Bcr-Abl is a leukemia-inducing protein, which causes oncogenic transformation of myeloid progenitors in Philadelphia chromosome (Ph)-positive chronic myeloid leukemia (CML) and lymphoid progenitors in Ph+ acute lymphoid leukemia (ALL). Oncogenic transformation of hematopoietic cells by the Bcr-Abl oncoprotein directly involves the activation Jak2 tyrosine kinase and the Stat5 transcription factor. Both proteins are normally linked to the IL-3/GM-CSF receptors for growth and survival. Since fibroblastic cells are not targets of BCR-ABL induced oncogenesis, we determined whether forced expression of the IL-3 receptor would allow oncogenic transformation of NIH 3T3 fibroblasts known to be resistant to transformation by BCR-ABL. NIH 3T3 cells transduced with the human IL-3 receptor a and b chains were highly susceptible to oncogenic transformation by expression of BCR-ABL. Forced expression of both receptor chains but not either one alone allowed efficient foci formation of NIH 3T3 cells expressing BCR-ABL, and these cells formed colonies in soft agar whereas BCR-ABL positive NIH 3T3 cells lacking IL-3 receptor expression did not. The Bcr-Abl kinase inhibitor imatinib mesylate (1 mM) and the Jak kinase inhibitor AG490 (10 mM) strongly inhibited agar colony formation. A small molecule inhibitor of Jak2 kinase, 1,2,3,4,5,6-hexabromocyclohexane reported to be specific for Jak2 (Sandberg et al. J. Med. Chem, 2005)-significantly reduced the phosphorylation of Gab2 at the YxxM motif, which is needed for activation of the PI-3 kinase and Akt, two proteins that are part of the Bcr-Abl/Jak2 Network (Samanta et al. Cancer Res, 2006). These findings indicate that Bcr-Abl oncoprotein requires the IL-3 receptor/Jak2/Stat5 pathways for oncogenic transformation of NIH 3T3 fibroblasts, and may explain partially why Bcr-Abl oncogenesis is restricted to hematopoietic malignancies. Furthermore, this cell system in fibroblastic and other cell lineages will provide a model to probe the detailed steps that require IL-3 receptor and Jak2 for Bcr-Abl induced leukemia.

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