Effect of Lipoproteins and Platelets on Macrophage Cholesterol Metabolism

Fosinopril reduces ADP-induced platelet aggregation in hypertensive patients

Platelets are intimately involved in atherosclerosis, and hypertension is a known risk factor for coronary artery disease. The angiotensin-converting enzyme (ACE) inhibitors were demonstrated to reduce hypertension and attenuate atherosclerosis. Because increased platelet aggregation was shown in hypertensive patients, the effect of a new ACE inhibitor, fosinopril, on platelet aggregation was studied. Fosinopril therapy (10 mg/day for 4 weeks) in 18 male hypertensive patients showed > or = 31% reduction in ADP-induced platelet aggregation. In vitro studies showed that fosinopril had similar inhibitory effect on ADP-induced platelet aggregation. No inhibitory effect could be detected with collagen as the aggregating agent. Finally, inhibition of platelet aggregation by fosinopril was less effective in platelets derived from hypertensive patients as compared with platelets derived from normal subjects. We conclude that fosinopril possesses a significant inhibitory activity on ADP-induced platelet aggregation both in vitro and in vivo.

Angiotensin II-modified LDL is taken up by macrophages via the scavenger receptor, leading to cellular cholesterol accumulation

The incidence of myocardial infarction is significantly higher in hypertensive patients with increased plasma concentration of angiotensin (Ang) II. Ang II was shown to bind to LDL in vitro, and in the present study we showed its binding to LDL in vivo. Ang II (10(-7) mol/L) was incubated with LDL for 3 hours at 37 degrees C, followed by reseparation of the modified lipoprotein (Ang II-LDL) and its incubation with J-774 A.1 macrophages. Binding of Ang II to LDL significantly increased the lipoprotein protein degradation (by 25%) and its cell association (by 75%) compared with nontreated LDL. Unlike Ang II-LDL, both Ang I-LDL and Ang III-LDL were taken up by macrophages similar to native LDL. The lipid composition and size of Ang II-LDL were similar to those of native LDL, and it was not aggregated. Ang II-LDL was not oxidized, as the contents of malondialdehyde and peroxides were not different from those found in native LDL. On heparin-Sepharose column chromatography, Ang II-LDL was eluted in the void volume, like acetylated LDL (Ac-LDL) and unlike native LDL, which binds to heparin. The cellular degradation of Ang II-125I-labeled LDL by J-774 A.1 macrophages of Ang II-125I-labeled LDL by J-774 A.1 macrophages was studied in the presence of a 50-fold excess of nonlabeled native LDL, Ang II-LDL, Ac-LDL, or oxidized LDL (Ox-LDL). Whereas native LDL had no effect on the degradation of Ang II-125I-LDL by the macrophages, Ac-LDL, Ox-LDL, and Ang II-LDL reduced the cellular uptake of the lipoprotein by 77%, 82%, and 87%, respectively. Similarly, fucoidin but not free Ang II reduced macrophage degradation of the labeled Ang II-LDL. We conclude that Ang II can modify LDL to a form that is not oxidized or aggregated but is still taken up at an enhanced rate by macrophages via the scavenger 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.

Functional morphological alterations of human blood platelets induced by oxidized low density lipoprotein

The effect of oxidized low density lipoprotein (LDL) on the functional morphology of human platelets in vitro was studied by means of transmission electron microscopy. The washed platelets, stimulated by oxidized LDL (50-300 micrograms protein/ml), showed disc-sphere transformation, centralization of granules and complete degranulation in a dose- and time-dependent manner. A cytodamage in platelet membrane was induced by oxidized LDL leading to a lower electron density of cytoplasm compared to control. The morphological observations were supported by an analysis of the platelet shape-change parameter. Since the shape change, induced by oxidized LDL (50 micrograms/ml), was inhibited by a preincubation of platelets with staurosporine (10 nM), the protein kinase C was probably involved in the platelet activation initiated by oxidized LDL. The present results suggest that oxidized LDL could contribute to pathological thrombosis and atherogenesis by activating platelets.

Involvement of the macrophage low density lipoprotein receptor-binding domains in the uptake of oxidized low density lipoprotein

Macrophages, unlike most other cells, possess both low density lipoprotein (LDL) and scavenger receptors. The scavenger receptor has been shown to mediate the uptake of oxidized LDL (ox-LDL), which ultimately leads to cholesterol loading of the macrophages. The present study was undertaken to define epitopes on ox-LDL that are important for lipoprotein binding to macrophages and to ascertain whether ox-LDL can bind to the LDL receptor. Monoclonal antibodies (Mabs) directed against several epitopes along the apolipoprotein B-100 (apo B-100) molecule were used. LDL (300 micrograms/ml) was oxidized by incubation with 10 microM CuSO4 for 24 hours. Ox-LDL, as opposed to acetylated LDL (ac-LDL), reacted with Mabs directed against the LDL receptor-binding domains (Mabs B1B6 and B1B3). Similarly, uptake of ox-LDL but not ac-LDL by a murine J774 macrophage-like cell line was inhibited by as much as 40% after using Mab B1B6. The anti-LDL receptor antibody IgG-C7 also inhibited 125I-ox-LDL uptake by macrophages by 60%. Chromatography on heparin-Sepharose columns of LDL that was partially oxidized for only 3 hours resulted in two fractions: an unbound fraction with characteristics similar to those of ox-LDL and a bound fraction similar to native LDL. Macrophage degradation of the unbound fraction was inhibited by Mab IgG-C7 and Mab B1B6, which are directed toward the LDL receptor and the LDL receptor-binding domains on apo B-100, respectively. When incubated with three types of macrophages, J774 macrophage cells, mouse peritoneal macrophages, and human monocyte-derived macrophages, excess amounts of unlabeled ox-LDL, like native LDL but unlike ac-LDL, substantially suppressed the uptake and degradation of 125I-labeled LDL. Similar studies with fibroblasts, however, revealed that unlabeled LDL but not unlabeled ox-LDL or ac-LDL competed with 125I-LDL for cellular uptake and degradation. Mab directed against epitopes on the amino terminus domain of apo B-100 (C14) demonstrates a similar immunoreactivity with ox-LDL and native LDL but a much lower reactivity with ac-LDL. Mab C14 inhibited macrophage degradation of ox-LDL by 34% but had no inhibitory effect on the uptake of native LDL or ac-LDL. Thus, the ac-LDL and LDL receptor-binding domains as well as a unique epitope on the amino terminus of apo B-100 may be involved in macrophage binding of ox-LDL.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypolipidemic drugs reduce lipoprotein susceptibility to undergo lipid peroxidation: in vitro and ex vivo studies

Oxidized LDL, which has been discovered in vivo in areas of proximity to the atherosclerotic lesion, has been shown to enhance macrophage cholesterol accumulation. We studied the anti-oxidant potential of pravastatin, bezafibrate and cholestyramine in 18 patients with hypercholesterolemia. In addition, we examined the electrophoretic mobility and the uptake of LDL by macrophages before and after drug therapy. Pravastatin lowered plasma levels of LDL cholesterol by 57%, cholestyramine by 27% and bezafibrate by 25%. Pravastatin and bezafibrate also altered the composition of LDL as evidenced by the reduction of its cholesterol/apo B100 ratio. Pravastatin and bezafibrate reduced plasma triglyceride levels by 45% and 25%, respectively, whereas cholestyramine raised plasma triglyceride concentrations by 28%. LDL propensity for in vitro oxidation was analyzed following lipoprotein incubation with 10 microM copper ions and determination of LDL malondialdehyde (MDA), peroxides (PD) and conjugated dienes (CD) content. All drugs inhibited the susceptibility to in vitro oxidation of LDL isolated after drug therapy in comparison to LDL isolated before commencing drug therapy. Pravastatin reduced MDA content by 22%, PD by 18% and CD by 20%. Cholestyramine reduced LDL content of MDA by 41%, PD by 25% and CD by 63%. Bezafibrate reduced MDA by 41%, PD by 38% and CD by 45%. LDL vitamin E content was reduced after treatment with bezafibrate, pravastatin and cholestyramine by 49%, 36% and 8%, respectively. The electrophoretic mobility of LDL after all drug therapies was reduced in comparison to LDL obtained before therapy. Macrophage uptake of LDL assessed by either the cellular cholesterol esterification rate or by lipoprotein degradation was not affected by drug therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma lipoproteins and monocyte-macrophages in a peroxisome-deficient system: study of a patient with infantile refsum disease

Hypocholesterolaemia in infantile Refsum disease (IRD) may link peroxisomes and lipoprotein metabolism. In our patient, plasma cholesterol levels were reduced to 26% and 29% of control in LDL and HDL fractions, respectively. Plasma apolipoproteins B-100 and A-I levels were 52% and 66% of controls, respectively. In the kindred, plasma cholesterol concentration was 61-73% of controls. The HDL-cholesterol/apo A-I ratios were: patient 0.12; kindred 0.17; controls 0.28. Analysis of the IRD patient's lipoprotein revealed compositional abnormalities in all fractions. The patient's LDL demonstrated a substantial reduction in its lipid-to-protein ratio. Alterations in plasma lipoproteins affect their interaction with macrophages. Upon incubation of the patient's LDL with J-774 macrophages, its cellular uptake, measured as cholesterol esterification rate, was only 66% of a control rate. The abnormal LDL of the IRD patient showed also only 25% of control susceptibility to in vitro oxidation. Studies of cellular cholesterol metabolism in the patient's monocyte-derived macrophages (MDM) showed 57% increased cholesterol esterification rate in comparison to normal MDM. The possible link between lipoprotein abnormalities and monocyte-macrophage cholesterol metabolism is discussed.

The contribution of the macrophage receptor for oxidized LDL to its cellular uptake

Oxidized LDL (Ox-LDL) was shown to be taken up by macrophages via several receptors including the acetyl-LDL(Ac-LDL), the LDL, and the Ox-LDL receptors. Cellular uptake and degradation of Ox-LDL could be dissociated from that of LDL and Ac-LDL as demonstrated by using macrophages that lack the LDL or the Ac-LDL receptors. In J-774 A.1 macrophage-like cell line unlabeled Ox-LDL reduced the 125I-Ox-LDL by up to degradation of 91% whereas unlabeled Ac-LDL and native LDL reduced 125I-Ox-LDL degradation by only 51% and 23%, respectively. Analysis of macrophage degradation of 125I-Ox-LDL in the presence of 30-fold excess concentration of LDL + Ac-LDL (to block uptake of 125I-Ox-LDL via the LDL and the Ac-LDL receptors) revealed that cellular degradation via the Ox-LDL receptor could account for 45% of the macrophage uptake of Ox-LDL.