IRF1 up-regulates isg15 gene expression in dsRNA stimulation or CSFV infection by targeting nucleotides −487 to −325 in the 5′ flanking region

•IRF1 and ISG15 are up-regulated after stimulation with dsRNA or CSFV.•IRF1 over-expression resultes in ISG15 up-regulation.•IRF1 is imported into the nucleus following dsRNA stimulation.•Reporter assays confirmed that IRF1 mediates ISG15 up-regulation.•IRF1 binds to nucleotides in the 5′ fr of isg15 gene. Interferon (IFN)-stimulated gene 15 (ISG15) encodes a ubiquitin-like protein that is heavily involved in immune response elicitation. As an important member of interferon regulatory factor (IRF) family, IRF1 can activate the expression of multiple genes, including the human optineurin gene (Sudhakar et al., 2013). In this study, a sequence in the promoter region of the optineurin gene was compared to the 5′ flanking region of the porcine isg15 gene. Porcine IRF1 also possesses antiviral activity against several swine viruses (Li et al., 2015), but the mechanism is not well understood. Herein, we report that porcine IRF1 and ISG15 were up-regulated in porcine kidney (PK-15) cells following stimulation with double-stranded RNA (dsRNA) or classical swine fever virus (CSFV) infection. We also found that siRNA-mediated knockdown of IRF1 expression resulted in lower ISG15 expression in response to polyinosinic:polycytidylic acid [poly(I:C)] or CSFV infection. The overexpression of IRF1 resulted in ISG15 up-regulation. IRF1 was shown to translocate to the nucleus in response to dsRNA stimulation. To further identify the functional domain of the isg15 gene that promotes IRF1 transcriptional activity, firefly luciferase and ISG15 reporter systems were constructed. The results of the firefly luciferase and ISG15 reporter assay suggested that IRF1 mediates the up-regulation of ISG15. Nucleotides −487 to −325, located in the 5′ flanking region of the isg15 gene, constituted the promoter region of IRF1. ChIP assay indicated that IRF1 protein was able to interact with the DNA in the 5′fr of isg15 gene in cells. As an innate immune response protein with broad-spectrum antiviral activity, the up-regulation of ISG15 mediated by IRF1 in porcine cells is reported for the first time. These results warrant further investigation into the antiviral activity of porcine IRF1 against reported swine viruses.