Estrogens are known to regulate the proliferation of breast cancer cells and to alter their cytoarchitectural and phenotypic properties, but the gene networks and pathways by which estrogenic hormones regulate these events are only partially understood. We used global gene expression profiling by Affymetrix GeneChip microarray analysis, with quantitative PCR verification in many cases, to identify patterns and time courses of genes that are either stimulated or inhibited by estradiol (E2) in estrogen receptor (ER)-positive MCF-7 human breast cancer cells. Of the >12,000 genes queried, over 400 showed a robust pattern of regulation, and, notably, the majority (70%) were down-regulated. We observed a general up-regulation of positive proliferation regulators, including survivin, multiple growth factors, genes involved in cell cycle progression, and regulatory factor-receptor loops, and the down-regulation of transcriptional repressors, such as Mad4 and JunB, and of antiproliferative and proapoptotic genes, including B cell translocation gene-1 and -2, cyclin G2, BCL-2 antagonist/killer 1, BCL 2-interacting killer, caspase 9, and TGFbeta family growth inhibitory factors. These together likely contribute to the stimulation of proliferation and the suppression of apoptosis by E2 in these cells. Of interest, E2 appeared to modulate its own activity through the enhanced expression of genes involved in prostaglandin E production and signaling, which could lead to an increase in aromatase expression and E2 production, as well as the decreased expression of several nuclear receptor coactivators that could impact ER activity. Our studies highlight the diverse gene networks and metabolic and cell regulatory pathways through which this hormone operates to achieve its widespread effects on breast cancer cells.

Estrogen is of great importance in the regulation of uterine function. The aim of this study was to examine the individual physiological roles of each of the two receptors for estradiol, estrogen receptor (ER) alpha and ERbeta, and their potential comodulatory effects on gene expression and uterine growth using recently developed ER subtype-selective agonist ligands. The use of ER subtype-selective ligands provides an alternative, complementary approach to the use of receptor knockout mice. Administration of the ERalpha-selective ligand propyl pyrazole triol (PPT) to immature mice resulted in a significant increase in uterine weight, as well as bromodeoxyuridine incorporation and proliferating cell nuclear antigen expression in luminal epithelial cells. PPT also increased complement component 3, lactoferrin, and glucose-6-phosphate dehydrogenase (G6PDH), and decreased androgen receptor (AR) and progesterone receptor (PR) mRNA levels in uterine tissue, as did estradiol (E(2)). However, when compared with E(2), PPT was less effective in stimulating uterine weight, complement component 3, and G6PDH expression but was as effective as E(2) in regulating lactoferrin, AR, and PR expression. In contrast to the action of the ERalpha agonist PPT, the ERbeta agonist diarylpropionitrile (DPN) did not increase uterine weight or luminal epithelial cell proliferation at a dose that reduced G6PDH and elicited a decrease in PR and AR mRNA and protein expression. Interestingly, DPN reduced the uterine weight stimulation by PPT, and enhanced the effect of PPT in decreasing uterine PR and AR mRNA. These findings with ER subtype-selective ligands indicate that ERalpha is the major regulator of estrogen function in the uterus, but that ERbeta does exert effects on some uterine markers of estrogen action. In addition, ERbeta can modulate ERalpha activity in a response-specific and dose-dependent manner.

ABSTRACT

C ellular activity is precisely regulated by a finely tuned balance between coactivator and corepressor proteins that control transcriptional networks during development and in normal and cancerous states (1). The corepressors N-CoR (nuclear receptor corepressor) and SMRT (silencing mediator of retinoid and thyroid hormone receptors) play crucial roles in transcriptional repression by multiple classes of transcription factors, including some nuclear hormone receptors. Repression by N-CoR and SMRT results from their association with large histone deacetylase complexes, which lead to histone deacetylation, altered chromatin structure, and decreased gene transcription (reviewed in ref. 2). Although few studies have demonstrated that N-CoR expression is regulated at the level of gene transcription, several examples of posttranslational regulation of N-CoR expression and activity have been described. Recent studies have shown that N-CoR interaction with transcription factors, such as NF-B and estrogen receptor (ER), can be reduced through translocation of N-CoR out of the nucleus and into the cytoplasm (3-5). N-CoR has also been shown to be regulated by ubiquitin-mediated protein degradation through an interaction with the E3 ubiquitin ligase Siah2, a mammalian homolog of Drosophila seven in absentia (Sina), which leads to the ubiquitination and degradation of N-CoR by the 26S proteasome (6).The majority of estrogen's effects on its numerous target tissues are mediated by its two receptors, ER␣ and ER␤, which act primarily as ligand-dependent transcription factors. Upon binding to its ligand, the ER associates with DNA either directly at estrogen response elements or through tethering to other transcription factors, leading to the recruitment of transcriptional coregulators and chromatin-modifying complexes and the regulation of gene expression (7). We and others (8-17) have used microarray gene expression profiling to identify estrogen target genes in breast cancer cells, where estrogen has been shown to stimulate proliferation and suppress apoptosis through the regulation of multiple genes. These studies have demonstrated that, as expected, estrogen up-regulates many cell-cycle regulators, growth factors, and antiapoptotic genes but also down-regulates a number of cell-cycle inhibitors and proapoptotic genes.Estrogen also regulates the mRNA expression of important transcriptional regulators, both transcription factors and transcriptional coactivators and corepressors (12). Evidence supporting the idea that 17␤-estradiol (E2) is capable of regulating the expression of coregulators has grown in the past few years. For example, this hormone has been shown to up-regulate mRNA levels for the corepressors RIP140 (12, 18), SHP (19), and SHARP (20) and also to down-regulate mRNA levels for the coactivators 21). In addition to the regulation of mRNA for some coregulators, the activity of these proteins can be modulated by hormone by changing the protein's state of phosphorylation, as observed for the coactivator SRC3͞AIB1...