TGF-β sensitizes heptocytes for damage by xenobiotics

Development and progression of chronic liver disease is associated with induction, activation and signal transduction of TGF-β. While its role in activation of hepatic stellate cells and subsequent extracellular matrix deposition is well established, not many details are currently known about TGF-β effects in hepatocytes. TGF-β causes oxidative stress by increasing reactive oxygen species and reducing cellular glutathione. mRNA expression profiling of mouse hepatocytes treated for 24 hours with TGF-β indicates participation of the hepatocyte detoxification system in this process by showing an up-regulation of phase I and down-regulation of phase II enzymes. This altered expression profile was confirmed on the protein level by Western blot and fluorescent based activity assays in mouse and human hepatocytes. Up-regulation of phase I enzymes was most pronounced for cytochrome P450 families 1–3, being responsible for xenobiotic metabolism. Therefore, we measured toxicity (Alamar blue and SRB staining) in hepatocytes pre-treated or not with TGF-β for 24h. For substances causing direct toxicity, e.g. DMSO, isopropanol or mercury chloride, there was no significant difference in the obtained EC₅₀ values, whereas those for substances causing metabolism dependent toxicity, e.g. caffeine (CYP1A2), verapamil (CYP1A2 and 3A4), digoxine or amiodarone (CYP3A), were significantly reduced in TGF-β pre-treated cells. Based on this contrary regulation of phase I and phase II enzymes, we speculate that presence of TGF-β results in accumulation of intermediate products from the detoxification process, sensitizing primary hepatocytes for damage by xenobiotics.