Aberrant Ras regulation and reduced p190 tyrosine phosphorylation in cells lacking p120-Gap

p120 RasGAP and ZO-2 are essential for Hippo signaling and tumor-suppressor function mediated by p190A RhoGAP

ARHGAP35, which encodes p190A RhoGAP (p190A), is a major cancer gene. p190A is a tumor suppressor that activates the Hippo pathway. p190A was originally cloned via direct binding to p120 RasGAP (RasGAP). Here, we determine that interaction of p190A with the tight-junction-associated protein ZO-2 is dependent on RasGAP. We establish that both RasGAP and ZO-2 are necessary for p190A to activate large tumor-suppressor (LATS) kinases, elicit mesenchymal-to-epithelial transition, promote contact inhibition of cell proliferation, and suppress tumorigenesis. Moreover, RasGAP and ZO-2 are required for transcriptional modulation by p190A. Finally, we demonstrate that low ARHGAP35 expression is associated with shorter survival in patients with high, but not low, transcript levels of TJP2 encoding ZO-2. Hence, we define a tumor-suppressor interactome of p190A that includes ZO-2, an established constituent of the Hippo pathway, and RasGAP, which, despite strong association with Ras signaling, is essential for p190A to activate LATS kinases.

Diverse p120RasGAP interactions with doubly phosphorylated partners EphB4, p190RhoGAP, and Dok1

RasGAP (p120RasGAP), the founding member of the GTPase-activating protein (GAP) family, is one of only nine human proteins to contain two SH2 domains and is essential for proper vascular development. Despite its importance, its interactions with key binding partners remains unclear. In this study we provide a detailed viewpoint of RasGAP recruitment to various binding partners and assess their impact on RasGAP activity. We reveal the RasGAP SH2 domains generate distinct binding interactions with three well-known doubly phosphorylated binding partners: p190RhoGAP, Dok1, and EphB4. Affinity measurements demonstrate a 100-fold weakened affinity for RasGAP-EphB4 binding compared to RasGAP-p190RhoGAP or RasGAP-Dok1 binding, possibly driven by single versus dual SH2 domain engagement with a dominant N-terminal SH2 interaction. Small-angle X-ray scattering reveals conformational differences between RasGAP-EphB4 binding and RasGAP-p190RhoGAP binding. Importantly, these interactions do not impact catalytic activity, implying RasGAP utilizes its SH2 domains to achieve diverse spatial-temporal regulation of Ras signaling in a previously unrecognized fashion.

p120 RasGAP and ZO-2 are essential for Hippo signaling and tumor suppressor function mediated by p190A RhoGAP

ARHGAP35 , which encodes p190A RhoGAP (p190A), is a major cancer gene. p190A is a tumor suppressor that activates the Hippo pathway. p190A was originally cloned via direct binding to p120 RasGAP (RasGAP). Here, we determine that a novel interaction of p190A with the tight junction-associated protein ZO-2 is dependent on RasGAP. We establish that both RasGAP and ZO-2 are necessary for p190A to activate LATS kinases, elicit mesenchymal-to-epithelial transition, promote contact inhibition of cell proliferation and suppress tumorigenesis. Moreover, RasGAP and ZO-2 are required for transcriptional modulation by p190A. Finally, we demonstrate that low ARHGAP35 expression is associated with shorter survival in patients with high, but not low, transcript levels of TJP2 encoding ZO-2. Hence, we define a tumor suppressor interactome of p190A that includes ZO-2, an established constituent of the Hippo pathway, and RasGAP, which despite strong association with Ras signaling, is essential for p190A to activate LATS kinases.

Crosstalk of the Caspase Family and Mammalian Target of Rapamycin Signaling

Cell can integrate the caspase family and mammalian target of rapamycin (mTOR) signaling in response to cellular stress triggered by environment. It is necessary here to elucidate the direct response and interaction mechanism between the two signaling pathways in regulating cell survival and determining cell fate under cellular stress. Members of the caspase family are crucial regulators of inflammation, endoplasmic reticulum stress response and apoptosis. mTOR signaling is known to mediate cell growth, nutrition and metabolism. For instance, over-nutrition can cause the hyperactivation of mTOR signaling, which is associated with diabetes. Nutrition deprivation can inhibit mTOR signaling via SH3 domain-binding protein 4. It is striking that Ras GTPase-activating protein 1 is found to mediate cell survival in a caspase-dependent manner against increasing cellular stress, which describes a new model of apoptosis. The components of mTOR signaling-raptor can be cleaved by caspases to control cell growth. In addition, mTOR is identified to coordinate the defense process of the immune system by suppressing the vitality of caspase-1 or regulating other interferon regulatory factors. The present review discusses the roles of the caspase family or mTOR pathway against cellular stress and generalizes their interplay mechanism in cell fate determination.

The GTPase-activating protein p120RasGAP has an evolutionarily conserved "FLVR-unique" SH2 domain

The Src homology 2 (SH2) domain has a highly conserved architecture that recognizes linear phosphotyrosine motifs and is present in a wide range of signaling pathways across different evolutionary taxa. A hallmark of SH2 domains is the arginine residue in the conserved FLVR motif that forms a direct salt bridge with bound phosphotyrosine. Here, we solve the X-ray crystal structures of the C-terminal SH2 domain of p120RasGAP (RASA1) in its apo and peptide-bound form. We find that the arginine residue in the FLVR motif does not directly contact pTyr1087 of a bound phosphopeptide derived from p190RhoGAP; rather, it makes an intramolecular salt bridge to an aspartic acid. Unexpectedly, coordination of phosphotyrosine is achieved by a modified binding pocket that appears early in evolution. Using isothermal titration calorimetry, we find that substitution of the FLVR arginine R377A does not cause a significant loss of phosphopeptide binding, but rather a tandem substitution of R398A (SH2 position βD4) and K400A (SH2 position βD6) is required to disrupt the binding. These results indicate a hitherto unrecognized diversity in SH2 domain interactions with phosphotyrosine and classify the C-terminal SH2 domain of p120RasGAP as "FLVR-unique."

Crystal structures of p120RasGAP N-terminal SH2 domain in its apo form and in complex with a p190RhoGAP phosphotyrosine peptide

The Rho and Ras pathways play vital roles in cell growth, division and motility. Cross-talk between the pathways amplifies their roles in cell proliferation and motility and its dysregulation is involved in disease pathogenesis. One important interaction for cross-talk occurs between p120RasGAP (RASA1), a GTPase activating protein (GAP) for Ras, and p190RhoGAP (p190RhoGAP-A, ARHGAP35), a GAP for Rho. The binding of these proteins is primarily mediated by two SH2 domains within p120RasGAP engaging phosphorylated tyrosines of p190RhoGAP, of which the best studied is pTyr-1105. To better understand the interaction between p120RasGAP and p190RhoGAP, we determined the 1.75 Å X-ray crystal structure of the N-terminal SH2 domain of p120RasGAP in the unliganded form, and its 1.6 Å co-crystal structure in complex with a synthesized phosphotyrosine peptide, EEENI(p-Tyr)SVPHDST, corresponding to residues 1100–1112 of p190RhoGAP. We find that the N-terminal SH2 domain of p120RhoGAP has the characteristic SH2 fold encompassing a central beta-sheet flanked by two alpha-helices, and that peptide binding stabilizes specific conformations of the βE-βF loop and arginine residues R212 and R231. Site-directed mutagenesis and native gel shifts confirm phosphotyrosine binding through the conserved FLVR motif arginine residue R207, and isothermal titration calorimetry finds a dissociation constant of 0.3 ± 0.1 μM between the phosphopeptide and SH2 domain. These results demonstrate that the major interaction between two important GAP proteins, p120RasGAP and p190RhoGAP, is mediated by a canonical SH2-pTyr interaction.

The NF2 tumor suppressor merlin interacts with Ras and RasGAP, which may modulate Ras signaling

Inactivation of the tumor suppressor NF2/merlin underlies neurofibromatosis type 2 (NF2) and some sporadic tumors. Previous studies have established that merlin mediates contact inhibition of proliferation; however, the exact mechanisms remain obscure and multiple pathways have been implicated. We have previously reported that merlin inhibits Ras and Rac activity during contact inhibition, but how merlin regulates Ras activity has remained elusive. Here we demonstrate that merlin can directly interact with both Ras and p120RasGAP (also named RasGAP). While merlin does not increase the catalytic activity of RasGAP, the interactions with Ras and RasGAP may fine-tune Ras signaling. In vivo, loss of RasGAP in Schwann cells, unlike the loss of merlin, failed to promote tumorigenic growth in an orthotopic model. Therefore, modulation of Ras signaling through RasGAP likely contributes to, but is not sufficient to account for, merlin’s tumor suppressor activity. Our study provides new insight into the mechanisms of merlin-dependent Ras regulation and may have additional implications for merlin-dependent regulation of other small GTPases.

p120RasGAP Protein Mediates Netrin-1 Protein-induced Cortical Axon Outgrowth and Guidance

The receptor deleted in colorectal cancer (DCC) mediates the attraction of growing axons to netrin-1 during brain development. In response to netrin-1 stimulation, DCC becomes a signaling platform to recruit proteins that promote axon outgrowth and guidance. The Ras GTPase-activating protein (GAP) p120RasGAP inhibits Ras activity and mediates neurite retraction and growth cone collapse in response to repulsive guidance cues. Here we show an interaction between p120RasGAP and DCC that positively regulates netrin-1-mediated axon outgrowth and guidance in embryonic cortical neurons. In response to netrin-1, p120RasGAP is recruited to DCC in growth cones and forms a multiprotein complex with focal adhesion kinase and ERK. We found that Ras/ERK activities are elevated aberrantly in p120RasGAP-deficient neurons. Moreover, the expression of p120RasGAP Src homology 2 (SH2)-SH3-SH2 domains, which interact with the C-terminal tail of DCC, is sufficient to restore netrin-1-dependent axon outgrowth in p120RasGAP-deficient neurons. We provide a novel mechanism that exploits the scaffolding properties of the N terminus of p120RasGAP to tightly regulate netrin-1/DCC-dependent axon outgrowth and guidance.

RasGAP Promotes Autophagy and Thereby Suppresses Platelet-Derived Growth Factor Receptor-Mediated Signaling Events, Cellular Responses, and Pathology

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) make profound contributions to both physiology and pathology. While it is widely believed that direct (PDGF-mediated) activation is the primary mode of activating PDGFRs, the discovery that they can also be activated indirectly begs the question of the relevance of the indirect mode of activating PDGFRs. In the context of a blinding eye disease, indirect activation of PDGFRα results in persistent signaling, which suppresses the level of p53 and thereby promotes viability of cells that drive pathogenesis. Under the same conditions, PDGFRβ fails to undergo indirect activation. In this paper, we report that RasGAP (GTPase-activating protein of Ras) prevented indirect activation of PDGFRβ. RasGAP, which associates with PDGFRβ but not PDGFRα, reduced the level of mitochondrion-derived reactive oxygen species, which are required for enduring activation of PDGFRs. Furthermore, preventing PDGFRβ from associating with RasGAP allowed it to signal enduringly and drive pathogenesis of a blinding eye disease. These results indicate a previously unappreciated role of RasGAP in antagonizing indirect activation of PDGFRβ, define the underlying mechanism, and raise the possibility that PDGFRβ-mediated diseases involve indirect activation of PDGFRβ.

A review of platelet derived growth factor playing pivotal role in bone regeneration

This article is focused on the literature review and study of recent advances in the field of bone grafting, which involves platelet-derived growth factor (PDGF) as one of the facilitating factors in bone regeneration. This article includes a description of the mechanism of PDGF for use in surgeries where bone grafting is required, which promotes future application of PDGF for faster bone regeneration or inhibition of bone growth if required as in osteosarcoma. The important specific activities of PDGF include mitogenesis (increase in the cell populations of healing cells), angiogenesis (endothelial mitoses into functioning capillaries), and macrophage activation (debridement of the wound site and a second phase source of growth factors for continued repair and bone regeneration). Thus PDGF can be utilized in wound with bone defect to conceal the wound with repair of bony defect.

p120RasGAP is a mediator of rho pathway activation and tumorigenicity in the DLD1 colorectal cancer cell line

KRAS is mutated in ∼40% of colorectal cancer (CRC), and there are limited effective treatments for advanced KRAS mutant CRC. Therefore, it is crucial that downstream mediators of oncogenic KRAS continue to be studied. We identified p190RhoGAP as being phosphorylated in the DLD1 CRC cell line, which expresses a heterozygous KRAS G13D allele, and not in DKO4 in which the mutant allele has been deleted by somatic recombination. We found that a ubiquitous binding partner of p190RhoGAP, p120RasGAP (RasGAP), is expressed in much lower levels in DKO4 cells compared to DLD1, and this expression is regulated by KRAS. Rescue of RasGAP expression in DKO4 rescued Rho pathway activation and partially rescued tumorigenicity in DKO4 cells, indicating that the combination of mutant KRAS and RasGAP expression is crucial to these phenotypes. We conclude that RasGAP is an important effector of mutant KRAS in CRC.

Nonredundant functions for Ras GTPase-activating proteins in tissue homeostasis

Inactivation of the small guanosine triphosphate-binding protein Ras during receptor signal transduction is mediated by Ras guanosine triphosphatase (GTPase)-activating proteins (RasGAPs). Ten different RasGAPs have been identified and have overlapping patterns of tissue distribution. However, genetic analyses are revealing critical nonredundant functions for each RasGAP in tissue homeostasis and as regulators of disease processes in mouse and man. Here, we discuss advances in understanding the role of RasGAPs in the maintenance of tissue integrity.

RASA1 maintains the lymphatic vasculature in a quiescent functional state in mice

RASA1 (also known as p120 RasGAP) is a Ras GTPase-activating protein that functions as a regulator of blood vessel growth in adult mice and humans. In humans, RASA1 mutations cause capillary malformation-arteriovenous malformation (CM-AVM); whether it also functions as a regulator of the lymphatic vasculature is unknown. We investigated this issue using mice in which Rasa1 could be inducibly deleted by administration of tamoxifen. Systemic loss of RASA1 resulted in a lymphatic vessel disorder characterized by extensive lymphatic vessel hyperplasia and leakage and early lethality caused by chylothorax (lymphatic fluid accumulation in the pleural cavity). Lymphatic vessel hyperplasia was a consequence of increased proliferation of lymphatic endothelial cells (LECs) and was also observed in mice in which induced deletion of Rasa1 was restricted to LECs. RASA1-deficient LECs showed evidence of constitutive activation of Ras in situ. Furthermore, in isolated RASA1-deficient LECs, activation of the Ras signaling pathway was prolonged and cellular proliferation was enhanced after ligand binding to different growth factor receptors, including VEGFR-3. Blockade of VEGFR-3 was sufficient to inhibit the development of lymphatic vessel hyperplasia after loss of RASA1 in vivo. These findings reveal a role for RASA1 as a physiological negative regulator of LEC growth that maintains the lymphatic vasculature in a quiescent functional state through its ability to inhibit Ras signal transduction initiated through LEC-expressed growth factor receptors such as VEGFR-3.

Differential Regulation of RasGAPs in Cancer

Ever since their discovery as cellular counterparts of viral oncogenes more than 25 years ago, much progress has been made in understanding the complex networks of signal transduction pathways activated by oncogenic Ras mutations in human cancers. The activity of Ras is regulated by nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), and much emphasis has been put into the biochemical and structural analysis of the Ras/GAP complex. The mechanisms by which GAPs catalyze Ras-GTP hydrolysis have been clarified and revealed that oncogenic Ras mutations confer resistance to GAPs and remain constitutively active. However, it is yet unclear how cells coordinate the large and divergent GAP protein family to promote Ras inactivation and ensure a certain biological response. Different domain arrangements in GAPs to create differential protein-protein and protein-lipid interactions are probably key factors determining the inactivation of the 3 Ras isoforms H-, K-, and N-Ras and their effector pathways. In recent years, in vitro as well as cell- and animal-based studies examining GAP activity, localization, interaction partners, and expression profiles have provided further insights into Ras inactivation and revealed characteristics of several GAPs to exert specific and distinct functions. This review aims to summarize knowledge on the cell biology of RasGAP proteins that potentially contributes to differential regulation of spatiotemporal Ras signaling.

A FAK-p120RasGAP-p190RhoGAP complex regulates polarity in migrating cells

Directional motility is a complex process requiring the spatiotemporal integration of signals that regulate cytoskeletal changes, and the establishment of an anteroposterior or polarized cell axis. Focal adhesion kinase (FAK) promotes cell migration, but a molecular role for FAK in promoting cell polarity remains undefined. Here, using wound healing and Golgi-reorientation analyses, we show that fibroblast, endothelial and carcinoma polarity during cell migration requires FAK and is associated with a complex between FAK, p120RasGAP and p190RhoGAP (p190A), leading to p190A tyrosine phosphorylation. Fibronectin-integrin-mediated FAK activation and phosphorylation promote SH2-mediated binding of p120RasGAP to FAK and FAK-mediated p190A tyrosine phosphorylation. The association of p120RasGAP with FAK facilitates the formation of a FAK-p120RasGAP-p190A complex targeted to leading-edge focal adhesions by FAK. Knockdown of p120RasGAP, mutation of FAK Y397 or inhibition of FAK activity prevent the association of FAK with p190A and subsequent tyrosine phosphorylation of p190A, and result in the loss of cell polarity. Because reconstitution of FAK-null fibroblasts with FAK or a Pyk2-FAK chimera restore the normal decrease in RhoA GTP binding upon cell spreading on fibronectin, our studies support a model whereby FAK activity facilitates the recruitment and stabilization of a p120RasGAP-p190A complex at leading-edge focal adhesions connected to the transient inhibition of RhoA activity and the regulation of cell polarity.

Overexpression of E-cadherin on melanoma cells inhibits chemokine-promoted invasion involving p190RhoGAP/p120ctn-dependent inactivation of RhoA

Melanoma cells express the chemokine receptor CXCR4 that confers high invasiveness upon binding to its ligand CXCL12. Melanoma cells at initial stages of the disease show reduction or loss of E-cadherin expression, but recovery of its expression is frequently found at advanced phases. We overexpressed E-cadherin in the highly invasive BRO lung metastatic cell melanoma cell line to investigate whether it could influence CXCL12-promoted cell invasion. Overexpression of E-cadherin led to defective invasion of melanoma cells across Matrigel and type I collagen in response to CXCL12. A decrease in individual cell migration directionality toward the chemokine and reduced adhesion accounted for the impaired invasion. A p190RhoGAP-dependent inhibition of RhoA activation was responsible for the impairment in chemokine-stimulated E-cadherin melanoma transfectant invasion. Furthermore, we show that p190RhoGAP and p120ctn associated predominantly on the plasma membrane of cells overexpressing E-cadherin, and that E-cadherin-bound p120ctn contributed to RhoA inactivation by favoring p190RhoGAP-RhoA association. These results suggest that melanoma cells at advanced stages of the disease could have reduced metastatic potency in response to chemotactic stimuli compared with cells lacking E-cadherin, and the results indicate that p190RhoGAP is a central molecule controlling melanoma cell invasion.

p120-Ras GTPase activating protein (RasGAP): a multi-interacting protein in downstream signaling

p120-RasGAP (Ras GTPase activating protein) plays a key role in the regulation of Ras-GTP bound by promoting GTP hydrolysis via its C-terminal catalytic domain. The p120-RasGAP N-terminal part contains two SH2, SH3, PH (pleckstrin homology) and CaLB/C2 (calcium-dependent phospholipid-binding domain) domains. These protein domains allow various functions, such as anti-/pro-apoptosis, proliferation and also cell migration depending of their distinct partners. The p120-RasGAP domain participates in protein-protein interactions with Akt, Aurora or RhoGAP to regulate functions described bellow. Here, we summarize, in angiogenesis and cancer, the various functional roles played by p120-RasGAP domains and their effector partners in downstream signaling.

Annexin A6 inhibits Ras signalling in breast cancer cells

Overexpression of epidermal growth factor receptor (EGFR) is associated with enhanced activation of wild-type (hyperactive) Ras in breast cancer. Little is known about the regulation of Ras inactivation and GTPase-activating proteins (GAPs), such as p120GAP, in cells with hyperactive Ras. Recently, we showed that in EGFR-overexpressing A431 cells, which lack endogenous Annexin A6 (AnxA6), ectopic expression of AnxA6 stimulates membrane recruitment of p120GAP to modulate Ras signalling. We now demonstrate that, AnxA6 is downregulated in a number of EGFR-overexpressing and estrogen receptor (ER)-negative breast cancer cells. In these cells, AnxA6 overexpression promotes Ca(2+)- and EGF-inducible membrane targeting of p120GAP. In ER-negative MDA-MB-436 cells, overexpression of p120GAP, but not CAPRI or a p120GAP mutant lacking the AnxA6-binding domain inhibits Ras/MAPK activity. AnxA6 knockdown in MDA-MB-436 increases Ras activity and cell proliferation in anchorage-independent growth assays. Furthermore, AnxA6 co-immunoprecipitates with H-Ras in a Ca(2+)- and EGF-inducible manner and fluorescence resonance energy transfer (FRET) microscopy confirmed that AnxA6 is in close proximity of active (G12V), but not inactive (S17N) H-Ras. Thus, association of AnxA6 with H-Ras-containing protein complexes may contribute to regulate p120GAP/Ras assembly in EGFR-overexpressing and ER-negative breast cancer cells.

Suppressor of cytokine signaling 1 inhibits IFN-gamma inflammatory signaling in human keratinocytes by sustaining ERK1/2 activation

IFN-gamma is a pleiotropic cytokine importantly involved in the development of skin inflammatory responses. Epidermal keratinocytes are extremely susceptible to IFN-gamma action, but, once transduced with the suppressors of cytokine signaling (SOCS)1 molecule, they can no longer express a number of IFN-gamma-inducible signal transducer and activator of transcription (STAT)1-dependent genes. Extracellular-signal-regulated kinase (ERK)1/2 pathway is also involved in the protection of keratinocytes from the proinflammatory effect of IFN-gamma. Here we show that, after IFN-gamma stimulation, SOCS1 inhibited IFN-gamma receptor and STAT1 phosphorylation but maintained ERK1/2 activation. SOCS1 was also necessary for the IFN-gamma-induced RAS and Raf-1 activities in keratinocytes. The enhanced ERK1/2 pathway in SOCS1-overexpressing keratinocytes was in part responsible for their inability to respond to IFN-gamma, in terms of CXCL10 and CCL2 production, and for the high production of CXCL8. Moreover, SOCS1 interacted with the RAS inhibitor p120 RasGAP and promoted its degradation after IFN-gamma stimulation. We hypothesize that SOCS1 functions as suppressor of IFN-gamma signaling, not only by inhibiting STAT1 activation but also by sustaining ERK1/2-dependent antiinflammatory pathways.

Generation of mice with a conditional allele of the p120 Ras GTPase-activating protein

p120 Ras GTPase-activating protein (RasGAP) encoded by the rasa1 gene in mice is a prototypical member of the RasGAP family of proteins involved in negative-regulation of the p21 Ras proto-oncogene. RasGAP has been implicated in signal transduction through a number of cell surface receptors. In humans, inactivating mutations in the coding region of the RASA1 gene cause capillary malformation arteriovenous malformation. In mice, generalized disruption of the rasa1 gene results in early embryonic lethality associated with defective vasculogenesis and increased apoptosis of neuronal cells. The early lethality in this mouse model precludes its use to further study the importance of RasGAP as a regulator of cell function. Therefore, to circumvent this problem, we have generated a conditional rasa1 knockout mouse. In this mouse, an exon that encodes a part of the RasGAP protein essential for catalytic activity has been flanked by loxP recognition sites. With the use of different constitutive and inducible Cre transgenic mouse lines, we show that deletion of this exon from the rasa1 locus results in effective loss of expression of catalytically-active RasGAP from a variety of adult tissues. The conditional rasa1 mouse will be useful for the analysis of the role of RasGAP in mature cell types.

Genome-wide discovery of Pax7 target genes during development

Pax7 plays critical roles in development of brain, spinal cord, neural crest, and skeletal muscle. As a sequence-specific DNA-binding transcription factor, any direct functional role played by Pax7 during development is mediated through target gene selection. Thus, we have sought to identify genes targeted by Pax7 during embryonic development using an unbiased chromatin immunoprecipitation (ChIP) cloning assay to isolate cis-regulatory regions bound by Pax7 in vivo. Sequencing and genomic localization of a library of chromatin-DNA fragments bound by Pax7 has identified 34 candidate Pax7 target genes, with occupancy of a selection confirmed with independent chromatin enrichment tests (ChIP-PCR). To assess the capacity of Pax7 to regulate transcription from these loci, we have cloned alternate transcripts of Pax7 (differing significantly in their DNA binding domain) into expression vectors and transfected cultured cells with these constructs, then analyzed target gene expression levels using RT-PCR. We show that Pax7 directly occupies sites within genes encoding transcription factors Gbx1 and Eya4, the neurogenic cytokine receptor ciliary neurotrophic factor receptor, the neuronal potassium channel Kcnk2, and the signal transduction kinase Camk1d in vivo and regulates the transcriptional state of these genes in cultured cells. This analysis gives us greater insight into the direct functional role played by Pax7 during embryonic development.

Neurofibroma-associated growth factors activate a distinct signaling network to alter the function of neurofibromin-deficient endothelial cells

Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.

Neurofibromin is a novel regulator of RAS-induced signals in primary vascular smooth muscle cells

Neurofibromatosis type I (NF1) is a genetic disorder caused by mutations in the NF1 tumor suppressor gene. Neurofibromin is encoded by NF1 and functions as a negative regulator of Ras activity. NF1 patients develop renal artery stenosis and arterial occlusions resulting in cerebral and visceral infarcts. Further, NF1 patients develop vascular neurofibromas where tumor vessels are invested in a dense pericyte sheath. Although it is well established that aberrations in Ras signaling lead to human malignancies, emerging data generated in genetically engineered mouse models now implicate perturbations in the Ras signaling axis in vascular smooth muscular cells (VSMCs) as central to the initiation and progression of neointimal hyperplasia and arterial stenosis. Despite these observations, the function of neurofibromin in regulating VSMC function and how Ras signals are terminated in VSMCs is virtually unknown. Utilizing VSMCs harvested from Nf1+/- mice and primary human neurofibromin-deficient VSMCs, we identify a discrete Ras effector pathway, which is tightly regulated by neurofibromin to limit VSMC proliferation and migration. Thus, these studies identify neurofibromin as a novel regulator of Ras activity in VSMCs and provide a framework for understanding cardiovascular disease in NF1 patients and a mechanism by which Ras signals are attenuated for maintaining VSMC homeostasis in blood vessel walls.

Molecular mechanisms involved in Ras inactivation: the annexin A6–p120GAP complex

In mammalian cells, a complex network of signaling pathways tightly regulates a variety of cellular processes, such as proliferation and differentiation. New insights from one of the most-important signaling cascades involved in oncogenesis, the Ras-Raf-MAPK pathway, suggest that the subcellular localisation and assembly of signaling modules of this pathway is crucial to control the biological response. This commonly requires membrane targeting events that are mediated by adaptor/scaffold proteins. Of particular interest is the translocation and complex formation of GTPase-activating proteins (GAPs), such as p120GAP, at the plasma membrane to inactivate Ras. Recent studies indicate that one member of the annexin family, annexin A6 acts as a targeting protein for p120GAP. This review discusses how annexin A6 modulates the involvement of negative regulators of the Ras-Raf-MAPK pathway contributing to Ras inactivation.

Mouse models of K-ras-initiated carcinogenesis

Activating mutations of the oncogene K-ras are found in one third of all human cancers. Much of our knowledge on K-ras signal transduction and its influence on tumor initiation and progression comes from in vitro studies with cell lines. However, mouse models of human cancer allow a much more faithful recapitulation of the human disease, and the in vivo perspective is crucial for our understanding of neoplasia. In recent years, several new murine models for K-ras-induced tumorigenesis have been described. They allow new insights into the specific role that oncogenic K-ras proteins play in different solid tumors, and they permit the molecular dissection of the pathways that are initiated by somatic mutations in subsets of cells. Key advances have been made by the use of tissue-specific and inducible control of expression, which is achieved by the Cre/LoxP technology or the tetracycline system. from these sophisticated models, a common picture emerges: The effects of K-ras on tumor initiation depend strongly on the cellular context, and different tissues vary in their susceptibility to K-ras transformation.

Impaired Akt activity down-modulation, caspase-3 activation, and apoptosis in cells expressing a caspase-resistant mutant of RasGAP at position 157

RasGAP bears two caspase-3 cleavage sites that are used sequentially as caspase activity increases in cells. When caspase-3 is mildly activated, RasGAP is first cleaved at position 455. This leads to the production of an N-terminal fragment, called fragment N, that activates the Ras-PI3K-Akt pathway and that promotes cell survival. At higher caspase activity, RasGAP is further cleaved at position 157 generating two small N-terminal fragments named N1 and N2. We have now determined the contribution of this second cleavage event in the regulation of apoptosis using cells in which the wild-type RasGAP gene has been replaced by a cDNA encoding a RasGAP mutant that cannot be cleaved at position 157. Our results show that cleavage of fragment N at position 157 leads to a marked reduction in Akt activity. This is accompanied by efficient processing of caspase-3 that favors cell death in response to various apoptotic stimuli. In nontumorigenic cells, fragments N1 and N2 do not modulate apoptosis. Therefore, the role of the second caspase-mediated cleavage of RasGAP is to allow the inactivation of the antiapoptotic function of fragment N so that caspases are no longer hampered in their ability to kill cells.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.

Partial cleavage of RasGAP by caspases is required for cell survival in mild stress conditions

Tight control of apoptosis is required for proper development and maintenance of homeostasis in multicellular organisms. Cells can protect themselves from potentially lethal stimuli by expressing antiapoptotic factors, such as inhibitors of apoptosis, FLICE (caspase 8)-inhibitory proteins, and members of the Bcl2 family. Here, we describe a mechanism that allows cells to survive once executioner caspases have been activated. This mechanism relies on the partial cleavage of RasGAP by caspase 3 into an amino-terminal fragment called fragment N. Generation of this fragment leads to the activation of the antiapoptotic Akt kinase, preventing further amplification of caspase activity. Partial cleavage of RasGAP is required for cell survival under stress conditions because cells expressing an uncleavable RasGAP mutant cannot activate Akt, cannot prevent amplification of caspase 3 activity, and eventually undergo apoptosis. Executioner caspases therefore control the extent of their own activation by a feedback regulatory mechanism initiated by the partial cleavage of RasGAP that is crucial for cell survival under adverse conditions.

v-Src induces Shc binding to tyrosine 63 in the cytoplasmic domain of the LDL receptor-related protein 1

We recently observed that the LDL receptor-related protein 1 (LRP-1) is tyrosine phosphorylated in v-Src-transformed cells. Using a GST-fusion protein containing the cytoplasmic domain of LRP-1, we show that LRP-1 is a direct substrate for v-Src in vitro. To study LRP-1 phosphorylation in vivo, we constructed an LRP-1 minireceptor composed of the beta chain linked at the amino-terminus to a Myc epitope (Myc-LRPbeta). When expressed together with v-Src, Myc-LRPbeta becomes phosphorylated on tyrosine. Of the four tyrosine residues present in the cytoplasmic domain of LRP-1, only Tyr 63 is phosphorylated by v-Src in vivo or in vitro. Using fibroblasts deficient in Src, Yes and Fyn, we were able to show that there are multiple kinases present in the cell that can phosphorylate LRP-1. Tyrosine-phosphorylated LRP-1 associates with Shc, a PTB and SH2 domain containing signaling protein that is involved in the activation of Ras. Binding of the purified Shc PTB domain to Tyr 63 containing peptides shows that the interaction between LRP-1 and Shc is direct. We found that DAB, a PTB domain containing signaling protein that is involved in signaling by LDL receptor-related proteins in the nervous system, did not bind to full-length LRP-1. Our observations suggest that LRP-1 may be involved in normal and malignant signal transduction through a direct interaction with Shc adaptor proteins.

Deregulation of the Egfr/Ras signaling pathway induces age-related brain degeneration in the Drosophila mutant vap

Ras signaling has been shown to play an important role in promoting cell survival in many different tissues. Here we show that upregulation of Ras activity in adult Drosophila neurons induces neuronal cell death, as evident from the phenotype of vacuolar peduncle (vap) mutants defective in the Drosophila RasGAP gene, which encodes a Ras GTPase-activating protein. These mutants show age-related brain degeneration that is dependent on activation of the EGF receptor signaling pathway in adult neurons, leading to autophagic cell death (cell death type 2). These results provide the first evidence for a requirement of Egf receptor activity in differentiated adult Drosophila neurons and show that a delicate balance of Ras activity is essential for the survival of adult neurons.

Deregulation of the Egfr/Ras signaling pathway induces age-related brain degeneration in the Drosophila mutant vap

Ras signaling has been shown to play an important role in promoting cell survival in many different tissues. Here we show that upregulation of Ras activity in adult Drosophila neurons induces neuronal cell death, as evident from the phenotype of vacuolar peduncle (vap) mutants defective in the Drosophila RasGAP gene, which encodes a Ras GTPase-activating protein. These mutants show age-related brain degeneration that is dependent on activation of the EGF receptor signaling pathway in adult neurons, leading to autophagic cell death (cell death type 2). These results provide the first evidence for a requirement of Egf receptor activity in differentiated adult Drosophila neurons and show that a delicate balance of Ras activity is essential for the survival of adult neurons.

Lymphoproliferative defects in mice lacking the expression of neurofibromin: functional and biochemical consequences of Nf1 deficiency in T-cell development and function

Ras plays an essential role in lymphocyte development and function. However, in vivo consequence(s) of regulation of Ras activity by guanosine triphosphatase (GTPase)-activating proteins (GAPs) on lymphocyte development and function are not known. In this study we demonstrate that neurofibromin, the protein encoded by the NF1 tumor suppressor gene functions as a GAP for Ras in T cells. Loss of Nf1 in T cells results in enhanced Ras activation, which is associated with thymic and splenic hyperplasia, and an increase in the absolute number of immature and mature T-cell subsets compared with control mice. Interestingly, in spite of a profound T-cell expansion and higher thymidine incorporation in unstimulated Nf1-deficient T cells, T-cell receptor and interleukin-2 receptor-mediated proliferation of thymocytes and mature T cells was substantially reduced compared with control mice. Collectively, these results identify neurofibromin as a GAP for Ras in T cells for maintaining immune homeostasis in vivo.

Targeted expression of oncogenic K-ras in intestinal epithelium causes spontaneous tumorigenesis in mice

BACKGROUND & AIMS

Ras oncoproteins are mutated in about 50% of human colorectal cancers, but their precise role in tumor initiation or progression is still unclear.

METHODS

This study presents transgenic mice that express K-ras(V12G), the most frequent oncogenic mutation in human tumors, under control of the murine villin promoter in epithelial cells of the large and small intestine.

RESULTS

More than 80% of the transgenic animals displayed single or multiple intestinal lesions, ranging from aberrant crypt foci (ACF) to invasive adenocarcinomas. Expression of K-ras(V12G) caused activation of the MAP kinase cascade, and the tumors were frequently characterized by deregulated cellular proliferation. Unexpectedly, we obtained no evidence of inactivating mutations of the tumor suppressor gene Apc, the "gatekeeper" in colonic epithelial proliferation. However, spontaneous mutation of the tumor-suppressor gene p53, a frequent feature in the human disease, was found in 3 of 7 tumors that were tested.

CONCLUSIONS

This animal model recapitulates the stages of tumor progression as well as a part of the genetic alterations found in human colorectal cancer. Furthermore, it indicates that activation of K-ras in concert with mutations in p53 may constitute a route to digestive tumor formation and growth, underlining the fact that the pathway to intestinal cancer is not necessarily a single road.

Characterization of G3BPs: tissue specific expression, chromosomal localisation and rasGAP(120) binding studies

The G3BP (ras-GTPase-Activating Protein SH3-Domain-Binding Protein) family of proteins has been implicated in both signal transduction and RNA-metabolism. We have previously identified human G3BP-1, G3BP-2, and mouse G3BP-2. Here, we report the cloning of mouse G3BP-1, the discovery of two alternatively spliced isoforms of mouse, and human G3BP-2 (G3BP-2a and G3BP-2b), and the chromosomal localisation of human G3BP-1 and G3BP-2, which map to 5q14.2-5q33.3 and 4q12-4q24 respectively. We mapped the rasGAP(120) interactive region of the G3BP-2 isoforms and show that both G3BP-2a and G3BP-2b use an N-terminal NTF2-like domain for rasGAP(120) binding rather than several available proline-rich (PxxP) motifs found in members of the G3BPs. Furthermore, we have characterized the protein expression of both G3BP-1 and G3BP-2a/b in adult mouse tissues, and show them to be both tissue and isoform specific.

RasGAP-associated endoribonuclease G3Bp: selective RNA degradation and phosphorylation-dependent localization

Mitogen activation of mRNA decay pathways likely involves specific endoribonucleases, such as G3BP, a phosphorylation-dependent endoribonuclease that associates with RasGAP in dividing but not quiescent cells. G3BP exclusively cleaves between cytosine and adenine (CA) after a specific interaction with RNA through the carboxyl-terminal RRM-type RNA binding motif. Accordingly, G3BP is tightly associated with a subset of poly(A)(+) mRNAs containing its high-affinity binding sequence, such as the c-myc mRNA in mouse embryonic fibroblasts. Interestingly, c-myc mRNA decay is delayed in RasGAP-deficient fibroblasts, which contain a defective isoform of G3BP that is not phosphorylated at serine 149. A G3BP mutant in which this serine is changed to alanine remains exclusively cytoplasmic, whereas a glutamate for serine substitution that mimics the charge of a phosphorylated serine is translocated to the nucleus. Thus, a growth factor-induced change in mRNA decay may be modulated by the nuclear localization of a site-specific endoribonuclease such as G3BP.

Tyrosine phosphorylation of p190 RhoGAP by Fyn regulates oligodendrocyte differentiation

During development of the central nervous system, oligodendrocyte progenitor cells differentiate into mature myelinating cells. The molecular signals that promote this process, however, are not well defined. One molecule that has been implicated in oligodendrocyte differentiation is the Src family kinase Fyn. In order to probe the function of Fyn in this system, a yeast two hybrid screen was performed. Using Fyn as bait, p190 RhoGAP was isolated in the screen of an oligodendrocyte cDNA library. Coimmunoprecipitation and in vitro binding assays verified that p190 RhoGAP bound to the Fyn SH2 domain. Phosphorylation of p190 required active Fyn tyrosine kinase and was increased threefold upon differentiation of primary oligodendrocytes. Moreover, complex formation between p190 and p120 RasGAP occurred in differentiated oligodendrocytes. p190 RhoGAP activity is known to regulate the RhoGDP:RhoGTP ratio. Indeed, expression of dominant negative Rho in primary oligodendrocytes caused a hyperextension of processes. Conversely, constitutively activated Rho caused reduced process formation. These findings define a pathway in which Fyn activity regulates the phosphorylation of p190, leading to an increase in RhoGAP activity with a subsequent increase in RhoGDP, which in turn, regulates the morphological changes that accompany oligodendrocyte differentiation.

Adaptor proteins in protein kinase C-mediated signal transduction

Spatial and temporal organization of signal transduction is essential in determining the speed and precision by which signaling events occur. Adaptor proteins are key to organizing signaling enzymes near their select substrates and away from others in order to optimize precision and speed of response. Here, we describe the role of adaptor proteins in determining the specific function of individual protein kinase C (PKC) isozymes. These isozyme-selective proteins were called collectively RACKs (receptors for activated C-kinase). The role of RACKs in PKC-mediated signaling was determined using isozyme-specific inhibitors and activators of the binding of each isozyme to its respective RACK. In addition to anchoring activated PKC isozymes, RACKs anchor other signaling enzymes. RACK1, the anchoring protein for activated betaIIPKC, binds for example, Src tyrosine kinase, integrin, and phosphodiesterase. RACK2, the epsilonPKC-specific RACK, is a coated-vesicle protein and thus is involved in vesicular release and cell-cell communication. Therefore, RACKs are not only adaptors for PKC, but also serve as adaptor proteins for several other signaling enzymes. Because at least some of the proteins that bind to RACKs, including PKC itself, regulate cell growth, modulating their interactions with RACKs may help elucidate signaling pathways leading to carcinogenesis and could result in the identification of novel therapeutic targets.

Regulation of Raf by Akt controls growth and differentiation in vascular smooth muscle cells

The stimulation of platelet-derived growth factor (PDGF) receptors shifts vascular smooth muscle (VSM) cells toward a more proliferative phenotype. Thrombin activates the same signaling cascades in VSM cells, namely the Ras/Raf/MEK/ERK and the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathways. Nonetheless, thrombin was not mitogenic, but rather increased the expression of the smooth muscle-specific myosin heavy chain (SM-MHC) indicative of an in vitro re-differentiation of VSM cells. A more detailed analysis of the temporal pattern and relative signal intensities revealed marked differences. The strong and biphasic phosphorylation of ERK1/2 in response to thrombin correlated with its ability to increase the activity of the SM-MHC promoter whereas Akt was only partially and transiently phosphorylated. By contrast, PDGF, a potent mitogen in VSM cells, induced a short-lived ERK1/2 phosphorylation but a complete and sustained phosphorylation of Akt. The phosphorylated form of Akt physically interacted with Raf. Moreover, Akt phosphorylated Raf at Ser(259), resulting in a reduced Raf kinase activity and a termination of MEK and ERK1/2 phosphorylation. Disruption of the PI 3-kinase signaling prevented the PDGF-induced Akt and Raf-Ser(259) phosphorylation. Under these conditions, PDGF elicited a more sustained MEK and ERK phosphorylation and increased SM-MHC promoter activity. Consistently, in cells that express dominant negative Akt, PDGF increased SM-MHC promoter activity. Furthermore, expression of constitutively active Akt blocked the thrombin-stimulated SM-MHC promoter activity. Thus, we present evidence that the balance and cross-regulation between the PI 3-kinase/Akt and Ras/Raf/MEK signaling cascades determine the temporal pattern of ERK1/2 phosphorylation and may thereby guide the phenotypic modulation of vascular smooth muscle cells.

RhoA inactivation by p190RhoGAP regulates cell spreading and migration by promoting membrane protrusion and polarity

The binding of extracellular matrix proteins to integrins triggers rearrangements in the actin cytoskeleton by regulating the Rho family of small GTPases. The signaling events that mediate changes in the activity of Rho proteins in response to the extracellular matrix remain largely unknown. We have demonstrated in previous studies that integrin signaling transiently suppresses RhoA activity through stimulation of p190RhoGAP. Here, we investigated the biological significance of adhesion-dependent RhoA inactivation by manipulating p190RhoGAP signaling in Rat1 fibroblasts. The inhibition of RhoA activity that is induced transiently by adhesion was antagonized by expression of dominant negative p190RhoGAP. This resulted in impaired cell spreading on a fibronectin substrate, reduced cell protrusion, and premature assembly of stress fibers. Conversely, overexpression of p190RhoGAP augmented cell spreading. Dominant negative p190RhoGAP elevated RhoA activity in cells on fibronectin and inhibited migration, whereas overexpression of the wild-type GAP decreased RhoA activity, promoted the formation of membrane protrusions, and enhanced motility. Cells expressing dominant negative p190RhoGAP, but not control cells or cells overexpressing the wild-type GAP, were unable to establish polarity in the direction of migration. Taken together, these data demonstrate that integrin-triggered RhoA inhibition by p190RhoGAP enhances spreading and migration by regulating cell protrusion and polarity.

RACK1, a protein kinase C scaffolding protein, interacts with the PH domain of p120GAP

The Ras GTPase-activating protein p120GAP is a multidomain protein consisting of a variety of noncatalytic domains that may be involved in its regulation. RACK1 is a membrane-associated protein that binds the C2 domain of PKC and is related in sequence to the beta subunit of heterotrimeric G-proteins which has been implicated in binding to PH domains. Because p120GAP contains both PH and C2/CaLB domains we determined whether it is also a RACK1 binding protein. Coimmunoprecipitation experiments indicate that p120GAP associates with RACK1, whereas PH or C2/CaLB domain deletion mutants do not. A fusion protein containing the GAP PH domain bound to endogenous RACK1 in lysates in a concentration-dependent manner and directly associated with recombinant RACK1. Finally, serine/threonine phosphorylation appears to be involved in regulating this association. These results suggest that p120GAP and RACK1 interact in vivo in a manner dependent upon both the PH and C2/CaLB domains of GAP.

Tyrosine-phosphorylated low density lipoprotein receptor-related protein 1 (Lrp1) associates with the adaptor protein SHC in SRC-transformed cells

v-Src transforms fibroblasts in vitro and causes tumor formation in the animal by tyrosine phosphorylation of critical cellular substrates. Exactly how v-Src interacts with these substrates remains unknown. One of its substrates, the adaptor protein Shc, is thought to play a crucial role during cellular transformation by v-Src by linking v-Src to Ras. We used Shc proteins with mutations in either the phosphotyrosine binding (PTB) or Src homology 2 domain to determine that phosphorylation of Shc in v-Src-expressing cells depends on the presence of a functional PTB domain. We purified a 100-kDa Shc PTB-binding protein from Src-transformed cells that was identified as the beta chain of the low density lipoprotein receptor-related protein LRP1. LRP1 acts as an import receptor for a variety of proteins and is involved in clearance of the beta-amyloid precursor protein. This study shows that LRP1 is tyrosine-phosphorylated in v-Src-transformed cells and that tyrosine-phosphorylated LRP1 binds in vivo and in vitro to Shc. The association between Shc and LRP1 may provide a mechanism for recruitment of Shc to the plasma membrane where it is phosphorylated by v-Src. It is at the membrane that Shc is thought to be involved in Ras activation. These observations further suggest that LRP1 could function as a signaling receptor and may provide new avenues to investigate its possible role during embryonal development and the onset of Alzheimer's disease.

Ablation of NF1 function in neurons induces abnormal development of cerebral cortex and reactive gliosis in the brain

Neurofibromatosis type 1 (NF1) is a prevalent genetic disorder that affects growth properties of neural-crest-derived cell populations. In addition, approximately one-half of NF1 patients exhibit learning disabilities. To characterize NF1 function both in vitro and in vivo, we circumvent the embryonic lethality of NF1 null mouse embryos by generating a conditional mutation in the NF1 gene using Cre/loxP technology. Introduction of a Synapsin I promoter driven Cre transgenic mouse strain into the conditional NF1 background has ablated NF1 function in most differentiated neuronal populations. These mice have abnormal development of the cerebral cortex, which suggests that NF1 has an indispensable role in this aspect of CNS development. Furthermore, although they are tumor free, these mice display extensive astrogliosis in the absence of conspicuous neurodegeneration or microgliosis. These results indicate that NF1-deficient neurons are capable of inducing reactive astrogliosis via a non-cell autonomous mechanism.

The Ras/p120 GTPase-activating protein (GAP) interaction is regulated by the p120 GAP pleckstrin homology domain

Pleckstrin homology domains are structurally conserved functional domains that can undergo both protein/protein and protein/lipid interactions. Pleckstrin homology domains can mediate inter- and intra-molecular binding events to regulate enzyme activity. They occur in numerous proteins including many that interact with Ras superfamily members, such as p120 GAP. The pleckstrin homology domain of p120 GAP is located in the NH(2)-terminal, noncatalytic region of p120 GAP. Overexpression of the noncatalytic domains of p120 GAP may modulate Ras signal transduction pathways. Here, we demonstrate that expression of the isolated pleckstrin homology domain of p120 GAP specifically inhibits Ras-mediated signaling and transformation but not normal cellular growth. Furthermore, we show that the pleckstrin homology domain binds the catalytic domain of p120 GAP and interferes with the Ras/GAP interaction. Thus, we suggest that the pleckstrin homology domain of p120 GAP may specifically regulate the interaction of Ras with p120 GAP via competitive intra-molecular binding.

Single cell Ras-GTP analysis reveals altered Ras activity in a subpopulation of neurofibroma Schwann cells but not fibroblasts

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by multiple neurofibromas, peripheral nerve tumors containing mainly Schwann cells and fibroblasts. The NF1 gene encodes neurofibromin, a tumor suppressor postulated to function in part as a Ras GTPase-activating protein. The roles of different cell types and of elevated Ras-GTP in neurofibroma formation are unclear. To determine which neurofibroma cell type has altered Ras-GTP regulation, we developed an immunocytochemical assay for active, GTP-bound Ras. In NIH 3T3 cells, the assay detected overexpressed, constitutively activated K-, N-, and Ha-Ras and insulin-induced endogenous Ras-GTP. In dissociated neurofibroma cells from NF1 patients, Ras-GTP was elevated in Schwann cells but not fibroblasts. Twelve to 62% of tumor Schwann cells showed elevated Ras-GTP, unexpectedly revealing neurofibroma Schwann cell heterogeneity. Increased basal Ras-GTP did not correlate with increased cell proliferation. Normal human Schwann cells, however, did not demonstrate elevated basal Ras activity. Furthermore, compared with cells from wild type littermates, Ras-GTP was elevated in all mouse Nf1(-/-) Schwann cells but never in Nf1(-/-) mouse fibroblasts. Our results indicate that Ras activity is detectably increased in only some neurofibroma Schwann cells and suggest that neurofibromin is not an essential regulator of Ras activity in fibroblasts.

Role of p120 Ras-GAP in directed cell movement

We have used cell lines deficient in p120 Ras GTPase activating protein (Ras-GAP) to investigate the roles of Ras-GAP and the associated p190 Rho-GAP (p190) in cell polarity and cell migration. Cell wounding assays showed that Ras-GAP-deficient cells were incapable of establishing complete cell polarity and migration into the wound. Stimulation of mutant cells with growth factor rescued defects in cell spreading, Golgi apparatus fragmentation, and polarized vesicular transport and partially rescued migration in a Ras-dependent manner. However, for directional movement, the turnover of stress fibers and focal adhesions to produce an elongate morphology was dependent on the constitutive association between Ras-GAP and p190, independent of Ras regulation. Disruption of the phosphotyrosine-mediated Ras-GAP/p190 complex by microinjecting synthetic peptides derived from p190 sequences in wild-type cells caused a suppression of actin filament reorientation and migration. From these observations we suggest that although Ras-GAP is not directly required for motility per se, it is important for cell polarization by regulating actin stress fiber and focal adhesion reorientation when complexed with 190. This observation suggests a specific function for Ras-GAP separate from Ras regulation in cell motility.

Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta receptor (betaPDGFR) is not the only tyrosine kinase able to associate with the GTPase-activating protein of Ras (RasGAP). The interaction of non-betaPDGFR kinase(s) with RasGAP was dependent on stimulation with platelet-derived growth factor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Because the tyrosine phosphorylation site of RasGAP is in a sequence context that is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasGAP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immunodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP results in the formation of a complex between RasGAP and Src. To begin to address the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that did not activate Src was unable to efficiently mediate the tyrosine phosphorylation of phospholipase Cgamma (PLCgamma). Taken together, these observations support the following hypothesis. When RasGAP is recruited to the betaPDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLCgamma. This hypothesis offers a mechanistic explanation for our previously published findings that the recruitment of RasGAP to the betaPDGFR attenuates the tyrosine phosphorylation of PLCgamma. Finally, these findings suggest a novel way in which RasGAP negatively regulates signal relay by the betaPDGFR.

Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta receptor (betaPDGFR) is not the only tyrosine kinase able to associate with the GTPase-activating protein of Ras (RasGAP). The interaction of non-betaPDGFR kinase(s) with RasGAP was dependent on stimulation with platelet-derived growth factor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Because the tyrosine phosphorylation site of RasGAP is in a sequence context that is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasGAP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immunodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP results in the formation of a complex between RasGAP and Src. To begin to address the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that did not activate Src was unable to efficiently mediate the tyrosine phosphorylation of phospholipase Cgamma (PLCgamma). Taken together, these observations support the following hypothesis. When RasGAP is recruited to the betaPDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLCgamma. This hypothesis offers a mechanistic explanation for our previously published findings that the recruitment of RasGAP to the betaPDGFR attenuates the tyrosine phosphorylation of PLCgamma. Finally, these findings suggest a novel way in which RasGAP negatively regulates signal relay by the betaPDGFR.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.