In vitro activation of a transcription factor by gamma interferon requires a membrane-associated tyrosine kinase and is mimicked by vanadate

Vaccinia virus blocks gamma interferon signal transduction: viral VH1 phosphatase reverses Stat1 activation

We have analyzed the effects of vaccinia virus (VV) on gamma interferon (IFN-gamma) signal transduction. Infection of cells with VV 1 to 2 h prior to treatment with IFN-gamma inhibits phosphorylation and nuclear translocation of Stat1 and consequently blocks accumulation of mRNAs normally induced by IFN-gamma. While phosphorylation of other proteins in the IFN-gamma pathway was not affected, activation of Stat1 by other ligand-receptor systems was also blocked by VV. This block of Stat1 activation was dose dependent, and although viral protein synthesis was not required, entry and uncoating of viral cores appear to be needed to block the accumulation of phosphorylated Stat1. These results suggest that a virion component is responsible for the effect. VV virions contain a phosphatase (VH1) that is sensitive to the phosphatase inhibitor Na(3)VO(4) but not to okadaic acid. Addition of Na(3)VO(4) but not okadaic acid restored normal Stat1 phosphorylation levels in VV-infected cells. Moreover, virions containing reduced levels of VH1 were unable to block the IFN-gamma signaling pathway. In vitro studies show that the phosphatase can bind and dephosphorylate Stat1, indicating that this transcription factor can be a substrate for VH1. Our results reveal a novel mechanism by which VV interferes with the onset of host immune responses by blocking the IFN-gamma signal cascade through the dephosphorylating activity of the viral phosphatase VH1.

Nuclear export signal located within theDNA-binding domain of the STAT1transcription factor

Latent signal transducers and activators of transcription (STATs) reside in the cytoplasm but rapidly accumulate in the nucleus following cytokine stimulation. Nuclear accumulation requires specific tyrosine phosphorylation and STAT dimerization. The presence of STATs in the nucleus is transient, however, and within hours the STATs reappear in the cytoplasm. Results indicate that STAT1 can be dephosphorylated in the nucleus and actively exported by the chromosome region maintenance 1 (CRM1) export receptor. CRM1 recognizes a specific amino acid sequence located within the DNA-binding domain of STAT1. This region shares sequence and functional properties of characterized nuclear export signals. The location of this sequence within STAT1 suggests that it is not accessible to CRM1 when STAT1 is bound to DNA. Evidence is presented to support a model in which STAT1 is tyrosine dephosphorylated in the nucleus and dissociates from DNA, allowing recognition by CRM1 and nuclear export. The regulated export of STAT1 may contribute to silencing of the signal pathway and/or to re-establish STAT1 in the cytoplasm to monitor activity of receptor-kinase signals.

Regulation of STAT1 nuclear export by Jak1

Signal transducer and activator of transcription 1 (STAT1) mediates gene expression in response to cytokines and growth factors. Activation of STAT1 is achieved through its tyrosine phosphorylation, a process that involves Jak tyrosine kinases. Here we show that STAT1, although phosphorylated on Y701, is unable to localize in the nucleus in the absence of Jak1 or Jak1 kinase activity. In contrast, the nuclear accumulation of STAT1 in Tyk2-deficient cells remains intact. Nuclear presence of tyrosine-phosphorylated STAT1 could be restored in Jak1-deficient cells by leptomycin B, an inhibitor of nuclear export. Amino acids 197 to 205 of STAT1 were found to encode a leucine-rich nuclear export signal (NES). An L-->A mutation within the NES restored nuclear retention of STAT1 in Jak1-deficient cells. Impaired binding of the transcriptional coactivator CBP to tyrosine-phosphorylated STAT1 derived from Jak1-deficient cells offers a model for the intermolecular regulation of the nuclear export sequence.

Shp-2 tyrosine phosphatase functions as a negative regulator of the interferon-stimulated Jak/STAT pathway

Shp-2 is an SH2 domain-containing protein tyrosine phosphatase. Although the mechanism remains to be defined, substantial experimental data suggest that Shp-2 is primarily a positive regulator in cell growth and development. We present evidence here that Shp-2, while acting to promote mitogenic signals, also functions as a negative effector in interferon (IFN)-induced growth-inhibitory and apoptotic pathways. Treatment of mouse fibroblast cells lacking a functional Shp-2 with IFN-alpha or IFN-gamma resulted in an augmented suppression of cell viability compared to that of wild-type cells. To dissect the molecular mechanism, we examined IFN-induced activation of signal transducers and activators of transcription (STATs) by electrophoretic mobility shift assay, using a specific DNA probe (hSIE). The amounts of STAT proteins bound to hSIE upon IFN-alpha or IFN-gamma stimulation were significantly increased in Shp-2(-/-) cells. Consistently, tyrosine phosphorylation levels of Stat1 upon IFN-gamma treatment and, to a lesser extent, upon IFN-alpha stimulation were markedly elevated in mutant cells. Furthermore, IFN-gamma induced a higher level of caspase 1 expression in Shp-2(-/-) cells than in wild-type cells. Reintroduction of wild-type Shp-2 protein reversed the hypersensitivity of Shp-2(-/-) fibroblasts to the cytotoxic effect of IFN-alpha and IFN-gamma. Excessive activation of STATs by IFNs was also diminished in mutant cells in which Shp-2 had been reintroduced. Together, these results establish that Shp-2 functions as a negative regulator of the Jak/STAT pathway. We propose that Shp-2 acts to promote cell growth and survival through two mechanisms, i.e., the stimulation of growth factor-initiated mitogenic pathways and the suppression of cytotoxic effect elicited by cytokines, such as IFNs.

A mutant cell line defective in response to double-stranded RNA and in regulating basal expression of interferon-stimulated genes

Although much progress has been made in identifying the signaling pathways that mediate the initial responses to interferons (IFNs), much less is known about how IFN-stimulated genes (ISGs) are kept quiescent in untreated cells, how the response is sustained after the initial induction, and how ISG expression is down-regulated, even in the continued presence of IFN. We have used the cell sorter to isolate mutant cells with constitutively high ISG expression. A recessive mutant, P2.1, has higher constitutive ISG levels than the parental U4C cells, which do not respond to any IFN. Unexpectedly, P2.1 cells also are deficient in the expression of ISGs in response to double-stranded RNA (dsRNA). Electrophoretic mobility-shift assays revealed that the defect is upstream of the activation of the transcription factors NFkappaB and IFN regulatory factor 1. Analysis of the pivotal dsRNA-dependent serine/threonine kinase PKR revealed that the wild-type kinase is present and is activated normally in response to dsRNA in P2.1 cells. Together, these data suggest that the defect in P2.1 cells is either downstream of PKR or in a component of a distinct pathway that is involved both in activating multiple transcription factors in response to dsRNA and in regulating the basal expression of ISGs.

Antagonistic Action of IFN-β and IFN-γ on High Affinity Fcγ Receptor Expression in Healthy Controls and Multiple Sclerosis Patients

Monocyte-macrophage activation by IFN-γ is characterized by a pronounced increase of high affinity Fc receptors for IgG (FcγRI), capable of triggering respiratory burst, phagocytosis, Ab-dependent cytotoxicity, and release of proinflammatory cytokines. In view of the antagonism of IFN-β on IFN-γ action, of interest in the chronic inflammatory disorder multiple sclerosis, we examined the possible effect of IFN-β on IFN-γ induction of FcγRI gene expression. We found that IFN-β significantly down-regulated IFN-γ-induced FcγRI surface expression in peripheral blood monocytes from healthy donors, in a dose- and time-dependent manner. This down-regulation of FcγRI surface levels did not correspond to a decrease in FcγRI mRNA, suggesting a posttranscriptional effect of IFN-β. Down-regulation of FcγRI surface expression correlated with diminished cellular signaling through FcγRI, since the IFN-γ-induced increase in Fcγ receptor-triggered respiratory burst was nearly completely abrogated by simultaneous addition of IFN-β. Finally, the same antagonism between both IFNs on FcγRI surface expression was observed in peripheral blood monocytes derived from multiple sclerosis patients; inhibition by IFN-β was even increased (82 ± 11%), as compared with healthy controls (67 ± 4%). These results may partially help explain the beneficial effect of IFN-β in multiple sclerosis.

Receptor-associated constitutive protein tyrosine phosphatase activity controls the kinase function of JAK1

Exposure of cells to protein tyrosine phosphatase (PTP) inhibitors causes an increase in the phosphotyrosine content of many cellular proteins. However, the level at which the primary signaling event is affected is still unclear. We show that Jaks are activated by tyrosine phosphorylation in cells that are briefly exposed to the PTP inhibitor pervanadate (PV), resulting in tyrosine phosphorylation and functional activation of Stat6 (in addition to other Stats). Mutant cell lines that lack Jak1 activity fail to support PV-mediated [or interleukin 4 (IL-4)-dependent] activation of Stat6 but can be rescued by complementation with functional Jak1. The docking sites for both Jak1 and Stat6 reside in the cytoplasmic domain of the IL-4 receptor alpha-chain (IL-4Ralpha). The glioblastoma-derived cell lines T98G, GRE, and M007, which do not express the IL-4Ralpha chain, fail to support Stat6 activation in response to either IL-4 or PV. Complementation of T98G cells with the IL-4Ralpha restores both PV-mediated and IL-4-dependent Stat6 activation. Murine L929 cells, which do not express the gamma common chain of the IL-4 receptor, support PV-mediated but not IL-4-dependent Stat6 activation. Thus, Stat6 activation by PV is an IL-4Ralpha-mediated, Jak1-dependent event that is independent of receptor dimerization. We propose that receptor-associated constitutive PTP activity functions to down-regulate persistent, receptor-linked kinase activity. Inhibition or deletion of PTP activity results in constitutive activation of cytokine signaling pathways.

Enhancement of antiproliferative activity of gamma interferon by the specific inhibition of tyrosine dephosphorylation of Stat1

Gamma interferon (IFN-gamma) signals to the nucleus through the activation, by tyrosine phosphorylation, of the latent cytoplasmic transcription factor Stat1 (signal transducer and activator of transcription). It has been demonstrated that the activity of Stat1 is dependent on tyrosine phosphorylation which is regulated by Jak tyrosine kinases as well as by the as-yet-unidentified protein tyrosine phosphatase. We report that the N-terminal domain of Stat1, which is highly conserved among all STAT family members, is required for its tyrosine dephosphorylation. A single amino acid substitution (Arg-31 to Ala) in the Stat1 N-terminal domain inhibited Stat1 tyrosine dephosphorylation. The deletion of the Stat1 N-terminal domain resulted in a mutant Stat1 protein which was constitutively phosphorylated on Tyr-701. Upon IFN-gamma stimulation, the tyrosine phosphorylation of this mutant protein was further enhanced but was not down-regulated by protein tyrosine phosphatase in vivo. When expressed in NIH 3T3 cells, this mutant protein greatly enhanced the antiproliferative activity of IFN-gamma. We suggest that the N-terminal domains of STATs are crucial for modulating STAT activities through regulating the tyrosine dephosphorylation of STATs.

Inhibition of alpha interferon but not gamma interferon signal transduction by phorbol esters is mediated by a tyrosine phosphatase

Previous studies have indicated that the expression of viral oncoproteins, cell transformation, or phorbol ester treatment of cells can inhibit alpha/beta interferon (IFN-alpha/beta)-induced gene expression. The mechanisms by which these promoters of cell growth exert their inhibitory effects vary, but in most instances they involve a disruption of the IFN-alpha/beta-induced transcription complex ISGF3 such that the DNA-binding component of this complex (the 48-kDa ISGF3gamma protein) does not bind to the interferon-stimulated response element (ISRE). In this report, we demonstrated that phorbol ester treatment of human peripheral blood monocytes dramatically inhibits activation of IFN-alpha/B-stimulated early response genes but by a mechanism which does not involve abrogation of the ISRE binding of ISGF3gamma. Phorbol ester treatment of monocytes inhibited IFN alpha-stimulated tyrosine phosphorylation of the transcription factors Stat1alpha, Stat2, and Stat3 and of the tyrosine kinase Tyk2 but had no effect on IFN-gamma activation of Stat1alpha. IFNalpha-stimulated tyrosine phosphorylation of Jak1 and the alpha subunit of the IFN-alpha receptor were unaffected by phorbol 12-myristate 13-acetate (PMA). Moreover, PMA caused the dephosphorylation of Tyk2 but not of Jak1, which was activated by IFN. Pretreatment of cells with vanadate prevented the effects of PMA with regard to PMA-induced Tyk2 dephosphorylation. These observations suggest that PMA exerts its inhibitory effects by activation of a tyrosine phosphatase which selectively regulates Tyk2 but not Jak1 activity.

Differential regulation of the alpha/beta interferon-stimulated Jak/Stat pathway by the SH2 domain-containing tyrosine phosphatase SHPTP1

Interferons (IFNs) induce early-response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat transcription factors. Previous studies implicated protein-tyrosine phosphatase (PTP) activity in the control of IFN-regulated Jak/Stat signaling, but the specific PTPs responsible remained unidentified. We have found that SH2 domain-containing PTP1 (SHPTP1; also called PTP1C, HCP, or SHP) reversibly associates with the IFN-alpha receptor complex upon IFN addition. Compared with macrophages from normal littermate controls, macrophages from motheaten mice, which lack SHPTP1, show dramatically increased Jak1 and Stat1 alpha tyrosine phosphorylation, whereas Tyk2 and Stat2 activation is largely unaffected. These findings correlate with selectively increased complex formation on a gamma response element, but not an IFN-stimulated response element, in motheaten macrophages. Our results establish that SHPTP1 selectively regulates distinct components of Jak/Stat signal transduction pathways in vivo.

Ligand-dependent and -independent activation of the transcription factor gamma RF-1 in a cell-free system

gamma RF-1 is a recently identified transcription factor induced by interferon-gamma (IFN-gamma) which binds to a unique palindromic enhancer, gamma RE-1, in the promoter of the mig gene. This paper describes the ligand-dependent and ligand-independent activation of gamma RF-1 in a cell-free system. gamma RF-1 activity was induced by IFN-gamma in a time-dependent manner from 5 to 60 min in lysates prepared from the human monocytic leukaemia line THP-1 and the human epidermoid carcinoma line A431. The activation of gamma RF-1 in vitro required both ATP and an inhibitor of tyrosine phosphatases (sodium orthovanadate or pervanadate). In the presence of limiting concentrations (micromolar) of ATP, activation was also dependent upon stimulation with IFN-gamma, whereas at millimolar concentrations of ATP, gamma RF-1 was activated by either sodium orthovanadate or pervanadate in the absence of ligand. Based on cell fractionation studies, both membrane and cytosol components were essential for activation of gamma RF-1 in vitro. Consistent with a role for one or more tyrosine kinases in the activation of gamma RF-1, its DNA binding activity was blocked by monoclonal anti-phosphotyrosine antibodies and by the tyrosine kinase inhibitors genistein, lavendustin A and herbimycin A. A comparison with recently described pathways of IFN-mediated transcription factor regulation indicates that the in vitro activation of gamma RF-1 is unique, requiring both membrane and cytosol fractions and inhibition of endogenous tyrosine phosphatase activity.

Prolactin induces phosphorylation of Tyr694 of Stat5 (MGF), a prerequisite for DNA binding and induction of transcription

Mammary gland factor (MGF) is a transcription factor discovered initially in the mammary epithelial cells of lactating animals. It confers the lactogenic hormone response to the milk protein genes. We reported recently the isolation of the cDNA encoding MGF. MGF is a novel member of the cytokine-regulated transcription factor gene family. Members of this gene family mediate interferon alpha/beta and interferon gamma induction of gene transcription, as well as the response to epidermal growth factor and interleukin-6, and have been named signal transducers and activators of transcription (Stat). The name Stat5 has been assigned to MGF. We studied the mechanisms involved in the prolactin activation of Stat5 in COS cells co-transfected with cDNA encoding Stat5 and the prolactin receptor. Prolactin treatment of the transfected cells caused activation of Stat5 within 5-10 min. This activation does not require ongoing protein synthesis. Tyrosine kinase inhibitors prevent Stat5 activation in transfected COS cells. Treatment of recombinant Stat5 with a tyrosine-specific protein phosphatase in vitro abolishes its DNA binding activity. Prolactin stimulation of transfected cells induces Stat5 phosphorylation on tyrosine. Phosphorylation of in vitro transcribed and translated Stat5 with the Jak2 tyrosine kinase, but not with fyn, lyn or lck, confers DNA binding activity. The prolactin response of the beta-casein milk protein gene promoter can be observed in COS cells transfected with cDNA vectors encoding Stat5 and the long form of the prolactin receptor. The short form of the prolactin receptor is unable to promote Stat5 phosphorylation and confer transcriptional induction in COS cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of the transcription factor PU.1/Spi-1 in myeloid cell-restricted expression of an interferon-inducible gene encoding the human high-affinity Fc gamma receptor

Induction by gamma interferon (IFN-gamma) of the gene encoding the human high-affinity Fc gamma receptor (Fc gamma R1) in myeloid cells requires an IFN-gamma response region (GRR) and a myeloid cell-activating transcription element (MATE). GRR and MATE interact with factors to form, respectively, an IFN-gamma-activating complex (GIRE-BP), depending on the phosphorylation of the 91-kDa protein (subunit of ISGF3), and a cell-type-specific complex (MATE-BP). Although GIRE-BP is detected in cells of different origins after IFN-gamma treatment, the presence of MATE-BP was found to be restricted to B- and myeloid cell lines. Sequence analysis of a cDNA encoding a polypeptide recognizing specifically the MATE motif led to the identification of this product as the proto-oncogene PU.1/Spi-1, a transcriptional activator expressed in myeloid and B cells. Expression of this factor in nonhematopoietic cells allowed IFN-gamma-induced expression of a reporter gene under control of the GRR and MATE sequences. The presence of these motifs in other gene promoters indicates that the binding of PU.1/Spi-1 and IFN regulatory proteins to their respective motifs could be part of a general mechanism leading to cell-type-restricted and IFN-induced gene expression.

Involvement of the transcription factor PU.1/Spi-1 in myeloid cell-restricted expression of an interferon-inducible gene encoding the human high-affinity Fc gamma receptor

Induction by gamma interferon (IFN-gamma) of the gene encoding the human high-affinity Fc gamma receptor (Fc gamma R1) in myeloid cells requires an IFN-gamma response region (GRR) and a myeloid cell-activating transcription element (MATE). GRR and MATE interact with factors to form, respectively, an IFN-gamma-activating complex (GIRE-BP), depending on the phosphorylation of the 91-kDa protein (subunit of ISGF3), and a cell-type-specific complex (MATE-BP). Although GIRE-BP is detected in cells of different origins after IFN-gamma treatment, the presence of MATE-BP was found to be restricted to B- and myeloid cell lines. Sequence analysis of a cDNA encoding a polypeptide recognizing specifically the MATE motif led to the identification of this product as the proto-oncogene PU.1/Spi-1, a transcriptional activator expressed in myeloid and B cells. Expression of this factor in nonhematopoietic cells allowed IFN-gamma-induced expression of a reporter gene under control of the GRR and MATE sequences. The presence of these motifs in other gene promoters indicates that the binding of PU.1/Spi-1 and IFN regulatory proteins to their respective motifs could be part of a general mechanism leading to cell-type-restricted and IFN-induced gene expression.

Prolactin activates the interferon-regulated p91 transcription factor and the Jak2 kinase by tyrosine phosphorylation

The prolactin (PRL) receptor is a member of the family of cytokine receptors that lack intrinsic tyrosine kinase activity but contain two conserved cysteines in their N-terminal regions and a WSXWS motif adjacent to their transmembrane domains. In a manner similar to the interferons (IFNs), exposure of cells to PRL results in tyrosine phosphorylation of several cellular proteins and the rapid transcriptional induction of the IFN regulatory factor 1 gene. In this communication, we demonstrate that treatment of rat Nb2 lymphoma cells with PRL activates a latent protein factor so that it binds to an enhancer in the IFN regulatory factor 1 gene. This enhancer has been shown to be required for IFN-gamma-activated expression of this gene. PRL-induced assembly of the DNA binding complex, PRL-stimulated factor, required tyrosine phosphorylation. PRL-stimulated factor contained at least one protein that was antigenically similar to the p91 transcription factor, a component of several transcription complexes required for cytokine-activated gene expression. PRL not only induced the tyrosine phosphorylation of p91 but also induced tyrosine phosphorylation of Jak2, a tyrosine kinase required for IFN-gamma-activated gene expression. These results provide evidence for a signaling mechanism, some of whose components are shared by both PRL and IFN-gamma receptors, that results in the expression of early response genes.

Inhibitors of serine/threonine phosphatases enhance phosphorylation of the interferon-gamma receptor while selectively attenuating interferon-gamma-induced gene expression in human peripheral-blood monocytes

Since many events following ligand-induced receptor clustering are controlled by serine and threonine (Ser/Thr) phosphorylation, we initiated an investigation into the role of Ser/Thr phosphatases in both phosphorylation of the interferon-gamma (IFN-gamma) receptor and IFN gamma-induced gene expression in human peripheral-blood monocytes. Whereas IFN gamma alone did not enhance phosphorylation of the IFN gamma receptor, treatment of monocytes with the Ser/Thr phosphatase inhibitors, okadaic acid and calyculin A, resulted in increased phosphorylation of the IFN gamma receptor. However, when these cells were analysed for IFN gamma-induced IP-10 gene expression, there was profound inhibition. Using three IFN gamma-induced early-response genes, IP-10, the Fc gamma receptor type I (Fc gamma RI) and ISG-54, we found selective sensitivity to pretreatment with okadaic acid and calyculin A. Whereas IFN gamma induction of IP-10 was blocked by both inhibitors, only calyculin A prevented Fc gamma RI-gene expression. Neither inhibitor prevented ISG-54 induction by IFN gamma. IFN-gamma-activated formation of the DNA-binding-protein complex FcRF gamma (which binds to the promoter of the Fc gamma RI gene) remained unaffected by okadaic acid or calyculin A. Therefore these data suggest that Ser/Thr phosphatases have no major part in IFN gamma-initiated signal transduction across the membrane, but selectively control the ultimate transcription of a set of early-response genes.

Ligand-induced IFN gamma receptor tyrosine phosphorylation couples the receptor to its signal transduction system (p91)

Herein we report that interferon-gamma (IFN gamma) induces the rapid and reversible tyrosine phosphorylation of the IFN gamma receptor. Using a panel of receptor intracellular domain mutants, we show that a membrane-proximal LPKS sequence (residues 266-269) is required for ligand-induced tyrosine kinase activation and/or kinase-receptor association and biological responsiveness, and a functionally critical membrane-distal tyrosine residue (Y440) is a target of the activated enzyme. The biological significance of Y440 phosphorylation was demonstrated by showing that a receptor-derived nonapeptide corresponding to receptor residues 436-444 and containing phosphorylated Y440 bound specifically to p91, blocked p91 phosphorylation and inhibited the generation of an active p91-containing transcription factor complex. In contrast, nonphosphorylated wild-type, phosphorylated mutant, or phosphorylated irrelevant peptides did not. Moreover, the phosphorylated Y440-containing peptide did not interact with a related but distinct latent transcription factor (p113) which is activatible by IFN alpha but not IFN gamma. These results thus document the specific and inducible association of p91 with the phosphorylated IFN gamma receptor and thereby elucidate the mechanism by which ligand couples the IFN gamma receptor to its signal transduction system.

Growth hormone and erythropoietin differentially activate DNA-binding proteins by tyrosine phosphorylation

Binding of growth hormone (GH) and erythropoietin (EPO) to their respective receptors results in receptor clustering and activation of tyrosine kinases that initiate a cascade of events resulting not only in the rapid tyrosine phosphorylation of several proteins but also in the induction of early-response genes. In this report, we show that GH and EPO induce the tyrosine phosphorylation of cellular proteins with molecular masses of 93 kDa and of 91 and 84 kDa, respectively, and that these proteins form DNA-binding complexes which recognize an enhancer that has features in common with several rapidly induced genes such as c-fos. Assembly of the protein complexes required tyrosine phosphorylation, which occurred within minutes after addition of ligand. The activated complexes translocated from the cytoplasm to the nucleus. The protein activated by GH is antigenically similar to p91, a protein common to several transcription complexes that are activated by interferons and other cytokines. In contrast, the proteins activated by EPO are distinct from p91. These findings establish the outlines for a cytokine-induced intracellular signaling pathway, which begins with ligand-induced receptor clustering that activates one or more tyrosine kinases. These data are the first to demonstrate that GH- and EPO-activated tyrosine-phosphorylated proteins can specifically recognize a well-defined enhancer and therefore provide a mechanism for rapidly transducing signals from the membrane to the nucleus.

Growth hormone and erythropoietin differentially activate DNA-binding proteins by tyrosine phosphorylation

Binding of growth hormone (GH) and erythropoietin (EPO) to their respective receptors results in receptor clustering and activation of tyrosine kinases that initiate a cascade of events resulting not only in the rapid tyrosine phosphorylation of several proteins but also in the induction of early-response genes. In this report, we show that GH and EPO induce the tyrosine phosphorylation of cellular proteins with molecular masses of 93 kDa and of 91 and 84 kDa, respectively, and that these proteins form DNA-binding complexes which recognize an enhancer that has features in common with several rapidly induced genes such as c-fos. Assembly of the protein complexes required tyrosine phosphorylation, which occurred within minutes after addition of ligand. The activated complexes translocated from the cytoplasm to the nucleus. The protein activated by GH is antigenically similar to p91, a protein common to several transcription complexes that are activated by interferons and other cytokines. In contrast, the proteins activated by EPO are distinct from p91. These findings establish the outlines for a cytokine-induced intracellular signaling pathway, which begins with ligand-induced receptor clustering that activates one or more tyrosine kinases. These data are the first to demonstrate that GH- and EPO-activated tyrosine-phosphorylated proteins can specifically recognize a well-defined enhancer and therefore provide a mechanism for rapidly transducing signals from the membrane to the nucleus.

A unique palindromic element mediates gamma interferon induction of mig gene expression

To define the molecular mechanisms involved in the action of gamma interferon (IFN-gamma), we have analyzed the transcriptional regulation of the mig (monokine induced by gamma interferon) gene, a member of the platelet factor 4-interleukin-8 cytokine family that is expressed in murine macrophages specifically in response to IFN-gamma. Analysis of mig/CAT chimeric constructs transiently transfected into the RAW 264.7 mouse monocytic cell line revealed a unique IFN-gamma-responsive element (gamma RE-1). The sequence of this cis regulatory element defined by deletion analysis contains an imperfect inverted repeat extending 27 bp. Examination of mig/CAT constructs with mutations in gamma RE-1 revealed that the palindromic positions in the element were essential for activity. Consistent with its function as an enhancer, a single copy of gamma RE-1 conferred IFN-gamma inducibility to a heterologous (herpes simplex virus thymidine kinase) promoter. Exonuclease III protection assays demonstrated symmetrical protection of a mig promoter fragment centered about the gamma RE-1 palindromic sequence. Using the gel electrophoretic mobility shift assay, we identified a factor (gamma RF-1) present in nuclear extracts prepared from IFN-gamma-stimulated RAW 264.7 cells which binds to gamma RE-1. The activation of gamma RF-1 occurred rapidly (within 1 min) in response to IFN-gamma and was independent of protein synthesis. Similar to the expression of mig mRNA, the formation of gamma RF-1 was selectively induced by IFN-gamma and not IFN-alpha. The regulation of gene expression through gamma RF-1 and gamma RE-1 may explain the preferential activation of a subset of interferon-inducible genes by IFN-gamma.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

A unique palindromic element mediates gamma interferon induction of mig gene expression

To define the molecular mechanisms involved in the action of gamma interferon (IFN-gamma), we have analyzed the transcriptional regulation of the mig (monokine induced by gamma interferon) gene, a member of the platelet factor 4-interleukin-8 cytokine family that is expressed in murine macrophages specifically in response to IFN-gamma. Analysis of mig/CAT chimeric constructs transiently transfected into the RAW 264.7 mouse monocytic cell line revealed a unique IFN-gamma-responsive element (gamma RE-1). The sequence of this cis regulatory element defined by deletion analysis contains an imperfect inverted repeat extending 27 bp. Examination of mig/CAT constructs with mutations in gamma RE-1 revealed that the palindromic positions in the element were essential for activity. Consistent with its function as an enhancer, a single copy of gamma RE-1 conferred IFN-gamma inducibility to a heterologous (herpes simplex virus thymidine kinase) promoter. Exonuclease III protection assays demonstrated symmetrical protection of a mig promoter fragment centered about the gamma RE-1 palindromic sequence. Using the gel electrophoretic mobility shift assay, we identified a factor (gamma RF-1) present in nuclear extracts prepared from IFN-gamma-stimulated RAW 264.7 cells which binds to gamma RE-1. The activation of gamma RF-1 occurred rapidly (within 1 min) in response to IFN-gamma and was independent of protein synthesis. Similar to the expression of mig mRNA, the formation of gamma RF-1 was selectively induced by IFN-gamma and not IFN-alpha. The regulation of gene expression through gamma RF-1 and gamma RE-1 may explain the preferential activation of a subset of interferon-inducible genes by IFN-gamma.

A nuclear tyrosine phosphatase downregulates interferon-induced gene expression

Alpha and gamma interferons rapidly induce several early response genes in primary human diploid fibroblasts. The transcription rates of these genes are maximal after 1 h of interferon treatment and return to basal levels within 8 h. Three different interferon-activated DNA-binding complexes (ISGF3, GAF, and FcRF gamma) that are responsible for transcriptional activation of cellular genes have been characterized. Assembly of these complexes requires tyrosine phosphorylation of one or more of the protein components. In this report, we demonstrate that a nuclear tyrosine phosphatase is responsible for the deactivation of these interferon-regulated transcription factors and the subsequent transcriptional downregulation of the corresponding genes. Furthermore, tyrosine phosphorylation is required for nuclear localization of the 91-kDa protein that is part of all three interferon-induced transcription complexes. These results provide the first evidence for a nuclear tyrosine phosphatase activity as a mechanism of transcriptional regulation.

A nuclear tyrosine phosphatase downregulates interferon-induced gene expression

Alpha and gamma interferons rapidly induce several early response genes in primary human diploid fibroblasts. The transcription rates of these genes are maximal after 1 h of interferon treatment and return to basal levels within 8 h. Three different interferon-activated DNA-binding complexes (ISGF3, GAF, and FcRF gamma) that are responsible for transcriptional activation of cellular genes have been characterized. Assembly of these complexes requires tyrosine phosphorylation of one or more of the protein components. In this report, we demonstrate that a nuclear tyrosine phosphatase is responsible for the deactivation of these interferon-regulated transcription factors and the subsequent transcriptional downregulation of the corresponding genes. Furthermore, tyrosine phosphorylation is required for nuclear localization of the 91-kDa protein that is part of all three interferon-induced transcription complexes. These results provide the first evidence for a nuclear tyrosine phosphatase activity as a mechanism of transcriptional regulation.

Complementation of a mutant cell line: central role of the 91 kDa polypeptide of ISGF3 in the interferon-alpha and -gamma signal transduction pathways

Mutants in complementation group U3, completely defective in the response of all genes tested to interferons (IFNs) alpha and gamma, do not express the 91 and 84 kDa polypeptide components of interferon-stimulated gene factor 3 (ISGF3), a transcription factor known to play a primary role in the IFN-alpha response pathway. The 91 and 84 kDa polypeptides are products of a single gene. They result from differential splicing and differ only in a 38 amino acid extension at the C-terminus of the 91 kDa polypeptide. Complementation of U3 mutants with cDNA constructs expressing the 91 kDa product at levels comparable to those observed in induced wild-type cells completely restored the response to both IFN-alpha and -gamma and the ability to form ISGF3. Complementation with the 84 kDa component similarly restored the ability to form ISGF3 and, albeit to a lower level, the IFN-alpha response of all genes tested so far. It failed, however, to restore the IFN-gamma response of any gene analysed. The precise nature of the DNA motifs and combination of factors required for the transcriptional response of all genes inducible by IFN-alpha and -gamma remains to be established. The results presented here, however, emphasize the apparent general requirement of the 91 kDa polypeptide in the primary transcriptional response to both types of IFN.