Estrogen inhibits ATR signaling to cell cycle checkpoints and DNA repair

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.

Estrogen modulates metabolic pathway adaptation to available glucose in breast cancer cells

Most cancers use glucose as substrate for aerobic glycolysis in preference to oxidative phosphorylation. However, variable glucose concentrations within the in-vivo tumor microenvironment may necessitate metabolic plasticity. Furthermore, little information exists on a role for estrogen receptors in modulating possible metabolic adaptations in breast cancer cells. Here we find that MCF-7 cells switch between metabolic pathways depending on glucose availability and 17β-estradiol (E(2)) potentiates adaptation. In high glucose conditions E(2) up-regulates glycolysis via enhanced AKT kinase activity and suppresses tricarboxylic acid cycle activity. After a decrease in extracellular glucose, mitochondrial pathways are activated in preference to glycolysis. In this setting, E(2) suppresses glycolysis and rescues cell viability by stimulating the tricarboxylic acid cycle via the up-regulation of pyruvate dehydrogenase (PDH) activity. E(2) also increases ATP in low glucose-cultured cells, and the novel phosphorylation of PDH by AMP kinase is required for these metabolic compensations. Capitalizing on metabolic vulnerability, knockdown of PDH in the low-glucose state strongly potentiates ionizing radiation-induced apoptosis and reverses the cell survival effects of E(2). We propose that lowering glucose substrate and inhibiting PDH may augment adjuvant therapies for estrogen receptor-positive breast cancer.

The role of master regulators in the metabolic/transcriptional coupling in breast carcinomas

Metabolic transformations have been reported as involved in neoplasms survival. This suggests a role of metabolic pathways as potential cancer pharmacological targets. Modulating tumor's energy production pathways may become a substantial research area for cancer treatment. The significant role of metabolic deregulation as inducing transcriptional instabilities and consequently whole-system failure, is thus of foremost importance. By using a data integration approach that combines experimental evidence for high-throughput genome wide gene expression, a non-equilibrium thermodynamics analysis, nonlinear correlation networks as well as database mining, we were able to outline the role that transcription factors MEF2C and MNDA may have as main master regulators in primary breast cancer phenomenology, as well as the possible interrelationship between malignancy and metabolic dysfunction. The present findings are supported by the analysis of 1191 whole genome gene expression experiments, as well as probabilistic inference of gene regulatory networks, and non-equilibrium thermodynamics of such data. Other evidence sources include pathway enrichment and gene set enrichment analyses, as well as motif comparison with a comprehensive gene regulatory network (of homologue genes) in Arabidopsis thaliana. Our key finding is that the non-equilibrium free energies provide a realistic description of transcription factor activation that when supplemented with gene regulatory networks made us able to find deregulated pathways. These analyses also suggest a novel potential role of transcription factor energetics at the onset of primary tumor development. Results are important in the molecular systems biology of cancer field, since deregulation and coupling mechanisms between metabolic activity and transcriptional regulation can be better understood by taking into account the way that master regulators respond to physicochemical constraints imposed by different phenotypic conditions.

Suppression of Jab1/CSN5 induces radio- and chemo-sensitivity in nasopharyngeal carcinoma through changes to the DNA damage and repair pathways

Nasopharyngeal carcinoma (NPC) is an Epstein-Barr virus-associated malignancy most common in East Asia and Africa. Radiotherapy and cisplatin-based chemotherapy are the main treatment options. Unfortunately, disease response to concurrent chemoradiotherapy varies among patients with NPC, and many cases are resistant to cisplatin. Increased DNA damage repair is one of the mechanisms contributing to this resistance. Jab1/CSN5 is a multifunctional protein that participates in controlling cell proliferation and the stability of multiple proteins. Jab1 overexpression has been found to correlate with poor prognosis in several tumor types. However, the biological significance of Jab1 activity in response to cancer treatment is unclear. In this study, we used three NPC cell lines (CNE1, CNE2, and HONE1) to investigate the hypothesis that Jab1 positively regulates the DNA repair protein Rad51 and, in turn, cellular response to treatment with DNA-damaging agents such as cisplatin, ionizing radiation (IR) and ultraviolet (UV). We found that Jab1 was overexpressed in two relatively cisplatin-, IR- and UV-resistant NPC cell lines, and knocking down its expression conferred sensitivity to cisplatin, IR and UV. By contrast, exogenous Jab1 expression enhanced the resistance of NPC cells to cisplatin, IR and UV Moreover, we provide a mechanism by which Jab1 positively regulated Rad51 through p53-dependent pathway, and increased ectopic expression of Rad51 conferred cellular resistance to cisplatin, IR and UV in Jab1-deficient cells. Taken together, our findings suggest that Jab1 plays an important role in the cellular response to cisplatin and irradiation by regulating DNA damage and repair pathways. Therefore, Jab1 is a novel biomarker for predicting the outcome of patients with NPC who are treated with DNA-damaging agents.

Inhibition of ERβ induces resistance to cisplatin by enhancing Rad51-mediated DNA repair in human medulloblastoma cell lines

Cisplatin is one of the most widely used and effective anticancer drugs against solid tumors including cerebellar tumor of the childhood, Medulloblastoma. However, cancer cells often develop resistance to cisplatin, which limits therapeutic effectiveness of this otherwise effective genotoxic drug. In this study, we demonstrate that human medulloblastoma cell lines develop acute resistance to cisplatin in the presence of estrogen receptor (ER) antagonist, ICI182,780. This unexpected finding involves a switch from the G2/M to G1 checkpoint accompanied by decrease in ATM/Chk2 and increase in ATR/Chk1 phosphorylation. We have previously reported that ERβ, which is highly expressed in medulloblastomas, translocates insulin receptor substrate 1 (IRS-1) to the nucleus, and that nuclear IRS-1 binds to Rad51 and attenuates homologous recombination directed DNA repair (HRR). Here, we demonstrate that in the presence of ICI182,780, cisplatin-treated medulloblastoma cells show recruitment of Rad51 to the sites of damaged DNA and increase in HRR activity. This enhanced DNA repair during the S phase preserved also clonogenic potential of medulloblastoma cells treated with cisplatin. In conclusion, inhibition of ERβ considered as a supplemental anticancer therapy, has been found to interfere with cisplatin–induced cytotoxicity in human medulloblastoma cell lines.

Akt: a double-edged sword in cell proliferation and genome stability

The Akt family of serine/threonine protein kinases are key regulators of multiple aspects of cell behaviour, including proliferation, survival, metabolism, and tumorigenesis. Growth-factor-activated Akt signalling promotes progression through normal, unperturbed cell cycles by acting on diverse downstream factors involved in controlling the G1/S and G2/M transitions. Remarkably, several recent studies have also implicated Akt in modulating DNA damage responses and genome stability. High Akt activity can suppress ATR/Chk1 signalling and homologous recombination repair (HRR) via direct phosphorylation of Chk1 or TopBP1 or, indirectly, by inhibiting recruitment of double-strand break (DSB) resection factors, such as RPA, Brca1, and Rad51, to sites of damage. Loss of checkpoint and/or HRR proficiency is therefore a potential cause of genomic instability in tumor cells with high Akt. Conversely, Akt is activated by DNA double-strand breaks (DSBs) in a DNA-PK- or ATM/ATR-dependent manner and in some circumstances can contribute to radioresistance by stimulating DNA repair by nonhomologous end joining (NHEJ). Akt therefore modifies both the response to and repair of genotoxic damage in complex ways that are likely to have important consequences for the therapy of tumors with deregulation of the PI3K-Akt-PTEN pathway.

Novel insight into KLF4 proteolytic regulation in estrogen receptor signaling and breast carcinogenesis

Krüppel-like factor 4 (KLF4), a zinc finger-containing transcriptional factor, is a pivotal regulator of cellular fate. KLF4 has attracted considerable attention for its opposing effect in carcinogenesis as tumor suppressor (e.g. colorectal cancer) or oncoprotein (e.g. breast cancer), depending on tissue context, with the underlying mechanism remaining largely unknown. Here we report that KLF4 mediates estrogen signaling in breast cancer formation. Accumulation of KLF4 by inhibiting its turnover triggers estrogen-induced transactivation. We identified Von Hippel-Lindau, pVHL, as the protein that governs KLF4 turnover in breast cancer cells and demonstrated that estrogen-induced down-regulation of pVHL facilitates accumulation of KLF4. We provide mechanistic insights into KLF4 steady-state degradation as well as its elevation in the presence of estrogen and show that elevated levels of pVHL or depletion of KLF4 attenuates the estrogen-induced transactivation and cell growth. Finally, immunohistochemical staining revealed reduced concentration of pVHL and accumulation of KLF4 in breast cancer tissues. We thus propose that suppression of pVHL in response to estrogen signaling results in elevation of KLF4, which mediates estrogen-induced mitogenic effect.

P53 mediates estradiol induced activation of apoptosis and DNA repair in non-malignant colonocytes

Clinical and animal studies have shown a strong link between estrogen status in women and decreased risk of colon cancer. However, little research has been done into the mechanism of protection that estrogen provides. Our laboratory has demonstrated that estradiol (E₂) inhibits the development of pre-neoplastic lesions through an estrogen receptor β (ERβ) mediated mechanism in mice. Our data also suggest that the primary protective role of E₂ treatment is increased apoptosis in non-malignant colonocytes that are damaged and at risk of becoming cancerous. The p53 protein plays a crucial role in the cellular response to stress by inducing cell cycle arrest, DNA repair mechanisms, and/or apoptosis. Due to the observed induction of apoptosis in response to E₂, we are investigating the role of p53 in this chemo-protective mechanism. E₂ suppressed growth of young adult mouse colonocytes (YAMCs) by inducing apoptosis and these physiological responses were completely lost in YAMCs lacking a functional p53 protein. Western blot analysis demonstrated increases in p53 protein levels in YAMCs after treatment with E₂ likely due to protein stabilization. E₂ was shown to enhance the transcriptional activity of p53, resulting in up-regulation of pro-apoptotic p53 target genes (Bax, Noxa, and PUMA). Finally, repair of DNA double stranded breaks was shown to be increased by E₂ treatment. Collectively, these data are the first to demonstrate that p53 is a primary mediator of the protective actions of E₂ in the colon.

DHHC-7 and -21 are palmitoylacyltransferases for sex steroid receptors

Extranuclear sex steroid receptors require palmitoylation to traffic to the plasma membrane, where they activate signal transduction cascades. We identify DHHC-7 and -21 palmitoylacyltransferases as conserved enzymes for the three classes of sex steroid receptors.

Classical estrogen, progesterone, and androgen receptors (ERs, PRs, and ARs) localize outside the nucleus at the plasma membrane of target cells. From the membrane, the receptors signal to activate kinase cascades that are essential for the modulation of transcription and nongenomic functions in many target cells. ER, PR, and AR trafficking to the membrane requires receptor palmitoylation by palmitoylacyltransferase (PAT) protein(s). However, the identity of the steroid receptor PAT(s) is unknown. We identified the DHHC-7 and -21 proteins as conserved PATs for the sex steroid receptors. From DHHC-7 and -21 knockdown studies, the PATs are required for endogenous ER, PR, and AR palmitoylation, membrane trafficking, and rapid signal transduction in cancer cells. Thus the DHHC-7 and -21 proteins are novel targets to selectively inhibit membrane sex steroid receptor localization and function.

Cracking the estrogen receptor's posttranslational code in breast tumors

Estrogen signaling pathways, because of their central role in regulating the growth and survival of breast tumor cells, have been identified as suitable and efficient targets for cancer therapies. Agents blocking estrogen activity are already widely used clinically, and many new molecules have entered clinical trials, but intrinsic or acquired resistance to treatment limits their efficacy. The basic molecular studies underlying estrogen signaling have defined the critical role of estrogen receptors (ER) in many aspects of breast tumorigenesis. However, important knowledge gaps remain about the role of posttranslational modifications (PTM) of ER in initiation and progression of breast carcinogenesis. Whereas major attention has been focused on the phosphorylation of ER, many other PTM (such as acetylation, ubiquitination, sumoylation, methylation, and palmitoylation) have been identified as events modifying ER expression and stability, subcellular localization, and sensitivity to hormonal response. This article will provide an overview of the current and emerging knowledge on ER PTM, with a particular focus on their deregulation in breast cancer. We also discuss their clinical relevance and the functional relationship between PTM. A thorough understanding of the complete picture of these modifications in ER carcinogenesis might not only open new avenues for identifying new markers for prognosis or prediction of response to endocrine therapy but also could promote the development of novel therapeutic strategies.

High RAD51 mRNA expression characterize estrogen receptor-positive/progesteron receptor-negative breast cancer and is associated with patient's outcome

Mutations in DNA double-strand breaks (DSB) repair genes are involved in the pathogenesis of hereditary mammary tumors, it is, however, still unclear whether defects in this pathway may play a role in sporadic breast cancer. In this study, we initially determined mRNA expression of 15 DSB related genes by reverse transcription quantitative polymerase chain reaction in paired normal tissue and cancer specimen from 20 breast cancer cases to classify them into homogeneous clusters. G22P1/ku70, ATR and RAD51 genes were differentially expressed in the three branches recognized by clustering analysis. In particular, a breast cancer subgroup characterized by high RAD51 mRNA levels and estrogen receptor (ER)-positive/progesteron receptor (PR)-negative phenotype was identified. This result was confirmed by the analysis of G22P1/ku70, ATR and RAD51 mRNA levels on paired normal and tumor specimens from an extended breast cancer cohort (n = 75). RAD51 mRNA levels were inversely associated with PR status (p = 0.02) and the highest levels were, indeed, detected in ER-positive/PR-negative tumors (p = 0.03). RAD51 immunostaining of a tissue microarray confirmed the inverse relationship between high RAD51 expression and negative PR status (p = 0.002), as well as, the association with ER-positive/PR-negative phenotype (p = 0.003). Interestingly, the analysis of microarray expression data from 295 breast cancers indicate that RAD51 increased mRNA expression is associated with higher risk of tumor relapse, distant metastases and worst overall survival (p = 0.015, p = 0.009 and p = 0.013 respectively). Our results suggest that RAD51 expression determination could contribute to a better molecular classification of mammary tumors and may represent a novel tool for evaluating postoperative adjuvant therapy for breast cancer patients.

Estrogen receptor α-mediated transcription induces cell cycle-dependent DNA double-strand breaks

Prolonged exposure to estrogen increases breast cancer risk. Estrogen is known to induce chromosomal aberrations, yet the mechanisms by which estrogen promotes genomic instability are not fully understood. Here, we show that exposure of MCF-7 cells to 17β-estradiol (E2) induces DNA double-strand breaks (DSBs), as determined by the formation of γH2AX foci. Foci formation was dependent upon estrogen receptor-α (ERα) and the catalytic activity of the type II topoisomerase, topoisomerase IIβ (topoIIβ). Moreover, we show by chromatin immunoprecipitation that topoIIβ-dependent E2-induced γH2AX localizes to the promoter of the estrogen-inducible gene, trefoil factor 1. E2-induced foci were associated with cyclin A expression and inhibited by pre-incubation with the DNA polymerase inhibitor aphidicolin suggesting that E2-induced DSBs are mediated by progression through S phase. Furthermore, E2-induced γH2AX foci colocalized with Rad51, suggesting that E2-induced DSBs are repaired by homologous recombination. We propose that DNA DSBs formed by the strand-cleaving activity of the topoIIβ-DNA cleavage complex at estrogen-inducible genes can present a barrier to DNA replication, leading to persistent DNA DSBs in ERα-positive breast cancer cells.

Bisphenol A impairs the double-strand break repair machinery in the germline and causes chromosome abnormalities

Bisphenol A (BPA) is a highly prevalent constituent of plastics that has been associated with diabetes, cardiovascular disease, and an increased risk of miscarriages in humans. In mice, BPA exposure disrupts the process of meiosis; however, analysis of the affected molecular pathways is lagging and has been particularly challenging. Here we show that exposure of the nematode Caenorhabditis elegans to BPA, at internal concentrations consistent with mammalian models, causes increased sterility and embryonic lethality. BPA exposure results in impaired chromosome synapsis and disruption of meiotic double-strand break repair (DSBR) progression. BPA carries an anti-estrogenic activity in the germline and results in germline-specific down-regulation of DSBR genes, thereby impairing maintenance of genomic integrity during meiosis. C. elegans therefore constitutes a model of remarkable relevance to mammals with which to assess how our chemical landscape affects germ cells and meiosis.

Minireview: Extranuclear steroid receptors: roles in modulation of cell functions

Steroid receptors existing outside the nucleus are increasingly being recognized in many organs and cell types, impacting the biology of bone, the heart and blood vessels, and the central nervous system. Some controversy exists as to the nature of the receptors at the plasma membrane. However, compelling evidence has been advanced that at least some classical steroid receptors mediate steroid ligand actions originating as signaling from the cell surface. Here I review the recent findings in this evolving field emphasizing the in vivo impact of these receptor pools with a focus on estrogen receptors.

Endothelin

Endothelin-1 is the most potent vasoconstrictor agent currently identified, and it was originally isolated and characterized from the culture media of aortic endothelial cells. Two other isoforms, termed endothelin-2 and endothelin-3, were subsequently identified, along with structural homologues isolated from the venom of Actractapis engaddensis known as the sarafotoxins. In this review, we will discuss the basic science of endothelins, endothelin-converting enzymes, and endothelin receptors. Only concise background information pertinent to clinical physician is provided. Next we will describe the pathophysiological roles of endothelin-1 in pulmonary arterial hypertension, heart failure, systemic hypertension, and female malignancies, with emphasis on ovarian cancer. The potential intervention with pharmacological therapeutics will be succinctly summarized to highlight the exciting pre-clinical and clinical studies within the endothelin field. Of note is the rapid development of selective endothelin receptor antagonists, which has led to an explosion of research in the field.

Comprehensive screen of genetic variation in DNA repair pathway genes and postmenopausal breast cancer risk

Mistakes in DNA repair can result in sustained damage and genetic instability. We comprehensively evaluated common variants in DNA repair pathway genes for their association with postmenopausal breast cancer risk with and without respect to estrogen receptor (ER) and progesterone receptor (PR) subtypes. In this study of 1,145 prospectively ascertained breast cancer cases and 1,142 matched controls from the Nurses' Health Study Cancer Genetic Markers of Susceptibility project, we evaluated 1,314 common genetic variants in 68 candidate genes. These variants were chosen to represent five DNA repair pathways including base excision repair, nucleotide excision repair, double-strand break repair (homologous recombination and non-homologous end-joining), direct reversal repair, and mismatch repair, along with candidate DNA polymerases, Fanconi Anemia complementation groups, and other genes relevant to DNA damage recognition and response. Main effects, pathway effects, and pair-wise interactions were evaluated using Logistic Regression, and the Admixture Maximum Likelihood (AML) and Kernel Machine tests. Eight linked loci within XRCC4 were associated with susceptibility to PR- breast cancer (main effect p-values corrected for multiple testing at the within-gene level < 0.04). These loci drove the association between the non-homologous end-joining pathway, and PR- breast cancer (AML p-value for the full pathway = 0.002; p-value when the eight loci were removed = 0.86). A Kernel machine test of no linear or quadratic effects, or pairwise interaction, yielded a p-value of 0.85. Common variation alone in DNA repair genes plays at most a small role in determining postmenopausal breast cancer risk among women of European ancestry.

Plasma membrane estrogen receptors

It is now firmly established that estrogen and all sex steroid receptors exist in discrete cellular pools outside the nucleus. Estrogen receptors (ER) have been localized to the plasma membrane where both ERalpha and ERbeta function in a wide variety of cells and organs. ERs have also been found in discrete cytoplasmic organelles including mitochondria and the endoplasmic reticulum. In ligand-dependent fashion, each ER pool contributes to the overall, integrated effects of estrogens producing biological outcomes. This review highlights the recent work establishing new roles and targets of membrane ER signaling. Such actions include prevention of vascular injury or cardiac hypertrophy, sexual behavior and pain perception mediated through the central nervous system, osteoblast survival, and fluid resorption in the colon.

Membrane oestrogen receptor alpha signalling to cell functions

Conservation of steroid hormone action outside the nucleus occurs from plants that make brassinosteroids to higher metazoans (primates). In plants, steroid hormone action occurs when the brassinosteroids bind a membrane tyrosine kinase receptor. Ligated receptors for all sex steroids exist at the plasma membrane and rapidly signal through G proteins to second messengers including calcium, cAMP and cGMP, activating proximal and more distal kinases. These signal cascades impact many functions of steroid hormones, responsible for the biological actions of these molecules. Support also exists for membrane-localized receptors of other members of the steroid superfamily, responding to glucocorticoids, mineralocorticoids, thyroid hormone, and vitamin D. The nature of these receptors is in some cases unclear. Steroid receptors also exist in discrete cytoplasmic organelles, most notably the mitochondria, although the functions of these receptors are poorly understood. In this review, I highlight the essential elements of the membrane oestrogen receptor alpha, noting where conserved aspects exist for other steroid receptors.