Collagen-stimulated unidirectional translocation of cholesterol in human platelet membranes

When human platelets are stimulated with collagen or thrombin, the asymmetr ic distribution of membrane lipids is disrupted as phosphatidylserine and p hosphatidylethanolamine translocate from the inner monolayer to the outer m onolayer, Coincident with the stimulus-dependent rearrangement of membrane phospholipids is a rapid redistribution of cholesterol from the outer to th e inner membrane monolayer, This redistribution of cholesterol was observed when the stimulus was collagen or ADP, The data presented here show that e pinephrine stimulation does not promote cholesterol translocation but does potentiate collagen-promoted movement of cholesterol, To investigate the pr ocess of cholesterol translocation, experiments were performed to determine whether collagen stimulated reverse cholesterol movement; i.e. from the in ner to the outer monolayer, For this study, the fluorescent sterol cholesta trienol (C-3) was incorporated into platelet membranes by exchange from cho lesterol-containing phosphatidylcholine small unilamellar vesicles. C-3 was then removed selectively from the outer monolayer by treatment of the plat elets with bovine serum albumin (BSA). During the subsequent incubation of BSA-treated platelets, C-3 moved spontaneously into the outer from the inne r monolayer, This translocation had an apparent halftime of approximately 2 5 min and was unaltered by the presence of collagen. These results suggest that collagen treatment of platelets selectively facilitates the inward mov ement of the sterol, We have hypothesized that cholesterol translocation ma y be thermodynamically driven as a result of an unfavorable entropy, result ing in cholesterol translocation out of an environment becoming enriched in phosphatidylethanolamine. The unidirectional nature of collagen-promoted c holesterol movement from the phosphatidylethanolamine-rich outer monolayer is consistent with this interpretation.