Ligand activation causes a phosphorylation-dependent change in platelet-derived growth factor receptor conformation

Transactivation of PDGFRbeta by dopamine D4 receptor does not require PDGFRbeta dimerization

Growth factor-induced receptor dimerization and cross-phosphorylation are hallmarks of signal transduction via receptor tyrosine kinases (RTKs). G protein-coupled receptors (GPCRs) can activate RTKs through a process known as transactivation. The prototypical model of RTK transactivation involves ligand-mediated RTK dimerization and cross-phosphorylation. Here, we show that the platelet-derived growth factor receptor β (PDGFRβ) transactivation by the dopamine receptor D4 (DRD4) is not dependent on ligands for PDGFRβ. Furthermore, when PDGFRβ dimerization is inhibited and receptor phosphorylation is suppressed to near basal levels, the receptor maintains its ability to be transactivated and is still effective in signaling to ERK1/2. Hence, the DRD4-PDGFRβ-ERK1/2 pathway can occur independently of a PDGF-like ligand, PDGFRβ cross-phosphorylation and dimerization, which is distinct from other known forms of transactivation of RTKs by GPCRs.

Aptamer-functionalized gold nanoparticles for turn-on light switch detection of platelet-derived growth factor

An aptamer modified gold nanoparticles (Apt-AuNPs) based molecular light switching sensor has been demonstrated for the analysis of breast cancer markers (platelet-derived growth factors (PDGFs) and their receptors) in homogeneous solutions. The PDGF binding aptamer has a unique structure with triple-helix conformation that allows N,N-dimethyl-2,7-diazapyrenium dication (DMDAP) and PDGF bindings. The fluorescence of DMDAP is almost completely quenched by Apt-AuNPs when it intercalates with the aptamers. Owing to high magnitudes of increases (up to 40-fold) in the turn-on fluorescence signals of DMDAP/Apt-AuNP upon PDGFs binding, the approach is highly sensitive for the detection of PDGFs. The DMDAP/Apt-AuNP probe specifically and sensitively detected PDGFs under optimal concentrations of salts and DMDAP. We also demonstrated that the Apt-AuNPs are effective selectors for enrichment of PDGF-AA from large-volume samples. The approach allows detection of PDGF-AA at a concentration down to 8 pM, showing better sensitivity than other signal aptamers. By conducting a competitive assay, we demonstrated the determination of PDGF receptor-alpha with LOD of 0.25 nM when using the DMDAP/Apt-AuNP as a probe.

Aptamer-modified gold nanoparticles for colorimetric determination of platelet-derived growth factors and their receptors

We have developed a highly specific sensing system for platelet-derived growth factors (PDGFs) and platelet-derived growth factor receptors (PDGFR) that uses gold nanoparticles (GNPs). We synthesized GNPs modified with an aptamer (Apt-GNPs) that is specific to PDGFs and used them to detect PDGFs by monitoring the changes in the color and extinction of the Apt-GNPs that occur as a result of aggregation. The color of the Apt-GNPs changes from red to purple at low concentrations (<400 nM), but changes only slightly at higher concentrations (>400 nM). We found that the sensitivity of the Apt-GNPs for the three PDGFs is highly salt-dependent, with an optimum condition of 200 mM NaCl. We obtained biphasic curves when plotting of the ratios of the extinction coefficients of the Apt-GNPs at 650 and 530 nm against the concentrations of PDGF-AA at various concentrations of Apt-GNPs. The linear ranges of the increases and decreases in this extinction ratio are 2.5-10 and 10-20 nM, respectively, for 0.42 nM Apt-GNPs and 25-75 and 75-200 nM, respectively, for 8.4 nM Apt-GNPs. When using 8.4 nM Apt-GNPs, the corresponding linear ranges of the increases and decreases in this extinction ratio are 15-100 and 100-400 nM, respectively, for PDGF-AB and 35-150 and 150-400 nM, respectively, for PDGF-BB. In addition, we have developed a homogeneous assay to detect the PDGF receptor-beta (PDGFR-beta) at concentrations as low as 3.2 nM, on the basis of the competition between the Apt-GNPs and PDGFR-beta for PDGF-BB. The results we present in this paper imply that there are practical applications of Apt-GNPs in protein analysis and cancer diagnosis.

Biphasic calcium response of platelet-derived growth factor stimulated glioblastoma cells is a function of cell confluence

BACKGROUND

Previous reports have linked the spiking or two-phased character of calcium transients evoked by platelet-derived growth factor (PDGF) to the position of cells in the cell cycle without regard to cell-cell contact and communication. Because cell confluence can regulate growth factor receptor expression and dephosphorylation, we investigated the effect of cell culture confluence and cell cycle on calcium responses of PDGF-BB-stimulated A172 glioblastoma cells.

METHODS

Digital imaging cytometry was used to correlate the peak and duration of calcium response with bromodeoxyuridine positivity and DNA content and with culture confluence on a cell-by-cell basis.

RESULTS

In serum-starved cultures, complete two-phase calcium signals and shorter, lower spikes occurred independent of cell cycle phase. However, the confluence of cell culture seemed essential for inducing a complete response because cells in sparse cultures exhibited mostly short spikes with lower peaks or no transients at all.

CONCLUSION

Because cell confluence, by virtue of cell-cell contacts, is assumed to be an important regulator of proliferation, one is tempted to speculate that in transformed cells the ability to produce stronger growth signals upon reaching confluence and facing contact inhibition could provide a proliferative advantage.

Type-specific antibodies to the platelet-derived growth factor receptors: role in elucidating the structural and functional characteristics of receptor types

Two types of platelet-derived growth factor receptors have been cloned and sequenced. Both are glycoproteins with similar molecular weights. We have earlier established the ligand binding specificity, ligand-induced dimerization, and kinase activation of these two receptor types [Bishayee et al. (1989) J. Biol. Chem. 264, 11699-11705; Kanakaraj et al. (1991) Biochemistry 30, 1761-1767]. In the present studies, we have investigated the biosynthesis, processing, and glycosylation of the alpha-receptor and compared its structural and functional characteristics to those of the beta-receptor. Unlike an anti-peptide antibody, AbP2 (amino acid residues 964-979), to the human beta-receptor which detects a phosphorylation-specific conformation of the receptor, an antibody, AbP alpha 2 (amino acid residues 956-971), to the corresponding region of the human alpha-receptor failed to do so. However, our studies revealed that the stability of the alpha-receptor is comparable to that of the beta-receptor. In addition, N-linked glycosylation of the alpha-receptor, like that of the beta-receptor, is not important in kinase activation. We have exploited the lack of an effect of N-linked oligosaccharides on the functioning of the alpha-receptor to develop a simple and rapid method for direct demonstration of ligand-induced noncovalently linked alpha-beta-receptor heterodimer formation. This method is based on the interaction between functionally active short and the long forms of two receptor types which can be resolved by denaturing gel electrophoresis.

Early events in M-CSF receptor signaling

The M-CSF receptor (M-CSFR) is expressed in monocytes-macrophages and their progenitors, and drives growth and development of this blood cell lineage. The M-CSFR is a member of a small family of growth factor receptors exhibiting related structures but distinct tissue-specific functions. This review discusses the early molecular events in the M-CSF signaling mechanisms, positive signals, negative signals, the possible organization of individual signaling pathways, and the problem of achieving specificity in the signal transduction mechanism.

Bidirectional transmembrane modulation of integrin alphaIIbbeta3 conformations

Activation of blood platelets by physiological stimuli (e.g. thrombin, ADP) at sites of vascular injury induces inside-out signaling, resulting in a conformational change of the prototype integrin alphaIIbbeta3 from an inactive to an active state competent to bind soluble fibrinogen. Furthermore, ligand occupancy of alphaIIbbeta3 initiates outside-in signaling and additional conformational changes of the receptor, leading to the exposure of extracellular neoepitopes termed ligand-induced binding sites (LIBS), which are recognized by anti-LIBS monoclonal antibodies. To date, the mechanism of bidirectional transmembrane signaling of alphaIIbbeta3 has not been established. In this study, using our newly developed anti-LIBScyt1 monoclonal antibody, we showed that extracellular ligand binding to alphaIIbbeta3 on blood platelets induces a transmembrane conformational change in alphaIIbbeta3, thereby exposing the LIBScyt1 epitope in the alphaIIb cytoplasmic sequence between Lys994 and Asp1003. In addition, a point mutation at this site (P998A/P999A) renders alphaIIbbeta3 constitutively active to bind extracellular ligands, resulting in fibrinogen-dependent cell-cell aggregation. Taken collectively, these results demonstrated that the extracellular ligand-binding site and a cytoplasmic LIBS epitope in integrin alphaIIbbeta3 are conformationally and functionally coupled. Such bidirectional modulation of alphaIIbbeta3 conformation across the cell membrane may play a key role in inside-out and outside-in signaling via this integrin.

Inhibition of platelet-derived growth factor receptor tyrosine kinase by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) is known to suppress platelet-derived growth factor (PDGF)-stimulated proliferation of rat cultured vascular smooth muscle cells. The present study examined whether ANP inhibits the PDGF receptor (PDGFR) tyrosine kinase activation, an initial event for PDGF cellular signaling. ANP reduced the in vivo tyrosine phosphorylation of PDGFR stimulated by PDGF in a dose-dependent manner. This effect was not due to the reduction in PDGFR protein as detected by immunoblot analysis. 8-Bromo-cyclic GMP, a membrane-permeable 3',5'-cyclic monophosphate (cGMP) derivative, mimicked the action of ANP. HS-142-1, an antagonist for guanylate cyclase A (GC-A) and B, co-incubated with ANP, restored the PDGF-induced PDGFR autophosphorylation. The effect of ANP was also observed in the presence of a protein tyrosine phosphatase inhibitor, sodium orthovanadate. To confirm that ANP exerts its action by inhibiting protein tyrosine kinase (PTK), an in vitro kinase assay was performed. Cyclic GMP inhibited PTK activity of PDGFR partially purified by lectin affinity chromatography. In contrast, PTK activity in immobilized PDGFR immunocomplexes was not inhibited by cGMP. However, exogenous cGMP dependent protein kinase (PKG) reduced the PTK activity in the presence of cGMP. These results demonstrate that ANP suppresses PDGFR PTK through GC-A probably by activating PKG. This may be an important mechanism by which ANP exerts its anti-proliferative action antagonizing PDGF.

SH2 domain protein interaction and possibilities for pharmacological intervention

SH2 domain containing proteins play a key role in the process of intracellular transmission of signalling events initiated at the cell surface. As a pre-requisite in the fulfillment of this function, these proteins bind to a variety of phospho-tyrosine (pY) containing target molecules. Delineation of these binding sites as essentially short linear peptides (both structurally and functionally) has led to the suggestion that the activity of these signalling complexes may be manipulated by the development of relatively simple peptide reagents. This review examines the range of possibilities open on this approach and the extent to which positive results have already been realised.

Identification of tyrosine 489 in the carboxy terminus of the Tpr-Met oncoprotein as a major site of autophosphorylation

The Met receptor tyrosine kinase is the receptor for hepatocyte growth factor/scatter factor. HGF/SF is a multifunctional cytokine that can stimulate proliferation, motility, and morphogenesis in epithelial and endothelial cells. Oncogenic activation of the Met receptor occurs through a genomic rearrangement that generates a hybrid protein in which tpr sequences are directly fused amino terminal to the met receptor kinase domain. The resultant Tpr-Met hybrid protein possesses tyrosine kinase activity, is constitutively phosphorylated on tyrosine residues in vivo, and transforms fibroblasts in culture. We have identified two tyrosine residues within the catalytic domain of the Tpr-Met oncoprotein (Y365, Y366) and Met receptor (Y1234, Y1235) that are phosphorylated and essential for both the catalytic and biological activity of the oncoprotein and receptor. However, a detailed analysis of phosphorylation in these proteins has not been undertaken. In order to determine the sites of tyrosine phosphorylation in the Tpr-Met oncoprotein, in vitro mutagenesis, phosphopeptide mapping, and dephosphorylation protection assays were performed. Here we identify that a single tyrosine (Y489) in the carboxy terminus of the Tpr-Met oncoprotein is highly phosphorylated and is essential for biological activity. In contrast, additional tyrosines (Y482, Y498) located in the carboxy terminus are not phosphorylated at detectable levels and are not essential for the biological activity of the oncoprotein.

Characterization of a novel epidermal-growth-factor-receptor-related 200-kDa tyrosine kinase in tumor cells

We have detected a tyrosine-phosphorylated 200-kDa protein in two human tumor cell lines, A1235 glioma and A172 glioblastoma. The protein is an integral plasma membrane sialoglycoprotein with tyrosine kinase activity. The interesting characteristic of this protein (gp200) is that it is recognized by a number of monoclonal and polyclonal antibodies to the 170-kDa epidermal-growth-factor (EGF) receptor; however, it lacks detectable EGF-binding activity. gp200 differs from three other EGF-receptor-related proteins, erb-B-2, erb-B-3 and erb-B-4 gene products, and hence appears to be yet another member of the EGF-receptor family of proteins. This is further strengthened by the fact that both gp200 and the EGF receptor contain a common epitope which is recognized by an anti-peptide IgG to the beta-type platelet-derived-growth-factor (PDGF) receptor. Our previous studies [Bishayee, S., Majumdar, S., Scher, C.D. & Khan, S. (1988) Mol. Cell. Biol. 8, 3696-3702] have demonstrated that this epitope in the PDGF receptor is highly susceptible to the phosphorylation state of the receptor and that such a conformational change appears to be important in biological message transmission. The expression of gp200, which appears to have tyrosine kinase activity and is immunologically related to the EGF receptor in tumor cells, suggests its possible involvement in cell growth.

A conformation-specific anti-peptide antibody to the beta-type platelet-derived growth factor receptor also recognizes the activated epidermal growth factor receptor

Earlier we reported the generation of a conformation-specific antibody (Ab P2) to the beta-type platelet-derived growth factor receptor (Bishayee, S., Majumdar, S., Scher, C. D., and Khan, S. (1988) Mol. Cell. Biol. 8, 3696-3702). Ab P2 is directed to a 16-amino acid peptide (Glu-Gly-Tyr-Lys-Lys-Lys-Tyr-Gln-Gln-Val-Asp-Glu-Glu-Phe-Leu-Arg) of the cytoplasmic domain of the receptor, and it recognizes the phosphorylated platelet-derived growth factor receptor but not the unphosphorylated receptor. We now report that Ab P2 also interacts with the epidermal growth factor receptor and that the recognition is specific for a conformation induced by phosphorylation of the receptor; however, Ab P2 is not directed to phosphotyrosine. Studies conducted with P2-derived peptides suggest that the conformation-specific antibody is directed to an acidic tripeptide, Asp-Glu-Glu, and this sequence is also present in the cytoplasmic domain of the epidermal growth factor receptor. With respect to the C terminus amino acid or ATP-binding site, Asp-Glu-Glu is located in different regions in these receptors; nevertheless, this tripeptide along with the surrounding amino acids is cryptic in the unphosphorylated receptor, and tyrosine phosphorylation uncovers this site. This suggests that the Asp-Glu-Glu sequence may be important in receptor functions.

Signal transduction by the PDGF receptors

The three isoforms of PDGF bind with different affinities to two related tyrosine kinase receptors, denoted the PDGF alpha- and beta-receptors. Ligand binding induces receptor dimerization, creating receptor homo- or heterodimers. Dimerization is accompanied by, and might be a prerequisite for, receptor autophosphorylation and kinase activation. Receptor autophosphorylation serves to regulate the kinase activity and to create binding sites on the receptor molecule for downstream signalling components. The activities of the signalling components are ultimately manifested as specific biological responses. All the currently described PDGF receptor-binding components, e.g. phospholipase C-gamma, members of the src family of cytoplasmic tyrosine kinases, the rasGT-Pase activating protein and p85, the regulatory subunit of phosphatidylinositol 3' kinase, contain a conserved src homology 2-domain, through which the association with the receptor takes place. The receptor-binding components appear to either possess an intrinsic enzymatic activity, or they function as adaptors, which may complex with catalytically active components. For most receptor-binding components, there is insufficient understanding of how binding to the receptor affects the catalytic function. Certain of these components become tyrosine-phosphorylated, i.e. they are substrates for the receptor tyrosine kinase. Moreover, the change in subcellular localization, which most of the receptor binding components undergo in conjunction with receptor binding, could play a critical role. The current efforts of many laboratories are aimed at delineating different PDGF receptor signal transduction pathways and what roles the different receptor-binding components play in the establishment of these pathways.

Growth hormone signal transduction

Growth hormone (GH) promotes animal growth by stimulating bone and cartilage cell proliferation, and influences carbohydrate and lipid metabolism. Some of these effects are brought about indirectly via somatomedin induction in hepatocytes, others by acting directly on the target cells. In either case, GH first binds to specific receptors on cells to trigger a sequence of biochemical events culminating in a biological response. Recently much has been learnt about the molecular structure of GH receptor, its binding to ligand, and the ensuing signal transduction events.

The insulin receptor C-terminus is involved in regulation of the receptor kinase activity

During the insulin receptor activation process, ligand binding and autophosphorylation induce two distinct conformational changes in the C-terminal domain of the receptor beta-subunit. To analyze the role of this domain and the involvement of the C-terminal autophosphorylation sites (Tyr1316 and Tyr1322) in receptor activation, we used (i) antipeptide antibodies against three different C-terminal sequences (1270-1281, 1294-1317, and 1309-1326) and (ii) an insulin receptor mutant (Y/F2) where Tyr1316 and Tyr1322 have been replaced by Phe. We show that the autophosphorylation-induced C-terminal conformational change is preserved in the Y/F2 receptor, indicating that this change is not induced by phosphorylation of the C-terminal sites but most likely by phosphorylation of the major sites in the kinase domain (Tyr1146, Tyr1150, and Tyr1151). Binding of antipeptide antibodies to the C-terminal domain modulated (activated or inhibited) both mutant and wild-type receptor-mediated phosphorylation of poly(Glu/Tyr). In contrast to the wild-type receptor, Y/F2 exhibited the same C-terminal configuration before and after insulin binding, evidencing that mutation of Tyr1316 and Tyr1322 introduced conformational changes in the C-terminus. Finally, the mutant receptor was 2-fold more active than the wild-type receptor for poly(Glu/Tyr) phosphorylation. In conclusion, the whole C-terminal region of the insulin receptor beta-subunit is likely to exert a regulatory influence on the receptor kinase activity. Perturbations of the C-terminal region, such as binding of antipeptides or mutation of Tyr1316 and Tyr1322, provoke alterations at the receptor kinase level, leading to activation or inhibition of the enzymic activity.

Antibodies against highly conserved sites in the epidermal growth factor receptor tyrosine kinase domain as probes for structure and function

We generated anti-peptide antibodies against four highly conserved sequences in the kinase domain and against two nonconserved sequences surrounding autophosphorylation sites in the carboxyl-terminal domain of the epidermal growth factor receptor (EGFR). These antibodies were used to examine topology and function in catalysis of specific sequences. Two of the highly conserved sites, HRD (residues 811-818) and DFG (residues 827-838), appeared to participate in catalysis since alpha HRD and alpha DFG but not the other anti-peptide antibodies inhibited EGFR kinase activity. Examination of the topology of the six sites revealed that epitopes in all except the HRD site appeared to be exposed to antibody binding in the EGFR. The conditions that caused increased exposure of the HRD site to interaction with antibody included autophosphorylation, addition of the ionic detergent sodium dodecyl sulfate (SDS), and elevation in temperature from 4 to 34 degrees C.

Effect of suramin on the mitogenic response of the human prostate carcinoma cell line PC-3

BACKGROUND

Suramin is an anthelmintic drug that recently has been shown to have clinical efficacy in the treatment of patients with some advanced malignancies, including prostate carcinoma. The current study was done to assess the effect of suramin at clinically relevant doses on the growth in culture of a human prostatic carcinoma cell line, PC-3.

METHODS

The antiproliferative effect of varying doses of suramin on PC-3 was assessed. Northern blot analysis was done to assess the potential changes in genetic expression at different times after the initiation of treatment.

RESULTS

Suramin inhibited the proliferation of PC-3 in a dose-related manner (concentration range, 30-300 microM). Compared with fetal calf serum 2%, when the cells were grown in fetal calf serum 10%, higher concentrations of suramin were required to inhibit tritiated thymidine incorporation. When grown in RPMI without supplement, the PC-3 cell number remained the same. When 100 microM suramin was included, the cell number decreased. By contrast, when RPMI was supplemented with insulin, transferrin, and selenium (ITS), PC-3 grew well. The inhibition of the proliferation of PC-3 cells by suramin was decreased when ITS were added to the cells grown under serum-free conditions.

CONCLUSIONS

These results were consistent with the hypothesis that in vitro inhibition of the growth of PC-3 cells by suramin may be caused, at least in part, by the growth factor antagonism of the drug. In fetal calf serum 2%, the suramin inhibition was reversible after 3 days. If the treatment was extended to 6 days, however, the PC-3 cells were unable to recover. Cell-cycle analysis revealed that, after 6 days of treatment, there was a decrease in the number of cells in G1 that corresponded with an increased number of cells in G2/M. This suggested that critical antineoplastic events were occurring during this time. Molecular analysis did not detect any altered expression of actin, transforming growth factors alpha or beta, or histone compared with untreated control samples.

Disulfide-linked and noncovalent dimers of p185HER-2 in human breast carcinoma cells

Enhanced levels of disulfide-linked dimers of the neu oncogene product have been suggested to be associated with the transformed state [Weiner DB, Liu J, Cohen JA, Williams WV, Greene MI: Nature 338:230-231, (1989)]. We, therefore, investigated the properties of the dimeric forms of p185HER-2/neu from the human breast carcinoma cell line, SK-BR-3. We found disulfide-linked dimers as well as noncovalently associated dimers that were detected by cross-linking with bis(sulfosuccinimidyl) suberate (BS3). However, the disulfide-linked dimers did not exist in intact cells, since they were eliminated when the cells were lysed in the presence of the alkylating agent, sodium iodoacetate. Moreover, the disulfide-linked dimeric molecules were not the activated form of p185HER-2 since they incorporated about the same level of phosphate in an in vitro kinase reaction as the monomeric molecules. In contrast, the noncovalent dimers appeared to be present on the surface of intact cells and were phosphorylated at levels at least tenfold higher than monomers in an in vitro kinase reaction.

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.

Calcium channels in PDGF-stimulated A172 cells open after intracellular calcium release and are not voltage-dependent

Using laser image cytometry and Indo-1 fluorescence, we investigated the intracellular free Ca2+ concentration ([Ca2+]i) of confluent A172 human glioblastoma cells stimulated by the BB homodimer of platelet-derived growth factor (PDGF-BB). The shape of the calcium transients and the delay time between stimulation and the beginning of the transient varied considerably. The percentage of responsive cells, the peak [Ca2+]i and the duration of the response were directly related to PDGF-BB dose, while the delay time was inversely related; the maximal response occurred at a PDGF-BB concentration of 20 ng/ml. Studies with EGTA and inorganic calcium-channel blockers (Ni2+, La3+) showed that the increase of [Ca2+]i resulted from initial release of intracellular stores and subsequent calcium influx across the plasma membrane. Opening of calcium channels in the plasma membrane, monitored directly by studying Mn2+ quenching of Indo-1 fluorescence, was stimulated by PDGF-BB and blocked by La3+; the opening occurred 55 +/- 10 s after the initial increase in [Ca2+]i. Therefore, in these tumor cells, intracellular release always occurs before channel opening in the plasma membrane. Depolarization of cells with high extracellular [K+] did not generally induce calcium transients but did decrease calcium influx. L-type calcium-channel blockers (verapamil, nifedipine, and diltiazem) had little or no effect on the calcium influx induced by PDGF-BB. These results indicate that PDGF-BB induces calcium influx by a mechanism independent of voltage-sensitive calcium channels in A172 human glioblastoma cells.

Inhibition of PDGF beta receptor signal transduction by coexpression of a truncated receptor

A mutated form of the platelet-derived growth factor (PDGF) beta receptor lacking most of its cytoplasmic domain was tested for its ability to block wild-type PDGF receptor function. PDGF induced the formation of complexes consisting of wild-type and truncated receptors. Such complexes were defective in autophosphorylation. When truncated receptors were expressed in excess compared to wild-type receptors, stimulation by PDGF of receptor autophosphorylation, association of phosphatidylinositol-3 kinase with the receptor, and calcium mobilization were blocked. Thus, a truncated receptor can inactivate wild-type receptor function by forming ligand-dependent receptor complexes (probably heterodimers) that are incapable of mediating the early steps of signal transduction.

cDNA cloning of a novel 85 kd protein that has SH2 domains and regulates binding of PI3-kinase to the PDGF beta-receptor

Using immobilized PDGF receptor as an affinity reagent, we purified an 85 kd protein (p85) from cell lysates and we cloned its cDNA. The protein contains an SH3 domain and two SH2 domains that are homologous to domains found in several receptor-associated enzymes. Recombinant p85 overexpressed in mammalian cells inhibited the binding of endogenous p85 and a 110 kd protein to the receptor and also blocked the association of PI3-kinase activity with the receptor. Experiments with receptor mutants and with short peptides derived from the kinase insert region of the PDGF receptor showed that the recombinant p85 binds to a well-defined phosphotyrosine-containing sequence of the receptor. p85 appears to be the subunit of PI3-kinase that links the enzyme to the ligand-activated receptor.

Attachment of A172 human glioblastoma cells affects calcium signalling: a comparison of image cytometry, flow cytometry, and spectrofluorometry

The intracellular free calcium concentration ([Ca2+]i) of indo-1 loaded A172 human glioblastoma cells stimulated by platelet-derived growth factor (PDGF) was studied in cell suspensions by flow cytometry and spectrofluorometry and in confluent monolayers by laser image cytometry and spectrofluorometry. With all three techniques, the percentage of responsive cells, peak [Ca2+]i, and the duration of response were directly related, and the delay time was inversely related to PDGF dose. The maximum response occurred at a PDGF concentration of about 20 ng/ml. Basal and peak [Ca2+]i did not differ significantly from method to method even though different calibration procedures were used. Cells in suspension monitored by both spectrofluorometry and flow cytometry displayed significantly shorter calcium responses than attached cells. This did not appear to be a direct effect of trypsinization. Spectral analysis of indo-1 in cytoplasm, 40% glycerol, and aqueous solutions showed significant differences in the isosbestic point and quantum efficiency. Calibration of [Ca2+]i with spectrofluorometry is more accurate using the ratio of fluorescence intensities than the fluorescence intensities measured at either 405 or 485 nm.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Association between the PDGF receptor and members of the src family of tyrosine kinases

We have examined the interaction between the platelet-derived growth factor (PDGF) receptor and three src family tyrosine kinases, pp60c-src, p59fyn, and pp62c-yes. The kinase activities of all three enzymes were elevated after PDGF stimulation of quiescent fibroblasts, coincident with association of the src family kinases with the PDGF receptor and other proteins. The presence of a protein of 81-85 kd in these complexes correlated with the detection of phosphatidylinositol (PI) kinase activity (previously described to associate with both the PDGF receptor and pp60c-src-middle T antigen). These results suggest that the physiological response to PDGF involves interaction of the receptor not only with serine/threonine and lipid kinases and a phospholipase, but also with other tyrosine kinases.

PDGF beta-receptor stimulates tyrosine phosphorylation of GAP and association of GAP with a signaling complex

Platelet-derived growth factor (PDGF) stimulated the tyrosine phosphorylation of the GTPase activating protein (GAP) in 3T3 cells and in CHO cells expressing wild-type PDGF receptors, but not in several CHO cell lines expressing mutant receptors defective in transmitting mitogenic signals. Following PDGF treatment of cells, GAP physically associated with the PDGF receptor and with Raf-1, phospholipase c-gamma, and PI-3 kinase, suggesting that PDGF induced the formation of complexes of signaling molecules. The association of GAP with the PDGF receptor and the phosphorylation of GAP with the PDGF receptor and the phosphorylation of GAP were reconstituted in vitro using purified protein and in insect cells expressing murine PDGF receptor and human GAP. However, in cells transformed by activated c-Ha-ras, which are defective in certain responses to PDGF, GAP failed to associate with the PDGF receptor or increase its phosphotyrosine content in response to PDGF. The association of GAP with ligand-activated PDGF receptors may directly link PDGF and ras signaling pathways.

PECAM-1 (CD31) cloning and relation to adhesion molecules of the immunoglobulin gene superfamily

An antibody to a platelet integral membrane glycoprotein was found to cross-react with the previously identified CD31 myelomonocytic differentiation antigen and with hec7, an endothelial cell protein that is enriched at intercellular junctions. This antibody identified a complementary DNA clone from an endothelial cell library. The 130-kilodalton translated sequence contained six extracellular immunoglobulin (Ig)-like domains and was most similar to the cell adhesion molecule (CAM) subgroup of the Ig superfamily. This is the only known member of the CAM family on platelets. Its cell surface distribution suggests participation in cellular recognition events.

Direct activation of the serine/threonine kinase activity of Raf-1 through tyrosine phosphorylation by the PDGF beta-receptor

We have examined the interaction between the serine/threonine kinase proto-oncogene product Raf-1 and the tyrosine kinase PDGF beta-receptor. Raf-1 tyrosine phosphorylation and kinase activity were increased by PDGF treatment of 3T3 cells or CHO cells expressing wild-type PDGF receptors but not mutant receptors defective in transmitting mitogenic signals, suggesting that the increase in Raf-1 kinase activity is a significant event in PDGF-induced mitogenesis. Concurrent with these increases, Raf-1 associated with the ligand-activated PDGF receptor. Furthermore, both mammalian Raf-1 and Raf-1 expressed using a recombinant baculoviral vector, associated in vitro with baculoviral-expressed PDGF receptor. This association was markedly decreased by prior phosphatase treatment of the receptor. Following incubation of partially purified baculoviral-expressed PDGF receptor with partially purified Raf-1, Raf-1 became phosphorylated on tyrosine and its serine/threonine kinase activity increased 4- to 6-fold. This is the first demonstration of the direct modulation of a protein activity by a growth factor receptor tyrosine kinase.

Phospholipase C-gamma is a substrate for the PDGF and EGF receptor protein-tyrosine kinases in vivo and in vitro

Phospholipase C-gamma (PLC-gamma) was rapidly phosphorylated on tyrosines and serines following PDGF and EGF treatment of quiescent 3T3 mouse fibroblasts and A431 human epidermoid cells, respectively, PDGF treatment increased PLC-gamma phosphorylation within 30 sec. This lasted for up to 1 hr, and occurred at high stoichiometry. Continuous receptor occupancy was required to maintain this phosphorylation. Three major sites of tyrosine phosphorylation were detected in PLC-gamma, two of which were phosphorylated in EGF-treated A431 cells. Under certain conditions PDGF receptor coimmunoprecipitated with PLC-gamma, suggesting that PDGF receptor can phosphorylate PLC-gamma directly. Indeed, purified PDGF or EGF receptor phosphorylated purified PLC-gamma on tyrosines identical to those phosphorylated in vivo. Tyrosine phosphorylation of PLC-gamma was not induced by bombesin, TPA, or insulin. Stimulation of PLC-gamma tyrosine phosphorylation and the reported ability of PDGF and EGF to induce phosphatidylinositol turnover in different cells were strongly correlated. We propose that tyrosine phosphorylation of PLC-gamma by PDGF and EGF receptors leads to its activation, and a consequent increase in phosphatidylinositol turnover.

Signal transduction by the platelet-derived growth factor receptor

When platelet-derived growth factor (PDGF) binds to its receptor on a quiescent fibroblast or smooth muscle cell, it stimulates a remarkably diverse group of biochemical responses, including changes in ion fluxes, activation of several kinases, alterations in cell shape, increased transcription of a number of genes, and stimulation of enzymes that regulate phospholipid metabolism. These and other reactions culminate, hours later, in DNA replication and cell division. How does the receptor for PDGF recognize and bind its specific ligand and then transduce this signal across the cell membrane via a single membrane-spanning region? Which of the immediate cellular responses are directly involved in the biochemical pathways that lead to DNA synthesis? How does the PDGF receptor trigger a diverse group of responses? Recent studies of the PDGF receptor have provided insight into these issues.