LOW-DENSITY-LIPOPROTEIN UPTAKE BY AN ENDOTHELIAL-SMOOTH MUSCLE-CELL BILAYER

To study the interaction of endothelial and smooth muscle cells, and the means by which such interaction may affect lipid permeability of the arterial wall, cell bilayers were established by use of a transwell culture system. After confluent growth of both cell types had been achieved, iodine 125 bound to low-density lipoprotein (10 ng protein/ml) was added to the media of the upper well. After a 3-hour incubation period, the iodine 125-bound low-density lipoprotein content of the upper and lower media demonstrated an impedance to lipoprotein movement across the endothelial cell monolayer as compared to the bare porous polycarbonate filter of the transwell (p < 10(-6)). The presence of smooth muscle cells in the bottom well significantly enhanced the permeability of the endothelial cell layer (p < 10(-6)). This effect remained unchanged over a 9-day time course. Membrane binding and cellular uptake of low-density lipoprotein by endothelial cells was not altered by smooth muscle cells, indicating that this change in permeability could not be easily attributed to changes in receptor-mediated transport or transcytosis. Membrane binding (p < 0.02) and cellular uptake (p < 10(-6)) of low-density lipoprotein by smooth muscle cells in the bilayer, when adjusted for counts available in the smooth muscle cell media, were both reduced in the early incubation period as compared to isolated smooth muscle cells. The disproportionate reduction in uptake as compared to binding would suggest that this was not entirely a receptor-dependent process. Over time, the uptake of lipoprotein by smooth muscle cells alone declined, with no significant overall change in membrane binding. In contrast, smooth muscle cell uptake of low-density lipoprotein in the bilayer showed a progressive and sustained increase (p < 10(-5); analysis of variance [ANOVA]), which was associated with a less dramatic increase in membrane binding (p < 0.05). This finding might be due to a delayed alteration of the low-density lipoprotein molecule by the endothelial cell, which could enhance its uptake by smooth muscle cells. These studies indicate an interaction between endothelial cells and smooth muscle cells, which alter lipoprotein uptake by a cellular bilayer and which could potentially influence lipid accumulation and atherosclerotic plaque formation in the arterial wall.