ATF4 and Parkinson disease: Compared to the SAH group, the expression levels of ER stress-related proteins including glucose-regulated protein 78 (GRP78), phosphorylated protein kinase R-like endoplasmic reticulum kinase (p-PERK), phosphorylated eukaryotic initiation factor 2α (p-eIF2α), activating transcription factor 4 (ATF4), and C/EBP homologous protein (CHOP), were significantly lower in the PD-treated group.