Flavonoids found in common vegetables, fruits, and legumes have been shown to possess antioxidant property. This study is the first to demonstrate that one member of the flavonoid family, genistein, can induce the expression of metallothionein (a metal-binding protein with antioxidant property). We found the effect of genistein to be time- and dose-dependent (10-100 microM). The effect can be observed at both protein and mRNA levels and was synergistic to that of 30 microM zinc. Genistein was shown previously to interact with the estrogen receptor and induce gene expression similar to estrogens at a lower affinity. We thus tested the hypothesis that the effect of genistein on metallothionein expression was mediated through the steroid hormone pathway. We found that various glucocorticoids do not affect metallothionein expression in Caco-2 cells. 17Beta-estradiol at 10-100 microM (concentrations much higher than needed to activate the estrogen response element) induced metallothionein expression in Caco-2 cells. However, a synthetic estrogen, diethylstilbestrol, did not increase metallothionein level at 10 microM. 17Beta-estradiol also did not act synergistically with zinc. Thus, genistein may enhance metallothionein expression through an uncharacterized mechanism. Further studies are needed to delineate the molecular mechanism and to determine whether the expression of other genes is also affected by genistein.