Central Regulatory Role for SIN1 in Interferon γ (IFNγ) Signaling and Generation of Biological Responses

Abstract

Edited by Charles E. SamuelThe precise signaling mechanisms by which type II IFN receptors control expression of unique genes to induce biological responses remain to be established. We provide evidence that Sin1, a known element of the mammalian target of rapamycin complex 2 (mTORC2), is required for IFN␥-induced phosphorylation and activation of AKT and that such activation mediates downstream regulation of mTORC1 and its effectors. These events play important roles in the assembly of the eukaryotic translation initiation factor 4F (eIF4F) and mRNA translation of IFN-stimulated genes. Interestingly, IFN␥-induced tyrosine phosphorylation of STAT1 is reduced in cells with targeted disruption of Sin1, leading to decreased transcription of several IFN␥-inducible genes in an mTORC2-independent manner. Additionally, our studies establish that Sin1 is essential for generation of type II IFN-dependent antiviral effects and antiproliferative responses in normal and malignant hematopoiesis. Together, our findings establish an important role for Sin1 in both transcription and translation of IFN-stimulated genes and type II IFN-mediated biological responses, involving both mTORC2-dependent and -independent functions.Interferons (IFNs) are pleiotropic cytokines that possess antiviral, immunomodulatory, growth-inhibitory, and cytotoxic properties (1-13). IFNs were among the earliest biologic therapeutic agents used to treat viral infections, certain solid tumors, hematologic cancers, and some autoimmune disorders (1-13). IFN␥, the only type II IFN, signals through a multimeric receptor complex, IFNGR, consisting of two different chains: the IFN␥ receptor binding subunit (IFNGR1) and a transmembrane accessory factor (IFNGR2) (14,15). Engagement of IFN␥ with IFNGR leads to activation of the Janus kinases, JAK1 and JAK2, leading to tyrosine phosphorylation and activation of STAT1 (14 -18). Upon activation, STAT1 homodimers bind DNA at IFN␥-activated site elements, leading to transcription of IFN-stimulated genes (ISGs) 6 (15-18). There is evidence for different signaling pathways that are activated by IFNGR. These include protein kinase C (PKC), MAPK, Mnk kinase, PI3K/AKT, and mammalian target of rapamycin complex 1 (mTORC1) and mTORC2 signaling cascades (19 -29). mTOR pathways promote mRNA translation in an mTOR/4E-BP1-dependent manner (25,(27)(28)(29)(30)(31)(32)(33). In previous studies, our group showed that AKT/mTOR pathways are engaged in IFN␥ signaling and control initiation of mRNA translation of ISGs (25,27,28,30). The 289-kDa mTOR kinase contains binding sites for multiple proteins regulating its activity or mediating its signals and consists of at least two independent multiprotein complexes: mTORC1 and mTORC2 (34 -38). mTORC1 is formed by mTOR, mLST8/G␤L (mammalian lethal with Sec13 protein 8/G-protein ␤-protein subunit-like), Raptor (rapamycin-sensitive companion of mTOR), Pras40 (Akt/PKB substrate 40 kDa), and Deptor (DEP domain-containing mTOR-interacting protein) and is sensitive to allosteric inhibitors, such ...

IFN-γ–inducible antiviral responses require ULK1-mediated activation of MLK3 and ERK5

“…IFNγ initiates its biological activities by binding to its cognate heterodimeric cell surface receptor, composed of two ligand binding IFN-gamma receptor 1 (IFNGR1) subunit chains and two signal-transducing IFNGR2 chains (4). This interaction triggers activation of several downstream signaling events, including the canonical Janus kinase (JAK)-STAT1 pathway, and non-STAT pathways, such as mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), mechanistic target of rapamycin complex 2 (mTORC2), calcium/calmodulin-dependent protein kinase II (CaMKII), and nuclear factor kappa-B (NF-kB) cellular pathways (3,5,6,7,8). Although IFNγ-mediated activation of STAT1 is required for several IFNγ-dependent antiviral responses, some of these activities are STAT1-independent, which implies that there may be activation of alternate and complementary pathways by IFNγ stimulation during a viral infection (3).…”

Folliculin regulates mTORC1/2 and WNT pathways in early human pluripotency

“…5f). mTORC2-independent roles of SIN1 have been described 43,44 . We therefore generated KD of another member of mTORC2, RICTOR in hESC and found that a decrease of RICTOR expression also downregulated P-AKT(S473) and P-S6 during primed transition, mimicking SIN1 mutant phenotype in cells exiting the naïve state (Fig.…”

“…However, we cannot rule out that the binding of FLCN to SIN1 in naïve hESC could also regulate mTORC2-independent functions of SIN1 (refs. 43,44 ). It is possible that FLCN may not directly interact with mTORC2 but rather may sequester SIN1 from this complex.…”

Sin1-mediated mTOR signaling in cell growth, metabolism and immune response

“…In addition, targeted disruption of Sin1 led to decreased activation of the STAT1 signaling pathway and type I IFN-induced gene transcription in antiproliferative responses [ 90 ]. More recently, it was found that Sin1 could regulate IFNγ-induced genes and type II IFN-mediated biological responses via both Akt/mTORC1 activation and tyrosine phosphorylation of STAT1 [ 91 ].…”

“…Apart from mediating the development of immune cells, Sin1 also contributes to the formation of a specific microenvironment in bone marrow as knockdown of Sin1 at mRNA level reduces low-dose irradiation-induced Akt Ser473 phosphorylation and subsequent responses in mouse osteoblasts, which inhibit osteoblast differentiation [ 107 ]. Sin1 may also regulate multiple cytokine-induced pathways since dysregulation of Sin1 led to decreased STAT1 and suppressed the transcription of several IFN-γ-induced genes [ 91 ]. As discussed above, Sin1 could also regulate type I IFN production and its antitumor responses [ 89 ].…”