Expression of dominant-negative JNK (JNKdn) inhibits proliferation of HCD57 cells. (A) Northern blot analysis of HCD57 cells either untransfected (lane 1) or transfected with JNK1dn (lane 2). Expression of the JNK1dn is indicated (arrow). Ethidium bromide–stained 28S RNA is shown to indicate equal loading of RNA (bottom). (B) In vitro kinase assay of JNK1 immunoprecipitates isolated from untransfected HCD57 cells (lane 1) or HCD57-JNK1dn transfected cells (lane 2). Phospho-cjun (top) and total JNK1 (bottom) are indicated. (C) Proliferation of HCD57 (♦) or HCD57-JNK1dn (▴) cultured in the presence of EPO for the number of hours indicated as assessed by MTT assay. (D) Cell-cycle progression of HCD57 (♦) and HCD57-JNK1dn (▪) as determine by BrdU pulse-chase labeling. The percentage of the BrdU⁺ cells entering G₁ (i), exiting S phase (ii), or entering G₂/M (iii) are plotted against time. Graphs represent triplicate samples and error bars represent standard deviation from the mean.