Mediator Of Unfolded Protein Response Research Articles

IRE1α regulates ROS and immune responses after UVB irradiation

Objective UV irradiation of the skin induces photo damage and generates cytotoxic intracellular reactive oxygen species (ROS), activating the unfolded protein response (UPR) to adapt or reduce these UVB-mediated damages. This study was designed to understand the role of the UPR mediator IRE1α in the antioxidant response following UVB irradiation of mouse skin and keratinocytes. Methods We used mice with an epidermal deletion of IRE1α and primary mouse keratinocytes to examine effects of UV on different parameters of the antioxidant response in the presence and absence of functional IRE1α. Results In the absence of IRE1α, PERK activity and protein levels are significantly compromised following UVB irradiation. Additionally, the loss of IRE1α suppressed phosphorylation of the PERK target, nuclear factor erythroid-2-related factor 2 (NRF2), and NRF2-dependent antioxidant gene expression after UVB irradiation. Interestingly, IRE1α-deficient keratinocytes exhibit elevated basal ROS levels, while a robust ROS induction upon UVB exposure is abolished. Because UVB-induced ROS plays an essential role in regulating skin inflammation, we analyzed recruited immune cell populations and the expression of pro-inflammatory cytokines, Il-6 and Tnfα, in mice with epidermally targeted deletion of Ire1α. Following UVB irradiation, there was significantly less recruitment of neutrophils and leukocytes and reduced expression of pro-inflammatory cytokine genes in the skin of mice lacking IRE1α. Furthermore, keratinocyte proliferation was also significantly reduced after chronic UVB exposure in the skin of these mice. Conclusion Collectively, our findings indicate that IRE1α is essential for basal and UVB-induced oxidative stress response, UV-induced skin immune responses, and keratinocyte proliferation. Significance statement These findings shed new light on the protective function of IRE1α in the response to UV. IRE1α plays an important role in the regulation of ROS, PERK stability, and antioxidant gene expression in response to UVB in mouse keratinocytes and epidermis.

Kisspeptin-10 Improves Testicular Redox Status but Does Not Alter the Unfolded Protein Response (UPR) That Is Downregulated by Hypothyroidism in a Rat Model.

Hypothyroidism compromises the testicular redox status and is associated with reduced sperm quality and infertility in men. In this regard, studies have demonstrated the antioxidant potential of kisspeptin in reproductive and metabolic diseases. In this study, we evaluate the effects of kisspeptin-10 (Kp10) on the testicular redox, as well as mediators of the unfolded protein response (UPR) in adult rats with hypothyroidism. Adult male Wistar rats were randomly separated into the Control (n = 15), Hypo (n = 13) and Hypo + Kp10 (n = 14) groups, and hypothyroidism was induced with 6-propyl-2-thiouracil (PTU) for three months. In the last month, half of the hypothyroid animals received Kp10. Testis samples were collected for enzymatic, immunohistochemical and/or gene evaluation of mediators of oxidative stress (TBARs, lipid hydroperoxides (LOOH), ROS, peroxynitrite, SOD, CAT and GPX), endoplasmic reticulum stress (GRP78, ATF6, PERK, CHOP, HO-1 and sXBP1) and antiapoptocytes (BCL-2). Hypothyroidism increased apoptosis index, TBARS and LOOH concentrations, and reduced testicular gene expression of Sod1, Sod2 and Gpx1, as well as the expression of Grp78, Atf6, Ho1 and Chop. Treatment with Kp10, in turn, reduced testicular apoptosis and the production of peroxynitrite, while increased SOD1 and GPX ½ expression, and enzymatic activity of CAT, but did not affect the lower expression of UPR mediators caused by hypothyroidism. This study demonstrated that hypothyroidism causes oxidative stress and dysregulated the UPR pathway in rat testes and that, although Kp10 does not influence the low expression of UPR mediators, it improves the testicular redox status, configuring it as an important antioxidant factor in situations of thyroid dysfunction.

The plasma membrane-associated transcription factor NAC091 regulates unfolded protein response in Arabidopsis thaliana

Adverse environmental stresses may cause the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER), and the unfolded protein response (UPR) pathway is initiated to mitigate the ER stress. Previous studies demonstrate that NAC062, a plasma membrane-associated transcription factor, plays important roles in promoting cell survival under ER stress conditions in Arabidopsis thaliana. In this study, we identified another plasma membrane-associated transcription factor, NAC091 (also known as ANAC091/TIP), as an important UPR mediator. ER stress induces the expression of NAC091, which is mainly dependent on the ER stress regulators bZIP60 and bZIP28. In addition, NAC091 has transcriptional activation activity, and the truncated form of NAC091 devoid of the C-terminal transmembrane domain (TMD) forms a homodimer in the nucleus. Under ER stress conditions, NAC091 relocates from the plasma membrane to the nucleus and regulates the expression of canonical UPR genes involved in cell survival. Further, the loss-of-function mutant of NAC091 confers impaired ER stress tolerance. Together, these results reveal the important role of NAC091 in ER stress response in Arabidopsis, and demonstrate that NAC091 relays the ER stress signal from the plasma membrane to the nucleus to alleviate ER stress and promote cell survival in plants.

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.

Myeloid-specific deletion of activating transcription factor 6 alpha increases CD11b+ macrophage subpopulations and aggravates lung fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, fibrotic interstitial lung disease of unknown etiology. The accumulation of macrophages is associated with disease pathogenesis. The unfolded protein response (UPR) has been linked to macrophage activation in pulmonary fibrosis. To date, the impact of activating transcription factor 6 alpha (ATF6α), one of the UPR mediators, on the composition and function of pulmonary macrophage subpopulations during lung injury and fibrogenesis is not fully understood. We began by examining the expression of Atf6α in IPF patients' lung single-cell RNA sequencing dataset, archived surgical lung specimens, and CD14+ circulating monocytes. To assess the impact of ATF6α on pulmonary macrophage composition and pro-fibrotic function during tissue remodeling, we conducted an in vivo myeloid-specific deletion of Atf6α. Flow cytometric assessments of pulmonary macrophages were carried out in C57BL/6 and myeloid specific ATF6α-deficient mice in the context of bleomycin-induced lung injury. Our results demonstrated that Atf6α mRNA was expressed in pro-fibrotic macrophages found in the lung of a patient with IPF and in CD14+ circulating monocytes obtained from blood of a patient with IPF. After bleomycin administration, the myeloid-specific deletion of Atf6α altered the pulmonary macrophage composition, expanding CD11b+ subpopulations with dual polarized CD38+ CD206+ expressing macrophages. Compositional changes were associated with an aggravation of fibrogenesis including increased myofibroblast and collagen deposition. A further mechanistic ex vivo investigation revealed that ATF6α was required for CHOP induction and the death of bone marrow-derived macrophages. Overall, our findings suggest a detrimental role for the ATF6α-deficient CD11b+ macrophages which had altered function during lung injury and fibrosis.

Redox profile and mediators of the unfolded protein response (UPR) in the placenta of rats during pregnancy.

Proliferation, differentiation, migration and apoptosis of trophoblastic cells are influenced by hypoxia, as well as adequate modulation of oxidative stress and the unfolded protein response (UPR) pathway. We aimed to evaluate the expression profile of redox and UPR mediators in the placenta of rats throughout pregnancy. Placental expression of hypoxia-inducible factor 1α (HIF1α), 8-Hydroxy-2'-deoxyguanosine (8-OHdG), superoxide dismutase 1 (SOD1), glutathione peroxidase (GPX), catalase (Cat), activating transcription factor 6 (ATF6), protein kinase RNA-like endoplasmic reticulum kinase (PERK), 78 kD glucose-regulated protein (GRP78) and C/EBP-homologous protein (CHOP), as well as reactive oxygen species (ROS) and peroxynitrite production, were evaluated in Wistar rats on the 10th, 12th, 14th, 16th and 18th day of pregnancy (DP). Increased immunostaining of HIF1α was observed on the 16th and 18th DP, while 8-OHdG and ROS production were greater on the 14th DP. SOD1 and Cat had increased immunostaining on the 14th and 18th DP, while staining of GPX1/2, GRP78 and CHOP was greater on the 18th DP. With regard to gene expression, Hif1α and Sod1 showed increased mRNA expression on the 12th and 16th DP, while Gpx1 had increased expression on the 10th and 16th DP. Cat, Perk and Grp78 gene expression was greater on the 14th DP, unlike Atf6, which showed greater expression on the 12th DP. In contrast, Chop maintained increased expression from the 12th to the 18th DP. The placental expression of redox and UPR mediators in rats is influenced by gestational age, with greater expression in periods of greater HIF1α and 8-OHdG expression and at the end of the pregnancy. This study provides data on the physiological modulation of redox and UPR mediators during placental development in rats.

Sel1L, an Endoplasmic Reticulum Associated Degradation Adaptor, Regulates CD8+ Tcell Fate

CD8+ T cells are critical players in protective immunity against intracellular pathogens and cancer. Upon activation with cognate antigen, CD8+ T cells undergo marked increase in protein synthesis and significant proteome remodeling as they clonally expand and acquire effector function. However, little is known about the mechanisms by which CD8+ T cells maintain protein homeostasis (proteostasis) during T cell differentiation and which proteostatic pathways regulate T cell fate. Proteostasis is maintained through two main pathways: ER-associated degradation (ERAD) and the unfolded protein response (UPR). While UPR mediators have been implicated in CD8+ T cell immune responses, nothing is known about the role of ERAD in CD8+ T cell immunity. To interrogate proteostasis following CD8+ T cell activation in vivo, we utilized the murine model of acute viral infection LCMV-Armstrong (LCMV) to evaluate endoplasmic reticulum (ER) stress and proteostatic pathway activation over the course of initial effector differentiation. We found that endogenous LCMV-specific CD8+ T cells experience dynamic ER stress as measured by an increase in ER size [fold change (F.C.) ER Tracker median fluorescence intensity (MFI) relative to D0: D5 7.48± 4.91 *p<0.05; D8: 0.86±1.78, n.s., one-way ANOVA Dunnett's multiple comparisons test] and protein aggregation (Proteostat F.C. relative to D0: D5 1.95± 0.24 *p<0.05; D8 1.6 ±0.11, n.s.) over the course of infection with a peak at day 5 post-infection (p.i.), largely resolving by day 8 p.i. Furthermore, we assessed activation of the UPR pathways and found induction of UPR mediators XBP1s (F.C. relative to D0 D5: 3.96 ± 1.88 **p<0.01 D8 1.60±0.36 n.s.) and CHOP (F.C. relative to D0: D5 12.47±0.32 ***p<0.001; D8 2.58±1.5, n.s.) in LCMV-specific CD8+ T cells at the highest levels on day 5 p.i., which largely resolved by day 8 p.i.. Together, these data suggest that activated virus-specific CD8+ T cells experience dynamic ER stress and upregulation of proteostatic pathways during the course of CD8+ T cell differentiation. To test the cell-intrinsic role of ERAD in CD8+ T cell immune responses, we utilized TCR-transgenic CD8+ T cells that were specific for LCMV (termed P14 cells) and were deficient in Sel1L, a critical adaptor of the ERAD complex that recognizes misfolded proteins in ER and recruits them to be retro-translocated to cytosol for proteasomal degradation. We adoptively transferred Sel1L-deficient P14 cells mixed in a 1:1 ratio with wildtype (WT) P14 cells into cogenically disparate mice that were subsequently infected with LCMV. By day 15 p.i., Sel1L-deficient P14 represented only 29.50% of donor cells in peripheral blood, a fraction which was reduced to 7.5% by day 45 p.i.. These data suggest that Sel1L/ERAD is required for CD8+ T cell survival and memory formation following acute viral infection. To investigate the mechanism by which Sel1L/ERAD regulated CD8+T cell fate, we performed RNA-sequencing on WT and Sel1L-deficient P14 cells at day 8 p.i.. Pathway analysis revealed a reduction in pathways corresponding to T-cell metabolism, specifically 'mitochondrial respiratory chain' (FDR 1.59E-03) and 'oxidative phosphorylation'(FDR 2.45E-08), as well as an increase in apoptotic terms. Furthermore, immune signature analysis suggested Sel1L-deficient P14 cells lack memory precursors CD8+ T cells due to enrichment in immune signature of KLRG1hi effector CD8+ T cells (FDR 1.34E-18). Functional validation by flow cytometry revealed that Sel1L-deficient P14 cells are significantly reduced in their ability to generate memory precursor CD8+ T cells (9.05%±3.84 vs. 6.12%±4.28 ***p<0.001, paired t-test) compared to WT on day 8 p.i. and that D6 in-vitro activated Sel1L-deficient CD8+ T cells exhibit reduced mitochondrial membrane potential relative to WT (TMRM MFI FC relative to WT 0.39±0.29 *p<0.05, unpaired t-test) Our studies demonstrate that virus-specific CD8+ T cells experience dynamic ER stress in vivo during an acute viral infection and that Sel1L/ERAD is required for antigen-specific CD8+ T cell persistence following an infection. Ongoing mechanistic studies will help determine how Sel1L/ERAD promotes CD8+ T cell survival and persistence.

Maternal hypothyroidism causes oxidative stress and endoplasmic reticulum stress in the maternal-fetal interface of rats.

Maternal hypothyroidism is associated with pre-eclampsia and intrauterine growth restriction, gestational diseases involving oxidative stress (OS) and endoplasmic reticulum stress (ERS) in the placenta. However, it is not known whether hypothyroidism also causes OS and ERS at the maternal-fetal interface. The aim was to evaluate the fetal-placental development and the expression of mediators of OS and of the unfolded protein response (UPR) in the maternal-fetal interface of hypothyroid rats. Hypothyroidism was induced in Wistar rats with propylthiouracil and the fetal-placental development and placental and decidual expression of antioxidant, hypoxia, and UPR mediators were analyzed at 14 and 18 days of gestation (DG), as well the expression of 8-OHdG and MDA, and reactive oxygen species (ROS) and peroxynitrite levels. Hypothyroidism reduced fetal weight at 14 and 18 DG, in addition to increasing the percentage of fetal death and reducing the weight of the uteroplacental unit at 18 DG. At 14 DG, there was greater decidual and/or placental immunostaining of Hif1α, 8-OHdG, MDA, SOD1, GPx1/2, Grp78 and CHOP in hypothyroid rats, while there was a reduction in placental and/or decidual gene expression of Sod1, Gpx1, Atf6, Perk, Ho1, Xbp1, Grp78 and Chop in the same gestational period. At 18 DG, hypothyroidism increased the placental ROS levels and the decidual and/or placental immunostaining of HIF1α, 8-OHdG, MDA, ATF4, GRP78 and CHOP, while it reduced the immunostaining and enzymatic activity of SOD1, CAT, GST. Hypothyroidism increased the placental mRNA expression of Hifα, Nrf2, Sod2, Gpx1, Cat, Perk, Atf6 and Chop at 18 DG, while decreasing the decidual expression of Sod2, Cat and Atf6. These findings demonstrated that fetal-placental restriction in female rats with hypothyroidism is associated with hypoxia and dysregulation in placental and decidual expression of UPR mediators and antioxidant enzymes, and activation of oxidative stress and endoplasmic reticulum stress at the maternal-fetal interface.

A Non-Canonical Role for IRE1α Links ER and Mitochondria as Key Regulators of Astrocyte Dysfunction: Implications in Methamphetamine use and HIV-Associated Neurocognitive Disorders

Astrocytes are one of the most numerous glial cells in the central nervous system (CNS) and provide essential support to neurons to ensure CNS health and function. During a neuropathological challenge, such as during human immunodeficiency virus (HIV)-1 infection or (METH)amphetamine exposure, astrocytes shift their neuroprotective functions and can become neurotoxic. Identifying cellular and molecular mechanisms underlying astrocyte dysfunction are of heightened importance to optimize the coupling between astrocytes and neurons and ensure neuronal fitness against CNS pathology, including HIV-1-associated neurocognitive disorders (HAND) and METH use disorder. Mitochondria are essential organelles for regulating metabolic, antioxidant, and inflammatory profiles. Moreover, endoplasmic reticulum (ER)-associated signaling pathways, such as calcium and the unfolded protein response (UPR), are important messengers for cellular fate and function, including inflammation and mitochondrial homeostasis. Increasing evidence supports that the three arms of the UPR are involved in the direct contact and communication between ER and mitochondria through mitochondria-associated ER membranes (MAMs). The current study investigated the effects of HIV-1 infection and chronic METH exposure on astrocyte ER and mitochondrial homeostasis and then examined the three UPR messengers as potential regulators of astrocyte mitochondrial dysfunction. Using primary human astrocytes infected with pseudotyped HIV-1 or exposed to low doses of METH for 7 days, astrocytes had increased mitochondrial oxygen consumption rate (OCR), cytosolic calcium flux and protein expression of UPR mediators. Notably, inositol-requiring protein 1α (IRE1α) was most prominently upregulated following both HIV-1 infection and chronic METH exposure. Moreover, pharmacological inhibition of the three UPR arms highlighted IRE1α as a key regulator of astrocyte metabolic function. To further explore the regulatory role of astrocyte IRE1α, astrocytes were transfected with an IRE1α overexpression vector followed by activation with the proinflammatory cytokine interleukin 1β. Overall, our findings confirm IRE1α modulates astrocyte mitochondrial respiration, glycolytic function, morphological activation, inflammation, and glutamate uptake, highlighting a novel potential target for regulating astrocyte dysfunction. Finally, these findings suggest both canonical and non-canonical UPR mechanisms of astrocyte IRE1α. Thus, additional studies are needed to determine how to best balance astrocyte IRE1α functions to both promote astrocyte neuroprotective properties while preventing neurotoxic properties during CNS pathologies.

A multi-omics approach identifies pancreatic cancer cell extracellular vesicles as mediators of the unfolded protein response in normal pancreatic epithelial cells.

Although cancer‐derived extracellular vesicles (cEVs) are thought to play a pivotal role in promoting cancer progression events, their precise effect on neighbouring normal cells is unknown. In this study, we investigated the impact of pancreatic cancer ductal adenocarcinoma (PDAC) derived EVs on recipient non‐tumourigenic pancreatic normal epithelial cells upon internalization. We demonstrate that cEVs are readily internalized and induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in treated normal pancreatic epithelial cells within 24 h. We further show that PDAC cEVs increase cell proliferation, migration, and invasion and that these changes are regulated at least in part, by the UPR mediator DDIT3. Subsequently, these cells release several inflammatory cytokines. Leveraging a layered multi‐omics approach, we analysed EV cargo from a panel of six PDAC and two normal pancreas cell lines, using multiple EV isolation methods. We found that cEVs were enriched for an array of biomolecules which can induce or regulate ER stress and the UPR, including palmitic acid, sphingomyelins, metabolic regulators of tRNA charging and proteins which regulate trafficking and degradation. We further show that palmitic acid, at doses relevant to those found in cEVs, is sufficient to induce ER stress in normal pancreas cells. These results suggest that cEV cargo packaging may be designed to disseminate proliferative and invasive characteristics upon internalization by distant recipient normal cells, hitherto unreported. This study is among the first to highlight a major role for PDAC cEVs to induce stress in treated normal pancreas cells that may modulate a systemic response leading to altered phenotypes. These findings highlight the importance of EVs in mediating disease aetiology and open potential areas of investigation toward understanding the role of cEV lipids in promoting cell transformation in the surrounding microenvironment.

Pumilio protects Xbp1 mRNA from regulated Ire1-dependent decay

The unfolded protein response (UPR) maintains homeostasis of the endoplasmic reticulum (ER). Residing in the ER membrane, the UPR mediator Ire1 deploys its cytoplasmic kinase-endoribonuclease domain to activate the key UPR transcription factor Xbp1 through non-conventional splicing of Xbp1 mRNA. Ire1 also degrades diverse ER-targeted mRNAs through regulated Ire1-dependent decay (RIDD), but how it spares Xbp1 mRNA from this decay is unknown. Here, we identify binding sites for the RNA-binding protein Pumilio in the 3′UTR Drosophila Xbp1. In the developing Drosophila eye, Pumilio binds both the Xbp1unspliced and Xbp1spliced mRNAs, but only Xbp1spliced is stabilized by Pumilio. Furthermore, Pumilio displays Ire1 kinase-dependent phosphorylation during ER stress, which is required for its stabilization of Xbp1spliced. hIRE1 can phosphorylate Pumilio directly, and phosphorylated Pumilio protects Xbp1spliced mRNA against RIDD. Thus, Ire1-mediated phosphorylation enables Pumilio to shield Xbp1spliced from RIDD. These results uncover an unexpected regulatory link between an RNA-binding protein and the UPR.

The Ca2+ -binding protein sorcin stimulates transcriptional activity of the unfolded protein response mediator ATF6.

Sorcin is a calcium-binding protein involved in maintaining endoplasmic reticulum (ER) Ca2+ stores. We have previously shown that overexpressing sorcin under the rat insulin promoter was protective against high-fat diet-induced pancreatic beta-cell dysfunction invivo. Activating transcription factor 6 (ATF6) is a key mediator of the unfolded protein response (UPR) that provides cellular protection during the progression of ER stress. Here, using nonexcitable HEK293 cells, we show that sorcin overexpression increased ATF6 signalling, whereas sorcin knock out caused a reduction in ATF6 transcriptional activity and increased ER stress. Altogether, our data suggest that sorcin downregulation during lipotoxic stress may prevent full ATF6 activation and a normal UPR during the progression of obesity and insulin resistance.

Expression of cancer cell-associated Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) regulates protective anti-tumor immunity in melanoma

Abstract Activation of moderated endoplasmic reticulum (ER) stress in cancer cells promotes cellular adaptation and survival in response to the adverse conditions of the tumor microenvironment (TME). However, how modulation of ER stress in cancer cells culminates in the development of an immunosuppressive TME remains poorly understood and therapeutically unexploited. Upon disruption of ER homeostasis, mediator proteins IRE1a, ATF6 and PERK induce activation of the unfolded protein response (UPR). Although maladaptive priming of UPR mediators in immune cells deactivates their capacity for anti-tumor immunity, the immunosuppressive potential of UPR mediators in cancer cells, in particular PERK, remains unknown. Thus, in this study, we hypothesized that activation of PERK in tumor cells shapes the formation of the immunosuppressive TME and impairs the development of anti-tumor immunity. Elimination of PERK in cancer cells impaired survival upon chemically-induced ER stress and promoted caspase-independent cell death. Interestingly, in vivo ablation of PERK in melanoma impaired tumor growth, which surprisingly occurred in a T cell-dependent manner. Additionally we observed that ablation of PERK shifted the TME from an immunosuppressive to immunostimulatory environment, enhanced accumulation of effector T cells and directed reprogramming of myeloid derived suppressor cells (MDSCs) into immunostimulatory cells. Additional experiments indicated that ablation of PERK induced expression of mediators associated with immunogenic cell death. Collectively, our findings demonstrate the immunoregulatory role of PERK in tumor cells and highlight the therapeutic potential of targeting UPR mediators and tumor adaptive processes.

Endoplasmic reticulum stress: New insights into the pathogenesis and treatment of retinal degenerative diseases.

Physiological equilibrium in the retina depends on coordinated work between rod and cone photoreceptors and can be compromised by the expression of mutant proteins leading to inherited retinal degeneration (IRD). IRD is a diverse group of retinal dystrophies with multifaceted molecular mechanisms that are not fully understood. In this review, we focus on the contribution of chronically activated unfolded protein response (UPR) to inherited retinal pathogenesis, placing special emphasis on studies employing genetically modified animal models. As constitutively active UPR in degenerating retinas may activate pro-apoptotic programs associated with oxidative stress, pro-inflammatory signaling, dysfunctional autophagy, free cytosolic Ca2+ overload, and altered protein synthesis rate in the retina, we focus on the regulatory mechanisms of translational attenuation and approaches to overcoming translational attenuation in degenerating retinas. We also discuss current research on the role of the UPR mediator PERK and its downstream targets in degenerating retinas and highlight the therapeutic benefits of reprogramming PERK signaling in preclinical animal models of IRD. Finally, we describe pharmacological approaches targeting UPR in ocular diseases and consider their potential applications to IRD.

The Unfolded Protein Response Mediator PERK Governs Myeloid Cell-Driven Immunosuppression in Tumors through Inhibition of STING Signaling

The primary mechanisms supporting immunoregulatory polarization of myeloid cells upon infiltration into tumors remain largely unexplored. Elucidation of these signals could enable better strategies to restore protective anti-tumor immunity. Here, we investigated the role of the intrinsic activation of the PKR-like endoplasmic reticulum (ER) kinase (PERK) in the immunoinhibitory actions of tumor-associated myeloid-derived suppressor cells (tumor-MDSCs). PERK signaling increased in tumor-MDSCs, and its deletion transformed MDSCs into myeloid cells that activated CD8+ Tcell-mediated immunity against cancer. Tumor-MDSCs lacking PERK exhibited disrupted NRF2-driven antioxidant capacity and impaired mitochondrial respiratory homeostasis. Moreover, reduced NRF2 signaling in PERK-deficient MDSCs elicited cytosolic mitochondrial DNA elevation and, consequently, STING-dependent expression of anti-tumor type I interferon. Reactivation of NRF2 signaling, conditional deletion of STING, or blockade of type I interferon receptor I restored the immunoinhibitory potential of PERK-ablated MDSCs. Our findings demonstrate the pivotal role of PERK in tumor-MDSC functionality and unveil strategies to reprogram immunosuppressive myelopoiesis in tumors to boost cancer immunotherapy.

Spliced XBP1 Levels Determine Sensitivity of Multiple Myeloma Cells to Proteasome Inhibitor Bortezomib Independent of the Unfolded Protein Response Mediator GRP78.

Background: Mechanisms mediating resistance against the proteasome inhibition by bortezomib (BTZ) in multiple myeloma (MM) cells are still unclear. We analyzed the activation of the unfolded protein response (UPR), induction of prosurvival, and apoptotic pathways after proteasome inhibition in BTZ-sensitive and -resistant cells. Thereafter, these findings from tissue culture were proofed on MM cells of BTZ-sensitive and BTZ-refractory patients.Methods: Proteasomal and ABC transporter activities were measured in sensitive and resistant cell lines by the use of the respective substrates. TP53 gene loss and mutations were determined by cytogenetics and targeted NGS. UPR pathways, proteasome subunit levels and protein secretion were studied by Western Blot analysis, and apoptosis was determined by flow cytometry. MM cell lines were stably transfected with inducible GRP78 expression to study unfolded protein expression. Transient knock-down of GRP78 was done by RNA interference. Splicing of XBP1 and expression of GRP78 was studied by real-time PCR in CD138-enriched MM primary cells of BTZ-refractory and -sensitive patients.Results: BTZ-sensitive cells displayed lower basal proteasomal activities. Similar activities of all three major ABC transporter proteins were detected in BTZ-sensitive and resistant cells. Sensitive cells showed deficiencies in triggering canonical prosurvival UPR provoked by endoplasmic reticulum (ER) stress induction. BTZ treatment did not increase unfolded protein levels or induced GRP78-mediated UPR. BTZ-resistant cells and BTZ-refractory patients exhibited lower sXBP1 levels. Apoptosis of BTZ-sensitive cells was correlating with induction of p53 and NOXA. Tumor cytogenetics and NGS analysis revealed more frequent TP53 deletions and mutations in BTZ-refractory MM patients.Conclusions: We identified low sXBP1 levels and TP53 abnormalities as factors correlating with bortezomib resistance in MM. Therefore, determination of sXBP1 levels and TP53 status prior to BTZ treatment in MM may be beneficial to predict BTZ resistance.

SEC24A identified as an essential mediator of thapsigargin-induced cell death in a genome-wide CRISPR/Cas9 screen

Endoplasmic reticulum (ER) stress from accumulated misfolded proteins in the ER can activate the unfolded protein response (UPR). The UPR acts either to restore proteostasis or to activate cell death pathways if the stress cannot be resolved. The key downstream effectors in these pathways have been studied extensively. However, in comparison, stressor-specific key mediators are not as well characterized. In this study, we sought to identify and compare the genes that are necessary for cell death induced by three classic pharmacological ER stressors with different mechanisms of action: thapsigargin, tunicamycin, and brefeldin A. We conducted genome-wide CRISPR/Cas9-based loss-of-function screens against these agents in HAP1 cells, which are a near-haploid cell line. Our screens confirmed that MFSD2A and ARF4, which were identified in previous screens, are necessary for tunicamycin- and brefeldin A-induced cytotoxicity, respectively. We identified a novel gene, SEC24A, as an essential gene for thapsigargin-induced cytotoxicity in HAP1 cells. Further experiments showed that the ability of SEC24A to facilitate ER stress-induced cell death is specific to thapsigargin and that SEC24A acts upstream of the UPR. These findings show that the genes required for ER stress-induced cell death are specific to the agent used to induce ER stress and that the resident ER cargo receptor protein SEC24A is an essential mediator of thapsigargin-induced UPR and cell death.

Crosstalk between inflammatory mediators and endoplasmic reticulum stress in liver diseases

An excessive inflammatory response is frequently associated with cellular dysfunction and cell death. The latter may cause single and multiple organ failure. The most susceptible organs are liver, lung, kidney, heart and intestine. This review will focus on the liver as a target organ for an excessive inflammatory response. It is commonly accepted that organ failure is caused by the action of inflammatory cytokines released in excess during the inflammatory response. It has been suggested that inflammation mediated liver failure is not due to an increased death rate of parenchymal cells, but due to an intracellular metabolic disorder. This metabolic disorder is associated with mitochondrial and endoplasmic reticulum (ER) dysfunction during the acute phase response elicited by systemic inflammation. An overproduction of acute phase proteins in the liver as well as elevated reactive oxygen species (ROS) generation induce ER stress, triggering the unfolded protein response (UPR), which may initiate or aggravate inflammation. It is known that certain inflammatory mediators, such as the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α induce ER stress. These findings suggest that ER stress and the subsequent UPR on the one hand, and the inflammatory response on the other create a kind of feed forward loop, which can be either beneficial (e.g., elimination of the pathogen and restoration of tissue homeostasis) or deleterious (e.g., excessive cell dysfunction and cell death). This review aims to unfurl the different pathways contributing to this loop and to highlight the relevance of UPR signaling (IRE1α, ATF6, and PERK) and mediators of the inflammatory response (NF-κB, STAT3, IL-1β, IL-6, TLR) which have a particular role as pathophysiological triggers in the liver.

HIV-1 Tat-induced bystander apoptosis in Jurkat cells involves unfolded protein responses.

The HIV transactivator protein (Tat) is a multifunctional protein that plays a critical role in viral replication and contributes to several pathological symptoms of HIV-1 infection, which has the loss of CD4+ T lymphocytes as one of its hallmark features. It has been shown that endoplasmic reticulum (ER) stress, including viral infections, is implicated in cellular dysfunction and cell death through activation of the unfolded protein response (UPR). Here, we demonstrate that the bystander stimulus of Tat on Jurkat cells resulted in time-dependent overexpression of major UPR markers including ER chaperone BiP, ER stress sensors ATF6, PERK, and IRE1, as well as an increase in levels of downstream mediators eIF2α, ATF4, XBP-1, and proapoptotic factors CHOP, GADD34, and BIM. This upregulation of UPR mediators was accompanied by decreased cell viability and increased apoptosis as evidenced by blue trypan dye exclusion and flow cytometry assays, respectively. Furthermore, we found that the Tat-associated apoptosis of Jurkat cells led to the loss of mitochondrial membrane potential and caspase-12 and -3 activation. Taken together, these results suggest that the exposure of HIV-1 Tat leads to ER stress/UPR triggering which in turn contributes to apoptotic death in Jurkat cells. SIGNIFICANCE OF THE STUDY: In the present work, we provide a substantial insight into the link between ER impairment and apoptotic death following a bystander HIV Tat stimulus, revealing an underlying ER-mediated apoptotic mechanism which could explain the continuous depletion of uninfected CD4+ T lymphocytes observed in HIV-related disease. Our findings reinforce the relevance of ER stress molecular responses in the course of HIV infection and may afford valuable information for the development of new therapeutic strategies to avoid CD4+ T lymphocyte loss and other disorders induced by circulant Tat.

IRE1α governs cytoskeleton remodelling and cell migration through a direct interaction with filamin A.

Maintenance of endoplasmic reticulum (ER) proteostasis is controlled by a signalling network known as the unfolded protein response (UPR). Here, we identified filamin A as a major binding partner of the ER stress transducer IRE1α. Filamin A is an actin crosslinking factor involved in cytoskeleton remodelling. We show that IRE1α controls actin cytoskeleton dynamics and affects cell migration upstream of filamin A. The regulation of cytoskeleton dynamics by IRE1α is independent of its canonical role as a UPR mediator, serving instead as a scaffold that recruits and regulates filamin A. Targeting IRE1α expression in mice affected normal brain development, generating a phenotype resembling periventricular heterotopia, a disease linked to the loss of function of filamin A. IRE1α also modulated cell movement and cytoskeleton dynamics in fly and zebrafish models. This study unveils an unanticipated biological function of IRE1α in cell migration, whereby filamin A operates as an interphase between the UPR and the actin cytoskeleton.