Lysophosphatidylcholine induces apoptotic and non-apoptotic death in vascular smooth muscle cells: in comparison with oxidized LDL

Lipoprotein modification by secretory phospholipase A(2) enzymes contributes to the initiation and progression of atherosclerosis

PURPOSE OF REVIEW

Secretory phospholipase A2s (sPLA2s) are considered to be important enzymes in the initiation and progression of atherosclerosis. In this review, we discuss the various mechanisms by which the direct action of the sPLA2s on LDL particles in the arterial intima may contribute to atherogenesis.

RECENT FINDINGS

A wealth of evidence, both in vitro and in vivo, supports a role for the sPLA2s in atherogenesis. Very recently, systemic inhibition of sPLA2s was found to reduce measures of arterial inflammation. The mechanisms behind this inhibition, however, are largely unknown. Here, we discuss the consequences of sPLA2 action on LDL in the arterial intima and address the recent findings regarding the effects of the lipolytic products of sPLA2, lysophosphatidylcholine, and fatty acids on intimal cells. LDL modified by sPLA2 can accumulate in the arterial intima both extracellularly and intracellularly. Importantly, the lipolytic products promote atherosclerosis by monocyte/macrophage recruitment, by enhancing the production of proretentive molecules by vascular smooth muscle cells, and by inducing cell death.

SUMMARY

Recent findings on sPLA2s support the idea that the enzymes contribute to human atherogenesis not only as initiating agents but also in maintaining plaque inflammation.

Expression of lipoprotein-associated phospholipase A(2) in carotid artery plaques predicts long-term cardiac outcome

AIMS

The aim was to test the hypothesis that carotid artery plaque expression of lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) predicts cardiac events.

METHODS AND RESULTS

Prospective cohort study of 162 consecutive patients undergoing elective carotid endarterectomy. Lipoprotein-associated phospholipase A(2) content was quantified by immunoblotting and lysophosphatidylcholine (lysoPC) by liquid chromatography tandem mass spectrometry. Additional biomolecular profiling by immunoblotting included C-reactive protein, p67phox, and matrix metalloproteinase-2 and -9. Macrophage plaque content was determined by quantitative immunostaining, plaque collagen content by quantitative Sirius red staining. Follow-up for cardiac death and non-fatal acute myocardial infarction was accomplished over a period of 48 +/- 14 months. Expression of Lp-PLA(2) and lysoPC was higher in carotid plaques of patients with than without cardiac events [median 1.6 (25th, 75th percentile 0.9, 2.5) vs. 0.8 (0.5, 2.0), P = 0.01 and 413 (281, 443) vs. 226 (96, 351) mmol/L, P = 0.03]. Smoking and point increase in carotid Lp-PLA(2) expression but no other traditional cardiovascular risk factor, histological or molecular marker remained predictive of cardiac events in the multivariate Cox proportional hazard analyses [HR 3.65 (1.36-9.83), P = 0.01 and HR 1.34 (1.01-1.77), P = 0.039]. Carotid plaque Lp-PLA(2) expression above the median constituted a more than three times higher risk for cardiac events [HR 3.39 (1.13-10.17), P = 0.03].

CONCLUSION

Lipoprotein-associated phospholipase A(2) expression in carotid artery plaques is a predictor of long-term cardiac outcome. The current study supports the concept of atherosclerosis as a systemic disease with multi-focal complications and personalized medicine.

Lysophosphatidylcholine induces endothelial cell injury by nitric oxide production through oxidative stress

OBJECTIVE

To determine whether lysophosphatidylcholine (LPC) induces endothelial cell injury by altering the production of nitric oxide (NO) and thereby increasing reactive oxygen species (ROS).

METHODS

Human umbilical vein endothelial cells (HUVECs) were cultured and exposed to LPC, LPC with N(G)-nitro-l-arginine methyl ester (L-NAME), LPC with antioxidants. LPC-induced cell injury and viability were determined using LDH and Resazurin assays. The Mann-Whitney U test was used for statistical analysis.

RESULTS

LPC induced HUVEC injury in a concentration-dependent manner. LPC induced the overproduction of NO and ROS in HUVECs and LPC-induced HUVEC injury is significantly inhibited by the eNOS inhibitor (L-NAME) and the antioxidants (p < 0.05).

CONCLUSIONS

These findings suggest that LPC induces the overproduction of NO, which may increase the oxidative stress on endothelial cells and lead to endothelial cell injury.

Metabolism and atherogenic disease association of lysophosphatidylcholine

Lysophosphatidylcholine (LPC) is a major plasma lipid that has been recognized as an important cell signalling molecule produced under physiological conditions by the action of phospholipase A(2) on phosphatidylcholine. LPC transports glycerophospholipid components such as fatty acids, phosphatidylglycerol and choline between tissues. LPC is a ligand for specific G protein-coupled signalling receptors and activates several second messengers. LPC is also a major phospholipid component of oxidized low-density lipoproteins (Ox-LDL) and is implicated as a critical factor in the atherogenic activity of Ox-LDL. Hence, LPC plays an important role in atherosclerosis and acute and chronic inflammation. In this review we focus in some detail on LPC function, biochemical pathways, sources and signal-transduction system. Moreover, we outline the detection of LPC by mass spectrometry which is currently the best method for accurate and simultaneous analysis of each individual LPC species and reveal the pathophysiological implication of LPC which makes it an interesting target for biomarker and drug development regarding atherosclerosis and cardiovascular disorders.

Relationship of lipoprotein-associated phospholipase A2 and oxidized low density lipoprotein in carotid atherosclerosis

Plasma levels of lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) and oxidized low density lipoprotein (oxLDL) have been identified as risk factors for cardiovascular disease. Lp-PLA(2) is the sole enzyme responsible for the hydrolysis of oxidized phospholipids on LDL particles in atherosclerotic plaques. We have studied the relationship between Lp-PLA(2) and oxLDL in carotid endarterectomy (CEA) tissues and in matched plasmas. In extracts from CEA anatomical segments, the levels of oxLDL were significantly associated with the levels of Lp-PLA(2) protein (r = 0.497) and activity (r = 0.615). OxLDL and Lp-PLA(2) mass/activity were most abundant in the carotid bifurcation and internal segments where plaque was most abundant. In extracts from CEA atheroma, the levels of oxLDL and Lp-PLA(2) were significantly correlated (r = 0.634). In matched plasma and atheroma extracts, the levels of Lp-PLA(2) were negatively correlated (r = - 0.578). The ratio of Lp-PLA(2) to oxLDL was higher in atheromatous tissue (277:1) than in normal tissue (135:1) and plasma (13:1). Immunohistochemical experiments indicated that in plaques, oxLDL and Lp-PLA(2) existed in overlapping but distinctly different distribution. Fluorescence microscopy showed both oxLDL and Lp-PLA(2) epitopes on the same LDL particle in plasma but not in plaque. These results suggest that the relationship between Lp-PLA(2) and oxLDL in the atherosclerotic plaque is different from that in the plasma compartment.

The signal molecule lysophosphatidylcholine in Eschscholzia californica is rapidly metabolized by reacylation

In cultured cells of California poppy (Eschscholzia californica), lysophosphatidylcholine (LPC) triggers a signal path that finally induces alkaloid biosynthesis. LPC is transiently generated by elicitor-activated phospholipase A(2) of the plasma membrane. Externally added LPC is rapidly acylated by a membrane-bound enzyme that shows the highest specific activity in the purified plasma membrane. The fatty acid incorporated into the sn-2 position of LPC is preferentially linoleic (18:2), which is the most abundant acyl component in the PC species of Eschscholzia cells, but a minor component of the pool of free fatty acids. The fatty acid at the sn-1 position of LPC is less important for substrate specificity. The capacity of LPC acylation by intact cells or isolated plasma membranes by far exceeds the rate of LPC generation by activated phospholipase A(2) and is not limited by the availability of acyl donors. Metabolites other than phosphatidylcholine (PC) were not significantly produced from labeled LPC within 20 min, indicating that lysophospholipases are not significantly contributing to the short-time metabolism of LPC. It is concluded that reacylation to PC is the dominating process in the detoxication of LPC and ensures the transient character of its steady state concentrations, even at maximum phospholipase A(2) activities.

Enhanced expression of Lp-PLA2 and lysophosphatidylcholine in symptomatic carotid atherosclerotic plaques

BACKGROUND AND PURPOSE

Circulating lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) has emerged as a novel biomarker for cardiovascular diseases. However, the correlation between the plaque expression of Lp-PLA(2) and plaque oxidative stress, inflammation, and stability as well as the clinical presentation remains poorly defined, especially for cerebrovascular disease. Therefore, this study was performed to test the hypothesis that Lp-PLA(2) expression is higher in symptomatic than in asymptomatic carotid plaques of patients undergoing carotid endarterectomy.

METHODS

The expression of Lp-PLA(2) in 167 carotid artery plaques was determined by immunoblotting and immunostaining. Plaque oxidative stress, inflammation, and stability were quantified by NAD(P)H oxidase p67phox and MMP-2 immunoblotting, oxidized LDL (oxLDL) immunoreactivity, macrophage and Sirius red collagen staining. Lysophosphatidylcholine 16:0 (lysoPC) concentration was measured in 55 plaques using liquid chromatography tandem mass spectrometry.

RESULTS

Lp-PLA(2) expression was significantly higher in plaques of symptomatic patients than asymptomatic patients (1.66+/-0.19 versus 1.14+/-0.10, P<0.05) and localized mainly to shoulder and necrotic lipid core areas in colocalization with oxLDL and macrophage content. Similarly, Lp-PLA(2) expression was related to collagen content, which was lower in plaques from symptomatic patients than in plaques from asymptomatic patients (9.1+/-2.2 versus 18.5+/-1.7% of staining/field, P<0.001). LysoPC plaque concentration was significantly higher in plaques of symptomatic than asymptomatic patients (437.0+/-57.91 versus 228.84+/-37.00 mmol/L, P<0.05).

CONCLUSIONS

Symptomatic carotid artery plaques are characterized by increased levels of Lp-PLA(2) and its product lysoPC in correlation with markers of tissue oxidative stress, inflammation, and instability. These findings strongly support a role for Lp-PLA2 in the pathophysiology and clinical presentation of cerebrovascular disease.

Mediation of apoptosis by oxidized phospholipids

Free radical-mediated oxidation of (poly)unsaturated glycerophospholipids in membranes and lipoproteins leads to the formation of a plethora of products. Some of these oxidized phospholipids, especially the truncated forms, induce apoptosis depending on their chemical structure, concentration and cell type. Depending on the phospholipid and the cell type, two pathways have so far been identified for the intracellular transmission of the apoptotic signals. One pathway involves activation of acid sphingomyelinase, which gives rise to the formation of ceramide and is followed by phosphorylation of pro-apoptotic mitogen-activated protein kinases. Alternatively, oxidized phospholipids act directly on mitochondria leading to efflux of pro-apoptotic effectors in endothelial cells. During the execution of the apoptotic program additional oxidized phospholipids are generated. The apoptotic cascade itself leads to oxidation and exposure of e.g. membrane phosphatidylserine. Oxidized phospholipids on the outer leaflet of the plasma membrane can form surface lipid patterns that specifically bind to phagocytic cells, e.g. macrophages.In this manuscript we review the recent literature reporting on apoptosis-inducing glycerophospholipids. In addition, we describe the cellular processes that lead to phospholipid oxidation as part of the apoptotic mode of cell death and are likely to enhance the recognition of apoptotic cells by phagocytic macrophages.

Proanthocyanidin from grape seeds inactivates the PI3-kinase/PKB pathway and induces apoptosis in a colon cancer cell line

The aim of this investigation was to evaluate the chemopreventative/antiproliferative potential of a grape seed proanthocyanidin extract (GSPE) against colon cancer cells (CaCo2 cells) and to investigate its mechanism of action. GSPE (10-100 microg/ml) significantly inhibited cell viability and increased apoptosis in CaCo2 cells, but did not alter viability in the normal colon cell line (NCM460). The increased apoptosis observed in GSPE-treated CaCo2 cells correlated with an attenuation of PI3-kinase (p110 and p85 subunits) and decreased PKB Ser(473) phosphorylation. GSPE might thus exert its beneficial effects by means of increased apoptosis and suppression of the important PI3-kinase survival-related pathway.

Protective role of a novel erythrocyte-derived depressing factor on blood vessels of renovascular hypertensive rats

1. We have isolated a novel human erythrocyte-derived depressing factor (EDDF) that has a significant antihypertensive effect in various rat models of hypertension. The aim of the present study was to examine the mechanisms of action of EDDF on vascular function in two-kidney, one-clip (2K1C) renovascular hypertensive rats. 2. The EDDF was prepared from human erythrocytes. Experiments were performed in 18 male Wistar rats. The vascular ring perfusion assay and a two-photon laser scanning fluorescence microscope (TMP) were used to evaluate the vascular contractile response. The effects of EDDF on phenylephrine (PE)- and noradrenaline (NA)-induced vascular contraction were evaluated in 2K1C hypertensive rats. The proliferation and DNA synthesis in vascular smooth muscle cells (VSMC) were determined using the [3H]-TdR (thymidine) incorporation and 3-(4,5-dimethyl-2 thiazoyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays. Flow cytometry, reverse transcription-polymerase chain reaction and western blots were used to measure cell cycle and apoptotic profiles, platelet-derived growth factor (PDGF)-A expression and the activity of extracellular signal-regulated kinase (ERK)-1/2, as well as the expression of cyclin D1 and cyclin-dependent kinase (CDK) 4. 3. At 10(-5) g/mL, EDDF significantly decreased the PE- and NA-induced hypertensive vascular contraction. In addition, EDDF inhibited DNA synthesis in primary VSMC from 2K1C rats. The mRNA expression of PDGF-A in VSMC was twofold higher in 2K1C rats compared with control rats, whereas EDDF significantly inhibited the increment in PDGF-A mRNA expression. In addition, EDDF inhibited the phosphorylation of ERK1/2 and decreased the expression of cyclin D1 and CDK4; p21 (Cip1) levels were increased after treatment with EDDF. 4. In conclusion, EDDF inhibits VSMC proliferation in 2K1C rats through G0/G1 cell cycle arrest. The effects may be mediated, in part, by enhanced expression of p21 (Cip1) and the inhibition of ERK1/2 phosphorylation and the expression of cyclin D1/CDK4 and PDGF-A.

Phospholipid Transfer Protein Augments Apoptosis in THP-1–Derived Macrophages Induced by Lipolyzed Hypertriglyceridemic Plasma

OBJECTIVE

Lipolysis of triglyceride-rich lipoproteins (TGRLPs) generates phospholipid-rich surface remnants and induces cytotoxic effects in adjacent vascular cells. We hypothesized that by integrating surface remnants into HDL, phospholipid transfer protein (PLTP) alleviates cytotoxicity.

METHODS AND RESULTS

To test this hypothesis and gain insight into cytotoxicity during the postprandial phase in vivo, we injected normo-TG and hyper-TG human volunteers after a standardized fat meal (postprandial sample) with heparin, thereby stimulating lipolysis (postprandial heparinized sample). Incubation of (primary) human macrophages and primary human endothelial cells with postprandial heparinized hyper-TG plasma induced pronounced cytotoxic effects that were dose dependent on the TG content of the sample. No such effects were seen with normo-TG and postprandial hyper-TG plasma. In vitro lipolysis of VLDL and chylomicrons indicated that both lipoprotein fractions can cause cytotoxicity. Interestingly, in experiments with THP-1-derived macrophages stably transfected with PLTP, PLTP substantially augmented both net phospholipid uptake and apoptotic cell death due to postprandial heparinized hyper-TG plasma. We observed that activation of caspase-3/7, poly-ADP-ribose polymerase, and enhanced bioactivity of acid sphingomyelinase may all contribute to this augmented apoptosis.

CONCLUSIONS

Our data show that lipolysis of TGRLPs and their remodelling by PLTP interact to disturb cellular phospholipid flux and intracellular signaling processes, ultimately leading to apoptosis in human macrophages and endothelial cells.

OxLDL-induced gene expression patterns in CASMC are mimicked in apoE-/- mice aortas

Oxidized low density lipoprotein (oxLDL) contributes to the pathophysiology of atherosclerosis, partly by altering gene expression in vascular cells. Here, we show 221 genes differentially regulated by oxLDL in coronary artery smooth muscle cells (CASMC), using oligonucleotide microarrays. These genes were classified into 14 functional groups. A comparable gene expression pattern was detected in apoE(-/-) mice. OxLDL induced an oxidative stress response in CASMC, but not the unfolded protein response. OxLDL also caused CASMC death which was accompanied by increased expression of FasL, Bax, and p53 but was caspase-independent. This approach provides further insight into disease pathology and prognosis.

H2O2 but not O2- elevated by oxidized LDL enhances human aortic smooth muscle cell proliferation

The oxidation of low density lipoprotein (LDL) as a key event in atherosclerosis suggests that antioxidant interventions may reduce the risk of atherosclerosis. However, the better strategies among antioxidant remedies for atherosclerosis remains difficult to be determined. Here, we show that oxidized LDL increases the steady-state level of intracellular hydrogen peroxide through stimulating the protein expressions of Nox1 and Cu/Zn superoxide dismutase (SOD) in human aortic smooth muscle cells (SMCs). The intracellular content of hydrogen peroxide rather than superoxide is a key modulator for vascular SMC (VSMC) proliferation, implying that without co-expression of catalase, increased Cu/Zn-SOD activity alone may not be beneficial to reduce the growth of VSMC in an atherosclerotic plaque.

Albumin inhibits cytotoxic activity of lysophosphatidylcholine by direct binding

Fetal bovine serum (FBS) was found to protect Jurkat T cells from LPC-induced cytotoxicity. Twenty micromolar LPC-induced cytotoxicity of 80-90% of the cells in media without FBS for 3 h, whereas 50-70% in media with 0.5% FBS. However, LPC-induced cytotoxicity was not observed in the presence of 5% FBS in media. The cytotoxicity was specific for LPC among lysophospholipids tested and significantly observed with palmitoyl (C16:0) LPC, stearoyl (C18:0) LPC, and oleoyl (C18:1) LPC among 11 synthetic LPCs. Furthermore, the cytoprotective effect of FBS was observed only when it was added before the treatment, but not after the treatment of LPC, and premixing of FBS and LPC before addition to the cells ameliorated LPC-induced cytotoxicity. Finally, albumin, a major constituent of FBS, prevented completely LPC-induced cytotoxicity even at as low as 3 microM concentration. We also found that five molecules of LPC could sequentially bind to one BSA using isothermal titration calorimetry. The above results suggest that the cytotoxic activity of LPC could be attenuated by albumin in blood. Finally, it should be cautioned that, when experiments are conducted with LPC dissolved in assay buffers containing albumin, the albumin in the buffer could influence the results.

Antitumor action and immune activation through cooperation of bee venom secretory phospholipase A2 and phosphatidylinositol-(3,4)-bisphosphate

We evaluated tumor cell growth modulation by bee venom secretory phospholipase A2 (bv-sPLA2) and phosphatidylinositol-(3,4)-bisphosphate as well as potential cooperative effects. In addition, the immunomodulatory impact of tumor cell treatment was examined by monitoring changes in phenotype and function of monocyte-derived dendritic cells (moDCs) cocultured with pretreated tumor cells. Bv-sPLA2 or phosphatidylinositol-(3,4)-bisphosphate alone displayed moderate effects on the proliferation of A498 renal cell carcinoma cells, T-47D breast cancer cells, DU145 prostate cancer cells and BEAS-2B transformed lung cells. However, when bv-sPLA2 was coadministered with phosphatidylinositol-(3,4)-bisphosphate a potent inhibition of [3H] thymidine incorporation into all tested cell lines occurred. This inhibition was due to massive cell lysis that reduced the number of cells with proliferative capacity. Importantly, tumor cell lysates generated with bv-sPLA2 plus phosphatidylinositol-(3,4)-bisphosphate induced maturation of human moDCs demonstrated by enhanced expression of CD83 and improved stimulation in allogeneic mixed leukocyte reactions. Our data demonstrate that bv-sPLA2 and phosphatidylinositol-(3,4)-bisphosphate synergistically generate tumor lysates which enhance the maturation of immunostimulatory human monocyte-derived dendritic cells. Such tumor lysates which represent complex mixtures of tumor antigens and simultaneously display potent adjuvant properties meet all requirements of a tumor vaccine.

Different cytotoxic injuries induced by lysophosphatidylcholine and 7-ketocholesterol in mouse endothelial cells

Lysophosphatidylcholine (LPC) and 7-ketocholesterol (7-KC) are two key components of oxidized low-density lipoprotein (oxLDL) and have been shown to injure endothelial cells derived from various species. This report examines LPC- and 7-KC-induced cell death in mouse aorta endothelial cells (MAECs). The presence and the mechanism of cell death were assessed with morphological criteria, Hoechst 33342 and propidium iodide fluorescence staining, and caspase-3 activity. The authors observed that 7-KC induced cell shrinkage, nuclear condensation, and caspase-3 activity. In contrast, LPC induced membrane rupture, nuclear expansion, and cell lysis. In addition, 7-KC induced a transient increase, whereas LPC induced a sustained increase in intracellular Ca2+ levels and production of reactive oxygen species (ROS). Antioxidants and calcium antagonists attenuated both 7-KC- and LPC-induced cell death. These findings suggest that 7-KC and LPC injure MAECs through differential mechanisms; LPC induces necrosis, 7-KC induces apoptosis, and the increase in intracellular Ca2+ levels and production of ROS are common mechanisms for these cytotoxic injuries.

The oxidized phospholipids POVPC and PGPC inhibit growth and induce apoptosis in vascular smooth muscle cells

Oxidized phospholipids, including 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) are typically present in oxidatively modified low density lipoprotein (oxLDL) and have been found in atherosclerotic lesions. These compounds are gaining increasing importance as inducers of different cellular responses like inflammation, proliferation, or cell death. The aim of this study was to elicit the type and outcome of the cellular response of vascular smooth muscle cells (VSMC) upon treatment with POVPC and PGPC. Both oxidized phospholipids led to inhibition of cell proliferation and showed cytotoxic effects in VSMC. Several morphological criteria, the presence of typical DNA fragments, and a phosphatidylserine shift towards the outer leaflet of the cell membrane revealed that apoptosis was the predominant mode of cell death. In all experiments, POVPC was found to be a more potent inducer of apoptosis than PGPC. Interestingly, in the presence of high levels of serum in the growth media the proapoptotic but not the antiproliferative effects of both oxidized phospholipids were abolished. Thus, we conclude that under low serum conditions both intact POVPC and PGPC are proapoptotic mediators. Under high serum conditions, these lipids are hydrolyzed and the resultant lipid mixture containing the degradation products is no longer apoptotic but antiproliferative. Thus, the direct and indirect effects of POVPC and PGPC on cell viability may account for the detrimental actions of oxLDL on VSMC.

Synthetic peptide (P-21) derived from Asp-hemolysin inhibits the induction of apoptosis on HUVECs by lysophosphatidylcholine

Lysophosphatidylcholine (LPC), formed during low-density lipoprotein (LDL) oxidation and located within atherosclerotic plaques, regulates a variety of cellular functions, some of which could be construed to promote atherosclerotic lesion development, including vascular muscle cell proliferation, monocyte attraction, and endothelial cell apoptosis. We have previously reported that the synthetic peptide derived from Asp-hemolysin, named P-21, inhibits oxidized LDL (OxLDL)-induced macrophage proliferation through binding of P-21 to OxLDL. In this study, to clarify the interaction between P-21 and LPC as a typical lipid moiety of OxLDL, we examined the influence of P-21 on LPC-induced apoptosis in human umbilical vein endothelial cells (HUVECs). Based on flow cytometric analysis, using annexin V-fluorescein isothiocyanate and propidium iodide as probes to assess apoptosis, LPC induced the apoptosis of HUVECs, and P-21 significantly inhibited this activity by 82.4%. Furthermore, dissociation-enhanced lanthanide fluorometric immunoassay indicated that LPC inhibited the binding of P-21 to OxLDL in a dose-dependent manner. A 50% inhibition dose was estimated to be 4.65 microM of LPC. These results suggest that P-21 inhibits LPC-induced HUVEC apoptosis through binding of P-21 to LPC.

Lysophosphatidylcholine (LPC) attenuates macrophage-mediated oxidation of LDL

We have previously shown that paraoxonase 1 action on macrophages produced lysophosphatidylcholine (LPC) and significantly decreased cell-mediated LDL oxidation. Thus, in the present study, we questioned whether LPC can directly inhibit macrophage-mediated oxidation of LDL. Addition of increasing LPC concentrations (0-5 microM) to J774A.1 macrophages, mouse peritoneal macrophages (MPM), or to human monocytes-derived macrophages (HMDM) resulted in up to 83%, 67%, and 75% inhibition in cell-mediated oxidation of LDL, respectively. The mechanism for this LPC effect involves up to 60% inhibition of superoxide anion release from MPM in response to phorbol ester (PMA), 26% inhibition of PMA-induced NADPH oxidase activation (p47phox translocation from the cytosol to the plasma membrane), and a 2-fold stimulation of the macrophage paraoxonase 2 (PON2) lactonase activity. We thus conclude that inhibition of macrophage-mediated oxidation of LDL by LPC can contribute to attenuation of macrophage foam cell formation and atherosclerotic lesion development.

Lymphocyte resistance to lysophosphatidylcholine mediated apoptosis in atherosclerosis

OBJECTIVE

Apoptosis is being increasingly regarded as a key component in the development and progression of atherosclerosis. Since it has become apparent that the immune system plays a predominant role in mediating atherogenesis, there has been a growing recognition that the evaluation of lymphocyte apoptosis may contribute to understanding a persistent altered immune and inflammatory response. The aim of the present study was to evaluate the apoptotic effect of lysophosphatidylcholine (LPC) on peripheral blood lymphocytes (PBL) derived from unstable angina (UA) patients, as compared to healthy donors.

METHODS

PBL isolated from 27 healthy donors and 25 age matched UA patients were examined. Early apoptotic events induced by LPC in resting and phytohemagglutinin (PHA)-activated lymphocytes were evaluated by several apoptotic assays. The levels of intracellular reactive oxygen species (ROS) and the expression of apoptotic regulated proteins (Bcl-2 and Bax) were measured.

RESULTS

LPC was found to induce apoptosis in normal activated lymphocytes, in a dose- and time-dependent manner, in association with an increase in intracellular ROS. In UA patients, an exposure of PHA-activated PBL to LPC triggered neither an increase in ROS generation, nor in the apoptotic manifestations, and was associated with a significantly lower ratio of Bax/Bcl-2 expression.

CONCLUSION

Our results indicate that PBL isolated from UA patients may be resistant to apoptosis induction by LPC, resulting from oxidative stress challenge and dysregulation of apoptosis-related protein expression.

Effect of lysophosphatidylcholine on vasomotor functions of porcine coronary arteries

BACKGROUND

Lysophosphatidylcholine (LPC) is a product of phosphatidylcholine hydrolysis by phospholipase A(2) and a mediator of the lipid-induced atherosclerotic changes. In this study, we determined the effects of LPC on vasomotor functions, oxidative stress, and endothelial nitric oxide synthase (eNOS) expression in porcine coronary arteries.

METHODS

Porcine coronary arteries were cut into 5-mm rings and were treated with LPC or antioxidant selenomethionine (SeMet). For the vasomotor studies, we used a myograph tension system. Levels of superoxide anion (O(2)(-)) were detected by the lucigenin-enhanced chemiluminescence method. The eNOS protein level was studied by immunohistochemistry with avidin-biotin complex immunoperoxidase procedure.

RESULTS

Endothelium-dependent relaxation in response to bradykinin was reduced by 36% and 81% for the rings treated with 12.5 and 25 mum of LPC, respectively, as compared with controls (P < 0.05). Endothelium-independent relaxation in response to sodium nitroprusside also was reduced by 63% after treatment with 25 mum LPC (P < 0.05). The O(2)(-) level was increased in the porcine arteries treated with 25 mum of LPC by 41% as compared with controls (P < 0.05). The antioxidant SeMet reversed the effects of LPC on vascular relaxation and O(2)(-) production. Immunoreactivity of eNOS in LPC-treated vessel rings also was reduced substantially.

CONCLUSIONS

LPC impairs endothelium-dependent and endothelium-independent vasorelaxation. This effect is associated with increased superoxide radical production and decreased eNOS activity and is practically reversed with the use of the antioxidant SeMet.

Lipoprotein-associated phospholipase A2: a novel marker of cardiovascular risk and potential therapeutic target

Although the clinical benefit of statins is well established, these agents reduce the risk of cardiovascular events by only 20 - 40%, and the residual risk for high-risk patients is considerable. The recognition of atherosclerosis as an inflammatory disease has opened the door to numerous complementary therapeutic approaches to further reduce risk and the overall burden of cardiovascular disease. Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is a novel inflammatory marker of cardiovascular risk that is being evaluated as a potential therapeutic target. The biological function of this enzyme in atherosclerosis has been controversial but recent evidence supports its pro-atherogenic role. The enzyme is predominantly bound to low-density lipoprotein cholesterol particles in humans, and its activity produces bioactive lipid mediators that promote inflammatory processes present at every stage of atherogenesis, from atheroma initiation to plaque destabilisation and rupture. Initial clinical studies suggest that the inhibitors of Lp-PLA(2) can block enzyme activity in plasma and within atherosclerotic plaques. However, more studies are needed to determine the potential clinical benefits of inhibiting Lp-PLA(2).

Role of lipoprotein-associated phospholipase A2 in atherosclerosis: biology, epidemiology, and possible therapeutic target

The development of atherosclerotic vascular disease is invariably linked to the formation of bioactive lipid mediators and accompanying vascular inflammation. Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an enzyme that is produced by inflammatory cells, co-travels with circulating low-density lipoprotein (LDL), and hydrolyzes oxidized phospholipids in LDL. Its biological role has been controversial with initial reports purporting atheroprotective effects of Lp-PLA2 thought to be a consequence of degrading platelet-activating factor and removing polar phospholipids in modified LDL. Recent studies, however, focused on pro-inflammatory role of Lp-PLA2 mediated by products of the Lp-PLA2 reaction (lysophosphatidylcholine and oxidized nonesterified fatty acids). These bioactive lipid mediators, which are generated in lesion-prone vasculature and to a lesser extent in the circulation (eg, in electronegative LDL), are known to elicit several inflammatory responses. The proinflammatory action of Lp-PLA2 is also supported by a number of epidemiology studies suggesting that the circulating level of the enzyme is an independent predictor of cardiovascular events, despite some attenuation of the effect by inclusion of LDL, the primary carrier of Lp-PLA2, in the analysis. These observations provide a rationale to explore whether inhibiting Lp-PLA2 activity and consequent interference with the formation of bioactive lipid mediators will abrogate inflammation associated with atherosclerosis, produce favorable changes in intermediate cardiovascular end points (eg, biomarkers, imaging, and endothelial function), and ultimately reduce cardiovascular events in high-risk patients.

Impact of exogenous lysolipids on sensitive and multidrug resistant K562 cells: 1H NMR studies

The ability of lysolipids to enter into a membrane bi-layer and disturb the membrane structure was used to study the behavior of K562 erythroleukemic cells, K562 wild type (K562wt) as well as the multidrug resistant cells K562adr. Both types of cells, when analyzed by proton NMR spectroscopy exhibit the high resolution signals assigned to so-called "mobile lipid" signals, which, in most cases, are located outside the lipid bi-layer as lipid droplets. In order to perform these studies, the K562wt and K562adr cells were treated for 48h with lysophosphatidylcholine oleoyl (LPC18), lysophosphatidylcholine palmitoyl (LPC16) and L-alpha-lysophosphatidyslerine (LPS). After evaluating toxicity of lysolipids, proton NMR of whole treated cells was used to analyze the mobile lipid content. Nile red staining and fluorescence microscopy were used to detect the presence of intracellular lipid droplets. Membrane lipid asymmetry perturbation was estimated by annexin V staining with use of flow cytometry. Using fluorescence spectroscopy the functioning of P-glycoprotein (P-gp) responsible for multidrug resistance was also evaluated after the treatment with lysolipids. Lysolipids were found to be more toxic for K562wt than for K562adr cells. LPS and LPC16 produced an increased of a mobile lipid NMR signal and amount of lipid droplets in K562wt cells only. LPC18, with the lowest toxicity, has shown more intense effects on NMR spectra with a large increase of lipid NMR signal without changes in lipid droplet staining. The functioning of the P-gp pump and membrane asymmetry were not modified by any of the lysolipids used.

Natural antibodies and the autoimmunity of atherosclerosis

In recent years, the subject of natural antibodies has been revisited and the immunobiological roles of these humoral factors are being better defined. These antibodies are secreted by distinct sets of innate-like B cells, B-1 cells and marginal zone B cells, which arise early in development to become the sources of "natural immune memory". Due to their interactions with a variety of self-determinants, natural antibodies have previously been postulated to play roles in the maintenance of host homeostasis. A central paradigm has recently been developed from the demonstration that oxidation derived epitopes on apoptotic cells and oxidized low-density lipoproteins are recognized by the phosphorylcholine-specific germline encoded B-1 cell natural antibody, T15, which has provided important insights into possible "house-keeping" functions under both normal and pathological conditions. In this review, the potential functions of natural antibodies in the pathogenesis and progression of the chronic inflammatory condition of atherosclerosis are discussed, as well as their capacities for apoptotic cell binding and clearance. These interactions of natural antibodies and oxidation-epitopes from phospholipids appear to provide a dynamic immunobiological connection linking host responses in infection, autoimmunity and atherosclerosis.

The Action Mode of Lysophosphatidylcholine in Human Monocytes

To elucidate the action and signal transduction of lysophosphatidylcholine (LPC), we challenged a set of LPC on U937 human monocytes and found that LPC mobilized Ca(2+). The Ca(2+) response was not blocked by pertussis toxin, an inhibitor of G(i/o) proteins, or by U73122, a phospholipase C inhibitor. Furthermore, the response was totally blocked by addition of EGTA to the extracellular media, suggesting that Ca(2+) influx across the plasma membrane was the only source of LPC-induced Ca(2+) response in the U937 cells. 16:0 and 18:0 LPC induced similar responses. Recently it has been suggested that two G protein-coupled receptors function as LPC receptors in the plasma membrane. RT-PCR analysis indicated that neither the G2A receptor nor the GPR4 receptor is expressed in the U937 monocytes. Our data suggests that another action mechanism of LPC may be involved in the LPC response in the U937 cells.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Temporary membrane distortion of vascular smooth muscle cells is responsible for their apoptosis induced by platelet-activating factor-like oxidized phospholipids and their degradation product, lysophosphatidylcholine

To obtain information about the mechanism of apoptosis induced by oxidized low density lipoproteins (oxLDL) in atherosclerotic plaques, we examined the effects of lysophosphatidylcholine (LPC) and platelet-activating factor (PAF)-like lipids (PAF-LL), which can be derived from oxLDL, on rat vascular smooth muscle cells (VSMC). All the lipids with different structures examined induced apoptosis of VSMC, so we studied the mechanism of induction of apoptosis by LPC. LPC-induced apoptosis was inhibited by alpha-tocopherol (alpha-T) and cholesterol (Chol), but not by other antioxidants such as palmitoyl ascorbic acid and PAF receptor antagonist. The cells temporarily became spherical and highly permeable before induction of apoptosis, and their change in shape was prevented by alpha-T and Chol. From these results, we suggest that the apoptosis induced by oxLDL-derived phospholipids in VSMC is caused by temporary membrane distortion, not through specific receptors.

Parallel phosphatidylinositol-3 kinase and p42/44 mitogen-activated protein kinase signaling pathways subserve the mitogenic and antiapoptotic actions of insulin-like growth factor I in osteoblastic cells

IGF-I is an endocrine and paracrine regulator of skeletal homeostasis, principally by virtue of its anabolic effects on osteoblastic cells. In the current study, we examined the intracellular signaling pathways by which IGF-I promotes proliferation and survival in SaOS-2 human osteoblastic cells. Inhibition of each of the phosphatidylinositol-3 kinase (PI-3 kinase), p42/44 MAPK, and p70s6 kinase pathways partially inhibited the ability of IGF-I to stimulate osteoblast proliferation and survival. Because activation of p70s6 kinase is downstream of both PI-3 kinase and p42/44 MAPK activation in osteoblasts treated with IGF-I, this ribosomal kinase represents a convergence point for IGF-I-induced PI-3 kinase and p42/44 MAPK signaling in osteoblastic cells. In addition, abrogation of PI-3 kinase-dependent Akt signaling, which does not inhibit IGF-I-induced p70s6 kinase phosphorylation, also inhibited the antiapoptotic effects of IGF-I in osteoblasts. Finally, interruption of G beta gamma signaling partially abrogated the ability of IGF-I to promote osteoblast survival, without inhibiting signaling through PI-3 kinase/Akt, p42/44 MAPKs, or p70s6 kinase. These data suggest that IGF-I signals osteoblast mitogenesis and survival through parallel, partly overlapping intracellular pathways involving PI-3 kinase, p42/44 MAPKs, and G beta gamma subunits.

Oxidized phospholipids in minimally modified low density lipoprotein induce apoptotic signaling via activation of acid sphingomyelinase in arterial smooth muscle cells

Oxidized phospholipids, including 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) and 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC), typically present in minimally modified low density lipoprotein, have been found in atherosclerotic lesions. These compounds are gaining increasing importance as inducers of different cellular responses (inflammation, proliferation, or cell death). It was the aim of this study to understand their impact on intracellular signal transduction pathways that are responsible for these biological effects. We found that in arterial smooth muscle cells, PGPC and POVPC activated sphingomyelinases, in particular the acid isoform, which is known to participate in the very early phase of apoptotic stress responses. In addition, mitogen-activated protein kinases, which are involved in induction of stress response and apoptosis were phosphorylated (activated). Finally, activation of caspase 3 was observed, showing that stimulation of smooth muscle cells with POVPC and PGPC is associated with apoptosis. Stimulation of all these enzymes by the oxidized phospholipids almost perfectly matched their activation by minimally modified LDL. Consequently, these phospholipids seem to be responsible for the effect of this particle on cell signaling. Survival and proliferation pathways including NF-kappa B or AKT kinase were not induced by POVPC and PGPC. Experiments with a specific inhibitor of acid sphingomyelinase named NB6 showed that this enzyme plays a central role in mediating the apoptotic effects of the oxidized lipids. Thus, we conclude that modified phospholipids induce signaling cascades via activation of acid sphingomyelinase finally leading to apoptosis of smooth muscle cells, which is a detrimental process in the development of atherosclerosis.

The lysophospholipid receptor G2A activates a specific combination of G proteins and promotes apoptosis

G2A, a G protein-coupled receptor for which lysophosphatidylcholine (LPC) is a high affinity ligand, belongs to a newly defined lysophospholipid receptor subfamily. Expression of G2A is transcriptionally up-regulated by stress-inducing and cell-damaging agents, and ectopic expression of G2A leads to growth inhibition. However, the G proteins that functionally couple to G2A have not been elucidated in detail. We report here that G2A ligand independently stimulates the accumulation of both inositol phosphates and cAMP. LPC does not further enhance inositol phosphate accumulation but dose-dependently augments intracellular cAMP concentration. Expression of G alpha(q) and G alpha(13) with G2A potentiates G2A-mediated activation of a NF-kappa B-luciferase reporter. These results demonstrate that G2A differentially couples to multiple G proteins including G alpha(s), G alpha(q), and G alpha(13), depending on whether it is bound to ligand. G2A-transfected HeLa cells display apoptotic signs including membrane blebbing, nuclear condensation, and reduction of mitochondrial membrane potential. Furthermore, G2A-induced apoptosis can be rescued by the caspase inhibitors, z-vad-fmk and CrmA. Although apoptosis occurs without LPC stimulation, LPC further enhances G2A-mediated apoptosis and correlates with its ability to induce cAMP elevation in both HeLa cells and primary lymphocytes. Rescue from G2A-induced apoptosis was achieved by co-expression of a G alpha(12/13)-specific inhibitor, p115RGS (regulator of G protein signaling), in combination with 2',5'-dideoxyadenosine treatment. These results demonstrate the ability of G2A to activate a specific combination of G proteins, and that G2A/LPC-induced apoptosis involves both G alpha(13)- and G alpha(s)-mediated pathways.

Mildly oxidized LDL particle subspecies are distinct in their capacity to induce apoptosis in endothelial cells: role of lipid hydroperoxides

The risk of atherosclerosis is intimately related to the heterogeneity of low-density lipoprotein (LDL) particles. The potential relationship between oxidative modification of distinct LDL subspecies and induction of apoptosis in arterial wall cells is indeterminate. The capacity of light LDL3 versus dense LDL5 to induce cytotoxicity in endothelial cells as a function of the degree of copper-mediated oxidation was compared. Mildly oxidized LDL3 (oxLDL3) exerted potent cytotoxicity, which was intimately related to both the degree of oxidation and the oxLDL3 concentration based on either cholesterol content or particle number. In contrast, dense LDL5 particles exerted a minor effect on cell viability. Cells incubated with oxLDL3 exhibited apoptotic features, with cytoplasmic condensation, cell or nuclear fragmentation, and accumulation of DNA fragments. OxLDL3-induced apoptosis involved cytoplasmic release of cytochrome c, with a concomitant increase in caspase-3-like protease activity. OxLDL3 particles were uniquely distinct from oxLDL5 particles in their elevated content of lipid hydroperoxides. Hydroperoxide removal by NaBH4 markedly reduced oxLDL3-induced cytotoxicity, leading to an increase in cell viability. Lipid hydroperoxide content of oxidatively modified LDL subclasses is therefore a major determinant of the induction of apoptosis in endothelial cells. These data are highly relevant to atherogenic hypercholesterolemia, in which the LDL phenotype is dominated by elevated concentrations of light LDL3.

Oxidized low-density lipoprotein-induced apoptosis

Cultured cells are able to oxidize low-density lipoproteins (LDL) and oxidized LDL (oxLDL), which are present in atherosclerosis areas, exhibit a variety of biological properties potentially involved in atherogenesis. This review is focused on the toxicity of oxLDL, more precisely on the toxic compounds generated during LDL oxidation, the features and the mechanisms of cell death (apoptosis or necrosis) induced by oxLDL. After internalization, toxic oxidized lipids, namely lipid peroxides, oxysterols and aldehydes, induce modifications of cell proteins, elicit oxidative stress, lipid peroxidation and alter various signaling pathways and gene expression. These events may participate in the toxic effect, and converge to trigger an intense, delayed and sustained calcium peak which elicits either apoptosis or necrosis processes. OxLDL-induced apoptosis involves both mitochondrial and death-receptor (Fas/FasL) apoptotic pathways, thereby activating the classical caspase cascade and subsequent biochemical and morphological apoptotic features. When apoptosis is blocked by overexpression of Bcl-2, oxLDL trigger necrosis through a calcium-dependent pathway. Apoptosis occurring in atherosclerotic areas is potentially involved in endothelial cell lining defects, necrotic core formation and plaque rupture or erosion which may trigger atherothrombotic events. However, the precise role of oxLDL in apoptosis/necrosis occurring in vivo in atherosclerotic plaques remains to be clarified.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Inhibition of lipoprotein-associated phospholipase A2 diminishes the death-inducing effects of oxidised LDL on human monocyte-macrophages

The death of macrophages contributes to atheroma formation. Oxidation renders low-density lipoprotein (LDL) cytotoxic to human monocyte-macrophages. Lipoprotein-associated phospholipase A2 (Lp-PLA2), also termed platelet-activating factor acetylhydrolase, hydrolyses oxidised phospholipids. Inhibition of Lp-PLA2 by diisopropyl fluorophosphate or Pefabloc (broad-spectrum serine esterase/protease inhibitors), or SB222657 (a specific inhibitor of Lp-PLA2) did not prevent LDL oxidation, but diminished the ensuing toxicity and apoptosis induction when the LDL was oxidised, and inhibited the rise in lysophosphatidylcholine levels that occurred in the inhibitors' absence. Hydrolysis products of oxidised phospholipids thus account for over a third of the cytotoxic and apoptosis-inducing effects of oxidised LDL on macrophages.