Cloning and promoter analysis of the chicken interferon regulatory factor-3 gene

Interferon regulatory factors (IRFs) are a family of DNA-binding proteins involved in mediating the cellular response to interferons (IFNs) and viral infection. Although extensively studied in mammals, IRFs of other vertebrates have been less well characterized. Previously, we cloned chicken interferon regulatory factor-3 (chIRF-3) mRNA, which is rapidly and transiently induced by double-stranded (ds)RNA. The chIRF-3 mRNA encodes a protein distinct from any known mammalian IRF. Here, we show that chIRF-3 is activated additively by type I and type II IFNs. To delineate the sequence elements required to regulate chIRF-3 expression, we cloned chlRF-3 and 0.48 kb of 5' flanking sequence. Computer analysis of the proximal promoter revealed three putative binding sites for nuclear factor (NF)-kappaB, two overlapping interferon-stimulated response elements (ISREs), and an interferon gamma activating sequence (GAS). The presence of both GAS and ISRE consensus sequences in the chIRF-3 promoter is unique among IRF family members. Both type I and II IFNs, as well as dsRNA and IRF-1, trans-activate the promoter in short-term transfection experiments. Mutational analysis of the promoter demonstrated that the putative NF-kappaB binding sites are needed for stimulation by dsRNA but not by either type I or type II IFN and that both the overlapping ISREs and GAS are required for full induction by type I or type II IFN.     

IFN regulatory factor family members differentially regulate the expression of type III IFN (IFN-lambda) genes

Virus replication induces the expression of antiviral type I (IFN-alphabeta) and type III (IFN-lambda1-3 or IL-28A/B and IL-29) IFN genes via TLR-dependent and -independent pathways. Although type III IFNs differ genetically from type I IFNs, their similar biological antiviral functions suggest that their expression is regulated in a similar fashion. Structural and functional characterization of the IFN-lambda1 and IFN-lambda3 gene promoters revealed them to be similar to IFN-beta and IFN-alpha genes, respectively. Both of these promoters had functional IFN-stimulated response element and NF-kappaB binding sites. The binding of IFN regulatory factors (IRF) to type III IFN promoter IFN-stimulated response element sites was the most important event regulating the expression of these genes. Ectopic expression of the components of TLR7 (MyD88 plus IRF1/IRF7), TLR3 (Toll/IL-1R domain-containing adapter-inducing factor), or retinoic acid-inducible gene I (RIG-I) signal transduction pathways induced the activation of IFN-lambda1 promoter, whereas the IFN-lambda3 promoter was efficiently activated only by overexpression of MyD88 and IRF7. The ectopic expression of Pin1, a recently identified suppressor for IRF3-dependent antiviral response, decreased the IFN promoter activation induced by any of these three signal transduction pathways, including the MyD88-dependent one. To conclude, the data suggest that the IFN-lambda1 gene is regulated by virus-activated IRF3 and IRF7, thus resembling that of the IFN-beta gene, whereas IFN-lambda2/3 gene expression is mainly controlled by IRF7, thus resembling those of IFN-alpha genes.