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Abstract: Lignans are plant polyphenols which may possess anticancer, antioxidant, antimicrobial, anti-inflammatory and immunomodulatory activities. In particular, the lignan 7-hydroxymatairesinol (HMR/lignan™, HMR) is a novel precursor of the mammalian lignan enterolactone (EL). In the present study, we investigated the estrogenicity of HMR and of EL in comparison to estradiol (E2), by measuring their effects on growth and apoptotic markers in the human estrogen-sensitive cell line MCF-7. HMR, EL and E2 concentration-dependently increased the percentage of MCF-7 cells in the S phase of the cell cycle, with the following relative potencies: E2≅EL≫HMR, and efficacies: E2>HMR≫EL. Treatment of MCF-7 cells with either HMR, EL or E2 also increased the Bcl-2/Bax mRNA ratio. The effects of HMR and EL were reduced in the presence of the estrogen receptor (ER) antagonist tamoxifene. We conclude that both HMR and its metabolite EL are endowed with estrogenic activity, which is likely to be exerted through the contribution of ER-dependent pathways and to target the same intracellular mechanisms acted upon by E2. The estrogenicity of HMR and EL is however milder than that of E2, as indicated by the lower potencies and efficacies of both lignans. The present results support the notion that dietary supplementation with HMR may result in a mild estrogenic activity, both directly and by providing a suitable source for endogenous EL. [Copyright &y& Elsevier]