Influence of ST1926 and CD437 on the phosphorylation of MAP kinases and effect of MAP kinase inhibitors on RRM-dependent apoptosis. NB4 cells were treated for the indicated amount of time with vehicle and the indicated concentrations of ST1926 and CD437. Total cellular extracts were subjected to Western blot analysis using polyclonal antibodies recognizing ERK (A) and p38 (B). In the case of panel C, we measured the level of JNK kinase activity on JNK immunoprecipitates using c-Jun as a substrate. Equivalent amounts of immunoprecipitates were electrophoresed on SDS-PAGE, blotted on nitrocellulose, and challenged with antibodies recognizing the phosphorylated form of c-JUN (serine/63) or JNK. The blots shown are representative of at least 2 independent experiments. NB4 cells were treated for 6 hours with vehicle (DMSO), ST1926 (1 μM), CD437 (1 μM), the indicated concentrations of the ERK inhibitor, U0126, and the indicated combinations (D), the p38 inhibitor, PD169316, and the indicated combinations (E), or the JNK inhibitor, SP600125, and the indicated combinations (F). Aliquots of the extracts were used for the determination of the apoptotic index (left-most panels) and the number of viable cells (rightmost panels). The numbers above the columns indicate percentages of viable cells. Less than 100 = below the limit of detection of the assay. Results are the mean ± SD of 3 separate culture dishes and are representative of at least 2 independent experiments. *Significantly higher or lower than the corresponding vehicle-treated group (P < .01 according to the Student t test).