Differential enhancement of gamma-glutamyl transpeptidase and gamma-glutamylcysteine synthetase by tert-butylhydroquinone in rat lung epithelial L2 cells.
Sublethal quinone-mediated oxidative stress stimulates increases in the activities and mRNA levels of gamma-glutamyl transpeptidase (GGT) and gamma-glutamylcysteine synthetase (GCS) in rat lung epithelial L2 cells [Kugelman, A. et al. 1994. Am. J. Respir. Cell Mol. Biol. 11:586-592; Shi, M. M. et al. 1994. J. Biol. Chem. 269:26512-26517]. The present study demonstrated that the quinone-induced increases in these two enzymes were differentially regulated. L2 cells were exposed to various concentrations of tertiary-butylhydroquinone (TBHQ) for different periods of times. TBHQ increased the activities and the mRNAs for GGT and the catalytic subunit of GCS; however, the time- and concentration-dependencies differed. With 50 microM TBHQ, GCS activity increased significantly by 6 h whereas the activity of GGT was not increased until later. Under the same conditions, the highest GCS-mRNA level observed was at 6 h whereas the mRNA level of GGT increased after 6 h, reached a higher level at 12 h, and then returned to the control level by 24 h. Differences were also observed in the concentration-dependence of mRNA increases between the GGT and GCS. Actinomycin D (an inhibitor of RNA synthesis) abolished the increase of GCS-mRNA but not the increase in GGT-mRNA, suggesting a difference in regulation by TBHQ between these two genes. Nuclear run-on experiments confirmed that the increase of GCS-mRNA, but not GGT-mRNA was due to increased transcription. The increase in GGT-mRNA probably results from a decreased degradation rate. The differences between these two enzymes demonstrate how cells can use multiple mechanisms for regulating gene expression in response to oxidative stress.
Liu
R M
RM
Department of Molecular Pharmacology and Toxicology, University of Southern California, Los Angeles 90033, USA.
Hu
H
H
Robison
T W
TW
Forman
H J
HJ
D000818:Animals; D000975:Antioxidants / Q000494:pharmacology*; D002478:Cells, Cultured / Q000201:enzymology; D004848:Epithelium / Q000187:drug effects / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D005721:Glutamate-Cysteine Ligase / Q000235:genetics* / Q000378:metabolism; D006873:Hydroquinones / Q000494:pharmacology*; D008168:Lung / Q000166:cytology; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D014158:Transcription, Genetic / Q000187:drug effects; D005723:gamma-Glutamyltransferase / Q000235:genetics* / Q000378:metabolism
D000975:Antioxidants; D006873:Hydroquinones; D012333:RNA, Messenger; C018855:2-tert-butylhydroquinone; D005723:gamma-Glutamyltransferase; D005721:Glutamate-Cysteine Ligase