The CrkL adapter protein is required for type I interferon-dependent gene transcription and activation of the small G-protein Rap1

Type I interferon and cancer

Type I interferon (IFN) is a class of proinflammatory cytokines with a dual role on malignant transformation, tumor progression, and response to therapy. On the one hand, robust, acute, and resolving type I IFN responses have been shown to mediate prominent anticancer effects, reflecting not only their direct cytostatic/cytotoxic activity on (at least some) malignant cells, but also their pronounced immunostimulatory functions. In line with this notion, type I IFN signaling has been implicated in the antineoplastic effects of various immunogenic therapeutics, including (but not limited to) immunogenic cell death (ICD)-inducing agents and immune checkpoint inhibitors (ICIs). On the other hand, weak, indolent, and non-resolving type I IFN responses have been demonstrated to support tumor progression and resistance to therapy, reflecting the ability of suboptimal type I IFN signaling to mediate cytoprotective activity, promote stemness, favor tolerance to chromosomal instability, and facilitate the establishment of an immunologically exhausted tumor microenvironment. Here, we review fundamental aspects of type I IFN signaling and their context-dependent impact on malignant transformation, tumor progression, and response to therapy.

An Essential Role for Perforin-2 in Type I IFN Signaling

Type I IFNs play a complex role in determining the fate of microbial pathogens and may also be deleterious to the host during bacterial and viral infections. Upon ligand binding, a receptor proximal complex consisting of IFN-α and -β receptors 1 and 2 (IFNAR1, IFNAR2, respectively), tyrosine kinase 2 (Tyk2), Jak1, and STAT2 are assembled and promote the phosphorylation of STAT1 and STAT2. However, how the IFNARs proximal complex is assembled upon binding to IFN is poorly understood. In this study, we show that the membrane-associated pore-forming protein Perforin-2 (P2) is critical for LPS-induced endotoxic shock in wild-type mice. Type I IFN-mediated JAK-STAT signaling is severely impaired, and activation of MAPKs and PI3K signaling pathways are delayed in P2-deficient mouse bone marrow-derived macrophages, mouse embryonic fibroblasts (MEFs), and human HeLa cells upon IFN stimulation. The P2 N-glycosylated extracellular membrane attack complex/perforin domain and the P2 domain independently associate with the extracellular regions of IFNAR1 and IFNAR2, respectively, in resting MEFs. In addition, the P2 cytoplasmic tail domain mediated the constitutive interaction between STAT2 and IFNAR2 in resting MEFs, an interaction that is dependent on the association of the extracellular regions of P2 and IFNAR2. Finally, the constitutive association of P2 with both receptors and STAT2 is critical for the receptor proximal complex assembly and reciprocal transphosphorylation of Jak1 and Tyk2 as well as the phosphorylation and activation of STAT1 and STAT2 upon IFN-β stimulation.

Differential Regulation of Type I and Type III Interferon Signaling

Interferons (IFNs) are very powerful cytokines, which play a key role in combatting pathogen infections by controlling inflammation and immune response by directly inducing anti-pathogen molecular countermeasures. There are three classes of IFNs: type I, type II and type III. While type II IFN is specific for immune cells, type I and III IFNs are expressed by both immune and tissue specific cells. Unlike type I IFNs, type III IFNs have a unique tropism where their signaling and functions are mostly restricted to epithelial cells. As such, this class of IFN has recently emerged as a key player in mucosal immunity. Since the discovery of type III IFNs, the last 15 years of research in the IFN field has focused on understanding whether the induction, the signaling and the function of these powerful cytokines are regulated differently compared to type I IFN-mediated immune response. This review will cover the current state of the knowledge of the similarities and differences in the signaling pathways emanating from type I and type III IFN stimulation.

Differential induction of interferon stimulated genes between type I and type III interferons is independent of interferon receptor abundance

It is currently believed that type I and III interferons (IFNs) have redundant functions. However, the preferential distribution of type III IFN receptor on epithelial cells suggests functional differences at epithelial surfaces. Here, using human intestinal epithelial cells we could show that although both type I and type III IFNs confer an antiviral state to the cells, they do so with distinct kinetics. Type I IFN signaling is characterized by an acute strong induction of interferon stimulated genes (ISGs) and confers fast antiviral protection. On the contrary, the slow acting type III IFN mediated antiviral protection is characterized by a weaker induction of ISGs in a delayed manner compared to type I IFN. Moreover, while transcript profiling revealed that both IFNs induced a similar set of ISGs, their temporal expression strictly depended on the IFNs, thereby leading to unique antiviral environments. Using a combination of data-driven mathematical modeling and experimental validation, we addressed the molecular reason for this differential kinetic of ISG expression. We could demonstrate that these kinetic differences are intrinsic to each signaling pathway and not due to different expression levels of the corresponding IFN receptors. We report that type III IFN is specifically tailored to act in specific cell types not only due to the restriction of its receptor but also by providing target cells with a distinct antiviral environment compared to type I IFN. We propose that this specific environment is key at surfaces that are often challenged with the extracellular environment.

IL-7-induced phosphorylation of the adaptor Crk-like and other targets

IL-7 is required for T cell differentiation and mature T cell homeostasis and promotes pro-B cell proliferation and survival. Tyrosine phosphorylation plays a central role in IL-7 signaling. We identified by two-dimensional electrophoresis followed by anti-phosphotyrosine immunoblotting and mass spectrometry sixteen tyrosine phosphorylated proteins from the IL-7-dependent cell line D1. IL-7 stimulation induced the phosphorylation of the proteins STI1, ATIC and hnRNPH, involved in pathways related to survival, proliferation and gene expression, respectively, and increased the phosphorylation of CrkL, a member of a family of adaptors including the highly homologous Crk isoforms CrkII and CrkI, important in multiple signaling pathways. We observed an increased phosphorylation of CrkL in murine pro-B cells and in murine and human T cells. In addition, IL-7 increased the association of CrkL with the transcription factor Stat5, essential for IL-7 pro-survival activity. The selective tyrosine kinase inhibitor Imatinib. counteracted the IL-7 pro-survival effect in D1 cells and decreased CrkL phosphorylation. These data suggested that CrkL could play a pro-survival role in IL-7-mediated signaling. We observed that pro-B cells also expressed, in addition to CrkL, the Crk isoforms CrkII and CrkI and therefore utilized pro-B cells conditionally deficient in all three to evaluate the role of these proteins. The observation that the IL-7 pro-survival effect was reduced in Crk/CrkL conditionally-deficient pro-B cells further pointed to a pro-survival role of these adaptors. To further evaluate the role of these proteins, gene expression studies were performed in Crk/CrkL conditionally-deficient pro-B cells. IL-7 decreased the transcription of the receptor LAIR1, which inhibits B cell proliferation, in a Crk/CrkL-dependent manner, suggesting that the Crk family of proteins may promote pro-B cell proliferation. Our data contribute to the understanding of IL-7 signaling and suggest the involvement of Crk family proteins in pathways promoting survival and proliferation.

The Effector TepP Mediates Recruitment and Activation of Phosphoinositide 3-Kinase on Early Chlamydia trachomatis Vacuoles

Chlamydia trachomatis delivers multiple type 3 secreted effector proteins to host epithelial cells to manipulate cytoskeletal functions, membrane dynamics, and signaling pathways. TepP is the most abundant effector protein secreted early in infection, but its molecular function is poorly understood. In this report, we provide evidence that TepP is important for bacterial replication in cervical epithelial cells, activation of type I IFN genes, and recruitment of class I phosphoinositide 3-kinases (PI3K) and signaling adaptor protein CrkL to nascent pathogen-containing vacuoles (inclusions). We also show that TepP is a target of tyrosine phosphorylation by Src kinases but that these modifications do not appear to influence the recruitment of PI3K or CrkL. The translocation of TepP correlated with an increase in the intracellular pools of phosphoinositide-(3,4,5)-triphosphate but not the activation of the prosurvival kinase Akt, suggesting that TepP-mediated activation of PI3K is spatially restricted to early inclusions. Furthermore, we linked PI3K activity to the dampening of transcription of type I interferon (IFN)-induced genes early in infection. Overall, these findings indicate that TepP can modulate cell signaling and, potentially, membrane trafficking events by spatially restricted activation of PI3K. IMPORTANCE This article shows that Chlamydia recruits PI3K, an enzyme important for host cell survival and internal membrane functions, to the pathogens inside cells by secreting a scaffolding protein called TepP. TepP enhances Chlamydia replication and dampens the activation of immune responses.

Nuclear Translocation of Crk Adaptor Proteins by the Influenza A Virus NS1 Protein

The non-structural protein-1 (NS1) of many influenza A strains, especially those of avian origin, contains an SH3 ligand motif, which binds tightly to the cellular adaptor proteins Crk (Chicken tumor virus number 10 (CT10) regulator of kinase) and Crk-like adapter protein (CrkL). This interaction has been shown to potentiate NS1-induced activation of the phosphatidylinositol 3-kinase (PI3K), but additional effects on the host cell physiology may exist. Here we show that NS1 can induce an efficient translocation of Crk proteins from the cytoplasm into the nucleus, which results in an altered pattern of nuclear protein tyrosine phosphorylation. This was not observed using NS1 proteins deficient in SH3 binding or engineered to be exclusively cytoplasmic, indicating a physical role for NS1 as a carrier in the nuclear translocation of Crk. These data further emphasize the role of Crk proteins as host cell interaction partners of NS1, and highlight the potential for host cell manipulation gained by a viral protein simply via acquiring a short SH3 binding motif.

Biochemical monitoring of the early endocytic traffic of the type I interferon receptor

The type I interferon (IFN) receptor consists of two transmembrane chains IFNAR1 and IFNAR2, associated with the tyrosine kinases Tyk2 and Jak1, respectively. Binding of IFN to this receptor complex induces activation of Jak/Stat and non-Stat signaling pathways. Ligand binding also drives receptor internalization and sorting toward degradation or recycling. To gain insights into receptor trafficking and its relation to signaling, we performed subcellular organelle fractionation from IFN-stimulated Daudi cells and defined biochemically an early endosomal antigen-1 (EEA1)-positive compartment bearing the activated IFN receptor. Endosomes were thus purified by immunoaffinity isolation on anti-EEA1 antibodies-coated beads. The content of these purified endosomal fractions was analyzed by Western blot and proteomics. Shortly after IFN stimulation, robustly ubiquitinated IFNAR1 and a small amount of IFNAR2 were found in this endosomal compartment, which also contained tyrosine-phosphorylated Tyk2 and Jak1. These data strongly point to the prolonged interaction during traffic of the tyrosine kinases, still in an activated configuration, with the receptors. Among the major constituents of this EEA1-positive compartment, some proteins that have been implicated in IFN signaling were identified. Altogether, these observations suggest that trafficking of the IFN receptor through endosomes may regulate signaling pathways.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

Stats: multifaceted regulators of transcription

The high-affinity binding interactions between interferons (IFNs) and their cognate cell surface receptors lead to the activation of receptor-associated Janus protein tyrosine kinases (Jaks) and subsequent phosphorylation and activation of a group of transcription factors, the signal transducers and activators of transcription (Stats). Upon IFN-induced activation, these Stat proteins form homodimeric and heterodimeric complexes that translocate to the nucleus and bind specific elements within the promoters of IFN-stimulated genes (ISGs). In addition to the well-studied IFN-induced ISG factor 3 (ISGF3) and Stat1:1 complexes, IFNs induce the formation of a number of other Stat-containing complexes, including Stat3:3 and Stat5:5 homodimers, as well as Stat2:1 and Stat5:CrkL heterodimers, that also mediate gene transcription. Moreover, emerging evidence suggests that particular amino acid residues within the individual Stat proteins contribute to different aspects of Stat function. These residues modulate the transcriptional activation potential of Stat-containing complexes and thereby influence the expression of ISGs. Indeed, the Stat proteins function in a multifaceted manner to regulate the expression of proteins that mediate IFN responses.

Characterization of p87C3G, a novel, truncated C3G isoform that is overexpressed in chronic myeloid leukemia and interacts with Bcr-Abl

A novel C3G isoform, designated p87C3G, lacking the most amino terminal region of the cognate protein has been found to be overexpressed in two CML cell lines, K562 and Boff 210, both expressing Bcr-Abl p210. p87C3G expression is also highly augmented in patients diagnosed with chronic myeloid leukemia (CML) Ph+, in comparison with healthy individuals, and returns to basal levels after treatment with STI571. p87C3G co-immunoprecipitates with both CrkL and Bcr-Abl in CML cell lines and co-immunoprecipitation between p87C3G and Bcr-Abl was also detected in primary cells from CML patients. These interactions have been confirmed by in vitro pull down experiments. The interaction between p87C3G and Bcr-Abl involves the SH3-binding domain of p87C3G and the SH3 domain of Abl and depends mostly on the first polyproline region of p87C3G. Furthermore, we also demonstrated that p87C3G is phosphorylated in vitro by a Bcr-Abl-dependent mechanism. These results indicate that p87C3G overexpression is linked to CML phenotype and that p87C3G may exert productive functional interactions with Bcr-Abl signaling components suggesting the implication of this C3G isoform in the pathogenesis of chronic myeloid leukemia.

Analysis of NOD2-mediated proteome response to muramyl dipeptide in HEK293 cells

NOD2, a cytosolic receptor for the bacterial proteoglycan fragment muramyl dipeptide (MDP), plays an important role in the recognition of intracellular pathogens. Variants in the bacterial sensor domain of NOD2 are genetically associated with an increased risk for the development of Crohn disease, a human chronic inflammatory bowel disease. In the present study, global protein expression changes after MDP stimulation were analyzed by two-dimensional PAGE of total protein extracts of human cultured cells stably transfected with expression constructs encoding for wild type NOD2 (NOD2(WT)) or the disease-associated NOD2 L1007fsinsC (NOD2(SNP13)) variant. Differentially regulated proteins were identified by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) peptide mass fingerprinting and MALDI MS/MS. The limited overlap in the responses of the NOD2-overexpressing cell lines to MDP included a down-regulation of heat shock 70-kDa protein 4. A complex pro-inflammatory program regulated by NOD2(WT) that encompasses a regulation of key genes involved in protein folding, DNA repair, cellular redox homeostasis, and metabolism was observed both under normal growth conditions and after stimulation with MDP. By using the comparison of NOD2(WT) and disease-associated NOD2(SNP13) variant, we have identified a proteomic signature pattern that may further our understanding of the influence of genetic variations in the NOD2 gene in the pathophysiology of chronic inflammatory bowel disease.

Multiple roles of Rap1 in hematopoietic cells: complementary versus antagonistic functions

Small G proteins serve as critical control points in signal transduction, integrating a wide range of stimuli to dictate discrete cellular outcomes. The outcomes of small G-protein signaling can both potentiate and antagonize one another. Studies in hematopoietic cells have uncovered multiple functions for the small G protein, Rap1 (Ras-proximate-1). Because Rap1 can regulate cell proliferation, differentiation, and adhesion through distinct mechanisms, it serves as a paradigm for the need for tight cellular control of small G-protein function. Rap1 has received recent attention for its role in enhancing integrin-dependent signals. This action of Rap1 augments a variety of processes that characterize hematopoietic-cell function, including aggregation, migration, extravasation, and homing to target tissues. Rap1 may also regulate cellular differentiation and proliferation via pathways that are distinct from those mediating adhesion, and involve regulation of the mitogen-activated protein (MAP) kinase or ERK (extracellular signal-regulated kinase) cascade. These actions of Rap1 occur in selected cell types to enhance or diminish ERK signaling, depending on the expression pattern of the MAP kinase kinase kinases of the Raf family: Raf-1 and B-Raf. This review will examine the functions of Rap1 in hematopoietic cells, and focus on 3 cellular scenarios where the multiple actions of Rap1 function have been proposed. Recent studies implicating Rap1 in the maturation of megakaryocytes, the pathogenesis of chronic myelogenous leukemia (CML), and activation of peripheral T cells will receive particular attention.

Mechanisms of type-I- and type-II-interferon-mediated signalling

Interferons are cytokines that have antiviral, antiproliferative and immunomodulatory effects. Because of these important properties, in the past two decades, major research efforts have been undertaken to understand the signalling mechanisms through which these cytokines induce their effects. Since the original discovery of the classical JAK (Janus activated kinase)-STAT (signal transducer and activator of transcription) pathway of signalling, it has become clear that the coordination and cooperation of multiple distinct signalling cascades - including the mitogen-activated protein kinase p38 cascade and the phosphatidylinositol 3-kinase cascade - are required for the generation of responses to interferons. It is anticipated that an increased understanding of the contributions of these recently identified pathways will advance our current thinking about how interferons work.

Activation of mitogen-activated protein kinase kinase (MKK) 3 and MKK6 by type I interferons

There is accumulating evidence that the p38 MAP kinase pathway plays important roles in Type I interferon (IFN) signaling, but the mechanisms regulating p38 activation during engagement of the Type I IFN receptor remain to be defined. We sought to identify the events that lead to activation of the p38 MAP kinase in response to Type I IFNs. Our data demonstrate that treatment of sensitive cell lines with IFNalpha results in activation of both MAP kinase kinase 3 (MKK3) and MAP kinase kinase 6 (MKK6). Such IFN-inducible activation of MKK3 and MKK6 is essential for downstream phosphorylation and activation of the p38 MAP kinase, as shown by studies using mouse embryonic fibroblasts (MEFs) with targeted disruption of the Mkk3 and Mkk6 genes (MKK3-/- MKK6-/-). Similarly, IFN-dependent activation of the downstream effectors of p38, MAPKAPK-2 and MAPKAPK-3, is not detectable in cells lacking Mkk3 and Mkk6, demonstrating that the function of these MAP kinase kinases is required for full activation of the p38 pathway. To define the functional relevance of MKK3/6 engagement in Type I IFN signaling, IFN-inducible gene transcription was evaluated in the MKK3/MKK6 double knock-out cells. IFNalpha- and IFNbeta-dependent transcription via either interferon-stimulated response element or IFNgamma activated site elements was defective in MKK3 -/-/MKK6 -/- MEFs in luciferase reporter assays. In addition, IFN-dependent induction of two genes known to be of importance in the generation of IFN responses, Isg15 and Irf-9, was diminished in the absence of Mkk3 and Mkk6. The effects of Mkk3 and Mkk6 on IFN-dependent transcription were unrelated to any effects on the phosphorylation and activation of STAT proteins, indicating the presence of a STAT-independent mechanism. Altogether, our findings demonstrate that MKK3 and MKK6 are rapidly activated during engagement of the Type I IFN receptor and play important roles in Type I IFN signaling and the generation of IFN responses.

Phosphorylated guanine nucleotide exchange factor C3G, induced by pervanadate and Src family kinases localizes to the Golgi and subcortical actin cytoskeleton

BACKGROUND

The guanine nucleotide exchange factor C3G (RapGEF1) along with its effector proteins participates in signaling pathways that regulate eukaryotic cell proliferation, adhesion, apoptosis and embryonic development. It activates Rap1, Rap2 and R-Ras members of the Ras family of GTPases. C3G is activated upon phosphorylation at tyrosine 504 and therefore, determining the localization of phosphorylated C3G would provide an insight into its site of action in the cellular context.

RESULTS

C3G is phosphorylated in vivo on Y504 upon coexpression with Src or Hck, two members of the Src family tyrosine kinases. Here we have determined the subcellular localization of this protein using antibodies specific to C3G and Tyr 504 phosphorylated C3G (pY504 C3G). While exogenously expressed C3G was present mostly in the cytosol, pY504 C3G formed upon Hck or Src coexpression localized predominantly at the cell membrane and the Golgi complex. Tyrosine 504-phosphorylated C3G showed colocalization with Hck and Src. Treatment of Hck and C3G transfected cells with pervanadate showed an increase in the cytosolic staining of pY504 C3G suggesting that tyrosine phosphatases may be involved in dephosphorylating cytosolic phospho-C3G. Expression of Src family kinases or treatment of cells with pervanadate resulted in an increase in endogenous pY504 C3G, which was localized predominantly at the Golgi and the cell periphery. Endogenous pY504 C3G at the cell periphery colocalized with F-actin suggesting its presence at the subcortical actin cytoskeleton. Disruption of actin cytoskeleton by cytochalasin D abolished phospho-C3G staining at the periphery of the cell without affecting its Golgi localization.

CONCLUSIONS

These findings show that tyrosine kinases involved in phosphorylation of C3G are responsible for regulation of its localization in a cellular context. We have demonstrated the localization of endogenous C3G modified by tyrosine phosphorylation to defined subcellular domains where it may be responsible for restricted activation of signaling pathways.

Epstein-Barr virus latent membrane protein LMP-2A is sufficient for transactivation of the human endogenous retrovirus HERV-K18 superantigen

Superantigens are microbial proteins that strongly stimulate T cells. We described previously that the Epstein-Barr virus (EBV) transactivates a superantigen encoded by the human endogenous retrovirus, HERV-K18. We now report that the transactivation is dependent upon the EBV latent cycle proteins. Moreover, LMP-2A is sufficient for induction of HERV-K18 superantigen activity.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Functional and genomic changes in the mouse ocular motor system in response to light deprivation from birth

Previous studies have suggested that abnormal visual experience early in life induces ocular motor abnormalities. The purpose of this study was to determine how visual deprivation alters the function and gene expression profile of the ocular motor system in mice. We measured the effect of dark rearing on eye movements, gene expression in the oculomotor nucleus, and contractility of isolated extraocular muscles. In vivo eye movement recordings showed decreased gains for optokinetic and vestibulo-ocular reflexes, confirming an effect of dark rearing on overall ocular motor function. Saccade peak velocities were preserved, however, arguing that the quantitative changes in these reflexes were not secondary to limitations in force generation. Using microarrays and quantitative PCR, we found that dark rearing shifted the oculomotor nucleus transcriptome to a state of delayed/arrested development. The expression of 132 genes was altered by dark rearing; these genes fit in various functional categories (signal transduction, transcription/translation control, metabolism, synaptic function, cytoskeleton), and some were known to be associated with neuronal development and plasticity. Extraocular muscle contractility was impaired by dark rearing to a greater extent than expected from the in vivo ocular motility studies: changes included decreased force and shortening speed and evidence of abnormal excitability. The results indicate that normal development of the mouse ocular motor system and its muscles requires visual experience. The transcriptional pattern of arrested development may indicate that vision is required to establish the adult pattern, but it also may represent the plastic response of oculomotor nuclei to abnormal extraocular muscles.

Role of p38α Map Kinase in Type I Interferon Signaling

Multiple signaling pathways are activated during engagement of the Type I interferon (IFN) receptor to mediate biological responses, including the Jak-Stat and Rac1/p38 Map kinase signaling cascades. In the present study we sought to determine the functional relevance of the p38alpha isoform in IFN signaling, using cells from mouse embryos with targeted disruption of the p38alpha gene. Our data demonstrate that p38alpha activation is essential for Type I IFN-dependent transcriptional regulation via ISRE or GAS elements. On the other hand, the function of p38alpha is not required for IFN-dependent Ser727 or Tyr701 phosphorylation of Stat1 and does not impact on the formation of ISGF3 or SIF nuclear binding complexes. In efforts to identify downstream effectors of p38 that may mediate IFN-dependent transcriptional responses, we found that IFNalpha activates the kinase Msk1, a known regulator of histone phosphorylation and chromatin remodeling. In other studies, we demonstrate that Type I IFN-dependent activation of the kinases MapKapK-2 and MapKapK-3 is defective in the absence of p38alpha, while Type I IFN-dependent antiviral properties are decreased in cells with targeted disruption of the MapKapK-2 gene. Altogether, our data establish that the p38alpha Map kinase pathway regulates activation of downstream effectors that participate in the induction of IFN-dependent gene transcription, to mediate IFN-responses.

Interferons: mechanisms of action and clinical applications

PURPOSE OF REVIEW

Interferons are pleiotropic cytokines that exhibit important biologic activities, including antiviral, antitumor, and immunomodulatory effects. These cytokines have found important applications in clinical medicine, including the treatment of certain malignancies. The purpose of this review is to provide an update on basic and clinical research in the interferon field.

RECENT FINDINGS

Significant advances have recently occurred in the field of type I interferon signal transduction. It is well known that the interferons transduce signals via activation of multiple signaling cascades, involving Jak kinases, signal transducer and activator of transcription proteins, Map kinases, and IRS and Crk proteins. Recent evidence indicates that the p38 Map kinase pathway plays an important role in type I interferon signaling in malignant cells and that its function is required for type I interferon-dependent gene transcription and generation of the antiproliferative of type I interferons. In clinical oncology, interferon-alpha remains an active and useful agent in the treatment of several malignant disorders, and efforts are underway to improve its efficacy by using different schedules and combinations with other agents.

SUMMARY

This review summarizes the mechanisms of signal transduction of interferons and the emerging new concepts in this area. An update on the clinical applications of interferons in oncology is also provided, and potential translational applications, reflecting recent advances in the field, are discussed.

Role of Stat5 in type I interferon-signaling and transcriptional regulation

Type I interferons are pleiotropic cytokines that transduce signals via activation of multiple downstream signaling cascades, including the Jak-Stat pathway. Although the roles of Stat1 and Stat2 in Type I interferon signaling are well established, the roles that other Stat-family members play in the induction of IFN-responses remain to be defined. In previous studies, we have shown that Stat5 associates with the CrkL adapter and forms a signaling complex that binds DNA. In the present study, we provide evidence that Stat5 is phosphorylated on serines 725/730 in an IFNalpha- and IFNbeta-dependent manner, providing direct evidence that serine phosphorylation of the protein is a component of an interferon signaling cascade. Such serine phosphorylation of Stat5 is Map kinase- and PI 3(')-kinase independent, while the activation of the serine kinase that phosphorylates Stat5 is regulated by upstream tyrosine kinase activity. Using mouse embryonic fibroblasts with targeted disruption of the Stat5a and Stat5b genes, we demonstrate that full activation of Stat5 is required for Type I interferon-dependent gene transcription via GAS elements. Altogether, our data provide evidence that Stat5 plays an important role in IFN-signaling and participates in the induction of Type I IFN-dependent responses. Furthermore, our results strongly suggest that, in addition to phosphorylation on tyrosine residues, phosphorylation on serine residues exhibits regulatory effects on the transcriptional capacity of Stat5.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

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.

Review: IFN-alpha/beta receptor interactions to biologic outcomes: understanding the circuitry

Type I interferons (IFNs), which include the IFN-alphas, IFN-beta, IFN-omega, IFN-kappa, and IFN-tau, are an evolutionarily conserved group of secreted cytokines that serve as potent extracellular mediators of host defense and homeostasis. Binding of IFNs to specific cell surface receptors results in the activation of multiple intracellular signaling cascades, leadingto the synthesis of proteins that mediate antiviral, growth inhibitory and immunomodulatory responses. In the past decade, considerable information has accumulated pertaining to the different signalingpathways that are activated by the type I IFNs. Although many of the literature findings are specific to defined cell systems or are tissue restricted, the intent of this review is to place these signaling cascades and their effectors in the context of distinct biologic outcomes.