Activation of PKC inhibits cell entry to S phase and cell growth. A: TPA inhibits cell entry to S phase. RWPE1 (left) and PC3 (right) cells were synchronized at G0/G1 phase by serum starvation. These cells were treated with or without 1 mg/mL TPA, labeled with BrdU, and stimulated with 10% fetal bovine serum. Cell cycles and DNA synthesis were then quantified 4 hours after serum stimulation. *P < 0.01. B: Knocking down of RACK1 enhances TPA-mediated cell growth arrest. RWPE1 (left) and PC3 (right) cells were synchronized at G0/G1 phase by serum starvation. These cells were silenced with siRACK, or siPKCβii, or Scr control. These cells were then treated with or without 1 mg/mL TPA, labeled with BrDU, and stimulated with 10% fetal bovine serum. Cell cycles and DNA synthesis were then quantified 4 hours after serum stimulation. *P < 0.01. C: Colony formation of RWPE1 (left) and PC3 (right) cells treated with or without TPA, in the presence of siRACK or Scr. *P < 0.05.