Polymyxin B complexes with and cationizes low density lipoproteins. The cause of polymyxin B-induced enhancement of endocytotic catabolism of low density lipoproteins.

We previously reported that polymyxin B (PMB) enhances cellular catabolism of low density lipoproteins (LDLs) through a non-LDL receptor-mediated endocytotic pathway. These data were obtained mainly by using Hep G2 cells, a well differentiated human hepatoma cell line. In the current study, we explore the mechanisms of PMB-mediated endocytotic catabolism of LDL. We found that PMB enhanced LDL catabolism also in homozygous familial hypercholesterolemia fibroblasts, thereby establishing that PMB-mediated cellular catabolism of LDL does not involve LDL receptors. By using [14C]sucrose, and ligands for the asialoglycoprotein (ASGP) receptors, possibilities were excluded that PMB enhances cellular endocytosis of LDL, by inducing a general increase of cellular pinocytic activity or by causing endocytosis of LDL via the ASGP receptors in Hep G2 cells. We further show, by using polymyxin B coupled Sepharose 4B (PMB-Sepharose 4B) beads, that PMB binds to LDL to form a complex. This binding was tight, and changes in pH and salt concentrations had no significant effect on the binding, but unlabelled LDL competed with 125I-LDL to bind to PMB-Sepharose 4B. Urea and endotoxins decreased this binding, suggesting that PMB binds to LDL at least partially through hydrophobic interactions. Agarose gel electrophoresis of PMB-LDL indicates that PMB cationizes LDL. In conclusion, PMB binds to LDL to form a PMB-LDL complex presumably through interactions between lipid groups. This endows LDL with positive charges, which enhances LDL binding to negatively charged cell membranes, and such bound LDL is rapidly internalized through absorptive endocytosis.