogen storage disease type Ia (GSD-Ia) patients manifest a pro-atherogenic lipid profile but are not at elevated risk for developing atherosclerosis. Serum phospholipid, which correlates positively with the scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux, and apolipoprotein A-IV and E, acceptors for ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol transport, are increased in GSD-Ia mice. Importantly, sera from GSD-Ia mice are more efficient than sera from control littermates in promoting SR-BI- and ABCA1-mediated cholesterol effluxes. As the first step in reverse cholesterol transport, essential for cholesterol homeostasis, these observations provide one explanation why GSD-Ia patients are apparently protected against premature atherosclerosis.

Paraoxonase 1 (PON1) enhances HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter in association with increased HDL binding to the cells: a possible role for lysophosphatidylcholine

We investigated the role of HDL-associated paraoxonase 1 (PON1) in HDL-mediated macrophage cholesterol efflux by using HDL derived from wild type mice (Control-HDL), from human PON1-transgenic mice (HDL-PON1Tg) or from PON1-knockout mice (HDL-PON1(0)). Cholesterol efflux from mouse peritoneal macrophages (MPM) or from J774 A.1 macrophage cell line by HDL-PON1Tg, was significantly increased (by 60%) compared to HDL-PON1(0). We demonstrated that this PON1 effect was associated with an increased HDL binding to the cells, as the binding of HDL-PON1Tg (or HDL-PON1(0) that was enriched with PON1) was increased by 50% compared to that of HDL-PON1(0). Using either a cAMP analogue, to increase ABCA1 receptor expression, or rabbit anti-mouse SR-BI specific antibody to block the SR-BI receptor, PON1 stimulation of HDL binding and of HDL-mediated macrophage cholesterol efflux, were both found to involve the ABCA1 transporter. Studies with PON1 specific inhibitors revealed that PON1 activity was required for its stimulation of HDL-mediated macrophage cholesterol efflux. Upon incubation of macrophages with Control-HDL or with HDL-PON1Tg, macrophage lysophosphatidylcholine (LPC) content was increased by 3.7- and 7.5-fold, respectively. Such an LPC enrichment of macrophages resulted in up to 60% increased HDL binding to the cells, and a 41% increased HDL-mediated cholesterol efflux. Similarly, macrophage loading with LPC (by either adding LPC, or PON1 or phospholipase A(2)) significantly increased apolipoprotein A-I (apoA-I) mediated cholesterol efflux by 104, 65 and 56%, respectively, in ABCA1 overexpressing macrophages. We conclude that HDL-associated PON1 may contribute to the attenuation of atherosclerosis development by its ability to act on macrophage phospholipids, to form LPC, in turn, stimulates HDL binding and HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter.

Paraoxonases 1, 2, and 3, oxidative stress, and macrophage foam cell formation during atherosclerosis development

Paraoxonases PON1 and PON3, which are both associated in serum with HDL, protect the serum lipids from oxidation, probably as a result of their ability to hydrolyze specific oxidized lipids. The activity of HDL-associated PON1 seems to involve an activity (phospholipase A2-like activity, peroxidase-like activity, lactonase activity) which produces LPC. To study the possible role of PON1 in macrophage foam cell formation and atherogenesis we used macrophages from control mice, from PON1 knockout mice, and from PON1 transgenic mice. Furthermore, we analyzed PON1-treated macrophages and PON1-transfected cells to demonstrate the contribution of PON1 to the attenuation of macrophage cholesterol and oxidized lipid accumulation and foam cell formation. PON1 was shown to inhibit cholesterol influx [by reducing the formation of oxidized LDL (Ox-LDL), increasing the breakdown of specific oxidized lipids in Ox-LDL, and decreasing macrophage uptake of Ox-LDL]. PON1 also inhibits cholesterol biosynthesis and stimulates HDL-mediated cholesterol efflux from macrophages. PON2 and PON3 protect against oxidative stress, with PON2 acting mainly at the cellular level. Whereas serum PON1 and PON3 were inactivated under oxidative stress, macrophage PON2 expression and activity were increased under oxidative stress, probably as a compensatory mechanism against oxidative stress. Intervention to increase the paraoxonases (cellular and humoral) by dietary or pharmacological means can reduce macrophage foam cell formation and attenuate atherosclerosis development.

Tibolone activates nitric oxide synthesis in human endothelial cells

After the unexpected findings of the Women's Health Initiative trial, indicating that traditional cardiovascular risk markers fail to predict the effects of hormone replacement therapy, it is of interest to characterize how steroids act on vascular cells. This is particularly important for tissue-specific drugs such as tibolone, whose actions may differ from other preparations. Because nitric oxide (NO) is a key regulator of vascular tone and atherogenesis, we studied its regulation by tibolone and its metabolites on human endothelial cells. Tibolone and its estrogenic metabolites (3alpha- and 3beta-OH tibolone) activate NO synthesis by recruiting functional estrogen receptors, whereas the progestogenic/androgenic metabolite (Delta(4) isomer) has no effect. During prolonged exposures, tibolone and the estrogenic compounds enhance the expression of endothelial NO synthase (eNOS). In addition, tibolone is able to induce rapid activation of eNOS, leading to rapid increases in the release of NO. Relevant for its clinical effects, the sulfated metabolites of tibolone are also effective in activating eNOS. Different from estrogen, rapid activation of eNOS does not rely on recruitment of phosphatidylinositol-3 kinase but rather on MAPK-dependent cascades. These results help to understand the mechanisms of action of tibolone on the cardiovascular system and have relevant clinical implications.

Estrogens, lipoproteins, and cardiovascular risk factors: an update following the randomized placebo-controlled trials of hormone-replacement therapy

PURPOSE OF REVIEW

The effects of hormone-replacement therapy on cardiovascular risk factors are examined. In an attempt to explain the results of recent randomized controlled trials in which no benefit of hormone-replacement therapy for postmenopausal women has been observed,

RECENT FINDINGS

Changes in lipoproteins in response to hormone-replacement therapy have now been analysed for both primary and secondary prevention studies. In none of the large randomized controlled trials was there any effect of hormone-induced changes in low-density lipoprotein, high-density lipoprotein, or triglyceride on clinical outcome. Further detailed studies of lipoprotein metabolism have not revealed any adverse effect of hormone-replacement therapy. Recent analysis of the Heart Estrogen/Progestin-Replacement Study data suggests hormone-replacement therapy reduces the risk of developing diabetes. The effect of hormone-replacement therapy on inflammatory markers and on flow-mediated dilatation is largely beneficial, although the effect on flow-mediated dilatation is modulated according to endothelial function, which is adversely affected by known risk factors, including age and presence of atherosclerosis. In this respect the work on polymorphisms of estrogen receptor-alpha may in due course help to define those women who would benefit most from use of estrogen. Crucially, oral but not transdermal hormone-replacement therapy increases activated protein C resistance independently of the presence of factor V Leiden. This effect increases the risk of venous thromboembolic events, which is reflected in the results of a hospital case control study of thromboembolism.

SUMMARY

Despite the outcome of the hormone-replacement therapy trials, recent work has confirmed the putative antiatherogenic effects of hormone-replacement therapy on lipoprotein metabolism. Metabolic differences of route of administration of estrogen, particularly on haemostatic variables, may explain this clinical paradox, which continues to be an important research area.