INHIBITION OF SCAVENGER RECEPTOR-MEDIATED MODIFIED LOW-DENSITY-LIPOPROTEIN ENDOCYTOSIS IN CULTURED BOVINE AORTIC ENDOTHELIAL-CELLS BY THE GLYCOPROTEIN PROCESSING INHIBITOR CASTANOSPERMINE
Ea. Sprague et al., INHIBITION OF SCAVENGER RECEPTOR-MEDIATED MODIFIED LOW-DENSITY-LIPOPROTEIN ENDOCYTOSIS IN CULTURED BOVINE AORTIC ENDOTHELIAL-CELLS BY THE GLYCOPROTEIN PROCESSING INHIBITOR CASTANOSPERMINE, Biochemistry, 32(34), 1993, pp. 8888-8895
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.