Purification of human platelet-derived growth factor

Electrospun silk fibroin fibers for storage and controlled release of human platelet lysate

Human platelet lysate (hPL) is a pool of growth factors and cytokines able to induce regeneration of different tissues. Despite its good potentiality as therapeutic tool for regenerative medicine applications, hPL has been only moderately exploited in this field. A more widespread adoption has been limited because of its rapid degradation at room temperature that decreases its functionality. Another limiting factor for its extensive use is the difficulty of handling the hPL gels. In this work, silk fibroin-based patches were developed to address several points: improving the handling of hPL, enabling their delivery in a controlled manner and facilitating their storage by creating a device ready to use with expanded shelf life. Patches of fibroin loaded with hPL were synthesized by electrospinning to take advantage of the fibrous morphology. The release kinetics of the material was characterized and tuned through the control of fibroin crystallinity. Cell viability assays, performed with primary human dermal fibroblasts, demonstrated that fibroin is able to preserve the hPL biological activity and prolong its shelf-life. The strategy of storing and preserving small active molecules within a naturally-derived, protein-based fibrous scaffold was successfully implemented, leading to the design of a biocompatible device, which can potentially simplify the storage and the application of the hPL on a human patient, undergoing medical procedures such as surgery and wound care.

STATEMENT OF SIGNIFICANCE

Human platelets lysate (hPL) is a mixture of growth factors and cytokines able to induce the regeneration of damaged tissues. This study aims at enclosing hPL in a silk fibroin electrospun matrix to expand its utilization. Silk fibroin showed the ability to preserve the hPL activity at temperature up to 60 °C and the manipulation of fibroin's crystallinity provided a tool to modulate the hPL release kinetic. This entails the possibility to fabricate the hPL silk fibroin patches in advance and store them, resulting in an easy and fast accessibility and an expanded use of hPL for wound healing.

Vascular precursor cells in tissue injury repair

Vascular precursor cells include stem cells and progenitor cells giving rise to all mature cell types in the wall of blood vessels. When tissue injury occurs, local hypoxia and inflammation result in the generation of vasculogenic mediators which orchestrate migration of vascular precursor cells from their niche environment to the site of tissue injury. The intricate crosstalk among signaling pathways coordinates vascular precursor cell proliferation and differentiation during neovascularization. Establishment of normal blood perfusion plays an essential role in the effective repair of the injured tissue. In recent years, studies on molecular mechanisms underlying the regulation of vascular precursor cell function have achieved substantial progress, which promotes exploration of vascular precursor cell-based approaches to treat chronic wounds and ischemic diseases in vital organ systems. Verification of safety and establishment of specific guidelines for the clinical application of vascular precursor cell-based therapy remain major challenges in the field.

Human malignant fibrous histiocytomas in vitro: growth characteristics and their association with expression of mRNA for platelet-derived growth factor, transforming growth factor-alpha and their receptors

Eight human malignant fibrous histiocytomas were examined in vitro, in order to relate their growth properties to mRNA expression for platelet-derived growth factor (PDGF), PDGF receptor (PDGF-R), transforming growth factor-alpha (TGF-alpha) and the epidermal growth factor receptor (EGF-R). Reverse transcriptase-polymerase chain reaction (RT-PCR) showed that all cell lines expressed mRNA for PDGF-R alpha and/or PDGF-R beta; six cell lines expressed mRNA for the PDGF-A chain, with one cell line coexpressing PDGF-B chain mRNA; seven cell lines expressed mRNA for TGF-alpha whereas six cell lines expressed EGF-R mRNA. Conditioned medium from three cell lines contained PDGF; none of the cell lines released TGF-alpha. Two cell lines grew without serum requirements; whereas both expressed mRNA for PDGF, PDGF-R, TGF-alpha and EGF-R, other cell lines, unable to grow without serum, showed the same combination of growth factor/growth factor receptor expression. The two cell lines able to grow without serum were also shown to be stimulated by the addition of PDGF-BB. These findings show that simultaneous expression of mRNA for a growth factor and its receptor does not necessarily imply an autocrine or paracrine loop. However, two of our cell lines fulfil the requirements of possible PDGF-related autocrine and paracrine regulation.

Nicotine stimulates DNA synthesis and proliferation in vascular endothelial cells in vitro

Nicotine is a major component of cigarette smoke and has been postulated to play an important role in atherogenesis and malignancy. Endothelial cell growth may be regulated by nicotine, yet operative mechanisms at the endothelial level are poorly understood. We studied the effects of nicotine (10(-14)-10(-4) M) on endothelial DNA synthesis, DNA repair, proliferation, and cytotoxicity by using cultures of bovine pulmonary artery endothelial cells. Assays were performed on cells incubated with nicotine in the presence and absence of hydroxyurea (an inhibitor of scheduled DNA synthesis), serum, human platelet-poor plasma, and platelet-derived growth factor and endothelial cell growth factor (PDGF and PDECGF, respectively). Nicotine significantly stimulated endothelial cell DNA synthesis and proliferation at concentrations lower than those obtained in blood after smoking (<10(-8) M). The stimulatory effects of nicotine were enhanced by serum (0.5%) and PDECGF and were blocked by the nicotinic-receptor antagonist hexamethonium. The response to nicotine was bimodal because cytotoxicity was observed at higher concentrations (>10(-6) M). This study has implications for understanding cellular mechanisms of nicotine action. The results may be important in tumor angiogenesis, atherogenesis, and vascular dysfunction in smokers.

Growth-promoting effects of substance P on endothelial cells in vitro. Synergism with calcitonin gene-related peptide, insulin, and plasma factors

The purpose of this study was to determine the effects of the vasoactive perivascular neuropeptide substance P (SP) on the growth and function of vascular endothelial cells in serum-free culture conditions with cells quiescent in the G0-G1 phase of the cell cycle and to characterize the response. In addition, interactions between SP and other growth factors and neuropeptides including insulin, platelet factors, neurokinin A, neurokinin B, and calcitonin gene-related peptide (CGRP) were studied on endothelial cell growth and compared. Growth effects were determined by stimulation of tritiated thymidine incorporation into DNA and cell proliferation. SP exhibited differential effects on cell growth that were a function of concentration, incubation time, interaction with other growth factors, and cell culture conditions. DNA synthesis in response to SP showed a bell-shaped distribution with a maximal effect that was 10.5-fold over control at 500 micrograms/mL of SP after 48 hours of incubation. The effect showed marked synergism with insulin (10 micrograms/mL) and with CGRP (0.01 to 10 micrograms/mL), which is colocalized with SP in vivo. Insulin and CGRP alone had no significant effect on endothelial cell growth. Furthermore, no synergism was observed between SP and platelet-derived growth factor or platelet-derived endothelial cell growth factor. Endothelial cell proliferation increased in response to SP to 2.6-fold over control at 48 hours, was maximal at 10 micrograms/mL SP, and also demonstrated synergism with insulin (10 micrograms/mL). Our studies indicate that neuropeptides play a significant role in regulating endothelial cell growth and proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin-like glycosaminoglycans influence growth and phenotype of human arterial smooth muscle cells in vitro. I. Evidence for reversible binding and inactivation of the platelet-derived growth factor by heparin

We have investigated the effects of interactions between growth factors and heparin-like glycosaminoglycans on untransformed human arterial smooth muscle cells (hASMC) in vitro. The results indicate that heparin in the presence of serum mitogens prevents the cells from entering the S phase of the cell cycle by binding and inactivating reversibly some serum mitogen(s). Our results suggest that platelet-derived growth factor (PDGF) is one of them and that it is the most potent stimulator of hASMC growth in vitro. Thymidine incorporation as well as increase in DNA content was inhibited not only by the presence of heparin in serum-containing medium but also when serum was chromatographed on Heparin-Sepharose at physiologic salt concentrations before exposure to the cells. The mitogenic activity of the unretained serum fraction was restored by the addition of PDGF AA, AB, or BB dimers or of a fraction (RF I) that dissociated from Heparin-Sepharose at 0.2 to 0.6 M NaCl. Radiolabeled recombinant PDGF (c-sis) dissociated from Heparin-Sepharose within a concentration range of NaCl similar to that of RF I. Neither the unretained material nor the RF I or PDGF dimers were effective alone. The effect of RF I was significantly decreased by the addition of an anti-PDGF IgG that is known to neutralize the PDGF mitogenic activity partially. Addition of heparin abolished DNA-synthesis when the PDGF dimers or RF I were combined with the unretained fraction. A second fraction (RF II) bound strongly to Heparin-Sepharose and eluted between 1.1 and 1.6 M NaCl. The RF II also induced DNA synthesis but was neither as efficient as RF I nor depending on other serum fractions for growth promotion and it was not inhibited by anti-PDGF IgG. A similar strong affinity for Heparin-Sepharose was found for labeled basic fibroblast growth factor and we cannot exclude the possibility that RF II represent fibroblast growth factor. Under these culture conditions, inhibition of hASMC proliferation was directly correlated with the expression of smooth muscle specific alpha actin isoforms in stress fibers and the suppression of a proliferating cell-specific nuclear antigen. Conversely, stimulation of hASMC proliferation was associated with the opposite phenomenon. We conclude that heparin-like glycosaminoglycans influence growth and phenotype of hASMCs in vitro by binding and inactivating PDGF. Inasmuch as heparin-like substances constitute a significant proportion of the proteoglycan-associated glycosaminoglycans of the arterial wall, such mechanisms might be important for the development of atherosclerotic lesions.

Acute lung injury. Pathogenesis of intraalveolar fibrosis

In patients dying with acute lung injury, interstitial mesenchymal cells migrate into the airspace where they replicate and deposit connective tissue. We therefore hypothesized that peptides capable of promoting mesenchymal cell migration and replication would be present in the alveolar airspace. To examine this hypothesis, patients with severe acute diffuse lung injury (n = 26) underwent bronchoalveolar lavage. Acutely ill patients without lung injury served as controls (n = 12). Recovered effluent was examined for mesenchymal cell growth-promoting and migration-promoting activity. Lavage cell supernates from both patients and controls were devoid of bioactivity. However, substantial growth-promoting and migration-promoting activity was present in lavage fluid from nearly every patient, whereas little or none was present in fluid from controls. Characterization of the bioactivity indicated a significant proportion consisted of three peptides related to PDGF: (a) a 14-kD peptide that shared with PDGF several biophysical, biochemical, receptor-binding, and antigenic properties; (b) a 29-kD peptide that appeared identical to PDGF of platelet origin; and (c) a 38-kD peptide that was biophysically and antigenically similar to PDGF. These data indicate that peptide moieties are present in the airspace of patients after acute lung injury that can signal mesenchymal cell migration and replication.

Manipulations of cholinesterase gene expression modulate murine megakaryocytopoiesis in vitro

Megakaryocytopoiesis was selectively inhibited in cultured murine bone marrow cells by a 15-mer oligodeoxynucleotide complementary to the initiator AUG region in butyrylcholinesterase mRNA. Furthermore, conditioned medium from Xenopus oocytes producing recombinant butyrylcholinesterase stimulated megakaryocytopoiesis. These observations implicate butyrylcholinesterase in megakaryocytopoiesis and suggest application of oligodeoxynucleotides for modulating bone marrow development.

Manipulations of cholinesterase gene expression modulate murine megakaryocytopoiesis in vitro

Megakaryocytopoiesis was selectively inhibited in cultured murine bone marrow cells by a 15-mer oligodeoxynucleotide complementary to the initiator AUG region in butyrylcholinesterase mRNA. Furthermore, conditioned medium from Xenopus oocytes producing recombinant butyrylcholinesterase stimulated megakaryocytopoiesis. These observations implicate butyrylcholinesterase in megakaryocytopoiesis and suggest application of oligodeoxynucleotides for modulating bone marrow development.

Changes in expression and organization of smooth-muscle-specific alpha-actin during fibronectin-mediated modulation of arterial smooth muscle cell phenotype

The spreading of freshly isolated arterial smooth muscle cells on a substrate of fibronectin is mediated by an integrin receptor on the cell surface. It is associated with organization of actin filaments in stress fibers and marked changes in cell morphology and function, collectively referred to as a transition from a contractile to a synthetic phenotype. To study further how extracellular matrix components affect smooth muscle phenotype, we have analyzed the expression and organization of smooth-muscle-specific alpha-actin in freshly isolated rat aortic smooth muscle cells cultured on a substrate of fibronectin under serum-free conditions. Northern-blot analysis showed that the expression of mRNA for smooth muscle alpha-actin, but not for nonmuscle actin, was strongly repressed during primary culture. On the other hand, the cellular content of alpha-actin was only moderately changed during the same period. Indirect immunofluorescence staining revealed that nonmuscle actin was rapidly organized in stress fibers, which did not stain with a monoclonal antibody against smooth muscle alpha-actin. Filament bundles containing alpha-actin were most prominent in the central parts of the cytoplasm and gradually disappeared as the spreading of the cells progressed. In contrast to the situation with nonmuscle actin, there was no apparent overlap in the staining for alpha-actin and the fibronectin receptor (alpha 5 beta 1), indicating that this receptor interacted with nonmuscle actin during the initial spreading process. Taken together, the results show that the expression and organization of smooth muscle alpha-actin are changed during interaction of the cells with fibronectin early in primary culture. They support the notion that integrin-mediated interactions between extracellular matrix components and arterial smooth muscle cells take part in the control of smooth muscle phenotype.

Expression of PDGF A-chain and beta-receptor genes during rat myoblast differentiation

L6J1 rat myoblasts and rat skeletal muscle were studied for expression of mRNAs encoding PDGF A-chain, PDGF B-chain, PDGF alpha-receptor, and PDGF beta-receptor during in vitro and in vivo myoblast differentiation. RNA blot hybridizations demonstrated expression of the PDGF A-chain gene and the PDGF beta-receptor gene in L6J1 myoblasts and in crude muscle tissue isolated from developing rats. Transcripts of the PDGF A-chain were identified at all examined stages of in vitro and in vivo myogenic differentiation. Expression of the PDGF beta-receptor gene decreased in differentiated myotubes of L6J1 cells and in rat adult muscle tissue. Receptor binding assays demonstrated specific binding of PDGF-BB, but not -AA, to exponentially proliferating L6J1 myoblasts and to terminally differentiated L6J1 myotubes. The binding per cell nucleus was higher in exponentially proliferating myoblasts than in differentiated L6J1 myotubes. In serum free medium PDGF-BB was shown to increase c-fos protooncogene immunoreactivity in L6J1 myoblasts. In the presence of 0.5% FCS, PDGF-BB increased DNA synthesis in L6J1 myoblasts, while PDGF-AA showed no such effect. Differentiation, as monitored by myotube formation, was reduced in PDGF-BB-treated cultures. The possible role of PDGF in myoblast proliferation and differentiation is discussed.

Expression of PDGF alpha- and beta-receptors in rat arterial smooth muscle cells is phenotype and growth state dependent

Adult rat arterial smooth muscle cells were shown to express mRNA for the platelet-derived growth factor (PDGF) alpha- and beta-receptors and to bind radioiodinated PDGF-AA and PDGF-BB in a phenotype-dependent and growth state-dependent manner. PDGF alpha-receptor mRNA was not detected in the intact aortic media, but appeared as the cells converted from a contractile to a synthetic phenotype during serum-free primary culture. PDGF beta-receptor mRNA was expressed already in vivo, and increased further as the cells were isolated and cultured in vitro. Exposure of the cells to human platelet PDGF resulted in increased PDGF alpha-receptor mRNA levels, decreased PDGF beta-receptor mRNA levels, and decreased binding of both PDGF-AA and PDGF-BB. Following removal of the exogenous mitogen, the content of PDGF alpha- and beta-receptor mRNA increased, as did the binding of PDGF-AA and PDGF-BB. Subsequently, the content of PDGF A-chain mRNA started to rise, and the cells retained a high rate of DNA synthesis in a serum-free medium. As a result of this autocrine stimulation, the PDGF receptors were down-regulated. Although smooth muscle cells in serum-free primary cultures bound the different PDGF isoforms to a varying extent (AA less than AB less than BB), the replicative response was of a similar magnitude. Subcultured cells bound the different PDGF isoforms in similar proportions as the primary cells. Contrary to the situation in primary cells, there was a direct correlation between the binding level and the DNA synthetic response. Moreover, the subcultured cells did not replicate in a serum-free medium. These observations support the idea that the phenotypic modulation of arterial smooth muscle cells in primary culture prepares the cells to activate autocrine growth mechanisms. When stimulated with an exogenous mitogen, they enter the cell cycle and are thereafter able to stimulate their own growth in an autocrine manner by production of PDGF-AA or a closely related molecule.

Medulloblastoma cell line secretes platelet-derived growth factor

Medulloblastoma is the most common malignant childhood brain tumor in which aggressive growth produces recurrence in approximately 50% of appropriately treated cases and metastases along the neuraxis in 30%. To date, no studies exist concerning the production of autocrine growth factors by this brain tumor type. Malignant brain tumors in adults often produce platelet-derived growth factor (PDGF). A medulloblastoma cell line, TE-671, has been used for many years in pediatric neuro-oncologic studies. We assayed this medulloblastoma cell line for the production of PDGF. PDGF is produced by medulloblastoma cells grown in monolayer tissue culture and stimulates PDGF-sensitive 3T3 fibroblasts to incorporate tritiated thymidine in a dose-dependent fashion. This biologic activity is blocked by PDGF antibodies in a dose-dependent relationship. We postulate that PDGF produced by medulloblastoma cells plays a role in the growth of this tumor by stimulating mitogenic activity.

A B-type PDGF receptor lacking most of the intracellular domain escapes degradation after ligand binding

The characteristics of the human B-type platelet-derived-growth-factor (PDGF) receptor expressed in Chinese hamster ovary (CHO) cells, were compared with those of a mutant receptor lacking all but 19 amino acids of the intracellular domain. The transfected wild-type receptor was synthesized as a 160-kDa precursor that was processed to 190 kDa. Each CHO cell expressed 30,000-100,000 receptors which bound PDGF-BB with a Kd of about 0.5 nM. Analysis of PDGF-AB binding yielded non-linear Scatchard plots; the major part of the binding sites had a Kd of 6 nM. PDGF-AA was not bound. The receptors expressed in CHO cells were down-regulated after binding of PDGF-BB, and mediated degradation of 125I-PDGF-BB with similar efficiency as PDGF-B-type receptors in human fibroblasts. The transfected receptor also transduced a mitogenic signal. The mutant receptor was synthesized as a 90-kDa precursor and was processed to 120 kDa with a slightly faster rate than the wild-type receptor. Cells expressing the mutant receptor generally had around 10(6) ligand-binding sites/cell, with a Kd for binding of PDGF-BB of 3 nM. The mutant receptor, which did not transduce a mitogenic response, mediated degradation of 125I-PDGF-BB, albeit less efficiently compared to the wild-type receptor. In contrast to the wild-type receptor, it was down-regulated only to a limited extent and not degraded in response to ligand binding. These findings indicate a role for the intracellular part of the receptor, not only in mitogenic signaling, but also in receptor internalization and intracellular routing.

Identification and purification of truncated insulin-like growth factor I from porcine uterus. Evidence for high biological potency

We report the completion of the purification of uterine-derived growth factors (UDGF) described previously by this laboratory [Ikeda, T., & Sirbasku, D.A. (1984) J. Biol. Chem. 259, 4049-4064]. During isolation, the mitogenic activity was monitored by using the human MCF-7 breast cancer cells in serum-free Ham's F12 and Dulbecco's modified Eagle's medium (1:1, v/v) containing 15 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (pH 7.2), 200 micrograms/mL bovine serum albumin, and 10 micrograms/mL human transferrin. This medium sustained growth for several days in response to a single addition of growth factor. The isolation of UDGF began with acetic acid extraction followed by sulfopropyl-Sephadex chromatography, Bio-Gel P-10 molecular sieve fractionation, and a series of reverse-phase high-pressure liquid chromatography separations. Purifications [[(1.0-8.5) X 10(6)]-fold] of three mitogens (5-20 ng each) were achieved. The mitogens were shown by protein microsequencing to be DES 1----3 to DES 1----6 forms of insulin-like growth factor I (truncated IGF-I). An Mr estimated by 125I labeling, urea-sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and autoradiography was consistent with a DES 1----3(4) N alpha truncation. Immunoadsorption and radioimmunoassay confirmed immunological properties equivalent to IGF-I. Radioreceptor assays showed truncated IGF-I was functionally equivalent to recombinant IGF-I. The ED50 values of DES 1----3 IGF-I and recombinant IGF-I for MCF-7 cell growth were 0.8-6.0 and 30-150 pg/mL, respectively. With Balb/c 3T3 mouse fibroblasts, the ED50 of DES 1----3 IGF-I was 100 times lower than that of IGF-I. We conclude that the major acid-stable low-Mr mitogenic activities isolated from uterus are very potent forms of truncated IGF-I capable of stimulating growth of epithelial and mesenchymal cells.

cDNA cloning and expression of a human platelet-derived growth factor (PDGF) receptor specific for B-chain-containing PDGF molecules

The structure of the human receptor for platelet-derived growth factor (PDGF) has been deduced through cDNA cloning. A 5.45-kilobase-pair cDNA clone predicts a 1,106-amino-acid polypeptide, including the cleavable signal sequence. The overall amino acid sequence similarity with the murine PDGF receptor is 85%. After transcription of the cDNA and translation in vitro, a PDGF receptor antiserum was used to immunoprecipitate a product of predicted size, which also could be phosphorylated in vitro. Stable introduction of the cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 190-kilodalton component, which was immunoprecipitated by the PDGF receptor antiserum; this most probably represents the mature PDGF receptor. Binding assays with different 125I-labeled dimeric forms of PDGF A and B chains showed that the PDGF receptor expressed in CHO cells bound PDGF-BB and, to a lesser extent, PDGF-AB, but not PDGF-AA.

High-yield purification of platelet-derived endothelial cell growth factor: structural characterization and establishment of a specific antiserum

Platelet-derived endothelial cell growth factor (PD-ECGF) is a 45-kDa protein that stimulates the growth of endothelial cells [Miyazono, K., et al. (1987) J. Biol. Chem. 262, 4098-4103]. Here, we describe a method to purify large quantities of PD-ECGF from human platelet lysate at a high yield (14% overall recovery). The purification method involves five steps, using high-performance liquid chromatography grade hydroxylapatite and hydrophobic chromatographies as the two final steps. The purified material contained two major components of apparent molecular weight values of 46,000 and 44,000. These components coeluted in a high-resolving reversed-phase chromatography and were found to give similar peptide maps after treatments with staphylococcal V8 protease, suggesting that the 44-kDa form is related to the 46-kDa molecule. Partial tryptic digestion of native PD-ECGF revealed that the molecule contains a trypsin-resistant domain of 37-39 kDa. A rabbit antiserum was produced against the purified material and was found to specifically recognize PD-ECGF in immunoblotting. When added to the cell culture medium, an immunoglobulin fraction of the antiserum neutralized the activity of purified PD-ECGF. Furthermore, it completely neutralized the endothelial cell mitogenic activity of platelet lysate, indicating that PD-ECGF is the only mitogen in platelet lysate for this cell type.

Expression of three recombinant homodimeric isoforms of PDGF in Saccharomyces cerevisiae: evidence for difference in receptor binding and functional activities

Three recombinant homodimeric isoforms of platelet-derived growth factor (PDGF) were produced and purified in milligram quantities by expression of PDGF A- and B-chains in yeast cells. Structural analysis of the purified short and long variants of PDGF-AA (PDGF-AAS and PDGF-AAL) and PDGF-BB showed that they had been properly processed and assembled into dimers. PDGF-AAS and PDGF-AAL were found to bind only to the PDGF A-type receptor on human fibroblasts, with affinities of 0.1 and 0.2 nM, respectively. PDGF-BB bound to cells with A- and B-type receptors and to cells with B-type receptor only with affinities of 0.6 nM in both cases. Each fibroblast appeared to express about 4-5 times more B-type receptors than A-type receptors. The maximal mitogenic response to PDGF-BB of human fibroblasts was almost 2-fold higher than that induced by either of the two PDGF-AA forms. The three isoforms of PDGF also stimulated growth in soft agar of human fibroblasts with PDGF-BB inducing a higher maximal response.

Induction of cyclic AMP synthesis by forskolin is followed by a reduction in the expression of c-myc messenger RNA and inhibition of 3H-thymidine incorporation in human fibroblasts

We have studied the effect of increased intracellular levels of cyclic AMP on the growth response to platelet-derived growth factor (PDGF) of human foreskin fibroblasts in culture. It was found that forskolin, a potent stimulator of adenylate cyclase activity, inhibits the stimulatory effect of PDGF on 3H-thymidine incorporation with a dose dependence similar to that observed with regard to cyclic AMP formation. A time-course study indicated that forskolin has no effect on ongoing DNA synthesis but affects events in the prereplicative phase. The cell-cycle block induced by forskolin was found to be reversible; after removal of the drug, DNA synthesis was initiated after a lag period, similar to that of the prereplicative phase of control cells. Forskolin had no effect on PDGF binding, receptor autophosphorylation, or c-fos mRNA expression. However, a reduction in PDGF-induced c-myc mRNA expression was observed in cultures given forskolin. Forskolin was also found to have a marked stimulatory effect on the expression of interferon-beta 2 mRNA expression. However, we were unable to demonstrate that the growth-inhibitory effect of forskolin is mediated by interferon-beta. In conclusion, an increase in cAMP levels leads to a reversible inhibition of PDGF-induced DNA synthesis in human fibroblasts, which may be related to an inhibition of c-myc mRNA expression.

Aberrant expression of receptors for platelet-derived growth factor in an anaplastic thyroid carcinoma cell line

Receptors for platelet-derived growth factor (PDGF) have previously only been found on cells of mesenchymal and glial origin. This study shows PDGF receptors on an anaplastic thyroid carcinoma cell line, C 643, that was found to express thyroglobulin mRNA, confirming its origin from thyroid epithelium. Northern blot analysis of poly(A)+ RNA hybridized with a human PDGF B-type receptor cDNA probe revealed a 5.4-kilobase transcript in the C 643 cells. The existence of receptor protein on the cell surface was shown by immunofluorescence microscopy with a PDGF receptor monoclonal antibody. Binding experiments with 125I-labeled dimeric forms of PDGF indicated that the cells contain B-, but not A-, type PDGF receptors. The addition of PDGF to C 643 membranes in the presence of [32P]ATP induced phosphorylation of the receptor. A polyclonal PDGF B-type receptor peptide antiserum was used to immunoprecipitate a receptor protein from metabolically labeled C 643 cells; the receptor was found to be 5-10 kDa larger than that in normal human fibroblasts. Removal of N-linked carbohydrates using endoglycosidase H resulted in deglycosylated receptor proteins of similar size in C 643 cells and fibroblasts, indicating differences in glycosylation patterns of the two receptor proteins. The aberrant expression of receptors might be crucial in tumor development by conferring a selective growth advantage to the cancer cells.

Induction of platelet-derived growth factor receptor expression in smooth muscle cells and fibroblasts upon tissue culturing

The expression of platelet-derived growth factor (PDGF) receptors in porcine uterus and human skin in situ, was compared with that of cultured primary cells isolated from the same tissues. PDGF receptor expression was examined by monoclonal antibodies specific for the B type PDGF receptor and by RNA/RNA in situ hybridization with a probe constructed from a cDNA clone encoding the B type PDGF receptor. In porcine uterus tissue both mRNA and the protein product for the PDGF receptor were detected in the endometrium; the myometrium, in contrast, contained much lower amounts. Moreover, freshly isolated myometrial cells were devoid of PDGF receptors. However, after 1 d in culture receptors appeared, and after 2 wk of culturing essentially all of the myometrial cells stained positively with the anti-PDGF receptor antibodies and contained PDGF receptor mRNA. Similarly, B type PDGF receptors were not detected in normal human skin, but fibroblast-like cells from explant cultures of human skin possessed PDGF receptors. When determined by immunoblotting, porcine uterus myometrial membranes contained approximately 20% of the PDGF receptor antigen compared with the amount found in endometrial membranes. In addition, PDGF stimulated the phosphorylation of a 175-kD component, most likely representing autophosphorylation of the B type PDGF receptor in endometrial membranes, whereas only a marginal phosphorylation was seen in myometrial membranes. Taken together, these results demonstrate that PDGF receptor expression varies in normal tissues and that fibroblasts and smooth muscle cells do not uniformly express the receptor in situ. Furthermore, fibroblasts and smooth muscle cells that are released from tissues are induced to express PDGF receptors in response to cell culturing. The data suggest that, in addition to the availability of the ligand, PDGF-mediated cell growth in vivo is dependent on factors regulating expression of the receptor.

Expression of recombinant platelet-derived growth factor A- and B-chain homodimers in rat-1 cells and human fibroblasts reveals differences in protein processing and autocrine effects

The autocrine effects of platelet-derived growth factor (PDGF) A- and B-chain homodimers (PDGF-AA and PDGF-BB) on rat-1 cells and human fibroblasts have been investigated by using human PDGF A- and B-chain cDNA clones expressed in a retroviral vector. Infection with replication-defective virus carrying the B-chain cDNA resulted in a phenotypical transformation resembling that induced by simian sarcoma virus. The resulting cells were focus forming in monolayer cultures, grew to high saturation densities, and formed large colonies in soft agar. The PDGF A-chain transfectants showed no transformed morphology and lacked focus-forming activity but grew to high saturation density in monolayer culture and formed small colonies in soft agar. A similar but weaker effect was obtained with an A-chain cDNA variant containing a 69-base-pair insertion in the 3' end of the protein-coding domain. A- and B-chain transfectants released PDGF receptor-competing activity into the medium, but only the medium conditioned by the B-chain transfectants possessed potent mitogenic activity on human fibroblasts. Both types of transfectants had downregulated levels of PDGF receptors; however, the B-chain transfectants were downregulated to significantly lower levels. Metabolic labeling and immunoprecipitations with PDGF antiserum showed that the PDGF B-chain protein was processed to a 24-kilodalton cell-associated and a 30-kilodalton secreted dimeric protein. The A-chain protein was rapidly secreted as a 31-kilodalton dimeric protein. The present study shows a marked difference in the autocrine effects of PDGF-AA and -BB expressed under the control of a retroviral promoter and suggests that different biological properties may be assigned to these two PDGF isoforms.

A human glioma cell line secretes three structurally and functionally different dimeric forms of platelet-derived growth factor

A human malignant glioma cell line, U-343 MGa Cl2:6, has previously been shown to secrete platelet-derived-growth-factor(PDGF)-like activity [Nistér, M., Heldin, C.-H., Wasteson, A. and Westermark, B. (1984) Proc. Natl Acad. Sci. USA 81, 926-930]. We report here that this activity consists of three different molecules separable by reversed-phase chromatography and immobilized-metal-ion affinity chromatography. HPLC reversed-phase chromatography resolved two peaks of activity, which were denoted glioma-derived growth factor-I (GDGF-I) and GDGF-II. GDGF-I was purified to greater than 90% purity; in SDS gel electrophoresis, it appeared as a 31-kDa component which by reduction was converted to 17 kDa. Immunoprecipitation of radiolabeled, reduced and alkylated GDGF-I with antisera made against peptides from the A and B chains of PDGF, gave a specific signal only with antiserum against the A chain. Furthermore, when reduced and alkylated GDGF-I was analyzed by reversed-phase HPLC, it eluted at the position of PDGF A chains. We conclude that GDGF-I is a homodimer of a polypeptide similar to the A chain of PGDF. GDGF-II was found to have higher mitogenic activity than GDGF-I. Analysis by immunoprecipitation with PDGF-chain-specific antisera revealed that GDGF-II contained a polypeptide similar to the B chain of PDGF. Immobilized-metal-ion affinity chromatography revealed that 95% of the mitogenic activity of GDGF-II consisted of a heterodimer of one A and one B chain, whereas 5% consisted of a B-chain homodimer. Thus, U-343 MGa Cl 2:6 cells secrete all three possible dimeric forms of PDGF.

cDNA cloning and expression of a human platelet-derived growth factor (PDGF) receptor specific for B-chain-containing PDGF molecules

The structure of the human receptor for platelet-derived growth factor (PDGF) has been deduced through cDNA cloning. A 5.45-kilobase-pair cDNA clone predicts a 1,106-amino-acid polypeptide, including the cleavable signal sequence. The overall amino acid sequence similarity with the murine PDGF receptor is 85%. After transcription of the cDNA and translation in vitro, a PDGF receptor antiserum was used to immunoprecipitate a product of predicted size, which also could be phosphorylated in vitro. Stable introduction of the cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 190-kilodalton component, which was immunoprecipitated by the PDGF receptor antiserum; this most probably represents the mature PDGF receptor. Binding assays with different 125I-labeled dimeric forms of PDGF A and B chains showed that the PDGF receptor expressed in CHO cells bound PDGF-BB and, to a lesser extent, PDGF-AB, but not PDGF-AA.

Effect on platelet-derived growth factor-induced mitogenesis of double-stranded RNA: evidence for an autocrine growth inhibition mediated by interferon-beta

Stimulation of normal human foreskin fibroblasts with platelet-derived growth factor (PDGF) was inhibited by the addition of the synthetic double-stranded RNA polyinosinic-polycytidylic acid (poly-I:C) as measured by incorporation of 3H-thymidine (3H-TdR). Single-stranded polycytidylic or polyinosinic acid had no effect. Double-stranded RNA is an inducer of interferon-beta (IFN-beta) in fibroblasts. On the mRNA level, an expression of IFN-beta 2 but not of IFN-beta 1 was seen after addition of PDGF and/or poly-I:C. The inhibition of PDGF-induced mitogenesis was completely blocked by an antiserum to IFN-beta. Poly-I:C did not interfere with PDGF binding to its receptor, nor did it block protein synthesis, indicating that the inhibition is not due to a nonspecific toxic effect of the double-stranded RNA but rather is mediated by IFN-beta. The present study implies that the IFN-beta system in fibroblasts is a very potent autocrine inhibitory pathway.

Diverse effects of fibronectin and laminin on phenotypic properties of cultured arterial smooth muscle cells

Plasma fibronectin promotes modulation of rat arterial smooth muscle cells from a contractile to a synthetic phenotype during the first few days in primary culture. This process includes cell adhesion and spreading, loss of myofilaments, and formation of a widespread rough endoplasmic reticulum and a prominent Golgi complex. The structural reorganization is accompanied by activation of overall RNA and protein synthesis. Moreover, the cells gain the ability to replicate their DNA and divide in response to platelet-derived growth factor. Here, it is demonstrated that the power of fibronectin to bring about this change in the differentiated properties of the smooth muscle cells resides in a 105-kD cell-binding fragment, whereas a 70-kD collagen-binding fragment and a 31-kD heparin-binding fragment are inactive in this respect. Laminin, another adhesive glycoprotein and a component of the basement membrane that normally surrounds arterial smooth muscle, was contrarily found to maintain the cells in a contractile phenotype. However, with increasing time more and more cells went through the modulation into a synthetic phenotype. This "catch-up" was counteracted by a peptide that contained the cell-attachment sequence of fibronectin (Arg-Gly-Asp-Ser). Hence, it is possible that the delayed modulation on laminin was due to production of fibronectin by the cells themselves. In support of this notion, fibronectin isolated from smooth muscle cultures was found to be as effective as plasma fibronectin in stimulating the phenotypic modulation. Moreover, using a combination of chemical, immunochemical, and immunocytochemical methods, it was demonstrated that the cells secreted fibronectin as well as laminin at an increasing rate during the first 4 d in primary culture and, notably, cells cultured on laminin produced more fibronectin than cells cultured on fibronectin. Newly synthesized fibronectin was incorporated into a network of pericellular and intercellular fibrils, whereas laminin formed a more diffuse layer covering the cells in a basement membrane-like manner. Taken together, the findings suggest diverse roles for fibronectin and laminin in the control of the differentiated properties of arterial smooth muscle cells. They further indicate that the ability of arterial smooth muscle cells to produce fibronectin and laminin early in primary culture is not directly related to the phenotypic state as determined morphologically and by measurement of overall rates of RNA and protein synthesis. This may be due to the cells being able to sense the macromolecular composition of the pericellular matrix and to modify their secretory activity accordingly.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of recombinant platelet-derived growth factor A- and B-chain homodimers in rat-1 cells and human fibroblasts reveals differences in protein processing and autocrine effects

The autocrine effects of platelet-derived growth factor (PDGF) A- and B-chain homodimers (PDGF-AA and PDGF-BB) on rat-1 cells and human fibroblasts have been investigated by using human PDGF A- and B-chain cDNA clones expressed in a retroviral vector. Infection with replication-defective virus carrying the B-chain cDNA resulted in a phenotypical transformation resembling that induced by simian sarcoma virus. The resulting cells were focus forming in monolayer cultures, grew to high saturation densities, and formed large colonies in soft agar. The PDGF A-chain transfectants showed no transformed morphology and lacked focus-forming activity but grew to high saturation density in monolayer culture and formed small colonies in soft agar. A similar but weaker effect was obtained with an A-chain cDNA variant containing a 69-base-pair insertion in the 3' end of the protein-coding domain. A- and B-chain transfectants released PDGF receptor-competing activity into the medium, but only the medium conditioned by the B-chain transfectants possessed potent mitogenic activity on human fibroblasts. Both types of transfectants had downregulated levels of PDGF receptors; however, the B-chain transfectants were downregulated to significantly lower levels. Metabolic labeling and immunoprecipitations with PDGF antiserum showed that the PDGF B-chain protein was processed to a 24-kilodalton cell-associated and a 30-kilodalton secreted dimeric protein. The A-chain protein was rapidly secreted as a 31-kilodalton dimeric protein. The present study shows a marked difference in the autocrine effects of PDGF-AA and -BB expressed under the control of a retroviral promoter and suggests that different biological properties may be assigned to these two PDGF isoforms.

Binding of different dimeric forms of PDGF to human fibroblasts: evidence for two separate receptor types

The binding of the three dimeric forms of platelet-derived growth factor (PDGF), PDGF-AA, PDGF-AB and PDGF-BB, to human fibroblasts was studied. Cross-competition experiments revealed the existence of two different PDGF receptor classes: the type A PDGF receptor bound all three dimeric forms of PDGF, whereas the type B PDGF receptor bound PDGF-BB with high affinity and PDGF-AB with lower affinity, but not PDGF-AA. The sizes of the two receptors were estimated with affinity labeling techniques; the A type receptor appeared as a major component of 125 kd and a minor of 160 kd, and the B type receptor as two components of 160 and 175 kd. A previously established PDGF receptor monoclonal antibody, PDGFR-B2, was shown to react with the B type receptor only. The different abilities of the three dimeric forms of PDGF to stimulate incorporation of [3H]TdR into human fibroblasts indicated that the major mitogenic effect of PDGF is mediated via the B type receptor.

A glioma-derived PDGF A chain homodimer has different functional activities from a PDGF AB heterodimer purified from human platelets

Glioma-derived growth factor I (GDGF-I) is structurally similar to a platelet-derived growth factor (PDGF) A chain homodimer, whereas PDGF purified from human platelets is a heterodimer of one A and one B chain. Binding experiments revealed that GDGF-I and PDGF bound to a common receptor on human fibroblasts, but also suggested the presence of a second receptor type recognizing only PDGF. In contrast to PDGF, GDGF-I had only a limited mitogenic activity, a low ability to stimulate receptor autophosphorylation and actin reorganization, and no chemotactic activity. GDGF-I did, however, cause transmodulation of EGF receptors, suggesting that it, like PDGF, activates protein kinase C in fibroblasts. These data indicate that different PDGF-like growth factors have different functional activities, which are possibly mediated via different receptors.

Arterial smooth muscle cells express platelet-derived growth factor (PDGF) A chain mRNA, secrete a PDGF-like mitogen, and bind exogenous PDGF in a phenotype- and growth state-dependent manner

Adult rat arterial smooth muscle cells are shown to express platelet-derived growth factor (PDGF) A chain mRNA, to secrete a PDGF-like mitogen, and to bind exogenous PDGF in a phenotype- and growth state-dependent manner. In the intact aortic media, where the cells are in a contractile phenotype, only minute amounts of PDGF A chain and no B chain (c-sis) RNA were detected. After cultivation and modulation of the cells into a synthetic phenotype, the A chain gene was distinctly expressed, whereas the B chain gene remained unexpressed. Cells kept in serum-free medium on a substrate of plasma fibronectin showed high levels of A chain RNA and high PDGF receptor activity, but did not secrete detectable amounts of PDGF-like mitogen. After exposure to PDGF, which is itself sufficient to initiate DNA synthesis and mitosis in these cells, a PDGF-like mitogen was released into the extracellular medium. Concomitantly, the amount of A chain transcripts per cell and the ability of the cells to bind radioactive PDGF decreased. Similarly, smooth muscle cells initially grown in the presence of serum released more PDGF-like mitogen, contained fewer A chain transcripts, and bound more radioactive PDGF in proliferating than in stationary cultures. The findings confirm the notion that adult rat arterial smooth muscle cells are able to promote their own growth in an autocrine or paracrine manner. Furthermore, they reveal some basic principles in the control of this process.

Density-dependent inhibition of cell growth by transforming growth factor-beta 1 in normal human fibroblasts

We report here that transforming growth factor-beta 1 (TGF-beta 1) inhibits platelet-derived growth factor (PDGF)-induced DNA synthesis in normal human fibroblasts in a cell density-dependent manner; no inhibition was seen in sparse cultures, approximately 50% inhibition in confluent cell cultures, and an almost total inhibition in dense cultures. The PDGF-inducible genes c-myc and c-fos were induced also by TGF-beta 1. Simultaneous addition of TGF-beta 1 and PDGF resulted in sustained, rather than transient, expression of c-fos mRNA; c-fos mRNA was detected as long as 24 hr after addition of PDGF and TFG-beta 1. TGF-beta 1 also induced mRNA for the A chain, but not the B chain, of PDGF. Conversely, PDGF induced TGF-beta 1 mRNA in sparse but not in dense cultures. These data indicate the existence of a complex interdependent regulation of PDGF and TGF-beta mRNA expression which is influenced by the cell density.

Biosynthesis and intracellular transport of the receptor for platelet-derived growth factor

The biosynthesis of the receptor for platelet-derived growth factor (PDGF) was examined in metabolically labeled human foreskin fibroblasts. The receptor was synthesized as a 145-kDa precursor, which, when incubated with endo-beta-N-acetylglucosaminidase H (endo H), underwent a 15-kDa decrease in molecular mass. This indicates that the size of the core protein is about 130 kDa and that the 145-kDa form represents a receptor precursor carrying high-mannose N-linked oligosaccharide groups. Within 15 min after synthesis, the receptor was converted to a 165-kDa form. This form was entirely resistant to endo H treatment and probably represents a receptor molecule that has undergone further posttranslational modification, including O-linked glycosylation. Subsequently, within 30 min, a molecule of 170 kDa--i.e., the size of the mature receptor--appeared. A slightly larger molecule, of 175 kDa, which could be immunoprecipitated from PDGF-stimulated 32P-labeled cells, probably represents a receptor further modified by autophosphorylation. The 170-kDa molecule had an isoelectric point of about 4.5. Addition of PDGF increased the turnover rate of the 170-kDa PDGF receptor.