Beyond high-density lipoprotein cholesterol levels evaluating high-density lipoprotein function as influenced by novel therapeutic approaches

Abstract

A number of therapeutic strategies targeting high-density lipoprotein (HDL) cholesterol and reverse cholesterol transport are being developed to halt the progression of atherosclerosis or even induce regression. However, circulating HDL cholesterol levels alone represent an inadequate measure of therapeutic efficacy. Evaluation of the potential effects of HDL-targeted interventions on atherosclerosis requires reliable assays of HDL function and surrogate markers of efficacy. Promotion of macrophage cholesterol efflux and reverse cholesterol transport is thought to be one of the most important mechanisms by which HDL protects against atherosclerosis, and methods to assess this pathway in vivo are being developed. Indexes of monocyte chemotaxis, endothelial inflammation, oxidation, nitric oxide production, and thrombosis reveal other dimensions of HDL functionality. Robust, reproducible assays that can be performed widely are needed to move this field forward and permit effective assessment of the therapeutic potential of HDL-targeted therapies.

Figure 1. Reverse Cholesterol Transport

High-density lipoprotein (HDL) cholesterol promotes and facilitates the process of reverse cholesterol transport (RCT), whereby excess macrophage cholesterol is effluxed to HDL and ultimately returned to the liver for excretion. Efflux to nascent and mature HDL occurs via the transporters ABCA1 and ABCG1, respectively. The HDL cholesterol is returned to the liver via the hepatic receptor SR-BI or by transfer to apolipoprotein (apo) B–containing lipoproteins by the action of cholesteryl ester transfer protein (2). Figure illustrations by Rob Flewell. CETP = cholesterol ester transfer protein; LDL = low-density lipoprotein; LDL-R = low-density lipoprotein receptor; SR-BI = scavenger receptor class B-type I; VLDL = very low-density lipoprotein.

Figure 2. Macrophage-Specific Reverse Cholesterol Transport In Vivo

(A) To assay macrophage-specific RCT, the critical atheroprotective pathway, a recently developed technique (33) follows the fate of radiolabeled cholesterol from prepared macrophages after injection. Macrophages are cholesterol-loaded with acetylated low-density lipoprotein (LDL) to become foam cells and labeled with ³H-cholesterol. Loaded and labeled macrophages are then injected intraperitoneally into mice, where they remain in the peritoneal cavity bathed in peritoneal fluid containing HDL acceptor particles. Subsequently, blood is sampled at several time points, and feces and plasma are collected continuously over 48 h and assayed for ³H-steroid. The fecal excretion of ³H-steroid is a measure of macrophage-to-feces RCT. (B) C57BL/6 mice were injected intravenously with apoA-I adenovirus (n = 10) or control (null) adenovirus vector (n = 10). Three days after vector injection, ³H-cholesterol-labeled J774 foam cells were injected. Compared with control subjects, mice overexpressing apoA-I demonstrated significantly higher levels of ³H-tracer in plasma at 48 h and in liver and feces. *p < 0.05 between control and apoA-I adenovirus groups. Reprinted, with permission, from Zhang et al. (33). Figure illustrations by Rob Flewell. Abbreviations as in Figure 1.

Figure 3. Kinetic Modeling of Cholesterol Efflux From Tissues In Vivo by Isotope Dilution

The flow of cholesterol from tissue to plasma, which encompasses the first step in RCT, can be assessed in vivo by noting the dilution of exogenous labeled cholesterol infused directly into the plasma compartment until steady-state levels are attained (47,48). The tracer abundance in plasma is diluted by the efflux of endogenous (unlabeled) cholesterol from tissues into plasma. At the isotopic plateau, the rate of appearance of endogenous cholesterol, representing tissue cholesterol efflux, equals the rate of tracer infusion divided by the plasma molar percent enrichment with labeled cholesterol. Figure illustrations by Rob Flewell. Abbreviations as in Figure 1.

Figure 4. HDL Functions Other Than Reverse Cholesterol Transport

High-density lipoprotein exerts a number of potentially antiatherogenic effects independent of cholesterol efflux and centripetal transport, including inhibiting lipid oxidation, impairing leukocyte adhesion and monocyte activation, promoting nitric oxide (NO) production and flow-induced vasodilation, preventing endothelial cell damage and death, and inhibiting activation of platelets and the coagulation cascade. The clinical significance of these varied functions remains unclear. Figure illustrations by Rob Flewell. Abbreviations as in Figures 1 and 2.

Figure 5. Lipid Oxidation

(A) Dichlorofluorescein (DCF), a fluorescent marker of lipid oxidation products, is added to test HDL isolated from patient serum (85). Oxidized phospholipids found in mildly oxidized HDL, such as 1-palmitoyl-2(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC) and 1-palmitoyl-2(5,6-epoxyisoprostane E2)-sn-glycero-3-phosphorylcholine (PEIPC), are added to assess the ability of HDL to inactivate biologically active phospholipids. To assess the ability of HDL to prevent phospholipid oxidation, HDL is added to a mixture of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) from unoxidized LDL and the endothelium-derived oxidants hydroperoxyoctadecadienoic acid (HPODE) or hydroperoxyeicosatetraenoic acid (HPETE). After the reagents are combined, spectroscopy permits quantification of net oxidation, with diminished fluorescence intensity signaling fewer oxidized phospholipids and suggesting a more antiatherogenic HDL. (B) The “inflammatory index” is derived by dividing net antioxidant activity in the presence of HDL by that observed in the absence of HDL. The HDL obtained from coronary artery disease (CAD) patients exhibits an impaired ability to antagonize monocyte chemotaxis and lipid oxidation compared with control subjects (75). The atheroprotective effect of HDL is partially restored following statin therapy. Figure illustrations by Rob Flewell. Abbreviations as in Figures 1 and 2.

Figure 6. Endothelial Cell NO Generation and Vasodilation

(A) Basal endothelial production of nitrite and nitrate is generated by incubating endothelial cells with L-arginine, the substrate of nitric oxide synthase (NOS), in the presence of test serum/HDL (50,88-90). Stimulated production is achieved through simultaneous addition of an NOS agonist, such as A23187. To stimulate endothelial production of free radicals, endothelial cells are incubated with an NOS inhibitor, such as L-nitroarginine-methylester (L-NAME), thereby inhibiting free-radical scavenging capacity, and the concentration of ${\mathrm{O}}_2^{-}$ is measured. Using this cell-based assay, enhanced NOS activity, nitric oxide (NO) production, and diminished ${\mathrm{O}}_2^{-}$ generation achieved by the test serum/HDL suggest restorative effects on endothelium and thus an antiatherogenic benefit. (B) The apoA-I mimetic L-4F restores NO production in cultured endothelial cells coincubated with LDL in the presence or absence of the NOS agonist A23187 (90). Reprinted, with permission, from Ou et al. (90). Figure illustrations by Rob Flewell. eNOS = endothelial nitric oxide synthase; other abbreviations as in Figures 1 and 2.