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.

The Importance of Oxidation or Glycosylation of Low-density Lipoproteins in Relation to Platelet Activation

Low-density lipoproteins (LDL) are thought to influence directly the sensitivity of platelets, but this may only be the case when the LDL are modified by oxidation. In diabetes, LDL are known to be modified by non-enzymatic glycosylation, especially when the blood glucose concentrations are poorly controlled: platelet activation is also concomitantly increased as is the concentration of plasma lipid peroxides. In this study we found that mild oxidation of LDL in vitro is more potent than strongly oxidised LDL in terms of the activation of platelets. Glycosylation of LDL per se has little effect on the aggregation of isolated platelets.

The influence of organic peroxides on platelet aggregation and sensitivity to nitric oxide

The effects of oxidative stress, induced by water-soluble and lipid peroxides, on platelet reactivity and platelet sensitivity to nitric oxide were investigated. Hydrogen peroxide and cumene hydroperoxide potentiated thrombin-induced platelet aggregation. In contrast, 15(S)-hydroperoxyeicosatetraenoic acid had no such effect, while 12(S)-hydroperoxyeicosatetraenoic acid inhibited platelet reactivity. All of the peroxides tested were found to decrease platelet sensitivity to nitric oxide, although the mechanisms by which the various peroxides altered platelet sensitivity to nitric oxide were different. The water-soluble peroxides opposed the actions of nitric oxide without affecting cyclic GMP levels, while 15(S)-hydroperoxyeicosatetraenoic acid caused a significant reduction in the concentration of cyclic GMP formed in response to NO. The data from this study demonstrate that water-soluble and lipid peroxides both affect platelet reactivity and regulation, but by different mechanisms. Thus, caution should be exercised when selecting peroxides to be used as models of oxidative stress.

Abnormal nitric oxide metabolism in systemic sclerosis: increased levels of nitrated proteins and asymmetric dimethylarginine

OBJECTIVES

Endothelial dysfunction is a primary event in systemic sclerosis; however, the aetiology of events and the role of nitric oxide (NO) is still unclear. The aim of the present study is to investigate whether there are abnormalities in NO metabolism in plasma from patients with primary Raynaud's phenomenon (RP) and in the pathogenesis of systemic sclerosis (SSc): limited SSc (lSSc) and diffuse (dSSc). We also wanted to investigate the effect of factors within patients' SSc serum on NO metabolism in human microvascular endothelial cells (HMECs).

METHODS

Plasma (n=89) or serum (n=80) was assayed for total nitrate and nitrite (NOx), nitration of proteins and the NO inhibitor asymmetric dimethylarginine (ADMA). HMECs were treated with patients' SSc serum and assayed for indicators of NO metabolism.

RESULTS

Plasma NOx was elevated in patients with RP or lSSc (P<0.002), but not in patients with dSSc, compared with controls. Nitrated proteins in plasma, however, were found to be very high in dSSc patients (P<0.03), compared with RP, lSSc or controls. Patients with dSSc also showed increased levels of serum ADMA (P<0.05). The high level of nitrated proteins in dSSc was strongly associated with the severity and duration of dSSc disease. Skin biopsy sections from dSSc patients also showed enhanced nitrotyrosine staining compared with controls. In HMECs, pre-incubation with SSc serum impaired the activity of nitric oxide synthase (NOS) but not the expression of inducible or endothelial NOS. SSc serum also induced a reduction in intracellular cGMP synthesis, and NOx production in the cell culture medium, but was not associated with increased cell cytotoxicity.

CONCLUSIONS

NO formation is increased in patients with primary RP or lSSc, but nitration of proteins and elevated ADMA is a particular feature of dSSc and may reflect abnormal NO regulation and/or contribute to endothelial dysfunction in SSc.

Circulating levels of the chemokines JE and KC in female C3H apolipoprotein-E-deficient and C57BL apolipoprotein-E-deficient mice as potential markers of atherosclerosis development

We have investigated serum chemokines for their suitability as markers of atherosclerosis development in apoE (apolipoprotein E)-deficient (−/−) mice. Female C3H apoE−/− and C57BL apoE−/− mice were fed on either diet W (Western diet; 6 weeks) or normal rodent diet (12 weeks). Serum lipids (0, 6 and 12 weeks) and terminal chemokine levels were measured using commercially available assays, whereas the lesion area was determined using Oil-Red O-stained aortic sections. Serum lipids were higher in C3H apoE−/− mice for both diets throughout the study; however, lesions were significantly larger in C57BL apoE−/− mice fed on either diet. Chemokine levels were significantly lower in C3H apoE−/− mice fed on the normal diet, but no difference was observed between the two groups fed on diet W. We conclude that serum chemokine levels are potential markers for atherosclerosis susceptibility in C3H and C57BL apoE−/− mice fed on a normal rodent diet.

Enhanced relaxation of diabetic rabbit cavernosal smooth muscle in response to nitric oxide: potential relevance to erectile dysfunction

New Zealand white rabbit cavernosal smooth muscle strips (n=6) were mounted in organ baths. Relaxations to nitric oxide (10(-7)-10(-4) mol/l) were measured and the same procedure was repeated on strips from rabbits 6 months after alloxan-induced diabetes (n=6). Transverse cavernosal sections were obtained from the same penises. Low and high resolution autoradiographs were prepared using [(3)H]-L-N(G)-nitroarginine (an index of nitric oxide binding sites) and analysed densitometrically. Histochemical analysis was performed on adjacent sections using NADPH diaphorase (an index of nitric oxide synthase activity). Nitric oxide relaxed control rabbit cavernosal smooth muscle strips in a concentration-dependent manner. Diabetic rabbit cavernosal smooth muscle strips were significantly (P<0.03) more sensitive to nitric oxide (mean IC(50)=3.9 x 10(-6) mol/l). Nitric oxide synthase binding sites were localised to the cavernosal endothelium and smooth muscle. Nitric oxide synthase activity was increased in 6 month diabetic cavernosal smooth muscle. These findings suggest impairments in the L-arginine-nitric oxide pathway may play a role in the pathophysiology of diabetic erectile dysfunction.

Inhibition of tissue factor activity reduces the density of cellular network formation in an in vitro model of angiogenesis

Tissue factor (TF) is a transmembrane glycoprotein that was originally recognized for its ability to initiate the extrinsic pathway of coagulation. More recently, additional functions of TF in cellular signalling have emerged, notably the role of TF in vasculogenesis and angiogenesis. We have described previously the ability of a peptide derived from the apolipoprotein B100 (apoB100) moiety of low-density lipoproteins (KRAD14) to inhibit the procoagulant function of TF. In this study, we demonstrate the ability of the KRAD14 peptide to attenuate the density of cellular network structures of T24 cells grown on specialized matrix (Matrigel). In addition, an alternative inhibitor of TF activity, the TF8 5G9 antibody, also reduces the density of cellular network formation. Targeted use of a stable structural equivalent of the KRAD14 peptide may thus prove useful in the prophylactic treatment of diseases whose pathologies feature the formation of neovascular tissue, e.g. tumour growth and metastasis, rupture of atherosclerotic plaques and retinopathy secondary to diabetes.

Stimulation of progesterone production in human granulosa-lutein cells by lipoproteins: evidence for cholesterol-independent actions of high-density lipoproteins

Low-density lipoproteins (LDL) have been consistently reported to stimulate ovarian steroidogenesis, apparently by the provision of cholesterol as a steroidogenic substrate. Recent studies suggest that high-density lipoproteins (HDL) can also deliver cholesterol to support progesterone synthesis in human granulosa-lutein cells. Therefore, this study investigated the contributions of (i) cholesterol delivery, (ii) cyclic AMP and (iii) protein kinase C (PKC) in the steroidogenic responses of human granulosa-lutein cells to HDL and LDL. Over a 24-h treatment incubation, HDL stimulated a larger increase in progesterone output than did LDL at equivalent cholesterol concentrations. Moreover, at equal protein concentrations (100 microg protein/ml), HDL doubled progesterone production by cells co-treated with a maximally effective concentration of 22R-hydroxycholesterol, whereas LDL had no effect on the progesterone response to this membrane-permeable sterol. These observations indicate that the progesterone response to HDL is not solely due to the delivery of cholesterol as a steroidogenic substrate. Over 24 h, the stimulation of progesterone synthesis by HDL was additive with the response to a maximally effective concentration of dibutyryl-cAMP, but was unaffected by the down-regulation of PKC activity (by chronic pre-treatment with a tumour-promoting phorbol ester). We have concluded that HDL appears to stimulate progesterone production in human granulosa-lutein cells by a mechanism not solely reliant on cholesterol delivery.

The nitration of proteins in platelets

Nitric oxide has many important physiological functions, but it may also form an important oxidant, peroxynitrite, as a consequence of its reaction with superoxide anions. Peroxynitrite is capable of nitrating the aromatic amino acids in proteins, particularly tyrosine. Nitrated proteins are found in tissues of a variety of diseases where inflammation occurs. However, our recent work suggests that more selective nitration of specific proteins may occur during normal physiological processes, such as platelet activation by collagen. It is not yet clear what role this may play in the normal cell biology, but there is potential to be a role in signal transduction mechanisms, possibly by influencing tyrosine phosphorylation or dephosphorylation.

Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: the roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H(2)O(2) was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine(157). This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H(2)O(2) were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H(2)O(2) did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H(2)O(2) at low concentrations. The results suggest that the action on VASP may be one site of action of H(2)O(2) but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

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.

The nitration of platelet cytosolic proteins during agonist-induced activation of platelets

The nitration of protein tyrosine residues by peroxynitrous acid has been associated with pathological conditions. Here it is shown, using a sensitive competitive enzyme-linked immunosorbent assay and immunoblotting for nitrotyrosine, that spontaneous nitration of specific proteins occurs during a physiological process, the activation of platelets by collagen. One of the main proteins nitrated is vasodilator-stimulated phosphoprotein. Endogenous synthesis of nitric oxide and activity of cyclo-oxygenase were required for the nitration of tyrosine. The nitration was mimicked by addition of peroxynitrite to unstimulated platelets, although the level of nitrotyrosine formation was greater and its distribution among the proteins was less specific.

Relaxation of rabbit lower urinary tract smooth muscle by nitric oxide and carbon monoxide: modulation by hydrogen peroxide

Recent studies suggest that the body produces two gaseous messengers, nitric oxide (NO) and carbon monoxide (CO), both of which activate soluble guanylyl cyclase and thus modulate the activity of smooth muscle cells. In the present study, the effects of NO and CO on the smooth muscle of the lower urinary tract were compared. In addition, the modulation of tissue NO- and CO-induced relaxation by hydrogen peroxide was examined. NO, produced endogenously by electrical field stimulation (EFS) or applied exogenously as a solution, induced a concentration-dependent relaxation of rabbit cavernosal and urethral smooth muscle strips, but not of bladder tissues. The cavernosal tissue was found to be three times more sensitive to the actions of NO than the urethra. CO also induced relaxation of both tissue types, but with no apparent difference in sensitivity between the tissues. However, CO was much less potent than NO with respect to smooth muscle relaxation. The mechanism of action of the two mediators was cyclic guanosine monophosphate (cGMP)-dependent, as evidenced by enhanced formation of cGMP and inhibition of relaxation by the guanylyl cyclase inhibitor, oxadiazoloquinoxaline-1-one (ODQ.) The data suggests that NO is the dominant messenger in these tissues, but does not exclude a role for CO. In the presence of hydrogen peroxide, the relaxation responses induced by both NO and CO were significantly increased, regardless of tissue type. The mechanism for this effect is unclear, but evidence points to a requirement for the activation of guanylyl cyclase and enhanced formation of cGMP, since potentiation by the peroxide was blocked by a specific guanylyl cyclase inhibitor. We suggest that H(2)O(2) may play a positive role in the amplification or NO and CO-mediated responses.

The role of the C-terminal domain in the inhibitory functions of tissue factor pathway inhibitor

Tissue factor pathway inhibitor (TFPI) inhibits the activity of coagulation factors VIIa and Xa through Kunitz domains, thereby inhibiting the activity of tissue factor. However, it has been shown that the C-terminal of this inhibitor is essential for the maximal anticoagulant activity of TFPI. We have investigated the endogenous ability of the C-terminal of TFPI to influence coagulation. A synthetic peptide corresponding to residues 254-265 within the C-terminal of TFPI was prepared and shown to be capable of inhibiting tissue factor pathway by preventing the activation of factor VII. Mutational analysis of the peptide revealed the identity of the key lysine residues.

Moderate supplementation with natural alpha-tocopherol decreases platelet aggregation and low-density lipoprotein oxidation

Previous studies have shown that oral administration of 300 mg alpha-tocopherol/day to healthy volunteers decreases platelet function and enhances their sensitivity to the platelet inhibitor, prostaglandin E(1), when full dose-response curves to a range of agonist concentrations are made. In this study, the effects of oral doses of natural alpha-tocopherol (75, 200 and 400 IU/day) were studied in order to determine whether the same effects might be achieved with lower intakes of vitamin E and whether inhibition is related to the platelet levels of the antioxidant in platelet membranes. Twenty two subjects undertook the supplementation regime, divided into three units of 2 weeks, each cycling through each of the dosages. The results show that uptake of vitamin E by the platelets was optimal at 75 IU/day, correlating with the maximal influence on platelet aggregation and platelet responsiveness to inhibition by PGE1, increased supplemental levels exerting no greater effects.

Alterations in the nitric oxide synthase binding sites and non-adrenergic, non-cholinergic mediated smooth muscle relaxation in the diabetic rabbit bladder outlet: possible relevance to the pathogenesis of diabetic cystopathy

PURPOSE

To investigate the effect of diabetes mellitus (DM) on the density and distribution of nitric oxide synthase (NOS) and the smooth muscle responses to non-adrenergic, non-cholinergic (NANC) nerve stimulation and exogenous nitric oxide (NO) in the rabbit lower urinary tract.

MATERIALS AND METHODS

Transverse sections of detrusor, bladder neck and urethra, from control and six months alloxan-induced DM New Zealand White rabbits were incubated with a radioligand for NOS ([3H]-L-N(G)-nitroarginine). Densitometric analysis was performed on the autoradiographs. NADPH diaphorase histochemistry was also used as a marker for NOS activity. Responses to NANC nerve stimulation (5 to 20 Hz) and to NO (10(-6) to 3x10(-4) M.) on smooth muscle strips from detrusor, bladder neck and urethra were measured in organ baths.

RESULTS

NOS binding sites were significantly (p<0.03) more dense in the bladder neck than in the detrusor in both DM and control groups. In DM bladder neck, NOS binding sites were significantly (p<0.04) increased compared with the controls. NADPH diaphorase activity appeared markedly increased in the detrusor, bladder neck and urethra of DM animals compared with controls. The mean IC50 for exogenous NO in control versus DM were not statistically different in the bladder neck (1.03x10(-4) M versus 9.8x10(-5) M) and urethra (8.1x10(-5) M versus 8.8x10(-5) M), but the relaxations to 5x10(-6) M of NO were significantly impaired (p<0.04) in the DM urethral smooth muscle. NANC nerve-mediated relaxations were significantly impaired (p<0.001) in the DM urethral smooth muscle.

CONCLUSIONS

Alterations of both the NOS binding sites and functional responses to NANC nerve stimulation suggest that NO may have a pathophysiological role in the urinary bladder dysfunction associated with DM.

Comparison of the inhibitory effects of ApoB100 and tissue factor pathway inhibitor on tissue factor and the influence of lipoprotein oxidation

The procoagulant activity of tissue factor is regulated by circulating inhibitors such as tissue factor pathway inhibitor (TFPI) and LDL. These 2 inhibitors also readily associate making the distinction between their activities difficult. We have examined the relative contributions of intact and C-terminal truncated TFPI and ApoB100. By following the inhibitory potential of the preparations, over a period of 120 minutes, it was demonstrated that TFPI and LDL-resembling particles inhibited tissue factor at different rates. TFPI was found to be a short, fast-acting inhibitor, whereas the action of LDL-resembling particles was more prolonged but slower. The oxidation of LDL has been closely associated with the development of cardiovascular disease, including atherosclerosis and thrombosis. Positively charged amino acids, particularly lysine residues, are prone to alterations via the formation of adducts by lipid peroxidation products. These residues are important in the inhibition of tissue factor activity by ApoB100. They also play an important role in the inhibitory Kunitz domains of TFPI. We have shown that the decline in the ability of LDL to inhibit tissue factor was as a result of modifications in LDL arising from oxidation. By examining the effects of oxidation on full-length and C-terminal truncated TFPI bound to LDL-resembling particles, we found that TFPI is only affected when in close association with ApoB100. C-terminal truncated TFPI was not affected significantly by oxidation. Finally, chemical modification of lysine and arginine residues reduced the overall inhibition of tissue factor by TFPI. We propose that TFPI and LDL act separately to inhibit tissue factor in vivo. However, the oxidation of LDL can alter both the endogenous activity of ApoB100 and reduce that of closely associated TFPI, compromising normal hemostasis.

Probucol improves symptoms and reduces lipoprotein oxidation susceptibility in patients with Raynaud's phenomenon

OBJECTIVE

Reactive oxygen species have been implicated in the pathogenesis of inflammatory and vascular disease. We have undertaken a controlled trial to evaluate probucol, a synthetic antioxidant, as a potential therapy for Raynaud's phenomenon.

METHODS

The study cohort included patients with systemic sclerosis (SSc; n = 20), primary Raynaud's phenomenon (n = 15) or 'autoimmune Raynaud's' (n = 5). Patients were allocated to receive either probucol (500 mg daily) or nifedipine (20 mg daily) for 12 weeks. Clinical and biochemical variables at baseline were compared with those at completion of treatment. Evaluation included assessment of Raynaud's attack frequency and severity by visual analogue scale, measurement of low-density lipoprotein (LDL) oxidation lag time, and plasma concentrations of cholesterol, triglyceride, vitamin E and vitamin C.

RESULTS

There was a significant reduction of both the frequency and severity of Raynaud's attacks in the patients who received probucol, but not in the control group. LDL oxidation lag time, reflecting in vitro susceptibility to oxidation, was also increased by probucol therapy and serum cholesterol levels were significantly reduced. Similar changes were observed in both SSc- and non-SSc-associated Raynaud's cases.

CONCLUSION

These data suggest that probucol may be useful for the symptomatic treatment of Raynaud's phenomenon and also reduces LDL oxidation susceptibility. Since oxidized lipoproteins may mediate vascular damage in SSc, the use of probucol could have additional disease-modifying benefits. Based upon the results of this pilot study, further evaluation of this novel form of therapy is warranted.

Identification of a domain in apolipoprotein B-100 that inhibits the procoagulant activity of tissue factor

The ability of low-density lipoprotein (LDL) to inhibit the procoagulant activity of tissue factor is mediated by a direct protein-protein interaction involving apolipoprotein (apo) B-100. A lysine-rich sequence within apo B-100 (residues 3121-3217), which we have termed lysine-rich apo B-100-derived (KRAD)-98 peptide, may be responsible for its activity. Within this region, residues 3147-3160 (KRAD-14) contain an exceptionally high proportion of positive amino acids. Both recombinant KRAD-98 and KRAD-14 peptides inhibited the procoagulant activity of tissue factor by preventing the activation of factor VII. KRAD-14 also inhibited the prothrombinase components, factors Xa and V. In comparison with the parent protein (apo B-100), KRAD-14 peptide displayed a 20-fold enhancement in the rate of inhibition, whereas KRAD-98 peptide exhibited a rate closer to that of apo B-100. Mutational analysis of KRAD-14 peptide revealed three adjacent amino acids, alteration of which greatly reduced the inhibitory potential of this peptide. A peptide derived from tissue factor (residues 58-66) was found to act co-operatively with tissue factor itself, but also augmented the inhibition of tissue-factor activity by apo B-100. In conclusion, LDL may be a physiological regulator of haemostatic mechanisms through the interactions of lysine-rich domains of apo B-100 with tissue factor.

Modification of tissue factor by peroxynitrite influences its procoagulant activity

Peroxynitrite, a reactive oxidising species resulting from a reaction between nitric oxide and the superoxide anion, modifies proteins by nitration of certain amino acids such as tyrosine. Tissue factor (TF), a transmembrane protein, is expressed on cells under inflammatory conditions and initiates the coagulation cascade. The extracellular domain of TF is rich in tyrosine. Exposure of recombinant TF and cellular TF to peroxynitrite was associated with a reduction in procoagulant activity. This was accompanied by an elevated level of nitrotyrosine residues. Peroxynitrite may have a protective role by attenuation of the thrombotic properties of TF.

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.

Modulatory role of lipoprotein oxidation on platelet-vessel wall interactions

Oxidation of plasma lipoproteins may play an important role in atherogenesis, but there are also indications that that this process of oxidation may influence other facets of cardiovascular disease, namely blood flow and thrombosis. The evidence that oxidation of low-density lipoproteins may modulate the action of the native lipoproteins with respect to endothelium-dependent relaxation, platelet activation and tissue factor activity is reviewed.

Physiology and biochemistry of endothelial function in children with chronic renal failure

Premature atherosclerosis is a major cause of morbidity and mortality in chronic renal failure (CRF). Endothelial dysfunction is a key early event in atherogenesis. The aim of this study was to assess the effect of CRF on endothelial function using physiological and biochemical measures. To focus on the effect of CRF itself, 23 children (matched with 23 controls for age and vessel diameter) were selected because they were normotensive, had normal total cholesterol (TC) levels, and were not on vasoactive drugs. Their mean (range) age was 12.0 (7.8 to 17.0) years; GFR 17.5 (8.8 to 34.5) ml/min/1.73 m2. The physiology of endothelial function in the brachial artery was assessed using high resolution ultrasound by measuring its diameter at rest, during reactive hyperemia (endothelium dependent dilation) and after sublingual glyceryl trinitrate (GTN; endothelium independent dilation). Nitric oxide (NO) metabolites and endogenous NO synthetase (eNOS) inhibitors were measured as an assessment of endothelial metabolism. Brachial artery dilation to flow [FMD, mean (SEM)%] was reduced in CRF to 4.9 (0.6) and controls 8.6 (0.6), P < 0.0001. In contrast, the response to GTN was similar in both groups: CRF 25.1 (1.6), controls 23.3 (1.2), P = 0.31. There was no difference in TC, low density lipoprotein (LDL) or high density lipoprotein (HDL) between the patients and the controls. Triglycerides (TG) were higher in the patients but within the normal range. Antibodies against oxidized LDL (ox-LDL) were high in CRF. Endogenous NOS inhibitors were high in CRF, and intermediate NO metabolites were low. There was no correlation between FMD of the brachial artery and lipid subfractions, or with NO metabolites or eNOS inhibitors. Endothelium dependent dilation of the brachial artery is impaired in children with CRF who do not have co-existing risk factors for atherosclerosis. This may represent early evidence of atherogenic vascular disease.

Effect of fresh fruit consumption on lung function and wheeze in children

BACKGROUND

Fresh fruit consumption and vitamin C intake have been associated with improved lung function in adults. Whether this is due to enhancement of lung growth, to a reduction in lung function decline, or to protection against bronchospasm is unclear.

METHODS

In a cross-sectional school based survey of 2650 children aged 8-11 from 10 towns in England and Wales the main outcome measure was forced expiratory volume in one second (FEV1) standardised for body size and sex. Exposure was assessed by a food frequency questionnaire to parents and by measurement of plasma levels of vitamin C in a subsample of 278 children.

RESULTS

FEV1 was positively associated with frequency of fresh fruit consumption. After adjustment for possible confounding variables including social class and passive smoking, those who never ate any fresh fruit had an estimated FEV1 some 79 ml (4.3%) lower than those who ate these items more than once a day (95% CI 22 to 136 ml). The association between FEV1 and fruit consumption was stronger in subjects with wheeze than in non-wheezers (p = 0.020 for difference in trend), though wheeze itself was not related to fresh fruit consumption. Frequency of consumption of salads and of green vegetables were both associated with FEV1 but the relationships were weaker than for fresh fruit. Plasma vitamin C levels were unrelated to FEV1 (r = -0.01, p = 0.92) or to wheeze and were only weakly related to fresh fruit consumption (r = 0.13, p = 0.055).

CONCLUSIONS

Fresh fruit consumption appears to have a beneficial effect on lung function in children. Further work is needed to confirm whether the effect is restricted to subjects who wheeze and to identify the specific nutrient involved.

Alterations in the structure of apolipoprotein B-100 determine the behaviour of LDL towards thromboplastin

Apolipoprotein B-100 acts as an inhibitor of thromboplastin activity independently of the tissue factor pathway inhibitor (TFPI) associated with plasma lipoproteins. Analysis of the primary structure of Apo B-100 showed a higher than expected occurrence of lysine groups in the receptor-binding region. In order to demonstrate the participation of lysine groups of Apo B-100 in the inhibition of thromboplastin, thromboplastin and Apo B-100 were incubated together in the presence of poly-L-lysine, poly-L-arginine, lysine and arginine monomers. The inhibition of thromboplastin by Apo B-100 was completely suppressed in the presence of poly-L-lysine. Poly-L-arginine was found to be less effective and neither lysine or arginine monomers had any significant effect on the inhibitory effect of Apo B-100. Alterations in the structure of Apo B-100 reconstituted in lipid vesicles resembling LDL, brought about by lipid peroxidation and lipid loading were examined by means of Fourier transform infra-red spectroscopy. It was found that, upon oxidation without the addition of cupric ions, the apolipoprotein attains a more exposed conformation with an increase in alpha-helical structure. This increase occurred at the expense of beta-structure. On lipid loading, an increase in beta-structure at the expense of the alpha-helix, was demonstrated. It is therefore proposed that the variable action of LDL towards thromboplastin derives from alterations in the secondary structure of the Apo B-100, particularly the receptor-binding region.

Modulation of vascular tone by low density lipoproteins: effects on L-arginine transport and nitric oxide synthesis

Low density lipoprotein (LDL) plays an important role in atherogenesis. Focal accumulation within the arterial intima of excess amounts of cholesterol-rich LDL leads to the migration and recruitment of monocytes, which then differentiate into macrophages after taking up large amounts of oxidatively modified LDL via their scavenger receptors and become lipid-laden 'foam cells' within the subendothelial space. It is generally accepted that oxidized LDL and hyperlipidaemia impair endothelial-dependent vascular relaxation, yet the existing literature on the effects of oxidatively modified LDL on endothelium-derived nitric oxide (NO) and prostacyclin (PGI2) release is inconclusive, since oxidized LDL has been reported to enhance or reduce NO and PGI2 production. Our studies using cultured human endothelial and smooth muscle cells have established that basal rates of L-arginine (NO precursor) transport, NO and PGI2 production and soluble guanylyl cyclase activity are unaffected by pretreatment (for 1 or 24 h) with native LDL, or with mildly or highly oxidized LDL. In contrast, highly oxidized LDL inhibited histamine-stimulated release of NO and PGI2 from human endothelial cells and induced an adaptive increase in the level of intracellular glutathione in human smooth muscle cells, a response which was prevented by the chain-breaking antioxidant alpha-tocopherol. Although initial rates of L-arginine transport and basal NO and PGI2 release from human endothelium are unaffected by oxidized LDL, agonist-stimulated release of these vasodilators is markedly attenuated. Elucidation of the mechanisms regulating these responses and their sensitivity to dietary antioxidants could lead to alternative strategies for reducing atherogenesis.

The influence of antioxidant nutrients on platelet function in healthy volunteers

There is mounting evidence that antioxidants may help to prevent coronary heart disease and modulate some thrombotic events such a platelet adhesion. However, the effects of antioxidant supplementation on platelet function in vivo are controversial. A double-blind, randomised, placebo-controlled study was performed on 40 healthy volunteers (20-50 years) supplemented daily with vitamin E (300 mg), vitamin C (250 mg) or beta-carotene (15 mg) for 8 weeks. Platelet function was assessed by platelet aggregation induced by ADP, arachidonic acid or collagen, platelet responsiveness to the inhibitor PGE1, beta-thromboglobulin release and ATP secretion. Supplementation with vitamin E resulted in a significant increase in platelet alpha-tocopherol level (+68%) reflecting closely the increase in plasma alpha-tocopherol level (+69%). Platelet function was significantly decreased by vitamin E as revealed by the decreased platelet aggregation in response to ADP and arachidonic acid, the increased sensitivity to inhibition by PGE1, the decreased plasma beta-thromboglobulin concentration and the decreased ATP secretion. Supplementation with vitamin C did not affect platelet function significantly although a trend towards a decreased platelet aggregability and an increased sensitivity to the inhibitor PGE1 were observed. No significant changes in platelet function occurred after supplementation with beta-carotene. In conclusion, supplementation of healthy volunteers with vitamin E decreased platelet function whereas supplementation with vitamin C or beta-carotene had no significant effects.

Time-course studies by synchrotron X-ray solution scattering of the structure of human low-density lipoprotein during Cu(2+)-induced oxidation in relation to changes in lipid composition

Low-density lipoproteins (LDLs) in plasma are constructed from a single molecule of apolipoprotein B-100 (apoB) (M(r) 512,000) in association with lipid [approximate M(r) (2-3) x 10(6)]. LDL oxidation is an important process in the development of atherosclerosis, and can be imitated by the addition of Cu2+ ions. Synchrotron X-ray scattering of LDL yields curves without radiation damage effects at concentrations close to physiological. The radius of gyration RG for preparations of LDL from different donors ranged between 12.1 and 16.0 nm, with a mean of 13.9 nm. At 4 degrees C, the distance distribution curve P(r) indicated a maximum dimension of 25-27 nm for LDL, a peak at 19.5 nm which corresponds to a surface shell of protein and phospholipid head groups in LDL, and submaxima between 1.7 and 13.5 nm, which correspond to an ordered lipid core in LDL. LDL from different donors exhibited distinct P(r) curves. For oxidation studies of LDL by X-rays, data are best obtained at 4 degrees C at a concentration of > or = 2 mg of LDL protein/ml together with controls based on non-oxidized LDL. LDL oxidation (2 mg of apoB/ml) was studied at 37 degrees C in the presence of 6.4, 25.6 and 51.2 mu of Cu2+/g of apoB. Large changes in P(r) were reproducibly observed in the inter-particle distance range between 13 and 16 nm shortly after initiation of oxidation. This corresponds to the phospholipid hydrocarbon in LDL, which has either increased in electron density during oxidation or become increasingly disordered. After 25 h, the structural changes subsequently spread to regions of the P(r) curves assigned to surface apoB and the central core of cholesteryl esters and triacyl-glycerols. Lipid analyses were carried out under the same solution conditions. The alpha-tocopherol and beta-carotene antioxidant contents of LDL were consumed within 1-2 h. Analyses of the formation of thiobarbituric acid-reactive substances and lipid hydroperoxides indicated that arachidonic acid was preferentially oxidized before the maximal formation of lipid hydroperoxides at 8-12 h after initiation of oxidation. High-performance TLC showed that phosphatidylcholine was continuously converted into lysophosphatidylcholine during oxidation, which is consistent with the early changes in the X-ray P(r) curves. The neutral core lipids became modified only after 12-15 h of oxidation. The combination of X-ray scattering structural analyses with biochemical analyses shows that the oxidation of LDL first affects the outer shell of surface phospholipid, then it spreads towards damage of apoB and the internal neutral lipid core of LDL.

Oxidized low-density lipoproteins inhibit endothelium-dependent relaxations in isolated large and small rabbit coronary arteries

1. Previous studies suggest that oxidatively modified low-density lipoproteins (oxLDL) contribute to the impairment of endothelium-dependent vasodilation in the large arteries of hypercholesterolaemic animals, whereas this may not be the case with regard to the impairment of coronary resistance vessels. For this reason, the effect of lipoproteins on coronary resistance arteries has been examined in this study. 2. The influence of lipoproteins on endothelium-dependent relaxation induced by acetylcholine (ACh) or sodium nitroprusside in PGF2 alpha-preconstricted rings from the large (1st order branch) and small coronary arteries (3rd order branch) and the aorta of New Zealand White rabbits, was investigated. 3. The sensitivity to ACh was greater in the large compared with the small diameter coronary arteries. 4. Endothelium-dependent relaxations were unaffected by native LDL. Oxidized LDL (0.5 and 1 mg protein mL-1) caused a reversible inhibition of relaxations in both preconstricted small and large coronary arteries which was overcome at high ACh concentrations. Similar inhibitions were found in the aorta. 5. Endothelium-independent relaxations elicited by sodium nitroprusside in the large and small coronary arteries were unaffected by the oxidized lipoproteins, indicating that soluble guanylate cyclase activity was unaltered. 6. It is concluded that inhibition of endothelium-dependent relaxation in the small diameter coronary arteries in hypercholesterolaemia may arise from products of oxidative modification of LDL present in the artery itself or released upstream from proximal lesions.

Practical approaches to low density lipoprotein oxidation: whys, wherefores and pitfalls

The purpose of this review is to bring together the different approaches for studying the oxidation of low density lipoproteins and try to identify some critical factors which will permit greater comparability between laboratories. These issues are discussed both in terms of the variety of exogenous mediators of oxidation applied (transition metal ions, haem proteins, azo initiators, peroxynitrite, cells etc.) and their raisons d'être, as well as the methodologies (formation of conjugated dienes, hydroperoxides, decomposition products of lipid peroxidation, altered surface charge, macrophage uptake) applicable to the different stages of the oxidation and the factors underlying their accurate execution and interpretation.

The synergism of hydrogen peroxide with plasma S-nitrosothiols in the inhibition of platelet activation

Earlier studies have shown that inhibition of aggregation of washed platelets (WP) by NO was enhanced almost 100-fold by H2O2. In the present study, the interactions of H2O2 with nitrosothiols, the influence of the presence of plasma and the mechanism of the synergism were investigated. H2O2 strongly enhanced the inhibitory effects of S-nitrosoglutathione (GSNO) on thrombin-induced aggregation of WP. S-Nitrosoalbumin also inhibited platelets, and this was similarly enhanced by H2O2. The synergism with H2O2 was demonstrable for both exogenous GSNO and NO in the presence of plasma when platelets were stimulated with collagen. The inhibition of platelets by GSNO and H2O2 was completely inhibited by guanylate cyclase inhibitors. Synergism was also observed whether the H2O2 was added simultaneously or 1 min before or after the GSNO (or NO). This suggests that the action of H2O2 follows the occupation by NO of haem sites in guanylate cyclase and that a prior reaction between NO and H2O2 was not required. In the absence of exogenous GSNO or NO, H2O2 inhibited activation of platelets in plasma, an effect abolished by guanylate cyclase inhibitors. This suggested that endogenous NO donors in plasma or NO synthesized in platelets may interact with H2O2. Addition of NG-nitro-L-arginine methyl ester (hydrochloride) (L-NAME) decreased the effects of the H2O2 by 25%, indicating that the major endogenous source of NO in platelet-rich plasma was not derived from platelet synthesis of NO but from NO donors in plasma, such as nitrosothiols. Inhibition by H2O2 was also enhanced by beta-mercaptosuccinate, a glutathione peroxidase inhibitor that protects the H2O2. These results suggest a potent synergism of H2O2 with endogenous plasma nitrosothiols that inhibit platelet function through an intracellular mechanism involving guanylate cyclase.

The mechanism of inhibition of factor III (thromboplastin) activity by apolipoprotein B-100. Protein-protein interactions

Factor III (thromboplastin) activity is inhibited by apoB-100, but the mechanism of inhibition is unknown. By examining the effect of purified apoB-100 on factor III activity, we showed that apoB-100 can inhibit factor III via a different mechanism from that caused by the issue-factor pathway-inhibitor, which is mainly carried on the surface of lipoproteins. Although the presence of calcium ions and factors X and VII may enhance the rate of inhibition, they are not a prerequisite for the inhibition of factor III by apoB-100. In addition, by investigating the changes in the UV spectra of apoB-100 on interaction with factor III and factors X and VII and by assigning the shifts in absorption spectra to particular amino acids, we showed that these interactions involve negative and positive residues within these proteins. By following the rates of interactions between apoB-100 and either factors III, X, VII, a two-step mechanism for the inhibition process involving factors X and VII was postulated. In this mechanism, the primary interaction of apoB-100 with factor III is followed by a rate-limiting step that can be accelerated by the presence of either factor X or VII and leads to the inhibition of factor III. Furthermore, a computer-based analysis of the sequences of factor III revealed a possible binding site for apoB-100.

Antioxidants, lipoprotein oxidation, and arterial function

Lipoproteins have an important influence on arterial tissues. High levels of low density lipoproteins (LDL) may inhibit arterial function in terms of the release of nitric oxide from the endothelium. This inhibition is enhanced by the oxidation of these lipoproteins which may occur during the development of atherosclerosis. Many of these effects are due to lipid oxidation products. The impairment of the release of nitric oxide and vasodilatation may be partially attenuated by antioxidants. LDL may also enhance the activity of blood platelets, especially when they are mildly oxidized. Oral administration of antioxidants may decrease the activation of platelets to a limited extent. Lipid peroxides may play a role in these processes.

Levels of antioxidant nutrients in plasma and low density lipoproteins: a human volunteer supplementation study

A human supplementation study was undertaken in order to investigate the correlation between the intake of individual daily dosages of vitamin E (300 mg), vitamin C (250 mg), or beta-carotene (15 mg) of eight week duration and their uptake in vivo in plasma and LDL. The effects of a combined supplement of vitamin E, vitamin C and beta-carotene (Redoxon protector-75 mg, 150 mg, 15 mg respectively) were also investigated. The results show that on supplementation with the individual antioxidants the increases in plasma alpha-tocopherol:cholesterol levels lie in the 1.5-2 fold range and the beta-carotene:cholesterol ratios give a mean 3.5 fold enhancement. The combined supplement containing the same level of beta-carotene as the single dosage achieved comparative levels of uptake in plasma. The level of plasma vitamin C appears to be maximal at about 100 microM regardless of the pre-supplementation level.

Oxidized lipoproteins

The understanding of the role of lipoprotein oxidation is still incomplete. Much is still to be learned about the mechanism of action of oxidized lipoproteins on different types of cell, as well as the origin of the oxidation process, and how it links to the situation in vivo. The benefits or otherwise of anti-oxidant therapy or dietary advice will be solved only by long-term studies, some of which have been begun or are planned. It is useful to recall that information about the effects of cholesterol-lowering therapy with statins are only now becoming available after several years of use. Convincing data about the efficacy of anti-oxidants are some way off, even though, on balance, the results so far appear encouraging.