ATF4-CREB1 interactions. (A) Western blotting analysis. HepG2 cells were grown and transfected with ATF4 and CREB1 siRNA or negative control siRNA for 24 h as described in Methods. After another 48 hr of culture in regular medium, the cell extracts were collected and analyzed by western blot. Protein expression levels were quantified and displayed as fold-changes as compared to the loading control in the bar graph. Data are presented as means ± SD from three independent experiments: *p < 0.05 and **p < 0.01 vs. the control. (B) Potential model of ATF4-CREB1 interactions. (C) and (D) The simulation is based on the modified model that incorporates the results of Figures 4 and 5. The simulation results shows the predicted dynamic profile with (C) active CREB1 protein binding to the ATF4 promoter to induce ATF4 transcription, and (D) ATF4 protein binding to its own promoter to induce transcription.