Role of contact inhibition in the regulation of receptor-mediated uptake of low density lipoprotein in cultured vascular endothelial cells.

Abstract

Bovine vascular endothelial cells during logarithmic growth bind, internalize, and degrade low density lipoprotein (LDL) via a receptor-mediated pathway. However, contact-inhibited (confluent) monolayers bind but do not internalize LDL. This is in contrast to aortic smooth muscle cells or endothelial cells that have lost the property of contact inhibition. These cells internalize and degrade LDL at both high and low cell densities. The LDL receptors of smooth muscle and sparse endothelial cells down-regulate in response to LDL. In contrast, normal endothelial cells at confluency show little response. When contact inhibition in endothelial monolayers was locally released by wounding, and LDL was present, only cells released from contact inhibition accumulated LDL cholesterol. In smooth muscle cells under the same conditions, the entire culture interiorized lipid. It thus appears that in endothelial cells, unlike smooth muscle cells, contact inhibition is the major factor regulating cellular uptake of LDL cholesteryl ester. Reversal of contact inhibition by wounding provides a mechanism by which the endothelium could be the primary initiator of the atherosclerotic plaque.A number of recent'studies have identified a pathway in several cultured cell lines by which low density lipoprotein (LDL) is taken up into the cells via a specific receptor (1-3y. After internalization, the lipoprotein is catabolized in the lysosomes. The lipoprotein apoprotein is degraded to low molecular weight material and the cholesteryl ester content is hydrolyzed by lysosomal cholesterol esterase and re-esterified by the acylCoA:cholesterol acyl-transferase pathway (1). Most of the studies reported so far have been carried out with cultured fibroblasts and smooth muscle cells. These cells are exposed under physiological conditions to only low concentrations of plasma lipoproteins such as are present in the intercellular lymph (4 The vascular endothelium in vivo reveals a characteristic appearance as a single layer of highly flattened, contact-inhibited cells. The use of the peptide fibroblast growth factor (FGF) (5) has now permitted the long-term maintenance-of pure endothelial clones from a variety of sources, including the coronary bed and aortic arch of several~species (6). These cells retain the in vivo properties of endothelium and have been used in the present research to define the significance of the LDL receptor-mediated pathway in vascular endothelium.The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. MATERIALS AND METHODSPreparation of LDL and Lipoprotein-Deficient Serum. Human and bovine LDL (1.019 < density < 1.063 g/cm3), human very low density lipoprotein (VLDL) (density < 1.006 g/cm3), human high density lipoprotein (HDL) (1.063 < density < 1.21 g/cm3), and the homologous lipoprotein-deficient sera (LPDS) (density > 1.21 g/cm3) were obtained from...

Interaction of human endothelial cells with elevated glucose concentrations and native and glycosylated low density lipoproteins

“…This is in contrast with the cessation of LDL internalization observed in 221 established lines of bovine endothelial cells upon development of contact inhibition [11] and cannot be attributed to the lack of fibroblast growth factor in the culture medium, since Coetzee et al have reported similar results for human endothelial cells cultured in the presence of fibroblast growth factor [12]. Because our cultures were exposed to lipoprotein-deficient serum prior to testing, and may thus have increased their number of LDL receptors [3], our results may overestimate the magnitude of LDL receptor activity expressed by endothelial cells in vivo.…”

“…The inverse relationship between degree of confluency and rate of LDL degradation described in bovine aortic endothelium [11] was also demonstrated in human endothelial cells (Fig.l) (Fig. 1).…”

“…This was observed using glucose concentrations and duration of exposure reported to result in appreciable non-enzymatic glycosylation of various tissues [20,21] and proteins [22]. Thus, possible biochemical changes induced by protracted elevation of ambient glucose do not appear to affect the function of LDL receptors on endothelial cells nor to modify the characteristics of contact inhibition that mediate the decreased internalization of LDL in confluent monolayers of endothelial cells [11]. However, when exposure to high glucose had modified the LDL ligand, significant impairment of interaction with the receptor was observed, resulting in reduced cellular uptake and degradation of LDL.…”

“…We have investigated whether prolonged exposure of vascular endothelial cells to high glucose levels alters the function of specific membrane recognition sites, particularly high-affinity receptors for low-density lipoproteins [10,11]. In our experiments, human endothelial cells cultured in high glucose concentrations maintained an unaltered dose-response for LDL uptake and degradation and the same inverse relationship between the state of confluency and LDL degradation exhibited by control cells.…”

“…Yet it is the endothelium that is in contact with the blood and first participates in LDL metabolism. Endothelial cells take up and degrade LDL through both receptor-dependent and independent mechanisms [10][11][12]. We sought to determine whether protracted elevation of ambient glucose may alter these interactions.…”