Role of Tyrosine 441 of Interferon-γ Receptor Subunit 1 in SOCS-1-mediated Attenuation of STAT1 Activation

Abstract

Suppressor of cytokine signaling (SOCS)-1, the key negative regulator of interferon (IFN)-␥-dependent signaling, is induced in response to IFN␥. SOCS-1 binds to and inhibits the IFN␥ receptor-associated kinase Janusactivated kinase (JAK) 2 and inhibits its function in vitro, but the mechanism by which SOCS-1 inhibits IFN␥-dependent signaling in vivo is not clear. Upon stimulation, mouse IFN␥ receptor subunit 1 (IFNGR1) is phosphorylated on several cytoplasmic tyrosine residues, and Tyr 419 is required for signal transducer and activator of transcription (STAT) 1 activation in mouse embryo fibroblasts. However, the functions of the other three cytoplasmic tyrosine residues are not known. Here we show that Tyr 441 is required to attenuate STAT1 activation in response to IFN␥. Several tyrosine to phenylalanine mutants of IFNGR1, expressed at normal levels in stable pools of IFNGR1-null cells, were analyzed for the phosphorylation of STAT1 during a 48-h period, and antiviral activity in response to IFN␥ was also measured. Stronger activation of STAT1 was observed in cells expressing all IFNGR1 variants mutated at Tyr 441 , and, consistently, stronger antiviral activity was also observed in these cells. Interferon (IFN)1 -␥ plays key roles in mediating antiviral and antigrowth responses and in modulating immune responses (1). The major signal transduction pathway activated by IFN␥ has been elucidated through both biochemical and genetic studies. The IFN␥ receptor complex consists of two receptor subunits, IFNGR1 and IFNGR2, and the tyrosine kinases Janus-activated kinase (JAK) 1 and JAK2, which bind to IFNGR1 and IFNGR2, respectively. IFN␥ induces the oligomerization of the receptor subunits, leading to the activation of JAK1 and JAK2, which then phosphorylate tyrosine residues within the cytoplasmic domain of IFNGR1. Signal transducer and activator of transcription (STAT) 1 is then recruited to the receptor complex and phosphorylated on Tyr 701 , allowing it to be released, form homodimers, translocate to the nucleus, and bind to ␥-activated sequences to activate the transcription of interferon-stimulated genes (ISGs) (1-3).The activation of STAT1 by IFN␥ is tightly controlled by several mechanisms (4). The SH2-containing phosphatase 2 binds to IFNGR1 and inhibits STAT1 activation without inhibiting the phosphorylation of IFNGR1 (5). Protein inhibitor of activated STAT 1 (PIAS-1) binds to STAT1 and prevents its association with target DNA (6). Both genetic and biochemical studies have shown that suppressor of cytokine signaling (SOCS)-1 is the most potent inhibitor of IFN␥ signaling (7). Mice lacking SOCS-1 develop a complex fatal neonatal disease (8 -10), and the mortality, which results from hypersensitivity to IFN␥, is largely prevented by administration of anti-IFN␥. In addition, premature death does not occur in mice lacking both 11). In response to IFN␥, STAT1 activation is much stronger in cells lacking SOCS-1 than in wild-type cells (11,12). The constitutive expression of SOCS-1 blocks IFN␥-mediated antivir...

Preassembly and ligand-induced restructuring of the chains of the IFN-γ receptor complex: the roles of Jak kinases, Stat1 and the receptor chains

“…Furthermore, when Tyr-457 is mutated, the resulting receptor is still capable of tyrosine phosphorylation of Jak1, Jak2, and Tyr-479 of IFN-γR1, which creates an evolutionarily conserved (Figure 3) recruitment site for SOCS-1 [48]. All except the last activity are known to require the presence and/or activity of Jak kinases [22,46].…”

Modulation of the activation of Stat1 by the interferon-γ receptor complex

“…Among these genes is SOCS-1 [11,17,18]. SOCS-1 binds to IFN-γR1 and the activation loop of Jak2 (and possibly also Jak1) and prevents either the recruitment of additional Stat proteins or the phosphorylation of target proteins by the Jak kinases [8,19]; thus SOCS-1 can repress signaling by not only IFN-γ but by other cytokines using the Jak/Stat pathway. Another notable gene induced by Stat1 is IRF-1, whose induction is necessary for the upregulation of MHC Class I and Class II [20,21], FasL [22], and caspases-1 and -8 [23,24] by IFN-γ.…”

“…The interaction of IFN-γ with this integral membrane receptor complex results in structural rearrangements in the receptor complex and the activation of Jak1 and Jak2 tyrosine kinases bound to the intracellular domains of IFN-γR1 and IFN-γR2 respectively [4][5][6]. Activation of Jak kinases results in the tyrosine phosphorylation of Jak1 and Jak2 on their kinase domains, and of the COOH-terminus of IFN-γR1 in two locations, which allow the recruitment of latent Stat1 and SOCS-1 [4, [7][8]. Translocation of Stat1 to the nucleus allows Stat1 to modify the transcription of a fraction of IFN-γ-sensitive genes [9,10].…”

RNA interference-mediated silencing of SOCS-1 via lentiviral vector promotes apoptosis of alveolar epithelial cells in vitro

“…Among them, SOCS1 was initially recognized as a repressor factor of IFN-γ, which is a critical molecule involved in the initiation of inflammation and is required for normal postnatal growth and survival (4)(5)(6). Knockout studies in mice suggest the role of SOCS1 as a modulator of IFN-γ action (7). Further studies have shown that SOCS1 can be induced by the activated janus kinase family (JAK) and the signaling transduction and activators of transcription family (STATs), and may downregulate the JAK/Stats signaling pathway (8), which plays an important role in the initiation and activation of inflammation, through inhibition of the phosphorylation of kinase mentioned above.…”