Respective contribution exerted by AF‐1 and AF‐2 transactivation functions in estrogen receptor α induced transcriptional activity by isoflavones and equol: Consequence on breast cancer cell proliferation

Abstract

Estrogens used in hormone replacement therapy regimens may increase the risk of developing breast cancer. Paradoxically, high consumption of plant-derived phytoestrogens, particularly soybean isoflavones, is associated with a low incidence of breast cancer. To explore the molecular basis for these potentially different experimental/clinical outcomes, we investigated whether soybean isoflavones elicit distinct transcriptional actions from estrogens by performing transient transfections in different cell lines. Our results demonstrate that 17beta estradiol (E2), isoflavones, and equol (EQ) effectively trigger the transcriptional activation with both estrogen receptors (ER), ER alpha and ER beta. ER alpha transcriptional activity is mediated through two transactivation domains AF-1 and AF-2, whose activity is tightly regulated in a cell-type and promoter-specific manner. Isoflavones, genistein, and daidzein (DAI), and EQ, the main estrogenic metabolite of DAI, are ER alpha agonists for transcriptional activation. The molecular mechanisms for ER alpha-induced transcriptional activity by isoflavones and EQ involve their capacity to act mainly through AF-1 regardless of the cell type. Therefore, our data indicate that estrogenic ligands, such as isoflavones and EQ, exert their effects on ER alpha transactivation similarly to the endogenous ligand E2, and suggest that the risk of estrogen-related diseases might not be reduced by soy-rich food and/or isoflavone- or EQ-based supplements.

Comparative Effects of R- and S-equol and Implication of Transactivation Functions (AF-1 and AF-2) in Estrogen Receptor-Induced Transcriptional Activity

“…Racemic equol and R- equol (10 µM) induce a similar transactivation of both ERα and ERαΔAF-1 (Figure 4B), indicating that deletion of AF-1 has no effect on ERα-induced transactivation for both compounds. Similar results are obtained in CHO cells (data not shown) with racemic equol presenting a similar transactivation profile as E2, as previously described [30]. In contrast, S- equol does not induce similar transcriptional activation of ERα and ERαΔAF-1 in HeLa cells.…”

“…We have previously demonstrated that the ability of phytoestrogens, such as genistein, daidzein and racemic equol to act as ER agonists is independent of the cellular context (AF-1 or AF-2 permissive) [30]. Therefore, it was of particular interest to determine the mechanisms of action of enantiomeric forms of equol on ER transcriptional activation in epithelial cell lines, which have different AF permissiveness.…”

“…We conclude that racemic, R- and S- equol exert distinct effects on ER transcriptional activity, which cannot be solely explained by their differential affinities for the ER subtypes [13], but may be due to the differential effects on the transactivation functionalities of the receptor and the cell differentiation stage [30,34,37,40,41,42]. Given the pleiotropic actions of phytoestrogenic compounds, it is possible that they are also affecting other biological processes in addition to these mechanisms, such as cellular metabolism, relative expression of each ER subtype, non-genomic activities of ER or other signal transduction pathways (such as MAPK).…”

Low dietary soy isoflavonoids increase hippocampal spine synapse density in ovariectomized rats

“…While DPN has approximately 70 fold higher affinity for ERα than for ERβ (Meyers et al, 2001), genistein is less selective: the ratio of ERβ to ERα affinity for genistein is approximately 16 (Muthyala et al, 2004). Daidzien is extensively converted by gut bacteria to S-equol (Setchell et al, 2005), a compound with binding affinities for ERα and ERβ comparable to those of genistein (Muthyala et al, 2004), but different biological activity with respect to the activation of cellular estrogen response pathways (Carreau et al, 2009; Liu et al, 2014). Thus, variations in the extent of metabolism of dietary isoflavonoids to equol could contribute significantly to their biological effects.…”

Bioavailability of glycitein relatively to other soy isoflavones in healthy young Caucasian men

“…As such they have been shown to exert preventive effects on bone resorption both in humans (Micke et al, 2006) and in rodent models (Mathey et al, 2004). They are capable of interacting with estradiol receptors (ERs) (Carreau, Flouriot, Bennetau-Pelissero, & Potier, 2009). Glycitein, like other isoflavones, has been shown to exhibit oestrogenic activity in the uterotrophic test in mice (Song et al, 1998).…”