Prevention of in vitro low-density lipoprotein oxidation by an albumin-containing Lp A-I subfraction

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

Alterations in lipoprotein defense against oxidative stress in metabolic syndrome

Metabolic syndrome (MetS) is a high-risk condition for premature atherosclerotic vascular disease. Patients with MetS display a lipoprotein profile in which dense low-density lipoproteins (LDL), which are more susceptible to oxidation, predominate. Oxidation of lipoproteins can be attenuated in vivo by enzymatic and nonenzymatic antioxidant defenses, but high-density lipoproteins (HDL) play a key role in the protection of LDL from oxidation. Such activity depends on the presence of apolipoproteins (apoA-I, apoA-II, apoA-IV, apoE) and enzymes (paraoxonase 1, platelet activating factor-acetylhydrolase, lecithin:cholesterol acyltransferase, glutathione peroxidase). The impairment of HDL antioxidative activity in MetS is partly related to an enrichment of small HDL in triglycerides and their depletion in cholesteryl esters, to the replacement of apoA-I by serum amyloid A, and to glycation and oxidation of apoA-I. Therapeutic normalization of the quantity and the quality of HDL particles may constitute a novel approach to attenuate atherosclerosis and cardiovascular risk in MetS.

Therapeutic elevation of HDL-cholesterol to prevent atherosclerosis and coronary heart disease

Innovative pharmacological approaches to raise anti-atherogenic high-density lipoprotein-cholesterol (HDL-C) are currently of considerable interest, particularly in atherogenic dyslipidemias characterized by low levels of HDL-C, such as type 2 diabetes, the metabolic syndrome, and mixed dyslipidemia, but equally among individuals with or at elevated risk for premature cardiovascular disease (CVD). Epidemiological and observational studies first demonstrated that HDL-C was a strong, independent predictor of coronary heart disease (CHD) risk, and suggested that raising HDL-C levels might afford clinical benefit. Accumulating data from clinical trials of pharmacological agents that raise HDL-C levels have supported this concept. In addition to the pivotal role that HDL-C plays in reverse cholesterol transport and cellular cholesterol efflux, HDL particles possess a spectrum of anti-inflammatory, anti-oxidative, anti-apoptotic, anti-thrombotic, vasodilatory and anti-infectious properties, all of which potentially contribute to their atheroprotective nature. Significantly, anti-atherogenic properties of HDL particles are attenuated in common metabolic diseases that are characterized by subnormal HDL-C levels, such as type 2 diabetes and metabolic syndrome. Inhibition of cholesteryl ester transfer protein (CETP), a key player in cholesterol metabolism and transport, constitutes an innovative target for HDL-C raising. In lipid efficacy trials, 2 CETP inhibitors-JTT-705 and torcetrapib-induced marked elevation in HDL-C levels, with torcetrapib displaying greater efficacy. Moreover, both agents attenuate aortic atherosclerosis in cholesterol-fed rabbits. Clinical trial data demonstrating the clinical benefits of these drugs on atherosclerosis and CHD are eagerly awaited.

Metabolic syndrome is associated with elevated oxidative stress and dysfunctional dense high-density lipoprotein particles displaying impaired antioxidative activity

A metabolic syndrome (MetS) phenotype is characterized by insulin-resistance, atherogenic dyslipidemia, oxidative stress, and elevated cardiovascular risk and frequently involves subnormal levels of high-density lipoprotein (HDL) cholesterol. We evaluated the capacity of physicochemically distinct HDL subfractions from MetS subjects to protect low-density lipoprotein against oxidative stress.MetS subjects presented an insulin-resistant phenotype, with central obesity and elevation in systolic blood pressure and plasma triglyceride, LDL-cholesterol, apolipoprotein B, glucose, and insulin levels. Systemic oxidative stress, assessed as plasma 8-isoprostanes, was significantly higher (3.7-fold) in MetS subjects (n = 10) compared with nonobese normolipidemic controls (n = 11). In MetS, small, dense HDL3a, 3b, and 3c subfractions possessed significantly lower specific antioxidative activity (up to -23%, on a unit particle mass basis) than their counterparts in controls. In addition, HDL2a and 3a subfractions from MetS patients possessed lower total antioxidative activity (up to -41%, at equivalent plasma concentrations). The attenuated antioxidative activity of small, dense HDL subfractions correlated with systemic oxidative stress and insulin resistance and was associated with HDL particles exhibiting altered physicochemical properties (core triglyceride enrichment and cholesteryl ester depletion). We conclude that antioxidative activity of small, dense HDL subfractions of altered chemical composition is impaired in MetS and associated with elevated oxidative stress and insulin resistance. Induction of selective increase in the circulating concentrations of dense HDL subfractions may represent an innovative therapeutic approach for the attenuation of high cardiovascular risk in MetS.

HDL: a recipe for longevity

The concentration of high density lipoprotein cholesterol (HDL-C) has been found to be a powerful negative predictor of premature coronary heart disease (CHD) and stroke in human prospective population studies. Evidence of the protective properties of HDLs has also been documented in the elderly and their offspring. HDLs mediate several functions that provide an insight into their potential anti-atherogenic mechanisms. Intervention strategies to prevent CHD have generally focused on lowering low-density lipoprotein cholesterol (LDL-C). However, several lifestyle and pharmacological interventions have the capacity to raise the level of HDL-C. As data accumulate on the protective role of HDLs, there is growing support for interventions that act to raise HDL-C concentrations.

Validation of a novel ELISA for measurement of MDA-LDL in human plasma

The involvement of oxidatively modified low density lipoprotein (LDL) in the development of CHD is widely described. We have produced two antibodies, recognizing the lipid oxidation product malondialdehyde (MDA) on whole LDL or ApoB-100. The antibodies were utilized in the development of an ELISA for quantitation of MDA-LDL in human plasma. Intra- and inter-assay coefficients of variation (% CV) were measured as 4.8 and 7.7%, respectively, and sensitivity of the assay as 0.04 micro g/ml MDA-LDL. Recovery of standard MDA-LDL from native LDL was 102%, indicating the ELISA to be specific with no interference from other biomolecules. Further validation of the ELISA was carried out against two established methods for measurement of lipid peroxidation products, MDA by HPLC and F(2)-isoprostanes by GC-MS. Results indicated that MDA-LDL is formed at a later stage of oxidation than either MDA or F(2)-isoprostanes. In vivo analysis demonstrated that the ELISA was able to determine steady-state concentrations of plasma MDA-LDL (an end marker of lipid peroxidation). A reference range of 34.3 +/- 8.8 micro g/ml MDA-LDL was established for healthy individuals. Further, the ELISA was used to show significantly increased plasma MDA-LDL levels in subjects with confirmed ischemic heart disease, and could therefore possibly be of benefit as a diagnostic tool for assessing CHD risk.

High density lipoproteins (HDLs) and atherosclerosis; the unanswered questions

The concentration of high density lipoprotein-cholesterol (HDL-C) has been found consistently to be a powerful negative predictor of premature coronary heart disease (CHD) in human prospective population studies. There is also circumstantial evidence from human intervention studies and direct evidence from animal intervention studies that HDLs protect against the development of atherosclerosis. HDLs have several documented functions, although the precise mechanism by which they prevent atherosclerosis remains uncertain. Nor is it known whether the cardioprotective properties of HDL are specific to one or more of the many HDL subpopulations that comprise the HDL fraction in human plasma. Several lifestyle and pharmacological interventions have the capacity to raise the level of HDL-C, although it is not known whether all are equally protective. Indeed, despite the large body of information identifying HDLs as potential therapeutic targets for the prevention of atherosclerosis, there remain many unanswered questions that must be addressed as a matter of urgency before embarking wholesale on HDL-C-raising therapies as strategies to prevent CHD. This review summarises what is known and highlights what we still need to know.

Kinetics of lipid peroxidation in mixtures of HDL and LDL, mutual effects

In view of the proposed central role of LDL oxidation in atherogenesis and the established role of HDL in reducing the risk of atherosclerosis, several studies were undertaken to investigate the possible effect of HDL on LDL peroxidation. Since these investigations yielded contradictory results, we have conducted systematic kinetic studies on the oxidation in mixtures of HDL and LDL induced by different concentrations of copper, 2, 2'-azo bis (2-amidinopropane) hydrochloride (AAPH) and myeloperoxidase (MPO). These studies revealed that oxidation of LDL induced either by AAPH or MPO is inhibited by HDL under all the studied conditions, whereas copper-induced oxidation of LDL is inhibited by HDL at low copper/lipoprotein ratio but accelerated by HDL at high copper/lipoprotein ratio. The antioxidative effects of HDL are only partially due to HDL-associated enzymes, as indicated by the finding that reconstituted HDL, containing no such enzymes, inhibits peroxidation induced by low copper concentration. Reduction of the binding of copper to LDL by competitive binding to the HDL also contributes to the antioxidative effect of HDL. The acceleration of copper-induced oxidation of LDL by HDL may be attributed to the hydroperoxides formed in the "more oxidizable" HDL, which migrate to the "less oxidizable" LDL and enhance the oxidation of the LDL lipids induced by bound copper. This hypothesis is supported by the results of experiments in which native LDL was added to oxidizing lipoprotein at different time points. When the native LDL was added prior to decomposition of the hydroperoxides in the oxidizing lipoprotein, the lag preceding oxidation of the LDL was much shorter than the lag observed when the native LDL was added at latter stages, after the level of hydroperoxides became reduced due to their copper-catalyzed decomposition. The observed dependence of the interrelationship between the oxidation of HDL and LDL on the oxidative stress should be considered in future investigations regarding the oxidation of lipoprotein mixtures.

Kinetic analysis of copper-induced peroxidation of HDL, autoaccelerated and tocopherol-mediated peroxidation

Comparison of the kinetic profiles of copper-induced peroxidation of HDL and LDL at different copper concentrations reveals that under all the studied experimental conditions HDL is more susceptible to oxidation than LDL. The mechanism responsible for HDL oxidation is a complex function of the copper/HDL ratio and of the tocopherol content of the HDL. At high copper concentrations, the kinetic profiles were similar to those observed for LDL oxidation, namely, relatively rapid accumulation of oxidation products, via an autoaccelerated, noninhibited mechanism, was preceded by an initial "lag phase." Under these conditions, the maximal peroxidation rate (V(max)) of HDL and LDL depended similarly on the molar ratio of bound copper/lipoprotein. Analysis of this dependency in terms of the binding characteristics of copper to lipoprotein, yielded similar dissociation constant (K = 10(-6) M) but different maximal binding capacities for the two lipoproteins (8 Cu(+2)/HDL as compared to 17 Cu(+2)/LDL). Given the size difference between HDL and LDL, these results imply that the maximal surface density of bound copper is at least 2-fold higher for HDL than for LDL. This difference may be responsible for the higher susceptibility of HDL to copper-induced oxidation in the presence of high copper concentrations. At relatively low copper concentrations, the kinetic profile of HDL oxidation was biphasic, similar to but more pronounced than the biphasic kinetics observed for the oxidation of LDL lipids at the same concentration of copper. Our results are consistent with the hypothesis that the first phase of rapid oxidation occurs via a tocopherol-mediated-peroxidation (TMP) mechanism. Accordingly, enrichment of HDL with tocopherol resulted in enhanced accumulation of hydroperoxides during the first phase of copper-induced oxidation. Notably, the maximal accumulation during the first phase decreased upon increasing the ratio of bound copper/HDL. This behavior can be predicted theoretically for peroxidation via a TMP mechanism, in opposition to autoaccelerated peroxidation. The possible pathophysiological significance of these findings is discussed.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

Glutathione-dependent ascorbate recycling activity of rat serum albumin

An efficient regeneration of vitamin C (ascorbate) from its oxidized byproduct, dehydroascorbate (DHAA), is necessary to maintain sufficient tissue levels of the reduced form of the vitamin. Additionally, the recycling may be more significant in mammals, such as guinea pigs and humans, who have lost the ability to synthesize ascorbate de novo, than it is in most other mammals who have retained the ability to synthesize the vitamin from glucose. Both a chemical and an enzymatic reduction of DHAA to ascorbate have been proposed. Several reports have appeared in which proteins, including thioltransferase, protein disulfide isomerase, and 3-alpha-hydroxysteroid dehydrogenase, characterized for other activities have been identified as having DHAA reductase activity in vitro. Whether these previously characterized proteins catalyze the reduction of DHAA in vivo is unclear. In the present study, a 66 kD protein was purified strictly on the basis of its DHAA-reductase activity and was identified as rat serum albumin. The protein was further characterized and results support the suggestion that serum albumin acts as an antioxidant and exerts a significant glutathione-dependent DHAA-reductase activity that may be important in the physiologic recycling of ascorbic acid.

Susceptibility to oxidation of copper-induced plasma lipoproteins from Japanese eel: protective effect of vitellogenin on the oxidation of very low density lipoprotein

The susceptibility to oxidation of copper-induced plasma lipoproteins from Japanese eel Anguilla japonica was examined with the guidance of thiobarbituric acid-reactive substances (TBARS). The TBARS values of copper-induced plasma lipoproteins increased with increasing the lipid-to-apolipoprotein ratios and very low density lipoprotein (VLDL) exhibited the highest TBARS value. On the other hand, vitellogenin, estrogen-induced precursor of egg yolk proteins, was resistant to copper-induced oxidation and seemed to chelate low concentrations of copper ion. Vitellogenin also protected the copper-induced oxidation of VLDL because of its antioxidant function. Vitellogenin seemed to serve as transition metals-binding lipoprotein by which free-radical reactions in the oocytes were extensively depressed.

Oxidation of low-density lipoprotein and atherosclerosis in chronic renal failure

The major cause of death in patients with end-stage renal failure receiving renal replacement therapy is cardiovascular disease. Oxidation of low-density lipoprotein (LDL) is recognized as a key early stage in the development of atherosclerosis, leading to uptake of LDL by the macrophage scavenger receptor and hence to foam cell formation. However, several studies have suggested that the susceptibility of LDL to oxidation is not increased in chronic renal failure. We propose a number of mechanisms which may lead to increased lipoprotein oxidation in vivo, and hence contribute to increased atherosclerosis in renal failure.

Increased selective uptake in vivo and in vitro of oxidized cholesteryl esters from high-density lipoprotein by rat liver parenchymal cells

Oxidation of low-density lipoprotein (LDL) leads initially to the formation of LDL-associated cholesteryl ester hydroperoxides (CEOOH). LDL-associated CEOOH can be transferred to high-density lipoprotein (HDL), and HDL-associated CEOOH are rapidly reduced to the corresponding hydroxides (CEOH) by an intrinsic peroxidase-like activity. We have now performed in vivo experiments to quantify the clearance rates and to identify the uptake sites of HDL-associated [3H]Ch18:2-OH in rats. Upon injection into rats, HDL-associated [3H]Ch18:2-OH is removed more rapidly from the circulation than HDL-associated [3H]Ch18:2. Two minutes after administration of [3H]Ch18:2-OH-HDL, 19.6 +/- 2.6% (S.E.M.; n = 4) of the label was taken up by the liver as compared with 2.4 +/- 0.25% (S.E.M.; n = 4) for [3H]Ch18:2-HDL. Organ distribution studies indicated that only the liver and adrenals exhibited preferential uptake of [3H]Ch18:2-OH as compared with [3H]Ch18:2, with the liver as the major site of uptake. A cell-separation procedure, employed 10 min after injection of [3H]Ch18:2-OH-HDL or [3H]Ch18:2-HDL, demonstrated that within the liver only parenchymal cells take up HDL-CE by the selective uptake pathway. Selective uptake by parenchymal cells of [3H]Ch18:2-OH was 3-fold higher than that of [3H]Ch18:2, while Kupffer and endothelial cell uptake of the lipid tracers reflected HDL holoparticle uptake (as analysed with iodinated versus cholesteryl ester-labelled HDL). The efficient uptake of [3H]Ch18:2-OH by parenchymal cells was coupled to a 3-fold increase in rate of radioactive bile acid secretion from [3H]Ch18:2-OH-HDL as compared with [3H]Ch18:2-HDL. In vitro studies with freshly isolated parenchymal cells showed that the association of [3H]Ch18:2-OH-HDL at 37 degrees C exceeded [3H]Ch18:2-HDL uptake almost 4-fold. Our results indicate that HDL-associated CEOH are efficiently and selectively removed from the blood circulation by the liver in vivo. The selective liver uptake is specifically exerted by parenchymal cells and coupled to a rapid biliary secretion pathway. The liver uptake and biliary secretion route may allow HDL to function as an efficient protection system against potentially atherogenic CEOOH.

Oxidation of lipoprotein(a) and low density lipoprotein containing density gradient ultracentrifugation fractions

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for atherosclerotic cardiovascular disease. It is thought that the atherogenicity of Lp(a) is mediated both through its LDL-like properties and its plasminogen-like properties. In this study we have investigated the LDL-like atherogenic properties of Lp(a) by comparing the susceptibility to in vitro oxidation of Lp(a) and LDL isolated from the same subject. The subjects studied varied widely in plasma Lp(a) concentration (331-1829 mg/l) and Lp(a) phenotype (from B to S4). Lipoproteins are notoriously unstable in vitro, consequently differences in in vitro handling could influence oxidizability. Therefore, the isolation and handling of Lp(a) and LDL were performed in an identical fashion. Lp(a) and LDL containing fractions were obtained by density gradient ultracentrifugation. Separate fractions containing various amounts of Lp(a) and LDL, quantitated by measuring both Lp(a) and apo B-100, were subsequently oxidized on equimolar apo B-100 basis. Despite large differences in the Lp(a)/apo B-100 ratio of the various fractions (ranging from 5.3 +/- 1.7 to 0.2 +/- 0.1) they showed quite similar oxidation characteristics. The most dense Lp(a) containing fractions showed an aberrant susceptibility to oxidation. Subsequent gel filtration and reconstitution experiments showed that this was due to protein (i.e., albumin) contamination. Removal of excess protein revealed an oxidation pattern similar to that of LDL. It is concluded that the susceptibility of Lp(a) to lipid-peroxidation is similar to that of LDL when isolated simultaneously and in the same way from the same subject. Thus, lipid-peroxidation of Lp(a) is not influenced by the presence of its distinguishing apolipoprotein(a).

High density lipoproteins and coronary heart disease

An inverse relationship between the concentration of high density lipoprotein (HDL) cholesterol and the development of coronary heart disease (CHD) is well established. It is unclear from the human studies whether this relationship reflects an ability of HDLs to protect against coronary disease or whether a low HDL in coronary patients is simply an epiphenomenon. Recent studies of transgenic mice, however, indicate that HDLs are directly antiatherogenic. The mechanism of the protection is unknown but may relate both to an involvement of HDLs in plasma cholesterol transport and to a range of non-lipid transport functions of HDLs. It is also unclear from human studies whether specific HDL subpopulations have differing abilities to protect against CHD, although such specificity is suggested from studies of transgenic mice. There is circumstantial evidence that elevating the concentration of HDL cholesterol in human subjects translates into a reduced coronary risk, although it should be stressed that there are still no reports of studies designed specifically to address this issue.