Simak Ali

Endocrine-responsive breast cancer and strategies for combating resistance

Deaths from breast cancer have fallen markedly over the past decade due, in part, to the use of endocrine agents that reduce the levels of circulating oestrogens or compete with oestrogen for binding to its receptor. However, many breast tumours either fail to respond or become resistant to endocrine therapies. By understanding the mechanisms that underlie this resistance, we might be able to develop strategies for overcoming or bypassing it.

Human estrogen receptor beta binds DNA in a manner similar to and dimerizes with estrogen receptor alpha.

The cloning of a novel estrogen receptor β (denoted ERβ) has recently been described (Kuiper, G. G. J. M., Enmark, E., Pelto-Huikko, M., Nilsson, S., and Gustafsson, J-A. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 5925–5930 and Mosselman, S., Polman, J., and Dijkema, R. (1996)FEBS Lett. 392, 49–53). ERβ is highly homologous to the “classical” estrogen receptor α (here referred to as ERα), has been shown to bind estrogens with an affinity similar to that of ERα, and activates expression of reporter genes containing estrogen response elements in an estrogen-dependent manner. Here we describe functional studies comparing the DNA binding abilities of human ERα and β in gel shift assays. We show that DNA binding by ERα and β are similarly affected by elevated temperature in the absence of ligand or in the presence of 17β-estradiol and the partial estrogen agonist 4-hydroxy-tamoxifen. In the absence of ligand, DNA binding by ERα and β is rapidly lost at 37 °C, while in the presence of 17β-estradiol and 4-hydroxy-tamoxifen, the loss in DNA binding at elevated temperature is much more gradual. We show that the loss in DNA binding is not due to degradation of the receptor proteins. However, while the complete antagonist ICI 182,780 does not “protect” human ERα (hERα) from loss of DNA binding at elevated temperaturein vitro, it does appear to protect human ERβ (hERβ), suggestive of differences in the way ICI 182,780 acts on hERα and β. We further report that ERα and β can dimerize with each other, the DNA binding domain of hERα being sufficient for dimerization with hERβ. Cell and promoter-specific transcription activation by ERα has been shown to be dependent on the differential action of the N- and C-terminal transcription activation functions AF-1 and AF-2, respectively. The existence of a second estrogen receptor gene and the dimerization of ERα and β add greater levels of complexity to transcription activation in response to estrogens.

Regulation of ERBB2 by oestrogen receptor–PAX2 determines response to tamoxifen

Crosstalk between the oestrogen receptor (ER) and ERBB2/HER-2 pathways has long been implicated in breast cancer aetiology and drug response, yet no direct connection at a transcriptional level has been shown. Here we show that oestrogen–ER and tamoxifen–ER complexes directly repress ERBB2 transcription by means of a cis-regulatory element within the ERBB2 gene in human cell lines. We implicate the paired box 2 gene product (PAX2), in a previously unrecognized role, as a crucial mediator of ER repression of ERBB2 by the anti-cancer drug tamoxifen. We show that PAX2 and the ER co-activator AIB-1/SRC-3 compete for binding and regulation of ERBB2 transcription, the outcome of which determines tamoxifen response in breast cancer cells. The repression of ERBB2 by ER-PAX2 links these two breast cancer subtypes and suggests that aggressive ERBB2-positive tumours can originate from ER-positive luminal tumours by circumventing this repressive mechanism. These data provide mechanistic insight into the molecular basis of endocrine resistance in breast cancer.

Phosphorylation of Human Estrogen Receptor α by Protein Kinase A Regulates Dimerization

ABSTRACT Phosphorylation provides an important mechanism by which transcription factor activity is regulated. Estrogen receptor α (ERα) is phosphorylated on multiple sites, and stimulation of a number of growth factor receptors and/or protein kinases leads to ligand-independent and/or synergistic increase in transcriptional activation by ERα in the presence of estrogen. Here we show that ERα is phosphorylated by protein kinase A (PKA) on serine-236 within the DNA binding domain. Mutation of serine-236 to glutamic acid prevents DNA binding by inhibiting dimerization by ERα, whereas mutation to alanine has little effect on DNA binding or dimerization. Furthermore, PKA overexpression or activation of endogenous PKA inhibits dimerization in the absence of ligand. This inhibition is overcome by the addition of 17β-estradiol or the partial agonist 4-hydroxy tamoxifen. Interestingly, treatment with the complete antagonist ICI 182,780 does not overcome the inhibitory effect of PKA activation. Our results indicate that in the absence of ligand ERα forms dimers through interaction between DNA binding domains and that dimerization mediated by the ligand binding domain only occurs upon ligand binding but that the complete antagonist ICI 182,780 prevents dimerization through the ligand-binding domain. Heterodimer formation between ERα and ERβ is similarly affected by PKA phosphorylation of serine 236 of ERα. However, 4-hydroxytamoxifen is unable to overcome inhibition of dimerization by PKA. Thus, phosphorylation of ERα in the DNA binding domain provides a mechanism by which dimerization and thereby DNA binding by the estrogen receptor is regulated.

Activation of Estrogen Receptor α by S118 Phosphorylation Involves a Ligand-Dependent Interaction with TFIIH and Participation of CDK7

Phosphorylation of the estrogen receptor alpha (ERalpha) N-terminal transcription activation function AF1 at serine 118 (S118) modulates its activity. We show here that human ERalpha is phosphorylated by the TFIIH cyclin-dependent kinase in a ligand-dependent manner. Furthermore, the efficient phosphorylation of S118 requires a ligand-regulated interaction of TFIIH with AF2, the activation function located in the ligand binding domain (LBD) of ERalpha. This interaction involves (1) the integrity of helix 12 of the LBD/AF2 and (2) p62 and XPD, two subunits of the core TFIIH. These findings are suggestive of a novel mechanism by which nuclear receptor activity can be regulated by ligand-dependent recruitment of modifying activities, such as kinases.

Phosphorylation of human estrogen receptor α at serine 118 by two distinct signal transduction pathways revealed by phosphorylation-specific antisera

Estrogen receptor α (ERα) is a transcription factor that regulates expression of target genes in a ligand-dependent manner. Activation of gene expression is mediated by two transcription activation functions AF-1 and AF-2, which act in a promoter- and cell-specific manner. Whilst AF-2 activity is regulated by estrogen (E2) binding, the activity of AF-1 is additionally modulated by phosphorylation at several sites. One of these phosphorylation sites, serine 118 (S118) is of particular interest as its mutation significantly reduces ERα activity. Previous studies have shown that S118 can be phosphorylated by the ERK1/2 mitogen activated protein kinases (MAPK) and by the cyclin-dependent protein kinase Cdk7. In this study we use antisera that specifically recognize ERα phosphorylated at S118 to demonstrate that MAPK phosphorylates S118 in a ligand-independent manner, whereas Cdk7 mediates E2-induced phosphorylation of S118. E2 stimulation of S118 phosphorylation was observed within 10 min of its addition and was maximal at 10−7 M E2. S118 phosphorylation was maximal at 30 min but then declined, such that by 180 min following E2 addition little S118 phosphorylation was evident. S118 phosphorylation was also induced by the partial estrogen antagonist 4-hydroxytamoxifen, but not by the complete antagonist ICI 182, 780. S118 phosphorylation upon addition of the MAPK inducers EGF or PMA followed the expected time courses. Finally, we show that ERα is phosphorylated at S118 in vivo using immunoblotting of extracts prepared from a series of ERα-positive breast tumours.

Critical research gaps and translational priorities for the successful prevention and treatment of breast cancer

IntroductionBreast cancer remains a significant scientific, clinical and societal challenge. This gap analysis has reviewed and critically assessed enduring issues and new challenges emerging from recent research, and proposes strategies for translating solutions into practice.MethodsMore than 100 internationally recognised specialist breast cancer scientists, clinicians and healthcare professionals collaborated to address nine thematic areas: genetics, epigenetics and epidemiology; molecular pathology and cell biology; hormonal influences and endocrine therapy; imaging, detection and screening; current/novel therapies and biomarkers; drug resistance; metastasis, angiogenesis, circulating tumour cells, cancer ‘stem’ cells; risk and prevention; living with and managing breast cancer and its treatment. The groups developed summary papers through an iterative process which, following further appraisal from experts and patients, were melded into this summary account.ResultsThe 10 major gaps identified were: (1) understanding the functions and contextual interactions of genetic and epigenetic changes in normal breast development and during malignant transformation; (2) how to implement sustainable lifestyle changes (diet, exercise and weight) and chemopreventive strategies; (3) the need for tailored screening approaches including clinically actionable tests; (4) enhancing knowledge of molecular drivers behind breast cancer subtypes, progression and metastasis; (5) understanding the molecular mechanisms of tumour heterogeneity, dormancy, de novo or acquired resistance and how to target key nodes in these dynamic processes; (6) developing validated markers for chemosensitivity and radiosensitivity; (7) understanding the optimal duration, sequencing and rational combinations of treatment for improved personalised therapy; (8) validating multimodality imaging biomarkers for minimally invasive diagnosis and monitoring of responses in primary and metastatic disease; (9) developing interventions and support to improve the survivorship experience; (10) a continuing need for clinical material for translational research derived from normal breast, blood, primary, relapsed, metastatic and drug-resistant cancers with expert bioinformatics support to maximise its utility. The proposed infrastructural enablers include enhanced resources to support clinically relevant in vitro and in vivo tumour models; improved access to appropriate, fully annotated clinical samples; extended biomarker discovery, validation and standardisation; and facilitated cross-discipline working.ConclusionsWith resources to conduct further high-quality targeted research focusing on the gaps identified, increased knowledge translating into improved clinical care should be achievable within five years.

Estrogen receptor alpha in human breast cancer : occurrence and significance

Estrogens have long been recognized as being important for stimulating the growth of a large proportion of breast cancers. Now it is recognized that estrogen action is mediated by two receptors, and the presence of estrogen receptor α (ERα)3 correlates with better prognosis and the likelihood of response to hormonal therapy. Over half of all breast cancers overexpress ERα and around 70% of these respond to anti-estrogen (for example tamoxifen) therapy. In addition, the presence of elevated levels of ERα in benign breast epithelium appears to indicate an increased risk of breast cancer, suggesting a role for ERα in breast cancer initiation, as well as progression. However, a proportion of ERα-positive tumors does not respond to endocrine therapy and the majority of those that do respond eventually become resistant. Most resistant tumors remain ERα-positive and frequently respond to alternative endocrine treatment, indicative of a continued role for ERα in breast cancer cell proliferation. The problem of resistance has resulted in the search for and the development of diverse hormonal therapies designed to inhibit ERα action, while research on the mechanisms which underlie resistance has shed light on the cellular mechanisms, other than ligand binding, which control ERα function.

Preoperative gefitinib versus gefitinib and anastrozole in postmenopausal patients with oestrogen-receptor positive and epidermal-growth-factor-receptor-positive primary breast cancer: a double-blind placebo-controlled phase II randomised trial.

BACKGROUND Some oestrogen-receptor (ER) positive breast cancers express epidermal growth factor receptor (EGFR), but whether inhibition of EGFR can suppress proliferation of breast cancer cells and ER function is not known. METHODS In a double-blind, placebo-controlled randomised trial of 56 postmenopausal patients with ER-positive and EGFR-positive primary breast cancer, 27 women were randomly assigned to the tyrosine-kinase inhibitor of EGFR gefitinib (250 mg given orally once a day) and the aromatase inhibitor anastrozole (1 mg given orally once a day), and 29 women to gefitinib (250 mg given orally once a day) and placebo of identical appearance to anastrozole given orally once a day, all given for 4-6 weeks before surgery. Primary outcome was inhibition of tumour-cell proliferation, as measured by Ki67 antigen labelling index. Secondary outcomes were reduction in EGFR phosphorylation at Tyr 845, reduction in ER phosphorylation at Ser 118, tumour size, and toxic effects. Analyses were by intention to treat. FINDINGS Patients assigned gefitinib and anastrozole had a greater reduction from pretreatment values in proliferation-related Ki67 labelling index than did those assigned gefitinib alone (mean % reduction 98.0 [95% CI 96.1-98.9] vs 92.4 [85.1-96.1]; difference between groups 5.6% [5.1-6.0], p=0.0054). Tumour size was reduced by 30-99% (partial response) in 14 of 28 patients assigned gefitinib and [corrected]in 12 of 22 assigned gefitinib, as assessed by ultrasonography. Reduction in phosphorylation of ER at Ser 118 was similar for both groups. Treatment was well tolerated and much the same for both groups. INTERPRETATION Single-agent gefitinib and gefitinib combined with anastrozole are well-tolerated and effective treatments for reducing the size of breast tumours and levels of ER phosphorylation when given as neoadjuvant therapy.

Regulation of ERBB2 by oestrogen receptor-PAX2 determines response to tamoxifen [Letter]

Crosstalk between the oestrogen receptor (ER) and ERBB2/HER-2 pathways has long been implicated in breast cancer aetiology and drug response, yet no direct connection at a transcriptional level has been shown. Here we show that oestrogen–ER and tamoxifen–ER complexes directly repress ERBB2 transcription by means of a cis-regulatory element within the ERBB2 gene in human cell lines. We implicate the paired box 2 gene product (PAX2), in a previously unrecognized role, as a crucial mediator of ER repression of ERBB2 by the anti-cancer drug tamoxifen. We show that PAX2 and the ER co-activator AIB-1/SRC-3 compete for binding and regulation of ERBB2 transcription, the outcome of which determines tamoxifen response in breast cancer cells. The repression of ERBB2 by ER-PAX2 links these two breast cancer subtypes and suggests that aggressive ERBB2-positive tumours can originate from ER-positive luminal tumours by circumventing this repressive mechanism. These data provide mechanistic insight into the molecular basis of endocrine resistance in breast cancer.