Effect of inhibition of JNK activity on primary murine erythroid cells. Murine bone marrow cells were incubated in the presence of EPO with no addition (–), DMSO vehicle (V), or SP600125 inhibitor in semisolid media as indicated. The number of BFU-e's (A) and CFU-e's (B) were counted 8 days and 2 days after the start of the experiment, respectively, and are expressed as number of colonies detected per 35-mM plate. Whereas the number of BFU-e's was reduced approximately 5-fold in the presence of 25 μM SP600125 inhibitor, the number of CFU-e's was not significantly affected by the presence of the inhibitor. (C) Bone marrow cells were pretreated with DMSO vehicle (columns 2 and 6), 12.5 μM SP600125 (columns 3 and 7), or 25 μM SP600125 (columns 4 and 8) for 6 hours prior to addition of nothing (columns 1-4) or 10 μM ara-C (columns 5-8) for 1 hour. The cells were then plated on methylcellulose media containing EPO but no inhibitors. ara-C–induced cell death of BFU-e's (lanes 5 and 6) and pretreatment with the SP600125 inhibitor prevented this cell death (columns 7 and 8), indicating slowing of proliferation. Error bars represent standard deviation from the mean.