Transforming growth factor-β1 induces hepatocyte apoptosis by a c-Jun independent mechanism

Background. During hepatic regeneration, transforming growth factor (TGF)-β1 messenger RNA increases after the initial cycle of DNA synthesis, and it may control hepatocyte growth by inducing apoptosis. TGF-β1 also induces c-Jun, a potential proapoptotic transcription factor. We hypothesized that autocrine expression of activated TGF-β1 (Ad5aTGF-β1) would increase c-jun expression in rat liver and limit hepatic regeneration by inducing apoptosis. Methods. Male rats (175 to 200 g) received portal venous injections with adenoviruses expressing either luciferase (Ad5Luc), as a control, or Ad5aTGF-β1 at a dose of 6 × 10⁹ plaque-forming units. Livers were harvested 24 or 48 hours after injection and nuclear extracts and total RNA isolated. TGF-β1 expression was confirmed by Northern blot analysis in all TGF-β1-injected rats. Results. A 2.5-fold increase in c-jun mRNA expression .was detected in Ad5aTGF-β1-infected rats compared with control rats. Transcriptional activity was assessed with an AP-1-responsive-reporter gene that increased 3-fold in rat primary hepatocytes infected with Ad5aTGF-β1. C-Jun N-terminal kinase activity also increased 6- to 7-fold in Ad5aTGF-β1-treated rats 24 and 48 hours after injection. Ad5aTGF-β1-injected rats demonstrated increased AP-1 binding activity compared with Ad5Luc rats. Hepatocytes infected in vitro with Ad5aTGF-β1 demonstrated increased apoptosis compared with Ad5Luc-infected hepatocytes (47% vs 27%) 36 hours after infection. Dual adenoviral infection with Ad5aTGF-β1 and a dominant-negative c-Jun (Ad5TAM67) decreased AP-1-induced Ad5Luc activity but not hepatocyte apoptosis (46% with dominant-negative c-Jun and 47% without). Conclusions. These data demonstrate that TGF-β1 induces c-Jun, but c-Jun is not proapoptotic in hepatocytes.