Platelet-derived growth factor stimulates growth factor receptor binding protein-2 association with Shc in vascular smooth muscle cells

Circ_0033596 depletion ameliorates oxidized low-density lipoprotein-induced human umbilical vein endothelial cell damage

BACKGROUND

Previous data have shown that circ_0033596 is involved in the pathogenesis of atherosclerosis (AS). The study aims to reveal the detailed mechanism of circ_0033596 in AS.

METHODS

Human umbilical vein endothelial cells (HUVECs) were treated with oxidized low-density lipoprotein (ox-LDL) to establish an AS cell model. Quantitative real-time polymerase chain reaction and western blot were implemented to detect the expression of circ_0033596, miR-637, growth factor receptor bound protein2 (GRB2), BCL2-associated x protein (Bax) and B-cell lymphoma-2 (Bcl-2). Cell viability, proliferation, apoptosis and tube formation were investigated by cell counting kit-8, EdU assay, flow cytometry and tube formation assay, respectively. The production of interleukin (IL-6) and tumor necrosis factor-α (TNF-α) was evaluated by enzyme-linked immunosorbent assay. Oxidative stress was evaluated by lipid peroxidation malondialdehyde assay kit and superoxide dismutase activity assay kit. Dual-luciferase reporter assay, RNA pull-down assay and RIP assay were performed to identify the associations among circ_0033596, miR-637 and GRB2.

RESULTS

The expression of circ_0033596 and GRB2 was significantly increased, while miR-637 was decreased in the blood of AS patients and ox-LDL-induced HUVECs compared with controls. Ox-LDL treatment inhibited HUVEC viability, proliferation and angiogenic ability and induced cell apoptosis, inflammation and oxidative stress, while these effects were attenuated after circ_0033596 knockdown. Circ_0033596 interacted with miR-637 and regulated ox-LDL-induced HUVEC damage by targeting miR-637. In addition, GRB2, a target gene of miR-637, participated in ox-LDL-induced HUVEC injury by combining with miR-637. Importantly, circ_0033596 activated GRB2 by interacting with miR-637.

CONCLUSION

Circ_0033596 depletion protected against ox-LDL-induced HUVEC injury by miR-637/GRB2 pathway, providing a therapeutic target for AS.

Neurotrophins are expressed in giant cell arteritis lesions and may contribute to vascular remodeling

INTRODUCTION

Giant cell arteritis (GCA) is characterized by intimal hyperplasia leading to ischaemic manifestations that involve large vessels. Neurotrophins (NTs) and their receptors (NTRs) are protein factors for growth, differentiation and survival of neurons. They are also involved in the migration of vascular smooth muscle cells (VSMCs). Our aim was to investigate whether NTs and NTRs are involved in vascular remodelling of GCA.

METHODS

We included consecutive patients who underwent a temporal artery biopsy for suspected GCA. We developed an enzymatic digestion method to obtain VSMCs from smooth muscle cells in GCA patients and controls. Neurotrophin protein and gene expression and functional assays were studied from these VSMCs. Neurotrophin expression was also analysed by immunohistochemistry in GCA patients and controls.

RESULTS

Whereas temporal arteries of both GCA patients (n = 22) and controls (n = 21) expressed nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin receptor kinase B (TrkB) and sortilin, immunostaining was more intense in GCA patients, especially in the media and intima, while neurotrophin-3 (NT-3) and P75 receptor (P75NTR) were only detected in TA from GCA patients. Expression of TrkB, a BDNF receptor, was higher in GCA patients with ischaemic complications. Serum NGF was significantly higher in GCA patients (n = 28) vs. controls (n = 48), whereas no significant difference was found for BDNF and NT-3. NGF and BDNF enhanced GCA-derived temporal artery VSMC proliferation and BDNF facilitated migration of temporal artery VSMCs in patients with GCA compared to controls.

CONCLUSIONS

Our results suggest that NTs and NTRs are involved in vascular remodelling of GCA. In GCA-derived temporal artery VSMC, NGF promoted proliferation and BDNF enhanced migration by binding to TrkB and p75NTR receptors. Further experiments are needed on a larger number of VSMC samples to confirm these results.

Small GTPase and regulation of inflammation response in atherogenesis

Small GTPases are key signal transducers from extracellular stimuli to the nucleus that regulate a variety of cellular responses, including changes in gene expression and cell adhesion and migration. Accumulating data have demonstrated that abnormal activation of these small GTPases plays a critical role in the atherosclerosis characterized by vascular abnormalities, especially endothelial dysfunction and inflammation. Here, we discuss the linkage between small GTPases, inflammation, and atherogenesis. First, small GTPases affect gene expression of inflammatory cytokines through proinflammatory signaling pathways, such as nuclear factor-κB, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, interlukin-8, and monocyte chemoattractant protein-1. Then, these molecules regulate the vascular inflammation through cell adhesion and migration. In turn, small GTPases are also regulated by extracellular stimuli, such as L-selectin, thrombin, oxidized phospholipids, and interleukins. Thus, these inflammatory cytokines generate a vicious cycle for small GTPases and inflammatory responses in the atherogenesis.

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.

PYK2 signaling is required for PDGF-dependent vascular smooth muscle cell proliferation

Aberrant vascular smooth muscle cell (VSMC) growth is associated with many vascular diseases including atherosclerosis, hypertension, and restenosis. Platelet-derived growth factor-BB (PDGF) induces VSMC proliferation through control of cell cycle progression and protein and DNA synthesis. Multiple signaling cascades control VSMC growth, including members of the mitogen-activated protein kinase (MAPK) family as well as phosphatidylinositol 3-kinase (PI3K) and its downstream effector AKT/protein kinase B (PKB). Little is known about how these signals are integrated by mitogens and whether there are common receptor-proximal signaling control points that synchronize the execution of physiological growth functions. The nonreceptor proline-rich tyrosine kinase 2 (PYK2) is activated by a variety of growth factors and G protein receptor agonists in VSMC and lies upstream of both PI3K and MAPK cascades. The present study investigated the role of PYK2 in PDGF signaling in cultured rat aortic VSMC. PYK2 downregulation attenuated PDGF-dependent protein and DNA synthesis, which correlated with inhibition of AKT and extracellular signal-regulated kinases 1 and 2 (ERK1/2) but not p38 MAPK activation. Inhibition of PDGF-dependent protein kinase B (AKT) and ERK1/2 signaling by inhibitors of upstream kinases PI3K and MEK, respectively, as well as downregulation of PYK2 resulted in modulation of the G(1)/S phase of the cell cycle through inhibition of retinoblastoma protein (Rb) phosphorylation and cyclin D(1) expression, as well as p27(Kip) upregulation. Cell division kinase 2 (cdc2) phosphorylation at G(2)/M was also contingent on PDGF-dependent PI3K-AKT and ERK1/2 signaling. These data suggest that PYK2 is an important upstream mediator in PDGF-dependent signaling cascades that regulate VSMC proliferation.

FRS2 via fibroblast growth factor receptor 1 is required for platelet-derived growth factor receptor beta-mediated regulation of vascular smooth muscle marker gene expression

Vascular smooth muscle cells (VSMC) exhibit phenotypic plasticity and change from a quiescent contractile phenotype to a proliferative synthetic phenotype during physiological arteriogenesis and pathological conditions such as atherosclerosis and restenosis. Platelet-derived growth factor (PDGF)-BB is a potent inducer of the VSMC synthetic phenotype; however, much less is known about the role of fibroblast growth factor-2 (FGF2) in this process. Here, we show using signal transduction mutants of FGF receptor 1 (FGFR1) expressed in rat VSMC that the adaptor protein FRS2 is essential for FGFR1-mediated phenotypic modulation and down-regulation of VSMC smooth muscle alpha-actin (SMA) gene expression. In addition, we show that PDGF-BB and FGF2 act synergistically to induce cell proliferation and down-regulate SMA and SM22alpha in VSMC. Furthermore, we show that PDGF-BB induces tyrosine phosphorylation of FGFR1 and that this phosphorylation is mediated by PDGF receptor-beta (PDGFRbeta), but not c-Src. We demonstrate that FRS2 co-immunoprecipitates with PDGFRbeta in a complex that requires FGFR1 and that both the extracellular and the intracellular domains of FGFR1 are required for association with PDGFRbeta, whereas the cytoplasmic domain of FGFR1 is required for FRS2 association with the FGFR1-PDGFRbeta complex. Knockdown of FRS2 in VSMC by RNA interference inhibited PDGF-BB-mediated down-regulation of SMA and SM22alpha without affecting PDGF-BB mediated cell proliferation or ERK activation. Together, these data support the notion that PDGFRbeta down-regulates SMA and SM22alpha through formation of a complex that requires FGFR1 and FRS2 and prove novel insight into VSMC phenotypic plasticity.

Ras induces vascular smooth muscle cell senescence and inflammation in human atherosclerosis

BACKGROUND

Vascular cells have a finite cell lifespan and eventually enter an irreversible growth arrest, cellular senescence. The functional changes associated with cellular senescence are thought to contribute to human aging and age-related vascular disorders. Ras, an important signaling molecule involved in atherogenic stimuli, is known to promote aging in yeast and cellular senescence in primary human fibroblasts. The aim of this study was to investigate the role of Ras-induced vascular smooth muscle cell (VSMC) senescence in atherogenesis.

METHODS AND RESULTS

We introduced an activated ras allele (H-rasV12) into human VSMCs using retroviral infection. Introduction of H-rasV12 induced a growth arrest with phenotypic characteristics of cellular senescence, such as enlarged cell shapes and increases in expression of cyclin-dependent kinase inhibitors and senescence-associated beta-galactosidase (SA-beta-gal) activity. Activation of Ras drastically increased expression of proinflammatory cytokines, in part through extracellular signal-regulated kinase activation. To determine whether Ras activation induces cellular senescence in vivo, we transduced the adenoviral vector encoding H-rasV12 into rat carotid arteries injured by a balloon catheter. Introduction of Ras into the arteries enhanced vascular inflammation and senescence compared with mock-infected injured arteries. Moreover, SA-beta-gal-positive VSMCs were detected in the intima of advanced human atherosclerotic lesions and exhibited increased levels of extracellular signal-regulated kinase activity and proinflammatory cytokine expression.

CONCLUSIONS

Our results suggest that atherogenic stimuli mediated by Ras induce VSMC senescence and vascular inflammation, thereby contributing to atherogenesis. This novel mechanism of atherogenesis may provide insights into a new antisenescence treatment for atherosclerosis.

Grb2 is required for the development of neointima in response to vascular injury

OBJECTIVE

Neointima formation occurs in arteries in response to mechanical or chemical injury and is responsible for substantial morbidity. In this work, the role of the intracellular linker protein Grb2 in the pathogenesis of neointima formation was examined. Grb2 is a critical signaling protein that facilitates the activation of the small GTPase ras by receptor tyrosine kinases.

METHODS AND RESULTS

Cultured rat aortic smooth muscle cells were treated with an antisense morpholino to Grb2 and these cells showed a reduced proliferative response to platelet-derived growth factor stimulation. Grb2-/- mice do not survive embryonic development. Grb2+/- mice appear normal at birth and are fertile but have defective signaling in several tissues. Cultured smooth muscle cells derived from Grb2+/- mice grew at a much slower rate than cells derived from Grb2+/+ mice. Grb2+/- and Grb2+/+ mice were subjected to carotid injury. After 21 days, Grb2+/+ mice developed robust neointima formation that, in some cases, resulted in an occlusive lesion. In contrast, Grb2+/- mice were resistant to the development of neointima

CONCLUSIONS

Grb2 is an essential component of the signaling cascade resulting in neointima formation after arterial injury.

Dual role of SRC homology domain 2-containing inositol phosphatase 2 in the regulation of platelet-derived growth factor and insulin-like growth factor I signaling in rat vascular smooth muscle cells

Src homology domain 2 (SH2)-containing inositol phosphatase 2 (SHIP2) possesses 5-phosphatase activity and an SH2 domain. The role of SHIP2 in platelet-derived growth factor (PDGF) and IGF-I signaling was studied by expressing wild-type (WT-) and a catalytically defective (Delta IP-) SHIP2 into rat aortic smooth muscle cells by adenovirus-mediated gene transfer. PDGF- and IGF-I-induced tyrosine phosphorylation of their respective receptors and phosphatidylinositol 3-kinase (PI3-kinase) activity were not affected by the expression of either WT- or Delta IP-SHIP2. SHIP2 possessed 5'-phosphatase activity to hydrolyze the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate in vivo. Akt and glycogen synthase kinase 3beta are known to be downstream molecules of PI3-kinase, leading to the antiapoptotic effect. Overexpression of WT-SHIP2 inhibited PDGF- and IGF-I-induced phosphorylation of these molecules and the protective effect of poly(ADP-ribose) polymerase degradation, whereas these phosphorylations and the protective effect were enhanced by the expression of Delta IP-SHIP2, which functions in a dominant negative fashion. Regarding the Ras-MAPK pathway, PDGF- and IGF-I-induced tyrosine phosphorylation of Shc was not affected by the expression of either WT- or Delta IP-SHIP2, whereas both expressed SHIP2 associated with Shc. Importantly, PDGF and IGF-I stimulation of Shc/Grb2 binding, MAPK activation, and 5-bromo-2'-deoxyuridine incorporation were all decreased in both WT- and Delta IP-SHIP2 expression. These results indicate that SHIP2 plays a negative regulatory role in PDGF and IGF-I signaling in vascular smooth muscle cells. As the bifunctional role, our results suggest that SHIP2 regulates PDGF- and IGF-I-mediated signaling downstream of PI3-kinase, leading to the antiapoptotic effect via 5-phosphatase activity, and that SHIP2 regulates the growth factor-induced Ras-MAPK pathway mainly via the SH2 domain.

Mitogenic activity of dermatofibrosarcoma protuberans is mediated via an extracellular signal related kinase dependent pathway

Dermatofibrosarcoma protuberans is a malignant mesenchymal tumor originating in the dermis. Although it is locally aggressive and recurs unless completely excised, it only rarely metastasizes. In this study, we investigated the mechanisms of increased proliferation of dermatofibrosarcoma protuberans cells. The cells showed increased DNA synthesis in serum-free medium, which was demonstrated by the incorporation of [3H]-thymidine. Increased DNA synthesis of dermatofibrosarcoma protuberans cells was abolished by genistein, a tyrosine kinase inhibitor, or by PD98059, a specific extracellular signal related kinase pathway inhibitor, but not by calphostin C, a protein kinase C inhibitor. Immunoblotting analysis of dermatofibrosarcoma protuberans cells using a specific antibody against phosphorylated extracellular signal related kinase (Thr202/Tyr204) showed that extracellular signal related kinase was expressed as constitutively phosphorylated molecules in dermatofibrosarcoma protuberans cells. Immunofluorescence analysis showed that the kinase was constitutively located in the nucleus of the cells. Furthermore, transfection of the dominant negative mutant extracellular signal related kinase into dermatofibrosarcoma protuberans cells abolished the increased mitogenic activity of the cells. These results suggest that an extracellular signal related kinase dependent pathway is implicated in the increased mitogenic activity of dermatofibrosarcoma protuberans cells.

Increased insulin sensitivity in IGF-I receptor--deficient brown adipocytes

Immortalized brown adipocyte cell lines have been generated from fetuses of mice deficient in the insulin-like growth factor I receptor gene (IGF-IR(-/-)), as well as from fetuses of wild-type mice (IGF-IR(+/+)). These cell lines maintained the expression of adipogenic- and thermogenic-differentiation markers and show a multilocular fat droplets phenotype. IGF-IR(-/-) brown adipocytes lacked IGF-IR protein expression; insulin receptor (IR) expression remained unchanged as compared with wild-type cells. Insulin-induced tyrosine autophosphorylation of the IR beta-chain was augmented in IGF-IR--deficient cells. Upon insulin stimulation, tyrosine phosphorylation of (insulin receptor substrate-1) IRS-1 was much higher in IGF-IR(-/-) brown adipocytes, although IRS-1 protein content was reduced. In contrast, tyrosine phosphorylation of IRS-2 decreased in IGF-IR--deficient cells; its protein content was unchanged as compared with wild-type cells. Downstream, the association IRS-1/growth factor receptor binding protein-2 (Grb-2) was augmented in the IGF-IR(-/-) brown adipocyte cell line. However, SHC expression and SHC tyrosine phosphorylation and its association with Grb-2 were unaltered in response to insulin in IGF-IR--deficient brown adipocytes. These cells also showed an enhanced activation of mitogen-activated protein kinase (MAPK) kinase (MEK1/2) and p42/p44 mitogen-activated protein kinase (MAPK) upon insulin stimulation. In addition, the lack of IGF-IR in brown adipocytes resulted in a higher mitogenic response (DNA synthesis, cell number, and proliferating cell nuclear antigen expression) to insulin than wild-type cells. Finally, cells lacking IGF-IR showed a much lower association between IR or IRS-1 and phosphotyrosine phosphatase 1B (PTP1B) and also a decreased PTP1B activity upon insulin stimulation. However, PTP1B/Grb-2 association remained unchanged in both cell types, regardless of insulin stimulation. Data presented here provide strong evidence that IGF-IR--deficient brown adipocytes show an increased insulin sensitivity via IRS-1/Grb-2/MAPK, resulting in an increased mitogenesis in response to insulin.

The TEL/PDGFbetaR fusion in chronic myelomonocytic leukemia signals through STAT5-dependent and STAT5-independent pathways

The TEL/PDGFbetaR gene, which encodes a fusion protein containing the ETS-family member TEL fused to the protein-tyrosine kinase domain of the platelet-derived growth factor receptor-beta (PDGFbetaR), confers interleukin 3 (IL-3)-independent growth on Ba/F3 hematopoietic cells. TEL/PDGFbetaR mutants have been generated that contain tyrosine-to-phenylalanine (Tyr-->Phe) substitutions at phosphorylation sites present in the native PDGFbetaR to assess the role of these sites in cell transformation by TEL/PDGFbetaR. Similar to previous findings in a murine bone marrow transplantation model, full transformation of Ba/F3 cells to IL-3-independent survival and proliferation required the TEL/PDGFbetaR juxtamembrane and carboxy terminal phosphorylation sites. In contrast to previous reports concerning comparable mutants in the native PDGFbetaR, each of the TEL/PDGFbetaR mutants is fully active as a protein-tyrosine kinase. Expression of the TEL/PDGFbetaR fusion protein causes hyperphosphorylation and activation of signal transducer and activator of transcription (STAT5), and this activation of STAT5 requires the juxtamembrane Tyr579 and Tyr581 in the TEL/PDGFbetaR fusion. Hyperphosphosphorylation of phospholipase Cgamma (PLCgamma) and the p85 subunit of phosphatidylinositol 3-kinase (PI3K) requires the carboxy terminal tyrosine residues of TEL/PDGFbetaR. Thus, full transformation of Ba/F3 cells by TEL/PDGFbetaR requires engagement of PI3K and PLCgamma and activation of STAT5. Taken together with the growth properties of cells transformed by the TEL/PDGFbetaR variants, these findings indicate that a minimal combination of these signaling intermediates contributes to hematopoietic transformation by the wild-type TEL/PDGFbetaR fusion. (Blood. 2001;98:3390-3397)

Glucosamine enhances platelet-derived growth factor-induced DNA synthesis via phosphatidylinositol 3-kinase pathway in rat aortic smooth muscle cells

Vascular smooth muscle cells play a key role in the development of atherosclerosis. Culture of vascular smooth muscle A10 cells with high glucose for 4 weeks enhanced platelet-derived growth factor (PDGF)-induced BrdU incorporation. Since a long period of high glucose incubation was required for the effect, and it was inhibited by co-incubation with azaserine, the role of hexosamine biosynthesis in the development of atherosclerosis in diabetes was studied in A10 cells. Addition of glucosamine to the culture media enhanced PDGF-stimulated BrdU incorporation, and PDGF-induced tyrosine phosphorylation of the PDGF beta-receptor was increased by glucosamine treatment. Of the subsequent intracellular signaling pathways, PDGF-induced PDGF beta-receptor association with PLC gamma was not affected, whereas tyrosine phosphorylation of Shc, subsequent association of Shc with Grb2, and MAP kinase activation were relatively decreased. In contrast, PDGF-induced PDGF beta-receptor association with the p85 regulatory subunit of PI3-kinase and PI3-kinase activation were increased by 20% (P<0.01) and 36% (P<0.01), respectively. The intracellular signaling molecules responsible for the glucosamine effect were further examined using pharmacological inhibitors. Pretreatment with PLC inhibitor (U73122) had negligible effects, and MEK1 inhibitor (PD98059) showed only a slight inhibitory effect on the PDGF-induced BrdU incorporation. In contrast, pretreatment with PI3-kinase inhibitor (LY294002) significantly inhibited glucosamine enhancement of PDGF-induced BrdU incorporation. These findings suggest that glucosamine is involved in the development of atherosclerosis by enhancing PDGF-induced mitogenesis specifically via the PI3-kinase pathway.

Stimulation of phosphatidylinositol 3-kinase by fibroblast growth factor receptors is mediated by coordinated recruitment of multiple docking proteins

The docking protein FRS2 is a major downstream effector that links fibroblast growth factor (FGF) and nerve growth factor receptors with the Ras/mitogen-activated protein kinase signaling cascade. In this report, we demonstrate that FRS2 also plays a pivotal role in FGF-induced recruitment and activation of phosphatidylinositol 3-kinase (PI3-kinase). We demonstrate that tyrosine phosphorylation of FRS2alpha leads to Grb2-mediated complex formation with the docking protein Gab1 and its tyrosine phosphorylation, resulting in the recruitment and activation of PI3-kinase. Furthermore, Grb2 bound to tyrosine-phosphorylated FRS2 through its SH2 domain interacts primarily via its carboxyl-terminal SH3 domain with a proline-rich region in Gab1 and via its amino-terminal SH3 domain with the nucleotide exchange factor Sos1. Assembly of FRS2alpha:Grb2:Gab1 complex induced by FGF stimulation results in activation of PI3-kinase and downstream effector proteins such as the S/T kinase Akt, whose cellular localization and activity are regulated by products of PI3-kinase. These experiments reveal a unique mechanism for generation of signal diversity by growth factor-induced coordinated assembly of a multidocking protein complex that can activate the Ras/mitogen-activated protein kinase cascade to induce cell proliferation and differentiation, and PI3-kinase to activate a mediator of a cell survival pathway.

Effects of active and negative mutants of Ras on rat arterial neointima formation

BACKGROUND

Ras protein is a key signal transducer in the cause of cell proliferation. We studied the effects of active and negative mutants of the Ras gene on arterial neointimal formation in rats, with the aim of elucidating the molecular mechanisms regulating restenosis following percutaneous transluminal coronary angioplasty.

MATERIALS AND METHODS

AdRasV12 and AdRasN17, the recombinant adenoviruses containing a constitutively active mutant and a dominant negative mutant of Ras, respectively, were used to determine whether Ras is necessary and sufficient to modulate the smooth muscle cell proliferation and neointima formation. Following balloon injury, rat common carotid arteries were treated in their distal half with AdRasV12, AdRasN17, or AdLacZ, with the proximal half used as uninfected control.

RESULTS

In rat arteries subjected to balloon injury, either uninfected or treated with AdLacZ, there were pronounced SMC proliferation and neointima formation. These changes were markedly augmented by AdRasV12 and reduced by AdRasN17.

CONCLUSION

Ras is necessary and sufficient for SMC proliferation and neointima formation and may play a critical role in restenosis following balloon angioplasty.

Actinomycin D as a novel SH2 domain ligand inhibits Shc/Grb2 interaction in B104-1-1 (neu*-transformed NIH3T3) and SAA (hEGFR-overexpressed NIH3T3) cells

Actinomycins, a family of bicyclic chromopeptide lactones with strong antineoplastic activity, were screened as inhibitors of Shc/Grb2 interaction in in vitro assay systems. To investigate the effects of actinomycin D on Shc/Grb2 interaction in cell-based experiments, we used SAA (normal hEGFR-overexpressed NIH3T3) cells and B104-1-1 (neu*-transformed NIH3T3) cells, because a large number of the Shc/Grb2 complexes were detected. Associated protein complexes containing Shc were immunoprecipitated from actinomycin D-treated cell lysates with polyclonal anti-Shc antibody. Then the association with Grb2 was assessed by immunoblotting with monoclonal anti-Grb2 antibody. The result of the immunoblotting experiment revealed that actinomycin D inhibited Shc/Grb2 interaction in a dose-dependent manner in both B104-1-1 and EGF-stimulated SAA cells. The inhibition of Shc/Grb2 interaction by actinomycin D in B104-1-1 cells also reduced tyrosine phosphorylation of MAP kinase (Erk1/Erk2), one of the major components in the Ras-MAP kinase signaling pathway. These results suggest that actinomycin D could be a non-phosphorylated natural and cellular membrane-permeable SH2 domain antagonist.

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.

Trapidil inhibits platelet-derived growth factor-stimulated mitogen-activated protein kinase cascade

Trapidil, an antiplatelet drug, has been shown to reduce restenosis after angioplasty. It exerts its action, at least in part, by inhibiting vascular smooth muscle cell proliferation, antagonizing platelet-derived growth factor (PDGF). We examined its site of action on PDGF cellular signaling. Exposure of cultured rat vascular smooth muscle cells to increasing concentrations of trapidil for 18 hours resulted in a dose-dependent reduction in PDGF-BB-stimulated [3H] thymidine incorporation. Trapidil (400 microg/mL) increased PDGF beta-receptor protein by 28+/-8%, whereas PDGF-induced tyrosine phosphorylation of PDGF beta-receptor remained unchanged. PDGF-induced tyrosine phosphorylation of phospholipase Cgamma, the p85 regulatory subunit of phosphatidyl-inositol 3 kinase, Ras GTPase-activating protein, and an adaptor molecule Shc were also not altered. On the other hand, trapidil inhibited PDGF-stimulated mitogen-activated protein kinase (MAP kinase) activity by 35+/-7% at 10 minutes and by 32+/-10% at 6 hours. Activation of Raf-1, an upstream activator of MAP kinase, by PDGF was also attenuated by trapidil. Moreover, protein content of MAP kinase phosphatase-1, which inactivates MAP kinase, was elevated in trapidil-treated cells. These actions of trapidil may be mediated by cAMP. Thus, there was a 1.9-fold increase in cellular cAMP generation in trapidil-treated cells. The present results demonstrate that trapidil antagonizes PDGF-induced mitogenesis and MAP kinase activation in vascular smooth muscle cells, probably through cAMP.

The 66-kDa Shc isoform is a negative regulator of the epidermal growth factor-stimulated mitogen-activated protein kinase pathway

In addition to tyrosine phosphorylation of the 66-, 52-, and 46-kDa Shc isoforms, epidermal growth factor (EGF) treatment of Chinese hamster ovary cells expressing the human EGF receptor also resulted in the serine/threonine phosphorylation of approximately 50% of the 66-kDa Shc proteins. The serine/threonine phosphorylation occurred subsequent to tyrosine phosphorylation and was prevented by pretreatment of the cells with the MEK-specific inhibitor PD98059. Surprisingly, only the gel-shifted 66-kDa Shc isoform (serine/threonine phosphorylated) was tyrosine phosphorylated and associated with Grb2. In contrast, only the non-serine/threonine-phosphorylated fraction of 66-kDa Shc was associated with the EGF receptor. To assess the relationship between the three Shc isoforms in EGF-stimulated signaling, the cDNA encoding the 66-kDa Shc species was cloned from a 16-day-old mouse embryo library. Sequence alignment confirmed that the 66-kDa Shc cDNA resulted from alternative splicing of the primary Shc transcript generating a 110-amino acid extension at the amino terminus. Co-immunoprecipitation of Shc and Grb2 from cells overexpressing the 52/46-kDa Shc isoforms versus the 66-kDa Shc species directly demonstrated a competition of binding for a limited pool of Grb2 proteins. Furthermore, expression of the 66-kDa Shc isoform markedly accelerated the inactivation of ERK following EGF stimulation. Together, these data indicate that the serine/threonine phosphorylation of 66-kDa Shc impairs its ability to associate with the tyrosine-phosphorylated EGF receptor and can function in a dominant-interfering manner by inhibiting EGF receptor downstream signaling pathways.

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.

Roles of vasodilatory prostaglandins in mitogenesis of vascular smooth muscle cells

Vasodilatory prostaglandins (PGI2, PGE1) and synthetic prostacyclin mimetics inhibit smooth muscle cell proliferation in vitro after stimulation by growth factors. Similar results are obtained in vivo after endothelial injury, suggesting that vasodilatory prostaglandins might also control smooth muscle cell proliferation in vivo. However, available data from clinical trials are conflicting and currently do not support the concept that these compounds might be successfully used to suppress excessive smooth muscle cell growth in response to tissue injury, specifically restenosis after PTCA. One possible explanation for these different results is an agonist-induced down-regulation of prostacyclin receptors in vascular smooth muscle cells. It is possible that enhanced endogenous prostacyclin biosynthesis, subsequent to induction of COX-2 and/or in relation to the formation of a neointima from media smooth muscle cells, might have a similar effect. There is still uncertainty regarding the cellular signal transduction pathways and their possibly complex interaction, although cAMP-dependent reactions are probably involved. In addition, vasodilatory prostaglandins might also interfere with the generation and action of other growth modulating factors, including PDGF, hepatocyte growth factor and nitric oxide. In conclusion, vasodilatory prostaglandins might be considered as growth modulating endogenous mediators in vascular smooth muscle cells.

Platelet-derived growth factor (PDGF): actions and mechanisms in vascular smooth muscle

1. PDGF is a highly hydrophilic cationic glycoprotein (M(r) 28-35kDa) produced by platelets, monocyte/macrophages, endothelial cells and vascular smooth muscle cells under some conditions. 2. Since its original description, PDGF has attracted much attention and it is currently believed to play a role in atherosclerosis and other vascular pathologies. 3. This review describes the vascular biology of PDGF. It particularly focuses on recent findings regarding the intracellular signals activated by PDGF in the context of vascular smooth muscle cell proliferation, migration and, contraction.

Interaction with the phosphotyrosine binding domain/phosphotyrosine interacting domain of SHC is required for the transforming activity of the FLT4/VEGFR3 receptor tyrosine kinase

The FLT4 gene encodes two isoforms of a tyrosine kinase receptor, which belongs to the family of receptors for vascular endothelial growth factor. As the result of an alternative processing of primary mRNA transcripts, the long isoform differs from the short isoform by an additional stretch of 65 amino acid residues located at the C terminus and containing three tyrosine residues, Tyr1333, Tyr1337, and Tyr1363. Only the long isoform is endowed with a transforming capacity in fibroblasts. We show that this activity is related to the capacity of the tyrosine 1337-containing sequence to interact with the phosphotyrosine binding domain of the SHC protein. This demonstrates that a functional property of this newly described domain includes relay of mitogenic signals. In addition, it shows that the same receptor can mediate different functions through the optional binding of the phosphotyrosine binding domain and that the alternative use of this domain is sufficient to direct the signal toward different pathways.

Dominant-negative mutants of Grb2 induced reversal of the transformed phenotypes caused by the point mutation-activated rat HER-2/Neu

To clarify the role of the Shc-Grb2-Sos trimer in the oncogenic signaling of the point mutation-activated HER-2/neu receptor tyrosine kinase (named p185), we interfered with the protein-protein interactions in the Shc.Grb2.Sos complex by introducing Grb2 mutants with deletions in either amino- (delta N-Grb2) or carboxyl-(delta C-Grb2) terminal SH3 domains into B104-1-1 cells derived from NIH3T3 cells expressing the point mutation-activated HER-2/neu. We found that the transformed phenotypes of the B104-1-1 cells were largely reversed by the delta N-Grb2. The effect of the delta C-Grb2 was much weaker. Biochemical analysis showed that the delta N-Grb2 was able to associate Shc but not p185 or Sos, while the delta C-Grb2 bound to Shc, p185, and Sos. The p185-mediated Ras activation was severely inhibited by the delta N-Grb2 but not the delta C-Grb2. Taken together, these data demonstrate that interruption of the interaction between Shc and the endogenous Grb2 by the delta N-Grb2 impairs the oncogenic signaling of the activated p185, indicating that (i) the delta N-Grb2 functions as a strong dominant-negative mutant, and (ii) Shc/Grb2/Sos pathway plays a major role in mediating the oncogenic signal of the activated p185. Unlike the delta N-Grb2, delta C-Grb2 appears to be a relatively weak dominant-negative mutant, probably due to its ability to largely fulfill the biological functions of the wild-type Grb2.

Analysis of interleukin-2-dependent signal transduction through the Shc/Grb2 adapter pathway. Interleukin-2-dependent mitogenesis does not require Shc phosphorylation or receptor association

The interleukin (IL)-2 receptor system has previously been shown to signal through the association and tyrosine phosphorylation of Shc. This study demonstrates that the IL-2 receptor beta (IL-2R beta) chain is the critical receptor component required to mediate this effect. The use of IL-2R beta chain deletion mutants transfected into a Ba/F3 murine cell model describes a requirement for the IL-2R beta "acid-rich" domain between amino acids 315 and 384 for Shc tyrosine phosphorylation and receptor association. COS cell co-transfection studies of IL-2R beta chain constructs containing point mutations of tyrosine to phenylalanine along with the tyrosine kinase Jak-1 and a hemagglutinin-tagged Shc revealed that the motif surrounding phosphorylated tyrosine 338 within the acid-rich domain of the IL-2R beta is a binding site for Shc. Deletion of this domain has previously been shown to abrogate the ability of IL-2 to activate Ras but does not affect IL-2-dependent mitogenesis in the presence of serum. Proliferation assays of Ba/F3 cells containing IL-2R beta chain deletion mutants in serum-free medium with or without insulin shows that deletion of the acid-rich domain does not affect IL-2-driven mitogenesis regardless of the culture conditions. This study thus defines the critical domain within the IL-2R beta chain required to mediate Shc binding and Shc tyrosine phosphorylation and further shows that Shc binding and phosphorylation are not required for IL-2-dependent mitogenesis. Neither serum nor insulin is required to supplement the loss of induction of the Shc adapter or Ras pathways, which therefore suggests a novel mechanism for mitogenic signal transduction mediated by this hematopoietin receptor.

Src homologous and collagen (Shc) protein binds to F-actin and translocates to the cytoskeleton upon nerve growth factor stimulation in PC12 cells

Immunoprecipitates of metabolically labeled PC12 cells consistently contained a 43-kDa protein that was associated with Shc, a signal-transducing protein with a single SH2 domain. Following affinity chromatography with immobilized recombinant glutathione S-transferase (GST)-Shc fusion protein, the 43-kDa protein was identified as actin by mass spectrometry and immunoblotting. Cosedimentation experiments using purified actin and GST-Shc showed that Shc binds directly to F-actin, confirming Shc-actin interaction in vivo. Various GST-truncated Shc fusion proteins were prepared and used in actin cosedimentation assays. Constructs containing the SH2 and collagen homology domains were not precipitated, and those containing the amino-terminal domain were. Thus, Shc-actin interactions do not occur in the region of tyrosine phosphorylation and leave the SH2 domain free to bind to other tyrosine-phosphorylated molecules. Although the major pool of Shc in unstimulated PC12 cells is soluble, two other pools are associated with the cytoskeleton and the submembranous cytoskeleton. Upon nerve growth factor stimulation, approximately 50% of the soluble Shc translocates to both cytoskeleton environments within 2 min, decreasing thereafter. When cells were pretreated with cytochalasin D, a drug that disrupts actin filaments, Shc translocation to the cytoskeleton was abolished. However, in the submembranous fraction, the Shc level was elevated in resting cells following cytochalasin D treatment. The kinetics of translocation, compared to mitogen-activated protein kinase activation, and the nature of the Shc-actin interaction suggest that the cytoskeletal association of Shc, induced by growth factors, may be related to membrane ruffling and actin fiber reorganization.

Shc isoform-specific tyrosine phosphorylation by the insulin and epidermal growth factor receptors

Insulin stimulation of Chinese hamster ovary cells expressing the human insulin and epidermal growth factor (EGF) receptors (CHO/IR/ER) resulted in the tyrosine phosphorylation of the 52-kDa Shc isoform with a relatively low extent of 46-kDa Shc tyrosine phosphorylation. In contrast, EGF stimulation resulted in the tyrosine phosphorylation of both the 52- and 46-kDa Shc isoforms. Consistent with these differences, Grb2 predominantly bound to the 52-kDa Shc isoform following insulin stimulation, whereas Grb2 associated with both the 52- and 46-kDa Shc isoforms after EGF stimulation. Further, in vitro kinetic analysis demonstrated that the insulin receptor has a 4-fold greater Vmax with no significant difference in the Km for the purified 52-kDa Shc isoform compared with the 46-kDa Shc isoform. However, the EGF receptor displayed the identical Vmax and Km for tyrosine phosphorylation of both of these species. In direct contrast to the EGF receptor, we also observed significant differences in binding interactions between the insulin receptor with the 52- and 46-kDa Shc isoforms in vitro. These data demonstrate that the predominant insulin-dependent Shc signaling pathway occurs via the 52-kDa Shc isoform, whereas the EGF receptor can effectively use both the 52- and 46-kDa Shc species.

Convergence of angiotensin II and platelet-derived growth factor receptor signaling cascades in vascular smooth muscle cells

Signaling cascades elicited by angiotensin II resemble those characteristic of growth factor stimulation. In this report, we demonstrate that angiotensin II converges with platelet-derived growth factor (PDGF) beta-receptor signaling cascades, independent of PDGF. Stimulation of smooth muscle cells with angiotensin II resulted in tyrosine phosphorylation on Shc proteins and subsequent complex formation between Shc and growth factor receptor binding protein-2 (GRB2). A 180-kDa protein co-precipitating with Shc.GRB2 complexes also demonstrated increased phosphorylation in response to angiotensin II. Immunoblot analyses and proteolytic digests failed to distinguish this 180-kDa protein from authentic PDGF beta-receptors. Corresponding with Shc and PDGF receptor phosphorylation induced by angiotensin II was the recruitment and phosphorylation of c-Src. Autocrine release of platelet-derived growth factor failed to account for Shc complex formation at the PDGF receptor following angiotensin II treatment, and a specific angiotensin II type I receptor antagonist, losartan, abolished the response. These results support a novel model for cross-talk between the G-protein-linked angiotensin II receptor and the PDGF receptor tyrosine kinase in vascular smooth muscle cells. Communication with the PDGF receptor may account for the ability of angiotensin II to elicit responses typical of growth factor signal transduction.