IRE1 signaling increases PERK expression during chronic ER stress

The Unfolded Protein Response (UPR) is an essential cellular process activated by the accumulation of unfolded proteins within the Endoplasmic Reticulum (ER), a condition referred to as ER stress. Three ER anchored receptors, IRE1, PERK and ATF6 act as ER stress sensors monitoring the health of the ER. Upon detection of ER stress, IRE1, PERK and ATF6 initiate downstream signaling pathways collectively referred to as the UPR. The overarching aim of the UPR is to restore ER homeostasis by reducing ER stress, however if that is not possible, the UPR transitions from a pro-survival to a pro-death response. While our understanding of the key signaling pathways central to the UPR is well defined, the same is not true of the subtle signaling events that help fine tune the UPR, supporting its ability to adapt to varying amplitudes or durations of ER stress. In this study, we demonstrate cross talk between the IRE1 and PERK branches of the UPR, wherein IRE1 via XBP1s signaling helps to sustain PERK expression during prolonged ER stress. Our findings suggest cross talk between UPR branches aids adaptiveness thereby helping to support the plasticity of UPR signaling responses.

DUBing Primary Tumors of the Central Nervous System: Regulatory Roles of Deubiquitinases

The ubiquitin proteasome system (UPS) utilizes an orchestrated enzymatic cascade of E1, E2, and E3 ligases to add single or multiple ubiquitin-like molecules as post-translational modification (PTM) to proteins. Ubiquitination can alter protein functions and/or mark ubiquitinated proteins for proteasomal degradation but deubiquitinases (DUBs) can reverse protein ubiquitination. While the importance of DUBs as regulatory factors in the UPS is undisputed, many questions remain on DUB selectivity for protein targeting, their mechanism of action, and the impact of DUBs on the regulation of diverse biological processes. Furthermore, little is known about the expression and role of DUBs in tumors of the human central nervous system (CNS). In this comprehensive review, we have used publicly available transcriptional datasets to determine the gene expression profiles of 99 deubiquitinases (DUBs) from five major DUB families in seven primary pediatric and adult CNS tumor entities. Our analysis identified selected DUBs as potential new functional players and biomarkers with prognostic value in specific subtypes of primary CNS tumors. Collectively, our analysis highlights an emerging role for DUBs in regulating CNS tumor cell biology and offers a rationale for future therapeutic targeting of DUBs in CNS tumors.

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.

New insights into IRE1α activation and function in anti-tumor immunity

Logue, Gorman, and Samali highlight a study by Guttman and colleagues (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202111068) that shows exogenous antigen peptides imported into the ER can activate the ER stress sensor IRE1α, attenuating cross-presentation by dendritic cells.

The stressosome, a caspase-8-activating signalling complex assembled in response to cell stress in an ATG5-mediated manner

Stress-induced apoptosis is mediated primarily through the intrinsic pathway that involves caspase-9. We previously reported that in caspase-9-deficient cells, a protein complex containing ATG5 and Fas-associated death domain (FADD) facilitated caspase-8 activation and cell death in response to endoplasmic reticulum (ER) stress. Here, we investigated whether this complex could be activated by other forms of cell stress. We show that diverse stress stimuli, including etoposide, brefeldin A and paclitaxel, as well as heat stress and gamma-irradiation, caused formation of a complex containing ATG5-ATG12, FADD and caspase-8 leading to activation of downstream caspases in caspase-9-deficient cells. We termed this complex the 'stressosome'. However, in these cells, only ER stress and heat shock led to stressosome-dependent cell death. Using in silico molecular modelling, we propose the structure of the stressosome complex, with FADD acting as an adaptor protein, interacting with pro-caspase-8 through their respective death effector domains (DEDs) and interacting with ATG5-ATG12 through its death domain (DD). This suggests that the complex could be regulated by cellular FADD-like interleukin-1β-converting enzyme-inhibitory protein (cFLIP_L ), which was confirmed experimentally. This study provides strong evidence for an alternative mechanism of caspase-8 activation involving the stressosome complex.

Multiplex Serum Biomarker Assays Improve Prediction of Renal and Mortality Outcomes in Chronic Kidney Disease

BACKGROUND

We investigated the predictive value of 11 serum biomarkers for renal and mortality end points in people with CKD.

METHODS

Adults with CKD (n=139) were enrolled from outpatient clinics between February 2014 and November 2016. Biomarker quantification was performed using two multiplex arrays on a clinical-grade analyzer. Relationships between biomarkers and renal and mortality end points were investigated by random forests and Cox proportional hazards regression.

RESULTS

The cohort was 56% male. The mean age was 63 years and median (IQR) CKD-EPI eGFR was 33 (24-51) ml/min per BSA. A total of 56 (40%) people developed a composite end point defined as ≥40% decline in eGFR, doubling of serum creatinine, RRT, or death over median (IQR) follow-up of 5.4 (4.7-5.7) years. Prediction of the composite end point was better with random forests trained on serum biomarkers compared with clinical variables (area under the curve of 0.81 versus 0.78). The predictive performance of biomarkers was further enhanced when considered alongside clinical variables (area under the curve of 0.83 versus 0.81 for biomarkers alone). Patients (n=27, 19%) with high soluble TNF receptor-1 (≥3 ng/ml) and neutrophil gelatinase-associated lipocalin (≥156 ng/ml), coupled with low complement 3a des-arginine (<2368 ng/ml), almost universally (96%) developed the composite renal and mortality end point. C-reactive protein (adjusted hazard ratio, 1.4; 95% CI, 1.1 to 1.8), neutrophil gelatinase-associated lipocalin (adjusted hazard ratio, 2.8; 95% CI, 1.3 to 6.1) and complement 3a desarginine (adjusted hazard ratio, 0.6; 95% CI, 0.4 to 0.96) independently predicted time to the composite end point.

CONCLUSIONS

Outpatients with the triad of high soluble TNF receptor-1 and neutrophil gelatinase-associated lipocalin coupled with low complement 3a des-arginine had high adverse event rates over 5-year follow-up. Incorporation of serum biomarkers alongside clinical variables improved prediction of CKD progression and mortality. Our findings require confirmation in larger, more diverse patient cohorts.

An Emerging Role for the Unfolded Protein Response in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is the most common form of pancreatic cancer and one of the leading causes of cancer-associated deaths in the world. It is characterised by dismal response rates to conventional therapies. A major challenge in treatment strategies for PDAC is the presence of a dense stroma that surrounds the tumour cells, shielding them from treatment. This unique tumour microenvironment is fuelled by paracrine signalling between pancreatic cancer cells and supporting stromal cell types including the pancreatic stellate cells (PSC). While our molecular understanding of PDAC is improving, there remains a vital need to develop effective, targeted treatments. The unfolded protein response (UPR) is an elaborate signalling network that governs the cellular response to perturbed protein homeostasis in the endoplasmic reticulum (ER) lumen. There is growing evidence that the UPR is constitutively active in PDAC and may contribute to the disease progression and the acquisition of resistance to therapy. Given the importance of the tumour microenvironment and cytokine signalling in PDAC, and an emerging role for the UPR in shaping the tumour microenvironment and in the regulation of cytokines in other cancer types, this review explores the importance of the UPR in PDAC biology and its potential as a therapeutic target in this disease.

Tumour Cell Secretome in Chemoresistance and Tumour Recurrence

Chemoresistance is a major factor driving tumour relapse and the high rates of cancer-related deaths. Understanding how cancer cells overcome chemotherapy-induced cell death is critical in promoting patient survival. One emerging mechanism of chemoresistance is the tumour cell secretome (TCS), an array of protumorigenic factors released by tumour cells. Chemotherapy exposure can also alter the composition of the TCS, known as therapy-induced TCS, and can promote tumour relapse and the formation of an immunosuppressive tumour microenvironment (TME). Here, we outline how the TCS can protect cancer cells from chemotherapy-induced cell death. We also highlight recent evidence describing how therapy-induced TCS can impact cancer stem cell (CSC) expansion and tumour-associated immune cells to enable tumour regrowth and antitumour immunity.

Inhibition of IRE1α RNase activity reduces NLRP3 inflammasome assembly and processing of pro-IL1β

The inflammasome is a multiprotein complex assembled in response to Pathogen Associated Molecular Patterns (PAMPs) and Danger Associated Molecular Patterns (DAMPs). Inflammasome activation occurs through a two-step mechanism, with the first signal facilitating priming of inflammasome components while the second signal triggers complex assembly. Once assembled, the inflammasome recruits and activates pro-caspase-1, which in turn processes pro-interleukin (IL)-18 and pro-IL-1β into their bio-active forms. Owing to its key role in the regulation of innate immune responses, the inflammasome has emerged as a therapeutic target for the treatment of inflammatory conditions. In this study we demonstrate that IRE1α, a key component of the Unfolded Protein Response, contributes to assembly of the NLRP3 inflammasome. Blockade of IRE1α RNase signaling lowered NLRP3 inflammasome assembly, caspase-1 activation and pro-IL-1β processing. These results underscore both the importance and potential therapeutic relevance of targeting IRE1α signaling in conditions of excessive inflammasome formation.

The role of the unfolded protein response in cancer progression: From oncogenesis to chemoresistance

Tumour cells endure both oncogenic and environmental stresses during cancer progression. Transformed cells must meet increased demands for protein and lipid production needed for rapid proliferation and must adapt to exist in an oxygen- and nutrient-deprived environment. To overcome such challenges, cancer cells exploit intrinsic adaptive mechanisms such as the unfolded protein response (UPR). The UPR is a pro-survival mechanism triggered by accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER), a condition referred to as ER stress. IRE1, PERK and ATF6 are three ER anchored transmembrane receptors. Upon induction of ER stress, they signal in a coordinated fashion to re-establish ER homoeostasis, thus aiding cell survival. Over the past decade, evidence has emerged supporting a role for the UPR in the establishment and progression of several cancers, including breast cancer, prostate cancer and glioblastoma multiforme. This review discusses our current knowledge of the UPR during oncogenesis, tumour growth, metastasis and chemoresistance.

The Unfolded Protein Response in Breast Cancer

In 2018, in the US alone, it is estimated that 268,670 people will be diagnosed with breast cancer, and that 41,400 will die from it. Since breast cancers often become resistant to therapies, and certain breast cancers lack therapeutic targets, new approaches are urgently required. A cell-stress response pathway, the unfolded protein response (UPR), has emerged as a promising target for the development of novel breast cancer treatments. This pathway is activated in response to a disturbance in endoplasmic reticulum (ER) homeostasis but has diverse physiological and disease-specific functions. In breast cancer, UPR signalling promotes a malignant phenotype and can confer tumours with resistance to widely used therapies. Here, we review several roles for UPR signalling in breast cancer, highlighting UPR-mediated therapy resistance and the potential for targeting the UPR alone or in combination with existing therapies.

Inhibition of IRE1 RNase activity modulates the tumor cell secretome and enhances response to chemotherapy

Triple-negative breast cancer (TNBC) lacks targeted therapies and has a worse prognosis than other breast cancer subtypes, underscoring an urgent need for new therapeutic targets and strategies. IRE1 is an endoplasmic reticulum (ER) stress sensor, whose activation is predominantly linked to the resolution of ER stress and, in the case of severe stress, to cell death. Here we demonstrate that constitutive IRE1 RNase activity contributes to basal production of pro-tumorigenic factors IL-6, IL-8, CXCL1, GM-CSF, and TGFβ2 in TNBC cells. We further show that the chemotherapeutic drug, paclitaxel, enhances IRE1 RNase activity and this contributes to paclitaxel-mediated expansion of tumor-initiating cells. In a xenograft mouse model of TNBC, inhibition of IRE1 RNase activity increases paclitaxel-mediated tumor suppression and delays tumor relapse post therapy. We therefore conclude that inclusion of IRE1 RNase inhibition in therapeutic strategies can enhance the effectiveness of current chemotherapeutics.

IRE1/XBP-1 activation has a major role in Triple negative breast cancer (TNBC). Here, the authors show that inhibition of IRE1’s RNase activity attenuates autocrine and paracrine signaling of pro-tumorigenic cytokines and synergizes with paclitaxel to confer potent anti-tumor effects in TNBC.

RIP2 enhances cell survival by activation of NF-ĸB in triple negative breast cancer cells

Receptor-interacting protein 2 (RIP2) is an essential mediator of inflammation and innate immunity, but little is known about its role outside the immune system. Recently, RIP2 has been linked to chemoresistance of triple negative breast cancer (TNBC), the most aggressive breast cancer subtype for which there is an urgent need for targeted therapies. In this study we show that high expression of RIP2 in breast tumors correlates with a worse prognosis and a higher risk of recurrence. We also demonstrate that RIP2 confers TNBC cell resistance against paclitaxel and ceramide-induced apoptosis. Overexpression of RIP2 lead to NF-κB activation, which contributed to higher expression of pro-survival proteins and cell survival. Conversely, RIP2 knockdown inhibited NF-κB signaling, reduced levels of anti-apoptotic proteins and sensitized cells to drug treatment. Together, these data show that RIP2 promotes survival of breast cancer cells through NF-κB activation and that targeting RIP2 may be therapeutically beneficial for treatment of TNBC.

The Janus Face of Death Receptor Signaling during Tumor Immunoediting

Cancer immune surveillance is essential for the inhibition of carcinogenesis. Malignantly transformed cells can be recognized by both the innate and adaptive immune systems through different mechanisms. Immune effector cells induce extrinsic cell death in the identified tumor cells by expressing death ligand cytokines of the tumor necrosis factor ligand family. However, some tumor cells can escape immune elimination and progress. Acquisition of resistance to the death ligand-induced apoptotic pathway can be obtained through cleavage of effector cell expressed death ligands into a poorly active form, mutations or silencing of the death receptors, or overexpression of decoy receptors and pro-survival proteins. Although the immune system is highly effective in the elimination of malignantly transformed cells, abnormal/dysfunctional death ligand signaling curbs its cytotoxicity. Moreover, DRs can also transmit pro-survival and pro-migratory signals. Consequently, dysfunctional death receptor-mediated apoptosis/necroptosis signaling does not only give a passive resistance against cell death but actively drives tumor cell motility, invasion, and contributes to consequent metastasis. This dual contribution of the death receptor signaling in both the early, elimination phase, and then in the late, escape phase of the tumor immunoediting process is discussed in this review. Death receptor agonists still hold potential for cancer therapy since they can execute the tumor-eliminating immune effector function even in the absence of activation of the immune system against the tumor. The opportunities and challenges of developing death receptor agonists into effective cancer therapeutics are also discussed.

Deficiency in the mitochondrial apoptotic pathway reveals the toxic potential of autophagy under ER stress conditions

Endoplasmic reticulum (ER) stress-induced cell death is normally associated with activation of the mitochondrial apoptotic pathway, which is characterized by CYCS (cytochrome c, somatic) release, apoptosome formation, and caspase activation, resulting in cell death. In this study, we demonstrate that under conditions of ER stress cells devoid of CASP9/caspase-9 or BAX and BAK1, and therefore defective in the mitochondrial apoptotic pathway, still undergo a delayed form of cell death associated with the activation of caspases, therefore revealing the existence of an alternative stress-induced caspase activation pathway. We identified CASP8/caspase-8 as the apical protease in this caspase cascade, and found that knockdown of either of the key autophagic genes, ATG5 or ATG7, impacted on CASP8 activation and cell death induction, highlighting the crucial role of autophagy in the activation of this novel ER stress-induced death pathway. In line with this, we identified a protein complex composed of ATG5, FADD, and pro-CASP8 whose assembly coincides with caspase activation and cell death induction. Together, our results reveal the toxic potential of autophagy in cells undergoing ER stress that are defective in the mitochondrial apoptotic pathway, and suggest a model in which the autophagosome functions as a platform facilitating pro-CASP8 activation. Chemoresistance, a common problem in the treatment of cancer, is frequently caused by the downregulation of key mitochondrial death effector proteins. Alternate stress-induced apoptotic pathways, such as the one described here, may become of particular relevance for tackling the problem of chemoresistance in cancer cells.

BCL-2 modulates the unfolded protein response by enhancing splicing of X-box binding protein-1

Accumulation of unfolded proteins within the endoplasmic reticulum (ER) triggers a highly conserved stress response mechanism termed the unfolded protein response (UPR). The UPR is a complex series of signaling pathways controlled by ER localized transmembrane receptors, PERK, ATF6 and IRE1α. Following activation IRE1α splices XBP-1 mRNA facilitating the formation of a potent transcription factor, spliced XBP-1. The BCL-2 family members, BAX and BAK, in addition to the mitochondrion also localize to the ER and have been demonstrated to directly interact with IRE1α promoting its activity. In this study we show that in addition to BAX and BAK, the anti-apoptotic BCL-2 protein can regulate IRE1α activity. Enhanced splicing of XBP-1 was observed in BCL-2 overexpressing cells implicating BCL-2 in the complex regulation of IRE1α activity.

Stressed to death - mechanisms of ER stress-induced cell death

The endoplasmic reticulum (ER) is a highly dynamic organelle of fundamental importance present in all eukaryotic cells. The majority of synthesized structural and secreted proteins undergo post-translational modification, folding and oligomerization in the ER lumen, enabling proteins to carry out their physiological functions. Therefore, maintenance of ER homeostasis and function is imperative for proper cellular function. Physiological and pathological conditions can disturb ER homeostasis and thus negatively impact upon protein folding, resulting in an accumulation of unfolded proteins. Examples include hypoxia, hypo- and hyperglycemia, acidosis, and fluxes in calcium levels. Increased levels of unfolded/misfolded proteins within the ER lumen triggers a condition commonly referred to as 'ER stress'. To combat ER stress, cells have evolved a highly conserved adaptive stress response referred to as the unfolded protein response (UPR). UPR signaling affords the cell a 'window of opportunity' for stress resolution however, if prolonged or excessive the UPR is insufficient and ER stress-induced cell death ensues. This review discusses the role of ER stress sensors IRE1, PERK and ATF6, describing their role in ER stress-induced death signaling with specific emphasis placed upon the importance of the intrinsic cell death pathway and Bcl-2 family regulation.

New directions in ER stress-induced cell death

Endoplasmic reticulum (ER) stress has been implicated in the pathophysiology of many diseases including heart disease, cancer and neurodegenerative diseases such as Alzheimer's and Huntington's. Prolonged or excessive ER stress results in the initiation of signaling pathways resulting in cell death. Over the past decade much research investigating the onset and progression of ER stress-induced cell death has been carried out. Owing to this we now have a better understanding of the signaling pathways leading to ER stress-mediated cell death and have begun to appreciate the importance of ER localized stress sensors, IRE1α, ATF6 and PERK in this process. In this article we provide an overview of the current thinking and concepts concerning the various stages of ER stress-induced cell death, focusing on the role of ER localized proteins in sensing and triggering ER stress-induced death signals with particular emphasis on the contribution of calcium signaling and Bcl-2 family members to the execution phase of this process. We also highlight new and emerging directions in ER stress-induced cell death research particularly the role of microRNAs, ER-mitochondria cross talk and the prospect of mitochondria-independent death signals in ER stress-induced cell death.

Disruption of microRNA biogenesis confers resistance to ER stress-induced cell death upstream of the mitochondrion

Global downregulation of microRNAs (miRNAs) is a common feature of human tumors and has been shown to enhance cancer progression. Several components of the miRNA biogenesis machinery (XPO5, DICER and TRBP) have been shown to act as haploinsufficient tumor suppressors. How the deregulation of miRNA biogenesis promotes tumor development is not clearly understood. Here we show that loss of miRNA biogenesis increased resistance to endoplasmic reticulum (ER) stress-induced cell death. We observed that HCT116 cells with a DICER hypomorphic mutation (Exn5/Exn5) or where DICER or DROSHA were knocked down were resistant to ER stress-induced cell death. Extensive analysis revealed little difference in the unfolded protein response (UPR) of WT compared to Exn5/Exn5 HCT116 cells upon ER stress treatment. However, analysis of the intrinsic apoptotic pathway showed that resistance occurred upstream of the mitochondria. In particular, BAX activation and dissipation of mitochondrial membrane potential was attenuated, and there was altered expression of BCL-2 family proteins. These observations demonstrate a key role for miRNAs as critical modulators of the ER stress response. In our model, downregulation of miRNA biogenesis delays ER stress-induced apoptosis. This suggests that disrupted miRNA biogenesis may contribute to cancer progression by inhibiting ER stress-induced cell death.

Fas/CD95-induced chemokines can serve as "find-me" signals for apoptotic cells

Apoptosis is commonly thought to represent an immunologically silent or even anti-inflammatory mode of cell death, resulting in cell clearance in the absence of explicit activation of the immune system. However, here we show that Fas/CD95-induced apoptosis is associated with the production of an array of cytokines and chemokines, including IL-6, IL-8, CXCL1, MCP-1, and GMCSF. Fas-induced production of MCP-1 and IL-8 promoted chemotaxis of phagocytes toward apoptotic cells, suggesting that these factors serve as "find-me" signals in this context. We also show that RIPK1 and IAPs are required for optimal production of cytokines and chemokines in response to Fas receptor stimulation. Consequently, a synthetic IAP antagonist potently suppressed Fas-dependent expression of multiple proinflammatory mediators and inhibited Fas-induced chemotaxis. Thus, in addition to provoking apoptosis, Fas receptor stimulation can trigger the secretion of chemotactic factors and other immunologically active proteins that can influence immune responsiveness toward dying cells.

Inhibitor of apoptosis proteins (IAPs) and their antagonists regulate spontaneous and tumor necrosis factor (TNF)-induced proinflammatory cytokine and chemokine production

Inhibitor of apoptosis proteins (IAPs) play a major role in determining whether cells undergo apoptosis in response to TNF as well as other stimuli. However, TNF is also highly proinflammatory through its ability to trigger the secretion of multiple inflammatory cytokines and chemokines, which is arguably the most important role of TNF in vivo. Indeed, deregulated production of TNF-induced cytokines is a major driver of inflammation in several autoimmune conditions such as rheumatoid arthritis. Here, we show that IAPs are required for the production of multiple TNF-induced proinflammatory mediators. Ablation or antagonism of IAPs potently suppressed TNF- or RIPK1-induced proinflammatory cytokine and chemokine production. Surprisingly, IAP antagonism also led to spontaneous production of chemokines, particularly RANTES, in vitro and in vivo. Thus, IAPs play a major role in influencing the production of multiple inflammatory mediators, arguing that these proteins are important regulators of inflammation in addition to apoptosis. Furthermore, small molecule IAP antagonists can modulate spontaneous as well as TNF-induced inflammatory responses, which may have implications for use of these agents in therapeutic settings.

Caspase-1 promiscuity is counterbalanced by rapid inactivation of processed enzyme

Members of the caspase family of cysteine proteases coordinate the highly disparate processes of apoptosis and inflammation. However, although hundreds of substrates for the apoptosis effector caspases (caspase-3 and caspase-7) have been identified, only two confirmed substrates for the key inflammatory protease (caspase-1) are known. Whether this reflects intrinsic differences in the substrate specificity of inflammatory versus apoptotic caspases or their relative abundance in vivo is unknown. To address this issue, we have compared the specificity of caspases-1, -3, and -7 toward peptide and protein substrates. Contrary to expectation, caspase-1 displayed concentration-dependent promiscuity toward a variety of substrates, suggesting that caspase-1 specificity is maintained by restricting its abundance. Although endogenous concentrations of caspase-1 were found to be similar to caspase-3, processed caspase-1 was found to be much more labile, with a half-life of ∼9 min. This contrasted sharply with the active forms of caspase-3 and caspase-7, which exhibited half-lives of 8 and 11 h, respectively. We propose that the high degree of substrate specificity displayed by caspase-1 is maintained through rapid spontaneous inactivation of this protease.

Expression, purification and use of recombinant annexin V for the detection of apoptotic cells

Apoptosis is a mode of programmed cell death that is widely used to eliminate cells during development, tissue homeostasis, infection or in response to injury. Alterations to the plasma membranes of apoptotic cells trigger recognition and engulfment of such cells by phagocytes. Measurement of plasma membrane phosphatidylserine externalization, using fluorescently labeled annexin V, is widely used for the detection of apoptotic cells. Here we describe protocols for bacterial expression, purification and FITC labeling of recombinant annexin V. By following the method outlined in this protocol, it is possible to produce milligram amounts of recombinant annexin V within 3 d. We also describe a method for the assessment of annexin V binding to cell populations by flow cytometry or fluorescence microscopy.

Distinct mechanisms of cardiomyocyte apoptosis induced by doxorubicin and hypoxia converge on mitochondria and are inhibited by Bcl-xL

Hypoxia and doxorubicin can cause cardiotoxicity and loss of myocardial function. These effects are due, in part, to an induction of apoptosis. Herein we identify the apoptotic pathways activated in H9c2 cells in response to hypoxia (O(2)/N(2)/CO(2), 0.5:94.5:5) and doxorubicin (0.5 muM). Although the apoptosis induced was accompanied by induction of Fas and Fas ligand, the death receptor pathway was not critical for caspase activation by either stimulus. Hypoxia induced the expression of endoplasmic reticulum (ER) stress mediators and processed ER-resident pro-caspase-12 whereas doxorubicin did not induce an ER stress response. Most importantly, both stimuli converged on mitochondria to promote apoptosis. Accumulation of cytochrome c in the cytosol coincided with the processing of pro-caspase-9 and -3. Increasing the expression of the anti-apoptotic protein Bcl-x(L), either by dexamethasone or adenovirus-mediated transduction, protected H9c2 cells from doxorubicin- and hypoxia-induced apoptosis. Bcl-x(L) attenuated mitochondrial cytochrome crelease and reduced downstream pro-caspase processing and apoptosis. These data demonstrate that two distinct cardiomyocyte-damaging stimuli converge on mitochondria thus presenting this organelle as a potentially important therapeutic target for anti-apoptotic strategies for cardiovascular diseases.

Mediators of endoplasmic reticulum stress-induced apoptosis

The efficient functioning of the endoplasmic reticulum (ER) is essential for most cellular activities and survival. Conditions that interfere with ER function lead to the accumulation and aggregation of unfolded proteins. ER transmembrane receptors detect the onset of ER stress and initiate the unfolded protein response (UPR) to restore normal ER function. If the stress is prolonged, or the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, as ER stress-induced apoptosis is implicated in the pathophysiology of several neurodegenerative and cardiovascular diseases. In this review, we examine the role of the molecules that are activated during the UPR in order to identify the molecular switch from the adaptive phase to apoptosis. We discuss how the activation of these molecules leads to the commitment of death and the mechanisms that are responsible for the final demise of the cell.

ER stress contributes to ischemia-induced cardiomyocyte apoptosis

Myocardial ischemia is a severe stress condition that leads to loss of cardiomyocytes. The cell loss is attributed to apoptosis, although the exact mechanisms involved are only partially defined, which limits therapeutic opportunities. Here, we show caspase activation and apoptosis in neonatal rat cardiomyocyte cultures subjected to simulated ischemia by serum, glucose, and oxygen deprivation (SGO). Caspase activation was preceded by endoplasmic reticulum (ER) stress and the activation of the unfolded protein response (UPR), detected by the induction of Grp78, induction and splicing of XBP1, and phosphorylation of eukaryotic initiation factor 2-alpha (eIF2alpha). At a later time the ER stress response switched from UPR and cytoprotective response to a pro-apoptotic response as demonstrated by the upregulation of CHOP and processing of pro-caspase-12. Thus, we provide evidence that the ER can generate and propagate apoptotic signals in response to ischemic stress and this pathway is therefore a novel target for prevention of ischemia-mediated cardiomyocyte loss.

Response to myocardial ischemia/reperfusion injury involves Bnip3 and autophagy

Ischemia and reperfusion (I/R) injury is associated with extensive loss of cardiac myocytes. Bnip3 is a mitochondrial pro-apoptotic Bcl-2 protein which is expressed in the adult myocardium. To investigate if Bnip3 plays a role in I/R injury, we generated a TAT-fusion protein encoding the carboxyl terminal transmembrane deletion mutant of Bnip3 (TAT-Bnip3DeltaTM) which has been shown to act as a dominant negative to block Bnip3-induced cell death. Perfusion with TAT-Bnip3DeltaTM conferred protection against I/R injury, improved cardiac function, and protected mitochondrial integrity. Moreover, Bnip3 induced extensive fragmentation of the mitochondrial network and increased autophagy in HL-1 myocytes. 3D rendering of confocal images revealed fragmented mitochondria inside autophagosomes. Enhancement of autophagy by ATG5 protected against Bnip3-mediated cell death, whereas inhibition of autophagy by ATG5K130R enhanced cell death. These results suggest that Bnip3 contributes to I/R injury which triggers a protective stress response with upregulation of autophagy and removal of damaged mitochondria.

Ischemia/reperfusion injury at the intersection with cell death

Since its description in myocardial ischemia/reperfusion a decade ago, the process of programmed cell death has been the subject of intense study by investigators in cardiology. Comparison with other systems has identified parallels as well as features that are unique to the heart. An understanding of the molecular events that transpire during ischemia should inform our analysis of the relevant pathways of cell death. The intersection of ischemic preconditioning and cell death can provide illumination to both signal transduction pathways.

Apoptosis repressor with caspase recruitment domain protects against cell death by interfering with Bax activation

Myocardial ischemia/reperfusion (I/R) is associated with an extensive loss of myocardial cells. The apoptosis repressor with caspase recruitment domain (ARC) is a protein that is highly expressed in heart and skeletal muscle and has been demonstrated to protect the heart against I/R injury (Gustafsson, A. B., Sayen, M. R., Williams, S. D., Crow, M. T., and Gottlieb, R. A. (2002) Circulation 106, 735-739). In this study, we have shown that transduction of TAT-ARCL31F, a mutant of ARC in the caspase recruitment domain, did not reduce creatine kinase release and infarct size after I/R. TAT-ARCL31F also failed to protect against hydrogen peroxide-mediated cell death in H9c2 cells, suggesting that the caspase recruitment domain is important in mediating ARC's protective effects. In addition, we report that ARC co-immunoprecipitated with the pro-apoptotic protein Bax, which causes cytochrome c release when activated. TAT-ARC, but not TAT-ARCL31F, prevented Bax activation and cytochrome c release in hydrogen peroxide-treated H9c2 cells. TAT-ARC was also effective in blocking cytochrome c release after ischemia and reperfusion, whereas TAT-ARCL31F had no effect on cytochrome c release. In addition, recombinant ARC protein abrogated Bax-induced cytochrome c release from isolated mitochondria. This suggests that ARC can protect against cell death by interfering with activation of the mitochondrial death pathway through the interaction with Bax, preventing mitochondrial dysfunction and release of pro-apoptotic factors.

Development of a PCR-SSOP approach capable of defining the natural killer cell inhibitory receptor (KIR) gene sequence repertoires

A molecular typing method based on polymerase chain reaction (PCR) amplification of three different target domains (immunoglobulin domains 1 and 3, and the transmembrane-cytoplasmic domain), followed by hybridisation with 26 digoxigenin-labelled sequence-specific oligonucleotide probes (SSOP) has been established for the polymorphic killer inhibitory receptor (KIR) genes. In addition to identifying the 12 KIR subfamilies, our PCR-SSOP typing approach could also distinguish the putative alleles, NKB1 and NKAT3, that comprise the KIR3DL1 subfamily. Ninety unrelated blood donors and 13 families (52 individuals), including both parents, were subjected to our KIR PCR-SSOP typing approach. All 12 KIR subfamilies, including a 2DS5 variant sequence, were present in the 90 individuals and displayed varied phenotype frequencies: 2DL1 (0.96), 2DL2 (0.31), 2DL3 (0.95), 2DS1 (0.56) 2DS2 (0.51), 2DS3 (0.27), 2DS4 (0.96), 2DS5v (0.35), 3DS1 (0.47), 3DL1 (0.96), 3DL2 (1.0) and 2DL4 (1.0). A total of 23 different KIR phenotypes were defined in this study, and 10 of these were only found on one occasion in one individual, indicating considerable diversity in the KIR phenotype profiles within the Irish population. Most individuals (93%) possessed the complement of inhibitory KIR specificities for the three well-defined HLA-B and -C ligands. An unusual probe pattern for 3DS1 was observed in 3 individuals indicating a variant 3DS1 gene sequence with changes at nucleotide positions 1185-1186, within the cytoplasmic domain. Sequencing analysis revealed a new single nucleotide polymorphism in exon 3 of 3DL1 NKB1(195, G-A) and a 22-bp deletion polymorphism in exon 5 of 2DS4 (nucleotides 777-798 deleted). A number of strong KIR associations were observed, namely 2DL1 with 2DL3, 2DS4 with 3DL1, 2DL2 with 2DS1/2DS2/2DS3, 2DS1 with 2DS3/2DS5v/3DS1, 2DS2 with 2DS3 and 2DS5v with 3DS1. Analysis of the KIR segregation observed in the 13 families confirmed these strong associations and permitted the definition of a number of partial KIR haplotypes, e.g. 2DL2-2DS1-2DS2-2DS3-3DL1. The segregation analysis concluded that at least 3 distinct gene loci encode 2DL1-4 and at least 4 gene loci encode the non-inhibitory KIR2DS1-2DS5. In the case of 3DL1-2 and 3DS1, our data suggests 3 gene loci, one for each subfamily.