Oxidized low density lipoprotein inhibits platelet plasma membrane Ca(2+)-ATPase

Oxidised Low-Density Lipoprotein-Induced Platelet Hyperactivity—Receptors and Signalling Mechanisms

Dyslipidaemia leads to proatherogenic oxidative lipid stress that promotes vascular inflammation and thrombosis, the pathologies that underpin myocardial infarction, stroke, and deep vein thrombosis. These prothrombotic states are driven, at least in part, by platelet hyperactivity, and they are concurrent with the appearancxe of oxidatively modified low-density lipoproteins (LDL) in the circulation. Modified LDL are heterogenous in nature but, in a general sense, constitute a prototype circulating transporter for a plethora of oxidised lipid epitopes that act as danger-associated molecular patterns. It is well-established that oxidatively modified LDL promote platelet activation and arterial thrombosis through a number of constitutively expressed scavenger receptors, which transduce atherogenic lipid stress to a complex array of proactivatory signalling pathways in the platelets. Stimulation of these signalling events underlie the ability of modified LDL to induce platelet activation and blunt platelet inhibitory pathways, as well as promote platelet-mediated coagulation. Accumulating evidence from patients at risk of arterial thrombosis and experimental animal models of disease suggest that oxidised LDL represents a tangible link between the dyslipidaemic environment and increased platelet activation. The aim of this review is to summarise recent advances in our understanding of the pro-thrombotic signalling events induced in platelets by modified LDL ligation, describe the contribution of individual platelet scavenger receptors, and highlight potential future challenges of targeting these pathways.

Association Between Low-Density Lipoprotein Cholesterol and Platelet Distribution Width in Acute Ischemic Stroke

Objective: Elevated low-density lipoprotein cholesterol (LDL-C) is an established risk factor for ischemic stroke; however, whether LDL-C affects the platelet deformation function in the peripheral blood circulation in patients with acute ischemic stroke (AIS) is unknown. The present study aimed to investigate the relationship between LDL-C and platelet distribution width (PDW) in AIS patients. Methods: We conducted a cross-sectional hospitalized-based study of consecutive 438 patients with AIS within 24 h. Blood samples were collected upon admission and prior to drug administration, and LDL-C and PDW (a parameter that reflects the heterogeneity of platelet volume) were assessed. The relationship between LDL-C and PDW were analyzed by linear curve fitting analyses. Crude and adjusted beta coefficients of LDL-C for PDW with 95% confidence intervals were analyzed using multivariate-adjusted linear regression models. Results: The PDW was significantly higher in the high LDL-C group compared with those in the normal LDL-C group (16.28 ± 0.37 fl vs. 16.08 ± 0.37 fl, p < 0.001). Adjusted smoothed plots suggested that there are linear relationships between LDL-C and PDW, and the Pearson's correlation coefficient (95%) was 0.387 (0.304-0.464, p < 0.001). The beta coefficients (95% CI) between LDL-C and PDW were 0.15 (0.12-0.18, p < 0.001) and 0.14 (0.11-0.18, p < 0.001), respectively, in AIS patients before and after adjusting for potential confounders. Conclusion: Our study suggested that the elevated LDL-C level was related to increased PDW among AIS patients.

Different Effect of Statins on Platelet Oxidized-LDLReceptor (CD36 and LOX-1) Expressionin Hypercholesterolemic Subjects

Hydroxymethyl-glutaryl-CoA-reductase inhibitors (statins) reduce cardiovascular mortality by decreasing cholesterol as well as by non-lipid-related actions. Oxidized low-density lipoproteins (ox-LDL) are pro-atherogenic molecules and potent platelet agonists. CD36 and lectin-like ox-LDL receptor-1 (LOX-1) are specific ox-LDL receptors also expressed in platelets. This study was planned to address whether treatment with atorvastatin 10 mg/day, pravastatin 40 mg/day or simvastatin 20 mg/day could affect platelet CD36 and LOX-1 expression. Twenty-four patients for each treatment were evaluated after 3, 6, and 9 days and at 6 weeks for complete lipid profile (chromogenic), ox-LDL (ELISA), platelet P-selectin (P-sel), CD36, LOX-1 (FACS), and intracellular citrullin recovery (iCit) (HPLC). Data show hyperactivated platelets (P-sel absolute values, percent variation in activated cells, all p < 0.001), and CD36 and LOX-1 overexpression (all p < 0.001) in patients at baseline. P-sel, CD36, and LOX-1 were significantly decreased by atorvastatin and simvastatin (all p < 0.01) and related with iCit increase (r = 0.58,p < 0.001) and platelet-associated ox-LDL (r = 0.51, p < 0.01) at 9 days. Pravastatin reduced LOX-1 and P-sel (p < 0.05) at 6 weeks in relation with decreased LDL and ox-LDL (r = 0.39, p < 0.01 and r = 0.37, p < 0.01, respectively). These data suggest that atorvastatin and simvastatin reduce platelet activity by exposure of CD36 and LOX-1 before significant LDL reduction, whereas pravastatin action is detected later and in relation with LDL and ox-LDL lowering. Rapid and consistent reduction of CD36 and LOX-1 could be considered a direct anti-atherothrombotic mechanism related to the role of ox-LDL in platelet activation, platelet-endothelium interactions, and NO synthase activity.

Na(+)/H(+) exchanger in the regulation of platelet activation and paradoxical effects of cariporide

Platelets are anucleated cell fragments derived from mature megakaryocytes and function in hemostasis when the endothelium is injured. Hemostasis involving platelets can be divided into four phases: adhesion, activation, secretion, and aggregation. Platelet activation requires a rise in intracellular Ca(2+) concentrations and results in both a morphological change and the secretion of platelet granule contents. Na(+)/H(+) exchanger isoform 1 (NHE1) regulates the intracellular pH (pHi) and the volume of platelets. In addition, NHE1 plays a large role in platelet activation. Thrombus generation involves NHE1 activation and an increase in [Ca(2+)]i, which results from NHE1-mediated Na(+) overload and the reversal of the Na(+)/Ca(2+) exchanger. Cariporide (HOE-642), a potent NHE1 inhibitor, has inhibitory effects on the degranulation of human platelets, the formation of platelet-leukocyte-aggregates, and the activation of the GPIIb/IIIa receptor (PAC-1). However, despite the demonstrated protection against myocardial infarction as mediated by cariporide in patients undergoing coronary artery bypass graft surgery, the EXPEDITION clinical trial revealed that cariporide treatment increased mortality due to thromboembolic stroke. These findings suggest that a better understanding of NHE1 and its effect on platelet function and procoagulant factor regulation is warranted in order to develop therapies using NHE inhibitors.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

Atorvastatin induces associated reductions in platelet P-selectin, oxidized low-density lipoprotein, and interleukin-6 in patients with coronary artery diseases

The development and progression of atherosclerosis comprises various processes, such as endothelial dysfunction, chronic inflammation, thrombus formation, and lipid profile modification. Statins are 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors that have pleiotropic effects in addition to cholesterol-lowering properties. However, the mechanisms of these effects are not completely understood. Here, we investigated whether atorvastatin affects the levels of malondialdehyde-modified low-density lipoprotein (MDALDL), an oxidized LDL, the proinflammatory cytokine interleukin-6 (IL-6), or platelet P-selectin, a marker of platelet activation, relative to that of LDL cholesterol (LDL-C). Forty-eight patients with coronary artery disease and hyperlipidemia were separated into two groups that were administered with (atorvastatin group) or without (control group) atorvastatin. The baseline MDA-LDL level in all participants significantly correlated with LDL-C (r = 0.71, P < 0.01) and apolipoprotein B levels (r = 0.66, P < 0.01). Atorvastatin (10 mg/day) significantly reduced the LDL-C level within 4 weeks and persisted for a further 8 weeks of administration. Atorvastatin also reduced the MDA-LDL level within 4 weeks and further reduced it over the next 8 weeks. Platelet P-selectin expression did not change until 4 weeks of administration and then significantly decreased at 12 weeks, whereas the IL-6 level was gradually, but not significantly, reduced at 12 weeks. In contrast, none of these parameters significantly changed in the control group within these time frames. The reduction (%) in IL-6 between 4 and 12 weeks after atorvastatin administration significantly correlated with that of MDALDL and of platelet P-selectin (r = 0.65, P < 0.05 and r = 0.70, P < 0.05, respectively). These results suggested that the positive effects of atorvastatin on the LDL-C oxidation, platelet activation and inflammation that are involved in atherosclerotic processes are exerted in concert after lowering LDL-C.

Lectin-like oxidized-LDL receptor-1 (LOX-1) polymorphisms influence cardiovascular events rate during statin treatment

BACKGROUND

Oxidized-LDL (ox-LDL) are involved in atherothrombosis by induction of endothelial dysfunction and thrombosis. The specific receptor lectin-like oxidized-LDL receptor-1 (LOX-1) is expressed in endothelial cells, monocytes and platelets. LOX-1 gene allelic variants (3'UTR/T) have been related with cardiovascular events and reduced anti-platelet activity induced by statins.

OBJECTIVES

To detect whether LOX-1 polymorphisms could affect statins effectiveness in cardiovascular prevention.

PATIENTS/METHODS

The present was a retrospective study performed in 751 white hypercholesterolemic subjects treated with increasing doses of atorvastatin (n=382, 247 male, 135 female) or simvastatin (n=369, 244 male, 125 female) up to 4 years, whose LDL target was 3.36 mmol/L according to the National Cholesterol Education Program, Adult Treatment Panel III (NCEP-ATPIII). Single nucleotide polymorphism were evaluated by allelic discrimination assays (PCR), lipid profile by enzymatic-colorimetric methods and C-reactive protein (CRP) by a nephelometric technique.

RESULTS

Twenty-three non-ST elevation (NSTEMI) and eleven ST-elevation myocardial infarction (STEMI) were encountered in the observational period without differences between treatments (p=0.175) and sex (p=0.139). Each symptomatic subject (10 reaching the appropriate LDL target and 24 with still undesirable LDL) had the 3'UTR/T allelic variant (adjusted O.R. 4.63, 95% C.I. 3.46-6.70, p<0.0001). Among patients not reaching LDL target the C allele resulted protective with respect to T carriers (p<0.00001). Also, similar changes of CRP resulted in different event rate between T and C carriers (p<0.001) in the whole cohort.

CONCLUSIONS

In the studied population LOX-1 genetic variants influence cardiovascular risk reduction induced by statins also in patients not reaching the LDL target. The previously described LOX-1-related antithrombotic actions of both statins employed could have a specific role in what observed, suggesting a genetic influence in statins LDL-lowering partially related actions.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

Plasma very-low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein oxidative modification induces procoagulant profiles in endogenous hypertriglyceridemia

This study was to investigate whether oxidatively modified lipoproteins were associated with changes of pro- and anticoagulant profiles in hypertriglyceridemic subjects. Plasma VLDL, LDL, and HDL were isolated with the one-step density gradient ultracentrifugation method. The oxidation of the lipoproteins was identified. Prothrombin time (PT) and activated partial thrombplastin time (APTT), tissue plasminogen activator and plasminogen activator inhibitor-1, and platelet aggregation rate were determined with a reaction system consisting of mixed fresh normal plasma, in endogenous hypertriglyceridemic (HTG) patients, in in vitro modified lipoproteins from a normolipidemic donor, and in experimental rats. The results indicated that oxVLDL, oxLDL, and oxHDL occurred in the plasma of HTG patients. Compared with the control group, PT and APTT, incubated with plasma VLDL, LDL, or HDL from HTG patients, respectively, were significantly reduced, while platelet maximal aggregation rates were significantly higher (P < 0.05-0.01). Similar procoagulant profiles were observed in in vitro modified lipoprotein components and in rats with intrinsic hypertriglyceridemia as well. These results support our previous finding that LDL, VLDL, and HDL were all oxidatively modified in vivo in the subjects with HTG, and suggest that procoagulation state may result from the abnormal plasma lipoprotein oxidative modification in vivo.

Resveratrol, a polyphenolic compound in red wine, protects against oxidized LDL-induced cytotoxicity in endothelial cells

BACKGROUND

Resveratrol, a polyphenolic constituent of red wine, has antioxidant effects. However, its protective effects against oxLDL-induced endothelial injury remained unclarified.

METHODS

Primary human umbilical vein endothelial cell cultures (HUVECs) treated with oxLDL (200 microg/ml) were used to explore the protective effect of resveratrol. Cytotoxicity of oxLDL on HUVECs was studied by measuring lactate dehydrogenase (LDH) release, methylthiazol tetrazolium (MTT) and apoptotic cell death as characterized by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) stain. We also measured the production of reactive oxygen species (ROS) by using the fluorescnet probe 2', 7'-dichlorofluorescein acetoxymethyl ester (DCF-AM), and observed the activity of antioxidant enzymes. Furthermore, several apoptotic signaling pathway with increased cytosolic calcium, alteration of mitochondrial membrane potential, cytochrome c release and activation of caspase 3 were also investigated.

RESULTS

Resveratrol attenuated oxLDL-induced cytotoxicity, apoptotic features, generation of ROS and intracellular calcium accumulation. OxLDL-induced mitochondria membrane potential collapase, cytochrome c release and activation of caspase 3 in HUVECs were also suppressed by resveratrol pretreatment.

CONCLUSIONS

Red wine intake may protect against oxLDL-induced dysfunction of endothelial cells.

Atorvastatin reduces platelet-oxidized-LDL receptor expression in hypercholesterolaemic patients

BACKGROUND

Oxidized-LDL (ox-LDL) are proatherogenic and platelet-activating molecules. Atorvastatin reduces platelet activity before cholesterol-lowering action. CD36 and lectin-like oxidized-LDL receptor-1 (LOX-1) are specific ox-LDL receptors expressed also in platelets. This study was planned to address whether the possible rapid effect of atorvastatin on platelets could be related to modulation of ox-LDL receptors.

MATERIALS AND METHODS

Forty-eight hypercholesterolaemic subjects requiring statin treatment (atorvastatin 20 mg day(-1)) after an ineffective diet regimen were evaluated for complete lipid-profile (chromogenic); P-selectin (P-sel), CD36 and LOX-1 expression (cytofluorimetric detection); circulating and platelet-associated ox-LDL (ox- and Pox-LDL, ELISA); and intracellular citrullin recovery (iCit, HPLC) at baseline and 3, 6 and 9 days after inclusion in the study. Moreover, we studied 48 normal controls matched for sex and age.

RESULTS

Platelet activity expressed by P-sel (in resting and thrombin-activated cells), CD36 and LOX-1 were increased in hypercholesterolaemic subjects (all P < 0.01). Atorvastatin induced a reduction of CD36 at 6 days (P < 0.05); and P-sel in resting (P < 0.001) and activated cells (P < 0.001) and LOX-1 were reduced at 9 days (all P < 0.001) in association with decreased Pox-LDL (P < 0.001) and increased iCit (P < 0.01). All data were obtained before a significant reduction of LDL and ox-LDL was achieved (P = 0.109 and 0.113).

DISCUSSION

Present data suggest that platelet deactivation by atorvastatin is related to CD36 and LOX-1 expression reduction before significant LDL changes. Moreover, the modulation of LOX-1 can be considered a self-relevant antiatherothrombotic action of atoravastin owing to the important role of this receptor in the ox-LDL-mediated vascular damage.

Early changes in pancreatic acinar cell calcium signaling after pancreatic duct obstruction

Intracellular Ca(2+)-changes not only participate in important signaling pathways but have also been implicated in a number of disease states including acute pancreatitis. To investigate the underlying mechanisms in an experimental model mimicking human gallstone-induced pancreatitis, we ligated the pancreatic duct of Sprague-Dawley rats and NMRI mice for up to 6 h and studied intrapancreatic changes including the dynamics of [Ca(2+)](i) in isolated acini. In contrast to bile duct ligation, pancreatic duct obstruction induced intra-pancreatic trypsinogen activation, leukocytosis, hyperamylasemia, and pancreatic edema and increased lung myeloperoxidase activity. Although resting [Ca(2+)](i) in isolated acini rose by 45% to 205 +/- 7 nmol, the acetylcholine- and cholecystokinin (CCK)-stimulated calcium peaks as well as the amylase secretion declined, but neither the [Ca(2+)](i)-signaling pattern nor the amylase output in response to the Ca(2+)-ATPase inhibitor thapsigargin nor the secretin-stimulated amylase release were impaired by pancreatic duct ligation. On the single cell level pancreatic duct ligation reduced the percentage of cells in which submaximal secretagogue stimulation was followed by a physiological response (i.e. Ca(2+) oscillations) and increased the percentage of cells with a pathological response (i.e. peak plateau or absent Ca(2+) signal). Moreover, it reduced the frequency and amplitude of Ca(2+) oscillation as well as the capacitative Ca(2+) influx in response to secretagogue stimulation. Serum pancreatic enzyme elevation as well as trypsinogen activation was significantly reduced by pretreatment of animals with the calcium chelator BAPTA-AM. These experiments suggest that pancreatic duct obstruction rapidly changes the physiological response of the exocrine pancreas to a Ca(2+)-signaling pattern that has been associated with premature digestive enzyme activation and the onset of pancreatitis, both of which can be prevented by administration of an intracellular calcium chelator.

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.

Time-dependent effect of statins on platelet function in hypercholesterolaemia

BACKGROUND

Reduction of platelet activity induced by statins has been described as a positive effect exerted by such molecules on vascular thrombotic events. However, the relations among cholesterol (LDL-C) reduction, the timing of the antiplatelet effect, the involved mechanisms and the doses of each statin able to reduce platelet function are not actually well known. The aim of our study was to evaluate the impact of simvastatin (20 mg day-1), atorvastatin (10 mg day-1), fluvastatin (40 mg day-1) and pravastatin (40 mg day-1) on platelet function in hypercholesterolaemic subjects with relation to (LDL-C), oxidized-LDL (ox-LDL) and antiport mechanism modifications.

MATERIALS AND METHODS

Sixteen subjects were assigned to each treatment (40 males, 24 females, mean age 48.7 +/- 13.4, LDL-C 5.13 +/- 0,23 mmol L-1) and evaluated for platelet surface P-selectin (P-sel), lipid profile, ox-LDL, platelet-associated ox-LDL (Pox-LDL), platelet cholesterol content, antiport mechanisms, and intracellular and systemic NO synthase every 7 days for one month.

RESULTS

Our data show a strong relation between enhanced P-sel and Pox-LDL (r = 0.68, P < 0.01). Simvastatin, atorvastatin, fluvastatin and pravastatin reduce platelet activity after 1, 2, 3 and 4 weeks of treatment, respectively (P < 0.001, P < 0.001, P < 0.01, P < 0.05). Pox-LDL are modulated early by simvastatin, atorvastatin and fluvastatin Pox-LDL (r = 0.66, 0.65 and 0.52; P < 0.001, 0.001 and 0.01, respectively) whereas LDL-C and ox-LDL reductions associated to modifications of antiport activity act later. Moreover, they are the most relevant finding in pravastatin-related subjects.

CONCLUSIONS

Our data suggest a different impact of several statins on platelet function, which is initially related to interference with Pox-LDL rather than LDL-C reduction.

The influence of platelet-smooth muscle cell interaction on the oxidative modification of low-density lipoprotein

OBJECTIVE

In view of the recognized association between thrombosis and atherosclerosis, it is hypothesized that exposure of arterial smooth muscle cells (SMC) to thrombogenic agents such as platelets and thrombin will alter the oxidation of low-density lipoprotein (LDL) and that this effect may be diminished by thrombin inhibition.

METHODS

Quiescent human aortic SMC in culture were exposed to LDL (40 microg protein/ml) alone or with washed human platelets (5 x 10(6)/ml), thrombin (40 units/ml), or a combination of these agents for 48 h. The media were removed, and both media and cell lysate fractions were assayed for malondialdehyde (MDA) content as an index of oxidation. Isolated platelets exposed to LDL and thrombin were studied in a similar manner to determine their individual oxidative activity. Finally, SMC and platelets were incubated with LDL and varying concentrations of thrombin (10-80 units/ml), both alone and in the presence of the thrombin inhibitors hirudin (u/u), and heparin (u/u), and MDA was measured.

RESULTS

SMC and platelets each demonstrated an ability to oxidize LDL, increasing MDA concentrations by 1.8- (P < 0.05) and 4- (P < 0.01) fold, respectively, compared to lipid-free media. Both platelets (P < 0.05) and thrombin (P < 0.001) enhanced the oxidation of LDL by SMC, while a combination of these two agents resulted in an additive effect (P < 0.001). The SMC lysate fraction showed an increase in oxidative products following exposure to platelets (P < 0.01) but not thrombin, suggesting that platelets stimulated uptake of the oxidized lipid by the SMC. Isolated platelets responded to thrombin with an increase in MDA within the media (P < 0.001). Smooth muscle cells exposed to thrombin also showed a dose-dependent increase in LDL oxidation (P < 0.01). This effect was not altered by hirudin, but was significantly inhibited by heparin (P < 0.05).

CONCLUSIONS

These results indicate that the oxidative potential of SMC and platelets is enhanced by their coincubation and by their concurrent exposure to thrombin. Heparin appears to block thrombin-stimulated oxidation. This interaction could be relevant to the dynamic interaction between atherosclerosis and thrombogenesis.

Modification of protein moiety of human low density lipoprotein by hypochlorite generates strong platelet agonist

Conflicting reports exist about the effects of mildly or extensively oxidized low density lipoproteins (LDLs) on the reactivity of human platelets. This platelet response is mainly caused by modification of the protein and lipid moiety, giving rise to very differently modified species with hardly predictable properties. The aim of this study was to prepare oxidized LDL with modifications essentially restricted to the protein moiety and to determine the eventual platelet responses. We treated LDL at 0 degrees C for 10 minutes with a 60- to 1000-fold molar excess of sodium hypochlorite in borate buffer in the presence of the radical scavenger butylated hydroxytoluene. Under these conditions, neither fragmentation of apolipoprotein B-100 nor formation of LDL aggregates was observed, and lipid oxidation products did not exceed the amount present in untreated LDLs. The degree of modification and the respective effects on platelet function were highly reproducible. Hypochlorite-modified LDLs act as strong platelet agonists, inducing morphological changes, dense granule release, and irreversible platelet aggregation. The evoked platelet effects are completely suppressed by inhibitors of the phosphoinositide cycle but not by EDTA or acetylsalicylic acid. Most likely, these effects are transmitted via high-affinity binding to a single class of sites, which does not recognize native or acetylated LDL. Obviously, modified lysines, and the secondary lipid modifications derived from them, are not essential for this interaction. We conclude that bioactive oxidized lipids are not directly involved in the stimulation of platelets by hypochlorite-modified LDLs.

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.

Stimulating effect of biologically modified low density lipoproteins on ADP-induced aggregation of washed platelets persists in absence of specific binding

Oxidized low density lipoproteins are closely associated with atherosclerosis and also might be directly involved in thrombosis because they have been shown to mediate a stimulating effect on human platelets. In this work, we used biologically modified low density lipoproteins (i.e., low density lipoproteins sufficiently oxidized to show specificity for the macrophage scavenger receptor system) to examine if specific binding of the oxidized apolipoprotein moiety to the platelet surface is a prerequisite for the platelet-stimulating effects reported by other authors. We find that biologically modified low density lipoproteins show specific binding to human platelets (K(d)=5.83+/-0.4 microg/mL, 3850+/-620 sites/platelet) and strongly augment both ADP- and thrombin-induced aggregation of washed platelets. Maleylated albumin, an antagonist of oxidized low density lipoproteins binding to all currently classified scavenger receptors, is able to reduce platelet oxidized low density lipoproteins binding to background levels. Nevertheless, maleylated albumin is not able to exert any kind of normalizing effect on the augmented ADP-induced aggregation response observed in the presence of biologically modified low density lipoproteins. From these data, we conclude that specific binding of oxidatively modified apolipoprotein B to the platelet surface is not essential to the process of platelet stimulation. Therefore, we conclude that these stimulating effects may be mediated by unidentified compounds formed in the lipid phase of the lipoproteins.

Pharmacological and immunohistochemical characterization of calmodulin-stimulated (Ca(2+)+Mg(2+))-ATPase in cultured porcine aortic endothelial cells

Plasma membrane (Ca(2+)+Mg(2+))-ATPase and Ca(2+) transport activities, best characterized in human erythrocytes, are stimulated by calmodulin and thought to play a crucial role in the termination of cellular Ca(2+) signaling in all cells. In plasma membranes isolated from cultured porcine aortic endothelial cells, the (Ca(2+)+Mg(2+))-ATPase was not readily measured. This is in part because of an overabundance of nonspecific Ca(2+)- and/or Mg(2+)-activated ecto-5'-nucleotide phosphohydrolases. Moreover, addition of exogenous calmodulin (10(-9) to 10(-6) mol/L) produced no measurable stimulation of ATPase activities, suggesting a permanently activated state or, alternatively, a complete lack thereof. To establish and verify the presence of a calmodulin-regulated (Ca(2+)+Mg(2+))-ATPase activity in these endothelial cells, immunohistochemical localization using a monoclonal mouse anti-(Ca(2+)+Mg(2+))-ATPase antibody (clone 5F10) was applied to intact pig aorta endothelium, cultured endothelial monolayers, and isolated endothelial plasma membrane fractions. This approach clearly demonstrated Ca(2+) pump immunoreactivity in each of these preparations. To confirm functional calmodulin stimulation of the (Ca(2+)+Mg(2+))-ATPase, 10(-5) mol/L calmidazolium (R24571) was added to the isolated plasma membrane preparation, which lowered the (Ca(2+)+Mg(2+))-ATPase activity from 143.0 to 78.15 nmol P(i)/mg protein x min(-1). This calmidazolium-reduced activity could then be stimulated 113.1+/-0.8% in a concentration-dependent manner by the addition of exogenous calmodulin (10(-7) to 2 x 10(-6) mol/L) with an EC(50) of 3.45+/-0.04 x 10(-7) mol/L (n=4). This represents a competitive lowering of the apparent calmodulin affinity by approximately 100 compared with other unopposed calmodulin-stimulated processes. Together, these findings support evidence for the presence of a calmodulin-stimulated plasma membrane (Ca(2+)+Mg(2+))-ATPase activity in cultured porcine aortic endothelial cells.

Effects of hypochlorite-modified low-density and high-density lipoproteins on intracellular Ca2+ and plasma membrane Ca(2+)-ATPase activity of human platelets

The presence of hypochlorite-modified lipoproteins in atherosclerotic lesions suggests that HOCl, a naturally occurring oxidant formed by the myeloperoxidase-catalyzed reaction of H2O2 and Cl-, is a candidate for generation of modified lipoproteins in vivo. We have previously demonstrated that Cu(2+)-oxidized LDL inhibits platelet plasma membrane Ca(2+)-ATPase (PMCA) in isolated membranes and causes an increase in cytosolic Ca2+ in resting whole platelets. However, Cu(2+)-oxidized LDL may not be identical in structure and function to the physiologically modified lipoprotein. Since platelet function may be affected by native and modified lipoproteins, the effect of HOCl-modified LDL and HDL3 on platelet PMCA and on the free intracellular Ca2+ concentration ([Ca2+]i) of whole platelets has been investigated. We demonstrate that in contrast to Cu(2+)-oxidized LDL, HOCl-modified LDL and HDL3 stimulate platelet PMCA activity in isolated membranes and that this effect results in a decrease of [Ca2+]i in vivo. Thus, HOCl-oxidation produces modified lipoproteins with the potential for altering platelet function and with properties different from those of the Cu(2+)-oxidized counterparts.

Vitamin E (alpha-tocopherol) does not inhibit platelet stimulation by oxidized low density lipoprotein in vitro

Platelet-rich plasma were treated with increasing concentrations of vitamin E (alpha-tocopherol). Washed platelets were exposed to oxidized low density lipoprotein (LDL) and examined by aggregometry and electron microscopy. The treatment of washed platelets by oxidized LDL induced morphological signs of activation like pseudopodia formation and an increase in light transmission. Alpha-tocopherol in a range of 0.001-1.0 mmol had no inhibiting influences on platelet activation by oxidized LDL. These results indicate that the free radical scavenger vitamin E cannot directly inhibit platelet activation by oxidized LDL. It may be supposed that platelet activation by oxidized LDL does not occur in a radical-dependent mechanism.

Lipid oxidation products and vascular function

The endothelium and blood platelets are intimately involved in both the maintenance of vascular tone and in haemostasis. They are also exposed to high concentrations of lipoproteins, either in the plasma or in the sub-endothelial region of the artery wall, and the biological activity of these cells has been shown to be modulated. Oxidative modification of these lipoproteins results in further variations in the properties of these particles in relation to the activities of the endothelium and platelets. These effects and how the work of Hermann Esterbauer on the details of lipoprotein oxidation permitted rapid progress in understanding these phenomena are discussed in this review.

Diabetes-induced alterations in platelet metabolism

OBJECTIVES

This review summarizes the recent findings on some aspects of platelet metabolism that appear to be affected as a consequence of diabetes mellitus. The metabolites include glutathione, L-Arginine/nitric oxide, as well as the ATP-dependent exchange of Na+/K+ and Ca2+.

CONCLUSIONS

Several aspects of platelet metabolism are altered in diabetics. These metabolic events give rise to a platelet that has less antioxidants, and higher levels of peroxides. The direct consequence of this is the overproduction platelet agonists. In addition, there is evidence for altered Ca2+ and Na+ transport across the plasma membrane. Recent evidence indicates that plasma ATPases in diabetic platelets are not damaged instead their activities are likely to be modulated by oxidized LDL. Finally, platelet inhibitory mechanisms regulated by NO appear to be perturbed in the diabetes disease-state. The combined production of NO and superoxide by NOS isoforms in the platelet could be a major contributory factor to platelet pathogenesis in diabetes mellitus.