In this article we have reviewed the evidence that implicates the organization and incorporation of mural thrombi as a significant component of atherosclerotic plaque growth in man. It has been emphasized that there is little or no evidence for a pathogenic role for thrombosis in plaque initiation or for the development of fatty streaks. We have suggested that the rapidly progressive category of atherosclerosis in man, as described by DeBakey, may well reflect a heightened propensity for mural or occlusive thrombosis in these patients. A broad spectrum of experimental studies examining the role of thrombosis in atherogenesis has been critically reviewed. These studies have established that experimental thrombi can become transformed into arterial fibrofatty plaques having many of the morphologic features of atherosclerosis. We have provided evidence, however, that the evolution of thrombi to fibrofatty lesions is dependent on the initial composition of the thrombi and that thrombi with a paucity of platelets and consisting predominantly of fibrin result only in fibrous intimal thickenings. The presence of hypercholesterolemia has been shown to influence the transformation of experimental thrombi. In particular, it slows the rate of thrombolysis, enhances the lipid content of the fibrofatty plaques, increases the numbers of macrophage-derived foam cells, and the frequency and extent of lesion calcification. Detailed lipid compositional studies of organizing thrombi in normolipidemic animals have shown that their lipid composition does not evolve toward the profile characteristic of atherosclerotic lesions and that the macrophage uptake of interstitial lipoproteins is probably a necessary component for the full biochemical development of the lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

This article has addressed the roles of the monocyte-macrophage in atherogenesis and factors influencing monocyte recruitment to the intima. The diversity of the secretory products of the macrophage and their putative participatory roles in pathogenesis have been reviewed and discussed. Additionally, we have presented summary data on the monocyte chemoattractant peptide SMC-CF and on the differentiation of the monocyte-derived foam cell. Discussion has centered on the concept of atherosclerosis as an inflammatory process.

Castanospermine, a plant alkaloid that inhibits the glycoprotein processing enzyme glucosidase I, has been used to inhibit N-linked oligosaccharide modification, resulting in the production of glycoproteins having Glc3Man7-9(GlcNAc)2 oligosaccharides. This alkaloid caused a significant inhibition of LDL endocytosis in cultured primate smooth muscle cells and human skin fibroblasts. At an optimum concentration of 250 micrograms/mL, castanospermine caused a 40% decrease in cell surface receptor-mediated LDL binding at 4 degrees C, with no apparent change in affinity. Further, the inhibitor had no direct effect on LDL metabolism. This inhibition of LDL receptor expression and function occurred only when the drug was present during de novo receptor synthesis, i.e., during up-regulation. Although the number of cell surface LDL receptors was significantly reduced in the presence of castanospermine, the total number of receptors in the cell was only slightly reduced, indicating that castanospermine induced a redistribution rather than a reduction in the number of receptors. Similarly, subcellular fractionation studies confirmed that castanospermine treatment of fibroblasts results in an altered distribution of receptor activity compared with controls. These findings are consistent with the conclusion that the decrease in specific LDL binding to cells grown in the presence of castanospermine is due to intracellular redistribution of the LDL receptor so that more receptor remains in internal compartments as a result of a diminished rate of transport.

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This synopsis emphasizes the inappropriateness of a "single stimulus-single response" approach in understanding the response of the arterial wall to injury and repair. The outcome of any injurious stimulus is a series of interactive cascades among the endogenousand exogenous cellular and non-cellular components of the arterial wall, and the cellular and non-cellular elements of the blood. Both genetic and hemodynamic factors can further influencethis response. The more prominent of the cellular and non-cellular components have beendiscussed.These include: I) the vascularendothelium, its dynamic interaction with macromolecules and formed elements of the blood, its role in the transport of plasma proteins, its influenceon the function of arterial smooth muscle cellsand the recruitment of blood-born monocytes; 2) the arterial smooth muscle cell, its role in the vasomotor function of the artery wall, arterial repair and reconstruction, metabolism of lipids and the secretion of cytokines regulating monocyte recruitment; 3) the mononuclear phagocyte, its role in arterial debridement, metabolism of modified LDLs, a precursor of the cholesteryl ester-rich foam cell, and the secretion of neutral hydrolases, bioactive lipids and cytokines; 4) lymphocytes, as mediators of the inflammatoryresponseand possibleautoimmune reactions;5)platelets, their rolesin hemostasis,thrombosis, atherogenesis, and the repair processand 6) plasma LDLs, their oxidative modification by cells of the vessel walland their roles in the injury process.The interactive processes among arterial and circulatingcomponents in both injury and repair is emphasized.

The experiments reported in this paper were undertaken to explore the interaction of tritiated H2DIDS (4,4'-diisothiocyano-1,2,diphenyl ethane-2,2'-disulfonic acid) with Ehrlich ascites tumor cells. Addition of (3H)H2DIDS to tumor cell suspension at 21 degrees C, pH 7.3, resulted in: (i) rapid reversible binding which increased with time and (ii) inhibition of sulfate transport. Tightly bound H2DIDS i.e., reagent not removed by cell washing, also increased with time. Binding of 0.02 nmol H2DIDS/mg dry mass or less did not affect sulfate transport, but, at greater than 0.02 nmol and up to 0.15 nmol the relationship between tight binding and inhibition of transport is linear. The fact that H2DIDS could bind to the cell and yet not affect anion transport suggests that binding sites exist unrelated to those concerned with the regulation of anion permeability. Support for this is the observation that H2DIDS is spontaneously released from cells even after extensive washings by a temperature-sensitive process. The most important source of released H2DIDS is the cell surface coat which labels rapidly (within 1 min) and is then spontaneously released into the medium. A second source is derived from H2DIDS that slowly entered the cells. Consequently, at least four modes of interaction exist between H2DIDS and ascites tumor cells. These include both reversible and irreversible binding to membrane components which regulate anion permeability, irreversible binding to cell surface proteins or glycocalyx, and finally incorporation of H2DIDS into the intracellular phase.

Sialic acids, occupying a terminal position in cell surface glycoconjugates, are major contributors to the net negative charge of the vascular endothelial cell surface. As integral membrane glycoproteins, LDL receptors also bear terminal sialic acid residues. Pretreatment of near-confluent, cultured bovine aortic endothelial cells (BAEC) with neuraminidase (50 mU/ml, 30 min, 37 degrees C) stimulated a significant increase in receptor-mediated 125I-LDL internalization and degradation relative to PBS-treated control cells. Binding studies at 4 degrees C revealed an increased affinity of LDL receptor sites on neuraminidase-treated cells compared to control BAEC (6.9 vs. 16.2 nM/10(6) BAEC) without a change in receptor site number. This enhanced LDL endocytosis in neuraminidase-treated cells was dependent upon the enzymatic activity of the neuraminidase and the removal of sialic acid from the cell surface. Furthermore, enhanced endocytosis due to enzymatic alteration of the 125I-LDL molecules was excluded. In contrast to BAEC, neuraminidase pretreatment of LDL receptor-upregulated cultured normal human fibroblasts resulted in an inhibition of 125I-LDL binding, internalization, and degradation. Specifically, a significant inhibition in 125I-LDL internalization was observed at 1 hr after neuraminidase treatment, which was associated with a decrease in the number of cell surface LDL receptor sites. Like BAEC, neuraminidase pretreatment of human umbilical vein endothelial cells resulted in enhanced receptor-mediated 125I-LDL endocytosis. These results indicate that sialic acid associated with either adjacent endothelial cell surface molecules or the endothelial LDL receptor itself may modulate LDL receptor-mediated endocytosis and suggest that this regulatory mechanism may be of particular importance to endothelial cells.

Castanospermine (1,6,7,8-tetrahydroxyoctahydroindolizidine) is a plant alkaloid that inhibits alpha-glucosidases, including the glycoprotein processing glucosidase I. When endothelial cells were grown for 48 h, or longer, in the presence of this alkaloid, they produced scavenger receptors for modified low-density lipoproteins (LDL) that had mostly Glc3Man7-9(GlcNAc)2 structures rather than the usual complex types of oligosaccharides. Furthermore, growth in the presence of castanospermine resulted in a substantial inhibition in degradation of endocytosed 125I-acetylated LDL, as well as a dose-dependent inhibition of 125I-acetylated LDL binding to these cells. Scatchard analysis of binding curves indicated that the diminished binding was due to a decrease in the number of scavenger receptor molecules at the cell surface rather than to a change in the affinity of the receptors for their ligand. Since castanospermine-treated cells had the same total number of cellular receptor molecules as did controls cells, it seemed likely that castanospermine caused an alteration in receptor targeting, rather than an inhibition in receptor synthesis or a stimulation in receptor degradation. Density gradient fractionation of cell homogenates showed that castanospermine-treated cells did have a much greater percentage of scavenger LDL receptor molecules in the endoplasmic reticulum-Golgi fraction and fewer receptors in the plasma membrane fraction, whereas normal cells showed the opposite distribution.