Progesterone receptor directly inhibits β-casein gene transcription in mammary epithelial cells through promoting promoter and enhancer repressive chromatin modifications

Mutation of Signal Transducer and Activator of Transcription 5 (STAT5) Binding Sites Decreases Milk Allergen αS1-Casein Content in Goat Mammary Epithelial Cells

α_S1-Casein (encoded by the CSN1S1 gene) is associated with food allergy more than other milk protein components. Milk allergy caused by α_S1-casein is derived from cow milk, goat milk and other ruminant milk. However, little is known about the transcription regulation of α_S1-casein synthesis in dairy goats. This study aimed to investigate the regulatory roles of signal transducer and activator of transcription 5 (STAT5) on α_S1-casein in goat mammary epithelial cells (GMEC). Deletion analysis showed that the core promoter region of CSN1S1 was located at −110 to −18 bp upstream of transcription start site, which contained two putative STAT5 binding sites (gamma-interferon activation site, GAS). Overexpression of STAT5a gene upregulated the mRNA level and the promoter activity of the CSN1S1 gene, and STAT5 inhibitor decreased phosphorylated STAT5 in the nucleus and CSN1S1 transcription activity. Further, GAS site-directed mutagenesis and chromatin immunoprecipitation (ChIP) assays revealed that GAS1 and GAS2 sites in the CSN1S1 promoter core region were binding sites of STAT5. Taken together, STAT5 directly regulates CSN1S1 transcription by GAS1 and GAS2 sites in GMEC, and the mutation of STAT5 binding sites could downregulate CSN1S1 expression and decrease α_S1-casein synthesis, which provide the novel strategy for reducing the allergic potential of goat milk and improving milk quality in ruminants.

Redundant and non-redundant cytokine-activated enhancers control Csn1s2b expression in the lactating mouse mammary gland

Enhancers are transcription factor platforms that synergize with promoters to control gene expression. Here, we investigate enhancers that activate gene expression several hundred-fold exclusively in the lactating mouse mammary gland. Using ChIP-seq for activating histone marks and transcription factors, we identify two candidate enhancers and one super-enhancer in the Csn1s2b locus. Through experimental mouse genetics, we dissect the lactation-specific distal enhancer bound by the mammary-enriched transcription factors STAT5 and NFIB and the glucocorticoid receptor. While deletions of canonical binding motifs for NFIB and STAT5, individually or combined, have a limited biological impact, a non-canonical STAT5 site is essential for enhancer activity during lactation. In contrast, the intronic enhancer contributes to gene expression only in late pregnancy and early lactation, possibly by interacting with the distal enhancer. A downstream super-enhancer, which physically interacts with the distal enhancer, is required for the functional establishment of the Csn1s2b promoter and gene activation. Lastly, NFIB binding in the promoter region fine-tunes Csn1s2b expression. Our study provides comprehensive insight into the anatomy and biology of regulatory elements that employ the JAK/STAT signaling pathway and preferentially activate gene expression during lactation.

Enhancers and promoters work together to actively regulate gene expression affecting several biological processes. Here, the authors provide molecular insights into the regulation of enhancers and super-enhancers in the Csn1s2b locus during lactation.

Bisphenol A and benzophenone-3 exposure alters milk protein expression and its transcriptional regulation during functional differentiation of the mammary gland in vitro

The plastic monomer and plasticizer bisphenol A (BPA), and the UV-filter benzophenone-3 (BP3) have been shown to have estrogenic activities that could alter mammary gland development. Our aim was to analyze whether BPA or BP3 direct exposure affects the functional differentiation of the mammary gland using an in vitro model. Mammary organoids were obtained and isolated from 8 week-old virgin female C57BL/6 mice and were differentiated on Matrigel with medium containing lactogenic hormones and exposed to: a) vehicle (0.01% ethanol); b) 1 × 10^-9 M or 1 × 10^-6 M BPA; or c) 1 × 10^-12 M, 1 × 10^-9 M or 1 × 10^-6 M BP3 for 72 h. The mRNA and protein expression of estrogen receptor alpha (ESR1) and progesterone receptor (PR) were assessed. In addition, mRNA levels of PR-B isoform, glucocorticoid receptor (GR), prolactin receptor (PRLR) and Stat5a, and protein expression of pStat5a/b were evaluated at 72 h. The mRNA and protein expression of milk proteins and their DNA methylation status were also analyzed. Although mRNA level of PRLR and GR was similar between treatments, mRNA expression of ESR1, total PR, PR-B and Stat5a was increased in organoids exposed to 1 × 10^-9 M BPA and 1 × 10^-12 M BP3. Total PR expression was also increased with 1 × 10^-6 M BPA. Nuclear ESR1 and PR expression was observed in all treated organoids; whereas nuclear pStat5a/b alveolar cells was observed only in organoids exposed to 1 × 10^-9 M BPA and 1 × 10^-12 M BP3. The beta-casein mRNA level was increased in both BPA concentrations and 1 × 10^-12 M BP3, which was associated with hypomethylation of its promoter. The beta-casein protein expression was only increased with 1 × 10^-9 M BPA or 1 × 10^-12 M BP3. In contrast, BPA exposure decreased alpha-lactalbumin mRNA expression and increased DNA methylation level in different methylation-sensitive sites of the gene. Also, 1 × 10^-9 M BPA decreased alpha-lactalbumin protein expression. Our results demonstrate that BPA or BP3 exposure alters milk protein synthesis and its transcriptional regulation during mammary gland differentiation in vitro.

Reciprocal fine-tuning of progesterone and prolactin-regulated gene expression in breast cancer cells

Progesterone and prolactin are two key hormones involved in development and remodeling of the mammary gland. As such, both hormones have been linked to breast cancer. Despite the overlap between biological processes ascribed to these two hormones, little is known about how co-expression of both hormones affects their individual actions. Progesterone and prolactin exert many of their effects on the mammary gland through activation of gene expression, either directly (progesterone, binding to the progesterone receptor [PR]) or indirectly (multiple transcription factors being activated downstream of prolactin, most notably STAT5). Using RNA-seq in T47D breast cancer cells, we characterized the gene expression programs regulated by progestin and prolactin, either alone or in combination. We found significant crosstalk and fine-tuning between the transcriptional programs executed by each hormone independently and in combination. We divided and characterized the transcriptional programs into four broad categories. All crosstalk/fine-tuning shown to be modulated by progesterone was dependent upon the expression of PR. Moreover, PR was recruited to enhancer regions of all regulated genes. Interestingly, despite the canonical role for STAT5 in transducing prolactin-signaling in the normal and lactating mammary gland, very few of the prolactin-regulated transcriptional programs fine-tuned by progesterone in this breast cancer cell line model system were in fact dependent upon STAT5. Cumulatively, these data suggest that the interplay of progesterone and prolactin in breast cancer impacts gene expression in a more complex and nuanced manner than previously thought, and likely through different transcriptional regulators than those observed in the normal mammary gland. Studying gene regulation when both hormones are present is most clinically relevant, particularly in the context of breast cancer.

The glucocorticoid receptor interferes with progesterone receptor-dependent genomic regulation in breast cancer cells

The glucocorticoid and progesterone receptors (GR and PR) are closely related members of the steroid receptor family. Despite sharing similar structural and functional characteristics; the cognate hormones display very distinct physiological responses. In mammary epithelial cells, PR activation is associated with the incidence and progression of breast cancer, whereas the GR is related to growth suppression and differentiation. Despite their pharmacological relevance, only a few studies have compared GR and PR activities in the same system. Using a PR⁺/GR⁺ breast cancer cell line, here we report that either glucocorticoid-free or dexamethasone (DEX)-activated GR inhibits progestin-dependent gene expression associated to epithelial-mesenchymal-transition and cell proliferation. When both receptors are activated with their cognate hormones, PR and GR can form part of the same complex according to co-immunoprecipitation, quantitative microscopy and sequential ChIP experiments. Moreover, genome-wide studies in cells treated with either DEX or R5020, revealed the presence of several regions co-bound by both receptors. Surprisingly, GR also binds novel genomic sites in cells treated with R5020 alone. This progestin-induced GR binding was enriched in REL DNA motifs and located close to genes coding for chromatin remodelers. Understanding GR behavior in the context of progestin-dependent breast cancer could provide new targets for tumor therapy.

Perinatal exposure to bisphenol A modifies the transcriptional regulation of the β-Casein gene during secretory activation of the rat mammary gland

With the aim to analyze whether bisphenol A (BPA) modifies β-Casein (β-Cas) synthesis and transcriptional regulation in perinatally exposed animals, here, pregnant F0 rats were orally exposed to 0, 0.6 or 52 μg BPA/kg/day from gestation day 9 until weaning. Then, F1 females were bred and mammary glands were obtained on lactation day 2. Perinatal BPA exposure decreased β-Cas expression without modifying the activation of prolactin receptor. It also decreased the expression of glucocorticoid receptor in BPA52-exposed dams and β1 and α6 integrins as well as dystroglycan in both BPA groups. In addition, BPA exposure altered the expression of histone-modifying enzymes and induced histone modifications and DNA methylation in the promoter, enhancer and exon VII of the β-Cas gene. An impaired crosstalk between the extracellular matrix and lactogenic hormone signaling pathways and epigenetic modifications of the β-Cas gene could be the molecular mechanisms by which BPA decreased β-Cas expression.

Regeneration of Bovine Mammary Gland in Immunodeficient Mice by Transplantation of Bovine Mammary Epithelial Cells Mixed with Matrigel

Background and Objectives

With the global demand for dairy protein for consumption growing annually, there has been increasing activity in the research field of dairy protein synthesis and production. From a manipulation perspective, it is more difficult to use live cattle for laboratory studies on the production of milk as well as of dairy protein such as casein, as compared with using laboratory animals like rodents. Therefore, we aimed to develop a mouse model of bovine mammary alveolar ducts for laboratory-scale studies. We studied the formation of the bovine mammary gland ductal structure by transplanting the MAC-T bovine alveolar cell line into mice.

Methods and Results

MAC-T cells (1×10⁷) were suspended in Matrigel and injected into the dorsal tissue of 8-week-old male BALB/C nude mice. Histological analysis of tissue dissected from the MAC-T cell-transplanted mice after 6 weeks showed the typical morphology of the tubuloalveolar female gland, as well as glands made up of branching ducts that were surrounded by smooth muscle with small alveoli budding off the ducts. In addition, the epithelial markers CK14 and CK18 were expressed within the duct-like structure. Prolactin was detected in the duct interior in these CK14+ and CK18+ cells but not in the non-transplanted MAC-T cells.

Conclusions

These results showed that duct-like tissue had been successfully formed after 6 weeks of transplantation of the CK14+ and CK18+ MAC-T cells into mice dorsal tissue. This mouse model will be a useful tool for further research on the bovine mammary gland.

Steroid hormone receptors silence genes by a chromatin-targeted mechanism similar to those used for gene activation

How genes are repressed by steroid hormones remains a matter of debate, and several indirect mechanisms have been proposed. We found that the ligand-activated progesterone receptor recruits to the promoter of downregulated genes a repressor complex composed of HP1γ, the lysine demethylase LSD1, histone deacetylases, coREST, the RNA SRA, and the ATPase BRG1. BRG1 is needed for chromatin remodeling and facilitates the deposition of linker histone variant H1.2, which compacts chromatin and hinders RNA polymerase loading and transcription. Thus, steroid hormone receptors can repress genes in ways reminiscent of those used for gene induction, namely by directly targeting factors that remodel chromatin. But while PR-dependent gene induction in T47D cells is mainly achieved by potentiating enhancer activity, repression acts at the level of gene promoters.

Regulatory roles of Oct proteins in the mammary gland

The expression of Oct-1 and -2 and their binding to the octamer motif in the mammary gland are developmentally and hormonally regulated, consistent with the expression of milk proteins. Both of these transcription factors constitutively bind to the proximal promoter of the milk protein gene β-casein and might be involved in the inhibition or activation of promoter activity via interactions with other transcription factors or cofactors at different developmental stages. In particular, the lactogenic hormone prolactin and glucocorticoids induce Oct-1 and Oct-2 binding and interaction with both the signal transducer and activator of transcription 5 (STAT5) and the glucocorticoid receptor on the β-casein promoter to activate β-casein expression. In addition, increasing evidence has shown the involvement of another Oct factor, Oct-3/4, in mammary tumorigenesis, making Oct-3/4 an emerging prognostic marker of breast cancer and a molecular target for the gene-directed therapeutic intervention, prevention and treatment of breast cancer. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

DNA methylation and transcription in a distal region upstream from the bovine AlphaS1 casein gene after once or twice daily milking

Once daily milking (ODM) induces a reduction in milk production when compared to twice daily milking (TDM). Unilateral ODM of one udder half and TDM of the other half, enables the study of underlying mechanisms independently of inter-individual variability (same genetic background) and of environmental factors. Our results show that in first-calf heifers three CpG, located 10 kb upstream from the CSN1S1 gene were methylated to 33, 34 and 28%, respectively, after TDM but these levels were higher after ODM, 38, 38 and 33%, respectively. These methylation levels were much lower than those observed in the mammary gland during pregnancy (57, 59 and 50%, respectively) or in the liver (74, 78 and 61%, respectively). The methylation level of a fourth CpG (CpG4), located close by (29% during TDM) was not altered after ODM. CpG4 methylation reached 39.7% and 59.5%, during pregnancy or in the liver, respectively. CpG4 is located within a weak STAT5 binding element, arranged in tandem with a second high affinity STAT5 element. STAT5 binding is only marginally modulated by CpG4 methylation, but it may be altered by the methylation levels of the three other CpG nearby. Our results therefore shed light on mechanisms that help to explain how milk production is almost, but not fully, restored when TDM is resumed (15.1±0.2 kg/day instead of 16.2±0.2 kg/day, p<0.01). The STAT5 elements are 100 bp away from a region transcribed in the antisense orientation, in the mammary gland during lactation, but not during pregnancy or in other reproductive organs (ovary or testes). We now need to clarify whether the transcription of this novel RNA is a consequence of STAT5 interacting with the CSN1S1 distal region, or whether it plays a role in the chromatin structure of this region.

Cyclin A2 and its associated kinase activity are required for optimal induction of progesterone receptor target genes in breast cancer cells

A role for the cell cycle protein cyclin A2 in regulating progesterone receptor (PR) activity is emerging. This study investigates the role of cyclin A2 in regulating endogenous PR activity in T47D breast cancer cells by depleting cyclin A2 expression and measuring PR target genes using q-RT-PCR. Targets examined included genes induced by the PR-B isoform more strongly than PR-A (SGK1, FKBP5), a gene induced predominantly by PR-A (HEF1), genes induced via PR tethering to other transcription factors (p21, p27), a gene induced in part via extra-nuclear PR signaling mechanisms (cyclin D1) and PR-repressed genes (DST, IL1R1). Progestin induction of target genes was reduced following cyclin A2 depletion. However, cyclin A2 depletion did not diminish progestin target gene repression. Furthermore, inhibition of the associated Cdk2 kinase activity of cyclin A2 also reduced progestin induction of target genes, while Cdk2 enhanced the interaction between PR and cyclin A2. These results demonstrate that cyclin A2 and its associated kinase activity are important for progestin-induced activation of endogenous PR target genes in breast cancer cells.

Underlying mechanisms involved in the decrease of milk secretion during Escherichia coli endotoxin induced mastitis in lactating mice

Mastitis, the inflammation of mammary glands resulting from bacterial infection, disrupts milk production in lactating mammary glands. In this study, we injected lipopolysaccharide (LPS), one of the endotoxins from Escherichia coli into mouse mammary glands to disrupt milk production, and we investigated the influence of LPS on nutrient uptake, synthesis, and secretion processes for milk component production in alveolar epithelial cells (AEC). The expression of genes relevant to the three-staged milk component production process (nutrient uptake, synthesis, and secretion of milk components) were down-regulated within 12 h after LPS injection in AEC. The internalization of glucose transporter 1 (GLUT-1) from the basolateral membrane to the cytoplasm occurred in accordance with the down-regulation of gene expression 3 h after LPS injection. The abnormal localization of adipophilin and beta-casein was also observed in the LPS-injected mammary glands. SLC7A1, an amino acid transporter, was up-regulated 3 and 6 h after LPS injection. Furthermore, the inactivation of signal transducer and activator of transcription 5 (STAT5) and the activation of STAT3 and nuclear factor-kappa B (NFkappaB) occurred 3 h after LPS injection. These results indicate that the nutrient uptake, synthesis, and secretion of milk components in AEC are rapidly shut down in the lactating mammary glands after LPS injection.

Progesterone receptor and Stat5 signaling cross talk through RANKL in mammary epithelial cells

Progesterone (P4) stimulates proliferation of the mammary epithelium by a mechanism that involves paracrine signaling mediated from progesterone receptor (PR)-positive to neighboring PR-negative cells. Here we used a primary mouse mammary epithelial cell (MEC) culture system to define the molecular mechanism by which P4 regulates the expression of target gene effectors of proliferation including the paracrine factor receptor and activator of nuclear factor κB ligand (RANKL). MECs from adult virgin mice grown and embedded in three-dimensional basement-membrane medium resemble mammary ducts in vivo structurally and with respect to other properties including a heterogeneous pattern of PR expression, P4 induction of RANKL and other target genes in a PR-dependent manner, and a proliferative response to progestin. RANKL was demonstrated to have multiple functional P4-responsive enhancers that bind PR in a hormone-dependent manner as detected by chromatin immunoprecipitation assay. P4 also stimulated recruitment of signal transducer and activator of transcription (Stat)5a to RANKL enhancers through an apparent tethering with PR. Analysis of primary MECs from Stat5a knockout mice revealed that P4 induction of RANKL and a broad range of other PR target genes required Stat5a, as did P4-stimulated cell proliferation. In the absence of Stat5a, PR binding was lost at selective RANKL enhancers but was retained with others, suggesting that Stat5a acts to facilitate PR DNA binding at selective sites and to function as a coactivator with DNA-bound PR at others. These results show that RANKL is a direct PR target gene and that Stat5a has a novel role as a cofactor in PR-mediated transcriptional signaling in the mammary gland.

Buffalo alpha S1-casein gene 5'-flanking region and its interspecies comparison

The expression of milk protein genes is tightly regulated in a spatio-temporal manner through the combinatorial interaction of lactogenic hormones and a set of transcription factors mediating developmental and tissue-specific gene expression. The recruitment of a unique set of transcription factors is determined by the cis-regulatory motifs present in the gene promoter region. Here, we report the isolation, sequencing, structural analysis and interspecies comparison of the 5'cis-regulatory region of the buffalo alpha S1 (αS1)-casein gene. The proximal promoter region of the buffalo αS1-casein gene harbored the insertion of a 72-bp fragment of long interspersed nuclear element of the L1_BT retrotransposon family. Among the core and vertebrate-specific promoter elements, the motifs for the binding of Brn POU domain factors (BRNF), Lim homeodomain factors (LHXF), NK6 homeobox transcription factors (NKX6), nuclear factor kappa B/c-rel (NFKB), AT-rich interactive domain factor (ARID), Brn POU domain factor 5 (BRN5), pancreatic and intestinal homeodomain transcription factor (PDX1), Distal-less homeodomain transcription factors (DLXF), T-cell factor/lymphoid enhancer-binding factor-1 (LEFF) and GHF-1 pituitary-specific POU domain transcription factor (PIT1) were over-represented in the αS1-casein gene regulatory region (Z score >4.0). The Multiple EM for Motif elicitation predicted three motifs which consisted of the sequences known to bind mammary gland factor/signal transducer and activator of transcription 5 (MGF/STAT5), estrogen receptor-related alpha (ERα), steroidogenic factor 1 (SF1) and glucocorticoid receptor (GR), indicating their potential role in the mammary gland-specific gene expression. The interspecies comparison of the proximal promoter region revealed conserved sequences for TATA boxes and MGF/STAT5 in all species, whereas activator protein 1 (AP1), pregnancy-specific mammary nuclear factor (PMF), CCAAT/enhancer binding protein (C/EBP), double-stranded and single-stranded DNA-binding protein 1 (DS1 and SS), ying and yang factor 1 (YY1), and GR half-sites were among ruminants. The functional significance of the L1_BT retrotransposon insertion on the buffalo αS1-casein gene expression needs to be experimentally validated.

The requirement for p42/p44 MAPK activity in progesterone receptor-mediated gene regulation is target gene-specific

Recent studies have suggested that progestins play a role in the etiology of breast cancer; however, the mechanisms by which progestins promote tumor formation/progression have not been defined. Progestin action, in target tissues such as the breast, is mediated by the progesterone receptor (PR). PR signaling is complex and PR regulates transcription of target genes through a variety of mechanisms. Many cell signaling pathways are activated inappropriately in breast cancer cells and these pathways can regulate PR activity. For example, the p42/p44 MAPK pathway can regulate PR function by altering phosphorylation of PR, as well as its coregulators. We found that inhibition of the p42/p44 MAPK signaling pathway with a MEK inhibitor (U0126) impairs PR-mediated gene induction, but not gene repression. In addition, the effects of U0126 on PR-mediated gene transcription are much greater with long-term versus short-term inhibition and are gene-specific. Finally, treatment with U0126 delays phosphorylation of Ser294, but does not block phosphorylation completely, suggesting that p42/p44 MAPK kinase is not the dominant kinase responsible for phosphorylating this site. Collectively, these studies suggest that in addition to the p42/p44 MAPK pathway, other signaling pathways are also important for PR transcriptional activity in breast cancer cells. The integration of PR transcriptional effects and cell signaling pathways has implications for the initiation or progression of breast cancer. Understanding how these pathways interact may aid in the development of prevention and/or treatment strategies for the disease.

Hormone-sensing cells require Wip1 for paracrine stimulation in normal and premalignant mammary epithelium

INTRODUCTION

The molecular circuitry of different cell types dictates their normal function as well as their response to oncogene activation. For instance, mice lacking the Wip1 phosphatase (also known as PPM1D; protein phosphatase magnesium-dependent 1D) have a delay in HER2/neu (human epidermal growth factor 2), but not Wnt1-induced mammary tumor formation. This suggests a cell type-specific reliance on Wip1 for tumorigenesis, because alveolar progenitor cells are the likely target for transformation in the MMTV(mouse mammary tumor virus)-neu but not MMTV-wnt1 breast cancer model.

METHODS

In this study, we used the Wip1-knockout mouse to identify the cell types that are dependent on Wip1 expression and therefore may be involved in the early stages of HER2/neu-induced tumorigenesis.

RESULTS

We found that alveolar development during pregnancy was reduced in Wip1-knockout mice; however, this was not attributable to changes in alveolar cells themselves. Unexpectedly, Wip1 allows steroid hormone-receptor-positive cells but not alveolar progenitors to activate STAT5 (signal transducer and activator of transcription 5) in the virgin state. In the absence of Wip1, hormone-receptor-positive cells have significantly reduced transcription of RANKL (receptor activator of nuclear factor kappa-B ligand) and IGF2 (insulin-like growth factor 2), paracrine stimulators of alveolar development. In the MMTV-neu model, HER2/neu activates STAT5 in alveolar progenitor cells independent of Wip1, but HER2/neu does not override the defect in STAT5 activation in Wip1-deficient hormone-sensing cells, and paracrine stimulation remains attenuated. Moreover, ERK (extracellular signal-regulated kinase) activation by HER2/neu in hormone-sensing cells is also Wip1 dependent.

CONCLUSIONS

We identified Wip1 as a potentiator of prolactin and HER2/neu signaling strictly in the molecular context of hormone-sensing cells. Furthermore, our findings highlight that hormone-sensing cells convert not only estrogen and progesterone but also prolactin signals into paracrine instructions for mammary gland development. The instructive role of hormone-sensing cells in premalignant development suggests targeting Wip1 or prolactin signaling as an orthogonal strategy for inhibiting breast cancer development or relapse.

The biology of progesterone receptor in the normal mammary gland and in breast cancer

This paper reviews work on progesterone and the progesterone receptor (PR) in the mouse mammary gland that has been used extensively as an experimental model. Studies have led to the concept that progesterone controls proliferation and morphogenesis of the luminal epithelium in a tightly orchestrated manner at distinct stages of development by paracrine signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL) as a major paracrine factor. Progesterone also drives expansion of stem cells by paracrine signals to generate progenitors required for alveologenesis. During mid-to-late pregnancy, progesterone has another role to suppress secretory activation until parturition mediated in part by crosstalk between PR and prolactin/Stat5 signaling to inhibit induction of milk protein gene expression, and by inhibiting tight junction closure. In models of hormone-dependent mouse mammary tumors, the progesterone/PR signaling axis enhances pre-neoplastic progression by a switch from a paracrine to an autocrine mode of proliferation and dysregulation of the RANKL signaling pathway. Limited experiments with normal human breast show that progesterone/PR signaling also stimulates epithelial cell proliferation by a paracrine mechanism; however, the signaling pathways and whether RANKL is a major mediator remains unknown. Work with human breast cancer cell lines, patient tumor samples and clinical studies indicates that progesterone is a risk factor for breast cancer and that alteration in progesterone/PR signaling pathways contributes to early stage human breast cancer progression. However, loss of PR expression in primary tumors is associated with a less differentiated more invasive phenotype and worse prognosis, suggesting that PR may limit later stages of tumor progression.

Histone demethylation and steroid receptor function in cancer

Steroid receptors recruit various cofactors to form multi-protein complexes which locally alter chromatin structure and control DNA accessibility in order to regulate gene transcription. Some of these factors are enzymes that add or remove histone marks in the vicinity of regulatory regions of target genes. Numerous histone modifications added by specific writer enzymes and removed by eraser enzymes have been identified. Histone methylation is a modification with a complex outcome, as it can lead to gene activation or repression, depending on the modified residue and the context. Methylation marks are added by different enzyme families displaying exquisite substrate specificity. Lysine methylation is reversible and two different demethylase families have been identified in humans, the Jumonji C and the lysine-specific demethylase families. A regulatory role of histone demethylases in fine-tuning the function of steroid receptors, especially the androgen receptor and estrogen receptor, has emerged in recent years. This is mostly inferred from in vitro studies, but more recently first in vivo data have further supported this concept. This and the deregulated expression observed for several histone demethylases suggest a role in tumours such as prostate and breast cancer.

The chromatin landscape of the casein gene locus

For several decades, the regulation of casein gene expression by the lactogenic hormones, prolactin and glucocorticoids, has provided an excellent model system in which to study how steroid and peptide hormones regulate gene expression. Early studies of casein gene regulation defined conserved sequence elements in the 5' flanking region of these genes, including one of which was identified as a γ-interferon activation sequence (GAS). Although this site was thought to interact with a mammary gland-specific factor, purification and cloning of this factor by Bernd Groner and his colleagues revealed it was instead a new member of the signal transducers and activators of transcription family, Stat5, which was expressed in many tissues. The exquisite tissue-specific expression of the casein genes was subsequently shown to depend not on a single transcription factor but on composite response elements that interacted with a number of ubiquitous transcription factors in response to the combinatorial effects of peptide and steroid hormone signaling. More recent studies have defined cooperative effects of prolactin and glucocorticoids as well as antagonistic effects of progesterone on the chromatin structure of both the casein gene proximal promoter region as well as a distal enhancer. Local chromatin modifications as well as long-range interactions facilitated by DNA looping are required for the hormonal regulation of β-casein gene expression. The casein genes are part of a large gene cluster, and the chromatin landscape of the entire cluster is regulated in a tissue-specific and developmental manner. Finally, newly discovered large non coding RNAs, such as the pregnancy-induced non coding RNA (PINC) may play an important role in the epigenetic regulation of mammary gland differentiation.