Phytoestrogens as natural prodrugs in cancer prevention: towards a mechanistic model

It has been widely acknowledged that regular consumption of fresh fruits and vegetables is linked with a relatively low incidence of cancers (e.g. breast, cervix, and colon). Notably, dietary polyphenolic compounds that show some structural similarity to human estrogen, e.g. isoflavones, coumestans, lignans, flavones, have been proposed to play a role in cancer prevention. However, at present there is no satisfactory explanation for the cancer preventative properties of this group of compounds. Whereas polyphenolic compounds have been shown to inhibit proliferation of tumour cells in vitro, the results of in vivo tests have mostly been disappointing in this respect. It seems that mammalian phase II detoxification mechanisms make that dietary polyphenols are rapidly and effectively removed from the body, i.e. their concentration in the blood plasma hardly ever reaches levels high enough to have a possible effect on tumour growth. The polymethoxyflavones nobiletin and tangeretin, common constituents of Citrus peel, are better absorbed than polyhydroxy flavonoids, and maintain their biological activity for a longer period of time. The compounds are known to be substrates for the estrogen-converting cytochrome P450 enzymes CYP1A1 and CYP1B1, which are typically over-expressed in a range of tumour tissues. The enzymes catalyse regioselective hydroxylation and dealkylation of the polymethoxyflavones, resulting in reaction products that appear to inhibit cell proliferation via interference with the MAPK/ERK cell signalling pathway.