Immune response - IFN gamma signaling pathway

Interferon-gamma signaling

Interferons (IFNs) are pleiotropic cytokines that mediate anti-viral responses, inhibit proliferation and participate in immune surveillance and tumor suppression by inducing the transcription of a number of IFN-stimulated genes. The IFN family includes two main classes of related cytokines, type I IFNs and type II IFN. There are many type I IFNs: interferon-alpha, interferon-beta and many others. By contrast, there is only one type II IFN, interferon-gamma (IFN-gamma) that is produced by activated T cells and natural killer (NK) cells [1].

IFN-gamma exerts its effects on cells by interacting with the specific IFN-gamma receptor that is composed of two subunits, IFNGR1 and IFNGR2. IFN-gamma receptor is expressed on surfaces of nearly all cells. Binding of IFN-gamma to its receptor induces oligomerization of the receptor and activation, via trans-phosphorylation, of the receptor-associated Janus kinases 1 and 2 (JAK1 and JAK2). The activated JAKs phosphorylate the intracellular domain of the receptor (e.g., tyrosine 440 of human IFNGR1) that serves as a docking site for Signal transducer and activator of transcription 1 (STAT1). STAT1 is phosphorylated on tyrosine 701, undergoes dimerization, translocates to the nucleus and regulates gene expression by binding to gamma-activated sequence (GAS) elements in the promoters of IFN-gamma-regulated genes [2].

Some kinases can phosphorylate STAT1 at serine 727 (Ser727). This phosphorylation is not required for STAT1 translocation to the nucleus or for its binding to the promoters. However, it is essential for the full transcriptional activation. These kinases include Protein kinase C delta (PKC-delta) and Calcium/calmodulin-dependent protein kinase II (CaMK II) [3], [4].

Precise mechanisms of IFN-gamma-induced activation of these kinases are not clear. However, it was shown that IFN-gamma activates Phosphatidylinositol 3-kinase (PI3K)/ v-AKT murine thymoma viral oncogene homolog (AKT) signaling pathway, perhaps via the adapter Cas-Br-M ecotropic retroviral transforming sequence (c-Cbl) that binds regulatory subunit of PI3K (PI3K reg class 1A). PKC-delta is an effector of the PI3K pathway [5]. Although the mechanism of PI3K-dependent PKC-delta activation is unclear, PI3K-dependent phosphorylation of PKC-delta by 3-Phosphoinositide dependent protein kinase-1 (PDK (PDPK1)) was demonstrated [6].

Also IFN-gamma induces c-Cbl mediated activation of v-CRK avian sarcoma virus CT10 oncogene homolog-like (CrkL). This provides a link between the IFN-gamma receptor and the Rap guanine nucleotide exchange factor 1 (C3G) and results in the IFN-gamma-dependent activation of RAP1A, member of RAS oncogene family (Rap1A), a protein known to exhibit tumor suppressor activity and mediate growth inhibitory responses [7].

IFN-gamma also induces phosphorylation of Phospholipase C gamma 2 (PLC-gamma 2) by JAK1/2. Diacylglycerol (DAG) is the product of the enzymatic activity of the PLC-gamma 2. It can activate some of the protein kinase C isoforms of, including PKC-alpha. The PKC-alpha can stimulate tyrosine-protein kinase SRC-1 (c-Src) activity. Although PKC-alpha can phosphorylate the c-Src directly, Actin filament associated protein (AFAP) is essential for this c-Src activation [8]. c-Src in its turn activates STAT1 by phosphorylation on tyrosine 701. This IFN-gamma-induced PLC-gamma 2/ PKC-alpha/ c-Src/ STAT1 pathway leads to the expression of Intercellular adhesion molecule 1 (ICAM-1) gene [9].

There are many known STAT1-targets in IFN-gamma-mediated signaling. These are SMAD family member 7 (SMAD7), Interferon regulatory factor 1 (IRF1) and proteins involved in cell cycle regulation, e.g., v-Myc myelocytomatosis viral oncogene homolog (c-Myc) and Cyclin-dependent kinase inhibitor 1A (p21) [10], [11].

IRF1 participates in the activation of the Suppressor of cytokine signaling-1 (SOCS-1). The SOCS-1 protein is critical for inhibiting IFN-gamma responses [12]. IFN-gamma induces expression of SOCS1 indirectly, by inducing the expression of the IRF-1 transcription factor via STAT1. IRF-1 in turn stimulates transcription of the SOCS-1 gene [13].

Several proteins interact with STAT1 and modulate its transcriptional activity: CREB-binding proteins (CBP and p300), Minichromosome maintenance protein 5 (MCM5) and Breast cancer susceptibility gene 1 (BRCA1). CBP and p300 possess histone acetyl transferase activity and function as co-activators. MCM5 and BRCA1 associate with phosphorylated STAT1 and enhance its transcriptional activity [2].

In addition, IFN-gamma may activate JAK-STAT-independent pathways.

Calcium-dependent tyrosine kinase PTK2B protein tyrosine kinase 2 beta (Pyk2(FAK2)) is a substrate for JAK2. Pyk2(FAK2) phosphorylates Mitogen-activated protein kinase kinase kinase 4 (MEKK4). Phosphorylated MEKK4 in turn phosphorylates Mitogen-activated protein kinase kinase 6 (MEK6). Subsequently, MEK6 phosphorylates p38 MAPK that phosphorylates and activates Activating transcription factor 2 (ATF-2). Protein-tyrosine phosphatase 2C (SHP-2) regulates this signaling pathway by dephosphorylating MEKK4 and its activating kinase, Pyk2(FAK2) [14].

Another pathway stimulated by IFN-gamma involves Mitogen-activated protein kinase kinase kinase 1 (MEKK1), Mitogen-activated protein kinase kinase 1 (MEK1) and Mitogen-activated protein kinases 1 and 3 (ERK1/2). MEKK1/ MEK1/ ERK1/2 cascade regulates activity of CCAAT/enhancer binding protein beta (C/EBP-beta) and C/EBP-beta-driven expression of Interferon regulatory factor 9 (IRF9) gene. IRF9 is a subunit of ISGF3 transcription complex that participates in interferon signaling [15].

References:

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