Thrombospondin production and thrombospondinmediated adhesion in U937 cells

U937 cells have low levels of surface thrombospondin (TSP) under control conditions but express higher lev-els after treatment for 1 day with 100 n&f phorbol my-ristate acetate (PMA). Increased surface expression is due, in part, to increased biosynthesis. Untreated U937 cells do not adhere to TSP-coated plastic culture dishes but adhere strongly to TSP after stimulation with PMA. Untreated U937 cells also adhere weakly to endo-thelial cell monolayers while PMA-treated U937 cells attach strongly to monolayers of rat pulmonary artery endothelial cells. Endothelial cell adhesion appears to be mediated, in part, by TSP since antibodies to TSP partially inhibit. c 19Sl Academic Press, Inc