Transforming growth factor  (TGF-) has been shown to induce apoptotic cell death in normal and transformed hepatocytes. However, the exact mechanism through which TGF- induces cell death is still unknown. We examined a potential role of various death receptor/ ligand systems in TGF--induced apoptosis and identified the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) as a mediator of TGF--induced apoptosis in hepatoma cells. TGF--induced apoptosis is significantly impaired upon blockage of TRAIL. We show that TRAIL is upregulated in hepatoma cells upon treatment with TGF-, whereas TRAIL receptor levels remain unchanged. In conclusion, our results provide evidence that the TRAIL system is critically involved in TGF--induced cell death in liver pathology. (HEPATOLOGY 2005;42:183-192.) T ransforming growth factor  (TGF-) is a multifunctional cytokine whose numerous cell and tissue activities include cell-cycle control, regulation of early development, differentiation, extracellular matrix formation, hematopoiesis, angiogenesis, chemotaxis, immune functions, and induction of apoptosis. 1 The apoptosis-inducing capacity of TGF- has been investigated in many cell types. In hepatocytes, TGF- 1 has been shown to inhibit cell proliferation and to induce apoptosis in vitro, and to control the excessive growth and mainte-nance of liver size in vivo. 2 In addition, TGF- 1 has been demonstrated to trigger apoptotic cell death in human and rat hepatoma cell lines. Transgenic mice overexpressing TGF- 1 suffer from continuing apoptotic cell death of hepatocytes and consequently develop hepatic fibrosis. 3,4 Exogenous administration of TGF- in rodents also results in a significant increase in hepatic cell death. 5 These data strongly suggest that apoptosis induced by TGF- may be involved in various hepatic lesions. Given its implication in these processes, it is important to understand the biochemical mechanism of initiation of TGF--induced apoptosis. So far, this issue has been addressed in several studies examining the role of different and known apoptosis signaling proteins. Analyses of TGF--induced cell death in hepatoma cell lines have confirmed that apoptosis is accompanied by the activation of caspases and cleavage of the caspase-3-specific substrate poly(ADP-ribose)polymerase (PARP). Analysis of the apoptotic process in primary rat hepatocytes has revealed that transcriptional activation is necessary for cell death induction. The application of cycloheximide, which blocks de novo protein synthesis, results in the inhibition of TGF--mediated apoptosis induction. 6 However, it remains unclear which target genes are crucial for TGF--induced apoptosis. In that respect, the role of different death receptor/ligand systems in TGF-induced apoptosis has not been thoroughly investigated. Essentially, these are tumor necrosis factor (TNF) and the CD95 (FAS/APO-1) and TNF-related apoptosis-inducing ligand (TRAIL) systems. TRAIL has attracted attention for its ability to preferentially kill a wide variety of tumor cell lines, 7,8 whereas most normal cells are resistant