A 5' extended IFN-stimulating response element is crucial for IFN-gamma-induced tripartite motif 22 expression via interaction with IFN regulatory factor-1.

Interferon-gamma is crucial for the noncytopathic clearance of hepatitis B virus. In our previous study, we demonstrated that an IFN-gamma-inducible molecule, tripartite motif (TRIM) 22, played an important role in antiviral immunity against hepatitis B virus. However, the molecular mechanism of TRIM22 induction by IFN-gamma is still unclear. In this study, we identified a novel cis-element termed 5' extended IFN-stimulating response element (5' eISRE) that was crucial for IFN-gamma inducibility of TRIM22 through transfection assays with luciferase reporter constructs and EMSAs. The 5' eISRE consists of an ISRE-like motif (ACTTTCGTTTCTC) and a 6-bp sequence (AATTTA) upstream of it, and all three thymine triplets of this cis-element (AATTTAACTTTCGTTTCTC) were revealed to contribute to the IFN-gamma inducibility of TRIM22 by site-directed mutagenesis. Further studies showed that upon IFN-gamma stimulation, the 5' eISRE could be bound by IFN regulatory factor-1 (IRF-1), but not by STAT1, as demonstrated by supershift analysis and an ELISA-based transcription factor assay. Moreover, overexpression of IRF-1 significantly induced TRIM22 expression, whereas silencing of IRF-1 with specific short interference RNA abolished IFN-gamma-induced TRIM22 expression in HepG2 cells, indicating an IRF-1-dependent expression of TRIM22. Taken together, it was demonstrated in this study that a novel cis-element, 5' eISRE, was crucial for the IFN-gamma-induced transcriptional activity of the TRIM22 gene via interaction with IRF-1.