The Role of IRE-XBP1 Pathway in Regulation of Retinal Pigment Epithelium Tight Junctions

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

IRE1α/XBP-1 promotes β-catenin signaling activation of airway epithelium in lipopolysaccharide-induced acute lung injury

BACKGROUND

Acute lung injury (ALI), along with the more severe condition--acute respiratory distress syndrome (ARDS), is a major cause of respiratory failure in critically ill patients with high morbidity and mortality. Inositol-requiring protein 1α (IRE1α)/X box protein-1 (XBP1) pathway was proved to regulate lipopolysaccharide (LPS)-induced lung injury and inflammation. Yet, its role on epithelial β-catenin in LPS-induced ALI remains to be elucidated.

METHODS

LPS-induced models were generated in mice (5 mg/kg) and Beas-2B cells (200 μg/mL). Two selective antagonists of IRE1α (4μ8c and STF-083010) were respectively given to LPS-exposed mice and cultured cells.

RESULTS

Up-regulated expression of endoplasmic reticulum (ER) stress markers immunoglobulin-binding protein (BIP) and spliced X box protein-1(XBP-1s) was detected after LPS exposure. Besides, LPS also led to a down-regulated total β-catenin level in the lung and Beas-2B cells, with decreased membrane distribution as well as increased cytoplasmic and nuclear accumulation, paralleled by extensively up-regulated downstream targets of the Wnt/β-catenin signaling. Treatment with either 4μ8c or STF-083010 not only significantly attenuated LPS-induced lung injury and inflammation, but also recovered β-catenin expression in airway epithelia, preserving the adhesive function of β-catenin while blunting its signaling activity.

CONCLUSION

These results illustrated that IRE1α/XBP1 pathway promoted the activation of airway epithelial β-catenin signaling in LPS-induced ALI.

Essential Role of XBP1 in Maintaining Photoreceptor Synaptic Integrity in Early Diabetic Retinopathy

Purpose

Diabetic retinopathy (DR) is a leading cause of blindness in working-age adults characterized by retinal dysfunction and neurovascular degeneration. We previously reported that deletion of X-box binding protein 1 (XBP1) leads to accelerated retinal neurodegeneration in diabetes; however, the mechanisms remain elusive. The goal of this study is to determine the role of XBP1 in the regulation of photoreceptor synaptic integrity in early DR.

Methods

Diabetes was induced by streptozotocin in retina-specific XBP1 conditional knockout (cKO) or wild-type (WT) mice to generate diabetic cKO (cKO/DM) or WT/DM mice for comparison with nondiabetic cKO (cKO/NDM) and WT/NDM mice. Retinal morphology, structure, and function were assessed by immunohistochemistry, optical coherence tomography, and electroretinogram (ERG) after 3 months of diabetes. The synapses between photoreceptors and bipolar cells were examined by confocal microscopy, and synaptic integrity was quantified using the QUANTOS algorithm.

Results

We found a thinning of the outer nuclear layer and a decline in the b-wave amplitude in dark- and light-adapted ERG in cKO/DM mice compared to all other groups. In line with these changes, cKO mice showed increased loss of synaptic integrity compared to WT mice, regardless of diabetes status. In searching for candidate molecules responsible for the loss of photoreceptor synaptic integrity in diabetic and XBP1-deficient retinas, we found decreased mRNA and protein levels of DLG4/PSD-95 in cKO/DM retina compared to WT/DM.

Conclusions

These findings suggest that XBP1 is a crucial regulator in maintaining synaptic integrity and retinal function, possibly through regulation of synaptic scaffold proteins.

Particulate Matter Elevates Ocular Inflammation and Endoplasmic Reticulum Stress in Human Retinal Pigmented Epithelium Cells

Because of their exposure to air, eyes can come into contact with air pollutants such as particulate matter (PM), which may cause severe ocular pathologies. Prolonged ocular PM exposure may increase inflammation and endoplasmic reticulum stress in the retina. Herein, we investigated whether PM exposure induces ocular inflammation and endoplasmic reticulum (ER) stress-related cellular responses in human retinal epithelium-19 (ARPE-19) cells. To understand how PM promotes ocular inflammation, we monitored the activation of the mitogen-activated protein kinase (MAPK)/nuclear factor kappa beta (NFκB) axis and the expression of key inflammatory mRNAs. We also measured the upregulation of signature components for the ER-related unfolded protein response (UPR) pathways, as well as intracellular calcium ([Ca²⁺]_i) levels, as readouts for ER stress induction following PM exposure. Ocular PM exposure significantly elevated the expression of multiple cytokine mRNAs and increased phosphorylation levels of NFκB-MAPK axis in a PM dose-dependent manner. Moreover, incubation with PM significantly increased [Ca²⁺]_i levels and the expression of UPR-related proteins, which indicated ER stress resulting from cell hypoxia, and upregulation of hypoxic adaptation mechanisms such as the ER-associated UPR pathways. Our study demonstrated that ocular PM exposure increased inflammation in ARPE-19 cells, by activating the MAPK/NFκB axis and cytokine mRNA expression, while also inducing ER stress and stress adaptation responses. These findings may provide helpful insight into clinical and non-clinical research examining the role of PM exposure in ocular pathophysiology and delineating its underlying molecular mechanisms.

Hyperlipidemia induces proinflammatory responses by activating STING pathway through IRE1α-XBP1 in retinal endothelial cells

Diabetic retinopathy (DR) is one of the most prevalent microvascular complications caused by diabetes mellitus. Previous studies demonstrate that microvascular endothelial inflammation caused by chronic hyperglycemia and hyperlipidemia plays a key role in the pathogenesis of DR. However, the detailed mechanisms on how endothelial inflammation contributes to DR are not fully understood. The STING pathway is an important innate immune signaling pathway. Although STING has been implicated in multiple autoimmune and metabolic diseases, it is not clear whether STING is involved in the pathogenesis of DR. Thus, re-analysis of the public single cell RNA sequencing (sc-RNAseq) data demonstrated that STING was highly expressed in mouse retinal vessels. Moreover, our results demonstrated that STING and p-TBK1 protein levels in retinal endothelial cells are significantly increased in mice fed with high fat diet compared with chow diet. In vitro, palmitic acid treatment on HRVECs induced mitochondrial DNA leakage into the cytosol, and augmented p-TBK1 protein and IFN-β mRNA levels. As STING is localized to the ER, we analyzed the relation between STING activation and ER stress. In HRVECs, STING pathway was shown to be activated under chemical-induced ER stress, but attenuated when IRE1α was abolished by genetic deletion or pharmacological inhibition. Taken together, our findings revealed that STING signaling plays an important role in mediating lipotoxicity-induced endothelial inflammatory and injury, and IRE1α-XBP1 signaling potentiated STING signaling. Thus, targeting the IRE1α or STING pathways to alleviate endothelial inflammation provides candidate therapeutic target for treating DR as well as other microvascular complications.

Vascular calcification: Molecular mechanisms and therapeutic interventions

Vascular calcification (VC) is recognized as a pathological vascular disorder associated with various diseases, such as atherosclerosis, hypertension, aortic valve stenosis, coronary artery disease, diabetes mellitus, as well as chronic kidney disease. Therefore, it is a life-threatening state for human health. There were several studies targeting mechanisms of VC that revealed the importance of vascular smooth muscle cells transdifferentiating, phosphorous and calcium milieu, as well as matrix vesicles on the progress of VC. However, the underlying molecular mechanisms of VC need to be elucidated. Though there is no acknowledged effective therapeutic strategy to reverse or cure VC clinically, recent evidence has proved that VC is not a passive irreversible comorbidity but an active process regulated by many factors. Some available approaches targeting the underlying molecular mechanism provide promising prospects for the therapy of VC. This review aims to summarize the novel findings on molecular mechanisms and therapeutic interventions of VC, including the role of inflammatory responses, endoplasmic reticulum stress, mitochondrial dysfunction, iron homeostasis, metabolic imbalance, and some related signaling pathways on VC progression. We also conclude some recent studies on controversial interventions in the clinical practice of VC, such as calcium channel blockers, renin-angiotensin system inhibitions, statins, bisphosphonates, denosumab, vitamins, and ion conditioning agents.

Polystyrene nanoplastics promote CHIP-mediated degradation of tight junction proteins by activating IRE1α/XBP1s pathway in mouse Sertoli cells

Microplastics (MPs) and nanoplastics (NPs) widely exist in human living environment and enter the body through water, food chain and breathing. Several studies have shown that MPs or NPs disrupt the blood-testis barrier in rodents. However, the molecular mechanism by which MPs and NPs damage the blood-testis barrier remains unclear. In the present study, our aim was to investigate the molecular mechanism of the destruction of blood-testis barrier induced by polystyrene (PS)-NPs. Mice were treated with 50 μg/kg·day PS-NPs by tail vein injection once daily for two consecutive days. The results showed that PS-NPs exposure significantly decreased the levels of tight junction (TJ) proteins ZO-2, occludin and claudin-11 in testis of mice. In vitro, we used TM4 Sertoli cells to explore the underlying mechanism of the decrease in TJ proteins induced by PS-NPs. We found that PS-NPs activated IRE1α and induced its downstream XBP1 splicing, which in turn elevated the expression of the E3 ubiquitin ligase CHIP, then CHIP triggers proteasomal degradation of ZO-2, occludin, and claudin-11 proteins. Our findings suggest that IRE1α/XBP1s/CHIP pathway is a pivotal mechanism of TJ proteins degradation induced by PS-NPs in mouse Sertoli cells. In conclusion, our results reveal that the degradation of TJ proteins is one of the mechanisms of blood-testis barrier destruction caused by acute exposure to PS-NPs.

Zinc transport from the endoplasmic reticulum to the cytoplasm via Zip7 is necessary for barrier dysfunction mediated by inflammatory signaling in RPE cells

Inflammatory signaling induces barrier dysfunction in retinal-pigmented epithelium (RPE) cells and plays a role in the pathology of age-related macular degeneration (AMD). We studied the role of Zn flux from the endoplasmic reticulum (ER) to the cytoplasm via Zip7 during inflammatory signaling in RPE cells. In ARPE-19 cells, Zip7 inhibition reduced impedance loss, FITC-dextran permeability and cytokine induction caused by challenge with IL-1β/TNF-α. Zip7 inhibition in iPS-derived RPE cells challenged with TNF- α reduced barrier loss in TER assays. In ARPE-19 cells, a Zn ionophore restored cytokine induction and barrier loss in cells challenged with IL-1 β /TNF- α despite Zip7 inhibition. A cell permeable Zn chelator demonstrated that Zn is essential for IL-1 β /TNF- α signaling. ER stress caused by Zip7 inhibition in ARPE-19 cells was found to partially contribute to reducing barrier dysfunction caused by IL-1 β /TNF- α. Overall, it was shown that Zn flux through Zip7 from the ER to the cytoplasm plays a critical role in driving barrier dysfunction caused by inflammatory cytokines in RPE cells.

Cellular stress signaling and the unfolded protein response in retinal degeneration: mechanisms and therapeutic implications

Background

The retina, as part of the central nervous system (CNS) with limited capacity for self-reparation and regeneration in mammals, is under cumulative environmental stress due to high-energy demands and rapid protein turnover. These stressors disrupt the cellular protein and metabolic homeostasis, which, if not alleviated, can lead to dysfunction and cell death of retinal neurons. One primary cellular stress response is the highly conserved unfolded protein response (UPR). The UPR acts through three main signaling pathways in an attempt to restore the protein homeostasis in the endoplasmic reticulum (ER) by various means, including but not limited to, reducing protein translation, increasing protein-folding capacity, and promoting misfolded protein degradation. Moreover, recent work has identified a novel function of the UPR in regulation of cellular metabolism and mitochondrial function, disturbance of which contributes to neuronal degeneration and dysfunction. The role of the UPR in retinal neurons during aging and under disease conditions in age-related macular degeneration (AMD), retinitis pigmentosa (RP), glaucoma, and diabetic retinopathy (DR) has been explored over the past two decades. Each of the disease conditions and their corresponding animal models provide distinct challenges and unique opportunities to gain a better understanding of the role of the UPR in the maintenance of retinal health and function.

Method

We performed an extensive literature search on PubMed and Google Scholar using the following keywords: unfolded protein response, metabolism, ER stress, retinal degeneration, aging, age-related macular degeneration, retinitis pigmentosa, glaucoma, diabetic retinopathy.

Results and conclusion

We summarize recent advances in understanding cellular stress response, in particular the UPR, in retinal diseases, highlighting the potential roles of UPR pathways in regulation of cellular metabolism and mitochondrial function in retinal neurons. Further, we provide perspective on the promise and challenges for targeting the UPR pathways as a new therapeutic approach in age- and disease-related retinal degeneration.

Intestinal epithelial tight junctions and permeability can be rescued through the regulation of endoplasmic reticulum stress by amniotic fluid stem cells during necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is one of the most severe gastrointestinal diseases affecting premature infants. It has been shown that NEC is associated with disrupted intestinal barrier and dysregulated endoplasmic reticulum (ER)-stress response. It has also been shown that stem cells derived from amniotic fluid (AFSC) rescued intestinal injury in experimental NEC. Herein, we hypothesized that the beneficial effects of AFSC in the injured intestine are due to the restoration of intestinal barrier function. We evaluated intestinal barrier function using an ex vivo intestinal organoid model of NEC. We found that AFSC restored the expression and localization of tight junction proteins in intestinal organoids, and subsequently decreased epithelial permeability. AFSC rescued tight junction expression by inducing a protective ER stress response that prevents epithelial cell apoptosis in injured intestinal organoids. Finally, we validated these results in our experimental mouse model of NEC and confirmed that AFSC induced sustained ER stress and prevented intestinal apoptosis. This response led to the restoration of tight junction expression and localization, which subsequently reduced intestinal permeability in NEC pups. These findings confirm that intestinal barrier function is disrupted during NEC intestinal injury, and further demonstrate the disruption can be reversed by the administration of AFSC through the activation of the ER stress pathway. This study provides insight into the pathogenesis of NEC and highlights potential therapeutic targets for the treatment of NEC.

The Impact of Endoplasmic Reticulum-Associated Protein Modifications, Folding and Degradation on Lung Structure and Function

The accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) causes ER stress and induces the unfolded protein response (UPR) and other mechanisms to restore ER homeostasis, including translational shutdown, increased targeting of mRNAs for degradation by the IRE1-dependent decay pathway, selective translation of proteins that contribute to the protein folding capacity of the ER, and activation of the ER-associated degradation machinery. When ER stress is excessive or prolonged and these mechanisms fail to restore proteostasis, the UPR triggers the cell to undergo apoptosis. This review also examines the overlooked role of post-translational modifications and their roles in protein processing and effects on ER stress and the UPR. Finally, these effects are examined in the context of lung structure, function, and disease.

Quercetin protects retina external barrier from oxidative stress injury by promoting autophagy

Purpose: This study aimed to investigate the protective effects of quercetin on the tight junction proteins of human retinal pigment epithelial cells (ARPE-19 cells) suffering from oxidative stress injury and explore the possible mechanism.Methods: H₂O₂ (300 μM) was used to establish an oxidative stress model of ARPE-19 cells. ARPE-19 cells were pretreated with different concentrations (0-80 μM) of quercetin before H₂O₂ exposure. The expression and distribution of tight junction proteins and autophagy-related proteins were detected by Western blot and immunostaining. ARPE-19 cells were pretreated with 5 mM 3-methyladenine (3-MA).Results: The cell viability weakened in the H₂O₂ group compared with the control group. However, it was preserved after pretreatment with quercetin. It was observed that the expression levels of occludin, claudin-1 were decreased in the H₂O₂ group. Quercetin treatment significantly enhanced the expression levels of them as compared to the H₂O₂ group. H₂O₂ alone strongly decreased the Zonula occludens protein 1 (ZO-1) expression in the cytomembrane. Quercetin supplementation enhanced the accumulation of ZO-1 in ARPE-19 cells. The expression levels of Beclin-1 and Microtubule associated protein light chain 3 II (LC-3II) increased, and that of P62 decreased in the quercetin protection group. The appearance of LC-3II, which examined by immunofluorescence experiments, enhanced in the quercetin protection group as compared with the control group. The expression levels of beclin-1 and LC-3II increased, and that of P62 increased in the autophagy-inhibited group compared with the quercetin protection group. The levels of occludin and claudin-1 also decreased.Conclusion: Quercetin prevents the loss of tight junction proteins by upregulating autophagy after oxidative stress in ARPE-19 cells.

Mitochondria Associated Membranes (MAMs): Emerging Drug Targets for Diabetes

MAMs, the physical association between the Endoplasmic Reticulum (ER) and mitochondria are, functional domains performing a significant role in the maintenance of cellular homeostasis. It is evolving as an important signaling center that coordinates nutrient and hormonal signaling for the proper regulation of hepatic insulin action and glucose homeostasis. Moreover, MAMs can be considered as hot spots for the transmission of stress signals from ER to mitochondria. The altered interaction between ER and mitochondria results in the amendment of several insulin-sensitive tissues, revealing the role of MAMs in glucose homeostasis. The development of mitochondrial dysfunction, ER stress, altered lipid and Ca2+ homeostasis are typically co-related with insulin resistance and β cell dysfunction. But little facts are known about the role played by these stresses in the development of metabolic disorders. In this review, we highlight the mechanisms involved in maintaining the contact site with new avenues of investigations for the development of novel preventive and therapeutic targets for T2DM.

TRPV4 Increases the Expression of Tight Junction Protein-Encoding Genes via XBP1 in Mammary Epithelial Cells

Simple Summary

Mammary glands are exocrine tissue, capable of secreting adequate amounts of milk protein during lactation. Each mammary gland is occupied by numerous alveoli. Each alveolus is composed of a single layer of mammary epithelial cells, adipose tissue, and ducts. Recent studies indicate that mild heat treatment of mammary epithelial cells at 39 °C has activated milk production. These results suggest that temperature may influence the physiological functions of mammary epithelial cells. In this study, we found that the temperature-sensitive transient receptor potential vanilloid 4 (TRPV4) was involved in the increase of β-casein and TJ protein-encoding gene expression in response to mild heat treatment. On the other hand, severe heat treatment (41 °C) reduced the cell viability. Moreover, the Trpv4 mRNA level was significantly increased at Day 15 of gestation when the mammary alveoli are formed. TRPV4 is activated not only by temperature but also by mechanical forces that guide mammary epithelial development in the normal mammary gland. Our data suggest that TRPV4 has a possible function in mammary gland development.

Abstract

Mild heat stress (39 °C–40 °C) can positively regulate cell proliferation and differentiation. Indeed, mild heat treatment at 39 °C enhances the less-permeable tight junctions (TJs) formation and milk production in mammary epithelial cells. However, the molecular mechanisms of this response have not yet been delineated. In this study, the involvement of temperature-sensitive transient receptor potential vanilloid 4 (TRPV4) in the increase of β-casein and TJ protein-encoding gene expression in response to mild heat treatment (39 °C) has been explored using HCll mouse mammary epithelial cells. Severe heat treatment (41 °C) induced the transcriptional level of Chop (C/EBP homologous protein; proapoptotic marker) and reduced the cell viability. It is speculated that the difference in unfolded protein response (UPR) gene expression upon stimulation at 39 °C vs. 41 °C controls cell survival vs. cell death. The accumulation of Trpv4 mRNA was significantly higher in 39 °C heat treatment cells. The β-casein, Zo-1 (zona occludens-1), Ocln (occludin), and Cldn3 (claudin 3) transcript levels were significantly increased in response to the addition of a selective TRPV4 channel agonist (GSK1016790A) at 37 °C. TRPV4 stimulation with GSK1016790A also increased the X-box-binding protein 1 splicing form (Xbp1s) at the transcript level. The increase in the mRNA levels of β-casein, Zo-1, Ocln, and Cldn3 in response to 39 °C heat treatment was suppressed by XBP1 knockdown. Moreover, the transcript level of Trpv4 was significantly increased at Day 15 of gestation, and its expression declined after 1 day of lactation. TRPV4 is activated not only by temperature but also by mechanical forces, such as cell stretching and shear stress, which guide mammary epithelial development in a normal mammary gland. These findings provide new insights of the possible function of TRPV4 in mammary gland development.

ER stress induced by ER calcium depletion and UVB irradiation regulates tight junction barrier integrity in human keratinocytes

BACKGROUND

Endoplasmic reticulum (ER) calcium depletion-induced ER stress is a crucial signal for keratinocyte differentiation and barrier homeostasis, but its effects on the epidermal tight junction (TJ) have not been characterized. Ultraviolet B (UVB) causes ER calcium release in keratinocytes and disrupts epidermal TJ, however, the involvement of ER stress in the UVB-induced TJ alterations remains unknown.

OBJECTIVES

To investigate the effect of ER stress by pharmacological ER calcium depletion or UVB on the TJ integrity in normal human epidermal keratinocytes (NHEK).

METHODS

NHEK were exposed to ER calcium pump inhibitor thapsigargin (Tg) or UVB. ER stress markers and TJ molecules expression, TJ and F-actin structures, and TJ barrier function were analyzed.

RESULTS

Tg or UVB exposure dose-dependently triggered unfolded protein response (UPR) in NHEK. Low dose Tg induced the IRE1α-XBP1 pathway and strengthened TJ barrier. Contrary, high dose Tg activated PERK phosphorylation and disrupted TJ by F-actin disorganization. UVB disrupted TJ and F-actin structures dose dependently. IRE1α RNase inhibition induced or exacerbated TJ and F-actin disruption in the presence of low dose Tg or UVB. High dose Tg increased RhoA activity. 4-PBA or Rho kinase (ROCK) inhibitor partially prevented the disruption of TJ and F-actin following high dose Tg or UVB.

CONCLUSIONS

ER stress has bimodal effects on the epidermal TJ depending on its intensity. The IRE1α pathway is critical for the maintenance of TJ integrity during mild ER stress. Severe ER stress-induced UPR or ROCK signalling mediates the disruption of TJ through cytoskeletal disorganization during severe ER stress.

Loss of XBP1 Leads to Early-Onset Retinal Neurodegeneration in a Mouse Model of Type I Diabetes

Retinal neuronal injury and degeneration is one of the primary manifestations of diabetic retinopathy, a leading cause of vision loss in working age adults. In pathological conditions, including diabetes and some physiological conditions such as aging, protein homeostasis can become disrupted, leading to endoplasmic reticulum (ER) stress. Severe or unmitigated ER stress can lead to cell death, which in retinal neurons results in irreversible loss of visual function. X-box binding protein 1 (XBP1) is a major transcription factor responsible for the adaptive unfolded protein response (UPR) to maintain protein homeostasis in cells undergoing ER stress. The purpose of this study is to determine the role of XBP1-mediated UPR in retinal neuronal survival and function in a mouse model of type 1 diabetes. Using a conditional retina-specific XBP1 knockout mouse line, we demonstrate that depletion of XBP1 in retinal neurons results in early onset retinal function decline, loss of retinal ganglion cells and photoreceptors, disrupted photoreceptor ribbon synapses, and Müller cell activation after induction of diabetes. Our findings suggest an important role of XBP1-mediated adaptive UPR in retinal neuronal survival and function in diabetes.

Commensal Escherichia coli Aggravates Acute Necrotizing Pancreatitis through Targeting of Intestinal Epithelial Cells

An increase of Escherichia-Shigella was previously reported in acute necrotizing pancreatitis (ANP). We investigated whether Escherichia coli MG1655, an Escherichia commensal organism, increased intestinal injury and aggravated ANP in rats. ANP was induced by retrograde injection of 3.5% sodium taurocholate into the biliopancreatic duct. Using gut microbiota-depleted rats, we demonstrated that gut microbiota was involved in the pancreatic injury and intestinal barrier dysfunction in ANP. Using 16S rRNA gene sequencing and quantitative PCR, we found intestinal dysbiosis and a significant increase of E. coli MG1655 in ANP. Afterward, administration of E. coli MG1655 by gavage to gut microbiota-depleted rats with ANP was performed. We observed that after ANP induction, E. coli MG1655-monocolonized rats presented more severe injury in the pancreas and intestinal barrier function than gut microbiota-depleted rats. Furthermore, Toll-like receptor 4 (TLR4)/MyD88/p38 mitogen-activated protein (MAPK) and endoplasmic reticulum stress (ERS) activation in intestinal epithelial cells were also increased more significantly in the MG1655-monocolonized ANP rats. In vitro, the rat ileal epithelial cell line IEC-18 displayed aggravated tumor necrosis factor alpha-induced inflammation and loss of tight-junction proteins in coculture with E. coli MG1655, as well as TLR4, MyD88, and Bip upregulation. In conclusion, our study shows that commensal E. coli MG1655 increases TLR4/MyD88/p38 MAPK and ERS signaling-induced intestinal epithelial injury and aggravates ANP in rats. Our study also describes the harmful potential of commensal E. coli in ANP.IMPORTANCE This study describes the harmful potential of commensal E. coli in ANP, which has not been demonstrated in previous studies. Our work provides new insights into gut bacterium-ANP cross talk, suggesting that nonpathogenic commensals could also exhibit adverse effects in the context of diseases.

Unravel the molecular mechanism of XBP1 in regulating the biology of cancer cells

Cancer cells are usually exposed to stressful environments, such as hypoxia, nutrient deprivation, and other metabolic dysfunctional regulation, leading to continuous endoplasmic reticulum (ER) stress. As the most conserved branch among the three un-folded protein response (UPR) pathways, Inositol-requiring enzyme 1α (IRE1α)-X-box-binding protein 1 (XBP1) signaling has been implicated in cancer development and progression. Active XBP1 with transactivation domain functions as a transcription factor to regulate the expression of downstream target genes, including many oncogenic factors. The regulatory activity of XBP1 in cell proliferation, apoptosis, metastasis, and drug resistance promotes cell survival, leading to tumorigenesis and tumor progression. In addition, the XBP1 peptides-based vaccination and/or combination with immune-modulatory drug administration have been developed for effective management for several cancers. Potentially, XBP1 is the biomarker of cancer development and progression and the strategy for clinical cancer management.

Regulation of Nrf2 by X Box-Binding Protein 1 in Retinal Pigment Epithelium

Normal function of the retinal pigment epithelium (RPE) is essential for maintaining the structural integrity of retinal photoreceptors and the visual process. Sustained oxidative damage of the RPE due to aging and other risk factors contributes to the development of age-related macular degeneration (AMD). The transcription factor NF-E2-related factor 2 (Nrf2) is a central regulator of cellular antioxidant and detoxification responses. Enhancing Nrf2 function protects RPE cells from oxidation-related apoptosis and cell death. Previously, we demonstrated that Nrf2 activation can be induced by endoplasmic reticulum (ER) stress; however, the mechanisms are not fully understood. In the present study, we examined the role of X box-binding protein 1 (XBP1), an ER stress-inducible transcription factor, in regulation of Nrf2 in the RPE. We found that RPE-specific XBP1 conditional knockout (cKO) mice exhibit a significant reduction in Nrf2 mRNA and protein levels, along with decreased expression of major Nrf2 target genes, in the RPE/choroid complex. Using primary RPE cells isolated from XBP1 cKO mice and human ARPE-19 cell line, we confirmed that loss of XBP1 gene or pharmacological inhibition of XBP1 splicing drastically reduces Nrf2 levels in the RPE. Conversely, overexpression of spliced XBP1 results in a modest but significant increase in cytosolic and nuclear Nrf2 protein levels without affecting the transcription of Nrf2 gene. Moreover, induction of ER stress by tunicamycin and thapsigargin markedly increases Nrf2 expression, which is abolished in cells pretreated with XBP1 splicing inhibitors 4μ8C and quinotrierixin. Mechanistic studies indicate that quinotrierixin reduces Nrf2 expression likely through inhibition of protein translation. Finally, we demonstrate that overexpression of Nrf2 protected RPE cells against oxidative injury but appeared to be insufficient to rescue from XBP1 deficiency-induced cell death. Taken together, our results indicate that XBP1 modulates Nrf2 activity in RPE cells and that XBP1 deficiency contributes to oxidative injury of the RPE.

Efavirenz and Efavirenz-like Compounds Activate Human, Murine, and Macaque Hepatic IRE1α-XBP1

Efavirenz (EFV), a widely used antiretroviral drug, is associated with idiosyncratic hepatotoxicity and dyslipidemia. Here we demonstrate that EFV stimulates the activation in primary hepatocytes of key cell stress regulators: inositol-requiring 1α (IRE1α) and X-box binding protein 1 (XBP1). Following EFV exposure, XBP1 splicing (indicating activation) was increased 35.7-fold in primary human hepatocytes. In parallel, XBP1 splicing and IRE1α phosphorylation (p-IRE1α, active IRE1α) were elevated 36.4-fold and 4.9-fold, respectively, in primary mouse hepatocytes. Of note, with EFV treatment, 47.2% of mouse hepatocytes were apoptotic; which was decreased to 23.9% in the presence of STF 083010, an inhibitor of XBP1 splicing. Experiments performed using pregnane X receptor (PXR)-null mouse hepatocytes revealed that EFV-mediated XBP1 splicing and hepatocyte death were not dependent on PXR, which is a nuclear receptor transcription factor that plays a crucial role in the cellular response to xenobiotics. Interestingly, incubation with the primary metabolite of EFV, 8-hydroxyefavirenz (8-OHEFV), only resulted in 10.3- and 2.9-fold increased XBP1 splicing in human and mouse hepatocytes and no change in levels of p-IRE1α in mouse hepatocytes. To further probe the structure-activity relationship of IRE1α-XBP1 activation by EFV, 16 EFV analogs were employed. Of these, an analog in which the EFV alkyne is replaced with an alkene and an analog in which the oxazinone oxygen is replaced by a carbon stimulated XBP1 splicing in human, mouse, and macaque hepatocytes. These data demonstrate that EFV and compounds sharing the EFV scaffold can activate IRE1α-XBP1 across human, mouse, and macaque species.

Quercetin protects against intestinal barrier disruption and inflammation in acute necrotizing pancreatitis through TLR4/MyD88/p38 MAPK and ERS inhibition

OBJECTIVE

To investigate the effects of quercetin on intestinal barrier disruption and inflammation in acute necrotizing pancreatitis (ANP) in rats, and its possible mechanism.

METHODS

ANP was established by retrograde injection of 3.5% sodium taurocholate into the biliopancreatic duct, and quercetin (50 mg/kg × 3) was administered by intraperitoneal injection prior to and after ANP induction. Pancreatitis was assessed by pancreatic histopathology, plasma amylase, pancreatic myeloperoxidase (MPO) activity, IL-1β, TNFα and IL-6 levels. Injury of the distal ileum was assessed by histological evaluation. The ultrastructural changes of ileal epithelial cells were examined by transmission electron microscope (TEM). Intestinal barrier function was estimated by plasma diamine oxidase (DAO), d-lactate, endotoxin; and intestinal tight junction proteins including zonula occludens-1 (ZO-1), claudin 1, occludin; and bacterial translocation. Intestinal inflammation was determined by IL-1β, TNFα and IL-17 A expression. TLR4, MyD88, pp38 MAPK, and endoplasmic reticulum stress (ERS)-related molecules (Bip, p-IRE1α, sXBP1, p-eIF2α, ATF6) were measured by immunohistochemistry and WB.

RESULTS

Quercetin intervention attenuated pancreatic and ileal pathological damages in ANP (P < 0.05), ameliorated intestinal barrier disruption and inflammation (P < 0.05). Meantime, QE significantly suppressed intestinal TLR4/MyD88/p38 MAPK pathway and ERS activation.

CONCLUSIONS

Quercetin plays a protective role against intestinal barrier disruption and inflammation in ANP, probably partly by inhibiting TLR4/MyD88/p38 MAPK and ERS activation.

Compromised phagosome maturation underlies RPE pathology in cell culture and whole animal models of Smith-Lemli-Opitz Syndrome

Treatment of rats with the cholesterol pathway inhibitor AY9944 produces an animal model of Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive disease caused by defective cholesterol synthesis. This SLOS rat model undergoes progressive and irreversible degeneration of the neural retina, with associated pathological features of the retinal pigmented epithelium (RPE). Here, we provide further insights into the mechanism involved in the RPE pathology. In the SLOS rat model, markedly increased RPE apical autofluorescence is observed, compared to untreated animals, which correlates with increased levels of A2E and other bisretinoids. Utilizing cultured human induced pluripotent stem cell (iPSC)- derived SLOS RPE cells, we found significantly elevated steady-state levels of 7-dehydrocholesterol (7DHC) and decreased cholesterol levels (key biochemical hallmarks of SLOS). Western blot analysis revealed altered levels of the macroautophagy/autophagy markers MAP1LC3B-II and SQSTM1/p62, and build-up of ubiquitinated proteins. Accumulation of immature autophagosomes was accompanied by inefficient degradation of phagocytized, exogenously supplied retinal rod outer segments (as evidenced by persistence of the C-terminal 1D4 epitope of RHO [rhodopsin]) in SLOS RPE compared to iPSC-derived normal human control. SLOS RPE cells exhibited lysosomal pH levels and CTSD activity within normal physiological limits, thus discounting the involvement of perturbed lysosomal function. Furthermore, 1D4-positive phagosomes that accumulated in the RPE in both pharmacological and genetic rodent models of SLOS failed to fuse with lysosomes. Taken together, these observations suggest that defective phagosome maturation underlies the observed RPE pathology. The potential relevance of these findings to SLOS and the requirement of cholesterol for phagosome maturation are discussed.

The inositol-requiring enzyme 1 (IRE1α) RNAse inhibitor, 4µ8C, is also a potent cellular antioxidant

Inositol-requiring enzyme 1 alpha (IRE1α) is an endoplasmic reticulum (ER)-transmembrane endonuclease that is activated in response to ER stress as part of the unfolded protein response (UPR). Chronic activation of the UPR has been implicated in the pathogenesis of many common diseases including diabetes, cancer, and neurological pathologies such as Huntington's and Alzheimer's disease. 7-Hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde (4µ8C) is widely used as a specific inhibitor of IRE1α ribonuclease activity (IC50 of 6.89 µM in cultured cells). However, in this paper, we demonstrate that 4µ8C acts as a potent reactive oxygen species (ROS) scavenger, both in a cell-free assay and in cultured cells, at concentrations lower than that widely used to inhibit IRE1α activity. In vitro we show that, 4µ8C effectively decreases xanthine/xanthine oxidase catalysed superoxide production with an IC50 of 0.2 µM whereas in cultured endothelial and clonal pancreatic β-cells, 4µ8C inhibits angiotensin II-induced ROS production with IC50 values of 1.92 and 0.29 µM, respectively. In light of this discovery, conclusions reached using 4µ8C as an inhibitor of IRE1α should be carefully evaluated. However, this unexpected off-target effect of 4µ8C may prove therapeutically advantageous for the treatment of pathologies that are thought to be caused by, or exacerbated by, both oxidative and ER stress such as endothelial dysfunction and/or diabetes.

Suppression of IgE-mediated mast cell activation and mouse anaphylaxis via inhibition of Syk activation by 8-formyl-7-hydroxy-4-methylcoumarin, 4μ8C

Mast cells trigger IgE-mediated allergic reactions by releasing various allergic mediators. 8-Formyl-7-hydroxy-4-methylcoumarin, also called 4μ8C, was originally known as an inositol-requiring enzyme 1 (IRE1) suppressant, but no study has examined its relationship with mast cells and allergic diseases. Therefore, the purpose of this study was to determine whether 4μ8C is effective in suppressing allergic reactions in mast cells and in IgE-mediated allergic animal model. 4μ8C suppressed the degranulation of IgE-mediated mast cells (IC₅₀=3.2μM) and the production of cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-4 (IL-4) in a dose-dependent manner. 4μ8C also suppressed passive cutaneous anaphylaxis (PCA) in mice (ED₅₀=25.1mg/kg). In an experiment on mast cell signaling pathways stimulated by antigen, the phosphorylation and activation of Syk was decreased by 4μ8C, and phosphorylation of downstream signaling molecules, such as linker for activated T cells (LAT), Akt, and the three MAP kinases, ERK, p38, and JNK, were suppressed. Mechanistic studies showed that 4μ8C inhibited the activity of Lyn and Fyn in vitro. Based on the results of those experiments, the suppressor mechanism of allergic reaction by 4μ8C involved reduced activity of Lyn and Fyn, which is pivotal in an IgE-mediated signaling pathway. In summary, for the first time, this study shows that 4μ8C inhibits Lyn and Fyn, thus suppressing allergic reaction by reducing the degranulation and the production of inflammatory cytokines. This suggests that 4μ8C can be used as a new medicinal candidate to control allergic diseases such as seasonal allergies and atopic dermatitis.

Comparative Proteomic Analysis of the Mitochondria-associated ER Membrane (MAM) in a Long-term Type 2 Diabetic Rodent Model

The mitochondria-associated ER membrane (MAM) plays a critical role in cellular energetics and calcium homeostasis; however, how MAM is affected under diabetic condition remains elusive. This study presented a comprehensive proteome profiling of isolated brain MAM from long-term type 2 diabetic mice vs. non-diabetic controls. MAM protein was extracted efficiently by a surfactant-aided precipitation/on-pellet digestion (SOD) method, and MAM proteome was quantified by an ion-current-based MS1 method combined with nanoLC-MS/MS. A total of 1,313 non-redundant proteins of MAM were identified, among which 144 proteins were found significantly altered by diabetes. In-depth IPA analysis identified multiple disease-relevant signaling pathways associated with the MAM proteome changes in diabetes, most significantly the unfolded protein response (UPR), p53, hypoxia-related transcription factors, and methyl CpG binding protein 2. Using immunofluorescence labeling we confirmed the activation of three UPR branches and increased ERp29 and calreticulin in diabetic retinas. Moreover, we found GRP75, a key MAM tethering protein, was drastically reduced by long-term diabetes. In vitro, acute high glucose treatment reduces ER-mitochondrial contact in retinal endothelial cells. This study provides first insight into the significant alterations in MAM proteome associated with activation of the UPR in diabetes, which may serve as novel benchmarks for the future studies of diabetic complications.

PERK and XBP1 differentially regulate CXCL10 and CCL2 production

Inflammation plays a key role in the pathogenesis of many retinal degenerative diseases related with photoreceptor dysfunction/degeneration. However the involvement of photoreceptor cells in inflammatory reactions is largely unknown as they are not considered as inflammatory cells. In this study, we assessed whether photoreceptor cells can produce CCL2 and CXCL10, two important players in inflammation during endoplasmic reticulum (ER) stress. After photoreceptor 661 W cells were treated with ER stress inducer thapsigargin (TG), induction of ER stress increased CXCL10 and CCL2 expression at both mRNA and protein levels, which was significantly blocked by an ER stress blocker 4-phenylbutyrate. ER stress contains three pathways: PERK, ATF6 and IRE1α. Knockdown of PERK attenuated TG-induced CXCL10 and CCL2 mRNA expression, associated with significant decreases in phosphorylation of NF-κB RelA and STAT3. In contrast to PERK, knockdown of XBP1, which is activated by IRE1α-mediated splicing, robustly enhanced TG-induced CXCL10 and CCL2 expression and phosphorylation of NF-κB RelA and STAT3. Blockade of NF-κB or STAT3 markedly diminished TG-induced CXCL10 and CCL2 expression. The specific roles of PERK and XBP1 in CXCL10 and CCL2 expression were further investigated by treating photoreceptor cells with advanced glycation end products (AGE) and high glucose (HG), two of the major contributors to diabetic complications. Similarly, AGE and HG induced CXCL10 and CCL2 expression in which PERK was a positive regulator while XBP1 was a negative regulator. These studies suggest that photoreceptors may be involved in retinal inflammation by expressing chemokines CXCL10 and CCL2. PERK and IRE1α/XBP1 in the unfolded protein response differentially regulate the expression of CXCL10 and CCL2 likely through modulation of ER stress-induced NF-κB RelA and STAT3 activation.