Species-specific high molecular weight forms of basic fibroblast growth factor

Oestrogens, via transforming growth factor alpha, modulate basic fibroblast growth factor synthesis in hypothalamic astrocytes: in vitro observations

The data presented here show that, in cultures of type 1 astrocytes obtained from the hypothalamus of neonatal female rat, 17beta-oestradiol is able to increase both the mRNA and the protein levels of basic fibroblast growth factor (bFGF). In particular, after 24 h of exposure to 17beta-oestradiol (10(-9) and 10(-10) m), an increase of messenger levels of bFGF appears in hypothalamic type 1 astrocytes. Similarly, an induction of bFGF protein is also evident at this time of exposure. The effect on the mRNA and protein levels of bFGF is blocked by the presence in the medium of an antibody raised against the transforming growth factor alpha (TGFalpha) receptor. This observation indicates that, TGFalpha, whose synthesis is modulated by oestrogens in hypothalamic astrocytes and which is able to increase, both the mRNA and the protein levels of bFGF in our experimental model, may act as the mediator of the oestrogenic induction of bFGF. Hypothalamic astrocytes, together with hypothalamic neurones synthesizing and secreting luteinizing hormone-releasing hormone (LHRH), form the LHRH network in conjunction with other neuronal systems. Gonadal steroids in general, and oestrogens in particular, play an important role in the control of the activity of this network. In addition, bFGF and TGFalpha, two growth factors released from astrocytes, are able to influence the activity of LHRH neurones. The present observations suggest that oestrogens may also act on LHRH neurones in an indirect fashion (i.e. by modulating the expression of bFGF and TGFalpha in glial cells).

CUG-initiated FGF-2 induces chromatin compaction in cultured cardiac myocytes and in vitro

Fibroblast growth factor-2 (FGF-2) is a mitogen found in CUG-initiated 21-25 kDa ("hi") or AUG-initiated 16-18 kDa ("lo") forms. Previously we demonstrated that "hi"-but not "lo"-FGF-2 caused a distinct nuclear phenotype characterized by apparently condensed chromatin present as separate clumps in the nucleus of cardiac myocytes. In this manuscript we investigated whether these effects were related to apoptosis or mitosis and whether they reflected a direct effect of "hi" FGF-2 on chromatin. Myocytes overexpressing "hi" FGF-2 and presenting the clumped chromatin phenotype: (i) were not labeled above background with antibodies to phosphorylated histones H1 and H3 used as indicators of mitotic chromatin condensation; (ii) did not stain positive for TUNEL; (iii) their nuclear lamina, visualized by anti-laminB immunofluorescence, appeared intact; (iv) neither caspase inhibitors, nor Bcl-2 or "lo" FGF-2 overexpression prevented the manifestation of the compacted nuclear phenotype. Purified recombinant "hi" FGF-2 was more potent than "lo" FGF-2 in promoting the condensation/aggregation of chick erythrocyte chromatin partially reconstituted with histone H1 in vitro. We conclude that the DNA phenotype induced by "hi" FGF-2 in cardiac myocytes likely reflects a direct effect on chromatin structure that does not require the engagement of mitosis or apoptosis. By affecting chromatin compaction "hi" FGF-2 may contribute to the regulation of gene expression.

Basic fibroblast growth factor regulates extracellular matrix and contractile protein expression independent of proliferation in vascular smooth muscle cells

Basic fibroblast growth factor (bFGF) can influence proliferation and differentiation in vascular smooth muscle cells. Basic FGF promotes some features of the synthetic phenotype (proliferation) but is known to inhibit others (collagen synthesis). Whether bFGF availability influences smooth muscle cell phenotype independent of proliferation is not known. The purpose of this study was to determine if the effects of bFGF on extracellular matrix and contractile protein expression are dependent on changes in proliferation. Basic FGF availability was manipulated by adding bFGF to cultured cells or by inhibiting bFGF expression using antisense RNA, and adjusting culture conditions such that proliferation was held constant. Compared to cells cultured in serum alone, smooth muscle alpha-actin and myosin heavy chain expression was markedly reduced by added bFGF, but was not influenced by antisense inhibition of bFGF expression. Under the same conditions, collagen synthesis was inhibited by added bFGF, and was stimulated by reduced bFGF expression. These consequences of altering bFGF availability were not associated with changes in FGF receptor expression. These findings demonstrate that alterations in bFGF availability can regulate smooth muscle cell phenotype independent of proliferation, which may be related to the regulation of smooth muscle cell phenotype in vivo.

Fibroblast growth factors are necessary for neural retina but not pigmented epithelium differentiation in chick embryos

During eye development, optic vesicles evaginate laterally from the neural tube and develop into two bilayered eye cups that are composed of an outer pigment epithelium layer and an inner neural retina layer. Despite their similar embryonic origin, the pigment epithelium and neural retina differentiate into two very distinct tissues. Previous studies have demonstrated that the developmental potential of the pigmented epithelial cells is not completely restricted; until embryonic day 4.5 in chick embryos, the cells are able to switch their phenotype and differentiate into neural retina when treated with fibroblast growth factors (FGF) (Park, C. M., and Hollenberg, M. J. (1989). Dev. Biol. 134, 201–205; Pittack, C., Jones, M., and Reh, T. A. 1991). Development 113, 577–588; Guillemot, F. and Cepko, C. L. (1992). Development 114, 743–754). These studies motivated us to test whether FGF is necessary for neural retina differentiation during the initial stages of eye cup development. Optic vesicles from embryonic day 1.5 chick were cultured for 24 hours as explants in the presence of FGF or neutralizing antibodies to FGF2. The cultured optic vesicles formed eye cups that contained a lens vesicle, neural retina and pigmented epithelium, based on morphology and expression of neural and pigmented epithelium-specific antigens. Addition of FGF to the optic vesicles caused the presumptive pigmented epithelium to undergo neuronal differentiation and, as a consequence, a double retina was formed. By contrast, neutralizing antibodies to FGF2 blocked neural differentiation in the presumptive neural retina, without affecting pigmented epithelial cell differentiation. These data, along with evidence for expression of several FGF family members and their receptors in the developing eye, indicate that members of the FGF family may be required for establishing the distinction between the neural retina and pigmented epithelium in the optic vesicle.

In vivo and in vitro effect of glucocorticoids on fibroblast growth factor (FGF)-2 and FGF receptor 1 expression

In order to clarify the physiological function of fibroblast growth factor (FGF-2) in the adrenal medulla the regulation of FGF-2 and FGF receptor 1 (FGFR1) was studied in vitro and in vivo in response to glucocorticoids. To assess the effects of glucocorticoids, in vivo extracts of adrenal medulla and adrenal cortex were analyzed by RNase protection assay and Western blot analysis. PC12 cells were chosen as a model system to study the effects of glucocorticoids in vitro. In PC12 cells, dexamethasone (DEX) was found to stimulate dramatically the expression of both FGF-2 mRNA and protein. Western blot analysis revealed that exclusively the 21-kDa FGF-2 isoform was enhanced. In contrast to the FGF-2 mRNA level FGFR1 was not affected by treatment with glucocorticoids. In vivo FGF-2 mRNA level and 21-kDa FGF-2 isoform level are significantly enhanced in the adrenal medulla 24 h after DEX injection. In vivo application of DEX leads to an increase of the medullary and cortical FGFR1 transcript levels. Glucocorticoid effects on FGF-2 expression were not found in adrenal cortex, heart, skeletal muscle, and kidney, respectively, in vivo and in L6 rat myoblasts in vitro. In addition to adrenal medullary cells glucocorticoids elevated the FGF-2 mRNA and protein level also in vivo in the brain and in vitro in immortalized Schwann cells. The present results suggest that the 21-kDa FGF-2 isoform mediates a physiological function specific for neuronal tissue which is modulated by glucocorticoids.

Antisense inhibition of basic fibroblast growth factor induces apoptosis in vascular smooth muscle cells

Basic fibroblast growth factor (bFGF), a potent mitogen for many cell types, is expressed by vascular smooth muscle cells and plays a prominent role in the proliferative response to vascular injury. Basic FGF has also been implicated as a survival factor for a variety of quiescent or terminally differentiated cells. Autocrine mechanisms could potentially mediate both proliferation and cell survival. To probe such autocrine pathways, endogenous bFGF production was inhibited in cultured rat vascular smooth muscle cells by the expression of antisense bFGF RNA. Inhibition of endogenous bFGF production induced apoptosis in these cells independent of proliferation, and apoptosis could be prevented by exogenous bFGF but not serum or epidermal growth factor. The induction of apoptosis was associated with an inappropriate entry into S phase. These data demonstrate that interruption of autocrine bFGF signaling results in apoptosis of vascular smooth muscle cells, and that the mechanism involves disruption of normal cell cycle regulation.

High and low molecular weight fibroblast growth factor-2 increase proliferation of neonatal rat cardiac myocytes but have differential effects on binucleation and nuclear morphology. Evidence for both paracrine and intracrine actions of fibroblast growth factor-2

Basic fibroblast growth factor (FGF-2) plays a vital role in the growth and differentiation of cardiac myocytes. It exists in high and low molecular weight forms because of the use of alternative initiation codons in the same mRNA. Higher levels of high molecular weight forms (molecular mass of 22 and 21.5 kD) are present in the rat heart during the neonatal stage, whereas the low molecular weight form (molecular mass of 18 kD) is predominant in the adult heart, suggesting different roles in development. Rat FGF-2 cDNAs that can preferentially express high or low molecular weight forms were introduced into neonatal rat ventricular myocyte cultures. Significant and comparable increases in overall cardiac myocyte DNA synthesis and proliferation were seen with 22/21.5- and 18-kD FGF-2 expression. A significantly higher mitotic index was seen in the vicinity of cardiac myocytes overexpressing high or low molecular weight forms of FGF-2 compared with nonoverexpressing cells. This increase was inhibited in the presence of neutralizing antibodies to FGF-2, pointing to a proximity-dependent paracrine effect of 22/21.5- and 18-kD FGF-2 on mitosis. By contrast, overexpression of high but not low molecular weight FGF-2 was associated with a significant increase in binucleation (approximately 36% of cardiac myocytes overexpressing 22/21.5-kD FGF-2 were binucleated compared with 9% of cardiac myocytes overexpressing 18-kD FGF-2), which was not affected by neutralizing antibodies to FGF-2. These results suggest that 22/21.5-kD FGF-2 and 18-kD FGF-2 have similar paracrine effects on proliferation but that 22-21.5-kD FGF-2 exerts a distinct intracrine effect on binucleation.

Production of heparin-binding epidermal growth factor-like growth factor (HB-EGF) at sites of thermal injury in pediatric patients

Fluids that accumulate at wound sites may be an important reservoir of growth factors that promote the normal wound healing response. The presence of heparin-binding growth factors was studied in burn wound fluid (BWF) from 45 pediatric patients who had sustained partial thickness burns. One of the growth factors present was similar to platelet-derived growth factor (PDGF) based on its heparin affinity, inhibition of bioactivity by a PDGF antiserum, and detection in a PDGF-AB enzyme-linked immunosorbent assay. A second growth factor was identified as heparin-binding epidermal growth factor-like growth factor (HB-EGF) based on its heparin affinity, competition with 125I-labeled epidermal growth factor (EGF) for EGF receptor binding, and recognition in biological assays and Western blots by two HB-EGF antisera. Amino acid sequence analysis of one form of this second growth factor verified its identity as an N-terminally truncated form of HB-EGF. Immunohistochemical analysis of partial thickness burns demonstrated the presence of HB-EGF in the advancing epithelial margin, islands of regenerating epithelium within the burn wound, and in the duct and proximal tubules of eccrine sweat glands. HB-EGF in the surface epithelium of burn wounds was uniformally distributed, whereas it was restricted to the basal epithelium in nonburned skin. These data support a role for PDGF and HB-EGF in burn wound healing and suggest that the response to injury includes deposition of HB-EGF and PDGF into blister fluid and a redistribution of HB-EGF in the surface epithelium near the wound site.

Abnormal bone growth and selective translational regulation in basic fibroblast growth factor (FGF-2) transgenic mice

Basic fibroblast growth factor (FGF-2) is a pleiotropic growth factor detected in many different cells and tissues. Normally synthesized at low levels, FGF-2 is elevated in various pathologies, most notably in cancer and injury repair. To investigate the effects of elevated FGF-2, the human full-length cDNA was expressed in transgenic mice under control of a phosphoglycerate kinase promoter. Overexpression of FGF-2 caused a variety of skeletal malformations including shortening and flattening of long bones and moderate macrocephaly. Comparison by Western blot of FGF-2 transgenic mice to nontransgenic littermates showed expression of human FGF-2 protein in all major organs and tissues examined including brain, heart, lung, liver, kidney, spleen, and skeletal muscle; however, different molar ratios of FGF-2 protein isoforms were observed between different organs and tissues. Some tissues preferentially synthesize larger isoforms of FGF-2 while other tissues produce predominantly smaller 18-kDa FGF-2. Translation of the high molecular weight isoforms initiates from unconventional CUG codons and translation of the 18-kDa isoform initiates from an AUG codon in the FGF-2 mRNA. Thus the Western blot data from the FGF-2 transgenic mice suggest that tissue-specific expression of FGF-2 isoforms is regulated translationally.

Identification of fibroblast growth factors in bovine cheese whey

Acidic and basic fibroblast growth factors (FGF) were identified in bovine cheese whey after partial purification using a two step procedure. Cation-exchange chromatography produced a mitogen-rich extract which was loaded on to a heparin-sepharose column and eluted stepwise with 0.8, 1.2 and 2.0 M-NH4HCO3. Mitogenic activity was found in all three fractions by cell growth assays using Balb/c-3T3 fibroblasts. Immunoblotting identified acidic FGF in the 1.2 M-eluate and basic FGF in the 2.0 M-eluate, but neither acidic nor basic FGF was detected in the 0.8 M-fraction. Quantitative radioreceptor assays indicated 5.8 ng of acidic FGF-like activity and 19.8 ng of basic FGF-like activity per 1 whey in the appropriate eluates. This study represents the first direct demonstration of FGF in milk.

Growth factors and canine flexor tendon healing: initial studies in uninjured and repair models

The role of growth factors in a variety of bone and soft tissue healing processes has been studied extensively in numerous recent models, yet little is known about the specific growth factors that may be playing a role in flexor tendon healing. We used a number of established protein purification techniques and bioassays to isolate and partially characterize a heparin-binding growth factor from unoperated canine tendons. Our data provide evidence that basic fibroblast growth factor, a potent angiogenic growth factor, is present in normal canine intrasynovial flexor tendons. We then studied repaired canine flexor tendons to further elucidate the role of growth factors in the tendon healing process. Heparin-sepharose elution profiles from three repair intervals (3, 10, and 17 days) were graphed and compared to known profiles of isolated growth factors. The three repair intervals demonstrated two elution profile peaks, consistent with varying amounts of platelet-derived growth factor and epidermal growth factor. Although additional experimentation is required to identify definitively the various protein isolates, these data provide compelling evidence that a variety of growth factors are present in uninjured and healing digital flexor tendons.

Characterization of cDNA encoding basic fibroblast growth factor of the marsupial Monodelphis domestica

We have isolated and characterized a 1,593-bp cDNA containing the coding region of the basic fibroblast growth factor (BFGF) gene of a marsupial, the opossum Monodelphis domestica. The encoded protein is 156 amino acids long. The BFGF gene of M. domestica is 82-87% identical to the BFGF genes of placental mammals at the nucleotide level and 92-93% identical to these genes at the level of the amino acids encoded. Regions of the BFGF molecule important in heparin binding, high-affinity receptor binding, and biologic function are highly conserved between placental mammals and this marsupial. There are several AUG and CUG codons in the 5' region of the marsupial cDNA that may serve as alternate sites of translation initiation; use of these sites would produce amino-terminally extended BFGF proteins. Amino-terminal extensions of BFGF in other species serve as nuclear localization signals. Conserved A+T-rich motifs in the 3' untranslated region of the marsupial mRNA probably serve to regulate mRNA stability. The high degree of evolutionary conservation of BFGF in mammals suggests that the molecule plays an important role in normal growth and development and that stringent control of its activity is essential.

Septal neuron cholinergic and GABAergic functions: differential regulation by basic fibroblast growth factor and epidermal growth factor

Numerous studies suggest that growth and trophic factors play roles in the development and mature function of brain neurons. Recently, growth factors whose actions were previously characterized on non-neuronal cells have been localized to the brain. We sought to determine whether these factors influence septal cholinergic function. Initially, we defined the effects of basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) on septal cholinergic cells in dissociated neuronal culture. Both factors elevated activity of the acetylcholine synthetic enzyme, choline acetyltransferase (CAT). To determine whether the factors acted directly on neurons or whether glia mediated the effects, a mitotic inhibitor, 5-fluorodeoxyuridine (FDUR), was added to the cultures to eliminate dividing glia. The action of EGF was completely blocked by the addition of FDUR. However, bFGF elevated CAT activity even in the presence of FDUR. Consequently, bFGF may regulate septal cholinergic function directly, whereas EGF may affect cholinergic cells indirectly through glia. To determine whether increases in CAT activity reflect increased enzyme activity per neuron or an increase in the number of cholinergic cells, bFGF-treated cultures were stained for acetylcholinesterase (AChE) to determine numbers of cholinergic cells. No differences in AChE-positive cells were noted, suggesting that bFGF increased CAT activity per cholinergic neuron. To determine whether bFGF regulates other populations in the septum, we examined GABAergic neurons by monitoring the activity of glutamic acid decarboxylase (GAD), a GABA synthetic enzyme. Basic FGF significantly increased GAD activity; however, the effect was completely abolished by addition of FDUR. Thus, bFGF may act directly on cholinergic neurons and indirectly on GABA cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Basic fibroblast growth factor (bFGF) acts intracellularly to cause the transdifferentiation of avian neural crest-derived Schwann cell precursors into melanocytes

We previously found that cultured neural crest-derived cells from embryonic quail peripheral nerves, which consist mostly of Schwann cell precursors, gave rise to melanocytes following treatment with basic fibroblast growth factor (bFGF) or 12-O-tetradecanoyl phorbol-13-acetate (TPA). Here, we show that antisense deoxyoligonucleotides targeted against two regions of the bFGF mRNA transcript blocked this TPA-induced transdifferentiation of Schwann cell precursors. Neither sense nor scrambled antisense control oligonucleotides had any effect in this regard. TPA increased bFGF protein expression in cell lysates but not in conditioned media from these cultures, and this expression was localized to the nucleus and cytoplasm. Furthermore, bFGF-neutralizing antibodies and inositol-hexakisphosphate (InsP6) both inhibited pigmentation caused by exogenous bFGF, but had no affect on TPA-induced melanogenesis, suggesting that bFGF is not released by these cells. These data indicate that bFGF is necessary for the TPA-induced transdifferentiation of Schwann cell precursors into melanocytes and that bFGF acts via an intracrine mechanism.

Distribution of acidic and basic fibroblast growth factors in ovine skin during follicle morphogenesis

Acidic and basic fibroblast growth factors (aFGF and bFGF) have been localized by immunochemistry in ovine skin during wool follicle morphogenesis. At 40 days of gestation, prior to the appearance of follicle primordia, bFGF immunoreactivity was detected in the intermediate and periderm layers of the epidermis and at the dermal-epidermal junction. Antibodies to aFGF did not bind to skin at this age. During early follicle formation, at 76 days of gestation, both FGFs were found in the epidermis and associated with the follicle primordia. Antibodies to aFGF, in particular, bound to the basal cells of the epidermis and the follicle cell aggregations. With the development of epidermal plugs, bFGF was confined to the intermediate layers of the epidermis and the dermal-epidermal junction, whereas aFGF staining was associated with the cells of the epidermis and the plugs. At 90 days, when many different stages of follicle development were in evidence, immunoreactivity for both FGFs was associated with the cells of the elongating epidermal column, particularly those adjacent to the dermal-epidermal junction. During follicle maturation, bFGF was found in the suprabasal layer of the epidermis, in the outer root sheath of the follicle and in the basement membrane zone surrounding the bulb matrix. Conversely, strong staining for aFGF was observed in the epidermis and pilary canal contiguous with the epidermis, and in cells of the upper bulb matrix of the follicle in the region of the keratogenous zone. Western blotting of extracts of mature follicles that had been isolated from the skin showed the presence of a major aFGF immunoreactive band with an apparent molecular mass of 27 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple molecular weight forms of basic fibroblast growth factor are developmentally regulated in the central nervous system

Basic fibroblast growth factor (bFGF) is a heparin-binding protein implicated in the differentiation, proliferation, and maintenance of cells in the central nervous system (CNS). It is not clear how bFGF achieves this multiplicity of effects. Multiple molecular weight forms of bFGF have recently been identified, however, and each form may have distinct activities during CNS development. We have examined the pattern of bFGF expression during CNS development using protein immunoblot and RNA blot analyses. RNA blot analysis detected a major bFGF transcript of 3.7 kb in embryonic and adult rat brain; however, this message decreased in abundance during development. Three bFGF protein forms were identified on immunoblots of adult rat brain extract with approximate molecular weights of 18, 21, and 22 kDa. Embryonic rat brain extracts also contained the 18- and 21-kDa bFGF protein forms, but lacked the 22-kDa form. Expression of the 22-kDa form was first detected in the neonate and then steadily increased to adult levels by 1 month of age. Immunoblots of adult human brain extracts also showed the presence of three bFGF protein forms with approximate molecular weights of 18, 22, and 24 kDa. In human second trimester fetal brain extracts, only the 18-kDa bFGF protein was detected. Comparison of bFGF proteins in developing rat spinal cord, cerebellum, and cortex demonstrated that distinct patterns of bFGF protein forms exist in different regions of the CNS. Therefore, the expression of individual bFGF protein forms is regulated in the CNS with regard to both developmental stage and location. These data support the idea that different forms of bFGF may be associated with specific developmental events during the maturation and organization of the nervous system.

Basic fibroblast growth factor in cultured cardiac myocytes

Distribution of basic-fibroblast-growth-factor-like peptides in immature cultured cardiac myocytes was investigated using specific antisera and immunolocalization. Basic FGF was detected in association with the external surface of the cell membrane, with specialized intercellular junctions and with the myofibril Z lines in the cytoplasm. Intense, punctate nuclear anti-bFGF labeling was observed in a fraction of interphase myocytes of near-confluent, proliferating cultures. This staining pattern persisted even after the dissolution of the nuclear envelope in prophase myocytes. The pattern of cellular localization of bFGF indicates a continuous participation of this factor in myocyte physiology as well as a role in the cell cycle. Furthermore, the identification of bFGF not only in cell lysates but also in culture media after gentle mechanical disruption suggests that cardiac myocytes may release bFGF in vivo following tissue damage.

Growth factors in the uterus: steroidal regulation and biological actions

Rapid progress has been made within the last 5-6 years in characterizing polypeptide growth factors in uterine tissues and fluids. There is convincing evidence that their synthesis and/or secretion is regulated by steroid hormones. The possibility that these growth factors play a central role in growth and development of the uterus or placenta is suggested by the presence of their receptors on uterine or placental cells and their stimulatory effects on these cells in vitro. Since growth factors interact synergistically and also have non-mitogenic functions, the presence of a variety of these factors in the uterus suggests that they probably regulate various aspects of uterine function through complex autocrine and paracine pathways. However, experimental models need to be designed that will permit a more detailed analysis of the actual role of these factors in utero. Fruitful approaches may be to administer neutralizing antibodies or blocking peptides so as to antagonize uterine growth factor action, or to develop appropriate transgenic animals. These and other lines of study should help us to understand the role of growth factors in development of the immature uterus, growth of the placenta or gravid uterus, repair and angiogenesis of the endometrium, and uterine pathology.