Advanced oxidation protein products induce apoptosis in podocytes through induction of endoplasmic reticulum stress

Abstract

Although podocyte apoptosis has been shown to be induced by the accumulation of advanced oxidation protein products (AOPPs), the mechanisms through which AOPPs trigger apoptosis in these cells remain unclear. In this study, we investigated the role of endoplasmic reticulum (ER) stress in AOPP-induced podocyte apoptosis. AOPP treatment induced overexpression of glucose-regulated protein 78 and CCAAT/enhancer-binding protein-homologous protein (CHOP) in podocytes, indicating that AOPPs induced ER stress. Notably, AOPP-induced increase in the rate of podocyte apoptosis was partly reversed by salubrinal, an ER stress inhibitor, whereas the AOPP effect was reproduced by an inducer of ER stress, thapsigargin, suggesting that AOPPs triggered podocyte apoptosis by inducing ER stress. Furthermore, AOPP-induced reactive oxygen species (ROS) generation, ER stress, and podocyte apoptosis were significantly inhibited by an nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, a ROS scavenger, or receptor of advanced glycation end products (RAGE) small interfering RNA (siRNA). Moreover, silencing of the three ER stress sensors, protein kinase-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol requiring 1 (IRE1), respectively, significantly lowered the apoptotic rate of the cells compared with that of the scramble siRNA-transfected cells. Lastly, our data suggested that CHOP- and caspase-12-dependent pathways were involved in ER stress-mediated podocyte apoptosis and that Bcl-2 suppression was involved in CHOP-mediated apoptosis. Collectively, our results indicate for the first time that AOPPs trigger podocyte apoptosis through induction of ER stress, which might be regulated by NADPH oxidase-dependent ROS through RAGE, and that this apoptosis is mediated by three unfolded protein response pathways, the PERK, ATF6, and IRE1 pathways, and the mediators, CHOP and caspase-12.