ABL Kinases Phosphorylate Tumor Suppressor Smad4 To Inhibit TGF-β Signaling Pathway In Chronic Myeloid Leukemia

As one of the most important multifunctional molecular pathways,transforming growth factor(TGF)-β signaling pathway mediates a variety of cellular processes.By upregulating the expression of tumor suppressing genes,such as p15,p21,p57,and downregulating the expression of oncogenes such as c-MYC,TGF-β signaling pathway plays a significant role in suppressing cell proliferation and maintaining cell homostasis.Therefore,mounting studies have associated the initiation and progression of cancer with the dysregulation of TGF-β signaling pathway,supporting its tumor-suppressive role.In leukemia,TGF-β signaling pathway is frequently dysregulated.Nevertheless,although mutations and deletions of TGF-β signaling components are frequently found in many cancers,they mostly focus on solid tumors with limited information available in leukemia.So how do leukemia cells escape the monitoring of TGF-β signaling pathway?Here,our study explored the detailed mechanism in chronic myeloid leukemia(CML).CML is a type of leukemia that starts in certain blood-forming cells of the bone marrow.Constitutively active BCR-ABL1 tyrosine kinase plays a key role in CML,which exists in about 95%of CML patients.In our study,we found Smad4,a central component in TGF-β signaling pathway,was phosphorylated by ABL kinases which can be detected both in CML cell line K562 and CML patient samples with Y195/Y301/Y322 being key phosphorylation sites.ABL1 overexpression attenuated TGF-β induced CDK inhibitor expression and cell proliferation inhibition,whereas Gleevec could enhance TGF-β-induced responses in K562,and the application of TGF-β could even sensitize K562 to Gleevec.Furthermore,Smad4 Y195/301/322F mutation in K562 had higher TGF-β signaling responses and Gleevec treatment sensitivity,whereas Smad4 Y195/301/322E mutation exerted opposite effects.Overall,this work identified Smad4 as a novel target of ABL kinases and uncovered the new underlying molecular mechanisms of CML and Gleevec activity,which provided an insightful guidance for CML therapy and Gleevec application.