Coordinated induction of the c- jun gene with genes encoding quinone oxidoreductases in response to xenobiotics and antioxidants

Abstract

Xenobiotics and antioxidants induce expression of detoxifying enzymes including NAD(P)H:quinone oxidoreductase (NQO1), NRH:quinone oxidoreductase (NQO2), and glutathione S-transferase Ya (GST Ya), presumably to provide protection to cells against electrophilic and oxidative stress. Antioxidant response elements (AREs) have been found in the promoter regions of the various detoxifying enzyme genes. An ARE is required for basal expression and induction of the various detoxifying enzyme genes in response to xenobiotics and antioxidants. In this study, we demonstrated that exposure of cells to xenobiotics [e.g. β-naphthoflavone (β-NF)] and antioxidants [e.g. tert-butyl hydroquinone ( t-BHQ)] also induced the expression of the proto-oncogene c- jun. The induction of c- jun gene expression followed kinetics similar to the induction of NQO1 and NQO2 genes with respect to the level and time of exposure. Sequence analysis of the c- jun gene promoter revealed the presence of an ARE between nucleotides -538 and -514. The c- jun ARE was highly homologous to the AREs from genes encoding NQO1, NQO2, and GST Ya. Constructs containing the c- jun ARE and 1.7 and 4.5 kb of the c- jun promoter ligated to the chloramphenicol acetyltransferase (CAT) gene, upon transfection in human hepatoblastoma (Hep-G2) cells, expressed the CAT gene, which was inducible with β-NF and t-BHQ. Band shift assays indicated binding of two specific nuclear protein complexes with the c- jun gene ARE. The faster running c- jun gene ARE–nuclear protein complex was specifically competed out by unlabeled NQO1 and GST Ya gene AREs. These results suggest that c- jun gene expression is coordinately induced and regulated with detoxifying enzyme genes in response to xenobiotics and antioxidants. The results also suggest involvement of an ARE-mediated mechanism of induction of c- jun gene expression. However, a comparison of fold induction of endogenous c- jun gene and transfected c- jun promoter/ARE-CAT constructs indicated involvement of another ARE upstream of the 4.5-kb promoter and/or additional mechanisms such as stabilization of c-Jun RNA in response to exposure to xenobiotics and antioxidants.