Platelet-derived growth factor (PDGF)-dependent association of phospholipase C-gamma with the PDGF receptor signaling complex

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Solution structure of tensin2 SH2 domain and its phosphotyrosine-independent interaction with DLC-1

Background

Src homology 2 (SH2) domain is a conserved module involved in various biological processes. Tensin family member was reported to be involved in tumor suppression by interacting with DLC-1 (deleted-in-liver-cancer-1) via its SH2 domain. We explore here the important questions that what the structure of tensin2 SH2 domain is, and how it binds to DLC-1, which might reveal a novel binding mode.

Principal Findings

Tensin2 SH2 domain adopts a conserved SH2 fold that mainly consists of five β-strands flanked by two α-helices. Most SH2 domains recognize phosphorylated ligands specifically. However, tensin2 SH2 domain was identified to interact with nonphosphorylated ligand (DLC-1) as well as phosphorylated ligand.

Conclusions

We determined the solution structure of tensin2 SH2 domain using NMR spectroscopy, and revealed the interactions between tensin2 SH2 domain and its ligands in a phosphotyrosine-independent manner.

Identification of Phospholipase C-γ1 as a Mitogen-activated Protein Kinase Substrate

The discovery of sequence motifs that mediate protein-protein interactions, coupled with the availability of protein amino acid sequence data, allows for the identification of putative protein binding pairs. The present studies were based on our identification of an amino acid sequence in phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) that fits the consensus sequence for a mitogen-activated protein kinase (MAPK) binding site, termed the D-domain. Extracellular signal-regulated kinase 2 (ERK2), an MAPK, and phospho-ERK2 were bound by an immobilized peptide sequence containing the identified PLC-gamma1 D-domain. Furthermore, a peptide containing the PLC-gamma1 D-domain was able to competitively inhibit the in vitro phosphorylation of recombinant PLC-gamma1 by recombinant phospho-ERK2, whereas a control peptide derived from a distant region of PLC-gamma1 was ineffective. Similarly, the peptide containing the PLC-gamma1 D-domain, but not the control peptide, competitively inhibited the in vitro phosphorylation of Elk-1 and c-Jun catalyzed by recombinant phospho-ERK2 and phospho-c-Jun N-terminal kinase 3 (phospho-JNK3), another type of MAPK, respectively. Incubation of anti-PLC-gamma1 immunocomplexes isolated from rat brain with recombinant phospho-ERK2 opposed the increase in PLC-gamma1-catalyzed hydrolysis of phosphatidylinositol 4,5-P(2) (PtdIns(4,5)P(2)), which was produced by a tyrosine kinase associated with the immunocomplexes, whereas in vitro phosphorylation of recombinant PLC-gamma1 by recombinant phospho-ERK2 did not alter PLC-gamma1-catalyzed PtdIns(4,5)P(2) hydrolysis. These studies have uncovered a previously unidentified mechanism for the integration of PLC-gamma1- and ERK2-dependent signaling.

Additive effects of PDGF receptor beta signaling pathways in vascular smooth muscle cell development

The platelet-derived growth factor β receptor (PDGFRβ) is known to activate many molecules involved in signal transduction and has been a paradigm for receptor tyrosine kinase signaling for many years. We have sought to determine the role of individual signaling components downstream of this receptor in vivo by analyzing an allelic series of tyrosine–phenylalanine mutations that prevent binding of specific signal transduction components. Here we show that the incidence of vascular smooth muscle cells/pericytes (v/p), a PDGFRβ-dependent cell type, can be correlated to the amount of receptor expressed and the number of activated signal transduction pathways. A decrease in either receptor expression levels or disruption of multiple downstream signaling pathways lead to a significant reduction in v/p. Conversely, loss of RasGAP binding leads to an increase in this same cell population, implicating a potential role for this effector in attenuating the PDGFRβ signal. The combined in vivo and biochemical data suggest that the summation of pathways associated with the PDGFRβ signal transduction determines the expansion of developing v/p cells.

Using both in vivo and biochemical approaches, the summation of pathways associated with the PDGFRβ signal transduction is shown to determine the expansion of a specific PDGFRβ-dependent cell type

A chemical genetic screen for direct v-Src substrates reveals ordered assembly of a retrograde signaling pathway

Using an ATP analog that is a specific substrate for an analog-specific allele of v-Src, we identified several novel cytoskeletal substrates that control actin assembly processes. A screen for less abundant v-Src substrates revealed the scaffolding protein Dok-1 as a direct substrate of v-Src. Further studies suggest that v-Src phosphorylation sites on Dok-1 are critical for its binding to RasGAP and Csk, negative regulators of Src signaling. This results in the downregulation of growth-promoting signals of the Src family kinases and the Ras pathway. Identification of the direct substrates of v-Src leads to a model for the precise order of assembly of a retrograde signaling pathway in v-Src-transformed cells and has provided new insight into the balance between those signals that promote cell transformation mediated by v-Src catalyzed tyrosine phosphorylation and those that inhibit it.

Calcium and growth responses of hyperresponsive airway smooth muscle to different isoforms of platelet-derived growth factor (PDGF)

Airway smooth muscle (ASM) mass is likely to be an important determinant of airway responsiveness. Highly inbred Fisher rats model innate hyperresponsiveness, and also have more ASM in vivo than control Lewis rats. Platelet derived growth factor (PDGF) is an important endogenous growth factor for ASM, and partially purified PDGF-AB causes enhanced growth of Fisher rat ASM cells, compared to Lewis cells. The aim of the present study was to determine the mitogenic effects of all three recombinant PDGF isoforms on ASM cells, and investigate the mechanisms of enhanced Fisher ASM growth responses. The potential mechanisms assessed include PDGF receptor expression and activation (tyrosine phoshorylation), and intracellular calcium (Ca2+) responses to PDGF isoforms. Fisher ASM cells had a greater mitogenic response to PDGF-AB and -AA, and a greater Ca2+ response to -BB than Lewis ASM cells. A Ca2+ response was not necessary for a mitogenic response, and the effects of PDGF isoforms on Ca2+ were not associated with their effects on growth. Therefore, we suggest that enhanced Fisher mitogenic response to PDGF-AA and -AB is not mediated by differences in Ca2+ signalling. Western analysis of the PDGF receptor indicated a similar expression of β-PDGF receptor in ASM cells from the two rat strains, but a greater expression of α-PDGF receptor in Fisher cells; however, phosphorylation of the PDGF receptor following growth stimulation did not differ between strains. This suggests a role for post-receptor signals, in addition to enhanced receptor expression, in the enhanced growth response of Fisher ASM cells to PDGF-AA and -AB.Key words: PDGF receptors, tyrosine phosphorylation, intracellular calcium, proliferation, airway smooth muscle cells.

Molecular mechanism of the impairment in activation signal transduction in CD4(+) T cells from old mice

It is well known that IL-2 production of CD4(+) T cells from old mice (old T cells) is impaired. In this study, we have examined TCR complex zeta chain expression of old T cells and their TCR downstream signal transduction pathways stimulated with anti-CD3. Activation of protein tyrosine kinases, Fyn and ZAP-70, and turnover of inositol phosphates stimulated with anti-CD3 were severely impaired in old T cells, although levels of these proteins were comparable to those in young T cells. Increase in intracellular Ca2+ concentration in old T cells was also impaired. Old T cells starting the Ca(2+) oscillation by the anti-CD3 stimulation were severely decreased in number and the oscillation waves were broader in shape. T cells with zeta-FcvarepsilonRgamma heterodimer in the TCR-CD3 complex were increased in proportion in old T cells with a concomitant decrease in the T cells with zeta-zeta homodimer. The density of the TCR-CD3 complex on old T cells was confirmed to be comparable to that on young T cells. The impairment in TCR downstream signal transduction pathways and the increase in zeta-FcvarepsilonRgamma heterodimer in the TCR-CD3 complex were confirmed to be the situation in Th1 clones established from old mice. These results indicate that old T cells are impaired in response to TCR stimulation, because T cells with the TCR-CD3 complex containing the zeta-FcvarepsilonRgamma heterodimer are increased in proportion in old T cells.

p130(Cas), an assembling molecule of actin filaments, promotes cell movement, cell migration, and cell spreading in fibroblasts

p130(Cas) (Cas) is an adaptor molecule which becomes tyrosine phosphorylated by v-Src- or v-Crk-triggered transformation and several physiological stimuli, such as cell attachment to fibronectin. We previously generated mice lacking Cas and demonstrated that Cas functions as an assembling molecule of actin filaments. To further explore Cas role in cellular function, we established Cas-deficient and Cas-re-expressing fibroblasts and compared their behaviors in response to several biological stimuli. We found that Cas-deficient fibroblasts showed significant defects in cell movement after mechanical wounding and in cell migration toward fibronectin as compared with Cas-re-expressing cells. In addition, when plated on fibronectin-coated dishes, Cas-deficient cells exhibited a significant delay in cell spreading as compared with Cas-re-expressing cells albeit that protein-tyrosine phosphorylation was similarly induced. These results demonstrated that Cas functions as a molecule promoting cell movement, cell migration, and cell spreading and suggest that Cas would be implicated in various physiological and pathological processes, such as would healing, chemotaxis, and tumor invasion.

Heart-specific activation of LTK results in cardiac hypertrophy, cardiomyocyte degeneration and gene reprogramming in transgenic mice

Leukocyte tyrosine kinase (LTK) is a receptor-type protein tyrosine kinase belonging to the insulin receptor superfamily. To elucidate its biological role, we generated transgenic mice expressing LTK under the control of cytomegarovirus enhancer and beta-actin promoter. The transgenic mice exhibited growth retardation and most of the transgenic mice died within several months after birth. Interestingly, although LTK was expressed in several major organs, the activation (tyrosine-phosphorylation, kinase activity, and multimerization) of LTK was observed selectively in the heart, where LTK was localized on intracellular membrane, presumably on endoplasmic reticulum. Echocardiography showed that the transgenic heart underwent severe concentric hypertrophy, which resulted in reduced cardiac output, low blood pressure, and increased heart rate. Histological examination of the heart exhibited focal degeneration of cardiomyocytes. These histological changes were considered to be due to apoptosis, based on the finding that the sarcolemmas of the degenerative cardiomyocytes were well preserved. In addition, expression of fetal genes, such as atrial natriuretic peptide and skeletal alpha-actin, was markedly induced in the transgenic heart. These results indicate that a certain tissue-specific mechanism of activating LTK exists in the heart and that the activated LTK resulted in cardiac hypertrophy, cardiomyocyte degeneration and gene reprogramming. These findings will provide novel insights into the activating mechanism and biological role of LTK in vivo.

APS, an adaptor protein containing PH and SH2 domains, is associated with the PDGF receptor and c-Cbl and inhibits PDGF-induced mitogenesis

Previously we cloned a novel adaptor protein, APS (adaptor molecules containing PH and SH2 domains) which was tyrosine phosphorylated in response to c-kit or B cell receptor stimulation. Here we report that APS was expressed in some human osteosarcoma cell lines, markedly so in SaOS-2 cells, and was tyrosine-phosphorylated in response to several growth factors, including platelet derived growth factor (PDGF), insulin-like growth factor (IGF), and granulocyte-macrophage colony stimulating factor (GM-CSF). Ectopic expression of the wild type APS, but not C-terminal truncated APS, in NIH3T3 fibroblasts suppressed PDGF-induced MAP kinase (Erk2) activation, c-fos and c-myc induction as well as cell proliferation. In vitro binding experiments suggest that APS bound to the beta type PDGF receptor, mainly via phosphotyrosine 1021 (pY1021). Indeed, tyrosine phosphorylation of PLC-gamma, which has been demonstrated to bind to pY1021, but not that of PI3 kinase and associated proteins, was reduced in APS transformants. PDGF induced phosphorylation of the tyrosine residue of APS close to the C-terminal end. In vitro and in vivo binding experiments indicate that the tyrosine phosphorylated C-terminal region of APS bound to c-Cbl, which has been shown to be a negative regulator of tyrosine kinases. Since coexpression of c-Cbl with wild type APS, but not C-terminal truncated APS, synergistically inhibited PDGF-induced c-fos promoter activation, c-Cbl could be a mechanism of inhibitory action of APS on PDGF receptor signaling.

Growth inhibition, enhancement of intercellular adhesion, and increased expression of carcinoembryonic antigen by overexpression of phosphoinositides-specific phospholipase C beta 1 in LS174T human colon adenocarcinoma cell line

By using a retrovirus-derived system we generated derivatives of the human colon adenocarcinoma cell line LS174T (ATCC CL 188) that stably overexpress a full-length cDNA encoding the beta 1 isoform of bovine phosphoinositides-specific phospholipase C (PI-PLC). This was confirmed by the elevated levels of catalytic activity to release phosphoinositides from phosphatidylinositol (PI-PLC) or phosphatidylinositol-bis-phosphate (PIP2-PLC), and the enhanced expressions of messenger RNA and protein. PI-PLC beta 1 overexpresser clones grew to form cell clumps floating in liquid medium, whereas the pMV7-introduced control clones displayed morphologic characteristics that were very similar to those of the parent LS174T cell line. Three individual PI-PLC beta 1 overexpresser cell lines displayed increased doubling time (18.0 h, 21.5 h, and 23.8 h) when compared with 4 individual pMV7-introduced control cell lines (13.1 h, 10.7 h, 12.9 h, and 9.3 h). Anchorage-independent growth ability in soft agar medium was dramatically suppressed by overexpression of PLC beta 1, and the ability of PLC-overproducer clones to form aggregates when cultured in liquid medium was dramatically enhanced when compared with that of pMV7-introduced control clones. Tumorigenicity of PLC beta 1-overproducers was much weaker than that of vector-transduced control clones. The spontaneous release of carcinoembryonic antigen from PLC beta 1-overproducer clones was much higher than that from pMV7 control clones. The ability of PLC beta 1-overproducer clones to form aggregates during suspension culture was much stronger than that of the control clones. These results provide the first evidence that elevated levels of endogenous PI-PLC beta 1 suppress tumor cell growth, but enhance the ability to form cell aggregates and to release carcinoembryonic antigen, an intercellular adhesion molecule.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Cardiovascular anomaly, impaired actin bundling and resistance to Src-induced transformation in mice lacking p130Cas

p130Cas (Cas), the protein encoded by the Crkas gene (also known as Cas), is an adaptor molecule with a unique structure that contains a Src homology (SH)-3 domain followed by multiple YXXP motifs and a proline-rich region. Cas was originally cloned as a highly tyrosine-phosphorylated protein in cells transformed by v-Src (refs 2,3) or v-Crk (ref. 4) and has subsequently been implicated in a variety of biological processes including cell adhesion, cell migration, growth factor stimulation, cytokine receptor engagement and bacterial infection. To determine its role in vivo, we generated mice lacking Cas. Cas-deficient embryos died in utero showing marked systemic congestion and growth retardation. Histologically, the heart was poorly developed and blood vessels were prominently dilated. Electron microscopic analysis of the heart revealed disorganization of myofibrils and disruption of Z-disks. In addition, actin stress fiber formation was severely impaired in Cas-deficient primary fibroblasts. Moreover, expression of activated Src in Cas-deficient primary fibroblasts did not induce a fully transformed phenotype, possibly owing to insufficient accumulation of actin cytoskeleton in podosomes. These findings have defined Cas function in cardiovascular development, actin filament assembly and Src-induced transformation.

Full activation of the platelet-derived growth factor beta-receptor kinase involves multiple events

Activation of receptor tyrosine kinases is thought to involve ligand-induced dimerization, which promotes receptor transphosphorylation and thereby increases the receptor's phosphotransferase activity. We used two platelet-derived growth factor beta-receptor (beta-PDGFR) mutants to identify events that are required for full engagement (autophosphorylation and activation of the kinase activity) of the beta-PDGFR kinase. The F79/81 receptor (Tyr to Phe substitution at 579 and 581 in the juxtamembrane domain of the receptor) was capable of only very modest ligand-dependent autophosphorylation and also failed to associate with numerous SH2 domain-containing proteins. Furthermore, stimulation with platelet-derived growth factor (PDGF) did not increase the kinase activity of the F79/81 mutant toward exogenous substrates. However, the F79/81 receptor had basal kinase activity and could be artificially stimulated by incubation with ATP. Because the low kinase activity of the F857 mutant (Tyr to Phe substitution at 857 in the putative activation loop) could not be increased by incubation with ATP, failure to phosphorylate Tyr-857 may be the reason why the F79/81 mutant has low kinase activity. Surprisingly, the F857 mutant underwent efficient PDGF-dependent autophosphorylation. Thus the PDGF-dependent increase in the kinase activity of the receptor is not required for autophosphorylation. We conclude that full activation of the beta-PDGFR kinase requires at least two, apparently distinct events.

Erythropoietin receptor-operated Ca2+ channels: activation by phospholipase C-gamma 1

Erythropoietin (EPO) increases Ca2+ influx in vascular smooth muscle cells and acts both as a direct vasoconstrictor and vascular growth factor (that is, angiogenesis). However, the mechanism by which EPO promotes extracellular Ca2+ entry in contractile cells has not been elucidated. In hematopoietic cells, EPO induces tyrosine kinase (TK)-dependent activation of phospholipase C (PLC)-gamma 1 and Ca2+ influx via a voltage-independent Ca2+ conductance. In contractile mesangial cells, we have recently characterized a voltage-independent, 1 pS Ca2+ channel that is dependent on both TK and PLC-gamma 1 activity. Therefore, we examined cultured rat glomerular mesangial cells after timed exposure to recombinant human EPO (20 U/ml). Erythropoietin increased the tyrosine phosphorylation of PLC-gamma 1, promoted membrane complex formation between PLC-gamma 1 and the EPO receptor itself, and raised the levels of intracellular inositol 1,4,5-trisphosphate and intracellular Ca2+. Consistent with our previous studies, 1 pS Ca2+ channel activity was extremely low under basal, unstimulated conditions in cell-attached patches, but was dramatically increased when EPO was present in the patch pipette. Tyrosine kinase inhibition with 100 micron genistein or 1 micron PP1 (Src; selective tyrosine kinase inhibitor) prevented all of these EPO-induced responses. We conclude that: (1) EPO-induced stimulation of 1 pS Ca2+ channels is mediated via a cytosolic Src TK in glomerular mesangial cells. (2) Stimulation of this Ca2(+)-activated, Ca2(+)-permeable channel is dependent on the tyrosine phosphorylation/activation of PLC-gamma 1. (3) This cascade provides a possible mechanism for the vasoconstriction and hypertension observed with clinical EPO use for the treatment of chronic anemias.

Angiotensin II stimulates tyrosine phosphorylation of phospholipase C-gamma-associated proteins. Characterization of a c-Src-dependent 97-kD protein in vascular smooth muscle cells

Stimulation of phospholipase C-gamma (PLC-gamma) is a critical event in angiotensin II (Ang II) signal transduction. We have previously shown that in rat aortic smooth muscle (RASM) cells Ang II stimulates tyrosine phosphorylation of PLC-gamma via activation of c-Src. Because we failed to demonstrate a direct association between c-Src and PLC-gamma, we hypothesized that a linker protein mediates the interaction between these molecules. To identify PLC-gamma-associated proteins, RASM cells were labeled with [32P]orthophosphate and stimulated with 100 nmol/L Ang II for 5 minutes. PLC-gamma was immunoprecipitated, and associated proteins were characterized by autoradiography and Western blotting with anti-phosphotyrosine antibodies. Ang II stimulated the phosphorylation of 47-, 60-, 84-, and 97-kD PLC-gamma-associated proteins. Because Ang II increased tyrosine phosphorylation of only the 97-kD protein, we characterized p97 further. An important role for Src in tyrosine phosphorylation of p97 was suggested by findings that p97 phosphorylation was inhibited by the selective Src-family kinase inhibitor CP-118,556, diminished in mouse aortic smooth muscle (MASM) cells from c-Src knockout mice compared with wild-type MASM cells, and increased in v-Src-transformed NIH-3T3 cells compared with wild-type NIH-3T3 cells. These studies are the first to define a PLC-gamma-associated protein that may be required for Ang II-mediated signal transduction.

Dissociation of tyrosine phosphorylation and activation of phosphoinositide phospholipase C induced by the protein kinase C inhibitor Ro-31-8220 in Swiss 3T3 cells treated with platelet-derived growth factor

Platelet-derived growth factor (PDGF) stimulates the hydrolysis of phosphatidylinositol 4,5-bisphosphate (Ptd InsP2) via phospholipase C-gamma1 (PLC-gamma1) in Swiss 3T3 cells. Treatment of cells with the protein kinase C (PKC) inhibitor Ro-31-8220 greatly decreased PDGF-induced tyrosine phosphorylation of PLC-gamma1, but paradoxically enhanced the production of inositol phosphates (InsPs). The inhibitor also caused an increase of PDGF receptor tyrosine phosphorylation at later times. The changes in phosphorylation of the receptor were correlated with alterations in PLC-gamma1 translocation to the particulate fraction. Thus, although activation of PLC-gamma1 was associated with phosphorylation of the receptor and translocation of the enzyme to the particulate fraction, it was dissociated from its tyrosine phosphorylation. A non-receptor-associated, cytosolic tyrosine kinase also was found to phosphorylate PLC-gamma1 in a PDGF-dependent manner, but was not inhibited by Ro-31-8220 in vitro. PKC depletion by phorbol ester treatment decreased the tyrosine phosphorylation of PLC-gamma1 induced by PDGF and slowed the translocation of PLC-gamma1, but Ro-31-8220 produced further effects. The effect of Ro-31-8220 to enhance the production of InsPs could not be attributed to inhibition of PKC since InsPs production with PDGF was decreased in PKC-depleted cells and a stimulatory effect of the inhibitor was still evident. Interestingly, Ro-31-8220 decreased the radioactivity in phosphatidylinositol and increased that in phosphatidylinositol 4-phosphate and PtdInsP2 in cells labeled with myo[3H]inositol. The increased synthesis of PtdInsP2 could contribute to the increased production of InsPs induced by Ro-31-8220. In summary, these results support the conclusion that the activation of PLC-gamma1 in response to PDGF requires autophosphorylation of the receptor and membrane association of PLC-gamma1, but not phosphorylation of the enzyme. Furthermore, the effects of Ro-31-8220 on the tyrosine phosphorylation and activity of PLC-gamma1, and on PtdInsP2 synthesis cannot be attributed to inhibition of PKC.

Platelet-derived growth factor stimulates sodium-dependent Pi transport in osteoblastic cells via phospholipase Cgamma and phosphatidylinositol 3' -kinase

Inorganic phosphate (Pi) is a major regulator of cell metabolism. The Pi transport activity in the plasma membrane is a main determinant of the intracellular level of this ion. In bone-forming cells, Pi transport is important for the calcification of the bone matrix. In this study, the effect of platelet-derived growth factor (PDGF) on Pi transport activity and the signaling mechanism involved in this cellular response were analyzed. The results indicate that PDGF is a potent and selective stimulator of sodium-dependent Pi transport in the mouse calvaria-derived MC3T3-E1 osteoblast-like cells. The change in Pi transport induced by PDGF-BB was dependent on translational processes and affected the Vmax of the Pi transport system. These observations suggested that enhanced Pi transport activity in response to PDGF resulted from insertion of newly synthesized Pi transporters in the plasma membrane. The role of activation of mitogen activated protein (MAP) kinase, phospholipase C (PLC)gamma or phosphatidylinositol 3-kinase (PI-3-kinase), in mediating this effect of PDGF, was investigated. A selective inhibitor of the PDGF receptor tyrosine kinase activity (CGP 53716) completely blocked PDGF-induced protein tyrosine phosphorylation of several proteins including the PDGF receptor, PLCgamma, MAP kinase, and association of the p85 subunit of PI-3'-kinase. Associated with this effect, the increase in Pi transport induced by PDGF was completely blunted by 5 microM CGP 53716. Inhibition of MAP kinase activity by cAMP agonists did not influence Pi transport stimulation induced by PDGF. However, inhibitors of protein kinase C completely blocked this response. A selective inhibitor of PI-3-kinase, LY294002, also significantly reduced this effect of PDGF. In summary, these results indicate that PDGF is a potent and selective stimulator of Pi transport in osteoblastic cells. The mechanism responsible for this effect is not mediated by MAP kinase but involves tyrosine phosphorylation-dependent activation of PLCgamma and PI-3-kinase.

A mutation at tyrosine 1062 in MEN2A-Ret and MEN2B-Ret impairs their transforming activity and association with shc adaptor proteins

Germ line mutations of the ret proto-oncogene are associated with the development of three dominantly inherited neoplastic disorders, multiple endocrine neoplasia (MEN) 2A, MEN 2B, and familial medullary thyroid carcinoma. It has been demonstrated that the mutations result in constitutive activation of the Ret protein, leading to transformation of NIH 3T3 cells. In the present study we investigated the role of tyrosine residues present in the carboxyl-terminal sequence for the transforming activity of Ret with the MEN 2A or MEN 2B mutation (MEN2A-Ret or MEN2B-Ret). Substitution of phenylalanine for tyrosine 1062 (designated Y1062F) markedly impaired the transforming activity of both MEN2A-Ret and MEN2B-Ret, whereas substitution or deletion for four other tyrosines (codons 981, 1015, 1090, and 1096) did not affect their activity. The Shc adaptor proteins bound to the MEN2A-Ret and MEN2B-Ret proteins and were phosphorylated on tyrosine in the transfectants. The binding of Shc to the Y1062F mutant proteins was reduced by approximately 80%, indicating that tyrosine 1062 is a major binding site for Shc. In addition, phosphopeptide analysis of MEN2A-Ret demonstrated that tyrosine 1062 represents an autophosphorylation site of the mutant Ret proteins.

Direct binding of C-terminal region of p130Cas to SH2 and SH3 domains of Src kinase

p130Cas is a major tyrosine-phosphorylated protein that tightly binds v-Crk in v-crk-transformed cells and v-Src in v-src-transformed cells. The "substrate domain" of p130Cas contains 15 possible Src homology (SH) 2-binding motifs, most of which conform to the binding motif for the Crk SH2 domain. Another region near its C terminus contains possible binding motifs for the Src SH2 domain and proline-rich sequences that are candidates for SH3-binding sites. Using GST fusion proteins, we revealed that both SH2 and SH3 domains of Src bind p130Cas, whereas v-Crk binds p130Cas through its SH2 domain. We located the binding site of p130Cas for the Src SH3 domain at the sequence RPLPSPP in the region near its C terminus. Mutations within this sequence or at Tyr762 of p130Cas caused a significant reduction in the association of p130Cas with Src, and no association was detected when both of them were deleted. The kinase activity in v-Crk-transformed cells was also associated with p130Cas through this region. On the other hand, the deletion of the substrate domain abolished the binding with v-Crk. The association through the C-terminal region of p130Cas with Src kinase may facilitate effective hyperphosphorylation of tyrosine residues in the substrate domain of p130Cas, resulting in the binding of SH2-containing molecules to p130Cas.

Requirement for phosphatidylinositol 3'-kinase activity in platelet-derived growth factor-stimulated tyrosine phosphorylation of p125 focal adhesion kinase and paxillin

The role of phosphatidylinositol 3'-kinase (PI 3'-kinase) activity in platelet-derived growth factor (PDGF)-stimulated tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin has been examined. The tyrosine phosphorylation of p125FAK and paxillin in response to PDGF was markedly inhibited by wortmannin in a dose-dependent manner. PDGF-stimulated PI 3'-kinase activity, membrane ruffle formation, and tyrosine phosphorylation of p125FAK and paxillin were all inhibited by the same low concentrations of wortmannin (>90% inhibition at 40nM). In contrast, tyrosine phosphorylation of p125FAK and paxillin in response to bombesin, endothelin, and phorbol 12,13-dibutyrate was not inhibited by wortmannin in these cells. Furthermore, LY294002, an inhibitor of PI 3'-kinase structurally unrelated to wortmannin, also inhibited PDGF-stimulated p125FAK tyrosine phosphorylation. PDGF was shown to stimulate the tyrosine phosphorylation of p125FAK in porcine aortic endothelial (PAE) cells transfected with the wild type PDGF-beta receptors, but not in PAE cells transfected with PDGF-beta receptors in which the PI 3'-kinase binding sites (Tyr-740/751) were replaced by phenylalanine. PDGF-stimulated, PI 3'-kinase-dependent tyrosine phosphorylation of p125FAK was not inhibited by rapamycin, and thus it was dissociated from the activation of p70 S6 kinase, previously identified as a molecular downstream target of PI 3'-kinase. Thus, we have identified a PI 3'-kinase-dependent signal transduction pathway in the action of PDGF, which leads to the phosphorylation of p125FAK and paxillin.

EGF or PDGF receptors activate atypical PKClambda through phosphatidylinositol 3-kinase

Overexpression of a TPA-insensitive PKC member, an atypical protein kinase C (aPKClambda), results in an enhancement of the transcriptional activation of TPA response element (TRE) in cells stimulated with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). EGF or PDGF also caused a transient increase in the in vivo phosphorylation level and a change in the intracellular localization of aPKClambda from the nucleus to the cytosol, indicating the activation of aPKClambda in response to this growth factor stimulation. These immediate signal-dependent changes in aKPClambda were observed for a PDGF receptor add-back mutant (Y40/51) that possesses only two of the five major autophosphorylation sites and binds PI3-kinase, and were inhibited by wortmannin, an inhibitor of PI3-kinase. Furthermore, an N-terminal fragment of the catalytic subunit of PI3-kinase, p110alpha, inhibited aPKClambda-dependent activation of TRE in Y40/51 cells stimulated with PDGF. Overexpression of p110alpha resulted in an enhancement of TRE expression in response to PDGF and the regulatory domain of aPKClambda inhibited this TRE activation in Y40/51 cells. These results provide the first in vivo evidence supporting the presence of a novel signalling pathway from receptor tyrosine kinases to aPKClambda through PI3-kinase.

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.

A new phospholipase C delta 4 is induced at S-phase of the cell cycle and appears in the nucleus

To discover a new phospholipase C (PLC) related to cell growth, we screened a cDNA library prepared from regenerating rat liver. A novel PLC (PLC delta 4) encoding a polypeptide of 770 amino acids with structural similarity to PLC delta-type isozymes was isolated. PLC delta 4 mRNA is expressed more remarkably in regenerating liver than in normal resting liver. It is also distributed abundantly in tumor cells such as hepatoma and src-transformed cells. Furthermore, its expression can be induced markedly by serum treatment and reaches a maximum at 8 h. Western blot analysis and immunocytochemical staining showed that PLC delta 4 is dominantly present in nucleus. Nuclear PLC delta 4 dramatically increases at the transition from G1- to S-phase, and the high content continues to the end of M-phase. PLC delta 4 almost disappears when cells re-enter the next G1-phase. On the other hand, the contents of PLC beta 1, PLC gamma 1, and PLC delta 1 do not change significantly during the cell cycle. These results suggest that PLC delta 4 is expressed in nucleus in response to mitogenic stimulation and plays important roles in cell growth as one of the early genes expressed during the transition from G1- to S-phase in the cell cycle.

Association of signaling proteins with a nonmitogenic heterodimeric complex composed of epidermal growth factor receptor and kinase-inactive p185c-neu

The functional consequences of heterodimer formation between the epidermal growth factor receptor (EGFr) and the p185c-neu receptor tyrosine kinase include increased mitogenic and transformation potencies. To determine the possible alteration of signal transduction pathways resulting from this heteromeric complex, the capacity of several signaling proteins to associate with the heterodimeric receptors has been assayed. The in vivo interaction with the EGFr/p185c-neu heterodimer of several signal transduction proteins, including phospholipase C-gamma 1 (PLC-gamma 1), the p85 subunit of phosphotidylinositol 3-kinase, the ras GTPase activating protein, SHC, NCK, p72RAF, and the tyrosine phosphatase SHPTP2, was measured by coimmunoprecipitation. The binding of these signaling proteins to a complex composed of EGFr and a kinase-inactive form of p185 (p185K757M) was not impaired, even though the mitogenic and transformation activity of this complex had been abrogated. In addition, the EGF-induced phosphorylation of GAP, p85, and PLC-gamma 1 did not correlate with the dominant-negative action of p185K757M on EGFr function. Thus, substrate association and phosphorylation do not correlate stringently with the mitogenic and transforming activity of this receptor complex, suggesting additional pathways or mechanisms vital to EGFr/p185c-neu heterodimeric signaling.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium butyrate inhibits platelet-derived growth factor-induced proliferation of vascular smooth muscle cells

Sodium butyrate (SB), a naturally occurring short-chain fatty acid, was investigated for its therapeutic value as an antiproliferative agent for vascular smooth muscle cells (SMCs). At 5-mmol/L concentration, SB had no significant effect on rat SMC proliferation. However, at the same concentration, SB inhibited platelet-derived growth factor (PDGF)-AA-, -AB-, and -BB-induced proliferation of SMCs. Exposure of SMCs to PDGF-BB resulted in activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of beta-PDGF-receptor (beta-PDGFR). The activated beta-PDGFR physically associated and phosphorylated signaling molecules such as ras-GTPase activating protein (GAP) and phospholipase C gamma (PLC gamma). SB, in the absence of PDGF-BB, caused neither beta-PDGFR tyrosine phosphorylation nor phosphorylation and association of GAP and PLC gamma with beta-PDGFR. PDGF-BB-enhanced activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of tyrosine residues of beta-PDGFR were unaffected by SB irrespective of whether SMCs were preincubated with SB before exposure to PDGF-BB plus SB or incubated concomitantly with PDGF-BB plus SB. Likewise, phosphorylation and association of GAP and PLC gamma with PDGF-BB-activated beta-PDGFR were unaffected. In addition, SB did not block PDGF-BB-stimulated, PLC gamma-mediated production of inositol triphosphate. Similarly, PDGF-BB-induced beta-PDGFR degradation was unaffected when SMCs were exposed to PDGF-BB plus SB, and SB by itself had no influence on beta-PDGFR degradation. Unlike beta-PDGFR kinase activity, mitogen-activated protein kinase (MAP-kinase) activity was stimulated by SB by about 2.7-fold. Exposure of SMCs to PDGF-BB caused an approximately 11.4-fold increase in MAP-kinase activity and this increase in activity was not significantly affected when cells were coincubated with PDGF-BB and SB (10.3-fold). However, pretreatment of SMCs with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB abolished most of the PDGF-BB-induced MAP-kinase activity (4.6-fold). Transcription of growth response genes such as c-fos, c-jun, and c-myc were induced by PDGF-BB, and their induction was suppressed, particularly c-myc, by incubating SMCs with PDGF-BB plus SB. Similarly, preincubation of cells with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB diminished PDGF-BB-induced transcription of c-fos, c-jun, and c-myc. However, SB by itself had no significant effect on c-fos, c-jun, and c-myc transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunoprecipitation of a phospholipase D activity with antiphosphotyrosine antibodies

When granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated human neutrophils were challenged with the chemotactic factor fMet-Leu-Phe, it was possible to detect a time-dependent increase in the hydrolytic (as measured by the production of phosphatidic acid, PA) and the transphosphatidylation (as measured by the production of phosphatidylethanol, PEt) activities of phospholipase D in intact cells prelabeled with a radioactive fatty acid. Both activities were inhibited by preincubation of cells with genistein. Appropriate conditions were developed to test the PLD transphosphatidylation activity against exogenous phosphatidylcholine (PCho) in an in vitro system. As in intact cells, increased PLD activity could be detected in cell lysates obtained from fMet-Leu-Phe-treated cells compared with controls. When lysates were immunoprecipitated with antiphosphotyrosine antibodies, a PLD activity was found only in immune complexes that were prepared from fMet-Leu-Phe-treated cells. Conversely, no activity was found in lysates immunoprecipitated with an irrelevant antibody (GTPase-activating protein, GAP) that nevertheless was able to recognize a tyrosylphosphorylated form of GAP, as demonstrated by western blotting. These data suggest that a PCho-PLD, or a tightly bound protein, is tyrosine phosphorylated during cell activation.

Rapid and long-term effects on protein kinase C on receptor tyrosine kinase phosphorylation and degradation

Rapid and long term effects of protein kinase C alpha activation on receptor tyrosine kinase signaling parameters were investigated in human 293 embryonic fibroblasts and mouse NIH 3T3 cells. Within minutes of phorbol 12-myristate 13-acetate treatment, epidermal growth factor receptor and HER2 tyrosine phosphorylation was decreased, while platelet-derived growth factor receptor and insulin receptor autophosphorylation was upregulated. These effects are not mediated by protein kinase C-dependent receptor tyrosine kinase phosphorylation but apparently by activation or inactivation of receptor tyrosine kinase-specific phosphatases, as indicated by neutralization of these phenomena upon treatment of cells with sodium orthovanadate. In contrast to these short term effects, sustained activation of protein kinase C alpha by phorbol 12-myristate 13-acetate results in translocation of protein kinase C from the cytosol to the membrane fraction where it forms stable complexes with all receptor tyrosine kinases investigated. Ligand-induced receptor tyrosine kinase/protein kinase C association in NIH 3T3 fibroblasts is accompanied by a mobility shift of the receptor, indicating phosphorylation by activated protein kinase C. This phenomenon correlates with the disappearance of receptor tyrosine kinases from the cell surface, implying that this interaction plays a role in the process of receptor internalization and degradation. Interestingly, ligand-stimulated receptor down-regulation is also enhanced by overexpression of phospholipase C gamma, which strongly indicates a role for this common receptor tyrosine kinase substrate in negative regulation of growth factor signals.

Induction of toxin sensitivity in insect cells by infection with baculovirus encoding diphtheria toxin receptor

The diphtheria toxin receptor (DTR) has been identified as the precursor of heparin-binding epidermal growth factor-like growth factor, which may interact with other membrane proteins to form the functional receptor. To test if mammalian DTR is able to confer toxin sensitivity onto phylogenetically distant cells, we expressed monkey DTR in the baculovirus system and tested infected insect cells for toxin sensitivity. cDNA encoding an epitope-tagged heparin-binding epidermal growth factor-like growth factor precursor (DTRB3) was inserted into the virus genome by allelic replacement to construct the recombinant virus vAc-DTRB3. SF9 cells infected with vAc-DTRB3 expressed functional DTR, which could be precipitated from the solubilized membrane fraction of infected cells with Sepharose-immobilized diphtheria toxin. The highest level of expression (about 5 x 10(6) receptors/cell) was observed 48 h after infection, at which time the infected cells were highly sensitive to diphtheria toxin. Uninfected SF9 cells and cells infected with the wild type virus were resistant to the toxin. The presence of heparin increased both the binding and the toxin sensitivity of vAc-DTRB3-infected SF9 cells. Translocation of toxin A fragment was induced when cells with surface-bound toxin were exposed to low pH, and the translocation was optimal at pH < or = 5.5. It was approximately 100 times more efficient at 24 degrees C than at 4 degrees C. The data indicate that monkey DTR is fully functional when expressed in insect cells.

Physiological levels of beta-amyloid increase tyrosine phosphorylation and cytosolic calcium

The a beta peptide is a neurotoxic peptide that accumulates in the brains of Alzheimer patients, but is also present in body fluids at subnanomolar levels. The potential effects of these low levels of a beta are unclear. We now show that one such action is to increase tyrosine phosphorylation in PC12 cells and olfactory neuroblasts. Application of a beta 25-35 or a beta 1-40 induces a dose-dependent increase in the tyrosine phosphorylation in both whole cells and in vitro. The increase in tyrosine phosphorylation is both rapid and sensitive, being stimulated by picomolar doses of a beta and occurring within 1 min of application. Calcium imaging experiments provide further support for the role of tyrosine phosphorylation in the action of a beta. While a beta does not alter calcium metabolism under basal conditions, the addition of a beta induces a rapid increase in cytoplasmic calcium in olfactory neuroblasts that have been treated with the tyrosine phosphatase inhibitor, sodium orthovanadate or in PC12 cells treated with nerve growth factor. These responses could be blocked by the tyrosine kinase inhibitor, herbimycin. These calcium responses displayed an obligate requirement for the presence of matrix proteins. The identification of a rapid, sensitive assay for the action of a beta may facilitate investigations of its mechanism of action.

Expression of phospholipases gamma 1, beta 1, and delta 1 in primary human colon carcinomas and colon carcinoma cell lines

The levels of expression of phosphoinositide-specific phospholipase Cs (PLCs) were examined in a series of primary human colon carcinomas and in eight colon carcinoma cell lines by using monoclonal antibodies and cDNA probes for PLC gamma 1, PLC beta 1, and PLC delta 1. Western and northern blot analyses of PLC gamma 1 revealed elevated expression of this isozyme at both the protein and mRNA levels in most tumors when compared with paired adjacent normal mucosa samples (in 11 of 13 pairs in the western blots and 8 of 9 pairs in the northern blots). On the other hand, decreased levels of the PLC delta 1 protein were seen in most colon carcinomas (12 of 13 paired samples). The levels of PLC beta 1 protein were too low to detect possible differences between the carcinoma and normal mucosa samples. Relatively high expression of PLC gamma 1 was found in almost all of the eight human colon carcinoma cell lines at both the protein and mRNA levels. Only weak expression of PLC beta 1 was detected in these cell lines, by both western and northern blot analyses, and PLC delta 1 protein was not detected in any of the carcinoma cell lines. These findings provide evidence that colon carcinomas display altered expression of individual isoforms of PLCs and suggest that increased expression of PLC gamma 1 may play an important role in colon carcinogenesis.

Beta PDGF receptor mutants defective for mitogenesis promote neurite outgrowth in PC12 cells

BACKGROUND

Platelet-derived growth factor (PDGF) promotes mitogenesis in fibroblast cell lines but stimulates neurite outgrowth in PC12 cells that ectopically express the beta PDGF receptor. To determine which substrates must associate with this receptor protein-tyrosine kinase in order to promote neurite outgrowth, we introduced into PC12 pheochromocytoma cells three mutant forms of the beta PDGF receptor that no longer associate with specific substrate proteins. We then assayed the ability of these receptor mutants to affect neurite extension.

RESULTS

Receptors lacking the kinase-insert domain did not associate with either phosphatidylinositol 3-kinase (PI 3-kinase) or Ras GTPase-activating protein (Ras-GAP) in PC12 cells. A carboxy-terminal truncation of the beta PDGF receptor eliminated the association of phospholipase C-gamma 1 (PLC-gamma 1) with the receptor and prevented phosphorylation of PLC-gamma 1 in PC12 cells. Finally, beta PDGF receptors that have tyrosine-to-phenylalanine point mutations at positions 708, 719, 977 and 989 did not associate with either PI 3-kinase or PLC-gamma 1. All three mutant forms of the beta PDGF receptor promoted PDGF-dependent neurite outgrowth in PC12 cells and elicited activation of mitogen-activated protein (MAP) kinases.

CONCLUSIONS

PC12 cells expressing the beta PDGF receptor extend neurites in response to PDGF in the absence of signalling through PI 3-kinase, RasGAP, and PLC-gamma 1. This contrasts with the requirements for mitogenesis for epithelial and fibroblast cell lines, in which the association of PI 3-kinase with the beta PDGF receptor is essential. This receptor protein-tyrosine kinase therefore phosphorylates and activates a similar set of intracellular signalling molecules in the context of both mitogenesis and differentiation, but the importance of particular pathways for each phenotypic response is distinct.

Regulation of protein kinase C and role in cancer biology

Protein kinase C (PKC) is a family of closely related lipid-dependent and diacyglycerol-activated isoenzymes known to play an important role in the signal transduction pathways involved in hormone release, mitogenesis and tumor promotion. Reversible activation of PKC by the second messengers diacylglycerol and calcium is an established model for the short term regulation of PKC in the immediate events of signal transduction. PKC can also be modulated long term by changes in the levels of activators or inhibitors for a prolonged period or by changes in the levels of functional PKC isoenzymes in the cell during development or in response to hormones and/or differentiation factors. Indeed, studies have indicated that the sustained activation or inhibition of PKC activity in vivo may play a critical role in regulation of long term cellular events such as proliferation, differentiation and tumorigenesis. In addition, these regulatory events are important in colon cancer, where a decrease in PKC activators and activity suggests PKC acts as an anti-oncogene, in breast cancer, where an increase in PKC activity suggests an oncogenic role for PKC, and in multidrug resistance (MDR) and metastasis where an increase in PKC activity correlates with increased resistance and metastatic potential. These studies highlight the importance and significance of regulation of PKC activity in vivo.

Functional interaction between the epidermal growth factor receptor and c-Src kinase activity

To study the relationship between the tyrosine kinase c-Src and the epidermal growth factor receptor (EGF-R), we used the breast cancer cell line ZR75-1, which was transfected with the EGF-R. The EGF-R transfected cell line expressed 60 times more EGF-R than a control cell line transfected with the empty vector. In the presence of EGF, the EGF-R over-expressing cell line grew much faster than the control cell line. Both cell lines expressed approximately equal amounts of c-Src. However, the cell line over-expressing the EGF-R showed a twofold enhancement of c-Src kinase activity after EGF stimulation. The activation of c-Src kinase by EGF was confirmed in other EGF-R expressing cell types.

A rapid bioassay for platelet-derived growth factor beta-receptor tyrosine kinase function

We have extended a melanophore-based bioassay for G-protein coupled receptors to include the functional expression of the murine platelet-derived growth factor (PDGF) beta-receptor. The homodimeric ligand PDGF-BB induced activation of the transiently expressed receptor in melanophore cells. This led to dose dependent pigment dispersion whereas it did not induce pigment dispersion in wild type cells. The effective concentration of PDGF-BB giving half-maximal pigment dispersion (EC50) was 1nM after 30 minutes exposure. PDGF-AA had no ability to induce pigment dispersion in melanophore cells transiently expressing the beta-PDGF receptor. PDGF-BB-induced pigment dispersion could be blocked by the bis-indolylmaleimide Ro 31-8220 which is an inhibitor of protein kinase C isoenzymes. Functional expression of the PDGF beta-receptor extends the use of the pigment translocation assay to include transmembrane signaling receptor tyrosine kinases. It opens the opportunity for the discovery of potent agonists and antagonists through massive drug screening and investigations of functional ligand-receptor interactions for single transmembrane domain receptors.

Calcium signaling in endothelin- and platelet-derived growth factor-stimulated chondrocytes

In bovine articular chondrocytes, endothelin (ET) and platelet-derived growth factor (PDGF) receptors mediate agonist-induced increases in inositol phosphate hydrolysis, cytoplasmic calcium concentration ([Ca2+]i), and mitogenesis. In most cells, ET stimulated nonoscillatory [Ca2+]i elevations with dose-dependent increases in both spike and plateau amplitudes. However, about 15% of cells showed oscillatory Ca2+ responses with a constant frequency and variable shape and duration of spiking. ET-1 and ET-2 were more potent than ET-3 in stimulating [Ca2+]i responses in inhibiting the specific binding of 125I-ET-1 and 125I-ET-3 and in promoting internalization of the receptor-ligand complex, consistent with actions through endothelin ETA receptors. Similar nonoscillatory and oscillatory patterns of Ca2+ responses were observed in PDGF-stimulated cells. In cells showing nonoscillatory Ca2+ responses to ET-1, subsequent stimulation with PDGF was frequently followed by the development of an oscillatory Ca2+ response. Nonoscillatory responses to both agonists were only slightly reduced in Ca(2+)-deficient medium, but the oscillatory responses were critically dependent on Ca2+ entry. Ca2+ spiking was not altered in the presence of the voltage-sensitive Ca2+ channel blocker, nifedipine; also, depolarization of chondrocytes by high K+ did not induce [Ca2+]i responses, confirming that voltage-sensitive calcium channels are not expressed in these cells. At high agonist concentrations, ET- but not PDGF-stimulated cells underwent rapid desensitization. Activation of ETA and PDGF receptors was associated with differential stimulation of thymidine incorporation; ET-1 induced a low-amplitude bell-shaped dose-response curve; PDGF induced a sustained sigmoidal and dose-dependent rise. These data indicate that two distinct types of Ca(2+)-mobilizing receptors initiate similar patterns of [Ca2+]i responses but have different capacities to maintain and reinitiate the Ca2+ signaling, as well as to promote mitogenesis.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Chemotaxis is an important component of wound healing, development, immunity and metastasis, yet the signalling pathways that mediate chemotaxis are poorly understood. Platelet-derived growth factor (PDGF) acts both as a mitogen and a chemoattractant. Upon stimulation, the tyrosine kinase PDGF receptor-beta (PDGFR-beta) autophosphorylates and forms a complex that includes SII2(Src homology 2)-domain-containing proteins such as the phosphatidylinositol-specific phospholipase C-gamma, Ras-GTPase-activating protein (GAP), and phosphatidylinositol-3-OH kinase. Specific tyrosine-to-phenylalanine substitutions in the PDGFR-beta can prevent binding of one SH2-domain-containing protein without affecting binding of other receptor-associated proteins. Here we use phospholipase C-gamma and PDGFR-beta mutants to map specific tyrosines involved in both positive and negative regulation of chemotaxis towards the PDGF-BB homodimer. Our results indicate that a delicate balance of migration-promoting (phospholipase C-gamma and phosphatidylinositol-3-OH kinase) and migration-suppressing (GAP) activities are recruited by the PDGFR-beta to drive chemotaxis towards PDGF-BB.

Receptor tyrosine kinases and their targets

One of the ways in which higher eukaryotes receive messages from the environment is via cell surface receptor tyrosine kinases. These are transmembrane proteins with an extracellular binding domain that specifies the growth factor with which it will interact, and an intracellular domain that encodes the tyrosine kinase. The mechanism by which receptor tyrosine kinases direct intracellular signal relay appears to involve receptor autophosphorylation that permits the stable binding of SH2 domain containing signal transduction enzymes. Some of the more recent advances are summarized in this review.