The macrophage scavenger receptor is a trimeric membrane glycoprotein with unusual ligand-binding properties which has been implicated in the development of atherosclerosis. The trimeric structure of the bovine type I scavenger receptor, deduced by complementary DNA cloning, contains three extracellular C-terminal cysteine-rich domains connected to the transmembrane domain by a long fibrous stalk. This stalk structure, composed of an alpha-helical coiled coil and a collagen-like triple helix, has not previously been observed in an integral membrane protein.

Therapies that raise levels of HDL, which is thought to exert atheroprotective effects via effects on endothelium, are being examined for the treatment or prevention of coronary artery disease (CAD). However, the endothelial effects of HDL are highly heterogeneous, and the impact of HDL of patients with CAD on the activation of endothelial eNOS and eNOS-dependent pathways is unknown. Here we have demonstrated that, in contrast to HDL from healthy subjects, HDL from patients with stable CAD or an acute coronary syndrome (HDL CAD ) does not have endothelial antiinflammatory effects and does not stimulate endothelial repair because it fails to induce endothelial NO production. Mechanistically, this was because HDL CAD activated endothelial lectin-like oxidized LDL receptor 1 (LOX-1), triggering endothelial PKCβII activation, which in turn inhibited eNOS-activating pathways and eNOS-dependent NO production. We then identified reduced HDL-associated paraoxonase 1 (PON1) activity as one molecular mechanism leading to the generation of HDL with endothelial PKCβII-activating properties, at least in part due to increased formation of malondialdehyde in HDL. Taken together, our data indicate that in patients with CAD, HDL gains endothelial LOX-1-and thereby PKCβII-activating properties due to reduced HDL-associated PON1 activity, and that this leads to inhibition of eNOS-activation and the subsequent loss of the endothelial antiinflammatory and endothelial repair-stimulating effects of HDL.

Background-High-density lipoprotein (HDL)-raising therapies are currently under intense evaluation, but the effects of HDL may be highly heterogeneous. We therefore compared the endothelial effects of HDL from healthy subjects and from patients with type 2 diabetes mellitus and low HDL (meeting the criteria for metabolic syndrome), who are frequently considered for HDL-raising therapies. Moreover, in diabetic patients, we examined the impact of extended-release (ER) niacin therapy on the endothelial effects of HDL. Methods and Results-HDL was isolated from healthy subjects (nϭ10) and patients with type 2 diabetes (nϭ33) by sequential ultracentrifugation. Effects of HDL on endothelial nitric oxide and superoxide production were characterized by electron spin resonance spectroscopy analysis. Effects of HDL on endothelium-dependent vasodilation and early endothelial progenitor cell-mediated endothelial repair were examined. Patients with diabetes were randomized to a 3-month therapy with ER niacin (1500 mg/d) or placebo, and endothelial effects of HDL were characterized. HDL from healthy subjects stimulated endothelial nitric oxide production, reduced endothelial oxidant stress, and improved endothelium-dependent vasodilation and early endothelial progenitor cell-mediated endothelial repair. In contrast, these beneficial endothelial effects of HDL were not observed in HDL from diabetic patients, which suggests markedly impaired endothelial-protective properties of HDL. ER niacin therapy improved the capacity of HDL to stimulate endothelial nitric oxide, to reduce superoxide production, and to promote endothelial progenitor cell-mediated endothelial repair. Further measurements suggested increased lipid oxidation of HDL in diabetic patients, and a reduction after ER niacin therapy. Conclusions-HDL from patients with type 2 diabetes mellitus and metabolic syndrome has substantially impaired endothelial-protective effects compared with HDL from healthy subjects. ER niacin therapy not only increases HDL plasma levels but markedly improves endothelial-protective functions of HDL in these patients, which is potentially more important.

R educed plasma levels of high-density lipoprotein (HDL) cholesterol are associated with an increased risk of coronary artery disease (CAD).1 Moreover, in patients with CAD that are treated with statin and have low levels of low-density lipoprotein (LDL) cholesterol, reduced HDL cholesterol levels were predictive of major cardiovascular events.2 Besides promoting reverse cholesterol transport, 3,4 HDL has been demonstrated to exert antiatherosclerotic effects, including antiinflammatory properties and stimulation of endothelial nitric oxide (NO) production. [5][6][7][8][9] However, these effects of HDL have been observed to be highly heterogenous in patients with CAD or diabetes mellitus. 10-12 Editorial see p 868 Clinical Perspective on p 904Endothelial dysfunction and injury are thought to contribute importantly to the progression of CAD. [13][14][15] Experimental studies have indicated that atherosclerotic lesion-prone vascular regions are characterized by a high endothelial cell turnover, 16 which has been attributed to an increased rate of endothelial cell apoptosis. Moreover, superficial atherosclerotic plaque erosion with the loss of an intact endothelial cell monolayer is observed quite frequently in patients with an acute coronary syndrome (ACS) based on pathological studies, 17,18 and on Background-Endothelial dysfunction and injury are thought to play an important role in the progression of coronary artery disease (CAD). High-density lipoprotein from healthy subjects (HDL Healthy ) has been proposed to exert endothelial antiapoptotic effects that may represent an important antiatherogenic property of the lipoprotein. The present study therefore aimed to compare effects of HDL CAD and HDL Healthy on the activation of endothelial anti-and proapoptotic pathways and to determine which changes of the lipoprotein are relevant for these processes. Methods and Results-HDL was isolated from patients with stable CAD (HDL sCAD ), an acute coronary syndrome (HDL ACS ), and healthy subjects. HDL Healthy induced expression of the endothelial antiapoptotic Bcl-2 protein Bcl-xL and reduced endothelial cell apoptosis in vitro and in apolipoprotein E-deficient mice in vivo. In contrast, HDL sCAD and HDL ACS did not inhibit endothelial apoptosis, failed to activate endothelial Bcl-xL, and stimulated endothelial proapoptotic pathways, in particular, p38-mitogen-activated protein kinase-mediated activation of the proapoptotic Bcl-2 protein tBid. Endothelial antiapoptotic effects of HDL Healthy were observed after inhibition of endothelial nitric oxide synthase and after delipidation, but not completely mimicked by apolipoprotein A-I or reconstituted HDL, suggesting an important role of the HDL proteome. HDL proteomics analyses and subsequent validations and functional characterizations suggested a reduced clusterin and increased apolipoprotein C-III content of HDL sCAD and HDL ACS as mechanisms leading to altered effects on endothelial apoptosis. Conclusions-The present study demonstrates for the first time that HDL CAD d...

The macrophage scavenger receptor, which has been implicated in the pathogenesis of atherosclerosis, has an unusually broad binding specificity. Ligands include modified low-density lipoprotein and some polyanions (for example, poly(I) but not poly(C]. The scavenger receptor type I (ref. 3) has three principal extracellular domains that could participate in ligand binding: two fibrous coiled-coil domains (alpha-helical coiled-coil domain IV and collagen-like domain V), and the 110-amino-acid cysteine-rich C-terminal domain VI. We have cloned complementary DNAs encoding a second scavenger receptor which we have termed type II. This receptor is identical to the type I receptor, except that the cysteine-rich domain is replaced by a six-residue C terminus. Despite this truncation, the type II receptor mediates endocytosis of chemically modified low-density lipoprotein with high affinity and specificity, similar to that of the type I receptor. Therefore one or both of the extracellular fibrous domains are responsible for the unusual ligand-binding specificity of the receptor.