A protective role of unfolded protein response in mouse ischemic acute kidney injury

Novel insights into the protective effects of leonurine against acute kidney injury: Inhibition of ER stress-associated ferroptosis via regulating ATF4/CHOP/ACSL4 pathway

Acute kidney injury (AKI) is a common and serious global health problem with high risks of mortality and the development of chronic kidney diseases. Leonurine is a unique bioactive component from Leonurus japonicus Houtt. and exerts antioxidant, antiapoptotic or anti-inflammatory properties. This study aimed to explore the benefits of leonurine on AKI and the possible mechanisms involved, with a particular foc on the regulation of ferroptosis and endoplasmic reticulum (ER) stress. Our results showed that leonurine exhibited prominent protective effects against AKI, as evidenced by the amelioration of histopathological alterations and reduction of renal dysfunction. In addition, leonurine significantly suppressed ferroptosis in AKI both in vivo and in vitro by effectively restoring ultrastructural abnormalities in mitochondria, decreasing ASCL4 and 4-HNE levels, scavenging reactive oxygen species (ROS), as well as increasing GPX4 and GSH levels. In parallel, leonurine also markedly mitigated ER stress via down-regulating PERK, eIF-2α, ATF4, CHOP and CHAC1. Further studies suggested that ER stress was closely involved in erastin-induced ferroptosis, and leonurine protected tubular epithelial cells in vitro by inhibiting ER stress-associated ferroptosis via regulating ATF4/CHOP/ASCL4 signalling pathway. Mechanistically, ATF4 silencing in vitro regulated CHOP and ACSL4 expressions, ultimately weakening both ER stress and ferroptosis. Notably, analyses of single-cell RNA sequencing data revealed that ATF4, CHOP and ASCL4 in renal tubular cells were all abnormally upregulated in patients with AKI compared to healthy controls, suggesting their contributions to the pathogenesis of AKI. Altogether, these findings suggest that leonurine alleviates AKI by inhibiting ER stress-associated ferroptosis via regulating ATF4/CHOP/ASCL4 signalling pathway, thus providing novel mechanisms for AKI treatment.

Exploring the therapeutic potential of quercetin: A focus on its sirtuin-mediated benefits

As the global population ages, preventing lifestyle- and aging-related diseases is increasing, necessitating the search for safe and affordable therapeutic interventions. Among nutraceuticals, quercetin, a flavonoid ubiquitously present in various plants, has garnered considerable interest. This review aimed to collate and analyze existing literature on the therapeutic potentials of quercetin, especially its interactions with SIRTs and its clinical applicability based on its bioavailability and safety. This narrative review was based on a literature survey spanning from 2015 to 2023 using PUBMED. The keywords and MeSH terms used were: "quercetin" AND "bioavailability" OR "metabolism" OR "metabolites" as well as "quercetin" AND "SIRTuin" OR "SIRT*" AND "cellular effects" OR "pathway" OR "signaling" OR "neuroprotective" OR "cardioprotective" OR "nephroprotective" OR "antiatherosclerosis" OR "diabetes" OR "antidiabetic" OR "dyslipidemia" AND "mice" OR "rats". Quercetin demonstrates multiple therapeutic activities, including neuroprotective, cardioprotective, and anti-atherosclerotic effects. Its antioxidant, anti-inflammatory, antiviral, and immunomodulatory properties are well-established. At a molecular level, it majorly interacts with SIRTs, particularly SIRT1 and SIRT6, and modulates numerous signaling pathways, contributing to its therapeutic effects. These pathways play roles in reducing oxidative stress, inflammation, autophagy regulation, mitochondrial biogenesis, glucose utilization, fatty acid oxidation, and genome stability. However, clinical trials on quercetin's effectiveness in humans are scarce. Quercetin exhibits a wide range of SIRT-mediated therapeutic effects. Despite the compelling preclinical data, more standardized clinical trials are needed to fully understand its therapeutic potential. Future research should focus on addressing its bioavailability and safety concerns.

Acute kidney injury: exploring endoplasmic reticulum stress-mediated cell death

Acute kidney injury (AKI) is a global health problem, given its substantial morbidity and mortality rates. A better understanding of the mechanisms and factors contributing to AKI has the potential to guide interventions aimed at mitigating the risk of AKI and its subsequent unfavorable outcomes. Endoplasmic reticulum stress (ERS) is an intrinsic protective mechanism against external stressors. ERS occurs when the endoplasmic reticulum (ER) cannot deal with accumulated misfolded proteins completely. Excess ERS can eventually cause pathological reactions, triggering various programmed cell death (autophagy, ferroptosis, apoptosis, pyroptosis). This article provides an overview of the latest research progress in deciphering the interaction between ERS and different programmed cell death. Additionally, the report consolidates insights into the roles of ERS in AKI and highlights the potential avenues for targeting ERS as a treatment direction toward for AKI.

Isoliquiritigenin pretreatment regulates ER stress and attenuates cisplatin-induced nephrotoxicity in male Wistar rats

Cisplatin (CP) is a chemotherapeutic drug used to treat solid tumors. However, studies have revealed its nephrotoxic effect. Oxidative stress, endoplasmic reticulum (ER) stress, and mitochondrial dysfunction are involved in CP-induced renal damage. Thus, preconditioning (hormetic effect) of ER stress is a strategy to prevent CP-induced renal damage. On the other hand, isoliquiritigenin (IsoLQ) is recognized as a flavonoid with antioxidant properties and an inducer of ER stress. Therefore, we evaluated the ER stress-inducing capacity of IsoLQ and its possible protective effect against CP-induced nephrotoxicity in adult male Wistar rats. The findings reflected that IsoLQ pretreatment might decrease renal damage by reducing plasma creatinine and blood urea nitrogen levels in animals with CP-induced nephrotoxicity. These may be associated with IsoLQ activating ER stress and unfolded protein response (UPR). We found increased messenger RNA levels of the ER stress marker glucose-related protein 78 kDa (GRP78). In addition, we also found that pretreatment with IsoLQ reduced the levels of CCAAT/enhancer-binding protein-homologous protein (CHOP) and X-box-binding protein 1 (XBP1) in the renal cortex, reflecting that IsoLQ can regulate the UPR and activation of the apoptotic pathway. Moreover, this preconditioning with IsoLQ of ER stress had oxidative stress-regulatory effects, as it restored the activity of glutathione peroxidase and glutathione reductase enzymes. Finally, IsoLQ modifies the protein expression of mitofusin 2 (Mfn-2) and voltage-dependent anion channel (VDAC). In conclusion, these data suggest that IsoLQ pretreatment has a nephroprotective effect; it could functionally regulate the ER and mitochondria and reduce CP-induced renal damage by attenuating hormesis-mediated ER stress.

Research progress on endoplasmic reticulum homeostasis in kidney diseases

The endoplasmic reticulum (ER) plays important roles in biosynthetic and metabolic processes, including protein and lipid synthesis, Ca²⁺ homeostasis regulation, and subcellular organelle crosstalk. Dysregulation of ER homeostasis can cause toxic protein accumulation, lipid accumulation, and Ca²⁺ homeostasis disturbance, leading to cell injury and even death. Accumulating evidence indicates that the dysregulation of ER homeostasis promotes the onset and progression of kidney diseases. However, maintaining ER homeostasis through unfolded protein response, ER-associated protein degradation, autophagy or ER-phagy, and crosstalk with other organelles may be potential therapeutic strategies for kidney disorders. In this review, we summarize the recent research progress on the relationship and molecular mechanisms of ER dysfunction in kidney pathologies. In addition, the endogenous protective strategies for ER homeostasis and their potential application for kidney diseases have been discussed.

XBP1 modulates endoplasmic reticulum and mitochondria crosstalk via regulating NLRP3 in renal ischemia/reperfusion injury

The functional status of mitochondria and the endoplasmic reticulum are central to renal ischemia/reperfusion injury (IRI). X-box binding protein 1 (XBP1) is an important transcription factor in endoplasmic reticulum stress. NLR family pyrin domain containing-3 (NLRP3) inflammatory bodies are closely related to renal IRI. In vivo and in vitro, we examined the molecular mechanisms and functions of XBP1-NLRP3 signaling in renal IRI, which influences ER-mitochondrial crosstalk. In this study, mice were subjected to 45 min of unilateral renal warm ischemia, the other kidney resected, and reperfusion was performed for 24 h in vivo. In vitro, murine renal tubular epithelial cells (TCMK-1) were exposed to hypoxia for 24 h and reoxygenation for 2 h. Tissue or cell damage was evaluated by measuring blood urea nitrogen and creatinine levels, histological staining, flow cytometry, terminal deoxynucleotidyl transferase-mediated nick-end labeling, diethylene glycol staining, and transmission electron microscopy (TEM). Western blotting, immunofluorescence staining, and ELISA were used to analyze protein expression. Whether XBP1 regulates the NLRP3 promoter was evaluated using a luciferase reporter assay. Kidney damage was reduced with decreasing blood urea nitrogen, creatinine, interleukin-1β, and interleukin-18 levels. XBP1 deficiency reduced tissue damage and cell apoptosis, protecting the mitochondria. Disruption of XBP1 was associated with reduced NLRP3 and cleaved caspase-1 levels and markedly improved survival. In vitro in TCMK-1 cells, XBP1 interference inhibited caspase-1-dependent mitochondrial damage and reduced the production of mitochondrial reactive oxygen species. The luciferase assay showed that spliced XBP1 isoforms enhanced the activity of the NLRP3 promoter. These findings reveal that XBP1 downregulation suppresses the expression of NLRP3, a potential regulator of endoplasmic reticulum mitochondrial crosstalk in nephritic injury and a potential therapeutic target in XBP1-mediated aseptic nephritis.

Emerging links between endoplasmic reticulum stress responses and acute kidney injury

All cell types must maintain homeostasis under periods of stress. To prevent the catastrophic effects of stress, all cell types also respond to stress by inducing protective pathways. Within the cell, the endoplasmic reticulum (ER) is exquisitely stress-sensitive, primarily because this organelle folds, posttranslationally processes, and sorts one-third of the proteome. In the 1990s, a specialized ER stress response pathway was discovered, the unfolded protein response (UPR), which specifically protects the ER from damaged proteins and toxic chemicals. Not surprisingly, UPR-dependent responses are essential to maintain the function and viability of cells continuously exposed to stress, such as those in the kidney, which have high metabolic demands, produce myriad protein assemblies, continuously filter toxins, and synthesize ammonia. In this mini-review, we highlight recent articles that link ER stress and the UPR with acute kidney injury (AKI), a disease that arises in ∼10% of all hospitalized individuals and nearly half of all people admitted to intensive care units. We conclude with a discussion of prospects for treating AKI with emerging drugs that improve ER function.

Inhibition of histone deacetylation with vorinostat does not prevent tunicamycin-mediated acute kidney injury

Endoplasmic reticulum (ER) stress is associated with acute kidney injury (AKI) caused by various mechanisms, including antibiotics, non-steroidal anti-inflammatory drugs, cisplatin, and radiocontrast. Tunicamycin (TM) is a nucleoside antibiotic that induces ER stress and is a commonly used model of AKI. 4-phenylbutyrate (4-PBA) is a chemical chaperone and histone deacetylase (HDAC) inhibitor and has been shown to protect the kidney from ER stress, apoptosis, and structural damage in a tunicamycin model of AKI. The renal protection provided by 4-PBA is attributed to its ability to prevent misfolded protein aggregation and inhibit ER stress; however, the HDAC inhibitor effects of 4-PBA have not been examined in the TM-induced model of AKI. As such, the main objective of this study was to determine if histone hyperacetylation provides any protective effects against TM-mediated AKI. The FDA-approved HDAC inhibitor vorinostat was used, as it has no ER stress inhibitory effects and therefore the histone hyperacetylation properties alone could be investigated. In vitro work demonstrated that vorinostat inhibited histone deacetylation in cultured proximal tubular cells but did not prevent ER stress or protein aggregation induced by TM. Vorinostat induced a significant increase in cell death, and exacerbated TM-mediated total cell death and apoptotic cell death. Wild type male mice were treated with TM (0.5 mg/kg, intraperitoneal injection), with or without vorinostat (50 mg/kg/day) or 4-PBA (1 g/kg/day). Mice treated with 4-PBA or vorinostat exhibited similar levels of histone hyperacetylation. Expression of the pro-apoptotic protein CHOP was induced with TM, and not inhibited by vorinostat. Further, vorinostat did not prevent any renal damage or decline in renal function caused by tunicamycin. These data suggest that the protective mechanisms found by 4-PBA are primarily due to its molecular chaperone properties, and the HDAC inhibitors used did not provide any protection against renal injury caused by ER stress.

Endoplasmic Reticulum Stress Induced Proliferation Remains Intact in Aging Mouse β-Cells

Aging is associated with loss of proliferation of the insulin-secreting β-cell, a possible contributing factor to the increased prevalence of type 2 diabetes in the elderly. Our group previously discovered that moderate endoplasmic reticulum (ER) stress occurring during glucose exposure increases the adaptive β-cell proliferation response. Specifically, the ATF6α arm of the tripartite Unfolded Protein Response (UPR) promotes β-cell replication in glucose excess conditions. We hypothesized that β-cells from older mice have reduced proliferation due to aberrant UPR signaling or an impaired proliferative response to ER stress or ATF6α activation. To investigate, young and old mouse islet cells were exposed to high glucose with low-dose thapsigargin or activation of overexpressed ATF6α, and β-cell proliferation was quantified by BrdU incorporation. UPR pathway activation was compared by qPCR of target genes and semi-quantitative Xbp1 splicing assay. Intriguingly, although old β-cells had reduced proliferation in high glucose compared to young β-cells, UPR activation and induction of proliferation in response to low-dose thapsigargin or ATF6α activation in high glucose were largely similar between young and old. These results suggest that loss of UPR-led adaptive proliferation does not explain the reduced cell cycle entry in old β-cells, and raise the exciting possibility that future therapies that engage adaptive UPR could increase β-cell number through proliferation even in older individuals.

Therapeutic Approaches Targeting Proteostasis in Kidney Disease and Fibrosis

Pathological insults usually disturb the folding capacity of cellular proteins and lead to the accumulation of misfolded proteins in the endoplasmic reticulum (ER), which leads to so-called “ER stress”. Increasing evidence indicates that ER stress acts as a trigger factor for the development and progression of many kidney diseases. The unfolded protein responses (UPRs), a set of molecular signals that resume proteostasis under ER stress, are thought to restore the adaptive process in chronic kidney disease (CKD) and renal fibrosis. Furthermore, the idea of targeting UPRs for CKD treatment has been well discussed in the past decade. This review summarizes the up-to-date literature regarding studies on the relationship between the UPRs, systemic fibrosis, and renal diseases. We also address the potential therapeutic possibilities of renal diseases based on the modulation of UPRs and ER proteostasis. Finally, we list some of the current UPR modulators and their therapeutic potentials.

ER stress preconditioning ameliorates liver damage after hemorrhagic shock and reperfusion

The mismatch of oxygen supply and demand during hemorrhagic shock disturbs endoplasmic reticulum (ER) homeostasis. The resulting accumulation of unfolded proteins in the ER lumen, which is a condition that is defined as ER stress, triggers the unfolded protein response (UPR). Since the UPR influences the extent of organ damage following hemorrhagic shock/reperfusion (HS/R) and mediates the protective effects of stress preconditioning before ischemia-reperfusion injury, the current study investigated the mechanisms of ER stress preconditioning and its impact on post-hemorrhagic liver damage. Male C56BL/6-mice were injected intraperitoneally with the ER stress inductor tunicamycin (TM) or its drug vehicle 48 h prior to being subjected to a 90 min pressure-controlled hemorrhagic shock (30±5 mmHg). A period of 14 h after hemorrhagic shock induction, mice were sacrificed. Hepatocellular damage was quantified by analyzing hepatic transaminases and hematoxylin-eosin stained liver tissue sections. Additionally, the topographic expression patterns of the ER stress marker binding immunoglobulin protein (BiP), UPR signaling pathways, and the autophagy marker Beclin1 were evaluated. TM injection significantly increased BiP expression and modified the topographic expression patterns of the UPR signaling proteins. In addition, immunohistochemical analysis of Beclin1 revealed an increased pericentral staining intensity following TM pretreatment. The histologic analysis of hepatocellular damage demonstrated a significant reduction in cell death areas in HS/R+TM (P=0.024). ER stress preconditioning influences the UPR and alleviates post-hemorrhagic liver damage. The beneficial effects were, at least partially, mediated by the upregulation of BiP and autophagy induction. These results underscore the importance of the UPR in the context of HS/R and may help identify novel therapeutic targets.

Isoliquiritigenin Pretreatment Induces Endoplasmic Reticulum Stress-Mediated Hormesis and Attenuates Cisplatin-Induced Oxidative Stress and Damage in LLC-PK1 Cells

Isoliquiritigenin (IsoLQ) is a flavonoid with antioxidant properties and inducer of endoplasmic reticulum (ER) stress. In vitro and in vivo studies show that ER stress-mediated hormesis is cytoprotective; therefore, natural antioxidants and ER stress inducers have been used to prevent renal injury. Oxidative stress and ER stress are some of the mechanisms of damage involved in cisplatin (CP)-induced nephrotoxicity. This study aims to explore whether IsoLQ pretreatment induces ER stress and produces hormesis to protect against CP-induced nephrotoxicity in Lilly Laboratories Cell-Porcine Kidney 1 (LLC-PK1) cells. During the first stage of this study, both IsoLQ protective concentration and pretreatment time against CP-induced toxicity were determined by cell viability. At the second stage, the effect of IsoLQ pretreatment on cell viability, ER stress, and oxidative stress were evaluated. IsoLQ pretreatment in CP-treated cells induces expression of glucose-related proteins 78 and 94 kDa (GRP78 and GRP94, respectively), attenuates CP-induced cell death, decreases reactive oxygen species (ROS) production, and prevents the decrease in glutathione/glutathione disulfide (GSH/GSSG) ratio, free thiols levels, and glutathione reductase (GR) activity. These data suggest that IsoLQ pretreatment has a moderately protective effect on CP-induced toxicity in LLC-PK1 cells, through ER stress-mediated hormesis, as well as by the antioxidant properties of IsoLQ.

Promising therapeutic effect of thapsigargin nanoparticles on chronic kidney disease through the activation of Nrf2 and FoxO1

Pathophysiological states cause misfolded protein accumulation in the endoplasmic reticulum (ER). Then, ER stress and the unfolded protein response (UPR) are activated. Targeting ER stress may enhance the adaptive UPR and then protect the cell against pathogenic environments. In the present study, we utilized nanotechnology to synthesize thapsigargin nanoparticles (TG NPs) which induced ER stress and the UPR pathway, to study the role of ER stress and autophagy in chronic kidney disease (CKD). We found that the mRNA levels of ER stress- and autophagy-related molecules were elevated in the renal tissue of CKD patients compared to those of healthy individuals. Furthermore, TG NPs induced the UPR pathway and autophagy in HK-2 human kidney tubular epithelial cells. TG NPs protected HK-2 cells against oxidative stress-induced cell death through the activation of Nrf2 and FoxO1. The siRNA-mediated inhibition of Nrf2 or FoxO1 resulted in enhanced oxidative stress-induced cytotoxicity in HK-2 cells. In a mouse model of adenine diet-induced CKD, TG NPs and KIM-1-TG NPs ameliorated renal injury through the stimulation of ER stress and its downstream pathways. Our findings suggest that the induction of ER stress using pharmacological agents may offer a promising therapeutic strategy for preventing or interfering with CKD progression.

Renal tubular cell spliced X-box binding protein 1 (Xbp1s) has a unique role in sepsis-induced acute kidney injury and inflammation

Sepsis is a systemic inflammatory state in response to infection, and concomitant acute kidney injury (AKI) increases mortality significantly. Endoplasmic reticulum stress is activated in many cell types upon microbial infection and modulates inflammation. The role of endoplasmic reticulum signaling in the kidney during septic AKI is unknown. Here we tested the role of the spliced X-box binding protein 1 (Xbp1s), a key component of the endoplasmic reticulum stress-activated pathways, in the renal response to sepsis in the lipopolysaccharide (LPS) model. Xbp1s was increased in the kidneys of mice treated with LPS but not in other models of AKI, or several chronic kidney disease models. The functional significance of Xbp1s induction was examined by genetic manipulation in renal tubules. Renal tubule-specific overexpression of Xbp1s caused severe tubule dilation and vacuolation with expression of the injury markers Kim1 and Ngal, the pro-inflammatory molecules interleukin-6 (Il6) and Toll-like receptor 4 (Tlr4), decreased kidney function and 50% mortality in five days. Renal tubule-specific genetic ablation of Xbp1 had no phenotype at baseline. However, after LPS, Xbp1 knockdown mice displayed lower renal NGAL, pro-apoptotic factor CHOP, serum creatinine levels, and a tendency towards lower Tlr4 compared to LPS-treated mice with intact Xbp1s. LPS treatment in Xbp1s-overexpressing mice caused a mild increase in NGAL and CHOP compared to LPS-treated mice without genetic Xbp1s overexpression. Thus, increased Xbp1s signaling in renal tubules is unique to sepsis-induced AKI and contributes to renal inflammation and injury. Inhibition of this pathway may be a potential portal to alleviate injury.

Acute kidney injury overview: From basic findings to new prevention and therapy strategies

Acute kidney injury (AKI) is defined as a decrease in kidney function within hours, which encompasses both injury and impairment of renal function. AKI is not considered a pathological condition of single organ failure, but a syndrome in which the kidney plays an active role in the progression of multi-organ dysfunction. The incidence rate of AKI is increasing and becoming a common (8-16% of hospital admissions) and serious disease (four-fold increased hospital mortality) affecting public health costs worldwide. AKI also affects the young and previously healthy individuals affected by infectious diseases in Latin America. Because of the multifactorial pathophysiological mechanisms, there is no effective pharmacological therapy that prevents the evolution or reverses the injury once established; therefore, renal replacement therapy is the only current alternative available for renal patients. The awareness of an accurate and prompt recognition of AKI underlying the various clinical phenotypes is an urgent need for more effective therapeutic interventions to diminish mortality and socio-economic impacts of AKI. The use of biomarkers as an indicator of the initial stage of the disease is critical and the cornerstone to fulfill the gaps in the field. This review discusses emerging strategies from basic science toward the anticipation of features, treatment of AKI, and new treatments using pharmacological and stem cell therapies. We will also highlight bioartificial kidney studies, addressing the limitations of the development of this innovative technology.

Proteostasis and Beyond: ATF6 in Ischemic Disease

Endoplasmic reticulum (ER) stress is a pathological hallmark of numerous ischemic diseases, including stroke and myocardial infarction (MI). In these diseases, ER stress leads to activation of the unfolded protein response (UPR) and subsequent adaptation of cellular physiology in ways that dictate cellular fate following ischemia. Recent evidence highlights a protective role for the activating transcription factor 6 (ATF6) arm of the UPR in mitigating adverse outcomes associated with ischemia/reperfusion (I/R) injury in multiple disease models. This suggests ATF6 as a potential therapeutic target for intervening in diverse ischemia-related disorders. Here, we discuss the evidence demonstrating the importance of ATF6 signaling in protecting different tissues against ischemic damage and discuss preclinical results focused on defining the potential for pharmacologically targeting ATF6 to intervene in such diseases.

Endoplasmic reticulum stress in ischemic and nephrotoxic acute kidney injury

Acute kidney injury (AKI) is a medical condition characterized by kidney damage with a rapid decline of renal function, which is associated with high mortality and morbidity. Recent research has further established an intimate relationship between AKI and chronic kidney disease. Perturbations of kidney cells in AKI result in the accumulation of unfolded and misfolded proteins in the endoplasmic reticulum (ER), leading to unfolded protein response (UPR) or ER stress. In this review, we analyze the role and regulation of ER stress in AKI triggered by renal ischemia-reperfusion and cisplatin nephrotoxicity. The balance between the two major components of UPR, the adaptive pathway and the apoptotic pathway, plays a critical role in determining the cell fate in ER stress. The adaptive pathway is evoked to attenuate translation, induce chaperones, maintain protein homeostasis and promote cell survival. Prolonged ER stress activates the apoptotic pathway, resulting in the elimination of dysfunctional cells. Therefore, regulating ER stress in kidney cells may provide a therapeutic target in AKI. KEY MESSAGES Perturbations of kidney cells in acute kidney injury result in the accumulation of unfolded and misfolded proteins in ER, leading to unfolded protein response (UPR) or ER stress. The balance between the adaptive pathway and the apoptotic pathway of UPR plays a critical role in determining the cell fate in ER stress. Modulation of ER stress in kidney cells may provide a therapeutic strategy for acute kidney injury.

Novel Role of ER Stress and Mitochondria Stress in Serum-deprivation Induced Apoptosis of Rat Mesenchymal Stem Cells

The poor survival of mesenchymal stem cells (MSCs) compromises the efficacy of stem cell therapy. Growth factor deprivation is one of the important factors that have challenged the survival of donor MSCs in cell therapy. In this study, the aim was to evaluate the effect of serum deprivation on the cell death of MSCs and to investigate the underlying mechanisms. Apoptosis of MSCs was evaluated with Hoechst 33342/PI staining. Signaling pathways involved in serum-deprivation induced apoptosis were analyzed using Western blotting. The results revealed that serum deprivation induced apoptosis in MSCs within 72 h of treatment. Serum deprivation was shown to lead to protein expression alterations in Bax, Bcl-2, casepase-3, casepase-8, GRP78, and CHOP during experiments. The data suggested that the mitochondria death pathway, the extrinsic apoptotic pathway and the endoplastic reticulum(ER) stress pathway were all involved in MSCs apoptosis. The increase in expression of CHOP and the simultaneous decrease in Bcl-2 expression suggest a synergistic effect in apoptosis induction in both the mitochondrion and the ER.

DBP-induced endoplasmic reticulum stress in male germ cells causes autophagy, which has a cytoprotective role against apoptosis in vitro and in vivo

Recently, spermatogenic cell apoptosis was shown to play a key role in the induction of testicular atrophy by dibutyl phthalate (DBP), thus causing reproductive toxicology. However, the molecular events induced by DBP in apoptotic germ cells remain unclear. In the present study, the mouse spermatocyte-derived GC-2 cell line was exposed to different doses of DBP. We found that DBP induced marked apoptosis in GC-2 cells. The levels of the major endoplasmic reticulum (ER) stress markers GRP-78, ATF-6, and p-EIF2α were elevated when GC-2 cells were exposed to 25 μM DBP and increased in a dose-dependent manner at higher concentrations. Furthermore, at a concentration that resulted in significant apoptosis (100 μM), CHOP, which plays a convergent role in ER stress-mediated apoptosis and is regulated by various upstream ER stress signals, was activated and partially contributed to the DBP-induced apoptosis. However, inhibition of ER stress by 4-PBA, a chemical with chaperone-like activities, augmented the GC-2 cell apoptosis induced by DBP. Further experiments demonstrated that DBP-induced ER stress additionally had a protective role, mediated through the activation of autophagy. These results were confirmed in prepubertal rat testis germ cells; DBP treatment significantly induced testicular atrophy, accompanied by apoptosis, ER stress, and autophagy. Inhibition of ER stress and autophagy significantly aggravated the DBP-induced damage to the germ cells and testes. Taken together, our data suggest that DBP-induced ER stress in germ cells has a cytoprotective effect that is mediated through autophagy activation. These findings provide novel clues regarding the molecular events involved in DBP-induced germ cell apoptosis.

Autophagy in acute kidney injury

Autophagy is a conserved multistep pathway that degrades and recycles damaged organelles and macromolecules to maintain intracellular homeostasis. The autophagy pathway is upregulated under stress conditions including cell starvation, hypoxia, nutrient and growth-factor deprivation, endoplasmic reticulum stress, and oxidant injury, most of which are involved in the pathogenesis of acute kidney injury (AKI). Recent studies demonstrate that basal autophagy in the kidney is vital for the normal homeostasis of the proximal tubules. Deletion of key autophagy proteins impaired renal function and increased p62 levels and oxidative stress. In models of AKI, autophagy deletion in proximal tubules worsened tubular injury and renal function, highlighting that autophagy is renoprotective in models of AKI. In addition to nonselective sequestration of autophagic cargo, autophagy can facilitate selective degradation of damaged organelles, particularly mitochondrial degradation through the process of mitophagy. Damaged mitochondria accumulate in autophagy-deficient kidneys of mice subjected to ischemia-reperfusion injury, but the precise mechanisms of regulation of mitophagy in AKI are not yet elucidated. Recent progress in identifying the interplay of autophagy, apoptosis, and regulated necrosis has revived interest in examining shared pathways/molecules in this crosstalk during the pathogenesis of AKI. Autophagy and its associated pathways pose potentially unique targets for therapeutic interventions in AKI.

Endoplasmic Reticulum Stress-Induced Autophagy Provides Cytoprotection from Chemical Hypoxia and Oxidant Injury and Ameliorates Renal Ischemia-Reperfusion Injury

We examined whether endoplasmic reticulum (ER) stress-induced autophagy provides cytoprotection from renal tubular epithelial cell injury due to oxidants and chemical hypoxia in vitro, as well as from ischemia-reperfusion (IR) injury in vivo. We demonstrate that the ER stress inducer tunicamycin triggers an unfolded protein response, upregulates ER chaperone Grp78, and activates the autophagy pathway in renal tubular epithelial cells in culture. Inhibition of ER stress-induced autophagy accelerated caspase–3 activation and cell death suggesting a pro-survival role of ER stress-induced autophagy. Compared to wild-type cells, autophagy-deficient MEFs subjected to ER stress had enhanced caspase–3 activation and cell death, a finding that further supports the cytoprotective role of ER stress-induced autophagy. Induction of autophagy by ER stress markedly afforded cytoprotection from oxidants H₂O₂ and tert-Butyl hydroperoxide and from chemical hypoxia induced by antimycin A. In contrast, inhibition of ER stress-induced autophagy or autophagy-deficient cells markedly enhanced cell death in response to oxidant injury and chemical hypoxia. In mouse kidney, similarly to renal epithelial cells in culture, tunicamycin triggered ER stress, markedly upregulated Grp78, and activated autophagy without impairing the autophagic flux. In addition, ER stress-induced autophagy markedly ameliorated renal IR injury as evident from significant improvement in renal function and histology. Inhibition of autophagy by chloroquine markedly increased renal IR injury. These studies highlight beneficial impact of ER stress-induced autophagy in renal ischemia-reperfusion injury both in vitro and in vivo.

Change of protein kinase R-like endoplasmic reticulum kinase/eukaryotic translation initiator factor-2α signaling pathway in nonalcoholic fatty liver disease in rats

AIM: To observe the change of the protein kinase R-like endoplasmic reticulum kinase/eukaryotic translation initiator factor-2α (PERK/eIF-2α) signaling pathway in nonalcoholic fatty liver disease (NAFLD) and to discuss the possible clinical significance.

METHODS: Thirty Wistar rats were randomly and equally divided into two groups: a normal control group (group 1) and an NAFLD group (group 2). NAFLD was induced by feeding a high-fat, high-glucose diet for 16 wk, and group 1 was given an ordinary diet at the same time. Serum levels of total cholesterol (TC), triglyeride (TG), low-density lipoprotein (LDL) and high-density lipoprotein (HDL) were measured in all rats. The mRNA expression of PERK, eIF-2α, C/EBP homology protein (CHOP) and glucose regulated protein 78 kDa (GRP78) in the liver was detected by real-time PCR, while Western blot was used to detect the expression of p-PERK, p-eIF-2α, CHOP and GRP78 proteins. Cell apoptosis was tested by flow cytometry.

RESULTS: Levels of TC, TG, and LDL in group 1 were significantly higher than those in group 2 (P < 0.05), but the level of HDL was significantly lower in group 1 (P < 0.05). Compared with group 1, the expression of p-PERK, p-eIF-2α, CHOP and GRP78 protein in group 2 was significantly increased (P < 0.05), and the expression of PERK, eIF-2α, CHOP and GRP78 mRNAs was also significantly elevated in group 2 (P < 0.05 or P < 0.01). Flow cytometry analysis suggested that apoptosis in group 2 was increased.

CONCLUSION: The PERK/eIF-2α signaling pathway is involved in the development and progression of NAFLD, which may cause endoplasmic reticulum stress and induce apoptosis.

Acute kidney injury: preclinical innovations, challenges, and opportunities for translation

BACKGROUND

Acute kidney injury (AKI) is a clinically important condition that has attracted a great deal of interest from the biomedical research community. However, acute kidney injury AKI research findings have yet to be translated into significant changes in clinical practice.

OBJECTIVE

This article reviews many of the preclinical innovations in acute kidney injury AKI treatment, and explores challenges and opportunities to translate these finding into clinical practice.

SOURCES OF INFORMATION

MEDLINE, ISI Web of Science.

FINDINGS

This paper details areas in biomedical research where translation of pre-clinical findings into clinical trials is ongoing, or nearing a point where trial design is warranted. Further, the paper examines ways that best practice in the management of AKI can reach a broader proportion of the patient population experiencing this condition.

LIMITATIONS

This review highlights pertinent literature from the perspective of the research interests of the authors for new translational work in AKI. As such, it does not represent a systematic review of all of the AKI literature.

IMPLICATIONS

Translation of findings from biomedical research into AKI therapy presents several challenges. These may be partly overcome by targeting populations for interventional trials where the likelihood of AKI is very high, and readily predictable. Further, specific clinics to follow-up with patients after AKI events hold promise to provide best practice in care, and to translate therapies into treatment for the broadest possible patient populations.

Endoplasmic Reticulum Stress in the Diabetic Kidney, the Good, the Bad and the Ugly

Diabetic kidney disease is the leading worldwide cause of end stage kidney disease and a growing public health challenge. The diabetic kidney is exposed to many environmental stressors and each cell type has developed intricate signaling systems designed to restore optimal cellular function. The unfolded protein response (UPR) is a homeostatic pathway that regulates endoplasmic reticulum (ER) membrane structure and secretory function. Studies suggest that the UPR is activated in the diabetic kidney to restore normal ER function and viability. However, when the cell is continuously stressed in an environment that lies outside of its normal physiological range, then the UPR is known as the ER stress response. The UPR reduces protein synthesis, augments the ER folding capacity and downregulates mRNA expression of genes by multiple pathways. Aberrant activation of ER stress can also induce inflammation and cellular apoptosis, and modify signaling of protective processes such as autophagy and mTORC activation. The following review will discuss our current understanding of ER stress in the diabetic kidney and explore novel means of modulating ER stress and its interacting signaling cascades with the overall goal of identifying therapeutic strategies that will improve outcomes in diabetic nephropathy.

mTOR contributes to ER stress and associated apoptosis in renal tubular cells

ER stress has been implicated in the pathogenesis of both acute and chronic kidney diseases. However, the molecular regulation of ER stress in kidney cells and tissues remains poorly understood. In this study, we examined tunicamycin-induced ER stress in renal proximal tubular cells (RPTC). Tunicamycin induced the phosphorylation and activation of PERK and eIF2α within 2 h in RPTC, which was followed by the induction of GRP78 and CHOP. Consistently, tunicamycin also induced apoptosis in RPTC. Interestingly, mTOR was activated rapidly during tunicamycin treatment, as indicated by phosphorylation of both mTOR and p70S6K. Inhibition of mTOR with rapamycin partially suppressed the phosphorylation of PERK and eIF2a and the induction of CHOP and GRP78 induction during tunicamycin treatment. Rapamycin also inhibited apoptosis during tunicamycin treatment and increased cell survival. Collectively, the results suggest that mTOR plays a regulatory role in ER stress, and inhibition of mTOR may have potential therapeutic effects in ER stress-related renal diseases.

Down-regulation of GRP78 enhances apoptosis via CHOP pathway in retinal ischemia-reperfusion injury

Ischemia/reperfusion (I/R) injury is the main cause of retinal apoptosis. But the mechanism remains elusive. During I/R injury, the intracellular calcium levels increase, resulting in the generation of reactive oxygen species, which have been shown to cause endoplasmic reticulum (ER) stress. However, little is known about the correlation between apoptosis and ER stress in retinal I/R injury. In the present study, we demonstrated that ER stress was activated in the retina of rat I/R models. The transcriptional expression of ER stress-associated molecules, glucose-regulated protein-78 (GRP78) and C/EBP-homologous protein (CHOP) were significantly increased in I/R retinas in a time-dependent manner. Partial inhibition of the endogenous expression of GRP78 with antisense oligonucleotide resulted in significant retinal damage and apoptosis in I/R injury rats. Also, the transcriptional expression of CHOP was persistently increased. Our findings indicate that ER stress may play a critical role in I/R injury induced retinal damage, and GRP78 may exert anti-apoptotic actions in I/R retina. Importantly, the persistent high expression of CHOP might serve as a possible mechanism that contributes to the enhanced the I/R-induced apoptosis after GRP78 down-regulation. These results may provide insight into the pathology of retinal I/R injury.

Activation of the miR-17 family and miR-21 during murine kidney ischemia-reperfusion injury

BACKGROUND

Ischemia-reperfusion (I/R) is the main cause of acute kidney injury (AKI) in patients. We investigated renal microRNA (miRNA) expression profiles and the time course of changes in selected miRNA expressions after renal I/R to characterize the miRNA network activated during development and recovery from AKI.

METHODS AND RESULTS

One day after lethal (30 minutes) and sublethal (20 minutes) renal ischemia, AKI was verified by renal histology (tubular necrosis, regeneration), blood urea nitrogen (BUN) level, renal mRNA expression, and plasma concentration of neutrophil gelatinase-associated lipocalin (NGAL) in C57BL/6J mice. On the first day after 30-minute, lethal I/R miR-21, miR-17-5p, and miR-106a were elevated out of the 21 miRNAs successfully profiled on the Luminex multiplex assay. After 20-minute, sublethal I/R, renal miR-17-5p and miR-106a expressions were elevated on the first and second days of reperfusion, while miR-21 expression increased later and lasted longer. Renal miR-17-5p and miR-21 expressions correlated with each other. Renal function returned to normal on the fourth day after sublethal I/R.

CONCLUSIONS

Our results demonstrate that besides miR-21, miR-17-5p, and miR-106a are additionally activated during the maintenance and recovery phases of renal I/R injury. Furthermore, a correlation between renal miR-17-5p and miR-21 expressions warrants further investigation of how they may influence each other and the outcome of renal ischemia-reperfusion injury.

4-Phenylbutyrate inhibits tunicamycin-induced acute kidney injury via CHOP/GADD153 repression

Different forms of acute kidney injury (AKI) have been associated with endoplasmic reticulum (ER) stress; these include AKI caused by acetaminophen, antibiotics, cisplatin, and radiocontrast. Tunicamycin (TM) is a nucleoside antibiotic known to induce ER stress and is a commonly used inducer of AKI. 4-phenylbutyrate (4-PBA) is an FDA approved substance used in children who suffer from urea cycle disorders. 4-PBA acts as an ER stress inhibitor by aiding in protein folding at the molecular level and preventing misfolded protein aggregation. The main objective of this study was to determine if 4-PBA could protect from AKI induced by ER stress, as typified by the TM-model, and what mechanism(s) of 4-PBA's action were responsible for protection. C57BL/6 mice were treated with saline, TM or TM plus 4-PBA. 4-PBA partially protected the anatomic segment most susceptible to damage, the outer medullary stripe, from TM-induced AKI. In vitro work showed that 4-PBA protected human proximal tubular cells from apoptosis and TM-induced CHOP expression, an ER stress inducible proapoptotic gene. Further, immunofluorescent staining in the animal model found similar protection by 4-PBA from CHOP nuclear translocation in the tubular epithelium of the medulla. This was accompanied by a reduction in apoptosis and GRP78 expression. CHOP(-/-) mice were protected from TM-induced AKI. The protective effects of 4-PBA extended to the ultrastructural integrity of proximal tubule cells in the outer medulla. When taken together, these results indicate that 4-PBA acts as an ER stress inhibitor, to partially protect the kidney from TM-induced AKI through the repression of ER stress-induced CHOP expression.

Arsenic induces reactive oxygen species-caused neuronal cell apoptosis through JNK/ERK-mediated mitochondria-dependent and GRP 78/CHOP-regulated pathways

Arsenic (As), a well-known high toxic metal, is an important environmental and industrial contaminant, and it induces oxidative stress, which causes many adverse health effects and diseases in humans, particularly in inorganic As (iAs) more harmful than organic As. Recently, epidemiological studies have suggested a possible relationship between iAs exposure and neurodegenerative disease development. However, the toxicological effects and underlying mechanisms of iAs-induced neuronal cell injuries are mostly unknown. The present study demonstrated that iAs significantly decreased cell viability and induced apoptosis in Neuro-2a cells. iAs also increased oxidative stress damage (production of malondialdehyde (MDA) and ROS, and reduction of Nrf2 and thioredoxin protein expression) and induced several features of mitochondria-dependent apoptotic signals, including: mitochondrial dysfunction, the activations of PARP and caspase cascades, and the increase in caspase-3 activity. Pretreatment with the antioxidant N-acetylcysteine (NAC) effectively reversed these iAs-induced responses. iAs also increased the phosphorylation of JNK and ERK1/2, but did not that p38-MAPK, in treated Neuro-2a cells. NAC and the specific JNK inhibitor (SP600125) and ERK1/2 inhibitor (PD98059) abrogated iAs-induced cell cytotoxicity, caspase-3/-7 activity, and JNK and ERK1/2 activation. Additionally, exposure of Neuro-2a cells to iAs triggered endoplasmic reticulum (ER) stress identified through several key molecules (GRP 78, CHOP, XBP-1, and caspase-12), which was prevented by NAC. Transfection with GRP 78- and CHOP-specific si-RNA dramatically suppressed GRP 78 and CHOP expression, respectively, and attenuated the activations of caspase-12, -7, and -3 in iAs-exposed cells. Therefore, these results indicate that iAs induces ROS causing neuronal cell death via both JNK/ERK-mediated mitochondria-dependent and GRP 78/CHOP-triggered apoptosis pathways.

Ischemia-reperfusion induces renal tubule pyroptosis via the CHOP-caspase-11 pathway

The apoptotic or necrotic death of renal tubule epithelial cells is the main pathogenesis of renal ischemia-reperfusion-induced acute kidney injury (AKI). Pyroptosis is a programmed cell death pathway that depends on the activation of the caspase cascade and IL-1 cytokine family members. However, the role of pyroptosis in AKI induced by ischemia-reperfusion remains unclear. In this study, we found that the levels of the pyroptosis-related proteins, including caspase-1, caspase-11, and IL-1β, were significantly increased after 6 h of renal ischemia-reperfusion injury (IRI) and peaked at 12 h after IRI. Enhanced pyroptosis was accompanied by elevated renal structural and functional injury. Similarly, hypoxia-reoxygenation injury (HRI) also induced pyroptosis in renal tubule epithelial NRK-52E cells, which was characterized by increased pore formation and elevated lactate dehydrogenase release. In addition, obvious upregulation of the endoplasmic reticulum (ER) stress biomarkers glucose-regulated protein 78 and C/EBP homologous protein (CHOP) preceded the incidence of pyroptosis in cells treated with IRI or HRI. Pretreatment with a low dose of tunicamycin, an inducer of ER stress, relieved IRI-induced pyroptosis and renal tissue injury. Silencing of CHOP by small interfering RNA significantly decreased HRI-induced pyroptosis of NRK-52E cells, as evidenced by reduced caspase-11 activity and IL-1β generation. Therefore, we conclude that pyroptosis of renal tubule epithelial cells is a key event during IRI and that CHOP-caspase-11 triggered by overactivated ER stress may be an essential pathway involved in pyroptosis.

The nephroprotective effect of tauroursodeoxycholic acid on ischaemia/reperfusion-induced acute kidney injury by inhibiting endoplasmic reticulum stress

The incidence of acute kidney injury (AKI) is very high, and multiple physiopathological processes are involved, including endoplasmic reticulum stress (ERS). Tauroursodeoxycholic acid (TUDCA) is an endogenous bile acid derivative that has been reported to inhibit ERS. To determine whether TUDCA had a nephroprotective effect on AKI and to explore the exact mechanism, an ischaemia/reperfusion (I/R)-induced AKI mouse model and a tunicamycin-pre-treated TCMK-1 cell model were established. It was found that the renal tubular necrosis score and cell apoptosis index reached their peak 24 hr after I/R. GRP78 and C/EBP homologous protein (CHOP) expression and Caspase 12 activation were enhanced, reaching their peaks at 4 and 12 hr, respectively. TUDCA intervention not only decreased the renal tubular necrosis score and the cell apoptosis index but also down-regulated GRP78 and CHOP expression and Caspase 12 activation. The survival rate of TCMK-1 cells pre-treated with TUDCA was significantly higher than that of TCMK-1 cells without TUDCA pre-treatment. In conclusion, TUDCA had a nephroprotective effect on IR-induced AKI by inhibiting ERS and by blocking GRP78 and CHOP expression, reducing Caspase 12 activation and inhibiting cell apoptosis.

Protein misfolding induces hypoxic preconditioning via a subset of the unfolded protein response machinery

Prolonged cellular hypoxia results in energy failure and ultimately cell death. However, less-severe hypoxia can induce a cytoprotective response termed hypoxic preconditioning (HP). The unfolded protein response pathway (UPR) has been known for some time to respond to hypoxia and regulate hypoxic sensitivity; however, the role of the UPR, if any, in HP essentially has been unexplored. We have shown previously that a sublethal hypoxic exposure of the nematode Caenorhabditis elegans induces a protein chaperone component of the UPR (L. L. Anderson, X. Mao, B. A. Scott, and C. M. Crowder, Science 323:630-633, 2009). Here, we show that HP induces the UPR and that the pharmacological induction of misfolded proteins is itself sufficient to stimulate a delayed protective response to hypoxic injury that requires the UPR pathway proteins IRE-1, XBP-1, and ATF-6. HP also required IRE-1 but not XBP-1 or ATF-6; instead, GCN-2, which is known to suppress translation and induce an adaptive transcriptional response under conditions of UPR activation or amino acid deprivation, was required for HP. The phosphorylation of the translation factor eIF2α, an established mechanism of GCN-2-mediated translational suppression, was not necessary for HP. These data suggest a model where hypoxia-induced misfolded proteins trigger the activation of IRE-1, which along with GCN-2 controls an adaptive response that is essential to HP.

[Involvement of endoplasmic reticulum stress in solid organ transplantation]

Endoplasmic reticulum (ER) stress is a situation caused by the accumulation of unfolded proteins in the endoplasmic reticulum, triggering an evolutionary conserved adaptive response termed the unfolded protein response. When adaptation fails, excessive and prolonged ER stress triggers cell suicide. Important roles for ER-initiated cell death pathways have been recognized for several diseases, including diabetes, hypoxia, ischemia/reperfusion injury, neurodegenerative and heart diseases. The implication of the ER stress is not well recognized in solid organ transplantation, but increasing evidence suggests its implication in mediating allograft injury. The purpose of this review is to summarize the mechanisms of ER stress and to discuss its implication during tissue injury in solid organ transplantation. The possible implications of the ER stress in the modifications of cell functional properties and phenotypic changes are also discussed beyond the scope of adaptation and cell death. Increasing the understanding of the cellular and molecular mechanisms of acute and chronic allograft damages could lead to the development of new biomarkers and to the discovery of new therapeutic strategies to prevent the initiation of graft dysfunction or to promote the tissue regeneration after injury.

Mediators of inflammation in acute kidney injury

Acute kidney injury (AKI) remains to be an independent risk factor for mortality and morbidity. Inflammation is now believed to play a major role in the pathopathophysiology of AKI. It is hypothesized that in ischemia, sepsis and nephrotoxic models that the initial insult results in morphological and/or functional changes in vascular endothelial cells and/or in tubular epithelium. Then, leukocytes including neutrophils, macrophages, natural killer cells, and lymphocytes infiltrate into the injured kidneys. The injury induces the generation of inflammatory mediators like cytokines and chemokines by tubular and endothelial cells which contribute to the recruiting of leukocytes into the kidneys. Thus, inflammation has an important role in the initiation and extension phases of AKI. This review will focus on the mediators of inflammation contributing to the pathogenesis of AKI.

The protective effect of radicicol against renal ischemia--reperfusion injury in mice

Overexpression of heat shock protein 70 kDa (HSP70) is known to confer cellular protection against ischemia-reperfusion (I/R) injury. Radicicol, a HSP90 inhibitor, has been reported to induce the expression of HSP70 protein. Here we studied whether radicicol attenuated renal I/R injury in vivo. Treatment of mice with radicicol ameliorated renal I/R injury and increased renal HSP70 mRNA and protein. Administration of radicicol with quercetin, an inhibitor of HSP70 induction, eliminated the renoprotective effect of radicicol. Our results suggest that the up-regulation of renal HSP70 protein by radicicol leads to a novel drug therapy against renal I/R injury.

The role of endoplasmic reticulum stress-related unfolded protein response in the radiocontrast medium-induced renal tubular cell injury

Contrast medium (CM) induces a direct toxic effect on renal tubular cells. This toxic effect may have a role in the pathophysiology of CM-induced nephropathy. CM has been shown to affect the endoplasmic reticulum (ER)-related capacity. Unfolded protein response (UPR) is known as a prosurvival response to reduce the accumulation of unfolded proteins and restore normal ER function. However, the role of ER stress-related UPR in the CM-induced renal cell injury still remains unclear. In this study, we examined whether UPR participates in urografin (an ionic CM)-induced renal tubular cells apoptosis. Treatment with urografin in normal rat renal tubular cell line (NRK52E) markedly increased cell apoptosis and decreased cell viability with a dose- and time-dependent manner. The cell necrosis was not increased in urografin-treated cells. Urografin also enhance the induction of ER stress-related markers in NRK52E cells, including glucose-regulated protein (GRP)78 and GRP94 expressions, procaspase-12 cleavage, phosphorylation of PERK (PKR [double-stranded RNA-activated protein kinase]-like ER kinase), and eukaryotic initiation factor 2alpha (eIF2alpha). Salubrinal, a selective inhibitor of eIF2alpha dephosphorylation, effectively decreased urografin-induced cell apoptosis. Furthermore, transfection of GRP78-small interfering RNA in NRK52E cells significantly enhanced urografin-induced cell apoptosis. These results suggest that GRP78/eIF2alpha-related signals play a protective role during UPR, and the activation of ER stress-related UPR may play an important regulative role in urografin-induced renal tubular injury.

Endoplasmic reticulum stress: an unrecognized actor in solid organ transplantation

Endoplasmic reticulum (ER) stress is an adaptive response to the accumulation of misfolded proteins within the ER, which can trigger cell dedifferentiation and cell suicide. Increasing evidences suggest its implication in mediating allograft injury. Herein, we summarize the mechanisms of ER stress and discuss its implication in allograft injury. Increasing our understanding of the cellular and molecular mechanisms of acute and chronic allograft damages could lead to the development of new biomarkers and to the discovery of new therapeutic strategies to prevent the initiation of graft dysfunction or to promote the tissue regeneration after injury.

Endoplasmic reticulum stress and renal disease

The accumulation of unfolded proteins in the endoplasmic reticulum (ER), leading to ER stress, is caused by a wide range of physiologic and pathologic conditions. Cells respond to ER stress by activating a series of integrative stress pathways termed the unfolded protein response (UPR). This either may be adaptive and promote cell survival, or if the ER stress is chronic or excessive, may lead to cell death. The role of ER stress in the pathophysiology of both acute and chronic kidney diseases has been gaining increasing interest. This review highlights the current knowledge of ER stress in renal disease, with emphasis on more recent advances. Potential therapeutic options targeting ER stress are discussed.

Ischemia activates the ATF6 branch of the endoplasmic reticulum stress response

Stresses that perturb the folding of nascent endoplasmic reticulum (ER) proteins activate the ER stress response. Upon ER stress, ER-associated ATF6 is cleaved; the resulting active cytosolic fragment of ATF6 translocates to the nucleus, binds to ER stress response elements (ERSEs), and induces genes, including the ER-targeted chaperone, GRP78. Recent studies showed that nutrient and oxygen starvation during tissue ischemia induce certain ER stress response genes, including GRP78; however, the role of ATF6 in mediating this induction has not been examined. In the current study, simulating ischemia (sI) in a primary cardiac myocyte model system caused a reduction in the level of ER-associated ATF6 with a coordinate increase of ATF6 in nuclear fractions. An ERSE in the GRP78 gene not previously shown to be required for induction by other ER stresses was found to bind ATF6 and to be critical for maximal ischemia-mediated GRP78 promoter induction. Activation of ATF6 and the GRP78 promoter, as well as grp78 mRNA accumulation during sI, were reversed upon simulated reperfusion (sI/R). Moreover, dominant-negative ATF6, or ATF6-targeted miRNA blocked sI-mediated grp78 induction, and the latter increased cardiac myocyte death upon simulated reperfusion, demonstrating critical roles for endogenous ATF6 in ischemia-mediated ER stress activation and cell survival. This is the first study to show that ATF6 is activated by ischemia but inactivated upon reperfusion, suggesting that it may play a role in the induction of ER stress response genes during ischemia that could have a preconditioning effect on cell survival during reperfusion.

A subcytotoxic dose of subtilase cytotoxin prevents lipopolysaccharide-induced inflammatory responses, depending on its capacity to induce the unfolded protein response

Subtilase cytotoxin (SubAB) is the prototype of a newly identified family of AB(5) cytotoxins produced by Shiga toxigenic Escherichia coli. SubAB specifically cleaves the essential endoplasmic reticulum (ER) chaperone BiP (GRP78), resulting in the activation of ER stress-induced unfolded protein response (UPR). We have recently shown that the UPR following ER stress can suppress cellular responses to inflammatory stimuli during the later phase, in association with inhibition of NF-kappaB activation. These findings prompted us to hypothesize that SubAB, as a selective UPR inducer, might have beneficial effects on inflammation-associated pathology via a UPR-dependent inhibition of NF-kappaB activation. The pretreatment of a mouse macrophage cell line, RAW264.7, with a subcytotoxic dose of SubAB-triggered UPR and inhibited LPS-induced MCP-1 and TNF-alpha production associated with inhibition of NF-kappaB activation. SubA(A272)B, a SubAB active site mutant that cannot induce UPR, did not show such effects. In addition, pretreatment with a sublethal dose of SubAB, but not SubA(A272)B, protected the mice from LPS-induced endotoxic lethality associated with reduced serum MCP-1 and TNF-alpha levels and also prevented the development of experimental arthritis induced by LPS in mice. Collectively, although SubAB has been identified originally as a toxin associated with the pathogenesis of hemolytic uremic syndrome, the unique ability of SubAB to selectively induce the UPR may have the potential to prevent LPS-associated inflammatory pathology under subcytotoxic conditions.

The protective effect of a newly developed molecular chaperone-inducer against mouse ischemic acute kidney injury

Activation of the unfolded protein response (UPR) has been suggested to attenuate renal ischemia-reperfusion (I/R) injury. We recently found a compound, namely BIX, that activated the UPR selectively through the activating transcription factor 6 pathway. This study examined the effect of BIX on renal I/R injury in mice. BIX selectively up-regulated renal BiP mRNA and protein. Pretreatment with BIX significantly ameliorated renal I/R injury. Co-administration of BIX and tunicamycin, a non-selective UPR inducer, provided no additional protection. Our results suggest that the UPR activation by BIX leads to a novel drug therapy against renal I/R injury.