Drug-induced transmembrane lipid scrambling in erythrocytes and in liposomes requires the presence of polyanionic phospholipids

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Abstract
The asymmetric transmembrane distribution of phospholipids between the two bilayer halves of erythrocyte can be modified upon addition of cationic amphiphilic drugs, such as chlorpromazine or verapamil. We studied this phenomenon in erythrocytes and in lipid vesicles using spin-labelled analogues of the endogenous phospholipids. The extent of the rapid disappearance of the analogues from the erythrocyte outer leaflet depended on the concentration of the drug. Up to 40% of spin-labelled sphingomyelin moved to the inner erythrocyte leaflet in 10 min in the presence of 1.5 mm chlorpromazine. Verapamil or vinblastine gave similar results. On the other hand, the inside-outside movement of the aminophospholipid analogues was less evident, and did not exceed 10%. This apparent discrepancy between inward and outward movements could result from the formation of an endovesicle which is known to occur upon drug addition at high concentration. A fraction of lipids would be trapped in the intravesicular leaflet, corresponding to the cell outer leaflet, and be inaccessible both from the cytoplasm and the extracellular medium. In cells submitted to a metabolic depletion of cellular ATP the intensity of the scrambling induced by the amphipaths was drastically lowered. We attribute this effect to the important reduction of the membrane content in phosphatidylinositol-4,5-bisphosphate (PIP2). The involvement of the latter lipid in triggering scrambling was partly confirmed by experiments carried out with artificial membranes. Indeed, in large unilamellar vesicles PIP2 is required in order to obtain a rapid redistribution of phospholipids between the two leaflets upon addition of drugs. However, the extent of phospholipid redistribution was limited to 15-20%. This redistribution was also induced when the vesicle membrane contained di-anionic phospholipids (phosphatidylinositol-4-monophosphate or diphosphatidylglycerol), but did not occur when it contained mono-anionic phospholipids (phosphatidylserine or phosphatidylinositol). Some drugs such as methochlorpromazine, active in artificial membranes, were ineffective in erythrocyte membranes, probably because they could not cross the membrane and reach PIP2 molecules at the cytoplasmic leaflet.

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