The addition of EPO to EPO-deprived cells produces activation of ERKs, inactivation of JNKs and p38 MAPK, and rescue of HCD-57 cells from apoptosis. (A and B) After 4 hours of EPO-starvation, HCD-57 cells were cultured in complete medium containing 2 U/mL EPO for the indicated periods of time. Cells were lysed for Western blot analyses with the indicated antibodies (A) or cell growth was measured as an index of cell viability (B). (•) Cells were cultured in the complete medium with EPO; (▵) cells were EPO-starved for 4 hours and then were cultured in EPO-containing medium; (○) cells were continually incubated in EPO-free medium. (C and D) After 16 hours of EPO-starvation, HCD-57 cells were cultured in complete medium containing 2 U/mL EPO for 24 hours and then placed in EPO-free medium and incubated for another 16 hours. Analyses of JNK1/2, p38 MAPK, and ERK1/2s (C) and assays of apoptosis (D) were performed as described in Fig 1. Lane C in (C) denotes control cells grown in the presence of EPO.