Fig. 5.

Depletion of Cdc25A causes loss of cyclin E. (a) Loss of Cdc25A by lentivirus-based RNAi resulted in a significant reduction in cyclin E expression without significantly altering the level of cyclin D. MCF10A cells were infected with lentivirus-targeting Cdc25A (lane 2) or with control virus (lane 1) and synchronized by serum starvation. The levels of Cdc25A, cyclin E1, and cyclin D1 were analyzed by Western blot 8 h after addition of serum. The level of Erk2 is included as a loading control. (b) Loss of cyclin E in MCF10A cells treated with lentivirus-based RNAi targeting Cdc25A is not caused by loss of Plk3. MCF10A cells were treated with lentivirus-targeting Cdc25A (lane 2) or with control virus (lane 1) and synchronized by serum starvation as above. The levels of Cdc25A and Plk3 were analyzed by Western blot 8 h after addition of serum. The level of Erk2 is included as a loading control. (c) Cyclin-dependent kinase complexes and Plk3 promote cell cycle progression. Cyclin E is required to restart the cell cycle after serum starvation (21). Depletion of Plk3 or Cdc25A attenuates expression of cyclin E and blocks cell cycle progression.