Both thapsigargin- and tunicamycin-induced endoplasmic reticulum stress increases expression of Hrd1 in IRE1-dependent fashion

E3 ubiquitin ligase SYVN1 as a promising therapeutic target for diverse human diseases

Numerous studies conducted in recent years indicate that mammalian E3 ubiquitin ligases serve as key regulators in the maintenance of cellular homeostasis by targeting the ubiquitination of substrate proteins and activating downstream signaling pathways. SYVN1, an E3 ubiquitin ligase, is characterized by its significant functions in regulating various biological processes, including molecular mechanisms related to gene expression, signaling pathways, and cell death, among others. Consequently, SYVN1 plays a crucial role in both normal human physiology and the pathogenesis of various diseases, such as oncogenesis, cardiovascular disorders, immune regulation, skeletal anomalies, and neurological diseases. This review synthesizes recent findings regarding the physiological and pathophysiological roles of SYVN1, offering new insights into potential strategies for the prevention and treatment of human diseases, as well as suggesting avenues for future drug development. In this Review, we summarize the latest findings regarding the physiological and pathophysiological roles of SYVN1, elucidating the mechanisms by which SYVN1 can regulate the progression of various diseases in humans. These important findings provide new avenues for further investigation of SYVN1 protein, new insights into potential strategies to prevent and treat human diseases, and new directions for future drug development.

Acute exposure to polystyrene nanoplastics induces unfolded protein response and global protein ubiquitination in lungs of mice

Inhaling microplastics (MPs) and nanoplastics (NPs) in the air can damage lung function. Xenobiotics in the body can cause endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) activation alleviates ER stress. Degradation of unfolded or misfolded proteins is an important pathway for recovering cellular homeostasis. The UPR and protein degradation induced by MPs/NPs in lung tissues are not well understood. Here, we investigated the UPR and protein ubiquitination in the lungs of mice exposed to polystyrene (PS)-NPs and their possible molecular mechanisms leading to protein ubiquitination. Mice were intratracheally administered with 5.6, 17, and 51 mg/kg PS-NPs once for 24 h. Exposure to PS-NPs elevated protein ubiquitination in the lungs of mice in a dose-dependent manner. PS-NPs activated three branches of UPR including inositol-requiring protein 1α (IRE1α), eukaryotic translation initiator factor 2α (eIF2α), and activating transcription factor 6α (ATF6α) in the lungs of mice. However, activated IRE1α did not trigger X-box binding protein 1 (XBP1) mRNA splicing. Exposure to PS-NPs induced an increase in the levels of E3 ubiquitin ligase hydroxymethyl glutaryl-coenzyme A reductase degradation protein 1 (HRD1) and carboxy terminus of Hsc70 interacting protein (CHIP) in the lungs of mice and BEAS-2B cells. ATF6α siRNA inhibited the levels of HRD1 and CHIP proteins induced by PS-NPs in BEAS-2B cells. These results suggest that ATF6α plays a critical role in increasing ubiquitination of unfolded or misfolded proteins by alleviating PS-NPs induced ER stress through UPR to achieve ER homeostasis in the lungs of mice.

Human neural stem cell secretome relieves endoplasmic reticulum stress-induced apoptosis and improves neuronal functions after traumatic brain injury in a rat model

Neural stem cell secretome (NSC-S) plays an important role in neuroprotection and recovery. Studies have shown that endoplasmic reticulum stress (ER stress) is involved in the progression of traumatic brain injury (TBI) and is a crucial cause of secondary damage and neuronal death after brain injury. Whether NSC-S is engaged in ER stress and ER stress-mediated neuronal apoptosis post-TBI has not been investigated. In the study, the Feeney SD male rat model was established. The results showed that NSC-S treatment significantly improved the behavior of rats with TBI. In addition, NSC-S relieved ER stress in TBI rats and was observed by transmission electron microscopy and western blot. The specific mechanism was further elucidated that restoration was achieved by alleviating the PERK-eIF2α pathway and thus protecting neurons from apoptosis. Notably, the discovery of calumenin (CALU) in NSC-S by liquid chromatography-tandem mass spectrometry (LC-MS/MS/MS) may be related to the protective effect of NSC-S on ER stress in neurons. Also, the mechanism by which it functions may be related to ubiquitination. In summary, NSC-S improved prognosis and ER stress in TBI rats and might be a promising treatment for relieving TBI.

Involvement of Proteasomal and Endoplasmic Reticulum Stress in Neurodegeneration After Global Brain Ischemia

A brief period of transient global brain ischemia leads to selective ischemic neurodegeneration associated with death of hippocampal CA1 pyramidal neurons days after reperfusion. The mechanism of such selective and delayed neurodegeneration is still uncertain. Our work aimed to study the involvement of proteasomal and endoplasmic reticulum (ER) stress in ischemic neurodegeneration. We have performed laser scanning confocal microscopy analysis of brain slices from control and experimental animals that underwent global brain ischemia for 15 min and varying times of reperfusion. We have focused on ubiquitin, PUMA, a proapoptotic protein of the Bcl-2 family overexpressed in response to both proteasomal and ER stress, and p53, which controls expression of PUMA. We have also examined the expression of HRD1, an E3 ubiquitin ligase that was shown to be overexpressed after ER stress. We have also examined potential crosstalk between proteasomal and ER stress using cellular models of both proteasomal and ER stress. We demonstrate that global brain ischemia is associated with an appearance of distinct immunoreactivity of ubiquitin, PUMA and p53 in pyramidal neurons of the CA1 layer of the hippocampus 72 h after ischemic insults. Such changes correlate with a delay and selectivity of ischemic neurodegeneration. Immunoreactivity of HRD1 observed in all investigated regions of rat brain was transiently absent in both CA1 and CA3 pyramidal neurones 24 h after ischemia in the hippocampus, which does not correlate with a delay and selectivity of ischemic neurodegeneration. We do not document significant crosstalk between proteasomal and ER stress. Our results favour dysfunction of the ubiquitin proteasome system and consequent p53-induced expression of PUMA as the main mechanisms responsible for selective and delayed degeneration of pyramidal neurons of the hippocampal CA1 layer in response to global brain ischemia.

STF-083010 an inhibitor of IRE1α endonuclease activity affects mitochondrial respiration and generation of mitochondrial membrane potential

STF-083010 is an inhibitor of endonuclease activity of inositol requiring-enzyme 1α (IRE1α) that is involved in activation of IRE1α-XBP1 axis of the unfolded protein response after ER stress. STF-083010 was tested as a possible antitumor agent in some previous studies exhibiting the ability either to induce death of tumour cells or to increase sensitivity of tumours cells to other neoplastic agents. STF-083010 exhibits also hepatoprotective effects in different models of liver injury and hepatic steatohepatitis. We have shown that STF-083010 has significant impact on mitochondrial functions that is not dependent on the way of STF-083010 application. We have observed that STF-083010 decrease of both maximal respiration (representing maximal electron transfer capacity of mitochondrial respiratory chain) and spare respiratory capacity after either incubation of the SH-SY5Y cells with STF-083010 or direct addition of STF-083010 to the respiration medium. In addition, we have documented impact of STF-083010 on generation of mitochondrial membrane potential (ΔΨm) that could be a result of decreased mitochondrial substrate level phosphorylation. Finally, increased sensitivity of ΔΨm to uncoupler in the presence of STF-083010 was documented. Our results indicate that STF-083010 has important impact on mitochondrial functions independently of its ability to inhibit endonuclease activity of IRE1α that is involved in activation of IRE1α-XBP1 axis of the unfolded protein response after ER stress. The impact of STF-083010 on mitochondrial functions could be associated with its possible off-target effect.

The Unfolded Protein Response: A Double-Edged Sword for Brain Health

Efficient brain function requires as much as 20% of the total oxygen intake to support normal neuronal cell function. This level of oxygen usage, however, leads to the generation of free radicals, and thus can lead to oxidative stress and potentially to age-related cognitive decay and even neurodegenerative diseases. The regulation of this system requires a complex monitoring network to maintain proper oxygen homeostasis. Furthermore, the high content of mitochondria in the brain has elevated glucose demands, and thus requires a normal redox balance. Maintaining this is mediated by adaptive stress response pathways that permit cells to survive oxidative stress and to minimize cellular damage. These stress pathways rely on the proper function of the endoplasmic reticulum (ER) and the activation of the unfolded protein response (UPR), a cellular pathway responsible for normal ER function and cell survival. Interestingly, the UPR has two opposing signaling pathways, one that promotes cell survival and one that induces apoptosis. In this narrative review, we discuss the opposing roles of the UPR signaling pathways and how a better understanding of these stress pathways could potentially allow for the development of effective strategies to prevent age-related cognitive decay as well as treat neurodegenerative diseases.

Botulinum toxin type A activates protective autophagy by modulating endoplasmic reticulum stress in hypoxia/reoxygenation-treated endothelial cells

Botulinum toxin type A (BTXA) previously protected endothelial cells in free skin flaps from ischemia/reperfusion injury by inducing autophagy. Endoplasmic reticulum (ER) stress-autophagy activation may have a role in BTXA-antagonized ischemia/reperfusion damage in human dermal microvascular endothelial cells (HDMECs), however, this has yet to be proven. HDMECs were pretreated with BTXA at various concentrations for 12 h before being subjected to hypoxia and reoxygenation (H/R). Cell Count Kit 8 (CCK8) and Western blot (WB) data showed that H/R treatment significantly increased the expression of ER stress (GRP78, CHOP) and autophagy (LC3II/I, Beclin-1) proteins. The optimal BTXA pretreatment concentration was 1.6 U/mL, which resulted in the highest levels of cell survival and expression of ER stress and autophagy. Following pretreatment with 1.6 U/mL BTXA and the addition of the ER stress inducer Thapsigargin (Tg), the ER stress inhibitor 4-phenylbutyrate (4-PBA), and the inhibitor of autophagy Bafilomycin A1 (Baf A1), respectively, HDMECs were then subjected to H/R treatment. Cell survival and the percentage of ethynyldeoxyuridine-labeled cells in the BTXA pretreatment groups were reduced by the addition of Tg, 4-PBA, and Baf A1. While the expression of GRP78, CHOP, and LC3 was upregulated by Tg and Baf A1, it was downregulated by 4-PBA. The findings showed that ER stress produced by BTXA pretreatment triggers protective autophagy and protects HDMECs from H/R damage. There were no cytoprotective effects from either excessive activation or reduction of ER stress. Our results are consistent with the notion that autophagy and moderate ER stress are critical for cellular longevity and, consequently, functional integrity and may represent a potential therapeutic target.

The endoplasmic reticulum stress and unfolded protein response in Alzheimer's disease: a calcium dyshomeostasis perspective

Protein misfolding is prominent in early cellular pathology of Alzheimer's disease (AD), implicating pathophysiological significance of endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and highlighting it as a target for drug development. Experimental data from animal AD models and observations on human specimens are, however, inconsistent. ER stress and associated UPR are readily observed in in vitro AD cellular models and in some AD model animals. In the human brain, components and markers of ER stress as well as UPR transducers are observed at Braak stages III-VI associated with severe neuropathology and neuronal death. The picture, however, is further complicated by the brain region- and cell type-specificity of the AD-related pathology. Terms 'disturbed' or 'non-canonical' ER stress/UPR were used to describe the discrepancies between experimental data and the classic ER stress/UPR cascade. Here we discuss possible 'disturbing' or 'interfering' factors which may modify ER stress/UPR in the early AD pathogenesis. We focus on the dysregulation of the ER Ca²⁺ homeostasis, store-operated Ca²⁺ entry, and the interaction between the ER and mitochondria. We suggest that a detailed study of the CNS cell type-specific alterations of Ca²⁺ homeostasis in early AD may deepen our understanding of AD-related dysproteostasis.

HRD1 in human malignant neoplasms: Molecular mechanisms and novel therapeutic strategy for cancer

In tumor cells, the endoplasmic reticulum (ER) plays an essential role in maintaining cellular proteostasis by stimulating unfolded protein response (UPR) underlying stress conditions. ER-associated degradation (ERAD) is a critical pathway of the UPR to protect cells from ER stress-induced apoptosis and the elimination of unfolded or misfolded proteins by the ubiquitin-proteasome system (UPS). 3-Hydroxy-3-methylglutaryl reductase degradation (HRD1) as an E3 ubiquitin ligase plays an essential role in the ubiquitination and dislocation of misfolded protein in ERAD. In addition, HRD1 can target other normal folded proteins. In various types of cancer, the expression of HRD1 is dysregulated, and it targets different molecules to develop cancer hallmarks or suppress the progression of the disease. Recent investigations have defined the role of HRD1 in drug resistance in types of cancer. This review focuses on the molecular mechanisms of HRD1 and its roles in cancer pathogenesis and discusses the worthiness of targeting HRD1 as a novel therapeutic strategy in cancer.

Role of sulforaphane in endoplasmic reticulum homeostasis through regulation of the antioxidant response

Nowadays, the nutraceutical agent sulforaphane (SFN) shows great versatility in turning on different cellular responses. Mainly, this isothiocyanate acts as a master regulator of cellular homeostasis due to its antioxidant response and cytoplasmic, mitochondrial, and endoplasmic reticulum (ER) protein modulation. Even more, SFN acts as an effective strategy to counteract oxidative stress, apoptosis, and ER stress, among others as seen in different injury models. Particularly, ER stress is buffered by the unfolded protein response (UPR) activation, which is the first instance in orchestrating the recovery of ER function. Interestingly, different studies highlight a close interrelationship between ER stress and oxidative stress, two events driven by the accumulation of reactive oxygen species (ROS). This response inevitably perpetuates itself and acts as a vicious cycle that triggers the development of different pathologies, such as cardiovascular diseases, neurodegenerative diseases, and others. Accordingly, it is vital to target ER stress and oxidative stress to increase the effectiveness of clinical therapies used to treat these diseases. Therefore, our study is focused on the role of SFN in preserving cellular homeostasis balance by regulating the ER stress response through the Nrf2-modulated antioxidant pathway.

Deep Learning-Based Morphological Classification of Endoplasmic Reticulum Under Stress

Endoplasmic reticulum stress (ER stress) is a condition that is defined by abnormal accumulation of unfolded proteins. It plays an important role in maintaining cellular protein, lipid, and ion homeostasis. By triggering the unfolded protein response (UPR) under ER stress, cells restore homeostasis or undergo apoptosis. Chronic ER stress is implicated in many human diseases. Despite extensive studies on related signaling mechanisms, reliable image biomarkers for ER stress remain lacking. To address this deficiency, we have validated a morphological image biomarker for ER stress and have developed a deep learning-based assay to enable automated detection and analysis of this marker for screening studies. Specifically, ER under stress exhibits abnormal morphological patterns that feature ring-shaped structures called whorls (WHs). Using a highly specific chemical probe for unfolded and aggregated proteins, we find that formation of ER whorls is specifically associated with the accumulation of the unfolded and aggregated proteins. This confirms that ER whorls can be used as an image biomarker for ER stress. To this end, we have developed ER-WHs-Analyzer, a deep learning-based image analysis assay that automatically recognizes and localizes ER whorls similarly as human experts. It does not require laborious manual annotation of ER whorls for training of deep learning models. Importantly, it reliably classifies different patterns of ER whorls induced by different ER stress drugs. Overall, our study provides mechanistic insights into morphological patterns of ER under stress as well as an image biomarker assay for screening studies to dissect related disease mechanisms and to accelerate related drug discoveries. It demonstrates the effectiveness of deep learning in recognizing and understanding complex morphological phenotypes of ER.

Ginsenoside Rb1 alleviates colitis in mice via activation of endoplasmic reticulum-resident E3 ubiquitin ligase Hrd1 signaling pathway

Endoplasmic reticulum (ER) homeostasis is regulated by ER-resident E3 ubiquitin ligase Hrd1, which has been implicated in inflammatory bowel disease (IBD). Ginsenoside Rb1 (GRb1) is the major ginsenoside in ginseng with multiple pharmacological activities. In this study we investigated the role of Hrd1 in IBD and its regulation by GRb1. Two mouse colitis models were established to mimic human IBD: drinking water containing dextran sodium sulfate (DSS) as well as intra-colonic infusion of 2, 4, 6-trinitrobenzene sulfonic acid (TNBS). Colitis mice were treated with GRb1 (20, 40 mg·kg^-1·d^-1, ig) or a positive control drug sulfasalazine (500 mg·kg^-1·d^-1, ig) for 7 days. The model mice showed typical colitis symptoms and pathological changes in colon tissue. In addition to significant inflammatory responses and cell apoptosis in colon tissue, colon epithelial expression of Hrd1 was significantly decreased, the expression of ER stress markers GRP78, PERK, CHOP, and caspase 12 was increased, and the expression of Fas was increased (Fas was removed by Hrd1-induced ubiquitination). These changes were partially, or completely, reversed by GRb1 administration, whereas injection of Hrd1 inhibitor LS102 (50 mg·kg^-1· d^-1, ip, for 6 days) exacerbated colitis symptoms in colitis mice. GRb1 administration not only normalized Hrd1 expression at both the mRNA and protein levels, but also alleviated the ER stress response, Fas-related apoptosis, and other colitis symptoms. In intestinal cell line IEC-6, the expression of Hrd1 was significantly decreased by LPS treatment, but was normalized by GRb1 (200 μM). GRb1 alleviated LPS-induced ER stress and cell apoptosis in IEC-6 cells, and GRb1 action was inhibited by knockdown of Hrd1 using small interfering RNA. In summary, these results reveal a pathological role of Hrd1 in colitis, and provide a novel insight into alternative treatment of colitis using GRb1 activating Hrd1 signaling pathway.

Selective AMPK activator leads to unfolded protein response downregulation and induces breast cancer cell death and autophagy

AIMS

AMPK plays a critical role regulating cell metabolism, growth and survival. Interfering with this enzyme activity has been extensively studied as putative mechanism for cancer therapy. The present work aims to identify a specific AMPK activator for cancer cells among a series of novel heterocyclic compounds.

MATERIALS AND METHODS

A series of novel hybrid heterocyclic compounds, namely naphtoquinone-4-oxoquinoline and isoquinoline-5,8-quinone-4-oxoquinoline derivatives, were synthesized via Michael reaction and their structures confirmed by spectral data: infrared; ¹H and ¹³C NMR spectroscopy (COSY, HSQC, HMBC); and high-resolution mass spectrometry (HRMS). The novel compounds were screened and tested for antitumoral activity and have part of their mechanism of action scrutinized.

KEY FINDINGS

Here, we identified a selective AMPK activator among the new hybrid heterocyclic compounds. This new compound presents selective cytotoxicity on breast cancer cells but not on non-cancer counterparts. We identified that by specifically activating AMPK in cancer cells, the drug downregulates unfolded protein response pathway, as well as inhibits mTOR signaling.

SIGNIFICANCE

These effects, that are selective for cancer cells, lead to activation of autophagy and, ultimately, to cancer cells death. Taken together, our data support the promising anticancer activity of this novel compound which is a strong modulator of metabolism.

Differential impact of imipramine on thapsigargin- and tunicamycin-induced endoplasmic reticulum stress and mitochondrial dysfunction in neuroblastoma SH-SY5Y cells

The aim of our work was to study effect of antidepressant imipramine on both thapsigargin- and tunicamycin-induced ER stress and mitochondrial dysfunction in neuroblastoma SH-SY5Y cells. ER stress in SH-SY5Y cells was induced by either tunicamycin or thapsigargin in the presence or absence of imipramine. Cell viability was tested by the MTT assay. Splicing of XBP1 mRNA was studied by RT-PCR. Finally, expression of Hrd1 and Hsp60 was determined by Western blot analysis. Our findings provide evidence that at high concentrations imipramine potentiates ER stress-induced death of SH-SY5Y cells. The effect of imipramine on ER stress-induced death of SH-SY5Y cells was stronger in combination of imipramine with thapsigargin. In addition, we have found that treatment of SH-SY5Y cells with imipramine in combination of either thapsigargin or tunicamycin is associated with the alteration of ER stress-induced IRE1α-XBP1 signalling. Despite potentiation of ER stress-induced XBP1 splicing, imipramine suppresses both thapsigargin- and tunicamycin-induced expression of Hrd1. Finally, imipramine in combination with thapsigargin, but not tunicamycin, aggravates ER stress-induced mitochondrial dysfunction without significant impact on intracellular mitochondrial content as indicated by the unaltered expression of Hsp60. Our results indicate the possibility that chronic treatment with imipramine might be associated with a higher risk of development and progression of neurodegenerative disorders, in particular those allied with ER stress and mitochondrial dysfunction like Parkinson's and Alzheimer's disease.

Calreticulin promotes EMT in pancreatic cancer via mediating Ca2+ dependent acute and chronic endoplasmic reticulum stress

Background

Our previous study showed that calreticulin (CRT) promoted EGF-induced epithelial-mesenchymal transition (EMT) in pancreatic cancer (PC) via Integrin/EGFR-ERK/MAPK signaling. We next investigated the novel signal pathway and molecular mechanism involving the oncogenic role of CRT in PC.

Methods

We investigated the potential role and mechanism of CRT in regulating intracellular free Ca²⁺ dependent acute and chronic endoplasmic reticulum stress (ERS)-induced EMT in PC in vitro and vivo.

Results

Thapsigargin (TG) induced acute ERS via increasing intracellular free Ca²⁺ in PC cells, which was reversed by CRT silencing. Additionally, CRT silencing inhibited TG-induced EMT in vitro by reversing TG-induced changes of the key proteins in EMT signaling (ZO-1, E-cadherin and Slug) and ERK/MAPK signaling (pERK). TG-promoted cell invasion and migration was also rescued by CRT silencing but enhanced by IRE1α silencing (one of the key stressors in unfolded protein response). Meanwhile, CRT was co-immunoprecipitated and co-localized with IRE1α in vitro and its silencing led to the chronic ERS via upregulating IRE1α independent of IRE1-XBP1 axis. Moreover, CRT silencing inhibited IRE1α silencing-promoted EMT, including inhibiting the activation of EMT and ERK/MAPK signaling and the promotion of cell mobility. In vivo, CRT silencing decreased subcutaneous tumor size and distant liver metastasis following with the increase of IRE1α expression. A negative relationship between CRT and IRE1α was also observed in clinical PC samples, which coordinately promoted the advanced clinical stages and poor prognosis of PC patients.

Conclusions

CRT promotes EMT in PC via mediating intracellular free Ca²⁺ dependent TG-induced acute ERS and IRE1α-mediated chronic ERS via Slug and ERK/MAPK signaling.