Regulation of AP-1 binding and transcriptional activity is TAK1-dependent. The effect of TAK1 deficiency on IL-1β-induced AP-1 activation was evaluated by electrophoretic mobility shift assay. Five days after small interfering RNA transfection, cultured fibroblast-like synoviocytes were stimulated with IL-1β (2 ng/ml) for 60 minutes. Nuclear extracts were obtained and evaluated for AP-1 and NF-κB binding activity. A representative experiment is shown (n = 3). IL-1β-induced AP-1 activity was significantly decreased in the absence of TAK1 (84 ± 8% inhibition compared to control, p = 0.03). NF-κB binding, on the other hand, was not significantly decreased with TAK1 deficiency (34 ± 7% inhibition, p = 0.21). Cold oligo, non-radioactive oligo; Sc, scrambled control.