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Transforming growth factor-beta (TGF-beta) (reviewed in refs 1-3) is a family of molecules that are made up as disulphide-bonded dimers of at least three different types of homologous polypeptides. The active molecules are cleaved from the C termini of precursors. TGF-beta 1, like other forms of TGF-beta, is synthesized and secreted in a latent high relative molecular mass form (L-TGF-beta 1) from which active TGF-beta 1 can be released by transient and probably unphysiological acidification. The latent complex from human platelets contains one dimeric TGF-beta 1 molecules, which is noncovalently associated with a disulphide-bonded complex of one dimeric remnant of the precursor and a single molecule of the so-called TGF-beta 1 binding protein (TGF-beta 1-BP). We report here that enzymatic removal in vitro of the carbohydrate structures in the remnant of the TGF-beta 1 precursor produces biologically active TGF-beta 1 from the latent complex, suggesting that carbohydrate structures are of importance in rendering TGF-beta 1 inactive in the complex in vivo.

Platelet-derived endothelial cell growth factor (PD-ECGF) is a 45-kDa endothelial cell mitogen which has angiogenic properties in vivo. We report here that human foreskin fibroblasts, a human squamous cell carcinoma cell line, and 2 out of the 3 human thyroid carcinoma cell lines investigated produce PD-ECGF, whereas 21 other cell lines examined do not. The positive cell lines contained a 1.8-kilobase PD-ECGF mRNA, and a 45-kDa protein could be demonstrated in lysates of the cell lines by immunoblotting and immunoprecipitation using a specific antiserum against PD-ECGF. Furthermore, the cell lysates contained mitogenic activity for endothelial cells that was neutralized by the PD-ECGF antiserum. PD-ECGF was found to be secreted only slowly from the producer cells, consistent with the previous finding that the primary translation product lacks a signal sequence. The restricted expression and intracellular sequestration of PD-ECGF imply a strictly controlled function in endothelial cell proliferation and angiogenesis. Aberrant production of PD-ECGF may play a role in tumor angiogenesis.

Human platelet-derived growth factor (PDGF) occurs as three isoforms which are made up of disulfide-bonded A and B chains. The isoforms bind with different affinities to two different but structurally related cell surface receptors. The A type receptor binds all three isoforms (PDGF-AA, PDGF-AB, PDGF-BB) with high affinity, whereas the B type receptor binds PDGF-BB with high affinity, PDGF-AB with lower affinity but does not appear to bind PDGF-AA. We have utilized the differential effects of the three isoforms on actin reorganization and membrane ruffling in human foreskin fibroblasts to probe the idea that ligand-induced receptor dimerization is associated with receptor activation. Actin reorganization was found to be induced only by PDGF-AB and PDGF-BB and is therefore likely to be mediated by the B type receptor. Simultaneous addition of PDGF-AA, or downregulation of the A type receptor blocked the effect of PDGF-AB but not that of PDGF-BB. This is compatible with a model by which PDGF-AB binds to and dimerizes one A and one B type receptor; PDGF-AB therefore requires A type receptors in order to be functionally active at physiological concentrations. In cells with down-regulated A type receptors, high concentrations of PDGF-AB inhibited the effect of PDGF-BB on actin reorganization. We believe that this is due to a monovalent binding of PDGF-AB to the B type receptors which prevents PDGF-BB from dimerizing the receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Platelet-derived growth factor (PDGF) has previously been shown to bind to a specific high affinity receptor on human foreskin fibroblasts. The present study was carried out to characterize some of the cellular events resulting from the interaction of the ligand with its receptor. Radiolabeled PDGF was rapidly internalized and degraded after binding to the cells. The degradation was complete and was inhibited by low concentrations of the lysosomotropic agents, chloroquine, ammonium chloride, or methylamine, suggesting that the degradation occurs in the lysosomes. The cellular binding capacity for PDGF decreased after exposure of the cells to PDGF at 37 degrees C. This down regulation of the PDGF receptor was optimal after a 60-min incubation at 37 degrees C and half-maximal at 0.5 nM concentration of PDGF. The binding capacity was restored when the PDGF-containing medium was changed to medium without PDGF; the binding capacity increased from 40 to 80% od the initial value after a 4-h incubation at 37 degrees C. The reappearance on the cell surface of PDGF-binding sites was dependent on protein synthesis and totally blocked by cycloheximide (20 micrograms/ml). Thus, either the receptor has to be resynthesized after internalization or, alternatively, any step in the recycling of "used" receptors is dependent on protein synthesis. Exposure of the cells to PDGF also caused a dose-dependent decrease in the binding capacity for epidermal growth factor which has a distinct receptor on these cells. In contrast, epidermal growth factor did not modulate the PDGF binding capacity, lending no support to the idea that the receptors for epidermal growth factor and PDGF are processed in a common pathway.

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.

Surface binding and uptake of platelet-derived growth factor (PDGF) in human fibroblasts cultivated in vitro were studied by quantitative electron microscopic autoradiography using 125I-labeled PDGF and by indirect immunofluorescence using PDGF antibodies. After 120 min at 12 degrees C PDGF was found preferentially in coated regions of the plasma membrane. Warming the cells to 37 degrees C initiated rapid ingestion of the factor via small vesicles, usually lacking a membrane coat. After 10 min PDGF started to appear in lysosomes, and it showed maximal concentration within these organelles after 30 min. There were also signs of passage of PDGF through the Golgi complex, but only after 60 min. Treatment of the cells with chloroquine, a weak base that inhibits intralysosomal degradation, prevented disappearance of tracer from the lysosomes. The observations indicate that PDGF was internalized via coated regions of the plasma membrane and carried to lysosomes for degradation. Subsequent appearance of tracer within the Golgi complex could reflect receptor-ligand complexes that escaped degradation and were recirculated back to the cell surface.

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.

An endothelial cell growth factor has been purified about 1,000,000-fold to homogeneity from human platelets by a seven-step procedure. The purified product has an apparent Mr on sodium dodecyl sulfate-polyacrylamide gels of 45,000. The mobility in sodium dodecyl sulfate gel electrophoresis was similar in the presence or absence of reducing agents, indicating that the factor consists of a single polypeptide chain. Maximal stimulation by the purified protein was achieved at a concentration of about 20 ng/ml (440 pM). Heparin did not potentiate the activity, nor did the factor bind to heparin immobilized on Sepharose. The purified factor was heat- and acid-labile; it was active on porcine and human endothelial cells, but not on human foreskin fibroblasts. Chromatofocusing revealed that the pI of the factor was 4.6. The structural and functional characteristics of the platelet-derived endothelial cell growth factor are distinct from previously characterized endothelial cell mitogens with affinities for heparin.

The effect of various growth factors on the synthesis of hyaluronan in human fibroblasts was investigated. When tested in medium containing 0.5% fetal calf serum, platelet-derived growth factor (PDGF)-BB was found to stimulate hyaluronan synthesis; the maximal response was equal to or higher than that obtained with 10% fetal calf serum. PDGF-AA gave only a limited effect, indicating that the stimulatory effect of PDGF on hyaluronan synthesis was mainly transduced via the B-type PDGF receptor. Epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)-beta 1 also stimulated hyaluronan synthesis; their effects were less than that of PDGF-BB, but combinations of factors produced potent stimulatory effects on hyaluronan synthesis. All factors stimulated hyaluronan synthesis in sparse as well as dense cultures. The effects of the factors on hyaluronan synthesis did not correlate with their mitogenic activities; PDGF-BB, EGF and bFGF are equipotent mitogens, but PDGF-BB had a much more potent effect on hyaluronan synthesis, and TGF-beta actually inhibits the growth of fibroblasts under the conditions of the assay.

A cationic protein that stimulates DNA synthesis in human cultured cells was isolated from human platelets by ion exchange chromatography, hydrophobic chromatography, gel chromatography, and gel electrophoresis in sodium dodecyl sulfate. The electrophoretic behavior of biologically active or radioiodinated and reduced growth factor indicated that the native protein (approximately 30,000 daltons) was composed of two different polypeptides (approximately 13,000-14,000 and 16,000-17,000 daltons, respectively) linked via reduction-susceptible bonds. The stimulatory activity on human glial cells of the purified product at a concentration of approximately 4 ng/ml (0.13 nM) was equal to that of 1% human serum.

The finding that PDGF production is common in normal as well as transformed cells indicate that PDGF has a function in autocrine and paracrine stimulation of cells in several physiological and pathological conditions. The expression of mRNA for the two chains of PDGF are independently regulated. The fact that the different dimeric forms of PDGF have different functional effects, indicate that the cellular response to PDGF may be more complex than previously thought, and may involve binding to more than one receptor. The cellular effects ascribed to PDGF, growth stimulation, ruffling and chemotaxis, seems to be mainly associated with B chain containing dimers. The function of PDGF-AA remains to be elucidated.

As human fibroblasts in culture senesce their response to platelet-derived growth factor (PDGF) becomes attenuated. To clarify at which level such cells are blocked in the pre-replicative part of the cell cycle, we have analysed PDGF-induced pre-replicative events in senescent (phase III) cultures. We found that phase III cells retain a normal number of PDGF receptors and that these are functional with regard to PDGF-induced receptor autophosphorylation. Phase III cells also respond to PDGF by rapid actin reorganization and increased levels of c-fos and c-myc mRNA, similar to growth-arrested phase II fibroblasts. However, the expression of the nuclear antigen K-67, which in phase II cell is induced in S-phase and continues to be expressed throughout the cell cycle, is not induced in phase III cells in response to PDGF. We conclude that phase III human fibroblasts, although blocked with regard to proliferation, still retain a functional growth factor receptor system, and display early responses when exposed to growth factors, such as changes in the cytoskeleton and the expression of proto-oncogenes.

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.

Platelet-derived growth factor was purified from fresh platelets by a large-scale procedure not involving the use of SDS (sodium dodecyl sulphate). The product, 0.5 mg of platelet-derived growth factor, obtained from about 3 x 10(13) platelets migrated as a single component in analytical gel electrophoresis in the presence of SDS and showed no inhomogeneity on sedimentation-equilibrium analysis in the ultracentrifuge. It had a high specific activity, 2 ng of platelet-derived growth factor/ml (70pM) being equivalent to 1% (v/v) human serum in an assay for multiplication-stimulating activity. Amino acid analysis revealed that platelet-derived growth factor contains all the common amino acids, except tryptophan, but no hexosamine. The molecular weight of platelet-derived growth factor, as determined by sedimentation-equilibrium analysis, was about 33 000. A similar value was obtained by gel electrophoresis in SDS under non-reducing conditions. In the presence of reducing agents the factor molecule was converted into two distinct components of lower molecular weight (17 000 and 14 000 respectively), as demonstrated by protein staining. The molecular model implicated by these findings is that platelet-derived growth factor consists of two different polypeptides chains, linked by disulphide bridges.

Epidermal growth factor (EGF), which can be purified from the mouse submaxillary gland or from pregnant human urine, is a potent multiplication-stimulating factor for several types of cultured cells, including human fibroblasts and glial cells. The molecule binds with high affinity and saturation kinetics to a cell-surface receptor, is subsequently internalised and finally degraded. The binding event is accompanied by a reduction in the number of EGF receptors. This phenomenon--'receptor down-regulation'--has been demonstrated with several hormones and may be a general principle for the modulation of binding groups on the outer cell surface. Further, it has been proposed that receptor loss acts to regulate the cellular response to the binding ligand. The present study provides direct experimental support for this hypothesis. It demonstrates that down-regulation of EGF receptors on glial cells causes desensitisation of the mitogenic response of these cells to subsequent stimulation with EGF.

The expression of growth factors for mesenchymal cells was investigated in 10 different human mammary carcinoma cell lines. Expression of mRNA for platelet-derived growth factor (PDGF) A-chain, PDGF B-chain, transforming growth factor-alpha, and insulin-like growth factor II were found in eight, nine, five, and two of the cell lines, respectively. The production of PDGF-like growth factors by the mammary carcinoma cell lines was investigated in detail. PDGF receptor competing activity and mitogenic activity, which could be neutralized by PDGF antibodies, were found in the conditioned medium of almost all the cell lines. A PDGF-like growth factor was partially purified from the conditioned medium of one of the cell lines, MCF7. A Mr 31,000 component, which was reduced to Mr 17,000, was furthermore precipitated by a PDGF antiserum from the conditioned medium of metabolically labeled MCF7 cells. These results indicate that growth factors for mesenchymal cells are frequently expressed in human mammary carcinoma cell lines. This is of interest in relation to the fact that mammary carcinoma tumors often contain a high proportion of proliferating connective tissue cells.

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 cellular receptor for platelet-derived growth factor (PDGF) was demonstrated by incubation of 125I-labeled PDGF with human foreskin fibroblast cultures followed by liberation of cell-bound radioactivity with Triton X-100. The cellular binding of labeled PDGF in the presence of increasing amounts of unlabeled PDGF showed saturation; Scatchard analysis of binding data indicated a single class of receptors having kd = 1 X 10(-9) M. The number of PDGF binding sites was approximately 3 X 10(5)/cell. Labeled PDGF binding reached an apparent equilibrium after 3 hr at 4 degrees C. At 37 degrees C, it passed a maximum after 30 min and then decreased with time due to degradation of the tracer. A large excess of unlabeled PDGF reduced labeled PDGF binding by more than 90% whereas similar doses of epidermal growth factor, fibroblast growth factor, or insulin had no effect. It was concluded that PDGF did not share receptors with these factors. PDGF receptors were found on skin fibroblasts, normal and malignant glial cells, smooth muscle cells, and 3T3 cells but not on epithelial-derived cells, neuroblastoma cells, endothelial cells, or peripheral lymphocytes.l As only the receptor-positive cells--i.e., the connective tissue- and glia-derived cells--are responsive to stimulation with PDGF, these findings imply a functional significance of the PDGF receptor.

Two cell lines (U-343 MG and U-343 MGa) with different phenotypic characteristics were established from the same human glioblastoma multiforme biopsy. Previous studies have shown that a clonal derivative (Cl 2) of the U-343 MGa line produces a PDGF-like growth factor. In the present investigation glioma PDGF production and 125I-PDGF binding were found to be differently expressed in U-343 MG, U-343 MGa, and U-343 MGa Cl 2 cultures, providing evidence for a clonal variation in these properties. In order to investigate this point further, several clones were derived from low (23 clones) and high (30 clones) passage U-343 MGa cultures, as well as from U-343 MGa Cl 2 cells (30 clones). The clones could be divided into 4 groups according to morphology and growth pattern. A determination of the amount of PDGF receptor competing activity in serum-free conditioned media gave evidence for a clonal variation in the production of glioma PDGF, corresponding to 0-87 ng of authentic PDGF per ml. There was also a considerable range in 125I-PDGF binding (0-44 fmol of tracer bound per 10(6) cells). Scatchard plots performed on two clones confirmed the presence of saturable, high affinity PDGF receptors. High passage cultures were found to give rise to a higher number of high producing clones than did low passage cultures. There appeared to be a negative correlation between production of glioma PDGF and binding of 125I-PDGF, probably due to the receptor blocking activity of the endogenous growth factor. However, the presence of clones, apparently devoid of both glioma PDGF production and 125I-PDGF binding, suggests a true clonal variation in these two parameters. The growth rate in serum-free medium was found to correlate fairly well to the extent of glioma PDGF production. Production of glioma PDGF was found to have a morphological correlate and be most prominent among clones of "immature" looking, tightly growing cells. Clones that had large star-shaped cells with some resemblance to normal glia-like cells in culture were found to have a low production and a high 125I-PDGF binding capacity.

The expression of both a PDGF-like growth factor and functional PDGF receptors within a clonal human osteosarcoma cell line (U-2 OS Cl 6) is demonstrated. These molecules are able to interact and induce tyrosine-specific phosphorylation and early actin reorganization in the osteosarcoma cells, effects similar to those that PDGF induces in normal responsive cells. Furthermore, immunoprecipitation with an antiserum against phosphotyrosine revealed that a 115 kd protein was constitutively phosphorylated in U-2 OS Cl 6 cells. A phosphorylated protein of similar apparent molecular weight has been found in human fibroblasts, but only after stimulation with PDGF. These data indicate that the PDGF-receptor-dependent pathway is constitutively activated in this cell line. Extracellularly added PDGF antibodies did not, however, affect the transformed properties or growth rate of U-2 OS Cl 6 cells in vitro. This indicates that autocrine PDGF receptor activation may be insignificant for maintaining the transformed state of this tumor cell line, or that autocrine receptor activation occurs in a compartment where it is inaccessible to extracellularly added antibodies.

Chemical modification of a histidine and lysine residue inactivates pea seed nucleoside diphosphate kinase (NDP kinase). Thus there seems to be a reactive lysine residue, at the active site of pea seed NDP kinase, in addition to the histidine residue phosphorylated by the substrate ATP as a consequence of the enzyme reaction. The presence of a reactive lysine at the active site of the enzyme could explain why a small amount of N-epsilon-phospholysine, as well as 1-phosphohistidine and 3-phosphohistidine, is formed on alkaline hydrolysis of the enzyme.

Normal cell replication is regulated by growth factors such as epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) that act through binding to specific surface receptors on target cells. Oncogenes may exert their transforming activity by encoding proteins that mimic the function of the normal regulatory factors along the mitogenic pathway, growth factors, their receptors or elements along the postreceptor signaling system. This may be exemplified by the human malignant glioma, in which the sis gene (encoding a growth factor homologous to PDGF) and the erb B gene (encoding a membrane protein homologous to the EGF receptor) have been implicated.

Previous studies have indicated that the oncogene v-sis of simian sarcoma virus (SSV) encodes a growth factor that is structurally and functionally similar to platelet-derived growth factor (PDGF). In the present investigation we have analysed the phenotypic characteristics of human foreskin fibroblasts transformed by SSV. It was found that the PDGF receptors were extensively down-regulated. This finding is consistent with a high, local, extracellular concentration of a PDGF-like factor, synthesized by the transformed cell. The receptors were up-regulated by suramin, a drug that is known to dissociate PDGF and the v-sis product from the PDGF receptors. A cell-associated v-sis product of mol. wt 24,000 was identified by immunoprecipitation with PDGF antibodies; release of this component was induced by a high concentration of exogenous PDGF, indicating that a fraction of the product is associated with the PDGF receptors. SSV was not found to be an immortalizing virus; when serially passaged, SSV-transformed cells had essentially the same life-span as their non-transformed counterparts. Moreover, SSV did not induce growth in soft agar beyond the level afforded by exogenously added PDGF. Thus, the present study favors the notion that SSV transformation is mediated by a growth factor that mimics PDGF but has no further cellular effects.

It has been shown previously that cultures of rat optic nerve contain three types of macroglial cells--oligodendrocytes and two types of astrocytes. Type-1 astrocytes develop from their own precursor cells beginning before birth, while oligodendrocytes and type-2 astrocytes develop postnatally from a common bipotential precursor called the O-2A progenitor cell. Proliferating O-2A progenitor cells give rise to postmitotic oligodendrocytes beginning around birth, and to type-2 astrocytes beginning in the second postnatal week. Studies in vitro have suggested that platelet-derived growth factor (PDGF), secreted by type-1 astrocytes, plays an important part in timing oligodendrocyte development: PDGF seems to keep O-2A progenitor cells proliferating until an intrinsic clock in the progenitor cells initiates the process leading to oligodendrocyte differentiation. The clock apparently determines when a progenitor cell becomes unresponsive to PDGF, at which point the cell stops dividing and, as a consequence, automatically differentiates into an oligodendrocyte. Here we have used radiolabelled PDGF to show that O-2A progenitor cells have PDGF receptors, suggesting that these cells respond directly to PDGF. The receptors resemble the type A PDGF receptor previously described on human fibroblasts and are initially retained when progenitor cells stop dividing and develop in vitro into oligodendrocytes. The latter finding indicates that receptor loss is not the reason that progenitor cells initially become mitotically unresponsive to PDGF.

Insulins from the hystricomorphs (guinea pig, porcupine, coypu, and casiragua) at high concentration stimulate DNA synthesis in human fibroblasts to a greater level than other mammalian insulins or insulin-like growth factors (IGFs). 125I-Labeled guinea pig insulin binds to a specific receptor and this binding is competed for by hystricomorph insulins but not by porcine insulin or IGFs. Fetal bovine serum also inhibits the binding of 125I-labeled guinea pig insulin and is more potent than fetal bovine plasma, in concordance with their relative potencies for growth stimulation in human fibroblasts. Of several other known growth factors tested, only platelet-derived growth factor (PDGF) inhibits binding of 125I-labeled guinea pig insulin. Four preparations of PDGF that vary in purity and potency for the stimulation of DNA synthesis in human fibroblasts over a 1,000-fold range compete with binding of 125I-labeled guinea pig insulin in proportion to their biological potencies. The purest preparation of PDGF is able to inhibit binding of 125I-labeled guinea pig insulin by 50% at 15 ng/ml (0.25 nM). Biologically, guinea pig insulin, like PDGF, exhibits a synergistic effect with plasma in initiating DNA synthesis in human fibroblasts; this effect is not observed with other mammalian insulins or IGFs. Thus, hystricomorph insulins appear to be mediating their growth-promoting effect through a different receptor and mechanism than other mammalian insulins or IGFs. Further, hystricomorph insulins may be sharing the mechanism of action for their growth effects with PDGF, perhaps suggesting some relationship between these peptides from very different sources.

Platelet-derived growth factor (PDGF) is a mitogen for connective tissue cells and occurs as disulphide-bonded homodimers or heterodimers of related polypeptide chains. Recent data indicate that the isoforms have different functional activities due to the fact that they bind with different affinities to two distinct receptor types. The frequent expression of PDGF and PDGF receptors in normal as well as transformed cells, suggests roles for PDGF in autocrine and paracrine stimulation of cell growth in vivo.

Two monoclonal antibodies against the receptor for platelet-derived growth factor (PDGF) were obtained by immunizing mice with pure PDGF receptor preparations derived from porcine uterus. The antibodies, denoted PDGFR-B1 and PDGFR-B2, both bound to the external domain of the receptor, as demonstrated by indirect immunofluorescence and binding of 125I-labeled antibodies to intact human fibroblasts. Both antibodies precipitated pure 175-kDa 32P-labeled autophosphorylated porcine PDGF receptor as well as a Mr 175,000 glycoprotein from metabolically labeled cells. The monoclonal antibodies did not inhibit binding of 125I-PDGF to human fibroblasts and did not stimulate these cells to undergo mitosis. Both antibodies induced clustering and down-regulation of their antigen. However, this resulted in only a partial loss of cell surface binding sites for PDGF itself, consistent with the conclusion that the monoclonals recognized only one of two or several receptors for PDGF. Clustering and down-regulation were not seen when the cells were incubated with monovalent Fab' fragments of the PDGFR-B2 antibody. The antibodies also stimulated autophosphorylation of pure PDGF receptor, and PDGFR-B2 was shown to stimulate phosphorylation of phosphofructokinase, an exogenous substrate for the PDGF receptor kinase. High concentrations of PDGFR-B2 antibody, or Fab' fragments thereof, failed to enhance the PDGF receptor kinase activity, compatible with the possibility that dimerization was of importance in the antibody-stimulated kinase activity of purified PDGF receptors.

Adult rat arterial smooth muscle cells (SMC) in primary culture modulate from contractile to synthetic phenotype. This process includes partial loss of myofilaments and formation of an extensive rough endoplasmic reticulum and a large Golgi complex. It gives the cells the ability to initiate DNA synthesis and actively proliferate when stimulated with serum or isolated growth factors. After a few divisions, growth becomes partly independent of exogenous mitogens and does not cease until multiple cell layers have been formed. Here, it is demonstrated that serum-free conditioned medium from primary cultures of adult rat arterial SMC contains a factor that initiates DNA synthesis in growth-arrested secondary cultures of SMC. The mitogenic activity was neutralized by antibodies to platelet-derived growth factor (PDGF), and no mitogenic activity occurred in conditioned medium from cultures pretreated with actinomycin D, excluding release into the medium of PDGF adsorbed to the plastic vessels during the initial culture in serum-containing medium. Exposure of human fibroblasts to samples of the conditioned medium at 4 degrees C inhibited subsequent binding of 125I-labeled PDGF. It was further shown that the SMC of the primary cultures were able to initiate DNA synthesis in a chemically defined medium lacking PDGF and other growth factors. During the early, most active, and partly autonomous growth phase, the SMC had a low binding capacity for 125I-labeled PDGF and responded but little to stimulation with exogenous PDGF. Later on, with increasing cell density and decreasing growth rate, the ability to bind and respond to exogenous PDGF increased. Taken together, the observations suggest that modulation of SMC from contractile to synthetic phenotype is accompanied by production of a PDGF-like protein and autocrine or possibly by mitogen-independent initiation of DNA synthesis. Functionally, this may be important during wound healing and in the development of atherosclerotic lesions.

The mRNA levels of two proto-oncogenes, c-fos and c-myc, were determined in human foreskin fibroblasts exposed to epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) in a serum-free, defined medium (MCDB 104). Untreated, quiescent cells were found to have low or undetectable levels of c-fos and c-myc mRNA. Within 10 min after the addition of EGF or PDGF the c-fos mRNA level increased, reached a peak at 30 min, and then declined to the control level after 60 min. The level of c-myc mRNA increased somewhat later and peaked after 8 h in cultures treated with either of the growth factors. The c-myc mRNA level remained elevated throughout the 24 h of investigation. The concentrations of EGF and PDGF required for a maximal effect on c-fos or c-myc expression were found to be similar to those that give maximal effect on cell proliferation. Both c-fos and c-myc mRNA expression were super-induced by the addition of cycloheximide. The addition of neutralizing PDGF antibodies to cultures that had received PDGF 4 h earlier inhibited the subsequent increase in the c-myc mRNA level, indicating that the effect of PDGF on c-myc expression is not caused by a "hit and run," mechanism. Density-inhibited cells responded to EGF and PDGF by an increase in c-fos and c-myc mRNA levels in the absence of any mitogenic response. The present results conform to the view that the c-fos and c-myc proto-oncogenes may be important (or necessary) but not sufficient for the initiation of DNA synthesis. Moreover, the finding that both EGF and PDGF increase c-fos and c-myc expression supports our previous suggestion that these two growth factors may in part act via a common intracellular pathway in the prereplicative phase of human fibroblasts.

The primary structure of the human A-type receptor for platelet-derived growth factor (PDGF) has been determined. A 6.5-kilobase (kb) transcript was identified through low-stringency hybridization with a probe derived from the B-type PDGF receptor cDNA. The sequence of a cDNA clone corresponding to the 6.5-kb transcript contains an open reading frame that predicts a 1089-amino acid growth factor receptor-like molecule, which displays 44% overall amino acid similarity with the PDGF B-type receptor. The two receptors have a similar domain organization, with five immunoglobulin-like domains extracellularly and an intracellular split protein tyrosine kinase domain. Transfection of the new cDNA into COS cells led to the expression of a protein specifically recognized by an antiserum previously shown to react with the PDGF A-type receptor. The expressed protein was shown to display high-affinity binding of all three 125I-labeled dimeric forms of PDGF A and B chains in a manner that is characteristic for the PDGF A-type receptor.

The tissue distribution of the receptor for platelet-derived growth factor (PDGF) was investigated by immunohistochemistry on frozen sections from normal and inflamed synovial tissue using monoclonal antibodies to the receptor. Non-inflamed synovial tissue showed no staining, indicating that PDGF receptor expression is low or absent in normal tissue. In contrast, tissue from synovitis with prominent neovascularization showed a strong staining in the tunica media of the proliferating blood vessels as well as on connective tissue cells in the stroma. Tissue from synovitis with prominent proliferation of synovial lining showed intense staining for PDGF receptors in fibroblast-like cells of the lining and a less intense staining on vascular and connective tissue cells deeper in the stroma. Staining for PDGF receptors was also intense in the pannus tissue close to infiltrated bone and cartilage. In all these forms of synovitis, PDGF receptor staining was associated with increased HLA-DR staining and infiltration of macrophages and T lymphocytes. The finding that PDGF receptor expression is induced in conjunction with the chronic synovial inflammation associated with rheumatoid arthritis and some other forms of arthritides suggests that PDGF may play a role in the stimulation of mesenchymal cell proliferation that often accompanies chronic inflammatory disease.

Previous studies have shown that platelet-derived growth factor (PDGF) stimulates the phosphorylation of two components in membranes prepared from human fibroblasts in the presence of Ca2+. One of these represents the 185-kDa PDGF receptor, which undergoes autophosphorylation, and the other has an Mr of 130 000. We show in this communication that the 130-kDa component is derived from the 185-kDa receptor via proteolysis by an endogenous Ca2+-dependent protease, which is dependent on a reduced -SH group for its activity. The 130-kDa fragment contains several of the characteristics of the receptor, such as the PDGF-binding site and the major autophosphorylation sites. Furthermore, the cleaved receptor retains tyrosine kinase activity.

U-1810, a human large-cell lung cancer line, was found to express a PDGF-like growth factor. 35S-cysteine labelling and immunoprecipitation revealed the synthesis and secretion of a 31-kDa PDGF-like protein. Serum-free conditioned medium contained PDGF-receptor-competing and mitogenic activity when tested on human fibroblasts. Whereas the receptor-competing activity was fully neutralized by anti-PDGF antibodies, the mitogenic activity was only partially affected. We therefore probed U-1810 mRNA with a panel of growth-factor DNA clones. We found expression of the genes for PDGF A- and B-chains, TGF-alpha, TGF-beta and IGF-II but not EGF or IGF-I. U-1810 cells lacked specific binding sites for PDGF but showed specific binding of EGF and expressed EGF-receptor transcripts. Thus, U-1810 is an example of a human tumor cell line that expresses multiple growth factor genes; in the intact tumor the corresponding growth factors may operate in autocrine stimulation of the tumor cells as well as in paracrine growth reactions (i.e. stroma recruitment).

SV40-transformed BHK cells were shown to express two transcripts, of 3.5 kb and 2.0 kb, that hybridised to a human c-sis probe. Antibodies directed against human PDGF specifically recognized a 31 kDa protein in SV40/BHK cell conditioned medium, which upon reduction was split into 16 kDa species. Unfractionated conditioned medium and one of two growth factors isolated from SV40/BHK cells competed with 125I-PDGF for binding to its receptor. The present communication thus provides compelling evidence that an SV40/BHK cell-derived growth factor is a hamster equivalent to human PDGF.

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.

The effects of platelet-derived growth factor (PDGF) on DNA replication and release of insulin and somatomedin-C/insulin-like growth factor I (SM-C/IGF-I) from cultured fetal rat islets have been studied. In medium containing 1% fetal calf serum and 16.7 mM glucose, both PDGF (2-10 ng/ml) and SM-C/IGF-I (100 ng/ml) stimulated DNA replication 2-fold. The growth stimulatory effects of the two peptides were additive. In the absence of serum, SM-C/IGF-I, but not PDGF, stimulated DNA replication. Under conditions of PDGF-stimulated DNA replication there was no increased release of either insulin or SM-C/IGF-I into the culture medium. An antibody against SM-C/IGF-I which inhibited SM-C/IGF-I-stimulated DNA replication did not affect PDGF-stimulated DNA replication. Similarly, an antibody against PDGF did not affect DNA replication stimulated by SM-C/IGF-I. It is concluded that PDGF stimulates islet cell DNA replication. This is the first demonstration of a tissue of nonmesodermal origin responding to PDGF. Stimulation of DNA replication appears to be independent of SM-C/IGF-I release, and furthermore, the results indicate that the islets do not produce PDGF-like substances themselves. It is suggested that PDGF is of importance for fetal islet development.

The receptor for platelet-derived growth factor has been purified to homogeneity on a large scale from porcine uterus. The purification procedure utilizes solubilization of uterus membranes by Triton X-100, followed by sequential chromatographies on wheat germ agglutinin-Sepharose, fast protein liquid chromatography Mono-Q, and anti-phosphotyrosine-Sepharose. About 160 micrograms of homogeneous and functionally active 170-kDa receptor could be purified from 5 kg of uterus tissue. The pure receptor responded to platelet-derived growth factor stimulation by autophosphorylation, indicating that the receptor has a kinase domain as an integral part of the molecule. A rabbit antiserum was produced against the pure receptor, which specifically recognizes the intact 170-kDa receptor.

Immunohistochemical staining revealed the presence of platelet-derived growth factor (PDGF) B-type receptors on capillaries of normal rat brain. Furthermore, capillary endothelial cells isolated from rat brain and grown in tissue culture bound [125I]PDGF-BB but not [125I]PDGF-AA, suggesting that they expressed B-type, but not A-type, PDGF receptors. PDGF-BB and PDGF-AB, but not PDGF-AA, also stimulated incorporation of [3H]thymidine into these cells. Thus, rat brain capillary endothelial cells have functional B-type receptors, and thereby differ from endothelial cells derived from large blood vessels, that do not express PDGF receptors. Our data suggest a possible role for PDGF-BB as an angiogenic factor.

First trimester human placentas actively express the sis proto-oncogene, the structural gene for the B chain of platelet-derived growth factor (PDGF). Using the in situ hybridization technique, the 4.2 kb c-sis transcript has been localized to the cytotrophoblastic component, especially the highly proliferative and invasive cytotrophoblastic shell, paralleling the distribution of c-myc transcripts in early placenta. Explants of first trimester placenta release significant levels of PDGF-like activity into the medium under apparent developmental control. Moreover, cultured trophoblasts display abundant high-affinity PDGF receptors and respond to exogenous authentic PDGF by an activation of the c-myc gene and DNA synthesis. The developing human placenta may therefore represent a case of autocrine growth regulation in a normal tissue, in which cells bearing receptors for a growth factor can also synthesize and respond to that factor.