Tumor necrosis factor (TNF) triggers distinct pathways in liver cells through TNF receptor 1 (TNF-R1) via adapter molecules, including the intracellular cascades leading to apoptosis, nuclear factor-kappa B (NF-kappa B), and Jun kinase (JNK) activation. TNF-dependent activation of NF-kappa B induces the transcription of antiapoptotic genes that renders liver cells resistant against TNF-induced apoptosis. In contrast, the role of JNK during TNF-induced apoptosis is less clear, so we studied its role during this process. Hepatoma cells treated with TNF and cycloheximide undergo apoptosis, which is proceeded by a strong activation of JNK. Adenoviral vectors (adv) were tested to block TNF-dependent JNK activation selectively. An adv expressing dominant-negative (dn) TRAF2 inhibited only JNK and not ERK or NF-kappa B activation. However, the effect of inhibiting JNK activation with a dn TAK1 virus was also specific but was stronger than that via dn TRAF2. In further experiments, the inhibitory effect of dn TAK1 on JNK was used to define its role during TNF-dependent apoptosis. Inhibition of JNK by adv dn TAK1 resulted in an earlier and stronger induction of apoptosis. Interestingly, TAM67, a dn form of c-Jun, did not mediate the JNK-dependent effect on TNF-dependent apoptosis, indicating that other molecular targets are essential to confer this mechanism. However, the modified apoptosis pattern could be inhibited by adv expressing Bcl-2 or dn FADD. In conclusion, we define TAK1 as a kinase specifically involved in TNF-induced JNK activation in hepatoma cells and show that JNK transduces antiapoptotic signals, which modulate the strength and time course of FADD-dependent cell death involving mitochondrial permeability transfer.