Dependence of Dbl and Dbs transformation on MEK and NF-kappaB activation

Altered Rho GTPase signaling pathways in breast cancer cells

The Rho family of GTPases have emerged as key players in regulating a diverse set of biological activities including actin organization, focal complex/adhesion assembly, cell motility, cell polarity, gene transcription and cell-cycle progression. Some Rho GTPases and their signaling components are overexpressed and/or are hyperactive in breast cancer and recent studies have shown a requirement for Rho GTPases in breast cancer cell metastasis in vivo. Herein we describe the contribution of Rho GTPase to the malignant phenotype of breast cancer cells and the role of these pathways as potential targets for breast cancer therapy. Rho GTPases promote cell-cycle progression through cyclin D1, and cyclin D1 in turn reduces cellular adhesion and promotes migration, an example of 'inside-out' signaling by cyclin D1. As cyclin D1 overexpression correlates with metastatic cancer, the 'inside-out' signaling function of cyclin D1 to promote cell migration may represent a useful new therapeutic target.

Crystal Structure of the DH/PH Fragment of Dbs without Bound GTPase

Dbl proteins are guanine nucleotide exchange factors for Rho GTPases, containing adjacent Dbl homology (DH) and pleckstrin homology (PH) domains. This domain architecture is virtually invariant and typically required for full exchange potential. Several structures of DH/PH fragments bound to GTPases implicate the PH domain in nucleotide exchange. To more fully understand the functional linkage between DH and PH domains, we have determined the crystal structure of the DH/PH fragment of Dbs without bound GTPase. This structure is generally similar to previously determined structures of Dbs bound to GTPases albeit with greater apparent mobility between the DH and PH domains. These comparisons suggest that the DH and PH domains of Dbs are spatially primed for binding GTPases and small alterations in intradomain conformations that may be elicited by subtle biological responses, such as altered phosphoinositide levels, are sufficient to enhance exchange by facilitating interactions between the PH domain and GTPases.

Rho GTPases: potential candidates for anticancer therapy

Low molecular weight Rho GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell proliferation, apoptosis, lipid metabolism, cytoarchitecture, adhesion, migration, cell polarity, and transcriptional regulation. The high incidence of overexpression of some members of the Rho family of GTPases in human tumors suggests that these proteins are important in the carcinogenic process, and therefore potential candidates for a therapeutic intervention. In recent years, the characterization of downstream effectors to Rho GTPases has increased our understanding of the general cellular effects that permit aberrant proliferation and motility of tumor cells. In addition, several transcription factors have been identified to play important roles at various levels of Rho-induced tumorigenesis. Accordingly, drugs that specifically alter Rho signaling display antineoplastic properties both at the level of tumor growth and tumor metastasis. In this review, a brief summary of the progress made in understanding the biological functions elicited by Rho GTPases that contribute to tumor biology will be made. In addition, a description of new drugs available targeted to specific elements of Rho signaling with antineoplastic or antimetastatic activity is included.

The RhoA- and CDC42-specific exchange factor Dbs promotes expansion of immature thymocytes and deletion of double-positive and single-positive thymocytes

Specific members of the Rho family of GTPases exert unique influences on thymocyte proliferation, differentiation and deletion. Dbs is a guanine nucleotide exchange factor which is expressed throughout thymocyte development and is able to activate the Rho family GTPases CDC42, RhoA and RhoG. Transgenic mice expressing an activated form of Dbs had increased numbers of double-negative thymocytes. The Dbs transgene promoted expansion of double-negative thymocytes in the absence of pre-TCR, but had no effect on pre-TCR-dependent differentiation of double-negative thymocytes into double-positive thymocytes. Transgenic double-positive thymocytes were proliferative in vivo, but were also susceptible to apoptosis in vivo and in vitro. The transgenic single-positive thymocytes had attenuated proliferative responses following TCR ligation, and were depleted rather than expanded during culture in the presence of anti-CD3. When expressing a positively selectable TCR, transgenic double-positive thymocytes were increased in number and activated, but the output of single-positive thymocytes was reduced. Transgenic double-positive thymocytes were acutely sensitive to deletion by TCR ligation in vivo. These results indicate that activation of Dbs has the potential to promote proliferation throughout thymocyte development, but also sensitizes double-positive and single-positive thymocytes to deletion.

Pleckstrin Homology Domain-mediated Activation of the Rho-specific Guanine Nucleotide Exchange Factor Dbs by Rac1

Dbs is a Rho-specific guanine nucleotide exchange factor that was identified in a screen for proteins whose expression causes deregulated growth in NIH 3T3 mouse fibroblasts. Although Rac1 has not been shown to be a substrate for Dbs in either in vitro or in vivo assays, the Rat ortholog of Dbs (Ost) has been shown to bind specifically to GTP.Rac1 in vitro. The dependence of the Rac1/Dbs interaction on GTP suggests that Dbs may in fact be an effector for Rac1. Here we show that the interaction between activated Rac1 and Dbs can be recapitulated in mammalian cells and that the Rac1 docking site resides within the pleckstrin homology domain of Dbs. This interaction is specific for Rac1 and is not observed between Rac1 and several other members of the Rho-specific guanine nucleotide exchange factor family. Co-expression of Dbs with activated Rac1 causes enhanced focus forming activity and elevated levels of GTP.RhoA in NIH 3T3 cells, indicating that Dbs is activated by the interaction. Consistent with this, activated Rac1 co-localizes with Dbs in NIH 3T3 cells, and natively expressed Rac1 relocalizes in response to Dbs expression. To summarize, we have characterized a surprisingly direct pleckstrin homology domain-mediated mechanism through which Rho GTPases can become functionally linked.

Involvement of Rho family GTPases in p19Arf- and p53-mediated proliferation of primary mouse embryonic fibroblasts

The Rho family GTPases Rac1, RhoA, and Cdc42 function as molecular switches that transduce intracellular signals regulating gene expression and cell proliferation as well as cell migration. p19(Arf) and p53, on the other hand, are tumor suppressors that act both independently and sequentially to regulate cell proliferation. To investigate the functional interaction and cooperativeness of Rho GTPases with the p19(Arf)-p53 pathway, we examined the contribution of Rho GTPases to the gene transcription and cell proliferation unleashed by deletion of p19Arf or p53 in primary mouse embryo fibroblasts. We found that (i) p19(Arf) or p53 deficiency led to a significant increase in PI 3-kinase activity, which in turn upregulated RhoA and Rac1 activities; (ii) deletion of p19Arf or p53 led to an increase in cell growth rate that was in part dependent on RhoA, Rac1, and Cdc42 activities; (iii) p19(Arf) or p53 deficiency caused an enhancement of the growth-related transcription factor NF-kappa B and cyclin D1 activities that are partly dependent on RhoA or Cdc42 but not on Rac1; (iv) forced expression of the activating mutants of Rac1, RhoA, or Cdc42 caused a hyperproliferative phenotype of the p19Arf(-/-) and p53(-/-) cells and promoted transformation of both cells; (v) RhoA appeared to contribute to p53-regulated cell proliferation by modulating cell cycle machinery, while hyperactivation of RhoA further suppressed a p53-independent apoptotic signal; and (vi) multiple pathways regulated by RhoA, including that of Rho-kinase, were required for RhoA to fully promote the transformation of p53(-/-) cells. Taken together, these results provide strong evidence indicating that signals through the Rho family GTPases can both contribute to cell growth regulation by p19Arf and p53 and cooperate with p19Arf or p53 deficiency to promote primary cell transformation.

Cell migration: Rho GTPases lead the way

Rho GTPases control signal transduction pathways that link cell surface receptors to a variety of intracellular responses. They are best known as regulators of the actin cytoskeleton, but in addition they control cell polarity, gene expression, microtubule dynamics and vesicular trafficking. Through these diverse functions, Rho GTPases influence many aspects of cell behavior. This review will focus specifically on their role in cell migration.

EphA4-mediated Rho activation via Vsm-RhoGEF expressed specifically in vascular smooth muscle cells

Rho-kinase, an effector of Rho GTPase, increases the contractility of vascular smooth muscle by phosphorylating myosin light chain (MLC) and by inactivating MLC phosphatase. A wide variety of extracellular stimuli activate RhoA via G protein-coupled receptors. In the present study, we demonstrate a novel cell-cell interaction-mediated Rho activation signaling pathway in vascular smooth muscle cells (VSMCs). Among many receptor tyrosine kinases, the Eph family receptors are unique in that they require cell-cell interaction to engage their ligands, ephrin. We found that a novel VSMC-specific guanine nucleotide exchange factor (GEF) for Rho (Vsm-RhoGEF/KIAA0915) was expressed specifically in VSMCs of several organs including the heart, aorta, liver, kidney, and spleen, as examined by the immunohistochemical analysis using a specific antibody against Vsm-RhoGEF. Based on the association of Vsm-RhoGEF with EphA4 in quiescent cells, we tested whether EphA4 and Vsm-RhoGEF were expressed in the same tissue and further studied the molecular mechanism of Vsm-RhoGEF regulation by EphA4. Immunohistochemical analysis showed that EphA4 and Vsm-RhoGEF expression overlapped in VSMCs. Additionally, tyrosine phosphorylation of Vsm-RhoGEF induced by EphA4 upon ephrin-A1 stimulation enhanced the Vsm-RhoGEF activity for RhoA. The requirement of Vsm-RhoGEF for ephrin-A1-induced assembly of actin stress fibers in VSMCs was shown by the overexpression of a dominant-negative form of VSM-RhoGEF and by the depletion of Vsm-RhoGEF using RNA interference. These results suggested that ephrin-A1-triggered EphA4-Vsm-RhoGEF-RhoA pathway is involved in the cell-cell interaction-mediated RhoA activation that regulates vascular smooth muscle contractility.

ROCK and nuclear factor-kappaB-dependent activation of cyclooxygenase-2 by Rho GTPases: effects on tumor growth and therapeutic consequences

Rho GTPases are overexpressed in a variety of human tumors contributing to both tumor proliferation and metastasis. Recently, several studies demonstrate an essential role of transcriptional regulation in Rho GTPases-induced oncogenesis. Herein, we demonstrate that RhoA, Rac1, and Cdc42 promote the expression of cyclooxygenase-2 (COX-2) at the transcriptional level by a mechanism that is dependent on the transcription factor nuclear factor-kappaB (NF-kappaB), but not Stat3, a transcription factor required for RhoA-induced tumorigenesis. With respect to RhoA, this effect is dependent on ROCK, but not PKN. Treatment of RhoA-, Rac1-, and Cdc42-transformed epithelial cells with Sulindac and NS-398, two well-characterized nonsteroid antiinflammatory drugs (NSAIDs), results in growth inhibition as determined by cell proliferation assays. Accordingly, tumor growth of RhoA-expressing epithelial cells in syngeneic mice is strongly inhibited by NS-398 treatment. The effect of NSAIDs over RhoA-induced tumor growth is not exclusively dependent on COX-2 because DNA-binding of NF-kappaB is also abolished upon NSAIDs treatment, resulting in complete loss of COX-2 expression. Finally, treatment of RhoA-transformed cells with Bay11-7083, a specific NF-kappaB inhibitor, leads to inhibition of cell proliferation. We suggest that treatment of human tumors that overexpress Rho GTPases with NSAIDs and drugs that target NF-kappaB could constitute a valid antitumoral strategy.

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

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

Multifunctional roles for the PH domain of Dbs in regulating Rho GTPase activation

Dbl family members are guanine nucleotide exchange factors specific for Rho guanosine triphosphatases (GTPases) and invariably possess tandem Dbl (DH) and pleckstrin homology (PH) domains. Dbs, a Dbl family member specific for Cdc42 and RhoA, exhibits transforming activity when overexpressed in NIH 3T3 mouse fibroblasts. In this study, the PH domain of Dbs was mutated to impair selectively either guanine nucleotide exchange or phosphoinositide binding in vitro and resulting physiological alterations were assessed. As anticipated, substitution of residues within the PH domain of Dbs integral to the interface with GTPases reduced nucleotide exchange and eliminated the ability of Dbs to transform NIH 3T3 cells. More interestingly, substitutions within the PH domain that prevent interaction with phosphoinositides yet do not alter in vitro activation of GTPases also do not transform NIH 3T3 cell and fail to activate RhoA in vivo despite proper subcellular localization. Therefore, the PH domain of Dbs serves multiple roles in the activation of GTPases and cannot be viewed as a simple membrane-anchoring device. In particular, the data suggest that binding of phosphoinositides to the PH domain within the context of membrane surfaces may direct orientations or conformations of the linked DH and PH domains to regulate GTPases activation.

A Rac/Cdc42-specific exchange factor, GEFT, induces cell proliferation, transformation, and migration

The Rho family of small GTPases, including Rho, Rac, and Cdc42, play essential roles in diverse cellular functions. The ability of Rho family GTPases to participate in signaling events is determined by the ratio of inactive (GDP-bound) and active (GTP-bound) forms in the cell. The activation of Rho family proteins requires the exchange of bound GDP for GTP, a process catalyzed by the Dbl family of guanine nucleotide exchange factors (GEFs). The GEFs have high affinity for the guanine nucleotide-free state of the GTPases and are thought to promote GDP release by stabilizing an intermediate transition state. In this study, we have identified and characterized a new Rac/Cdc42-specific Dbl family guanine nucleotide exchange factor, named GEFT. GEFT is highly expressed in the excitable tissues, including brain, heart, and muscle. Low or very little expression was detected in other nonexcitable tissues. GEFT has specific exchange activity for Rac and Cdc42 in our in vitro GTPase exchange assays and glutathione S-transferase-PAK pull-down assays with GTP-bound Rac1 and Cdc42. Overexpression of GEFT leads to changes in cell morphology and actin cytoskeleton re-organization, including the formation of membrane microspikes, filopodia, and lamilliopodia. Furthermore, expression of GEFT in NIH3T3 cells promotes foci formation, cell proliferation, and cell migration, possibly through the activation of transcriptional factors involved in cell growth and proliferation. Together, our data suggest that GEFT is a Rac/Cdc42-specific GEF protein that regulates cell morphology, cell proliferation, and transformation.

Role of the pleckstrin homology domain in intersectin-L Dbl homology domain activation of Cdc42 and signaling

Intersectin-long (ITSN-L) contains the invariant Dbl homology (DH) and pleckstrin homology (PH) domain structure characteristic of the majority of Dbl family proteins. This strict domain topography suggests that the PH domain serves an essential, conserved function in the regulation of the intrinsic guanine nucleotide exchange activity of the DH domain. We evaluated the role of the PH domain in regulating the DH domain function of ITSN-L. Surprisingly, we found that the PH domain was dispensable for guanine nucleotide exchange activity on Cdc42 in vitro, yet the PH domain enhanced the ability of the DH domain to activate Cdc42 signaling in vivo. PH domains can interact with phosphoinositide substrates and products of phosphatidylinositol 3-kinase (PI3K). However, PI3K activation did not modulate ITSN-L DH domain function in vivo.

Intersectin 1L guanine nucleotide exchange activity is regulated by adjacent src homology 3 domains that are also involved in endocytosis

Intersectin 1L is a scaffolding protein involved in endocytosis that also has guanine nucleotide exchange activity for Cdc42. In the context of the full-length protein, the catalytic exchange activity of the DH domain is repressed. Here we use biochemical methods to dissect the mechanism for this inhibition. We demonstrate that the intersectin 1L SH3 domains, which bind endocytic proteins, directly inhibit the activity of the DH domain in assays for both binding and exchange of Cdc42. This inhibitory mechanism seems to act through steric hindrance of Cdc42 binding by an intramolecular interaction between the intersectin 1L SH3 domain region and the adjacent DH domain. Surprisingly, the mode of SH3 domain binding is other than through the proline peptide binding pocket. The dual role of the SH3 domains in endocytosis and repression of exchange activity suggests that the intersectin 1L exchange activity is regulated by endocytosis. We show that the endocytic protein, dynamin, competes for binding to the SH3 domains with the neural Wiskott-Aldrich Syndrome protein, an actin filament nucleation protein that is a substrate for activated Cdc42. Swapping of SH3 domain binding partners might act as a switch controlling the actin nucleation activity of intersectin 1L.

NF-kappaB activation in cancer: a challenge for ubiquitination- and proteasome-based therapeutic approach

Nuclear factor-kappa B (NF-kappaB) activation relies primarily on ubiquitin-mediated degradation of its inhibitor IkappaB. NF-kappaB plays an important role in many aspects of tumor development, progression, and therapy. Some types of cancer are characterized by constitutive NF-kappaB activity, whereas in others such activity is induced following chemotherapy. NF-kappaB-harboring tumors are generally resistant to chemotherapy and their eradication requires NF-kappaB inhibition. Here we describe the mechanisms of NF-kappaB activation in normal and tumor cells, review prevalent notions regarding the factor's contribution to tumorigenicity and discuss present and future options for NF-kappaB inhibition in cancer. The ubiquitination-mediated activation of NF-kappaB is intersected by another cancer-associated protein, beta-catenin. We, therefore, compare the related activation pathways and discuss the possibility of differential targeting of the involved ubiquitination machinery.

STAT5a activation mediates the epithelial to mesenchymal transition induced by oncogenic RhoA

The involvement of Rho GTPases in signal transduction pathways leading to transcription activation is one of the major roles of this family of GTPases. Thus, the identification of transcription factors regulated by Rho GTPases and the understanding of the mechanisms of their activation and its biological outcome are of great interest. Here, we provide evidence that Rho GTPases modulate Stat5a, a transcription factor of the family of signal transducers and activators of transcription. RhoA triggers tyrosine phosphorylation (Y696) of Stat5a via a JAK2-dependent mechanism and promotes DNA-binding activity of Stat5a. Tyrosine phosphorylation of Stat5a is also stimulated physiologically by lysophosphatidic acid (LPA) in a Rho-dependent manner. Simultaneously, RhoA reduces serine phosphorylation of Stat5a at both serine residues S726 and S780, resulting in a further increase of activity as defined by mutagenesis experiments. Furthermore, serine dephosphorylation of Stat5a by RhoA does not take place by down-modulation of either JNK1, MEK1, or p38 MAP kinases, as determined by transfection experiments or chemical inhibition of both MEK1, p38, and JNK serine kinases. Thus, RhoA regulates Stat5a via tyrosine phosphorylation and via a yet to be determined novel down-modulating pathway that involves serine dephosphorylation. Finally, we provide evidence for a role of Stat5a in RhoA-induced epithelial-to-mesenchymal transition with concomitant increase in vimentin expression, E-cadherin down-regulation, and cell motility.

Functional analysis of cdc42 residues required for Guanine nucleotide exchange

Guanine nucleotide exchange factors (GEFs) directly engage small GTPases to facilitate the exchange of bound GDP for GTP, leading to GTPase activation. Several recent crystal structures of GEFs in complex with Rho family GTPases highlight the conserved interactions and conformational alterations necessary for catalyzing exchange. In the present study, functional roles were defined for specific residues within Cdc42 implicated by the crystal structures as important for physiological exchange of guanine nucleotides within Rho GTPases. In particular, this study highlights the paramount importance of the phosphate-binding loop and interactions with the magnesium co-factor as critical for proper regulation of RhoGEF-catalyzed exchange. Other conformational alterations of the GTPases affecting interactions with the sugar and base of guanine nucleotides are also important but are secondary. Of particular note, substitution of alanine for cysteine at position 18 of Cdc42 leads to a fast cycling phenotype for Cdc42 with heightened affinity for RhoGEFs and produces a dominant negative form of Cdc42 capable of inhibiting RhoGEFs both in vitro and in vivo.

Critical role of the pleckstrin homology and cysteine-rich domains in Vav signaling and transforming activity

Vav family proteins are members of the Dbl family of guanine nucleotide exchange factors and activators of Rho family small GTPases. In addition to the Dbl homology (DH) domain important for guanine nucleotide exchange factor catalytic function, all Dbl family proteins contain an adjacent pleckstrin homology (PH) domain that serves to regulate DH domain activity. Although the role of the PH domain in Vav function has been evaluated extensively, its precise role and whether it serves a distinct role in different Vav proteins remain unresolved. Additionally, the precise role of an adjacent cysteine-rich domain (CRD) in regulating DH domain function is also unclear. In this study, we evaluated the contribution of these putative protein-protein or protein-lipid interaction domains to Vav signaling and transforming activity. In contrast to previous observations, we found that the PH domain is critical for Vav transforming activity. Similarly, the CRD was also essential and served a function distinct from that of the PH domain. Although mutation of either domain reduced Vav membrane association, addition of plasma membrane targeting sequences to either the CRD or PH domain mutant proteins did not restore Vav transforming activity. This result contrasts with other Dbl family proteins, where a membrane targeting sequence alone was sufficient to restore the loss of function caused by mutation of the PH domain. Furthermore, green fluorescent protein fusion proteins containing the PH domain or CRD, or both, failed to target to the plasma membrane, suggesting that these two domains also serve regulatory functions independent of promoting membrane localization. Finally, we found that phosphatidylinositol 3-kinase activation may promote Vav membrane association via phosphatidylinositol 3,4,5-triphosphate binding to the PH domain.

RhoGEF specificity mutants implicate RhoA as a target for Dbs transforming activity

Dbs is a Rho-specific guanine nucleotide exchange factor (RhoGEF) that exhibits transforming activity when overexpressed in NIH 3T3 mouse fibroblasts. Like many RhoGEFs, the in vitro catalytic activity of Dbs is not limited to a single substrate. It can catalyze the exchange of GDP for GTP on RhoA and Cdc42, both of which are expressed in most cell types. This lack of substrate specificity, which is relatively common among members of the RhoGEF family, complicates efforts to determine the molecular basis of their transforming activity. We have recently determined crystal structures of several RhoGEFs bound to their cognate GTPases and have used these complexes to predict structural determinants dictating the specificities of coupling between RhoGEFs and GTPases. Guided by this information, we mutated Dbs to alter significantly its relative exchange activity for RhoA versus Cdc42 and show that the transformation potential of Dbs correlates with exchange on RhoA but not Cdc42. Supporting this conclusion, oncogenic Dbs activates endogenous RhoA but not endogenous Cdc42 in NIH 3T3 cells. Similarly, a competitive inhibitor that blocks RhoA activation also blocks Dbs-mediated transformation. In conclusion, this study highlights the usefulness of specificity mutants of RhoGEFs as tools to genetically dissect the multiple signaling pathways potentially activated by overexpressed or oncogenic RhoGEFs. These ideas are exemplified for Dbs, which is strongly implicated in the transformation of NIH 3T3 cells via RhoA and not Cdc42.

Loss of the Lkb1 tumour suppressor provokes intestinal polyposis but resistance to transformation

Germline mutations in LKB1 (also known as STK11) are associated with Peutz-Jeghers syndrome (PJS), a disorder with predisposition to gastrointestinal polyposis and cancer. PJS polyps are unusual neoplasms characterized by marked epithelial and stromal overgrowth but have limited malignant potential. Here we show that Lkb1(+/-) mice develop intestinal polyps identical to those seen in individuals affected with PJS. Consistent with this in vivo tumour suppressor function, Lkb1 deficiency prevents culture-induced senescence without loss of Ink4a/Arf or p53. Despite compromised mortality, Lkb1(-/-) mouse embryonic fibroblasts show resistance to transformation by activated Ha-Ras either alone or with immortalizing oncogenes. This phenotype is in agreement with the paucity of mutations in Ras seen in PJS polyps and suggests that loss of Lkb1 function as an early neoplastic event renders cells resistant to subsequent oncogene-induced transformation. In addition, the Lkb1 transcriptome shows modulation of factors linked to angiogenesis, extracellular matrix remodelling, cell adhesion and inhibition of Ras transformation. Together, our data rationalize several features of PJS polyposis--notably its peculiar histopathological presentation and limited malignant potential--and place Lkb1 in a distinct class of tumour suppressors.

Activation of rac and cdc42 video imaged by fluorescent resonance energy transfer-based single-molecule probes in the membrane of living cells

Rho family G proteins, including Rac and Cdc42, regulate a variety of cellular functions such as morphology, motility, and gene expression. We developed fluorescent resonance energy transfer-based probes which monitored the local balance between the activities of guanine nucleotide exchange factors and GTPase-activating proteins for Rac1 and Cdc42 at the membrane. These probes, named Raichu-Rac and Raichu-Cdc42, consisted of a Cdc42- and Rac-binding domain of Pak, Rac1 or Cdc42, a pair of green fluorescent protein mutants, and a CAAX box of Ki-Ras. With these probes, we video imaged the Rac and Cdc42 activities. In motile HT1080 cells, activities of both Rac and Cdc42 gradually increased toward the leading edge and decreased rapidly when cells changed direction. Under a higher magnification, we observed that Rac activity was highest immediately behind the leading edge, whereas Cdc42 activity was most prominent at the tip of the leading edge. Raichu-Rac and Raichu-Cdc42 were also applied to a rapid and simple assay for the analysis of putative guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) in living cells. Among six putative GEFs and GAPs, we identified KIAA0362/DBS as a GEF for Rac and Cdc42, KIAA1256 as a GEF for Cdc42, KIAA0053 as a GAP for Rac and Cdc42, and KIAA1204 as a GAP for Cdc42. In conclusion, use of these single-molecule probes to determine Rac and Cdc42 activity will accelerate the analysis of the spatiotemporal regulation of Rac and Cdc42 in a living cell.

Rho GTPases in transformation and metastasis

During the development and progression of human cancer, cells undergo numerous changes in morphology, proliferation, and transcriptional profile. Over the past couple of decades there have been intense efforts to understand the molecular mechanisms involved, and members of the Ras superfamily of small GTPases have emerged as important players. Mutated versions of the Ras genes were first identified in human cancers some 20 years ago, but more recently, the Rho branch of the family has been receiving increased attention. In addition to the experimental evidence implicating Rho GTPase signaling in promoting malignant transformation, genetic analysis of human cancers has now revealed a few examples of direct alterations in the genes encoding regulators of Rho GTPases. In this review, we discuss the evidence implicating Rho GTPases in transformation and metastasis, as well as the progress made toward identifying their biochemical mechanism of action.

Role of Dbl's big sister in the anti-mitogenic pathway from alpha1B-adrenergic receptor to c-Jun N-terminal kinase

We previously reported that the alpha1B-adrenergic receptor leads to activation of Rho family small GTPases, and in turn, c-Jun N-terminal kinase (JNK), which results in the inhibition of cell proliferation. Here, we show the involvement of the Rho family guanine nucleotide exchange factor (GEF) Dbl's Big Sister (Dbs) in the signaling pathway. Transfection of a Dbl-homology (DH) and pleckstrin-homology (PH) domain-deficient form of Dbs into cells blocked the alpha1B-adrenergic receptor-induced activation of JNK. Conversely, transfection of an isolated DH domain of Dbs induced JNK activation. Stimulation of the alpha1B-adrenergic receptor enhanced an intrinsic Cdc42-GEF activity of Dbs in a manner dependent on Src family tyrosine kinases. Additionally, DH and PH domain deficient Dbs blocked the receptor-induced inhibition of cell proliferation, while DH domain of Dbs inhibited cell proliferation via the JNK-dependent pathway. Taken together, Dbs may play an important role in the anti-mitogenic JNK pathway downstream of the alpha1B-adrenergic receptor.

NF-kappaB1 can inhibit v-Abl-induced lymphoid transformation by functioning as a negative regulator of cyclin D1 expression

Mounting evidence implicates deregulated Rel/NF-kappaB signaling as a common feature of lymphoid malignancies. Despite the fact that they promote the survival and proliferation of normal lymphocytes, the underlying mechanisms by which various Rel/NF-kappaB proteins with different transcriptional regulatory capacities might facilitate transformation remain to be established. Here we show that the proliferation and tumorigenicity of Abelson murine leukemia virus (A-MuLV)-transformed pre-B cells are enhanced in the absence of NF-kappaB1 and that this coincides with elevated levels of cyclin D1. Support for a link between cyclin D1 expression and v-Abl transformation came from the finding that proliferation of transformed pre-B cells was reduced in the absence of cyclin D1, while enforced cyclin D1 expression increased the proliferation and tumorigenicity of wild-type transformants. A reduction in endogenous cyclin D1 levels that coincided with NF-kappaB1 transgene reversal of enhanced nfkb1(-/-) pre-B-cell transformation, coupled with NF-kappaB1 inhibition of v-Abl-induced kappaB-dependent murine cyclin D1 transcription, lends support to a model in which v-Abl-induced cyclin D1 transcription in transformed pre-B cells is controlled by Rel/NF-kappaB dimers with different activities.

p38 MAPK-mediated activation of NF-kappaB by the RhoGEF domain of Bcr

The oncogenic fusion protein p210 Bcr-Abl is causally associated with virtually all cases of chronic myelogenous leukemia. The wild-type Bcr product has several recognizable structural and functional motifs including a domain that contains guanine nucleotide exchange activity for Rho family GTPases (DH/PH domain). Although this domain is retained within p210 Bcr-Abl, it has no known signaling activities in vivo. Here we report that a fragment of Bcr that encodes the isolated DH/PH domain is a potent activator of the NF-kappaB transcription factor. Within the context of full length Bcr, this activity is regulated by proximal flanking sequences that suppress the DH/PH domain encoded guanine nucleotide exchange activity. NF-kappaB activation by Bcr is not mediated by nuclear translocation, but rather by p38 mitogen-activated protein kinase (MAPK)-dependent modification of the RelA/p65 transactivation domain. Although we were able to demonstrate that Bcr can function as an exchange factor for Cdc42 in vivo, NF-kappaB activation appears to occur via a Cdc42-independent mechanism. These studies constitute direct evidence that the Bcr RhoGEF domain can function in vivo, and identify a new signaling activity that may contribute to the transforming potential of p210 Bcr-Abl.

Rel/NF-kappa B/I kappa B signal transduction in the generation and treatment of human cancer

The Rel/NF-kappa B family is a group of structurally-related, tightly-regulated transcription factors that control the expression of a multitude of genes involved in key cellular and organismal processes. The Rel/NF-kappa B signal transduction pathway is misregulated in a variety of human cancers, especially ones of lymphoid cell origin, due either to genetic changes (such as chromosomal rearrangements, amplifications, and mutations) or to chronic activation of the pathway by epigenetic mechanisms. Constitutive activation of the Rel/NF-kappa B pathway can contribute to the oncogenic state in several ways, for example, by driving proliferation, by enhancing cell survival, or by promoting angiogenesis or metastasis. In many cases, inhibition of Rel/NF-kappa B activity reverses all or part of the malignant state. Thus, the Rel/NF-kappa B pathway has received much attention as a focal point for clinical intervention.

Regulation of proto-Dbl by intracellular membrane targeting and protein stability

The pleckstrin homology (PH) domain of onco-Dbl, a guanine nucleotide exchange factor (GEF) for Cdc42 and RhoA GTPases, interacts with phosphoinositides (PIPs). This interaction modulates both the GEF activity and the targeting to the plasma membrane of onco-Dbl. Conversely, we have previously shown that in proto-Dbl an intramolecular interaction between the N-terminal domain and the PH domain imposes a negative regulation on both the DH and PH functions, suppressing its transforming activity. Here we have further investigated the mode of regulation of proto-Dbl by generating proto-Dbl mutants deleted of the last C-terminal 50 amino acids, which contain a PEST motif, and/or unable to bind to PIPs due to substitutions of the positively charged residues of the PH domain. The PH mutants of proto-Dbl retained a relative weak GEF activity toward Cdc42 and RhoA in vitro, but their RhoA activating potential was impaired in vivo. Further, these mutants lost both the plasma membrane targeting and the transforming activities, contrary to the PH mutants of onco-Dbl that retained the exchange activity both in vitro and in vivo and showed significant, but partially, reduced transforming activity. Deletion of the C-terminal sequences from onco-Dbl did not affect its function, whereas similar deletion of proto-Dbl led to an increase of transforming activity. Analysis of the half-life of the proto-Dbl mutants revealed that deletion of the C-terminal sequences increases the stability of the protein. Overall, the transformation potential of proto-Dbl mutants was associated with an augmented localization of the protein to the plasma membrane and a strong activation of Jun N-terminal kinase activity and transcription of cyclin D1. Together with previous observations, these data suggest that the biological activity of proto-Dbl is tightly regulated by a combination of mechanisms that involve intramolecular interaction, PH binding to PIPs, and the N- and C-terminal domain-dependent turnover of the protein.

Defective dendrite elongation but normal fertility in mice lacking the Rho-like GTPase activator Dbl

Dbl is the prototype of a large family of GDP-GTP exchange factors for small GTPases of the Rho family. In vitro, Dbl is known to activate Rho and Cdc42 and to induce a transformed phenotype. Dbl is specifically expressed in brain and gonads, but its in vivo functions are largely unknown. To assess its role in neurogenesis and gametogenesis, targeted deletion of the murine Dbl gene was accomplished in embryonic stem cells. Dbl-null mice are viable and did not show either decreased reproductive performances or obvious neurological defects. Histological analysis of mutant testis showed normal morphology and unaltered proliferation and survival of spermatogonia. Dbl-null brains indicated a correct disposition of the major neural structures. Analysis of cortical stratification indicated that Dbl is not crucial for neuronal migration. However, in distinct populations of Dbl-null cortical pyramidal neurons, the length of dendrites was significantly reduced, suggesting a role for Dbl in dendrite elongation.

The Rho exchange factor Net1 is regulated by nuclear sequestration

Net1 is a guanine nucleotide exchange factor specific for the small GTPase Rho. Oncogenic activation of Net1 occurs by truncation of the N-terminal part of the protein, which functions as a negative regulatory domain. Here, we have investigated the mechanism of Net1 regulation via its N terminus. We find that Net1 localizes to the nucleus, whereas oncogenic Net1 is found in the cytoplasm. Nuclear import of Net1 is mediated by two nuclear localization signals present in the N terminus of the protein, and forced cytoplasmic localization of Net1 is sufficient to activate Rho. In addition, the pleckstrin homology (PH) domain of Net1 acts as a nuclear export signal. Because an amino acid substitution in the PH domain that inhibits guanine nucleotide exchange factor activity does not inhibit nuclear export, we conclude that this PH domain has at least two functions. Together, our results suggest that Net1 can shuttle in and out of the nucleus, and that activation of Rho by Net1 is controlled by changes in its subcellular localization.

Synergism of aminobisphosphonates and farnesyl transferase inhibitors on tumor metastasis

Aminobisphosphonates have shown significant antitumor activity in vitro and in vivo with selective pharmacodistribution to bone, and an established role in the treatment of malignant bone disease. Given that the mode of action of aminobisphosphonates involves decreasing the prenylation of the Rho family of proteins, through decreasing the availability of prenyl groups (farnesyl and geranylgeranyl isoprenoids), the authors sought the inhibition of Rho protein prenylation at two points, by using an aminobiphosphonate (alendronate) in conjunction with a prenyl transferase inhibitor (R115777, a specific farnesyl transferase inhibitor with limited effects in geranylgeranyl transferase). The authors show synergistic inhibition of the prenylation dependent membrane association and migratory function of Rho proteins, translating into a suppressive effect on in vitro tumor cell invasiveness and in vivo metastasis. The findings support the use of aminobisphosphonates in conjunction with farnesyl transferase inhibitors in the prevention of metastatic progression and suggest that metastatic progression is a valid end point in assessing the antitumor activity of farnesyl transferase inhibitors.

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

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

The role of Rho GTPases in disease development

The functionality and efficacy of Rho GTPase signaling is pivotal for a plethora of biological processes. Due to the integral nature of these molecules, the dysregulation of their activities can result in diverse aberrant phenotypes. Dysregulation can, as will be described below, be based on an altered signaling strength on the level of a specific regulator or that of the respective GTPase itself. Alternatively, effector pathways emanating from a specific Rho GTPase may be under- or overactivated. In this review, we address the role of the Rho-type GTPases as a subfamily of the Ras-superfamily of small GTP-binding proteins in the development of various disease phenotypes. The steadily growing list of genetic alterations that specifically impinge on proper Rho GTPase function corresponds to pathological categories such as cancer progression, mental disabilities and a group of quite diverse and unrelated disorders. We will provide an overview of disease-rendering mutations in genes that have been positively correlated with Rho GTPase signaling and will discuss the cellular and molecular mechanisms that may be affected by them.

A crystallographic view of interactions between Dbs and Cdc42: PH domain-assisted guanine nucleotide exchange

Dbl-related oncoproteins are guanine nucleotide exchange factors (GEFs) specific for Rho guanosine triphosphatases (GTPases) and invariably possess tandem Dbl (DH) and pleckstrin homology (PH) domains. While it is known that the DH domain is the principal catalytic subunit, recent biochemical data indicate that for some Dbl-family proteins, such as Dbs and Trio, PH domains may cooperate with their associated DH domains in promoting guanine nucleotide exchange of Rho GTPases. In order to gain an understanding of the involvement of these PH domains in guanine nucleotide exchange, we have determined the crystal structure of a DH/PH fragment from Dbs in complex with Cdc42. The complex features the PH domain in a unique conformation distinct from the PH domains in the related structures of Sos1 and Tiam1.Rac1. Consequently, the Dbs PH domain participates with the DH domain in binding Cdc42, primarily through a set of interactions involving switch 2 of the GTPase. Comparative sequence analysis suggests that a subset of Dbl-family proteins will utilize their PH domains similarly to Dbs.

Dematin interacts with the Ras-guanine nucleotide exchange factor Ras-GRF2 and modulates mitogen-activated protein kinase pathways

Erythroid dematin is a major component of red blood cell junctional complexes that link the spectrin-actin cytoskeleton to the overlying plasma membrane. Transcripts of dematin are widely distributed including human brain, heart, lung, skeletal muscle, and kidney. In vitro, dematin binds and bundles actin filaments in a phosphorylation-dependent manner. The primary structure of dematin consists of a C-terminal domain homologous to the 'headpiece' domain of villin, an actin-binding protein of the brush border cytoskeleton. Except filamentous actin, no other binding partners of dematin have been identified. To investigate the physiological function of dematin, we employed the yeast two-hybrid assay to identify dematin-interacting proteins in the adult human brain. Here, we show that dematin interacts with the guanine nucleotide exchange factor Ras-GRF2 by yeast two-hybrid assay, and this interaction is further confirmed by blot overlay, surface plasmon resonance, co-transfection, and co-immunoprecipitation assays. Human Ras-GRF2 is expressed in a variety of tissues and, similar to other guanine nucleotide exchange factors (GEFs), displays anchorage independent growth in soft agar. Co-transfection and immunoblotting experiments revealed that dematin blocks transcriptional activation of Jun by Ras-GRF2 and activates ERK1 via a Ras-GRF2 independent pathway. Because much of the present evidence has centered on the identification of the Rho family of GTPases as key regulators of the actin cytoskeleton, the direct association between dematin and Ras-GRF2 may provide an alternate mechanism for regulating the activation of Rac and Ras GTPases via the actin cytoskeleton.

Quantitative analysis of the effect of phosphoinositide interactions on the function of Dbl family proteins

Normally, Rho GTPases are activated by the removal of bound GDP and the concomitant loading of GTP catalyzed by members of the Dbl family of guanine nucleotide exchange factors (GEFs). This family of GEFs invariantly contain a Dbl homology (DH) domain adjacent to a pleckstrin homology (PH) domain, and while the DH domain usually is sufficient to catalyze nucleotide exchange, possible roles for the conserved PH domain remain ambiguous. Here we demonstrate that the conserved PH domains of three distinct Dbl family proteins, intersectin, Dbs, and Tiam1, selectively bind lipid vesicles only when phosphoinositides are present. While the PH domains of intersectin and Dbs promiscuously bind several multiphosphorylated phosphoinositides, Tiam1 selectively interacts with phosphatidylinositol 3-phosphate (K(D) approximately 5-10 microm). In addition, and in contrast to recent reports, catalysis of nucleotide exchange on nonprenylated Rac1 provided by various extended portions of Tiam1 is not influenced by (a) soluble phosphoinositide head groups, (b) dibutyl versions of phosphoinositides, or (c) lipid vesicles containing phosphoinositides. Likewise, GEF activity afforded by DH/PH fragments of intersectin and Dbs are also not altered by phosphoinositide interactions. These results strongly suggest that unless all relevant components are localized to a lipid membrane surface, Dbl family GEFs generally are not intrinsically modulated by binding phosphoinositides.

Dbl family guanine nucleotide exchange factors

The Dbl family of guanine nucleotide exchange factors are multifunctional molecules that transduce diverse intracellular signals leading to the activation of Rho GTPases. The tandem Dbl-homology and pleckstrin-homology domains shared by all members of this family represent the structural module responsible for catalyzing the GDP-GTP exchange reaction of Rho proteins. Recent progress in genomic, genetic, structural and biochemical studies has implicated Dbl family members in diverse biological processes, including growth and development, skeletal muscle formation, neuronal axon guidance and tissue organization. The detailed pictures of their autoregulation, agonist-controlled activation and mechanism of interaction with Rho GTPase substrates, have begun to emerge.

The gene for a new brain specific RhoA exchange factor maps to the highly unstable chromosomal region 1p36.2-1p36.3

Guanine nucleotide exchange factors from the Dbl family are proto-oncogenic proteins that activate small GTPases of the Rho family. Here we report the characterization of GEF720, a novel Dbl-like protein related to p115Rho-GEF. GEF720 activated RhoA both in our recently developed Yeast Exchange Assay and in biochemical in vitro exchange assays. GEF720 induced RhoA dependent assembly of actin stress fibers in REF52 fibroblastic cells. In NIH3T3 cells this Dbl-like protein elicited formation of transformation foci with a morphology similar to RhoA-V14 induced foci. In the PC12 neuron-like cell line, expression of GEF720, whose mRNA is brain specific, inhibited NGF-induced neurite outgrowth. Finally, GEF720 gene is located on human chromosome 1 on band 1p36, between Tumor Protein 73 and Tumor Necrosis Factor Receptor 12, two genes rearranged in many neuroblastoma cell lines. Together, these results show that this new Dbl related protein, GEF720, is an exchange factor that can directly activate RhoA in vivo and is potentially involved in the control of neuronal cell differentiation. GEF720 is also a new candidate gene involved in the progression of neuroblastoma and developmental abnormalities associated with rearrangements in the 1p36 chromosomal region.

Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

The Ras superfamily of low-molecular-weight GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell growth and development, apoptosis, lipid metabolism, cytoarchitecture, membrane trafficking, and transcriptional regulation. More than 100 genes of this superfamily grouped in six subfamilies have been described so far, pointing to the complexities and specificities of their cellular functions. Dysregulation of members of at least two of these families (the Ras and the Rho families) is involved in the events that lead to the uncontrolled proliferation and invasiveness of human tumors. In recent years, the cloning and characterization of downstream effectors for Ras and Rho proteins have given crucial clues to the specific pathways that lead to aberrant cellular growth and ultimately to tumorigenesis. A direct link between the functions of some of these effectors with the appearance of transformed cells and their ability to proliferate and invade surrounding tissues has been made. Accordingly, drugs that specifically alter their functions display antineoplasic properties, and some of these drugs are already under clinical trials. In this review, we survey the progress made in understanding the underlying molecular connections between carcinogenesis and the specific cellular functions elicited by some of these effectors. We also discuss new drugs with antineoplastic or antimetastatic activity that are targeted to specific effectors for Ras or Rho proteins.

Dbl and the Rho GTPases activate NF kappa B by I kappa B kinase (IKK)-dependent and IKK-independent pathways

Dbl is a guanine nucleotide exchange factor that activates the Rho family GTPases Cdc42, Rac, and Rho. Dbl and all three GTPases are strong activators of transcription factor NF kappa B, which has been shown to have an important role in Dbl-induced oncogenic transformation. Here we show that although Dbl activation of NF kappa B requires Cdc42, Rac, and Rho, the different GTPases activate NF kappa B by different mechanisms. Whereas Rac stimulates the activity of the I kappa B kinase IKK beta, Cdc42 and Rho activate NF kappa B without activating either IKK alpha or IKK beta. Like Dbl, Rac activation of IKK beta is mediated by the serine/threonine kinases NIK but not MEKK. This differs from Rac activation of the JNK pathway, which was previously shown to be mediated by MEKK. The pathway leading from Rho and Cdc42 to NF kappa B is more elusive, but our results suggest that it involves an IKK alpha/IKK beta-independent mechanism. Finally, we show that the signaling enzymes that mediate NF kappa B activation by Dbl and the Rho GTPases are also necessary for malignant transformation induced by oncogenic Dbl.

Cdc42, but not RhoA, regulates cyclin D1 expression in bovine tracheal myocytes

We previously demonstrated that Rac1 increased cyclin D1 promoter activity in an extracellular signal-regulated kinase (ERK)-independent, antioxidant-sensitive manner. Here, we examined the regulation of cyclin D1 expression by Cdc42 and RhoA. Overexpression of active Cdc42, but not of RhoA, induced transcription from the cyclin D1 promoter. Furthermore, dominant negative Cdc42, but not RhoA, attenuated platelet-derived growth factor-mediated activation of the cyclin D1 promoter. Overexpression of active Cdc42 increased cyclin D1 protein abundance in COS cells. Cdc42-induced cyclin D1 promoter activation was independent of ERK as evidenced by insensitivity to PD-98059, an inhibitor of mitogen-activated protein kinase/ERK kinase (MEK). Furthermore, Cdc42 was neither sufficient nor required for activation of ERK. Similar to Rac1-induced cyclin D1 expression, pretreatment with the antioxidants catalase and ebselen inhibited Cdc42-mediated transcription from the cyclin D1 promoter. Finally, like Rac1, active Cdc42 induced transactivation of the cyclin D1 promoter cAMP response element binding protein/activating transcription factor-2 binding site. Together, these data suggest that in airway smooth muscle cells, Cdc42 and Rac1 share a common signaling pathway to cyclin D1 promoter activation.

Rho signals to cell growth and apoptosis

Ras and Rho GTPases are among the best studied signaling molecules in molecular biology. Essential cellular processes, such as cell growth, lipid metabolism, cytoarchitecture, membrane trafficking, transcriptional regulation, apoptosis, and response to genotoxic agents, are directly modulated by different members of this superfamily of proteins. Not until recently have we begun to understand the physiological implications of Ras and Rho GTPases, linking them to processes such as embryonic development, tissue remodeling, tumorigenesis and metastasis. In this sense, uncontrolled activation, due to overexpression of different members of the Rho family in a variety of tissues, leads to uncontrolled proliferation and invasiveness of human tumors. In this review, an attempt to briefly integrate recent findings in transcriptional regulation by Rho GTPases in the context of carcinogenesis and metastasis as well as apoptosis is made.

Autoinhibition mechanism of proto-Dbl

The dbl oncogene encodes a prototype member of the Rho GTPase guanine nucleotide exchange factor (GEF) family. Oncogenic activation of proto-Dbl occurs through truncation of the N-terminal 497 residues. The C-terminal half of proto-Dbl includes residues 498 to 680 and 710 to 815, which fold into the Dbl homology (DH) domain and the pleckstrin homology (PH) domain, respectively, both of which are essential for cell transformation via the Rho GEF activity or cytoskeletal targeting function. Here we have investigated the mechanism of the apparent negative regulation of proto-Dbl imposed by the N-terminal sequences. Deletion of the N-terminal 285 or C-terminal 100 residues of proto-Dbl did not significantly affect either its transforming activity or GEF activity, while removal of the N-terminal 348 amino acids resulted in a significant increase in both transformation and GEF potential. Proto-Dbl displayed a mostly perinuclear distribution pattern, similar to a polypeptide derived from its N-terminal sequences, whereas onco-Dbl colocalized with actin stress fibers, like the PH domain. Coexpression of the N-terminal 482 residues with onco-Dbl resulted in disruption of its cytoskeletal localization and led to inhibition of onco-Dbl transforming activity. The apparent interference with the DH and PH functions by the N-terminal sequences can be rationalized by the observation that the N-terminal 482 residues or a fragment containing residues 286 to 482 binds specifically to the PH domain, limiting the access of Rho GTPases to the catalytic DH domain and masking the intracellular targeting function of the PH domain. Taken together, our findings unveiled an autoinhibitory mode of regulation of proto-Dbl that is mediated by the intramolecular interaction between its N-terminal sequences and PH domain, directly impacting both the GEF function and intracellular distribution.

Oligomerization of DH domain is essential for Dbl-induced transformation

The dbl oncogene product (onco-Dbl) is the prototype member of a family of guanine nucleotide exchange factors (GEFs) for Rho GTPases. The Dbl homology (DH) domain of onco-Dbl is responsible for the GEF catalytic activity, and the DH domain, together with the immediately adjacent pleckstrin homology (PH) domain, constitutes the minimum module bearing transforming function. In the present study, we demonstrate that the onco-Dbl protein exists in oligomeric form in vitro and in cells. The oligomerization is mostly homophilic in nature and is mediated by the DH domain. Mutagenesis studies mapped the region involved in oligomerization to the conserved region 2 of the DH domain, which is located at the opposite side of the Rho GTPase interacting surface. Residue His556 of this region, in particular, is important for this activity, since the H556A mutant retained the GEF catalytic capability and the binding activity toward Cdc42 and RhoA in vitro but was deficient in oligomer formation. Consequently, the Rho GTPase activating potential of the H556A mutant was significantly reduced in cells. The focus-forming and anchorage-independent growth activities of onco-Dbl were completely abolished by the His556-to-Ala mutation, whereas the abilities to stimulate cell growth, activate Jun N-terminal kinase, and cause actin cytoskeletal changes were retained by the mutant. The ability of onco-Dbl to oligomerize allowed multiple Rho GTPases to be recruited to the same signaling complex, and such an ability is defective in the H556A mutant. Taken together, these results suggest that oligomerization of onco-Dbl through the DH domain is essential for cellular transformation by providing the means to generate a signaling complex that further augments and/or coordinates its Rho GTPase activating potential.

Genetic screens in mammalian cells by enhanced retroviral mutagens

Genetic approaches such as retrovirus-mediated mutagenesis and cDNA expression libraries have contributed greatly to our understanding of signal transduction in mammalian cells. However, previously described methods for retroviral insertional mutagenesis are hindered by low mutagenesis rates and difficulties in cloning mutated genes. cDNA expression library methods are usually cell-type dependent and bias towards abundant and short messages. With the near completion of the genome projects, alternative genetic methods are needed where large numbers of genes can be more easily isolated and biochemically studied. We have developed a novel retrovirus-mediated genetic screening method in cultured cells. To achieve efficient and regulated mutagenesis, we constructed Enhanced Retroviral Mutagen (ERM) vectors that contained several engineered sequences (e.g., an ERM Tag and a splice donor) controlled by a tetracycline-responsive promoter. Endogenous genes can thus be randomly activated and tagged in a conditional system. NIH3T3 cells were used to screen for focus-forming genes using the ERM strategy. We showed that these added sequences increased the screening efficiency by >10-fold, and allowed more direct identification of the genes targeted. Sequence analysis of approximately 10% of the >600 focus clones recovered revealed both known oncogenes and novel factors such as protein kinases and GTP/GDP exchange proteins. The ERM strategy should help to facilitate large-scale gene identification in diverse pathways and integrate both genetic (with the completion of the genome projects) and functional information more readily.