A model for the extracellular release of PAF: the influence of plasma membrane phospholipid asymmetry.

Recent studies suggesting that cellular activation leads to enhanced transbilayer movement of phospholipids and loss of plasma membrane phospholipid asymmetry lead us to hypothesize that such events may govern the release of PAF, a potent, but variably release, lipid mediator synthesized by numerous inflammatory cells. To model these membrane events, we studied the transbilayer movement of PAF across the human erythrocyte and erythrocyte ghost plasma membrane, membranes with documented phospholipid asymmetry which can be deliberately manipulated. Utilizing albumin to extract outer leaflet PAF, transbilayer movement of PAF was shown to be significantly enhanced in erythrocytes and ghosts altered to lose membrane asymmetry when compared to movement in those with native membrane asymmetry. Verification of membrane changes was demonstrated using merocyanine 540 (MC540), a dye which preferentially stains loosely packed or hydrophobic membranes, and acceleration of the modified Russell's viper venom clotting assay by externalized anionic phospholipids. Utilizing the erythrocyte ghost loaded with PAF in either the outer or the inner leaflet, enhanced transbilayer movement to the opposite leaflet was seen to accompany loss of membrane asymmetry. Studies utilizing ghosts loaded with albumin intracellularly demonstrated that 'acceptor' molecules binding PAF further influence the disposition of PAF across the plasma membrane. Taken together, these findings suggest that the net release of PAF from activated inflammatory cells will depend on localization of PAF to the plasma membrane, transbilayer movement, which is facilitated by alteration of membrane phospholipid asymmetry, and removal from the membrane by extracellular and intracellular 'acceptor' molecules.