The biological effects of (-)-epigallocatechin-3-gallate (EGCG) have been extensively investigated in cell lines, but its stability and interactions with cells under culture conditions are unclear. In the present study, the stability, uptake, biotransformation, and efflux of [(3)H]EGCG in HT-29 human colon adenocarcinoma cells were investigated. EGCG was unstable in McCoy's 5A culture media with a half-life of less than 30 min, and the half-life increased to 130 min in the presence of cells. The major oxidative products were theasinensin (M(r) 914) and another dimer with M(r) 884. Addition of EGCG (50 micro M) to cell culture media caused the production of H(2)O(2) (up to 25 micro M), and the amount was lower and gradually decreased in the presence of cells. The uptake of EGCG was concentration dependent and did not plateau, even at 640 micro M, suggesting a passive diffusion process. Approximately 75% of the [(3)H]EGCG was found in the cytoplasmic fraction when the cells were incubated with 0.5-20 micro M [(3)H]EGCG for 15 min. The membrane-associated radioactivity increased with time, apparently because of the binding of dimers to the membrane. The accumulation of [(3)H]EGCG in the cells was significantly higher at 4 degrees C than at 37 degrees C. Multidrug-resistant protein inhibitors, such as indomethacin and probenecid, effectively increased the accumulation of EGCG 4"-glucuronide and 4"-methyl EGCG in the cell. These results suggest that EGCG is metabolized in the cell and that the metabolites are pumped out by MRPs. The present study provides fundamental information on the stability, uptake, biotransformation, and efflux of EGCG under cell culture conditions and suggests the need for careful interpretation of related results on the biological activities of EGCG.