A phosphotyrosine switch determines the antitumor activity of ERβ

Estrogen receptors in breast carcinogenesis and endocrine therapy

Excessive exposure to estrogen has long been associated with an increased risk for developing breast cancer and anti-estrogen therapy is the gold standard of care in the treatment of estrogen receptor (ER) α-positive breast cancers. However, there are several mysteries concerning both anti-estrogen, tamoxifen, and estrogen. The most important of these are: (1) some ERα-positive breast cancers do not respond to tamoxifen; (2) some ERα-negative breast cancers do respond to tamoxifen; (3) initial or acquired resistance to tamoxifen occurs with recurrent tumors; (4) estrogen can cause marked tumor regression in long-term tamoxifen-resistant ERα-positive breast cancer. These mysteries indicate that we do not know enough about estrogen signaling to understand the effects of targeting these receptors in cancer. The discovery of ERβ, the second estrogen receptor, has added another level of complexity to estrogen signaling. This review summarizes recent publications and makes an updated portrait of ERα and ERβ in breast carcinogenesis and endocrine cancer therapy.

Multiple Functions of the Eya Phosphotyrosine Phosphatase

Eyes absent (Eya), a protein conserved from plants to humans and best characterized as a transcriptional coactivator, is also the prototype for a novel class of eukaryotic aspartyl protein tyrosine phosphatases. This minireview discusses recent breakthroughs in elucidating the substrates and cellular events regulated by Eya's tyrosine phosphatase function and highlights some of the complexities, new questions, and surprises that have emerged from efforts to understand how Eya's unusual multifunctionality influences developmental regulation and signaling.

Methyltransferase-like 17 physically and functionally interacts with estrogen receptors

Estrogen exerts its physiological and pathological functions through two estrogen receptors (ERs), ERα and ERβ, which act as transcription factors. Coregulators, including coactivators and corepressors, have been shown to be crucial for regulation of ER transcriptional activity. Although many coregulators have been identified to regulate activities of ERs, novel coregulators are still needed to be investigated. Here, we show that human methyltransferase-like 17 (METTL17), whose function is unknown, physically interacts with ERα and ERβ, and functionally acts as a coactivator for ERs. METTL17 interacts with ER in vitro and in yeast and mammalian cells. Activation function-1 (AF1) and AF2 domains of ERs are responsible for the interaction between METTL17 and ERs. Knockdown of METTL17 reduces transcriptional activities of ERα and ERβ in breast cancer cells, whereas METTL17 overexpression increases ERα and ERβ transcriptional activities. Inhibition of METTL17 expression decreases mRNA and protein levels of ER target genes, including PR, cathepsin D, and pS2. Moreover, METTL17 knockdown reduces breast cancer cell growth. These results indicate that METTL17 is a novel coactivator of ERs and may play a role in breast tumorigenesis.

Estrogen receptor mutations and functional consequences for breast cancer

A significant number of estrogen receptor α (ERα)-positive breast tumors develop resistance to endocrine therapy and recur with metastatic disease. Several mechanisms of endocrine resistance have been proposed, including genetic alterations that lead to ERs with altered protein sequence. By altering the conformation of the protein and increasing the interaction with coactivators, point mutations in ESR1, the gene encoding ERα, promote an active form of the receptor in the absence of hormone that assists tumor cells to evade hormonal treatments. Recent studies have confirmed that ESR1 point mutations frequently occur in metastatic breast tumors that are refractory to endocrine therapy, and suggest the development of novel strategies that may be more effective in controlling ER signaling and benefit patients with recurrent and metastatic disease.

The DACH/EYA/SIX gene network and its role in tumor initiation and progression

The functional abnormality of developmental genes is a common phenomenon in cancer initiation and progression. The retinal determination gene network (RDGN) is a key signal in Drosophila eye specification, and this conservative pathway is also required for the development of multiple organs in mammalian species. Recent studies demonstrated that aberrant expressions of RDGN components in vertebrates, mainly Dach, Six, and Eya, represent a novel tumor signal. RDGN regulates proliferation, apoptosis, tumor growth and metastasis through interactions with multiple signaling pathways in a co-ordinated fashion; Dach acts as a tumor suppressor, whereas Six and Eya function as oncogenes. Clinical analyses demonstrated that the expression levels of RDGN correlate with tumor stage, metastasis and survival, suggesting that combinational detection of this pathway might be used as a promising biomarker for the stratification of therapy and for the prediction of the prognosis of cancer patients.

Interplay of retinal determination gene network with TGF-β signaling pathway in epithelial-mesenchymal transition

As a fundamental event in the generation of tissues and organs during embryogenesis, the epithelial-mesenchymal transition (EMT) has also been implicated in cancer progression by its ability to alter the plasticity of epithelial cells to acquire invasive properties. Evidence is mounting that ectopic activation of transforming growth factors β (TGF-β)/bone morphogenetic protein (BMP) superfamily members to enhance tumorigenesis and metastasis. In this respect, the Retinal Determination Gene Network (RDGN), which was identified to govern the normal initiation of the morphogenetic furrow in Drosophila, has now been found to be de-regulated in various types of cancers, and the key members of this network, DACH, SIX, and EYA, have emerged as novel co-regulators of TGF- signaling during EMT. Understanding the molecular mechanism by which RDGN regulates TGF-β/BMP signaling to influence EMT may lead to novel strategies for targeted therapies.

ERα down-regulation plays a key role in silibinin-induced autophagy and apoptosis in human breast cancer MCF-7 cells

The estrogen receptor alpha (ERα) has been proven to be one of the most important therapeutic targets in breast cancer over the last 30 years. Previous studies pointed out that a natural flavonoid, silibinin, induced apoptosis in human breast cancer MCF-7 cells. In the present study we report that exposure of MCF-7 cells to silibinin led to cell death through the down-regulation of ERα expression. Silibinin-induced apoptosis of MCF-7 cells through up-regulation of caspase 6 due to ERα signalling repression was further boosted by ERα antagonist. Moreover, up-regulation of autophagy induced by silibinin accounted for apoptotic exacerbation, being further enhanced by ERα inhibition. Upon ERα activation, series of downstream signalling pathways can be activated. We found that silibinin reduced the expressions of Akt/mTOR and extracellular-signal-related kinase (ERK), which respectively accounted for the induction of autophagy and apoptosis. These effects were further augmented by co-treatment with ERα inhibitor. We conclude that the treatment with silibinin of ERα-positive MCF-7 cells down-regulates the expression of ERα, and subsequently mTOR and ERK signaling pathways, ERα downstream, finally resulting in induction of autophagy and apoptosis.

Efficient exploration of pan-cancer networks by generalized covariance selection and interactive web content

Statistical network modeling techniques are increasingly important tools to analyze cancer genomics data. However, current tools and resources are not designed to work across multiple diagnoses and technical platforms, thus limiting their applicability to comprehensive pan-cancer datasets such as The Cancer Genome Atlas (TCGA). To address this, we describe a new data driven modeling method, based on generalized Sparse Inverse Covariance Selection (SICS). The method integrates genetic, epigenetic and transcriptional data from multiple cancers, to define links that are present in multiple cancers, a subset of cancers, or a single cancer. It is shown to be statistically robust and effective at detecting direct pathway links in data from TCGA. To facilitate interpretation of the results, we introduce a publicly accessible tool (cancerlandscapes.org), in which the derived networks are explored as interactive web content, linked to several pathway and pharmacological databases. To evaluate the performance of the method, we constructed a model for eight TCGA cancers, using data from 3900 patients. The model rediscovered known mechanisms and contained interesting predictions. Possible applications include prediction of regulatory relationships, comparison of network modules across multiple forms of cancer and identification of drug targets.

Prostate tumorigenesis induced by PTEN deletion involves estrogen receptor β repression

The role of ERβ in prostate cancer is unclear, although loss of ERβ is associated with aggressive disease. Given that mice deficient in ERβ do not develop prostate cancer, we hypothesized that ERβ loss occurs as a consequence of tumorigenesis caused by other oncogenic mechanisms and that its loss is necessary for tumorigenesis. In support of this hypothesis, we found that ERβ is targeted for repression in prostate cancer caused by PTEN deletion and that loss of ERβ is important for tumor formation. ERβ transcription is repressed by BMI-1, which is induced by PTEN deletion and important for prostate tumorigenesis. This finding provides a mechanism for how ERβ expression is regulated in prostate cancer. Repression of ERβ contributes to tumorigenesis because it enables HIF-1/VEGF signaling that sustains BMI-1 expression. These data reveal a positive feedback loop that is activated in response to PTEN loss and sustains BMI-1.

The SIX1-EYA transcriptional complex as a therapeutic target in cancer

INTRODUCTION

The SIX homeodomain proteins and the eyes absent (EYA) family of co-activators form a bipartite transcription factor complex that promotes the proliferation and survival of progenitor cells during organogenesis and is down-regulated in most adult tissues. Abnormal over-expression of SIX1 and EYA in adult tissue is associated with the initiation and progression of diverse tumor types. Importantly, SIX1 and EYA are often co-overexpressed in tumors, and the SIX1-EYA2 interaction has been shown to be critical for metastasis in a breast cancer model. The EYA proteins also contain protein tyrosine phosphatase activity, which plays an important role in breast cancer growth and metastasis as well as directing cells to the repair pathway upon DNA damage.

AREAS COVERED

This review provides a summary of the SIX1/EYA complex as it relates to development and disease and the current efforts to therapeutically target this complex.

EXPERT OPINION

Recently, there have been an increasing number of studies suggesting that targeting the SIX1/EYA transcriptional complex will potently inhibit tumor progression. Although current attempts to develop inhibitors targeting this complex are still in the early stages, continued efforts toward developing better compounds may ultimately result in effective anti-cancer therapies.