RESUMO
Glutathione (GSH) is an abundant cellular non-protein sulfhydryl that functions as an important protectant against reactive oxygen species and electrophiles, is involved in the detoxification of xenobiotics, and contributes to the maintenance of cellular redox balance. The rate-limiting enzyme in the de novo synthesis of glutathione is gamma-glutamylcysteine synthetase (GCS), a heterodimer consisting of heavy and light subunits expressing catalytic and regulatory functions, respectively. Exposure of HepG2 cells to beta-naphthoflavone (beta-NF) resulted in a time- and dose-dependent increase in the steady-state mRNA levels for both subunits. In order to identify sequences mediating the constitutive and induced expression of the heavy subunit gene, a series of deletion mutants created from the 5'-flanking region (-3802 to +465) were cloned into a luciferase reporter vector (pGL3-Basic) and transfected into HepG2 cells. Constitutive expression was maximally directed by sequences between -202 and +22 as well as by elements between -3802 to -2752. The former sequence contains a consensus TATA box. Increased luciferase expression following exposure to 10 microM beta-NF was only detected in cells transfected with a reporter vector containing the full-length -3802:+465 fragment. Hence, elements directing constitutive and induced expression of the GCS heavy subunit are present in the distal portion of the 5'-flanking region, between positions -3802 and -2752. Sequence analysis revealed the presence of several putative consensus response elements in this region, including two potential antioxidant response elements (ARE3 and ARE4), separated by 34 base pairs. When cloned into the thymidine kinase-luciferase vector, pT81-luciferase, and transfected into HepG2 cells, both ARE3 and ARE4 increased basal luciferase expression approximately 20-fold. When cloned in tandem in their native arrangement the increase in luciferase activity was in excess of 100-fold, suggesting a strong interaction between the two sequences. Luciferase expression was elevated in beta-NF-treated cells transfected with the ARE4-tk-luciferase vector and all DNA fragments containing ARE4. In contrast, ARE3 did not direct increased luciferase expression in response to beta-NF nor did it significantly modify the magnitude of induction directed by ARE4. The influence of the ARE4 oligonucleotide on constitutive and induced expression was eliminated by introduction of a single base mutation, converting the core ARE sequence in ARE4 from 5'-GTGACTCAGCG-3' to 5'-GGGACTCAGCG-3'. When introduced into the full-length -3802:+465 segment, the same single base mutation also eliminated both functions. Collectively the data indicate that the constitutive and beta-NF-induced expression of the human GCS heavy subunit gene is mediated by a distal ARE sequence containing an embedded tetradecanoylphorbol-13-acetate-responsive element.
