Extracellular matrix regulates ovarian hormone-dependent proliferation of mouse mammary epithelial cells

Generation of Tailored Extracellular Matrix Hydrogels for the Study of In Vitro Folliculogenesis in Response to Matrisome-Dependent Biochemical Cues

While ovarian tissue cryopreservation (OTC) is an important fertility preservation option, it has its limitations. Improving OTC and ovarian tissue transplantation (OTT) must include extending the function of reimplanted tissue by reducing the extensive activation of primordial follicles (PMFs) and eliminating the risk of reimplanting malignant cells. To develop a more effective OTT, we must understand the effects of the ovarian microenvironment on folliculogenesis. Here, we describe a method for producing decellularized extracellular matrix (dECM) hydrogels that reflect the protein composition of the ovary. These ovarian dECM hydrogels were engineered to assess the effects of ECM on in vitro follicle growth, and we developed a novel method for selectively removing proteins of interest from dECM hydrogels. Finally, we validated the depletion of these proteins and successfully cultured murine follicles encapsulated in the compartment-specific ovarian dECM hydrogels and these same hydrogels depleted of EMILIN1. These are the first, optically clear, tailored tissue-specific hydrogels that support follicle survival and growth comparable to the "gold standard" alginate hydrogels. Furthermore, depleted hydrogels can serve as a novel tool for many tissue types to evaluate the impact of specific ECM proteins on cellular and molecular behavior.

Quercetin improves postpartum hypogalactia in milk-deficient mice via stimulating prolactin production in pituitary gland

Postpartum dysgalactia is a common clinical problem for lactating women. Seeking out the safe and efficient phytoestrogens will be a promising strategy for postpartum dysgalactia therapy. In this study, the postpartum mice within four groups, including control group, the model group, and the treatment groups intragastrically administrated with normal saline, bromocriptine, bromocriptine plus 17α-ethinyl estradiol, and bromocriptine plus quercetin, respectively, were used. The results showed that quercetin, a kind of natural phytoestrogen, could efficiently promote lactation yield and mammary gland development in the agalactosis mice produced by bromocriptine administration. Mechanically, quercetin, such as 17α-ethinyl estradiol, significantly stimulated prolactin (PRL) production and deposition in the mammary gland in the agalactosis mice determined by western blotting, quantitative polymerase chain reaction, and enzyme-linked immunosorbent assay, respectively. Furthermore, quercetin could increase the expression of β-casein, stearoyl-CoA desaturase, fatty acid synthase, and α-lactalbumin in the breast tissues that are responsible for the production of fatty acid, lactose, and galactose in the milk at the transcriptional level determined by quantitative polymerase chain reaction. Specifically, quercetin promoted primary mammary epithelial cell proliferation and stimulated prolactin receptor (PRLR) expression probably via AKT activation in vitro. In conclusion, this study indicates that estrogen-like quercetin promotes mammary gland development and lactation yield in milk-deficient mice, probably via stimulating PRL expression and release from the pituitary gland, as well as induces PRLR expression in primary mammary epithelial cells.

Mammary gland development--It's not just about estrogen

The mammary gland (MG) is one of a few organs that undergoes most of its growth after birth. Much of this development occurs concurrently with specific reproductive states, such that the ultimate goal of milk synthesis and secretion is coordinated with the nutritional requirements of the neonate. Central to the reproductive-MG axis is its endocrine regulation, and pivotal to this regulation is the ovarian secretion of estrogen (E). Indeed, it is widely accepted that estrogens are essential for growth of the MG to occur, both for ductal elongation during puberty and for alveolar development during gestation. As the factors regulating MG development continually come to light from the fields of developmental biology, lactation physiology, and breast cancer research, a growing body of evidence serves as a reminder that the MG are not as exclusively dependent on estrogens as might have been thought. The objective of this review is to summarize the state of information regarding our understanding of how estrogen (E) has been implicated as the key regulator of MG development, and to highlight some of the alternative E-independent mechanisms that have been discovered. In particular, we review our findings that dietary trans-10,cis-12 conjugated linoleic acid promotes ductal elongation and that the combination of progesterone (P) and prolactin (PRL) can stimulate branching morphogenesis in the absence of E. Ultimately, these examples stand as a healthy challenge to the question of just how important estrogens are for MG development. Answers to this question, in turn, increase our understanding of MG development across all mammals and the ways in which it can affect milk production.

A Progesterone-CXCR4 Axis Controls Mammary Progenitor Cell Fate in the Adult Gland

Progesterone drives mammary stem and progenitor cell dynamics through paracrine mechanisms that are currently not well understood. Here, we demonstrate that CXCR4, the receptor for stromal-derived factor 1 (SDF-1; CXC12), is a crucial instructor of hormone-induced mammary stem and progenitor cell function. Progesterone elicits specific changes in the transcriptome of basal and luminal mammary epithelial populations, where CXCL12 and CXCR4 represent a putative ligand-receptor pair. In situ, CXCL12 localizes to progesterone-receptor-positive luminal cells, whereas CXCR4 is induced in both basal and luminal compartments in a progesterone-dependent manner. Pharmacological inhibition of CXCR4 signaling abrogates progesterone-directed expansion of basal (CD24⁺CD49f^hi) and luminal (CD24⁺CD49f^lo) subsets. This is accompanied by a marked reduction in CD49b⁺SCA-1⁻ luminal progenitors, their functional capacity, and lobuloalveologenesis. These findings uncover CXCL12 and CXCR4 as novel paracrine effectors of hormone signaling in the adult mammary gland, and present a new avenue for potentially targeting progenitor cell growth and malignant transformation in breast cancer.

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Progesterone induces distinct molecular programs in mammary cell compartments

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CXCR4 induction occurs in lobuloalveoli and is progesterone dependent

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CXCR4 inhibition abrogates luminal progenitor expansion and mammopoiesis

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Targeting of the CXCL12-CXCR4 axis may limit mammary progenitor cell transformation

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

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

A potential role of progestin-induced laminin-5/α6-integrin signaling in the formation of side branches in the mammary gland

Mammary organoids from adult mice produce tubules, analogous to mammary ducts in vivo, in response to hepatocyte growth factor (HGF) when cultured in collagen gels. The combination of HGF plus progestin (R5020) causes reduced tubule number and length. We hypothesized that the inhibitory effect on tubulogenesis was due to progestin-mediated alteration of HGF/c-Met signaling. Using molecular inhibitors and short hairpin RNA, it was determined that HGF activation of Ras-related C3 botulinum toxin substrate (Rac1) was required for the formation of cytoplasmic extensions, the first step of tubulogenesis, and that Rac1 activity was Src kinase (Src) and focal adhesion kinase (FAK) dependent. The highly novel finding was that R5020 reduced tubulogenesis by up-regulating and increasing extracellular laminin and α6-integrin ligation to reduce activation of the Src, focal adhesion kinase, and Rac1 pathway. Receptor activator of nuclear factor-κB ligand, another progesterone-induced paracrine factor, did not replicate this effect of R5020. The inhibitory effect of R5020 on tubulogenesis was likely mediated through progesterone receptor (PR) isoform A (PRA), because PRA is the predominant PR isoform expressed in the organoids, and the progestin-induced effect was prevented by the PR antagonist RU486. These results provide a plausible mechanism that explains progestin/PRA-mediated blunting of HGF-induced tubulogenesis in vitro and is proposed to be relevant to progesterone/PRA-induced side-branching in vivo during pregnancy.

TGF-beta biology in mammary development and breast cancer

Transforming growth factor-β1 (TGF-β) was first implicated in mammary epithelial development by Daniel and Silberstein in 1987 and in breast cancer cells and hormone resistance by Lippman and colleagues in 1988. TGF-β is critically important for mammary morphogenesis and secretory function through specific regulation of epithelial proliferation, apoptosis, and extracellular matrix. Differential TGF-β effects on distinct cell types are compounded by regulation at multiple levels and the influence of context on cellular responses. Studies using controlled expression and conditional-deletion mouse models underscore the complexity of TGF-β biology across the cycle of mammary development and differentiation. Early loss of TGF-β growth regulation in breast cancer evolves into fundamental deregulation that mediates cell interactions and phenotypes driving invasive disease. Two outstanding issues are to understand the mechanisms of biological control in situ and the circumstances by which TGF-β regulation is subverted in neoplastic progression.

The contribution of adhesion signaling to lactogenesis

The mammary gland undergoes hormonally controlled cycles of pubertal maturation, pregnancy, lactation, and involution, and these processes rely on complex signaling mechanisms, many of which are controlled by cell-cell and cell-matrix adhesion. The adhesion of epithelial cells to the extracellular matrix initiates signaling mechanisms that have an impact on cell proliferation, survival, and differentiation throughout lactation. The control of integrin expression on the mammary epithelial cells, the composition of the extracellular matrix and the presence of secreted matricellular proteins all contribute to essential adhesion signaling during lactogenesis. In vitro and in vivo studies, including the results from genetically engineered mice, have shed light on the regulation of these processes at the cell and tissue level and have led to increased understanding of the essential signaling components that are regulated in temporal and cell specific manner during lactogenesis. Recent studies suggest that a secreted matricellular protein, CTGF/CCN2, may play a role in lactogenic differentiation through binding to β1 integrin complexes, enhancing the production of extracellular matrix components and contributions to cell adhesion signaling.

Isolation of mammary-specific extracellular matrix to assess acute cell-ECM interactions in 3D culture

Studies of mammary epithelial cells (MECs) cultured with reconstituted basement membrane proteins derived from EHS tumors have contributed greatly to the understanding of both normal physiology and transformation. Only when plated on such biologically relevant substratum are MECs able to form morphologically correct, differentiated structures, highlighting a critical role for extracellular matrix (ECM) proteins in MEC organization and function. Here, we describe methods modified from the original EHS matrix protocol for isolating tissue-specific ECM from rat mammary glands, and for subsequent use in short-term 3D cell culture models designed to assess acute cell-ECM interactions. Using this protocol, the final matrix is enriched up to 58-fold for ECM proteins such as fibronectin and laminin, while cellular proteins such as GAPDH are reduced 98-fold. We have previously shown that MECs plated in mammary-specific ECM form more elaborate duct-like and alveolar-like structures compared to MECs plated in Matrigel™, demonstrating the biological relevance of tissue-specific ECM. Use of mammary-specific ECM in 3D cell culture models will further our ability to study the intricate interplay between a cell and its microenvironment, and permit identification of modifying factors.

Bidirectional extracellular matrix signaling during tissue morphogenesis

Normal tissue development and function are regulated by the interplay between cells and their surrounding extracellular matrix (ECM). The ECM provides biochemical and mechanical contextual information that is conveyed from the cell membrane through the cytoskeleton to the nucleus to direct cell phenotype. Cells, in turn, remodel the ECM and thereby sculpt their local microenvironment. Here we review the mechanisms by which cells interact with, respond to, and influence the ECM, with particular emphasis placed on the role of this bidirectional communication during tissue morphogenesis. We also discuss the implications for successful engineering of functional tissues ex vivo.

Loss of TGF-beta or Wnt5a results in an increase in Wnt/beta-catenin activity and redirects mammary tumour phenotype

Introduction

The tumour-suppressive effects of transforming growth factor-beta (TGF-β) are well documented; however, the mechanistic basis of these effects is not fully understood. Previously, we showed that a non-canonical member of the Wingless-related protein family, Wnt5a, is required for TGF-β-mediated effects on mammary development. Several lines of evidence support the hypothesis that Wnt5a acts as a tumour suppressor. In addition, it has been shown that Wnt5a can antagonise canonical Wnt/β-catenin signalling in various cell types. Here we test the hypothesis that TGF-β and Wnt5a can antagonise Wnt/β-catenin signalling and redirect mammary tumour phenotype. The results provide a new mechanism for the tumour-suppressive effects of TGF-β.

Methods

Wnt/β-catenin signalling was measured in tumours with altered TGF-β (dominant-negative TGF-β type II receptor, DNIIR) or Wnt5a (Wnt5a^-/-) signalling as the accumulation of nuclear β-catenin using both confocal microscopy and cell fractionation. RT-PCR was used to measure the expression of Wnt/β-catenin target genes. Sca1 expression was determined by western blot and keratin (K) 6- and K14-positive populations were determined by immunohistochemistry.

Results

Loss of TGF-β or Wnt5a signalling resulted in stabilisation of nuclear β-catenin and expression of Wnt/β-catenin target genes suggesting that TGF-β and Wnt5a act to inhibit Wnt/β-catenin signalling in mammary epithelium. Increased expression of Sca-1 was observed in developing DNIIR and Wnt5a-/- mammary glands. DNIIR and Wnt5a-/- tumours demonstrated an expanded population of K6- and K14-expressing cells typically seen in Wnt/β-catenin-induced tumours.

Conclusions

The key findings here are that: TGF-β and Wnt5a regulate Wnt/β-catenin activity; and loss of TGF-β and Wnt5a redirect the phenotype of tumours so that they resemble tumours induced by activation of Wnt/β-catenin. The findings suggest a new mechanism for the tumour-suppressive effects of TGF-β.

Tamoxifen induces pleiotrophic changes in mammary stroma resulting in extracellular matrix that suppresses transformed phenotypes

INTRODUCTION

The functional unit of the mammary gland has been defined as the epithelial cell plus its microenvironment, a hypothesis that predicts changes in epithelial cell function will be accompanied by concurrent changes in mammary stroma. To test this hypothesis, the question was addressed of whether mammary stroma is functionally altered by the anti-oestrogen drug tamoxifen.

METHODS

Forty female rats at 70 days of age were randomised to two groups of 20 and treated with 1.0 mg/kg tamoxifen or vehicle subcutaneously daily for 30 days, followed by a three-day wash out period. Mammary tissue was harvested and effects of tamoxifen on mammary epithelium and stroma determined.

RESULTS

As expected, tamoxifen suppressed mammary alveolar development and mammary epithelial cell proliferation. Primary mammary fibroblasts isolated from tamoxifen-treated rats displayed a three-fold decrease in motility and incorporated less fibronectin in their substratum in comparison to control fibroblasts; attributes indicative of fibroblast quiescence. Immunohistochemistry analysis of CD68, a macrophage lysosomal marker, demonstrated a reduction in macrophage infiltration in mammary glands of tamoxifen-treated rats. Proteomic analyses by mass spectrometry identified several extracellular matrix (ECM) proteins with expression levels with tamoxifen treatment that were validated by Western blot. Mammary tissue from tamoxifen-treated rats had decreased fibronectin and increased collagen 1 levels. Further, ECM proteolysis was reduced in tamoxifen-treated rats as detected by reductions in fibronectin, laminin 1, laminin 5 and collagen 1 cleavage fragments. Consistent with suppression in ECM proteolysis with tamoxifen treatment, matrix metalloproteinase-2 levels and activity were decreased. Biochemically extracted mammary ECM from tamoxifen-treated rats suppressed in vitro macrophage motility, which was rescued by the addition of proteolysed collagen or fibronectin. Mammary ECM from tamoxifen-treated rats also suppressed breast tumour cell motility, invasion and haptotaxis, reduced organoid size in 3-dimensional culture and blocked tumour promotion in an orthotopic xenograft model; effects which could be partially reversed by the addition of exogenous fibronectin.

CONCLUSIONS

These data support the hypothesis that mammary stroma responds to tamoxifen treatment in concert with the epithelium and remodels to a microenvironment inhibitory to tumour cell progression. Reduced fibronectin levels and reduced ECM turnover appear to be hallmarks of the quiescent mammary microenvironment. These data may provide insight into attributes of a mammary microenvironment that facilitate tumour dormancy.

Estradiol and tamoxifen differently affects the inhibitory effects of vitamin A and their metabolites on the proliferation and expression of alpha2beta1 integrins in MCF-7 breast cancer cells

PURPOSE

Retinoids are well known inhibitors of estrogen-dependent breast cancer cell growth and differentiation. alpha2beta1 integrins are involved in the normal growth and differentiation of breast cells, they also take part in many pathological processes including malignancies. The aim of the study was to evaluate the effect of estradiol and tamoxifen on the inhibitory action of retinoids on the proliferation of MCF-7 breast cancer cells and alpha2beta1 integrin expression.

MATERIALS AND METHODS

Evaluation was based on [3H]thymidine incorporation and the proliferative activity of PCNA- and Ki 67-positive cells. Expression of alpha2beta1 was assessed through immunocytochemical analysis.

RESULTS

Treatment of cancer cells with the examined compounds and tamoxifen (10 microM) revealed that only 13-cis retinoic acid (13-cis RA) and all-trans retinoic acid (ATRA) (10(-5) M) decreased cells proliferation compared to the tamoxifen group (30.84%+/-3.32, p<0.01 and 31.05%+/-4.67, p<0.01, respectively). The lowest fraction of PCNA positive cells was also observed after the simultaneous addition ATRA (10(-5) M) and tamoxifen (10 microM) (30.75%+/-0.95, p<0.01, compared to the tamoxifen group). Our results showed that the decrease of alpha2beta1 integrin expression by 13-cis RA (10(-5) M, 49.6+/-3.25%) and ATRA (10-9 M, 15.0%+/-5.0) was augmented by tamoxifen and to a lesser extent by estradiol, particularly in the case of ATRA at 10(-7) or 10(-9) M.

CONCLUSIONS

This data suggest that tamoxifen augments the inhibitory effect of retinoids on proliferation and alpha2beta1 integrin expression in MCF-7 cells.

KIBRA interacts with discoidin domain receptor 1 to modulate collagen-induced signalling

Mammary gland development is coupled to reproductive events by hormonal cues of ovarian and pituitary origin, which activate a genomic regulatory network. Identification of the components and regulatory links that comprise this network will provide the basis for defining the network's dynamic response during normal development and its perturbation during breast carcinogenesis. In this study KIBRA was identified as a transcript showing decreased expression associated with failed mammary gland development in Prlr knockout mammary epithelium. It is strongly up-regulated during pregnancy, falls during lactation and is again up-regulated during involution of the gland at weaning. A bioinformatic approach was undertaken to identify potential binding partners which interact with the WW domains of KIBRA. We show that KIBRA binds to a WW domain binding motif, PPxY, in the tyrosine kinase receptor DDR1, and dissociates upon treatment with the DDR1 ligands collagen type I or IV. In addition we show that KIBRA and DDR1 also interact with PKCz to form a trimeric complex. Finally, overexpression and knockdown studies demonstrate that KIBRA promotes the collagen-stimulated activation of the MAPK cascade. Thus KIBRA may play a role in how the reproductive state influences the mammary epithelial cell to respond to changing cell-context information, such as experienced during the tissue remodeling events of mammary gland development.

Cyclin-dependent kinase inhibitors and basement membrane interact to regulate breast epithelial cell differentiation and acinar morphogenesis

OBJECTIVE

The cyclin-dependent kinase inhibitors (CDKIs), p21(CIP1) and p27(KIP1) regulate growth and differentiation in diverse tissue types. We aimed to determine whether p21(CIP1) or p27(KIP1) could induce a terminally differentiated phenotype in breast cells, and to examine if CDKI expression is regulated by basement membrane interactions.

MATERIALS AND METHODS

Effects of increased CDKI expression on the phenotype of MCF-10A breast epithelial cells were examined by retroviral transduction of p21(CIP1) or p27(KIP1) cDNA.

RESULTS

Overexpression of p21(CIP1) or p27(KIP1) reduced MCF-10A growth rates in monolayer cultures, altered cellular morphology and stimulated accumulation of neutral lipid droplets, suggesting partial lactational differentiation. However, markers of luminal differentiation (oestrogen and progesterone receptors, alpha-lactalbumin, beta-casein and adipophilin) were absent when examined by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Cell-basement membrane contacts are known to be essential for full mammary epithelial cell differentiation and therefore parental MCF-10A cells were cultured on a basement membrane preparation (Matrigel) in which they form acini. Immunocytochemistry showed that Ki67, the cell proliferation marker, was initially expressed at high levels and as growth decreased p27(KIP1) expression steadily increased. Surprisingly, p21(CIP1) was highest at the early stages of acinus growth and was detected in proliferating cells, as demonstrated by colocalization in dual Ki67/p21(CIP1) immunofluorescence. Overexpression of p21(CIP1) or p27(KIP1) impaired formation of acini, whereas their knockdown, using siRNA, increased acinus formation.

CONCLUSION

We conclude that both p21(CIP1) and p27(KIP1) induce partial secretory differentiation of mammary cells in monolayer, but during acinus morphogenesis in 3D culture they have a highly regulated temporal expression pattern.

Wnt5a is required for proper mammary gland development and TGF-beta-mediated inhibition of ductal growth

Transforming growth factor-beta (TGF-beta) plays an essential role in growth and patterning of the mammary gland, and alterations in its signaling have been shown to illicit biphasic effects on tumor progression and metastasis. We demonstrate in mice that TGF-beta (Tgfbeta) regulates the expression of a non-canonical signaling member of the wingless-related protein family, Wnt5a. Loss of Wnt5a expression has been associated with poor prognosis in breast cancer patients; however, data are lacking with regard to a functional role for Wnt5a in mammary gland development. We show that Wnt5a is capable of inhibiting ductal extension and lateral branching in the mammary gland. Furthermore, Wnt5a(-/-) mammary tissue exhibits an accelerated developmental capacity compared with wild-type tissue, marked by larger terminal end buds, rapid ductal elongation, increased lateral branching and increased proliferation. Additionally, dominant-negative interference of TGF-beta signaling impacts not only the expression of Wnt5a, but also the phosphorylation of discoidin domain receptor 1 (Ddr1), a receptor for collagen and downstream target of Wnt5a implicated in cell adhesion/migration. Lastly, we show that Wnt5a is required for TGF-beta-mediated inhibition of ductal extension in vivo and branching in culture. This study is the first to show a requirement for Wnt5a in normal mammary development and its functional connection to TGF-beta.

Extracellular matrix, Rac1 signaling, and estrogen-induced proliferation in MCF-7 breast cancer cells

Estrogen receptor positive (ER+), estrogen (E) responsive MCF-7 breast cancer cells cultured on the extracellular matrix (ECM) protein laminin (LM), exhibit significantly reduced E-induced proliferation compared with cells cultured on collagen I (Col I) that is not due to a loss of ER. Based on reported differences in integrin-activated pathways on Col I vs. LM, we investigated the potential role of Rac1/c-jun-N-terminal kinase (JNK) activation and downstream regulation of cyclin D1 by E on Col I vs.LM. E-induced proliferation was increased on LM in MCF-7 cells expressing constitutively active Rac1 (CA Rac1) and decreased in dominant negative Rac1-(DN Rac1) expressing cells on Col I. siRNA knockdown established the specificity and requirement for Rac1 activation for E-induced regulation of cyclin D1. More robust c-Jun activation occurred on Col I than on LM and E-induced proliferation was abolished after treatment with a JNK inhibitor. These results provide evidence that Rac1/JNK/c-Jun activation promotes E-induced proliferation on Col I and reduced Rac1/JNK/c-Jun activation on LM contributes significantly to reduced E-induced proliferation in MCF-7 cells on LM. These results identify a novel role for extracellular matrix (ECM)-integrin regulation of Rac1-JNK pathway in E-regulated proliferation in ER+ breast cancer cells. These findings suggest that tumor stromal environment, i.e., ECM composition, may contribute to loss of E regulation in ER+ breast cancers. Defining molecular markers for early identification of ER+ tumors that are ER+ but antiestrogen resistant would allow the design and use of alternative therapies to inhibit tumor growth and improve survival.

The extracellular matrix as an adhesion checkpoint for mammary epithelial function

The development of the mammary gland is spatially regulated by the interaction of the mammary epithelium with the extracellular matrix (ECM). Cells receive cues from the ECM through a family of adhesion receptors called integrins, consisting of α- and β-chain dimers. Integrins assist cells in sensing their appropriate developmental context in response to both hormones and growth factors. Here we argue that cell adhesion to the ECM plays a key role in specific developmental checkpoints, particularly in alveolar survival, morphogenesis and function. Specific ablation of αβ1-integrins in the luminal epithelium of the mammary gland shows that this sub-type of receptors is required for proliferation, accurate morphological organisation, as well as milk secretion. Downstream, small Rho GTPases mediate cellular polarisation and differentiation. Current challenges in studying the integration of signals in checkpoints of mammary gland development are discussed.

Regulation of pituitary hormones and cell proliferation by components of the extracellular matrix

The extracellular matrix is a three-dimensional network of proteins, glycosaminoglycans and other macromolecules. It has a structural support function as well as a role in cell adhesion, migration, proliferation, differentiation, and survival. The extracellular matrix conveys signals through membrane receptors called integrins and plays an important role in pituitary physiology and tumorigenesis. There is a differential expression of extracellular matrix components and integrins during the pituitary development in the embryo and during tumorigenesis in the adult. Different extracellular matrix components regulate adrenocorticotropin at the level of the proopiomelanocortin gene transcription. The extracellular matrix also controls the proliferation of adrenocorticotropin-secreting tumor cells. On the other hand, laminin regulates the production of prolactin. Laminin has a dynamic pattern of expression during prolactinoma development with lower levels in the early pituitary hyperplasia and a strong reduction in fully grown prolactinomas. Therefore, the expression of extracellular matrix components plays a role in pituitary tumorigenesis. On the other hand, the remodeling of the extracellular matrix affects pituitary cell proliferation. Matrix metalloproteinase activity is very high in all types of human pituitary adenomas. Matrix metalloproteinase secreted by pituitary cells can release growth factors from the extracellular matrix that, in turn, control pituitary cell proliferation and hormone secretion. In summary, the differential expression of extracellular matrix components, integrins and matrix metalloproteinase contributes to the control of pituitary hormone production and cell proliferation during tumorigenesis.

Collagen-IV and laminin-1 regulate estrogen receptor alpha expression and function in mouse mammary epithelial cells

The expression level and functional activity of estrogen receptor alpha is an important determinant of breast physiology and breast cancer treatment. However, it has been difficult to identify the signals that regulate estrogen receptor because cultured mammary epithelial cells generally do not respond to estrogenic signals. Here, we use a combination of two- and three-dimensional culture systems to dissect the extracellular signals that control endogenous estrogen receptor alpha. Its expression was greatly reduced when primary mammary epithelial cells were placed on tissue culture plastic; however, the presence of a reconstituted basement membrane in combination with lactogenic hormones partially prevented this decrease. Estrogen receptor alpha expression in primary mammary fibroblasts was not altered by these culture conditions, indicating that its regulation is cell type specific. Moreover, estrogen receptor-dependent reporter gene expression, as well as estrogen receptor alpha levels, were increased threefold in a functionally normal mammary epithelial cell line when reconstituted basement membrane was added to the medium. This regulatory effect of reconstituted basement membrane was reproduced by two of its components, collagen-IV and laminin-1, and it was blocked by antibodies against alpha2, alpha6 and beta1 integrin subunits. Our results indicate that integrin-mediated response to specific basement membrane components, rather than cell rounding or cell growth arrest induced by reconstituted basement membrane, is critical in the regulation of estrogen receptor alpha expression and function in mammary epithelial cells.

Effects of a progestogen on normal human breast epithelial cell apoptosis in vitro and in vivo

Many investigators have reported cyclic proliferation of normal human breast epithelial cells. A delicate balance between proliferation and apoptosis (programmed cell death) ensures breast homeostasis. Both the follicular and luteal phases of the menstrual cycle are characterized by proliferation, whereas apoptosis occurs only at the end of the latter phase. In this study, we observed that the withdrawal of a synthetic progestin (nomegestrol acetate or NOMAC), but not continuous treatment with it, induced apoptosis of normal human breast epithelial cells in vitro and in women who applied NOMAC gel to their breasts. Furthermore, this apoptotic response was specific to normal breast cells, since withdrawal of NOMAC did not induce apoptosis of tumoral T47D cells in vitro or of fibroadenoma cells in women. These observations open up new perspectives in the prevention of hyperplasia and breast cancer.

Estrogen mediates mammary epithelial cell proliferation in serum-free culture indirectly via mammary stroma-derived hepatocyte growth factor

Epithelial-stromal cell interactions are important for normal development and function of the mouse mammary gland. The steroid hormone estrogen is required for epithelial cell proliferation and ductal development in vivo. Recent studies of estrogen receptor alpha knockout mice indicate that estrogen-induced proliferation is dependent upon the presence of estrogen receptor in mammary stromal cells, but not in epithelial cells. The purpose of the present study was to identify the underlying mechanism of estrogen-dependent stroma-derived effects on mammary epithelium. We have developed a minimally supplemented serum-free medium, collagen gel primary mammary coculture system to address the issue of stroma-derived, estrogen-dependent effects on epithelial cell proliferation. Conditioned medium from mammary fibroblasts or coculture with mammary fibroblasts caused increased epithelial cell proliferation and produced tubular/ductal morphology. Hepatocyte growth factor (HGF) was identified as the mediator of this effect, as the proliferative activity in fibroblast-conditioned medium was completely abolished by neutralizing antibody to HGF, whereas neutralizing antibodies to either epidermal growth factor or IGF-I had no effect. Treatment of mammary fibroblasts with estrogen increased the production of HGF. From these results we conclude that estrogen may indirectly mediate mammary epithelial cell proliferation via the regulation of HGF in mammary stromal cells and that HGF plays a crucial role in estrogen-induced proliferation in vivo.

Hepatocyte growth factor is required for progestin-induced epithelial cell proliferation and alveolar-like morphogenesis in serum-free culture of normal mammary epithelial cells

The steroid hormones, estrogen and progesterone, are required for mammary epithelial cell proliferation and alveolar morphogenesis in vivo. We have developed a minimally supplemented, serum-free medium, collagen gel primary mammary culture system to determine the mechanism of progestin-induced proliferation and alveolar morphogenesis. In epithelial cells cultured alone, treatment with progestin (R5020) alone produced a lumen within the epithelial organoids, but did not stimulate epithelial cell proliferation. The formation of lumens was associated with increased apoptosis, targeted within the organoids. We have previously reported that in our culture system hepatocyte growth factor (HGF) increases epithelial cell proliferation and induces a tubulo-ductal morphological response. In the present report we show that treatment with HGF and progestin (R5020) further increases epithelial proliferation above that with HGF alone and also produces an alveolar-like morphology similar to that observed in vivo in response to progestin treatment. To the best of our knowledge this is the first in vitro demonstration of both progestin-induced proliferation and alveolar-like morphogenesis of normal nonpregnant mouse mammary epithelial cells in vitro. These results suggest that HGF may play a crucial role in progestin-induced proliferation and morphogenesis in vivo.

Mammary gland architecture as a determining factor in the susceptibility of the human breast to cancer

The developmental pattern of the breast can be assessed by determining the composition of the breast in specific lobular structures, which are designated as lobules type 1 (Lob 1), lobules type 2 (Lob 2), and lobules type 3 (Lob 3), with Lob 1 being the less developed and Lob 3 being the most differentiated or with the highest number of ductules per lobular unit. In the present work, the patient population consisted of three groups of women who underwent surgical procedures: The first group included women who underwent reduction mammoplasty (RM) for cosmetic reasons. The second group included women who underwent prophylactic subcutaneous mastectomy after genetic counseling for either carrying the BRCA-1 gene or belonging to a pedigree with familial breast cancer (FAM), and the third group included women who underwent modified radical mastectomy (MRM) for the diagnosis of invasive carcinoma. The RM group consisted of 33 women, of whom 9 were nulliparous and 24 were parous. The FAM group consisted of 17 women, of whom 8 were nulliparous and 9 were parous. The MRM group consisted of 43 women, of whom 7 were nulliparous and 36 were parous. The analysis of the lobular composition of all of the samples from the RM group, which is considered the control group, revealed that Lob 1 represented 22%, Lob 2 represented 37%, and Lob 3 represented 38%, whereas the tissue examined from the FAM and MRM groups contained a preponderance of Lob 1 at 48% and 74%, respectively, over Lob 3, which was 10% and 3%, respectively. When the results of the analysis of breast tissue were separated according to the pregnancy history of the donor, it was found that in the control group or RM, there was a significant difference in lobular composition. Nulliparous women of the RM group showed a preponderance of Lob 1 (46%) over parous women, which contained only 17%, whereas the percentage of Lob 3 in the nulliparous group was significantly lower (7%) than the parous group (48%). In the breast tissues obtained from FAM and MRM, no significant differences in lobular composition were observed, as all of the samples contained a higher concentration of Lob 1, independent of the pregnancy history. The breast tissue of FAM and MRM of parous women had a developmental pattern that was similar to that of nulliparous women of the same group and that was less developed than the breast of parous women of the control group. An important difference between the Lob 1 of the FAM group versus the control (RM) and the MRM group was that most of these lobules had thin ductules with an increase in hyalinization of the intralobular stroma manifested in the whole-mount preparation as an alteration in the branching pattern. The data suggest that the breast tissue of women with invasive cancer, as well as those from a background of familial breast cancer, have an architectural pattern different from the control or normal tissues and that the BRCA-1 or related genes may have a functional role in the branching pattern of the breast during lobular development, mainly in the epithelial stroma interaction.

Fibronectin and the alpha(5)beta(1) integrin are under developmental and ovarian steroid regulation in the normal mouse mammary gland

Extracellular matrix (ECM) proteins have been shown to regulate mammary epithelial cell proliferation, differentiation, and apoptosis in vitro. However, little is known about the hormonal regulation and functional role of ECM proteins and integrins during mammary gland development in vivo. We examined the temporal and spatial localization and hormone regulation of collagen I, collagen IV, laminin, and fibronectin. Among these ECM proteins only fibronectin changed appreciably. Fibronectin levels increased 3-fold between the onset of puberty and sexual maturity, remaining high during pregnancy and lactation. This increase occurred specifically in the epithelial basement membrane. Fibronectin was decreased 70% by ovariectomy and increased 1.5- and 2-fold by estrogen or estrogen plus progesterone treatment, respectively. The fibronectin-specific integrin, alpha(5)beta(1), was localized in myoepithelial cells; it increased 2.2-fold between puberty and sexual maturity and decreased in late pregnancy and lactation. The basal localization of alpha(5)beta(1) was notably increased in pubertal and adult virgin mice. alpha(5)beta(1) concentrations decreased 40-50% after ovariectomy in pubertal and adult mice, which was reversed by estrogen plus progesterone treatment in adult mice. The high basal expression of alpha(5)beta(1) during active proliferation and the low expression in nonproliferating and lactating glands indicate that fibronectin signaling may be required for hormone-dependent proliferation in the mammary gland.

Reciprocal regulation of extracellular matrix proteins and ovarian steroid activity in the mammary gland

Despite the critical importance of ovarian steroids in the treatment of breast cancer, little is known about the acquisition or loss of estrogen and progesterone responsiveness in either the normal or neoplastic mammary gland. This review focuses on the interactions among mammary stroma-derived extracellular matrix (ECM) proteins, integrins and ovarian hormone-dependent proliferation in normal and neoplastic mammary cells both in vivo and in vitro. In vitro studies show that fibronectin is required for progesterone-induced proliferation of normal mammary epithelial cells and that specific ECM proteins also regulate interactions between growth factors and ovarian hormones. Studies with human breast cancer cell lines have shown that laminin inhibits estrogen-induced proliferation and estrogen-response-element-mediated transcription in vitro and also inhibits estrogen-induced proliferation in vivo. Reciprocally, ovarian steroids regulate the expression of ECM proteins and their cellular receptors, integrins, during mammary gland development in vivo. The fibronectin-specific integrin, alpha5beta1 is regulated by ovarian steroids and its expression is positively correlated with developmental stages of peak proliferation. These studies suggest that the coordinated regulation of ovarian hormone responsiveness and ECM/integrin expression may be critical to normal mammary gland development and breast cancer growth and progression.

Proliferation of mouse mammary epithelial cells in vitro: interactions among epidermal growth factor, insulin-like growth factor I, ovarian hormones, and extracellular matrix proteins

The purpose of the present study was to investigate the role of extracellular matrix proteins (ECMs; collagens I and IV, fibronectin, and laminin) in modulating proliferative responses of normal mammary epithelial cells in serum-free culture to epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I). As EGF and IGF-I can alter steroid responses, the interactions among growth factors, estrogen, and R5020 were also investigated. We report the novel finding that all ECMs tested, but not a nonspecific attachment factor, poly-L-lysine (PL), promoted a highly synergistic proliferative response to EGF plus IGF-I. EGF receptors were significantly increased with culture time on all ECMs, but not on PL. IGF receptor expression was significantly 2- to 4-fold higher on all ECMs compared with PL. EGF decreased IGF-binding protein-2 (IGFBP-2) and IGFBP-3 by more than 50% in the presence of IGF-I on PL or collagen I. These results indicate that ECM-specific IGF-I/EGF synergism occurs in response to ECM up-regulation of growth factor receptors and EGF down-regulation of inhibitory IGFBPs. Growth factors did not synergize with estrogen and/or R5020. Instead, estrogen plus R5020 decreased EGF-plus IGF-I-induced proliferation in an ECM-dependent manner. These studies demonstrate that proliferation of normal mammary epithelial cells involves complex interactions among steroids, growth factors, binding proteins, and ECMs.

Transforming growth factor-beta and breast cancer: Mammary gland development

Transforming growth factor (TGF)-beta1 is a pluripotent cytokine that profoundly inhibits epithelial proliferation, induces apoptosis, and influences morphogenesis by mediating extracellular matrix deposition and remodeling. The physiologic roles of the action of TGF-beta in mammary gland, indeed in most tissues, are poorly understood. In order to understand the actions of TGF-beta, we need to take into account the complexity of its effects on different cell types and the influence of context on cellular responses. This task is further compounded by multiple mechanisms for regulating TGF-beta transcription, translation, and activity. One of the most significant factors that obscures the action of TGF-beta is that it is secreted as a stable latent complex, which consists of the 24-kDa cytokine and the 80-kDa dimer of its prepro region, called latency-associated peptide. Latency imposes a critical restraint on TGF-beta activity that is often overlooked. The extracellular process known as activation, in which TGF-beta is released from the latent complex, is emphasized in the present discussion of the role of TGF-beta in mammary gland development. Definition of the spatial and temporal patterns of latent TGF-beta activation in situ is essential for understanding the specific roles that TGF-beta plays during mammary gland development, proliferation, and morphogenesis.

Apoptosis in the estrous and menstrual cycles

In the absence of pregnancy, the adult mammary gland is subjected to cyclic fluctuations of hormonal stimulation that constitute the estrous and menstrual cycles. The mammary epithelium responds to these systemic hormonal changes by regional proliferation, differentiation and cell death by apoptosis. The fact that the mammary epithelial response involves only a minor subset of the epithelial cells implies a delicate local control of epithelial cell fate resulting in either cell death or survival. Evidence gleaned from descriptive data suggests that the apoptosis-related genes of the Bcl-2 gene family, tissue remodeling genes, protein tyrosine kinases and master genes of the homeotic gene cluster may be involved in determining epithelial cell fate during the estrous cycle.

Role of stromal and epithelial estrogen receptors in vaginal epithelial proliferation, stratification, and cornification

Estradiol 17-beta (E2) induces epithelial proliferation, stratification, and cornification in vaginal epithelium. Our aim was to determine the respective roles of epithelial and stromal estrogen receptor-alpha (ER alpha) in these E2-induced events. Vaginal epithelium (E) and stroma (S) from adult ER alpha knockout (ko) and wild-type (wt) neonatal Balb/c mice were enzymatically separated and used to produce four types of tissue recombinants in which epithelium, stroma, or both lack functional ER alpha. Tissue recombinants were grafted into female nude mice, which were subsequently ovariectomized and treated with oil or E2. In response to E2 treatment, grafts prepared with wt-S (wt-S + wt-E and wt-S + ko-E) showed similar large increases in epithelial labeling index, indicating that E2 stimulated epithelial proliferation despite a lack of epithelial ER alpha in wt-S + ko-E tissue recombinants. Conversely, in tissue recombinants prepared with ko-S (ko-S + wt-E and ko-S + ko-E), epithelial labeling index remained at baseline levels after E2 or oil treatment, even though epithelial ER alpha were detected in ko-S + wt-E grafts. Epithelial cornification was present in wt-S + wt-E grafts from E2-treated hosts, whereas epithelium in all other tissue recombinants failed to cornify. Grafts composed of wt-S + wt-E from E2-treated hosts had highly stratified epithelium, whereas epithelial thickness was reduced almost 60% in wt-S + ko-E tissue recombinants grown in E2-treated hosts and was atrophic in all other tissue recombinants. In addition, cytokeratin 10, a marker of epithelial differentiation, was strongly expressed in wt-S + wt-E tissue recombinants grown in E2-treated hosts but was markedly reduced or absent in all other tissue recombinants. These results indicate that E2-induced vaginal epithelial proliferation is mediated indirectly through stromal ER alpha, consistent with our recent findings in uterus. Conversely, both epithelial and stromal ER alpha are required for E2-induced cornification and normal epithelial stratification. These are the first known functions attributed to epithelial ER alpha in vivo and the first time any epithelial response to E2 has been shown to involve both stromal and epithelial ER alpha.

The role of mammary stroma in modulating the proliferative response to ovarian hormones in the normal mammary gland

Postnatal mammary gland development is highly dependent on the ovarian steroids, estrogen and progesterone. However, evidence from both in vitro and in vivo studies indicates that steroid-induced development occurs indirectly, requiring stromal cooperation in epithelial proliferation and morphogenesis. Stromal cells appear to influence epithelial cell behavior by secretion of growth factors and/or by altering the composition of the extracellular matrix in which epithelial cells reside. This review will discuss the requirement for stromal tissue in modulating proliferative responses to ovarian hormones during postnatal development and the potential role of the EGF, IGF, HGF and FGF3 growth factor families. Additionally, the roles of extracellular matrix proteins, including fibronectin, collagens and laminin, will be summarized.

Mammary gland growth and development from the postnatal period to postmenopause: ovarian steroid receptor ontogeny and regulation in the mouse

Ovarian steroid hormones play a critical role in regulating mammary gland growth and development. The mammary gland sequentially acquires and cyclically exhibits proliferative responses to estrogen and/or progesterone from birth to postmenopause. The focus of this review is to present our current understanding of estrogen and progesterone receptor distribution in epithelial and stromal cells and their functions in relation to mammary gland development. Insights gained from the study of the normal mammary gland are relevant to our understanding of the conditions which may predispose women to the development of breast cancer as well as to alterations in hormonal regulation that occur in breast cancer.