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This article summarizes available data on the chemopreventive
efficacies of tea polyphenols, curcumin and ellagic acid in various model
systems. Emphasis is placed upon the anticarcinogenic activity of these
polyphenols and their proposed mechanism(s) of action. Tea is grown in
about 30 countries and, next to water, is the most widely consumed
beverage in the world. Tea is manufactured as either green, black, or
oolong; black tea represents approximately 80% of tea products.
Epidemiological studies, though inconclusive, suggest a protective effect
of tea consumption on human cancer. Experimental studies of the
antimutagenic and anticarcinogenic effects of tea have been conducted
principally with green tea polyphenols (GTPs). GTPs exhibit antimutagenic
activity in vitro, and they inhibit carcinogen-induced skin, lung,
forestomach, esophagus, duodenum and colon tumors in rodents. In addition,
GTPs inhibit TPA-induced skin tumor promotion in mice. Although several
GTPs possess anticarcinogenic activity, the most active is
(-)-epigallocatechin-3-gallate (EGCG), the major constituent in the GTP
fraction. Several mechanisms appear to be responsible for the
tumor-inhibitory properties of GTPs, including enhancement of antioxidant
(glutathione peroxidase, catalase and quinone reductase) and phase II
(glutathione-S-transferase) enzyme activities; inhibition of chemically
induced lipid peroxidation; inhibition of irradiation- and TPA-induced
epidermal ornithine decarboxylase (ODC) and cyclooxygenase activities;
inhibition of protein kinase C and cellular proliferation;
antiinflammatory activity; and enhancement of gap junction intercellular
communication. Curcumin is the yellow coloring agent in the spice tumeric.
It exhibits antimutagenic activity in the Ames Salmonella test and has
anticarcinogenic activity, inhibiting chemically induced preneoplastic
lesions in the breast and colon and neoplastic lesions in the skin,
forestomach, duodenum and colon of rodents. In addition, curcumin inhibits
TPA-induced skin tumor promotion in mice. The mechanisms for the
anticarcinogenic effects of curcumin are similar to those of the GTPs.
Curcumin enhances glutathione content and glutathione-S-transferase
activity in liver; and it inhibits lipid peroxidation and arachidonic acid
metabolism in mouse skin, protein kinase C activity in TPA-treated NIH 3T3
cells, chemically induced ODC and tyrosine protein kinase activities in
rat colon, and 8-hydroxyguanosine formation in mouse fibroblasts. Ellagic
acid is a polyphenol found abundantly in various fruits, nuts and
vegetables. Ellagic acid is active in antimutagenesis assays, and has been
shown to inhibit chemically induced cancer in the lung, liver, skin and
esophagus of rodents, and TPA-induced tumor promotion in mouse
skin.(ABSTRACT TRUNCATED AT 400 WORDS) |