Myeloperoxidase impairs ABCA1-dependent cholesterol efflux through methionine oxidation and site-specific tyrosine chlorination of apolipoprotein A-I

Role of redox regulation and lipid rafts in macrophages during Ox-LDL-mediated foam cell formation

Hyperlipidemias and small dense LDLs in patients with high-triglyceride low-HDL syndromes lead to a prolonged half life of apoB-containing particles. This is associated with reactive oxygen species (ROS) activation and leads to formation of oxidized LDL (Ox-LDL). Generators of ROS in macrophages (MACs) include myeloperoxidase (MPO)-mediated respiratory burst and raft-associated NADPH-oxidase. The intracellular oxidant milieu is involved in cellular signaling pathways, like ion-transport systems, protein phosphorylation, and gene expression. Lipid oxidation through ROS can amplify foam cell formation through Ox-LDL uptake, leading to formation of ceramide (Cer)-rich lipid membrane microdomains, and is associated with expansion of the lysosomal compartment and an upregulation of ABCA1 and other genes of the AP3 secretory pathway. Ox-LDL may also affect cell-surface turnover of Cer-backbone sphingolipids and apoE-mediated uptake by LRP-family members. In contrast, HDL-mediated lipid efflux causes disruption of lipid membrane microdomains and prevents foam cell formation. Oxidation of HDL through MPO leads to a failure of lipid efflux and enhancement of MAC loading. Therefore, lipid rafts and oxidation processes are important in regulation of MAC foam cell formation and atherosclerosis, and the balance between oxidant and antioxidant intracellular systems is critically important for efficient MAC function.

Advanced glycation of apolipoprotein A-I impairs its anti-atherogenic properties

AIMS/HYPOTHESIS

AGE contribute to the pathogenesis of diabetic complications, including dyslipidaemia and atherosclerosis. However, the precise mechanisms remain to be established. In the present study, we examined whether AGE modification of apolipoprotein A-I (apoA-I) affects its functionality, thus altering its cardioprotective profile.

MATERIALS AND METHODS

The ability of AGE-modified apoA-I to facilitate cholesterol and phospholipid efflux, stabilise ATP-binding cassette transporter A1 (ABCA1) and inhibit expression of adhesion molecules in human macrophages and monocytes was studied.

RESULTS

The ability of AGE-modified apoA-I to promote cholesterol efflux from THP-1 macrophages, isolated human monocytes and from ABCA1-transfected HeLa cells was significantly reduced (>70%) compared with unmodified apoA-I. This effect was reversed by preventing AGE formation with aminoguanidine or reversing AGE modification using the cross-link breaker alagebrium chloride. AGE-modification of HDL also reduced its capacity to promote cholesterol efflux. AGE-apoA-I was also less effective than apoA-I in stabilising ABCA1 in THP-1 cells as well as in inhibiting expression of CD11b in human monocytes.

CONCLUSIONS/INTERPRETATION

AGE modification of apoA-I considerably impairs its cardioprotective, antiatherogenic properties, including the ability to promote cholesterol efflux, stabilise ABCA1 and inhibit the expression of adhesion molecules. These findings provide a rationale for targeting AGE in the management of diabetic dyslipidaemia.

Serum Myeloperoxidase Levels Are Associated With the Future Risk of Coronary Artery Disease in Apparently Healthy Individuals

OBJECTIVES

We evaluated whether serum myeloperoxidase (MPO) levels are associated with the risk of future development of coronary artery disease (CAD) in apparently healthy individuals.

BACKGROUND

An enzyme of the innate immune system, MPO exhibits a wide array of proatherogenic effects. These include induction of oxidative damage to low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol and promotion of plaque vulnerability. Recent studies revealed that MPO independently predicts adverse outcomes in patients with chest pain or suspected acute coronary syndrome.

METHODS

Myeloperoxidase was measured in baseline samples of a case-control study nested in the prospective EPIC (European Prospective Investigation into Cancer and Nutrition)-Norfolk population study. Case subjects (n = 1,138) were apparently healthy men and women who developed CAD during 8-year follow-up. Control subjects (n = 2,237), matched for age, gender, and enrollment time, remained free of CAD.

RESULTS

The MPO levels were significantly higher in case subjects than in control subjects and correlated with C-reactive protein (CRP) (rho = 0.25; p < 0.001) and white blood cell count (rho = 0.33; p < 0.001). Risk of future CAD increased in consecutive quartiles of MPO concentration, with an odds ratio (OR) of 1.49 in the top versus bottom quartile (95% confidence interval [CI] 1.20 to 1.84; p < 0.001). After adjustment for traditional risk factors, the OR in the top quartile remained significant at 1.36 (95% CI 1.07 to 1.73). Elevated MPO levels (>728 pmol/l) similarly predicted increased risk of future CAD among participants with either LDL-cholesterol <130 mg/dl, HDL-cholesterol >50 mg/dl, or CRP <2.0 mg/l (OR 1.52 [95% CI 1.21 to 1.91], 1.59 [95% CI 1.24 to 2.05], and 1.42 [95% CI 1.14 to 1.77)], respectively).

CONCLUSION

Elevated MPO levels predict future risk of CAD in apparently healthy individuals. This study suggests that inflammatory activation precedes the onset of overt CAD by many years.

HDL proteomics: pot of gold or Pandora's box?

In this issue of the JCI, Vaisar et al. studied the proteome of HDL (see the related article beginning on page 746). They reveal, quite unexpectedly, that HDL is enriched in several proteins involved in the complement cascade, as well as in a variety of protease inhibitors, supporting the concept that HDL plays a role in innate immunity and in the regulation of proteolytic cascades involved in inflammatory and coagulation processes. The protein makeup of HDL also appears to be altered in patients with coronary artery disease. HDL proteomics is in its infancy, and preliminary findings will need to be confirmed using standardized approaches in larger clinical samples. However, this approach promises to better elucidate the relationship of HDL to atherosclerosis and its complications and could eventually help in the development of biomarkers to predict the outcome of interventions that alter HDL levels and functions.

The strange case of Dr HDL and Mr HDL: Does a NO’s story illuminate the mystery of HDL’s dark side uncovered by Dr HDL’s drug targeting CETP?

Recently, the first large-scale morbidity and mortality trial (ILLUMINATE) to evaluate the cardiovascular end points of a CETP inhibitor (torcetrapib) has been prematurely stopped because the mortality was significantly increased in the treated group. Why torcetrapib caused excess death is not known. Based on the fact that HDL interacts with endothelial nitric oxyde synthase (eNOS) and nitric oxide (NO) secretion, which partly controlled blood pressure and than torcetrapib could increase blood pressure among some patients, we hypothesize that CETP inhibition could have significantly inhibit eNOS. CETP inhibition would have enlarged HDL size resulting in a deficit in the interaction between HDL and the Scavenger Receptor class B type I (SR-BI), which is an important link between HDL and eNOS activation. We suggest than the deficit in NO secretion would have been sufficient among all patients to induce a destabilization of the plaques of atheroma, but could have induced a pathogenic increase in blood pressure only in patients whose eNOS activity was naturally weak due to genetic polymorphisms of this enzyme. We also hypothesize that the increase in HDL levels, induced by CETP inhibition, coupled with the capacity of HDL to induce endothelin-1 secretion would have aggravated the cardiovascular risks under this CETP inhibitor treatment.

Substrates of the methionine sulfoxide reductase system and their physiological relevance

Posttranslational modifications can change a protein's structure, function, and solubility. One specific modification caused by reactive oxygen species is the oxidation of the sulfur atom in the methionine (Met) side chain. This modified amino acid is denoted as methionine sulfoxide (MetO). MetOs in proteins are of considerable interest as they are involved in early posttranslational modification events. Thus, various organisms produce specific enzymes that can reverse these modifications. MetO reductases, known collectively as the methionine sulfoxide reductase (Msr) system, are the only known enzymes that can reduce MetOs. The current research field of Met redox cycles is consumed with elucidating its role in regulation, redox homeostasis, prevention of irreversible modifications, pathogenesis, and the aging process. Substrates of the Msr system can be loosely classified by the overall effect of the MetO on the protein. Regulated substrates utilize Met as a molecular switch to modulate activation; scavenging substrates use Mets to detoxify oxidants and protect important regions of the protein; and modified substrates are altered by Met oxidation resulting in various changes in their properties, including function, activity, structure, and degradation resistance.

Haptoglobin genotype is a regulator of reverse cholesterol transport in diabetes in vitro and in vivo

Two common alleles exist at the haptoglobin (Hp) locus, and the Hp2 allele is associated with an increased incidence of cardiovascular disease, specifically in diabetes mellitus (DM). Oxidative stress is increased in Hp2 mice and humans with DM. Oxidative modification of the apolipoprotein A-I inhibits reverse cholesterol transport. We sought to test the hypothesis that reverse cholesterol transport is impaired in Hp2 DM mice and humans. In vitro, using serum from non-DM and DM individuals, we measured cholesterol efflux from (3)H-cholesterol-labeled macrophages. In vivo, we injected (3)H-cholesterol-loaded macrophages intraperitoneally into non-DM and DM mice with the Hp1-1 or Hp2-2 genotype and monitored (3)H-tracer levels in plasma, liver, and feces. In vitro, in DM individuals only, we observed significantly decreased cholesterol efflux from macrophages incubated with serum from Hp2-1 or Hp2-2 as compared with Hp1-1 individuals (P<0.01). The interaction between Hp type and DM was recapitulated using purified Hp and glycated Hb. In vivo, DM mice loaded with (3)H-cholesterol-labeled macrophages had a 40% reduction in (3)H-cholesterol in plasma, liver, and feces as compared with non-DM mice (P<0.01). The reduction in reverse cholesterol transport associated with DM was significantly greater in Hp2-2 mice as compared with Hp1-1 mice (54% versus 25% in plasma; 52% versus 27% in liver; 57% versus 32% in feces; P<0.03). reverse cholesterol transport is decreased in Hp2-2 DM. This may explain in part the increased atherosclerotic burden found in Hp2-2 DM individuals.