Progestins initiate a luminal to myoepithelial switch in estrogen-dependent human breast tumors without altering growth

Respective roles of Pik3ca mutations and cyproterone acetate impregnation in mouse meningioma tumorigenesis

Despite their rarity, PIK3CA mutations in meningiomas have raised interest as potentially targetable, ubiquitous mutations owing to their presence in sporadic benign and malignant tumors but also in hormone-related cases. Using new genetically engineered mouse models, we here demonstrate that Pik3ca mutations in postnatal meningeal cells are sufficient to promote meningioma formation but also tumor progression in mice. Conversely, hormone impregnation, whether alone or in association with Pik3ca and Nf2 mutations, fails to induce meningioma tumorigenesis while promoting breast tumor formation. We then confirm in vitro the effect of Pik3ca mutations but not hormone impregnation on the proliferation of primary cultures of mouse meningeal cells. Finally, we show by exome analysis of breast tumors and meninges that hormone impregnation promotes breast tumor formation without additional somatic oncogenic mutation but is associated with an increased mutational burden on Pik3ca-mutant background. Taken together, these results tend to suggest a prominent role of Pik3ca mutations over hormone impregnation in meningioma tumorigenesis, the exact effect of the latter is still to be discovered.

Progesterone receptors in normal breast development and breast cancer

Progesterone receptors (PR) play a pivotal role in many female reproductive tissues such as the uterus, the ovary, and the mammary gland (MG). Moreover, PR play a key role in breast cancer growth and progression. This has led to the development and study of different progestins and antiprogestins, many of which are currently being tested in clinical trials for cancer treatment. Recent reviews have addressed the role of PR in MG development, carcinogenesis, and breast cancer growth. Thus, in this review, in addition to making an overview on PR action in normal and tumor breast, the focus has been put on highlighting the still unresolved topics on hormone treatment involving PR isoforms and breast cancer prognosis.

Cytokeratin 5 alters β-catenin dynamics in breast cancer cells

Estrogen receptor (ER) positive breast cancers often contain subpopulations of cells that express the intermediate filament protein cytokeratin 5 (CK5). CK5+ cells are enriched in cancer stem cell (CSC) properties, can be induced by progestins, and predict poor prognosis in ER+ breast cancer. We established through CK5 knockout and overexpression in ER+ breast cancer cell lines that CK5 is important for tumorsphere formation, prompting us to speculate that CK5 has regulatory activity in CSCs. To interrogate CK5 interacting proteins that may be functionally cooperative, we performed immunoprecipitation-mass spectrometry for CK5 in ER+ breast cancer cells. Focusing on proteins with signaling activity, we identified β-catenin, a key transcription factor of the Wnt signaling pathway and cell adhesion molecule, as a CK5 interactor, which we confirmed by co-immunoprecipitation in several breast cancer models. We interrogated the dual functions of β-catenin in relation to CK5. Knockout or knockdown of CK5 ablated β-catenin transcriptional activity in response to progestins and Wnt stimuli. Conversely, CK5 induced by progestins or overexpression was sufficient to promote loss of β-catenin at the cell membrane and total E-cadherin loss. A breast cancer patient-derived xenograft showed similar loss of membrane β-catenin and E-cadherin in CK5+ but not intratumoral CK5− cells and single cell RNA sequencing found the top enriched pathways in the CK5+ cell cluster were cell junction remodeling and signaling. This report highlights that CK5 actively remodels cell morphology and that blockade of CK5-β-catenin interaction may reverse the detrimental properties of CK5+ breast cancer cells.

Breast Cancer Stem Cells and Sex Steroid Hormones

Breast cancer stem cells (BCSCs) are a small population of tumor-initiating cells that express stem cell-associated markers. In recent years, their properties and mechanisms of regulation have become the focus of intense research due to their intrinsic resistance to conventional cancer therapies. This review describes breast cancer stem cell origin, signaling pathways involved in self-renewal, such as Wnt, Notch and Hedgehog, biomarkers linked to stemness, and the role of sex steroid hormones in BCSC regulation.

Molecular mechanisms underlying progesterone receptor action in breast cancer: Insights into cell proliferation and stem cell regulation

The ovarian steroid hormone progesterone and its nuclear receptor, the Progesterone Receptor (PR), play an essential role in the regulation of cell proliferation and differentiation in the mammary gland. In addition, experimental and clinical evidence demonstrate their critical role in controlling mammary gland tumorigenesis and breast cancer development. When bound to its ligand, the main action of PR is as a transcription factor, which regulates the expression of target genes networks. PR also activates signal transduction pathways through a rapid or non-genomic mechanism in breast cancer cells, an event that is fully integrated with its genomic effects. This review summarizes the molecular mechanisms of the ligand-activated PR actions that drive epithelial cell proliferation and the regulation of the stem cell population in the normal breast and in breast cancer.

Phosphorylated Progesterone Receptor Isoforms Mediate Opposing Stem Cell and Proliferative Breast Cancer Cell Fates

Progesterone receptors (PRs) are key modifiers of estrogen receptor (ER) target genes and drivers of luminal breast cancer progression. Total PR expression, rather than isoform-specific PR expression, is measured in breast tumors as an indicator of functional ER. We identified phenotypic differences between PR-A and PR-B in luminal breast cancer models with a focus on tumorsphere biology. Our findings indicated that PR-A is a dominant driver of cancer stem cell (CSC) expansion in T47D models, and PR-B is a potent driver of anchorage-independent proliferation. PR-A+ tumorspheres were enriched for aldehyde dehydrogenase (ALDH) activity, CD44+/CD24-, and CD49f+/CD24- cell populations relative to PR-B+ tumorspheres. Progestin promoted heightened expression of known CSC-associated target genes in PR-A+ but not PR-B+ cells cultured as tumorspheres. We report robust phosphorylation of PR-A relative to PR-B Ser294 and found that this residue is required for PR-A-induced expression of CSC-associated genes and CSC behavior. Cells expressing PR-A S294A exhibited impaired CSC phenotypes but heightened anchorage-independent cell proliferation. The PR target gene and coactivator, FOXO1, promoted PR phosphorylation and tumorsphere formation. The FOXO1 inhibitor (AS1842856) alone or combined with onapristone (PR antagonist), blunted phosphorylated PR, and tumorsphere formation in PR-A+ and PR-B+ T47D, MCF7, and BT474 models. Our data revealed unique isoform-specific functions of phosphorylated PRs as modulators of distinct and opposing pathways relevant to mechanisms of late recurrence. A clear understanding of PR isoforms, phosphorylation events, and the role of cofactors could lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target CSCs in luminal breast cancer.

Development of Novel Patient-Derived Xenografts from Breast Cancer Brain Metastases

Brain metastases are an increasing burden among breast cancer patients, particularly for those with HER2+ and triple negative (TN) subtypes. Mechanistic insight into the pathophysiology of brain metastases and preclinical validation of therapies has relied almost exclusively on intracardiac injection of brain-homing cells derived from highly aggressive TN MDA-MB-231 and HER2+ BT474 breast cancer cell lines. Yet, these well characterized models are far from representing the tumor heterogeneity observed clinically and, due to their fast progression in vivo, their suitability to validate therapies for established brain metastasis remains limited. The goal of this study was to develop and characterize novel human brain metastasis breast cancer patient-derived xenografts (BM-PDXs) to study the biology of brain metastasis and to serve as tools for testing novel therapeutic approaches. We obtained freshly resected brain metastases from consenting donors with breast cancer. Tissue was immediately implanted in the mammary fat pad of female immunocompromised mice and expanded as BM-PDXs. Brain metastases from 3/4 (75%) TN, 1/1 (100%) estrogen receptor positive (ER+), and 5/9 (55.5%) HER2+ clinical subtypes were established as transplantable BM-PDXs. To facilitate tracking of metastatic dissemination using BM-PDXs, we labeled PDX-dissociated cells with EGFP-luciferase followed by reimplantation in mice, and generated a BM-derived cell line (F2-7). Immunohistologic analyses demonstrated that parental and labeled BM-PDXs retained expression of critical clinical markers such as ER, progesterone receptor, epidermal growth factor receptor, HER2, and the basal cell marker cytokeratin 5. Similarly, RNA sequencing analysis showed clustering of parental, labeled BM-PDXs and their corresponding cell line derivative. Intracardiac injection of dissociated cells from BM-E22-1, resulted in magnetic resonance imaging-detectable macrometastases in 4/8 (50%) and micrometastases (8/8) (100%) mice, suggesting that BM-PDXs remain capable of colonizing the brain at high frequencies. Brain metastases developed 8-12 weeks after ic injection, located to the brain parenchyma, grew around blood vessels, and elicited astroglia activation characteristic of breast cancer brain metastasis. These novel BM-PDXs represent heterogeneous and clinically relevant models to study mechanisms of brain metastatic colonization, with the added benefit of a slower progression rate that makes them suitable for preclinical testing of drugs in therapeutic settings.

Breast Cancer Suppression by Progesterone Receptors Is Mediated by Their Modulation of Estrogen Receptors and RNA Polymerase III

Greater than 50% of estrogen receptor (ER)-positive breast cancers coexpress the progesterone receptor (PR), which can directly and globally modify ER action to attenuate tumor growth. However, whether this attenuation is mediated only through PR-ER interaction remains unknown. To address this question, we assessed tumor growth in ER/PR-positive patient-derived xenograft models of breast cancer, where both natural and synthetic progestins were found to antagonize the mitogenic effects of estrogens. Probing the genome-wide mechanisms by which this occurs, we documented that chronic progestin treatment blunted ER-mediated gene expression up to 2-fold at the level of mRNA transcripts. Unexpectedly, <25% of all ER DNA binding events were affected by the same treatment. The PR cistrome displayed a bimodal distribution. In one group, >50% of PR binding sites were co-occupied by ER, with a propensity for both receptors to coordinately gain or lose binding in the presence of progesterone. In the second group, PR but not ER was associated with a large fraction of RNA polymerase III-transcribed tRNA genes, independent of hormone treatment. Notably, we discovered that PR physically associated with the Pol III holoenzyme. Select pre-tRNAs and mature tRNAs with PR and POLR3A colocalized at their promoters were relatively decreased in estrogen + progestin-treated tumors. Our results illuminate how PR may indirectly impede ER action by reducing the bioavailability of translational molecules needed for tumor growth. Cancer Res; 77(18); 4934-46. ©2017 AACR.

Progesterone receptor antagonism inhibits progestogen-related carcinogenesis and suppresses tumor cell proliferation

PURPOSE

Blockade of the progestogen-progesterone receptor (PR) axis is a novel but untested strategy for breast cancer prevention. We report preclinical data evaluating telapristone acetate (TPA), ulipristal acetate (UPA), and mifepristone.

METHODS

Tumors were induced with medroxyprogesterone acetate (MPA) plus 7,12-dimethylbenz[a]anthracene (DMBA) in mice, and MPA or progesterone plus N-methyl-N-nitrosourea (MNU) in rats. Mammary gland histology, tumor incidence, latency, multiplicity, burden and histology were evaluated, along with immunohistochemical labeling of pHH3 (proliferation), CD34 (angiogenesis), and estrogen and progesterone receptors (ER and PR). A concentration gradient of TPA, UPA, and mifepristone was tested for growth inhibition of T47D spheroids.

RESULTS

In mouse mammary glands, no tumors formed, but TPA opposed the pro-hyperplastic effects of MPA (p = 0.002). In rats, TPA decreased tumor incidence (p = 0.037 for MPA + TPA vs. MPA, and p = 0.032 for progesterone + TPA vs. progesterone) and tumor burden (p = 0.042 for progesterone + TPA vs. progesterone), with significant decreases in pHH3 and CD34 positive cells. TPA and UPA were superior to mifepristone in growth inhibition of T47D spheroids.

CONCLUSION

TPA has consistent anti-tumorigenic effects in several models, which are accompanied by decreases in cell proliferation, angiogenesis, and hormone receptor expression.

Minireview: Progesterone Regulation of Proliferation in the Normal Human Breast and in Breast Cancer: A Tale of Two Scenarios?

Progesterone (P), which signals through the P receptor (PR), is critical in normal development of the breast, but its signaling axis is also a major driver of breast cancer risk. Here we review recent advances in the understanding of P signaling in the normal human breast, with a focus on the importance of the balance between autocrine and paracrine signaling. To date, most data (which derive largely from mouse models or human breast cancer cell line studies) have demonstrated that the vast majority of PR+ cells appear to act as "sensor" cells, which respond to P stimulation by translating these hormonal cues into paracrine signals. However, growing evidence suggests that, dependent on the cellular context, P may also signal in an autocrine manner in a subset of cells in the normal mouse mammary gland and human breast. It has been suggested that it may be dysregulation of this autocrine signaling, resulting in a "switch" from a predominance of paracrine signaling to autocrine signaling in PR+ cells, which is an early event during breast tumorigenesis. This review summarizes current evidence in the literature that demonstrates the mechanisms through which P acts in the normal human breast, as well as highlighting the important questions that remain unanswered.

Steroid induction of therapy-resistant cytokeratin-5-positive cells in estrogen receptor-positive breast cancer through a BCL6-dependent mechanism

Therapy resistance remains a major problem in estrogen receptor-α (ERα)-positive breast cancer. A subgroup of ERα-positive breast cancer is characterized by mosaic presence of a minor population of ERα-negative cancer cells expressing the basal cytokeratin-5 (CK5). These CK5-positive cells are therapy resistant and have increased tumor-initiating potential. Although a series of reports document induction of the CK5-positive cells by progestins, it is unknown if other 3-ketosteroids share this ability. We now report that glucocorticoids and mineralocorticoids effectively expand the CK5-positive cell population. CK5-positive cells induced by 3-ketosteroids lacked ERα and progesterone receptors, expressed stem cell marker, CD44, and displayed increased clonogenicity in soft agar and broad drug-resistance in vitro and in vivo. Upregulation of CK5-positive cells by 3-ketosteroids required induction of the transcriptional repressor BCL6 based on suppression of BCL6 by two independent BCL6 small hairpin RNAs or by prolactin. Prolactin also suppressed 3-ketosteroid induction of CK5+ cells in T47D xenografts in vivo. Survival analysis with recursive partitioning in node-negative ERα-positive breast cancer using quantitative CK5 and BCL6 mRNA or protein expression data identified patients at high or low risk for tumor recurrence in two independent patient cohorts. The data provide a mechanism by which common pathophysiological or pharmacologic elevations in glucocorticoids or other 3-ketosteroids may adversely affect patients with mixed ERα+/CK5+ breast cancer. The observations further suggest a cooperative diagnostic utility of CK5 and BCL6 expression levels and justify exploring efficacy of inhibitors of BCL6 and 3-ketosteroid receptors for a subset of ERα-positive breast cancers.

Steroid hormones, steroid receptors, and breast cancer stem cells

The ovarian hormones progesterone and estrogen play important roles in breast cancer etiology, proliferation, and treatment. Androgens may also contribute to breast cancer risk and progression. In recent years, significant advances have been made in defining the roles of these steroid hormones in stem cell homeostasis in the breast. Stem cells are potential origins of breast cancer and may dictate tumor phenotype. At least a portion of breast cancers are proposed to be driven by cancer stem cells (CSCs), cells that mimic the self-renewing and repopulating properties of normal stem cells, and can confer drug resistance. Progesterone has been identified as the critical hormone regulating normal murine mammary stem cell (MaSC) populations and normal human breast stem cells. Synthetic progestins increase human breast cancer risk; one theory speculates that this occurs through increased stem cells. Progesterone treatment also increases breast CSCs in established breast cancer cell lines. This is mediated in part through progesterone regulation of transcription factors, signal transduction pathways, and microRNAs. There is also emerging evidence that estrogens and androgens can regulate breast CSC numbers. The evolving concept that a breast CSC phenotype is dynamic and can be influenced by cell signaling and external cues emphasizes that steroid hormones could be crucial players in controlling CSC number and function. Here we review recent studies on steroid hormone regulation of breast CSCs, and discuss mechanisms by which this occurs.

Impact of progesterone on stem/progenitor cells in the human breast

The epithelium of the human breast is made up of a branching ductal-lobular system, which is lined by a single layer of luminal cells surrounded by a contractile basal cell layer. The co-ordinated development of stem/progenitor cells into these luminal and basal cells is fundamentally important for breast morphogenesis. The ovarian steroid hormone, progesterone, is critical in driving proliferation and normal breast development, yet progesterone analogues have also been shown to be a major driver of breast cancer risk. Studies in recent years have revealed an important role for progesterone in stimulating the mammary stem cell compartment in the mouse mammary gland, and growing evidence supports the notion that progesterone also stimulates progenitor cells in both the normal human breast and in breast cancer cells. As changes in cell type composition are one of the hallmark features of breast cancer progression, these observations have critical implications in discerning the mechanisms of how progesterone increases breast cancer risk. This review summarises recent work regarding the impact of progesterone action on the stem/progenitor cell compartment of the human breast.

Luminal breast cancer metastases and tumor arousal from dormancy are promoted by direct actions of estradiol and progesterone on the malignant cells

Introduction

Luminal, estrogen receptor-positive (ER⁺) breast cancers can metastasize but lie dormant for years before recurrences prove lethal. Understanding the roles of estrogen (E) or progestin (P) in development of luminal metastases or in arousal from dormancy is hindered by few preclinical models. We have developed such models.

Methods

Immunocompromised, ovariectomized (ovx’d) mice were intracardiac-injected with luminal or basal human breast cancer cells. Four lines were tested: luminal ER⁺PR⁺ cytokeratin 5-negative (CK5⁻) E3 and MCF-7 cells, basal ER⁻PR⁻CK5⁺ estrogen withdrawn-line 8 (EWD8) cells, and basal ER⁻PR⁻CK5⁻ MDA-MB-231 cells. Development of micrometastases or macrometastases was quantified in ovx’d mice and in mice supplemented with E or P or both. Metastatic deposits were analyzed by immunohistochemistry for luminal, basal, and proliferation markers.

Results

ER⁻PR⁻ cells generated macrometastases in multiple organs in the absence or presence of hormones. By contrast, ovx’d mice injected with ER⁺PR⁺ cells appeared to be metastases-free until they were supplemented with E or E+P. Furthermore, unlike parental ER⁺PR⁺CK5⁻ cells, luminal metastases were heterogeneous, containing a significant (6% to 30%) proportion of non-proliferative ER⁻PR⁻CK5⁺ cells that would be chemotherapy-resistant. Additionally, because these cells lack receptors, they would also be endocrine therapy-resistant. With regard to ovx’d control mice injected with ER⁺PR⁺ cells that appeared to be metastases-free, systematic pathologic analysis of organs showed that some harbor a reservoir of dormant micrometastases that are ER⁺ but PR⁻. Such cells may also be endocrine therapy- and chemotherapy-resistant. Their emergence as macrometastases can be triggered by E or E+P restoration.

Conclusions

We conclude that hormones promote development of multi-organ macrometastases in luminal disease. The metastases display a disturbing heterogeneity, containing newly emergent ER⁻PR⁻ subpopulations that would be resistant to endocrine therapy and chemotherapy. Similar cells are found in luminal metastases of patients. Furthermore, lack of hormones is not protective. While no overt metastases form in ovx’d mice, luminal tumor cells can seed distant organs, where they remain dormant as micrometastases and sheltered from therapies but arousable by hormone repletion. This has implications for breast cancer survivors or women with occult disease who are prescribed hormones for contraception or replacement purposes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-014-0489-4) contains supplementary material, which is available to authorized users.

Progesterone downregulation of miR-141 contributes to expansion of stem-like breast cancer cells through maintenance of progesterone receptor and Stat5a

Progesterone (P4) has emerged as an important hormone regulating mammary stem cell populations. In breast cancer, P4 and synthetic analogs increase the number of stem-like cells within luminal estrogen receptor (ER) and progesterone receptor (PR) positive breast cancers. These cells gain expression of de-differentiated cell markers CD44 and cytokeratin 5 (CK5), lose luminal markers ER and PR, and are more therapy resistant. We previously described that P4-downregulation of microRNA (miR)-29a contributes to the expansion of CD44^high and CK5⁺ cells. Here we investigated P4-downregulation of miR-141, a member of the miR-200 family of tumor suppressors, in facilitating an increase in stem-like breast cancer cells. miR-141 was the sole member of the miR-200 family P4-downregulated at the mature miRNA level in luminal breast cancer cell lines. Stable inhibition of miR-141 alone increased the CD44^high population, and potentiated P4-mediated increases in both CD44^high and CK5⁺ cells. Loss of miR-141 enhanced both mammosphere formation and tumor initiation. miR-141 directly targeted both PR and Stat5a, transcription factors important for mammary stem cell expansion. miR-141 depletion increased PR protein levels, even in cells lines where PR expression is estrogen-dependent. Stat5a suppression via siRNA or a small molecule inhibitor reduced the P4-dependent increase in CK5⁺ and CD44^high cells. These data support a mechanism by which P4-triggered loss of miR-141 facilitates breast cancer cell de-differentiation through deregulation of PR and Stat5a, two transcription factors important for controlling mammary cell fate.

Rapid estrogen signaling negatively regulates PTEN activity through phosphorylation in endometrial cancer cells

Hyperestrogenicity is a risk factor for endometrial cancer. 17β-estradiol (E2) is known to stimulate both genomic and nongenomic estrogen receptor-α (ERα) actions in a number of reproductive tissues. However, the contributions of transcription-independent ERα signaling on normal and malignant endometrium are not fully understood. Phosphatase and tensin homolog (PTEN) is a tumor suppressor that decreases cellular mitosis primarily through negative regulation of the phosphoinositide 3-kinase/AKT signaling axis. PTEN levels are elevated during the E2 dominated, mitotically active, proliferative phase of the menstrual cycle, indicating possible hormonal regulation of PTEN in the uterus. In order to determine if rapid E2 signaling regulates PTEN, we used ERα-positive, PTEN-positive, endometrial cells. We show that cytosolic E2/ERα signaling leads to increased phosphorylation of PTEN at key regulatory residues. Importantly, E2 stimulation decreased PTEN lipid phosphatase activity and caused consequent increases in phospho-AKT. We further demonstrate that cytosolic ERα forms a complex with PTEN in an E2-dependent manner, and that ERα constitutively complexes with protein kinase2-α (CK2α), a kinase previously shown to phosphorylate the C-terminal tail of PTEN. These results provide mechanistic support for an E2-dependent, ERα cytosolic signaling complex that negatively regulates PTEN activity through carboxy terminus phosphorylation. Using an animal model, we show that sustained E2 signaling results in increased phospho-PTEN (S380, T382, and T383), total PTEN, and phospho-AKT (S473). Taken together, we provide a novel mechanism in which transcription-independent E2/ERα signaling may promote a pro-tumorigenic environment in the endometrium.

Generation of breast cancer stem cells by steroid hormones in irradiated human mammary cell lines

Exposure to ionizing radiation was shown to result in an increased risk of breast cancer. There is strong evidence that steroid hormones influence radiosensitivity and breast cancer risk. Tumors may be initiated by a small subpopulation of cancer stem cells (CSCs). In order to assess whether the modulation of radiation-induced breast cancer risk by steroid hormones could involve CSCs, we measured by flow cytometry the proportion of CSCs in irradiated breast cancer cell lines after progesterone and estrogen treatment. Progesterone stimulated the expansion of the CSC compartment both in progesterone receptor (PR)-positive breast cancer cells and in PR-negative normal cells. In MCF10A normal epithelial PR-negative cells, progesterone-treatment and irradiation triggered cancer and stemness-associated microRNA regulations (such as the downregulation of miR-22 and miR-29c expression), which resulted in increased proportions of radiation-resistant tumor-initiating CSCs.

Prolactin suppresses a progestin-induced CK5-positive cell population in luminal breast cancer through inhibition of progestin-driven BCL6 expression

Prolactin controls the development and function of milk-producing breast epithelia but also supports growth and differentiation of breast cancer, especially luminal subtypes. A principal signaling mediator of prolactin, Stat5, promotes cellular differentiation of breast cancer cells in vitro, and loss of active Stat5 in tumors is associated with antiestrogen therapy failure in patients. In luminal breast cancer, progesterone induces a cytokeratin-5 (CK5)-positive basal cell-like population. This population possesses characteristics of tumor stem cells including quiescence, therapy resistance and tumor-initiating capacity. Here we report that prolactin counteracts induction of the CK5-positive population by the synthetic progestin (Pg) R5020 in luminal breast cancer cells both in vitro and in vivo. CK5-positive cells were chemoresistant as determined by fourfold reduced rate of apoptosis following docetaxel exposure. Pg-induction of CK5 was preceded by marked upregulation of BCL6, an oncogene and transcriptional repressor critical for the maintenance of leukemia-initiating cells. Knockdown of BCL6 prevented induction of CK5-positive cell population by Pg. Prolactin suppressed Pg-induced BCL6 through Jak2-Stat5 but not Erk- or Akt-dependent pathways. In premenopausal but not postmenopausal patients with hormone receptor-positive breast cancer, tumor protein levels of CK5 correlated positively with BCL6, and high BCL6 or CK5 protein levels were associated with unfavorable clinical outcome. Suppression of Pg-induction of CK5-positive cells represents a novel prodifferentiation effect of prolactin in breast cancer. The present progress may have direct implications for breast cancer progression and therapy as loss of prolactin receptor-Stat5 signaling occurs frequently and BCL6 inhibitors currently being evaluated for lymphomas may have value for breast cancer.

Genomic signatures of pregnancy-associated breast cancer epithelia and stroma and their regulation by estrogens and progesterone

Pregnancy-associated breast cancers (PABC) generally present at advanced stages and have a poor prognosis. The reasons are unclear but we hypothesized that the continuous high levels of estrogens and progesterone were involved. We have now carried out a detailed analysis of PABC compared to tumors of age-matched nonpregnant (non-PABC) women. Malignant epithelia and tumor-associated stroma of PABC and non-PABC were isolated by laser capture microdissection and gene expression profiled. Additionally, normal breast epithelia and stroma adjacent to the two tumor types were analyzed. Lastly, subsets of previously identified E- and P-regulated genes were defined in all tissues. We find that PABC signatures cluster with established breast cancer subtypes. Major hormone-regulated genes whose expression correlated with epithelia of PABC dealt with regulation of cell proliferation, metabolism, and tumor aggressiveness, including genes used to predict tumor recurrence. Compared to normal epithelia, a significant number of genes associated with cell cycle processes were enriched in PABC, many of which are hormone regulated. Thus, compared to normal epithelia, many of the genes that were differentially expressed in epithelia of PABC were distinct from those differentially expressed in non-PABC. With regard to the tumor microenvironment, immune-related genes were enriched in tumor-associated stroma of PABC. Compared to normal stroma, PABC-associated stroma overexpressed immune response genes, while genes involved in angiogenesis and extracellular matrix deposition were more commonly downregulated. This suggests that the heightened aggressiveness of PABC may involve a predisposition to metastasis through extracellular matrix degradation, plus angiogenesis independence. Moreover, genes encoding cell proliferative factors, signaling, immunomodulators and cell death, were hormone regulated in stroma. In sum, these analyses demonstrate complex patterns of enrichment and hormonal regulation of genes in PABC and suggest that it may have a distinct biological nature.

Role of KLF5 in hormonal signaling and breast cancer development

Steroid hormones, including ovarian steroid hormones progesterone and estrogen and androgen, play vital roles in the development of normal mammary gland and breast cancer via their receptors. How these hormones regulate these physiological and pathological processes remains to be elucidated. Krüppel-like factor 5 (KLF5) is a transcription factor playing significant roles in breast carcinogenesis, whose expression has been shown to be regulated by hormones. In this review, the relationships among hormonal signaling, KLF5, and breast cancer are summarized and discussed.

Progestin modulates the lipid profile and sensitivity of breast cancer cells to docetaxel

Progestins induce lipid accumulation in progesterone receptor (PR)-positive breast cancer cells. We speculated that progestin-induced alterations in lipid biology confer resistance to chemotherapy. To examine the biology of lipid loaded breast cancer cells, we used a model of progestin-induced lipid synthesis. T47D (PR-positive) and MDA-MB-231 (PR-negative) cell lines were used to study progestin response. Oil red O staining of T47D cells treated with progestin showed lipid droplet formation was PR dependent, glucose dependent and reduced sensitivity to docetaxel. This protection was not observed in PR-negative MDA-MB-231 cells. Progestin treatment induced stearoyl CoA desaturase-1 (SCD-1) enzyme expression and chemical inhibition of SCD-1 diminished lipid droplets and cell viability, suggesting the importance of lipid stores in cancer cell survival. Gas chromatography/mass spectroscopy analysis of phospholipids from progestin-treated T47D cells revealed an increase in unsaturated fatty acids, with oleic acid as most abundant. Cells surviving docetaxel treatment also contained more oleic acid in phospholipids, suggesting altered membrane fluidity as a potential mechanism of chemoresistance mediated in part by SCD-1. Lastly, intact docetaxel molecules were present within progestin induced lipid droplets, suggesting a protective quenching effect of intracellular lipid droplets. Our studies suggest the metabolic adaptations produced by progestin provide novel metabolic targets for future combinatorial therapies for progestin-responsive breast cancers.

Progesterone and estrogen receptors segregate into different cell subpopulations in the normal human breast

Progesterone is critical in normal breast development and its synthetic derivatives are emerging as major drivers of breast cancer risk. The recent demonstration that progesterone regulates the stem cell compartment in the murine mammary gland, despite the absence of progesterone receptor (PR) in mammary stem cells, highlights the fact that PR distribution in progenitor cell subsets in the human breast remains to be conclusively shown. By utilising two independent cell sorting strategies to fractionate cells into distinct subpopulations enriched for different cell lineage characteristics, we have demonstrated a consistent enrichment of PR transcripts, relative to estrogen receptor transcripts, in the bipotent progenitor subfraction in the normal human breast. We have also shown co-expression of both steroid hormone receptors with basal markers in a subset of human breast cells, and finally we have demonstrated that PR+ bipotent progenitor cells are estrogen-insensitive, and that estrogen regulates PR in mature luminal cells only.

Patient-derived luminal breast cancer xenografts retain hormone receptor heterogeneity and help define unique estrogen-dependent gene signatures

Bypassing estrogen receptor (ER) signaling during development of endocrine resistance remains the most common cause of disease progression and mortality in breast cancer patients. To date, the majority of molecular research on ER action in breast cancer has occurred in cell line models derived from late stage disease. Here we describe patient-derived ER+ luminal breast tumor models for the study of intratumoral hormone and receptor action. Human breast tumor samples obtained from patients post surgery were immediately transplanted into NOD/SCID or NOD/SCID/ILIIrg(-/-) mice under estrogen supplementation. Five transplantable patient-derived ER+ breast cancer xenografts were established, derived from both primary and metastatic cases. These were assessed for estrogen dependency, steroid receptor expression, cancer stem cell content, and endocrine therapy response. Gene expression patterns were determined in select tumors ±estrogen and ±endocrine therapy. Xenografts morphologically resembled the patient tumors of origin, and expressed similar levels of ER (5-99 %), and progesterone and androgen receptors, over multiple passages. Four of the tumor xenografts were estrogen dependent, and tamoxifen or estrogen withdrawal (EWD) treatment abrogated estrogen-dependent growth and/or tumor morphology. Analysis of the ER transcriptome in select tumors revealed notable differences in ER mechanism of action, and downstream activated signaling networks, in addition to identifying a small set of common estrogen-regulated genes. Treatment of a naïve tumor with tamoxifen or EWD showed similar phenotypic responses, but relatively few similarities in estrogen-dependent transcription, and affected signaling pathways. Several core estrogen centric genes were shared with traditional cell line models. However, novel tumor-specific estrogen-regulated potential target genes, such as cancer/testis antigen 45, were uncovered. These results evoke the importance of mapping both conserved and tumor-unique ER programs in breast cancers. Furthermore, they underscore the importance of primary xenografts for improved understanding of ER+ breast cancer heterogeneity and development of personalized therapies.

Progestin suppression of miR-29 potentiates dedifferentiation of breast cancer cells via KLF4

The female hormone progesterone (P4) promotes the expansion of stem-like cancer cells in estrogen receptor (ER)- and progesterone receptor (PR)-positive breast tumors. The expanded tumor cells lose expression of ER and PR, express the tumor-initiating marker CD44, the progenitor marker cytokeratin 5 (CK5) and are more resistant to standard endocrine and chemotherapies. The mechanisms underlying this hormone-stimulated reprogramming have remained largely unknown. In the present study, we investigated the role of microRNAs in progestin-mediated expansion of this dedifferentiated tumor cell population. We demonstrate that P4 rapidly downregulates miR-29 family members, particularly in the CD44(+) cell population. Downregulation of miR-29 members potentiates the expansion of CK5(+) and CD44(+) cells in response to progestins, and results in increased stem-like properties in vitro and in vivo. We demonstrate that miR-29 directly targets Krüppel-like factor 4 (KLF4), a transcription factor required for the reprogramming of differentiated cells to pluripotent stem cells, and for the maintenance of breast cancer stem cells. These results reveal a novel mechanism, whereby progestins increase the stem cell-like population in hormone-responsive breast cancers, by decreasing miR-29 to augment PR-mediated upregulation of KLF4. Elucidating the mechanisms whereby hormones mediate the expansion of stem-like cells furthers our understanding of the progression of hormone-responsive breast cancers.

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.

Active allies: hormones, stem cells and the niche in adult mammopoiesis

Adult stem cells are recruited in response to specific physiological demands to regenerate, repair or maintain essential cellular components of tissues, while preserving self-renewal capacity. Signals that activate adult stem cells are not simply cell autonomous and stem cells are part of a larger dynamic framework, the stem cell 'niche', which integrates systemic and local cues to sustain stem cell functionality. The mammary stem cell niche responds readily to hormonal stimuli, generating pertinent signals that activate stem cells, culminating in stem cell expansion and tissue growth. We review here current knowledge of the mammary stem cell niche with attention to the potent stimulation rendered by ovarian hormones, relevant cellular and molecular players, and the implication of a deregulated niche, for breast cancer risk.

PI3K/AKT pathway regulates phosphorylation of steroid receptors, hormone independence and tumor differentiation in breast cancer

Using a model of medroxyprogesterone acetate (MPA)-induced mouse mammary tumors that transit through different stages of hormone dependence, we previously reported that the activation of the phosphatidylinositol 3-kinase (PI3K)/AKT (protein kinase B) pathway is critical for the growth of hormone-independent (HI) mammary carcinomas but not for the growth of hormone-dependent (HD) mammary carcinomas. The objective of this work was to explore whether the activation of the PI3K/AKT pathway is responsible for the changes in tumor phenotype and for the transition to autonomous growth. We found that the inhibition of the PI3K/AKT/mTOR (mammalian target of rapamycin) pathway suppresses HI tumor growth. In addition, we were able to induce mammary tumors in mice in the absence of MPA by inoculating HD tumor cells expressing a constitutively active form of AKT1, myristoylated AKT1 (myrAKT1). These tumors were highly differentiated and displayed a ductal phenotype with laminin-1 and cytokeratin 8 expression patterns typical of HI tumors. Furthermore, myrAKT1 increased the tumor growth of IBH-6 and IBH-7 human breast cancer cell lines. In the estrogen-dependent IBH-7 cell line, myrAKT1 induced estrogen-independent growth accompanied by the expression of the adhesion markers focal adhesion kinase and E-cadherin. Finally, we found that cells expressing myrAKT1 exhibited increased phosphorylation of the progesterone receptor at Ser190 and Ser294 and of the estrogen receptor α at Ser118 and Ser167, independently of exogenous MPA or estrogen supply. Our results indicate that the activation of the PI3K/AKT/mTOR pathway promotes tissue architecture remodeling and the activation of steroid receptors.

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.

Synthetic progestins induce growth and metastasis of BT-474 human breast cancer xenografts in nude mice

OBJECTIVE

Previous studies have shown that sequential exposure to estrogen and progesterone or medroxyprogesterone acetate (MPA) stimulates vascularization and promotes the progression of BT-474 and T47-D human breast cancer cell xenografts in nude mice (Liang et al, Cancer Res 2007, 67:9929). In this follow-up study, the effects of progesterone, MPA, norgestrel (N-EL), and norethindrone (N-ONE) on BT-474 xenograft tumors were compared in the context of several different hormonal environments. N-EL and N-ONE were included in the study because synthetic progestins vary considerably in their biological effects and the effects of these two progestins on the growth of human tumor xenografts are not known.

METHODS

Estradiol-supplemented intact and ovariectomized immunodeficient mice were implanted with BT-474 cells. Progestin pellets were implanted simultaneously with estradiol pellets either 2 days before tumor cell injection (ie, combined) or 5 days after tumor cell injections (ie, sequentially).

RESULTS

Progestins stimulated the growth of BT-474 xenograft tumors independent of exposure timing and protocol, MPA stimulated the growth of BT-474 xenograft tumors in ovariectomized mice, and progestins stimulated vascular endothelial growth factor elaboration and increased tumor vascularity. Progestins also increased lymph node metastasis of BT-474 cells. Therefore, progestins, including N-EL and N-ONE, induce the progression of breast cancer xenografts in nude mice and promote tumor metastasis.

CONCLUSIONS

These observations suggest that women who ingest progestins for hormone therapy or oral contraception could be more at risk for developing breast cancer because of proliferation of existing latent tumor cells. Such risks should be considered in the clinical setting.

Cytokeratin 5 positive cells represent a steroid receptor negative and therapy resistant subpopulation in luminal breast cancers

A majority of breast cancers are estrogen receptor (ER) positive and have a luminal epithelial phenotype. However, these ER⁺ tumors often contain heterogeneous subpopulations of ER⁻ tumor cells. We previously identified a population of cytokeratin 5 (CK5) positive cells within ER⁺ and progesterone receptor positive (PR⁺) tumors that is both ER⁻PR⁻ and CD44⁺, a marker of breast tumor-initiating cells (TICs). These CK5⁺ cells have properties of TICs in luminal tumor xenografts, and we speculated that they are more resistant to chemo- and anti-ER-targeted therapies than their ER⁺ neighbors. To test this, we used ER⁺PR⁺ T47D and MCF7 breast cancer cells. CK5⁺ cells had lower proliferative indices than CK5⁻ cells, were less sensitive to 5-fluorouracil and docetaxel, and cultures became enriched for CK5⁺ cells after treatments. CK5⁺ cells were less prone to drug-induced apoptosis than CK5⁻ cells. In cells treated with 17β-estradiol (E) plus anti-estrogens tamoxifen or fulvestrant, ER protein levels decreased, and CK5 protein levels increased, compared to controls treated with E alone. In ER⁺ tumors from patients treated with neoadjuvant endocrine therapies ER gene expression decreased, and CK5 gene expression increased in post compared to pre-treatment tumors. The number of CK5⁺ cells in tumors also increased in post- compared to pre-treatment tumors. We conclude that an ER⁻PR⁻CK5⁺ subpopulation found in many luminal tumors is resistant to standard endocrine and chemotherapies, relative to the majority ER⁺PR⁺CK5⁻ cells. Compounds that effectively target these cells are needed to improve outcome in luminal breast cancers.

Progestagen component in combined hormone replacement therapy in postmenopausal women and breast cancer risk: a debated clinical issue

The relevance of the progestagen component in combined hormone replacement therapy (HRT) for breast cancer risk has been long debated. In vitro studies have shown that progestins exert both genomic transcriptional and non-genomic effects that can enhance the proliferation, invasiveness and spread of breast cancer cells. According to a novel hypothesis, progestins can still activate cancer stem cells in patients with pre-existing, clinically undetected breast cancer. However, some experimental and clinical data suggest that different progestins may have a different impact on the pathophysiology of malignant breast cells. In vitro studies on estrogen receptor (ER)+ breast cancer cells have shown that the addition of medroxyprogesterone acetate (MPA) to estradiol (E(2)) produces a significantly higher increase of the mRNA levels and activities of estrogen-activating enzymes aromatase, 17beta hydroxysteroid dehydrogenase type-1 and sulfatase when compared with progesterone plus E(2). In randomised trial performed on ovariectomised adult female monkeys, oral E(2) plus MPA have resulted in a significantly greater proliferation of breast lobular and ductal epithelium when compared with placebo, whereas E(2) plus micronised progesterone have not. In the same experimental model, oral E(2) plus MPA have been found to induce the expression of genes encoding epidermal growth factor receptor (EGFR) ligands and downstream targets, whereas E(2) alone or E(2) plus micronised progesterone had no or modest effects on EGFR-related genes. In last years, some clinical studies on HRT users have shown that androgenic progestin- or MPA-based formulations are associated with an increased breast cancer incidence, whereas micronised progesterone- or dydrogesterone-based formulations are not. Further basic and clinical investigations on this topic are strongly warranted to elucidate whether the choice of the progestagen component in combined HRT could be of clinical relevance as for breast cancer risk.

Progesterone and breast cancer

Progesterone is an ovarian steroid hormone that is essential for normal breast development during puberty and in preparation for lactation and breastfeeding. The actions of progesterone are primarily mediated by its high-affinity receptors, which include the classical progesterone receptor (PR)-A and -B isoforms, located in diverse tissues, including the brain, where progesterone controls reproductive behavior, and the breast and reproductive organs. Progestins are frequently prescribed for contraception or during postmenopausal hormone replacement therapy, in which progestins are combined with estrogen as a means to block estrogen-induced endometrial growth. The role of estrogen as a potent breast mitogen is undisputed, and inhibitors of the estrogen receptor and estrogen-producing enzymes (aromatases) are effective first-line cancer therapies. However, PR action in breast cancer is grossly understudied and remains controversial. Herein, we review existing evidence and discuss the challenges to defining a role for progesterone in breast cancer.

Comparative proteomics and molecular mechanical analysis in CDA-II induced therapy of LCI-D20 hepatocellular carcinoma model

PURPOSE

To investigate the differential proteins and related molecular mechanism of CDA-II (cell differentiation agent-II) induced therapy on a human hepatocellular carcinoma model in nude mice with high metastatic potential (LCI-D20).

METHODS

After tumors were transplanted 11 days, mice were intraperitoneally injected with CDA-II (1,800 mg/kg) for 20 days continuously. The tumor growth-inhibitory efficiency in CDA-II treated groups was calculated. Proteins extracted from tumor tissue were separated by two-dimensional gel electrophoresis (2DE) and the differential proteins were identified by matrix assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS). Western blotting (WB) was performed to verify the expression of certain candidate proteins. Reverse transcription-polymerase chain reaction (RT-PCR) was engaged to study the molecular mechanism of the therapy.

RESULTS

CDA-II suppressed the growth and metastasis of tumor. The tumor growth-inhibitory efficiency was 41.8%. In total, 27 differentially expressed proteins were identified, including HSP27, UGDH, CK8, Hsp60, ENOA and AnxA5, with functions involved in oncogene expression and/or cell differentiation. In addition, apparent alternations of HSP60 and beta-actin expression levels and their different posttranslational modifications (PTMs) were investigated. RT-PCR analysis confirmed that the cancer related genes c-myc, N-ras and MMP-9 were significantly down-regulated.

CONCLUSION

Our results demonstrate that CDA-II presence can change the proteome profiling and favors of the tumor suppression in LCI-D20 cell differentiation. Our results also suggest that the dynamic PTM of HSP60 expression levels could be used to predict HCC and might be a promising and useful biomarker to prognosticate CDA-II therapeutic efficacy.

The Year in Basic Science: update of estrogen plus progestin therapy for menopausal hormone replacement implicating stem cells in the increased breast cancer risk

This transcript is based on my The Year in Basic Science lecture at ENDO 2008. I reviewed current data surrounding hormone replacement therapy and the relationship between systemic estrogen plus progestin (E+P) treatment and increased breast cancer risk, and I explored the hypothesis that women who develop breast cancer while on E+P had occult, undiagnosed disease before they started therapy. Beginning with recent hormone replacement therapy data focusing on E+P and its association with breast cancer to set the stage, the lecture then reviewed our newly published data that progestins expand breast cancer stem cells. Finally, the issues of occult or undiagnosed breast cancer in presumably healthy women, and of tumor dormancy in breast cancer survivors, were brought to bear on the discussion. Taken together, these apparently disparate themes allowed me to suggest the idea that systemic progestins have the ability to reawaken cancers that were presumed to be either nonexistent or cured. To avoid this potentially devastating outcome while retaining the benefits of E+P, I advocated the use of local P delivery methods, rather than the currently popular systemic routes.

Challenges to defining a role for progesterone in breast cancer

Progesterone is an ovarian steroid hormone that is essential for normal breast development during puberty and in preparation for lactation. The actions of progesterone are primarily mediated by its high affinity receptors, including the classical progesterone receptor (PR) -A and -B isoforms, located in diverse tissues such as the brain where progesterone controls reproductive behavior, and the breast and reproductive organs. Progestins are frequently prescribed as contraceptives or to alleviate menopausal symptoms, wherein progestin is combined with estrogen as a means to block estrogen-induced endometrial growth. Estrogen is undisputed as a potent breast mitogen, and inhibitors of the estrogen receptor (ER) and estrogen producing enzymes (aromatases) are effective first-line cancer therapies. However, PR action in breast cancer remains controversial. Herein, we review existing evidence from in vitro and in vivo models, and discuss the challenges to defining a role for progesterone in breast cancer.

Progestins in hormone replacement therapies reactivate cancer stem cells in women with preexisting breast cancers: a hypothesis

Why does hormone replacement therapy (HRT) with estrogens plus progestins increase the risk of breast cancer? First, experimental estrogen receptor-positive (ER+) and progesterone receptor-positive (PR+) human breast cancers contain a rare subpopulation of ER(-), PR(-) cancer stem cells. Especially in small, nascent ER+, PR+ tumor colonies, progestins, but not estrogens, reactivate cells with ER(-), PR(-) stem-like properties. Second, there is a reservoir of occult, undetected, preinvasive breast cancer in some women who are candidates for HRT. We propose that women who develop breast cancer while on estrogens plus progestins harbor undiagnosed nascent disease before the start of therapy. The progestin component, in a nonproliferative step, reactivates receptor-negative cancer stem cells within such germinal, perhaps even dormant tumors. After reacquiring receptors, these tumor cells are expanded by the mitogenic properties of estrogens. We argue that screening methods need to be improved to detect small, preexisting malignancies prior to the start of HRT. Women harboring such disease should be excluded from regimens that include systemic progestins.

Rare steroid receptor-negative basal-like tumorigenic cells in luminal subtype human breast cancer xenografts

There are two major subtypes of human breast cancers: the luminal, estrogen, and progesterone receptor-positive, cytokeratin 18-positive (ER(+)PR(+)CK18(+)) subtype, and the basal ER(-)PR(-)CK18(-)CK5(+) subtype. Tumor-initiating cells (CD44(+)) have been described for human breast cancers; whether these are common to the two subtypes is unknown. We have identified a rare population of cells that are both CD44(+) and ER(-)PR(-)CK5(+) in luminal-like ER(+)PR(+) T47D human breast tumor xenografts. The tumor-isolated CD44(+) cell fraction was highly enriched for clonogenic (in vitro culture) and tumorigenic (in vivo reimplantation) cells compared with the CD44(-) cell fraction. Rare ER(-)PR(-)CK5(+) cells were present within CD44(+)-derived colonies. Tumor-isolated cells placed in minimal media also contained rare ER(-)PR(-)CK5(+) cells at early time points (<10 cells); however, this population did not expand with increasing colony size. The number of ER(+)PR(+)CK5(-) cells, conversely, increased linearly with colony growth. Similary, tumors originating in vivo from CD44(+) cells contained a rare static ER(-)PR(-)CK5(+) population, an intermediate ER(-)PR(-)CK5(-) population, and an expanding ER(+)PR(+)CK5(-) population. Putative ER(+)PR(+)CK5(+) transitional cells could be seen only in colonies or tumors treated with a progestin. We propose that luminal ER(+)PR(+) breast tumors contain a minor ER(-)PR(-)CK5(+) population that has the capacity to generate the majority of ER(+)PR(+)CK18(+)CK5(-) cells. Luminal breast cancers are treated with endocrine therapies that target ER. The rare ER(-)PR(-)CK5(+) progenitor cells would escape such treatments and survive to repopulate the tumor.

Progesterone receptor isoform functions in normal breast development and breast cancer

Progesterone acting through two isoforms of the progesterone receptor (PR), PRA and PRB, regulates proliferation and differentiation in the normal mammary gland in mouse, rat, and human. Progesterone and PR have also been implicated in the etiology and pathogenesis of human breast cancer. The focus of this review is recent advances in understanding the role of the PR isoform-specific functions in the normal breast and in breast cancer. Also discussed is information obtained from rodent studies and their relevance to our understanding of the role of progestins in breast cancer etiology.

Progesterone receptor action: translating studies in breast cancer models to clinical insights

Progesterone receptors (PR) are useful prognostic indicators of breast cancers likely to respond to anti-estrogen receptor (ER) therapies. However, the role of progesterone, therapeutic progestins, or unliganded or liganded PRin breast cancer development or progression remains controversial. PR are ligand-activated transcription factors that act in concert with intracellular signaling pathways as "sensors" of multiple growth factor inputs to hormonally regulated tissues, such as the breast. The recently defined induction of rapid signaling events upon progestin-binding to PR-B provides a means to ensure that receptors and coregulators are appropriately phosphorylated as part of optimal transcription complexes. PR-activated kinase cascades may provide additional avenues for progestin-regulated gene expression independent of PR nuclear action. Herein, we present an overview ofprogesterone/PR and signaling cross-talk in breast cancer models and discuss the potential significance ofprogestin/PR action in breast cancer biology using examples from both in vitro and in vivo models, as well as limited clinical data. Kinases are emerging as key mediators of PR action. Cross-talk between PR and membrane-initiated signaling events suggests a mechanism for coordinated regulation ofgene subsets by mitogenic stimuli in hormonally responsive normal tissues. Dysregulation of this cross-talk mechanism may contribute to breast cancer biology; further studies are needed to address the potential for targeting PR in addition to ER and selected protein kinases as part of more effective breast cancer therapies.

Progestin-dependent progression of human breast tumor xenografts: a novel model for evaluating antitumor therapeutics

Recent clinical trials indicate that synthetic progestins may stimulate progression of breast cancer in postmenopausal women, a result that is consistent with studies in chemically-induced breast cancer models in rodents. However, progestin-dependent progression of breast cancer tumor xenografts has not been shown. This study shows that xenografts obtained from BT-474 and T47-D human breast cancer cells without Matrigel in estrogen-supplemented nude mice begin to regress within days after tumor cell inoculation. However, their growth is resumed if animals are supplemented with progesterone. The antiprogestin RU-486 blocks progestin stimulation of growth, indicating involvement of progesterone receptors. Exposure of xenografts to medroxyprogesterone acetate, a synthetic progestin used in postmenopausal hormone replacement therapy and oral contraception, also stimulates growth of regressing xenograft tumors. Tumor progression is dependent on expression of vascular endothelial growth factor (VEGF); growth of progestin-dependent tumors is blocked by inhibiting synthesis of VEGF or VEGF activity using a monoclonal anti-VEGF antibody (2C3) or by treatment with PRIMA-1, a small-molecule compound that reactivates mutant p53 into a functional protein and blocks VEGF production. These results suggest a possible model system for screening potential therapeutic agents for their ability to prevent or inhibit progestin-dependent human breast tumors. Such a model could potentially be used to screen for safer antiprogestins, antiangiogenic agents, or for compounds that reactivate mutant p53 and prevent progestin-dependent progression of breast disease.

Spontaneous fusion with, and transformation of mouse stroma by, malignant human breast cancer epithelium

Adenocarcinoma cells from the pleural effusion of a patient with breast cancer were injected into the mammary glands of nude mice and grown into solid tumors. A cell line derived from these tumors expressed alpha-smooth muscle actin but not human cytokeratin 7, indicating "activated" stroma of mouse origin. Cells in mitosis exhibited mainly polyploid mouse karyotypes, but 30% had mixed mouse and human chromosomes, among which 8% carried mouse/human translocations. Nuclei of interphase cells were 64% hybrid. Hybrid mouse/human nuclei were also detected in the primary xenograft. Thus, synkaryons formed in the solid tumor by spontaneous fusion between the malignant human epithelium and the surrounding normal host mouse stroma. The transformed stroma-derived cells are tumorigenic with histopathologic features of malignancy, suggesting a new mechanism for tumor progression.

Progesterone receptor isoforms and proliferation in the rat mammary gland during development

Progesterone (P), acting through progesterone receptor (PR) isoforms A and B, plays an important role in normal mammary gland development and is implicated in the etiology of breast cancer. Because of significant similarities between human and rat mammary gland development and hormonal responsiveness of mammary cancers, we investigated P action in the rat mammary gland. By immunohistochemical methods we determined PRA and PRB expression at puberty, sexual maturity, pregnancy, and lactation and after postlactational involution and their functional roles in the regulation of proliferation. PRA expression was restricted to luminal epithelial cells, whereas PRB was expressed in both luminal and myoepithelial cells, indicating a novel role of PRB in myoepithelial cell regulation. The majority of PRA-positive (PRA+) cells coexpressed PRB. In the pubertal and adult virgin mammary gland, PRA+PRB+ cells also expressed nuclear cyclin D1 but did not contain the proliferation marker bromodeoxyuridine. Based on a lack of phosphorylated retinoblastoma protein expression and the expression patterns of the cyclin-dependent kinase inhibitors p21 and p27 in these cells, we conclude that PRA+PRB+ cells appear to be cell cycle arrested and do not proliferate. PRA+ cells were decreased in the adult gland and during and after pregnancy. The percentage of PRB+ cells was relatively constant throughout development, and in a significant proportion of cells, only PRB was detected. During development, and especially during pregnancy, a high percentage of PRB+ cells were positive for bromodeoxyuridine. From this observation, we conclude that these cells proliferate and that P acting through PRB may directly stimulate proliferation.

Keratin 6 is not essential for mammary gland development

INTRODUCTION

Keratin 6 (K6) has previously been identified as a marker of early mammary gland development and has also been proposed to be a marker of mammary gland progenitor cells. However, the function of K6 in the mammary gland was not known, so we examined the expression pattern of the protein during both embryonic and postnatal mammary development, as well as the mammary gland phenotype of mice that were null for both K6a and K6b isoforms.

METHOD

Immunostaining was performed to determine the expression pattern of K6a throughout mammary gland development, from the embryonic mammary bud to lactation. Double immunofluorescence was used to co-localize K6 with known markers of mammary gland development. Wild-type and K6ab-null mammary tissues were transplanted into the cleared fat pads of nude mice and the outgrowths were analyzed for morphology by whole-mount staining and for markers of mammary epithelium by immunostaining. Finally, progesterone receptor (PR) and bromodeoxyuridine co-localization was quantified by double immunofluorescence in wild-type and K6ab-null mammary outgrowths.

RESULTS

Here we report that K6 is expressed earlier than described previously, by embryonic day 16.5. K6a is the predominant isoform expressed in the mammary gland, localized in the body cells and luminal epithelial cells but not in the cap cells or myoepithelial cells. Co-localization studies showed that most K6a-positive cells express steroid receptors but do not proliferate. When both the K6a and K6b genes are deleted, mammary gland development appears normal, with similar expression of most molecular markers examined in both the pubertal gland and the mature gland. Loss of K6a and K6b, however, leads to an increase in the number of steroid-receptor-positive cells, and increased co-localization of steroid receptor expression and proliferation was observed.

CONCLUSION

Although K6a was not essential for mammary gland development, loss of both K6a and K6b resulted in an increase in PR-positive mammary epithelial cells and decreased proliferation after exposure to steroid hormones. There was also increased co-localization of PR and bromodeoxyuridine, suggesting alterations in patterning events important for normal lobuloalveolar development.

Estradiol regulates different genes in human breast tumor xenografts compared with the identical cells in culture

In breast cancers, estrogen receptor (ER) levels are highly correlated with response to endocrine therapies. We sought to define mechanisms of estrogen (E) signaling in a solid breast tumor model using gene expression profiling. ER(+) T47D-Y human breast cancer cells were grown as xenografts in ovariectomized nude mice under four conditions: 1) 17beta-estradiol for 8 wk (E); 2) without E for 8 wk (control); 3) E for 7 wk followed by 1 wk of E withdrawal (Ewd); or 4) E for 8 wk plus tamoxifen for the last week. E-regulated genes were defined as those that differed significantly between control and E and/or between E and Ewd or control and Ewd. These protocols generated 188 in vivo E-regulated genes that showed two major patterns of regulation. Approximately 46% returned to basal states after Ewd (class I genes); 53% did not (class II genes). In addition, more than 70% of class II-regulated genes also failed to reverse in response to tamoxifen. These genes may be interesting for the study of hormone-resistance issues. A subset of in vivo E-regulated genes appears on lists of clinical ER discriminator genes. These may be useful therapeutic targets or markers of E activity. Comparison of in vivo E-regulated genes with those regulated in identical cells in vitro after 6 and 24 h of E treatment demonstrate only 11% overlap. This indicates the extent to which gene expression profiles are uniquely dependent on hormone-treatment times and the cellular microenvironment.