Platelet LDL receptor recognizes with the same apparent affinity both oxidized and native LDL. Evidence that the receptor-ligand complexes are not internalized

Binding properties of recombinant LDL receptor and LOX-1 receptor to LDL measured using bio-layer interferometry and atomic force microscopy

Oxidation of low-density lipoproteins (LDLs) triggers a recognition by scavenger receptors such as lectin-like oxidized LDL receptor-1 (LOX-1) and is related to inflammation and cardiovascular diseases. Although LDLs that are recognized by LOX-1 can be risk-related LDLs, conventional LDL detection methods using commercially available recombinant receptors remain undeveloped. Using a bio-layer interferometry (BLI), we investigated the binding of recombinant LOX-1 (reLOX-1) and LDL receptors to the oxidized LDLs. The recombinant LDL receptor preferably bound minimally modified LDLs, while the reLOX-1 recognized extensively oxidized LDLs. An inversed response of the BLI was observed during the binding in the case of reLOX-1. AFM study showed that the extensively oxidized LDLs and aggregates of LDLs were observed on the surface, supporting the results. Altogether, a combined use of these recombinant receptors and the BLI method is useful in detecting high-risk LDLs such as oxidized LDLs and modified LDLs.

Colchicine as a Modulator of Platelet Function: A Systematic Review

The microtubule inhibitor and anti-inflammatory agent colchicine is used to treat a range of conditions involving inflammasome activation in monocytes and neutrophils, and is now known to prevent coronary and cerebrovascular events. In vitro studies dating back more than 50 years showed a direct effect of colchicine on platelets, but as little contemporary attention has been paid to this area, we have critically reviewed the effects of colchicine on diverse aspects of platelet biology in vitro and in vivo. In this systematic review we searched Embase, Medline, and PubMed for articles testing platelets after incubation with colchicine and/or reporting a clinical effect of colchicine treatment on platelet function, including only papers available in English and excluding reviews and conference abstracts. We identified 98 relevant articles and grouped their findings based on the type of study and platelet function test. In vitro, colchicine inhibits traditional platelet functions, including aggregation, clotting, degranulation, and platelet-derived extracellular vesicle formation, although many of these effects were reported at apparently supraphysiological concentrations. Physiological concentrations of colchicine inhibit collagen- and calcium ionophore-induced platelet aggregation and internal signaling. There have been limited studies of in vivo effects on platelets. The colchicine-platelet interaction has the potential to contribute to colchicine-mediated reduction in cardiovascular events, but there is a pressing need for high quality clinical research in this area.

Differential effects of native and oxidatively modified low-density lipoproteins on platelet function

Low-density lipoproteins (LDL) have been various reported to induce platelet aggregation independently and/or sensitise platelets to other agonists. In these earlier studies the extent of oxidation of LDL was not always reported or addressed. We have now investigated the effects of native, minimally modified and fully oxidised LDL (0-1gapolipoproteinB(100)/l on platelet function using platelet aggregometry and fluorescence activated flow cytometry. Native LDL did not activate isolated platelets but inhibited ADP- and thrombin-induced aggregation of isolated platelets by 51 % in the presence or absence of added fibrinogen. Longer pre-incubations were required to produce a comparable inhibition by native LDL on platelets in plasma. Flow cytometric analysis showed that native LDL inhibited ADP-induced fibrinogen binding by up to 38%. In contrast, minimally modified LDL induced primary platelet aggregation and fibrinogen binding in the absence of other agonists, enhanced both submaximal (1-2mumol/l) ADP-induced aggregation, fibrinogen binding and degranulation (CD63 and P-selectin expression). Fully oxidised LDL, however, inhibited ADP-induced platelet aggregation and fibrinogen binding. The effects of minimally modified LDL on platelet aggregation could be reproduced partially by adding 15-hydroperoxy-eicosatetraenoic acid to native LDL. These data indicate that the extent of oxidation of LDL is critical in determining their effects on platelet function. Native LDL did not activate platelets, whilst minimally modified LDL exerted a pro-aggregatory effect, possibly due to the presence of lipid hydroperoxides near to the concentration range found in pathological states.

A new rapid method to measure human platelet cholesterol: a pilot study

INTRODUCTION

Platelet cholesterol (PC) could be used to assess "tissue" cholesterol of patients with vascular disease. However, the methods available so far to measure PC involve a complex extraction process. We developed a rapid method to measure PC and assessed its correlation with serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), LDL-C/HDL-C ratio, triglycerides (TG), and non-HDL-C.

METHODS

We assessed repeatability (20 times, 3 participants) and reproducibility (8 times, 2 participants). A group of 47 healthy participants was studied. Blood was collected to analyze serum TC, LDL-C, HDL-C, and TG. Citrated blood was used to prepare a platelet pellet. A "clear soup" was produced (by disrupting this pellet using freeze-thaw and sonication cycles) and used to measure PC.

RESULTS

Repeatability of PC showed a coefficient of variation (CV) of 4.8%. The reproducibility of PC over a period of 2 months was CV 7.5% and 8.1% (8 measurements for 2 participants). The PC of participants with serum LDL-C >2.6 mmol/L (treatment goal recommended by the National Cholesterol Education Program Adult Treatment Panel III) was 377 ± 120 μmol/10(12) platelets (n = 25). There was a significant correlation (Spearman, correlation coefficient) of PC (n = 25) with serum LDL-C (r(s) = 0.45, P = .02), LDL-C/HDL-C (r(s) = 0.45, P = .02), TG (r(s) = 0.43, P = .03), and non-HDL-C (r(s) = 0.53, P = .007).

CONCLUSION

This technique of measuring PC has the advantage of being reproducible, fast, and simpler than previous methods. Thus, it may be useful for multiple sampling when investigating changes in PC in hypercholesterolemic patients. More extensive evaluation is necessary.

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.

Inhibitory mechanisms of low concentrations of oxidized low-density lipoprotein on platelet aggregation

The intracellular mechanisms underlying oxidized low-density lipoprotein (oxLDL)-signaling pathways in platelets are not yet completely understood. Therefore, the aim of this study was to further examine the effects of oxLDL in prevention of platelet aggregation. In this study, oxLDL concentration-dependently (40-120 microg/ml) inhibited platelet aggregation in human platelet-rich plasma stimulated by agonists. Moreover, oxLDL (40 and 80 microg/ml) markedly decreased the fluorescence intensity of platelet membranes tagged with diphenylhexatriene. Rapid phosphorylation of a protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by PDBu (150 nM). This phosphorylation was markedly inhibited by oxLDL (40 and 80 microg/ml) in phosphorus-32-labeled platelets. In addition, oxLDL (40 and 80 microg/ml) markedly increased levels of cyclic AMP and cyclic AMP-induced vasodilator-stimulated phosphoprotein (VASP) Ser(157) phosphorylation. The thrombin-evoked increase in pHi was inhibited in the presence of oxLDL (40 and 80 microg/ml). These results indicate that the antiplatelet activity of oxLDL may involve the following pathways. (1) oxLDL may initially induce conformational changes in platelet membranes, leading to inhibition of the activation of protein kinase C, followed by inhibition of P47 protein phosphorylation, and intracellular Ca(2+) mobilization. (2) oxLDL also activated formation of cyclic AMP and cyclic AMP-induced VASP Ser(157) phosphorylation, resulting in inhibition of the Na(+)/H(+)exchanger; this leads to reduced intracellular Ca(2+) mobilization, and ultimately to inhibition of platelet aggregation. This study further provides new insights concerning the effects of low concentrations of oxLDL on platelet aggregation.

Low concentration of oxidized low density lipoprotein suppresses platelet reactivity in vitro: an intracellular study

The intracellular mechanisms underlying oxidized low density lipoprotein (oxLDL)-signaling pathways in platelets remain obscure and findings have been controversial. Therefore, we examined the influence of oxLDL in washed human platelets. In this study, oxLDL concentration-dependently (20-100 microg/mL) inhibited platelet aggregation in human platelets stimulated by collagen (1 microg/mL) and arachidonic acid (60 microM), but not by thrombin (0.02 U/mL). The activity of oxLDL was greater at 24 h in inhibiting platelet aggregation than at 12 h. At 24 h, oxLDL concentration-dependently inhibited intracellular Ca2+ mobilization and thromboxane B2 formation in human platelets stimulated by collagen. In addition, at 24 h oxLDL (40 and 80 microg/mL) significantly increased the formation of cyclic AMP, but not cyclic GMP or nitrate. In an ESR study, 24 h-oxLDL (40 microg/mL) markedly reduced the ESR signal intensity of hydroxyl radicals (OH(-)) in both collagen (2 microg/mL)-activated platelets and Fenton reaction (H2O2 + Fe2+). The inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca2+ mobilization, followed by inhibition of thromboxane A2 formation, thereby leading to increased cyclic AMP formation and finally inhibited platelet aggregation. This study provides new insights concerning the effect of oxLDL in platelet aggregation.

Characterization of simple and reproducible vascular stenosis model in hypercholesterolemic hamsters

The importance of low-density lipoprotein (LDL) in the etiology of atherosclerosis is well recognized. We have established a reproducible stenosis model in hypercholesterolemic hamsters, and the process of arterial stenosis by thrombus or neointima was studied and compared with that in normal hamsters. The level of plasma LDL was 4.6 times higher in hamsters fed a high-cholesterol diet than in hamsters fed normal food. Endothelial injury in right common carotid arteries was induced using a modified catheter. Arterial blood flow was monitored continuously using a Doppler flow probe. Arterial patency after the initiation of injury in high-cholesterol hamsters was significantly changed as compared with that of normal hamsters. Neointima was observed 2 wk after the vascular injury. The neointimal area of high-cholesterol hamsters was significantly larger than that of normal hamsters. To characterize the stenosis in hypercholesterolemic hamsters, we measured platelet aggregation, thrombin time, activated partial thromboplastin time, and proliferating smooth muscle cells (SMC) in vitro and in vivo. The half-maximal inhibitory concentration value for platelet aggregation induced by thrombin or collagen, the DNA synthesis stimulated by platelet-derived growth factor (PDGF)-BB, and 5-bromo-2-deoxy-uridine labeling indices (proliferating index of SMC in vivo) in high-cholesterol hamsters were each significantly higher than the comparable value from normal hamsters. However, specific binding of PDGF-BB in SMC was not different between the two types of hamsters. Furthermore, we investigated the inhibitory effects of probucol or losartan on neointima formation using this model. Probucol, but not losartan, significantly reduced the neointimal area in hypercholesterolemic hamsters. These findings indicated that high levels of plasma LDL strongly contributed to the development of thrombus and neointima formation via both up-regulation of platelet aggregation and the enhancement of SMC proliferation. This stenosis model may be useful for the investigation of hypercholesterolemia-associated cardiovascular diseases.

Low-density lipoprotein (LDL) binds to a G-protein coupled receptor in human platelets. Evidence that the proaggregatory effect induced by LDL is modulated by down-regulation of binding sites and desensitization of its mediated signaling

We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein. LDL stimulation induced cytosolic [Ca2+]i mobilization, increase in inositol 1,4,5-triphosphate (IP3) formation and a rapid cytosol-to-membrane translocation of protein kinase C (PKC) enzymatic activity. Pertussis toxin inhibited all the stimulatory effects, whereas cholera toxin had no effect. Using ligand-binding assays, we demonstrated that exposing platelet LDL receptors to high concentrations of LDL (1.5 g/l) caused a rapid down-regulation and desensitization, as shown by the reduction in the Bmax, intracellular [Ca2+]i mobilization and IP3 formation to 65, 73 and 63%, respectively. The inhibitory effects were reversible and dose and time dependent. Furthermore, VLDL (0.2 g/l) and IDL (0.07 g/l) induced similar desensitization effects. However, HDL3 (up to 1.5 g/l), chylomicrons (up to 0.5 g/l) and cyclohexandione-modified LDL (which does not bind to platelets) had no significant effects. Protein kinase C inhibitors (150 nmol/l staurosporine, 100 micromol/l H-7, and 10 nmol/l bisindolylmaleimide) inhibited desensitization to 71%, on average. Sequestration blocking agents (0.30 g/l, concanavalin A) had no significant effect if phosphorylation was operative. However, there was a complete blockade with the concurrent inhibition of both pathways. In contrast, cAMP-dependent protein kinase inhibitors (PKI, 1 micromol/l) or beta2-adrenergic receptor kinase inhibitors (100 nmol/l, heparin), had no effect. Overall results indicate that LDL binds to a pertussis sensitive G-protein coupled receptor and that high levels of lipoproteins down-regulate the number of receptors and desensitize its mediated response by a mechanism that involves PKC-phosphorylation and sequestration of binding sites. This new regulatory mechanism may have implications for the thrombogenicity in hyperlipidemia and for effects of lipid lowering therapy.

Hypochlorite modified LDL are a stronger agonist for platelets than copper oxidized LDL

Experimental low density lipoprotein (LDL) oxidation is usually performed using trace copper, although the in vivo relevance of this method has been called into question. Such LDL augment adenosine 5'-diphosphate (ADP) induced platelet aggregation, presumably by the action of lipid derived compounds. In striking contrast, we find that LDL oxidized to a comparable extent by hypochlorite, an in vivo occurring oxidant, reveal themselves to be potent promoters of platelet aggregation. Interestingly, hypochlorite modified LDL seem to mediate their influence on human platelets by means of the modified apolipoprotein B-100 (apoB) moiety. Also, the finding that hypochlorite modified albumin is able to trigger platelet aggregation suggests an essential role for hypochlorite modified protein(s) in the process of platelet activation.

Molecular requirements in the recognition of low-density lipoproteins (LDL) by specific platelet membrane receptors

We have demonstrated that platelet low-density lipoprotein (LDL) receptors differ from classic LDL receptors of nucleated cells. Although positively charged Arg and Lys residues of apoprotein B-100 are known to play a key role in LDL recognition by classic LDL receptors, there are no conclusive data on platelet LDL receptors. This study investigated the molecular requirements of LDL particle recognition by platelet LDL receptors. The involvement of lipid and protein fractions was determined by displacement studies of the binding of 125I-LDL to platelets and fibroblasts (used as a classical LDL receptor model). The role of the protein moiety was evaluated by chemically modifying positively charged apoB residues (Lys, Arg, and Tyr) via copper-induced oxidation, cyclohexanedione, and tetranitromethane, respectively. The involvement of the lipid fraction was determined by ligand binding assays using 125I-LDL particles that had previously been delipidated and subjected to apoB solubilization. The degree of particle modification was analyzed by agarose/acrylamide gel electrophoresis and anion exchange chromatography. Modifying the amino acid residues increased particle electronegativity in the following order of potency: CHD-LDL>TNM-LDL>ox-LDL>native LDL. The results obtained by displacement studies in fibroblasts suggested that the gain in the LDL negative charge was the most important factor in the loss of receptor affinity. The chemical models of protein modification used in our study greatly affected LDL binding to the classical fibroblast receptor. In contrast, there was very slight difference in the displacement capacity on platelet 125I-LDL binding, which suggests that the protein fraction does not play a major role in the interaction of LDL with its platelet receptor. On the other hand, whereas modifying the lipid moiety did not alter the ability of solubilized 125I-apoB to interact with the classical fibroblast LDL receptor, platelet LDL receptors were unable to recognize these particles. In conclusion, our results confirm that the protein fraction plays a key role in the fibroblast LDL-receptor recognition process, whereas the lipid fraction appears to have a more relevant role in platelet LDL-receptor recognition.

Oxidized low-density lipoprotein (LDL) enhances thromboxane A(2) synthesis by platelets, but lysolecithin as a product of LDL oxidation has an inhibitory effect

Oxidation of low-density lipoprotein (LDL) by copper sulfate led to a significant increase in lysophosphatidylcholine (lyso PC) at the expense of phosphatidylcholine. Incubation of different concentrations of oxidized LDL (oxLDL) (32-650 microg protein/ml) with platelets for 1 h at 37 degrees C increased lyso PC content. The increase was dependent on oxLDL concentration. Incubation of platelets with various concentrations of lyso PC in solution for 5 or 15 min showed that lyso PC percentage was increased in the platelet membrane and the increase was dose dependent. Platelets incubated with various concentrations of lyso PC (2-100 microM) for 5 or 15 min and then triggered with thrombin also showed a significant decrease of thromboxane A(2) (TXA(2)) release as lyso PC concentration reached 10 microM or 6 microM, respectively. The decrease of TXA(2) release was more significant as lyso PC concentration was increased. The present study showed that this inhibition of TXA(2) release by lyso PC was due to 1) inhibition of phospholipase A(2) and the decrease of free arachidonic acid liberation from platelet phospholipid and 2) inhibition of cyclooxygenase. These inhibitory effects of lyso PC were discussed in relation to its effect on membrane fluidity. Lyso PC at concentrations of 30, 50, and 100 microM caused a sudden drop in TXA(2) release and a sudden increase of lactic dehydrogenase loss from the platelets due to their lysis and inhibition of cyclooxygenase enzyme. The present study shows that oxLDL contains high levels of lyso PC that are transferable to the platelets and can weaken their responsiveness to thrombin and decrease TXA(2) release. In our previous study, we found that oxLDL also contained high levels of oxysterols and thiobarbituric acid reactive substances (TBARS), which enhanced platelet reactivity to thrombin and increased TXA(2) release. We conclude that the net effect of oxLDL on platelets will depend on its degree of oxidation and the ratio between oxysterols plus TBARS/lyso PC. Variations in this ratio may explain some of the contradictions cited in the literature concerning the effect of oxLDL on platelet activation.

Mildly oxidized low density lipoprotein rapidly stimulates via activation of the lysophosphatidic acid receptor Src family and Syk tyrosine kinases and Ca2+ influx in human platelets

In contrast to native low density lipoprotein (LDL), mildly oxidized LDL (mox-LDL) induced platelet shape change and stimulated during shape change the tyrosine phosphorylation of specific proteins including Syk; the translocation of Src, Fyn, and Syk to the cytoskeleton; and the increase of cytosolic Ca(2+) due to mainly Ca(2+) entry. The stimulation of these early signal pathways by mox-LDL was inhibited by desensitization of the lysophosphatidic acid (LPA) receptor and specific LPA receptor antagonists, was independent of the alpha(IIb)beta(3)-integrin, and was mimicked by LPA. Stimulation of tyrosine phosphorylation and Syk activation were independent of the increase of cytosolic Ca(2+) and were suppressed by genistein and two specific inhibitors of the Src family tyrosine kinases, PP1 and PD173956. In contrast to PP1 and PD 173956, genistein prevented shape change by mox-LDL. The results indicate that mox-LDL, through activation of the LPA receptor, stimulates two separate early signal pathways, (a) Src family and Syk tyrosine kinases, and (b) Ca(2+) entry. The activation of these early signaling pathways by mox-LDL probably plays a role in platelet responses subsequent to shape change. The inhibition of mox-LDL-induced platelet activation by LPA receptor antagonists or dietary isoflavonoids such as genistein could have implications in the prevention and therapy of cardiovascular diseases.

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.

Human platelets exclusively bind oxidized low density lipoprotein showing no specificity for acetylated low density lipoprotein

The widely studied macrophage scavenger receptor system is known to bind both acetylated low density lipoprotein and oxidized low density lipoprotein. Although only the latter ligand has been shown to occur in vivo, acetylated low density lipoprotein is often used to evaluate the contribution of scavenger receptors to different (patho)physiologic processes, assuming that all existing subtypes of scavenger receptors recognise both lipoproteins. In the present work, we identify human platelets as the first natural cell type to bind oxidized low density lipoprotein without showing specificity for acetylated low density lipoprotein. Consequently, platelets possess exclusive receptor(s) for oxidized low density lipoprotein distinct from the 'classical' scavenger receptor AI/AII. From the data presented in this work, we conclude that the class B scavenger receptor CD36 (GPIV) is responsible for this exclusive oxidized low density lipoprotein binding.

Mechanisms for regulating platelet high density lipoprotein type3 binding sites: evidence that binding sites are downregulated by a protein kinase C-dependent mechanism

The mechanisms for regulating platelet HDL3 binding sites were investigated. HDL3 binding was rapid (T(1/2) association=4 minutes) and completely reversible (T(1/2) dissociation=14.5 minutes) at 4 degrees C, 22 degrees C, and 37 degrees C, and kinetic analysis yielded forward and reverse constants of 7.3x10(-4) x s(-1) and 7.13x10(3) x s(-1) x M(-1), respectively. Nevertheless, neither inhibitors of binding sites recycling or of pinocytosis, such as ammonium chloride, chloroquine, monensin, colchicine, and sodium azide, modified the binding characteristics. Moreover, when platelets were loaded with cholesterol, binding sites were not regulated (up or down). However, when exposed to high concentrations of HDL3 (1.5 g/L), apoE-free HDL (1.5 g/L), HDL2 (0.5 g/L), apoE-rich HDL (0.5 g/L), and VLDL (0.3 g/L) there was rapid downregulation of the number of binding sites in isolated permeabilized platelets, as shown by the reduction of Bmax to 66%, 58%, 45%, 53%, and 51%, respectively. Downregulation was rapid, reversible, and dose and time dependent. In contrast, LDL (up to 2.0 g/L), IDL (up to 0.1 g/L), and chylomicrons (up to 0.5 g/L) had no effect. Protein kinase C inhibitors (150 nmol/L staurosporine, 100 micromol/L H-7, and 10 nmol/L bisindolylmaleimide) inhibited downregulation up to 62% (as average value). The role of the PKC activation in regulating the activity of HDL3 binding sites also was analyzed by determining the cytosol-to-membrane translocation of enzymatic activity. Downregulation mediated by HDL3 rapidly translocated PKC activity (21% +/- 11 of total PKC activity was membrane-associated in control platelets vs. 55+/-8% in downregulated platelets, mean+/-SEM, n=3). However, agents that block sequestration (0.30 g/L, concanavalin A), and other protein kinase inhibitors, such a cAMP-dependent protein kinase inhibitors (1 micromol/L, PKI), and beta2-adrenergic receptor kinase inhibitors (100 nmol/L, heparin) had no effect. The results show that neither endocytotic response nor cholesterol-dependent mechanisms participate in the modulation of platelet HDL3 binding sites. However, a new regulatory mechanism that involves PKC-dependent downregulation of the number of binding sites may be an important pathway to regulate the thrombogenicity of lipoproteins and their effects on platelet reactivity.

Oxysterols and TBARS are among the LDL oxidation products which enhance thromboxane A2 synthesis by platelets

In this study, we compared the effects of normal LDL (nLDL) and oxidized LDL (oxLDL) on thromboxane (TXA2) release by platelets triggered by low concentration of thrombin, and we determined which component of oxLDL is responsible for that activation. After oxidation of LDL with copper sulfate, the small molecular weight fraction (< 10 kDa) which was high in TBARS was removed; using Amicon Centriprep-10 concentrator membrane. More than 67% of TBARS in the oxLDL preparation was found in solution while the remaining was covalently attached to the oxLDL particles. OxLDL contained significantly higher levels of oxysterols and TBARS than the nLDL. Platelets preincubated with low concentrations of oxLDL (33-132 micrograms protein/mL) produced significantly higher TXA2 than platelets preincubated with equivalent concentrations of nLDL when triggered with thrombin. Platelets treated with oxLDL also contained significantly higher levels of oxysterols than platelets treated with nLDL. Platelets preincubated with pure cholestanetriol (10 micrograms/mL) contained a high level of cholestanetriol in the membrane, and TXA2 release was significantly increased in these platelets compared to the control platelets. The TBARS in solution also was very potent in enhancing TXA2 release by thrombin-treated platelets. These results indicate that oxysterols and the free TBARS either in solution or covalently attached to the oxLDL particles are partly responsible for the stimulatory effect of oxLDL on TXA2 release by platelets. The present study also showed that this enhancement of TXA2 release was due to activation of phospholipase A2 and to the increase of arachidonic acid liberation from the platelet phospholipids.

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

Oxidized low density lipoprotein (LDL) has been shown to enhance platelet activation. Since platelet activation is accompanied by an increase in cytosolic calcium, the effects of oxidized LDL on plasma membrane Ca(2+)-ATPase, plasma membrane fluidity and cytoplasmic calcium were studied in human platelets and purified platelet plasma membranes. Our results demonstrate that oxidized LDL, but not native LDL, inhibits the activity of Ca(2+)-ATPase in purified platelet plasma membranes (P < 0.01). Addition of the free radical scavenger alpha-tocopherol had no effect on the ability of oxidized LDL to inhibit the Ca(2+)-ATPase. An increased cytoplasmic calcium level in whole platelets was induced by oxidized LDL (P < 0.01), indicating that the plasma membrane Ca(2+)-extrusion pump may also be inhibited in vivo by oxidized LDL, although other mechanisms for the increase in cytoplasmic calcium are possible. Since no change in membrane fluidity was observed in platelet plasma membranes exposed to oxidized or native LDL as estimated by steady state trimethylammonium diphenylhexatriene (TMA-DPH) anisotropy, oxidized LDL does not affect the Ca(2+)-ATPase by grossly changing the membrane environment. The present results suggest that exposure of platelets to oxidized LDL causes inhibition of the plasma membrane Ca(2+)-ATPase which contributes to the observed increase in cytoplasmic calcium and increased sensitivity to agonists.

Proteolytic susceptibility of platelet low density lipoprotein receptor

In order to further characterize low density lipoprotein (LDL)-platelet interaction, we investigated the effect of protease pretreatment of human platelets on the subsequent binding of iodinated LDL (125I-LDL). Our results showed that the platelet LDL receptor had a proteolytic susceptibility different from that of both classical LDL receptors and the fibrinogen receptor. Platelet pretreatment with chymotrypsin, trypsin, and pronase (at 50 micrograms/mL) had no effect on 125I-LDL binding, whereas fibroblast 125I-LDL binding was markedly reduced. Mild proteolytic digestion, however (up to 1 mg/mL), was helpful in characterizing the platelet LDL receptor. Scatchard analysis showed that chymotrypsin did not modify LDL binding characteristics, whereas trypsin and pronase altered maximal number of binding sites (Bmax) without variation in dissociation constant. Trypsin increased Bmax approximately twofold (2156 +/- 327 binding sites on control platelets vs. 5246 +/- 296 on treated platelets, P < 0.001, mean +/- SEM, n = 5), but pronase decreased Bmax about 50% (2017 +/- 275 control vs. 1153 +/- 195 treated, P < 0.001). A minimum of 30 min preincubation was required to detect significant effects, and apparent equilibrium was reached by 60 min. Maximal increase in platelet LDL binding sites induced by trypsin was observed at a protein concentration of 1 mg/mL at 37 degrees C, whereas at 4 degrees C no effect was found. In contrast, maximal pronase-inhibitory effect also was observed at 37 degrees C but at higher protein concentration (10 mg/mL). Aprotinin, phenylmethylsulfonylfluoride, and soybean trypsin inhibitor were capable of fully blocking both the stimulation and the inhibition of platelet LDL binding induced by trypsin and pronase, respectively. Platelet pretreatment with both chymotrypsin and pronase (0.5 mg/mL) activated fibrinogen binding sites to a similar extent as ADP (100 microM). Furthermore, LDL (at a protein concentration of 0.3 mg/mL) increased by 81 +/- 6% the binding of fibrinogen to both protease- and ADP-stimulated platelets, but was unable to activate fibrinogen binding sites in unstimulated platelets. Overall, the results suggest that platelet LDL receptor presents a different proteolytic susceptibility in comparison with both "classical" LDL receptor and fibrinogen receptor.

Decreased adhesion of oxidized LDL-stimulated platelets caused by cytochalasin D

The adhesion of human blood platelets is studied with an in vitro model using reflection contrast microscopy and an image analysis system. The adhesive feature is promoted by oxidatively modified low density lipoprotein, which also induces functional morphological changes of platelets. However, when washed platelets are pretreated with 0.05 mM cytochalasin D, oxidized low density lipoprotein (100 micrograms/ml) causes a slower increase of the adhesion area (11.6 microns 2/min) compared to untreated platelets (15.7 microns 2/min) or platelets treated by oxidized low density lipoprotein alone (20.5 microns 2/min, P < 0.01). These results are supported by light transmission analysis and by transmission electron microscopy. Our experiments suggest that cytochalasin D inhibits the change of platelets in shape induced by oxidized low density lipoprotein, hinders the adhesion, but does not prevent the adhesion entirely.