Activation of CCAAT/enhancer-binding protein β by 2′-amino-3′-methoxyflavone (PD98059) leads to the induction of glutathione S-transferase A2

Abstract

The induction of glutathione S-transferases by flavonoids is associated with cancer chemopreventive effects. We reported that 2′-amino-3′-methoxyflavone (PD98059), an MKK1 inhibitor, induces glutathione S-transferase A2 (rGSTA2). This report comparatively examines the role of CCAAT/enhancer-binding protein (C/EBP) and Nrf2 in the induction of rGSTA2 by PD98059. We first assessed whether the MKK1/ERK1/2 pathway regulated rGSTA2 induction. Northern and western blot analyses showed that PD98059 at the concentrations effective for the inhibition of MKK1 increased the rGSTA2 protein and mRNA levels in H4IIE cells. PD98059 also induced rGSTA2 in cells stably transfected with dominant-negative mutant of MKK1(-), which provided evidence that the inhibition of MKK1/ERK1/2 by PD98059 was not responsible for rGSTA2 induction. Gel shift assay and immunoblot analysis of subcellular fractions revealed that PD98059 caused nuclear translocation of C/ EBPβ and increased C/EBP DNA binding, which was super-shifted with anti-C/EBPβ antibody. Nrf2 was not activated by PD98059. PD98059 increased the luciferase reporter gene activity in cells transfected with the C/EBP-containing -1.65 kb flanking region of the rGSTA2 gene. Deletion of the C/EBP-binding site or over-expression of dominant-negative mutant of C/EBP abolished the reporter gene activity. Flavone, a backbone structure of PD98059, also induced nuclear translocation of C/EBPβ and C/EBP-mediated rGSTA2 gene induction. Inhibition of phosphatidylinositol 3-kinase abolished C/EBPβ-mediated rGSTA2 induction by PD98059. These results provide evidence that PD98059 and flavone induce nuclear translocation of C/EBPβ and activate the C/EBP-binding site in the rGSTA2 gene, which constitutes the distinct pathway for the enzyme induction irrespective of the inhibition of MKK1/ERK activity.