Appearance of basic fibroblast growth factor receptors upon differentiation of rat mammary epithelial to myoepithelial-like cells in culture

Selectivity in glycosaminoglycan binding dictates the distribution and diffusion of fibroblast growth factors in the pericellular matrix

The range of biological outcomes generated by many signalling proteins in development and homeostasis is increased by their interactions with glycosaminoglycans, particularly heparan sulfate (HS). This interaction controls the localization and movement of these signalling proteins, but whether such control depends on the specificity of the interactions is not known. We used five fibroblast growth factors with an N-terminal HaloTag (Halo-FGFs) for fluorescent labelling, with well-characterized and distinct HS-binding properties, and measured their binding and diffusion in pericellular matrix of fixed rat mammary 27 fibroblasts. Halo-FGF1, Halo-FGF2 and Halo-FGF6 bound to HS, whereas Halo-FGF10 also interacted with chondroitin sulfate/dermatan sulfate, and FGF20 did not bind detectably. The distribution of bound FGFs in the pericellular matrix was not homogeneous, and for FGF10 exhibited striking clusters. Fluorescence recovery after photobleaching showed that FGF2 and FGF6 diffused faster, whereas FGF1 diffused more slowly, and FGF10 was immobile. The results demonstrate that the specificity of the interactions of proteins with glycosaminoglycans controls their binding and diffusion. Moreover, cells regulate the spatial distribution of different protein-binding sites in glycosaminoglycans independently of each other, implying that the extracellular matrix has long-range structure.

Transport of fibroblast growth factor 2 in the pericellular matrix is controlled by the spatial distribution of its binding sites in heparan sulfate

The heparan sulfate (HS) chains of proteoglycans are a key regulatory component of the extracellular matrices of animal cells, including the pericellular matrix around the plasma membrane. In these matrices they regulate transport, gradient formation, and effector functions of over 400 proteins central to cell communication. HS from different matrices differs in its selectivity for its protein partners. However, there has been no direct test of how HS in the matrix regulates the transport of its partner proteins. We address this issue by single molecule imaging and tracking in fibroblast pericellular matrix of fibroblast growth factor 2 (FGF2), stoichiometrically labelled with small gold nanoparticles. Transmission electron microscopy and photothermal heterodyne imaging (PHI) show that the spatial distribution of the HS-binding sites for FGF2 in the pericellular matrix is heterogeneous over length scales ranging from 22 nm to several µm. Tracking of individual FGF2 by PHI in the pericellular matrix of living cells demonstrates that they undergo five distinct types of motion. They spend much of their time in confined motion (∼110 nm diameter), but they are not trapped and can escape by simple diffusion, which may be slow, fast, or directed. These substantial translocations (µm) cover distances far greater than the length of a single HS chain. Similar molecular motion persists in fixed cells, where the movement of membrane PGs is impeded. We conclude that FGF2 moves within the pericellular matrix by translocating from one HS-binding site to another. The binding sites on HS chains form non-random, heterogeneous networks. These promote FGF2 confinement or substantial translocation depending on their spatial organisation. We propose that this spatial organisation, coupled to the relative selectivity and the availability of HS-binding sites, determines the transport of FGF2 in matrices. Similar mechanisms are likely to underpin the movement of many other HS-binding effectors.

The heparan sulfate co-receptor and the concentration of fibroblast growth factor-2 independently elicit different signalling patterns from the fibroblast growth factor receptor

Background

The fibroblast growth factor receptor (FGFR) interprets concentration gradients of FGF ligands and structural changes in the heparan sulfate (HS) co-receptor to generate different cellular responses. However, whether the FGFR generates different signals is not known.

Results

We have previously shown in rat mammary fibroblasts that in cells deficient in sulfation, and so in HS co-receptor, FGF-2 can only stimulate a transient phosphorylation of p42/44 ^MAPK and so cannot stimulate DNA synthesis. Here we demonstrate that this is because in the absence of HS, FGF-2 fails to stimulate the phosphorylation of the adaptor FGFR substrate 2 (FRS2). In cells possessing the HS co-receptor, FGF-2 elicits a bell-shaped dose response: optimal concentrations stimulate DNA synthesis, but supramaximal concentrations (≥ 100 ng/mL) have little effect. At optimal concentrations (300 pg/mL) FGF-2 stimulates a sustained dual phosphorylation of p42/44 ^MAPK and tyrosine phosphorylation of FRS2. In contrast, 100 ng/mL FGF-2 only stimulates a transient early peak of p42/44 ^MAPK phosphorylation and fails to stimulate appreciably the phosphorylation of FRS2 on tyrosine.

Conclusions

These results suggest that the nature of the FGFR signal produced is determined by a combination of the HS co-receptor and the concentration of FGF ligand. Both the phosphorylation of the adaptor FRS2, the kinetics (sustained or transient) of phosphorylation of p42/44(MAPK) are varied, and so differing cellular responses are produced.

N-glycosylation of fibroblast growth factor receptor 1 regulates ligand and heparan sulfate co-receptor binding

The regulation of cell function by fibroblast growth factors (FGF) occurs through a dual receptor system consisting of a receptor-tyrosine kinase, FGFR and the glycosaminoglycan heparan sulfate (HS). Mutations of some potential N-glycosylation sites in human fgfr lead to phenotypes characteristic of receptor overactivation. To establish how N-glycosylation may affect FGFR function, soluble- and membrane-bound recombinant receptors corresponding to the extracellular ligand binding domain of FGFR1-IIIc were produced in Chinese Hamster Ovary cells. Both forms of FGFR1-IIIc were observed to be heavily N-glycosylated and migrated on SDS-PAGE as a series of multiple bands between 50 and 75 kDa, whereas the deglycosylated receptors migrated at 32 kDa, corresponding to the expected molecular weight of the polypeptides. Optical biosensor and quartz crystal microbalance-dissipation binding assays show that the removal of the N-glycans from FGFR1-IIIc caused an increase in the binding of the receptor to FGF-2 and to heparin-derived oligosaccharides, a proxy for cellular HS. This effect is mediated by N-glycosylation reducing the association rate constant of the receptor for FGF-2 and heparin oligosaccharides. N-Glycans were analyzed by mass spectrometry, which demonstrates a predominance of bi- and tri-antennary core-fucosylated complex type structures carrying one, two, and/or three sialic acids. Modeling of such glycan structures on the receptor protein suggests that at least some may be strategically positioned to interfere with interactions of the receptor with FGF ligand and/or the HS co-receptor. Thus, the N-glycans of the receptor represent an additional pathway for the regulation of the activity of FGFs.

An in vitro model of epithelial cell growth stimulation in the rodent mammary gland

Mouse mammary epithelial cell cultures previously described bring about extensive proliferation and a cell population with the appropriate markers for luminal ductal epithelial cells, and also the ability to form normal tissue after implantation into mice. This success may result from a culture environment that resembles certain aspects of the environment in the mammary gland. Mouse mammary epithelial cells, whose proliferation is limited when plated alone, can be stimulated to multiply by contact with lethally irradiated cells of the LA7 rat mammary tumour line. Most of the proliferative stimulus is imparted by direct cell contact between LA7 and mouse mammary cells. Junctions, including adherens junctions, form among all cells in the culture, much as junctions form in the mammary gland. LA7 cells secrete TGFalpha and bFGF, factors found in the mammary gland, and factors to which mouse mammary cells respond in culture. Mouse mammary cells express keratins 8 and 18, markers for luminal cells of the mammary duct. LA7 cells express keratin 14 and vimentin, markers for myoepithelial cells. These facts, taken together, fit a model of cell replacement in an epithelial tissue and also imitate the relationship between luminal ductal cells and myoepithelial cells in the mammary gland. This method of culturing cells is useful, not only for in vitro-in vivo carcinogenesis studies, but also for the study of mechanisms by which growth signals are imparted from one cell to another.

Fibroblast growth factor-2 binds to small heparin-derived oligosaccharides and stimulates a sustained phosphorylation of p42/44 mitogen-activated protein kinase and proliferation of rat mammary fibroblasts

We examine the relationship between the chain length of heparin-derived oligosaccharides, fibroblast growth factor (FGF)-2 binding kinetics and the ability of the oligosaccharides to allow FGF-2-induced proliferation of chlorate-treated rat mammary fibroblasts. First, using an optical biosensor, we show that FGF-2 did not bind disaccharides, but definitively bound to tetrasaccharides. As the chain length increased from tetrasaccharide to octasaccharide, there was a substantial increase in k(ass) (564000 M(-1) x s(-1) to 2000000 M(-1) x s(-1), respectively) and affinity (K(d) 77 nM to 11 nM, respectively) for FGF-2. From decasaccharides and longer, the k(ass) and affinity for FGF-2 was reduced substantially (tetradecasaccharide k(ass) 470000 M(-1) x s(-1), K(d) 30 nM). In chlorate-treated, and hence sulphated, glycosaminoglycan-deficient cells, FGF-2 alone or in the presence of disaccharides did not stimulate DNA synthesis and it only elicited an early transient dual phosphorylation of p42/44 mitogen-activated protein kinase (MAPK). In the same cells FGF-2 in the presence of tetrasaccharides and longer oligosaccharides was able to restore DNA synthesis and enable the sustained dual phosphorylation of p42/44(MAPK). However, the oligosaccharides from tetrasaccharides to octasaccharides were less potent in proliferation assays than deca- and longer oligosaccharides. Therefore, there was no correlation between the binding parameters and the potency of the oligosaccharides in DNA synthesis assays. These results demonstrate that tetrasaccharides are able to bind FGF-2 and enable FGF-2 to stimulate cell proliferation, which sets important boundary conditions for models of the FGF-2-heparan sulphate-FGF receptor complex.

Fibroblast growth factor-2 stimulation of p42/44MAPK phosphorylation and IkappaB degradation is regulated by heparan sulfate/heparin in rat mammary fibroblasts

Fibroblast growth factor-2 (FGF-2) interacts with a dual receptor system consisting of tyrosine kinase receptors and heparan sulfate proteoglycans (HSPGs). In rat mammary fibroblasts, FGF-2 stimulated DNA synthesis and induced a sustained phosphorylation of p42/44(MAPK) and of its downstream target, p90(RSK). Moreover, FGF-2 also stimulated the transient degradation of IkappaBalpha and IkappaBbeta. PD098059, a specific inhibitor of p42/44(MAPK) phosphorylation, inhibited FGF-2-stimulated DNA synthesis, phosphorylation of p42/44(MAPK) and p90(RSK), and degradation of IkappaBbeta. In contrast, in chlorate-treated and hence sulfated glycosaminoglycan-deficient cells, FGF-2 was unable to stimulate DNA synthesis. However, FGF-2 was able to trigger a transient phosphorylation of both p42/44(MAPK) and p90(RSK), which peaked at 15 min and returned to control levels at 30 min. In these sulfated glycosaminoglycan-deficient cells, no degradation of IkappaBalpha and IkappaBbeta was observed after FGF-2 addition. However, in chlorate-treated cells, the addition of heparin or purified HSPGs simultaneously with FGF-2 restored DNA synthesis, the sustained phosphorylation of p42/44(MAPK) and p90(RSK), and the degradation of IkappaBalpha and IkappaBbeta. These results suggest that the HSPG receptor for FGF-2 not only influences the outcome of FGF-2 signaling, e.g. cell proliferation, but importantly regulates the immediate-early signals generated by this growth factor.

In vivo and in vitro evidence of basic fibroblast growth factor action in mouse mammary gland development

Basic fibroblast growth factor (bFGF) stimulated [3H]thymidine incorporation at all stages of development, although the magnitude of this effect was the greatest in cells derived from pregnant mice. Cells primed with insulin and bFGF synthesized more casein than cells not exposed to either hormone. bFGF inhibited casein synthesis and decreased the amounts of beta-casein and alpha-lactalbumin transcripts in cells from pregnant animals simultaneously incubated with insulin, hydrocortisone and prolactin. bFGF content in mammary gland increased with puberty and pregnancy, but decreased markedly in lactation; the number of bFGF receptors in epithelial cells changed in parallel. These data suggest that bFGF may have a physiological role both in stimulating growth and in inhibiting functional differentiation of normal mouse mammary epithelial cells.

Expression and localization of members of the fibroblast growth factor family in the bovine mammary gland

The goal of the study was to examine the expression and localization of members of the fibroblast growth factor family in the bovine mammary gland during different developmental and functional stages. Mammary tissue was obtained from German Brown Swiss cows (n = 23) during defined stages of mammogenesis (before and during pregnancy), lactogenesis, peak and late lactation, and involution. Extracted mRNA was analyzed by reverse transcription polymerase chain reaction and RNase protection assay. The tissue content of fibroblast growth factor-1 and fibroblast growth factor-2 was determined by radioimmunoassay, and the localization of fibroblast growth factor-2 was determined by immunohistochemistry. The highest mRNA concentration for fibroblast growth factor-1, -2, and -7 and their receptors was detected in the glands of virgin heifers or primigravid heifers during involution; less abundant fibroblast growth factor mRNA was detected during lactogenesis and galactopoiesis. Tissue protein concentrations of fibroblast growth factor-1 and fibroblast growth factor-2 showed similar tendencies. Immunoreactive fibroblast growth factor-2 was observed during mammogenesis and involution in endothelial cells, ductal epithelial cells, myoepithelial cells, and some alveolar cells. After positive staining, fibroblast growth factor-2 could only be observed in endothelial and myoepithelial cells during lactogenesis and could only be observed in myoepithelial cells during galactopoiesis. Expression, tissue concentration, and distinct localization suggest that fibroblast growth factors may be important in the local regulation of the bovine mammary gland.

Stem cells in breast epithelia

The rodent and human nonpregnant mammary glands contain epithelial, intermediate and myoepithelial cells which have all been isolated as cell lines in vitro. Transforming growth factor-alpha (TGF alpha) and basic fibroblast growth factor (bFGF) are produced by myoepithelial cells and can stimulate the growth of intermediate stem cells in vitro. Epithelial and intermediate cells behave like stem cells in vitro, since they can differentiate into alveolar-like an myoepithelial cells. The myoepithelial differentiation pathway is associated with the early expression of a calcium-binding regulatory protein called p9Ka and the protease, Cathepsin D. Myoepithelial cells are also present in benign lesions but not in malignant mammary carcinomas of rats or humans, whose resultant cell lines fail to differentiate completely along the myoepithelial cell pathway. Loss of the myoepithelial cell in some invasive carcinomas may be compensated, at least in part, by changes in malignant cells. Over-expression of TGF alpha and/or erbB receptors may reduce the requirement for TGF alpha, whilst ectopic production of bFGF and its receptors and p9Ka/Cathespin D may assist in tumorigenesis and in metastasis, respectively. Thus compensation for, or retention of, molecules potentially involved in the differentiation of mammary cells may be a mechanism by which malignancy progresses in some human invasive carcinomas.

Interactions of putative heparin-binding domains of basic fibroblast growth factor and its receptor, FGFR-1, with heparin using synthetic peptides

We have examined structure-function relationships that have been proposed to account for the heparin-binding properties of basic fibroblast growth factor and its receptor, FGFR-1, using synthetic peptides, DNA synthesis assays and binding assays in a resonant mirror biosensor. The results suggest that the interaction of FGFR-1 with heparin may not be physiologically relevant and that the site of interaction of the polysaccharide on bFGF is more complex than has been anticipated.

A paracrine role for myoepithelial cell-derived FGF2 in the normal human breast

We have studied separated normal human breast epithelial and myoepithelial cells for the presence of basic fibroblast growth factor (FGF2) and its receptors, both low (heparan sulfate proteoglycans) and high affinity (FGFR1), and for the effects of FGF2 on the proliferation of both cell types. Our results indicate that these cells differ markedly in their synthesis and response to FGF2. We found, using PCR of purified cell populations, mRNA for FGF2 only in the myoepithelial cells, whereas immunostaining and Western blotting results demonstrated the presence of FGF2 protein in both epithelial and myoepithelial cells. FGF2 had no effect on the proliferation of myoepithelial cells, but it did maintain the survival of the separated epithelial cells in low serum and stimulate their growth in 5% and 10% FCS. Immunostainable FGFR1 was present in epithelial cells and, to a lesser extent, in myoepithelial cells. Low-affinity binding sites for FGF2 were synthesized by epithelial and myoepithelial cells, but myoepithelial cells possessed a greater proportion of higher-affinity heparan sulfate proteoglycans. These results indicate that myoepithelial cell-derived FGF2 may be an important paracrine factor controlling epithelial cell survival and growth in the normal human breast.

Acidic and basic fibroblast growth factor mRNA and protein in mouse mammary glands

Previous studies suggest that members of the fibroblast growth factor (FGF) family are mitogenic to mammary epithelium. In order to determine expression of acidic and basic FGF (aFGF and bFGF) during mammary development, mice were euthanized as virgins, early pregnant, mid-pregnant, late-pregnant, or during early lactation. Mammary expression of both aFGF and bFGF mRNA increased through pregnancy. Acidic FGF mRNA continued to increase during early lactation, but basic FGF message level decreased drastically during early lactation. Western blots probed with anti-aFGF showed four immunoreactive bands approx 30, 48, 52, and 55-kDa in size. The 30-, 48-, and 55-kDa bands for aFGF were expressed at low levels during virgin and early pregnant stages but were more prominent during the later stages. The 52-kDa band was high during the virgin and early pregnant stages and low in mid-pregnancy through early lactation. Blots probed with anti-bFGF showed two bands approx 30 and 50 kDa in size. Both bands increased through early-pregnancy, but during late-pregnancy there was a decrease in immunoreactive protein levels, which remained low during early lactation. Experiments to determine where FGF mRNAs are produced in the mammary gland suggest that both FGFs may be produced in the stroma, leading to the hypothesis that aFGF and bFGF are stromally produced growth factors and probably act on the epithelial component of the gland in a paracrine fashion.

Hormonal regulation of acidic and basic fibroblast growth factor production and expression in mouse mammary gland

Based on previous results showing developmental regulation of aFGF and bFGF, we evaluated the effect of various hormones on aFGF and bFGF mRNA levels, in mammary gland. Northern blots indicated that estrogen alone increased aFGF but had no effect on bFGF messenger RNA level. Progesterone alone increased aFGF and bFGF mRNA levels. Estrogen and progesterone together increased aFGF mRNA level in mammary gland, but the increase was no greater than that caused by either hormone alone. However, the combination of estrogen and progesterone had no effect on bFGF message level. PRL or GH, when administered with estrogen and progesterone, increased aFGF, but did not have any effect on bFGF message level. However, when PRL and GH were administered together with estrogen and progesterone, they increased bFGF messenger RNA level. Ovarian steroid withdrawal increased aFGF, but did not have any effect on bFGF mRNA accumulation. PRL alone (in the absence of estrogen and progesterone) decreased aFGF, whereas it had no effect on bFGF message level. Hydrocortisone alone decreased aFGF, but increased bFGF mRNA level. However, PRL and hydrocortisone increased aFGF, but did not have any significant effect on bFGF message level. In the overall model, during growth of the mammary gland, ovarian steroids cause an increase in aFGF mRNA. During lactogenesis, ovarian steroid withdrawal causes an increase in aFGF messenger RNA levels. Lactogenic hormones together cause a further increase in aFGF message levels.

Effect on tumorigenicity and metastasis of transfection of a diploid benign rat mammary epithelial cell line with DNA corresponding to the mRNA for basic fibroblast growth factor

To examine the potential role of fibroblast growth factors (FGF) in tumorigenesis and metastasis, plasmid constructs containing the human basic FGF (bFGF) gene, with or without fusion to a secretory signal peptide (IgbFGF), were transfected into the diploid rat mammary epithelial cell line Rama 37. All transfectants possessed multiple copies of the transfected cDNA, which was expressed as the corresponding mRNA and the protein. The amount of bFGF protein was usually greater than the bFGF growth-stimulatory activity that could be recovered from the transfected cells. Nevertheless, the amount of bFGF growth-stimulatory activity secreted by the IgbFGF transfectants (0.08-0.8 ng/ml/24 hr) was sufficient to induce growth in responsive cells. However, the transfectants themselves were refractory to stimulation by exogenously added bFGF, despite possessing a small number of high-affinity receptors for bFGF. When the bFGF or the IgbFGF transfectants were inoculated into the mammary fat pads of syngeneic rats, the tumour incidence was low (0-50%). However, when cells cultured from these tumours were inoculated into the fat pad of syngeneic rats, the tumour incidence was 100%. Tumours were in all cases benign and no metastases were observed. Our results suggest that the role of bFGF in metastasis is not simply one of autocrine/paracrine stimulation of cell growth and that other events may also be required.

Expression of growth factor receptors in arterial smooth muscle cells. Dependency on cell phenotype and serum factors

The effects of modulation of rabbit aortic smooth muscle cells (SMCs) from the 'contractile' phenotype on surface membrane receptors binding epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and platelet-derived growth factor-BB (PDGF-BB), as well as their responsiveness to these growth factors was investigated in cell culture. Cells predominantly of the 'contractile' phenotype expressed low numbers of high affinity EGF and bFGF receptors (EGFr: 1.09 +/- 0.18 fmol/10(6) cells; bFGFr: 0.32 +/- 0.07 fmol/10(6) cells). Upon modulation from the 'contractile' phenotype, the expression of these cell surface receptors increased greatly: 8- and 11-fold with respect to EGF and bFGF receptors. Cell surface receptors binding [125I]-PDGF-BB were largely unaltered. The elevated bFGF receptor number appeared dependent on SMC modulation from the 'contractile' phenotype and serum; the latter factor did not influence EGF receptor numbers. In both instances the increase in receptor numbers was independent of the proliferation status of the cells. Cells expressing high levels of the growth factor receptors also rapidly entered the cell cycle, proliferated, and exhibited growth factor-specific changes in shape in the presence of these growth factors. Because the effects on growth factor receptor numbers were observed in confluent cells, such alterations, are likely to play a significant role in vessel remodelling following balloon catheter angioplasty, in atherosclerotic vessels and the vascular hypertrophy associated with hypertension.

Growth factors and their receptors in neoplastic mammary glands

Control of the growth of mammary glands is largely exerted in vivo by systemic hormones and locally-produced growth factors, whereas malignant tumours gradually lose the ability to respond to both types of control in vivo. However, the systemic hormones have little direct effect on stimulating the growth of rat or human mammary cell lines in vitro. Estrogens are thought to work by stimulating locally-produced growth factors and/or their receptors, eg transferrin, TGF alpha and IGF-1, and prolactin by a contaminating pituitary mammary growth factor (PMGF). Mammary stem cells intermediate between epithelial and myoepithelial cells are thought to be retained in malignant carcinomas, whereas the TGF alpha and bFGF-producing myoepithelial cells are lost. Hormonal autonomy of carcinomas may develop by overproduction of the locally-produced growth factors, their receptors (including related receptors, eg c-erbB-2) and/or by stem cells differentiating sufficiently to utilise normal control mechanisms, eg refractivity to PMGF and autocrine/paracrine response to bFGF. The failure of the stem cells to differentiate completely to myoepithelial cells in carcinomas greatly reduces the heparan sulphate proteoglycan sink used to sequester to bFGF in normal glands and also removes the possibility of eliminating cells by terminal differentiation, both processes possibly contributing to the uncontrolled growth of the malignant breast cell.

Expression of the FGFR1 gene in human breast-carcinoma cells

Fibroblast growth factors (FGF) constitute a family of at least 9 members which act through high-affinity tyrosine-kinase receptors encoded by 4 distinct genes. In humans, the FGFR1 gene is located in chromosomal region 8p12. Its amplification and expression were examined in a panel of 110 breast carcinoma samples by Southern- and Northern-blot analyses. FGFR1 was amplified in 9% and overexpressed in about 15% of the tumors studied. In situ hybridization experiments were performed on tissue sections of normal breast and tumors with a high level of FGFR1 expression. In both normal and tumoral tissues, FGFR1 RNA was detected in the epithelial cells. Overexpression of FGFR1 seems to be associated with small, well-differentiated diploid tumors.

Alternative splicing in fibroblast growth factor receptor 2 is associated with induced epithelial-mesenchymal transition in rat bladder carcinoma cells

We described previously that acidic fibroblast growth factor (aFGF), but not basic fibroblast growth factor (bFGF), can induce the rat carcinoma cell line NBT-II to undergo a rapid and reversible transition from epithelial to mesenchymal phenotype (EMT). We now find that NBT-II EMT is stimulated by keratinocyte growth factor (KGF) in cells grown at low density. Accordingly, a high-affinity receptor showing 98% homology to mouse FGF receptor 2b/KGF receptor was cloned and sequenced from NBT-II cells. Northern analysis indicated that mRNA for FGF receptor 2b/KGF receptor was drastically down-regulated within 1 wk in aFGF-induced mesenchymal NBT-II cells. This decrease coincided with an up-regulation of FGF receptor 2c/Bek, a KGF-insensitive, alternatively spliced form of FGF receptor 2b/KGF receptor. Functional studies confirmed that KGF could not maintain EMT induction on mesenchymal NBT-II cells. FGF receptor 1 and FGF receptor 2c/Bek could also support EMT induction when transfected into NBT-II cells in response to aFGF or bFGF. Such transfected cells could bind bFGF as well as aFGF. Therefore, EMT can be induced through different FGF receptors, but EMT may also regulate FGF receptor expression itself.

Secretion of transforming growth factor alpha and expression of its receptor in human mammary cell lines

The secretion of transforming growth factor alpha (TGF alpha) and the expression of cell-surface receptors for epidermal growth factor (EGF) were measured in a series of human mammary cell lines. The amount of TGF alpha secreted by the cells did not correlate with the phenotype of the cells (epithelial or myoepithelial), the mechanism of immortalization of the cells (SV40 or spontaneous) or the source of the cells (normal mammary gland, benign hyperplastic lesion, malignant tumour). The level of expression of cell-surface receptors for EGF was markedly increased as a consequence of SV40-immortalization of mammary cells, but otherwise did not correlate with the phenotype of the cells or the source of the cells. Much of the increase was accounted for by the appearance of a large number of low-affinity receptors for EGF in the SV40-immortalized cells. It is suggested that one of the mechanisms whereby SV40-immortalization suppresses the senescence of primary cultures of human mammary epithelial cells involves increasing the level of expression of receptors for EGF. In contrast the level of secretion of TGF alpha by cells in culture is probably a consequence of the mechanisms of adaptation of each cell line to culture conditions, and does not reflect the level of secretion of TGF alpha by cells in vivo.

Fibroblast growth factors and their receptors: an information network controlling tissue growth, morphogenesis and repair

The stimulation of cellular metabolism by the nine fibroblast growth factors (FGFs) is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases (FGFR) and heparan sulphate proteoglycans (HSPG). The stimulation of cell division by FGFs has an obligate requirement for both partners of the dual-receptor system. The binding of the nine FGFs to the FGFRs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFR proteins. Thus many of the FGFR isoforms bind several FGFs. It is likely that each FGF requires a different pattern of sulphation within the heparan sulphate chains for binding. Therefore, the HSPG receptors may provide additional specificity, allowing a cell to fine tune its response to the FGFs present in the extracellular milieu. The HSPG receptors also control the availability of FGFs and hence regulate the transport of FGFs within a tissue. FGF-stimulated cell division would appear to have a mandatory requirement for the FGFs to be translocated to the nucleus via the cytosol after interacting with the dual-receptor system. The consequences of the potential direct action of FGFs in stimulating cell division are examined in the light of current models of signal transduction.

The expression of basic fibroblast growth factor and its receptor in cell lines derived from normal human mammary gland and a benign mammary lesion

mRNA for basic Fibroblast Growth Factor (bFGF) was expressed in a series of SV40-transformed human mammary cell lines as molecules of 7.1, 3.6, 2.0 and 1.2 kb. This expression was much weaker in those lines of epithelial morphology than in myoepithelial-like cell lines derived from them. It was confirmed, using northern hybridization to single-stranded RNA probes, that the multiple mRNAs were transcribed from the coding strand for bFGF. bFGF activity was detected in extracts of the cells and the relative amounts of activity corresponded in general to the amounts of mRNA found. Similar results were obtained from spontaneously transformed cell lines derived from a human benign breast lesion. The presence of bFGF protein in the extracts was confirmed by western blotting, which showed a band of 18–19 kDa, migrating in the same position as authentic bFGF; in addition, the myoepithelial-like cells showed prominent bands of bFGF at 24 and 26 kDa. No FGF receptor was detectable by the binding of 125I-bFGF to the SV40-transformed cell lines or to the epithelial cell lines from the benign breast lesion, but both high- and low-affinity receptors were found on myoepithelial-like cells derived from the latter. The results indicate that differentiation to the human myoepithelial-like phenotype in culture is associated with the enhanced expression of bFGF, and it is suggested that bFGF, immunocytochemically detected in the basement membrane of the human breast, may arise, at least in part, from the myoepithelial cells of the mammary parenchyma.

Ectopic production of heparin-binding growth factors and receptors for basic fibroblast growth factor by rat mammary epithelial cell lines derived from malignant metastatic tumours

A rat mammary (Rama) epithelial cell line, Rama 704, derived from normal rat mammary gland does not possess any detectable cell-surface receptors for basic fibroblast growth factor (bFGF), produces a barely detectable level of bFGF mRNA and does not contain detectable levels of bFGF-like activity. Similar results have been obtained with the Rama 37 epithelial cells derived from benign tumours. However, 4 independently isolated epithelial cell lines derived from malignant rat mammary tumours and their metastases possess receptors for bFGF and contain between 2 ng and 9 ng heparin-binding, growth-stimulatory activity per 10(6) cells. The weakly metastatic Rama 600 cells possess high- and low-affinity receptors for bFGF, (KD 20 pM and 8 nM, respectively), while the moderately metastatic Rama 800 cells possess only high-affinity receptors (KD 40 pM). The moderately metastatic C18PLN and 267LU cells, derived from metastases arising from benign Rama 37 cells which had been transfected with DNA from the malignant Rama 800 cells, also possess only high-affinity receptors (KD 36 pM and 80 pM, respectively). Our results show that within the Rama system there is a correlation between the appearance of heparin-binding growth factors and of high-affinity but not low-affinity receptors for bFGF with the malignant phenotype.

Basic fibroblast growth factor (bFGF): mitogenic activity and binding sites in human breast cancer

We investigated binding characteristics of basic fibroblast growth factor (bFGF) on membranes prepared from 4 human breast cancer cell lines and 38 primary BC biopsies. Competitive binding experiments were performed and analyzed using the "Ligand" program. Furthermore bFGF mitogenic activity was measured by [3H]thymidine incorporation into DNA from breast cancer cell lines. The presence of high-affinity binding sites was demonstrated in each cell type (MCF-7: Kd = 0.60 nM; T-47D: Kd = 0.55 nM; BT-20: Kd = 0.77 nM; MDA-MB-231: Kd = 0.34 nM). The presence of these high-affinity binding sites was confirmed with saturation experiments. A second class of low-affinity binding sites was detected in the 2 hormone-independent cells (BT-20: Kd = 2.9 nM; MDA-MB-231: Kd = 2.7 nM). bFGF stimulated the proliferation of MCF-7, T-47D, BT-20 but not MDA-MB-231 cell lines. With competition experiments, binding sites were detectable in 36/38 breast cancers; high-affinity binding sites (Kd less than 1 nM) were present in 19/36 cases and low-affinity binding sites (Kd greater than 2 nM) were present in 29/36 cases (the two classes of binding sites were present in 12 breast cancers). No relation between bFGF binding sites and node involvement, histologic type or grading of the tumor was evidenced. There were negative correlations (Spearman test) between total bFGF binding sites and estradiol receptor (P = 0.05) or progesterone receptor (P = 0.009). The demonstration of (1) bFGF specific binding sites in breast cancer membranes, and (2) bFGF growth stimulation of some breast cancer cell lines indicates that this factor may be involved directly in the growth of some breast cancers.

Cloning and high level expression of a synthetic gene for human basic fibroblast growth factor

A gene encoding human basic fibroblast growth factor has been chemically synthesized, cloned and expressed in Escherichia coli as a biologically active protein. The 465 bp gene was assembled by enzymatic ligation of 6 pairs of oligonucleotides and cloned in the expression vector pLCII downstream from the strong PL promoter. This promoter directed the synthesis of a fusion protein between a 31 amino acids fragment of the lambda phage cII protein and bFGF. A four amino acid recognition sequence for the site-specific protease fXa was introduced in the plasmid construct and this allowed cleavage of the fusion protein at the boundary between cII and bFGF. bFGF was purified close to homogeneity using a Heparin-Sepharose column and Mono S cation exchange chromatography. The use of the pLCII expression system resulted in the accumulation of 20 to 25 mg of purified bFGF per l of bacterial culture. The recombinant bFGF was mitogenic for mouse 3T3 fibroblasts and the dose-response curve was similar to the one for native bFGF.

High and low affinity membrane binding sites for fibroblast growth factors in the developing chick brain

Acidic and basic fibroblast growth factors (aFGF and bFGF), two mitogenic, neurotrophic and angiogenic molecules, are present in the embryonic chick brain but their function remains unclear. In order to approach the biological activity of FGFs during brain development, we have looked for their receptors and studied their regulation through chick brain development. Competitive binding studies realized on brain membranes indicated the presence of two classes of FGF binding sites: high affinity binding sites (dissociation constant, Kd = 100 pM) and low affinity binding sites (Kd = 20 nM). Cross-competition experiments show that these two classes of binding sites both interact with aFGF and bFGF. The number of sites in these two classes of binding sites changes during embryogenesis. On the one hand, the membrane capacity of high affinity sites decreases from E7 (1 +/- 0.2 pmol/mg of protein) to E15 (0.5 +/- 0.2 pmol/mg of protein); on the other hand, the membrane capacity of low affinity sites increases from E15 (25 +/- 4 pmol/mg of protein) to P1 (75 +/- 20 pmol/mg of protein). Cross-linking experiments revealed the presence of two putative receptor forms of molecular masses of about 130 and 95 kDa. These results suggest that the biological activity of aFGF and bFGF during brain embryogenesis could be regulated by the expression of high and low affinity binding sites for these growth factors.

Rat mammary myoepithelial-like cells in culture possess kinetically distinct low-affinity receptors for fibroblast growth factor that modulate growth stimulatory responses

The rat mammary myoepithelial-like cell line Rama 401 possesses 46,000 high-affinity receptors (Kd 52 pM) and 2.8 x 10(6) low-affinity receptors (Kd 24 nM) for basic fibroblast growth factor (bFGF) per cell. Heparin or heparinase pretreatment of the cells inhibits the specific binding of [125I]-bFGF by over 70%, and abolishes binding to the low-affinity sites. Dissociation experiments suggest that there are three kinetically distinct low-affinity receptors, with dissociation rate constants of 3.8 s-1, 0.067 s-1 and 0.0018 s-1. Consistent with the presence of low-affinity receptors possessing a slow dissociation rate constant, exogenously added bFGF bound to the low-affinity receptor can stimulate DNA synthesis in Rama 401 cells without being released into the bulk of the culture medium. These results suggest that the low-affinity receptors on Rama 401 cells are heparan sulfate glycosaminoglycans (HSGAGs) and that their ability to modulate the action of bFGF may result from their diverse range of dissociation rate constants. A cell line, Rama 401ts, derived from Rama 401 by transformation with a temperature sensitive src gene, deposits less extracellular matrix at the permissive temperature of 34 degrees C than at the non-permissive temperature of 41 degrees C. Whilst the binding of [125I]-bFGF to Rama 401ts cells at 41 degrees C is identical to that observed with the parental Rama 401 cells, at 34 degrees C there are fewer low-affinity receptors. These results suggest the (HSGAGs) low-affinity receptors on Rama 401 cells are associated at least in part with the extracellular matrix.

Relationship of growth factors and differentiation in normal and neoplastic development of the mammary gland

The different mammary cell lines described herein appear to be representative of the cell types found in both normal glands and benign tumors of rats and humans. The epithelial cell lines can differentiate to both alveolar-like and myoepithelial-like cells in culture. The epithelial cell lines and particularly those cell lines representing intermediate stages in the myoepithelial differentiation pathway are candidates for the epithelial stem cells found in rat and possibly in human terminal ductal structures. The systemic mammatrophic hormones that are thought to control the growth of the mammary gland in vivo have little or no stimulatory effect alone on the growth of normal and neoplastic rat mammary cells in culture. The pituitary growth factors (fibroblast growth factor [FGF] and pituitary-derived mammary growth factor [PMGF],) and the growth factors released from the different cell lines, (stromal prostaglandin E2 [PGE2] and myoepithelial transforming growth factor alpha [TGF-alpha]) are much more potent mitogenic agents for the mammary cell lines. The ability of FGF and epidermal growth factor (EGF) -related molecules to simulate growth of the different mammary cell types in culture correlates with the presence of their high-affinity receptors. Thus these growth factors are promising candidates for some of the primary effectors of mammary growth in vivo. Malignant mammary epithelial cells have a greatly reduced rate of growth compared to their normal and benign counterparts. They also fail to differentiate or to respond to PMGF but can still respond to PGE2 and TGF-alpha. In addition, highly malignant variants appear capable of adapting to a new growth environment in vivo. This suggests that simple molecular explanations based solely on the autostimulation of cell growth may not be sufficient to explain some of the properties of the slowly growing, highly malignant cells.

Stromal influences on transformation of human mammary epithelial cells overexpressing c-myc and SV40T

The proto-oncogene c-myc and the oncogene SV40T, both of which have been implicated in the process of cellular immortalization in vitro, have been introduced via amphotropic retroviral expression vectors into the human mammary epithelial cell (HMEC) line 184A1N4 (A1N4). Two stable cell lines were established by growth in selective medium and were found to overexpress either c-myc (A1N4-myc) or SV40T antigen (A1N4-T). Neither the A1N4, A1N4-myc, or A1N4-T cells will grow in soft agar or form tumors in nude mice. However, A1N4-T or A1N4-myc cells, but not the parental A1N4 cells, form colonies in soft agar in response to either epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha), or basic fibroblast growth factor (bFGF). Like EGF and TGF alpha, bFGF is moderately mitogenic for the anchorage-dependent growth (ADG) of all three cell lines. Further, co-cultivation of A1N4-T or A1N4-myc cells with primary diploid mammary fibroblasts can also induce the anchorage-independent growth (AIG) and stimulate the ADG of A1N4-T or A1N4-myc. In addition, conditioned medium obtained from these mammary fibroblasts also stimulated the AIG of the A1N4-T and A1N4-myc cells and was found to contain immunoreactive TGF alpha and bioactive FGF. The mammary fibroblasts express specific mRNA transcripts for bFGF and acidic FGF (aFGF). These results suggest that growth factors such as TFG alpha or FGF, which may be derived from the adjacent mammary stroma, might influence in a paracrine manner the phenotypic characteristics of a population of human mammary epithelial cells toward transformation.

High-level production of human acidic fibroblast growth factor in E. coli cells: inhibition of DNA synthesis in rat mammary fibroblasts at high concentrations of growth factor

Recombinant human acidic fibroblast growth factor has been produced in E. coli cells at a level of at least 50 mg/l culture. The recombinant and natural acidic fibroblast growth factors are almost identical to one another when tested on rat mammary fibroblasts for their ability to stimulate DNA synthesis, to bind to the high-affinity surface receptors of the cells and to inhibit DNA synthesis when present in the culture medium at high concentrations. The recombinant acidic fibroblast growth factor binds to two cell-surface polypeptides of molecular masses 160 kDa and 140 kDa, which are the same size as the receptors for basic fibroblast growth factor, and it binds preferentially to the smaller polypeptide.

Synthesis of basic fibroblast growth factor upon differentiation of rat mammary epithelial to myoepithelial-like cells in culture

Acidic fibroblast growth factor (aFGF) mRNA was detected in a rat mammary fibroblastic cell line, but not in rat mammary epithelial cell lines or myoepithelial-like cell lines. Basic FGF (bFGF) mRNA was detected in both the fibroblasts and the myoepithelial-like cells, but was absent from the epithelial cells. A series of cell lines representing stages in the differentiation pathway of epithelial cells to a myoepithelial-like morphology showed an increase in the amount of bFGF mRNA and activity present and the FGF from the myoepithelial-like rat mammary 29 cells was able to displace [125I]-bFGF specifically bound to rat mammary fibroblasts. FGF activity was also present in an extract of rat mammary gland. Analysis of cell extracts and conditioned medium indicated that FGF activity was cell-associated. The cell-associated bFGF was resistant to degradation by trypsin. Extraction of myoepithelial-like cells with Triton X-100 and 2 M NaCl showed that 50-65% of the cell-associated bFGF was in a detergent-resistant but 2 M NaCl-labile structure. Thus, the synthesis of bFGF is developmentally regulated in rat mammary cell lines, and at least 50% is present in the extracellular matrix.