Characterization of low density lipoprotein-like particle in the human aorta from grossly normal and atherosclerotic regions

Endothelial Dysfunction: From Pathophysiology to Novel Therapeutic Approaches

The vascular endothelium is an active tissue that plays a crucial role in the maintenance of vascular homeostasis [...].

Clinical Significance of Electronegative Low-Density Lipoprotein Cholesterol in Atherothrombosis

Despite the numerous risk factors for atherosclerotic cardiovascular diseases (ASCVD), cumulative evidence shows that electronegative low-density lipoprotein (L5 LDL) cholesterol is a promising biomarker. Its toxicity may contribute to atherothrombotic events. Notably, plasma L5 LDL levels positively correlate with the increasing severity of cardiovascular diseases. In contrast, traditional markers such as LDL-cholesterol and triglyceride are the therapeutic goals in secondary prevention for ASCVD, but that is controversial in primary prevention for patients with low risk. In this review, we point out the clinical significance and pathophysiological mechanisms of L5 LDL, and the clinical applications of L5 LDL levels in ASCVD can be confidently addressed. Based on the previously defined cut-off value by receiver operating characteristic curve, the acceptable physiological range of L5 concentration is proposed to be below 1.7 mg/dL. When L5 LDL level surpass this threshold, clinically relevant ASCVD might be present, and further exams such as carotid intima-media thickness, pulse wave velocity, exercise stress test, or multidetector computed tomography are required. Notably, the ultimate goal of L5 LDL concentration is lower than 1.7 mg/dL. Instead, with L5 LDL greater than 1.7 mg/dL, lipid-lowering treatment may be required, including statin, ezetimibe or PCSK9 inhibitor, regardless of the low-density lipoprotein cholesterol (LDL-C) level. Since L5 LDL could be a promising biomarker, we propose that a high throughput, clinically feasible methodology is urgently required not only for conducting a prospective, large population study but for developing therapeutics strategies to decrease L5 LDL in the blood.

Calcium deposition within coronary atherosclerotic lesion: Implications for plaque stability

Atherosclerotic lesion progression is associated with intimal calcification. The earliest lesion that shows calcification is pathologic intimal thickening in which calcifications appear as microcalcifications that vary in size from <0.5 to 15 μm. The calcifications become larger as plaques progress, becoming punctate (>15 μm to 1 mm in diameter), fragmented (>1 mm), and eventually sheet-like calcification (>3 mm). When stratified by plaque type, maximum calcifications are observed in fibrocalcific plaques, followed by healed plaque ruptures. Lesions of acute thrombi, i.e., plaque rupture and erosions, which are the most frequent causes of acute coronary syndromes, show much less calcification than stable fibrocalcific plaques. Conversely, a calcified nodule, the least common lesion of acute thrombosis, occurs in highly calcified lesions. Pro-inflammatory cytokines observed in unstable plaques may provoke an early phase of osteogenic differentiation of smooth muscle cells (SMCs), a release of calcifying extracellular matrix vesicles, and/or induce apoptosis of macrophages and SMCs, which also calcify. Recent pathologic and imaging based studies indicate that lesions with dense calcifications are more likely to be stable plaques (fibrocalcific plaques), while micro, punctate, or fragmented calcifications are associated with either early stage plaques or unstable lesions (plaque rupture or erosion). Clinical non-invasive computed tomography (CT) studies show that the greater the calcium score, the higher the likelihood of patients developing future acute coronary events. This appears contradictory with the findings from pathologic autopsy studies. However, CT analysis of calcium subtypes is limited by resolution and blooming artifacts. Thus, areas of heavy calcification may not be the cause of future events as pathologic studies suggest. Rather, calcium may be an overall marker for the extent of disease. These types of discrepancies can perhaps be resolved by invasive or non-invasive high resolution imaging studies carried out at intervals in patients who present with acute coronary syndromes versus stable angina patients. Coronary calcium burden is greater in stable plaques than unstable plaques and there is a negative correlation between necrotic core area and area of calcification. Recent clinical studies have demonstrated that statins can reduce plaque burden by demonstrating a reduction in percent and total atheroma volume. However, calcification volume increases. In summary, pathologic studies show that sheet calcification is highly prevalent in stable plaques, while microcalcifications, punctate, and fragmented calcifications are more frequent in unstable lesions. Both pathologic and detailed analysis of imaging studies in living patients can resolve some of the controversies in our understanding of coronary calcification.

Transport of Apolipoprotein B-Containing Lipoproteins through Endothelial Cells Is Associated with Apolipoprotein E-Carrying HDL-Like Particle Formation

Passage of apolipoprotein B-containing lipoproteins (apoB-LPs), i.e., triglyceride-rich lipoproteins (TRLs), intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs), through the endothelial monolayer occurs in normal and atherosclerotic arteries. Among these lipoproteins, TRLs and IDLs are apoE-rich apoB-LPs (E/B-LPs). Recycling of TRL-associated apoE has been shown to form apoE-carrying high-density lipoprotein (HDL)-like (HDL_E) particles in many types of cells. The current report studied the formation of HDL_E particles by transcytosis of apoB-LPs through mouse aortic endothelial cells (MAECs). Our data indicated that passage of radiolabeled apoB-LPs, rich or poor in apoE, through the MAEC monolayer is inhibited by filipin and unlabeled competitor lipoproteins, suggesting that MAECs transport apoB-LPs via a caveolae-mediated pathway. The cholesterol and apoE in the cell-untreated E/B-LPs, TRLs, IDLs, and LDLs distributed primarily in the low-density (LD) fractions (d ≤ 1.063). A substantial portion of the cholesterol and apoE that passed through the MAEC monolayer was allotted into the high-density (HD) (d > 1.063) fractions. In contrast, apoB was detectable only in the LD fractions before or after apoB-LPs were incubated with the MAEC monolayer, suggesting that apoB-LPs pass through the MAEC monolayer in the forms of apoB-containing LD particles and apoE-containing HD particles.

[Coronary atherosclerosis and progression to unstable plaques : Histomorphological and molecular aspects]

Atherosclerosis causes clinical symptoms through luminal narrowing by stenosis or by precipitating thrombi that obstruct blood flow to the myocardium (coronary artery disease), central nervous system (ischemic stroke) or lower extremities (peripheral vascular disease). The most common of these manifestations of atherosclerosis is coronary artery disease, clinically presenting as either stable angina or acute coronary syndromes. Atherosclerosis is a mainly lipoprotein-driven disease, which is associated with the formation of atherosclerotic plaques at specific sites of the vascular system through inflammation, necrosis, fibrosis and calcification. In most cases, plaque rupture of a so-called thin-cap fibroatheroma leads to contact of the necrotic core material of the underlying atherosclerotic plaque with blood, resulting in the formation of a thrombus with acute occlusion of the affected (coronary) artery. The atherosclerotic lesions that can cause acute coronary syndromes by formation of a thrombotic occlusion encompass (1) thin-cap fibroatheroma, (2) plaque erosion and (3) so-called calcified nodules in calcified and tortuous arteries of aged individuals. The underlying pathomechanisms remain incompletely understood so far. In this review, the mechanisms of atherosclerotic plaque initiation and progression are discussed.

Pathophysiology of atherosclerosis plaque progression

Atherosclerotic plaque rupture with luminal thrombosis is the most common mechanism responsible for the majority of acute coronary syndromes and sudden coronary death. The precursor lesion of plaque rupture is thought to be a thin cap fibroatheroma (TCFA) or "vulnerable plaque". TCFA is characterised by a necrotic core with an overlying thin fibrous cap (≤65 μm) that is infiltrated by macrophages and T-lymphocytes. Intraplaque haemorrhage is a major contributor to the enlargement of the necrotic core. Haemorrhage is thought to occur from leaky vasa vasorum that invades the intima from the adventitia as the intima enlarges. The early atherosclerotic plaque progression from pathologic intimal thickening (PIT) to a fibroatheroma is thought to be the result of macrophage infiltration. PIT is characterised by the presence of lipid pools which consist of proteoglycan with lipid insudation. The conversion of the lipid pool to a necrotic core is poorly understood but is thought to occur as a result of macrophage infiltration which releases matrix metalloproteinase (MMPs) along with macrophage apoptosis that leads to the formation of a acellular necrotic core. The fibroatheroma has a thick fibrous cap that begins to thin over time through macrophage MMP release and apoptotic death of smooth muscle cells converting the fibroatheroma into a TCFA. Other causes of thrombosis include plaque erosion which is less frequent than plaque rupture but is a common cause of thrombosis in young individuals especially women <50 years of age. The underlying lesion morphology in plaque erosion consists of PIT or a thick cap fibroatheroma. Calcified nodule is the least frequent cause of thrombosis, which occurs in older individuals with heavily calcified and tortious arteries.

Terrein: a new melanogenesis inhibitor and its mechanism

Terrein is a bioactive fungal metabolite whose effects are almost unknown. In this study, we found for the first time that terrein has a strong hypopigmentary effect in a spontaneously immortalized mouse melanocyte cell line, Mel-Ab. Treatment of Mel-Ab cells with terrein (10-100 microM) for 4 days significantly reduced melanin levels in a dose-dependent manner. In addition, terrein at the same concentration also reduced tyrosinase activity. We then investigated whether terrein influences the extracellular signal-regulated protein kinase (ERK) pathway and the expression of microphthalmia-associated transcription factor (MITF), which is required for tyrosinase expression. Terrein was found to induce sustained ERK activation and MITF down-regulation, and luciferase assays showed that terrein inhibits MITF promoter activity in a dose-dependent manner. To elucidate the correlation between ERK pathway activation and a decreased MITF transcriptional level, PD98059, a specific inhibitor of the ERK pathway, was applied before terrein treatment and found to abrogate the terrein-induced MITF attenuation. Terrein also reduced the tyrosinase protein level for at least 72 h. These results suggest that terrein reduces melanin synthesis by reducing tyrosinase production via ERK activation, and that this is followed by MITF down-regulation.

Arterial heparan sulfate is negatively associated with hyperglycemia and atherosclerosis in diabetic monkeys

Background

Arterial proteoglycans are implicated in the pathogenesis of atherosclerosis by their ability to trap plasma lipoproteins in the arterial wall and by their influence on cellular migration, adhesion and proliferation. In addition, data have suggested an anti-atherogenic role for heparan sulfate proteoglycans and a pro-atherogenic role for dermatan sulfate proteoglycans. Using a non-human primate model for human diabetes, studies examined diabetes-induced changes in arterial proteoglycans that may increase susceptibility to atherosclerosis.

Methods

Control (n = 7) and streptozotocin-induced diabetic (n = 8) cynomolgous monkeys were assessed for hyperglycemia by measurement of plasma glycated hemoglobin (GHb). Thoracic aortas obtained at necropsy, were extracted with 4 M guanidine HCL and proteoglycans were measured as hexuronic acid. Atherosclerosis was measured by enzymatic analysis of extracted tissue cholesterol. Glycosaminoglycan chains of arterial proteoglycans were released with papain, separated by agarose electrophoresis and analysed by scanning densitometry.

Results

Tissue cholesterol was positively associated with hexuronic acid content in diabetic arteries (r = .82, p < .025) but not in control arteries. Glycosaminoglycan chain analysis demonstrated that dermatan sulfate was associated with increased tissue cholesterol in both control (r = .8, p < 0.05) and diabetic (r = .8, p < .025) arteries, whereas a negative relationship was observed between heparan sulfate and tissue cholesterol in diabetic arteries only (r = -.7, p < .05). GHb, which was significantly higher in diabetic animals (8.2 ± 0.9 vs 3.8 ± 0.2%, p < .0005) was negatively associated with heparan sulfate in diabetic arteries (r = -.7, p < .05).

Conclusions

These data implicate hyperglycemia induced modifications in arterial proteoglycans that may promote atherosclerosis.

Glycation of plasma low density lipoproteins increases interaction with arterial proteoglycans

The increased susceptibility to atherosclerosis of diabetic individuals, may result from diabetes-associated modification in plasma low density lipoproteins (LDL) which enhance their interaction with arterial extracellular matrix proteoglycans. Using a nonhuman primate model for human diabetes, studies were conducted to examine diabetes-induced changes in LDL. Plasma LDL were isolated from control (n = 4) and streptozotocin-induced diabetic (n = 3) cynomolgus macaques by differential ultracentrifugation. An in vitro binding assay was used to measure LDL interaction with arterial proteoglycans. Significantly more diabetic LDL bound to proteoglycans than control LDL (12.9+/-0.7 microg LDL cholesterol/microg proteoglycan versus 8.9+/-0.5 microg LDL cholesterol/microg proteoglycan (mean +/- S.E.M.), P < 0.005). Glycation of LDL, determined by fructosamine content, was significantly enhanced in diabetic versus control animals (37+/-3.1 versus 20+/-1.5 micromol/l (mean +/- S.E.M.) P < 0.005). The correlation coefficient between fructosamine content of LDL and its binding to arterial proteoglycans was 0.95. No LDL compositional variables other than glycation correlated with proteoglycan binding. Removal of the glycated portion of LDL from diabetic animals returned LDL proteoglycan binding to normal. These data demonstrate that the diabetes induced glycation of LDL increases its proteoglycan binding properties: thus, a critical mechanism in atherosclerosis, enhanced LDL interaction with arterial proteoglycans, may be accelerated by the diabetic state.

Isradipine lowers human arterial low density lipoprotein retention in vivo

During the recent past it has been discussed that calcium antagonists may exert antiatherosclerotic actions at the vessel wall. Apolipoprotein B containing lipoproteins were isolated by immunoaffinity chromatography and radiolabeled with 123-iodine. The effect of 2 x 2.5 mg isradipine on the low density lipoproteins (LDL) entry into the carotid and femoral arteries of 12 hypertensive patients with primary hyperlipoproteinemia (total cholesterol >6.5 mmol/l [250 mg/dL) was examined. Cholesterol -1.7% (P< 0.05 664), high density lipoprotein (HDL) cholesterol +4.5% (P< 0.01 123), and LDL cholesterol -1% (P< 0.01 563) did not change, nor did any of the safety parameters. The types of entry kinetics reflecting vascular surface lining did not change while the LDL retention 20 h after tracer application was depressed by up to 23.5%. The data were comparable in the carotid and femoral artery segments, the significance level ranging up to 0.0009. These results indicate a decreased LDL retention in the arterial wall of hypertensive patients induced by isradipine. The clinical implications of the findings ought to be pursued in properly designed clinical trials.

Delivery of benzoporphyrin derivative, a photosensitizer, into atherosclerotic plaque of Watanabe heritable hyperlipidemic rabbits and balloon-injured New Zealand rabbits

In this study we compared the plasma distribution and arterial accumulation of a photosensitizer, benzoporphyrin derivative (BPD), in two models of atherosclerosis: the spontaneous lesions of the Watanabe heritable hyperlipidemic (WHHL) rabbit and induced lesions of the balloon-injured, cholesterol-fed New Zealand white (NZW) rabbit. Selective uptake and retention of a photosensitizer by the abnormal portion of a vessel is a necessity in order for photodynamic therapy to become a successful modality for inhibition of intimal hyperplasia, selective removal of atherosclerotic tissue or imaging of diseased arteries. Liposome-based formulations were compared to freshly isolated native low density lipoprotein (LDL) and acetylated-LDL (Ac-LDL) as delivery vehicles for BPD. Plasma distribution of the photosensitizer was analyzed by KBr density gradient ultracentrifugation. Although the delivery vehicle influenced plasma distribution immediately postinjection, BPD subsequently partitioned according to the plasma concentration of the lipoproteins. Photosensitizer level in plaque and normal artery specimens was determined by ethyl acetate extraction and spectrofluorometric measurement. The measurement of BPD in normal and atherosclerotic arterial tissue demonstrated a selective accumulation in atherosclerotic tissue. Preassociation with LDL and Ac-LDL enhanced accumulation of BPD in atherosclerotic tissue when compared with normal artery (mean ratios of 2.8 and 4.1 were achieved, respectively). These results indicate that the preferential uptake of BPD by atherosclerotic plaque can be enhanced by preassociation with plasma lipoproteins, suggesting that light activation could lead to a highly selective destruction of diseased vascular tissue.

The role of triglyceride-rich lipoprotein families in the progression of atherosclerotic lesions as determined by sequential coronary angiography from a controlled clinical trial

We have demonstrated previously in a subset of Monitored Atherosclerosis Regression Study (MARS) subjects with hypercholesterolemia (190 to 295 mg/dL) and documented coronary artery disease that lovastatin significantly reduces cholesterol-rich lipoprotein B (LpB) but has little effect on complex, triglyceride-rich apolipoprotein (apo) B-containing LpBc (the sum of LpB:C, LpB:C:E and LpA-II:B:C:D:E) particles defined by their apolipoprotein composition. This differential effect of lovastatin on apoB-containing lipoprotein families offered the opportunity to determine in the same subset of MARS subjects the independent relationship of LpB and LpBc with the progression of coronary artery disease. Subjects randomized to either lovastatin (40 mg twice daily) or matching placebo were evaluated by coronary angiography before randomization and after 2 years of treatment, and the overall coronary status was judged by a coronary global change score. In the lovastatin-treated group, there were 22 nonprogressors (69%) and 10 progressors (31%), while in the placebo group 13 subjects (42%) were nonprogressors and 18 (58%) were progressors (P < .03). In the lovastatin-treated group, lipid and lipoprotein parameters did not differ between progressors and nonprogressors except for LpBc and LpA-II:B:C:D:E particle levels, which were statistically higher in progressors (P = .02). In the placebo-treated group, progressors differed from nonprogressors by having significantly higher levels of triglycerides (P = .03) and apoC-III in VLDL + LDL (P = .05), the characteristic constituents of triglyceride-rich lipoproteins. In the placebo- and lovastatin-treated groups combined, progressors had significantly higher on-trial levels of triglycerides (P = .003), VLDL cholesterol (P = .005), apoC-III in VLDL + LDL (P = .008), apoC-III (P = .01), apoB (P = .03), and total cholesterol (P = .04) than nonprogressors. Even after adjustment for treatment group, progressors in the combined placebo- and lovastatin-treated groups had significantly higher levels of LpBc, LpA-II:B:C:D:E, triglycerides, and apoC-III in VLDL + LDL than nonprogressors. Progressors in the placebo-treated, lovastatin-treated, and combined treatment groups had lower levels of LpA-1 but not LpA-I:A-II than non-progressors, and this difference reached statistical significance (P = .047) in the combined sample adjusted for treatment group. Results of this study show that elevated levels of triglyceride-rich LpBc in general and LpA-II:B:C:D:E in particular contribute significantly to the progression of coronary artery disease. Furthermore, they provide additional evidence for the potentially protective role of LpA-I particles in the atherogenic process and suggest that apolipoprotein-defined lipoprotein families may be more specific prognosticators of coronary artery atherosclerosis progression than lipids and apolipoproteins.

Similarity between naturally occurring modified desialylated, electronegative and aortic low density lipoprotein

The sialic acid content of electronegative low density lipoprotein (LDL) and LDL isolated from human aortic intima was measured. Sialic acid level in electronegative LDL of healthy subjects was 1.7-fold lower than in native LDL. Sialic acid content in electronegative LDL of coronary atherosclerosis patients was 3-fold lower than in native LDL. Lipoproteins isolated from grossly normal human aortic intima and from fatty streaks contained 20-56% less sialic acid as compared to blood plasma LDL. A negative correlation was established between the ability of electronegative and aortic LDL to stimulate lipid accumulation in cells cultured from uninvolved human aortic intima and lipoprotein sialic acid content. The results obtained indicate that electronegative and aortic LDLs have a low sialic acid content, i.e., are desialylated lipoproteins. Considered together with the fact that all known atherogenic LDLs have similar characteristics, our findings suggest that modified LDLs are the same lipoprotein particles subjected to multiple modification.

Transfer of low density lipoprotein into the arterial wall and risk of atherosclerosis

The aim of the review is to summarize the present knowledge on determinants of transfer of low density lipoprotein (LDL) into the arterial wall, particularly in relation to the risk of development of atherosclerosis. The flux of LDL into the arterial wall (in moles of LDL per surface area per unit of time) has two major determinants, i.e. the LDL concentration in plasma and the arterial wall permeability. LDL enters the arterial wall as intact particles by vesicular ferrying through endothelial cells and/or by passive sieving through pores in or between endothelial cells. Estimates in vivo of the LDL permeability of a normal arterial wall vary between 5 and 100 nl/cm2/h. In laboratory animals, the regional variation in the arterial wall permeability predicts the pattern of subsequent dietary induced atherosclerosis. Moreover, mechanical or immunological injury of the arterial wall increases the LDL permeability and is accompanied by accelerated development of experimental atherosclerosis. This supports the idea that an increased permeability to LDL, like an increased plasma LDL concentration, increases the risk of atherosclerosis. Hypertension, smoking, genetic predisposition, atherosclerosis, and a small size of LDL may all increase the arterial wall permeability to LDL and in this way increase the risk of accelerated development of atherosclerosis. The hypothesis that atherosclerosis risk can be reduced by improving the barrier function of the arterial wall towards the entry of LDL remains to be investigated; agents which directly modulate the LDL permeability of the arterial wall in vivo await identification.

External imaging of atherosclerosis in rabbits using an 123I-labeled synthetic peptide fragment

The oligopeptide fragment of apolipoprotein B, SP-4, has demonstrated pronounced uptake in the healing edges of balloon-injured rabbit aortic endothelium. To assess 123I-labeled SP-4 for identification of atherosclerotic plaques by gamma camera imaging, 14 Watanabe heritable hyperlipidemic (WHHL) and 5 normal rabbits were imaged 5 minutes and 12 and 24 hours after intravenous injection of 123I-SP-4. In addition, two WHHL and two normal rabbits were injected with 125I-SP-4 for autoradiography. Twelve of the 14 WHHL, but none of the normal, rabbits had visually apparent focal radioiodine accumulation in the region of the aorta. Focus-to-lung and focus-to-heart count ratios were 2.4 +/- 1.3 and 1.0 +/- 0.4, respectively. Five of the visually positive WHHL rabbits were reimaged 4 and 8 weeks later with 123I-NaI and 123I-SP-2 (an apo E peptide), respectively, as negative controls. Perceptible, but faint, aortic localization of 123I-NaI and of 123I-SP-2 was seen in only one animal each. The distributions of atherosclerotic lesions on photographs of the opened WHHL aortas and of film blackening on 125I-SP-4 autoradiograms were identical. In contrast, the two normal rabbit aortas did not exhibit plaques on photographs or film blackening on autoradiograms. Thus, in an animal model closely simulating human atherosclerotic disease, SP-4 localizes specifically in aortic atherosclerotic lesions.

Potential of 99mTc-LDLs labeled by two different methods for scintigraphic detection of experimental atherosclerosis in rabbits

In this study we evaluated two different 99mTc-labeling techniques to produce 99mTc-low density lipoprotein (99mTc-LDL) suitable for the scintigraphic delineation of experimental atherosclerotic lesions. The two methods are 1) a procedure that uses stannous chloride and sodium borohydride (borohydride method) and 2) a procedure that uses sodium dithionite as a reducing agent and that has been successfully applied in previous scintigraphic atherosclerosis detection (dithionite method). 99mTc-LDL produced by either method was injected into New Zealand White rabbits with diet-induced atherosclerotic plaques and in control rabbits. Scintigraphic images were taken 10 minutes (t = 0) and 1, 4, 8, 16, and 24 hours after injection. Clearance of plasma radioactivity was also studied. Stability of the 99mTc-LDL complex in the circulation was examined by size exclusion chromatography of plasma samples. After scintigraphy, the animals were killed, and the biodistribution of radioactivity was determined. The thoracic and abdominal aortas appeared in scintigraphic images to accumulate 99mTc over their entire length with either 99mTc-LDL preparation. The sparse imaging of focal atherosclerosis was found to be due to the fact that the aortas were covered with confluent atherosclerotic lesions. Scintigraphic image analysis showed that 24 hours after injection, the accumulated radioactivity in the abdominal aorta of the atherosclerotic rabbits was 57% and 54%, respectively, of the accumulated radioactivity in the abdominal aorta at t = 0 when the borohydride versus the dithionite method was used. In the control animals this value was 25% for the dithionite method, whereas in the borohydride method the aortas could not be detected in the images at t = 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo quantification of cholesterol content in human carotid arteries by quantitative gamma-camera imaging after injection of autologous low density lipoproteins (LDL)

Low density lipoproteins (LDL) were isolated by immunoaffinity chromatography from 18 patients (31-70 years) suffering from primary hypercholesterolemia with angiographically proven atherosclerosis of either one or both carotid arteries. LDL were labeled with 123I (1 mCi/mg LDL) by the iodine monochloride method followed by purification with dialysis and immediately reinjected thereafter. Gamma-camera serial controls over carotid regions allowed visual detection of uptake of the radiocompound uptake in 12 out of the 18 patients. The lipid entry ratio (LER; counts over the vascular region/pixel as compared to the contralateral side after background subtraction) confirmed the visual findings. Whole body images performed until 20 h after reinjection showed 3 different kinetic types of LDL-influx into the vessel wall: decreasing (type I), increasing and then decreasing (type II) and continuously increasing (type III) with time. Four patients underwent endarterectomy within 2-7 weeks after gamma-camera imaging. Histological control revealed an extensive amount of "foam cells" in tissue samples derived during surgery and an absence of endothelial lining in samples belonging to patients with type II kinetics.

Reduced proteoglycan binding of low density lipoproteins from monkeys (Macaca fascicularis) fed a fish oil versus lard diet

We examined the effect of isocaloric substitution of dietary fish oil for lard on the properties of low density lipoproteins (LDL) important in binding to arterial proteoglycans (PG). Cynomolgus monkeys (n = 10) were fed atherogenic diets enriched in fish oil or lard in a crossover study consisting of two 15-week phases of atherogenic diet separated by a 6-week monkey chow "wash-out period." LDL were isolated from plasma during each dietary phase, characterized for chemical and physical properties, and assessed for their ability to interact in vitro with arterial PG. Plasma LDL cholesterol was similar during fish oil and lard consumption (356 +/- 34 and 331 +/- 17 mg/dl, mean +/- SEM), but during fish-oil feeding relative to that of lard, LDL size was smaller (4.2 +/- 0.1 versus 4.9 +/- 0.1 g/mumol) and LDL particles differed in chemical composition. When animals were fed fish oil, significantly fewer (p less than 0.05) LDL particles bound to PG in both dietary phases: 1.00 +/- 0.27 (x10(12)) versus 5.31 +/- 0.83 (x10(12)) particles/micrograms PG in phase 1 and 3.56 +/- 0.67 (x10(12)) versus 6.00 +/- 0.52 (x10(12)) in phase 2 for LDL from animals fed fish oil and lard, respectively. These studies indicate that dietary fat-induced changes in LDL particles lead to altered in vitro interactions with artery wall PG and suggest a novel mechanism for the protective effect of fish oil against atherosclerosis.

Lesion-derived low density lipoprotein and oxidized low density lipoprotein share a lability for aggregation, leading to enhanced macrophage degradation

In this study we assessed whether low density lipoproteins (LDL) isolated from minced aortic atherosclerotic plaques obtained at autopsy (A-LDL) shared structural and functional properties with LDL oxidized by incubation with Cu2+ for 8-18 hours at 20 degrees C (Ox-LDL). Although both A-LDL and Ox-LDL represented monomeric particles about the size of LDL, both differed from LDL in that they showed an increase in electrophoretic mobility relative to LDL, an increase in cholesterol to protein ratio, and an increase in reactivity with a monoclonal antibody that recognizes epitopes on malondialdehyde (MDA)-modified proteins. In addition, both showed an increase in fluorescence at 360 nm excitation, 430 nm emission, an increase in fragmentation of apolipoprotein B with patterns that were quite similar, and an increase in recognition by the scavenger receptor on mouse peritoneal macrophages (MPMs) based on competition of 125I-A-LDL and 125I-Ox-LDL degradation by excess acetylated LDL. In addition, inhibition of degradation by MPMs of 125I-A-LDL and 125I-Ox-LDL by excess unlabeled Ox-LDL and A-LDL were similar. When MDA was added in increasing amounts to labeled LDL and A-LDL, less MDA was required to modify A-LDL than LDl to obtain ligands that were degraded by MPMs to the same degree. Finally, both A-LDL and Ox-LDL but not LDL underwent aggregation (increased metastability) when concentrated to levels exceeding 1 mg protein/ml and showed enhanced macrophage uptake via phagocytosis (inhibition by cytochalasin D). These results demonstrate that A-LDL and Ox-LDL share properties additional to those previously reported, suggesting that oxidation may be a major mode of modification of LDL accumulating in atherosclerotic lesions. This could lead to lipid loading of macrophages induced by phagocytosis of aggregated particles, in addition to unregulated uptake via the scavenger receptor of monomeric particles.

Macrophage degradation of LDL extracted from human aortic plaques: effect of isolation conditions

Several laboratories have recently reported on the structural and functional characteristics of an LDL fraction isolated from atherosclerotic lesions, designated A-LDL. Given the wide variety of tissue sources and isolation conditions that have been employed, we have addressed whether several procedures currently used affect the interaction of A-LDL with macrophages, and, if so, by what mechanisms. We isolated A-LDL from human aortic plaques by ultracentrifugation and gel filtration chromatography. Although some differences in the chromatographic elution profiles on gel filtration were apparent between homogenized and nonhomogenized extracts, A-LDL isolated from the same pool of plaque minces with or without homogenization showed no differences in macrophage degradation or inhibition of this degradation by excess acetyl-LDL. A-LDL isolated from plaques obtained at surgery or at autopsy less than 12 hr after death also showed no major differences in macrophage recognition, suggesting that post-mortem changes were probably not affecting cell recognition. However, A-LDL particles underwent aggregation when subjected to concentration, when stored for periods of 2 weeks or more, or when subjected to vortexing. The aggregated A-LDL was degraded more readily by macrophages than unaggregated A-LDL, and inhibition of degradation of aggregated A-LDL by excess acetyl-LDL was less than for unaggregated A-LDL. Collectively, these studies show that although post-mortem changes and tissue homogenization do not appreciably affect the interaction of A-LDL with macrophages in culture, other isolation and preparation conditions have dramatic effects which could explain some of the diversity of A-LDL metabolism reported in the literature.

Nuclear medicine and atherosclerosis

Although the pathomechanisms of atherosclerosis are well known, their radioisotopic monitoring is still in its early childhood. The current radioisotope techniques are of only limited value for contributing to the clinical diagnosis of atherosclerosis. The limited reaction time of cellular blood constituents (platelets, monocytes) with the vascular surface at the injury site makes it very difficult to catch the point of injury. Lipoproteins excellently allow receptor imaging, while vascular monitoring is only of scientific interest at present. Labelling and subsequent imaging of components of the coagulation cascade have not succeeded so far, nor have attempts using unspecific labels such as porphyrin, polyclonal IgG and Fc fragments, for example. Preliminary evidence indicates that radioisotopic techniques may be of great benefit in the future in elucidating functional aspects of the disease, while they do not contribute to examining the stage and extent of atherosclerosis.

Interaction of glycosaminoglycans with lipoproteins

Apolipoproteins B-100 and E are protein constituents of human plasma chylomicrons, very low (VLDL), and low density lipoproteins (LDL). The interaction of lipoproteins with cell receptors is mediated by apoB and E. Lipoproteins also bind to the extracellular matrix, such as glycosaminoglycans (GAG), forming insoluble complexes in the presence of Ca2+. The purpose of this study was to identify the GAG-binding domains in apoB and E. By a combination of fragmentation of the intact proteins, peptide synthesis and quantitative GAG-binding, domains in apoB and apoE were identified and are shown below. These domains contain clusters of basic amino acids that we suggest are required for GAG-binding. table; see text.

Low density lipoprotein interaction with artery derived proteoglycan: the influence of LDL particle size and the relationship to atherosclerosis susceptibility

An in vitro binding system was used to determine whether increases in LDL particle size and altered LDL chemical composition accompanying increased plasma cholesterol concentrations result in greater association of LDL with artery proteoglycan (PG) and whether the binding is related to atherosclerosis. LDL isolated from hypercholesterolemic atherosclerosis-susceptible White Carneau and resistant Show Racer pigeons was complexed to purified White Carneau pigeon aorta-derived high molecular weight PG under conditions whereby PG monomers were saturated. Using LDL of molecular weight greater than 5.0 x 10(6) daltons from both pigeon breeds, an inverse correlation between LDL size and the number of LDL particles bound per micrograms PG was demonstrated (r = 0.87, P less than 0.01). This relationship was attributed to the increased size of the LDL particle rather than any modification in chemical composition known to occur when LDL size increases, suggesting the major effect was attributed to steric hindrance. White Carneau pigeons with high molecular weight LDL had more severe atherosclerosis and the PG-LDL complexes contained excess cholesterol but no relationship was seen between atherosclerosis and number of LDL complexed. In animals with LDL between 3.6 x 10(6) and 4.8 x 10(6) daltons, considerable variability in PG binding was apparent, but this also was not related to LDL chemical composition. In this group of pigeons, which were all White Carneau, the positive relationship of PG-LDL binding and aorta cholesterol concentration was significant (r = 0.67, P less than 0.05). These results suggest that factors other than chemical composition (perhaps surface charge or apoprotein conformation changes) influence PG-LDL binding and that the assessment of PG-LDL binding is useful in predicting atherosclerosis in animals that do not respond to hypercholesterolemia by increasing LDL size.

Isolation of low density lipoprotein from atherosclerotic vascular tissue of Watanabe heritable hyperlipidemic rabbits

Atherogenic properties of low density lipoproteins (LDL) in vivo may reflect modification of lipoproteins associated with endothelial translocation and exposure to extracellular matrix and interstitial fluid. To examine whether modifications of LDL occur in vivo, lipoproteins were isolated from plasma and vascular tissue of Watanabe heritable hyperlipidemic (WHHL) rabbits. LDL was extracted from vascular tissue (LDL-V) by homogenization in the presence of a sodium carbonate buffer. Control experiments demonstrated that modifications did not occur under the preparative conditions used to release LDL from tissue. LDL-V contained less esterified cholesterol, but more cholesterol esters, than did LDL from plasma (LDL-P). The diameters of both LDL-V and LDL-P followed gaussian distributions, but LDL-V particles were smaller (20.3 +/- 0.1 and 26.3 +/- 0.1 nm). Mild lipid peroxidation was evident in LDL-V. The sphingomyelin content was increased in LDL-V, with less phosphatidylcholine than in LDL-P. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that apolipoprotein B was depleted in LDL-V, but Western blot analyses identified lower molecular weight proteins antigenically related to apolipoprotein B. LDL-V markedly stimulated cholesterol esterification in mouse peritoneal macrophages and also in rabbit alveolar macrophages, a cell type that did not respond to acetylated LDL. LDL-V was not recognized by cultured rabbit skin fibroblasts. Thus, LDL isolated from atherosclerotic vascular tissue in vivo was modified in a fashion that could confer atherogenic properties reflected by augmentation of cholesterol esterification in macrophages in vitro.

Analytical isoelectric focusing of apolipoprotein B of human plasma low-density lipoproteins in the presence of a nonionic and a zwitterionic detergent

A method for the analytical isoelectric focusing of Nonidet-P40-delipidated apolipoprotein B of human plasma low-density lipoproteins has been developed. Isoelectric focusing was performed in the presence of the zwitterionic nondenaturing detergent Chaps, 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate, and the nonionic surfactant Nonidet-P40, polyoxyethyleneglycol p-t-octylphenol with a mean of 9.0 ethylene oxide units per molecule. Low-density lipoprotein (LDL) apolipoprotein B (apo-B) entered 3.75% polyacrylamide gels without precipitation at the sites of sample application, permitting apoprotein recoveries of greater than 90% in the migrating bands. LDL apo-B exhibited 10 distinguishable bands with apparent isoelectric points of 7.34 (band 1), 7.27 (band 2), 7.16 (band 3), 7.02 (band 4), 6.88 (band 5), 6.70 (band 6), 6.61 (band 7), 6.48 (band 8), 6.40 (band 9), and 6.24 (band 10), respectively. Bands 3 and 4, 6 and 7, as well as 8 and 9 could be identified as major double bands. When the focused apo-B was run in a second dimension by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the same relative molecular weight of B-100 was obtained for all focused bands. After electrotransfer to nitrocellulose paper, all bands reacted with polyclonal anti-human LDL antibody. Furthermore, the detergent-solubilized apo-B retained the immunological properties of native low-density lipoproteins when tested by double immunodiffusion against polyvalent anti-human LDL sera.

Role of superoxide in endothelial-cell modification of low-density lipoproteins

Cultured endothelial cells and arterial smooth muscle cells have been shown to modify LDL in a way that leads to rapid uptake by macrophages. Previous studies have demonstrated that this modification involves free radical peroxidation of LDL, and that the role of the cells was to accelerate oxidation under conditions where it otherwise would occur slowly. The objective of the present study was to determine whether the modification was mediated by oxygen-derived free radicals, and whether the ability of a given cell type of line to modify LDL was related to its secretion rate of O2- or H2O2. The results showed that modification required the presence of oxygen, and could be specifically inhibited by superoxide dismutase but not by catalase or by mannitol, a hydroxyl radical scavenger. Rabbit aortic endothelial cells, rabbit arterial smooth muscle cells, monkey arterial smooth muscle cells and human skin fibroblasts were all found to modify LDL, and all of these cell types generated more O2- (superoxide dismutase-inhibitable cytochrome c reduction) than a line of bovine aortic endothelial cells that did not modify LDL. The content of superoxide dismutase and catalase was higher in bovine aortic endothelial cells than in the cell lines that modified LDL, but glutathione peroxidase levels were not different. It was concluded that cells that were capable of modifying LDL produced superoxide or a substance that could be converted to superoxide in the medium, and that superoxide was an important, though possibly indirect, mediator of the modification of LDL by cells.

Extracts of human atherosclerotic lesions can modify low density lipoproteins leading to enhanced uptake by macrophages

Plasma low density lipoproteins (LDL) and/or other lipoproteins containing apo B that accumulate in atherosclerotic lesions of human aortas exhibit structural changes that are associated with enhanced uptake in an unregulated fashion by macrophages in culture, resulting in the formation of foam cells in vitro. In an attempt to better characterize the structure-function modifications, we have incubated plasma LDL with extracts of human atherosclerotic plaques obtained at surgery, and determined whether such plaque-modified LDL also demonstrates enhanced uptake by cultured mouse peritoneal macrophages (MPM). Enhanced uptake was found which was linear over a concentration range of 100 micrograms lipoprotein protein/ml, as assessed by enhanced degradation of [125I]LDL and by stimulation of cholesterol esterification. Extracts of non-arterial human tissue were unable to induce this modification, suggesting tissue specificity. When delipidated apo B from tissue-treated [125I]LDL was subjected to SDS-PAGE, autoradiograms demonstrated, in addition to the B-100 band of apo B, a doublet of higher molecular weight than B-100 and a band just entering the gel, both at the expense of the B-100 band. No lower molecular weight bands suggestive of apo B degradation were seen. Modest increases in LDL electrophoretic mobility and thiobarbituric acid reactive substances were found following the incubation of LDL with plaque extracts. These changes could be inhibited by butylated hydroxytoluene (BHT), suggesting that free radical-induced lipid peroxidation was responsible for these modifications. However, since BHT did not inhibit the uptake of the tissue-incubated LDL by macrophages, the actual modification responsible for enhanced macrophage recognition did not appear to be free radical-induced. Uptake of plaque-modified [125I]LDL was inhibited by only 22% by a 20-fold excess of acetyl LDL or plaque-modified LDL. If the latter did not represent a mixture of modified and unmodified particles, this result would suggest that uptake was not mediated by the scavenger receptor. It is possible that foam cells are formed in vivo when LDL particles, which have been modified by interacting with components of the arterial wall, are taken up by tissue macrophages.

Modified plasma-derived lipoproteins in human atherosclerotic plaques

Low density lipoproteins extracted from surgical specimens of human atherosclerotic plaques (A-LDL) showed altered electrophoretic mobility indicating a greater negative charge than that of plasma LDL (P-LDL). A-LDL but not P-LDL showed high affinity binding/degradation by human monocyte-derived macrophages; this was inhibited by acetylated LDL but not by native P-LDL. Following injection of 125I-labelled autologous P-LDL prior to reconstructive arterial surgery, polyacrylamide and agarose gel electrophoresis of A-LDL extracted from arterial intima showed that the A-LDL and its apolipoprotein B moiety were derived from P-LDL; the electrophoretic mobility of the product A-LDL was greater than that of native P-LDL. The compositions of arterial intermediate density lipoprotein (A-IDL) and A-LDL differed from those obtained from human plasma intermediate density lipoprotein (P-IDL) and P-LDL. A-IDL showed a reduced triglyceride content and increased esterified and unesterified cholesterol. Although the total cholesterol content of A-LDL was similar to that of P-LDL, there was an increase in unesterified cholesterol and a decrease of cholesteryl ester. These studies indicate that LDL extracted from human atherosclerotic plaque is derived from and modified from P-LDL in vivo. Compared with native P-LDL, A-LDL showed differences in charge and composition, associated with its high affinity binding by the acetyl LDL receptor of human macrophages.

Isolation and characterization of four heparin-binding cyanogen bromide peptides of human plasma apolipoprotein B

Apolipoprotein B-100 (apoB-100) is the major protein constituent of human plasma low-density lipoproteins (LDL). On the basis of its amino acid sequence [Chen, S.-H., Yang, C.-Y., Chen, P.-F., Setzer, D., Tanimura, M., Li, W.-H., Gotto, A. M., Jr., & Chan, L. (1986) J. Biol. Chem. 261, 12918-12921], apo B-100 is one of the largest monomeric proteins known with a calculated molecular weight of 512937. Heparin binds to the LDL surface by interacting with positively charged amino acid residues of apoB-100, forming soluble complexes in the absence of divalent metals and insoluble complexes in their presence. The purpose of this study was to isolate and characterize the heparin-binding domain(s) of apoB-100. Human plasma LDL were fragmented with cyanogen bromide (CNBr). After delipidation and reduction-carboxymethylation, the CNBr peptides were fractionated by sequential chromatography on DEAE-Sephacel, Mono S, and high reactive heparin (HRH) AffiGel-10; HRH was purified by chromatography of crude bovine lung heparin on LDL AffiGel-10. Heparin-binding peptides were further purified by reverse-phase high-performance liquid chromatography. Heparin-binding activity was monitored by a dot-blot assay with 125I-HRH. The amino-terminal sequences of four CNBr heparin-binding peptides (CNBr-I-IV) were determined. CNBr-I-IV correspond to residues 2016-2151, 3109-3240, 3308-3394, and 3570-3719, respectively, of the amino acid sequence of apoB-100. Each CNBr peptide contains a domain(s) of basic amino acid residues which we suggest accounts for their heparin-binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake by mouse peritoneal macrophages of large cholesteryl ester-rich particles isolated from human atherosclerotic lesions

We have previously shown that a lipoprotein fraction consisting of large cholesteryl ester-rich particles can be isolated from homogenates of human aortic plaques by gel exclusion chromatography. This fraction was recognized by a high-affinity binding site on mouse peritoneal macrophages (MPM) resulting in unregulated uptake, stimulation of cholesterol esterification, and massive accumulation of cholesteryl esters. In this report we have further characterized such a fraction, designated lipid-protein complex (LP), which can be isolated from the void volume fraction of a Bio-Gel A-150m column following chromatography of plaque extracts. LP possessed a mean cholesterol-to-protein ratio of 2.3; it was heterogeneous in size and structure as observed by electron microscopy after negative staining, and it stimulated cholesterol esterification in MPM in a linear fashion over a 48-hr time interval, suggesting that the binding site on MPM recognizing LP was not down-regulated by intracellular cholesterol content. This uptake resulted in the presence of oil red O-positive intracellular droplets and numerous vacuoles containing electron-dense structures, whereas MPM incubated without lipoprotein showed few vacuoles or lipid droplets. Using SDS-PAGE and immunoblot and dot-blot techniques, we found that the major proteins associated with LP were albumin and fibronectin, whereas apoB and apoE were present in lower amounts. These proteins may be responsible for opsonization of LP, making it recognizable to receptors on MPM and facilitating LP uptake by MPM. LP isolated from tissue extracts without homogenization had the same structural and functional characteristics, suggesting that homogenization per se was not responsible for creating a particle that was recognized by MPM. However, homogenization yielded two to three times more LP. MPM uptake of LP derived from lysed foam cells may represent one of the mechanisms by which fatty streak lesions may grow to larger atherosclerotic lesions.

Orientational order of cholesteryl oleate in low-density lipoprotein observed by 2H-NMR

Cholesteryl oleate, selectively deuterated at various positions along the acyl chain, has been incorporated into fresh human serum low-density lipoprotein (LDL2). Temperature-dependent 2H-NMR spectra were recorded between 15 and 45 degrees C. For deuterons at C-2' and C-5' of the acyl chain, two 2H-NMR spectral components, a broad and a narrow signal, are observed. This is interpreted as reflecting the coexistence of two cholesteryl ester regions in the LDL2 core which possess different degrees of order. The C-2H bond order parameters, SCD, are approx. 0.12-0.20 for the more ordered region and approx. 0.04-0.06 for the less ordered region. Longitudinal relaxation times, T1, of deuterated cholesteryl oleate are found to increase between C-8' and the terminal -C2H3 group, which is consistent with an increased rate of chain motion toward the free ends of the ester acyl chains.

Lipoproteins containing apo B extracted from human aortas. Structure and function

We have isolated, by anti-LDL affinity chromatography, apo B-containing lipoproteins from homogenates of atherosclerotic plaques excised from the human aorta. This fraction, called A-LP, has similarities with plasma LDL, such as having similar size and relative lipid composition, along with containing apo B. However, the fraction also contains some particles larger than LDL, it is more electronegative than LDL, the relative protein content is less than in LDL, and its apo B is highly degraded. A-LP is recognized by a high affinity binding site on mouse peritoneal macrophages (MPM), as suggested by dose-response curves of stimulation of cholesterol esterification. The interaction is inhibited by negatively-charged carbohydrates such as fucoidin, but excess A-LP did not inhibit the degradation of labeled acetyl-LDL by MPM, suggesting that the binding site recognizing A-LP may not be the scavenger receptor. Finally, stimulation of cholesterol esterification by A-LP in MPM is unregulated over a 48 hr time interval, leading to massive accumulations of cholesteryl esters and a transition of these MPM to a morphology characteristic of foam cells. It is possible that when monocytes enter the arterial intima at specific sites and become tissue macrophages, they internalize A-LP in an unregulated fashion. This, in turn, would make the monocyte-macrophage lipid-laden, and could explain the etiology of foam cells in fatty streak lesions. The modification in A-LP relative to P-LDL responsible for the enhanced recognition still needs to be elucidated.

Reduction in tissue LDL accumulation during coronary artery regression in cynomolgus macaques

The aim of this study was to determine whether the amount of LDL that had accumulated in the coronary arteries of cynomolgus monkeys during an 18-month period on a hypercholesterolemic (H) diet was reduced during subsequent periods of 6 and 12 months on a normolipemic (N) diet. This was performed by assessing the accumulation of LDL in the left anterior descending (LAD) branch of the left coronary artery within an area 4 mm from its origin, since this region contained the largest lesions in the LAD. LDL accumulation was estimated by measuring the percent cross-sectional area of artery occupied by reaction product depicting apo B by an immunoperoxidase procedure. The following reduction in mean (+/- SD) percent cross-sectional area occupied by reaction product was found in 8 animals on the progression diet (group I), 7 animals on the 6-month regression diet (group II), and 9 animals on the 12-month regression diet (group III), respectively: 21.7 +/- 4.7, 6.9 +/- 5.5, and 2.3 +/- 1.3. Differences between group I and either group II or III were statistically significant (using the Wilcoxon signed-ranks test). In group I, LDL was localized primarily in the necrotic core and around pools of foam cells. In groups II and III fewer foam cells and smaller pools of extracellular debris were qualitatively evident, and LDL was localized closer to the lumen and along collagen fibers. These results suggest that lowering of the plasma LDL level following termination of a hypercholesterolemic diet also induces a decrease in the LDL content in coronary artery lesions, even without significant reductions in lesion size, and that this decrease might be responsible for the decrease in foam cells.

Putative role of cholesteryl ester transfer protein in removal of cholesteryl ester from vascular interstitium, studied in a model system in cell culture

A model system to study the putative role of cholesteryl ester transfer protein in the egress of interstitial cholesteryl ester is described. Confluent cultures of bovine aortic smooth muscle cells were labeled for 24 h with [3H]cholesteryl linoleyl ether and [14C]cholesteryl linoleate by incubation with bovine milk lipoprotein lipase. This method of labeling results in the transfer of cholesteryl linoleyl ether and cholesteryl ester to three compartments: a trypsin-releasable, trypsin-resistant and catabolic compartment (Stein, O., Halperin, G., Leitersdorf, E., Olivecrona, T. and Stein, Y. (1984) Biochim. Biophys. Acta 795, 47-59). The efflux of labeled cholesteryl linoleyl ether and cholesteryl ester from the extracellular and cell-surface related compartments into a serum-free culture medium containing 1% bovine serum albumin was studied during 24 h of postincubation. The efflux was expressed as a percentage of pulse value, i.e., radioactivity retained by the cell culture at the end of the labeling period. The efflux of [3H]cholesteryl linoleyl ether, [14C]cholesteryl ester and 14C-labeled free cholesterol (formed by cellular hydrolysis of cholesterol ester) into the culture medium with 1% bovine serum albumin was about 5% of the pulse value. Addition of human lipoprotein-deficient serum resulted in a 3-10-fold increase in the efflux of [3H]cholesteryl linoleyl ether and [14C]cholesteryl ester, but did not change markedly the efflux of 14C-labeled free cholesterol. Rat lipoprotein-deficient serum which does not contain cholesteryl ester transfer protein did not increase the efflux of [3H]cholesteryl linoleyl ether or [14C]cholesteryl ester. The rate of cholesteryl ester efflux in the presence of human lipoprotein-deficient serum was linear for about 6 h and increased further up to 24 h. Addition of Intralipid to medium containing human lipoprotein-deficient serum further enhanced the efflux of [3H]cholesteryl linoleyl ether and, to a lesser extent, that of cholesteryl ester. A similar effect was observed also by addition of rat VLDL to medium containing human lipoprotein-deficient serum. Inhibition of cholesteryl linoleyl ether and cholesteryl ester efflux and marked enhancement of free cholesterol efflux occurred when rat HDL was added to medium containing human lipoprotein-deficient serum, while human HDL was only slightly inhibitory. The results obtained with human lipoprotein-deficient serum were reproduced with partially purified cholesteryl ester transfer protein. Using the partially purified cholesteryl ester transfer protein, the efflux of cholesteryl linoleate was compared to that of cholesteryl oleate and was found to be the same.

Role of the low density lipoprotein receptor in penetration of low density lipoprotein into rabbit aortic wall

The present study was designed to determine whether binding of low density lipoprotein (LDL) to endothelial LDL receptors contributes significantly to the penetration of LDL into the normal rabbit aorta. Initial flux rate was used as a measure of uptake of LDL. Reductive methylation of LDL is known to block its recognition by the LDL receptor. Therefore, the difference in flux rates of native LDL and reductively methylated LDL (methyl-LDL) was assumed to represent the receptor-dependent uptake. Native LDL and methyl-LDL were labeled with different isotopes (125I or 131I) and both were injected simultaneously into the same rabbit. After 30 to 60 minutes, trichloroacetic acid-precipitable counts were determined in aortic specimens. The initial flux rates, expressed as plasma clearance (nl/g/hr), were 1787 for native LDL and 1924 for methyl-LDL. The difference was not significant, which suggests that the flux of LDL into the aorta is not significantly dependent upon, or regulated by, endothelial LDL receptors, but is mediated by other mechanisms.

Increase of electrophoretic mobility and of content of soluble proteins of human plasma beta-lipoproteins by incubation of plasma in vitro

Incubation of human plasma at 37 degrees C was shown to induce an increase in the electrophoretic mobility of beta-lipoproteins. beta-Lipoproteins were isolated by density gradient ultracentrifugation after such incubation and their composition was analysed. Incubation decreased the content of free cholesterol and increased that of soluble apolipoproteins. The soluble peptides appearing in LDL upon incubation showed 3 major and 2 minor bands upon polyacrylamide disc gel electrophoresis. Two of the major bands corresponded to apolipoprotein C-III-1 and C-III-2 and one of the minor to apolipoprotein C-II. The addition of an inhibitor to lecithin:cholesterol acyltransferase (LCAT) abolished the increase in electrophoretic mobility and to a large extent diminished the other reported effects of incubation. The hypothesis is put forward that during incubation of plasma at 37 degrees C the free cholesterol in the surface of LDL is removed through the action of LCAT, lipid transferases and exchange processes and is replaced in the surface shell by peptides which cause the change in electrophoretic mobility.

Measurement in vivo of irreversible degradation of low density lipoprotein in the rabbit aorta. Predominance of intimal degradation

The development of a highly sensitive method for assessing, tissue by tissue, the rates of irreversible protein degradation in vivo has allowed us to quantify low density lipoprotein degradation in the normal rabbit aorta and to localize it. The method depends upon the fact that tyramine-cellobiose, like sucrose used in previous studies, can be covalently attached to proteins, enter cells with them, and then remains trapped within the cell after the remainder of the protein molecule has been degraded. Rabbit LDL (d = 1.02 to d = 1.06 g/ml) was labeled with 125I-tyramine-cellobiose and injected into rabbits. Aortic 125I content 24 hours later served as a cumulative measure of degraded LDL (after appropriate corrections for any intact, nondegraded LDL present). Calculated aortic degradation of LDL averaged 9.4 X 10(-3) percent of the plasma pool per g aortic wet weight per day (n = 6). Intimal cells, obtained by gentle swabbing, accounted for fully 40% of total aortic degradation even though the intima represented less than 5% of the aortic mass. Autoradiography confirmed the high concentration of label in the intima. Degradation of unmodified and reductively methylated LDL were compared. The fractional rate of degradation of methylated LDL by the intima was 50% to 60% of that for native LDL, indicating that 40% to 50% of LDL degradation in the intima, predominantly endothelial cells, is mediated by LDL receptors.

Apoprotein B quantification in rhesus and cynomolgus monkey atherosclerotic lesions

The concept that much of the cholesterol deposition in atherosclerotic plaque development is provided by ingress of blood-derived apo B-rich lipoproteins into the arterial intima is given support by the study of arterial apo B accumulation. To compare the arterial wall level of immunoreactive apo B during the progression of diet-induced atherosclerosis in two widely used animal models of atherosclerosis, rhesus and cynomolgus monkeys were fed an atherogenic diet for 4, 8, and 12 months and their abdominal aortas quantitated for apo B. Apo B was extracted from aortic intima-media homogenates in two forms: Tris-buffer extractable or 'loosely bounds' and detergent (Triton X-100) extractable or 'tightly bound'. The aortic extracts were quantitated for apo B by radial immunodiffusion, using goat anti-rhesus apo B along with serum LDL standards of the appropriate species diluted in the two extract solutions. The control monkeys' aortas contained only buffer-extractable apo B. The atherosclerotic aortas of both species of monkeys progressively increased their levels of loosely bound and tightly bound apo B through 4, 8, and 12 months of atherogenic diet feeding, with the 8- and 12-month cynomolgus aortas containing much larger amounts of apo B than the rhesus aortas. These differences in aortic apo B content could be accounted for by the greater rate at which the cynomolgus atherosclerotic lesions developed at the later time points. When the total lesion apo B levels were correlated with representative morphometrically-quantitated histopathologic sections of the homogenized aortas, a highly significant correlation was seen between the total aortic apo B values and both the absolute area of the intimal lesions and the total area of oil red O stainable lipid in the lesions (P less than 0.001). These data indicate that as atherosclerotic lesions become larger and richer in lipid with progression of the disease, the amount of apo B-associated lipoproteins which are deposited unmetabolized in the lesions increases. These lipoproteins are increased in both the tightly bound and loosely bound forms.

Atherosclerosis: pathology, pathogenesis, and role of risk factors

Atherosclerosis is a complex disease that represents the end product of the interaction of many different causative agents. Those that originate external to the arterial wall usually are called primary risk factors. Many other influences, the secondary risk factors, modulate the primary factors. The penetrance of the secondary factors is variable. They can have a major effect in some people but not in others. The idea of risk factor is important because it provides the conceptual framework upon which to build an intervention program for prevention of atherosclerosis. The development of atherosclerosis can be viewed as a two-step process (Table 2). The first is injury to the arterial wall. The second is response to injury. The primary risk factors can be regarded as the injurious agents. Examples are factors causing endothelial damage, influx of plasma lipoproteins, toxic products of smoking, hemodynamic injury of hypertension, and perhaps microvascular injury from diabetes mellitus. The response to injury represents typical pathologic changes--proliferation of smooth muscle cells, mononuclear infiltration, phagocytosis of products of injury, secretion of connective tissue elements, neovascularization, and necrosis. Regulation of these latter processes is poorly understood and is a worthy subject for future research. Modulation of the primary injurious factors through alteration of secondary risk factors is currently the only significant approach to prevention of atherosclerosis. Future investigation may provide more direct ways to prevent or retard atherogenesis, either by more effective modification of primary factors or by reducing the magnitude of response to these factors.