Estrogenic activity of bovine milk high or low in equol using immature mouse uterotrophic responses and an estrogen receptor transactivation assay

Background: Cowʹs milk contain phytoestrogens especially equol depending on the composition of the feed ration. However, it is unknown whether milk differing in equol exhibits different estrogenicity in model systems and thereby potentially in humans as milk consumers. Methods: The estrogenicity of high and low equol milk (HEM and LEM, respectively) and purified equol was investigated in immature female mice including mRNA expression of six estrogen-sensitive genes in uterine tissue. Extracts of HEM and LEM were also tested for estrogenicity in vitro in an estrogen receptor (ER) reporter gene assay with MVLN cells. Results: The total content of phytoestrogens was approximately 10 times higher in HEM compared with LEM, but levels of endogenous milk estrone and 17β-estradiol were similar in the two milk types (503–566 and 60–64.6 pg/ml, respectively). There was no difference in uterine weight between mice receiving LEM and HEM, and no difference from controls. Equol (50 times the concentration in HEM) was not uterotrophic. The ERβ mRNA expression was down-regulated in the uteri of HEM mice compared with LEM and controls, but there was no difference between milk types for any of the other genes. Extracts of HEM showed a higher estrogenicity than extracts of LEM in MVLN cells, and there was a dose-dependent increase in estrogenicity by equol. Conclusion: The higher in vitro estrogenicity of HEM was not reflected as a higher uterine weight in vivo although the down-regulation of ERβ in uterine tissue of HEM mice could suggest some estrogenic activity of HEM at the gene expression level.