BRCA1 in hormone-responsive cancers

In Silico Identification of a BRCA1:miR-29:DNMT3 Axis Involved in the Control of Hormone Receptors in BRCA1-Associated Breast Cancers

Germline inactivating mutations in the BRCA1 gene lead to an increased lifetime risk of ovarian and breast cancer (BC). Most BRCA1-associated BC are triple-negative tumors (TNBC), aggressive forms of BC characterized by a lack of expression of estrogen and progesterone hormone receptors (HR) and HER2. How BRCA1 inactivation may favor the development of such a specific BC phenotype remains to be elucidated. To address this question, we focused on the role of miRNAs and their networks in mediating BRCA1 functions. miRNA, mRNA, and methylation data were retrieved from the BRCA cohort of the TCGA project. The cohort was divided into a discovery set (Hi-TCGA) and a validation set (GA-TCGA) based on the platform used for miRNA analyses. The METABRIC, GSE81002, and GSE59248 studies were used as additional validation data sets. BCs were differentiated into BRCA1-like and non-BRCA1-like based on an established signature of BRCA1 pathway inactivation. Differential expression of miRNAs, gene enrichment analysis, functional annotation, and methylation correlation analyses were performed. The miRNAs downregulated in BRCA1-associated BC were identified by comparing the miRNome of BRCA1-like with non-BRCA1-like tumors from the Hi-TCGA discovery cohort. miRNAs:gene-target anticorrelation analyses were then performed. The target genes of miRNAs downregulated in the Hi-TCGA series were enriched in the BRCA1-like tumors from the GA-TCGA and METABRIC validation data sets. Functional annotation of these genes revealed an over-representation of several biological processes ascribable to BRCA1 activity. The enrichment of genes related to DNA methylation was particularly intriguing, as this is an aspect of BRCA1 functions that has been poorly explored. We then focused on the miR-29:DNA methyltransferase network and showed that the miR-29 family, which was downregulated in BRCA1-like tumors, was associated with poor prognosis in these BCs and inversely correlated with the expression of the DNA methyltransferases DNMT3A and DNMT3B. This, in turn, correlated with the methylation extent of the promoter of HR genes. These results suggest that BRCA1 may control the expression of HR via a miR-29:DNMT3:HR axis and that disruption of this network may contribute to the receptor negative phenotype of tumors with dysfunctional BRCA1.

Association of long-term particulate matter exposure with all-cause mortality among patients with ovarian cancer: A prospective cohort

BACKGROUND

Evidence of the association between particles with a diameter of 2.5 μm or less (PM_2.5) in long term and ovarian cancer (OC) mortality is limited.

METHODS

This prospective cohort study analyzed data collected between 2015 and 2020 from 610 newly diagnosed OC patients, aged 18-79 years. The residential average PM_2.5 concentrations 10 years before the date of OC diagnosis were assessed by random forest models at a 1 km × 1 km resolution. Cox proportional hazard models fully adjusted for the covariates (including age at diagnosis, education, physical activity, kitchen ventilation, FIGO stage, and comorbidities) and distributed lag non-linear models were used to estimate the hazard ratios (HRs) and 95 % confidence intervals (CIs) of PM_2.5 and all-cause mortality of OC.

RESULTS

During a median follow-up of 37.6 months (interquartile: 24.8-50.5 months), 118 (19.34 %) deaths were confirmed among 610 OC patients. One-year PM_2.5 exposure levels before OC diagnosis was significantly associated with an increase in all-cause mortality among OC patients (single-pollutant model: HR = 1.22, 95 % CI: 1.02-1.46; multi-pollutant models: HR = 1.38, 95 % CI: 1.10-1.72). Furthermore, during 1 to 10 years prior to diagnosis, the lag-specific effect of long-term PM_2.5 exposure on the all-cause mortality of OC had a risk increase for lag 1-6 years, and the exposure-response relationship was linear. Of note, significant interactions between several immunological indicators as well as solid fuel use for cooking and ambient PM_2.5 concentrations were observed.

CONCLUSION

Higher ambient PM_2.5 concentrations were associated with an increased risk of all-cause mortality among OC patients, and there was a lag effect in long-term PM_2.5 exposure.

[Therapeutic strategies for the treatment of endocrine resistant hormone receptor positive advanced breast cancer]

HR+ breast cancers are defined by the prominence of signaling pathways dependent on the estrogen receptor. Endocrine therapy is the standard treatment for these advanced diseases. Resistance to these treatments, called hormone resistance, appears invariably with biological mechanisms that have led to the development of therapeutic opportunities. An exhaustive literature review was carried out concerning the biology of the hormone resistance pathways, the therapeutic options before the era of CDK4/6 inhibitors, the rise of CDK4/6 inhibitors and the therapeutic prospects in a situation of hormone resistance. Various biological abnormalities have been identified in the mechanisms of hormone resistance such as changes in the estrogen receptor, mutations in the ESR1 gene, aberrant activation of the PI3K pathway or cell cycle deregulations. Historical strategies for circumventing this hormone resistance have been based on hormonal manipulation, on the development of new endocrine therapy such as fulvestrant (selective estrogen receptor inhibitor, SERD), on combinations of treatments such as everolimus, a mTOR inhibitor. This strategy combining endocrine therapy and targeted therapy has led to the development of combinations with CDK4/6 inhibitors which have now become a standard treatment in the hormone resistance phase. The future of this therapeutic era remains to be written with new combinations of hormone therapy and targeted therapy such as PI3K inhibitors or even with the positioning of new SERDs in clinical development.

PRMT1, a Key Modulator of Unliganded Progesterone Receptor Signaling in Breast Cancer

The progesterone receptor (PR) is a key player in major physiological and pathological responses in women, and the signaling pathways triggered following hormone binding have been extensively studied, particularly with respect to breast cancer development and progression. Interestingly, growing evidence suggests a fundamental role for PR on breast cancer cell homeostasis in hormone-depleted conditions, with hormone-free or unliganded PR (uPR) involved in the silencing of relevant genes prior to hormonal stimulation. We herein identify the protein arginine methyltransferase PRMT1 as a novel actor in uPR signaling. In unstimulated T47D breast cancer cells, PRMT1 interacts and functions alongside uPR and its partners to target endogenous progesterone-responsive promoters. PRMT1 helps to finely tune the silencing of responsive genes, likely by promoting a proper BRCA1-mediated degradation and turnover of unliganded PR. As such, PRMT1 emerges as a key transcriptional coregulator of PR for a subset of relevant progestin-dependent genes before hormonal treatment. Since women experience periods of hormonal fluctuation throughout their lifetime, understanding how steroid receptor pathways in breast cancer cells are regulated when hormones decline may help to determine how to override treatment failure to hormonal therapy and improve patient outcome.

BRCA1: An Endocrine and Metabolic Regulator

The breast and ovarian cancer susceptibility gene (BRCA1) is a tumor suppressor whose mutation has been associated with the development of breast, ovarian and, probably, other malignancies at young ages. The BRCA1 gene product participates in multiple biological pathways including the DNA damage response, transcriptional control, cell growth and apoptosis. Inactivating germline mutations of the BRCA1 gene can be detected in a substantial portion of families with inherited breast and/or ovarian cancer. While the genomic and cancer-related actions of BRCA1 have been extensively investigated, not much information exists regarding the cellular and circulating factors involved in regulation of BRCA1 expression and action. The present review article dissects the emerging role of BRCA1 as an important regulator of various endocrine and metabolic axes. Experimental and clinical evidence links BRCA1 with a number of peptide and steroid hormones. Furthermore, comprehensive analyses identified complex interactions between the insulin/insulin-like growth factor-1 (IGF1) signaling axis and BRCA1. The correlation between metabolic disorders, including diabetes and the metabolic syndrome, and BRCA1 mutations, are discussed in this article.

A comprehensive focus on global spectrum of BRCA1 and BRCA2 mutations in breast cancer

Breast cancer (BC) is the most common cancer of women all over the world. BRCA1 and BRCA2 gene mutations comprise the most important genetic susceptibility of BC. Except for few common mutations, the spectrum of BRCA1 and BRCA2 mutations is heterogeneous in diverse populations. 185AGdel and 5382insC are the most important BRCA1 and BRCA2 alterations which have been encountered in most of the populations. After those Ashkenazi founder mutations, 300T>G also demonstrated sparse frequency in African American and European populations. This review affords quick access to the most frequent alterations among various populations which could be helpful in BRCA screening programs.

A DNA repair BRCA1 estrogen receptor and targeted therapy in breast cancer

BRCA1 is a key mediator of DNA repair pathways and participates in the maintenance of the genomic integrity of cells. The control of DNA damage repair mechanisms by BRCA1 is of great interest since molecular defects in this pathway may reflect a predictive value in terms of a cell’s sensitivity to DNA damaging agents or anticancer drugs. BRCA1 has been found to exhibit a hormone-dependent pattern of expression in breast cells. Wild-type BRCA1 is required for the inhibition of the growth of breast tumor cells in response to the pure steroidal ERα antagonist fulvestrant. Also a loss of BRCA1-mediated transcriptional activation of ERα expression results in increased resistance to ERα antagonists. Platinum-based drugs, poly(ADP-ribose) polymerase (PARP) inhibitors, and their combination are currently included in chemotherapy regimens for breast cancer. Preclinical and clinical studies in a BRCA1-defective setting have recently indicated a rationale for the use of these compounds against hereditary breast cancers. Initial findings indicate that neoadjuvant use of cisplatin results in high rates of complete pathological response in patients with breast cancer who have BRCA1 mutations. Cisplatin produces a better response in triple-negative breast cancer (TNBC) than in non-TNBC diseases in both the neoadjuvant and adjuvant settings. This implies that TNBC cells may harbor a dysfunctional BRCA1 repair pathway.

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.

Quantitative proteomic identification of the BRCA1 ubiquitination substrates

Mutation of the BRCA1 tumor suppressor gene predisposes women to hereditary breast and ovarian cancers. BRCA1 forms a heterodimer with BARD1. The BRCA1/BARD1 heterodimer has ubiquitin ligase activity, considered to play crucial roles in tumor suppression and DNA damage response. Nevertheless, relevant BRCA1 substrates are poorly defined. We have developed a new approach to systematically identify the substrates of ubiquitin ligases by identifying proteins that display an enhanced incorporation of His-tagged ubiquitin upon ligase coexpression; using this method, we identified several candidate substrates for BRCA1. These include scaffold attachment factor B2 (SAFB2) and Tel2 as well as BARD1. BRCA1 was found to enhance SAFB protein expression and induce Tel2 nuclear translocation. Identification of the ubiquitination substrates has been a major obstacle to understanding the functions of ubiquitin ligases. The quantitative proteomics approach we devised for the identification of BRCA1 substrates will facilitate the identification of ubiquitin ligase-substrate pairs.

Detoxification: a novel function of BRCA1 in tumor suppression?

Our studies found that BRCA1 levels negatively correlate with DNA adducts induced by Benzo(a)pyrene (BaP). Pulse-chase experiments showed that the increase in BaP-induced DNA adducts in BRCA1 knockdown cells may not be associated with BRCA1’s function in nucleotide excision repair activity; rather, it may be associated with its function in modulating transcriptional regulation. BRCA1 knockdown in MCF-10A cells significantly attenuated the induction of CYP1A1 following BaP treatment indicating that the increase in BaP-induced adducts in BRCA1 knockdown cells is not CYP1A1 dependent. However, our study shows that BRCA1 defective cells may still be able to biotransform BaP by regulating other CYP enzymes, including CYP1B1. Knockdown of BRCA1 also severely affected the expression levels of two types of uridine diphosphate glucorunyltransferase (UGT1A1 and UGT1A9) and NRF2. Both UGTs are known as BaP-specific detoxification enzymes, and NRF2 is a master regulator of antioxidant and detoxification genes. Thus, we concluded that the increased amount of BaP-induced DNA adducts in BRCA1 knockdown cells is strongly associated with its loss of functional detoxification. Chromatin immunoprecipitation assay revealed that BRCA1 is recruited to the promoter/enhancer sequences of UGT1A1, UGT1A9, and NRF2. Regulation of UGT1A1 and UGT1A9 expression showed that the induction of DNA adducts by BaP is directly affected by their expression levels. Finally, overexpression of UGTs, NRF2, or ARNT significantly decreased the amount of BaP-induced adducts in BRCA1-deficient cells. Overall, our results suggest that BRCA1 protects cells by reducing the amount of BaP-induced DNA adducts possibly via transcriptional activation of detoxification gene expression.

The ordering of expression among a few genes can provide simple cancer biomarkers and signal BRCA1 mutations

Background

A major challenge in computational biology is to extract knowledge about the genetic nature of disease from high-throughput data. However, an important obstacle to both biological understanding and clinical applications is the "black box" nature of the decision rules provided by most machine learning approaches, which usually involve many genes combined in a highly complex fashion. Achieving biologically relevant results argues for a different strategy. A promising alternative is to base prediction entirely upon the relative expression ordering of a small number of genes.

Results

We present a three-gene version of "relative expression analysis" (RXA), a rigorous and systematic comparison with earlier approaches in a variety of cancer studies, a clinically relevant application to predicting germline BRCA1 mutations in breast cancer and a cross-study validation for predicting ER status. In the BRCA1 study, RXA yields high accuracy with a simple decision rule: in tumors carrying mutations, the expression of a "reference gene" falls between the expression of two differentially expressed genes, PPP1CB and RNF14. An analysis of the protein-protein interactions among the triplet of genes and BRCA1 suggests that the classifier has a biological foundation.

Conclusion

RXA has the potential to identify genomic "marker interactions" with plausible biological interpretation and direct clinical applicability. It provides a general framework for understanding the roles of the genes involved in decision rules, as illustrated for the difficult and clinically relevant problem of identifying BRCA1 mutation carriers.

Tumor suppressor BRCA1 is expressed in prostate cancer and controls insulin-like growth factor I receptor (IGF-IR) gene transcription in an androgen receptor-dependent manner

PURPOSE

The insulin-like growth factor (IGF) system plays an important role in prostate cancer. The BRCA1 gene encodes a transcription factor with tumor suppressor activity. The involvement of BRCA1 in prostate cancer, however, has not yet been elucidated. The purpose of the present study was to examine the functional correlations between BRCA1 and the IGF system in prostate cancer.

EXPERIMENTAL DESIGN

An immunohistochemical analysis of BRCA1 was done on tissue microarrays comprising 203 primary prostate cancer specimens. In addition, BRCA1 levels were measured in prostate cancer xenografts and in cell lines representing early stages (P69 cells) and advanced stages (M12 cells) of the disease. The ability of BRCA1 to regulate IGF-I receptor (IGF-IR) expression was studied by coexpression experiments using a BRCA1 expression vector along with an IGF-IR promoter-luciferase reporter.

RESULTS

We found significantly elevated BRCA1 levels in prostate cancer in comparison with histologically normal prostate tissue (P<0.001). In addition, an inverse correlation between BRCA1 and IGF-IR levels was observed in the androgen receptor (AR)-negative prostate cancer-derived P69 and M12 cell lines. Coexpression experiments in M12 cells revealed that BRCA1 was able to suppress IGF-IR promoter activity and endogenous IGF-IR levels. On the other hand, BRCA1 enhanced IGF-IR levels in LNCaP C4-2 cells expressing an endogenous AR.

CONCLUSIONS

We provide evidence that BRCA1 differentially regulates IGF-IR expression in AR-positive and AR-negative prostate cancer cells. The mechanism of action of BRCA1 involves modulation of IGF-IR gene transcription. In addition, immunohistochemical data are consistent with a potential survival role of BRCA1 in prostate cancer.

Liver X receptor ligands suppress ubiquitination and degradation of LXRalpha by displacing BARD1/BRCA1

Liver X receptor (LXR) is a ligand-activated transcription factor that plays important roles in cholesterol and lipid homeostasis. However, ligand-induced posttranslational modification of LXR is largely unknown. Here, we show that ligand-free LXRalpha is rapidly degraded by ubiquitination. Without ligand, LXRalpha interacts with an ubiquitin E3-ligase protein complex containing breast and ovarian cancer susceptibility 1 (BRCA1)-associated RING domain 1 (BARD1). Interestingly, LXR ligand represses ubiquitination and degradation of LXRalpha, and the interaction between LXRalpha and BARD1 is inhibited by LXR ligand. Consistently, T0901317, a synthetic LXR ligand, increased the level of LXRalpha protein in liver. Moreover, overexpression of BARD1/BRCA1 promoted the ubiquitination of LXRalpha and reduced the recruitment of LXRalpha to the target gene promoters, whereas BARD1 knockdown reversed such effects. Taken together, these data suggest that LXR ligand prevents LXRalpha from ubiquitination and degradation by detaching BARD1/BRCA1, which might be critical for the early step of transcriptional activation of ligand-stimulated LXRalpha through a stable binding of LXRalpha to the promoters of target genes.

Isoflavones and the prevention of breast and prostate cancer: new perspectives opened by nutrigenomics

Epidemiological evidence together with preclinical data from animal and in vitro studies strongly support a correlation between soy isoflavone consumption and protection towards breast and prostate cancers. The biological processes modulated by isoflavones, and especially by genistein, have been extensively studied, yet without leading to a clear understanding of the cellular and molecular mechanisms of action involved. This review discusses the existing gaps in our knowledge and evaluates the potential of the new nutrigenomic approaches to improve the study of the molecular effects of isoflavones. Several issues need to be taken into account for the proper interpretation of the results already published for isoflavones. Too often knowledge on isoflavone bioavailability is not taken into account; supra-physiological doses are frequently used. Characterization of the individual variability as defined by the gut microflora composition and gene polymorphisms may also help to explain the discrepancies observed so far in the clinical studies. Finally, the complex inter-relations existing between tissues and cell types as well as cross-talks between metabolic and signalling pathways have been insufficiently considered. By appraising critically the abundant literature with these considerations in mind, the mechanisms of action that are the more likely to play a role in the preventive effects of isoflavones towards breast and prostate cancers are reviewed. Furthermore, the new perspectives opened by the use of genetic, transcriptomic, proteomic and metabolomic approaches are highlighted.

The tumor suppressor activity induced by adenovirus-mediated BRCA1 overexpression is not restricted to breast cancers

The BRCA1 (breast cancer 1) breast cancer susceptibility gene is recognized as responsible for most familial breast and ovarian cancers and is suggested to be a tissue-specific tumor suppressor gene. In this report, we investigated the tissue specificity of tumor inhibitory activities induced by a recombinant adenovirus coding for wild-type BRCA1 (wtAdBRCA1). We demonstrated a pronounced in vitro antiproliferative effect on H1299 lung and HT29 colon cells upon infection with AdBRCA1. We describe a prolonged G1 cell cycle arrest associated with a decrease in the hyperphosphorylated form of Rb, suggesting that the Rb/E2F pathway is implicated in BRCA1-induced cell growth arrest. We also observed a significant antitumor effect in these pre-established subcutaneous tumors after in situ delivery of AdBRCA1, although these two tumors do not express wt p53, and also estrogen alpha and beta, progesterone and androgen receptors. Moreover, BRCA1 can induce a strong prolonged cell cycle arrest and apoptotic cell death but no significant antiangiogenic effect in H1299 tumors. Finally, our data indicate that intratumor administration of wtAdBRCA1 significantly inhibits growth of lung and colon steroid hormone-independent tumors.

Identification of BAF57 mutations in human breast cancer cell lines

Accumulating genetic and biochemical evidences support a role for the SWI/SNF chromatin-remodeling complex in cancer development and multiple core subunits of these complexes have been found to function as tumor suppressor genes. The core SWI/SNF subunit BAF57 mediates direct interactions with estrogen and androgen receptors (ER and AR) regulating their transcriptional activity. BAF57 gene maps to chromosome band 17 q21 in close proximity to the BRCA1 gene. This locus has been associated with frequent loss of heterozygosity (LOH) and allelic imbalance in breast cancers; however, BRCA1 mutations are rare events in sporadic breast cancer with LOH in the region, suggesting that another tumor suppressor gene resides in this area. All these reasons prompted us to screen for mutations in the BAF57 gene using a panel of the most commonly used human breast cancer cell lines. All cell lines analysed contain wild-type copies of BAF57 gene with the only exception of the breast ductal carcinoma cell line BT549. Sequencing of genomic DNA and cDNA generated from BT549 mRNA demonstrated the presence of a CA dinucleotide insertion in exon 5 of BAF57. The absence of wild-type BAF57 alleles indicates that this is a biallelic inactivating mutation that causes a frameshift and as a consequence a premature stop codon leading to a truncated BAF57 protein. A functional characterisation of the truncated BAF57 showed that it has lost the ability to bind to ER but still binds to the nuclear receptor coactivator SRC1e. Furthermore, we observed that the expression of the truncated BAF57 increased the ability of SRC1e to potentiate transcriptional activation by ERalpha, suggesting that mutations in BAF57 could contribute to the oncogenic transformation in breast cancer cells.

DNA methylation markers - an opportunity to further individualize therapy in breast cancer?

Over the last few decades, a wealth of treatment options have become available for breast cancer. To specifically direct those therapies to patients with the highest need who will receive the greatest benefit, biomarkers are urgently needed. Two specific needs seem to be most pressing: first is the need for prognostic markers, which would determine which group of patients may recover without adjuvant chemotherapy. Second, predictive markers for specific treatments, such as different endocrine treatments, chemotherapies or targeted drugs, are expected to play a major role in the near future. Ideally, such markers should be strong single markers, or low-complexity marker panels containing only a few markers, to allow for easier assay development and improved reproducibility. The possibility to measure the marker(s) in formalin-fixed specimens would greatly facilitate integration into routine clinical practice. A common and early event in breast cancer is aberrant DNA methylation within gene regulatory regions, affecting a variety of genes with different functions. Data from recently published studies indicate that altered DNA methylation carries prognostic as well as predictive information in breast cancer. Together with the technical advantages of a DNA-based marker, DNA methylation may well constitute the ideal biomarker to further individualize breast cancer treatment. Here the recent literature is reviewed and the most interesting markers, which have the potential to significantly change breast cancer treatment and, therefore, warrant further systematic clinical validation, are highlighted.

The joint effect of smoking and AIB1 on breast cancer risk in BRCA1 mutation carriers

Women with BRCA1 mutations are at an elevated risk for breast cancer. AIB1 (NCOA3/SRC3) genotype and smoking may alter this risk. We examined the differences in breast cancer risk by AIB1 polyglutamine repeat polymorphism and pre-diagnosis smoking habits for BRCA1 mutation carriers to determine if there was an interaction between smoking and AIB1 genotype. Multivariate Cox proportional hazards regression was used with 316 female BRCA1 mutation carriers to model breast cancer risk. Ever having smoked was associated with a decreased breast cancer risk [Hazard Ratio (HR) = 0.63, 95% CI, 0.47-0.87]. A dose-response relationship between number of pack-years smoked and breast cancer risk was also found for women who smoked <20 pack years of cigarettes (HR = 0.72, 95% CI, 0.52-1.00) and for women who smoked >/=20 pack years (HR = 0.41, 95% CI, 0.23-0.71; P for trend = 0.0007). Women with a 28 repeat allele for AIB1 had a significantly reduced risk of breast cancer (HR = 0.72, 95% CI, 0.51-1.00). Women who smoked >/=20 pack-years with a 28 repeat allele had an even greater reduced risk of breast cancer (HR = 0.19, 95% CI, 0.07-0.54) compared to women who were never smokers with no 28 allele. Since AIB1 appears to modulate the effect of endogenous hormones via the estrogen receptor, and smoking affects circulating hormone levels, these results support evidence that steroid hormones play an important role in breast carcinogenesis in BRCA1 mutation carriers, and suggest mechanisms for developing novel cancer prevention strategies for BRCA1 mutation carriers.

Interaction Between Estrogen Receptor Alpha, Ionizing Radiation and (anti-) Estrogens in Breast Cancer Cells

PURPOSE

Estrogen receptor alpha (ERalpha) plays a major role in breast cancer development. It acts as ligand-inducible transcription factor which determines growth, survival and differentiation of breast cancer cells. The aim of this study is to evaluate the potential interference between radiotherapy and estrogen receptor responsiveness. Materials and methods. The effect of ionizing radiation was assessed on the estrogen receptor alpha status, growth (proliferation and apoptosis) and sensitivity of MCF-7 breast cancer cells to estrogenic (17beta-estradiol (E2)), selective estrogen receptor modulator (SERM) and anti-estrogenic compounds. Results. We have observed a ligand-independent decrease in ERalpha expression after radiation, resulting from a specific reduction in mRNA level and protein synthesis. This ERalpha disappearance occurred 72 h post-irradiation at 8 Gy and decreased the transcriptional activity in ERalpha of these cells. On the other hand, E2 impedes the growth inhibitory effects (essentially on proliferation) of ionizing radiation in MCF-7 cells, which potentially decreases radiosensitivity of these cells. This effect was totally blocked by SERM and anti-estrogenic treatments. Moreover, this growth effect of concurrent anti-estrogenic drugs and ionizing radiation appeared to be strongly synergistic.

CONCLUSIONS

This study may increase general comprehension of ERalpha modulation by radiotherapy and improve adjuvant therapeutic approaches based on co-administration of radiation and endocrine therapy.

The breast cancer susceptibility gene BRCA1 regulates progesterone receptor signaling in mammary epithelial cells

The progesterone receptor (PR) plays roles in normal mammary development and breast cancer formation, where it may exert both stimulatory and inhibitory actions. Previously, the breast cancer susceptibility gene product BRCA1 was found to interact with and inhibit the transcriptional activity of estrogen receptor-alpha. In this study, we found that exogenous wild-type BRCA1 inhibited the activity of the PR in transient transfection assays utilizing a mouse mammary tumor virus-Luc reporter. Wild-type BRCA1 inhibited the activity of endogenous PR in human breast cancer cells (T47D and MCF-7) and inhibited the activity of exogenous PR-A, PR-B, and [PR-A plus PR-B] isoforms. On the other hand, knockdown of endogenous BRCA1 using small interfering RNA enhanced the progesterone-stimulated activity of the PR by about 4-fold. We documented an in vivo association of the endogenous BRCA1 with PR isoforms A and B and a direct in vitro interaction between BRCA1 and PR, which was partially mapped. Whereas down-regulation of the coactivator p300 contributes to the BRCA1-mediated repression of estrogen receptor-alpha, this mechanism does not contribute to inhibition of PR activity, because exogenous p300 did not rescue the BRCA1 repression of PR activity. The BRCA1-PR interaction has functional consequences. Thus, we showed that BRCA1 inhibits the expression of various endogenous progesterone-responsive genes and inhibits progesterone-stimulated proliferation of T47D cells. Finally, exogenous progesterone caused an exaggerated proliferative response in the mammary glands of mice harboring a mammary-targeted conditional deletion of the full-length isoform of Brca1. These findings suggest that BRCA1 regulates the activity of progesterone, a major hormone of pregnancy that may also participate in mammary carcinogenesis.

BRCA1 regulation of transcription

BRCA1, a tumor suppressor gene on chromosome 17q21, was identified in 1994 based on its linkage to hereditary breast and ovarian cancer syndromes. The BRCA1 gene encodes a 220 kDa nuclear phosphoprotein. Studies aimed at elucidating the mechanisms of its tumor suppressor activity have revealed, in part, that BRCA1 participates in the DNA damage response and acts to maintain the integrity of the genome. This activity is generic and does not account for the propensity of BRCA1 mutation carriers to develop specific tumor types rather than a broad spectrum of cancers. In addition to genome maintenance, BRCA1 has been found to broadly regulate gene transcription, even though it is not itself a sequence-specific DNA-binding transcription factor. The ability of BRCA1 to function as a coregulator of transcription may underlie some of its tumor suppressor activity and may explain the tissue-specific nature of this activity. This review will focus on how BRCA1 selectively regulates transcription and how this regulatory function may relate to tumor suppression.

Regulation of the estrogen-inducible gene expression profile by the breast cancer susceptibility gene BRCA1

The tumor suppressor gene BRCA1 functions in part as a caretaker in preserving the integrity of the genome, but also exhibits tissue-specific function by inhibiting estrogen receptor activity. Because estrogen (E2) induces a wide range of gene expression changes (by nongenomic and several transcriptional pathways), we sought to determine how comprehensive is the BRCA1-mediated inhibition of E2-induced gene expression alterations. Using cDNA-spotted microarrays, we identified a relatively large number of gene expression alterations (both increased and decreased expression) in MCF-7 cells caused by E2, some of which have been reported in previous studies. However, in the presence of exogenous wild-type BRCA1 (wtBRCA1), the response to E2 was severely blunted, with only about 10% the number of gene expression changes as that found in the absence of wtBRCA1. Examples of these findings were confirmed by semiquantitative and quantitative RT-PCR assays. In contrast to wtBRCA1, the induction by E2 of several E2-responsive genes was not inhibited by a full-length tumor-associated mutant BRCA1 protein [T300G (or (61)Cys-->Gly)]. For three E2-responsive genes whose induction by E2 was inhibited by wtBRCA1, wtBRCA1 had little or no effect on the mRNA half-life in the presence of E2. Consistent with these findings, wtBRCA1 inhibited E2-stimulated proliferation of MCF-7 cells, but wtBRCA1 failed to inhibit the proliferation of MCF-7 cells stimulated by IGF-I. Our findings suggest that BRCA1 globally inhibits the response to estrogen in a dose- and time-dependent fashion. The implications of these findings for understanding how BRCA1 may act to restrain E2 action in vivo are considered.

Expression profiling of the estrogen responsive genes in response to phytoestrogens using a customized DNA microarray

Here, we examined phytoestrogens, isoflavones (genistein, daidzein, glycitein, biochanin A and ipriflavone), flavones (chrysin, luteolin and apigenin), flavonols (kaempferol and quercetin), and a coumestan, a flavanone and a chalcone (coumestrol, naringenin and phloretin, respectively) by means of a DNA microarray assay. A total of 172 estrogen responsive genes were monitored with a customized DNA microarray and their expression profiles for the above phytoestrogens were compared with that for 17beta-estradiol (E2) using correlation coefficients, or R values, after a correlation analysis by linear regression. While R values indicate the similarity of the response by the genes, we also examined the genes by cluster analysis and by their specificity to phytoestrogens (specific to genistein, daidzein or glycitein) or gene functions. Several genes were selected from p53-related genes (CDKN1A, TP53I11 and CDC14), Akt2-related genes (PRKCD, BRCA1, TRIB3 and APPL), mitogen-activated protein kinase-related genes (RSK and SH3BP5), Ras superfamily genes (RAP1GA1, RHOC and ARHGDIA) and AP-1 family and related genes (RIP140, FOS, ATF3, JUN and FRA2). We further examined the extracts from two local crops of soy beans (Kuro-daizu or Mochi-daizu) by comparing the gene expression profiles with those of E2 or phytoestrogens as a first step in utilizing the expression profiles for various applications.

COBRA1 inhibits AP-1 transcriptional activity in transfected cells

Mutations in the breast cancer susceptibility gene (BRCA1) account for a significant proportion of hereditary breast and ovarian cancers. Cofactor of BRCA1 (COBRA1) was isolated as a BRCA1-interacting protein and exhibited a similar chromatin reorganizing activity to that of BRCA1. However, the biological role of COBRA1 remains largely unexplored. Here, we report that ectopic expression of COBRA1 inhibited activator protein 1 (AP-1) transcriptional activity in transfected cells in a dose-dependent manner, whereas reduction of endogenous COBRA1 with a small interfering RNA significantly enhanced AP-1-mediated transcriptional activation. COBRA1 physically interacted with the AP-1 family members, c-Jun and c-Fos, and the middle region of COBRA1 bound to c-Fos. Lack of c-Fos binding site in the COBRA1 completely abolished the COBRA1 inhibition of AP-1 trans-activation. These findings suggest that COBRA1 may directly modulate AP-1 pathway and, therefore, may play important roles in cell proliferation, differentiation, apoptosis, and oncogenesis.

Gene Expression Profiling of Breast Cancer in Relation to Estrogen Receptor Status and Estrogen-Metabolizing Enzymes: Clinical Implications

Interactions between luminal epithelial cells and their surrounding microenvironment govern the normal development and function of the mammary gland. Estradiol plays a key role in abnormal intracellular signaling, which contributes to the development and progression of breast tumors. The present article summarizes the results from a microarray whole genome gene expression analysis as well as a quantitative analysis of the mRNA expression of members of the estradiol metabolic and signaling pathways in the tumors of postmenopausal breast cancer patients. The analysis of the variation in whole genome gene expression resulted in a tumor classification comprising several distinct groups with distinct expression of the estrogen receptor (ER). The parallel study on the expression of only nine mRNA transcripts of members of the estradiol pathways resulted in two main clusters, representing ER− and ER tumors. The mRNA expression of the estradiol-metabolizing enzymes did not follow the expression of the ER in all cases, leading to the recognition of several further subclasses of tumors. When the tumor classes obtained by whole genome gene expression analysis were compared with those obtained by independent quantitation of the estradiol-metabolizing enzymes, a statistically significant association between both classification groups was observed. These findings point to a possible association between development of a tumor with a particular expression profile and its capacity to synthesize estradiol as measured by the expression of the transcripts for the necessary key enzymes. Further, whole genome expression patterns were studied in 12 patients treated with anastrozole. Using significance analysis of microarrays, we identified 298 genes significantly differently expressed between partial response and progressive disease groups.