Transforming growth factor-1 (TGF-1) plays a vital role in controlling liver size by inducing apoptotic cell death in hepatocytes (Oberhammer et al, 1992a; 1992b). Previous work in our group and others has suggested that TGF-1-induced apoptosis involves a caspase cascade (Cain et al, 1996; Inayat-Hussain et al, 1997; Choi et al, 1998; Shima et al, 1999), however the mechanism by which TGF-1 induces caspase activation and cell death is unknown. In the current study, I have extended these observations by directly showing the activation and processing of caspases-2, 3, 7 and 8 and the caspase-6 substrate Lamin B1 during TGF-1-induced apoptosis in rat hepatoma cells. Furthermore, I have shown that TGF-1 is unlikely to be inducing caspase activation via a typical death receptor-mediated mechanism. Rather, that TGF-1 may activate the caspase cascade by causing cytochrome c release, which induces assembly of an ~700-kD apoptosome caspase-processing complex, which recruits and activates the caspases. In addition, the work in this study provides the first evidence that TGF-1-induced apoptosis is associated with the coordinated induction and repression of specific groups of genes and their functional protein products. Most notably, TGF-1 causes the early and sustained down-regulation of genes encoding proteins involved in anti-oxidant pathways. Importantly, the generation of reactive oxygen species (ROS) has been previously demonstrated during TGF-1-induced apoptosis (Sanchez et al, 1996; Sanchez et al, 1997), and TGF-1-induced oxidative stress and apoptosis can be blocked by anti-oxidants (Sanchez et al, 1996; Ribeiro et al, 1999). At later time-points, there was also a specific up-regulation of apoptogenic proteins, such as caspase-8, Bad, and Bak1, which may lead to an amplification of the apoptotic response. Hence, taken together the data in this study suggests that TGF-1 may trigger apoptosis by specifically down-regulating anti-oxidants, thus leading to an increase in ROS, cytochrome c release and the activation of caspases via the apoptosome complex.