Biotransformation and Bioavailability of Tea Polyphenols: Implications for Cancer Prevention Research

摘要

Consumption of tea (Camellia sinensis) has been suggested to prevent cancer, heart disease, and other diseases. Animal studies have shown that tea and tea constituents inhibit carcinogenesis at a number of organ sites including the skin, lung, oral cavity, esophagus, stomach, liver, intestine, colon, and prostate. A number of potential cancerprevention mechanisms for the tea polyphenols have been proposed based mainly on studies with human cancer cells. These include protection from oxidative stress, induction of oxidative stress, inhibition of enzymes (MAP kinases, cyclin-dependent kinases, telomerase, etc.), and inhibition of growth factor-related cell signaling (epidermal growth factor and others). Whereas some studies report effects of epigallocatechin-3-gallate (EGCG) at submicromolar levels, most experiments require concentrations of greater than 10 or 20 uM to demonstrate the effect. In humans, mice, and rats, tea polyphenols undergo glucuronidation, sulfation, methylation, and ring fission. Recent reports also suggest that EGCG and other catechins may be substrates for active efflux. The peak plasma concentrations of EGCG, epigallocatechin (EGC), and epicatechin (EC) following oral administration of green tea are 0.04 -1 uM, 0.3-5 uM, and 0.1-2.5 uM, respectively. The plasma levels of theaflavins are much lower (~ 2 nM). The present chapter reviews the literature concerning the biotransformation and bioavailability of tea polyphenols. It is intended to serve as a guide for designing future bioavailabiiity experiments and for interpreting mechanistic data regarding the actions of tea polyphenols in vitro and in vivo.