Transforming growth factor (TGF-) signaling is pivotal for the progression of specific types of tumors at certain stages. However, the mechanism by which TGF- is regulated by other factors remains unclear. In this study, the involvement of SP600125, an inhibitor of c-Jun N-terminal kinase (JNK), in TGF--induced apoptosis of the RBE human cholangiocarcinoma cell line was investigated. Exogenous TGF-1 activated Smad and non-Smad signaling pathways, including the JNK pathway in RBE cells, and induced apoptosis, which was inhibited by knockdown of Smad4 expression. SP600125 increased the TGF-1-induced phosphorylation of Smad2 and Smad3, which enhanced the TGF-1-induced transcriptional response and apoptosis in RBE cells. The effect of SP600125 on the transcriptional response and apoptosis was reduced by knockdown of Smad4 expression. In addition, TGF-1-induced apoptosis was abrogated using the pan-caspase inhibitor Z-VAD-fmk. SP600125 promoted the TGF-1-induced caspase cleavage, while knockdown of Smad4 expression counteracted this effect. These results indicate that SP600125 enhances TGF--induced apoptosis of RBE cells through a Smad-dependent pathway that involves Smad-dependent caspase activation. SP600125 is hypothesized to be an ideal therapeutic candidate for treating human cholangiocarcinoma.