ATF4 and colonic neoplasm: Similarly, pretreatment of colon cancer cell with an ER stress inhibitor, 4-phenylbutyric acid (4-PBA), markedly attenuated PD induced cell death (Figure 2B; Supplementary Figure S1D) and the expressions of ATF4 and CHOP (Figure 2D), suggesting that ROS-mediated ER stress plays a key role in the PD-mediated cell death.