Bcr-Abl Inhibitors Induce Expression of HoxA10, Playing a Role as an Inhibitor of the Proliferation through PI3K/PKB Pathway in Chronic Myelogenous Leukemia Cells.

[Background] Homeobox (Hox) genes are grouped together in 4 clusters, A to D. Recent studies has shown that the Hox proteins are important in the control of differentiation and proliferation in hematopoietic cells. It is reported that HoxA10 has been expressed in all types acute myeloid leukemias (except for APL) but not in acute lymphoid leukemias, implicating an important role as a regulator of myeloid progenitors. Moreover, overexpression of HoxA10 induses the proliferation of early progenitor cells and leads to the development of myeloid leukemias. However, we found that Bcr-Abl inhibitors increased the expression of HoxA10 gene in CML cells. In this study, we investigated the mechanism controlling HoxA10 in CML cells treated with Bcr-Abl inhibitors.

[Methods] The cells used in this study were Ph1 positive CML cells, K562 and Meg01, and U937 cells for a Ph1 negative cell. We used AMN107 and BMS354825 for Bcr-Abl inhibitors, LY294002 for a PI3K inhibitor, PP2 for a Src kinase inhibitor, and SB203580 for a p38 MAP kinase inhibitor. For analysis of HoxA10 mRNA and protein, RT-PCR and western blot were performed in K562, Meg-01 and U937 cells, which untreated or treated with AMN107, BMS354825, LY294002, PP2, or SB203580 respectively. We then attempted to localize the intracellular locations of HoxA10 in K562 and Meg01, which untreated or treated with AMN107, BMS354825, or LY294002 by using conforcal fluorescence microscopy. Finally, for analysis of proliferation in K562, Meg-01 and U937 transfected with siRNA HoxA10, MTT assays were performed in untransfected or transfected K562, Meg-01 and U937 treated with or without AMN107, BMS354825, orLY294002.

[Results] Both K562 and Meg01 cells expressed HoxA10 mRNA and protein at lower level compared to U937 cells. Interestingly, treatment with AMN107, BMS354825, or LY294002 increased the expression of HoxA10 mRNA and protein in both K562 and Meg01 cells. In contrast, treatment with PP2 or SB203580 did not affect the expression of HoxA10 mRNA and protein in both K562 and Meg01 cells. The fluorescence of HoxA10 was more strongly observed in the area corresponding to the cell’s cytoplasm than nucleus, and the treatment with AMN107, BMS354825, or LY294002 increased the fluorescence in nucleus of K562 and Meg01 cells in a time-dependent manner. In K562 and Meg01 cells transfected with the siRNA HoxA10, treatment with AMN107 or BMS354825 slightly inhibited the proliferation compared to K562 and Meg01 transfected with control siRNA.

[Conclusions] In this study, we showed that Bcr-Abl inhibitors induced the expression of HoxA10, and HoxA10 was regulated by PI3K/P[Kappa[B pathway in CML cells. This finding indicates a new insight in the regulation of cell proliferation via the PI3K/PKB signal pathway in CML cells.