A role for PKR in hematologic malignancies

Luteolin protects DYT-PRKRA cells from apoptosis by suppressing PKR activation

DYT-PRKRA is a movement disorder caused by mutations in the PRKRA gene, which encodes for PACT, the protein activator of interferon-induced, double-stranded RNA (dsRNA)-activated protein kinase PKR. PACT brings about PKR's catalytic activation by a direct binding in response to stress signals and activated PKR phosphorylates the translation initiation factor eIF2α. Phosphorylation of eIF2α is the central regulatory event that is part of the integrated stress response (ISR), an evolutionarily conserved intracellular signaling network essential for adapting to environmental stresses to maintain healthy cells. A dysregulation of either the level or the duration of eIF2α phosphorylation in response to stress signals causes the normally pro-survival ISR to become pro-apoptotic. Our research has established that the PRKRA mutations reported to cause DYT-PRKRA lead to enhanced PACT-PKR interactions causing a dysregulation of ISR and an increased sensitivity to apoptosis. We have previously identified luteolin, a plant flavonoid, as an inhibitor of the PACT-PKR interaction using high-throughput screening of chemical libraries. Our results presented in this study indicate that luteolin is markedly effective in disrupting the pathological PACT-PKR interactions to protect DYT-PRKRA cells against apoptosis, thus suggesting a therapeutic option for using luteolin to treat DYT-PRKRA and possibly other diseases resulting from enhanced PACT-PKR interactions.

Innate Immunity: A Balance between Disease and Adaption to Stress

Since first being documented in ancient times, the relation of inflammation with injury and disease has evolved in complexity and causality. Early observations supported a cause (injury) and effect (inflammation) relationship, but the number of pathologies linked to chronic inflammation suggests that inflammation itself acts as a potent promoter of injury and disease. Additionally, results from studies over the last 25 years point to chronic inflammation and innate immune signaling as a critical link between stress (exogenous and endogenous) and adaptation. This brief review looks to highlight the role of the innate immune response in disease pathology, and recent findings indicating the innate immune response to chronic stresses as an influence in driving adaptation.

Glycogen synthase kinase (GSK)-3 and the double-strand RNA-dependent kinase, PKR: When two kinases for the common good turn bad

Glycogen synthase kinase (GSK)-3α/β and the double-stranded RNA-dependent kinase PKR are two sentinel kinases that carry-out multiple similar yet distinct functions in both the cytosol and the nucleus. While these kinases belong to separate signal transduction cascades, they demonstrate an uncanny propensity to regulate many of the same proteins either through direct phosphorylation or by altering transcription/translation, including: c-MYC, NF-κB, p53 and TAU, as well as each another. A significant number of studies centered on the GSK3 kinases have led to the identification of the GSK3 interactome and a number of substrates, which link GSK3 activity to metabolic control, translation, RNA splicing, ribosome biogenesis, cellular division, DNA repair and stress/inflammatory signaling. Interestingly, many of these same pathways and processes are controlled by PKR, but unlike the GSK3 kinases, a clear picture of proteins interacting with PKR and a complete listing of its substrates is still missing. In this review, we take a detailed look at what is known about the PKR and GSK3 kinases, how these kinases interact to influence common cellular processes (innate immunity, alternative splicing, translation, glucose metabolism) and how aberrant activation of these kinases leads to diseases such as Alzheimer's disease (AD), diabetes mellitus (DM) and cancer.

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GSK3α/β and PKR are major regulators of cellular homeostasis and the response to stress/inflammation and infection.

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GSK3α/β and PKR interact with and/or modify many of the same proteins and affect the expression of similar genes.

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A balance between AKT and PKR nuclear signaling may be responsible for regulating the activation of nuclear GSK3β.

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GSK3α/β- and PKR-dependent signaling influence major molecular mechanisms of the cell through similar intermediates.

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Aberrant activation of GSK3α/β and PKR is highly involved in cancer, metabolic disorders, and neurodegenerative diseases.

Naturally Occurring and Engineered Alphaviruses Sensitive to Double-Stranded-RNA-Activated Protein Kinase Show Restricted Translation in Mammalian Cells, Increased Sensitivity to Interferon, and Marked Oncotropism

Alphaviruses are insect-borne viruses that alternate between replication in mosquitoes and vertebrate species. Adaptation of some alphaviruses to vertebrate hosts has involved the acquisition of an RNA structure (downstream loop [DLP]) in viral subgenomic mRNAs that confers translational resistance to protein kinase (PKR)-mediated eIF2α phosphorylation. Here, we found that, in addition to promoting eIF2-independent translation of viral subgenomic mRNAs, presence of the DLP structure also increased the resistance of alphavirus to type I interferon (IFN). Aura virus (AURAV), an ecologically isolated relative of Sindbis virus (SV) that is poorly adapted to replication in vertebrate cells, displayed a nonfunctional DLP structure and dramatic sensitivity to type I IFN. Our data suggest that an increased resistance to IFN emerged during translational adaptation of alphavirus mRNA to vertebrate hosts, reinforcing the role that double-stranded RNA (dsRNA)-activated protein kinase (PKR) plays as both a constitutive and IFN-induced antiviral effector. Interestingly, a mutant SV lacking the DLP structure (SV-ΔDLP) and AURAV both showed a marked oncotropism for certain tumor cell lines that have defects in PKR expression and/or activation. AURAV selectively replicated in and killed some cell lines derived from human hepatocarcinoma (HCC) that lacked PKR response to infection or poly(I·C) transfection. The oncolytic activities of SV-ΔDLP and AURAV were also confirmed using tumor xenografts in mice, showing tumor regression activities comparable to wild-type SV. Our data show that translation of alphavirus subgenomic mRNAs plays a central role in IFN susceptibility and cell tropism, suggesting an unanticipated oncolytic potential that some naive arboviruses may have in virotherapy.IMPORTANCE Interferons (IFNs) induce the expression of a number of antiviral genes that protect the cells of vertebrates against viruses and other microbes. The susceptibility of cells to viruses greatly depends on the level and activity of these antiviral effectors but also on the ability of viruses to counteract this antiviral response. Here, we found that the level of one of the main IFN effectors in the cell, the dsRNA-activated protein kinase (PKR), greatly determines the permissiveness of cells to alphaviruses that lack mechanisms to counteract its activation. These naive viruses also showed a hypersensitivity to IFN, suggesting that acquisition of IFN resistance (even partial) has probably been involved in expanding the host range of alphaviruses in the past. Interestingly, some of these naive viruses showed a marked oncotropism for some tumor cell lines derived from human hepatocarcinoma (HCC), opening the possibility of their use in oncolytic therapy to treat human tumors.

Expression of the double-stranded RNA-dependent kinase PKR influences osteosarcoma attachment independent growth, migration, and invasion

Osteosarcoma (OS) is a rare, insidious tumor of mesenchymal origin that most often affects children, adolescents, and young adults. While the primary tumor can be controlled with chemotherapy and surgery, it is the lung metastases that are eventually fatal. Multiple studies into the initial drivers of OS development have been undertaken, but few of these have examined innate immune/inflammatory signaling. A central figure in inflammatory signaling is the innate immune/stress-activated kinase double-stranded RNA-dependent protein kinase (PKR). To characterize the role of PKR in OS, U2OS, and SaOS-2 osteosarcoma cell lines were stably transfected with wild-type or dominant-negative (DN) PKR. Overexpression of PKR enhanced colony formation in soft agar (U2OS and SaOS-2), enhanced cellular migration (U2OS), and invasive migration (SaOS-2). In contrast, overexpression of DN-PKR inhibited attachment-independent growth, migration and/or invasion. These data demonstrate a role for inflammatory signaling in OS formation and migration/invasion and suggest the status of PKR expression/activation may have prognostic value.

Signal Transduction in Ribosome Biogenesis: A Recipe to Avoid Disaster

Energetically speaking, ribosome biogenesis is by far the most costly process of the cell and, therefore, must be highly regulated in order to avoid unnecessary energy expenditure. Not only must ribosomal RNA (rRNA) synthesis, ribosomal protein (RP) transcription, translation, and nuclear import, as well as ribosome assembly, be tightly controlled, these events must be coordinated with other cellular events, such as cell division and differentiation. In addition, ribosome biogenesis must respond rapidly to environmental cues mediated by internal and cell surface receptors, or stress (oxidative stress, DNA damage, amino acid depletion, etc.). This review examines some of the well-studied pathways known to control ribosome biogenesis (PI3K-AKT-mTOR, RB-p53, MYC) and how they may interact with some of the less well studied pathways (eIF2α kinase and RNA editing/splicing) in higher eukaryotes to regulate ribosome biogenesis, assembly, and protein translation in a dynamic manner.

Accumulation of RNA-dependent protein kinase (PKR) in the nuclei of lung cancer cells mediates radiation resistance

We have previously demonstrated that radiation induced cell death in PKR (−/−) deficient mouse embryo fibroblasts (MEFs) but not in PKR (+/+) wild type MEFs. Our study indicated that PKR can also be involved in survival pathways following radiation therapy through activation of the AKT survival pathways in these MEFs is mediated in part through PKR. The role of PKR on radiation sensitivity in cancer cells has not been evaluated. In this study, we demonstrated that radiation treatment causes nuclear translocation of PKR in human lung cancer cells. The transduction of lung cancer cells with a dominant negative adenoviral PKR vector blocks nuclear translocation of PKR and leads to the reversal of radiation resistance. Plasmid transduction of lung cancer cells with nuclear targeted wild type PKR vectors also increased radiation resistance. This effect is selectively abrogated by plasmid transduction of dominant negative PKR vectors which restore radiation sensitivity. These findings suggest a novel role for PKR in lung cancer cells as a mediator of radiation resistance possibly through translocation of the protein product to the nucleus.

RNA-dependent protein kinase (PKR) depletes nutrients, inducing phosphorylation of AMP-activated kinase in lung cancer

We have demonstrated that RNA-dependent protein kinase (PKR) and its downstream protein p-eIF2α are independent prognostic markers for overall survival in lung cancer. In the current study, we further investigate the interaction between PKR and AMPK in lung tumor tissue and cancer cell lines. We examined PKR protein expression in 55 frozen primary lung tumor tissues by Western blotting and analyzed the association between PKR expression and expresson of 139 proteins on tissue samples examined previously by Reverse Phase Protein Array (RPPA) from the same 55 patients. We observed that biomarkers were either positively (phosphorylated AMP-activated kinase^T172 [p-AMPK]) or negatively (insulin receptor substrate 1, meiotic recombination 11, ATR interacting protein, telomerase, checkpoint kinase 1, and cyclin E1) correlated with PKR. We further confirmed that induction of PKR with expression vectors in lung cancer cells causes activation of the AMPK protein independent of the LKB1, TAK1, and CaMKKβ pathway. We found that PKR causes nutrient depletion, which increases AMP levels and decreases ATP levels, causing AMPK phosphorylation. We further demonstrated that inhibiting AMPK expression with compound C or siRNA enhanced PKR-mediated cell death. We next explored the combination of PKR and p-AMPK expression in NSCLC patients and observed that expression of p-AMPK predicted a poor outcome for adenocarcinoma patients with high PKR expression and a better prognosis for those with low PKR expression. These findings were consistent with our in vitro results. AMPK might rescue cells facing metabolic stresses, such as ATP depletion caused by PKR. Our data indicate that PKR causes nutrient depletion, which induces the phosphorylation of AMPK. AMPK might act as a protective response to metabolic stresses, such as nutrient deprivation.

Variable maternal methylation overlapping the nc886/vtRNA2-1 locus is locked between hypermethylated repeats and is frequently altered in cancer

Cancer is as much an epigenetic disease as a genetic one; however, the interplay between these two processes is unclear. Recently, it has been shown that a large proportion of DNA methylation variability can be explained by allele-specific methylation (ASM), either at classical imprinted loci or those regulated by underlying genetic variants. During a recent screen for imprinted differentially methylated regions, we identified the genomic interval overlapping the non-coding nc886 RNA (previously known as vtRNA2-1) as an atypical ASM that shows variable levels of methylation, predominantly on the maternal allele in many tissues. Here we show that the nc886 interval is the first example of a polymorphic imprinted DMR in humans. Further analysis of the region suggests that the interval subjected to ASM is approximately 2 kb in size and somatically acquired. An in depth analysis of this region in primary cancer samples with matching normal adjacent tissue from the Cancer Genome Atlas revealed that aberrant methylation in bladder, breast, colon and lung tumors occurred in approximately 27% of cases. Hypermethylation occurred more frequently than hypomethylation. Using additional normal-tumor paired samples we show that on rare occasions the aberrant methylation profile is due to loss-of-heterozygosity. This work therefore suggests that the nc886 locus is subject to variable allelic methylation that undergoes cancer-associated epigenetic changes in solid tumors.

The impact of PKR activation: from neurodegeneration to cancer

An inverse association between cancer and neurodegeneration is plausible because these biological processes share several genes and signaling pathways. Whereas uncontrolled cell proliferation and decreased apoptotic cell death governs cancer, excessive apoptosis contributes to neurodegeneration. Protein kinase R (PKR), an interferon-inducible double-stranded RNA protein kinase, is involved in both diseases. PKR activation blocks global protein synthesis through eIF2α phosphorylation, leading to cell death in response to a variety of cellular stresses. However, PKR also has the dual role of activating the nuclear factor κ-B pathway, promoting cell proliferation. Whereas PKR is recognized for its negative effects on neurodegenerative diseases, in part, inducing high level of apoptosis, the role of PKR activation in cancer remains controversial. In general, PKR is considered to have a tumor suppressor function, and some clinical data show a correlation between suppressed or inactivated PKR and a poor prognosis for several cancers. However, other studies show high PKR expression and activation levels in various cancers, suggesting that PKR might contribute to neoplastic progression. Understanding the cellular factors and signals involved in the regulation of PKR in these age-related diseases is relevant and may have important clinical implications. The present review highlights the current knowledge on the role of PKR in neurodegeneration and cancer, with special emphasis on its regulation and clinical implications.

Hope and fear for interferon: the receptor-centric outlook on the future of interferon therapy

After several decades of intense clinical research, the great promise of Type I interferons (IFN1) as the anticancer wonder drugs that could cure or, at the very least, curb the progression of various oncological diseases has regrettably failed to deliver. Severe side effects and low efficacy of IFN1-based pharmaceutics greatly limited use of these drugs and further reduced the enthusiasm of clinical oncologists for future optimization of IFN1-based therapeutic modalities. Incredibly, extensive clinical studies to assess the efficacy of IFN1 alone or in combination with other anticancer drugs have not been paralleled by an equal scope in defining the determinants that confer cell sensitivity or refractoriness to IFN1. Given that all effects of IFN1 on malignant and benign cells alike are mediated by its receptor, the mechanisms regulating these receptor cell surface levels should play a paramount role in shaping the magnitude and duration of IFN1-elicited effects. These mechanisms and their role in controlling IFN1 responses, as well as an ability of a growing tumor to commandeer these events, are the focus of our review. We postulate that activation of numerous signaling pathways leading to elimination of IFN1 receptor occurs in cancer cells and benign cells that contribute to tumor tissue. We further hypothesize that activation of these eliminative pathways enables the escape from IFN1-driven suppression of tumorigenesis and elicits the primary refractoriness of tumor to the pharmaceutical IFN1.

PKR negatively regulates leukemia progression in association with PP2A activation, Bcl-2 inhibition and increased apoptosis

Reduced expression and activity of the proapoptotic, double-stranded RNA-dependent protein kinase, PKR (protein kinase R) is observed in breast, lung and various leukemias, suggesting that loss of PKR potentiates transformation. Now we report that decreased PKR activity inhibits chemotherapy-induced apoptosis of leukemia cells both in vitro and in vivo. Inhibition of PKR expression or activity reduces protein phosphatase 2A (PP2A) activity, a B-cell lymphoma 2 (Bcl-2) phosphatase, resulting in enhanced Bcl-2 phosphorylation. Thus, inhibition of PKR activity leads to hyperphosphorylation of Bcl-2, stabilization of Bcl-2/Bax interaction and decreased Bax insertion into the outer mitochondrial membrane. Treatment with the PP2A activator, FTY720, restores Bcl-2 dephosphorylation and apoptosis in cells with reduced PKR expression following stress. Significantly, xenografts of REH leukemic cells with reduced PKR display significantly increased tumor volume, increased resistance to doxorubicin treatment and shorter survival. Importantly, FTY720 treatment restores sensitivity to chemotherapy and prolongs overall survival of these mice. Collectively, these findings suggest that PP2A activation is a downstream target of PKR and the PKR/PP2A signaling axis is required for rapid and potent stress-induced apoptosis. Importantly, loss of PKR promotes leukemia progression and may serve as a biomarker for predicting chemosensitivity.

The multiple functions of TRBP, at the hub of cell responses to viruses, stress, and cancer

The TAR RNA binding protein (TRBP) has emerged as a key player in many cellular processes. First identified as a cellular protein that facilitates the replication of human immunodeficiency virus, TRBP has since been shown to inhibit the activation of protein kinase R (PKR), a protein involved in innate immune responses and the cellular response to stress. It also binds to the PKR activator PACT and regulates its function. TRBP also contributes to RNA interference as an integral part of the minimal RNA-induced silencing complex with Dicer and Argonaute proteins. Due to its multiple functions in the cell, TRBP is involved in oncogenesis when its sequence is mutated or its expression is deregulated. The depletion or overexpression of TRBP results in malignancy, suggesting that the balance of TRBP expression is key to normal cellular function. These studies show that TRBP is multifunctional and mediates cross talk between different pathways. Its activities at the molecular level impact the cellular function from normal development to cancer and the response to infections.

Anti-tumorigenic effects of Type 1 interferon are subdued by integrated stress responses

Viral and pharmacological inducers of protein kinase RNA-activated (PKR)-like ER kinase (PERK) were shown to accelerate the phosphorylation-dependent degradation of the IFNAR1 chain of the Type 1 interferon (IFN) receptor and to limit cell sensitivity to IFN. Here we report that hypoxia can elicit these effects in a PERK-dependent manner. The altered fate of IFNAR1 affected by signaling downstream of PERK depends on phosphorylation of eIF2α (eukaryotic translational initiation factor 2-α) and ensuing activation of p38α kinase. Activators of other eIF2α kinases such as PKR or GCN2 (general control nonrepressed-2) are also capable of eliminating IFNAR1 and blunting IFN responses. Modulation of constitutive PKR activity in human breast cancer cells stabilizes IFNAR1 and sensitizes these cells to IFNAR1-dependent anti-tumorigenic effects. Although downregulation of IFNAR1 and impaired IFNAR1 signaling can be elicited in response to amino-acid deficit, the knockdown of GCN2 in melanoma cells reverses these phenotypes. We propose that, in cancer cells and the tumor microenvironment, activation of diverse eIF2α kinases followed by IFNAR1 downregulation enables multiple cellular components of tumor tissue to evade the direct and indirect anti-tumorigenic effects of Type 1 IFN.

Allelic methylation levels of the noncoding VTRNA2-1 located on chromosome 5q31.1 predict outcome in AML

Deletions of chromosome 5q are associated with poor outcomes in acute myeloid leukemia (AML) suggesting the presence of tumor suppressor(s) at the locus. However, definitive identification of putative tumor suppressor genes remains controversial. Here we show that a 106-nucleotide noncoding RNA vault RNA2-1 (vtRNA2-1), previously misannotated as miR886, could potentially play a role in the biology and prognosis of AML. vtRNA2-1 is transcribed by polymerase III and is monoallelically methylated in 75% of healthy individuals whereas the remaining 25% of the population have biallelic hypomethylation. AML patients without methylation of VTRNA2-1 have a considerably better outcome than those with monoallelic or biallelic methylation (n = 101, P = .001). We show that methylation is inversely correlated with vtRNA2-1 expression, and that 5-azanucleosides induce vtRNA2-1 and down-regulate the phosphorylated RNA-dependent protein kinase (pPKR), whose activity has been shown to be modulated by vtRNA2-1. Because pPKR promotes cell survival in AML, the data are consistent with vtRNA2-1 being a tumor suppressor in AML. This is the first study to show that vtRNA2-1 might play a significant role in AML, that it is either mono- or biallelically expressed in the blood cells of healthy individuals, and that its methylation state predicts outcome in AML.