Double-stranded RNA-dependent protein kinase mediates c-Myc suppression induced by type I interferons

m6A Modification Prevents Formation of Endogenous Double-Stranded RNAs and Deleterious Innate Immune Responses during Hematopoietic Development

N6-methyladenosine (m6A) is the most abundant RNA modification, but little is known about its role in mammalian hematopoietic development. Here, we show that conditional deletion of the m6A writer METTL3 in murine fetal liver resulted in hematopoietic failure and perinatal lethality. Loss of METTL3 and m6A activated an aberrant innate immune response, mediated by the formation of endogenous double-stranded RNAs (dsRNAs). The aberrantly formed dsRNAs were long, highly m6A modified in their native state, characterized by low folding energies, and predominantly protein coding. We identified coinciding activation of pattern recognition receptor pathways normally tasked with the detection of foreign dsRNAs. Disruption of the aberrant immune response via abrogation of downstream Mavs or Rnasel signaling partially rescued the observed hematopoietic defects in METTL3-deficient cells in vitro and in vivo. Our results suggest that m6A modification protects against endogenous dsRNA formation and a deleterious innate immune response during mammalian hematopoietic development.

Protein kinase R and its cellular regulators in cancer: An active player or a surveillant?

Protein kinase R (PKR), originally known as an antiviral protein, senses various stresses as well as pathogen-driven double-stranded RNAs. Thereby activated PKR provokes diverse downstream events, including eIF2α phosphorylation and nuclear factor kappa-light-chain-enhancer of activated B cells activation. Consequently, PKR induces apoptosis and inflammation, both of which are highly important in cancer as much as its original antiviral role. Therefore, cellular proteins and RNAs should tightly control PKR activity. PKR and its regulators are often dysregulated in cancer and it is undoubted that such dysregulation contributes to tumorigenesis. However, PKR's precise role in cancer is still in debate, due to incomprehensible and even contradictory data. In this review, we introduce important cellular PKR regulators and discuss about their roles in cancer. Among them, we pay particular attention to nc886, a PKR repressor noncoding RNA that has been identified relatively recently, because its expression pattern in cancer can explain interesting yet obscure oncologic aspects of PKR. Based on nc886 and its regulation of PKR, we have proposed a tumor surveillance model, which reconciles contradictory data about PKR in cancer. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Small-Molecule targeting of translation initiation for cancer therapy

Translation initiation plays a critical role in the regulation of cell growth and tumorigenesis. We report here that inhibiting translation initiation through induction of eIF2α phosphorylation by small-molecular-weight compounds restricts the availability of the eIF2·GTP·Met-tRNA_i ternary complex and abrogates the proliferation of cancer cells in vitro and tumor growth in vivo. Restricting the availability of the ternary complex preferentially down-regulates the expression of growth-promoting proteins and up-regulates the expression of ER stress response genes in cancer cells as well as in tumors excised from either animal models of human cancer or cancer patients. These findings provide the first direct evidence for translational control of gene-specific expression by small molecules in vivo and indicate that translation initiation factors are bona fide targets for development of mechanism-specific anti-cancer agents.

Type I interferon regulation of natural killer cell function in primary and secondary infections

The priming of natural killer (NK) cells by type I interferon (IFN) is necessary for protection against primary and secondary viral infections. However, the pathway by which type I IFN activates NK cells to elicit antiviral responses is controversial. There is evidence to suggest that type I IFN priming of NK cells occurs through both direct and indirect pathways. As with many innate mechanisms, type I IFN and NK cells also orchestrate the adaptive immune response and thus aid in protection against secondary infections. Type I IFN can shape CD4(+) T cell, B cell and humoral memory formation. In addition, long-lived NK cells can perform specific and enhanced memory-like protection in secondary infections. This review outlines the different mechanisms underlying type I IFN regulation of NK cells and how type I IFN and NK cells can be used as a therapeutic target in vaccinations.

Beta interferon regulation of glucose metabolism is PI3K/Akt dependent and important for antiviral activity against coxsackievirus B3

An effective type I interferon (IFN)-mediated immune response requires the rapid expression of antiviral proteins that are necessary to inhibit viral replication and virus spread. We provide evidence that IFN-β regulates metabolic events important for the induction of a rapid antiviral response: IFN-β decreases the phosphorylation of AMP-activated protein kinase (AMPK), coincident with an increase in intracellular ATP. Our studies reveal a biphasic IFN-β-inducible uptake of glucose by cells, mediated by phosphatidylinositol 3-kinase (PI3K)/Akt, and IFN-β-inducible regulation of GLUT4 translocation to the cell surface. Additionally, we provide evidence that IFN-β-regulated glycolytic metabolism is important for the acute induction of an antiviral response during infection with coxsackievirus B3 (CVB3). Last, we demonstrate that the antidiabetic drug metformin enhances the antiviral potency of IFN-β against CVB3 both in vitro and in vivo. Taken together, these findings highlight an important role for IFN-β in modulating glucose metabolism during a virus infection and suggest that the use of metformin in combination with IFN-β during acute virus infection may result in enhanced antiviral responses.

IMPORTANCE

Type I interferons (IFN) are critical effectors of an antiviral response. These studies describe for the first time a role for IFN-β in regulating metabolism--glucose uptake and ATP production--to meet the energy requirements of a robust cellular antiviral response. Our data suggest that IFN-β regulates glucose metabolism mediated by signaling effectors similarly to activation by insulin. Interference with IFN-β-inducible glucose metabolism diminishes the antiviral response, whereas treatment with metformin, a drug that increases insulin sensitivity, enhances the antiviral potency of IFN-β.

STAT1 requirement for PKR-induced cell cycle arrest in vascular smooth muscle cells in response to heparin

Interferons (IFNs) are a family of cytokines that exhibit antiviral, antiproliferative, and immunomodulatory properties. PKR (protein kinase, RNA activated) is of central importance in mediating the antiproliferative actions of IFNs. Our research has established that PKR inhibits vascular smooth muscle cell (VSMC) proliferation by regulating G1 to S transition. Many cardiovascular diseases result from complications of atherosclerosis, a chronic and progressive inflammatory condition often characterized by excessive proliferation of VSMC. Thus, an effective method for inhibiting VSMC proliferation is likely to arrest atherosclerosis and restenosis at early stages. Our research establishes that PKR activation in VSMC leads to a G1 arrest brought about by an inhibition of cyclin-dependent kinase 2 (Cdk2) activity by p27(kip1). In quiescent VSMC, p27(kip1) levels are high and when stimulated by serum/growth factors, p27(kip1) levels drop by destabilization of the protein. Under conditions that lead to activation of PKR, there is a marked inhibition of p27(kip1) down-regulation due to increased stability of p27(kip1) protein. In order to understand the mechanism of heparin-induced stabilization of p27(kip1) in VSMC, we examined the involvement of the Signal Transducer and Activator of Transcription-1 (STAT1), which is an important player in mediating antiproliferative effects of IFNs. Our results demonstrate that PKR overexpression in VSMC leads to an increase in p27(kip1) protein levels and this increase requires the catalytic activity of PKR. PKR activation induced by antiproliferative agent heparin leads to phosphorylation of STAT1 on serine 727, which is essential for the cell cycle block. STAT1 null VSMCs are largely defective in heparin-induced cell cycle arrest and in PKR null cells the STAT1 phosphorylation in response to heparin was absent. These results establish that heparin causes STAT1 phosphorylation on serine 727 via activation of PKR and that this event is required for the G1 arrest in VSMC.

Interaction of IFNλR1 with TRAF6 regulates NF-κB activation and IFNλR1 stability

IFNλR1 is a member of the class II cytokine receptor family, and it associates with IL-10R2 to form a functional receptor complex, IFNλR. This receptor complex transduces signals from IFNλs (IFNλ1, IFNλ2, and IFNλ3), promoting antiviral and antiproliferative activities similar to those of type I IFNs. In an effort to further understand signal transduction through IFNλR1, we used bioinformatics analysis and identified a tumor necrosis factor receptor-associated factor 6 (TRAF6)-binding motif in the intracellular domain of IFNλR1. In subsequent immunoprecipitation and GST pull-down assays, IFNλR1 was shown to immunoprecipitate with TRAF6 and was pulled down by GST-TRAF6. Endogenous IFNλR1 and TRAF-6 interaction implies that these proteins really interact in the cells. This interaction was abrogated upon mutation of the TRAF6-binding motif in IFNλR1. Furthermore, the interaction between IFNλR1 and TRAF6 inhibited TRAF6-induced NF-κB activation, likely due to a reduction in TRAF6 autoubiquitination. Moreover, co-expression of IFNλR1 with TRAF6 significantly increased the stability of IFNλR1, thereby prolonging its half-life and enhancing its steady-state level in cultured cells.

A role for PKR in hematologic malignancies

The double-stranded RNA-dependent kinase PKR has been described for many years as strictly a pro-apoptotic kinase. Recent data suggest that the main purpose of this kinase is damage control and repair following stress and, if all else fails, apoptosis. Aberrant activation of PKR has been reported in numerous neurodegenerative diseases and cancer. Although a subset of myelodysplastic syndromes (MDS) and chronic lymphocytic leukemia contain low levels of PKR expression and activity, elevated PKR activity and/or expression have been detected in a wide range of hematologic malignancies, from bone marrow failure disorders to acute leukemia. With the recent findings that cancers containing elevated PKR activity are highly sensitive to PKR inhibition, we explore the role of PKR in hematologic malignancies, signal transduction pathways affected by PKR, and how PKR may contribute to leukemic transformation.

Inhibition of activation of dsRNA-dependent protein kinase and tumour growth inhibition

Inhibition of dsRNA-activated protein kinase (PKR), not only attenuates muscle atrophy in a murine model of cancer cachexia (MAC16), but it also inhibits tumour growth. In vitro the PKR inhibitor maximally inhibited growth of MAC16 tumour cells at a concentration of 200 nM, which was also maximally effective in attenuating phosphorylation of PKR and of eukaryotic initiation factor (eIF)2 on the alpha-subunit. There was no effect on the growth of the MAC13 tumour, which does not induce cachexia, even at concentrations up to 1,000 nM. There was constitutive phosphorylation of PKR and eIF2alpha in the MAC16, but not in the MAC13 tumour, while levels of total PKR and eIF2alpha were similar. There was constitutive upregulation of nuclear factor-kappaB (NF-kappaB) in the MAC16 tumour only, and this was attenuated by the PKR inhibitor, suggesting that it arose from activation of PKR. In MAC16 alone the PKR inhibitor also attenuated expression of the 20S proteasome. The PKR inhibitor potentiated the cytotoxicity of both 5-fluorouracil and gemcitabine to MAC16 cells in vitro. These results suggest that inhibitors of PKR may be useful therapeutic agents against tumours showing increased expression of PKR and constitutive activation of NF-kappaB, and may also prove useful in sensitising tumours to standard chemotherapeutic agents.

Upregulation of a Small Subset of Genes Drives Type I Interferon-Induced Antiviral Memory

Interferons (IFNs) stand in the frontline of defense against viral infections. In this study, we aimed at characterizing the gene expression profile specific to the antiviral effect out of the hundreds of genes involved also in other IFN activities. We found that the IFN-induced antiviral state is maintained for a prolonged time even after IFN occlusion. This was achieved through the active expression of a small set of <40 genes long after IFN was occluded, from which two groups are distinguished: one includes genes participating in direct inhibition of viral replication, such as Mx and OAS; the second group is related to antigen presentation, including all genes involved in the proteasome-to-immunoproteasome switch and class I MHC genes. Transcription of these genes continued after IFN removal and was Stat1 independent, suggesting the involvement of other signaling elements in addition to the canonical signal transduction pathway. Not less important were genes whose upregulation, in cases by many fold, is terminated once IFN is removed. Among these are viral sensing genes, such as retinoic acid-inducible gene-I protein (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and toll-like receptor (TLR), cytokines, and apoptotic-related genes. Our findings provide a systemwide depiction of prolonged intracellular antiviral protection without the need for ongoing IFN stimulation.

The role of type I interferons in TLR responses

Recent advances in unravelling the complexities of the signalling pathways that constitute innate immunity have highlighted type I interferon as a key component in the response to infection. Here we focus on the emerging field of pattern-recognition receptor signalling, specifically Toll-like receptors and retinoic acid inducible gene-like helicases, from the perspective of this 50-year-old cytokine. The type I interferon gene family encompasses more than 20 subtypes, whose nature and properties have been extensively studied during its relatively long history. In this review we update and integrate available data on the mechanics of activation of the interferon genes and the role of this cytokine family in the innate immune response.

Using interferon-alpha to block expression of cellular ribosome subunit S24 variant 2 in human fibroblasts inhibits translation of the poliovirus genome

Studies about the proteins induced by interferon (IFN-)-alpha stimulation have provided some data on their mechanism of antiviral effect. These proteins were confirmed to contribute to antiviral functions. In this study, IFN-alpha stimulation of human fibroblasts was shown to induce the inhibition of S24 variant 2 (a structural component of the ribosomal small subunit) at the mRNA and protein levels, implying a possible antiviral mechanism for IFN-alpha in human fibroblasts. The delay of poliovirus replication by IFN-alpha was partially compensated for by S24 variant 2 expressed in pcDNA vector-transfected cells, and the interference RNA of S24 variant 2 was able to induce mimetically, to some extent, this poliovirus replication delay. These observations revealed that S24 variant 2 could be involved in the antiviral effects of IFN-alpha in human fibroblasts.

The dsRNA protein kinase PKR: virus and cell control

The IFN-induced double-stranded RNA-dependent protein kinase (PKR) is one of the four mammalian serine-threonine kinases (the three others being HRI, GCN2 and PERK) that phosphorylate the eIF2 alpha translation initiation factor, in response to stress signals, mainly as a result of viral infections. eIF2 alpha phosphorylation results in arrest of translation of both cellular and viral mRNAs, an efficient way to inhibit virus replication. The particularity of PKR is to activate by binding to dsRNA through two N terminal dsRNA binding motifs (dsRBM). PKR activation during a viral infection represents a threat for several viruses, which have therefore evolved to express PKR inhibitors, such as the Vaccinia E3L and K3L proteins. The function of PKR can also be regulated by cellular proteins, either positively (RAX/PACT; Mda7) or negatively (p58IPK, TRBP, nucleophosmin, Hsp90/70). PKR can provoke apoptosis, in part through its ability to control protein translation, but the situation appears to be more complex, as NF-kappaB, ATF-3 and p53 have also been implicated. PKR-induced apoptosis involves mainly the FADD/caspase 8 pathway, while the mitochondrial APAF/caspase 9 pathway is also engaged. As a consequence of the effects of PKR on translation, transcription and apoptosis, PKR can function to control cell growth and cell differentiation, and its activity can be controlled by the action of several oncogenes.

Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action

The double-stranded RNA-dependent protein kinase PKR is a critical mediator of the antiproliferative and antiviral effects exerted by interferons. Not only is PKR an effector molecule on the cellular response to double-stranded RNA, but it also integrates signals in response to Toll-like receptor activation, growth factors, and diverse cellular stresses. In this review, we provide a detailed picture on how signaling downstream of PKR unfolds and what are the ultimate consequences for the cell fate. PKR activation affects both transcription and translation. PKR phosphorylation of the alpha subunit of eukaryotic initiation factor 2 results in a blockade on translation initiation. However, PKR cannot avoid the translation of some cellular and viral mRNAs bearing special features in their 5' untranslated regions. In addition, PKR affects diverse transcriptional factors such as interferon regulatory factor 1, STATs, p53, activating transcription factor 3, and NF-kappaB. In particular, how PKR triggers a cascade of events involving IKK phosphorylation of IkappaB and NF-kappaB nuclear translocation has been intensively studied. At the cellular and organism levels PKR exerts antiproliferative effects, and it is a key antiviral agent. A point of convergence in both effects is that PKR activation results in apoptosis induction. The extent and strength of the antiviral action of PKR are clearly understood by the findings that unrelated viral proteins of animal viruses have evolved to inhibit PKR action by using diverse strategies. The case for the pathological consequences of the antiproliferative action of PKR is less understood, but therapeutic strategies aimed at targeting PKR are beginning to offer promising results.

Genetic deletion of PKR abrogates TNF-induced activation of IkappaBalpha kinase, JNK, Akt and cell proliferation but potentiates p44/p42 MAPK and p38 MAPK activation

Double-stranded RNA-dependent protein kinase (PKR), a ubiquitously expressed serine/threonine kinase, has been implicated in the regulation or modulation of cell growth through multiple signaling pathways, but how PKR regulates tumor necrosis factor (TNF)-induced signaling pathways is poorly understood. In the present study, we used fibroblasts derived from PKR gene-deleted mice to investigate the role of PKR in TNF-induced activation of nuclear factor-kappaB (NF-kappaB), mitogen-activated protein kinases (MAPKs) and growth modulation. We found that in wild-type mouse embryonic fibroblast (MEF), TNF induced NF-kappaB activation as measured by DNA binding but deletion of PKR abolished this activation. This inhibition was associated with suppression of inhibitory subunit of NF-kappaB (IkappaB)alpha kinase (IKK) activation, IkappaBalpha phosphorylation and degradation, p65 phosphorylation and nuclear translocation, and NF-kappaB-dependent reporter gene transcription. TNF-induced Akt activation needed for IKK activation was also abolished by deletion of PKR. NF-kappaB activation was diminished in PKR-deleted cells transfected with TNF receptor (TNFR) 1, TNFR-associated death domain and TRAF2 plasmids; NF-kappaB activated by NF-kappaB-inducing kinase, IKK or p65, however, was minimally affected. Among the MAPKs, it was interesting that whereas TNF-induced c-Jun N-terminal kinase (JNK) activation was abolished, activation of p44/p42 MAPK and p38 MAPK was potentiated in PKR-deleted cells. TNF induced the expression of NF-kappaB-regulated gene products cyclin D1, c-Myc, matrix metalloproteinase-9, survivin, X-linked inhibitor-of-apoptosis protein (IAP), IAP1, Bcl-x(L), A1/Bfl-1 and Fas-associated death domain protein-like IL-1beta-converting enzyme-inhibitory protein in wild-type MEF but not in PKR-/- cells. Similarly, TNF induced the proliferation of wild-type cells, but this proliferation was completely suppressed in PKR-deleted cells. Overall, our results indicate that PKR differentially regulates TNF signaling; IKK, Akt and JNK were positively regulated, whereas p44/p42 MAPK and p38 MAPK were negatively regulated.

A new double-stranded RNA binding protein (DRBP-120) is associated with double-stranded RNA-activated protein kinase (PKR)

Double-stranded RNA-activated protein kinase (PKR) plays an important role in interferon-induced antiviral responses, and is also involved in intracellular signaling pathways, including the apoptosis, proliferation, and transcription pathways. In this study, a new 120-kDa PKR-associated protein designated double-stranded RNA binding protein (DRBP)-120 was identified using co-immunoprecipitation with anti-PKR antiserum and two-dimensional electrophoresis. Furthermore, DRBP-120 is a double-stranded RNA (dsRNA)-binding protein, and it was detected in both the cytoplasm and the nucleus of HeLa cells associated with PKR.

Defining the mechanism by which IFN-β dowregulates c-myc expression in human melanoma cells: pivotal role for human polynucleotide phosphorylase (hPNPaseold-35)

Type I interferons (IFN-alpha/-beta) are capable of suppressing c-myc mRNA expression by modulating post-transcriptional processing. However, the molecular mechanism of this phenomenon is poorly understood. We previously established that human polynucleotide phosphorylase (hPNPase(old-35)), a type I IFN-inducible 3',5' exoribonuclease involved in mRNA degradation, induces G1 cell cycle arrest and eventually apoptosis by specifically degrading c-myc mRNA. We now demonstrate a close association between IFN-beta-induced hPNPase(old-35) upregulation and c-myc downregulation in human melanoma cells. Employing stable melanoma cell clones expressing hPNPase(old-35) small inhibitory RNA, we demonstrate that hPNPase(old-35) is a key molecule coupled with IFN-beta-mediated downregulation of c-myc mRNA. Inhibition of hPNPase(old-35) or overexpression of c-myc protects melanoma cells from IFN-beta-mediated growth inhibition, emphasizing the importance of hPNPase(old-35) upregulation and consequent c-myc downregulation in IFN-beta-induced growth inhibition and apoptosis induction. In these contexts, targeted overexpression of hPNPase(old-35) might be a novel therapeutic strategy for c-myc-overexpressing and IFN-resistant tumors, such as melanomas.

Viral hepatitis and hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors in the world. The incidence of HCC varies considerably with the geographic area because of differences in the major causative factors. Chronic hepatitis B and C, mostly in the cirrhotic stage, are responsible for the great majority of cases of HCC worldwide. The geographic areas at the highest risk are South-East Asia and sub-Saharan Africa, here hepatitis B is highly endemic and is the main cause of HCC. In areas with an intermediate rate of HCC such as Southern Europe and Japan, hepatitis C is the predominant cause, whereas in low rate areas such as Northern Europe and the USA, HCC is often related to other factors as alcoholic liver disease. There is a rising incidence in HCC in developed countries during the last two decades, due to the increasing rate of hepatitis C infection and improvement of the clinical management of cirrhosis.

METHODS

This article reviews the literature on hepatitis and hepatocellular carcinoma. The Medline search was carried out using these key words and articles were selected on epidemiology, risk factors, screening, and prevention of hepatocellular carcinoma.

RESULTS

Screening of patients with advanced chronic hepatitis B and C with hepatic ultrasound and determination of serum alfa-fetoprotein may improve the detection of HCC, but further studies are needed whether screening improves clinical outcome. Hepatitis B and C viruses (HBV/HCV) can be implicated in the development of HCC in an indirect way, through induction of chronic inflammation, or directly by means of viral proteins or, in the case of HBV, by creation of mutations by integration into the genome of the hepatocyte.

CONCLUSION

The most effective tool to prevent HCC is avoidance of the risk factors such as viral infection. For HBV, a very effective vaccine is available. Preliminary data from Taiwan indicate a protective effect of universal vaccination on the development of HCC. Vaccination against HBV should therefore be a health priority. In patients with chronic hepatitis B or C, interferon-alfa treatment in a noncirrhotic stage is protective for HCC development in responders, probably by prevention of cirrhosis development. When cirrhosis is already present, the protective effect is less clear. For cirrhosis due to hepatitis B, a protective effect was demonstrated in Oriental, but not in European patients. For cirrhosis due to hepatitis C, interferon-alfa treatment showed to be protective in some studies, especially in Japan with a high incidence of HCC in untreated patients. Virological, but also merely biochemical response, seems to be associated with a lower risk of development of HCC. As most studies are not randomized controlled trials, no definitive conclusions on the long-term effects of interferon-alfa in HBV or HCV cirrhosis can be established. Especially in hepatitis C, prospective studies should be performed using the more potent reference treatments for cirrhotics, namely the combination of peginterferon and ribavirin.

The GRIMs: a new interface between cell death regulation and interferon/retinoid induced growth suppression

Cytokines and vitamins play a central role in controlling neoplastic cell growth. The interferon (IFN) family of cytokines regulates antiviral, anti-tumor, antimicrobial, differentiation, and immune responses in mammals. Significant advances have been made with respect to IFN-induced signal transduction pathways and antiviral responses. However, the IFN-induced anti-tumor actions are poorly defined. Although IFNs themselves inhibit tumor growth, combination of IFNs with retinoids (a class of Vitamin A related compounds) strongly potentiates the IFN-regulated anti-tumor action in a number of cell types. To define the molecular mechanisms involved in IFN/retinoid (RA)-induced apoptosis we have employed a genetic approach and identified several critical genes. In this review, I provide the current picture of IFN- RA- and IFN/RA-regulated growth suppressive pathways. In particular, I focus on a novel set of genes, the genes-associated with retinoid-interferon induced mortality (GRIM). GRIMs may be novel types of tumor suppressors, useful as biological response markers and potentially novel targets for drug development.

Reduction of double-stranded RNA-activated protein kinase in hepatocellular carcinoma associated with hepatitis B virus

Chronic hepatitis B virus (HBV) infection is a major cause of hepatocellular carcinoma (HCC) in Asia. Double-stranded RNA (dsRNA)-activated protein kinase (PKR) is an interferon-induced, serine/threonine protein kinase. Recent studies have suggested that PKR is involved in the pathogenesis of HCC with hepatitis virus C infection by inhibiting viral and cellular proteins related to cell growth and proliferation. In the present study, PKR was examined in both tumor and non-tumor tissues from HCC livers infected with HBV. The expression of PKR was determined by TaqMan real-time PCR and immunohistochemical methods. The level of PKR was also analyzed in relation to pathological changes observed in HCC. The result showed that PKR was reduced in tumor tissues of HCC from HBV carriers with low serum viral load (<0.7 x 10(6) copies/ml) compared to those with higher serum viral load. However, the overall PKR level was much lower in tumor tissues than that in non-tumor tissues, irrespective of HBV carrier status or serum viral load. PKR level tended to be lower in HCC samples with alpha-fetoprotein (AFP) more than 500 ng/ml (mean: 4024.2 ng/ml) than those with AFP less than 500 ng/ml (mean: 50.6 ng/ml). There was no significant difference in the expression of PKR between tumor tissues with well differentiation and those with poor or moderate differentiation. In conclusion, the level of PKR was reduced in HCC tumor tissues, suggesting a possible role of PKR in promoting the growth of tumor. HBV may participate in altering the level of PKR, but factors other than HBV should play a more determining role in the regulation of PKR in HCC. The association between PKR and AFP levels may offer an alternative tumor marker for HCC.

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.

Oncogenic Ki-ras inhibits the expression of interferon-responsive genes through inhibition of STAT1 and STAT2 expression

Endogenous interferon gamma (IFNgamma) promotes the host response to primary tumors, and IFNgamma-insensitive tumors display increased tumorigenicity and can evade tumor surveillance mechanisms. Here we demonstrate that activating mutations of Ki-ras are sufficient to inhibit the expression of STAT1 and STAT2, transcription factors required for signaling by IFNs, providing a potential mechanism for the insensitivity of tumors to IFNs. We demonstrated that colon cancer cell lines with Ki-ras mutations display reduced expression of IFN-responsive genes compared with the cell lines that have retained wild type Ras and that inactivation of the mutant Ki-ras allele in the HCT116 colon cancer cell line is sufficient to restore the expression of STAT1, STAT2, and IRF-9. Accordingly, the expression of 27 interferon-inducible genes was reduced in HCT116 cells compared with the isogenic clones with targeted deletion of the mutant Ki-ras allele, Hkh2 and Hke-3. The expression of IFNgamma receptors did not differ among the isogenic cell lines. IFNgamma stimulated transcription of a STAT1-dependent reporter gene was impaired by RasV12, demonstrating a transmodulation of IFN/STAT signaling by activated Ras. Finally, we demonstrated that the expression of RasV12 in 293T cells is sufficient to inhibit the endogenous expression of STAT1 and STAT2, confirming the negative regulation of IFN signaling by oncogenic Ras. Our data demonstrate that the signaling initiated by activated Ki-ras interferes with the IFN/STAT signaling pathway and modulates the responsiveness of cancer cells to interferons. Furthermore, the data suggest that tumors harboring activating Ki-ras mutations may escape tumor surveillance mechanisms due to reduced responsiveness to IFNgamma.

Critical roles for IFN-beta in lymphoid development, myelopoiesis, and tumor development: links to tumor necrosis factor alpha

We have generated mice null for IFN-beta and report the diverse consequences of IFN-beta for both the innate and adaptive arms of immunity. Despite no abnormalities in the proportional balance of CD4 and CD8 T cell populations in the peripheral blood, thymus, and spleen of IFN-beta-/- mice, activated lymph node and splenic T lymphocytes exhibit enhanced T cell proliferation and decreased tumor necrosis factor alpha production, relative to IFN-beta+/+ mice. Notably, constitutive and induced expression of tumor necrosis factor alpha is reduced in the spleen and bone marrow (BM) macrophages, respectively, of IFN-beta-/- mice. We also observe an altered splenic architecture in IFN-beta-/- mice and a reduction in resident macrophages. We identify a potential defect in B cell maturation in IFN-beta-/- mice, associated with a decrease in B220+ve/high/CD43-ve BM-derived cells and a reduction in BP-1, IgM, and CD23 expression. Circulating IgM-, Mac-1-, and Gr-1-positive cells are also substantially decreased in IFN-beta-/- mice. The decrease in the numbers of circulating macrophages and granulocytes likely reflects defective maturation of primitive BM hematopoiesis in mice, shown by the reduction of colony-forming units, granulocyte-macrophage. We proceeded to evaluate the in vivo growth of malignant cells in the IFN-beta-/- background and give evidence that Lewis lung carcinoma-specific tumor growth is more aggressive in IFN-beta-/- mice. Taken altogether, our data suggest that, in addition to the direct growth-inhibitory effects on tumor cells, IFN-beta is required during different stages of maturation in the development of the immune system.

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.

Interferon-gamma: an overview of signals, mechanisms and functions

Interferon-gamma (IFN-gamma) coordinates a diverse array of cellular programs through transcriptional regulation of immunologically relevant genes. This article reviews the current understanding of IFN-gamma ligand, receptor, signal transduction, and cellular effects with a focus on macrophage responses and to a lesser extent, responses from other cell types that influence macrophage function during infection. The current model for IFN-gamma signal transduction is discussed, as well as signal regulation and factors conferring signal specificity. Cellular effects of IFN-gamma are described, including up-regulation of pathogen recognition, antigen processing and presentation, the antiviral state, inhibition of cellular proliferation and effects on apoptosis, activation of microbicidal effector functions, immunomodulation, and leukocyte trafficking. In addition, integration of signaling and response with other cytokines and pathogen-associated molecular patterns, such as tumor necrosis factor-alpha, interleukin-4, type I IFNs, and lipopolysaccharide are discussed.

The cell death regulator GRIM-19 is an inhibitor of signal transducer and activator of transcription 3

GRIM-19 (gene associated with retinoid-IFN-induced mortality 19), isolated as a cell death activator in a genetic screen used to define mechanisms involved in IFN-beta- and retinoic acid-induced cell death, codes for a approximately 16-kDa protein that induces apoptosis in a number of cell lines. Antisense ablation of GRIM-19 caused resistance to cell death induced by IFN plus retinoic acid and conferred a growth advantage to cells. To understand the molecular bases for its cell death regulatory activity, we used a yeast two-hybrid screen and identified that the transcription factor STAT3 (signal transducer and activator of transcription 3) binds to GRIM-19. GRIM-19 inhibits transcription driven by activation of STAT3, but not STAT1. It neither inhibits the ligand-induced activation of STAT3 nor blocks its ability to bind to DNA. Mutational analysis indicates that the transactivation domain of STAT3, especially residue S727, is required for GRIM-19 binding. Because GRIM-19 does not bind significantly to other STATs, our studies identify a specific inhibitor of STAT3. Because constitutively active STAT3 up-regulates antiapoptotic genes to promote tumor survival, its inhibition by GRIM-19 also demonstrates an antioncogenic effect exerted by biological therapeutics.

Type I interferon differential therapy for erythroleukemia: specificity of STAT activation

Type I interferons (IFNs), pleiotropic cytokines with antiviral, antiproliferative, apoptotic, and immunoregulatory functions, are efficacious in the treatment of malignancies, viral infections, and autoimmune diseases. Binding of these cytokines to their cognate receptor leads to activation of the Jak-signal transducers and activators of transcription (STAT) signaling pathway and altered gene expression. This signal pathway has been intensely studied using human IFN-alpha 2 and IFN-beta. However, there are over 14 human IFN-alpha subtypes and over 10 murine IFN-alpha subtypes, with a single IFN-beta subtype in both species. J2E cells are immortalized at the proerythroblast stage of development and produce a rapid and fatal erythroleukemia in vivo. These cells retain the ability to respond to erythropoietin in vitro by proliferating, differentiating, and remaining viable in the absence of serum. Here, we show that J2E cells are also functionally regulated differentially by IFN subtype treatment in vitro. A novel finding was the selective activation of STAT and mitogen-activated protein kinase (MAPK) molecules by different subtypes binding the IFN receptor. These findings indicate distinct effects for individual type I IFN subtypes, which are able to differentially activate members of the STAT and MAPK family. Finally, we investigated the efficacy of IFN naked DNA therapy in treating J2E-induced erythroleukemia in athymic nude mice. IFN subtypes differentially regulated the onset of erythroleukemia with delayed onset and increased survival, possibly via a reduction in cell viability, and enhanced antiproliferative and apoptotic effects observed for IFNA6 and IFNA9 treatment, respectively. Moreover, these data highlight the necessity to choose the best IFN subtype in disease treatment.

Neoplastic progression in melanoma and colon cancer is associated with increased expression and activity of the interferon-inducible protein kinase, PKR

The interferon-inducible, double-stranded RNA (dsRNA)-activated protein kinase, PKR, plays key roles in regulation of cell growth and differentiation, and has been postulated as a tumor suppressor. Downstream effectors of PKR include the translation initiation factor, eIF2alpha, and the transcription factor, NF-kappaB. We found elevated levels of PKR protein, dsRNA-dependent PKR autophosphorylation activity, and phosphorylated eIF2alpha in melanoma cells compared to nontransformed melanocytes in culture. Treatment with interferon-alpha2b further induced PKR expression and activity. Immunohistochemical analysis of primary melanomas demonstrated minimal PKR immunoreactivity, but melanoma lymph node metastases expressed a high level of PKR protein. Furthermore, analysis of colon cancer specimens revealed that transformation from normal mucosa to adenomas and carcinomas was coincident with an increase in PKR expression. These data do not support the concept of PKR as a classic tumor suppressor but instead suggest that PKR upregulation occurs at defined steps in cancer progression, probably as a cellular response to neoplasia.

Interferon gamma inhibits growth of human pancreatic carcinoma cells via caspase-1 dependent induction of apoptosis

BACKGROUND AND AIMS

The poor prognosis of pancreatic cancer is partly due to resistance to a broad spectrum of apoptotic stimuli. To identify intact proapoptotic pathways of potential clinical relevance, we characterised the effects of interferon gamma (IFN-gamma) on growth and survival in human pancreatic cancer cells.

METHODS

IFN-gamma receptor expression and signal transduction were examined by reverse transcriptase-polymerase chain reaction (RT-PCR), immunoprecipitation, western blot analysis, and transactivation assays. Effects on cell growth and survival were evaluated in terms of cell numbers, colony formation, cell cycle analysis, DNA fragmentation, and poly(ADP ribose) polymerase (PARP) cleavage.

RESULTS

All four pancreatic cancer cell lines examined expressed functional IFN-gamma receptors and downstream effectors, including the putative tumour suppressor interferon regulatory factor 1 (IRF-1). IFN-gamma treatment profoundly inhibited anchorage dependent and independent growth of pancreatic cancer cells. Cell cycle analyses revealed subdiploid cells suggesting apoptosis, which was confirmed by demonstration of DNA fragmentation and PARP cleavage. Time and dose dependency of apoptosis induction and growth inhibition correlated closely, identifying apoptosis as the main, if not exclusive, mechanism responsible for growth inhibition. Apoptosis was preceded by upregulation of procaspase-1 and accompanied by proteolytic activation. Furthermore, the caspase inhibitor z-vad-fmk completely prevented IFN-gamma mediated apoptosis.

CONCLUSIONS

These results identify an intact proapoptotic pathway in pancreatic cancer cells and suggest that IRF-1 and/or procaspase-1 may represent potential therapeutic targets to be further explored.

Modulation of p53 dependent gene expression and cell death through thioredoxin-thioredoxin reductase by the Interferon-Retinoid combination

We have shown earlier that the IFN-beta and all-trans retinoic acid (RA) combination, but not the single agents, induces death in several tumor cell lines. Employing a genetic technique we have identified several Genes associated with Retinoid-IFN induced Mortality (GRIM). One of the GRIMs was human thioredoxin reductase (TR), a redox enzyme. Since the overexpressed TR augments IFN/RA stimulated cell death, we explored the mechanisms of TR-mediated death. Here we show that TR augments cell death by upregulating the transcriptional activity of p53 tumor suppressor. This process does not involve a physical increase in levels of p53. Using redox inactive mutants of TR and its substrate, thioredoxin (Trx), we demonstrate that IFN/RA-induced regulation of p53 dependent gene expression requires TR and Trx. In contrast-over-expression of wildtype TR or Trx augment the p53 dependent gene expression in response to IFN/RA treatment. Consistent with these results an increased DNA binding activity of p53 was noted in the presence of TR. These studies identify a novel mechanism of p53 mediated cell death regulation involving redox enzymes.

Regulation of interferon and retinoic acid-induced cell death activation through thioredoxin reductase

Interferons (IFNs) and retinoids are potent biological response modifiers. The IFN-beta and all-trans-retinoic acid combination, but not these single agents individually, induces death in several tumor cell lines. To elucidate the molecular basis for these actions, we have employed an antisense knockout approach to identify the gene products that mediate cell death and isolated several genes associated with retinoid-IFN-induced mortality (GRIMs). One of the GRIM cDNAs, GRIM-12, was identical to human thioredoxin reductase (TR). To define the functional relevance of TR to cell death and to define its mechanism of death-modulating functions, we generated mutants of TR and studied their influence on the IFN/RA-induced death regulatory functions of caspases. Wild-type TR activates cell death that was inhibited in the presence of caspase inhibitors or catalytically inactive caspases. A mutant TR, lacking the active site cysteines, inhibits the cell death induced by caspase 8. IFN/all-trans-retinoic acid-induced cytochrome c release from the mitochondrion was promoted in the presence of wild type and was inhibited in the presence of mutant TR. We find that TR modulates the activity of caspase 8 to promote death. This effect is in part caused by the stimulation of death receptor gene expression. These studies identify a new mechanism of cell death regulation by the IFN/all-trans-retinoic acid combination involving redox enzymes.

Inositol hexakisphosphate kinase 2 mediates growth suppressive and apoptotic effects of interferon-beta in ovarian carcinoma cells

Interferons (IFNs) regulate the expression of genes that mediate their antiviral, antitumor, and immunomodulatory actions. We have previously shown that IFN-beta suppresses growth of human ovarian carcinoma xenografts in vivo and induces apoptosis of ovarian carcinoma cells in vitro. To investigate mechanisms of IFN-beta-induced apoptosis we employed an antisense technical knockout approach to identify gene products that mediate cell death and have isolated several regulators of interferon-induced death (RIDs). In this investigation, we have characterized one of the RIDs, RID-2. Sequence analysis revealed that RID-2 was identical to human inositol hexakisphosphate kinase 2 (IP6K2). IP6K2 is post-transcriptionally induced by IFN-beta in ovarian carcinoma cells. A mutant IP6K2 with substitutions in the putative inositol phosphate binding domain abrogates IFN-beta-induced apoptosis. These studies identify a novel function for IP6K2 in cell growth regulation and apoptosis.

Theiler's murine encephalomyelitis virus induces apoptosis in gamma interferon-activated M1 differentiated myelomonocytic cells through a mechanism involving tumor necrosis factor alpha (TNF-alpha) and TNF-alpha-related apoptosis-inducing ligand

Infection of susceptible mice with the low-neurovirulence Theiler's murine encephalomyelitis virus strain BeAn results in an inflammatory demyelinating disease similar to multiple sclerosis. While the majority of virus antigen is detected in central nervous system macrophages (Mphis), few infiltrating Mphis are infected. We used the myelomonocytic precursor M1 cell line to study BeAn virus-Mphi interactions in vitro to elucidate mechanisms for restricted virus expression. We have shown that restricted BeAn infection of M1 cells differentiated in vitro (M1-D) results in apoptosis. In this study, BeAn infection of gamma interferon (IFN-gamma)-activated M1-D cells also resulted in apoptosis but with no evidence of virus replication or protein expression. RNase protection assays of M1-D cellular RNA revealed up-regulation of Fas and the p55 chain of the tumor necrosis factor alpha (TNF-alpha) receptor transcripts with IFN-gamma activation. BeAn infection of activated cells resulted in increased caspase 8 mRNA transcripts and the appearance of TNF-alpha-related apoptosis-inducing ligand (TRAIL) 4 h postinfection. Both unactivated and activated M1-D cells expressed TRAIL receptors (R1 and R2), but only activated cells were killed by soluble TRAIL. Activated cells were also susceptible to soluble FasL- and TNF-alpha-induced apoptosis. The data suggest that IFN-gamma-activated M1-D cell death receptors become susceptible to their ligands and that the cells respond to BeAn virus infection by producing the ligands TNF-alpha and TRAIL to kill the susceptible cells. Unactivated cells are not susceptible to FasL or TRAIL and require virus replication to initiate apoptosis. Therefore, two mechanisms of apoptosis induction can be triggered by BeAn infection: an intrinsic pathway requiring virus replication and an extrinsic pathway signaling through the death receptors.

Thioredoxin participates in a cell death pathway induced by interferon and retinoid combination

Interferons (IFNs) and retinoids are potent tumor growth suppressors. We have shown earlier that the IFN-beta and all-trans retinoic acid combination, but not the single agents, induces death in several tumor cell lines. Employing a genetic approach we have recently identified several Genes associated with Retinoid-IFN induced Mortality (GRIM) that mediate the cell death effect of IFN/RA combination. One of the GRIMs, GRIM-12, was identical to human thioredoxin reductase (TR), an enzyme that controls intracellular redox state. To define the participants of TR mediated death pathway we have examined the role of thioredoxin (Trx), its downstream substrate, and its influence on IFN/RA-induced death regulation. Inhibition of the thioredoxin expression by antisense RNA suppressed cell death. Similarly, a mutant Trx1 lacking the critical cysteine residues blocked cell death. In contrast, overexpression of wildtype thioredoxin augmented cell death. This effect of Trx1 was in part due to its ability to augment cell death via caspase-8. The redox inactive Trx1 mutant inhibits the cell death induced by caspase-8 but not caspase-3. These studies identify a novel mechanism of cell death regulation by IFN/RA combination involving redox enzymes.

Interferon and cytotoxic chemotherapy for the treatment of post-transplant lymphoproliferative disorder

Interferon-alpha and cytotoxic chemotherapy may be effective treatment modalities for the post-transplant lymphoproliferative disorder. Interferon-alpha may result in a complete response in up to 40% of patients, while chemotherapy may be effective in 75% of those failing local surgical excision, a reduction in immunosuppression, and an antiviral agent. Interferon may be used early after diagnosis in patients with relatively slowly growing tumors. Chemotherapy should be selected for patients with bulky, rapidly growing malignancies. The toxicity of chemotherapy may be minimized by discontinuing maintenance immunosuppression during chemotherapy, administering GCSF, and providing antimicrobial prophylaxis. Rejection is minimized by the reintroduction of maintenance immunosuppression when the patient is no longer neutropenic.

The interferon-beta and tamoxifen combination induces apoptosis using thioredoxin reductase

Interferons (IFNs) suppress cell growth by inducing cellular genes. The anti-estrogen tamoxifen (Tam), binds to estrogen receptor and inhibits transcription of estrogen stimulated genes. In cells resistant to IFN-induced growth suppression, IFN/Tam combination causes cell death. We previously reported that the combination of IFN-beta and Tam was a more potent growth suppressor of human tumor xenografts than either agent alone. The IFN/Tam combination acts in a manner similar to the IFN/retinoic acid combination. Using a genetic technique, we have recently identified several genes associated with retinoid-IFN-induced mortality (GRIM). One such gene, GRIM-12, was identical to human thioredoxin reductase (TR). In the present study we have examined whether the IFN/Tam combination also requires GRIM-12 for inducing cell death. We report here that GRIM-12 is necessary for mediating the cell death effects of IFN/Tam, and its expression is induced by IFN/Tam at a post-transcriptional stage. Repression of GRIM-12 levels either by antisense expression or by dominant negative inhibitors caused resistance to IFN/Tam induced death and promoted cell growth. Overexpression of GRIM-12 increased IFN/Tam induced apoptosis. Thus, these studies have identified a critical role for GRIM-12 (TR) in apoptosis.

Molecular mechanism of interferon alfa-mediated growth inhibition in human neuroendocrine tumor cells

BACKGROUND & AIMS

Although human neuroendocrine tumors respond to interferon (IFN)-alpha treatment in vivo, the underlying mechanisms of growth inhibition are poorly understood. To characterize the antiproliferative effects at a molecular level, we explored the growth-regulatory action of IFN-alpha in the human neuroendocrine tumor cell lines BON and QGP1.

METHODS

IFN-alpha receptor expression and signal transduction were examined by reverse-transcription polymerase chain reaction, immunoblotting, subcellular fractionation, and transactivation assays. Growth regulation was evaluated by cell numbers, soft agar assays, and cell cycle analysis using flow cytometry. Expression and activity of cell cycle-regulatory molecules were determined by immunoblotting and histone H1-kinase assays.

RESULTS

Both cell lines expressed IFN-alpha receptor mRNA transcripts. Ligand binding initiated phosphorylation of Jak kinases and Stat transcription factors, resulting in Stat activation, nuclear translocation, and transcription from an ISRE-reporter construct. Prolonged IFN-alpha treatment dose-dependently inhibited both anchorage-dependent and -independent growth. Cell cycle analysis of IFN-alpha-treated, unsynchronized cultures revealed an increased S-phase population, which was further substantiated in G(1) synchronized QGP1 cells. IFN-alpha-treated cells entered S phase in parallel to control cultures, but their progress into G(2)/M phase was delayed. Both cellular cyclin B levels and CDC 2 activity were substantially reduced. The extent and time course of this reduction corresponded to the observed S-phase accumulation.

CONCLUSIONS

IFN-alpha directly inhibits growth of human neuroendocrine tumor cells by specifically delaying progression through S phase and into G(2)/M. These cell cycle changes are associated with inhibition of cyclin B expression, resulting in reduced CDC2 activity.

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.

Regulation of c-myc expression by IFN-gamma through Stat1-dependent and -independent pathways

Interferons (IFNs) inhibit cell growth in a Stat1-dependent fashion that involves regulation of c-myc expression. IFN-gamma suppresses c-myc in wild-type mouse embryo fibroblasts, but not in Stat1-null cells, where IFNs induce c-myc mRNA rapidly and transiently, thus revealing a novel signaling pathway. Both tyrosine and serine phosphorylation of Stat1 are required for suppression. Induced expression of c-myc is likely to contribute to the proliferation of Stat1-null cells in response to IFNs. IFNs also suppress platelet-derived growth factor (PDGF)-induced c-myc expression in wild-type but not in Stat1-null cells. A gamma-activated sequence element in the promoter is necessary but not sufficient to suppress c-myc expression in wild-type cells. In PKR-null cells, the phosphorylation of Stat1 on Ser727 and transactivation are both defective, and c-myc mRNA is induced, not suppressed, in response to IFN-gamma. A role for Raf-1 in the Stat1-independent pathway is revealed by studies with geldanamycin, an HSP90-specific inhibitor, and by expression of a mutant of p50(cdc37) that is unable to recruit HSP90 to the Raf-1 complex. Both agents abrogated the IFN-gamma-dependent induction of c-myc expression in Stat1-null cells.

Sensitivity of an epstein-barr virus-positive tumor line, Daudi, to alpha interferon correlates with expression of a GC-rich viral transcript

The exquisite sensitivity of the Burkitt's lymphoma (BL)-derived cell line Daudi to type I interferons has not previously been explained. Here we show that expression of an Epstein-Barr virus (EBV) transcript, designated D-HIT (Y. Gao et al., J. Virol. 71:84-94, 1997), correlates with the sensitivity of different Daudi cell isolates (or that of other EBV-carrying cells, where known) to alpha interferon (IFN-alpha). D-HIT, transcribed from a GC-rich repetitive region (IR4) of the viral genome, is highly structured, responding to RNase digestion in a manner akin to double-stranded RNA. Comparing EBV-carrying BL cell lines with differing responses to IFN-alpha, we found the protein levels of the dsRNA-activated kinase, PKR, to be similar, whereas the levels of the autophosphorylated active form of PKR varied in a manner that correlated with endogenous levels of D-HIT expression. In a classical in vitro kinase assay, addition of either poly(I)-poly(C) or an in vitro-transcribed D-HIT homolog stimulated the autophosphorylation activity of PKR from IFN-alpha-treated cells in both EBV-positive and EBV-negative B lymphocytes. By transfection experiments, these RNAs were shown to reduce cell proliferation and to sensitize otherwise relatively insensitive Raji cells to IFN-alpha. The data lead to a model wherein the D-HIT viral RNA also serves as a possible transcriptional activator of IFN-alpha or cellular genes regulated by this cytokine.

PKR; a sentinel kinase for cellular stress

The double stranded RNA (dsRNA)-activated protein kinase PKR is a ubiquitously expressed serine/threonine protein kinase that is induced by interferon and activated by dsRNA, cytokine, growth factor and stress signals. It is essential for cells to respond adequately to different stresses including growth factor deprivation, products of the inflammatory response (TNF) and bacterial (lipopolysaccharide) and viral (dsRNA) products. As a vital component of the cellular antiviral response pathway, PKR is autophosphorylated and activated on binding to dsRNA. This results in inhibition of protein synthesis via the phosphorylation of eIF2alpha and also induces transcription of inflammatory genes by PKR-dependent signaling of the activation of different transcription factors. Along with RNaseL, PKR constitutes the antiviral arm of a group of mammalian stress response proteins that have counterparts in yeast. What began as adaptation to amino acid deprivation and sensing unfolded proteins in the endoplasmic reticulum has evolved into a family of sophisticated mammalian stress response proteins able to mediate cellular responses to both physical and biological stress.

Dissociation Between IFN-α-Induced Anti-Viral and Growth Signaling Pathways

The ability of IFN-α to induce an anti-viral state in a wide variety of cell types as well as to inhibit cellular growth has long been appreciated. It is less clear, however, whether both these effects lie downstream of a common signaling pathway. In this study we have taken advantage of an atypical human myeloma cell line (KAS-6/1) displaying a dramatic proliferative response to IFN-α in an effort to resolve the signaling requirements for IFN-α-induced anti-viral and growth regulatory effects. Thus, we have analyzed the ability of IFN-α to induce a number of known receptor-initiated events in this cell line and have compared these responses with those exhibited by a cell lineage- and maturation stage-matched myeloma cell line (ANBL-6) that displays typical IFN-α responsiveness. Despite the widely contrasting effects of IFN-α on cellular proliferation, IFN-α was shown to be comparable in its ability to induce the expression of early response genes as well as induce resistance to viral infection in both cell lines. By contrast, the effects of IFN-α on the activation of mitogen-activated protein kinase (MAPK) were strikingly distinct. Finally, although inhibition of MEK and MAPK activation had no effect on the induction of the anti-viral response, it completely blocked IFN-α-stimulated proliferation of the KAS-6/1 cells. In summary, our analysis of the role of the MAPK and anti-viral signaling pathways using these two cell lines suggests that the anti-viral and growth regulatory effects of IFN-α display a differential requirement for activation of the MAPK pathway.

Cell cycle regulation of the double stranded RNA activated protein kinase, PKR

The interferon (IFN)-induced, double stranded RNA (dsRNA)-activated serine/threonine kinase, PKR, is a potent negative regulator of cell growth when overexpressed in yeast or mammalian cells. To determine whether endogenous PKR plays a role in cell growth control, we have investigated the regulation of PKR levels and activity during the cell cycle in human glioblastoma T98G cells. The steady-state level of PKR mRNA in T98G cells was highest in growth arrested cells, dropped sharply within 3 h of serum stimulation then gradually increased as cells progressed through G1, reaching a plateau in early S phase. PKR protein level increased following serum stimulation reaching a peak at the G2+M boundary and declining thereafter. In contrast, PKR kinase activity exhibited two peaks, in early G1 and at the G1/S boundary, declining sharply in early S phase. Thus, the activity profile did not follow the protein profile indicating a tight regulation of PKR at the level of activity. In T98G cells expressing the catalytically inactive PKRK296R the dsRNA-induced activation of NF-kappaB and IRF-1 was suppressed and the mutant cells exhibited resistance to stress induced apoptosis. Cell cycle distribution analysis showed that the mutant expressing cells exhibited longer G1 phase and fewer cells engaged in S phase. Furthermore, early passage mouse embryo fibroblasts derived from PKR knockout mice grew more slowly compared with the control cells. Taken together these results suggest that PKR may play a role in cell cycle progression.

Thioredoxin reductase mediates cell death effects of the combination of beta interferon and retinoic acid

Interferons (IFNs) and retinoids are potent biological response modifiers. By using JAK-STAT pathways, IFNs regulate the expression of genes involved in antiviral, antitumor, and immunomodulatory actions. Retinoids exert their cell growth-regulatory effects via nuclear receptors, which also function as transcription factors. Although these ligands act through distinct mechanisms, several studies have shown that the combination of IFNs and retinoids synergistically inhibits cell growth. We have previously reported that IFN-beta-all-trans-retinoic acid (RA) combination is a more potent growth suppressor of human tumor xenografts in vivo than either agent alone. Furthermore, the IFN-RA combination causes cell death in several tumor cell lines in vitro. However, the molecular basis for these growth-suppressive actions is unknown. It has been suggested that certain gene products, which mediate the antiviral actions of IFNs, are also responsible for the antitumor actions of the IFN-RA combination. However, we did not find a correlation between their activities and cell death. Therefore, we have used an antisense knockout approach to directly identify the gene products that mediate cell death and have isolated several genes associated with retinoid-IFN-induced mortality (GRIM). In this investigation, we characterized one of the GRIM cDNAs, GRIM-12. Sequence analysis suggests that the GRIM-12 product is identical to human thioredoxin reductase (TR). TR is posttranscriptionally induced by the IFN-RA combination in human breast carcinoma cells. Overexpression of GRIM-12 causes a small amount of cell death and further enhances the susceptibility of cells to IFN-RA-induced death. Dominant negative inhibitors directed against TR inhibit its cell death-inducing functions. Interference with TR enzymatic activity led to growth promotion in the presence of the IFN-RA combination. Thus, these studies identify a novel function for TR in cell growth regulation.

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.

p130, p107, and pRb are differentially regulated in proliferating cells and during cell cycle arrest by alpha-interferon

We have determined how the phosphorylation of the retinoblastoma family (pRb, p107, and p130) is governed in individual cell cycle phases of Daudi B-cells during cell cycle exit triggered by alpha-interferon (alpha-IFN). alpha-IFN causes dephosphorylation of pRb and loss of p130 phosphorylated Form 3. However, the change in p130 phosphorylation in response to alpha-IFN occurs before dephosphorylation of pRb is complete because loss of p130 Form 3 occurs throughout the cell cycle prior to complete arrest in G1, whereas pRb is dephosphorylated only in G1. In contrast, p107 is dephosphorylated and is then depleted from cells as they exit the cell cycle. p130, predominantly in Form 1, and hypophosphorylated pRb bind an E2F DNA binding site; p130 complexes E2F-4, whereas pRb binds both E2F-4 and E2F-1. The phosphorylated forms of E2F-4 that bind to the E2F DNA site are different from hyperphosphorylated E2F-4, which predominates in primary hemopoietic cells in G0. We conclude that although cell cycle arrest induced by alpha-IFN may be mediated in part by formation of a complex containing p130 and E2F-4, alpha-IFN does not induce hyperphosphorylation of E2F-4, which characterizes primary hemopoietic cells in G0.

Ultrastructural nucleolar alterations induced by an ametantrone--poly r(A-U) complex

The current study has documented changes in the ultrastructure as well as in the intranucleolar distribution of rDNA and rRNA in RT4 (human transitional cell bladder carcinoma) cell nucleoli following a 3-h exposure to toxic doses of 50 microM ametantrone (AMT), 200 microM poly (adenylate-uridylate) (poly r(A-U) or an AMT/poly r(A-U) combination with an AMT/polyribonucleotide ratio of 1:4 and a poly r(A-U) concentration of 200 microM. While the main nucleolar components (fibrillar center (F), dense fibrillar component (D), granular component (G) and interstices (I) can be discerned following all treatments, the nucleoli exhibit: compaction, segregation, a decrease in the number of F, an increase in the size of remaining F, margination of intranucleolar chromatin and retention of intranucleolar pre-rRNA and rRNA. The relative abilities of the test agents to induce nucleolar compaction are AMT/poly r(A-U) > poly r(A-U) > AMT > sham-treated, while the abilities of the test agents to induce the remaining nucleolar changes are AMT/poly r(A-U) > or = AMT > poly r(A-U) > sham-treated cells. Poly r(A-U) and the induced interferon induce nucleolar compaction, while AMT produces nucleolar segregation. These results are consistent with a model in which the poly r(A-U) and/or the AMT inhibit DNA transcription and rRNA processing as well as the release of nascent preribosomes from the nucleolus.

RNA binding and modulation of PKR activity

PKR is an RNA-dependent protein kinase that is induced in mammalian cells by interferon treatment. It is present in a latent or inactive form in mammalian cells and is activated by very low concentrations of double-stranded (ds) RNA. Activated PKR phosphorylates eIF2, an essential initiation factor of protein synthesis, as well as other substrates including histone IIA, a 90-kDa protein from rabbit reticulocytes, the inhibitor, IkappaB, of the transcription factor, NF-kappaB, and the HIV-1 Tat protein. PKR interacts with several cellular and viral products and these interactions modulate its activation by dsRNA. Here we describe methods that are used to study the activation or inhibition of PKR by RNA modulators. Specifically, we detail (1) the purification of PKR from interferon-treated mammalian cells, (2) functional assays for PKR activation and inhibition in vitro, using purified enzyme or crude cell lysates, and (3) assays allowing evaluation of the binding of dsRNA and single-stranded RNA to PKR.