Rapid detection of inducers of enzymes that protect against carcinogens.

Abstract

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 wellestablished 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 H 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.

Coordinate regulation of enzyme markers for inflammation and for protection against oxidants and electrophiles

“…After the Griess reaction assays for nitrite, the culture media were discarded, and cells were washed three times with DPBS. Cell lysates were prepared, and the specific activities of NQO1 were determined (11,54). Cells treated with LPS or IFN-␥ and TNF-␣ but without test compounds were used as controls.…”

“…When inducers modify specific and highly reactive cysteine residues of Keap1, it loses the ability to repress Nrf2, which translocates to the nucleus, binds to the antioxidant response elements (ARE) of phase 2 genes, and activates their transcription (5)(6)(7)9). Much evidence points to the powerful protective effects of phase 2 genes: (i) their up-regulation protects cells, animals, and humans against a wide variety of damaging agents including oxygen-and nitrogen-based oxidants, toxicities of carcinogens and other electrophiles, and radiation (5-7, 10); (ii) when the phase 2 induction mechanism is disrupted, cells are much more susceptible to the damaging toxicities; and (iii) numerous anticarcinogens have been identified and isolated from natural sources by bioassays that monitor induction of Nrf2-dependent enzymes such as NAD(P)H:quinone oxidoreductase (NQO1) (11)(12)(13). The successful strategy is exemplified by the phase 2 bioassay-guided isolation of the isothiocyanate sulforaphane from broccoli (12).…”

Modulation of Glutathione S-Transferase Subunits A2, M1, and P1 Expression by Interleukin-1β in Rat Hepatocytes in Primary Culture

“…Furthermore, the modulation of carcinogen-inactivating GST expression by cytokines may bear relevance to the process of chemical carcinogenesis and may contribute to interindividual variations in cancer susceptibility. In recent years, considerable attention has been paid to the development of chemointervention strategies involving induction of GSTs (45). For example, OPZ has been proposed as a good candidate for chemoprotection because of its ability to prevent against aflatoxin B 1 -induced hepatocarcinogenesis in the rat (9).…”

NAD(P)H:quinone oxidoreductase 1 reduces the mutagenicity of DNA caused by NADPH:P450 reductase-activated metabolites of benzo(a)pyrene quinones

“…The presence of the unrepaired DNA damage during DNA replication process results in cellular genetic changes influencing the expression of growth-regulatory genes, culminating in the genesis of cancer (Harris, 1991). As metabolism of the procarcinogens is the pivotal step determining the ultimate genotoxic and therefore the carcinogenic potential of the chemicals, selective detoxification of the procarcinogenic chemicals by modulating the activities of the detoxification enzymes has attracted much attention as a feasible chemopreventive mechanism (Talalay et al, 1988;Prochaska et al, 1992;Morse et al, 1993).…”