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Transposable elements (TEs) are mobile genetic modules of viral derivation that have been co-opted to become modulators of mammalian gene expression. TEs are a major source of endogenous dsRNAs, signaling molecules able to coordinate inflammatory responses in various physiological processes. Here, we provide evidence for a positive involvement of TEs in inflammation-driven bone repair and mineralization. In newly fractured mice bone, we observed an early transient upregulation of repeats occurring concurrently with the initiation of the inflammatory stage. In human bone biopsies, analysis revealed a significant correlation between repeats expression, mechanical stress and bone mineral density. We investigated a potential link between LINE-1 (L1) expression and bone mineralization by delivering a synthetic L1 RNA to osteoporotic patient-derived mesenchymal stem cells and observed a dsRNA-triggered protein kinase (PKR)-mediated stress response that led to strongly increased mineralization. This response was associated with a strong and transient inflammation, accompanied by a global translation attenuation induced by eIF2α phosphorylation. We demonstrated that L1 transfection reshaped the secretory profile of osteoblasts, triggering a paracrine activity that stimulated the mineralization of recipient cells. Synopsis: Transposable elements (TEs) are selfish genetic modules that have, in certain contexts, evolved into modulators of mammalian gene expression. This study provides evidence for a role of TEs in promoting an inflammatory response that is required for bone repair and mineralization via dsRNA sensing-dependent PKR activation. In mice, TE expression is transiently upregulated shortly after a bone fracture concurrently with the onset of the inflammatory process. In humans, mechanically loaded bones upregulate TE expression, which correlates with local bone mineral density. Transfection with LINE-1 (L1) TE RNA stimulates mineralization activity in human osteoblasts in vitro. Cytoplasmic L1 RNA accumulation in differentiating osteoblasts triggers PKR-dependent translational shutdown, inflammation, and reshaping of the secretome. The secretome of L1 RNA-primed osteoblasts induces the mineralization activity of recipient osteoblasts. Bone fracture is associated with upregulation of transient transposable element RNA expression triggering bone mineralization via a paracrine, secretome mediated process. [ABSTRACT FROM AUTHOR]

The innate immune cell network detects specific microbes and damages to cell integrity in order to coordinate and polarize the immune response against invading pathogens. In recent years, a cross-talk between microbial-sensing pathways and endoplasmic reticulum (ER) homeostasis has been discovered and have attracted the attention of many researchers from the inflammation field. Abnormal accumulation of proteins in the ER can be seen as a sign of cellular malfunction and triggers a collection of conserved emergency rescue pathways. These signalling cascades, which increase ER homeostasis and favour cell survival, are collectively known as the unfolded protein response (UPR). The induction or activation by microbial stimuli of several molecules linked to the ER stress response pathway have led to the conclusion that microbe sensing by immunocytes is generally associated with an UPR, which serves as a signal amplification cascade favouring inflammatory cytokines production. Induction of the UPR alone was shown to promote inflammation in different cellular and pathological models. Here we discuss how the innate immune and ER-signalling pathways intersect. Moreover, we propose that the induction of UPR-related molecules by microbial products does not necessarily reflect ER stress, but instead is an integral part of a specific transcription programme controlled by innate immunity receptors. [ABSTRACT FROM AUTHOR]

Oncogenic viruses frequently target the pathways controlled by tumor suppressor genes, suggesting an extra function for these proteins as antiviral factors. The control exerted by the tumor suppressor Arf on cellular proliferation is crucial to restrict tumor development; however, a potential contribution of Arf to prevent viral infectivity has remained unexplored. In the present study, we investigated the consequences of loss or increased expression of Arf on viral infection. Our results reveal that ARF expression is induced by interferon and after viral infection. Furthermore, we show that ARF protects against viral infection in a gene dosage-dependent manner, and that this antiviral action is mediated in part by PKR through a mechanism that involves ARF-induced release of PKR from nucleophosmin complexes. Finally, Arf-null mice were hypersensitive to viral infection compared to wild-type mice. Together, our results reveal a novel and unexpected role for the tumor suppressor ARF in viral infection surveillance. [ABSTRACT FROM AUTHOR]

Various types of stress, such as disruption of calcium homeostasis, inhibition of protein glycosylation and reduction of disulfide bonds, result in accumulation of misfolded proteins in the endoplasmic reticulum (ER). The initial cellular response involves removal of such proteins by the ER, but excessive and/or long-term stress results in apoptosis. In this study, we used a randomized ribozyme library and ER stress-mediated apoptosis (tunicamycin-induced apoptosis) in SK-N-SH human neuroblastoma cells as a selective phenotype to identify factors involved in this process. We identified a double-stranded RNA-dependent protein kinase (PKR) as one of the participants in this process. The level of nuclear PKR was elevated, but the level of cytoplasmic PKR barely changed in tunicamycin-treated SK-N-SH cells. Furthermore, tunicamycin also raised levels of phosphorylated PKR in the nucleus. We also detected the accumulation of phosphorylated PKR in the nuclei of autopsied brain tissues in Alzheimer's disease. Thus, PKR might play a role in ER stress-induced apoptosis and in Alzheimer's disease. [ABSTRACT FROM AUTHOR]

PKR, a member of the eukaryotic initiation‐factor 2α (eIF‐2α) kinase family, mediates the host antiviral response and is implicated in tumor suppression and apoptosis. Here we show that PKR is regulated by the heat shock protein 90 (Hsp90) molecular chaperone complex. Mammalian PKR expressed in budding yeast depends on several components of the Hsp90 complex for accumulation and activity. In mammalian cells, inhibition of Hsp90 function with geldanamycin (GA) during de novo synthesis of PKR also interferes with its accumulation and activity. Hsp90 and its co‐chaperone p23 bind to PKR through its N‐terminal double‐stranded (ds) RNA binding region as well as through its kinase domain. Both dsRNA and GA induce the rapid dissociation of Hsp90 and p23 from mature PKR, activate PKR both in vivo and in vitro and within minutes trigger the phosphorylation of the PKR substrate eIF‐2α. A short‐term exposure of cells to the Hsp90 inhibitors GA or radicicol not only derepresses PKR, but also activates the Raf–MAPK pathway. This suggests that the Hsp90 complex may more generally assist the regulatory domains of kinases and other Hsp90 substrates. [ABSTRACT FROM AUTHOR]

The protein kinase PKR (dsRNA‐dependent protein kinase) phosphorylates the eukaryotic translation initiation factor eIF2α to downregulate protein synthesis in virus‐infected cells. Two double‐stranded RNA binding domains (dsRBDs) in the N‐terminal half of PKR are thought to bind the activator double‐stranded RNA, mediate dimerization of the protein and target PKR to the ribosome. To investigate further the importance of dimerization for PKR activity, fusion proteins were generated linking the PKR kinase domain to heterologous dimerization domains. Whereas the isolated PKR kinase domain (KD) was non‐functional in vivo, expression of a glutathione S‐transferase–KD fusion, or co‐expression of KD fusions containing the heterodimerization domains of the Xlim‐1 and Ldb1 proteins, restored PKR activity in yeast cells. Finally, coumermycin‐mediated dimerization of a GyrB–KD fusion protein increased eIF2α phosphorylation and inhibited reporter gene translation in mammalian cells. These results demonstrate the critical importance of dimerization for PKR activity in vivo, and suggest that a primary function of double‐stranded RNA binding to the dsRBDs of native PKR is to promote dimerization and activation of the kinase domain. [ABSTRACT FROM AUTHOR]

The double-stranded RNA (dsRNA)-activated protein kinase PKR is an interferon (IFN)-induced enzyme that controls protein synthesis through phosphorylation of eukaryotic initiation factor 2α (eIF-2α). PKR also regulates signals initiated by diverse stimuli, including dsRNA, IFN-γ, tumor necrosis factor-α, interleukin-1 and lipopolysaccharide, to different transcription factors, resulting in pro-inflammatory gene expression. Stat3 plays an essential role in promoting cell survival and proliferation by different growth factors, including platelet-derived growth factor (PDGF). Here we show that PKR physically interacts with Stat3 and is required for PDGF-induced phosphorylation of Stat3 at Tyr705 and Ser727, resulting in DNA binding and transcriptional activation. PKR-mediated phosphorylation of Stat3 on Ser727 is indirect and channeled through Erks. Although PKR is pre-associated with the PDGF β-receptor, treatment with PDGF only modestly activates PKR. However, the induction of c-fos by PDGF is defective in PKR-null cells. Taken together, these results establish PKR as an upstream regulator of activation of Stat3 and as a common mediator of both growth-promoting and growth-inhibitory signals. [ABSTRACT FROM AUTHOR]

A key step in the activation of interferon‐inducible antiviral kinase PKR involves differential binding of viral double‐stranded RNA (dsRNA) to its two structurally similar N‐terminal dsRNA binding motifs, dsRBM1 and dsRBM2. We show here, using NMR spectroscopy, that dsRBM1 with higher RNA binding activity exhibits significant motional flexibility on a millisecond timescale as compared with dsRBM2 with lower RNA binding activity. We further show that dsRBM2, but not dsRBM1, specifically interacts with the C‐terminal kinase domain. These results suggest a dynamically tuned dsRNA binding mechanism for PKR activation, where motionally more flexible dsRBM1 anchors to dsRNA, thereby inducing a cooperative RNA binding for dsRBM2 to expose the kinase domain. [ABSTRACT FROM AUTHOR]

Protein kinase RNA-regulated (PKR) is an established component of innate antiviral immunity. Recently, PKR has been shown to be essential for signal transduction in other situations of cellular stress. The relationship between PKR and the stress-activated protein kinases (SAPKs), such as p38 mitogen­activated protein kinase (MAPK), is not clear. Using embryonic fibroblasts from PKR wild-type and null mice, we established a requirement for PKR in the activation of SAPKs by double­strunded RNA, lipopolysaccharide (LPS) and proinflammatory cytokines. This does not reflect a global failure to activate SAPKs in the PKR-null background as these kinases are activated normally by anisomycin and other physicochemical stress. Activation of p38 MAPK was restored in immortalized PKR-null cells by reconstitution with human PKR. We also show that LPS induction of interleukin-6 and interleukin-12 mRNA is defective in PKR-null cells, and that production of these cytokines is impaired in PKR-null mice challenged with LPS. Our findings indicate, for the first time, that PKR is required for p38 MAPK signaling and plays a potentially important role in the innate response against bacterial endotoxin. [ABSTRACT FROM AUTHOR]

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)-1α and IL-1β 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)-κB 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 IκB degradation or NF-κB nuclear localization. The inhibitory actions of dnPKR on dsRNA-induced macrophage activation can be overcome by treatment with interferon (IFN)-γ, an event associated with PKR degradation. Furthermore, dsRNA + IFN-γ stimulate inducible nitric oxide synthase expression, IκB degradation and NF-κB nuclear localization to similar levels in macrophages isolated from PKR-/- and PKR+/+ mice. These findings indicate that both NF-κB and PKR are required for dsRNA-induced macrophage activation; however, dsRNA-induced NF-κB activation occurs by PKR-independent mechanisms in macropbages. In addition, the PKR dependence of dsRNA-induced macrophage activation can be overcome by IFN-γ. [ABSTRACT FROM AUTHOR]

Interferons (IFNs) inhibit cell growth in a Stat1-dependent fashion that involves regulation of c-myc expression. IFN-γ suppresses c-myc in wild-type mouse embryo fibroblasts, but not in Stat1-null cells, where IFNs induce c-myc mRNA rapidly and transiently, thus revealing a novel signaling pathway. Both tyrosine and serine phosphorylation of Stat1 are required for suppression. Induced expression of c-myc is likely to contribute to the proliferation of Stat1-null cells in response to IFNs. IFNs also suppress platelet-derived growth factor (PDGF)-induced c-myc expression in wild-type but not in Stat1-null cells. A gamma-activated sequence element in the promoter is necessary but not sufficient to suppress c-myc expression in wild-type cells. In PKR-null cells, the phosphorylation of Stat1 on Ser727 and transactivation are both defective, and c-myc mRNA is induced, not suppressed, in response to IFN-γ. A role for Raf-1 in the Stat1-independent pathway is revealed by studies with geldanamycin, an HSPg0-specific inhibitor, and by expression of a mutant of p50cdc37 that is unable to recruit HSP90 to the Raf-1 complex. Both agents abrogated the IFN-γ-dependent induction of c-myc expression in Stat1-null cells. [ABSTRACT FROM AUTHOR]

Protein kinase PKR is an interferon-induced enzyme that plays a key role in the control of viral infections and cellular homeostasis. Compared with other known kinases, PKR is activated by a distinct mechanism that involves double-stranded RNA (dsRNA) binding in its N-terminal region in an RNA sequence-independent fashion. We report here the solution structure of the 20 kDa dsRNA-binding domain (dsRBD) of human PKR, which provides the first three-dimensional insight into the mechanism of its dsRNA-mediated activation. The structure of dsRBD exhibits a dumb-bell shape comprising two tandem linked dsRNA-binding motifs (dsRBMs) both with an α-β-β-β-α fold. The structure, combined with previous mutational and biochemical data, reveals a highly conserved RNA-binding site on each dsRBM and suggests a novel mode of protein—RNA recognition. The central linker is highly flexible, which may enable the two dsRBMs to wrap around the RNA duplex for cooperative and high-affinity binding, leading to the overall change of PKR conformation and its activation. [ABSTRACT FROM AUTHOR]

PKR, a latent protein kinase, mediates the antiviral actions of interferon. It is also involved in cellular signal transduction, apoptosis, growth regulation and differentiation. Although in virus-infected cells, viral double-stranded (ds) RNA can serve as a PKR activator, cellular activators have remained obscure. Here, we report the cloning of PACT, a cellular protein activator of PKR. PACT heterodimerized with PKR and activated it in vitro in the absence of dsRNA. In mammalian cells, overexpression of PACT caused PKR activation and, in yeast, co-expression of PACT enhanced the anti-growth effect of PKR. Thus, PACT has the hallmarks of a direct activator of PKR. [ABSTRACT FROM AUTHOR]