Modulation of guanine nucleotides bound to Ras in NIH3T3 cells by oncogenes, growth factors, and the GTPase activating protein (GAP)

Nucleotide Exchange on RAS Proteins Using Hydrolysable and Non-hydrolysable Nucleotides

Biochemical and biophysical assays using recombinant RAS require the protein to be in either the active or inactive state. Here we describe methods to exchange the nucleotide present in the purified RAS protein with either GDPβS, GppNHp, or GTP depending on the assay requirement. In addition, we also describe the HPLC method used to validate the exchange process and provide information on the efficiency of the nucleotide exchange.

K15 promoter-driven enforced expression of NKIRAS exhibits tumor suppressive activity against the development of DMBA/TPA-induced skin tumors

NKIRAS1 and NKIRAS2 (also called as κB-Ras) were identified as members of the atypical RAS family that suppress the transcription factor NF-κB. However, their function in carcinogenesis is still controversial. To clarify how NKIRAS acts on cellular transformation, we generated transgenic mice in which NKIRAS2 was forcibly expressed using a cytokeratin 15 (K15) promoter, which is mainly activated in follicle bulge cells. The ectopic expression of NKIRAS2 was mainly detected in follicle bulges of transgenic mice with NKIRAS2 but not in wild type mice. K15 promoter-driven expression of NKIRAS2 failed to affect the development of epidermis, which was evaluated using the expression of K10, K14, K15 and filaggrin. However, K15 promoter-driven expression of NKIRAS2 effectively suppressed the development of skin tumors induced by treatment with 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol 13-acetate (TPA). This observation suggested that NKIRAS seemed to function as a tumor suppressor in follicle bulges. However, in the case of oncogenic HRAS-driven cellular transformation of murine fibroblasts, knockdown of NKIRAS2 expression drastically suppressed HRAS-mutant-provoked cellular transformation, suggesting that NKIRAS2 was required for the cellular transformation of murine fibroblasts. Furthermore, moderate enforced expression of NKIRAS2 augmented oncogenic HRAS-provoked cellular transformation, whereas an excess NKIRAS2 expression converted its functional role into a tumor suppressive phenotype, suggesting that NKIRAS seemed to exhibit a biphasic bell-shaped enhancing effect on HRAS-mutant-provoked oncogenic activity. Taken together, the functional role of NKIRAS in carcinogenesis is most likely determined by not only cellular context but also its expression level.

Active GTPase Pulldown Protocol

Ras and its related small GTPases are important signalling nodes that regulate a wide variety of cellular functions. The active form of these proteins exists in a transient GTP bound state that mediates downstream signalling events. The dysregulation of these GTPases has been associated with the progression of multiple diseases, most prominently cancer and developmental syndromes known as Rasopathies. Determining the activation state of Ras and its relatives has hence been of paramount importance for the investigation of the biochemical functions of small GTPases in the cellular signal transduction network. This chapter describes the most broadly employed approach for the rapid, label-free qualitative and semi-quantitative determination of the Ras GTPase activation state, which can readily be adapted to the analysis of other related GTPases. The method relies on the affinity-based isolation of the active GTP-bound fraction of Ras in cellular extracts, followed by its visualization via western blotting. Specifically, we describe the production of the recombinant affinity probes or baits that bind to the respective active GTPases and the pulldown method for isolating the active GTPase fraction from adherent or non-adherent cells. This method allows for the reproducible measurement of active Ras or Ras family GTPases in a wide variety of cellular contexts.

Dysregulation of Rho GTPases in orofacial cleft patients-derived primary cells leads to impaired cell migration, a potential cause of cleft/lip palate development

Cleft lip and/or palate are a split in the lip, the palate or both. This results from the inability of lip buds and palatal shelves to properly migrate and assemble during embryogenesis. By extracting primary cells from a cleft patient, we aimed at offering a better understanding of the signaling mechanisms and interacting molecules involved in the lip and palate formation and fusion. With Rho GTPases being indirectly associated with cleft occurrence, we investigated the role of the latter in both. First, whole exome sequencing was conducted in a patient with cleft lip and palate. Primary fibroblastic cells originating from the upper right gingiva region were extracted and distinct cellular populations from two individuals were obtained: a control with no cleft phenotype and a patient with a cleft lip and palate. The genetic data showed three candidate variables in ARHGEF18, EPDR1, and CUL7. Next, the molecular data showed no significant change in proliferation rates between healthy patient cells and CL/P patient cells. However, CL/P patient cells showed decreased migration, increased adhesion and presented with a more elongated phenotype. Additionally, RhoA activity was upregulated in these cells, whereas Cdc42 activity was downregulated, resulting in loss of polarity. Our results are suggestive of a possible correlation between a dysregulation of Rho GTPases and the observed phenotype of cleft lip and palate patient cells. This insight into the intramolecular aspect of this disorder helps link the genetic defect with the observed phenotype and offers a possible mechanism by which CL/P occurs.

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

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

Ras history: The saga continues

Although the roots of Ras sprouted from the rich history of retrovirus research, it was the discovery of mutationally activated RAS genes in human cancer in 1982 that stimulated an intensive research effort to understand Ras protein structure, biochemistry and biology. While the ultimate goal has been developing anti-Ras drugs for cancer treatment, discoveries from Ras have laid the foundation for three broad areas of science. First, they focused studies on the origins of cancer to the molecular level, with the subsequent discovery of genes mutated in cancer that now number in the thousands. Second, elucidation of the biochemical mechanisms by which Ras facilitates signal transduction established many of our fundamental concepts of how a normal cell orchestrates responses to extracellular cues. Third, Ras proteins are also founding members of a large superfamily of small GTPases that regulate all key cellular processes and established the versatile role of small GTP-binding proteins in biology. We highlight some of the key findings of the last 28 years.

Endothelial signaling and the molecular basis of arteriovenous malformation

Arteriovenous malformations occur when abnormalities of vascular patterning result in the flow of blood from arteries to veins without an intervening capillary bed. Recent work has revealed the importance of the Notch and TGF-β signaling pathways in vascular patterning. Specifically, Notch signaling has an increasingly apparent role in arterial specification and suppression of branching, whereas TGF-β is implicated in vascular smooth muscle development and remodeling under angiogenic stimuli. These physiologic roles, consequently, have implicated both pathways in the pathogenesis of arteriovenous malformation. In this review, we summarize the studies of endothelial signaling that contribute to arteriovenous malformation and the roles of genes implicated in their pathogenesis. We further discuss how endothelial signaling may contribute to vascular smooth muscle development and how knowledge of signaling pathways may provide us targets for medical therapy in these vascular lesions.

Probing the GTPase cycle with real-time NMR: GAP and GEF activities in cell extracts

The Ras superfamily of small GTPases is a large family of switch-like proteins that control diverse cellular functions, and their deregulation is associated with multiple disease processes. When bound to GTP they adopt a conformation that interacts with effector proteins, whereas the GDP-bound state is generally biologically inactive. GTPase activating proteins (GAPs) promote hydrolysis of GTP, thus impeding the biological activity of GTPases, whereas guanine nucleotide exchange factors (GEFs) promote exchange of GDP for GTP and activate GTPase proteins. A number of methods have been developed to assay GTPase nucleotide hydrolysis and exchange, as well as the activity of GAPs and GEFs. The kinetics of these reactions are often studied with purified proteins and fluorescent nucleotide analogs, which have been shown to non-specifically impact hydrolysis and exchange. Most GAPs and GEFs are large multidomain proteins subject to complex regulation that is challenging to reconstitute in vitro. In cells, the activities of full-length GAPs or GEFs are typically assayed indirectly on the basis of nucleotide loading of the cognate GTPase, or by exploiting their interaction with effector proteins. Here, we describe a recently developed real-time NMR method to assay kinetics of nucleotide exchange and hydrolysis reactions by direct monitoring of nucleotide-dependent structural changes in an isotopically labeled GTPase. The unambiguous readout of this method makes it possible to precisely measure GAP and GEF activities from extracts of mammalian cells, enabling studies of their catalytic and regulatory mechanisms. We present examples of NMR-based assays of full-length GAPs and GEFs overexpressed in mammalian cells.

Mathematical investigation of how oncogenic ras mutants promote ras signaling

We have used a mathematical model of the Ras signaling network to link observable biochemical properties with cellular levels of RasGTP. Although there is abundant data characterizing Ras biochemistry, attributing specific changes in biochemical properties to observed phenotypes has been hindered by the scope and complexity of Ras regulation. A mathematical model of the Ras signaling module, therefore, appeared to be of value for this problem. The model described the core architecture shared by pathways that signal through Ras. Mass-action kinetics and ordinary differential equations were used to describe network reactions. Needed parameters were largely available in the published literature and resulted in a model with good agreement to experimental data. Computational analysis of the model resulted in several unanticipated predictions and suggested experiments that subsequently validated some of these predictions.

Dynamics of the Rho-family small GTPases in actin regulation and motility

The p21 Rho-family of small GTPases are master regulators of actin cytoskeleton rearrangements. Their functions have been well characterized in terms of their effects toward various actin-modulating protein targets. However, more recent studies have shown that the dynamics of Rho GTPase activities are highly complex and tightly regulated in order to achieve their specific subcellular localization. Furthermore, these localized effects are highly dynamic, often spanning the time-scale of seconds, making the interpretation of traditional biochemical approaches inadequate to fully decipher these rapid mechanisms in vivo. Here, we provide an overview of Rho family GTPase biology, and introduce state-of-the-art approaches to study the dynamics of these important signaling proteins that ultimately coordinate the actin cytoskeleton rearrangements during cell migration.

KappaB-Ras is a nuclear-cytoplasmic small GTPase that inhibits NF-kappaB activation through the suppression of transcriptional activation of p65/RelA

NF-κB is an important transcription factor involved in various biological responses, including inflammation, cell differentiation, and tumorigenesis. κB-Ras was identified as an IκB-interacting small GTPase and is reported to disturb cytokine-induced NF-κB activation. In this study, we established that κB-Ras is a novel type of nuclear-cytoplasmic small GTPase that mainly binds to GTP, and its localization seemed to be regulated by its GTP/GDP-binding state. Unexpectedly, the GDP-binding form of the κB-Ras mutant exhibited a more potent inhibitory effect on NF-κB activation, and this inhibitory effect seemed to be due to suppression of the transactivation of a p65/RelA NF-κB subunit. κB-Ras suppressed phosphorylation at serine 276 on the p65/RelA subunit, resulting in decreased interaction between p65/RelA and the transcriptional coactivator p300. Interestingly, the GDP-bound κB-Ras mutant exhibited higher interactive affinity with p65/RelA and inhibited the phosphorylation of p65/RelA more potently than wild-type κB-Ras. Taken together, these findings suggest that the GDP-bound form of κB-Ras in cytoplasm suppresses NF-κB activation by inhibiting its transcriptional activation.

Modeling membrane localization: case study of a Ras signaling model

Modeling a biological system requires the careful integration of experimental data. It is unclear how best to incorporate rate constants measured in three-dimensional solution for reactions that physiologically occur between reactants confined to the two-dimensional cell membrane. One method adjusts second order rate constants by a factor that is the ratio of the cytoplasmic volume to the volume of a shell which membrane bound proteins can access. The value for this factor has been estimated to be 250. We have previously used this method in our model of the Ras signaling network that made several experimentally confirmed predictions. Here, we investigate if the value of this parameter affects model based predictions. We find that many of our results are robust to the value used. Two predictions appear to be sensitive to the value of the parameter: predicted levels of WT RasGTP after transfection with WT Ras and the experimentally observed increased levels of WT RasGTP when a GTPase Accelerating Protein (GAP) insensitive Ras mutant is present. For these predictions that are sensitive to the value of the membrane localization parameter, we find that the theoretically derived value of 250 results in model predictions that most closely match experimental observations.

Many faces of Ras activation

Ras proteins were originally identified as the products of oncogenes capable of inducing cell transformation. Over the last twenty-five years they have been studied in great detail because mutant Ras proteins are associated with many types of human cancer. Wild type Ras proteins play a central role in the regulation of proliferation and differentiation of various cell types. They alternate between an active GTP-bound state and an inactive GDP-bound state. Their activation is catalysed by a specialized group of enzymes known as guanine nucleotide exchange factors (GEFs). To date, four subfamilies of GEF molecules have been identified. Although all of them are able to activate Ras, their structure, tissue expression and regulation are significantly diverse. In this review we will summarize the various mechanisms by which these exchange factors activate Ras.

Network analysis of oncogenic Ras activation in cancer

To investigate the unregulated Ras activation associated with cancer, we developed and validated a mathematical model of Ras signaling. The model-based predictions and associated experiments help explain why only one of two classes of activating Ras point mutations with in vitro transformation potential is commonly found in cancers. Model-based analysis of these mutants uncovered a systems-level process that contributes to total Ras activation in cells. This predicted behavior was supported by experimental observations. We also used the model to identify a strategy in which a drug could cause stronger inhibition on the cancerous Ras network than on the wild-type network. This system-level analysis of the oncogenic Ras network provides new insights and potential therapeutic strategies.

Quantitative model of Ras-phosphoinositide 3-kinase signalling cross-talk based on co-operative molecular assembly

In growth-factor-stimulated signal transduction, cell-surface receptors recruit PI3Ks (phosphoinositide 3-kinases) and Ras-specific GEFs (guanine nucleotide-exchange factors) to the plasma membrane, where they produce 3'-phosphorylated phosphoinositide lipids and Ras-GTP respectively. As a direct example of pathway networking, Ras-GTP also recruits and activates PI3Ks. To refine the mechanism of Ras-PI3K cross-talk and analyse its quantitative implications, we offer a theoretical model describing the assembly of complexes involving receptors, PI3K and Ras-GTP. While the model poses the possibility that a ternary receptor-PI3K-Ras complex forms in two steps, it also encompasses the possibility that receptor-PI3K and Ras-PI3K interactions are competitive. In support of this analysis, experiments with platelet-derived growth factor-stimulated fibroblasts revealed that Ras apparently enhances the affinity of PI3K for receptors; in the context of the model, this suggests that a ternary complex does indeed form, with the second step greatly enhanced through membrane localization and possibly allosteric effects. The apparent contribution of Ras to PI3K activation depends strongly on the quantities and binding affinities of the interacting molecules, which vary across different cell types and stimuli, and thus the model could be used to predict conditions under which PI3K signalling is sensitive to interventions targeting Ras.

Genomewide analysis of gene expression associated with Tcof1 in mouse neuroblastoma

Mutations in the Treacher Collins syndrome gene, TCOF1, cause a disorder of craniofacial development. We manipulated the levels of Tcof1 and its protein treacle in a murine neuroblastoma cell line to identify downstream changes in gene expression using a microarray platform. We identified a set of genes that have similar expression with Tcof1 as well as a set of genes that are negatively correlated with Tcof1 expression. We also showed that the level of Tcof1 and treacle expression is downregulated during differentiation of neuroblastoma cells into neuronal cells. Inhibition of Tcof1 expression by siRNA induced morphological changes in neuroblastoma cells that mimic differentiation. Thus, expression of Tcof1 and treacle synthesis play an important role in the proliferation of neuroblastoma cells and we have identified genes that may be important in this pathway.

Distinct mechanisms determine the patterns of differential activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by receptors for growth factors or antigen

Although GTPases of the Ras family have been implicated in many aspects of the regulation of cells, little is known about the roles of individual family members. Here, we analyzed the mechanisms of activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by two types of external stimuli, growth factors and ligation of the antigen receptors of B or T lymphocytes (BCRs and TCRs). The growth factors interleukin-3, colony-stimulating factor 1, and epidermal growth factor all preferentially activated M-Ras and K-Ras 4B over H-Ras or N-Ras. Preferential activation of M-Ras and K-Ras 4B depended on the presence of their polybasic carboxy termini, which directed them into high-buoyant-density membrane domains where the activated receptors, adapters, and mSos were also present. In contrast, ligation of the BCR or TCR resulted in activation of H-Ras, N-Ras, and K-Ras 4B, but not M-Ras. This pattern of activation was not influenced by localization of the Ras proteins to membrane domains. Activation of H-Ras, N-Ras, and K-Ras 4B instead depended on the presence of phospholipase C-gamma and RasGRP. Thus, the molecular mechanisms leading to activation of Ras proteins vary with the stimulus and can be influenced by either colocalization with activated receptors or differential sensitivity to the exchange factors activated by a stimulus.

p47 GTPases: regulators of immunity to intracellular pathogens

Activation of the innate immune system by interferon-gamma (IFN-gamma is crucial for host resistance to infection. IFN-gamma induces the expression of a wide range of mediators that undermine the ability of pathogens to survive in host cells, including a newly discovered family of 47-kDa GTPases. Elimination of different p47 GTPases in mice by gene targeting severely cripples IFN-gamma-regulated defence against Toxoplasma gondii, Listeria monocytogenes, Mycobacterium spp. and other pathogens. In this article, we review our understanding of the role of p47 GTPases in resistance to intracellular infection and discuss the present evidence concerning their mode of action.

Capillary malformation-arteriovenous malformation, a new clinical and genetic disorder caused by RASA1 mutations

Capillary malformation (CM), or "port-wine stain," is a common cutaneous vascular anomaly that initially appears as a red macular stain that darkens over years. CM also occurs in several combined vascular anomalies that exhibit hypertrophy, such as Sturge-Weber syndrome, Klippel-Trenaunay syndrome, and Parkes Weber syndrome. Occasional familial segregation of CM suggests that there is genetic susceptibility, underscored by the identification of a large locus, CMC1, on chromosome 5q. We used genetic fine mapping with polymorphic markers to reduce the size of the CMC1 locus. A positional candidate gene, RASA1, encoding p120-RasGAP, was screened for mutations in 17 families. Heterozygous inactivating RASA1 mutations were detected in six families manifesting atypical CMs that were multiple, small, round to oval in shape, and pinkish red in color. In addition to CM, either arteriovenous malformation, arteriovenous fistula, or Parkes Weber syndrome was documented in all the families with a mutation. We named this newly identified association caused by RASA1 mutations "CM-AVM," for capillary malformation-arteriovenous malformation. The phenotypic variability can be explained by the involvement of p120-RasGAP in signaling for various growth factor receptors that control proliferation, migration, and survival of several cell types, including vascular endothelial cells.

Identification of Aurora kinases as RasGAP Src homology 3 domain-binding proteins

The GTPase-activating protein RasGAP functions as both a negative regulator and an effector of Ras proteins. In tumor cells, RasGAP is no longer able to deactivate oncogenic Ras proteins, and its effector function becomes predominant. As RasGAP itself has no obvious enzymatic function that may explain this effector function, we looked for downstream RasGAP effectors that could fulfill this role. We looked for the existence of RasGAP Src homology 3 (SH3) domain partners as this domain is involved in the regulation of cell proliferation and has an anti-apoptotic effect. We report here the identification of a new RasGAP SH3 domain-binding protein, named Aurora. This Drosophila melanogaster Ser/Thr kinase has three human orthologs called Aurora/Ipl1-related kinase or HsAIRK-1, -2, and -3. Coimmunoprecipitation experiments in COS cells confirmed that HsAIRK-1 and HsAIRK-2 both interact with RasGAP. RasGAP pull-down experiments showed that it interacts with HsAIRK-1 in G(2)/M HeLa cells. We also demonstrated that RasGAP binds to the kinase domain of Aurora and that this interaction inhibits the kinase activity of HsAIRK-1 and HsAIRK-2. Finally we showed that RasGAP forms a ternary complex with HsAIRK and survivin. This complex may be involved in the regulation of the balance between cell division and apoptosis.

p60(v-src) and serum control cell shape and apoptosis via distinct pathways in quail neuroretina cells

We made use of QNR cells transformed by a thermosensitive (tsNY68) strain of the Rous sarcoma virus (RSV) to compare the effect of p60(v-src) and serum in cultured nerve cells. In this system, both p60(v-src) heat inactivation and serum removal resulted in growth arrest in G1. In both cases, growth arrest was reversible since cell proliferation was rapidly re-induced following respectively p60v-src renaturation or serum re-addition. However, cells did not fully recover their ability to grow in soft agar, suggesting that, in contrast to the cell cycle machinery, the transforming capacities of these cells have been irreversibly altered. We found that p60(v-src) kinase activity prevented detachment from the substratum and cell death following serum removal. Thermal inactivation of p60(v-src) at restrictive temperature (41.5 degrees C), but not serum removal, resulted in dramatic morphological changes, which occurred 4 h after temperature shift up to 41.5 degrees C. Later on, typical features of apoptotic cells could be observed. Cell death was greatly reduced by the caspase-3 inhibitor ZVAD.FMK, but not by the caspase-1 inhibitor Ac-YVAD.CHO. Together, these results suggested that p60(v-src) and serum factors act on distinct pathways, at least in part. In an attempt to identify the signalling pathways involved in the cell response to p60(v-src) down regulation, we found that Erk and Rac were rapidly inactivated following temperature shift up to 41.5 degrees C. Thus, the combined effects of p60(v-src) and serum factors on the cytoskeleton dynamics and the apoptosis machinery are essential for full neoplastic transformation of neuroretina cells.

Sensitivity of wild type and mutant ras alleles to Ras specific exchange factors: Identification of factor specific requirements

We have investigated the productive interaction between the four mammalian Ras proteins (H-, N-, KA- and KB-Ras) and their activators, the mammalian exchange factors mSos1, GRF1 and GRP, by using a modified Saccharomyces cerevisiae whose growth is dependent on activation of a mammalian Ras protein by its activator. All four mammalian Ras proteins were activated with similar efficiencies by the individual exchange factors. The H-Ras mutant V103E, which is competent for membrane localization, nucleotide binding, intrinsic and stimulated GTPase activity as well as intrinsic exchange, was defective for activation by all factors tested, suggesting that the integrity of this residue is necessary for catalyzed exchange. However, when other H-Ras mutants were studied, some distinct sensitivities to the exchange factors were observed. GRP-mediated, but not mSos1-mediated, exchange was blocked in additional mutants, suggesting different structural requirements for GRP. Analysis of Ras-mediated gene activation in murine fibroblasts confirmed these results.

Regulation of cell cycle molecules by the Ras effector system

Eukaryotic cell cycle progression is driven by an ordered array of phosphorylation events that are specifically catalyzed by members of CDK (cyclin-dependent kinase) family serine/threonine protein kinases, each consisting of a catalytic subunit CDK and a positive regulatory subunit cyclin. In mammalian somatic cells extracellular cues act mainly during the G1 phase to regulate the activity of D type cyclin-dependent CDKs, which, in turn, serve as key regulators of G1--S phase progression by phosphorylating and functionally inactivating the tumor suppressor retinoblastoma (Rb) protein. The small molecular weight G protein Ras has been implicated as a crucial molecule that transduces extracellular growth stimuli into intracellular signals. Recent studies, including our own, have demonstrated that maintained cellular Ras activity is required until late in the G1 phase for inactivation of the Rb protein and the G1/S transition and mediates both upregulation of cyclin D1 and downregulation of p27kip1 CDK inhibitor.

Oncogenic insertional mutations in the P-loop of Ras are overactive in MAP kinase signaling

Mutations of Ras with three extra amino acids inserted into the phosphate-binding (P) loop have been investigated both in vitro and in vivo. Such mutants have originally been detected as oncogenes both in the ras and the TC21 genes. Biochemical experiments reveal the molecular basis of their oncogenic potential: the mutants show a strongly attenuated binding affinity for nucleotides, most notably for GDP, leading to a preference for GTP binding. Furthermore, both the intrinsic as well as the GAP-stimulated GTP hydrolysis are drastically diminished. The binding interaction with GAP is reduced, whereas binding to the Ras-binding domain of the downstream effector c-Raf1 is not altered appreciably. Microinjection into PC12 cells shows the mutants to be as potent to induce neurite outgrowth as conventional oncogenic Ras mutants. Unexpectedly, their ability to stimulate the MAP kinase pathway as measured by a reporter gene assay in RK13 cells is much higher than that of the normal oncogenic mutant G12V. This characteristic was attributed to an increased stimulation of c-Raf1 kinase activity by the insertional Ras mutants.

Suppression of malignant growth potentials of v-Src-transformed human gallbladder epithelial cells by adenovirus-mediated dominant negative H-Ras

Although Src transformation of NIH3T3 mouse fibroblasts has been shown to be dependent on Ras function, the signaling mechanism whereby Src induces malignant transformation of human epithelial cells still remains unclear. In the present study, we analyzed the functional role of Ras, which acts downstream of Src in intracellular signaling, in the acquisition of fully neoplastic potentials by v-Src-transformed human gallbladder epithelial cells (HAG/src3-1) by infecting these cells with replication-defective adenovirus vector expressing dominant negative H-Ras (AdCARasY57). High efficiency of gene transduction was demonstrated with the adenovirus vector containing beta-gal gene insert (AdCALacZ). On infection with AdCARasY57, the activity of mitogen-activated protein (MAP) kinase, a major downstream event triggered by Ras, was markedly inhibited over 7 days, indicating that the inhibition of Ras function by AdCARasY57 remains active during this period. AdCARasY57 did not inhibit the monolayer growth of HAG-1 cells transfected with activated H-ras, but inhibited the HAG/src3-1 cells by 30%, as compared with cells infected with AdCALacZ as a control. This growth inhibition by AdCARasY57 was strengthened nearly twofold on surfaces coated with an antiadhesive polymer (poly 2-hydroxyethylmethacrylate) that can quantitate anchorage-independent growth, and was much more pronounced up to 95% when assayed in soft agar. The HAG/src3-1 cells transfected with beta-gal gene produced tumors in nude mice within 4 weeks after implantation, whereas cells infected with AdCARasY57 failed to form tumors during this period. These findings show that Ras function is essential for v-Src-induced anchorage-independent growth in vitro as well as tumorigenesis in vivo, and that mitogenic activity driven by v-Src is not solely dependent on MAP kinase pathway. Because anchorage-independent growth correlates with tumor growth in vivo as well as metastatic potential, targeting Ras would be potentially useful for the treatment of human tumors with elevated Src tyrosine kinase activity.

Ras-induced cellular events (review)

Ras is a crucial regulator of cell growth in eukaryotic cells. Activated Ras can stimulate signal transduction cascades, leading to cell proliferation, differentiation or apoptosis. It is also one of the most commonly mutated genes in both solid tumours and haematologic neoplasias. In leukaemia and tumours, aberrant Ras signalling can be induced directly by Ras mutation or indirectly by altering genes that associate with Ras or its signalling pathways. A requisite for Ras function is localization to the plasma membrane, which is induced by the post-translational modification farnesylation. Molecules that interfere with this Ras modification have been used as antitumour agents. Ras is emerging as a dual regulator of cell functions, playing either positive or negative roles in the control of proliferation or apoptosis. The diversity of Ras-mediated effects may be related in part to the differential involvement of Ras homologues in distinct cellular processes or to the expanding array of Ras effectors.

High Affinity IgG Receptor Activation of Src Family Kinases Is Required for Modulation of the Shc-Grb2-Sos Complex and the Downstream Activation of the Nicotinamide Adenine Dinucleotide Phosphate (Reduced) Oxidase

We used the U937 cell line to examine the modulation of adaptor protein interactions (Shc, Grb2, and Cbl) after high affinity IgG receptor (FcγRI) cross-linking, leading to the formation of the Grb2-Sos complex, the activation of Ras, and the regulation of the respiratory burst. Cross-linking of FcγRI induced the conversion of GDP-Ras to GTP-Ras reaching a maximum 5 min after stimulation. Concomitant with Ras activation, Sos underwent an electrophoretic mobility shift and the Sos-Grb2 association was increased (6-fold). The Grb2-Sos complex was present only in the membrane fraction and was augmented after FcγRI stimulation. Tyrosine-phosphorylated Shc, mainly the p52 isoform, was observed to transiently onload to the membrane Grb2-Sos complex on FcγRI stimulation. Cross-linking of FcγRI induces the tyrosine phosphorylation of Cbl, which forms a complex with Grb2 and Shc via the Cbl C terminus. Kinetic experiments confirm that Cbl-Grb2 is relatively stable, whereas Grb2-Sos, Grb2-Shc, and Cbl-Shc interactions are highly inducible. The Src family tyrosine kinase inhibitor, PP1, was shown to completely inhibit Shc tyrosine phosphorylation, the Shc-Grb2 interaction, and the FcγR-induced respiratory burst. Our results provide the first evidence that the upstream activation of Src kinases is required for the modulation of the Shc-Grb2 interaction and the myeloid NADPH oxidase response.

Focal adhesion kinase mediates the integrin signaling requirement for growth factor activation of MAP kinase

The mitogen-activated protein (MAP) kinase pathway is a critical regulator of cell growth, migration, and differentiation. Growth factor activation of MAP kinase in NIH 3T3 cells is strongly dependent upon integrin-mediated adhesion, an effect that contributes to the anchorage dependence of normal cell growth. We now show that expression of constructs that constitutively activate focal adhesion kinase (FAK) rescued the defect in serum activation of MAP kinase in suspended cells without directly activating MAP kinase. Dominant negative FAK blocked both the rescue of suspended cells by the activated construct and the serum activation of MAP kinase in adherent cells. MAP kinase in FAK(-/)- mouse embryo fibroblasts was adhesion-insensitive, and reexpression of FAK restored its adhesion dependence. MAP kinase activity in ras-transformed cells is still decreased in suspension, but expression of constructs that constitutively activate FAK enhanced their anchorage-independent growth without increasing adherent growth. V-src, which activates both Ras and FAK, induced MAP kinase activation that was insensitive to loss of adhesion, and that was blocked by a dominant negative FAK. These results demonstrate that FAK mediates the integrin requirement for serum activation of MAP kinase in normal cells, and that bypassing this mechanism contributes to anchorage-independent growth in transformed cells.

Formation of complexes involving RasGAP and p190 RhoGAP during morphogenetic events of the gastrulation in xenopus

In relation to the activation of the Src-family of tyrosine kinases during early morphogenetic events of gastrulation in Xenopus, we have identified two multiprotein complexes. The first complex, including RasGAP, p190 RhoGAP and p62, was previously characterized in murine fibroblasts overexpressing c-Src or transformed by v-Src and has been correlated with cytoskeleton remodelling. A second complex, not identified in other models includes tyrosine-phosphorylated p66SHC, Grb2, RasGAP and p190 RhoGAP. The association with p66SHC, considered as a negative regulator of ERK (extracellular signal-regulated kinase), p120RasGAP and p190RhoGAP, suggests a possible mechanism for coupling Ras and Rho signalling pathways. The interaction of RasGAP and p190 RhoGAP in two multiprotein complexes could constitute an additional level of Rho regulation during morphogenetic events of gastrulation.

Novel sensing mechanisms and targets for the cAMP-protein kinase A pathway in the yeast Saccharomyces cerevisiae

The cAMP-protein kinase A (PKA) pathway in the yeast Saccharomyces cerevisiae plays a major role in the control of metabolism, stress resistance and proliferation, in particular in connection with the available nutrient conditions. Extensive information has been obtained on the core section of the pathway, i.e. Cdc25, Ras, adenylate cyclase, PKA, and on components interacting directly with this core section, such as the Ira proteins, Cap/Srv2 and the two cAMP phosphodiesterases. Recent work has now started to reveal upstream regulatory components and downstream targets of the pathway. A G-protein-coupled receptor system (Gpr1-Gpa2) acts upstream of adenylate cyclase and is required for glucose activation of cAMP synthesis in concert with a glucose phosphorylation-dependent mechanism. Although a genuine signalling role for the Ras proteins remains unclear, they appear to mediate at least part of the potent stimulation of cAMP synthesis by intracellular acidification. Recently, several new targets of the PKA pathway have been discovered. These include the Msn2 and Msn4 transcription factors mediating part of the induction of STRE-controlled genes by a variety of stress conditions, the Rim15 protein kinase involved in stationary phase induction of a similar set of genes and the Pde1 low-affinity cAMP phosphodiesterase, which specifically controls agonist-induced cAMP signalling. A major issue that remains to be resolved is the precise connection between the cAMP-PKA pathway and other nutrient-regulated components involved in the control of growth and of phenotypic characteristics correlated with growth, such as the Sch9 and Yak1 protein kinases. Cln3 appears to play a crucial role in the connection between the availability of certain nutrients and Cdc28 kinase activity, but it remains to be clarified which nutrient-controlled pathways control Cln3 levels.

Growth factor-induced signal transduction in adherent mammalian cells is sensitive to gravity

Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in a wide variety of cells. This activation leads to increased cell proliferation in most cell types. Among the early effects evoked by EGF are receptor clustering, cell rounding, and early gene expression. The influence of gravity on EGF-induced EGF receptor clustering and gene expression as well as on actin polymerization and cell rounding have been investigated in adherent A431 epithelial cells with the use of sounding rockets to create microgravity conditions. EGF-induced c-fos and c-jun expression decreased in microgravity. This was caused by alteration of the EGF receptor and protein kinase C-mediated signal transduction pathways. In contrast, neither the binding of EGF to the receptor nor the receptor clustering were changed under microgravity conditions. Because cell morphology was also modulated under microgravity conditions, and the growth factor-induced signal transduction cascades have been demonstrated to be linked to the actin microfilament system, it is tempting to suggest that the actin microfilament system constitutes the gravity-sensitive cell component.

RAS and leukemia: from basic mechanisms to gene-directed therapy

PURPOSE AND DESIGN

The purpose of this review is to provide an overview of the literature linking Ras signaling pathways and leukemia and to discuss the biologic and potential therapeutic implications of these observations. A search of MEDLINE from 1966 to October 1998 was performed.

RESULTS

A wealth of data has been published on the role of Ras pathways in cancer. To be biologically active, Ras must move from the cytoplasm to the plasma membrane. Importantly, a posttranslational modification--addition of a farnesyl group to the Ras C-terminal cysteine--is a requisite for membrane localization of Ras. Farnesylation of Ras is catalyzed by an enzyme that is designated farnesyltransferase. Recently, several compounds have been developed that can inhibit farnesylation. Preclinical studies indicate that these molecules can suppress transformation and tumor growth in vitro and in animal models, with little toxicity to normal cells.

CONCLUSION

An increasing body of data suggests that disruption of Ras signaling pathways, either directly through mutations or indirectly through other genetic aberrations, is important in the pathogenesis of a wide variety of cancers. Molecules such as farnesyl transferase inhibitors that interfere with the function of Ras may be exploitable in leukemia (as well as in solid tumors) as novel antitumor agents.

Identification of Grb4/Nckbeta, a src homology 2 and 3 domain-containing adapter protein having similar binding and biological properties to Nck

Adapter proteins made up of Src homology (SH) domains mediate multiple cellular signaling events initiated by receptor protein tyrosine kinases. Here we report that Grb4 is an adapter protein closely related to but distinct from Nck that is made up of three SH3 domains and one SH2 domain. Northern analysis indicated that both genes are expressed in multiple tissues. Both Nck and Grb4 proteins could associate with receptor tyrosine kinases and the SH3-binding proteins PAK, Sos1, and PRK2, and they synergized with v-Abl and Sos to induce gene expression via the transcription factor Elk-1. Although neither protein was transforming on its own, both Nck and Grb4 cooperated with v-Abl to transform NIH 3T3 cells and influenced the morphology and anchorage-dependent growth of wild type Ras-transformed cells. Nck and Grb4 therefore appear to be functionally redundant.

Membrane interactions of a constitutively active GFP-Ki-Ras 4B and their role in signaling. Evidence from lateral mobility studies

Membrane anchorage of Ras proteins in the inner leaflet of the plasma membrane is an important factor in their signaling and oncogenic potential. Despite these important roles, the precise mode of Ras-membrane interactions is not yet understood. It is especially important to characterize these interactions at the surface of intact cells. To investigate Ras-membrane interactions in live cells, we employed studies on the lateral mobility of a constitutively active Ras isoform to characterize its membrane dynamics, and examined the effects of the Ras-displacing antagonist S-trans, trans-farnesylthiosalicylic acid (FTS) (Haklai, R., Gana-Weisz, M., Elad, G., Paz, A., Marciano, D., Egozi, Y., Ben-Baruch, G., and Kloog, Y. (1998) Biochemistry 37, 1306-1314) on these parameters. A green fluorescent protein (GFP) was fused to the N terminus of constitutively active Ki-Ras 4B(12V) to generate GFP-Ki-Ras(12V). When stably expressed in Rat-1 cells, this protein was preferentially localized to the plasma membrane and displayed transforming activity. The lateral mobility studies demonstrated that GFP-Ki-Ras(12V) undergoes fast lateral diffusion at the plasma membrane, rather than exchange between membrane-bound and unbound states. Treatment of the cells with FTS had a biphasic effect on GFP-Ki-Ras(12V) lateral mobility. At the initial phase, the lateral diffusion rate of GFP-Ki-Ras(12V) was elevated, suggesting that it is released from some constraints on its lateral mobility. This was followed by dislodgment of the protein into the cytoplasm, and a reduction in the diffusion rate of the fraction of GFP-Ki-Ras(12V) that remained associated with the plasma membrane. Control experiments with other S-prenyl analogs showed that these effects are specific for FTS. These results have implications for the interactions of Ki-Ras with specific membrane anchorage domains or sites.

Ras activation in astrocytomas and neurofibromas

Oncogenic mutations resulting in activated Ras Guanosine Triphosphate (GTP) are prevalent in 30% of all human cancers, but not primary nervous system tumors. Several growth factors/receptors are implicated in the pathogenesis of malignant astrocytomas including epidermal growth factor (EGFR) and platelet derived growth factor (PDGF-R) receptors, plus the highly potent and specific angiogenic vascular endothelial growth factor (VEGF). A significant proportion of these tumors also express a truncated EGFR, which is constitutively activated. Our work demonstrates that the mitogenic signals from both the normal PDGF-R and EGFR and the truncated EGFR activate Ras. Inhibition of Ras by genetic or pharmacological strategies leads to decreased astrocytoma tumorgenic growth in vitro and decreased expression of VEGF. This suggests that these agents may be potentially important as novel anti-proliferative and anti-angiogenic therapies for human malignant astrocytomas. In contrast to astrocytomas, where increased levels of activated Ras GTP results from transmitted signals from activated growth factor receptors, the loss of neurofibromin is postulated to lead to functional up-regulation of the Ras pathway in neurofibromatosis-1(NF-1). We have demonstrated that NF-1 neurofibromas and neurogenic sarcomas, compared to non-NF-1 Schwannomas, have markedly elevated levels of activated Ras GTP. Increased Ras GTP was associated with increased tumor vascularity in the NF-1 neurogenic sarcomas, perhaps related to increased VEGF secretion. The role of Ras inhibitors as potential therapy in this tumor is also under study.

Molecular markers of IGF-I-mediated mitogenesis

The aim of these investigations was to identify a number of molecular markers that correlate to growth stimulation by IGF-I. For this purpose, we have selected four cell lines that respond equally well to growth stimulation by serum, but differ in their proliferative response to IGF-I. Two cell lines (R503 and R600 cells) respond to IGF-I with both DNA synthesis and cell division, a third cell line (R508 cells) can enter S phase after IGF-I, but the cells do not divide, and a fourth one (R12 cells) totally fails to respond to IGF-I with growth. Using these cell lines, all of which had an intact mitogenic response program to serum, we show that: (1) an increase in GTP/GDP ratio is an early event that distinguishes cells capable of entering S phase after IGF-I from cells that do not; (2) all cells that are induced to synthesize DNA by IGF-I have increased phosphorylation of MAP kinases, regardless of their ability to divide; (3) the same cell lines display a similar increase in cyclin A and B expression at early times after stimulation; and (4) cyclin levels and cyclin B-associated cdc2 kinase activity remain elevated at later times only in cells that undergo cell division. These results establish certain parameters of IGF-I-mediated mitogenesis and clearly separate the occurrence of DNA synthesis from cell division in certain situations.

Comparison of the signal transduction pathways activated by gastrin in enterochromaffin-like and parietal cells

BACKGROUND & AIMS

Gastrin stimulates acid secretion from parietal cells and histamine release from enterochromaffin-like (ECL) cells through identical gastrin receptors. However, gastrin has been shown to have a trophic effect only on ECL cells. The aim of this study was to compare gastrin-induced signal transduction pathways in the ECL and parietal cells of Mastomys natalensis, an African rodent.

METHODS

Both ECL and parietal cells were isolated from the gastric mucosa of M. natalensis, and intracellular signal transduction events in response to gastrin were investigated.

RESULTS

Gastrin elicited histamine release from ECL cells and acid secretion from parietal cells in association with enhanced inositol phospholipid turnover. Although gastrin increased [3H]thymidine incorporation into ECL cells, it had no effect on parietal cells. Moreover, tyrosine phosphorylation and activation of mitogen-activated protein (MAP) kinase as well as c-fos and c-jun gene expression were augmented only in ECL cells. In addition, gastrin increased the formation of guanosine triphosphate-Ras with a simultaneous decrease in guanosine diphosphate-Ras levels in ECL but not in parietal cells.

CONCLUSIONS

Although gastrin receptors are present in both ECL and parietal cells, they activate the Ras-MAP kinase pathway only in ECL cells.

Molecular basis for tissue expansion: clinical implications for the surgeon

A wide variety of tissue expansion techniques have been used for breast reconstruction, craniofacial surgery, and burn care in plastic reconstructive surgery. However, the basic mechanism by which skin and surrounding tissue respond to mechanical expansion remains unclear. Recent studies have revealed the biomechanical aspects of cells subjected to strain and various factors involved in the stretch-induced signal transduction pathway. In this regard, we have reported previously that mechanical force increases keratinocyte growth and protein synthesis and alters cell morphology. The mechanism by which strain causes an enhancement of cellular growth appears to be a network of several integrated cascades, implicating growth factors, cytoskeleton, and the protein kinase family. Recently, additional evidence has accumulated that mechanical strain stimulates signal transduction pathways that could trigger a series of cascades eventually leading to a new skin production. For example, we have evidence suggesting a key role for protein kinase C (PKC) in mechanosignaling as PKC is activated and translocated in keratinocytes subjected to strain in an isoform-specific manner. In this report, molecular mechanisms leading to enhancement of skin surface area are reviewed, and possible future applications are discussed.

Insulin receptor-mediated dissociation of Grb2 from Sos involves phosphorylation of Sos by kinase(s) other than extracellular signal-regulated kinase

The Ras signaling pathway is rapidly activated and then down-regulated following stimulation of multiple cell-surface receptors including the insulin receptor (IR). Much recent attention has focused on elucidating the mechanism of Ras inactivation following IR engagement. Previous data suggest that IR-mediated serine/threonine phosphorylation of the Ras guanine nucleotide exchange factor Sos correlates with its decreased affinity for the adapter protein Grb2. This phosphorylation-induced disassembly of the Grb2.Sos complex is thought to be responsible, at least in part, for diminishing Ras activity in Chinese hamster ovary cells. In this report, we confirm the causal relationship between Sos phosphorylation and Grb2/Sos dissociation. We then examine several putative phosphorylation sites of Sos that could potentially regulate this event. Since a number of reports suggest that extracellular signal-regulated kinase (ERK) phosphorylates Sos, we generated a Sos mutant lacking all seven canonical phosphorylation sites for ERK. This mutant is a poor substrate of activated ERK in vitro and fails to undergo a change in its electrophoretic mobility following IR stimulation. It is, however, phosphorylated after IR stimulation when expressed in Chinese hamster ovary cells. Interestingly, the mutant protein still dissociates from Grb2 following insulin stimulation, suggesting that ERK is not the kinase responsible for regulating the stability of the Grb2.Sos complex.

What does insulin do to Ras?

The Ras pathway lies in the center of signalling cascades of numerous growth-promoting factors. The Ras pathway appears to connect signalling events that begin at the plasma membrane with nuclear events. Insulin is one of the major stimulants of the Ras signalling pathway. The influence of insulin on this pathway consists of five important events: (1) p21Ras activation is promoted by insulin stimulation of the guanine nucleotide exchange factor, Sos, resulting in increased GTP-loading of p21Ras; (2) p21Ras deactivation involves the hyperphosphorylation of Sos; (3) insulin increases farnesyltransferase (FTase) activity that farnesylates p21Ras; (4) increased amounts of farnesylated p21Ras translocate to the plasma membrane where they can be activated by other growth-promoting agents; and (5) cellular responses to other growth factors are potentiated by insulin-stimulated pre-loading of the plasma membrane with farnesylated p21Ras.

Alloantigen-induced unresponsiveness in cord blood T lymphocytes is associated with defective activation of Ras

Human umbilical cord blood T lymphocytes (CBTL) respond to primary allostimulation but they do not proliferate upon rechallenge with alloantigen. Using PKH-26-labeled cells created a proliferative block that was observed only in CBTL that have divided during primary stimulation (PKH-26(dim)) but not in unstimulated (PKH-26(bright)) CBTL. CBTL's secondary unresponsiveness resembles anergy and can be overcome by treatment with phorbol myristate acetate (PMA) and ionomycin or by high doses (50-100 units/ml) of interleukin 2. Addition of interleukin 2 to the primary cultures does not prevent the induction of secondary unresponsiveness. Defective Ras activation is detected in PKH-26(dim) CBTL during secondary response to alloantigen or after antibody-mediated T cell receptor stimulation whereas Ras is activated and proliferation is induced in CBTL during primary alloantigenic stimulation. Upon stimulation with PMA plus ionomycin, PMA plus alloantigen, but not alloantigen plus ionomycin, Ras is activated in PKH-26(dim) CBTL, and the block in proliferation is overcome. Correction of PKH-26(dim) CBTL's proliferative defect correlates with PMA-induced Ras activation, suggesting a defect in the signaling pathway leading to Ras. Ras-independent signals, necessary but not sufficient to induce PKH-26(dim) CBTL proliferation, are provided by alloantigen exposure, as evident by the ability of PMA plus alloantigen but not PMA alone to overcome the proliferative block. Functional signal transduction through CD28 in PKH-26(dim) CBTL is supported by detectable CD28-mediated PI-3 kinase activation after PKH-26(dim) CBTL's exposure to alloantigen or CD28 cross-linking. These results suggest that defective activation of Ras plays a key role in PKH-26(dim) CBTL's secondary unresponsiveness and point to a defect along the T cell receptor rather than the CD28 signaling pathway.

Asexual sporulation in Aspergillus nidulans

The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidiophore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

The putative "switch 2" domain of the Ras-related GTPase, Rab1B, plays an essential role in the interaction with Rab escort protein

Posttranslational modification of Rab proteins by geranylgeranyltransferase type II requires that they first bind to Rab escort protein (REP). Following prenylation, REP is postulated to accompany the modified GTPase to its specific target membrane. REP binds preferentially to Rab proteins that are in the GDP state, but the specific structural domains involved in this interaction have not been defined. In p21 Ras, the alpha2 helix of the Switch 2 domain undergoes a major conformational change upon GTP hydrolysis. Therefore, we hypothesized that the corresponding region in Rab1B might play a key role in the interaction with REP. Introduction of amino acid substitutions (I73N, Y78D, and A81D) into the putative alpha2 helix of Myc-tagged Rab1B prevented prenylation of the recombinant protein in cell-free assays, whereas mutations in the alpha3 and alpha4 helices did not. Additionally, upon transient expression in transfected HEK-293 cells, the Myc-Rab1B alpha2 helix mutants were not efficiently prenylated as determined by incorporation of [3H]mevalonate. Metabolic labeling studies using [32P]orthophosphate indicated that the poor prenylation of the Rab1B alpha2 helix mutants was not directly correlated with major disruptions in guanine nucleotide binding or intrinsic GTPase activity. Finally, gel filtration analysis of cytosolic fractions from 293 cells that were coexpressing T7 epitope-tagged REP with various Myc-Rab1B constructs revealed that mutations in the alpha2 helix of Rab1B prevented the association of nascent (i.e., nonprenylated) Rab1B with REP. These data indicate that the Switch 2 domain of Rab1B is a key structural determinant for REP interaction and that nucleotide-dependent conformational changes in this region are largely responsible for the selective interaction of REP with the GDP-bound form of the Rab substrate.

Novel nicotinamide adenine dinucleotide analogues as potential anticancer agents: quest for specific inhibition of inosine monophosphate dehydrogenase

Synthetic nicotinamide adenine dinucleotide (NAD) analogues containing 5-beta-D-ribofuranosylnicotinamide (C-NAD), 6-beta-D-ribofuranosylpicolinamide (C-PAD), 3-beta-D-ribofuranosylbenzamide (BAD), and 2-beta-D-ribofuranosylthiazole-4-carboxamide (TAD) in place of the nicotinamide riboside moiety are described and evaluated as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). TAD and BAD showed potent inhibitory activity against the enzyme in the form of pyrophosphates, as well as metabolically stable methylene- and difluoromethylenebis(phosphonate)s. Fluorination at the C2' (ribo and arabino configuration) and C3' (ribo) of the adenosine moiety of TAD afforded analogues highly potent against IMPDH, but weakly active against alcohol dehydrogenase. With the exception of the methylenebis(phosphonate) analogue of TAD compounds containing a methylene bridge were poor inhibitors of growth of K562 cells. On the other hand, NAD analogues containing difluoromethylene linkage were highly effective in inhibition of K562 cell growth, as well as potent inducers of K562 cell differentiation. Such compounds, therefore, may be of potential therapeutic interest.

Chemotactic peptide-induced activation of Ras in human neutrophils is associated with inhibition of p120-GAP activity

The monomeric G-protein Ras is now considered to function as an initial regulator of multiple signaling pathways in both normal and transformed cell types. Adhesion and chemoattractant receptors are known to trigger activation of Ras in human neutrophils, but the signaling mechanism that activates Ras has only been partially elucidated. The present results show that in neutrophils, a time- and dose-dependent f-Met-Leu-Phe (FMLP)-induced activation of Ras is mediated by Gi2-proteins, because such activation is inhibited by pertussis toxin and because direct stimulation of heterotrimeric G-proteins with AlF4- is sufficient to activate Ras. Pretreatment of neutrophils with tyrosine kinase inhibitors, i.e. genistein or erbstatin that completely block FMLP-stimulated protein tyrosine phosphorylations, did not affect the FMLP-induced activation of Ras. Moreover, FMLP did not induce any detectable translocation of Grb2 and Sos to the plasma membrane of neutrophils. Other signaling molecules, such as protein kinase C, phosphatidylinositol 3-kinase and Ca2+, do not appear to be involved in the FMLP-induced Ras activation. Instead, stimulation of neutrophils with FMLP or C5a, the latter of which also activates Gi2-proteins, resulted in transient inhibition of the activity of Ras GTPase-activating proteins (GAP) with kinetics that correlated well with the kinetics of Ras activation. Moreover, decreased Ras-GAP activity was found in p120-GAP but not in neurofibromin immunoprecipitates of FMLP-stimulated cells. These results suggest that tyrosine kinase-dependent Ras exchange factors do not contribute to the FMLP-induced activation of Ras but that such activation is mediated via inhibition of p120-GAP in neutrophils.