Adenovirus-mediated gene transfer of Lp-PLA2 reduces LDL degradation and foam cell formation in vitro

Vascular endothelial growth factor modified macrophages transdifferentiate into endothelial-like cells and decrease foam cell formation

Macrophages are largely involved in the whole process of atherosclerosis from an initiation lesion to an advanced lesion. Endothelial disruption is the initial step and macrophage-derived foam cells are the hallmark of atherosclerosis. Promotion of vascular integrity and inhibition of foam cell formation are two important strategies for preventing atherosclerosis. How can we inhibit even the reverse negative role of macrophages in atherosclerosis? The present study was performed to investigate if overexpressing endogenous human vascular endothelial growth factor (VEGF) could facilitate transdifferentiation of macrophages into endothelial-like cells (ELCs) and inhibit foam cell formation. We demonstrated that VEGF-modified macrophages which stably overexpressed human VEGF (hVEGF₁₆₅) displayed a high capability to alter their phenotype and function into ELCs in vitro. Exogenous VEGF could not replace endogenous VEGF to induce the transdifferentiation of macrophages into ELCs in vitro. We further showed that VEGF-modified macrophages significantly decreased cytoplasmic lipid accumulation after treatment with oxidized LDL (ox-LDL). Moreover, down-regulation of CD36 expression in these cells was probably one of the mechanisms of reduction in foam cell formation. Our results provided the in vitro proof of VEGF-modified macrophages as atheroprotective therapeutic cells by both promotion of vascular repair and inhibition of foam cell formation.

Advances in the Study of the Antiatherogenic Function and Novel Therapies for HDL

The hypothesis that raising high-density lipoprotein cholesterol (HDL-C) levels could improve the risk for cardiovascular disease (CVD) is facing challenges. There is multitudinous clear clinical evidence that the latest failures of HDL-C-raising drugs show no clear association with risks for CVD. At the genetic level, recent research indicates that steady-state HDL-C concentrations may provide limited information regarding the potential antiatherogenic functions of HDL. It is evident that the newer strategies may replace therapeutic approaches to simply raise plasma HDL-C levels. There is an urgent need to identify an efficient biomarker that accurately predicts the increased risk of atherosclerosis (AS) in patients and that may be used for exploring newer therapeutic targets. Studies from recent decades show that the composition, structure and function of circulating HDL are closely associated with high cardiovascular risk. A vast amount of data demonstrates that the most important mechanism through which HDL antagonizes AS involves the reverse cholesterol transport (RCT) process. Clinical trials of drugs that specifically target HDL have so far proven disappointing, so it is necessary to carry out review on the HDL therapeutics.

Phospholipase A2 regulation of lipid droplet formation

The classical regard of lipid droplets as mere static energy-storage organelles has evolved dramatically. Nowadays these organelles are known to participate in key processes of cell homeostasis, and their abnormal regulation is linked to several disorders including metabolic diseases (diabetes, obesity, atherosclerosis or hepatic steatosis), inflammatory responses in leukocytes, cancer development and neurodegenerative diseases. Hence, the importance of unraveling the cell mechanisms controlling lipid droplet biosynthesis, homeostasis and degradation seems evident Phospholipase A2s, a family of enzymes whose common feature is to hydrolyze the fatty acid present at the sn-2 position of phospholipids, play pivotal roles in cell signaling and inflammation. These enzymes have recently emerged as key regulators of lipid droplet homeostasis, regulating their formation at different levels. This review summarizes recent results on the roles that various phospholipase A2 forms play in the regulation of lipid droplet biogenesis under different conditions. These roles expand the already wide range of functions that these enzymes play in cell physiology and pathophysiology.

A macrophage-specific synthetic promoter for therapeutic application of adiponectin

Foam cell formation from macrophage is a major cause of atherosclerosis. An efficient macrophage-specific promoter is required for the targeting to macrophages. In this study, we develop a macrophage-specific synthetic promoter for the therapeutic application of adiponectin (APN), an antiatherogenic gene. Synthetic promoter-146 (SP146), registered on the NCBI website (http://www.ncbi.nlm.nih.gov/nuccore/DQ107383), was tested for promoter activities in two non-macrophage cell lines (293 T, HeLa) and a macrophage cell line (RAW264.7, bone marrow-derived macrophages). To enforce macrophage specificity, partial elements of p47(phox) including the PU.1 site with various lengths (-C1, -C2 and -C3) were inserted next to the synthetic promoters. SP146-C1 showed the highest specificity and efficacy in RAW264.7 cells and was selected for development of an APN-carrying macrophage-specific promoter. Green fluorescent protein (GFP)- or APN-expressing lentivirus under SP146-C1 (Lenti-SP-GFP or Lenti-SP-APN, respectively) showed the highest expression efficacy in RAW264.7 cells compared with the non-macrophage cell lines. APN overexpression in RAW264.7 cells successfully inhibited intracellular lipid accumulation, and atherosclerotic lesions and lipid accumulation were significantly reduced by Lenti-SP-APN in ApoE-/- atherosclerosis mice. In conclusion, the synthetic promoter SP146-C1, combined with a p47(phox) promoter element, was successfully developed to target macrophage, and macrophage-specific introduction of APN under SP146-C1 was shown to ameliorate the atherosclerotic pathology.

PLAC™ test for identification of individuals at increased risk for coronary heart disease

Recent advances in cardiovascular research point to a critical role of inflammatory processes in the etiology of cardiovascular disease. This has led to the discovery of novel inflammatory biomarkers, which may be useful as additional screening tools for the identification of individuals at increased risk of coronary heart disease. One such novel inflammatory biomarker is lipoprotein-associated phospholipase A(2). This review discusses the recent development of a US Food and Drug Administration-approved blood test for lipoprotein-associated phospholipase A(2) (PLAC test, diaDexus, Inc.) and its efficacy as a predictive biomarker of risk for cardiovascular disease. More specifically, the article addresses the potential target group most likely to benefit from this new screening test and provides a prospective scenario for its implementation.

Oxidatively truncated phospholipids are required agents of tumor necrosis factor α (TNFα)-induced apoptosis

TNFα generates reactive oxygen species (ROS) at the cell surface that induce cell death, but how ROS communicate to mitochondria and their specific apoptotic action(s) are both undefined. ROS oxidize phospholipids to hydroperoxides that are friable and fragment adjacent to the (hydro)peroxide function, forming truncated phospholipids, such as azelaoyl phosphatidylcholine (Az-PC). Az-PC is relatively soluble, and exogenous Az-PC rapidly enters cells to damage mitochondrial integrity and initiate intrinsic apoptosis. We determined whether this toxic phospholipid is formed within cells during TNFα stimulation in sufficient quantities to induce apoptosis and if they are essential in TNFα-induced cytotoxicity. We found that TNFα induced ROS formation and phospholipid peroxidation in Jurkat cells, and either chemical interference with NADPH oxidase activity or siRNA suppression of the NADPH oxidase-4 subunit blocked ROS accumulation and phospholipid peroxidation. Mass spectrometry showed that phospholipid peroxides and then Az-PC increased after TNFα exposure, whereas ROS inhibition abolished Az-PC accumulation and TNFα-induced cell death. Glutathione peroxidase-4 (GPx4), which specifically metabolizes lipid hydroperoxides, fell in TNFα-stimulated cells prior to death. Ectopic GPx4 overcame this, reduced peroxidized phospholipid accumulation, blocked Az-PC accumulation, and prevented death. Conversely, GPx4 siRNA knockdown enhanced phospholipid peroxidation, increasing TNFα-stimulated Az-PC formation and apoptosis. Truncated phospholipids were essential elements of TNFα-induced apoptosis because overexpression of PAFAH2 (a phospholipase A(2) that selectively hydrolyzes truncated phospholipids) blocked TNFα-induced Az-PC accumulation without affecting phospholipid peroxidation. PAFAH2 also abolished apoptosis. Thus, phospholipid oxidation and truncation to apoptotic phospholipids comprise an essential element connecting TNFα receptor signaling to mitochondrial damage and apoptotic death.

Implication of lipoprotein associated phospholipase A2 activity in oxLDL uptake by macrophages

Recognition and uptake of oxidized LDL (oxLDL) by scavenger receptors of macrophages and foam cell formation are mediated by the oxidatively modified apolipoprotein B (ApoB) and lipid moiety of oxLDL. A great amount of oxidized phosphatidylcholine (oxPC) of oxLDL is hydrolyzed at the sn-2 position by lipoprotein associated phospholipase A(2) (Lp-PLA(2)) to lysophosphatidylcholine and small oxidation products. This study examines the involvement of Lp-PLA(2) in the uptake of oxLDL by mouse peritoneal macrophages. LDL with intact Lp-PLA(2) activity [LDL(+)] and LDL with completely inhibited Lp-PLA(2) activity [LDL(-)] were subjected to oxidation with 5 microM CuSO(4) for 6 h [moderately oxLDL (MoxLDL)], or 24 h [heavily oxLDL (HoxLDL)] and peritoneal macrophages were incubated with these preparations. The uptake of MoxLDL(-) was about 30% increased compared with that of MoxLDL(+), and HoxLDL(-) uptake was about 20% increased compared with that of HoxLDL(+). Inhibition of Lp-PLA(2) activity had no effect on the uptake of ApoB-liposomes conjugates with ApoB isolated from MoxLDL(-), MoxLDL(+), HoxLDL(-), and HoxLDL(+). Liposomes prepared from the lipid extract of MoxLDL(-), MoxLDL(+), HoxLDL(-), and HoxLDL(+) exhibited a similar pattern to that observed in the uptake of the corresponding intact lipoproteins. This study suggests that the progressive inactivation of Lp-PLA(2) during LDL oxidation leads to an increased uptake of oxLDL by macrophages, which could be primarily attributed to the increased uptake of the oxidized phospholipids enriched lipid moiety of oxLDL.

Lipoprotein-associated phospholipase A2: a cardiovascular risk predictor and a potential therapeutic target

Lipoprotein-associated phospholipase A2 (Lp-PLA2), present in the circulation and in atherosclerotic plaque, is an inflammatory marker with potential use as a predictor of cardiovascular risk and as a therapeutic target. Although Lp-PLA2 is associated with both LDL and HDL, it is important to determine whether Lp-PLA2 has a predominantly pro- or anti-atherogenic effect. Increasing evidence suggests a proatherogenic role for Lp-PLA2. ©iEpidemiologic and clinical evidence suggests Lp-PLA2 is an independent predictor of risk and may be superior to other inflammatory markers owing to its specificity and minimal biovariation. Lp-PLA2 inhibitors currently being investigated in clinical trials are promising novel anti-inflammatory agents with a specificity for the vascular bed and a potential for decreasing plaque vulnerability.

Biology of platelet-activating factor acetylhydrolase (PAF-AH, lipoprotein associated phospholipase A2)

INTRODUCTION

This article is focused on platelet-activating factor acetylhydrolase (PAF-AH), a lipoprotein bound, calcium-independent phospholipase A(2) activity also referred to as lipoprotein-associated phospholipase A(2) or PLA(2)G7. PAF-AH catalyzes the removal of the acyl group at the sn-2 position of PAF and truncated phospholipids generated in settings of inflammation and oxidant stress.

DISCUSSION

Here, I discuss current knowledge related to the structural features of this enzyme, including the molecular basis for association with lipoproteins and susceptibility to oxidative inactivation. The circulating form of PAF-AH is constitutively active and its expression is upregulated by mediators of inflammation at the transcriptional level. This mechanism is likely responsible for the observed up-regulation of PAF-AH during atherosclerosis and suggests that increased expression of this enzyme is a physiological response to inflammatory stimuli. Administration of recombinant forms of PAF-AH attenuate inflammation in a variety of experimental models. Conversely, genetic deficiency of PAF-AH in defined human populations increases the severity of atherosclerosis and other syndromes. Recent advances pointing to an interplay among oxidized phospholipid substrates, Lp(a), and PAF-AH could hold the key to a number of unanswered questions.

The emerging roles of PAF acetylhydrolase

Platelet-activating factor (PAF), a phospholipid autacoid with potent effects throughout the innate immune system, is selectively degraded by two small families of PAF acetylhydrolases (PAF-AHs). These Ca2+-independent phospholipases A2 display remarkable specificity for the length of the sn-2 residue, but this selectivity is lost as the residue gains oxygen functions. Two of the PAF-AHs therefore are specific oxidized phospholipid phospholipases that reduce inflammation, but also remove oxidatively truncated phospholipids that induce apoptosis. The roles of these enzymes are manifold, and their separate and combined functions are now being addressed in model systems and clinical studies.

The role of phospholipid oxidation products in inflammatory and autoimmune diseases: evidence from animal models and in humans

Since the discovery of oxidized phospholipids (OxPL) and their implication as modulators of inflammation in cardiovascular disease, roles for these lipid oxidation products have been suggested in many other disease settings. Lipid oxidation products accumulate in inflamed and oxidatively damaged tissue, where they are derived from oxidative modification of lipoproteins, but also from membranes of cells undergoing apoptosis. Thus, increased oxidative stress as well as decreased clearance of apoptotic cells has been implied to contribute to accumulation of OxPL in chronically inflamed tissues.A central role for OxPL in disease states associated with dyslipedemia, including atherosclerosis, diabetes and its complications, metabolic syndrome, and renal insufficiency, as well as general prothrombotic states, has been proposed. In addition, in organs which are constantly exposed to oxidative stress, including lung, skin, and eyes, increased levels of OxPL are suggested to contribute to inflammatory conditions. Moreover, accumulation of OxPL causes general immunmodulation and may lead to autoimmune diseases. Evidence is accumulating that OxPL play a role in lupus erythematosus, antiphospholipid syndrome, and rheumatoid arthritis. Last but not least, a role for OxPL in neurological disorders including multiple sclerosis (MS), Alzheimer's and Parkinson's disease has been suggested.This chapter will summarize recent findings obtained in animal models and from studies in humans that indicate that formation of OxPL represents a general mechanism that may play a major role in chronic inflammatory and autoimmune diseases.

Antioxidant gene therapy for cardiovascular disease: current status and future perspectives

Excessive production of reactive oxygen species has been implicated to play an important role in a number of cardiovascular pathologies, including hypertension, atherosclerosis, myocardial infarction, ischemia/reperfusion injury, and restenosis after angioplasty or venous bypass grafting. The formation of reactive oxygen species is balanced out by antioxidant defenses, and augmenting this defense by antioxidant therapies could therefore provide a potential means to treat conditions in which the formation of reactive oxygen species exceeds the capability of natural protective mechanisms. In this review, we summarize the studies in which antioxidant gene therapy has been used successfully to treat cardiovascular diseases. We also discuss the current limitations of antioxidant gene therapy and envision future therapeutic targets and methodological approaches for an improved outcome.

Comprehensive evaluation of genetic and environmental factors influencing the plasma lipoprotein-associated phospholipase A2 activity in a Japanese population

The lipoprotein-associated phospholipase A2 (Lp-PLA2) metabolizes oxidized phospholipids, generating lysophosphatidylcholine. The activity of the enzyme is known to be influenced largely by a single-nucleotide polymorphism, G994T, in the Lp-PLA2 gene. Interestingly, this polymorphism is much more prevalent in Japanese than Caucasians. The purpose of the current study was to evaluate the effects of the G994T, several environmental factors, and their interactions on the Lp-PLA2 activity in a large Japanese cohort. Participants (1,110 males and 908 females) of a health-screening examination were recruited for this study. Genotyping of the G994T was done using allele-specific polymerase chain reaction (PCR). The Lp-PLA2 activity was measured using commercial kits. The minor allele (994T) frequency of the polymorphism was 0.17 in this study, which was consistent with previous reports. According to the multivariate linear regression analysis, the G994T was the most potent factor influencing the enzyme activity (standardized beta=0.76), followed by the low-density lipoprotein cholesterol (LDL-C) level (standardized beta=0.32) and the sex (standardized beta=0.13). The LDL-C level showed a significant interaction with the G994T genotype. By contrast, no significant interaction was observed between the LDL-C level and the sex. These observations should provide useful information for future clinical and epidemiological evaluations of the Lp-PLA2 activity in cardiovascular diseases in Japanese.

Lipoprotein-Associated Phospholipase A2

Lipoprotein-associated phospholipase A2 (LP-PLA2) is an emerging inflammatory marker that is used to assess the risk for cardiovascular disease (CVD) and associated events. Several epidemiologic studies have demonstrated an independent association between plasma Lp-PLA2 concentration and risk for cardiovascular events. HMG-CoA reductase inhibitors (statins) and fenofibrates can reduce Lp-PLA2 concentrations in plasma, and orally active, specific Lp-PLA2 inhibitors have been developed and are in clinical trials to evaluate the potential of Lp-PLA2 as a therapeutic target. This article reviews recent studies of Lp-PLA2 in the setting of CVD, discusses the proposed mechanisms of action of Lp-PLA2, and describes methods for measurement and their clinical application. Recent evidence that suggests Lp-PLA2's potential usefulness as a therapeutic target also is reviewed.

Plasma PAF-acetylhydrolase: an unfulfilled promise?

Plasma Platelet-activating-Factor (PAF)-acetylhydrolase (PAF-AH also named lipoprotein-PLA(2) or PLA(2)G7 gene) is secreted by macrophages, it degrades PAF and oxidation products of phosphatidylcholine produced upon LDL oxidation and/or oxidative stress, and thus is considered as a potentially anti-inflammatory enzyme. Cloning of PAF-AH has sustained tremendous promises towards the use of PAF-AH recombinant protein in clinical situations. The reason for that stems from the numerous animal models of inflammation, atherosclerosis or sepsis, where raising the levels of circulating PAF-AH either through recombinant protein infusion or through the adenoviral gene transfer showed to be beneficial. Unfortunately, neither in human asthma nor in sepsis the recombinant PAF-AH showed sufficient efficacy. One of the most challenging questions nowadays is as to whether PAF-AH is pro- or anti-atherogenic in humans, as PAF-AH may possess a dual pro- and anti-inflammatory role, depending on the concentration and the availability of potential substrates. It is equally possible that the plasma level of PAF-AH is a diagnostic marker of ongoing atherosclerosis.

Carboxyl Ester Lipase Expression in Macrophages Increases Cholesteryl Ester Accumulation and Promotes Atherosclerosis

Carboxyl ester lipase (CEL, also called cholesterol esterase or bile salt-dependent lipase) is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, triacylglycerols, and phospholipids in a trihydroxy bile salt-dependent manner but hydrolyzes ceramides and lysophospholipids via bile salt-independent mechanisms. Although CEL is synthesized predominantly in the pancreas, a low level of CEL expression was reported in human macrophages. This study used transgenic mice with macrophage CEL expression at levels comparable with that observed in human macrophages to explore the functional role and physiological significance of macrophage CEL expression. Peritoneal macrophages from CEL transgenic mice displayed a 4-fold increase in [(3)H]oleate incorporation into cholesteryl [(3)H]oleate compared with CEL-negative macrophages when the cells were incubated under basal conditions in vitro. When challenged with acetylated low density lipoprotein, cholesteryl ester accumulation was 2.5-fold higher in macrophages expressing the CEL transgene. The differences in cholesteryl ester accumulation were attributed to the lower levels of ceramide and lysophosphatidylcholine in CEL-expressing cells than in CEL-negative cells. CEL transgenic mice bred to an atherosclerosis susceptible apoE(-/-) background displayed an approximate 4-fold higher atherosclerotic lesion area than apoE(-/-) mice without the CEL transgene when both were fed a high fat/cholesterol diet. Plasma level of the atherogenic lysophosphatidylcholine was lower in the CEL transgenic mice, but plasma cholesterol level and lipoprotein profile were similar between the two groups. These studies documented that CEL expression in macrophages is pro-atherogenic and that the mechanism is because of its hydrolysis of ceramide and lysophosphatidylcholine in promoting cholesterol esterification and decreasing cholesterol efflux.

Phospholipid mediators in the vessel wall: involvement in atherosclerosis

PURPOSE OF REVIEW

This review provides a brief update on the involvement of major phospholipid mediators, with the emphasis on platelet-activating factor and its analogues generated upon the oxidation of lipoproteins in vascular pathology, including atherogenesis.

RECENT FINDINGS

Phospholipid mediators are produced during inflammation by various enzymes, mostly from pre-existing membrane phospholipids, and trigger cellular signaling via G-coupled receptors. A short description of lysophosphatidic acid, lysophosphocholine and sphingosine-1 phosphate receptors and their actions is given, but attention is focused mainly on platelet-activating factor and its analogues. The majority of these mediators participate in leukocyte adhesiveness to the endothelium, leukocyte transmigration into the vessel wall and the subsequent formation of various chemokines leading to foam cell formation and smooth muscle cell proliferation and dedifferentiation. Platelet-activating factor and platelet-activating factor-like phospholipids are degraded in plasma by the lipoprotein-bound enzyme of myeloid origin, PAF-acetylhydrolase, also known as LDL-PLA2. Although the overexpression of PAF-acetylhydrolase shows marked anti-atherogenic properties in animal models, epidemiological data in the Caucasian population have demonstrated that its level might be a risk factor for cardiovascular disease. Recent genetic studies have shown, however, that the A379V polymorphism of this gene, responsible for slightly higher enzymatic activity, exerts a protective effect, probably by modifying the enzyme function towards a less atherogenic form.

SUMMARY

Phospholipid-borne mediators are certainly key players in inflammation and thus in atherosclerosis. The generation of such biologically active molecules is possibly dependent on nutritional habits and the availability of antioxidants, including enzymes protective against oxidative damage, including PAF-acetylhydrolase.

Intravascular adenovirus-mediated lipoprotein-associated phospholipase A2 gene transfer reduces neointima formation in balloon-denuded rabbit aorta

Postangioplasty restenosis is a multifactorial process and involves mechanisms such as inflammation and stimulation of the expression of growth factors. Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) can modify inflammatory responses by hydrolyzing phospholipids with shortened and/or oxidized sn-2 residues. In this study, we tested a hypothesis that adenovirus-mediated Lp-PLA(2) gene transfer can reduce restenosis in rabbits. Aortas of cholesterol-fed NZW rabbits were balloon-denuded and intra-arterial gene transfer was performed using Dispatch catheter with Lp-PLA(2) or LacZ adenoviruses (1.15 x 10(10)pfu). Intima/media ratio (I/M), histology and cell proliferation were analyzed. Two weeks after the gene transfer I/M in the LacZ-transduced control group was 0.45+/-0.05 but Lp-PLA(2) gene transfer reduced I/M to 0.25+/-0.03. At four weeks time point I/M in the Lp-PLA(2) group (0.34+/-0.05) was also lower than in the LacZ group (0.53+/-0.06). Plasma Lp-PLA(2) activity was increased in the Lp-PLA(2) group (48.2+/-4.2) as compared to the LacZ group (33.6+/-3.51) at two weeks time point. Transgene expression was detected in the arterial wall two and four weeks after the procedure. Apoptosis was higher in the control vessels than in the Lp-PLA(2) group at two weeks time point. In conclusion, local adenovirus-mediated Lp-PLA(2) gene transfer resulted in a significant reduction in neointima formation in balloon-denuded rabbit aorta and may be useful for the prevention of restenosis after arterial manipulations.

Gene therapy of atherosclerosis

Atherosclerosis and related diseases are the leading cause of death in Western world. The disease process begins with the formation of fatty streaks already during the first decade of life but does not manifest clinically until several decades later. Gene therapy is a potential new way to target multiple factors playing a role in the development and progression of atherosclerosis. A great number of genes involved in the development of atherosclerosis have been identified and have been tested both in vitro and in vivo as potential new targets for therapy. Pre-clinical experiments have shown the feasibility and safety of several gene therapy applications for the treatment of atherosclerosis and clinical trials have also provided evidence for the applicability of gene therapy for the treatment of cardiovascular diseases. In this review we discuss vectors and potential gene therapy approaches for intervention and therapy of atherosclerosis.