Modulation of secreted proteins of mouse mammary epithelial cells by the collagenous substrata

Tissue architecture and function: dynamic reciprocity via extra- and intra-cellular matrices

Mammary gland development, functional differentiation, and homeostasis are orchestrated and sustained by a balance of biochemical and biophysical cues from the organ's microenvironment. The three-dimensional microenvironment of the mammary gland, predominantly 'encoded' by a collaboration between the extracellular matrix (ECM), hormones, and growth factors, sends signals from ECM receptors through the cytoskeletal intracellular matrix to nuclear and chromatin structures resulting in gene expression; the ECM in turn is regulated and remodeled by signals from the nucleus. In this chapter, we discuss how coordinated ECM deposition and remodeling is necessary for mammary gland development, how the ECM provides structural and biochemical cues necessary for tissue-specific function, and the role of the cytoskeleton in mediating the extra--to intracellular dialogue occurring between the nucleus and the microenvironment. When operating normally, the cytoskeletal-mediated dynamic and reciprocal integration of tissue architecture and function directs mammary gland development, tissue polarity, and ultimately, tissue-specific gene expression. Cancer occurs when these dynamic interactions go awry for an extended time.

Accessibility to the Fibronectin Synergy Site in a 3D Matrix Regulates Engagement of α 5β 1 versus α vβ 3 Integrin Receptors

Cell adhesion and migration on fibronectin (FN) extracellular matrix are mediated by integrin receptors. Integrins alpha5beta1 and alphavbeta3 require the RGD cell-binding sequence in FN, but alpha5beta1 also requires the nearby synergy site for maximal binding. In this study, we investigated how differences in the numbers of RGD or synergy sites within a three-dimensional (3D) FN-rich matrix influence cell adhesion and migration. CHO cell adhesion, spreading, and migration were reduced on 3D chimeric matrix containing FN lacking RGD (FN(RGD-)). Incorporation of FN with mutation of the synergy site (FN(syn-)), however, resulted in selective usage of integrins. CHO cells expressing alpha5beta1 showed decreased interactions with FN(syn-) chimeric matrix. In contrast, the presence of FN(syn-) had no effect on CHOalphavbeta3 cell migration. Interestingly, CHOalpha5/alphavbeta3 cells expressing both integrins selectively used alpha5beta1 for migration on wild type FN matrix but preferred alphavbeta3 for migration on FN(syn-) chimeric matrix. Thus sequestration or exposure of the FN synergy site within a 3D matrix may represent a novel mechanism for regulating cell functions through differential usage of integrin receptors. [Supplementary materials are available for this article. Go to the publisher's online edition of Cell Communication and Adhesion for the following free supplemental resource: a video recording shows migration of HT1080 cells on 3D matrix. HT1080 cells were allowed to attach to the matrix in serum-free DMEM for 2 h. FBS was then added to the medium to a final concentration of 10% and video recording was started. Images were taken every 5 min for 2 h. The video plays at 6 frames/s.].

Mechanotransduction in development: a growing role for contractility

Mechanotransduction research has focused historically on how externally applied forces can affect cell signalling and function. A growing body of evidence suggests that contractile forces that are generated internally by the actomyosin cytoskeleton are also important in regulating cell behaviour, and suggest a broader role for mechanotransduction in biology. Although the molecular basis for these cellular forces in mechanotransduction is being pursued in cell culture, researchers are also beginning to appreciate their contribution to in vivo developmental processes. Here, we examine the role for mechanical forces and contractility in regulating cell and tissue structure and function during development.

Organotypic modelling as a means of investigating epithelial-stromal interactions during tumourigenesis

The advent of co-culture approaches has allowed researchers to more accurately model the behaviour of epithelial cells in cell culture studies. The initial work on epidermal modelling allowed the development of reconstituted epidermis, growing keratinocytes on top of fibroblasts seeded in a collagen gel at an air-liquid interface to generate terminally differentiated 'skin equivalents'. In addition to developing ex vivo skin sheets for the treatment of burns victims, such cultures have also been used as a means of investigating both the development and repair of the epidermis, in more relevant conditions than simple two-dimensional culture, but without the use of animals. More recently, by varying the cell types used and adjusting the composition of the matrix components, this physiological system can be adapted to allow the study of interactions between tumour cells and their surrounding stroma, particularly with regards to how such interactions regulate invasion. Here we provide a summary of the major themes involved in tumour progression and consider the evolution of the approaches used to study cancer cell behaviour. Finally, we review how organotypic models have facilitated the study of several key pathways in cancer development and invasion, and speculate on the exciting future roles for these models in cancer research.

Cell response to matrix mechanics: focus on collagen

Many model systems and measurement tools have been engineered for observing and quantifying the effect of mechanics on cellular response. These have contributed greatly to our current knowledge of the molecular events by which mechanical cues affect cell biology. Cell responses to the mechanical properties of type 1 collagen gels are discussed, followed by a description of a model system of very thin, mechanically tunable collagen films that evoke similar responses from cells as do gel systems, but have additional advantages. Cell responses to thin films of collagen suggest that at least some of the mechanical cues that cells can respond to in their environment occur at the sub-micron scale. Mechanical properties of thin films of collagen can be tuned without altering integrin engagement, and in some cases without altering topology, making them useful in addressing questions regarding the roles of specific integrins in transducing or mitigating responses to mechanical cues. The temporal response of cells to differences in ECM may provide insight into mechanisms of signal transduction.

Unraveling the microenvironmental influences on the normal mammary gland and breast cancer

The normal mammary gland and invasive breast tumors are both complex 'organs' composed of multiple cell types as well as extracellular matrix in three-dimensional (3D) space. Conventionally, both normal and malignant breast cells are studied in vitro as two-dimensional monolayers of epithelial cells, which results in the loss of structure and tissue function. Many laboratories are now investigating regulation of signaling function in the normal mammary gland using 3D cultures. However, it is also important to assay malignant breast cells ex vivo in a physiologically relevant environment to more closely mimic tumor architecture, signal transduction regulation and tumor behavior in vivo. Here we present the potential of these 3D models for drug testing, target validation and guidance of patient selection for clinical trials. We also argue that in order to get full insight into the biology of the normal and malignant breast, and to create in vivo-like models for therapeutic approaches in humans, we need to continue to create more complex heterotypic models to approach the full context the cells encounter in the human body.

Historical perspectives of prolactin and growth hormone as mammogens, lactogens and galactagogues--agog for the future!

Around 80 years ago researchers first established that the pituitary gland regulates mammary gland function as demonstrated by the ability of its extracts to promote both mammogenesis and lactogenesis in animal models. Little did they realize that in fact two hormones, prolactin (PRL) and growth hormone (GH), were contributing to these effects. By the mid 1930s PRL had been purified as a distinct lactogen, while the galactopoietic effect of GH was confirmed after its purification in the 1940s. Interest in these hormones initially centered about their potential for increasing milk production, while in the latter half of the twentieth century it became obvious that these hormones also had the potential to influence mammary cancer development. During the past 50 years large strides have been made into understanding how these hormones signal to, and within, cells of the mammary gland, paralleling rapid developments in the fields of cellular and molecular biology. In compiling this review we have summarized the progress that has been made to date regarding roles for these hormones in the mammary gland, with a goal of ensuring that some of the seminal literature is not diluted or forgotten. In doing so it is clear that there are lessons to be learned from past experiences, where new methods and technologies will continue to present exciting new opportunities to revisit lingering questions regarding these fascinating hormones and this fascinating organ.

Characterization of Primary Breast Carcinomas Grown in Three-Dimensional Cultures

BACKGROUND

The process of progression and spread of cancer is not easily replicated in animal models and is difficult to examine in vitro. This is particularly true for human primary breast carcinoma cells, whose in vitro growth is shown to be limited to one or two passages in monolayer culture. Three-dimensional (3D) growth of breast cancer cells suggests that cell aggregates grown in this manner have many similarities to in vivo behavior.

MATERIALS AND METHODS

Primary tumors obtained from five breast cancer patients were grown in 3D cultures using the rotating-wall vessel bioreactor. Tumor aggregates were assessed for DNA ploidy, cell cycle kinetics, and expression of tumor markers and cytokines. Comparisons between fresh tumor cells and 3D aggregates were performed.

RESULTS

All five breast cancers were found to be aneuploid after 3D culture, with elevated S-phase fractions. Reverse transcription-polymerase chain reaction analyses revealed mRNA expression of HER2/neu, H-ras, K-ras, p53, transforming growth factor-alpha, transforming growth factor-beta, interleukin-1, and interleukin-6 in 3D-grown tumor cells; in most cases, expression appeared increased when compared with mRNA obtained from freshly isolated primary tumor cells.

CONCLUSIONS

After prolonged 3D growth in the rotating wall bioreactor, complex tissue-like constructs of primary breast tumor cells exhibited significantly increased proliferative activity in conjunction with oncogene activation and developed into aggressive aneuploid populations.

Architecture Is the Message: The role of extracellular matrix and 3-D structure in tissue-specific gene expression and breast cancer

I was honored to deliver the 2(nd) Stanley Korsmeyer memorial Lecture on May 9(th), 2007 in Padova, Italy. Stan will always occupy a very special place in my heart: I admired him greatly not only for his magnificent and original science but also for his integrity and his grace. This review, which summarizes my laboratory's contribution to cell and cancer biology in the last 30 years, is dedicated to Stan's memory, and to Elaine Fuchs, one of my most cherished friends without whose support this work would not have gained the degree of recognition it enjoys today. My thanks also to the Pezcoller Foundation for making that week in May, 2007 one of the most memorable in my scientific life.

RANK overexpression in transgenic mice with mouse mammary tumor virus promoter-controlled RANK increases proliferation and impairs alveolar differentiation in the mammary epithelia and disrupts lumen formation in cultured epithelial acini

RANK and RANKL, the key regulators of osteoclast differentiation and activation, also play an important role in the control of proliferation and differentiation of mammary epithelial cells during pregnancy. Here, we show that RANK protein expression is strictly regulated in a spatial and temporal manner during mammary gland development. RANK overexpression under the control of the mouse mammary tumor virus (MMTV) promoter in a transgenic mouse model results in increased mammary epithelial cell proliferation during pregnancy, impaired differentiation of lobulo-alveolar structures, decreased expression of the milk proteins beta-casein and whey acidic protein, and deficient lactation. We also show that treatment of three-dimensional in vitro cultures of primary mammary cells from MMTV-RANK mice with RANKL results in increased proliferation and decreased apoptosis in the luminal area, resulting in bigger acini with filled lumens. Taken together, these results suggest that signaling through RANK not only promotes proliferation but also inhibits the terminal differentiation of mammary epithelial cells. Moreover, the increased proliferation and survival observed in a three-dimensional culture system suggests a role for aberrant RANK signaling during breast tumorigenesis.

Antioxidant and Antibacterial Genes Are Upregulated in Early Involution of the Mouse Mammary Gland: Sharp Increase of Ceruloplasmin and Lactoferrin in Accumulating Breast Milk

The mammary gland develops mainly after birth, and shows a repeated cycle of pregnancy-triggered proliferation, differentiation for lactation, and a regressive phase after weaning known as involution. Compared to the proliferation and differentiation phases, the molecular mechanisms of involution are largely unknown. In the present study we screened genes that could play a potential role in early involution of the mouse mammary gland using fluorescent differential display followed by gene-specific reverse transcription-polymerase chain reaction. We found that five genes were upregulated more than twofold 48 h after weaning: ceruloplasmin, chemokine (CXC motif) ligand 4, epoxide hydrolase 1, lactoferrin, and properdin P factor. The products of these genes can be linked to defense against oxidative stress and/or infectious bacteria. Electrophoretic analysis and mass spectrometry of milk proteins showed that the concentrations of ceruloplasmin and lactoferrin in milk were increased fivefold and more than 38-fold, respectively, within 48 h after weaning. These increases were in contrast to the constant presence of other major proteins including albumin, caseins, transferrin, and whey acidic protein. Ceruloplasmin and lactoferrin may cooperate in the defense of the mammary gland in the postlactation period.

Cellular mechanotransduction: putting all the pieces together again

Analysis of cellular mechanotransduction, the mechanism by which cells convert mechanical signals into biochemical responses, has focused on identification of critical mechanosensitive molecules and cellular components. Stretch-activated ion channels, caveolae, integrins, cadherins, growth factor receptors, myosin motors, cytoskeletal filaments, nuclei, extracellular matrix, and numerous other structures and signaling molecules have all been shown to contribute to the mechanotransduction response. However, little is known about how these different molecules function within the structural context of living cells, tissues, and organs to produce the orchestrated cellular behaviors required for mechanosensation, embryogenesis, and physiological control. Recent work from a wide range of fields reveals that organ, tissue, and cell anatomy are as important for mechanotransduction as individual mechanosensitive proteins and that our bodies use structural hierarchies (systems within systems) composed of interconnected networks that span from the macroscale to the nanoscale in order to focus stresses on specific mechanotransducer molecules. The presence of isometric tension (prestress) at all levels of these multiscale networks ensures that various molecular scale mechanochemical transduction mechanisms proceed simultaneously and produce a concerted response. Future research in this area will therefore require analysis, understanding, and modeling of tensionally integrated (tensegrity) systems of mechanochemical control.

Of extracellular matrix, scaffolds, and signaling: tissue architecture regulates development, homeostasis, and cancer

The microenvironment influences gene expression so that the behavior of a cell is largely determined by its interactions with the extracellular matrix, neighboring cells, and soluble local and systemic cues. We describe the essential roles of context and organ structure in directing mammary gland development and differentiated function and in determining the response to oncogenic insults, including mutations. We expand on the concept of "dynamic reciprocity" to present an integrated view of development, cancer, and aging and posit that genes are like the keys on a piano: Although they are essential, it is the context that makes the music.

Modeling dynamic reciprocity: engineering three-dimensional culture models of breast architecture, function, and neoplastic transformation

In order to understand why cancer develops as well as predict the outcome of pharmacological treatments, we need to model the structure and function of organs in culture so that our experimental manipulations occur under physiological contexts. This review traces the history of the development of a prototypic example, the three-dimensional (3D) model of the mammary gland acinus. We briefly describe the considerable information available on both normal mammary gland function and breast cancer generated by the current model and present future challenges that will require an increase in its complexity. We propose the need for engineered tissues that faithfully recapitulate their native structures to allow a greater understanding of tissue function, dysfunction, and potential therapeutic intervention.

Use of three-dimensional collagen gels to study mechanotransduction in T47D breast epithelial cells

Several pathological and disease conditions can alter the mechanical properties of the extracellular matrix (ECM). Conversely, some diseases may arise from changes in the density or rigidity of the ECM. This necessitates the use and development of in vitro models to understand how both biophysical and biochemical signals regulate complex cellular behaviors. T47D breast epithelial cells will differentiate into duct-like tubules when cultured in a floating three-dimensional (3D) collagen gel, but not a 3D collagen gel that is left attached to the culture dish. This paper details several protocols we have developed for analyzing breast cell biology in 3D matrices, including culturing cells in 3D collagen gels, immunostaining cellular structures, and performing biochemical procedures directly from cells embedded in collagen gels.

Expression, location, and interactions of ErbB2 and its intramembrane ligand Muc4 (sialomucin complex) in rat mammary gland during pregnancy

Muc4 (also called Sialomucin complex) is a heterodimeric glycoprotein complex consisting of a peripheral O-glycosylated subunit ASGP-1 (ascites sialoglycoprotein-1) tightly but non-covalently bound to an N-glycosylated transmembrane subunit ASGP-2. Muc4/SMC can act as an intramembrane ligand for ErbB2 via an EGF-like domain present in the transmembrane subunit. The complex is developmentally regulated in normal rat mammary gland and overexpressed in a number of mammary tumors. Overexpression of Muc4/SMC has been shown to block cell-cell and cell-matrix interactions, protect tumor cells from immune surveillance, promote metastasis, and protect from apoptosis. We have investigated whether Muc4/SMC and ErbB2 are co-expressed and co-localized in normal rat mammary gland and whether Muc4/SMC-ErbB2 complex formation is developmentally regulated in this tissue. Muc4/SMC and ErbB2 have different expression patterns and regulatory mechanisms in the developing rat mammary gland, but both are maximally expressed during late pregnancy and lactation. The two proteins form a complex in lactating mammary gland which is not detected in the virgin gland. Moreover, this complex does not contain ErbB3. ErbB2 is co-localized with Muc4/SMC at the apical surfaces of ductal and alveolar cells in lactating gland; however, another form of ErbB2, recognized by a different antibody, localizes to the basolateral surfaces of these cells. ErbB2 phosphorylated on Tyr 1248 co-localized with Muc4/SMC at the apical surface but not at the basolateral surfaces of these cells. To investigate the function of Muc4 in the mammary gland, transgenic mice were derived using an MMTV-Muc4 construct. Interestingly, mammary gland development in the transgenic mice was aberrant, exhibiting a bifurcated pattern, including invasion down the blood vessel, similar to that exhibited by transgenic mice inappropriately expressing activated ErbB2 in the mammary gland. These data provide further evidence of the ability of Muc4/SMC to interact with ErbB2 and influence its behavior in normal epithelia.

Microenvironmental regulators of tissue structure and function also regulate tumor induction and progression: the role of extracellular matrix and its degrading enzymes

It is now widely accepted that elements of the cellular and tissue microenvironment are crucial regulators of cell behavior in culture and homeostasis in vivo, and that many of the same factors influence the course of tumor progression. Less well established is the extent to which extracellular factors actually cause cancer, and the circumstances under which this may occur. Using physiologically relevant three-dimensional culture assays and transgenic animals, we have explored how the environmental and architectural context of cells, tissues, and organs controls mammary-specific gene expression, growth regulation, apoptosis, and drug resistance and have found that loss of tissue structure is a prerequisite for cancer progression. Here we summarize this evidence and highlight two of our recent studies. Using mouse mammary epithelial cells, we show that exposure to matrix metalloproteinase-3 (MMP-3) stimulates production of reactive oxygen species (ROS) that destabilize the genome and induce epithelial-mesenchymal transition, causing malignant transformation. Using a human breast cancer progression series, we find that ADAM-dependent growth factor shedding plays a crucial role in acquisition of the malignant phenotype. These findings illustrate how normal tissue structure controls the response to extracellular signals so as to preserve tissue specificity and growth status.

Apoptosis and involution in the mammary gland are altered in mice lacking a novel receptor, beta1,4-Galactosyltransferase I

Receptor-mediated cell-extracellular matrix (ECM) interactions are critical regulators of cell survival, and perturbing these signaling pathways can disrupt cellular differentiation and function in a variety of tissues, including the mammary gland. One such receptor is the cell surface-associated, long isoform of beta1,4-galactosyltransferase I (GalT I). Deletion of long GalT I leads to increased mammary ductal branching morphogenesis [Dev. Biol., 244 (2002) 114]. Here, we show that this expansion in the mammary epithelial (ME) cell compartment is accomplished through decreased apoptosis during pregnancy and involution. Decreased apoptosis during involution is concomitant with delayed alveolar collapse, persistent expression of the milk protein gene alpha-lactalbumin and delayed expression of genes associated with the tissue-remodeling phase of involution. Using 3-dimensional in vitro cultures, we show that the decrease in apoptosis is dependent on laminin 1, a ligand for surface GalT I, suggesting that surface GalT I negatively influences ECM-dependent cell survival, a novel function for an ECM receptor. In the best-studied examples, ECM promotes survival through integrin receptor-mediated activation of focal adhesion kinase (FAK). Aggregation of surface GalT I also activates FAK, therefore, we asked if FAK activation was altered in ME from long GalT I null mice. Activated FAK was appropriately localized to focal adhesions in long GalT I null ME. However, FAK activation was constitutively reduced 4.5-fold in long GalT I nulls relative to wild type. Expression of the integrin beta1 subunit was not affected by loss of long GalT I. Collectively, these results suggest that surface GalT I might negatively regulate ME cell survival by linking integrin-independent FAK activation to apoptotic rather than survival signaling events.

Use of three-dimensional basement membrane cultures to model oncogene-induced changes in mammary epithelial morphogenesis

The development of breast carcinomas involves a complex set of phenotypic alterations in breast epithelial cells and the surrounding microenvironment. While traditional transformation assays provide models for investigating certain aspects of the cellular processes associated with tumor initiation and progression, they do not model alterations in tissue architecture that are critically involved in tumor development. In this review, we provide examples of how three-dimensional (3D) cell culture models can be utilized to dissect the pathways involved in the development of mammary epithelial structures and to elucidate the mechanisms responsible for oncogene-induced phenotypic alterations in epithelial behavior and architecture. Many normal mammary epithelial cell lines undergo a stereotypic morphogenetic process when grown in the presence of exogenous matrix proteins. This 3D morphogenesis culminates in the formation of well-organized, polarized spheroids, and/or tubules that are highly reminiscent of normal glandular architecture. In contrast, transformed cell lines isolated from mammary tumors exhibit significant deviations from normal epithelial behavior in 3D culture. We describe the use of 3D models as a method for both reconstructing and deconstructing the cell biological and biochemical events involved in mammary neoplasia.

Culture of large vessel endothelial cells on floating collagen gels promotes a phenotype characteristic of endothelium in vivo

The vascular endothelium in vivo is a remarkably quiescent cell layer that displays a highly differentiated and tissue-specific phenotype. Once established in culture, endothelial cells (EC) are phenotypically different from their in situ counterparts, displaying altered gene expression, increased mitotic index, and decreased cell density. To determine whether manipulating the microenvironment of cells in vitro would lead to a more differentiated phenotype, we cultured bovine aortic EC on floating collagen gels. EC cultured to confluence on floating gels for 24 or 48 hr display mitotic indices nearly identical to those of quiescent endothelium in vivo, nearly two log orders lower than that of EC cultured to confluence on plastic, and cell density on floating gels also resembles that observed for endothelium in vivo. Culture of EC on floating gels leads to decreased expression of platelet-derived growth factor-B, fibronectin, and fibronectin isoform ED-B, and increased levels of connexin40, relative to cells cultured on plastic. We conclude that culture of bovine aortic EC under standard culture conditions results in a phenotype reminiscent of development and/or wound healing, and that culturing them on a floating collagen gel leads to a more differentiated phenotype, reminiscent of that observed for large vessel EC in vivo.

Tumor reversion: correction of malignant behavior by microenvironmental cues

Cancer is characterized by unrestrained proliferation and loss of organization, a process that is intimately linked to, and controlled by, reciprocal signaling between the genetically altered tumor epithelium, the stroma, the components of the basement membrane and inflammatory mediators. Much work has been done to characterize the genetics of cancer cells. In this review, we describe the experiments that have been performed, which point to the significant role of the tissue microenvironment in the developmental regulation of normal and neoplastic cells. Using a variety of model systems, the works of a number of laboratories have converged on a hypothesis where the correction of 1 or 2 signaling defects can revert tumor cells to a normal phenotype, both in vivo and in culture, even when the tumor cells possess multiple genetic and epigenetic lesions. This paradigm has been successfully used to treat acute promyelocytic leukemia, and it remains the task of biomedical researchers to identify additional targets for the reversion of other human malignancies.

ROCK-generated contractility regulates breast epithelial cell differentiation in response to the physical properties of a three-dimensional collagen matrix

Breast epithelial cells differentiate into tubules when cultured in floating three-dimensional (3D) collagen gels, but not when the cells are cultured in the same collagen matrix that is attached to the culture dish. These observations suggest that the biophysical properties of collagenous matrices regulate epithelial differentiation, but the mechanism by which this occurs is unknown. Tubulogenesis required the contraction of floating collagen gels through Rho and ROCK-mediated contractility. ROCK-mediated contractility diminished Rho activity in a floating 3D collagen gel, and corresponded to a loss of FAK phosphorylated at Y397 localized to 3D matrix adhesions. Increasing the density of floating 3D collagen gels also disrupted tubulogenesis, promoted FAK phosphorylation, and sustained high Rho activity. These data demonstrate the novel finding that breast epithelial cells sense the rigidity or density of their environment via ROCK-mediated contractility and a subsequent down-regulation of Rho and FAK function, which is necessary for breast epithelial tubulogenesis to occur.

Cytoskeletal reorganization dependence of signaling by the gonadotropin-releasing hormone receptor

Activation of classical G protein-coupled receptors (GPCRs) like the mammalian gonadotropin-releasing hormone receptor (GnRHR) typically stimulates heterotrimeric G protein molecules that subsequently activate downstream effectors. Receptor activation of heterotrimeric G protein pathways primarily controls intermediary cell metabolism by elevation or diminution of soluble cytoplasmic second messenger molecules. We have demonstrated here that stimulation of the GnRHR also results in a dramatic change in both cell adhesion and superstructural morphology. Gonadotropin-releasing hormone (GnRH) receptor activation rapidly increases the capacity of HEK293 cells expressing the GnRHR to remain matrix-adherent in the face of fluid insults. Coinciding with this profound elevation in matrix adherence, we demonstrated a GnRH-induced alteration in both cell morphology and the de novo generation of polymerized actin structures. GnRH induction of cytoskeletal remodeling was correlated with significant increases in the tyrosine phosphorylation status of a series of cytoskeletal associated proteins, e.g. focal adhesion kinase (FAK), c-Src, and microtubule-associated protein kinase (MAPK or ERK1/2). The activation of the distal downstream effector ERK1/2 was demonstrated to be sensitive to the disrupters of cytoskeletal rearrangement, cytochalasin D and latrunculin B. In addition to the sensitivity of ERKs to cytoskeletal integrity, GnRH-induced FAK and c-Src kinase activation were sensitive to these agents and the fibronectin-integrin antagonistic RGDS peptide. Activation of ERK was dependent on its protein-protein assembly with FAK and c-Src at focal adhesion complexes. Induction of the cell remodeling event leading to this signaling complex assembly occurred primarily via GnRHR activation of the monomeric G protein Rac but not RhoA. These findings demonstrated a clear divergence of GnRHR signaling via the Rac monomeric G protein focal adhesion signaling complex assembly and cytoskeletal remodeling independent of the classical heterotrimeric G protein-controlled phospholipase C-beta pathway.

Integrin signaling and mammary cell function

The mammary gland is a highly organized tissue, containing ductal structures, secretory alveolar units, and a supporting stroma. The organization of the epithelial cells within the tissue depends upon cell-cell adhesion as well as cell interactions with the extracellular matrix that underlies the epithelial units and makes up most of the organization of the stroma. Adhesion to the extracellular matrix is mediated by a class of heterodimeric transmembrane receptors called integrins, which cluster at focal adhesions. Integrins link the matrix with an intracellular structural scaffold, the cytoskeleton, as well as with signaling enzymes that direct cell survival, proliferation, differentiation, and migration. Two key enzymes that are recruited to sites of integrin clustering are focal adhesion kinase and integrin-linked kinase. Both enzymes are involved with communication downstream of integrins and have key roles in regulating cell behavior. This review will focus on what is known about focal adhesion kinase and integrin-linked kinase signaling and will discuss current evidence about their role in mammary gland biology and neoplasia.

PEA3 transactivates the Muc4/sialomucin complex promoter in mammary epithelial and tumor cells

Sialomucin complex (SMC, rat Muc4) is a heterodimeric glycoprotein composed of two subunits, the mucin component ascites sialoglycoprotein ASGP-1 and the transmembrane subunit ASGP-2, which is aberrantly expressed on the surfaces of a variety of tumor cells. Up-regulation of the Muc4/SMC gene in the 13762 sublines of the rat mammary adenocarcinoma correlates with the overexpression of transcription factor PEA3 and the receptor tyrosine kinase ErbB2. Here we report that PEA3 is capable of transactivating the Muc4/SMC promoter in a dose-dependent manner via direct attachment to a PEA3 binding site. ERM and ER81, the other two members of the PEA3 subfamily of transcription factors, could not transactivate the Muc4/SMC promoter. Transcriptional activation of Muc4/SMC by PEA3 is potentiated by Ras and MEKK1 kinases. These data suggest that expression of PEA3 in mammary tumors leads to up-regulation of Muc4/SMC transcription, the gene product of which may contribute to the metastatic potential of mammary tumors.

The role of collagen structure in mitogen stimulation of ERK, cyclin D1 expression, and G1-S progression in rat hepatocytes

Adhesion to type 1 collagen can elicit different cellular responses dependent upon whether the collagen is in a fibrillar form (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread extensively, express cyclin D1, and increase DNA synthesis in response to epidermal growth factor, whereas hepatocytes adherent to collagen gel have increased differentiated function, but lower DNA synthesis. The signaling mechanisms by which different forms of type I collagen modulate cell cycle progression are unknown. When ERK MAP kinase activation was analyzed in hepatocytes attached to collagen film, two peaks of ERK activity were demonstrated. Only the second peak, which correlated with an increase of cyclin D1, was required for G1-S progression. Notably, this second peak of ERK activity was absent in cells adherent to collagen gel, but not required in the presence of exogenous cyclin D1. Expression of activated mutants of the Ras/Raf/MEK signaling pathway in cells adherent to collagen gel restored ERK phosphorylation and DNA synthesis, but differentially affected cell shape. Although Ras, Raf, and MEK all increased expression of cyclin D1 on collagen film, only Ras and Raf significantly up-regulated cyclin D1 levels on collagen gel. These results demonstrate that adhesion to polymerized collagen induces growth arrest by inhibiting the Ras/ERK-signaling pathway to cyclin D1 required in late G1.

The organizing principle: microenvironmental influences in the normal and malignant breast

The current paradigm for cancer initiation and progression rests on the groundbreaking discoveries of oncogenes and tumor suppressor genes. This framework has revealed much about the role of genetic alterations in the underlying signaling pathways central to normal cellular function and to tumor progression. However, it is clear that single gene theories or even sequential acquisition of mutations underestimate the nature of the genetic and epigenetic changes in tumors, and do not account for the observation that many cancer susceptibility genes (e.g. BRCA1, APC) show a high degree of tissue specificity in their association with neoplastic transformation. Therefore, the cellular and tissue context itself must confer additional and crucial information necessary for mutated genes to exert their influence. A considerable body of evidence now shows that cell-cell and cell-extracellular matrix (ECM) interactions are essential organizing principles that help define the nature of the tissue context, and play a crucial role in regulating homeostasis and tissue specificity. How this context determines functional integrity, and how its loss can lead to malignancy, appears to have much to do with tissue structure and polarity.

Sensing the environment: a historical perspective on integrin signal transduction

Cell adhesion mediated by integrin receptors has a critical function in organizing cells in tissues and in guiding haematopoietic cells to their sites of action. However, integrin adhesion receptors have broader functions in regulating cell behaviour through their ability to transduce bi-directional signals into and out of the cell and to engage in reciprocal interactions with other cellular receptors. This historical perspective traces the key findings that have led to our current understanding of these important functions of integrins.

Enhanced branching morphogenesis in mammary glands of mice lacking cell surface beta1,4-galactosyltransferase

Development of the mammary gland is influenced both by the systemic hormonal environment and locally through cell-cell and cell-extracellular matrix (ECM) interactions. We have previously demonstrated aberrant mammary gland morphogenesis in transgenic mice with elevated levels of the long isoform of beta1,4-galactosyltransferase 1 (GalT), a proportion of which is targeted to the plasma membrane, where it plays a role in cell-ECM interactions. Here, we show that mammary glands of mice lacking the long GalT isoform exhibit a complementary phenotype. Cell-surface GalT activity was reduced by over 60%, but because the short GalT isoform is intact, total GalT activity was reduced only slightly relative to wild type. Mammary glands from long GalT-null mice were characterized by excess branching, and this phenotype was accompanied by altered expression of laminin chains. Laminin alpha1 and alpha3 were reduced 2.4- and 3.0-fold, respectively, while expression of laminin gamma2 was elevated 2.3-fold. The expression and cleavage of laminin gamma2 have been correlated with branching and cell migration, and Western blotting revealed an altered pattern in gamma2 cleavage products in long GalT-null mammary glands. We then examined the expression of metalloproteases that cleave laminins or that have been shown to play a role in mammary gland morphogenesis. Expression of MT1-MMP, a membrane-bound protease that can cleave laminin gamma2, was elevated 5.5-fold in the long GalT-nulls. MMP 7 was also elevated 5.1-fold. Our results suggest that expression of surface GalT is important for the proper regulation of matrix expression and deposition, which in turn regulates the proper branching morphogenesis of the mammary epithelial ductal system.

Differential interactions of specific nuclear factor I isoforms with the glucocorticoid receptor and STAT5 in the cooperative regulation of WAP gene transcription

The distal region (-830 to -720 bp) of the rat whey acidic protein (WAP) gene contains a composite response element (CoRE), which has been demonstrated previously to confer mammary gland-specific and hormonally regulated WAP gene expression. Point mutations in the binding sites for specific transcription factors present within this CoRE have demonstrated the importance of both nuclear factor I (NFI) and STAT5 as well as cooperative interactions with the glucocorticoid receptor (GR) in the regulation of WAP gene expression in the mammary gland of transgenic mice. This study reports the characterization of NFI gene expression during mammary gland development and the identification and cloning of specific NFI isoforms (NFI-A4, NFI-B2, and NFI-X1) from the mouse mammary gland during lactation. Some but not all of these NFI isoforms synergistically activate WAP gene transcription in cooperation with GR and STAT5, as determined using transient cotransfection assays in JEG-3 cells. On both the WAP CoRE and the mouse mammary tumor virus long terminal repeat promoter, the NFI-B isoform preferentially activated gene transcription in cooperation with STAT5A and GR. In contrast, the NFI-A isoform suppressed GR and STAT cooperativity at the WAP CoRE. Finally, unlike their interaction with the NFI consensus binding site in the adenovirus promoter, the DNA-binding specificities of the three NFI isoforms to the palindromic NFI site in the WAP CoRE were not identical, which may partially explain the failure of the NFI-A isoform to cooperate with GR and STAT5A.

Three-dimensional extracellular matrix substrates for cell culture

In summary, the understanding of cell biology will be furthered as cell culture expands from 2-D to 3-D systems. In choosing which substrate, synthetic or biologically derived, is most well suited for a specific application, substrate composition and structure as well as cell type(s) must be carefully considered. In addition, optimization of seeding densities, medium conditions, growth factor supplements, and other culture parameters may be necessary. Finally, cytometric analyses of such 3-D culture systems will require concurrent innovations in 3-D imaging and methods for quantitating cell morphology, phenotype, and function.

PMC42, a novel model for the differentiated human breast

Cultured human breast carcinoma cell lines are important models for investigating the pathogenesis of breast cancer. Their use, however, is limited because of loss of expression of breast-specific markers and the development of a dedifferentiated phenotype after continuous culture. PMC42 is a unique human breast carcinoma line, previously shown to express secretory and myoepithelial markers. We have induced PMC42 cells to form hollow organoids in culture, similar to in vivo breast structures, using a combination of hormones including estrogen, progesterone, dexamethasone, insulin, and prolactin in combination with a permeable extracellular matrix. The organoids comprised polarized cells located around a central lumen. Expression of beta-casein was demonstrated in cells within organoids using reverse transcriptase-polymerase chain reaction, Western blot analysis, and confocal immunofluorescence. In this in vitro system, milk-specific gene expression was induced through hormone and matrix interactions which may be similar to those operating in vivo. PMC42 is a novel model for investigations into the molecular mechanisms of carcinogenesis and differentiation in the human breast.

Modulation of alpha2beta1 integrin changes during mammary gland development by beta-oestradiol

In order to study the role of cell-matrix interactions in mammary gland function, temporal changes in alpha2beta1 integrin, the major receptor for collagen and the influence of beta-oestradiol on its level and distribution in rat mammary gland at different stages of development were studied. The level of alpha2beta1 integrin determined by ELISA, was found to be high during different days of pregnancy, while in the lactating stage, it was significantly reduced. By immunocytochemical analysis, alpha2beta1 integrin was found to be localized towards the luminal side of acinar cells, both in the virgin and midpregnant stage, while it was not detected in the lactating stage. The possible role of hormones in modulating the level of integrin was examined in both in vitro and in vivo experiments using beta-oestradiol. Supplementing beta-oestradiol to isolated mammary epithelial cells from both virgin and lactating glands caused a concentration dependent increase in the incorporation of [35S]methionine into alpha2beta1 integrin associated with the cells. Administration of beta-oestradiol to virgin and lactating glands caused about 1.4-4-fold increase in the level of alpha2 integrin, indicating that upregulation of integrin during pregnancy may be due to oestrogen and as the oestrogen level falls during lactating phase, downregulation of alpha2beta1 integrin occurs. Treatment with beta-oestradiol also resulted in the appearance of alpha2beta1 integrin in the acinar region of the lactating tissue, while in the untreated controls no staining for integrin was seen. These results indicate that oestrogen, apart from directly affecting the cellular activity, can influence mammary tissue function by affecting cell-ECM interactions through the modulation of integrin receptors for matrix proteins.

Division of labor among the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor to signal morphogenesis and beta-casein expression in mammary epithelial cells

Contact of cultured mammary epithelial cells with the basement membrane protein laminin induces multiple responses, including cell shape changes, growth arrest, and, in the presence of prolactin, transcription of the milk protein beta-casein. We sought to identify the specific laminin receptor(s) mediating the multiple cell responses to laminin. Using assays with clonal mammary epithelial cells, we reveal distinct functions for the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor. Signals from laminin for beta-casein expression were inhibited in the presence of function-blocking antibodies against both the alpha6 and beta1 integrin subunits and by the laminin E3 fragment. The alpha6-blocking antibody perturbed signals mediated by the alpha6beta4 integrin, and the beta1-blocking antibody perturbed signals mediated by another integrin, the alpha subunit(s) of which remains to be determined. Neither alpha6- nor beta1-blocking antibodies perturbed the cell shape changes resulting from cell exposure to laminin. However, the E3 laminin fragment and heparin both inhibited cell shape changes induced by laminin, thereby implicating an E3 laminin receptor in this function. These results elucidate the multiplicity of cell-extracellular matrix interactions required to integrate cell structure and signaling and ultimately permit normal cell function.

Control of milk secretion and apoptosis during mammary involution

Lactation depends on regular suckling or milking of the mammary gland. Without this stimulus, milk secretion stops and mammary involution is induced. Involution caused by abrupt cessation of milk removal is characterized by de-differentiation and apoptosis of mammary epithelial cells, the extent and time course of the latter varying between species. Apoptosis is inhibited and milk secretion is restored by re-suckling, if milk stasis is of short duration. Mammary involution and apoptosis also occur during weaning, even in concurrently-pregnant animals when the interval between lactations is restricted, suggesting that tissue remodeling is essential for subsequent lactation. Declining milk production in ruminants after peak lactation is also associated with, and probably results from, net cell loss by apoptosis. Involution and apoptosis are controlled by changes in systemic galactopoietic hormone levels, and by intra-mammary mechanisms responsive to milk removal. Milk stasis precipitated by litter removal or cessation of milking may involve intra-mammary control related to physical distension of the epithelium. Local control of apoptosis in rodents during weaning, and after peak lactation in dairy animals, may be due to the actions of milk-borne survival factors or their inhibitors, and can be manipulated by frequency of milk removal.

Post-transcriptional regulation of a milk membrane protein, the sialomucin complex (Ascites sialoglycoprotein (ASGP)-1/ASGP-2, rat muc4), by transforming growth factor beta

Sialomucin complex (SMC, Rat Muc4) is a heterodimeric glycoprotein complex consisting of a mucin subunit ASGP-1 (ascites sialoglycoprotein-1) and a transmembrane subunit ASGP-2, which can act as a ligand for the receptor tyrosine kinase ErbB2. SMC is highly expressed on the surface of ascites 13762 rat mammary adenocarcinoma cells, approximately 100 times the level in lactating mammary gland and 10(4) times that in virgin mammary gland. SMC is sharply increased at mid-pregnancy in a manner similar to beta-casein. Unlike beta-casein, SMC appears to be regulated post-transcriptionally. Its transcript is present in both virgin and pregnant mammary tissue, and SMC synthesis is induced rapidly in cultured primary mammary epithelial cells from either normal pregnant or virgin rats. SMC protein, but not transcript, levels are significantly reduced when mammary cells are cultured in Matrigel, a reconstituted basement membrane which stimulates casein expression. SMC precursor is synthesized in Matrigel at a 10-fold lower rate. Matrigel has no effect on either the level of SMC or its transcript in cultured 13762 mammary tumor cells. The Matrigel effect on primary mammary and 13762 cells is mimicked by transforming growth factor beta, a component associated with this complex matrix. These results indicate that SMC is a novel product of normal mammary gland and milk, which is post-transcriptionally regulated by transforming growth factor beta in normal mammary gland, but not in 13762 mammary adenocarcinoma cells.

Expression of autoactivated stromelysin-1 in mammary glands of transgenic mice leads to a reactive stroma during early development

Extracellular matrix and extracellular matrix-degrading matrix metalloproteinases play a key role in interactions between the epithelium and the mesenchyme during mammary gland development and disease. In patients with breast cancer, the mammary mesenchyme undergoes a stromal reaction, the etiology of which is unknown. We previously showed that targeting of an autoactivating mutant of the matrix metalloproteinase stromelysin-1 to mammary epithelia of transgenic mice resulted in reduced mammary function during pregnancy and development of preneoplastic and neoplastic lesions. Here we examine the cascade of alterations before breast tumor formation in the mammary gland stroma once the expression of the stromelysin-1 transgene commences. Beginning in postpubertal virgin animals, low levels of transgene expression in mammary epithelia led to increased expression of endogenous stromelysin-1 in stromal fibroblasts and up-regulation of other matrix metalloproteinases, without basement membrane disruption. These changes were accompanied by the progressive development of a compensatory reactive stroma, characterized by increased collagen content and vascularization in glands from virgin mice. This remodeling of the gland affected epithelial-mesenchymal communication as indicated by inappropriate expression of tenascin-C starting by day 6 of pregnancy. This, together with increased transgene expression, led to basement membrane disruption starting by day 15 of pregnancy. We propose that the highly reactive stroma provides a prelude to breast epithelial tumors observed in these animals.

Control of normal mammary epithelial phenotype by integrins

Mammary epithelial cells contact a specialized extracellular matrix in vivo known as the basement membrane. Interactions with extracellular matrix are mediated through integrins. These cell surface receptors are involved with the formation of adhesion complexes, which link the extracellular matrix with the actin-based cytoskeleton, and are also associated with components of growth factor signaling pathways. Differentiation of breast epithelia into lactational cells requires appropriate hormones and integrin-mediated interactions with basement membrane. Integrins may regulate the ability of lactogenic hormones to trigger their intracellular signaling pathways.

Effect of substratum on growth, cell morphology and lactoferrin synthesis and secretion in bovine mammary cell culture

The role of extracellular matrix in morphology, growth and lactoferrin synthesis and secretion in bovine mammary cells from a developing gland is poorly defined. In this study, bovine mammary cells from a hormone-primed developing gland were isolated and cultured on plastic, collagen, embedded within collagen, or on EHS-matrix, with the hormones prolactin, insulin, and cortisol in the presence or absence of fetal calf serum. Mammary cells on plastic or collagen spread and formed confluent cells sheets, while those embedded within collagen or on EHS-matrix maintained their acinar-like structure. Histological and ultrastructural analysis of cells showed that cells on plastic and collagen grew in multilayers, while those embedded within collagen or on EHS-matrix lacked any lumen structure. The ultrastructure of cells on different substrata more resembled an undifferentiated phenotype. Mammary cells secreted lactoferrin in increasing concentrations throughout the culture period. The total amount secreted in culture was regulated by extracellular matrix and fetal calf serum. Cells embedded within collagen in serum-free cultures secreted the lowest amounts of lactoferrin (up to 619 ng/ml; day 14), while those on collagen and supplemented with fetal calf serum secreted up to 4920 ng/ml at day 14. Fetal calf serum induced higher lactoferrin secretion within each substratum on which the cells were cultured. No intracellular accumulation of lactoferrin was noted in cells on plastic or collagen or those embedded within collagen, whereas those on EHS-matrix accumulated more than 500 ng/ml of lactoferrin intracellularly/intracinarly. Furthermore, when cultured on a similar substratum, cells from a developing gland secreted higher lactoferrin than cells from a lactating gland.

Misregulation of stromelysin-1 expression in mouse mammary tumor cells accompanies acquisition of stromelysin-1-dependent invasive properties

Stromelysin-1 is a member of the metalloproteinase family of extracellular matrix-degrading enzymes that regulates tissue remodeling. We previously established a transgenic mouse model in which rat stromelysin-1 targeted to the mammary gland augmented expression of endogenous stromelysin-1, disrupted functional differentiation, and induced mammary tumors. A cell line generated from an adenocarcinoma in one of these animals and a previously described mammary tumor cell line generated in culture readily invaded both a reconstituted basement membrane and type I collagen gels, whereas a nonmalignant, functionally normal epithelial cell line did not. Invasion of Matrigel by tumor cells was largely abolished by metalloproteinase inhibitors, but not by inhibitors of other proteinase families. Inhibition experiments with antisense oligodeoxynucleotides revealed that Matrigel invasion of both cell lines was critically dependent on stromelysin-1 expression. Invasion of collagen, on the other hand, was reduced by only 40-50%. Stromelysin-1 was expressed in both malignant and nonmalignant cells grown on plastic substrata. Its expression was completely inhibited in nonmalignant cells, but up-regulated in tumor cells, in response to Matrigel. Thus misregulation of stromelysin-1 expression appears to be an important aspect of mammary tumor cell progression to an invasive phenotype.

Influence of several extracellular matrix components in primary cultures of bovine mammary epithelial cells

Mammary epithelial cells, obtained from lactating cows, were cultured onto inserts coated with several components of extracellular matrix. The influence of these components upon the maintenance of differentiation has been determinated. Every day, alpha S1-casein secretion was measured by radioimmunoassay (RIA) in apical and basal compartments. Reorganization of functional tight junctions was evaluated by measurement of transepithelial electrical resistance (TER). On EHS matrix, cells underwent alveolar structures and never established TER. alpha S1-casein secretion strongly fluctuated with the day of culture. When plated onto fibronectin, cells reorganized a typical pavement and established TER. Nevertheless, TER and casein secretion highly fluctuated. On laminin-coated inserts, a few cells bound to the substratum, dedifferentiated, and proliferated to confluency within 9 days. TER progressively increased to a stable level after 15 days. Casein was not recovered after 6 days. Cells on type I collagen-coated inserts reorganized an epithelial pavement within 2 days and quickly established a stable TER. They secreted apically high levels of casein during 2 weeks. As cells maintained their biochemical differentiation, the culture on type I collagen-coated inserts seems an efficient model for primary culture of bovine mammary epithelial cells and allows studies of polarized alpha S1-casein secretion.

Expression of a 74-kDa nuclear factor 1 (NF1) protein is induced in mouse mammary gland involution. Involution-enhanced occupation of a twin NF1 binding element in the testosterone-repressed prostate message-2/clusterin promoter

Testosterone repressed prostate message-2 (TRPM-2)/clusterin gene expression is rapidly induced in early involution of the mouse mammary gland, after weaning, and in the rat ventral prostate, after castration. A search for involution-enhanced DNaseI footprints in the proximal mouse TRPM-2/clusterin gene promoter led to the identification and characterization (by DNase I footprinting and EMSA) of a twin nuclear factor 1 (NF1) binding element at -356/-309, relative to the proposed transcription start site; nuclear extracts from 2-day involuting mouse mammary gland showed an enhanced footprint over the proximal NF1 element; extracts from involuting prostate showed enhanced occupancy of both NF1 binding elements. Subsequent EMSA and Western analysis led to the detection of a 74-kDa NF1 protein whose expression is triggered in early involution in the mouse mammary gland; such an induced protein is not found in the involuting rat ventral prostate. This protein was not found in lactation where three other NF1 proteins of 114, 68, and 46 kDa were detected. Reiteration of the epithelial cell apoptosis associated with early mammary gland involution, in vitro, in a primary cell culture system, triggered the appearance of the 74-kDa NF1. Overlaying the cells with laminin-rich extracellular matrix suppressed the apoptosis and the expression of the 74-kDa NF1 and, in the presence of lactogenic hormones, initiated milk protein gene expression and the expression of two of the lactation-associated NF1 proteins (68 and 46 kDa). This study, thus, identifies for the first time the occurrence of a switch in expression of different members of the family of NF1 transcription factors as mammary epithelial cells move from the differentiated to the involution/apoptotic state, and it is likely that the involution-specific 74-kDa NF1 accounts for the enhanced NF1 footprint detected on the TRPM-2/clusterin promoter with extracts of mouse mammary gland.

Mammary gland morphogenesis is inhibited in transgenic mice that overexpress cell surface beta1,4-galactosyltransferase

Mammary gland morphogenesis is facilitated by a precise sequence of cell-cell and cell-matrix interactions, which are mediated in part through a variety of cell surface receptors and their ligands (Boudreau, N., Myers, C. and Bissell, M. J. (1995). Trends in Cell Biology 5, 1–4). Cell surface beta1,4-galactosyltransferase (GalTase) is one receptor that participates in a variety of cell-cell and cell-matrix interactions during fertilization and development, including mammary epithelial cell-matrix interactions (Barcellos-Hoff, M. H. (1992). Exp. Cell Res. 201, 225–234). To analyze GalTase function during mammary gland morphogenesis in vivo, we created transgenic animals that overexpress the long isoform of GalTase under the control of a heterologous promoter. As expected, mammary epithelial cells from transgenic animals had 2.3 times more GalTase activity on their cell surface than did wild-type cells. Homozygous transgenic females from multiple independent lines failed to lactate, whereas transgenic mice overexpressing the Golgi-localized short isoform of GalTase lactated normally. Glands from transgenic females overexpressing surface GalTase were characterized by abnormal and reduced ductal development with a concomitant reduction in alveolar expansion during pregnancy. The phenotype was not due to a defect in proliferation, since the mitotic index for transgenic and wild-type glands was similar. Morphological changes were accompanied by a dramatic reduction in the expression of milk-specific proteins. Immunohistochemical markers for epithelia and myoepithelia demonstrated that both cell types were present. To better understand how overexpression of surface GalTase impairs ductal morphogenesis, primary mammary epithelial cultures were established on basement membranes. Cultures derived from transgenic mammary glands were unable to form anastomosing networks of epithelial cells and failed to express milk-specific proteins, unlike wild-type mammary cultures that formed epithelial tubules and expressed milk proteins. Our results suggest that cell surface GalTase is an important mediator of mammary cell interaction with the extracellular matrix. Furthermore, perturbing surface GalTase levels inhibits the expression of mammary-specific gene products, implicating GalTase as a component of a receptor-mediated signal transduction pathway required for normal mammary gland differentiation.

Three-dimensional mammary primary culture model systems

Model systems have been developed to investigate the complex and coordinated regulation of mammary gland development and transformation. Primary cultures, using newly isolated cells or tissue, are optimal for such studies since, in comparison to immortalized cell lines, the normal signal transduction pathways are presumed to be intact. Three such models are described, including whole organ culture, mammary epithelial cell (MEC) organoids, and MEC-stromal cocultures. Studies using whole-organ culture have the advantage that the normal glandular architecture remains intact, the MEC can undergo lobuloalveolar development and express milk proteins in a hormone dependent manner, and, following hormonal withdrawal, undergo involution. Moreover, transformation of the MEC is readily accomplished. Culture of isolated MEC organoids within an EHS-derived reconstituted basement membrane permits extensive proliferation, branching end bud and alveolar morphogenesis, and accumulation of milk protein and lipid in a physiologically relevant hormone- and growth factor-dependent manner. This model can thus be utilized to investigate the mechanism by which various modulators exert their direct effects on the epithelium. Finally, in view of compelling evidence for stromal-epithelial interactions during normal mammary gland development, and potentially also during the development of malignancy, models in which MEC can be cocultured with enriched populations of stroma offer considerable potential as a tool to understand the nature and mechanisms of the interactions that occur during the various developmental states, and how such interactions may go awry during carcinogenesis.

Self-complementary oligopeptide matrices support mammalian cell attachment

A new class of ionic self-complementary oligopeptides is described, two members of which have been designated RAD16 and EAK16. These oligopeptides consist of regular repeats of alternating ionic hydrophilic and hydrophobic amino acids and associate to form stable beta-sheet structures in water. The addition of buffers containing millimolar amounts of monovalent salts or the transfer of a peptide solution into physiological solutions results in the spontaneous assembly of the oligopeptides into a stable, macroscopic membranous matrix. The matrix is composed of ordered filaments which form porous enclosures. A variety of mammalian cell types are able to attach to both RAD16 and EAK16 membranous matrices. These matrices provide a novel experimental system for analysing mechanisms of in vitro cell attachment and may have applications in in vivo studies of tissue regeneration, tissue transplantation and would healing.

The spatial and temporal expression of the alpha 2 beta 1 integrin and its ligands, collagen I, collagen IV, and laminin, suggest important roles in mouse mammary morphogenesis

To begin to determine the role of the alpha 2 beta 1 integrin and its ligands, collagen I, collagen IV, and laminin, in mammary epithelial differentiation in vivo, we determined the expression of these molecules by in situ hybridization and immunofluorescence in the developing mouse mammary gland. Expression of collagen I, collagen IV, and laminin mRNAs in the mammary gland during puberty corresponded to the period of greatest growth of the gland, 4-7 weeks postnatally. Collagen I expression preceded collagen IV expression, both of which preceded laminin expression, suggesting an important temporal sequence of extracellular matrix (ECM) production. When growth of the epithelium ceased in the adult virgin gland, expression of all three mRNAs became undetectable. Following the onset of pregnancy these molecules were re-expressed with the same chronology observed during puberty. Collagen I, collagen IV, and laminin were expressed by stromal cells immediately surrounding the developing ductal epithelium. Surprisingly, we found no expression of ECM components in the epithelial cells, suggesting the mammary epithelium does not synthesize its own basement membrane. The distribution of collagen I was consistent with a role in duct formation, since collagen I was strikingly abundant around larger mammary ducts, but was sparse around growing endbuds or alveoli. Conversely, there was abundant laminin near growing endbuds and around alveoli, and less around large ducts, suggesting its role is different than collagen I. The alpha 2 beta 1 integrin was present on the basal, lateral, and apical surfaces of the mammary epithelium throughout postnatal development and pregnancy. The alpha 2 beta 1 integrin expression was strongest at midpregnancy, suggesting a role for alpha 2 beta 1 integrin in the alveolar formation that occurs at this time. The alpha 2 beta 1 integrin expression decreased dramatically in the lactating gland. Our results suggest that alpha 2 beta 1 integrin interactions with its temporally and spatially regulated ligands, collagen I, collagen IV, and laminin, could play an important role in mammary morphogenesis in vivo.

Extracellular matrix regulates whey acidic protein gene expression by suppression of TGF-alpha in mouse mammary epithelial cells: studies in culture and in transgenic mice

Whey acidic protein (WAP) is an abundant rodent milk protein. Its expression in mouse mammary epithelial cell cultures was previously found to require the formation of an extracellular matrix (ECM)-induced three-dimensional alveolar structure. In the absence of such structures, cells were shown to secrete diffusible factors leading to suppression of WAP expression. We demonstrate here that (a) TGF-alpha production and secretion by mammary cells is downregulated by the basement membrane-dependent alveolar structure, and (b) compared with beta-casein, WAP expression is preferentially inhibited both in culture and in transgenic mice when TGF-alpha is added or overexpressed. Thus, (c) the enhanced TGF-alpha production when cells are not in three-dimensional structures largely accounts for the WAP-inhibitory activity found in the conditioned medium. Since this activity can be abolished by incubating the conditioned medium with a function blocking antibody to TGF-alpha. The data suggest that ECM upregulates WAP by downregulating TGF-alpha production. We also propose that changes in TGF-alpha activity during mouse gestation and lactation could contribute to the pattern of temporal expression of WAP in the gland. These results provide a clear example of cooperation among lactogenic hormones, ECM, and locally acting growth factors in regulation of tissue-specific gene expression.