Interferon regulatory factor-3-mediated activation of the interferon-sensitive response element by toll-like receptor (TLR) 4 but not TLR3 requires the p65 subunit of NF-κB

Interferon regulatory factor ( IRF) 3 is a transcription factor that binds the interferon- sensitive response element ( ISRE) and is activated by Toll- like receptor 3 ( TLR3) and TLR4. We have found that a dominant negative form of I kappa B kinase 2 and a mutant form of I kappa B, which acts as a super- repressor of NF- kappa B, blocked activation of the ISRE by the TLR4 ligand lipopolysaccharide but not the TLR3 ligand poly( I- C). TLR4 failed to activate the ISRE in mouse embryonic fibroblasts bearing a targeted deletion of p65, whereas the response to TLR3 in these cells was normal. The p65 subunit of NF- kappa B was detected in the lipopolysaccharide- activated but not poly( I- C)- activated ISRE- binding complex. Finally, p65 promoted transactivation of gene expression by IRF- 3. These results therefore indicate that IRF- 3-mediated activation of the ISRE by TLR4 but not TLR3 requires the p65 subunit of NF- kappa B.