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Bruton's tyrosine kinase prevents activation of the anti‐apoptotic transcription factor STAT3 and promotes apoptosis in neoplastic B‐cells and B‐cell precursors exposed to oxidative stress

Bruton's tyrosine kinase (BTK) was previously demonstrated to be a mediator of oxidative stress‐induced apoptosis in irradiated neoplastic B‐cells and B‐cell precursors. Defective BTK expression in leukaemic B‐cell precursors from infants with t(4;11) acute lymphoblastic leukaemia has been associated with radiation resistance. The present study examined whether BTK mediates apoptosis during oxidative stress by interfering with the anti‐apoptotic function of signal transducer and activator of transcription 3 (STAT3). BTK physically associated with and tyrosine phosphorylated STAT3; this association was promoted by pervanadate (PV)‐induced oxidative stress. The BTK/STAT3 interaction appeared to prevent STAT3 response to oxidative stress, because PV‐induced STAT3 activation was markedly enhanced in DT40 chicken lymphoma B‐cells that were rendered BTK‐deficient by targeted disruption of the btk gene as well as in BTK‐deficient RAMOS‐1 human lymphoma B‐cells. These BTK‐deficient cells were highly resistant to oxidative stress‐induced apoptosis triggered by PV treatment. Reconstitution of BTK‐deficient DT40 cells with wild‐type human BTK gene eliminated the amplification of the STAT3 response and restored the PV‐induced apoptotic signal. Similarly, while the BTK‐positive NALM‐6 human leukaemic B‐cell precursor cell line showed no STAT3 activation after PV treatment and was exquisitely sensitive to PV‐induced apoptosis, PV failed to induce apoptosis in BTK‐deficient RAMOS‐1 human lymphoma B‐cells that showed a robust STAT3 response. These results provide unprecedented biochemical and genetic evidence for a unique mode of cross‐talk that occurs between BTK and STAT3 pathways during oxidative stress, whereby BTK may trigger apoptosis via negative regulation of the anti‐apoptotic STAT3 activity.