Oxidative tyrosylation of high density lipoprotein impairs biliary sterol secretion in rats

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

Preservation of biological function despite oxidative modification of the apolipoprotein A-I mimetic peptide 4F

Myeloperoxidase (MPO)-derived hypochlorous acid induces changes in HDL function via redox modifications at the level of apolipoprotein A-I (apoA-I). As 4F and apoA-I share structural and functional properties, we tested the hypothesis that 4F acts as a reactive substrate for hypochlorous acid (HOCl). 4F reduced the HOCl-mediated oxidation of the fluorescent substrate APF in a concentration-dependent manner (ED(50) ∼ 56 ± 3 μM). This reaction induced changes in the physical properties of 4F. Addition of HOCl to 4F at molar ratios ranging from 1:1 to 3:1 reduced 4F band intensity on SDS-PAGE gels and was accompanied by the formation of a higher molecular weight species. Chromatographic studies showed a reduction in 4F peak area with increasing HOCl and the formation of new products. Mass spectral analyses of collected fractions revealed oxidation of the sole tryptophan (Trp) residue in 4F. 4F was equally susceptible to oxidation in the lipid-free and lipid-bound states. To determine whether Trp oxidation influenced its apoA-I mimetic properties, we monitored effects of HOCl on 4F-mediated lipid binding and ABCA1-dependent cholesterol efflux. Neither property was altered by HOCl. These results suggest that 4F serves as a reactive substrate for HOCl, an antioxidant response that does not influence the lipid binding and cholesterol effluxing capacities of the peptide.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

Vasculoprotective Effects of Apolipoprotein Mimetic Peptides: An Evolving Paradigm In Hdl Therapy (Vascular Disease Prevention, In Press.)

Anti-atherogenic effects of high density lipoprotein (HDL) and its major protein component apolipoprotein A-I (apoA-I) are principally thought to be due to their ability to mediate reverse cholesterol transport. These agents also possess anti-oxidant properties that prevent the oxidative modification of low density lipoprotein (LDL) and anti-inflammatory properties that include inhibition of endothelial cell adhesion molecule expression. Results of the Framingham study revealed that a reduction in HDL levels is an independent risk factor for coronary artery disease (CAD). Accordingly, there has been considerable interest in developing new therapies that specifically elevate HDL cholesterol. However, recent evidence suggests that increasing circulating HDL cholesterol levels alone is not sufficient as a mode of HDL therapy. Rather, therapeutic approaches that increase the functional properties of HDL may be superior to simply raising the levels of HDL per se. Our laboratory has pioneered the development of synthetic, apolipoprotein mimetic peptides which are structurally and functionally similar to apoA-I but possess unique structural homology to the lipid-associating domains of apoA-I. The apoA-I mimetic peptide 4F inhibits atherogenic lesion formation in murine models of atherosclerosis. This effect is related to the ability of 4F to induce the formation of pre-β HDL particles that are enriched in apoA-I and paraoxonase. 4F also possesses anti-inflammatory and anti-oxidant properties that are independent of its effect on HDL quality per se. Recent studies suggest that 4F stimulates the expression of the antioxidant enzymes heme oxygenase and superoxide dismutase and inhibits superoxide anion formation in blood vessels of diabetic, hypercholesterolemic and sickle cell disease mice. The goal of this review is to discuss HDL-dependent and -independent mechanisms by which apoA-I mimetic peptides reduce vascular injury in experimental animal models.

Structural modifications of HDL and functional consequences

High density lipoproteins (HDL) are susceptible to structural modifications mediated by various mechanisms including oxidation, glycation, homocysteinylation or enzymatic degradation. Structural alterations of HDL may affect their functional and atheroprotective properties. Oxidants, such as hypochlorous acid, peroxyl radicals, metal ions, peroxynitrite, lipoxygenases and smoke extracts, can alter both surface and core components of HDL. The formation of lipid peroxidation derivatives, such as thiobarbituric acid reactive substances, conjugated dienes, lipid hydroperoxides and aldehydes, is associated with changes of physical properties (fluidity, molecular order) and of apoprotein conformation. Non-enzymatic glycation, generally associated with lipoxidation, leads to form irreversible complexes called advanced glycation end products. These HDL modifications are accompanied with altered biological activities of HDL and associated enzymes, including paraoxonase, CETP and LCAT. Homocysteine-induced modification of HDL is mediated by homocysteine-thiolactone, and can be prevented by a calcium-dependent thiolactonase/paraoxonase. Tyrosylation of HDL induces the formation of dimers and trimers of apo AI, and alters cholesterol efflux. Phospholipases and proteolytic enzymes can also modify HDL lipid and apoprotein structure. HDL modification induces generally the loss of their anti-inflammatory and cytoprotective properties. This could play a role in the pathogenesis of atherosclerosis and neurodegenerative diseases such as Alzheimer's disease.

Myeloperoxidase-mediated oxidation of high-density lipoproteins: Fingerprints of newly recognized potential proatherogenic lipoproteins

Substantial evidence supports the notion that oxidative processes participate in the pathogenesis of atherosclerotic heart disease. Major evidence for myeloperoxidase (MPO) as enzymatic catalyst for oxidative modification of lipoproteins in the artery wall has been suggested in numerous studies performed with low-density lipoprotein. In contrast to low-density lipoprotein, plasma levels of high-density lipoprotein (HDL)-cholesterol and apoAI, the major apolipoprotein of HDL, inversely correlate with the risk of developing coronary artery disease. These antiatherosclerotic effects are attributed mainly to HDL's capacity to transport excess cholesterol from arterial wall cells to the liver during 'reverse cholesterol transport'. There is now strong evidence that HDL is a selective in vivo target for MPO-catalyzed oxidation impairing the cardioprotective and antiinflammatory capacity of this antiatherogenic lipoprotein. MPO is enzymatically active in human lesion material and was found to be associated with HDL extracted from human atheroma. MPO-catalyzed oxidation products are highly enriched in circulating HDL from individuals with cardiovascular disease where MPO concentrations are also increased. The oxidative potential of MPO involves an array of intermediate-generated reactive oxygen and reactive nitrogen species and the ability of MPO to generate chlorinating oxidants-in particular hypochlorous acid/hypochlorite-under physiological conditions is a unique and defining activity for this enzyme. All these MPO-generated reactive products may affect structure and function of HDL as well as the activity of HDL-associated enzymes involved in conversion and remodeling of the lipoprotein particle, and represent clinically useful markers for atherosclerosis.

Oxidative tyrosylation of high density lipoproteins impairs cholesterol efflux from mouse J774 macrophages: role of scavenger receptors, classes A and B

Studies were designed to test whether tyrosylation of high-density lipoprotein (HDL(T)) modifies its metabolic features. HDL(T) was less effective than native HDL in promoting cholesterol efflux from J774-AI macrophages. Cell association with fluorescent HDL(T)-apolipoprotein and the uptake of HDL(T)-[(3)H]cholesteryl hexadecyl ether were enhanced by 50% in comparison with native HDL. In addition, neutral cholesterol ester hydrolase (nCEH) activity in J774-AI, which controls the hydrolysis of cholesteryl ester stores to provide free cholesterol for cellular release, declined in the presence of HDL(T). In vitro displacement experiments revealed the ability of HDL(T) to compete with oxidized and acetylated LDL, known as ligands of scavenger receptor (SR) class B type I/II. Similarly, treatment with a blocking antibody to SR-BI/II reduced the cell association of HDL(T) and native HDL by 50%. The addition of polyinosinic acid, an inhibitor of SR class A, reduced the cell association of HDL(T) without affecting that of native HDL. These findings provide evidence that HDL(T) can compete with modified LDL, bind SR-BI/BII and internalize cholesterol ester. Furthermore, the impaired capacity of HDL(T) in promoting cholesterol efflux from J774-AI was accompanied by diminished nCEH and enhanced recognition by SR-AI/II, which appears to involve the transport of cholesterol into cells.

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.