Kifunensine inhibits glycoprotein processing and the function of the modified LDL receptor in endothelial cells

Kifunensine is an alkaloid that is produced by the actinomycete Kitasatosporia kifunense and resembles the cyclic oxamide derivative of 1-aminodeoxymannojirimycin in structure. We previously showed that this compound was a potent inhibitor of the purified glycoprotein processing enzyme, mannosidase I, and caused an almost complete inhibition in the formation of complex types of oligosaccharides with the concurrent accumulation of N-linked oligosaccharides having Man₉(GlcNAc)₂ structures in influenza virus-infected Madin Darby canine kidney cells. Kifunensine, at concentrations of 1 μg/ml or higher in the culture medium, caused an almost complete inhibition in the formation of complex types of oligosaccharides by human skin fibroblasts or aortic endothelial cells, with the resulting accumulation of Man₉(GlcNAc)₂ oligosaccharides on the cell surface N-linked glycoproteins, and more specifically on the scavenger-LDL receptor. When endothelial cells were grown in the presence of 1 μg/ml of kifunensine, there was a 75% inhibition in the ability of these cells to degrade ¹²⁵I-labeled acetyl-LDL, but this inhibitor appeared to have little or no effect on the ability of either endothelial cells or fibroblasts to degrade ¹²⁵I-labeled LDL, even at kifunensine concentrations of 10 μg/ml. Kifunensine also decreased the binding of the labeled acetyl-LDL by the scavenger receptor of the endothelial cells, but the amount of this inhibition relative to controls was significantly less than that of the degradation, suggesting that kifunensine affects two different steps of acetyl-LDL metabolism in these cells. Endothelial cells grown in the presence of 10 μg/ml of kifunensine had only half the activity of the lysosomal enzymes, β-hexosaminidase, and proteases, as did control cells, although kifunensine did not affect [³H]leucine incorporation into protein. Thus, kifunensine apparently affects the activity of (some) lysosomal enzymes in an as yet undefined manner.