Imatinib Induces Bim Expression And Apoptosis In Chronic Myelogenous Leukemia Cells Via P38/H2AX Pathway

Imatinib mesylate, a specific trysine kinase inhibitor for BCR-ABL, hasbecome an effective first-line agent for the treatment of chronic myeloidleukemia (CML). Unfortunately, about30%patients have failed to respondto treatment with imatinib because of bcr-abl mutation or amplification.Therefore, it is urgently required to find new and more effective treatmentto replace or enhance the therapeutic effect of IM. Inspiringly, increasingevidence has suggested that histone H2AX plays a critical role in regulationof tumor cell apoptosis and acts as a novel human tumor suppressor protein.The cells lacking H2AX are resistant to apoptotic induction and susceptibleto tumor transformation. The previous data showed that histone H2AXphosphorylation at the C terminus (Ser139) is involved in various cellapoptosis. However, the action of H2AX in chronic myelogenous leukemia(CML) cells is unknown. Furthermore the detailed mechanism andepigenetic regulation by H2AX remain elusive in cancer cells.In this study, the underlying mechanisms of H2AX C-terminalphosphorylation and the effect of it on nuclear apoptosis-related geneexpression were elucidated in imatinib-induced apoptosis of K562cells invitro. The results not only demonstrated that H2AX can participate in theregulation of imatinib-induced CML cell apoptosis as in other cancer cells，but also illustrated that p38can play an important role in K562cellapoptosis, which is closely related to H2AX Ser139phosphorylation.Furthermore it’s found that H2AX-wt overexpression can enhanceapoptotic sensitivity of CML cells (K562) to imatinib. But overexpressionof Ser139-mutated H2AX(blocking phosphorylation) decreased sensitivity of apoptosis in K562cells. Similarly, knockdown of H2AX made K562cells resistant to apoptotic induction. These data revealed that the functionof H2AX involved in apoptosis is strictly related to its phosphorylation(Ser139). During this process, we also observed the changes of themembers of MAPK (mitogen activated protein kinase) family with westernblot. The results showed that imatinib may stimulate MAPK familymember p38, and H2AX phosphorylation followed a similar time course,suggesting a parallel response. In addition, H2AX phosphorylation andapoptosis can be blocked by p38siRNA or its pharmacological inhibitor,we found that H2AX phosphorylation and apoptosis were regulated by p38MAPK pathway in K562cells. However， mitogen-and stress-activatedprotein kinase-1and-2(MSK1/2), the p38downstream kinases with theability to phosphorylate histone H3at serine10, was not involved in H2AXphosphorylation. Finally, we provided epigenetic evidence showing thatH2AX phosphorylation regulated apoptosis-related gene Bim expression.Blocking of H2AX phosphorylation inhibited Bim gene expression. Thesame results were obtained from H2AX wild-type and knockout mouseembryonic fibroblasts (MEFs). Taken together, these data demonstratedthat H2AX phosphorylation regulated by p38is involved in Bim expressionand apoptosis in CML cells induced by imatinib.