The Phytoestrogen Daidzein Affects the Antioxidant Enzyme System of Rat Hepatoma H4IIE Cells

Phytoestrogens such as the soy isoflavonoid daidzein have potential health benefits. The antioxidant properties of phytoestrogens are considered to be responsible in part for their protective effects. The antioxidant enzyme (AOE) system plays an important role in the defense of cells against oxidative insults. To determine whether flavonoids can exert antioxidative effects not only directly but also indirectly by modulating the AOE system, we investigated the influence of the flavonoid daidzein on the expression of different AOE. Daidzein treatment of hepatoma H4IIE cells increased catalase mRNA expression two- to threefold. Expression levels of copper zinc superoxide dismutase (CuZnSOD) were not affected by exposure to daidzein. Manganese superoxide dismutase (MnSOD) mRNA expression levels decreased slightly and glutathione peroxidase (GPx) levels increased slightly after daidzein exposure. Changes in AOE mRNA expression levels were significant at 300 μmol/L daidzein. To elucidate the mechanisms underlying the strong increase in catalase mRNA, transfection experiments were performed. Transient transfection of hepatoma cells with reporter plasmids containing different parts of the upstream region of the catalase gene showed a significant one- to threefold increase in reporter gene activity after daidzein exposure. This indicates that daidzein can directly activate the rat catalase promoter region. Despite the increase in catalase mRNA, daidzein pretreatment of cells did not protect against oxidative stress resulting from H2O2 exposure. On the contrary, daidzein itself exerted a mild oxidative stress. In conclusion, the changes in the AOE system provoked by daidzein affected the oxidant rather than the antioxidant properties of daidzein.