DDIT3 and chronic obstructive pulmonary disease: Since cell proliferation was inhibited by CSE at 48 h (Figure 5), we investigated the protein levels of the different stress pathways (ATF6, IRE1, PERK, and CHOP) at a preceding timepoint (8 h) after exposure to different concentrations of CSE in lung fibroblasts from healthy and COPD subjects (Figure 6).