Transport Studies of Doxorubicin in Model Membranes Indicate a Difference in Passive Diffusion across and Binding at the Outer and Inner Leaflet of the Plasma Membrane
The kinetics of passive transport of the anticancer drug doxorubicin were analyzed in relation to membrane composition in large unilamellar vesicles in which DNA was enclosed. Special attention was paid to lipids that are typical for the inner and outer leaflet of the plasma membrane of mammalian cells: Phosphatidylethanolamine and anionic phosphatidylserine versus phosphatidylcholine, sphingomyelin, and cholesterol, respectively. The presence of anionic phospholipids results in a highly efficient incorporation of the drug into biological and model membranes [de Wolf, F. A., et al. (1993) Biochemistry 32, 6688-6695]. Therefore, the effect of drug binding on the amount of free, transportable drug was explicitly taken into account. However, even after correction for binding the permeability coefficient was about 35% lower in membranes containing 50 mol % of the anionic phosphatidylserine than in membranes consisting only of zwitterionic phospholipids (0.71-0.79 versus 1.18-1.25 microns s-1). This shows that drug binding and insertion also affect the intrinsic transport characteristics of the membranes. As compared to pure phosphatidylcholine, binding was not influenced by the incorporation of sphingomyelin or cholesterol, but equimolar amounts of sphingomyelin and cholesterol in phosphatidylcholine membranes decreased the rate of doxorubicin transport by 60% and 80%, respectively. The inhibitory effect of these two lipids is probably due to a closer packing of the membranes. In accordance, after the acyl chain order was decreased by adding the anaesthetic-like phenethyl alcohol (0.5% v/v), transport was stimulated more than 4-fold. The implications of our findings for the functioning and rate of drug pumping by the multidrug resistance-conferring P-glycoprotein in cancer cells are discussed.