ER Stress-Induced Secretion of Proteins and Their Extracellular Functions in the Heart

Membranal Expression of Calreticulin Induced by Unfolded Protein Response in Melanocytes: A Mechanism Underlying Oxidative Stress-Induced Autoimmunity in Vitiligo

Calreticulin (CRT), a damage-associated molecular pattern molecule, is reported to translocate from the endoplasmic reticulum to the membrane in melanocytes under oxidative stress. To investigate the potential role of CRT in the pathogenesis of vitiligo, we analyzed the correlation between CRT and ROS in serum and lesions of vitiligo, detected CRT and protein kinase RNA-like endoplasmic reticulum kinase (PERK) expression in vitiligo lesions, and studied the production of CRT and mediators of unfolded protein response (UPR) pathway and then tested the chemotactic migration of CD8+ T cells or CD11c+ CD86+ cells. Initially, we verified the overexpression of CRT in perilesional epidermis that was positively correlated with the disease severity of vitiligo. Furthermore, the PERK branch of UPR was confirmed to be responsible for the overexpression and membranal translocation of CRT in melanocytes under oxidative stress. We also found that oxidative stress-induced membranal translocation of CRT promoted the activation and migration of CD8+ T cells in vitiligo. In addition, dendritic cells from patients with vitiligo were also prone to maturation with the coincubation of melanocytes harboring membranal CRT. CRT could be induced on the membrane of melanocytes through UPR and might play a role in oxidative stress-triggered CD8+ T-cell response in vitiligo.

A FRET-based multifunctional fluorescence probe for the simultaneous detection of sulfite and viscosity in living cells

Viscosity and sulfur dioxide derivatives were significant indicators for the assessment of health threat and even cancers, therefore, on-site and real time detection of viscosity and sulfur dioxide derivatives has obtained considerable attentions. An FRET-based fluorescence probe JZX was designed and synthesized based on a novel energy donor of N,N-diethyl-4-(1H-phenanthro[9,10-d]imidazol-2-yl)benzamide fluorophore. JZX exhibited a large Stokes shift (230 nm), high energy transfer efficiency, wide emission channel gap (135 nm) and excellent stability and biocompatibility. JZX detected sulfur dioxide with low detection limit (55 nM), fast responding (16 min), high selectivity and sensitivity. Additionally, JZX tend to target endoplasmic reticulum of which normal metabolism will be disturbed by the abnormal levels of viscosity and sulfur dioxide derivatives. Prominently, JZX could concurrently detect viscosity and sulfur dioxide derivatives depending on different fluorescence signals in living cells for the screening of cancer cells. Hence, probe JZX will be a promising candidate for the detection of viscosity and sulfur dioxide derivatives, and even for the diagnosis of liver cancers.

Exposure to copper induces endoplasmic reticulum (ER) stress-mediated apoptosis in chicken (Gallus gallus) myocardium

Copper (Cu), an omnipresent environmental pollutant, can cause potential harm to the public and ecosystems. In order to study the cardiotoxicity caused by Cu, molecular biology techniques were used to analyze the effect of Cu on ER stress-mediated cardiac apoptosis. In vivo investigation, 240 1-day-old chickens were fed with Cu (11, 110, 220, and 330 mg/kg) diet for 7 weeks. The consequence showed that high-Cu can induce ER stress and apoptosis in heart tissue. The vitro experiments, the Cu treatment for 24 h could provoke ultrastructural damage and upregulate the apoptosis rate. Meanwhile, GRP78, GRP94, eIF2α, ATF6, XBP1, CHOP, Bax, Bak1, Bcl2, Caspase-12 and Caspase-3 genes levels, and GRP78, GRP94 and Caspase-3 proteins levels were increased, which indicated that ER stress and apoptosis in cardiomyocytes. But the mRNA level of Bcl2 were decreased after Cu exposure. Conversely, Cu-induced ER stress-mediated apoptosis can be alleviated by treatment with 4-PBA. These findings generally showed that Cu exposure can contribute to ER stress-mediated apoptosis in chicken myocardium, which clarifies the important mechanism link between ER stress and apoptosis, and provides a new perspective for Cu toxicology.

LRPAP1 is released from activated microglia and inhibits microglial phagocytosis and amyloid beta aggregation

Low-density lipoprotein receptor-related protein-associated protein 1 (LRPAP1), also known as receptor associated protein (RAP), is an endoplasmic reticulum (ER) chaperone and inhibitor of LDL receptor related protein 1 (LRP1) and related receptors. These receptors have dozens of physiological ligands and cell functions, but it is not known whether cells release LRPAP1 physiologically at levels that regulate these receptors and cell functions. We used mouse BV-2 and human CHME3 microglial cell lines, and found that microglia released nanomolar levels of LRPAP1 when inflammatory activated by lipopolysaccharide or when ER stressed by tunicamycin. LRPAP1 was found on the surface of live activated and non-activated microglia, and anti-LRPAP1 antibodies induced internalization. Addition of 10 nM LRPAP1 inhibited microglial phagocytosis of isolated synapses and cells, and the uptake of Aβ. LRPAP1 also inhibited Aβ aggregation in vitro. Thus, activated and stressed microglia release LRPAP1 levels that can inhibit phagocytosis, Aβ uptake and Aβ aggregation. We conclude that LRPAP1 release may regulate microglial functions and Aβ pathology, and more generally that extracellular LRPAP1 may be a physiological and pathological regulator of a wide range of cell functions.

The role of heat shock proteins in the pathogenesis of heart failure (Review)

The influence of heat shock proteins (HSPs) on protein quality control systems in cardiomyocytes is currently under investigation. The effect of HSPs on the regulated cell death of cardiomyocytes (CMCs) is of great importance, since they play a major role in the implementation of compensatory and adaptive mechanisms in the event of cardiac damage. HSPs mediate a number of mechanisms that activate the apoptotic cascade, playing both pro‑ and anti‑apoptotic roles depending on their location in the cell. Another type of cell death, autophagy, can in some cases lead to cell death, while in other situations it acts as a cell survival mechanism. The present review considered the characteristics of the expression of HSPs of different molecular weights in CMCs in myocardial damage caused by heart failure, as well as their role in the realization of certain types of regulated cell death.

ATF3 regulates SPHK1 in cardiomyocyte injury via endoplasmic reticulum stress

AIM

Endoplasmic reticulum (ER) stress is common in different human pathologies, including cardiac diseases. Sphingosine kinase-1 (SPHK1) represents an important player in cardiac growth and function. Nevertheless, its function in cardiomyocyte ER stress remains vague. This study sought to evaluate the mechanism through which SPHK1 might influence ER stress during myocardial infarction (MI).

METHODS

MI-related GEO data sets were queried to screen differentially expressed genes. Murine HL-1 cells exposed to oxygen-glucose deprivation (OGD) and mice with MI were induced, followed by gene expression manipulation using short hairpin RNAs and overexpression vectors. The activating transcription factor 3 (ATF3) and SPHK1 expression was examined in cells and tissues. Cell counting kit-8, TUNEL, DHE, HE, and Masson's staining were conducted in vitro and in vivo. The inflammatory factor concentrations in mouse serum were measured using ELISA. Finally, the transcriptional regulation of SPHK1 by ATF3 was validated.

RESULTS

ATF3 and SPHK1 were upregulated in vivo and in vitro. ATF3 downregulation reduced the SPHK1 transcription. ATF3 and SPHK1 downregulation increased the viability of OGD-treated HL-1 cells and decreased apoptosis, oxidative stress, and ER stress. ATF3 and SPHK1 downregulation narrowed the infarction area and attenuated myocardial fibrosis in mice, along with reduced inflammation in the serum and ER stress in the myocardium. In contrast, SPHK1 reduced the protective effect of ATF3 downregulation in vitro and in vivo.

CONCLUSIONS

ATF3 downregulation reduced SPHK1 expression to attenuate cardiomyocyte injury in MI.

Resveratrol triggers the ER stress-mediated intrinsic apoptosis of neuroblastoma cells coupled with suppression of Rho-dependent migration and consequently prolongs mouse survival

Neuroblastoma, the most common childhood tumor, are highly malignant and fatal because neuroblastoma cells extremely defend against apoptotic targeting. Traditional treatments for neuroblastomas are usually ineffective and lead to serious side effects and poor prognoses. In this study, we investigated the molecular mechanisms of resveratrol-induced insults to neuroblastoma cells and survival extension of nude mice with neuroblastomas, especially in the endoplasmic reticular (ER) stress-intracellular reactive oxygen species (iROS) axis-mediated signals. Resveratrol specifically killed neuroblastoma cells mainly via apoptosis and autophagy rather than necrosis. As to the mechanisms, resveratrol time-dependently triggered productions of Grp78 protein and iROS in neuroblastoma cells. Attenuating the ER stress-iROS signaling axis significantly suppressed resveratrol-induced autophagy, DNA damage, and cell apoptosis. Successively, resveratrol decreased phosphorylation of retinoblastoma protein and induced cell cycle arrest at the S phase, translocation of Bak protein to mitochondria, a reduction in the mitochondrial membrane potential, cascade activation of caspases-9, -3, and -6, and DNA fragmentation. Moreover, weakening the ER stress-iROS axis concomitantly overcome resveratrol-induced decreases in translocation of Rho protein to membranes and succeeding cell migration. Interestingly, administration of resveratrol did not cause significant side effects but could protect the neuroblastoma-bearing nude mice from body weight loss and consequently extended the animal survival. In parallel, resveratrol elevated levels of Grp78 and then induced cell apoptosis in neuroblastoma tissues. This study has shown that resveratrol could kill neuroblastoma cells and extend survival of animals with neuroblastomas by triggering the ER stress-iROS-involved intrinsic apoptosis and suppression of Rho-dependent cell migration. Our results imply the potential of resveratrol as a drug candidate for chemotherapy of neuroblastoma patients.

Cardioprotective Properties of Kaempferol: A Review

Cardiac diseases, such as myocardial infarction and heart failure, have become a major clinical problem globally. The accumulating data demonstrate that bioactive compounds with antioxidant and anti-inflammatory properties have favorable effects on clinical problems. Kaempferol is a flavonoid found in various plants; it has demonstrated cardioprotective properties in numerous cardiac injury models. This review aims to collate updated information regarding the effects of kaempferol on cardiac injury. Kaempferol improves cardiac function by alleviating myocardial apoptosis, fibrosis, oxidative stress, and inflammation while preserving mitochondrial function and calcium homeostasis. However, the mechanisms of action of its cardioprotective properties remain unclear; therefore, elucidating its action could provide insight into directions for future studies.

Relevance of serum levels of the endoplasmic reticulum stress protein GRP78 (glucose-regulated protein 78 kDa) as biomarker in pulmonary diseases

Cellular stress and inflammation contribute to the initiation and progression of a variety of pulmonary diseases. Endoplasmic reticulum (ER) stress and its main regulator GRP78 (glucose-regulated protein 78 kDa) appear to be involved in the pathogenesis of pulmonary diseases, and GRP78 was found to be a biomarker in a wide range of inflammatory diseases. The aim of this study was to investigate the relevance of serum GRP78 in pulmonary disorders.In this prospective cohort study, 78 consecutive patients with chronic obstructive pulmonary disease (COPD, n = 28), asthma (n = 38) or interstitial lung disease (ILD, n = 12) underwent measurement of serum GRP78 levels by ELISA.The mean age of patients was 59.8 ± 12.4 years, 48.7% were female. Patients with elevated GRP78 levels (> median) offered a significantly better oxygenation status (capillary pO2: 75.3 ± 11.7 mmHg vs. 67.8 ± 15.9 mmHg, p = 0.02). Significant correlations were observed between GRP78, on the one hand, and haemoglobin, high-sensitivity C-reactive protein (hs-CRP) and eosinophil counts, on the other hand (haemoglobin: Pearson's r = -0.25, hs-CRP: r = 0.30, eosinophils: r = 0.63).Subsequently, we evaluated GRP78 measurements in function of severity stratifiers of the specific underlying pulmonary disease. ILD patients with a severe diffusion impairment (DLCO< 40% of predicted), exhibited a significant decrease in GRP78 levels (p = 0.01). In COPD and asthma, both characterized by obstructive ventilatory defects, a forced expiratory volume in one second (FEV1) <30% of predicted was accompanied by significantly lower GRP78 (p = 0.0075).In both obstructive and restrictive pulmonary disorders, GRP78 protein concentrations were reduced with increasing disease severity. These data suggest a prevalent role of GRP78 in the presently studied pulmonary disorders.

Metal-Binding Proteins Cross-Linking with Endoplasmic Reticulum Stress in Cardiovascular Diseases

The endoplasmic reticulum (ER), an essential organelle in eukaryotic cells, is widely distributed in myocardial cells. The ER is where secreted protein synthesis, folding, post-translational modification, and transport are all carried out. It is also where calcium homeostasis, lipid synthesis, and other processes that are crucial for normal biological cell functioning are regulated. We are concerned that ER stress (ERS) is widespread in various damaged cells. To protect cells' function, ERS reduces the accumulation of misfolded proteins by activating the unfolded protein response (UPR) pathway in response to numerous stimulating factors, such as ischemia or hypoxia, metabolic disorders, and inflammation. If these stimulatory factors are not eliminated for a long time, resulting in the persistence of the UPR, it will aggravate cell damage through a series of mechanisms. In the cardiovascular system, it will cause related cardiovascular diseases and seriously endanger human health. Furthermore, there has been a growing number of studies on the antioxidative stress role of metal-binding proteins. We observed that a variety of metal-binding proteins can inhibit ERS and, hence, mitigate myocardial damage.

Focus on Alzheimer's Disease: The Role of Fibroblast Growth Factor 21 and Autophagy

Alzheimer's disease (AD) is a disorder of the central nervous system that is typically marked by progressive cognitive impairment and memory loss. Amyloid β plaque deposition and neurofibrillary tangles with hyperphosphorylated tau are the two hallmark pathologies of AD. In mammalian cells, autophagy clears aberrant protein aggregates, thus maintaining proteostasis as well as neuronal health. Autophagy affects production and metabolism of amyloid β and accumulation of phosphorylated tau proteins, whose malfunction can lead to the progression of AD. On the other hand, defective autophagy has been found to induce the production of the neuroprotective factor fibroblast growth factor 21 (FGF21), although the underlying mechanism is unclear. In this review, we highlight the significance of aberrant autophagy in the pathogenesis of AD, discuss the possible mechanisms by which defective autophagy induces FGF21 production, and analyze the potential of FGF21 in the treatment of AD. The findings provide some insights into the potential role of FGF21 and autophagy in the pathogenesis of AD.

The Role of the Mesencephalic Astrocyte-Derived Neurotrophic Factor in Patients in Intensive Care Units Receiving Voriconazole Therapy

Recent publications regarding the role of mesencephalic astrocyte-derived neurotrophic factor (MANF) in various metabolic and degenerative disorders suggest that MANF is both a marker of disease and a possible therapeutic agent. We investigate the role of plasma MANF levels in patients in intensive care units (ICUs) receiving voriconazole (VCZ) therapy while also comparing MANF levels in healthy individuals. A single-center prospective study was conducted. The plasma MANF level in patients in ICU was found to have high interindividual variability and was significantly higher than that in healthy controls (P < .01). Compared with patients using VCZ only, patients using both VCZ and amikacin had 3-fold lower MANF concentrations (P < .05). The MANF concentrations also decreased when alkaline phosphatase (ALP) and serum creatinine levels were above the upper limits of the normal range (P < .05) and the estimated glomerular filtration rate (eGFR) was below the lower limit of the normal range (P < .01). Receiver operating characteristic curve analysis indicated that low MANF levels were associated with high ALP levels, high creatinine levels, and low eGFR. The cut-off value of MANF for ALP levels higher than 126 U/L was 0.35 ng/mL (area under curve, AUC = 0.62, 95%CI = 0.50-0.74, P = .044); for serum creatinine levels higher than 104 μmol/L, the cut-off value was 0.41 ng/mL (AUC = 0.74, 95%CI = 0.62-0.87, P = .001); and for eGFR below 80 mL/min, the cut-off value was 0.75 ng/mL (AUC = 0.70, 95%CI = 0.59-0.81, P = .002). Monitoring plasma MANF levels may be of value for clinical decision-making regarding the choice of antibiotics and the prediction of impaired liver function and renal function in patients admitted to an ICU.

Upregulated PD-1 signaling antagonizes glomerular health in aged kidneys and disease

With an aging population, kidney health becomes an important medical and socioeconomic factor. Kidney aging mechanisms are not well understood. We previously showed that podocytes isolated from aged mice exhibit increased expression of programmed cell death protein 1 (PD-1) surface receptor and its 2 ligands (PD-L1 and PD-L2). PDCD1 transcript increased with age in microdissected human glomeruli, which correlated with lower estimated glomerular filtration rate and higher segmental glomerulosclerosis and vascular arterial intima-to-lumen ratio. In vitro studies in podocytes demonstrated a critical role for PD-1 signaling in cell survival and in the induction of a senescence-associated secretory phenotype. To prove PD-1 signaling was critical to podocyte aging, aged mice were injected with anti-PD-1 antibody. Treatment significantly improved the aging phenotype in both kidney and liver. In the glomerulus, it increased the life span of podocytes, but not that of parietal epithelial, mesangial, or endothelial cells. Transcriptomic and immunohistochemistry studies demonstrated that anti-PD-1 antibody treatment improved the health span of podocytes. Administering the same anti-PD-1 antibody to young mice with experimental focal segmental glomerulosclerosis (FSGS) lowered proteinuria and improved podocyte number. These results suggest a critical contribution of increased PD-1 signaling toward both kidney and liver aging and in FSGS.

Follistatin-like 1 promotes proliferation of matured human hypoxic iPSC-cardiomyocytes and is secreted by cardiac fibroblasts

The human heart has limited regenerative capacity. Therefore, patients often progress to heart failure after ischemic injury, despite advances in reperfusion therapies generally decreasing mortality. Depending on its glycosylation state, Follistatin-like 1 (FSTL1) has been shown to increase cardiomyocyte (CM) proliferation, decrease CM apoptosis, and prevent cardiac rupture in animal models of ischemic heart disease. To explore its therapeutic potential, we used a human in vitro model of cardiac ischemic injury with human induced pluripotent stem cell-derived CMs (iPSC-CMs) and assessed regenerative effects of two differently glycosylated variants of human FSTL1. Furthermore, we investigated the FSTL1-mediated interplay between human cardiac fibroblasts (cFBs) and iPSC-CMs in hypoxia. Both FSTL1 variants increased viability, while only hypo-glycosylated FSTL1 increased CM proliferation post-hypoxia. Human fetal cardiac fibroblasts (fcFBs) expressed and secreted FSTL1 under normoxic conditions, while FSTL1 secretion increased by iPSC-cFBs upon hypoxia but decreased in iPSC-CMs. Co-culture of iPSC-CMs and cFBs increased FSTL1 secretion compared with cFB mono-culture. Taken together, we confirm that FSTL1 induces iPSC-CM proliferation in a human cardiac in vitro hypoxia damage model. Furthermore, we show hypoxia-related FSTL1 secretion by human cFBs and indications for FSTL1-mediated intercellular communication between cardiac cell types in response to hypoxic conditions.

Endoplasmin regulates differentiation of tonsil-derived mesenchymal stem cells into chondrocytes through ERK signaling

It is well-known that some species of lizard have an exceptional ability known as caudal autotomy (voluntary self-amputation of the tail) as an anti-predation mechanism. After amputation occurs, they can regenerate their new tails in a few days. The new tail section is generally shorter than the original one and is composed of cartilage rather than vertebrae bone. In addition, the skin of the regenerated tail distinctly differs from its original appearance. We performed a proteomics analysis for extracts derived from regenerating lizard tail tissues after amputation and found that endoplasmin (ENPL) was the main factor among proteins up-regulated in expression during regeneration. Thus, we performed further experiments to determine whether ENPL could induce chondrogenesis of tonsil-derived mesenchymal stem cells (T-MSCs). In this study, we found that chondrogenic differentiation was associated with an increase of ENPL expression by ER stress. We also found that ENPL was involved in chondrogenic differentiation of T-MSCs by suppressing extracellular signal-regulated kinase (ERK) phosphorylation.

Stress-responsive regulation of extracellular proteostasis

Genetic, environmental, and aging-related insults can promote the misfolding and subsequent aggregation of secreted proteins implicated in the pathogenesis of numerous diseases. This has led to considerable interest in understanding the molecular mechanisms responsible for regulating proteostasis in extracellular environments such as the blood and cerebrospinal fluid (CSF). Extracellular proteostasis is largely dictated by biological pathways comprising chaperones, folding enzymes, and degradation factors localized to the ER and extracellular space. These pathways limit the accumulation of nonnative, potentially aggregation-prone proteins in extracellular environments. Many reviews discuss the molecular mechanisms by which these pathways impact the conformational integrity of the secreted proteome. Here, we instead focus on describing the stress-responsive mechanisms responsible for adapting ER and extracellular proteostasis pathways to protect the secreted proteome from pathologic insults that challenge these environments. Further, we highlight new strategies to identify stress-responsive pathways involved in regulating extracellular proteostasis and describe the pathologic and therapeutic implications for these pathways in human disease.

Sacubitril Ameliorates Cardiac Fibrosis Through Inhibiting TRPM7 Channel

Heart failure caused by cardiac fibrosis has become a major challenge of public health worldwide. Cardiomyocyte programmed cell death (PCD) and activation of fibroblasts are crucial pathological features, both of which are associated with aberrant Ca²⁺ influx. Transient receptor potential cation channel subfamily M member 7 (TRPM7), the major Ca²⁺ permeable channel, plays a regulatory role in cardiac fibrosis. In this study, we sought to explore the mechanistic details for sacubitril, a component of sacubitril/valsartan, in treating cardiac fibrosis. We demonstrated that sacubitril/valsartan could effectively ameliorate cardiac dysfunction and reduce cardiac fibrosis induced by isoprotereno (ISO) in vivo. We further investigated the anti-fibrotic effect of sacubitril in fibroblasts. LBQ657, the metabolite of sacubitril, could significantly attenuate transforming growth factor-β 1 (TGF-β1) induced cardiac fibrosis by blocking TRPM7 channel, rather than suppressing its protein expression. In addition, LBQ657 reduced hypoxia-induced cardiomyocyte PCD via suppression of Ca²⁺ influx regulated by TRPM7. These findings suggested that sacubitril ameliorated cardiac fibrosis by acting on both fibroblasts and cardiomyocytes through inhibiting TRPM7 channel.

HA15 alleviates bone loss in ovariectomy-induced osteoporosis by targeting HSPA5

The imbalance between osteogenesis and adipogenesis in the bone marrow is the main characteristic of osteoporosis (OP). Thus, exploring regulation of the differentiation of bone marrow stromal cells (BMSCs) into osteoblasts and adipocytes is important to identify novel targets for the treatment of OP. In the present study, the master regulator of endoplasmic reticulum (ER) stress, heat shock protein family A (Hsp70) member 5 (HSPA5) was shown to significantly accumulate in osteoblasts and adipocytes, but not in osteoclasts in bone sections from aged and postmenopausal OP mice. In vitro study revealed that HSPA5 negatively modulated osteogenic differentiation and positively promoted adipogenic differentiation, and that targeting HSPA5 with its inhibitor HA15 enhanced osteogenic differentiation and inhibited adipogenic differentiation. Also, HA15 treatment induces ER stress and autophagy, and decreases apoptosis in cells. We constructed a postmenopausal OP model in mice with ovariectomy surgery, and treated the mice with HA15. The results showed that HA15 treatment induced appropriate ER stress, activated autophagy and decreased apoptosis in osteoblasts, thereby alleviating bone loss in vivo. Our results indicated that HSPA5 participated in OP pathogenesis by regulating the differentiation of BMSCs. HSPA5 may serve as a new target for the treatment of OP, and targeting HSPA5 with HA15 prevents the progression of OP and provides a candidate therapeutic molecule for postmenopausal OP.

iPLA2β Contributes to ER Stress-Induced Apoptosis during Myocardial Ischemia/Reperfusion Injury

Both calcium-independent phospholipase A2 beta (iPLA2β) and endoplasmic reticulum (ER) stress regulate important pathophysiological processes including inflammation, calcium homeostasis and apoptosis. However, their roles in ischemic heart disease are poorly understood. Here, we show that the expression of iPLA2β is increased during myocardial ischemia/reperfusion (I/R) injury, concomitant with the induction of ER stress and the upregulation of cell death. We further show that the levels of iPLA2β in serum collected from acute myocardial infarction (AMI) patients and in samples collected from both in vivo and in vitro I/R injury models are significantly elevated. Further, iPLA2β knockout mice and siRNA mediated iPLA2β knockdown are employed to evaluate the ER stress and cell apoptosis during I/R injury. Additionally, cell surface protein biotinylation and immunofluorescence assays are used to trace and locate iPLA2β. Our data demonstrate the increase of iPLA2β augments ER stress and enhances cardiomyocyte apoptosis during I/R injury in vitro and in vivo. Inhibition of iPLA2β ameliorates ER stress and decreases cell death. Mechanistically, iPLA2β promotes ER stress and apoptosis by translocating to ER upon myocardial I/R injury. Together, our study suggests iPLA2β contributes to ER stress-induced apoptosis during myocardial I/R injury, which may serve as a potential therapeutic target against ischemic heart disease.

Glucose-regulated protein (GRP78) is an important cell surface receptor for viral invasion, cancers, and neurological disorders

The 78 kDa glucose-regulated protein (GRP78) is an endoplasmic reticulum (ER)-resident molecular chaperone. GRP78 is a member of the 70 kDa heat shock family of proteins involved in correcting and clearing misfolded proteins in the ER. In response to cellular stress, GRP78 escapes from the ER and moves to the plasma membrane where it (a) functions as a receptor for many ligands, and (b) behaves as an autoantigen for autoantibodies that contribute to human disease and cancer. Cell surface GRP78 (csGRP78) associates with the major histocompatibility complex class I (MHC-I), and is the port of entry for several viruses, including the predictive binding of the novel SARS-CoV-2. Furthermore, csGRP78 is found in association with partners as diverse as the teratocarcinoma-derived growth factor 1 (Cripto), the melanocortin-4 receptor (MC4R) and the DnaJ-like protein MTJ-1. CsGRP78 also serves as a receptor for a large variety of ligands including activated α₂ -macroglobulin (α₂ M*), plasminogen kringle 5 (K5), microplasminogen, the voltage-dependent anion channel (VDAC), tissue factor (TF), and the prostate apoptosis response-4 protein (Par-4). In this review, we discuss the mechanisms involved in the translocation of GRP78 from the ER to the cell surface, and the role of secreted GRP78 and its autoantibodies in cancer and neurological disorders.

Seipin Deficiency Accelerates Heart Failure Due to Calcium Handling Abnormalities and Endoplasmic Reticulum Stress in Mice

Seipin deficiency can induce hypertrophic cardiomyopathy and heart failure, which often leads to death in humans. To explore the effects and the possible mechanisms of Seipin deficiency in myocardial remodeling, Seipin knockout (SKO) mice underwent transverse aortic constriction (TAC) for 12 weeks. We found a more severe left ventricular hypertrophy and diastolic heart failure and increases in inflammatory cell infiltration, collagen deposition, and apoptotic bodies in the SKO group compared to those in the wild type (WT) group after TAC. Electron microscopy also showed a more extensive sarcoplasmic reticulum expansion, deformation of microtubules, and formation of mitochondrial lesions in the cardiomyocytes of SKO mice than in those of WT mice after TAC. Compared with the WT group, the SKO group showed increases in endoplasmic reticulum (ER) stress-, inflammation-, and fibrosis-related gene expression, while calcium ion-related factors, such as Serca2a and Ryr, were decreased in the SKO group after TAC. Increased levels of the ER stress-related protein GRP78 and decreased SERCA2a and P-RYR protein levels were detected in the SKO group compared with the WT group after TAC. Slowing of transient Ca²⁺ current decay and an increased SR Ca²⁺ content in myocytes were detected in the cardiomyocytes of SKO mice. Adipose tissue transplantation could not rescue the cardiac hypertrophy after TAC in SKO mice. In conclusion, we found that Seipin deficiency could promote cardiac hypertrophy and diastolic heart failure after TAC in mice. These changes may be related to the impairment of myocardial calcium handling, ER stress, inflammation, and apoptosis.

The Role of Proteostasis Derailment in Cardiac Diseases

The incidence and prevalence of cardiac diseases, which are the main cause of death worldwide, are likely to increase because of population ageing and changes in lifestyle. Prevailing theories about the mechanisms of cardiac disease onset feature the gradual derailment of cellular protein homeostasis (proteostasis) and loss of the protein quality control as central factors. In the heart, loss of protein patency, due to flaws in design (genetically) or environmentally-induced wear and tear, may overwhelm protein quality control, thereby triggering derailment of proteostasis and contributing to cardiac disease onset.