Extracellular matrix and hormones transcriptionally regulate bovine beta-casein 5' sequences in stably transfected mouse mammary cells

Expression analysis of the individual bovine beta-, alpha s2- and kappa-casein genes in transgenic mice

To identify cis-acting regulatory elements involved in the regulation of expression of the casein genes, the bovine beta-, alpha s2- and kappa-casein genes were isolated from cosmid libraries and introduced into the murine germline. Bovine casein expression was analysed at the RNA and protein level. The bovine beta-casein gene, including 16 kb of 5'- and 8 kb of 3'-flanking region, appeared to be expressed in all 12 transgenic mouse lines analysed. In 50% of these lines expression levels in milk exceeded 1 mg/ml. Three lines displayed expression levels comparable with or well above (20 mg/ml) the beta-casein levels in bovine milk. Transgene expression was restricted to the mammary gland. Strong induction of expression occurred at parturition and thus resembled the bovine rather than the murine pattern. In spite of this high-level tissue-specific and developmentally regulated expression, beta-casein expression levels were integration-site-dependent, suggesting that not all elements involved in regulation of expression were included in this beta-casein clone. Neither the bovine alpha s2- nor the kappa-casein gene, including 8 kb and 5 kb of 5'- and 1.5 kb and 19 kb of 3'-flanking sequences respectively, were properly expressed in transgenic mice. However, they were transcribed in stably transfected mouse mammary epithelial cells. This indicates that regulatory elements required for high-level, mammary gland-specific expression are not present in the alpha s2- and kappa-casein clones used in this study and are probably located elsewhere in the casein gene locus.

Prolactin-mediated gene activation in mammary epithelial cells

Mammary epithelial cells grow and develop with the onset of sexual maturity. In addition, lobular alveolar structures are formed during pregnancy, and quiescent differentiated cells secrete high levels of milk proteins after parturition. These events are governed by multiple hormones and growth factors and involve the sequential and synergistic action of functionally distinct signal transduction pathways. Milk protein genes have been analyzed and composite response elements have been identified in the promoter sequences. Transcription factors, which relay the hormonal signals, bind to these sequences. The factor that confers prolactin simulation to milk protein gene transcription has recently been identified. MGF/Stat5 is a latent transcription factor that becomes activated by a tyrosine-specific protein kinase, Jak2, associated with the prolactin receptor. Tyrosine phosphorylation converts the latent factor into one with DNA-binding and transcriptional activation potential. The regulation of MGF/Stat5 in vitro and in vivo indicates that it is a central component of the lactogenic hormone signaling pathway. Involvement of MGF/Stat5 in the signaling by other cytokines indicates that the same factor might be involved in regulation of growth-promoting genes, primarily in hematopoietic cells.

A hierarchy of ECM-mediated signalling regulates tissue-specific gene expression

A dynamic and reciprocal flow of information between cells and the extracellular matrix contributes significantly to the regulation of form and function in developing systems. Signals generated by the extracellular matrix do not act in isolation. Instead, they are processed within the context of global signalling hierarchies whose constituent inputs and outputs are constantly modulated by all the factors present in the cell's surrounding microenvironment. This is particularly evident in the mammary gland, where the construction and subsequent destruction of such a hierarchy regulates changes in tissue-specific gene expression, morphogenesis and apoptosis during each developmental cycle of pregnancy, lactation and involution.

Stat5 as a target for regulation by extracellular matrix

Transcription of tissue-specific genes in mammary gland requires signals from both prolactin and basement membrane. Here we address the mechanism by which this specialized extracellular matrix regulates transcription. Using mammary cell cultures derived from transgenic mice harboring the ovine beta-lactoglobulin gene, we show that either a basement membrane extract, or purified laminin-1, induced high levels of beta-lactoglobulin synthesis. It is known that prolactin signals through Stat5 (signal transducer and activator of transcription). This transcription factor interacts with gamma-interferon activation site-related motifs within the beta-lactoglobulin promoter, which we show are required for matrix dependence of beta-lactoglobulin expression. The DNA binding activity of Stat5 was present only in extracts of mammary cells cultured on basement membrane, indicating that the activation state of Stat5 is regulated by the type of substratum the cell encounters. Thus, basement membrane controls transcription of milk protein genes through the Stat5-mediated prolactin signaling pathway, providing a molecular explanation for previous studies implicating extracellular matrix in the control of mammary differentiation.

Activation of STAT factors by prolactin, interferon-gamma, growth hormones, and a tyrosine phosphatase inhibitor in rabbit primary mammary epithelial cells

In numerous studies on mammary epithelial cell lines multiple factors, added to the medium or contained in the serum, were required for casein gene expression. It has been shown in these systems that the mammary gland factor (MGF) is implicated in the activation of the beta-casein gene promoter. In the present study, we determined the relationship between known agents that affect casein gene expression and MGF activity using the properties of rabbit primary mammary epithelial cells to respond to PRL alone, when cultured in chemically defined medium. We demonstrate that MGF is rapidly activated by PRL alone or by human growth hormone, a natural ligand of many PRL receptors (PRL-Rs), in the cytoplasm and accumulated in the nucleus. The MGF activation by PRL occurred in the absence of endogenous extracellular matrix, a condition where casein synthesis is known to be markedly reduced. Different inhibitors of protein-tyrosine kinases, which have been shown to reduce casein mRNA synthesis, but not of protein kinase C, decrease the MGF activity. A tyrosine phosphatase inhibitor, sodium pervanadate, induced two GAS-binding complexes related to MGF and STAT1. Our data show that MGF is a latent cytoplasmic factor rapidly activated in mammary epithelial cells, by a mechanism involving a tyrosine kinase and a tyrosine phosphatase.

CCAAT/enhancer-binding protein isoforms beta and delta are expressed in mammary epithelial cells and bind to multiple sites in the beta-casein gene promoter

Lactogenic hormone-dependent expression of the rat beta-casein gene in mammary epithelial cells is controlled via a complex regulatory region in the promoter. The sequence between -176 and -82 is the minimal region to confer the response to glucocorticoid hormone and prolactin on a heterologous promoter. The response is further enhanced by the region between -282 and -176. DNase I footprinting experiments and electromobility shift assays revealed the presence of four binding sites for CCAAT/enhancer-binding protein (C/EBP) isoforms in the hormone response region between -220 and -132. In nuclear extracts from mammary epithelial cells, the prevalent C/EBP isoform binding to these sites is beta (C/EBP-beta). C/EBP-delta is also present in mammary epithelial cells, whereas C/EBP-alpha is not detectable. The C/EBP sites are located in close proximity to the previously characterized binding sites for the prolactin-inducible mammary gland factor/signal transducer and activator of transcription-5, the nuclear factor YY1, and the glucocorticoid receptor. The importance of the two proximal C/EBP binding sites at the 5' border of the minimal region was tested by mutational analysis. Mutations of each site were found to inhibit strongly both the basal and the lactogenic hormone-induced transcription of a beta-casein gene promoter chloramphenicol acetyltransferase construct. The results implicate C/EBPs as important regulators of beta-casein gene expression in the mammary epithelium.

Integrin-mediated tyrosine phosphorylation and cytokine message induction in monocytic cells. A possible signaling role for the Syk tyrosine kinase

Activation of cytoplasmic tyrosine kinases is an important aspect of signal transduction mediated by integrins. In the human monocytic cell line THP-1, either integrin-dependent cell adhesion to fibronectin or ligation of beta 1 integrins with antibodies causes a rapid and intense tyrosine phosphorylation of two sets of proteins of about 65-75 and 120-125 kDa. In addition, integrin ligation leads to nuclear translocation of the p50 and p65 subunits of the NF-kappa B transcription factor, to activation of a reporter gene driven by a promoter containing NF-kappa B sites, and to increased levels of mRNAs for immediate-early genes, including the cytokine interleukin (IL)-1 beta. The tyrosine kinase inhibitors genistein and herbimycin A block both integrin-mediated tyrosine phosphorylation and increases in IL-1 beta message levels, indicating a causal relationship between the two events. The components tyrosine phosphorylated subsequent to cell adhesion include paxillin, pp125FAK, and the SH2 domain containing tyrosine kinase Syk. In contrast, integrin ligation with antibodies induces tyrosine phosphorylation of Syk but not of FAK or paxillin. In adhering cells, pre-treatment with cytochalasin D suppresses tyrosine phosphorylation of FAK and paxillin but not of Syk, while IL-1 beta message induction is unaffected. These observations indicate that the Syk tyrosine kinase may be an important component of an integrin signaling pathway in monocytic cells, leading to activation of NF-kappa B and to increased levels of cytokine messages.

Proteoglycan biosynthesis is required in BC3H1 myogenic cells for modulation of vascular smooth muscle alpha-actin gene expression in response to microenvironmental signals

Induction of vascular smooth muscle (VSM) alpha-actin mRNA expression during cytodifferentiation of mouse BC3H1 myogenic cells coincides with the accumulation of cell surface- and extracellular matrix-associated sulfated proteoglycans. Inhibition of proteoglycan biosynthesis in myogenic cells using an artificial beta-D-xyloside glycosaminoglycan acceptor was accompanied by a reduction in cell surface/extracellular matrix proteoglycans and VSM alpha-actin mRNA expression while enhancing the secretion of free chondroitin sulfate/dermatan sulfate and heparan sulfate glycosaminoglycans into the culture medium. Maximum inhibition of VSM alpha-actin mRNA expression required that proteoglycan biosynthesis be blocked during the early phase of cytodifferentiation when myoblasts were fully confluent and quiescent. The inhibitory effect of beta-D-xyloside on alpha-actin mRNA expression resulted from attenuation at both the transcriptional and post-transcriptional control points. Sustained proteoglycan biosynthesis was required for induction of VSM alpha-actin mRNA in quiescent myoblasts in response to cytodifferentiation-permissive, substrate-associated macromolecules (SAM) or upon exposure to soluble serum factors capable of transiently stimulating VSM alpha-actin gene transcription. The results suggested that efficient myoblast cytodifferentiation and modulation of VSM alpha-actin mRNA levels depended on intact cell surface proteoglycans to convey signals generated as a consequence of cellular interaction with substrate components and serum factors.

Dynamic reciprocity revisited: a continuous, bidirectional flow of information between cells and the extracellular matrix regulates mammary epithelial cell function

Interactions between cells and the extracellular matrix (ECM) generate two classes of signals, mechanical and biochemical. In the case of the mammary epithelial cell, both are required to initiate ECM-dependent expression of the abundant milk protein beta-casein. Mechanical signals induce a cellular rounding, while functional biochemical signals are associated with an increase in tyrosine phosphorylation. These individual components are part of a complex signalling hierarchy that leads to the emergence of the fully functional lactational phenotype. Interestingly, both the assembly and disassembly of this hierarchy, which occur cyclically in vivo, are constantly modulated by dynamic and reciprocal interactions that take place within a functional unit composed of both the cell and the ECM.

Components of the nuclear signaling cascade that regulate collagenase gene expression in response to integrin-derived signals

We have shown previously that the expression of collagenase is upregulated in rabbit synovial fibroblasts cultured on a substrate of antibody to the alpha 5 chain of the alpha 5 beta 1 integrin fibronectin receptor or on the 120-kD cell-binding chymotryptic fragment of plasma fibronectin, but remains at basal levels in cells plated on intact plasma fibronectin. We now have identified some of the components of a signaling pathway that couples the fibronectin receptor to the induction of collagenase transcription. We studied the control of collagenase gene expression in cells adhering to the 120-kD fragment of fibronectin, to antifibronectin receptor antibody, or to plasma fibronectin by transiently introducing promoter-reporter constructs into rabbit synovial fibroblasts before plating cells on these matrices. The constructs contained segments of the human collagenase promoter regulating transcription of chloramphenicol acyl transferase. Expression of constructs containing the -1200/-42-bp segment or the -139/-42-bp segment of the collagenase promoter inserted upstream from the reporter gene was induced to similar extents in cells plated on the 120-kD fragment of fibronectin or on anti-fibronectin receptor antibody, relative to that in fibroblasts plated on fibronectin. The expression of the construct containing the -66/-42-bp segment of the promoter was not regulated and was similar to that of the parent pBLCAT2 plasmid, suggesting that the -139/-67 region of the collagenase promoter, which contains PEA3- and AP1-binding sites, regulates the transcription of collagenase caused by integrin-derived signals. Expression of a reporter construct containing only the PEA3 and AP1 sites in the collagenase promoter (-90/-67) also increased in cells plated on the 120-kD fragment of fibronectin or on anti-fibronectin receptor antibody, relative to that in cells plated on fibronectin. Mutations in either the AP1 or PEA3 site of this minimal promoter abrogated its activity in cells plated on these inductive ligands. Expression of c-fos mRNA increased within 1 h of plating cells on the 120-kD fibronectin fragment or on anti-fibronectin receptor antibody, relative to that in cells plated on fibronectin. c-Fos protein accumulated in the nuclei of fibroblasts within 10 min of plating on the 120-kD fibronectin fragment. The increase in c-Fos was required for the increase in collagenase in cells plated on the 120-kD fibronectin fragment: incubation of cells with antisense, but not sense, c-fos oligonucleotides diminished both basal and induced expression of the -139/-42 collagenase promoter-reporter construct and decreased expression of the endogenous collagenase gene.(ABSTRACT TRUNCATED AT 400 WORDS)

Mammary epithelial cell differentiation in vitro is regulated by an interplay of EGF action and tenascin-C downregulation

Expression of the extracellular matrix glycoprotein tenascin-C in the mammary gland is associated with cellular proliferation and cell motility during organogenesis and tumorigenesis. Because the source and the regulation of tenascin-C in these tissues are unclear, we have used tenascin-C cDNA, FITC-immunofluorescence and immuno-precipitation to examine tenascin-C expression of mammary epithelial cells. Using several mammary epithelial cell lines we could show that tenascin-C can be produced and secreted by epithelial cells. However it was found that tenascin-C synthesis was inversely correlated with the polarized epithelial phenotype. Among three mouse mammary epithelial cell clones, tenascin-C expression was most abundant in HC-11 cells, the least differentiated cell type. Expression levels were high during the growth phase but were nearly abolished when cells were grown to confluence and induced to express milk proteins. Downregulation of tenascin-C by EGF apparently commits HC-11 cells to respond to lactogenic hormones and consequently, hormone induced levels of beta-casein mRNA decreased significantly when HC-11 cells were grown on a tenascin-C substrate. On the other hand, TGF-beta, another growth factor involved in coordinated growth and differentiation of the mammary gland in vivo was found to be a very potent inducer of tenascin-C. The generation of fully polarized and tight epithelium affected the levels of tenascin-C expression. In contrast to HC-11 cells, which do not form epithelial domes in vitro, highly polarized and dome forming EpH4 and Fos-ER cells nearly lacked tenascin-C. Similarly, induction of dome formation in the rat mammary stem cell line Rama 25 by the differentiation inducer dimethylsulfoxide caused a loss of TN-C-transcripts. The inability of Fos-ER cells to develop domes in the presence of soluble tenascin-C also suggests its interference with induction and maintenance of mammary epithelial cell differentiation.

Suppression of mammary epithelial cell differentiation by the helix-loop-helix protein Id-1

Cell proliferation and differentiation are precisely coordinated during the development and maturation of the mammary gland, and this balance invariably is disrupted during carcinogenesis. Little is known about the cell-specific transcription factors that regulate these processes in the mammary gland. The mouse mammary epithelial cell line SCp2 grows well under standard culture conditions but arrests growth, forms alveolus-like structures, and expresses beta-casein, a differentiation marker, 4 to 5 days after exposure to basement membrane and lactogenic hormones (differentiation signals). We show that this differentiation entails a marked decline in the expression of Id-1, a helix-loop-helix (HLH) protein that inactivates basic HLH transcription factors in other cell types. SCp2 cells stably transfected with an Id-1 expression vector grew more rapidly than control cells under standard conditions, but in response to differentiation signals, they arrested growth and formed three-dimensional structures similar to those of control cells. Id-1-expressing cells did not, however, express beta-casein. Moreover, 8 to 10 days after receiving differentiation signals, they lost three-dimensional organization, invaded the basement membrane, and then resumed growth. SCp2 cells expressing an Id-1 antisense vector grew more slowly than controls; in response to differentiation signals, they remained stably growth arrested and fully differentiated, as did control cells. We suggest that Id-1 renders cells refractory to differentiation signals and receptive to growth signals by inactivating one or more basic HLH proteins that coordinate growth and differentiation in the mammary epithelium.

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

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

Laminin mediates tissue-specific gene expression in mammary epithelia

Tissue-specific gene expression in mammary epithelium is dependent on the extracellular matrix as well as hormones. There is good evidence that the basement membrane provides signals for regulating beta-casein expression, and that integrins are involved in this process. Here, we demonstrate that in the presence of lactogenic hormones, laminin can direct expression of the beta-casein gene. Mouse mammary epithelial cells plated on gels of native laminin or laminin-entactin undergo functional differentiation. On tissue culture plastic, mammary cells respond to soluble basement membrane or purified laminin, but not other extracellular matrix components, by synthesizing beta-casein. In mammary cells transfected with chloramphenicol acetyl transferase reporter constructs, laminin activates transcription from the beta-casein promoter through a specific enhancer element. The inductive effect of laminin on casein expression was specifically blocked by the E3 fragment of the carboxy terminal region of the alpha 1 chain of laminin, by antisera raised against the E3 fragment, and by a peptide corresponding to a sequence within this region. Our results demonstrate that laminin can direct tissue-specific gene expression in epithelial cells through its globular domain.

Suppression of ICE and apoptosis in mammary epithelial cells by extracellular matrix

Apoptosis (programmed cell death) plays a major role in development and tissue regeneration. Basement membrane extracellular matrix (ECM), but not fibronectin or collagen, was shown to suppress apoptosis of mammary epithelial cells in tissue culture and in vivo. Apoptosis was induced by antibodies to beta 1 integrins or by overexpression of stromelysin-1, which degrades ECM. Expression of interleukin-1 beta converting enzyme (ICE) correlated with the loss of ECM, and inhibitors of ICE activity prevented apoptosis. These results suggest that ECM regulates apoptosis in mammary epithelial cells through an integrin-dependent negative regulation of ICE expression.

Alteration of collagen-dependent adhesion, motility, and morphogenesis by the expression of antisense alpha 2 integrin mRNA in mammary cells

Although integrins are known to mediate adhesive binding of cells to the extracellular matrix, their role in mediating cellular growth, morphology, and differentiation is less clear. To determine more directly the role of the alpha 2 beta 1 integrin, a collagen and laminin receptor, in mediating the collagen-dependent differentiation of mammary cells, we reduced expression of the integrin by the well differentiated human breast carcinoma cell line, T47D, by stably expressing alpha 2 integrin antisense mRNA. Flow cytometry demonstrated that the antisense-expressing clones had levels of alpha 2 beta 1 integrin on their surfaces that were decreased by 30–70%. Adhesion of antisense-expressing clones to both collagens I and IV was decreased relative to controls in a manner that correlated with the level of cell surface alpha 2 beta 1 integrin expression. Adhesion to fibronectin and laminin were not affected. Motility across collagen-coated filters in haptotaxis assays was increased for only those clones that exhibited intermediate levels of adhesion to collagen, suggesting that an intermediate density of cell-surface alpha 2 beta 1 integrin optimally supports cell motility. When cultured in three-dimensional collagen gels, T47D cells organized in a manner suggestive of a glandular epithelium. In contrast, antisense-expressing clones with decreased alpha 2 beta 1 integrin were not able to organize in three-dimensional collagen gels. The growth rate of T47D cells was reduced when the cells were cultured in three-dimensional collagen gels. Unlike adhesion, motility, and morphogenesis, growth rates were unaffected by reduction of alpha 2 beta 1 integrin expression. Our results suggest that adhesive interactions mediated by a critical level of surface alpha 2 beta 1 integrin expression are key determinants of the collagen-dependent morphogenetic capacity of mammary epithelial cells.

Tenascin-C inhibits extracellular matrix-dependent gene expression in mammary epithelial cells. Localization of active regions using recombinant tenascin fragments

The physiological role of tenascin in vivo has remained obscure. Although tenascin is regulated in a stage and tissue-dependent manner, knock-out mice appear normal. When tenascin expression was examined in the normal adult mouse mammary gland, little or none was present during lactation, when epithelial cells actively synthesize and secrete milk proteins in an extracellular matrix/lactogenic hormone-dependent manner. In contrast, tenascin was prominently expressed during involution, a stage characterized by the degradation of the extracellular matrix and the subsequent loss of milk production. Studies with mammary cell lines indicated that tenascin expression was high on plastic, but was suppressed in the presence of the laminin-rich, Engelbreth-Holm-Swarm (EHS) tumour biomatrix. When exogenous tenascin was added together with EHS to mammary epithelial cells, beta-casein protein synthesis and steady-state mRNA levels were inhibited in a concentration-dependent manner. Moreover, this inhibition by tenascin could be segregated from its effects on cell morphology. Using two beta-casein promoter constructs attached to the chloramphenicol acetyltransferase reporter gene we showed that tenascin selectively suppressed extracellular matrix/prolactin-dependent transcription of the beta-casein gene in three-dimensional cultures. Finally, we mapped the active regions within the fibronectin type III repeat region of the tenascin molecule that are capable of inhibiting beta-casein protein synthesis. Our data are consistent with a model where both the loss of a laminin-rich basement membrane by extracellular matrix-degrading enzymes and the induction of tenascin contribute to the loss of tissue-specific gene expression and thus the involuting process.

Matrix/integrin interaction activates the mitogen-activated protein kinase, p44erk-1 and p42erk-2

Cell adhesion to extracellular matrix proteins is a dynamic process leading to dramatic changes in the cell phenotype. Integrins are one of the major receptor families that mediate cell-matrix contact. Evidence that integrins can act as signal transducing molecules has accumulated over the past few years. We report here that p44erk-1 and p42erk-2 mitogen-activated protein (MAP) kinases are rapidly phosphorylated on tyrosine residues upon adhesion of human skin fibroblasts to fibronectin or upon cross-linking of beta 1 integrins with antibody. The tyrosine phosphorylation of both kinases is associated with increased enzymatic activity. Pretreatment of the cells with cytochalasin D, which selectively disrupts the network of the actin filaments, completely inhibits this adhesion-mediated MAP kinase activation. Thus, our findings indicate that ligation of beta 1 integrins induces an increase in both tyrosine phosphorylation and enzymatic activity of p44erk-1 and p42erk-2 MAP kinases, and that the integrity of the actin cytoskeleton is essential in this process. Since MAP kinase behaves as a convergence point for diverse receptor-initiated signaling events at the plasma membrane, this serine/threonine kinase plays a key role and helps to account for the diversity of integrin-dependent cell functions.

Extracellular matrix-dependent tissue-specific gene expression in mammary epithelial cells requires both physical and biochemical signal transduction

Extracellular matrix (ECM) profoundly influences the growth and differentiation of the mammary gland epithelium, both in culture and in vivo. Utilizing a clonal population of mouse mammary epithelial cells that absolutely requires an exogenous ECM for function, we developed a rapid assay to study signal transduction by ECM. Two components of the cellular response to a basement membrane overlay that result in the expression of the milk protein beta-casein were defined. The first component of this response involves a rounding and clustering of the cells that can be physically mimicked by plating the cells on a nonadhesive substratum. The second component is biochemical in nature, and it is associated with beta 1 integrin clustering and increased tyrosine phosphorylation. The second component is initiated in a morphology-independent manner, but the proper translation of this biochemical signal into a functional response requires cell rounding and cell clustering. Thus, physical and biochemical signal transduction events contribute to the ECM-dependent regulation of tissue-specific gene expression in mouse mammary epithelial cells.

Integrin binding and cell spreading on extracellular matrix act at different points in the cell cycle to promote hepatocyte growth

This study was undertaken to determine the importance of integrin binding and cell shape changes in the control of cell-cycle progression by extracellular matrix (ECM). Primary rat hepatocytes were cultured on ECM-coated dishes in serum-free medium with saturating amounts of growth factors (epidermal growth factor and insulin). Integrin binding and cell spreading were promoted in parallel by plating cells on dishes coated with fibronectin (FN). Integrin binding was separated from cell shape changes by culturing cells on dishes coated with a synthetic arg-gly-asp (RGD)-peptide that acts as an integrin ligand but does not support hepatocyte extension. Expression of early (junB) and late (ras) growth response genes and DNA synthesis were measured to determine whether these substrata induce G0-synchronized hepatocytes to reenter the growth cycle. Cells plated on FN exhibited transient increases in junB and ras gene expression (within 2 and 8 h after plating, respectively) and synchronous entry into S phase. Induction of junB and ras was observed over a similar time course in cells on RGD-coated dishes, however, these round cells did not enter S phase. The possibility that round cells on RGD were blocked in mid to late G1 was confirmed by the finding that when trypsinized and replated onto FN-coated dishes after 30 h of culture, they required a similar time (12-15 h) to reenter S phase as cells that had been spread and allowed to progress through G1 on FN. We have previously shown that hepatocytes remain viable and maintain high levels of liver-specific functions when cultured on these RGD-coated dishes. Thus, these results suggest that ECM acts at two different points in the cell cycle to regulate hepatocyte growth: first, by activating the G0/G1 transition via integrin binding and second, by promoting the G1/S phase transition and switching off the default differentiation program through mechanisms related to cell spreading.

Transcriptional activation by viral enhancers: critical dependence on extracellular matrix-cell interactions in mammary epithelial cells

Extracellular matrix (ECM)-cell interactions are essential for the regulation of many genes in differentiated cell types. A number of expression vectors that work well in cells cultured on tissue-culture plastic appear to be inactive or sporadically active in vivo. We reasoned that these responses also may be influenced by the ECM. We therefore examined three commonly used viral enhancers and found that they all responded either positively or negatively to the presence of exogenous ECM. Using mouse mammary epithelial cells, we found that a mouse mammary tumor virus enhancer linked to its own promoter or to a truncated (and by itself inactive) beta-casein promoter drove transcription efficiently only when the cells were in contact with an ECM (more than a 100-fold induction over tissue-culture plastic). Similarly, the cytomegalovirus enhancer was more active in cells in contact with ECM. In contrast, the simian virus 40 enhancer, linked to the beta-casein promoter, was 12-fold more active in cells on tissue-culture plastic. This activity was strongly reduced when the cells interacted with ECM. Thus, we conclude that different enhancers can respond to ECM by either activating or suppressing transcription. This observation has important implications for understanding the mechanisms of promoter action and for designing expression systems for use in gene therapy.

YY1 represses beta-casein gene expression by preventing the formation of a lactation-associated complex

Site-specific mutagenesis of the highly conserved milk box (-140 to -110) region suggested that beta-casein expression is regulated by a hormone-mediated relief of repression (M. Schmitt-Ney, W. Doppler, R. K. Ball, and B. Groner, Mol. Cell. Biol. 11:3745-3755, 1991). However, when this sequence was placed upstream of a heterologous thymidine kinase promoter, it activated reporter gene expression. This apparent paradox was resolved when the trans-acting factor YY1, capable of acting as both a positive and negative regulator, was shown to interact with the milk box region, using bacterially expressed YY1 and specific oligonucleotide and antibody competition experiments. Second, it was demonstrated that extracts prepared from several cell types contained a protein(s) interacting with the mammary gland-specific factor (MGF) binding site, previously shown to be required for beta-casein promoter activity (Schmitt-Ney et al., Mol. Cell. Biol. 11:3745-3755, 1991). Sequence analysis of this site revealed similarity to the gamma interferon-activated sequence, suggesting that MGF may be related to the stat91 signaling protein. Finally, using an oligonucleotide encompassing both the YY1 and MGF sites, we detected a slow-mobility complex only in extracts from mammary glands at late pregnancy and lactation (lactation-associated complex [LAC]). Site-specific mutation of the YY1 binding site led to an enhancement in LAC DNA binding activity, while mutation of the MGF site decreased detectable LAC. These results support a model in which lactogenic stimuli lead to a decrease in YY1 binding, and subsequent increased formation of LAC at a nearby binding site, to stimulate beta-casein transcription.

YY1 represses beta-casein gene expression by preventing the formation of a lactation-associated complex

Site-specific mutagenesis of the highly conserved milk box (-140 to -110) region suggested that beta-casein expression is regulated by a hormone-mediated relief of repression (M. Schmitt-Ney, W. Doppler, R. K. Ball, and B. Groner, Mol. Cell. Biol. 11:3745-3755, 1991). However, when this sequence was placed upstream of a heterologous thymidine kinase promoter, it activated reporter gene expression. This apparent paradox was resolved when the trans-acting factor YY1, capable of acting as both a positive and negative regulator, was shown to interact with the milk box region, using bacterially expressed YY1 and specific oligonucleotide and antibody competition experiments. Second, it was demonstrated that extracts prepared from several cell types contained a protein(s) interacting with the mammary gland-specific factor (MGF) binding site, previously shown to be required for beta-casein promoter activity (Schmitt-Ney et al., Mol. Cell. Biol. 11:3745-3755, 1991). Sequence analysis of this site revealed similarity to the gamma interferon-activated sequence, suggesting that MGF may be related to the stat91 signaling protein. Finally, using an oligonucleotide encompassing both the YY1 and MGF sites, we detected a slow-mobility complex only in extracts from mammary glands at late pregnancy and lactation (lactation-associated complex [LAC]). Site-specific mutation of the YY1 binding site led to an enhancement in LAC DNA binding activity, while mutation of the MGF site decreased detectable LAC. These results support a model in which lactogenic stimuli lead to a decrease in YY1 binding, and subsequent increased formation of LAC at a nearby binding site, to stimulate beta-casein transcription.

The nuclear factor YY1 participates in repression of the beta-casein gene promoter in mammary epithelial cells and is counteracted by mammary gland factor during lactogenic hormone induction

Expression of the beta-casein milk protein gene in the mammary epithelial cell line HC11 is primarily regulated at the transcriptional level. A 338-bp segment of promoter sequence 5' of the transcription start site is sufficient to confer inducibility by the lactogenic hormones insulin, glucocorticoid hormone, and prolactin. Positively and negatively acting promoter elements and specific DNA binding proteins have been identified. The binding of the mammary gland factor MGF to a site between -80 and -100 is indispensable for hormonal induction of transcription. Binding of MGF activity to DNA is greatly enhanced by the action of the lactogenic hormones. Repression of transcription in the uninduced state is mediated by a promoter element located adjacent to the MGF binding site at positions -110 to -150. This repressor element consists of two interacting protein binding sites. A nuclear factor that binds specifically to the proximal site between positions -110 and -120 has been characterized and found to be identical with the nuclear factor YY1 (delta, NF-E1). YY1 does not bind to the distal site. The simultaneous mutation in the proximal and the distal sites results in high, hormone-independent transcription. This finding suggests that YY1 plays a functional role in the repression and acts in conjunction with a second DNA binding protein. Comparison of YY1 DNA binding activity in uninduced and hormone-induced cells showed that relief of repression is not mediated by changes in the concentration or binding affinity of YY1. Infection of HC11 cells with a YY1-expressing recombinant retrovirus resulted in overexpression of YY1 but did not suppress hormonal induction. The addition of purified MGF decreased YY1 binding to its DNA recognition site in vitro. This finding indicates that MGF regulates the DNA binding activity of YY1 and thereby may cause the relief of transcriptional repression.

The nuclear factor YY1 participates in repression of the beta-casein gene promoter in mammary epithelial cells and is counteracted by mammary gland factor during lactogenic hormone induction

Expression of the beta-casein milk protein gene in the mammary epithelial cell line HC11 is primarily regulated at the transcriptional level. A 338-bp segment of promoter sequence 5' of the transcription start site is sufficient to confer inducibility by the lactogenic hormones insulin, glucocorticoid hormone, and prolactin. Positively and negatively acting promoter elements and specific DNA binding proteins have been identified. The binding of the mammary gland factor MGF to a site between -80 and -100 is indispensable for hormonal induction of transcription. Binding of MGF activity to DNA is greatly enhanced by the action of the lactogenic hormones. Repression of transcription in the uninduced state is mediated by a promoter element located adjacent to the MGF binding site at positions -110 to -150. This repressor element consists of two interacting protein binding sites. A nuclear factor that binds specifically to the proximal site between positions -110 and -120 has been characterized and found to be identical with the nuclear factor YY1 (delta, NF-E1). YY1 does not bind to the distal site. The simultaneous mutation in the proximal and the distal sites results in high, hormone-independent transcription. This finding suggests that YY1 plays a functional role in the repression and acts in conjunction with a second DNA binding protein. Comparison of YY1 DNA binding activity in uninduced and hormone-induced cells showed that relief of repression is not mediated by changes in the concentration or binding affinity of YY1. Infection of HC11 cells with a YY1-expressing recombinant retrovirus resulted in overexpression of YY1 but did not suppress hormonal induction. The addition of purified MGF decreased YY1 binding to its DNA recognition site in vitro. This finding indicates that MGF regulates the DNA binding activity of YY1 and thereby may cause the relief of transcriptional repression.

Structure and function of milk protein genes

Interspecies comparisons of cDNA and mosaic milk protein genes have confirmed their high rate of evolution, but the overall gene organization has been conserved. The three Ca-sensitive casein genes, which share common motifs in the promoter region and contain similar sequences that encode signal peptide and multiple phosphorylation sites, probably derived from a common ancestor. alpha s1- and alpha s2-casein genes, divided into many small exons, undergo complex splicing, and the deleted caseins arise from exon skipping. The four bovine casein genes are clustered on 200 kb of chromosome 6. alpha-Lactalbumin and beta-lactoglobulin pseudogenes occur in ruminants. Study of the expression of native and modified milk protein genes in mammary cell lines and transgenic animals and DNA footprinting have shown the occurrence of important regulatory motifs in the proximal 5' flanking region, including one recognized by a specific mammary nuclear factor. Good stage- and tissue-specific expression has been obtained in transgenic animals with milk protein genes having less than a 3-kb 5' flanking region. Better knowledge of both the structure and function of milk protein genes, which has already allowed the use of powerful techniques for the rapid identification of alleles, offers the potential for the genetic modification of milk composition.

Transgenic expression of COL1A1-chloramphenicol acetyltransferase fusion genes in bone: differential utilization of promoter elements in vivo and in cultured cells

To directly compare the patterns of collagen promoter expression in cells and tissues, the activity of COL1A1 fusion genes in calvariae of neonatal transgenic mice and in primary bone cell cultures derived by sequential digestion of transgenic calvariae was measured. ColCAT3.6 contains 3.6 kb (positions -3521 to +115) of the rat COL1A1 gene ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. ColCAT2.3 and ColCAT1.7 are 5' deletion mutants which contain 2,296 and 1,672 bp, respectively, of COL1A1 DNA upstream from the transcription start site. ColCAT3.6 activity was 4- to 6-fold lower in primary bone cell cultures than in intact calvariae, while ColCAT2.3 activity was at least 100-fold lower in primary bone cells than in calvariae. These changes were accompanied by a threefold decrease in collagen synthesis and COL1A1 mRNA levels in primary bone cells compared with collagen synthesis and COL1A1 mRNA levels in freshly isolated calvariae. ColCAT3.6 and ColCAT2.3 activity was maintained in calvariae cultured in the presence or absence of serum for 4 to 7 days. Thus, when bone cells are removed from their normal microenvironment, there is parallel downregulation of collagen synthesis, collagen mRNA levels, and ColCAT3.6 activity, with a much greater decrease in ColCAT2.3. These data suggest that a 624-bp region of the COL1A1 promoter between positions -2296 and -1672 is active in intact and cultured bone but inactive in cultured cells derived from the bone. We suggest that the downregulation of COL1A1 activity in primary bone cells may be due to the loss of cell shape or to alterations in cell-cell and/or cell-matrix interactions that normally occur in intact bone.

Transgenic expression of COL1A1-chloramphenicol acetyltransferase fusion genes in bone: differential utilization of promoter elements in vivo and in cultured cells

To directly compare the patterns of collagen promoter expression in cells and tissues, the activity of COL1A1 fusion genes in calvariae of neonatal transgenic mice and in primary bone cell cultures derived by sequential digestion of transgenic calvariae was measured. ColCAT3.6 contains 3.6 kb (positions -3521 to +115) of the rat COL1A1 gene ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. ColCAT2.3 and ColCAT1.7 are 5' deletion mutants which contain 2,296 and 1,672 bp, respectively, of COL1A1 DNA upstream from the transcription start site. ColCAT3.6 activity was 4- to 6-fold lower in primary bone cell cultures than in intact calvariae, while ColCAT2.3 activity was at least 100-fold lower in primary bone cells than in calvariae. These changes were accompanied by a threefold decrease in collagen synthesis and COL1A1 mRNA levels in primary bone cells compared with collagen synthesis and COL1A1 mRNA levels in freshly isolated calvariae. ColCAT3.6 and ColCAT2.3 activity was maintained in calvariae cultured in the presence or absence of serum for 4 to 7 days. Thus, when bone cells are removed from their normal microenvironment, there is parallel downregulation of collagen synthesis, collagen mRNA levels, and ColCAT3.6 activity, with a much greater decrease in ColCAT2.3. These data suggest that a 624-bp region of the COL1A1 promoter between positions -2296 and -1672 is active in intact and cultured bone but inactive in cultured cells derived from the bone. We suggest that the downregulation of COL1A1 activity in primary bone cells may be due to the loss of cell shape or to alterations in cell-cell and/or cell-matrix interactions that normally occur in intact bone.

Regulation of expression of mouse mammary tumor virus through sequences located in the hormone response element: involvement of cell-cell contact and a negative regulatory factor

Mouse mammary tumor virus (MMTV) is a latently oncogenic retrovirus responsible for the neoplastic transformation of mammary epithelial cells. Its expression is regulated by steroids, polypeptide growth factors, and cell-type-specific factors. Using GR mouse mammary cells and NIH 3T3 fibroblasts stably transfected with chimeric constructs of the long terminal repeat region of MMTV, we have demonstrated a novel mechanism of cell-type-specific expression of this virus. In confluent monolayer cultures that permit cell-cell interaction, MMTV long terminal repeat expression is positively regulated by sequences within the hormone response element (HRE) that bind the transcription factors CTF/NFI and OTFI. Although these factors are present in NIH 3T3 cells, MMTV expression in these cells is not regulated by cell density. This is partially due to a negative regulatory factor that binds sequences between -164 and -151 in the HRE. Mutations that destroy the binding site for this factor restored in part the cell density-regulated expression of MMTV to NIH 3T3 fibroblasts. The HRE is thus a central coordinator of regulatory pathways that positively or negatively influence the expression of MMTV.

Extracellular matrix regulates expression of the TGF-beta 1 gene

Transforming growth factor-beta (TGF-beta) is a potent regulator of cell proliferation and modulates the interactions of cells with their extracellular matrix (ECM), in part by inducing the synthesis of various ECM proteins. Three different isoforms of TGF-beta are synthesized in a defined pattern in specific cell populations in vivo. In the specific case of TGF-beta 1, this well-defined and limited expression stands in sharp contrast to its synthesis by virtually all cells in culture. Using mammary epithelial cells as a model system, we evaluated the substratum dependence of the expression of TGF-beta 1. The level of TGF-beta 1 expression is high in cells on plastic, but is strongly downregulated when cells are cultured on a reconstituted basement membrane matrix. In contrast, TGF-beta 2 mRNA levels in cells on either substratum remain unchanged. Using the chloramphenicol acetyl transferase gene as reporter gene under the control of the TGF-beta 1 promoter, we show that transcription from this promoter is suppressed when the cells are in contact with either endogenously synthesized or exogenously administered basement membrane. TGF-beta 1 promoter activity is strongly induced by the absence of basement membrane, i.e., by direct contact of the cells with plastic. This modulation of transcription from the TGF-beta 1 promoter occurs in the absence of lactogenic hormones which allow full differentiation. Our results thus indicate that basement membrane is an important regulator of TGF-beta 1 synthesis, and explain why most cells in culture on plastic express TGF-beta 1 in contrast with the more restricted TGF-beta 1 synthesis in vivo. We propose that there is a feedback loop whereby TGF-beta 1-induced synthesis of basement membrane components is repressed once a functional basement membrane is present. Finally, these results together with our current knowledge of regulation of TGF-beta 1 and TGF-beta 2 synthesis, suggest that, in vivo, TGF-beta 1 may play a major role in regulating the ECM synthesis and the cell-ECM interactions, whereas TGF-beta 2 may be more important in morphogenetic processes.

Coordinated expression of extracellular matrix-degrading proteinases and their inhibitors regulates mammary epithelial function during involution

Extracellular matrix (ECM) plays an important role in the maintenance of mammary epithelial differentiation in culture. We asked whether changes in mouse mammary specific function in vivo correlate with changes in the ECM. We showed, using expression of beta-casein as a marker, that the temporal expression of ECM-degrading proteinases and their inhibitors during lactation and involution are inversely related to functional differentiation. After a lactation period of 9 d, mammary epithelial cells maintained beta-casein expression up to 5 d of involution. Two metalloproteinases, 72-kD gelatinase (and its 62-kD active form), and stromelysin, and a serine proteinase tissue plasminogen activator were detected by day four of involution, and maintained expression until at least day 10. The expression of their inhibitors, the tissue inhibitor of metalloproteinases (TIMP) and plasminogen activator inhibitor-1, preceded the onset of ECM-degrading proteinase expression and was detected by day two of involution, and showed a sharp peak of expression centered on days 4-6 of involution. When involution was accelerated by decreasing lactation to 2 d, there was an accelerated loss of beta-casein expression evident by day four and a shift in expression of ECM-remodeling proteinases and inhibitors to a focus at 2-4 d of involution. To further extend the correlation between mammary-specific function and ECM remodeling we initiated involution by sealing just one gland in an otherwise hormonally sufficient lactating animal. Alveoli in the sealed gland contained casein for at least 7 d after sealing, and closely resembled those in a lactating gland. The relative expression of TIMP in the sealed gland increased, whereas the expression of stromelysin was much lower than that of a hormone-depleted involuting gland, indicating that the higher the ratio of TIMP to ECM-degrading proteinases the slower the process of involution. To test directly the functional role of ECM-degrading proteinases in the loss of tissue-specific function we artificially perturbed the ECM-degrading proteinase-inhibitor ratio in a normally involuting gland by maintaining high concentrations of TIMP protein with the use of surgically implanted slow-release pellets. In a concentration-dependent fashion, involuting mammary glands that received TIMP implants maintained high levels of casein and delayed alveolar regression. These data suggest that the balance of ECM-degrading proteinases and their inhibitors regulates the organization of the basement membrane and the tissue-specific function of the mammary gland.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term expression of a retrovirally introduced beta-galactosidase gene in rodent cells implanted in vivo using biodegradable polymer meshes

Grafts of various types of cells have been performed using bioresorbable polymer matrices. These synthetic fibers are degraded by hydrolysis into normal metabolic intermediates and induce a number of events that are conductive to healing and/or repair, the most important of which may be angiogenesis. The use of biodegradable meshes to deliver genetically altered cells was studied. A beta-galactosidase gene was inserted into Long-Evans rat bone marrow stromal (BMS) cells or fibroblasts derived from C57BL/6J mouse embryos using the retroviral vector LNL-SLX beta gal. Expression was monitored using X-gal staining. X-gal+ cells from monolayer cultures were seeded onto either polyglycolic acid (PGA) or polyglactin (PGL) biodegradable meshes and grown to confluence. Two types of grafts were performed: (1) embryonic C57BL/6J mouse fibroblasts (EMF) into either nude mice or adult C57BL/6J mice, and (2) Long-Evans rat BMS into Long-Evans rats. Beta-Galactosidase activity was found for up to 152 days for EMF in nude mice, 123 days for EMF in adult C57BL/6J mice, and 90 days for grafts of syngeneic BMS cells into Long-Evans rats. Noninfected cells grafted using the same methods did not stain with X-gal.

A novel transcriptional enhancer is involved in the prolactin- and extracellular matrix-dependent regulation of beta-casein gene expression

Lactogenic hormones and extracellular matrix (ECM) act synergistically to regulate beta-casein expression in culture. We have developed a functional subpopulation of the mouse mammary epithelial cell strain COMMA-1D (designated CID 9), which expresses high level of beta-casein, forms alveolar-like structures when plated onto the EHS tumor-derived matrix, and secretes beta-casein unidirectionally into a lumen. We have further shown that ECM- and prolactin-dependent regulations of beta-casein occur mainly at the transcriptional level and that 5' sequences play an important role in these regulations. To address the question of the nature of the DNA sequence requirements for such regulation, we analyzed the bovine beta-casein gene promoter in these cells. We now have located a 160-bp transcriptional enhancer (BCE1) within the 5' flanking region of the beta-casein gene. Using functional assays, we show that BCE1 contains responsive elements for prolactin- and ECM-dependent regulation. BCE1 placed upstream of a truncated and inactive beta-casein promoter (the shortest extending from -89 to +42 bp with regard to the transcription start site) reconstitutes a promoter even more potent than the intact promoter, which contains BCE1 in its normal context more than 1.5 kb upstream. This small fusion promoter also reconstitutes the normal pattern of regulation, including a requirement for both prolactin and ECM and a synergistic action of prolactin and hydrocortisone. By replacing the milk promoter with a heterologous viral promoter, we show that BCE1 participates in the prolactin- and ECM-mediated regulation.

6-Dimethyl amino purine and 2-amino purine inhibit the induction of expression of milk protein genes by prolactin

Two protein kinase-inhibitors, 6-dimethyl amino purine and 2-amino purine inhibited induction of beta-casein synthesis by prolactin when added to the culture medium of rabbit mammary explant and cells. The accumulation of the mRNA for alpha s1- and beta-caseins and for whey acidic protein did not take place in the presence of the inhibitors whereas beta-actin mRNA concentration was not altered. In the same experimental conditions, H7, an inhibitor of protein kinase C and, to a lower extent, of protein kinase A did not prevent prolactin from acting. These data suggest for the first time that specific protein kinases are involved in the transduction of the prolactin signal to milk protein genes.

High expression of the caprine beta-casein gene in transgenic mice

An 18-kb caprine genomic DNA fragment, comprising the beta-casein transcription unit with about 3-kb 5' and 6-kb 3' flanking regions, was microinjected into fertilized one-cell murine eggs. All nine lines of transgenic mice obtained expressed the transgene in their mammary glands, as demonstrated by Northern blot analysis of mRNA in miscellaneous tissues, and qualitative and quantitative analysis of caprine beta-casein in milk, using SDS/PAGE, Western blotting and rocket immunoelectrophoresis. Two lines produced milk containing up to 21-24 mg of the exogenous protein/ml, a yield which is roughly twice that found in goat milk. The yield reached at least 40 mg/ml in some progeny of crossbred G1 transgenic mice. Thus, the investigated gene appears to be a good candidate for making hybrid constructs that might promote an efficient production of valuable foreign proteins in milk.

Biological mechanisms of fluorosis and level and timing of systemic exposure to fluoride with respect to fluorosis

Enamel fluorosis can occur following either an acute or chronic exposure to fluoride during tooth formation. Fluorosed enamel is characterized by a retention of amelogenins in the early-maturation stage, and by the formation of a more porous enamel with a subsurface hypomineralization. The mechanisms by which fluoride affects enamel development include specific effects on both the ameloblasts and on the developing enamel matrix. Maturation-stage ameloblast modulation is more rapid in fluorosed enamel as compared with control enamel, and proteolytic activity in fluorosed early-maturation enamel is reduced as compared with controls. Secretory enamel appears to be more susceptible to the effects of fluoride following acute fluoride exposure, such as may occur with the use of fluoride supplements. However, both human and animal studies show that the transition/early-maturation stage of enamel formation is most susceptible to the effects of chronic fluoride ingestion at above-optimal levels of fluoride in drinking water.

High-level expression of the rat whey acidic protein gene is mediated by elements in the promoter and 3' untranslated region

The high-level expression of the rat whey acidic protein (WAP) gene in transgenic mice depends on the interaction of 5'-flanking promoter sequences and intragenic sequences. Constructs containing 949 bp of promoter sequences and only 70 bp of 3'-flanking DNA were expressed at uniformly high levels, comparable to or higher than that of the endogenous gene. Although this WAP transgene was developmentally regulated, it was expressed earlier during pregnancy than was the endogenous WAP gene. Replacement of 3' sequences, including the WAP poly(A) addition site, with simian virus 40 late poly(A) sequences resulted in an approximately 20-fold reduction in the expression of WAP mRNA in the mammary gland during lactation. Nevertheless, position-independent expression of the transgene was still observed. Further deletion of 91 bp of conserved WAP 3' untranslated region (UTR) led to integration site-dependent expression. Position independence was restored following reinsertion of the WAP 3' UTR into the deleted construct at the same location, but only when the insertion was in the sense orientation. The marked differences observed between the expression levels of the 3'-end deletion constructs in transgenic mice were not seen in transfected CID 9 mammary epithelial cells. In these cells, expression of the endogenous WAP gene was dependent on the interaction of these cells with a complex extracellular matrix. In contrast, the transfected WAP constructs were not dependent on extracellular matrix for expression. Thus, both the abnormal expression of WAP in cells cultured on plastic and the precocious developmental expression of WAP in transgenic mice may reflect the absence of a negative control element(s) within these recombinant constructs.

High-level expression of the rat whey acidic protein gene is mediated by elements in the promoter and 3' untranslated region

The high-level expression of the rat whey acidic protein (WAP) gene in transgenic mice depends on the interaction of 5'-flanking promoter sequences and intragenic sequences. Constructs containing 949 bp of promoter sequences and only 70 bp of 3'-flanking DNA were expressed at uniformly high levels, comparable to or higher than that of the endogenous gene. Although this WAP transgene was developmentally regulated, it was expressed earlier during pregnancy than was the endogenous WAP gene. Replacement of 3' sequences, including the WAP poly(A) addition site, with simian virus 40 late poly(A) sequences resulted in an approximately 20-fold reduction in the expression of WAP mRNA in the mammary gland during lactation. Nevertheless, position-independent expression of the transgene was still observed. Further deletion of 91 bp of conserved WAP 3' untranslated region (UTR) led to integration site-dependent expression. Position independence was restored following reinsertion of the WAP 3' UTR into the deleted construct at the same location, but only when the insertion was in the sense orientation. The marked differences observed between the expression levels of the 3'-end deletion constructs in transgenic mice were not seen in transfected CID 9 mammary epithelial cells. In these cells, expression of the endogenous WAP gene was dependent on the interaction of these cells with a complex extracellular matrix. In contrast, the transfected WAP constructs were not dependent on extracellular matrix for expression. Thus, both the abnormal expression of WAP in cells cultured on plastic and the precocious developmental expression of WAP in transgenic mice may reflect the absence of a negative control element(s) within these recombinant constructs.

Control of mammary epithelial differentiation: basement membrane induces tissue-specific gene expression in the absence of cell-cell interaction and morphological polarity

Functional differentiation in mammary epithelia requires specific hormones and local environmental signals. The latter are provided both by extracellular matrix and by communication with adjacent cells, their action being intricately connected in what appears to be a cascade of events leading to milk production. To distinguish between the influence of basement membrane and that of cell-cell contact in this process, we developed a novel suspension culture assay in which mammary epithelial cells were embedded inside physiological substrata. Single cells, separated from each other, were able to assimilate information from a laminin-rich basement membrane substratum and were induced to express beta-casein. In contrast, a stromal environment of collagen I was not sufficient to induce milk synthesis unless accompanied by cell-cell contact. The expression of milk proteins did not depend on morphological polarity since E-cadherin and alpha 6 integrin were distributed evenly around the surface of single cells. In medium containing 5 microM Ca2+, cell-cell interactions were impaired in small clusters and E-cadherin was not detected at the cell surface, yet many cells were still able to produce beta-casein. Within the basement membrane substratum, signal transfer appeared to be mediated through integrins since a function-blocking anti-integrin antibody severely diminished the ability of suspension-cultured cells to synthesize beta-casein. These results provide evidence for a central role of basement membrane in the induction of tissue-specific gene expression.

The extracellular matrix coordinately modulates liver transcription factors and hepatocyte morphology

The extracellular matrix (ECM) promotes tissue morphogenesis, cell migration, and the differentiation of a variety of cell types. However, the mechanisms by which ECM causes differentiated gene expression have been unknown. In this report, we show that culturing the hepatocyte-derived cell line H2.35 on an ECM gel changes cell morphology and selectively stimulates the transcription of a subset of liver-specific genes, including serum albumin. Transcriptional activation by ECM also occurs with transfected plasmids bearing the transcriptional enhancer of the albumin gene. ECM substrates of different composition activated the albumin enhancer only when the ECM promoted a cuboidal, differentiated cell morphology. Enhancer activation by the ECM was mediated by two liver transcription factors, HNF3 alpha and eH-TF, which appear to be regulated differently by matrix. Specifically, we found that a collagen gel substratum caused a selective increase in the factor HNF3 alpha at the levels of mRNA accumulation and DNA-binding activity in nuclear extracts, both in H2.35 cells and in the hepatoma cell line HepG2. We conclude that the ECM can stimulate cell differentiation by selectively activating transcriptional regulatory factors and that such regulation occurs coordinately with ECM-promoted changes in cell shape.