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Microbiology and Molecular Biology Reviews, December 2006, p. 1032-1060, Vol. 70, No. 4
1092-2172/06/$08.00+0 doi:10.1128/MMBR.00027-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Department of Molecular and Cellular Biology, Centro Nacional de Biotecnología, Ciudad Universitaria Cantoblanco, 28049 Madrid, Spain,1 Cell Proliferation Group, MRC Clinical Sciences Centre, Imperial College Faculty of Medicine, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, United Kingdom,2 Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, Madrid 28049, Spain,3 Department of Microbiology II, Universidad Complutense de Madrid, Plaza Ramón y Cajal s/n, Madrid 28040, Spain4
SUMMARY INTRODUCTION TRANSLATION REGULATION BY PKR eIF-2alpha Recognition and Phosphorylation Impact of PKR Activation on Translation PKR ENGAGEMENT BY DIFFERENT SIGNAL TRANSDUCTION PATHWAYS PKR Activation by dsRNA PKR Activation by TLRs PKR Activation by Growth Receptors and Cytokines PKR Activation in Response to Cell Stress REGULATION OF SIGNAL TRANSDUCTION BY PKR IRF-1 STAT Tumor Suppressor p53 MAPK Activation by PKR ATF-3 NF-kappaB MECHANISM OF NF-kappaB ACTIVATION BY PKR PKR Activates the IKK Complex TRAFs Link PKR to IKK Activation PKR-Independent NF-kappaB Activation by dsRNA IDENTIFICATION OF NOVEL GENES INDUCED IN RESPONSE TO PKR Genes Induced by PKR in the Absence of Viral Infection Genes Induced by PKR during Viral Infection MODULATION OF PKR ACTION BY CELL AND VIRAL PROTEINS Modulation by Cell Proteins p58IPK. TRBP. Glycoprotein p67. NPM. MDA7. Heat shock proteins Hsp90 and Hsp70. PACT/RAX. Modulation of PKR Activation by Mammalian Viruses Herpesviruses. (i) HSV-1. (ii) Human herpesvirus 8 (KSHV). (iii) EBV. Poxviruses. (i) VV. (ii) MCV. Orthomyxovirus: influenza virus. Retrovirus: HIV-1. Flavivirus: HCV. Reovirus. ROLE OF PKR IN APOPTOSIS INDUCTION PKR Mediates Apoptosis Induced by Different Stimuli Role of PKR Effectors in Apoptosis Induction Connections between PKR and the Apoptotic Machinery PKR REGULATION OF CELL GROWTH AND DIFFERENTIATION Regulation of PKR Activity during the Cell Cycle PKR Control of Cell Differentiation Effects of PKR-Inhibitory Proteins on Cell Growth Tumor Suppressors and Oncogenes That Regulate PKR Action PKR Regulation of Transcription Factors Role of PKR in Human Tumorigenesis PKR as a Target in Cancer Therapy CONCLUDING REMARKS ACKNOWLEDGMENTS REFERENCES
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B. In particular, how PKR triggers a cascade of events involving IKK phosphorylation of IB and NF-B nuclear translocation has been intensively studied. At the cellular and organism levels PKR exerts antiproliferative effects, and it is a key antiviral agent. A point of convergence in both effects is that PKR activation results in apoptosis induction. The extent and strength of the antiviral action of PKR are clearly understood by the findings that unrelated viral proteins of animal viruses have evolved to inhibit PKR action by using diverse strategies. The case for the pathological consequences of the antiproliferative action of PKR is less understood, but therapeutic strategies aimed at targeting PKR are beginning to offer promising results. | INTRODUCTION |
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PKR was discovered after it was observed at the National Institute for Medical Research, London, United Kingdom, by the groups of Metz and Kerr that cell extracts prepared from IFN-treated vaccinia virus-infected cells, which are known to have restricted translation of viral and cellular mRNAs in cultured cells (244), were exquisitely sensitive to a translational block after addition to a cell-free system of the exogenous mRNA (99) and of the synthetic form of dsRNA, poly(rI) · poly(rC) (pIC) (190). These seminal studies led to the identification of a protein with dsRNA-dependent kinase activity (296, 317), now known as PKR (43), which was later cloned (245) after the laboratory of Hovanessian prepared specific antibodies for PKR purification and partial sequencing (211). The research attempting to define the mechanism by which dsRNA inhibits protein synthesis led to the discovery of another important enzyme, the 2',5'-oligoadenylate synthetase (158). PKR is the most-studied member of the alpha subunit of eukaryotic initiation factor 2 (eIF-2
)-specific kinase subfamily (74). It is a serine/threonine kinase, characterized by two distinct kinase activities: autophosphorylation, which represents the activation reaction, and the phosphorylation of eIF-2 (101, 156), which impairs eIF-2 activity, resulting in inhibition of protein synthesis (293). In addition to its translational regulatory function, PKR has a role in signal transduction and transcriptional control through the IB/NF-B pathway (201). PKR, which is expressed constitutively in mammalian cells, has also been implicated in the control of cell growth and proliferation with tumor suppressor function (198, 224, 246).
PKR is encoded from a single gene located on human chromosome 2p21-22 and mouse chromosome 17E2 (12, 200, 331). The human Pkr gene consists of 17 exons, whereas the mouse gene has 16 (200, 350). In mice, three PKR transcripts with tissue-specific differential expression have been described, but their molecular basis and functional significance remain unresolved (160, 188). Expression of PKR varies in a time- and tissue-specific manner during human fetal development; levels are imperceptible in blastema and immature mesenchymal cells but high in a variety of more differentiated tissues such as epithelial cells (141). In adult tissues, PKR levels are low in the proliferating immature zone of squamous mucosa and increase progressively in the nonproliferating mature keratinocytes (141).
Due to its intrinsic properties, PKR has been studied extensively to document its relevance as a first-line defense mechanism against infection and as a cell growth regulator. Numerous reviews on PKR action have been published over the years (10, 11, 118, 125, 170, 171, 180, 183, 184, 280, 305, 324, 368, 376-378). In this review, we provide an in-depth analysis of current knowledge of this important serine/threonine protein kinase. We summarize PKR structure and function, with particular emphasis on the impact of PKR activation on translation and the signal transduction pathways it mediates. We review several effectors regulated by PKR, with special attention to the NF-B-PKR activation pathway and approaches to the identification of new effectors, regulators, and targets of PKR activation. We also examine its numerous cellular and viral regulators, highlighting the importance of PKR regulation under normal and stress conditions. We discuss the mechanisms and pathways involved in PKR-induced apoptosis and review the experimental findings that support or do not support PKR involvement as a tumor suppressor, and we discuss how it might be used as a target in anticancer therapies.
| TRANSLATION REGULATION BY PKR |
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in response to stress signals, mainly as a result of viral infections (11, 78, 146, 376). Phosphorylation of eIF-2 at residue S51 prevents the recycling of this factor that is required for ongoing translation, leading to general inhibition of translation. In addition to the kinase domain (KD) shared by the other eIF-2 kinases, PKR also has a dsRNA-binding domain (dsRBD) that regulates its activity. As a consequence of dsRNA accumulation in infected cells, PKR-triggered eIF-2 phosphorylation also inhibits translation of viral mRNA (7, 214, 216, 335); this constitutes the basic mechanism by which PKR exerts its antiviral activity on a wide spectrum of DNA and RNA viruses. A number of recent reports have provided insights into the mechanism of PKR activation and eIF-2 phosphorylation, consisting of a three-step pathway in which KD dimerization triggers autophosphorylation, in turn promoting specific recognition of eIF-2. Recognition and Phosphorylation crystals revealed the determinants for the exquisite specificity of PKR for its natural substrate. eIF-2 recognition involves bipartite interactions with the G helix and the PKR phospho-acceptor-binding site located in the catalytic pocket between lobes (Fig. 1). The G helix of PKR in the surface of the C lobe of KD interacts initially with eIF-2 to promote a conformational change in this factor that brings the phosphorylatable S51 residue close to the PKR phospho-acceptor site for catalysis. This change in eIF-2 involves local unfolding of S51, which affords this residue full accessibility to the catalytic cleft of PKR (61, 80, 355). Other eIF-2 kinases also have the G helix in a similar orientation, suggesting a fully conserved mechanism for eIF-2 recognition. This allosteric mechanism of catalysis explains earlier observations that short peptides derived from the eIF-2 sequence containing unfold S51 were much poorer substrates than complete eIF-2. Given this high-order substrate recognition mechanism, the existence of another PKR substrate(s) apart from eIF-2 should be considered with caution. Recently, phosphorylation at tyrosine residues in PKR upon activation has been reported (337). Phosphorylation at these tyrosine residues (Y101, Y162, and Y293) appears to influence the binding to dsRNA, autophosphorylation, and eIF-2 phosphorylation. These data support the idea that PKR is a kinase of dual specificity that also requires tyrosine phosphorylation for full-scale activation.
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); it binds the Met-tRNAi in a GTP-dependent manner to form the ternary complex, which joins the 40S subunit (153, 236). Once Met-tRNAi is delivered, eIF-2 is released from the 48S initiation complex after eIF-5-promoted GTP hydrolysis (153, 236). Inactive eIF-2-GDP complexes are continuously regenerated by GDP-to-GTP exchange in a process catalyzed by the GTP exchange factor eIF-2B. eIF-2 activity is regulated by phosphorylation at S51 of its subunit. As a consequence of eIF-2 phosphorylation, eIF-2 affinity for eIF-2B increases up to 100-fold, leading to competitive inhibition of eIF-2B and the resulting inhibition of translation initiation (338). Since eIF-2B is present in limited amounts with respect to eIF-2, small increases in eIF-2 phosphorylation can thus lead to an exacerbated effect on protein synthesis (153).
eIF-2 phosphorylation has emerged not only as the main regulation point in protein synthesis but also as a critical trigger of the stress response. This is illustrated by the existence of four eIF-2 kinases (PKR, GCN2, PERK, and HRI) that sensitize mammalian cells to different stress signals and allow them to respond to adverse situations (78). Although translation of most cell and viral mRNAs is inhibited by eIF-2 phosphorylation, translation of few mRNAs involved in the stress response is enhanced by limited eIF-2 phosphorylation. This is the case for yeast GCN4 and mammalian activating transcription factor 4 (ATF-4), ATF-3, and CAT-1 mRNAs (79, 134, 232, 386). Under nonstress conditions, translation of these mRNAs is inhibited by the presence of upstream short open reading frames (ORFs) that attract ribosomes to translate short peptides, restricting the flow of scanning ribosomes to the bona fide GCN4 and ATF-4 ORFs (78). Phosphorylation of eIF-2 limits the number of active 43S complexes, promoting reinitiation from GCN4 and ATF-4 ORFs. Although for most viruses, translation of their mRNAs also requires the participation of eIF-2, translation in some insect viruses (cricket paralysis virus) and alphaviruses (Sindbis virus [SV] and Semliki Forest virus) can proceed in the absence of functional eIF-2 (363, 375). Alphaviruses are a case apart from the rest of the viruses, since complete eIF-2 phosphorylation is detected in infected cells (363). Translation of SV and Semliki Forest virus subgenomic mRNAs resists eIF-2 phosphorylation by the existence of a stable stem-loop structure in the RNA downstream of the AUG codon that stalls the ribosomes on the correct site to initiate translation (Fig. 2). To promote this, alphaviruses can alternatively use eIF-2A to deliver the Met-tRNAi and initiate translation in the presence of high levels of phosphorylated eIF-2 (363).
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phosphorylation in response to viral infection, preventing translation of viral mRNA without affecting the overall translation of the cell mRNAs. Restricted activation of PKR to sites of viral RNA synthesis and translation could thus specifically prevent accumulation of viral proteins. Although direct experimental evidence for such spatial regulation of PKR activity is still lacking, the availability of specific antibodies to PKR will allow the localization of active PKR during viral infections or other stress. In addition, PKR has been also detected in the nuclei of human and murine cells, specially when the kinase is overexpressed from transfected plasmids (172, 269). The biological significance of PKR translocation to the nucleus is unknown, but recent data suggest that it could be involved in stress-induced apoptosis, since accumulation of phosphorylated PKR has been detected in tunicamycin-treated cultured cells and in neurons from patients with Alzheimer's disease (269). | PKR ENGAGEMENT BY DIFFERENT SIGNAL TRANSDUCTION PATHWAYS |
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B pathway (365) was a step towards the identification of a role for PKR as a signal transducer in that pathway (201), although its participation was found to be more complex than initially thought, as we discuss below. In addition to mediating a critical role in response to dsRNA, thus acting as a sensor of viral infections, PKR is switched on by a set of other activators, such as proinflammatory stimuli, growth factors, cytokines, and oxidative stress. In addition, PKR integrates and transmits these signals not only to eIF-2 and the translational machinery but also to various factors such as STAT, interferon regulatory factor 1 (IRF-1), p53, Jun N-terminal protein kinase (JNK), and p38, as well as engaging the NF-B pathway (364, 376, 378).
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-ß-ß- folds, whereas the 20-amino-acid linker is entirely in a random-coil conformation (258, 259). This allows the dsRBD to wrap around the dsRNA molecule for optimal protein-RNA interactions, and it offers a satisfactory explanation for length requirements for dsRNA molecules to be effective PKR activators. dsRNA molecules shorter than 30 base pairs thus fail to bind or activate PKR (238). This explains why generally PKR is not activated by short interfering RNAs (siRNAs) of 19 to 29 nucleotides in length. However, experiments have to be carried out with caution, as PKR activation upon siRNA treatment has been reported as a potential nonspecific effect under certain circumstances (312, 329). PKR is activated by dsRNA of greater than 30 bp, but optimal PKR activation is achieved with dsRNA of 80 bp or longer, with no sequence requirements. This indicates that dsRBDs have intrinsic affinity for the A conformation of dsRNA and explains why PKR can be activated by dsRNAs of diverse origins.
Most natural dsRNA activators of PKR are synthesized in virus-infected cells as by-products of viral replication or transcription. For RNA viruses, dsRNA replicative forms are obligatory intermediates for the synthesis of new genomic RNA copies. Complex DNA viruses such as vaccinia virus (VV), adenovirus, or herpes simplex virus (HSV) have ORFs in opposite orientation; they produce overlapping mRNA transcripts that can fold to form dsRNA stretches responsible for PKR activation in infected cells (169, 210, 240). One of the most striking enzymatic properties of PKR is that high dsRNA concentrations inhibit enzyme activity (238, 365). Some endogenous molecules reported to be PKR inhibitors, such as Alu RNA, can thus efficiently activate the kinase in vitro at lower concentrations (57). More-specific structures, such as a pseudoknot present at the 5' end of IFN- mRNA, are reported to activate PKR, although the role of this interaction in the control of IFN- synthesis remains to be analyzed (18).
After binding dsRNA, PKR undergoes a number of conformational changes that relieve the autoinhibitory interactions of the enzyme and allow subsequent substrate recognition. Biochemical and genetic data have underscored the importance of homodimerization in PKR activation (80). Thus, replacement of the dsRBD with an unrelated domain that is able to dimerize, such as glutathione S-transferase, constitutively activates PKR, both in vitro and in vivo (359). Recent crystallographic data show a dimer interphase on the N-terminal lobe of the KD, so that PKR monomers associate in a back-to-back conformation in which the R262-D266 salt bridge and the Y293-D289-Y323H bond triad are involved in interlobe contacts (80). After homodimerization, PKR undergoes rapid autophosphorylation in a stretch of amino acids termed the activation segment. Among others, residues Thr446 and Thr451 in this segment are consistently phosphorylated during activation (80, 353, 399). This further stabilizes PKR dimerization, which in turn increases the catalytic activity of the kinase. In contrast to the case for receptor tyrosine kinases, the paradigm archetype of kinase activation, autophosphorylation of PKR monomers appears to occur in cis or through the action of one PKR dimer on another PKR dimer or monomer (61, 80).
Whether of viral origin or pIC, dsRNA thus not only induces effects on translation but also influences various signal transduction pathways that affect different transcriptional activities. As such, PKR mediates the dsRNA-induced transcription of many genes (84, 134, 190).
B are activated. As a result, cytokines such as type I IFN or interleukin-10 (IL-10) are secreted (343), leading to boosting of antiviral activities. Studies of PKR-deficient mice and cells derived from these animals showed impaired responses to different TLR ligands and reduced production of proinflammatory cytokines in response to LPS, suggesting that PKR is an intermediary in TLR signaling (127). PKR interacts with TIRAP and is phosphorylated in LPS-stimulated wild-type (wt) macrophages, suggesting that PKR is a component of the TLR4 signaling pathway (155).
In addition, PKR is phosphorylated and activated in response to CpG (155). CpG engages a different TLR family member, TLR9, which uses a different adapter molecule, MyD88 (155). PKR is also engaged in dsRNA-activated TLR3 signaling, although it does not interact directly with TLR3; it is recruited by a TAK1-containing complex in response to dsRNA binding to the TLR3 receptor (174). PKR is therefore a common component, integrating at least three TLR family members (Fig. 4).
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, and IL-1. The interferons are known transcriptional inducers of PKR, and type I IFN (IFN- and IFN-ß) are better PKR inducers than IFN- (212). As mentioned above, however, transcription of human IFN- mRNA is in turn self-regulated through a pseudoknot that results in local PKR activation (18). In addition to its direct transcriptional activation by IFN-, PKR mediates the IFN--triggered NF-B activation and c-myc expression that is STAT1 independent (72, 290). Another context in which PKR has a role downstream of IFN- signaling is in preneural cells, where it synergizes with TNF- to activate NF-B by affecting IBß stability (54). Overall, PKR function as a downstream mediator of IFN- signaling is revealed clearly by the reduced antiviral response observed in PKR–/– mice treated with IFN- (390).
Analysis of TNF- signaling in cells with altered PKR levels (using antisense strategies to suppress PKR expression or in PKR–/– mouse embryo fibroblasts [MEF]) showed a slight decrease in NF-B activation in the absence of PKR (202, 239). It is noteworthy that, in contrast to the profound defects observed in response to pIC, ablation of PKR results in slight although sustained and reproducible defects in NF-B activation by TNF-.
PKR has also been implicated in the signal cascades initiated by several growth factors. PKR is phosphorylated in response to PDGF, activating expression of immediate-early genes such as c-fos (73, 254). Moreover, transcriptional induction of c-fos is impaired in PDGF-treated PKR-null MEF compared to wt MEF (73). Thus, it seems that once activated by PDGF, PKR plays a critical role in mediating phosphorylation of STAT3 on its Ser727 residue (73). This phosphorylation is extracellular signal-regulated kinase-1/2 (ERK1/2) dependent, as it can be inhibited by treatment with a specific chemical inhibitor. Overall, PKR is a downstream mediator of PDGF action, integrating both growth-promoting and growth-inhibitory signals.
PKR also mediates IL-1 signaling, although the evidence suggests that it does so in a cell type-specific manner. For example, PKR–/– MEF show a defect in p38 activation in response to IL-1 (127). In contrast, IL-1 treatment does not activate PKR in endothelial cells (265). PKR also regulates the transcription of cytokines such as IL-1, IL-1ß, and TNF-. Therefore, PKR is an important mediator for regulating the coordinated induction of cytokine responses. Further investigation is needed to assign roles to different cytokines in PKR activation.
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or pIC treatment is defective in PKR–/– mice (202). , correlating with the ability of PKR to induce p53 (392). In U937 cells overexpressing PKR, inhibition of p53 expression by antisense techniques prevents TNF--induced apoptosis, and p53 overexpression confers susceptibility to apoptosis. Another study showed that PKR could interact directly with the C-terminal part of p53 and phosphorylate p53 on the Ser392 residue (48). The ability of p53 to cause cell cycle arrest and regulate transcription of target genes is impaired in PKR–/– MEF. In these cells, a minor induction of mdm2 and p21 transcripts correlates with defective phosphorylation of Ser18 in p53 (the equivalent to Ser15 in human p53). Experiments using PKR–/– MEF also hint at a role for PKR in modulating p53 function in response to adriamycin or gamma irradiation. A role for phosphatidylinositol 3-kinase in this process is suggested by studies with chemical inhibitors that diminished Ser18 phosphorylation on p53 (48). Further studies are needed to clarify the link between PKR and p53.
JNK is expressed ubiquitously and can be activated by many types of stress, such as UV and gamma irradiation, protein synthesis inhibitors (anisomycin), hyperosmolarity, toxins, ischemia/reperfusion injury, heat shock, chemotherapeutic drugs, ceramide, T-cell receptor stimulation, peroxide, and inflammatory cytokines such as TNF. p38, on the other hand, is activated in response to cytokines such as IFN-, IL-1, or TNF- or cell stress such as UV irradiation, osmotic shock, heat shock, LPS, and others (207).
PKR is an activator for signaling cascades involving stress-activated protein kinases and is described as mediating JNK and p38 activation in response to specific stimuli (58, 127). For full activation in response to LPS or cytokines such as IFN-, IL-1, or TNF-, both p38 and JNK are dependent on PKR (127) (Fig. 4). A study using a chemical inhibitor of p38 MAPK (SB203580) showed that p38 MAPK has a critical role in IFN signaling. p38 MAPK is needed for activation of phospholipase A2 and for phosphorylation of Ser727 in STAT1 (98) (Fig. 4). Using MEF derived from PKR-null mice, it was shown that PKR is required for p38 MAPK activation in response to dsRNA, LPS, and proinflammatory cytokines but not in response to other forms of stress. p38 MAPK is integrated in a signaling cascade and is activated directly by phosphorylation by MKK3 or MKK6 (289), whereas JNK is downstream of MKK4. Experiments using PKR–/– cells showed that PKR is upstream of MKK3 or MKK6 and MKK4 (Fig. 4). The requirement for PKR in p38 activation is maintained in immortalized cell lines, in contrast to the case for JNK, which can be activated independently of PKR in immortalized cells (58, 127).
An interesting insight into the role of PKR in p38 activation came from analysis of the specific requirements in response to different stimuli. The catalytic function of PKR is required in response to LPS and dsRNA, but in response to TNF-, PKR seems to have a merely structural role, as a noncatalytic mutant (K296R) mimics the effect of PKR in p38 signaling. As p38 MAPK is a master regulator that controls several transcription factors, such as NF-B, ATF-2, and STAT1, these transcriptional pathways are also influenced by PKR. At least to a certain extent, p38 MAPK is thus central in PKR control of these factors and in response to certain specific stimuli. Finally, although solid evidence has yet to be produced, PKR might also activate ERK, as suggested by its role in mediating STAT1 and STAT3 phosphorylation in residues located in ERK consensus sequences (73).
E3L) triggered an increase in ATF-3 expression that was not observed in PKR–/– cells. Since protein synthesis was severely reduced at 16 h after VV-PKR infection, the increase in ATF-3 protein levels is possible because its translation is not affected by eIF-2 phosphorylation due to the special 5' untranslated region of the ATF-3 mRNA (134). ATF-3 can also be induced by PERK and GCN4 (173), and is involved in PKR-induced apoptosis (134). BB family of transcription factors controls the expression of genes involved in immune and inflammatory responses, cell differentiation, and apoptosis, among others (115). The human family includes NF-B1 (p50), NF-B2 (p52), RelA (p65), RelB, c-Rel, and the proteins p100 and p105. The key mechanism that regulates NF-B activation is its cytoplasmic retention mediated by interaction with inhibitory molecules of the IB family (364). IB proteins are phosphorylated by the IKK complex at two close serine residues in response to a variety of stimuli, which tags them for ubiquitin-proteasome-mediated degradation (297). This event allows NF-B translocation to the nucleus, where it regulates transcription (115). The IKK complex contains a structural protein termed IKK or NEMO and two kinase subunits, IKK and IKKß (114). Another pathway that regulates NF-B activation involves NIK phosphorylation of a complex containing IKK, which phosphorylates p100, which once processed can activate p52/RelB target genes (286).
The first clues suggesting a role for PKR in NF-B activation arose from observations that dsRNA could induce NF-B activity in different cell lines (365). Subsequent experiments using the kinase inhibitor 2-aminopurine suggested a role for PKR in this process. Additional evidence came from analysis of NF-B activation following dsRNA treatment in cells lacking PKR expression. When PKR expression was downregulated using 2-5A antisense oligonucleotides, diminished NF-B activation was observed in response to dsRNA, with no significant change in the response to TNF- (239). Similarly, experiments performed with PKR–/– MEF showed that NF-B activation was impaired in response to pIC treatment (390). As a consequence of the NF-B activation impairment, PKR–/– MEF show defects in IFN production compared with wt MEF (390). Finally, experiments performed with human cells and VV recombinants expressing PKR in an inducible manner showed the ability of PKR to activate NF-B in the context of viral infection (116, 117). Because the NF-B pathway plays a pivotal role in the regulation of cell growth, next we discuss mechanistic aspects of NF-B when is activated by PKR.
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MECHANISM OF NF-B ACTIVATION BY PKR
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B directly in response to dsRNA, based on in vivo and in vitro evidence (201). Although PKR appears to be necessary for transducing this signal (202, 239, 390), later evidence pointed to an indirect role for PKR in IB phosphorylation. Mutant cells lacking IKK were unable to induce NF-B in response to pIC treatment (387). Leaman et al. isolated a mutant cell line defective in dsRNA-induced NF-B activation, in which PKR activation was normal and NF-B could be activated in response to other stimuli (212). B activation in response to PKR showed that IB phosphorylation on serines 32 and 36 precedes its degradation and translocation of NF-B to the nucleus (116, 117). As phosphorylation on these residues is the hallmark of IKK kinase activity, a role for the IKK complex in the process seemed obvious. The effect of IKK on PKR activation of NF-B, induced either by dsRNA or by infection with vesicular stomatitis virus (VSV), was confirmed independently by several studies using MEF lacking expression of different IKK subunits or dominant negative versions of the IKK proteins (26, 58, 116, 397). Although NIK was initially proposed to be downstream of PKR in the IKK activation process (397), the participation of NIK in this pathway seems dubious by virtue of current knowledge of IKK signaling. Although the evidence is consistent with a role for PKR in activating IKK in response to dsRNA, the nature of the PKR effect is unclear. PKR interacts physically with the IKK complex in a way similar to that observed for other kinases upstream of IKK (26, 116, 397). Association with the IKK complex seems to involve the PKR catalytic domain, as suggested by mutational analysis (124). Whether this interaction is direct or indirect nonetheless remains to be adequately defined.
It needs to be determined whether PKR catalytic activity is required for dsRNA activation of the IKK complex or whether PKR is merely a structural component in this process. Catalytically inactive PKR mutants (K296R) can be coimmunoprecipitated with IKK (124). Experiments with NIH 3T3 cells suggested that a catalytically inactive PKR mutant is a poor IKK activator, but when PKR is expressed at high levels it can activate IKK efficiently (58). Purified PKR, either wt or mutant K296R, activates the recombinant IKKß protein, suggesting that PKR catalytic activity is not needed in the process. These results support the hypothesis that protein-protein interaction, but not PKR catalytic activity, is needed to activate IKK.
In contrast, experiments using PKR–/– cells suggested that PKR catalytic activity is needed for IKK activation. Complementation of PKR–/– MEF with wt PKR, but not with a catalytically inactive mutant, restored appropriate NF-B (and IRF-1) activation (202). Similar results were obtained upon expression of a battery of PKR mutants in PKR–/– cells by using VV recombinants (124). In this setting, a direct relationship was clearly drawn between the catalytic ability of the PKR mutants and NF-B activation (124). Based on the experiments described above, it is reasonable to consider that under normal circumstances, PKR catalytic activity is necessary to signal dsRNA-dependent activation of IKK. It is noteworthy that results that indicate only a protein-protein interaction between PKR and IKK to be necessary for activating the IKK complex were obtained under experimental conditions in which endogenous wt PKR was still present; this could confuse interpretation of the findings, as both endogenous wt PKR and the overexpressed exogenous mutants would form heterotypic complexes in association with IKK. In any event, the question as to whether PKR catalytic activity is needed for activating IKK clearly remains to be settled.
B (122).
The PKR/TRAF relationship does not end here, and it probably has a role in linking PKR with other signaling pathways, such as some TLR-related pathways. A complex containing TRAF6 has been identified downstream of PKR in the signaling cascades triggered by TLR3 and TLR4 (155, 174) (Fig. 3). It was recently reported that TRAF3 is required to activate NF-B and to produce type I IFN downstream of TLR3, -4, -7, and -9 (138, 266). In addition, TRAF3 is involved in TLR-independent antiviral responses and associates physically with PKR (266). TRAF3 is also involved in PKR-dependent type I IFN induction, a process that requires NF-B activation as shown by the fact that TRAF3–/– MEF do not produce type I IFN in response to VSV infection (266) (Fig. 3). A putative complex involving TRAF3 downstream of PKR would therefore be a convergence point in the regulation of NF-B activation and type I IFN production and is consistent with the proposed model of a functional relationship between TRAF and PKR.
B Activation by dsRNAB activation. dsRNA is generated as a step or a by-product during viral replication and hence can be used as a perfect reporter for cells to detect viral infections. Different cell signaling pathways are activated in response to dsRNA, which eventually result in the triggering of a robust antiviral response through the production of type I IFN. At the molecular level, IFN production is boosted through the activation of several transcription factors, such as IRF-3, IRF-7, or NF-B (167).
We have already discussed the prominent role of PKR in dsRNA-dependent NF-B activation. The impaired NF-B activation in response to viral infection or pIC observed in PKR–/– MEF exemplifies the effect of PKR (202). Although NF-B activation was largely suppressed, some NF-B activity can still be triggered by dsRNA in the absence of PKR. In addition, work with PKR–/– mice showed that they remain susceptible to VSV infection, depending on the route of virus inoculation. These observations suggest that there could be cell-specific, PKR-independent mechanisms of NF-B activation and of IFN production (7, 58).
The search for a dsRNA receptor that senses viral infection independently of PKR resulted in the identification of one member of the TLR family, TLR3 (3). TLR3–/– mice show impaired responses to dsRNA and pIC, whereas TLR3 expression conferred dsRNA responsiveness in dsRNA-insensitive 293 cells. The activation of NF-B triggered by dsRNA and mediated by TLR3 is PKR independent (3). Given the tissue distribution of TLR3 (179), the presence of additional pathways to activate NF-B and produce IFN in response to dsRNA, like the recently discovered RIGI and MDA5 (181, 393), cannot be excluded.
| IDENTIFICATION OF NOVEL GENES INDUCED IN RESPONSE TO PKR |
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B-dependent genes induced by PKR. Interestingly, they also showed distinct PKR-mediated signaling. Upregulation of survival genes such as those for c-IAP1, c-IAP2, and A20, which are NF-B dependent, occurs soon after PKR activation, but cells ultimately die by PKR-triggered apoptosis, concomitant with upregulation of other mRNAs such as those of GADD34/MYD116 and GADD153/CHOP, whose induction is dependent on eIF-2 phosphorylation. Similar studies by Ung et al., who used human cells expressing a Gyrb-PKR fusion that dimerizes and is activated following coumermycin addition (359), identified 22 induced genes, some of which encode proteins involved in apoptosis, such as GADD45A, GADD45B, BIK, and IRF-1, highlighting the importance of transcriptional regulation in PKR-induced apoptosis. Although Hsp70 is known to be induced following VV infection (175, 313), after VV-PKR infection the microarray data showed downregulation of several heat shock protein family genes, including those for Hsp10, Hsp40, and Hsp70. Hsp70 is an antiapoptotic chaperone protein (251) that inhibits mitochondrial release of cytochrome c and blocks procaspase 9 recruitment to the apoptosome complex (16). These results suggest that PKR might induce apoptosis by upregulating apoptotic genes and downregulating antiapoptotic genes (133). The identification of nine upregulated genes with roles in apoptosis concurs with a role for PKR in the apoptotic response to viral infection. One of these genes is that for ATF-3, as we described above. These findings provide new insights into specific mechanisms used by PKR and indicate that during infection PKR produces transcriptional alteration in genes of various pathways, coordinating PKR-regulated apoptotic functions.
| MODULATION OF PKR ACTION BY CELL AND VIRAL PROTEINS |
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p58IPK.
p58IPK is a member of the tetratricopeptide repeat family and was the first reported cellular inhibitor of PKR (218-220). p58IPK interacts directly with PKR and inhibits its kinase activity by preventing dimerization. Influenza virus partially evades the host antiviral response by recruiting p58IPK to repress PKR-mediated eIF-2 phosphorylation (219, 220). In the absence of viral infection, p58IPK overexpression results in malignant transformation (14). Although the exact mechanism has not been defined, it has been suggested that p58IPK transforms cells by interfering with PKR-regulated pathways. PKR inhibition by p58IPK can stimulate cell growth by disrupting PKR-dependent control of mRNA translation and by blocking PKR-dependent apoptosis (102).
TRBP. The trans-activation response (TAR) RNA-binding protein (TRBP) is a cellular RNA-binding protein isolated by its ability to bind human immunodeficiency virus type 1 (HIV-1) TAR RNA (110, 111). Proposed TRBP functions include inhibition of PKR activation, regulation of cell proliferation, PKR-independent translational activation, modulation of HIV-1 gene expression through its association with TAR, and the control of mRNA translation. (9, 19, 86, 274, 327). TRBP facilitates VV protein synthesis in cells infected with a virus mutant lacking the E3L gene (274). Like TRBP, PKR also binds TAR RNA. This RNA activates and inhibits PKR at low and high concentrations, respectively. However, why TAR RNA activates PKR in some studies (35, 88, 193, 318) but not in others (135, 136) remains unexplained. These discrepancies are probably related to the purity of the TAR RNA preparations. TRBP is thought to inhibit PKR function by competing for common RNA substrates. In addition, TRBP and PKR form a complex through direct protein-protein interaction through their dsRBDs (60), preventing PKR activation. Recent data indicate that low TRBP levels support an innate HIV-1 resistance in astrocytes by enhancing the PKR antiviral response, which suggests a major role for TRBP in viral expression (268). Like that of the p58 PKR inhibitor, TRBP overexpression results in malignant transformation (19). Interestingly, TRBP is described as a dsRBD protein partner of human Dicer, which is required for optimal RNA silencing mediated by siRNA and endogenous micro RNA (137).
Glycoprotein p67.
The glycoprotein p67 was identified first as a component that copurified with eIF-2 fractions (63) and later as a methionine aminopeptidase 2 (358); recent studies with mammalian cells demonstrate a link between these two properties (65, 68). p67 inhibits eIF-2 phosphorylation from its kinases HRI, PERK, GCN2, and PKR, although the molecular mechanisms involved are still unclear (66). p67 has several O-linked N-acetyl-ß-D-glucosamine (GlcNAc) residues that are critical for its inhibition of eIF-2 phosphorylation (67). There is correlative evidence that p67 has a role in control of protein synthesis (52). p67 expression rescues BSC-40 cells from the antiviral effects of PKR induction following VV infection, and the PKR-mediated translational block is specifically abrogated by p67. These effects correlate with p67 protection of eIF-2 phosphorylation and, in part, with inhibition of PKR-mediated activation of NF-B, supporting the concept that p67 is an in vivo modifier of PKR activity (120).
NPM.
Nucleophosmin (NPM) (also known as B23) is an abundant and ubiquitously expressed nucleolar phosphoprotein implicated in ribosome biogenesis (163, 267, 382). It binds nucleic acids (87), has intrinsic RNase activity (310), and also acts as a molecular chaperone (339) shuttling between the nucleus and cytoplasm (24). NPM has also been implicated in the acute response to environmental stress and controls cell proliferation (44, 163, 382). NPM is frequently overexpressed in tumors of diverse origin (50, 262), and it is translocated in lymphomas and leukemias (96, 250). NPM interacts with PKR, inhibiting eIF-2 phosphorylation and PKR-mediated apoptosis (273). It was suggested that the capacity of NPM to inhibit PKR activation could explain how NPM promotes cell proliferation and suppresses the apoptosis pathway (273). The Arf/mdm2/p53 pathway was recently linked to PKR, since NPM was suggested to mediate the antiviral activity of the tumor suppressor ARF via PKR (108).
MDA7. Melanoma differentiation-associated gene-7 (Mda7) is a tumor suppressor gene with limited homology to the pleiotropic homodimeric cytokine IL-10 (40, 97). Pataer et al. reported that MDA7 protein interacts physically with PKR, leading to the rapid induction of PKR and activation of its downstream targets, resulting in apoptosis induction in human lung cancer cells (276, 277). Overexpression of Mda7 with an adenoviral vector induces apoptosis in cancer cells, but not in normal cells, through activation of multiple signal transduction pathways (247, 275, 276, 306). Direct interaction between PKR and MDA7 may be important for PKR activation and apoptosis induction, probably through MDA7 phosphorylation or activation of other downstream targets.
Heat shock proteins Hsp90 and Hsp70. Correct PKR folding depends on the chaperone 90 and its cochaperone p23, but subsequent binding of PKR to Hsp90 has an inhibitory effect on its activation (83). The anticancer drug geldanamycin, an inhibitor of Hsp90, disrupts the interaction of the PKR-Hsp90-p23 complex, allowing PKR activation. Similar to Hsp90, Hsp70 binds to PKR, inhibits PKR phosphorylation, and prevents apoptosis. In stressed cells, Hsp70 binds to the Fanconi's anemia complementation group C (FANCC) protein and forms a ternary complex with PKR. FANCC detaches from the PKR-Hsp70 complex and activates PKR (272). Hematopoietic cells with FANCC mutations or downregulated Hsp70 show constitutive PKR activation and sensitivity to various cell stress signals as well as to IFN therapy.
PACT/RAX. As we have seen, the list of PKR inhibitors is long. The mouse protein RAX and its human orthologue PACT are nonetheless the only known cellular activators of PKR. PACT is a ubiquitously expressed protein (165, 279) that belongs to the family of dsRNA-binding proteins and has three dsRNA-binding domains. Domains 1 and 2, located at the N-terminal side of the protein, not only mediate dsRNA binding but also are involved in the direct interaction of PKR with the N-terminal domain (282). In contrast, PACT domain 3, located at its C terminus, does not bind dsRNA but binds weakly to the PKR kinase domain and activates it (229). Although in vitro analysis show
