INDUCTION AND ENHANCEMENT OF NORMAL HUMAN MEGAKARYOCYTE POLYPLOIDIZATION ARE CONCOMITANT WITH PERTURBATION IN THE ACTIN METABOLISM

Background Megakaryocyte polyploidization results from the lack of cyt oplasmic separation while the nucleus keeps dividing. Methods To inves tigate the role of actin in the megakaryocyte polyploidization, three human cell lines with megakaryocytic properties (DAMI, HEL and K562) w ere incubated in the presence of cytochalasin B, an inhibitor of actin polymerization. These data were then compared with normal megakaryocy tes. Results Compared with control conditions, cells cultured in the p resence of cytochalasin B revealed an augmentation of cell size and pl oidy and an arrest of cell proliferation. The expression of platelet m embrane glycoproteins Ib, IIb/IIIa, IIIa and thrombospondin and transf errin receptors was augmented after treatment with cytochalasin B. Phy siologically, the role of actin in inducing polyploidization could be related to an imbalance between G- and F-actins. To test this hypothes is, we measured G- F- and total actin in cytochalasin B-treated cells. Actin was found to be increased significantly in cytochalasin B-treat ed DAMI and HEL cell lines. In contrast, the G/F-actin ratio was not a ffected by cytochalasin B. To confirm these actin changes in physiolog ical megakaryocytopoiesis, G- and F-actin contents were then estimated in normal megakaryocytes. The G- and F-actin contents of megakaryocyt es from eight normal patients exponentially decreased from 2 to 128n, whereas the total actin content per cell kept increasing. The G/F rati o was unaffected. Conclusion Polyploidization of human megakaryocytes results from either a diminution of actin synthesis or an increased ac tin turnover, which in turn possibly abrogates the formation of the ac tin cleavage furrow in telophasis.