Interaction of the transcriptional activator Stat-2 with the type I interferon receptor

Protein phosphorylation profiling of peripheral nerve regeneration after autologous nerve grafting

Autologous nerve grafting is the golden standard therapeutic approach of peripheral nerve injury. However, the clinical effect of autologous nerve grafting is still unsatisfying. To achieve better clinical functional recovery, it is of an impending need to expand our understanding of the dynamic cellular and molecular changes after nerve transection and autologous nerve transplantation. To address this aim, in the current study, rats were subjected to sciatic nerve transection and autologous nerve grafting. Rat sciatic nerve segments were collected at 4, 7, and 14 days after surgery and subjected to antibody array analysis to determine phosphoprotein profiling patterns. Compared with rats that underwent sham surgery, a total of 48, 19, and 75 differentially expressed phosphoproteins with fold changes > 2 or < -2 were identified at 4, 7, and 14 days after autologous nerve grafting, respectively. Several phosphoproteins, including STAM2 (Phospho-Tyr192) and Tau (Phospho-Ser422), were found to be differentially expressed at multiple time points, suggesting the importance of the phosphorylation of these proteins. Western blot validation of the expression patterns of STAM2 (Phospho-Tyr192) indicated the accuracy of antibody array assay. Bioinformatic analysis of these differentially expressed proteins suggested that cellular behavior and organ morphology were significantly involved biological functions while cell behavior and immune response-related signaling pathways were significantly involved canonical signaling pathways. These outcomes contributed to the illumination of the molecular mechanisms underlying autologous nerve grafting from the phosphoprotein profiling perspective.

Measles Virus-Based Treatments Trigger a Pro-inflammatory Cascade and a Distinctive Immunopeptidome in Glioblastoma

Glioblastoma is an aggressive primary brain tumor with bad prognosis. On the other hand, oncolytic measles virus (MeV) therapy is an experimental glioma treatment strategy with clinical safety and first evidence of anti-tumoral efficacy. Therefore, we investigated the combination of MeV with conventional therapies by cytotoxic survival assays in long-term glioma cell lines LN229, LNZ308, and glioma stem-like GS8 cells, as well as the basal viral infectivity in primary glioblastoma cultures T81/16, T1094/17, and T708/16. We employed Chou-Talalay analysis to identify the synergistic treatment sequence chemotherapy, virotherapy, and finally radiotherapy (CT-VT-RT). RNA sequencing and immunopeptidome analyses were used to delineate treatment-induced molecular and immunological profiles. CT-VT-RT displayed synergistic anti-glioma activity and initiated a type 1 interferon response, along with canonical Janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling, and downstream interferon-stimulated genes were induced, resulting in apoptotic cascades. Furthermore, antigen presentation along with immunostimulatory chemokines was increased in CT-VT-RT-treated glioma cells, indicating a treatment-induced pro-inflammatory phenotype. We identified novel treatment-induced viral and tumor-associated peptides through HLA ligandome analysis. Our data delineate an actionable treatment-induced molecular and immunological signature of CT-VT-RT, and they could be exploited for the design of novel tailored treatment strategies involving virotherapy and immunotherapy.

[Endosomal control of intracellular signaling]

Membrane receptors control essential processes such as cell growth, adhesion, differentiation and metabolism through the activation of specific signaling pathways. Nowadays, these receptors are not only known to signal from the plasma membrane but also from intracellular compartments. Indeed, after being internalized with their ligands via different endocytic pathways, some membrane receptors can initiate signal only after reaching the sorting endosome where they associate with specific protein partners. This review illustrates how this spatio-temporal regulation of signal transduction can occur, with several examples, including interferon receptors which activate JAK/STAT signaling pathways. The literature presented here explains why this control of signaling pathways occuring at the endosomal level creates a higher degree of tuning for the affected cellular processes.

Protein kinase C activation mediates interferon-β-induced neuronal excitability changes in neocortical pyramidal neurons

BACKGROUND

Cytokines are key players in the interactions of the immune and nervous systems. Recently, we showed that such interplay is mediated by type I interferons (IFNs), which elevate the excitability of neocortical pyramidal neurons. A line of indirect evidence suggested that modulation of multiple ion channels underlies the effect. However, which currents are principally involved and how the IFN signaling cascade is linked to the respective ion channels remains elusive.

METHODS

We tested several single and combined ionic current modulations using an in silico model of a neocortical layer 5 neuron. Subsequently we investigated resulting predictions by whole-cell patch-clamp recordings in layer 5 neurons of ex vivo neocortical rat brain slices pharmacologically reproducing or prohibiting neuronal IFN effects.

RESULTS

The amount and type of modulation necessary to replicate IFN effects in silico suggested protein kinase C (PKC) activation as link between the type I IFN signaling and ion channel modulations. In line with this, PKC activation with 4β-phorbol 12-myristate 13-acetate (4β-PMA) or Bryostatin1 augmented the excitability of neocortical layer 5 neurons comparable to IFN-β in our ex vivo recordings. In detail, both PKC activators attenuated the rheobase and increased the input-output gain as well as the input resistance, thereby augmenting the neuronal excitability. Similar to IFN-β they also left the threshold of action potential generation unaffected. In further support of PKC mediating type I IFN effects, IFN-β, 4β-PMA and Bryostatin1 reduced the amplitude of post-train after-hyperpolarizations in a similar manner. In conjunction with this finding, IFN-β reduced M-currents, which contribute to after-hyperpolarizations and are modulated by PKC. Finally, blocking PKC activation with GF109203X at the catalytic site or calphostin C at the regulatory site prevented the main excitatory effects of IFN-β.

CONCLUSION

Multiple ion channel modulations underlie the neuromodulatory effect of type I IFNs. PKC activation is both sufficient and necessary for mediating the effect, and links the IFN signaling cascade to the intrinsic ion channels. Therefore, we regard PKC activation as unitary mechanism for the neuromodulatory potential of type I IFNs in neocortical neurons.

Interferon induction and function at the mucosal surface

Interferons (IFNs) are produced in response to virus infection and induce an antiviral state in virtually all cell types. In addition to upregulating the transcription of genes that inhibit virus replication, type I (or -α/β) IFNs also act to orchestrate the adaptive immune response to virus infection. Recently a new family of antiviral cytokines, the type III (or -λ) IFNs, has been identified that activate the same antiviral pathways via a distinct receptor. Although the identical transcription factor, IFN-stimulated gene factor 3 is activated by either IFN-α/β or IFN-λ signaling, differences in the induction and action of these two cytokine families are beginning to be appreciated. In this article, we review this emerging body of literature on the differing roles these cytokines play in host defense of the mucosal surface. Although many viruses enter the body through the respiratory and gastrointestinal tracts, we have focused the discussion on influenza A virus, respiratory syncytial virus, and rotavirus, three ubiquitous human pathogens that target the epithelial lining and are associated with a major disease burden.

Induction and control of the type I interferon pathway by Bluetongue virus

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A general review describing the current knowledge on the type I IFN pathway.

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Description of several mechanisms evolved by viruses to counteract this antiviral response.

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An up-to-date review on the interaction of BTV and the type I IFN pathway in vivo and in vitro.

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Description of the cellular sensors involved in the induction of IFN-α/β synthesis upon BTV infection in haematopoietic and non-haematopoietic cells.

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Description of the strategies evolved by BTV to counteract this cellular antiviral response.

The innate immune response is the first line of defence against viruses, involving the production of type I IFN (IFN-α/β) and other pro-inflammatory cytokines that control the infection. It also shapes the adaptive immune response generated by both T and B cells. Production of type I IFN occurs both in vivo and in vitro in response to Bluetongue virus (BTV), an arthropod-borne virus. However, the mechanisms responsible for the production of IFN-β in response to BTV remained unknown until recently and are still not completely understood. In this review, we describe the recent advances in the identification of cellular sensors and signalling pathways involved in this process. The RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) were shown to be involved in the expression of IFN-β as well as in the control of BTV infection in non-haematopoietic cells. In contrast, induction of IFN-α/β synthesis in sheep primary plasmacytoid dendritic cells (pDCs) required the MyD88 adaptor independently of the Toll-like receptor 7 (TLR7), as well as the kinases dsRNA-activated protein kinase (PKR) and stress-activated protein kinase (SAPK)/Jun N-terminal protein kinase (JNK). As type I IFN is essential for the establishment of an antiviral cellular response, most of viruses have elaborated counteracting mechanisms to hinder its action. This review also addresses the ability of BTV to interfere with IFN-β synthesis and the recent findings describing the non-structural viral protein NS3 as a powerful antagonist of the host cellular response.

Molecular cloning, subcelluar location and expression profile of signal transducer and activator of transcription 2 (STAT2) from turbot, Scophthalmus maximus

Signal transducer and activator of transcription 2 (STAT2) is an important molecule involved in the type I interferon signalling pathway. To date, little STAT2 homologue is available in fish except Atlantic salmon and goldfish. In this paper, STAT2 was firstly cloned and characterized from turbot, a marine flatfish with high economic value. Briefly, turbot STAT2 cDNA is 3206 bp in length encoding a predicted protein of 793 amino acids. The phylogenetic tree shows that turbot STAT2 protein shared the closest relationship with Atlantic salmon. Analysis of subcellular distribution indicates that STAT2 is mainly present in the cytoplasm of TK cells. Stat2 mRNA is constitutively expressed in widespread tissues and induced by several folds in turbot tissues and TK cells after stimulation with Vibrio anguillarum and lymphocystis disease virus (LCDV). Unlike the higher vertebrate STAT2, turbot STAT2 nuclear export signal (NES) exists not in the C-terminal 79 amino acids but in N-terminal 137-312 amino acids (STAT_alpha domain). The nuclear translocation of turbot STAT2 after Poly(I:C) treatment proved its transcription activity in TK cells. All these results suggested that STAT2 may be involved in the immune response in turbot as a transcription factor.

HSV-2 inhibits type-I interferon signaling via multiple complementary and compensatory STAT2-associated mechanisms

Type-I interferon (IFN)-mediated responses are a crucial first line of defense against viral infections and are critical for generating both innate and adaptive immunity. Therefore, viruses have necessarily evolved mechanisms to impede the IFN response. HSV-2 was found to completely abolish type-1 IFN-mediated signaling via multiple STAT2-associated mechanisms. Although the extent and kinetics of this inactivation were indistinguishable between the various cell-lines examined, there were distinct differences in the mechanisms HSV-2 employed to subvert IFN-signaling among the cell-lines. These mechanistic differences could be segregated into two categories dependent on the phase of the HSV replicative cycle that was responsible for this inhibition: (1) early phase-inhibited cells which exhibited abrogation of IFN-signaling prior to viral DNA replication; (2) late phase-inhibited cells where early phase inhibition mechanisms were not functional, but viral functions expressed following DNA replication compensated for their ineffectiveness. In early phase-inhibited cells, HSV-2 infection targeted STAT2 protein for proteosomal degradation and prevented de novo expression of STAT2 by degrading its mRNA. In contrast, HSV-2 infected late phase-inhibited cells exhibited no apparent changes in STAT2 transcript or protein levels. However, in these cells STAT2 was not activated by phosphorylation and failed to translocate to the cell nucleus, thereby preventing transactivation of antiviral genes. In primary human fibroblasts, HSV-2 failed to fully degrade STAT2 and therefore, both early and late phase mechanisms functioned cooperatively to subvert IFN-mediated antiviral gene expression. Taken together, these results indicate the importance that HSV-2 has assigned to STAT2, investing significant genomic currency throughout its replicative lifecycle for continuous targeted destruction and inhibition of this protein.

Herpes simplex virus-1 infection causes the secretion of a type I interferon-antagonizing protein and inhibits signaling at or before Jak-1 activation

Host cells respond to viral infection by the production of type I interferons (IFNs), which induce the expression of antiviral genes. Herpes simplex virus I (HSV-1) encodes many mechanisms that inhibit the type I IFN response, including the ICP27-dependent inhibition of type I IFN signaling. Here we show inhibition of Stat-1 nuclear accumulation in cells that express ICP27. ICP27 expression also induces the secretion of a small, heat-stable type I IFN antagonizing protein that inhibits Stat-1 nuclear accumulation. We show that the inhibition of IFN-induced Stat-1 phosphorylation occurs at or upstream of Jak-1 phosphorylation. Finally, we show that ISG15 expression is induced after IFNalpha treatment in mock-infected cells, but not cells infected with WT HSV-1 or ICP27(-) HSV-1. These data suggest that HSV-1 has evolved multiple mechanisms to inhibit IFN signaling not only in infected cells, but also in neighboring cells, thereby allowing for increased viral replication and spread.

Palmitoylation of interferon-alpha (IFN-alpha) receptor subunit IFNAR1 is required for the activation of Stat1 and Stat2 by IFN-alpha

Type I interferons (IFNs) bind IFNAR receptors and activate Jak kinases and Stat transcription factors to stimulate the transcription of genes downstream from IFN-stimulated response elements. In this study, we analyze the role of protein palmitoylation, a reversible post-translational lipid modification, in the functional properties of IFNAR. We report that pharmacological inhibition of protein palmitoylation results in severe defects of IFN receptor endocytosis and signaling. We generated mutants of the IFNAR1 subunit of the type I IFN receptor, in which each or both of the two cysteines present in the cytoplasmic domain are replaced by alanines. We show that cysteine 463 of IFNAR1, the more proximal of the two cytoplasmic cysteines, is palmitoylated. A thorough microscopic and biochemical analysis of the palmitoylation-deficient IFNAR1 mutant revealed that IFNAR1 palmitoylation is not required for receptor endocytosis, intracellular distribution, or stability at the cell surface. However, the lack of IFNAR1 palmitoylation affects selectively the activation of Stat2, which results in a lack of efficient Stat1 activation and nuclear translocation and IFN-alpha-activated gene transcription. Thus, receptor palmitoylation is a previously undescribed mechanism of regulating signaling activity by type I IFNs in the Jak/Stat pathway.

Regulation of XAF1 expression in human colon cancer cell by interferon beta: activation by the transcription regulator STAT1

XIAP-associated factor 1 (XAF1) is a novel tumor suppressor and interferon stimulated gene (ISG). Interferon beta (IFNbeta) exerts anti-proliferative effect and induces apoptosis through the Jak-Stat signaling cascade by the type I Interferon receptor (IFN-R), which initiates gene transcription of those biological effectors of IFNbeta. The aim of this study is to determine the effect of IFNbeta on XAF1 expression and the putative mechanisms mediated by the critical role of signal transducers and activators of transcription 1 (Stat1). Gene expression was detected by RT-PCR and Western blot analysis. The promoter activity of XAF1 was examined by luciferase reporter assay. The activity of interferon stimulated response element (ISRE) was assessed by electrophoretic mobility shift assay (EMSA) and quantitative chromatin immunoprecipitation assay (Q-ChIP). Results showed that IFNbeta stimulated XAF1 promoter activity in colon cancer cell line DLD1 in a time- and dose-dependent manner. A high affinity ISRE binding element (ISRE-XAF1) was located in -55 to -66 nt upstream of the first ATG site of XAF1 gene. Deletion of ISRE-XAF1 completely abrogated basal and IFNbeta-induced promoter activity. IFNbeta-induced XAF1 expression was mediated by Stat1 through the interaction with ISRE-XAF1. Knocking down of the Stat1 expression and blocking its phosphorylation decreased IFNbeta-induced XAF1 expression. Results suggested that induction of an immediate early response gene-XAF1 by IFNbeta was mediated by the transcription regulator Stat1 through the ISRE site within the promoter region of XAF1 gene in colon cancer.

Inhibition of interferon signaling by rabies virus phosphoprotein P: activation-dependent binding of STAT1 and STAT2

Rabies virus (RV) phosphoprotein P is an interferon (IFN) antagonist counteracting transcriptional activation of type I IFN (K. Brzózka, S. Finke, and K. K. Conzelmann, J. Virol 79:7673-7681, 2005). We here show that RV P in addition is responsible for preventing IFN-alpha/beta- and IFN-gamma-stimulated JAK-STAT signaling in RV-infected cells by the retention of activated STATs in the cytoplasm. Expression of IFN-stimulated response element- and gamma-activated sequence-controlled genes was severely impaired in cells infected with RV SAD L16 or in cells expressing RV P protein from transfected plasmids. In contrast, a recombinant RV expressing small amounts of P had lost the ability to interfere with JAK-STAT signaling. IFN-mediated tyrosine phosphorylation of STAT1 and STAT2 was not impaired in RV P-expressing cells; rather, a defect in STAT recycling was suggested by distinct accumulation of tyrosine-phosphorylated STATs in cell extracts. In the presence of P, activated STAT1 and STAT2 were unable to accumulate in the nucleus. Notably, STAT1 and STAT2 were coprecipitated with RV P only from extracts of cells previously stimulated with IFN-alpha or IFN-gamma, whereas in nonstimulated cells no association of P with STATs was observed. This conditional, IFN activation-dependent binding of tyrosine-phosphorylated STATs by RV P is unique for a viral IFN antagonist. The 10 C-terminal residues of P are required for counteracting JAK-STAT signaling but not for inhibition of transcriptional activation of IFN-beta, thus demonstrating two independent functions of RV P in counteracting the host's IFN response.

Mechanisms of type-I interferon signal transduction

Interferons regulate a number of biological functions including control of cell proliferation, generation of antiviral activities and immumodulation in human cells. Studies by several investigators have identified a number of cellular signaling cascades that are activated during engagement of interferon receptors. The activation of multiple signaling cascades by the interferon receptors appears to be critical for the generation of interferon-mediated biological functions and immune surveillance. The present review summarizes the existing knowledge on the multiple signaling cascades activated by Type I interferons. Recent developments in this research area are emphasized and the implications of these new discoveries on our understanding of interferon actions are discussed.

Role of protein kinase C-δ (PKC-δ) in the generation of the effects of IFN-α in chronic myelogenous leukemia cells

OBJECTIVE

The mechanisms by which interferon alpha (IFN-alpha) induces antileukemic responses in chronic myelogenous leukemia (CML) cells are not known. We examined whether a member of the protein kinase C (PKC) family of proteins, PKC-delta, is activated during treatment of BCR-ABL cells with IFN-alpha and participates in the induction of interferon responses.

METHODS

Immunoblots and immune complex kinase assays were performed to study the phosphorylation and activation of PKC-delta in response to IFN-alpha in CML-derived cell lines. The effects of pharmacological inhibition of PKC-delta on the suppressive effects of IFN-alpha on leukemic CFU-GM progenitors from CML patients were assessed by clonogenic assays in methylcellulose.

RESULTS

IFN-alpha treatment of the sensitive CML-derived KT-1 cell line resulted in phosphorylation of PKC-delta and activation of its kinase domain. Such phosphorylation/activation of PKC-delta was required for phosphorylation of Stat1 on serine 727, as inhibition of PKC-delta activity blocked the IFN-alpha-dependent serine phosphorylation of Stat1 and IFN-alpha-inducible gene transcription. IFN-alpha treatment strongly inhibited leukemic CFU-GM progenitor colony formation from bone marrow or peripheral blood of patients with CML, and such inhibition was reversed by concomitant treatment of the cells with the PKC-delta pharmacologic inhibitor rottlerin.

CONCLUSION

Taken altogether, our data demonstrate that PKC-delta plays a critical role in Type I IFN signaling in BCR-ABL expressing cells, acting as a serine kinase for Stat1, to regulate transcriptional activation of interferon-regulated genes and induction of antileukemic responses.

Differential regulation of the p70 S6 kinase pathway by interferon alpha (IFNalpha) and imatinib mesylate (STI571) in chronic myelogenous leukemia cells

The precise mechanisms by which imatinib mesylate (STI571) and interferon alpha (IFNalpha) exhibit antileukemic effects are not known. We examined the effects of IFNs or imatinib mesylate on signaling pathways regulating initiation of mRNA translation in BCR-ABL-expressing cells. Treatment of IFN-sensitive KT-1 cells with IFNalpha resulted in phosphorylation/activation of mammalian target of rapamycin (mTOR) and downstream activation of p70 S6 kinase. The IFN-activated p70 S6 kinase was found to regulate phosphorylation of S6 ribosomal protein, which regulates translation of mRNAs with oligopyrimidine tracts in the 5'-untranslated region. In addition, IFNalpha treatment resulted in an mTOR- and/or phosphatidyl-inositol 3'(PI 3') kinase-dependent phosphorylation of 4E-BP1 repressor of mRNA translation on sites that are required for its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF4E) complex. In contrast to the effects of IFNs, imatinib mesylate suppressed p70 S6 kinase activity, consistent with inhibition of BCR-ABL-mediated activation of the mTOR/p70 S6 kinase pathway. Moreover, the mTOR inhibitor rapamycin enhanced the suppressive effects of imatinib mesylate on primary leukemic granulocyte macrophage-colony-forming unit (CFU-GM) progenitors from patients with chronic myelogenous leukemia (CML). Taken altogether, our data demonstrate that IFNs and imatinib mesylate differentially regulate PI 3' kinase/mTOR-dependent signaling cascades in BCR-ABL-transformed cells, consistent with distinct effects of these agents on pathways regulating mRNA translation. They also support the concept that combined use of imatinib mesylate with mTOR inhibitors may be an appropriate future therapeutic strategy for the treatment of CML.

Engagement of Protein Kinase C-θ in Interferon Signaling in T-cells

Protein kinase C-theta (PKC-theta) plays important roles in the activation and survival of lymphocytes and is the predominant PKC isoform expressed in T-cells. Interferons regulate T-cell function and activation, but the precise signaling mechanisms by which they mediate such effects have not been elucidated. We determined whether PKC-theta is engaged in interferon (INF) signaling in T-cells. Both Type I (alpha, beta) and Type II (gamma) IFNs induced phosphorylation of PKC-theta in human T-cell lines and primary human T-lymphocytes. Such phosphorylation of PKC-theta resulted in activation of its kinase domain, suggesting that this kinase plays a functional role in interferon signaling. Consistent with this, inhibition of PKC-theta protein expression using small interfering RNAs (siRNA) abrogated IFN-alpha- and IFN-gamma-dependent gene transcription via GAS elements. Similarly, blocking of PKC-theta kinase activity by overexpression of a dominant-negative PKC-theta mutant also blocked GAS-driven transcription, further demonstrating a requirement for PKC-theta in IFN-dependent transcriptional activation. The effects of PKC-theta on IFN-dependent gene transcription were not mediated by regulation of the IFN-activated STAT pathway, as siRNA-mediated PKC-theta knockdown had no effects on STAT1 phosphorylation and binding of STAT1-containing complexes to SIE/GAS elements. On the other hand, siRNA-mediated PKC-theta inhibition blocked phosphorylation/activation of MKK4, suggesting that interferon-dependent PKC-theta activation regulates downstream engagement of MAP kinase pathways. Altogether, these findings demonstrate that PKC-theta is an interferon-inducible kinase and strongly suggest that it plays an important role in the generation of interferon-responses in T-cells.

Activation of protein kinase C delta by all-trans-retinoic acid

All-trans-retinoic acid (RA) is a potent inhibitor of leukemia cell proliferation and induces differentiation of acute promyelocytic leukemia cells in vitro and in vivo. For RA to induce its biological effects in target cells, binding to specific retinoic acid nuclear receptors is required. The resulting complexes bind to RA-responsive elements (RAREs) in the promoters of RA-inducible genes to initiate gene transcription and to generate protein products that mediate the biological effects of RA. In this report, we provide evidence that a member of the protein kinase C (PKC) family of proteins, PKC delta, is activated during RA treatment of the NB-4 and HL-60 acute myeloid leukemia cell lines as well as the MCF-7 breast cancer cell line. Such RA-dependent phosphorylation was also observed in primary acute promyelocytic leukemia cells and resulted in activation of the kinase domain of PKC delta. In studies aimed at understanding the functional relevance of PKC delta in the induction of RA responses, we found that pharmacological inhibition of PKC delta (but not of other PKC isoforms) diminished RA-dependent gene transcription via RAREs. On the other hand, overexpression of a constitutively active form of the kinase strongly enhanced RA-dependent gene transcription via RAREs. Gel shift assays and chromatin immunoprecipitation studies demonstrated that PKC delta associated with retinoic acid receptor-alpha and was present in an RA-inducible protein complex that bound to RAREs. Pharmacological inhibition of PKC delta activity abrogated the induction of cell differentiation and growth inhibition of NB-4 blast cells, demonstrating that its function is required for such effects. Altogether, our data provide strong evidence that PKC delta is activated in an RA-dependent manner and plays a critical role in the generation of the biological effects of RA in malignant cells.

The p38 mitogen-activated protein kinase pathway and its role in interferon signaling

Interferons (IFNs) are pleiotropic cytokines that exhibit multiple biological effects on cells and tissues. IFN receptors are expressed widely in mammalian cells and virtually all different cell types express them on their surface. The Type I IFN receptor has a multichain structure, composed of at least two distinct receptor subunits, IFNalphaR1 and IFNalphaR2. Two Jak-kinases, Tyk-2 and Jak-1, associate with the different receptor subunits and are activated in response to IFNalpha or IFNbeta to regulate engagement of multiple downstream signaling cascades. These include the Stat-pathway, whose function is essential for transcriptional activation of IFN-sensitive genes, and the insulin receptor substrate pathway, which regulates downstream activation of the phosphatidyl-inositol-3' kinase. Members of the Map family of kinases are also activated by the Type I IFN receptor and participate in the generation of IFN signals. The p38 Map kinase pathway appears to play a very important role in the induction of IFN responses. p38 is rapidly activated during engagement of the Type I IFN receptor, and such an activation is regulated by the small G-protein Rac1, which functions as its upstream effector in a tyrosine kinase-dependent manner. The activated form of p38 regulates downstream activation of other serine kinases, notably MapKapK-2 and MapKapK-3, indicating the existence of Type I IFN-dependent signaling cascades activated downstream of p38. Extensive studies have shown that p38 plays a critical role in Type I IFN-dependent transcriptional regulation, without modifying activation of the Stat-pathway. It is now well established that the function of p38 is essential for gene transcription via ISRE or GAS elements, but has no effects on the phosphorylation of Stat-proteins, the formation of Stat-complexes, and their binding to the promoters of IFN-sensitive genes. As Type I IFNs regulate gene expression for proteins with antiviral properties, it is not surprising that pharmacological inhibition of the p38 pathway blocks induction of IFNalpha-antiviral responses. In addition, pharmacological inhibition of p38 abrogates the suppressive effects of Type I IFNs on normal human hematopoietic progenitors, indicating a critical role for this signaling cascade in the induction of the regulatory effects of Type I IFNs on hematopoiesis. p38 is also activated during IFNalpha-treatment of primary leukemia cells from patients with chronic myelogenous leukemia. Such activation is required for IFNalpha-dependent suppression of leukemic cell progenitor growth, indicating that this pathway plays a critical role in the induction of the antileukemic effects of IFNalpha.

Expression of interferon receptor subunits, IFNAR1 and IFNAR2, in the ovine uterus

Interferon-tau (IFN-tau) is the antiluteolytic factor released by concepti of ruminant ungulate species prior to implantation. All type I interferons, including IFN-tau, exert their action through a common receptor, which consists of two subunits, IFNAR1 and IFNAR2c, but the distribution of the two polypeptides in uterine endometrium has not been examined. In situ hybridization and immunohistochemistry on sections from pregnant and nonpregnant ovine uteri at Days 14 and 15 after estrus and mating showed that both IFNAR1 and IFNAR2 mRNA and protein were strongly expressed in endometrial luminal epithelium (LE), superficial glandular epithelium (GE), and stromal cells, within but not outside caruncles. Similar staining patterns were noted in pregnant and nonpregnant uteri for both subunits. Western blot analysis of membrane fractions from cell lines derived from endometrial LE, GE, and stromal cells, and affinity cross-linking experiments with radioactively labeled IFN-tau performed on crude endometrial membranes indicated the presence of both high ( approximately 110 kDa) and low (75-80 kDa) molecular mass forms of the two receptor subunits. To localize where IFN-tau binds when it is introduced into the uterine lumen, immunohistochemistry with an antiserum against IFN-tau was performed on sections of uteri from Day 14 nonpregnant ewes whose uteri had previously been infused with IFN-tau. Staining was concentrated on the LE and superficial GE cells, and was absent from the deeper regions of the glands and from the stromal tissues. These studies demonstrate the heavy concentration of IFNAR1 and IFNAR2 in cells of the LE and superficial GE, which appear to be the main targets for IFN-tau.

Simultaneous measurement of multiple active kinase states using polychromatic flow cytometry

Intracellular assays of signaling systems have been limited by an inability to correlate functional subsets of cells in complex populations on the basis of active kinase states. Such correlations could be important in distinguishing changes in signaling status that arise in rare cell subsets during functional activation or in disease manifestation. Here we demonstrate the ability to simultaneously detect activated kinase members of the mitogen-activated protein kinases family (p38 MAPK, p44/42 MAPK, JNK/SAPK), members of cell survival pathways (AKT/PKB), and members of T-cell activation pathways (TYK2), among others, in subpopulations of complex cell populations by multiparameter flow-cytometric analysis. We demonstrate the utility of these probes in identifying distinct signaling cascades for (1) both artificial and physiological stimulatory conditions of peripheral blood mononuclear cells (PBMCs), (2) cytokine stimulation in human memory and naïve lymphocyte subsets as identified by five differentiation markers, and (3) ordering of kinase activation in potential signaling hierarchies. Polychromatic flow-cytometric active kinase measurements demonstrate that multidimensional analysis of signaling pathways can provide functional signaling pathway assessment on a single-cell level and allow for potential correlation with biological and clinical parameters.

IFN-gamma activates the C3G/Rap1 signaling pathway

IFN-gamma transduces signals by activating the IFN-gamma receptor-associated Jak-1 and Jak-2 kinases and by inducing tyrosine phosphorylation and activation of the Stat-1 transcriptional activator. We report that IFN-gamma activates a distinct signaling cascade involving the c-cbl protooncogene product, CrkL adapter, and small G protein Rap1. During treatment of NB-4 human cells with IFN-gamma, c-cbl protooncogene product is rapidly phosphorylated on tyrosine and provides a docking site for the src homology 2 domain of CrkL, which also undergoes IFN-gamma-dependent tyrosine phosphorylation. CrkL then regulates activation of the guanine exchange factor C3G, with which it interacts constitutively via its N terminus src homology 3 domain. This results in the IFN-gamma-dependent activation of Rap1, a protein known to exhibit tumor suppressor activity and mediate growth inhibitory responses. In a similar manner, Rap1 is also activated in response to treatment of cells with type I IFNs (IFN-alpha, IFN-beta), which also engage CrkL in their signaling pathways. On the other hand, IFN-gamma does not induce formation of nuclear CrkL-Stat5 DNA-binding complexes, which are induced by IFN-alpha and IFN-beta, indicating that pathways downstream of CrkL are differentially regulated by different IFN subtypes. Taken altogether, our data demonstrate that, in addition to activating the Stat pathway, IFN-gamma activates a distinct signaling cascade that may play an important role in the generation of its growth inhibitory effects on target cells.

The human interferon alpha species and receptors

Interferon (IFN) was approved by the U.S. Food and Drug Administration on June 5, 1986. As the first biotherapeutic approved, IFN-alpha paved the way for development of many other cytokines and growth factors. Nevertheless, we have just touched the surface of understanding the multitude of human IFNs. This paper reviews the history of the purification of human leukocyte IFN and key aspects of our current state of knowledge of human interferon alpha genes, proteins, and receptors.

Signaling pathways activated by interferons

Interferons are pleiotropic cytokines that exhibit negative regulatory effects on the growth of normal and malignant hematopoietic cells in vitro and in vivo. There are two different classes of interferons, Type I (alpha, beta, and omega) and Type II (gamma) interferons. Although the precise mechanisms by which these cytokines exhibit their potent effects on hematopoiesis remain unknown, there has been considerable progress in our understanding of the cellular changes that occur upon engagement of interferon receptors. It is now well established that Type I interferons activate multiple signaling pathways in hematopoietic cells, a finding consistent with their pleiotropic biological effects. One major pathway in Type I IFN signaling involves activation of Stat- proteins and formation of complexes that translocate to the nucleus and bind to specific elements to regulate gene transcription. The activation of this pathway (Jak-Stat pathway) is apparently regulated by members of the Jak-family of kinases, which are constitutively associated with the Type I IFN receptor. In addition to the Jak-Stat pathway, multiple other Jak-kinase-dependent signaling cascades are activated, including the IRS-PI 3'-kinase pathway, a pathway involving the vav proto-oncogene product, and a pathway involving adaptor proteins of the Crk-family (CrkL and CrkII). The only Type II interferon, IFNgamma, also activates multiple Jak-kinase-dependent signaling cascades, including the Stat and Crk pathways. Recent evidence suggests that non-Stat pathways play a critical role in the generation of signals for both Type I and Type II interferons and may be the primary mediators of their growth inhibitory effects on hematopoietic cells.

The STAT family of proteins in cytokine signaling

It has now been well established that the STAT family of proteins play important roles in cytokine-mediated specific gene activation. Although significant progress has been made toward the understanding of the structure and function of STATs as well as the regulation of STAT signaling pathways, many important questions remain to be answered. STAT PTPase(s) and STAT serine kinase(s) which play important roles in regulating STAT activity have not been identified. In addition, the molecular mechanisms of the negative regulation of STAT signaling by recently discovered protein inhibitors and the crosstalk between STAT and other signal transduction pathways have not been understood. The JAK/STAT field remains to be challenging and exciting.

The intracellular domain of interferon-alpha receptor 2c (IFN-alphaR2c) chain is responsible for Stat activation

Type I IFNs activate the Jak-Stat signal transduction pathway. The IFN-alpha receptor 1 (IFN-alphaR1) subunit and two splice variants of the IFN-alphaR2 subunit, IFN-alphaR2c and IFN-alphaR2b, are involved in ligand binding. All these receptors have been implicated in cytokine signaling and, specifically, in Stat recruitment. To evaluate the specific contribution of each receptor subunit to Stat recruitment we employed chimeric receptors with the extracellular domain of either IFN-gammaR2 or IFN-gammaR1 fused to the intracellular domains of IFN-alphaR1, IFN-alphaR2b, and IFN-alphaR2c. These chimeric receptors were expressed in hamster cells. Because human IFN-gamma exhibits no activity on hamster cells, the use of the human IFN-gamma receptor extracellular domains allowed us to avoid the variable cross-species activity of the type I IFNs and eliminate the possibility of contributions of endogenous type I IFN receptors into the Stat recruitment process. We demonstrate that Stat recruitment is solely a function of the IFN-alphaR2c intracellular domain. When chimeric receptors with the human IFN-gammaR1 extracellular domain and various human IFN-alpha receptor intracellular domains were expressed in hamster cells carrying the human IFN-gammaR2 subunit, only the IFN-alphaR2c subunit was capable of supporting IFN-gamma signaling as measured by MHC class I induction, antiviral protection, and Stat activation. Neither the IFN-alphaR2b nor the IFN-alphaR1 intracellular domain was able to recruit Stats or support IFN-gamma-induced biological activities. Thus, the IFN-alphaR2c intracellular domain is necessary and sufficient to activate Stat1, Stat2, and Stat3 proteins.

Identification of a domain in the beta subunit of the type I interferon (IFN) receptor that exhibits a negative regulatory effect in the growth inhibitory action of type I IFNs

Expression of human alpha and long form of the beta (betaL) subunits of type I interferon receptor (IFN-R) in mouse cells is sufficient to activate the Jak-Stat pathway and to elicit an antiviral state in response to human IFNalpha2 and IFNbeta. We demonstrate herein, however, that these cells respond to the antiproliferative effects of murine IFNalphabeta but not human type I IFNs. These results suggest that an unknown species-specific component is required for the antiproliferative effect of human type I IFNs. The absence of this component can be complemented by expressing the human betaL chain truncated at amino acid 346. Thus, the distal region of betaL appears to function as a negative regulator of the growth inhibitory effects of type I IFNs. Further studies looking for possible targets of the betaL regulatory domain demonstrated that this region associates with a tyrosine phosphatase. These results suggest that a protein associated with the negative regulatory domain of betaL, likely a tyrosine phosphatase, plays a role in regulating the growth inhibitory effects of human type I IFNs.

The type I interferon receptor mediates tyrosine phosphorylation of the CrkL adaptor protein

Interferon (IFN) alpha induces rapid and transient tyrosine phosphorylation of the Src homology 2/Src homology 3 (SH2/SH3)-containing CrkL adaptor protein in a time- and dose-dependent manner. Such phosphorylation is most likely regulated by the Type I interferon receptor (IFNR)-associated Tyk-2 kinase, as suggested by the detection of Type I IFN-dependent tyrosine kinase activity in anti-CrkL immunoprecipitates and the IFNalpha-dependent association of CrkL with Tyk-2 in intact cells. Two other Type I IFNs, IFNbeta and IFNomega, also induce tyrosine phosphorylation of CrkL, suggesting that the protein is involved in the signaling pathways of several different Type I IFNs. In the IFNalpha-sensitive U-266 and Daudi cell lines, CrkL interacts via its N terminus SH3 domain with the guanine exchange factor C3G that regulates activation of Rap-1, a small G-protein that exhibits tumor suppressor activity. Thus, tyrosine phosphorylation of CrkL links the functional Type I IFNR complex to the C3G-Rap-1 signaling cascade that mediates growth inhibitory responses.

Interaction of p59fyn with interferon-activated Jak kinases

During IFN alpha stimulation, p59(fyn) associates with the Type I IFNR-associated Tyk-2 kinase in several human hematopoietic cell lines in vivo. This interaction is direct, and is mediated by the SH2 domain in p59(fyn), as shown by binding studies using glutathione-S-transferase fusion proteins and far western blots. Furthermore, in response to IFN alpha-treatment of cells, the SH2 domain of Fyn interacts with the Tyk-2-associated c-cbl proto-oncogene product. In a similar manner, during IFN gamma stimulation, p59(fyn) associates via its SH2 domain with the activated form of the IFN gamma-dependent Jak-2 kinase. These data suggest that p59(fyn) is a common element in IFN alpha and IFN gamma signaling, and is selectively engaged by the Type I or II IFN receptors via specific interactions with distinct Jak kinases.

The state of the STATs: recent developments in the study of signal transduction to the nucleus

The STATs (signal transducers and activators of transcription) are latent cytoplasmic proteins that, upon activation by cell surface bound polypeptide ligands, move to the nucleus to direct transcription. A variety of protein-protein interactions that affect the function of STATs has been recently recognized. It has become clear that the STATs are functional mosaics, or mixtures of signal transduction and transcription modules.

Differences in interferon alpha and beta signaling. Interferon beta selectively induces the interaction of the alpha and betaL subunits of the type I interferon receptor

All Type I interferons (IFNalpha, IFNbeta, IFNomega) bind to the Type I IFN receptor (IFNR) and elicit a common set of signaling events, including activation of the Jak/Stat and IRS pathways. However, IFNbeta selectively induces the association of the alpha subunit of the Type I IFNR with p100, a tyrosyl phosphoprotein, to transduce IFNbeta-specific signals. Using antibodies raised against the different components of the Type I IFNR, we identified p100 as the long form of the beta subunit (betaL subunit) of the Type I IFNR. This was also confirmed in experiments with mouse L-929 cells transfected with truncated forms of betaL. Thus, IFNbeta stimulation of human cells or mouse L-929 transfectants expressing the human alpha and betaL subunits, selectively induces the formation of a signaling complex containing the alpha and betaL subunits of the receptor. The IFNbeta-regulated interaction of the alpha and betaL chains is rapid and transient and follows a similar time course with the tyrosine phosphorylation of these receptor components. These data demonstrate that the signaling specificity for different Type I IFNs is established early in the signaling cascade, at the receptor level, and results from distinct interactions between components of the Type I IFNR.

The type-I interferon receptor. The long and short of it

The type-I interferon receptor is a multisubunit receptor of the cytokine receptor superfamily. The production of specific monoclonal antibodies against the receptor and the cloning of different receptor subunits have contributed to understanding the type-I interferon receptor structure and function. The present article analyzes these new advances and the role of the different receptor subunits in type-I interferon signaling.