Platelets induce endoplasmic reticulum stress in macrophages in vitro

BACKGROUND

Endoplasmic reticulum (ER) stress is a key feature of lipid-laden macrophages and contributes to the development of atherosclerotic plaques. Blood platelets are known to interact with macrophages and fine-tune effector functions such as inflammasome activation and phagocytosis. However, the effect of platelets on ER stress induction is unknown.

OBJECTIVES

The objective of this study is to elucidate the potential of platelets in regulating ER stress in macrophages in vitro.

METHODS

Bone marrow-derived macrophages and RAW 264.7 cells were incubated with isolated murine platelets, and ER stress and inflammation markers were determined by reverse transcription-quantitative polymerase chain reaction, Western blotting, and enzyme-linked immunosorbent assay. ER morphology was investigated by electron microscopy. Cell viability, lipid accumulation, and activation were measured by flow cytometry. To gain mechanistic insights, coincubation experiments were performed with platelet decoys/releasates as well as lipopolysaccharide, blocking antibodies, and TLR4 inhibitors.

RESULTS

Coincubation of platelets and macrophages led to elevated levels of ER stress markers (BIP, IRE1α, CHOP, and XBP1 splicing) in murine and human macrophages, which led to a pronounced enlargement of the ER. Macrophage ER stress was accompanied by increased release of proinflammatory cytokines and intracellular lipid accumulation, but not cell death. Platelet decoys, but not platelet releasates or lysate from other cells, phenocopied the effect of platelets. Blocking TLR4 inhibited inflammatory activation of macrophages but did not affect ER stress induction by platelet coincubation.

CONCLUSION

To our knowledge, this study is the first to demonstrate that platelets induce ER stress and unfolded protein response in macrophages by heat-sensitive membrane proteins, independent of inflammatory activation of macrophages.

Mechanism of Qili Qiangxin Capsule for Heart Failure Based on miR133a-Endoplasmic Reticulum Stress

OBJECTIVE

To investigate the pharmacological mechanism of Qili Qiangxin Capsule (QLQX) improvement of heart failure (HF) based on miR133a-endoplasmic reticulum stress (ERS) pathway.

METHODS

A left coronary artery ligation-induced HF after myocardial infarction model was used in this study. Rats were randomly assigned to the sham group, the model group, the QLQX group [0.32 g/(kg·d)], and the captopril group [2.25 mg/(kg·d)], 15 rats per group, followed by 4 weeks of medication. Cardiac function such as left ventricular ejection fraction (EF), fractional shortening (FS), left ventricular systolic pressure (LVSP), left ventricular end diastolic pressure (LVEDP), the maximal rate of increase of left ventricular pressure (+dp/dt max), and the maximal rate of decrease of left ventricular pressure (-dp/dt max) were monitored by echocardiography and hemodynamics. Hematoxylin and eosin (HE) and Masson stainings were used to visualize pathological changes in myocardial tissue. The mRNA expression of miR133a, glucose-regulated protein78 (GRP78), inositol-requiring enzyme 1 (IRE1), activating transcription factor 6 (ATF6), X-box binding protein1 (XBP1), C/EBP homologous protein (CHOP) and Caspase 12 were detected by RT-PCR. The protein expression of GRP78, p-IRE1/IRE1 ratio, cleaved-ATF6, XBP1-s (the spliced form of XBP1), CHOP and Caspase 12 were detected by Western blot. TdT-mediated dUTP nick-end labeling (TUNEL) staining was used to detect the rate of apoptosis.

RESULTS

QLQX significantly improved cardiac function as evidenced by increased EF, FS, LVSP, +dp/dt max, -dp/dt max, and decreased LVEDP (P<0.05, P<0.01). HE staining showed that QLQX ameliorated cardiac pathologic damage to some extent. Masson staining indicated that QLQX significantly reduced collagen volume fraction in myocardial tissue (P<0.01). Results from RT-PCR and Western blot showed that QLQX significantly increased the expression of miR133a and inhibited the mRNA expressions of GRP78, IRE1, ATF6 and XBP1, as well as decreased the protein expressions of GRP78, cleaved-ATF6 and XBP1-s and decreased p-IRE1/IRE1 ratio (P<0.05, P<0.01). Further studies showed that QLQX significantly reduced the expression of CHOP and Caspase12, resulting in a significant reduction in apoptosis rate (P<0.05, P<0.01).

CONCLUSION

The pharmacological mechanism of QLQX in improving HF is partly attributed to its regulatory effect on the miR133a-IRE1/XBP1 pathway.

Hypoxia-activated XBP1s recruits HDAC2-EZH2 to engage epigenetic suppression of ΔNp63α expression and promote breast cancer metastasis independent of HIF1α

Hypoxia is a hallmark of cancer development. However, the molecular mechanisms by which hypoxia promotes tumor metastasis are not fully understood. In this study, we demonstrate that hypoxia promotes breast cancer metastasis through suppression of ΔNp63α in a HIF1α-independent manner. We show that hypoxia-activated XBP1s forms a stable repressor protein complex with HDAC2 and EZH2 to suppress ΔNp63α transcription. Notably, H3K27ac is predominantly occupied on the ΔNp63 promoter under normoxia, while H3K27me3 on the promoter under hypoxia. We show that XBP1s binds to the ΔNp63 promoter to recruit HDAC2 and EZH2 in facilitating the switch of H3K27ac to H3K27me3. Pharmacological inhibition or the knockdown of either HDAC2 or EZH2 leads to increased H3K27ac, accompanied by the reduced H3K27me3 and restoration of ΔNp63α expression suppressed by hypoxia, resulting in inhibition of cell migration. Furthermore, the pharmacological inhibition of IRE1α, but not HIF1α, upregulates ΔNp63α expression in vitro and inhibits tumor metastasis in vivo. Clinical analyses reveal that reduced p63 expression is correlated with the elevated expression of XBP1, HDAC2, or EZH2, and is associated with poor overall survival in human breast cancer patients. Together, these results indicate that hypoxia-activated XBP1s modulates the epigenetic program in suppression of ΔNp63α to promote breast cancer metastasis independent of HIF1α and provides a molecular basis for targeting the XBP1s/HDAC2/EZH2-ΔNp63α axis as a putative strategy in the treatment of breast cancer metastasis.

Transcriptomics analysis of LINC02202/XBP1 axis in melanoma: Implications for drug targeting and PD-1 monoclonal antibody efficacy

Malignant melanoma (MM) is a highly aggressive and deadly form of skin cancer, primarily caused by recurrence and metastasis. Therefore, it is crucial to investigate the regulatory mechanisms underlying melanoma recurrence and metastasis. Our study has identified a potential targeted regulatory relationship between LINC02202, miR-526b-3p and XBP1 in malignant melanoma. Through the regulation of the miR-526b-3p/XBP1 signalling pathway, LINC02202 may play a role in tumour progression and immune infiltration and inhibiting the expression of LINC02202 can increase the efficacy of immunotherapy for melanoma. Our findings shed light on the impact of LINC02202/XBP1 on the phenotype and function of malignant melanoma cells. Furthermore, this study provides a theoretical foundation for the development of novel immunotherapy strategies for malignant melanoma.

Innate IRE1α-XBP1 activation by viral single-stranded RNA and its influence on lung cytokine production during SARS-CoV-2 pneumonia

The utilization of host-cell machinery during SARS-CoV-2 infection can overwhelm the protein-folding capacity of the endoplasmic reticulum and activate the unfolded protein response (UPR). The IRE1α-XBP1 arm of the UPR could also be activated by viral RNA via Toll-like receptors. Based on these premises, a study to gain insight into the pathogenesis of COVID-19 disease was conducted using nasopharyngeal exudates and bronchioloalveolar aspirates. The presence of the mRNA of spliced XBP1 and a high expression of cytokine mRNAs were observed during active infection. TLR8 mRNA showed an overwhelming expression in comparison with TLR7 mRNA in bronchioloalveolar aspirates of COVID-19 patients, thus suggesting the presence of monocytes and monocyte-derived dendritic cells (MDDCs). In vitro experiments in MDDCs activated with ssRNA40, a synthetic mimic of SARS-CoV-2 RNA, showed induction of XBP1 splicing and the expression of proinflammatory cytokines. These responses were blunted by the IRE1α inhibitor MKC8866, the TLR8 antagonist CU-CPT9a, and knockdown of TLR8 receptor. In contrast, the IRE1α-XBP1 activator IXA4 enhanced these responses. Based on these findings, the TLR8/IRE1α system seems to play a significant role in the induction of the proinflammatory cytokines associated with severe COVID-19 disease and might be a druggable target to control cytokine storm.

Methylparaben induces hepatic glycolipid metabolism disorder by activating the IRE1α-XBP1 signaling pathway in male mice

Methylparaben (MP), a preservative widely used in daily supplies, exists in both the environment and the human body. However, the potential health risks posed by MP remain unclear. This study aimed to unravel the mechanisms by which MP disrupts glucose and lipid homeostasis. For this, we administered MP to mice and observed changes in glucose and lipid metabolism. MP exposure led to hyperglycemia, hyperlipidemia, visceral organ injury, and hepatic lipid accumulation. RNA sequencing results from mice livers indicated a close association between MP exposure and endoplasmic reticulum (ER) stress, inflammatory response, and glucose and lipid homeostasis. Western blotting and quantitative reverse transcription-polymerase chain reaction revealed that MP activated ER stress, particularly the inositol-requiring enzyme 1 (IRE1)/X-box binding protein 1 (XBP1) pathway, which further promoted the activation of the nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways. The activation of these pathways phosphorylated insulin receptor substrate-1 (IRS1) (ser 307), resulting in decreased phosphorylation of protein kinase B (Akt) (ser 473), leading to insulin resistance. Additionally, MP exposure promoted lipogenesis through ER stress. To explore potential remedies, we administered the ER stress inhibitor 4-phenylbutyric acid (4-PBA) and the IRE1α-XBP1 pathway inhibitor toyocamycin to mice, both of which protected against metabolic disorders and organ injury caused by MP. These findings suggest that MP induces disruptions in glucose and lipid metabolism through ER stress, primarily through the IRE1α-XBP1 pathway.

Hepatocellular carcinoma cell-derived exosomal miR-21-5p promotes the polarization of tumor-related macrophages (TAMs) through SP1/XBP1 and affects the progression of hepatocellular carcinoma

BACKGROUND

Tumor-associated macrophages (TAMs) have unique functions in the development of hepatocellular carcinoma (HCC). The tumor microenvironment is in a complex state in chronic disease. As a major participant in tumor-associated inflammation, TAMs have a unique effect on promoting tumor cell proliferation, angiogenesis and immunosuppression. The in-depth study of TAMs has important scientific and clinical value and provides new ideas for the treatment of cancer.

METHODS

Bioinformatics analysis, dual-luciferase reporter assays, RT-qPCR and clinical samples were used to analyze the potential mechanism of the miR-21-5p/SP1/XBP1 molecular axis in HCC. In this study, miR-21-5p was highly expressed in HCC exosomes compared with normal hepatocyte exosomes, and HCC exosomes containing miR-21-5p promoted the proliferation and migration of HCC cells and inhibited cell apoptosis. In addition, this treatment promoted the M2 polarization of macrophages, induced the expression of transcription factor-specific protein 1 (SP1), and inhibited the expression of X-box binding protein 1 (XBP1). However, these expression trends were reversed after inhibition of miR-21-5p expression in exosomes of hepatoma cells, and the effects of exosomal miR-21-5p were partially restored after overexpression of SP1. Animal experiments also verified that exosomal miR-21-5p in HCC cells affected the expression level of the SP1/XBP1 protein and promoted M2 polarization of TAMs.

CONCLUSION

Exosomal miR-21-5p in HCC cells can affect the development of HCC cells by regulating SP1/XBP1 and promoting the M2 polarization of TAMs, thereby affecting the adverse prognostic response of HCC patients.

Identification and functional coordination analysis of gene co-expression networks in different tissues of XBP1 cartilage-specific deficient mice

Abnormal differentiation and proliferation of chondrocytes leads to various diseases related to growth and development. The process of chondrocyte differentiation involves a series of complex cellular and molecular interactions. X-box binding protein 1 (XBP1), an essential molecule of the unfolded protein response (UPR) in Endoplasmic Reticulum (ER) stress, participated in cartilage development and causes other related diseases. We previously reported that XBP1 deficiency in cartilage impacts the function and associated diseases of many different tissues including cartilage. However, how differential expression of genes modulates the roles of cartilage and other tissues when XBP1 is lack of in chondrocytes remains unclear. We aimed to screen for differentially expressed (DE) genes in cartilage, brain, heart, and muscle by high-throughput sequencing in XBP1 cartilage-specific knockout (CKO) mice. Further, gene co-expression networks were constructed by weighted gene co-expression network analysis (WGCNA) algorithm and pivot genes were identified in the above four tissues. Protein detection, Hematoxylin-eosin (HE) staining and immunohistochemistry (IHC) experiments have proved that these differentially co-expressed genes participate in the downstream regulatory pathway of different tissues and affect tissue function.Significantly differentially expressed mRNAs [differentially expressed genes (DEGs)] were identified between XBP1 CKO mice and controls in cartilage, brain, heart, and muscle tissues, including 610, 126, 199 and 219 DEGs, respectively. 39 differentially co-expressed genes were identified in the above four tissues, and they were important pivot genes. Comprehensive analysis discovered that XBP1 deficiency in cartilage influences the difference of co-expressed genes between cartilage and other different tissues. These differentially co-expressed genes participate in downstream regulatory pathways of different tissues and affect tissue functions. Collectively, our conclusions may contribute potential biomarkers and molecular mechanisms for the mutual modulation between cartilage and different tissues and the diagnosis and treatment of diseases caused by abnormalities in different tissues. The analysis also provides meaningful insights for future genetic discoveries.

Angiotensin II type-2 receptor attenuates liver fibrosis progression by suppressing IRE1α-XBP1 pathway

The renin-angiotensin system (RAS) has been recognized as a crucial contributor to the development of liver fibrosis, and AT2R, an essential component of RAS, is involved in the progression of liver fibrosis. However, the underlying mechanisms by which AT2R modulates liver fibrosis remain elusive. Here, we report that AT2R was induced to be highly expressed during the progression of liver fibrosis, and the elevated AT2R attenuates liver fibrosis by suppressing IRE1α-XBP1 pathway. In this study, we found that AT2R is not expressed in the no cirrhotic adult liver, but is induced expression during liver fibrosis in both cirrhotic patients and fibrotic mice models. Upregulated AT2R inhibits the activation and proliferation of hepatic stellate cells (HSCs). In addition, our study showed that during liver fibrosis, AT2R deletion increased the dimerization activation of IRE1α and promoted XBP1 splicing, and the spliced XBP1s could promote their transcription by binding to the AT2R promoter and repress the IRE1α-XBP1 axis, forming an AT2R-IRE1α-XBP1 negative feedback loop. Importantly, the combination treatment of an AT2R agonist and an endoplasmic reticulum stress (ER stress) alleviator significantly attenuated liver fibrosis in a mouse model of liver fibrosis. Therefore, we conclude that the AT2R-IRE1α signaling pathway can regulate the progression of liver fibrosis, and AT2R is a new potential therapeutic target for treating liver fibrosis.

Loss of function of XBP1 splicing activity of IRE1α favors B cell tolerance breakdown

Anti-nuclear antibodies are the hallmark of autoimmune diseases such as systemic lupus erythematosus (SLE) and scleroderma. However, the molecular mechanisms of B cell tolerance breakdown in these pathological contexts are poorly known. The study of rare familial forms of autoimmune diseases could therefore help to better describe common biological mechanisms leading to B cell tolerance breakdown. By Whole-Exome Sequencing, we identified a new heterozygous mutation (p.R594C) in ERN1 gene, encoding IRE1α (Inositol-Requiring Enzyme 1α), in a multiplex family with several members presenting autoantibody-mediated autoimmunity. Using human cell lines and a knock-in (KI) transgenic mouse model, we showed that this mutation led to a profound defect of IRE1α ribonuclease activity on X-Box Binding Protein 1 (XBP1) splicing. The KI mice developed a broad panel of autoantibodies, however in a subclinical manner. These results suggest that a decrease of spliced form of XBP1 (XBP1s) production could contribute to B cell tolerance breakdown and give new insights into the function of IRE1α which are important to consider for the development of IRE1α targeting strategies.

Expression and Clinical Significance of IRE1-XBP1s, p62, and Caspase-3 in Colorectal Cancer Patients

Background

Three main cell signaling pathways including the endoplasmic reticulum stress (ERS) response, autophagy, and apoptosis play critical roles in both cell survival and death. They were found to crosstalk with one another during tumorigenesis and cancer progression. This study aimed to investigate the expression of the spliced form of X-box binding protein 1 (XBP1s), p62, and caspase-3, as the essential biomarkers of ERS, autophagy, and apoptosis in patients with colorectal cancer (CRC), as well as the correlation between their expression and clinicopathological data.

Methods

This retrospective study was conducted on formalin-fixed paraffin-embedded (FFPE) blocks, which were collected from patients and their tumor margins, from the tumor bank of Imam Khomeini Hospital (Tehran, Iran) from 2017 to 2019. Tissue microarray (TMA) was used to measure the XBP1s, p62, and caspase-3 biomarkers. Data were analyzed using SPSS software version 20, and P≤0.05 was considered statistically significant.

Results

Evaluating the total of 91 patients, a significant relationship was found between XBP1s expression and TNM stage (P=0.003), primary tumor (pT) (P=0.054), and the degree of differentiation (P=0.006); and between caspase-3 with pT (P=0.004), and lymphovascular invasion (P=0.02). However, no significant correlation was found between p62 and clinicopathological data. Furthermore, a positive relationship between XBP1s and p62 was confirmed (correlation coefficient: 22.2% and P=0.05).

Conclusion

Our findings indicated that XBP1s could be considered as a target for therapy in personalized medicine.

miRNA-378 Is Downregulated by XBP1 and Inhibits Growth and Migration of Luminal Breast Cancer Cells

X-box binding protein 1 (XBP1) is a transcription factor that plays a crucial role in the unfolded protein response (UPR), a cellular stress response pathway involved in maintaining protein homeostasis in the endoplasmic reticulum (EnR). While the role of XBP1 in UPR is well-characterised, emerging evidence suggests its involvement in endocrine resistance in breast cancer. The transcriptional activity of spliced XBP1 (XBP1s) is a major component of its biological effects, but the targets of XBP1s in estrogen receptor (ER)-positive breast cancer are not well understood. Here, we show that the expression of miR-378 and PPARGC1B (host gene of miR-378) is downregulated during UPR. Using chemical and genetic methods, we show that XBP1s is necessary and sufficient for the downregulation of miR-378 and PPARGC1B. Our results show that overexpression of miR-378 significantly suppressed cell growth, colony formation, and migration of ER-positive breast cancer cells. Further, we found that expression of miR-378 sensitised the cells to UPR-induced cell death and anti-estrogens. The expression of miR-378 and PPARGC1B was downregulated in breast cancer, and higher expression of miR-378 is associated with better outcomes in ER-positive breast cancer. We found that miR-378 upregulates the expression of several genes that regulate type I interferon signalling. Analysis of separate cohorts of breast cancer patients showed that a gene signature derived from miR-378 upregulated genes showed a strong association with improved overall and recurrence-free survival in breast cancer. Our results suggest a growth-suppressive role for miR-378 in ER-positive breast cancer where downregulation of miR-378 by XBP1 contributes to endocrine resistance in ER-positive breast cancer.

Impaired T cell IRE1α/XBP1 signaling directs inflammation in experimental heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is a widespread syndrome with limited therapeutic options and poorly understood immune pathophysiology. Using a 2-hit preclinical model of cardiometabolic HFpEF that induces obesity and hypertension, we found that cardiac T cell infiltration and lymphoid expansion occurred concomitantly with cardiac pathology and that diastolic dysfunction, cardiomyocyte hypertrophy, and cardiac phospholamban phosphorylation were T cell dependent. Heart-infiltrating T cells were not restricted to cardiac antigens and were uniquely characterized by impaired activation of the inositol-requiring enzyme 1α/X-box-binding protein 1 (IRE1α/XBP1) arm of the unfolded protein response. Notably, selective ablation of XBP1 in T cells enhanced their persistence in the heart and lymphoid organs of mice with preclinical HFpEF. Furthermore, T cell IRE1α/XBP1 activation was restored after withdrawal of the 2 comorbidities inducing HFpEF, resulting in partial improvement of cardiac pathology. Our results demonstrated that diastolic dysfunction and cardiomyocyte hypertrophy in preclinical HFpEF were T cell dependent and that reversible dysregulation of the T cell IRE1α/XBP1 axis was a T cell signature of HFpEF.

IRE1α protects against osteoarthritis by regulating progranulin-dependent XBP1 splicing and collagen homeostasis

Osteoarthritis (OA) is a full-joint, multifactorial, degenerative and inflammatory disease that seriously affects the quality of life of patients due to its disabling and pain-causing properties. ER stress has been reported to be closely related to the progression of OA. The inositol-requiring enzyme 1α/X-box-binding protein-1 spliced (IRE1α/XBP1s) pathway, which is highly expressed in the chondrocytes of OA patients, promotes the degradation and refolding of abnormal proteins during ER stress and maintains the stability of the ER environment of chondrocytes, but its function and the underlying mechanisms of how it contributes to the progression of OA remain unclear. This study investigates the role of IRE1α/ERN1 in OA. Specific deficiency of ERN1 in chondrocytes spontaneously resulted in OA-like cartilage destruction and accelerated OA progression in a surgically induced arthritis model. Local delivery of AdERN1 relieved degradation of the cartilage matrix and prevented OA development in an ACLT-mediated model. Mechanistically, progranulin (PGRN), an intracellular chaperone, binds to IRE1α, promoting its phosphorylation and splicing of XBP1u to generate XBP1s. XBP1s protects articular cartilage through TNF-α/ERK1/2 signaling and further maintains collagen homeostasis by regulating type II collagen expression. The chondroprotective effect of IRE1α/ERN1 is dependent on PGRN and XBP1s splicing. ERN1 deficiency accelerated cartilage degeneration in OA by reducing PGRN expression and XBP1s splicing, subsequently decreasing collagen II expression and triggering collagen structural abnormalities and an imbalance in collagen homeostasis. This study provides new insights into OA pathogenesis and the UPR and suggests that IRE1α/ERN1 may serve as a potential target for the treatment of joint degenerative diseases, including OA.

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.

XBP1 promotes NRASG12D pre-B acute lymphoblastic leukaemia through IL-7 receptor signalling and provides a therapeutic vulnerability for oncogenic RAS

Activating point mutations of the RAS gene act as driver mutations for a subset of precursor-B cell acute lymphoblastic leukaemias (pre-B ALL) and represent an ambitious target for therapeutic approaches. The X box-binding protein 1 (XBP1), a key regulator of the unfolded protein response (UPR), is critical for pre-B ALL cell survival, and high expression of XBP1 confers poor prognosis in ALL patients. However, the mechanism of XBP1 activation has not yet been elucidated in RAS mutated pre-B ALL. Here, we demonstrate that XBP1 acts as a downstream linchpin of the IL-7 receptor signalling pathway and that pharmacological inhibition or genetic ablation of XBP1 selectively abrogates IL-7 receptor signalling via inhibition of its downstream effectors, JAK1 and STAT5. We show that XBP1 supports malignant cell growth of pre-B NRASG12D ALL cells and that genetic loss of XBP1 consequently leads to cell cycle arrest and apoptosis. Our findings reveal that active XBP1 prevents the cytotoxic effects of a dual PI3K/mTOR pathway inhibitor (BEZ235) in pre-B NRASG12D ALL cells. This implies targeting XBP1 in combination with BEZ235 as a promising new targeted strategy against the oncogenic RAS in NRASG12D -mutated pre-B ALL.

The Role of XBP1 in bone metabolism

Bone is a dynamic organ that, once formed, undergoes a constant remodeling process that includes bone resorption and synthesis. Osteoclasts and osteoblasts are primarily responsible for controlling this process. X-box binding protein 1 (XBP1), a transcription factor, affects the metabolism of bones in various ways. In recent years, numerous studies have revealed that XBP1 plays a vital role in bone metabolism, including osteoclast and osteoblast development, as well as in regulating immune cell differentiation that affects the immune microenvironment of bone remodeling. In this review, we highlight the regulatory mechanisms of XBP1 on osteoclasts and osteoblasts, how XBP1 affects the immune microenvironment of bone remodeling by influencing the differentiation of immune cells, and predict the possible future research directions of XBP1 to provide new insights for the treatment of bone-related metabolic diseases.

The unfolded protein response components IRE1α and XBP1 promote human coronavirus infection

The cellular processes that support human coronavirus replication and contribute to the pathogenesis of severe disease remain incompletely understood. Many viruses, including coronaviruses, cause endoplasmic reticulum (ER) stress during infection. IRE1α is a component of the cellular response to ER stress that initiates non-conventional splicing of XBP1 mRNA. Spliced XBP1 encodes a transcription factor that induces the expression of ER-related targets. Activation of the IRE1α-XBP1 pathway occurs in association with risk factors for severe human coronavirus infection. In this study, we found that the human coronaviruses HCoV-OC43 (human coronavirus OC43) and SARS-CoV-2 (severe acute respiratory syndrome coronavirus-2) both robustly activate the IRE1α-XBP1 branch of the unfolded protein response in cultured cells. Using IRE1α nuclease inhibitors and genetic knockdown of IRE1α and XBP1, we found that these host factors are required for optimal replication of both viruses. Our data suggest that IRE1α supports infection downstream of initial viral attachment and entry. In addition, we found that ER stress-inducing conditions are sufficient to enhance human coronavirus replication. Furthermore, we found markedly increased XBP1 in circulation in human patients with severe coronavirus disease 2019 (COVID-19). Together, these results demonstrate the importance of IRE1α and XBP1 for human coronavirus infection. IMPORTANCE There is a critical need to understand the cellular processes co-opted during human coronavirus replication, with an emphasis on identifying mechanisms underlying severe disease and potential therapeutic targets. Here, we demonstrate that the host proteins IRE1α and XBP1 are required for robust infection by the human coronaviruses, SARS-CoV-2 and HCoV-OC43. IRE1α and XBP1 participate in the cellular response to ER stress and are activated during conditions that predispose to severe COVID-19. We found enhanced viral replication with exogenous IRE1α activation, and evidence that this pathway is activated in humans during severe COVID-19. Together, these results demonstrate the importance of IRE1α and XBP1 for human coronavirus infection.

Novel XBP1s-independent function of IRE1 RNase in HIF-1α-mediated glycolysis upregulation in human macrophages upon stimulation with LPS or saturated fatty acid

In obesity, adipose tissue infiltrating macrophages acquire a unique pro-inflammatory polarization, thereby playing a key role in the development of chronic inflammation and Type 2 diabetes. Increased saturated fatty acids (SFAs) levels have been proposed to drive this specific polarization. Accordingly, we investigated the immunometabolic reprogramming in SFA-treated human macrophages. As expected, RNA sequencing highlighted a pro-inflammatory profile but also metabolic signatures including glycolysis and hypoxia as well as a strong unfolded protein response. Glycolysis upregulation was confirmed in SFA-treated macrophages by measuring glycolytic gene expression, glucose uptake, lactate production and extracellular acidification rate. Like in LPS-stimulated macrophages, glycolysis activation in SFA-treated macrophages was dependent on HIF-1α activation and fueled the production of pro-inflammatory cytokines. SFAs and LPS both induced IRE1α endoribonuclease activity, as demonstrated by XBP1 mRNA splicing, but with different kinetics matching HIF-1α activation and the glycolytic gene expression. Interestingly, the knockdown of IRE1α and/or the pharmacological inhibition of its RNase activity prevented HIF-1α activation and significantly decreased glycolysis upregulation. Surprisingly, XBP1s appeared to be dispensable, as demonstrated by the lack of inhibiting effect of XBP1s knockdown on glycolytic genes expression, glucose uptake, lactate production and HIF-1α activation. These experiments demonstrate for the first time a key role of IRE1α in HIF-1α-mediated glycolysis upregulation in macrophages stimulated with pro-inflammatory triggers like LPS or SFAs through XBP1s-independent mechanism. IRE1 could mediate this novel function by targeting other transcripts (mRNA or pre-miRNA) through a mechanism called regulated IRE1-dependent decay or RIDD. Deciphering the underlying mechanisms of this novel IRE1 function might lead to novel therapeutic targets to curtail sterile obesity- or infection-linked inflammation.

RSV replication modifies the XBP1s binding complex on the IRF1 upstream enhancer to potentiate the mucosal anti-viral response

Introduction

The unfolded protein response (UPR) has emerged as an important signaling pathway mediating anti-viral defenses to Respiratory Syncytial Virus (RSV) infection. Earlier we found that RSV replication predominantly activates the evolutionarily conserved Inositol Requiring Enzyme 1α (IRE1α)-X-Box Binding Protein 1 spliced (XBP1s) arm of the Unfolded Protein Response (UPR) producing inflammation, metabolic adaptation and cellular plasticity, yet the mechanisms how the UPR potentiates inflammation are not well understood.

Methods

To understand this process better, we examined the genomic response integrating RNA-seq and Cleavage Under Targets and Release Using Nuclease (CUT&RUN) analyses. These data were integrated with an RNA-seq analysis conducted on RSV-infected small airway cells ± an IRE1α RNAse inhibitor.

Results

We identified RSV induced expression changes in ~3.2K genes; of these, 279 required IRE1α and were enriched in IL-10/cytokine signaling pathways. From this data set, we identify those genes directly under XBP1s control by CUT&RUN. Although XBP1s binds to ~4.2 K high-confidence genomic binding sites, surprisingly only a small subset of IL10/cytokine signaling genes are directly bound. We further apply CUT&RUN to find that RSV infection enhances XBP1s loading on 786 genomic sites enriched in AP1/Fra-1, RELA and SP1 binding sites. These control a subset of cytokine regulatory factor genes including IFN response factor 1 (IRF1), CSF2, NFKB1A and DUSP10. Focusing on the downstream role of IRF1, selective knockdown (KD) and overexpression experiments demonstrate IRF1 induction controls type I and -III interferon (IFN) and IFN-stimulated gene (ISG) expression, demonstrating that ISG are indirectly regulated by XBP1 through IRF1 transactivation. Examining the mechanism of IRF1 activation, we observe that XBP1s directly binds a 5' enhancer sequence whose XBP1s loading is increased by RSV. The functional requirement for the enhancer is demonstrated by targeting a dCas9-KRAB silencer, reducing IRF1 activation. Chromatin immunoprecipitation shows that XBP1 is required, but not sufficient, for RSV-induced recruitment of activated phospho-Ser2 Pol II to the enhancer.

Discussion

We conclude that XBP1s is a direct activator of a core subset of IFN and cytokine regulatory genes in response to RSV. Of these IRF1 is upstream of the type III IFN and ISG response. We find that RSV modulates the XBP1s binding complex on the IRF1 5' enhancer whose activation is required for IRF1 expression. These findings provide novel insight into how the IRE1α-XBP1s pathway potentiates airway mucosal anti-viral responses.

Emodin, an Emerging Mycotoxin, Induces Endoplasmic Reticulum Stress-Related Hepatotoxicity through IRE1α-XBP1 Axis in HepG2 Cells

Emodin, an emerging mycotoxin, is known to be hepatotoxic, but its mechanism remains unclear. We hypothesized that emodin could induce endoplasmic reticulum (ER) stress through the inositol-requiring enzyme 1 alpha (IRE1α)-X-box-binding protein 1 (XBP1) pathway and apoptosis, which are closely correlated and contribute to hepatotoxicity. To test this hypothesis, a novel IRE1α inhibitor, STF-083010, was used. An MTT assay was used to evaluate metabolic activity, and quantitative PCR and western blotting were used to investigate the gene and protein expression of ER stress or apoptosis-related markers. Apoptosis was evaluated with flow cytometry. Results showed that emodin induced cytotoxicity in a dose-dependent manner in HepG2 cells and upregulated the expression of binding immunoglobulin protein (BiP), C/EBP homologous protein (CHOP), IRE1α, spliced XBP1, the B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 ratio, and cleaved caspase-3. Cotreatment with emodin and STF-083010 led to the downregulation of BiP and upregulation of CHOP, the Bax/Bcl-2 ratio, and cleaved caspase-3 compared with single treatment with emodin. Furthermore, the apoptosis rate was increased in a dose-dependent manner with emodin treatment. Thus, emodin induced ER stress in HepG2 cells by activating the IRE1α-XBP1 axis and induced apoptosis, indicating that emodin can cause hepatotoxicity.

Activation of XBP1 but not ATF6α rescues heart failure induced by persistent ER stress in medaka fish

The unfolded protein response is triggered in vertebrates by ubiquitously expressed IRE1α/β (although IRE1β is gut-specific in mice), PERK, and ATF6α/β, transmembrane-type sensor proteins in the ER, to cope with ER stress, the accumulation of unfolded and misfolded proteins in the ER. Here, we burdened medaka fish, a vertebrate model organism, with ER stress persistently from fertilization by knocking out the AXER gene encoding an ATP/ADP exchanger in the ER membrane, leading to decreased ATP concentration-mediated impairment of the activity of Hsp70- and Hsp90-type molecular chaperones in the ER lumen. ER stress and apoptosis were evoked from 4 and 6 dpf, respectively, leading to the death of all AXER-KO medaka by 12 dpf because of heart failure (medaka hatch at 7 dpf). Importantly, constitutive activation of IRE1α signaling-but not ATF6α signaling-rescued this heart failure and allowed AXER-KO medaka to survive 3 d longer, likely because of XBP1-mediated transcriptional induction of ER-associated degradation components. Thus, activation of a specific pathway of the unfolded protein response can cure defects in a particular organ.

XBP1 as a novel molecular target to attenuate drug resistance in hepatocellular carcinoma

INTRODUCTION

Despite improvements in clinical management of hepatocellular carcinoma (HCC), prognosis remains poor with a 5-year survival rate less than 40%. Drug resistance in HCC makes it challenging to treat; therefore, it is imperative to develop new therapeutic strategies. Higher expression of X-box binding protein 1 (XBP1) in tumor cells is highly correlated with poor prognosis. In tumor cells, XBP1 modulates the unfolded protein response (UPR) to restore homeostasis in endoplasmic reticulum. Targeting XBP1 could be a promising therapeutic strategy to overcome HCC resistance and improve the survival rate of patients.

AREAS COVERED

This review provides the recent evidence that indicates XBP1 is involved in HCC drug resistance via DNA damage response, drug inactivation, and inhibition of apoptosis. In addition, the potential roles of XBP1 in inducing resistance in HCC cells were highlighted, and we showed how its inhibition could sensitize tumor cells to controlled cell death.

EXPERT OPINION

Due to the diversity in molecular mechanism of multidrug-resistance, targeting one specific pathway is inadequate. XBP1 inhibition could be a potential therapeutic target to overcome verity of resistance mechanisms. The main function of this transcription factor in HCC treatment response is an attractive area for further studies and should be discussed more.

The IRE1α-XBP1 Signaling Axis Promotes Glycolytic Reprogramming in Response to Inflammatory Stimuli

Immune cells must be able to adjust their metabolic programs to effectively carry out their effector functions. Here, we show that the endoplasmic reticulum (ER) stress sensor Inositol-requiring enzyme 1 alpha (IRE1α) and its downstream transcription factor X box binding protein 1 (XBP1) enhance the upregulation of glycolysis in classically activated macrophages (CAMs). The IRE1α-XBP1 signaling axis supports this glycolytic switch in macrophages when activated by lipopolysaccharide (LPS) stimulation or infection with the intracellular bacterial pathogen Brucella abortus. Importantly, these different inflammatory stimuli have distinct mechanisms of IRE1α activation; while Toll-like receptor 4 (TLR4) supports glycolysis under both conditions, TLR4 is required for activation of IRE1α in response to LPS treatment but not B. abortus infection. Though IRE1α and XBP1 are necessary for maximal induction of glycolysis in CAMs, activation of this pathway is not sufficient to increase the glycolytic rate of macrophages, indicating that the cellular context in which this pathway is activated ultimately dictates the cell's metabolic response and that IRE1α activation may be a way to fine-tune metabolic reprogramming. IMPORTANCE The immune system must be able to tailor its response to different types of pathogens in order to eliminate them and protect the host. When confronted with bacterial pathogens, macrophages, frontline defenders in the immune system, switch to a glycolysis-driven metabolism to carry out their antibacterial functions. Here, we show that IRE1α, a sensor of ER stress, and its downstream transcription factor XBP1 support glycolysis in macrophages during infection with Brucella abortus or challenge with Salmonella LPS. Interestingly, these stimuli activate IRE1α by independent mechanisms. While the IRE1α-XBP1 signaling axis promotes the glycolytic switch, activation of this pathway is not sufficient to increase glycolysis in macrophages. This study furthers our understanding of the pathways that drive macrophage immunometabolism and highlights a new role for IRE1α and XBP1 in innate immunity.

Tumor-intrinsic IRE1α signaling controls protective immunity in lung cancer

IRE1α-XBP1 signaling is emerging as a central orchestrator of malignant progression and immunosuppression in various cancer types. Employing a computational XBP1s detection method applied to TCGA datasets, we demonstrate that expression of the XBP1s mRNA isoform predicts poor survival in non-small cell lung cancer (NSCLC) patients. Ablation of IRE1α in malignant cells delays tumor progression and extends survival in mouse models of NSCLC. This protective effect is accompanied by alterations in intratumoral immune cell subsets eliciting durable adaptive anti-cancer immunity. Mechanistically, cancer cell-intrinsic IRE1α activation sustains mPGES-1 expression, enabling production of the immunosuppressive lipid mediator prostaglandin E2. Accordingly, restoring mPGES-1 expression in IRE1αKO cancer cells rescues normal tumor progression. We have developed an IRE1α gene signature that predicts immune cell infiltration and overall survival in human NSCLC. Our study unveils an immunoregulatory role for cancer cell-intrinsic IRE1α activation and suggests that targeting this pathway may help enhance anti-tumor immunity in NSCLC.

XBP1 Activation Reduces Severity of Polycystic Kidney Disease due to a Nontruncating Polycystin-1 Mutation in Mice

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in Pkd1 and Pkd2. They encode the polytopic integral membrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively, which are expressed on primary cilia. Formation of kidney cysts in ADPKD starts when a somatic second hit mechanism inactivates the wild-type Pkd allele. Approximately one quarter of families with ADPDK due to Pkd1 have germline nonsynonymous amino acid substitution (missense) mutations. A subset of these mutations is hypomorphic, retaining some residual PC1 function. Previous studies have shown that the highly conserved Ire1 α -XBP1 pathway of the unfolded protein response can modulate levels of functional PC1 in the presence of mutations in genes required for post-translational maturation of integral membrane proteins. We examine how activity of the endoplasmic reticulum chaperone-inducing transcription factor XBP1 affects ADPKD in a murine model with missense Pkd1 .

METHODS

We engineered a Pkd1 REJ domain missense murine model, Pkd1 R2216W , on the basis of the orthologous human hypomorphic allele Pkd1 R2220W , and examined the effects of transgenic activation of XBP1 on ADPKD progression.

RESULTS

Expression of active XBP1 in cultured cells bearing PC1 R2216W mutations increased levels and ciliary trafficking of PC1 R2216W . Mice homozygous for Pkd1 R2216W or heterozygous for Pkd1 R2216Win trans with a conditional Pkd1 fl allele exhibit severe ADPKD following inactivation in neonates or adults. Transgenic expression of spliced XBP1 in tubule segments destined to form cysts reduced cell proliferation and improved Pkd progression, according to structural and functional parameters.

CONCLUSIONS

Modulating ER chaperone function through XBP1 activity improved Pkd in a murine model of PC1, suggesting therapeutic targeting of hypomorphic mutations.

Inositol-requiring enzyme 1α/X-box protein 1 pathway expression is impaired in pediatric cholestatic liver disease explants

BACKGROUND

Increased intrahepatic bile acids cause endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) is activated to maintain homeostasis. UPR dysregulation, including the inositol-requiring enzyme 1α/X-box protein 1 (IRE1α/XBP1) pathway, is associated with adult liver diseases but has not been characterized in pediatric liver diseases. We evaluated hepatic UPR expression in pediatric cholestatic liver disease (CLD) explants and hypothesize that an inability to appropriately activate the hepatic IRE1α/XBP1 pathway is associated with the pathogenesis of CLD.

METHODS

We evaluated 34 human liver explants, including: pediatric CLD (Alagille, ALGS, and progressive familial intrahepatic cholestasis, PFIC), pediatric non-cholestatic liver disease controls (autoimmune hepatitis, AIH), adult CLD, and normal controls. We performed RNA-seq, quantitative PCR, and western blotting to measure expression differences of the hepatic UPR and other signaling pathways.

RESULTS

Pathway analysis demonstrated that the KEGG 'protein processing in ER' pathway was downregulated in pediatric CLD compared to normal controls. Pediatric CLD had decreased hepatic IRE1α/XBP1 pathway gene expression and decreased protein expression of phosphorylated IRE1α compared to normal controls. IRE1α/XBP1 pathway gene expression was also decreased in pediatric CLD compared to AIH disease controls.

CONCLUSIONS

Pediatric CLD explants have decreased expression of the protective IRE1α/XBP1 pathway and down-regulated KEGG protein processing in the ER pathways. IRE1α/XBP1 pathway expression differences occur when compared to both normal and non-cholestatic disease controls. Attenuated expression of the IRE1α/XBP1 pathway is associated with cholestatic diseases and may be a target for future therapeutics.

Effects of bisphenol A on uterine leiomyoma: In vitro and in vivo evaluation with mechanistic insights related to XBP1

The incidence rate of human uterine leiomyomas is over 70% in the women of childbearing age, which has caused serious health and financial burden. Our previous study confirmed that Bisphenol A (BPA)，representative environmental estrogen, promoted the proliferation of human uterine leiomyomas and up-regulated the expression of cell proliferation-related genes. In this study, by combining ChIP-seq and RNA-seq, it was shown that after BPA intervention, H3K27ac modification levels and gene expression levels were altered in uterine leiomyomas cells. Moreover experimental verification found that BPA can regulate ITGA2 through the transcription factor XBP1, activate the downstream PI3K/AKT signaling pathway, eventually promote the proliferation of uterine leiomyomas. The present study provides new insights into the pathogenesis associated with exposure to BPA and other endocrine disruptors with similar effects by defining XBP1 as an important regulator, and which may act as an intervention and treatment target for uterine leiomyomas.

Unfolded protein response IRE1/XBP1 signaling is required for healthy mammalian brain aging

Aging is a major risk factor to develop neurodegenerative diseases and is associated with decreased buffering capacity of the proteostasis network. We investigated the significance of the unfolded protein response (UPR), a major signaling pathway activated to cope with endoplasmic reticulum (ER) stress, in the functional deterioration of the mammalian brain during aging. We report that genetic disruption of the ER stress sensor IRE1 accelerated age-related cognitive decline. In mouse models, overexpressing an active form of the UPR transcription factor XBP1 restored synaptic and cognitive function, in addition to reducing cell senescence. Proteomic profiling of hippocampal tissue showed that XBP1 expression significantly restore changes associated with aging, including factors involved in synaptic function and pathways linked to neurodegenerative diseases. The genes modified by XBP1 in the aged hippocampus where also altered. Collectively, our results demonstrate that strategies to manipulate the UPR in mammals may help sustain healthy brain aging.

Enteroviruses Manipulate the Unfolded Protein Response through Multifaceted Deregulation of the Ire1-Xbp1 Pathway

Many viruses are known to trigger endoplasmic reticulum (ER) stress in host cells, which in turn can develop a protective unfolded protein response (UPR). Depending on the conditions, the UPR may lead to either cell survival or programmed cell death. One of three UPR branches involves the upregulation of Xbp1 transcription factor caused by the unconventional cytoplasmic splicing of its mRNA. This process is accomplished by the phosphorylated form of the endoribonuclease/protein kinase Ire1/ERN1. Here, we show that the phosphorylation of Ire1 is up-regulated in HeLa cells early in enterovirus infection but down-regulated at later stages. We also find that Ire1 is cleaved in poliovirus- and coxsackievirus-infected HeLa cells 4-6 h after infection. We further show that the Ire1-mediated Xbp1 mRNA splicing is repressed in infected cells in a time-dependent manner. Thus, our results demonstrate the ability of enteroviruses to actively modulate the Ire1-Xbp1 host defensive pathway by inducing phosphorylation and proteolytic cleavage of the ER stress sensor Ire1, as well as down-regulating its splicing activity. Inactivation of Ire1 could be a novel mode of the UPR manipulation employed by viruses to modify the ER stress response in the infected cells.

Circular RNA circ_0079593 facilitates glioma development via modulating miR-324-5p/XBP1 axis

Glioma is a common brain tumor with high mortality. Circular RNAs (circRNAs) play crucial roles in tumor occurrence and development. However, the function and molecular basis of circ_0079593 in glioma remain unknown. Quantitative real-time PCR (qPCR) and Western blot were used for expression determination of circ_0079593, microRNA-324-5p (miR-324-5p) and X-box binding protein 1 (XBP1). Cell Counting Kit-8 (CCK-8), colony formation, flow cytometry, transwell assays, and tube formation assay were employed to evaluate cell functions. Glycolysis was determined via detecting glucose consumption, lactate production and ATP level. The binding relationship between miR-324-5p and circ_0079593 or XBP1 was validated by dual-luciferase reporter assay and RNA Immunoprecipitation (RIP) assay. Besides, xenograft assay was applied to test tumor growth in vivo. Circ_0079593 and XBP1 levels were elevated, while miR-324-5p level was declined in glioma. Silencing of circ_0079593 restrained proliferation, mobility, angiogenesis and glycolysis and induced apoptosis in glioma cells. Circ_0079593 accelerated glioma progression via sequestering miR-324-5p, one of the targets of circ_0079593. XBP1 was a target gene of miR-324-5p, and miR-324-5p alleviated the malignant growth of glioma by repressing XBP1. Furthermore, silence of circ_0079593 hindered tumor growth in vivo. Circ_0079593 contributed to the malignant evolution of glioma via modulating miR-324-5p and downstream XBP1 gene, suggesting that circ_0079593 might be a promising therapeutic target for glioma. Circ_0079593 was boosted in glioma. Circ_0079593 depletion restrained glioma progression. Circ_0079593 triggered glioma development via miR-324-5p/XBP1 axis. Circ_0079593 silence suppressed glioma tumorigenesis in vivo.

XBP1 variant 1 promotes mitosis of cancer cells involving upregulation of the polyglutamylase TTLL6

XBP1 variant 1 (Xv1) is the most abundant XBP1 variant and is highly enriched across cancer types but nearly none in normal tissues. Its expression is associated with poor patients' survival and is specifically required for survival of malignant cells, but the underlying mechanism is not known. Here we report that Xv1 upregulates the polyglutamylase tubulin tyrosine ligase-like 6 (TTLL6) and promotes mitosis of cancer cells. Like the canonical XBP1, Xv1 mRNA undergoes unconventional splicing by IRE1α under endoplasmic reticulum stress, but it is also constitutively spliced by IRE1β. The spliced Xv1 mRNA encodes the active form of Xv1 protein (Xv1s). RNA sequencing in HeLa cells revealed that Xv1s overexpression regulates expression of genes that are not involved in the canonical unfolded protein response, including TTLL6 as a highly upregulated gene. Gel shift assay and chromatin immunoprecipitation revealed that Xv1s bind to the TTLL6 promoter region. Knockdown of TTLL6 caused death of cancer cells but not benign and normal cells, similar to the effects of knocking down Xv1. Moreover, overexpression of TTLL6 partially rescued BT474 cells from apoptosis induced by either TTLL6 or Xv1 knockdown, supporting TTLL6 as an essential downstream effector of Xv1 in regulating cancer cell survival. TTLL6 is localized in the mitotic spindle of cancer cells. Xv1 or TTLL6 knockdown resulted in decreased spindle polyglutamylation and interpolar spindle, as well as congression failure, mitotic arrest and cell death. These findings suggest that Xv1 is essential for cancer cell mitosis, which is mediated, at least in part, by increasing TTLL6 expression.

Newly synthesized mRNA escapes translational repression during the acute phase of the mammalian unfolded protein response

Endoplasmic Reticulum (ER) stress, caused by the accumulation of misfolded proteins in the ER, elicits a homeostatic mechanism known as the Unfolded Protein Response (UPR). The UPR reprograms gene expression to promote adaptation to chronic ER stress. The UPR comprises an acute phase involving inhibition of bulk protein synthesis and a chronic phase of transcriptional induction coupled with the partial recovery of protein synthesis. However, the role of transcriptional regulation in the acute phase of the UPR is not well understood. Here we analyzed the fate of newly synthesized mRNA encoding the protective and homeostatic transcription factor X-box binding protein 1 (XBP1) during this acute phase. We have previously shown that global translational repression induced by the acute UPR was characterized by decreased translation and increased stability of XBP1 mRNA. We demonstrate here that this stabilization is independent of new transcription. In contrast, we show XBP1 mRNA newly synthesized during the acute phase accumulates with long poly(A) tails and escapes translational repression. Inhibition of newly synthesized RNA polyadenylation during the acute phase decreased cell survival with no effect in unstressed cells. Furthermore, during the chronic phase of the UPR, levels of XBP1 mRNA with long poly(A) tails decreased in a manner consistent with co-translational deadenylation. Finally, additional pro-survival, transcriptionally-induced mRNAs show similar regulation, supporting the broad significance of the pre-steady state UPR in translational control during ER stress. We conclude that the biphasic regulation of poly(A) tail length during the UPR represents a previously unrecognized pro-survival mechanism of mammalian gene regulation.

Unfolded Protein Response Differentially Modulates the Platelet Phenotype

BACKGROUND

Unfolded protein response (UPR) is a multifaceted signaling cascade that alleviates protein misfolding. Although well studied in nucleated cells, UPR in absence of transcriptional regulation has not been described. Intricately associated with cardiovascular diseases, platelets, despite being anucleate, respond rapidly to stressors in blood. We investigate the UPR in anucleate platelets and explore its role, if any, on platelet physiology and function.

METHODS

Human and mouse platelets were studied using a combination of ex vivo and in vivo experiments. Platelet lineage-specific knockout mice were generated independently for each of the 3 UPR pathways, PERK (protein kinase RNA [PKR]-like endoplasmic reticulum kinase), XBP1 (X-binding protein), and ATF6 (activating transcription factor 6). Diabetes patients were prospectively recruited, and platelets were evaluated for activation of UPR under chronic pathophysiological disease conditions.

RESULTS

Tunicamycin induced the IRE1α (inositol-requiring enzyme-1alpha)-XBP1 pathway in human and mouse platelets, while oxidative stress predominantly activated the PERK pathway. PERK deletion significantly increased platelet aggregation and apoptosis and phosphorylation of PLCγ2, PLCβ3, and p38 MAPK. Deficiency of XBP1 increased platelet aggregation, with higher PLCβ3 and PKCδ activation. ATF6 deletion mediated a relatively modest effect on platelet phenotype with increased PKA (protein kinase A). Platelets from diabetes patients exhibited a positive correlation between disease severity, platelet activation, and protein aggregation, with only IRE1α-XBP1 activation. Moreover, IRE1α inhibition increased platelet aggregation, while clinically approved chemical chaperone, sodium 4-phenylbutyrate reduced the platelet hyperactivation.

CONCLUSIONS

We show for the first time, that UPR activation occurs in platelets and can be independent of genomic regulation, with selective induction being specific to the source and severity of stress. Each UPR pathway plays a key role and can differentially modulate the platelet activation pathways and phenotype. Targeting the specific arms of UPR may provide a new antiplatelet strategy to mitigate thrombotic risk in diabetes and other cardiovascular diseases.

Role of XBP1 in regulating the progression of non-alcoholic steatohepatitis

BACKGROUND & AIMS

Non-alcoholic steatohepatitis (NASH) is associated with the dysregulation of lipid metabolism and hepatic inflammation, though the underlying mechanisms remain unclear. We aimed to investigate the role of X-box binding protein-1 (XBP1) in the progression of NASH.

METHODS

Human liver tissues obtained from patients with NASH and controls were used to assess XBP1 expression. NASH models were developed in hepatocyte-specific Xbp1 knockout (Xbp1ΔHep), macrophage-specific Xbp1 knockout (Xbp1ΔMf), macrophage-specific Nlrp3 knockout, and wild-type (Xbp1FL/FL or Nlrp3FL/FL) mice fed with a high-fat diet for 26 weeks or a methionine/choline-deficient diet for 6 weeks.

RESULTS

The expression of XBP1 was significantly upregulated in liver samples from patients with NASH. Hepatocyte-specific Xbp1 deficiency inhibited the development of steatohepatitis in mice fed the high-fat or methionine/choline-deficient diets. Meanwhile, macrophage-specific Xbp1 knockout mice developed less severe steatohepatitis and fibrosis than wild-type Xbp1FL/FL mice in response to the high-fat or methionine/choline-deficient diets. Macrophage-specific Xbp1 knockout mice showed M2 anti-inflammatory polarization. Xbp1-deleted macrophages reduced steatohepatitis by decreasing the expression of NLRP3 and secretion of pro-inflammatory cytokines, which mediate M2 macrophage polarization in macrophage-specific Xbp1 knockout mice. Steatohepatitis was less severe in macrophage-specific Nlrp3 knockout mice than in wild-type Nlrp3FL/FL mice. Xbp1-deleted macrophages prevented hepatic stellate cell activation by decreasing expression of TGF-β1. Less fibrotic changes were observed in macrophage-specific Xbp1 knockout mice than in wild-type Xbp1FL/FL mice. Inhibition of XBP1 suppressed the development of NASH.

CONCLUSION

XBP1 regulates the development of NASH. XBP1 inhibitors protect against steatohepatitis. Thus, XBP1 is a potential target for the treatment of NASH.

LAY SUMMARY

XBP1 is a transcription factor that is upregulated in liver tissues of patients with non-alcoholic steatohepatitis (NASH). Conditional knockout of Xbp1 in hepatocytes resulted in decreased lipid accumulation in mice, while genetic deletion of Xbp1 in macrophages ameliorated nutritional steatohepatitis and fibrosis in mice. Pharmacological inhibition of XBP1 protects against steatohepatitis and fibrosis, highlighting a promising therapeutic strategy for NASH.

Type I but Not Type II Calreticulin Mutations Activate the IRE1α/XBP1 Pathway of the Unfolded Protein Response to Drive Myeloproliferative Neoplasms

Approximately 20% of patients with myeloproliferative neoplasms (MPN) harbor mutations in the gene calreticulin (CALR), with 80% of those mutations classified as either type I or type II. While type II CALR-mutant proteins retain many of the Ca2+ binding sites present in the wild-type protein, type I CALR-mutant proteins lose these residues. The functional consequences of this differential loss of Ca2+ binding sites remain unexplored. Here, we show that the loss of Ca2+ binding residues in the type I mutant CALR protein directly impairs its Ca2+ binding ability, which in turn leads to depleted endoplasmic reticulum (ER) Ca2+ and subsequent activation of the IRE1α/XBP1 pathway of the unfolded protein response. Genetic or pharmacologic inhibition of IRE1α/XBP1 signaling induces cell death in type I mutant but not type II mutant or wild-type CALR-expressing cells, and abrogates type I mutant CALR-driven MPN disease progression in vivo.

SIGNIFICANCE

Current targeted therapies for CALR-mutated MPNs are not curative and fail to differentiate between type I- versus type II-driven disease. To improve treatment strategies, it is critical to identify CALR mutation type-specific vulnerabilities. Here we show that IRE1α/XBP1 represents a unique, targetable dependency specific to type I CALR-mutated MPNs. This article is highlighted in the In This Issue feature, p. 265.

XBP1 maintains beta cell identity, represses beta-to-alpha cell transdifferentiation and protects against diabetic beta cell failure during metabolic stress in mice

AIMS/HYPOTHESIS

Pancreatic beta cell dedifferentiation, transdifferentiation into other islet cells and apoptosis have been implicated in beta cell failure in type 2 diabetes, although the mechanisms are poorly defined. The endoplasmic reticulum stress response factor X-box binding protein 1 (XBP1) is a major regulator of the unfolded protein response. XBP1 expression is reduced in islets of people with type 2 diabetes, but its role in adult differentiated beta cells is unclear. Here, we assessed the effects of Xbp1 deletion in adult beta cells and tested whether XBP1-mediated unfolded protein response makes a necessary contribution to beta cell compensation in insulin resistance states.

METHODS

Mice with inducible beta cell-specific Xbp1 deletion were studied under normal (chow diet) or metabolic stress (high-fat diet or obesity) conditions. Glucose tolerance, insulin secretion, islet gene expression, alpha cell mass, beta cell mass and apoptosis were assessed. Lineage tracing was used to determine beta cell fate.

RESULTS

Deletion of Xbp1 in adult mouse beta cells led to beta cell dedifferentiation, beta-to-alpha cell transdifferentiation and increased alpha cell mass. Cell lineage-specific analyses revealed that Xbp1 deletion deactivated beta cell identity genes (insulin, Pdx1, Nkx6.1, Beta2, Foxo1) and derepressed beta cell dedifferentiation (Aldh1a3) and alpha cell (glucagon, Arx, Irx2) genes. Xbp1 deletion in beta cells of obese ob/ob or high-fat diet-fed mice triggered diabetes and worsened glucose intolerance by disrupting insulin secretory capacity. Furthermore, Xbp1 deletion increased beta cell apoptosis under metabolic stress conditions by attenuating the antioxidant response.

CONCLUSIONS/INTERPRETATION

These findings indicate that XBP1 maintains beta cell identity, represses beta-to-alpha cell transdifferentiation and is required for beta cell compensation and prevention of diabetes in insulin resistance states.

miR-148a targets XBP1 to regulate trophoblast apoptosis induced by plasma reticulum stress in preeclampsia

To study the relationship between miR-148a and preeclampsia (PE), and clarify that miR-148a can regulate the endoplasmic reticulum stress (ERS) of placental trophoblasts by targeting the ERS protein X box binding protein 1 (XBP1).Fifty patients with hypertension during pregnancy, patients with mild PE, patients with severe PE, and normal pregnant women were selected, and their placental tissues were collected. RT-polymerase chain reaction was used to detect the expression of miR-148a in placental tissues, and Western blot was used to detect XBP1 in placental tissues. Compare the expression differences of miR-148a and XBP1 in each group, and analyze the correlation between the expressions of the two.Compared with the Neg-miR group, MTT experiment result in pre-miR-148a group was decreased. MTT experiment result in anti-miR-148a group was increased. Cell cycle test result in pre-miR-148a group [G1 (%)] was increased. Cell cycle test result in anti-miR-148a group [S (%)] was increased. Apoptosis test result in pre-miR-148a group [early apoptotic cells (%), late apoptotic cells (%)] was increased. Apoptosis test result in anti-miR-148a group [early apoptotic cells (%), late apoptotic cells (%)] was decreased. XBP1 expression result in pre-miR-148a group was increased. XBP1 expression result in anti-miR-148a group was decreased. Compared with the normal population, XBP1 is expressed in hypertension, mild eclampsia, severe eclampsia increased. GRP78, CHOP, and caspase-12 expression result in pre-miR-148a group was increased. GRP78, CHOP, and caspase-12 expression result in anti-miR-148a group was decreased.miR-148a can regulate the ERS-mediated apoptosis by targeting XBP1, thereby intervening in the occurrence and development of PE.

FOXK2 promotes ovarian cancer stemness by regulating the unfolded protein response pathway

Understanding the regulatory programs enabling cancer stem cells (CSCs) to self-renew and drive tumorigenicity could identify new treatments. Through comparative chromatin-state and gene expression analyses in ovarian CSCs versus non-CSCs, we identified FOXK2 as a highly expressed stemness-specific transcription factor in ovarian cancer. Its genetic depletion diminished stemness features and reduced tumor initiation capacity. Our mechanistic studies highlight that FOXK2 directly regulated IRE1α (encoded by ERN1) expression, a key sensor for the unfolded protein response (UPR). Chromatin immunoprecipitation and sequencing revealed that FOXK2 bound to an intronic regulatory element of ERN1. Blocking FOXK2 from binding to this enhancer by using a catalytically inactive CRISPR/Cas9 (dCas9) diminished IRE1α transcription. At the molecular level, FOXK2-driven upregulation of IRE1α led to alternative XBP1 splicing and activation of stemness pathways, while genetic or pharmacological blockade of this sensor of the UPR inhibited ovarian CSCs. Collectively, these data establish what we believe is a new function for FOXK2 as a key transcriptional regulator of CSCs and a mediator of the UPR, providing insight into potentially targetable new pathways in CSCs.

Genetic and pharmacological inhibition of XBP1 protects against APAP hepatotoxicity through the activation of autophagy

Acetaminophen (APAP) hepatotoxicity induces endoplasmic reticulum (ER) stress which triggers the unfolded protein response (UPR) in hepatocytes. However, the mechanisms underlying ER stress remain poorly understood, thus reducing the options for exploring new pharmacological therapies for patients with hyperacute liver injury. Eight-to-twelve-week-old C57BL/6J Xbp1-floxed (Xbp1f/f) and hepatocyte-specific knockout Xbp1 mice (Xbp1∆hepa) were challenged with either high dose APAP [500 mg/kg] and sacrificed at early (1-2 h) and late (24 h) stages of hepatotoxicity. Histopathological examination of livers, immunofluorescence and immunohistochemistry, Western blot, real time (RT)-qPCR studies and transmission electron microscopy (TEM) were performed. Pharmacological inhibition of XBP1 using pre-treatment with STF-083010 [STF, 75 mg/kg] and autophagy induction with Rapamycin [RAPA, 8 mg/kg] or blockade with Chloroquine [CQ, 60 mg/kg] was also undertaken in vivo. Cytoplasmic expression of XBP1 coincided with severity of human and murine hyperacute liver injury. Transcriptional and translational activation of the UPR and sustained activation of JNK1/2 were major events in APAP hepatotoxicity, both in a human hepatocytic cell line and in a preclinical model. Xbp1∆hepa livers showed decreased UPR and JNK1/2 activation but enhanced autophagy in response to high dose APAP. Additionally, blockade of XBP1 splicing by STF, mitigated APAP-induced liver injury and without non-specific off-target effects (e.g., CYP2E1 activity). Furthermore, enhanced autophagy might be responsible for modulating CYP2E1 activity in Xbp1∆hepa animals. Genetic and pharmacological inhibition of Xbp1 specifically in hepatocytes ameliorated APAP-induced liver injury by enhancing autophagy and decreasing CYP2E1 expression. These findings provide the basis for the therapeutic restoration of ER stress and/or induction of autophagy in patients with hyperacute liver injury.

Pharmacologic IRE1/XBP1s activation promotes systemic adaptive remodeling in obesity

In obesity, signaling through the IRE1 arm of the unfolded protein response exerts both protective and harmful effects. Overexpression of the IRE1-regulated transcription factor XBP1s in liver or fat protects against obesity-linked metabolic deterioration. However, hyperactivation of IRE1 engages regulated IRE1-dependent decay (RIDD) and TRAF2/JNK pro-inflammatory signaling, which accelerate metabolic dysfunction. These pathologic IRE1-regulated processes have hindered efforts to pharmacologically harness the protective benefits of IRE1/XBP1s signaling in obesity-linked conditions. Here, we report the effects of a XBP1s-selective pharmacological IRE1 activator, IXA4, in diet-induced obese (DIO) mice. IXA4 transiently activates protective IRE1/XBP1s signaling in liver without inducing RIDD or TRAF2/JNK signaling. IXA4 treatment improves systemic glucose metabolism and liver insulin action through IRE1-dependent remodeling of the hepatic transcriptome that reduces glucose production and steatosis. IXA4-stimulated IRE1 activation also enhances pancreatic function. Our findings indicate that systemic, transient activation of IRE1/XBP1s signaling engenders multi-tissue benefits that integrate to mitigate obesity-driven metabolic dysfunction.

Xbp1 and Brachyury establish an evolutionarily conserved subcircuit of the notochord gene regulatory network

Gene regulatory networks coordinate the formation of organs and structures that compose the evolving body plans of different organisms. We are using a simple chordate model, the Ciona embryo, to investigate the essential gene regulatory network that orchestrates morphogenesis of the notochord, a structure necessary for the proper development of all chordate embryos. Although numerous transcription factors expressed in the notochord have been identified in different chordates, several of them remain to be positioned within a regulatory framework. Here, we focus on Xbp1, a transcription factor expressed during notochord formation in Ciona and other chordates. Through the identification of Xbp1-downstream notochord genes in Ciona, we found evidence of the early co-option of genes involved in the unfolded protein response to the notochord developmental program. We report the regulatory interplay between Xbp1 and Brachyury, and by extending these results to Xenopus, we show that Brachyury and Xbp1 form a cross-regulatory subcircuit of the notochord gene regulatory network that has been consolidated during chordate evolution.

Epithelial X-Box Binding Protein 1 Coordinates Tumor Protein p53-Driven DNA Damage Responses and Suppression of Intestinal Carcinogenesis

BACKGROUND & AIMS

Throughout life, the intestinal epithelium undergoes constant self-renewal from intestinal stem cells. Together with genotoxic stressors and failing DNA repair, this self-renewal causes susceptibility toward malignant transformation. X-box binding protein 1 (XBP1) is a stress sensor involved in the unfolded protein response (UPR). We hypothesized that XBP1 acts as a signaling hub to regulate epithelial DNA damage responses.

METHODS

Data from The Cancer Genome Atlas were analyzed for association of XBP1 with colorectal cancer (CRC) survival and molecular interactions between XBP1 and p53 pathway activity. The role of XBP1 in orchestrating p53-driven DNA damage response was tested in vitro in mouse models of chronic intestinal epithelial cell (IEC) DNA damage (Xbp1/H2bfl/fl, Xbp1ΔIEC, H2bΔIEC, H2b/Xbp1ΔIEC) and via orthotopic tumor organoid transplantation. Transcriptome analysis of intestinal organoids was performed to identify molecular targets of Xbp1-mediated DNA damage response.

RESULTS

In The Cancer Genome Atlas data set of CRC, low XBP1 expression was significantly associated with poor overall survival and reduced p53 pathway activity. In vivo, H2b/Xbp1ΔIEC mice developed spontaneous intestinal carcinomas. Orthotopic tumor organoid transplantation revealed a metastatic potential of H2b/Xbp1ΔIEC-derived tumors. RNA sequencing of intestinal organoids (H2b/Xbp1fl/fl, H2bΔIEC, H2b/Xbp1ΔIEC, and H2b/p53ΔIEC) identified a transcriptional program downstream of p53, in which XBP1 directs DNA-damage-inducible transcript 4-like (Ddit4l) expression. DDIT4L inhibits mechanistic target of rapamycin-mediated phosphorylation of 4E-binding protein 1. Pharmacologic mechanistic target of rapamycin inhibition suppressed epithelial hyperproliferation via 4E-binding protein 1.

CONCLUSIONS

Our data suggest a crucial role for XBP1 in coordinating epithelial DNA damage responses and stem cell function via a p53-DDIT4L-dependent feedback mechanism.

Analysis of XBP1 Contribution to Hyperosmolarity-Induced Lipid Synthesis

The unfolded protein response (UPR) is a complex network of intracellular pathways that transmits signals from ER lumen and/or ER bilayer to the nuclear compartment in order to activate gene transcription. UPR is activated by the loss of ER capacities, known as ER stress, and occurs to restore ER properties. In this regard, glycerolipid (GL) synthesis activation contributes to ER membrane homeostasis and IRE1α-XBP1, one UPR pathway, has a main role in lipogenic genes transcription. Herein, we describe the strategy and methodology used to evaluate whether IRE1α-XBP1 pathway regulates lipid metabolism in renal epithelial cells subjected to hyperosmolar environment. XBP1s activity was hindered by blocking IRE1α RNAse activity and by impeding its expression; under these conditions, we determined GL synthesis and lipogenic enzymes expression.

In Vitro Stimulation of IRE1α/XBP1-Deficient B Cells with LPS

During immune responses, pathogen-specific B cells differentiate into plasma cells. Plasma cells synthesize and secrete large amounts of immunoglobulin (Ig) molecules which play a central role in immunity against pathogens. The synthesis, proper folding, and secretion of these Ig molecules require expansion of the extensive endoplasmic reticulum (ER) network. Accumulation of unfolded or misfolded proteins in the ER is sensed by three sensors: IRE1/XBP1, PERK, and ATF6, which coordinate with each other and initiate the unfolded protein response (UPR) pathway to expand the ER network and its protein folding and secretion capability. The expansion and maintenance of the ER network in plasma cells is triggered by activation of the IRE1/XBP1 branch of the UPR pathway. Here, we discuss the methods to stimulate the differentiation of B cells into plasma cells, measure the activation of the XBP1 pathway, and quantify the ER network.

Effect of Denervation on XBP1 in Skeletal Muscle and the Neuromuscular Junction

Denervation of skeletal muscle is a debilitating consequence of injury of the peripheral nervous system, causing skeletal muscle to experience robust atrophy. However, the molecular mechanisms controlling the wasting of skeletal muscle due to denervation are not well understood. Here, we demonstrate that transection of the sciatic nerve in Sprague–Dawley rats induced robust skeletal muscle atrophy, with little effect on the neuromuscular junction (NMJ). Moreover, the following study indicates that all three arms of the unfolded protein response (UPR) are activated in denervated skeletal muscle. Specifically, ATF4 and ATF6 are elevated in the cytoplasm of skeletal muscle, while XBP1 is elevated in the nuclei of skeletal muscle. Moreover, XBP1 is expressed in the nuclei surrounding the NMJ. Altogether, these results endorse a potential role of the UPR and, specifically, XBP1 in the maintenance of both skeletal muscle and the NMJ following sciatic nerve transection. Further investigations into a potential therapeutic role concerning these mechanisms are needed.

The Endoplasmic Reticulum Stress Sensor IRE1α Regulates the UV DNA Repair Response through the Control of Intracellular Calcium Homeostasis

The unfolded protein response is activated by UVB irradiation, but the role of a key mediator, IRE1α, is not clear. In this study, we show that mice with an epidermal IRE1α deletion are sensitized to UV with increased apoptosis, rapid loss of UV-induced cyclopyrimidine dimer‒positive keratinocytes, and sloughing of the epidermis. In vitro, Ire1α-deficient keratinocytes have increased UVB sensitivity, reduced cyclopyrimidine dimer repair, and reduced accumulation of γH2AX and phosphorylated ATR, suggesting defective activation of nucleotide excision repair. Knockdown of XBP1 or pharmacologic inhibition of the IRE1α ribonuclease did not phenocopy Ire1α deficiency. The altered UV response was linked to elevated intracellular calcium levels and ROS, and this was due to dysregulation of the endoplasmic reticulum calcium channel InsP3R. Pharmacologic, genetic, and biochemical studies linked the regulation of the Ins3PR, intracellular calcium, and normal UV DNA damage response to CIB1 and the IRE1α‒TRAF2‒ASK1 complex. These results suggest a model where IRE1α activation state drives CIB1 binding either to the InsP3R or ASK1 to regulate endoplasmic reticulum calcium efflux, ROS, and DNA repair responses after UV irradiation.

Proteomic Analysis of Marinesco-Sjogren Syndrome Fibroblasts Indicates Pro-Survival Metabolic Adaptation to SIL1 Loss

Marinesco-Sjogren syndrome (MSS) is a rare multisystem pediatric disorder, caused by loss-of-function mutations in the gene encoding the endoplasmic reticulum cochaperone SIL1. SIL1 acts as a nucleotide exchange factor for BiP, which plays a central role in secretory protein folding. SIL1 mutant cells have reduced BiP-assisted protein folding, cannot fulfil their protein needs, and experience chronic activation of the unfolded protein response (UPR). Maladaptive UPR may explain the cerebellar and skeletal muscle degeneration responsible for the ataxia and muscle weakness typical of MSS. However, the cause of other more variable, clinical manifestations, such as mild to severe mental retardation, hypogonadism, short stature, and skeletal deformities, is less clear. To gain insights into the pathogenic mechanisms and/or adaptive responses to SIL1 loss, we carried out cell biological and proteomic investigations in skin fibroblasts derived from a young patient carrying the SIL1 R111X mutation. Despite fibroblasts not being overtly affected in MSS, we found morphological and biochemical changes indicative of UPR activation and altered cell metabolism. All the cell machineries involved in RNA splicing and translation were strongly downregulated, while protein degradation via lysosome-based structures was boosted, consistent with an attempt of the cell to reduce the workload of the endoplasmic reticulum and dispose of misfolded proteins. Cell metabolism was extensively affected as we observed a reduction in lipid synthesis, an increase in beta oxidation, and an enhancement of the tricarboxylic acid cycle, with upregulation of eight of its enzymes. Finally, the catabolic pathways of various amino acids, including valine, leucine, isoleucine, tryptophan, lysine, aspartate, and phenylalanine, were enhanced, while the biosynthetic pathways of arginine, serine, glycine, and cysteine were reduced. These results indicate that, in addition to UPR activation and increased protein degradation, MSS fibroblasts have profound metabolic alterations, which may help them cope with the absence of SIL1.

Xbp1s-Ddit3 promotes MCT-induced pulmonary hypertension

Pulmonary hypertension (PH) is a life-threatening disease characterized by vascular remodeling. Exploring new therapy target is urgent. The purpose of the present study is to investigate whether and how spliced x-box binding protein 1 (xbp1s), a key component of endoplasmic reticulum stress (ERS), contributes to the pathogenesis of PH. Forty male SD rats were randomly assigned to four groups: Control, Monocrotaline (MCT), MCT+AAV-CTL (control), and MCT+AAV-xbp1s. The xbp1s protein levels were found to be elevated in lung tissues of the MCT group. Intratracheal injection of adeno-associated virus serotype 1 carrying xbp1s shRNA (AAV-xbp1s) to knock down the expression of xbp1s effectively ameliorated the MCT-induced elevation of right ventricular systolic pressure (RVSP), total pulmonary resistance (TPR), right ventricular hypertrophy and medial wall thickness of muscularized distal pulmonary arterioles. The abnormally increased positive staining rates of proliferating cell nuclear antigen (PCNA) and Ki67 and decreased positive staining rates of terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) in pulmonary arterioles were also reversed in the MCT+AAV-xbp1s group. For mechanistic exploration, bioinformatics prediction of the protein network was performed on the STRING database, and further verification was performed by qRT-PCR, Western blots and co-immunoprecipitation (Co-IP). DNA damage-inducible transcript 3 (Ddit3) was identified as a downstream protein that interacted with xbp1s. Overexpression of Ddit3 restored the decreased proliferation, migration and cell viability caused by silencing of xbp1s. The protein level of Ddit3 was also highly consistent with xbp1s in the animal model. Taken together, our study demonstrated that xbp1s-Ddit3 may be a potential target to interfere with vascular remodeling in PH.