Analysis on the mechanisms of growth, differentiation and malignant transformation <of megakaryocytic cells

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In this project, we examined the mechanisms of growth, differentiation, and malignant transformation of megakaryocytic cells.During the late phase of megakaryopoiesis, megakaryocytes undergo polyploidization characterized by DNA duplication without concomitant cell division. We initially examined the roles of AIM-1 which is a member of Aurora/Ipl1 serine threonine kinase family and essential for mitosis in megakaryocytic polpyploidization. In a proliferative hematopoietic cells, the expression of AIM-1 mRNA was restrictedly observed at G_2/M phase of cell cycle. In contras its expression was continuously repressed during polyploidization in normal megakaryocytes as in erythro/megakaryocytic cell lines (F-36P, K562, and CMK). Supplement of AIM-1 activities by the induced express on of wild-type AIM-1 canceled TPA-induced polyploidizaTion of K562.Moreover, suppression of AIM-1 activities by the induced expression dominant-negative(DN) AIM-1 led to polyploidization of K562 and CMK. These results suggested that down-regulation of AIM-1 may be involved in polyploid formation of megakaryocytes.Next, we examined the mechanism of the growth and survival of a BCR/ABL-positive erythroid/magakaryocytic cell line, K562. We inducibly expressed DN Ras (N17), phosphatidylinositol 3-kinase(PI3-K)(Δp85) and STAT5 (694F) alone or ill combination in K562 The inducibly expressed N17, 694F and Δp85 inhibited the growth by 90% 55% and 40%, respectively. In addition, N17 induced apoptosis in a small proportion of K562, whereas 694F and Δp85 were hardly effective In contrast, coexpression of two DN mutants in any combinations induced severe apoptosis. Furthermore, although K562 was resistant to IFN-α- and dexamethasone-induced apoptosis, disruption of one pathway by N17 694F or Δp85 sensitized K562 to these reagents. These results suggest that cooperation among these molecules is required for full leukemogenic activities of BCR/ABL.