Platelet-derived growth factor: mechanism of action and possible in vivo function

Differential effects of SPARC and cationic SPARC peptides on DNA synthesis by endothelial cells and fibroblasts

SPARC (secreted protein, acidic and rich in cysteine), also known as osteonectin, is an extracellular Ca+2-binding glycoprotein that inhibits the incorporation of [3H]-thymidine and delays the onset of S-phase in synchronized cultures of bovine aortic endothelial (BAE) cells. This effect appears not to be dependent on the functional properties of SPARC associated with changes in cell shape or inhibition of cell spreading. In this study we investigate the conditions under which cell cycle modulation occurs in different types of cells. Human umbilical vein endothelial cells, a transformed fetal BAE cell line, and bovine capillary endothelial cells exhibited a sensitivity to SPARC and a cationic peptide from a non-Ca+2-binding region of SPARC (peptide 2.1, 0.2-0.8 mM) similar to that observed in BAE cells. In contrast, human foreskin fibroblasts and fetal bovine ligament fibroblasts exhibited an increase in the incorporation of [3H]-thymidine in the presence of 25 microM-0.2 mM peptide 2.1; inhibition was observed at concentrations in excess of 0.4 mM. This biphasic modulation could be further localized to a sequence of 10 amino acids comprising the N-terminal half of peptide 2.1. A synthetic peptide from another cationic region of SPARC (peptide 2.3) increased [3H]-thymidine incorporation by BAE cells and fibroblasts in a dose-dependent manner. In endothelial cells, a stimulation of 50% was observed at a concentration of 0.01 mM; fibroblasts required approximately 100-fold more peptide 2.3 for levels of stimulation comparable to those obtained in endothelial cells. The observation that SPARC and unique SPARC peptides can differentially influence the growth of fibroblasts and endothelial cells in a concentration-dependent manner suggests that SPARC might regulate proliferation of specific cells during wound repair and remodeling.

Tyrosines 1021 and 1009 are phosphorylation sites in the carboxy terminus of the platelet-derived growth factor receptor beta subunit and are required for binding of phospholipase C gamma and a 64-kilodalton protein, respectively

Binding of platelet-derived growth factor (PDGF) to the PDGF receptor (PDGFR) beta subunit triggers receptor tyrosine phosphorylation and the stable association of a number of signal transduction molecules, including phospholipase C gamma (PLC gamma), the GTPase activating protein of ras (GAP), and phosphatidylinositol-3 kinase (PI3K). Previous reports have identified three PDGFR tyrosine phosphorylation sites in the kinase insert domain that are important for stable association of GAP and PI3K. Two of them, tyrosine (Y) 740, and Y-751 are required for the stable association of PI3K, while Y-771 is required for binding of GAP. Here we present data for two additional tyrosine phosphorylation sites, Y-1009 and Y-1021, that are both in the carboxy-terminal region of the PDGFR. Characterization of PDGFR mutants in which these phosphorylation sites are substituted with phenylalanine (F) indicated that Y-1021 and Y-1009 were required for the stable association of PLC gamma and a 64-kDa protein, respectively. An F-1009/F-1021 double mutant selectively failed to bind both PLC gamma and the 64-kDa protein, whereas all of the carboxy-terminal mutants bound wild-type levels of GAP and PI3K. The carboxy terminus encodes the complete binding site for PLC gamma, since a phosphorylated carboxy-terminal fusion protein selectively bound PLC gamma. To determine the biological consequences of failure to associate with PLC gamma, we measured PDGF-dependent inositol phosphate production and initiation of DNA synthesis. The PDGFR mutants that failed to associate with PLC gamma were not able to mediate the PDGF-dependent production of inositol phosphates. Since tyrosine phosphorylation of PLC gamma enhances its enzymatic activity, we speculated that PDGFR mutants that failed to activate PLC gamma were unable to mediate its tyrosine phosphorylation. Surprisingly, the F-1021 receptor mediated readily detectable levels of PDGF-dependent PLC gamma tyrosine phosphorylation. Thus, the production of inositol phosphates requires not only PLC gamma tyrosine phosphorylation but also its association with the PDGFR. Comparison of the mutant PDGFRs' abilities to initiate PDGF-dependent DNA synthesis indicated that failure to associate with PLC gamma and produce inositol phosphates diminished the mitogenic response by 30%. In contrast, preventing the PDGFR from binding the 64-kDa protein did not compromise PDGF-triggered DNA synthesis at saturating concentrations of PDGF. Thus, it appears that phosphorylation of the PDGFR at Y-1021 is required for the stable association of PLC gamma to the receptor's carboxy terminus, the production of inositol phosphates, and initiation of the maximal mitogenic response.

Platelet-derived growth factor. Structure, function and implications in normal and malignant cell growth

Platelet-derived growth factor (PDGF) is a potent mitogen for a variety of cell types. PDGF is made up as dimers of A and B polypeptide chains which are combined to generate the three isoforms of PDGF (AA, AB, BB). These bind with different specificities and affinities to two types of cell surface receptors (the alpha-receptor and the beta-receptor), both being members of the protein tyrosine kinase family of growth factor receptors. A number of human tumor cell lines, particularly those established from glioma and sarcoma, have been shown to produce PDGF and express the cognate receptor type. In these instances, tumor cell growth may be enhanced by an autocrine receptor activation. In other tumor cell types, where PDGF is produced in the absence of receptor expression, the growth factor may act in a paracrine fashion. This view is supported by our recent finding that human melanoma cells that have been stably transfected with a PDGF B-chain cDNA, elicit a stroma response when transplanted to nude mice.

The participation of growth factors in simulating the quiescent, proliferative, and differentiative stages of rat granulosa cells grown in a serum-free medium

Ovarian granulosa cells from small antral follicles from immature rats were cultured in a serum-free medium on an extracellular matrix for 10 days with growth factors in an effort to simulate the metabolic states they experience during their differentiation. During in vivo differentiation granulosa cells are initially quiescent, later proliferate and subsequently commence differentiation. With the production of androstenedione by the vascularized theca interna they produce estrogen and when the follicle reaches the preovulatory stage, granulosa cells produce both estrogen and progesterone. Culturing granulosa cells in serum-free medium plus FSH, PDGF, or FSH plus PDGF, the cells remain quiescent. The cells proliferate most consistently (as assessed by DNA quantitation) when cultured in FSH, PDGF, TGF alpha, TGF beta and GH, and undergo the first level of differentiation by producing estrogen (assessed by RIA) when cultured in FSH, PDGF, TGF beta, IGF-I and delta 4-A. Further differentiation is achieved in the presence of FSH, PDGF, TGF alpha, bFGF and delta 4-A when the cells produce both estrogen and progesterone similar to their production in preovulatory follicles. Phase contrast photomicrographs were made to monitor cellular shape changes. Electron microscopic analysis of the quiescent and proliferative cells reveal them to contain the normally occurring organelles. After 8 days in culture, cells producing estrogen, and estrogen and progesterone, contain endoplasmic reticulum of the smooth variety, an organelle which, in cooperation with mitochondria, is known to be involved in the production of steroids such as estrogen and progesterone. Therefore, with the addition of one or more growth factors and androstenedione to FSH-containing serum free medium, the simulated conditions are partially reminiscent of the follicular microenvironment, in which granulosa cells cultured on extracellular matrix can exhibit characteristics of growth and differentiation similar to folliculogenesis.

Platelet-derived growth factor promotes survival of rat and human mesencephalic dopaminergic neurons in culture

The effect of two isoforms of platelet-derived growth factor (PDGF), PDGF-AA and PDGF-BB, was tested on dissociated cell cultures of ventral mesencephalon from rat and human embryos. PDGF-BB but not PDGF-AA reduced the progressive loss of tyrosine hydroxylase- (TH)-positive neurons in rat and human cell cultures. The mean number of TH-positive cells in the PDGF-BB-treated rat culture was 64% and 106% higher than in the control cultures after 7 and 10 days in vitro, respectively. Corresponding figures for human TH-positive neurons were 90% and 145%. The influence of PDGF-BB was specific for TH-positive neurons and not a general trophic effect, since no change of either total cell number or metabolic activity was found. In PDGF-BB-treated cultures of human but not rat tissue the TH-positive neurons had longer neurites than observed in control or PDGF-AA-treated cultures. These data indicate that PDGF-BB may act as a trophic factor for mesencephalic dopaminergic neurons and suggest that administration of PDGF-BB could ameliorate degeneration and possibly promote axonal sprouting of these neurons in vivo.

Differential effects of PDGF isoforms on proliferation of normal rat kidney cells

The effects of the PDGF isoforms AA, AB and BB have been studied on the proliferation of normal rat kidney cells, a non-tumorigenic fibroblast cell line which contains both type alpha and type beta PDGF receptors. On monolayer cells made quiescent by serum deprivation, PDGF-AA is a relatively poor mitogen compared to PDGF-AB and PDGF-BB. When these cells are made density-arrested following continuous incubation with epidermal growth factor, however, they can be restimulated to proliferate by all three PDGF isoforms with similar activity when added at sufficiently high concentration, resulting in phenotypic cellular transformation. Binding of radiolabelled isoforms to confluent NRK monolayers obeys the predictions of an induced receptor dimerisation model, and increases in the order AA < AB < BB. Upon preincubation of the cells with PDGF-AA, the dose-response curve for mitogenic activity of PDGF-AB is shifted to higher concentrations, indicating that PDGF-AA can partly antagonize the growth stimulating activity of PDGF-AB, as has also been observed in ligand binding studies. This observation has subsequently been confirmed using fluorescence cytometric analysis. PDGF-AB is highly active in inducing anchorage-independent proliferation of NRK cells, but in all such assays PDGF-AA is at least as potent as PDGF-BB. Intriguingly, PDGF-BB is almost devoid of activity in inducing soft agar growth of these cells, in contrast to PDGF-AA. When compared to substrate-attached cells, enhanced expression of the type alpha PDGF receptor was observed under anchorage-independent conditions. These results show that the relative potency of the three PDGF isoforms to stimulate proliferation of NRK cells is different for quiescent cells in monolayer, density-arrested cells and anchorage-independent cells. Moreover it is shown that the biological activity of PDGFs can be impaired by the additional presence of other isoforms.

Expression of functional beta-platelet-derived growth factor receptors on hematopoietic cell lines

The beta-type receptor of platelet-derived growth factor (beta PDGFR) is a class III transmembrane receptor with tyrosine kinase activity. The beta PDGFR gene is located on mouse chromosome 18 close to the c-fms gene which codes for the colony stimulating factor-1 receptor (CSF-1R). We previously reported that in a high percentage of myeloblastic leukemias induced by the Friend helper murine leukemia virus (F-MuLV), proviruses were integrated in the first intron of the c-fms gene leading to an enhanced expression of c-fms mRNA. Since activation by proviral insertion can act at long distance, we studied beta PDGF receptor gene expression in murine myeloblastic leukemias. This gene was found to be frequently expressed but the level of beta PDGF receptor mRNA was weak and not related to proviral activation. High affinity binding sites were expressed on myeloblastic cells and ligand binding induced cell proliferation. To determine whether beta PDGFR expression is a common feature in hematopoietic cells, we tested cell lines belonging to other hematopoietic lineages. We found that multipotent stem and mast cell lines also expressed the beta PDGF receptor gene. This suggests that PDGF, known as a mitogen for connective tissue cells, could also play a role in normal hematopoiesis.

The regulation of megakaryocyte polyploidization and its implications for coronary artery occlusion

Polyploidization is a distinctive feature of megakaryocyte differentiation. The physiological meaning and the regulation of this process are obscure. Megakaryocyte ploidy varies in normal biology and in disease. Here we review the evidence suggesting that ploidy changes may have a role in the determination of platelet reactivity and in the aetiology of coronary artery occlusion. We also present a hypothesis that may serve as a framework to explore the regulation of megakaryocyte polyploidization at the molecular level and also may provide a rational basis to explain the occurrence of ploidy changes in ischaemic heart disease.

Renal extracellular matrix accumulation in acute puromycin aminonucleoside nephrosis in rats

Progressive renal fibrosis is considered to be the final common pathway leading to chronic renal insufficiency. In this study, the authors examined some of the cellular and molecular mechanisms regulating the renal accumulation of extracellular matrix (ECM) proteins using rats with puromycin amino-nucleoside (PAN) nephrosis as an acute model system. Puromycin aminonucleoside rats developed reversible nephrotic syndrome accompanied by an interstitial infiltrate of monocytes. The number of interstitial fibroblasts expressing ST4 antigen did not increase. During the first 4 days, steady-state mRNA levels for all genes examined remained at or below control levels. At 1 week, nephrotic syndrome and interstitial inflammation were established, and a period of renal cell proliferation occurred, identified by increased histone mRNA levels and localized by tritiated thymine autoradiography to tubular epithelial cells and occasional interstitial cells. Transforming growth factor-beta (TGF-beta) steady-state mRNA levels were increased eightfold, but returned to control levels by 3 weeks. At week 1, there was a 10- to 20-fold increase in kidney steady-state mRNA levels for genes encoding interstitial matrix proteins collagen I and fibronectin and basement membrane collagen IV. By in situ hybridization, alpha 1(I) procollagen mRNA was localized to interstitial cells. Immunofluorescence microscopy demonstrated focal accumulation of ECM proteins in the tubulointerstitial compartment at 2 and 3 weeks, but by 6 weeks, kidney immunohistology was normal again. Steady-state mRNA levels for the matrix degrading metalloproteinase stromelysin remained at control values, whereas the levels for interstitial collagenase were normal at week 1 and increased twofold to threefold at 2 and 3 weeks. Steady-state mRNA levels for the tissue inhibitor of metalloproteinases (TIMP) increased fivefold at 1 week and returned to baseline values over the next 2 weeks. The results of this study suggest that tubulointerstitial ECM accumulation occurs in rats with acute PAN nephrosis because of the activation of genes encoding several matrix proteins and inhibition of matrix degradation mediated by TIMP. These events are reversed during the phase of recovery from nephrotic syndrome. Increased mRNA levels for TGF-beta, possibly originating from inflammatory interstitial monocytes, are likely to be one of the mediators of the molecular events observed.

Effects of homodimeric isoforms of platelet-derived growth factor (PDGF-AA and PDGF-BB) on wound healing in rat

Platelet-derived growth factor (PDGF) has been suggested to have a significant role in wound healing. The present work was aimed at studying the effects of PDGF-AA and PDGF-BB homodimers on developing granulation tissue in rats. Subcutaneously implanted hollow cylindrical cellulose sponges were used as an inductive matrix for the ingrowth of granulation tissue. Fifty microliters of solutions containing 0, 5, 50, or 500 ng of PDGF-AA or PDGF-BB homodimers was injected daily into the sponges; 7 days after implantation the granulation tissue in the sponge cylinders was analyzed. Five hundred nanograms of PDGF-BB stimulated significantly the accumulation of collagen, indicated by the elevated hydroxyproline content of the sponge (+34%, P < 0.001). Similarly, the amounts of RNA-ribose, nitrogen, hexosamines, and uronic acids were significantly higher, reflecting a PDGF-BB-induced increase in the accumulation of RNA, protein, and glycosaminoglycans. Analyses of wound fluid showed no essential changes in the composition of different cell types after PDGF-BB-treatment. The PDGF-AA-treatment increased significantly the mean amount of RNA but there were no significant changes in other parameters. In vitro both PDGF-AA and PDGF-BB increased significantly the number of rat granulation tissue derived fibroblasts in culture at concentrations of 10 and 30 ng/ml. This proliferative effect resulted in a lowered level of protein synthesis per cell. To conclude, PDGF-BB accelerates granulation tissue formation both in vitro and in vivo, whereas PDGF-AA is effective in vitro but it is clearly less effective in vivo.

Capillary endothelial cells secrete a heparin-binding mitogen for pericytes

The cells of the retinal microvasculature consist predominantly of mesodermally derived pericytes and endothelial cells, and the regulatory factors which govern their co-ordinated growth and define their phenotypic characteristics in vivo may be regarded as key elements of the angiogenic process. An investigation of these cells in co-culture experiments has led to the identification of a potent mitogen for pericytes in medium conditioned by retinal endothelial cells (EC-FBS). EC-FBS activity was shown to be non-dialyzable, and stable to both heat and acid treatment. EC-FBS was inactivated by passage over a heparin-Agarose column. The column-bound activity could be eluted as a single peak at approximately 1.0 M NaCl. Stimulation of pericyte growth was also achieved with platelet-derived growth factor (PDGF), acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) and could be blocked by using the appropriate antiserum (anti-PDGF or anti-aFGF). Neither antisera, however, blocked the activity of EC-FBS. The EC-FBS mitogen markedly altered the phenotypic behavior of pericytes compared with PDGF and the FGFs; yet, unlike them, it failed to stimulate the growth of smooth muscle cells (SMC) and Balb/c 3T3 cells.

Splicing of the platelet-derived-growth-factor A-chain mRNA in human malignant mesothelioma cell lines and regulation of its expression

Platelet-derived-growth-factor (PDGF) A-chain transcripts differing in the presence or absence of an alternative exon-derived sequence have been described. In some publications, the presence of PDGF A-chain transcripts with this exon-6-derived sequence was suggested to be tumour specific. However, in this paper it was shown by reverse-transcription polymerase-chain-reaction (PCR) analysis that both normal mesothelial cells and malignant mesothelioma cell lines predominantly express the PDGF A-chain transcript without the exon-6-derived sequence. This sequence encodes a cell-retention signal, which means that the PDGF A-chain protein is most likely to be secreted by both cell types. In cultured normal mesothelial cells, the secreted PDGF A-chain protein might be involved in autocrine growth stimulation via PDGF alpha receptors. However, human malignant mesothelioma cell lines only possess PDGF beta receptors. If this also holds true in vivo, the PDGF A-chain protein produced and secreted by malignant mesothelial cells might have a paracrine function. In a previous paper, we described elevated expression of the PDGF A-chain transcript in human malignant mesothelioma cell lines, compared to normal mesothelial cells. In this paper, the possible reason for this elevation was studied. First, alterations at the genomic level were considered, but cytogenetic and Southern-blot analysis revealed neither consistent chromosomal aberrations, amplification nor structural rearrangement of the PDGF A-chain gene in the malignant cells. Possible differences in transcription rate of the PDGF A-chain gene, and stability of the transcript between normal and malignant cells, were therefore studied. The presence of a protein-synthesis inhibitor, cycloheximide, in the culture medium did not significantly influence the PDGF A-chain mRNA level in normal mesothelial and malignant mesothelioma cell lines. Furthermore, nuclear run-off analysis showed that nuclear PDGF A-chain mRNA levels varied in both cell types to the same extent as the levels observed in Northern blots. Taken together, this suggests that increased transcription is the most probable mechanism for the elevated mRNA level of the PDGF A-chain gene in human malignant mesothelioma cell lines.

Stable association between the bovine papillomavirus E5 transforming protein and activated platelet-derived growth factor receptor in transformed mouse cells

The 44-amino acid E5 transforming protein of bovine papillomavirus is the shortest protein known to induce tumorigenic transformation of fibroblasts. We showed previously that expression of the E5 protein activates the cellular beta receptor for platelet-derived growth factor (PDGF) and proposed that the activated receptor transmits the transforming signal to the cell. Here we use coimmunoprecipitation analysis to show that the E5 protein and the activated PDGF receptor exist in a stable complex in transformed mouse C127 cells. These results suggest a distinct mechanism of growth factor receptor activation and provide further evidence that the PDGF receptor is an important target of the E5 protein.

PDGF-AA and PDGF-BB biosynthesis: proprotein processing in the Golgi complex and lysosomal degradation of PDGF-BB retained intracellularly

Platelet-derived growth factor is a potent mitogen for cells of mesenchymal origin. It is made up of two polypeptide chains (A and B) combined in three disulfide-linked dimeric forms (AA, AB, and BB). Here, the biosynthesis and proteolytic processing of the two homodimeric forms of PDGF (AA and BB) were studied in CHO cells stably transfected with A-chain (short splice version) or B-chain cDNA. PDGF-AA was processed to a 30-kD molecule which was secreted from the cells. In contrast, PDGF-BB formed two structurally distinct end products; a minor secreted 30-kD form and a major cell-associated 24-kD form. Immunocytochemical studies at light- and electron-microscopical levels revealed presence of PDGF in the Golgi complex, in lysosomes, and to a smaller extent in the ER. From analysis of cells treated with brefeldin A, an inhibitor of ER to Golgi transport, it was concluded that dimerization occurs in the ER, whereas the proteolytic processing of PDGF-AA and PDGF-BB precursors normally occurs in a compartment distal to the ER. Exposure of the cultures to the lysosomal inhibitor chloroquine led to an increased cellular accumulation of PDGF-BB, as determined both by metabolic labeling experiments and immunocytochemical methods, indicating that the retained form of PDGF-BB is normally degraded in lysosomes. Structural analysis of the two end products of PDGF-BB revealed that the secreted 30-kD form is a dimer of peptides processed as the B-chain of PDGF purified from human platelets, and that the retained 24-kD form is made up of subunits additionally processed in the NH2-terminus. Also, the 24-kD form was shown to be composed of proteolytic fragments held together by disulfide bridges. Taken together these findings suggest that the newly synthesized PDGF A- and B-chains are dimerized in the ER and thereafter transferred to the Golgi complex for proteolytic processing. From there, PDGF-AA is carried in vesicles to the cell surface for release extracellularly by exocytosis. A smaller part of PDGF-BB (the 30-kD form) is handled in a similar way, whereas the major part (the 24-kD form) is generated by additional proteolysis in the Golgi complex, from which it is slowly carried over to lysosomes for degradation.

Platelet-derived growth factor B-chain-like immunoreactivity in the developing and adult rat brain

Platelet-derived growth factors (PDGFs) are growth-regulatory molecules that stimulate chemotaxis, proliferation and increased metabolism, primarily of connective tissue cells. In our previous paper, we have demonstrated the ubiquitous localization of PDGF B-chain-containing proteins in neurons and expression of transcripts for PDGF A-chain, B-chain and the two forms of the PDGF receptor in the brains of non-human primates. In the present study, the cellular localization of PDGF B-chain in developing and adult rat brains was analyzed using immunocytochemistry with a PDGF B-chain-specific monoclonal antibody. Intense PDGF B-chain immunoreactivity (PDGFB-I) was distributed around the continuously regenerating primary olfactory neurons at all stages of development from embryo to adult. The major part of PDGFB-I associated with neurons appeared some time after birth and increased with age. PDGFB-I appeared in several nerve fiber systems during earlier stages of development and gradually decreased with age. In conjunction with other data showing the existence of functional PDGF receptor beta-subunits in the neurons, these data suggest a possible role for PDGF B-chain as a neurotrophic or neuroregulatory factor in both developing and mature brains.

GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

Hydrocortisone-induced increase of PDGF beta-receptor expression in a human malignant mesothelioma cell line

The effect of hydrocortisone (HC) on PDGF beta-receptor expression was studied in the human malignant mesothelioma cell line Mero-14. HC was found to induce a time- and dose-dependent increase of PDGF beta-receptor mRNA. Nuclear run off analysis revealed that HC induced increased transcription of the PDGF beta-receptor gene. The expression of PDGF beta-receptor protein was also elevated by HC as demonstrated with an immunoblotting assay. However, the number of PDGF-BB binding sites on the cell surface of Mero-14 remained unchanged upon HC treatment. These results suggest that steroid hormones can regulate PDGF receptor expression in vivo.

A novel endothelial cell surface receptor tyrosine kinase with extracellular epidermal growth factor homology domains

Endothelial cell surfaces play key roles in several important physiological and pathological processes such as blood clotting, angiogenic responses, and inflammation. Here we describe the cloning and characterization of tie, a novel type of human endothelial cell surface receptor tyrosine kinase. The extracellular domain of the predicted tie protein product has an exceptional multidomain structure consisting of a cluster of three epidermal growth factor homology motifs embedded between two immunoglobulinlike loops, which are followed by three fibronectin type III repeats next to the transmembrane region. Additionally, a cDNA form lacking the first of the three epidermal growth factor homology domains was isolated, suggesting that alternative splicing creates different tie-type receptors. Cells transfected with tie cDNA expression vector produce glycosylated polypeptides of 117 kDa which are reactive to antisera raised against the tie carboxy terminus. The tie gene was located in chromosomal region 1p33 to 1p34. Expression of the tie gene appeared to be restricted in some cell lines; large amounts of tie mRNA were detected in endothelial cell lines and in some myeloid leukemia cell lines with erythroid and megakaryoblastoid characteristics. In addition, mRNA in situ studies further indicated the endothelial expression of the tie gene. The tie receptor tyrosine kinase may have evolved for multiple protein-protein interactions, possibly including cell adhesion to the vascular endothelium.

A novel endothelial cell surface receptor tyrosine kinase with extracellular epidermal growth factor homology domains

Endothelial cell surfaces play key roles in several important physiological and pathological processes such as blood clotting, angiogenic responses, and inflammation. Here we describe the cloning and characterization of tie, a novel type of human endothelial cell surface receptor tyrosine kinase. The extracellular domain of the predicted tie protein product has an exceptional multidomain structure consisting of a cluster of three epidermal growth factor homology motifs embedded between two immunoglobulinlike loops, which are followed by three fibronectin type III repeats next to the transmembrane region. Additionally, a cDNA form lacking the first of the three epidermal growth factor homology domains was isolated, suggesting that alternative splicing creates different tie-type receptors. Cells transfected with tie cDNA expression vector produce glycosylated polypeptides of 117 kDa which are reactive to antisera raised against the tie carboxy terminus. The tie gene was located in chromosomal region 1p33 to 1p34. Expression of the tie gene appeared to be restricted in some cell lines; large amounts of tie mRNA were detected in endothelial cell lines and in some myeloid leukemia cell lines with erythroid and megakaryoblastoid characteristics. In addition, mRNA in situ studies further indicated the endothelial expression of the tie gene. The tie receptor tyrosine kinase may have evolved for multiple protein-protein interactions, possibly including cell adhesion to the vascular endothelium.

Characterization of the molecular mechanism involved in the activation of hyaluronan synthetase by platelet-derived growth factor in human mesothelial cells

The molecular mechanism involved in the stimulation of hyaluronan synthetase in normal human mesothelial cells was investigated. Exposure of mesothelial cells to platelet-derived growth factor (PDGF)-BB stimulated hyaluronan synthetase activity, measured in isolated membrane preparations, as well as hyaluronan secretion into the medium. The effect on hyaluronan synthetase was maximal after 6 h of treatment. In contrast, the stimulatory effect of transforming growth factor-beta 1 reached a maximum after 24 h. The stimulatory effect of PDGF-BB was inhibited by cycloheximide. The phosphotyrosine phosphatase inhibitor vanadate was found to stimulate hyaluronan synthetase activity, and to potentiate the effect of PDGF-BB. The protein kinase C (PKC) stimulator phorbol 12-myristate 13-acetate (PMA) also stimulated hyaluronan synthetase; furthermore, depletion of PKC by preincubation of the cells with PMA led to an inhibition of the PDGF-BB-induced stimulation of hyaluronan synthetase activity. Thus the PDGF-BB-induced stimulation of hyaluronan synthetase activity is dependent on protein synthesis and involves tyrosine phosphorylation and activation of PKC.

Inhibition of platelet-derived growth factor-induced mitogenesis and tyrosine kinase activity in cultured bone marrow fibroblasts by tyrphostins

Tyrphostins, which block protein tyrosine kinase activity, were studied for their inhibitory action on platelet-derived growth factor (PDGF)-induced proliferation of human bone marrow fibroblasts. Of the seven tryphostins examined, tyrphostin AG370 was found to be the most potent blocker against PDGF-induced mitogenesis (IC50 = 20 microM). This PTK blocker also blocks mitogenesis induced by epidermal growth factor (IC50 = 50 microM) and human serum (IC50 = 50 microM), but with lower efficacy. In digitonin-permeabilized fibroblasts as well as in intact fibroblasts, tyrphostin AG370 inhibits PDGF receptor autophosphorylation and the tyrosine phosphorylation of intracellular protein substrates (pp120, pp85, and pp75) which coprecipitate with the PDGF receptor. In comparison to AG370, AG18, a potent EGF receptor blocker, was less efficient in inhibiting PDGF-induced proliferation of fibroblasts and phosphorylation of the intracellular protein substrates. Under the conditions in which AG370 inhibits PDGF-induced mitogenesis and phosphorylation, it does not affect [125I]PDGF internalization and enhance [125I]PDGF binding. These findings suggest that AG370, which is an indole tyrphostin, may serve as a model for developing analogues with a therapeutic potential for treatment of diseases which involve abnormal cellular proliferation induced by PDGF.

Differential effects of platelet-derived growth factors on fetal hippocampal and cortical grafts: evidence from intraocular transplantation in rats

Effects of platelet-derived growth factor-AA (PDGF-AA) and platelet-derived growth factor-BB (PDGF-BB) on developing parietal cortex (E16) and hippocampal (E18-E19) grafts were studied using the in vivo method of intraocular transplantation. Survival and growth of grafts in the anterior eye chamber of adult host rats under the influence of regular treatments with 0.5 ng (in a 100 ng/ml concentration) PDGF-AA or PDGF-BB was followed and compared to those receiving vehicle solution alone (0.5 mg HSA/ml Hanks). Both PDGF-AA and PDGF-BB increased the volume of transplanted cortical grafts. PDGF-BB also exerted trophic effects on grafted hippocampal tissue whereas PDGF-AA seemed to inhibit hippocampal growth. Histological and immunohistochemical studies revealed an increase in the density of astroglial elements in PDGF-AA- and PDGF-BB-treated cortical grafts whereas the PDGF-AA- and PDGF-BB-treated hippocampal grafts maintained a cytoarchitecture closely resembling that of control grafts. These findings support in vitro experiments showing that developing glial cells are stimulated by PDGFs and we further propose regional differences of action of PDGFs in the developing central nervous system.

Expression of platelet-derived growth factor and its receptors in proliferative disorders of fibroblastic origin

Platelet-derived growth factor (PDGF) is known to stimulate the proliferation of connective tissue-derived cells in vitro. Less is known about its functions in vivo, and the role of PDGF in the development of human tumors has not been clarified. The authors have investigated the occurrence of PDGF and PDGF receptors in a series of proliferative disorders of fibroblastic origin using immunohistochemical and in situ hybridization techniques. High expression of PDGF beta-receptor mRNA and protein was found in the malignant tumors, and also in some benign lesions, such as dermatofibroma. In all these cases, benign as well as malignant, the PDGF B-chain mRNA, and less clearly, the PDGF A-chain mRNA, were coexpressed with the beta-receptor. In contrast, high expression of PDGF alpha-receptor mRNA was only found in fully malignant lesions, i.e., malignant fibrous histiocytoma. These data indicate that an autocrine growth stimulation via the PDGF beta-receptor could occur in an early phase of tumorigenesis, and may be a necessary but insufficient event for the progression into fully malignant human connective tissue lesions.

The extracellular glycoprotein SPARC interacts with platelet-derived growth factor (PDGF)-AB and -BB and inhibits the binding of PDGF to its receptors

Interactions among growth factors, cells, and extracellular matrix are critical to the regulation of directed cell migration and proliferation associated with development, wound healing, and pathologic processes. Here we report the association of PDGF-AB and -BB, but not PDGF-AA, with the extracellular glycoprotein SPARC. Complexes of SPARC and 125I-labeled PDGF-BB or -AB were specifically immunoprecipitated by anti-SPARC immunoglobulins. 125I-PDGF-BB and -AB also bound specifically to SPARC that was immobilized on microtiter wells or bound to nitrocellulose after transfer from SDS/polyacrylamide gels. The binding of PDGF-BB to SPARC was pH-dependent; significant binding was detectable only above pH 6.6. The interaction of SPARC with specific dimeric forms of PDGF affected the activity of this mitogen. SPARC inhibited the binding of PDGF-BB and PDGF-AB, but not PDGF-AA, to human dermal fibroblasts in a dose-dependent manner. The expression of SPARC and PDGF was minimal in most normal adult tissues but was increased after injury. Enhanced expression of both PDGF-B chain and SPARC was seen in advanced lesions of atherosclerosis. We suggest that the coordinate expression of SPARC and PDGF-B-containing dimers following vascular injury may regulate the activity of specific dimeric forms of PDGF in vivo.

Autocrine ligand binding to cell receptors. Mathematical analysis of competition by solution "decoys"

Autocrine ligands have been demonstrated to regulate cell proliferation, cell adhesion, and cell migration in a number of different systems and are believed to be one of the underlying causes of malignant cell transformation. Binding of these ligands to their cellular receptors can be compromised by diffusive transport of ligand away from the secreting cell. Exogenous addition of antibodies or solution receptors capable of competing with cellular receptors for these autocrine ligands has been proposed as a means of inhibiting autocrine-stimulated cell behavioral responses. Such "decoys" complicate cellular binding by offering alternative binding targets, which may also be capable of aiding or abating transport of the ligand away from the cell surface. We present a mathematical model incorporating autocrine ligand production and the presence of competing cellular and solution receptors. We elucidate effects of key system parameters including ligand diffusion rate, binding rate constants, cell density, and secretion rate on the ability of solution receptors to inhibit cellular receptor binding. Both plated and suspension cell systems are considered. An approximate analytical expression relating the key parameters to the critical concentration of solution "decoys" required for inhibition is derived and compared to the numerical calculations. We find that in order to achieve essentially complete inhibition of surface receptor binding, the concentration of decoys may need to be as much as four to eight orders of magnitude greater than the equilibrium disociation constant for ligand binding to surface receptors.

Characterization of the mouse PDGF A-chain gene. Evolutionary conservation of gene structure, nucleotide sequence and alternative splicing

The mouse platelet-derived growth factor (PDGF) A-chain gene has been structurally characterized and compared with its human counterpart. The organization of the two genes is similar. Both consist of 7 exons spaced by 6 introns of corresponding sizes. As in the human gene, exon 6 encodes a sequence which is alternatively spliced. When present, it codes for an alternative C-terminus of the A-chain. In intron 5, conserved stretches of nucleotides, potentially involved in the regulation of the alternative splicing, are identified. The untranslated sequences show a high degree of nucleotide sequence identity and several conserved consensus binding sites for transcription factors are identified within the 5' untranslated as well as in the 5' flanking region.

Malignant transformation of NIH 3T3 fibroblasts by human c-sis is dependent upon the level of oncogene expression

High-level expression of the c-sis oncogene, which encodes the beta chain of platelet-derived growth factor, transforms immortalized rodent fibroblasts in vitro to a malignant phenotype. c-sis gene expression has been demonstrated in a variety of human tumors, although generally at levels much lower than those shown to transform cells in vitro. We examined the effect of lower levels of c-sis expression on the phenotype of NIH 3T3 fibroblasts. Clones with various levels of c-sis expression were generated by transfecting NIH 3T3 cells with a plasmid that expressed the human c-sis cDNA and the TN5 neomycin-resistance gene. G418-resistant clones, which expressed the c-sis cDNA, were selected and characterized. Alterations in the phenotype of the clones that expressed c-sis ranged from increased growth in soft agar to malignant tumor formation in nude and syngeneic mice. Increased levels of c-sis cDNA expression correlated with the acquisition of features of transformation in a dose-dependent manner and altered the cellular phenotype in a manner consistent with the progression of cells towards malignancy. These data support a model in which low levels of sis gene expression in tumors contribute to the acquisition of some features of transformation but require complementation by other genes or factors to produce a fully malignant phenotype.

A chimera between platelet-derived growth factor beta-receptor and fibroblast growth factor receptor-1 stimulates pancreatic beta-cell DNA synthesis in the presence of PDGF-BB

This study was undertaken to characterize the expression of a chimeric growth factor receptor composed of the extracellular and transmembrane domains of the platelet-derived growth factor (PDGF) beta-receptor (PDGFR-beta) fused to the intracellular domain of the fibroblast growth factor receptor-1 (FGFR-1) and to assess its effect on the growth potential of pancreatic islet cells. For this purpose rat pancreatic islets or monolayers of pancreatic islet cells were transfected with recombinant DNA constructs coding for the PDGF B-chain, the PDGFR-beta, the FGFR-1 and the chimera between PDGFR-beta and FGFR-1. DNA synthesis, monitored as the percentage of labelled nuclei and [3H]thymidine incorporation, was stimulated in pancreatic islet cells cotransfected with the constructs coding for the PDGF B-chain and the PDGFR-beta or the chimeric PDGFR-beta/FGFR-1 as compared with that determined after transfection with control plasmid. PDGF-BB stimulated DNA synthesis when islet cells had been transfected with PDGFR-beta or PDGFR-beta/FGFR-1. Cotransfection of the PDGFR-beta and the chimeric PDGFR-beta/FGFR-1 constructs attenuated the stimulation of DNA synthesis in response to PDGF-BB. Receptor binding studies showed binding with a Kd of 0.7 nM to the chimeric receptor. The present findings show that when the chimeric PDGFR-beta/FGFR-1 construct is expressed in beta-cells it is efficient in increasing DNA synthesis when stimulated with ligand.

Inhibition of growth factor binding and intracellular Ca2+ signalling by dextran sulfates of different sizes and degrees of sulfation

The ability of dextran sulfates of varying molecular sizes (5-500 kDa) and degrees of sulfate substitution (0.3-1.9) to inhibit the binding of platelet-derived growth factor (PDGF) to intact Swiss 3T3 fibroblasts and to inhibit inositol(1,4,5)trisphosphate-dependent release of Ca2+ in permeabilized Swiss 3T3 cells was examined in the present study. Significant correlations were found between increased molecular size of the dextran sulfates and inhibition of both PDGF binding (r = 0.77) and Ca2+ release (r = 0.72). The degree of sulfate substitution did not correlate with inhibition of either activity.

Ligand-induced homo- and hetero-dimerization of platelet-derived growth factor alpha- and beta-receptors in intact cells

Porcine aortic endothelial cells expressing platelet-derived growth factor (PDGF) alpha- or beta-receptors after transfection of the corresponding cDNAs, were used to investigate whether PDGF receptor dimerization occurs in intact cells after ligand binding. Using three different methods--covalent cross-linking of 125I-labeled ligand, cross-linking of metabolically labeled cells after ligand-binding followed by immunoprecipitation, and immunoblotting of cells after ligand binding and cross-linking--it was demonstrated that alpha- as well as beta-receptors form ligand-induced dimeric complexes. Dimerization correlated with induction of receptor kinase activity, measured as receptor autophosphorylation. Heterodimeric complexes could furthermore be induced by PDGF-AB, when added to a mixture of lysates from the alpha- and beta-receptor expressing cell lines, or when added to human fibroblasts which express both receptor types.

Inhibition of growth factor binding, Ca2+ signaling and cell growth by polysulfonated azo dyes related to the antitumor agent suramin

The ability of the polysulfonated antitumor drug suramin and six related polysulfonated azo dyes to inhibit the cell growth, platelet-derived growth factor (PDGF)-receptor binding, and intracellular Ca2+ signaling of Swiss 3T3 fibroblasts was studied. Some of the azo dyes were more potent inhibitors of PDGF binding than was suramin. The concentration giving 50% inhibition (IC50) of PDGF binding was 0.5 microM for the most potent azo dye as compared with 10 microM for suramin. The azo dyes were generally more potent inhibitors of nonmitochondrial Ca2+ uptake and of inositol(1,4,5)trisphosphate-mediated Ca2+ release in permeabilized Swiss 3T3 cells than was suramin, and they were more potent inhibitors of PDGF-induced Ca2+ signaling in intact Swiss 3T3 cells. The azo dyes were only as effective as or less effective than suramin in inhibiting the growth of Swiss 3T3 cells, with IC50 values of between 74 and 361 microM being noted for the dyes as compared with 70 microM for suramin. The difference between the growth-inhibitory activity of the azo dyes and that of suramin could not be explained by metabolism of the compounds, which was not detectable in either Swiss 3T3 cells or human liver slice preparations. The results suggest that suramin and some of the azo dyes have actions on cell growth in addition to inhibition of growth factor binding and of Ca2+ signaling.

Molecular mechanisms for generation of neural diversity and specificity: roles of polypeptide factors in development of postmitotic neurons

Development of postmitotic neurons is influenced by two groups of polypeptide factors. Neurotrophic factors promote neuronal survival both in vivo and in vitro. Neuronal differentiation factors influence transmitter phenotypes without affecting neuronal survival. The list of neurotrophic factors is increasing partly because certain growth factors and cytokines have been shown to possess neurotrophic activities and also because new neurotrophic factors including new members of the nerve growth factor (NGF) family have been identified at the molecular level. In vitro assays using recombinant neurotrophic factors and distributions of their mRNAs and proteins have indicated that members of a neurotrophic gene family may play sequential and complementary roles during development and in the adult nervous system. Most of the receptors for neurotrophic factors contain tyrosine kinase domains, suggesting the importance of tyrosine phosphorylation and subsequent signal transduction for their effects. Molecules such as LIF (leukemia inhibitory factor) and CNTF (ciliary neurotrophic factor) have been identified as neuronal differentiation factors in vitro. At the moment, however, it remains to be determined whether or not the receptors for a group of neuronal differentiation factors constitute a gene family or contain domains of kinase or phosphatase activity. Synergetic combinations of neurotrophic and neuronal differentiation factors as well as their receptors may contribute to the generation of neural specificity and diversity.

The expression of PDGF alpha- and beta-receptors in subpopulations of PDGF-producing cells implicates autocrine stimulatory loops in the control of proliferation in cytotrophoblasts that have invaded the maternal endometrium

In order to explain the high proliferative potential of human placental cytotrophoblasts, we have addressed the potential involvement of platelet-derived growth factor (PDGF) ligand and receptors. Although PDGF is usually described as a mitogen for cells of mesenchymal origin, we show in this report that extra-villous term placental cytotrophoblasts express the PDGF alpha- and beta-receptor genes, both in vivo and in vitro. In addition, cytotrophoblasts produce significant amounts of PDGF-B protein. By immunohistochemical analysis of receptor expression, we found that the PDGF alpha-receptors could be detected at the cell surface, while the PDGF beta-receptors were only detected intracellularly. In addition, double immunostaining analysis showed that the PDGF alpha- and beta-receptor molecules are expressed in different subpopulations of cytotrophoblasts. The addition of PDGF-AA and PDGF-BB homodimers to cytotrophoblast primary cultures induced a significant increase in DNA synthesis. We conclude, therefore, that PDGF is a growth factor for placental cytotrophoblasts and suggest that the growth of cytotrophoblasts can partly be explained by a PDGF autostimulatory loop, limited by the number of receptor-positive cytotrophoblasts.