Activation of estrogen signaling pathways collaborates with loss of Brca1 to promote development of ERalpha-negative and ERalpha-positive mammary preneoplasia and cancer

Overexpression of Estrogen Receptor α in Mammary Glands of Aging Mice Is Associated with a Proliferative Risk Signature and Generation of Estrogen Receptor α-Positive Mammary Adenocarcinomas

Age is a risk factor for human estrogen receptor-positive breast cancer, with highest prevalence following menopause. While transcriptome risk profiling is available for human breast cancers, it is not yet developed for prognostication for primary or secondary breast cancer development utilizing at-risk breast tissue. Both estrogen receptor α (ER) and aromatase overexpression have been linked to human breast cancer. Herein, conditional genetically engineered mouse models of estrogen receptor 1 (Esr1) and cytochrome P450 family 19 subfamily A member 1 (CYP19A1) were used to show that induction of Esr1 overexpression just before or with reproductive senescence and maintained through age 30 months resulted in significantly higher prevalence of estrogen receptor-positive adenocarcinomas than CYP19A1 overexpression. All adenocarcinomas tested showed high percentages of ER+ cells. Mammary cancer development was preceded by a persistent proliferative transcriptome risk signature initiated within 1 week of transgene induction that showed parallels to the Prosigna/Prediction Analysis of Microarray 50 human prognostic signature for early-stage human ER+ breast cancer. CYP19A1 mice also developed ER+ mammary cancers, but histology was more divided between adenocarcinoma and adenosquamous, with one ER- adenocarcinoma. Results demonstrate that, like humans, generation of ER+ adenocarcinoma in mice was facilitated by aging mice past the age of reproductive senescence. Esr1 overexpression was associated with a proliferative estrogen pathway-linked signature that preceded appearance of ER+ mammary adenocarcinomas.

Expression and activation of nuclear hormone receptors result in neuronal differentiation and favorable prognosis in neuroblastoma

BACKGROUND

Neuroblastoma (NB), a childhood tumor derived from the sympathetic nervous system, presents with heterogeneous clinical behavior. While some tumors regress spontaneously without medical intervention, others are resistant to therapy, associated with an aggressive phenotype. MYCN-amplification, frequently occurring in high-risk NB, is correlated with an undifferentiated phenotype and poor prognosis. Differentiation induction has been proposed as a therapeutic approach for high-risk NB. We have previously shown that MYCN maintains an undifferentiated state via regulation of the miR-17 ~ 92 microRNA cluster, repressing the nuclear hormone receptors (NHRs) estrogen receptor alpha (ERα) and the glucocorticoid receptor (GR).

METHODS

Cell viability was determined by WST-1. Expression of differentiation markers was analyzed by Western blot, RT-qPCR, and immunofluorescence analysis. Metabolic phenotypes were studied using Agilent Extracellular Flux Analyzer, and accumulation of lipid droplets by Nile Red staining. Expression of angiogenesis, proliferation, and neuronal differentiation markers, and tumor sections were assessed by immunohistochemistry. Gene expression from NB patient as well as adrenal gland cohorts were analyzed using GraphPad Prism software (v.8) and GSEA (v4.0.3), while pseudo-time progression on post-natal adrenal gland cells from single-nuclei transcriptome data was computed using scVelo.

RESULTS

Here, we show that simultaneous activation of GR and ERα potentiated induction of neuronal differentiation, reduced NB cell viability in vitro, and decreased tumor burden in vivo. This was accompanied by a metabolic reprogramming manifested by changes in the glycolytic and mitochondrial functions and in lipid droplet accumulation. Activation of the retinoic acid receptor alpha (RARα) with all-trans retinoic acid (ATRA) further enhanced the differentiated phenotype as well as the metabolic switch. Single-cell nuclei transcriptome analysis of human adrenal glands indicated a sequential expression of ERα, GR, and RARα during development from progenitor to differentiated chromaffin cells. Further, in silico analysis revealed that patients with higher combined expression of GR, ERα, and RARα mRNA levels had elevated expression of neuronal differentiation markers and a favorable outcome.

CONCLUSION

Together, our findings suggest that combination therapy involving activation of several NHRs could be a promising pharmacological approach for differentiation treatment of NB patients.

Bilateral Salpingo-oophorectomy and Breast Cancer Risk for BRCA1 and BRCA2 Mutation Carriers: Assessing the Evidence

Without preventive interventions, women with germline pathogenic variants in BRCA1 or BRCA2 have high lifetime risks for breast cancer and tubo-ovarian cancer. The increased risk for breast cancer starts at a considerably younger age than that for tubo-ovarian cancer. Risk-reducing bilateral salpingo-oophorectomy (rrBSO) is effective in reducing tubo-ovarian cancer risk for BRCA1 and BRCA2 mutation carriers, but whether it reduces breast cancer risk is less clear. All studies of rrBSO and breast cancer risk are observational in nature and subject to various forms of bias and confounding, thus limiting conclusions that can be drawn about causation. Early studies supported a statistically significant protective association for rrBSO on breast cancer risk, which is reflected by several international guidelines that recommend consideration of premenopausal rrBSO for breast cancer risk reduction. However, these historical studies were hampered by the presence of several important biases, including immortal person-time bias, confounding by indication, informative censoring, and confounding by other risk factors, which may have led to overestimation of any protective benefit. Contemporary studies, specifically designed to reduce some of these biases, have yielded contradictory results. Taken together, there is no clear and consistent evidence for a role of premenopausal rrBSO in reducing breast cancer risk in BRCA1 or BRCA2 mutation carriers.

Disruption of STAT5A and NMI signaling axis leads to ISG20-driven metastatic mammary tumors

Molecular dynamics of developmental processes are repurposed by cancer cells to support cancer initiation and progression. Disruption of the delicate balance between cellular differentiation and plasticity during mammary development leads to breast cancer initiation and metastatic progression. STAT5A is essential for differentiation of secretory mammary alveolar epithelium. Active STAT5A characterizes breast cancer patients for favorable prognosis. N-Myc and STAT Interactor protein (NMI) was initially discovered as a protein that interacts with various STATs; however, the relevance of these interactions to normal mammary development and cancer was not known. We observe that NMI protein is expressed in the mammary ductal epithelium at the onset of puberty and is induced in pregnancy. NMI protein is decreased in 70% of patient specimens with metastatic breast cancer compared to primary tumors. Here we present our finding that NMI and STAT5A cooperatively mediate normal mammary development. Loss of NMI in vivo caused a decrease in STAT5A activity in normal mammary epithelial as well as breast cancer cells. Analysis of STAT5A mammary specific controlled genetic program in the context of NMI knockout revealed ISG20 (interferon stimulated exonuclease gene 20, a protein involved in rRNA biogenesis) as an unfailing negatively regulated target. Role of ISG20 has never been described in metastatic process of mammary tumors. We observed that overexpression of ISG20 is increased in metastases compared to matched primary breast tumor tissues. Our observations reveal that NMI-STAT5A mediated signaling keeps a check on ISG20 expression via miR-17–92 cluster. We show that uncontrolled ISG20 expression drives tumor progression and metastasis.

Non-Genomic Actions of Estrogens on the DNA Repair Pathways Are Associated With Chemotherapy Resistance in Breast Cancer

Estrogens have been implicated in the etiology of breast cancer for a long time. It has been stated that long-term exposure to estrogens is associated with a higher incidence of breast cancer, since estradiol (E₂) stimulates breast cell growth; however, its effect on DNA damage/repair is only starting to be investigated. Recent studies have documented that estrogens are able to modify the DNA damage response (DDR) and DNA repair mechanisms. On the other hand, it has been proposed that DDR machinery can be altered by estrogen signaling pathways, that can be related to cancer progression and chemoresistance. We have demonstrated that E₂ promotes c-Src activation and breast cancer cell motility, through a non-genomic pathway. This review discusses scientific evidence supporting this non-genomic mechanism where estrogen modifies the DNA repair pathways, and its relationship to potential causes of chemoresistance.

Environmental Exposures during Puberty: Window of Breast Cancer Risk and Epigenetic Damage

During puberty, a woman’s breasts are vulnerable to environmental damage (“window of vulnerability”). Early exposure to environmental carcinogens, endocrine disruptors, and unhealthy foods (refined sugar, processed fats, food additives) are hypothesized to promote molecular damage that increases breast cancer risk. However, prospective human studies are difficult to perform and effective interventions to prevent these early exposures are lacking. It is difficult to prevent environmental exposures during puberty. Specifically, young women are repeatedly exposed to media messaging that promotes unhealthy foods. Young women living in disadvantaged neighborhoods experience additional challenges including a lack of access to healthy food and exposure to contaminated air, water, and soil. The purpose of this review is to gather information on potential exposures during puberty. In future directions, this information will be used to help elementary/middle-school girls to identify and quantitate environmental exposures and develop cost-effective strategies to reduce exposures.

Environmental exposures during windows of susceptibility for breast cancer: a framework for prevention research

Background

The long time from exposure to potentially harmful chemicals until breast cancer occurrence poses challenges for designing etiologic studies and for implementing successful prevention programs. Growing evidence from animal and human studies indicates that distinct time periods of heightened susceptibility to endocrine disruptors exist throughout the life course. The influence of environmental chemicals on breast cancer risk may be greater during several windows of susceptibility (WOS) in a woman’s life, including prenatal development, puberty, pregnancy, and the menopausal transition. These time windows are considered as specific periods of susceptibility for breast cancer because significant structural and functional changes occur in the mammary gland, as well as alterations in the mammary micro-environment and hormone signaling that may influence risk. Breast cancer research focused on these breast cancer WOS will accelerate understanding of disease etiology and prevention.

Main text

Despite the plausible heightened mechanistic influences of environmental chemicals on breast cancer risk during time periods of change in the mammary gland’s structure and function, most human studies of environmental chemicals are not focused on specific WOS. This article reviews studies conducted over the past few decades that have specifically addressed the effect of environmental chemicals and metals on breast cancer risk during at least one of these WOS. In addition to summarizing the broader evidence-base specific to WOS, we include discussion of the NIH-funded Breast Cancer and the Environment Research Program (BCERP) which included population-based and basic science research focused on specific WOS to evaluate associations between breast cancer risk and particular classes of endocrine-disrupting chemicals—including polycyclic aromatic hydrocarbons, perfluorinated compounds, polybrominated diphenyl ethers, and phenols—and metals. We outline ways in which ongoing transdisciplinary BCERP projects incorporate animal research and human epidemiologic studies in close partnership with community organizations and communication scientists to identify research priorities and effectively translate evidence-based findings to the public and policy makers.

Conclusions

An integrative model of breast cancer research is needed to determine the impact and mechanisms of action of endocrine disruptors at different WOS. By focusing on environmental chemical exposure during specific WOS, scientists and their community partners may identify when prevention efforts are likely to be most effective.

Peroxisome proliferator-activated receptor gamma and BRCA1

Peroxisome proliferator-activated receptor gamma agonists have been proposed as breast cancer preventives. Individuals who carry a mutated copy of BRCA1, DNA repair-associated gene, are at increased risk for development of breast cancer. Published data in the field suggest there could be interactions between peroxisome proliferator-activated receptor gamma and BRCA1 that could influence the activity of peroxisome proliferator-activated receptor gamma agonists for prevention. This review explores these possible interactions between peroxisome proliferator-activated receptor gamma, peroxisome proliferator-activated receptor gamma agonists and BRCA1 and discusses feasible experimental directions to provide more definitive information on the potential connections.

Proliferation and ovarian hormone signaling are impaired in normal breast tissues from women with BRCA1 mutations: benefit of a progesterone receptor modulator treatment as a breast cancer preventive strategy in women with inherited BRCA1 mutations

Women with inherited BRCA1 mutations have an elevated risk (40-80%) for developing breast and ovarian cancers. Reproductive history has been reported to alter this risk, suggesting a relationship between ovarian hormone signaling and BRCA1-related tumor development. BRCA1 interactions with estrogen receptor (ER) and progesterone receptor (PR) signaling were previously described in human breast cancer cell lines and mouse models. However, few studies have examined the effect of ovarian hormone regulation in normal human breast tissues bearing a heterozygous BRCA1 mutation. This study compares the proliferation level (Ki67) and the expression of ER, PR, and of the PR target gene, fatty acid synthase (FASN), in histologically normal breast tissues from women with BRCA1 mutations (BRCA1+/mut, n=23) or without BRCA1 mutations (BRCA1+/+, n=28). BRCA1+/mut tissues showed an increased proliferation and impaired hormone receptor expression with a marked loss of the PR isoform, PR-B. Responses to estradiol and progesterone treatments in BRCA1+/mut and BRCA1+/+ breast tissues were studied in a mouse xenograft model, and showed that PR and FASN expression were deregulated in BRCA1+/mut breast tissues. Progesterone added to estradiol treatment increased the proliferation in a subset of BRCA1+/mut breast tissues. The PR inhibitor, ulipristal acetate (UPA), was able to reverse this aberrant progesterone-induced proliferation. This study suggests that a subset of women with BRCA1 mutations could be candidates for a UPA treatment as a preventive breast cancer strategy.

BRCA1-mimetic compound NSC35446.HCl inhibits IKKB expression by reducing estrogen receptor-α occupancy in the IKKB promoter and inhibits NF-κB activity in antiestrogen-resistant human breast cancer cells

PURPOSE

We previously identified small molecules that fit into a BRCA1-binding pocket within estrogen receptor-alpha (ERα), mimic the ability of BRCA1 to inhibit ERα activity ("BRCA1-mimetics"), and overcome antiestrogen resistance. One such compound, the hydrochloride salt of NSC35446 ("NSC35446.HCl"), also inhibited the growth of antiestrogen-resistant LCC9 tumor xenografts. The purpose of this study was to investigate the down-stream effects of NSC35446.HCl and its mechanism of action.

METHODS

Here, we studied antiestrogen-resistant (LCC9, T47DCO, MCF-7/RR, LY2), ERα-negative (MDA-MB-231, HCC1806, MDA-MB-468), and antiestrogen-sensitive (MCF-7) cell lines. Techniques utilized include RNA-seq, qRT-PCR, cell growth analysis, cell-cycle analysis, Western blotting, luciferase reporter assays, TUNEL assays, in silico analysis of the IKKB gene, and ChIP assays.

RESULTS

SC35446.HCl inhibited proliferation and induced apoptosis in antiestrogen-resistant LCC9, T47DCO, MCF-7/RR, and LY2 cells but not in ERα-negative breast cancer cell lines. IKKB (IKKβ, IKBKB), an upstream activator of NF-κB, was identified as a BRCA1-mimetic-regulated gene based on an RNA-seq analysis. NSC35446.HCl inhibited IKKB, IKKA, and IKKG/NEMO mRNA and protein expression in LCC9 cells. NSC35446.HCl also inhibited NF-κB activity and expression of NF-κB target genes. In silico analysis of the IKKB promoter identified nine estrogen response element (ERE) half-sites and one ERE-like full-site. ChIP assays revealed that ERα was recruited to the ERE-like full-site and five of the nine half-sites and that ERα recruitment was inhibited by NSC35446.HCl in LCC9 and T47DCO cells.

CONCLUSIONS

These studies identify functional EREs in the IKKB promoter and identify IKKB as an ERα and NSC35446.HCl-regulated gene, and they suggest that NF-κB and IKKB, which were previously linked to antiestrogen resistance, are targets for NSC35446.HCl in reversing antiestrogen resistance.

Responsiveness of Brca1 and Trp53 Deficiency-Induced Mammary Preneoplasia to Selective Estrogen Modulators versus an Aromatase Inhibitor in Mus musculus

An intervention study initiated at age 4 months compared the impact of tamoxifen (25 mg), raloxifene (22.5 mg), and letrozole (2.5 mg) administered by 60-day release subcutaneous pellet on mammary preneoplasia prevalence at age 6 months in conditional genetically engineered mouse models with different Breast cancer 1 (Brca1) gene dosages targeted to mammary epithelial cells and germline Tumor protein P53 (Trp53) haploinsufficiency (10-16/cohort). The proportion of unexposed control mice demonstrating mammary preneoplasia at age 6 months was highest in Brca1^{fl11/fl11/Cre/p53-/+} (54%) mice followed by Brca1^{WT/fl11/Cre/p53-/+} mice (30%). By age 12 months, invasive mammary cancers appeared in 80% of Brca1^{fl11/fl11/Cre/p53-/+} and 42% of Brca1^{WT/fl11/Cre/p53-/+} control unexposed mice. The spectrum of cancer histology was similar in both models without somatic mutation of the nongenetically engineered Brca1, Trp53, Brca2, or Death-associated protein kinase 3 (Dapk3) alleles. Two-month exposure to tamoxifen, raloxifene, and letrozole significantly reduced estrogen-mediated tertiary branching by 65%, 71%, and 78%, respectively, in Brca1^{fl11/fl11/Cre/p53-/+} mice at age 6 months. However, only letrozole significantly reduced hyperplastic alveolar nodules (HAN) prevalence (by 52%) and number (by 30%) and invasive cancer appeared despite tamoxifen exposure. In contrast, tamoxifen significantly reduced HAN number by 95% in Brca1^{WT/fl11/Cre/p53-/+} mice. Control mice with varying combinations of the different genetically modified alleles and MMTV-Cre transgene demonstrated that the combination of Brca1 insufficiency and Trp53 haploinsufficiency was required for appearance of preneoplasia and no individual genetic alteration confounded the response to tamoxifen. In summary, although specific antihormonal approaches showed effectiveness, with Brca1 gene dosage implicated as a possible modifying variable, more effective chemopreventive approaches for Brca1 mutation-induced cancer may require alternative and/or additional agents. Cancer Prev Res; 10(4); 244-54. ©2017 AACR.

Wnt signaling in cancer

Wnt signaling is one of the key cascades regulating development and stemness, and has also been tightly associated with cancer. The role of Wnt signaling in carcinogenesis has most prominently been described for colorectal cancer, but aberrant Wnt signaling is observed in many more cancer entities. Here, we review current insights into novel components of Wnt pathways and describe their impact on cancer development. Furthermore, we highlight expanding functions of Wnt signaling for both solid and liquid tumors. We also describe current findings how Wnt signaling affects maintenance of cancer stem cells, metastasis and immune control. Finally, we provide an overview of current strategies to antagonize Wnt signaling in cancer and challenges that are associated with such approaches.

Primary cancer cell culture: mammary-optimized vs conditional reprogramming

The impact of different culture conditions on biology of primary cancer cells is not always addressed. Here, conditional reprogramming (CRC) was compared with mammary-optimized EpiCult-B (EpiC) for primary mammary epithelial cell isolation and propagation, allograft generation, and genome-wide transcriptional consequences using cancer and non-cancer mammary tissue from mice with different dosages of Brca1 and p53 Selective comparison to DMEM was included. Primary cultures were established with all three media, but CRC was most efficient for initial isolation (P<0.05). Allograft development was faster using cells grown in EpiC compared with CRC (P<0.05). Transcriptome comparison of paired CRC and EpiC cultures revealed 1700 differentially expressed genes by passage 20. CRC promoted Trp53 gene family upregulation and increased expression of epithelial differentiation genes, whereas EpiC elevated expression of epithelial-mesenchymal transition genes. Differences did not persist in allografts where both methods yielded allografts with relatively similar transcriptomes. Restricting passage (<7) reduced numbers of differentially expressed genes below 50. In conclusion, CRC was most efficient for initial cell isolation but EpiC was quicker for allograft generation. The extensive culture-specific gene expression patterns that emerged with longer passage could be limited by reducing passage number when both culture transcriptomes were equally similar to that of the primary tissue. Defining impact of culture condition and passage on the transcriptome of primary cells could assist experimental design and interpretation. For example, differences that appear with passage and culture condition are potentially exploitable for comparative studies targeting specific biological networks in different transcriptional environments.

MicroRNA-206 is differentially expressed in Brca1-deficient mice and regulates epithelial and stromal cell compartments of the mouse mammary gland

Depletion of Brca1 leads to defects in mouse mammary gland development and mammary tumors in humans and mice. To explore the role of microRNAs (miRNAs) in this process, we examined the mammary glands of MMTV-Cre Brca1^Co/Co mice for differential miRNA expression using a candidate approach. Several miRNAs were differentially expressed in mammary tissue at day 1 of lactation and in mammary epithelial cell lines in which Brca1 messenger RNA (mRNA) levels have been reduced. Functional studies revealed that several of these miRNAs regulate mammary epithelial cell function in vitro, including miR-206. Creation and analysis of MMTV-miR-206 transgenic mice showed no effect on lactational mammary development and no tumors, but indicates a role in mammary tissue remodeling in mature mice, potentially involving Igf-1 and Sfrp1. These results indicate the potential of miRNAs to mediate the consequences of Brca1 loss and suggest a novel function for miR-206.

A new class of small molecule estrogen receptor-alpha antagonists that overcome anti-estrogen resistance

Previous studies indicate that BRCA1 protein binds to estrogen receptor-alpha (ER) and inhibits its activity. Here, we found that BRCA1 over-expression not only inhibits ER activity in anti-estrogen-resistant LCC9 cells but also partially restores their sensitivity to Tamoxifen. To simulate the mechanism of BRCA1 inhibition of ER in the setting of Tamoxifen resistance, we created a three-dimensional model of a BRCA1-binding cavity within the ER/Tamoxifen complex; and we screened a pharmacophore database to identify small molecules that could fit into this cavity. Among the top 40 “hits”, six exhibited potent ER inhibitory activity in anti-estrogen-sensitive MCF-7 cells and four of the six exhibited similar activity (IC₅₀ ≤ 1.0 μM) in LCC9 cells. We validated the model by mutation analysis. Two representative compounds (4631-P/1 and 35466-L/1) inhibited ER-dependent cell proliferation in Tamoxifen-resistant cells (LCC9 and LCC2) and partially restored sensitivity to Tamoxifen. The compounds also disrupted the association of BRCA1 with ER. In electrophoretic mobility shift assays, the compounds caused dissociation of ER from a model estrogen response element. Finally, a modified form of compound 35446 (hydrochloride salt) inhibited growth of LCC9 tumor xenografts at non-toxic concentrations. These results identify a novel group of small molecules that can overcome Tamoxifen resistance.

Small-molecule "BRCA1-mimetics" are antagonists of estrogen receptor-α

CONTEXT

Resistance to conventional antiestrogens is a major cause of treatment failure and, ultimately, death in breast cancer.

OBJECTIVE

The objective of the study was to identify small-molecule estrogen receptor (ER)-α antagonists that work differently from tamoxifen and other selective estrogen receptor modulators.

DESIGN

Based on in silico screening of a pharmacophore database using a computed model of the BRCA1-ER-α complex (with ER-α liganded to 17β-estradiol), we identified a candidate group of small-molecule compounds predicted to bind to a BRCA1-binding interface separate from the ligand-binding pocket and the coactivator binding site of ER-α. Among 40 candidate compounds, six inhibited estradiol-stimulated ER-α activity by at least 50% in breast carcinoma cells, with IC50 values ranging between 3 and 50 μM. These ER-α inhibitory compounds were further studied by molecular and cell biological techniques.

RESULTS

The compounds strongly inhibited ER-α activity at concentrations that yielded little or no nonspecific toxicity, but they produced only a modest inhibition of progesterone receptor activity. Importantly, the compounds blocked proliferation and inhibited ER-α activity about equally well in antiestrogen-sensitive and antiestrogen-resistant breast cancer cells. Representative compounds disrupted the interaction of BRCA1 and ER-α in the cultured cells and blocked the interaction of ER-α with the estrogen response element. However, the compounds had no effect on the total cellular ER-α levels.

CONCLUSIONS

These findings suggest that we have identified a new class of ER-α antagonists that work differently from conventional antiestrogens (eg, tamoxifen and fulvestrant).

Comparison of tamoxifen and letrozole response in mammary preneoplasia of ER and aromatase overexpressing mice defines an immune-associated gene signature linked to tamoxifen resistance

Response to breast cancer chemoprevention can depend upon host genetic makeup and initiating events leading up to preneoplasia. Increased expression of aromatase and estrogen receptor (ER) is found in conjunction with breast cancer. To investigate response or resistance to endocrine therapy, mice with targeted overexpression of Esr1 or CYP19A1 to mammary epithelial cells were employed, representing two direct pathophysiological interventions in estrogen pathway signaling. Both Esr1 and CYP19A1 overexpressing mice responded to letrozole with reduced hyperplastic alveolar nodule prevalence and decreased mammary epithelial cell proliferation. CYP19A1 overexpressing mice were tamoxifen sensitive but Esr1 overexpressing mice were tamoxifen resistant. Increased ER expression occurred with tamoxifen resistance but no consistent changes in progesterone receptor, pSTAT3, pSTAT5, cyclin D1 or cyclin E levels in association with response or resistance were found. RNA-sequencing (RNA-seq) was employed to seek a transcriptome predictive of tamoxifen resistance using these models and a second tamoxifen-resistant model, BRCA1 deficient/Trp53 haploinsufficient mice. Sixty-eight genes associated with immune system processing were upregulated in tamoxifen-resistant Esr1- and Brca1-deficient mice, whereas genes related to aromatic compound metabolic process were upregulated in tamoxifen-sensitive CYP19A1 mice. Interferon regulatory factor 7 was identified as a key transcription factor regulating these 68 immune processing genes. Two loci encoding novel transcripts with high homology to human immunoglobulin lambda-like polypeptide 1 were uniquely upregulated in the tamoxifen-resistant models. Letrozole proved to be a successful alternative to tamoxifen. Further study of transcriptional changes associated with tamoxifen resistance including immune-related genes could expand our mechanistic understanding and lead to biomarkers predictive of escape or response to endocrine therapies.

Genetically engineered ERα-positive breast cancer mouse models

The majority of human breast cancers are estrogen receptor-positive (ER+), but this has proven challenging to model in genetically engineered mice. This review summarizes information on 21 mouse models that develop ER+ mammary cancer. Where available, information on cancer pathology and gene expression profiles is referenced to assist in understanding which histological subtype of ER+ human cancer each model might represent. ESR1, CCDN1, prolactin, TGFα, AIB1, ESPL1, and WNT1 overexpression, PIK3CA gain of function, as well as loss of P53 (Trp53) or STAT1 are associated with ER+ mammary cancer. Treatment with the PPARγ agonist efatutazone in a mouse with Brca1 and p53 deficiency and 7,12-dimethylbenz(a)anthracene exposure in combination with an activated myristoylated form of AKT1 also induce ER+ mammary cancer. A spontaneous mutant in nude mice that develops metastatic ER+ mammary cancer is included. Age of cancer development ranges from 3 to 26 months and the percentage of cancers that are ER+ vary from 21 to 100%. Not all models are characterized as to their estrogen dependency and/or response to anti-hormonal therapy. Strain backgrounds include C57Bl/6, FVB, BALB/c, 129S6/SvEv, CB6F1, and NIH nude. Most models have only been studied on one strain background. In summary, while a range of models are available for studies of pathogenesis and therapy of ER+ breast cancers, many could benefit from further characterization, and opportunity for development of new models remains.

BRCA1 and estrogen/estrogen receptor in breast cancer: where they interact?

BRCA1 mainly acts as a tumor suppressor and BRCA1 mutation correlates with increased cancer risk. Although it is well recognized that BRCA1 related tumorigenesis is mainly caused by the increased DNA damage and decreased genome stability, it is not clear that why BRCA1 related patients have higher risk for cancer development mainly in estrogen responsive tissues such as breast and ovary. Recent studies suggested that BRCA1 and E-ER (estrogen and estrogen receptor) signaling synergistically regulate the mammary epithelial cell proliferation and differentiation. In this current presentation, we reviewed the correlation between mammary gland epithelial cell transformation and the status of BRCA1 and ER. Then the mechanisms of BRCA1 and E-ER interaction at both gene transcription level and protein-protein interaction level are discussed. Furthermore, the tumorigenic mechanisms are discussed by focusing on the synergistic effect of BRCA1 and E-ER on cell metabolism, ROS management, and antioxidant activity in mammary gland epithelial cells. Also, the possibility of cell de-differentiation promoted by coordinated effect between BRCA1 mutation and E-ER signal is explored. Together, the currently available evidences suggest that BRCA1 mutation and E-ER signal together, contribute to breast tumorigenesis by providing the metabolic support for cancer cell growth and even may directly be involved in promoting the de-differentiation of cancer-prone epithelial cells.

Estrogen signaling and the DNA damage response in hormone dependent breast cancers

Estrogen is necessary for the normal growth and development of breast tissue, but high levels of estrogen are a major risk factor for breast cancer. One mechanism by which estrogen could contribute to breast cancer is via the induction of DNA damage. This perspective discusses the mechanisms by which estrogen alters the DNA damage response (DDR) and DNA repair through the regulation of key effector proteins including ATM, ATR, CHK1, BRCA1, and p53 and the feedback on estrogen receptor signaling from these proteins. We put forward the hypothesis that estrogen receptor signaling converges to suppress effective DNA repair and apoptosis in favor of proliferation. This is important in hormone-dependent breast cancer as it will affect processing of estrogen-induced DNA damage, as well as other genotoxic insults. DDR and DNA repair proteins are frequently mutated or altered in estrogen responsive breast cancer, which will further change the processing of DNA damage. Finally, the action of estrogen signaling on DNA damage is also relevant to the therapeutic setting as the suppression of a DDR by estrogen has the potential to alter the response of cancers to anti-hormone treatment or chemotherapy that induces DNA damage.

Lower mitotic activity in BRCA1/2-associated primary breast cancers occurring after risk-reducing salpingo-oophorectomy

Risk-reducing salpingo-oophorectomy (RRSO) is associated with 50% reduction of BRCA1/2-associated breast cancer (BC) risk, possibly through decreased growth activity. In this pilot study, tumor characteristics and growth rates of BRCA1/2-associated primary BCs (PBCs) detected after RRSO were compared with those of PBCs originating without RRSO. From a cohort of 271 women with BRCA1/2-associated screen detected BC, we selected 20 patients with PBC detected ≥12 months after RRSO (RRSO group). Controls were 36 BRCA1/2 mutation carriers with PBC detected without RRSO (non-RRSO group) matched for age at diagnosis (± 2.5 y) and for BRCA1 or BRCA2 mutation. Pathology samples were revised for histological subtype, tumor differentiation grade, mitotic activity index (MAI), estrogen receptor (ER), progesterone receptor (PR), and HER2 status. Tumor growth rates, expressed as tumor volume doubling times (DT), were calculated from revised magnetic resonance and mammographic images. Median age at PBC diagnosis was 52 y (range 35-67). PBCs after RRSO had lower MAIs (12 vs. 22 mitotic counts/2 mm, P = 0.02), were smaller (11 vs. 17 mm, P = 0.01), and tend to be PR-positive more often than PBCs without RRSO (38% vs. 13%, P = 0.07). Differentiation grade, ER, and HER2 status were not different. Median DT was 124 d (range 89-193) in the RRSO group and 93 days (range 54-253) in the non-RRSO group (P = 0.47). BC occurring after RRSO in BRCA mutation carriers features a lower MAI, suggesting a less aggressive biological phenotype. When confirmed in larger series, this may have consequences for BC screening protocols after RRSO.

Mouse models of BRCA1 and their application to breast cancer research

Germline mutations of human breast cancer-associated gene 1 (BRCA1) predispose women to breast and ovarian cancers. In mice, over 20 distinct mutations, including null, hypomorphic, isoform, conditional, and point mutations, have been created to study functions of Brca1 in mammary development and tumorigenesis. Analyses using these mutant mice have yielded an enormous amount of information that greatly facilitates our understanding of the gender- and tissue-specific tumor suppressor functions of BRCA1, as well as enriches our insights into applying these preclinical models of disease to breast cancer research. Here, we review features of these mutant mice and their applications to cancer prevention and therapeutic treatment.

Tamoxifen and risk of contralateral breast cancer for BRCA1 and BRCA2 mutation carriers

PURPOSE

To determine whether adjuvant tamoxifen treatment for breast cancer (BC) is associated with reduced contralateral breast cancer (CBC) risk for BRCA1 and/or BRCA2 mutation carriers.

METHODS

Analysis of pooled observational cohort data, self-reported at enrollment and at follow-up from the International BRCA1, and BRCA2 Carrier Cohort Study, Kathleen Cuningham Foundation Consortium for Research into Familial Breast Cancer, and Breast Cancer Family Registry. Eligible women were BRCA1 and BRCA2 mutation carriers diagnosed with unilateral BC since 1970 and no other invasive cancer or tamoxifen use before first BC. Hazard ratios (HRs) for CBC associated with tamoxifen use were estimated using Cox regression, adjusting for year and age of diagnosis, country, and bilateral oophorectomy and censoring at contralateral mastectomy, death, or loss to follow-up.

RESULTS

Of 1,583 BRCA1 and 881 BRCA2 mutation carriers, 383 (24%) and 454 (52%), respectively, took tamoxifen after first BC diagnosis. There were 520 CBCs over 20,104 person-years of observation. The adjusted HR estimates were 0.38 (95% CI, 0.27 to 0.55) and 0.33 (95% CI, 0.22 to 0.50) for BRCA1 and BRCA2 mutation carriers, respectively. After left truncating at recruitment to the cohort, adjusted HR estimates were 0.58 (95% CI, 0.29 to 1.13) and 0.48 (95% CI, 0.22 to 1.05) based on 657 BRCA1 and 426 BRCA2 mutation carriers with 100 CBCs over 4,392 person-years of prospective follow-up. HRs did not differ by estrogen receptor status of the first BC (missing for 56% of cases).

CONCLUSION

This study provides evidence that tamoxifen use is associated with a reduction in CBC risk for BRCA1 and BRCA2 mutation carriers. Further follow-up of these cohorts will provide increased statistical power for future prospective analyses.

The PPARγ agonist efatutazone increases the spectrum of well-differentiated mammary cancer subtypes initiated by loss of full-length BRCA1 in association with TP53 haploinsufficiency

Peroxisome proliferator-activated receptor gamma (PPARγ) agonists have anticancer activity and influence cell differentiation. We examined the impact of the selective PPARγ agonist efatutazone on mammary cancer pathogenesis in a mouse model of BRCA1 mutation. Mice with conditional loss of full-length BRCA1 targeted to mammary epithelial cells in association with germline TP53 insufficiency were treated with efatutazone through the diet starting at age 4 months and were euthanized at age 12 months or when palpable tumor reached 1 cm(3). Although treatment did not reduce percentage of mice developing invasive cancer, it significantly reduced prevalence of noninvasive cancer and total number of cancers per mouse and increased prevalence of well-differentiated cancer subtypes not usually seen in this mouse model. Invasive cancers from controls were uniformly estrogen receptor α negative and undifferentiated, whereas well-differentiated estrogen receptor α-positive papillary invasive cancers appeared in efatutazone-treated mice. Expression levels of phosphorylated AKT and CDK6 were significantly reduced in the cancers developing in efatutazone-treated mice. Efatutazone treatment reduced rates of mammary epithelial cell proliferation and development of hyperplastic alveolar nodules and increased expression levels of the PPARγ target genes Adfp, Fabp4, and Pdhk4 in preneoplastic mammary tissue. Intervention efatutazone treatment in mice with BRCA1 deficiency altered mammary cancer development by promoting development of differentiated invasive cancer and reducing prevalence of noninvasive cancer and preneoplastic disease.

Time-lapse imaging of primary preneoplastic mammary epithelial cells derived from genetically engineered mouse models of breast cancer

Time-lapse imaging can be used to compare behavior of cultured primary preneoplastic mammary epithelial cells derived from different genetically engineered mouse models of breast cancer. For example, time between cell divisions (cell lifetimes), apoptotic cell numbers, evolution of morphological changes, and mechanism of colony formation can be quantified and compared in cells carrying specific genetic lesions. Primary mammary epithelial cell cultures are generated from mammary glands without palpable tumor. Glands are carefully resected with clear separation from adjacent muscle, lymph nodes are removed, and single-cell suspensions of enriched mammary epithelial cells are generated by mincing mammary tissue followed by enzymatic dissociation and filtration. Single-cell suspensions are plated and placed directly under a microscope within an incubator chamber for live-cell imaging. Sixteen 650 μm x 700 μm fields in a 4x4 configuration from each well of a 6-well plate are imaged every 15 min for 5 days. Time-lapse images are examined directly to measure cellular behaviors that can include mechanism and frequency of cell colony formation within the first 24 hr of plating the cells (aggregation versus cell proliferation), incidence of apoptosis, and phasing of morphological changes. Single-cell tracking is used to generate cell fate maps for measurement of individual cell lifetimes and investigation of cell division patterns. Quantitative data are statistically analyzed to assess for significant differences in behavior correlated with specific genetic lesions.

Derailed estrogen signaling and breast cancer: an authentic couple

Estrogen or 17β-estradiol, a steroid hormone, plays a critical role in the development of mammary gland via acting through specific receptors. In particular, estrogen receptor-α (ERα) acts as a transcription factor and/or a signal transducer while participating in the development of mammary gland and breast cancer. Accumulating evidence suggests that the transcriptional activity of ERα is altered by the action of nuclear receptor coregulators and might be responsible, at least in part, for the development of breast cancer. In addition, this process is driven by various posttranslational modifications of ERα, implicating active participation of the upstream receptor modifying enzymes in breast cancer progression. Emerging studies suggest that the biological outcome of breast cancer cells is also influenced by the cross talk between microRNA and ERα signaling, as well as by breast cancer stem cells. Thus, multiple regulatory controls of ERα render mammary epithelium at risk for transformation upon deregulation of normal homeostasis. Given the importance that ERα signaling has in breast cancer development, here we will highlight how the activity of ERα is controlled by various regulators in a spatial and temporal manner, impacting the progression of the disease. We will also discuss the possible therapeutic value of ERα modulators as alternative drug targets to retard the progression of breast cancer.

Atbf1 regulates pubertal mammary gland development likely by inhibiting the pro-proliferative function of estrogen-ER signaling

ATBF1 is a candidate tumor suppressor that interacts with estrogen receptor (ER) to inhibit the function of estrogen-ER signaling in gene regulation and cell proliferation control in human breast cancer cells. We therefore tested whether Atbf1 and its interaction with ER modulate the development of pubertal mammary gland, where estrogen is the predominant steroid hormone. In an in vitro model of cell differentiation, i.e., MCF10A cells cultured in Matrigel, ATBF1 expression was significantly increased, and knockdown of ATBF1 inhibited acinus formation. During mouse mammary gland development, Atbf1 was expressed at varying levels at different stages, with higher levels during puberty, lower during pregnancy, and the highest during lactation. Knockout of Atbf1 at the onset of puberty enhanced ductal elongation and bifurcation and promoted cell proliferation in both ducts and terminal end buds of pubertal mammary glands. Enhanced cell proliferation primarily occurred in ER-positive cells and was accompanied by increased expression of ER target genes. Furthermore, inactivation of Atbf1 reduced the expression of basal cell markers (CK5, CK14 and CD44) but not luminal cell markers. These findings indicate that Atbf1 plays a role in the development of pubertal mammary gland likely by modulating the function of estrogen-ER signaling in luminal cells and by modulating gene expression in basal cells.

Uptake of a randomized breast cancer prevention trial comparing letrozole to placebo in BRCA1/2 mutations carriers: the LIBER trial

Women with germline BRCA1 or BRCA2 (BRCA1/2) mutations are considered as an extreme risk population for developing breast cancer. Prophylactic mastectomy provides a valid option to reduce such risk, impacting however, the quality of life. Medical prevention by aromatase inhibitor that has also recently shown to have preventive effect may thus be considered as an alternative. LIBER is an ongoing double-blind, randomized phase III trial to evaluate the efficacy of 5-year letrozole versus placebo to decrease breast cancer incidence in post-menopausal BRCA1/2 mutation carriers (NCT00673335). We present data on the uptake of this trial. We compared characteristics of women in the LIBER trial (n = 113) to those of women enrolled in the prospective ongoing national GENEPSO cohort (n = 1,505). Uptake was evaluated through a survey sent to all active centres, with responses obtained from 17 to the 20 (85%) centres. According to the characteristics of the women enrolled in the GENEPSO cohort and the survey, approximately one-third of BRCA1/2 mutation carriers were eligible for the trial. Five hundred and thirty-four women eligible from chart review have been informed by mail about the prevention trial and were invited to an oral information by participating centres. Forty-four percentage of them came to the dedicated medical visit. Uptake of drug prevention trial was 32% among women informed orally and 15% of all the eligible women. The main reasons of refusal were: potential side effects, probability to receive the placebo and lack of support from their physicians. Additionally, we noticed that prior prophylactic oophorectomy and previous unilateral breast cancer were more frequent in women enrolled in the LIBER trial than in the French cohort (93% vs. 60% and 50% vs. 39%, respectively). Based on an overall 15% uptake among all eligible subjects, greater and wider information of the trial should be offered to women with BRCA1/2 mutation to improve recruitment. Women with previous unilateral breast cancer or prior prophylactic oophorectomy are more likely to enter a medical prevention trial.

BRCA1 promoter methylation status does not predict response to tamoxifen in sporadic breast cancer patients

The purpose of this study is to investigate whether BRCA1 promoter methylation is associated with poorer outcome in sporadic breast cancer cases treated with tamoxifen. BRCA1 promoter methylation was determined by bisulfite pyrosequencing in two groups of sporadic breast cancer patients, systemically untreated (N = 497) and treated with adjuvant tamoxifen (N = 497). Associations of BRCA1 promoter methylation with clinopathological characteristics and the effect of BRCA1 promoter methylation on time to first recurrence (TTR) and overall survival (OS) were examined. No significant differences were observed between BRCA1 promoter methylation and clinopathological characteristics in untreated and tamoxifen-treated groups. Cut point analysis did not find any promising cut point for BRCA1 promoter methylation that would differentially influence TTR and OS in untreated and tamoxifen-treated group. Using the median (2.53 %) and an arbitrary value of 10 % as a cut point for methylation, we still found no significant effect of BRCA1 promoter methylation on TTR and OS in untreated and tamoxifen-treated group. Despite data suggesting that BRCA1 levels impact estrogen receptor response to tamoxifen, our results indicate that BRCA1 promoter methylation is not associated with poorer outcome in sporadic breast cancer cases treated with tamoxifen.

A phenotypic mouse model of basaloid breast tumors

Chemotherapeutic strategies that target basal-like breast tumors are difficult to design without understanding their cellular and molecular basis. Here, we induce tumors in mice by carcinogen administration, creating a phenocopy of tumors with the diagnostic and functional aspects of human triple negative disease (including EGFR expression/lack of erbB, estrogen-independent growth and co-clustering of the transcriptome with other basaloid models). These tumor strains are a complement to established mouse models that are based on mutations in Brca1 and/or p53. Tumors comprise two distinct cell subpopulations, basal and luminal epithelial cells. These cell fractions were purified by flow cytometry, and only basal cell fractions found to have tumor initiating activity (cancer stem cells). The phenotype of serially regenerated tumors was stable, and irrespective of tumor precursor cell. Tumors were passaged entirely in vivo and serial generations tested for their phenotypic stability. The relative chemo-sensitivity of basal and luminal cells were evaluated. Upon treatment with anthracycline, tumors were effectively de-bulked, but recurred; this correlated with maintenance of a high rate of basal cell division throughout the treatment period. Thus, these tumors grow as robust cell mixtures of basal bipotential tumor initiating cells alongside a luminal majority, and the cellular response to drug administration is dominated by the distinct biology of the two cell types. Given the ability to separate basal and luminal cells, and the discovery potential of this approach, we propose that this mouse model could be a convenient one for preclinical studies.

Mouse models of estrogen receptor-positive breast cancer

Breast cancer is the most frequent malignancy and second leading cause of cancer-related deaths among women. Despite advances in genetic and biochemical analyses, the incidence of breast cancer and its associated mortality remain very high. About 60 – 70% of breast cancers are Estrogen Receptor alpha (ER-α) positive and are dependent on estrogen for growth. Selective estrogen receptor modulators (SERMs) have therefore provided an effective targeted therapy to treat ER-α positive breast cancer patients. Unfortunately, development of resistance to endocrine therapy is frequent and leads to cancer recurrence. Our understanding of molecular mechanisms involved in the development of ER-α positive tumors and their resistance to ER antagonists is currently limited due to lack of experimental models of ER-α positive breast cancer. In most mouse models of breast cancer, the tumors that form are typically ER-negative and independent of estrogen for their growth. However, in recent years more attention has been given to develop mouse models that develop different subtypes of breast cancers, including ER-positive tumors. In this review, we discuss the currently available mouse models that develop ER-α positive mammary tumors and their potential use to elucidate the molecular mechanisms of ER-α positive breast cancer development and endocrine resistance.

Assessing estrogen signaling aberrations in breast cancer risk using genetically engineered mouse models

Aberrations in estrogen signaling increase breast cancer risk. Molecular mechanisms that impact breast cancer initiation, promotion, and progression can be investigated using genetically engineered mouse models. Increasing estrogen receptor alpha (ERα) expression levels twofold is sufficient to initiate and promote breast cancer progression. Initiation and promotion can be increased by p53 haploinsufficiency and by coexpressing the nuclear coactivators amplified in breast cancer 1 (AIB1) or the splice variant AIB1Δ3. Progression to invasive cancer is found with coexpression of these nuclear coactivators as well as following a single dose of 7,12-dimethylbenz(a)anthracene. Loss of signal transducer and activator of transcription 5a reduces the prevalence of initiation and promotion but does not protect from invasive cancer development. Cyclin D1 loss completely interrupts mammary epithelial proliferation and survival when ERα is overexpressed. Loss of breast cancer gene 1 increases estrogen signaling and cooperates with ERα overexpression in initiation, promotion, and progression of mammary cancer.

Choosing a mouse model: experimental biology in context--the utility and limitations of mouse models of breast cancer

Genetically engineered mice are critical experimental models for the study of breast cancer biology. Transgenic mice, employing strong mammary epithelial promoters to drive oncogenes, develop carcinomas with phenotypes corresponding to the molecular pathway activated. Gene-targeted (knockout) mice, in which tumor suppressors are deleted, develop mammary neoplasms with phenotypes primarily including patterns seen in spontaneous mouse mammary tumors, albeit at higher rates. Improved genetic engineering, using inducible gene expression, somatic gene transduction, conditional alleles, and crossbreeding for combined/compound genetic engineering yields precise molecular models with exquisite experimental control and phenotypes with comparative pathologic validity. Mammary gland transplantation technology adds a practical and validated method for assessing biologic behavior of selected mammary tissues. Overall, the many mouse models available are a rich resource for experimental biology with phenocopies of breast cancer subtypes, and a variety of practical advantages. The challenge is matching the model to the experimental question.

RINGs of good and evil: RING finger ubiquitin ligases at the crossroads of tumour suppression and oncogenesis

The ubiquitin-proteasome system has numerous crucial roles in physiology and pathophysiology. Fundamental to the specificity of this system are ubiquitin-protein ligases (E3s). Of these, the majority are RING finger and RING finger-related E3s. Many RING finger E3s have roles in processes that are central to the maintenance of genomic integrity and cellular homeostasis, such as the anaphase promoting complex/cyclosome (APC/C), the SKP1-cullin 1-F-box protein (SCF) E3s, MDM2, BRCA1, Fanconi anaemia proteins, CBL proteins, von Hippel-Lindau tumour suppressor (VHL) and SIAH proteins. As a result, many RING finger E3s are implicated in either the suppression or the progression of cancer. This Review summarizes current knowledge in this area.

Protective effects of prepubertal genistein exposure on mammary tumorigenesis are dependent on BRCA1 expression

This study investigated whether prepubertal dietary exposure to genistein reduces mammary tumorigenesis by upregulating Brca1 expression in mice. Heterozygous Brca1(+/-) mice and their wild-type (WT) littermates were fed control AIN93G diet or 500 ppm genistein-supplemented AIN93G diet from postnatal day (PND) 15 to PND30 and then switched to AIN93G diet. Prepubertal dietary exposure to genistein reduced 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary incidence (P = 0.029) and aggressiveness of the tumors (P < 0.001) in the WT mice and upregulated the expression of Brca1 in their mammary glands (P = 0.04). In contrast, prepubertal genistein diet neither significantly reduced mammary tumorigenesis or tumor aggressivity nor increased Brca1 mRNA expression in the Brca1(+/-) mice. These results may be related to the opposing effects of prepubertal genistein diet on the expression of Rankl and CK5/CK18 ratio (marker of luminal epithelial cell differentiation) in the mammary gland and estrogen receptor (ER-α) and progesterone receptor (PgR) protein levels in the mammary tumor: these all were reduced in the WT mice or increased in Brca1(+/-) mice. Both the WT and Brca1(+/-) mice exhibited reduced levels of amphiregulin, CK5, and CK18, delayed ductal elongation and a reduction in terminal end bud number in the normal mammary gland, and reduced HER-2 protein levels in the mammary tumors; however, these effects were not sufficient to significantly reduce mammary tumorigenesis in Brca1(+/-) mice. Our results show that upregulation of Brca1 may be required for prepubertal dietary genistein exposure to reduce later mammary tumorigenesis, perhaps because in the absence of this upregulation, mice do not exhibit genistein-induced downregulation of ER-α, PgR, and Rankl.

Overexpression of ligase defective E6-associated protein, E6-AP, results in mammary tumorigenesis

E6-associated protein (E6-AP) is a dual function protein. It acts as an E3 ubiquitin-protein ligase enzyme and coactivator of steroid hormone receptors such as estrogen (ERα) and progesterone (PR) receptors. It promotes the degradation of ERα and PR through the ubiquitin-proteasome pathway. Furthermore, it has been shown that the levels of E6-AP are inversely associated with that of ERα in human breast tumors. But the role of wild-type human E6-AP and its ubiquitin-protein ligase activity in mammary tumorigenesis is still unknown. To investigate this role, the authors utilized transgenic mice lines that specifically overexpress either the wild-type human E6-AP (E6-AP(WT)) or the ubiquitin-protein ligase defective E6-AP that contains C833S mutation (E6-AP(C833S)) in the mammary gland. To further substantiate the role of E6-AP in the development of breast tumorigenesis, it was also examined the expression of E6-AP in a large cohort of human breast cancer samples. The transgenic mice that overexpress wild-type E6-AP (E6-AP(WT)) fail to develop mammary tumors. Unlike the E6-AP(WT) mice, the E6-AP(C833S) mice that overexpress ubiquitin-protein ligase defective E6-AP protein develop mammary hyperplasia with a median latency of 18 months. These observations suggest that the inactivation of the ubiquitin-protein ligase function of E6-AP is sufficient to initiate the process of mammary tumor development. Furthermore, the data also suggests that E6-AP exerts its effects on target cells by modulating the protein levels and functions of ERα and PR. In addition, it was found in human breast cancer patients that the level of E6-AP is decreased in invasive breast tumors compared to normal breast tissue. Moreover, the authors also show that the survival patterns for E6-AP negative patients were worse compared to E6-AP positive patients. Taken together, these data suggests that E6-AP may act as a tumor suppressor in breast.

Altered AIB1 or AIB1Δ3 expression impacts ERα effects on mammary gland stromal and epithelial content

Amplified in breast cancer 1 (AIB1) (also known as steroid receptor coactivator-3) is a nuclear receptor coactivator enhancing estrogen receptor (ER)α and progesterone receptor (PR)-dependent transcription in breast cancer. The splice variant AIB1Δ3 demonstrates increased ability to promote ERα and PR-dependent transcription. Both are implicated in breast cancer risk and antihormone resistance. Conditional transgenic mice tested the in vivo impact of AIB1Δ3 overexpression compared with AIB1 on histological features of increased breast cancer risk and growth response to estrogen and progesterone in the mammary gland. Combining expression of either AIB1 or AIB1Δ3 with ERα overexpression, we investigated in vivo cooperativity. AIB1 and AIB1Δ3 overexpression equivalently increased the prevalence of hyperplastic alveolar nodules but not ductal hyperplasia or collagen content. When AIB1 or AIB1Δ3 overexpression was combined with ERα, both stromal collagen content and ductal hyperplasia prevalence were significantly increased and adenocarcinomas appeared. Overexpression of AIB1Δ3, especially combined with overexpressed ERα, led to an abnormal response to estrogen and progesterone with significant increases in stromal collagen content and development of a multilayered mammary epithelium. AIB1Δ3 overexpression was associated with a significant increase in PR expression and PR downstream signaling genes. AIB1 overexpression produced less marked growth abnormalities and no significant change in PR expression. In summary, AIB1Δ3 overexpression was more potent than AIB1 overexpression in increasing stromal collagen content, inducing abnormal mammary epithelial growth, altering PR expression levels, and mediating the response to estrogen and progesterone. Combining ERα overexpression with either AIB1 or AIB1Δ3 overexpression augmented abnormal growth responses in both epithelial and stromal compartments.

Abnormal Mammary Adipose Tissue Environment of Brca1 Mutant Mice Show a Persistent Deposition of Highly Vascularized Multilocular Adipocytes

A major challenge to breast cancer research is the identification of alterations in the architecture and composition of the breast that are associated with breast cancer progression. The aim of the present investigation was to characterize the mammary adipose phenotype from Brca1 mutant mice in the expectation that this would shed light on the role of the mammary tissue environment in the early stages of breast tumorigenesis. We observed that histological sections of mammary tissue from adult Brca1 mutant mice abnormally display small, multilocular adipocytes that are reminiscent of brown adipose tissue (BAT) as compared to wildtype mice. Using a marker for BAT, the uncoupling protein 1 (UCP1), we demonstrated that these multilocular adipose regions in Brca1 mutant mice stain positive for UCP1. Transcriptionally, UCP1 mRNA levels in the Brca1 mutant mice were elevated greater than 50-fold compared to age-matched mammary glands from wildtype mice. Indeed, BAT has characteristics that are favorable for tumor growth, including high vascularity. Therefore, we also demonstrated that the multilocular brown adipose phenotype in the mammary fat pad of Brca1 mutant mice displayed regions of increased vascularity as evidenced by a significant increase in the protein expression of CD31, a marker for angiogenesis. This Brca1 mutant mouse model should provide a physiologically relevant context to determine whether brown adipose tissue can play a role in breast cancer development.

Loss of BRCA1 leads to an increased sensitivity to Bisphenol A

Humans are chronically exposed to the plasticizer, Bisphenol A (BPA), that can adversely affect the normal hormonal regulation of cellular functions by mimicking the actions of estrogen. This biological response to BPA may vary according to an individual's genetic characteristics (e.g., BRCA1 mutations or deletion). In this study, both cell culture and mouse models were used to elucidate whether the loss of BRCA1 function could affect BPA-mediated cell proliferation. In studies using BPA levels comparable to human exposures, we found that loss of BRCA1 enhances BPA-induced cell proliferation in both systems. In vitro, we found that loss of BRCA1 enhances BPA-induced ERα signaling. In vivo, we found that BPA administration stimulates mammary gland epithelial tissue/cell proliferation leading to hyperplasia in Brca1 mutant mice compared to wild-type control mice. These results suggest that the biological responses in BRCA1-deficient cells may depend on environmental exposures, specifically BPA.

AKT regulates BRCA1 stability in response to hormone signaling

The observation that inherited mutations within BRCA1 result in breast and ovarian cancers suggests a functional relationship may exist between hormone signaling and BRCA1 function. We demonstrate that AKT activation promotes the expression of BRCA1 in response to estrogen and IGF-1 receptor signaling, and the rapid increase in BRCA1 protein levels appears to occur independently of new protein synthesis. Further, we identify a novel AKT phosphorylation site in BRCA1 at S694 which is responsive to activation of these signaling pathways. These data suggest AKT phosphorylation of BRCA1 increases total protein expression by preventing proteasomal degradation. AKT activation also appears to support nuclear localization of BRCA1, and co-expression of activated AKT with BRCA1 decreases radiation sensitivity, suggesting this interaction has functional consequences for BRCA1's role in DNA repair. Targets within this pathway could provide strategies for modulation of BRCA1 protein, which may prove therapeutically beneficial for breast and ovarian cancer treatment.

Role of ERalpha in the differential response of Stat5a loss in susceptibility to mammary preneoplasia and DMBA-induced carcinogenesis

Deregulated estrogen signaling is evidently linked to breast cancer pathophysiology, although the role of signal transducer and activator of transcription (Stat)5a, integral to normal mammary gland development, is less clear. A mouse model of mammary epithelial cell-targeted deregulated estrogen receptor alpha (ERalpha) expression [conditional ERalpha in mammary epithelium (CERM)] was crossed with mice carrying a germ line deletion of Stat5a [Stat5a-/-] to investigate interactions between ERalpha and Stat5a in mammary tissue. CERM, CERM/Stat5a+/-, CERM/Stat5a-/-, Stat5a+/-, Stat5a-/- and wild-type (WT) mice were generated to test the roles of ERalpha and Stat5a on pubertal differentiation and cancer progression with and without exposure to the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). Only CERM/Stat5a-/- mice demonstrated delayed pubertal terminal end bud differentiation. Without DMBA exposure, Stat5a loss abrogated ERalpha-initiated hyperplastic alveolar nodule (HAN) development and, similarly, Stat5a-/- mice did not develop HANs. However, although Stat5a loss still reduced ERalpha-initiated HAN prevalence following DMBA exposure, Stat5a loss without deregulated ERalpha was associated with an increased HAN prevalence compared with WT. Progression to ERalpha(+) and ERalpha(-) adenocarcinoma was found in all CERM-containing genotypes (CERM, CERM/Stat5a+/-, CERM/Stat5a-/-) and ERalpha(+) adenocarcinoma in the Stat5a-/- genotype. The mammary epithelial cell proliferative index was increased only in CERM mice independent of Stat5a loss. No differences in apoptotic indices were found. In summary, Stat5a cooperated with deregulated ERalpha in retarding pubertal mammary differentiation and contributed to ERalpha-initiated preneoplasia, but its loss did not prevent development of invasive cancer. Moreover, in the absence of deregulated ERalpha, Stat5a loss was associated with development of both HANs and invasive cancer following DMBA exposure.

BRCA1 deficient mouse models to study pathogenesis and therapy of triple negative breast cancer

Genetically engineered mice along with allograft and xenograft models can be used to effectively model triple negative breast cancer both for studies of pathophysiology as well as preclinical prevention and therapeutic drug studies. In this review eight distinct genetically engineered mouse models of BRCA1 deficiency are discussed in relationship to the generation of triple negative mammary cancer. Allograft models derived from some of these genetically engineered mice are considered and xenograft models derived from breast cancers that developed from BRCA1 mutation are presented. Examples of the use of genetically engineered, allograft and xenografts models for preventive and therapeutic studies are presented.

BRCA1 regulates acetylation and ubiquitination of estrogen receptor-alpha

Inherited mutations of the breast cancer susceptibility gene BRCA1 confer a high risk for breast cancer development. The (300)RXKK and (266)KXK motifs have been identified previously as sites for acetylation of the estrogen receptor-alpha (ER-alpha), and (302)K was also found to be a site for BRCA1-mediated mono-ubiquitination of ER-alpha in vitro. Here we show that ER-alpha proteins with single or double lysine mutations of these motifs (including K303R, a cancer-associated mutant) are resistant to inhibition by BRCA1, even though the mutant ER-alpha proteins retain the ability to bind to BRCA1. We also found that BRCA1 overexpression reduced and knockdown increased the level of acetylated wild-type ER-alpha, without changing the total ER-alpha protein level. Increased acetylation of ER-alpha due to BRCA1 small interfering RNA was dependent upon phosphatidylinositol 3-kinase/Akt signaling and on up-regulation of the coactivator p300. In addition, using an in vitro acetylation assay, we found that in vitro-translated wild-type BRCA1 but not a cancer-associated point mutant (C61G) inhibited p300-mediated acetylation of ER-alpha. Furthermore, BRCA1 overexpression increased the levels of mono-ubiquitinated ER-alpha protein, and a BRCA1 mutant that is defective for ubiquitin ligase activity but retains other BRCA1 functions (I26A) did not ubiquitinate ER-alpha or repress its activity in vivo. Finally, ER-alpha proteins with mutations of the (300)RXKK or (266)KXK motifs showed modest or no BRCA1-induced ubiquitination. We propose a model in which BRCA1 represses ER-alpha activity, in part, by regulating the relative degree of acetylation vs. ubiquitination of ER-alpha.

A mammary-specific, long-range deletion on mouse chromosome 11 accelerates Brca1-associated mammary tumorigenesis

We engineered a mammary-specific knockout model for Brca1 deficiency that also lacks the majority of one chromosome 11 to determine whether tumor susceptibility loci reside on this chromosome that cooperate with the loss of Brca1 during mammary cancer formation. Brca1-deficient females that are haploinsufficient in 60 cM of chromosome 11 exhibited accelerated mammary tumorigenesis in comparison to Brca1 conditional knockout mice. On the histopathologic level, these tumors were either adenocarcinomas or benign, inflammatory lesions. Like human BRCA1-associated breast cancers, mammary carcinomas in this new mouse model were ERalpha-negative and of basal epithelial origin. Brca1 deficiency and haploinsufficiency in 60 cM of chromosome 11 caused widespread genome instability as determined by spectral karyotyping analysis. In addition to the verification of the long-range deletion event, the spectral karyotyping analysis revealed that the duplication of the genome and higher degree of aneuploidy occur rather late in tumor progression. Despite chromosomal rearrangements near the Trp53 locus as determined by fluorescence in situ hybridization, the Trp53 gene was transcriptionally active. The analysis of the coding sequence and expression pattern of p53 and p21 suggests that loss-of-heterozygosity of Trp53 caused by somatic mutations contributes to accelerated mammary tumorigenesis in this model.

Growth hormone and insulin-like growth factor-I in the transition from normal mammary development to preneoplastic mammary lesions

Adult female mammary development starts at puberty and is controlled by tightly regulated cross-talk between a group of hormones and growth factors. Although estrogen is the initial driving force and is joined by luteal phase progesterone, both of these hormones require GH-induced IGF-I in the mammary gland in order to act. The same group of hormones, when experimentally perturbed, can lead to development of hyperplastic lesions and increase the chances, or be precursors, of mammary carcinoma. For example, systemic administration of GH or IGF-I causes mammary hyperplasia, and overproduction of IGF-I in transgenic animals can cause the development of usual or atypical hyperplasias and sometimes carcinoma. Although studies have clearly demonstrated the transforming potential of both GH and IGF-I receptor in cell culture and in animals, debate remains as to whether their main role is actually instructive or permissive in progression to cancer in vivo. Genetic imprinting has been shown to occur in precursor lesions as early as atypical hyperplasia in women. Thus, the concept of progression from normal development to cancer through precursor lesions sensitive to hormones and growth factors discussed above is gaining support in humans as well as in animal models. Indeed, elevation of estrogen receptor, GH, IGF-I, and IGF-I receptor during progression suggests a role for these pathways in this process. New agents targeting the GH/IGF-I axis may provide a novel means to block formation and progression of precursor lesions to overt carcinoma. A novel somatostatin analog has recently been shown to prevent mammary development in rats via targeted IGF-I action inhibition at the mammary gland. Similarly, pegvisomant, a GH antagonist, and other IGF-I antagonists such as IGF binding proteins 1 and 5 also block mammary gland development. It is, therefore, possible that inhibition of IGF-I action, or perhaps GH, in the mammary gland may eventually play a role in breast cancer chemoprevention by preventing actions of both estrogen and progesterone, especially in women at extremely high risk for developing breast cancer such as BRCA gene 1 or 2 mutations.

BRCA1, hormone, and tissue-specific tumor suppression

Germline mutations of BRCA1 predispose women to breast and ovarian cancers. Elucidating molecular mechanism of tissue- and gender-specific phenomena in BRCA1-related tumors is a key to our understanding of BRCA1 function in tumor suppression. This review summarizes studies in recent years on the link between BRCA1 and estrogen/progesterone signaling pathways, as well as discusses various models underscoring a triangle relationship among BRCA1, estrogen and genome instability.