p38 MAPK inhibition can account for most of the overall antiapoptotic effect mediated by BCR-ABL expression in hematopoietic progenitors.

(A) BCR-ABL expression suppresses p38 MAPK activity in both ES-derived hematopoietic progenitors and BaF3 cells. Conditions are identical to Figure 5 with an activation site–specific p38 MAPK phospho-antibody and total p38 MAPK antibody. This molecular pathway was analyzed 3 times from 3 independent experiments with similar results. (B) The p38 MAPK small molecule inhibitor SB 203580 expands ES-derived hematopoietic progenitors in a dose-dependent manner. Doses of 0.1, 1.0, and 10 μM of SB 203580 or control DMSO were added daily from day 5 to day 7 of ES in vitro differentiation cultures. ES-differentiated day 7 total cell numbers on SB 203580 addition were compared to those in the presence and absence of BCR-ABL induction. Results represent the mean ± SEM of an experiment conducted in triplicate. Similar results were obtained in another independent experiment done in triplicate cultures. Statistical analysis (paired t test) of BCR-ABL OFF versus BCR-ABL ON total cell population wasP < .01. Statistical analysis (paired t test) of BCR-ABL OFF plus 10 μM SB 203580 versus BCR-ABL OFF total cell population was P < .01. (C) The p38 MAPK small molecule inhibitor SB 203580 suppresses apoptosis in ES-derived hematopoietic progenitors similar to BCR-ABL expression. SB 203580 (10 μM) or control DMSO (10 μM) was added daily from day 5 to day 7 of ES in vitro differentiation cultures. The level of apoptosis as measured by annexin V levels (refer to Figure 3) on SB 203580 addition was compared to that in the presence and absence of BCR-ABL induction. A representative result of an experiment done in triplicate cultures is shown. Similar results were obtained in 2 independent experiments done in triplicate cultures.