Prenyl group identification of rap2 proteins: a ras superfamily member other than ras that is farnesylated

The RIT1 C-terminus associates with lipid bilayers via charge complementarity

RIT1 is a member of the Ras superfamily of small GTPases involved in regulation of cellular signaling. Mutations to RIT1 are involved in cancer and developmental disorders. Like many Ras subfamily members, RIT1 is localized to the plasma membrane. However, RIT1 lacks the C-terminal prenylation that helps many other subfamily members adhere to cellular membranes. We used molecular dynamics simulations to examine the mechanisms by which the C-terminal peptide (CTP) of RIT1 associates with lipid bilayers. We show that the CTP is unstructured and that its membrane interactions depend on lipid composition. While a 12-residue region of the CTP binds strongly to anionic bilayers containing phosphatidylserine lipids, the CTP termini fray from the membrane allowing for accommodation of the RIT1 globular domain at the membrane-water interface.

Beyond the Mevalonate Pathway: Control of Post-Prenylation Processing by Mutant p53

Missense mutations in the TP53 gene are among the most frequent alterations in human cancer. Consequently, many tumors show high expression of p53 point mutants, which may acquire novel activities that contribute to develop aggressive tumors. An unexpected aspect of mutant p53 function was uncovered by showing that some mutants can increase the malignant phenotype of tumor cells through alteration of the mevalonate pathway. Among metabolites generated through this pathway, isoprenoids are of particular interest, since they participate in a complex process of posttranslational modification known as prenylation. Recent evidence proposes that mutant p53 also enhances this process through transcriptional activation of ICMT, the gene encoding the methyl transferase responsible for the last step of protein prenylation. In this way, mutant p53 may act at different levels to promote prenylation of key proteins in tumorigenesis, including several members of the RAS and RHO families. Instead, wild type p53 acts in the opposite way, downregulating mevalonate pathway genes and ICMT. This oncogenic circuit also allows to establish potential connections with other metabolic pathways. The demand of acetyl-CoA for the mevalonate pathway may pose limitations in cell metabolism. Likewise, the dependence on S-adenosyl methionine for carboxymethylation, may expose cells to methionine stress. The involvement of protein prenylation in tumor progression offers a novel perspective to understand the antitumoral effects of mevalonate pathway inhibitors, such as statins, and to explore novel therapeutic strategies.

Rap2B promotes angiogenesis via PI3K/AKT/VEGF signaling pathway in human renal cell carcinoma

Human renal cell carcinoma which is a highly vascular tumor is the leading cause of death from urologic cancers. Angiogenesis has a pivotal role in oncogenesis and in the viability and expansion of renal cell carcinoma. Rap2B, as a small guanosine triphosphate–binding protein of the Ras family, was first discovered in the early 1990s during the screening of a platelet complementary DNA library. Previous studies have shown that Rap2B aberrantly expressed in human carcinogenesis and promoted the development of tumors via multiple signaling pathways. However, the function of Rap2B in tumor angiogenesis that is necessary for tumor growth and metastasis remains unknown. In this study, we examined the role of Rap2B in angiogenesis in renal cell carcinoma by Western blot, quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, human umbilical vascular endothelial cells growth assay, and endothelial cell tube formation assay. We found that Rap2B promoted angiogenesis in vitro and in vivo. Moreover, our data illustrated that phosphoinositide 3-kinase/AKT signaling pathway is involved in Rap2B-mediated upregulation of vascular endothelial growth factor and renal cell carcinoma angiogenesis. Taken together, these results revealed that Rap2B promotes renal cell carcinoma angiogenesis via phosphoinositide 3-kinase/AKT/vascular endothelial growth factor signaling pathway, which suggests that Rap2B is a novel therapeutic target for renal cell carcinoma anti-angiogenesis therapy.

Guanine nucleotide exchange factor Epac2-dependent activation of the GTP-binding protein Rap2A mediates cAMP-dependent growth arrest in neuroendocrine cells

First messenger-dependent activation of MAP kinases in neuronal and endocrine cells is critical for cell differentiation and function and requires guanine nucleotide exchange factor (GEF)-mediated activation of downstream Ras family small GTPases, which ultimately lead to ERK, JNK, and p38 phosphorylation. Because there are numerous GEFs and also a host of Ras family small GTPases, it is important to know which specific GEF-small GTPase dyad functions in a given cellular process. Here we investigated the upstream activators and downstream effectors of signaling via the GEF Epac2 in the neuroendocrine NS-1 cell line. Three cAMP sensors, Epac2, PKA, and neuritogenic cAMP sensor-Rapgef2, mediate distinct cellular outputs: p38-dependent growth arrest, cAMP response element-binding protein-dependent cell survival, and ERK-dependent neuritogenesis, respectively, in these cells. Previously, we found that cAMP-induced growth arrest of PC12 and NS-1 cells requires Epac2-dependent activation of p38 MAP kinase, which posed the important question of how Epac2 engages p38 without simultaneously activating other MAP kinases in neuronal and endocrine cells. We now show that the small GTP-binding protein Rap2A is the obligate effector for, and GEF substrate of, Epac2 in mediating growth arrest through p38 activation in NS-1 cells. This new pathway is distinctly parcellated from the G protein-coupled receptor → G_s → adenylate cyclase → cAMP → PKA → cAMP response element-binding protein pathway mediating cell survival and the G protein-coupled receptor → G_s → adenylate cyclase → cAMP → neuritogenic cAMP sensor-Rapgef2 → B-Raf → MEK → ERK pathway mediating neuritogenesis in NS-1 cells.

Rap2B GTPase: structure, functions, and regulation

Rap2B GTPase, a member of Ras-related protein superfamily, was first discovered from a platelet cDNA library in the early 1990s. Since then, it has been reported to play an important role in regulating cellular processes including cytoskeletal organization, cell growth, and proliferation. It can be stimulated and suppressed by a wide range of external and internal inducers, circulating between GTP-bound active state and GDP-bound inactive state. Increasing focus on Ras signaling pathway reveals critical effects of Rap2B on tumorigenesis. In particular, Rap2B behaves in a p53-dependent manner in regulation of apoptosis and migration. Apart from being an oncogenic activator, Rap2B has been found to participate in many other physiological events via diverse downstream effectors. In this review, we present recent studies on the structure, regulation, and multiple biological functions of Rap2B, shedding light on its potential status in treatment of cancer as well as other diseases.

Structure, functional regulation and signaling properties of Rap2B

The Ras family small guanosine 5'-triphosphate (GTP)-binding protein Rap2B is is a member of the Ras oncogene family and a novel target of p53 that regulates the p53-mediated pro-survival function of cells. The Rap2B protein shares ~90% homology with Rap2A, and its sequence is 70% identical to other members of the Rap family such as RaplA and RaplB. As a result, Rap2B has been theorized to have similar signaling effectors to the GTPase-binding protein Rap, which mediates various biological functions, including the regulation of sterile 20/mitogen-activated proteins. Since its identification in the early 1990s, Rap2B has elicited a considerable interest. Numerous studies indicate that Rap2B exerts specific biological functions, including binding and stimulating phospholipase C-ε and interferon-γ. In addition, downregulation of Rap2B affects the growth of melanoma cells. The present review summarizes the possible effectors and biological functions of Rap2B. Increasing evidence clearly supports the association between Rap2B function and tumor development. Therefore, it is conceivable that anticancer drugs targeting Rap2B may be generated as novel therapies against cancer.

Analogs of farnesyl diphosphate alter CaaX substrate specificity and reactions rates of protein farnesyltransferase

Attempts to identify the prenyl-proteome of cells or changes in prenylation following drug treatment have used 'clickable' alkyne-modified analogs of the lipid substrates farnesyl- and geranylgeranyl-diphosphate (FPP and GGPP). We characterized the reactivity of four alkyne-containing analogs of FPP with purified protein farnesyltransferase and a small library of dansylated peptides using an in vitro continuous spectrofluorimetric assay. These analogs alter prenylation specificity and reactivity suggesting that in vivo results obtained using these FPP analogs should be interpreted cautiously.

Palmitoylation pilots ras to recycling endosomes

We recently showed that palmitoylated Ras proteins (H-Ras and N-Ras) localize intracellularly at recycling endosomes (REs) and that REs act as a way-station for Ras proteins as they move along the post-Golgi exocytic pathway to the plasma membrane (PM). Palmitoylation is essential for H-Ras/N-Ras targeting to REs. H-Ras requires two palmitoyl groups for RE targeting. A lack of either or both palmitoyl groups causes H-Ras to be mislocalized to the endoplasmic reticulum (ER), the Golgi apparatus, or the PM. In this commentary, we summarize recent progress about the Ras trafficking cycle between the endomembranes (endosomes/ER/Golgi) and the PM. We further discuss (1) the critical determinants of RE targeting of lipidated proteins and (2) possible Ras-mediated signaling pathways that originate from REs.

Mechanisms of isoform specific Rap2 signaling during enterocytic brush border formation

Brush border formation during polarity establishment of intestinal epithelial cells is uniquely governed by the Rap2A GTPase, despite expression of the other highly similar Rap2 isoforms (Rap2B and Rap2C). We investigated the mechanisms of this remarkable specificity and found that Rap2C is spatially segregated from Rap2A signaling as it is not enriched at the apical membrane after polarization. In contrast, both Rap2A and Rap2B are similarly located at Rab11 positive apical recycling endosomes and inside the brush border. However, although Rap2B localizes similarly it is not equally activated as Rap2A during brush border formation. We reveal that the C-terminal hypervariable region allows selective activation of Rap2A, yet this selectivity does not originate from the known differential lipid modifications of this region. In conclusion, we demonstrate that Rap2 specificity during brush border formation is determined by two distinct mechanisms involving segregated localization and selective activation.

The GDI-like solubilizing factor PDEδ sustains the spatial organization and signalling of Ras family proteins

We identify a role for the GDI-like solubilizing factor (GSF) PDEδ in modulating signalling through Ras family G proteins by sustaining their dynamic distribution in cellular membranes. We show that the GDI-like pocket of PDEδ binds and solubilizes farnesylated Ras proteins, thereby enhancing their diffusion in the cytoplasm. This mechanism allows more effective trapping of depalmitoylated Ras proteins at the Golgi and polycationic Ras proteins at the plasma membrane to counter the entropic tendency to distribute these proteins over all intracellular membranes. Thus, PDEδ activity augments K/Hras signalling by enriching Ras at the plasma membrane; conversely, PDEδ down-modulation randomizes Ras distributions to all membranes in the cell and suppresses regulated signalling through wild-type Ras and also constitutive oncogenic Ras signalling in cancer cells. Our findings link the activity of PDEδ in determining Ras protein topography to Ras-dependent signalling.

Rap2 function requires palmitoylation and recycling endosome localization

Rap2A, Rap2B, and Rap2C are Ras-like small G proteins. The role of their post-translational processing has not been investigated due to the lack of information on their downstream signaling. We have recently identified the Traf2- and Nck-interacting kinase (TNIK), a member of the STE20 group of mitogen-activated protein kinase kinase kinase kinases, as a specific Rap2 effector. Here we report that, in HEK293T cells, Rap2A (farnesylated) and Rap2C (likely farnesylated), but not Rap2B (geranylgeranylated), require palmitoylation for membrane-association and TNIK activation, whereas all Rap2 proteins, including Rap2B, require palmitoylation for induction of TNIK-mediated phenotype, the suppression of cell spreading. Furthermore, we report for the first time that, in COS-1 cells, Rap2 proteins localize, and recruit TNIK, to the recycling endosomes, but not the Golgi nor the endoplasmic reticulum, in a palmitoylation-dependent manner. These observations implicate the involvement of palmitoylation and recycling endosome localization in cellular functions of Rap2 proteins.

Protein prenylation in an insect cell-free protein synthesis system and identification of products by mass spectrometry

To evaluate the ability of an insect cell-free protein synthesis system to carry out proper protein prenylation, several CAIX (X indicates any C-terminal amino acid) sequences were introduced into the C-terminus of truncated human gelsolin (tGelsolin). Tryptic digests of these mutant proteins were analyzed by MALDI-TOF MS and MALDI-quadrupole-IT-TOF MS. The results indicated that the insect cell-free protein synthesis system possesses both farnesyltransferase (FTase) and geranylgeranyltransferase (GGTase) I, as is the case of the rabbit reticulocyte lysate system. The C-terminal amino acid sequence requirements for protein prenylation in this system showed high similarity to those observed in rat prenyltransferases. In the case of rhoC, which is a natural geranylgeranylated protein, it was found that it could serve as a substrate for both prenyltransferases in the presence of either farnesyl or geranylgeranyl pyrophosphate, whereas geranylgeranylation was only observed when both prenyl pyrophosphates were added to the in vitro translation reaction mixture. Thus, a combination of the cell-free protein synthesis system with MS is an effective strategy to analyze protein prenylation.

Biochemical characterization of RGS14: RGS14 activity towards G-protein alpha subunits is independent of its binding to Rap2A

RGS (regulators of G-protein signalling) modulate signalling by acting as GAPs (GTPase-activating proteins) for alpha subunits of heterotrimeric G-proteins. RGS14 accelerates GTP hydrolysis by G(ialpha) family members through its RGS domain and suppresses guanine nucleotide dissociation from G(ialpha1) and G(ialpha3) subunits through its C-terminal GoLoco domain. Additionally, RGS14 binds the activated forms of the small GTPases Rap1 and Rap2 by virtue of tandem RBDs (Raf-like Ras/Rap binding domains). RGS14 was identified in a screen for Rap2 effectors [Traver, Splingard, Gaudriault and De Gunzburg (2004) Biochem. J. 379, 627-632]. In the present study, we tested whether Rap binding regulates RGS14's biochemical activities. We found that RGS14 activity towards heterotrimeric G-proteins, as either a GAP or a GDI (guanine nucleotide dissociation inhibitor), was unaffected by Rap binding. Extending our biochemical characterization of RGS14, we also examined whether RGS14 can suppress guanine nucleotide exchange on G(ialpha1) in the context of the heterotrimer. We found that a heterotrimer composed of N-myristoylated G(ialpha1) and prenylated G(betagamma) is resistant to the GDI activity of the GoLoco domain of RGS14. This is consistent with models of GoLoco domain action on free G(alpha) and suggests that RGS14 alone cannot induce subunit dissociation to promote receptor-independent activation of G(betagamma)-mediated signalling pathways.

Farnesylation or geranylgeranylation? Efficient assays for testing protein prenylation in vitro and in vivo

BACKGROUND

Available in vitro and in vivo methods for verifying protein substrates for posttranslational modifications via farnesylation or geranylgeranylation (for example, autoradiography with 3H-labeled anchor precursors) are time consuming (weeks/months), laborious and suffer from low sensitivity.

RESULTS

We describe a new technique for detecting prenyl anchors in N-terminally glutathione S-transferase (GST)-labeled constructs of target proteins expressed in vitro in rabbit reticulocyte lysate and incubated with 3H-labeled anchor precursors. Alternatively, hemagglutinin (HA)-labeled constructs expressed in vivo (in cell culture) can be used. For registration of the radioactive marker, we propose to use a thin layer chromatography (TLC) analyzer. As a control, the protein yield is tested by Western blotting with anti-GST- (or anti-HA-) antibodies on the same membrane that has been previously used for TLC-scanning. These protocols have been tested with Rap2A, v-Ki-Ras2 and RhoA (variant RhoA63L) including the necessary controls. We show directly that RasD2 is a farnesylation target.

CONCLUSION

Savings in time for experimentation and the higher sensitivity for detecting 3H-labeled lipid anchors recommend the TLC-scanning method with purified GST- (or HA-) tagged target proteins as the method of choice for analyzing their prenylation capabilities in vitro and in vivo and, possibly, also for studying the myristoyl and palmitoyl posttranslational modifications.

Thematic review series: lipid posttranslational modifications. Structural biology of protein farnesyltransferase and geranylgeranyltransferase type I

More than 100 proteins necessary for eukaryotic cell growth, differentiation, and morphology require posttranslational modification by the covalent attachment of an isoprenoid lipid (prenylation). Prenylated proteins include members of the Ras, Rab, and Rho families, lamins, CENPE and CENPF, and the gamma subunit of many small heterotrimeric G proteins. This modification is catalyzed by the protein prenyltransferases: protein farnesyltransferase (FTase), protein geranylgeranyltransferase type I (GGTase-I), and GGTase-II (or RabGGTase). In this review, we examine the structural biology of FTase and GGTase-I (the CaaX prenyltransferases) to establish a framework for understanding the molecular basis of substrate specificity and mechanism. These enzymes have been identified in a number of species, including mammals, fungi, plants, and protists. Prenyltransferase structures include complexes that represent the major steps along the reaction path, as well as a number of complexes with clinically relevant inhibitors. Such complexes may assist in the design of inhibitors that could lead to treatments for cancer, viral infection, and a number of deadly parasitic diseases.

The small GTP-binding protein, Rhes, regulates signal transduction from G protein-coupled receptors

The Ras homolog enriched in striatum, Rhes, is the product of a thyroid hormone-regulated gene during brain development. Rhes and the dexamethasone-induced Dexras1 define a novel distinct subfamily of proteins within the Ras family, characterized by an extended variable domain in the carboxyl terminal region. We have carried this study because there is a complete lack of knowledge on Rhes signaling. We show that in PC12 cells, Rhes is targeted to the plasma membrane by farnesylation. We demonstrate that about 30% of the native Rhes protein is bound to GTP and this proportion is unaltered by typical Ras family nucleotide exchange factors. However, Rhes is not transforming in murine fibroblasts. We have also examined the role of Rhes in cell signaling. Rhes does not stimulate the ERK pathway. By contrast, it binds to and activates PI3K. On the other hand, we demonstrate that Rhes impairs the activation of the cAMP/PKA pathway by thyroid-stimulating hormone, and by an activated beta2 adrenergic receptor by a mechanism that suggests uncoupling of the receptor to its cognate heterotrimeric complex. Overall, our results provide the initial insights into the role in signal transduction of this novel Ras family member.

Farnesyl transferase inhibitors as anticancer agents

Protein farnesylation catalysed by the enzyme farnesyl protein transferase involves the addition of a 15-carbon farnesyl group to conserved amino acid residues at the carboxyl terminus of certain proteins. Protein substrates of farnesyl transferase include several G-proteins, which are critical intermediates of cell signalling and cytoskeletal organisation such as Ras, Rho, PxF and lamins A and B. Activated Ras proteins trigger a cascade of phosphorylation events through sequential activation of the PI3 kinase/AKT pathway, which is critical for cell survival, and the Raf/Mek/Erk kinase pathway that has been implicated in cell proliferation. Ras mutations which encode for constitutively activated proteins are found in 30% of human cancers. Because farnesylation of Ras is required for its transforming and proliferative activity, the farnesyl protein transferase inhibitors were designed as anticancer agents to abrogate Ras function. However, current evidence suggests that the anticancer activity of the farnesyl transferase inhibitors may not be simply due to Ras inhibition. This review will discuss available clinical data on three of these agents that are currently undergoing clinical trials.

GTP-binding proteins and regulated exocytosis

Regulated exocytosis, which occurs in response to stimuli, is a two-step process involving the docking of secretory granules (SGs) at specific sites on the plasma membrane (PM), with subsequent fusion and release of granule contents. This process plays a crucial role in a number of tissues, including exocrine glands, chromaffin cells, platelets, and mast cells. Over the years, our understanding of the proteins involved in vesicular trafficking has increased dramatically. Evidence from genetic, biochemical, immunological, and functional assays supports a role for ras-like monomeric GTP-binding proteins (smgs) as well as heterotrimeric GTP-binding protein (G-protein) subunits in various steps of the vesicular trafficking pathway, including the transport of secretory vesicles to the PM. Data suggest that the function of GTP-binding proteins is likely related to their localization to specific cellular compartments. The presence of both G-proteins and smgs on secretory vesicles/granules implicates a role for these proteins in the final stages of exocytosis. Molecular mechanisms of exocytosis have been postulated, with the identification of a number of proteins that modify, regulate, and interact with GTP-binding proteins, and with the advent of approaches that assess the functional importance of GTP-binding proteins in downstream, exocytotic events. Further, insight into vesicle targeting and fusion has come from the characterization of a SNAP receptor (SNARE) complex composed of vesicle, PM, and soluble membrane trafficking components, and identification of a functional linkage between GTP-binding and SNARES.

Farnesylcysteine analogues inhibit store-regulated Ca2+ entry in human platelets: evidence for involvement of small GTP-binding proteins and actin cytoskeleton

We have investigated the mechanism of Ca(2+) entry into fura-2-loaded human platelets by preventing the prenylation of proteins such as small GTP-binding proteins. The farnesylcysteine analogues farnesylthioacetic acid (FTA) and N-acetyl-S-geranylgeranyl-L-cysteine (AGGC), which are inhibitors of the methylation of prenylated and geranylgeranylated proteins respectively, significantly decreased thrombin-evoked increases in intracellular free Ca(2+) concentration ([Ca(2+)](i)) in the presence, but not in the absence, of external Ca(2+), suggesting a relatively selective inhibition of Ca(2+) entry over internal release. Both these compounds and N-acetyl-S-farnesyl-L-cysteine, which had similar effects to those of FTA, also decreased Ca(2+) entry evoked by the depletion of intracellular Ca(2+) stores with thapsigargin. The inactive control N-acetyl-S-geranyl-L-cysteine was without effect. Patulin, an inhibitor of prenylation that is inert with respect to methyltransferases, also decreased store-regulated Ca(2+) entry. Cytochalasin D, an inhibitor of actin polymerization, significantly decreased store-regulated Ca(2+) entry in a time-dependent manner. Both cytochalasin D and the farnesylcysteine analogues FTA and AGGC inhibited actin polymerization; however, when evoking the same extent of decrease in actin filament formation, FTA and AGGC showed greater inhibitory effects on Ca(2+) entry, indicating a cytoskeleton-independent component in the regulation of Ca(2+) entry by small GTP-binding-protein. These findings suggest that prenylated proteins such as small GTP-binding proteins are involved in store-regulated Ca(2+) entry through actin cytoskeleton-dependent and cytoskeleton-independent mechanisms in human platelets.

The p21-Ras signal transduction pathway and growth regulation in human high-grade gliomas

Deregulated p21-Ras function, as a result of mutation, overexpression or growth factor-induced overactivation, contributes to at least 30% of human cancer. This article reviews the potential role of the p21-Ras family of GTPases in the regulation of growth of high-grade gliomas and describes how targeting this oncoprotein clinically may provide a novel strategy to counteract glioma proliferation. The application of strategies directed at selectively opposing the deregulated signal transduction pathway of high-grade gliomas may be of potential therapeutic benefit and may offer a whole new arsenal of antineoplastic agents to be included in the multimodal treatment of these challenging neoplasms.

Isoprenylation of polypeptides in the nematode Caenorhabditis elegans

Covalent modification of eucaryotic proteins, involving addition of isoprenyl groups, is a widespread phenomenon. Here we provide direct evidence for this form of covalent modification in the free-living nematode, Caenorhabditis elegans. Following incubation in the presence of [3H]mevalonolactone, specific C. elegans polypeptides became labelled in both aqueous and detergent (Triton X-114)-enriched extracts. Chemical and GC-MS analysis of modifying groups, cleaved from C. elegans polypeptides, revealed that geranylgeranylation and, to a lesser extent, farnesylation of target polypeptides occurred. Immunoblot analysis provided preliminary evidence that the ras-like let-60 polypeptide was a target for isoprenylation in C. elegans.

Newly synthesized Rho A, not Ras, is isoprenylated and translocated to membranes coincident with progression of the G1 to S phase of growth-stimulated rat FRTL-5 cells

Ras and Rho are involved in the regulation of signal transduction events governing cell growth and cell proliferation. Protein prenylation is essential for the activation and/or the translocation of these small GTPases; however, protein geranylgeranylation rather than farnesylation is required for G1/S transition. We studied prenylation and translocation of Ras and Rho A during G1/S progression in growth-stimulated rat thyroid FRTL-5 cells. Immunoblot analysis revealed that both Ras and Rho A were detected in membrane fractions at G0. Rho A was eliminated from the membrane fraction during G1 and was not detected on the membrane at mid-G1. Translocation of Rho A from the cytoplasm back to the membranes was observed during late G1 phase. In contrast, Ras remains in the membrane fraction through the cell cycle progression from G1 to S phase. The immunoprecipitation of Rho A from the membrane fraction demonstrated that newly synthesized Rho A, labeled by pulsing cells with [35S]methionine and [35S]cysteine, was geranylgeranylated and associated with the membrane in late G1. These results indicate that Rho A, not Ras, was eliminated from membrane fraction during G1 progression and that newly synthesized Rho A is geranylgeranylated and translocated to membranes during G1/S progression in growth-stimulated FRTL-5 cells.

Signal transduction pathways and their relevance in human astrocytomas

Aberrations in a number of signal transduction pathways have been identified as playing a key role in the molecular pathogenesis of astrocytomas and their progression to high grade glioblastoma multiforme (GBM). GBMs are characterized by overexpression of the Platelet Derived Growth Factor Receptors (PDGFR) and their ligands (PDGF), as well as the Epidermal Growth Factor Receptor (EGF-R). These receptors activate the Ras pathway, a key cellular signal transduction pathway, culminating in the activation of a wide range of Ras-dependent cellular events. GBMs have also been found to either overexpression or lose expression of various Protein Kinase C (PKC) isoforms. Major strides are being made in developing pharmacological agents which specifically inhibit these growth factor receptors and intracellular signal transduction pathways. Elucidating the role of these pathways in GBMs is thus of major clinical importance, as these novel molecularly-targeted agents may prove of use in the clinical management of GBMs in the future.

The Ras-related protein Rheb is farnesylated and antagonizes Ras signaling and transformation

Presently, nothing is known about the function of the Ras-related protein Rheb. Since Rheb shares significant sequence identity with the core effector domains of Ras and KRev-1/Rap1A, it may share functional similarities with these two structurally related, yet functionally distinct, small GTPases. Furthermore, since like Ras, Rheb terminates with a COOH terminus that is likely to signal for farnesylation, it may be a target for the farnesyltransferase inhibitors that block Ras processing and function. To compare Rheb function with those of Ras and KRev-1, we introduced mutations into Rheb that generate constitutively active or dominant negative forms of Ras and Ras-related proteins and were designated Rheb(64L) and Rheb(20N), respectively. Expression of wild type or mutant Rheb did not alter the morphology or growth properties of NIH 3T3 cells. Thus, aberrant Rheb function is distinct from that of Ras and fails to cause cellular transformation. Instead, similar to KRev-1, co-expression of Rheb antagonized oncogenic Ras transformation and signaling. In vitro and in vivo analyses showed that like Ras, Rheb proteins are farnesylated and are sensitive to farnesyltransferase inhibition. Thus, it is possible that Rheb function may be inhibited by farnesyltransferase inhibitors treatment and, consequently, may contribute to the ability of these inhibitors to impair Ras transformation.

B cell antigen receptor signaling induces the formation of complexes containing the Crk adapter proteins

Crk proteins are Src homology (SH) 2/SH3-containing adapter proteins that can mediate the formation of signaling complexes. We show that engaging the B cell antigen receptor (BCR) on the RAMOS B cell line caused both Crk-L and Crk II to associate with several tyrosine-phosphorylated proteins. We identified two of these phosphoproteins as Cas and Cbl and showed that both bound to the Crk SH2 domain after BCR engagement. BCR ligation also increased the amount of Crk proteins in the particulate fraction of the cells and induced the formation of Crk.Cas and Crk.Cbl complexes in the particulate fraction. We propose that tyrosine phosphorylation of membrane-associated Cas and Cbl creates binding sites for the Crk SH2 domain and recruits Crk complexes to cellular membranes. Thus, Crk proteins may participate in BCR signaling by using their SH2 domains to direct the interactions and subcellular localization of proteins that bind to their SH3 domains. In RAMOS cells, we found that the SH3 domains of Crk-L and Crk II bound C3G. Since C3G activates Rap, a negative regulator of the Ras pathway, Crk proteins may participate in regulation of Ras signaling by the BCR.

Glycoprotein IIb-IIIa and the translocation of Rap2B to the platelet cytoskeleton

The stimulation of human platelets with physiological agonists results in the incorporation of several proteins into the cytoskeleton, fibrinogen binding, and platelet aggregation. We recently demonstrated that the Ras-related low molecular weight GTP-binding protein Rap2B associates with the cytoskeleton in activated platelets and that this interaction requires platelet aggregation. In the present study we demonstrate that agonist-induced actin polymerization is necessary for the translocation of Rap2B to the cytoskeleton, suggesting that Rap2B interacts with the newly formed actin filaments. Moreover, the association of Rap2B with Triton X-100-insoluble material from platelets was totally blocked by treatment of intact platelets with monoclonal antibodies against the fibrinogen receptor glycoprotein IIb-IIIa. Platelets from patients affected by Glanzmann thrombastenia, a genetic disorder in which platelet plasma membranes lack glycoprotein IIb-IIIa but possess normal levels of Ras-related proteins, failed to incorporate Rap2B into the cytoskeleton upon activation by thrombin. Comparative immunoblotting revealed that the translocation of Rap2B to the cytoskeleton during platelet aggregation was accompanied by the simultaneous translocation of glycoprotein IIb-IIIa. Moreover, the cytoskeleton from aggregated platelets contained Rap2B and glycoprotein IIb-IIIa in comparable amounts. These results demonstrate the association of Rap2B and glycoprotein IIb-IIIa and their translocation to the cytoskeleton in aggregated human platelets.

Association of the low molecular weight GTP-binding protein rap2B with the cytoskeleton during platelet aggregation

The intracellular distribution of the low-molecular-weight GTP-binding protein rap2B was investigated in resting and agonist-activated human platelets. In both cases, platelets were lysed by Triton X-100, and cell fractions were obtained by differential centrifugations. Using a specific polyclonal antiserum, we found that rap2B in resting platelets was completely detergent-soluble. When platelets were aggregated with thrombin, the thromboxane analogue U46619, or the Ca(2+)-ATPase inhibitor thapsigargin, a significant amount of rap2B became associated with the cytoskeleton. This association was paralleled by a decrease of rap2B in the Triton X-100-soluble fraction. Translocation of rap2B to the cytoskeleton strictly depended on platelet aggregation, and maximal incorporation was found when approximately 50% aggregation was measured. Inhibition of fibrinogen binding to the glycoprotein IIb-IIIa complex completely prevented the interaction of rap2B with the cytoskeleton. These results clearly demonstrate that changes in the intracellular localization of rap2B occur during platelet activation and represent evidence that this low molecular weight GTP-binding protein may be involved in platelet function.