Internalization of phospholipids from the plasma membrane of human osteoblasts depends on the lipid head group

The redistribution of spin- or fluorescence-labeled phospholipid analogs ac ross the plasma membrane of human osteoblast cells, either in suspension or grown as monolayers, was investigated. After incorporation into the outer membrane leaflet, analogs of the aminophospholipids phosphatidylserine and phosphatidylethanolamine moved rapidly to the inner monolayer, whereas the choline-containing analogs of phosphatidylcholine and sphingomyelin disappe ared more slowly from the outer leaflet. The fast inward movement of the am inophospholipids became reduced after lowering the intracellular ATP, sugge sting the presence of an aminophospholipid translocase activity in the plas ma membrane of these cells. From these data, a transverse phospholipid asym metry in osteoblasts can be inferred with the aminophospholipids mainly con centrated in the inner monolayer and the choline-containing phospholipids i n the outer leaflet. A similar pattern of phospholipid internalization was inferred for osteoblasts from human osteoporotic bones and for a human oste osarcoma cell line. The relevance of the enrichment of phosphatidylserine i n the cytoplasmic membrane leaflet for calcification in skeletal tissues is emphasized.