Abstract 4593: Molecular characterization of the androgen metabolite 3βAdiol as a partial agonist of ERα

Abstract

Abstract Previously we have shown that androgens induce the growth of estrogen-dependent, estrogen receptor (ER)-positive breast cancer cell lines via metabolism to estrogen-like steroids, independent of CYP19 aromatase. One pathway involves the 3β-hydroxysteroid dehydrogenase (3β-HSD) mediated metabolism of 5α-dihydrotestosterone (DHT) to 5α-androstane-3β,17β-diol (3βAdiol). 3βAdiol induces the growth of ER+ breast cancer cells by directly activating ER. However, in MCF-7 cells, maximal growth induction by 3βAdiol was only ∼75% of maximal growth induced by 17β-estradiol (E2), suggesting that 3βAdiol acts as a weak or partial agonist of ERα. In this study, we characterize 3βAdiol as a partial agonist of ERα using cell growth assays and an in vitro receptor binding assay. We use cDNA microarrays to examine gene expression induced by the partial agonist 3βAdiol vs. the full agonist E2. We tested if 3βAdiol as a partial agonist could antagonize E2-induced breast cancer cell growth. MCF-7 cells in estrogen-free conditions were treated with 100pM E2 (∼EC90) and increasing concentrations of 3βAdiol. Increasing 3βAdiol suppressed growth induced by E2 to the maximal level induced by 3βAdiol alone. These data confirm that 3βAdiol acts as a partial agonist of ERα. Additionally, we performed an in vitro, florescence polarization-based receptor binding assay to determine the relative affinities of 3βAdiol and E2 for ERα. Based on their ability to compete with a labeled ligand for binding to ERα, 3βAdiol bound ERα with an approximately 10-fold lower affinity than E2. This decreased affinity of 3βAdiol for ERα is consistent with the reduced potency of 3βAdiol versus E2 observed in growth assays. To compare changes in gene expression induced by either the full agonist E2 or partial agonist 3βAdiol, samples were analyzed with Affymetrix HG-U133 Plus 2.0 microarrays. MCF-7 cells were treated for 24 hours with 10nM of steroid or vehicle control. To evaluate overall changes in gene expression induced by the steroids, probe sets were selected that had >2-fold change in expression versus the control array, with at least one sample having an expression value >26. 1273 probe sets were differentially expressed between the E2 and control arrays, and 1318 between the 3βAdiol and control arrays, however, 1030 of these were common to both steroids. Genes in canonical estrogen signaling pathways were similarly regulated by both E2 and 3βAdiol, including GREB1, Bcl-2, TFF1 and CXCL12. Only 45 probe sets were differentially expressed between the E2 and 3βAdiol arrays. These data demonstrate that 3βAdiol is a partial agonist of ERα in our breast cancer cell model. The majority of changes in gene expression are similar between the full and partial agonist. These data suggest that the differential growth induction by the full and partial agonist are likely due to differential levels or time courses of gene induction, rather than the induction of a novel set of genes. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4593.