Activation of the UPR sensor ATF6α is regulated by its redox-dependent dimerization and ER retention by ERp18

Unraveling the functional and molecular interplay between cellular senescence and the unfolded protein response

Senescence is a complex cellular state that can be considered as a stress response phenotype. A decade ago, we suggested the intricate connections between unfolded protein response (UPR) signaling and the development of the senescent phenotype. Over the past ten years, significant advances have been made in understanding the multifaceted role of the UPR in regulating cellular senescence, highlighting its contribution to biological processes such as oxidative stress and autophagy. In this updated review, we expand these interconnections with the benefit of new insights, and we suggest that targeting specific components of the UPR could provide novel therapeutic strategies to mitigate the deleterious effects of senescence, with significant implications for age-related pathologies and geroscience.

Glucose sensing and the unfolded protein response

The unfolded protein response (UPR) is activated primarily upon alteration of protein folding in the endoplasmic reticulum (ER). This occurs under physiological situations that cause an abrupt increase in protein synthesis, or under redox and metabolic stresses. Among the latter, hyperglycemia and glucose scarcity have been identified as major modulators of UPR signaling. Indeed, the first mammalian UPR effector, the glucose-regulated protein 78, also known as BiP, was identified in response to glucose deprivation. Tunicamycin, arguably the most commonly used drug to induce ER stress responses in vitro and in vivo, is an inhibitor of N-glycosylation. We compile here evidence that the UPR is activated upon physiological and pathological conditions that alter glucose levels and that this is mostly mediated by alterations of protein N-glycosylation, ATP levels, or redox balance. The three branches of the UPR transduced by PERK/ATF4, IRE1/XBP1s, and ATF6, as well as non-canonical ER sensors such as SCAP/SREBP, sense ER protein glycosylation status driven by glucose and other glucose-derived metabolites. The outcomes of UPR activation range from restoring protein N-glycosylation and protein folding flux to stimulating autophagy, organelle recycling, and mitochondrial respiration, and in some cases, cell death. Anabolic responses to glucose levels are also stimulated by glucose through components of the UPR. Therefore, the UPR should be further studied as a potential biomarker and mediator of glucose-associated diseases.

Phenylhydrazone-based Endoplasmic Reticulum Proteostasis Regulator Compounds with Enhanced Biological Activity

Pharmacological enhancement of endoplasmic reticulum (ER) proteostasis is an attractive strategy to mitigate pathology linked to etiologically-diverse protein misfolding diseases. However, despite this promise, few compounds have been identified that enhance ER proteostasis through defined mechanisms of action. We previously identified the phenylhydrazone-based compound AA263 as a compound that promotes adaptive ER proteostasis remodeling through mechanisms including activation of the ATF6 signaling arm of the unfolded protein response (UPR). However, the protein target(s) of AA263 and the potential for further development of this class of ER proteostasis regulators had not been previously explored. Here, we employ chemical proteomics to demonstrate that AA263 covalently targets a subset of ER protein disulfide isomerases, revealing a molecular mechanism for the activation of ATF6 afforded by this compound. We then use medicinal chemistry to establish next-generation AA263 analogs showing improved potency and efficacy for ATF6 activation, as compared to the parent compound. Finally, we show that treatment with these AA263 analogs enhances secretory pathway proteostasis to correct the pathologic protein misfolding and trafficking of both a destabilized, disease-associated α1-antitrypsin (A1AT) variant and an epilepsy-associated GABA A receptor variant. These results establish AA263 analogs with enhanced potential for correcting imbalanced ER proteostasis associated with etiologically-diverse protein misfolding disorders.

IRE1 is a promising therapeutic target in pancreatic cancer

[Figure: see text].

Homeostasis control in health and disease by the unfolded protein response

Cells rely on the endoplasmic reticulum (ER) to fold and assemble newly synthesized transmembrane and secretory proteins - essential for cellular structure-function and for both intracellular and intercellular communication. To ensure the operative fidelity of the ER, eukaryotic cells leverage the unfolded protein response (UPR) - a stress-sensing and signalling network that maintains homeostasis by rebalancing the biosynthetic capacity of the ER according to need. The metazoan UPR can also redirect signalling from cytoprotective adaptation to programmed cell death if homeostasis restoration fails. As such, the UPR benefits multicellular organisms by preserving optimally functioning cells while removing damaged ones. Nevertheless, dysregulation of the UPR can be harmful. In this Review, we discuss the UPR and its regulatory processes as a paradigm in health and disease. We highlight important recent advances in molecular and mechanistic understanding of the UPR that enable greater precision in designing and developing innovative strategies to harness its potential for therapeutic gain. We underscore the rheostatic character of the UPR, its contextual nature and critical open questions for its further elucidation.

Thiol isomerase ERp18 enhances platelet activation and arterial thrombosis

Background

Thiol isomerases regulate the thiol-disulfide exchange of functional proteins in cells. Using genetically modified mouse models and inhibitors, we and others demonstrated that 7 thiol isomerases (ERp57, protein diisulfide isomerase, ERp72, ERp46, ERp5, TMX4, and TMX1) participate in thrombosis. There are 21 thiol isomerases in mammals, but whether other enzymes of this family also contribute to thrombosis remains unknown.

Objectives

Investigate whether and how ERp18 participates in arterial thrombosis.

Methods

ERp18 knockout mice and arterial thrombosis models were used to determine the role of ERp18 in thrombosis. Platelets from ERp18 knockout mice were used to detect aggregation, activation, spreading, and clot retraction. Finally, flow cytometry and immunoprecipitation were used to detect the binding between ERp18 and αIIbβ3.

Results

The mice lacking ERp18 exhibited a prolonged tail bleeding time and decreased platelet thrombus formation in FeCl3-induced carotid arterial injury and laser-induced cremaster artery injury models. ERp18 deficiency inhibited platelet aggregation, adenosine triphosphate release, integrin αIIbβ3 activation, P-selectin expression, platelet adhesion, as well as clot retraction. Flow cytometry and coimmunoprecipitation analyses revealed that ERp18 binds to the platelet surface via interaction with integrin αIIbβ3. Moreover, the ERp18 protein promoted the binding of integrin αIIbβ3 to fibrinogen and platelet aggregation. Furthermore, the recombinant ERp18 protein exhibited reductase activity and cleaved integrin αIIbβ3 disulfides.

Conclusion

ERp18 participates in platelet activation and thrombosis. Its function is, at least in part, through the regulation of integrin αIIbβ3 function. This finding expands our understanding of the role of thiol isomerases in the redox regulation of thrombosis and platelet function.

Reactive oxygen species in hypertension

Hypertension is a leading risk factor for stroke, heart disease and chronic kidney disease. Multiple interacting factors and organ systems increase blood pressure and cause target-organ damage. Among the many molecular elements involved in the development of hypertension are reactive oxygen species (ROS), which influence cellular processes in systems that contribute to blood pressure elevation (such as the cardiovascular, renal, immune and central nervous systems, or the renin-angiotensin-aldosterone system). Dysregulated ROS production (oxidative stress) is a hallmark of hypertension in humans and experimental models. Of the many ROS-generating enzymes, NADPH oxidases are the most important in the development of hypertension. At the cellular level, ROS influence signalling pathways that define cell fate and function. Oxidative stress promotes aberrant redox signalling and cell injury, causing endothelial dysfunction, vascular damage, cardiovascular remodelling, inflammation and renal injury, which are all important in both the causes and consequences of hypertension. ROS scavengers reduce blood pressure in almost all experimental models of hypertension; however, clinical trials of antioxidants have yielded mixed results. In this Review, we highlight the latest advances in the understanding of the role and the clinical implications of ROS in hypertension. We focus on cellular sources of ROS, molecular mechanisms of oxidative stress and alterations in redox signalling in organ systems, and their contributions to hypertension.

A portrait of the infected cell: how SARS-CoV-2 infection reshapes cellular processes and pathways

Positive-sense single-stranded RNA (+ssRNA) viruses exert a profound influence on cellular organelles and metabolic pathway by usurping host processes to promote their replication. In this review, we present a portrait of selected cellular pathways perturbed in SARS-CoV-2 infection: the effect of viral translation, replication and assembly on the morphology and function of the ER, the remodelling of degradative pathways with a focus on the autophagic processes, and the alterations affecting cellular membranes and lipid metabolism. For each of these cellular processes, we highlight the specific viral and host factors involved and their interplay in this microscopic tug-of-war between pro-viral and anti-viral effects that ultimately tip the scale toward the propagation or the resolution of the infection.

The Tumor Suppressor TPD52-Governed Endoplasmic Reticulum Stress is Modulated by APCCdc20

Aberrant regulation of unfolded protein response (UPR)/endoplasmic reticulum (ER) stress pathway is associated with cancer development, metastasis, and relapse, and the UPR signal transducer ATF6 has been proposed as a diagnostic and prognostic marker for many cancers. However, a causal molecular link between ATF6 activation and carcinogenesis is not established. Here, it is found that tumor protein D52 (TPD52) integrates ER stress and UPR signaling with the chaperone machinery by promoting S2P-mediated cleavage of ATF6. Although TPD52 has been generally considered as an oncogene, TPD52 is identified as a novel tumor suppressor in bladder cancer. Significantly, attenuation of the ER stress via depletion of TPD52 facilitated tumorigenesis in a subset of human carcinomas. Furthermore, the APCCdc20 E3 ligase is validated as the upstream regulator marking TPD52 for polyubiquitination-mediated proteolysis. In addition, inactivation of Cdc20 sensitized cancer cells to treatment with the ER stress inducer in a TPD52-dependent manner. Thus, the study suggests that TPD52 is a novel Cdc20 substrate that may modulate ER stress to prevent tumorigenesis.

Modulation of Protein Disulfide Isomerase Functions by Localization: The Example of the Anterior Gradient Family

Significance: Oxidative folding within the endoplasmic reticulum (ER) introduces disulfide bonds into nascent polypeptides, ensuring proteins' stability and proper functioning. Consequently, this process is critical for maintaining proteome integrity and overall health. The productive folding of thousands of secretory proteins requires stringent quality control measures, such as the unfolded protein response (UPR) and ER-Associated Degradation (ERAD), which contribute significantly to maintaining ER homeostasis. ER-localized protein disulfide isomerases (PDIs) play an essential role in each of these processes, thereby contributing to various aspects of ER homeostasis, including maintaining redox balance, proper protein folding, and signaling from the ER to the nucleus. Recent Advances: Over the years, there have been increasing reports of the (re)localization of PDI family members and other ER-localized proteins to various compartments. A prime example is the anterior gradient (AGR) family of PDI proteins, which have been reported to relocate to the cytosol or the extracellular environment, acquiring gain of functions that intersect with various cellular signaling pathways. Critical Issues: Here, we summarize the functions of PDIs and their gain or loss of functions in non-ER locations. We will focus on the activity, localization, and function of the AGR proteins: AGR1, AGR2, and AGR3. Future Directions: Targeting PDIs in general and AGRs in particular is a promising strategy in different human diseases. Thus, there is a need for innovative strategies and tools aimed at targeting PDIs; those strategies should integrate the specific localization and newly acquired functions of these PDIs rather than solely focusing on their canonical roles.

The unfolded protein response regulates ER exit sites via SNRPB-dependent RNA splicing and contributes to bone development

Splicing and endoplasmic reticulum (ER)-proteostasis are two key processes that ultimately regulate the functional proteins that are produced by a cell. However, the extent to which these processes interact remains poorly understood. Here, we identify SNRPB and other components of the Sm-ring, as targets of the unfolded protein response and novel regulators of export from the ER. Mechanistically, The Sm-ring regulates the splicing of components of the ER export machinery, including Sec16A, a component of ER exit sites. Loss of function of SNRPB is causally linked to cerebro-costo-mandibular syndrome (CCMS), a genetic disease characterized by bone defects. We show that heterozygous deletion of SNRPB in mice resulted in bone defects reminiscent of CCMS and that knockdown of SNRPB delays the trafficking of type-I collagen. Silencing SNRPB inhibited osteogenesis in vitro, which could be rescued by overexpression of Sec16A. This rescue indicates that the role of SNRPB in osteogenesis is linked to its effects on ER-export. Finally, we show that SNRPB is a target for the unfolded protein response, which supports a mechanistic link between the spliceosome and ER-proteostasis. Our work highlights components of the Sm-ring as a novel node in the proteostasis network, shedding light on CCMS pathophysiology.

A new perspective on liver diseases: Focusing on the mitochondria-associated endoplasmic reticulum membranes

The pathogenesis of liver diseases is multifaceted and intricate, posing a persistent global public health challenge with limited therapeutic options. Therefore, further research into liver diseases is imperative for better comprehension and advancement in treatment strategies. Numerous studies have confirmed the endoplasmic reticulum (ER) and mitochondria as key organelles driving liver diseases. Notably, the mitochondrial-associated ER membranes (MAMs) establish a physical and functional connection between the ER and mitochondria, highlighting the importance of inter-organelle communication in maintaining their functional homeostasis. This review delves into the intricate architecture and regulative mechanism of the integrated MAM that facilitate the physiological transfer of signals and substances between organelles. Additionally, we also provide a detailed overview regarding the varied pathogenic roles of malfunctioning MAM in liver diseases, focusing on its involvement in the progression of ER stress and mitochondrial dysfunction, the regulation of mitochondrial dynamics and Ca2+ transfer, as well as the disruption of lipid and glucose homeostasis. Furthermore, the current challenges and prospects associated with MAM in liver disease research are thoroughly discussed. In conclusion, elucidating the specific structure and function of MAM in different liver diseases may pave the way for novel therapeutic strategies.

Endoplasmic Reticulum Membrane Homeostasis and the Unfolded Protein Response

The endoplasmic reticulum (ER) is the key organelle for membrane biogenesis. Most lipids are synthesized in the ER, and most membrane proteins are first inserted into the ER membrane before they are transported to their target organelle. The composition and properties of the ER membrane must be carefully controlled to provide a suitable environment for the insertion and folding of membrane proteins. The unfolded protein response (UPR) is a powerful signaling pathway that balances protein and lipid production in the ER. Here, we summarize our current knowledge of how aberrant compositions of the ER membrane, referred to as lipid bilayer stress, trigger the UPR.

A genome-wide CRISPR/Cas9 screen identifies calreticulin as a selective repressor of ATF6α

Activating transcription factor 6 (ATF6) is one of three endoplasmic reticulum (ER) transmembrane stress sensors that mediate the unfolded protein response (UPR). Despite its crucial role in long-term ER stress adaptation, regulation of ATF6 alpha (α) signalling remains poorly understood, possibly because its activation involves ER-to-Golgi and nuclear trafficking. Here, we generated an ATF6α/Inositol-requiring kinase 1 (IRE1) dual UPR reporter CHO-K1 cell line and performed an unbiased genome-wide CRISPR/Cas9 mutagenesis screen to systematically profile genetic factors that specifically contribute to ATF6α signalling in the presence and absence of ER stress. The screen identified both anticipated and new candidate genes that regulate ATF6α activation. Among these, calreticulin (CRT), a key ER luminal chaperone, selectively repressed ATF6α signalling: Cells lacking CRT constitutively activated a BiP::sfGFP ATF6α-dependent reporter, had higher BiP levels and an increased rate of trafficking and processing of ATF6α. Purified CRT interacted with the luminal domain of ATF6α in vitro and the two proteins co-immunoprecipitated from cell lysates. CRT depletion exposed a negative feedback loop implicating ATF6α in repressing IRE1 activity basally and overexpression of CRT reversed this repression. Our findings indicate that CRT, beyond its known role as a chaperone, also serves as an ER repressor of ATF6α to selectively regulate one arm of the UPR.

Endoplasmic reticulum: Monitoring and maintaining protein and membrane homeostasis in the endoplasmic reticulum by the unfolded protein response

The endoplasmic reticulum (ER) regulates essential cellular processes, including protein folding, lipid synthesis, and calcium homeostasis. The ER homeostasis is maintained by a conserved set of signaling cascades called the Unfolded Protein Response (UPR). How the UPR senses perturbations in ER homeostasis has been the subject of active research for decades. In metazoans, the UPR consists of three ER-membrane embedded sensors: IRE1, PERK and ATF6. These sensors detect the accumulation of misfolded proteins in the ER lumen and adjust protein folding capacity according to cellular needs. Early work revealed that the ER-resident chaperone BiP binds to all three UPR sensors in higher eukaryotes and BiP binding was suggested to regulate their activity. More recent data have shown that in higher eukaryotes the interaction of the UPR sensors with a complex network of chaperones and misfolded proteins modulates their activation and deactivation dynamics. Furthermore, emerging evidence suggests that the UPR monitors ER membrane integrity beyond protein folding defects. However, the mechanistic and structural basis of UPR activation by proteotoxic and lipid bilayer stress in higher eukaryotes remains only partially understood. Here, we review the current understanding of novel protein interaction networks and the contribution of the lipid membrane environment to UPR activation.

Gradual ER calcium depletion induces a progressive and reversible UPR signaling

The unfolded protein response (UPR) is a widespread signal transduction pathway triggered by endoplasmic reticulum (ER) stress. Because calcium (Ca2+) is a key factor in the maintenance of ER homeostasis, massive Ca2+ depletion of the ER is a potent inducer of ER stress. Although moderate changes in ER Ca2+ drive the ubiquitous Ca2+ signaling pathways, a possible incremental relationship between UPR activation and Ca2+ changes has yet to be described. Here, we determine the sensitivity and time-dependency of activation of the three ER stress sensors, inositol-requiring protein 1 alpha (IRE1α), protein kinase R-like ER kinase (PERK), and activating transcription factor 6 alpha (ATF6α) in response to controlled changes in the concentration of ER Ca2+ in human cultured cells. Combining Ca2+ imaging, fluorescence recovery after photobleaching experiments, biochemical analyses, and mathematical modeling, we uncover a nonlinear rate of activation of the IRE1α branch of UPR, as compared to the PERK and ATF6α branches that become activated gradually with time and are sensitive to more important ER Ca2+ depletions. However, the three arms are all activated within a 1 h timescale. The model predicted the deactivation of PERK and IRE1α upon refilling the ER with Ca2+. Accordingly, we showed that ER Ca2+ replenishment leads to the complete reversion of IRE1α and PERK phosphorylation in less than 15 min, thus revealing the highly plastic character of the activation of the upstream UPR sensors. In conclusion, our results reveal a dynamic and dose-sensitive Ca2+-dependent activation/deactivation cycle of UPR induction, which could tightly control cell fate upon acute and/or chronic stress.

Mild endoplasmic reticulum stress alleviates FB1-triggered intestinal pyroptosis via the Sec62-PERK pathway

Fumonisin B1 (FB1), a water-soluble mycotoxin released by Fusarium moniliforme Sheld, is widely present in corn and its derivative products, and seriously endangers human life and health. Recent studies have reported that FB1 can lead to pyroptosis, however, the mechanisms by which FB1-induced pyroptosis remain indistinct. In the present study, we aim to investigate the mechanisms of pyroptosis in intestinal porcine epithelial cells (IPEC-J2) and the relationship between FB1-induced endoplasmic reticulum stress (ERS) and pyroptosis. Our experimental results showed that the pyroptosis protein indicators in IPEC-J2 were significantly increased after exposure to FB1. The ERS markers, including glucose-regulated Protein 78 (GRP78), PKR-like ER kinase protein (PERK), and preprotein translocation factor (Sec62) were also significantly increased. Using small interfering RNA silencing of PERK or Sec62, the results demonstrated that upregulation of Sec62 activates the PERK pathway, and activation of the PERK signaling pathway is upstream of FB1-induced pyroptosis. After using the ERS inhibitor 4-PBA reduced the FB1-triggered intestinal injury by the Sec62-PERK pathway. In conclusion, we found that FB1 induced pyroptosis by upregulating Sec62 to activate the PERK pathway, and mild ERS alleviates FB1-triggered damage. It all boils down to one fact, the study provides a new perspective for further, and improving the toxicological mechanism of FB1.

Insights into the Activation of Unfolded Protein Response Mechanism during Coronavirus Infection

Coronaviruses represent a significant class of viruses that affect both animals and humans. Their replication cycle is strongly associated with the endoplasmic reticulum (ER), which, upon virus invasion, triggers ER stress responses. The activation of the unfolded protein response (UPR) within infected cells is performed from three transmembrane receptors, IRE1, PERK, and ATF6, and results in a reduction in protein production, a boost in the ER's ability to fold proteins properly, and the initiation of ER-associated degradation (ERAD) to remove misfolded or unfolded proteins. However, in cases of prolonged and severe ER stress, the UPR can also instigate apoptotic cell death and inflammation. Herein, we discuss the ER-triggered host responses after coronavirus infection, as well as the pharmaceutical targeting of the UPR as a potential antiviral strategy.

Unraveling the Connection: Pain and Endoplasmic Reticulum Stress

Pain is a complex and multifaceted experience. Recent research has increasingly focused on the role of endoplasmic reticulum (ER) stress in the induction and modulation of pain. The ER is an essential organelle for cells and plays a key role in protein folding and calcium dynamics. Various pathological conditions, such as ischemia, hypoxia, toxic substances, and increased protein production, may disturb protein folding, causing an increase in misfolding proteins in the ER. Such an overload of the folding process leads to ER stress and causes the unfolded protein response (UPR), which increases folding capacity in the ER. Uncompensated ER stress impairs intracellular signaling and cell function, resulting in various diseases, such as diabetes and degenerative neurological diseases. ER stress may be a critical universal mechanism underlying human diseases. Pain sensations involve the central as well as peripheral nervous systems. Several preclinical studies indicate that ER stress in the nervous system is enhanced in various painful states, especially in neuropathic pain conditions. The purpose of this narrative review is to uncover the intricate relationship between ER stress and pain, exploring molecular pathways, implications for various pain conditions, and potential therapeutic strategies.

ER stress signaling at the interphase between MASH and HCC

HCC is the most frequent primary liver cancer with an extremely poor prognosis and often develops on preset of chronic liver diseases. Major risk factors for HCC include metabolic dysfunction-associated steatohepatitis, a complex multifactorial condition associated with abnormal endoplasmic reticulum (ER) proteostasis. To cope with ER stress, the unfolded protein response engages adaptive reactions to restore the secretory capacity of the cell. Recent advances revealed that ER stress signaling plays a critical role in HCC progression. Here, we propose that chronic ER stress is a common transversal factor contributing to the transition from liver disease (risk factor) to HCC. Interventional strategies to target the unfolded protein response in HCC, such as cancer therapy, are also discussed.

Endoplasmic reticulum homeostasis-From molecules to organisms: Report on the 14th International Calreticulin Workshop, Saint Malo, France

The Calreticulin Workshop, initiated in 1994 by Marek Michalak in Banff (Alberta, Canada), was first organized to be an informal scientific meeting attended by researchers working on diverse biological questions related to functions associated with the endoplasmic reticulum (ER)-resident lectin-like chaperone and applied to a wide range of biological systems and models. Since then, this workshop has broadened the range of topics to cover all ER-related functions, has become international and has been held in Canada, Chile, Denmark, Italy, Switzerland, UK, USA, Greece and this year in France. Each conference, which is organized every other year (pending world-wide pandemic), generally attracts between 50 and 100 participants, including both early career researchers and international scientific leaders to favour discussions and exchanges. Over the years, the International Calreticulin Workshop has become an important gathering of the calreticulin and ER communities as a whole. The 14th International Calreticulin Workshop occurred from May 9-12 in St-Malo, Brittany, France, and has been highlighted by its rich scientific content and open-minded discussions held in a benevolent atmosphere. The 15th International Calreticulin Workshop will be organized in 2025 in Brussels, Belgium.

Zn2+-dependent functional switching of ERp18, an ER-resident thioredoxin-like protein

ERp18 is an endoplasmic reticulum (ER)-resident thioredoxin (Trx) family protein, similar to cytosolic Trx1. The Trx-like domain occupies a major portion of the whole ERp18 structure, which is postulated to be an ER paralog of cytosolic Trx1. Here, we elucidate that zinc ion (Zn2+) binds ERp18 through its catalytic motif, triggering oligomerization of ERp18 from a monomer to a trimer. While the monomeric ERp18 has disulfide oxidoreductase activity, the trimeric ERp18 acquires scavenger activity for hydrogen peroxide (H2O2) in the ER. Depletion of ERp18 thus causes the accumulation of H2O2, which is produced during the oxidative folding of nascent polypeptides in the ER. ERp18 knockdown in C. elegans without Prx4 and GPx7/8, both of which are also known to have H2O2 scavenging activity in the ER, shortened the lifespan, suggesting that ERp18 may form a primitive and essential H2O2 scavenging system for the maintenance of redox homeostasis in the ER.

Increased cellular protein modification by methylglyoxal activates endoplasmic reticulum-based sensors of the unfolded protein response

The unfolded protein response (UPR) detects increased misfolded proteins and activates protein refolding, protein degradation and inflammatory responses. UPR sensors in the endoplasmic reticulum, IRE1α and PERK, bind and are activated by proteins with unexpected surface hydrophobicity, whereas sensor ATF6 is activated by proteolytic cleavage when released from complexation with protein disulfide isomerases (PDIs). Metabolic dysfunction leading to the formation of misfolded proteins with surface hydrophobicity and disruption of ATF6-PDI complexes leading to activation of UPR sensors remains unclear. The cellular concentration of reactive dicarbonyl metabolite, methylglyoxal (MG), is increased in impaired metabolic health, producing increased MG-modified cellular proteins. Herein we assessed the effect of high glucose concentration and related increased cellular MG on activation status of IRE1α, PERK and ATF6. Human aortal endothelial cells and HMEC-1 microvascular endothelial cells were incubated in low and high glucose concentration to model blood glucose control, with increase or decrease of MG by silencing or increasing expression of glyoxalase 1 (Glo1), which metabolizes MG. Increased MG induced by high glucose concentration activated IRE1α, PERK and ATF6 and related downstream signalling leading to increased chaperone, apoptotic and inflammatory gene expression. Correction of increased MG by increasing Glo1 expression prevented UPR activation. MG modification of proteins produces surface hydrophobicity through arginine-derived hydroimidazolone MG-H1 formation, with related protein unfolding and preferentially targets PDIs and chaperone pathways for modification. It thereby poses a major challenge to proteostasis and activates UPR sensors. Pharmacological decrease of MG with Glo1 inducer, trans-resveratrol and hesperetin in combination, offers a novel treatment strategy to counter UPR-related cell dysfunction, particularly in hyperglycemia associated with diabetes.

Oxidative Stress and Endoplasmic Reticular Stress Interplay in the Vasculopathy of Hypertension

Under physiologic conditions, reactive oxygen species (ROS) function as signalling molecules that control cell function. However, in pathologic conditions, increased generation of ROS triggers oxidative stress, which plays a role in vascular changes associated with hypertension, including endothelial dysfunction, vascular reactivity, and arterial remodelling (termed the vasculopathy of hypertension). The major source of ROS in the vascular system is NADPH oxidase (NOX). Increased NOX activity drives vascular oxidative stress in hypertension. Molecular mechanisms underlying vascular damage in hypertension include activation of redox-sensitive signalling pathways, post-translational modification of proteins, and oxidative damage of DNA and cytoplasmic proteins. In addition, oxidative stress leads to accumulation of proteins in the endoplasmic reticulum (ER) (termed ER stress), with consequent activation of the unfolded protein response (UPR). ER stress is emerging as a potential player in hypertension as abnormal protein folding in the ER leads to oxidative stress and dysregulated activation of the UPR promotes inflammation and injury in vascular and cardiac cells. In addition, the ER engages in crosstalk with exogenous sources of ROS, such as mitochondria and NOX, which can amplify redox processes. Here we provide an update of the role of ROS and NOX in hypertension and discuss novel concepts on the interplay between oxidative stress and ER stress.

Divergent Proteome Reactivity Influences Arm-Selective Activation of the Unfolded Protein Response by Pharmacological Endoplasmic Reticulum Proteostasis Regulators

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the unfolded protein response (UPR) has proven useful for ameliorating proteostasis deficiencies in cellular and mouse models of numerous etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino-p-cresol substructure affording a quinone methide, which then covalently modifies a subset of endoplasmic reticulum (ER) protein disulfide isomerases (PDIs). Another compound identified in this screen, AA132, also contains a 2-amino-p-cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent modification of proteins including multiple PDIs. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. However, the extent of PDI labeling by AA147 approaches a plateau more rapidly than PDI labeling by AA132. These observations together suggest that AA132 can access a larger pool of proteins for covalent modification, possibly because its activated form is less susceptible to quenching than activated AA147. In other words, the lower reactivity of activated AA132 allows it to persist longer and modify more PDIs in the cellular environment. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and enables selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Unfolding the Interactions between Endoplasmic Reticulum Stress and Oxidative Stress

Oxidative stress is caused by an imbalance in cellular redox state due to the accumulation of reactive oxygen species (ROS). While homeostatic levels of ROS are important for cell physiology and signaling, excess ROS can induce a variety of negative effects ranging from damage to biological macromolecules to cell death. Additionally, oxidative stress can disrupt the function of redox-sensitive organelles including the mitochondria and endoplasmic reticulum (ER). In the case of the ER, the accumulation of misfolded proteins can arise due to oxidative stress, leading to the onset of ER stress. To combat ER stress, cells initiate a highly conserved stress response called the unfolded protein response (UPR). While UPR signaling, within the context of resolving ER stress, is well characterised, how UPR mediators respond to and influence oxidative stress is less defined. In this review, we evaluate the interplay between oxidative stress, ER stress and UPR signaling networks. Specifically, we assess how UPR signaling mediators can influence antioxidant responses.

The endoplasmic reticulum kinase PERK interacts with the oxidoreductase ERO1 to metabolically adapt mitochondria

Endoplasmic reticulum (ER) homeostasis requires molecular regulators that tailor mitochondrial bioenergetics to the needs of protein folding. For instance, calnexin maintains mitochondria metabolism and mitochondria-ER contacts (MERCs) through reactive oxygen species (ROS) from NADPH oxidase 4 (NOX4). However, induction of ER stress requires a quick molecular rewiring of mitochondria to adapt to new energy needs. This machinery is not characterized. We now show that the oxidoreductase ERO1⍺ covalently interacts with protein kinase RNA-like ER kinase (PERK) upon treatment with tunicamycin. The PERK-ERO1⍺ interaction requires the C-terminal active site of ERO1⍺ and cysteine 216 of PERK. Moreover, we show that the PERK-ERO1⍺ complex promotes oxidization of MERC proteins and controls mitochondrial dynamics. Using proteinaceous probes, we determined that these functions improve ER-mitochondria Ca²⁺ flux to maintain bioenergetics in both organelles, while limiting oxidative stress. Therefore, the PERK-ERO1⍺ complex is a key molecular machinery that allows quick metabolic adaptation to ER stress.

Divergent Proteome Reactivity Influences Arm-Selective Activation of Pharmacological Endoplasmic Reticulum Proteostasis Regulators

Pharmacological activation of the activating transcription factor 6 (ATF6) arm of the Unfolded Protein Response (UPR) has proven useful for ameliorating proteostasis deficiencies in a variety of etiologically diverse diseases. Previous high-throughput screening efforts identified the small molecule AA147 as a potent and selective ATF6 activating compound that operates through a mechanism involving metabolic activation of its 2-amino- p -cresol substructure affording a quinone methide, which then covalently modifies a subset of ER protein disulfide isomerases (PDIs). Intriguingly, another compound identified in this screen, AA132, also contains a 2-amino- p -cresol moiety; however, this compound showed less transcriptional selectivity, instead globally activating all three arms of the UPR. Here, we show that AA132 activates global UPR signaling through a mechanism analogous to that of AA147, involving metabolic activation and covalent PDI modification. Chemoproteomic-enabled analyses show that AA132 covalently modifies PDIs to a greater extent than AA147. Paradoxically, activated AA132 reacts slower with PDIs, indicating it is less reactive than activated AA147. This suggests that the higher labeling of PDIs observed with activated AA132 can be attributed to its lower reactivity, which allows this activated compound to persist longer in the cellular environment prior to quenching by endogenous nucleophiles. Collectively, these results suggest that AA132 globally activates the UPR through increased engagement of ER PDIs. Consistent with this, reducing the cellular concentration of AA132 decreases PDI modifications and allows for selective ATF6 activation. Our results highlight the relationship between metabolically activatable-electrophile stability, ER proteome reactivity, and the transcriptional response observed with the enaminone chemotype of ER proteostasis regulators, enabling continued development of next-generation ATF6 activating compounds.

Anterior gradient proteins in gastrointestinal cancers: from cell biology to pathophysiology

Most of the organs of the digestive tract comprise secretory epithelia that require specialized molecular machines to achieve their functions. As such anterior gradient (AGR) proteins, which comprise AGR1, AGR2, and AGR3, belong to the protein disulfide isomerase family, and are involved in secretory and transmembrane protein biogenesis in the endoplasmic reticulum. They are generally expressed in epithelial cells with high levels in most of the digestive tract epithelia. To date, the vast majority of the reports concern AGR2, which has been shown to exhibit various subcellular localizations and exert pro-oncogenic functions. AGR2 overexpression has recently been associated with a poor prognosis in digestive cancers. AGR2 is also involved in epithelial homeostasis. Its deletion in mice results in severe diffuse gut inflammation, whereas in inflammatory bowel diseases, the secretion of AGR2 in the extracellular milieu participates in the reshaping of the cellular microenvironment. AGR2 thus plays a key role in inflammation and oncogenesis and may represent a therapeutic target of interest. In this review, we summarize the already known roles and mechanisms of action of the AGR family proteins in digestive diseases, their expression in the healthy digestive tract, and in digestive oncology. At last, we discuss the potential diagnostic and therapeutic implications underlying the biology of AGR proteins.

New insights into the role of the Golgi apparatus in the pathogenesis and therapeutics of human diseases

The Golgi apparatus is an essential cellular organelle that mediates homeostatic functions, including vesicle trafficking and the post-translational modification of macromolecules. Its unique stacked structure and dynamic functions are tightly regulated, and several Golgi proteins play key roles in the functioning of unconventional protein secretory pathways triggered by cellular stress responses. Recently, an increasing number of studies have implicated defects in Golgi functioning in human diseases such as cancer, neurodegenerative, and immunological disorders. Understanding the extraordinary characteristics of Golgi proteins is important for elucidating its associated intracellular signaling mechanisms and has important ramifications for human health. Therefore, analyzing the mechanisms by which the Golgi participates in disease pathogenesis may be useful for developing novel therapeutic strategies. This review articulates the structural features and abnormalities of the Golgi apparatus reported in various diseases and the suspected mechanisms underlying the Golgi-associated pathologies. Furthermore, we review the potential therapeutic strategies based on Golgi function.

Endoplasmic reticulum stress in innate immune cells - a significant contribution to non-alcoholic fatty liver disease

Liver disease and its complications affect millions of people worldwide. NAFLD (non-alcoholic fatty liver disease) is the liver disease associated with metabolic dysfunction and consists of four stages: steatosis with or without mild inflammation (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. With increased necroinflammation and progression of liver fibrosis, NAFLD may progress to cirrhosis or even hepatocellular carcinoma. Although the underlying mechanisms have not been clearly elucidated in detail, what is clear is that complex immune responses are involved in the pathogenesis of NASH, activation of the innate immune system is critically involved in triggering and amplifying hepatic inflammation and fibrosis in NAFLD/NASH. Additionally, disruption of endoplasmic reticulum (ER) homeostasis in cells, also known as ER stress, triggers the unfolded protein response (UPR) which has been shown to be involved to inflammation and apoptosis. To further develop the prevention and treatment of NAFLD/NASH, it is imperative to clarify the relationship between NAFLD/NASH and innate immune cells and ER stress. As such, this review focuses on innate immune cells and their ER stress in the occurrence of NAFLD and the progression of cirrhosis.

Understanding ER homeostasis and the UPR to enhance treatment efficacy of acute myeloid leukemia

Protein biogenesis, maturation and degradation are tightly regulated processes that are governed by a complex network of signaling pathways. The endoplasmic reticulum (ER) is responsible for biosynthesis and maturation of secretory proteins. Circumstances that alter cellular protein homeostasis, determine accumulation of misfolded and unfolded proteins in the ER, a condition defined as ER stress. In case of stress, the ER activates an adaptive response called unfolded protein response (UPR), a series of pathways of major relevance for cancer biology. The UPR plays a preeminent role in adaptation of tumor cells to the harsh conditions that they experience, due to high rates of proliferation, metabolic abnormalities and hostile environment scarce in oxygen and nutrients. Furthermore, the UPR is among the main adaptive cell stress responses contributing to the development of resistance to drugs and chemotherapy. Clinical management of Acute Myeloid Leukemia (AML) has improved significantly in the last decade, thanks to development of molecular targeted therapies. However, the emergence of treatment-resistant clones renders the rate of AML cure dismal. Moreover, different cell populations that constitute the bone marrow niche recently emerged as a main determinant leading to drug resistance. Herein we summarize the most relevant literature regarding the role played by the UPR in expansion of AML and ability to develop drug resistance and we discuss different possible modalities to overturn this adaptive response against leukemia. To this aim, we also describe the interconnection of the UPR with other cellular stress responses regulating protein homeostasis. Finally, we review the newest findings about the crosstalk between AML cells and cells of the bone marrow niche, under physiological conditions and in response to therapies, discussing in particular the importance of the niche in supporting survival of AML cells by favoring protein homeostasis.

Pathways Linking Nicotinamide Adenine Dinucleotide Phosphate Production to Endoplasmic Reticulum Protein Oxidation and Stress

The endoplasmic reticulum (ER) lumen is highly oxidizing compared to other subcellular compartments, and maintaining the appropriate levels of oxidizing and reducing equivalents is essential to ER function. Both protein oxidation itself and other essential ER processes, such as the degradation of misfolded proteins and the sequestration of cellular calcium, are tuned to the ER redox state. Simultaneously, nutrients are oxidized in the cytosol and mitochondria to power ATP generation, reductive biosynthesis, and defense against reactive oxygen species. These parallel needs for protein oxidation in the ER and nutrient oxidation in the cytosol and mitochondria raise the possibility that the two processes compete for electron acceptors, even though they occur in separate cellular compartments. A key molecule central to both processes is NADPH, which is produced by reduction of NADP+ during nutrient catabolism and which in turn drives the reduction of components such as glutathione and thioredoxin that influence the redox potential in the ER lumen. For this reason, NADPH might serve as a mediator linking metabolic activity to ER homeostasis and stress, and represent a novel form of mitochondria-to-ER communication. In this review, we discuss oxidative protein folding in the ER, NADPH generation by the major pathways that mediate it, and ER-localized systems that can link the two processes to connect ER function to metabolic activity.