The interplay of matrix metalloproteinases, morphogens and growth factors is necessary for branching of mammary epithelial cells

The expression of novel membrane-type matrix metalloproteinase isoforms is required for normal development of zebrafish embryos

Matrix metalloproteinases (MMPs) play important roles in the turnover of components of extracellular matrix (ECM) and in the processing of active and latent-signaling molecules bound to the ECM or associated with the cell surface. Through such actions, MMPs regulate a variety of cellular and developmental processes. Membrane-type matrix metalloproteinases (MT-MMPs) are of particular importance because they function in the immediate pericellular environment that modulates both cell-cell and cell-ECM interactions. In this study, we utilized zebrafish as a developmental model to study the role of MT-MMPs during early embryogenesis. We successfully isolated two isoforms of a MT-MMP homologue that are structurally similar to MT1-MMP. They have been named zebrafish MT-MMPalpha and beta. Zebrafish MT-MMPbeta is unique among vertebrate MT-MMPs in that it contains an Arg-Glu-Asp (RED) multiple-repeat motif in its linker region. Whole mount in situ analysis, RT-PCR, immunofluorescence, reporter analysis, Western blot analysis, and zymography indicated that MT-MMPalpha and beta were expressed through at least the first 72 h of development and that this expression was targeted to the cell surface. Functional studies using injection of either mRNA or morpholino antisense oligonucleotides resulted in a truncation of the cranial to caudal axis as monitored through 72 h post fertilization, indicating that zebrafish MT-MMPalpha and beta had an important role in embryonic development. Axis markers indicated that these effects likely involved processes occurring later than 10 h of embryogenesis.

Binding to EGF receptor of a laminin-5 EGF-like fragment liberated during MMP-dependent mammary gland involution

Extracellular matrix (ECM) fragments or cryptic sites unmasked by proteinases have been postulated to affect tissue remodeling and cancer progression. Therefore, the elucidation of their identities and functions is of great interest. Here, we show that matrix metalloproteinases (MMPs) generate a domain (DIII) from the ECM macromolecule laminin-5. Binding of a recombinant DIII fragment to epidermal growth factor receptor stimulates downstream signaling (mitogen-activated protein kinase), MMP-2 gene expression, and cell migration. Appearance of this cryptic ECM ligand in remodeling mammary gland coincides with MMP-mediated involution in wild-type mice, but not in tissue inhibitor of metalloproteinase 3 (TIMP-3)-deficient mice, supporting physiological regulation of DIII liberation. These findings indicate that ECM cues may operate via direct stimulation of receptor tyrosine kinases in tissue remodeling, and possibly cancer invasion.

Characterization of mammary tumor cell lines from wild type and vitamin D3 receptor knockout mice

1,25-Dihydroxyvitamin D(3) (1,25D(3)), the active metabolite of vitamin D(3), inhibits breast cancer cell growth in vivo and in vitro. To examine mechanisms of 1,25D(3) induced growth arrest and apoptosis, cell lines were established from DMBA induced mammary tumors derived from vitamin D(3) receptor knockout (VDRKO) and wild type (WT) mice. Two VDRKO (KO240, KO288) and two WT (WT145, WT276) cell lines were selected and characterized. All four cell lines express cytokeratins indicative of an epithelial origin, as well as vimentin, which is expressed in many transformed cell lines. The tumorigenicity of the cells was confirmed in vivo as all four cell lines form estrogen responsive tumors in nude mice. Both WT cell lines express the VDR protein and are sensitive to growth inhibition by 1,25D(3) at doses as low as 1 nM. Flow cytometric analysis indicated that 1,25D(3) induces G(0)/G(1) arrest and apoptosis in the WT cell lines. In contrast, both cell lines established from tumors that developed in VDRKO mice lack VDR mRNA and protein. Cells from WT mice exhibit 1,25D(3) inducible transcriptional activity, as measured by reporter gene assays, but cells from VDRKO mice do not. Cells from VDRKO mice are also completely resistant to 1,25D(3) mediated growth arrest and apoptosis over the range of 0.01-100 nM 1,25D(3). VDRKO cells are also resistant to the synthetic vitamin D(3) analogs EB1089 and CB1093 that are more potent growth inhibitors than 1,25D(3) in WT cells. This data conclusively demonstrate that the induction of cell cycle arrest and apoptosis in breast cancer cells by 1,25D(3), EB1089 and CB1093 is dependent on the nuclear VDR. Cells lacking VDR remain sensitive to growth arrest mediated by 9-cis retinoic acid, a ligand for the retinoid x receptor which can heterodimerize with the VDR. Sensitivity to apoptosis induced by the DNA damaging agent etoposide is not altered in VDRKO cells, indicating that VDR ablation does not impair apoptotic pathways in general. All four cell lines display equal sensitivity to tamoxifen induced growth arrest. These estrogen responsive, transformed cell lines which differentially express the VDR provide a novel model system for identification of the mechanisms by which 1,25D(3) regulates proliferation and apoptosis in breast cancer cells.

HGF/SF-met signaling in the control of branching morphogenesis and invasion

Hepatocyte growth factor/Scatter factor (HGF/SF) is a multifunctional growth factor which can induce diverse biological events. In vitro, these include scattering, invasion, proliferation and branching morphogenesis. In vivo, HGF/SF is responsible for many processes during embryonic development and a variety of activities in adults, and many of these normal activities have been implicated in its role in tumorgenesis and metastasis. The c-Met receptor tyrosine kinase is the only known receptor for HGF/SF and mediates all HGF/SF induced biological activities. Upon HGF/SF stimulation, the c-Met receptor is tyrosine-phosphorylated which is followed by the recruitment of a group of signaling molecules and/or adaptor proteins to its cytoplasmic domain and its multiple docking sites. This action leads to the activation of several different signaling cascades that form a complete network of intra and extracellular responses. Different combinations of signaling pathways and signaling molecules and/or differences in magnitude of responses contribute to these diverse series of HGF/SF-Met induced activities and most certainly are influenced by cell type as well as different cellular environments. In this review, we focus on HGF/SF-induced branching morphogenesis and invasion, and bring together recent new findings which provide insight into how HGF/SF, via c-Met induces this response.

Tube or not tube: remodeling epithelial tissues by branching morphogenesis

Branching morphogenesis involves the restructuring of epithelial tissues into complex and organized ramified tubular networks. Early rounds of branching are controlled genetically in a hardwired fashion in many organs, whereas later, branching is stochastic, responding to environmental cues. We discuss this sequential process from formation of an organ anlage and invagination of the epithelium to branch initiation and outgrowth in several model systems including Drosophila trachea and mammalian lung, mammary gland, and kidney.

Host microenvironment in breast cancer development: epithelial-mesenchymal transition in breast cancer development

The epithelial-mesenchymal transition (EMT) is a developmental mechanism of crucial importance in establishing the body plan in many multicellular organisms. Several transduction pathways controlling the various steps of the morphological transition have been identified by molecular analyses of this process in cell lines and in vivo. The newly formed mesenchymal cells can exhibit locomotory and invasive phenotypes, suggesting that EMTs contribute to the progression of carcinoma. Diverse evidence indicates that EMT subprograms are involved in the appearance of different breast carcinoma types. Several normal and malignant breast cell lines are currently being analyzed to define key steps in EMT and to identify candidate genes. DNA profiling technology is also being applied to uncover pathways that lead to a metastatic phenotype.

Cancer as a disease of epithelial-mesenchymal interactions and extracellular matrix regulation

Carcinogenesis - the process of cancer formation - is commonly discussed in terms of genetic alterations that lead to deregulation of cell growth. Recently, there has been a resurgence of interest in epigenetic factors and, in particular, the role of the stromal microenvironment and angiogenesis in tumor formation. In this article, cancer is presented as a disease of the developmental processes that govern how cells organize into tissues and tissues into organs. This histogenetic perspective raises the possibility that epithelial-mesenchymal interactions and the extracellular matrix (basement membrane) that is deposited through these interactions may actively contribute to the carcinogenic process. Experimental work is reviewed that confirms that extracellular matrix plays a key role in normal histodifferentiation during both epitheliogenesis and angiogenesis, and that epigenetic deregulation of cell-matrix interactions may actively promote tumor initiation and progression. The contributions of integrins, cytoskeleton, tensegrity and local variations in extracellular matrix mechanics to these processes are discussed, as are the implications of this work for future studies on cancer formation.

EGF controls the in vivo developmental potential of a mammary epithelial cell line possessing progenitor properties

The bilayered mammary epithelium comprises a luminal layer of secretory cells and a basal layer of myoepithelial cells. Numerous data suggest the existence of self-renewing, pluripotent mammary stem cells; however, their molecular characteristics and differentiation pathways are largely unknown. BC44 mammary epithelial cells in culture, display phenotypic characteristics of basal epithelium, i.e., express basal cytokeratins 5 and 14 and P-cadherin, but no smooth muscle markers. In vivo, after injection into the cleared mammary fat pad, these cells gave rise to bilayered, hollow, alveolus-like structures comprising basal cells expressing cytokeratin 5 and luminal cells positive for cytokeratin 8 and secreting beta-casein in a polarized manner into the lumen. The persistent stimulation of EGF receptor signaling pathway in BC44 cells in culture resulted in the loss of the in vivo morphogenetic potential and led to the induction of active MMP2, thereby triggering cell scattering and motility on laminin 5. These data (a) suggest that BC44 cells are capable of asymmetric division for self-renewal and the generation of a differentiated progeny restricted to the luminal lineage; (b) clarify the function of EGF in the control of the BC44 cell phenotypic plasticity; and (c) suggest a role for this phenomenon in the mammary gland development.

Crk synergizes with epidermal growth factor for epithelial invasion and morphogenesis and is required for the met morphogenic program

Activation of the Met receptor tyrosine kinase through its ligand, hepatocyte growth factor, stimulates cell spreading, cell dispersal, and the inherent morphogenic program of various epithelial cell lines. Although both hepatocyte growth factor and epidermal growth factor (EGF) can activate downstream signaling pathways in Madin-Darby canine kidney epithelial cells, EGF fails to promote the breakdown of cell-cell junctional complexes and initiate an invasive morphogenic program. We have undertaken a strategy to identify signals that synergize with EGF in this process. We provide evidence that the overexpression of the CrkII adapter protein complements EGF-stimulated pathways to induce cell dispersal in two-dimensional cultures and cell invasion and branching morphogenesis in three-dimensional collagen gels. This finding correlates with the ability of CrkII to promote the breakdown of adherens junctions in stable cell lines and the ability of EGF to stimulate enhanced Rac activity in cells overexpressing CrkII. We have previously shown that the Gab1-docking protein is required for branching morphogenesis downstream of the Met receptor. Consistent with a role for CrkII in promoting EGF-dependent branching morphogenesis, the binding of Gab1 to CrkII is required for the branching morphogenic program downstream of Met. Together, our data support a role for the CrkII adapter protein in epithelial invasion and morphogenesis and underscores the importance of considering the synergistic actions of signaling pathways in cancer progression.

Do different branching epithelia use a conserved developmental mechanism?

Formation of branching epithelial trees from unbranched precursors is a common process in animal organogenesis. In humans, for example, this process gives rise to the airways of the lungs, the urine-collecting ducts of the kidneys and the excretory epithelia of the mammary, prostate and salivary glands. Branching in these different organs, and in different animal classes and phyla, is morphologically similar enough to suggest that they might use a conserved developmental programme, while being dissimilar enough not to make it obviously certain that they do. In this article, I review recent discoveries about the molecular regulation of branching morphogenesis in the best-studied systems, and present evidence for and against the idea of there being a highly conserved mechanism. Overall, I come to the tentative conclusion that key mechanisms are highly conserved, at least within vertebrates, but acknowledge that more work needs to be done before the case is proved beyond reasonable doubt.

Cell migration in development and disease

A recent meeting at the Max Delbrück Center in Berlin, Germany provided a forum to discuss the molecular mechanisms of cell migration in a broad range of contexts including chemotaxis, development, immunity, and cancer.