Plasma membrane recruitment of RalGDS is critical for Ras-dependent Ral activation

Development of Son of Sevenless Homologue 1 (SOS1) Modulators To Treat Cancers by Regulating RAS Signaling

Son of sevenless homologue 1 (SOS1) protein is universally expressed in cells and plays an important role in the RAS signaling pathway. Specifically, this protein interacts with RAS in response to upstream stimuli to promote guanine nucleotide exchange in RAS and activates the downstream signaling pathways. Thus, targeting SOS1 is a new approach for treating RAS-driven cancers. In this Perspective, we briefly summarize the structural and functional aspects of SOS1 and focus on recent advances in the discovery of activators, inhibitors, and PROTACs that target SOS1. This review aims to provide a timely and updated overview on the strategies for targeting SOS1 in cancer therapy.

RILP inhibits proliferation, migration, and invasion of PC3 prostate cancer cells

RILP (Rab-interacting lysosomal protein) is a key regulator of lysosomal transport and a potential tumor suppressor. However, the role of RILP in prostate cancer and the underlying mechanism of RILP in regulating the proliferation, migration, and invasion of prostate cancer cells remain to be studied. In this study, we confirmed RalGDS (Ral guanine nucleotide dissociation stimulator) as the interaction partner of RILP in PC3 prostate cancer cells. Immunofluorescence microscopy showed that RILP recruits RalGDS to the lysosomal compartment. We found that RILP inhibits the activation of RalA and downstream effector RalBP1, and negatively regulates the downstream molecular phosphorylation of Ras. We showed that RILP inhibits the proliferation, migration, and invasion of PC3 prostate cancer cells, which may give rise to novel ideas for cancer treatment.

RAS GTPase signalling to alternative effector pathways

RAS GTPases are fundamental regulators of development and drivers of an extraordinary number of human cancers. RAS oncoproteins constitutively signal through downstream effector proteins, triggering cancer initiation, progression and metastasis. In the absence of targeted therapeutics to mutant RAS itself, inhibitors of downstream pathways controlled by the effector kinases RAF and PI3K have become tools in the treatment of RAS-driven tumours. Unfortunately, the efficacy of this approach has been greatly minimized by the prevalence of acquired drug resistance. Decades of research have established that RAS signalling is highly complex, and in addition to RAF and PI3K these small GTPase proteins can interact with an array of alternative effectors that feature RAS binding domains. The consequence of RAS binding to these effectors remains relatively unexplored, but these pathways may provide targets for combinatorial therapeutics. We discuss here three candidate alternative effectors: RALGEFs, RASSF5 and AFDN, detailing their interaction with RAS GTPases and their biological significance. The metastatic nature of RAS-driven cancers suggests more attention should be granted to these alternate pathways, as they are highly implicated in the regulation of cell adhesion, polarity, cell size and cytoskeletal architecture.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Methods to Investigate the Roles of β-Arrestin-Dependent RalGDS Activation in GPCR-Stimulated Membrane Blebbing

G protein-coupled receptors (GPCRs) comprise the largest family of integral membrane proteins, which in addition to signaling via heterotrimeric G proteins can activate small G proteins both directly and indirectly. The activation of a variety of GPCRs leads to the translocation of Ral GDP dissociation stimulator (RalGDS) to the plasma membrane, where it functions as a guanine nucleotide exchange factor of RalA to promote membrane blebbing. The translocation of RalGDS is β-arrestin-dependent and can be inhibited by either the expression of the β-arrestin1 amino-terminal domain or the expression of RalGDS clone 284 (amino acid residues 616-768 of RalGDS). We describe here a methodology for assessing GPCR-dependent stimulation of RalGDS plasma membrane translocation.

Single-molecule fluorescence imaging of RalGDS on cell surfaces during signal transduction from Ras to Ral

RalGDS is one of the Ras effectors and functions as a guanine nucleotide exchange factor for the small G-protein, Ral, which regulates membrane trafficking and cytoskeletal remodeling. The translocation of RalGDS from the cytoplasm to the plasma membrane is required for Ral activation. In this study, to understand the mechanism of Ras–Ral signaling we performed a single-molecule fluorescence analysis of RalGDS and its functional domains (RBD and REMCDC) on the plasma membranes of living HeLa cells. Increased molecular density of RalGDS and RBD, but not REMCDC, was observed on the plasma membrane after EGF stimulation of the cells to induce Ras activation, suggesting that the translocation of RalGDS involves an interaction between the GTP-bound active form of Ras and the RBD of RalGDS. Whereas the RBD played an important role in increasing the association rate constant between RalGDS and the plasma membrane, the REMCDC domain affected the dissociation rate constant from the membrane, which decreased after Ras activation or the hyperexpression of Ral. The Y64 residue of Ras and clusters of RalGDS molecules were involved in this reduction. From these findings, we infer that Ras activation not merely increases the cell-surface density of RalGDS, but actively stimulates the RalGDS–Ral interaction through a structural change in RalGDS and/or the accumulation of Ral, as well as the GTP–Ras/RalGDS clusters, to induce the full activation of Ral.

Regulation of Ras signaling and function by plasma membrane microdomains

Together H-, N- and KRAS mutations are major contributors to ~30% of all human cancers. Thus, Ras inhibition remains an important anti-cancer strategy. The molecular mechanisms of isotypic Ras oncogenesis are still not completely understood. Monopharmacological therapeutics have not been successful in the clinic. These disappointing outcomes have led to attempts to target elements downstream of Ras, mainly targeting either the Phosphatidylinositol 3-Kinase (PI3K) or Mitogen-Activated Protein Kinase (MAPK) pathways. While several such approaches are moderately effective, recent efforts have focused on preclinical evaluation of combination therapies to improve efficacies. This review will detail current understanding of the contributions of plasma membrane microdomain targeting of Ras to mitogenic and tumorigenic signaling and tumor progression. Moreover, this review will outline novel approaches to target Ras in cancers, including targeting schemes for new drug development, as well as putative re-purposing of drugs in current use to take advantage of blunting Ras signaling by interfering with Ras plasma membrane microdomain targeting and retention.

A Gα-Stimulated RapGEF Is a Receptor-Proximal Regulator of Dictyostelium Chemotaxis

Chemotaxis, or directional movement toward extracellular chemical gradients, is an important property of cells that is mediated through G-protein-coupled receptors (GPCRs). Although many chemotaxis pathways downstream of Gβγ have been identified, few Gα effectors are known. Gα effectors are of particular importance because they allow the cell to distinguish signals downstream of distinct chemoattractant GPCRs. Here we identify GflB, a Gα2 binding partner that directly couples the Dictyostelium cyclic AMP GPCR to Rap1. GflB localizes to the leading edge and functions as a Gα-stimulated, Rap1-specific guanine nucleotide exchange factor required to balance Ras and Rap signaling. The kinetics of GflB translocation are fine-tuned by GSK-3 phosphorylation. Cells lacking GflB display impaired Rap1/Ras signaling and actin and myosin dynamics, resulting in defective chemotaxis. Our observations demonstrate that GflB is an essential upstream regulator of chemoattractant-mediated cell polarity and cytoskeletal reorganization functioning to directly link Gα activation to monomeric G-protein signaling.

Endocytosis and serpentine filopodia drive blebbishield-mediated resurrection of apoptotic cancer stem cells

The blebbishield emergency program helps to resurrect apoptotic cancer stem cells (CSCs) themselves. Understanding the mechanisms behind this program is essential to block resurrection of CSCs during cancer therapy. Here we demonstrate that endocytosis drives serpentine filopodia to construct blebbishields from apoptotic bodies and that a VEGF-VEGFR2-endocytosis-p70S6K axis governs subsequent transformation. Disengagement of RalGDS from E-cadherin initiates endocytosis of RalGDS and its novel interaction partners cdc42, VEGFR2, cleaved β-catenin, and PKC-ζ as well as its known interaction partner K-Ras. We also report novel interactions of p45S6K (cleaved p70S6K) and PKM-ζ with PAK-1 filopodia-forming machinery specifically in blebbishields. Thus, a RalGDS-endocytosis-filopodia-VEGFR2-K-Ras-p70S6K axis drives the blebbishield emergency program, and therapeutic targeting of this axis might prevent resurrection of CSCs during cancer therapy.

RILP suppresses invasion of breast cancer cells by modulating the activity of RalA through interaction with RalGDS

RILP (Rab7-interacting lysosomal protein) is a key regulator for late endosomal/lysosomal trafficking, and probably a tumor suppressor in prostate cancer. However, the role of RILP in other cancers and the underlying mechanism for RILP in regulating the invasion of cancer cells remain to be investigated. In this study, we showed that overexpression of RILP in breast cancer cells inhibits the migration and invasion, whereas the depletion of RILP by RNAi-mediated knockdown promotes the migration and invasion. We identified RalGDS (Ral guanine nucleotide dissociation stimulator) as a novel interacting partner for RILP, and truncation analysis revealed the N-terminal region of RILP is responsible for interacting with the guanine nucleotide exchange factor (GEF) domain of RalGDS. Immunofluorescence microscopy revealed that RalGDS can be recruited to the late endosomal compartments by RILP. Further investigations indicated that the overexpression of RILP inhibits the activity of RalA, a downstream target of RalGDS. Our data suggest that RILP suppresses the invasion of breast cancer cells by interacting with RalGDS to inhibit its GEF activity for RalA.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Contributions of KRAS and RAL in non-small-cell lung cancer growth and progression

INTRODUCTION

KRAS mutations are poor prognostic markers for patients with non-small-cell lung cancer (NSCLC). RALA and RALB GTPases lie downstream of RAS and are implicated in RAS-mediated tumorigenesis. However, their biological or prognostic role in the context of KRAS mutation in NSCLC is unclear.

METHODS

Using expression analysis of human tumors and a panel of cell lines coupled with functional in vivo and in vitro experiments, we evaluated the prognostic and functional importance of RAL in NSCLC and their relationship to KRAS expression and mutation.

RESULTS

Immunohistochemical (N = 189) and transcriptomic (N = 337) analyses of NSCLC patients revealed high RALA and RALB expression was associated with poor survival. In a panel of 14 human NSCLC cell lines, RALA and RALB had higher expression in KRAS mutant cell lines whereas RALA but not RALB activity was higher in KRAS mutant cell lines. Depletion of RAL paralogs identified cell lines that are dependent on RAL expression for proliferation and anchorage independent growth. Overall, growth of NSCLC cell lines that carry a glycine to cystine KRAS mutation were more sensitive to RAL depletion than those with wild-type KRAS. The use of gene expression and outcome data from 337 human tumors in RAL-KRAS interaction analysis revealed that KRAS and RAL paralog expression jointly impact patient prognosis.

CONCLUSION

RAL GTPase expression carries important additional prognostic information to KRAS status in NSCLC patients. Simultaneously targeting RAL may provide a novel therapeutic approach in NSCLC patients harboring glycine to cystine KRAS mutations.

The role of RAS effectors in BCR/ABL induced chronic myelogenous leukemia

BCR/ABL is the causative agent of chronic myelogenous leukemia (CML). Through structure/function analysis, several protein motifs have been determined to be important for the development of leukemogenesis. Tyrosine177 of BCR is a Grb2 binding site required for BCR/ABL-induced CML in mice. In the current study, we use a mouse bone marrow transduction/transplantation system to demonstrate that addition of oncogenic NRAS (NRASG12D) to a vector containing a BCR/ABL(Y177F) mutant "rescues" the CML phenotype rapidly and efficiently. To further narrow down the pathways downstream of RAS that are responsible for this rescue effect, we utilize well-characterized RAS effector loop mutants and determine that the RAL pathway is important for rapid induction of CML. Inhibition of this pathway by a dominant negative RAL is capable of delaying disease progression. Results from the present study support the notion of RAL inhibition as a potential therapy for BCR/ABL-induced CML.

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.

Negative regulation of the RalGAP complex by 14-3-3

RGC1 and RGC2 comprise a functional RalGAP complex (RGC) that suppresses RalA activity. The PI3-kinase/Akt signaling pathway activates RalA through phosphorylation-mediated inhibition of the RGC. Here we identify a novel phosphorylation-dependent interaction between 14-3-3 and the RGC. 14-3-3 binds to the complex through an Akt-phosphorylated residue, threonine 715, on RGC2. Interaction with 14-3-3 does not alter in vitro activity of the GTPase-activating protein complex. However, blocking the interaction between 14-3-3 and RGC2 in cells increases suppression of RalA activity by the RGC, suggesting that 14-3-3 inhibits the complex through a non-catalytic mechanism. Together, these data show that 14-3-3 negatively regulates the RGC downstream of the PI3-kinase/Akt signaling pathway.

Detection of Rho GEF and GAP activity through a sensitive split luciferase assay system

Rho GTPases regulate the assembly of cellular actin structures and are activated by GEFs (guanine-nucleotide-exchange factors) and rendered inactive by GAPs (GTPase-activating proteins). Using the Rho GTPases Cdc42, Rac1 and RhoA, and the GTPase-binding portions of the effector proteins p21-activated kinase and Rhophilin1, we have developed split luciferase assays for detecting both GEF and GAP regulation of these GTPases. The system relies on purifying split luciferase fusion proteins of the GTPases and effectors from bacteria, and our results show that the assays replicate GEF and GAP specificities at nanomolar concentrations for several previously characterized Rho family GEFs (Dbl, Vav2, Trio and Asef) and GAPs [p190, Cdc42 GAP and PTPL1-associated RhoGAP]. The assay detected activities associated with purified recombinant GEFs and GAPs, cell lysates expressing exogenous proteins, and immunoprecipitates of endogenous Vav1 and p190. The results demonstrate that the split luciferase system provides an effective sensitive alternative to radioactivity-based assays for detecting GTPase regulatory protein activities and is adaptable to a variety of assay conditions.

KRAS mutant colorectal tumors: past and present

The treatment of metastatic colorectal cancer (mCRC) remains one of the largest hurdles in cancer therapeutics to date. The most advanced treatment option for mCRC patients are anti-epidermal growth factor receptor (EGFR) monoclonal antibodies (mAbs) that bind to and inhibit the activity of EGFR. While the use of anti-EGFR mABs has had great impact in the treatment of mCRC, it has now been widely accepted that mCRC tumors with a mutation in the small GTPase KRAS do not respond to these therapies. KRAS mutations allow for EGFR independent activation of various oncogenic signaling cascades. In attempts to inhibit KRAS mutant tumor growth, BRAF, MEK and farsenyltransferase inhibitors have been used, however, their clinical efficacy is still accruing in the setting of CRC. Recent data suggests that various other inhibitors, including inhibitors of Src family kinases (SFK) and hepatocyte growth factor receptor (MET), may have potential preclinical and clinical success in KRAS mutant tumors. Additionally, it is becoming increasingly clear that different KRAS missense mutations may have varied biological responses to cetuximab, suggesting that cetuximab may still be a potential therapeutic option in some KRAS mutant tumors. In this review, we highlight the importance for both improved multimodality approaches for treating KRAS mutant mCRC tumors and stratification of KRAS mutations in response to different treatment regimes in order to optimize the best possible care for mCRC patients.

MEK1/2 inhibitors AS703026 and AZD6244 may be potential therapies for KRAS mutated colorectal cancer that is resistant to EGFR monoclonal antibody therapy

Epidermal growth factor receptor (EGFR) monoclonal antibodies (mAb) are used widely to treat metastatic colorectal cancer (mCRC) patients, but it is now clear that patients harboring K-ras mutation are resistant to EGFR mAbs such as cetuximab (Erbitux) and panitumumab (Vectibix). For this reason, current recommendations for patient care involve diagnosing the K-ras mutational status of patients prior to EGFR mAb therapy. In this study, we investigated the ability of two MEK inhibitors currently in clinical trials, AS703026 and AZD6244, to address the challenge posed by the resistance of K-ras mutated colorectal cancers to EGFR mAb. AS703026 and AZD6244 were tested in various cell-based assays and tumor xenograft studies, focusing on isogenic human colorectal tumor cell lines that expressed only WT or mutant K-Ras (D-WT or D-MUT). The EGFR mAb cetuximab inhibited the Ras-ERK pathway and proliferation of D-WT cells in vitro and in vivo, but it did not inhibit proliferation of D-MUT cells in either setting. In contrast, AS703026 and AZD6244 effectively inhibited the growth of D-MUT cells in vitro and in vivo by specific inhibition of the key MEK downstream target kinase ERK. Inhibition of MEK by AS703026 or AZD6244 also suppressed cetuximab-resistant colorectal cancer cells attributed to K-ras mutation both in vitro and in vivo. Our findings offer proof-of-concept for the use of MEK inhibitors as an effective therapy in K-ras mutated CRC.

Aberrant overexpression of the Rgl2 Ral small GTPase-specific guanine nucleotide exchange factor promotes pancreatic cancer growth through Ral-dependent and Ral-independent mechanisms

Our recent studies established essential and distinct roles for RalA and RalB small GTPase activation in K-Ras mutant pancreatic ductal adenocarcinoma (PDAC) cell line tumorigencity, invasion, and metastasis. However, the mechanism of Ral GTPase activation in PDAC has not been determined. There are four highly related mammalian RalGEFs (RalGDS, Rgl1, Rgl2, and Rgl3) that can serve as Ras effectors. Whether or not they share distinct or overlapping functions in K-Ras-mediated growth transformation has not been explored. We found that plasma membrane targeting to mimic persistent Ras activation enhanced the growth-transforming activities of RalGEFs. Unexpectedly, transforming activity did not correlate directly with total cell steady-state levels of Ral activation. Next, we observed elevated Rgl2 expression in PDAC tumor tissue and cell lines. Expression of dominant negative Ral, which blocks RalGEF function, as well as interfering RNA suppression of Rgl2, reduced PDAC cell line steady-state Ral activity, growth in soft agar, and Matrigel invasion. Surprisingly, the effect of Rgl2 on anchorage-independent growth could not be rescued by constitutively activated RalA, suggesting a novel Ral-independent function for Rgl2 in transformation. Finally, we determined that Rgl2 and RalB both localized to the leading edge, and this localization of RalB was dependent on endogenous Rgl2 expression. In summary, our observations support nonredundant roles for RalGEFs in Ras-mediated oncogenesis and a key role for Rgl2 in Ral activation and Ral-independent PDAC growth.

RNA interference suppressing PLCE1 gene expression decreases invasive power of human bladder cancer T24 cell line

Mutational activation of the ras proto-oncogenes is frequently found in cancers. The phospholipase C epsilon gene (PLCE1) encodes a novel ras-related protein (R-Ras) effector mediating the effects of R-Ras on the actin cytoskeleton and membrane protrusion, because R-Ras is coprecipitated with the PLCE1 protein and can increase its activity. The nature of downstream signaling pathways from Ras involved in bladder cancer remains poorly understood. We aimed to construct a small hairpin RNA (shRNA) expression plasmid against the PLCE1 gene and to observe the inhibition of human bladder carcinoma cell T24 migration by RNA interference suppressing the expression of PLCE1. Two PLCE1 plasmids (P1 and P2) were constructed and inserted into T24 cells. Reverse transcriptase-polymerase chain reaction and Western blot analyses were performed to investigate inhibition of PLCE1 expression after plasmid transfection. Invasive power of the T24 cell line was measured before and after transfection by a membrane invasion culture system (transwell chamber), gelatin enzymography, and immunocytochemistry of cells. The RT-PCR analysis of BCL2 mRNA levels among different groups of T24 cell line indicated that expression of BCL2 mRNA was lower in the two positive plasmid-transfected cell groups than in the blank control or HK-A groups. Silencing of PLCE1 might downregulate the level of MMP and BCL2 gene expression, decreasing the invasive power of bladder cancer T24 cells and thus inhibiting tumor development.

RalGDS family members couple Ras to Ral signalling and that's not all

Ras proteins function as molecular switches that are activated in response to signalling pathways initiated by various extracellular stimuli and subsequently bind to numerous effector proteins leading to the activation of several signalling cascades within the cell. Ras and Ras-related proteins belong to a large superfamily of small GTPases characterized by significant sequence and function similarities. Several evidence indicate the existence of complex signalling networks that link Ras with its relatives in the family. A key role in this cross-talk is played by guanine nucleotide exchange factors (GEFs) that serve both as regulators and as effectors of Ras family proteins. The members of the RalGDS family, RalGDS, RGL, RGL2/Rlf and RGL3, can interact with activated Ras through their Ras Binding Domain (RBD), but may function as effectors for other Ras family members. They possess a REM-CDC25 homology region like RasGEFs, but specifically activate only RalA and RalB and not Ras or other Ras-related small GTPases. In this review we provide an update on this recently discovered family of GEFs, highlighting their crucial role in coupling activated Ras to activation of Ral, thus regulating several fundamental cell processes, and also discussing some evidence supporting Ras-independent additional functions of RalGDS proteins.

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.

Beta-arrestin/Ral signaling regulates lysophosphatidic acid-mediated migration and invasion of human breast tumor cells

The lipid mediator lysophosphatidic acid (LPA) plays a role in cancer progression and signals via specific G protein-coupled receptors, LPA(1-3). LPA has been shown to enhance the metastasis of breast carcinoma cells to bone. However, the mechanisms by which LPA receptors regulate breast cancer cell migration and invasion remain unclear. Breast cancer cell proliferation has been shown to be stimulated by Ral GTPases, a member of the Ras superfamily. Ral activity can be regulated by the multifunctional protein beta-arrestin. We now show that HS578T and MDA-MB-231 breast cancer cells and MDA-MB-435 melanoma cells have higher expression of beta-arrestin 1 mRNA compared with the nontumorigenic mammary MCF-10A cells. Moreover, we found that the mRNA levels of LPA1, LPA2, beta-arrestin 2, and Ral GTPases are elevated in the advanced stages of breast cancer. LPA stimulates the migration and invasion of MDA-MB-231 cells, but not of MCF-10A cells, and this is mediated by pertussis toxin-sensitive G proteins and LPA1. However, ectopic expression of LPA1 in MCF-10A cells caused these cells to acquire an invasive phenotype. Gene knockdown of either beta-arrestin or Ral proteins significantly impaired LPA-stimulated migration and invasion. Thus, our data show a novel role for beta-arrestin/Ral signaling in mediating LPA-induced breast cancer cell migration and invasion, two important processes in metastasis.

Guanine exchange factor RalGDS mediates exocytosis of Weibel-Palade bodies from endothelial cells

The small GTP-binding protein Ral has been implicated in regulated exocytosis via its interaction with the mammalian exocyst complex. We have previously demonstrated that Ral is involved in exocytosis of Weibel-Palade bodies (WPBs). Little is known about intracellular signaling pathways that promote activation of Ral in response to ligand binding of G protein-coupled receptors. Here we show that RNAi-mediated knockdown of RalGDS, an exchange factor for Ral, results in inhibition of thrombin- and epinephrine-induced exocytosis of WPBs, while overexpression of RalGDS promotes exocytosis of WPBs. A RalGDS variant lacking its exchange domain behaves in a dominant negative manner by blocking release of WPBs. We also provide evidence that RalGDS binds calmodulin (CaM) via an amino-terminal CaM-binding domain. RalGDS association to CaM is required for Ral activation because a cell-permeable peptide comprising this RalGDS CaM-binding domain inhibits Ral activation and WPB exocytosis. Together our findings suggest that RalGDS plays a vital role in the regulation of Ral-dependent WPB exocytosis after stimulation with Ca(2+)- or cAMP-raising agonists.

Small GTP-binding proteins and their regulators in cardiac hypertrophy

Small GTP-binding proteins (small G proteins) act as GDP-GTP-regulated molecular switches and are activated by guanine nucleotide exchange factors (GEFs) in response to diverse extracellular stimuli. During this last decade, numerous molecular and cellular studies, as well as genetically-modified animal models, have highlighted the role of small G proteins in the regulation of cardiac hypertrophy. The growing interest in small G protein signalling comes from the fact that chronic hypertrophic response is considered maladaptive and predisposes individuals to heart failure. Although some of the hypertrophic signalling pathways involving small G proteins have now been identified, a central question deals with the identity of the GEFs that modulate small G protein activation in the context of cardiac hypertrophy. Here, we discuss the precise regulation of Ras and Rho subfamilies of GTPases by GEFs and other regulatory proteins during cardiac hypertrophy. In addition, we summarize recent published data, mainly those describing the role of small G proteins in the development of myocardial hypertrophy and we further present the importance of their downstream effectors in myocardial remodelling.

Ras proteins: paradigms for compartmentalised and isoform-specific signalling

Ras GTPases mediate a wide variety of cellular processes by converting a multitude of extracellular stimuli into specific biological responses including proliferation, differentiation and survival. In mammalian cells, three ras genes encode four Ras isoforms (H-Ras, K-Ras4A, K-Ras4B and N-Ras) that are highly homologous but functionally distinct. Differences between the isoforms, including their post-translational modifications and intracellular sorting, mean that Ras has emerged as an important model system of compartmentalised signalling and membrane biology. Ras isoforms in different subcellular locations are proposed to recruit distinct upstream and downstream accessory proteins and activate multiple signalling pathways. Here, we summarise data relating to isoform-specific signalling, its role in disease and the mechanisms promoting compartmentalised signalling. Further understanding of this field will reveal the role of Ras signalling in development, cellular homeostasis and cancer and may suggest new therapeutic approaches.

Activated Ras induces cytoplasmic vacuolation and non-apoptotic death in glioblastoma cells via novel effector pathways

Expression of activated H-Ras induces a unique form of non-apoptotic cell death in human glioblastoma cells and other specific tumor cell lines. The major cytopathological features of this form of death are the accumulation of large phase-lucent, LAMP1-positive, cytoplasmic vacuoles. In this study we sought to determine if induction of cytoplasmic vacuolation a) depends on Ras farnesylation, b) is specific to H-Ras, and c) is mediated by signaling through the major known Ras effector pathways. We find that the unusual effects of activated H-Ras depend on farnesylation and membrane association of the GTPase. Both H-Ras(G12V) and K-Ras4B(G12V) stimulate vacuolation, but activated forms of Cdc42 and RhoA do not. Amino acid substitutions in the Ras effector domain, which are known to selectively impair its interactions with Raf kinase, class-I phosphatidylinositide 3-kinase (PI3K), or Ral nucleotide exchange factors, initially pointed to Raf as a possible mediator of cell vacuolation. However, the MEK inhibitor, PD98059, did not block the induction of vacuoles, and constitutively active Raf-Caax did not mimic the effects of Ras(G12V). Introduction of normal PTEN together with H-Ras(G12V) into U251 glioblastoma cells reduced the PI3K-dependent activation of Akt, but had no effect on vacuolation. Finally, co-expression of H-Ras(G12V) with a dominant-negative form of RalA did not suppress vacuolation. Taken together, the observations indicate that Ras activates non-conventional and perhaps unique effector pathways to induce cytoplasmic vacuolation in glioblastoma cells. Identification of the relevant signaling pathways may uncover specific molecular targets that can be manipulated to activate non-apoptotic cell death in this type of cancer.

mTOR-independent translational control of the extrinsic cell death pathway by RalA

Oncogenic potential is associated with translational regulation, and the prevailing view is that oncogenes use mTOR-dependent pathways to up-regulate the synthesis of proteins critical for transformation. In this study, we show that RalA, a key mediator of Ras transformation, is also linked to the translational machinery. At least part of this linkage, however, is independent of mTOR and acts through RalBP1 to suppress cdc42-mediated activation of S6 kinase and the translation of the antiapoptotic protein FLIP(S). This action, rather than contributing to transformation, opens a latent tumor-suppressive mechanism that can be activated by tumor necrosis factor-related apoptosis-inducing ligand. These results show that the translational machinery is linked to tumor suppression as well as cell-proliferative pathways and that the reestablishment of cell death pathways by activation of the Ral oncogenic program provides a means for selective therapeutic targeting of Ral-driven malignancies.

Oog1, an oocyte-specific protein, interacts with Ras and Ras-signaling proteins during early embryogenesis

We previously identified an oocyte-specific gene, Oogenesin 1 (Oog1), that encodes 326 amino acids containing a leucine zipper structure and a leucine-rich repeat. In the present study, to identify the interacting proteins of Oog1, we performed a yeast two-hybrid screening using a GV-oocyte cDNA library and found that Ral guanine nucleotide dissociation stimulator (RalGDS) is the binding partner of Oog1. Coimmunoprecipitation assay confirmed the interaction between Oog1 and RalGDS proteins. Colocalization experiments provide the evidence that the nuclear localization of RalGDS depends on the expression of Oog1. Interestingly, RalGDS protein localized in the nucleus rather than the cytoplasm between late 1-cell and early 2-cell stages, the time when Oog1 localizes in the nucleus. We also examined the interaction between Oog1 and Ras by GST pull-down assay and revealed that Oog1 interacts with Ras in a GTP-dependent manner. These findings suggest a role of Oog1 as a Ras-binding protein.

The RAP1 guanine nucleotide exchange factor Epac2 couples cyclic AMP and Ras signals at the plasma membrane

Epac-1 and -2 (exchange proteins directly activated by cyclic AMP) are guanine-nucleotide exchange factors for the GTPases Rap1 and -2. Epac2 but not Epac1 was found to possess a RA (Ras association) domain similar to that found in the Ras effector Ral-GDS. This domain specifically bound Ras-GTP, enabling oncogenic Ras to translocate Epac2 from the cytosol to the plasma membrane. Consequently, a small pool of plasma membrane-bound Rap1 was activated at the expense of bulk Rap1 located on intracellular organelles. Whereas translocation of Epac2 was not mimicked by challenge with epidermal growth factor alone, costimulation with forskolin, prostaglandin E2, or an Epac-selective cyclic AMP analog-induced rapid relocation of GFP-Epac2 but not -Epac1 to the plasma membrane in a Ras-dependent manner. Deletion of the cyclic AMP-binding domain overcame the need for nucleotide, suggesting that this domain normally masked the RA domain in the resting GEF. Thus, Epac2 can respond to costimulation by agonists that jointly elevate Ras-GTP and cyclic AMP levels, activating a specific pool of Rap1 at the plasma membrane. Therefore, despite its previous description as a Ras antagonist or independently functioning GTPase, Rap1/Krev-1 may additionally act downstream of Ras in cells that express the cyclic AMP-regulated GEF, Epac2.

Activation of RalA is critical for Ras-induced tumorigenesis of human cells

RalGEFs were recently shown to be critical for Ras-mediated transformed and tumorigenic growth of human cells. We now show that the oncogenic activity of these proteins is propagated by activation of one RalGEF substrate, RalA, but blunted by another closely related substrate, RalB, and that the oncogenic signaling requires binding of the RalBP1 and exocyst subunit effector proteins. Knockdown of RalA expression impeded, if not abolished, the ability of human cancer cells to form tumors. RalA was also commonly activated in a panel of cell lines from pancreatic cancers, a disease characterized by activation of Ras. Activation of RalA signaling thus appears to be a critical step in Ras-induced transformation and tumorigenesis of human cells.

Bioluminescence resonance energy transfer identify scaffold protein CNK1 interactions in intact cells

Connector enhancer of KSR (CNK) proteins have been proposed to act as scaffolds in the Ras-MAPK pathway. In this work, using in vivo bioluminescence resonance energy transfer (BRET) assays and in vitro co-immunoprecipitation, we show that hCNK1 interacts with the active form of Rho A (G14V) proteins. The domain of hCNK1 that allows binding to Rho proteins involves the C-terminal PH domain. Overexpression of hCNK1 does not affect the actin cytoskeleton and does not modify the appearance of stress fibers in cells overexpressing a constitutively active form of RhoA. In contrast, hCNK1 was able to significantly decrease the RhoA-induced transcriptional activity of the serum response element (SRE) without effect on the Ras-induced SRE activation. These results identify hCNK1 as a specific partner of Rho proteins both in vitro and in vivo and suggest a role of hCNK1 in the signal transduction of Rho proteins.

Identification of a novel domain of Ras and Rap1 that directs their differential subcellular localizations

The small GTPase Ha-Ras and Rap1A exhibit high mutual sequence homology and share various target proteins. However, they exert distinct biological functions and exhibit differential subcellular localizations; Rap1A is predominantly localized in the perinuclear region including the Golgi apparatus and endosomes, whereas Ha-Ras is predominantly localized in the plasma membrane. Here, we have identified a small region in Rap1A that is crucial for its perinuclear localization. Analysis of a series of Ha-Ras-Rap1A chimeras shows that Ha-Ras carrying a replacement of amino acids 46-101 with that of Rap1 exhibits the perinuclear localization. Subsequent mutational studies indicate that Rap1A-type substitutions within five amino acids at positions 85-89 of Ha-Ras, such as NNTKS85-89TAQST, NN85-86TA, and TKS87-89QST, are sufficient to induce the perinuclear localization of Ha-Ras. In contrast, substitutions of residues surrounding this region, such as FAI82-84YSI and FEDI90-93FNDL, have no effect on the plasma membrane localization of Ha-Ras. A chimeric construct consisting of amino acids 1-134 of Rap1A and 134-189 of Ha-Ras, which harbors both the palmitoylation and farnesylation sites of Ha-Ras, exhibits the perinuclear localization like Rap1A. Introduction of a Ha-Ras-type substitution into amino acids 85-89 (TAQST85-89NNTKS) of this chimeric construct causes alteration of its predominant subcellular localization site from the perinuclear region to the plasma membrane. These results indicate that a previously uncharacterized domain spanning amino acids 85-89 of Rap1A plays a pivotal role in its perinuclear localization. Moreover, this domain acts dominantly over COOH-terminal lipid modification of Ha-Ras, which has been considered to be essential and sufficient for the plasma membrane localization.

Control of embryonicXenopusmorphogenesis by a Ral-GDS/Xral branch of the Ras signalling pathway

Ras proteins mediate biological responses through various effectors and play a key role in relaying the Fibroblast Growth Factor (FGF) mesoderm induction signal during embryogenesis of the frog, Xenopus laevis. One Ras effector pathway involves the activation of the small G protein Ral. In the present study, we have investigated the role of key components in the Ral branch of FGF and Ras signalling during early Xenopus development. Treatment of animal caps with bFGF, which converts prospective ectoderm to mesoderm, activates Xral. The Ras mutant 12V37G, which can bind to Ral-GDS but not Raf, also activates Xral as well as causing developmental defects and cortical F-actin disassembly. A similar phenotype is induced by Ral-GDS itself. FGF-induced expression of several signature mesodermal genes, by contrast, is independent of Xral signalling. This and other data suggest that the RalB branch of Ras and FGF signalling regulates the actin cytoskeleton and morphogenesis in a transcriptionally independent manner. We also find Xral to be specifically activated in the marginal zone of Xenopus embryos, and find that disruption of the Ral pathway in this region prevents closure of the blastopore during gastrulation. We conclude that Ral signalling is autonomously required by mesodermal cells to effect essential morphogenetic changes during Xenopus gastrulation.

Ral-GTPases: approaching their 15 minutes of fame

Andy Warhol, the famous pop artist, once claimed that "in the future everyone will be famous for 15 minutes". The same, it seems, can be said of proteins, because at any given time some proteins become more "fashionable" to study than others. But most proteins have been highly conserved throughout millions of years of evolution, which implies that they all have essential roles in cell biology. Thus, each one will no doubt enter the limelight if the right experiment in the right cell type is done. A good example of this is the Ras-like GTPases (Ral-GTPases), which until recently existed in the shadow of their close cousins--the Ras proto-oncogenes. Recent studies have yielded insights into previously unappreciated roles for Ral-GTPases in intensively investigated disciplines such as vesicle trafficking, cell morphology, transcription and possibly even human oncogenesis.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

Ral GDP dissociation stimulator and Ral GTPase are involved in myocardial hypertrophy

Ras-related GTPase (Ral) is converted to the GTP-bound form by Ral GDP dissociation stimulator (Ral-GDS), a putative effector protein of Ras. Although a number of studies indicate that Ras induces cardiac hypertrophy, the functional role of Ral-GDS/Ral signaling pathway is as yet unknown in cardiac myocytes. We investigated the role of the Ral-GDS/Ral pathway in cardiac hypertrophy. Transfection of Ral-GDS and constitutively active mutant of Ral (RalG23V) in cultured rat neonatal myocytes stimulated promoter activity of c-fos (5.4-fold and 2.6-fold, P<0.01), alpha-skeletal actin (2.7-fold and 2.1-fold, P<0.01), and beta-myosin heavy chain-luciferase (2.8-fold and 2.3-fold, P<0.01). Ral-GDS-induced or RalG23V-induced promoter activation was increased synergistically with activated Ras (RasG12V). Dominant-negative mutant of Ral (RalS28N) partially inhibited RasG12V induced promoter activation. Cardiac myocytes transfected with RalG23V showed increased cell size compared with nontransfected or vector-transfected cells (2.1-fold, P<0.01). Cardiotrophin-1 (CT-1) upregulated Ral-GDS mRNA expression and induced Ral activation. CT-1-induced Ral-GDS mRNA expression was inhibited by overexpression of the dominant-negative mutant of STAT3. Moreover, Ral activity was elevated in hypertrophied hearts (2.1-fold, P<0.01) by mechanical stress in association with increased CT-1 expression and signal transducer and activator of transcription 3 (STAT3) phosphorylation in the rat aortic banding model. Ral-GDS/Ral pathway is involved in a wide range of gene expressions and is activated by hypertrophic stimuli in vitro and in vivo. SATA3 may play a key role in Ral-GDS expression and Ral activation. Our data provide evidence that the Ral-GDS/Ral signaling pathway is a link to the process of cardiac hypertrophy.

Mechanism of the spatio-temporal regulation of Ras and Rap1

Epidermal growth factor (EGF) activates Ras and Rap1 at distinct intracellular regions. Here, we explored the mechanism underlying this phenomenon. We originally noticed that in cells expressing Epac, a cAMP-dependent Rap1 GEF (guanine nucleotide exchange factor), cAMP activated Rap1 at the perinuclear region, as did EGF. However, in cells expressing e-GRF, a recombinant cAMP-responsive Ras GEF, cAMP activated Ras at the peripheral plasma membrane. Based on the uniform cytoplasmic expression of Epac and e-GRF, GEF did not appear to account for the non-uniform increase in the activities of Ras and Rap1. In contrast, when we used probes with reduced sensitivity to GTPase-activating proteins (GAPs), both Ras and Rap1 appeared to be activated uniformly in the EGF-stimulated cells. Furthermore, we calculated the local rate constants of GEFs and GAPs from the video images of Ras activation and found that GAP activity was higher at the central plasma membrane than the periphery. Thus we propose that GAP primarily dictates the spatial regulation of Ras family G proteins, whereas GEF primarily determines the timing of Ras activation.

A Ral guanine exchange factor-Ral pathway is conserved in Drosophila melanogaster and sheds new light on the connectivity of the Ral, Ras, and Rap pathways

Ras GTPases are central to many physiological and pathological signaling pathways and act via a combination of effectors. In mammals, at least three Ral exchange factors (RalGEFs) contain a Ras association domain and constitute a discrete subgroup of Ras effectors. Despite their ability to bind activated Rap as well as activated Ras, they seem to act downstream of Ras but not downstream of Rap. We have revisited the Ras/Rap-Ral connections in Drosophila melanogaster by using iterative two-hybrid screens with these three GTPases as primary baits and a subsequent genetic approach. We show that (i) the Ral-centered protein network appears to be extremely conserved in human and flies, (ii) in this network, RGL is a functional Drosophila orthologue of RalGEFs, and (iii) the RGL-Ral pathway functionally interacts with both the Ras and Rap pathways. Our data do not support the paradigmatic model where Ral is in the effector pathway of Ras. They reveal a signaling circuitry where Ral is functionally downstream of the Rap GTPase, at odds with the pathways described for mammalian cell lines. Thus, in vivo data show variations in the connectivity of pathways described for cell lines which might display only a subset of the biological possibilities.

A growing family of guanine nucleotide exchange factors is responsible for activation of Ras-family GTPases

GTPases of the Ras subfamily regulate a diverse array of cellular-signaling pathways, coupling extracellular signals to the intracellular response machinery. Guanine nucleotide exchange factors (GEFs) are primarily responsible for linking cell-surface receptors to Ras protein activation. They do this by catalyzing the dissociation of GDP from the inactive Ras proteins. GTP can then bind and induce a conformational change that permits interaction with downstream effectors. Over the past 5 years, approximately 20 novel Ras-family GEFs have been identified and characterized. These data indicate that a variety of different signaling mechanisms can be induced to activate Ras, enabling tyrosine kinases, G-protein-coupled receptors, adhesion molecules, second messengers, and various protein-interaction modules to relocate and/or activate GEFs and elevate intracellular Ras-GTP levels. This review discusses the structure and function of the catalytic or CDC25 homology domain common to almost all Ras-family GEFs. It also details our current knowledge about the regulation and function of this rapidly growing family of enzymes that include Sos1 and 2, GRF1 and 2, CalDAG-GEF/GRP1-4, C3G, cAMP-GEF/Epac 1 and 2, PDZ-GEFs, MR-GEF, RalGDS family members, RalGPS, BCAR3, Smg GDS, and phospholipase C(epsilon).

Differential roles of Ras and Rap1 in growth factor-dependent activation of phospholipase C epsilon

Phospholipase C epsilon is a phosphoinositide-specific phospholipase C that selectively associates with Ras and Rap small GTPases as a target. Here we explored the molecular basis of the Rap1- as well as Ras-mediated regulation of phospholipase C epsilon upon platelet-derived growth factor stimulation by using a receptor mutant deficient in its ability to phosphorylate and activate phospholipase C gamma. Following platelet-derived growth factor treatment, this receptor induces persistent activation of ectopically expressed PLC epsilon through activation of Ras and Rap1. The rapid and initial phase of the activation is mediated by Ras, whereas Rap1 is responsible for the prolonged activation. We further demonstrate that the CDC25 homology domain, which exhibits guanine nucleotide exchange factor activity toward Rap1, but not Ras, is critical for the prolonged activation of phospholipase C epsilon. Platelet-derived growth factor prevented the hematopoietic BaF3 cells containing the mutant receptor from undergoing apoptosis, and enabled these cells to proliferate, only when phospholipase C epsilon was expressed. Therefore, the phospholipase C signal is suggested to be critical for survival and growth of BaF3 cells.

Exoenzyme S shows selective ADP-ribosylation and GTPase-activating protein (GAP) activities towards small GTPases in vivo

Intracellular targeting of the Pseudomonas aeruginosa toxins exoenzyme S (ExoS) and exoenzyme T (ExoT) initially results in disruption of the actin microfilament structure of eukaryotic cells. ExoS and ExoT are bifunctional cytotoxins, with N-terminal GTPase-activating protein (GAP) and C-terminal ADP-ribosyltransferase activities. We show that ExoS can modify multiple GTPases of the Ras superfamily in vivo. In contrast, ExoT shows no ADP-ribosylation activity towards any of the GTPases tested in vivo. We further examined ExoS targets in vivo and observed that ExoS modulates the activity of several of these small GTP-binding proteins, such as Ras, Rap1, Rap2, Ral, Rac1, RhoA and Cdc42. We suggest that ExoS is the major ADP-ribosyltransferase protein modulating small GTPase function encoded by P. aeruginosa. Furthermore, we show that the GAP activity of ExoS abrogates the activation of RhoA, Cdc42 and Rap1.

Beta-arrestins regulate a Ral-GDS Ral effector pathway that mediates cytoskeletal reorganization

beta-Arrestins are important in chemoattractant receptor-induced granule release, a process that may involve Ral-dependent regulation of the actin cytoskeleton. We have identified the Ral GDP dissociation stimulator (Ral-GDS) as a beta-arrestin-binding protein by yeast two-hybrid screening and co-immunoprecipitation from human polymorphonuclear neutrophilic leukocytes (PMNs). Under basal conditions, Ral-GDS is localized to the cytosol and remains inactive in a complex formed with beta-arrestins. In response to formyl-Met-Leu-Phe (fMLP) receptor stimulation, beta-arrestin Ral-GDS protein complexes dissociate and Ral-GDS translocates with beta-arrestin from the cytosol to the plasma membrane, resulting in the Ras-independent activation of the Ral effector pathway required for cytoskeletal rearrangement. The subsequent re-association of beta-arrestin Ral-GDS complexes is associated with the inactivation of Ral signalling. Thus, beta-arrestins regulate multiple steps in the Ral-dependent processes that result in chemoattractant-induced cytoskeletal reorganization.

Involvement of phosphatidylinositol 3-kinase, but not RalGDS, in TC21/R-Ras2-mediated transformation

Oncogenic Ras and activated forms of the Ras-related protein TC21/R-Ras2 share similar abilities to alter cell proliferation. However, in contrast to Ras, we found previously that TC21 fails to activate the Raf-1 serine/threonine kinase. Thus, TC21 must utilize non-Raf effectors to regulate cell function. In this study, we determined that TC21 interacts strongly with some (RalGDS, RGL, RGL2/Rlf, AF6, and the phosphatidylinositol 3-kinase (PI3K) catalytic subunit p110delta), and weakly with other Ras small middle dotGTP-binding proteins. In addition, library screening identified novel TC21-interacting proteins. We also determined that TC21, similar to Ras, mediates activation of phospholipase Cepsilon. We then examined if RalGDS, a RalA guanine nucleotide exchange factor, or PI3K are effectors for TC21-mediated signaling and cell proliferation in murine fibroblasts. We found that overexpression of full-length RalGDS reduced the focus forming activity of activated TC21. Furthermore, expression of activated Ras, but not TC21, enhanced GTP loading on RalA. In fact, TC21 attenuated insulin-stimulated RalA small middle dotGTP formation. In contrast, like Ras, expression of activated TC21 resulted in membrane translocation and an increase in the PI3K-dependent phosphorylation of Akt, and inhibition of PI3K activity interfered with TC21 focus formation. Finally, unlike Ras, TC21 did not activate the Rac small GTPase, indicating that Ras may not activate Rac by PI3K. Taken together, these results suggest that PI3K, but not RalGDS, is an important mediator of cell proliferation by TC21.

PDK1 mediates growth factor-induced Ral-GEF activation by a kinase-independent mechanism

Ras proteins transduce extracellular signals to intracellular signaling pathways by binding to and promoting the activation of at least three classes of downstream signaling molecules: Raf kinases, phosphoinositide-3-kinase (PI3-K) and Ral guanine nucleotide exchange factors (Ral-GEFs). Previous work has demonstrated that epidermal growth factor (EGF) activates Ral-GEFs, at least in part, by a Ras-mediated redistribution of the GEFs to their target, Ral-GTPases, in the plasma membrane. Here we show that Ral-GEF stimulation by EGF involves an additional mechanism, PI3-K-dependent kinase 1 (PDK1)-induced enhancement of Ral-GEF catalytic activity. Remarkably, this PDK1 function is not dependent upon its kinase activity. Instead, the non-catalytic N-terminus of PDK1 mediates the formation of an EGF-induced complex with the N-terminus of the Ral-GEF, Ral-GDS, thereby relieving its auto-inhibitory effect on the catalytic domain of Ral-GDS. These results elucidate a novel function for PDK1 and demonstrate that two Ras effector pathways cooperate to promote Ral-GTPase activation.