Characterization of the estrogenic activities of zearalenone and zeranol in vivo and in vitro

In the present study, we compared the estrogenic activity of zearalenone (ZEN) and zeranol (ZOL) by determining their relative receptor binding affinities for human ERα and ERβ and also by determining their uterotropic activity in ovariectomized female mice. ZOL displayed a much higher binding affinity for human ERα and ERβ than ZEN did. The IC50 values of ZEN and ZOL for binding to human ERα were 240.4 and 21.79 nM, respectively, and the IC50 values for binding to ERβ were 165.7 and 42.76 nM, respectively. In ovariectomized female ICR mice, s.c. administration of ZEN at doses ≥2 mg/kg/day for 3 consecutive days significantly increased uterine wet weight compared with the control group, and administration of ZOL increased the uterine wet weight at lower doses (≥0.5 mg/kg/day for 3 days). Based on available X-ray crystal structures of human ERα and ERβ, we have also conducted molecular modeling studies to probe the binding characteristics of ZEN and ZOL for human ERα and ERβ. Our data revealed that ZEN and ZOL were able to occupy the active site of the human ERα and ERβ in a strikingly similar manner as 17β-estradiol, such that the phenolic rings of ZEN and ZOL occupied the same receptor region as occupied by the A-ring of 17β-estradiol. The primary reason that ZOL and ZEN is less potent than 17β-estradiol is likely because 17β-estradiol could bind to the receptor pocket without significantly changing its conformation, while ZOL or ZEN would require considerable conformational alterations upon binding to the estrogen receptors (ERs).