| ABSTRACT: |
|
Early studies on the mechanism of the cancer blocking effects of BHA
(tert-butyl-hydroxyanisole) in rodents showed that this and related
antioxidant food additives produced large elevations (by enhanced
transcription) of glutathione transferases (GSTs) and other Phase 2
enzymes (eg, UDP-glucuronosyltransferases, epoxide hydrolase, and
NAD(P)H:quinone reductase [QR]), as well as increases in the levels of
glutathione, in the livers and peripheral tissues of rodents. These
findings led to the suggestion that other 'monofunctional' inducers of
Phase 2 enzymes would block chemical carcinogenesis. The prediction of
anticarcinogenic activity of some chemical compounds, and the isolation of
several anticarcinogens from plant sources, based solely on their effects
as enzyme inducers, strongly support this conclusion. For instance: (a)
Two anticarcinogenic terpenoids were isolated from green coffee beans
based on their ability to raise GSTs in mice; (b) the anticarcinogenic
effects of oltipraz (now in clinical protection trials) and other
1,2-dithiole-3-thiones were predicted solely on this basis; (c) dimethyl
fumarate, a classical Michael reaction acceptor, was found to be a Phase 2
enzyme inducer and shown to inhibit the development of spontaneous hepatic
tumors in obese yellow (Avy/A) mice (Zhang and Talalay, unpublished
observations); (d) isolation of the isothiocyanate sulforaphane
(1-isothiocyanato-4-(methylsulfinyl)butane) as the principal and very
potent Phase 2 enzyme inducer from broccoli, and demonstration of its
ability to block DMBA-induced mammary tumors in rats; (e) anticarcinogenic
structural analogs of sulforaphane were designed and synthesized based on
monitoring enzyme inducer potency. We have developed a simple system for
detecting and quantitating the potency of Phase 2 enzyme inducers based on
measuring the elevation of the specific activities of quinone reductase (a
typical Phase 2 enzyme that is easy to measure) in murine hepatoma cells
grown in microtiter plates. High Phase 2 inducer activity is present in
organic solvent extracts of a number of Cruciferae. We have concentrated
on those belonging to the genus Brassica (broccoli, cauliflower, cabbage)
and genus Raphanus (radishes, daikons). In these plants, isothiocyanates
are responsible for nearly all of the inducer activity. Efforts are under
way to maximize such inducer activity and to develop edible plants
suitable for chemoprotection against cancer. Edible plants contain many
non-nutritive, minor constituents that have been shown to block chemical
carcinogenesis. These minor components are secondary plant products such
as: terpenes (eg, limonene), isothiocyanates (and their glucosinolate
precursors), cinnamates, coumarins, flavonoids, 1,2-dithiole-3-thiones,
organic sulfides and disulfides (which are especially rich in plants of
the Allium family), curcumin, ellagic acid, phenols and polyphenols. It is
interesting and probably significant that nearly all of these secondary
plant products can modulate enzymes of xenobiotic metabolism. The
molecular mechanisms of Phase 2 enzyme induction are being examined. A
common 41-bp enhancer element (containing the Antioxidant Response Element
[ARE], or Electrophile Response Element [EpRE]) found in the 5'-upstream
region of the genes of certain GSTs and QRs regulates the synthesis of
these Phase 2 enzymes. This enhancer region was inserted into a plasmid
together with a minimal promoter derived from the mouse GST Ya gene and a
growth hormone reporter gene. When hepatoma cells were transfected with
these constructs, the growth hormone reporter gene responded not only to
all of the chemical types of inducers of Phase 2 enzymes, but the response
was quantitatively closely correlated with the enzyme inducer potencies of
the same compounds when tested for QR expression in the same hepatic
cells. |