Activity of three selective estrogen receptor modulators on hormone-dependent responses in the mouse uterus and mammary gland
Introduction
Estradiol (E2) is a hormone that plays a key role in reproduction and in the homeostasis of a variety of mammalian systems. When estrogen levels decline at menopause, many women experience vasomotor instability and vaginal atrophy, and are at increased risk for developing osteoporosis and heart disease. Hormone therapy can address many of these consequences of reduced ovarian productivity, but not all women are candidates for these medications.
Classically, estrogens act via binding to intracellular estrogen receptors (ER), two of which have been described (ERα and ERβ). Upon binding hormone, these receptors modulate transcription of target genes. Known ER ligands encompass a large class of steroidal and non-steroidal compounds (Anstead et al., 1997), and within this group is a subset of ligands called SERMs that can mimic the effects of E2 in some situations and block its effects in others. The mechanisms responsible for this activity are poorly understood but known to be cell type and gene promoter dependent (Miller, 2002). The cell-type dependent action of this class of compounds is clinically useful. For example, tamoxifen, a triphenylethylene, was the first SERM that was originally identified for use in breast cancer due to strong antiestrogenic activity in the mammary gland but was later discovered to prevent bone mineral density loss (Jordan et al., 1987). Tamoxifen also has significant uterotropic activity, and its clinical use is associated with an increased incidence of uterine cancer (Bernstein et al., 1999, Fisher et al., 1998, van Leeuwen et al., 1994). In rats, lasofoxifene (LAS), a tetrahydronaphthalene, has been shown to prevent bone loss with no effect on the uterine epithelium (Ke et al., 2004, Ke et al., 1998, Ke et al., 2000), and in an N-methylurea-induced model of rat mammary carcinoma, LAS demonstrated chemopreventive and therapeutic potential (Cohen et al., 2001). Raloxifene (RAL), a benzothiophene-based SERM, also has antiestrogenic activity in the mammary gland and maintains bone mineral density in the postmenopausal population without introducing the uterotrophic liability seen with tamoxifen (Delmas et al., 1997). Preclinically, RAL caused regression of mammary gland development in intact rats and antagonized estrogen-stimulated mammary gland proliferation in ovariectomized (OVX) rats (Buelke-Sam et al., 1998). A more recently developed SERM is bazedoxifene acetate (BZA). Like other SERMs, BZA preserves bone mineral density and biomechanical parameters in a rodent model of osteopenia, has no effect on uterine epithelium and antagonizes E2-dependent MCF-7 breast cancer cell proliferation in vitro (Komm et al., 2005).
In mammals, the development of the mammary gland and uterus is dependent on various hormonal contributions. Estrogens and progestins are well-established modulators of reproductive function in the uterus and play significant roles during pregnancy and parturition. In the mammary gland, these hormones support the development of a branched ductal system and secretory lobuloalveoli during pregnancy. Specifically, estrogens facilitate ductal elongation into the fat pad, whereas progestins enable ductal side branching and morphogenesis of the lobuloalveoli (Silberstein, 2001).
We previously described development of a short-term in vivo mouse model to measure uterine and mammary gland responses to estrogens and progestins using both gross tissue responses as well as molecular markers (Crabtree et al., 2006). In this report, we describe characterization of three SERMs (BZA, LAS and RAL) in this model and examine their effect on endpoints dependent on the co-administration of E2 and progesterone (P) in addition to those requiring E2 alone.
Section snippets
Animal care and treatment regimen
Seven-week old c57BL/6 female mice were ovariectomized, rested for 1 week prior to treatment and dosed as previously described with 6 mice/group (Crabtree et al., 2006). Briefly, E2 and/or SERMs were administered on all 7 days, whereas co-treatment with P occurred only on days 4–7. As relevant, vehicle was administered in control groups and in groups receiving only SERM, SERM + E2 or SERM + P. E2 (Sigma, St. Louis, MA; 1 μg/kg) and P (Sigma; 30 mg/kg) were delivered by subcutaneous injection in a
Effect of SERMs on uterine wet weight
One of the hallmarks of an ER agonist effect on the uterus is an increase in uterine wet weight. Thus, we evaluated BZA, LAS and RAL on this endpoint in agonist and antagonist modes. As seen in Fig. 1, all SERMs increased uterine wet weight compared to vehicle when dosed alone at 3 mg/kg. The slight increase in uterine weight with SERMs was not unexpected given some previously published reports from studies done in rodents with RAL and LAS (Onoe et al., 2000, Wang et al., 2006). This ER agonist
Discussion
As a class of compounds, SERMs display a continuum of ER agonist and antagonist activity, depending both on the compound and the endpoint measured. In vitro microarray and peptide interaction profiling studies support the conclusion that each SERM has its own molecular signature that is unique to the test paradigm (Frasor et al., 2004, Gadal et al., 2005, Iannone et al., 2004, Scafoglio et al., 2006, Tee et al., 2004). This body of literature demonstrates the individual impact of SERMs on
Acknowledgement
We thank the Wyeth Research BioResources group for in vivo support.
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