The influence of cell shape on the induction of functional differentiation in mouse mammary cells in vitro

The molecular basis of mammary gland development and epithelial differentiation

Our understanding of the molecular events underpinning the development of mammalian organ systems has been increasing rapidly in recent years. With the advent of new and improved next-generation sequencing methods, we are now able to dig deeper than ever before into the genomic and epigenomic events that play critical roles in determining the fates of stem and progenitor cells during the development of an embryo into an adult. In this review, we detail and discuss the genes and pathways that are involved in mammary gland development, from embryogenesis, through maturation into an adult gland, to the role of pregnancy signals in directing the terminal maturation of the mammary gland into a milk producing organ that can nurture the offspring. We also provide an overview of the latest research in the single-cell genomics of mammary gland development, which may help us to understand the lineage commitment of mammary stem cells (MaSCs) into luminal or basal epithelial cells that constitute the mammary gland. Finally, we summarize the use of 3D organoid cultures as a model system to study the molecular events during mammary gland development. Our increased investigation of the molecular requirements for normal mammary gland development will advance the discovery of targets to predict breast cancer risk and the development of new breast cancer therapies.

RNA sequencing of long-term label-retaining colon cancer stem cells identifies novel regulators of quiescence

Recent data suggest that therapy-resistant quiescent cancer stem cells (qCSCs) are the source of relapse in colon cancer. Here, using colon cancer patient-derived organoids and xenografts, we identify rare long-term label-retaining qCSCs that can re-enter the cell cycle to generate new tumors. RNA sequencing analyses demonstrated that these cells display the molecular hallmarks of quiescent tissue stem cells, including expression of p53 signaling genes, and are enriched for transcripts common to damage-induced quiescent revival stem cells of the regenerating intestine. In addition, we identify negative regulators of cell cycle, downstream of p53, that we show are indicators of poor prognosis and may be targeted for qCSC abolition in both p53 wild-type and mutant tumors. These data support the temporal inhibition of downstream targets of p53 signaling, in combination with standard-of-care treatments, for the elimination of qCSCs and prevention of relapse in colon cancer.

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Colon tumors contain therapy-resistant quiescent cancer stem cells (qCSCs)

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qCSC gene expression mirrors that of quiescent stem cells of the regenerating gut

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qCSCs are enriched for p53 signaling genes

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qCSC elimination may be achieved by inhibiting downstream targets of p53 signaling

Special review series on 3D organotypic culture models: Introduction and historical perspective

Three dimensional (3D) organ-like (organotypic) culture models are a rapidly advancing area of in vitro biological science. In contrast to monolayer cell culture methods which were developed to achieve proliferation of animal cells in the beginning of in vitro biology, the advancements in 3D culture methods are designed to promote cellular differentiation, and to achieve in vivo-like 3D structure and organotypic functions. This project was conceived through the Society for In Vitro Biology to draw on the expertise of individual scientists with special expertise in organotypic cultures of selected tissues or associated interrogation methods to prepare individual-focused reviews in this series. This introductory manuscript will review the early achievements of animal cell culture in monolayer culture and the limitations of that approach to reproduce functioning organ systems. Among these are the nature and 3D architecture of the substrate on which or in which the cells are grown, physical and mechanical clues from the substrate, cell-cell interactions, and defined biochemical factors that trigger the induction of the 3D organotypic differentiation. The organoid culture requires a source of cells with proliferative capacity (ranging from tissue-derived stem or immortalized cells to the iPSC cultures), a suitable substrate or matrix with the mechanical and stimulatory properties appropriate for the organotypic construct and the necessary stimulation of the culture to drive differentiation of the cell population to form the functioning organotypic construct. Details for each type of organotypic construct will be provided in the following papers.

In vitro models of the human endometrium: evolution and application for women's health

The endometrium is the inner lining of the uterus that undergoes complex regeneration and differentiation during the human menstrual cycle. The process of endometrial shedding, regeneration, and differentiation is driven by ovarian steroid hormones and prepares the endometrium and intrauterine environment for embryo implantation and pregnancy establishment. Endometrial glands and their secretions are essential for pregnancy establishment, and cross talk between the glandular epithelium and stromal cells appears vital for decidualization and placental development. Despite being crucial, the biology of the human endometrium during pregnancy establishment and most of pregnancy is incomplete, given the ethical and practical limitations of obtaining and studying endometrium from pregnant women. As such, in vitro models of the human endometrium are required to fill significant gaps in understanding endometrial biology. This review is focused on the evolution and development of in vitro three-dimensional models of the human endometrium and provides insight into the challenges and promises of those models to improve women's reproductive health.

Collective Cell Behavior in Mechanosensing of Substrate Thickness

Extracellular matrix stiffness has a profound effect on the behavior of many cell types. Adherent cells apply contractile forces to the material on which they adhere and sense the resistance of the material to deformation-its stiffness. This is dependent on both the elastic modulus and the thickness of the material, with the corollary that single cells are able to sense underlying stiff materials through soft hydrogel materials at low (<10 μm) thicknesses. Here, we hypothesized that cohesive colonies of cells exert more force and create more hydrogel deformation than single cells, therefore enabling them to mechanosense more deeply into underlying materials than single cells. To test this, we modulated the thickness of soft (1 kPa) elastic extracellular-matrix-functionalized polyacrylamide hydrogels adhered to glass substrates and allowed colonies of MG63 cells to form on their surfaces. Cell morphology and deformations of fluorescent fiducial-marker-labeled hydrogels were quantified by time-lapse fluorescence microscopy imaging. Single-cell spreading increased with respect to decreasing hydrogel thickness, with data fitting to an exponential model with half-maximal response at a thickness of 3.2 μm. By quantifying cell area within colonies of defined area, we similarly found that colony-cell spreading increased with decreasing hydrogel thickness but with a greater half-maximal response at 54 μm. Depth-sensing was dependent on Rho-associated protein kinase-mediated cellular contractility. Surface hydrogel deformations were significantly greater on thick hydrogels compared to thin hydrogels. In addition, deformations extended greater distances from the periphery of colonies on thick hydrogels compared to thin hydrogels. Our data suggest that by acting collectively, cells mechanosense rigid materials beneath elastic hydrogels at greater depths than individual cells. This raises the possibility that the collective action of cells in colonies or sheets may allow cells to sense structures of differing material properties at comparatively large distances.

Photoresponsive Hydrogels with Photoswitchable Stiffness: Emerging Platforms to Study Temporal Aspects of Mesenchymal Stem Cell Responses to Extracellular Stiffness Regulation

An extensive number of cell-matrix interaction studies have identified matrix stiffness as a potent regulator of cellular properties and behaviours. Perhaps most notably, matrix stiffness has been demonstrated to regulate mesenchymal stem cell (MSC) phenotype and lineage commitment. Given the therapeutic potential for MSCs in regenerative medicine, significant efforts have been made to understand the molecular mechanisms involved in stiffness regulation. These efforts have predominantly focused on using stiffness-defined polyacrylamide (PA) hydrogels to culture cells in 2D and have enabled elucidation of a number of mechano-sensitive signalling pathways. However, despite proving to be a valuable tool, these stiffness-defined hydrogels do not reflect the dynamic nature of living tissues, which are subject to continuous remodelling during processes such as development, ageing, disease and regeneration. Therefore, in order to study temporal aspects of stiffness regulation, researchers have developed and exploited novel hydrogel substrates with in situ tuneable stiffness. In particular, photoresponsive hydrogels with photoswitchable stiffness are emerging as exciting platforms to study MSC stiffness regulation. This chapter provides an introduction to the use of PA hydrogel substrates, the molecular mechanisms of mechanotransduction currently under investigation and the development of these emerging photoresponsive hydrogel platforms.

Thermogel-Coated Poly(ε-Caprolactone) Composite Scaffold for Enhanced Cartilage Tissue Engineering

A three-dimensional (3D) composite scaffold was prepared for enhanced cartilage tissue engineering, which was composed of a poly(ε-caprolactone) (PCL) backbone network and a poly(lactide-co-glycolide)-block-poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PLGA⁻PEG⁻PLGA) thermogel surface. The composite scaffold not only possessed adequate mechanical strength similar to native osteochondral tissue as a benefit of the PCL backbone, but also maintained cell-friendly microenvironment of the hydrogel. The PCL network with homogeneously-controlled pore size and total pore interconnectivity was fabricated by fused deposition modeling (FDM), and was impregnated into the PLGA⁻PEG⁻PLGA solution at low temperature (e.g., 4 °C). The PCL/Gel composite scaffold was obtained after gelation induced by incubation at body temperature (i.e., 37 °C). The composite scaffold showed a greater number of cell retention and proliferation in comparison to the PCL platform. In addition, the composite scaffold promoted the encapsulated mesenchymal stromal cells (MSCs) to differentiate chondrogenically with a greater amount of cartilage-specific matrix production compared to the PCL scaffold or thermogel. Therefore, the 3D PCL/Gel composite scaffold may exhibit great potential for in vivo cartilage regeneration.

Evaluation of early stage human bone marrow stromal proliferation, cell migration and osteogenic differentiation on μ-MIM structured stainless steel surfaces

It is well established that surface topography greatly affect cell-surface interactions. In a recent study we showed that microstructured stainless steel surfaces characterized by the presence of defined hexagonally arranged hemisphere-like structures significantly affected cell architecture (shape and focal adhesion size) of primary human bone mesenchymal stromal cells. This study aimed at further investigating the influence these microstructures (microcline protruding hemispheres) on critical aspects of cell behaviour namely; proliferation, migration and osteogenic differentiation. As with previously reported data, we used primary human bone mesenchymal stromal cells to investigate such effects at an early stage in vitro. Cells of different patients were utilised for cell migration studies. Our data showed that an increase in cell proliferation was exhibited as a function of surface topography (hemispheres). Cell migration velocity also varied as a function of surface topography on patient specific basis and seems to relate to the differentiated state of the seeded cell population (as demonstrated by bALP positivity). Osteogenic differentiation, however, did not exhibit significant variations (both up and down-regulation) as a function of both surface topography and time in culture.

A systems biology approach to cancer: fractals, attractors, and nonlinear dynamics

Cancer begins to be recognized as a highly complex disease, and advanced knowledge of the carcinogenic process claims to be acquired by means of supragenomic strategies. Experimental data evidence that tumor emerges from disruption of tissue architecture, and it is therefore consequential that the tissue level should be considered the proper level of observation for carcinogenic studies. This paradigm shift imposes to move from a reductionistic to a systems biology approach. Indeed, cell phenotypes are emergent modes arising through collective nonlinear interactions among different cellular and microenvironmental components, generally described by a phase space diagram, where stable states (attractors) are embedded into a landscape model. Within this framework cell states and cell transitions are generally conceived as mainly specified by the gene-regulatory network. However, the system's dynamics cannot be reduced to only the integrated functioning of the genome-proteome network, and the cell-stroma interacting system must be taken into consideration in order to give a more reliable picture. As cell form represents the spatial geometric configuration shaped by an integrated set of cellular and environmental cues participating in biological functions control, it is conceivable that fractal-shape parameters could be considered as "omics" descriptors of the cell-stroma system. Within this framework it seems that function follows form, and not the other way around.

Isolation, culture and analysis of mouse mammary epithelial cells

Limited understanding of the cell biology of the breast and breast cancer hampers our ability to develop new therapeutic approaches. Mouse models of mammary gland development and tumourigenesis are key to developing new insights into the biology of both the normal and diseased tissues. Recent advances have enabled the isolation, molecular characterisation and functional analysis of mouse mammary epithelial cell subpopulations from the normal gland, including subpopulations enriched for stem cell behaviour. Application of these techniques both to the normal mammary gland and to tumour models will promote a better understanding of the nature of the different epithelial cell types in the mammary gland, the origins of mammary tumours and the role of tumour stem cells.

Geometric control of tissue morphogenesis

Morphogenesis is the dynamic and regulated change in tissue form that leads to creation of the body plan and development of mature organs. Research over the past several decades has uncovered a multitude of genetic factors required for morphogenesis in animals. The behaviors of individual cells within a developing tissue are determined by combining these genetic signals with information from the surrounding microenvironment. At any point in time, the local microenvironment is influenced by macroscale tissue geometry, which sculpts long range signals by affecting gradients of morphogens and mechanical stresses. The geometry of a tissue thus acts as both a template and instructive cue for further morphogenesis.

Rate of increase of osmolality determines osmotic tolerance of mouse inner medullary epithelial cells

Renal inner medullary cells survive and function despite interstitial osmolality of 600-1,700 mosmol/kgH(2)O or more. In contrast, much smaller changes kill cells in tissue culture. Using mouse inner medullary epithelial cells at passage 2, we defined factors that might account for the difference. Most of the factors that we tested, including addition of hormones (insulin-like growth factor I, epidermal growth factor, or deamino-8-D-arginine vasopressin), growth on porous supports, and presence of matrix proteins (collagen I, collagen IV, fibronectin, laminin, or fibrillar collagen I), have no significant effect. However, the time course of the change makes a major difference. When osmolality is increased from 640 to 1,640 mosmol/kgH(2)O by addition of NaCl and urea in a single step, only 30% of cells survive for 24 h. However, when the same increase is made linearly over 20 h, 89% of the cells remain viable 24 h later. We conclude that gradual changes in osmolality, e.g., in vivo, allow cells to survive much greater changes than do the step changes routinely used in cell culture experiments.

Behavior of a cell line derived from normal human hepatocytes on non-physiological and physiological-type substrates: evidence for enhancement of secretion of liver-specific proteins by a three-dimensional growth pattern

The behavior of a recently described cell line, HH25, derived from normal human hepatocytes, has been investigated on several different substrates--tissue-culture plastic, glass, a thin layer of rat-tail collagen I, and thin layers or thick gels of extracellular matrix derived from the Engelbreth-Holm-Swarm murine sarcoma (EHS matrix). Cellular morphology, proliferation, and secretion of three hepatocyte-specific proteins (albumin, alpha1 acid glycoprotein, and alpha1 antitrypsin) have been examined. There were no differences in morphology, proliferation, or differentiated function in the cells on either plastic, glass, collagen, I, or a thin layer of EHS matrix, but on a thick EHS matrix gel the cells altered their morphology (forming three-dimensional colonies with canalicular-like structures) and their production of albumin and alpha1 acid glycoprotein was enhanced. This suggests that the enhanced differentiated function is associated with the morphological change (occurring only on the thick EHS gel) rather than with receptor-mediated cell-matrix interactions (which can also occur on the thin layer of EHS matrix). This cell line is therefore a good in vitro cellular model for the investigation of the roles of morphological changes and of cell-cell and cell-matrix interactions in the control of human hepatocyte behavior without the need for an extensive source of primary tissue.

Hormone-dependent expression of gamma-casein mRNA in mouse mammary epithelial cells cultured on floating collagen gels

Mammary epithelial cells isolated from pregnant mice were plated on a collagen gel matrix and cultured in a serum-free medium supplemented with combinations of insulin, dexamethasone and prolactin (PRL). After the cells formed a monolayer, the collagen gel was detached from the culture dish and allowed to float in the medium. Quantification of gamma-casein mRNA by a competitive PCR method revealed that the cells on the floating gel accumulated considerably larger amounts of gamma-casein mRNA than the cells on the gel remained attached to the dish. Under these floating collagen gel culture conditions, addition of both dexamethasone and PRL to the insulin-supplemented basal medium maximally stimulated the accumulation of gamma-casein mRNA. These observations strongly suggest that the status of the extracellular matrix as well as hormones controls the differentiation of mouse mammary epithelial cells at the mRNA level.

Cell renewal and functional morphology of human lactating breast: presentation of a new mode of cell death ('magentosis') characterized by nuclear periodic acid-Schiff reactivity

Cell kinetics and functional morphology of the human lactating breast were analyzed using 21 paraffin-embedded specimens of the lactating breast obtained by biopsy or surgery and two of the 'resting' breast. Four types (I-IV) of lactating lobules were categorized histologically, and were well correlated with their functional status demonstrated by immunohistochemistry for epithelial membrane antigen, beta-casein, lipase, lactoferrin, secretory component and IgA. Type I corresponded to a pre-lactating state, type II to an actively lactating state, type III to an early stage of regression, and type IV to an advanced stage of regression. Cell proliferation monitored by Ki-67 (MIB-1) immunostaining was at the highest level in type I lobules followed by type IV, while the labeling indices were low in types II and III. Apoptosis demonstrated by terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick end-labeling (TUNEL) method showed labeling profiles comparable with the proliferative activity. Holocrine-type desquamative cells with intact-appearing nuclei were frequently recognized in type II lobules. In type III, a peculiar form of cell death, designated 'magentosis', was demonstrated. These degenerative cells occasionally shedding into the acinar lumen possessed homogeneous pale, TUNEL-negative nuclei with diastase-resistant periodic acid-Schiff (PAS) reactivity. 'Magentosis' specifically identified in an early phase of involution of the human lactating breast might represent a unique mode of cell death distinguishable from apoptosis and necrosis.

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.

Vectorial release of poliovirus from polarized human intestinal epithelial cells

Polarized epithelial cells represent the primary barrier to virus infection of the host, which must also be traversed prior to virus dissemination from the infected organism. Although there is considerable information available concerning the release of enveloped viruses from such cells, relatively little is known about the processes involved in the dissemination of nonenveloped viruses. We have used two polarized epithelial cell lines, Vero C1008 (African green monkey kidney epithelial cells) and Caco-2 (human intestinal epithelial cells), infected with poliovirus and investigated the process of virus release. Release of poliovirus was observed to occur almost exclusively from the apical cell surface in Caco-2 cells, whereas infected Vero C1008 cells exhibited nondirectional release. Structures consistent with the vectorial transport of virus contained within vesicles or viral aggregates were observed by electron microscopy. Treatment with monensin or ammonium chloride partially inhibited virus release from Caco-2 cells. No significant cell lysis was observed at the times postinfection when extracellular virus was initially detected, and transepithelial resistance and vital dye uptake measurements showed only a moderate decrease. Brefeldin A was found to significantly and specifically inhibit poliovirus biosynthetic processes by an as yet uncharacterized mechanism. The vectorial release of poliovirus from the apical (or luminal) surface of human intestinal epithelial cells has significant implications for viral pathogenesis in the human gut.

Virus infection of polarized epithelial cells

This chapter focuses on the interaction of viruses with epithelial cells. The role of specific pathways of virus entry and release in the pathogenesis of viral infection is examined together with the mechanisms utilized by viruses to circumvent the epithelial barrier. Polarized epithelial cells in culture, which can be grown on permeable supports, provide excellent systems for investigating the events in virus entry and release at the cellular level, and much information is being obtained using such systems. Much remains to be learned about the precise routes by which many viruses traverse the epithelial barrier to initiate their natural infection processes, although important information has been obtained in some systems. Another area of great interest for future investigation is the process of virus entry and release from other polarized cell types, including neuronal cells.

Bidirectional entry of poliovirus into polarized epithelial cells

The interactions of viruses with polarized epithelial cells are of some significance to the pathogenesis of disease because these cell types comprise the primary barrier to many virus infections and also serve as the sites for virus release from the host. Poliovirus-epithelial cell interactions are of particular interest since this virus is an important enteric pathogen and the host cell receptor has been identified. In this study, poliovirus was observed to adsorb to both the apical and basolateral surfaces of polarized monkey kidney (Vero C1008) and human intestinal (Caco-2) epithelial cells but exhibited preferential binding to the basolateral surfaces of both cell types. Localization of the poliovirus receptor by a receptor-specific monoclonal antibody (D171) revealed a similar distribution predominantly on basolateral membranes, and treatment of cells with antibody D171 inhibited virus adsorption to both membrane surfaces. Poliovirus was able to initiate infection with similar efficiency following adsorption to either surface, and infection was blocked at both surfaces by D171, indicating that functional receptor molecules are expressed on both surfaces at sufficient density to mediate efficient infection at the apical and basolateral plasma membranes. Poliovirus infection resulted in a decrease in transepithelial resistance which was inhibited by prior treatment with monoclonal antibody D171 and occurred prior to other visible cytopathic effects. These results have interesting implications for viral pathogenesis in the human gut.

Changes in the extracellular matrix of the normal human breast during the menstrual cycle

The normal human mammary gland undergoes a well defined sequence of histological changes in both epithelial and stromal compartments during the menstrual cycle. Studies in vitro have suggested that the extracellular matrix surrounding the individual cells plays a central role in modulating a wide variety of cellular events, including proliferation, differentiation and gene expression. We therefore investigated the distribution of a number of extracellular matrix molecules in the normal breast during the menstrual cycle. By use of indirect immunofluorescence, with specific antibodies, we demonstrated that laminin, heparan sulphate proteoglycan, type IV collagen, type V collagen, chondroitin sulphate and fibronectin undergo changes in distribution during the menstrual cycle, whereas collagen types I, III, VI and VII remain unchanged. These changes were most marked in the basement membrane, sub-basement membrane zone and delimiting layer of fibroblasts surrounding the ductules where basement membrane markers such as laminin, heparan sulphate proteoglycan, and type IV and V collagens appear greatly reduced during the mid-cycle period (days 8 to 22). These results suggest that some extracellular matrix molecules may act as mediators in the hormonal control of the mammary gland, whereas others may have a predominantly structural role.

A fibrin matrix modulates the proliferation, hormone secretion and morphologic differentiation of cultured human placental trophoblast

Term placental trophoblast epithelialize fibrin deposits attached to villi in vitro and trophoblast cultured on a fibrin matrix form an epithelial bilayer typical of the trophoblast layer on term villi. We compared the morphology of cells grown on fibrin with cells grown on substrates of type IV collagen, laminin, type I collagen, or Matrigel. We also used autoradiography, hormone assays, electron microscopy, and immunofluorescence to determine what functional activities were influenced by trophoblast-fibrin interactions. Cultured cellular trophoblast from term placentae differentiated to form syncytial trophoblast and to secrete estrogen, progesterone, and hCG in the presence or absence of matrices. Trophoblast proliferation was lower in cells grown on matrices and was inversely related to cell height after 24 h in culture. Cells grown on fibrin remained the tallest and had the lowest labelling index. Cells grown for 72 h on fibrin had the most dilated rough endoplasmic reticulum but the lowest media hormone levels. Only cells grown on a fibrin matrix formed a basal lamina-like structure at the trophoblast-substrate interface, and only a fibrin matrix facilitated trophoblast to form an epithelial bilayer in culture. However, this histology was not accompanied by a change in the amount of syncytial trophoblast formed by the cells grown on fibrin. The results suggest that a fibrin matrix uniquely modulates the trophoblast phenotype, away from the secretion of placental specific products like hCG in favour of a repair-oriented phenotype that forms basement membrane and a trophoblast bilayer.

Morphogenetic behavior of simian virus 40-transformed human mammary epithelial stem cell lines on collagen gels

Transformation of primary cultures of human breast cells with simian virus 40 and clonal selection has yielded single-cell-cloned, epithelial cell lines, as well as myoepithelial-related cell lines. When grown on floating collagen gels, the epithelial cell lines give rise to branching rays of cells, thick fingerlike protrusions, saclike structures, and degenerating areas. The myoepithelial-related cell lines give rise only to the branching rays. Epidermal growth factor stimulates the production of the thick protrusions, whereas cholera toxin stimulates the production of the degenerating areas. Immunocytochemical staining of these cultures using reagents directed against the cell surface-extracellular matrix or the cellular cytoskeleton confirms the epithelial and myoepithelial nature of the cells, and demonstrates that the degenerating areas are undergoing squamous metaplasia. The fingerlike protrusions consist of cords of cells composed of inner, epithelial and outer, myoepithelial-related cells sometimes surrounding a central lumen reminiscent of ducts. The saclike structures resemble alveoli. Ultrastructural analysis confirms the identification of the basic cell types and also identifies indeterminate cells possessing features of both epithelial and myoepithelial cells. It is suggested that the epithelial cell lines represent human mammary stem cells that can undergo processes of morphogenesis and differentiation in vitro to form many of the three-dimensional structures found within the breast.

Branchings and course of the larger ducts and accompanying structures within the rat submandibular salivary gland

In the rat submandibular gland, the larger ducts, vessels, and autonomic elements are surrounded by appreciable collagen to form an extensive, ramifying core for the bulk of the parenchyma. This entity is analogous to the armature within a clay sculpture because it provides a framework supporting softer surroundings. Its largest portion, the trunk, begins dorsorostrally at the hilus. Along its caudal course, the trunk emits limbs that, in turn, branch. Rostrally, the trunk and major limbs have their greatest diameters and contain lymphatic vessels and autonomic ganglia that are fewer or absent in the smaller divisions. Four to six orders of branching commonly occur. This architectural device constitutes a physical gradient which peaks at the hilus and is minimal where its components undergo the transition to the lobular parenchyma. The presence of an appreciable amount of connective tissue, nerves, and larger vessels would make this entity firmer than the highly hydrated parenchyma. The major divisions of this grouping are contacted by septa that extend from the organ capsule and demarcate lobules into which smaller divisions penetrate. The parenchyma is consequently ensconced between several strong structures containing abundant connective tissue. The dense collagen encompassing individual components of the biological armature would stabilize duct epithelium and its intercellular channels as well as brace vessels' walls. As a result of this architectural arrangement, the patency of all acinar, duct, and vascular structures is likely to persist despite neck and jaw movements that would otherwise distort them and thus hinder the formation and flow of saliva during feeding. This mechanism may be widespread among organs.

Modulation of milk protein synthesis through alteration of the cytoskeleton in mouse mammary epithelial cells cultured on a reconstituted basement membrane

Recent studies indicate that the cytoskeleton may be involved in modulating tissue-specific gene expression in mammalian cells. We have studied the role of the cytoskeleton in regulating milk protein synthesis and secretion by primary mouse mammary epithelial cells cultured on a reconstituted basement membrane that promotes differentiation. After 8 days in culture, cells were treated with cytochalasin D (CD) (0.5-1 micrograms/ml) to alter actin filaments or acrylamide (Ac) (5 mM) to alter intermediate filaments (cytokeratins). CD inhibited synthesis of most proteins in a concentration-dependent manner, with beta-casein being the first affected. In contrast, Ac increased protein synthesis and secretion by 17-31% after a 12 hr treatment. Polyacrylamide gel electrophoresis of total secreted proteins indicates that synthetic rates of most proteins were increased equally by Ac treatment. This increase is apparently controlled at the level of translation, because control and Ac-treated cells contained the same amount of poly-A+ RNA, and neither CD nor Ac altered mRNA levels for beta-casein. There was also no indication that either CD or Ac can induce the expression of milk proteins in quiescent cells cultured on a plastic substratum. In conjunction with the biochemical studies, changes in cytoskeletal morphology caused by the drug treatments were analyzed by immunofluorescence microscopy. As has been observed in other cell types, low concentrations of CD caused cells to round up by disrupting actin filaments. Ac treatment slightly decreased the intensity of actin staining, but no changes in microfilament organization were observed. Ac-treated cells showed slight disorganization of the cytokeratin filaments, with some peripheral interfibrillar bundling, but the cytokeratin network did not collapse and no retraction of cell extensions or breakdown of cell-cell contacts was observed. These results confirm previous reports that the actin cytoskeleton may play a role in regulating tissue-specific protein synthesis. How Ac stimulates protein synthesis is unknown, but it is unlikely that this effect is directly mediated through intermediate filaments.

Primary culture of normal rat mammary epithelial cells within a basement membrane matrix. II. Functional differentiation under serum-free conditions

A serum-free primary culture system is described which allows normal rat mammary epithelial cells (RMECs) embedded within a reconstituted basement membrane to undergo extensive growth and functional differentiation as detected by synthesis and secretion of the milk products casein and lipid. RMECs isolated from mammary glands of immature virgin rats were seeded within an extracellular matrix preparation derived from the Engelbreth-Holm-Swarm sarcoma and cultured in a serum-free medium consisting of Dulbecco's modified Eagle's medium-F12 containing insulin, prolactin, progesterone, hydrocortisone, epidermal growth factor, bovine serum albumin, transferrin, and ascorbic acid. Casein synthesis and secretion were documented at the electron microscopic level as well as by an enzyme-linked immunosorbent assay (ELISA) assay using a polyclonal antibody against total rat caseins. Numerous secretory vesicles with casein micelles were noted near the apical surface of the RMECs, and secreted casein was observed in the lumen. These ultrastructural data were confirmed by the ELISA assay which showed that microgram amounts of casein per well were synthesized by the RMECs and that the amount of casein increased with time in culture. Using immunoblot analysis it was demonstrated that the full complement of casein proteins was synthesized. In addition to casein protein, beta-casein mRNA levels were shown to increase with time. Synthesized lipid was detected at both the light and electron microscopic levels. Phase contrast photomicrographs demonstrated extensive intracellular lipid accumulation within the ductal and lobuloalveolarlike colonies, and at the electron micrograph level, lipid droplets were predominantly localized near the apical surface of the RMECs. The lipid nature of these droplets was verified by oil red O staining. Results from this study demonstrate that RMECs from immature virgin rats proliferate extensively and rapidly develop the capacity to synthesize and secrete casein and lipid when grown within a reconstituted basement membrane under defined serum-free conditions. This unique system should thus serve as an excellent model in which the regulation of mammary development and gene expression can be investigated.

Primary culture of normal rat mammary epithelial cells within a basement membrane matrix. I. Regulation of proliferation by hormones and growth factors

A serum-free primary culture system has been developed which allows for three-dimensional growth and differentiation of normal rat mammary epithelial cells (RMECs) within an extracellular matrix preparation. RMECs were isolated from mammary glands of immature 50- to 60-d-old rats and the organoids embedded within a reconstituted basement membrane matrix prepared from the Engelbreth-Holm-Swarm sarcoma. Cells grown in a serum-free media consisting of phenol red-free Dulbecco's modified Eagle's medium-F12 culture medium containing 10 micrograms/ml insulin, 1 microgram/ml prolactin, 1 microgram/ml progesterone, 1 microgram/ml hydrocortisone, 10 ng/ml epidermal growth factor (EGF), 1 mg/ml fatty-acid-free bovine serum albumin (BSA), 5 micrograms/ml transferrin, and 5 microM ascorbic acid proliferated extensively (15- to 20-fold increase in cell number as quantitated using the MTT dye assay) over a 2- to 3-wk culture period and remained viable for months in culture. Several types of colonies were observed including the alveolarlike budding cluster which predominates at later times in culture, units with no or various degrees of ductal-like projections, stellate colonies, and two- and three-dimensional web units. Optimal proliferation required insulin, prolactin, progesterone, EGF, and bovine serum albumin. Hydrocortisone was not required for proliferation, but the colonies developing in its absence were morphologically altered, with a high frequency of colonies that formed an extensively branched network with many fine projections. Cell proliferation was also dependent on substratum, with significantly less growth and development occurring in RMECs grown within a type I collagen gel matrix compared to RMECs grown within the reconstituted basement membrane. In conjunction with other studies demonstrating extensive differentiation as well as proliferation, it is concluded that this model should prove to be an important tool to study the hormonal regulation of the growth and development of rat mammary cells.

Effects of substrata on the polarization of bovine endometrial epithelial cells in vitro

Epithelial-cell function requires cellular polarity in which apical membrane surfaces have unique characteristics and cellular organelles are stratified. Physiological investigations of endometrial epithelial cells would be enhanced greatly by the ability of a method to polarize cells in culture. This study investigates the effects of different substrata on polarization of cultured bovine endometrial epithelial cells. Fetal bovine endometrial epithelial-cell lines were developed from explant outgrowth. Epithelial monolayers were subcultured onto amniotic membranes, Millicell-HA membranes, or Millicell-CM membranes coated with rat-tail collagen, Matrigel, laminin, Vitrogen, or fibronectin. Cultures on these substrata were maintained at the air/liquid interface. Cells grown on either collagen-coated or uncoated Millicell membranes also were maintained submerged in medium. Excellent polarized morphology was attained in cultures grown at the air/liquid interface on amniotic membranes and rat-tail collagen-coated membranes. Lectin-binding patterns to apical membranes of polarized epithelial cell cultures paralleled patterns of binding to bovine endometrial surfaces in vivo. Cultures on rat-tail collagen were maintained for several weeks. These methods provide a valuable system for studying the endometrium in vitro.

Influence of mammary cell differentiation on the expression of proteins encoded by endogenous BALB/c mouse mammary tumor virus genes

The interactions between differentiation-associated cellular events in the intact mammary gland or in cultured mammary cells and the post-transcriptional activity of the endogenous mouse mammary tumor virus (MMTV) loci were investigated. The transcriptional activities of the endogenous MMTV proviruses of the BALB/c mouse strain (Mtv-6, Mtv-8 and Mtv-9) appear to be regulated differentially during pregnancy-induced mammary gland development (J.E. Knepper, D. Medina and J.S. Butel, J. Virol. 59, 518-521, 1986). Analysis of MMTV-specific proteins at various stages of mammary gland development (virgin, midpregnant, lactating, regressing) established the presence of steady-state levels of a 67,000-Mr env precursor-type polypeptide at all physiological stages. However, processing to lower-molecular-weight env-specific proteins, including a predominant 50,000-Mr species, was detected only with the transition to the functional mammary gland phenotype. The contributions of cell proliferation, cell-matrix interactions, and modulation of functional activity to the pattern of endogenous MMTV protein expression were investigated using a 3-dimensional collagen type I culture system. Growth and cell-matrix interactions (cell polarization, lumen formation) leading to formation of 3-dimensional duct-like structures were permissive for the synthesis and processing of MMTV-specific proteins; accumulation of high levels of the 50,000-Mr env-specific polypeptide was associated with the onset of the fully functional mammary cell phenotype. Expression of MMTV-specific proteins was not due to amplification of a specific cell subpopulation. The potential of the full-length Mtv-8 and Mtv-9 proviruses to be transcribed, as indicated by their methylation status, was not dramatically different between differentiated and undifferentiated mammary cells in culture. This study indicates that MMTV transcriptional activity is reflected at the protein level in mammary tissue of BALB/c mice and that viral protein synthesis and processing may serve as important markers of different physiological stages of mammary epithelial cells. These observations also suggest a general approach to the examination of potential modulatory effects of cellular interactions (cell-cell, cell-matrix or both) known to be important in various differentiated epithelial cell systems for the expression of viral genes.

Human endometrial cells grown on an extracellular matrix form simple columnar epithelia and glands

Normal human endometrial cells were grown on an extracellular matrix containing type IV collagen, laminin, heparan sulfate proteoglycan, and entactin (Matrigel). On the extracellular matrix, dispersed endometrial cells remained rounded, and aggregated to form mounds of cells, which continued to grow in this arrangement. At 10 d, light microscopy demonstrated that these mounds were comprised of an eosinophilic substance, containing individual fusiform stromal cells. About 50% of the mounds were covered with a single layer of polarized cuboidal to columnar cells with basal nuclei, whereas 60% contained columnar cells forming glandular structures with open lumina. These polarized cuboidal and columnar cells were epithelial, based on their positive staining for cytokeratins and the possession of microvilli, tonofilaments, abundant glycogen, ribosomes, and primitive junctional complexes. Kreyberg's stain showed the presence of mucin within the lumina of the glands, indicating that they were functional. Thus, human endometrial cells grown on an extracellular matrix form a simple cuboidal to columnar epithelium, a stromal component, and glandular structures, thereby mimicking the in vivo morphology of the endometrium.

Expression of extracellular matrix components is regulated by substratum

Reconstituted basement membranes and extracellular matrices have been demonstrated to affect, positively and dramatically, the production of milk proteins in cultured mammary epithelial cells. Here we show that both the expression and the deposition of extracellular matrix components themselves are regulated by substratum. The steady-state levels of the laminin, type IV collagen, and fibronectin mRNAs in mammary epithelial cells cultured on plastic dishes and on type I collagen gels have been examined, as has the ability of these cells to synthesize, secrete, and deposit laminin and other, extracellular matrix proteins. We demonstrate de novo synthesis of a basement membrane by cells cultured on type I collagen gels which have been floated into the medium. Expression of the mRNA and proteins of basement membranes, however, are quite low in these cultures. In contrast, the levels of laminin, type IV collagen, and fibronectin mRNAs are highest in cells cultured on plastic surfaces, where no basement membrane is deposited. It is suggested that the interaction between epithelial cells and both basement membrane and stromally derived matrices exerts a negative influence on the expression of mRNA for extracellular matrix components. In addition, we show that the capacity for lactational differentiation correlates with conditions that favor the deposition of a continuous basement membrane, and argue that the interaction between specialized epithelial cells and stroma enables them to create their own microenvironment for accurate signal transduction and phenotypic function.

Myogenesis and histogenesis of skeletal muscle on flexible membranes in vitro

Primary muscle cell cultures consisting of single myocytes and fibroblasts are grown on flexible, optically clear biomembranes. Muscle cell growth, fusion and terminal differentiation are normal. A most effective membrane for these cultures is commercially available Saran Wrap. Muscle cultures on Saran will, once differentiated, contract vigorously and will deform the Saran which is pinned to a Sylgard base. At first, the muscle forms a two-dimensional network which ultimately detaches from the Saran membrane allowing an undergrowth of fibroblasts so that these connective tissue cells completely surround groups of muscle fibers. A three-dimensional network is thus formed, held in place through durable adhesions to stainless steel pins. This three-dimensional, highly contractile network is seen to consist of all three connective tissue compartments seen in vivo, the endomysium, perimysium and epimysium. Finally, this muscle shows advanced levels of maturation in that neonatal and adult isoforms of myosin heavy chain are detected together with high levels of myosin fast light chain 3.

Effect of a reconstituted basement membrane on expression of surfactant apoproteins in cultured adult rat alveolar type II cells

Pulmonary surfactant, which is composed of phospholipids and three lung-specific apoproteins, is synthesized and secreted by alveolar type II cells. Previous work from this laboratory (Biochim. Biophys. Acta 1987; 931:143-156) has shown that cell-extracellular matrix interactions and cuboidal cell shape affect both the ultrastructural appearance and pattern of phospholipids synthesized by cultured rat type II cells. In the present study, we have examined the effects of cell-matrix interactions and cell shape on the ability of adult rat type II cells to express the surfactant apoproteins in culture. Isolated adult rat type II cells were cultured for 2, 4, and 8 days on either tissue culture plastic, on an extract of the Engelbreth-Holm-Swarm (EHS) tumor, or on laminin-coated plastic dishes. Expression of surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) was evaluated by Northern analysis using specific rat cDNA probes for these mRNAs. SP-A content was determined by enzyme-linked immunosorbent assay using a polyclonal antibody raised against rat SP-A purified from lavage. Type II cells cultured on plastic dishes assumed an attenuated morphology soon after being placed in culture. Except for an occasional positive signal on day 2 of culture, these cells were uniformly negative for the presence of mRNA for SP-A, SP-B, or SP-C. Type II cells cultured on plastic did not contain SP-A. In contrast, type II cells cultured on EHS gels formed three-dimensional aggregates on the surface of the substratum; these aggregates were composed of polarized cells that had their apical surfaces directed inward. Type II cells cultured on this substratum showed a positive signal for mRNA for all three surfactant proteins; the abundance of these mRNAs, however, was significantly below that seen in freshly isolated type II cells. While the abundance of mRNA for SP-A and SP-B steadily increased with time in culture under these conditions, the abundance of SP-C mRNA decreased, suggesting that SP-C is regulated independently of SP-A and SP-B. These cultures were also positive for SP-A content, which increased with increasing time in culture. Type II cells cultured on laminin-coated dishes initially spread more slowly across the culture surface than cells on plastic, but were extremely attenuated by day 8 in culture. These cells contained neither SP-A, nor mRNA for any of the three surfactant proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Casein accumulation in distended rough endoplasmic reticulum of collagen gel-cultivated mouse mammary epithelia

Mouse mammary epithelial cells cultivated on collagen gels synthesize and secrete casein in a hormone-dependent manner. Fine-structure electron microscopy of secretory cultures revealed numerous cytoplasmic structures surrounded by membrane that is studded with ribosomes. The structures appear to be distended rough endoplasmic reticulum (RER). Electron microscope protein A-colloidal gold immunolocalization showed casein antiserum-specific deposition of gold particles over the RER cytoplasmic vesicles in cells provided insulin, prolactin, and hydrocortisone (IPF). Nonimmune antiserum showed no gold particle deposition over these cytoplasmic structures. Epithelia provided only insulin showed no such cytoplasmic vesicles nor any specific deposition of gold particles. Immunoblot analysis of cell lysate and culture medium showed casein only in IPF-treated cultures. It appears that the casein secretory pathway in collagen gel cultured mammary epithelia is blocked at the step that fuses RER vesicles to Golgi membrane. The data raise questions regarding the processing and maturation of casein and the mechanism of casein secretion in these cultures.

Growth of epithelium from a preneoplastic mammary outgrowth in response to mammary adipose tissue

We investigated the effects of conditioned media derived from mouse mammary fat pads on the proliferation of CL-S1 cells, an epithelial cell line originally isolated from a preneoplastic mammary outgrowth line. Cell proliferation in vitro in serum-free defined medium was compared to that in this medium conditioned using intact mammary fat pad pieces or isolated fat pad adipocytes. Culture medium was conditioned by incubating the conditioning material in defined culture medium for 24 h at 37 degrees C. Conditioned medium induced CL-S1 proliferation as much as 10- to 20-fold above the minimal levels of growth in control cultures after 13 d of culture. The growth-stimulatory factor(s) had an apparent molecular weight of greater than 10 kDa. This growth-stimulatory activity was both heat and trypsin stable. Because the role of adipose tissue is to store and release lipids, we next tested whether lipids are released during medium conditioning. The lipid composition of the fat pad conditioned medium was characterized using both thin layer and gas liquid chromatography. These lipid analyses indicated that the fat pad pieces released significant amounts of fatty acids and phospholipids into the medium during the conditioning period. The free fatty acid composition included both saturated and unsaturated molecules, and about 80% of the total fatty acids consisted of palmitate, stearate, oleate, and linoleate. These same fatty acids were a structural component of the majority of phospholipid found in the medium. The addition of palmitate or stearate to defined medium had no effect or was inhibitory for CL-S1 proliferation, depending on the concentration used. Defined medium supplemented with oleate, arachidonate, or linoleate induced CL-S1 proliferation, and the inhibitory effects of palmitate and stearate were overcome by addition of oleate and linoleate. These data indicate that both unsaturated and saturated fatty acids are released from intact adipose cells of the mouse mammary fat pad and that fatty acids can influence the growth of preneoplastic mouse mammary epithelium. Thus, unsaturated fatty acids, perhaps in conjunction with other substances released simultaneously, are candidate molecules for the substances that mediate the effect of adipose tissue on growth of epithelium.

Synthesis of calelectrins and calpactin I during cytochalasin mediated cell spreading inhibition

Mammary epithelial cell spreading on collagen gels has previously been shown to be correlated with the synthesis of a group of calcium-binding proteins (CBPs) which we have identified as the calcium-binding proteins termed calelectrins and calpactin I monomer/p36. To determine whether cell spreading per se is required for CBP synthesis, we examined the effect of cytochalasin D on these two events. Concentrations of cytochalasin D that did not reduce total protein synthesis, caused inhibition of cell spreading in a dose-dependent manner, but did not cause inhibition of CBP synthesis. Synthesis of collagen also continued during cytochalasin inhibition of cell spreading. Removal of the inhibitor from the cultures initiated cell spreading and CBP synthesis continued. Membrane-cytoskeleton complexes from control and CD treated cells were identical in regard to binding CBPs in a calcium-dependent manner. Colchicine, which inhibited cell spreading, was shown to be toxic to general protein synthesis at 75 nM. The data clearly indicate that mere inhibition of epithelial cell spreading does not automatically suppress CBP synthesis.

Both cell substratum regulation and hormonal regulation of milk protein gene expression are exerted primarily at the posttranscriptional level

The mechanism by which individual peptide and steroid hormones and cell-substratum interactions regulate milk protein gene expression has been studied in the COMMA-D mammary epithelial cell line. In the presence of insulin, hydrocortisone, and prolactin, growth of COMMA-D cells on floating collagen gels in comparison with that on a plastic substratum resulted in a 2.5- to 3-fold increase in the relative rate of beta-casein gene transcription but a 37-fold increase in beta-casein mRNA accumulation. In contrast, whey acidic protein gene transcription was constitutive in COMMA-D cells grown on either substratum, but its mRNA was unstable and little intact mature mRNA was detected. Culturing COMMA-D cells on collagen also promoted increased expression of other genes expressed in differentiated mammary epithelial cells, including those encoding alpha- and gamma-casein, transferrin, malic enzyme, and phosphoenolpyruvate carboxykinase but decreased the expression of actin and histone genes. Using COMMA-D cells, we defined further the role of individual hormones in influencing beta-casein gene transcription. With insulin alone, a basal level of beta-casein gene transcription was detected in COMMA-D cells grown on floating collagen gels. Addition of prolactin but not hydrocortisone resulted in a 2.5- to 3.0-fold increase in beta-casein gene transcription, but both hormones were required to elicit the maximal 73-fold induction in mRNA accumulation. This posttranscriptional effect of hormones on casein mRNA accumulation preceded any detectable changes in the relative rate of transcription. Thus, regulation by both hormones and cell substratum of casein gene expression is exerted primarily at the post transcriptional level.

Both cell substratum regulation and hormonal regulation of milk protein gene expression are exerted primarily at the posttranscriptional level

The mechanism by which individual peptide and steroid hormones and cell-substratum interactions regulate milk protein gene expression has been studied in the COMMA-D mammary epithelial cell line. In the presence of insulin, hydrocortisone, and prolactin, growth of COMMA-D cells on floating collagen gels in comparison with that on a plastic substratum resulted in a 2.5- to 3-fold increase in the relative rate of beta-casein gene transcription but a 37-fold increase in beta-casein mRNA accumulation. In contrast, whey acidic protein gene transcription was constitutive in COMMA-D cells grown on either substratum, but its mRNA was unstable and little intact mature mRNA was detected. Culturing COMMA-D cells on collagen also promoted increased expression of other genes expressed in differentiated mammary epithelial cells, including those encoding alpha- and gamma-casein, transferrin, malic enzyme, and phosphoenolpyruvate carboxykinase but decreased the expression of actin and histone genes. Using COMMA-D cells, we defined further the role of individual hormones in influencing beta-casein gene transcription. With insulin alone, a basal level of beta-casein gene transcription was detected in COMMA-D cells grown on floating collagen gels. Addition of prolactin but not hydrocortisone resulted in a 2.5- to 3.0-fold increase in beta-casein gene transcription, but both hormones were required to elicit the maximal 73-fold induction in mRNA accumulation. This posttranscriptional effect of hormones on casein mRNA accumulation preceded any detectable changes in the relative rate of transcription. Thus, regulation by both hormones and cell substratum of casein gene expression is exerted primarily at the post transcriptional level.

A clonal derivative of mammary epithelial cell line COMMA-D retains stem cell characteristics of unique morphological and functional heterogeneity

The COMMA-D cell line derived from mammary epithelial cells of midpregnant mice was shown previously to be heterogeneous as determined by phase-contrast microscopy, immunocytochemical staining, DNA content, and oncogenic potential (K.D. Danielson et al. (1984) Proc. Natl. Acad. Sci. USA 81, 3756; D. Medina et al. (1986) J. Natl. Cancer Inst. 76, 1143). Clonal subpopulations of COMMA-D cells have now been isolated by both transfection and selection using a dominant-selectable gene transfer vector and by limiting dilution. Despite their clonal origin, these subpopulations in many cases retained the heterogeneity of the parental COMMA-D line. Of 18 clonal lines assayed, only 5 were able to express beta-casein mRNA. Pooled populations of G418-resistant cells expressed substantially higher levels of beta-casein mRNA than the clonal lines. One of the expressing clonal lines, BNW-7, was characterized further, using immunocytochemical techniques. Approximately 10% of BNW-7 cells expressed casein under the appropriate hormonal and cell-substratum conditions by indirect immunofluorescent staining. Casein immunoperoxidase staining of BNW-7 cells on floating collagen gels revealed that casein-producing cells were localized in small alveolar structures, which were formed in a non-hormone-dependent fashion. The cells in these alveolar structures were cuboidal with basally located nuclei, expressed keratin intermediate filament proteins preferentially, and comprised approximately 18% of the total cells. Cells elsewhere on the surface of the gel displayed a flattened morphology, and expressed vimentin intermediate filament proteins preferentially. A proportion of COMMA-D cells, therefore, appeared to have some of the characteristics of mammary stem cells, and retained the ability to differentiate and form phenotypically heterogeneous cell populations in vitro.

Role of the cytoskeleton in laminin induced mammary gene expression

The differentiation of rat mammary epithelial cells is characterized both by morphologic changes and by the expression of a group of milk protein genes. We have previously shown that by culturing these cells on the basement membrane glycoprotein laminin, the synthesis of the milk proteins, transferrin, alpha-casein, and alpha-lactalbumin is induced. In order to determine if this effect is mediated through the cytoskeleton, we have treated these cells with cytochalasin D and colchicine. Treatment with cytochalasin D or colchicine for 24 h inhibits the accumulation of alpha-casein, transferrin, and alpha-lactalbumin without significant effect on general protein synthesis. Pulse chase studies show that cytochalasin D does not alter the intracellular turnover of alpha-casein or transferrin. Additionally, treatment with cytochalasin D causes an early (within 1 h) increase in secretion of alpha-casein and transferrin suggesting that the actin cytoskeleton provides a meshwork for secretory vesicles. The disruption of this network enhances the secretion of preformed proteins. However, long term (24 h) treatment with cytochalasin D inhibits synthesis of these milk proteins. Northern blot analysis indicates that treatment with cytochalasin D or colchicine inhibits the laminin induced increase in alpha-casein, alpha-lactalbumin, and transferrin mRNAs. These studies indicate that the major effect of the cytoskeleton on laminin induced milk protein gene expression occurs at the level of accumulation of mRNAs for these proteins. We conclude that the expression of laminin induced milk protein gene expression in primary rat mammary cultures depends on the integrity of the actin and microtubule cytoskeleton.

In vitro cytodifferentiation of perinatal rat islet cells within a tridimensional matrix of collagen

Cell suspensions prepared by collagenase digestion of pancreases obtained from rat fetuses (21.5 d old) and newborns (2.5 d old) were mixed with a collagen solution and inoculated on a collagen base layer. At the onset of the culture, most acinar cells became necrotic, whereas other epithelial cells proliferated. Most of the cell clusters arranged themselves into simple polarized structures composed of epithelial cells forming hollow spheres, and from these budded neoformed endocrine islets. Scarce fibroblasts were located close to these structures. Immunocytochemical localization of insulin and glucagon, as well as ultrastructural characteristics of the cell types revealed an intrainsular distribution similar to the in vivo localization. Tridimensional matrix of collagen offers, to perinatal pancreatic cells in culture, an environment close to the in vivo conditions: cells reorganize themselves in tissuelike structures and cell interactions concerned in the cytodifferentiation of pancreatic islets occur. This system allows for the study of undifferentiated epithelial cells--the presumed stem cells--differentiating and differentiated endocrine cells in the same preparation.

Mammary gland morphogenesis in vitro: extracellular requirements for the formation of tubules in collagen gels by a cloned rat mammary epithelial cell line

The mechanism of induction of tubular outgrowths in vitro on floating collagen gels and the influence of extracellular factors on this process have been investigated using the clonal rat mammary epithelial cell line, Rama 25. Growth of Rama 25 on such floating gels causes their contraction. Contraction of the gel is accompanied by a 10-fold increase in the number of cells per unit area, a change in cell shape, and a convolution of the epithelial cell sheet. Gels folded over manually show an 11-times higher incidence of tubules along the folds than on the flat surface. Tubular formation begins when cords of cells develop from local proliferations of the cell sheet and become canalized. Tubules follow wrinkles in the gel and branch to yield monopodial, dichotomous, or lobular architecture. Hydrocortisone and insulin, in the presence of serum, stimulate both narrow and thick tubular structures on folded gels, whereas extra additions of 1 ng/ml cholera toxin or 100 ng/ml epidermal growth factor preferentially stimulate thick tubular structures. Floating glutaraldehyde-fixed gels, very thick collagen gels, and collagen gels prepared on the top of rigid steel grids fail to support the formation of tubules, suggesting that flexibility and access of the medium to basal surfaces are important to their genesis. Incorporation of hyaluronic acid into the gel matrix preferentially inhibits the thick tubular outgrowths. Thus, the branching tubular structures generated by Rama 25 can be influenced in different ways by various extracellular factors in the medium and in the gel.

Regulation of rat mammary gene expression by extracellular matrix components

In the mammary gland the induction and maintenance of differentiation are dependent on both lactogenic hormones and the extracellular matrix (ECM). Since mammary epithelial cells differentiate on a basement membrane in vivo we have examined the effects of basement membrane components on the expression of milk protein genes in primary rat mammary cultures. We examined the effects of a basement membrane gel derived from the Englebreth-Holm-Swarm tumor as well as its major component, laminin, on the expression of a group of milk protein genes. We demonstrate that the basement membrane gel induces alpha-casein and alpha-lactalbumin (alpha-LA) accumulation up to 160- and 70-fold, respectively, of that on tissue culture plastic. Laminin, a major component of the basement membrane, also caused significant induction of these same proteins. In order to determine whether these ECM effects occurred at a translational or post-translational level, pulse-chase experiments were performed. These experiments demonstrated that a laminin substratum selectively effects milk protein turnover and secretion. In order to demonstrate whether ECM effects occurred at the level of steady state accumulation of mRNA we performed dot blot and Northern analyses using cloned cDNA probes for alpha-, beta-, and gamma-caseins and alpha-LA. These studies demonstrated that ECM components induced alpha- and beta-caseins up to 10-fold, and alpha-LA up to 3-fold, with no significant effect on gamma-casein. These results demonstrate that milk protein genes are not coordinately regulated by ECM components. Furthermore, since the amount of induction of milk proteins exceeds the amount of induction of mRNAs for these proteins, we conclude that in our system a major effect of ECM components is at the translational and/or post-translational levels. Based on these findings we propose a model in which basement membrane components effect mammary gene expression at multiple levels.

Functional differentiation of alveolar type II epithelial cells in vitro: effects of cell shape, cell-matrix interactions and cell-cell interactions

Alveolar type II epithelial cells rapidly lose characteristics of differentiated function when cultured on plastic dishes. We have attempted to circumvent this problem by culturing type II cells under conditions that might better reproduce their environment in vivo. Cell-matrix interactions were studied by culturing isolated adult rat type II cells on Engelbreth-Holm-Swarm (EHS) tumor basement membrane. Aggregates of type II cells formed on the surface of the matrix during 4 days in culture. Microscopic examination of these aggregates revealed cuboidal cells that retained more characteristics of differentiated type II cells than did cells cultured on plastic. Type II cells cultured on EHS matrix incorporated a higher percentage of acetate into phosphatidylcholine (PC) than did cells on plastic, and a higher percentage of this PC was saturated. Phosphatidylglycerol (PG) synthesis by these cells was no different from that seen in cells on plastic. The effects of cell-cell interactions and cell shape were evaluated by culturing type II cells on feeder layers that in turn were grown on collagen gels. The feeder layer cells included fetal rat lung fibroblasts, adult rat lung fibroblasts, fetal rat skin fibroblasts, bovine aortic endothelial cells, and rat mammary tumor epithelial cells. One-half of the gels remained attached to the culture dish and one-half of the gels were detached after 24 h and allowed to float free in the medium. Type II cells grown in association with any of the attached feeder layers became flattened and lost their differentiated phenotype. These cells incorporated no greater percentage of acetate into PC than did cells on plastic. Saturated PC synthesis was modestly increased. PG synthesis declined in parallel with that seen in cells cultured on plastic. Type II cells cultured on feeder layers that were detached assumed their native cuboidal shape and also exhibited many morphological characteristics of differentiated function. These cells incorporated a significantly greater percentage of acetate into PC compared to cells on either plastic or attached feeder layers. Saturated PC synthesis also increased markedly. These cells, however, incorporated no greater percentage of acetate into PG than did cells on plastic or attached feeder layers. These data suggest an important role for cell shape and cell-matrix interactions and maintenance of type II cell differentiation. The effects of cell-cell interactions, while beneficial, appear to be non-specific.

A novel system for mammary epithelial cell culture

A method is described for the isolation and density gradient enrichment of mammary epithelial fragments from pregnant, nonlactating bovine tissue. Immunocytochemical analysis prior to and following culture revealed specific staining with antibodies to keratin, indicating that these cells are epithelial in nature. Fragments enriched for epithelium could be stored in liquid nitrogen for extended periods prior to culture. When cast within a three-dimensional matrix of collagen gel, the mammary fragments grew as branching, duct-like structures and displayed a 4-fold increase in cell number during 10 to 12 d of culture.

Casein gene expression in mouse mammary epithelial cell lines: dependence upon extracellular matrix and cell type

The COMMA-D mammary cell line exhibits mammary-specific functional differentiation under appropriate conditions in cell culture. The cytologically heterogeneous COMMA-D parental line and the clonal lines DB-1, TA-5, and FA-1 derived from the COMMA-D parent were examined for similar properties of functional differentiation. In monolayer cell culture, the cell lines DB-1, TA-5, FA-1, and MA-4 were examined for expression of mammary-specific and epithelial-specific proteins by an indirect immunofluorescence assay. The clonal cell lines were relatively homogeneous in their respective staining properties and seemed to represent three subpopulations found in the heterogeneous parental COMMA-D line. None of the four clonal lines appeared to represent myoepithelial cells. The cell lines were examined for expression of beta-casein mRNA in the presence or absence of prolactin. The heterogeneous COMMA-D line, but none of the clonal lines, was induced by the presence of prolactin to produce significantly increased levels of beta-casein MRNA. The inducibility of beta-casein in the COMMA-D cell line was further enhanced by a reconstituted basement membrane preparation enriched in laminin, collagen IV, and proteoglycans. Individual matrix components of laminin, fibronectin, heparan sulfate, heparan, or hyaluronic acid were not effective as substrata for the induction of beta-casein mRNA. These results support the hypothesis that the functional response of inducible mammary cell populations is a result of interaction among hormones, multiple extracellular matrix components, and specific cell types.