We have previously shown that oval cells harboring a genetically inactivated Met tyrosine kinase (Met-/- oval cells) are more sensitive to TGF-ß-induced apoptosis than cells expressing a functional Met (Metflx/flx), demonstrating that the HGF/Met axis plays a pivotal role in oval cell survival. Here, we have examined the mechanism behind this effect and have found that TGF-ß induced a mitochondria-dependent apoptotic cell death in Metflx/flx and Met-/- oval cells, associated with a marked increase in levels of the BH3-only proteins Bim and Bmf. Bmf plays a key role during TGF-ß-mediated apoptosis since knocking down of BMF significantly diminished the apoptotic response in Met-/- oval cells. TGF-ß also induced oxidative stress accompanied by NADPH oxidase 4 (Nox4) mRNA up-regulation and decreased protein levels of antioxidant enzymes. Antioxidants inhibit both TGF-ß-induced caspase 3 activity and Bmf up-regulation, revealing an oxidative stress-dependent Bmf regulation by TGF-ß. Notably, oxidative stress-related events were strongly amplified in Met-/- oval cells, emphasizing the critical role of Met in promoting survival. Pharmacological inhibition of PI3K did impair HGF-driven protection from TGF-ß-induced apoptosis and increased sensitivity of Metflx/flx oval cells to TGF-ß by enhancing oxidative stress, reaching apoptotic indices similar to those obtained in Met-/- oval cells. Interestingly, both PI3K inhibition and/or knockdown itself resulted in caspase-3 activation and loss of viability in Metflx/flx oval cells, whereas no effect was observed in Met-/- oval cells. Altogether, results presented here provide solid evidences that both paracrine and autocrine HGF/Met signaling requires PI3K to promote mouse hepatic oval cell survival against TGF-ß-induced oxidative stress and apoptosis.

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