Metabolic Cooperation between Vascular Endothelial and Smooth Muscle Cells in Co-culture: Changes in Low Density Lipoprotein Metabolism Cells

A microcarrier co-culture system for aortic endothelial cells and smooth muscle cells (SMCs) was developed as a model for metabolic interactions between cells of the vessel wall. Low density lipoprotein (LDL) metabolism in SMCs was significantly influenced by coculture with endothelium. The numbers of high affinity receptors for LDL was increased more than twofold (range, 2.1-5.6), with concomitant increases in LDL receptor-mediated endocytosis and degradation. These effects reached a plateau at an endothelial celI/SMC ratio of 1. Kinetic analysis of the endocytic pathway for LDL in SMCs indicated that, in co-culture with endothelium, there was no alteration in the binding affinity of LDL to its receptors but that the internalization rate constant declined and the rate constant for degradation increased. This analysis suggested that the formation and migration of endocytic vesicles was the rate-limiting step of enhanced LDL metabolism under co-culture conditions. Two mechanisms by which endothelial cells influenced smooth muscle LDL metabolism were identified. First, mitogen(s) derived from endothelial cells stimulated entry of SMCs into the growth cycle, and the changes in LDL metabolism occurred as a consequence of G1-S transition. Second, SMC lipoprotein metabolism was stimulated in the absence of mitogens by a low molecular weight (<3,500) factor or factors. Co-culture was a required condition for the latter effect, suggesting that the mediator(s) may be unstable or that cell-cell communication was necessary for expression. These results (a) demonstrate that vascular cell interactions can modify LDL metabolism in SMCs, (b) provide some insights into the mechanisms responsible, and (c) identify co-culture as an experimental approach appropriate to certain aspects of vascular cell biology. Vascular endothelial cells and smooth muscle cells (SMCs) ~ co-exist in close apposition to each other in all blood vessels except capillaries. Investigations of the metabolic interactions that may occur between these cells are essential to an understanding of vascular homeostasis and the pathogenesis of vascular diseases such as atherosclerosis. A number of recent studies have drawn attention to the importance of vascular cell interactions. Pharmacological studies of endothelial cell~.4bbreviations used in this paper: ECCM, endothelial cell-conditioned medium; EDGF, endothelial cell-derived growth factor; PDS, plasma-derived serum; SMC, smooth muscle cell. SMC interactions by Furchgott and Zawadzki have shown that endothelial cells are essential for the relaxation of arterial SMCs by acetylcholine in isolated blood vessel preparations (1). In tissue culture, Hajjar et al. (2) have measured changes in the activities of intracellular enzymes of cholesterol metabolism in SMCs when they were overlaid with a monolayer of endothelial cells. In the area of cellular growth control, vascular cell interactions have been investigated primarily by the use ofceU-conditioned media (3-5) or by co-cultures in which endothelium and SMCs were plated together (3, 6, 7). In attempts to dissect such interactions, it is necessary to separate effects that may be mediated via cell-cell contact THE JOURNAL OF CELL BIoLoGY VOLUME 101 SEPtemBEr 1985 871-879 8 7 1 © The Rockefeller University Press 0021-9525/8510910871109 $1.00 on A ril 3, 2017 D ow nladed fom Published September 1, 1985