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

Signaling by the cytokine receptor superfamily: JAKs and STATs

A variety of cytokines, lymphokines and growth factors function by interacting with receptors that are members of the cytokine receptor superfamily. These receptors share extracellular motifs and have limited similarity in their cytoplasmic domains. Although lacking catalytic domains, this family of receptors couples ligand binding with the induction of tyrosine phosphorylation. Recent studies have shown that this is mediated by members of the Janus kinase (JAK) family of cytoplasmic protein tyrosine kinases. JAKs physically associate with the membrane-proximal region of the ligand-bound receptor, leading to their tyrosine phosphorylation and activation. The activated JAKs phosphorylate the receptors as well as cytoplasmic proteins belonging to a family of transcription factors called the signal transducers and activators of transcription (STATs), providing a novel signaling pathway that is shared by all members of the cytokine receptor superfamily.

Molecular cloning of APRF, a novel IFN-stimulated gene factor 3 p91-related transcription factor involved in the gp130-mediated signaling pathway

Acute-phase response factor (APRF) is a transcription factor that binds to the interleukin-6 (IL-6)-responsive elements identified in the promoters of various acute-phase protein genes. We report here the purification and cloning of APRF. APRF exhibits a 52.5% overall homology at the amino acid level with p91, a component of the interferon (IFN)-stimulated gene factor 3 complexes. The cloned APRF protein is tyrosine phosphorylated and translocated into the nucleus in response to IL-6, but not in response to IFN-gamma. Tyrosine phosphorylation was also observed in response to other cytokines, such as leukemia inhibitory factor, oncostatin M, and ciliary neurotrophic factor, whose receptors share the IL-6 receptor signal transducer gp130. In contrast, we observed that p91 is not tyrosine phosphorylated in response to IL-6. These results suggest that this novel p91-related protein may play a major role in the gp130-mediated signaling pathway and that selective activation of p91-related factors may explain the diversity of cellular responses to different cytokines.

Molecular cloning of rat JAK3, a novel member of the JAK family of protein tyrosine kinases

We have cloned and sequenced a cDNA (JAK3) encoding a novel member of the JAK family of protein tyrosine kinases. JAK3 was identified by RT-PCR of rat mesangial cells using degenerate oligonucleotide primers, and a full-length clone was isolated from a rat spleen cDNA library. The primary structure of JAK3 showed cDNA with an open reading frame of 1,100 amino acids which comprises the PTK catalytic domain and a second kinase-related domain characteristic for JAK kinase. JAK3 was phylogenetically shown to be most closely related to JAK2 among the previously known JAK family members, JAK1, JAK2 and Tyk2. Southern analysis revealed that JAK3 is a single copy gene and well conserved in the vertebral genome. Northern analysis indicated that the 4.0 kb mRNA was transcribed in a variety of tissues including spleen, lung, kidney and intestine.

Signaling by the cytokine receptor superfamily just another kinase story

Many cytokines initiate cellular responses through their interaction with members of the cytokine receptor superfamily. This family of receptors contains no catalytic domains in the cytoplasmic domain, but all couple ligand binding to tyrosine phosphorylation, and this activity requires a membrane-proximal region that contains some similarity among the receptors. Recent studies have shown that members of the JAK family of protein tyrosine kinases associate with the membraneproximal region, are rapidly tyrosine-phosphorylated following ligand binding, and their in vitro kinase activity is activated. The JAK family of kinases is characterized by two kinase domains, only one of which contains all of the hallmarks of active kinases. This family of 130-kD kinases lacks SH2 or SH3 domains, but family members contain extensive homology in the large amino terminal region. Individual receptors associate with, or require, one or more of the three known family members including JAKI, JAK2, and tyk2. Putative substrates of the JAK family of kinases include the 91-kD and 113-kD proteins of the interferon-stimulated transcription complex ISGF3 that, when tyrosinephosphorylated, migrate to the nucleus and participate in the activation of gene transcription. Recent evidence suggests that the 91- and 113-kD proteins are members of a large family of genes that are potential substrates o f JAK family members and may regulate a variety o f genes involved in cell growth, differentiation, or function. Together the data provide a new, generalized model for the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily regulate cellular activity.

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.

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.

Structural and functional basis for hormone binding and receptor oligomerization

Most single-pass transmembrane receptors undergo a change in oligomeric state upon hormone binding. Recent mutational, biophysical and structural studies of the human growth hormone and tumor necrosis factor receptor complexes have revealed much about the mechanisms and molecular bases for binding and oligomerization. Principles learned from these examples and others should apply to many other hormone-receptor complexes.

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.

Activation of JAK2 kinase mediated by the interleukin 6 signal transducer gp130

The interleukin 6 receptor-associated signal transducer, gp130, is shared by receptor complexes for leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and interleukin 11. We show here that JAK2 kinase is rapidly tyrosine phosphorylated in mouse embryonic stem cells whose pluripotentiality is maintained only by gp130-sharing cytokines after stimulation that is known to induce gp130 homodimerization. JAK1 is also tyrosine phosphorylated, but to a lesser extent, under the same conditions. Comparable results are obtained with hemopoietic lineage cells such as myeloid leukemic M1 cells and pro-B-cell line-derived transfectants expressing gp130, the former of which differentiate into macrophages after stimulation of gp130 and the latter of which initiate DNA synthesis. gp130-dimerizing stimulus upregulates kinase activity of JAK2 as revealed by immunocomplex kinase assay. Deletion or point mutation in the membrane-proximal cytoplasmic motifs in gp130 that are conserved in the hemopoietic cytokine receptor family results in the loss of tyrosine phosphorylation of JAK2, which coincides with the lack of signal transducing capability of gp130 mutants.

A novel member of the interferon receptor family complements functionality of the murine interferon gamma receptor in human cells

Expression of the human interferon gamma receptor (IFN-gamma R) in mouse cells is not sufficient to confer biological responsiveness to human IFN-gamma and vice versa. An additional species-specific component is required for signal transduction. We identified this cofactor by expression cloning in simian COS cells stably transfected with the nonfunctional murine IFN-gamma R and a IFN-gamma-inducible reporter construct encoding the human Tac antigen (interleukin-2 receptor alpha chain, CD25). A cDNA clone was obtained that, upon stable transfection, rendered human HEp-2 cells expressing the murine IFN-gamma R fully responsive to murine IFN-gamma. This cDNA encodes a novel 332 amino acid type I transmembrane protein that belongs to the IFN receptor family and that we designate IFN-gamma R beta chain.

Identification and sequence of an accessory factor required for activation of the human interferon gamma receptor

Human chromosomes 6 and 21 are both necessary to confer sensitivity to human interferon gamma (Hu-IFN-gamma), as measured by induction of class I human leukocyte antigen (HLA) and protection against encephalomyocarditis virus (EMCV) infection. Whereas human chromosome 6 encodes the Hu-IFN-gamma receptor, human chromosome 21 encodes accessory factors for generating biological activity through the Hu-IFN-gamma receptor. Probes from a genomic clone were used to identity cDNA clones expressing a species-specific accessory factor. These cDNA clones are able to substitute for human chromosome 21 to reconstitute the Hu-IFN-gamma receptor-mediated induction of class I HLA antigens. However, the factor encoded by the cDNA does not confer full antiviral protection against EMCV, confirming that an additional factor encoded on human chromosome 21 is required for reconstitution of antiviral activity against EMCV. We conclude that this accessory factor belongs to a family of such accessory factors responsible for different actions of IFN-gamma.

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.

Association and activation of Jak-Tyk kinases by CNTF-LIF-OSM-IL-6 beta receptor components

A recently defined family of cytokines, consisting of ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), oncostatin M (OSM), and interleukin-6 (IL-6), utilize the Jak-Tyk family of cytoplasmic tyrosine kinases. The beta receptor components for this cytokine family, gp130 and LIF receptor beta, constitutively associate with Jak-Tyk kinases. Activation of these kinases occurs as a result of ligand-induced dimerization of the receptor beta components. Unlike other cytokine receptors studied to date, the receptors for the CNTF cytokine family utilize all known members of the Jak-Tyk family, but induce distinct patterns of Jak-Tyk phosphorylation in different cell lines.

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.

Interferon-induced nuclear signalling by Jak protein tyrosine kinases

Interferons IFN-alpha/beta and IFN-gamma act through independent cell-surface receptors, inducing gene expression through tyrosine phosphorylation of cytoplasmic transcription factors . IFN-alpha stimulates phosphorylation and nuclear localization of the 84/91K and 113K subunits of latent ISGF3 (interferon-stimulated gene factor 3), which combine with the 48K DNA-binding subunit to bind regulatory elements of IFN-alpha-responsive genes. IFN-gamma activates p91 alone, inducing IFN-gamma-responsive genes through a distinct DNA element. Genetic complementation studies implicated the tyrosine kinase Tyk2 in IFN-alpha signalling and, more recently, the related Jak2 kinase in IFN-gamma signalling. We now present biochemical evidence for Jak-family kinase involvement in IFN signal transduction. Jak1 was activated in response to IFN-alpha and IFN-gamma; Jak2 responded exclusively to IFN-gamma. Overexpression of either Jak1 or Jak2 stimulated p91 DNA-binding activity and p91-dependent transcription. Overexpression also activated endogenous Jak kinases, suggesting that interactions between Jak kinases are required during interferon signalling.

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.

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.