Activation of genomic and/or nongenomic signaling pathways contributes to the regulation of cell proliferation and differentiation [3]. Estrogens control the production and activity of components in the cell cycle progression, including cyclines, cyclin-dependent kinases, and their inhibitors [4]. Additionally, direct cancerogenic effects of estrogens can occurs via formation of electrophilic, redox-active estrogen ortho-quinones from catechol estrogens. The concurrent formation of reactive oxygen species and

superoxide anions can damage DNA and cellular proteins [5]. In serum and tissues like the female breast, estrogens are mainly present Inhibitors,research,lifescience,medical in their inactive sulfated form [5, 6]. The important precursor for E2 in the “sulfate pathway” is inactive estrone-3-sulfate (E1S). This is the most abundant estrogen in women at all ages as well as in men. Levels of E1S in blood are 5–10-fold higher than that of unconjugated estrogens, estrone (E1), estradiol (E2), and estriol (E3). As it has also a longer half-life than E2, it is considered Inhibitors,research,lifescience,medical as storage form for estrogens in some organs, for example, breast, from where active E1 is liberated Inhibitors,research,lifescience,medical by removal of the sulfate through STS [7, 8]. To create E2, E1S is taken up into the cells. There, after the removal of sulfate, E1 is reduced by reductive members of the superfamily of 17beta-hydroxysteroid dehydrogenases (17beta-HSDs) to form E2. Oxidative 17β-HSDs catalyze the conversion

of E2 to E1. Reductive 17beta-HSDs Inhibitors,research,lifescience,medical also inactivate androgens and catalyze also the formation of other estrogens, for example, 5alpha-androstenediol. Since 17beta-HSDs modulate the concentration of active estrogens and androgens, inhibitors of these enzymes may be applied in cancer therapy [9, 10] (Figure 1). Figure 1 Estrone sulfate (E1S), androstenediol-sulfate (Adione-S), and dehydroepiandrosterone-sulfate (DHEA-S) are taken up into cells by organic anion transporting polypeptides (OATPs) and other transporters from the SLC-family. The

“sulfatase pathway,” … Polar estrogen sulfates, particularly, E1S, are taken up into cells by specific transport proteins from different families Inhibitors,research,lifescience,medical of SLC transporters including the family of Anacetrapib organic anion transporters SLC21 or organic anion transporting polypeptides (OATPs). Within this concept, transporters from the OATP (SLC21) family such as OATP1A2, OATP1B3, OATP2B1, and OATP3A1 contribute to the cellular accumulation of E1S [11, 12], while ABC-efflux pumps from the MRP-family (ABCC1 and ABCC2), and the breast-cancer resistance protein (BCRP, ABCG2) mediates the efflux of E1S from the cells [13] (Figure 2). Uptake, biotransformation and excretion are transcriptionally regulated by nuclear receptors, for example, the pregnane X receptor. Furthermore, the variability in the expression levels and gene variants of transporters and enzymes can affect expression and function. These mechanisms may therefore influence the susceptibility of individuals to certain malignancies [14, 15].