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

Involvement of the IRF-1 transcription factor in antiviral responses to interferons

The mechanisms underlying interferon (IFN)-induced antiviral states are not well understood. Interferon regulatory factor-1 (IRF-1) is an IFN-inducible transcriptional activator, whereas IRF-2 suppresses IRF-1 action. The inhibition of encephalomyocarditis virus (EMCV) replication by IFN-alpha and especially by IFN-gamma was impaired in cells from mice with a null mutation in the IRF-1 gene (IRF-1-/- mice). The IRF-1-/- mice were less resistant than normal mice to EMCV infection, as revealed by accelerated mortality and a larger virus titer in target organs. The absence of IRF-1 did not clearly affect replication of two other types of viruses. Thus, IRF-1 is necessary for the antiviral action of IFNs against some viruses, but IFNs activate multiple activation pathways through diverse target genes to induce the antiviral state.

Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins

Through the study of transcriptional activation in response to interferon alpha (IFN-alpha) and interferon gamma (IFN-gamma), a previously unrecognized direct signal transduction pathway to the nucleus has been uncovered: IFN-receptor interaction at the cell surface leads to the activation of kinases of the Jak family that then phosphorylate substrate proteins called STATs (signal transducers and activators of transcription). The phosphorylated STAT proteins move to the nucleus, bind specific DNA elements, and direct transcription. Recognition of the molecules involved in the IFN-alpha and IFN-gamma pathway has led to discoveries that a number of STAT family members exist and that other polypeptide ligands also use the Jak-STAT molecules in signal transduction.

Signaling pathways activated by protein tyrosine phosphorylation in lymphocytes

Antigen and cytokine receptors induce rapid tyrosine phosphorylation of receptor subunits, other membrane proteins, and signaling components. Each receptor induces phosphorylation of a number of proteins. Although there is often overlap between targets of different receptors, any given receptor only induces phosphorylation of a subset of possible targets. How this choice of targets is achieved for these receptors is not yet understood. The cellular events downstream of some signaling components are beginning to come into view. Recent progress in these areas is discussed.

Receptor to nucleus signaling via tyrosine phosphorylation of the p91 transcription factor

Recent studies on signal transduction stimulated by interferons have defined pathways that link cell surface receptors to target genes in the nucleus. After interferon binding, nonreceptor tyrosine kinases are activated that phosphorylate components of latent DNA-binding factors in the cytoplasm. These phosphorylated factors form multimeric complexes that translocate to the nucleus and bind to specific DNA sequences in the promoters of induced genes. A 91-kD factor (p91 or Stat 91) is activated by interferons and serves as a subunit partner in the composition of diverse transcription factors.

Cytokine signal transduction and the JAK family of protein tyrosine kinases

Cytokine receptors fall into two basic classes: those with their own intrinsic protein tyrosine kinase (PTK) domain, and those lacking a PTK domain. Nonetheless, PTK activity plays a fundamental role in the signal transduction processes lying downstream of both classes of receptor. It now seems likely that many of those cytokine receptors that lack their own PTK domain use members of the JAK family of PTKs to propagate their intracellular signals. Moreover, the involvement of the JAK kinases in a newly defined pathway which links membrane receptors directly to the activation of nuclear genes, via latent cytoplasmic transcription factors known as STATs (for Signal Transducers and Activators of Transcription), appears to be a theme common to cytokine receptors of both classes.

Interferon-activated signal transduction to the nucleus

Recent studies on gene activation by interferons have uncovered a direct signaling pathway from the cell-surface receptor for interferons to the nucleus. Activation of JAK tyrosine kinases by interferons leads to activation, by direct tyrosine phosphorylation, of a family of latent cytoplasmic transcription factors named STATs (for signal transducers and activators of transcription), which then translocate to the nucleus to direct transcriptional activation.

Interferon activation of the transcription factor Stat91 involves dimerization through SH2-phosphotyrosyl peptide interactions

Stat91 (a 91 kd protein that acts as a signal transducer and activator of transcription) is inactive in the cytoplasm of untreated cells but is activated by phosphorylation on tyrosine in response to a number of polypeptide ligands, including interferon alpha (IFN-alpha) and IFN-gamma. We report here that the inactive Stat91 in the cytoplasm of untreated cells is a monomer and that upon IFN-gamma-induced phosphorylation it forms a stable homodimer. Only the dimer is capable of binding to a specific DNA sequence directing transcription. Through dissociation and reassociation assays, we show that dimerization of Stat91 is mediated through SH2-phosphotyrosyl peptide interactions. Dimerization involving SH2 recognition of specific phosphotyrosyl peptides may well provide a prototype for interactions among family members of STAT proteins to form different transcription complexes.

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.

Polypeptide signalling to the nucleus through tyrosine phosphorylation of Jak and Stat proteins

Binding of interferons IFN-alpha and IFN-gamma to their cell surface receptors promptly induces tyrosine phosphorylation of latent cytoplasmic transcriptional activators (or Stat proteins, for signal transducers and activators of transcription). Interferon-alpha activates both Stat91 (M(r) 91,000; ref. 1) and Stat113 (M(r) 113,000; ref. 2) whereas IFN-gamma activates only Stat91 (refs 3, 4). The activated proteins then move into the nucleus and directly activate genes induced by IFN-alpha and IFN-gamma. Somatic cell genetics experiments have demonstrated a requirement for tyrosine kinase-2 (Tyk2) in the IFN-alpha response pathway and for Jak2 (ref. 6), a kinase with similar sequence, in the IFN-gamma response pathway. Here we investigate the tyrosine phosphorylation events on Stat and Jak proteins after treatment of cells with IFNs alpha and gamma and with epidermal growth factor (EGF). Stat91 is phosphorylated on Tyr701 after cells are treated with IFN-alpha and EGF, as it was after treatment with IFN-gamma (ref. 8). We find that Jak1 also becomes phosphorylated on tyrosine after cells are treated with these same three ligands, although each ligand is shown to activate at least one other different kinase. Jak1 may therefore be the enzyme that phosphorylates Tyr 701 in Stat91.

Complementation by the protein tyrosine kinase JAK2 of a mutant cell line defective in the interferon-gamma signal transduction pathway

Interferons (IFNs) alpha/beta (type I) and gamma (type II) bind to distinct cell surface receptors, inducing transcription of overlapping sets of genes by intracellular pathways that have recently attracted much attention. Previous studies using cell lines selected for their inability to respond to IFN-alpha (ref. 4) have shown that the protein kinase Tyk2 plays a central role in the IFN alpha/beta response. Here we report the isolation of the cell line gamma 1A, selected for its inability to express IFN-gamma-inducible cell-surface markers, that is deficient in all aspects of the IFN-gamma response tested, but responds normally to IFNs alpha and beta. The mutant cells can be complemented by the expression of another member of the JAK family of protein tyrosine kinases, JAK2 (refs 6-9). Unlike IFNs alpha and beta, IFN-gamma induces rapid tyrosine phosphorylation of JAK2 in wild-type cells, and JAK2 immunoprecipitates from these cells show tyrosine kinase activity. These responses are absent in gamma 1A cells. JAK2 is therefore required for the response to IFN-gamma but not to IFNs alpha and beta.

The protein tyrosine kinase JAK1 complements defects in interferon-alpha/beta and -gamma signal transduction

We have produced a cell line which lacks the protein tyrosine kinase JAK1 and is completely defective in interferon response. Complementation of this mutant with JAK1 restored the response, establishing the requirement for JAK1 in both the interferon-alpha/beta and -gamma signal transduction pathways. The reciprocal interdependence between JAK1 and Tyk2 activities in the interferon-alpha pathway, and between JAK1 and JAK2 in the interferon-gamma pathway, may reflect a requirement for these kinases in the correct assembly of interferon receptor complexes.