A NOVEL INHIBITORY-ACTION OF WHEAT-GERM-AGGLUTININ ON PHOSPHOLIPASE-CIN HEL AND MEG-O1 CELL-LINES

Stimulation of HEL megakaryocytic cells by Fc gamma RIIA crosslinking is associated with tyrosine phosphorylation of syk and phospholipase C gamma 2(PLC gamma 2) and is accompanied by formation of inositol phos phates and release of intracellular Ca2+ These responses are inhibited by the kinase inhibitors, staurosporine and ST271. In contrast, the G -protein receptor agonist, thrombin induces formation of inositol phos phates and release of intracellular calcium without an increase in tyr osine phosphorylation. The plant lectin wheat germ agglutinin (WGA) st imulates tyrosine phosphorylation of syk and PLC gamma 2 but surprisin gly does not stimulate formation of inositol phosphates and induce rel ease of intracellular Ca2+. WGA also inhibited formation of inositol p hosphates and release of intracellular Ca2+ by Fc gamma RIIA crosslink ing and thrombin-stimulation. A similar inhibitory effect of WGA was o bserved against elevation of Ca2+ by the same two stimuli in MEG-01 me gakaryotic cells. The results demonstrate a novel pathway of inhibitio n of PLC on crosslinking of cell surface proteins that is not present in platelets.