PKR, apoptosis and cancer

Engineered measles virus as a novel oncolytic viral therapy system for hepatocellular carcinoma

The oncolytic measles virus Edmonston strain (MV-Edm), a nonpathogenic virus targeting cells expressing abundant CD46, selectively destroys neoplastic tissue. Clinical development of MV-Edm would benefit from noninvasive monitoring strategies to determine the speed and extent of the spread of the virus in treated patients and the location of virus-infected cells. We evaluated recombinant MV-Edm expressing carcinoembryonic antigen (CEA) or the human sodium iodide symporter (hNIS) for oncolytic potential in hepatocellular carcinoma (HCC) and efficiency in tracking viruses in vivo by noninvasive monitoring. CD46 expression in human HCC and primary hepatocytes was assessed by flow cytometry and immunohistochemistry. Infectivity, syncytium formation, and cytotoxicity of recombinant MV-Edm in HCC cell lines were evaluated by fluorescence microscopy, crystal violet staining, and the MTS assay. Transgene expression in HCC cell lines after infection with recombinant MV-Edm in vitro and in vivo was assessed by CEA concentration, 125I-uptake, and 123I-imaging studies. Toxicology studies were performed in Ifnar(KO)xCD46 transgenic mice. The CD46 receptor was highly expressed in HCC compared to nonmalignant hepatic tissue. Recombinant MV-Edm efficiently infected HCC cell lines, resulting in extensive syncytium formation followed by cell death. Transduction of HCC cell lines and subcutaneous HCC xenografts with recombinant MV-Edm resulted in high-level expression of transgenes in vitro and in vivo. MV-Edm was nontoxic in susceptible mice. Intratumoral and intravenous therapy with recombinant MV-Edm resulted in inhibition of tumor growth and prolongation of survival with complete tumor regression in up to one third of animals. In conclusion, engineered MV-Edm may be a potent and novel cancer gene therapy system for HCC. MV-Edm expressing CEA or hNIS elicited oncolytic effects in human HCC cell lines in vitro and in vivo, enabling the spread of the virus to be monitored in a noninvasive manner.

Expression of PACT is regulated by Sp1 transcription factor

PACT is a stress-modulated, cellular activator of interferon (IFN)-induced double-stranded (ds) RNA-activated protein kinase (PKR) and is an important regulator of PKR-dependent signaling pathways. The research presented here is aimed at understanding the regulation of PACT expression in mammalian cells. PACT is expressed ubiquitously in different cell types at varying abundance. We have characterized the sequence elements in PACT promoter region that are required for its expression. Using deletion analysis of the promoter we have identified the minimal basal promoter of PACT to be within 101 nucleotides upstream of its transcription start site. Further mutational analyses within this region, followed by electrophoretic mobility shift analyses (EMSAs) and chromatin immunoprecipitation (ChiP) analysis have shown that Specificity protein 1 (Sp1) is the major transcription factor responsible for PACT promoter activity.

Enhancement of adenoviral MDA-7-mediated cell killing in human lung cancer cells by geldanamycin and its 17-allyl- amino-17-demethoxy analogue

Our previous studies demonstrated that adenovirus-mediated overexpression of melanoma differentiation-associated gene-7 (Ad-mda7) leads to rapid induction of double-stranded RNA-dependent protein kinase (PKR) and activation of its downstream targets, resulting in apoptosis induction in human lung cancer cells. Here, we report that Ad-mda7 and the benzoquinone ansamycin geldanamycin (GA) interact in a highly synergistic manner to induce cell death in human lung cancer cells. Co-administration of Ad-mda7 and GA did not modify expression of MDA-7, and was not associated with further PKR induction and activation; instead the enhanced cytotoxicity of this combination was associated with inactivation of AKT by GA. By surface staining using anti-E-cadherin monoclonal antibody and flow cytometry, we found that treatment with the combination of Ad-mda7 and GA increased E-cadherin levels in these cancer cells. Ad-mda7 and GA cotreatment also inhibited lung cancer cell motility by increasing the beta-catenin/E-cadherin association. Moreover, combination of GA derivative 17-allyl-amino, 17-demethoxygeldanamycin (17AAG), with Ad-mda7 resulted in enhancement of cell death in A549 and H460 human lung cancer cells.

Oncolytic measles virus targets high CD46 expression on multiple myeloma cells

OBJECTIVE

Multiple myeloma (MM) is an incurable B cell malignancy and novel therapeutics are urgently needed. Live attenuated measles virus (MV) has potent oncolytic activity against MM tumor xenografts. The virus is tumor selective and preferentially targets cells that express high levels of CD46 receptors. However, CD46 levels on MM have not previously been evaluated. In this study, we investigated the potential of CD46 as a target for MM therapy and correlated surface levels of CD46 on MM cells with their susceptibility to MV-induced cytopathic effects.

MATERIALS AND METHODS

CD46 expression on neoplastic plasma cells (PCs) and nonplasma cells (NPCs) from 38 MM patients was analyzed by flow cytometry and receptor numbers were quantitated using BD QuantiBRITE PE beads.

RESULTS

Results showed that malignant PCs expressed significantly higher levels of CD46 receptors compared to NPCs (p < 0.0001). The mean CD46 receptor numbers on PCs and NPCs were 49,130/cell and 7,340/cell, respectively. Potent cytopathic effects of extensive intercellular fusion were observed in measles-infected PCs but not in NPCs. The extent of MV-induced cytopathic effects of cell fusion correlated with CD46 expression levels on the MM cells. Normal plasma cells do not overexpress CD46 and colony-forming assays demonstrated that MV was not cytotoxic to normal bone marrow progenitor cells.

CONCLUSION

The present study establishes CD46 as a surface antigen that is expressed more abundantly on primary MM cells compared to normal hematopoietic cells of various lineages in the bone marrow, making CD46 a promising surface marker for targeted cytoreductive therapy of MM.

Human Gene Profiling in Response to the Active Protein Kinase, Interferon-induced Serine/threonine Protein Kinase (PKR), in Infected Cells

The interferon-induced serine/threonine protein kinase (PKR) has an essential role in cell survival and cell death after viral infection and under stress conditions, but the host genes involved in these processes are not well defined. We used human cDNA microarrays to identify, in infected cells, genes differentially expressed after PKR expression and analyzed the requirement of catalytic activity of the enzyme. To express PKR, we used vaccinia virus (VV) recombinants producing wild type PKR (VV-PKR) and the catalytically inactive mutant K296R (VV-PKR-K296R). Most regulated genes were classified according to biological function, including apoptosis, stress, defense, and immune response. Transcriptional changes detected by microarray analysis were confirmed for selected genes by quantitative real time reverse transcription PCR. A total of 111 genes were regulated specifically by PKR catalytic activity. Of these, 97 were up-regulated, and 14 were down-regulated. The ATF-3 transcription factor, involved in stress-induced beta-cell apoptosis, was up-regulated. Activation of endogenous PKR with a VV mutant lacking the viral protein E3L (VVDeltaE3L), a PKR inhibitor, triggered an increase in ATF-3 expression that was not observed in PKR(-/-) cells. Using null cells for ATF-3 and for the p65 subunit of NF-kappaB, we showed that induction of apoptosis by PKR at late times of infection was dependent on ATF-3 expression and regulated by NF-kappaB activation. Here, we identified human genes selectively induced by expression of active PKR in infected cells and linked ATF-3 to a novel mechanism used by PKR to induce apoptosis.

Nucleophosmin and cancer

NPM1 is a crucial gene to consider in the context of the genetics and biology of cancer. NPM1 is frequently overexpressed, mutated, rearranged and deleted in human cancer. Traditionally regarded as a tumour marker and a putative proto-oncogene, it has now also been attributed with tumour-suppressor functions. Therefore, NPM can contribute to oncogenesis through many mechanisms. The aim of this review is to analyse the role of NPM in cancer, and examine how deregulated NPM activity (either gain or loss of function) can contribute to tumorigenesis.

Double-stranded RNA-dependent protein kinase phosphorylation of the alpha-subunit of eukaryotic translation initiation factor 2 mediates apoptosis

As the molecular processes of complex cell stress signaling pathways are defined, the subsequent challenge is to elucidate how each individual event influences the final biological outcome. Phosphorylation of the translation initiation factor 2 (eIF2alpha)atSer(51) is a molecular signal that inhibits translation in response to activation of any of four diverse eIF2alpha stress kinases. We used gene targeting to replace the wild-type Ser(51) allele with an Ala in the eIF2alpha gene to test the hypothesis that translational control through eIF2alpha phosphorylation is a central death stimulus in eukaryotic cells. Homozygous eIF2alpha mutant mouse embryo fibroblasts were resistant to the apoptotic effects of dsRNA, tumor necrosis factor-alpha, and serum deprivation. TNFalpha treatment induced eIF2alpha phosphorylation and activation of caspase 3 primarily through the dsRNA-activated eIF2alpha kinase PKR. In addition, expression of a phospho-mimetic Ser(51) to Asp mutant eIF2alpha-activated caspase 3, indicating that eIF2alpha phosphorylation is sufficient to induce apoptosis. The proapoptotic effects of PKR-mediated eIF2alpha phosphorylation contrast with the anti-apoptotic response upon activation of the PKR-related endoplasmic reticulum eIF2alpha kinase, PERK. Therefore, divergent fates of death and survival can be mediated through phosphorylation at the same site within eIF2alpha. We propose that eIF2alpha phosphorylation is fundamentally a death signal, yet it may promote either death or survival, depending upon coincident signaling events.

Aristolochic acid I-induced DNA damage and cell cycle arrest in renal tubular epithelial cells in vitro

DNA damage is a critical event preceding cellular apoptosis or necrosis. This study was carried out to investigate the effect of aristolochic acid I (AAI) on DNA damage and cell cycle in porcine proximal tubular epithelial cell lines (LLC-PK1 cells). LLC-PK1 cells were stimulated with AAI at the concentrations of 80, 320, and 1,280 ng/ml for 24 h. DNA damage was examined by comet assay and the cell cycle was assayed by flow cytometry (FCM), cellular apoptosis and lysis were examined simultaneously. Cellular nuclear changes were observed by electron microscopy and the expression of wild-type p53 protein and mRNA were measured by FCM and RT-PCR. We found that AAI-induced DNA damage prior to apoptosis and lysis in LLC-PK1 cells in a dose-dependent manner (P<0.01). The percentage of cells in the G2/M phase that were treated with AAI (320 and 1,280 ng/ml) for 24 h increased significantly (P<0.01). Electron micrographs showed the nuclear abnormalities in AAI-treated cells. The expression of p53 protein and mRNA did not change in the AAI-treated cells. AAI may cause DNA damage and cell cycle arrest in LLC-PK1 cells through a wild-type p53-independent pathway, prior to apoptosis or necrosis. This study on the molecular mechanism of AAI-induced toxicity may explain why tubular epithelial cells present limited proliferation and regeneration abilities in the clinical presentation of AAI-associated nephrotoxicity.

Interferon-inducible protein IFIXalpha1 functions as a negative regulator of HDM2

The 200-amino-acid repeat (HIN-200) gene family with the hematopoietic interferon (IFN)-inducible nuclear protein encodes highly homologous proteins involved in cell growth, differentiation, autoimmunity, and tumor suppression. IFIX is the newest member of the human HIN-200 family and is often downregulated in breast tumors and breast cancer cell lines. The expression of the longest isoform of IFIX gene products, IFIXalpha1, is associated with growth inhibition, suppression of transformation, and tumorigenesis. However, the mechanism underlying the tumor suppression activity of IFIXalpha1 is not well understood. Here, we show that IFIXalpha1 downregulates HDM2, a principal negative regulator of p53, at the posttranslational level. IFIXalpha1 destabilizes HDM2 protein and promotes its ubiquitination. The E3 ligase activity of HDM2 appears to be required for this IFIXalpha1 effect. Importantly, HDM2 downregulation is required for the IFIXalpha1-mediated increase of p53 protein levels, transcriptional activity, and nuclear localization, suggesting that IFIXalpha1 positively regulates p53 by acting as a negative regulator of HDM2. We found that IFIXalpha1 interacts with HDM2. Interestingly, the signature motif of the HIN-200 gene family, i.e., the 200-amino-acid HIN domain of IFIXalpha1, is sufficient not only for binding HDM2 but also for downregulating it, leading to p53 activation. Finally, we show that IFIX mediates HDM2 downregulation in an IFN-inducible system. Together, these results suggest that IFIXalpha1 functions as a tumor suppressor by repressing HDM2 function.

The Catalytic Activity of the Eukaryotic Initiation Factor-2α Kinase PKR Is Required to Negatively Regulate Stat1 and Stat3 via Activation of the T-cell Protein-tyrosine Phosphatase

Tyrosine phosphorylation of the transcription factors Stat1 and Stat3 is required for them to dimerize, translocate to the nucleus, and induce gene transcription. Nuclear Stat1 and Stat3 are dephosphorylated and deactivated by the T-cell protein-tyrosine phosphatase (TC-PTP), which facilitates the return of both proteins to the cytoplasm. The protein kinase PKR plays an important role in translational control through the modulation of eukaryotic initiation factor-2alpha phosphorylation. Previous data have implicated PKR in cell signaling via regulation of Stat1 and Stat3, but the molecular mechanisms underlying these events have remained elusive. Using PKR(-/-) mouse embryonic fibroblasts and a conditionally active form of human PKR, we demonstrate herein that tyrosine (but not serine) phosphorylation of either Stat1 or Stat3 is impaired in cells with activated kinase. This reduction in Stat1 and Stat3 tyrosine phosphorylation by active PKR proceeds through TC-PTP, which is a substrate of the eukaryotic initiation factor-2alpha kinase both in vitro and in vivo. TC-PTP phosphorylation alone is insufficient to increase its in vivo phosphatase activity unless accompanied by the inhibition of protein synthesis as a result of PKR activation. These data reveal a novel function of PKR as a negative regulator of Stat1 and Stat3 with important implications in cell signaling.

Gene therapy with E2F-1 up-regulates the protein kinase PKR and inhibits growth of leiomyosarcoma in vivo

Overexpression of the transcription factor E2F-1 induces apoptosis in a variety of carcinoma cells and inactivates murine double minute protein 2, a factor associated with poor prognosis in soft tissue sarcomas. We have shown previously that the double-stranded RNA-activated protein kinase PKR plays an important role in mediating this apoptotic response in carcinoma cells to E2F-1. We sought to evaluate the potential of E2F-1 gene therapy in soft tissue sarcomas and to study the involvement of PKR in the response to E2F-1 overexpression in mesenchymal cells. A replication-deficient adenovirus carrying the E2F-1 gene (Ad5E2F) was used to induce E2F-1 overexpression in the p53 mutated leiomyosarcoma cell line, SKLMS-1. Western blot analysis confirmed E2F-1 overexpression and up-regulation of the antiapoptotic factor Bcl-2 48 hours following infection with Ad5E2F. Apoptosis in Ad5E2F-treated cells was confirmed by fluorescence-activated cell sorting analysis and by poly(ADP-ribose) polymerase cleavage and DNA fragmentation assays. Vector-dependent up-regulation of PKR correlated with the amount of Ad5E2F-induced apoptosis. In vivo treatment of SKLMS-1 tumor-bearing BALB/c mice with intratumoral injections of Ad5E2F at a dose of 2 x 10(10) viral particles resulted in significant inhibition in tumor growth compared with control-treated animals (P < 0.016). Complete disappearance of all tumors was seen in two of seven mice in the Ad5E2F-treated animals. Immunohistochemical analysis of tumor specimens showed overexpression of E2F-1 and up-regulation of PKR in Ad5E2F-treated tumors. These findings show that adenovirus-mediated overexpression of E2F-1 results in up-regulation of PKR and significant growth suppression of leiomyosarcomas in vivo. Taken together, these data suggest that E2F-1 gene therapy and PKR modulation might be a promising treatment strategy for these tumors that are highly resistant to conventional therapies.

Okadaic acid induces tyrosine phosphorylation of IkappaBalpha that mediated by PKR pathway in human osteoblastic MG63 cells

Treatment of human osteosarcoma cell line MG 63 cells with okadaic acid stimulated phosphorylation of IkappaBalpha, as judged from the results of Western blot analysis and a lambda protein phosphatase dephosphorylation assay. The stimulated phosphorylation of IkappaBalpha was both time- and dose-dependent. The phosphorylation sites of IkappaBalpha were taken to be tyrosine residues because the anti-phospho-tyrosine antibody bound to the samples immunoprecipitated with the anti-IkappaBalpha antibody. In the cells treated with 100 nM okadaic acid consequential translocation of NF-kappaB p65 from the cytosol to the nucleus occurred. Double-stranded RNA-dependent protein kinase (PKR) is a player in the cellular antiviral response and is involved in transcriptional stimulation through activation of NF-kappaB. We investigated the functional relationship between PKR and IkappaBalpha phosphorylation by constructing MG 63 PKR K/R cells that produced a catalytically inactive mutant PKR. NF-kappaB p65 was detected in the nucleus of these cells, even in the unstimulated cells. Although IkappaBalpha was degraded phosphorylation of eIF-2 alpha, a substrate of PKR, did not occur in the mutant cells treated with okadaic acid. Our results suggest that okadaic acid-induced tyrosine phosphorylation of IkappaBalpha was mediated by PKR kinase activity, thus indicating the involvement of this kinase in the control mechanism governing the activation of NF-kappaB.

Interaction of viruses with the mammalian RNA interference pathway

It has been known for some time that plants and insects use RNA interference (RNAi) as nucleic acid-based immunity against viral infections. However, it was unknown whether mammalian cells employ the RNA interference pathway as an antiviral mechanism as well. Over the past years, it has become clear that a variety of viruses, first in plants but recently in insect and mammalian viruses as well, encode suppressors of the RNAi pathway arguing for an antiviral role of this machinery. More recent findings have revealed that certain viruses encode their own microRNAs or microRNA-like RNA molecules, which are processed by the mammalian RNAi machinery. Furthermore, host-encoded microRNAs have been shown to both silence and enhance intracellular levels of viral RNAs. These findings argue that interactions between the RNAi pathway and viral genomes can profoundly affect the outcomes of the viral life cycles and contribute to the pathogenic signatures of the infectious agents.

Tyrosine phosphorylation acts as a molecular switch to full-scale activation of the eIF2alpha RNA-dependent protein kinase

Phosphorylation of the alpha-subunit of translation eukaryotic initiation factor-2 (eIF2) leads to the inhibition of protein synthesis in response to diverse conditions of stress. Serine/threonine RNA-dependent protein kinase (PKR) is an eIF2alpha kinase family member induced by type I IFN and activated in response to dsRNA or virus infection. Herein, we demonstrate that human PKR is a dual specificity kinase phosphorylated at Y101, Y162 and Y293 in vitro and in vivo. Site-specific tyrosine phosphorylation is essential for efficient dsRNA-binding, dimerization, kinase activation and eIF2alpha phosphorylation of PKR. Biologically, tyrosine phosphorylation of PKR mediates the antiviral and antiproliferative properties of the kinase through its ability to control translation. Our data demonstrate an important role of tyrosine phosphorylation in biochemical and biological processes caused or mediated by the activation of the eIF2alpha kinase PKR.

Unshackling the links between reovirus oncolysis, Ras signaling, translational control and cancer

Reovirus has an inherent preference for replicating in cells with dysregulated growth factor signaling cascades that comprise Ras activation. Precisely how reovirus exploits the host cell Ras pathway is unclear, but there is evidence suggesting that activated Ras signaling is important for efficient viral protein synthesis. Defining the molecular mechanism of reovirus oncolysis will shed light on reovirus replication and important aspects of cellular transformation, Ras signaling cascades and regulation of protein translation.

Expression of double-stranded RNA-activated protein kinase (PKR) and its prognostic significance in lymph node negative rectal cancer

OBJECTIVE

The interferon-induced, double-stranded RNA-activated, protein kinase (PKR) is a key regulator of translational initiation, and plays an important role in the regulation of cell proliferation, apoptosis and transformation. The aim of this study was to evaluate the prognostic significance of PKR in lymph node negative rectal cancer.

METHODS

Forty-three patients with stage II rectal carcinoma who underwent potentially curative resection followed by post-operative adjuvant chemoradiation and 5-fluorouracil-based chemotherapy were investigated immunohistochemically using the monoclonal antibody TJ4C4. Overall scores for PKR expression were calculated based on staining intensity and immunoreactive tumor cell fraction. Clinical information, including tumor grade, carcinoembryonic antigen (CEA), disease-free survival (DFS) and overall survival (OS) was evaluated and compared with the degree of PKR expression.

RESULTS

The median follow-up duration was 53.2 months, and median patient age was 55 years (range 33-73). No relationships were found between PKR score and age, sex, tumor grade or CEA level; however, smaller tumors (< or =5 cm) were associated with high PKR score (P = 0.025). When patients were subdivided into two groups based on the PKR score, the relapse rate was lower for those with a high PKR score (7.4 versus 43.8%, P = 0.008), and a significant difference was found between these two groups in terms of 5 year DFS (92.6 versus 55.6%, P = 0.0072) and 5 year OS (92.6 versus 57.7%, P = 0.0459). Other clinicopathologic variables were not related to clinical outcome.

CONCLUSION

PKR expression levels were associated with disease recurrence, DFS and OS in lymph node negative rectal cancer patients.

A cellular screening assay to test the ability of PKR to induce cell death in mammalian cells

Long double-stranded RNA (>30 bp), usually expressed in cells infected with RNA viruses, triggers antiviral responses that induce apoptosis of the infected cells. PKR can be selectively activated in glioblastoma cells by in situ generation of dsRNA following introduction of antisense RNA complementary to an RNA expressed specifically in these cells. Harnessing PKR for the selective killing of cancer cells is potentially a powerful strategy for treating cancer, but we were unable to induce apoptosis by this approach in a T cell lymphoma. We therefore established a cellular screening assay to test the ability of PKR to induce death in cell lines, especially those originating from human cancers. This "PKR killing screen" is based on the infection of cells with an adenoviral vector encoding GyrB-PKR, followed by coumermycin treatment. Cancers represented by cell lines in which PKR activation leads to cell death are good candidates for the dsRNA killing approach, using antisense to RNA molecules specifically expressed in these cells. The PKR killing screen may also serve as a tool for exploring PKR signaling and other related pathways, by identifying new cases in which PKR signaling is inhibited or impaired.

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

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

Identification of the heparin-binding domains of the interferon-induced protein kinase, PKR

PKR is an interferon-induced serine-threonine protein kinase that plays an important role in the mediation of the antiviral and antiproliferative actions of interferons. PKR is present at low basal levels in cells and its expression is induced at the transcriptional level by interferons. PKR's kinase activity stays latent until it binds to its activator. In the case of virally infected cells, double-stranded (ds) RNA serves as PKR's activator. The dsRNA binds to PKR via two copies of an evolutionarily conserved motif, thus inducing a conformational change, unmasking the ATP-binding site and leading to autophosphorylation of PKR. Activated PKR then phosphorylates the alpha-subunit of the protein synthesis initiation factor 2 (eIF2alpha) thereby inducing a general block in the initiation of protein synthesis. In addition to dsRNA, polyanionic agents such as heparin can also activate PKR. In contrast to dsRNA-induced activation of PKR, heparin-dependent PKR activation has so far remained uncharacterized. In order to understand the mechanism of heparin-induced PKR activation, we have mapped the heparin-binding domains of PKR. Our results indicate that PKR has two heparin-binding domains that are nonoverlapping with its dsRNA-binding domains. Although both these domains can function independently of each other, they function cooperatively when present together. Point mutations created within these domains rendered PKR defective in heparin-binding, thereby confirming their essential role. In addition, these mutants were defective in kinase activity as determined by both in vitro and in vivo assays.

Comparative immunomodulatory properties of a chitosan-MDP adjuvant combination following intranasal or intramuscular immunisation

As the precise functions of adjuvants become clearer, opportunities are presented in their complementary use for the induction of tailored immune responses to subunit vaccines. Here we comparatively investigate the immunological outcome following intranasal or intramuscular immunisation with Helicobacter pylori urease admixed to a chitosan and muramyl di-peptide (MDP) combination. MDP appeared to limit the antigenicity of rUre by either administration route. Nasal administration of the combined adjuvant formulation resulted in an up-regulation of type I recall responses in splenocytes as opposed to adjuvantisation with chitosan alone. In contrast, intramuscular immunisation appeared to limit the responsiveness to the antigen when adjuvanted with chitosan and even more so when chitosan was combined with MDP, suggesting that the mechanism of adjuvantisation and adjuvant synergy differed depending on the immunisation route. Recognising the benefit of improved delivery of MDP intranasally due to the specific physiological effects of chitosan, we discuss the impact of the newly identified pathogen associated molecular pattern (PAMP) role of MDP with respect to the adjuvanticity of proposed chemical variants of this peptide adjuvant.

Potential applications of RNA interference technology in the treatment of cancer

Inhibition of growth and progression of cancer cells is a challenge with major potential impact. RNA interference (RNAi) technology has been rapidly developed as a laboratory tool for the downregulation of the expression of a gene of interest. Moreover, RNAi offers a new potential for gene therapy of particular neoplasms by the specific inhibition of a cancer-associated target. This article will briefly describe the mechanism and application possibilities of RNAi, and illustrate the therapeutic potential in cancer gene therapy. The utilization of RNAi technology as a potential therapeutic tool for the treatment of cancer will be discussed in detail for two specific targets; the Bcr-Abl tyrosine kinase and the multidrug transporter MDR1/P-glycoprotein.

Gene Transfer–Mediated Pre-mRNA Segmental Trans-splicing As a Strategy to Deliver Intracellular Toxins for Cancer Therapy

Virus-mediated transfer of genes coding for intracellular toxins holds promise for cancer therapy, but the inherent toxicity of such vectors make them a risk to normal tissues and a challenge to produce due to the intrinsic dilemma that expression of toxin molecules kills producer cells. We employed pre-mRNA segmental trans-splicing (STS), in which two engineered DNA fragments coding for 5′ “donor” and 3′ “acceptor” segments of a toxin gene, respectively, are expressed by viral vectors. When co-delivered to target cells, the two vectors generate two toxin pre-mRNA fragments which are spliced by the target cell machinery to produce functional mRNA and toxin. To test this approach, we used an enzymatic fragment of Shigatoxin1A1 (STX1A1) known to provoke apoptotic cell death. Two adenovirus vectors, Shigatoxin1A1 donor (AdStx1A1Do) and Shigatoxin1A1 acceptor (AdStx1A1Ac), respectively, were used to deliver the Stx1A1 gene fragments. HeLa, HEp2, and A549 cells transfected with AdStx1A1Do and AdStx1A1Ac had a dose-dependent reduction in viability and inhibition of protein synthesis. Intratumoral injection of AdStx1A1Do and AdStx1A1Ac into preexisting HeLa, Hep2, and A549 tumors in immunodeficient mice revealed significant inhibition of tumor growth. There was no evidence of liver damage, suggesting that there was no leakage of vector or toxin from the site of injection following intratumoral injection of AdStx1A1Do and AdStx1A1Ac. These results suggest that the obstacles preventing gene transfer of intracellular toxins for local cancer therapy could be overcome by pre-mRNA segmental trans-splicing.

New pharmacokinetic and pharmacodynamic tools for interferon-alpha (IFN-alpha) treatment of human cancer

Interferon alpha (IFN-alpha) has been widely used in the treatment of human solid and haematologic malignancies. Although the antitumour activity of IFN-alpha is well recognised at present, no major advances have been achieved in the last few years. Recent findings have provided new information on the molecular mechanisms of the antitumour activity of the cytokine. In fact, IFN-alpha appears to block cell proliferation, at least in part, through the induction of apoptotic effects. This cytokine can also regulate the progression of tumour cells through the different phases of the cell cycle inducing an increase of the expression of the cyclin-dependent kinase inhibitors p21 and p27. However, it must be considered that IFN-alpha is a physiologic molecule with ubiquitously expressed receptors that is likely to activate survival mechanisms in the cell. We have recently identified an epidermal growth factor (EGF) Ras-dependent protective response to the apoptosis induced by IFN-alpha in epidermoid cancer cells. The identification of tissue- and/or tumour-specific survival pathways and their selective targeting might provide a new approach to improve the efficacy of IFN-alpha-based treatment of human cancer. Moreover, new pegylated species of IFN-alpha are now available with a more favourable pharmacokinetic profile. We will review these achievements, and we will specifically address the topic of IFN-alpha-based molecularly targeted combinatory antitumour approaches.

eIF2 and the control of cell physiology

Eukaryotic initiation factor eIF2 and its 'exchange factor' eIF2B play a key role in the regulation of protein synthesis in eukaryotes from yeast to mammals. Phosphorylation of eIF2 inhibits eIF2B and thus translation initiation. Four eIF2 kinases are now known in mammalian cells and these are activated in response to specific stress conditions. While phosphorylation of eIF2 serves to impair general protein synthesis, it causes upregulation of the translation of certain specific mRNAs that encode transcription factors. It can, therefore, exert effects on gene expression at multiple levels. The importance of correct control of eIF2 and eIF2B for normal physiology is exemplified by data from transgenic mice carrying knock-in or knock-out mutations and by the fact that mutations in the genes for the eIF2 kinase PERK or for eIF2B give rise to serious human diseases.

Glucose-stimulated protein synthesis in pancreatic beta-cells parallels an increase in the availability of the translational ternary complex (eIF2-GTP.Met-tRNAi) and the dephosphorylation of eIF2 alpha

In pancreatic beta-cells, glucose causes a rapid increase in the rate of protein synthesis. However, the mechanism by which this occurs is poorly understood. In this report, we demonstrate, in the pancreatic beta-cell line MIN6, that glucose stimulates the recruitment of ribosomes onto the mRNA, indicative of an increase in the rate of the initiation step of protein synthesis. This increase in the rate of initiation is not mediated through an increase in the availability of the initiation complex eIF4F, because glucose is unable to stimulate eIF4F assembly or, in the absence of amino acids, modulate the phosphorylation status of 4E-BP1. Moreover, in MIN6 cells and isolated islets of Langerhans, rapamycin, an inhibitor of the mammalian target of rapamycin, only partially inhibited glucose-stimulated protein synthesis. However, we show that glucose stimulates the dephosphorylation of eIF2 alpha in MIN6 cells and the assembly of the translational ternary complex, eIF2-GTP.Met-tRNAi, in both MIN6 cells and islets of Langerhans. The changes in the phosphorylation of eIF2 alpha are not mediated by the PKR-like endoplasmic reticulum eIF2 alpha kinase (PERK), because PERK is not phosphorylated at low glucose concentrations and overexpression of a dominant negative form of PERK has no significant effect on either glucose-stimulated protein synthesis or the phosphorylation of eIF2 alpha. Taken together, these results indicate that glucose-stimulated protein synthesis in pancreatic beta-cells is regulated by a mechanism largely independent of the activity of mammalian target of rapamycin, but which is likely to be dependent on the availability of the translational ternary complex, regulated by the phosphorylation status of eIF2 alpha.

High CD46 receptor density determines preferential killing of tumor cells by oncolytic measles virus

Live attenuated Edmonston B strain of measles virus (MV-Edm) is a potent and specific oncolytic agent, but the mechanism underlying its tumor selectivity is unknown. The virus causes cytopathic effects (CPEs) of extensive syncytial formation in tumor cells but minimal damage or cell killing in normal cells. The CPE is dependent on expression of viral proteins and the presence of CD46, the major cellular receptor of MV-Edm. Using a virally encoded soluble marker peptide to provide a quantitative readout of the level of viral gene expression, we determined that tumor cells and normal cells expressed comparable levels of viral proteins. CD46 mediates virus attachment, entry, and virus-induced cell-to-cell fusion. Using engineered cells expressing a range of CD46 densities, we determined that whereas virus entry increased progressively with CD46 density, cell fusion was minimal at low receptor densities but increased dramatically above a threshold density of CD46 receptors. It is well established that tumor cells express abundant CD46 receptors on their surfaces compared with their normal counterparts. Thus, at low CD46 densities typical of normal cells, infection occurs, but intercellular fusion is negligible. At higher densities typical of tumor cells, infection leads to extensive cell fusion. Intercellular fusion also results in enhancement of viral gene expression through recruitment of neighboring uninfected cells into the syncytium, further amplifying the CPE. Discrimination between high and low CD46 receptor density provides a compelling basis for the oncolytic specificity of MV-Edm and establishes MV-Edm as a promising CD46-targeted cancer therapeutic agent.

Small ISGs coming forward

Interferons (IFNs) were first characterized as antiviral proteins. Since then, IFNs have proved to be involved in malignant, angiogenic, inflammatory, immune, and fibrous diseases and, thus, possess a broad spectrum of pathophysiologic properties. IFNs activate a cascade of intracellular signaling pathways leading to upregulation of more than 1000 IFN-stimulated genes (ISGs) within the cell. The function of some of the IFN-induced proteins is well described, whereas that of many others remain poorly characterized. This review focuses on three families of small intracellular and intrinsically nonsecreted proteins (10-20 kDa) separated into groups according to their amino acid sequence similarity: the ISG12 group (6-16, ISG12, and ISG12-S), the 1-8 group (9-27/Leu13, 1-8U, and 1-8D), and the ISG15 group (ISG15/UCRP). These IFN-induced genes are abundantly and widely expressed and mainly induced by type I IFN. ISG15 is very well described and is a member of the ubiquitin-like group of proteins. 9-27/Leu-13 associates with CD81/TAPA-1 and plays a role in B cell development. The functions of 1-8U, 1-8D, 6-16, ISG12, and ISG12-S proteins are unknown at present.

The Fanconi anemia proteins functionally interact with the protein kinase regulated by RNA (PKR)

Protein kinase regulated by RNA (PKR) plays critical roles in cell growth and apoptosis and is implicated as a potential pathogenic factor of Alzheimer's, Parkinson's, and Huntington's diseases. Here we report that this proapoptotic kinase is also involved in Fanconi anemia (FA), a disease characterized by bone marrow (BM) failure and leukemia. We have used a BM extract to show that three FA proteins, FANCA, FANCC, and FANCG, functionally interact with the PKR kinase, which in turn regulates translational control. By using a combined immunoprecipitation and reconstituted kinase assay, in which an active PKR kinase complex was captured from a normal cell extract, we demonstrated functional interactions between the FA proteins and the PKR kinase. In primary human BM cells, mutations in the FANCA, FANCC, and FANCG genes markedly increase the amount of PKR bound to FANCC, and this PKR accumulation is correlated with elevated PKR activation and hypersensitivity of BM progenitor cells to growth repression mediated by the inhibitory cytokines interferon-gamma and tumor necrosis factor-alpha. Specific inhibition of PKR by 2-aminopurine in these FA BM cells attenuates PKR activation and apoptosis induction. In lymphoblasts derived from an FA-C patient, overexpression of a dominant negative mutant PKR (PKRK296R) suppressed PKR activation and apoptosis induced by interferon-gamma and tumor necrosis factor-alpha. Furthermore, by using genetically matched wild-type and PKR-null cells, we demonstrated that forced expression of a patient-derived FA-C mutant (FANCCL554P) augmented double-stranded RNA-induced PKR activation and cell death. Thus, inappropriate activation of PKR as a consequence of certain FA mutations might play a role in bone marrow failure that frequently occurred in FA.

Control of alpha subunit of eukaryotic translation initiation factor 2 (eIF2 alpha) phosphorylation by the human papillomavirus type 18 E6 oncoprotein: implications for eIF2 alpha-dependent gene expression and cell death

Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) at serine 51 inhibits protein synthesis in cells subjected to various forms of stress including virus infection. The human papillomavirus (HPV) E6 oncoprotein contributes to virus-induced pathogenicity through multiple mechanisms including the inhibition of apoptosis and the blockade of interferon (IFN) action. We have investigated a possible functional relationship between the E6 oncoprotein and eIF2alpha phosphorylation by an inducible-dimerization form of the IFN-inducible protein kinase PKR. Herein, we demonstrate that HPV type 18 E6 protein synthesis is rapidly repressed upon eIF2alpha phosphorylation caused by the conditional activation of the kinase. The remainder of E6, however, can rescue cells from PKR-mediated inhibition of protein synthesis and induction of apoptosis. E6 physically associates with GADD34/PP1 holophosphatase complex, which mediates translational recovery, and facilitates eIF2alpha dephosphorylation. Inhibition of eIF2alpha phosphorylation by E6 mitigates eIF2alpha-dependent responses to transcription and translation of proapoptotic genes. These findings demonstrate, for the first time, a role of the oncogenic E6 in apoptotic signaling induced by PKR and eIF2alpha phosphorylation. The functional interaction between E6 and the eIF2alpha phosphorylation pathway may have important implications for HPV infection and associated pathogenesis.

Selectivity of an oncolytic herpes simplex virus for cells expressing the DF3/MUC1 antigen

Replication-conditional viruses destroy tumors in a process referred to as viral oncolysis. An important prerequisite for this cancer therapy strategy is use of viruses that replicate preferentially in neoplastic cells. In this study the DF3/MUC1 promoter/enhancer sequence is used to regulate expression of gamma(1)34.5 to drive replication of a Herpes simplex virus 1 (HSV-1) mutant (DF3gamma34.5) preferentially in DF3/MUC1-positive cells. HSV-1 gamma(1)34.5 functions to dephosphorylate elongation initiation factor 2alpha, which is an important step for robust HSV-1 replication. After DF3gamma34.5 infection of cells, elongation initiation factor 2alpha phosphatase activity and viral replication were observed preferentially in DF3/MUC1-positive cells but not in DF3/MUC1-negative cells. Regulation of gamma(1)34.5 function results in preferential replication in cancer cells that express DF3/MUC1, restricted biodistribution in vivo, and less toxicity as assessed by LD(50). Preferential replication of DF3gamma34.5 was observed in DF3/MUC1-positive liver tumors after intravascular perfusion of human liver specimens. DF3gamma34.5 was effective against carcinoma xenografts in nude mice. Regulation of gamma(1)34.5 by the DF3/MUC1 promoter is a promising strategy for development of HSV-1 mutants for viral oncolysis.

Targets and mechanisms for the regulation of translation in malignant transformation

There is increasing evidence that deregulation of gene expression at the level of mRNA translation can contribute to cell transformation and the malignant phenotype. Two steps in the pathway of polypeptide chain initiation, viz. the assembly of the 43S initiation complex catalysed by polypeptide chain initiation factor eIF2 and the binding of eIF4E to eIF4G during the recruitment of mRNA to the ribosome, have been shown to be likely targets for changes associated with tumorigenesis. The activity of eIF2 is controlled by changes in phosphorylation of the alpha subunit of this factor. The availability of eIF4E for binding to eIF4G is regulated by the phosphorylation of a small family of eIF4E-binding proteins (the 4E-BPs). The activities of the protein kinases and/or phosphatases responsible for the (de)phosphorylation of these substrates may in turn be controlled by cellular and viral oncogenes and tumour-suppressor genes. This review will describe recent aspects of the mechanisms involved, with particular emphasis on the regulation of the eIF2 alpha kinase PKR and the control of 4E-BP phosphorylation by viral gene products, growth-inhibitory cytokines and the tumour-suppressor protein p53.

Ribosomal protein L22 inhibits regulation of cellular activities by the Epstein-Barr virus small RNA EBER-1

Epstein-Barr virus (EBV) is a potent mitogenic and antiapoptotic agent for B lymphocytes and is associated with several different types of human tumour. The abundantly expressed small viral RNA, EBER-1, binds to the growth inhibitory and pro-apoptotic protein kinase R (PKR) and blocks activation of the latter by double-stranded RNA. Recent evidence has suggested that expression of EBER-1 alone in EBV-negative B cells promotes a tumorigenic phenotype and that this may be related to inhibition of the pro-apoptotic effects of PKR. The ribosomal protein L22 binds to EBER-1 in virus-infected cells, but the significance of this has not previously been established. We report here that L22 and PKR compete for a common binding site on EBER-1. As a result of this competition, L22 interferes with the ability of the small RNA to inhibit the activation of PKR by dsRNA. Transient expression of EBER-1 in murine embryonic fibroblasts stimulates reporter gene expression and partially reverses the inhibitory effect of PKR. However, EBER-1 is also stimulatory when transfected into PKR knockout cells, suggesting an additional, PKR-independent, mode of action of the small RNA. Expression of L22 prevents both the PKR-dependent and -independent effects of EBER-1 in vivo. These results suggest that the association of L22 with EBER-1 in EBV-infected cells can attenuate the biological effects of the viral RNA. Such effects include both the inhibition of PKR and additional mechanism(s) by which EBER-1 stimulates gene expression.

An RNA-dependent protein kinase is involved in tunicamycin-induced apoptosis and Alzheimer's disease

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

Mechanisms of dengue virus-induced cell death

The outcome of virus infection depends on viral and host factors. The interactions between flaviviruses and their target cells must be investigated if we are to understood the pathogenicity of these RNA viruses. Host cells are thought to respond to viral infection by initiation of apoptotic cell death. Apoptosis is an active process of cellular self-destruction with distinctive morphological and biochemical features. There is mounting evidence that dengue (DEN) virus can trigger the host cell to undergo apoptosis in a cell-dependent manner. Virally induced apoptosis contributes directly to the cytopathogenic effects of DEN virus in cultured cells. The induction of apoptosis involves the activation of intracellular signaling systems. Although the underlying molecular processes that trigger apoptosis are not well characterized, our knowledge regarding the cellular mechanisms and viral determinants of the outcome of DEN virus infection of target cells is improving. The cellular factors that regulate cell death, such as Bcl-2 family members, can modulate the outcome of DEN virus infection in cultured cells. Apoptosis inhibitors delay DEN virus-induced apoptosis, thereby providing a suitable environment for the virus. During DEN virus infection, cell death is also modulated by the virulence of the infecting strains. The purpose of this review is to present recent information on the cellular mechanisms and viral proteins associated with apoptosis in response to DEN virus. This knowledge may provide new insights into the viral pathogenicity.

Genetically Targeted Cancer Therapy

The is a double-stranded RNA-activated protein kinase (PKR) has been largely investigated for its key role in viral host defense. Although best characterized by its function in mediating the antiviral and antiproliferative effects of interferon (IFN), PKR is also implicated in transcriptional regulation, cell differentiation, signal transduction, and tumor suppression. However, recent findings identifying PKR as an important effector of apoptosis have led to an increased interest in PKR modulation as an antitumor strategy. PKR can either be up-regulated through direct induction by the transcription factor E2F-1, or it can be activated through direct protein-protein interactions with the melanoma differentiation-associated gene-7 (MDA7, IL-24). Additionally, the intracellular formation of double-stranded RNA by transfection with antisense RNA complementary to tumor-specific RNA sequences can induce PKR activation and apoptosis selective to these tumor cells. The growing application of viral vector-based gene therapies and oncolytic, replicating viruses that must elude viral defense in order to be effective, has also drawn attention to PKR. Oncolytic viruses, like the attenuated herpes simplex virus R3616, the vesicular stomatitis virus, or reovirus, specifically replicate in tumor cells only because the viral host defense in the permissive cells is suppressed. In this article we review the role of PKR as an effector of apoptosis and a target for tumor treatment strategies and discuss the potential of PKR-modifying agents to treat patients with cancer. Targeted gene therapy against cancer can be approached by activation of PKR with the down-regulation of protein synthesis and induction of apoptosis, or by suppression of PKR with the propagation of oncolytic virus. Since the PKR pathway can be modified by many routes, antitumor therapies combining oncolytic virus, gene therapies, and chemotherapy with PKR modifiers are likely to emerge in the near future as therapeutic options in the treatment of patients with cancer.

Expression analysis and genomic characterization of human melanoma differentiation associated gene-5, mda-5: a novel type I interferon-responsive apoptosis-inducing gene

Melanoma differentiation associated gene-5 (mda-5) was identified by subtraction hybridization as a novel upregulated gene in HO-1 human melanoma cells induced to terminally differentiate by treatment with IFN-beta+MEZ. Considering its unique structure, consisting of a caspase recruitment domain (CARD) and an RNA helicase domain, it was hypothesized that mda-5 contributes to apoptosis occurring during terminal differentiation. We have currently examined the expression pattern of mda-5 in normal tissues, during induction of terminal differentiation and after treatment with type I IFNs. In addition, we have defined its genomic structure and chromosomal location. IFN-beta, a type I IFN, induces mda-5 expression in a biphasic and dose-dependent manner. Based on its temporal kinetics of induction and lack of requirement for prior protein synthesis mda-5 is an early type I IFN-responsive gene. The level of mda-5 mRNA is in low abundance in normal tissues, whereas expression is induced in a spectrum of normal and cancer cells by IFN-beta. Expression of mda-5 by means of a replication incompetent adenovirus, Ad.mda-5, induces apoptosis in HO-1 cells as confirmed by morphologic, biochemical and molecular assays. Additionally, the combination of Ad.mda-5+MEZ further augments apoptosis as observed in Ad.null or uninfected HO-1 cells induced to terminally differentiate by treatment with IFN-beta+MEZ. The mda-5 gene is located on human chromosome 2q24 and consists of 16 exons, without pseudogenes, and is conserved in the mouse genome. Present data documents that mda-5 is a novel type I IFN-inducible gene, which may contribute to apoptosis induction during terminal differentiation and during IFN treatment. The conserved genomic and protein structure of mda-5 in human and mouse will permit analysis of the evolution and developmental aspects of this gene.

Upstream signaling pathways leading to the activation of double-stranded RNA-dependent serine/threonine protein kinase in beta-amyloid peptide neurotoxicity

One of the hallmarks of Alzheimer's disease is extracellular accumulation of senile plaques composed primarily of aggregated beta-amyloid (Abeta) peptide. Treatment of cultured neurons with Abeta peptide induces neuronal death in which apoptosis is suggested to be one of the mechanisms. We have demonstrated previously that Abeta peptide induces activation of double-stranded RNA-dependent serine/threonine protein kinase (PKR) and phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) in neurons in vitro. Degenerating neurons in brain tissues from Alzheimer's disease patients also displayed high immunoreactivity for phosphorylated PKR and eIF2alpha. Our previous data have also indicated that PKR plays a significant role in mediating Abeta peptide-induced neuronal death, because neurons from PKR knockout mice and neuroblastoma SH-SY5Y cells stably transfected with dominant negative mutant of PKR are less susceptible to Abeta peptide toxicity. Therefore, it is important to understand how PKR is activated by Abeta peptide. We report here that inhibition of caspase-3 activity reduces phosphorylation of PKR and to a certain extent, cleavage of PKR and eIF2alpha in neurons exposed to Abeta peptide. Calcium release from the endoplasmic reticulum and activation of caspase-8 are the upstream signals modulating the caspase-3-mediated activation of PKR by Abeta peptide. Although in other systems HSP90 serves as a repressor for PKR, it is unlikely the candidate for caspase-3 to affect PKR activation in neurons after Abeta peptide exposure. Elucidation of the upstream pathways for PKR activation can help us to understand how this kinase participates in Abeta peptide neurotoxicity and to develop effective neuroprotective strategy.

Tumor specific activation of PKR as a non-toxic modality of cancer treatment

Over the past decade progress has been made in the development of therapies against cancer. Small molecules, mainly tyrosine kinase inhibitors (tyrphostins) like Gleevec, Iressa targeting CML and EGFR overexpressing tumors have entered the clinic, where a large number of other tyrphostins are at various stages of clinical development. In parallel a few antibodies like Herceptin targeting breast cancer overexpressing Her-2 and Rituxan targeting B cell malignancies are utilized in the clinic. In all these cases success is moderate and restricted to a narrow population of patients, except for Gleevec which is effective for a long duration for chronic CML. The cancer community agrees that this is actually a unique exception that proves the rule. Over the past few years a few modalities of cancer gene therapies have emerged. In this short review we shall summarize our efforts to develop methods to activate PKR selectively in cancer cells.

Nucleophosmin interacts with and inhibits the catalytic function of eukaryotic initiation factor 2 kinase PKR

In normal cells the protein kinase PKR effects apoptosis in response to various extra and intracellular cues and can also function to suppress the neoplastic phenotype. Because most neoplastic cells are resistant to certain apoptotic cues, we reasoned that an early molecular event in carcinogenesis or leukemogenesis might be the inactivation of PKR by expression or activation of intracellular PKR inhibitors. Seeking novel PKR-modulating proteins we report here that nucleophosmin (NPM), a protein frequently overexpressed in a variety of human malignancies, binds to PKR, and inhibits its activation. Co-immunoprecipitation and in vitro binding experiments showed that NPM associated with PKR. Kinase assays demonstrated that recombinant NPM inhibited PKR activation in a dose-dependent manner. In addition, purified recombinant NPM was phosphorylated by activated PKR. Most importantly, overexpression of NPM suppressed PKR activity, enhanced protein synthesis, and inhibited apoptosis. Lymphoblasts from patients with Fanconi anemia (FA) expressed low levels of NPM, which correlated with high ground-state activation of PKR and cellular hypersensitivity to apoptotic cues, but enforced expression of NPM in these mutant cells reduced aberrant apoptotic responses. Inhibition of PKR by NPM may be one mechanism by which neoplastic clones evolve in sporadic malignancies and in neoplastic cells arising in the context of the cancer predisposition syndrome, Fanconi anemia.

A novel role for RAX, the cellular activator of PKR, in synergistically stimulating SV40 large T antigen-dependent gene expression

The double-stranded (ds) RNA-binding protein RAX was discovered as a stress-induced cellular activator of the dsRNA-dependent protein kinase (PKR), a key regulator of protein synthesis in response to viral infection and cellular stress. We now report a novel function of RAX, independent of PKR, to enhance SV40 promoter (origin)/enhancer-dependent gene expression. Several mammalian cell lines including COS-7, CV-1, and HeLa cells were tested. Results reveal that the SV40 large T antigen is required for RAX-mediated, synergistic enhancement of gene expression. RAX augments SV40 regulatory element-dependent DNA replication and transcription. The mechanism requires the SV40 enhancer, a viral transcriptional element that is necessary for efficient SV40 DNA replication in vivo. Mutational analysis reveals that the dsRNA-binding domains of RAX are required for the gene expression enhancing function. Thus, in addition to stimulating PKR activity, RAX can positively regulate both SV40 large T antigen-dependent DNA replication and transcription in a mechanism that may alter the interaction of the cellular factor(s) with the SV40 enhancer via the dsRNA-binding domains of RAX. This novel function of RAX may have implications for regulation of mammalian DNA replication and transcription because of the many similarities between the viral and cellular processes.

The facts of death

Cell death is a physiological process critical for organismal development and required for the removal of damaged cells. Apoptosis, the purposeful killing of cells, is highly regulated, and abnormalities in apoptotic control mechanisms contribute to an assortment of human diseases. Depending on the nature of the death signal, separate independent signaling pathways become activated that coordinately participate in the execution of apoptosis via a common death effector machinery. This article reviews the specific morphological and biochemical hallmarks of apoptosis, the major participating players, and the various signaling pathways involved in triggering this highly complex form of cell death.

Small molecule inhibitors of the RNA-dependent protein kinase

The RNA-dependent protein kinase (PKR) is an interferon-induced serine/threonine protein kinase that phosphorylates the alpha subunit of the eukaryotic initiation factor 2 in response to viral infection. Classical genetic approaches for studying the role of PKR in cell signaling have their limitations due to overlapping but non-redundant pathways. Small molecule inhibitors of PKR will be useful in this regard. We report here, the discovery of a small molecule inhibitor of the kinase reaction of PKR. The inhibitor was discovered by screening a library of 26 different ATP-binding site directed inhibitors of varying structure. We also describe the development of a high-throughput assay for screening a large number of compounds for a PKR inhibitor using a rabbit reticulocyte lysate system and luciferase mRNA. The assay takes advantage of the fact that the reticulocyte lysate is rich in components of the translational machinery, of which PKR is an integral part. This assay can be carried out with added exogenous human PKR to study the effect of various compounds in their ability to rescue the translational block imposed by human PKR.

Involvement of protein phosphatase 2A in interferon-alpha-2b-induced apoptosis in K562 human chronic myelogenous leukaemia cells

Interferon-alpha (IFN-alpha)-2b is known to have antiproliferative effects on hematological malignant cells, especially chronic myelogenous leukaemia (CML). However, it can induce cytogenetical remissions in a very small percentage of the patients. Also during interferon therapy, resistance can emerge in the CML clones. K562 is an in vitro model cell line transformed from a Ph positive CML patient. It can be induced to differentiate to granulocytic and/or monocytic lineages with certain molecules. IFN-alpha-2b generally exerts its effects on CML cells by Janus family kinases (Jak/Stat) pathway, mostly through tyrosine kinase system. However, there is almost no data on the relevance of serine/threonine (Ser/Thr) protein phosphatase (PP) system in the interferon induced signal transduction pathways. In this study, we investigated serine/threonine protein phosphatases in the IFN-alpha-2b induced K562 cytotoxicity. Trypan blue dye exclusion test and MTT assay were utilised for determining cytotoxicity. IC(50) of IFN-alpha-2b on K562 cells was found to be 600IU/ml. However, no differentiation was determined by analysis of cell surface antigen expressions. Serine/threonine protein phosphatase inhibitors calyculin A (Cal A) and okadaic acid (OKA) augmented the IFN-alpha-2b induced cytotoxicity. Apoptosis assay by the mono-oligonucleosome detection and acridine orange/propidium iodide dye revealed marked apoptosis underlying cytotoxicity. Phosphatase enzyme assay revealed a gradual increase in protein phosphatase 2A (PP2A) activity during interferon induced cytotoxicity. Conversely, immunoblots showed no change in the expression of PP2A catalytic and regulatory subunits. In conclusion, PP2A plays a role in IFN-alpha-2b induced apoptosis of K562 cells and should be investigated as a new window furthermore.

Expression of double-stranded RNA-activated protein kinase in keratinocytes and keratinocytic neoplasia

Double-stranded RNA-activated protein kinase (PKR) is a interferon-induced protein initially known for its inhibitory effects on cellular and viral protein synthesis. In recent studies, PKR has been shown to be an important participant in a broad array of cellular processes, including signal transduction, differentiation, apoptosis, cell growth, and tumorigenesis. The expression of PKR in normal human keratinocytes (NHEK) was examined, and its expression in several skin lesions was compared immunohistochemically with that of proliferating cell nuclear antigen (PCNA). Expression of PKR mRNA was detected in NHEK without IFNgamma treatment; the level of PKR mRNA increased with IFNgamma treatment for two hours. Immunoblot analysis revealed that the monoclonal anti-PKR antibody reacted specifically with a 68kDa PKR protein in extracts from NHEK. Immunohistochemistry revealed that PKR protein was expressed in normal epidermis and mucosa. PKR expression was not restricted only to suprabasal cells but was also observed in basal cells positive for PCNA. In psoriatic plaques, PKR expression was lower in basal and parabasal keratinocytes and comparable in suprabasal keratinocytes to the levels in normal skin. PKR was partially detected in atypical cells in non-invasive keratinocytic neoplasia but was completely absent from undifferentiated tumor cells of squamous cell carcinoma. The present study demonstrated that PKR protein is constitutively expressed in epidermal and epithelial keratinocytes of normal skin and mucosa and indicated that a loss of PKR is not associated with the malignant transformation itself but with the increased cell proliferative activity and the altered differentiation of keratinocytes.

A nine-nucleotide deletion and splice variation in the coding region of the interferon induced ISG12 gene

Interferons (IFNs) are a family of cytokines with growth inhibitory, and antiviral functions. IFNs exert their biological actions through the expression of more than 1000 IFN stimulated genes, ISGs. ISG12 is an IFN type I induced gene encoding a protein of M(r) 12,000. We have identified a novel, IFN inducible splice variant of ISG12 lacking exon 2 leading to a putative truncated protein isoform of M(r) 7400, ISG12-S. In cells from blood and cervical cytobrush material from healthy women, the level of ISG12-S expression was higher than ISG12 expression, whereas the expression pattern was more evenly distributed between ISG12 and ISG12-S in breast carcinoma cells, in cancer cell lines and in cervical cytobrush material with neoplastic lesions. In addition, we have found a nine-nucleotide deletion situated in exon 4 of the ISG12 gene. This deletion leads to a three-amino-acid deletion (AMA) in the putative ISG12 gene products, ISG12Delta and ISG12-SDelta. We have determined the prevalence of the deletion ISG12Delta in normal and neoplastic cells. Homozygosity ISG12(0/0) and ISG12(Delta/Delta), and heterozygosity ISG12(0/Delta) were found, although the ISG12(Delta/Delta) genotype was rare. In heterozygous cells from cytobrush material with neoplastic lesions, we found a preference for expression of the ISG12(0) allele.

Epigenetic silencing of multiple interferon pathway genes after cellular immortalization

Abrogating cellular senescence is a necessary step in the formation of a cancer cell. Promoter hypermethylation is an epigenetic mechanism of gene regulation known to silence gene expression in carcinogenesis. Treatment of spontaneously immortal Li-Fraumeni fibroblasts with 5-aza-2'-deoxycytidine (5AZA-dC), an inhibitor of DNA methyltransferase (DNMT), induces a senescence-like state. We used microarrays containing 12 558 genes to determine the gene expression profile associated with cellular immortalization and also regulated by 5AZA-dC. Remarkably, among 85 genes with methylation-dependent downregulation (silencing) after immortalization, 39 (46%) are known to be regulated during interferon signaling, a known growth-suppressive pathway. This work indicates that gene silencing may be associated with an early event in carcinogenesis, cellular immortalization.

Interferons and apoptosis

The interferons (IFNs), in addition to their well-known antiviral activities, have important roles in the control of cell proliferation and are effective agents for the treatment of a limited number of malignant diseases. IFNs not only regulate cell growth and division but also influence cell survival through their effects on apoptosis. This review describes the current state of knowledge about the mechanisms of action of these cytokines on the apoptotic machinery, with particular emphasis on the synergism that exists between the IFNs and other proapoptotic agents, such as members of the tumor necrosis factor (TNF) family. The review also discusses the physiologic and clinical implications of the effects of the IFNs on apoptosis for regulation of viral infection and tumor growth.

Induction of caspase-dependent apoptosis by betanodaviruses GGNNV and demonstration of protein alpha as an apoptosis inducer

Betanodaviruses, members of the Nodaviridae family, are the causative agents of viral nervous necrosis in fish and infection by which cause high mortality in larvae and juveniles in a wide range of marine fish species in Asia, Europe, Australia, Martinique, and Tahit. Greasy grouper (Epinephelus tauvina) nervous necrosis viruses (GGNNV) were investigated for their apoptotic activity in culture cells. GGNNV infection of sea bass (SB) cells appeared to induce a typical cytopathic effect (CPE), i.e., cytoplasmic vacuolation, thinning, rounding up, detachment of infected cells from the cultured dish, and eventually cell lysis and death. The infected SB cells underwent DNA fragmentation and stained positive in terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) assay, suggesting that GGNNV infection induced apoptosis in SB cells. In addition, GGNNV-infected SB cells showed an increased activity of caspase-8-like proteases (IETDase) and caspase-3-like proteases (IETDase), whereas inhibitor of caspase-8 and caspase-3 reduced GGNNV-induced apoptosis. This suggests that GGNNV may promote apoptosis via the extrinsic pathway in SB cells. Protein alpha, the precursor of GGNNV capsid proteins, was transiently expressed in SB and Cos-7 cells. The DNA fragmentation and TUNEL positive signal were apparent in SB and Cos-7 cells expressing protein alpha, suggesting that protein alpha may serve as an apoptotic inducer in these cells. Moreover, expression of protein alpha resulted in the activation of caspase-3-like proteases in both cells, which could be inhibited by a caspase-3-like protease specific inhibitor DEVD-CHO peptide. These results suggest that fish caspases are important elements in GGNNV-meditated apoptosis.