Regulation of mouse mammary gland development and tumorigenesis by the ERBB signaling network

A Basic Review on Estrogen Receptor Signaling Pathways in Breast Cancer

Breast cancer is the most common cancer and the deadliest among women worldwide. Estrogen signaling is closely associated with hormone-dependent breast cancer (estrogen and progesterone receptor positive), which accounts for two-thirds of tumors. Hormone therapy using antiestrogens is the gold standard, but resistance to these treatments invariably occurs through various biological mechanisms, such as changes in estrogen receptor activity, mutations in the ESR1 gene, aberrant activation of the PI3K pathway or cell cycle dysregulations. All these factors have led to the development of new therapies, such as selective estrogen receptor degraders (SERDs), or combination therapies with cyclin-dependent kinases (CDK) 4/6 or PI3K inhibitors. Therefore, understanding the estrogen pathway is essential for the treatment and new drug development of hormone-dependent cancers. This mini-review summarizes current literature on the signalization, mechanisms of action and clinical implications of estrogen receptors in breast cancer.

Neuregulin 4 Boosts the Efficacy of Anti-ERBB2 Neutralizing Antibodies

ERBB4 is a tyrosine kinase receptor reported to exert both oncogenic and tumor suppressor activities. These paradoxical effects were suggested to stem from different ERBB4 homo-/hetero-dimers and/or isoforms. By stratifying breast cancer patients for clinical and molecular subtypes and ERBB4 mRNA abundance, we here report that higher ERBB4 levels correlate with longer relapse-free survival in breast cancer patients of HER2-enriched and luminal A molecular subtypes, proposing a cancer-protecting role for this receptor in these specific subgroups. We also observed that HER2-enriched breast cancers express intermediate ERBB4 mRNA levels compared to luminal and triple-negative/basal-like subgroups, which displayed the highest and the lowest levels, respectively. Inspired by these clinical data, we tested the activation of ERBB4 by Neuregulins as a potential anticancer strategy for HER2+ breast cancers. To this end, we employed two HER2+ breast cancer cellular models (BT474 and SKBR3), which express intermediate/high and low ERBB4 levels, respectively. Cell proliferation and motility were evaluated on these cellular models following treatments with Neuregulin 1 (NRG1), which activates both ERBB3 and ERBB4, or Neuregulin 4 (NRG4), which specifically activates ERBB4. Both NRG1 and NRG4 were used alone or in combination with anti-ERBB2 neutralizing antibodies, namely trastuzumab and pertuzumab. In vitro treatment with NRG1 on BT474 cells restrained cell growth and reduced the anti-proliferative efficacy of trastuzumab. In contrast, treatment with NRG1 on SKBR3 cells increased cell proliferation and migration, and partially or completely impaired the anti-proliferative/anti-migratory action of trastuzumab and/or pertuzumab. Importantly, in both the cell lines, treatment with NRG4 robustly potentiated the anti-proliferative action of trastuzumab and pertuzumab. Collectively, our data in HER2+ breast cancer cells highlight that NRG1 may exert both pro- and anti-proliferative effects, and may reduce the efficacy of anti-HER2 agents, whereas NRG4 may boost the anti-proliferative effects of anti-ERBB2 agents. We propose a provocative paradigm shift in the field of growth factors in cancer progression, suggesting the administration of ERBB4 ligands, such as Neuregulin 4, as a strategy to improve the efficacy of anti-ERBB2 agents.

Role of zonula occludens in gastrointestinal and liver cancers

A growing body of evidence suggests that tight junction (TJ) proteins play a crucial role in the pathogenesis of various diseases, including gastrointestinal (GI) cancer and hepatocellular carcinoma (HCC). TJ proteins primarily maintain the epithelial and endothelial cells intact together through integral proteins however, recent reports suggest that they also regulate gene expression necessary for cell proliferation, angiogenesis, and metastasis through adapter proteins such as zonula occludens (ZO). ZO proteins are membrane-associated cytosolic scaffolding proteins that modulate cell proliferation by interacting with several transcription factors. Reduced ZO proteins in GI cancer and HCC are correlated with tumor development and poor prognosis. Pubmed has searched for using the keyword ZO and gastric cancer, ZO and cancer, and ZO and HCC for the last ten years to date. This review summarized the role of ZO proteins in cell proliferation and their expression in GI cancer and HCC. Furthermore, therapeutic interventions targeting ZO in GI and liver cancers are reviewed.

Human milk extracellular vesicle miRNA expression and associations with maternal characteristics in a population-based cohort from the Faroe Islands

Human milk plays a critical role in infant development and health, particularly in cognitive, immune, and cardiometabolic functions. Milk contains extracellular vesicles (EVs) that can transport biologically relevant cargo from mother to infant, including microRNAs (miRNAs). We aimed to characterize milk EV-miRNA profiles in a human population cohort, assess potential pathways and ontology, and investigate associations with maternal characteristics. We conducted the first study to describe the EV miRNA profile of human milk in 364 mothers from a population-based mother-infant cohort in the Faroe Islands using small RNA sequencing. We detected 1523 miRNAs with ≥ one read in 70% of samples. Using hierarchical clustering, we determined five EV-miRNA clusters, the top three consisting of 15, 27 and 67 miRNAs. Correlation coefficients indicated that the expression of many miRNAs within the top three clusters was highly correlated. Top-cluster human milk EV-miRNAs were involved in pathways enriched for the endocrine system, cellular community, neurodevelopment, and cancers. miRNA expression was associated with time to milk collection post-delivery, maternal body mass index, and maternal smoking, but not maternal parity. Future studies investigating determinants of human EV-miRNAs and associated health outcomes are needed to elucidate the role of human milk EV-miRNAs in health and disease.

Icaritin enhances the efficacy of cetuximab against triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) has a greater risk of recurrence and metastasis along with a worse prognosis compared with other subtypes of breast cancer. Studies have revealed that mitogenic estrogen signaling is involved in the malignant proliferation of TNBC cells through a novel variant of the estrogen receptor, estrogen receptor α-36 (ER-α36). The results of the present study demonstrated that knockdown of ER-α36 expression in TNBC cells using short hairpin RNA inhibited rapid estrogen signaling bypass activation of the PI3K/AKT signaling pathway. Moreover, the ER-α36 modulator icaritin inhibited the proliferation of TNBC cells both in vitro and in vivo. Here, it was revealed that the combination of icaritin and cetuximab, a therapeutic epidermal growth factor receptor (EGFR) neutralizing antibody, induced apoptosis and inhibited cell proliferation synergistically in TNBC cells. The results of the present study improved the understanding of the underlying mechanisms of TNBC progression and supported the therapeutic potential of combined treatment targeting the ER-α36 and EGFR.

Mechanism of trastuzumab resistance caused by HER-2 mutation in breast carcinomas

Background: Trastuzumab is an effective drug for the treatment of Her2-positive breast cancer. But, primary or secondary resistances to trastuzumab have become an important factor influencing the curative effect. The mechanisms of trastuzumab resistance are somewhat complex. The present work aims to explore the mechanism of trastuzumab resistance caused by HER-2 mutation in breast carcinomas.

Methods: Firstly, the HER2 wild type (WT) and HER2 mutant (HER2 Q429R, HER2 Q429H and HER2 T798M are the commonest 3 types of mutations) MCF7 cell lines were established. Cell proliferation inhibition was then assessed by the Cell Counting Kit-8 assay and BrdU assay. Transwell invasion assays were also conducted to investigate the metastatic potential influenced by the HER2 mutation. Furthermore, Western blotting and co-immunoprecipitation were conducted to detect protein levels and the physical interaction of HER2 and trastuzumab.

Results: The results showed that the mutant MCF7 cells were less sensitive to trastuzumab than the WTMCF7 cells. The mutation of HER2 almost had no influence on the expression of HER2 and the interaction of HER2 and trastuzumab. Finally, the mutation of HER2 weakened the inhibition of trastuzumab in the PI3K/AKT pathways. In addition, the inhibition of PI3K/AKT signaling-pathway increased the trastuzumab-sensitivity of HER2-mutant MCF7 cells.

Conclusions: Dysregulation of the PI3K-AKT signaling-pathway was a key mechanism inducing the trastuzumab-resistance to HER2 mutant breast cancer cells.

Glucocorticoid Receptor Modulates EGFR Feedback upon Acquisition of Resistance to Monoclonal Antibodies

Evidences of a crosstalk between Epidermal Growth Factor Receptor (EGFR) and Glucocorticoid Receptor (GR) has been reported, ranging from the modulation of receptor levels or GR mediated transcriptional repression of EGFR target genes, with modifications of epigenetic markers. The present study focuses on the involvement of EGFR positive and negative feedback genes in the establishment of cetuximab (CTX) resistance in metastatic Colorectal Cancer (CRC) patients. We evaluated the expression profile of the EGFR ligands TGFA and HBEGF, along with the pro-inflammatory cytokines IL-1B and IL-8, which were previously reported to be negatively associated with monoclonal antibody response, both in mice and patient specimens. Among EGFR negative feedback loops, we focused on ERRFI1, DUSP1, LRIG3, and LRIG1. We observed that EGFR positive feedback genes are increased in CTX-resistant cells, whereas negative feedback genes are reduced. Next, we tested the expression of these genes in CTX-resistant cells upon GR modulation. We unveiled that GR activation leads to an increase in ERRFI1, DUSP1, and LRIG1, which were shown to restrict EGFR activity, along with a decrease in the EGFR activators (TGFA and IL-8). Finally, in a cohort of xenopatients, stratified for response to cetuximab, we observed an inverse association between the expression level of LRIG1 and CRC progression upon CTX treatment. Our model implies that combining GR modulation to EGFR inhibition may yield an effective treatment strategy in halting cancer progression.

Stromal PTEN determines mammary epithelial response to radiotherapy

The importance of the tumor-associated stroma in cancer progression is clear. However, it remains uncertain whether early events in the stroma are capable of initiating breast tumorigenesis. Here, we show that in the mammary glands of non-tumor bearing mice, stromal-specific phosphatase and tensin homolog (Pten) deletion invokes radiation-induced genomic instability in neighboring epithelium. In these animals, a single dose of whole-body radiation causes focal mammary lobuloalveolar hyperplasia through paracrine epidermal growth factor receptor (EGFR) activation, and EGFR inhibition abrogates these cellular changes. By analyzing human tissue, we discover that stromal PTEN is lost in a subset of normal breast samples obtained from reduction mammoplasty, and is predictive of recurrence in breast cancer patients. Combined, these data indicate that diagnostic or therapeutic chest radiation may predispose patients with decreased stromal PTEN expression to secondary breast cancer, and that prophylactic EGFR inhibition may reduce this risk.

FGFR1 and HER1 or HER2 co-amplification in breast cancer indicate poor prognosis

Human epidermal growth factor receptor 1 or 2 (HER1/2), and fibroblast growth factor receptor 1 (FGFR1) signaling serve critical roles in the progression of breast cancer; however, cross-talk between HER1/2 and FGFR1 signaling has not been extensively studied. In the present study, the copy number variation status of FGFR1 and HER1/2, and the clinical implications and prognostic relevance of this, were evaluated in invasive ductal breast cancer (IDC) tissue samples. Quantitative polymerase chain reaction and fluorescence in situ hybridization were used to assess gene copy number variation in IDC samples, and the clinical characteristics and survival curves of patients with IDC were analyzed. The amplification of FGFR1 was identified in 16.0% of the samples (12 of 75), of HER1 in 26.7% (20 of 75), of HER2 in 37.3% (28 of 75), and of FGFR1 and HER1/2 simultaneously in 8.0% (6 of 75). FGFR1 and HER1/2 co-amplification were significantly correlated with distant metastasis (P=0.035), recurrence (P=0.026) and decreased disease-free survival time (P=0.042). This was the case for patients undergoing endocrine therapy (P=0.002) and chemotherapy (P=0.044). Taken together, the results indicate that patients with FGFR1 and HER1/2 co-amplification may exhibit a less favorable prognosis compared with patients with either FGFR1, HER1/2 amplification or without amplification.

Aberrantly high expression of the CUB and zona pellucida-like domain-containing protein 1 (CUZD1) in mammary epithelium leads to breast tumorigenesis

The peptide hormone prolactin (PRL) and certain members of the epidermal growth factor (EGF) family play central roles in mammary gland development and physiology, and their dysregulation has been implicated in mammary tumorigenesis. Our recent studies have revealed that the CUB and zona pellucida-like domain-containing protein 1 (CUZD1) is a critical factor for PRL-mediated activation of the transcription factor STAT5 in mouse mammary epithelium. Of note, CUZD1 controls production of a specific subset of the EGF family growth factors and consequent activation of their receptors. Here, we found that consistent with this finding, CUZD1 overexpression in non-transformed mammary epithelial HC11 cells increases their proliferation and induces tumorigenic characteristics in these cells. When introduced orthotopically in mouse mammary glands, these cells formed adenocarcinomas, exhibiting elevated levels of STAT5 phosphorylation and activation of the EGF signaling pathway. Selective blockade of STAT5 phosphorylation by pimozide, a small-molecule inhibitor, markedly reduced the production of the EGF family growth factors and inhibited PRL-induced tumor cell proliferation in vitro Pimozide administration to mice also suppressed CUZD1-driven mammary tumorigenesis in vivo Analysis of human MCF7 breast cancer cells indicated that CUZD1 controls the production of the same subset of EGF family members in these cells as in the mouse. Moreover, pimozide treatment reduced the proliferation of these cancer cells. Collectively, these findings indicate that overexpression of CUZD1, a regulator of growth factor pathways controlled by PRL and STAT5, promotes mammary tumorigenesis. Blockade of the STAT5 signaling pathway downstream of CUZD1 may offer a therapeutic strategy for managing these breast tumors.

A Living Biobank of Breast Cancer Organoids Captures Disease Heterogeneity

Breast cancer (BC) comprises multiple distinct subtypes that differ genetically, pathologically, and clinically. Here, we describe a robust protocol for long-term culturing of human mammary epithelial organoids. Using this protocol, >100 primary and metastatic BC organoid lines were generated, broadly recapitulating the diversity of the disease. BC organoid morphologies typically matched the histopathology, hormone receptor status, and HER2 status of the original tumor. DNA copy number variations as well as sequence changes were consistent within tumor-organoid pairs and largely retained even after extended passaging. BC organoids furthermore populated all major gene-expression-based classification groups and allowed in vitro drug screens that were consistent with in vivo xeno-transplantations and patient response. This study describes a representative collection of well-characterized BC organoids available for cancer research and drug development, as well as a strategy to assess in vitro drug response in a personalized fashion.

Evidence that gene expression of ovarian follicular tight junction proteins is regulated in vivo and in vitro in cattle

Tight junctions (TJ) are common paracellular sealing structures that control the transport of water, ions, and macromolecules across cell layers. Because the role of TJ in bovine follicular development is unknown, we investigated the developmental and hormonal regulation of the transmembrane TJ protein, occludin (OCLN), and the cytoplasmic TJ proteins, TJ protein 1 (TJP1) and cingulin (CGN) in bovine granulosa cells (GC) and theca cells (TC). For this purpose, bovine GC and TC were isolated from large (>8 mm) and/or small (1 to 5 mm) follicles and either extracted for real-time PCR (qPCR) or cultured in vitro. The abundances of both and mRNA were greater ( < 0.05) in TC than GC, whereas the mRNA abundance was greater ( < 0.05) in GC than TC. The abundance of mRNA in both GC and TC was greater ( < 0.05) in small follicles compared with large follicles, whereas the GC of large follicles had less ( < 0.05) mRNA abundance than the GC of small follicles. The abundance of mRNA in GC or TC did not differ ( > 0.10) among follicle sizes. In vitro treatment with various growth factors known to affect ovarian folliculogenesis indicated that , , and were hormonally regulated. Fibroblast growth factor 9 (FGF9) decreased ( < 0.05) the and mRNA abundances. Tumor necrosis factor α (TNFα) and vascular endothelial growth factor A (VEGFA) increased ( < 0.05) the mRNA abundance but decreased ( < 0.05) the mRNA abundance. Dexamethasone (DEX) increased ( < 0.05) and mRNA abundances. Epidermal growth factor (EGF) decreased ( < 0.05) and dihydrotestosterone (DHT) increased ( < 0.05) the abundances of , , and mRNA. We propose that the downregulation of OCLN and other TJ proteins during follicular development could reduce barrier function, thereby participating in increasing follicle size by allowing for an increase in the volume of follicular fluid as well as by allowing additional serum factors into the follicular fluid that potentially may directly impact GC functions. The results of the current study indicate the following in cattle: 1) gene expression of TJ proteins (i.e., , , and ) differs between GC and TC and changes with follicle size, and 2) autocrine, paracrine, and endocrine regulators, such as FGF9, EGF, DHT, TNFα, and glucocorticoids, modulate , , and mRNA abundance in TC in vitro.

Efficacy of recombinant bovine epidermal growth factor in the treatment of experimental subclinical Staphylococcus aureus mastitis in a ewe model

Staphylococcus aureus is the most common contagious mastitis pathogen of dairy cattle. Antimicrobial treatment of infected cattle results in variable cure rates. Epidermal growth factor (EGF) plays an important role in the modulation of host innate immune responses and the regulation of mammary epithelial regeneration, indicating that EGF may be useful as a treatment for mastitis. A pilot study was conducted to evaluate the efficacy of recombinant bovine EGF (rbEGF) for the treatment of S aureus intramammary infection (IMI) using an ovine model. Each ewe was experimentally infected with S aureus in both udder halves. One udder half of each ewe received one of two treatments: EGF (n=13) or pirlimycin (n=13). The contralateral udder half of each ewe received sterile saline as a control. The bacteriological cure rate following rbEGF was significantly lower (15 per cent) than that attained with pirlimycin hydrochloride (61 per cent) and did not differ from that following treatment with sterile saline. Cure rates following treatment with rbEGF were not significantly different to those following sterile saline. Given that EGF is associated with modulation of host immunity and wound healing, future studies into EGF should not focus on whether EGF increases cure rates of S aureus IMI.

Role of peptidylarginine deiminase 2 (PAD2) in mammary carcinoma cell migration

BACKGROUND

Penetration of the mammary gland basement membrane by cancer cells is a crucial first step in tumor invasion. Using a mouse model of ductal carcinoma in situ, we previously found that inhibition of peptidylarginine deiminase 2 (PAD2, aka PADI2) activity appears to maintain basement membrane integrity in xenograft tumors. The goal of this investigation was to gain insight into the mechanisms by which PAD2 mediates this process.

METHODS

For our study, we modulated PAD2 activity in mammary ductal carcinoma cells by lentiviral shRNA-mediated depletion, lentiviral-mediated PAD2 overexpression, or PAD inhibition and explored the effects of these treatments on changes in cell migration and cell morphology. We also used these PAD2-modulated cells to test whether PAD2 may be required for EGF-induced cell migration. To determine how PAD2 might promote tumor cell migration in vivo, we tested the effects of PAD2 inhibition on the expression of several cell migration mediators in MCF10DCIS.com xenograft tumors. In addition, we tested the effect of PAD2 inhibition on EGF-induced ductal invasion and elongation in primary mouse mammary organoids. Lastly, using a transgenic mouse model, we investigated the effects of PAD2 overexpression on mammary gland development.

RESULTS

Our results indicate that PAD2 depletion or inhibition suppresses cell migration and alters the morphology of MCF10DCIS.com cells. In addition, we found that PAD2 depletion suppresses the expression of the cytoskeletal regulatory proteins RhoA, Rac1, and Cdc42 and also promotes a mesenchymal to epithelial-like transition in tumor cells with an associated increase in the cell adhesion marker, E-cadherin. Our mammary gland organoid study found that inhibition of PAD2 activity suppresses EGF-induced ductal invasion. In vivo, we found that PAD2 overexpression causes hyperbranching in the developing mammary gland.

CONCLUSION

Together, these results suggest that PAD2 plays a critical role in breast cancer cell migration. Our findings that EGF treatment increases protein citrullination and that PAD2 inhibition blocks EGF-induced cell migration suggest that PAD2 likely functions within the EGF signaling pathway to mediate cell migration.

CUZD1 is a critical mediator of the JAK/STAT5 signaling pathway that controls mammary gland development during pregnancy

In the mammary gland, genetic circuits controlled by estrogen, progesterone, and prolactin, act in concert with pathways regulated by members of the epidermal growth factor family to orchestrate growth and morphogenesis during puberty, pregnancy and lactation. However, the precise mechanisms underlying the crosstalk between the hormonal and growth factor pathways remain poorly understood. We have identified the CUB and zona pellucida-like domain-containing protein 1 (CUZD1), expressed in mammary ductal and alveolar epithelium, as a novel mediator of mammary gland proliferation and differentiation during pregnancy and lactation. Cuzd1-null mice exhibited a striking impairment in mammary ductal branching and alveolar development during pregnancy, resulting in a subsequent defect in lactation. Gene expression profiling of mammary epithelium revealed that CUZD1 regulates the expression of a subset of the EGF family growth factors, epiregulin, neuregulin-1, and epigen, which act in an autocrine fashion to activate ErbB1 and ErbB4 receptors. Proteomic studies further revealed that CUZD1 interacts with a complex containing JAK1/JAK2 and STAT5, downstream transducers of prolactin signaling in the mammary gland. In the absence of CUZD1, STAT5 phosphorylation in the mammary epithelium during alveologenesis was abolished. Conversely, elevated expression of Cuzd1 in mammary epithelial cells stimulated prolactin-induced phosphorylation and nuclear translocation of STAT5. Chromatin immunoprecipitation confirmed co-occupancy of phosphorylated STAT5 and CUZD1 in the regulatory regions of epiregulin, a potential regulator of epithelial proliferation, and whey acidic protein, a marker of epithelial differentiation. Collectively, these findings suggest that CUZD1 plays a critical role in prolactin-induced JAK/STAT5 signaling that controls the expression of key STAT5 target genes involved in mammary epithelial proliferation and differentiation during alveolar development.

In the mammary gland, genetic circuits controlled by the hormones, estrogen, progesterone and prolactin, act in concert with pathways regulated by members of the epidermal growth factor family to orchestrate growth and morphogenesis during puberty, pregnancy and lactation. We have identified CUZD1 as a novel mediator of prolactin signaling in the steroid hormone-primed mouse mammary gland during pregnancy and lactation. Cuzd1-null mice exhibited a striking impairment in ductal branching and alveolar development during pregnancy, resulting in a subsequent defect in lactation. Administration of prolactin failed to induce proliferation of the mammary epithelium in Cuzd1-null mice. Protein binding studies revealed that CUZD1 interacts with downstream transducers of prolactin signaling, JAK1/JAK2 and STAT5. Additionally, elevated expression of Cuzd1 in mammary epithelial cells stimulated phosphorylation and nuclear translocation of STAT5. CUZD1, therefore, is a critical mediator of prolactin that controls mammary alveolar development.

Hormonal Modulation of Breast Cancer Gene Expression: Implications for Intrinsic Subtyping in Premenopausal Women

Clinics are increasingly adopting gene-expression profiling to diagnose breast cancer subtype, providing an intrinsic, molecular portrait of the tumor. For example, the PAM50-based Prosigna test quantifies expression of 50 key genes to classify breast cancer subtype, and this method of classification has been demonstrated to be superior over traditional immunohistochemical methods that detect proteins, to predict risk of disease recurrence. However, these tests were largely developed and validated using breast cancer samples from postmenopausal women. Thus, the accuracy of such tests has not been explored in the context of the hormonal fluctuations in estrogen and progesterone that occur during the menstrual cycle in premenopausal women. Concordance between traditional methods of subtyping and the new tests in premenopausal women is likely to depend on the stage of the menstrual cycle at which the tissue sample is taken and the relative effect of hormones on expression of genes versus proteins. The lack of knowledge around the effect of fluctuating estrogen and progesterone on gene expression in breast cancer patients raises serious concerns for intrinsic subtyping in premenopausal women, which comprise about 25% of breast cancer diagnoses. Further research on the impact of the menstrual cycle on intrinsic breast cancer profiling is required if premenopausal women are to benefit from the new technology of intrinsic subtyping.

Understanding the Impact of ErbB Activating Events and Signal Transduction on Antigen Processing and Presentation: MHC Expression as a Model

Advances in molecular pathology have changed the landscape of oncology. The ability to interrogate tissue samples for oncogene amplification, driver mutations, and other molecular alterations provides clinicians with an enormous level of detail about their patient's cancer. In some cases, this information informs treatment decisions, especially those related to targeted anti-cancer therapies. However, in terms of immune-based therapies, it is less clear how to use such information. Likewise, despite studies demonstrating the pivotal role of neoantigens in predicting responsiveness to immune checkpoint blockade, it is not known if the expression of neoantigens impacts the response to targeted therapies despite a growing recognition of their diverse effects on immunity. To realize the promise of 'personalized medicine', it will be important to develop a more integrated understanding of the relationships between oncogenic events and processes governing anti-tumor immunity. One area of investigation to explore such relationships centers on defining how ErbB/HER activation and signal transduction influences antigen processing and presentation.

Deregulation of the Hippo pathway in mouse mammary stem cells promotes mammary tumorigenesis

The Hippo-YAP pathway mediates organ size control, contact inhibition, and tumorigenesis. It is a kinase cascade that inhibits the nuclear localization and transcriptional activities of YAP and TAZ. E-cadherin, cell junctions, polarity proteins, and the merlin/NF2 tumor suppressor activate the pathway to inhibit YAP/TAZ activity, while growth factor signaling inhibits the pathway to activate YAP/TAZ in the nucleus. We examined its role in the development of mouse mammary glands and tumor formation using gland reconstitution by transplantation of genetically modified mammary stem cells (MaSCs). Knockdown of YAP and TAZ with shRNA in MaSCs did not inhibit gland reconstitution. In contrast, knockdown of β-catenin blocked gland reconstitution, consistent with the known role of Wnt signaling in mammary gland development. However, we find that Hippo signaling is involved in mammary tumor formation. Expression of a constitutively active form of YAP caused rapid formation of large tumors. Moreover, knockdown of YAP/TAZ slowed the development of tumors in polyoma middle T transgenic mice, a well-studied mammary tumor model involving activation of several signaling pathways. YAP accumulated in nuclei of mammary glands in ErbB2/EGFR-transgenic mice, suggesting that EGFR signaling affects YAP in vivo similar to cell culture. ErbB2/EGFR-transgenic mice develop mammary tumors in 7-8 months, but surprisingly, MaSCs from these mice did not form tumors when transplanted into host mice. Nonetheless, expression of dominant-negative Lats, which inhibits Hippo signaling, leads to tumor formation in ErbB2-transgenic mice, suggesting that Hippo signaling is involved in EGFR-induced mammary tumorigenesis.

Roles of the ER-α36-EGFR/HER2 positive regulatory loops in tamoxifen resistance

Tamoxifen provided a successful treatment for ER-positive breast cancer for the past four decades. However, most breast tumors are eventually resistant to tamoxifen therapy. Extensive researches were conducted to understand the molecular mechanisms involved in tamoxifen resistance, and have revealed that multiple signaling molecules and pathways such as EGFR and HER2 are involved in tamoxifen resistance. Currently, the mechanisms by which tamoxifen sensitive breast cancer cells acquire these signaling pathways and develop tamoxifen resistance have not been elucidated. The identification of ER-α36, a variant of ER-α, that is able to mediate agonist activity of tamoxifen provided great insights into the underlying mechanisms of tamoxifen resistance. In this review, we will discuss the biological function and the possible underlying mechanisms of ER-α36 in tamoxifen resistance and specifically illustrate a novel cross-talk mechanism; positive regulatory loops between the ER-α36 and EGFR/HER2 in tamoxifen resistance. The function and the underlying mechanisms of ER-α36 in tamoxifen resistance of the breast cancer stem/progenitor cells will also be discussed. Finally, we will postulate a novel approach to restore tamoxifen sensitivity in tamoxifen resistant breast cancer cells.

Role of 3'-5'-cyclic adenosine monophosphate on the epidermal growth factor dependent survival in mammary epithelial cells

Epidermal growth factor (EGF) has been suggested to play a key role in the maintenance of epithelial cell survival during lactation. Previously, we demonstrated that EGF dependent activation of PI3K pathway prevents apoptosis in confluent murine HC11 cells cultured under low nutrient conditions. The EGF protective effect is associated with increased levels of the antiapoptotic protein Bcl-XL. Here, we identify the EGF-dependent mechanism involved in cell survival that converges in the regulation of bcl-X expression by activated CREB. EGF induces Bcl-XL expression through activation of a unique bcl-X promoter, the P1; being not only the PI3K/AKT signaling pathway but also the increase in cAMP levels and the concomitant PKA/CREB activation necessary for both bcl-XL upregulation and apoptosis avoidance. Results presented in this work suggest the existence of a novel connection between the EGF receptor and the adenylate cyclase that would have an impact in preventing apoptosis under low nutrient conditions.

p21-activated kinases in Erbb2-positive breast cancer: A new therapeutic target?

The activation of receptor tyrosine kinases, particularly ErbB2, has been linked to the genesis and progression of breast cancer. Two of the central signaling pathways activated by ErbB2 are the Ras/Raf-1/Mek/Erk pathway, which plays an important role in tumor cell growth and migration, and the PI3K/Akt pathway, which plays an important role in cell survival. Recently, we and others have shown that signaling through the Ras-Erk pathway can be influenced by p21-activated kinase 1 (Pak1), an effector of the Rho family GTP ases Rac and Cdc42. Expression of activated forms of Rac promotes activation of Erk through mechanisms involving Pak1 phosphorylation of Raf-1 and Mek1. In addition, Pak1 has also been implicated in the activation of Akt. However, our understanding regarding the degree to which Rho GTPases, and their effectors such as Pak1, contribute to ErbB2-mediated signaling is very limited.Recent results from our laboratory indicate that ErbB2 expression correlates with Pak activation in estrogen receptor negative human breast tumor samples. Using a three-dimensional (3D) culture of human MCF-10A mammary epithelial cells, we found that activation of Rac-Pak pathway by ErbB2 induces growth factor independent proliferation and promotes disruption of acini-like structures through the activation of the Erk and Akt pathways. We also observed that blocking Pak1 activity by small molecule inhibitors impeded the ability of activated ErbB2 to transform these cells and to activate its associated downstream signaling targets. In addition, we found that suppressing Pak activity in ErbB2-amplified breast cancer cells delayed tumor formation and downregulated Erk and Akt signaling in vivo. These results support a model in which Pak, by activating Erk and Akt, cooperates with ErbB2 in transforming mammary epithelial cells.

Diurnal suppression of EGFR signalling by glucocorticoids and implications for tumour progression and treatment

Signal transduction by receptor tyrosine kinases (RTKs) and nuclear receptors for steroid hormones is essential for body homeostasis, but the cross-talk between these receptor families is poorly understood. We observed that glucocorticoids inhibit signalling downstream of EGFR, an RTK. The underlying mechanism entails suppression of EGFR's positive feedback loops and simultaneous triggering of negative feedback loops that normally restrain EGFR. Our studies in mice reveal that the regulation of EGFR's feedback loops by glucocorticoids translates to circadian control of EGFR signalling: EGFR signals are suppressed by high glucocorticoids during the active phase (night-time in rodents), while EGFR signals are enhanced during the resting phase. Consistent with this pattern, treatment of animals bearing EGFR-driven tumours with a specific kinase inhibitor was more effective if administered during the resting phase of the day, when glucocorticoids are low. These findings support a circadian clock-based paradigm in cancer therapy.

Stromal-epithelial cell interactions and alteration of branching morphogenesis in macromastic mammary glands

True macromastia is a rare but disabling condition characterized by massive breast growth. The aetiology and pathogenic mechanisms for this disorder remain largely unexplored because of the lack of in vivo or in vitro models. Previous studies suggested that regulation of epithelial cell growth and development by oestrogen was dependent on paracrine growth factors from the stroma. In this study, a co-culture model containing epithelial and stromal cells was used to investigate the interactions of these cells in macromastia. Epithelial cell proliferation and branching morphogenesis were measured to assess the effect of macromastic stromal cells on epithelial cells. We analysed the cytokines secreted by stromal cells and identified molecules that were critical for effects on epithelial cells. Our results indicated a significant increase in cell proliferation and branching morphogenesis of macromastic and non-macromastic epithelial cells when co-cultured with macromastic stromal cells or in conditioned medium from macromastic stromal cells. Hepatocyte growth factor (HGF) is a key factor in epithelial–stromal interactions of macromastia-derived cell cultures. Blockade of HGF with neutralizing antibodies dramatically attenuated epithelial cell proliferation in conditioned medium from macromastic stromal cells. The epithelial–stromal cell co-culture model demonstrated reliability for studying interactions of mammary stromal and epithelial cells in macromastia. In this model, HGF secreted by macromastic stromal cells was found to play an important role in modifying the behaviour of co-cultured epithelial cells. This model allows further studies to investigate basic cellular and molecular mechanisms in tissue from patients with true breast hypertrophy.

Crosstalk between HER2 signaling and angiogenesis in breast cancer: molecular basis, clinical applications and challenges

PURPOSE OF REVIEW

Angiogenesis is an essential hallmark of cancer. Targeting angiogenesis has proven its efficacy in the modern therapeutic paradigm. HER2 positive breast cancer, in particular, is a challenging disease in which resistance to standard therapy has been attributed to parallel and downstream signaling cascades including angiogenesis. This review explores the molecular mechanisms underlying crosstalk between HER2 signaling and angiogenesis. It highlights the role of angiogenesis in the emerging resistance to anti-HER2 therapy. It surveys the current repertoire of clinical trials involving use of combination of anti-HER2 and antiangiogenic therapies. Finally, it entertains the hopes and challenges posed by this novel therapeutic approach.

RECENT FINDINGS

HER2 signaling upregulates angiogenesis at different levels and by different mechanisms. A large number of clinical trials were conducted in attempt to exploit the potential benefit of the combination. Results of early phase trials were promising. However, in the late phase clinical trials, the AVEREL trial did not demonstrate a consistent benefit for bevacizumab in the HER2 positive breast cancer patient population. The BETH trial is ongoing and recruiting patients. Safety issues regarding cardiovascular toxicity of the combination have been already raised. Negative experience of dual EGFR and VEGF targeting in colon cancer cannot be overlooked.

SUMMARY

Angiogenesis and HER2 signaling are closely related at the molecular level. Appraisal of efficacy of antiangiogenic therapies requires revisit of the current literature as well as following the results of ongoing trials.

Progressive increase of glucose transporter-3 (GLUT-3) expression in estrogen-induced breast carcinogenesis

INTRODUCTION

Increased glucose uptake and glycolysis are main metabolic characteristics of malignant cells. A family of glucose transporters (GLUTs) facilitates glucose movement across the plasma membranes in a tumor-specific manner. Glucose transporter-1 (GLUT-1), GLUT-3 and recently GLUT-12, have been previously shown in breast cancer cells and are found to be associated with poor prognosis. In addition, it has been shown that estrogen plays critical roles in GLUT regulation, however, the stage-specific GLUT regulation of mammary carcinogenesis is unclear.

METHODS

GLUT expression patterns were investigated in an in vitro-in vivo progressive, estrogen-induced, mammary carcinogenesis model which consisted of four cell lines, with same genetic background. In this model, different stages of tumor initiation and progression are represented, MCF-10F being the normal stage, E2 cells the transformed stage by estrogen, C5 cells, the invasive stage, and T4 cells the tumorigenic stage. In addition, loss of ductulogenesis and solid mass formation in collagen matrix and invasiveness of the cells were counted.

RESULTS

Real time PCR showed that GLUT1 expression was downregulated in MCF10F after treatment with 17β-estradiol (E2), and in the invasive cell type (C5), but not in the tumor cells (T4), which had no changes compared to MCF10F. C5 and T4 cells showed the highest rate of GLUT-3 expression. These cells were also found to be associated with loss of ductulogenesis, solid mass formation and higher invasive capacity, whereas, GLUT-12 was downregulated in C5 and T4 cells.

CONCLUSION

Estrogen-induced malignant transformation is associated with remarkable and progressive GLUT-3 expression, GLUT-1 re-expression at further stages, as well as GLUT-12 downregulation.

Uncoupling of PI3K from ErbB3 impairs mammary gland development but does not impact on ErbB2-induced mammary tumorigenesis

The formation of ErbB2/ErbB3 heterodimers plays a critical role in ErbB2-mediated signaling in both normal mammary development and mammary tumor progression. Through 7 phosphoinositide 3-kinase (PI3K) phosphotyrosine-binding sites, ErbB3 is able to recruit PI3K and initiate the PI3K/AKT signaling pathway. To directly explore the importance of the ErbB3/PI3K pathway in mammary development and tumorigenesis, we generated a mouse model that carries a mutant ErbB3 allele lacking the seven known PI3K-binding sites (ErbB3(Δ85)). Mice homozygous for the ErbB3(Δ85) allele exhibited an initial early growth defect and a dramatic impairment of mammary epithelial outgrowth. Although homozygous adult mice eventually recovered from the growth defect, their mammary glands continued to manifest the mammary outgrowth and lactation defects throughout their adult life. Interestingly, despite the presence of a profound mammary gland defect, all of the female ErbB3Δ85 mice developed metastatic ErbB2-induced mammary tumors secondary to mammary epithelial expression of an activated ErbB2 oncogene capable of compensatory PI3K signaling from both EGF receptor and ErbB2. Our findings therefore indicate that, although ErbB3-associated PI3K activity is critical for mammary development, it is dispensable for ErbB2-induced mammary tumor progression.

Apc mutation enhances PyMT-induced mammary tumorigenesis

The Adenomatous Polyposis Coli (APC) tumor suppressor gene is silenced by hypermethylation or mutated in up to 70% of human breast cancers. In mouse models, Apc mutation disrupts normal mammary development and predisposes to mammary tumor formation; however, the cooperation between APC and other mutations in breast tumorigenesis has not been studied. To test the hypothesis that loss of one copy of APC promotes oncogene-mediated mammary tumorigenesis, Apc^Min/+ mice were crossed with the mouse mammary tumor virus (MMTV)-Polyoma virus middle T antigen (PyMT) or MMTV-c-Neu transgenic mice. In the PyMT tumor model, the Apc^Min/+ mutation significantly decreased survival and tumor latency, promoted a squamous adenocarcinoma phenotype, and enhanced tumor cell proliferation. In tumor-derived cell lines, the proliferative advantage was a result of increased FAK, Src and JNK signaling. These effects were specific to the PyMT model, as no changes were observed in MMTV-c-Neu mice carrying the Apc^Min/+ mutation. Our data indicate that heterozygosity of Apc enhances tumor development in an oncogene-specific manner, providing evidence that APC-dependent pathways may be valuable therapeutic targets in breast cancer. Moreover, these preclinical model systems offer a platform for dissection of the molecular mechanisms by which APC mutation enhances breast carcinogenesis, such as altered FAK/Src/JNK signaling.

In vivo evidence for epidermal growth factor receptor (EGFR)-mediated release of prolactin from the pituitary gland

Members of the epidermal growth factor receptor (EGFR/ERBB) system are essential local regulators of mammary gland development and function. Emerging evidence suggests that EGFR signaling may also influence mammary gland activity indirectly by promoting the release of prolactin from the pituitary gland in a MAPK and estrogen receptor-α (ERα)-dependent manner. Here, we report that overexpression of the EGFR ligand betacellulin (BTC) causes a lactating-like phenotype in the mammary gland of virgin female mice including the major hallmarks of lactogenesis. BTC transgenic (BTC-tg) females showed reduced levels of prolactin in the pituitary gland and increased levels of the hormone in the circulation. Furthermore, treatment of BTC-tg females with bromocriptine, an inhibitor of prolactin secretion, blocked the development of the lactation-like phenotype, suggesting that it is caused by central release of prolactin rather than by local actions of BTC in the mammary gland. Introduction of the antimorphic Egfr allele Wa5 also blocked the appearance of the mammary gland alterations, revealing that the phenotype is EGFR-dependent. We detected an increase in MAPK activity, but unchanged phosphorylation of ERα in the pituitary gland of BTC-tg females as compared with control mice. These results provide the first functional evidence in vivo for a role of the EGFR system in regulating mammary gland activity by modulating prolactin release from the pituitary gland.

The PI3 kinase/mTOR blocker NVP-BEZ235 overrides resistance against irreversible ErbB inhibitors in breast cancer cells

Resistance against first and second generation (irreversible) ErbB inhibitors is an unsolved problem in clinical oncology. The purpose of this study was to examine the effects of the irreversible ErbB inhibitors pelitinib and canertinib on growth of breast and ovarian cancer cells. Although in vitro growth-inhibitory effects of both drugs exceeded by far the effects of all reversible ErbB blockers tested (lapatinib, erlotinib, and gefitinib), complete growth inhibition was usually not reached. To define the mechanism of resistance, we examined downstream signaling pathways in drug-exposed cells by Western blot analysis. Although ErbB phosphorylation was reduced by pelitinib and canertinib, activation of the AKT/mTOR pathway remained essentially unaltered in drug-resistant cells. Correspondingly, transfection of tumor cells with constitutively activated AKT was found to promote resistance against all ErbB inhibitors tested, whereas dominant negative AKT reinstalled sensitivity in drug-resistant cells. In a next step, we applied PI3K/AKT/mTOR blockers including the dual PI3K/mTOR kinase inhibitor NVP-BEZ235. These agents were found to cooperate with pelitinib and canertinib in producing in vitro growth inhibition in cancer cells resistant against ErbB-targeting drugs. In conclusion, our data show that ErbB drug-refractory activation of the PI3K/AKT/mTOR pathway plays a crucial role in resistance against classical and second-generation irreversible ErbB inhibitors, and NVP-BEZ235 can override this form of resistance against pelitinib and canertinib.

E3 ubiquitin ligases in ErbB receptor quantity control

Signaling through ErbB family growth factor receptor tyrosine kinases is necessary for the development and homeostasis of a wide variety of tissue types. However, the intensity of receptor-mediated cellular signaling must fall within a precise range; insufficient signaling can lead to developmental abnormalities or tissue atrophy, while over-signaling can lead to hyperplastic and ultimately neoplastic events. While a plethora of mechanisms have been described that regulate downstream signaling events, it appears that cells also utilize various mechanisms to regulate their ErbB receptor levels. Such mechanisms are collectively termed "ErbB receptor quantity control." Notably, studies over the past few years have highlighted roles for post-transcriptional processes, particularly protein degradation, in ErbB quantity control. Here the involvement of ErbB-directed E3 ubiquitin ligases is discussed, including Nrdp1-mediated ErbB3 degradation, ErbB4 degradation mediated by Nedd4 family E3 ligases, and CHIP-mediated ErbB2 degradation. The hypothesis is forwarded that protein degradation-based ErbB quantity control mechanisms play central roles in suppressing receptor overexpression in normal cells, and that the loss of such mechanisms could facilitate the onset or progression of ErbB-dependent tumors.

ErbB/EGF signaling and EMT in mammary development and breast cancer

Activation of the ErbB family of receptor tyrosine kinases via cognate Epidermal Growth Factor (EGF)-like peptide ligands constitutes a major group of related signaling pathways that control proliferation, survival, angiogenesis and metastasis of breast cancer. In this respect, clinical trials with various ErbB receptor blocking antibodies and specific tyrosine kinase inhibitors have proven to be partially efficacious in the treatment of this heterogeneous disease. Induction of an embryonic program of epithelial-to-mesenchymal transition (EMT) in breast cancer, whereupon epithelial tumor cells convert to a more mesenchymal-like phenotype, facilitates the migration, intravasation, and extravasation of tumor cells during metastasis. Breast cancers which exhibit properties of EMT are highly aggressive and resistant to therapy. Activation of ErbB signaling can regulate EMT-associated invasion and migration in normal and malignant mammary epithelial cells, as well as modulating discrete stages of mammary gland development. The purpose of this review is to summarize current information regarding the role of ErbB signaling in aspects of EMT that influence epithelial cell plasticity during mammary gland development and tumorigenesis. How this information may contribute to the improvement of therapeutic approaches in breast cancer will also be addressed.

Epidermal growth factor and hepatocyte growth factor receptors collaborate to induce multiple biological responses in bovine mammary epithelial cells

The aim of this work was to explore whether epidermal growth factor (EGF) and hepatocyte growth factor (HGF) could increase the biological responses of a mammary epithelial cell line of bovine origin when added simultaneously. We also investigated a possible molecular mechanism underlying this cooperation. The development of mammary gland requires several circulating and locally produced hormones. Hepatocyte growth factor and its tyrosine kinase receptor, mesenchymal-epithelial transition factor (MET), are expressed and temporally regulated during mammary development and differentiation. Epidermal growth factor receptor and its ligands have also been implicated in the growth and morphogenesis of the mammary epithelium. Both EGF and HGF seem to exert a morphogenic program in this tissue; therefore, we hypothesized that these cytokines could act cooperatively in bovine mammary epithelial cells. We have already shown that the bovine BME-UV cell line, a nontumorigenic mammary epithelial line, expresses both MET and EGF receptor. Simultaneous treatment with HGF and EGF elicited an increase in proliferation, dispersion, degradation of extracellular matrix, and motility. Following EGF treatment, BME-UV mammary cells exhibited an increase in MET expression at both the mRNA and protein levels. Long-term treatment of BME-UV cells with HGF and EGF together increased the level of activation of the extracellular signal-regulated kinase 1/2 and protein kinase B signaling pathways when compared with HGF or EGF alone. These data outline a possible cooperative role of the EGF and HGF pathways and indicate that cross-talk between their respective receptors may modulate mammary gland development in the cow.

Heritable variation of ERBB2 and breast cancer risk

Amplification of the epithelial growth factor receptor gene ERBB2 (HER2, NEU) in breast cancer is associated with a poor clinical prognosis. In mammary gland development, this receptor plays a role in ductal and lobuloalveolar differentiation. We conducted a systematic investigation of the role of genetic variation of the ERBB2 gene in breast cancer risk in a study of 842 histologically confirmed invasive breast cancer cases and 1,108 controls from the Shanghai Breast Cancer Study. We observed that the ERBB2 gene resides within a locus of high linkage disequilibrium, composed of three major ancestral haplotypes in the study population. These haplotypes are marked by simple tandem repeat and single nucleotide polymorphisms, including the missense variants I655V and P1170A. We observed a risk-modifying effect of a highly polymorphic simple tandem repeat within an evolutionarily conserved region, 4.4 kb upstream from the ERBB2 transcription start site. Under a dominant genetic model, the age-adjusted odds ratio was 1.74 (95% confidence interval, 1.27-2.37). Its association with breast cancer, and with breast cancer stratified by histology, by histologic grade, and by stage, remained significant after correction for multiple comparisons. In contrast, we observed no association of ERBB2 single nucleotide polymorphism haplotypes with breast cancer predisposition.

Tight junctions and the regulation of gene expression

Cell adhesion is a key regulator of cell differentiation. Cell interactions with neighboring cells and the extracellular matrix regulate gene expression, cell proliferation, polarity and apoptosis. Apical cell-cell junctions participate in these processes using different types of proteins, some of them exhibit nuclear and junctional localization and are called NACos for Nuclear Adhesion Complexes. Tight junctions are one type of such cell-cell junctions and several signaling complexes have been identified to associate with them. In general, expression of tight junction components suppresses proliferation to allow differentiation in a coordinated manner with adherens junctions and extracellular matrix adhesion. These tight junction components have been shown to affect several signaling and transcriptional pathways, and changes in the expression of tight junction proteins are associated with several disease conditions, such as cancer. Here, we will review how tight junction proteins participate in the regulation of gene expression and cell proliferation, as well as how they are regulated themselves by different mechanisms involved in gene expression and cell differentiation.

Transcriptome analysis of mammary epithelial subpopulations identifies novel determinants of lineage commitment and cell fate

Background

Understanding the molecular control of cell lineages and fate determination in complex tissues is key to not only understanding the developmental biology and cellular homeostasis of such tissues but also for our understanding and interpretation of the molecular pathology of diseases such as cancer. The prerequisite for such an understanding is detailed knowledge of the cell types that make up such tissues, including their comprehensive molecular characterisation. In the mammary epithelium, the bulk of the tissue is composed of three cell lineages, namely the basal/myoepithelial, luminal epithelial estrogen receptor positive and luminal epithelial estrogen receptor negative cells. However, a detailed molecular characterisation of the transcriptomic differences between these three populations has not been carried out.

Results

A whole transcriptome analysis of basal/myoepithelial cells, luminal estrogen receptor negative cells and luminal estrogen receptor positive cells isolated from the virgin mouse mammary epithelium identified 861, 326 and 488 genes as highly differentially expressed in the three cell types, respectively. Network analysis of the transcriptomic data identified a subpopulation of luminal estrogen receptor negative cells with a novel potential role as non-professional immune cells. Analysis of the data for potential paracrine interacting factors showed that the basal/myoepithelial cells, remarkably, expressed over twice as many ligands and cell surface receptors as the other two populations combined. A number of transcriptional regulators were also identified that were differentially expressed between the cell lineages. One of these, Sox6, was specifically expressed in luminal estrogen receptor negative cells and functional assays confirmed that it maintained mammary epithelial cells in a differentiated luminal cell lineage.

Conclusion

The mouse mammary epithelium is composed of three main cell types with distinct gene expression patterns. These suggest the existence of a novel functional cell type within the gland, that the basal/myoepithelial cells are key regulators of paracrine signalling and that there is a complex network of differentially expressed transcription factors controlling mammary epithelial cell fate. These data will form the basis for understanding not only cell fate determination and cellular homeostasis in the normal mammary epithelium but also the contribution of different mammary epithelial cell types to the etiology and molecular pathology of breast disease.

Prolactin and ErbB4/HER4 signaling interact via Janus kinase 2 to induce mammary epithelial cell gene expression differentiation

Differentiation of mammary epithelium in vivo requires signaling through prolactin and ErbB4/HER4-dependent mechanisms. Although stimulation of either the prolactin receptor or ErbB4/HER4 results in activation of the transcription factor signal transducer and activator of transcription 5A (STAT5A) and induction of lactogenic differentiation, how these pathways intersect is unknown. We show herein that prolactin signaling in breast cells cooperates with and is substantially enhanced by the receptor tyrosine kinase ErbB4/HER4. Prolactin and the ErbB4/HER4 ligand heparin-binding epidermal growth factor each induced STAT5A tyrosine phosphorylation and nuclear translocation; each pathway required the intracellular tyrosine kinase Janus kinase 2 (JAK2). We found that full prolactin-mediated STAT5A activation and binding to the endogenous beta-casein promoter required ErbB4/HER4 but did not require ErbB1/epidermal growth factor receptor. For example, prolactin-induced STAT5A activity was markedly diminished in cells overexpressing kinase inactive HER4, in cells transfected with small interfering RNAs to specifically knock down endogenous ErbB4/HER4 expression and in cells treated with a small molecule inhibitor that targets ErbB4 kinase. Interestingly, prolactin caused ErbB4/HER4 tyrosine phosphorylation in a JAK2 kinase-dependent manner. Finally, prolactin receptor, ErbB4/HER4, and JAK2 were coimmunoprecipitated from prolactin-treated but not untreated cells. These results suggest that prolactin signaling engages the ErbB4 pathway via JAK2 and that ErbB4 provides an important component of STAT5A-dependent lactogenic differentiation; this pathway integration may help explain the similar deficit in mammary development observed in gene-targeted mice deficient in prolactin receptor, JAK2, ErbB4, or STAT5A.

A splice variant of HER2 corresponding to Herstatin is expressed in the noncancerous breast and in breast carcinomas

Herstatin (HST) is an alternatively spliced HER2 product with growth-inhibitory properties in experimental cancer systems. The role of HST in adult human tissues and disease remains unexplored. Here, we investigated HST expression at the mRNA and protein (immunohistochemistry [IHC]) level in parallel with parameters reflecting HER activation in 187 breast carcinomas and matched noncancerous breast tissues (NCBT). Noncancerous breast tissues demonstrated the highest HST/HER2 transcript ratios corresponding to a few positive epithelial and stromal cells by IHC. Although HST/HER2 transcript ratios in tumors were inversely associated with HER2 IHC grading (P = .0048 for HER2 IHC-1+ and P = .0006 for HER2 IHC-2+ vs HER2-negative tumors), relative HST expression within the same tumor/NCBT system remained constant. HST/HER2 ratios did not predict the presence of HST protein, which was found in 46 (25%) of 187 tumors. A subgroup of HER2 IHC-3+ tumors exhibited high HST/HER2 transcript ratios, strong HST protein positivity, and cytoplasmic phospho-Akt/PKB and p21(CIP1/WAF1) localization. In conclusion, HST may act as a paracrine factor in the adult breast. Because HST is described as an endogenous pan-HER inhibitor, the presence of this protein in breast carcinomas may portent the inefficiency of exogenous efforts to block HER2 dimerization, whereas its absence may justify such interventions.

ErbB4/HER4: role in mammary gland development, differentiation and growth inhibition

The ErbB receptor tyrosine kinase family has often been associated with increased growth of breast epithelial cells, as well as malignant transformation and progression. In contrast, ErbB4/HER4 exhibits unique attributes from a two step proteolytic cleavage which releases an 80 kilodalton, nuclear localizing, tyrosine kinase to a signal transduction mechanism that slows growth and stimulates differentiation of breast cells. This review provides an overview of ErbB4/HER4 in growth and differentiation of the mammary epithelium, including its physiologic role in development, the contrasting growth inhibition/tumor suppression and growth acceleration of distinct ErbB4/HER4 isoforms and a description of the unique cell cycle regulated pattern of nuclear HER4 ubiquitination and destruction.

Gefitinib prevents cancer progression in mice expressing the activated rat HER2/neu

We tested the efficacy of gefitinib in the prevention of HER2/neu-mediated breast cancer development in BALB-NeuT transgenic mice. Oral administration of gefitinib to female transgenic mice from 5 to 14 weeks of age reduced tumor multiplicity from 9.6 +/- 0.82 to 0.58 +/- 1.1 (83%). We observed a decrease in the number and size of lobules and lobular nodules in treated mice with a reduction in the overall disease burden per gland. Normal duct development in the mammary glands was not affected by gefitinib. The development of acinic cell carcinoma in the parotid glands of these animals was also reduced coincident with decreased stromal involvement during progression. Gefitinib eliminated phosphorylation of HER2 and HER3 and signaling through MAPK and Akt in lobular hyperplasias and carcinomas. At the same time MAPK activity and cytokine production in splenocytes and lymph nodes was increased in gefitinib-treated animals coincident with an increase in lymph node size. Delaying gefitinib treatment until mammary glands exhibited atypical lobular hyperplasias reduced efficacy. These studies demonstrate the critical role of HER2 signal transduction in the onset and progression of HER2/neu-dependent breast cancer and suggest a role for specific inhibitors to prevent the outgrowth of early hyperplastic disease.

Transgenic models to study actions of prolactin in mammary neoplasia

Transgenic models to explore the role of prolactin and its interactions with other factors in mammary oncogenesis have begun to reveal the dynamic contributions of prolactin to the development and progression of this disease. Targeting prolactin to mammary epithelial cells mimics the local production of this hormone that is prominent in women, and permits studies in the absence of effects on the ovarian steroid milieu. These models have demonstrated that local production of prolactin is sufficient to induce mammary tumors after a long latency. Prolactin also can potentiate actions of other oncogenic stimuli, decreasing tumor latency and increasing incidence in several models. Augmented proliferation, without alteration of apoptosis, is a consistent feature. Pathways in addition to the well-characterized Jak2-Stat5 pathway, including ERK1/2 and Akt1/2, are implicated in these actions. These studies have also revealed a complex relationship with estrogen; while prolactin increases ERalpha expression, it does not require estrogenic ligand for lesion development, and indeed, in combination with the EGFR ligand, TGFalpha, prolactin can contribute to estrogen insensitivity. These studies highlight the utility of these models to decipher the interplay between prolactin and other oncogenic factors in breast cancer, with implications for preventative and therapeutic strategies.

Prolactin regulation of mammary gland development

Mammary morphogenesis is orchestrated with other reproductive events by pituitary-driven changes to the systemic hormone environment, initiating the formation of a mammary ductal network during puberty and the addition of secretory alveoli during pregnancy. Prolactin is the major driver of development during pregnancy via regulation of ovarian progesterone production (in many species) and direct effects on mammary epithelial cells (in all species). Together these hormones regulate two aspects of development that are the subject of intense interest: (1) a genomic regulatory network that integrates many additional spatial and temporal cues to control gene expression and (2), the activity of a stem and progenitor cell hierarchy. Amalgamation of these two aspects will increase our understanding of cell proliferation and differentiation within the mammary gland, with clear application to our attempts to control breast cancer. Here we focus on providing an over-view of prolactin action during development of the model murine mammary gland.

Mechanistic aspects of crosstalk between GH and PRL and ErbB receptor family signaling

Growth hormone (GH) and prolactin (PRL) are anterior pituitary hormones that have multiple roles in growth and metabolism. Both hormones are important in mammary development and breast cancer. The epidermal growth factor (EGF) family of peptides and the receptors that they activate (the ErbB family) are also major players in mammary biology and pathophysiology. Recent studies in signal transduction have highlighted the interplay between signaling pathways referred to as crosstalk. In this review, cell biological and signaling studies related to crosstalk between GH and PRL and the ErbB family are discussed. In particular, the role of GH- and PRL-induced phosphorylation of ErbB receptors in regulating EGF responsiveness is highlighted with attention to potential pathophysiological relevance.

Alterations of gene expression in the development of early hyperplastic precursors of breast cancer

Enlargement of normal terminal duct lobular units (TDLUs) by hyperplastic columnar epithelial cells is one of the most common abnormalities of growth in the adult female human breast. These hyperplastic enlarged lobular units (HELUs) are important clinically as the earliest histologically identifiable potential precursor of breast cancer. The causes of the hyperplasia are unknown but may include estrogen-simulated growth mediated by estrogen receptor-alpha, which is highly elevated in HELUs and may be fundamental to their development. The present study used DNA microarray technology and RNA from microdissected pure epithelial cells to examine changes in gene expression and molecular pathways associated with the development of HELUs from TDLUs. The results suggest that HELUs evolve from TDLUs primarily by reactivation of pathways involved in embryonic development and suppression of terminal differentiation. Changes in ERBB genes were particularly prominent, including a uniform switch in ligands for the ERBB1 receptor (14-fold decrease in epidermal growth factor and 10-fold increase in amphiregulin, respectively) in HELUs compared with TDLUs. Epidermal growth factor regulates terminal differentiation in adult breast and amphiregulin is critical to normal embryonic breast development. Because HELUs are such early potential precursors of breast cancer, targeting some of these alterations may be especially promising strategies for breast cancer prevention.

The HER4 cytoplasmic domain, but not its C terminus, inhibits mammary cell proliferation

Unlike the proliferative action of other epidermal growth factor (EGF) receptor family members, HER4/ErbB4 is often associated with growth-inhibitory and differentiation signaling. These actions may involve HER4 two-step proteolytic processing by intramembraneous gamma-secretase, releasing the soluble, intracellular 80-kDa HER4 cytoplasmic domain, s80HER4. We demonstrate that pharmacological inhibition of either gamma-secretase activity or HER4 tyrosine kinase activity blocked heregulin-dependent growth inhibition of SUM44 breast cancer cells. We next generated breast cell lines stably expressing GFP-s80HER4 [green fluorescent protein (GFP) fused to the N terminus of the HER4 cytoplasmic domain, residues 676-1308], GFP-CT(HER4) (GFP fused to N terminus of the HER4 C-terminus distal to the tyrosine kinase domain, residues 989-1308), or GFP alone. Both GFP-s80HER4 and GFP-CTHER4 were found in the nucleus, but GFP-s80HER4 accumulated to a greater extent and sustained its nuclear localization. s80HER4 was constitutively tyrosine phosphorylated, and treatment of cells with a specific HER family tyrosine kinase inhibitor 1) blocked tyrosine phosphorylation; 2) markedly diminished GFP-s80HER4 nuclear localization; and 3) reduced signal transducer and activator of transcription (STAT)5A tyrosine phosphorylation and nuclear localization as well as GFP-s80HER4:STAT5A interaction. Multiple normal mammary and breast cancer cell lines, stably expressing GFP-s80HER4 (SUM44, MDA-MB-453, MCF10A, SUM102, and HC11) were growth inhibited compared with the same cell line expressing GFP-CTHER4 or GFP alone. The s80HER4-induced cell number reduction was due to slower growth because rates of apoptosis were equivalent in GFP-, GFP-CTHER4-, and GFP-s80HER4-expressing cells. Lastly, GFP-s80HER4 enhanced differentiation signaling as indicated by increased basal and prolactin-dependent beta-casein expression. These results indicate that surface HER4 tyrosine phosphorylation and ligand-dependent release of s80HER4 are necessary, and s80HER4 signaling is sufficient for HER4-dependent growth inhibition.