Two cis-DNA elements involved in myeloid-cell-specific expression and gamma interferon (IFN-gamma) activation of the human high-affinity Fc gamma receptor gene: a novel IFN regulatory mechanism

Temporal gene profiling of the 5XFAD transgenic mouse model highlights the importance of microglial activation in Alzheimer's disease

BACKGROUND

The 5XFAD early onset mouse model of Alzheimer's disease (AD) is gaining momentum. Behavioral, electrophysiological and anatomical studies have identified age-dependent alterations that can be reminiscent of human AD. However, transcriptional changes during disease progression have not yet been investigated. To this end, we carried out a transcriptomic analysis on RNAs from the neocortex and the hippocampus of 5XFAD female mice at the ages of one, four, six and nine months (M1, M4, M6, M9).

RESULTS

Our results show a clear shift in gene expression patterns between M1 and M4. At M1, 5XFAD animals exhibit region-specific variations in gene expression patterns whereas M4 to M9 mice share a larger proportion of differentially expressed genes (DEGs) that are common to both regions. Analysis of DEGs from M4 to M9 underlines the predominance of inflammatory and immune processes in this AD mouse model. The rise in inflammation, sustained by the overexpression of genes from the complement and integrin families, is accompanied by an increased expression of transcripts involved in the NADPH oxidase complex, phagocytic processes and IFN-γ related pathways.

CONCLUSIONS

Overall, our data suggest that, from M4 to M9, sustained microglial activation becomes the predominant feature and point out that both detrimental and neuroprotective mechanisms appear to be at play in this model. Furthermore, our study identifies a number of genes already known to be altered in human AD, thus confirming the use of the 5XFAD strain as a valid model for understanding AD pathogenesis and for screening potential therapeutic molecules.

Antigen targeting to myeloid-specific human Fc gamma RI/CD64 triggers enhanced antibody responses in transgenic mice

Besides their phagocytic effector functions, myeloid cells have an essential role as accessory cells in the induction of optimal humoral immune responses by presenting captured antigens and activating lymphocytes. Antigen presentation by human monocytes was recently found to be enhanced in vitro through the high-affinity Fc receptor for IgG (Fc gamma RI; CD64), which is exclusively present on myeloid cells. To evaluate a comparable role of Fc gamma RI in antigen presentation in vivo, we generated human Fc gamma RI transgenic mice. Under control of its endogenous promoter, human Fc gamma RI was selectively expressed on murine myeloid cells at physiological expression levels. As in humans, expression was properly regulated by the cytokines IFN-gamma, G-CSF, IL-4, and IL-10, and was up-regulated during inflammation. The human receptor expressed by murine macrophages bound monomeric human IgG and mediated particle phagocytosis and IgG complex internalization. To evaluate whether specific targeting of antigens to Fc gamma RI can induce enhanced antibody responses, mice were immunized with an anti-human Fc gamma RI antibody containing antigenic determinants. Transgenic mice produced antigen-specific antibody responses with high IgG1 titers and substantial IgG2a and IgG2b responses. These data demonstrate that human Fc gamma RI on myeloid cells is highly active in mediating enhanced antigen presentation in vivo, and show that anti-Fc gamma RI mAbs are promising vaccine adjuvants.

Differentiation-regulated serine phosphorylation of STAT1 promotes GAF activation in macrophages

Gamma interferon (IFN-gamma), a macrophage-activating cytokine, modulates gene expression through the activity of a transcription factor designated IFN-gamma activation factor (GAF). GAF is formed after phosphorylation on tyrosine and dimerization of the 91-kDa protein STAT1. We have recently reported that differentiation of the promonocytic cell line U937 into monocytes increases the amount of cellular GAF after IFN-gamma treatment and at the same time increases the phosphorylation of STAT1. Here we show that activation of the JAK family kinases, which are instrumental in mediating STAT1 phosphorylation on tyrosine, did not increase upon monocytic U937 differentiation. Consistent with this finding, levels of STAT1 tyrosine phosphorylation were virtually identical in promonocytic and monocytic U937 cells. Analysis of STAT1 phosphoamino acids and mapping of phosphopeptides showed an IFN-gamma-dependent increase in Ser phosphorylation in differentiated cells. Analyses of STAT1 isoforms by two-dimensional gel electrophoresis demonstrated a differentiation-induced shift toward more acidic isoforms. All isoforms were equally sensitive to subsequent tyrosine phosphorylation, as indicated by a sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobility shift typical for tyrosine-phosphorylated STAT1. Consistent with the importance of Ser phosphorylation for high-affinity binding to the IFN-gamma activation site sequence, phosphatase 2A treatment strongly reduced the formation of IFN-gamma activation site-GAF complexes in an electrophoretic mobility shift assay. Our data indicate that the activity of GAF is modulated by STAT1 serine kinases/phosphatases and suggest that this mechanism is employed in the developmental control of macrophage responsiveness to IFN-gamma.

Interferon regulatory factor 1 is required for mouse Gbp gene activation by gamma interferon

Full-scale transcriptional activation of the mouse Gbp genes by gamma interferon (IFN-gamma) requires protein synthesis in embryonic fibroblasts. Although the Gbp-1 and Gbp-2 promoters contain binding sites for transcription factors Stat1 and IFN regulatory factor 1 (IRF-1), deletion analysis revealed that the Stat1 binding site is dispensable for IFN-gamma inducibility of Gbp promoter constructs in transfected fibroblasts. However, activation of the mouse Gbp promoter by IFN-gamma requires transcription factor IRF-1. Transient overexpression of IRF-1 cDNA in mouse fibroblasts resulted in high-level expression of Gbp promoter constructs. Unlike wild-type cells, IRF-1% embryonic stem cells lacking functional transcription factor IRF-1 contained very low levels of Gbp transcripts that were not increased in response to differentiation or treatment with IFN-gamma. Treatment of IRF-1% mice with IFN-gamma resulted in barely detectable levels of Gbp RNA in spleens, lungs, and livers, whereas such treatment induced high levels of Gbp RNA in the organs of wild-type mice. These observations suggest two alternative pathways for transcriptional induction of genes in response to IFN-gamma: immediate response that results from activation of preformed Stat1 and delayed response that results from induced de novo synthesis of transcription factor IRF-1.

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.

In vivo footprinting of the IRF-1 promoter: inducible occupation of a GAS element next to a persistent structural alteration of the DNA

GAS (gamma activated sequence) and GAS-like elements are found in a rapidly growing number of genes. Data from EMSA (electromobility shift assay) and transient transfection assays using heterologous promoter systems do not necessarily reflect transcriptional involvement and protein occupation of a binding site in vivo. This has been shown recently by in vivo footprinting of the NF-kappa B site at -40 in the interferon regulatory factor-1 (IRF-1) promoter. Here we show by in vivo footprinting using dimethylsulfate (DMS) that the GAS of the IRF-1 promoter, which also contains an overlapping putative NF-kappa B site, is occupied upon treatment with gamma-interferon (IFN gamma) but not with phorbol 12-myristate 13-acetate (PMA). Irrespective of induction, we detect a very strong DMS hypersensitivity at a guanosine just adjacent to GAS and a less persistent minor DMS hypersensitivity at a central cytosine. Our data confirm the crucial role of GAS in transcriptional activation by IFN gamma and are consistent with induced binding of p91 to GAS. In addition, our data suggest a major conformational distortion of the DNA at the GAS element of the IRF-1 promoter and that this GAS element is not involved in transcriptional activation by PMA.

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.

Restriction of interferon gamma responsiveness and basal expression of the myeloid human Fc gamma R1b gene is mediated by a functional PU.1 site and a transcription initiator consensus

The restricted expression of the human Fc gamma R1b gene to myeloid cells is likely to be regulated by a combination of transcription factors that may not be solely expressed in myeloid cells, but act together to restrict the expression of the gene to myeloid cells. Low basal expression of the human Fc gamma R1b gene is specifically upregulated by interferon gamma (IFN-gamma). A 181-bp region of 5' flanking sequence contains several key regulatory motifs that include the extended gamma response region (XGRR) and the PIE region. The XGRR contains the 39-bp gamma response region originally defined in the highly homologous Fc gamma R1a gene. The XGRR is in close proximity to the 21-bp PIE motif that is conserved in the promoters of some other myeloid genes. The PIE motif contains a consensus site for the macrophage and B cell transcription factor, PU.1, and is adjacent to the cluster of transcription start sites. An active transcription initiator, Inr, consensus spans the start sites and appears to direct transcription initiation of this TATA-less gene. In this study, we demonstrate that the PIE region contains a functional PU.1 site that binds a human PU.1-like protein and that associated factors present in myeloid extracts also bind in this PIE region. Mutational analysis reveals an absolute requirement for an intact PU.1 box for both basal and IFN-gamma inducible expression of this gene. In addition, mutations in the Inr greatly reduce basal and inducible transcription. Insertion of a strong TATA box downstream from the Inr or at -30 bp from the transcription start sites restores basal and inducible activity in the presence of a mutated PU.1 site. We also demonstrate that indeed, when the XGRR is positioned in the context of a heterologous TATA containing promoter, it is able to respond equivalently to either IFN-alpha or IFN-gamma. However, IFN-alpha responsiveness does not occur in the context of the physiological Fc gamma R1b TATA-less basal promoter. Our results suggest that a human PU.1-like factor acts as a "bridging factor" between the upstream IFN-gamma enhancer and the Inr dependent preinitiation complex. These findings indicate that the structure of the basal promoter in combination with restricted activators like PU.1 are important in regulating the expression of this gene.

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.

Induction by interleukin-6 of interferon regulatory factor 1 (IRF-1) gene expression through the palindromic interferon response element pIRE and cell type-dependent control of IRF-1 binding to DNA

The effects of interleukin-6 (IL-6) on interferon regulatory factor 1 (IRF-1) gene expression were studied in B-hybridoma B9 cells which are growth-stimulated by IL-6 and breast carcinoma T47D cells which are growth-inhibited. IL-6 induced the production of IRF-1 mRNA and protein in both cell types, but IRF-1 binding activity to its target DNA sequence was induced only in T47D cells. With B9 cells, there was no IRF-1 binding but instead strong constitutive binding of the IRF-2 repressor, indicating that binding of IRF-1 to DNA is an important regulatory step. The IRF-1 gene promoter element, palindromic IFN-response element (pIRE), was found to respond to IL-6 with high efficiency as compared with IFN-gamma or IFN-beta. On this palindromic TTC...GAA sequence, two protein complexes (pIRE-a and pIRE-b) were induced within minutes by IL-6. pIRE-b is similar to the main complex induced by IFN-gamma and contains the Stat91 protein. pIRE-a predominantly induced by IL-6 is a slowly migrating complex which does not contain Stat91 and has low affinity for IFN-gamma activated sequence (GAS)-type sequences. Comparison of the relative effects of IL-6 and IFN-gamma shows that pIRE enhancers are differently regulated than GAS elements. Distinct transcription complexes, forming in ratios dependent on the inducer, help explain how various cytokines sharing effects through Stat91 on related enhancers can produce specific patterns of gene expression. Activation of the pIRE-a factors defines a novel transcriptional activity of IL-6 in epithelial and lymphoid cells.

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 factor induced by differentiation signals in cells of the macrophage lineage binds to the gamma interferon activation site

Rapid transcriptional induction of genes in response to gamma interferon (IFN-gamma) is mediated by the IFN-gamma activation site (GAS) and its cognate protein, the IFN-gamma activation factor (GAF). We describe a GAS-associated, differentiation-induced factor (DIF) as a potential molecular link between the activities of IFN-gamma and of growth and differentiation factors. DIF DNA binding was activated by colony-stimulating factor 1 in murine macrophages and also during tetradecanoyl phorbol acetate-induced differentiation or IFN-gamma treatment in myeloid U937 cells. IFN-gamma activation of DIF decreased significantly upon monocytic differentiation. DIF binding to DNA was inhibited by antiphosphotyrosine antibodies and could be induced by treatment of U937 cells with vanadate. Unlike GAF, DIF-DNA complexes did not contain the 91-kDa protein (p91) from ISGF-3. DIF bound with high affinity to GAS from the promoters of the IFP 53/tryptophanyl-tRNA synthetase and Fc gamma RI genes, intermediate affinity to the Ly6A/E GAS, and low affinity to the guanylate-binding protein GAS. DIF may belong to a family of cytokine- or growth factor-induced factors binding with variable affinities to GAS-related elements: the interleukin-6-responsive acute-phase response factor associated with GAS from different IFN-inducible promoters but with a different preference of binding compared with DIF. The sis-inducible element of the c-fos promoter bound GAF but not DIF. However, the sis-inducible element could be changed by point mutation to compete for GAF and DIF binding. Our data show DIF to be a novel DNA-binding protein which is activated in response to differentiating signals. Moreover, they suggest that a family of cytokine- or growth factor-regulated proteins integrates and coordinates the responses to cytokines and to growth and differentiation factors by binding to GAS-related elements.

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 factor induced by differentiation signals in cells of the macrophage lineage binds to the gamma interferon activation site

Rapid transcriptional induction of genes in response to gamma interferon (IFN-gamma) is mediated by the IFN-gamma activation site (GAS) and its cognate protein, the IFN-gamma activation factor (GAF). We describe a GAS-associated, differentiation-induced factor (DIF) as a potential molecular link between the activities of IFN-gamma and of growth and differentiation factors. DIF DNA binding was activated by colony-stimulating factor 1 in murine macrophages and also during tetradecanoyl phorbol acetate-induced differentiation or IFN-gamma treatment in myeloid U937 cells. IFN-gamma activation of DIF decreased significantly upon monocytic differentiation. DIF binding to DNA was inhibited by antiphosphotyrosine antibodies and could be induced by treatment of U937 cells with vanadate. Unlike GAF, DIF-DNA complexes did not contain the 91-kDa protein (p91) from ISGF-3. DIF bound with high affinity to GAS from the promoters of the IFP 53/tryptophanyl-tRNA synthetase and Fc gamma RI genes, intermediate affinity to the Ly6A/E GAS, and low affinity to the guanylate-binding protein GAS. DIF may belong to a family of cytokine- or growth factor-induced factors binding with variable affinities to GAS-related elements: the interleukin-6-responsive acute-phase response factor associated with GAS from different IFN-inducible promoters but with a different preference of binding compared with DIF. The sis-inducible element of the c-fos promoter bound GAF but not DIF. However, the sis-inducible element could be changed by point mutation to compete for GAF and DIF binding. Our data show DIF to be a novel DNA-binding protein which is activated in response to differentiating signals. Moreover, they suggest that a family of cytokine- or growth factor-regulated proteins integrates and coordinates the responses to cytokines and to growth and differentiation factors by binding to GAS-related elements.

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.

Tyrosine phosphorylated p91 binds to a single element in the ISGF2/IRF-1 promoter to mediate induction by IFN alpha and IFN gamma, and is likely to autoregulate the p91 gene

ISGF2 was initially identified, purified and cloned as an interferon-alpha (IFN alpha) induced transcription factor that binds to the IFN-stimulated response element (ISRE) of IFN alpha/beta-stimulated genes (ISGs). It was reported to be transcriptionally regulated by several cytokines including IFN alpha and IFN gamma. IFN alpha and IFN gamma inducibility is mediated by a single element: a high affinity, nearly palindromic version of the IFN gamma activation site (GAS). The ISGF2 GAS is bound specifically by p91, which was previously identified as a subunit of the ISG activator ISGF3, and shown to mediate IFN gamma induction of the GBP gene via a GAS. Tyrosine phosphorylation and DNA binding activity of p91 parallel transcription of ISGF2 in response to IFN alpha and/or IFN gamma, consistent with induction mediated by only a GAS. Transcription of the genes that encode p91 and p113, another subunit of ISGF3, is activated only by IFN alpha. This result suggests induction mediated by an ISRE, and implies autoregulation, requiring the products of both genes. Specificity of the ISRE is the basis for the previous conclusion. In contrast, it appears likely that the ISGF2 GAS, and p91 or related factors, also mediate induction of ISGF2 by IL-6 and prolactin. Convergence of signalling pathways from at least four cytokines on this single site would thus be a key aspect of a general role for ISGF2 in cellular growth control.

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.