A normotriglyceridemic, low HDL-cholesterol phenotype is characterised by elevated oxidative stress and HDL particles with attenuated antioxidative activity

Abstract

Objective:

Low levels of high density lipoprotein-cholesterol (HDL-C) are highly prevalent in subjects presenting premature atherosclerosis. It is indeterminate as to whether high cardiovascular risk in low HDL-C subjects occurs concomitantly with elevated oxidative stress and/or with biologically dysfunctional HDL particles.

Methods and results:

Systemic oxidative stress (as plasma 8-isoprostanes) was 2.3-fold elevated (p < 0.05) in normocholesterolemic, normotriglyceridemic, normoglycemic low HDL-C subjects (plasma HDL-C, <40 mg/dL; n = 8) as compared to normolipidemic controls (n = 15). HDL subfractions (HDL2b, 2a, 3a, 3b and 3c) isolated by density gradient ultracentrifugation from low HDL-C subjects displayed significantly lower (−21 to −43%, p < 0.05) specific antioxidative activity (sAA; capacity to protect LDL from oxidation on a unit particle mass or on a particle number basis) as compared to controls. Altered chemical composition (core triglyceride enrichment, cholesteryl ester depletion) paralleled antioxidative dysfunction of HDL subfractions. Plasma 8-isoprostane levels negatively correlated with sAA of HDL subfractions and positively correlated with the total cholesterol/HDL-C ratio, which was significantly elevated in the low HDL-C phenotype.

Conclusions:

Low HDL-C subjects display elevated oxidative stress and possess HDL particle subspecies with attenuated intrinsic antioxidative activity which is intimately related to their altered chemical composition.

Introduction

Epidemiological studies have identified low circulating levels of high density lipoprotein (HDL) as a strong, independent risk factor for premature atherosclerosis and coronary heart disease (CHD) [1], [2]. Low HDL states are the most common form of dyslipidemia in CHD patients [3]. Genetic deficiency of apolipoprotein A-I (apoA-I) [4], a major HDL apolipoprotein, or elevated activities of plasma cholesteryl ester transfer protein (CETP) and/or hepatic lipase (HL) [5], or reduced activities of lecithin-cholesterol acyltransferase (LCAT) [6] or lipoprotein lipase (LPL) [7], may equally underlie a low HDL-C phenotype.

In contrast to low HDL-C phenotypes, elevated HDL levels are frequently associated with low cardiovascular (CV) risk [1], [2]. Indeed, plasma HDL particles exert potent antiatherogenic and anti-inflammatory activities, including antioxidative activity, which endows them with the capacity to protect low density lipoprotein (LDL) against oxidative stress [8]. In vivo, endothelial dysfunction and local inflammation in the arterial wall give rise to the production of a spectrum of prooxidants which act to oxidise LDL [9]. However, HDL particles transport enzymes exerting antioxidative activity, including paraoxonase (PON) [10], platelet-activating factor acetylhydrolase (PAF-AH) [11] and LCAT [12], which may inhibit LDL oxidation. These enzymes hydrolyse diverse molecular species of oxidised lipids, preventing their accumulation in LDL. ApoA-I inhibits LDL oxidation by removing oxidised lipids from LDL [13]; in addition, the intrinsic physicochemical properties of HDL particles can contribute to HDL antioxidative activity [14].

Plasma HDL particles are highly heterogeneous in their structure, metabolism and biological function. Small, dense HDL3 possess higher capacities to accept cholesterol [15], to inhibit expression of adhesion molecules [16] and to exert protection of LDL from oxidation [14] as compared to large, light HDL2. Indeed, specific association was observed between elevated plasma levels of HDL3 and diminished CV risk in the VA-HIT trial [17].

The physicochemical properties and biological activity of HDL particle subfractions may be altered in low HDL-C dyslipidemias; for example, in hypertriglyceridemia associated with low HDL-C levels, small, triglyceride (TG)-rich HDL particles accumulate and display diminished capacity for cellular cholesterol efflux [18] together with a reduced plasma residence time [19]. Similarly, HDL are defective acceptors of cellular cholesterol in familial HDL deficiency [20]. Moreover, in double apoA-I- and LDL receptor-deficient mice, low HDL-C levels result in markedly increased atherosclerosis and elevated oxidative stress [21], strongly indicating a link between oxidative stress and HDL functionality. These studies provoke the working hypothesis that HDL antioxidative function may be attenuated in human dyslipidemias involving low HDL-C levels and oxidative stress.

The potential relationships between oxidative stress and the antioxidative function of HDL particles is however indeterminate in low HDL-C dyslipidemias. We, therefore, evaluated the relationship of systemic oxidative stress to the capacity of HDL particle subfractions to protect LDL from oxidation in normocholesterolemic, normotriglyceridemic, normoglycemic subjects displaying low HDL-C levels. Our data reveal that the intrinsic antioxidative activity of HDL subfractions is deficient in this low HDL-C phenotype, that this deficient activity is amplified by subnormal numbers of circulating HDL particles and that this HDL dysfunctionality is intimately related to elevated oxidative stress monitored as breakdown products of arachidonic acid, i.e. plasma 8-isoprostanes [22].