Nrf2 and c-Jun regulation of antioxidant response element (ARE)-mediated expression and induction of γ-glutamylcysteine synthetase heavy subunit gene

Abstract

γ-Glutamylcysteine synthetase (γ-GCS) is a rate-limiting enzyme in the de novo synthesis of glutathione, a known scavenger of electrophiles and reactive oxygen species (ROS). The γ-GCS gene is expressed ubiquitously and induced coordinately with NAD(P)H:quinone oxidoreductase 1 ( NQO1) and glutathione S-transferase Ya ( GST Ya) in response to xenobiotics and antioxidants. The antioxidant response element (ARE) is required for expression and induction of these genes. In the current report, we demonstrated that ARE-mediated γ-GCS gene expression and induction is regulated by similar Nrf and Jun factors as reported earlier for the NQO1 and GST Ya genes. The γ-GCS gene ARE competed with the binding of nuclear proteins (Nrf + Jun) to the NQO1 gene ARE (hARE). In addition, the overexpression of Nrf2 and Nrf1 with c-Jun significantly up-regulated γ-GCS ARE-mediated basal expression and β-naphthoflavone induction of the chloramphenicol acetyltransferase gene in transfected HepG2 cells. Interestingly, Nrf2 + c-Jun was more effective than Nrf1 + c-Jun in the regulation of ARE-mediated γ-GCS gene expression. Further experiments demonstrated that the c-Jun level within the cells is an important determinant of the level of ARE-mediated γ-GCS gene expression. Therefore, at higher concentrations of c-Jun, γ-GCS gene expression is repressed, presumably due to generation of a sufficient amount of c-Jun + c-Fos complex that interferes with the binding of Nrf2 + c-Jun complex to the ARE.