Binding of transition metals by apolipoprotein A-I-containing plasma lipoproteins: inhibition of oxidation of low density lipoproteins

Association of ferritin and lipids with C-reactive protein

C-reactive protein (CRP) and lipids (e.g., low-density lipoprotein [LDL]) are both markers of cardiovascular disease risk, yet they are not highly correlated. Oxidative stress of lipids induced by iron may play a role in vascular inflammation, as indicated by CRP. The purpose of this study was to examine, in a representative sample of United States adults, the relation between ferritin, lipids, and CRP. We analyzed data on adults (aged > or =25 years) in the National Health and Nutrition Examination Survey III, a national public-use data set collected between 1988 and 1994. Ferritin, total cholesterol, LDL, high-density lipoprotein, and ferritin-lipid combinations were analyzed in relation to CRP in age-, gender-, and race-adjusted models as well as models with other potential confounding variables. In adjusted models, neither elevated ferritin (odds ratio [OR] 1.11, 95% confidence interval [CI] 0.94 to 1.32) nor elevated LDL was significantly associated with elevated CRP (OR 1.03, 95% CI 0.79 to 1.33). Patients with elevated ferritin and elevated LDL were more likely to have elevated CRP (OR 1.68; 95% CI 1.06 to 2.68). Patients with elevated ferritin and low high-density lipoprotein were also more likely to have elevated CRP (OR 1.71; 95% CI 1.28 to 2.27). These results suggest that both iron and lipids induce inflammation. Future research needs to focus on preventive medicine to decrease iron in patients with elevated lipids.

Serum oxidized low-density lipoproteins in rheumatoid arthritis

OBJECTIVE

The aim of this study was to measure serum oxidized low-density lipoprotein (Ox-LDL) levels in patients with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE) and to identify any association with clinical variables.

METHODS

We studied 126 female patients (27 with active RA, 27 with inactive RA, 72 with SLE). One hundred fifteen age-matched healthy women (76 for RA, 39 for SLE) with no clinical or laboratory evidence of disease served as normal controls. Serum Ox-LDL levels were measured with a commercial enzyme-linked immunosorbent assay kit (Mercodia, Sweden).

RESULTS

The serum Ox-LDL levels were significantly higher in patients with active RA (P<0.05) or SLE (P<0.01) than age-matched controls and significantly higher in patients with active RA than with inactive RA (P<0.01). The levels of serum total cholesterol and LDL were significantly lower in patients with RA than in age-matched controls (P<0.01). There was no correlation among serum Ox-LDL levels and inflammatory markers (erythrocyte sedimentation rate, C-reactive protein) or rheumatoid factors in patients with RA.

CONCLUSIONS

Compared with healthy women, those with SLE or active RA had increased serum Ox-LDL levels, which may contribute to the increased risk of cardiovascular disease in this patient group.

Copper-induced oxidative damage on astrocytes: protective effect exerted by human high density lipoproteins

In the present study, we confirmed that copper ions induce oxidative damage in human astrocytes in culture, as demonstrated by the significant increase in the levels of hydroperoxides and in the fluorescence intensity of the fluorescent probe dichloro-dihydrofluorescein diacetate (H(2)DCFDA). The compositional changes were associated with a significant decrease in cell viability in astrocytes treated with 10 microM Cu(++) with respect to control cells. Astrocytes incubated with copper ions in the presence of high density lipoproteins (HDL) isolated from plasma of normolipemic subjects showed lower levels of hydroperoxides and a higher cell viability with respect to cells oxidized alone. Moreover, a significant decrease in the levels of hydroperoxides was observed in oxidized astrocytes treated with HDL. These results demonstrate that HDL exert a protective role against lipid peroxidation. The protective effect could be related to the ability of HDL to bind metal ions at the lipoprotein surface and/or to a stimulation of the efflux of lipid hydroperoxides from cell membranes as demonstrated in other cell types. Oxidative damage of astrocytes was induced at a copper concentration similar to that observed in cerebrospinal fluid (CSF) of patients affected by neurodegenerative diseases such as Alzheimer's (AD) and Parkinson's diseases (PD). Lipoprotein particles similar for density and chemical composition to plasma HDL were recently isolated in human CSF, therefore, the protective role exerted by HDL against Cu(++)-induced oxidative damage of astrocytes could be of physiological relevance.

Small, dense HDL particles exert potent protection of atherogenic LDL against oxidative stress

OBJECTIVE

The relationship of the structural and functional heterogeneity of HDL particles to protection of LDL against oxidative stress is indeterminate.

METHODS AND RESULTS

HDL subfractions of defined physicochemical properties were isolated by density gradient ultracentrifugation from normolipidemic human serum (n=8), and their capacity to protect LDL from oxidation was evaluated. Under mild oxidative stress induced by AAPH or Cu(II), HDL subfractions (at equal cholesterol or protein concentration or equal particle number) significantly decreased LDL oxidation rate (-20% to -85%) in the propagation phase (234 nm), which was prolonged by up to 82% with decreased maximal diene formation. Antioxidative activity of HDL subfractions increased with increment in density, as follows: HDL2b<HDL2a<HDL3a<HDL3b<HDL3c (confirmed by thiobarbituric acid-reactive substance content and LDL electrophoretic mobility). Concordantly, antioxidative activity of small HDL prepared by FPLC was significantly higher (+56%) than that of large HDL. Antioxidative action of HDL subfractions was primarily associated with inactivation of LDL lipid hydroperoxides. The potent protective activity of small HDL could not be accounted for exclusively by enzymatic activities (PON1, platelet-activating factor acetylhydrolase, and lecithin-cholesterol acyltransferase).

CONCLUSIONS

Small, dense HDL exhibit potent antioxidant activity, which may arise from synergy in inactivation of oxidized LDL lipids by enzymatic and nonenzymatic mechanisms, in part reflecting distinct intrinsic physicochemical properties.

[From organophosphate compound toxicity to atherosclerosis: role of paraoxonase 1]

PURPOSE

Paraoxonase 1 is an ubiquitous human serum and tissue esterase known to hydrolyse organophosphorous compounds. It seems to be implicated in various vascular diseases.

CURRENT KNOWLEDGE AND KEY POINTS

Recently paraoxonase has been located on the surface of High Density Lipoproteins (HDL) which has directed studies towards its involvement in atherosclerosis. An antioxidant effect has been suggested from its structure rich in reducing amino acids (cysteine), which was confirmed on low density lipoproteins (LDL) first in vitro and then in vivo. Paraoxonase 1 hydrolyses an arachidonic acid derivative found on the surface of oxidised LDL known to participate in the essential initial step of atherogenesis. Clinically paraoxonase 1 activity is low when pathological vascular ageing occurs early (myocardial infarction) and when cardiovascular risk is high (diabetes mellitus, chronic renal failure, analphalipoproteinemia).

FUTURE PROSPECTS AND PROJECTS

The genetic polymorphism of this enzyme is one of the determinants of serum paraoxonase 1 activity variations. It could explain sensitivity differences in chronic organophosphate intoxications and has been suspected as a risk factor of vascular injury. A decrease of this enzyme activity with ageing could play a part in the high prevalence of cardiovascular diseases in the aged.

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.

Contribution of HDL-apolipoproteins to the inhibition of low density lipoprotein oxidation and lipid accumulation in macrophages

High-density lipoprotein (HDL) is known as a protective factor against atherosclerosis. However, whether HDL-apolipoproteins (apo-HDL) contribute to the protection in arterial cells remains unclear. The localization patterns of human apolipoproteins in atherosclerotic arteries were determined using immunohistochemical examination. The results indicate that several apolipoproteins are retained in component cells of the coronary artery walls. To elucidate the possible roles of apo-HDL in the protection of atherosclerotic lesion formation, we investigated the effects of apo-HDL on the formation of conjugated diene (CD) in a cell-free system and thiobarbituric acid-reactive substances (TBARS) in the medium of a macrophage-mediated LDL oxidation system. The results showed that apo-HDL significantly exerted an inhibitory effect on LDL lipid oxidation in vitro. In addition, apo-HDL decreased cholesterol influx but enhanced cholesterol efflux from J774 macrophages in a dose-dependent manner. These results are consistent with the notion that there is reduced intracellular lipid accumulation in apo-HDL treated macrophages. These data provide a direct evidence for apo-HDL in protecting LDL from oxidative modification and in reducing the accumulation of cholesterol and lipid droplets by J774 macrophages.

Homocysteine promotes the LDL oxidase activity of ceruloplasmin

Ceruloplasmin (CP) oxidises low density lipoprotein (LDL). The oxidising potential depends on the formation of Cu(+)-CP which is redox-cycled during oxidation. Homocysteine (HCY) reduces free Cu(2+), potentiating its cell-damaging property. We show that HCY enhanced LDL oxidation by CP, but did not activate the LDL oxidising potential of Cu(2+)-diamine oxidase. Selective removal of the redox-active Cu(2+) abolished the LDL oxidase activity of CP. However, HCY partially restored the LDL oxidase activity of redox-copper depleted CP, indicating that the remaining six copper atoms in CP may also be involved in the process. Spectroscopic and oxidation inhibition studies using the Cu(+)-reagent bathocuproine revealed that HCY induced Cu(+)-CP formation, thus promoting its LDL oxidase activity.

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.

Recombinant lipoproteins: lipoprotein-like lipid particles for drug targeting

Lipoproteins are endogenous particles that transport lipids through the blood to various cell types, where they are recognised and taken up via specific receptors. These particles are, therefore, excellent candidates for the targeted delivery of drugs to various tissues. For example, the remnant receptor and the asialoglycoprotein receptor (ASGPr), which are uniquely localised on hepatocytes, recognise chylomicrons and lactosylated high density lipopoteins (HDL), respectively. In addition, tumour cells of various origins overexpress the low density lipoprotein (LDL) receptor that recognises apolipoprotein E (apoE) on small triglyceride-rich particles and apoB-100 on LDL. Being endogenous, lipoproteins are biodegradable, do not trigger immune reactions, and are not recognised by the reticuloendothelial system (RES). However, their endogenous nature also hampers large-scale pharmaceutical application. In the past two decades, various research groups have successfully synthesised recombinant lipoproteins from commercially available natural and synthetic lipids and serum-derived or recombinant apolipoproteins, which closely mimic the metabolic behaviour of their native counterparts in animal models as well as humans. In this paper, we will summarise the studies that led to the development of these recombinant lipoproteins, and we will address the possibility of using these lipidic particles to selectively deliver a wide range of lipophilic, amphiphilic, and polyanionic compounds to hepatocytes and tumour cells. In addition, the intrinsic therapeutic activities of recombinant chylomicrons and HDL in sepsis and atherosclerosis will be discussed.

Serum paraoxonase is reduced in type 1 diabetic patients compared to non-diabetic, first degree relatives; influence on the ability of HDL to protect LDL from oxidation

Paraoxonase is a serum enzyme with an anti-oxidant function, protecting low density lipoproteins (LDL) from oxidative modifications. Diabetic patients are suggested to be at greater risk of oxidative stress, which may contribute to the significantly higher incidence of vascular disease in this population. Less efficient protection mechanisms may be one feature of the greater susceptibility to oxidation in diabetes. In this context, the present study examined the hypothesis that serum paraoxonase is reduced in type 1 (insulin-dependent) diabetic patients and that the reduction can affect the anti-oxidant capacity of HDL. Serum paraoxonase concentrations and activities were compared in type 1 patients and first degree, non-diabetic relatives with particular attention paid to the confounding effects of paraoxonase gene polymorphisms. In addition, the ability of HDL-paraoxonase to protect low density lipoproteins from oxidation was analysed in an in vitro system. Serum concentrations and enzyme activities of paraoxonase were significantly lower in type 1 patients compared to non-diabetic, first degree relatives. The differences were independent of promoter and coding region polymorphisms, which influence serum concentrations and activities of the enzyme. Overall, paraoxonase concentrations were a mean 13.3+/-4.5% lower (P<0.02) in type 1 patients. Specific activities did not differ between diabetic and non-diabetic groups. The concentration ratios of LDL cholesterol:paraoxonase (1.37+/-0.51 vs. 1.18+/-0.37, P=0.003) and apolipoprotein B:paraoxonase (0.84+/-0.33 vs. 0.71+/-0.40; P=0.012) were significantly higher in diabetic patients, consistent with a reduced capacity to protect LDL from oxidation. In vitro oxidation studies showed that a significantly higher level of lipid hydroperoxides was generated in LDL in the presence of HDL, containing paraoxonase levels equivalent to those of type 1 patients, compared to HDL containing paraoxonase levels equivalent to those of control subjects (mean difference 8.1%, P<0.05). The study demonstrates that serum concentrations of the antioxidant enzyme paraoxonase are significantly lower in type 1 (insulin-dependent) diabetic patients compared to non-diabetic, first-degree relatives, independently of known gene polymorphisms. Concentrations are reduced to an extent that can affect its anti-oxidant capacity. The results are consistent with the contention that modifications to serum paraoxonase in type 1 patients can increase risk of lipoprotein oxidation and, consequently, risk of vascular disease.

Lactoferrin in infant formulas: effect on oxidation

Lactoferrin is an iron transport protein present in human milk at an average concentration of 1.4 mg/mL. Commercially modified infant formulas based on cow's milk contain much lower amounts of lactoferrin (0.1 mg/mL lactoferrin) and soy based formulas have none. In addition to its role in iron transport, lactoferrin has bacteriostatic and bactericidal activities. Infant formulas are supplemented with relatively large amounts of iron (up to 12 mg/L). The effect of various concentrations of added lactoferrin and supplemental iron on lipid oxidation was tested in two different infant formulas. The extent of oxidation in the formulas as a function of time was determined by formation of hydroperoxides, production of hexanal, and fluorescence. On the basis of all three of these determinations, lactoferrin acted as an antioxidant in the absence and presence of different concentrations of supplemented iron. Lactoferrin inhibited oxidation in a concentration-dependent manner even at concentrations beyond its capacity to bind iron at its two high affinity binding sites. Lactoferrin can be used, therefore, as a dual purpose additive in infant formulas and similar food products for its antioxidant and its antimicrobial properties.

Infection and inflammation induce LDL oxidation in vivo

Epidemiological studies have shown an increased incidence of coronary artery disease in patients with chronic infections and inflammatory disorders. Because oxidative modification of lipoproteins plays a major role in atherosclerosis, the present study was designed to test the hypothesis that the host response to infection and inflammation induces lipoprotein oxidation in vivo. Lipoprotein oxidation was measured in 3 distinct models of infection and inflammation. Syrian hamsters were injected with bacterial lipopolysaccharide (LPS), zymosan, or turpentine to mimic acute infection, acute systemic inflammation, and acute localized inflammation, respectively. Levels of oxidized fatty acids in serum and lipoprotein fractions were measured by determining levels of conjugated dienes, thiobarbituric acid-reactive substances, and lipid hydroperoxides. Our results demonstrate a significant increase in conjugated dienes and thiobarbituric acid-reactive substances in serum in all 3 models. Moreover, LPS and zymosan produced a 4-fold to 6-fold increase in conjugated diene and lipid hydroperoxide levels in LDL fraction. LPS also produced a 17-fold increase in LDL content of lysophosphatidylcholine that is formed during the oxidative modification of LDL. Finally, LDL isolated from animals treated with LPS was significantly more susceptible to ex vivo oxidation with copper than LDL isolated from saline-treated animals, and a 3-fold decrease occurred in the lag phase of oxidation. These results demonstrate that the host response to infection and inflammation increases oxidized lipids in serum and induces LDL oxidation in vivo. Increased LDL oxidation during infection and inflammation may promote atherogenesis and could be a mechanism for increased incidence of coronary artery disease in patients with chronic infections and inflammatory disorders.

Infection and inflammation-induced proatherogenic changes of lipoproteins

Epidemiologic studies suggest a link between infection/inflammation and atherosclerosis. During the acute-phase response to infection and inflammation, cytokines induce tissue and plasma events that lead to changes in lipoprotein. Many of these changes are similar to those proposed to promote atherogenesis. The changes of lipoproteins during infection and inflammation are reviewed with a focus on those that are potentially proatherogenic. Hypertriglyceridemia, elevated triglyceride-rich lipoproteins, the appearance of small dense low-density lipoproteins, increased platelet-activating factor acetylhydrolase activity, and secretory phospholipase A(2), sphingolipid-enriched lipoproteins, and decreased high-density lipoprotein (HDL) cholesterol are changes that could promote atherogenesis. Moreover, alterations of proteins associated with HDL metabolism (e.g., paraoxonase, apolipoprotein A-I, lecithin:cholesterol acyltransferase, cholesterol ester transfer protein, hepatic lipase, phospholipid transfer protein, and serum amyloid A) could decrease the ability of HDL to protect against atherogenesis through antioxidation and reverse cholesterol transport mechanisms. These proatherogenic changes of lipoproteins may contribute to the link between infection/inflammation and atherosclerosis.

Smoking is associated with reduced serum paraoxonase activity and concentration in patients with coronary artery disease

BACKGROUND

Paraoxonase is an HDL-associated enzyme that protects lipoproteins from oxidative modifications. Smoking is a major cardiovascular risk factor that promotes lipid peroxidation. Cigarette smoke has been shown in vitro to inhibit paraoxonase. The present study examined the hypothesis that smoking is associated with modulated serum activities and concentrations of paraoxonase.

METHODS AND RESULTS

Coronary artery disease was assessed with the use of coronary arteriography in participants recruited from a hospital cardiology division. Medical and lifestyle data were obtained, and a fasting blood sample was provided. Three smoking categories were established (never, ex-smokers, and current smokers), and serum paraoxonase variables were compared among them. The activities and concentrations of paraoxonase were significantly lower in current than in never smokers. Ex-smokers had values comparable to those of never smokers. Ex-smokers who had recently stopped (<3 months) had activities and concentrations comparable to those of current smokers; values returned to the levels of never smokers within 2 years of cessation of smoking. Smoking status was an independent determinant of paraoxonase activity and concentration in multivariate analysis. Finally, lower paraoxonase was associated with more severe coronary disease and a reduced capacity to protect LDL from oxidation.

CONCLUSIONS

Smoking is independently associated with significant decreases in serum paraoxonase activities and concentrations, which normalize within a relatively short time of cessation. Lower serum paraoxonase is linked to more severe coronary artery disease and a lower antioxidant capacity. The data are consistent with the hypothesis that smoking modifies serum paraoxonase such that there is an increased risk of coronary artery disease due to a diminished capacity to protect lipoproteins from oxidative stress.

Presence of CuZn superoxide dismutase in human serum lipoproteins

It has previously been demonstrated that CuZn-superoxide dismutase (SOD) is secreted by several human cell lines. This suggests that the circulating enzyme derives from both hemolysis and peripheral tissues as a result of cellular secretion. In the present report, we evaluated the presence of CuZn-SOD in human serum lipoproteins by both enzyme-linked immunosorbent assay and Western blot analysis of immunoprecipitated lipoprotein samples. The distribution of CuZn-SOD activity among the different lipoprotein fractions was also determined by the xanthine/xanthine oxidase method. The results demonstrated that CuZn-SOD is noticeably present in serum lipoproteins and mainly in low and high density lipoproteins (LDL and HDL). Moreover, experiments performed by incubating CuZn-SOD with a lipid emulsion and subsequent separation of the lipid fraction by ultracentrifugation showed that this enzyme associates in a saturable manner with lipids. The CuZn-SOD bound to LDL and HDL could exert a physiological protective role against oxidative damage of these lipoprotein classes that carry out a crucial role in the cholesterol transport.

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.

High density lipoproteins (HDL) and the oxidative hypothesis of atherosclerosis

The oxidative hypothesis of atherosclerosis classically implies a central role for low density lipoprotein (LDL) oxidation. However, new antiatherogenic properties have been recognized for high density lipoproteins (HDL), apart from their ability to reverse cholesterol transport. Indeed, native HDL could protect LDL from oxidation, thereby minimizing the deleterious consequences of this process. Several mechanisms have been suggested to explain this protective role. Two HDL-associated enzymes, paraoxonase and PAF-acetylhydrolase, detoxify oxidized phospholipids produced by lipid peroxidation. In addition, HDL could reduce hydroperoxides to their corresponding hydroxides. It has also been suggested that HDL could inhibit oxidized LDL-induced transduction signals. However, in vivo HDL oxidation in the subendothelial space would favor the atherosclerotic process. Indeed, atherogenic properties of these oxidized HDL partly result from some loss of their cholesterol effluxing capacity and from an inactivation of the lecithin-cholesterol acyltransferase, which is a HDL-associated enzyme involved in reverse cholesterol transport. Finally, oxidized HDL could induce cholesterol accumulation in macrophages. Further in-depth investigation is needed to assess these antagonistic effects and their consequences for the atherosclerotic process.

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.

Increased atherosclerosis in ApoE and LDL receptor gene knock-out mice as a result of human cholesteryl ester transfer protein transgene expression

The plasma cholesteryl ester transfer protein (CETP) plays a major role in the catabolism of HDL cholesteryl ester (CE). CETP transgenic mice have decreased HDL cholesterol levels and have been reported to have either increased or decreased early atherosclerotic lesions. To evaluate the impact of CETP expression on more advanced forms of atherosclerosis, we have cross-bred the human CETP transgene into the apoE knock-out (apoE0) background with and without concomitant expression of the human apo A-I transgene. In this model the CETP transgene is induced to produce plasma CETP levels 5 to 10 times normal human levels. CETP expression resulted in moderately reduced HDL cholesterol (34%) in apoE0 mice and markedly reduced HDL cholesterol (76%) in apoE0/apoA1 transgenic mice. After injection of radiolabeled HDL CE, the CETP transgene significantly delayed the clearance of CE radioactivity from plasma in apoE0 mice, but accelerated the clearance in apoE0/apoA1 transgenic mice. ApoE0/CETP mice displayed an increase in mean atherosclerotic lesion area on the chow diet (approximately 2-fold after 2 to 4 months, and 1.4- to 1.6-fold after 7 months) compared with apoE0 mice (P<0.02). At 7 months apoA1 transgene expression resulted in a 3-fold reduction in mean lesion area in apoE0 mice (P<0.001). In the apoE0/apoA1 background, CETP produced an insignificant 1.3- to 1.7-fold increase in lesion area. In further studies the CETP transgene was bred onto the LDL receptor knock-out background (LDLR0). After 3 months on the Western diet, the mean lesion area was increased 1.8-fold (P<0.01) in LDLR0/CETP mice, compared with LDLR0 mice. These studies indicate that CETP expression leads to a moderate increase in atherosclerosis in apoE0 and LDLR0 mice, and suggest a proatherogenic effect of CETP activity in metabolic settings in which clearance of remnants or LDL is severely impaired. However, apoA1 overexpression has more dramatic protective effects on atherosclerosis in apoE0 mice, which are not significantly reversed by concomitant expression of CETP.

Scavenger receptor BI mediates the selective uptake of oxidized cholesterol esters by rat liver

High density lipoprotein (HDL) can protect low density lipoprotein (LDL) against oxidation. Oxidized cholesterol esters from LDL can be transferred to HDL and efficiently and selectively removed from the blood circulation by the liver and adrenal in vivo. In the present study, we investigated whether scavenger receptor BI (SR-BI) is responsible for this process. At 30 min after injection, the selective uptake of oxidized cholesterol esters from HDL for liver and adrenal was 2.3- and 2.6-fold higher, respectively, than for native cholesterol esters, whereas other tissues showed no significant difference. The selective uptake of oxidized cholesterol esters from HDL by isolated liver parenchymal cells could be blocked for 75% by oxidized LDL and for 50% by phosphatidylserine liposomes, both of which are known substrates of SR-BI. In vivo uptake of oxidized cholesterol esters from HDL by parenchymal cells decreased by 64 and 81% when rats were treated with estradiol and a high cholesterol diet, respectively, whereas Kupffer cells showed 660 and 475% increases, respectively. These contrasting changes in oxidized cholesterol ester uptake were accompanied by similar contrasting changes in SR-BI expression of parenchymal and Kupffer cells. The rates of SR-BI-mediated selective uptake of oxidized and native cholesterol esters were analyzed in SR-BI-transfected Chinese hamster ovary cells. SR-BI-mediated selective uptake was 3.4-fold higher for oxidized than for native cholesterol esters (30 min of incubation). It is concluded that in addition to the selective uptake of native cholesterol esters, SR-BI is responsible for the highly efficient selective uptake of oxidized cholesterol esters from HDL and thus forms an essential mediator in the HDL-associated protection system for atherogenic oxidized cholesterol esters.

Relationship between myometrial resistance artery behavior and circulating lipid composition

OBJECTIVES

The study investigated whether an inducible alteration in endothelium-dependent relaxation in myometrial vessels could be correlated with plasma lipid composition.

STUDY DESIGN

Myometrial resistance vessels were obtained from 10 women with normal pregnancy undergoing elective cesarean delivery. Paired vessels were incubated with plasma samples from patients with preeclampsia or from women with normal pregnancy and mounted on a wire myograph. After contraction with vasopressin, the degree of relaxation in response to bradykinin was observed. Plasma samples were assayed for cholesterol, triglycerides, apolipoprotein A1, and apolipoprotein B.

RESULTS

A significant reduction in endothelium-dependent relaxation with respect to control values was observed in vessels incubated in plasma samples from patients with preeclampsia (P =.0001). Although no significant difference was noted between the lipid profiles of the 2 subgroups, a significant correlation was found between the vessel relaxation and the plasma content of apolipoprotein A1 (R2 = 0.36, P = .025).

CONCLUSION

Plasma samples from women with pregnancies complicated by preeclampsia are capable of altering endothelium-dependent myometrial vessel relaxation. A significant relationship between the apolipoprotein A1 concentration and endothelial behavior supports the suggestion that aberrant lipid metabolism may be involved in the endothelial dysfunction characteristic of preeclampsia.

Recombinant apolipoproteins for the treatment of vascular diseases

The protein components of human lipoproteins, apolipoproteins, allow the redistribution of cholesterol from the arterial wall to other tissues and exert beneficial effects on systems involved in the development of arterial lesions, like inflammation and hemostasis. Because of these properties, the antiatherogenic apolipoproteins, particularly apo A-I and apo E, may provide an innovative approach to the management of vascular diseases. The recent availability of extractive or biosynthetic molecules is allowing a detailed overview of their therapeutic potential in a number of animal models of arterial disease. Infusions of apo E, or more dramatically, of apo A-I, both recombinant or extractive, cause a direct reduction of the atherosclerotic burden in experimental animals. Naturally, as the apo A-I(Milano) (apo A-I(M)) dimer, or engineered recombinant apolipoproteins with prolonged permanence in plasma and improved function may offer an even better approach to the therapeutic handling of arterial disease. This progress will go on in parallel with innovations in the technologies for direct, non invasive assessments of human atherosclerosis, thus allowing closer monitoring of this potential new approach to therapy.

Mechanism of high-density lipoprotein subfractions inhibiting copper-catalyzed oxidation of low-density lipoprotein

OBJECTIVE

To investigate the role of HDL subfractions, HDL2 and HDL3, on the oxidation of LDL catalyzed by 5 microM Cu2+ ion, and to illustrate the mechanism of the generation of conjugated diene and thiobarbituric acid reactive substances (TBARS) during LDL oxidation.

METHODS

LDL was incubated for 8 h with 5 microM Cu2+ ion in phosphate-buffered saline (PBS) alone, or in the presence of HDL2, HDL3, HSA, BSA, or transferrin. Meantime, LDL was incubated for 24 h with 10 microM Ni2+ ions in PBS. The amount of conjugated diene and TBARS in each sample of LDL were measured.

RESULTS

(a) HDL2 and HDL3 could inhibit the generation of conjugated diene, but could not inhibit the generation of TBARS; (b) the transferrin containing HDL3 shows the ability of inhibiting the generation of both conjugated diene and TBARS; (c) the transferrin presented in blood exhibits the inhibitory effect on the generation of conjugated diene and TBARS, however, when the transferrin is saturated with Fe3+ ion, it could not inhibit the generation of TBARS; (d) HSA and BSA could prevent the generation of conjugated diene and TBARS; (e) Ni2+ ion could induce the generation of conjugated diene, but the amount of TBARS was much smaller than that induced by Cu2+ ion.

CONCLUSION

HDL2 and HDL3 play important role in the copper-catalyzed oxidation of LDL; it is absolutely necessary to require chelation of Cu2+ ion for inhibiting generation of TBARS; whereas, inhibition of conjugated diene can be fulfilled either by chelating Cu2+ ion, or the free radicals scavenger.

The relationship between apolipoprotein E and serum oxidation-related variables is apolipoprotein E phenotype dependent

To examine the relationship between apolipoprotein E and serum oxidation status, we assayed apolipoprotein E level, apolipoprotein E phenotype, and levels of lipid peroxides and transition metal ions and their binding proteins in sera from apparently healthy individuals. The study group included 129 women aged 22-63 years and 53 men aged 22-56 years. Among subjects with apolipoprotein E 4/3 phenotype, lipid peroxide levels were higher compared with E 3/2 phenotype (786 +/- 182 nmol/l vs. 659 +/- 174 nmol/l, P = 0.015), and ceruloplasmin levels were slightly higher compared with apolipoprotein E 3/3 phenotype (0.28 +/- 0.08 mg/l vs. 0.26 +/- 0.06 mg/l, P = 0.035). In the study group as a whole, there were significant associations between serum apolipoprotein E level, and serum levels of ceruloplasmin (r = 0.266, P < 0.001) and ferritin (r = 0.2, P < 0.007). Among subjects with apolipoprotein E 4/3 phenotype, there was a significant association between serum apolipoprotein E and lipid peroxide levels (r = 0.470, P < 0.01), which was not apparent among subjects with E 3/3 or E 3/2 phenotypes. In multivariate analysis, apolipoprotein E phenotype was a small but significant independent contributor to variation in serum lipid peroxide levels. These data suggest that there may be heterogeneity among apolipoprotein E phenotypes in their relationships with serum lipid oxidation status.

The atherogenic lipoprotein phenotype and vascular endothelial dysfunction

There is accumulating evidence that elevated plasma triglycerides and related abnormalities constitute an independent cardiovascular risk factor. Although the pathogenetic basis for the apparent relationship between elevated triglyceride-rich lipoproteins and CAD is still uncertain, evidence is accumulating to suggest that endothelial dysfunction is involved. There is evidence to suggest that triglyceride-rich particles may be directly damaging to the endothelium; this may be principally via oxidative mechanisms. Triglyceride-rich particles can cross the endothelial barrier and enter the arterial wall, thus placing them in a position to promote direct endothelial damage. These particles stimulate endothelial expression of adhesion molecules and the prothrombotic factor PAI-1. By reducing LDL size and HDL cholesterol concentrations, thereby further increasing the endothelial oxidative burden, triglyceride-rich particles may indirectly promote endothelial dysfunction. In addition, free fatty acids, which are the major substrates for endogenous synthesis of triglyceride-rich particles, are also potentially damaging to the endothelium. This occurs via oxidative stress, by facilitating transfer of LDL across the endothelium, and by enhancing toxicity of triglyceride-rich particles. Finally, there is recent strong evidence to suggest that increased postprandial circulating concentrations of triglyceride-rich particles and remnant particles may be deleterious to the endothelium.

Oxidative modification of HDL3 in vitro and its effect on PLTP-mediated phospholipid transfer

The oxidation of HDL3 by Cu(II) and its effect on the ability of these particles to act as phospholipid acceptors in human plasma phospholipid transfer protein (PLTP)-mediated lipid transfer were investigated. Oxidation of HDL3 was monitored by measuring the following parameters: (i) formation of conjugated dienes, (ii) production of thiobarbituric acid reactive substances (TBARS), (iii) decrease in reactive lysine and (iv) tryptophan residues, (v) change in particle charge and (vi) diameter, and (vii) oligomerisation of apoA-I and apoA-II. Formation of conjugated dienes was the parameter responding to the oxidative treatment with the fastest kinetics. The appearance of TBARS and modification of apolipoprotein tryptophan residues were detected simultaneously but required higher Cu(II) concentrations for maximal kinetics. Cross-linking of the major protein constituents of HDL3, apoA-I and apoA-II, represented later steps of the oxidation process. Further, the oxidative modification was accompanied by a progressive change in HDL3 particle charge and a minor increase in particle diameter. PLTP-mediated phospholipid transfer to the oxidized particles was investigated using an assay measuring the transfer of fluorescent, pyrene-labeled PC. The transfer was significantly inhibited, but only after extensive modification of the HDL proteins, suggesting that the HDL oxidative modifications occurring in vivo do not essentially impair its phospholipid acceptor function. A similar but less pronounced inhibition was observed when two other phospholipid transfer proteins, the nonspecific lipid transfer protein (ns-LTP) and the phosphatidylcholine transfer protein (PC-TP), were studied in parallel. This indicates that the inhibition was partly due to unspecific effects of the modification on acceptor particle surface properties, but included an aspect specific for PLTP.

Effects of recombinant apolipoprotein A-I(Milano) on aortic atherosclerosis in apolipoprotein E-deficient mice

BACKGROUND

We previously reported marked inhibitory effects of recombinant apolipoprotein (apo) A-I(Milano)/phospholipid complex (A-I[Milano]/PC) on neointimal lesions in balloon-injured iliofemoral arteries of hypercholesterolemic rabbits. In this study, we tested the hypothesis that apo A-I(Milano)/PC would inhibit aortic atherosclerosis in apo E-deficient mice.

METHODS AND RESULTS

Thirty-five apo E-deficient mice fed a high-cholesterol diet were included in the study. Control mice were killed at 20 (n=8) or 25 (n=7) weeks. Treated mice received 18 injections of either 40 mg/kg apo A-I(Milano)/PC (n=15) or PC only (n=5) intravenously every other day from 20 weeks until death at 25 weeks. Aortic atherosclerosis was identified with Sudan IV staining. Lipid and macrophage contents of the aortic sinus plaques were measured after oil-red O and Mac-1 antibody staining, respectively, and quantified with computed morphometry. In control mice, from 20 to 25 weeks, aortic atherosclerosis increased by 59% (11 +/- 1% versus 17 +/- 5% of the aortic surface, P=.002), and lipid content increased by 45% (22 +/- 8% versus 32 +/- 6% of plaque area, P=.02) without a significant change in macrophage content (10.8 +/- 2% versus 13.2 +/- 6%). Compared with 20-week-old untreated control mice, PC only-treated mice at 25 weeks demonstrated a 32% increase in aortic atherosclerosis (11 +/- 1% versus 15 +/- 4%, P=.01) and an increase in lipid content (22 +/- 8% versus 47 +/- 3%, P<.0001) without a change in macrophage content (10.8 +/- 2% versus 11 +/- 2%). In comparison with 20-week-old untreated control mice, 25-week-old apo A-I(Milano)/PC-treated mice demonstrated no increase in aortic atherosclerosis (11 +/- 1% versus 10 +/- 4%, P=NS), a 40% reduction in lipid content (22 +/- 8% versus 13 +/- 8%, P=.01), and a 46% reduction in macrophage content (10.8 +/- 2% versus 5.8 +/- 2.9%; P=.03). Serum cholesterol levels were markedly elevated in all groups and did not change significantly with apo A-I(Milano)/PC or PC only. In vitro, apo A-I(Milano)/PC stimulated cholesterol efflux from cholesterol-loaded FU5AH hepatoma cell lines in a dose-dependent manner, whereas PC only or PC-free apo A-I(Miano) had no effect.

CONCLUSIONS

Recombinant A-I(Milano)/PC prevented progression of aortic atherosclerosis and reduced lipid and macrophage content of plaques in apo E-deficient mice despite severe hypercholesterolemia. Thus, A-I(Milano)/PC may have a role in inhibiting progression and promoting stabilization of atherosclerosis.

Evidence for a paraoxonase-independent inhibition of low-density lipoprotein oxidation by high-density lipoprotein

One mechanism by which plasma high-density lipoprotein (HDL) may protect against atherogenesis is by inhibiting the oxidation of low-density lipoprotein (LDL). Recent evidence suggests that paraoxonase, an HDL-associated, calcium-dependent enzyme, may be responsible for the antioxidant action of HDL (Mackness et al., Atherosclerosis 1993;104:129; Mackness et al., FEBS Lett 1991;286:152; Watson et al., J Clin Invest 1995;96:2882; Navab et al., Arterio Thromb Vasc Biol 1996;16:831); in particular, paraoxonase activity inhibits the formation of 'minimally oxidized' LDL by hydrolyzing biologically active oxidized phospholipids (Watson et al., J Clin Invest 1995;96:2882; Navab et al., Arterio Thromb Vasc Biol 1996;16:831). However, antioxidant effects of HDL have also been demonstrated under calcium-free conditions, arguing that this enzyme may not be the only mechanism by which HDL inhibits LDL oxidation (Tribble et al., J Lipid Res 1995;36:2580). Here we have evaluated the role of paraoxonase in prevention of LDL oxidation by using HDL subfractions, isolated from human serum or EDTA-plasma, which display markedly different levels of paraoxonase activity; the abilities of modified forms of HDL to prevent LDL oxidation by cultured human (THP-1) macrophages were also assessed. Paraoxonase activity was substantially lower in HDL prepared from plasma compared to serum HDL; moreover, virtually all of the lipoprotein-associated paraoxonase activity was located in the HDL3 fraction, with HDL2 retaining only 1-5% of the total activity. Despite possessing 5-fold differences in paraoxonase activity, HDL3 isolated from plasma or serum was equally effective in inhibiting LDL oxidation by THP-1 macrophages; furthermore, although plasma HDL3 was more protective than plasma HDL2, the latter did significantly inhibit LDL oxidation. Non-paraoxonase antioxidant constituents of plasma HDL3 were investigated further. ApoHDL3, the totally delipidated form of HDL3, was much less effective than native HDL3; when examined individually, purified apolipoprotein A-II gave greater protection than apo A-I, although this effect was not evident in apo A-II-enriched HDL3. Partial delipidation of HDL3, which removes both neutral lipids and alpha-tocopherol, did not significantly diminish its ability to inhibit LDL oxidation by THP-1 macrophages; phospholipid vesicles prepared from partially delipidated HDL3 also inhibited LDL oxidation effectively. We conclude that, in this model of cellular LDL oxidation, the phospholipid fraction of HDL exerts inhibitory effects which are independent of HDL paraoxonase activity.

Premature coronary artery disease associated with a disruptive mutation in the estrogen receptor gene in a man

BACKGROUND

While estrogens protect against coronary artery disease in women, it is unclear whether they influence cardiovascular function in men. The present report describes coronary vascular abnormalities and the lipoprotein profile of a male patient with estrogen insensitivity caused by a disruptive mutation in the estrogen-receptor gene.

METHODS AND RESULTS

Stress thallium scintigraphy, echocardiography, and electron-beam computed tomography (CT) scanning of the coronary arteries and detailed lipoprotein analysis were performed. Electron-beam CT scanning of the coronary arteries showed calcium in the left anterior descending artery. Lipoprotein analysis showed relatively low levels of total (130 mg/dL), LDL (97 mg/dL), and HDL (34 mg/dL) cholesterol; apolipoprotein A-I (91.7 mg/dL); and lipoprotein(a) (4.1 nmol/L), but normal levels of triglycerides (97 mg/dL) and pre-beta-1-HDL cholesterol (61 microg/mL).

CONCLUSIONS

The absence of functional estrogen receptors may be a novel risk factor for coronary artery disease in men.

Cholesterol-mediated changes of neutral cholesterol esterase activity in macrophages. Mechanism for mobilization of cholesteryl esters in lipid droplets by HDL

Cholesteryl esters (CE) in lipid droplets undergo a continual cycle of hydrolysis and reesterification by neutral cholesterol esterase (N-CEase) and acyl CoA:cholesterol acyltransferase (ACAT), respectively. The mechanism by which HDL mobilizes CE from lipid droplets in J774 A.1 cells was investigated, focusing on N-CEase activity. We asked whether HDL enhances the activity and, if so, what signals induce the change of the activity. An incubation of cells with HDL enhanced the decline of cholesteryl-[l-14C]-oleate in foam cells and increased N-CEase activity in the supernatant of cell homogenate in a concentration-dependent manner, whereas incubation with LDL decreased the activity. In addition, N-CEase activity was fivefold higher when cells were cultured in 10% lipoprotein-deficient serum (LPDS) medium (2 micrograms cholesterol/mL) than when cultured in 10% fetal calf serum medium (31 micrograms cholesterol/mL), suggesting that changes in N-CEase activity are mediated by cholesterol. An addition of cholesterol (0 to 30 micrograms/mL) in LPDS medium markedly inhibited N-CEase activity with a concomitant increase in cellular cholesterol concentration. This inhibitory effect of cholesterol was also observed in mouse peritoneal macrophages. In vitro addition of cholesterol did not affect N-CEase activity. Treatment of cells with HMG-CoA reductase inhibitors enhanced N-CEase activity, whereas ACAT inhibitor decreased the activity. Northern blot analysis of N-CEase mRNA showed that the expression was not altered by the presence of cholesterol in LPDS medium. These results suggest that cholesterol downregulates N-CEase activity, probably through cholesterol-dependent appearance of some factors.

Apolipoprotein L, a new human high density lipoprotein apolipoprotein expressed by the pancreas. Identification, cloning, characterization, and plasma distribution of apolipoprotein L

In this study, we have identified and characterized a new protein present in human high density lipoprotein that we have designated apolipoprotein L. Using a combination of liquid-phase isoelectrophoresis and high resolution two-dimensional gel electrophoresis, apolipoprotein L was identified and partially sequenced from immunoisolated high density lipoprotein (Lp(A-I)). Expression was only detected in the pancreas. The cDNA sequence encoding the full-length protein was cloned using reverse transcription-polymerase chain reaction. The deduced amino acid sequence contains 383 residues, including a typical signal peptide of 12 amino acids. No significant homology was found with known sequences. The plasma protein is a single chain polypeptide with an apparent molecular mass of 42 kDa. Antibodies raised against this protein detected a truncated form with a molecular mass of 39 kDa. Both forms were predominantly associated with immunoaffinity-isolated apoA-I-containing lipoproteins and detected mainly in the density range 1.123 < d < 1.21 g/ml. Free apoL was not detected in plasma. Anti-apoL immunoaffinity chromatography was used to purify apoL-containing lipoproteins (Lp(L)) directly from plasma. Nondenaturing gel electrophoresis of Lp(L) showed two major molecular species with apparent diameters of 12.2-17 and 10.4-12.2 nm. Moreover, Lp(L) exhibited both pre-beta and alpha electromobility. Apolipoproteins A-I, A-II, A-IV, and C-III were also detected in the apoL-containing lipoprotein particles.

Plasma LDL and HDL subspecies are heterogenous in particle content of tocopherols and oxygenated and hydrocarbon carotenoids. Relevance to oxidative resistance and atherogenesis

Epidemiological data indicate that dietary tocopherols and carotenoids can exert cardioprotective effects, which may be mediated by their antioxidant actions. The oxidative modification of LDL underlies the atherogenicity of these cholesterol-rich particles. The resistance of LDL to oxidation is influenced by several endogenous factors, among which the content of tocopherols and carotenoids is prominent. Of the exogenous factors, HDL inhibits oxidation of LDL via several mechanisms. In view of the paucity of data on the distribution of diverse tocopherol and carotenoid components among the apoB- and apoA-I-containing lipoproteins of human plasma, we evaluated the quantitative and qualitative features of the LDL and HDL particle subspecies in normolipidemic subjects. The bulk of tocopherols and hydrocarbon carotenoids (lycopene, alpha- and beta-carotene) was transported in LDL (45% and 76%, respectively), in contrast to the oxygenated carotenoids (lutein/zeaxanthin, canthaxanthin, and beta-cryptoxanthin), which were equally distributed between LDL and HDL. alpha-Tocopherol content was independently associated with lipid core size (cholesteryl ester and triglyceride) in VLDL, LDL, and HDL (P < .005); by contrast, the particle content of the oxygenated carotenoids lutein/zeaxanthin and canthaxanthin was strongly related to that of phospholipids. A significant and progressive decrease in the molar content of alpha- and gamma-tocopherols was found with increase in density from light to dense LDL subspecies (LDL1 to LDL5); a similar trend was observed in HDL subspecies. Furthermore, particle contents of lutein/zeaxanthin, beta-cryptoxanthin, beta-carotene, and lycopene were markedly reduced in small, dense LDL (LDL5, d = 1.050 to 1.065 g/mL). We conclude that diminished contents in such carotenoids as well as in tocopherols could underlie not only the diminished oxidative resistance of small, dense LDL but also reduced tissue targeting of antioxidants in subjects with a dense LDL phenotype.

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.

Neurotrophic effects of transferrin on embryonic chick brain and neural retinal cell cultures

The viability and differentiation promoting effects of various transferrins [iron-saturated (holo) and iron-depleted (apo) human and chick ovo (conalbumin)-transferrins, and bovine apo-transferrin] were studied, using serum-free, flat-sedimented cell cultures of embryonic chick brain and neural retina. The effects of transferrin (Tf) on the cell cultures depended on the type of Tf used and the parameter measured. Significant differences between brain and neural retina cultures in the effects of apo-ovoTf and iron [supplemented as ammonium-iron (III) citrate] were detected. Maximal levels of mitochondrial activity were observed in the presence of 2 mg/l apo-ovoTf in neural retina cell cultures. In brain cell cultures, 40 mg ovoTf/l were needed to achieve maximal levels. In brain, but not in neural, retina cell cultures ovoTf and optimal concentrations of Fe3+ exhibited similar effects on biochemical parameters of cell function and differentiation. Although, in the absence of ovoTf, neuronal outgrowth on areas not covered by glial cells was inhibited in both cell cultures, the differences were more prominent in neural retina cell cultures. Our data strongly suggest that Tf plays a key role in processes not connected directly with its iron transport capability.

Apolipoprotein E allele-specific antioxidant activity and effects on cytotoxicity by oxidative insults and beta-amyloid peptides

The apolipoprotein E (APOE) E4 allele is associated with Alzheimer's disease, cardiovascular disease, and decreased longevity. To probe the mechanism of these associations, cell lines were created which secrete each apoE isoform. ApoE conditioned media, purified apoE, and commercially obtained apoE protected B12 cells from hydrogen peroxide cytotoxicity with E2 > E3 > E4. Physiological levels of apoE protected cells from beta-amyloid peptides, while higher doses of apoE led to increased cytotoxicity. E2 > E3 > E4 possessed antioxidant activity, and apoE bound certain metal ions. The decreased antioxidant activity of E4 could contribute to its association with Alzheimer's disease, cardiovascular disease and decreased longevity.

Lipoprotein composition in NIDDM: effects of dietary oleic acid on the composition, oxidisability and function of low and high density lipoproteins

Oxidation of low density lipoprotein (LDL) plays an important role in the pathogenesis of atherosclerosis and is related to the fatty acid composition which is altered in diabetes mellitus. This study examines the relationship between the fatty acid composition of LDL and high density lipoprotein (HDL) and lipoprotein oxidation. A group of nine non-insulin-dependent diabetic (NIDDM) patients were compared to seven healthy control subjects before and after a high monounsaturated diet. Lipoproteins were isolated and oxidisability was measured by conjugated diene formation and lipid peroxide analysis. Serum HDL cholesterol was significantly lower in the diabetic patients. LDL cholesteryl ester linoleic acid in the diabetic patients was significantly higher at baseline and decreased after diet (p < 0.05) while oleic acid increased in both diabetic and non-diabetic subjects (p < 0.05). HDL cholesteryl ester oleic acid was lower in the diabetic patients compared with control subjects (p < 0.05) before diet and it increased significantly after diet (p < 0.05). LDL lipid peroxides and conjugated diene formation were related to LDL glycation (r = 0.46, p < 0.05 and r = 0.49, p < 0.05, respectively). Both decreased following diet (lipid peroxides for diabetic patients from 476 +/- 30 to 390 +/- 20 nmol/mg protein p < 0.05 and for control subjects from 350 +/- 36 to 198 +/- 30 nmol/mg protein p < 0.05). HDL conjugated diene formation decreased in both groups after diet but only significantly in the control group (55.4 +/- 7.5 to 53.2 +/- 6.7 nmol/mg protein for diabetic patients and 45.8 +/- 6.4 to 31.6 +/- 4.8 nmol/mg protein p < 0.05 for control subjects). There was a positive correlation between LDL lipid peroxide formation and percentage of cholesteryl ester linoleic acid in LDL from diabetic patients (r = 0.61, p < 0.05) and control subjects (r = 0.91, p < 0.01). Fatty acid composition of LDL was reflected in the composition of HDL. In the presence of HDL lipoprotein peroxidation decreased. This decrease in lipoprotein peroxidation was positively related to the percentage of linoleic acid in LDL (r = 0.71, p < 0.05). This study confirms the close relationship between the fatty acid composition of LDL and HDL and demonstrates the importance of the fatty acid composition of the cholesteryl ester fraction in relation to LDL oxidation in diabetes. Linoleic acid in HDL appears to be a protecting factor against oxidation.

Alternative splicing of the human cholesteryl ester transfer protein gene in transgenic mice. Exon exclusion modulates gene expression in response to dietary or developmental change

The plasma cholesteryl ester transfer protein (CETP) mediates the transfer of cholesteryl ester from high density lipoprotein to other lipoproteins. The human DETP gene produces two forms of mRNA, with or without exon 9 (E9)-derived sequences. To study the function and regulation of alternative splicing the CETP gene, transgenic mice were prepared 1) with the metallothionein (mT) promoter driving an E9-deleted construct (mT.CETP(-E9) transgene), and 2) with the natural flanking regions (NFR) controlling expression of genomic sequences which permit alternative splicing of E9 (NFR.CETP(+/-E9) transgene). With zinc induction, the mT.CETP(-E9) transgene gave rise to abundant E9-deleted CETP mRNA in liver and small intestine, but only relatively small amounts of E9-deleted protein were found in plasma. The E9-deleted form of CETP was inactive in lipid transfer and produced no changes in plasma lipoprotein profile. The NFR.CETP(+/-E9) transgene gave rise to full-length (FL) and E9-deleted forms of CETP mRNA in liver and spleen. In response to hypercholesterolemia induced by diet and breeding into an apoE gene knock-out background, the FL CETP mRNA was induced more than the E9-deleted mRNA, resulting in a 2-fold increase in ratio of FL/E9-deleted mRNA. The expression of CETP mRNA was found to be developmentally regulated. In NFR.CETP(+/-E9) transgenic mice CETP mRNA levels were markedly increased in the liver and small intestine in the perinatal period and decreased in adult mice, whereas CETP mRNA in the spleen was low in perinatal mice and increased in adults. The developmental increase in CETP mRNA in the liver and spleen was preceded by an increased ratio of FL/E9-deleted forms. Thus, the E9-deleted mRNA appears to be poorly translated and/or secreted, and the cognate protein is inactive in lipid transfer and lipoprotein metabolism. CETP gene expression was found to be highly regulated in a tissue-specific fashion during development. Increased CETP gene expression during development or in response to hypercholesterolemia is associated with preferential accumulation of the full-length CETP mRNA.

Oxidation-related analytes and lipid and lipoprotein concentrations in healthy subjects

The relations between oxidation-related analytes and lipoprotein risk factors for coronary heart disease are poorly understood. To address this issue, ceruloplasmin, copper, iron, ferritin, cotinine, lipid peroxides, cholesterol, triglyceride, apoB, apoA-I, and lipoprotein(a) levels were measured in sera from apparently healthy subjects (51 men and 115 women). Pairwise comparisons revealed strong positive associations (P < .001) of copper and ceruloplasmin with lipid peroxides, total cholesterol, triglycerides and apoB, of transferrin with apoA-I and cholesterol, and of ferritin with triglycerides. Serum levels of oxidation-related analytes did not differ between smokers and nonsmokers. In multivariate analysis, serum copper was the major independent determinant of serum lipid peroxide level, accounting for 15% of the variability in concentration (ferritin accounted for 1.6%). Copper and ceruloplasmin accounted for 20.5% of the variation in triglyceride levels; triglycerides and apoB accounted for 12% of the variability in ferritin levels; apoB and apoA-I accounted for 9% of the variability in transferrin levels. The data suggest that serum copper contributes to lipid peroxidation in vivo. There are significant associations between lipoprotein and transition metal-related analytes, and further work is needed to elucidate the physiological basis for these relations.

Effect of in-vivo supplementation with low-dose vitamin E on susceptibility of low-density lipoprotein and high-density lipoprotein to oxidative modification

OBJECTIVES

We have examined the effects of in-vivo supplementation with low-dose vitamin E on the susceptibility of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) to oxidative modification, and compared the oxidizability of HDL with that of LDL.

METHODS

Normal humans (n = 8) ingested vitamin E (150 mg/day for 1 week, followed by 300 mg/day for 3 weeks) in divided doses with meals. The subjects did not use any medications or vitamins before being enrolled in this study. Fasting blood samples were drawn before and at the end of supplementation. LDL and HDL were separated by sequential ultracentrifugation and susceptability to copper-mediated oxidation was measured.

RESULTS

After vitamin E supplementation, vitamin E content of LDL increased 1.9-fold and that of HDL increased 1.8-fold. Lag time before initiation of LDL oxidation lengthened significantly (+20%, p < 0.01), and the propagation rate of LDL decreased significantly (-10%, p < 0.05). The lag time of HDL oxidation did not change significantly, but the propagation rate of HDL oxidation decreased significantly (-24%, p < 0.001). The lag time of HDL oxidation was shorter than that of LDL. HDL contained the same or higher concentrations of vitamin E relative to lipid mass as LDL, but contained lower concentration of CoQ10 relative to lipid mass and fewer molecules of vitamin E and beta-carotene per particle than LDL.

CONCLUSIONS

We conclude that in-vivo supplementation of low-dose vitamin E protects LDL against oxidative modification and decreases the propagation rate of HDL oxidation significantly. We suggest that supplementation with low-dose vitamin E would be beneficial for ameliorating atherosclerosis.

Peroxidation of model lipoprotein solutions sensitized by photoreduction of ferritin by 365 nm radiation

A mechanistic study involving the 365 nm irradiation of aerated, phosphate-buffered solutions of human high-density lipoproteins (HDL3 fraction) and ferritin was undertaken. The 365 nm irradiation of phosphate-buffered horse spleen ferritin solutions induces the release of Fe2+ in the medium. The initial quantum yield of Fe2+ release on irradiation is 0.002. This quantum yield is oxygen independent. The 365 nm irradiation of mixtures of HDL and ferritin leads to alterations in apolipoproteins as revealed by tryptophan (Trp) oxidation and electrophoretic pattern modification. In parallel with protein damage, lipid peroxidation is induced as shown by hydroperoxide and thiobarbituric acid reactive substances (TBARS) formation. These peroxidations are strongly reduced in 0.1 M formate solution, which suggests chain initiation by .OH radicals or subsequent radicals produced by .OH. They are completely inhibited by desferrioxamine, consistent with propagation by Fe2+ ion. By contrast incubation of HDL in the presence of ferritin and FeSO4 induces only poor auto-oxidation. The biological relevance of this study is discussed.

Lipopolysaccharide (LPS)-binding protein is carried on lipoproteins and acts as a cofactor in the neutralization of LPS

Lipoproteins isolated from normal human plasma can bind and neutralize bacterial lipopolysaccharide (LPS) and may represent an important mechanism in host defense against gram-negative septic shock. Recent studies have shown that experimentally elevating the levels of circulating high-density lipoproteins (HDL) provides protection against death in animal models of endotoxic shock. We sought to define the components of HDL that are required for neutralization of LPS. To accomplish this we have studied the functional neutralization of LPS by native and reconstituted HDL using a rapid assay that measures the CD14-dependent activation of leukocyte integrins on human neutrophils. We report here that reconstituted HDL particles (R-HDL), prepared from purified apolipoprotein A-I (apoA-I) combined with phospholipid and free cholesterol, are not sufficient to neutralize the biologic activity of LPS. However, addition of recombinant LPS binding protein (LBP), a protein known to transfer LPS to CD14 and enhance responses of cells to LPS, enabled prompt binding and neutralization of LPS by R-HDL. Thus, LBP appears capable of transferring LPS not only to CD14 but also to lipoprotein particles. In contrast with R-HDL, apoA-I containing lipoproteins (LpA-I) isolated from plasma by selected affinity immunosorption (SAIS) on an anti-apoA-I column, neutralized LPS without addition of exogenous LBP. Several lines of evidence demonstrated that LBP is a constituent of LpA-I in plasma. Passage of plasma over an anti-apoA-I column removed more than 99% of the LBP detectable by ELISA, whereas 31% of the LBP was recovered by elution of the column. Similarly, the ability of plasma to enable activation of neutrophils by LPS (LBP/Septin activity) was depleted and recovered by the same process. Furthermore, an immobilized anti-LBP monoclonal antibody coprecipitated apoA-I. The results described here suggest that in addition to its ability to transfer LPS to CD14, LBP may also transfer LPS to lipoproteins. Since LBP appears to be physically associated with lipoproteins in plasma, it is positioned to play an important role in the neutralization of LPS.