Evidence for involvement of protein kinase C in the cellular response to interferon alpha

Interplay between Janus Kinase/Signal Transducer and Activator of Transcription Signaling Activated by Type I Interferons and Viral Antagonism

Interferons (IFNs), which were discovered a half century ago, are a group of secreted proteins that play key roles in innate immunity against viral infection. The major signaling pathway activated by IFNs is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, which leads to the expression of IFN-stimulated genes (ISGs), including many antiviral effectors. Viruses have evolved various strategies with which to antagonize the JAK/STAT pathway to influence viral virulence and pathogenesis. In recent years, notable progress has been made to better understand the JAK/STAT pathway activated by IFNs and antagonized by viruses. In this review, recent progress in research of the JAK/STAT pathway activated by type I IFNs, non-canonical STAT activation, viral antagonism of the JAK/STAT pathway, removing of the JAK/STAT antagonist from viral genome for attenuation, and the potential pathogenesis roles of tyrosine phosphorylation-independent non-canonical STATs activation during virus infection are discussed in detail. We expect that this review will provide new insight into the understanding the complexity of the interplay between JAK/STAT signaling and viral antagonism.

Changes in Susceptibility to Oncolytic Vesicular Stomatitis Virus during Progression of Prostate Cancer

A major challenge to oncolytic virus therapy is that individual cancers vary in their sensitivity to oncolytic viruses, even when these cancers arise from the same tissue type. Variability in response may arise due to differences in the initial genetic lesions leading to cancer development. Alternatively, susceptibility to viral oncolysis may change during cancer progression. These hypotheses were tested using cells from a transgenic mouse model of prostate cancer infected with vesicular stomatitis virus (VSV). Primary cultures from murine cancers derived from prostate-specific Pten deletion contained a mixture of cells that were susceptible and resistant to VSV. Castration-resistant cancers contained a higher percentage of susceptible cells than cancers from noncastrated mice. These results indicate both susceptible and resistant cells can evolve within the same tumor. The role of Pten deletion was further investigated using clonal populations of murine prostate epithelial (MPE) progenitor cells and tumor-derived Pten(-/-) cells. Deletion of Pten in MPE progenitor cells using a lentivirus vector resulted in cells that responded poorly to interferon and were susceptible to VSV infection. In contrast, tumor-derived Pten(-/-) cells expressed higher levels of the antiviral transcription factor STAT1, activated STAT1 in response to VSV, and were resistant to VSV infection. These results suggest that early in tumor development following Pten deletion, cells are primarily sensitive to VSV, but subsequent evolution in tumors leads to development of cells that are resistant to VSV infection. Further evolution in castration-resistant tumors leads to tumors in which cells are primarily sensitive to VSV.

IMPORTANCE

There has been a great deal of progress in the development of replication-competent viruses that kill cancer cells (oncolytic viruses). However, a major problem is that individual cancers vary in their sensitivity to oncolytic viruses, even when these cancers arise from the same tissue type. The experiments presented here were to determine whether both sensitive and resistant cells are present in prostate cancers originating from a single genetic lesion in transgenic mice, prostate-specific deletion of the gene for the tumor suppressor Pten. The results indicate that murine prostate cancers are composed of both cells that are sensitive and cells that are resistant to oncolytic vesicular stomatitis virus (VSV). Furthermore, androgen deprivation led to castration-resistant prostate cancers that were composed primarily of cells that were sensitive to VSV. These results are encouraging for the use of VSV for the treatment of prostate cancers that are resistant to androgen deprivation therapy.

STAT2 phosphorylation and signaling

STAT2 is an essential transcription factor in type I IFN mediated anti-viral and anti-proliferative signaling. STAT2 function is regulated by tyrosine phosphorylation, which is the trigger for STAT-dimerization, subsequent nuclear translocation, and transcriptional activation of IFN stimulated genes. Evidence of additional STAT2 phosphorylation sites has emerged as well as novel roles for STAT2 separate from the classical ISGF3-signaling. This review aims to summarize knowledge of phosphorylation-mediated STAT2-regulation and future avenues of related STAT2 research.

Proteome-wide overexpression of host proteins for identification of factors affecting tombusvirus RNA replication: an inhibitory role of protein kinase C

To identify host genes affecting replication of Tomato bushy stunt virus (TBSV), a small model positive-stranded RNA virus, we overexpressed 5,500 yeast proteins individually in Saccharomyces cerevisiae, which supports TBSV replication. In total, we identified 141 host proteins, and overexpression of 40 of those increased and the remainder decreased the accumulation of a TBSV replicon RNA. Interestingly, 36 yeast proteins were identified previously by various screens, greatly strengthening the relevance of these host proteins in TBSV replication. To validate the results from the screen, we studied the effect of protein kinase C1 (Pkc1), a conserved host kinase involved in many cellular processes, which inhibited TBSV replication when overexpressed. Using a temperature-sensitive mutant of Pkc1p revealed a high level of TBSV replication at a semipermissive temperature, further supporting the idea that Pkc1p is an inhibitor of TBSV RNA replication. A direct inhibitory effect of Pkc1p was shown in a cell-free yeast extract-based TBSV replication assay, in which Pkc1p likely phosphorylates viral replication proteins, decreasing their abilities to bind to the viral RNA. We also show that cercosporamide, a specific inhibitor of Pkc-like kinases, leads to increased TBSV replication in yeast, in plant single cells, and in whole plants, suggesting that Pkc-related pathways are potent inhibitors of TBSV in several hosts.

Tyrosine phosphorylation of protein kinase D2 mediates ligand-inducible elimination of the Type 1 interferon receptor

Type 1 interferons (including IFNα/β) activate their cell surface receptor to induce the intracellular signal transduction pathways that play an important role in host defenses against infectious agents and tumors. The extent of cellular responses to IFNα is limited by several important mechanisms including the ligand-stimulated and specific serine phosphorylation-dependent degradation of the IFNAR1 chain of Type 1 IFN receptor. Previous studies revealed that acceleration of IFNAR1 degradation upon IFN stimulation requires activities of tyrosine kinase TYK2 and serine/threonine protein kinase D2 (PKD2), whose recruitment to IFNAR1 is also induced by the ligand. Here we report that activation of PKD2 by IFNα (but not its recruitment to the receptor) depends on TYK2 catalytic activity. PKD2 undergoes IFNα-inducible tyrosine phosphorylation on specific phospho-acceptor site (Tyr-438) within the plekstrin homology domain. Activated TYK2 is capable of facilitating this phosphorylation in vitro. Tyrosine phosphorylation of PKD2 is required for IFNα-stimulated activation of this kinase as well as for efficient serine phosphorylation and degradation of IFNAR1 and ensuing restriction of the extent of cellular responses to IFNα.

Genetic polymorphisms, their allele combinations and IFN-beta treatment response in Irish multiple sclerosis patients

INTRODUCTION

IFN-beta is widely used as first-line immunomodulatory treatment for multiple sclerosis. Response to treatment is variable (30-50% of patients are nonresponders) and requires a long treatment duration for accurate assessment to be possible. Information about genetic variations that predict responsiveness would allow appropriate treatment selection early after diagnosis, improve patient care, with time saving consequences and more efficient use of resources.

MATERIALS & METHODS

We analyzed 61 SNPs in 34 candidate genes as possible determinants of IFN-beta response in Irish multiple sclerosis patients. Particular emphasis was placed on the exploration of combinations of allelic variants associated with response to therapy by means of a Markov chain Monte Carlo-based approach (APSampler).

RESULTS

The most significant allelic combinations, which differed in frequency between responders and nonresponders, included JAK2-IL10RB-GBP1-PIAS1 (permutation p-value was p(perm) = 0.0008), followed by JAK2-IL10-CASP3 (p(perm) = 0.001).

DISCUSSION

The genetic mechanism of response to IFN-beta is complex and as yet poorly understood. Data mining algorithms may help in uncovering hidden allele combinations involved in drug response versus nonresponse.

Interferon-alpha induces transient upregulation of c-FLIP through NF-kappaB activation

Interferon-alpha (IFN-alpha) induces apoptosis in some cell types and promotes cell survival in other cell types, but the molecular mechanisms underlying distinct IFN-alpha-induced cell behaviours remain poorly understood. In the present study, we show that IFN-alpha induced the cellular FLICE (FADD-like interleukin-1 beta-converting enzyme) inhibitory protein (c-FLIP), which serves as a promoter of cell survival in human B lymphoma cells. IFN-alpha induction of transient upregulation of c-FLIP was partially abrogated by the NF-kappaB inhibitor BAY11-7082 (BAY). Pretreatment with BAY sensitized both Daudi and U266 cells to the IFN-alpha-induced loss of mitochondrial membrane potential (DeltaPsi(m)). IFN-alpha phosphorylated the PKC isoform PKCalpha at a threonine residue, and the PKCalpha/betaI inhibitor Go6976 abrogated upregulation of IFN-alpha-induced NF-kappaB activity, leading to sensitization of cells to IFN-alpha-induced apoptosis. To analyze the role of PKCalpha in the IFN-alpha-induced signaling, Daudi cells overexpressing a constitutively active mutant of PKCalpha (caPKCalpha) were used. The caPKCalpha-expressing Daudi cells were partially resistant to the IFN-alpha-induced loss of DeltaPsi(m), concomitant with elevated levels of c-FLIP protein. Together, these results demonstrate that IFN-alpha causes a transient upregulation of c-FLIP expression, at least through PKCalpha-mediated activation of NF-kappaB. The balance between IFN-alpha-induced pro-apoptotic and survival signals determines the cell fate. Thus, therapeutic intervention in this balance may be effective for treatment of patients with IFN-alpha-refractory tumours.

Interferon alpha activates NF-kappaB in JAK1-deficient cells through a TYK2-dependent pathway

In addition to activating members of the STAT transcription factor family, interferon alpha/beta (IFNalpha/beta) activates the NF-kappaB transcription factor. To determine the role of the Janus tyrosine kinase (JAK)-STAT pathway in NF-kappaB activation by IFN, we examined NF-kappaB activation in JAK1-deficient mutant human fibrosarcoma cells. In wild-type fibrosarcoma cells (2fTGH), IFN activates STAT1, STAT2, and STAT3, as well as NF-kappaB complexes comprised of p50 and p65. In contrast, in JAK1-deficient cells, IFN induces NF-kappaB activation and NF-kappaB dependent gene transcription but does not activate these STAT proteins and has no effect on STAT-dependent gene transcription. Expression of a catalytically inactive TYK2 tyrosine kinase in JAK1-deficient cells, as well as in the highly IFN-sensitive Daudi lymphoblastoid cell line, abrogates NF-kappaB activation by IFN. Moreover, IFN does not promote NF-kappaB activation in TYK2-deficient mutant fibrosarcoma cells. Our results demonstrate a dichotomy between the classical JAK-STAT pathway and the NF-kappaB signaling pathway. In the IFN signaling pathway leading to STAT activation, both JAK1 and TYK2 are essential, whereas NF-kappaB activation requires only TYK2.

Differential STAT5 signaling by ligand-dependent and constitutively active cytokine receptors

Many leukemia and cancer cells exhibit constitutive activation of STAT5, which was suggested to provide an anti-apoptotic advantage. Transformation of cytokine-dependent hematopoietic cells, such as Ba/F3 cells to autonomous growth and tumorigenicity equally results in selection for constitutive activation of STAT5. We compared STAT5 signaling between erythropoietin(Epo)-dependent cells and cells that were transformed by oncogenic activation of the erythropoietin receptor (EpoR) by coexpression of the gp55-P envelope protein of the spleen focus forming virus or by expression of the R129C constitutively active EpoR mutant. In transformed cells it was mainly STAT5B that was constitutively activated. In contrast, Epo stimulation activated both STAT5A and STAT5B. In transformed cells, chromatin immunoprecipitation (ChIP) showed STAT5 to be physically bound to promoters of STAT5 target genes, such as Bcl(XL), and to be able to promote transactivation of the Bcl(XL) promoter in a constitutive fashion. Sequencing of native sequences after ChIP with anti-STAT5 antibodies in Epo-dependent and -transformed cells indicated that in gp55-transformed cells, STAT5B bound in the chromatin not only to N3 high affinity, but also to low affinity N4 GAS sites. Transactivation for N3 GAS sites in luciferase reporters was specific to gp55 transformation. Because we also found preferential constitutive STAT5B activation after transformation of cells by a truncated form of the G-CSF-R that produces severe neutropenia (Kostmann syndrome) and favors leukemia in humans, we discuss the potential role of STAT5B in oncogenic transformation of hematopoietic cells.

Interferon alpha /beta promotes cell survival by activating nuclear factor kappa B through phosphatidylinositol 3-kinase and Akt

Interferons (IFNs) play critical roles in host defense by modulating gene expression via activation of signal transducer and activator of transcription (STAT) factors. IFN-alpha/beta also activates another transcription factor, nuclear factor kappaB (NF-kappaB), which protects cells against apoptotic stimuli. NF-kappaB activation requires the IFN-dependent association of STAT3 with the IFNAR1 chain of the IFN receptor. IFN-dependent NF-kappaB activation involves the sequential activation of a serine kinase cascade involving phosphatidylinositol 3-kinase (PI-3K) and Akt. Whereas constitutively active PI-3K and Akt induce NF-kappaB activation, Ly294002 (a PI-3K inhibitor), dominant-negative PI-3K, and kinase-dead Akt block IFN-dependent NF-kappaB activation. Moreover, dominant-negative PI-3K blocks IFN-promoted degradation of kappaBox alpha. Ly294002, a dominant-negative PI-3K construct, and kinase-dead Akt block IFN-promoted cell survival, enhancing apoptotic cell death. Therefore, STAT3, PI-3K, and Akt are components of an IFN signaling pathway that promotes cell survival through NF-kappaB activation.

MHC class I gene expression and regulation

Major histocompatibility complex (MHC) is a conglomerate of genes that play an important role in recognition of self and nonself. These genes are under tight control. In this review we have discussed the transcription processes regulating MHC gene expression. Various biological or chemical modulators can modulate MHC gene expression. The promoter region of class I genes can be activated through several pathways. Hence, these genes are not typical "domestic" genes. Extensive studies on regulation of MHC class I expression, using transfection techniques and transgenic animal models, have resulted in identification of various cis-acting sequences involved in positive and negative regulation of class I genes. Work is in progress to identify the transacting proteins that bind to these sites and to delineate the mechanisms that regulate constitutive and inducible expression of class I genes in normal and diseased cells. It has been seen that various biological molecules (IFN, GM-CSF, IL-2) and other chemicals up-regulate the MHC expression. If the exact mechanisms are known by which the expression of class I genes is up regulated, the efforts can be made to balance the beneficial and toxic effects of biological molecules with one another, which may facilitate the use of combination of these molecules in subpharmacological doses (to eliminate toxicity) for early and better management of neoplastic diseases, as it is well-known that during malignancy MHC gene expression is down-regulated. In the future, the use of transgenic and knockout mice will be useful in acquiring a better understanding, which may further help in cancer therapy.

Receptor for activated C-kinase (RACK-1), a WD motif-containing protein, specifically associates with the human type I IFN receptor

The cytoplasmic domain of the human type I IFN receptor chain 2 (IFNAR2c or IFN-alphaRbetaL) was used as bait in a yeast two-hybrid system to identify novel proteins interacting with this region of the receptor. We report here a specific interaction between the cytoplasmic domain of IFN-alphaRbetaL and a previously identified protein, RACK-1 (receptor for activated C kinase). Using GST fusion proteins encoding different regions of the cytoplasmic domain of IFN-alphaRbetaL, the minimum site for RACK-1 binding was mapped to aa 300-346. RACK-1 binding to IFN-alphaRbetaL did not require the first 91 aa of RACK-1, which includes two WD domains, WD1 and WD2. The interaction between RACK-1 and IFN-alphaRbetaL, but not the human IFN receptor chain 1 (IFNAR1 or IFN-alphaRalpha), was also detected in human Daudi cells by coimmunoprecipitation. RACK-1 was shown to be constitutively associated with IFN-alphaRbetaL, and this association was not effected by stimulation of Daudi cells with type I IFNs (IFN-beta1b). RACK-1 itself did not become tyrosine phosphorylated upon stimulation of Daudi cells with IFN-beta1b. However, stimulation of cells with either IFN-beta1b or PMA did result in an increase in detectable immunofluorescence and intracellular redistribution of RACK-1.

Potential role for protein kinases in regulation of bidirectional endoplasmic reticulum-to-Golgi transport revealed by protein kinase inhibitor H89

Recent evidence suggests a regulatory connection between cell volume, endoplasmic reticulum (ER) export, and stimulated Golgi-to-ER transport. To investigate the potential role of protein kinases we tested a panel of protein kinase inhibitors for their effect on these steps. One inhibitor, H89, an isoquinolinesulfonamide that is commonly used as a selective protein kinase A inhibitor, blocked both ER export and hypo-osmotic-, brefeldin A-, or nocodazole-induced Golgi-to-ER transport. In contrast, H89 did not block the constitutive ER Golgi-intermediate compartment (ERGIC)-to-ER and Golgi-to-ER traffic that underlies redistribution of ERGIC and Golgi proteins into the ER after ER export arrest. Surprisingly, other protein kinase A inhibitors, KT5720 and H8, as well as a set of protein kinase C inhibitors, had no effect on these transport processes. To test whether H89 might act at the level of either the coatomer protein (COP)I or the COPII coat protein complex we examined the localization of betaCOP and Sec13 in H89-treated cells. H89 treatment led to a rapid loss of Sec13-labeled ER export sites but betaCOP localization to the Golgi was unaffected. To further investigate the effect of H89 on COPII we developed a COPII recruitment assay with permeabilized cells and found that H89 potently inhibited binding of exogenous Sec13 to ER export sites. This block occurred in the presence of guanosine-5'-O-(3-thio)triphosphate, suggesting that Sec13 recruitment is inhibited at a step independent of the activation of the GTPase Sar1. These results identify a requirement for an H89-sensitive factor(s), potentially a novel protein kinase, in recruitment of COPII to ER export sites, as well as in stimulated but not constitutive Golgi-to-ER transport.

Transcriptional induction of p69 2'-5'-oligoadenylate synthetase by interferon-alpha is stimulated by 12-O-tetradecanoyl phorbol-13-acetate through IRF/ISRE binding motifs

Protein kinase C (PKC) is required for transcriptional induction of 2'-5'-oligoadenylate (2-5A) synthetases by interferon (IFN)-alpha. Regulatory elements located in the 5'-flanking region of the p69 2-5A synthetase gene have been identified which are required for transcriptional stimulation by PKC. The region from -366 to -117 bp, relative to the translational start site, contains three sequence motifs that resemble interferon stimulated response elements/interferon regulatory factor elements (ISRE/IRF-E), which are required for stimulation of the IFN-alpha-response by the PKC activator, 12-O-tetradecanoyl phorbol-13-acetate (TPA). Constructs which have a deletion of the region containing IRF-Es located at -361 bp to -280 and at -246 to -172 bp do not respond to TPA treatment. Likewise, introduction of point mutations into either of these IRF-Es decreases stimulation of IFN-alpha induction by TPA and constructs containing point mutations in both upstream IRF-Es are nonresponsive to TPA. Binding of the inducible factor to the ISRE is abrogated in cells depleted of PKC by prolonged treatment with TPA. PKC appears to function as a signaling component in an IFN-independent pathway that increases the activity of IFN-alpha-regulated transcription factors in the nucleus.

Effects of interferons on cortisol production in bovine adrenal fasciculata cells stimulated by adrenocorticotropin

The effects of interferons (IFNs) IFN-alpha, IFN-beta and IFN-gamma on the production of cortisol in bovine adrenal fasciculata cells have been investigated. Pretreatment of the fasciculata cells with recombinant interferon-alpha-2b from man (over 300 units mL(-1)), but not with fibroblast IFN-beta or recombinant IFN-gamma from man, reduced the production of cortisol in cells stimulated with adrenocorticotropin (ACTH) (1 nM). IFN-alpha-2b inhibited ACTH-induced cortisol production in a concentration- (300-15000 units mL(-1)) and time- (2-24h) dependent manner. The inhibitory effect of IFN-alpha-2b on the production was abolished when the cells were simultaneously treated with anti-IFN-alpha antibody, and it was reversible. IFN-alpha-2b also inhibited dibutyryl cyclic AMP-induced production of cortisol but not pregnenolone-induced production. The effect of IFN-alpha-2b was not influenced by increases in external ACTH and Ca2+ concentrations and IFN-alpha-2b did not affect the ACTH-induced increase in cyclic AMP level in the cells. These results strongly suggest that IFN-alpha-2b reduces ACTH-induced production of cortisol in bovine adrenal fasciculata cells by affecting the early process of cortisol synthesis. The results also indicate that IFNs might not directly affect steroidogenesis in the adrenal cortex in-vivo, because of the requirement of high concentrations of IFN-alpha-2b for inhibition, and because of the ineffectiveness of IFN-beta and IFN-gamma.

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.

Adenovirus induction of an interferon-regulatory factor during entry into the late phase of infection

Virus infection of animal cells can induce intracellular antiviral responses mediated by the induction of interferon-regulatory transcription factors (IRFs), which bind to and control genes directed by the interferon-stimulated response element (ISRE). The purpose of this study was to determine whether adenovirus (Ad) induces IRFs during infection, because they might play a role in promoting viral pathogenesis. Here we show that after the late phase of infection, Ad induces a transcription factor related to the IRF family of factors. The IRF is induced shortly after Ad entry into late phase and is shown to stimulate ISRE-directed transcription, to require activation by protein tyrosine kinase signalling, and to be induced several hours prior to the inhibition of cell protein synthesis. Inhibition of tyrosine kinase activity blocks Ad induction and activation of the IRF. Attempts to identify the Ad-induced factor immunologically and by photo-UV cross-linking indicate that it is likely a novel member of the IRF family. Finally, several independent lines of evidence also suggest that Ad induction of the IRF might correlate with the ability of the virus to block host cell protein synthesis later during infection.

Complex induction of bovine uterine proteins by interferon-tau

Interferon-tau (IFN-tau) is released by the conceptus and induces two uterine proteins during early pregnancy: ubiquitin cross-reactive protein (UCRP) and granulocyte chemotactic protein-2 (GCP-2). The present experiments were designed to determine whether detection (Western blot) of cytosolic UCRP and release of GCP-2 could be used to examine IFN-tau signal transduction in cultured endometrial explants and primary epithelial cells. Recombinant (r) type 1 IFNs (rboIFN-tau and rboIFN-alpha; 5, 25, 100 nM) induced UCRP, but only rboIFN-tau induced GCP-2 in explant culture. Recombinant boIFN-tau and conceptus secretory proteins containing native IFN-tau induced UCRP and GCP-2 in cultured primary epithelial cells. All concentrations of rboIFN-alpha (25, 50, 100 nM) induced UCRP, but only the highest concentration induced GCP-2 in cultured primary epithelial cells. Interestingly, phorbol ester (100, 500, 1000 ng/ml) induced GCP-2, but it had no effect on UCRP. Because type 1 IFNs induce UCRP, IFN-tau probably interacts with the janus kinase (Jak)-associated IFN-alpha receptor to phosphorylate signal transducers and activators of transcription (STAT) and/or interferon regulatory factor-1 (IRF-1). However, IFN-tau-specific induction of GCP-2 may involve a variant type 1 receptor subunit or activators of transcription that are associated with protein kinase C and the Jak/STAT/IRF-1 pathway.

STAT3 complements defects in an interferon-resistant cell line: evidence for an essential role for STAT3 in interferon signaling and biological activities

STAT proteins play critical roles in the signal transduction pathways for various cytokines. The type I interferons (IFNalpha/beta) promote the DNA-binding activity of the transcription factors STAT1, STAT2, and STAT3. Although the requirement for STAT1 and STAT2 in IFNalpha/beta signaling and action is well documented, the biological importance of STAT3 to IFN action has not yet been addressed. We found that STAT3 plays a critical role in signal transduction by IFNalpha/beta. A human cell line that is resistant to the antiviral and antiproliferative activities of IFN but is still IFN-responsive by virtue of STAT1 and STAT2 activation was found to be defective in STAT3 activation and in induction of NF-kappaB DNA-binding activity. Expression of STAT3 in these resistant cells complemented these signaling defects and also markedly increased cellular sensitivity to the antiviral and antiproliferative effects of IFN. Because STAT3 is involved in the induction of NF-kappaB DNA-binding activity and in the induction of antiviral and antiproliferative activity, our results place STAT3 as an important upstream element in type I IFN signal transduction and in the induction of biological activities. Therefore, our results indicate that STAT1 and STAT2 are not the only STATs required for the expression of the key biological activities of IFNalpha/beta.

Protein kinase C is required for induction of 2',5'-oligoadenylate synthetases

Induction of the p40/46 and p69/71 isoforms of the 2',5'-oligoadenylate (2-5A) synthetase by interferon-alpha (IFN-alpha) is variable among six different Burkitt lymphoma cell lines with Ramos cells expressing among the highest levels of these enzymes. Inhibitors of protein kinase C (PKC) block induction of mRNAs encoding both isoforms; however, induction of the p69/71 isoform is more sensitive to these inhibitors. Down-regulation of PKC by prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) also blocks induction of 2-5A synthetase mRNAs and decreases both constitutive and IFN-alpha-induced enzymatic activity. Cotreatment of cells with TPA and IFN-alpha increases induction of 2-5A synthetase mRNAs above that seen in cells treated with IFN-alpha alone. IFN-alpha does not directly activate PKC-alpha or PKC-delta, the two most abundant PKC isoforms present in Ramos cells, suggesting that PKC activation by another signaling pathway is necessary for maximal induction of 2-5A synthetases by IFN-alpha.

STAT3 as an adapter to couple phosphatidylinositol 3-kinase to the IFNAR1 chain of the type I interferon receptor

STAT (signal transducers and activators of transcription) proteins undergo cytokine-dependent phosphorylation on serine and tyrosine. STAT3, a transcription factor for acute phase response genes, was found to act as an adapter molecule in signal transduction from the type I interferon receptor. STAT3 bound to a conserved sequence in the cytoplasmic tail of the IFNAR1 chain of the receptor and underwent interferon-dependent tyrosine phosphorylation. The p85 regulatory subunit of phosphatidylinositol 3-kinase, which activates a series of serine kinases, bound to phosphorylated STAT3 and subsequently underwent tyrosine phosphorylation. Thus, STAT3 acts as an adapter to couple another signaling pathway to the interferon receptor.

Modulation of Apo-1/Fas (CD95)-induced programmed cell death in myeloma cells by interferon-alpha 2

The Apo-1/Fas (CD95) antigen is known to be involved in the process of T cell-mediated target cell killing and has recently been shown to be expressed on myeloma cell lines and native malignant plasma cells. Several cytokines have been reported to interfere with spontaneous and even Apo-1/Fas-induced apoptosis, but no attempt has been made yet to investigate these interactions and the possible underlying mechanisms in myeloma cells. Since in myeloma patients Interferon (IFN)-alpha2 displays a profound therapeutic effect in vivo, which is usually attributed to its growth inhibitory and/or immunomodulatory capacity, we set out to study the potential interference of IFN-alpha2 with Apo-1/Fas-induced apoptosis. Contrary to expectations, IFN-alpha2 reduced the degree of apoptosis caused by the treatment of five Apo-1/Fas-sensitive myeloma cell lines with a Fas monoclonal antibody (mAb). Simultaneous application of IFN-alpha2 and Fas mAb was superior to the prolonged (i.e. >8 h) preincubation with the cytokine as far as inhibition of Apo-1/Fas-induced apoptosis was concerned. This effect of IFN-alpha2 was neither explained by a down-regulation of the Apo-1/Fas receptor nor caused by modulation of the expression levels of c-myc, bcl-2-, bcl-xL, bax- or p53 genes. IFN-alpha2 did not alter the Apo-1/Fas-induced activity of Mitogen-activated protein kinase (MAPK) 1 and did not inhibit the Apo-1/Fas-mediated proteolytic cleavage of ADP-ribosyltransferase, a substrate of Interleukin-beta1 converting enzyme (ICE) and homologues. However, activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) mimicked the effects of IFN-alpha2. Furthermore, the bis-indolylmaleimide GF 109203X, a specific inhibitor of PKC, inhibited the effect of PMA as well as that of IFN-alpha2 on Apo-1/Fas-induced apoptosis. These results point to a PKC-dependent mechanism of transient interaction between the intracellular signaling along the IFN-alpha2 and the Apo-1/Fas pathway (downstream of MAPK signaling as well as of ICE homologues), which becomes exhausted by prolonged stimulation with the cytokine. According to our data IFN-alpha2, applied continuously and in high doses resembling the therapeutic situation in vivo, inhibits myeloma growth. However, based on the observed inhibitory effect of IFN-alpha2 on Apo-1/Fas-induced apoptosis, a partial inhibition of the natural immune surveillance on myeloma cells by endogenous IFN-alpha2 present in the bone marrow microenvironment of this malignancy should be investigated.

Regulation of interferon-gamma-activated STAT1 by the ubiquitin-proteasome pathway

STAT proteins (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that are phosphorylated by Janus kinases in response to cytokines. Phosphorylated STAT proteins translocate to the nucleus, where they transiently turn on specific sets of cytokine-inducible genes. The mechanism that controls the amounts of activated STAT proteins is not understood. STAT1 proteins activated by interferon-gamma treatment in HeLa cells were shown to be stabilized by a proteasome inhibitor and ubiquitinated in vivo. Thus, the amount of activated STAT1 may be negatively regulated by the ubiquitin-proteasome pathway.

Activation of protein kinase A inhibits interferon induction of the Jak/Stat pathway in U266 cells

Activation of early response genes by interferons (IFNs) requires tyrosine phosphorylation of the Stat transcription factors and is mediated by the Jak family of tyrosine kinases. Recent evidence suggests that ERK2 serine/threonine kinase modulates the IFN-stimulated Jak/Stat pathway. In this report we show that in the myeloma cell line U266 protein kinase A specifically interacts with the cytoplasmic domain of the IFNalpha/beta receptor. Treatment of cells with the adenylate cyclase activator forskolin inhibits IFNbeta-, IFNgamma-, and hydrogen peroxide/vanadate-induced formation of complexes that bind to enhancers known to stimulate the expression of IFN-regulated genes. Immunoprecipitations followed by anti-phosphotyrosine immunoblots indicate that tyrosine phosphorylation of the alpha chain of the IFNalpha/beta receptor, Jak1, Tyk2, as well as Stat1 and Stat2 is reduced as a consequence of incubation of cells with forskolin. In contrast, dideoxyforskolin, which fails to activate adenylate cyclase, has no effect on IFN induction of the Jak/Stat pathway. These results indicate a novel regulatory mechanism by which protein kinase A can modulate the Jak/Stat signaling cascade.

Involvement of an arachidonic-acid-dependent pathway in the interferon-beta-mediated expression of C202 gene in Ehrlich-ascites-tumor cells

We have investigated the signal transduction mechanism of the expression of the C202 gene mediated by interferon beta (IFN-beta) in the murine Ehrlich's ascites tumor cell line. We have shown that treatment of cells with IFN-beta transiently enhances within minutes the release of free arachidonic acid through membrane phospholipase activity. Furthermore, prior treatment with either p-bromophenacyl bromide, an antagonist of both cytosolic and secretory phospholipase A2, or neomycin, which blocks phospholipase C activity, significantly decreased the activation of the murine IFN-beta-inducible gene, C202. Moreover, an increase of the expression of the C202 gene was observed after blocking of both the cyclooxygenase and lipoxygenase pathways. This suggests that further metabolism of arachidonic acid to epoxides via epoxygenase-catalysed pathways may be a mechanism by which second messengers for IFN-beta-mediated effects on C202 gene expression are generated. Taken together, these results indicate that lipids as second messengers may be important mediators in the IFN-beta-based activation of C202 gene expression.

Requirement for MAP kinase (ERK2) activity in interferon alpha- and interferon beta-stimulated gene expression through STAT proteins

Activation of early response genes by interferons (IFNs) requires tyrosine phosphorylation of STAT (signal transducers and activators of transcription) proteins. It was found that the serine-threonine kinase mitogen-activated protein kinase (MAPK) [specifically, the 42-kilodalton MAPK or extracellular signal-regulated kinase 2 (ERK2)] interacted with the alpha subunit of IFN-alpha/beta receptor in vitro and in vivo. Treatment of cells with IFN-beta induced tyrosine phosphorylation and activation of MAPK and caused MAPK and Stat1 alpha to coimmunoprecipitate. Furthermore, expression of dominant negative MAPK inhibited IFN-beta-induced transcription. Therefore, MAPK appears to regulate IFN-alpha and IFN-beta activation of early response genes by modifying the Jak-STAT signaling cascade.

Inhibition of protein phosphorylation modulates expression of the Jak family protein tyrosine kinases

Treatment of murine Friend cells with a dose of the protein kinase inhibitor staurosporine, which is able to block the response of the cells to interferons, appears to inhibit phosphorylation of Jak proteins and, interestingly, to specifically reduce tyk2 and Jak1 expression and to increase Jak2 both in the presence and in the absence of interferons. Therefore, a potential role for phosphorylation events in the regulation of expression of the Jak family members is suggested.

Cellular protein kinase C isoform zeta regulates human parainfluenza virus type 3 replication

Phosphorylation of the P proteins of nonsegmented negative-strand RNA viruses is critical for their function as transactivators of the viral RNA polymerases. Using unphosphorylated P protein of human parainfluenza virus type 3 (HPIV3) expressed in Escherichia coli, we have shown that the cellular protein kinase that phosphorylates P in vitro is biochemically and immunologically indistinguishable from cellular protein kinase C isoform zeta (PKC-zeta). Further, PKC-zeta is specifically packaged within the progeny HPIV3 virions and remains tightly associated with the ribonucleoprotein complex. The P protein seems also to be phosphorylated intracellularly by PKC-zeta, as shown by the similar protease digestion pattern of the in vitro and in vivo phosphorylated P proteins. The growth of HPIV3 in CV-1 cells is completely abrogated when a PKC-zeta-specific inhibitor pseudosubstrate peptide was delivered into cells. These data indicate that PKC-zeta plays an important role in HPIV3 gene expression by phosphorylating P protein, thus providing an opportunity to develop antiviral agents against an important human pathogen.

Role of protein kinase C in the inhibitory action of trophoblast interferons on expression of the oxytocin receptor in sheep endometrium

PhosphoIipid/Ca(2+) -dependent protein kinase C (PKC) and oxytocin receptor were measured in sheep endometrial explants after culture for up to 96 h. Oxytocin receptor binding and PKC activity were reduced by up to 90% in explants exposed to recombinant ovine trophoblast interferon (rolFN-τ), recombinant bovine IFN-α(1) or ovine conceptus secretory proteins (a source of IFN-τ). Inhibition occurred in both caruncular and intercaruncular endometrium taken between days 7 and 10 of the oestrous cycle and in intercaruncular (but not caruncular) endometrium on day 6. Down-regulation of PKC by continued exposure of expiants to 4β-phorbol myristate acetate, or treatment with PKC inhibitors reduced both oxytocin receptor binding and PKC activity by up to 70%. Tyrosine kinase inhibitors were ineffective. Addition of oxytocin or progesterone, which reduce oxytocin receptor bindingin vivo, also lowered oxytocin receptor bindingin vitro in the absence of any effect on PKC. The data indicate that IFN-τ inhibits oxytocin receptor synthesis by a mechanism involving PKC inhibition, but that a non-PKC pathway also operates to control oxytocin receptor binding in non-pregnant animals. These conclusions were supported by measuring PKC activity and oxytocin receptor binding in endometrium without culture. Prolonged exposure of the endometrium to IFN-τin vivo may lead to PKC down regulation by a mechanism analogous to that involved in the action of continuous activation by agonist, and this may represent one function of the prolonged secretion of IFN-τ over a 10-day period in early pregnancy.

Signal transduction and activation of gene transcription by interferons

Advances in the field of interferon research have identified a signal transduction pathway that initiates at a cell surface receptor and culminates at target genes in the nucleus. The binding of interferon to a transmembrane receptor stimulates the concomitant activation of tyrosine kinases of the Janus kinase (JAK) family. Subsequently, latent cytoplasmic transcription factors are activated by tyrosine phosphorylation and function as signal transducers and activators of transcription (STATs). The STATs form homomeric or heteromeric protein complexes that translocate to the nucleus to bind to specific DNA sequences in the promoters of stimulated genes. The discovery of this regulated pathway in the interferon system served as a paradigm for receptor to nucleus signal transmission by a variety of cytokines.

Interferon alpha and intracytoplasmic free calcium in hairy cell leukemia cells

Hairy cell leukemia (HCL) is a B-cell tumor affecting the pre-plasma stage of B cell differentiation. One of the most striking characteristics of this disease is its remarkable responsiveness to alpha-interferon (IFN-alpha) therapy. Interferons constitute a heterologous family of multifunctional cytokines displaying anti-viral, anti-proliferative and immunoregulatory properties. These activities have been extensively studied in hairy cells, but the mechanism of action of IFN-alpha in hairy cell leukemia remains unknown. Our approach to investigate the mode action of IFN-alpha in HCL has been to identify abnormalities which occur in these tumor cells and then to ascertain whether these abnormalities can be rectified by IFN-alpha treatment. A high level of free Ca2+ in the cytoplasm of hairy cells was identified. Increases in cytosolic Ca2+ are believed to be a pivotal signal in regulating cell proliferation, cell differentiation and cell death. These high Ca2+ levels in hairy cells could be reduced upon treatment with IFN-alpha either in vitro or in vivo, probably acting by reducing Ca2+ influx into the leukemic cells. Moreover, the effect of IFN-alpha on [Ca2+]i seems to be correlated with down-regulation of CD20 phosphorylation, a B cell specific phosphoprotein involved in Ca2+ influx across the plasma membrane. The possible origins and implications of Ca2+ deregulation and the possible mechanisms or sites of action of IFN-alpha in tumor cells from HCL are explored in this review.

Biologic agents as biochemical modulators: pharmacologic basis for the interaction of cytotoxic chemotherapeutic drugs and interferon

Biochemical modulation of cytotoxic cancer chemotherapeutic agents is one means of enhancing the activity and selectivity of antitumor drugs. Traditionally this approach has utilized detailed information regarding a particular enzymatic reaction or biochemical pathway to develop potential modulating agents. In contrast, the reported clinical therapeutic activity of IFN in combination with cytotoxic agents has prompted a reexamination of the biochemical actions of the cytokine. Interferon elicits a number of cellular actions that might contribute to its pharmacologic activity, including both direct antitumor effects and host-mediated actions. The best understood are those related to the cytotoxicity of the fluoropyrimidine antimetabolites and include enzymatic reactions involved in fluoropyrimidine metabolic activation, catabolism, and interaction with its target enzyme. However, even in this instance, a mechanistic association of a specific pharmacologic action with therapeutic activity remains to be determined. These studies demonstrate that cytokines and other biologic agents may exert specific biochemical modulations that augment (or potentially attenuate) the activity of the cytotoxic chemotherapeutic agents.

Transcription termination/polyadenylation occurs at multiple sites in the human type I interferon receptor gene

Based on the previously reported sequence, we isolated an independent cDNA clone encoding a binding component of the human type I interferon receptor (IFN-R). This cDNA is identical to the published sequence except that it lacks 62 bases of 5' untranslated sequence and terminates at the first of two potential polyadenylation sites. In Northern blot analyses of poly(A)+RNAs from both IFN-sensitive and IFN-resistant Daudi cells, this cloned cDNA hybridized to a predominant mRNA of 2.4 kb, as well as to mRNAs of 1.8, 4.8, and 5.6 kb, and occasionally 6.9 kb. These various transcripts, which were also observed at similar levels in Raji B cells and two T-cell lines, Jurkat and MOLT-4, were detected after high-stringency washes, and by alternate probes corresponding to subfragments of the cDNA. In contrast, only the 4.8- and 5.6-kb transcripts hybridized to a polymerase chain reaction (PCR)-derived probe that corresponded to genomic sequences immediately down-stream from the second polyadenylation site. These results indicate that the latter transcripts arise from the same gene as the predominant 2.4-kb mRNA due to incomplete transcription termination at either of the known polyadenylation sites. Finally, Northern blot analysis of total RNAs revealed the presence of the predominant 2.4-kb type I IFN-R transcript in numerous tissues from second trimester human fetuses, suggesting that the type I IFN-R gene is constitutively expressed in multiple cell types.

Interferon alpha induces protein kinase C-epsilon (PKC-epsilon) gene expression and a 4.7-kb PKC-epsilon-related transcript

Protein kinases play key roles in the induction by human interferon alpha (IFN-alpha) of specific gene expression and biological activity in various human cell lines. We now report that IFN-alpha increased the 7-kb transcript for the epsilon isotype of protein kinase C (PKC-epsilon) and the cellular content of PKC-epsilon 24 and 48 hr after IFN-alpha addition (a 2-fold and 6-fold increase, respectively). Furthermore, IFN-alpha markedly induced a 4.7-kb transcript that hybridized to a PKC-epsilon-specific, but not to a PKC-eta-specific, cDNA probe. The induction of the 4.7-kb PKC-epsilon-related mRNA by IFN-alpha had the following properties reported for the classical IFN-alpha-stimulated genes: rapid kinetics of induction, high maintained levels in IFN-alpha-sensitive but not in IFN-alpha-resistant cell lines, protein synthesis-independent induction, and high sensitivity to inhibitors of protein tyrosine kinase activity. These results show that the regulation of gene expression by IFN-alpha include not only the classical IFN-alpha-stimulated genes but also the coordinated regulation of two PKC-epsilon-related transcripts that appeared to be highly relevant to the biological actions of IFN-alpha.

Double-stranded RNA activates novel factors that bind to the interferon-stimulated response element

Infection of cells with adenovirus or transfection of cells with double-stranded RNA (dsRNA) activates transcription of the alpha/beta interferon-stimulated genes (ISGs). Induction of ISG expression by adenovirus appears to be mediated through the same DNA target that is responsive to alpha/beta interferons, the interferon-stimulated response element (ISRE). Transcriptional induction by alpha/beta interferons has been shown previously to be mediated by the activation of a latent cytoplasmic transcription factor, ISGF3, that translocates to the nucleus and binds to the ISRE. However, ISG expression induced by adenovirus or dsRNA appears to be mediated by unique dsRNA-activated factors (DRAFs) that bind to the ISRE. The activation of these preexisting factors by dsRNA does not require new protein synthesis. Two DRAFs, DRAF1 and DRAF2, have been identified in our studies as ISRE-binding complexes in gel mobility shift assays. The ISRE-binding specificity of DRAF1 is similar to that of ISGF3; however, the ISRE-binding specificity of DRAF2 is distinct. Activation of DRAF1 and DRAF2 is independent of interferon action since it occurs in cells that are nonresponsive to interferon and in cells that lack the alpha/beta interferon locus. The activation pathway of DRAF1 and DRAF2 is blocked by the protein kinase inhibitors staurosporine and genistein. This is analogous to the interferon signal transduction pathway and suggests that phosphorylation, possibly tyrosine phosphorylation, is involved in activation of these factors.

Double-stranded RNA activates novel factors that bind to the interferon-stimulated response element

Infection of cells with adenovirus or transfection of cells with double-stranded RNA (dsRNA) activates transcription of the alpha/beta interferon-stimulated genes (ISGs). Induction of ISG expression by adenovirus appears to be mediated through the same DNA target that is responsive to alpha/beta interferons, the interferon-stimulated response element (ISRE). Transcriptional induction by alpha/beta interferons has been shown previously to be mediated by the activation of a latent cytoplasmic transcription factor, ISGF3, that translocates to the nucleus and binds to the ISRE. However, ISG expression induced by adenovirus or dsRNA appears to be mediated by unique dsRNA-activated factors (DRAFs) that bind to the ISRE. The activation of these preexisting factors by dsRNA does not require new protein synthesis. Two DRAFs, DRAF1 and DRAF2, have been identified in our studies as ISRE-binding complexes in gel mobility shift assays. The ISRE-binding specificity of DRAF1 is similar to that of ISGF3; however, the ISRE-binding specificity of DRAF2 is distinct. Activation of DRAF1 and DRAF2 is independent of interferon action since it occurs in cells that are nonresponsive to interferon and in cells that lack the alpha/beta interferon locus. The activation pathway of DRAF1 and DRAF2 is blocked by the protein kinase inhibitors staurosporine and genistein. This is analogous to the interferon signal transduction pathway and suggests that phosphorylation, possibly tyrosine phosphorylation, is involved in activation of these factors.

Differentiation-dependent activation of interferon-stimulated gene factors and transcription factor NF-kappa B in mouse embryonal carcinoma cells

We have recently shown that the adenovirus E1A gene products block interferon-alpha-induced signal transduction and transcription factor NF-kappa B-mediated gene induction. Here we report that the same responses are also blocked in undifferentiated F9 teratocarcinoma cells. The block was removed upon cellular differentiation and regained upon the introduction of viral E1A into the differentiated cells. In undifferentiated cells, interferon-beta failed to induce the transcription of interferon-responsive genes because of a lack of activation of the cognate trans-acting factors. As a result, in these cells, virus replication was not inhibited by interferon. Similarly, in undifferentiated but not in differentiated F9 cells, tumor necrosis factor alpha failed to stimulate NF-kappa B-mediated transcription of a reporter gene because of a failure in the activation of NF-kappa B trans-acting factor. These results suggest that a cellular E1A-like activity, present in undifferentiated F9 cells, and adenoviral E1A use similar mechanisms for repressing the expression of specific cellular genes.

Stimulation of MHC class I transcription by interferon-gamma involves a non-A, non-C kinase in addition to protein kinase C

The signal pathways by which interferon-gamma (IFN-gamma) is able to up-regulate major histocompatibility complex (MHC) class I transcription were studied in two human hematopoietic tumor cell lines, K562 and Ramos. These studies suggest that the IFN-gamma signal is transduced via an H7- and staurosporine-sensitive kinase that is distinct from protein kinase C (PKC) and protein kinase A (PKA) in both cell types. Ramos cells appear to utilize an additional pathway involving double-stranded RNA-dependent protein kinase. PKC and possibly PKA appear to be involved in one or more intersecting pathways by which agonists of these kinases are able to act synergistically with IFN-gamma, but activation of these latter pathways is neither necessary nor sufficient for induction of MHC class I transcription. Modulation of G-protein- and Ca2+-calmodulin-associated pathways and arachidonic acid metabolism had no effect on constitutive or IFN-gamma-stimulated class I transcription. The class I stimulatory factor produced in response to IFN-gamma treatment appears to have a short t1/2. The identity of this factor is unknown, but is likely to be distinct from known mediators of IFN-stimulated transcription. Gene and cell-type specificity in the signal transduction pathways utilized by IFN-gamma implies that such pathways may be useful targets for experimental and therapeutic manipulation.

Activation of transcription by IFN-gamma: tyrosine phosphorylation of a 91-kD DNA binding protein

Interferon-gamma (IFN-gamma) induces the transcription of the gene encoding a guanylate binding protein by activating a latent cytoplasmic factor, GAF (gamma-activated factor). GAF is translocated to the nucleus and binds a DNA element, the gamma-activated site. Through cross-linking and the use of specific antibodies GAF was found to be a 91-kilodalton DNA binding protein that was previously identified as one of four proteins in interferon-stimulated gene factor-3 (ISGF-3), a transcription complex activated by IFN-alpha. The IFN-gamma-dependent activation of the 91-kilodalton DNA binding protein required cytoplasmic phosphorylation of the protein on tyrosine. The 113-kilodalton ISGF-3 protein that is phosphorylated in response to IFN-alpha was not phosphorylated nor translocated to the nucleus in response to IFN-gamma. Thus the two different ligands result in tyrosine phosphorylation of different combinations of latent cytoplasmic transcription factors that then act at different DNA binding sites.

Tyrosine phosphorylation is required for activation of an alpha interferon-stimulated transcription factor

The signal transduction pathway of alpha interferon utilizes tyrosine phosphorylation to transmit a signal generated at the cell surface to the transcriptional machinery in the nucleus. Activation of the interferon pathway initiates with the binding of alpha interferon to its cell surface receptor. The ligand-receptor complex signals the activation of a latent cytoplasmic transcription factor. The active form of the interferon-stimulated gene factor (ISGF3) is phosphorylated on tyrosine residues. ISGF3 subsequently translocates to the nucleus and binds to a DNA sequence, the interferon-stimulated response element, found within the promoter of inducible genes. ISGF3 is a multicomponent factor consisting of four proteins of 113 kDa, 91 kDa, 84 kDa, and 48 kDa. Three proteins consistent with sizes of 113 kDa, 91 kDa, and 84 kDa copurify with ISGF3 and are phosphorylated on tyrosine residues after stimulation by alpha interferon. Tyrosine phosphorylation is essential for activation of ISGF3. Genistein, a tyrosine kinase inhibitor, blocks the appearance of ISGF3 and blocks the transcriptional stimulation of interferon-induced genes. This study shows that tyrosine phosphorylation provides a link between the interferon-receptor complex at the plasma membrane and specific activation of gene expression in the nucleus.

Modulation of the antigenic phenotype of human melanoma cells by differentiation-inducing and growth-suppressing agents

Tumor cells often display alterations in their normal program of cellular differentiation. A promising approach for the treatment of cancer involves the induction of terminal differentiation and a loss of proliferative capacity in cancer cells. In human melanoma cells, the combination of mezerein (MEZ) and fibroblast interferon (IFN-beta), results in a rapid and irreversible suppression of cell growth with a concomitant increase in the synthesis of melanin. The induction of terminal differentiation is associated with alterations in the expression of several cellular genes, including fibronectin, ISG-15 and ISG-54, and changes in the expression of specific cell surface antigens, including intercellular adhesion molecule-1 (ICAM-1) and HLA Class I antigens. In the HO-1 human melanoma cell line, induction of terminal differentiation by MEZ plus IFN-beta results in an induction and/or increased expression of ICAM-1, HLA Class I antigens and HLA Class II antigens. IFN-beta and MEZ alone can modulate expression of these antigens to a lower extent than does the combination of compounds. Induction of terminal differentiation and the irreversible suppression of cell growth is not a prerequisite for antigenic modulation in HO-1 cells. This is indicated by the inability of immune interferon (IFN-gamma), a strong inducer of ICAM-1, HLA Class I antigens and HLA Class II antigens synthesis, or the combination of IFN-beta plus IFN-gamma which synergistically but reversibly suppresses HO-1 growth, to induce melanin synthesis or terminal differentiation in HO-1 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of protein kinase C in induction of gene expression and inhibition of cell proliferation by interferon alpha

Recent studies have suggested that protein kinase C (PKC) may be involved in the mechanism of signal transduction by which members of the interferon (IFN) family regulate gene expression and cell phenotype. We have investigated the role of PKC in the control of cell growth and gene expression by IFN alpha in Daudi cells. Treatment of these cells with two analogues of staurosporine, which are potent inhibitors of PKC, completely blocked the induction by IFN alpha of the mRNA for 2',5'-oligoadenylate synthetase and the 6-16 gene. These compounds also inhibited cell proliferation and thymidine incorporation in this system. In contrast, the protein kinase inhibitor 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H7) did not significantly inhibit the induction of these genes by IFN alpha and had no effect on Daudi cell growth or thymidine incorporation in the presence or absence of IFN alpha. No effect of IFN alpha on total PKC activity could be observed, and there were no significant changes in the overall levels of individual PKC isoforms or their mRNA following IFN alpha treatment. In contrast, treatment of Daudi cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, which also inhibits cell proliferation, strongly down-regulated PKC. These data suggest that the activity of a PKC species, or a closely related enzyme, may be required both for continued cell proliferation and the response to IFN alpha in Daudi cells, but that IFN-induced growth inhibition does not involve overall down-regulation or change in activity of PKC.

Subunit of an alpha-interferon-responsive transcription factor is related to interferon regulatory factor and Myb families of DNA-binding proteins

Alpha interferon stimulates transcription by converting the positive transcriptional regulator ISGF3 from a latent to an active form. This receptor-mediated event occurs in the cytoplasm, with subsequent translocation of the activated factor to the nucleus. ISGF3 has two components, termed ISGF3 alpha and ISGF3 gamma. ISGF3 gamma serves as the DNA recognition subunit, while ISGF3 alpha, which appears to consist of three polypeptides, is a target for alpha interferon signaling and serves as a regulatory component whose activation is required to form ISGF3. ISGF3 gamma DNA-binding activity was identified as a 48-kDa polypeptide, and partial amino acid sequence has allowed isolation of cDNA clones. ISGF3 gamma translated in vitro from recombinant clones bound DNA with a specificity indistinguishable from that of ISGF3 gamma purified from HeLa cells. Sequencing of ISGF3 gamma cDNA clones revealed significant similarity to the interferon regulatory factor (IRF) family of DNA binding proteins in the amino-terminal 117 residues of ISGF3 gamma. The other IRF family proteins bind DNA with a specificity related to but distinct from that of ISGF3 gamma. We note sequence similarities between the related regions of IRF family proteins and the imperfect tryptophan repeats which constitute the DNA-binding domain of the c-myb oncoprotein. These sequence similarities suggest that ISGF3 gamma and IRF proteins and the c-myb oncoprotein use a common structural motif for DNA recognition. Recombinant ISGF3 gamma, like the natural protein, interacted with HeLa cell ISGF3 alpha to form the mature ISGF3 DNA-binding complex. We suggest that other IRF family members may participate in signaling pathways by interacting with as yet unidentified regulatory subunits analogous to ISGF3 alpha.