Brian J. Van Lenten

The Yin and Yang of Oxidation in the Development of the Fatty Streak A Review Based on the 1994 George Lyman Duff Memorial Lecture

Recent data support the hypothesis that the fatty streak develops in response to specific phospholipids contained in LDL that become trapped in the artery wall and become oxidized as a result of exposure to the oxidative waste of the artery wall cells. The antioxidants present within both LDL and the microenvironments in which LDL is trapped function to prevent the formation of these biologically active, oxidized lipids. Enzymes associated with LDL and HDL (eg, platelet activating factor acetylhydrolase) or with HDL alone (eg, paraoxonase) destroy these biologically active lipids. The regulation and expression of these enzymes are determined genetically and are also significantly modified by environmental influences, including the acute-phase response or an atherogenic diet. The balance of these multiple factors leads to an induction or suppression of the inflammatory response in the artery wall and determines the clinical course.

HDL and cardiovascular disease: atherogenic and atheroprotective mechanisms

The lipoprotein HDL has two important roles: first, it promotes reverse cholesterol transport, and second, it modulates inflammation. Epidemiological studies show that HDL-cholesterol levels are inversely correlated with the risk of cardiovascular events. However, many patients who experience a clinical event have normal, or even high, levels of HDL cholesterol. Measuring HDL-cholesterol levels provides information about the size of the HDL pool, but does not predict HDL composition or function. The main component of HDL, apolipoprotein A-I (apo A-I), is largely responsible for reverse cholesterol transport through the macrophage ATP-binding cassette transporter ABCA1. Apo A-I can be damaged by oxidative mechanisms, which render the protein less able to promote cholesterol efflux. HDL also contains a number of other proteins that are affected by the oxidative environment of the acute-phase response. Modification of the protein components of HDL can convert it from an anti-inflammatory to a proinflammatory particle. Small peptides that mimic some of the properties of apo A-I have been shown in preclinical models to improve HDL function and reduce atherosclerosis without altering HDL-cholesterol levels. Robust assays to evaluate the function of HDL are needed to supplement the measurement of HDL-cholesterol levels in the clinic.

High-Density Lipoprotein Loses Its Anti-Inflammatory Properties During Acute Influenza A Infection

BackgroundViruses have been identified as one of a variety of potential agents that are implicated in atherogenesis. Methods and ResultsC57BL/6J mice were killed before or 2, 3, 5, 7, or 9 days after intranasal infection with 105 plaque-forming units (pfu) of Influenza A strain WSN/33. Peak infectivity in lungs was reached by 72 hours, and it returned to baseline by 9 days. No viremia was observed at any time. The activities of paraoxonase and platelet-activating factor acetylhydrolase in HDL decreased after infection and reached their lowest levels 7 days after inoculation. The ability of HDL from infected mice to inhibit LDL oxidation and LDL-induced monocyte chemotactic activity in human artery wall cell cocultures decreased with time after inoculation. Moreover, as the infection progressed, LDL more readily induced monocyte chemotaxis. Peak interleukin-6 and serum amyloid A plasma levels were observed at 2 and 7 days after inoculation. HDL apoA-I levels did not change. ApoJ and ceruloplasmin levels in HDL peaked 3 days after infection. Ceruloplasmin remained elevated throughout the time course, whereas apoJ levels decreased toward baseline after the third day. ConclusionsWe conclude that alterations in the relative levels of paraoxonase, platelet-activating factor acetylhydrolase, ceruloplasmin, and apoJ in HDL occur during acute influenza infection, causing HDL to lose its anti-inflammatory properties.

Mechanisms of Disease: proatherogenic HDL—an evolving field

It is well known that, in large populations, HDL-cholesterol levels are inversely related to the risk of atherosclerotic clinical events; however, in an individual, the predictive value of an HDL-cholesterol level is far from perfect. As a result, other HDL-associated factors have been investigated, including the quality and function of HDL in contradistinction to the level of HDL-cholesterol. Regarding their quality, HDL particles are highly heterogeneous and contain varying levels of antioxidants or pro-oxidants, which results in variation in HDL function. It has been postulated that HDL functions to promote reverse cholesterol transport. Recent studies support this role for HDL but also indicate that HDL is a modulator of systemic inflammation. In the absence of inflammation, HDL has a complement of antioxidant enzymes that work to maintain an anti-inflammatory state. In the presence of systemic inflammation, these antioxidant enzymes can be inactivated and HDL can accumulate oxidized lipids and proteins that make it proinflammatory. Under these conditions the main protein of HDL, apolipoprotein A-I, can be modified by reactive oxygen species. This modification impairs the ability of HDL to promote cholesterol efflux by the ATP-binding cassette transporter A-1 pathway. Animal studies and small-scale human studies suggest that measures of the quality and novel functions of HDL might provide an improved means of identifying subjects at increased risk for atherosclerotic events, compared with the current practice of only measuring HDL-cholesterol levels. The quality and function of HDL are also attractive targets for emerging therapies.

Influenza Infection Promotes Macrophage Traffic Into Arteries of Mice That Is Prevented by D-4F, an Apolipoprotein A-I Mimetic Peptide

Background—We reported that HDL loses its antiinflammatory properties during acute influenza A infection in mice, and we hypothesized that these changes might be associated with increased trafficking of macrophages into the artery wall. The present study tested this hypothesis. Methods and Results—D-4F, an apolipoprotein A-I mimetic peptide, or vehicle in which it was dissolved (PBS) was administered daily to LDL receptor–null mice after a Western diet and after influenza infection. D-4F treatment increased plasma HDL cholesterol and paraoxonase activity compared with PBS and inhibited increases in LDL cholesterol and peak levels of interleukin-6 after infection. Lung viral titers were reduced by 50% in mice receiving D-4F. Injection of female mice with male macrophages, which were detected with real-time polymerase chain reaction to measure the male Sry gene, revealed a marked increase in macrophage traffic into the aortic arch and innominate arteries after infection that was prevented by administration of D-4F. Conclusions—We conclude that loss of antiinflammatory properties of HDL after influenza infection in mice is associated with increased arterial macrophage traffic that can be prevented by administration of D-4F.

The double jeopardy of HDL.

The ability of high‐density lipoprotein (HDL) to promote cholesterol efflux is thought to be important in its protection against cardiovascular disease. Anti‐inflammatory properties of HDL have emerged as additional properties that may also be important. HDL appears to have evolved as part of the innate immune system functioning to inhibit inflammation in the absence of an acute phase response (APR) but functioning to increase inflammation in the presence of an APR. Inbred strains of mice that are genetically susceptible to atherosclerosis have pro‐inflammatory HDL, while inbred strains that are resistant to atherosclerosis have anti‐inflammatory HDL. In one small study, humans with coronary heart disease (CHD) or CHD equivalents had pro‐inflammatory HDL prior to statin therapy and about half continued to have pro‐inflammatory HDL after statin therapy despite a profound decrease in plasma lipids. Pro‐inflammatory HDL was relatively weak in its ability to promote cholesterol efflux while anti‐inflammatory HDL was better in promoting cholesterol efflux. In other studies, oxidative alterations of the major protein of HDL, apolipoprotein A‐I (apoA‐I), impaired the ability of the apoA‐I to promote cholesterol efflux. Thus, HDL structure and function may be more important than HDL‐cholesterol levels in predicting risk for cardiovascular disease.

Apolipoprotein A-I (apoA-I) and apoA-I mimetic peptides inhibit tumor development in a mouse model of ovarian cancer

We examined whether reduced levels of Apolipoprotein A-I (apoA-I) in ovarian cancer patients are causal in ovarian cancer in a mouse model. Mice expressing a human apoA-I transgene had (i) increased survival (P < 0.0001) and (ii) decreased tumor development (P < 0.01), when compared with littermates, following injection of mouse ovarian epithelial papillary serous adenocarcinoma cells (ID-8 cells). ApoA-I mimetic peptides reduced viability and proliferation of ID8 cells and cis-platinum–resistant human ovarian cancer cells, and decreased ID-8 cell-mediated tumor burden in C57BL/6J mice when administered subcutaneously or orally. Serum levels of lysophosphatidic acid, a well-characterized modulator of tumor cell proliferation, were significantly reduced (>50% compared with control mice, P < 0.05) in mice that received apoA-I mimetic peptides (administered either subcutaneously or orally), suggesting that binding and removal of lysophosphatidic acid is a potential mechanism for the inhibition of tumor development by apoA-I mimetic peptides, which may serve as a previously unexplored class of anticancer agents.

Oxidized lipids as mediators of coronary heart disease.

Purpose of review To summarize the recent evidence on the physiological relevance of the view that LDL lipid oxidation may play a major role in the inflammatory reaction that leads to or amplifies atherogenesis. Oxidation of LDL phospholipids containing arachidonic acid at the sn-2 position occurs when a critical concentration of ‘seeding molecules’ derived from the lipoxygenase pathway is reached in LDL. This generates a series of biologically active, oxidized phospholipids that mediate the cellular events seen in the developing fatty streak. Recent findings We have observed that LDL from mice that are genetically predisposed to diet-induced atherosclerosis is highly proinflammatory when the mice are maintained on an atherogenic diet, when they are injected with LDL-derived oxidized phospholipids, or once they are infected with influenza A virus. Patients with coronary atherosclerosis also had highly proinflammatory LDL, despite having normal blood lipid levels or normal plasma HDL levels. Summary We and others have hypothesized that HDL and LDL-derived oxidized phospholipids may be part of a system of nonspecific innate immunity. We therefore propose that determination of HDL capacity against LDL oxidation and the detection of proinflammatory HDL may be a useful marker of susceptibility to atherosclerosis.

Lipoprotein inflammatory properties and serum amyloid A levels but not cholesterol levels predict lesion area in cholesterol-fed rabbits

Rabbits on a 1% cholesterol diet received injections of vehicle with or without D-4F or L-4F. After 1 month, the percent of aorta with atherosclerotic lesions was 24 ± 15% (vehicle), 10 ± 6% (D-4F) (P < 0.01 vs. vehicle), and 13 ± 9% (L-4F) (P < 0.05 vs. vehicle). Inflammatory indexes for HDL and LDL were determined by measuring monocyte chemotactic activity after adding rabbit lipoproteins to human endothelial cells. HDL-inflammatory index (HII) and LDL-inflammatory index (LII), respectively, were 1.39 ± 0.24; 1.35 ± 0.29 (vehicle), 0.67 ± 0.26; 0.63 ± 0.38 (D-4F) (P < 0.001 vs. vehicle), and 0.67 ± 0.2; 0.68 ± 0.32 (L-4F) (P < 0.01 vs. vehicle). Serum amyloid A (SAA) levels were 95 ± 39, 8 ± 22, and 7 ± 19 μg/ml, respectively, for vehicle, D-4F, and L-4F (P < 0.001 vs. vehicle). There was no correlation between lesion area and total plasma or HDL-cholesterol levels. In contrast, there was a positive correlation with HII, LII, and SAA (P = 0.002, P = 0.0026, P = 0.0079, respectively). HII correlated closely with SAA levels (r = 0.6616; r2 = 0.4377, P < 0.0001). Thus, HII, LII, and SAA are better predictors of lesion area than are total plasma or HDL-cholesterol levels in cholesterol-fed rabbits.

An Oral ApoJ Peptide Renders HDL Antiinflammatory in Mice and Monkeys and Dramatically Reduces Atherosclerosis in Apolipoprotein E–Null Mice

Objective—To determine the properties of a peptide synthesized from D-amino acids corresponding to residues 113 to 122 in apolipoprotein (apo) J. Methods and Results—In contrast to D-4F, D- [113–122]apoJ showed minimal self-association and helicity in the absence of lipids. D-4F increased the concentration of apoA-I with pre-&bgr; mobility in apoE-null mice whereas D- [113–122]apoJ did not. After an oral dose D- [113–122]apoJ more slowly associated with lipoproteins and was cleared from plasma much more slowly than D-4F. D- [113–122]apoJ significantly improved the ability of plasma to promote cholesterol efflux and improved high-density lipoprotein (HDL) inflammatory properties for up to 48 hours after a single oral dose in apoE-null mice, whereas scrambled D- [113–122]apoJ did not. Oral administration of 125 &mgr;g/mouse/d of D- [113–122]apoJ reduced atherosclerosis in apoE-null mice (70.2% reduction in aortic root sinus lesion area, P=4.3×10−13; 70.5% reduction by en face analysis, P=1.5×10−6). In monkeys, oral D- [113–122]apoJ rapidly reduced lipoprotein lipid hydroperoxides (LOOH) and improved HDL inflammatory properties. Adding 250 ng/mL of D-[113–122]apoJ (but not scrambled D- [113–122]apoJ) to plasma in vitro reduced LOOH and increased paraoxonase activity. Conclusions—Oral D- [113–122]apoJ significantly improves HDL inflammatory properties in mice and monkeys and inhibits lesion formation in apoE-null mice.