E2F1 induces hPTTG1 expression. E2F1-induced hPTTG1 mRNA (A) and protein expression (B) in H1299 cells but not in HCT116 cells (D and E). Cells were transfected with empty vector pcDNA3.1 or E2F1. hPTTG1 mRNA levels were detected by real-time PCR (A and D); myc-tagged E2F1 and hPTTG1 protein levels were measured by Western blot (B and E). Control is empty vector pcDNA3.1. E2F1-induced hPTTG1 was abolished by E2F1-DN in H1299 cells (C) and not observed in cells in the presence of p53 (E) but detected in HCT116 p21−/− cells (F). G, Impairing endogenous Rb expression concordantly elevated E2F1 and hPTTG1 protein in H1299 cells. H1299 cells were transfected with negative control siRNA or Rb siRNA 106038 or 106039 for 48 h. H, Suppressing endogenous E2F1 by siRNA attenuated hPTTG1 expression in both H1299 and HCT116 cells. H1299 and HCT116 cells were transfected with negative control siRNA, E2F1 siRNA 114509, 107658, or 107660 and harvested after 24 or 48 h, respectively. I, Suppressing endogenous E2F1 by siRNA attenuated hPTTG1 expression in mouse pituitary AtT-20 cells. AtT-20 cells were transfected with negative control siRNA, mouse E2F1 siRNA 61093, 160617, or 160619. Scrambled is negative control siRNA. Rb, E2F1, and hPTTG1 protein were detected by Western blot. β-Actin was used as an internal control, and proliferating cell nuclear antigen was used as a late G1/S indicator of the cell cycle.