Sensitivity to carcinogenesis is increased and chemoprotective efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice
- Minerva Ramos-Gomez*,
- Mi-Kyoung Kwak*,
- Patrick M. Dolan*,
- Ken Itoh†,
- Masayuki Yamamoto†,
- Paul Talalay‡, and
- Thomas W. Kensler*,‡,§
- Departments of *Environmental Health Sciences and ‡Pharmacology and Molecular Sciences, The Johns Hopkins University, Baltimore, MD 21205; and †Center for TARA, Tsukuba University, Tsukuba 305-8577, Japan
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Contributed by Paul Talalay
Abstract
Induction of phase 2 enzymes, which neutralize reactive electrophiles and act as indirect antioxidants, appears to be an effective means for achieving protection against a variety of carcinogens in animals and humans. Transcriptional control of the expression of these enzymes is mediated, at least in part, through the antioxidant response element (ARE) found in the regulatory regions of their genes. The transcription factor Nrf2, which binds to the ARE, appears to be essential for the induction of prototypical phase 2 enzymes such as glutathione S-transferases (GSTs) and NAD(P)H:quinone oxidoreductase (NQO1). Constitutive hepatic and gastric activities of GST and NQO1 were reduced by 50–80% in nrf2-deficient mice compared with wild-type mice. Moreover, the 2- to 5-fold induction of these enzymes in wild-type mice by the chemoprotective agent oltipraz, which is currently in clinical trials, was almost completely abrogated in the nrf2-deficient mice. In parallel with the enzymatic changes, nrf2-deficient mice had a significantly higher burden of gastric neoplasia after treatment with benzo[a]pyrene than did wild-type mice. Oltipraz significantly reduced multiplicity of gastric neoplasia in wild-type mice by 55%, but had no effect on tumor burden in nrf2-deficient mice. Thus, Nrf2 plays a central role in the regulation of constitutive and inducible expression of phase 2 enzymes in vivo and dramatically influences susceptibility to carcinogenesis. Moreover, the total loss of anticarcinogenic efficacy of oltipraz in the nrf2-disrupted mice highlights the prime importance of elevated phase 2 gene expression in chemoprotection by this and similar enzyme inducers.
Footnotes
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↵ § To whom reprint requests should be addressed: Department of Environmental Health Sciences, The Johns Hopkins School of Hygiene and Public Health, 615 N. Wolfe Street, Baltimore, MD 21201. E-mail: tkensler{at}jhsph.edu.
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See commentary on page 2941.
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↵ ¶ The metabolism of many carcinogens and other xenobiotics may be considered as resulting from the activities of two families of ubiquitous, versatile, and inducible mammalian enzymes. Phase 1 enzymes (principally cytochromes P450) introduce functional groups by oxidations and reductions and generally lead to nonelectrophilic metabolites, but they may also produce highly reactive electrophiles that avidly bind to nucleophilic centers of DNA and initiate carcinogenesis. The products of phase 1 metabolism are efficiently detoxified by phase 2 enzymes that promote a broad range of reactions, including conjugation with endogenous ligands, e.g., glutathione (glutathione transferases) or glucuronic acid (glucuronosyltransferases) or by disabling reactive functional groups (e.g., reduction of quinones to hydroquinones by NQO1 or hydrolysis of epoxides to diols by epoxide hydrolase). Thus, many carcinogens are innocuous until activated by phase 1 enzymes, and the balance of phase 1 to phase 2 enzymes controls the fate and toxicological effects of carcinogens. Although in the past much emphasis has been placed on the importance of phase 2 enzymes for disarming electrophiles, many phase 2 enzymes are also efficient antioxidants. Thus, glutathione transferases reduce a wide variety of products of oxidative stress and reactive oxygen species. Inducers of phase 2 enzymes are appropriately regarded as “indirect” antioxidants (6) and thereby play major protective roles.
- Abbreviations:
- GST,
- glutathione S-transferase;
- NQO1,
- NAD(P)H:quinone oxidoreductase;
- ARE,
- antioxidant response element;
- CYP,
- cytochrome P450
- Copyright © 2001, The National Academy of Sciences
