Many faces of Ras activation

Mammalian son of sevenless Guanine nucleotide exchange factors: old concepts and new perspectives

The Son of Sevenless (Sos) factors were originally discovered 2 decades ago as specialized Ras activators in signaling pathways controlling the process of R7 cell development in the eye of Drosophila melanogaster. The 2 known members of the mammalian Sos family (Sos1 and Sos2) code for ubiquitously expressed, highly homologous (69% overall) proteins involved in coupling signals originated by cell surface receptor tyrosine kinases (RTKs) to downstream, Ras-dependent mitogenic signaling pathways. Mechanistically, the Sos proteins function as enzymatic factors interacting with Ras proteins in response to upstream stimuli to promote guanine nucleotide exchange (GDP/GTP) and subsequent formation of the active Ras-GTP complex. In this review, we summarize current knowledge on structural, regulatory, and functional aspects of the Sos family, focusing on specific aspects of Sos biology such as structure-function relationship, crosstalk with different signaling pathways, and in vivo functional significance as deduced from phenotypic characterization of Sos knockout mice and human genetic syndromes caused by germline hSos1 mutations.

Regulation of Neuronal Function by Ras-GRF Exchange Factors

Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2) constitute a family of guanine nucleotide exchange factors (GEFs). The main isoforms, p140-GRF1 and p135-GRF2, have 2 GEF domains that give them the capacity to activate both Ras and Rac GTPases in response to signals from a variety of neurotransmitter receptors. GRF1 and GRF2 proteins are found predominantly in adult neurons of the central nervous system, although they can also be detected in a limited number of other tissues. p140-GRF1 and p135-GRF2 contain calcium/calmodulin-binding IQ domains that allow them to act as calcium sensors to mediate the actions of NMDA-type and calcium-permeable AMPA-type glutamate receptors. p140-GRF1 also mediates the action of dopamine receptors that signal through cAMP. Although p140-GRF1 and p135-GRF2 have similar functional domains, studies of GRF knockout mice show that they can play strikingly different roles in regulating MAP kinase family members, neuronal synaptic plasticity, specific forms of learning and memory, and behavioral responses to psychoactive drugs. In addition, the function of GRF proteins may vary in different regions of the brain. Alternative splice variants yielding smaller GRF1 gene isoforms with fewer functional domains also exist; however, their distinct roles in neurons have not been revealed. Continuing studies of these proteins should yield important insights into the biochemical basis of brain function as well as novel concepts to explain how complex signal transduction proteins, like Ras-GRFs, integrate multiple upstream signals into specific downstream outputs to control brain function.

Differential Regulation of RasGAPs in Cancer

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

Ehrlichia chaffeensis induces monocyte inflammatory responses through MyD88, ERK, and NF-κB but not through TRIF, interleukin-1 receptor 1 (IL-1R1)/IL-18R1, or toll-like receptors

Human monocytic ehrlichiosis, an influenza-like illness accompanied by signs of hepatitis, is caused by infection of monocytes/macrophages with a lipopolysaccharide-deficient bacterium, Ehrlichia chaffeensis. The E. chaffeensis strain Wakulla induces diffuse hepatitis with neutrophil infiltration in mice with severe combined immunodeficiency, which is accompanied by strong CXCL2 (mouse functional homolog of interleukin-8 [IL-8]) and tumor necrosis factor alpha (TNF-α) expression in the liver. In this study, we found that expression of IL-1β, CXCL2, and TNF-α was induced by strain Wakulla in mouse bone marrow-derived macrophages; this expression was dependent on MyD88, but not on TRIF, TLR2/4, IL-1R1/IL-18R1, or endosome acidification. When the human leukemia cell line THP-1 was exposed to E. chaffeensis, significant upregulation of IL-8, IL-1β, and TNF-α mRNA and extracellular regulated kinase 2 (ERK2) activation were detected. U0126 (inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 [MEK1/2] upstream of ERK), manumycin A (Ras inhibitor), BAY43-9006 (Raf-1 inhibitor), and NS-50 (inhibitor of NF-κB nuclear translocation) inhibited the cytokine gene expression. A luciferase reporter assay using HEK293 cells, which lack Toll-like receptors (TLRs), showed activation of both the IL-8 promoter and NF-κB by E. chaffeensis. Activation of the IL-8 promoter in transfected HEK293 cells was inhibited by manumycin A, BAY43-9006, U0126, and transfection with a dominant-negative Ras mutant. These results indicate that the E. chaffeensis Wakulla strain can induce inflammatory responses through MyD88-dependent NF-κB and ERK pathways, without the involvement of TRIF and TLRs.

G alpha i2 and ZAP-70 mediate RasGRP1 membrane localization and activation of SDF-1-induced T cell functions

RasGRP1, a Ras guanine-nucleotide exchange factor, critically mediates T cell development and function and controls immunodeficiency and autoimmunity. In this study, we describe a unique mechanism of mobilization and activation of RasGRP1 in response to SDF-1, a chemokine that signals via the G protein-coupled receptor CXCR4. Depletion of RasGRP1 impaired SDF-1-stimulated human T cell migration, expression of the activation marker CD69, and activation of the ERK MAPK pathway, indicating that RasGRP1 mediates SDF-1 functions. SDF-1 treatment caused RasGRP1 to localize to the plasma membrane to activate K-Ras and to the Golgi to activate N-Ras. These events were required for cellular migration and for ERK activation that mediates downstream transcriptional events in response to SDF-1. SDF-1-dependent localization of RasGRP1 did not require its diacylglycerol-binding domain, even though diacyglycerol was previously shown to mediate localization of RasGRP1 in response to Ag stimulation. This domain was, however, required for activity of RasGRP1 after its localization. Intriguingly, SDF-1 treatment of T cells induced the formation of a novel molecular signaling complex containing RasGRP1, Gαi2, and ZAP-70. Moreover, SDF-1-mediated signaling by both Gi proteins and ZAP-70 was required for RasGRP1 mobilization. In addition, RasGRP1 mobilization and activation in response to SDF-1 was dependent on TCR expression, suggesting that CXCR4 heterodimerizes with the TCR to couple to ZAP-70 and mobilize RasGRP1. These results increase understanding of the molecular mechanisms that mediate SDF-1 effects on T cells and reveal a novel mechanism of RasGRP1 regulation. Other G protein-coupled receptors may similarly contribute to regulation of RasGRP1.

Regulation of Ras proteins by reactive nitrogen species

Ras GTPases have been a subject of intense investigation since the early 1980s, when single point mutations in Ras were shown to cause deregulated cell growth control. Subsequently, Ras was identified as the most prevalent oncogene found in human cancer. Ras proteins regulate a host of pathways involved in cell growth, differentiation, and apoptosis by cycling between inactive GDP-bound and active GTP-bound states. Regulation of Ras activity is controlled by cellular factors that alter guanine nucleotide cycling. Oncogenic mutations prevent protein regulatory factors from down-regulating Ras activity, thereby maintaining Ras in a chronically activated state. The central dogma in the field is that protein modulatory factors are the primary regulators of Ras activity. Since the mid-1990s, however, evidence has accumulated that small molecule reactive nitrogen species (RNS) can also influence Ras guanine nucleotide cycling. Herein, we review the basic chemistry behind RNS formation and discuss the mechanism through which various RNS enhance nucleotide exchange in Ras proteins. In addition, we present studies that demonstrate the physiological relevance of RNS-mediated Ras activation within the context of immune system function, brain function, and cancer development. We also highlight future directions and experimental methods that may enhance our ability to detect RNS-mediated activation in cell cultures and in vivo. The development of such methods may ultimately pave new directions for detecting and elucidating how Ras proteins are regulated by redox species, as well as for targeting redox-activated Ras in cancer and other disease states.

An orthosteric inhibitor of the Ras-Sos interaction

Mimics of α-helices on protein surfaces have emerged as powerful reagents for antagonizing protein-protein interactions, which are difficult to target with small molecules. Herein we describe the design of a cell-permeable synthetic α-helix based on the guanine nucleotide exchange factor Sos that interferes with Ras-Sos interaction and downregulates Ras signaling in response to receptor tyrosine kinase activation.

Targeted Sos1 deletion reveals its critical role in early T-cell development

Activation of the small G protein Ras is required for thymocyte differentiation. In thymocytes, Ras is activated by the Ras guanine exchange factors (RasGEFs) Sos1, Sos2, and RasGRP1. We report the development of a floxed allele of sos1 to assess the role of Sos1 during thymocyte development. Sos1 was required for pre-T-cell receptor (pre-TCR)- but not TCR-stimulated developmental signals. Sos1 deletion led to a partial block at the DN-to-DP transition. Sos1-deficient thymocytes showed reduced pre-TCR-stimulated proliferation, differentiation, and ERK phosphorylation. In contrast, TCR-stimulated positive selection, and negative selection under strong stimulatory conditions, remained intact in Sos1-deficient mice. Comparison of RasGEF expression at different developmental stages showed that relative to Sos2 and RasGRP1, Sos1 is most abundant in DN thymocytes, but least abundant in DP thymocytes. These data reveal that Sos1 is uniquely positioned to affect signal transduction early in thymocyte development.

NO-released zinc supports the simultaneous binding of Raf-1 and PKCγ cysteine-rich domains to HINT1 protein at the mu-opioid receptor

In the brain, the mu-opioid receptor (MOR) activates neural nitric oxide synthase (nNOS) through the PI3K/Akt pathway. The resulting nitric oxide (NO) enhances the function of the glutamate N-methyl-d-aspartate receptor (NMDAR)/calcium and calmodulin-dependent serine/threonine kinase (CaMKII), which subsequently diminishes MOR signaling strength. Because the ERK1/2 cascade is implicated in opioid tolerance, we analyzed the role of morphine-generated NO in this negative regulation. We found that NO-released endogenous zinc ions recruit the Ras/Raf-1/ERK1/2 cassette to histidine triad nucleotide-binding protein 1 (HINT1). A-Raf and B-Raf showed little or no MOR association. The zinc ions bridge the Raf-1 cysteine-rich domain (CRD) with HINT1 at the MOR C-terminus. Morphine also recruits PKCγ via NO/zinc to the MOR-HINT1 complex. Both Raf-1 and PKCγ CRDs bind simultaneously to HINT1, enabling PKCγ to enhance Raf-1 function to intensify MEK/ERK1/2 activation. Thus, through attached HINT1, the MOR facilitates the cross-talk of two NO- and zinc-regulated signal-transduction pathways, PKC/Src and Raf-1/ERK1/2, implicated in the negative control of morphine effects. This study reveals new aspects of ERK1/2 regulation by the MOR without requiring the transactivation of a receptor tyrosine kinase.

Targeting Ras-RAF-ERK and its interactive pathways as a novel therapy for malignant gliomas

Malignant gliomas are the most common and the deadliest brain malignancies in adults. Despite the lack of a complete understanding of the biology of these tumors, significant advances have been made in the past decades. One of the key discoveries made in the area of malignant gliomas is that these tumors can be induced and maintained by aberrant signaling networks. In this context, the Ras pathway has been extensively exploited, from both basic and translational perspectives. Although somatic oncogenic mutations of Ras genes are frequent in several cancer types, early investigations on gliomas revealed disappointing facts that the Ras mutations are nearly absent in malignant gliomas and that the BRAF mutations are present in a very small percentage of gliomas. Therefore, the observed deregulation of the Ras-RAF-ERK signaling pathway in gliomas is attributed to its upstream positive regulators, including, EGFR and PDGFR known to be highly active in the majority of malignant gliomas. In contrast to the initial negative results on the somatic mutations of H-Ras, K-Ras and BRAF, recent breakthrough studies on pediatric low-grade astrocytomas uncovered genetic alterations of the BRAF gene involving copy number gains and rearrangements. The 7q34 rearrangements result in a novel in-frame KIAA1549:BRAF fusion gene that possesses constitutive BRAF kinase activity resembling oncogenic BRAF (V600E). In light of the earlier findings and recent breakthroughs, this review summarizes our current understanding of the Ras-RAF-ERK signaling pathway in gliomas and the outcome of preclinical and clinical studies that evaluated the efficacy of Ras-targeted therapy in malignant gliomas.

A RasGRP, C. elegans RGEF-1b, couples external stimuli to behavior by activating LET-60 (Ras) in sensory neurons

RasGRPs, which load GTP onto Ras and Rap1, are expressed in vertebrate and invertebrate neurons. The functions, regulation, and mechanisms of action of neuronal RasGRPs are unknown. Here, we show how C. elegans RGEF-1b, a prototypical neuronal RasGRP, regulates a critical behavior. Chemotaxis to volatile odorants was disrupted in RGEF-1b-deficient (rgef-1⁻/⁻) animals and wild-type animals expressing dominant-negative RGEF-1b in AWC sensory neurons. AWC-specific expression of RGEF-1b-GFP restored chemotaxis in rgef-1⁻/⁻ mutants. Signals disseminated by RGEF-1b in AWC neurons activated a LET-60 (Ras)-MPK-1 (ERK) signaling cascade. Other RGEF-1b and LET-60 effectors were dispensable for chemotaxis. A bifunctional C1 domain controlled intracellular targeting and catalytic activity of RGEF-1b and was essential for sensory signaling in vivo. Chemotaxis was unaffected when Ca²+-binding EF hands and a conserved phosphorylation site of RGEF-1b were inactivated. Diacylglycerol-activated RGEF-1b links external stimuli (odorants) to behavior (chemotaxis) by activating the LET-60-MPK-1 pathway in specific neurons.

Constitutively active Harvey Ras confers resistance to epidermal growth factor receptor-targeted therapy with cetuximab and gefitinib

Kirsten Ras (K-Ras) mutations have been implicated as a key predictive marker of resistance to therapies targeting the epidermal growth factor receptor (EGFR). To determine whether Harvey Ras (H-Ras) mutations also can confer resistance to EGFR-targeted therapy, we expressed a constitutively active H-Ras (Ras G12V) in A431 human vulvar squamous carcinoma cells. Compared with corresponding control cells, A431-Ras cells exhibited marked resistance to the EGFR inhibitors cetuximab and gefitinib, reducing inhibition of Akt and Erk phosphorylation, inhibition of HIF-1α expression and transcriptional activity, and antitumor effects in vitro and in vivo. Our data indicate that constitutively active H-Ras can also confer resistance to anti-EGFR therapy in cancer cells.

The RalGEF-Ral Effector Signaling Network: The Road Less Traveled for Anti-Ras Drug Discovery

The high frequency of RAS mutations in human cancers (33%) has stimulated intense interest in the development of anti-Ras inhibitors for cancer therapy. Currently, the major focus of these efforts is centered on inhibitors of components involved in Ras downstream effector signaling. In particular, more than 40 inhibitors of the Raf-MEK-ERK mitogen-activated protein kinase cascade and phosphoinositide 3-kinase-AKT-mTOR effector signaling networks are currently under clinical evaluation. However, these efforts are complicated by the fact that Ras can utilize at least 9 additional functionally distinct effectors, with at least 3 additional effectors with validated roles in Ras-mediated oncogenesis. Of these, the guanine nucleotide exchange factors of the Ras-like (Ral) small GTPases (RalGEFs) have emerged as important effectors of mutant Ras in pancreatic, colon, and other cancers. In this review, we summarize the evidence for the importance of this effector pathway in cancer and discuss possible directions for therapeutic inhibition of aberrant Ral activation and signaling.

Reactive oxygen species (ROS) play a critical role in the cAMP-induced activation of Ras and the phosphorylation of ERK1/2 in Leydig cells

Activation of the LH receptor (LHR) in Leydig cells results in the phosphorylation of ERK1/2 by cAMP-dependent and cAMP-independent pathways. Here we examine the mechanisms by which cAMP stimulates ERK1/2 phosphorylation. We show that the stimulation of steroidogenesis is not necessary or sufficient to stimulate the phosphorylation of ERK1/2 but that other cAMP-dependent mitochondrial functions are involved. Using MA-10 cells as a model, we showed that cAMP analogs increase reactive oxygen species (ROS) formation and that an uncoupler of oxidative phosphorylation and a ROS scavenger prevent this increase. These two compounds also inhibit the increase in ERK1/2 phosphorylation provoked by cAMP analogs, thus suggesting that the cAMP-induced phosphorylation of ERK1/2 is mediated by mitochondrial ROS. In agreement with this hypothesis we also show that a reduction in glutathione levels, which alters the redox state of MA-10 cells, potentiates the effect of cAMP on ERK1/2 phosphorylation. Measurements of the dephosphorylation of ERK and the activation of Ras showed that the ROS scavenger prevents the cAMP-provoked activation of Ras and that cAMP, with or without a ROS scavenger, has little or no effect on the dephosphorylation of ERK. Lastly, we show that the uncoupler of oxidative phosphorylation and the ROS scavenger also prevent the ability of cAMP analogs to increase ERK1/2 phosphorylation in primary cultures of mouse Leydig cells. We conclude that, in Leydig cells, cAMP enhances the phosphorylation of ERK1/2 via a mitochondria-derived, ROS-dependent activation of Ras.

Phosphoinositide 3-kinase regulates plasma membrane targeting of the Ras-specific exchange factor RasGRP1

Receptor-induced targeting of exchange factors to specific cellular membranes is the predominant mechanism for initiating and compartmentalizing signal transduction by Ras GTPases. The exchange factor RasGRP1 has a C1 domain that binds the lipid diacylglycerol and thus can potentially mediate membrane localization in response to receptors that are coupled to diacylglycerol-generating phospholipase Cs. However, the C1 domain is insufficient for targeting RasGRP1 to the plasma membrane. We found that a basic/hydrophobic cluster of amino acids within the plasma membrane-targeting domain of RasGRP1 is instead responsible for plasma membrane targeting. This basic/hydrophobic cluster binds directly to phospholipid vesicles containing phosphoinositides via electrostatic interactions with polyanionic phosphoinositide headgroups and insertion of a tryptophan into the lipid bilayer. B cell antigen receptor ligation and other stimuli induce plasma membrane targeting of RasGRP1 by activating the phosphoinositide 3-kinase signaling pathway, which generates phosphoinositides within the plasma membrane. Direct detection of phosphoinositides by the basic/hydrophobic cluster of RasGRP1 provides a novel mechanism for coupling and co-compartmentalizing phosphoinositide 3-kinase and Ras signaling and, in coordination with diacylglycerol detection by the C1 domain, gives RasGRP1 the potential to serve as an integrator of converging signals from the phosphoinositide 3-kinase and phospholipase C pathways.

Ras superfamily GEFs and GAPs: validated and tractable targets for cancer therapy?

There is now considerable and increasing evidence for a causal role for aberrant activity of the Ras superfamily of small GTPases in human cancers. These GTPases function as GDP-GTP-regulated binary switches that control many fundamental cellular processes. A common mechanism of GTPase deregulation in cancer is the deregulated expression and/or activity of their regulatory proteins, guanine nucleotide exchange factors (GEFs) that promote formation of the active GTP-bound state and GTPase-activating proteins (GAPs) that return the GTPase to its GDP-bound inactive state. In this Review, we assess the association of GEFs and GAPs with cancer and their druggability for cancer therapeutics.

Demonstration of angiotensin II-induced Ras activation in the trans-Golgi network and endoplasmic reticulum using bioluminescence resonance energy transfer-based biosensors

Previous studies have demonstrated that molecules of the Ras signaling pathway are present in intracellular compartments, including early endosomes, the endoplasmic reticulum (ER), and the Golgi, and suggested that mitogens can regulate Ras activity in these endomembranes. In this study, we investigated the effect of angiotensin II (AngII) on intracellular Ras activity in living HEK293 cells expressing angiotensin type 1 receptors (AT(1)-Rs) using newly developed bioluminescence resonance energy transfer biosensors. To investigate the subcellular localization of AngII-induced Ras activation, we targeted our probes to various intracellular compartments, such as the trans-Golgi network (TGN), the ER, and early endosomes. Using these biosensors, we detected AngII-induced Ras activation in the TGN and ER, but not in early endosomes. In cells expressing a cytoplasmic tail deletion AT(1)-R mutant, the AngII-induced response was enhanced, suggesting that receptor internalization and β-arrestin binding are not required for AngII-induced Ras activation in endomembranes. Although we were able to demonstrate EGF-induced Ras activation in the plasma membrane and TGN, but not in other endomembranes, AG1478, an EGF receptor inhibitor, did not affect the AngII-induced response, suggesting that the latter is independent of EGF receptor transactivation. AngII was unable to stimulate Ras activity in the studied compartments in cells expressing a G protein coupling-deficient AT(1)-R mutant ((125)DRY(127) to (125)AAY(127)). These data suggest that AngII can stimulate Ras activity in the TGN and ER with a G protein-dependent mechanism, which does not require β-arrestin-mediated signaling, receptor internalization, and EGF receptor transactivation.

A mathematical framework to determine the temporal sequence of somatic genetic events in cancer

Human cancer is caused by the accumulation of genetic alterations in cells. Of special importance are changes that occur early during malignant transformation because they may result in oncogene addiction and represent promising targets for therapeutic intervention. Here we describe a computational approach, called Retracing the Evolutionary Steps in Cancer (RESIC), to deduce the temporal sequence of genetic events during tumorigenesis from cross-sectional genomic data of tumors at their fully transformed stage. When applied to a dataset of 70 advanced colorectal cancers, our algorithm accurately predicts the sequence of APC, KRAS, and TP53 mutations previously defined by analyzing tumors at different stages of colon cancer formation. We further validate the method with glioblastoma and leukemia sample data and then apply it to complex integrated genomics databases, finding that high-level EGFR amplification appears to be a late event in primary glioblastomas. RESIC represents the first evolutionary mathematical approach to identify the temporal sequence of mutations driving tumorigenesis and may be useful to guide the validation of candidate genes emerging from cancer genome surveys.

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.

Genetic analysis of Ras signalling pathways in cell proliferation, migration and survival

We have used mouse embryonic fibroblasts (MEFs) devoid of Ras proteins to illustrate that they are essential for proliferation and migration, but not for survival, at least in these cells. These properties are unique to the Ras subfamily of proteins because ectopic expression of other Ras-like small GTPases, even when constitutively active, could not compensate for the absence of Ras proteins. Only constitutive activation of components of the Raf/Mek/Erk pathway was sufficient to sustain normal proliferation and migration of MEFs devoid of Ras proteins. Activation of the phosphatidylinositol 3-kinase (PI3K)/PTEN/Akt and Ral guanine exchange factor (RalGEF)/Ral pathways, either alone or in combination, failed to induce proliferation or migration of Rasless cells, although they cooperated with Raf/Mek/Erk signalling to reproduce the full response mediated by Ras signalling. In contrast to current hypotheses, Ras signalling did not induce proliferation by inducing expression of D-type Cyclins. Rasless MEFs had normal levels of Cyclin D1/Cdk4 and Cyclin E/Cdk2. However, these complexes were inactive. Inactivation of the pocket proteins or knock down of pRb relieved MEFs from their dependence on Ras signalling to proliferate.

A Flt3- and Ras-dependent pathway primes B cell development by inducing a state of IL-7 responsiveness

Ras plays an important role in B cell development. However, the stage at which Ras governs B cell development remains unclear. Moreover, the upstream receptors and downstream effectors of Ras that govern B cell differentiation remain undefined. Using mice that express a dominant-negative form of Ras, we demonstrate that Ras-mediated signaling plays a critical role in the development of common lymphoid progenitors. This developmental block parallels that found in flt3(-/-) mice, suggesting that Flt3 is an important upstream activator of Ras in early B cell progenitors. Ras inhibition impaired proliferation of common lymphoid progenitors and pre-pro-B cells but not pro-B cells. Rather, Ras promotes STAT5-dependent pro-B cell differentiation by enhancing IL-7Ralpha levels and suppressing socs2 and socs3 expression. Our results suggest a model in which Flt3/Ras-dependent signals play a critical role in B cell development by priming early B cell progenitors for subsequent STAT5-dependent B cell differentiation.

Utilizing ras signaling pathway to direct selective replication of herpes simplex virus-1

Re-engineering the tropism of viruses is an attractive translational strategy for targeting cancer cells. The Ras signal transduction pathway is a central hub for a variety of pro-oncogenic events with a fundamental role in normal and neoplastic physiology. In this work we were interested in linking Ras activation to HSV-1 replication in a direct manner in order to generate a novel oncolytic herpes virus which can target cancer cells. To establish such link, we developed a mutant HSV-1 in which the expression of ICP4 (infected cell protein-4, a viral protein necessary for replication) is controlled by activation of ELK, a transcription factor down-stream of the Ras pathway and mainly activated by ERK (extracellular signal-regulated kinase, an important Ras effector pathway). This mutant HSV-1 was named as Signal-Smart 1 (SS1). A series of prostate cells were infected with the SS1 virus. Cells with elevated levels of ELK activation were preferentially infected by the SS1 virus, as demonstrated by increased levels of viral progeny, herpetic glycoprotein C and overall SS1 viral protein production. Upon exposure to SS1, the proliferation, invasiveness and colony formation capabilities of prostate cancer cells with increased ELK activation were significantly decreased (p<0.05), while the rate of apoptosis/necrosis in these cells was increased. Additionally, high Ras signaling cells infected with SS1 showed a prominent arrest in the G1 phase of the cell cycle as compared to cells exposed to parental HSV-1. The results of this study reveal the potential for re-modeling the host-herpes interaction to specifically interfere with the life of cancer cells with increased Ras signaling. SS1 also serves as a "prototype" for development of a family of signal-smart viruses which can target cancer cells on the basis of their signaling portfolio.

Phosphatidic acid signaling regulation of Ras superfamily of small guanosine triphosphatases

Phosphatidic acid (PA) has been increasingly recognized as an important signaling lipid regulating cell growth and proliferation, membrane trafficking, and cytoskeletal reorganization. Recent studies indicate that the signaling PA generated from phospholipase D (PLD) and diacylglycerol kinase (DGK) plays critical roles in regulating the activity of some members of Ras superfamily of small guanosine triphosphatases (GTPases), such as Ras, Rac and Arf. Change of PA levels regulates the activity of small GTPases by modulating membrane localization and activity of small GTPase regulatory proteins, guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). In addition, PA also targets some small GTPases to membranes by direct binding. This review summarizes the roles of PLD and DGK in regulating the activity of several Ras superfamily members and cellular processes they control. Some future directions and the implication of PA regulation of Ras small GTPases in pathology are also discussed.