Activation of the JNK pathway promotes phosphorylation and degradation of BimEL--a novel mechanism of chemoresistance in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemias (T-ALLs) are highly malignant tumors with 20% of patients continues to fail therapy, in part due to chemoresistance of T-ALL cells via largely unknown mechanisms. Here, we showed that lack of Bcl-2-interacting mediator of cell death (Bim) EL protein expression, a BH3-only member of the Bcl-2 family proteins, conferred resistance of a T-ALL cell line, Sup-T1, to etoposide-induced apoptosis. Overexpression of Bim EL significantly restored its sensitivity to etoposide-induced caspase activation and poly(ADP-ribose) polymerase cleavage. Surprisingly, we found that constitutive activation of the c-Jun N-terminal kinase (JNK) pathway in Sup-T1 cells promoted phosphorylation and degradation of Bim EL via the proteosome. Blocking with a proteosome inhibitor yielded an elevated level of Bim EL and accumulation of Bim EL species phosphorylated at Ser 69 . Pretreatment of Sup-T1 cells with a specific JNK inhibitor, SP600125, also increased the Bim EL level and resensitized the cells to etoposide-induced apoptosis. Together, our findings suggest that the JNK activation status may correlate with the Bim EL level and in turn can control the sensitivity of T-ALL cells to chemotherapeutic agents.