A major inducer of anticarcinogenic protective enzymes from broccoli: isolation and elucidation of structure

Significance of an unusually low Km for glutathione in glutathione transferases of the alpha, mu and pi classes

1. Interactions of glutathione transferases (GST) of the alpha, mu and pi classes with glutathione (GSH) and glutathione conjugates (GS-X) are in contrast with those of a GST of the theta class (GST5-5). 2. GST 5-5 has a Km for GSH of approx. 5 mM. Thus Km/ambient [GSH] is approx. 1, within the range of Km/ambient [s] of glycolytic enzymes. GSTs of the alpha, mu and pi classes yield much lower values of Km for GSH (approx. 0.1 mM) hence Km/ambient [s] is significantly lower than those of most (non-GST) enzymes (p < 0.025). 3. GSTs of the alpha, mu and pi classes are sensitive to inhibition by GS-X (i.e. product) and GS-X analogues. GST 5-5 is not. 4. Rate enhancements up to 10(10), similar to an average enzyme (10(8)-10(12)), are seen in catalysis by GST 5-5, but not in catalysis by GSTs of alpha, mu and pi classes (> 10(7)). 5. Comparisons of primary structure indicate that theta class GSTs may have a decreased binding of the glu-alpha-amino- and gly-COO(-)-groups of GSH compared with GSTs of the other classes. 6. It is concluded that GSTs of alpha, mu and pi classes have evolved towards increased product binding at the expense of catalytic efficiency. Thus GSH is uniquely utilized both as a nucleophile and a 'tag' which can be used to bind and sequester product particularly during GSH-depletion. This interpretation unifies the catalytic and binding properties of these GSTs and alters their perceived role in detoxication.

Strategies of antioxidant defense

Cellular protection against the deleterious effects of reactive oxidants generated in aerobic metabolism, called oxidative stress, is organized at multiple levels. Defense strategies include three levels of protection; prevention, interception, and repair. Regulation of the antioxidant capacity includes the maintenance of adequate levels of antioxidant and the localization of antioxidant compounds and enzymes. Short-term and long-term adaptation and cell specialisation in these functions are new areas of interest. Control over the activity of prooxidant enzymes, such as NADPH oxidase and NO synthases, is crucial. Synthetic antioxidants mimic biological strategies.

NAD(P)H:quinone oxidoreductase1 (DT-diaphorase) expression in normal and tumor tissues

NAD(P)H:Quinone Oxidoreductase1 (NQO1) also known as DT-diaphorase is a flavoprotein that catalyzes the two-electron reduction of quinones, quinone imines and azo-dyes and thereby protects cells against mutagenicity and carcinogenicity resulting from free radicals and toxic oxygen metabolites generated by the one-electron reductions catalyzed by cytochromes P450 and other enzymes. High levels of NQO1 gene expression have been observed in liver, lung, colon and breast tumors as compared to normal tissues of the same origin. The transcription of the NQO1 gene is activated in response to exposure to bifunctional (e.g. beta-naphthoflavone (beta-NF), 2, 3, 7, 8 tetrachorodibenzo-p-dioxin (TCDD)) and monofunctional (phenolic antioxidants/chemoprotectors e.g. 2(3)-tert-butyl-4-hydroxy-anisole (BHA)) inducers. The high level of expression of the NQO1 gene and its induction by beta-NF and BHA require the presence of an AP1 binding site contained within the human Antioxidant Response Element (hARE) and are mediated by products of proto-oncogenes, Jun and Fos. Induction of NQO1 gene expression involves transfer of a redox signal from xenobiotics to unknown 'redox protein(s)' which in turn, modify the Jun and Fos proteins for greater affinity towards the AP1 site of the NQO1 gene and activates transcription. The expression and regulation of the NQO1 gene is complex as many additional cis-elements have been identified in the promoter region and is a subject of great future interest. In addition to established tumors, NQO1 gene expression is also increased in developing tumors, indicating a role in cellular defense during tumorigenesis. It has been proposed that low molecular weight substance(s) can diffuse from tumor cells into surrounding normal cells and activate the expression of the NQO1 gene. Purification and characterization of such substance(s) may provide important information in regard to the mechanism of activation of NQO1 gene expression and the role of increased NQO1 expression in tumor development. In view of the general consensus that NQO1 is over-expressed in tumor cells and the realization that NQO1 may either activate or detoxify xenobiotics, it is important to establish the role of NQO1 in the activation, and the detoxification of xenobiotics and drugs and in the intrinsic sensitivity of tumors to bioreductive alkylating aziridinyl benzoquinones such as diaziquone (AZQ), mitomycin C (MMC), and indoloquinone EO9, as well as to the dinitrophenyl aziridine, CB1954, and the benzotriazine-di-N-oxide, SR 4233.

Oltipraz, an inhibitor of human immunodeficiency virus type 1 replication

Glutathione depletion may play a pivotal role in the pathogenesis of human immunodeficiency virus type-1 (HIV-1) infection. Since certain compounds prevent experimental carcinogenesis by elevating the levels of glutathione and phase II detoxication enzymes, we compared the potencies of several inducers with their ability to inhibit basal levels of HIV-1 replication in H9 cutaneous T-cell lymphoma cells. All monofunctional inducers tested elevated the levels of glutathione and quinone reductase, a marker for phase II enzyme induction. However, only oltipraz [4-methyl-5-(2-pyrazinyl)-1,2-dithiole-3-thione] was effective at inhibiting HIV-1 replication (IC50 = 14.8 +/- 3.1 microM). The antiviral effect of oltipraz was potentiated by 3'-azido-3'-deoxythymidine. Thus, 1,2-dithiole-3-thiones represent a hitherto unrecognized class of anti-HIV-1 agents. Oltipraz behaves kinetically as an irreversible inhibitor of HIV-1 reverse transcriptase in the template-primer binding domain. Oltipraz has been used to treat schistosomiasis in humans and is undergoing clinical evaluation as an anticarcinogen. Thus, oltipraz (and other 1,2-dithiole-3-thiones) may have therapeutic utility in HIV-1-infected individuals, not only because of their antiretroviral activity, but also by preventing the development of HIV-1-associated neoplasms.

Chemical and molecular regulation of enzymes that detoxify carcinogens

Inductions of detoxication (phase 2) enzymes, such as glutathione transferases and NAD(P)H:(quinone-acceptor) oxidoreductase, are a major mechanism for protecting animals and their cells against the toxic and neoplastic effects of carcinogens. These inductions result from enhanced transcription, and they are evoked by diverse chemical agents: oxidizable diphenols and phenylenediamines; Michael reaction acceptors; organic isothiocyanates; other electrophiles--e.g., alkyl and aryl halides; metal ions--e.g., HgCl2 and CdCl2; trivalent arsenic derivatives; vicinal dimercaptans; organic hydroperoxides and hydrogen peroxide; and 1,2-dithiole-3-thiones. The molecular mechanisms of these inductions were analyzed with the help of a construct containing a 41-bp enhancer element derived from the 5' upstream region of the mouse liver glutathione 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 Hep G2 human hepatoma cells, the concentrations of 28 compounds (from the above classes) required to double growth hormone production, and the concentrations required to double quinone reductase specific activities in Hepa 1c1c7 cells, spanned a range of four orders of magnitude but were closely linearly correlated. Six compounds tested were inactive in both systems. A 26-bp subregion of the above enhancer oligonucleotide (containing the two tandem "AP-1-like" sites but lacking the preceding ETS protein binding sequence) was considerably less responsive to the same inducers. We conclude that the 41-bp enhancer element mediates most, if not all, of the phase 2 enzyme inducer activity of all of these widely different classes of compounds.

Glutathione S-transferases: gene structure and regulation of expression

The current knowledge about the structure of GST genes and the molecular mechanisms involved in regulation of their expression are reviewed. Information derived from the study of rat and mouse GST Alpha-class, Ya genes, and a rat GST Pi-class gene seems to indicate that a single cis-regulatory element, composed of two adjacent AP-1-like binding sites in the 5'-flanking region of these GST genes, is responsible for their basal and xenobiotic-inducible activity. The identification of Fos/Jun (AP-1) complex as the trans-acting factor that binds to this element and mediates the basal and inducible expression of GST genes offers a basis for an understanding of the molecular processes involved in GST regulation. The induction of expression of Fos and Jun transcriptional regulatory proteins by a variety of extracellular stimuli is known to mediate the activation of target genes via the AP-1 binding sites. The modulation of the AP-1 activity may account for the changes induced by growth factors, hormones, chemical carcinogens, transforming oncogenes, and cellular stress-inducing agents in the pattern of GST expression. Recent observations implying reactive oxygen as the transduction signal that mediates activation of c-fos and c-jun genes are presently considered to provide an explanation for the induction of GST gene expression by chemical agents of diverse structure. The possibility that these agents may all induce conditions of oxidative stress by various pathways to activate expression of GST genes that are regulated by the AP-1 complex is discussed.

Rapid detection of inducers of enzymes that protect against carcinogens

Dietary composition is a major determinant of cancer risk in humans and experimental animals. Major and minor components of the diet may enhance or suppress the development of malignancy. Many dietary constituents also modify the metabolism of carcinogens by induction of enzymes involved in xenobiotic metabolism, and this is one well-established mechanism for modulating the risk of cancer. We have developed a simple system for rapid detection and measurement of the induction of enzymes that detoxify carcinogens (phase II enzymes), based on the direct assay of the activity of quinone reductase [NAD(P)H:(quinone-acceptor) oxidoreductase, EC 1.6.99.2] in murine hepatoma cells grown in microtiter plate wells. Survey of extracts of a variety of commonly consumed, organically grown vegetables for quinone reductase inducer activity identified crucifers (and particularly those of the genus Brassica) as singularly rich sources. It is therefore of interest that high consumption of these types of vegetables has been correlated with decreased cancer risk in humans. The assay system also measures toxicity, which was unrelated to inducer potency among the vegetable extracts examined. By use of mutant hepatoma cells (defective in regulation of certain cytochrome P-450 enzymes) selective (monofunctional) inducers of protective phase II enzymes can be distinguished from (bifunctional) inducers that also elevate cytochromes P-450 (phase I enzymes) and thereby pose the risk of carcinogen activation. The assay system therefore permits not only rapid detection of inducers of anticarcinogenic enzymes in the human diet but also elucidation of effects of storage and processing on inducer activities.