Pathways of Ras function: connections to the actin cytoskeleton

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

Neurofibromatosis type 1 as a model system to study molecular mechanisms of autism spectrum disorder symptoms

Neurofibromatosis type 1 (NF1) is monogenic neurodevelopmental disorder caused by mutation of NF1 gene, which leads to increased susceptibility to various tumors formations. Additionally, majority of patients with NF1 are experience high incidence of cognitive deficits. Particularly, we review the growing number of reports demonstrated a higher incidence of autism spectrum disorder (ASD) in individuals with NF1. In this review we also discuss face validity of preclinical Nf1 mouse models. Then we describe discoveries from these animal models that have uncovered the deficiencies in the regulation of Ras and other intracellular pathways as critical mechanisms underlying the Nf1 cognitive problems. We also summarize and interpret recent preclinical and clinical studies that point toward potential pharmacological therapies for NF1 patients.

Both Phenolic and Non-phenolic Green Tea Fractions Inhibit Migration of Cancer Cells

Green tea consumption is associated with chemoprevention of many cancer types. Fresh tea leaves are rich in polyphenolic catechins, which can constitute up to 30% of the dry leaf weight. While the polyphenols of green tea have been well investigated, it is still largely unknown, whether or not non-phenolic constituents also reveal chemopreventive and anti-metastatic effects. In this study, we investigated the effects of a fraction of green tea rich in phenolic compounds (PF), a non-phenolic fraction (NPF), which contains glyceroglycolipids (GGL), and a pure glyceroglycolipid compound isolated from the non-phenolic fraction in human cancer. Dried green tea leaves were extracted and applied to a Sephadex LH-20 column. The resazurin reduction assay was used to investigate the cytotoxicity of green tea samples toward human HepG2 hepatocellular carcinoma and normal AML12 hepatocytes cells. Gene expression profiling was performed by mRNA microarray hybridization and the microarray results were validated by RT-PCR. The scratch migration assay was used to investigate the effects of green tea samples on cell migration in vitro. The changes of microtubule dynamics were observed using fluorescence microscopy. PF and NPF were prepared from methanol extract of green tea. A GGL was isolated from NPF. All three green tea samples did not show significant cytotoxic activity up to 10 μg/mL in both HepG2 and AML12 cells, whereas cytotoxicity of the control drug doxorubicin was observed with both cell lines (IC₅₀ on AML12: 0.024 μg/mL, IC₅₀ on HepG2: 2.103 μg/mL). We identified three sets of genes differentially expressed upon treatment with the green tea samples. The genes were associated with cytoskeleton formation, cellular movement, and morphology. The correlation coefficients between mRNA expression values determined by microarray and RT-PCR were R = 0.94. HepG2 and U2OS cells treated with green tea extracts showed the delayed closures. Besides, the number of distinct tubulin filaments decreased upon treatment with green tea samples. We identified not only PF, but also glyceroglycolipids in NPF as contributing factors to the chemopreventive effects of green tea. Both PF and NPF of green tea inhibited cancer cell migration by the disassembly of microtubules, even though they were not cytotoxic.

Smooth muscle α-actin is a direct target of PLZF: effects on the cytoskeleton and on susceptibility to oncogenic transformation

Changes in cell morphology and rearrangements of the actin cytoskeleton are common features accompanying cell transformation induced by various oncogenes. In this study, we show that promyelocytic leukemia zinc finger protein (PLZF) binds to the promoter of smooth muscle α-actin, reducing mRNA and protein levels encoded by this gene and resulting in a reorganization of the actin cytoskeleton. In cultures of chicken embryo fibroblasts (CEF), this effect on α-actin expression is correlated with a change in cellular phenotype from spindle shaped to polygonal and flattened. This morphological change is dependent on Ras function. The polygonal, flattened CEF show a high degree of resistance to the transforming activity of several oncoproteins. Our results support the conclusion that reorganization of the actin cytoskeleton plays an important role in tumor suppression by PLZF.

RAS: target for cancer therapy

The RAS protein controls signaling pathway are major player in cell growth, its regulation and malignant transformation. Any activation in RAS brings alteration in upstream or downstream signaling component. Activating mutation in RAS is found in approximately 30% of human cancer. RAS plays essential role in tumor maintenance and is therefore an appropriate target for anticancer therapy. Among the anti-RAS strategies that are under evaluation in the clinic are pharmacologic inhibitors designed to prevent: (1) association with the plasma membrane (prenylation and post prenylation inhibitors). (2) Downstream signaling (kinase inhibitor), (3) upstream pathway (kinase inhibitor and monoclonal antibody), (4) Expression of RAS or other component of pathway (siRNA and antisense oligonucleotide). Several of these new therapeutic agents are showing promising result in the clinic and many more are on the way. Here, we review the current status and new hopes for targeting RAS as an anticancer drug.

Dominant active Rac and dominant negative Rac revert the dominant active Ras phenotype in Colletotrichum trifolii by distinct signalling pathways

The small G-protein superfamily is an evolutionarily conserved group of GTPases that regulate diverse signalling pathways including pathways for growth and development in eukaryotes. Previously, we showed that dominant active mutation in the unique Ras gene (DARas) of the fungal phytopathogen Colletotrichum trifolii displays a nutrient-dependent phenotype affecting polarity, growth and differentiation. Signalling via the MAP kinase pathway is significantly impaired in this mutant as well. Here we describe the cloning and functional characterization of Rac (Ct-Rac1), a member of the Rho family of G proteins. Ct-Rac1 expression is downregulated by DARas under limiting nutrition. Co-expression of DARas with dominant active Rac (DARac) stimulates MAPK activation and restores the wild-type phenotype. Inhibition of MAPK activation suppresses phenotypic restoration suggesting Rac-mediated MAPK activation is responsible for reversion to the wild-type phenotype. We also examined the role of reactive oxygen species (ROS) in these genetic backgrounds. The DARas mutant strain generates high levels of ROS as determined by DCFH-DA fluorescence. Co-expression with DNRac decreases ROS generation to wild-type levels and restores normal fungal growth and development. Pretreatment of DARas with antioxidants or a cytosolic phospholipase A2 inhibitor also restores the wild-type phenotype. These findings suggest that Ras-mediated ROS generation is dependent on a Rac-cPLA(2)-linked signalling pathway. Taken together, this study provides evidence that Rac functions to restore the hyphal morphology of DARas by regulating MAPK activation and intracellular ROS generation.

Farnesyltransferase inhibition: a novel method of immunomodulation

Farnesyltransferase inhibitors (FTIs) are anticancer compounds that inhibit Ras GTPases. Since Ras GTPases play key roles in T cell activation and function, we hypothesized that FTIs have immunomodulatory properties and are potential antirejection agents. An investigation was performed on a potent FTI to evaluate this hypothesis in the in vitro setting. The in vitro effects of the FTI A-228839 were evaluated. Lectin- or antigen presenting cell (APC)-induced lymphocyte proliferation in the presence of A-228839 was measured. The effects of A-228839 on 1E5 T cell polarity were assessed by microscopy. Intracellular calcium ([Ca(2+)](i)) kinetics of lectin-activated lymphocytes was monitored by flow cytometry. The effects of A-228839 on peripheral blood mononuclear cell (PBMC) cytokine production was assessed by a cytometric bead array method. Activation-induced apoptosis was measured with an annexin V staining assay.A-228839 inhibited lectin-induced proliferation (IC(50)=0.24+/-0.11 microM). The inhibitory effects of A-228839 on lectin induced lymphocyte proliferation were additive to those of CsA. A-228839 was more effective in inhibiting APC-induced T cell proliferation (IC(50)=0.10+/-0.09 microM). A-228839 significantly disrupted the polarized shape of 1E5 T cells at physiologic concentrations. A-228839 altered PBMC baseline [Ca(2+)](i) but did not affect [Ca(2+)](i) kinetics during lectin-induced lymphocyte activation. A-228839 inhibited lymphocyte Th1 cytokine production at submicromolar levels and promoted apoptosis in lectin-activated lymphocytes.A-228839 potently inhibits lymphocyte activation and function. Our results suggest that FTIs may represent a new class of clinically useful immunomodulatory agents. A-228839 has potent in vitro immunomodulatory properties that warrant in vivo evaluation as an antirejection agent.

Inhibiting signal transduction: recent advances in the development of receptor tyrosine kinase and Ras inhibitors

Since aberrant cell signaling is implicated in the initiation, growth, and progression of cancer, proteins involved in signal transduction are rational therapeutic targets. Receptor tyrosine kinases (RTK) and Ras oncoprotein are examples of critical signaling proteins that mediate the processes of cellular growth and differentiation. Agents presently being evaluated as inhibitors of signal transduction include both natural and synthetic compounds, monoclonal antibodies, and antisense oligonucleotides. Preclinical studies of compounds which inhibit RTK and Ras have shown that these targets can be blocked, while side effects in animal models are minimal. Early clinical trials reveal that, in general, treatment with these compounds is both feasible and tolerable. However, many issues about STI remain unresolved including how to optimize schedule, how long to continue treatment, specific mechanisms of action, and how to optimize combinations of STI with standard therapeutic modalities. Addressing these issues may require a shift in the traditional paradigm of drug development, as conventional endpoints may not adequately capture the potential benefits from agents believed to act in a cytostatic vs. cytotoxic manner. This review will discuss the rationale and application of inhibiting signal transduction using inhibitors of RTK and Ras as prototypes of this class of agents.

Mutations in ras proto-oncogenes are associated with lower mdr1 gene expression in adult acute myeloid leukaemia

Mutations in ras genes have been found to be the most frequent genetic aberrations in adult myeloid leukaemia (AML). Some reports have shown an improved outcome of ras-mutated AML. In order to understand the biology of ras mutation in AML, we studied a cohort of patients treated in a prospective multicentre trial for ras mutational status and resistance gene expression. Blast samples from 162 adult patients with de novo or secondary AML were examined for resistance gene expression (mdr1, mrp1 and lrp) and ras mutations using reverse transcription-polymerase chain reaction and protein nucleic acid-competitive polymerase chain reaction strategies respectively. Ras mutations were confirmed using DNA sequencing. Ras mutations leading to an exchange of amino acids were found in 40 (25%) patients. Thirty AML patients had N-ras mutations and nine patients had K-ras mutations. One patient showed both N-ras and K-ras mutations. Resistance gene expression was positive for mdr1 in 30%, for mrp1 in 43% and for lrp in 62% of patients. There was a strong inverse correlation between the presence of ras mutation and mdr1 expression (P = 0.005). However, no significant difference was seen between patients with or without ras mutations and mrp1 or lrp expression. Whereas mdr1 expression was associated with a lower complete remission rate (P < 0.04), ras mutations had no significant influence on remission status. Neither ras mutation nor mdr1 expression had a significant impact on overall or disease-free survival to date. For the first time, there is evidence that activated ras genes are associated with lower mdr1 expression in AML.

Major role for active extension in the formation of processes by ras-transformed fibroblasts

Expression of constitutively active Ras protein in fibroblasts results in enhanced cell motility, invasion competence and morphological changes including the formation of elongate cellular processes. These processes have been shown to resemble retraction tails formed passively behind nontransformed cells by movement relative to sites of cell-substrate attachment. However, analysis presented here reveals that active extension mechanisms also play a role in the formation of these processes. Extension of distal process ends occurs at 0.42+/-0.44 microm/min in ras-transformed fibroblasts and accounts for 63.6+/-27.5% of observed process lengths. Active process extension by ras-transformed fibroblasts also persists in the presence of cell-cell contacts. Studies conducted using actin or microtubule antagonists, and correlation of process behavior followed by fixation and immunostaining reveal that process extension requires intact actin and microtubule networks. Other analyses reveal that active extension plays a significantly smaller role in the formation of processes by non-transformed control fibroblasts. These observations demonstrate that constitutively active Ras enhances process extension in fibroblasts and is a causal factor in process extension by fibroblasts in the presence of cell-cell contacts. Moreover, these studies demonstrate that process extension by ras-transformed fibroblasts is accomplished through mechanisms similar to those thought to drive active extension of processes by other cell types including neurons. These findings suggest that extension of cellular processes could play an important role in the metastatic behavior of ras-transformed fibroblasts as well as the response of untransformed fibroblasts to receptor mediated signal transduction events.

Growth inhibition of astrocytoma cells by farnesyl transferase inhibitors is mediated by a combination of anti-proliferative, pro-apoptotic and anti-angiogenic effects

While 25% of human cancers harbor oncogenic Ras mutations, such mutations are not found in astrocytomas. We have previously demonstrated that the activation of receptor tyrosine kinases expressed by malignant human astrocytoma cells and specimens results in functional upregulation of the Ras signalling pathway and increased levels of activated Ras*GTP. Farnesyl transferase inhibitors (FTIs) are promising anti-cancer agents in early clinical trials, which may exert their effect through pharmacological inhibition of the Ras signalling pathway. In this study we establish the anti-tumorigenic properties of the FTI L-744,832 against a panel of malignant human astrocytoma cell lines. Furthermore, we demonstrate the multiple mechanisms by which L-744,832 exerts its effect. L-744,832 demonstrates both cytostatic and cytotoxic effects on astrocytoma cells, and cells expressing a truncated constitutively phosphorylated Epidermal Growth Factor Receptor common in high-grade astrocytomas (EGFRvIII/p140EGF-R) demonstrate increased sensitivity to the agent. L-744,832 is capable of inducing apoptosis in astrocytoma cells under anchorage-dependent conditions; this process occurs in a p53-independent manner and is associated with increased expression of Bax and Bak. L-744,832 also induces cell cycle arrest at both the G1/M and G2/S checkpoints; this process is also independent of p53 mutational status. Cell cycle arrest in drug-treated cells can be accompanied by induction of p21WAF1/CIP1, but this induction is not necessary for the cell cycle inhibitory effects, nor is it dependent on functional p53. Finally, angiogenesis in astrocytomas has been shown to be dependent on secretion of Vascular Endothelial Growth Factor (VEGF) by tumour cells, particularly under hypoxic conditions. L-744,832 potently inhibits the secretion of VEGF under hypoxic conditions. These combinations of mechanisms suggest that these tumours, despite the absence of oncogenic Ras mutations, will be amenable to growth inhibition by FTIs, through a combination of anti-proliferative, pro-apoptotic, and anti-angiogenic effects.

Ras protein farnesyltransferase: A strategic target for anticancer therapeutic development

Ras proteins are guanine nucleotide-binding proteins that play pivotal roles in the control of normal and transformed cell growth and are among the most intensively studied proteins of the past decade. After stimulation by various growth factors and cytokines, Ras activates several downstream effectors, including the Raf-1/mitogen-activated protein kinase pathway and the Rac/Rho pathway. In approximately 30% of human cancers, including a substantial proportion of pancreatic and colon adenocarcinomas, mutated ras genes produce mutated proteins that remain locked in an active state, thereby relaying uncontrolled proliferative signals. Ras undergoes several posttranslational modifications that facilitate its attachment to the inner surface of the plasma membrane. The first-and most critical-modification is the addition of a farnesyl isoprenoid moiety in a reaction catalyzed by the enzyme protein farnesyltransferase (FTase). It follows that inhibiting FTase would prevent Ras from maturing into its biologically active form, and FTase is of considerable interest as a potential therapeutic target. Different classes of FTase inhibitors have been identified that block farnesylation of Ras, reverse Ras-mediated cell transformation in human cell lines, and inhibit the growth of human tumor cells in nude mice. In transgenic mice with established tumors, FTase inhibitors cause regression in some tumors, which appears to be mediated through both apoptosis and cell cycle regulation. FTase inhibitors have been well tolerated in animal studies and do not produce the generalized cytotoxic effects in normal tissues that are a major limitation of most conventional anticancer agents. There are ongoing clinical evaluations of FTase inhibitors to determine the feasibility of administering them on dose schedules like those that portend optimal therapeutic indices in preclinical studies. Because of the unique biologic aspects of FTase, designing disease-directed phase II and III evaluations of their effectiveness presents formidable challenges.

Role of RhoA activation in the growth and morphology of a murine prostate tumor cell line

Prostate cancer cells derived from transgenic mice with adenocarcinoma of the prostate (TRAMP cells) were treated with the HMG-CoA reductase inhibitor, lovastatin. This caused inactivation of the small GTPase RhoA, actin stress fiber disassembly, cell rounding, growth arrest in the G1 phase of the cell cycle, cell detachment and apoptosis. Addition of geranylgeraniol (GGOL) in the presence of lovastatin, to stimulate protein geranylgeranylation, prevented lovastatin's effects. That is, RhoA was activated, actin stress fibers were assembled, the cells assumed a flat morphology and cell growth resumed. The following observations support an essential role for RhoA in TRAMP cell growth: (1) TRAMP cells expressing dominant-negative RhoA (T19N) mutant protein displayed few actin stress fibers and grew at a slower rate than controls (35 h doubling time for cells expressing RhoA (T19N) vs 20 h for untransfected cells); (2) TRAMP cells expressing constitutively active RhoA (Q63L) mutant protein displayed a contractile phenotype and grew faster than controls (13 h doubling time). Interestingly, addition of farnesol (FOL) with lovastatin, to stimulate protein farnesylation, prevented lovastatin-induced cell rounding, cell detachment and apoptosis, and stimulated cell spreading to a spindle shaped morphology. However, RhoA remained inactive and growth arrest persisted. The morphological effects of FOL addition were prevented in TRAMP cells expressing dominant-negative H-Ras (T17N) mutant protein. Thus, it appears that H-Ras is capable of inducing cell spreading, but incapable of supporting cell proliferation, in the absence of geranylgeranylated proteins like RhoA.

H-ras-transformed NRK-52E renal epithelial cells have altered growth, morphology, and cytoskeletal structure that correlates with renal cell carcinoma in vivo

We studied the effect of the ras oncogene on the growth kinetics, morphology, cytoskeletal structure, and tumorigenicity of the widely used NRK-52E rat kidney epithelial cell line and two H-ras oncogene-transformed cell lines, H/1.2-NRK-52E (H/1.2) and H/6.1-NRK-52E (H/6.1). Population doubling times of NRK-52E, H/1.2, and H/6.1 cells were 28, 26, and 24 h, respectively, with the transformed cells reaching higher saturation densities than the parent cells. NRK-52E cells had typical epithelial morphology with growth in colonies. H/1.2 and H/6.1 cell colonies were more closely packed, highly condensed, and had increased plasma membrane ruffling compared to parent cell colonies. NRK-52E cells showed microfilament, microtubule, and intermediate filament networks typical of epithelial cells, while H/1.2 and H/6.1 cells showed altered cytoskeleton architecture, with decreased stress fibers and increased microtubule and intermediate filament staining at the microtubule organizing center. H/1.2 and H/6.1 cells proliferated in an in vitro soft agar transformation assay, indicating anchorage-independence, and rapidly formed tumors in vivo with characteristics of renal cell carcinoma, including mixed populations of sarcomatoid, granular, and clear cells. H/6.1 cells consistently showed more extensive alterations of growth kinetics, morphology, and cytoskeleton than H/1.2 cells, and formed tumors of a more aggressive phenotype. These data suggest that analysis of renal cell characteristics in vitro may have potential in predicting tumor behavior in vivo, and significantly contribute to the utility of these cell lines as in vitro models for examining renal epithelial cell biology and the role of the ras proto-oncogene in signal transduction involving the cytoskeleton.

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

PURPOSE AND DESIGN

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

RESULTS

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

CONCLUSION

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

N-Ras induces alterations in Golgi complex architecture and in constitutive protein transport

Aberrant glycosylation of proteins and lipids is a common feature of many tumor cell types, and is often accompanied by alterations in membrane traffic and an anomalous localization of Golgi-resident proteins and glycans. These observations suggest that the Golgi complex is a key organelle for at least some of the functional changes associated with malignant transformation. To gain insight into this possibility, we have analyzed changes in the structure and function of the Golgi complex induced by the conditional expression of the transforming N-Ras(K61) mutant in the NRK cell line. A remarkable and specific effect associated with this N-Ras-induced transformation was a conspicuous rearrangement of the Golgi complex into a collapsed morphology. Ultrastructural and stereological analyses demonstrated that the Golgi complex was extensively fragmented. The collapse of the Golgi complex was also accompanied by a disruption of the actin cytoskeleton. Functionally, N-Ras-transformed KT8 cells showed an increase in the constitutive protein transport from the trans-Golgi network to the cell surface, and did not induce the appearance of aberrant cell surface glycans. The Golgi complex collapse, the actin disassembly, and the increased constitutive secretion were all partially inhibited by the phospholipase A2 inhibitor 4-bromophenylacyl bromide. The results thus suggest the involvement of the actin cytoskeleton in the shape of the Golgi complex, and intracellular phospholipase A2 in its architecture and secretory function.

Evidence that atypical protein kinase C-lambda and atypical protein kinase C-zeta participate in Ras-mediated reorganization of the F-actin cytoskeleton

Expression of transforming Ha-Ras L61 in NIH3T3 cells causes profound morphological alterations which include a disassembly of actin stress fibers. The Ras-induced dissolution of actin stress fibers is blocked by the specific PKC inhibitor GF109203X at concentrations which inhibit the activity of the atypical aPKC isotypes lambda and zeta, whereas lower concentrations of the inhibitor which block conventional and novel PKC isotypes are ineffective. Coexpression of transforming Ha-Ras L61 with kinase-defective, dominant-negative (DN) mutants of aPKC-lambda and aPKC-zeta, as well as antisense constructs encoding RNA-directed against isotype-specific 5' sequences of the corresponding mRNA, abrogates the Ha-Ras-induced reorganization of the actin cytoskeleton. Expression of a kinase-defective, DN mutant of cPKC-alpha was unable to counteract Ras with regard to the dissolution of actin stress fibers. Transfection of cells with constructs encoding constitutively active (CA) mutants of atypical aPKC-lambda and aPKC-zeta lead to a disassembly of stress fibers independent of oncogenic Ha-Ras. Coexpression of (DN) Rac-1 N17 and addition of the phosphatidylinositol 3'-kinase (PI3K) inhibitors wortmannin and LY294002 are in agreement with a tentative model suggesting that, in the signaling pathway from Ha-Ras to the cytoskeleton aPKC-lambda acts upstream of PI3K and Rac-1, whereas aPKC-zeta functions downstream of PI3K and Rac-1. This model is supported by studies demonstrating that cotransfection with plasmids encoding L61Ras and either aPKC-lambda or aPKC-zeta results in a stimulation of the kinase activity of both enzymes. Furthermore, the Ras-mediated activation of PKC-zeta was abrogated by coexpression of DN Rac-1 N17.

Multiple stages and genetic alterations in immortalization, malignant transformation, and tumor progression of human skin keratinocytes

An in vitro carcinogenesis model of human skin keratinocytes has been developed based on the spontaneously immortalized keratinocyte cell line HaCaT. Immortalization, the initial stage in human carcinogenesis in vitro, was induced by ultraviolet-type mutations in the p53 gene followed by further genetic alterations leading to the loss of senescence genes, in particular on chromosome 3p. Despite multiple genetic changes, the HaCaT cell line sustained its genomic balance up to high passage levels and maintained a non-tumorigenic phenotype. Tumorigenic transformation was induced by ras oncogene transfection but also by culture stress and elevated temperature, resulting in benign and malignant tumorigenic clones. Malignant conversion was associated with the loss of a copy of chromosome 15, leading to a decrease in thrombospondin-1 (TSP-1) expression. Heat-induced malignant conversion was associated with a gain of material on chromosome 11, including the cyclin D1 gene. The microenvironment plays a major role in tumorigenic transformation and the control of malignant cells. Overexpression of platelet-derived growth factor in HaCaT cells caused mesenchyme activation and formation of benign tumors. Halting tumor angiogenesis completely prevented invasion of malignant cells and induced a benign tumor phenotype. Transfer of a normal chromosome 15 or TSP-1 transfection into a skin carcinoma line resulted in tumor suppression due to TSP-1-blocked tumor vascularization. Because of the reduced TSP-1 expression, blood vessels infiltrated the tumor, and it expanded. Progression to more aggressive tumor phenotypes required the in vivo environment and was caused by selection of a subpopulation and further genetic modifications. The improved autonomous growth of these cells was associated with new expression of granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, which acted in an autocrine manner to stimulate proliferation and migration. With this in vitro skin carcinogenesis model we were able to demonstrate multiple stages in the transformation process that were associated with different genetic and phenotypic characteristics. In addition, we documented that modulation of the tumor stroma plays an important and decisive role in tumor development and progression. From this we hypothesize that the growth restraints of the microenvironment are increasingly lost with advancing stages of carcinogenesis but can be restored by modulation of the tumor stroma.

Induction of cyclooxygenase-2 by activated Ha-ras oncogene in Rat-1 fibroblasts and the role of mitogen-activated protein kinase pathway

Elevated cyclooxygenase-2 (COX-2) expression and activity have been observed in several different transformed cell types that express mutated ras genes. To investigate the mechanism of increased COX-2 expression following Ras-mediated transformation, Rat-1:iRas cell line was transfected with an Ha-RasVal-12 cDNA expression vector that is under the transcriptional control of the lac operon and is inducible with isopropyl-1-thio-beta-D-galactopyranoside (IPTG). IPTG treatment caused parallel increases in the levels of Ha-Ras and COX-2 proteins in Rat-1:iRas cells. The increased expression of COX-2 was accompanied by increased prostaglandin E2 production. Selective inhibition of COX-2 activity suppressed the production of prostaglandin E2 by >90% but did not alter the progress of the morphological transformation. The level of COX-2 mRNA was up-regulated by activated Ha-Ras. Induction of Ras increased the transcription of COX-2 by 44.3 +/- 10.1% and increased the half-life of COX-2 mRNA by approximately 3.5-fold. A specific mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) inhibitor (PD 98059) caused a delay in both the activation of ERK1/2 and the induction of COX-2 in IPTG-induced Rat-1:iRas cells. Inhibition of ERK activity by PD 98059 also suppressed the induction of COX-2 by epidermal growth factor in intestinal epithelial cells and significantly reduced the expression of COX-2 in Ha-Ras-transformed rat intestinal epithelial cells. ERK activity appears to be required for induction of COX-2 by Ras.

Signal transduction pathways and their relevance in human astrocytomas

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

Nucleotide hydrolysis-dependent conformational changes in p21(ras) as studied using ESR spectroscopy

We have employed ESR spectroscopy using guanine nucleotides that contain a spin label at the 2',3'-position of the ribose to investigate structural changes in the proto-oncogene product p21(ras) that are dependent on nucleotide hydrolysis. The three nucleotide analogs used were 2',3'-(2,2,5, 5-tetramethyl-3-pyrroline-1-oxyl-3-carboxylic acid ester (SL) GTP, SL-GDP, and the non-hydrolyzable analog SL-guanylylimidodiphosphate. SL-GTP was hydrolyzed by p21 with rates similar to those for GTP hydrolysis and appears to be an excellent substrate analog. The ESR spectra of SL-GTP and SL-GDP in complex with p21 differ significantly when acquired at 0 degrees C or 5 degrees C indicating different environments (conformations) of the protein-bound radicals depending on the phosphorylation state of the bound nucleotide. We calculated the rate constant for the conformational change as deduced from the changes in the corresponding ESR spectra upon incubation of the p21.SL-GTP complex at 25 degrees C and compared it to the rate constant of hydrolysis of SL-GTP at the same temperature. The rate constant deduced from the ESR method was similar to that determined by a high performance liquid chromatography technique. The data are in agreement with the idea that a conformational change during GTP hydrolysis by p21 occurs simultaneously with the actual hydrolysis step.

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

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

The highly conserved skb1 gene encodes a protein that interacts with Shk1, a fission yeast Ste20/PAK homolog

The Shk1 protein kinase, a homolog of Saccharomyces cerevisiae Ste20 and mammalian p21Cdc42/Rac-activated kinases, is an essential component of a Ras- and Cdc42-dependent signaling cascade required for cell viability, normal morphology, and mitogen-activated protein kinase-mediated sexual responses in the fission yeast, Schizosaccharomyces pombe. To identify S. pombe proteins that modulate or mediate Shk1 functions, we conducted a two-hybrid screen for Shk1-interacting proteins. One of the genes identified as a result of this screen was skb1. We show that Skb1 interacts with a region of the N-terminal regulatory domain of Shk1 distinct from that to which Cdc42 binds, and that Shk1, Cdc42, and Skb1 are able to form a ternary complex in vivo. S.pombe cells carrying an skb1 null mutation are less elongate in morphology than wild-type cells and exhibit a moderate growth defect. The morphology defect of the skb1 deletion mutant is suppressed by overexpression of Shk1. Overexpression of Skb1 causes wild-type S. pombe cells to become hyperelongated. Additional genetic analyses described herein suggest that Skb1 is a component of the morphology control branch of the Ras signaling cascade in S. pombe and that it positively modulates Shk1 function. Homologs of Skb1 are encoded by open reading frames in the genomes of S. cerevisiae and Caenorhabditis elegans and by an uncharacterized human cDNA sequence. Thus, skb1 may be the first well-characterized member of a highly conserved family of genes encoding potential p21Cdc42/Rac-activated kinase regulators.

TC21 causes transformation by Raf-independent signaling pathways

Although the Ras-related protein TC21/R-Ras2 has only 55% amino acid identity with Ras proteins, mutated forms of TC21 exhibit the same potent transforming activity as constitutively activated forms of Ras. Therefore, like Ras, TC21 may activate signaling pathways that control normal cell growth and differentiation. To address this possibility, we determined if regulators and effectors of Ras are also important for controlling TC21 activity. First, we determined that Ras guanine nucleotide exchange factors (SOS1 and RasGRF/CDC25) synergistically enhanced wild-type TC21 activity in vivo and that Ras GTPase-activating proteins (GAPs; p120-GAP and NF1-GAP) stimulated wild-type TC21 GTP hydrolysis in vitro. Thus, extracellular signals that activate Ras via SOS1 activation may cause coordinate activation of Ras and TC21. Second, we determined if Raf kinases were effectors for TC21 transformation. Unexpectedly, yeast two-hybrid binding analyses showed that although both Ras and TC21 could interact with the isolated Ras-binding domain of Raf-1, only Ras interacted with full-length Raf-1, A-Raf, or B-Raf. Consistent with this observation, we found that Ras- but not TC21-transformed NIH 3T3 cells possessed constitutively elevated Raf-1 and B-Raf kinase activity. Thus, Raf kinases are effectors for Ras, but not TC21, signaling and transformation. We conclude that common upstream signals cause activation of Ras and TC21, but activated TC21 controls cell growth via distinct Raf-independent downstream signaling pathways.

Activation of the Raf-1/MAP kinase cascade is not sufficient for Ras transformation of RIE-1 epithelial cells

The potent transforming activity of membrane-targeted Raf-1 (Raf-CAAX) suggests that Ras transformation is triggered primarily by a Ras-mediated translocation of Raf-1 to the plasma membrane. However, whereas constitutively activated mutants of Ras [H-Ras(61L) and K-Ras4B(12V)] and Raf-1 (DeltaRaf-22W and Raf-CAAX) caused indistinguishable morphologic and growth (in soft agar and nude mice) transformation of NIH 3T3 fibroblasts, only mutant Ras caused morphologic transformation of RIE-1 rat intestinal cells. Furthermore, only mutant Ras-expressing RIE-1 cells formed colonies in soft agar and developed rapid and progressive tumors in nude mice. We also observed that activated Ras, but not Raf-1, caused transformation of IEC-6 rat intestinal and MCF-10A human mammary epithelial cells. Although both Ras- and DeltaRaf-22W-expressing RIE-1 cells showed elevated Raf-1 and mitogen-activated protein (MAP) kinase activities, only Ras-transformed cells produced secreted factors that promoted RIE-1 transformation. Incubation of untransformed RIE-1 cells in the presence of conditioned medium from Ras-expressing, but not DeltaRaf-22W-expressing, cells caused a rapid and stable morphologic transformation that was indistinguishable from the morphology of Ras-transformed RIE-1 cells. Thus, induction of an autocrine growth mechanism may distinguish the transforming actions of Ras and Raf. In summary, our observations demonstrate that oncogenic Ras activation of the Raf/MAP kinase pathway alone is not sufficient for full tumorigenic transformation of RIE-1 epithelial cells. Thus, Raf-independent signaling events are essential for oncogenic Ras transformation of epithelial cells, but not fibroblasts.

Mechanism of action of platelet-derived growth factor

More than 20 years ago, platelet-derived growth factor (PDGF) was identified and later purified. Through recent years of intense research, a large body of information has been collected on how PDGF transduces its biological effects to responding cells. Two homologous receptors, the PDGF alpha- and beta-receptors, have been identified, which are receptor tyrosine kinases. Binding of PDGF leads to activation of the kinase and autophosphorylation. Particularly in the PDGF beta-receptor, a considerable number of autophosphorylation sites have been identified, which allow for physical interaction with signal transduction molecules. The signal transduction molecules are often enzymes, which undergo activity changes in conjunction with binding to the receptor. Other signal transduction molecules function as adaptors, which can couple to subunits equipped with catalytic activity. Through the activity changes of inherent or directly coupled catalytic activities, a signal is propagated, which ultimately results in a cellular response. PDGF is known to induce migration, proliferation and differentiation of different cells types. An array of signal transduction molecules has been shown to interact with the PDGF beta-receptor; several appear to contribute to the generation of the proliferative response, indicating the existence of parallel pathways for this response, which are utilized by many different growth factor receptors. Migration of cells towards PDGF appears to be more strictly dependent on activation of phosphatidylinositol 3' kinase. Interestingly, the PDGF alpha-receptor emits negative signals that inhibit simultaneous positive signals for migration induced by this receptor, or by other receptors, such as the PDGF beta-receptor. Virtually nothing is known about signal transduction initiated by PDGF, which generates differentiation responses. Since PDGF appears to play a role in different physiological and pathological processes, it is important to continue delineation of signal transduction pathways initiated through activation of the PDGF receptors.

Induced mitotic death of HeLa cells by abnormal expression of c-H-ras

When HeLa cells were selected for stable expression of a neo gene, linked either to mutated or wt c-H-ras genes, morphological examination of selected clones from several experiments revealed formation of giant multinucleated cells. These morphological alterations culminate in cell death, as a consequence of mitotic catastrophe (or mitotic death). Although clones expressing the mutated gene produced significantly larger numbers of these giant cells, those transfected with the normal allele were also found to produce significantly more giant multinucleated cells than non-transfected HeLa cells. Northern blot analysis of mRNA revealed overexpression of the normal H-ras gene in these clones. Chromatin structure analysis of these clones showed gross alterations, including the presence of micronuclei and heteroploid nuclei. Interestingly, odd numbers of nuclei were found in colonies of these giant cells. In addition, alterations in cell cycle parameters were observed, including the appearance of a subpopulation of cells with an abnormal content of DNA, probably representing dying cells. Our data support the notion that abnormal expression of H-ras contributes to mitotic catastrophe and death of a subpopulation of HeLa cells.

Farnesyltransferase inhibitors and anti-Ras therapy

The oncoprotein encoded by mutant ras genes is initially synthesized as a cytoplasmic precursor which requires posttranslational processing to attain biological activity; farnesylation of the cysteine residue present in the CaaX motif located at the carboxy-terminus of all Ras proteins is the critical modification. Once farnesylated and further modified, the mature Ras protein is inserted into the cell's plasma membrane where it participates in the signal transduction pathways that control cell growth and differentiation. The farnesylation reaction that modifies Ras and other cellular proteins having an appropriate CaaX motif is catalyzed by a housekeeping enzyme termed farnesyl-protein transferase (FPTase). Inhibitors of this enzyme have been prepared by several laboratories in an effort to identify compounds that would block Ras-induced cell transformation and thereby function as Ras-specific anticancer agents. A variety of natural products and synthetic organic compounds were found to block farnesylation of Ras proteins in vitro. Some of these compounds exhibit antiproliferative activity in cell culture, block the morphological alterations associated with Ras-transformation, and can block the growth of Ras-transformed cell lines in tumor colony-forming assays. By contrast, these compounds do not affect the growth or morphology of cells transformed by the Raf or Mos oncoproteins, which do not require farnesylation to achieve biological activity. The efficacy and lack of toxicity observed with FPTase inhibitors in an animal tumor model suggest that specific FPTase inhibitors may be useful for the treatment of some types of cancer.

Massive programmed cell death in intestinal epithelial cells induced by three-dimensional growth conditions: suppression by mutant c-H-ras oncogene expression

Deregulation of molecular pathways controlling cell survival and death, including programmed cell death, are thought to be important factors in tumor formation, disease progression, and response to therapy. Studies devoted to analyzing the role of programmed cell death in cancer have been carried out primarily using conventional monolayer cell culture systems. However the majority of cancers grow as three-dimensional solid tumors. Because gene expression, and possibly function, can be significantly altered under such conditions, we decided to analyze the control and characteristics of cell death using a compatible three-dimensional tissue culture system (multicellular spheroids) and compare the results obtained to those using two-dimensional monolayer cell culture. To do so we selected for study an immortalized, but nontumorigenic line of rat intestinal epithelial cells, called IEC-18, and several tumorigenic variants of IEC-18 obtained by transfection with a mutant (activated) c-H-ras oncogene. The rationale for choosing these cell lines was based in part on the fact that intestinal epithelial cells grow in vivo in a monolayer-like manner and form solid tumors only after sustaining certain genetic mutations, including those involving the ras gene family. We found that the IEC-18 cells, which grow readily and survive in monolayer cell culture, undergo massive cell death within 48-72 h when cultured as multicellular spheroids on a nonadhesive surface. This process was accompanied by a number of features associated with programmed cell death including chromatin condensation (Hoechst 33258 staining) apoptotic morphology, DNA degradation, and a virtual complete loss of colony forming (clonogenic) ability in the absence of apparent membrane damage as well as accumulation of lipid containing vacuoles in the cytoplasm. Moreover, enforced over-expression of a transfected bcl-2 gene could prevent this cell death process from taking place. In marked contrast, three different stably transfected ras clones of IEC-18 survived when grown as multicellular spheroids. In addition, an IEC cell line (called clone 25) carrying its mutant transfected ras under a glucocorticoid inducible promoter survived in three-dimensional culture only when the cells were exposed to dexamethasone. If exposure to dexamethasone was delayed for as long as 48 h the cells nevertheless survived, whereas the cells became irreversibly committed to programmed cell death (PCD) if exposed to dexamethasone after 72 h. These results suggest that intestinal epithelial cells may be programmed to activate a PCD pathway upon detachment from a physiologic two-dimensional monolayer configuration, and that this process of adhesion regulated programmed cell death (ARPCD) can be substantially suppressed by expression of a mutant ras oncogene.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of Rac1, RhoA, and mitogen-activated protein kinases is required for Ras transformation

Although substantial evidence supports a critical role for the activation of Raf-1 and mitogen-activated protein kinases (MAPKs) in oncogenic Ras-mediated transformation, recent evidence suggests that Ras may activate a second signaling pathway which involves the Ras-related proteins Rac1 and RhoA. Consequently, we used three complementary approaches to determine the contribution of Rac1 and RhoA function to oncogenic Ras-mediated transformation. First, whereas constitutively activated mutants of Rac1 and RhoA showed very weak transforming activity when transfected alone, their coexpression with a weakly transforming Raf-1 mutant caused a greater than 35-fold enhancement of transforming activity. Second, we observed that coexpression of dominant negative mutants of Rac1 and RhoA reduced oncogenic Ras transforming activity. Third, activated Rac1 and RhoA further enhanced oncogenic Ras-triggered morphologic transformation, as well as growth in soft agar and cell motility. Finally, we also observed that kinase-deficient MAPKs inhibited Ras transformation. Taken together, these data support the possibility that oncogenic Ras activation of Rac1 and RhoA, coupled with activation of the Raf/MAPK pathway, is required to trigger the full morphogenic and mitogenic consequences of oncogenic Ras transformation.

Aberrant function of the Ras signal transduction pathway in human breast cancer

Although ras mutations are infrequent (approximately 5%) in breast cancers, there is considerable evidence that suggests that the pathways which Ras services may still be deregulated in breast cancer cells. The recent identification of many of the components of the Ras signal transduction pathway has defined a network of proto-oncogene proteins controlling diverse signaling events that regulate cell growth and differentiation. Consequently, mutations that perturb the function of any one component of this signal pathway may trigger the same oncogenic events as mutation of ras itself. Moreover, several Ras-related proteins have recently been demonstrated to possess the ability to trigger malignant transformation via signaling pathways shared with Ras proteins. Thus, it is possible that the aberrant function of Ras-related proteins may contribute to breast cancer development. Consequently, it is important not to dismiss the Ras pathway in the development of breast cancer merely because of the infrequent detection of mutations in ras itself, but rather to consider the influence of aberrations upstream or downstream of Ras and of certain Ras-related proteins in the development of breast cancer. Finally, the critical importance of components upstream and downstream of Ras provides additional targets for rational drug design approaches to block the aberrant function of Ras signaling in human tumors.

Shk1, a homolog of the Saccharomyces cerevisiae Ste20 and mammalian p65PAK protein kinases, is a component of a Ras/Cdc42 signaling module in the fission yeast Schizosaccharomyces pombe

We describe a protein kinase, Shk1, from the fission yeast Schizosaccharomyces pombe, which is structurally related to the Saccharomyces cerevisiae Ste20 and mammalian p65PAK protein kinases. We provide genetic evidence for physical and functional interaction between Shk1 and the Cdc42 GTP-binding protein required for normal cell morphology and mating in S. pombe. We further show that expression of the STE20 gene complements the shk1 null mutation and that Shk1 is capable of signaling to the pheromone-responsive mitogen-activated protein kinase cascade in S. cerevisiae. Our results lead us to propose that signaling modules composed of small GTP-binding proteins and protein kinases related to Shk1, Ste20, and p65PAK, are highly conserved in evolution and participate in both cytoskeletal functions and mitogen-activated protein kinase signaling pathways.

Dynamics of active lamellae in cultured epithelial cells: effects of expression of exogenous N-ras oncogene

We examined the functional consequences of cellular transformation of rat IAR-2 epithelial cells, by a mutant N-ras oncogene, on the dynamics of active lamellae, structures that play an important role in cell motility, adhesion, and surface-receptor capping. Lamellar activity was assessed by measuring the rate of outer-edge pseudopodial activity and by analyzing the motility of Con A-coated beads placed on lamellar surfaces with optical tweezers. Although transformation dramatically affected the shape and size of active cellular lamellae, there was little detectable effect on either pseudopodial activity or bead movement. To investigate the potential relationship between functional lamellar activity and the microtubule cytoskeleton, lamellar activity was examined in nontransformed and transformed cells treated with the microtubule-disrupting drug nocodazole. In the absence of microtubules, transformed cells were less polarized and possessed decreased rates of pseudopodial and bead motility. On the basis of these observations, it is suggested that ras-induced transformation of epithelial cells consists of two cytoskeletal modifications: overall diminished actin cytoskeletal dynamics in lamellae and reorganization of the microtubule cytoskeleton that directs pseudopodial activity to smaller polarized lamellae.

Multisubunit receptors in the immune system and their association with the cytoskeleton: in search of functional significance

Various multisubunit receptors of the immune system share similarities in structure and induce closely related signal transduction pathways upon ligand binding. Examples include the T cell antigen receptor (TCR), the B cell antigen receptor (BCR), and the high-affinity receptor for immunoglobulin E (Fc epsilon RI). Although these receptors are devoid of intrinsic kinase activity, they can associate with a similar array of intracellular kinases, phosphatases and other signaling molecules. Furthermore, these receptor complexes all form an association with the cytoskeletal matrix. In this review, we compare the structural and functional characteristics of the TCR, BCR and Fc epsilon RI. We examine the role of the cytoskeleton in regulating receptor-mediated signal transduction, as analyzed in other well-characterized receptors, including the epidermal growth factor receptor and integrin receptors. On the basis of this evidence, we review the current data depicting a cytoskeletal association for multisubunit immune system receptors and explore the potential bearing of this interaction on signaling function.

Dbl and Vav mediate transformation via mitogen-activated protein kinase pathways that are distinct from those activated by oncogenic Ras

Vav and Dbl are members of a novel class of oncogene proteins that share significant sequence identity in a approximately 250-amino-acid domain, designated the Dbl homology domain. Although Dbl functions as a guanine nucleotide exchange factor (GEF) and activator of Rho family proteins, recent evidence has demonstrated that Vav functions as a GEF for Ras proteins. Thus, transformation by Vav and Dbl may be a consequence of constitutive activation of Ras and Rho proteins, respectively. To address this possibility, we have compared the transforming activities of Vav and Dbl with that of the Ras GEF, GRF/CDC25. As expected, GRF-transformed cells exhibited the same reduction in actin stress fibers and focal adhesions as Ras-transformed cells. In contrast, Vav- and Dbl-transformed cells showed the same well-developed stress fibers and focal adhesions observed in normal or RhoA(63L)-transformed NIH 3T3 cells. Furthermore, neither Vav- or Dbl-transformed cells exhibited the elevated levels of Ras-GTP (60%) observed with GRF-transformed cells. Finally, GRF, but not Vav or Dbl, induced transcriptional activation from Ras-responsive DNA elements (ets/AP-1, fos promoter, and kappa B). However, like Ras- and GRF-transformed cells, both Vav- and Dbl-transformed cells exhibited constitutively activated mitogen-activated protein kinases (MAPKs) (primarily p42MAPK/ERK2). Since kinase-deficient forms of p42MAPK/ERK2 and p44MAPK/ERK1 inhibited Dbl transformation, MAPK activation may be an important component of its transforming activity. Taken together, our observations indicate that Vav and Dbl transformation is not a consequence of Ras activation and instead may involve the constitutive activation of MAPKs.

Dbl and Vav mediate transformation via mitogen-activated protein kinase pathways that are distinct from those activated by oncogenic Ras

Vav and Dbl are members of a novel class of oncogene proteins that share significant sequence identity in a approximately 250-amino-acid domain, designated the Dbl homology domain. Although Dbl functions as a guanine nucleotide exchange factor (GEF) and activator of Rho family proteins, recent evidence has demonstrated that Vav functions as a GEF for Ras proteins. Thus, transformation by Vav and Dbl may be a consequence of constitutive activation of Ras and Rho proteins, respectively. To address this possibility, we have compared the transforming activities of Vav and Dbl with that of the Ras GEF, GRF/CDC25. As expected, GRF-transformed cells exhibited the same reduction in actin stress fibers and focal adhesions as Ras-transformed cells. In contrast, Vav- and Dbl-transformed cells showed the same well-developed stress fibers and focal adhesions observed in normal or RhoA(63L)-transformed NIH 3T3 cells. Furthermore, neither Vav- or Dbl-transformed cells exhibited the elevated levels of Ras-GTP (60%) observed with GRF-transformed cells. Finally, GRF, but not Vav or Dbl, induced transcriptional activation from Ras-responsive DNA elements (ets/AP-1, fos promoter, and kappa B). However, like Ras- and GRF-transformed cells, both Vav- and Dbl-transformed cells exhibited constitutively activated mitogen-activated protein kinases (MAPKs) (primarily p42MAPK/ERK2). Since kinase-deficient forms of p42MAPK/ERK2 and p44MAPK/ERK1 inhibited Dbl transformation, MAPK activation may be an important component of its transforming activity. Taken together, our observations indicate that Vav and Dbl transformation is not a consequence of Ras activation and instead may involve the constitutive activation of MAPKs.

Farnesyltransferase inhibition causes morphological reversion of ras-transformed cells by a complex mechanism that involves regulation of the actin cytoskeleton

A potent and specific small molecule inhibitor of farnesyl-protein transferase, L-739,749, caused rapid morphological reversion and growth inhibition of ras-transformed fibroblasts (Rat1/ras cells). Morphological reversion occurred within 18 h of L-739,749 addition. The reverted phenotype was stable for several days in the absence of inhibitor before the transformed phenotype reappeared. Cell enlargement and actin stress fiber formation accompanied treatment of both Rat1/ras and normal Rat1 cells. Significantly, inhibition of Ras processing did not correlate with the initiation or maintenance of the reverted phenotype. While a single treatment with L-739,749 was sufficient to morphologically revert Rat1/ras cells, repetitive inhibitor treatment was required to significantly reduce cell growth rate. Thus, the effects of L-739,749 on transformed cell morphology and cytoskeletal actin organization could be separated from effects on cell growth, depending on whether exposure to a farnesyl-protein transferase inhibitor was transient or repetitive. In contrast, L-739,749 had no effect on the growth, morphology, or actin organization of v-raf-transformed cells. Taken together, the results suggest that the mechanism of morphological reversion is complex and may involve farnesylated proteins that control the organization of cytoskeletal actin.

Farnesyltransferase inhibition causes morphological reversion of ras-transformed cells by a complex mechanism that involves regulation of the actin cytoskeleton

A potent and specific small molecule inhibitor of farnesyl-protein transferase, L-739,749, caused rapid morphological reversion and growth inhibition of ras-transformed fibroblasts (Rat1/ras cells). Morphological reversion occurred within 18 h of L-739,749 addition. The reverted phenotype was stable for several days in the absence of inhibitor before the transformed phenotype reappeared. Cell enlargement and actin stress fiber formation accompanied treatment of both Rat1/ras and normal Rat1 cells. Significantly, inhibition of Ras processing did not correlate with the initiation or maintenance of the reverted phenotype. While a single treatment with L-739,749 was sufficient to morphologically revert Rat1/ras cells, repetitive inhibitor treatment was required to significantly reduce cell growth rate. Thus, the effects of L-739,749 on transformed cell morphology and cytoskeletal actin organization could be separated from effects on cell growth, depending on whether exposure to a farnesyl-protein transferase inhibitor was transient or repetitive. In contrast, L-739,749 had no effect on the growth, morphology, or actin organization of v-raf-transformed cells. Taken together, the results suggest that the mechanism of morphological reversion is complex and may involve farnesylated proteins that control the organization of cytoskeletal actin.

Ras regulatory interactions: novel targets for anti-cancer intervention?

Advances in the understanding of Ras oncoprotein function suggest novel points for anti-tumor intervention. First, upstream-acting guanine nucleotide exchange factors and SH2/SH3 domain-containing adaptor proteins that link Ras with growth factor receptor tyrosine kinases have recently been characterized. Second, work on downstream-acting Ras effector functions including the Ras GTPase-activating protein (p120GAP) and the Raf kinase has revealed direct biochemical interactions that are functionally required for oncogenic Ras signalling. We summarize progress in these areas and discuss the potential for novel applications to anti-cancer chemotherapy.