A paracrine role for the epithelial progesterone receptor in mammary gland development

ER and PR signaling nodes during mammary gland development

The ovarian hormones estrogen and progesterone orchestrate postnatal mammary gland development and are implicated in breast cancer. Most of our understanding of the molecular mechanisms of estrogen receptor (ER) and progesterone receptor (PR) signaling stems from in vitro studies with hormone receptor-positive cell lines. They have shown that ER and PR regulate gene transcription either by binding to DNA response elements directly or via other transcription factors and recruiting co-regulators. In addition they cross-talk with other signaling pathways through nongenomic mechanisms. Mouse genetics combined with tissue recombination techniques have provided insights about the action of these two hormones in vivo. It has emerged that hormones act on a subset of mammary epithelial cells and relegate biological functions to paracrine factors. With regards to hormonal signaling in breast carcinomas, global gene expression analyses have led to the identification of gene expression signatures that are characteristic of ERα-positive tumors that have stipulated functional studies of hitherto poorly understood transcription factors. Here, we highlight what has been learned about ER and PR signaling nodes in these different systems and attempt to lay out in which way the insights may converge.

Reprogramming non-mammary and cancer cells in the developing mouse mammary gland

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the "niche" and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells, including mouse and human cancer cells, may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.

Interplay between progesterone and prolactin in mammary development and implications for breast cancer

Progesterone and prolactin remodel mammary morphology during pregnancy by acting on the mammary epithelial cell hierarchy. The roles of each hormone in mammary development have been well studied, but evidence of signalling cross-talk between progesterone and prolactin is still emerging. Factors such as receptor activator of NFkB ligand (RANKL) may integrate signals from both hormones to orchestrate their joint actions on the epithelial cell hierarchy. Common targets of progesterone and prolactin signalling are also likely to integrate their pro-proliferative actions in breast cancer. Therefore, a thorough understanding of the interplay between progesterone and prolactin in mammary development may reveal therapeutic targets for breast cancer. This review summarises our understanding of Pg and PRL action in mammary gland development before focusing on molecular mechanisms of signalling cross-talk and the implications for breast cancer.

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.

Paracrine signaling by progesterone

Steroid hormones coordinate and control the development and function of many organs and are implicated in many pathological processes. Progesterone signaling, in particular, is essential for several important female reproductive functions. Physiological effects of progesterone are mediated by its cognate receptor, expressed in a subset of cells in target tissues. Experimental evidence has accumulated that progesterone acts through both cell intrinsic as well as paracrine signaling mechanisms. By relegating the hormonal stimulus to paracrine signaling cascades the systemic signal gets amplified locally and signaling reaches different cell types that are devoid of hormone receptors. Interestingly, distinct biological responses to progesterone in different target tissues rely on several tissue-specific and some common paracrine factors that coordinate biological responses in different cell types. Evidence is forthcoming that the intercellular signaling pathways that control development and physiological functions are important in tumorigenesis.

Progesterone regulation of stem and progenitor cells in normal and malignant breast

Progesterone plays an important, if not controversial, role in mammary epithelial cell proliferation and differentiation. Evidence supports that progesterone promotes rodent mammary carcinogenesis under some conditions, progesterone receptors (PR) are necessary for murine mammary gland tumorigenesis, and exogenous progestin use in post-menopausal women increases breast cancer risk. Thus, the progesterone/PR signaling axis can promote mammary tumorigenesis, albeit in a context-dependent manner. A mechanistic basis for the tumor promoting actions of progesterone has thus far remained unknown. Recent studies, however, have identified a novel role for progesterone in controlling the number and function of stem and progenitor cell populations in the normal human and mouse mammary glands, and in human breast cancers. These discoveries promise to reshape our perception of progesterone function in the mammary gland, and have spawned new hypotheses for how progestins may increase the risk of breast cancer. Here we review studies on progesterone regulation of mammary stem cells in normal and malignant tissue, and their implications for breast cancer risk, tumorigenesis, and tumor behavior.

From the ranks of mammary progesterone mediators, RANKL takes the spotlight

Whether during the diestrus phase of the estrous cycle or with pregnancy onset, the mitogenic effects of progesterone are well-established in the murine mammary epithelium. Importantly, progesterone-induced mitogenicity is critical for mammary tumor promotion, providing one explanation for the increase in breast cancer-risk observed with prolonged progestin-based hormone therapy. At the cellular level, progesterone projects its mitogenic influence through an evolutionary conserved paracrine mechanism of action. In this regard, recent studies provide compelling support for receptor activator of NF-kB ligand (RANKL) as a key paracrine mediator of the progesterone mitogenic signal. Induction of RANKL is sufficient to elicit mammary ductal side-branching and alveologenesis, the very morphogenetic responses elicited by progesterone during pregnancy and at diestrus. Significantly, the proliferative and pro-survival signals triggered by RANKL are also required for progestin-promotion of mammary tumorigenesis, underscoring a dual role for RANKL in progesterone-dependent mammary morphogenesis and tumorigenesis. Recently, RANKL has been shown to be critical for progesterone-induced expansion of the mammary stem cell population (and its lineal descendents), thereby advancing our conceptual understanding not only of RANKL's involvement in normal mammary morphogenesis but also in breast cancer risk associated with sustained hormone exposure. Finally, these studies together suggest that chemotherapeutic intervention of RANKL signaling represents a feasible approach for the effective prevention and/or treatment of hormone-responsive breast cancers.

Intrinsic cues and hormones control mouse mammary epithelial tree size

Organ size control is a long-standing question in biology. In mammals, using conditional cell ablation, two mutually exclusive mechanisms involving either intrinsic or extrinsic programs have been described to control organ size. The mammary gland is an ideal model for such studies, since it undergoes size and morphological changes during puberty and pregnancy. The role of stem cells in controlling mammary epithelial tree size is unclear, although mammary stem cells are able to reconstitute a functional organ on transplantation. Here, we show that mammary gland cellularity was strictly dependent on mammary stem cell number, even following a 20-fold expansion of the mammary stem cell pool at puberty and transient 3-fold expansions with each pregnancy. In addition, the expansion of the mammary stem cell pool was hormone dependent, as demonstrated by female bilateral ovariectomies during puberty and transplants of male-derived cells into female recipients. In these transplants, apart from a mammary stem cell expansion, we also observed the donor cells reconstituting functional mammary glands, developing alveolar structures, and secreting milk after the recipient's parturition. Taken together, these data suggest that in the mammary gland, there is a third organ size control mechanism, combining intrinsic cues throughout the organism's lifetime, with extrinsic hormone signals at particular developmental windows (puberty, pregnancy), where an expansion of the mammary stem cell pool occurs. This mechanism might have strong implications for the understanding of mammary tumorigenesis, since the expansion of the mammary stem cell pool precedes the generation of breast tumors.

Elf5, hormones and cell fate

Recent elucidation of the stem and progenitor cell hierarchies that operate during normal tissue and organ development has provided a foundation for the development of new insights into the disease process. These hierarchies are established by genetic mechanisms, which specify and determine cell fate and act as cell-clade gatekeepers, upon which all multicellular organisms depend for viability. Perturbation of this gatekeeper function characterizes developmentally based diseases, such as cancer. Here, the emerging gatekeeper and master regulatory roles of the ETS transcription factor Elf5 in several diverse developmental scenarios is reviewed, and how this function intersects with hormonal and growth factor mediated regulation of these processes is shown.

The biology of human breast epithelial progenitors

Current evidence suggests that similar to other tissues in the human body mammary epithelia cells are being maintained by the unique properties of stem cells, undifferentiated as well as lineage-restricted progenitors. Because of their longevity, proliferation and differentiation potentials these primitive breast epithelial cells are likely targets of transforming mutations that can cause them to act as cancer initiating cells. In this context, understanding the molecular mechanisms that regulate the normal functions of the human breast epithelial stem cells and progenitors and how alterations to these same mechanisms can confer a cancer stem cell phenotype on these rare cell populations is crucial to the development of new and more effective therapies again breast cancer. This review article will examine the current state of knowledge about the isolation and characterization of human breast epithelial progenitors and their relevance to breast cancer research.

Accumulation of multipotent progenitors with a basal differentiation bias during aging of human mammary epithelia

Women older than 50 years account for 75% of new breast cancer diagnoses, and the majority of these tumors are of a luminal subtype. Although age-associated changes, including endocrine profiles and alterations within the breast microenvironment, increase cancer risk, an understanding of the cellular and molecular mechanisms that underlies these observations is lacking. In this study, we generated a large collection of normal human mammary epithelial cell strains from women ages 16 to 91 years, derived from primary tissues, to investigate the molecular changes that occur in aging breast cells. We found that in finite lifespan cultured and uncultured epithelial cells, aging is associated with a reduction of myoepithelial cells and an increase in luminal cells that express keratin 14 and integrin-α6, a phenotype that is usually expressed exclusively in myoepithelial cells in women younger than 30 years. Changes to the luminal lineage resulted from age-dependent expansion of defective multipotent progenitors that gave rise to incompletely differentiated luminal or myoepithelial cells. The aging process therefore results in both a shift in the balance of luminal/myoepithelial lineages and to changes in the functional spectrum of multipotent progenitors, which together increase the potential for malignant transformation. Together, our findings provide a cellular basis to explain the observed vulnerability to breast cancer that increases with age.

FOXA1: a transcription factor with parallel functions in development and cancer

When aberrant, factors critical for organ morphogenesis are also commonly involved in disease progression. FOXA1 (forkhead box A1), also known as HNF3α (hepatocyte nuclear factor 3α), is required for postnatal survival due to its essential role in controlling pancreatic and renal function. In addition to regulating a variety of tissues during embryogenesis and early life, rescue experiments have revealed a specific role for FOXA1 in the postnatal development of the mammary gland and prostate. Activity of the nuclear hormone receptors ERα (oestrogen receptor α) and AR (androgen receptor) is also required for proper development of the mammary gland and prostate respectively. FOXA1 modulates ER and AR function in breast and prostate cancer cells, supporting the postulate that FOXA1 is involved in ER and AR signalling under normal conditions, and that some carcinogenic processes in these tissues stem from hormonally regulated developmental pathways gone awry. In addition to broadly reviewing the function of FOXA1 in various aspects of development and cancer, this review focuses on the interplay of FOXA1/ER and FOXA1/AR, in normal and cancerous mammary and prostate epithelial cells. Given the hormone dependency of both breast and prostate cancer, a thorough understanding of FOXA1's role in both cancer types is critical for battling hormone receptor-positive disease and acquired anti-hormone resistance.

On the shoulders of giants: a historical perspective of unique experimental methods in mammary gland research

While most organs undergo development in utero, the mouse mammary gland orchestrates five major developmental stages following birth: pre-puberty, puberty, pregnancy, lactation, and involution. Induced by both local and systemic factors, these five developmental stages transpire with dramatic alterations in glandular morphology and cellular function. As an experimental system, the mammary gland provides remarkable accessibility to processes regulating stem cell function, hormone response, and epithelial-stromal-extracellular matrix interactions. This review will provide a historical perspective of the unique in vitro and in vivo techniques used to study the mammary gland and how these methods have provided valuable insight into the biology of this organ.

Mammary adipocytes bioactivate 25-hydroxyvitamin D₃ and signal via vitamin D₃ receptor, modulating mammary epithelial cell growth

The vitamin D(3) receptor (VDR) is present in all microenvironments of the breast, yet it is hypothesized to signal through the epithelium to regulate hormone induced growth and differentiation. However, the influence or contribution of the other microenvironments within the breast that express VDR, like the breast adipose tissue, are yet to be investigated. We hypothesized that the breast adipocytes express the signaling components necessary to participate in vitamin D(3) synthesis and signaling via VDR, modulating ductal epithelial cell growth and differentiation. We utilized human primary breast adipocytes and VDR wild type (WT) and knockout (KO) mice to address whether breast adipocytes participate in vitamin D(3) -induced growth regulation of the ductal epithelium. We report in this study that breast primary adipocytes express VDR, CYP27B1 (1α-hydroxylase, 1α-OHase), the enzyme that generates the biologically active VDR ligand, 1α,25-dihydroxyvitamin D(3) (1,25D(3) ), and CYP24 (24-hydroxylase, 24-OHase), a VDR-1,25D(3) induced target gene. Furthermore, the breast adipocytes participate in bioactivating 25-hydroxyvitamin D(3) (25D(3) ) to the active ligand, 1,25D(3) , and secreting it to the surrounding microenvironment. In support of this concept, we report that purified mammary ductal epithelial fragments (organoids) from VDR KO mice, co-cultured with WT breast adipocytes, were growth inhibited upon treatment with 25D(3) or 1,25D(3) compared to vehicle alone. Collectively, these results demonstrate that breast adipocytes bioactivate 25D(3) to 1,25D(3) , signal via VDR within the adipocytes, and release an inhibitory factor that regulates ductal epithelial cell growth, suggesting that breast adipose tissue contributes to vitamin D(3) -induced growth regulation of ductal epithelium.

Notch-induced mammary tumorigenesis does not involve the lobule-limited epithelial progenitor

The mouse mammary epithelial cell hierarchy contains both multipotent stem cell as well as lineage-limited duct and lobular progenitor cell functions. The latter-also termed parity-identified mammary epithelial cells (PI-MECs)-are marked by beta-galactosidase (β Gal) expression following pregnancy and involution in whey acidic protein promoter (WAP)-Cre/Rosa26-flox-stop-flox-lacZ (WC/R26) mice, and are the targets of tumorigenic transformation in mouse mammary tumor virus-erbB2 transgenic mice. In this study, we demonstrate that an epithelial population distinct from PI-MECs is transformed during WAP-Int3 tumorigenesis. As expected, WAP-Int3/WC/R26 triple-transgenic mice failed to undergo secretory alveolar development, failed to lactate and developed mammary tumors. Following pregnancy and involution, β Gal+ mammary epithelial cells were found in the normal mammary tissue, but the resulting mammary tumors were all β Gal-. WAP-Int3/WC/R26 mammary glands contained ample estrogen receptor alpha (ERα)+ MECs, but only rare (<1%) progesterone receptor (PR)+ and RANKL+ cells. In addition, dissociated MECs from WAP-Int3/WC/R26 glands failed to regenerate a mammary tree upon transplantation into a cleared fat-pad of a nu/nu recipient mouse. However, when mixed with normal MECs, PI-MECs from WAP-Int3/WC/R26 mice contributed progeny to the resulting functional outgrowth. The WAP-Int3/WC/R26-derived PI-MECs displayed all of the properties of fully functional lobular progenitors including giving rise to ERα+, PR+, smooth muscle actin+ and RANKL+ epithelial progeny. These results demonstrate that WAP-Int3 has no oncogenic effect upon PI-MECs and that the expansion of functional lobular progenitors is required for secretory alveolar development and lactation. Furthermore, lobular progenitor function is ultimately controlled by signals within its microenvironment.

Perinatal exposure to bisphenol a increases adult mammary gland progesterone response and cell number

Bisphenol A [BPA, 2,2,-bis (hydroxyphenyl) propane] is one of the highest-volume chemicals produced worldwide. It is detected in body fluids of more than 90% of the human population. Originally synthesized as an estrogenic compound, it is currently utilized to manufacture food and beverage containers resulting in uptake with food and drinks. There is concern that exposure to low doses of BPA, defined as less than or equal to 5 mg/kg body weight /d, may have developmental effects on various hormone-responsive organs including the mammary gland. Here, we asked whether perinatal exposure to a range of low doses of BPA is sufficient to alter mammary gland hormone response later on in life, with a possible impact on breast cancer risk. To mimic human exposure, we added BPA to the drinking water of C57/Bl6 breeding pairs. Analysis of the mammary glands of their daughters at puberty showed that estrogen-dependent transcriptional events were perturbed and the number of terminal end buds, estrogen-induced proliferative structures, was altered in a dose-dependent fashion. Importantly, adult females showed an increase in mammary epithelial cell numbers comparable to that seen in females exposed to diethylbestrol, a compound exposure to which was previously linked to increased breast cancer risk. Molecularly, the mRNAs encoding Wnt-4 and receptor activator of nuclear factor κB ligand, two key mediators of hormone function implicated in control of mammary stem cell proliferation and carcinogenesis, showed increased induction by progesterone in the mammary tissue of exposed mice. Thus, perinatal exposure to environmentally relevant doses of BPA alters long-term hormone response that may increase the propensity to develop breast cancer.

Functional characterization of stem cell activity in the mouse mammary gland

Any portion of the mouse mammary gland is capable of recapitulating a clonally derived complete and functional mammary tree upon transplantation into an epithelial divested mammary fat-pad of a recipient host. As such, it is an ideal model tissue for the study somatic stem cell function. This review will outline what is known regarding the function of stem/progenitor cells in the mouse mammary gland, including how progenitor populations can be functionally defined, the evidence for and potential role of selective DNA strand segregation, and the role of the niche in maintaining and controlling stem cell function.

Wnt5a as an effector of TGFβ in mammary development and cancer

Wnt5a is a member of the Wingless-related/MMTV-integration family of secreted growth factors, which are involved in a wide range of cellular processes. Wnt signaling can be broadly divided into two categories the canonical, ß-catenin-dependent pathway and the non-canonical ß-catenin-independent pathway. Wnt5a is a non-canonical signaling member of the Wnt family. Loss of Wnt5a is associated with early relapse of invasive breast cancer, increased metastasis, and poor survival in humans. It has been shown that TGF-ß directly regulates expression of Wnt5a in mammary gland and that Wnt5a mediates the effects of TGF-ß on branching during mammary gland development. Here we review the evidence suggesting Wnt5a acts as an effector of TGF-ß actions in breast cancer. It is suggested that the tumor suppressive functions of TGF-ß involve Wnt5a-mediated antagonism of Wnt/ß-catenin signaling and limiting the stem cell population. Interactions between TGF-ß and Wnt5a in metastasis appear to be more complex, and may depend on specific cues from the microenvironment as well as activation of specific intracellular signaling pathways.

The mammary gland microenvironment directs progenitor cell fate in vivo

The mammary gland is a unique organ that continually undergoes postnatal developmental changes. In mice, the mammary gland is formed via signals from terminal end buds, which direct ductal growth and elongation. Intriguingly, it is likely that the entire cellular repertoire of the mammary gland is formed from a single antecedent cell. Furthermore, in order to produce progeny of varied lineages (e.g., luminal and myoepithelial cells), signals from the local tissue microenvironment influence mammary stem/progenitor cell fate. Data have shown that cells from the mammary gland microenvironment reprogram adult somatic cells from other organs (testes, nerve) into cells that produce milk and express mammary epithelial cell proteins. Similar results were found for human tumorigenic epithelial carcinoma cells. Presently, it is unclear how the deterministic power of the mammary gland microenvironment controls epithelial cell fate. Regardless, signals generated by the microenvironment have a profound influence on progenitor cell differentiation in vivo.

The induction of KLF5 transcription factor by progesterone contributes to progesterone-induced breast cancer cell proliferation and dedifferentiation

Progesterone (Pg) promotes normal breast development during pregnancy and lactation and increases the risk of developing basal-type invasive breast cancer. However, the mechanism of action of Pg has not been fully understood. In this study, we demonstrate that the mRNA and protein expression of Klf5, a pro-proliferation transcription factor in breast cancer, was dramatically up-regulated in mouse pregnant and lactating mammary glands. Pg, but not estrogen and prolactin, induced the expression of Krüpple-like factor 5 (KLF5) in multiple Pg receptor (PR)-positive breast cancer cell lines. Pg induced the KLF5 transcription through PR in the PR-positive T47D breast cancer cells. Pg-activated PR increased the KLF5 promoter activity likely through binding to a Pg response element at the KLF5 promoter. Importantly, Pg failed to promote T47D cell proliferation when the KLF5 induction was blocked by small interfering RNA. KLF5 is essential for Pg to up-regulate the expression of cell cycle genes, including CyclinA, Cdt1, and E2F3. In addition, KLF5 overexpression was sufficient to induce the cytokeratin 5 (CK5) expression, and the induction of CK5 by Pg was significantly reduced by KLF5 small interfering RNA. Consistently, the expression of KLF5 was positively correlated with that of CK5 in a panel of breast cancer cell lines. Taken together, we conclude that KLF5 is a Pg-induced gene that contributes to Pg-mediated breast epithelial cell proliferation and dedifferentiation.

Mammary stem cells and their regulation by steroid hormones

Sustained exposure to estrogen and progesterone is a well-established risk factor for breast cancer. These hormones play a central role in the female reproductive cycle, in which they control morphogenesis of the mammary gland during puberty, ovulatory cycles and pregnancy. Mouse mammary stem cells (MaSCs) have recently been discovered to be highly responsive to female hormones, despite lacking expression of the estrogen and progesterone receptors. The inhibition of MaSCs by hormone receptor antagonists further suggests that these cells contribute to oncogenesis. Identification of paracrine mediators of hormone signaling to MaSCs may lead to the development of novel inhibitors that drive MaSCs into a more quiescent state. In this context, inhibition of the receptor activator of NF-κB/receptor activator of NF-κB ligand signaling pathway has profound implications for the prevention of breast cancer.

Molecular regulators of pubertal mammary gland development

The pubertal mammary gland is an ideal model for experimental morphogenesis. The primary glandular branching morphogenesis occurs at this time, integrating epithelial cell proliferation, differentiation, and apoptosis. Between birth and puberty, the mammary gland exists in a relatively quiescent state. At the onset of puberty, rapid expansion of a pre-existing rudimentary mammary epithelium generates an extensive ductal network by a process of branch initiation, elongation, and invasion of the mammary mesenchyme. It is this branching morphogenesis that characterizes pubertal mammary gland growth. Tissue-specific molecular networks interpret signals from local cytokines/growth factors in both the epithelial and stromal microenvironments. This is largely orchestrated by secreted ovarian and pituitary hormones. Here, we review the major molecular regulators of pubertal mammary gland development.

Progesterone receptor action: defining a role in breast cancer

The ovarian steroid hormones, estradiol and progesterone, and their nuclear receptors (estrogen receptor [ER] and progesterone receptor [PR]), are involved in breast cancer development. As ER-positive/PR-positive tumors progress, they are likely to become steroid hormone-resistant/independent, yet often retain expression of their steroid receptors. Notably, up to 40% of women with steroid receptor-positive tumors exhibit de novo resistance or eventually fail on estrogen- or ERα-blocking therapies (acquired resistance). Indeed, most of the research on this topic has centered on mechanisms of ER 'escape' from endocrine therapy and the design of better ER-blocking strategies; signaling pathways that mediate endocrine (i.e., anti-estrogen) resistance are also excellent therapeutic targets. However, serious consideration of PR isoforms as important drivers of early breast cancer progression and ER modulators is timely and significant. Indeed, progress has been hindered by ER-centric experimental approaches. This article will focus on defining a role for PR in breast cancer with hopes of providing a refreshing PR-focused perspective.

ck2-dependent phosphorylation of progesterone receptors (PR) on Ser81 regulates PR-B isoform-specific target gene expression in breast cancer cells

Progesterone receptors (PR) are critical mediators of mammary gland development and contribute to breast cancer progression. Progestin-induced rapid activation of cytoplasmic protein kinases leads to selective regulation of growth-promoting genes by phospho-PR species. Herein, we show that phosphorylation of PR Ser81 is ck2 dependent and progestin regulated in intact cells but also occurs in the absence of PR ligands when cells enter the G(1)/S phase of the cell cycle. T47D breast cancer cells stably expressing a PR-B mutant receptor that cannot be phosphorylated at Ser79/81 (S79/81A) formed fewer soft agar colonies. Regulation of selected genes by PR-B, but not PR-A, also required Ser79/81 phosphorylation for basal and/or progestin-regulated (BIRC3, HSD11β2, and HbEGF) expression. Additionally, wild-type (wt) PR-B, but not S79/81A mutant PR, was robustly recruited to a progesterone response element (PRE)-containing transcriptional enhancer region of BIRC3; abundant ck2 also associated with this region in cells expressing wt but not S79/81A PR. We conclude that phospho-Ser81 PR provides a platform for ck2 recruitment and regulation of selected PR-B target genes. Understanding how ligand-independent PRs function in the context of high levels of kinase activities characteristic of breast cancer is critical to understanding the basis of tumor-specific changes in gene expression and will speed the development of highly selective treatments.

Mammary gland reprogramming: metalloproteinases couple form with function

The adult mammary structure provides for the rapid growth, development, and immunological protection of the live-born young of mammals through its production of milk. The dynamic remodeling of the branched epithelial structure of the mammary gland in response to physiological stimuli that allow its programmed branching morphogenesis at puberty, cyclical turnover during the reproductive cycle, differentiation into a secretory organ at parturition, postlactational involution, and ultimately, regression with age is critical for these processes. Extracellular metalloproteinases are essential for the remodeling programs that operate in the tissue microenvironment at the interface of the epithelium and the stroma, coupling form with function. Deregulated proteolytic activity drives the transition of a physiological mammary microenvironment into a tumor microenvironment, facilitating malignant transformation.

Breast cancer stem cells and their role in resistance to endocrine therapy

Developmentally, tumours can be viewed as aberrant versions of normal tissues. For example, tumours often retain differentiation markers of their tissue of origin. In addition, there is evidence that they contain cancer stem-like cells (CSCs) that drive tumourigenesis. In this review, we summarise current evidence that breast CSCs may partially explain endocrine resistance in breast cancer. In normal breast, the stem cells are known to possess a basal phenotype and to be mainly oestrogen receptor-α-negative (ER-). If the hierarchy in breast cancer reflects this, the breast CSC may be endocrine resistant because it expresses very little ER and can only respond to treatment by virtue of paracrine signalling from neighbouring, differentiated ER+ tumour cells. Normal breast epithelial stem cells are regulated by the epidermal growth factor receptor and other growth factor receptor signals. The observed increase in growth factor receptor expression in endocrine-resistant breast cancers may reflect a bigger proportion of CSCs selected by endocrine therapies. There is evidence from a number of studies that breast CSCs are ER- and EGR+/HER2+, which would support this view. It is reported that CSCs express mesenchymal genes, which are suppressed by ER expression, further indicating the mutual exclusion between ER+ cells and the CSCs. As we learn more about CSCs, differentiation and the expression and functional activity of the ER in these cells in diverse breast tumour sub-types, it is hoped that our understanding will lead to new modalities to overcome the problem of endocrine resistance in the clinic.

The normal mammary microenvironment suppresses the tumorigenic phenotype of mouse mammary tumor virus-neu-transformed mammary tumor cells

The microenvironment of the mammary gland has been shown to exert a deterministic control over cells from different normal organs during murine mammary gland regeneration in transplantation studies. When mouse mammary tumor virus (MMTV)-neu-induced tumor cells were mixed with normal mammary epithelial cells (MECs) in a dilution series and inoculated into epithelium-free mammary fat pads, they were redirected to non-carcinogenic cell fates by interaction with untransformed MECs during regenerative growth. In the presence of non-transformed MECs (50:1), tumor cells interacted with MECs to generate functional chimeric outgrowths. When injected alone, tumor cells invariably produced tumors. Here, the normal microenvironment redirects MMTV-neu-transformed tumorigenic cells to participate in the regeneration of a normal, functional mammary gland. In addition, the redirected tumor cells show the capacity to differentiate into normal mammary cell types, including luminal, myoepithelial and secretory. The results indicate that signals emanating from a normal mammary microenvironment, comprised of stromal, epithelial and host-mediated signals, combine to suppress the cancer phenotype during glandular regeneration. Clarification of these signals offers improved therapeutic possibilities for the control of mammary cancer growth.

Murine mammary epithelial stem cells: discovery, function, and current status

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may comprise the progeny of a single cellular antecedent, i.e., may be generated from a single mammary epithelial stem cell. Early studies showed that any portion of an intact murine mammary gland containing epithelium could recapitulate an entire mammary epithelial tree on transplantation into an epithelium-free mammary fat pad. More recent studies have shown that a hierarchy of mammary stem/progenitor cells exists among the mammary epithelium and that their behavior and maintenance is dependent on signals generated both locally and systemically. In this review, we have attempted to develop the scientific saga surrounding the discovery and characterization of the murine mammary stem/progenitor cell hierarchy and to suggest further approaches that will enhance our knowledge and understanding of these cells and their role in both normal development and neoplasia.

From milk to malignancy: the role of mammary stem cells in development, pregnancy and breast cancer

Adult stem cells of the mammary gland (MaSCs) are a highly dynamic population of cells that are responsible for the generation of the gland during puberty and its expansion during pregnancy. In recent years significant advances have been made in understanding how these cells are regulated during these developmentally important processes both in humans and in mice. Understanding how MaSCs are regulated is becoming a particularly important area of research, given that they may be particularly susceptible targets for transformation in breast cancer. Here, we summarize the identification of MaSCs, how they are regulated and the evidence for their serving as the origins of breast cancer. In particular, we focus on how changes in MaSC populations may explain both the increased risk of developing aggressive ER/PR(-) breast cancer shortly after pregnancy and the long-term decreased risk of developing ER/PR(+) tumors.

TGF-beta biology in mammary development and breast cancer

Transforming growth factor-β1 (TGF-β) was first implicated in mammary epithelial development by Daniel and Silberstein in 1987 and in breast cancer cells and hormone resistance by Lippman and colleagues in 1988. TGF-β is critically important for mammary morphogenesis and secretory function through specific regulation of epithelial proliferation, apoptosis, and extracellular matrix. Differential TGF-β effects on distinct cell types are compounded by regulation at multiple levels and the influence of context on cellular responses. Studies using controlled expression and conditional-deletion mouse models underscore the complexity of TGF-β biology across the cycle of mammary development and differentiation. Early loss of TGF-β growth regulation in breast cancer evolves into fundamental deregulation that mediates cell interactions and phenotypes driving invasive disease. Two outstanding issues are to understand the mechanisms of biological control in situ and the circumstances by which TGF-β regulation is subverted in neoplastic progression.

The role of estrogen receptors, erbB receptors, vascular endothelial growth factor and its receptors, and vascular endothelial growth inhibitor in the development of the rat mammary gland

We identified the localization and distribution of cell-specific epidermal growth factor receptors (EGFRs: erbB-1, erbB-2, erbB-3, erbB-4), vascular endothelial growth factor (VEGF), VEGF receptors [VEGFRs: VEGF-R1 (flt-1), VEGF-R2 (flk-1/KDR), VEGF-R3 (flt-4)], vascular endothelial growth inhibitor (VEGI), and estrogen receptor (ER), and determined whether or not these growth factors in rat mammary glands are functional. Thirty-five adult female Spraque-Dawley rats were randomly divided into five groups, each of which were at the 7th, 14th, and 21st day of pregnancy; 7th day post-delivery; and 7th day after weaning. It was determined that erbB, VEGF and its receptors, VEGI, and ER stained at different intensities. Intense staining was observed, in particular, in erbB receptors during pregnancy and involution, and also in VEGF and its receptors during lactation, while ER stained during the last periods of pregnancy and lactation. In conclusion, the expression of erbB, VEGF and its receptors, and ER were determined at varying intensities at different sites of the mammary gland during pregnancy, lactation, and involution periods.

Wnt and mammary stem cells: hormones cannot fly wingless

The mammary stem cell and its local microenvironment are central for the maintenance of proper tissue homeostasis during normal development. Defining the hierarchical organization of the epithelial subtypes in the mammary gland and the molecular pathways guiding their development has begun to provide a framework for understanding how cancer stem cells sustain the progression and heterogeneity of breast cancers. The Wnt pathway plays a fundamental role in multiple adult stem cells, as well as in orchestrating proper mammary gland development and maintenance. These processes are intricately guided by the influence of systemic hormones and local factors. Alterations in Wnt signaling can skew the homeostatic balance of the mammary epithelium to drive malignant progression; however, complexities of Wnt pathway components present a challenge in understanding their physiological function.

Key signaling nodes in mammary gland development and cancer: β-catenin

β-Catenin plays important roles in mammary development and tumorigenesis through its functions in cell adhesion, signal transduction and regulation of cell-context-specific gene expression. Studies in mice have highlighted the critical role of β-catenin signaling for stem cell biology at multiple stages of mammary development. Deregulated β-catenin signaling disturbs stem and progenitor cell dynamics and induces mammary tumors in mice. Recent data showing deregulated β-catenin signaling in metaplastic and basal-type tumors suggest a similar link to reactivated developmental pathways and human breast cancer. The present review will discuss β-catenin as a central transducer of numerous signaling pathways and its role in mammary development and breast cancer.

RANK ligand mediates progestin-induced mammary epithelial proliferation and carcinogenesis

RANK ligand (RANKL), a TNF-related molecule, is essential for osteoclast formation, function and survival through interaction with its receptor RANK. Mammary glands of RANK- and RANKL-deficient mice develop normally during sexual maturation, but fail to form lobuloalveolar structures during pregnancy because of defective proliferation and increased apoptosis of mammary epithelium. It has been shown that RANKL is responsible for the major proliferative response of mouse mammary epithelium to progesterone during mammary lactational morphogenesis, and in mouse models, manipulated to induce activation of the RANK/RANKL pathway in the absence of strict hormonal control, inappropriate mammary proliferation is observed. However, there is no evidence so far of a functional contribution of RANKL to tumorigenesis. Here we show that RANK and RANKL are expressed within normal, pre-malignant and neoplastic mammary epithelium, and using complementary gain-of-function (mouse mammary tumour virus (MMTV)-RANK transgenic mice) and loss-of function (pharmacological inhibition of RANKL) approaches, define a direct contribution of this pathway in mammary tumorigenesis. Accelerated pre-neoplasias and increased mammary tumour formation were observed in MMTV-RANK transgenic mice after multiparity or treatment with carcinogen and hormone (progesterone). Reciprocally, selective pharmacological inhibition of RANKL attenuated mammary tumour development not only in hormone- and carcinogen-treated MMTV-RANK and wild-type mice, but also in the MMTV-neu transgenic spontaneous tumour model. The reduction in tumorigenesis upon RANKL inhibition was preceded by a reduction in pre-neoplasias as well as rapid and sustained reductions in hormone- and carcinogen-induced mammary epithelial proliferation and cyclin D1 levels. Collectively, our results indicate that RANKL inhibition is acting directly on hormone-induced mammary epithelium at early stages in tumorigenesis, and the permissive contribution of progesterone to increased mammary cancer incidence is due to RANKL-dependent proliferative changes in the mammary epithelium. The current study highlights a potential role for RANKL inhibition in the management of proliferative breast disease.

The progesterone receptor hinge region regulates the kinetics of transcriptional responses through acetylation, phosphorylation, and nuclear retention

Progesterone receptors (PRs) are critical regulators of mammary gland development and contributors to breast cancer progression. Posttranslational modifications of PR have been shown to alter hormone responsiveness. Site-directed mutagenesis demonstrated that upon hormone binding, PR is acetylated at the consensus sequence, KXKK (amino acids 638-641), located within the hinge region. We created an acetylation-deficient (K-A) mutant as well as acetylation mimics (K-Q or K-T). Interestingly, similar to K-A PR, PR acetylation mimics (K-Q or K-T) displayed delayed phosphorylation and nuclear entry relative to wild-type (wt) PR-B, indicative of disruption of PR nuclear-cytoplasmic shuttling. Wt PR-B, but not K-mutant PRs, induced c-myc at 1 h of progestin treatment. However, at 6 h of treatment, c-myc induction was comparable with levels induced by wt PR-B, suggesting that the precise timing of PR phosphorylation and nuclear retention are critical for cells to rapidly initiate robust transcriptional programs. In contrast to c-myc, progestin-induced serum- and glucocorticoid-regulated kinase (SGK) expression displayed sensitivity to PR acetylation but not nuclear entry. Namely, in the presence of progestin, acetylation-deficient (K-A) mutant PR-B up-regulated SGK mRNA relative to wt PR; progesterone response element-luciferase assays confirmed this result. However, K-Q and K-T acetylation mimics only weakly induced SGK expression independently of nuclear retention. These data reveal the ability of PR acetylation to alter the magnitude of transcriptional response at selected (slow response) promoters (SGK), whereas the hinge region dictates the kinetics of the transcriptional response to hormone at other (rapid response) promoters (c-myc). In sum, the PR hinge region is multifunctional. Understanding the ability of this region to couple acetylation, phosphorylation, and nuclear entry may provide clues to mechanisms of altered hormone responsiveness.

Isoform-specific degradation of PR-B by E6-AP is critical for normal mammary gland development

E6-associated protein (E6-AP), which was originally identified as an ubiquitin-protein ligase, also functions as a coactivator of estrogen (ER-α) and progesterone (PR) receptors. To investigate the in vivo role of E6-AP in mammary gland development, we generated transgenic mouse lines that either overexpress wild-type (WT) human E6-AP (E6-AP(WT)) or ubiquitin-protein ligase-defective E6-AP (E6-AP(C833S)) in the mammary gland. Here we show that overexpression of E6-AP(WT) results in impaired mammary gland development. In contrast, overexpression of E6-AP(C833S) or loss of E6-AP (E6-AP(KO)) increases lateral branching and alveolus-like protuberances in the mammary gland. We also show that the mammary phenotypes observed in the E6-AP transgenic and knockout mice are due, in large part, to the alteration of PR-B protein levels. We also observed alteration in ER-α protein level, which might contribute to the observed mammary phenotype by regulating PR expression. Furthermore, E6-AP regulates PR-B protein levels via the ubiquitin-proteasome pathway. Additionally, we also show that E6-AP impairs progesterone-induced Wnt-4 expression by decreasing the steady state level of PR-B in both mice and in human breast cancer cells. In conclusion, we present the novel observation that E6-AP controls mammary gland development by regulating PR-B protein turnover via the ubiquitin proteasome pathway. For the first time, we show that the E3-ligase activity rather than the coactivation function of E6-AP plays an important role in the mammary gland development, and the ubiquitin-dependent PR-B degradation is not required for its transactivation functions. This mechanism appears to regulate normal mammogenesis, and dysregulation of this process may be an important contributor to mammary cancer development and progression.

The normal microenvironment directs mammary gland development

Normal development of the mammary gland is a multidimensional process that is controlled in part by its mammary microenvironment. The mammary microenvironment is a defined location that encompasses mammary somatic stem cells, neighboring signaling cells, the basement membrane and extracellular matrix, mammary fibroblasts as well as the intercellular signals produced and received by these cells. These dynamic signals take numerous forms including growth factors, steroids, cell-cell or cell-basement membrane physical interactions. Cellular growth and differentiation of the mammary gland throughout the developmental stages are regulated by changes in these signals and interactions. The purpose of this review is to summarize current information and research regarding the role of the mammary microenvironment during normal glandular development.

Hormone action in the mammary gland

A woman's breast cancer risk is affected by her reproductive history. The hormonal milieu also influences the course of the disease. The female reproductive hormones, estrogens, progesterone, and prolactin, have a major impact on breast cancer and control postnatal mammary gland development. Analysis of hormone receptor mutant mouse strains combined with tissue recombination techniques and proteomics revealed that sequential activation of hormone signaling in the mammary epithelium is required for progression of morphogenesis. Hormones impinge on a subset of luminal mammary epithelial cells (MECs) that express hormone receptors and act as sensor cells translating and amplifying systemic signals into local stimuli. Proliferation is induced by paracrine mechanisms mediated by distinct factors at different stages. Tissue and stage specificity of hormonal signaling is achieved at the molecular level by different chromatin contexts and differential recruitment of coactivators and corepressors.

Recombinant PBD-1 (porcine beta-defensin 1) expressed in the milk by transplanting transgenic mES-like-derived cells into mouse mammary gland

ES (embryonic stem)-derived cells have been investigated in many animal models of severe injury and degenerative disease. However, few studies have examined the ability of ES-derived cells to improve functional outcome following partially damaged breast and also the modification of mammary tissue to produce costly proteins. This study investigates the feasibility of implanting mES-dK (mouse ES-derived keratinocytes-like) cells stably transfected with a mammary gland special expression vector for the PBD-1 (porcine beta-defensin 1) in developing mammary glands. Our aim was to assess the ability of cell grafting to improve functional outcome following partial damage of the breast, also on the breast modification mammary tissue in mice for the production of PBD-1 protein secreted in the milk. Our results showed that the ratios of the surviving cells labelled with the myoepithelial or luminal cell markers, EMA (epithelial membrane antigen) and CALLA, were 41.7 +/- 15.2% and 28.4 +/- 9.6%, respectively, which revealed that transplanted mES-dK cells survived, integrated in vivo and differentiated into myoepithelial or luminal cells. In addition, Western blot analysis showed that 37.5% (3 out of 8) female transplanted mice had PBD-1 expression in their milk and reached 0.4998, 0.5229 and 0.5195 microg/ml, respectively.

The ERalpha coactivator, HER4/4ICD, regulates progesterone receptor expression in normal and malignant breast epithelium

The HER4 intracellular domain (4ICD) is a potent estrogen receptor (ERalpha) coactivator with activities in breast cancer and the developing mammary gland that appear to overlap with progesterone receptor (PgR). In fact, 4ICD has recently emerged as an important regulator and predictor of tamoxifen response, a role previously thought to be fulfilled by PgR. Here we investigated the possibility that the 4ICD coactivator regulates PgR expression thereby providing a mechanistic explanation for their partially overlapping activities in breast cancer. We show that 4ICD is both sufficient and necessary to potentiate estrogen stimulation of gene expression. Suppression of HER4/4ICD expression in the MCF-7 breast tumor cell line completely eliminated estrogen stimulated expression of PgR. In addition, the HER4/4ICD negative MCF-7 variant, TamR, failed to express PgR in response to estrogen. Reintroduction of wild-type HER4 but not the gamma-secretase processing mutant HER4V673I into the TamR cell line restored PgR expression indicating that 4ICD is an essential PgR coactivator in breast tumor cells. These results were substantiated in vivo using two different physiologically relevant experimental systems. In the mouse mammary gland estrogen regulates expression of PgR-A whereas expression of PgR-B is estrogen independent. Consistent with a role for 4ICD in estrogen regulated PgR expression in vivo, PgR-A, but not PgR-B, expression was abolished in HER4-null mouse mammary glands during pregnancy. Coexpression of PgR and 4ICD is also commonly observed in ERalpha positive breast carcinomas. Using quantitative AQUA IHC technology we found that 4ICD potentiated PgR expression in primary breast tumors and the highest levels of PgR expression required coexpression of ERalpha and the 4ICD coactivator. In summary, our results provide compelling evidence that 4ICD is a physiologically important ERalpha coactivator and 4ICD cooperates with ERalpha to potentiate PgR expression in the normal and malignant breast. We propose that direct coupling of these signaling pathways may have important implications for mammary development, breast carcinogenesis, and patient response to endocrine therapy.

Progesterone induces adult mammary stem cell expansion

Reproductive history is the strongest risk factor for breast cancer after age, genetics and breast density. Increased breast cancer risk is entwined with a greater number of ovarian hormone-dependent reproductive cycles, yet the basis for this predisposition is unknown. Mammary stem cells (MaSCs) are located within a specialized niche in the basal epithelial compartment that is under local and systemic regulation. The emerging role of MaSCs in cancer initiation warrants the study of ovarian hormones in MaSC homeostasis. Here we show that the MaSC pool increases 14-fold during maximal progesterone levels at the luteal dioestrus phase of the mouse. Stem-cell-enriched CD49fhi cells amplify at dioestrus, or with exogenous progesterone, demonstrating a key role for progesterone in propelling this expansion. In aged mice, CD49fhi cells display stasis upon cessation of the reproductive cycle. Progesterone drives a series of events where luminal cells probably provide Wnt4 and RANKL signals to basal cells which in turn respond by upregulating their cognate receptors, transcriptional targets and cell cycle markers. Our findings uncover a dynamic role for progesterone in activating adult MaSCs within the mammary stem cell niche during the reproductive cycle, where MaSCs are putative targets for cell transformation events leading to breast cancer.

FOXA1 is an essential determinant of ERalpha expression and mammary ductal morphogenesis

FOXA1, estrogen receptor alpha (ERalpha) and GATA3 independently predict favorable outcome in breast cancer patients, and their expression correlates with a differentiated, luminal tumor subtype. As transcription factors, each functions in the morphogenesis of various organs, with ERalpha and GATA3 being established regulators of mammary gland development. Interdependency between these three factors in breast cancer and normal mammary development has been suggested, but the specific role for FOXA1 is not known. Herein, we report that Foxa1 deficiency causes a defect in hormone-induced mammary ductal invasion associated with a loss of terminal end bud formation and ERalpha expression. By contrast, Foxa1 null glands maintain GATA3 expression. Unlike ERalpha and GATA3 deficiency, Foxa1 null glands form milk-producing alveoli, indicating that the defect is restricted to expansion of the ductal epithelium, further emphasizing the novel role for FOXA1 in mammary morphogenesis. Using breast cancer cell lines, we also demonstrate that FOXA1 regulates ERalpha expression, but not GATA3. These data reveal that FOXA1 is necessary for hormonal responsiveness in the developing mammary gland and ERalpha-positive breast cancers, at least in part, through its control of ERalpha expression.

Human breast cell proliferation and its relationship to steroid receptor expression

The steroidal regulation of proliferation and differentiation in the rodent mammary gland is well described, but how ovarian hormones regulate these processes in the human remains poorly understood. To investigate this, we developed the athymic nude mouse model in which intact normal human breast tissue is grafted subcutaneously and treated with estrogen and/or progesterone at human physiological serum levels. We demonstrated, first, that estrogen and not progesterone is the major epithelial cell mitogen in the adult non-pregnant, non-lactating breast, second, that estrogen induces progesterone receptor (PR) expression and, third, that PR expression is maximally induced at low estrogen concentrations while a higher amount of estrogen was required to induce proliferation. These data raised the question of whether one cell type possessed differential responses to high and low estrogen concentrations or whether PR expression and proliferation occurred in two cell populations. Using double-label immunofluorescence, we demonstrated that steroid receptor expression and cell proliferation (Ki67 antigen) occurred in separate cell populations in normal human breast epithelium, and that cells expressing the estrogen receptor-alpha (ERalpha) invariably contained the PR. We also found that this dissociation between steroid receptor expression and cell proliferation in normal epithelium was disrupted at an early stage in breast tumor formation. Recent findings presented herein support the proposal that some ERalpha/PR-positive epithelial cells are quiescent breast stem cells that act as 'steroid hormone sensors'. Such hormone sensor cells are likely to secrete positive or negative paracrine/juxtacrine factors dependent on the prevailing estrogen or progesterone concentration to influence the proliferative activity of adjacent ERalpha/PR-negative epithelial cells.

A mouse model to dissect progesterone signaling in the female reproductive tract and mammary gland

Considering the regulatory complexities of progesterone receptor (PR) action throughout the female reproductive axis and mammary gland, we generated a mouse model that enables conditional ablation of PR function in a spatiotemporal specific manner. Exon 2 of the murine PR gene was floxed to generate a conditional PR allele (PR(flox)) in mice. Crossing the PR(flox/flox) mouse with the ZP3-cre transgenic demonstrated that the PR(flox) allele recombines to a PR null allele (PR(d)). Mice homozygous for the recombined null PR allele (PR(d/d)) exhibit uterine, ovarian, and mammary gland defects that phenocopy those of our previously described PR knockout (PRKO) model. Therefore, this conditional mouse model for PR ablation represents an invaluable resource with which to further define in a developmental and/or reproductive stage-specific manner the individual and integrative roles of distinct PR populations resident in multiple progesterone-responsive target sites.

Control of mammary stem cell function by steroid hormone signalling

The ovarian hormones oestrogen and progesterone profoundly influence breast cancer risk, underpinning the benefit of endocrine therapies in the treatment of breast cancer. Modulation of their effects through ovarian ablation or chemoprevention strategies also significantly decreases breast cancer incidence. Conversely, there is an increased risk of breast cancer associated with pregnancy in the short term. The cellular mechanisms underlying these observations, however, are poorly defined. Here we demonstrate that mouse mammary stem cells (MaSCs) are highly responsive to steroid hormone signalling, despite lacking the oestrogen and progesterone receptors. Ovariectomy markedly diminished MaSC number and outgrowth potential in vivo, whereas MaSC activity increased in mice treated with oestrogen plus progesterone. Notably, even three weeks of treatment with the aromatase inhibitor letrozole was sufficient to reduce the MaSC pool. In contrast, pregnancy led to a transient 11-fold increase in MaSC numbers, probably mediated through paracrine signalling from RANK ligand. The augmented MaSC pool indicates a cellular basis for the short-term increase in breast cancer incidence that accompanies pregnancy. These findings further indicate that breast cancer chemoprevention may be achieved, in part, through suppression of MaSC function.

Two distinct mechanisms underlie progesterone-induced proliferation in the mammary gland

The mouse mammary gland develops postnatally under the control of female reproductive hormones. Estrogens and progesterone trigger morphogenesis by poorly understood mechanisms acting on a subset of mammary epithelial cells (MECs) that express their cognate receptors, estrogen receptor alpha (ERalpha) and progesterone receptor (PR). Here, we show that in the adult female, progesterone drives proliferation of MECs in two waves. The first, small wave, encompasses PR(+) cells and requires cyclin D1, the second, large wave, comprises mostly PR(-) cells and relies on the tumor necrosis factor (TNF) family member, receptor activator of NF-kappaB-ligand (RANKL). RANKL elicits proliferation by a paracrine mechanism. Ablation of RANKL in the mammary epithelium blocks progesterone-induced morphogenesis, and ectopic expression of RANKL in MECs completely rescues the PR(-/-) phenotype. Systemic administration of RANKL triggers proliferation in the absence of PR signaling, and injection of a RANK signaling inhibitor interferes with progesterone-induced proliferation. Thus, progesterone elicits proliferation by a cell-intrinsic and a, more important, paracrine mechanism.

Loss of protein kinase C delta alters mammary gland development and apoptosis

As apoptotic pathways are commonly deregulated in breast cancer, exploring how mammary gland cell death is regulated is critical for understanding human disease. We show that primary mammary epithelial cells from protein kinase C delta (PKCδ) -/- mice have a suppressed response to apoptotic agents in vitro. In the mammary gland in vivo, apoptosis is critical for ductal morphogenesis during puberty and involution following lactation. We have explored mammary gland development in the PKCδ -/- mouse during these two critical windows. Branching morphogenesis was altered in 4- to 6-week-old PKCδ -/- mice as indicated by reduced ductal branching; however, apoptosis and proliferation in the terminal end buds was unaltered. Conversely, activation of caspase-3 during involution was delayed in PKCδ -/- mice, but involution proceeded normally. The thymus also undergoes apoptosis in response to physiological signals. A dramatic suppression of caspase-3 activation was observed in the thymus of PKCδ -/- mice treated with irradiation, but not mice treated with dexamethasone, suggesting that there are both target- and tissue-dependent differences in the execution of apoptotic pathways in vivo. These findings highlight a role for PKCδ in both apoptotic and nonapoptotic processes in the mammary gland and underscore the redundancy of apoptotic pathways in vivo.

Influence of Hoxa5 on p53 tumorigenic outcome in mice

Hox genes encode transcription factors of crucial importance in the pattern formation of a large spectrum of species. Several studies have now proposed a role for these developmental genes in cancer biology. It has been suggested that HOXA5 possesses growth-suppressive properties through activation of p53 expression in human breast tissue. To assess the genetic cooperation that may exist between Hoxa5 and p53 in tumorigenesis, we generated Hoxa5/p53 compound mutant mice. The presence of Hoxa5 null alleles increased the susceptibility of p53(-/-) mice to develop tumors with a high prevalence for thymic lymphoma, suggesting that the loss of function of the two genes collaborate in tumor formation. To extend our analysis to mammary tumorigenesis, we performed Hoxa5/p53 whole mammary gland transplantations into wild-type hosts. In the p53(-/-) background, the presence of one Hoxa5 mutant allele had no impact on mammary tumor formation. In contrast, the complete loss of Hoxa5 function influenced the tumorigenic outcome of p53(+/-) mammary glands. However, the collaborative nature of this interaction did not depend on the transcriptional regulation of p53 by Hoxa5. Altogether, our data establish that Hoxa5 and p53 cooperate in mammary tumorigenesis in vivo.