E2F1 regulates Trib2 mRNA and protein expression levels. (A) (Left) E2F1^−/− (KO) MEFs, (center) 293T, or (right) K562 cells transfected with E2F1, pcDNA control, or ICNX as indicated were measured for Trib2 mRNA expression by real-time PCR analyses. Data are presented relative to control pcDNA transfected cells ± SD of duplicate cultures. *P < .05, **P < .005, and ***P < .001. (B) WT and E2F1^−/− (KO) SV40 MEFs were measured for (left) E2F1 and (right) Trib2 mRNA expression by real-time PCR analyses. (C) Western blot analyses of E2F1 and Trib2 protein levels in WT and E2F1^−/− (KO) MEFs. Actin shown as protein loading control. (D) (Left) E2F1^−/− (KO) MEFs or (right) K562 cells were transfected with pcDNA3 empty vector, E2F1, ICNX, or Trib2 as indicated, and western blot analyses were performed for Trib2, E2F1, and actin as a loading control. (E) 293T, (F) HeLa, and (G) K562 cells were transfected with scrambled control siRNA and siE2F1 (1 or 2 different E2F1 siRNAs as indicated), and real-time PCR analysis was performed for E2F1 and Trib2. Data are presented relative to control transfected cells and representative of 2 independent experiments. Error bars denote ± SD of each sample measured in triplicate. *P < .05, **P < .005, and ***P < .001.