Dynamic Expression of Erbb Pathway Members during Early Mammary Gland Morphogenesis

Organoid Cultures for the Study of Mammary Biology and Breast Cancer: The Promise and Challenges

During the last decade, biomedical research has experienced a resurgence in the use of three-dimensional culture models for studies of normal and cancer biology. This resurgence has been driven by the development of models in which primary cells are grown in tissue-mimicking media and extracellular matrices to create organoid or organotypic cultures that more faithfully replicate the complex architecture and physiology of normal tissues and tumors. In addition, patient-derived tumor organoids preserve the three-dimensional organization and characteristics of the patient tumors ex vivo, becoming excellent preclinical models to supplement studies of tumor xenografts transplanted into immunocompromised mice. In this perspective, we provide an overview of how organoids are being used to investigate normal mammary biology and as preclinical models of breast cancer and discuss improvements that would enhance their utility and relevance to the field.

ALK inhibitors suppress HCC and synergize with anti-PD-1 therapy and ABT-263 in preclinical models

Hepatocellular carcinoma (HCC) currently lacks effective therapies, leaving a critical need for new treatment options. A previous study identified the anaplastic lymphoma kinase (ALK) amplification in HCC patients, raising the question of whether ALK inhibitors could be a viable treatment. Here, we showed that both ALK inhibitors and ALK knockout effectively halted HCC growth in cell cultures. Lorlatinib, a potent ALK inhibitor, suppressed HCC tumor growth and metastasis across various mouse models. Additionally, in an advanced immunocompetent humanized mouse model, when combined with an anti-PD-1 antibody, lorlatinib more potently suppressed HCC tumor growth, surpassing individual drug efficacy. Lorlatinib induced apoptosis and senescence in HCC cells, and the senolytic agent ABT-263 enhanced the efficacy of lorlatinib. Additional studies identified that the apoptosis-inducing effect of lorlatinib was mediated via GGN and NRG4. These findings establish ALK inhibitors as promising HCC treatments, either alone or in combination with immunotherapies or senolytic agents.

The Mammary Gland: Basic Structure and Molecular Signaling during Development

The mammary gland is a compound, branched tubuloalveolar structure and a major characteristic of mammals. The mammary gland has evolved from epidermal apocrine glands, the skin glands as an accessory reproductive organ to support postnatal survival of offspring by producing milk as a source of nutrition. The mammary gland development begins during embryogenesis as a rudimentary structure that grows into an elementary branched ductal tree and is embedded in one end of a larger mammary fat pad at birth. At the onset of ovarian function at puberty, the rudimentary ductal system undergoes dramatic morphogenetic change with ductal elongation and branching. During pregnancy, the alveolar differentiation and tertiary branching are completed, and during lactation, the mature milk-producing glands eventually develop. The early stages of mammary development are hormonal independent, whereas during puberty and pregnancy, mammary gland development is hormonal dependent. We highlight the current understanding of molecular regulators involved during different stages of mammary gland development.

Is It the Twilight of BACE1 Inhibitors?

β-secretase (BACE1) has been regarded as a prime target for the development of amyloid beta (Aβ) lowering drugs in the therapy of Alzheimer´s disease (AD). Although the enzyme was discovered in 1991 and helped to formulate the Aβ hypothesis as one of the very important features of AD etiopathogenesis, progress in AD treatment utilizing BACE1 inhibitors has remained limited. Moreover, in the last years, major pharmaceutical companies have discontinued clinical trials of five BACE1 inhibitors that had been strongly perceived as prospective. In our review, the Aβ hypothesis, the enzyme, its functions, and selected substrates are described. BACE1 inhibitors are classified into four generations. Those that underwent clinical trials displayed adverse effects, including weight loss, skin rashes, worsening of neuropsychiatric symptoms, etc. Some inhibitors could not establish a statistically significant risk-benefit ratio, or even scored worse than placebo. We still believe that drugs targeting BACE1 may still hide some potential, but a different approach to BACE1 inhibition or a shift of focus to modulation of its trafficking and/or post-translational modification should now be followed.

RNA-Seq Reveals the Expression Profiles of Long Non-Coding RNAs in Lactating Mammary Gland from Two Sheep Breeds with Divergent Milk Phenotype

Simple Summary

Long non-coding RNAs (lncRNAs) play a key role in regulating the expression level of mRNAs. The expression profiles of ovine mammary gland were investigated in two sheep breeds with divergent milk phenotype using RNA-Seq. A total of 1894 lncRNAs were found to be expressed and 68 of these were differentially expressed between the two breeds. Some important Gene Ontogeny (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways that were related to lactation and mammary gland morphogenesis were found for the target genes of differentially expressed lncRNAs. This study can improve our understanding of the functions of lncRNAs in the regulation of lactation, milk yield, and milk components in sheep.

Abstract

Long non-coding RNAs (lncRNAs) are a kind of non-coding RNA with >200 nucleotides in length. Some lncRNAs have been proven to have clear regulatory functions in many biological processes of mammals. However, there have been no reports on the roles of lncRNAs in ovine mammary gland tissues. In the study, the expression profiles of lncRNAs were studied using RNA-Seq in mammary gland tissues from lactating Small-Tailed Han (STH) ewes and Gansu Alpine Merino (GAM) ewes with different milk yield and ingredients. A total of 1894 lncRNAs were found to be expressed. Compared with the GAM ewes, the expression levels of 31 lncRNAs were significantly up-regulated in the mammary gland tissues of STH ewes, while 37 lncRNAs were remarkably down-regulated. Gene Ontogeny (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis found that the target genes of differentially expressed lncRNAs were enriched in the development and proliferation of mammary epithelial cells, morphogenesis of mammary gland, ErbB signaling pathway, and Wnt signaling pathway. Some miRNA sponges of differentially expressed lncRNAs, reported to be associated with lactation and mammary gland morphogenesis, were found in a lncRNA-miRNA network. This study reveals comprehensive lncRNAs expression profiles in ovine mammary gland tissues, thereby providing a further understanding of the functions of lncRNAs in the lactation and mammary gland development of sheep.

Is Neuregulin-4 a predictive marker of microvascular complications in type 2 diabetes mellitus?

BACKGROUND AND AIMS

Neuregulin-4 (Nrg-4) is a new adipokine released from brown adipose tissue. It plays pivotal role in regulating systemic energy balance, glucose and lipid metabolism and in reducing chronic inflammation. We aimed to investigate the relation between diabetic microvascular complications (DMC) and serum (Nrg-4) levels in patients with type 2 diabetes mellitus.

METHODS

Patients with type 2 diabetes mellitus were divided into DMC and diabetic patients without microvascular complications (non-DMC). Nrg-4 levels of the patients were compared.

RESULTS

Fifty and 29 patients enrolled to the DMC and non-DMC groups, respectively. Nrg-4 was 1.23 (0.02-5.1) ng/mL and 2.5 (0.21-6.01) ng/mL in DMC and in non-DMC groups, respectively (P < .001). In patients with DMC, FPG was 189.5 (89-446) mg/ dL, whereas it was 128 (95-278) mg/dL in non-DMC diabetic patients (P < .001). HbA1c was also significantly higher in the DMC group than in the non-DMC group (P < .001). Negative correlation was found between Nrg-4 and FPG (r = -0.231, P = .03), HBA1c (r = -0.312, P = .003) and microalbuminuria (r = -0.277, P = .009). Logistic regression analysis showed a 1-unit decrease in Nrg-4 to increase the presence of DMC by 1.9 times. The best cut-off value of Nrg-4 was 1.56 ng/mL with 82.1% sensitivity and 64% specificity, in predicting DMC.

CONCLUSION

In patients with diabetes, Nrg-4 levels may be a good predictor of early detection of one or more DMC, as microvascular dysfunction in an organ system is considered to be an initial onset of subclinical systemic damage.

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.

A systematic SNP selection approach to identify mechanisms underlying disease aetiology: linking height to post-menopausal breast and colorectal cancer risk

Data from GWAS suggest that SNPs associated with complex diseases or traits tend to co-segregate in regions of low recombination, harbouring functionally linked gene clusters. This phenomenon allows for selecting a limited number of SNPs from GWAS repositories for large-scale studies investigating shared mechanisms between diseases. For example, we were interested in shared mechanisms between adult-attained height and post-menopausal breast cancer (BC) and colorectal cancer (CRC) risk, because height is a risk factor for these cancers, though likely not a causal factor. Using SNPs from public GWAS repositories at p-values < 1 × 10⁻⁵ and a genomic sliding window of 1 mega base pair, we identified SNP clusters including at least one SNP associated with height and one SNP associated with either post-menopausal BC or CRC risk (or both). SNPs were annotated to genes using HapMap and GRAIL and analysed for significantly overrepresented pathways using ConsensuspathDB. Twelve clusters including 56 SNPs annotated to 26 genes were prioritised because these included at least one height- and one BC risk- or CRC risk-associated SNP annotated to the same gene. Annotated genes were involved in Indian hedgehog signalling (p-value = 7.78 × 10⁻⁷) and several cancer site-specific pathways. This systematic approach identified a limited number of clustered SNPs, which pinpoint potential shared mechanisms linking together the complex phenotypes height, post-menopausal BC and CRC.

Prenatal Mammary Gland Development in the Mouse: Research Models and Techniques for Its Study from Past to Present

Mammary gland development starts during prenatal life, when at designated positions along the ventrolateral boundary of the embryonic or fetal trunk, surface ectodermal cells coalesce to form primordia for mammary glands, instead of differentiating into epidermis. With the wealth of genetically engineered mice available as research models, our understanding of the prenatal phase of mammary development has recently greatly advanced. This understanding includes the recognition of molecular and mechanistic parallels between prenatal and postnatal mammary morphogenesis and even tumorigenesis, much of which can moreover be extrapolated to human. This makes the murine embryonic mammary gland a useful model for a myriad of questions pertaining to normal and pathological breast development. Hence, unless indicated otherwise, this review describes embryonic mammary gland development in mouse only, and lists mouse models that have been examined for defects in embryonic mammary development. Techniques that originated in the field of developmental biology, such as explant culture and tissue recombination, were adapted specifically to research on the embryonic mammary gland. Detailed protocols for these techniques have recently been published elsewhere. This review describes how the development and adaptation of these techniques moved the field forward from insights on (comparative) morphogenesis of the embryonic mammary gland to the understanding of tissue and molecular interactions and their regulation of morphogenesis and functional development of the embryonic mammary gland. It is here furthermore illustrated how generic molecular biology and biochemistry techniques can be combined with these older, developmental biology techniques, to address relevant research questions. As such, this review should provide a solid starting point for those wishing to familiarize themselves with this fascinating and important subdomain of mammary gland biology, and guide them in designing a relevant research strategy.

Wnt and Neuregulin1/ErbB signalling extends 3D culture of hormone responsive mammary organoids

The development of in vitro culture systems quantitatively and qualitatively recapitulating normal breast biology is key to the understanding of mammary gland biology. Current three-dimensional mammary culture systems have not demonstrated concurrent proliferation and functional differentiation ex vivo in any system for longer than 2 weeks. Here, we identify conditions including Neuregulin1 and R-spondin 1, allowing maintenance and expansion of mammary organoids for 2.5 months in culture. The organoids comprise distinct basal and luminal compartments complete with functional steroid receptors and stem/progenitor cells able to reconstitute a complete mammary gland in vivo. Alternative conditions are also described that promote enrichment of basal cells organized into multiple layers surrounding a keratinous core, reminiscent of structures observed in MMTV-Wnt1 tumours. These conditions comprise a unique tool that should further understanding of normal mammary gland development, the molecular mechanism of hormone action and signalling events whose deregulation leads to breast tumourigenesis.

Protein-Trap Insertional Mutagenesis Uncovers New Genes Involved in Zebrafish Skin Development, Including a Neuregulin 2a-Based ErbB Signaling Pathway Required during Median Fin Fold Morphogenesis

Skin disorders are widespread, but available treatments are limited. A more comprehensive understanding of skin development mechanisms will drive identification of new treatment targets and modalities. Here we report the Zebrafish Integument Project (ZIP), an expression-driven platform for identifying new skin genes and phenotypes in the vertebrate model Danio rerio (zebrafish). In vivo selection for skin-specific expression of gene-break transposon (GBT) mutant lines identified eleven new, revertible GBT alleles of genes involved in skin development. Eight genes—fras1, grip1, hmcn1, msxc, col4a4, ahnak, capn12, and nrg2a—had been described in an integumentary context to varying degrees, while arhgef25b, fkbp10b, and megf6a emerged as novel skin genes. Embryos homozygous for a GBT insertion within neuregulin 2a (nrg2a) revealed a novel requirement for a Neuregulin 2a (Nrg2a) – ErbB2/3 – AKT signaling pathway governing the apicobasal organization of a subset of epidermal cells during median fin fold (MFF) morphogenesis. In nrg2a mutant larvae, the basal keratinocytes within the apical MFF, known as ridge cells, displayed reduced pAKT levels as well as reduced apical domains and exaggerated basolateral domains. Those defects compromised proper ridge cell elongation into a flattened epithelial morphology, resulting in thickened MFF edges. Pharmacological inhibition verified that Nrg2a signals through the ErbB receptor tyrosine kinase network. Moreover, knockdown of the epithelial polarity regulator and tumor suppressor lgl2 ameliorated the nrg2a mutant phenotype. Identifying Lgl2 as an antagonist of Nrg2a – ErbB signaling revealed a significantly earlier role for Lgl2 during epidermal morphogenesis than has been described to date. Furthermore, our findings demonstrated that successive, coordinated ridge cell shape changes drive apical MFF development, making MFF ridge cells a valuable model for investigating how the coordinated regulation of cell polarity and cell shape changes serves as a crucial mechanism of epithelial morphogenesis.

Neuregulin-3 regulates epithelial progenitor cell positioning and specifies mammary phenotype

Mutation of Neuregulin-3 (Nrg3) results in defective embryonic mammary gland development. Here, we investigate functions of Nrg3 signaling in embryonic mammary morphogenesis. Nrg3 regulates the distribution of epithelial progenitor cells within the presumptive mammary-forming region during early mammary morphogenesis. Basal and suprabasal epithelial cells are significantly smaller within the hypoplastic mammary primordium (MP) that forms in Nrg3 mutants, indicative of failure to acquire mammary epithelial cell (MEC) morphological phenotype. Activation of Erbb4 JM-a CYT-1, an Erbb4 isoform expressed in the developing MP, leads to MEC spreading and migration. Nrg3 promotes the accumulation of epithelial progenitor cells at the MP site in embryo explant cultures. Our results implicate Nrg3 signaling in mediating key events of mammary mesenchyme specification, including mesenchymal condensation, mitosis, and induction of mammary marker expression. Taken together, our results show Nrg3 has a major role in conferring specification of the mammary phenotype to both epithelial and mesenchymal progenitor cells.

Stem cells and the developing mammary gland

The mammary gland undergoes dynamic changes throughout life. In the mouse, these begin with initial morphogenesis of the gland in the mid-gestation embryo followed by hormonally regulated changes during puberty and later in adulthood. The adult mammary gland contains a hierarchy of cell types with varying potentials for self-maintenance and differentiation. These include cells able to produce complete, functional mammary glands in vivo and that contain daughter cells with the same remarkable regenerative potential, as well as cells with more limited clonogenic activity in vitro. Here we review how applying in vitro and in vivo methods for quantifying these cells in adult mammary tissue to fetal mammary cells has enabled the first cells fulfilling the functional criteria of transplantable, isolated mammary stem cells to be identified a few days before birth. Thereafter, the number of these cells increases rapidly. Populations containing these fetal stem cells display growth and gene expression programs that differ from their adult counterparts but share signatures characteristic of certain types of breast cancer. Such observations reinforce growing evidence of important differences between tissue-specific fetal and adult cells with stem cell properties and emphasize the merits of investigating their molecular basis.

Neuregulin 3 and erbb signalling networks in embryonic mammary gland development

We review the role of Neuregulin 3 (Nrg3) and Erbb receptor signalling in embryonic mammary gland development. Neuregulins are growth factors that bind and activate its cognate Erbb receptor tyrosine kinases, which form a signalling network with established roles in breast development and breast cancer. Studies have shown that Nrg3 expression profoundly impacts early stages of embryonic mammary development. Network analysis shows how Nrg/Erbb signals could integrate with other major regulators of embryonic mammary development to elicit the morphogenetic processes and cell fate decisions that occur as the mammary lineage is established.

Transcriptome analysis of embryonic mammary cells reveals insights into mammary lineage establishment

INTRODUCTION

The mammary primordium forms during embryogenesis as a result of inductive interactions between its constitutive tissues, the mesenchyme and epithelium, and represents the earliest evidence of commitment to the mammary lineage. Previous studies of embryonic mouse mammary epithelium indicated that, by mid-gestation, these cells are determined to a mammary cell fate and that a stem cell population has been delimited. Mammary mesenchyme can induce mammary development from simple epithelium even across species and classes, and can partially restore features of differentiated tissue to mouse mammary tumours in co-culture experiments. Despite these exciting properties, the molecular identity of embryonic mammary cells remains to be fully characterised.

METHODS

Here, we define the transcriptome of the mammary primordium and the two distinct cellular compartments that comprise it, the mammary primordial bud epithelium and mammary mesenchyme. Pathway and network analysis was performed and comparisons of embryonic mammary gene expression profiles to those of both postnatal mouse and human mammary epithelial cell sub-populations and stroma were made.

RESULTS

Several of the genes we have detected in our embryonic mammary cell signatures were previously shown to regulate mammary cell fate and development, but we also identified a large number of novel candidates. Additionally, we determined genes that were expressed by both embryonic and postnatal mammary cells, which represent candidate regulators of mammary cell fate, differentiation and progenitor cell function that could signal from mammary lineage inception during embryogenesis through postnatal development. Comparison of embryonic mammary cell signatures with those of human breast cells identified potential regulators of mammary progenitor cell functions conserved across species.

CONCLUSIONS

These results provide new insights into genetic regulatory mechanisms of mammary development, particularly identification of novel potential regulators of mammary fate and mesenchymal-epithelial cross-talk. Since cancers may represent diseases of mesenchymal-epithelial communications, we anticipate these results will provide foundations for further studies into the fundamental links between developmental, stem cell and breast cancer biology.

Understanding the HER family in breast cancer: interaction with ligands, dimerization and treatments

Breast carcinoma is the most frequent type of cancer affecting women. Among the recently described molecular and phenotypic classes of breast cancer, human epidermal growth factor receptor 2 (HER2)-positive tumours are associated with a poor prognosis. HER2 plays an important role in cancer progression being targeted to provide predictive and prognostic information. Moreover, HER2 is related to cancer resistance against a variety of therapies; however, trastuzumab (herceptin) has proved successful in treatment of this subgroup. Nevertheless, resistance to this drug may be acquired by patients after a period of treatment, which indicates that other molecular mechanisms might influence success of this therapy. Dimerization between members of the HER family may contribute to resistance against treatments due to different combinations that trigger different downstream pathways. This is promoted by ligands, which are expressed as transmembrane precursor protein molecules and have a conserved epidermal growth factor-like domain. Through resistance to trastuzumab, other drugs are being developed to interact in different domains of HER2 protein. It might be a good strategy to apply new drugs simultaneously to trastuzumab due to act in different domains of HER2. The study of interaction between receptors/ligands will characterize specifically their signalling pathway and understand which strategy to acquire.

Neuregulin 1 sustains the gene regulatory network in both trabecular and nontrabecular myocardium

RATIONALE

The cardiac gene regulatory network (GRN) is controlled by transcription factors and signaling inputs, but network logic in development and it unraveling in disease is poorly understood. In development, the membrane-tethered signaling ligand Neuregulin (Nrg)1, expressed in endocardium, is essential for ventricular morphogenesis. In adults, Nrg1 protects against heart failure and can induce cardiomyocytes to divide.

OBJECTIVE

To understand the role of Nrg1 in heart development through analysis of null and hypomorphic Nrg1 mutant mice.

METHODS AND RESULTS

Chamber domains were correctly specified in Nrg1 mutants, although chamber-restricted genes Hand1 and Cited1 failed to be activated. The chamber GRN subsequently decayed with individual genes exhibiting decay patterns unrelated to known patterning boundaries. Both trabecular and nontrabecular myocardium were affected. Network demise was spatiotemporally dynamic, the most sensitive region being the central part of the left ventricle, in which the GRN underwent complete collapse. Other regions were partially affected with graded sensitivity. In vitro, Nrg1 promoted phospho-Erk1/2-dependent transcription factor expression, cardiomyocyte maturation and cell cycle inhibition. We monitored cardiac pErk1/2 in embryos and found that expression was Nrg1-dependent and levels correlated with cardiac GRN sensitivity in mutants.

CONCLUSIONS

The chamber GRN is fundamentally labile and dependent on signaling from extracardiac sources. Nrg1-ErbB1/4-Erk1/2 signaling critically sustains elements of the GRN in trabecular and nontrabecular myocardium, challenging our understanding of Nrg1 function. Transcriptional decay patterns induced by reduced Nrg1 suggest a novel mechanism for cardiac transcriptional regulation and dysfunction in disease, potentially linking biomechanical feedback to molecular pathways for growth and differentiation.

Mammary gland growth factors: roles in normal development and in cancer

Normal development of the mammary gland proceeds via interactions between the epithelium and the mesenchyme that start during embryogenesis and continue during pubertal outgrowth and differentiation. The function of specific peptide growth factors that bind members of the receptor tyrosine kinase family and the cytokine receptor family are required at each stage. In many cases the peptides are produced in one compartment and act on receptors in the other compartment. One of the striking differences between normal development and cancer is the loss of this cross-talk. Mammary tumor cells often produce a peptide and express the receptor on the same cell leading to autocrine activation of signaling pathways, a mechanism that is characteristic for cancer cells. We will discuss different peptides in the context of normal development and cancer in this review.

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.

Embryonic mammary anlagen analysis using immunolabelling of whole mounts

The mouse mammary gland is a unique organ since although the mammary gland primordium forms during embryogenesis, the majority of development occurs postnatally upon hormonal stimulation at puberty and full functionality (i.e. lactation) is not achieved until after parturition. Since both the epidermis and mammary glands share the same developmental origin, many mouse models with epidermal phenotypes often also exhibit abnormal mammary gland development. However, since the most widely used methods to analyse mammary glands are laborious and time-consuming, many mouse models exist that have not been analysed for mammary phenotypes. We have developed a simplified method that allows rapid analysis of embryonic mammary anlagen using immunolabelling of whole mounts that should facilitate more comprehensive studies of mouse mammary glands.

The neuregulin family of genes and their multiple splice variants in breast cancer

The neuregulin family consists of four genes, NRG1-4 which can each encode products containing a domain related to the epidermal growth factor family of ligands. Each gene is subject to complex control of transcription and to splicing of their mRNA product to give many variant proteins. These do not contain secretory sequences but some, through their transmembrane sequence, are routed via the Golgi where they are glycosylated, to the cell surface. Here they may be released by regulated proteolysis to act as soluble proteins which can interact and activate members of the EGF receptor family of receptor tyrosine kinases. Other splice variants do not encode transmembrane sequences and these are found either in the cytoplasm or, if they encode a nuclear localisation sequence, in distinct compartments in the nucleoplasm. It has been shown that the variants containing a full EGF domain can act as receptor agonists but the function of the cytoplasmic and nuclear products is unknown as yet. All four neuregulin genes are expressed and play an important role in mammary gland development. They are also expressed at elevated levels in some cases of ductal carcinoma in situ of the breast and breast cancer. They seem to be active in this setting and their presence may affect the efficacy of treatment with endocrine agents or with signal transduction inhibitors directed at the EGF receptor family members. Much remains to be learned however of their normal function and their influence on breast cancer development, progression and response to therapy.

The role of NRG3 in mammary development

The Neuregulin gene family encodes EGF-containing ligands which mediate their effects by binding to the ERBB receptor tyrosine kinases, a signalling network with important roles in both mammary gland development and breast cancer. Neuregulin3 (NRG3), a ligand for ERBB4, promotes early mammary morphogenesis and acts during specification of the mammary placode, an aggregate of epithelial cells that forms during mid-embryogenesis. Recent studies have shown that NRG3 can alter the cell fate of other epidermal progenitor populations when NRG3 is mis-expressed throughout the basal layer of the developing epidermis with the K14 promoter. Here evidence for a key function for NRG3 in promoting early mammary morphogenesis and the implication for the role of NRG3 in breast cancer and establishment of the mammary lineage are discussed.