EFFECTS OF DMSO-INDUCED DIFFERENTIATION ON ARACHIDONATE MOBILIZATION IN THE HUMAN HISTIOCYTIC LYMPHOMA CELL-LINE U937 - RESPONSIVENESS TO SUBMICROMOLAR CALCIUM IONOPHORE A23187 AND PHORBOL ESTERS

The human histiocytic lymphoma cell line, U937, is a rich source for i solation and purification of the 85 kDa cytosolic phospholipase A(2) ( cPLA(2)). Recent studies suggest that this enzyme catalyzes the agonis t-stimulated release of arachidonate from membrane phospholipids, ther eby initiating eicosanoid synthesis. We therefore investigated in situ regulation of phospholipase A, activity in intact U937 cells. The res ults indicate that calcium ionophore A23187 stimulatable release in in tact undifferentiated U937 is low and only weakly dose dependent. Dime thyl sulfoxide (DMSO) differentiation of U937 cells results in a drama tic increase of A23187-stimulated arachidonate mobilization. Consisten t with the characteristics of cPLA, in vitro, A23187-stimulated arachi donate release in differentiated U937 cells is highly specific for ara chidonate and is activated by submicromolar A23187 concentrations. Pho rbol myristate acetate (PMA) further potentiates arachidonate release in differentiated U937 cells by 4-6-fold over A23187 alone. However, t reatment of differentiated U937 cells with PMA alone is an ineffective stimulus for arachidonate release, suggesting that a calcium transien t is necessary for in situ arachidonate mobilization. A23187-stimulate d arachidonate release increases during distinct temporal phases of di fferentiation (0-36 h, 84-96 h). By contrast PMA enhancement of the re sponse to A23187 develops early in differentiation, and is complete by 36 h. These results suggest that differentiation-induced alterations in cPLA, regulatory elements, such as intracellular free calcium and/o r phosphorylation, may regulate mobilization of arachidonate in U937 c ells.