The role of the interferon regulatory factors, IRF-1 and IRF-2, in LPS-induced cyclooxygenase-2 (COX-2) expression in vivo and in vitro

Cyclooxygenase (COX) exists as two isoforms: COX-1, which is constitutively expressed in most cell types; and COX-2, which is inducible by lipopolysaccharide (LPS) and cytokines in a variety of cell types. Although previous studies have implicated two DNA binding proteins, interferon regulatory factor (IRF)-1 and IRF-2, in the regulation of LPS- and IFN-γ-induced COX-2, their effects in vivo and in vitro are not well-defined. Using real-time PCR, COX-2 gene expression in the livers and lungs of mice challenged in vivo and in macrophages stimulated with LPS in vitro was investigated i n wild-type and in IRF-1 and IRF-2 knockout mice. In response to 35 mg/kg LPS, IRF-1-, but not IRF-2-deficient mice, exhibited much poorer induction of COX-2 gene expression in both the livers and lungs. In vitro, COX-2 mRNA levels were also poorly induced in IRF-1-deficient macrophages, while IRF-2-deficient macrophages exhibited higher levels than in normal macrophages. IRF-1 and IRF-2 were confirmed to activate and repress expression of the COX-2 promoter, respectively, in a transient transfection system and the role of specific DNA binding sites confirmed by site-specific mutagenesis. Collectively, these data provide evidence for an important role for IRF-1 in vivo and in vitro and for IRF-2 in vitro in the regulation of COX-2 expression by LPS.