IL-1β-induced JNK activation in fibroblast-like synoviocytes (FLSs) is TAK1-dependent. The effect of TAK1 deficiency on JNK activation was determined by western blot analysis. Three days after TAK1 or scrambled control (sc) small interfering RNA transfection, serum-starved FLSs were stimulated with IL-1β (2 ng/ml) for 15 minutes. Cell lysates were evaluated for P-JNK, P-MKK4, P-MKK7, P-p38, and actin. JNK, MKK4, and MKK7, but not p38 activation was significantly decreased in the absence of TAK1 (average inhibition: JNK, 58 ± 1%, p = 0.01; MKK4, 49 ± 3%, p = 0.01; MKK7, 49 ± 7%, p = 0.04). A representative experiment is shown (n = 3).