Svetlana Saveljeva

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.

Stress-induced self-cannibalism: on the regulation of autophagy by endoplasmic reticulum stress

Macroautophagy (autophagy) is a cellular catabolic process which can be described as a self-cannibalism. It serves as an essential protective response during conditions of endoplasmic reticulum (ER) stress through the bulk removal and degradation of unfolded proteins and damaged organelles; in particular, mitochondria (mitophagy) and ER (reticulophagy). Autophagy is genetically regulated and the autophagic machinery facilitates removal of damaged cell components and proteins; however, if the cell stress is acute or irreversible, cell death ensues. Despite these advances in the field, very little is known about how autophagy is initiated and how the autophagy machinery is transcriptionally regulated in response to ER stress. Some three dozen autophagy genes have been shown to be required for the correct assembly and function of the autophagic machinery; however; very little is known about how these genes are regulated by cellular stress. Here, we will review current knowledge regarding how ER stress and the unfolded protein response (UPR) induce autophagy, including description of the different autophagy-related genes which are regulated by the UPR.

Endoplasmic reticulum stress induces ligand-independent TNFR1-mediated necroptosis in L929 cells

Endoplasmic reticulum (ER) stress-induced cellular dysfunction and death is associated with several human diseases. It has been widely reported that ER stress kills through activation of the intrinsic mitochondrial apoptotic pathway. Here we demonstrate that ER stress can also induce necroptosis, an receptor-interacting protein kinase 1 (RIPK1)/RIPK3/mixed lineage kinase domain-like protein (MLKL)-dependent form of necrosis. Remarkably, we observed that necroptosis induced by various ER stressors in L929 cells is dependent on tumor necrosis factor receptor 1 (TNFR1), but occurs independently of autocrine TNF or lymphotoxin α production. Moreover, we found that repression of either TNFR1, RIPK1 or MLKL did not protect the cells from death but instead allowed a switch to ER stress-induced apoptosis. Interestingly, while caspase inhibition was sufficient to protect TNFR1- or MLKL-deficient cells from death, rescue of the RIPK1-deficient cells additionally required RIPK3 depletion, indicating a switch back to RIPK3-dependent necroptosis in caspase-inhibited conditions. The finding that ER stress also induces necroptosis may open new therapeutic opportunities for the treatment of pathologies resulting from unresolved ER stress.

Defective ATG16L1-mediated removal of IRE1α drives Crohn’s disease–like ileitis

ATG16L1T300A, a major risk polymorphism in Crohn’s disease (CD), causes impaired autophagy, but it has remained unclear how this predisposes to CD. In this study, we report that mice with Atg16l1 deletion in intestinal epithelial cells (IECs) spontaneously develop transmural ileitis phenocopying ileal CD in an age-dependent manner, driven by the endoplasmic reticulum (ER) stress sensor IRE1&agr;. IRE1&agr; accumulates in Paneth cells of Atg16l1&Dgr;IEC mice, and humans homozygous for ATG16L1T300A exhibit a corresponding increase of IRE1&agr; in intestinal epithelial crypts. In contrast to a protective role of the IRE1&bgr; isoform, hyperactivated IRE1&agr; also drives a similar ileitis developing earlier in life in Atg16l1;Xbp1&Dgr;IEC mice, in which ER stress is induced by deletion of the unfolded protein response transcription factor XBP1. The selective autophagy receptor optineurin interacts with IRE1&agr;, and optineurin deficiency amplifies IRE1&agr; levels during ER stress. Furthermore, although dysbiosis of the ileal microbiota is present in Atg16l1;Xbp1&Dgr;IEC mice as predicted from impaired Paneth cell antimicrobial function, such structural alteration of the microbiota does not trigger ileitis but, rather, aggravates dextran sodium sulfate–induced colitis. Hence, we conclude that defective autophagy in IECs may predispose to CD ileitis via impaired clearance of IRE1&agr; aggregates during ER stress at this site.

Endoplasmic reticulum stress-mediated induction of SESTRIN 2 potentiates cell survival

Upregulation of SESTRIN 2 (SESN2) has been reported in response to diverse cellular stresses. In this study we demonstrate SESTRIN 2 induction following endoplasmic reticulum (ER) stress. ER stress-induced increases in SESTRIN 2 expression were dependent on both PERK and IRE1/XBP1 arms of the unfolded protein response (UPR). SESTRIN 2 induction, post ER stress, was responsible for mTORC1 inactivation and contributed to autophagy induction. Conversely, knockdown of SESTRIN 2 prolonged mTORC1 signaling, repressed autophagy and increased ER stress-induced cell death. Unexpectedly, the increase in ER stress-induced cell death was not linked to autophagy inhibition. Analysis of UPR pathways identified prolonged eIF2α, ATF4 and CHOP signaling in SESTRIN 2 knockdown cells following ER stress. SESTRIN 2 regulation enables UPR derived signals to indirectly control mTORC1 activity shutting down protein translation thus preventing further exacerbation of ER stress.

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.

HSPB1 facilitates ERK-mediated phosphorylation and degradation of BIM to attenuate endoplasmic reticulum stress-induced apoptosis

BIM, a pro-apoptotic BH3-only protein, is a key regulator of the intrinsic (or mitochondrial) apoptosis pathway. Here, we show that BIM induction by endoplasmic reticulum (ER) stress is suppressed in rat PC12 cells overexpressing heat shock protein B1 (HSPB1 or HSP27) and that this is due to enhanced proteasomal degradation of BIM. HSPB1 and BIM form a complex that immunoprecipitates with p-ERK1/2. We found that HSPB1-mediated proteasomal degradation of BIM is dependent on MEK-ERK signaling. Other studies have shown that several missense mutations in HSPB1 cause the peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease, which is associated with nerve degeneration. Here we show that cells overexpressing CMT-related HSPB1 mutants exhibited increased susceptibility to ER stress-induced cell death and high levels of BIM. These findings identify a novel function for HSPB1 as a negative regulator of BIM protein stability leading to protection against ER stress-induced apoptosis, a function that is absent in CMT-associated HSPB1 mutants.

ER stress responses in the absence of apoptosome: a comparative study in CASP9 proficient vs deficient mouse embryonic fibroblasts.

Cells respond to endoplasmic reticulum (ER) stress through the unfolded protein response (UPR), autophagy and cell death. In this study we utilized casp9(+/+) and casp9(-/-) MEFs to determine the effect of inhibition of mitochondrial apoptosis pathway on ER stress-induced-cell death, UPR and autophagy. We observed prolonged activation of UPR and autophagy in casp9(-/-) cells as compared with casp9(+/+) MEFs, which displayed transient activation of both pathways. Furthermore we showed that while casp9(-/-) MEFs were resistant to ER stress, prolonged exposure led to the activation of a non-canonical, caspase-mediated mode of cell death.

ATG16L1 orchestrates interleukin-22-signaling in the intestinal epithelium via cGAS/STING

A coding variant of the inflammatory bowel disease (IBD) risk gene ATG16L1 has been associated with defective autophagy and deregulation of endoplasmic reticulum (ER) function. IL-22 is a barrier protective cytokine by inducing regeneration and antimicrobial responses in the intestinal mucosa. We show that ATG16L1 critically orchestrates IL-22 signaling in the intestinal epithelium. IL-22 stimulation physiologically leads to transient ER stress and subsequent activation of STING-dependent type I interferon (IFN-I) signaling, which is augmented in Atg16l1&Dgr;IEC intestinal organoids. IFN-I signals amplify epithelial TNF production downstream of IL-22 and contribute to necroptotic cell death. In vivo, IL-22 treatment in Atg16l1&Dgr;IEC and Atg16l1&Dgr;IEC/Xbp1&Dgr;IEC mice potentiates endogenous ileal inflammation and causes widespread necroptotic epithelial cell death. Therapeutic blockade of IFN-I signaling ameliorates IL-22–induced ileal inflammation in Atg16l1&Dgr;IEC mice. Our data demonstrate an unexpected role of ATG16L1 in coordinating the outcome of IL-22 signaling in the intestinal epithelium.