IRE1α inhibition attenuates neuronal pyroptosis via miR-125/NLRP1 pathway in a neonatal hypoxic-ischemic encephalopathy rat model

BACKGROUND

Inhibition of inositol-requiring enzyme-1 alpha (IRE1α), one of the sensor signaling proteins associated with endoplasmic reticulum (ER) stress, has been shown to alleviate brain injury and improve neurological behavior in a neonatal hypoxic-ischemic encephalopathy (HIE) rat model. However, there is no information about the role of IRE1α inhibitor as well as its molecular mechanisms in preventing neuronal pyroptosis induced by NLRP1 (NOD-, LRR- and pyrin domain-containing 1) inflammasome. In the present study, we hypothesized that IRE1α can degrade microRNA-125-b-2-3p (miR-125-b-2-3p) and activate NLRP1/caspased-1 pathway, and subsequently promote neuronal pyroptosis in HIE rat model.

METHODS

Ten-day old unsexed rat pups were subjected to hypoxia-ischemia (HI) injury, and the inhibitor of IRE1α, STF083010, was administered intranasally at 1 h after HI induction. AntimiR-125 or NLRP1 activation CRISPR was administered by intracerebroventricular (i.c.v) injection at 24 h before HI induction. Immunofluorescence staining, western blot analysis, reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR), brain infarct volume measurement, neurological function tests, and Fluoro-Jade C staining were performed.

RESULTS

Endogenous phosphorylated IRE1α (p-IRE1α), NLRP1, cleaved caspase-1, interleukin-1β (IL-1β), and interleukin-18 (IL-18) were increased and miR-125-b-2-3p was decreased in HIE rat model. STF083010 administration significantly upregulated the expression of miR-125-b-2-3p, reduced the infarct volume, improved neurobehavioral outcomes and downregulated the protein expression of NLRP1, cleaved caspase-1, IL-1β and IL-18. The protective effects of STF083010 were reversed by antimiR-125 or NLRP1 activation CRISPR.

CONCLUSIONS

IRE1α inhibitor, STF083010, reduced neuronal pyroptosis at least in part via miR-125/NLRP1/caspase-1 signaling pathway after HI.

Targeting the Unfolded Protein Response in Hormone-Regulated Cancers

Cancer cells exploit many of the cellular adaptive responses to support their survival needs. One of these is the unfolded protein response (UPR), a highly conserved signaling pathway that is mounted in response to endoplasmic reticulum (ER) stress. Recent work showed that steroid hormones, in particular estrogens and androgens, regulate the canonical UPR pathways in breast cancer (BCa) and prostate cancer (PCa). In addition, UPR has pleiotropic effects in advanced disease and development of therapy resistance. These findings implicate the UPR pathway as a novel target in hormonally regulated cancers in the clinic. Here, we review the potential therapeutic value of recently developed small molecule inhibitors of UPR in hormone regulated cancers.

The metastasis suppressor, NDRG1, differentially modulates the endoplasmic reticulum stress response

The metastasis suppressor, N-myc downstream regulated gene-1 (NDRG1), is a stress response protein that is involved in the inhibition of multiple oncogenic signaling pathways. Initial studies have linked NDRG1 and the endoplasmic reticulum (ER) stress response. Considering this, we extensively examined the mechanism by which NDRG1 regulates the ER stress response in pancreatic and colon cancer cells. We also examined the anti-cancer agent, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), which induces NDRG1 expression and causes ER stress. The expression of NDRG1 was demonstrated to regulate the three main arms of the ER stress response by: (1) increasing the expression of three major ER chaperones, binding immunoglobulin protein (BiP), calreticulin, and calnexin; (2) suppressing the protein kinase, RNA-activated (PKR)-like ER kinase (PERK); (3) inhibiting the inositol-requiring kinase 1α (IRE1α) arm; and (4) increasing the cleavage of activating transcription factor 6 (ATF6). An important finding was that NDRG1 enhances the anti-proliferative and anti-migratory activity of Dp44mT. This increased efficacy could be related to the following effects in the presence of Dp44mT and NDRG1, namely: markedly increased activation of the PERK target, eukaryotic translation initiation factor 2α (eIF2α); the maintenance of activating transcription factor 4 (ATF4) expression; high cytosolic Ca⁺² that increases the sensitivity of cells to apoptosis via activation of the calmodulin-dependent kinase II (CaMKII) signaling cascade; and increased pro-apoptotic C/EBP-homologous protein (CHOP) expression. Collectively, this investigation dissects the molecular mechanisms through which NDRG1 manipulates the ER stress response and its ability to potentiate the activity of the potent anti-cancer agent, Dp44mT.

Binding to an Unusual Inactive Kinase Conformation by Highly Selective Inhibitors of Inositol-Requiring Enzyme 1α Kinase-Endoribonuclease

A series of imidazo[1,2- b]pyridazin-8-amine kinase inhibitors were discovered to allosterically inhibit the endoribonuclease function of the dual kinase-endoribonuclease inositol-requiring enzyme 1α (IRE1α), a key component of the unfolded protein response in mammalian cells and a potential drug target in multiple human diseases. Inhibitor optimization gave compounds with high kinome selectivity that prevented endoplasmic reticulum stress-induced IRE1α oligomerization and phosphorylation, and inhibited endoribonuclease activity in human cells. X-ray crystallography showed the inhibitors to bind to a previously unreported and unusually disordered conformation of the IRE1α kinase domain that would be incompatible with back-to-back dimerization of the IRE1α protein and activation of the endoribonuclease function. These findings increase the repertoire of known IRE1α protein conformations and can guide the discovery of highly selective ligands for the IRE1α kinase site that allosterically inhibit the endoribonuclease.

Riluzole induces AR degradation via endoplasmic reticulum stress pathway in androgen-dependent and castration-resistant prostate cancer cells

BACKGROUND

Prostate cancer (PCa) is diagnosed at the highest rate of all non-cutaneous male cancers in the United States. The androgen-dependent (AD) transcription factor, androgen receptor (AR), drives PCa-but inhibiting AR or androgen biosynthesis induces remission for only a short time. At which point, patients acquire more aggressive castration-resistant (CR) disease with re-activated AR-dependent signaling. To combat treatment resistance, down-regulating AR protein expression has been considered as a potential treatment strategy for CR-PCa.

METHODS

AD- and CR-PCa cell lines were treated with the well-tolerated FDA-approved oral medicine, riluzole. Expression of full-length or wild-type AR (AR-FL) and constitutively active AR-splice variant 7 (AR-V7) was assessed by immunoblotting. AR-FL/AR-V7 activity was measured using qRT-PCR of AR-target genes. Cytoplasmic [Ca²⁺ ] levels were measured using a fluorescent Ca²⁺ indicator microplate assay. Markers of the endoplasmic reticulum stress (ERS) pathway and autophagy were assessed by immunoblotting. Direct interaction between AR and selective autophagy receptor p62 was demonstrated by co-immunoprecipitation.

RESULTS

We demonstrate that riluzole downregulates AR-FL, mutant ARs, and AR-V7 proteins expression by protein degradation through ERS pathway and selective autophagy. Riluzole also significantly inhibited AR transcription activity by decreasing its target genes expression (PSA, TMPRSS2, and KLK2).

CONCLUSIONS

We provide key mechanistic insights by which riluzole exerts its anti-tumorigenic effects and induces AR protein degradation via ERS pathways. Our findings support the potential utility of riluzole for treatment of PCa.

IRE1α-XBP1s pathway promotes prostate cancer by activating c-MYC signaling

Activation of endoplasmic reticulum (ER) stress/the unfolded protein response (UPR) has been linked to cancer, but the molecular mechanisms are poorly understood and there is a paucity of reagents to translate this for cancer therapy. Here, we report that an IRE1α RNase-specific inhibitor, MKC8866, strongly inhibits prostate cancer (PCa) tumor growth as monotherapy in multiple preclinical models in mice and shows synergistic antitumor effects with current PCa drugs. Interestingly, global transcriptomic analysis reveal that IRE1α-XBP1s pathway activity is required for c-MYC signaling, one of the most highly activated oncogenic pathways in PCa. XBP1s is necessary for optimal c-MYC mRNA and protein expression, establishing, for the first time, a direct link between UPR and oncogene activation. In addition, an XBP1-specific gene expression signature is strongly associated with PCa prognosis. Our data establish IRE1α-XBP1s signaling as a central pathway in PCa and indicate that its targeting may offer novel treatment strategies.

Small molecule inhibition of IRE1α kinase/RNase has anti-fibrotic effects in the lung

Endoplasmic reticulum stress (ER stress) has been implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF), a disease of progressive fibrosis and respiratory failure. ER stress activates a signaling pathway called the unfolded protein response (UPR) that either restores homeostasis or promotes apoptosis. The bifunctional kinase/RNase IRE1α is a UPR sensor/effector that promotes apoptosis if ER stress remains high and irremediable (i.e., a "terminal" UPR). Using multiple small molecule inhibitors against IRE1α, we show that ER stress-induced apoptosis of murine alveolar epithelial cells can be mitigated in vitro. In vivo, we show that bleomycin exposure to murine lungs causes early ER stress to activate IRE1α and the terminal UPR prior to development of pulmonary fibrosis. Small-molecule IRE1α kinase-inhibiting RNase attenuators (KIRAs) that we developed were used to evaluate the contribution of IRE1α activation to bleomycin-induced pulmonary fibrosis. One such KIRA-KIRA7-provided systemically to mice at the time of bleomycin exposure decreases terminal UPR signaling and prevents lung fibrosis. Administration of KIRA7 14 days after bleomycin exposure even promoted the reversal of established fibrosis. Finally, we show that KIRA8, a nanomolar-potent, monoselective KIRA compound derived from a completely different scaffold than KIRA7, likewise promoted reversal of established fibrosis. These results demonstrate that IRE1α may be a promising target in pulmonary fibrosis and that kinase inhibitors of IRE1α may eventually be developed into efficacious anti-fibrotic drugs.

Ethyl Acetate Extract of Scindapsus cf. hederaceus Exerts the Inhibitory Bioactivity on Human Non-Small Cell Lung Cancer Cells through Modulating ER Stress

Unfolded protein response (UPR) is a cytoprotective mechanism that alleviates the protein-folding burden in eukaryotic organisms. Moderate activation of UPR is required for maintaining endoplasmic reticulum (ER) homeostasis and profoundly contributes to tumorigenesis. Defects in UPR signaling are implicated in the attenuation of various malignant phenotypes including cell proliferation, migration, and invasion, as well as angiogenesis. This suggests UPR as a promising target in cancer therapy. The pharmacological effects of the plant Scindapsus cf. hederaceus on human cancer cell lines is not understood. In this study, we identified an ethyl acetate extract from Scindapsus cf. hederaceus (SH-EAE), which markedly altered the protein expression of UPR-related genes in human non-small cell lung cancer (NSCLC) cells. Treatment with the SH-EAE led to the dose-dependent suppression of colony forming ability of both H1299 and H460 cells, but not markedly in normal bronchial epithelial BEAS-2B cells. SH-EAE treatment also attenuated the migration and invasion ability of H1299 and H460 cells. Moreover, SH-EAE strikingly suppressed the protein expression of two ER stress sensors, including inositol requiring enzyme-1α (IRE-1α) and protein kinase R-like ER kinase (PERK), and antagonized the induction of C/EBP homologous protein (CHOP) expression by thapsigargin, an ER stress inducer. SH-EAE induced the formation of massive vacuoles which are probably derived from ER. Importantly, SH-EAE impaired the formation of intersegmental vessels (ISV) in zebrafish larvae, an index of angiogenesis, but had no apparent effect on the rate of larval development. Together, our findings demonstrate, for the first time, that the ability of SH-EAE specifically targets the two sensors of UPR, with significant anti-proliferation and anti-migration activities as a crude extract in human NSCLC cells. Our finding also indicates potential applications of SH-EAE in preventing UPR activation in response to Tg-induced ER stress. We suggest that SH-EAE attenuates UPR adaptive pathways for rendering the NSCLC cells intolerant to ER stress.

Pharmacologic inhibition of S1P attenuates ATF6 expression, causes ER stress and contributes to apoptotic cell death

Mammalian cells express unique transcription factors embedded in the endoplasmic reticulum (ER) membrane, such as the sterol regulatory element-binding proteins (SREBPs), that promote de novo lipogenesis. Upon their release from the ER, the SREBPs require proteolytic activation in the Golgi by site-1-protease (S1P). As such, inhibition of S1P, using compounds such as PF-429242 (PF), reduces cholesterol synthesis and may represent a new strategy for the management of dyslipidemia. In addition to the SREBPs, the unfolded protein response (UPR) transducer, known as the activating transcription factor 6 (ATF6), is another ER membrane-bound transcription factor that requires S1P-mediated activation. ATF6 regulates ER protein folding capacity by promoting the expression of ER chaperones such as the 78-kDa glucose-regulated protein (GRP78). ER-resident chaperones like GRP78 prevent and/or resolve ER polypeptide accumulation and subsequent ER stress-induced UPR activation by folding nascent polypeptides. Here we report that pharmacological inhibition of S1P reduced the expression of ATF6 and GRP78 and induced the activation of UPR transducers inositol-requiring enzyme-1α (IRE1α) and protein kinase RNA-like ER kinase (PERK). As a consequence, S1P inhibition also increased the susceptibility of cells to ER stress-induced cell death. Our findings suggest that S1P plays a crucial role in the regulation of ER folding capacity and also identifies a compensatory cross-talk between UPR transducers in order to maintain adequate ER chaperone expression and activity.

Small Molecule Targeted Recruitment of a Nuclease to RNA

The choreography between RNA synthesis and degradation is a key determinant in biology. Engineered systems such as CRISPR have been developed to rid a cell of RNAs. Here, we show that a small molecule can recruit a nuclease to a specific transcript, triggering its destruction. A small molecule that selectively binds the oncogenic microRNA(miR)-96 hairpin precursor was appended with a short 2'-5' poly(A) oligonucleotide. The conjugate locally activated endogenous, latent ribonuclease (RNase L), which selectively cleaved the miR-96 precursor in cancer cells in a catalytic and sub-stoichiometric fashion. Silencing miR-96 derepressed pro-apoptotic FOXO1 transcription factor, triggering apoptosis in breast cancer, but not healthy breast, cells. These results demonstrate that small molecules can be programmed to selectively cleave RNA via nuclease recruitment and has broad implications.

A novel mechanism of RNase L inhibition: Theiler's virus L* protein prevents 2-5A from binding to RNase L

The OAS/RNase L pathway is one of the best-characterized effector pathways of the IFN antiviral response. It inhibits the replication of many viruses and ultimately promotes apoptosis of infected cells, contributing to the control of virus spread. However, viruses have evolved a range of escape strategies that act against different steps in the pathway. Here we unraveled a novel escape strategy involving Theiler's murine encephalomyelitis virus (TMEV) L* protein. Previously we found that L* was the first viral protein binding directly RNase L. Our current data show that L* binds the ankyrin repeats R1 and R2 of RNase L and inhibits 2'-5' oligoadenylates (2-5A) binding to RNase L. Thereby, L* prevents dimerization and oligomerization of RNase L in response to 2-5A. Using chimeric mouse hepatitis virus (MHV) expressing TMEV L*, we showed that L* efficiently inhibits RNase L in vivo. Interestingly, those data show that L* can functionally substitute for the MHV-encoded phosphodiesterase ns2, which acts upstream of L* in the OAS/RNase L pathway, by degrading 2-5A.

Angiotensin II Causes β-Cell Dysfunction Through an ER Stress-Induced Proinflammatory Response

The metabolic syndrome is associated with an increase in the activation of the renin angiotensin system, whose inhibition reduces the incidence of new-onset diabetes. Importantly, angiotensin II (AngII), independently of its vasoconstrictor action, causes β-cell inflammation and dysfunction, which may be an early step in the development of type 2 diabetes. The aim of this study was to determine how AngII causes β-cell dysfunction. Islets of Langerhans were isolated from C57BL/6J mice that had been infused with AngII in the presence or absence of taurine-conjugated ursodeoxycholic acid (TUDCA) and effects on endoplasmic reticulum (ER) stress, inflammation, and β-cell function determined. The mechanism of action of AngII was further investigated using isolated murine islets and clonal β cells. We show that AngII triggers ER stress, an increase in the messenger RNA expression of proinflammatory cytokines, and promotes β-cell dysfunction in murine islets of Langerhans both in vivo and ex vivo. These effects were significantly attenuated by TUDCA, an inhibitor of ER stress. We also show that AngII-induced ER stress is required for the increased expression of proinflammatory cytokines and is caused by reactive oxygen species and IP3 receptor activation. These data reveal that the induction of ER stress is critical for AngII-induced β-cell dysfunction and indicates how therapies that promote ER homeostasis may be beneficial in the prevention of type 2 diabetes.

In vitro and in vivo effects of 2,4 diaminoquinazoline inhibitors of the decapping scavenger enzyme DcpS: Context-specific modulation of SMN transcript levels

C5-substituted 2,4-diaminoquinazoline inhibitors of the decapping scavenger enzyme DcpS (DAQ-DcpSi) have been developed for the treatment of spinal muscular atrophy (SMA), which is caused by genetic deficiency in the Survival Motor Neuron (SMN) protein. These compounds are claimed to act as SMN2 transcriptional activators but data underlying that claim are equivocal. In addition it is unclear whether the claimed effects on SMN2 are a direct consequence of DcpS inhibitor or might be a consequence of lysosomotropism, which is known to be neuroprotective. DAQ-DcpSi effects were characterized in cells in vitro utilizing DcpS knockdown and 7-methyl analogues as probes for DcpS vs non-DcpS-mediated effects. We also performed analysis of Smn transcript levels, RNA-Seq analysis of the transcriptome and SMN protein in order to identify affected pathways underlying the therapeutic effect, and studied lysosomotropic and non-lysosomotropic DAQ-DCpSi effects in 2B/- SMA mice. Treatment of cells caused modest and transient SMN2 mRNA increases with either no change or a decrease in SMNΔ7 and no change in SMN1 transcripts or SMN protein. RNA-Seq analysis of DAQ-DcpSi-treated N2a cells revealed significant changes in expression (both up and down) of approximately 2,000 genes across a broad range of pathways. Treatment of 2B/- SMA mice with both lysomotropic and non-lysosomotropic DAQ-DcpSi compounds had similar effects on disease phenotype indicating that the therapeutic mechanism of action is not a consequence of lysosomotropism. In striking contrast to the findings in vitro, Smn transcripts were robustly changed in tissues but there was no increase in SMN protein levels in spinal cord. We conclude that DAQ-DcpSi have reproducible benefit in SMA mice and a broad spectrum of biological effects in vitro and in vivo, but these are complex, context specific, and not the result of simple SMN2 transcriptional activation.

RraAS1 inhibits the ribonucleolytic activity of RNase ES by interacting with its catalytic domain in Streptomyces coelicolor

RraA is a protein inhibitor of RNase E, which degrades and processes numerous RNAs in Escherichia coli. Streptomyces coelicolor also contains homologs of RNase E and RraA, RNase ES and RraAS1/RraAS2, respectively. Here, we report that, unlike other RraA homologs, RraAS1 directly interacts with the catalytic domain of RNase ES to exert its inhibitory effect. We further show that rraAS1 gene deletion in S. coelicolor results in a higher growth rate and increased production of actinorhodin and undecylprodigiosin, compared with the wild-type strain, suggesting that RraAS1-mediated regulation of RNase ES activity contributes to modulating the cellular physiology of S. coelicolor.

Structural elucidation of a novel mechanism for the bacteriophage-based inhibition of the RNA degradosome

In all domains of life, the catalysed degradation of RNA facilitates rapid adaptation to changing environmental conditions, while destruction of foreign RNA is an important mechanism to prevent host infection. We have identified a virus-encoded protein termed gp37/Dip, which directly binds and inhibits the RNA degradation machinery of its bacterial host. Encoded by giant phage фKZ, this protein associates with two RNA binding sites of the RNase E component of the Pseudomonas aeruginosa RNA degradosome, occluding them from substrates and resulting in effective inhibition of RNA degradation and processing. The 2.2 Å crystal structure reveals that this novel homo-dimeric protein has no identifiable structural homologues. Our biochemical data indicate that acidic patches on the convex outer surface bind RNase E. Through the activity of Dip, фKZ has evolved a unique mechanism to down regulate a key metabolic process of its host to allow accumulation of viral RNA in infected cells.

XBP1: A Pivotal Transcriptional Regulator of Glucose and Lipid Metabolism

X-box binding protein 1 (XBP1) is a major, well-conserved component of the unfolded protein response (UPR) that is crucial for glucose homeostasis and lipid metabolism. Metabolic dysfunction has been associated with XBP1 transcriptional activity. Recently, compounds selectively targeting the IRE1α-XBP1 pathway have emerged as a potential approach for treatment of metabolic diseases.

DcpS is a transcript-specific modulator of RNA in mammalian cells

The scavenger decapping enzyme DcpS is a multifunctional protein initially identified by its property to hydrolyze the resulting cap structure following 3' end mRNA decay. In Saccharomyces cerevisiae, the DcpS homolog Dcs1 is an obligate cofactor for the 5'-3' exoribonuclease Xrn1 while the Caenorhabditis elegans homolog Dcs-1, facilitates Xrn1 mediated microRNA turnover. In both cases, this function is independent of the decapping activity. Whether DcpS and its decapping activity can affect mRNA steady state or stability in mammalian cells remains unknown. We sought to determine DcpS target genes in mammalian cells using a cell-permeable DcpS inhibitor compound, RG3039 initially developed for therapeutic treatment of spinal muscular atrophy. Global mRNA levels were examined following DcpS decapping inhibition with RG3039. The steady-state levels of 222 RNAs were altered upon RG3039 treatment. Of a subset selected for validation, two transcripts that appear to be long noncoding RNAs HS370762 and BC011766, were dependent on DcpS and its scavenger decapping catalytic activity and referred to as DcpS-responsive noncoding transcripts (DRNT) 1 and 2, respectively. Interestingly, only the increase in DRNT1 transcript was accompanied with an increase of its RNA stability and this increase was dependent on both DcpS and Xrn1. Importantly, unlike in yeast where the DcpS homolog is an obligate cofactor for Xrn1, stability of additional Xrn1 dependent RNAs were not altered by a reduction in DcpS levels. Collectively, our data demonstrate that DcpS in conjunction with Xrn1 has the potential to regulate RNA stability in a transcript-selective manner in mammalian cells.

Molecular mechanisms of human IRE1 activation through dimerization and ligand binding

IRE1 transduces the unfolded protein response by splicing XBP1 through its C-terminal cytoplasmic kinase-RNase region. IRE1 autophosphorylation is coupled to RNase activity through formation of a back-to-back dimer, although the conservation of the underlying molecular mechanism is not clear from existing structures. We have crystallized human IRE1 in a back-to-back conformation only previously seen for the yeast homologue. In our structure the kinase domain appears primed for catalysis but the RNase domains are disengaged. Structure-function analysis reveals that IRE1 is autoinhibited through a Tyr-down mechanism related to that found in the unrelated Ser/Thr protein kinase Nek7. We have developed a compound that potently inhibits human IRE1 kinase activity while stimulating XBP1 splicing. A crystal structure of the inhibitor bound to IRE1 shows an increased ordering of the kinase activation loop. The structures of hIRE in apo and ligand-bound forms are consistent with a previously proposed model of IRE1 regulation in which formation of a back-to-back dimer coupled to adoption of a kinase-active conformation drive RNase activation. The structures provide opportunities for structure-guided design of IRE1 inhibitors.

INHIBITION OF ERN1 SIGNALING ENZYME AFFECTS HYPOXIC REGULATION OF THE EXPRESSION OF E2F8, EPAS1, HOXC6, ATF3, TBX3 AND FOXF1 GENES IN U87 GLIOMA CELLS

Hypoxia as well as the endoplasmic reticulum stress are important factors of malignant tumor growth and control of the expression of genes, which regulate numerous metabolic processes and cell proliferation. Furthermore, blockade of ERN1 (endoplasmic reticulum to nucleus 1) suppresses cell proliferation and tumor growth. We studied the effect of hypoxia on the expression of genes encoding the transcription factors such as E2F8 (E2F transcription factor 8), EPAS1 (endothelial PAS domain protein 1), TBX3 (T-box 3), ATF3 (activating transcription factor 3), FOXF1 (forkhead box F), and HOXC6 (homeobox C6) in U87 glioma cells with and without ERN1 signaling enzyme function. We have established that hypoxia enhances the expression of HOXC6, E2F8, ATF3, and EPAS1 genes but does not change TBX3 and FOXF1 gene expression in glioma cells with ERNI function. At the same time, the expression level of all studied genes is strongly decreased, except for TBX3 gene, in glioma cells without ERN1 function. Moreover, the inhibition of ERN1 signaling enzyme function significantly modifies the effect of hypoxia on the expression of these transcription factor genes. removes or introduces this regulation as well as changes a direction or magnitude of hypoxic regulation. Present study demonstrates that fine-tuning of the expression of proliferation related genes depends upon hypoxia and ERN1-mediated endoplasmic reticulum stress signaling and correlates with slower proliferation rate of glioma cells without ERN1 function.

The first small-molecule inhibitors of members of the ribonuclease E family

The Escherichia coli endoribonuclease RNase E is central to the processing and degradation of all types of RNA and as such is a pleotropic regulator of gene expression. It is essential for growth and was one of the first examples of an endonuclease that can recognise the 5'-monophosphorylated ends of RNA thereby increasing the efficiency of many cleavages. Homologues of RNase E can be found in many bacterial families including important pathogens, but no homologues have been identified in humans or animals. RNase E represents a potential target for the development of new antibiotics to combat the growing number of bacteria that are resistant to antibiotics in use currently. Potent small molecule inhibitors that bind the active site of essential enzymes are proving to be a source of potential drug leads and tools to dissect function through chemical genetics. Here we report the use of virtual high-throughput screening to obtain small molecules predicted to bind at sites in the N-terminal catalytic half of RNase E. We show that these compounds are able to bind with specificity and inhibit catalysis of Escherichia coli and Mycobacterium tuberculosis RNase E and also inhibit the activity of RNase G, a paralogue of RNase E.

Allosteric inhibition of the IRE1α RNase preserves cell viability and function during endoplasmic reticulum stress

Depending on endoplasmic reticulum (ER) stress levels, the ER transmembrane multidomain protein IRE1α promotes either adaptation or apoptosis. Unfolded ER proteins cause IRE1α lumenal domain homo-oligomerization, inducing trans autophosphorylation that further drives homo-oligomerization of its cytosolic kinase/endoribonuclease (RNase) domains to activate mRNA splicing of adaptive XBP1 transcription factor. However, under high/chronic ER stress, IRE1α surpasses an oligomerization threshold that expands RNase substrate repertoire to many ER-localized mRNAs, leading to apoptosis. To modulate these effects, we developed ATP-competitive IRE1α Kinase-Inhibiting RNase Attenuators-KIRAs-that allosterically inhibit IRE1α's RNase by breaking oligomers. One optimized KIRA, KIRA6, inhibits IRE1α in vivo and promotes cell survival under ER stress. Intravitreally, KIRA6 preserves photoreceptor functional viability in rat models of ER stress-induced retinal degeneration. Systemically, KIRA6 preserves pancreatic β cells, increases insulin, and reduces hyperglycemia in Akita diabetic mice. Thus, IRE1α powerfully controls cell fate but can itself be controlled with small molecules to reduce cell degeneration.

Inhibition of ER stress-associated IRE-1/XBP-1 pathway reduces leukemic cell survival

Activation of the ER stress response is associated with malignant progression of B cell chronic lymphocytic leukemia (CLL). We developed a murine CLL model that lacks the ER stress-associated transcription factor XBP-1 in B cells and found that XBP-1 deficiency decelerates malignant progression of CLL-associated disease. XBP-1 deficiency resulted in acquisition of phenotypes that are disadvantageous for leukemic cell survival, including compromised BCR signaling capability and increased surface expression of sphingosine-1-phosphate receptor 1 (S1P1). Because XBP-1 expression requires the RNase activity of the ER transmembrane receptor IRE-1, we developed a potent IRE-1 RNase inhibitor through chemical synthesis and modified the structure to facilitate entry into cells to target the IRE-1/XBP-1 pathway. Treatment of CLL cells with this inhibitor (B-I09) mimicked XBP-1 deficiency, including upregulation of IRE-1 expression and compromised BCR signaling. Moreover, B-I09 treatment did not affect the transport of secretory and integral membrane-bound proteins. Administration of B-I09 to CLL tumor-bearing mice suppressed leukemic progression by inducing apoptosis and did not cause systemic toxicity. Additionally, B-I09 and ibrutinib, an FDA-approved BTK inhibitor, synergized to induce apoptosis in B cell leukemia, lymphoma, and multiple myeloma. These data indicate that targeting XBP-1 has potential as a treatment strategy, not only for multiple myeloma, but also for mature B cell leukemia and lymphoma.

ATF6 upregulates XBP1S and inhibits ER stress-mediated apoptosis in osteoarthritis cartilage

As we previously reported, transcription factor XBP1S enhances BMP2-induced chondrocyte differentiation and acts as a positive mediator of chondrocyte hypertrophy. The purpose of this study was to determine (1) whether XBP1S influences ER stress-mediated apoptosis in osteoarthritis (OA); (2) whether ATF6 regulates IRE1/XBP1 signal pathway in OA cartilage; (3) what are the associated molecules affecting apoptosis in osteoarthritis and the molecular events underlying this process. Herein, we examined and found that ER stress-associated molecules were activated in OA patients, specifically XBP1S splice and expression were increased markedly by TNF-α and IL-1β treatments. Transcription factor ATF6 can specifically bind to the promoter of XBP1 gene and enhance the expression of XBP1S spliced by IRE1α in osteoarthritis cartilage. Furthermore, siXBP1S can enhance ER stress-mediated apoptosis and main matrix degradation in osteoarthritis. Whereas AdXBP1S can inhibit ER stress-mediated apoptosis and TNFα induced nitrite production in OA cartilage. In a word, our observations demonstrate the importance of XBP1S in osteoarthritis. ATF6 and IRE1α can regulate endogenous XBP1S gene expression synergistically in OA cartilage. More significantly, XBP1S was a negative regulator of apoptosis in osteoarthritis by affecting caspase 3, caspase 9, caspase 12, p-JNK1, and CHOP.

Sustained IRE1 and ATF6 signaling is important for survival of melanoma cells undergoing ER stress

Apoptosis triggered by endoplasmic reticulum (ER) stress is associated with rapid attenuation of the IRE1α and ATF6 pathways but persistent activation of the PERK branch of the unfolded protein response (UPR) in cells. However, melanoma cells are largely resistant to ER stress-induced apoptosis, suggesting that the kinetics and durations of activation of the UPR pathways are deregulated in melanoma cells undergoing ER stress. We show here that the IRE1α and ATF6 pathways are sustained along with the PERK signaling in melanoma cells subjected to pharmacological ER stress, and that this is, at least in part, due to increased activation of the MEK/ERK pathway. In contrast to an initial increase followed by rapid reduction in activation of IRE1α and ATF6 signaling in control cells that were relatively sensitive to ER stress-induced apoptosis, activation of IRE1α and ATF6 by the pharmacological ER stress inducer tunicamycin (TM) or thapsigargin (TG) persisted in melanoma cells. On the other hand, the increase in PERK signaling lasted similarly in both types of cells. Sustained activation of IRE1α and ATF6 signaling played an important role in protecting melanoma cells from ER stress-induced apoptosis, as interruption of IRE1α or ATF6 rendered melanoma cells sensitive to apoptosis induced by TM or TG. Inhibition of MEK partially blocked IRE1α and ATF6 activation, suggesting that MEK/ERK signaling contributed to sustained activation of IRE1α and ATF6. Taken together, these results identify sustained activation of the IRE1α and ATF6 pathways of the UPR driven by the MEK/ERK pathway as an important protective mechanism against ER stress-induced apoptosis in melanoma cells.

The DcpS inhibitor RG3039 improves motor function in SMA mice

Spinal muscular atrophy (SMA) is caused by mutations of the survival motor neuron 1 (SMN1) gene, retention of the survival motor neuron 2 (SMN2) gene and insufficient expression of full-length survival motor neuron (SMN) protein. Quinazolines increase SMN2 promoter activity and inhibit the ribonucleic acid scavenger enzyme DcpS. The quinazoline derivative RG3039 has advanced to early phase clinical trials. In preparation for efficacy studies in SMA patients, we investigated the effects of RG3039 in severe SMA mice. Here, we show that RG3039 distributed to central nervous system tissues where it robustly inhibited DcpS enzyme activity, but minimally activated SMN expression or the assembly of small nuclear ribonucleoproteins. Nonetheless, treated SMA mice showed a dose-dependent increase in survival, weight and motor function. This was associated with improved motor neuron somal and neuromuscular junction synaptic innervation and function and increased muscle size. RG3039 also enhanced survival of conditional SMA mice in which SMN had been genetically restored to motor neurons. As this systemically delivered drug may have therapeutic benefits that extend beyond motor neurons, it could act additively with SMN-restoring therapies delivered directly to the central nervous system such as antisense oligonucleotides or gene therapy.