Rac regulates phosphorylation of the myosin-II heavy chain, actinomyosin disassembly and cell spreading.

GTPases of the Rho family regulate actinomyosin-based contraction in non-muscle cells. Activation of Rho increases contractility, leading to cell rounding and neurite retraction in neuronal cell lines. Activation of Rac promotes cell spreading and interferes with Rho-mediated cell rounding. Here we show that activation of Rac may antagonize Rho by regulating phosphorylation of the myosin-II heavy chain. Stimulation of PC12 cells or N1E-115 neuroblastoma cells with bradykinin induces phosphorylation of threonine residues in the myosin-II heavy chain; this phosphorylation is Ca2+ dependent and regulated by Rac. Both bradykinin-mediated and constitutive activation of Rac promote cell spreading, accompanied by a loss of cortical myosin II. Our results identify the myosin-II heavy chain as a new target of Rac-regulated kinase pathways, and implicate Rac as a Rho antagonist during myosin-II-dependent cell-shape changes.

Tiam1-deficiency impairs mammary tumor formation in MMTV-c-neu but not in MMTV-c-myc mice.

BACKGROUND: Rho-like small GTPases, including RhoA, Rac1 and Cdc42, are crucial for the regulation of a large variety of biological processes such as the cytoskeletal organization and gene transcription. The activities of Rho GTPases are predominantly controlled by guanine nucleotide exchange factors (GEFs), which activate GTPases by catalyzing the exchange of bound GDP for GTP. Earlier, we have identified the Tiam1 gene as an invasion-inducing gene that encodes a specific activator (GEF) of the Rac GTPase. We found that Tiam1-mediated Rac signaling functions in various aspects of tumorigenicity including the formation and progression of Ras-induced skin tumors and Wnt-induced intestinal tumors. Here, we further distinguish the oncogenic pathways that depend on Tiam1 signaling in the mammary gland. MATERIAL AND METHODS: We crossed Tiam1 knockout mice with MMTV-c-myc and MMTV-c-neu transgenic mice, in which the expression of both oncogenes is targeted to the mammary gland leading to mammary tumorigenesis. RESULTS: We found Tiam1 important for Neu-induced tumor formation and progression but not for Myc-induced tumors. Tiam1-deficiency delayed Neu-induced tumor initiation and reduced metastasis but had no effect on the growth of the MMTV-c-neu tumors. CONCLUSION: Our data indicate that the Rac activator Tiam1 contributes to tumorigenicity induced by specific oncogenic signaling pathways only.

Surface morphology and agglutinability with concanavalin A in normal and transformed murine fibroblasts.

The surface morphology of attached and suspended normal and transformed fibroblasts has been studied with the scanning electron microscope. Normal murine fibroblasts (3T3) grow in vitro with widely extended leading lamellae. During most parts of the cell cycle the surfaces of these cells are practically free of microvilli. When the cells round up for mitosis, their cell surfaces become adorned with many microvilli. In contrast, simian virus 40-transformed fibroblasts (SV3T3) grow more compact, and their cell surfaces remain smooth throughout the life cycle. When confluent 3T3 and SV3T3 cells are suspended with ethylenediaminetetraacetic acid (EDTA) for agglutination assays, similar differences in surface morphology are found: 3T3 cells always bear many microvilli, whereas most SV3T3 cells are essentially free of microvilli. The addition of concanavalin A (Con A) does not influence the surface morphology of the suspended cells. The morphological differences described here may be important for the agglutination process of the normal and transformed 3T3 cells, because they affect the real cell surface area and thus the density of Con A-binding sites.

The guanine nucleotide exchange factor Tiam1 affects neuronal morphology; opposing roles for the small GTPases Rac and Rho.

The invasion-inducing T-lymphoma invasion and metastasis 1 (Tiam1) protein functions as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac1. Differentiation-dependent expression of Tiam1 in the developing brain suggests a role for this GEF and its effector Rac1 in the control of neuronal morphology. Here we show that overexpression of Tiam1 induces cell spreading and affects neurite outgrowth in N1E-115 neuroblastoma cells. These effects are Rac-dependent and strongly promoted by laminin. Overexpression of Tiam1 recruits the alpha 6 beta 1 integrin, a laminin receptor, to specific adhesive contacts at the cell periphery, which are different from focal contacts. Cells overexpressing Tiam1 no longer respond to lysophosphatidic acid- induced neurite retraction and cell rounding, processes mediated by Rho, suggesting that Tiam1-induced activation of Rac antagonizes Rho signaling. This inhibition can be overcome by coexpression of constitutively active RhoA, which may indicate that regulation occurs at the level of Rho or upstream. Conversely, neurite formation induced by Tiam1 or Rac1 is further promoted by inactivating Rho. These results demonstrate that Rac- and Rho-mediated pathways oppose each other during neurite formation and that a balance between these pathways determines neuronal morphology. Furthermore, our data underscore the potential role of Tiam1 as a specific regulator of Rac during neurite formation and illustrate the importance of reciprocal interactions between the cytoskeleton and the extracellular matrix during this process.

Oncogenic Ras downregulates Rac activity, which leads to increased Rho activity and epithelial-mesenchymal transition.

Proteins of the Rho family regulate cytoskeletal rearrangements in response to receptor stimulation and are involved in the establishment and maintenance of epithelial cell morphology. We recently showed that Rac is able to downregulate Rho activity and that the reciprocal balance between Rac and Rho activity is a major determinant of cellular morphology and motility in NIH3T3 fibroblasts. Using biochemical pull-down assays, we analyzed the effect of transient and sustained oncogenic Ras signaling on the activation state of Rac and Rho in epithelial MDCK cells. In contrast to the activation of Rac by growth factor-induced Ras signaling, we found that sustained signaling by oncogenic RasV12 permanently downregulates Rac activity, which leads to upregulation of Rho activity and epithelial-mesenchymal transition. Oncogenic Ras decreases Rac activity through sustained Raf/MAP kinase signaling, which causes transcriptional downregulation of Tiam1, an activator of Rac in epithelial cells. Reconstitution of Rac activity by expression of Tiam1 or RacV12 leads to downregulation of Rho activity and restores an epithelial phenotype in mesenchymal RasV12- or RafCAAX-transformed cells. The present data reveal a novel mechanism by which oncogenic Ras is able to interfere with the balance between Rac and Rho activity to achieve morphological transformation of epithelial cells.

Induction of cell scattering by expression of beta1 integrins in beta1-deficient epithelial cells requires activation of members of the rho family of GTPases and downregulation of cadherin and catenin function.

Adhesion receptors, which connect cells to each other and to the surrounding extracellular matrix (ECM), play a crucial role in the control of tissue structure and of morphogenesis. In this work, we have studied how intercellular adhesion molecules and beta1 integrins influence each other using two different beta1-null cell lines, epithelial GE11 and fibroblast-like GD25 cells. Expression of beta1A or the cytoplasmic splice variant beta1D, induced the disruption of intercellular adherens junctions and cell scattering in both GE11 and GD25 cells. In GE11 cells, the morphological change correlated with the redistribution of zonula occluden (ZO)-1 from tight junctions to adherens junctions at high cell confluency. In addition, the expression of beta1 integrins caused a dramatic reorganization of the actin cytoskeleton and of focal contacts. Interaction of beta1 integrins with their respective ligands was required for a complete morphological transition towards the spindle-shaped fibroblast-like phenotype. The expression of an interleukin-2 receptor (IL2R)-beta1A chimera and its incorporation into focal adhesions also induced the disruption of cadherin-based adhesions and the reorganization of ECM-cell contacts, but failed to promote cell migration on fibronectin, in contrast to full-length beta1A. This indicates that the disruption of cell-cell adhesion is not simply the consequence of the stimulated cell migration. Expression of beta1 integrins in GE11 cells resulted in a decrease in cadherin and alpha-catenin protein levels accompanied by their redistribution from the cytoskeleton-associated fraction to the detergent-soluble fraction. Regulation of alpha-catenin protein levels by beta1 integrins is likely to play a role in the morphological transition, since overexpression of alpha-catenin in GE11 cells before beta1 prevented the disruption of intercellular adhesions and cell scattering. In addition, using biochemical activity assays for Rho-like GTPases, we show that the expression of beta1A, beta1D, or IL2R-beta1A in GE11 or GD25 cells triggers activation of both RhoA and Rac1, but not of Cdc42. Moreover, dominant negative Rac1 (N17Rac1) inhibited the disruption of cell-cell adhesions when expressed before beta1. However, all three GTPases might be involved in the morphological transition, since expression of either N19RhoA, N17Rac1, or N17Cdc42 reversed cell scattering and partially restored cadherin-based adhesions in GE11-beta1A cells. Our results indicate that beta1 integrins regulate the polarity and motility of epithelial cells by the induction of intracellular molecular events involving a downregulation of alpha-catenin function and the activation of the Rho-like G proteins Rac1 and RhoA.

Rac downregulates Rho activity: reciprocal balance between both GTPases determines cellular morphology and migratory behavior.

Using biochemical assays to determine the activation state of Rho-like GTPases, we show that the guanine nucleotide exchange factor Tiam1 functions as a specific activator of Rac but not Cdc42 or Rho in NIH3T3 fibroblasts. Activation of Rac by Tiam1 induces an epithelial-like morphology with functional cadherin-based adhesions and inhibits migration of fibroblasts. This epithelial phenotype is characterized by Rac-mediated effects on Rho activity. Transient PDGF-induced as well as sustained Rac activation by Tiam1 or V12Rac downregulate Rho activity. We found that Cdc42 also downregulates Rho activity. Neither V14Rho or N19Rho affects Rac activity, suggesting unidirectional signaling from Rac towards Rho. Downregulation of Rho activity occurs independently of Rac- induced cytoskeletal changes and cell spreading. Moreover, Rac effector mutants that are defective in mediating cytoskeleton changes or Jun kinase activation both downregulate Rho activity, suggesting that neither of these Rac signaling pathways are involved in the regulation of Rho. Restoration of Rho activity in Tiam1-expressing cells by expression of V14Rho results in reversion of the epithelioid phenotype towards a migratory, fibroblastoid morphology. We conclude that Rac signaling is able to antagonize Rho activity directly at the GTPase level, and that the reciprocal balance between Rac and Rho activity determines cellular morphology and migratory behavior in NIH3T3 fibroblasts.

Matrix-dependent Tiam1/Rac signaling in epithelial cells promotes either cell-cell adhesion or cell migration and is regulated by phosphatidylinositol 3-kinase.

We previously demonstrated that both Tiam1, an activator of Rac, and constitutively active V12Rac promote E-cadherin-mediated cell-cell adhesion in epithelial Madin Darby canine kidney (MDCK) cells. Moreover, Tiam1 and V12Rac inhibit invasion of Ras-transformed, fibroblastoid MDCK-f3 cells by restoring E-cadherin-mediated cell-cell adhesion. Here we show that the Tiam1/Rac-induced cellular response is dependent on the cell substrate. On fibronectin and laminin 1, Tiam1/Rac signaling inhibits migration of MDCK-f3 cells by restoring E-cadherin-mediated cell- cell adhesion. On different collagens, however, expression of Tiam1 and V12Rac promotes motile behavior, under conditions that prevent formation of E-cadherin adhesions. In nonmotile cells, Tiam1 is present in adherens junctions, whereas Tiam1 localizes to lamellae of migrating cells. The level of Rac activation by Tiam1, as determined by binding to a glutathione-S-transferase- PAK protein, is similar on fibronectin or collagen I, suggesting that rather the localization of the Tiam1/Rac signaling complex determines the substrate-dependent cellular responses. Rac activation by Tiam1 requires PI3-kinase activity. Moreover, Tiam1- but not V12Rac-induced migration as well as E-cadherin-mediated cell- cell adhesion are dependent on PI3-kinase, indicating that PI3-kinase acts upstream of Tiam1 and Rac.

Regulated membrane localization of Tiam1, mediated by the NH2-terminal pleckstrin homology domain, is required for Rac-dependent membrane ruffling and C-Jun NH2-terminal kinase activation.

Rho-like GTPases, including Cdc42, Rac, and Rho, regulate signaling pathways that control actin cytoskeletal structures and transcriptional activation. The Tiam1 gene encodes an activator of Rac1, and similarly to constitutively activated (V12)Rac1, overexpression of Tiam1 in fibroblasts induces the formation of membrane ruffles. Tiam1 contains a Dbl homology (DH) domain and adjacent pleckstrin homology (PH) domain, hallmarks for activators of Rho-like GTPases. Unique for Tiam1 are an additional PH domain and a Discs-large homology region in the NH2-terminal part of the protein. Here we show that both in fibroblasts and COS cells, membrane localization of Tiam1 is required for the induction of membrane ruffling. A detailed mutational analysis, in combination with confocal laser scanning microscopy and immunoelectron microscopy, demonstrates that the NH2-terminal PH domain of Tiam1, but not the DH-adjacent PH domain, is essential for membrane association. This NH2-terminal PH domain of Tiam1 can be functionally replaced by the myristoylated membrane localization domain of c-Src, indicating that the primary function of this PH domain is to localize the protein at the membrane. After serum starvation, both membrane association of Tiam1 and ruffling can be induced by serum, suggesting that receptor stimulation induces membrane translocation of Tiam1. Similar to V12Rac1, Tiam1 stimulates the activity of the c-Jun NH2-terminal kinase (JNK). This Rac-dependent stimulation of JNK also requires membrane association of Tiam1. We conclude that the regulated membrane localization of Tiam1 through its NH2-terminal PH domain determines the activation of distinct Rac-mediated signaling pathways.

Inhibition of invasion of epithelial cells by Tiam1-Rac signaling.

Tiam1 encodes an exchange factor for the Rho-like guanosine triphosphatase Rac. Both Tiam1 and activated RacV12 promote invasiveness of T lymphoma cells. In epithelial Madin-Darby canine kidney (MDCK) cells, Tiam1 localized to adherens junctions. Ectopic expression of Tiam1 or RacV12 inhibited hepatocyte growth factor-induced scattering by increasing E-cadherin-mediated cell-cell adhesion accompanied by actin polymerization at cell-cell contacts. In Ras-transformed MDCK cells, expression of Tiam1 or RacV12 restored E-cadherin-mediated adhesion, resulting in phenotypic reversion and loss of invasiveness. These data suggest an invasion-suppressor role for Tiam1 and Rac in epithelial cells.

Invasive and metastatic potential induced by ras-transfection into mouse BW5147 T-lymphoma cells.

Noninvasive, nonmetastatic BW5147 T-lymphoma cells were transfected with the activated human c-Ha-ras oncogene and were examined subsequently for the acquisition of invasive properties in vitro and of metastatic potential in vivo. It was found that several transfectants harboring the ras gene had become invasive in vitro, as assessed in hepatocyte cultures, and metastatic after tail vein injection into syngeneic AKR mice. The induced level of both invasive and metastatic potential appeared to depend on the level of expression of the transfected ras gene. Those transfectants exhibiting an elevated level of ras expression, mostly cells containing a high copy number of the ras gene, showed the highest invasiveness (up to 30-fold increase) and produced widespread metastasis in all mice tested. Transfectants with a low level of ras expression were less invasive and formed metastases in a few mice only, limited to a few organs or even to a single deposit in one organ. Untransfected BW cells, control transfected cells without the ras gene, and ras transfectants that did not express the gene were noninvasive and nonmetastatic. No changes in number of ras gene copies were found between isolated metastases and the transfectants from which they were derived. However, RNA analysis of the cells from the isolated metastases revealed similar, as well as elevated or diminished levels of ras transcription when compared to the corresponding cell lines prior to injection, suggesting that a persistent high expression of the ras gene is not necessarily needed for the independent growth at the secondary site. Our results indicate that the activated human ras oncogene may confer metastatic potential onto lymphoid tumor cells, probably due to the induction of invasiveness.

Decrease in 5'-nucleotidase activity in malignant transformed and normal stimulated cells.

Analysis of six different cell types of normal and transformed fibroblasts grown in vitro and of four different cell types of normal and leukemic lymphocytes grown in vivo have shown a marked decrease of 3- to 30-fold in the specific activity of 5'-nucleotidase in the malignant cells as compared to their normal parental cells. The results have also indicated that a serum stimulation of untransformed or normal fibroblasts and a stimulation of normal lymphocytes by concanavalin A resulted in a significant decrease in the specific activity of 5'-nucleotidase of the stimulated cultures as compared to the resting cells. In both the malignant cells and the stimulated normal cells, the decrease in 5'-nucleotidase activity was not accompanied by a similar decrease in the specific activity of acid phosphatase, indicating a specific enzyme alteration in the surface membranes of the transformed and the normal stimulated cells.

Invasiveness in hepatocyte and fibroblast monolayers and metastatic potential of T-cell hybridomas in mice.

Fusion of noninvasive, nonmetastatic BW5147 T-lymphoma cells with normal T-lymphocytes usually resulted in highly invasive and metastatic T-cell hybridomas, apparently due to properties derived from the normal T-cell. Occasionally hybrids arose that were non- or low invasive, probably by loss of relevant genes upon chromosome segregation, since these cells contained much less DNA than highly invasive hybrids. The metastatic potential of 20 representative T-cell hybridomas was tested by tail vein injection in syngeneic mice and cells were found to be either nonmetastatic (NM), low metastatic (LM), or high metastatic (HM). NM hybrids were tumorigenic but did not form metastases and HM hybridomas caused wide-spread metastasis. LM cells formed metastases in a limited number of mice and predominantly in lymphoid tissues. In hepatocyte cultures, NM cell lines were found to be the least invasive, HM cells the most, whereas LM hybrids exhibited intermediate levels. Invasiveness was not only measured in rat hepatocyte cultures but also in rat embryo fibroblast monolayers, and the relative invasive capacity in both model systems correlated well. Pertussis toxin inhibited invasion in both systems to 20-30% of control values. This suggests that the mechanisms of invasion into hepatocyte and fibroblast cultures are at least partially similar and that the fibroblast invasion assay is a relevant model to study aspects of lymphoma metastasis. We conclude that invasive potential is a prerequisite for T-cell hybridomas to colonize tissues from the bloodstream and that a minimum level of invasiveness is necessary for extensive and wide-spread metastasis formation.

Oncogene activation in human myeloid leukemia.

We have studied by means of DNA-mediated gene transfer the activation of protooncogenes in human myeloid leukemias that represent various stages of myeloid differentiation. DNA from three cell lines, HL-60 (promyelocytic leukemia), Rc2a (myelomonocytic leukemia), and KG-1 (acute myeloblastic leukemia), was capable of transforming NIH/3T3 cells. Hybridization analysis indicated that, in all three tumor cell lines, the N-ras oncogene was activated. The cell lines U-937 ("histiocytic lymphoma") and K-562 (erythroblastic leukemia) yielded no transforming DNA. Fresh leukemia cells derived from an acute myelomonocytic leukemia patient and from a juvenile chronic myelogenous leukemia patient contained an activated N-ras and c-Ki-ras oncogene, respectively. DNA from some other myelogenous leukemia patients was not able to transform NIH/3T3 cells. Our results indicate that hematopoietic tumors of the myeloid lineage may contain oncogenes active in NIH/3T3 cell transformation and that, in particular, the N-ras oncogene may be activated in tumors representing various stages of maturation.

Cell surface sialic acid and the invasive and metastatic potential of T-cell hybridomas.

T-cell hybridomas prepared by fusion of non-invasive non-metastatic BW5147 T-lymphoma cells and activated normal T-cells were found to be highly invasive in vitro and highly metastatic in vivo upon tail vein injection. By prolonged culturing and subcloning, non-invasive, non-metastatic hybrids were selected with modal DNA/cell contents close to the diploid value of both fusion partners. Since normal activated T-cells were invasive in vitro in hepatocyte cultures, these data suggest that invasiveness of the hybrids is derived from the parental normal T-cells and is one of the properties responsible for the metastatic potential of these cells. Analysis of a large panel of T-cell hybrids with fluorescein isothiocyanate conjugated lectins, specific for terminal galactose and/or N-acetylgalactosamine sugar residues, showed an inverse correlation between expression of lectin receptor sites and invasive and metastatic potential of the hybrids. Soybean agglutinin, as well as peanut agglutinin and Ricinus communis agglutinin, reacted strongly with non- or low-invasive hybrids but only weakly with invasive hybrids. The difference in lectin binding between both types of hybrids appeared to be due to masking of receptor sites by sialic acid. Removal of cell surface sialic acid by neuraminidase treatment unmasked the lectin receptor sites of invasive hybrids to the level of the corresponding sites of non- or low-invasive cells. This increase in active lectin binding sites was simultaneously accompanied by a striking decrease of invasiveness to the level of the low-invasive hybrids. Conversely, the blocking of R. communis agglutinin receptors by sialic acid allowed selection of invasive hybrids from segregating cell populations with the toxic lectin R. communis agglutinin. The results taken together indicate that sialylation of particular cell surface carbohydrate residues on the T-cell hybridomas is associated with the invasive and metastatic potential of these hybrids. The reduction of invasive potential after removal of cell surface sialic acid provides further evidence for a functional role of this sugar residue in invasiveness of the T-cell hybrids.

Invasiveness of T-cell hybridomas in vitro and their metastatic potential in vivo.

BW5147 lymphoma cells, which are noninvasive and nonmetastatic, were fused with normal T-lymphocytes. The invasiveness of the generated T-cell hybridomas was tested in hepatocyte cultures, and their metastatic potential was tested by tail vein injection. A total of 29 hybridomas generated from alloantigen-activated T-cells were all found to be invasive. One of these cell lines rapidly lost invasiveness in culture. Most hybridomas generated from nonstimulated spleen T-cells were also invasive, but 5 of 27 were not. Six invasive and four noninvasive hybridomas were injected into the tail vein of syngeneic mice. All invasive cell lines caused extensive and widespread tumor growth, particularly in the liver, which was usually severalfold enlarged; the spleen; kidneys; and ovaries. In contrast the noninvasive hybrids, which were tumorigenic upon s.c. injection, did not form any metastases. We conclude that properties derived from normal T-cells, when introduced into noninvasive T-lymphoma cells, cause them to become invasive as well as metastatic. Furthermore for this tumor cell type invasiveness as measured in hepatocyte cultures appears to be closely associated with the ability to colonize organs from the bloodstream.

Effect of cancer-related and drug-induced alterations in surface carbohydrates on the invasive capacity of mouse and rat cells.

The effect of alterations in cell surface carbohydrates on invasion of mouse and rat cells into embryonic chick heart fragments in organ culture was studied. Matching pairs of malignant and nonmalignant cells, including all categories of carcinogenic induction (i.e., viral, chemical, or oncogenic), were compared for their alterations in cell surface carbohydrates and invasive behavior. Glycopeptides derived from the surface of malignant cells expressed cancer-related changes in carbohydrate composition, demonstrated by gel filtration chromatography as a shift in size distribution in comparison with those from nonmalignant counterparts. This phenotypic property strictly correlated with the acquisition of the invasive capacity. Morphological transformation of cells without simultaneous alteration in surface carbohydrates was, however, insufficient for invasion. To test the possible mechanistic role of altered surface carbohydrates in the invasive capacity of cells, the surface molecules of noninvasive cells were modified by incubation with an alkyl-lysophospholipid (racemic 1-O-octadecyl-2-O-methyl glycero-3-phosphocholine). Alkyl-lysophospholipid induced an increase in surface sialylation resembling the changes found in malignant and invasive cells. After pretreatment with alkyl-lysophospholipid, morphologically transformed but nonmalignant and noninvasive cells became able to invade chick heart tissue. These findings indicate that alterations in cell surface carbohydrates, induced by entirely different mechanisms, endowed cells with invasive capacity.

Location of genes involved in invasion and metastasis on human chromosome 7.

We have obtained evidence for the existence of genes controlling invasion and metastasis by somatic cell fusion studies. Noninvasive, nonmetastatic mouse BW5147 T-lymphoma cells were fused with invasive human T-cells. The human cells were either activated normal peripheral blood lymphocytes or leukemic T-lymphoblasts. Both fusions resulted in highly invasive human-mouse T-cell hybrids which metastasized in nude mice. Thus the genes derived from either malignant or nonmalignant but inherently invasive cells enable the T-cell hybridomas to metastasize. By continued in vitro selection for invasive cells employing monolayers of rat embryo fibroblasts followed by subcloning, we were able to isolate invasive hybrids that had lost all human chromosomes except chromosome 7. We present evidence that one or more genes residing on human chromosome 7 are necessary and sufficient for both the establishment and maintenance of invasiveness and metastatic potential of the interspecies T-cell hybrids.

Sequence of the human invasion-inducing TIAM1 gene, its conservation in evolution and its expression in tumor cell lines of different tissue origin.

By means of proviral tagging in combination with in vitro selection for invasive T-lymphoma variants, we have previously identified the murine invasion- and metastasis-inducing Tiam1 gene. Tiam1 encodes a novel protein which shares a Dbl-homology (DH) domain with GDP dissociation stimulator-(GDS) proteins that activate Rho-like GTPases. We have cloned the human TIAM1 coding sequence and studied its evolutionary conservation and expression pattern. TIAM1 is highly conserved among vertebrates. The close similarity between human TIAM1 and the mouse homologue is indicated by 88% and 95% identity of nucleotides and predicted sequences, respectively. The murine gene is highly expressed in brain and testis and at low or moderate levels in almost all other normal tissues. Interestingly, Tiam1 transcripts were found in virtually all analysed tumor cell lines of human and rodent origin including B- and T-lymphomas, neuroblastomas, melanomas and carcinomas. The evolutionary conservation as well as the broad expression pattern of Tiam1 in most normal tissues, suggests a general function in cellular signaling processes presumably by activation of a Rho-like GTPase that regulates the cytoskeletal organization.

Oncogenic activity of Tiam1 and Rac1 in NIH3T3 cells.

We have recently identified the invasion-inducing Tiam1 gene by proviral insertional mutagenesis. The Tiam1 protein shares a Dbl homology (DH) domain with an increasing number of oncoproteins, some of which have been shown to function as GDP dissociation stimulators (GDS) for small GTPases of the Rho family. In vitro and in vivo analyses indicate that Tiam1 activates the Rho like GTPase Rac1. Here we have analysed the consequences of overexpression of several mutant Tiam1 proteins in NIH3T3 fibroblasts. Similar to other proteins containing a DH domain, N-terminal truncation of the Tiam1 protein activates its oncogenic potential, establishing Tiam1 as a proto-oncogene. In addition, we show the sequences N-terminal of the catalytic DH domain are required for morphological transformation accompanied by the formation of membrane ruffling, but not for the induction of an oncogenic phenotype. Overexpression of constitutively active Rac1 (V12Rac1) in NIH3T3 cells produces a similar oncogenic phenotype, suggesting that the oncogenic effects of Tiam1 are a consequence of Rac activation.