Biomed Environ Sci, 2018; 31(7): 555-559 555 Research Highlight The Role of Endoplasmic Reticulum Stress-related Apoptosis in Vascular Endothelium Pathogenesis* TAO Yong Kang1, SHI Jing2, YU Pu Lin2,#, and ZHANG Guo Qiang1,# Vascular endothelium refers to a single layer of processes in the ER lumen, lead to accumulation of endothelial cells that line the inner surface of blood unfolded or misfolded proteins, and result in a vessels, serving as barriers and transducers between cellular condition known as ER stress. The ER the circulating blood in the lumen and the rest of the responds to ER stress by further activating vessel wall. Endothelial cells play essential roles in intracellular signal transduction pathways, called the many aspects of vascular biology, such as barrier unfolded protein response (UPR). The UPR functions, thrombosis/fibrinolysis, inflammation, contributes to restoring ER homeostasis by reducing angiogenesis, vasoconstriction and vasodilation, and protein synthesis, increasing ER chaperone levels, repair of damaged organs. The endothelium also and clearing misfolded or unfolded proteins[3]. produces active factors, including nitric oxide (NO), Nevertheless, despite its protective role, when bradykinin and prostacyclin, endothelin, reactive strenuous perturbations are prolonged or excessive oxygen species (ROS), prostaglandin H2, for the compensation mechanism, the UPR then thromboxane A2, and angiotensin II[1]. Endothelial triggers intracellular signal cascades that lead to the dysfunction is a strong independent predictor of activation of inflammatory pathways and eventually both cardiovascular events and mortality in various cell apoptosis[4]. The ER stress also stimulates populations. Dysfunctional endothelium is also transmembrane stress sensors: protein kinase-like related to numerous pathological conditions, ER kinase (PERK), inositol-requiring kinase 1 α including aging, obesity, hypertension, and type 2 (IRE1α), and activating transcription factor 6 (ATF6), diabetes[2]. Since the roles of the endothelium are which bind to ER chaperone glucose-regulated critical, any noxious stimulating factor could impair protein 78 kD protein/immunoglobulin binding its normal function, of which the exact mechanisms protein (GRP78/BiP) under unstressed conditions[4]. have not yet been completely revealed. However, PERK is an ER transmembrane protein kinase several studies suggest that a disproportionate that oligomerizes to activate its kinase domain activation of the endoplasmic reticulum (ER) stress during ER stress. PERK inactivates the eukaryotic response may play a central role by inducing translation initiation factor 2 α-subunit (eIF2α) by endothelial apoptosis. serine phosphorylation. This inactivation globally shuts off the translation of 90% of cellular mRNAs, Molecular Mechanisms of the ER Stress Response [5] thus attenuating protein load to the ER . and Apoptosis Paradoxical to its suppression of global protein Pathways of ER Stress The ER is an essential translation, the phosphorylation of eIF2α leads to subcellular network of flattened, membrane- translation of several mRNAs containing open enclosed sacs or tube-like compartments. The ER is reading frames, such as activating transcription responsible for lipid biosynthesis, calcium factor-4 (ATF4). ATF4 binds to promoter/enhancer sequestration, and the synthesis and folding of regions and transcriptionally augments the almost one-third of proteins. Different physiological expression of a subset of UPR genes including C/EBP and pathological perturbations, such as oxidative homologous protein (CHOP), vascular endothelial stress, ischemia, and disturbance of calcium growth factor (VEGF) A, TRB3, E-selectin, and genes homeostasis could interfere with protein folding important in amino acid metabolism[6]. doi: 10.3967/bes2018.076 *This study was funded by Beijing Science and Technology Commission Foundation [Z151100004015071]. 1. Departments of Emergency, China-Japan Friendship Hospital, Beijing 10029, China; 2. Department of Deriatric Medicine, Beijing Hospital, National Center of Gerontology, Beijing 100730, China 556 Biomed Environ Sci, 2018; 31(7): 555-559 IRE1α is a transmembrane protein, which best known as well as the major branch. As contains both a Ser/Thr kinase domain and an mentioned before, IRE1α can interact with TRAF-2 to endoribonuclease domain. IRE1α oligomerizes and become IRE-1/TRAF-2 complex, which could activate autophosphorylates when dissociated from both ASK1 and JNK, hence causing apoptosis. GRP78/BiP during ER stress. Furthermore, misfolded Moreover, p38MAPK is also able to up-regulate or unfolded proteins may directly stimulate IRE1α by CHOP in the IRE1α pathway. Prolonged ER stress in the domain in the ER chamber. The activated vascular endothelial cells of the brain induces a endoribonuclease domain of IRE1α splices a 26-base significant flow of Ca2+ from the ER to the intron from X-box binding protein 1 (XBP1) mRNA, mitochondria, leading to mitochondrial Ca2+ rendering it competent for translation to produce overload, and then activation of 41-kDa XBP1 protein, a bZIP-family transcription mitochondria-dependent apoptosis. This last event is factor. Spliced XBP1 can promote the expression of rescued by blocking the mitochondrial Bax channel. UPR genes implicated in ER-associated protein Failure of Ca2+ homeostasis can increase production degradation. Moreover, IRE1α can interact with of ROS, the resulting elevated levels of ROS can tumor necrosis factor (TNF)-receptor-associated deregulate Ca2+ homeostasis, thereby leading to factor 2 (TRAF-2) as IRE1/TRAF-2 complex. The apoptosis[10-11]. Caspase-12 is included in complex IRE1/TRAF-2 recruits the inhibitor of kB (IkB) ER-stress-mediated apoptosis of rodent species. This kinase (IKK) and consequently activates nuclear mechanism involves the interaction of factor kB (NF-kB), which is a transcriptional factor pro-caspase-12 with the IRE1-TRAF-2 complex, that promotes the expression of inflammation genes. CHOP-ERO1α-IP3R-calcium-calpain pathway might Furthermore, TRAF-2 both promotes the activation also contribute to caspase-12 activation. In humans, of apoptosis-signaling-kinase 1 (ASK1) and enhances caspase-4 belongs to the caspase-1 subfamily, is an the activity of the stress kinase Jun-N-terminal analog of rodent caspase-12, and performs the kinase (JNK), which is known to induce insulin functions usually ascribed to rodent caspase-12 in resistance, inflammatory pathways, and apoptosis. the context of human ER stress[12]. ASK1 also promotes the activity of p38 mitogen-activated protein kinase (MAPK) that in turn ER Stress-related Apoptosis as an Underlying activates CHOP[7]. Molecular Mechanism in Endothelial Cell The third ER transmembrane sensor, ATF6, Pathogenesis acts as a potent transcription factor and performs a A series of experiments explored how ER stress different mechanism of protein activation when affected endothelial cells by different mechanisms. compared with both PERK and IRE1α. Upon In the sections below, recent studies about ER its dissociation from GRP78/BiP, ATF6 is stress-related apoptosis in endothelial pathogenesis transported to the Golgi apparatus and is cleaved by are reviewed. proteases. Then, the cleaved N-terminal cytosolic domain of ATF6 translocates to the nucleus and Hyperlipidemia upregulates genes involved in protein folding and degradation, including CHOP, NF-kB, GRP78/BiP, and Dickkopf1 (DKK1) expression is upregulated in XBP1[2]. response to ox-LDL treatment in a time- and concentration-dependent manner in human ER Stress-related Apoptosis umbilical vein endothelial cells (HUVECs). The ER stress can induce widespread pathologic interference of DKK1 reverses ox-LDL-induced apoptosis under prolonged noxious stimulation. apoptosis in HUVECs. The mechanism underlying this Non-selective ER stress-induced apoptosis is effect is DKK1’s activation of the JNK signal considered a crucial pathogenic factor in several transduction pathway and inhibition of canonical diseases, including neurodegenerative diseases, Wnt signaling, followed by activation of IRE1α and diabetes, atherosclerosis, and renal disease[8]. eif2α/CHOP pathways. DKK1 promotes plaque Activation of CHOP and IRE1α could induce formation and vulnerability partly by inducing apoptosis in cells under conditions of stress apoptosis in endothelial cells. Endothelial apoptosis overload[9]. As discussed above, CHOP could be is partly induced through the induction of the activated in three branches of the ER stress signaling JNK-endoplasmic reticulum stress pathway and the pathway, of which PERK-eIF2α-ATF4 pathway is the inhibition of canonical Wnt signaling[13]. ERS-related apoptosis in endothelium 557 Hyperlipidemic Apoe knockout mice with endothelial cell apoptosis and inflammation that is, endothelial-specific gain of transient receptor at least partly, mediated by ER stress. potential canonical 3 channel function Homocysteinemia (TgESTRPC3/ApoeKO) have increased burden of advanced aortic atherosclerosis with 16 weeks’ Zhang et al.[19] reported that homocysteine high-fat diet compared with nontransgenic ApoeKO induces apoptosis in HUVECs by activation of the littermate controls (non-Tg/ApoeKO), the livers of UPR and is signaled through IRE1α in 2001. TgESTRPC3/ApoeKO mice show steatosis, fibrosis, Homocysteine treatment decreases endothelial and altered hepatic enzymes compared with cell viability and increased apoptosis as verified by non-Tg/ApoeKO animals at an early stage. In vitro,