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

Regulatory functions of the coronary endothelium

In this brief review three functions of the coronary endothelium are surveyed: (a) its barrier and exchange function, (b) the prevention of coagulation and platelet aggregation, and (c) its role in vasoregulation. Impairment of these functions can occur in ischemia, hypertension, arteriosclerosis and inflammation.

Measurement of endothelial permeability to 125I-low density lipoproteins in rabbit arteries by use of en face preparations

A procedure of en face quantitative autoradiography of the endothelium (Hautchen preparations) was developed to examine regional variations in 125I-low density lipoprotein (125I-LDL) permeability in the arterial wall in vivo. Endothelial preparations from fixed arterial tissue and calibration standards consisting of known concentrations of 125I-albumin were dipped in nuclear emulsion, exposed for 1-3 months, developed, and stained with hematoxylin. Digital image analysis was used to analyze dark-field images of autoradiographs. Background grain densities on cold endothelial preparations were 30-100% higher than on glass, but the variability in grain densities on the two different surfaces was similar. Regression slopes of grain density versus concentration for calibration standards were the same for sections placed on cold tissue or glass. For 1-5-microns-thick calibration standards of the same concentration, the grain density was proportional to the total amount of radioactivity per unit area. The results indicated that errors arising from nonuniformities in preparation thickness were minimal, and permeabilities and intimal concentrations could be determined. Rabbits were killed 10 minutes after injection of 125I-LDL, and endothelial preparations were made. For regions of uniformly low grain density in the rabbit aorta, the 125I-LDL permeability was 1.9 +/- 0.8 x 10(-8) cm/sec, and the effective diffusion coefficient was 5.4 +/- 3.1 x 10(-10) cm2/sec. Errors in the estimated permeability arising from nonuniformities in tissue thickness were the same as the reported experimental variability. Analysis of elevated regions of permeability suggested that 125I-LDL was binding to the extracellular matrix. Approximately 25% of the sites of elevated grain density were associated with mitotic endothelial cells, and such regions had higher permeabilities than sites associated with nonmitotic cells. Around intercostal arteries, sites of highest permeability were distal and lateral to the vessels and occurred where lesions first develop in hypercholesterolemic rabbits.

Autoradiographic analysis of the distribution of 125I-tyramine-cellobiose-LDL in atherosclerotic lesions of the WHHL rabbit

It is well established that plasma lipoproteins enter the artery wall and play a role in the atherogenic process. However, it is still unclear where within developing atherosclerotic lesions lipoproteins accumulate and which arterial cells participate in the metabolism of these lipoproteins. For this reason, light and electron microscopic autoradiograms were prepared from sections of lesioned aortas of Watanabe heritable hyperlipidemic (WHHL) rabbits 44 hours after injection of 125I-tyramine cellobiose-low density lipoprotein (TC-LDL). After uptake of 125I-TC-LDL and intracellular degradation of the LDL protein, the nondegradable TC ligand remains trapped and thus demarcates the cells participating in the degradation of LDL. Results of other studies indicate that 48 hours after injection into WHHL rabbits, about one half of the 125I label present in lesions represents accumulated degradation products while the remaining 125I label is present as intact 125I-TC-LDL. The distribution of autoradiographic silver grains was analyzed at low resolution in fatty streaks, transitional lesions, and advanced atheroma. In all cases, the majority of silver grains were associated with superficially located subendothelial macrophage-derived foam cells. In more advanced lesions, labeling was predominant in foam cells situated within the lateral margins of the lesions. Morphometric quantification of the distribution of silver grains in electron photomicrographs of fatty streaks from two young WHHL rabbits strongly supported the data obtained at the light microscopic level. In early fatty streaks from the aortic arch and the thoracic and abdominal aortas, subendothelial macrophage-derived foam cells contained a high proportion of the silver grains (40-60% of the total) and accounted for between 30% and 40% of the lesion volume. In contrast, smooth muscle cells in the lesions contained only 7-10% of the total silver grains and accounted for approximately 20% of the lesion volume. Endothelial cells contained the most silver grains on a per-unit-volume basis by occupying only 1-2% of the lesion volume. However, the endothelium contained less than 5% of the total grains in lesions. The remaining silver grains (25-45%) were associated with the extracellular matrix, which constituted between 40% and 50% of the lesion volume. These data indicate that in the WHHL rabbit, subendothelial macrophage-derived foam cells avidly accumulate and metabolize LDL despite having few functional LDL receptors.

Influx in vivo of low density, intermediate density, and very low density lipoproteins into aortic intimas of genetically hyperlipidemic rabbits. Roles of plasma concentrations, extent of aortic lesion, and lipoprotein particle size as determinants

To compare the atherogenic potential of low density lipoprotein (LDL), intermediate density lipoprotein (IDL), and very low density lipoprotein (VLDL) under conditions where plasma levels of these lipoproteins are elevated, the influx of cholesterol in these lipoproteins into the aortic intima was measured in vivo in genetically hyperlipidemic rabbits from the St. Thomas's Hospital strain, an animal model that shares many of the features of the human disorder familial combined hyperlipidemia. Univariate linear regression showed that the arterial influx of LDL cholesterol (n = 25), IDL cholesterol (n = 14), and VLDL cholesterol (n = 10) was positively and linearly associated with plasma concentrations of LDL cholesterol in the range 0.2-6.4 mmol/l, of IDL cholesterol in the range 0.1-7.0 mmol/l, and of VLDL cholesterol in the range 0.7-8.5 mmol/l, respectively, and also with the extent of lesions in the arterial intima in the range 0-100% of the surface area. Multiple linear regression suggested that the arterial influx of LDL, IDL, and VLDL cholesterol was linearly dependent on plasma concentration, independent of lesion size. Furthermore, it appeared that the arterial influx of the three lipoproteins was linearly dependent on the extent of the lesions, independent of lipoprotein concentration. When influx was normalized for plasma concentration (intimal clearance) and for lesion size (compared within the same aorta), the intimal clearance of the larger IDL and VLDL particles was 15-35% less than that of the smaller LDL particles. These findings suggest that the quantitatively most important mechanism for transfer of plasma lipoproteins into the arterial intima involves nonspecific molecular sieving and that at elevated plasma levels, IDL and VLDL share with LDL the potential for causing atherosclerosis.

Lipid accumulation in rabbit aortic intima 2 hours after bolus infusion of low density lipoprotein. A deep-etch and immunolocalization study of ultrarapidly frozen tissue

The intima from aortas of normal New Zealand White rabbits was studied 2 hours after infusion of 320 mg human low density lipoprotein (LDL), resulting in a plasma concentration of at least five times and maximally 20 times the values found in normal rabbit serum. The following techniques were used: 1) ultrarapid freezing without chemical fixation, followed by freeze-etching; 2) immunofluorescence microscopy; and 3) postembedding immunogold-labeling electron microscopy. In the latter two methods MB47, a murine monoclonal antibody against human apolipoprotein B, was used as the primary antibody. Control rabbits were infused with the same volume of buffer only. Rotary-shadowed replicas of samples from the LDL-injected rabbits showed the deposition of lipidlike particles in the subendothelial-intimal space that were the size of the injected LDL (23 nm). In focal areas of the intima, groups of 23-nm-sized lipidlike particles and larger lipidlike structures were found enmeshed in the extracellular matrix. Control replicas were essentially free of lipid deposition. Immunofluorescence microscopy of frozen aortic cross sections showed an overall increase in apolipoprotein B in the intima of the LDL-injected rabbits. The presence of apolipoprotein B in the intima was also confirmed by immunoelectron microscopy. These in vivo results show that clustering of LDL-sized particles occurs in the intima within 2 hours of excessive LDL uptake. It also demonstrates the interaction of these LDL-sized particles with the filaments of the extracellular matrix. The clustering of the LDL-sized particles supports the possibility that LDL self-aggregation may occur in vivo and that components of the extracellular matrix are involved in this process.

Formation of a lipid gradient across the human aortic wall during ageing and the development of atherosclerosis

The smooth muscle cell invasion and macrophage stimulation within the intima during prolonged exposure to high blood levels of cholesterol esters contribute to increased production of connective tissue matrix. The thickened intima in turn immobilising more LDL derived lipid from the plasma. With damage to the internal elastic lamellae, from essential hypertension, the absorbed lipid can move down a concentration gradient into the medial tissue. This model was supported by our laboratory finding of a lipid gradient across the aorta wall. The gradient commenced shortly after completion of body growth, when the transmedial gradient became detectable. The slope of the gradient progressively increased during ageing. Association of the lipid medial gradient with the degree of atherosclerotic involvement suggested that the gradient influenced the development of intimal lesions. Accumulation of lipid within the medial tissue may then reduce the inward lipid transfer rate from the intima, promoting increased intimal retention and cause the formation of atherosclerotic plaques from the fat saturated intima.

Extravascular distribution of low-density lipoprotein and dextran in a high-permeability state

Previous studies from this laboratory indicated that in the early phase of a high-permeability state, low-density lipoprotein (LDL) crosses the microvascular barrier by porous pathways. To determine the extravascular distribution of LDL (3,500,000 MW) and a smaller reference macromolecule [20,000 MW dextran (D20)] image processing techniques were employed, and extravascular accumulation of solutes from different microvessels was compared to quantitative fluorescence microscopy. Frog mesenteric venular microvessels (n = 8) were cannulated and perfused with fluorescent-labeled LDL and D20 and extravascular distribution of both solutes was imaged at control and after permeability was increased with the calcium ionophore ionomycin (5 microM). At the peak increase in apparent permeability (2-4 min after ionophore), the processed images of the microvessels demonstrated that the extravascular distribution of fluorescent-labeled solute was not uniform and that D20 accumulated outside the microvessel wall in some areas where LDL did not accumulate. The patterns of extravascular accumulation of LDL and D20 in a high-permeability state imply a distribution of effective microvascular pore sizes and/or a distribution of resistances to solute flow in the pore or in the tissue surrounding the microvessel.

Low density lipoprotein transport across a microvascular endothelial barrier after permeability is increased

We investigated the pathways for low density lipoprotein (LDL) transport across an endothelial barrier in individually perfused microvessels before and after an increase in permeability. The divalent cation ionophore A23187 (5 microM) was used to increase microvessel permeability. LDL permeability coefficients (PsLDL) were measured using quantitative fluorescence microscopy. In the control state, PsLDL measured after 10-23 minutes of accumulation of fluorescent-labeled LDL outside the microvessel wall was 4.8 x 10(-8) cm/sec. The transvascular vesicular exchange of approximately 50 vesicles/sec would account for the measured flux. The flux of LDL across the microvessel wall increased as much as 170-fold at the peak of the permeability increase (2-4 minutes after ionophore infusion). Permeability returned toward control values 10 minutes after ionophore infusion but remained elevated for as long as ionophore was present in the perfusate. The effective PsLDL was similar in magnitude to the Ps for fluorescent-labeled dextran (MW 20,000) when permeability was increased. To investigate the nature of pathways for LDL in the high-permeability state, PsLDL was measured at a series of microvessel pressures. LDL transport increased as microvessel pressure increased, demonstrating coupling of LDL flux to transvascular water flow. Solvent drag accounted for more than 95% of the increased flux of LDL in the period 2-10 minutes after permeability increased. Our results conform to the hypothesis that porous pathways between adjacent endothelial cells contribute to LDL transport across an endothelial barrier when permeability is increased.

Initiation of atherosclerotic lesions in cholesterol-fed rabbits. II. Selective retention of LDL vs. selective increases in LDL permeability in susceptible sites of arteries

We asked if the arterial sites most prone to early lesions in cholesterol-fed rabbits have higher permeabilities to low density lipoprotein (LDL) in normolipidemic rabbits or if these sites become more permeable shortly after the onset of cholesterol feeding. We also considered whether the focal increases in the concentration of LDL within the arterial wall in lesion-susceptible sites before fatty streak formation can be explained by increased arterial permeability to LDL or by other mechanisms such as decreased rates of LDL efflux or degradation. 125I-tyramine cellobiose-labeled LDL was injected 1 hour before death to determine the initial rate of LDL entry into lesion-prone and lesion-resistant sites of aorta as a measure of permeability. This was studied in normal rabbits and in rabbits fed cholesterol for 4, 8, or 16 days. Combining this permeability data with the tracer data described in the accompanying article, we fit a kinetic model to calculate the mass and mean residence time of intact LDL within the artery and the fractional rates of LDL degradation and efflux from the artery. In normal rabbits, the permeability of lesion-susceptible branch sites of the abdominal aorta was about four times that of the lesion-resistant, nonbranched areas. However, the permeability of the aortic arch, a susceptible site, was similar to that of the lesion-resistant descending thoracic aorta. Permeability to LDL did not increase in any aortic site during the 16 days of cholesterol feeding, even in sites with the largest increases in arterial LDL concentrations. Plasma LDL cholesterol concentration increased substantially and total LDL cholesterol delivery into the artery increased many fold. Since there was no differential change in permeability between susceptible and resistant sites, the increased entry of LDL did not explain the selective increases in arterial LDL concentration in susceptible sites. Kinetic analysis indicated that the fractional rate of degradation of the arterial LDL pool was lower in lesion-prone sites than in lesion-resistant sites in all animals. Fractional rates of efflux of arterial LDL decreased in lesion-susceptible branch sites of the abdominal aorta and were low in the lesion-susceptible aortic arch. These results suggest that the focal increases in LDL concentration observed in all lesion-susceptible sites of cholesterol-fed rabbits before fatty streak formation are due to localized differences in LDL retention and diminished fractional rates of LDL degradation, not to selectively increased permeability.(ABSTRACT TRUNCATED AT 400 WORDS)

Passage of low density lipoproteins through monolayers of human arterial endothelial cells. Effects of vasoactive substances in an in vitro model

The endothelium controls the influx of lipoproteins into the arterial wall, a process that may be disturbed in arteriosclerotic blood vessels. We have used an in vitro model to investigate the characteristics of the passage of low density lipoproteins (LDL) through monolayers of human arterial endothelial cells. Umbilical artery, aorta, or carotid artery endothelial cells were cultured on polycarbonate filters and formed a tight monolayer in which the cells were connected by tight junctions. Passage of 125I-LDL through these monolayers proceeded linearly over a 24-hour period. It was threefold lower through monolayers of aorta or carotid artery cells than through monolayers of umbilical artery cells. The LDL passage process did not show saturation with LDL concentrations up to 800 micrograms/ml LDL-protein (i.e., 1.6 nmol/ml apolipoprotein B) between 2 and 4 hours after addition. However, during the first 30 to 60 minutes after addition of high concentrations of LDL, a reduction of the passage rate of both LDL and peroxidase, resulting in an apparent saturation of the passage process, was observed. The passage rate of the negatively charged acetylated LDL was twofold lower than that of native LDL. Addition of histamine to the endothelial monolayer resulted in a large, but transient, increase in permeability paralleled by a decrease in electrical resistance. The effects of histamine were mediated via an H1 receptor. Thrombin and Ca++ ionophore also induced an increase in permeability of the monolayer, while bradykinin did not. The effects of histamine and thrombin were paralleled by a rapid and marked increase in cytoplasmatic Ca++ concentration of the endothelial cells, while bradykinin induced only a small increase. Although the cyclic adenosine 5'-monophosphate-elevating agent, forskolin, markedly decreased the basal rate of LDL passage through the endothelial cell monolayers, it did not change the relative increase in permeability induced by histamine. Thus, histamine induces small, but significant, increases in the permeability of tight endothelial cell monolayers.

Role of LDL receptors in the in vitro uptake and degradation of LDL in the media of rabbit thoracic aorta

The possible role of plasma low-density protein (LDL) receptors in the uptake and degradation of LDL in the whole arterial wall was investigated by comparison of the in vitro uptake of 125I-native LDL (nLDL) and 131I-methylated LDL (mLDL) by the media of deendothelialized rabbit thoracic aorta excised at in vivo length and pressurized to 70 mm Hg, taking the advantage that mLDL is not recognized by the LDL receptor. The distribution of the relative concentrations of nLDL (Cn) and mLDL (Cm) across the wall was obtained using a serial frozen sectioning technique. The aorta was incubated under three different conditions for varying periods of incubation in order to analyze separately the processes of binding, binding-internalization, and degradation. At 39 degrees C, in which binding-internalization and degradation occurred, Cn was significantly higher than Cm at each position across the media. The mean medial Cn/Cm ratio was 1.36 +/- 0.15 (n = 5) after 1 hour of incubation, and decreased to 1.23 +/- 0.22 (n = 7) after 2 hours of incubation and to 1.13 +/- 0.11 (n = 5) after 4 hours of incubation. At 4 degrees C, in which internalization and degradation were blocked, the Cn/Cm ratio reflected the surface nLDL binding alone; the Cn/Cm ratio was 1.47 +/- 0.20 (n = 5) after 4 hours of incubation, higher than the value obtained at 39 degrees C. To investigate whether degradation of nLDL occurred after receptor binding, the interstitial LDL was washed out by an LDL-free solution after 2-hour incubation at 39 degrees C. After 30 minutes of washout, the Cn/Cm ratio decreased to 1.06 +/- 0.20 (n = 5) in the inner media and was unchanged in the outer media. After 1 hour of washout, the ratio declined to 0.57 +/- 0.18 (n = 7) in the inner part of the media and increased progressively to 1 at the media-adventitia boundary. The Cn/Cm ratio, at 0.67 +/- 0.12 (n = 5), was practically constant throughout the media after 2 hours of washout. The nLDL degradation rate across the media was obtained from the comparison of nLDL and mLDL before and after the washout. A steep decreasing gradient in nLDL degradation rate was observed from the luminal to the external surface. The mean medial nLDL degradation rate value was 9.6 +/- 4.5 microliters/cm3 wet tissue/hr. We concluded that functional LDL receptors participate in the uptake and degradation of LDL in the whole aorta.

Comparison of arterial intimal clearances of LDL from diabetic and nondiabetic cholesterol-fed rabbits. Differences in intimal clearance explained by size differences

Arterial intimal clearances of low density lipoproteins (LDL) from diabetic cholesterol-fed rabbits (D-LDL) and LDL from nondiabetic cholesterol-fed rabbits (N-LDL) were compared. In six experiments, D-LDL and N-LDL were isolated from a diabetic and a nondiabetic rabbit, were iodinated with 125I and 131I, respectively, were mixed, and were reinjected into the same two rabbits as well as into a normal rabbit. Fractional catabolic rates for D-LDL and N-LDL in normal rabbits were 0.065 and 0.074 h-1 (p less than 0.05), respectively. For five of the six pairs of LDL, the D-LDL was smaller than N-LDL as determined by gel filtration. The arterial permeability to N-LDL, when normalized for differences in arterial cholesterol content, did not appear to differ between diabetic and nondiabetic rabbits. The relative arterial intimal clearance (D-LDL/N-LDL) in arteries from diabetic and nondiabetic rabbits was inversely related to the relative molecular weight (D-LDL/N-LDL). For example, when the molecular weight of D-LDL was as low as 60% of that of N-LDL (i.e., the diameter of D-LDL was reduced 16%), the intimal clearance of D-LDL was 40% larger than that of N-LDL. When, on the other hand, molecular weights and diameters of the two LDL were similar, the intimal clearance was also quite similar. These results suggest that arterial intimal clearance of LDL from diabetic and nondiabetic cholesterol-fed rabbits is comparable unless the two types of LDL have a different size.

Quantification in vivo of increased LDL content and rate of LDL degradation in normal rabbit aorta occurring at sites susceptible to early atherosclerotic lesions

While the exact mechanisms that initiate atherosclerotic lesions are unknown, considerable evidence supports a role for low density lipoprotein (LDL). We investigated whether in the normal rabbit, LDL metabolism in areas of aorta that are destined to become lesioned during cholesterol feeding differed from the metabolism in adjacent lesion-resistant aorta. These studies took advantage of the predictable pattern of early atherosclerotic lesions in the cholesterol-fed rabbit. Early lesions occur diffusely in the aortic arch and ascending aorta and distal to branch orifices in the abdominal aorta and the descending thoracic aorta. Arterial rates of irreversible degradation of LDL and concentrations of intact LDL were measured in susceptible and resistant sites with homologous doubly labeled LDL. LDL was labeled directly with 131I and with 125I-tyramine cellobiose. The latter label provides a highly sensitive means to determine the sites and rates of lipoprotein degradation in vivo. The arterial concentration of intact LDL in the lesion-prone aortic arch was 3.12 +/- 0.45 micrograms LDL cholesterol/g (n = 14), 3.6 +/- 0.69 times that in the relatively lesion-resistant descending thoracic aorta (p less than 0.001). The rate of LDL degradation in the aortic arch was 3.14 +/- 0.41 micrograms LDL cholesterol/g/day, 2.14 +/- 0.24 times that in the descending thoracic aorta (p less than 0.001). In the abdominal aorta, the LDL (per gram wet weight) concentration and degradation rate (per square centimeter surface area) at branch sites exceeded that at nonbranch sites by 88 +/- 11% (p less than 0.001) and by 61 +/- 8% (p less than 0.001), respectively. These data provide evidence that in the normal rabbit, which does not develop atherosclerotic lesions, focal elevations of arterial LDL degradation rate and concentrations of intact LDL occur at sites that first develop atherosclerotic lesions in the hypercholesterolemic animal. These differences in LDL metabolism may be linked causally to the propensity to develop atherosclerotic lesions at these sites.

Influx and cellular degradation of low density lipoproteins in rabbit aorta determined in an in vitro perfusion system

The accumulation of 125I-low density lipoprotein (LDL) into normal and atherosclerotic arterial tissue and cellular uptake in arterial cells were studied in an in vitro perfusion system for rabbit aorta. The accumulation of 125I-LDL in normal tissue could be fitted to an inverse exponential function with an initial influx rate of 1.39 nl/mg wet weight/hour and an equilibration volume of about 2% of the tissue volume. The influx rate into atherosclerotic plaques was about 10 times faster and the equilibration volume, 50 times higher. In atherosclerotic tissue there was a steep concentration gradient between the plaque and the underlying media. The accumulation of 125I-LDL in the media under plaque and in normal tissue adjacent to plaques was similar to that seen in normal tissue. For studies of cellular uptake of LDL a trace label, 125I-tyramine-cellobiose (TC), was used. Normal or atherosclerotic rabbit aorta was perfused in vitro with medium containing 125I-TC-LDL. After perfusion the tissue was digested and the cells were isolated by density gradient centrifugation. Two main cell fractions with characteristics of smooth muscle cells and foam cells, respectively, were obtained. A 70-fold higher uptake was seen in the foam cells. In conclusion, these studies suggest a higher influx rate into atherosclerotic plaques, as well as a high LDL concentration in the plaque, compared with normal tissue or underlying media. We suggest that most of the cellular uptake of LDL in the arterial wall is caused by the foam cells.

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)

Rabbit beta-migrating very low density lipoprotein increases endothelial macromolecular transport without altering electrical resistance

Rabbit aortic endothelial cells (RAEC) were grown on micropore filters in a new device. This system allowed in situ measurement of transendothelial electrical resistance (TEER). The monolayers demonstrated a TEER of 14 +/- 1 omega X cm2 at confluence. No difference was seen in the transport of low density lipoproteins (LDL) across endothelial cell monolayers obtained from normal or Watanabe heritable hyperlipidemic rabbits, indicating that the LDL receptor was not involved in the LDL transport. TEER was inversely correlated with 22Na transport (r2 = 0.93, P = less than 0.001) but not with 125I-LDL transport. The amount of LDL transported at 15 degrees C or across glutaraldehyde-fixed monolayers was half that of the controls at 37 degrees C. Preincubation of the monolayers with rabbit beta-migrating very low density lipoproteins (beta-VLDL) increased cholesterol content by 65%, and the transport of albumin and LDL doubled without a change in TEER. Removal of beta-VLDL from the culture medium resulted in the return of cellular cholesterol content and LDL transport to control values. We conclude that preincubation of RAEC with beta-VLDL resulted in an increased permeability to LDL and albumin, and that beta-VLDL may promote increased transendothelial transport of macromolecules in cholesterol-fed rabbits.

The role of lipoproteins and receptor-mediated endocytosis in the transport of bacterial lipopolysaccharide

The addition of bacterial lipopolysaccharide (LPS) from Escherichia coli 0111:B4 to human monocyte-macrophages cultured in serum results in suppression of scavenger receptor activity. The present studies were performed to examine if the effect on scavenger receptor activity was mediated by LPS alone or by LPS in association with lipoproteins. Radioiodinated LPS (125I-LPS) was added to human plasma in vitro and to normal and hyperlipidemic rabbit plasma in vitro and in vivo to determine the distribution of 125I-LPS among the lipoprotein classes. It was found that all lipoprotein classes bound LPS in direct proportion to their plasma cholesterol concentration. LPS alone was compared to LPS bound to low density lipoprotein (LDL), high density lipoprotein, or reductively-methylated LDL for their abilities to suppress scavenger receptor activity in monocyte-macrophages in lipoprotein-free serum. Only LPS bound to LDL (LPS-LDL) demonstrated an effect similar to that observed when LPS was added to cells in serum. Either unlabeled LDL or unlabeled LPS-LDL complexes competed with the uptake of 125I-LPS-LDL complexes, which appeared to proceed by receptor-mediated endocytosis. In contrast to the uptake of 125I-LDL, the uptake of 125I-LPS-LDL by cultured monocyte-macrophages was not followed by its hydrolysis and the release of its radioactive degradation products into the medium. The association of LPS with lipoproteins was very stable and appeared to be mediated by a lipid-lipid interaction. We hypothesize that LPS bound to lipoproteins may be transported into the artery wall and may initiate the atherosclerotic reaction.

Participation of the endothelium in the development of the atherosclerotic plaque

In the past decade, initiated by the response-to-injury hypothesis of Ross and Glomset, the endothelium has been implicated in atherogenesis but as a passive participant--more involved through its absence than its presence. The hypothesis stated that endothelial desquamation due to an undefined injury led to platelet adhesion to the exposed basement membrane, and infiltration of serum lipoproteins. The subsequent release from the platelet alpha-granule of a potent smooth muscle cell mitogen and chemoattractant--the platelet-derived growth factor (PDGF)--was postulated to cause the intimal proliferative response that is known to be important in atherosclerotic plaque development. Recent evidence from several laboratories indicates that the endothelium has the potential to play a more active role in plaque development than simply contributing to pathological sequelae resulting from the loss of the nonthrombogenic surface provided by the endothelium. First, the endothelial cell (EC) is the site of attachment, and possibly activation, of blood-borne monocytes which enter the vessel wall as an early event in experimental atherogenesis. We have obtained in vitro evidence that the expression of monocyte binding sites on the surface of EC is a regulatable process and that increased EC turnover and certain exogenous agents acting on EC cause increased monocyte adhesion. Similar events may be responsible for focal adhesion of monocytes to the endothelium in vivo following hypercholesterolemia. Secondly, EC in culture are capable of chemically modifying low density lipoprotein (LDL) by a free radical oxidation process that renders the LDL toxic to proliferating cells and recognizable to the scavenger receptor of monocyte-derived macrophages. Thus, by oxidation of LDL, the EC have the potential to play an active role both in the formation of lipid-laden foam cells and in the accumulation of necrotic tissue which are hallmarks of the atherosclerotic lesion. Thirdly, cultured EC have been recently shown to secrete multiple mitogens for cultured smooth muscle cells. One of these mitogens appears to be closely related, if not identical, to PDGF using the criteria of receptor binding and biochemical and immunological similarity. Production of growth factors by EC is a regulatable process that is stimulated by exogenous agents such as endotoxin and phorbol esters which cause severe injury to cultured EC. Such a regulatory mechanism may participate in the in vivo proliferation of vascular SMC during the atherosclerotic process.(ABSTRACT TRUNCATED AT 400 WORDS)