Toll-like receptor-mediated IRE1α activation as a therapeutic target for inflammatory arthritis

Endoplasmic reticulum stress: A master regulator of metabolic syndrome

Endoplasmic reticulum (ER) stress, a change in the ER homeostasis, leads to initiation of the unfolded protein response (UPR). The primary functions of the UPR are to restore the ER's physiological activity and coordinate the apoptotic and adaptive responses. Pathophysiological conditions that augment ER stress include hypoxia, misfolded and/or mutated protein accumulation, and high glucose. Prolonged ER stress is a critical factor in the pathogenesis of metabolic syndrome including type 2 diabetes mellitus, cardiovascular diseases, atherosclerosis, obesity, and fatty liver disease. UPR is a complex homeostatic pathway between newly synthesized proteins and their maturation, although the regulatory mechanisms contributing to the UPR and the possible therapeutic strategies are yet to be clarified. Therefore, a comprehensive understanding of the underlying molecular mechanisms is necessary to develop therapeutic interventions targeting ER stress response. In this review, we discuss the role of ER stress and UPR signaling in the pathogenesis of metabolic syndrome, highlighting the main functions of UPR components. We have emphasized the use of novel small molecular chemical chaperones, considered as modulators of ER stress. The initial studies with these chemical chaperones are promising, but detailed studies are required to define their efficacy and adverse effects during therapeutic use in humans.

Peli1 induction impairs cardiac microvascular endothelium through Hsp90 dissociation from IRE1α

Ameliorating cardiac microvascular injury is the most effective means to mitigate diabetes-induced cardiovascular complications. Inositol-requiring 1α (IRE1α), a sensor of endoplasmic reticulum stress, is activated by Toll like receptors (TLRs), and then promotes cardiac microvascular injury. Peli1 is a master regulator of TLRs and activates IRE1α. This study aims to investigate whether Peli1 in endothelial cells promotes diabetes-induced cardiac microvascular injury through activating IRE1α. Here we found that Peli1 was markedly up-regulated in cardiac endothelial cells of both diabetic mice and in AGEs-treated cardiac microvascular endothelial cells (CMECs). Peli1 deficiency in endothelial cells significantly alleviated diabetes-induced cardiac microvascular permeability, promoted microvascular regeneration, and suppressed apoptosis, accompanied by the attenuation of adverse cardiac remodeling. Furthermore, Peli1 deletion in CMECs ameliorated AGEs-induced damages in vitro. We identified heat shock protein 90 (Hsp90) as a potential binding partner for Peli1, and the Ring domain of Peli1 directly bound with Hsp90 to enhance IRE1α phosphorylation. Our study suggests that blocking Peli1 in endothelial cells may protect against diabetes-induced cardiac microvascular injury by restraining ER stress.

T-2 toxin inhibits the production of mucin via activating the IRE1/XBP1 pathway

T-2 toxin is a trichothecene mycotoxin that widely contaminates food and has a variety of toxic effects. However, the underlying mechanism of T-2 toxin on intestinal mucin remains unclear. In present study, human intestinal Caco-2 cells and HT-29 cells were treated with 100 ng/mL T-2 toxin at one-quarter of the IC₅₀ for 24 h, which caused the inhibition of MUC2 and adhesion of E. coli O157:H7. We found T-2 toxin induced endoplasmic reticulum stress and activated the IRE1/XBP1 pathway, which may be related to the inhibition of MUC2. Interestingly, T-2 toxin activated IRE1α to inhibit IRE1β, which optimized mucin production. Furthermore, overexpression of IRE1β in the cells apparently alleviated the inhibition of MUC2 caused by T-2 toxin. IRE1α knock-down blocked the down-regulation of IRE1β and MUC2 induced by T-2 toxin. We revealed the critical role of IRE1α in the inhibition of intestinal mucin. This finding was confirmed in BALB/c mice which were exposed to T-2 toxin (0.5 mg/kg bw) for 4 weeks. T-2 toxin activated the IRE1/XBP1 pathway to disrupt intestinal mucin, which lead to the imbalance of gut microbiota and an increased risk of host infection by E. coli O157:H7. T-2 toxin exposure also increased the expressions of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α in mice, which might respond to IRE1α activation. Importantly, IRE1α activation was a therapeutic target for intestinal inflammation caused by T-2 toxin. This study provided a new perspective to understand the intestinal toxicity of T-2 toxin.

Vascular Smooth Muscle Cell Plasticity and Autophagy in Dissecting Aortic Aneurysms

Objective- Recent studies suggested the occurrence of phenotypic switching of vascular smooth muscle cells (VSMCs) during the development of aortic aneurysm (AA). However, lineage-tracing studies are still lacking, and the behavior of VSMCs during the formation of dissecting AA is poorly understood. Approach and Results- We used multicolor lineage tracing of VSMCs to track their fate after injury in murine models of Ang II (angiotensin II)-induced dissecting AA. We also addressed the direct impact of autophagy on the response of VSMCs to AA dissection. Finally, we studied the relevance of these processes to human AAs. Here, we show that a subset of medial VSMCs undergoes clonal expansion and that VSMC outgrowths are observed in the adventitia and borders of the false channel during Ang II-induced development of dissecting AA. The clonally expanded VSMCs undergo phenotypic switching with downregulation of VSMC differentiation markers and upregulation of phagocytic markers, indicative of functional changes. In particular, autophagy and endoplasmic reticulum stress responses are activated in the injured VSMCs. Loss of autophagy in VSMCs through deletion of autophagy protein 5 gene ( Atg5) increases the susceptibility of VSMCs to death, enhances endoplasmic reticulum stress activation, and promotes IRE (inositol-requiring enzyme) 1α-dependent VSMC inflammation. These alterations culminate in increased severity of aortic disease and higher incidence of fatal AA dissection in mice with VSMC-restricted deletion of Atg5. We also report increased expression of autophagy and endoplasmic reticulum stress markers in VSMCs of human dissecting AAs. Conclusions- VSMCs undergo clonal expansion and phenotypic switching in Ang II-induced dissecting AAs in mice. We also identify a critical role for autophagy in regulating VSMC death and endoplasmic reticulum stress-dependent inflammation with important consequences for aortic wall homeostasis and repair.

Linking the Endoplasmic Reticulum to Parkinson's Disease and Alpha-Synucleinopathy

Accumulation of misfolded proteins is a central paradigm in neurodegeneration. Because of the key role of the endoplasmic reticulum (ER) in regulating protein homeostasis, in the last decade multiple reports implicated this organelle in the progression of Parkinson's Disease (PD) and other neurodegenerative illnesses. In PD, dopaminergic neuron loss or more broadly neurodegeneration has been improved by overexpression of genes involved in the ER stress response. In addition, toxic alpha-synuclein (αS), the main constituent of proteinaceous aggregates found in tissue samples of PD patients, has been shown to cause ER stress by altering intracellular protein traffic, synaptic vesicles transport, and Ca²⁺ homeostasis. In this review, we will be summarizing evidence correlating impaired ER functionality to PD pathogenesis, focusing our attention on how toxic, aggregated αS can promote ER stress and cell death.

The E3 ligase Hrd1 stabilizes Tregs by antagonizing inflammatory cytokine-induced ER stress response

Treg differentiation, maintenance, and function are controlled by the transcription factor FoxP3, which can be destabilized under inflammatory or other pathological conditions. Tregs can be destabilized under inflammatory or other pathological conditions, but the underlying mechanisms are not fully defined. Herein, we show that inflammatory cytokines induce ER stress response, which destabilizes Tregs by suppressing FoxP3 expression, suggesting a critical role of the ER stress response in maintaining Treg stability. Indeed, genetic deletion of Hrd1, an E3 ligase critical in suppressing the ER stress response, leads to elevated expression of ER stress-responsive genes in Treg and largely diminishes Treg suppressive functions under inflammatory condition. Mice with Treg-specific ablation of Hrd1 displayed massive multiorgan lymphocyte infiltration, body weight loss, and the development of severe small intestine inflammation with aging. At the molecular level, the deletion of Hrd1 led to the activation of both the ER stress sensor IRE1α and its downstream MAPK p38. Pharmacological suppression of IRE1α kinase, but not its endoribonuclease activity, diminished the elevated p38 activation and fully rescued the stability of Hrd1-null Tregs. Taken together, our studies reveal ER stress response as a previously unappreciated mechanism underlying Treg instability and that Hrd1 is crucial for maintaining Treg stability and functions through suppressing the IRE1α-mediated ER stress response.

Molecular Insights Into the Relationship Between Autoimmune Thyroid Diseases and Breast Cancer: A Critical Perspective on Autoimmunity and ER Stress

The etiopathologies behind autoimmune thyroid diseases (AITDs) unravel misbehavior of immune components leading to the corruption of immune homeostasis where thyroid autoantigens turn foe to the self. In AITDs lymphocytic infiltration in the thyroid shows up a deranged immune system charging the follicular cells of the thyroid gland (thyrocytes) leading to the condition of either hyperthyroidism or hypothyroidism. The inflammation in AITDs consistently associate with ER function due to which disturbances in the ER protein homeostasis leads to unfolded protein response (UPR) that promotes pathogenesis of autoimmunity. The roles of ER stress in the instantaneous downregulation of MHC class I molecules on thyrocytes and the relevance of IFN γ in the pathogenesis of AITD has been well-documented. Thyroglobulin being the major target of autoantibodies in most of the AITDs is because of its unusual processing in the ER. Autoimmune disorders display a conglomeration of ER stress-induced UPR activated molecules. Several epidemiological data highlight the preponderance of AITDs in women as well as its concurrence with breast cancer. Both being an active glandular system displaying endocrine activity, thyroid as well as breast tissue show various commonalities in the expression pattern of heterogenous molecules that not only participate in the normal functioning but at the same time share the blame during disease establishment. Studies on the development and progression of breast carcinoma display a deranged and uncontrolled immune response, which is meticulously exploited during tumor metastasis. The molecular crosstalks between AITDs and breast tumor microenvironment rely on active participation of immune cells. The induction of ER stress by Tunicamycin advocates to provide a model for cancer therapy by intervening glycosylation. Therefore, this review attempts to showcase the molecules that are involved in feeding up the relationship between breast carcinoma and AITDs.

The IRE1α-XBP1 pathway function in hypoxia-induced pulmonary vascular remodeling, is upregulated by quercetin, inhibits apoptosis and partially reverses the effect of quercetin in PASMCs

Hypoxia is a common cause of pulmonary vascular remodeling and endoplasmic reticulum stress (ERS). Upon ER stress, the unfolded protein response (UPR) which activates the IRE1α, PERK and ATF6 signaling pathways is activated to cope with ERS in mammalian cells; however, the role of the three UPR arms in pulmonary vascular remodeling has not been defined. The present study showed that GRP78, a marker of ERS, was upregulated in hypoxic pulmonary artery smooth muscle cells (PASMCs). Among the three arms of the UPR, the IRE1α pathway was noticeably upregulated in hypoxic PASMCs. An inhibitor of IRE1α/XBP1 pathway, 4u8c, inhibited hypoxia-induced cell proliferation and migration and increased cell apoptosis by downregulating PCNA and MMP9 and activating mitochondrial apoptosis by enhancing the expression of BAX, activating caspase-9 and caspase-3, and eventually cleaving PARP. Quercetin affects ERS in many cell types and was shown to relieve hypoxic pulmonary hypertension (HPH) in our previous study. We demonstrated that quercetin evoked excessive GRP78 expression in hypoxic PASMCs compared with hypoxia alone by evaluating the expression of GRP78. The expression of IRE1α and XBP1s, a cleavage form of XBP1u, was upregulated by quercetin in a dose-dependent manner. Pretreatment with 4u8c reversed the apoptosis-promoting effect of quercetin by inhibiting mitochondrial apoptosis. However, 4u8c amplified the effect of quercetin on proliferation and migration in hypoxic PASMCs. In conclusion, the study demonstrated that the IRE1α-XBP1 pathway is involved in the process of hypoxia-induced pulmonary vascular remodeling; 4u8c could restrain hypoxia-induced cell proliferation and migration and reverse the hypoxia-induced apoptosis arrest, while quercetin excited excessive ERS and the IRE1α pathway in hypoxic PASMCs and promoted apoptosis. Our data suggest that intervening the IRE1α-XBP1 pathway may be useful for hypoxia-induced pulmonary arterial hypertension therapy.

Modulation of the sigma-1 receptor-IRE1 pathway is beneficial in preclinical models of inflammation and sepsis

Sepsis is an often deadly complication of infection in which systemic inflammation damages the vasculature, leading to tissue hypoperfusion and multiple organ failure. Currently, the standard of care for sepsis is predominantly supportive, with few therapeutic options available. Because of increased sepsis incidence worldwide, there is an urgent need for discovery of novel therapeutic targets and development of new treatments. The recently discovered function of the endoplasmic reticulum (ER) in regulation of inflammation offers a potential avenue for sepsis control. Here, we identify the ER-resident protein sigma-1 receptor (S1R) as an essential inhibitor of cytokine production in a preclinical model of septic shock. Mice lacking S1R succumb quickly to hypercytokinemia induced by a sublethal challenge in two models of acute inflammation. Mechanistically, we find that S1R restricts the endonuclease activity of the ER stress sensor IRE1 and cytokine expression but does not inhibit the classical inflammatory signaling pathways. These findings could have substantial clinical implications, as we further find that fluvoxamine, an antidepressant therapeutic with high affinity for S1R, protects mice from lethal septic shock and dampens the inflammatory response in human blood leukocytes. Our data reveal the contribution of S1R to the restraint of the inflammatory response and place S1R as a possible therapeutic target to treat bacterial-derived inflammatory pathology.

Treatment of established TH2 cells with 4μ8c, an inhibitor of IRE1α, blocks IL-5 but not IL-4 secretion

BACKGROUND

T cell activation induces ER stress and upregulates Inositol Requiring Enzyme 1 alpha (IRE1α), an activator of the unfolded protein response (UPR) pathway. Inhibition of IRE1α RNase activity in activated CD4⁺ splenocytes from naïve mice, via treatment of the cells with the commercially available drug 4μ8c upon activation, results in the reduction of the secretion of proteins IL-5, IL-4, and IL-13. Prior to this work, it was unknown if 4μ8c could inhibit TH2 cytokines in established TH2 cells, cells that are crucial in promoting disease in severe asthma.

RESULTS

Treatment of a mouse T helper (TH)2 cell line and differentiated human TH2 cells with 4μ8c resulted in inhibition of IL-5, but not IL-4, as measured by ELISA. The reduced cytokine expression was not due to differences in mRNA stability or mRNA levels; it appears to be due to a defect in secretion, as the cells produce cytokines IL-5 as measured by flow cytometry and western blot.

CONCLUSION

These data suggest that the inhibition of IL-5 was due to post-translational processes. IL-5 promotes chronic, inflammatory asthma, and 4μ8c blocks its expression in T cells in vitro. Future studies will determine if 4μ8c treatment can ameliorate the effects of the cytokine IL-5 in a disease model.

Stabilization of cytokine mRNAs in iNKT cells requires the serine-threonine kinase IRE1alpha

Activated invariant natural killer T (iNKT) cells rapidly produce large amounts of cytokines, but how cytokine mRNAs are induced, stabilized and mobilized following iNKT activation is still unclear. Here we show that an endoplasmic reticulum stress sensor, inositol-requiring enzyme 1α (IRE1α), links key cellular processes required for iNKT cell effector functions in specific iNKT subsets, in which TCR-dependent activation of IRE1α is associated with downstream activation of p38 MAPK and the stabilization of preformed cytokine mRNAs. Importantly, genetic deletion of IRE1α in iNKT cells reduces cytokine production and protects mice from oxazolone colitis. We therefore propose that an IRE1α-dependent signaling cascade couples constitutive cytokine mRNA expression to the rapid induction of cytokine secretion and effector functions in activated iNKT cells.

Invariant natural killer T (iNKT) cells rapidly enhance cytokine secretion and effector function following activation, but the underlying mechanism is still unclear. Here the authors show that an endoplasmic reticulum stress sensor, inositol-requiring enzyme 1α, activates the p38 kinase to stabilize cytokine mRNA for enhanced iNKT functions.

Discovery of tear biomarkers in children with chronic non-infectious anterior uveitis: a pilot study

BACKGROUND

Biomarkers in easily obtained specimens that accurately predict uveitis in children with juvenile idiopathic arthritis (JIA) are needed. Aqueous humor has been studied for biomarkers, but is not routinely available. We evaluated tears from children with chronic anterior uveitis (CAU) for biomarkers reported in aqueous humor. In this pilot study, we used Schirmer strips to collect tears from seven children (nine eyes); three children had JIA- associated uveitis (JIA-U) and four had idiopathic disease (I-CAU). Liquid chromatography-tandem mass spectrometry was used to identify and quantify tear proteins. The Mann-Whitney U test identified differential tear protein expression between children with JIA-U and those with I-CAU.

RESULTS

S100A9, LAP3, TTR, MIF, sCD14, S100A8, and SAA1 were detected in tears of all children; the same cytokines have been reported in aqueous humor of children with JIA-U. Tears from children with JIA-U had higher expression of proteins associated with inflammatory arthritis (SEMA3G, TIMP1, HEXB, ERN1, and SAA1) than tears from those with I-CAU. In addition, we found higher expression of sCD14, S100A8, and SAA1, but lower expression of S100A9, LAP3, TTR, and MIF, in tears from children with JIA-U compared to tears from those with I-CAU.

CONCLUSIONS

Tears contain similar cytokine profiles to aqueous humor in children with CAU and may be a clinically useful source of disease biomarkers. Tears from children with JIA-U also contain cytokines associated with inflammatory arthritis; furthermore, differential expression of other tear proteins as well may provide clues to intrinsic differences between JIA-U and I-CAU, despite their similar clinical phenotypes.

Cholestasis induced liver pathology results in dysfunctional immune responses after arenavirus infection

Immune responses are critical for defense against pathogens. However, prolonged viral infection can result in defective T cell immunity, leading to chronic viral infection. We studied immune activation in response to arenavirus infection during cholestasis using bile duct ligation (BDL). We monitored T cell responses, virus load and liver pathology markers after infection with lymphocytic choriomeningitis virus (LCMV). BDL mice failed to induce protective anti-viral immunity against LCMV and consequently exhibited chronic viral infection. BDL mice exhibited reduced anti-viral T cell immunity as well as reduced type 1 interferon production early after LCMV infection. Consistently, the presence of serum from BDL mice reduced the responsiveness of dendritic cell (DC) and T cell cultures when compared to Sham controls. Following fractionation and mass spectrometry analyses of sera, we identified several serum factors to be upregulated following BDL including bilirubin, bile acids, 78 kDa Glucose regulated protein (GRP78) and liver enzymes. Bilirubin and GRP78 were capable of inhibiting DC and T cell activation. In this work, we demonstrate that liver damage mediated by cholestasis results in defective immune induction following arenavirus infection.

Farnesoid X receptor signaling activates the hepatic X-box binding protein 1 pathway in vitro and in mice

Bile acids are endogenous ligands of the nuclear receptor, farnesoid X receptor (FXR), and pharmacological FXR modulators are under development for the treatment of several liver disorders. The inositol-requiring enzyme 1α/X-box binding protein 1 (IRE1α/XBP1) pathway of the unfolded protein response (UPR) is a protective cellular signaling pathway activated in response to endoplasmic reticulum (ER) stress. We investigated the role of FXR signaling in activation of the hepatic XBP1 pathway. Mice were treated with deoxycholic acid (DCA), cholestyramine, GW4064, or underwent bile duct ligation (BDL), and hepatic UPR activation was measured. Huh7-Ntcp and HepG2 cells were treated with FXR agonists, inhibitor, small interfering RNA (siRNA), or small heterodimer partner (SHP) siRNA to determine the mechanisms of IRE1α/XBP1 pathway activation. DCA feeding and BDL increased and cholestyramine decreased expression of hepatic XBP1 spliced (XBP1s). XBP1 pathway activation increased in Huh7-Ntcp and HepG2 cells treated with bile acids, 6α-ethyl-chenodeoxycholic acid (6-ECDCA) or GW4064. This effect decreased with FXR knockdown and treatment with the FXR inhibitor guggulsterone. FXR agonists increased XBP1 splicing and phosphorylated IRE1α (p-IRE1α) expression. Overexpression of SHP similarly increased XBP1 splicing, XBP1s, and p-IRE1α protein expression. SHP knockdown attenuated FXR agonist-induced XBP1s and p-IRE1α protein expression. Co-immunoprecipitation (Co-IP) assays demonstrate a physical interaction between overexpressed green fluorescent protein (GFP)-SHP and FLAG-IRE1α in HEK293T cells. Mice treated with GW4064 had increased, and FXR and SHP null mice had decreased, basal Xbp1s gene expression.

CONCLUSION

FXR signaling activates the IRE1α/XBP1 pathway in vivo and in vitro. FXR pathway activation increases XBP1 splicing and enhances p-IRE1α expression. These effects are mediated, at least in part, by SHP. IRE1α/XBP1 pathway activation by bile acids and pharmacological FXR agonists may be protective during liver injury and may have therapeutic implications for liver diseases. (Hepatology 2018;68:304-316).

IRE1α Implications in Endoplasmic Reticulum Stress-Mediated Development and Pathogenesis of Autoimmune Diseases

Inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α) is the most prominent and evolutionarily conserved endoplasmic reticulum (ER) membrane protein. This transduces the signal of misfolded protein accumulation in the ER, named as ER stress, to the nucleus as “unfolded protein response (UPR).” The ER stress-mediated IRE1α signaling pathway arbitrates the yin and yang of cell life. IRE1α has been implicated in several physiological as well as pathological conditions, including immune disorders. Autoimmune diseases are caused by abnormal immune responses that develop due to genetic mutations and several environmental factors, including infections and chemicals. These factors dysregulate the cell immune reactions, such as cytokine secretion, antigen presentation, and autoantigen generation. However, the mechanisms involved, in which these factors induce the onset of autoimmune diseases, are remaining unknown. Considering that these environmental factors also induce the UPR, which is expected to have significant role in secretory cells and immune cells. The role of the major UPR molecule, IRE1α, in causing immune responses is well identified, but its role in inducing autoimmunity and the pathogenesis of autoimmune diseases has not been clearly elucidated. Hence, a better understanding of the role of IRE1α and its regulatory mechanisms in causing autoimmune diseases could help to identify and develop the appropriate therapeutic strategies. In this review, we mainly center the discussion on the molecular mechanisms of IRE1α in the pathophysiology of autoimmune diseases.

IRE1α prevents hepatic steatosis by processing and promoting the degradation of select microRNAs

Obesity or a high-fat diet represses the endoribonuclease activity of inositol-requiring enzyme 1α (IRE1α), a transducer of the unfolded protein response (UPR) in cells under endoplasmic reticulum (ER) stress. An impaired UPR is associated with hepatic steatosis and nonalcoholic fatty liver disease (NAFLD), which is caused by lipid accumulation in the liver. We found that IRE1α was critical to maintaining lipid homeostasis in the liver by repressing the biogenesis of microRNAs (miRNAs) that regulate lipid mobilization. In mice fed normal chow, the endoribonuclease function of IRE1α processed a subset of precursor miRNAs in the liver, including those of the miR-200 and miR-34 families, such that IRE1α promoted their degradation through the process of regulated IRE1-dependent decay (RIDD). A high-fat diet in mice or hepatic steatosis in patients was associated with the S-nitrosylation of IRE1α and inactivation of its endoribonuclease activity. This resulted in an increased abundance of these miRNA families in the liver and, consequently, a decreased abundance of their targets, which included peroxisome proliferator-activated receptor α (PPARα) and the deacetylase sirtuin 1 (SIRT1), regulators of fatty acid oxidation and triglyceride lipolysis. IRE1α deficiency exacerbated hepatic steatosis in mice. The abundance of the miR-200 and miR-34 families was also increased in cultured, lipid-overloaded hepatocytes and in the livers of patients with hepatic steatosis. Our findings reveal a mechanism by which IRE1α maintains lipid homeostasis through its regulation of miRNAs, a regulatory pathway distinct from the canonical IRE1α-UPR pathway under acute ER stress.

ER Stress: A Therapeutic Target in Rheumatoid Arthritis?

Diverse physiological and pathological conditions that impact on protein folding of the endoplasmic reticulum (ER) cause ER stress. The unfolded protein response (UPR) and the ER-associated degradation (ERAD) pathway are activated to cope with ER stress. In rheumatoid arthritis (RA), inflammation and ER stress work in parallel by driving inflammatory cells to release cytokines that induce chronic ER stress pathways. This chronic ER stress may contribute to the pathogenesis of RA through synoviocyte proliferation and proinflammatory cytokine production. Therefore, ER stress pathways and their constituent elements are attractive targets for RA drug development. In this review, we integrate current knowledge of the contribution of ER stress to the overall pathogenesis of RA, and suggest some therapeutic implications of these discoveries.

The TLR4-IRE1α pathway activation contributes to palmitate-elicited lipotoxicity in hepatocytes

Lipotoxicity induced by saturated fatty acids (SFAs) plays a pathological role in the development of non‐alcoholic fatty liver disease (NAFLD); however, the exact mechanism(s) remain to be clearly elucidated. Toll‐like receptor (TLR) 4 plays a fundamental role in activating the innate immune system. Intriguingly, hepatocytes express TLR4 and machinery for TLR4 signalling pathway. That liver‐specific TLR4 knockout mice are protective against diet‐induced NAFLD suggests that hepatocyte TLR4 signalling pathway plays an important role in NAFLD pathogenesis. Herein, using cultured hepatocytes, we sought to directly examine the role of TLR4 signalling pathway in palmitate‐elicited hepatotoxicity and to elucidate underlying mechanism(s). Our data reveal that palmitate exposure up‐regulates TLR4 expression at both mRNA and protein levels in hepatocytes, which are associated with NF‐κB activation. The inhibition of TLR4 signalling pathway through both pharmacological and genetic approaches abolished palmitate‐induced cell death, suggesting that TLR4 signalling pathway activation contributes to palmitate‐induced hepatotoxicity. Mechanistic investigations demonstrate that inositol‐requiring enzyme 1α (IRE1α), one of three major signal transduction pathways activated during endoplasmic reticulum (ER) stress, is the downstream target of palmitate‐elicited TLR4 activation and mechanistically implicated in TLR4 activation‐triggered cell death in response to palmitate exposure. Collectively, our data identify that the TLR4‐IRE1α pathway activation contributes to palmitate‐elicited lipotoxicity in hepatocytes. Our findings suggest that targeting TLR4‐IRE1α pathway can be a potential therapeutic choice for the treatment of NAFLD as well as other metabolic disorders, with lipotoxicity being the principal pathomechanism.

Immunometabolic Signaling Pathways Contribute to Macrophage and Dendritic Cell Function

Understanding of antigen-presenting cell (APC) participation in tissue inflammation and metabolism has advanced through numerous studies using systems biology approaches. Previously unrecognized connections between these research areas have been elucidated in the context of inflammatory disease involving innate and adaptive immune responses. A new conceptual framework bridges APC biology, metabolism, and cytokines in the generation of effective T-cell responses. Exploring these connections is paramount to addressing the rising tide of multi-organ system diseases, particularly chronic diseases associated with metabolic syndrome, infection, and cancer. Focused research in these areas will aid the development of strategies to harness and manipulate innate immunology to improve vaccine development, anti-viral, anti-inflammatory, and anti-tumor therapies. This review highlights recent advances in APC "immunometabolism" specifically related to chronic viral and metabolic disease in humans. The goal of this review is to develop an abridged and consolidated outlook on recent thematic updates to APC immunometabolism in the areas of regulation and crosstalk between metabolic and inflammatory signaling and the integrated stress response and how these signals dictate APC function in providing T-cell activation Signal 3.

TRAF molecules in inflammation and inflammatory diseases

Purpose of Review

This review presents an overview of the current knowledge of TRAF molecules in inflammation with an emphasis on available human evidence and direct in vivo evidence of mouse models that demonstrate the contribution of TRAF molecules in the pathogenesis of inflammatory diseases.

Recent Findings

The tumor necrosis factor receptor (TNF-R)-associated factor (TRAF) family of cytoplasmic proteins was initially identified as signaling adaptors that bind directly to the intracellular domains of receptors of the TNF-R superfamily. It is now appreciated that TRAF molecules are widely employed in signaling by a variety of adaptive and innate immune receptors as well as cytokine receptors. TRAF-dependent signaling pathways typically lead to the activation of nuclear factor-κBs (NF-κBs), mitogen-activated protein kinases (MAPKs), or interferon-regulatory factors (IRFs). Most of these signaling pathways have been linked to inflammation, and therefore TRAF molecules were expected to regulate inflammation and inflammatory responses since their discovery in 1990s. However, direct in vivo evidence of TRAFs in inflammation and especially in inflammatory diseases had been lacking for many years, partly due to the difficulty imposed by early lethality of TRAF2^-/-, TRAF3^-/-, and TRAF6^-/- mice. With the creation of conditional knockout and lineage-specific transgenic mice of different TRAF molecules, our understanding about TRAFs in inflammation and inflammatory responses has rapidly advanced during the past decade.

Summary

Increasing evidence indicates that TRAF molecules are versatile and indispensable regulators of inflammation and inflammatory responses and that aberrant expression or function of TRAFs contributes to the pathogenesis of inflammatory diseases.

Senescence-associated secretory phenotype contributes to pathological angiogenesis in retinopathy

Pathological angiogenesis is the hallmark of diseases such as cancer and retinopathies. Although tissue hypoxia and inflammation are recognized as central drivers of vessel growth, relatively little is known about the process that bridges the two. In a mouse model of ischemic retinopathy, we found that hypoxic regions of the retina showed only modest rates of apoptosis despite severely compromised metabolic supply. Using transcriptomic analysis and inducible loss-of-function genetics, we demonstrated that ischemic retinal cells instead engage the endoplasmic reticulum stress inositol-requiring enzyme 1α (IRE1α) pathway that, through its endoribonuclease activity, induces a state of senescence in which cells adopt a senescence-associated secretory phenotype (SASP). We also detected SASP-associated cytokines (plasminogen activator inhibitor 1, interleukin-6, interleukin-8, and vascular endothelial growth factor) in the vitreous humor of patients suffering from proliferative diabetic retinopathy. Therapeutic inhibition of the SASP through intravitreal delivery of metformin or interference with effectors of senescence (semaphorin 3A or IRE1α) in mice reduced destructive retinal neovascularization in vivo. We conclude that the SASP contributes to pathological vessel growth, with ischemic retinal cells becoming prematurely senescent and secreting inflammatory cytokines that drive paracrine senescence, exacerbate destructive angiogenesis, and hinder reparative vascular regeneration. Reversal of this process may be therapeutically beneficial.

Palmitate inhibits arthritis by inducing t-bet and gata-3 mRNA degradation in iNKT cells via IRE1α-dependent decay

Long chain fatty acids (LCFAs) exert pro-inflammatory effects in vivo. However, little is known regarding the effect of LCFAs on invariant (i) NKT cell functions. Here, we report an inhibitory effect of saturated LCFAs on transcription factors in iNKT cells. Among the saturated LCFAs, palmitic acid (PA) specifically inhibited IL-4 and IFN-γ production and reduced gata-3 and t-bet transcript levels in iNKT cells during TCR-mediated activation. In iNKT cells, PA was localized and induced dilation in the endoplasmic reticulum and increased the mRNA levels of downstream molecules of IRE1α RNase. Moreover, PA increased the degradation rates of gata-3 and t-bet mRNA, which was restored by IRE1α inhibition or transfection with mutant gata-3 or t-bet, indicating that gata-3 and t-bet are cleaved via regulated IRE1α-dependent decay (RIDD). A PA-rich diet and PA injection suppressed IL-4 and IFN-γ production by iNKT cells in C57BL/6, but not Jα18 knockout mice, which was restored by injection of STF083010, an IRE1α-specific inhibitor. Furthermore, a PA-rich diet and PA injection attenuated arthritis in an iNKT cell-dependent manner. Taken together, our experiments demonstrate that a saturated LCFA induced RIDD-mediated t-bet and gata-3 mRNA degradation in iNKT cells, thereby suppressing arthritis.

Endoplasmic reticulum stress cooperates with Toll-like receptor ligation in driving activation of rheumatoid arthritis fibroblast-like synoviocytes

BACKGROUND

Endoplasmic reticulum (ER) stress has proinflammatory properties, and transgenic animal studies of rheumatoid arthritis (RA) indicate its relevance in the process of joint destruction. Because currently available studies are focused primarily on myeloid cells, we assessed how ER stress might affect the inflammatory responses of stromal cells in RA.

METHODS

ER stress was induced in RA fibroblast-like synoviocytes (FLS), dermal fibroblasts, and macrophages with thapsigargin or tunicamycin alone or in combination with Toll-like receptor (TLR) ligands, and gene expression and messenger RNA (mRNA) stability was measured by quantitative polymerase chain reaction. Cellular viability was measured using cell death enzyme-linked immunosorbent assays and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, and signaling pathway activation was analyzed by immunoblotting.

RESULTS

No cytotoxicity was observed in FLS exposed to thapsigargin, despite significant induction of ER stress markers. Screening of 84 proinflammatory genes revealed minor changes in their expression (fold change 90th percentile range 2.8-8.3) by thapsigargin alone, but the vast majority were hyperinduced during combined stimulation with thapsigargin and TLR ligands (35% greater than fivefold vs lipopolysaccharide alone). The synergistic response could not be explained by quantitative effects on nuclear factor-κB and mitogen-activated protein kinase pathways alone, but it was dependent on increased mRNA stability. mRNA stabilization was similarly enhanced by ER stress in dermal fibroblasts but not in macrophages, correlating with minimal cooperative effects on gene induction in macrophages.

CONCLUSIONS

RA FLS are resistant to apoptosis induced by ER stress, but ER stress potentiates their activation by multiple TLR ligands. Interfering with downstream signaling pathway components of ER stress may be of therapeutic potential in the treatment of RA.

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

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

Physiological/pathological ramifications of transcription factors in the unfolded protein response

Numerous environmental, physiological, and pathological insults disrupt protein-folding homeostasis in the endoplasmic reticulum (ER), referred to as ER stress. Eukaryotic cells evolved a set of intracellular signaling pathways, collectively termed the unfolded protein response (UPR), to maintain a productive ER protein-folding environment through reprogramming gene transcription and mRNA translation. The UPR is largely dependent on transcription factors (TFs) that modulate expression of genes involved in many physiological and pathological conditions, including development, metabolism, inflammation, neurodegenerative diseases, and cancer. Here we summarize the current knowledge about these mechanisms, their impact on physiological/pathological processes, and potential therapeutic applications.

Tumorigenic and Immunosuppressive Effects of Endoplasmic Reticulum Stress in Cancer

Malignant cells utilize diverse strategies that enable them to thrive under adverse conditions while simultaneously inhibiting the development of anti-tumor immune responses. Hostile microenvironmental conditions within tumor masses, such as nutrient deprivation, oxygen limitation, high metabolic demand, and oxidative stress, disturb the protein-folding capacity of the endoplasmic reticulum (ER), thereby provoking a cellular state of "ER stress." Sustained activation of ER stress sensors endows malignant cells with greater tumorigenic, metastatic, and drug-resistant capacity. Additionally, recent studies have uncovered that ER stress responses further impede the development of protective anti-cancer immunity by manipulating the function of myeloid cells in the tumor microenvironment. Here, we discuss the tumorigenic and immunoregulatory effects of ER stress in cancer, and we explore the concept of targeting ER stress responses to enhance the efficacy of standard chemotherapies and evolving cancer immunotherapies in the clinic.

Recent Insights into the Role of Unfolded Protein Response in ER Stress in Health and Disease

Unfolded stress response (UPR) is a conserved cellular pathway involved in protein quality control to maintain homeostasis under different conditions and disease states characterized by cell stress. Although three general schemes of and genes induced by UPR are rather well-established, open questions remain including the precise role of UPR in human diseases and the interactions between different sensor systems during cell stress signaling. Particularly, the issue how the normally adaptive and pro-survival UPR pathway turns into a deleterious process causing sustained endoplasmic reticulum (ER) stress and cell death requires more studies. UPR is also named a friend with multiple personalities that we need to understand better to fully recognize its role in normal physiology and in disease pathology. UPR interacts with other organelles including mitochondria, and with cell stress signals and degradation pathways such as autophagy and the ubiquitin proteasome system. Here we review current concepts and mechanisms of UPR as studied in different cells and model systems and highlight the relevance of UPR and related stress signals in various human diseases.

Targeting IRE1 with small molecules counteracts progression of atherosclerosis

Metaflammation, an atypical, metabolically induced, chronic low-grade inflammation, plays an important role in the development of obesity, diabetes, and atherosclerosis. An important primer for metaflammation is the persistent metabolic overloading of the endoplasmic reticulum (ER), leading to its functional impairment. Activation of the unfolded protein response (UPR), a homeostatic regulatory network that responds to ER stress, is a hallmark of all stages of atherosclerotic plaque formation. The most conserved ER-resident UPR regulator, the kinase/endoribonuclease inositol-requiring enzyme 1 (IRE1), is activated in lipid-laden macrophages that infiltrate the atherosclerotic lesions. Using RNA sequencing in macrophages, we discovered that IRE1 regulates the expression of many proatherogenic genes, including several important cytokines and chemokines. We show that IRE1 inhibitors uncouple lipid-induced ER stress from inflammasome activation in both mouse and human macrophages. In vivo, these IRE1 inhibitors led to a significant decrease in hyperlipidemia-induced IL-1β and IL-18 production, lowered T-helper type-1 immune responses, and reduced atherosclerotic plaque size without altering the plasma lipid profiles in apolipoprotein E-deficient mice. These results show that pharmacologic modulation of IRE1 counteracts metaflammation and alleviates atherosclerosis.

Cold-inducible RNA-binding protein (CIRP) causes sepsis-associated acute lung injury via induction of endoplasmic reticulum stress

Cold-inducible RNA-binding protein (CIRP), released into the circulation during sepsis, causes lung injury via an as yet unknown mechanism. Since endoplasmic reticulum (ER) stress is associated with acute lung injury (ALI), we hypothesized that CIRP causes ALI via induction of ER stress. To test this hypothesis, we studied the lungs of wild-type (WT) and CIRP knockout (KO) mice at 20 h after induction of sepsis by cecal ligation and puncture (CLP). WT mice had significantly more severe ALI than CIRP KO mice. Lung ER stress markers (BiP, pIRE1α, sXBP1, CHOP, cleaved caspase-12) were increased in septic WT mice, but not in septic CIRP KO mice. Effector pathways downstream from ER stress – apoptosis, NF-κB (p65), proinflammatory cytokines (IL-6, IL-1β), neutrophil chemoattractants (MIP-2, KC), neutrophil infiltration (MPO activity), lipid peroxidation (4-HNE), and nitric oxide (iNOS) – were significantly increased in WT mice, but only mildly elevated in CIRP KO mice. ER stress markers were increased in the lungs of healthy WT mice treated with recombinant murine CIRP, but not in the lungs of TLR4 KO mice. This suggests CIRP directly induces ER stress via TLR4 activation. In summary, CIRP induces lung ER stress and downstream responses to cause sepsis-associated ALI.

The burgeoning field of innate immune-mediated disease and autoinflammation

Immune-mediated autoinflammatory diseases are occupying an increasingly prominent position among the pantheon of debilitating conditions that afflict humankind. This review focuses on some of the key developments that have occurred since the original description of autoinflammatory disease in 1999, and focuses on underlying mechanisms that trigger autoinflammation. The monogenic autoinflammatory disease range has expanded considerably during that time, and now includes a broad spectrum of disorders, including relatively common conditions such as cystic fibrosis and subsets of systemic lupus erythematosus. The innate immune system also plays a key role in the pathogenesis of complex inflammatory disorders. We have proposed a new nomenclature to accommodate the rapidly increasing number of monogenic disorders, which predispose to either autoinflammation or autoimmunity or, indeed, combinations of both. This new terminology also encompasses a wide spectrum of genetically determined autoinflammatory diseases, with variable clinical manifestations of immunodeficiency and immune dysregulation/autoimmunity. We also explore some of the ramifications of the breakthrough discovery of the physiological role of pyrin and the search for identifiable factors that may serve to trigger attacks of autoinflammation. The evidence that pyrin, as part of the pyrin inflammasome, acts as a sensor of different inactivating bacterial modification Rho GTPases, rather than interacting directly with these microbial products, sets the stage for a better understanding of the role of microorganisms and infections in the autoinflammatory disorders. Finally, we discuss some of the triggers of autoinflammation as well as potential therapeutic interventions aimed at enhancing autophagy and proteasome degradation pathways. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The myeloid heat shock transcription factor 1/β-catenin axis regulates NLR family, pyrin domain-containing 3 inflammasome activation in mouse liver ischemia/reperfusion injury

Heat shock transcription factor 1 (HSF1) has been implicated in the differential regulation of cell stress and disease states. β-catenin activation is essential for immune homeostasis. However, little is known about the role of macrophage HSF1-β-catenin signaling in the regulation of NLRP3 inflammasome activation during ischemia/reperfusion (I/R) injury (IRI) in the liver. This study investigated the functions and molecular mechanisms by which HSF1-β-catenin signaling influenced NLRP3-mediated innate immune response in vivo and in vitro. Using a mouse model of IR-induced liver inflammatory injury, we found that mice with a myeloid-specific HSF1 knockout (HSF1^M-KO ) displayed exacerbated liver damage based on their increased serum alanine aminotransferase levels, intrahepatic macrophage/neutrophil trafficking, and proinflammatory interleukin (IL)-1β levels compared to the HSF1-proficient (HSF1^FL/FL ) controls. Disruption of myeloid HSF1 markedly increased transcription factor X-box-binding protein (XBP1), NLR family, pyrin domain-containing 3 (NLRP3), and cleaved caspase-1 expression, which was accompanied by reduced β-catenin activity. Knockdown of XBP1 in HSF1-deficient livers using a XBP1 small interfering RNA ameliorated hepatocellular functions and reduced NLRP3/cleaved caspase-1 and IL-1β protein levels. In parallel in vitro studies, HSF1 overexpression increased β-catenin (Ser552) phosphorylation and decreased reactive oxygen species (ROS) production in bone-marrow-derived macrophages. However, myeloid HSF1 ablation inhibited β-catenin, but promoted XBP1. Furthermore, myeloid β-catenin deletion increased XBP1 messenger RNA splicing, whereas a CRISPR/CRISPR-associated protein 9-mediated XBP1 knockout diminished NLRP3/caspase-1.

CONCLUSION

The myeloid HSF1-β-catenin axis controlled NLRP3 activation by modulating the XBP1 signaling pathway. HSF1 activation promoted β-catenin, which, in turn, inhibited XBP1, leading to NLRP3 inactivation and reduced I/R-induced liver injury. These findings demonstrated that HSF1/β-catenin signaling is a novel regulator of innate immunity in liver inflammatory injury and implied the therapeutic potential for management of sterile liver inflammation in transplant recipients. (Hepatology 2016;64:1683-1698).

Toll-like Receptor (TLR) Signaling Interacts with CREBH to Modulate High-density Lipoprotein (HDL) in Response to Bacterial Endotoxin

Bacterial endotoxin can induce inflammatory and metabolic changes in the host. In this study, we revealed a molecular mechanism by which a stress-inducible, liver-enriched transcription factor, cAMP-responsive element-binding protein hepatic-specific (CREBH), modulates lipid profiles to protect the liver from injuries upon the bacterial endotoxin lipopolysaccharide (LPS). LPS challenge can activate CREBH in mouse liver tissues in a toll-like receptor (TLR)/MyD88-dependent manner. Upon LPS challenge, CREBH interacts with TNF receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase that functions as a key mediator of TLR signaling, and this interaction relies on MyD88. Further analysis demonstrated that TRAF6 mediates K63-linked ubiquitination of CREBH to facilitate CREBH cleavage and activation. CREBH directly activates expression of the gene encoding Apolipoprotein A4 (ApoA4) under LPS challenge, leading to modulation of high-density lipoprotein (HDL) in animals. CREBH deficiency led to reduced production of circulating HDL and increased liver damage upon high-dose LPS challenge. Therefore, TLR/MyD88-dependent, TRAF6-facilitated CREBH activation represents a mammalian hepatic defense response to bacterial endotoxin by modulating HDL.

Toll-like receptors in the pathogenesis of autoimmune diseases: recent and emerging translational developments

Autoinflammatory diseases are defined as the loss of self-tolerance in which an inflammatory response to self-antigens occurs, which are a significant global burden. Toll-like receptors are key pattern recognition receptors, which integrate signals leading to the activation of transcription factors and ultimately proinflammatory cytokines. Recently, it has become apparent that these are at the nexus of autoinflammatory diseases making them viable and attractive drug targets. The aim of this review was to evaluate the role of innate immunity in autoinflammatory conditions alongside the role of negative regulation while suggesting possible therapeutic targets.

The ER membrane-anchored ubiquitin ligase Hrd1 is a positive regulator of T-cell immunity

Identification of positive regulators of T-cell immunity induced during autoimmune diseases is critical for developing novel therapies. The endoplasmic reticulum resident ubiquitin ligase Hrd1 has recently emerged as a critical regulator of dendritic cell antigen presentation, but its role in T-cell immunity is unknown. Here we show that genetic deletion of Hrd1 in mice inhibits T-cell proliferation, production of IL-2, and differentiation of Th1 and Th17 cells, and consequently protects mice from experimental autoimmune encephalomyelitis. Hrd1 facilitates T-cell proliferation by the destruction of cyclin-dependent kinase inhibitor p27(kip1), and deletion of p27(kip1) in Hrd1-null T-cells rescues proliferative capacity but not the production of cytokines, including IL-2, IFN-γ and IL-17. T-cell expression of Hrd1 is higher in patients with multiple sclerosis than in healthy individuals, and knockdown of Hrd1 in human CD4(+) T cells inhibits activation and differentiation to Th1 and Th17 cells. Our study identifies Hrd1 as a previously unappreciated positive regulator of T cells and implies that Hrd1 is a potential therapeutic target for autoimmune diseases.

IRE1α inhibition by natural compound genipin on tumour associated macrophages reduces growth of hepatocellular carcinoma

Accumulating evidences postulated the influential roles of macrophages in mediating hepatocellular carcinoma (HCC) initiation and progression. In this study, we demonstrate that a small molecule, genipin reduced HCC growth through suppressing IRE1α-mediated infiltration and priming of tumour associated macrophages (TAMs). Oral administration of genipin (30mg/kg/2days) suppressed orthotopic HCC tumour growth without challenging the viability and proliferation of HCC cells. Genipin reduced infiltration of inflammatory monocytes into liver and tumour thereby suppressed TAMs presence in HCC microenvironment. Suppression of HCC growth was diminished in HCC-implanted mice with depletion of TAMs by liposome clodronate. Genipin inhibited the TAMs migration, and reduced expression of TAMs-derived inflammatory cytokines that favors HCC proliferation. This is revealed by the in vivo deletion of IRE1α on TAMs in genipin-treated HCC-implanted mice. Diminishing IRE1α neutralised the inhibitory effect of genipin on TAMs. Silencing the expression of IRE1α greatly reduced TAMs migration and expression of inflammatory cytokines that prime HCC proliferation. Suppression of IRE1α led to reduced XBP-1 splicing and NF-κB activation. The reduced association of IRE1α with TRAF2 and IKK complex may be responsible for the genipin-mediated inactivation of NF-κB. The findings show the important role of TAMs in inhibitory effect of genipin on HCC, and TAMs-expressing IRE1α as a promising target for disrupting the tumour environment that favor of HCC development.

The unfolded protein response in immunity and inflammation

The unfolded protein response (UPR) is a highly conserved pathway that allows the cell to manage endoplasmic reticulum (ER) stress that is imposed by the secretory demands associated with environmental forces. In this role, the UPR has increasingly been shown to have crucial functions in immunity and inflammation. In this Review, we discuss the importance of the UPR in the development, differentiation, function and survival of immune cells in meeting the needs of an immune response. In addition, we review current insights into how the UPR is involved in complex chronic inflammatory diseases and, through its role in immune regulation, antitumour responses.

ENDOPLASMIC RETICULUM STRESS IN SEPSIS

Sepsis is an enormous public health issue and the leading cause of death in critically ill patients in intensive care units. Overwhelming inflammation, characterized by cytokine storm, oxidative threats, and neutrophil sequestration, is an underlying component of sepsis-associated organ failure. Despite recent advances in sepsis research, there is still no effective treatment available beyond the standard of care and supportive therapy. To reduce sepsis-related mortality, a better understanding of the biological mechanism associated with sepsis is essential. Endoplasmic reticulum (ER), a subcellular organelle, is responsible for the facilitation of protein folding and assembly and involved in several other physiological activities. Under stress and inflammatory conditions, ER loses homeostasis in its function, which is termed ER stress. During ER stress, unfolded protein response (UPR) is activated to restore ER function to its normal balance. However, once stress is beyond the compensatory capacity of UPR or protracted, apoptosis would be initiated by triggering cell injuries, even cell death. As such, ER stress and UPR are reported to be implicated in several pathological and inflammatory conditions. Although the detrimental role of ER stress during infections has been demonstrated, there is growing evidence that ER stress participates in the pathogenesis of sepsis. In this review, we summarize current research in the context of ER stress and UPR signaling associated with sepsis and its related clinical conditions, such as trauma-hemorrhage and ischemia/reperfusion injury. We also discuss the potential implications of ER stress as a novel therapeutic target and prognostic marker in patients with sepsis.

The role of ER stress in lipid metabolism and lipotoxicity

The endoplasmic reticulum (ER) is a cellular organelle important for regulating calcium homeostasis, lipid metabolism, protein synthesis, and posttranslational modification and trafficking. Numerous environmental, physiological, and pathological insults disturb ER homeostasis, referred to as ER stress, in which a collection of conserved intracellular signaling pathways, termed the unfolded protein response (UPR), are activated to maintain ER function for cell survival. However, excessive and/or prolonged UPR activation leads to initiation of self-destruction through apoptosis. Excessive accumulation of lipids and their intermediate products causes metabolic abnormalities and cell death, called lipotoxicity, in peripheral organs, including the pancreatic islets, liver, muscle, and heart. Because accumulating evidence links chronic ER stress and defects in UPR signaling to lipotoxicity in peripheral tissues, understanding the role of ER stress in cell physiology is a topic under intense investigation. In this review, we highlight recent findings that link ER stress and UPR signaling to the pathogenesis of peripheral organs due to lipotoxicity.

Protein misfolding in the endoplasmic reticulum as a conduit to human disease

In eukaryotic cells, the endoplasmic reticulum is essential for the folding and trafficking of proteins that enter the secretory pathway. Environmental insults or increased protein synthesis often lead to protein misfolding in the organelle, the accumulation of misfolded or unfolded proteins - known as endoplasmic reticulum stress - and the activation of the adaptive unfolded protein response to restore homeostasis. If protein misfolding is not resolved, cells die. Endoplasmic reticulum stress and activation of the unfolded protein response help to determine cell fate and function. Furthermore, endoplasmic reticulum stress contributes to the aetiology of many human diseases.

Tumor Necrosis Factor-α Attenuates the Osteogenic Differentiation Capacity of Periodontal Ligament Stem Cells by Activating PERK Signaling

BACKGROUND

Human periodontal ligament stem cells (PDLSCs) display efficient osteogenic differentiation capacity but fail to rescue bone breakdown associated with periodontitis. Endoplasmic reticulum (ER) stress and the unfolded protein response have recently been linked to inflammation and osteogenic differentiation. Therefore, the role of the double-stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK) pathway in the impaired osteogenic differentiation ability of PDLSCs treated with tumor necrosis factor (TNF)-α was investigated.

METHODS

PDLSCs were isolated and stimulated with osteogenic media containing 1, 10, or 20 ng/mL TNF-α. Assessment included: 1) expression of runt-related transcription factor 2 and osteocalcin; 2) mRNA expression and activity of alkaline phosphatase; and 3) formation of mineralization nodules. Furthermore, expression of PERK pathway-related factors: 1) glucose-regulated protein (GRP) 78; 2) PERK; 3) activating transcription factor (ATF) 4; and 4) CCAAT-enhancer-binding proteins (C/EBP) homologous protein were also measured. Osteogenic differentiation and inhibition of the PERK pathway were also examined in cells pretreated with an inhibitor of ER stress, 4-phenylbutyric acid (PBA), followed by TNF-α stimulation. Finally, PERK small interfering RNA was used to examine osteogenic differentiation attenuated by TNF-α.

RESULTS

Higher concentrations of TNF-α (10 and 20 ng/mL) impaired osteogenic differentiation of PDLSCs but activated the PERK pathway. Pretreatment of PDLSCs with lower concentrations of 4-PBA prevented the TNF-α-induced upregulation of GRP78, PERK, and ATF4 and recovered differentiation ability. Finally, PERK knockdown also restored osteogenic differentiation.

CONCLUSION

TNF-α attenuates osteogenic differentiation ability of PDLSCs through activation of the PERK pathway.

X-Box-Binding Protein 1 and Innate Immune Responses of Human Cystic Fibrosis Alveolar Macrophages

RATIONALE

Alveolar macrophages (AMs) play a key role in host defense to inhaled bacterial pathogens, in part by secreting inflammatory mediators. Cystic fibrosis (CF) airways exhibit a persistent, robust inflammatory response that may contribute to the pathophysiology of CF. Recent findings have linked endoplasmic reticulum stress responses mediated by inositol-requiring enzyme 1α-dependent messenger RNA splicing (activation) of X-box-binding protein-1 (XBP-1s) to inflammation in peripheral macrophages. However, the role of XBP-1s in CF AM function is not known.

OBJECTIVES

To evaluate inflammatory responses of AMs from chronically infected/inflamed human CF lungs and test whether XBP-1s is required for AM-mediated inflammation.

METHODS

Basal and LPS-induced inflammatory responses were evaluated in primary cultures of non-CF versus CF AMs. XBP-1s was measured and its function was evaluated in AMs using 8-formyl-7-hydroxy-4-methylcoumarin (4μ8C), an inhibitor of inositol-requiring enzyme 1α-dependent XBP-1s, and in THP-1 cells stably expressing XBP-1 shRNA, XBP-1s, or a dominant-negative XBP-1.

MEASUREMENTS AND MAIN RESULTS

CF AMs exhibited exaggerated basal and LPS-induced production of tumor necrosis factor-α and IL-6, and these responses were coupled to increased levels of XBP-1s. In non-CF and CF AMs, LPS-induced cytokine production was blunted by 4µ8C. A role for XBP-1s in AM inflammatory responses was further established by data from dTHP-1 cells indicating that expression of XBP-1 shRNA reduced XBP-1s levels and LPS-induced inflammatory responses; and LPS-induced inflammation was up-regulated by expression of XBP-1s and inhibited by dominant-negative XBP-1.

CONCLUSIONS

These findings suggest that AMs contribute to the robust inflammation of CF airways via an up-regulation of XBP-1s-mediated cytokine production.

Molecular Pathways: Immunosuppressive Roles of IRE1α-XBP1 Signaling in Dendritic Cells of the Tumor Microenvironment

The endoplasmic reticulum (ER) is a massive cytoplasmic membrane network that functions primarily to ensure proper folding and posttranslational modification of newly synthesized secreted and transmembrane proteins. Abnormal accumulation of unfolded proteins in this organelle causes a state of "ER stress," which is a hallmark feature of various diseases, including cancer, neurodegeneration, and metabolic dysfunction. Cancer cells exploit the IRE1α-XBP1 arm of the ER stress response to efficiently adjust their protein-folding capacity and ensure survival under hostile tumor microenvironmental conditions. However, we recently found that dendritic cells (DC) residing in the ovarian cancer microenvironment also experience sustained ER stress and demonstrate persistent activation of the IRE1α-XBP1 pathway. This previously unrecognized process disrupts metabolic homeostasis and antigen-presenting capacity in DCs, thereby crippling their natural ability to support the protective functions of infiltrating antitumor T cells. In this review, we briefly discuss some of the mechanisms that fuel ER stress in tumor-associated DCs, the biologic processes altered by aberrant IRE1α-XBP1 signaling in these innate immune cells, and the unique immunotherapeutic potential of targeting this pathway in cancer hosts. Clin Cancer Res; 22(9); 2121-6. ©2016 AACR.

Molecular and cellular mechanisms linking inflammation to insulin resistance and β-cell dysfunction

Obesity is a major public health problem worldwide, and it is associated with an increased risk of developing type 2 diabetes. It is now commonly accepted that chronic inflammation associated with obesity induces insulin resistance and β-cell dysfunction in diabetic patients. Obesity-associated inflammation is characterized by increased abundance of macrophages and enhanced production of inflammatory cytokines in adipose tissue. Adipose tissue macrophages are suggested to be the major source of local and systemic inflammatory mediators such as tumor necrosis factor α, interleukin (IL)-1β, and IL-6. These cytokines induce insulin resistance in insulin target tissues by activating the suppressors of cytokine signaling proteins, several kinases such as c-Jun N-terminal kinase, IκB kinase β, and protein kinase C, inducible nitric oxide synthase, extracellular signal-regulated kinase, and protein tyrosine phosphatases such as protein tyrosine phosphatase 1B. These activated factors impair the insulin signaling at the insulin receptor and the insulin receptor substrates levels. The same process most likely occurs in the pancreas as it contains a pool of tissue-resident macrophages. High concentrations of glucose or palmitate via the chemokine production promote further immune cell migration and infiltration into the islets. These events ultimately induce inflammatory responses leading to the apoptosis of the pancreatic β cells. In this review, the cellular and molecular players that participate in the regulation of obesity-induced inflammation are discussed, with particular attention being placed on the roles of the molecular players linking inflammation to insulin resistance and β-cell dysfunction.

Cross talk between ER stress, oxidative stress, and inflammation in health and disease

In mammals, endoplasmic reticulum (ER) stress, oxidative stress, and inflammatory responses compose the major defense networks that help the cells adapt to and survive stress conditions caused by biochemical, physiological and pathological stimuli. However, chronic ER stress, oxidative stress, or inflammation have been found to be associated with the initiation and progression of a variety of human diseases in the modern world. Under many pathophysiologic conditions, ER stress response, oxidative stress, and inflammatory responses are integrated and amplified in specialized cell types to facilitate the progression of disease. In the past few decades, ER stress response, oxidative stress, and inflammation as well as their interactive relationships have been hot research topics in biomedicine. In this review, we summarize the recent advance in our understanding of the cross talk between ER stress response, oxidative stress, and inflammation in immunity and in inflammatory and metabolic diseases.

Inhibition of host cell translation elongation by Legionella pneumophila blocks the host cell unfolded protein response

Cells of the innate immune system recognize bacterial pathogens by detecting common microbial patterns as well as pathogen-specific activities. One system that responds to these stimuli is the IRE1 branch of the unfolded protein response (UPR), a sensor of endoplasmic reticulum (ER) stress. Activation of IRE1, in the context of Toll-like receptor (TLR) signaling, induces strong proinflammatory cytokine induction. We show here that Legionella pneumophila, an intravacuolar pathogen that replicates in an ER-associated compartment, blocks activation of the IRE1 pathway despite presenting pathogen products that stimulate this response. L. pneumophila TLR ligands induced the splicing of mRNA encoding XBP1s, the main target of IRE1 activity. L. pneumophila was able to inhibit both chemical and bacterial induction of XBP1 splicing via bacterial translocated proteins that interfere with host protein translation. A strain lacking five translocated translation elongation inhibitors was unable to block XBP1 splicing, but this could be rescued by expression of a single such inhibitor, consistent with limitation of the response by translation elongation inhibitors. Chemical inhibition of translation elongation blocked pattern recognition receptor-mediated XBP1 splicing, mimicking the effects of the bacterial translation inhibitors. In contrast, host cell-promoted inhibition of translation initiation in response to the pathogen was ineffective in blocking XBP1 splicing, demonstrating the need for the elongation inhibitors for protection from the UPR. The inhibition of host translation elongation may be a common strategy used by pathogens to limit the innate immune response by interfering with signaling via the UPR.

The IRE1α/XBP1s Pathway Is Essential for the Glucose Response and Protection of β Cells

Although glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

Endoplasmic Reticulum Stress Activates the Inflammasome via NLRP3- and Caspase-2-Driven Mitochondrial Damage

Endoplasmic reticulum (ER) stress is observed in many human diseases, often associated with inflammation. ER stress can trigger inflammation through nucleotide-binding domain and leucine-rich repeat containing (NLRP3) inflammasome, which might stimulate inflammasome formation by association with damaged mitochondria. How ER stress triggers mitochondrial dysfunction and inflammasome activation is ill defined. Here we have used an infection model to show that the IRE1α ER stress sensor regulates regulated mitochondrial dysfunction through an NLRP3-mediated feed-forward loop, independently of ASC. IRE1α activation increased mitochondrial reactive oxygen species, promoting NLRP3 association with mitochondria. NLRP3 was required for ER stress-induced cleavage of caspase-2 and the pro-apoptotic factor, Bid, leading to subsequent release of mitochondrial contents. Caspase-2 and Bid were necessary for activation of the canonical inflammasome by infection-associated or general ER stress. These data identify an NLRP3-caspase-2-dependent mechanism that relays ER stress to the mitochondria to promote inflammation, integrating cellular stress and innate immunity.

Redox distress and genetic defects conspire in systemic autoinflammatory diseases

Inflammation is initiated by innate immune cell activation after contact with pathogens or tissue injury. An increasing number of observations have suggested that cellular stress, in the absence of infection or evident damage, can also induce inflammation. Thus, inflammation can be triggered by exogenous pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs)-so-called classic inflammation-or by endogenous stress resulting from tissue or cellular dysfunction. External triggers and cellular stress activate the same molecular pathways, possibly explaining why classic and stress-induced inflammation have similar clinical manifestations. In some systemic autoinflammatory diseases (SAIDs), inflammatory cells exhibit reduction-oxidation (redox) distress, having high levels of reactive oxygen species (ROS), which promote proinflammatory cytokine production and contribute to the subversion of mechanisms that self-limit inflammation. Thus, SAIDs can be viewed as a paradigm of stress-related inflammation, being characterized by recurrent flares or chronic inflammation (with no recognizable external triggers) and by a failure to downmodulate this inflammation. Here, we review SAID pathophysiology, focusing on the major cytokines and DAMPs, and on the key roles of redox distress. New therapeutic opportunities to tackle SAIDs by blocking stress-induced pathways and control the response to stress in patients are also discussed.