α,β-Unsaturated Aldehydes Increase GlutathioneS-Transferase mRNA and Protein: Correlation with Activation of the Antioxidant Response Element

Abstract

A series of α,β-unsaturated aldehydes was evaluated to determine if these compounds could mediate inducible expression of glutathioneS-transferase (GST) through the 5′-flanking antioxidant response element (ARE). The ARE from rGST A1 was subcloned into a luciferase reporter construct and used to transiently transfect rat Clone 9 hepatoma cells. Transfected cells were treated with 4-hydroxy-trans-2-nonenal (4-HNE),trans-2-hexenal (t-2-HE), 2-propenal (acrolein, 2-PE), and ethacrynic acid (EA), a control compound also containing an α,β-unsaturated carbonyl moiety. Each compound was evaluated for cytotoxicity to construct dosing regimens in transfection studies. IC50values for growth inhibition were measured using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide. IC50values in Clone 9 cells were: 4-HNE, 6.3 ± 0.7 μM;t-2-HE, 16.0 ± 0.7 μM; 2-PE, 2.2 ± 0.4 μM; and EA, 38.0 ± 1.6 μM. A dose-dependent increase in luciferase activity was observed in transfected cells with all four compounds tested, indicating that α,β-unsaturated aldehydes function as direct activators of the ARE. To determine whether or not the observed promoter activation led to increased transcriptional and translational induction of GST, cells were treated with the various compounds and assayed for increases in GST mRNA, protein, and enzyme activity. Studies in Clone 9 cells revealed increased steady-state message for GST A1 and A4, increased GST A4-4 protein by Western blotting, and increased GST activity toward 1-chloro-2,4-dinitrobenzene in response to treatment with all four compounds evaluated. Collectively, these studies demonstrate that EA and certain α,β-unsaturated aldehydes produced as a result of cellular membrane lipid peroxidation are activators of the ARE and efficient inducers of GST A1-1 and A4-4.