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Regulation of phase 2 enzyme induction by oltipraz and other dithiolethiones
P A Egner1, T W Kensler, T Prestera
1Department of Environmental Health Sciences, Johns Hopkins School of Hygiene and Public Health, Baltimore, MD 21205.
Abstract:
4-Methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz) and several other dithiolethiones protect against the acute toxicities of many xenobiotics and are effective inhibitors of experimental carcinogenesis. These protective effects are mediated, in part, through elevation of glutathione S-transferase, NAD(P)H: quinone reductase and UDP-glucuronosyltransferase activities in the liver and other target tissues. The induction of these phase 2 enzymes by oltiprax results from enhanced transcription. In the present study, the molecular mechanisms of these inductions were analyzed utilizing a construct containing a 41 bp enhancer element derived from the 5'-upstream region of the mouse liver glutathione S-transferase Ya subunit gene ligated to the 5' end of the isolated promoter region of this gene, and inserted into a plasmid containing a human growth hormone reporter gene. When this construct was transfected into murine Hepa 1c1c7 hepatoma cells, the concentrations of 25 dithiolethiones and related analogs required to double growth hormone production were determined and spanned a range nearly three orders of magnitude. Concentrations of dithiolethiones required to double the specific activity of NAD(P)H: quinone reductase were also determined in Hepa 1c1c7 cells. There was a positive correlation (r = 0.78) between the potencies of the 21 active compounds as inducers of both NAD(P)H: quinone reductase activity and growth hormone production. Moreover, no dithiolethiones were inactive in only one system. It is probable, therefore, that the induction of NAD(P)H: quinone reductase and other phase 2 enzymes by oltipraz and other dithiolethiones is mediated entirely through the 41 bp enhancer element.
Insights
Oltipraz and other dithiolethiones protect against xenobiotics by increasing phase 2 enzymes. This study shows their induction is likely mediated by a specific 41 bp enhancer element, crucial for detoxification. Keywords: oltipraz, dithiolethiones, phase 2 enzymes, xenobiotics, detoxification.
Area of Science:
- Biochemistry
- Toxicology
- Molecular Biology
Background:
- Dithiolethiones, including oltipraz, offer protection against xenobiotic toxicities and experimental carcinogenesis.
- These protective effects are partly due to increased activity of phase 2 detoxification enzymes like glutathione S-transferase and NAD(P)H: quinone reductase.
- Oltipraz induces these phase 2 enzymes via enhanced gene transcription.
Purpose of the Study:
- To investigate the molecular mechanisms behind the induction of phase 2 enzymes by dithiolethiones.
- To identify the specific DNA elements responsible for mediating these transcriptional changes.
Main Methods:
- A reporter construct was created using a 41 bp enhancer element from the mouse glutathione S-transferase Ya subunit gene promoter linked to a human growth hormone reporter gene.
- This construct was transfected into murine Hepa 1c1c7 hepatoma cells.
- The potency of 25 dithiolethiones and analogs in inducing reporter gene (human growth hormone) production and NAD(P)H: quinone reductase activity was measured.
Main Results:
- A significant positive correlation (r = 0.78) was observed between the potency of compounds in inducing NAD(P)H: quinone reductase activity and reporter gene production.
- All tested dithiolethiones that were active in one system were also active in the other, with no compounds showing selective activity.
- The study determined the concentrations of various dithiolethiones required to double reporter gene expression and enzyme activity.
Conclusions:
- The 41 bp enhancer element is likely the primary mediator for the induction of NAD(P)H: quinone reductase and other phase 2 enzymes by oltipraz and related dithiolethiones.
- This enhancer element plays a critical role in the cellular response to dithiolethiones, contributing to their chemoprotective effects.