A temporally-restricted pattern of endothelial cell collagen 4 alpha 1 expression during embryonic development determined with a novel knockin Col4a1-P2A-eGFP mouse line

Classical collagen type IV comprising of a heterotrimer of two collagen IV alpha 1 chains and one collagen IV alpha 2 chain is the principal type of collagen synthesized by endothelial cells (EC) and is a major constituent of vascular basement membranes. In mouse and man, mutations in genes that encode collagen IV alpha 1 and alpha 2 result in vascular dysfunction. In addition, mutations in genes that encode the Ephrin receptor B4 (EPHB4) and the p120 Ras GTPase-activating protein (RASA1) that cause increased activation of the Ras mitogen-activated protein kinase (MAPK) signaling pathway in EC result in vascular dysfunction as a consequence of impaired export of collagen IV. To understand the pathogenesis of collagen IV-related vascular diseases and phenotypes it is necessary to identify at which times collagen IV is actively synthesized by EC. For this purpose, we used CRISPR/Cas9 targeting in mice to include immediately after the terminal Col4a1 codon a sequence that specifies a P2A peptide followed by enhanced green fluorescent protein (eGFP). Analysis of eGFP expression in Col4a1-P2A-eGFP mice revealed active embryonic EC synthesis of collagen IV alpha 1 through mid to late gestation followed by a sharp decline before birth. These results provide a contextual framework for understanding the basis for the varied vascular abnormalities resulting from perturbation of EC expression and export of functional collagen IV.

Clear Cell Squamous Cell Carcinoma of the Maxillary Gingiva Associated with PIK3CA and HRAS Mutations: Report of a Case and Literature Review

BACKGROUND

Squamous cell carcinoma (SCC) is the most common oral malignancy, and somatic mutations in some driver genes have been implicated in SCC development. Clear cell SCC (CCSCC) is a rare histological variant of SCC, and various clear cell neoplasms must be considered in the differential diagnosis of CCSCC in the oral cavity. Based on a limited number of CCSCC cases reported in the oral cavity, CCSCC is considered an aggressive variant of SCC with a poor prognosis; however, its genetic characteristics remain unknown.

METHODS

A maxillary gingival tumor in an 89-year-old female was described and investigated using immunohistochemical staining, special staining, fluorescence in situ hybridization, and next-generation sequencing (NGS) with a custom panel of driver genes, including those associated with SCC and clear cell neoplasm development.

RESULTS

Histopathological examination revealed a proliferation of atypical epithelial cells with abundant clear cytoplasm and enlarged and centrally placed round nuclei. The tumor was exophytic with deep, penetrating proliferation. The atypical clear cells were continuous with the conventional SCC cells. Immunohistochemical analysis showed that the clear cells were positive for CK AE1/AE3 and CK5/6 and nuclear-positive for p63. In contrast, the clear cells were negative for αSMA, S100, HMB45, Melan-A, CD10, and p16. p53 immunoreactivity exhibited a wild-type expression pattern. Additionally, the clear cells were positive for periodic acid-Schiff (PAS) and negative for diastase-PAS, mucicarmine, and Alcian blue. Based on these results, the diagnosis of CCSCC was confirmed. Molecular analysis of the clear cells identified PIK3CA p.E542K (c.1624G>A) and HRAS p.G12A (c.35 G>C) somatic mutations classified as oncogenic. No pathogenic variants were identified in TP53, EWSR1, AKT1, PTEN, BRAF, KRAS, NRAS, RASA1, or MAML2.

CONCLUSIONS

We report a case of CCSCC of the oral cavity with PIK3CA and HRAS mutations. The identification of PIK3CA and/or HRAS mutations is rare in SCC; however, both mutations are important potential targets for antitumor therapy. A detailed analysis of gene mutations in CCSCC may lead to a better understanding of its biological behavior and an improved prognosis, as well as a differential diagnosis from other clear cell neoplasms.

The Role of Low Complexity Regions in Protein Interaction Modes: An Illustration in Huntingtin

Low complexity regions (LCRs) are very frequent in protein sequences, generally having a lower propensity to form structured domains and tending to be much less evolutionarily conserved than globular domains. Their higher abundance in eukaryotes and in species with more cellular types agrees with a growing number of reports on their function in protein interactions regulated by post-translational modifications. LCRs facilitate the increase of regulatory and network complexity required with the emergence of organisms with more complex tissue distribution and development. Although the low conservation and structural flexibility of LCRs complicate their study, evolutionary studies of proteins across species have been used to evaluate their significance and function. To investigate how to apply this evolutionary approach to the study of LCR function in protein-protein interactions, we performed a detailed analysis for Huntingtin (HTT), a large protein that is a hub for interaction with hundreds of proteins, has a variety of LCRs, and for which partial structural information (in complex with HAP40) is available. We hypothesize that proteins RASA1, SYN2, and KAT2B may compete with HAP40 for their attachment to the core of HTT using similar LCRs. Our results illustrate how evolution might favor the interplay of LCRs with domains, and the possibility of detecting multiple modes of LCR-mediated protein-protein interactions with a large hub such as HTT when enough protein interaction data is available.

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."

RASA1 inhibits the progression of renal cell carcinoma by decreasing the expression of miR-223-3p and promoting the expression of FBXW7

RAS p21 protein activator 1 (RASA1), also known as p120-RasGAP, is a RasGAP protein that functions as a signaling scaffold protein, regulating pivotal signal cascades. However, its biological mechanism in renal cell carcinoma (RCC) remains unknown. In the present study, RASA1, F-box/WD repeat-containing protein 7 (FBXW7), and miR-223-3p expression were assessed via quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western blot. Then, the targeted correlations of miR-223-3p with FBXW7 and RASA1 were verified via a dual-luciferase reporter gene assay. CCK-8, flow cytometry, and Transwell assays were implemented independently to explore the impact of RASA1 on cell proliferation, apoptosis, migration, and cell cycle progression. Finally, the influence of RASA1 on tumor formation in RCC was assessed in vivo through the analysis of tumor growth in nude mice. Results showed that FBXW7 and RASA1 expression were decreased in RCC tissues and cell lines, while miR-223-3p was expressed at a higher level. Additionally, FBXW7 and RASA1 inhibited cell proliferation but facilitated the population of RCC cells in the G0/G1 phase. Altogether, RASA1 may play a key role in the progression of RCC by decreasing miR-223-3p and subsequently increasing FBXW7 expression.

Changqin NO. 1 inhibits neuronal apoptosis via suppressing GAS5 expression in a traumatic brain injury mice model

The present study was designed to investigate the mechanism of the traditional Chinese medicine Changqin NO. 1 on the amelioration of traumatic brain injury (TBI). Adult male C57BL/6J mice and newborn mice were used to generate a mouse TBI model and harvest primary neurons, respectively. The localizations of specific neural markers neuropilin-1 (Nrp-1), growth-associated protein-43 (GAP-43) and microtubule-associated protein Tau (Tau) were examined in brain tissues by immunohistochemistry. Terminal deoxynucleotidyl transferase dUTP nick end labeling apoptotic cell detection in tissue sections and the CCK-8 cell viability assay were performed to examine neuronal apoptosis. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were also carried out in this study. The association between long non-coding RNA (lncRNA) growth-arrest specific 5 (GAS5), miR-335 and RAS p21 GTPase activating protein 1 (Rasa1) was disclosed using the dual-luciferase reporter assay. Changqin NO. 1 inhibited TBI-induced neuronal apoptosis in vivo and in vitro. GAS5 functioned as a competing endogenous RNA (ceRNA) by sponging miR-335 to upregulate Rasa1 expression in mouse neuronal cells. Further investigations demonstrated that GAS5 promoted neuronal apoptosis following TBI via the miR-335/Rasa1 axis. In vivo experiments indicated that Changqin NO. 1 exerted neuroprotection during TBI via the GAS5/miR-335/Rasa1 axis. Changqin NO. 1 promoted neuroprotective effects by inhibiting neuronal apoptosis via the GAS5/miR-335/Rasa1 axis in TBI.

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.

MicroRNA-223 Regulates Cardiac Fibrosis After Myocardial Infarction by Targeting RASA1

BACKGROUND/AIMS

Percutaneous coronary intervention reduces acute myocardial infarction (MI)-induced mortality to a great extent, but effective treatments for MI-induced cardiac fibrosis and heart failure are still lacking. MicroRNAs (miRNAs) play a variety of roles in cells and have thus been investigated extensively. MicroRNA-223 (miR-223) expression has been reported to be altered in post-MI heart failure in humans; however, the roles of miR-223 in MI remain unknown. Our study aimed to elucidate the roles of miR-223 in cardiac fibrosis.

METHODS

Cultured cardiac fibroblasts (CFs) were activated by TGF-β1 stimulation. Gain and loss of miR-223 and RAS p21 protein activator 1 (RASA1) knockdown in CFs were achieved by transfecting the cells with miR-223 mimics and inhibitors, as well as small interfering RNA-RASA1 (siRASA1), respectively. Quantitative real-time reverse transcriptase-polymerase chain reactions (qRT-PCR) was used to determine miR-223-3p and RASA1 expression levels, and Cell Counting Kit-8 (CCK-8), transwell migration and scratch assays were performed to assess CFs viability and migration, respectively. Western blotting was used to detect collagen I, collagen III, alpha-smooth muscle actin (a-SMA), RASA1, p-Akt/t-Akt, p-MEK1/2/t-MEK1/2, and p-ERK1/2/t-ERK1/2 protein expressions, and immunofluorescence assays were used to detect the expression of α-actin, vimentin and α-SMA. Luciferase assays were carried out to determine whether miR-223 binds to RASA1. Rat models of MI were established by the ligation of the left anterior descending (LAD) coronary artery. MiR-223 inhibition in vivo was achieved via intramyocardial injections of the miR-223 sponge carried by adeno-associated virus 9 (AAV9). The cardiac function was detected by echocardiography, and cardiac fibrosis was shown by Masson's trichrome staining.

RESULTS

miR-223 was increased in CFs compared to cardiomypcytes, and TGF-β1 treatment increased miR-223 expression in CFs. The miR-223 mimics enhanced cell proliferation and migration and collagen I, collagen III, and α-SMA protein expression in CFs, while the miR-223 inhibitors had contrasting effects and partially prevented the promoting effects of TGF-β1. qRT-PCR and western blotting revealed that miR-223 negatively regulated RASA1 expression, and the luciferase assays showed that miR-223 suppressed the luciferase activity of the RASA1 3' untranslated region (3'UTR), indicating that miR-223 binds directly to RASA1. Similar to transfection with the miR-223 mimics, RASA1 knockdown enhanced cell proliferation and migration and collagen I, collagen III, and α-SMA protein expression in CFs. Moreover, RASA1 knockdown partially reversed the inhibitory effects of the miR-223 inhibitor on cell proliferation and migration and collagen I, collagen III, and α-SMA protein expression, indicating that the effects of miR-223 in CFs are partially mediated by the regulation of RASA1 expression. Further exploration showed that miR-223 mimics and siRASA1 promoted MEK1/2, ERK1/2 and AKT phosphorylation, while the miR-223 inhibitors had contrasting effects. The in vivo experiments confirmed the results of the in vitro experiments and showed that miR-223 inhibition prevented cardiac functional deterioration and cardiac fibrosis.

CONCLUSIONS

miR-223 enhanced cell proliferation, migration, and differentiation in CFs, thus mediated cardiac fibrosis after MI partially via the involvement of RASA1.

A Transposon-based Analysis Reveals RASA1 Is Involved in Triple-Negative Breast Cancer

RAS genes are mutated in 20% of human tumors, but these mutations are very rare in breast cancer. Here, we used a mouse model to generate tumors upon activation of a mutagenic T2Onc2 transposon via expression of a transposase driven by the keratin K5 promoter in a p53+/- background. These animals mainly developed mammary tumors, most of which had transposon insertions in one of two RASGAP genes, neurofibromin1 (Nf1) and RAS p21 protein activator (Rasa1). Immunohistochemical analysis of a collection of human breast tumors confirmed that low expression of RASA1 is frequent in basal (triple-negative) and estrogen receptor negative tumors. Bioinformatic analysis of human breast tumors in The Cancer Genome Atlas database showed that although RASA1 mutations are rare, allelic loss is frequent, particularly in basal tumors (80%) and in association with TP53 mutation. Inactivation of RASA1 in MCF10A cells resulted in the appearance of a malignant phenotype in the context of mutated p53. Our results suggest that alterations in the Ras pathway due to the loss of negative regulators of RAS may be a common event in basal breast cancer. Cancer Res; 77(6); 1357-68. ©2017 AACR.

Cdc42 and k-Ras Control Endothelial Tubulogenesis through Apical Membrane and Cytoskeletal Polarization: Novel Stimulatory Roles for GTPase Effectors, the Small GTPases, Rac2 and Rap1b, and Inhibitory Influence of Arhgap31 and Rasa1

A critical and understudied property of endothelial cells is their ability to form lumens and tube networks. Although considerable information has been obtained concerning these issues, including the role of Cdc42 and Rac1 and their effectors such as Pak2, Pak4, Par6b, and co-regulators such as integrins, MT1-MMP and Par3; many key questions remain that are necessary to elucidate molecular and signaling requirements for this fundamental process. In this work, we identify new small GTPase regulators of EC tubulogenesis including k-Ras, Rac2 and Rap1b that act in conjunction with Cdc42 as well as the key downstream effectors, IQGAP1, MRCKβ, beta-Pix, GIT1, and Rasip1 (which can assemble into multiprotein complexes with key regulators including α2β1 integrin and MT1-MMP). In addition, we identify the negative regulators, Arhgap31 (by inactivating Cdc42 and Rac) and Rasa1 (by inactivating k-Ras) and the positive regulator, Arhgap29 (by inactivating RhoA) which play a major functional role during the EC tubulogenic process. Human EC siRNA suppression or mouse knockout of Rasip1 leads to identical phenotypes where ECs form extensive cord networks, but cannot generate lumens or tubes. Essential roles for these molecules during EC tubulogenesis include; i) establishment of asymmetric EC cytoskeletal polarization (subapical distribution of acetylated tubulin and basal membrane distribution of F-actin); and ii) directed membrane trafficking of pinocytic vacuoles or other intracellular vesicles along acetylated tubulin tracks to the developing apical membrane surface. Cdc42 co-localizes subapically with acetylated tubulin, while Rac1 and k-Ras strongly label vacuole/ vesicle membranes which accumulate and fuse together in a polarized, perinuclear manner. We observe polarized apical membrane and subapical accumulation of key GTPases and effectors regulating EC lumen formation including Cdc42, Rac1, Rac2, k-Ras, Rap1b, activated c-Raf and Rasip1 to control EC tube network assembly. Overall, this work defines novel key regulators and their functional roles during human EC tubulogenesis.

MiR-21/RASA1 axis affects malignancy of colon cancer cells via RAS pathways

AIM: To determine how the oncogene miR-21 regulates the RAS signaling pathways and affects colon cancer cell behaviors.

METHODS: RAS p21 GTPase activating protein 1 (RASA1) protein expression in six colon cancer cell lines was assessed by Western blot. Colon cancer RKO cells were chosen for transfection because they are KRAS wild type colon cancer cells whose RASA1 expression is significantly decreased. RKO cells were transfected with vectors overexpressing or down-regulating either miR-21 or RASA1. Furthermore, a luciferase reporter assay was used to determine whether RASA1 is a gene target of miR-21. Then, changes in mRNA and protein levels of RASA1, RAS-GTP, and other components of the RAS signaling pathways were assessed in transfected RKO cells by real-time quantitative reverse transcription-polymerase chain reaction, Western blot and immunoprecipitation. Finally, cell proliferation, apoptosis, invasion, and tumor formation ability were assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide dye assay, flow cytometry, transwell assay, and animal experiment, respectively.

RESULTS: RASA1 protein levels were significantly decreased in RKO cells compared with the other 5 colon cancer cell lines, and RASA1 was confirmed as a target gene of miR-21. Interestingly, RASA1 mRNA and protein levels in pre-miR-21-LV (up-regulation of miR-21) cells were lower than those in anti-miR-21-LV (down-regulation of miR-21) cells (P < 0.05). In addition, pre-miR-21-LV or siRASA1 (down-regulation of RASA1) cells showed higher cell proliferation, reduced apoptosis, increased expression of RAS-GTP, p-AKT, Raf-1, KRAS, and p-ERK1/2, and higher invasion and tumor formation ability, compared with control, anti-miR-21-LV or pcDNA3.1-RASA1 (up-regulation of RASA1) cells (P < 0.05).

CONCLUSION: RASA1 is a target gene of miR-21, which promotes malignant behaviors of RKO cells through regulation of RASA1 expression.

RASA1 functions in EPHB4 signaling pathway to suppress endothelial mTORC1 activity

Vascular malformations are linked to mutations in RAS p21 protein activator 1 (RASA1, also known as p120RasGAP); however, due to the global expression of this gene, it is unclear how these mutations specifically affect the vasculature. Here, we tested the hypothesis that RASA1 performs a critical effector function downstream of the endothelial receptor EPHB4. In zebrafish models, we found that either RASA1 or EPHB4 deficiency induced strikingly similar abnormalities in blood vessel formation and function. Expression of WT EPHB4 receptor or engineered receptors with altered RASA1 binding revealed that the ability of EPHB4 to recruit RASA1 is required to restore blood flow in EPHB4-deficient animals. Analysis of EPHB4-deficient zebrafish tissue lysates revealed that mTORC1 is robustly overactivated, and pharmacological inhibition of mTORC1 in these animals rescued both vessel structure and function. Furthermore, overexpression of mTORC1 in endothelial cells exacerbated vascular phenotypes in animals with reduced EPHB4 or RASA1, suggesting a functional EPHB4/RASA1/mTORC1 signaling axis in endothelial cells. Tissue samples from patients with arteriovenous malformations displayed strong endothelial phospho-S6 staining, indicating increased mTORC1 activity. These results indicate that deregulation of EPHB4/RASA1/mTORC1 signaling in endothelial cells promotes vascular malformation and suggest that mTORC1 inhibitors, many of which are approved for the treatment of certain cancers, should be further explored as a potential strategy to treat patients with vascular malformations.

Ras pathway inhibition prevents neovascularization by repressing endothelial cell sprouting

Vascular networks develop from a growing vascular front that responds to VEGF and other guidance cues. Angiogenesis is required for normal tissue function, but, under conditions of stress, inappropriate vascularization can lead to disease. Therefore, inhibition of angiogenic sprouting may prevent neovascularization in patients with blinding neovascular eye diseases, including macular degeneration. VEGF antagonists have therapeutic benefits but also can elicit off-target effects. Here, we found that the Ras pathway, which functions downstream of a wide range of cytokines including VEGF, is active in the growing vascular front of developing and pathological vascular networks. The endogenous Ras inhibitor p120RasGAP was expressed predominately in quiescent VEGF-insensitive endothelial cells and was ectopically downregulated in multiple neovascular models. MicroRNA-132 negatively regulated p120RasGAP expression. Experimental delivery of α-miR-132 to developing mouse eyes disrupted tip cell Ras activity and prevented angiogenic sprouting. This strategy prevented ocular neovascularization in multiple rodent models even more potently than the VEGF antagonist, VEGF-trap. Targeting microRNA-132 as a therapeutic strategy may prove useful for treating multiple neovascular diseases of the eye and for preventing vision loss regardless of the neovascular stimulus.

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.

Completion of proteomic data sets by Kd measurement using cell-free synthesis of site-specifically labeled proteins

The characterization of phosphotyrosine mediated protein-protein interactions is vital for the interpretation of downstream pathways of transmembrane signaling processes. Currently however, there is a gap between the initial identification and characterization of cellular binding events by proteomic methods and the in vitro generation of quantitative binding information in the form of equilibrium rate constants (K_d values). In this work we present a systematic, accelerated and simplified approach to fill this gap: using cell-free protein synthesis with site-specific labeling for pull-down and microscale thermophoresis (MST) we were able to validate interactions and to establish a binding hierarchy based on K_d values as a completion of existing proteomic data sets. As a model system we analyzed SH2-mediated interactions of the human T-cell phosphoprotein ADAP. Putative SH2 domain-containing binding partners were synthesized from a cDNA library using Expression-PCR with site-specific biotinylation in order to analyze their interaction with fluorescently labeled and in vitro phosphorylated ADAP by pull-down. On the basis of the pull-down results, selected SH2’s were subjected to MST to determine K_d values. In particular, we could identify an unexpectedly strong binding of ADAP to the previously found binding partner Rasa1 of about 100 nM, while no evidence of interaction was found for the also predicted SH2D1A. Moreover, K_d values between ADAP and its known binding partners SLP-76 and Fyn were determined. Next to expanding data on ADAP suggesting promising candidates for further analysis in vivo, this work marks the first K_d values for phosphotyrosine/SH2 interactions on a phosphoprotein level.

Role of mTOR, Bad, and Survivin in RasGAP Fragment N-Mediated Cell Protection

Partial cleavage of p120 RasGAP by caspase-3 in stressed cells generates an N-terminal fragment, called fragment N, which activates an anti-apoptotic Akt-dependent survival response. Akt regulates several effectors but which of these mediate fragment N-dependent cell protection has not been defined yet. Here we have investigated the role of mTORC1, Bad, and survivin in the capacity of fragment N to protect cells from apoptosis. Neither rapamycin, an inhibitor of mTORC1, nor silencing of raptor, a subunit of the mTORC1 complex, altered the ability of fragment N from inhibiting cisplatin- and Fas ligand-induced death. Cells lacking Bad, despite displaying a stronger resistance to apoptosis, were still protected by fragment N against cisplatin-induced death. Fragment N was also able to protect cells from Fas ligand-induced death in conditions where Bad plays no role in apoptosis regulation. Fragment N expression in cells did neither modulate survivin mRNA nor its protein expression. Moreover, the expression of cytoplasmic survivin, known to exert anti-apoptotic actions in cells, still occurred in UV-B-irradiated epidermis of mouse expressing a caspase-3-resistant RasGAP mutant that cannot produce fragment N. Additionally, survivin function in cell cycle progression was not affected by fragment N. These results indicate that, taken individually, mTOR, Bad, or Survivin are not required for fragment N to protect cells from cell death. We conclude that downstream targets of Akt other than mTORC1, Bad, or survivin mediate fragment N-induced protection or that several Akt effectors can compensate for each other to induce the pro-survival fragment N-dependent response.

Caveolin-1 protects B6129 mice against Helicobacter pylori gastritis

Caveolin-1 (Cav1) is a scaffold protein and pathogen receptor in the mucosa of the gastrointestinal tract. Chronic infection of gastric epithelial cells by Helicobacter pylori (H. pylori) is a major risk factor for human gastric cancer (GC) where Cav1 is frequently down-regulated. However, the function of Cav1 in H. pylori infection and pathogenesis of GC remained unknown. We show here that Cav1-deficient mice, infected for 11 months with the CagA-delivery deficient H. pylori strain SS1, developed more severe gastritis and tissue damage, including loss of parietal cells and foveolar hyperplasia, and displayed lower colonisation of the gastric mucosa than wild-type B6129 littermates. Cav1-null mice showed enhanced infiltration of macrophages and B-cells and secretion of chemokines (RANTES) but had reduced levels of CD25⁺ regulatory T-cells. Cav1-deficient human GC cells (AGS), infected with the CagA-delivery proficient H. pylori strain G27, were more sensitive to CagA-related cytoskeletal stress morphologies (“humming bird”) compared to AGS cells stably transfected with Cav1 (AGS/Cav1). Infection of AGS/Cav1 cells triggered the recruitment of p120 RhoGTPase-activating protein/deleted in liver cancer-1 (p120RhoGAP/DLC1) to Cav1 and counteracted CagA-induced cytoskeletal rearrangements. In human GC cell lines (MKN45, N87) and mouse stomach tissue, H. pylori down-regulated endogenous expression of Cav1 independently of CagA. Mechanistically, H. pylori activated sterol-responsive element-binding protein-1 (SREBP1) to repress transcription of the human Cav1 gene from sterol-responsive elements (SREs) in the proximal Cav1 promoter. These data suggested a protective role of Cav1 against H. pylori-induced inflammation and tissue damage. We propose that H. pylori exploits down-regulation of Cav1 to subvert the host's immune response and to promote signalling of its virulence factors in host cells.

Infection with the bacterium Helicobacter pylori (H. pylori) mainly affects children in the developing countries who are at risk to progress to gastric cancer (GC) as adults after many years of persistent infection, especially with strains which are positive for the oncogenic virulence factor CagA. Eradication of H. pylori by antibiotics is a treatment of choice but may also alter the susceptibility to allergies and other tumor types. Thus, novel diagnostic or prognostic markers are needed which detect early molecular changes in the stomach mucosa during the transition of chronic inflammation to cancer. In our study, we found that the tumor suppressor caveolin-1 (Cav1) is reduced upon infection with H. pylori, and CagA was sufficient but not necessary for this down-regulation. Loss of Cav1 was caused by H. pylori-dependent activation of sterol-responsive element-binding protein-1 (SREBP1), and this event abolished the interaction of Cav1 with p120 RhoGTPase-activating protein/deleted in liver cancer-1 (p120RhoGAP/DLC1), a second bona fide tumor suppressor in gastric tissue. Conclusively, Cav1 and DLC1 may constitute novel molecular markers in the H. pylori-infected gastric mucosa before neoplastic transformation of the epithelium.

[Construction of has-miR-335 lentiviral vector and verification of the target gene of miR-335]

OBJECTIVE

To construct a lentiviral vector of miR-335 gene and verify the target gene of miR-335.

METHODS

The precursor sequence of miR-335 gene was amplified from the genomic DNA by PCR and cloned into the lentiviral vector PLVTHM labeled with GFP. Real-time quantitative RT-PCR was used to detect miR-335 and RASA1 expression in different colorectal cancer cell lines. A recombinant vector psiCHECK-2-RASA1 containing RASA1 3'UTR was constructed followed by site-directed mutagenesis of RASA1 3'UTR to establish the vector psiCHECK-2-RASA1-Mut. Co-transfection of hsa-mir-335 or a NC with these recombined vectors in HEK293A and SW480 cells was performed, and dual-luciferase reporter assay was utilized to examine the changes in luciferase activities. The recombinant PLVTHM-miR335 plasmid was packaged into mature lentivirus by 293FT cells and used to infect SW620 cells. Flow cytometry was employed for sorting the GFP+ cells. The expression of miR-335 and RASA1 were determined by qRT-PCR, and Western blotting was used to detect the expression of RASA1 protein in SW620 cell lines.

RESULTS

The recombinant lentiviral vector PLVTHM-miR335, psiCHECK-2-RASA1 and the mutation expression vector psiCHECK-2-RASA1-Mut were successfully constructed. Dual-luciferase reporter assay showed that miR-335 decreased luciferase activity in cells co-transfected with psiCHECK-2-RASA1. The expression of miR-335 in SW620 cells infected with the lentivirus PLVTHM-miR335 was significantly increased, but the expression of RASA1 showed only slight changes. Overexpression of miR-335 suppressed the expression of RASA1 protein in SW620 cells.

CONCLUSION

We have successfully constructed the lentiviral vector containing mir-335 gene and a SW620 cell line with miR-335 overexpression. MiR-335 can suppress RASA1 gene expression by targeting the specific sequence of RASA1 3'UTR.

Integrin signaling through Arg activates p190RhoGAP by promoting its binding to p120RasGAP and recruitment to the membrane

The Rho family GTPases RhoA (Rho), Rac1, and Cdc42 are essential effectors of integrin-mediated cell attachment and spreading. Rho activity, which promotes formation of focal adhesions and actin stress fibers, is inhibited upon initial cell attachment to allow sampling of the new adhesive environment. The Abl-related gene (Arg) tyrosine kinase mediates adhesion-dependent inhibition of Rho through phosphorylation and activation of the Rho inhibitor p190RhoGAP-A (p190). p190 phosphorylation promotes its binding to p120RasGAP (p120). Here, we elucidate the mechanism by which p120 binding regulates p190 activation after adhesion. We show that p190 requires its p120-binding domain to undergo Arg-dependent activation in vivo. However, p120 binding does not activate p190RhoGAP activity in vitro. Instead, activation of p190 requires recruitment to the cell periphery. Integrin-mediated adhesion promotes relocalization of p190 and p120 to the cell periphery in wild-type fibroblasts, but not in arg(-/-) fibroblasts. A dominant-negative p120 fragment blocks p190:p120 complex formation, prevents activation of p190 by adhesion, and disrupts the adhesion-dependent recruitment of p190 to the cell periphery. Our results demonstrate that integrin signaling through Arg activates p190 by promoting its association with p120, resulting in recruitment of p190 to the cell periphery where it inhibits Rho.

p190-RhoGAP as an integral component of the Tiam1/Rac1-induced downregulation of Rho

The Rho family of small GTPases plays a central role in intracellular signal transduction, particularly in reorganization of the actin cytoskeleton. Rho activity induces cell contractility, whereas Rac promotes cellular protrusion, which counteracts Rho signaling. In this regard, the reciprocal balance between these GTPases determines cell morphology and migratory behavior. Here we demonstrate that Tiam1/Rac1 signaling is able to antagonize Rho activity directly at the GTPase level in COS-7 cells. p190-RhoGAP plays a central regulatory role in this signaling pathway. Interfering with its activation by Src-kinase-dependent tyrosine phosphorylation or its recruitment to the membrane through interaction with the SH2 domains of p120-RasGAP blocks the Tiam1-mediated rapid downregulation of Rho. This process is mediated by Rac1, but not by Rac2 or Rac3 isoforms. Our data provide evidence for a biochemical pathway of the reciprocal regulation of two related small GTPases, which are key elements in cell migration.

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.

Annexin A6 stimulates the membrane recruitment of p120GAP to modulate Ras and Raf-1 activity

Annexin A6 is a calcium-dependent membrane-binding protein that interacts with signalling proteins, including the GTPase-activating protein p120GAP, one of the most important inactivators of Ras. Since we have demonstrated that annexin A6 inhibits EGF- and TPA-induced Ras signalling, we investigated whether modulation of Ras activity by annexin A6 was mediated via altered subcellular localization of p120GAP. First, we exploited our observation that high-density lipoproteins (HDL) can activate the Ras/MAP kinase pathway. Expression of annexin A6 caused a significant reduction in HDL-induced activation of Ras and Raf-1. Annexin A6 promoted membrane binding of p120GAP in vitro, and plasma membrane targeting of p120GAP in living cells, both in a Ca(2+)-dependent manner, which is consistent with annexin A6 promoting the Ca(2+)-dependent assembly of p120GAP-Ras at the plasma membrane. We then extended these studies to other cell types and stimuli. Expression of annexin A6 in A431 cells reduced, while RNAi-mediated suppression of annexin A6 in HeLa cells enhanced EGF-induced Ras and Erk activation. Importantly, the enhancement of Ras activation following RNAi-mediated reduction in p120GAP levels was more marked in annexin A6-expressing A431 cells than controls, indicating that the effect of annexin A6 on Ras was mediated via p120GAP. Finally, we demonstrated that annexin A6 promotes plasma membrane targeting of p120GAP in A431 cells in response to a variety of stimuli, resulting in colocalization with H-Ras. These findings demonstrate an important role for annexin A6 in regulating plasma membrane localization of p120GAP and hence Ras activity.

PKCdelta regulates endothelial basal barrier function through modulation of RhoA GTPase activity

We have shown that PKCdelta enhanced microvascular endothelial basal barrier function, correlating with elevated RhoA GTPase activity and increased focal contact formation. In the current study, we investigated signaling pathways important in PKCdelta modulation of barrier function in unstimulated endothelial cell monolayers by assessing the effects of PKCdelta inhibition in endothelial cells (EC) derived from rat pulmonary artery (PAEC) and epididymus (FPEC). Rottlerin exposure or Ad PKCdeltadn infection significantly enhanced monolayer permeability in both EC. Immunofluorescence analyses demonstrated fewer stress fibers and focal contacts in rottlerin-treated or Ad PKCdeltadn-infected EC; yet, PKCdelta inhibition caused no significant changes in microtubule structures. These changes correlated with a reduction in both focal adhesion kinase (FAK) and RhoA GTPase activities. Microfilament stabilization significantly attenuated the focal contact and barrier disruptive effects of rottlerin. FAK overexpression did not blunt the effects of rottlerin-induced barrier dysfunction or stress fiber and focal contact disruption. Conversely, GFP-linked dominant active RhoA overexpression protected EC from stress fiber and focal contact disruption induced by both rottlerin exposure and overexpression of PKCdelta dominant negative protein. Additionally, PKCdelta immunoprecipitated with p190RhoGAP and p120RasGAP, modulators of RhoA activity. Thus, PKCdelta may regulate basal endothelial barrier function by stabilizing microfilaments and focal contacts by regulating RhoA GTPase activity through upstream modulators, p190RhoGAP and p120RasGAP.

Trap RACK1 with Ras to mobilize Src signaling at syndecan-2/p120-GAP upon transformation with oncogenic ras

HiTrap-syndecan-2/p120-GAP and HiTrap-syndecan-2/RACK1 affinity columns were applied to reveal that Src tyrosine kinase was highly expressed in BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus. Both columns were effective to isolate Src tyrosine kinase. The selective molecular affinity for Src was found to be stronger with HiTrap-syndecan-2/RACK1, as revealed with competitive RACK1 to dislodge Src from HiTrap-syndecan-2/p120-GAP. We thus challenged the syndecan-2/p120-GAP and syndecan-2/RACK1 with GTP-K(B)-Ras(Q(61)K). The reaction between RACK1 and syndecan-2 was sustained in the presence of mutant Ras proteins, but not the reaction between p120-GAP and syndecan-2. In the presence of syndecan-2, GTP-K(B)-Ras(Q(61)K) exhibited sufficient reactivity with p120-GAP to discontinue the reaction between p120-GAP and syndecan-2. But the interference of mutant Ras disappeared when Src tyrosine kinase was introduced to stabilize the syndecan-2/p120-GAP complex. On the other hand, in the absence of syndecan-2, GTP-K(B)-Ras(Q(61)K) was found to react with RACK1. The reaction between GTP-K(B)-Ras(Q(61)K) and RACK1 could provide a mechanism to deprive RACK1 for the organization of syndecan-2/RACK1 complex and to facilitate the formation of syndecan-2/p120-GAP complex, as well as to provide docking sites for Src signaling upon transformation with oncogenic ras.

GAPs in growth factor signalling

Approximately 2% of genes predicted by the sequenced human genome encode small GTPases and their regulators, highlighting the biological significance of regulated GTPase activity. Among the key GTPase regulators are the GTPase activating proteins (GAPs), which function to down-modulate active GTPases. Of the numerous identified GAPs, several have been implicated in signal transduction downstream of growth factors. In particular, GAPs for the Ras and Rho GTPases, which mediate a variety of receptor-transduced signals, appear to play an essential role in growth factor dependent GTPase regulation. Experimental studies of several of the GAPs have begun to elucidate mechanisms by which GAP activity is influenced by growth factor signaling, including direct phosphorylation, sub-cellular redistribution and protein degradation. Here, some of these mechanisms of GAP regulation in the context of signaling responses to growth factors are reviewed.

P120-GAP associated with syndecan-2 to function as an active switch signal for Src upon transformation with oncogenic ras

BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus were applied to reveal a complex of p120-GAP/syndecan-2 being highly expressed upon transformation. Of interest, most of the p120-GAP/syndecan-2 complex was localized at caveolae, a membrane microdomain enriched with caveolin-1. To confirm the molecular interaction between syndecan-2 and p120-GAP, we further purified p120-GAP protein from mouse brains by using an affinity column of HiTrap-RACK1 and expressed mouse RACK1-encoded fusion protein and mouse syndecan-2-encoded fusion protein in bacteria. We report molecular affinities exist between p120-GAP and RACK1, syndecan-2 and RACK1 as well as p120-GAP and syndecan-2. The selective affinity between p120-GAP and syndecan-2 was found to be sufficient to detach RACK1. The p120-GAP/syndecan-2 complex was demonstrated to keep Src tyrosine kinase in an activated form. On the other hand, the syndecan-2/RACK1 complex was found to have Src in an inactivated form. These data indicate that the p120-GAP/syndecan-2 complex at caveolae could provide a docking site for Src to transmit tyrosine signaling, implying that syndecan-2/p120-GAP functions as a tumor promoter upon transformation with oncogenic ras of shrimp P. japonicus.

BCL6 suppresses RhoA activity to alter macrophage morphology and motility

BCL6 is a potent transcriptional repressor that plays important roles in germinal center formation, T helper cell differentiation and lymphomagenesis and regulates expression of several chemokine genes in macrophages. In a further investigation of its role in macrophages, we show that BCL6 inactivation in primary bone marrow-derived macrophages leads to decreased polarization, motility and cell spreading accompanied by an increase in peripheral focal complexes, anchored F-actin bundles and cortical F-actin density. These changes were associated with excess RhoA activation. C3 transferase inhibition of RhoA activity reverted the adhesion structure phenotype, which was not affected by Rho kinase inhibitors, suggesting that other downstream effectors of Rho maintain this Bcl6–/– phenotype. Excess RhoA activation in BCL6-deficient macrophages is associated with a decrease in the p120RasGAP (RASA1)-mediated translocation of p190RhoGAP (GRLF1) to active RhoA at the plasma membrane and a reduction in cell surface expression of the CSF1R that has been reported to recruit RasGAP to the plasma membrane. Reconstitution of BCL6 expression in Bcl6–/– macrophages results in complete reversion of the morphological phenotype and a significant increase in cell surface CSF1R expression whereas overexpression of the CSF1R corrects the polarization and adhesion structure defects. These results demonstrate that BCL6 suppresses RhoA activity, largely through upregulation of surface CSF1R expression, to modulate cytoskeletal and adhesion structures and increase the motility of macrophages.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

Role of Dok-1 and Dok-2 in leukemia suppression

Chronic myelogenous leukemia (CML) is characterized by the presence of the chimeric p210bcr/abl oncoprotein that shows elevated and constitutive protein tyrosine kinase activity relative to the normal c-abl tyrosine kinase. Although several p210bcr/abl substrates have been identified, their relevance in the pathogenesis of the disease is unclear. We have identified a family of proteins, Dok (downstream of tyrosine kinase), coexpressed in hematopoietic progenitor cells. Members of this family such as p62dok (Dok-1) and p56dok-2 (Dok-2) associate with the p120 rasGTPase-activating protein (rasGAP) upon phosphorylation by p210bcr/abl as well as receptor and nonreceptor tyrosine kinases. Here, we report the generation and characterization of single and double Dok-1 or Dok-2 knockout (KO) mutants. Single KO mice displayed normal steady-state hematopoiesis. By contrast, concomitant Dok-1 and Dok-2 inactivation resulted in aberrant hemopoiesis and Ras/MAP kinase activation. Strikingly, all Dok-1/Dok-2 double KO mutants spontaneously developed transplantable CML-like myeloproliferative disease due to increased cellular proliferation and reduced apoptosis. Furthermore, Dok-1 or Dok-2 inactivation markedly accelerated leukemia and blastic crisis onset in Tec-p210bcr/abl transgenic mice known to develop, after long latency, a myeloproliferative disorder resembling human CML. These findings unravel the critical and unexpected role of Dok-1 and Dok-2 in tumor suppression and control of the hematopoietic compartment homeostasis.

The SPRY domain-containing SOCS box protein 1 (SSB-1) interacts with MET and enhances the hepatocyte growth factor-induced Erk-Elk-1-serum response element pathway

The suppressor of cytokine signaling (SOCS) protein family includes a SPRY (repeats in splA and RyR) domain-containing SOCS box protein (SSB) subfamily, which consists of four members, SSB-1, SSB-2, SSB-3, and SSB-4. These proteins contain a central SPRY domain and a C-terminal SOCS box. Although some of the SOCS protein subfamilies function as adaptors for a large family of ubiquitin-protein isopeptide ligases to regulate certain signaling pathways, the function of the SSB subfamily remains to be determined. In our previous studies, we have found that two SPRY domain-containing proteins, RanBP9 and RanBP10, interact with MET through the SPRY domain. In the present study, we explored the function of SSB proteins in the regulation of the hepatocyte growth factor (HGF)-MET signaling. Our results showed that all four SSB proteins also interacted with the MET. The MET interaction with SSB-1 was further investigated. We demonstrated that SSB-1 bound to MET tyrosine kinase domain through its SPRY domain. MET interacted with SSB-1 in both the absence and the presence of HGF, but HGF treatment resulted in the recruitment of more SSB-1 by MET. We showed that overexpression of SSB-1 but not other SSB proteins enhanced the HGF-induced serum response element (SRE)-luciferase activity. Overexpression of SSB-1 exhibited no effect on the basal level or epidermal growth factor-induced SRE-luciferase activity. SSB-1 also enhanced HGF-induced Erk phosphorylation. Suppression of SSB-1 by the RNA interference method down-regulated HGF-induced SRE-luciferase activity and decreased Elk-1 activation. These results suggest that SSB-1 may play an important role in enhancing the HGF-induced Erk-Elk-1-SRE pathway. Furthermore, we demonstrated that in response to HGF stimulation, the SSB-1 protein became phosphorylated at tyrosine residue 31. The phosphorylated SSB-1 protein bound to p120Ras-GTPase-activating protein (GAP) but did not promote the degradation of p120RasGAP, indicating that enhanced HGF-MET signaling by overexpression of SSB-1 was not dependent on p120RasGAP degradation.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

p120 Ras GTPase-activating protein associates with fibroblast growth factor receptors in Drosophila

Btl (breathless) and Htl (heartless), the two FGFRs (fibroblast growth factor receptors) in Drosophila melanogaster, control cell migration and differentiation in the developing embryo. These receptors signal through the conserved Ras/mitogen-activated protein kinase pathway, but how they regulate Ras activity is not known. The present study shows that there is a direct interaction between p120 RasGAP (Ras GTPase-activating protein), a negative regulator of Ras, and activated FGFRs in Drosophila. The interaction is dependent on the SH2 (Src homology 2) domains of RasGAP, which have been shown to interact with a phosphotyrosine residue within the consensus sequence (phospho)YXXPXD. A potential binding site that matches this consensus is found in both Btl and Htl, located between the transmembrane and kinase domains of each receptor. A peptide corresponding to this region was capable of binding RasGAP only when the tyrosine residue was phosphorylated. This tyrosine residue appears to be conserved in human FGFR-1 and mediates the association with the adapter protein CrkII, but no association between dCrk (Drosophila homologue of CrkII) and the activated FGFRs was detected. RasGAP was a substrate of the activated FGFR kinase domain, and mutation of the tyrosine residue within the potential binding site on the receptor prevented tyrosine phosphorylation of RasGAP. RasGAP attenuated FGFR signalling in vivo and this ability was dependent on both its SH2 domains and its GAP activity. On the basis of these results, we propose that RasGAP is directly recruited into activated FGFRs in Drosophila and plays a role in regulating the strength of signalling through Ras and the mitogen-activated protein kinase pathway.

N-cadherin activation substitutes for the cell contact control in cell cycle arrest and myogenic differentiation: involvement of p120 and beta-catenin

N-cadherin is expressed throughout skeletal myogenesis and has been proposed to be involved in the differentiation program of myogenic precursors. Here, we further characterize the N-cadherin involvement and its mechanism of action at the onset of differentiation, through controlled N-cadherin activation by plating isolated C2 myoblasts on surfaces coated with a chimeric Ncad-Fc homophilic ligand (N-cadherin ectodomain fused to the immunoglobulin G Fc fragment). We show that N-cadherin activation substitutes for the cell density in myogenic differentiation by promoting myogenin and troponin T expression. In addition, N-cadherin adhesion participates to the associated cell cycle arrest through the nuclear accumulation of cyclin-dependent kinase inhibitors p21 and p27. Mouse primary myoblast cultures exhibited similar responses to N-cadherin as C2 cells. RNA interference knockdowns of the N-cadherin-associated cytoplasmic proteins p120 and beta-catenin produced opposite effects on the differentiation pathway. p120 silencing resulted in a decreased myogenic differentiation, associated with a reduction in cadherin-catenin content, which may explain its action on myogenic differentiation. beta-Catenin silencing led to a stimulatory effect on myogenin expression, without any effect on cell cycle. Our results demonstrate that N-cadherin adhesion may account for cell-cell contact-dependent cell cycle arrest and differentiation of myogenic cells, involving regulation through p120 and beta-catenins.

Overexpression of FAK promotes Ras activity through the formation of a FAK/p120RasGAP complex in malignant astrocytoma cells

Focal adhesion kinase (FAK) signaling may be mediated through the modulation of Ras activity. We have shown previously that grade III malignant astrocytoma biopsy samples exhibit elevated levels of FAK, and that overexpression of FAK in U-251MG malignant astrocytoma cells promotes the phosphorylation of Shc, a potential upstream mediator of Ras activity. Here, we report that overexpression of FAK promotes Ras activity in U-251MG malignant astrocytoma cells cultured in aggregate suspension or as monolayers adherent to vitronectin. The overexpression of FAK also promoted the association of FAK with p120RasGAP, which is a negative regulator of Ras activity, in the U-251MG cells cultured in aggregate suspension, with this association being abrogated upon plating of the cells onto vitronectin. An association of FAK with p120RasGAP also was observed in malignant astrocytoma biopsy samples, but not in normal brain samples. As overexpression of FAK in U-251MG cells in aggregate suspension culture reduced the amount of p120RasGAP complexed with active Ras, we hypothesize that the association of FAK with p120 RasGAP may facilitate Ras activity. The overexpression of a mutated FAK in which the Y397 had been mutated to F did not result in the formation of the FAK/p120RasGAP complex and did not promote Ras activity, indicating that the Y397 residue of FAK plays a role in the formation of this complex and in the activation of Ras. Moreover, the overexpression of mutated FAK (397F) was found to inhibit anchorage-independent growth. These data provide the basis for a previously undescribed mechanism in which the elevated expression of FAK can promote Ras activity through its competitive recruitment of p120RasGAP, thereby diminishing the association of p120RasGAP with active Ras.

Complete polarization of single intestinal epithelial cells upon activation of LKB1 by STRAD

The LKB1 gene encodes a serine/threonine kinase that is mutated in the Peutz-Jeghers cancer syndrome. LKB1 is homologous to the Par-4 polarity genes in C. elegans and D. melanogaster. We have previously reported the identification and characterization of an LKB1-specific adaptor protein, STRAD, which activates LKB1 and translocates it from nucleus to cytoplasm. We have now constructed intestinal epithelial cell lines in which inducible STRAD activates LKB1. Upon LKB1 activation, single cells rapidly remodel their actin cytoskeleton to form an apical brush border. The junctional proteins ZO-1 and p120 redistribute in a dotted circle peripheral to the brush border, in the absence of cell-cell contacts. Apical and basolateral markers sort to their respective membrane domains. We conclude that LKB1 can induce complete polarity in intestinal epithelial cells. In contrast to current thinking on polarization of simple epithelia, these cells can fully polarize in the absence of junctional cell-cell contacts.

Normal p21Ras/MAP kinase pathway expression and function in PBMC from patients with polycystic ovary disease

Polycystic ovary disease (PCOD) is associated with insulin resistance and increased prevalence of type II diabetes mellitus (T2DM). The p21Ras/MAP kinase is a major intracellular signaling pathway mediating insulin signaling in insulin responsive tissues. The expression, regulation and function of the p21Ras/MAP kinase pathway in PCOD patients were examined. Peripheral blood mononuclear cells (PBMC) were isolated from ten patients with PCOD and ten controls. The expression of p21Ras and its regulatory proteins; hSOS1 and p120GAP were studied. The basal and phytohemaglutinin (PHA) or insulin stimulated phosphorylation of MAP kinase was determined. Expression of p21Ras, and its regulatory proteins hSOS1 and p120GAP were similar in PCOD patients and controls. Basal, PHA and insulin stimulated phosphorylation of MAP kinase, were also comparable in the two groups as well as their PBMC proliferative response. These data indicate that the expression and overall function of the p21Ras/MAP kinase pathway remain intact in non-diabetic patients with PCOD.

Variation in cadherins and catenins expression is linked to both proliferation and transformation of Rhabdomyosarcoma

Cadherins are a family of transmembrane glycoproteins that mediate Ca(2+)-dependent homophilic cell-cell adhesion and play a crucial role in cell differentiation. E-cadherin-mediated cell-cell adhesion is lost during the development of most epithelial cancers. This study examines cadherin-dependent adhesion in cell lines derived from rhabdomyosarcoma (RMS), a highly malignant soft-tissue tumor committed to the myogenic lineage, but arrested prior to terminal differentiation. We analysed the expression of cadherins and associated catenins at the mRNA and protein levels as well as their localization in nine RMS-derived cell lines relative to normal myoblasts. We show a decrease in the expression of cadherins and catenins in all RMS-derived cell lines compared to control cells. This decrease in the expression of N- and M-cadherin was confirmed in RMS biopsies. In contrast, R-cadherin is found expressed in RMS, whereas it is normally absent in normal myoblasts. We show that a decrease of R-cadherin expression using RNA interference inhibits cell proliferation of the RD cell line. In addition to their diminished expression, cadherins and catenins do not localize to intercellular contacts in embryonal RMS (ERMS), whereas specific persistent localization is seen in alveolar RMS (ARMS)-derived cell lines. Thus, RMS exhibit defects in the expression of molecules of the cadherin family. Defects in the localization of these adhesion molecules at the sites of cell-cell contact are specifically observed in the ERMS subtype. In addition, our data suggest that R-cadherin is a specific diagnostic marker for RMS and is also an important factor of RMS cell proliferation.

Cell-cycle-dependent three-dimensional redistribution of nuclear proteins, P 120, pKi-67, and SC 35 splicing factor, in the presence of the topoisomerase I inhibitor camptothecin

Topoisomerase I (Topo I) is mostly known for its role in DNA relaxation, which is required for duplication and transcription. Topo I acts as a protein kinase mainly directed to the mRNA splicing factor SC35. Camptothecin is one of the specific Topo I inhibitors and is effective on the two functions of the enzyme. In this study we demonstrated that treatment of KB cells with camptothecin for only 30 min induced the 3D reorganization and redistribution of three proteins involved in the nucleus machinery, P 120, pKi-67, and SC 35, and this occurred in a cell cycle-dependent manner. Our data were obtained from confocal microscopic studies after immunolabeling, 3D reconstruction, and measurement of the nuclear components volumes. In the presence of camptothecin, P 120, which occupied the nucleolar volume, lost its reticulation and pKi-67 was redistributed within the nucleoplasm and even into the cytoplasm. Finally, for SC 35 the fusion of its dots into bigger volumes was observed specifically during the G1 phase. Variations of volumes were also observed for the nucleolus and for the nucleus. These results pointed out that, depending on the cell cycle phase, Topo I functions were selective toward the three different proteins.

p120-independent modulation of E-cadherin adhesion activity by the membrane-proximal region of the cytoplasmic domain

Cadherins are transmembrane glycoproteins that function as Ca2+-dependent cell-cell adhesion molecules and are linked to the actin cytoskeleton via catenins. Previously, we showed that, although E-cadherin lacking its cytoplasmic tail is active in aggregation assays, partially truncated E-cadherin lacking the carboxyl-terminal catenin-binding site is not. Contrary to this observation, a similar N-cadherin construct is found to be functional. Chimeric constructs, in which the membrane-proximal region of the partially truncated E-cadherin was replaced by that of N-cadherin, are active in aggregation assays. N-cadherin constructs in the opposite manner are nonfunctional. Although deletion of the membrane-proximal region, which eliminates the binding site for p120, results in activation of the nonfunctional E-cadherin mutant polypeptides, amino acid substitutions in the membrane-proximal region, which uncouple p120 binding, do not. The p120 uncoupling could not activate a full-length E-cadherin construct, which was beta-catenin-uncoupled by amino acid substitutions in the catenin-binding site. These results indicate that the membrane-proximal region determines the activity of these cadherin constructs but that p120 does not seem directly involved in the modulation of E-cadherin activity.

The tetraspanin CD151 functions as a negative regulator in the adhesion-dependent activation of Ras

Transmembrane proteins of the tetraspanin superfamily are associated with integrins and are thought to regulate adhesion-dependent signaling. The molecular mechanisms of this regulation remain unknown. We used rat fibroblasts to analyze the contribution of the tetraspanin CD151 in the adhesion-dependent signaling. Expression of CD151 specifically attenuated adhesion-dependent activation of Ras. Furthermore, activation of PKB/c-Akt and ERK1/2, downstream targets in the Ras signaling pathway, was also diminished in cells expressing CD151. In contrast, adhesion-dependent activation of FAK and c-Src were not affected by CD151. The attenuation of Ras signaling did not correlate with phosphorylation of Tyr925-FAK, tyrosine phosphorylation of Shc, or with assembly of the p120RasGAP-p62Dok complex. Using mutants of CD151 we established that the cytoplasmic C-terminal portion is critical for activity of CD151 toward Ras. Taken together these results identify CD151 as a negative regulator of Ras and suggest a novel mechanism of adhesion-dependent regulation of Ras activity.

Structural fingerprints of the Ras-GTPase activating proteins neurofibromin and p120GAP

Ras specific GTPase activating proteins (GAPs), neurofibromin and p120GAP, bind GTP bound Ras and efficiently complement its active site. Here we present comparative data from mutations and fluorescence-based assays of the catalytic domains of both RasGAPs and interpret them using the crystal structures. Three prominent regions in RasGAPs, the arginine-finger loop, the phenylalanine-leucine-arginine (FLR) region and alpha7/variable loop contain structural fingerprints governing the GAP function. The finger loop is crucial for the stabilization of the transition state of the GTPase reaction. This function is controlled by residues proximal to the catalytic arginine, which are strikingly different between the two RasGAPs. These residues specifically determine the orientation and therefore the positioning of the arginine finger in the Ras active site. The invariant FLR region, a hallmark for RasGAPs, indirectly contributes to GTPase stimulation by forming a scaffold, which stabilizes Ras switch regions. We show that a long hydrophobic side-chain in the FLR region is crucial for this function. The alpha7/variable loop uses several conserved residues including two lysine residues, which are involved in numerous interactions with the switch I region of Ras. This region determines the specificity of the Ras-RasGAP interaction.

Dynamic regulation of the Ras pathway via proteolysis of the NF1 tumor suppressor

Mutations in the NF1 tumor suppressor underlie the familial tumor predisposition syndrome neurofibromatosis type I. Although its encoded protein, neurofibromin, functions as a Ras-GTPase activating protein (GAP), nothing is known about how it is normally regulated or its precise role in controlling Ras signaling pathways. We show here that neurofibromin is dynamically regulated by the ubiquitin-proteasome pathway. Degradation is rapidly triggered in response to a variety of growth factors and requires sequences adjacent to the catalytic GAP-related domain of neurofibromin. However, whereas degradation is rapid, neurofibromin levels are re-elevated shortly after growth factor treatment. Accordingly, Nf1-deficient mouse embryonic fibroblasts (MEFs) exhibit an enhanced activation of Ras, prolonged Ras and ERK activities, and proliferate in response to subthreshold levels of growth factors. Thus, the dynamic proteasomal regulation of neurofibromin represents an important mechanism of controlling both the amplitude and duration of Ras-mediated signaling. Furthermore, this previously unrecognized Ras regulatory mechanism may be exploited therapeutically.

Localized suppression of RhoA activity by Tyr31/118-phosphorylated paxillin in cell adhesion and migration

RhoA activity is transiently inhibited at the initial phase of integrin engagement, when Cdc42- and/or Rac1-mediated membrane spreading and ruffling predominantly occur. Paxillin, an integrin-assembly protein, has four major tyrosine phosphorylation sites, and the phosphorylation of Tyr31 and Tyr118 correlates with cell adhesion and migration. We found that mutation of Tyr31/118 caused enhanced activation of RhoA and premature formation of stress fibers with substantial loss of efficient membrane spreading and ruffling in adhesion and migration of NMuMG cells. These phenotypes were similar to those induced by RhoA(G14V) in parental cells, and could be abolished by expression of RhoA(T19N), Rac1(G12V), or p190RhoGAP in the mutant-expressing cells. Phosphorylated Tyr31/118 was found to bind to two src homology (SH)2 domains of p120RasGAP, with coprecipitation of endogenous paxillin with p120RasGAP. p190RhoGAP is known to be a major intracellular binding partner for the p120RasGAP SH2 domains. We found that Tyr31/118-phosphorylated paxillin competes with p190RhoGAP for binding to p120RasGAP, and provides evidence that p190RhoGAP freed from p120RasGAP efficiently suppresses RhoA activity during cell adhesion. We conclude that Tyr31/118-phosphorylated paxillin serves as a template for the localized suppression of RhoA activity and is necessary for efficient membrane spreading and ruffling in adhesion and migration of NMuMG cells.

Microaerophilic conditions permit to mimic in vitro events occurring during in vivo Helicobacter pylori infection and to identify Rho/Ras-associated proteins in cellular signaling

Molecular dissection of the mechanisms underlying Helicobacter pylori infection suffers from the lack of in vitro systems mimicking in vivo observations. A system was developed whereby human epithelial cells (Caco-2) grown as polarized monolayers and bacteria can communicate with each other under culture conditions optimal for each partner. Caco-2 cells grown on filter supports were inserted in a vertical position into diffusion chambers equilibrated with air and 5% CO(2) at their basolateral surface (aerophilic conditions) and 5% CO(2), 5% O(2), 90% N(2) (microaerophilic conditions) in the apical compartment. Remarkably, the epithelial polarized layer was stable under these asymmetric culture conditions for at least 24 h, and the presence of Caco-2 cells was necessary to maintain H. pylori growth. In contrast to previous studies conducted with non-polarized Caco-2 cells and other cell lines kept under aerophilic conditions, we found H. pylori-dependent stimulation of cytokine secretion (MCP-1 (monocyte chemoattractant protein-1), GRO-alpha (growth-regulated oncogene-alpha), RANTES (regulated on activation normal T cell expressed and secreted)). This correlated with nuclear translocation of NF-kappaB p50 and p65 subunits. Tyrosine phosphorylation of nine cellular proteins was induced or enhanced; we identified p120(RasGAP), p190(RhoGAP), p62dok (downstream of tyrosine kinases), and cortactin as H. pylori-inducible targets. Moreover, reduction of H. pylori urease expression was observed in adherent bacteria as compared with bacteria in suspension. In addition to mimicking several observations seen in the inflamed gastric mucosa, the novel in vitro system was allowed to underscore complex cellular events not seen in classical in vitro analyses of microaerophilic bacteria-epithelial cell cross-talk.

Constitutive up-regulated activity of MAP kinase is associated with down-regulated early p21Ras pathway in lymphocytes of SLE patients

Aberrant expression of the p21Ras proto-oncogene has been reported in lymphoid cells of SLE patients. We previously showed that the expression of the p21Ras stimulatory element, hSOS1, is reduced in PBMC from SLE patients with non-active disease. However, the significance of this finding regarding the regulation and function of the p21Ras pathway and its correlation to disease activity remained unclear. The expression, regulation and function of the p21Ras pathway were determined in 23 ambulatory SLE patients with active and non-active disease and eleven controls. Levels of p21Ras stimulatory element hSOS1 but not p21Ras and its inhibitory element p120GAP were significantly decreased in SLE patients. Early p21Ras signalling was down-regulated in SLE patients with active disease as indicated by the decreased membrane/cytoplasmic (M/C) ratios of the p21Ras regulatory elements hSOS1 and p120GAP and by the non-responsiveness of these ratios to cellular stimulation. Anchorage of p21Ras to the cellular membrane was also significantly decreased in these patients. In contrast, the late p21Ras signalling was up-regulated in SLE patients as indicated by the significantly higher constitutive activity of the p21Ras down stream key regulator enzyme MAP Kinase. Taken together, our data demonstrate for the first time a disease associated functional defect in p21Ras signalling in lymphocytes of SLE patients.

EphB ligand, ephrinB2, suppresses the VEGF- and angiopoietin 1-induced Ras/mitogen-activated protein kinase pathway in venous endothelial cells

Interaction between ephrinB2 and EphB4 in endothelial cells at the arterial-venous capillary interface is critical for proper embryonic capillary morphogenesis. However, the intracellular downstream signaling of ephrinB2-EphB in vascular endothelial cells is unknown. This study examined the effect of ephrinB2-induced activation of EphB kinases on vascular endothelial growth factor (VEGF)- and angiopoietin-1 (Ang1)-induced Ras/mitogen-activated protein kinase (MAPK) signaling cascades in human umbilical vein endothelial cells (HUVECs). Reverse transcriptase-polymer chain reaction results showed that HUVECs expressed three kinds of EphB kinases known to bind to ephrinB2: EphB2, EphB3, and EphB4. EphrinB2 not only increased the phosphorylation of EphB2 and EphB4 in a time-dependent manner but also increased recruitment of p120-Ras-GTPase-activating protein (p120-RasGAP) to EphB2 and EphB4. Accordingly, ephrinB2 inhibited VEGF- and Ang1-induced Ras-MAPK activities, whereas ephrinB2 did not alter VEGF-induced Flk phosphorylation or Ang1-induced Tie2 phosphorylation. Furthermore, ephrinB2 suppressed VEGF- and Ang1-induced proliferation and/or migration, which are mediated mainly through Ras/MAPK signaling cascades. From these results, we propose that ephrinB2-EphB, signaling through Ras/MAPK cascade, may be critical for proper morphogenesis of capillary endothelium through the arrest of endothelial cell proliferation and migration at the arterial-venous interface.

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.

Evidence for aberrant regulation of the p21Ras pathway in PBMCs of patients with chronic idiopathic urticaria

BACKGROUND

Recent data suggest that a subpopulation of patients with chronic urticaria has an autoimmune disorder. Aberrant expression and regulation of the p21Ras pathway has been reported in lymphoid cells in a variety of systemic autoimmune diseases but not in chronic idiopathic urticaria (CIU).

OBJECTIVES

The aim of this study was to examine the expression, regulation, and function of the p21Ras pathway in patients with CIU.

METHODS

Twenty-four patients with CIU and 14 control subjects were enrolled. All patients and 9 control subjects were intradermally injected with autologous serum. PBMCs were isolated, and the p21Ras and its regulatory proteins were studied.

RESULTS

We found increased expression of the p21ras proto-oncogene in patients with CIU. This was associated with a low expression of the p21Ras stimulatory element human son of sevenless (hSOS1) but normally expressed p21Ras inhibitory element p120GTPase-activating protein. The basal nonstimulated membrane/cytoplasmic ratio of hSOS1, which indicates the p21Ras pathway activity, was higher in patients compared with that seen in control subjects. Moreover, after stimulation, both patients and control subjects decreased their hSOS1 membrane/cytoplasmic ratio. The magnitude of this decrease was much higher in patients than in control subjects: 14- and approximately 2-fold, respectively. The basal and stimulated activities of the p21Ras downstream key regulatory enzyme mitogen-activated protein kinase were comparable in patients and control subjects, as was their in vitro mitogen-stimulated lymphocyte proliferation.

CONCLUSION

Our data demonstrate for the first time an aberrant signaling through the p21Ras pathway in lymphocytes of patients with CIU. This finding further supports the autoimmune basis of this disease.

Regulation of extracellular signal-regulated kinase activity by p120 RasGAP does not involve its pleckstrin homology or calcium-dependent lipid binding domains but does require these domains to regulate cell proliferation

The gene encoding for p120 RasGAP, has been disrupted in mice (M. Henkemeyer et al., Nature (Lond.), 377: 695-701, 1995). In this study, using fibroblasts derived from these mouse embryos (Gap-/-; P. van der Geer et al., Mol. Cell Biol., 17: 1840-1847, 1997), we demonstrate that mitogen-activated protein kinase (MAPK) activation is prolonged after epidermal growth factor (EGF), but not lysophosphatidic acid, stimulation as compared with wild-type cells. Furthermore, these cells exhibited a moderate increase in their proliferative rate and saturation density, as well as a limited ability to form colonies in soft agar. Stable cell lines expressing full-length p120GAP not only restored the ability to down-regulate MAPK after EGF stimulation but also lowered their saturation densities. Similarly, expression of p120GAP, missing either its pleckstrin homology (PH) or its calcium-dependent lipid binding (CaLB)/C2 domain, restored MAPK down-regulation and retained the ability to associate with p190 RhoGAP and to be phosphorylated by v-src but exhibited higher saturation densities similar to Gap-/- cells. Our results, therefore, suggest that p120GAP functions not only by down-regulating the Ras/MAPK pathway after growth factor stimulation but is also important in regulating cell proliferation that involves its PH and CaLB domains.

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.