In the present study, we studied downstream signals of BCR-ABL with regard to Src family kinases and YAP, a transcription cofactor and an effector of the Hippo pathway. We first checked the phosphorylation status of YAP and found that it was constitutively phosphorylated at tyrosine 357 in CML-derived cell lines (TCC-S and K562) but not in AML-derived cell lines (HL-60 and KG-1a). Treatment with imatinib or RK-20449 inhibited cell growth and decreased tyrosine phosphorylation of YAP in both CML lines. Expression of Survivin or Cyclin D1 was decreased in TCC-S, but not in either HL-60 or KG-1a. Furthermore, we established BCR-ABL stable transfectant and control empty vector transfectant from TF-1, a factor-dependent human erythroleukemia cell line, to verify our results obtained with CML cell lines. YAP was phosphorylated at Y357 constitutively in BCR-ABL stable transfectant but not in control transfectant, and treatment with imatinib or RK-20449, a Src family kinase-specific inhibitor, inhibited cell growth, YAP tyrosine phosphorylation, and expression of Cyclin D1 in BCR-ABL stable transfectant. These results suggest that BCR-ABL induces tyrosine phosphorylation of YAP presumably through Src family kinases, which results in expression of Survivin and Cyclin D leading to leukemogenesis in CML cells.
Keywords: BCR-ABL; Chronic myeloid leukemia; Src family kinase; Tyrosine phosphorylation; Yes-associated protein.