Trends in Immunology
ReviewStat1-dependent and -independent pathways in IFN-γ-dependent signaling
Section snippets
The Jak–Stat1 pathway in IFN-γ-dependent signaling
IFN-γ exerts its effects on cells by interacting with a specific receptor composed of two subunits, IFNGR1 and IFNGR2, that is expressed on nearly all cell surfaces. Binding of IFN-γ to its receptor induces receptor oligomerization and activation of the receptor-associated Janus kinases Jak1 and Jak2 by trans-phosphorylation. The activated Jaks phosphorylate the intracellular domain of the receptor (e.g. tyrosine 440 of human IFNGR1), which serves as a docking site for Stat1 [2]. Additional
Additional signal-transduction pathways activated by IFN–γ
In addition to the well known Jak–Stat pathway, IFN-γ activates several additional signal-transduction proteins, including the mitogen-activated protein kinases (MAPKs) Pyk2 and extracellular-signal-regulated kinase 1/2 (ERK1/2) [15]; the Src-family kinase Fyn [16]; the adaptor proteins c-Cbl, CrkL, CrkII and Vav 17, 18; the G-protein-linked signaling molecules C3G and Ras GTPase-activating protein 1 (Rap-1) [19]; and the SH2-domain-containing protein tyrosine phosphatases SHP-1 and SHP-2 20, 21
Stat1-independent pathways in IFN-γ-dependent signaling
IFN-γ regulates the expression of many genes in different cell types. The total number of IFN-γ-regulated genes is estimated to be ∼500 [28]. Microarray analysis has indicated that even in a single cell type - for example, 2fTGH fibrosarcoma cells - IFN-γ regulates the expression of ∼100 genes [29]. There is now considerable evidence for Stat1-independent pathways in IFN-γ-dependent signaling. IFN-γ suppresses the cell growth induced by growth factors and cytokines in wild-type cells but
Identification of Stat1-independent genes using microarrays
Microarrays (high-density arrays of oligonucleotides or cDNAs on silicone chips or glass slides) provide a means to monitor changes in the level of expression of thousands of transcripts simultaneously in response to a variety of stimuli [35]. We have performed microarray analyses with wild-type and Stat1-null bone-marrow-derived macrophages (BMMs), untreated or treated with IFN-γ for one hour [30]. The total number of mRNAs present in all the samples was similar, with an average of ∼5000
Patterns of gene expression in response to IFN-γ in macrophages
The expression of several genes, including IRF-1, IP-10, MIG and SOCS-1, is induced in wild-type cells but not in Stat1-null BMMs and thus, is Stat1-dependent. The expression of c-fos and KC (GRO/melanoma growth stimulating activity) is suppressed in wild-type BMMs only, indicating that Stat1 is required for this suppression. We distinguish two types of Stat1-independent pathways of gene regulation. In type I, the expression of some genes is induced [e.g. monocyte chemotactic protein 1 (MCP-1),
Stat1-independent regulation of gene expression in fibroblasts
There is increasing evidence that fibroblasts play an important role in the orchestration of immune responses [37]. We have performed a time-course analysis to discern the profile of IFN-γ-dependent regulation of gene expression in serum-starved Stat1-null fibroblasts [38]. The induced genes can be classified broadly into those encoding immediate-early proteins, transcription factors, and cytoplasmic regulatory and secreted proteins. Among the immediate-early proteins and transcription factors
Biological implications of Stat1-independent pathways
Studies of kinase-negative Jak mutants, as well as resistance to Sindbis virus and MCMV infection in Stat1-null mice suggest that the Jak1-dependent and Stat1-independent pathways regulate some antiviral responses. The products of the regulated genes promote important functions that are required for productive antiviral responses in vivo. Several viruses have the capacity to abrogate the function of Stat1 39, 40. Stat1-independent pathways provide the host with a redundant fail-safe mechanism
Conclusions
The demonstration that IFN-γ engages multiple signal-transduction pathways raises several important issues for the future, including defining the connections between specific signal-transduction molecules, transcription factors and Stat1-independent gene expression. Stat1-independent pathways contribute to antiviral responses, as well as to the control of proliferation and tumor suppression. The role of these pathways in a variety of physiological and pathological conditions remains to be
Acknowledgements
C.V.R. and M.P.G. contributed equally to this review.
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