Cross talk between p22phox and ATF4 in the endothelial unfolded protein response

Background: Cardiovascular diseases have been associated with stress in the endoplasmic reticulum (ER) and accumulation of unfolded proteins leading to the unfolded protein response (UPR). Reactive oxygen species (ROS) such as superoxide and H₂O₂ derived from NADPH oxidases have been implicated in the pathogenesis of cardiovascular diseases. ROS have also been associated with ER stress. The role NADPH oxidases in the UPR is, however, not completely resolved yet. Aim: In this study, we investigated the role of p22phox, an essential component of most NADPH oxidases, in the UPR of endothelial cells. Results: Induction of ER stress increased p22phox expression at the transcriptional level. p22phox was identified as novel target of the UPR transcription factor ATF4 (activator of transcription factor 4) under ER stress conditions by promoter analyses and ChIP. Depletion of ATF4 and p22phox diminished the levels of superoxide and H₂O₂ under ER stress conditions. On the contrary, p22phox was instrumental in increasing eIF2α phosphorylation and subsequent ATF4 expression on induction of ER stress by chemicals, oxysterols, or severe hypoxia in vitro and in vivo, leading to increased expression of CHOP and activation of effector caspases. Innovation: p22phox is a novel target of ATF4 in response to ER stress, which can promote the PERK-ATF4 branch of the UPR in vitro and in vivo. Conclusion: p22phox-dependent NADPH oxidases are important mediators of ER stress driving the UPR.