Effect of TAK1 gene silencing on LPS- and TNF-induced MAPK signaling in monocytes. Mouse monocytic cells (J774.1) were transiently transfected with siRNAs against TAK1 (siTAK1) or with a nontargeting control siRNA (siCT) as described in the Methods. Twenty-four hours after lipofection, the cells were treated with 1 μg/mL of LPS (A) or 20 ng/mL of TNF-α (B) for the indicated time periods. Whole-cell extracts were prepared, and the inhibition of TAK1 and the activation of JNK, p38, and ERK1/2 were determined by Western blot analyses. Knockdown of TAK1 leads to a reduction of TNF-α- and LPS-induced JNK activation, whereas no effect was evidenced on p38 and ERK activation, compared with siCT-treated cells.