INHIBITION OF SCAVENGER RECEPTOR-MEDIATED MODIFIED LOW-DENSITY-LIPOPROTEIN ENDOCYTOSIS IN CULTURED BOVINE AORTIC ENDOTHELIAL-CELLS BY THE GLYCOPROTEIN PROCESSING INHIBITOR CASTANOSPERMINE

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 castanospermi ne resulted in a substantial inhibition in degradation of endocytosed I-125-acetylated LDL, as well as a dose-dependent inhibition of I-125- acetylated LDL binding to these cells. Scatchard analysis of binding c urves indicated that the diminished binding was due to a decrease in t he number of scavenger receptor molecules at the cell surface rather t han to a change in the affinity of the receptors for their ligand. Sin ce castanospermine-treated cells had the same total number of cellular receptor molecules as did control cells, it seemed likely that castan ospermine caused an alteration in receptor targeting, rather than an i nhibition in receptor synthesis or a stimulation in receptor degradati on. Density gradient fractionation of cell homogenates showed that cas tanospermine-treated cells did have a much greater percentage of scave nger LDL receptor molecules in the endoplasmic reticulum-Golgi fractio n and fewer receptors in the plasma membrane fraction, whereas normal cells showed the opposite distribution.