AMINOPHOSPHOLIPID TRANSLOCASE ACTIVITY IN JEG-3 - A CHORIOCARCINOMA MODEL OF CYTOTROPHOBLAST DIFFERENTIATION

The plasma membrane is characterized by a non-symmetrical distribution of phospholipids; the outer monolayer of the plasma membrane consists primarily of phosphatidylcholine (PC), and the aminophospholipids, ph osphatidylserine (PS) and phosphatidylethanolamine (PE), preferentiall y reside in the inner monolayer. Asymmetry is maintained by a membrane associated ATP-dependent aminophospholipid translocase that preferent ially relocates PS and PE from the outer to the inner monolayer. Altho ugh in most cells the translocase minimizes expression of PS on the ou ter surface, differentiating trophoblasts express increasing levels of surface PS. One possible explanation of prolonged PS externalization is that trophoblasts lack an effective aminophospholipid translocase. To test this hypothesis, fluorescent PC and PS analogues, NBD-PC and N BD-PS, were introduced into the plasma membrane of a choriocarcinoma m odel of trophoblast, JEG-3 cells. After incubation, the fluorescent li pid remaining on the outer monolayer was removed by incubation with fe tal bovine serum. JEG-3 cells selectively translocated 80 per cent of the NBD-PS without significant translocation of NBD-PC. The process wa s significantly inhibited by N-ethylmaleimide (NEM) and vanadate. It i s concluded that this model of trophoblast contains an active aminopho spholipid translocase. (C) 1997 W. B. Saunders Company Ltd.