Nrf2 regulates antioxidative and liver regeneration promoting genes

The liver is frequently challenged by insults such as toxins and alcohol, viral infection or metabolic overload. In contrast to other organs, the liver has a unique capability to regenerate after injury. However, sustained presence of the damaging insult prolongs the regeneration response, resulting in liver fibrosis and cirrhosis or acute liver failure. Liver regeneration can be impaired by permanent oxidative stress mediated by the formation of reactive oxygen species (ROS). ROS are produced by metabolic intermediates of xenobiotics, mitochondrial leakage, induction of CYP2 family and activation of inflammatory cells. A crucial player in the defense against oxidative stress is the transcription factor NF-E2-related factor 2 (Nrf2). It has been shown that Nrf2 binds to the cis-acting enhancer element, antioxidant response element (ARE), controlling expression of many antioxidant proteins such as glutathione S-transferase and NADH quinone oxidoreductase 1. Furthermore it was demonstrated that Nrf2 plays a crucial liver regeneration after partial hepatectomy, ethanol or bile acid toxicity.

Additionally, liver regeneration associated protein ALR (Augmenter of Liver Regeneration) was reported to be upregulated in models of liver damage after tissue resection or caused by oxidative stress. Recent micro array analysis demonstrated that Nrf2 regulates several cytoprotective proteins including ALR. Furthermore, promoter analysis of ALR predicts the presence of ARE, the potential binding site for Nrf2. Therefore, we aimed to analyze whether Nrf2 is involved in the regulation of ALR expression mediating beneficial effect of Nrf2/ALR. Treatment of primary human hepatocytes and HepG2 cells with tBHQ, a well known ARE-activator, induced mRNA and protein expression of ALR. Furthermore, promoter activity of ALR was significantly induced after co-transfection of Nrf2 (caNrf2) over control and dominant negative mutant of Nrf2 (dnNrf2). The functional impact of increased promoter activity after Nrf2-transfection was underlined by elevated mRNA and protein levels of ALR. In addition, after partial hepatectomy we found in livers from Nrf2+/+ mice compared to Nrf2-/- KO mice increased anti-oxidant proteins such as GST and NQO1 as well as ALR expression.

Our results demonstrate that transcription factor Nrf2, known to control antioxidant proteins, additionally regulates the expression of ALR. Indeed a hepato-protective role of ALR against radical stress after irradiation, free fatty acid and H2O2 treatment was reported. Recently, we have shown that hepatitis B virus infection induces expression of ARE regulated genes by activation of Nrf2 and furthermore we found in this study enhanced ALR expression after HBV infection. ALR acting as liver regeneration and anti-oxidative protein is regulated by Nrf2 and therefore links oxidative stress to regeneration.