Of JAKs, STATs, blind watchmakers, jeeps and trains.

Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling is essential but not sufficient for full responses to the interferons (IFNs), most cytokines and some growth factors. The IFN-gamma and interleukin-6 (IL-6) response pathways have been used as model systems to investigate both the signals involved and their organisation. Activated STAT1 diffuses freely in the cytoplasmic and nuclear compartments of the cell providing a 'random walk' element in the IFN-gamma response. Completely foreign chimeric receptors and, remarkably, in the absence of STAT3, the endogenous IL-6 receptor can efficiently mediate an IFN-gamma-like response. Accordingly all of the signals required for an IFN-gamma response can be generated through physiological levels of a foreign ligand. JAK/STAT signalling, therefore, appears 'soft-wired', modular and highly flexible with substantial overlap between different response pathways. The data are consistent with a generic or 'core' set of signals from JAK/receptor complexes with 'add-on' modulation through specific receptor motifs. The cellular background likely profoundly affects the nature of the response.

The JAK/STAT pathway is not sufficient to sustain the antiproliferative response in an interferon-resistant human melanoma cell line.

The mechanism of resistance of malignant melanoma to treatment with interferon-alpha is unknown, and currently there is no reliable method of predicting response. Signalling via the JAK/STAT pathway is known to mediate many interferon-regulated events and has been implicated in mediating the antiproliferative response. The objective of this study was to determine whether defects in JAK/STAT signalling may be responsible for interferon resistance. The in vitro response to interferon was determined in a panel of established melanoma cell lines, and the components and functioning of the JAK/STAT pathway were examined in sensitive and resistant cell lines. Two melanoma cell lines, characterized as sensitive (MM418) and resistant (MeWo) to the antiproliferative effect of interferon, were both shown by Western blotting to possess all the protein components of the JAK/STAT pathway, and were shown to be capable of producing functional transcription factors using an electrophoretic mobility shift assay and a ribonuclease protection assay of known interferon-induced genes. In addition, both cell lines had intact antiviral and HLA upregulation responses. These data suggest that there is no defect in the JAK/STAT pathway per se in the MeWo cell line, and that the substantial resistance to interferon must be mediated through components either downstream or additional to this signalling pathway. Others have shown JAK/STAT defects to be responsible for interferon resistance in some melanoma cell lines. However, our results highlight the likely heterogeneity in the mechanisms leading to interferon resistance both in cell lines and tumours, and suggest that a clinical assay based on analysis of components of the JAK/STAT pathway may have only limited use as a predictor of interferon response.

P-STAT1 mediates higher-order chromatin remodelling of the human MHC in response to IFNgamma.

Transcriptional activation of the major histocompatibility complex (MHC) by IFNgamma is a key step in cell-mediated immunity. At an early stage of IFNgamma induction, chromatin carrying the entire MHC locus loops out from the chromosome 6 territory. We show here that JAK/STAT signalling triggers this higher-order chromatin remodelling and the entire MHC locus becomes decondensed prior to transcriptional activation of the classical HLA class II genes. A single point mutation of STAT1 that prevents phosphorylation is sufficient to abolish chromatin remodelling, thus establishing a direct link between the JAK/STAT signalling pathway and human chromatin architecture. The onset of chromatin remodelling corresponds with the binding of activated STAT1 and the chromatin remodelling enzyme BRG1 at specific sites within the MHC, and is followed by RNA-polymerase recruitment and histone hyperacetylation. We propose that the higher-order chromatin remodelling of the MHC locus is an essential step to generate a transcriptionally permissive chromatin environment for subsequent activation of classical HLA genes.

Signaling through a mutant IFN-gamma receptor.

Activation of STAT1 and the IFN-gamma response are thought to be mediated exclusively through the Y440 motif of the human IFNGR1 receptor subunit. Contrary to this accepted dogma, here it is shown that IFNGR1 with a mutant (Y440F) motif, when stably expressed in IFNGR1-negative human fibroblasts at levels similar to wild type, can sustain a substantial IFN-gamma response. The mutant receptor supports selective induction of IFN-gamma-inducible genes but is notably defective in the CIITA, class II HLA, suppressor of cytokine signaling and antiviral responses. Remarkably, similar selective defects are observed in human fibrosarcoma cells expressing a mutant JAK1. The phenotypes are novel and appear distinct from those observed in response to the inhibition of known additional pathways. Data from different cell types further emphasizes the importance of cellular background in determining the response.

Differential responses to IFN-alpha subtypes in human T cells and dendritic cells.

Type I IFNs (IFN-alphabeta) constitute a family of cytokines that have important antiviral and immunoregulatory properties and have been successfully used in the treatment of a wide variety of diseases. There are 12 functional human IFN-alpha subtypes and one IFN-beta subtype that signal through the common cell surface IFN-alphabetaR. To date, virtually no information is available on the specificity of IFN-alpha responses in immune cells. In this study, Janus kinase/STAT signaling and transcriptional responses to selected IFN-alpha subtypes in human T cells and dendritic cells were analyzed. Evidence for IFN-alpha subtype and cell type specificity was found. Also, differences between kinetics of expression of IFN-stimulated genes (ISGs) and in the requirements of individual ISGs for additional signaling pathways were observed. In particular, IFN-gamma-inducible protein-10 (IP-10), a key chemokine in Th1-type inflammatory diseases, was differentially regulated. In dendritic cells, it was highly induced by IFN-alpha2 and IFN-alpha21 but much less efficiently by IFN-alpha1. It was only marginally induced by these subtypes in T cells. In marked contrast to other ISGs analyzed, optimum induction of IP-10 was dependent on activation of p38 kinase(s). The observed variations (subtype-, cell type-, and ISG-related differentials) provide further insight into the complexity and plasticity of the IFN-alphabeta response. Furthermore, the novel observation that IFN-alpha1 poorly induces IP-10 is potentially of clinical importance, because this subtype may be more beneficial in cases where Th1-mediated side effects (e.g., exacerbation of autoimmune diseases) are not desirable.

The antiviral response to gamma interferon.

A role for alpha/beta interferon (IFN-alpha/beta) in the IFN-gamma antiviral response has long been suggested. Accordingly, possible roles for autocrine or double-stranded-RNA (dsRNA)-induced IFN-alpha/beta in the IFN-gamma response were investigated. Use was made of wild-type and a variety of mutant human fibrosarcoma cell lines, including mutant U5A cells, which lack a functional IFN-alpha/beta receptor and hence an IFN-alpha/beta response. IFN-gamma did not induce detectable levels of IFN-alpha/beta in any of the cell lines, nor was the IFN-gamma response per se dependent on autocrine IFN-alpha/beta. On the other hand, a number of responses to dsRNA [poly(I). poly(C)] and encephalomyocarditis virus were greatly enhanced by IFN-gamma pretreatment (priming) of wild-type cells or of mutant cells lacking an IFN-alpha/beta response; these include the primary induction of dsRNA-inducible mRNAs, including IFN-beta mRNA, and, to a lesser extent, the dsRNA-mediated activation of the p38 mitogen-activated protein (MAP) kinase(s). IFN-gamma priming of mRNA induction by dsRNA is dependent on JAK1 and shows biphasic kinetics, with an initial rapid (<30-min) response being followed by a more substantial effect on overnight incubation. The IFN-gamma-primed dsRNA responses appear to be subject to modulation through the p38, phosphatidylinositol 3-kinase, and ERK1/ERK2 MAP kinase pathways. It can be concluded that despite efficient priming of IFN-beta production, the IFN-alpha/beta pathways play no significant role in the primary IFN-gamma antiviral response in these cell-virus systems. The observed IFN-gamma priming of dsRNA responses, on the other hand, will likely play a significant role in combating virus infection in vivo.

Cell-type and donor-specific transcriptional responses to interferon-alpha. Use of customized gene arrays.

A sensitive, specific, reproducible, robust, and cost-effective customized cDNA array system based on established nylon membrane technology has been developed for convenient multisample expression profiling for several hundred genes of choice. The genes represented are easily adjusted (depending on the availability of corresponding cDNAs) and the method is accordingly readily applicable to a wide variety of systems. Here we have focused on the expression profiles for interferon-alpha2a, the most widely used interferon for the treatment of viral hepatitis and malignancies, in primary cells (peripheral blood mononuclear cells, T cells, and dendritic cells) and cell lines (Kit255, HT1080, HepG2, and HuH7). Of 150 genes studied, only six were consistently induced in all cell types and donors, whereas 74 genes were induced in at least one cell type. IRF-7 was identified as the only gene exclusively induced in the hematopoietic cells. No gene was exclusively induced in the nonhematopoietic cell lines. In T cells 12, and in dendritic cells, 25 genes were induced in all donors whereas 45 and 42 genes, respectively, were induced in at least one donor. The data suggest that signaling through IFN-alpha2 can be substantially modulated to yield significant cell-type and donor-specific qualitative and quantitative differences in gene expression in response to this cytokine under highly standardized conditions.

Mutational switch of an IL-6 response to an interferon-gamma-like response.

Signaling through Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) is central to the responses to the majority of cytokines and some growth factors, including the interferons (IFNs) and the IL-6 family of cytokines. The biological responses to stimulation through the widely distributed IL-6 and IFN-gamma receptors are, however, completely different. Remarkably, it is shown here that, in mouse embryo fibroblasts lacking STAT3, IL-6 mediates an IFN-gamma-like response including prolonged activation of STAT1, the induction of multiple IFN-gamma-inducible genes, the expression of class II MHC antigens, and an antiviral state. Normal cells exposed to IL-6 thus require a STAT3-dependent function(s) to down-regulate STAT1 activity and prevent an IFN-gamma-like response. The data encourage the view that the very disparate IFN-gamma and IL-6 JAK/receptor complexes mediate a common set of generic or "core" signals which are subject to STAT3-dependent modulation to provide IL-6 specificity. The switching of one cytokine response to one closely mimicking another as a result of the loss of a single signaling component has profound implications, for example, for the interpretation of the phenotypes of knockout mice and for the clinical use of inhibitors of signaling.