TGF-β promotes oxidative stress during chronic liver damage by interfering with MEOS and ADH metabolising systems

Aims: Fibrogenesis process involves cytokines, oxidative stress and complex interactions between liver cells. The hepatotoxic effect during chronic alcohol comsumption results from activating the alcohol dehydrogenase ADH and microsomal ethanol oxidating system MEOS. TGF-β is induced in livers of patients with alcoholic cirrhosis, suggesting that it is involved in the development of alcohol-induced liver damage but the process details remain to be understood.

Aims: Primary cultured mouse hepatocytes were used as a system to investigate a possible crosstalk between TGF-β signaling and alcohol damaging effects.

Methods: Using DNA microarray, a TGF-β dependent expression profile was established for hepatocytes. Results were confirmed by RT-PCR and Western blot analysis after cells treatment with TGF-β and/or ethanol.

Results: Microarray data implicate that TGF-β induces expression of cytochrome P450 enzymes of MEOS in hepatocytes. The candidate genes of fibrosis, Col11A1 or apoptosis markers as GADD45β were also induced. This finding was confirmed by RT-PCR and Western blot analysis, showing that TGF-β and alcohol stimulate expression of Cyp2E1 and Cyp21A1 enzymes to a similar extent. Co-stimulation with TGF-β and alcohol did not display enhanced Cyp2E1 expression, suggesting that TGF-β might be a downstream mediator of alcohol effects. The latter is currently investigated by interfering with TGF-β signaling using Smad7 overexpression or inhibitors for TGF-β receptors. Interestingly, while inducing MEOS activity, TGF-β downregulates ADH1 expression in mouse hepatocytes. At present, we aim to delineate the TGF-β signalling pathways leading to regulation of Cyp2E1, Cyp21A1 and ADH1 to find out if it is Smad dependent or not.

Conclusion: TGF-β regulates expression of enzymes involved in alcohol metabolism. Genes encoding enzymes of MEOS are upregulated, whereas ADH1 expression is decreased. By increasing the oxidative stress, inducing apoptosis and extracellular matrix deposition TGF-β actively contribute to development of liver damage. Future studies aim at delineating the intracellular mechanisms through which TGF-β is participating in alcohol-induced liver disease.