The PR-A and PR-B isoforms of progesterone receptors (PR) have different physiological functions, and their ratio varies widely in breast cancers. To determine whether the two PR regulate different genes, we used human breast cancer cell lines engineered to express one or the other isoform. Cells were treated with progesterone in triplicate, time-separated experiments, allowing statistical analyses of microarray gene expression data. Of 94 progesterone-regulated genes, 65 are uniquely regulated by PR-B, 4 uniquely by PR-A, and only 25 by both. Almost half the genes encode proteins that are membrane-bound or involved in membraneinitiated signaling. We also find an important set of progesterone-regulated genes involved in mammary gland development and/or implicated in breast cancer. This first, large scale study of PR gene regulation has important implications for the measurement of PR in breast cancers and for the many clinical uses of synthetic progestins. It suggests that it is important to distinguish between the two isoforms in breast cancers and that isoform-specific genes can be used to screen for ligands that selectively modulate the activity of PR-A or PR-B. Additionally, use of natural target genes, rather than "consensus" response elements, for transcription studies should improve our understanding of steroid hormone action.

The nuclear receptors belong to a superfamily of proteins, many of which are ligand-regulated, that bind to specific DNA sequences and control specific gene transcription. Recent data show that, in addition to contacting the basal transcription machinery directly, nuclear receptors inhibit or enhance transcription by recruiting an array of coactivator or corepressor proteins to the transcription complex. In this review we define the properties of these putative coregulatory factors; we describe the basal and coregulatory factors that are currently known to interact with nuclear receptors; we suggest various mechanisms by which coactivators and corepressors act; we discuss issues that are raised by the presence of multiple, perhaps competing, coregulatory factors; and we speculate how these additional regulatory layers may explain the heterogeneity of hormone responses that are observed in normal and malignant tissues.

Introduction: The androgen receptor (AR) is widely expressed in breast cancers and has been proposed as a therapeutic target in estrogen receptor alpha (ER) negative breast cancers that retain AR. However, controversy exists regarding the role of AR, particularly in ER + tumors. Enzalutamide, an AR inhibitor that impairs nuclear localization of AR, was used to elucidate the role of AR in preclinical models of ER positive and negative breast cancer.

The transcription factor ZEB1 is normally not expressed in epithelial cells. When inappropriately expressed in carcinomas, ZEB1 initiates epithelial to mesenchymal transition due to its ability to repress E-cadherin and other genes involved in polarity. Recently, ZEB1 and ZEB2 have been identified as direct targets of the microRNA-200c family. We find that miR-200c levels are high in well-differentiated endometrial, breast and ovarian cancer cell lines, but extremely low in poorly-differentiated cancer cells. Low or absent miR-200c results in aberrant expression of ZEB1 and consequent repression of E-cadherin. Reinstatement of miR-200c to such cells restores E-cadherin and dramatically reduces migration and invasion. Microarray profiling reveals that in addition to ZEB1 and ZEB2, other mesenchymal genes (such as FN1, NTRK2, and QKI), which are also predicted direct targets of miR-200c, are indeed inhibited by addition of exogenous miR-200c. One such gene, class IIIβ-tubulin (TUBB3), which encodes a tubulin isotype normally found only in neuronal cells, is a direct target of miR-200c. This finding is of particular significance because we show that restoration of miR-200c increases sensitivity to microtubule-targeting agents by up to 85%. Since expression of TUBB3 is a common mechanism of resistance to microtubule-binding chemotherapeutic agents in many types of solid tumors, the ability of miR-200c to restore chemosensitivity to such agents may be explained by its ability to reduce TUBB3. Because miR-200c is crucial for maintenance of epithelial identity, behavior, and sensitivity to chemotherapy, we propose that it warrants further investigation as a therapeutic strategy for aggressive, drug-resistant cancers.

Progesterone has biphasic effects on proliferation of breast cancer cells; it stimulates growth in the first cell cycle, then arrests cells at G 1 /S of the second cycle accompanied by up-regulation of the cyclin-dependent kinase inhibitor, p21. We now show that progesterone regulates transcription of the p21 promoter by an unusual mechanism. This promoter lacks a canonical progesterone response element. Instead, progesterone receptors (PRs) interact with the promoter through the transcription factor Sp1 at the third and fourth of six Sp1 binding sites located downstream of nucleotide 154. Mutation of Sp1 site 3 eliminates basal transcription, and mutation of sites 3 and 4 eliminates transcriptional up-regulation by progesterone. Progesterone-mediated transcription is further prevented by overexpression of E1A, suggesting that CBP/p300 is required. Indeed, in HeLa cells, Sp1 and CBP/p300 associate with stably integrated flagtagged PRs in a multiprotein complex. Since many signals converge on p21, cross-talk between PRs and other factors co-localized on the p21 promoter, may explain how progesterone can be either proliferative or differentiative in different target cells.Progesterone is a paradoxical hormone having either growth stimulatory effects or growth inhibitory and differentiative effects, depending on the tissue in question and the dose and treatment regimen (1, 2). In the uterus for example, progesterone inhibits epithelial growth and has differentiative effects (3). It is therefore used to counteract the proliferative and carcinogenic effects of unopposed estrogens in women prescribed hormone replacement therapy (4). In the breast, the role of progesterone is more complex. The hormone is required for terminal growth and differentiation of the mammary gland (2). Therefore, mice lacking progesterone receptors (PRs) 1 exhibit incomplete mammary gland ductal branching and failure of lobulo-alveolar development (5). In animal models of mammary carcinogenesis, progesterone, depending on the regimen used, can either inhibit or promote tumor formation (2). On the other hand, in animals with established PR-positive mammary tumors, progesterone is usually proliferative, and progesterone antagonists inhibit tumor growth (6). Despite this, in humans, second-line high dose progestin therapy effectively suppresses the growth of hormone-dependent PR-and estrogen receptorpositive breast cancers that have acquired resistance to the antiestrogen tamoxifen (6).How can these contradictory effects of progesterone be reconciled? Recent studies have dealt with the effects of progesterone on mitosis and key cell cycle regulatory proteins in cultured human breast cancer cells (1, 7-9). Treatment of such cells with progestins produces biphasic effects. Studies focusing on the initial growth stimulatory component show that progestin-induced entry of cells into S-phase is accompanied by transient increases of cyclin D1 and cyclin-dependent kinase 4 activity (1, 7). Indeed, cyclin D1 is a critical component of the mitogenic respon...