Involvement of endoplasmic reticulum stress response and IRE1-mediated ASK1/JNK/Mcl-1 pathways in silver nanoparticle-induced apoptosis of human retinal pigment epithelial cells

Silver nanoparticles (AgNPs) have cytotoxic effects on various human cell types. The endoplasmic reticulum (ER) is very sensitive to cytotoxic damage. Retina tissue is easily affected by internal and external stimuli. However, the effect of AgNPs on human retinal cells is not known. This study examined the effect of AgNPs on ER stress induction and their mechanism of action in human retinal pigment epithelium (RPE) ARPE-19 cells. We found that AgNPs significantly increased ARPE-19 cell cytotoxicity and stimulated caspase-3 and poly (ADP-ribose) polymerase (PARP) cleavage, as well as mitochondrial membrane potential (MMP) depolarization, in ARPE-19 cells in a dose-dependent manner (0.2−5 μg/mL for 18 h). AgNPs (5 μg/mL for 18 h) induced several signature ER stress markers, as indicated by the upregulated expressions of CCAAT/enhancer-binding protein–homologous protein (CHOP), phosphorylated protein kinase RNA-like ER kinase (PERK), eukaryotic initiation factor 2α (eIF2α), and inositol-requiring protein 1 (IRE1), and cleaved activating transcription factor 6 (ATF6). AgNPs also activated ASK1 and JNK in ARPE-19 cells, and induced increases in Bax and Puma expressions, as well as a decrease in Mcl-1 expression. However, inhibition of the ER stress response by pretreatment with 4-PBA included apparently and dose-dependently reduced levels of p-PERK, p-IRE1, CHOP, cleaved ATF6, p-ASK1, p-JNK, cleaved caspase-3, procaspase-12, and MMP depolarization in AgNP-treated ARPE-19 cells; it also led to significantly increased Mcl-1 protein levels in a dose-dependent manner in ARPE-19 cells. Pretreatment with JNK inhibitor SP600125 significantly attenuated caspase-3 cleavage and MMP depolarization and increased Mcl-1 protein levels in AgNPs-treated ARPE-19 cells in a dose-dependent manner. Hence, our study demonstrated that AgNPs induced apoptosis in human RPE ARPE-19 cells by ER stress response and ER stress-dependent mitochondrial apoptosis via the IRE1/ASK1/JNK/Mcl-1 pathways.