Endoplasmic reticulum (ER) homeostasis, stress, and the unfolded protein response (UPR). (A) ER homeostasis is mediated by 78-kDa glucose-regulated protein (GRP78). Under stress conditions, GRP78 dissociates from pancreatic endoplasmic reticulum kinase (PERK), inositol-requiring enzymes 1α (IRE1α), as well as the activating transcription factor 6 (ATF6), leading to activation of their downstream signaling cascades in order to restore ER homeostasis. (B) When ER homeostasis fails to be restored, excessive UPR could lead to apoptosis, primarily via upregulation of C/EBP homologous protein (CHOP). PM: cytoplasmic membrane; eIF2α: eukaryotic initiation factor 2α; ATF4: activating transcription factor 4; GADD34: DNA damage inducible protein 34; XPB1: X-box-binding protein (XBP1s: spliced form); ERO1α: endoplasmic reticulum oxidoreductase 1α; PDI: protein disulfide isomerase; DR5: death receptor 5; TRAIL: TNF related apoptosis-inducing ligand; IP3R: inositol 1,4,5-triphosphate receptor; BAP31: B cell receptor-associated protein 31; Bid: BH3 Interacting Domain Death Agonist; TRAF2: tumor necrosis factor receptor-associated factor 2; RIDD: regulated IRE1-dependent decay; ASK1: apoptosis signal-regulating kinase 1; JNK: JUN N-terminal kinase; E2F7: E2F transcription factor 7; E2F1: E2F transcription factor 1.