Estrogenic effects of silymarin in ovariectomized rats

The objective of this study was to test whether silymarin induces changes indicative of estrogenic effects in gonadal organs of ovariectomized (OVX) rats. Silymarin was administered in two experimental groups of OVX rats (n = 7 + 7) for 30 days at the doses of 25 or 50 mg per animal per day. OVX rats (n = 7) receiving 5 µg of 17b-estradiol (E2) for the last three days before killing and untreated OVX rats (n = 7) were used as the positive and the negative controls, respectively. Uterine and blood samples were collected immediately after killing. Compared with the negative controls, total and normalized uterine weights were significantly higher in the two experimental groups (P < 0.01 and P < 0.05, respectively). Uterotrophic effects of silymarin were also evident from increased heights of the luminal epithelium (P < 0.01) and the endometrium (P < 0.05). The response was dose-independent within the tested range. The strongest uterine response was observed in the OVX rats treated with E2. A highly significant decrease in mean density of estrogen receptor (ERa) immunostaining in the luminal and the glandular endometrial epithelia (P < 0.01) and a stronger ERa immunostaining in stromal cells were observed in the two experimental and the positive control groups. The activities of alkaline and acid phosphatases were significantly increased in the luminal (P < 0.05) and the glandular (P < 0.01) epithelia only in the rats treated with E2. Both silymarin and E2 induced an increase in thyroid hormone concentrations in blood serum. The rises of free T3 and T4 were significant (P < 0.05) in the group receiving 50 mg of silymarin per day. Hepatic oxidative metabolism of steroids was assumed to be another target of the action of silymarin. The mitochondrial cytochrome P450-dependent testosterone hydroxylase activity and the cytochrome P4501A-dependent 7-ethoxyresorufin O-deethylase activity were significantly increased (P < 0.05) in the group receiving 50 mg of silymarin per day and in the E2-treated control group, respectively. However, the modulations of the CYP enzymes played only a minor role in the overall estrogenic effect of silymarin. Histological and functional alterations in the OVX rats treated orally with silymarin for 30 days were consistent with those seen in E2-treated rats and were indicative of estrogenic effects of silymarin.