Activation of p38 and/or JNK/SAPK induces erythroid differentiation or apoptotic cell death to some extent, and activation of ERK inhibits stress-induced apoptotic cell death. (A) Addition of C2-ceramide or anisomycin for 1 hour induces erythroid differentiation. (B) Transient expression of MKK6 (Glu) induces erythroid differentiation. Lanes 1 and 2: transfectants of MKK6 (Glu) in pIND expression vector incubated with or without Epo. Lanes 3 and 4: transfectants incubated with or without muristerone A for 1 day. The percentage of hemoglobinized cells was counted after 4.5 days. Each value represents the mean of six independent clones. (C) A prolonged MKK6 expression slightly causes apoptotic cell death. Lanes 1 and 2: cell death rate of the transfectants with or without Epo. Lanes 3 and 4: cell death rate of the transfectants incubated with muristeron A for 4.5 days. (D) Expression of constitutively active MKK1 inhibits stress-induced apoptosis. The transfectants constitutively expressing MKK1 active mutant (▵N3-S218E/S222D) (○) or mock-transfectants (•) were treated with osmotic (left panel) or heat (right panel) shock for various periods as indicated, and the cell death rate (%) was measured. Values shown are the mean of five experiments.
