The apoptosis pathway triggered by the interferon-induced protein kinase PKR requires the third basic domain, initiates upstream of Bcl-2, and involves ICE-like proteases

Mumps virus can suppress the effective augmentation of HPC-induced apoptosis by IFN-gamma through disruption of IFN signaling in U937 cells

Cells of the human promonocytic cell line U937 were found to be sensitive to hexadecylphosphocholine (HPC), which is a potential anticancer drug. Induction of apoptosis was found in U937 cells after treatment with HPC for 24 to 48 hr. The apoptosis in U937 cells exposed to HPC was increased significantly in the presence of interferon-gamma (IFN-gamma). The augmentation of HPC-induced apoptosis by IFN-gamma is repressed in cells (U937-MP) persistently infected with mumps virus. A persistently infected cell line, U937-MP, showed poor induction of signal transducers and activators of transcription-1alpha (STAT-alpha), STAT-2, p48 and IFN-regulatory factor-1 (IRF-1), which are closely correlated with interferon-alpha (IFN-alpha) and IFN-gamma signaling pathways. Expression of MHC class-I or class-II was augmented by IFN-alpha or IFN-gamma in U937 cells, but not in persistently infected cells. Therefore, it is suggested that the IFN-gamma signaling pathway plays an important role in the augmentation of HPC-induced apoptosis. Mumps virus can suppress the IFN-gamma signaling pathway and subsequent development of apoptosis.

Inhibitory sequences in the N-terminus of the double-stranded-RNA-dependent protein kinase, PKR, are important for regulating phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha)

During viral infection, phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) by the interferon-induced RNA-dependent protein kinase, PKR, leads to inhibition of translation initiation and viral proliferation. Activation of PKR is mediated by association of virally encoded double-stranded RNAs (dsRNAs) with two dsRNA binding domains (dsRBDs) located in the N-terminus of PKR. To better understand the molecular mechanisms regulating PKR, we characterized the activities of wild-type and mutant versions of human PKR expressed and purified from yeast. The catalytic rate of eIF2alpha phosphorylation by our purified PKR was increased in response to dsRNA, but not single-stranded RNA or DNA, consistent with the properties previously described for PKR purified from mammalian sources. While both dsRBD1 and dsRBD2 were required for activation of PKR by dsRNA, only deletion of dsRBD1 severely reduced the basal eIF2alpha kinase activity. Removal of as few as 25 residues at the C-terminal junction of dsRBD2 dramatically increased eIF2alpha kinase activity and characterization of larger deletions that included dsRBD1 demonstrated that removal of these negative-acting sequences could bypass the dsRBD1 requirement for in vitro phosphorylation of eIF2alpha. Heparin, a known in vitro activator of PKR, enhanced eIF2alpha phosphorylation by PKR mutants lacking their entire N-terminal sequences, including the dsRBDs. The results indicate that induction of PKR activity is mediated by multiple mechanisms, one of which involves release of inhibition by negative-acting sequences in PKR.

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.

Impaired translational response and increased protein kinase PKR expression in T cells from lupus patients

Activation of peripheral blood T cells results in a rapid and substantial rise in translation rates and proliferation, but proliferation in response to mitogen stimulation is impaired in systemic lupus erythematosus (SLE). We have investigated translation rates and initiation factor activities in T cells from SLE patients in response to activating signals. Activation by PMA plus ionomycin strongly increased protein synthesis in control T cells but not in T cells from SLE patients. The rate of protein synthesis is known to be strongly dependent on the activity of two eukaryotic translation initiation factors, eIF4E and eIF2alpha. We show that following stimulation, eIF4E expression and phosphorylation increased equivalently in control and SLE T cells. Expression of eIF4E interacting proteins - eIF4G, an inducer, and 4E-BP1 and 4E-BP2, two specific repressors of eIF4E function - and the phosphorylation level of 4E-BP1, were all identical in control and SLE T cells. In contrast, the protein kinase PKR, which is responsible for the phosphorylation and consequent inhibition of eIF2alpha activity, was specifically overexpressed in activated SLE T cells, correlating with an increase in eIF2alpha phosphorylation. Therefore, high expression of PKR and subsequent eIF2alpha phosphorylation is likely responsible, at least in part, for impaired translational and proliferative responses to mitogens in T cells from SLE patients.

PACT, a stress-modulated cellular activator of interferon-induced double-stranded RNA-activated protein kinase, PKR

The interferon (IFN)-induced, double-stranded (ds)RNA-activated serine-threonine protein kinase, PKR, is a key mediator of the antiviral activities of IFNs. In addition, PKR activity is also involved in regulation of cell proliferation, apoptosis, and signal transduction. In virally infected cells, dsRNA has been shown to bind and activate PKR kinase function. Implication of PKR activity in normal cellular processes has invoked activators other than dsRNA because RNAs with perfectly duplexed regions of sufficient length that are able to activate PKR are absent in cellular RNAs. We have recently reported cloning of PACT, a novel protein activator of PKR. PACT heterodimerizes with PKR and activates it by direct protein-protein interaction. Overexpression of PACT in mammalian cells leads to phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2alpha), the cellular substrate for PKR, and leads to inhibition of protein synthesis. Here, we present evidence that endogenous PACT acts as a protein activator of PKR in response to diverse stress signals such as serum starvation, and peroxide or arsenite treatment. Following exposure of cells to these stress agents, PACT is phosphorylated and associates with PKR with increased affinity. PACT-mediated activation of PKR leads to enhanced eIF2alpha phosphorylation followed by apoptosis. Based on the results presented here, we propose that PACT is a novel stress-modulated physiological activator of PKR.

Building better vaccines: how apoptotic cell death can induce inflammation and activate innate and adaptive immunity

The immunological consequences of apoptosis have been hotly debated. Apoptosis was originally described as a set of cellular morphological changes that occur in the absence of inflammation but the term has been redefined on the basis of a set of conserved molecular events that include the activation of caspases. Though the apoptosis occurring during normal development is immunologically bland or even tolerizing, the apoptotic death after viral infection or after the ligation of Fas can trigger powerful innate and adaptive immune responses. The molecular machinery at the nexus of apoptosis and inflammation includes caspase-1 --an activator of IL-1beta and IL-18 - as well as the double-stranded-RNA-dependent protein kinase pathway and RNaseL pathway, which are key effectors of antiviral immunity. New proapoptotic vaccines induce immune responses that may be able to prevent or treat infectious disease and cancer.

Rubella virus capsid protein induces apoptosis in transfected RK13 cells

Rubella virus is an enveloped positive-strand RNA virus that can cause mild to severe birth defects or death in an infected fetus. RV induction of programmed cell death, demonstrated in cell culture, has been implicated in the pathogenesis. The timing of apoptosis, 48 h p.i., suggested that accumulation of RV structural proteins might induce cell death in infected cells. Expression of RV structural proteins, capsid, envelope glycoproteins E1 and E2, in transiently transfected RK13 cells was as potent an inducer of cell death as RV infection. Immunofluorescence microscopy revealed that RV structural protein transfected cells exhibited the condensed nuclei typical of apoptotic cell death. Transfection with the capsid protein construct, but not E2 and E1, resulted in as much cell death as joint expression of all three RV structural proteins. Capsid required a membrane-anchoring domain to induce cell death, but a heterologous polypeptide fused to the capsid membrane anchor did not cause apoptosis. Deletion mutants demonstrated that the apoptosis-inducing activity resides in the N-terminal 170 amino acids of capsid. Though apoptosis-inducing capsid constructs appear to have an ER sub-cellular localization, disruption of the ER calcium storage capacity does not correlate with cell death. Mechanisms consistent with these results are discussed.

The interferon-induced protein kinase (PKR), triggers apoptosis through FADD-mediated activation of caspase 8 in a manner independent of Fas and TNF-alpha receptors

The interferon-induced dsRNA-dependent protein kinase (PKR) induces apoptosis of mammalian cells. Apoptosis induction by PKR involves phosphorylation of the translational factor eIF-2alpha and activation of the transcriptional factor NF-kappaB, but caspase pathways activated by PKR are not known. Upregulation of Fas mRNA by PKR has been suggested to play a role in PKR-induced apoptosis. To learn how PKR induces apoptosis, we have analysed the role of molecules in death receptor pathways. We showed the involvement of the FADD-caspase 8 pathway on PKR-induced apoptosis based on four experimental findings: upregulation of caspase 8 activity during PKR-induced apoptosis, blocking of PKR-induced apoptosis by the use of a chemical inhibitor of caspase 8, and inhibition of PKR-induced apoptosis by expression of both a FADD dominant negative or a viral FLIP molecule. Significantly, despite the PKR-mediated upregulation of Fas mRNA expression, the Fas receptor-ligand pathway is not needed for PKR-induced apoptosis. Antibodies that inhibit TNFalpha-TNFR1 or Fas-FasL interactions were not able to block PKR-induced apoptosis. Taken together, our observations establish the involvement of caspase 8 in PKR-induced apoptosis and suggest that death receptors other than Fas or TNFR1 or, alternatively, a novel mechanism involving FADD independently of death receptors, are responsible for PKR-induced apoptosis.

Translation initiation factor modifications and the regulation of protein synthesis in apoptotic cells

The rate of protein synthesis is rapidly down-regulated in mammalian cells following the induction of apoptosis. Inhibition occurs at the level of polypeptide chain initiation and is accompanied by the phosphorylation of the alpha subunit of initiation factor eIF2 and the caspase-dependent cleavage of initiation factors eIF4G, eIF4B, eIF2alpha and the p35 subunit of eIF3. Proteolytic cleavage of these proteins yields characteristic products which may exert regulatory effects on the translational machinery. Inhibition of caspase activity protects protein synthesis from long-term inhibition in cells treated with some, but not all, inducers of apoptosis. This review describes the initiation factor modifications and the possible signalling pathways by which translation may be regulated during apoptosis. We discuss the significance of the initiation factor cleavages and other changes for protein synthesis, and the implications of these events for our understanding of the cellular changes associated with apoptosis.

Human breast cancer cells contain elevated levels and activity of the protein kinase, PKR

PKR is a double-stranded (ds) RNA activated protein kinase whose expression is induced by interferon. Activated PKR phosphorylates its cellular substrate, eIF2, an essential initiation factor of translation. Prior evidence from a murine model system suggested that PKR may act as a tumor suppressor, but the evidence from human tumors is equivocal. To study PKR function in human breast cancer, PKR activity was measured in mammary carcinoma cell lines and nontransformed mammary epithelial cell lines. If PKR functioned as a tumor suppressor in this system, its activity would be higher in nontransformed cells than in carcinoma cells. On the contrary, PKR autophosphorylation and the phosphorylation of its substrate, the alpha-subunit of eIF2, is 7 - 40-fold higher in lysates prepared from breast carcinoma cell lines than in those from nontransformed epithelial cell lines. Correspondingly, a larger proportion of eIF2alpha is present in a phosphorylated state in carcinoma cell lines than in nontransformed cell lines. Protein synthesis is not inhibited by the high eIF2alpha phosphorylation in carcinoma cells, probably because they contain higher levels of eIF2B, the initiation factor that is inhibited by eIF2alpha phosphorylation. The dramatically lower PKR activity in nontransformed cell lines is partially due to lower PKR protein levels (2 - 4-fold) as well as to the presence of a PKR inhibitor. The nontransformed cells contain P58, a known cellular inhibitor of PKR that physically interacts with PKR and may be responsible for the low PKR activity in these cells. Taken together, these observations call into question the role of PKR as a tumor suppressor and suggest a positive regulatory role of PKR in growth control of breast cancer cells.

Identification of caspase 3-mediated cleavage and functional alteration of eukaryotic initiation factor 2alpha in apoptosis

Induction of apoptosis in a variety of cell types leads to inhibition of protein synthesis. Recently, the cleavage of eukaryotic translation initiation factor 4G (eIF4G) by caspase 3 was described as a possible event contributing to translation inhibition. Here, we report the cleavage of another initiation factor in apoptotic cells, eIF2alpha, that could contribute to regulation of translation during apoptosis. This cleavage event could be completely inhibited by pretreatment of HeLa cells with Z-VAD-fmk. In vitro analysis using purified eIF2 and purified caspases showed cleavage of eIF2alpha by caspase 3, 6, 8, and 10 but not 9. Caspase 3 most efficiently cleaved eIF2alpha and this could be inhibited by addition of Ac-DEVD-CHO in vitro. Comparison of cleavage of phosphorylated versus nonphosphorylated eIF2alpha revealed a modest preference of the caspases for the nonphosphorylated form. When eIF2. 2B complex was used as substrate, only caspase 3 was capable of eIF2alpha cleavage, which was not affected by phosphorylation of the alpha subunit. The eIF2.GDP binary complex was cleaved much less efficiently by caspase 3. Sequence analysis of the cleavage fragment suggested that the cleavage site is located in the C-terminal portion of the protein. Analysis showed that after caspase cleavage, exchange of GDP bound to eIF2 was very rapid and no longer dependent upon eIF2B. Furthermore, in vitro translation experiments indicated that cleavage of eIF2alpha results in functional alteration of the eIF2 complex, which no longer stimulated upstream AUG selection on a mRNA containing a viral internal ribosome entry site and was no longer capable of stimulating overall translation. In conclusion, we describe here the cleavage of a translation initiation factor, eIF2alpha that could contribute to inhibition or alteration of protein synthesis during the late stages of apoptosis.

Induction of E-selectin expression by double-stranded RNA and TNF-alpha is attenuated in murine aortic endothelial cells derived from double-stranded RNA-activated kinase (PKR)-null mice

The adherence of leukocytes on the endothelium is mediated in part by the transient expression of the E-selectin adhesion molecule. Because we have previously shown that the dsRNA-activated kinase PKR mediates dsRNA induction of NF-kappaB, we used murine aortic endothelial (MuAE) cells isolated from wild-type and PKR-null mice to investigate the role of PKR in the induction of E-selectin expression by dsRNA (pIC) and TNF-alpha. E-selectin mRNA and protein expression was inducible by both pIC and TNF-alpha in wild-type MuAE cells, whereas induction of E-selectin expression by these agents was defective in PKR-null MuAE cells. Induction of E-selectin promoter activity and NF-kappaB DNA binding activity were substantially reduced in pIC- or TNF-alpha-treated PKR-null cells, indicating a role for PKR in both pIC and TNF-alpha induction of E-selectin via an NF-kappaB-dependent pathway. In PKR-null cells, pIC-mediated degradation of IkappaBbeta is deficient. Activation of this pathway requires the PKR-dependent degradation of the IkappaBbeta protein. Moreover, both phosphorylated and unphosphorylated activating transcription factor 2 DNA-binding activities were reduced in PKR-null aortic endothelial cells. These results indicate that the PKR is required for full activation of E-selectin expression by pIC and TNF-alpha in primary mouse aortic endothelial cells identifying activating transcription factor 2 as a new target for PKR-dependent regulation and suggest a role for PKR in leukocyte adhesion.

NF-kappaB activation by double-stranded-RNA-activated protein kinase (PKR) is mediated through NF-kappaB-inducing kinase and IkappaB kinase

The interferon (IFN)-inducible double-stranded-RNA (dsRNA)-activated serine-threonine protein kinase (PKR) is a major mediator of the antiviral and antiproliferative activities of IFNs. PKR has been implicated in different stress-induced signaling pathways including dsRNA signaling to nuclear factor kappa B (NF-kappaB). The mechanism by which PKR mediates activation of NF-kappaB is unknown. Here we show that in response to poly(rI). poly(rC) (pIC), PKR activates IkappaB kinase (IKK), leading to the degradation of the inhibitors IkappaBalpha and IkappaBbeta and the concomitant release of NF-kappaB. The results of kinetic studies revealed that pIC induced a slow and prolonged activation of IKK, which was preceded by PKR activation. In PKR null cell lines, pIC failed to stimulate IKK activity compared to cells from an isogenic background wild type for PKR in accord with the inability of PKR null cells to induce NF-kappaB in response to pIC. Moreover, PKR was required to establish a sustained response to tumor necrosis factor alpha (TNF-alpha) and to potentiate activation of NF-kappaB by cotreatment with TNF-alpha and IFN-gamma. By coimmunoprecipitation, PKR was shown to be physically associated with the IKK complex. Transient expression of a dominant negative mutant of IKKbeta or the NF-kappaB-inducing kinase (NIK) inhibited pIC-induced gene expression from an NF-kappaB-dependent reporter construct. Taken together, these results demonstrate that PKR-dependent dsRNA induction of NF-kappaB is mediated by NIK and IKK activation.

A mammalian homologue of GCN2 protein kinase important for translational control by phosphorylation of eukaryotic initiation factor-2alpha

A family of protein kinases regulates translation in response to different cellular stresses by phosphorylation of the alpha subunit of eukaryotic initiation factor-2 (eIF-2alpha). In yeast, an eIF-2alpha kinase, GCN2, functions in translational control in response to amino acid starvation. It is thought that uncharged tRNA that accumulates during amino acid limitation binds to sequences in GCN2 homologous to histidyl-tRNA synthetase (HisRS) enzymes, leading to enhanced kinase catalytic activity. Given that starvation for amino acids also stimulates phosphorylation of eIF-2alpha in mammalian cells, we searched for and identified a GCN2 homologue in mice. We cloned three different cDNAs encoding mouse GCN2 isoforms, derived from a single gene, that vary in their amino-terminal sequences. Like their yeast counterpart, the mouse GCN2 isoforms contain HisRS-related sequences juxtaposed to the kinase catalytic domain. While GCN2 mRNA was found in all mouse tissues examined, the isoforms appear to be differentially expressed. Mouse GCN2 expressed in yeast was found to inhibit growth by hyperphosphorylation of eIF-2alpha, requiring both the kinase catalytic domain and the HisRS-related sequences. Additionally, lysates prepared from yeast expressing mGCN2 were found to phosphorylate recombinant eIF-2alpha substrate. Mouse GCN2 activity in both the in vivo and in vitro assays required the presence of serine-51, the known regulatory phosphorylation site in eIF-2alpha. Together, our studies identify a new mammalian eIF-2alpha kinase, GCN2, that can mediate translational control.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

Identification of functional domains of the interferon-induced enzyme PKR in cells lacking endogenous PKR

The interferon (IFN)-induced, double-stranded RNA (dsRNA)-activated human protein kinase (PKR) has been shown to exert antiviral and antiproliferative effects. Activation of the enzyme in mammalian cells results in protein synthesis inhibition and cell death by apoptosis. Previous studies on the structure-function relationship of PKR have been based on vectors expressing the enzyme in mammalian cells containing endogenous PKR. As exogenously expressed PKR can form heterodimers with endogenous PKR, the results obtained on the functional characterization of mutant forms of PKR have been taken with caution. To address the natural consequences of heterodimer formation between endogenous and exogenous PKR, we have analyzed the structure-function relationship of PKR ectopically expressed from vaccinia virus (VV) recombinants in cells lacking the endogenous enzyme. We demonstrate that PKR-mediated inhibition of protein synthesis and induction of apoptosis is not dependent on the presence of endogenous PKR. Further, PKR activity is independent of the presence of dsRNA binding motifs (dsRBM). Moreover, single-point mutations of the third basic domain decreased PKR activation. Our findings demonstrate that PKR can be activated in the absence of its N-terminal domain (amino acids 1-232) and that the third basic domain is important for its biologic function.

Apoptosis and HIV infection: T-cells fiddle while the immune system burns

Enhancement of programmed cell death (apoptosis) of CD4 T-cells by human immunodeficiency virus (HIV) is thought to be an important factor in the pathogenesis of HIV disease. Recent studies have cast doubt on this concept, however, portraying apoptosis as a potent antiviral strategy to eliminate infected cells. These studies have shed new light on the role of apoptosis in HIV infection. While cellular and immunologic mechanisms of apoptosis purge the HIV-infected lymphoid cell population, HIV thwarts apoptosis in myeloid cells, particularly monocyte/macrophages. Although HIV protease inhibitor therapy partially reverses the lymphoid cell process, this therapeutic approach fails to counter the persistence of HIV infection in myeloid cells. Thus apoptosis of T-cells may be a futile host attempt to control the spread of HIV while the infection smoulders in monocyte/macrophages. In other words, the antiviral defense system fiddles while the immune system burns.

Induction of apoptosis by double-stranded-RNA-dependent protein kinase (PKR) involves the alpha subunit of eukaryotic translation initiation factor 2 and NF-kappaB

The double-stranded (ds) RNA-dependent protein kinase (PKR) is a key mediator of antiviral effects of interferon (IFN) and an active player in apoptosis induced by different stimuli. The translation initiation factor eIF-2alpha (alpha subunit of eukaryotic translation initiation factor 2) and IkappaBalpha, the inhibitor of the transcription factor NF-kappaB, have been proposed as downstream mediators of PKR effects. To evaluate the involvement of NF-kappaB and eIF-2alpha in the induction of apoptosis by PKR, we have used vaccinia virus (VV) recombinants that inducibly express PKR concomitantly with a dominant negative mutant of eIF-2alpha or a repressor form of IkappaBalpha. We found that while expression of PKR by a VV vector resulted in extensive inhibition of protein synthesis and induction of apoptosis, coexpression of PKR with a dominant negative mutant of eIF-2alpha (Ser-51-->Ala) reversed both the PKR-mediated translational block and PKR-induced apoptosis. Coexpression of PKR with a repressor form of IkappaBalpha (Ser-32, 36-Ala) also leads to the inhibition of apoptosis by abolishing NF-kappaB induction, while translation remains blocked. Treating cells with two different proteasome inhibitors which block IkappaBalpha degradation, prevented PKR-induced apoptosis, supporting results from coexpression studies. Biochemical analysis and transient assays revealed that PKR expression by a VV vector induced NF-kappaB binding and transactivation. In addition, upregulation of Fas mRNA transcription occurred during PKR activation. Our findings provide direct evidence for the involvement of eIF-2alpha and NF-kappaB in the induction of apoptosis by PKR.

Inhibition of double-stranded RNA- and tumor necrosis factor alpha-mediated apoptosis by tetratricopeptide repeat protein and cochaperone P58(IPK)

P58(IPK) is a tetratricopeptide repeat-containing cochaperone that is involved in stress-activated cellular pathways and that inhibits the activity of protein kinase PKR, a primary mediator of the antiviral and antiproliferative properties of interferon. To gain better insight into the molecular actions of P58(IPK), we generated NIH 3T3 cell lines expressing either wild-type P58(IPK) or a P58(IPK) deletion mutant, DeltaTPR6, that does not bind to or inhibit PKR. When treated with double-stranded RNA (dsRNA), DeltaTPR6-expressing cells exhibited a significant increase in eukaryotic initiation factor 2alpha phosphorylation and NF-kappaB activation, indicating a functional PKR. In contrast, both of these PKR-dependent events were blocked by the overexpression of wild-type P58(IPK). In addition, the P58(IPK) cell line, but not the DeltaTPR6 cell line, was resistant to dsRNA-induced apoptosis. Together, these findings demonstrate that P58(IPK) regulates dsRNA signaling pathways by inhibiting multiple PKR-dependent functions. In contrast, both the P58(IPK) and DeltaTPR6 cell lines were resistant to tumor necrosis factor alpha-induced apoptosis, suggesting that P58(IPK) may function as a more general suppressor of programmed cell death independently of its PKR-inhibitory properties. In accordance with this hypothesis, although PKR remained active in DeltaTPR6-expressing cells, the DeltaTPR6 cell line displayed a transformed phenotype and was tumorigenic in nude mice. Thus, the antiapoptotic function of P58(IPK) may be an important factor in its ability to malignantly transform cells.

Involvement of a p53-dependent pathway in rubella virus-induced apoptosis

In light of the important role of apoptotic cell death in the pathogenesis of several viral infections, we asked whether the cytopathogenicity evoked by rubella virus (RV) might also involve apoptotic mechanisms. The To-336 strain of RV induced apoptosis in Vero and RK-13 cells, but not in fibroblast cell lines. UV-inactivated RV virions did not elicit the apoptotic response, indicating that productive infection is required for the induction of cell death. Both p53 and p21 protein levels were highly elevated in RV-infected Vero cells. The level of p21 mRNA was increased, while expression of the p53 gene was unaffected by RV infection. A dominant-negative p53 mutant (p53(W248)) conferred partial protection from RV-induced apoptosis. These data implicate a p53-dependent apoptotic pathway in the cytopathogenicity of RV, thereby suggesting a mechanism by which RV exerts its teratogenic effects.

The emerging role of the interferon-induced PKR protein kinase as an apoptotic effector: a new face of death?

Recent research has thrown a spotlight on the interferon (IFN)-induced PKR protein kinase, implicating it as an important effector of apoptosis induced by several cellular stress conditions, including viral infection, cytokine treatment, and growth factor deprivation. In this review, we summarize the evidence for the role of PKR as a death accomplice and discuss how PKR might promote cell demise in light of current knowledge of the molecular mechanisms of apoptosis. Given its new found role and its established antiviral function, it is no wonder that PKR is a popular target for viral evasion of the host defense. PKR-dependent apoptosis may offer a novel cell-death pathway for specific manipulation in therapeutic strategies against apoptosis-related diseases.

Identification by two-dimensional gel electrophoresis of vaccinia virus and cellular phosphoproteins modified after inducible expression of the dsRNA-activated protein kinase

The interferon (IFN)-induced double-stranded (ds) RNA-activated protein kinase (PKR) is a serine/threonine kinase that plays an important role in the biology of IFN, exerting antiviral and anticellular actions. These effects have been correlated with phosphorylation of the eukaryotic initiation factor eIF-2alpha and the NF-kappaB inhibitor IkappaB, although it has not been demonstrated that IkappaB is a direct target of PKR in vivo. In view of the various biological effects of PKR, it is likely that other cellular substrates are involved in PKR action. To identify novel substrates of PKR, we have carried out a systematic study of the phosphorylated proteins from cultured cells following PKR activation using high-resolution two-dimensional gel electrophoresis (2D-PAGE). We have used metabolic labeling with [32P]orthophosphate of HeLa cells infected with vaccinia virus (VV) recombinants expressing wild type (wt) or the catalytically inactive mutant form (K296R) of PKR under regulation of the Escherichia coli lacI operator/repressor system. Upon induction of PKR in the presence of isopropyl-beta-D-thiogalactoside (IPTG), the 68-kDA wt enzyme and eIF-2alpha are phosphorylated. These events lead to changes in the phosphorylation state of viral and cellular proteins. A distinct set of VV-induced phosphoproteins remained phophorylated, while the labeling of other viral proteins decreased markedly, probably as a result of a PKR-dependent translational block. Five proteins of unknown origin (68, 26, 20, 19, 15-16 kDA) appeared to be newly phosphorylated after PKR activation. Expression of the catalytically inactive K296R mutant form of PKR did not induce changes in the phosphorylation of VV proteins. Thus, by 2D-PAGE, we identified cellular and VV-induced phosphoproteins modified after PKR activation. Some or all of the phosphoproteins appearing or increasing in amount after PKR activation might not be direct targets of PKR, but rather indirect consequences of PKR activation.

Augmentation of verotoxin-induced cytotoxicity/apoptosis by interferon is repressed in cells persistently infected with mumps virus

Verotoxin type 2 (VT2) produced by enterohemorrhagic Escherichia coli (EHEC) has been shown to have high cytotoxic potency toward several human B lymphoid cell lines with and without Epstein-Barr virus (EBV). Cell death, apoptosis induced by VT2, is closely correlated with the expression of receptor molecule Gb3/CD77, recognized by the toxin, but not with the infection or presence of EBV. Pretreatment of cells with interferon-alpha (IFN-alpha) for 24 h resulted in augmentation of apoptosis by VT2. Pretreatment within 8 h, however, was not effective. It has been reported that IFN-alpha-induced apoptosis is correlated with the induction of the 2',5'-OAS/RNase L system or dsRNA-activated protein kinase (PKR) or both. We have established persistent infection in both Akata and P3HR-1 cells with mumps virus. The persistently infected cell lines, P3HR-MP2 and Akata-MP2, showed poor induction of 2',5'-OAS and PKR in response to IFN-alpha. Augmentation of VT2-induced apoptosis by IFN-alpha was not found in the cell lines P3HR-MP2 and Akata-MP2. Therefore, these findings were interpreted to indicate that augmentation of VT2-induced apoptosis by IFN-alpha may be mediated by PKR and the 2',5'-OAS/RNaseL system. It is also suggested that mumps virus can suppress apoptosis and establish persistent infection.

The double-stranded RNA activated protein kinase PKR physically associates with the tumor suppressor p53 protein and phosphorylates human p53 on serine 392 in vitro

The tumor suppressor p53 is a multifunctional protein that plays a critical role in modulating cellular responses upon DNA damage or other stresses. These functions of p53 are regulated both by protein-protein interactions and phosphorylation. The double-stranded RNA activated protein kinase PKR is a serine/threonine kinase that modulates protein synthesis through the phosphorylation of translation initiation factor eIF-2alpha. PKR is an interferon (IFN)-inducible protein that is thought to mediate the anti-viral and anti-proliferative effects of IFN via its capacity to inhibit protein synthesis. Here we report that PKR physically associates with p53. The interaction of PKR with p53 is enhanced by IFNs and upon conditions that p53 acquires a wild type conformation. PKR/p53 complex formation in vitro requires the N-terminal regulatory domain of PKR and the last 30 amino acids of the C-terminus of human p53. In addition, p53 may function as a substrate of PKR since phosphorylation of human p53 on serine392 is induced by activated PKR in vitro. These novel findings raise the possibility of a functional interaction between PKR and p53 in vivo, which may account, at least in part, for the ability of each protein to regulate gene expression at both the transcriptional and the translational levels.

Regulatable expression of the interferon-induced double-stranded RNA dependent protein kinase PKR induces apoptosis and fas receptor expression

PKR is an interferon-induced dsRNA-dependent protein kinase involved in the antiviral response as well as in cell growth and differentiation. Studies using a transdominant negative mutant of PKR also have implicated the kinase in tumor suppression and apoptosis. However, functional studies of PKR have been hampered by the lack of a suitable expression system. In this study, we used a tetracycline-regulated inducible system in NIH3T3 cells to investigate the involvement of PKR in programmed cell death (apoptosis). We show that expression of wild-type PKR causes apoptosis and correlates with increased mRNA levels for the Fas receptor, a member of the tumor necrosis family of proteins. Expression of an inactive form of PKR (K296R) or the vector alone did not induce apoptosis or elevate Fas mRNA levels. Our results clearly demonstrate that expression of an active form of PKR triggers apoptosis, possibly through upregulation of the Fas receptor.

Double-stranded-RNA-activated protein kinase PKR enhances transcriptional activation by tumor suppressor p53

The tumor suppressor p53 plays a key role in inducing G1 arrest and apoptosis following DNA damage. The double-stranded-RNA-activated protein PKR is a serine/threonine interferon (IFN)-inducible kinase which plays an important role in regulation of gene expression at both transcriptional and translational levels. Since a cross talk between IFN-inducible proteins and p53 had already been established, we investigated whether and how p53 function was modulated by PKR. We analyzed p53 function in several cell lines derived from PKR+/+ and PKR-/- mouse embryonic fibroblasts (MEFs) after transfection with the temperature-sensitive (ts) mutant of mouse p53 [p53(Val135)]. Here we report that transactivation of transcription by p53 and G0/G1 arrest were impaired in PKR-/- cells upon conditions that ts p53 acquired a wild-type conformation. Phosphorylation of mouse p53 on Ser18 was defective in PKR-/- cells, consistent with an impaired transcriptional induction of the p53-inducible genes encoding p21(WAF/Cip1) and Mdm2. In addition, Ser18 phosphorylation and transcriptional activation by mouse p53 were diminished in PKR-/- cells after DNA damage induced by the anticancer drug adriamycin or gamma radiation but not by UV radiation. Furthermore, the specific phosphatidylinositol-3 (PI-3) kinase inhibitor LY294002 inhibited the induction of phosphorylation of Ser18 of p53 by adriamycin to a higher degree in PKR+/+ cells than in PKR-/- cells. These novel findings suggest that PKR enhances p53 transcriptional function and implicate PKR in cell signaling elicited by a specific type of DNA damage that leads to p53 phosphorylation, possibly through a PI-3 kinase pathway.

Apoptosis: an innate immune response to virus infection

Viruses can induce apoptosis of infected cells either directly, to assist virus dissemination, or by inadvertently triggering cellular sensors that initiate cell death. Cellular checkpoints that can function as 'alarm bells' to transmit pro-apoptotic signals in response to virus infections include death receptors, protein kinase R, mitochondrial membrane potential, p53 and the endoplasmic reticulum.

Characterization of transgenic mice with targeted disruption of the catalytic domain of the double-stranded RNA-dependent protein kinase, PKR

The interferon-inducible, double-stranded RNA-dependent protein kinase PKR has been implicated in anti-viral, anti-tumor, and apoptotic responses. Others have attempted to examine the requirement of PKR in these roles by targeted disruption at the amino terminal-encoding region of the Pkr gene. By using a strategy that aims at disruption of the catalytic domain of PKR, we have generated mice that are genetically ablated for functional PKR. Similar to the other mouse model of Pkr disruption, we have observed no consequences of loss of PKR on tumor suppression. Anti-viral response to influenza and vaccinia also appeared to be normal in mice and in cells lacking PKR. Cytokine signaling in the type I interferon pathway is normal but may be compromised in the erythropoietin pathway in erythroid bone marrow precursors. Contrary to the amino-terminal targeted Pkr mouse, tumor necrosis factor alpha-induced apoptosis and the anti-viral apoptosis response to influenza is not impaired in catalytic domain-targeted Pkr-null cells. The observation of intact eukaryotic initiation factor-2alpha phosphorylation in these Pkr-null cells provides proof of rescue by another eukaryotic initiation factor-2alpha kinase(s).

Double-stranded RNA-activated protein kinase (PKR) is negatively regulated by 60S ribosomal subunit protein L18

The double-stranded RNA (dsRNA)-activated protein kinase (PKR) provides a fundamental control step in the regulation of protein synthesis initiation through phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2alpha), a process that prevents polypeptide chain initiation. In such a manner, activated PKR inhibits cell growth and induces apoptosis, whereas disruption of normal PKR signaling results in unregulated cell growth. Therefore, tight control of PKR activity is essential for regulated cell growth. PKR is activated by dsRNA binding to two conserved dsRNA binding domains within its amino terminus. We isolated a ribosomal protein L18 by interaction with PKR. L18 is a 22-kDa protein that is overexpressed in colorectal cancer tissue. L18 competed with dsRNA for binding to PKR, reversed dsRNA binding to PKR, and did not directly bind dsRNA. Mutation of K64E within the first dsRNA binding domain of PKR destroyed both dsRNA binding and L18 interaction, suggesting that the two interactive sites overlap. L18 inhibited both PKR autophosphorylation and PKR-mediated phosphorylation of eIF-2alpha in vitro. Overexpression of L18 by transient DNA transfection reduced eIF-2alpha phosphorylation and stimulated translation of a reporter gene in vivo. These results demonstrate that L18 is a novel regulator of PKR activity, and we propose that L18 prevents PKR activation by dsRNA while PKR is associated with the ribosome. Overexpression of L18 may promote protein synthesis and cell growth in certain cancerous tissue through inhibition of PKR activity.

PKR, apoptosis and cancer

The double-stranded (ds) RNA-regulated serine/threonine protein kinase, PKR, is an interferon-inducible enzyme of widespread occurrence in mammalian cells. PKR is activated by dsRNA via a mechanism involving autophosphorylation. Once activated, the enzyme phosphorylates the alpha-subunit of protein synthesis initiation factor eIF2, thereby inhibiting translation. Accumulating data suggest that PKR has additional substrates, and that the kinase may also regulate gene transcription and signal transduction pathways. Although PKR plays an important role in mediating the antiviral effects of interferons, PKR is also implicated in regulating cell proliferation in uninfected cells and may have a tumor suppressor function under normal conditions. Studies of human malignancies and tumor cell lines suggest that, in general, patients bearing tumors with a higher PKR content have a more favorable prognosis. However, in human breast carcinoma cells, dysregulation of PKR may be associated with the establishment or maintenance of the transformed state.

Abnormal levels and minimal activity of the dsRNA-activated protein kinase, PKR, in breast carcinoma cells

The interferon induced, dsRNA-activated, protein kinase, PKR, is a key regulator of translational initiation, playing an important role in the regulation of cell proliferation, apoptosis and transformation. PKR levels correlate inversely with proliferative activity in several human tumor systems. This inverse relationship breaks down in human invasive ductal breast carcinomas which exhibit high levels of PKR (Haines et al., Tumor Biol. 17 (1996) 5-12). Consistent with the data from human tumors, the levels of PKR in several breast carcinoma cell lines, MCF7, T47D, BT20, MDAMB231 and MDAMB468, are paradoxically high compared to those found in the normal breast cell lines MCF10A and Hs578Bst. The activity of affinity- or immuno-purified PKR from MCF7, T47D, and BT20 cells appears to be severely attenuated, as judged by its ability to autophosphorylate, or phosphorylate eIF2 alpha. Furthermore, the activity of the kinase from breast carcinoma cells is refractory to stimulation by dsRNA or heparin. However, PKR from breast carcinoma cells remains functional with respect to its ability to bind dsRNA. The activity of PKR from MCF10A cells is reduced by prior incubation with extracts from MCF7 cells, suggesting that MCF7 extracts contain a transdominant inhibitor of PKR. Deregulation of PKR may therefore provide a mechanism for the development or maintenance of a transformed phenotype of human breast carcinomas, mimicking the effects of manipulation of PKR or eIF2 activity observed in experimental systems. Thus, breast carcinomas may provide the first indication of a role for PKR in the pathogenesis of a naturally occurring human cancer.

Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control

In response to various environmental stresses, eukaryotic cells down-regulate protein synthesis by phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2alpha). In mammals, the phosphorylation was shown to be carried out by eIF-2alpha kinases PKR and HRI. We report the identification and characterization of a cDNA from rat pancreatic islet cells that encodes a new related kinase, which we term pancreatic eIF-2alpha kinase, or PEK. In addition to a catalytic domain with sequence and structural features conserved among eIF-2alpha kinases, PEK contains a distinctive amino-terminal region 550 residues in length. Using recombinant PEK produced in Escherichia coli or Sf-9 insect cells, we demonstrate that PEK is autophosphorylated on both serine and threonine residues and that the recombinant enzyme can specifically phosphorylate eIF-2alpha on serine-51. Northern blot analyses indicate that PEK mRNA is expressed in all tissues examined, with highest levels in pancreas cells. Consistent with our mRNA assays, PEK activity was predominantly detected in pancreas and pancreatic islet cells. The regulatory role of PEK in protein synthesis was demonstrated both in vitro and in vivo. The addition of recombinant PEK to reticulocyte lysates caused a dose-dependent inhibition of translation. In the Saccharomyces model system, PEK functionally substituted for the endogenous yeast eIF-2alpha kinase, GCN2, by a process requiring the serine-51 phosphorylation site in eIF-2alpha. We also identified PEK homologs from both Caenorhabditis elegans and the puffer fish Fugu rubripes, suggesting that this eIF-2alpha kinase plays an important role in translational control from nematodes to mammals.

Activation of the dsRNA-dependent protein kinase, PKR, induces apoptosis through FADD-mediated death signaling

The dsRNA-dependent protein kinase (PKR) is considered to play a key role in interferon-mediated host defense against viral infection and conceivably malignant transformation. To investigate further the mechanisms of PKR-induced growth inhibition, we have developed tetracycline-inducible murine cell lines that express wild-type PKR or a catalytically inactive PKR variant, PKRdelta6. Following induction, the growth of the wild-type PKR-expressing cells was similar to that of cells transfected with vector alone, while cells expressing PKRdelta6 became malignantly transformed. Significantly, treatment with dsRNA caused the wild-type PKR-overexpressing cells to undergo programed cell death while, conversely, cells expressing PKRdelta6 were completely resistant. Our studies demonstrated that activation of PKR induces the expression of members of the tumor necrosis factor receptor (TNFR) family, including Fas (CD95/Apo-1) and pro-apopotic Bax. In contrast, transcripts representing Fas, TNFR-1, FADD (Fas-associated death domain), FLICE, Bad and Bax were ablated in cells expressing PKRdelta6. The involvement of the death receptors in PKR-induced apoptosis was underscored by demonstrating that murine fibroblasts lacking FADD were almost completely resistant to dsRNA-mediated cell death. Thus, PKR, a key cellular target for viral repression, is a receptor/inducer for the induction of pro-apoptotic genes by dsRNA and probably functions in interferon-mediated host defense to trigger cell death in response to virus infection and perhaps tumorigenesis.

The murine PKR tumor suppressor gene is rearranged in a lymphocytic leukemia

The double-stranded RNA-dependent kinase, PKR, is encoded by an interferon inducible gene and is largely responsible for the anti-viral effects of this cytokine. Recent studies have shown that PKR may also play a role in the regulation of normal cellular growth. Although numerous examples of viral strategies for inactivation of PKR exist, there is no evidence of PKR inactivation in tumors. We demonstrate here that the Tik gene, which encodes a dual-specificity kinase, is the murine homolog of PKR, the dsRNA-dependent kinase, and has undergone a rearrangement of one allele in a murine lymphocytic leukemia cell. We have cloned a cDNA that corresponds to a mutated transcript from the rearranged mPKR gene and show that while the mutated polypeptide retains its ability to dimerize and bind dsRNA, it is catalytically inactive. Although this mutated mPKR lacks apparent dominant-negative function, the net effect of reduced PKR activity in these cells may be significant.

How cells respond to interferons

Interferons play key roles in mediating antiviral and antigrowth responses and in modulating immune response. The main signaling pathways are rapid and direct. They involve tyrosine phosphorylation and activation of signal transducers and activators of transcription factors by Janus tyrosine kinases at the cell membrane, followed by release of signal transducers and activators of transcription and their migration to the nucleus, where they induce the expression of the many gene products that determine the responses. Ancillary pathways are also activated by the interferons, but their effects on cell physiology are less clear. The Janus kinases and signal transducers and activators of transcription, and many of the interferon-induced proteins, play important alternative roles in cells, raising interesting questions as to how the responses to the interferons intersect with more general aspects of cellular physiology and how the specificity of cytokine responses is maintained.

Apoptosis regulators from DNA viruses

The past year has seen an considerable expansion in knowledge about the field of apoptosis modulators expressed by DNA viruses. These diverse classes of virus-encoded regulators include caspase inhibitors, signal transduction effectors, Bcl-2 homologs, cell cycle control proteins, transcriptional regulators, reactive oxide scavengers, kinases, 'death factors' and novel host-range proteins.

Isolation of the gene encoding the Drosophila melanogaster homolog of the Saccharomyces cerevisiae GCN2 eIF-2alpha kinase

Genomic and cDNA clones homologous to the yeast GCN2 eIF-2alpha kinase (yGCN2) were isolated from Drosophila melanogaster. The identity of the Drosophila GCN2 (dGCN2) gene is supported by the unique combination of sequence encoding a protein kinase catalytic domain and a domain homologous to histidyl-tRNA synthetase and by the ability of dGCN2 to complement a deletion mutant of the yeast GCN2 gene. Complementation of Deltagcn2 in yeast by dGCN2 depends on the presence of the critical regulatory phosphorylation site (serine 51) of eIF-2alpha. dGCN2 is composed of 10 exons encoding a protein of 1589 amino acids. dGCN2 mRNA is expressed throughout Drosophila development and is particularly abundant at the earliest stages of embryogenesis. The dGCN2 gene was cytogenetically and physically mapped to the right arm of the third chromosome at 100C3 in STS Dm2514. The discovery of GCN2 in higher eukaryotes is somewhat unexpected given the marked differences between the amino acid biosynthetic pathways of yeast vs. Drosophila and other higher eukaryotes. Despite these differences, the presence of GCN2 in Drosophila suggests at least partial conservation from yeast to multicellular organisms of the mechanisms responding to amino acid deprivation.

Using genetic means to dissect homologous and heterologous protein-protein interactions of PKR, the interferon-induced protein kinase

The interferon-induced protein kinase, PKR, is a pivotal component of interferon (IFN)-induced cellular antiviral and antiproliferative response. The identification and characterization of proteins, of both viral and cellular origins, that interact with PKR have proven to be a valuable probe for unraveling the cellular regulation and function of PKR. Several studies have demonstrated that PKR forms dimers and that dimerization is likely to be required for activation and/or catalytic function. It is therefore important to elucidate the mechanism of PKR dimer formation and the role of PKR effectors in modulating kinase dimerization. Herein we describe the use of the two genetic approaches, the lambda repressor fusion and the yeast two-hybrid systems, to detect and analyze homo- and heterotypic interactions with PKR. We also describe several biochemical methodologies commonly used in our laboratory to validate the genetic results. Although the examples in this article focus on PKR, the techniques can easily be adapted to investigate protein-protein associations in a variety of experimental systems. Finally, given the important role of PKR as a mediator of IFN-induced antiviral and antiproliferative effects, these studies may provide clues to the development of reagents that target PKR to enhance the therapeutic use of IFN in the treatment of disease.

The short Sendai virus leader region controls induction of programmed cell death

The replication of nonsegmented minus-strand RNA genomes, like that of Sendai paramyxovirus (SeV), are controlled by the short leader regions present at each end of the linear genomes and antigenomes; the left and right promoters (PL and PR), respectively. Wild-type SeV is highly cytopathic in cell culture, because it induces programmed cell death (PCD). We have found that a recombinant SeV (rSeVGP42), in which the first 42 nt of le+ sequences at PL were replaced with the equivalent sequences of PR, and which produces infectious virus in amounts comparable to wild type, does not kill cells. Further, the increasing replacement of the terminal le+ sequences at PL with le- sequences led to a decreasing fraction of infected cells being apoptotic. This property (PCD-), moreover, is dominant in cells co-infected with SeVwt and rSeVGP42, and the mutant virus therefore appears to have gained a function which prevents PCD induced by SeVwt. Even though this virus has not been selected for naturally, it excludes SeVwt during co-infections of cultured cells or embryonated chicken eggs. The noncytopathic nature of cells infected or co-infected with rSeVGP42 leads automatically to stable, persistent infections. The mutation in rSeVGP42 is not in the protein coding regions of the viral genome, but in the 55-nt-long leader region which controls antigenome synthesis from genome templates. The SeV leader regions, which are expressed as short RNAs, thus appear to control the induction of PCD.

Molecular mechanisms of interferon resistance mediated by viral-directed inhibition of PKR, the interferon-induced protein kinase

The interferon (IFN)-induced cellular antiviral response is the first line of defense against viral infection within an animal host. In order to establish a productive infection, eukaryotic viruses must first overcome the IFN-induced blocks imposed on viral replication. The double-stranded RNA-activated protein kinase (PKR) is a key component mediating the antiviral actions of IFN. This IFN-induced protein kinase can restrict viral replication through its ability to phosphorylate the protein synthesis initiation factor eukaryotic initiation factor-2 alpha-subunit and reduce levels of viral protein synthesis. Viruses, therefore, must block the function of PKR in order to avoid these deleterious antiviral effects associated with PKR activity. Indeed, many viruses have developed effective measures to repress PKR activity during infection. This review will focus primarily on an overview of the different molecular mechanisms employed by these viruses to meet a common goal: the inhibition of PKR function, uncompromised viral protein synthesis, and unrestricted virus replication. The past few years have seen exciting new advances in this area. Rather unexpectedly, this area of research has benefited from the use of the yeast system to study PKR. Other recent advances include studies on PKR regulation by the herpes simplex viruses and data from our laboratory on the medically important hepatitis C viruses. We speculate that IFN is ineffective as a therapeutic agent against hepatitis C virus because the virus can effectively repress PKR function. Finally, we will discuss briefly the future directions of this PKR field.

Phosphorylation of eukaryotic translation initiation factor 2 mediates apoptosis in response to activation of the double-stranded RNA-dependent protein kinase

The interferon-inducible, double-stranded (ds) RNA-dependent serine/threonine protein kinase (PKR) plays a role in viral pathogenesis, cell growth, and differentiation and is implicated as a tumor suppressor gene. Expression of a trans-dominant negative, catalytically inactive mutant PKR protected NIH3T3 cells from apoptosis in response to either treatment with tumor necrosis factor alpha (TNF alpha), serum deprivation. In cells expressing mutant PKR, TNF alpha, but not dsRNA induced transcription from a nuclear factor kappa B-dependent promoter, demonstrating specificity for dsRNA in signaling through the PKR pathway. Serum or platelet-derived growth factor addition to serum-deprived mutant PKR-expressing cells induced transcription of the early response genes c-fos and c-jun, indicating that the immediate early response signaling was intact. Overexpression of wild-type PKR in a transient DNA transfection system was sufficient to induce apoptosis. TNF alpha-induced apoptosis correlated with increased phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2 alpha), the primary physiological substrate of the PKR. Furthermore, forced expression of a nonphosphorylatable S51A mutant eIF-2 alpha partially protected cells from TNF alpha-induced apoptosis, and expression of a S51D mutant eIF-2 alpha, a mutant that mimics phosphorylated eIF-2 alpha, was sufficient to induce apoptosis. Taken together, these studies identify a novel requirement for PKR in stress-induced apoptosis that is mediated through eIF-2 alpha phosphorylation.

Activation of the IFN-inducible enzyme RNase L causes apoptosis of animal cells

The interferon (IFN)-induced enzyme RNase L produced by a recombinant vaccinia virus (VV) causes death of mammalian cells with morphological and biochemical characteristics of apoptosis. Coexpression of 2-5A-synthetase enhances apoptosis induced by RNase L Activation of endogenous RNase L by infection with a VV ts mutant (ts22) or with wild-type virus in the presence of the antipoxvirus drug isatin-beta-thiosemicarbazone, a treatment known to significantly increase the amount of double-stranded RNA late during infection, also causes pronounced apoptosis of infected cells. The effects observed with recombinant virus-derived RNase L or with the endogenous enzyme are specific, since apoptosis also occurs in cells derived from mice lacking the IFN-induced protein kinase (PKR). The apoptosis antagonist Bcl-2 prevents induction of cell death by RNase L activation. Apoptosis of mammalian cells by RNase L activation could be a mechanism mediating anticellular actions of IFN.