The role of serum and the physiology of delivery in determining the bioactive fraction of estradiol and xenoestrogens

摘要

The bioactive fraction of estradiol in serum from adult men was examined by saturation analysis of estrogen receptors in intact MCF-7 cells. I compared the whole cell up-take of tritiated estradiol from serum-free medium (SFM) with the uptake of estradiol from serum; and the bioactive fraction was 2.39% $pm$ 0.017% (mean $pm$ SEM). This was indistinguishable from the free fraction of 2.46% $pm$ 0.08% measured by ultrafiltration dialysis. Since this method takes into account the interactions between: estradiol and serum proteins, serum proteins and their membrane receptors, and other potential cell uptake or exclusion mechanisms, these results indicate that in MCF-7 cells the bioactive fraction of estradiol in serum is the free fraction. To determine the bioactive fraction of xenoestrogens in adult male serum, I developed a competition assay (the relative binding affinity-serum modified access, RBA-SMA, assay). In this assay the ability of a xenoestrogen to inhibit the binding of tritiated estradiol to estrogen receptors within intact MCF-7 cells was measured in the presence of SFM or 100% serum to determine whether serum reduced or enhanced the xenoestrogen's bioactivity relative to estradiol.;I examined a wide variety of manmade and plant xenoestrogens in this assay, two of which were bisphenol A and octylphenol (used in plastics and other products). In SFM octylphenol (OP) was 10-fold more estrogenic than bisphenol A (BPA). However, in serum, BPA was more estrogenic than OP (the uptake of BPA was enhanced and the uptake of OP was inhibited relative to estradiol). Extrapolation of the relative bioactivities of OP and BPA measured in adult human serum led to the prediction that BPA would show over 500-fold greater estrogenic activity than OP in fetal mouse serum.;Bisphenol A and octylphenol were fed to pregnant mice at 2 and 20 $mu$g $$ kg per day, which were predicted to be within the bioactive range for bisphenol A but not octylphenol. Consistent with my prediction, male mice exposed to these doses of bisphenol A, but not octylphenol, had significantly increased adult prostate weights relative to control males (an effect previously seen after prenatal exposure to a low dose of estradiol).;I also measured the bioactive fraction of estradiol in human fetal cord serum collected at normal parturition to determine if serum protects the human fetus from the high levels of estrogen present during gestation, similar to effects of serum in fetal mice. In contrast to mice, it does not appear that serum binding proteins serve as a mechanism to limit the cell uptake of steroidal estrogens in human fetuses, and putative mechanisms to limit the saturation of estrogen receptors in human fetal tissues remain to be determined.