Thrombin-induced shape change in human megakaryoblastic leukemic cells, MEG-O1, is mediated by protein kinase C

We investigated the intracellular processes of the shape change in human megakaryoblastic leukemic cells, MEG-01, by platelet agonists. Thrombin induced the formation of many pseudopods. This shape change was also induced by phorbol 12-myristate 13 acetate (TPA) and weakly by Ca2+ ionophore, A23187, but not by ADP, collagen, or epinephrine. Electron microscopy and FITC-labeled phalloidin staining revealed thick submembranous microfilament bundles in the pseudopods of the shape-changed cells by thrombin. Shape change was inhibited by cytochalasin B. Since Ca2+-dependent phosphorylation reactions play central role on the initiation of shape change of platelet, we examined the effects of protein kinase C (PKC) inhibitor, H-7, and myosin light chain (MLC) kinase inhibitor, ML-9, on the shape change of MEG-01 cells induced by thrombin, and observed that H-7 potently inhibited thrombin-induced shape change, while ML-9 did not. These results suggest that thrombin-induced reorganization of microfilaments and shape change of MEG-01 cells are mediated by PKC, but not by MLC kinase.