Immune response IFN alpha/beta signaling pathway
IFN alpha/beta signaling pathway
Interferons (IFNs) are pleiotropic cytokines that exhibit important biologicactivities, including antiviral, antiproliferative, antitumor and immunomodulatoryeffects [1], [2].
IFNs are classified as either Type I or Type II. Type I IFNs include theIFN-alpha family of 13 subtypes, IFN-beta, IFN-omega, IFN-tau, IFN-kappa,IFN-lambda, and IFN-zeta. By contrast, there is only one Type-II IFN, IFN-gamma [3], [2], [4].
IFN-alpha and IFN-beta bind to the type I IFN receptor (IFN-alpha/beta receptor ) consisting of two subunits, Interferon (alpha, beta andomega) receptor 1 ( IFNAR1 ) and Interferon (alpha, beta and omega) receptor 2 (IFNAR2 ) [5].
IFN-alpha/beta receptor lacks intrinsic kinase activity and thus relies onassociated Janus kinases ( JAK1 and Tyk2 ) to phosphorylate receptor andsignal transducing molecules, such as Signal transducers and activators of transcription1 ( STAT1 and STAT2 ), after ligand-induced receptor clustering.IFNAR1 is pre-associated with Tyk2, and also binds STAT1 andSTAT2. IFNAR2 is pre-associated with JAK1, STAT1 andSTAT2 [4].
The tyrosine phosphorylation of STAT1 and STAT2 by JAK1 andTyk2 leads to the formation of transcriptional complexes that translocate to thenucleus to induce expression of certain genes [2].
An important transcriptional complex that is induced by Type-I IFNs is the ISGFactor-3 complex ( ISGF3 ). The mature ISGF3 complex is composed ofphosphorylated forms of STAT1 and STAT2 and Interferon regulatory factor 9( IRF9 ), which does not undergo tyrosine phosphorylation [2].ISGF3 is the only complex that binds specific elements known as IFN-stimulatedresponse elements (ISREs) that are present in the promoters of certain genes, such asPromyelocytic leukemia ( PML ), ISG15 ubiquitin-like modifier ( ISG15 ),Interferon-induced protein with tetratricopeptide repeats 2 ( ISG54 ) andInterferon alpha-inducible protein 6 ( IFI6 ) [6], [7],[8], [9].
In response to IFN-alpha, STAT1 and STAT2 can also form anothertranscriptional complex, STAT1/STAT2 heterodimer, that exhibits binding to thegamma-activated sequence (GAS) element of the Interferon regulatory factor 1 (IRF1 ) gene [10], [11]. IRF1, in turn, can alsoinduce the transcription of ISG15, ISG54 and IFI6 genes, whereasanother IFN-alpha-inducible factor, Interferon regulatory factor 2 ( IRF2 ), isinvolved in the repression of gene transcription [12], [13], [14], [15].
Arginine methylation of STAT1 by Protein arginine methyltransferase 1 (PRMT1 ) is an additional posttranslational modification that regulatestranscription factor function required for proper IFN-alpha/beta-induced transcription[16].
A number of negative regulatory molecules limit the extent of type I IFN signaling.Suppressor of cytokine signaling 1 ( SOCS1 ) inhibits type I IFN signaling viainteractions with IFNAR1, JAK1 and Tyk2 [17]. Proteintyrosine phosphatases non-receptor type 6 and 11 ( SHP-1 and SHP-2 )dephosphorylate JAK1 and STAT1 and suppress their signaling [18], [19]. Protein tyrosine phosphatase non-receptor type 1 (PTP-1B ) dephosphorylates Tyk2 and modulates signaling responses toIFN-alpha [20]. A type I IFN-inducible Ubiquitin specific peptidase 18( UBP43 ) binds directly to IFNAR2 and blocks the interaction betweenJAK1 and IFN-alpha/beta receptor [21].
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