Mammalian expression vectors for Ras family proteins: generation and use of expression constructs to analyze Ras family function

TRIP13 protects pancreatic cancer cells against intrinsic and therapy-induced DNA replication stress

Oncogene activation in normal untransformed cells induces DNA replication stress and creates a dependency on DNA damage response (DDR) mechanisms for cell survival. Different oncogenic stimuli signal via distinct mechanisms in every cancer setting. The DDR is also pathologically reprogrammed and deployed in diverse ways in different cancers. Because mutant KRAS is the driver oncogene in 90% of pancreatic ductal adenocarcinomas (PDACs), here we have investigated DDR mechanisms by which KRAS-induced DNA replication stress is tolerated in normal human pancreatic epithelial cells [human pancreatic nestin-expressing (HPNE) cells]. Using a candidate screening approach, we identify TRIP13 as a KRASG12V-induced messenger RNA that is also expressed at high levels in PDAC relative to normal tissues. Using genetic and pharmacological tools, we show that TRIP13 is necessary to sustain ongoing DNA synthesis and viability specifically in KRASG12V-expressing cells. TRIP13 promotes survival of KRASG12V-expressing HPNE cells in a homologous recombination (HR)-dependent manner. KRASG12V-expressing HPNE cells lacking TRIP13 acquire hallmark HR deficiency phenotypes, including sensitivity to inhibitors of translesion synthesis and poly-ADP ribose polymerase. Established PDAC cell lines are also sensitized to intrinsic DNA damage and therapy-induced genotoxicity following TRIP13 depletion. Taken together, our results expose TRIP13 as an attractive new and therapeutically tractable vulnerability of KRAS-mutant PDAC.

TRIP13 protects pancreatic cancer cells against intrinsic and therapy-induced DNA replication stress

Oncogene activation in normal untransformed cells induces DNA replication stress and creates a dependency on DNA Damage Response (DDR) mechanisms for cell survival. Different oncogenic stimuli signal via distinct mechanisms in every cancer setting. The DDR is also pathologically re-programmed and deployed in diverse ways in different cancers. Because mutant KRAS is the driver oncogene in 90% of Pancreatic Ductal Adenocarcinomas (PDAC), here we have investigated DDR mechanisms by which KRAS-induced DNA replication stress is tolerated in normal human pancreatic epithelial cells (HPNE). Using a candidate screening approach, we identify TRIP13 as a KRASG12V-induced mRNA that is also expressed at high levels in PDAC relative to normal tissues. Using genetic and pharmacological tools, we show that TRIP13 is necessary to sustain ongoing DNA synthesis and viability specifically in KRASG12V-expressing cells. TRIP13 promotes survival of KRASG12V-expressing HPNE cells in a Homologous Recombination (HR)-dependent manner. KRASG12V-expressing HPNE cells lacking TRIP13 acquire hallmark HR-deficiency (HRD) phenotypes including sensitivity to inhibitors of Trans-Lesion Synthesis (TLS) and Poly-ADP Ribose Polymerase (PARP). Established PDAC cell lines are also sensitized to intrinsic DNA damage and therapy-induced genotoxicity following TRIP13-depletion. Taken together our results expose TRIP13 as an attractive new and therapeutically-tractable vulnerability of KRAS-mutant PDAC.

PI3K Functions Downstream of Cdc42 to Drive Cancer phenotypes in a Melanoma Cell Line

Rho proteins are part of the Ras superfamily, which function to modulate cytoskeletal dynamics including cell adhesion and motility. Recently, an activating mutation in Cdc42, a Rho family GTPase, was found in a patient sample of melanoma. Previously, our work had shown the PI3K was important downstream of mutationally active Cdc42. Our present study sought to determine whether PI3K was a crucial downstream partner for Cdc42 in a melanoma cells line with a BRAF mutation, which is the most common mutation in cutaneous melanoma. In this work we were able to show that Cdc42 contributes to proliferation, anchorage-independent growth, cell motility and invasion. Treatment with a pan-PI3K inhibitor was able to effectively ameliorate all these cancer phenotypes. These data suggest that PI3K may be an important target downstream of Cdc42 in melanoma.

The X-Linked Intellectual Disability Gene Zdhhc9 Is Essential for Dendrite Outgrowth and Inhibitory Synapse Formation

Palmitoylation is a reversible post-translational lipid modification that facilitates vesicular transport and subcellular localization of modified proteins. This process is catalyzed by ZDHHC enzymes that are implicated in several neurological and neurodevelopmental disorders. Loss-of-function mutations in ZDHHC9 have been identified in patients with X-linked intellectual disability (XLID) and associated with increased epilepsy risk. Loss of Zdhhc9 function in hippocampal cultures leads to shorter dendritic arbors and fewer inhibitory synapses, altering the ratio of excitatory-to-inhibitory inputs formed onto Zdhhc9-deficient cells. While Zdhhc9 promotes dendrite outgrowth through the palmitoylation of the GTPase Ras, it promotes inhibitory synapse formation through the palmitoylation of another GTPase, TC10. Zdhhc9 knockout mice exhibit seizure-like activity together with increased frequency and amplitude of both spontaneous and miniature excitatory and inhibitory postsynaptic currents. These findings present a plausible mechanism for how the loss of ZDHHC9 function may contribute to XLID and epilepsy.

Endothelial Palmitoylation Cycling Coordinates Vessel Remodeling in Peripheral Artery Disease

RATIONALE

Peripheral artery disease, common in metabolic syndrome and diabetes mellitus, responds poorly to medical interventions and is characterized by chronic vessel immaturity leading to lower extremity amputations.

OBJECTIVE

To define the role of reversible palmitoylation at the endothelium in the maintenance of vascular maturity.

METHODS AND RESULTS

Endothelial knockout of the depalmitoylation enzyme APT-1 (acyl-protein thioesterase 1) in mice impaired recovery from chronic hindlimb ischemia, a model of peripheral artery disease. Endothelial APT-1 deficiency decreased fibronectin processing, disrupted adherens junctions, and inhibited in vitro lumen formation. In an unbiased palmitoylation proteomic screen of endothelial cells from genetically modified mice, R-Ras, known to promote vessel maturation, was preferentially affected by APT-1 deficiency. R-Ras was validated as an APT-1 substrate, and click chemistry analyses demonstrated increased R-Ras palmitoylation in cells with APT-1 deficiency. APT-1 enzyme activity was decreased in endothelial cells from db/db mice. Hyperglycemia decreased APT-1 activity in human umbilical vein endothelial cells, due, in part, to altered acetylation of the APT-1 protein. Click chemistry analyses demonstrated increased R-Ras palmitoylation in the setting of hyperglycemia. Altered R-Ras trafficking, increased R-Ras palmitoylation, and fibronectin retention were found in diabetes mellitus models. Loss of R-Ras depalmitoylation caused by APT-1 deficiency constrained R-Ras membrane trafficking, as shown by total internal reflection fluorescence imaging. To rescue cellular phenotypes, we generated an R-Ras molecule with an inserted hydrophilic domain to circumvent membrane rigidity caused by defective palmitoylation turnover. This modification corrected R-Ras membrane trafficking, restored fibronectin processing, increased adherens junctions, and rescued defective lumen formation induced by APT-1 deficiency.

CONCLUSIONS

These results suggest that endothelial depalmitoylation is regulated by the metabolic milieu and controls plasma membrane partitioning to maintain vascular homeostasis.

Endfoot regeneration restricts radial glial state and prevents translocation into the outer subventricular zone in early mammalian brain development

Neural stem cells, called radial glia, maintain epithelial structure during the early neocortical development. The prevailing view claims that when radial glia first proliferate, their symmetric divisions require strict spindle orientation; its perturbation causes precocious neurogenesis and apoptosis. Here, we show that despite this conventional view, radial glia at the proliferative stage undergo normal symmetric divisions by regenerating an apical endfoot even if it is lost by oblique divisions. We found that the Notch-R-Ras-integrin β1 pathway promotes the regeneration of endfeet, whose leading edge bears ectopic adherens junctions and the Par-polarity complex. However, this regeneration ability gradually declines during the subsequent neurogenic stage and hence oblique divisions induce basal translocation of radial glia to form the outer subventricular zone, a hallmark of the development of the convoluted brain. Our study reveals that endfoot regeneration is a temporally changing cryptic property, which controls the radial glial state and its shift is essential for mammalian brain size expansion.

Small GTPase R-Ras participates in neural tube formation in zebrafish embryonic spinal cord

Ras related (R-Ras), a small GTPase, is involved in the maintenance of apico-basal polarity in neuroepithelial cells of the zebrafish hindbrain, axonal collapse in cultured murine hippocampal neurons, and maturation of blood vessels in adult mice. However, the role of R-Ras in neural tube formation remains unknown. Using antisense morpholino oligonucleotides (AMOs), we found that in the spinal cord of zebrafish embryos, the lumen was formed bilaterally in rras morphants, whereas it was formed at the midline in control embryos. As AMO can cause off-target effects, we generated rras mutant zebrafish lines using CRISPR/Cas9 technology. Although these rras mutant embryos did not have a bilateral lumen in the spinal cord, the following findings suggest that the phenotype is unlikely due to an off-target effect of rras AMO: 1) The rras morphant phenotype was rescued by an injection of AMO-resistant rras mRNA, and 2) a bilaterally segregated spinal cord was not observed in rras mutant embryos injected with rras AMO. The results suggest that the function of other ras family genes may be redundant in rras mutants. Previous research reported a bilaterally formed lumen in the spinal cord of zebrafish embryos with a mutation in a planar cell polarity (PCP) gene, van gogh-like 2 (vangl2). In the present study, in cultured cells, R-Ras was co-immunoprecipitated with Vangl2 but not with another PCP regulator, Pricke1. Interestingly, the interaction between R-Ras and Vangl2 was stronger in guanine-nucleotide free point mutants of R-Ras than in wild-type or constitutively active (GTP-bound) forms of R-Ras. R-Ras may regulate neural tube formation in cooperation with Vangl2 in the developing zebrafish spinal cord.

NRAS destines tumor cells to the lungs

The lungs are frequently affected by cancer metastasis. Although NRAS mutations have been associated with metastatic potential, their exact role in lung homing is incompletely understood. We cross-examined the genotype of various tumor cells with their ability for automatic pulmonary dissemination, modulated NRAS expression using RNA interference and NRAS overexpression, identified NRAS signaling partners by microarray, and validated them using Cxcr1- and Cxcr2-deficient mice. Mouse models of spontaneous lung metastasis revealed that mutant or overexpressed NRAS promotes lung colonization by regulating interleukin-8-related chemokine expression, thereby initiating interactions between tumor cells, the pulmonary vasculature, and myeloid cells. Our results support a model where NRAS-mutant, chemokine-expressing circulating tumor cells target the CXCR1-expressing lung vasculature and recruit CXCR2-expressing myeloid cells to initiate metastasis. We further describe a clinically relevant approach to prevent NRAS-driven pulmonary metastasis by inhibiting chemokine signaling. In conclusion, NRAS promotes the colonization of the lungs by various tumor types in mouse models. IL-8-related chemokines, NRAS signaling partners in this process, may constitute an important therapeutic target against pulmonary involvement by cancers of other organs.

PIK3CA hotspot mutations differentially impact responses to MET targeting in MET-driven and non-driven preclinical cancer models

Background

The MET receptor tyrosine kinase represents a promising target in cancer. PIK3CA activating mutations are common in several tumor types and can potentially confer resistance to anti-receptor tyrosine kinase therapy.

Methods

MET and/or PI3K pathway inhibition was assessed in NIH3T3 cells harboring MET-activating point mutation with or without ectopic expression of PIK3CA^E545K and PIK3CA^H1047R, as well as in MET-expressing head and neck cancer cells with endogenous PIK3CA mutations. Endpoints included PI3K pathway activation, cell proliferation, colony-forming ability, cell death, wound-healing, and an in vivo model.

Results

PIK3CA^E545K and PIK3CA^H1047R confer resistance to MET inhibition in MET-driven models. PIK3CA^H1047R was more potent than PIK3CA^E545K at inducing resistance in PI3K pathway activation, cell proliferation, colony-forming ability, induction of cell death and wound-healing upon MET inhibition. Resistance to MET inhibition could be synergistically overcome by co-targeting PI3K. Furthermore, combined MET/PI3K inhibition led to enhanced anti-tumor activity in vivo in tumors harboring PIK3CA^H1047R. In head and neck cancer cells the combination of MET/PI3K inhibitors led to more-than-additive effects.

Conclusions

PIK3CA mutations can lead to resistance to MET inhibition, supporting future clinical evaluation of combinations of PI3K and MET inhibitors in common scenarios of malignant neoplasms featuring aberrant MET expression and PIK3CA mutations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-017-0660-5) contains supplementary material, which is available to authorized users.

Cellular model of neuronal atrophy induced by DYNC1I1 deficiency reveals protective roles of RAS-RAF-MEK signaling

Neuronal atrophy is a common pathological feature occurred in aging and neurodegenerative diseases. A variety of abnormalities including motor protein malfunction and mitochondrial dysfunction contribute to the loss of neuronal architecture; however, less is known about the intracellular signaling pathways that can protect against or delay this pathogenic process. Here, we show that the DYNC1I1 deficiency, a neuron-specific dynein intermediate chain, causes neuronal atrophy in primary hippocampal neurons. With this cellular model, we are able to find that activation of RAS-RAF-MEK signaling protects against neuronal atrophy induced by DYNC1I1 deficiency, which relies on MEK-dependent autophagy in neuron. Moreover, we further reveal that BRAF also protects against neuronal atrophy induced by mitochondrial impairment. These findings demonstrate protective roles of the RAS-RAF-MEK axis against neuronal atrophy, and imply a new therapeutic target for clinical intervention.

VPS35 binds farnesylated N-Ras in the cytosol to regulate N-Ras trafficking

Ras guanosine triphosphatases (GTPases) regulate signaling pathways only when associated with cellular membranes through their C-terminal prenylated regions. Ras proteins move between membrane compartments in part via diffusion-limited, fluid phase transfer through the cytosol, suggesting that chaperones sequester the polyisoprene lipid from the aqueous environment. In this study, we analyze the nature of the pool of endogenous Ras proteins found in the cytosol. The majority of the pool consists of farnesylated, but not palmitoylated, N-Ras that is associated with a high molecular weight (HMW) complex. Affinity purification and mass spectrographic identification revealed that among the proteins found in the HMW fraction is VPS35, a latent cytosolic component of the retromer coat. VPS35 bound to N-Ras in a farnesyl-dependent, but neither palmitoyl- nor guanosine triphosphate (GTP)-dependent, fashion. Silencing VPS35 increased N-Ras's association with cytoplasmic vesicles, diminished GTP loading of Ras, and inhibited mitogen-activated protein kinase signaling and growth of N-Ras-dependent melanoma cells.

The GTP-bound and Sumoylated Form of the rab17 Small Molecular Weight GTPase Selectively Binds Syntaxin 2 in Polarized Hepatic WIF-B Cells

A major focus for our laboratory is identifying the molecules and mechanisms that regulate polarized apical protein sorting in hepatocytes, the major epithelial cells of the liver. These trafficking pathways are regulated, in part, by small molecular weight rab GTPases. We chose to investigate rab17, whose expression is restricted to polarized epithelial cells, is enriched in liver, and has been implicated in regulating basolateral to apical transcytosis. To initiate our studies, we generated three recombinant adenoviruses expressing wild type, constitutively active (GTP bound), or dominant-negative (GDP bound) rab17. Immunoblotting revealed rab17 immunoreactive species at 25 kDa (the predicted rab17 molecular mass) and 40 kDa. We determined that mono-sumoylation of the 25-kDa rab17 is responsible for the shift in molecular mass, and that rab17 prenylation is required for sumoylation. We further determined that sumoylation selectively promotes interactions with syntaxin 2 (but not syntaxins 3 or 4) and that these interactions are nucleotide dependent. Furthermore, a K68R-mutated rab17 led to the redistribution of syntaxin 2 and 5' nucleotidase from the apical membrane to subapical puncta, whereas multidrug resistance protein 2 distributions were not changed. Together these data are consistent with the proposed role of rab17 in vesicle fusion with the apical plasma membrane and further implicate sumoylation as an important mediator of protein-protein interactions. The selectivity in syntaxin binding and apical protein redistribution further suggests that rab17 and syntaxin 2 mediate fusion of transcytotic vesicles at the apical surface.

The differential palmitoylation states of N-Ras and H-Ras determine their distinct Golgi subcompartment localizations

Despite a high degree of structural homology and shared exchange factors, effectors and GTPase activating proteins, a large body of evidence suggests functional heterogeneity among Ras isoforms. One aspect of Ras biology that may explain this heterogeneity is the differential subcellular localizations driven by the C-terminal hypervariable regions of Ras proteins. Spatial heterogeneity has been documented at the level of organelles: palmitoylated Ras isoforms (H-Ras and N-Ras) localize on the Golgi apparatus whereas K-Ras4B does not. We tested the hypothesis that spatial heterogeneity also exists at the sub-organelle level by studying the localization of differentially palmitoylated Ras isoforms within the Golgi apparatus. Using confocal, live-cell fluorescent imaging and immunogold electron microscopy we found that, whereas the doubly palmitoylated H-Ras is distributed throughout the Golgi stacks, the singly palmitoylated N-Ras is polarized with a relative paucity of expression on the trans Golgi. Using palmitoylation mutants, we show that the different sub-Golgi distributions of the Ras proteins are a consequence of their differential degree of palmitoylation. Thus, the acylation state of Ras proteins controls not only their distribution between the Golgi apparatus and the plasma membrane, but also their distribution within the Golgi stacks.

Cell cycle restriction is more important than apoptosis induction for RASSF1A protein tumor suppression

The Ras association domain family protein 1A (RASSF1A) is arguably one of the most frequently inactivated tumor suppressors in human cancer. RASSF1A modulates apoptosis via the Hippo and Bax pathways but also modulates the cell cycle. In part, cell cycle regulation appears to be dependent upon the ability of RASSF1A to complex with microtubules and regulate their dynamics. Which property of RASSF1A, apoptosis induction or microtubule regulation, is responsible for its tumor suppressor function is not known. We have identified a short conserved motif that is essential for the binding of RASSF family proteins with microtubule-associated proteins. By making a single point mutation in the motif, we were able to generate a RASSF1A variant that retains wild-type apoptotic properties but completely loses the ability to bind microtubule-associated proteins and complex with microtubules. Comparison of this mutant to wild-type RASSF1A showed that, despite retaining its proapoptotic properties, the mutant was completely unable to induce cell cycle arrest or suppress the tumorigenic phenotype. Therefore, it appears that the cell cycle/microtubule effects of RASSF1A are key to its tumor suppressor function rather than its apoptotic effects.

Ras regulates SCF(β-TrCP) protein activity and specificity via its effector protein NORE1A

Ras is the most frequently activated oncogene found in human cancer, but its mechanisms of action remain only partially understood. Ras activates multiple signaling pathways to promote transformation. However, Ras can also exhibit a potent ability to induce growth arrest and death. NORE1A (RASSF5) is a direct Ras effector that acts as a tumor suppressor by promoting apoptosis and cell cycle arrest. Expression of NORE1A is frequently lost in human tumors, and its mechanism of action remains unclear. Here we show that NORE1A forms a direct, Ras-regulated complex with β-TrCP, the substrate recognition component of the SCF(β-TrCP) ubiquitin ligase complex. This interaction allows Ras to stimulate the ubiquitin ligase activity of SCF(β-TrCP) toward its target β-catenin, resulting in degradation of β-catenin by the 26 S proteasome. However, the action of Ras/NORE1A/β-TrCP is substrate-specific because IκB, another substrate of SCF(β-TrCP), is not sensitive to NORE1A-promoted degradation. We identify a completely new signaling mechanism for Ras that allows for the specific regulation of SCF(β-TrCP) targets. We show that the NORE1A levels in a cell may dictate the effects of Ras on the Wnt/β-catenin pathway. Moreover, because NORE1A expression is frequently impaired in tumors, we provide an explanation for the observation that β-TrCP can act as a tumor suppressor or an oncogene in different cell systems.

CD52 is a molecular target in advanced systemic mastocytosis

Advanced systemic mastocytosis (SM) is an aggressive hematopoietic neoplasm with poor prognosis and short survival times. So far, no curative therapy is available for affected patients. We have identified the cell surface antigen CD52 (CAMPATH-1) as a molecular target expressed abundantly on the surface of primary neoplastic mast cells (MCs) in patients with advanced SM. In contrast, neoplastic MCs of patients with indolent SM and normal MCs expressed only low levels or did not express CD52. To study the mechanisms of CD52 expression and the value of this antigen as a potential therapeutic target, we generated a human MC cell line, designated MCPV-1, by lentiviral immortalization of cord blood-derived MC progenitor cells. Functional studies revealed that activated RAS profoundly promotes surface expression of CD52. The CD52-targeting antibody alemtuzumab induced cell death in CD52(+) primary neoplastic MCs obtained from patients with SM as well as in MCPV-1 cells. NSG mice xenotransplanted with MCPV-1 cells survived significantly longer after treatment with alemtuzumab (median survival: 31 d untreated vs. 46 d treated; P=0.0012). We conclude that CD52 is a novel marker and potential therapeutic target in neoplastic MCs in patients with advanced SM.

Selectivity of CDC25 homology domain-containing guanine nucleotide exchange factors

The Ras family of small G-proteins plays an essential role in the regulation of a variety of signal transduction processes, ranging from cell cycle control to the regulation of exocytosis. Signalling by the Ras G-proteins is initiated by the CDC25 homology domain (CDC25-HD) containing guanine nucleotide exchange factors (GEFs); each GEF, with its specific selectivity profile towards G-proteins, commonly acts on only a small subset of the Ras family members. Thus, GEFs play a pivotal part in establishing the activation of the downstream signalling routes. The structural basis for the establishment of selectivity in the GEF-G-protein interaction is only partially understood, and several controversies on the selectivity of GEFs are discussed in the literature. In the present study, we undertook a systematic approach to determine the selectivity of CDC25-HD for members of the Ras family. We generated a data set of 126 pairs using a standardised in vitro approach encompassing purified recombinant proteins, and a comprehensive mutational study analysed the basis of the selectivity. Together, these data highlight the distinct selectivity of various GEFs and allow for predictions of untested combinations of GEFs and G-proteins.

Global epigenetic changes induced by SWI2/SNF2 inhibitors characterize neomycin-resistant mammalian cells

Background

Previously, we showed that aminoglycoside phosphotransferases catalyze the formation of a specific inhibitor of the SWI2/SNF2 proteins. Aminoglycoside phosphotransferases, for example neomycin-resistant genes, are used extensively as selection markers in mammalian transfections as well as in transgenic studies. However, introduction of the neomycin-resistant gene is fraught with variability in gene expression. We hypothesized that the introduction of neomycin-resistant genes into mammalian cells results in inactivation of SWI2/SNF2 proteins thereby leading to global epigenetic changes.

Methodology

Using fluorescence spectroscopy we have shown that the inhibitor, known as Active DNA-dependent ATPase ADomain inhibitor (ADAADi), binds to the SWI2/SNF2 proteins in the absence as well as presence of ATP and DNA. This binding occurs via a specific region known as Motif Ia leading to a conformational change in the SWI2/SNF2 proteins that precludes ATP hydrolysis. ADAADi is produced from a plethora of aminoglycosides including G418 and Streptomycin, two commonly used antibiotics in mammalian cell cultures. Mammalian cells are sensitive to ADAADi; however, cells stably transfected with neomycin-resistant genes are refractory to ADAADi. In resistant cells, endogenous SWI2/SNF2 proteins are inactivated which results in altered histone modifications. Microarray data shows that the changes in the epigenome are reflected in altered gene expression. The microarray data was validated using real-time PCR. Finally, we show that the epigenetic changes are quantized.

Significance

The use of neomycin-resistant genes revolutionized mammalian transfections even though questions linger about efficacy. In this study, we have demonstrated that selection of neomycin-resistant cells results in survival of only those cells that have undergone epigenetic changes, and therefore, data obtained using these resistant genes as selection markers need to be cautiously evaluated.

RASSF1A and the rs2073498 Cancer Associated SNP

RASSF1A is one of the most frequently inactivated tumor suppressors yet identified in human cancer. It is pro-apoptotic and appears to function as a scaffolding protein that interacts with a variety of other tumor suppressors to modulate their function. It can also complex with the Ras oncoprotein and may serve to integrate pro-growth and pro-death signaling pathways. A SNP has been identified that is present in approximately 29% of European populations [rs2073498, A(133)S]. Several studies have now presented evidence that this SNP is associated with an enhanced risk of developing breast cancer. We have used a proteomics based approach to identify multiple differences in the pattern of protein/protein interactions mediated by the wild type compared to the SNP variant protein. We have also identified a significant difference in biological activity between wild type and SNP variant protein. However, we have found only a very modest association of the SNP with breast cancer predisposition.

Synaptotagmin 1 causes phosphatidyl inositol lipid-dependent actin remodeling in cultured non-neuronal and neuronal cells

Here we demonstrate that a dramatic actin polymerizing activity caused by ectopic expression of the synaptic vesicle protein synaptotagmin 1 that results in extensive filopodia formation is due to the presence of a lysine rich sequence motif immediately at the cytoplasmic side of the transmembrane domain of the protein. This polybasic sequence interacts with anionic phospholipids in vitro, and, consequently, the actin remodeling caused by this sequence is interfered with by expression of a phosphatidyl inositol (4,5)-bisphosphate (PIP2)-targeted phosphatase, suggesting that it intervenes with the function of PIP2-binding actin control proteins. The activity drastically alters the behavior of a range of cultured cells including the neuroblastoma cell line SH-SY5Y and primary cortical mouse neurons, and, since the sequence is conserved also in synaptotagmin 2, it may reflect an important fine-tuning role for these two proteins during synaptic vesicle fusion and neurotransmitter release.

The Ras effector RASSF2 controls the PAR-4 tumor suppressor

RASSF2 is a novel proapoptotic effector of K-Ras. Inhibition of RASSF2 expression enhances the transforming effects of K-Ras, and epigenetic inactivation of RASSF2 is frequently detected in mutant Ras-containing primary tumors. Thus, RASSF2 is implicated as a tumor suppressor whose inactivation facilitates transformation by disconnecting apoptotic responses from Ras. The mechanism of action of RASSF2 is not known. Here we show that RASSF2 forms a direct and endogenous complex with the prostate apoptosis response protein 4 (PAR-4) tumor suppressor. This interaction is regulated by K-Ras and is essential for the full apoptotic effects of PAR-4. RASSF2 is primarily a nuclear protein, and shuttling of PAR-4 from the cytoplasm to the nucleus is essential for its function. We show that RASSF2 modulates the nuclear translocation of PAR-4 in prostate tumor cells, providing a mechanism for its biological effects. Thus, we identify the first tumor suppressor signaling pathway emanating from RASSF2, we identify a novel mode of action of a RASSF protein, and we provide an explanation for the extraordinarily high frequency of RASSF2 inactivation we have observed in primary prostate tumors.

Radiosensitization of epidermal growth factor receptor/HER2-positive pancreatic cancer is mediated by inhibition of Akt independent of ras mutational status

PURPOSE

Epidermal growth factor receptor (EGFR) family members (e.g., EGFR, HER2, HER3, and HER4) are commonly overexpressed in pancreatic cancer. We investigated the effects of inhibition of EGFR/HER2 signaling on pancreatic cancer to elucidate the role(s) of EGFR/HER2 in radiosensitization and to provide evidence in support of further clinical investigations.

EXPERIMENTAL DESIGN

Expression of EGFR family members in pancreatic cancer lines was assessed by quantitative reverse transcription-PCR. Cell growth inhibition was determined by MTS assay. The effects of inhibition of EGFR family receptors and downstream signaling pathways on in vitro radiosensitivity were evaluated using clonogenic assays. Growth delay was used to evaluate the effects of nelfinavir on in vivo tumor radiosensitivity.

RESULTS

Lapatinib inhibited cell growth in four pancreatic cancer cell lines, but radiosensitized only wild-type K-ras-expressing T3M4 cells. Akt activation was blocked in a wild-type K-ras cell line, whereas constitutive phosphorylation of Akt and extracellular signal-regulated kinase (ERK) was seen in lines expressing mutant K-ras. Overexpression of constitutively active K-ras (G12V) abrogated lapatinib-mediated inhibition of both Akt phosphorylation and radiosensitization. Inhibition of MAP/ERK kinase/ERK signaling with U0126 had no effect on radiosensitization, whereas inhibition of activated Akt with LY294002 (enhancement ratio, 1.2-1.8) or nelfinavir (enhancement ratio, 1.2-1.4) radiosensitized cells regardless of K-ras mutation status. Oral nelfinavir administration to mice bearing mutant K-ras-containing Capan-2 xenografts resulted in a greater than additive increase in radiation-mediated tumor growth delay (synergy assessment ratio of 1.5).

CONCLUSIONS

Inhibition of EGFR/HER2 enhances radiosensitivity in wild-type K-ras pancreatic cancer. Nelfinavir, and other phosphoinositide 3-kinase/Akt inhibitors, are effective pancreatic radiosensitizers regardless of K-ras mutation status.

Regulation of Rnd3 localization and function by protein kinase C alpha-mediated phosphorylation

The Rnd proteins (Rnd1, Rnd2 and Rnd3/RhoE) form a distinct branch of the Rho family of small GTPases. Altered Rnd3 expression causes changes in cytoskeletal organization and cell cycle progression. Rnd3 functions to decrease RhoA activity, but how Rnd3 itself is regulated to cause these changes is still under investigation. Unlike other Rho family proteins, Rnd3 is regulated not by GTP/GDP cycling, but at the level of expression and by post-translational modifications such as prenylation and phosphorylation. We show in the present study that, upon PKC (protein kinase C) agonist stimulation, Rnd3 undergoes an electrophoretic mobility shift and its subcellular localization becomes enriched at internal membranes. These changes are blocked by inhibition of conventional PKC isoforms and do not occur in PKCalpha-null cells or to a non-phosphorylatable mutant of Rnd3. We further show that PKCalpha directly phosphorylates Rnd3 in an in vitro kinase assay. Additionally, we provide evidence that the phosphorylation status of Rnd3 has a direct effect on its ability to block signalling from the Rho-ROCK (Rho-kinase) pathway. These results identify an additional mechanism of regulation and provide clarification of how Rnd3 modulates Rho signalling to alter cytoskeletal organization.

Tools to study the function of the Ras-related, estrogen-regulated growth inhibitor in breast cancer

The Ras-related, estrogen-regulated growth inhibitor (Rerg) is a Ras-related small GTPase and candidate tumor suppressor. Rerg gene expression is stimulated by the estrogen receptor alpha (ERalpha), and Rerg gene expression is absent in ER-negative breast cancers. ER-negative breast cancers are highly invasive and metastastic and are typically more advanced than their ER-positive counterparts. Like Ras, Rerg binds and hydrolyzes GTP, but unlike Ras, Rerg has been shown to possess growth inhibitory activity in breast cancer cells. The precise role that Rerg loss plays in breast cancer growth and the mechanisms by which it does so are unknown. This chapter describes tools used to detect and manipulate the expression of Rerg in breast cancer cells. We validate use of an antibody to detect Rerg expression. We describe the generation of expression vectors that encode wild-type and mutants of Rerg that are altered in GDP/GTP regulation. We also describe the development of an inducible Rerg expression system and of a retrovirus-based RNA interference approach to repress Rerg expression. These tools will be invaluable in evaluating the biological function of Rerg in breast cancer.

RASSF6 is a novel member of the RASSF family of tumor suppressors

RASSF family proteins are tumor suppressors that are frequently downregulated during the development of human cancer. The best-characterized member of the family is RASSF1A, which is downregulated by promoter methylation in 40-90% of primary human tumors. We now identify and characterize a novel member of the RASSF family, RASSF6. Like the other family members, RASSF6 possesses a Ras Association domain and binds activated Ras. Exogenous expression of RASSF6 promoted apoptosis, synergized with activated K-Ras to induce cell death and inhibited the survival of specific tumor cell lines. Suppression of RASSF6 enhanced the tumorigenic phenotype of a human lung tumor cell line. Furthermore, RASSF6 is often downregulated in primary human tumors. RASSF6 shares some similar overall properties as other RASSF proteins. However, there are significant differences in biological activity between RASSF6 and other family members including a discrete tissue expression profile, cell killing specificity and impact on signaling pathways. Moreover, RASSF6 may play a role in dictating the degree of inflammatory response to the respiratory syncytial virus. Thus, RASSF6 is a novel RASSF family member that demonstrates the properties of a Ras effector and tumor suppressor but exhibits biological properties that are unique and distinct from those of other family members.

PRL tyrosine phosphatases regulate rho family GTPases to promote invasion and motility

Phosphatase found in regenerating liver (PRL)-1, PRL-2, and PRL-3 [also known as PTP4A1, PTP4A2, and PTP4A3, respectively] constitute a unique family of putative protein tyrosine phosphatases (PTPs) modified by farnesylation. PRL-3 is amplified and its message is up-regulated in colorectal carcinoma metastases. Its ectopic expression promotes invasive and metastatic properties, supporting a causal link between PRL-3 and late-stage cancer development. However, neither PRL phosphatase substrates nor their signaling pathways have been defined. To address possible mechanisms for the biological activity of PRL-3, we sought to identify its downstream targets, reasoning that regulators of motility and invasion, such as the Rho family of small GTPases, might be logical candidates. We found that levels of active RhoA and RhoC were increased 4- to 7-fold in SW480 colorectal carcinoma cells expressing exogenous PRL-1 and PRL-3, and that PRL-mediated motility and Matrigel invasion were blocked by pharmacologic inhibition of Rho kinase (ROCK), a key Rho effector. In contrast, the activity of Rac was reduced by PRL PTPs, whereas Cdc42 activity was unaffected. PRL-3 stimulated transcription driven by the serum response element in a Rho-dependent manner. We also confirmed that the ability of PRL PTPs to induce invasion and motility is dependent on farnesylation. Catalytic PRL-3 mutants (C104A or D72A) were impaired in PRL-3-induced invasion and Rho activation, indicating that these properties require phosphatase activity. We conclude that PRL PTPs stimulate Rho signaling pathways to promote motility and invasion. Characterization of PRL activity and regulatory pathways should enhance efforts to understand and interfere with PRL-mediated events in invasion and metastasis.

RASSF Family Proteins and Ras Transformation

There are six members of the RASSF gene family, with RASSF1 being the best characterized. All six genes produce proteins that contain Ras Association (RA) domains that can interact directly with activated Ras in overexpression studies. Their role in mediating the biological effects of Ras remains under investigation. However, they seem to modulate some of the growth inhibitory responses mediated by Ras. Moreover, evidence is accumulating that RASSF family members may serve as tumor suppressors that succumb to inactivation during the evolution of the transformed phenotype. Thus, RASSF proteins may be described as effector/tumor suppressors, in contrast to traditional Ras effectors such as Raf and PI-3 kinase, which may be considered to be effector/oncoproteins.

Using Inhibitors of Prenylation to Block Localization and Transforming Activity

The proper subcellular localization and biological activity of most Ras and Rho family small GTPases are dependent on their posttranslational modification by isoprenylation. Farnesyltransferase (FTase) and geranylgeranyl transferase I (GGTase I) are the prenyltransferases that catalyze the irreversible attachment of C15 farnesyl (Ras, Rnd) or C20 (R-Ras, Ral, Rap, Rho, Rac, Cdc42) isoprenoid lipid moieties to these small GTPases and other proteins. Therefore, pharmacological inhibitors of FTase (FTIs) and GGTase I (GGTIs) have been developed to prevent these modifications and thereby to block the lipid-mediated association of Ras and Rho proteins with cellular membranes and the consequent signaling and transforming activities. In addition, other small molecule inhibitors such as farnesyl thiosalicylic acid (FTS) can compete with the isoprenoid moiety of small GTPases for membrane binding sites. Finally, endogenous regulatory proteins such as RhoGDIs can bind to and mask the prenyl groups of small GTPases, leading to their sequestration from membranes. We describe here methods to use each of these categories of prenylation inhibitors to manipulate and investigate the subcellular localization patterns and transforming potential of these Ras and Rho family GTPases.

Rac3-Mediated Transformation Requires Multiple Effector Pathways

Our initial characterization of Rac3, a close relative of the small GTPase Rac1, established its ability to promote membrane ruffling, transformation, and activation of c-jun transcriptional activity. The finding that Rac3 is transforming, and its similarity to Rac1, a protein that has a well-established connection to many processes important for cancer progression, prompted further investigation into Rac3 transformation. We used effector domain mutants (EDMs) to explore the relationship among Rac signaling, transformation, and effector usage. All Rac3 EDMs tested (N26D, F37L, Y40C, and N43D) retained the ability to promote membrane ruffling and focus formation. In contrast, only the N43D mutant promoted anchorage independence. This differs from Rac1, where both N26D and N43D mutants were impaired in both types of transformation. To learn more about the signaling pathways involved, we did luciferase reporter assays and glutathione S-transferase pull-down assays for effector binding. We found evidence for a functional link between activation of phospholipase Cbeta2 by Rac3 and signaling to the serum response factor (SRF). Surprisingly, we also found that Rac3 binds poorly to the known Rac1 effectors mixed lineage kinases 2 and 3 (MLK2 and MLK3). Transcription of cyclin D1 was the only pathway that correlated with growth in soft agar. Our experiments show that activation of membrane ruffling and transcriptional activation of c-jun, SRF, or E2F are not sufficient to promote anchorage-independent growth mediated by Rac3. Instead, multiple effector pathways are required for Rac3 transformation, and these overlap partially but not completely with those used by Rac1.

Pathway- and expression level-dependent effects of oncogenic N-Ras: p27(Kip1) mislocalization by the Ral-GEF pathway and Erk-mediated interference with Smad signaling

Overactivation of Ras pathways contributes to oncogenesis and metastasis of epithelial cells in several ways, including interference with cell cycle regulation via the CDK inhibitor p27(Kip1) (p27) and disruption of transforming growth factor beta (TGF-beta) anti-proliferative activity. Here, we show that at high expression levels, constitutively active N-Ras induces cytoplasmic mislocalization of murine and human p27 via the Ral-GEF pathway and disrupts TGF-beta-mediated Smad nuclear translocation by activation of the Mek/Erk pathway. While human p27 could also be mislocalized via the phosphatidylinositol 3-kinase/Akt pathway, only Ral-GEF activation was effective for murine p27, which lacks the Thr157 Akt phosphorylation site of human p27. This establishes a novel role for the Ral-GEF pathway in regulating p27 localization. Interference with either Smad translocation or p27 nuclear localization was sufficient to disrupt TGF-beta growth inhibition. Moreover, expression of activated N-Ras or specific effector loop mutants at lower levels using retroviral vectors induced p27 mislocalization but did not inhibit Smad2/3 translocation, indicating that the effects on p27 localization occur at lower levels of activated Ras. These findings have important implications for the contribution of activated Ras to oncogenesis and for the conversion of TGF-beta from an inhibitory to a metastatic factor in some epithelial tumors.

Transforming activity of the Rho family GTPase, Wrch-1, a Wnt-regulated Cdc42 homolog, is dependent on a novel carboxyl-terminal palmitoylation motif

Wrch-1 is a Rho family GTPase that shares strong sequence and functional similarity with Cdc42. Like Cdc42, Wrch-1 can promote anchorage-independent growth transformation. We determined that activated Wrch-1 also promoted anchorage-dependent growth transformation of NIH 3T3 fibroblasts. Wrch-1 contains a distinct carboxyl-terminal extension not found in Cdc42, suggesting potential differences in subcellular location and function. Consistent with this, we found that Wrch-1 associated extensively with plasma membrane and endosomes, rather than with cytosol and perinuclear membranes like Cdc42. Like Cdc42, Wrch-1 terminates in a CAAX tetrapeptide (where C is cysteine, A is aliphatic amino acid, and X is any amino acid) motif (CCFV), suggesting that Wrch-1 may be prenylated similarly to Cdc42. Most surprisingly, unlike Cdc42, Wrch-1 did not incorporate isoprenoid moieties, and Wrch-1 membrane localization was not altered by inhibitors of protein prenylation. Instead, we showed that Wrch-1 is modified by the fatty acid palmitate, and pharmacologic inhibition of protein palmitoylation caused mislocalization of Wrch-1. Most interestingly, mutation of the second cysteine of the CCFV motif (CCFV > CSFV), but not the first, abrogated both Wrch-1 membrane localization and transformation. These results suggest that Wrch-1 membrane association, subcellular localization, and biological activity are mediated by a novel membrane-targeting mechanism distinct from that of Cdc42 and other isoprenylated Rho family GTPases.

Role of TC21/R-Ras2 in enhanced migration of neurofibromin-deficient Schwann cells

The neurofibromatosis type 1 tumor suppressor protein neurofibromin, is a GTPase activating protein for H-, N-, K-, R-Ras and TC21/R-Ras2 proteins. We demonstrate that Schwann cells derived from Nf1-null mice have enhanced chemokinetic and chemotactic migration in comparison to wild-type controls. Surprisingly, this migratory phenotype is not inhibited by a farnesyltransferase inhibitor or dominant-negative (dn) (N17)H-Ras (which inhibits H-, N-, and K-Ras activation). We postulated that increased activity of R-Ras and/or TC21/R-Ras2, due to loss of Nf1, contributes to increased migration. Mouse Schwann cells (MSCs) express R-Ras and TC21/R-Ras2 and their specific guanine exchange factors, C3G and AND-34. Infection of Nf1-null MSCs with a dn(43N)R-Ras adenovirus (to inhibit both R-Ras and TC21/R-Ras2 activation) decreases migration by approximately 50%. Conversely, expression of activated (72L)TC21/R-Ras2, but not activated (38V)R-Ras, increases migration, suggesting a role of TC21/R-Ras2 activation in the migration of neurofibromin-deficient Schwann cells. TC21/R-Ras2 preferentially couples to the phosphatidylinositol 3-kinase (PI3-kinase) and MAP kinase pathways. Treatment with a PI3-kinase or MAP kinase inhibitor reduces Nf1-null Schwann cell migration, implicating these TC21 effectors in Schwann cell migration. These data reveal a key role for neurofibromin regulation of TC21/R-Ras2 in Schwann cells, a cell type critical to NF1 tumor pathogenesis.

Enhanced cathepsin L expression is mediated by different Ras effector pathways in fibroblasts and epithelial cells

Ras expression induces increased expression and altered targeting of lysosomal proteases in multiple cell types, but the specific downstream cytoplasmic signaling pathways mediating these changes have not been identified. In this study, we compared the involvement of 3 major Ras effectors, Raf, phosphatidylinositol 3-kinase (PI3K) and Ral guanine nucleotide exchange factor (RalGEF) in the Ras-mediated alteration of lysosomal protease protein expression and targeting in rat 208F fibroblasts and rat ovarian surface epithelial (ROSE) cells. Effector domain mutants of Ras, constitutively activated variants of Raf, PI3K and RalGEF and pharmacologic inhibitors of MEK and PI3K were utilized to determine the role of these downstream pathways in mediating fibroblast transformation and lysosomal protease regulation in the fibroblasts and epithelial cells. We found that Raf activation of the ERK mitogen-activated protein kinase pathway alone was sufficient to cause morphologic and growth transformation of the fibroblasts and was necessary and sufficient to alter cathepsin L expression and targeting. In contrast, transformation and upregulation of cathepsin L expression in the epithelial cells required the activity of all 3 Ras effectors. Increased protease secretion from the epithelial cells was not observed on ectopic expression of Ras, as it was from the fibroblasts, consistent with the utilization of different signaling pathways in the 2 cell types. In neither cell type did Ras expression increase the expression, processing or secretion of 2 other major lysosomal proteases, cathepsin B and cathepsin D. Thus, Ras utilizes different effectors to mediate transformation and to deregulate cathepsin L expression and secretion in fibroblast and epithelial cells.

High affinity for farnesyltransferase and alternative prenylation contribute individually to K-Ras4B resistance to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs.

RASSF2 is a novel K-Ras-specific effector and potential tumor suppressor

Ras proteins regulate a wide range of biological processes by interacting with a broad assortment of effector proteins. Although activated forms of Ras are frequently associated with oncogenesis, they may also provoke growth-antagonistic effects. These include senescence, cell cycle arrest, differentiation, and apoptosis. The mechanisms that underlie these growth-inhibitory activities are relatively poorly understood. Recently, two related novel Ras effectors, NORE1 and RASSF1, have been identified as mediators of apoptosis and cell cycle arrest. Both of these proteins exhibit many of the properties normally associated with tumor suppressors. We now identify a novel third member of this family, designated RASSF2. RASSF2 binds directly to K-Ras in a GTP-dependent manner via the Ras effector domain. However, RASSF2 only weakly interacts with H-Ras. Moreover, RASSF2 promotes apoptosis and cell cycle arrest and is frequently down-regulated in lung tumor cell lines. Thus, we identify RASSF2 as a new member of the RASSF1 family of Ras effectors/tumor suppressors that exhibits a specificity for interacting with K-Ras.

The pro-apoptotic Ras effector Nore1 may serve as a Ras-regulated tumor suppressor in the lung

Ras oncoproteins mediate multiple biological effects by activating multiple effectors. Classically, Ras activation has been associated with enhanced cellular growth and transformation. However, activated forms of Ras may also inhibit growth by inducing senescence, apoptosis, and differentiation. Induction of apoptosis by Ras may be mediated by its effector RASSF1, which appears to function as a tumor suppressor. We now show that the Ras effector Nore1, which is structurally related to RASSF1, can also mediate a Ras-dependent apoptosis. Moreover, an analysis of Nore1 protein expression showed that it is frequently down-regulated in lung tumor cell lines and primary lung tumors. Like RASSF1, this correlates with methylation of the Nore1 promoter rather than gene deletion. Finally, re-introduction of Nore1, driven by its own promoter, impairs the growth in soft agar of a human lung tumor cell line. Consequently, we propose that the Ras effector Nore1 is a member of a family of Ras effector/tumor suppressors that includes RASSF1.

Critical role of the pleckstrin homology domain in Dbs signaling and growth regulation

Dbl family proteins act as guanine nucleotide exchange factors and positive regulators of Rho GTPase function by stimulating formation of the active, GTP-bound state. All Dbl family Rho guanine nucleotide exchange factors possess an invariant tandem domain structure consisting of a Dbl homology (DH) catalytic domain followed by a pleckstrin homology (PH) regulatory domain. We determined previously that the PH domain of Dbs was critical for the intrinsic catalytic activity of the DH domain in vitro and for Dbs transformation in vivo. In this study, we evaluated the role of phosphoinositide binding to the PH domain in regulating the DH domain function of Dbs in vitro and in vivo. We determined that mutation of basic amino acids located within the beta1-beta2 and beta3-beta4 loops of the PH domain resulted in impaired phospholipid binding in vitro, yet full guanine nucleotide exchange activity in vitro was retained for RhoA and Cdc42. Surprisingly, these mutants were compromised in their ability to activate Rho GTPases in vivo and to cause transformation of NIH 3T3 cells. However, Dbs subcellular localization was impaired by these PH domain mutations, supporting a role for phospholipid interactions in facilitating membrane association. Despite the importance of phospholipid binding for Dbs function in vivo, we found that Dbs signaling and transforming activity was not stimulated by phosphatidylinositol 3-kinase activation. We suggest that the PH domain of Dbs facilitates two distinct roles in the regulation of DH domain function, one critical for GTPase association and activation in vitro and one critical for phosphoinositide binding and GTPase interaction in vivo, that together promote Dbs association with membranes.

Rig is a novel Ras-related protein and potential neural tumor suppressor

The Ras superfamily consists of a large group of monomeric GTPases demonstrating homology to Ras oncoproteins. Although structurally similar, Ras-superfamily proteins are functionally diverse. Whereas some members exhibit oncogenic properties, others may serve as tumor suppressors. We have identified a novel Ras-related protein that suppresses cell growth and have designated it Rig (Ras-related inhibitor of cell growth). Overexpression of Rig inhibited Ras-mediated cellular transformation and activation of downstream signaling in NIH 3T3 cells. rig mRNA is expressed at high levels in normal cardiac and neural tissue. However, Rig protein expression is frequently lost or down-regulated in neural tumor-derived cell lines and primary human neural tumors. Moreover, expression of exogenous Rig in human astrocytoma cells suppressed growth. Rig has a C-terminal CAAX motif that codes for posttranslational modification by both farnesyl and geranylgeranyl isoprenoid lipids. Consequently, Rig may play a role in the cellular response to farnesyl transferase inhibitors. Rig bears 63% overall sequence homology to a recently described Ras-family member Noey2, a tumor suppressor in breast and ovarian tissue. Therefore, Rig and Noey2 may represent a new subfamily of Ras-like tumor suppressors.

Critical but distinct roles for the pleckstrin homology and cysteine-rich domains as positive modulators of Vav2 signaling and transformation

Vav2, like all Dbl family proteins, possesses tandem Dbl homology (DH) and pleckstrin homology (PH) domains and functions as a guanine nucleotide exchange factor for Rho family GTPases. Whereas the PH domain is a critical positive regulator of DH domain function for a majority of Dbl family proteins, the PH domains of the related Vav and Vav3 proteins are dispensable for DH domain activity. Instead, Vav proteins contain a cysteine-rich domain (CRD) critical for DH domain function. We evaluated the contribution of the PH domain and the CRD to Vav2 guanine nucleotide exchange, signaling, and transforming activity. Unexpectedly, we found that mutations of the PH domain impaired Vav2 signaling, transforming activity, and membrane association. However, these mutations do not influence exchange activity on Rac and only slightly affect exchange on RhoA and Cdc42. We also found that the CRD was critical for the exchange activity in vitro and contributed to Vav2 membrane localization. Finally, we found that phosphoinositol 3-kinase activation synergistically enhanced Vav2 transforming and signaling activity by stimulating exchange activity but not membrane association. In conclusion, the PH domain and CRD are mechanistically distinct, positive modulators of Vav2 DH domain function in vivo.

A distinct class of dominant negative Ras mutants: cytosolic GTP-bound Ras effector domain mutants that inhibit Ras signaling and transformation and enhance cell adhesion

Cytosolic GTP-bound Ras has been shown to act as a dominant negative (DN) inhibitor of Ras by sequestering Raf in non-productive cytosolic complexes. Nevertheless, this distinct class of DN mutants has been neither well characterized nor extensively used to analyze Ras signaling. In contrast, DN Ras17N, which functions by blocking Ras guanine nucleotide exchange factors, has been well characterized and is widely used. Cytosolic GTP-bound Ras mutants could be used to inhibit particular Ras effectors by introducing additional mutations (T35S, E37G or Y40C) that permit them to associate selectively with and inhibit Raf, RalGDS, or phosphoinositide 3-kinase, respectively. When the wild-type Ras effector binding region is used, cytosolic Ras should associate with all Ras effectors, even those that are not yet identified, making these DN Ras mutants effective inhibitors of multiple Ras functions. We generated cytosolic GTP-bound H-, N-, and K-Ras, and we assessed their ability to inhibit Ras-induced phenotypes. In fibroblasts, cytosolic H-, N-, and K-Ras inhibited Ras-induced Elk-1 activation and focus formation, induced a flattened cell morphology, and increased adhesion to fibronectin through modulation of a beta(1)-subunit-containing integrin, thereby demonstrating that DN activity is not limited to a subset of Ras isoforms. We also generated cytosolic GTP-bound Ras effector domain mutants (EDMs), each of which reduced the ability of cytosolic GTP-bound Ras proteins to inhibit Elk-1 activation and to induce cell flattening, implicating multiple pathways in these phenotypes. In contrast, Ras-induced focus formation, platelet-derived growth factor (PDGF)-, or Ras-induced phospho-Akt levels and cell adhesion to fibronectin were affected by T35S and Y40C EDMs, whereas PDGF- or Ras-induced phospho-Erk levels were affected only by the T35S EDM, implying that a more limited set of Ras-mediated pathways participate in these phenotypes. These data constitute the first extensive characterization of this functionally distinct class of DN Ras inhibitor proteins.

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.