JNK activity is elevated and essential for apoptotic cell death in HOIP-depleted cells. A, siRNA transfected HEK293 cells were treated with 25 μM cisplatin for the indicated times. Cell lysates were analyzed by immunoblotting. B, siCON or siHOIP transfected HEK293 cells were pre-treated with 3 μM JNK inhibitor or 10 μM ATM inhibitor before vehicle only (−) or 5 μM cisplatin treatment (+) as indicated. JNK kinases were immunoprecipitated from cell lysates, and kinase activity was measured using an in vitro kinase assay. Activity is presented as mU/mg cell lysate; * Student’s t-test was used to calculate significance: p=0.0396. C, siCON- or siHOIP-transfected HEK293 cells were pre-treated with vehicle only (−) or 3 μM JNK inhibitor (+) before treatment with 5 μM cisplatin for the indicated times. Cell lysates were analyzed by immunoblotting. D, Cell viability of siCON or siHOIP-transfected cells pre-treated with JNK inhibitor in response to cisplatin was measured by MTS cell proliferation assay. E, As (C) except that U2OS cells were used. Data in (B) and (D) are represented as mean ± SEM from three independent experiments.