Vascular endothelial cell growth-activated XBP1 splicing in endothelial cells is crucial for angiogenesis

An Overview of Unfolded Protein Response Signaling and Its Role in Cancer

Secretory and transmembrane proteins undergo post-translational modifications and folding in the subcellular organelle, that is, endoplasmic reticulum (ER) to become functionally active. Various factors such as high oxidative stress, low glucose, calcium imbalance, and viral infections interfere with the ER protein folding functions, leading to accumulation of unfolded and misfolded proteins that activate downstream signal transduction pathways, termed as unfolded protein response (UPR). This UPR signaling is adaptive and restored the normal function of cells by decreasing protein synthesis, increasing the folding capacity of ER and degradation of misfolded proteins. If the stress condition is overwhelmed, then UPR signaling shifts to apoptotic pathways. However, cancer cells utilized these UPR signaling for their survival and progression as an adaptive mechanism. In this review, the authors discuss about the overview of ER stress and subsequent UPR signaling and various aspects of cancer as survival, proliferation, and angiogenesis in relation to UPR. Understanding the UPR signaling in relation to cancer will be further helpful in designing therapeutics against cancer.

The interaction between XBP1 and eNOS contributes to endothelial cell migration

The X-box binding protein 1 (XBP1) is a pivotal transcription factor in the endoplasmic reticulum stress response. Our previous studies have proven that XBP1 is involved in vascular endothelial growth factor (VEGF)-mediated endothelial cell (EC) proliferation and angiogenesis. In this study, we used EC monolayer wound healing, tube formation and transwell migration models to explore the role of XBP1splicing in EC migration. We found that scratching on EC monolayer triggered XBP1splicing, which was attenuated by the presence of SU5416and LY294002, suggesting that VEGF signalling pathways may be involved. Over-expression of the spliced XBP1 (XBP1s) via Ad-XBP1s gene transfer increased while knockdown of IRE1αor XBP1 by ShRNA lentivirus suppressed EC migration. Over-expression of XBP1s up-regulated the nitric oxide synthase 3 (NOS3)mRNA through the 3'UTR-mediated stabilisation and increased eNOS protein translation. Further experiments demonstrated that miR-24 participated in the XBP1s-induced eNOSup-regulation and EC migration. Further co-IP and immunofluorescence staining assays revealed that protein kinase B (Akt), eNOS andXBP1s form a complex, resulting in Akt and eNOS nucleus relocation. These results suggest that XBP1 splicing can regulate eNOS expression and cellular location, leading to EC migration and therefore contributing to wound healing and angiogenesis.

Unspliced XBP1 Confers VSMC Homeostasis and Prevents Aortic Aneurysm Formation via FoxO4 Interaction

RATIONALE

Although not fully understood, the phenotypic transition of vascular smooth muscle cells exhibits at the early onset of the pathology of aortic aneurysms. Exploring the key regulators that are responsible for maintaining the contractile phenotype of vascular smooth muscle cells (VSMCs) may confer vascular homeostasis and prevent aneurysmal disease. XBP1 (X-box binding protein 1), which exists in a transcriptionally inactive unspliced form (XBP1u) and a spliced active form (XBP1s), is a key component in response to endoplasmic reticular stress. Compared with XBP1s, little is known about the role of XBP1u in vascular homeostasis and disease.

OBJECTIVE

We aim to investigate the role of XBP1u in VSMC phenotypic switching and the pathogenesis of aortic aneurysms.

METHODS AND RESULTS

XBP1u, but not XBP1s, was markedly repressed in the aorta during the early onset of aortic aneurysm in both angiotensin II-infused apolipoprotein E knockout (ApoE-/-) and CaPO4 (calcium phosphate)-induced C57BL/6J murine models, in parallel with a decrease in smooth muscle cell contractile apparatus proteins. In vivo studies revealed that XBP1 deficiency in smooth muscle cells caused VSMC dedifferentiation, enhanced vascular inflammation and proteolytic activity, and significantly aggravated both thoracic and abdominal aortic aneurysms in mice. XBP1 deficiency, but not an inhibition of XBP1 splicing, induced VSMC switching from the contractile phenotype to a proinflammatory and proteolytic phenotype. Mechanically, in the cytoplasm, XBP1u directly associated with the N terminus of FoxO4 (Forkhead box protein O 4), a recognized repressor of VSMC differentiation via the interaction and inhibition of myocardin. Blocking the XBP1u-FoxO4 interaction facilitated nuclear translocation of FoxO4, repressed smooth muscle cell marker genes expression, promoted proinflammatory and proteolytic phenotypic transitioning in vitro, and stimulated aortic aneurysm formation in vivo.

CONCLUSIONS

Our study revealed the pivotal role of the XBP1u-FoxO4-myocardin axis in maintaining the VSMC contractile phenotype and providing protection from aortic aneurysm formation.

Transdifferentiated Human Vascular Smooth Muscle Cells are a New Potential Cell Source for Endothelial Regeneration

Endothelial dysfunction is widely implicated in cardiovascular pathological changes and development of vascular disease. In view of the fact that the spontaneous endothelial cell (EC) regeneration is a slow and insufficient process, it is of great interest to explore alternative cell sources capable of generating functional ECs. Vascular smooth muscle cell (SMC) composes the majority of the vascular wall and retains phenotypic plasticity in response to various stimuli. The aim of this study is to test the feasibility of the conversion of SMC into functional EC through the use of reprogramming factors. Human SMCs are first dedifferentiated for 4 days to achieve a vascular progenitor state expressing CD34, by introducing transcription factors OCT4, SOX2, KLF4 and c-MYC. These SMC-derived progenitors are then differentiated along the endothelial lineage. The SMC-converted ECs exhibit typical endothelial markers expression and endothelial functions in vitro, in vivo and in disease model. Further comprehensive analysis indicates that mesenchymal-to-epithelial transition is requisite to initiate SMCs reprogramming into vascular progenitors and that members of the Notch signalling pathway regulate further differentiation of the progenitors into endothelial lineage. Together, we provide the first evidence of the feasibility of the conversion of human SMCs towards endothelial lineage through an intermediate vascular progenitor state induced by reprogramming.

eIF2α signaling regulates ischemic osteonecrosis through endoplasmic reticulum stress

Osteonecrosis of the femoral head (ONFH) primarily results from ischemia/hypoxia to the femoral head, and one of the cellular manifestations is the endoplasmic reticulum (ER) stress. To understand possible linkage of ischemic osteonecrosis to the ER stress, a surgery-induced animal model was employed and salubrinal was administered to evaluate the role of ER stress. Salubrinal is a synthetic chemical that inhibits de-phosphorylation of eIF2α, and it can suppress cell death from the ER stress at a proper dose. The results indicated that the ER stress was associated with ONFH and salubrinal significantly improved ONFH-induced symptoms such as osteonecrosis, bone loss, reduction in vessel perfusion, and excessive osteoclastogenesis in the femoral head. Salubrinal also protected osteoblast development by upregulating the levels of ATF4, ALP and RUNX2, and it stimulated angiogenesis of endothelial cells through elevating ATF4 and VEGF. Collectively, the results support the notion that the ER stress is an important pathological outcome in the surgery-induced ONFH model, and salubrinal improves ONFH symptoms by enhancing angiogenesis and bone healing via suppressing the ER stress.

Endoplasmic reticulum stress and the development of endothelial dysfunction

The vascular endothelium plays a critical role in cardiovascular homeostasis, and thus identifying the underlying causes of endothelial dysfunction has important clinical implications. In this regard, the endoplasmic reticulum (ER) has recently emerged as an important regulator of metabolic processes. Dysfunction within the ER, broadly termed ER stress, evokes the unfolded protein response (UPR), an adaptive pathway that aims to restore ER homeostasis. Although the UPR is the first line of defense against ER stress, chronic activation of the UPR leads to cell dysfunction and death and has recently been implicated in the pathogenesis of endothelial dysfunction. Numerous risk factors for endothelial dysfunction can induce ER stress, which may in turn disrupt endothelial function via direct effects on endothelium-derived vasoactive substances or by activating other pathogenic cellular networks such as inflammation and oxidative stress. This review summarizes the available data linking ER stress to endothelial dysfunction.

Inositol-Requiring Enzyme 1 Facilitates Diabetic Wound Healing Through Modulating MicroRNAs

Diabetic skin ulcers represent a challenging clinical problem with mechanisms not fully understood. In this study, we investigated the role and mechanism for the primary unfolded protein response (UPR) transducer inositol-requiring enzyme 1 (IRE1α) in diabetic wound healing. Bone marrow-derived progenitor cells (BMPCs) were isolated from adult male type 2 diabetic and their littermate control mice. In diabetic BMPCs, IRE1α protein expression and phosphorylation were repressed. The impaired diabetic BMPC angiogenic function was rescued by adenovirus-mediated expression of IRE1α but not by the RNase-inactive IRE1α or the activated X-box binding protein 1 (XBP1), the canonical IRE1α target. In fact, IRE1α RNase processes a subset of microRNAs (miRs), including miR-466 and miR-200 families, through which IRE1α plays an important role in maintaining BMPC function under the diabetic condition. IRE1α attenuated maturation of miR-466 and miR-200 family members at precursor miR levels through the regulated IRE1α-dependent decay (RIDD) independent of XBP1. IRE1α deficiency in diabetes resulted in a burst of functional miRs from miR-466 and miR-200 families, which directly target and repress the mRNA encoding the angiogenic factor angiopoietin 1 (ANGPT1), leading to decreased ANGPT1 expression and disrupted angiogenesis. Importantly, cell therapies using IRE1α-expressing BMPCs or direct IRE1α gene transfer significantly accelerated cutaneous wound healing in diabetic mice through facilitating angiogenesis. In conclusion, our studies revealed a novel mechanistic basis for rescuing angiogenesis and tissue repair in diabetic wound treatments.

Screening for angiogenic inhibitors in zebrafish to evaluate a predictive model for developmental vascular toxicity

Chemically-induced vascular toxicity during embryonic development may cause a wide range of adverse effects. To identify putative vascular disrupting chemicals (pVDCs), a predictive pVDC signature was constructed from 124 U.S. EPA ToxCast high-throughput screening (HTS) assays and used to rank 1060 chemicals for their potential to disrupt vascular development. Thirty-seven compounds were selected for targeted testing in transgenic Tg(kdrl:EGFP) and Tg(fli1:EGFP) zebrafish embryos to identify chemicals that impair developmental angiogenesis. We hypothesized that zebrafish angiogenesis toxicity data would correlate with human cell-based and cell-free in vitro HTS ToxCast data. Univariate statistical associations used to filter HTS data based on correlations with zebrafish angiogenic inhibition in vivo revealed 132 total significant associations, 33 of which were already captured in the pVDC signature, and 689 non-significant assay associations. Correlated assays were enriched in cytokine and extracellular matrix pathways. Taken together, the findings indicate the utility of zebrafish assays to evaluate an HTS-based predictive toxicity signature and also provide an experimental basis for expansion of the pVDC signature with novel HTS assays.

Causes and consequences of endoplasmic reticulum stress in rheumatic disease

Rheumatic diseases represent a heterogeneous group of inflammatory conditions, many of which involve chronic activation of both innate and adaptive immune responses by multiple genetic and environmental factors. These immune responses involve the secretion of excessive amounts of cytokines and other signalling mediators by activated immune cells. The endoplasmic reticulum (ER) is the cellular organelle that directs the folding, processing and trafficking of membrane-bound and secreted proteins, including many key components of the immune response. Maintaining homeostasis in the ER is critical to cell function and survival. Consequently, elaborate mechanisms have evolved to sense and respond to ER stress through three main signalling pathways that together comprise the unfolded protein response (UPR). Activation of the UPR can rapidly resolve the accumulation of misfolded proteins, direct permanent changes in the size and function of cells during differentiation, and critically influence the immune response and inflammation. Recognition of the importance of ER stress and UPR signalling pathways in normal and dysregulated immune responses has greatly increased in the past few years. This Review discusses several settings in which ER stress contributes to the pathogenesis of rheumatic diseases and considers some of the therapeutic opportunities that these discoveries provide.

Dual role for the unfolded protein response in the ovary: adaption and apoptosis

The endoplasmic reticulum (ER) is the principal organelle responsible for several specific cellular functions including synthesis and folding of secretory or membrane proteins, lipid metabolism, and Ca²⁺ storage. Different physiological as well as pathological stress conditions can, however, perturb ER homeostasis, giving rise to an accumulation of unfolded or misfolded proteins in the ER lumen, a condition termed ER stress. To deal with an increased folding demand, cells activate the unfolded protein response (UPR), which is initially protective but can become detrimental if ER stress is severe and prolonged. Accumulating evidence demonstrates a link between the UPR and ovarian development and function, including follicular growth and maturation, follicular atresia, and corpus luteum biogenesis. Additionally, ER stress and the UPR may also play an important role in the ovary under pathological conditions. Understanding the molecular mechanisms related to the dual role of unfolded protein response in the ovarian physiology and pathology may reveal the pathogenesis of some reproductive endocrine diseases and provide a new guidance to improve the assisted reproductive technology. Here we review the current literature and discuss concepts and progress in understanding the UPR, and we also analyze the role of ER stress and the UPR in the ovary.

Posttranscriptional and transcriptional regulation of endothelial nitric-oxide synthase during hypoxia: the role of microRNAs

Understanding the cellular pathways that regulate endothelial nitric oxide (eNOS, NOS3) expression and consequently nitric oxide (NO) bioavailability during hypoxia is a necessary aspect in the development of novel treatments for cardiovascular disorders. eNOS expression and eNOS-dependent NO cellular signaling during hypoxia promote an equilibrium of transcriptional and posttranscriptional molecular mechanisms that belong to both proapoptotic and survival pathways. Furthermore, NO bioavailability results not only from eNOS levels, but also relies on the presence of eNOS substrate and cofactors, the phosphorylation status of eNOS, and the presence of reactive oxygen species (ROS) that can inactivate eNOS. Since both NOS3 levels and these signaling pathways can also be a subject of posttranscriptional modulation by microRNAs (miRNAs), this class of short noncoding RNAs contribute another level of regulation for NO bioavailability. As miRNA antagomirs or specific target protectors could be used in therapeutic approaches to regulate NO levels, either by changing NOS3 mRNA stability or through factors governing eNOS activity, it is critical to understand their role in governing eNOS activity during hypoxa. In contrast to a large number of miRNAs reported to the change eNOS expression during hypoxia, only a few miRNAs modulate eNOS activity. Furthermore, impaired miRNA biogenesis leads to NOS3 mRNA stabilization under hypoxia. Here we discuss the recent studies that define miRNAs’ role in maintaining endothelial NO bioavailability emphasizing those miRNAs that directly modulate NOS3 expression or eNOS activity.

Autophagy in the eye: Development, degeneration, and aging

Autophagy is a catabolic pathway that promotes the degradation and recycling of cellular components. Proteins, lipids, and even whole organelles are engulfed in autophagosomes and delivered to the lysosome for elimination. In response to stress, autophagy mediates the degradation of cell components, which are recycled to generate the nutrients and building blocks required to sustain cellular homeostasis. Moreover, it plays an important role in cellular quality control, particularly in neurons, in which the total burden of altered proteins and damaged organelles cannot be reduced by redistribution to daughter cells through cell division. Research has only begun to examine the role of autophagy in the visual system. The retina, a light-sensitive tissue, detects and transmits electrical impulses through the optic nerve to the visual cortex in the brain. Both the retina and the eye are exposed to a variety of environmental insults and stressors, including genetic mutations and age-associated alterations that impair their function. Here, we review the main studies that have sought to explain autophagy's importance in visual function. We describe the role of autophagy in retinal development and cell differentiation, and discuss the implications of autophagy dysregulation both in physiological aging and in important diseases such as age-associated macular degeneration and glaucoma. We also address the putative role of autophagy in promoting photoreceptor survival and discuss how selective autophagy could provide alternative means of protecting retinal cells. The findings reviewed here underscore the important role of autophagy in maintaining proper retinal function and highlight novel therapeutic approaches for blindness and other diseases of the eye.

Deregulation of XBP1 expression contributes to myocardial vascular endothelial growth factor-A expression and angiogenesis during cardiac hypertrophy in vivo

Endoplasmic reticulum (ER) stress has been reported to be involved in many cardiovascular diseases such as atherosclerosis, diabetes, myocardial ischemia, and hypertension that ultimately result in heart failure. XBP1 is a key ER stress signal transducer and an important pro‐survival factor of the unfolded protein response (UPR) in mammalian cells. The aim of this study was to establish a role for XBP1 in the deregulation of pro‐angiogenic factor VEGF expression and potential regulatory mechanisms in hypertrophic and failing heart. Western blots showed that myocardial XBP1s protein was significantly increased in both isoproterenol (ISO)‐induced and pressure‐overload‐induced hypertrophic and failing heart compared to normal control. Furthermore, XBP1 silencing exacerbates ISO‐induced cardiac dysfunction along with a reduction of myocardial capillary density and cardiac expression of pro‐angiogenic factor VEGF‐A in vivo. Consistently, experiments in cultured cardiomyocytes H9c2 (2‐1) cells showed that UPR‐induced VEGF‐A upregulation was determined by XBP1 expression level. Importantly, VEGF‐A expression was increased in failing human heart tissue and blood samples and was correlated with the levels of XBP1. These results suggest that XBP1 regulates VEGF‐mediated cardiac angiogenesis, which contributes to the progression of adaptive hypertrophy, and might provide novel targets for prevention and treatment of heart failure.

Semaphorin 3A Induces Odontoblastic Phenotype in Dental Pulp Stem Cells

In cases of pulp exposure due to deep dental caries or severe traumatic injuries, existing pulp-capping materials have a limited ability to reconstruct dentin-pulp complexes and can result in pulpectomy because of their low potentials to accelerate dental pulp cell activities, such as migration, proliferation, and differentiation. Therefore, the development of more effective therapeutic agents has been anticipated for direct pulp capping. Dental pulp tissues are enriched with dental pulp stem cells (DPSCs). Here, the authors investigated the effects of semaphorin 3A (Sema3A) on various functions of human DPSCs in vitro and reparative dentin formation in vivo in a rat dental pulp exposure model. Immunofluorescence staining revealed expression of Sema3A and its receptor Nrp1 (neuropilin 1) in rat dental pulp tissue and human DPSC clones. Sema3A induced cell migration, chemotaxis, proliferation, and odontoblastic differentiation of DPSC clones. In addition, Sema3A treatment of DPSC clones increased β-catenin nuclear accumulation, upregulated expression of the FARP2 gene (FERM, RhoGEF, and pleckstrin domain protein 2), and activated Rac1 in DPSC clones. Furthermore, in the rat dental pulp exposure model, Sema3A promoted reparative dentin formation with dentin tubules and a well-aligned odontoblast-like cell layer at the dental pulp exposure site and with novel reparative dentin almost completely covering pulp tissue at 4 wk after direct pulp capping. These findings suggest that Sema3A could play an important role in dentin regeneration via canonical Wnt/β-catenin signaling. Sema3A might be an alternative agent for direct pulp capping, which requires further study.

Glioblastoma invasion and cooption depend on IRE1α endoribonuclease activity

IRE1α is an endoplasmic reticulum (ER)-resident transmembrane signaling protein and a cellular stress sensor. The protein harbors a cytosolic dual kinase/endoribonuclease activity required for adaptive responses to micro-environmental changes. In an orthotopic xenograft model of human glioma, invalidation of IRE1α RNase or/and kinase activities generated tumors with remarkably distinct phenotypes. Contrasting with the extensive angiogenesis observed in tumors derived from control cells, the double kinase/RNase invalidation reprogrammed mesenchymal differentiation of cancer cells and produced avascular and infiltrative glioblastomas with blood vessel co-option. In comparison, selective invalidation of IRE1α RNase did not compromise tumor angiogenesis but still elicited invasive features and vessel co-option. In vitro, IRE1α RNase deficient cells were also endowed with a higher ability to migrate. Constitutive activation of both enzymes led to wild-type-like lesions. The presence of IRE1α, but not its RNase activity, is therefore required for glioblastoma neovascularization, whereas invasion results only from RNase inhibition. In this model, two key mechanisms of tumor progression and cancer cell survival are functionally linked to IRE1α.

XBP1 splicing triggers miR-150 transfer from smooth muscle cells to endothelial cells via extracellular vesicles

The interaction between endothelial cells (ECs) and smooth muscle cells (SMCs) plays a critical role in the maintenance of vessel wall homeostasis. The X-box binding protein 1 (XBP1) plays an important role in EC and SMC cellular functions. However, whether XBP1 is involved in EC-SMC interaction remains unclear. In this study, In vivo experiments with hindlimb ischemia models revealed that XBP1 deficiency in SMCs significantly attenuated angiogenesis in ischemic tissues, therefore retarded the foot blood perfusion recovery. In vitro studies indicated that either overexpression of the spliced XBP1 or treatment with platelet derived growth factor-BB up-regulated miR-150 expression and secretion via extracellular vesicles (EVs). The XBP1 splicing-mediated up-regulation of miR-150 might be due to increased stability. The SMC-derived EVs could trigger EC migration, which was abolished by miR-150 knockdown in SMCs, suggesting miR-150 is responsible for SMC-stimulated EC migration. The SMC-derived miR-150-containing EVs or premiR-150 transfection increased vascular endothelial growth factor (VEGF)-A mRNA and secretion in ECs. Both inhibitors SU5416 and LY294002 attenuated EVs-induced EC migration. This study demonstrates that XBP1 splicing in SMCs can control EC migration via SMC derived EVs-mediated miR-150 transfer and miR-150-driven VEGF-A/VEGFR/PI3K/Akt pathway activation, thereby modulating the maintenance of vessel wall homeostasis.

Critical role of XBP1 in cancer signalling is regulated by PIN1

XBP1 (X-box-binding protein 1) is activated in cancer and has a pivotal role in tumorigenesis and progression of human cancer. In particular, the XBP1 transcriptional regulatory network is well known to drive cancer development, but little is known about whether the stability of XBP1 is regulated and, if so, what controls the stability of XBP1. In the present study we show that PIN1 prolyl isomerase interacts with the active form of XBP1 (XBP1s) in a phosphorylation-dependent manner and promotes XBP1s-induced cell proliferation and transformation through the regulation of XBP1 stability. By contrast, depletion of Pin1 in cancer cells reduced XBP1s expression, which subsequently inhibits cell proliferation and transformation. Interestingly, XBP1s activates multiple oncogenic pathways including NF-κB (nuclear factor κB), AP1 (activator protein 1) and Myc, and down-regulates PIN1 transcription via a negative-feedback mechanism through p53 induction. Ultimately, reciprocal regulation of Pin1 and XBP1s is associated with the activation of oncogenic pathways, and the relationship of PIN1 and XBP1 may be an attractive target for novel therapy in cancers.

Endoplasmic reticulum stress in the brain subfornical organ contributes to sex differences in angiotensin-dependent hypertension in rats

AIM

Endoplasmic reticulum (ER) stress in the brain subfornical organ (SFO), a key cardiovascular regulatory centre, has been implicated in angiotensin (ANG) II-induced hypertension in males; however, the contribution of ER stress to ANG II-induced hypertension in females is unknown. Female hormones have been shown to prevent ER stress in the periphery. We tested the hypothesis that females are less susceptible to ANG II-induced SFO ER stress than males, leading to sex differences in hypertension.

METHODS

Male, intact and ovariectomized (OVX) female rats received a continuous 2-week subcutaneous infusion of ANG II or saline. Additional male, intact and OVX female rats received intracerebroventricular (ICV) injection of ER stress inducer tunicamycin.

RESULTS

ANG II, but not saline, increased blood pressure (BP) in both males and females, but intact females exhibited smaller increase in BP and less depressor response to ganglionic blockade compared with males or OVX females. Molecular studies revealed that ANG II elevated expression of ER stress biomarkers and Fra-like activity in the SFO in both males and females; however, elevations in these parameters were less in intact females than in males or OVX females. Moreover, ICV tunicamycin induced smaller elevation in BP and less increase in expression of ER stress biomarkers in the SFO in intact females compared with males or OVX females.

CONCLUSION

The results suggest that differences in ANG II-induced brain ER stress between males and females contribute to sex differences in ANG II-mediated hypertension and that oestrogen protects females against ANG II-induced brain ER stress.

Endoplasmic Reticulum Stress and the Hallmarks of Cancer

Tumor cells are often exposed to intrinsic and external factors that alter protein homeostasis, thus producing endoplasmic reticulum (ER) stress. To cope with this, cells evoke an adaptive mechanism to restore ER proteostasis known as the unfolded protein response (UPR). The three main UPR signaling branches initiated by IRE1α, PERK, and ATF6 are crucial for tumor growth and aggressiveness as well as for microenvironment remodeling or resistance to treatment. We provide a comprehensive overview of the contribution of the UPR to cancer biology and the acquisition of malignant characteristics, thus highlighting novel aspects including inflammation, invasion and metastasis, genome instability, resistance to chemo/radiotherapy, and angiogenesis. The therapeutic potential of targeting ER stress signaling in cancer is also discussed.

Overexpression of X-Box Binding Protein 1 (XBP1) Correlates to Poor Prognosis and Up-Regulation of PI3K/mTOR in Human Osteosarcoma

Increasing evidence demonstrates that dysregulation of XBP1 function contributes to tumorigenesis in some cancers. However, little is known about the role of XBP1 in the progression of osteosarcoma (OS). The expression of XBP1 in OS samples was measured by quantitative RT-PCR and Western blotting assays. Cell cycle analysis and cell counting kit 8 (CCK8) assays were performed to determine the effects of XBP1 expression on cells growth capacity. Cell apoptosis coassay was applied to determine cell survival. The expression of genes affected by XBP1 was examined by quantitative RT-RCR and validated by Western blotting assays. XBP1 was overexpressed in OS clinical samples compared with corresponding non-cancerous tissues. Overexpression of XBP1 was significantly associated with advanced clinical stages, high degree of malignancy and low tumor necrosis rate. Furthermore, hypoxia activated XBP1, and silencing XBP1 significantly enhanced OS cell apoptosis. Knock-down of XBP1 resulted in inhibition of OS growth. Most importantly, knockdown of XBP1 led to down-regulation of PIK3R3 and mTOR. Taken together, XBP1 is up-regulated and has a pro-tumor effect in OS with activation of PI3K/mTOR signaling. Thus, targeting XBP1 may provide a new potential therapeutic method for OS.

MicroRNA regulation of unfolded protein response transcription factor XBP1 in the progression of cardiac hypertrophy and heart failure in vivo

Background

XBP1 is a key transcription factor of the unfolded protein response in mammalian cells, which is involved in several cardiovascular pathological progression including cardiac hypertrophy and myocardial infarction, but its expression trend, function and upstream regulate mechanism in the development of heart failure are unclear. In the present study, therefore, the potential role of miRNAs in the regulation of XBP1 expression in heart failure was examined.

Methods and results

First, western blots showed that cardiac expression of ER stress marker XBP1 were induced in the early adaptive phase, but decreased in the maladaptive phase in hypertrophic and failing heart, while there was no obvious change of upstream ATF6 and IRE1 activity in this progression. Interestingly, we further found that XBP1 and its downstream target VEGF were attenuated by miR-30* and miR-214 in cardiomyocyte. Moreover, we found that miR-30* was significantly reduced in the early phase of cardiac hypertrophic animal model and in human failing hearts, while both miR-214 and miR-30* were increased in the maladaptive diseased heart, thereby contribute to impairment of cardiac XBP1 and VEGF expression.

Conclusions

These results provide the first clear link between miRNAs and direct regulation of XBP1 in heart failure and reveal that miR-214 and miR-30* synergistically regulates cardiac VEGF expression and angiogenesis by targeting XBP1 in the progression from adaptive hypertrophy to heart failure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0725-4) contains supplementary material, which is available to authorized users.

ER Stress and Angiogenesis

Proper tissue vascularization is vital for cellular function as it delivers oxygen, nutrients, hormones, and immune cells and helps to clear cellular debris and metabolic waste products. Tissue angiogenesis occurs to satisfy energy requirements and cellular sensors of metabolic imbalance coordinate vessel growth. In this regard, the classical pathways of the unfolded protein response activated under conditions of ER stress have recently been described to generate angiomodulatory or angiostatic signals. This review elaborates on the link between angiogenesis and ER stress and discusses the implications for diseases characterized by altered vascular homeostasis, such as cancer, retinopathies, and atherosclerosis.

XBP 1-Deficiency Abrogates Neointimal Lesion of Injured Vessels Via Cross Talk With the PDGF Signaling

OBJECTIVE

Smooth muscle cell (SMC) migration and proliferation play an essential role in neointimal formation after vascular injury. In this study, we intended to investigate whether the X-box-binding protein 1 (XBP1) was involved in these processes.

APPROACH AND RESULTS

In vivo studies on femoral artery injury models revealed that vascular injury triggered an immediate upregulation of XBP1 expression and splicing in vascular SMCs and that XBP1 deficiency in SMCs significantly abrogated neointimal formation in the injured vessels. In vitro studies indicated that platelet-derived growth factor-BB triggered XBP1 splicing in SMCs via the interaction between platelet-derived growth factor receptor β and the inositol-requiring enzyme 1α. The spliced XBP1 (XBP1s) increased SMC migration via PI3K/Akt activation and proliferation via downregulating calponin h1 (CNN1). XBP1s directed the transcription of mir-1274B that targeted CNN1 mRNA degradation. Proteomic analysis of culture media revealed that XBP1s decreased transforming growth factor (TGF)-β family proteins secretion via transcriptional suppression. TGF-β3 but not TGF-β1 or TGF-β2 attenuated XBP1s-induced CNN1 decrease and SMC proliferation.

CONCLUSIONS

This study demonstrates for the first time that XBP1 is crucial for SMC proliferation via modulating the platelet-derived growth factor/TGF-β pathways, leading to neointimal formation.

MicroRNA-214 Is Upregulated in Heart Failure Patients and Suppresses XBP1-Mediated Endothelial Cells Angiogenesis

More and more miRNAs have been shown to regulate gene expression in the heart and dysregulation of their expression has been linked to cardiovascular diseases including the miR‐199a/214 cluster. However, the signature of circulating miR‐214 expression and its possible roles during the development of heart failure has been less well studied. In this study, we elucidated the biological and clinical significance of miR‐214 dysregulation in heart failure. Firstly, circulating miR‐214 was measured by quantitative PCR, and we found that miR‐214 was upregulated in the serum of chronic heart failure patients, as well as in hypertrophic and failing hearts of humans and mice. Adeno‐associated virus serotype 9 (AAV9)‐mediated miR‐214 silencing attenuates isoproterenol (ISO) infusion‐induced cardiac dysfunction and impairment of cardiac angiogenesis in mice. Mechanistically, miR‐214 overexpression reduces angiogenesis of HUVECs by targeting XBP1, an important transcription factor of unfolded protein response, and XBP1 silencing decreases HUVECs proliferation and angiogenesis similar to miR‐214 overexpression. Furthermore, ectopic expression of XBP1 enhances endothelial cells proliferation and tube formation, and reverses anti‐angiogenic effect of miR‐214 over expression. All these findings suggest that miR‐214 is an important regulator of angiogenesis in heart in vitro and in vivo, likely via regulating the expression of XBP1, and demonstrate that miR‐214 plays an essential role in the control/inhibition of cardiac angiogenesis. J. Cell. Physiol. 230: 1964–1973, 2015. © 2015 The Authors. Journal of Cellular Physiology published by Wiley Periodicals, Inc.

Spliced XBP1 promotes macrophage survival and autophagy by interacting with Beclin-1

Macrophage autophagy plays an important role in the development of atherosclerosis, but the precise mechanism mediating this process is unclear. The potential role of the X-box binding protein 1 (XBP1), a crucial transduction factor that is involved in endoplasmic reticulum stress and the unfolded protein response, in bone marrow-derived macrophage autophagy is unknown. This study mainly explores the roles of XBP1 mRNA splicing in bone marrow-derived macrophage autophagy. The present study shows that the transient overexpression of spliced XBP1 via adenovirus-mediated gene transfer induces autophagy and promotes proliferation in bone marrow-derived macrophages via the down-regulation of Beclin-1, but that the sustained overexpression of spliced XBP1 leads to apoptosis. When XBP1 is down-regulated in bone marrow-derived macrophages using siRNA, rapamycin-induced autophagosome formation is ablated. Furthermore, we have detected the overexpression of XBP1 in areas of atherosclerotic plaques in the arteries of ApoE-/- mice. These results demonstrate that XBP1 mRNA splicing plays an important role in maintaining the function of bone marrow-derived macrophages and provide new insight into the study and treatment of atherosclerosis.

Involvement of the IRE1α-XBP1 pathway and XBP1s-dependent transcriptional reprogramming in metabolic diseases

The X-box binding protein 1 (XBP1) is not only an important component of the unfolded protein response (UPR), but also an important nuclear transcription factor. Upon endoplasmic reticulum stress, XBP1 is spliced by inositol-requiring enzyme 1 (IRE1), thereby generating functional spliced XBP1 (XBP1s). XBP1s functions by translocating into the nucleus to initiate transcriptional programs that regulate a subset of UPR- and non-UPR-associated genes involved in the pathophysiological processes of various diseases. Recent reports have implicated XBP1 in metabolic diseases. This review summarizes the effects of XBP1-mediated regulation on lipid metabolism, glucose metabolism, obesity, and atherosclerosis. Additionally, for the first time, we present XBP1s-dependent transcriptional reprogramming in metabolic diseases under different conditions, including pathology and physiology. Understanding the function of XBP1 in metabolic diseases may provide a basic knowledge for the development of novel therapeutic targets for ameliorating these diseases.

The unfolded protein response in retinal vascular diseases: implications and therapeutic potential beyond protein folding

Angiogenesis is a complex, step-wise process of new vessel formation that is involved in both normal embryonic development as well as postnatal pathological processes, such as cancer, cardiovascular disease, and diabetes. Aberrant blood vessel growth, also known as neovascularization, in the retina and the choroid is a major cause of vision loss in severe eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity, and central and branch retinal vein occlusion. Yet, retinal neovascularization is causally and dynamically associated with vasodegeneration, ischemia, and vascular remodeling in retinal tissues. Understanding the mechanisms of retinal neovascularization is an urgent unmet need for developing new treatments for these devastating diseases. Accumulating evidence suggests a vital role for the unfolded protein response (UPR) in regulation of angiogenesis, in part through coordinating the secretion of pro-angiogenic growth factors, such as VEGF, and modulating endothelial cell survival and activity. Herein, we summarize current research in the context of endoplasmic reticulum (ER) stress and UPR signaling in retinal angiogenesis and vascular remodeling, highlighting potential implications of targeting these stress response pathways in the prevention and treatment of retinal vascular diseases that result in visual deficits and blindness.

Unspliced X-box-binding protein 1 (XBP1) protects endothelial cells from oxidative stress through interaction with histone deacetylase 3

It is well known that atherosclerosis occurs geographically at branch points where disturbed flow predisposes to the development of plaque via triggering of oxidative stress and inflammatory reactions. In this study, we found that disturbed flow activated anti-oxidative reactions via up-regulating heme oxygenase 1 (HO-1) in an X-box-binding protein 1 (XBP1) and histone deacetylase 3 (HDAC3)-dependent manner. Disturbed flow concomitantly up-regulated the unspliced XBP1 (XBP1u) and HDAC3 in a VEGF receptor and PI3K/Akt-dependent manner. The presence of XBP1 was essential for the up-regulation of HDAC3 protein. Overexpression of XBP1u and/or HDAC3 activated Akt1 phosphorylation, Nrf2 protein stabilization and nuclear translocation, and HO-1 expression. Knockdown of XBP1u decreased the basal level and disturbed flow-induced Akt1 phosphorylation, Nrf2 stabilization, and HO-1 expression. Knockdown of HDAC3 ablated XBP1u-mediated effects. The mammalian target of rapamycin complex 2 (mTORC2) inhibitor, AZD2014, ablated XBP1u or HDAC3 or disturbed flow-mediated Akt1 phosphorylation, Nrf2 nuclear translocation, and HO-1 expression. Neither actinomycin D nor cycloheximide affected disturbed flow-induced up-regulation of Nrf2 protein. Knockdown of Nrf2 abolished XBP1u or HDAC3 or disturbed flow-induced HO-1 up-regulation. Co-immunoprecipitation assays demonstrated that XBP1u physically bound to HDAC3 and Akt1. The region of amino acids 201 to 323 of the HDAC3 protein was responsible for the binding to XBP1u. Double immunofluorescence staining revealed that the interactions between Akt1 and mTORC2, Akt1 and HDAC3, Akt1 and XBP1u, HDAC3, and XBP1u occurred in the cytosol. Thus, we demonstrate that XBP1u and HDAC3 exert a protective effect on disturbed flow-induced oxidative stress via up-regulation of mTORC2-dependent Akt1 phosphorylation and Nrf2-mediated HO-1 expression.

The impact of the endoplasmic reticulum protein-folding environment on cancer development

The endoplasmic reticulum (ER) is an essential organelle in eukaryotic cells for the storage and regulated release of calcium and as the entrance to the secretory pathway. Protein misfolding in the ER causes accumulation of misfolded proteins (ER stress) and activation of the unfolded protein response (UPR), which has evolved to maintain a productive ER protein-folding environment. Both ER stress and UPR activation are documented in many different human cancers. In this Review, we summarize the impact of ER stress and UPR activation on every aspect of cancer and discuss outstanding questions for which answers will pave the way for therapeutics.

Modulation of endothelial cell migration by ER stress and insulin resistance: a role during maternal obesity?

Adverse microenvironmental stimuli can trigger the endoplasmic reticulum (ER) stress pathway, which initiates the unfolded protein response (UPR), to restore protein-folding homeostasis. Several studies show induction of ER stress during obesity. Chronic UPR has been linked to different mechanisms of disease in obese and diabetic individuals, including insulin resistance (IR) and impaired angiogenesis. Endothelial cell (EC) migration is an initial step for angiogenesis, which is associated with remodeling of existing blood vessels. EC migration occurs according to the leader–follower model, involving coordinated processes of chemotaxis, haptotaxis, and mechanotaxis. Thus, a fine-tuning of EC migration is necessary to provide the right timing to form the required vessels during angiogenesis. ER stress modulates EC migration at different levels, usually impairing migration and angiogenesis, although different effects may be observed depending on the tissue and/or microenvironment. In the context of pregnancy, maternal obesity (MO) induces IR in the offspring. Interestingly, several proteins associated with obesity-induced IR are also involved in EC migration, providing a potential link with the ER stress-dependent alterations observed in obese individuals. Different signaling cascades that converge on cytoskeleton regulation directly impact EC migration, including the Akt and/or RhoA pathways. In addition, ER is the main intracellular reservoir for Ca²⁺, which plays a pivotal role during EC migration. Therefore, ER stress-related alterations in Ca²⁺ signaling or Ca²⁺ levels might also produce distorted EC migration. However, the above findings have been studied in the context of adult obesity, and no information has been reported regarding the effect of MO on fetal EC migration. Here we summarize the state of knowledge about the possible mechanisms by which ER stress and IR might impact EC migration and angiogenesis in fetal endothelium exposed to MO during pregnancy.

A novel ER stress-independent function of the UPR in angiogenesis

Tumors rely on the unfolded protein response (UPR) and angiogenesis to survive the metabolic stress of hypoxia. Karali et al. (2014) revealed that VEGF signaling engages UPR sensors in an unconventional manner that is independent of endoplasmic reticulum (ER) stress, mediated by mTOR signaling to promote endothelial cell survival and angiogenesis.

Endoplasmic reticulum stress and angiogenesis: is there an interaction between them?

When cells are subjected to stress by changes in their extracellular environment, unfolded proteins accumulate in the endoplasmic reticulum (ER), causing ER stress. This initiates the unfolded protein response (UPR), a signal transduction cascade aiming at restoring cellular homeostasis. The UPR and angiogenesis are involved in the pathogenesis of many diseases such as cancer, pulmonary diseases and chronic liver diseases (CLDs) including alcoholic liver disease, non-alcoholic steatohepatitis and hepatitis B. This review summarizes the upcoming knowledge of the interaction between the UPR and angiogenesis in physiological angiogenesis and in different CLDs and other diseases.

JunB protects β-cells from lipotoxicity via the XBP1-AKT pathway

Diets rich in saturated fats may contribute to the loss of pancreatic β-cells in type 2 diabetes. JunB, a member of the activating protein 1 (AP-1) transcription factor family, promotes β-cell survival and mediates part of the beneficial effects of GLP-1 agonists. In this study we interrogated the molecular mechanisms involved in JunB-mediated β-cell protection from lipotoxicity. The saturated fatty acid palmitate decreased JunB expression, and this loss may contribute to β-cell apoptosis, as overexpression of JunB protected cells from lipotoxicity. Array analysis of JunB-deficient β-cells identified a gene expression signature of a downregulated endoplasmic reticulum (ER) stress response and inhibited AKT signaling. JunB stimulates XBP1 expression via the transcription factor c/EBPδ during ER stress, and forced expression of XBP1s rescued the viability of JunB-deficient cells, constituting an important antiapoptotic mechanism. JunB silencing inhibited AKT activation and activated the proapoptotic Bcl-2 protein BAD via its dephosphorylation. BAD knockdown reversed lipotoxic β-cell death potentiated by JunB siRNA. Interestingly, XBP1s links JunB and AKT signaling as XBP1 knockdown also reduced AKT phosphorylation. GLP-1 agonists induced cAMP-dependent AKT phosphorylation leading to β-cell protection against palmitate-induced apoptosis. JunB and XBP1 knockdown or IRE1 inhibition decreased AKT activation by cAMP, leading to β-cell apoptosis. In conclusion, JunB modulates the β-cell ER stress response and AKT signaling via the induction of XBP1s. The activation of the JunB gene network and the crosstalk between the ER stress and AKT pathway constitute a crucial defense mechanism by which GLP-1 agonists protect against lipotoxic β-cell death. These findings elucidate novel β-cell-protective signal transduction in type 2 diabetes.

Developmental competence of bovine early embryos depends on the coupled response between oxidative and endoplasmic reticulum stress

The stress produced by the coupling of reactive oxygen species (ROS) and endoplasmic reticulum (ER) has been explored extensively, but little is known regarding their roles in the early development of mammalian embryos. Here, we demonstrated that the early development of in vitro-produced (IVP) bovine embryos was governed by the cooperative action between ROS and ER stress. Compared with the tension produced by 5% O2, 20% O2 significantly decreased the blastocyst formation rate and cell survival, which was accompanied by increases in ROS and in levels of sXBP-1 transcript, which is an ER stress indicator. In addition, treatment with glutathione (GSH), a ROS scavenger, decreased ROS levels, which resulted in increased blastocyst formation and cell survival rates. Importantly, levels of sXBP-1 and ER stress-associated transcripts were reduced by GSH treatment in developing bovine embryos. Consistent with this observation, tauroursodeoxycholate (TUDCA), an ER stress inhibitor, improved blastocyst developmental rate, trophectoderm proportion, and cell survival. Moreover, ROS and sXBP-1 transcript levels were markedly decreased by supplementation with TUDCA, suggesting a possible mechanism governing the mutual regulation between ROS and ER stress. Interestingly, knockdown of XBP-1 transcripts resulted in both elevation of ROS and decrease of antioxidant transcripts, which ultimately reduced in vitro developmental competence of bovine embryos. Based on these results, in vitro developmental competence of IVP bovine embryos was highly dependent on the coupled response between oxidative and ER stresses. These results increase our understanding of the mechanism(s) governing early embryonic development and may improve strategies for the generation of IVP embryos with high developmental competence.

Robust rat pulmonary radioprotection by a lipophilic Mn N-alkylpyridylporphyrin, MnTnHex-2-PyP(5+)

With the goal to enhance the distribution of cationic Mn porphyrins within mitochondria, the lipophilic Mn(III)meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin, MnTnHex-2-PyP(5+) has been synthesized and tested in several different model of diseases, where it shows remarkable efficacy at as low as 50 µg/kg single or multiple doses. Yet, in a rat lung radioprotection study, at higher 0.6-1 mg/kg doses, due to its high accumulation and micellar character, it became toxic. To avoid the toxicity, herein the pulmonary radioprotection of MnTnHex-2-PyP(5+) was assessed at 50 µg/kg. Fischer rats were irradiated to their right hemithorax (28 Gy) and treated with 0.05 mg/kg/day of MnTnHex-2-PyP(5+) for 2 weeks by subcutaneously-implanted osmotic pumps, starting at 2 h post-radiation. The body weights and breathing frequencies were followed for 10 weeks post-radiation, when the histopathology and immunohistochemistry were assessed. Impact of MnTnHex-2-PyP(5+) on macrophage recruitment (ED-1), DNA oxidative damage (8-OHdG), TGF-β1, VEGF(A) and HIF-1α were measured. MnTnHex-2-PyP(5+) significantly decreased radiation-induced lung histopathological (H&E staining) and functional damage (breathing frequencies), suppressed oxidative stress directly (8-OHdG), or indirectly, affecting TGF-β1, VEGF (A) and HIF-1α pathways. The magnitude of the therapeutic effects is similar to the effects demonstrated under same experimental conditions with 120-fold higher dose of ~5000-fold less lipophilic Mn(III)meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, MnTE-2-PyP(5+).

The unfolded protein response and diabetic retinopathy

Diabetic retinopathy, a common complication of diabetes, is the leading cause of blindness in adults. Diabetes chronically damages retinal blood vessels and neurons likely through multiple pathogenic pathways such as oxidative stress, inflammation, and endoplasmic reticulum (ER) stress. To relieve ER stress, the cell activates an adaptive mechanism known as the unfolded protein response (UPR). The UPR coordinates the processes of protein synthesis, protein folding, and degradation to ensure proteostasis, which is vital for cell survival and activity. Emerging evidence suggests that diabetes can activate all three UPR branches in retinal cells, among which the PERK/ATF4 pathway is the most extensively studied in the development of diabetic retinopathy. X-box binding protein 1 (XBP1) is a major transcription factor in the core UPR pathway and also regulates a variety of genes involved in cellular metabolism, redox state, autophagy, inflammation, cell survival, and vascular function. The exact function and implication of XBP1 in the pathogenesis of diabetic retinopathy remain elusive. Focusing on this less studied pathway, we summarize recent progress in studies of the UPR pertaining to diabetic changes in retinal vasculature and neurons, highlighting the perspective of XBP1 as a potential therapeutic target in diabetic retinopathy.

The alternative heart: impact of alternative splicing in heart disease

Alternative splicing is the main driver of protein diversity and allows the production of different proteins from each gene in the genome. Changes in exon exclusion, intron retention or the use of alternative splice sites can alter protein structure, localisation, regulation and function. In the heart, alternative splicing of sarcomeric genes, ion channels and cell signalling proteins can lead to cardiomyopathies, arrhythmias and other pathologies. Also, a number of inherited conditions and heart-related diseases develop as a result of mutations affecting splicing. Here, we review the impact that changes in alternative splicing have on individual genes and on whole biological processes associated with heart disease. We also discuss promising therapeutic tools based on the manipulation of alternative splicing.