Castanospermine inhibits the function of the low-density lipoprotein receptor

Abstract

Castanospermine, a plant alkaloid that inhibits the glycoprotein processing enzyme glucosidase I, has been used to inhibit N-linked oligosaccharide modification, resulting in the production of glycoproteins having Glc3Man7-9(GlcNAc)2 oligosaccharides. This alkaloid caused a significant inhibition of LDL endocytosis in cultured primate smooth muscle cells and human skin fibroblasts. At an optimum concentration of 250 micrograms/mL, castanospermine caused a 40% decrease in cell surface receptor-mediated LDL binding at 4 degrees C, with no apparent change in affinity. Further, the inhibitor had no direct effect on LDL metabolism. This inhibition of LDL receptor expression and function occurred only when the drug was present during de novo receptor synthesis, i.e., during up-regulation. Although the number of cell surface LDL receptors was significantly reduced in the presence of castanospermine, the total number of receptors in the cell was only slightly reduced, indicating that castanospermine induced a redistribution rather than a reduction in the number of receptors. Similarly, subcellular fractionation studies confirmed that castanospermine treatment of fibroblasts results in an altered distribution of receptor activity compared with controls. These findings are consistent with the conclusion that the decrease in specific LDL binding to cells grown in the presence of castanospermine is due to intracellular redistribution of the LDL receptor so that more receptor remains in internal compartments as a result of a diminished rate of transport.

Glycoprotein Processing Inhibitors

“…Cell fractionation studies indicated that DNJ caused an accumulation of a1 -antitrypsin in the ER, suggesting that the presence of glucose on the N-linked oligosaccharide might retard the transfer of the glycoprotein from the ER to the Golgi apparatus [66]. Similar results were obtained with regard to the synthesis and targeting of the low density lipoprotein receptor in smooth muscle cells grown in the presence of castanospermine [67]. and in the synthesis and targeting of the insulin receptor in the presence of DNJ or castanospermine [68].…”

Transient, lectin-like association of calreticulin with folding intermediates of cellular and viral glycoproteins.

“…They share sequence motifs, bind calcium, and serve as molecular chaperones. Whereas their precise role in the maturation of glycoproteins has yet to be fully elucidated, the effects of glucosidase inhibitors on protein maturation indicate that they are important for the efficient and timely folding, retention, and possibly oligomeric assembly of glycoproteins (Edwards et al, 1989;Pal et al, 1989;Bergeron et al, 1994;Hammond and Helenius, 1994a). It will be interesting to determine whether other proteins sharing sequence homology with these chaperones (CNE1 of Saccharomyces cerevisiae, RALl of Onchocerca volvulus, and p407 of Aplysia californica or the surface protein of adherent melanoma cells (White et al, 1994)), might share similar lectin properties.…”

Export of the High Affinity IgE Receptor From the Endoplasmic Reticulum Depends on a Glycosylation-Mediated Quality Control Mechanism

“…In the latter study, a 24-h incubation period resulted in a 50% reduction of surface receptors. Similarly, a CST-induced decrease (40%) in expression of the integral membrane low-density lipoprotein receptor has been reported and ascribed to a mechanism involving receptor redistribution (52).…”