BCR/ABL Promotes Neutrophil Differentiation through Downregulation of c-jun Expression.

The BCR/ABL fusion protein transforms hematopoietic stem cells and causes chronic myeloid leukemia (CML). An increasing number of mature neutrophils is a characteristic feature in the chronic phase of CML. However, the mechanism by which stem cells transformed by Bcr/Abl differentiate mainly to mature neutrophils remains obscure. To investigate this mechanism, we compared the gene expression profile of CML neutrophils with that of normal neutrophils by microarray analysis. The genes encoding neutrophil granule proteins were upregulated in CML neutrophils, and C/EBPα and C/EBPε, critical transcription factors that regulate granulocytic differentiation, were also upregulated in these neutrophils. On the contrary, the expression of c-jun, a transcriptional factor that contributes to monopoiesis, was downregulated in CML neutrophils. Differences in the expressions of these genes were confirmed by quantitative RT-PCR. A BCR/ABL tyrosine kinase inhibitor, imatinib, released the downregulation of c-jun expression in primary CML neutrophils, showing that Bcr/Abl inhibited the expression of c-jun.

Next, to explore the roles of these transcriptional factors in the chronic phase of CML, we established sublines of KCL22, a cell line derived from CML blastic crisis, in which C/EBPα or C/EBPε expression was inducible (KCL22/α or KCL22/ε respectively). Overexpression of either C/EBP protein resulted in morphological changes, such as a reduction of the nuclear to cytoplasmic ratio, more condensed nuclear chromatin, and segmented nuclei, as well as the expression of differentiation specific markers including G-CSF receptor. These data indicate that C/EBPα/ε expression is sufficient to induce myeloid differentiation in BCR/ABL-positive CML cells. Imatinib treatment released the down regulation of c-jun in KCL22, KCL22/α, and KCL22/ε cells in a manner similar to that in primary cells. Interestingly, imatinib induces monocytic differentiation of KCL22/α cells instead of granulocytic differentiation. This effect of imatinib is independent from C/EBPα induction in KCL22/α cells. The monocytic differentiation and the inhibition of granulocytic differentiation in KCL22/α cells were accompanied by c-jun upregulation. To investigate whether these effects on differentiation of KCL22/α cells depend on releasing the downregulation of c-jun expression with imatinib, we knocked down c-jun expression with siRNA in KCL22/α cells after induction of C/EBPα protein and with imatinib treatment. In the fraction of KCL22/α cells with segmented nuclei, the G-CSF receptor increased when c-jun expression was inhibited with siRNA, indicating that the level of c-jun expression controlled the differentiation fate of CML cells. These findings suggest that Bcr/Abl promotes neutrophil differentiation through downregulation of c-jun accompanied by elevated expressions of C/EBPα/ε.