Protein kinase R is an innate immune sensor of proteotoxic stress via accumulation of cytoplasmic IL-24

Proteasome dysfunction can lead to autoinflammatory disease associated with elevated type I interferon (IFN-αβ) and NF-κB signaling; however, the innate immune pathway driving this is currently unknown. Here, we identified protein kinase R (PKR) as an innate immune sensor for proteotoxic stress. PKR activation was observed in cellular models of decreased proteasome function and in multiple cell types from patients with proteasome-associated autoinflammatory disease (PRAAS). Furthermore, genetic deletion or small-molecule inhibition of PKR in vitro ameliorated inflammation driven by proteasome deficiency. In vivo, proteasome inhibitor-induced inflammatory gene transcription was blunted in PKR-deficient mice compared with littermate controls. PKR also acted as a rheostat for proteotoxic stress by triggering phosphorylation of eIF2α, which can prevent the translation of new proteins to restore homeostasis. Although traditionally known as a sensor of RNA, under conditions of proteasome dysfunction, PKR sensed the cytoplasmic accumulation of a known interactor, interleukin-24 (IL-24). When misfolded IL-24 egress into the cytosol was blocked by inhibition of the endoplasmic reticulum-associated degradation pathway, PKR activation and subsequent inflammatory signaling were blunted. Cytokines such as IL-24 are normally secreted from cells; therefore, cytoplasmic accumulation of IL-24 represents an internal danger-associated molecular pattern. Thus, we have identified a mechanism by which proteotoxic stress is detected, causing inflammation observed in the disease PRAAS.

A cold-stress-inducible PERK/OGT axis controls TOM70-assisted mitochondrial protein import and cristae formation

The architecture of cristae provides a spatial mitochondrial organization that contains functional respiratory complexes. Several protein components including OPA1 and MICOS complex subunits organize cristae structure, but upstream regulatory mechanisms are largely unknown. Here, in vivo and in vitro reconstitution experiments show that the endoplasmic reticulum (ER) kinase PERK promotes cristae formation by increasing TOM70-assisted mitochondrial import of MIC19, a critical subunit of the MICOS complex. Cold stress or β-adrenergic stimulation activates PERK that phosphorylates O-linked N-acetylglucosamine transferase (OGT). Phosphorylated OGT glycosylates TOM70 on Ser94, enhancing MIC19 protein import into mitochondria and promoting cristae formation and respiration. In addition, PERK-activated OGT O-GlcNAcylates and attenuates CK2α activity, which mediates TOM70 Ser94 phosphorylation and decreases MIC19 mitochondrial protein import. We have identified a cold-stress inter-organelle PERK-OGT-TOM70 axis that increases cell respiration through mitochondrial protein import and subsequent cristae formation. These studies have significant implications in cellular bioenergetics and adaptations to stress conditions.

Sterile activation of invariant natural killer T cells by ER-stressed antigen-presenting cells

Invariant NKT (iNKT) cells have the unique ability to shape immunity during antitumor immune responses and other forms of sterile and nonsterile inflammation. Recent studies have highlighted a variety of classes of endogenous and pathogen-derived lipid antigens that can trigger iNKT cell activation under sterile and nonsterile conditions. However, the context and mechanisms that drive the presentation of self-lipid antigens in sterile inflammation remain unclear. Here we report that endoplasmic reticulum (ER)-stressed myeloid cells, via signaling events modulated by the protein kinase RNA-like ER kinase (PERK) pathway, increase CD1d-mediated presentation of immunogenic endogenous lipid species, which results in enhanced iNKT cell activation both in vitro and in vivo. In addition, we demonstrate that actin cytoskeletal reorganization during ER stress results in an altered distribution of CD1d on the cell surface, which contributes to enhanced iNKT cell activation. These results define a previously unidentified mechanism that controls iNKT cell activation during sterile inflammation.

PKR modulates abnormal brain signaling in experimental obesity

Metabolic disorders including obesity and type 2 diabetes are known to be associated with chronic inflammation and are obvious risk factors for Alzheimer's disease. Recent evidences concerning obesity and diabetes suggest that the metabolic inflammasome ("metaflammasome") mediates chronic inflammation. The double-stranded RNA-dependent protein kinase (PKR) is a central component of the metaflammasome. In wild type (WT) and PKR-/- mice, blood glucose, insulin and lipid levels and the brain expression of the phosphorylated components of the metaflammasome-PKR, JNK, IRS1 and IKKbeta-were studied after the induction of obesity by a high fat diet (HFD). The results showed significant increased levels of activated brain metaflammasome proteins in exposed WT mice but the changes were not significant in PKR-/- mice. In addition, gain weight was observed in WT mice and also in PKR-/- mice exposed to HFD. Increased blood insulin level was more accentuated in PKR -/- mice. The modulation of PKR activity could be an appropriate therapeutic approach, aimed at reducing abnormal brain metabolism and inflammation linked to metabolic disorders in order to reduce the risk of neurodegeneration.

A lentiviral vector bearing a reverse intron demonstrates superior expression of both proteins and microRNAs

While lentiviral expression vectors are widely used in many facets of molecular biology, due to their ability to stably express heterologous genes in both dividing and non-dividing cells, they suffer from the disadvantage that introns inserted into the vector genome are generally rapidly lost by splicing in packaging cell lines. The presence of an intron, if achievable, has the potential to facilitate the expression of transgene cDNAs, as splicing has been extensively shown to facilitate mRNA biogenesis and function. Moreover, if a stable intron could be introduced into a lentiviral vector, this could greatly facilitate the expression of microRNAs (miRNAs), and especially miRNA clusters, as the introduction of pri-miRNA stems into the exonic region of a lentiviral vector can strongly reduce both vector titer and the expression of any miRNA-linked indicator gene due to cleavage of the vector RNA genome by cellular Drosha. Here, we describe a novel lentiviral vector design in which transgenes and/or miRNAs are expressed using an antisense-orientated, inducible promoter driving an expression cassette bearing a functional intron. We demonstrate that this lentiviral vector, called pTREX, is able to express higher levels of both transgenes and pri-miRNA clusters when compared with a closely similar conventional lentiviral vector.

Partial reconstitution of the RNAi response in human cells using Drosophila gene products

While mammalian somatic cells are incapable of mounting an effective RNA interference (RNAi) response to viral infections, plants and invertebrates are able to generate high levels of viral short interfering RNAs (siRNAs) that can control many infections. In Drosophila, the RNAi response is mediated by the Dicer 2 enzyme (dDcr2) acting in concert with two cofactors called Loqs-PD and R2D2. To examine whether a functional RNAi response could be mounted in human somatic cells, we expressed dDcr2, in the presence or absence of Loqs-PD and/or R2D2, in a previously described human cell line, NoDice/ΔPKR, that lacks functional forms of human Dicer (hDcr) and PKR. We observed significant production of ∼21-nt long siRNAs, derived from a cotransfected double stranded RNA (dsRNA) expression vector, that were loaded into the human RNA-induced silencing complex (RISC) and were able to significantly reduce the expression of a cognate indicator gene. Surprisingly, dDcr2 was able to produce siRNAs even in the absence of Loqs-PD, which is thought to be required for dsRNA cleavage by dDcr2. This result may be explained by our finding that dDcr2 is able to bind the human Loqs-PD homolog TRBP when expressed in human cells in the absence of Loqs-PD. We conclude that it is possible to at least partially rescue the ability of mammalian somatic cells to express functional siRNAs using gene products of invertebrate origin.

PERK-eIF2α-ATF4 pathway mediated by endoplasmic reticulum stress response is involved in osteodifferentiation of human periodontal ligament cells under cyclic mechanical force

To prevent excess accumulation of unfolded proteins in endoplasmic reticulum (ER), eukaryotic cells have signaling pathways from the ER to the cytosol or nucleus. These processes are known as the endoplasmic reticulum stress (ERS) response. Protein kinase R like endoplasmic reticulum kinase (PERK) is a major transducer of the ERS response and it directly phosphorylate α-subunit of eukaryotic initiation factor 2 (eIF2α), resulting in translational attenuation. Phosphorylated eIF2α specifically promoted the translation of the activating transcription factor 4 (ATF4). ATF4 is a known important transcription factor which plays a pivotal role in osteoblast differentiation and bone formation. Furthermore, ATF4 is a downstream target of PERK. Studies have shown that PERK-eIF2α-ATF4 signal pathway mediated by ERS was involved in osteoblastic differentiation of osteoblasts. We have known that orthodontic tooth movement is a process of periodontal ligament cells (PDLCs) osteodifferentiation and alveolar bone remodeling under mechanical force. However, the involvement of PERK-eIF2α-ATF4 signal pathway mediated by ERS in osteogenic differentiation of PDLCs under mechanical force has not been unclear. In our study, we applied the cyclic mechanical force at 10% elongation with 0.5Hz to mimic occlusal force, and explored whether PERK-eIF2α-ATF4 signaling pathway mediated by ERS involved in osteogenic differentiation of PDLCs under mechanical force. Firstly, cyclic mechanical force will induce ERS and intensify several osteoblast marker genes (ATF4, OCN, and BSP). Next, we found that PERK overexpression increased eIF2α phosphorylation and expression of ATF4, furthermore induced BSP, OCN expression, thus it will promote osteodifferentiation of hPDLCs; mechanical force could promote this effect. However, PERK(-/-) cells showed the opposite changes, which will inhibit osteodifferentiation of hPDLCs. Taken together, our study proved that PERK-eIF2α-ATF4 signaling pathway mediated by ERS involved in osteoblast differentiation of PDLCs under cyclic mechanical force.

Deficient PKR in RAX/PKR Association Ameliorates Ethanol-Induced Neurotoxicity in the Developing Cerebellum

Ethanol-induced neuronal loss is closely related to the pathogenesis of fetal alcohol spectrum disorders. The cerebellum is one of the brain areas that are most sensitive to ethanol. The mechanism underlying ethanol neurotoxicity remains unclear. Our previous in vitro studies have shown that the double-stranded RNA (dsRNA)-activated protein kinase (PKR) regulates neuronal apoptosis upon ethanol exposure and ethanol activates PKR through association with its intracellular activator RAX. However, the role of PKR and its interaction with RAX in vivo have not been investigated. In the current study, by utilizing N-PKR-/- mice, C57BL/6J mice with a deficient RAX-binding domain in PKR, we determined the critical role of RAX/PKR association in PKR-regulated ethanol neurotoxicity in the developing cerebellum. Our data indicate that while N-PKR-/- mice have a similar BAC profile as wild-type mice, ethanol induces less brain/body mass reduction as well as cerebellar neuronal loss. In addition, ethanol promotes interleukin-1β (IL-1β) secretion, and IL-1β is a master cytokine regulating inflammatory response. Importantly, ethanol-promoted IL-1β secretion is inhibited in the developing cerebellum of N-PKR-/- mice. Thus, RAX/PKR interaction and PKR activation regulate ethanol neurotoxicity in the developing cerebellum, which may involve ethanol-induced neuroinflammation. Further, PKR could be a possible target for pharmacological intervention to prevent or treat fetal alcohol spectrum disorder (FASD).

TNF-α mediates PKR-dependent memory impairment and brain IRS-1 inhibition induced by Alzheimer's β-amyloid oligomers in mice and monkeys

Alzheimer's disease (AD) and type 2 diabetes appear to share similar pathogenic mechanisms. dsRNA-dependent protein kinase (PKR) underlies peripheral insulin resistance in metabolic disorders. PKR phosphorylates eukaryotic translation initiation factor 2α (eIF2α-P), and AD brains exhibit elevated phospho-PKR and eIF2α-P levels. Whether and how PKR and eIF2α-P participate in defective brain insulin signaling and cognitive impairment in AD are unknown. We report that β-amyloid oligomers, AD-associated toxins, activate PKR in a tumor necrosis factor α (TNF-α)-dependent manner, resulting in eIF2α-P, neuronal insulin receptor substrate (IRS-1) inhibition, synapse loss, and memory impairment. Brain phospho-PKR and eIF2α-P were elevated in AD animal models, including monkeys given intracerebroventricular oligomer infusions. Oligomers failed to trigger eIF2α-P and cognitive impairment in PKR(-/-) and TNFR1(-/-) mice. Bolstering insulin signaling rescued phospho-PKR and eIF2α-P. Results reveal pathogenic mechanisms shared by AD and diabetes and establish that proinflammatory signaling mediates oligomer-induced IRS-1 inhibition and PKR-dependent synapse and memory loss.

Deoxynivalenol-induced weight loss in the diet-induced obese mouse is reversible and PKR-independent

The trichothecene deoxynivalenol (DON), a potent ribotoxic mycotoxin produced by the cereal blight fungus Fusarium graminearum, commonly contaminates grain-based foods. Oral exposure to DON causes decreased food intake, reduced weight gain and body weight loss in experimental animals - effects that have been linked to dysregulation of hormones responsible for mediating satiety at the central nervous system level. When diet-induced obese (DIO) mice are fed DON, they consume less food, eventually achieving body weights of control diet-fed mice. Here, we extended these findings by characterizing: (1) reversibility of DON-induced body weight loss and anorexia in DIO mice and (2) the role of double-stranded RNA-activated protein kinase (PKR) which has been previously linked to initiation of the ribotoxic stress response. The results demonstrated that DON-induced weight loss was reversible in DIO mice and this effect corresponded to initiation of a robust hyperphagic response. When DIO mice deficient in PKR were exposed to DON, they exhibited weight suppression similar to DIO wild-type fed the toxin, suggesting the toxin's weight effects were not dependent on PKR. Taken together, DON's effects on food consumption and body weight are not permanent and, furthermore, PKR is not an essential signaling molecule for DON's anorectic and weight effects.

Chaperone insufficiency links TLR4 protein signaling to endoplasmic reticulum stress

Inflammation plays an important pathogenic role in a number of metabolic diseases such as obesity, type 2 diabetes, and atherosclerosis. The activation of inflammation in these diseases depends at least in part on the combined actions of TLR4 signaling and endoplasmic reticulum stress, which by acting in concert can boost the inflammatory response. Defining the mechanisms involved in this phenomenon may unveil potential targets for the treatment of metabolic/inflammatory diseases. Here we used LPS to induce endoplasmic reticulum stress in the human monocyte cell-line, THP-1. The unfolded protein response, produced after LPS, was dependent on CD14 activity but not on RNA-dependent protein kinase and could be inhibited by an exogenous chemical chaperone. The induction of the endoplasmic reticulum resident chaperones, GRP94 and GRP78, by LPS was of a much lower magnitude than the effect of LPS on TLR4 and MD-2 expression. In face of this apparent insufficiency of chaperone expression, we induced the expression of GRP94 and GRP78 by glucose deprivation. This approach completely reverted endoplasmic reticulum stress. The inhibition of either GRP94 or GRP78 with siRNA was sufficient to rescue the protective effect of glucose deprivation on LPS-induced endoplasmic reticulum stress. Thus, insufficient LPS-induced chaperone expression links TLR4 signaling to endoplasmic reticulum stress.

Role of ER stress response in photodynamic therapy: ROS generated in different subcellular compartments trigger diverse cell death pathways

We have analyzed the molecular mechanisms of photoinduced cell death using porphyrins with similar structure differing only in the position of the ethylene glycol (EG) chain on the phenyl ring. Meta- and para-positioned EG chains targeted porphyrins to different subcellular compartments. After photoactivation, both types of derivatives induced death of tumor cells via reactive oxygen species (ROS). Para derivatives pTPP(EG)4 and pTPPF(EG)4 primarily accumulated in lysosomes activated the p38 MAP kinase cascade, which in turn induced the mitochondrial apoptotic pathway. In contrast, meta porphyrin derivative mTPP(EG)4 localized in the endoplasmic reticulum (ER) induced dramatic changes in Ca(2+) homeostasis manifested by Ca(2+) rise in the cytoplasm, activation of calpains and stress caspase-12 or caspase-4. ER stress developed into unfolded protein response. Immediately after irradiation the PERK pathway was activated through phosphorylation of PERK, eIF2α and induction of transcription factors ATF4 and CHOP, which regulate stress response genes. PERK knockdown and PERK deficiency protected cells against mTPP(EG)4-mediated apoptosis, confirming the causative role of the PERK pathway.

Improved control of tuberculosis and activation of macrophages in mice lacking protein kinase R

Host factors that microbial pathogens exploit for their propagation are potential targets for therapeuic countermeasures. No host enzyme has been identified whose genetic absence benefits the intact mammalian host in vivo during infection with Mycobacterium tuberculosis (Mtb), the leading cause of death from bacterial infection. Here, we report that the dsRNA-dependent protein kinase (PKR) is such an enzyme. PKR-deficient mice contained fewer viable Mtb and showed less pulmonary pathology than wild type mice. We identified two potential mechanisms for the protective effect of PKR deficiency: increased apoptosis of macrophages in response to Mtb and enhanced activation of macrophages in response to IFN-gamma. The restraining effect of PKR on macrophage activation was explained by its mediation of a previously unrecognized ability of IFN-gamma to induce low levels of the macrophage deactivating factor interleukin 10 (IL10). These observations suggest that PKR inhibitors may prove useful as an adjunctive treatment for tuberculosis.

PERK (EIF2AK3) regulates proinsulin trafficking and quality control in the secretory pathway

OBJECTIVE

Loss-of-function mutations in Perk (EIF2AK3) result in permanent neonatal diabetes in humans (Wolcott-Rallison Syndrome) and mice. Previously, we found that diabetes associated with Perk deficiency resulted from insufficient proliferation of beta-cells and from defects in insulin secretion. A substantial fraction of PERK-deficient beta-cells display a highly abnormal cellular phenotype characterized by grossly distended endoplasmic reticulum (ER) and retention of proinsulin. We investigated over synthesis, lack of ER-associated degradation (ERAD), and defects in ER to Golgi trafficking as possible causes.

RESEARCH DESIGN AND METHODS

ER functions of PERK were investigated in cell culture and mice in which Perk was impaired or gene dosage modulated. The Ins2(+/Akita) mutant mice were used as a model system to test the role of PERK in ERAD.

RESULTS

We report that loss of Perk function does not lead to uncontrolled protein synthesis but impaired ER-to-Golgi anterograde trafficking, retrotranslocation from the ER to the cytoplasm, and proteasomal degradation. PERK was also shown to be required to maintain the integrity of the ER and Golgi and processing of ATF6. Moreover, decreasing Perk dosage surprisingly ameliorates the progression of the Akita mutants toward diabetes.

CONCLUSIONS

PERK is a positive regulator of ERAD and proteasomal activity. Reducing PERK activity ameliorates the progression of diabetes in the Akita mouse, whereas increasing PERK dosage hastens its progression. We speculate that PERK acts as a metabolic sensor in the insulin-secreting beta-cells to modulate the trafficking and quality control of proinsulin in the ER relative to the physiological demands for circulating insulin.

A small molecule inhibitor of endoplasmic reticulum oxidation 1 (ERO1) with selectively reversible thiol reactivity

Endoplasmic reticulum oxidation 1 (ERO1) is a conserved eukaryotic flavin adenine nucleotide-containing enzyme that promotes disulfide bond formation by accepting electrons from reduced protein disulfide isomerase (PDI) and passing them on to molecular oxygen. Although disulfide bond formation is an essential process, recent experiments suggest a surprisingly broad tolerance to genetic manipulations that attenuate the rate of disulfide bond formation and that a hyperoxidizing ER may place stressed cells at a disadvantage. In this study, we report on the development of a high throughput in vitro assay for mammalian ERO1alpha activity and its application to identify small molecule inhibitors. The inhibitor EN460 (IC(50), 1.9 mum) interacts selectively with the reduced, active form of ERO1alpha and prevents its reoxidation. Despite rapid and promiscuous reactivity with thiolates, EN460 exhibits selectivity for ERO1. This selectivity is explained by the rapid reversibility of the reaction of EN460 with unstructured thiols, in contrast to the formation of a stable bond with ERO1alpha followed by displacement of bound flavin adenine dinucleotide from the active site of the enzyme. Modest concentrations of EN460 and a functionally related inhibitor, QM295, promote signaling in the unfolded protein response and precondition cells against severe ER stress. Together, these observations point to the feasibility of targeting the enzymatic activity of ERO1alpha with small molecule inhibitors.

Enlarged spleens without enlarged lymph nodes in tlr3-/- pkr-/- mice

Initial phenotypic studies in a mouse containing mutations in both toll-like receptor 3 (TLR3) and RNA-de-pendent protein kinase R (PKR) revealed comparable spleen and bone marrow cell populations in tlr3(-/)-, pkr(-/-), and tlr3(-/-)pkr(-/-) mice to wild-type controls. Splenomegaly developing between 8 and 10 weeks of age was observed in tlr3(-/-) and tlr3(-/-)pkr(-/-) mice but not in wild-type or pkr(-/-) mice. Palpably enlarged cervical, axillary, and inguinal lymph nodes accompanied by enlarged spleens were observed in 12-18-week-old tlr3(-/-) mice at a higher frequency compared with other genotypes. The enlarged spleens and lymph nodes observed in tlr3(-/-) mice were accompanied by destruction of organ architecture and lymphocyte infiltration. However, the enlargement of these organs was not the result of clonal proliferation of one lymphocyte subset. It is likely this phenotype is a result of TLR3 deficiency in combination with an additional, uncharacterized genetic defect or the presence of an infectious agent. These data also suggest that PKR may have a role in preventing progression from splenomegaly to lymphadenopathy in these mice.

The integrated stress response prevents demyelination by protecting oligodendrocytes against immune-mediated damage

In response to ER stress, the pancreatic endoplasmic reticulum kinase (PERK) coordinates an adaptive program known as the integrated stress response (ISR) by phosphorylating the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha). IFN-gamma, which activates the ER stress response in oligodendrocytes, is believed to play a critical role in the immune-mediated CNS disorder multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE). Here we report that CNS delivery of IFN-gamma before EAE onset ameliorated the disease course and prevented demyelination, axonal damage, and oligodendrocyte loss. The beneficial effects of IFN-gamma were accompanied by PERK activation in oligodendrocytes and were abrogated in PERK-deficient animals. Our results indicate that IFN-gamma activation of PERK in mature oligodendrocytes attenuates EAE severity and suggest that therapeutic approaches to activate the ISR could prove beneficial in MS.

Salicylates trigger protein synthesis inhibition in a protein kinase R-like endoplasmic reticulum kinase-dependent manner

The non-steroidal anti-inflammatory drug aspirin and its metabolite, sodium salicylate, have profound effects on cellular protein synthesis and cell physiology. However, the underlying mechanism by which they cause these responses remains unclear. We show here that salicylates induce phosphorylation of the alpha-subunit of eukaryotic translation initiation factor 2 (eIF2alpha), resulting in the inhibition of mRNA translation in cells. Exposure of cells to acetyl salicylic acid resulted in strong activation of eIF2alpha stress-activated protein kinase R-like endoplasmic reticulum kinase (PERK). Analysis of fibroblasts with a targeted deletion of the perk gene revealed that PERK is indispensable for triggering the phosphorylation of eIF2alpha as well as the inhibition of protein synthesis induced by salicylates. Although salicylate treatment did not trigger activation of inositol-requiring enzyme 1, there was an increased expression of the pro-apoptotic transcription factor CHOP-(gadd153), a downstream event to eIF2alpha phosphorylation known to mediate endoplasmic reticulum stress-mediated responses. Thus, salicylates selectively trigger an endoplasmic reticulum stress-responsive signaling pathway initiated through activation of PERK to induce their cellular effects.

Maintenance of endoplasmic reticulum (ER) homeostasis in herpes simplex virus type 1-infected cells through the association of a viral glycoprotein with PERK, a cellular ER stress sensor

In the efforts of viruses to dominate and control critical cellular pathways, viruses generate considerable intracellular stress within their hosts. In particular, the capacity of resident endoplasmic reticulum (ER) chaperones to properly process the acute increase in client protein load is significantly challenged. Such alterations typically induce the unfolded protein response, one component of which acts through IRE1 to restore ER homeostasis by expanding the folding capabilities, whereas the other arm activates the eIF-2alpha (alpha subunit of eukaryotic initiation factor 2) kinase PERK to transiently arrest production of new polypeptide clientele. Viruses, such as herpes simplex virus type 1 (HSV-1), however, go to great lengths to prevent the inhibition of translation resulting from eIF-2alpha phosphorylation. Here, we establish that PERK, but not IRE1, resists activation by acute ER stress in HSV-1-infected cells. This requires the ER luminal domain of PERK, which associates with the viral glycoprotein gB. Strikingly, gB regulates viral protein accumulation in a PERK-dependent manner. This is the first description of a virus-encoded PERK-specific effector and defines a new strategy by which viruses are able to maintain ER homeostasis.

PERK EIF2AK3 control of pancreatic beta cell differentiation and proliferation is required for postnatal glucose homeostasis

Mutations in PERK (EIF2AK3) result in permanent neonatal diabetes as well as several other anomalies that underlie the human Wolcott-Rallison syndrome, and these anomalies are mirrored in Perk knockout mice. To identify the cause of diabetes in PERK-deficient mice, we generated a series of tissue- and cell-specific knockouts of the Perk gene and performed a developmental analysis of the progression to overt diabetes. We discovered that PERK is specifically required in the insulin-secreting beta cells during the fetal and early neonatal period as a prerequisite for postnatal glucose homeostasis. However, PERK expression in beta cells is not required at the adult stage to maintain beta cell functions and glucose homeostasis. We show that PERK-deficient mice exhibit severe defects in fetal/neonatal beta cell proliferation and differentiation, resulting in low beta cell mass, defects in proinsulin trafficking, and abrogation of insulin secretion that culminate in permanent neonatal diabetes.

[Stress granules: RNP-containing cytoplasmic bodies springing up under stress. The structure and mechanism of organization]

In this review recent data describing stress granules are summarized. Stress granules are specific RNA-containing structures in the cytoplasm of living cells which arise under stress conditions (e. g. heat shock, UV irradiation, energy depletion and oxidative stress). It became evident that stress granules accumulate non-canonical 48S initiation complexes and contain mRNA with associated proteins, small ribosomal subunits and some initiation factors. Stress granules are depleted with ternary complex and large ribosomal subunit. It's proposed that eIF2alpha phosphorylation and ternary complex decrease can be a trigger for stress granule formation. Shuttling nuclear and cytoplasmic RNA-binding protein TIA-1 plays a crucial role in this process. It's proposed that TIA-1 forms prion-like aggregates, and these aggregates are scaffolds for other components of stress granules. Cytoskeletal structures facilitate the accumulation of stress granule components in local cytoplasmic sites. Investigation of process of stress granule formation is important for understanding of cell reaction to stress and translation regulation mechanisms.

Perk-dependent translational regulation promotes tumor cell adaptation and angiogenesis in response to hypoxic stress

It has been well established that the tumor microenvironment can promote tumor cell adaptation and survival. However, the mechanisms that influence malignant progression have not been clearly elucidated. We have previously demonstrated that cells cultured under hypoxic/anoxic conditions and transformed cells in hypoxic areas of tumors activate a translational control program known as the integrated stress response (ISR). Here, we show that tumors derived from K-Ras-transformed Perk(-/-) mouse embryonic fibroblasts (MEFs) are smaller and exhibit less angiogenesis than tumors with an intact ISR. Furthermore, Perk promotes a tumor microenvironment that favors the formation of functional microvessels. These observations were corroborated by a microarray analysis of polysome-bound RNA in aerobic and hypoxic Perk(+/+) and Perk(-/-) MEFs. This analysis revealed that a subset of proangiogenic transcripts is preferentially translated in a Perk-dependent manner; these transcripts include VCIP, an adhesion molecule that promotes cellular adhesion, integrin binding, and capillary morphogenesis. Taken with the concomitant Perk-dependent translational induction of additional proangiogenic genes identified by our microarray analysis, this study suggests that Perk plays a role in tumor cell adaptation to hypoxic stress by regulating the translation of angiogenic factors necessary for the development of functional microvessels and further supports the contention that the Perk pathway could be an attractive target for novel antitumor modalities.

PKR and RNase L contribute to protection against lethal West Nile Virus infection by controlling early viral spread in the periphery and replication in neurons

West Nile virus (WNV) is a neurotropic, mosquito-borne flavivirus that can cause lethal meningoencephalitis. Type I interferon (IFN) plays a critical role in controlling WNV replication, spread, and tropism. In this study, we begin to examine the effector mechanisms by which type I IFN inhibits WNV infection. Mice lacking both the interferon-induced, double-stranded-RNA-activated protein kinase (PKR) and the endoribonuclease of the 2',5'-oligoadenylate synthetase-RNase L system (PKR(-/-) x RL(-/-)) were highly susceptible to subcutaneous WNV infection, with a 90% mortality rate compared to the 30% mortality rate observed in congenic wild-type mice. PKR(-/-) x RL(-/-) mice had increased viral loads in their draining lymph nodes, sera, and spleens, which led to early viral entry into the central nervous system (CNS) and higher viral burden in neuronal tissues. Although mice lacking RNase L showed a higher CNS viral burden and an increased mortality, they were less susceptible than the PKR(-/-) x RL(-/-) mice; thus, we also infer an antiviral role for PKR in the control of WNV infection. Notably, a deficiency in both PKR and RNase L resulted in a decreased ability of type I IFN to inhibit WNV in primary macrophages and cortical neurons. In contrast, the peripheral neurons of the superior cervical ganglia of PKR(-/-) x RL(-/-) mice showed no deficiency in the IFN-mediated inhibition of WNV. Our data suggest that PKR and RNase L contribute to IFN-mediated protection in a cell-restricted manner and control WNV infection in peripheral tissues and some neuronal subtypes.

Pathogenesis of Dugbe virus infection in wild-type and interferon-deficient mice

In 129 mice, infection with the nairovirus Dugbe virus (DUGV) was lethal following intracerebral but not intraperitoneal inoculation. Following both routes of inoculation, immunostaining of tissue sections demonstrated virus-positive cells in the brain, indicating that DUGV is neuroinvasive in mice. Many brain areas were affected and neurones were the main cell type infected. Infected cells showed punctate accumulations of viral nucleoprotein in the cytoplasm, indicative of virus replication sites. Immunostaining for activated caspase 3 demonstrated no evidence of apoptosis. The type I interferon (IFN) system plays a significant role in defence against DUGV, as 129 IFN-alpha/beta R(-/-) mice died rapidly following both intraperitoneal and intracerebral inoculations. Studies were undertaken to determine whether the IFN-inducible proteins, protein kinase R (PKR) and MxA, were important for protection; neither PKR nor constitutively expressed human MxA played significant roles.

Internal ribosome entry site-mediated translation of Apaf-1, but not XIAP, is regulated during UV-induced cell death

Components of the cellular translation machinery are targets of caspase-mediated cleavage during apoptosis that correlates with the inhibition of protein synthesis, which accompanies apoptosis. Paradoxically, protein synthesis is required for apoptosis to occur in many experimental settings. Previous studies showed that two proteins that regulate apoptosis by controlling caspase activity, XIAP and Apaf-1, are translated by a unique, cap-independent mechanism mediated by an internal ribosome entry site (IRES) that is used preferentially under conditions in which normal cap-dependent translation is repressed. We investigated the regulation of XIAP and Apaf-1 following UVC irradiation. We show that UVC irradiation leads to the inhibition of translation and cell death. Furthermore, IRES-mediated translation of Apaf-1, but not XIAP, is enhanced by UVC irradiation, and this increase in Apaf-1 translation correlated with cell death. The enhanced Apaf-1 IRES-mediated translation is caspase-independent but is negatively modulated by the eIF2alpha kinase protein kinase RNA-like endoplasmic reticulum kinase. These data suggest that progression of UV-induced apoptosis requires IRES-mediated translation of Apaf-1 to ensure continuous levels of Apaf-1 despite an overall suppression of protein synthesis.

Integrin alpha 11 is a novel type I interferon stimulated gene

Interferons (IFNs) are a family of cytokines that have many biological functions in the cell, including regulation of cellular growth, differentiation, immunomodulation, and viral replication by inducing a set of interferon stimulated genes (ISGs). Based on their structure and biological activities IFNs are subdivided into two groups: type I IFNs, which includes IFN-alpha and IFN-beta and type II IFNs, represented by IFN-gamma. The aim of this work was to investigate whether integrin alpha 11 (ITGA-11), a novel collagen-binding integrin, is responsive to type I IFN treatment. Our findings indicated that type I IFNs were able to induce the ITGA-11 mRNA levels in T98G cells. Increased levels of ITGA-11 protein were also observed in IFN-treated cells. The in vivo induction of ITGA-11 was detected in spleen and lungs of IFN-treated BALB/c mice. T98G cells infected with Murine encephalomyocarditis virus showed increased levels of ITGA-11 mRNA and protein. We observed that the ITGA-11 promoter has binding sites for transcriptional factors regulated by IFNs and the double-stranded RNA dependent protein kinase (PKR). Therefore we investigated the role of PKR in the induction of ITGA-11 by using a PKR deficient mouse embryo fibroblast cell line (MEFs). PKR(-/-) MEFs treated with IFN did not show increased levels of ITGA-11 protein nor mRNA although that could be promptly detected in wild type MEFs. Taken together our data suggest that ITGA-11 is a new interferon stimulated gene.

Animal models of type 2 diabetes with reduced pancreatic beta-cell mass

Type 2 diabetes is increasingly viewed as a disease of insulin deficiency due not only to intrinsic pancreatic beta-cell dysfunction but also to reduction of beta-cell mass. It is likely that, in diabetes-prone subjects, the regulated beta-cell turnover that adapts cell mass to body's insulin requirements is impaired, presumably on a genetic basis. We still have a limited knowledge of how and when this derangement occurs and what might be the most effective therapeutic strategy to preserve beta-cell mass. The animal models of type 2 diabetes with reduced beta-cell mass described in this review can be extremely helpful (a) to have insight into the mechanisms underlying the defective growth or accelerated loss of beta-cells leading to the beta-cell mass reduction; (b) to investigate in prospective studies the mechanisms of compensatory adaptation and subsequent failure of a reduced beta-cell mass. Furthermore, these models are of invaluable importance to test the effectiveness of potential therapeutic agents that either stimulate beta-cell growth or inhibit beta-cell death.

Critical role for the oligoadenylate synthetase/RNase L pathway in response to IFN-beta during acute ocular herpes simplex virus type 1 infection

We previously demonstrated that IFN-beta transgene treatment protects mouse trigeminal ganglia (TG) cells from acute HSV-1 infection in vitro. However, IFN-alpha6 transgene treatment does not provide protection against acute HSV-1 infection in vitro, even though equivalent levels of IFN are expressed with both transgene treatments. In the present study we show that IFN-beta transgene treatment before acute ocular HSV-1 infection protects mice from HSV-1-mediated mortality, whereas IFN-alpha6 transgene treatment does not reduce mortality. Treatment with the IFN-beta and IFN-alpha6 transgenes was associated with increased expression of oligoadenylate synthetase (OAS)1a mRNA in the eye. However, protein kinase R mRNA was not up-regulated in the eye. In TG, only IFN-beta transgene treatment reduced infectious virus levels. Furthermore, in the absence of a functional OAS pathway, corneal HSV-1 Ag expression was more widespread, and the ability of IFN-beta transgene treatment to reduce infectious HSV-1 in eyes and TG was lost. Along with selective up-regulation of OAS1a mRNA expression in TG from IFN-beta transgene-treated mice, we found increased levels of phospho-STAT1. Likewise, p38 MAPK phosphorylation was increased in TG from IFN-beta transgene-treated mice, compared with both IFN-alpha6 and vector-treated mice. We also observed a time-dependent increase in JNK phosphorylation in TG from IFN-beta transgene-treated vs IFN-alpha6 and vector-treated mice. Our results demonstrate that IFN-beta is a potent antiviral cytokine that exerts protection against ocular HSV-1 infection via selective up-regulation of OAS1a mRNA in TG and by altering the phosphorylation of proteins in antiviral signaling cascades.

PERK and GCN2 contribute to eIF2alpha phosphorylation and cell cycle arrest after activation of the unfolded protein response pathway

Exposure of cells to endoplasmic reticulum (ER) stress leads to activation of PKR-like ER kinase (PERK), eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation, repression of cyclin D1 translation, and subsequent cell cycle arrest in G1 phase. However, whether PERK is solely responsible for regulating cyclin D1 accumulation after unfolded protein response pathway (UPR) activation has not been assessed. Herein, we demonstrate that repression of cyclin D1 translation after UPR activation occurs independently of PERK, but it remains dependent on eIF2alpha phosphorylation. Although phosphorylation of eIF2alpha in PERK-/- fibroblasts is attenuated in comparison with wild-type fibroblasts, it is not eliminated. The residual eIF2alpha phosphorylation correlates with the kinetics of cyclin D1 loss, suggesting that another eIF2alpha kinase functions in the absence of PERK. In cells harboring targeted deletion of both PERK and GCN2, cyclin D1 loss is attenuated, suggesting GCN2 functions as the redundant kinase. Consistent with these results, cyclin D1 translation is also stabilized in cells expressing a nonphosphorylatable allele of eIF2alpha; in contrast, repression of global protein translation still occurs in these cells, highlighting a high degree of specificity in transcripts targeted for translation inhibition by phosphorylated eIF2alpha. Our results demonstrate that PERK and GCN2 function to cooperatively regulate eIF2alpha phosphorylation and cyclin D1 translation after UPR activation.

Melanoma differentiation-associated gene-7 protein physically associates with the double-stranded RNA-activated protein kinase PKR

We previously reported that adenoviral-mediated overexpression of the melanoma differentiation-associated gene-7 (Ad-mda7; approved gene symbol IL24) leads to the rapid induction of PKR and activation of its downstream targets, resulting in apoptosis induction in human lung cancer cells. To evaluate the mechanism by which Ad-mda7 activates PKR, we studied the interaction between MDA-7 and PKR proteins. Following Ad-mda7 transduction of lung cancer cells, intracellular and extracellular MDA-7 protein was generated, leading to dose- and time-dependent PKR induction. Purified MDA-7 protein administered extracellularly did not induce PKR or apoptosis, suggesting that Ad-mda7-mediated PKR activation and apoptosis were not dependent on extracellular MDA-7 protein. Following Ad-mda7 transduction, RT-PCR demonstrated no increase in PKR mRNA levels despite increased levels of PKR protein, suggesting posttranscriptional regulation of PKR by MDA-7. Immunofluorescence and coimmunoprecipitation studies demonstrated that MDA-7 protein physically interacts with PKR. Transduction of PKR+/+ and PKR-/- transformed MEFs with Ad-mda7 demonstrated phosphorylated MDA-7 and PKR proteins in the lysates of PKR+/+ but not PKR-/- cells. These findings identify the first binding partner for MDA-7 and suggest that direct interaction between PKR and MDA-7 may be important for PKR activation and apoptosis induction, possibly through MDA-7 phosphorylation or activation of other downstream targets.

RNase L and double-stranded RNA-dependent protein kinase exert complementary roles in islet cell defense during coxsackievirus infection

Coxsackievirus (CV) is an important human pathogen that has been linked to the development of autoimmunity. An intact pancreatic beta cell IFN response is critical for islet cell survival and protection from type 1 diabetes following CV infection. In this study, we show that IFNs trigger an antiviral state in beta cells by inducing the expression of proteins involved in intracellular antiviral defense. Specifically, we demonstrate that 2',5'-oligoadenylate synthetases (2-5AS), RNase L, and dsRNA-dependent protein kinase (PKR) are expressed by pancreatic islet cells and that IFNs (IFN-alpha and IFN-gamma) increase the expression of 2-5AS and PKR, but not RNase L. Moreover, our in vitro studies uncovered that these pathways play important roles in providing unique and complementary antiviral activities that critically regulate the outcome of CV infection. The 2-5AS/RNase L pathway was critical for IFN-alpha-mediated islet cell resistance from CV serotype B4 (CVB4) infection and replication, whereas an intact PKR pathway was required for efficient IFN-gamma-mediated repression of CVB4 infection and replication. Finally, we show that the 2-5AS/RNase L and the PKR pathways play important roles for host survival during a challenge with CVB4. In conclusion, this study has dissected the pathways used by distinct antiviral signals and linked their expression to defense against CVB4.

Distinctive roles for 2',5'-oligoadenylate synthetases and double-stranded RNA-dependent protein kinase R in the in vivo antiviral effect of an adenoviral vector expressing murine IFN-beta

To evaluate the anti-HSV-1 mechanisms of murine IFN-beta in ocular infection, mice were transduced with an adenoviral vector expressing murine IFN-beta (Ad:IFN-beta). Ocular transduction with Ad:IFN-beta resulted in enhanced survival following infection with HSV-1. The protective effect was associated with a reduction in 1) viral titer, 2) viral gene expression, 3) IFN-gamma levels, and 4) the percentage of CD8(+) T lymphocyte and NK cell infiltration in infected tissue. Expression of IFN-beta resulted in an elevation of the IFN-induced antiviral gene 2',5'-oligoadenylate synthetase (OAS1a) but not dsRNA-dependent protein kinase R (PKR) in the cornea and trigeminal ganglion (TG). Mice deficient in the downstream effector molecule of the OAS pathway, RNase L, were no more sensitive to ocular HSV-1 compared with wild-type controls in the TG based on measurements of viral titer. However, the efficacy of Ad:IFN-beta was transiently lost in the eyes of RNase L mice. By comparison, PKR-deficient mice were more susceptible to ocular HSV-1 infection, and the antiviral efficacy following transduction with Ad:IFN-beta was significantly diminished in the eye and TG. These results suggest that PKR is central in controlling ocular HSV-1 infection in the absence of exogenous IFN, whereas the OAS pathway appears to respond to exogenous IFN, contributing to the establishment of an antiviral environment in a tissue-restricted manner.

The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4

Macrophages are pivotal constituents of the innate immune system, vital for recognition and elimination of microbial pathogens. Macrophages use Toll-like receptors (TLRs) to detect pathogen-associated molecular patterns--including bacterial cell wall components, such as lipopolysaccharide or lipoteichoic acid, and viral nucleic acids, such as double-stranded (ds)RNA--and in turn activate effector functions, including anti-apoptotic signalling pathways. Certain pathogens, however, such as Salmonella spp., Shigellae spp. and Yersiniae spp., use specialized virulence factors to overcome these protective responses and induce macrophage apoptosis. We found that the anthrax bacterium, Bacillus anthracis, selectively induces apoptosis of activated macrophages through its lethal toxin, which prevents activation of the anti-apoptotic p38 mitogen-activated protein kinase. We now demonstrate that macrophage apoptosis by three different bacterial pathogens depends on activation of TLR4. Dissection of anti- and pro-apoptotic signalling events triggered by TLR4 identified the dsRNA responsive protein kinase PKR as a critical mediator of pathogen-induced macrophage apoptosis. The pro-apoptotic actions of PKR are mediated both through inhibition of protein synthesis and activation of interferon response factor 3.

Ribonuclear inclusions in skeletal muscle in myotonic dystrophy types 1 and 2

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are caused by genomic expansions of CTG or CCTG repeats. When transcribed, these mutations give rise to repeat expansion RNAs that form nuclear inclusions and compromise the function of myonuclei. Here, we have used in situ hybridization and immunofluorescence to compare DM1 and DM2 and search for proteins that associate with the RNA nuclear (ribonuclear) inclusions. Although muscle disease is generally more severe in DM1, the ribonuclear inclusions were 8- to 13-fold more intense in DM2, implying greater amounts of repeat expansion RNA. Expression of repeat expansion RNA in myoblasts has been implicated in the pathogenesis of congenital DM1. However, we found that repeat expansion RNA is also expressed in myoblasts in DM2, a disorder that has not been associated with a congenital phenotype. Of 10 putative CUG binding proteins tested for colocalization with mutant RNA, only proteins in the muscleblind family were recruited into ribonuclear inclusions. Previous studies have shown activation of the protein kinase, PKR, by expanded CUG repeats in vitro. However, breeding experiments utilizing PKR knockout mice indicate that this kinase is not required for disease pathogenesis in a transgenic mouse model of DM1. We conclude that ribonuclear inclusions are a key feature of the muscle pathology in DM and that sequestration of muscleblind proteins may have a direct role in the disease process.

Activation of NF-kappaB in cells productively infected with HSV-1 depends on activated protein kinase R and plays no apparent role in blocking apoptosis

Microarray data reported elsewhere indicated that herpes simplex virus 1 induces the up-regulation of nuclear factor kappaB (NF-kappaB)-regulated genes, including that of its inhibitor, IkappaBalpha, consistent with the reports that wild-type virus induces the activation of NF-kappaB. In this report we show that activation of NF-kappaB in infected cells is linked to the activation of protein kinase R (PKR). Specifically: (i) PKR is activated in infected cells although the effects of the activated enzyme on protein synthesis are negated by the viral gene gamma134.5, which encodes a protein phosphatase 1alpha accessory factor that enables the dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2. NF-kappaB is activated in wild-type murine embryonic fibroblasts but not in related PKR-null cells. (ii) In cells infected with a replication-competent Deltagamma134.5 mutant (R5104), but carrying a US11 gene expressed early in infection, eukaryotic translation initiation factor 2alpha is not phosphorylated, and in in vitro assays, PKR bound to the US11 protein is not phosphorylated on subsequent addition of double-stranded RNA. Here we report that this mutant does not activate PKR, has no effect on the accumulation of IkappaBalpha, and does not cause the translocation of NF-kappaB in infected cells. (iii) One hypothesis advanced for the activation of NF-kappaB is that it blocks apoptosis induced by viral gene products. The replication-competent R5104 mutant does not induce the programmed cell's death. We conclude that in herpes simplex virus 1-infected cells, activation of NF-kappaB depends on activation of PKR and that NF-kappaB is not required to block apoptosis in productively infected cells.

PERK eIF2alpha kinase regulates neonatal growth by controlling the expression of circulating insulin-like growth factor-I derived from the liver

Humans afflicted with the Wolcott-Rallison syndrome and mice deficient for PERK (pancreatic endoplasmic reticulum eIF2alpha kinase) show severe postnatal growth retardation. In mice, growth retardation in Perk-/- mutants is manifested within the first few days of neonatal development. Growth parameters of Perk-/- mice, including comparison of body weight to length and organ weights, are consistent with proportional dwarfism. Tibia growth plates exhibited a reduction in proliferative and hypertrophic chondrocytes underlying the longitudinal growth retardation. Neonatal Perk-/- deficient mice show a 75% reduction in liver IGF-I mRNA and serum IGF-I within the first week, whereas the expression of IGF-I mRNA in most other tissues is normal. Injections of IGF-I partially reversed the growth retardation of the Perk-/- mice, whereas GH had no effect. Transgenic rescue of PERK activity in the insulin- secreting beta-cells of the Perk-/- mice reversed the juvenile but not the neonatal growth retardation. We provide evidence that circulating IGF-I is derived from neonatal liver but is independent of GH at this stage. We propose that PERK is required to regulate the expression of IGF-I in the liver during the neonatal period, when IGF-I expression is GH-independent, and that the lack of this regulation results in severe neonatal growth retardation.

Regulation of cyclooxygenase-2 expression by macrophages in response to double-stranded RNA and viral infection

In this study the regulation of macrophage expression of cyclooxygenase-2 (COX-2) in response to dsRNA and virus infection was examined. Treatment of RAW 264.7 macrophages with dsRNA results in COX-2 mRNA accumulation and protein expression and the production of PGE(2). Similar to dsRNA, encephalomyocarditis virus (EMCV) infection of RAW 264.7 cells stimulates COX-2 expression and PGE(2) accumulation. The dsRNA-dependent protein kinase (PKR), which has been shown to participate in the regulation of gene expression in response to dsRNA and virus infection, does not appear to participate in the regulation of COX-2 expression by macrophages. Expression of dominant negative mutants of PKR in RAW 264.7 cells fails to attenuate dsRNA- and EMCV-induced COX-2 expression or PGE(2) production. Furthermore, dsRNA and EMCV stimulate COX-2 expression and PGE(2) accumulation to similar levels in macrophages isolated from wild-type and PKR-deficient mice. Recently, a novel PKR-independent role for the calcium-independent phospholipase A(2) (iPLA(2)) in the regulation of inducible NO synthase expression by macrophages in response to virus infection has been identified. The selective iPLA(2) suicide substrate inhibitor bromoenol lactone prevents dsRNA- and EMCV-stimulated inducible NO synthase expression; however, bromoenol lactone does not attenuate dsRNA- or EMCV-induced COX-2 expression by macrophages. In contrast, inhibition of NF-kappaB activation prevents dsRNA-stimulated COX-2 expression and PGE(2) accumulation by macrophages. These findings indicate that virus infection and treatment with dsRNA stimulate COX-2 expression by a mechanism that requires the activation of NF-kappaB and that is independent of PKR or iPLA(2) activation.

Endoplasmic reticulum stress and diabetes mellitus

Pancreatic beta-cells are strongly engaged in protein secretion and have highly developed endoplasmic reticulum (ER). Proper folding of polypeptide into a three-dimensional structure is essential for cellular function and protein malfolding can threaten cell survival. Various conditions can perturb the protein folding in the ER, which is collectively called ER stress. In order to adapt ER stress conditions, the cells respond in three distinct ways such as transcriptional induction of ER chaperones, translational attenuation, and ER-associated degradation (ERAD). However, when ER functions are severely impaired, the cell is eliminated by apoptosis via transcriptional induction of CHOP/GADD153, the activation of cJUN NH2-terminal kinase, and/or the activation of caspase-12. Recent studies have revealed that beta-cell is one of the most susceptible cells for ER stress, and ER stress-mediated apoptosis in beta-cells can be a cause of diabetes. A comprehensive understanding of the impact of the ER stress pathway in beta-cells and how it relates to the development of diabetes may contribute to provide new targets for the prevention and treatment of this disease.

Toll-like receptor 4 and Toll-IL-1 receptor domain-containing adapter protein (TIRAP)/myeloid differentiation protein 88 adapter-like (Mal) contribute to maximal IL-6 expression in macrophages

Previous studies have shown that engagement of Toll-like receptors (TLR) 2 and 4 can induce macrophages to express a variety of proinflammatory cytokines. We have recently demonstrated that TLR2 agonists poorly induce a subset of TLR4-inducible proinflammatory genes (e.g., inducible protein (IP)-10, inducible NO synthase (iNOS), monocyte chemoattractant protein-5, IL-12p40), due in part to differential activation of IFN-beta production and phosphorylation of the transcription factor STAT1. TLR4, but not TLR2, agonists can induce IFN-beta expression via a mechanism that requires the adapter protein Toll-IL-1R domain-containing adapter protein (TIRAP)/myeloid differentiation protein 88 (MyD88) adapter-like (Mal), but not the adapter protein MyD88. Thus, the failure of TLR2 agonists to induce STAT1-dependent genes results, in part, from their failure to induce the expression of IFN-beta. In this study, we show that IL-6 expression is also preferentially induced by activation of TLR4. TLR4-dependent induction of IL-6 expression did require Toll-IL-1R domain-containing adapter protein (TIRAP)/MyD88 adapter-like (Mal), but unlike iNOS and IP-10, it did not require the expression of IFN-beta. Although exogenous IFN-beta and IFN-gamma could synergize with TLR2 agonists to restore high levels of iNOS expression and NO production, these IFNs could not synergize with TLR2 agonists to induce high levels of IL-6. Similarly, neutralizing anti-IFN Abs could block iNOS gene expression in LPS-stimulated murine macrophages, whereas these Abs had little effect on IL-6 gene expression in these cells. Together, these studies demonstrate that IL-6, like iNOS and IP-10, is differentially expressed in macrophages stimulated via TLR2 vs TLR4, although these differences appear to arise from distinct signaling mechanisms.

Absence of PKR attenuates the anti-HSV-1 activity of an adenoviral vector expressing murine IFN-beta

A study was undertaken to evaluate the efficacy of an adenoviral vector containing the murine interferon-beta (IFN-beta) transgene (Ad:IFN-beta) against herpes simplex virus type 1 (HSV-1) infection in two transduced cell lines. The transduction of the adenoviral vector efficiency, ranging from 2% to 100%, was dependent on the multiplicity of infection (moi) (0.4-50 plaque-forming units [pfu]/cell). Supernatants from cells transduced with the Ad:IFN-beta but not the adenoviral null vector (Ad:Null) contained biologically active IFN-beta (6.6-106 U/ml depending on the moi). Cells transduced with the Ad:IFN-beta displayed up to 25-fold reduction in viral titers compared with cells transduced with the Ad:Null or nontransduced cell controls. The suppression in viral titer correlated with a reduction in viral gene (alpha, beta, and gamma) and protein expression. The expression of IFN beta-responsive genes, including protein kinase R (PKR) and 2',5'-oligoadenylate synthetase (OAS), were significantly elevated in the Ad:IFN-beta-transduced cells by 12-fold and 25-fold, respectively. However, after infection with HSV-1, a transient but significant drop in PKR but not OAS gene expression was observed 10 h postinfection. The absence of PKR but not RNase L significantly attenuated the antiviral efficacy of the transgene. Collectively, these results illustrate the feasibility of employing a viral vector to deliver a potent antiviral gene to targeted cells without any obvious detriment to the vector itself and support an important role for PKR as a mediator of the anti-HSV-1 activity of type I IFN.

The PERK eukaryotic initiation factor 2 alpha kinase is required for the development of the skeletal system, postnatal growth, and the function and viability of the pancreas

Phosphorylation of eukaryotic initiation factor 2 alpha (eIF-2 alpha) is typically associated with stress responses and causes a reduction in protein synthesis. However, we found high phosphorylated eIF-2 alpha (eIF-2 alpha[P]) levels in nonstressed pancreata of mice. Administration of glucose stimulated a rapid dephosphorylation of eIF-2 alpha. Among the four eIF-2 alpha kinases present in mammals, PERK is most highly expressed in the pancreas, suggesting that it may be responsible for the high eIF-2 alpha[P] levels found therein. We describe a Perk knockout mutation in mice. Pancreata of Perk(-/-) mice are morphologically and functionally normal at birth, but the islets of Langerhans progressively degenerate, resulting in loss of insulin-secreting beta cells and development of diabetes mellitus, followed later by loss of glucagon-secreting alpha cells. The exocrine pancreas exhibits a reduction in the synthesis of several major digestive enzymes and succumbs to massive apoptosis after the fourth postnatal week. Perk(-/-) mice also exhibit skeletal dysplasias at birth and postnatal growth retardation. Skeletal defects include deficient mineralization, osteoporosis, and abnormal compact bone development. The skeletal and pancreatic defects are associated with defects in the rough endoplasmic reticulum of the major secretory cells that comprise the skeletal system and pancreas. The skeletal, pancreatic, and growth defects are similar to those seen in human Wolcott-Rallison syndrome.

Interferon inhibits dengue virus infection by preventing translation of viral RNA through a PKR-independent mechanism

Previously, we demonstrated that pretreatment of cells with interferon (IFN) alpha + gamma or beta + gamma inhibited dengue virus (DV) replication. In this study, experiments were performed to better define the mechanism by which IFN blocks the accumulation of dengue virus (DV) RNA. Pretreatment of human hepatoma cells with IFN beta + gamma did not significantly alter virus attachment, viral entry, or nucleocapsid penetration into the cytoplasm. The inhibitory effect of IFN was retained even when naked DV RNA was transfected directly into cells, confirming that steps associated with viral entry were not the primary target of IFN action. Biosynthetic labeling experiments revealed that IFN abolished the translation of infectious viral RNA that occurred prior to RNA replication. Subcellular fractionation experiments demonstrated that IFN did not significantly alter the ability of viral RNA to attach to ribosomes. The antiviral effect of IFN appeared independent of the IFN-induced, double-stranded RNA-activated protein kinase (PKR) and RNase L, as genetically deficient PKR- RNase L- cells that were infected by DV retained sensitivity to inhibition by IFN. We conclude that IFN prevents DV infection by inhibiting translation of the infectious viral RNA through a novel, PKR-independent mechanism.

Double-stranded RNA-dependent protein kinase is not required for double-stranded RNA-induced nitric oxide synthase expression or nuclear factor-kappaB activation by islets

Environmental factors, such as viral infection, have been implicated in the destruction of beta-cells during the development of autoimmune diabetes. Double-stranded RNA (dsRNA), produced during viral replication, is an active component of a viral infection that stimulates antiviral responses in infected cells. Previous studies have shown that treatment of rat islets with dsRNA in combination with gamma-interferon (IFN-gamma) results in a nitric oxide-dependent inhibition of glucose-stimulated insulin secretion. This study examines the role of nuclear factor-kappaB (NF-kappaB) and the dsRNA-dependent protein kinase (PKR) in dsRNA + IFN-gamma-induced nitric oxide synthase (iNOS) expression and nitric oxide production by rat, mouse, and human islets. Treatment of rat and human islets with dsRNA in the form of polyinosinic-polycytidylic acid (poly IC) and IFN-gamma resulted in iNOS expression and nitric oxide production. Inhibitors of NF-kappaB activation-the proteasome inhibitor MG-132 and the antioxidant pyrrolidine-dithiocarbamate (PDTC)-prevented poly IC + IFN-gamma-induced iNOS expression and nitric oxide production. Incubation of rat islets for 3 h or human islets for 2 h with poly IC alone or poly IC + IFN-gamma resulted in NF-kappaB nuclear translocation and degradation of the NF-kappaB inhibitor protein, IkappaB, events that are prevented by MG-132. PKR has been shown to participate in dsRNA-induced NF-kappaB activation in a number of cell types, including mouse embryonic fibroblasts. However, poly IC stimulated NF-kappaB nuclear translocation and IkappaB degradation to similar levels in islets isolated from mice devoid of PKR (PKR-/-) and wild-type mice (PKR+/+). Furthermore, the genetic absence of PKR did not affect dsRNA + IFN-gamma-induced iNOS expression, nitric oxide production, or the inhibitory actions of these agents on glucose-stimulated insulin secretion. These results suggest that 1) NF-KB activation is required for dsRNA + IFN-gamma-induced iNOS expression, 2) PKR is not required for either dsRNA-induced NF-kappaB activation or dsRNA + IFN-y-induced iNOS expression by islets, and 3) PKR is not required for dsRNA + IFN-gamma-induced inhibition of glucose-stimulated insulin secretion by islets.

The double-stranded RNA-activated protein kinase PKR is dispensable for regulation of translation initiation in response to either calcium mobilization from the endoplasmic reticulum or essential amino acid starvation

The alpha-subunit of eukaryotic initiation factor eIF2 is a preferred substrate for the double-stranded RNA-activated protein kinase, PKR. Phosphorylation of eIF2alpha converts the factor from a substrate into a competitive inhibitor of the guanine nucleotide exchange factor, eIF2B, leading to a decline in mRNA translation. Early studies provided evidence implicating PKR as the kinase that phosphorylates eIF2alpha under conditions of cell stress such as the accumulation of misfolded proteins in the lumen of the endoplasmic reticulum, i.e., the unfolded protein response (UPR). However, the recent identification of a trans-microsomal membrane eIF2alpha kinase, termed PEK or PERK, suggests that this kinase, and not PKR, might be the kinase that is activated by misfolded protein accumulation. Similarly, genetic studies in yeast provide compelling evidence that a kinase termed GCN2 phosphorylates eIF2alpha in response to amino acid deprivation. However, no direct evidence showing activation of the mammalian homologue of GCN2 by amino acid deprivation has been reported. In the present study, we find that in fibroblasts treated with agents that promote the UPR, protein synthesis is inhibited as a result of a decrease in eIF2B activity. Furthermore, the reduction in eIF2B activity is associated with enhanced phosphorylation of eIF2alpha. Importantly, the magnitude of the change in each parameter is identical in wildtype cells and in fibroblasts containing a chromosomal deletion in the PKR gene (PKR-KO cells). In a similar manner, we find that during amino acid deprivation the inhibition of protein synthesis and extent of increase in eIF2alpha phosphorylation are identical in wildtype and PKR-KO cells. Overall, the results show that PKR is not required for increased eIF2alpha phosphorylation or inhibition of protein synthesis under conditions promoting the UPR or in response to amino acid deprivation.

Activation of NF-kappaB by double-stranded RNA (dsRNA) in the absence of protein kinase R and RNase L demonstrates the existence of two separate dsRNA-triggered antiviral programs

Double-stranded RNA (dsRNA) of viral origin triggers two programs of the innate immunity in virus-infected cells. One is intended to decrease the rate of host cell protein synthesis and thus to prevent viral replication. This program is mediated by protein kinase R (PKR) and by RNase L and contributes, eventually, to the self-elimination of the infected cell via apoptosis. The second program is responsible for the production of antiviral (type I) interferons and other alarmone cytokines and serves the purpose of preparing naive cells for the viral invasion. This second program requires the survival of the infected cell and depends on the expression of antiapoptotic genes through the activation of the NF-kappaB transcription factor. The second program therefore relies on ongoing transcription and translation. It has been proposed that PKR plays an essential role in the activation of NF-kappaB by dsRNA. Here we present evidence that the dsRNA-induced NF-kappaB activity and the expression of beta interferon and inflammatory cytokines do not require either PKR or RNase L. Our results indicate, therefore, that the two dsRNA-activated programs are separate and can function independently of each other.

Negative regulation of CD8+ T cell function by the IFN-induced and double-stranded RNA-activated kinase PKR

The IFN-induced and dsRNA-activated kinase (PKR) mediates the antiviral and antiproliferative effects of IFN-alpha and IFN-gamma. Despite these findings, PKR:(-/-) mice have no overt immunological phenotype. Here we tested the role of PKR in cellular immunity by determining the induction and elicitation of contact hypersensitivity in PKR:(-/-) mice, a model of T cell-mediated immunity. When compared with wild type, the magnitude of contact hypersensitivity responses in PKR:(-/-) mice were 2-fold higher and of extended duration. This was also observed when naive recipients of immune CD8(+) T cells from sensitized PKR:(-/-) and CD4(+) T cells from sensitized wild-type PKR:(+/+) or PKR:(-/-) mice were challenged with hapten, indicating a regulatory defect intrinsic to the CD8(+) T cell population. Isolated lymph node T cells from PKR:(-/-) mice were hyperproliferative during Con A-mediated stimulation. These results implicate PKR for the first time in the growth control of mature T lymphocytes and give insight into the negative regulation of CD8(+) T cell-mediated immune responses.

Potential role of PKR in double-stranded RNA-induced macrophage activation

In this study, the role of the double-stranded (ds) RNA-dependent protein kinase (PKR) in macrophage activation was examined. dsRNA [polyinosinic:polycytidylic acid (poly IC)]-stimulated inducible nitric oxide synthase, interleukin (IL)-1alpha and IL-1beta mRNA expression, nitrite formation and IL-1 release are attenuated in RAW264.7 cells stably expressing dominant negative (dn) mutants of PKR. The transcriptional regulator nuclear factor (NF)-kappaB is activated by dsRNA, and appears to be required for dsRNA-induced macrophage activation. While dnPKR mutants prevent macrophage activation, they fail to attenuate dsRNA-induced IkappaB degradation or NF-kappaB nuclear localization. The inhibitory actions of dnPKR on dsRNA-induced macrophage activation can be overcome by treatment with interferon (IFN)-gamma, an event associated with PKR degradation. Furthermore, dsRNA + IFN-gamma stimulate inducible nitric oxide synthase expression, IkappaB degradation and NF-kappaB nuclear localization to similar levels in macrophages isolated from PKR(-/-) and PKR(+/+) mice. These findings indicate that both NF-kappaB and PKR are required for dsRNA-induced macrophage activation; however, dsRNA-induced NF-kappaB activation occurs by PKR-independent mechanisms in macrophages. In addition, the PKR dependence of dsRNA-induced macrophage activation can be overcome by IFN-gamma.

Central role of double-stranded RNA-activated protein kinase in microbial induction of nitric oxide synthase

NO synthase 2 (NOS2) is induced in airway epithelium by influenza virus infection. NOS2 induction late in the course of viral infection may occur in response to IFN-gamma, but early in infection gene expression may be induced by the viral replicative intermediate dsRNA through the dsRNA-activated protein kinase (PKR). Since PKR activates signaling pathways important in NOS2 gene induction, we determined whether PKR is a component in the signal transduction pathway leading to NOS2 gene expression after viral infection of airway epithelium. We show that NOS2 gene expression in human airway epithelial cells occurs in response to influenza A virus or synthetic dsRNA. Furthermore, dsRNA leads to rapid activation of PKR, followed by activation of signaling components including NF-kappaB and IFN regulatory factor 1. NOS2 expression is markedly diminished and IFN regulatory factor 1 and NF-kappaB activation are substantially impaired in PKR null cells. Strikingly, NOS2 induction in response to LPS is abolished in PKR null cells, confirming a central role for PKR in the general signaling pathway to NOS2.

Effect of deficiency of the double-stranded RNA-dependent protein kinase, PKR, on antiviral resistance in the presence or absence of ribonuclease L: HSV-1 replication is particularly sensitive to deficiency of the major IFN-mediated enzymes

Control of viral replication by interferon (IFN) is thought to be principally mediated by the 2',5'-oligoadenylate synthetase (OAS)/RNAse L, double-stranded dependent protein kinase (PKR), and myxovirus resistance protein (Mx) pathways. In this study, we monitored the constitutive and IFN-induced antiviral activity in mouse embryo fibroblasts lines derived from mice with targeted disruption of either PKR or PKR/RNAse L genes. At high multiplicity of infection (moi = 10), the absence of PKR had no effect on replication of vesicular stomatitis virus (VSV) but moderately enhanced encephalomyocarditis virus (EMCV) growth and greatly increased replication of herpes simplex virus-1 (HSV-1). Replication of EMCV, HSV-1, and VSV was modestly higher in PKR-/- RNAse L-/- fibroblasts when compared with control cells. Although the antiviral action of IFN-alpha was unaffected by the absence of PKR, IFN action was significantly impaired in the double knockout cells but was dependent on the stage of the virus cycle. At early stages, it appeared that anti-EMCV and anti-HSV-1 action of IFN-alpha was significantly compromised, although weak residual antiviral activity was seen. The action of IFN-alpha against VSV was specifically compromised at a late stage of virus replication. The results showed that PKR is an important mediator in constitutive resistance against HSV-1 and that RNAse L is also necessary for the full antiviral activity of IFN against a variety of viruses. These results supported the existence of novel pathways aimed toward specific stages of the virus life cycle.

Specific phenotypic restoration of an attenuated virus by knockout of a host resistance gene

To produce disease, viruses must enter the host, multiply locally in host tissues, spread from the site of entry, and overcome or evade host immune responses. At each stage in this infectious process, specific microbial and host genes determine the ultimate virulence of the virus. Genetic approaches have identified many viral genes that play critical roles in virulence and are presumed to target specific components of the host innate and acquired immune response. However, formal proof that a virulence gene targets a specific protein in a host pathway in vivo has not been obtained. Based on cell culture studies, it has been proposed that the herpes simplex virus type 1 gene ICP34.5 (ICP, infected cell protein) enhances neurovirulence by negating antiviral functions of the IFN-inducible double-stranded RNA-dependent protein kinase R or PKR [Chou, J., Chen, J.J., Gross, M. & Roizman, B. (1995) Proc. Natl. Acad. Sci. USA 92, 10516-10520]. Herein, we show that a virus that has been attenuated by deletion of ICP34.5 exhibits wild-type replication and virulence in a host from which the PKR gene has been deleted. We show that restoration of virulence is specific to ICP34.5 and PKR by using additional host and viral mutants. The use of recombinant viruses to infect animals with null mutations in host defense genes provides a formal genetic test for identifying in vivo mechanisms and targets of microbial virulence genes.