Transforming Growth Factor-β Signaling in Hepatocytes Promotes Hepatic Fibrosis and Carcinogenesis in Mice With Hepatocyte-Specific Deletion of TAK1

BACKGROUND & AIMS: Transforming growth factor (TGF)-beta-activated kinase 1 (TAK1) is activated in different cytokine signaling pathways. Deletion of Tak1 from hepatocytes results in spontaneous development of hepatocellular carcinoma (HCC), liver inflammation, and fibrosis. TGF-beta activates TAK1 and Smad signaling, which regulate cell death, proliferation, and carcinogenesis. However, it is not clear whether TGF-beta signaling in hepatocytes, via TGF-beta receptor-2 (Tgfbr2), promotes HCC and liver fibrosis. METHODS: We generated mice with hepatocyte-specific deletion of Tak1 (Tak1 Delta Hep), as well as Tak1/Tgfbr2DHep and Tak1/Smad4 Delta Hep mice. Tak1flox/flox, Tgfbr2 beta Hep, and Smad4 Delta Hep mice were used as controls, respectively. We assessed development of liver injury, inflammation, fibrosis, and HCC. Primary hepatocytes isolated from these mice were used to assess TGF-beta-mediated signaling. RESULTS: Levels of TGF-beta, TGF-beta R2, and phospho-Smad2/3 were increased in HCCs from Tak1 Delta Hep mice, which developed liver fibrosis and inflammation by 1 month and HCC by 9 months. However, Tak1/Tgfbr2 Delta Hep mice did not have this phenotype, and their hepatocytes did not undergo spontaneous cell death or compensatory proliferation. Hepatocytes from Tak1 Delta Hep mice incubated with TGF-beta did not activate p38, c-Jun N-terminal kinase, or nuclear factor-beta B; conversely, TGF-beta-mediated cell death and phosphorylation of Smad2/3 were increased, compared with control hepatocytes. Blocking the Smad pathway inhibited TGF-beta-mediated death of Tak1-/- hepatocytes. Accordingly, disruption of Smad4 reduced the spontaneous liver injury, inflammation, fibrosis, and HCC that develops in Tak1 Delta Hep mice. Levels of the anti-apoptotic protein Bcl-xL, beta-catenin, connective tissue growth factor, and vascular endothelial growth factor were increased in HCC from Tak1 Delta Hep mice, but not in HCCs from Tak1/Tgfbr2 Delta Hep mice. Injection of N-nitrosodiethylamine induced HCC formation in wild-type mice, but less in Tgfbr2 Delta Hep mice. CONCLUSIONS: TGF-beta promotes development of HCC in Tak1 Delta Hep mice by inducing hepatocyte apoptosis and compensatory proliferation during early phases of tumorigenesis, and inducing expression of anti-apoptotic, pro-oncogenic, and angiogenic factors during tumor progression.