Distinct involvement of cdc42 and RhoA GTPases in actin organization and cell shape in untransformed and Dbl oncogene transformed NIH3T3 cells

RHOAL57V drives the development of diffuse gastric cancer through IGF1R-PAK1-YAP1 signaling

Cancer-associated mutations in the guanosine triphosphatase (GTPase) RHOA are found at different locations from the mutational hotspots in the structurally and biochemically related RAS. Tyr42-to-Cys (Y42C) and Leu57-to-Val (L57V) substitutions are the two most prevalent RHOA mutations in diffuse gastric cancer (DGC). RHOAY42C exhibits a gain-of-function phenotype and is an oncogenic driver in DGC. Here, we determined how RHOAL57V promotes DGC growth. In mouse gastric organoids with deletion of Cdh1, which encodes the cell adhesion protein E-cadherin, the expression of RHOAL57V, but not of wild-type RHOA, induced an abnormal morphology similar to that of patient-derived DGC organoids. RHOAL57V also exhibited a gain-of-function phenotype and promoted F-actin stress fiber formation and cell migration. RHOAL57V retained interaction with effectors but exhibited impaired RHOA-intrinsic and GAP-catalyzed GTP hydrolysis, which favored formation of the active GTP-bound state. Introduction of missense mutations at KRAS residues analogous to Tyr42 and Leu57 in RHOA did not activate KRAS oncogenic potential, indicating distinct functional effects in otherwise highly related GTPases. Both RHOA mutants stimulated the transcriptional co-activator YAP1 through actin dynamics to promote DGC progression; however, RHOAL57V additionally did so by activating the kinases IGF1R and PAK1, distinct from the FAK-mediated mechanism induced by RHOAY42C. Our results reveal that RHOAL57V and RHOAY42C drive the development of DGC through distinct biochemical and signaling mechanisms.

Mechanobiology of microvesicle release, uptake, and microvesicle-mediated activation

Microvesicles are small, membrane-bound vesicles that are shed from the plasma membrane of cells into the extracellular space. Microvesicles contain a variety of cargo not typically thought to be released from cells, including receptor tyrosine kinases, cytosolic signaling proteins, and microRNAs, which are transferred from donor cells to recipient cells. The transfer of microvesicle cargo can result in the transformation of recipient cells thereby supporting disease progression, including modified fibroblast metabolism, epithelial cell contractility, vascular remodeling, and immune cell inflammatory signaling. Additionally, microvesicles are believed to play prominent roles in cell-cell communication and disease progression as they are detected at elevated concentrations in diseased tissues. As microvesicle uptake by recipient cells can modulate cell function to promote disease progression, understanding the mechanisms and mechanosensitivity of microvesicle release, internalization, and the resulting signaling is crucial to fully comprehend their functions in disease. Here, we review recent advances in the understanding of actomyosin-regulated microvesicle biogenesis, microvesicle uptake via pinocytosis, and the resulting cellular transformation. We discuss the effects of altered cell contractility, mode of cell migration, and extracellular matrix compliance on microvesicle signaling, with direct implications in disease progression and identifying future therapeutic targets.

Paracrine signalling of AGR2 stimulates RhoA function in fibroblasts and modulates cell elongation and migration

The most prominent cancer-associated fibroblasts (CAFs) in tumor stroma is known to form a protective structure to support tumor growth. Anterior gradient-2 (AGR2), a tumor secretory protein is believed to play a pivotal role during tumor microenvironment (TME) development. Here, we report that extracellular AGR2 enhances fibroblasts elongation and migration significantly. The early stimulation of RhoA showed the association of AGR2 by upregulation of G1-S phase-regulatory protein cyclin D1 and FAK phosphorylation through fibroblasts growth factor receptor (FGFR) and vascular endothelial growth factor receptor (VEGFR). Our finding indicates that secretory AGR2 alters fibroblasts elongation, migration, and organization suggesting the secretory AGR2 as a potential molecular target that might be responsible to alter fibroblasts infiltration to support tumor growth.

Gain-of-Function RHOA Mutations Promote Focal Adhesion Kinase Activation and Dependency in Diffuse Gastric Cancer

Diffuse gastric cancer (DGC) is a lethal malignancy lacking effective systemic therapy. Among the most provocative recent results in DGC has been that of highly recurrent missense mutations in the GTPase RHOA. The function of these mutations has remained unresolved. We demonstrate that RHOA^Y42C, the most common RHOA mutation in DGC, is a gain-of-function oncogenic mutant, and that expression of RHOA^Y42C with inactivation of the canonical tumor suppressor Cdh1 induces metastatic DGC in a mouse model. Biochemically, RHOA^Y42C exhibits impaired GTP hydrolysis and enhances interaction with its effector ROCK. RHOA ^Y42C mutation and Cdh1 loss induce actin/cytoskeletal rearrangements and activity of focal adhesion kinase (FAK), which activates YAP-TAZ, PI3K-AKT, and β-catenin. RHOA^Y42C murine models were sensitive to FAK inhibition and to combined YAP and PI3K pathway blockade. These results, coupled with sensitivity to FAK inhibition in patient-derived DGC cell lines, nominate FAK as a novel target for these cancers. SIGNIFICANCE: The functional significance of recurrent RHOA mutations in DGC has remained unresolved. Through biochemical studies and mouse modeling of the hotspot RHOA^Y42C mutation, we establish that these mutations are activating, detail their effects upon cell signaling, and define how RHOA-mediated FAK activation imparts sensitivity to pharmacologic FAK inhibitors.See related commentary by Benton and Chernoff, p. 182.This article is highlighted in the In This Issue feature, p. 161.

Microvesicle Cargo and Function Changes upon Induction of Cellular Transformation

Extracellular vesicles (EVs), including exosomes and microvesicles (MVs), have emerged as a major form of intercellular communication, playing important roles in several physiological processes and diseases, including cancer. EVs generated by cancer cells contain a variety of proteins and RNA species that can be transferred between cancer cells as well as between cancer and non-transformed (normal) cells, thereby impacting a number of aspects of cancer progression. Here we show how oncogenic transformation influences the biogenesis and function of EVs using a mouse embryonic fibroblast (MEF) cell line that can be induced to express an oncogenic form of diffuse B cell lymphoma (Dbl). Although MEFs induced to express onco-Dbl generated a similar amount of MVs as uninduced control cells, we found that MVs isolated from onco-Dbl-transformed cells contain a unique signaling protein, the ubiquitously expressed non-receptor tyrosine kinase focal adhesion kinase. The addition of MVs isolated from MEFs expressing onco-Dbl to cultures of fibroblasts strongly promoted their survival and induced their ability to grow under anchorage-independent conditions, outcomes that could be reversed by knocking down focal adhesion kinase and depleting it from the MVs or by inhibiting its kinase activity using a specific inhibitor. We then showed the same to be true for MVs isolated from aggressive MDAMB231 breast cancer cells. Together, these findings demonstrate that the induction of oncogenic transformation gives rise to MVs, which uniquely contain a signaling protein kinase that helps propagate the transformed phenotype and thus may offer a specific diagnostic marker of malignant disease.

Mechanism by which a recently discovered allosteric inhibitor blocks glutamine metabolism in transformed cells

The mitochondrial enzyme glutaminase C (GAC) catalyzes the hydrolysis of glutamine to glutamate plus ammonia, a key step in the metabolism of glutamine by cancer cells. Recently, we discovered a class of allosteric inhibitors of GAC that inhibit cancer cell growth without affecting their normal cellular counterparts, with the lead compound being the bromo-benzophenanthridinone 968. Here, we take advantage of mouse embryonic fibroblasts transformed by oncogenic Dbl, which hyperactivates Rho GTPases, together with (13)C-labeled glutamine and stable-isotope tracing methods, to establish that 968 selectively blocks the enhancement in glutaminolysis necessary for satisfying the glutamine addiction of cancer cells. We then determine how 968 inhibits the catalytic activity of GAC. First, we developed a FRET assay to examine the effects of 968 on the ability of GAC to undergo the dimer-to-tetramer transition necessary for enzyme activation. We next demonstrate how the fluorescence of a reporter group attached to GAC provides a direct read-out of the binding of 968 and related compounds to the enzyme. By combining these fluorescence assays with newly developed GAC mutants trapped in either the monomeric or dimeric state, we show that 968 has the highest affinity for monomeric GAC and that the dose-dependent binding of 968 to GAC monomers directly matches its dose-dependent inhibition of enzyme activity and cellular transformation. Together, these findings highlight the requirement of tetramer formation as the mechanism of GAC activation and shed new light on how a distinct class of allosteric GAC inhibitors impacts the metabolic program of transformed cells.

ERG oncoprotein inhibits ANXA2 expression and function in prostate cancer

Overexpression of ERG in the prostate epithelium, due to chromosomal translocations, contributes to prostate tumorigenesis. Here, genomic analysis of ERG siRNA-treated prostate cells harboring the endogenous TMPRSS2-ERG fusion revealed an inverse relationship between ERG and Annexin A2 (ANXA2) expression at both the RNA and protein level. ANXA2, a Ca(2+)-dependent and phospholipid-binding protein, is involved in various cellular functions, including maintenance of epithelial cell polarity. Mechanistic studies defined the prostate-specific transcription start site of ANXA2 and showed that the recruitment of ERG to the ANXA2 promoter is required for transcriptional repression by ERG. Knockdown of ERG enhanced the apical localization of ANXA2, the bundling of actin filaments at cell-cell junctions and formation of a polarized epithelial phenotype. ERG overexpression disrupted ANXA2-mediated cell polarity and promoted epithelial-mesenchymal transition (EMT) by inhibiting CDC42 and RHOA, and by activating cofilin. Immunohistochemistry demonstrated a reciprocal relationship of ANXA2 and ERG expression in a large fraction of primary prostate cancer clinical specimens. ANXA2 was absent or markedly reduced in ERG(+) tumors, which were mostly well differentiated. ERG(-) tumors, meanwhile, expressed moderate to high levels of ANXA2, and were either poorly differentiated or displayed subsets of poorly differentiated cells. Taken together, the transcriptional repression of ANXA2 by ERG in prostate epithelial cells plays a critical role in abrogating differentiation, promoting EMT, and in the reciprocal correlation of ERG and ANXA2 expression observed in human prostate cancer.

IMPLICATIONS

ANXA2 is a new component of the ERG network with potential to enhance biologic stratification and therapeutic targeting of ERG-stratified prostate cancers.

The function of p120 catenin in filopodial growth and synaptic vesicle clustering in neurons

The signaling by p120 catenin via its downstream effector RhoA is essential for filopodial growth and synaptic vesicle clustering along spinal axons and contributes to the formation of the neuromuscular junction.

At the developing neuromuscular junction (NMJ), physical contact between motor axons and muscle cells initiates presynaptic and postsynaptic differentiation. Using Xenopus nerve–muscle cocultures, we previously showed that innervating axons induced muscle filopodia (myopodia), which facilitated interactions between the synaptic partners and promoted NMJ formation. The myopodia were generated by nerve-released signals through muscle p120 catenin (p120ctn), a protein of the cadherin complex that modulates the activity of Rho GTPases. Because axons also extend filopodia that mediate early nerve–muscle interactions, here we test p120ctn's function in the assembly of these presynaptic processes. Overexpression of wild-type p120ctn in Xenopus spinal neurons leads to an increase in filopodial growth and synaptic vesicle (SV) clustering along axons, whereas the development of these specializations is inhibited following the expression of a p120ctn mutant lacking sequences important for regulating Rho GTPases. The p120ctn mutant also inhibits the induction of axonal filopodia and SV clusters by basic fibroblast growth factor, a muscle-derived molecule that triggers presynaptic differentiation. Of importance, introduction of the p120ctn mutant into neurons hinders NMJ formation, which is observed as a reduction in the accumulation of acetylcholine receptors at innervation sites in muscle. Our results suggest that p120ctn signaling in motor neurons promotes nerve–muscle interaction and NMJ assembly.

Evidence for two CRIB domains in phospholipase D2 (PLD2) that the enzyme uses to specifically bind to the small GTPase Rac2

Phospholipase D (PLD) and small GTPases are vital to cell signaling. We report that the Rac2 and the PLD2 isoforms exist in the cell as a lipase-GTPase complex that enables the two proteins to elicit their respective functionalities. A strong association between the two molecules was demonstrated by co-immunoprecipitation and was confirmed in living cells by FRET with CFP-Rac2 and YFP-PLD2 fluorescent chimeras. We have identified the amino acids in PLD2 that define a specific binding site to Rac2. This site is composed of two CRIB (Cdc42-and Rac-interactive binding) motifs that we have named "CRIB-1" and "CRIB-2" in and around the PH domain in PLD2. Deletion mutants PLD2-ΔCRIB-1/2 negate co-immunoprecipitation with Rac2 and diminish the FRET signal in living cells. The PLD2-Rac2 association was further confirmed in vitro using affinity-purified recombinant proteins. Binding was saturable with an apparent K(d) of 3 nm and was diminished with PLD2-ΔCRIB mutants. Furthermore, PLD2 bound more efficiently to Rac2-GTP than to Rac2-GDP or to a GDP-constitutive Rac2-N17 mutant. Increasing concentrations of recombinant Rac2 in vitro and in vivo during cell adhesion inhibit PLD2. Conversely, Rac2 activity is increased in the presence of PLD2-WT but not in PLD2-ΔCRIB. We propose that in activated cells PLD2 affects Rac2 in an initial positive feedback, but as Rac2-GTP accumulates in the cell, this constitutes a "termination signal" leading to PLD2 inactivation.

Shotgun proteomics of Aspergillus niger microsomes upon D-xylose induction

Protein secretion plays an eminent role in cell maintenance and adaptation to the extracellular environment of microorganisms. Although protein secretion is an extremely efficient process in filamentous fungi, the mechanisms underlying protein secretion have remained largely uncharacterized in these organisms. In this study, we analyzed the effects of the d-xylose induction of cellulase and hemicellulase enzyme secretion on the protein composition of secretory organelles in Aspergillus niger. We aimed to systematically identify the components involved in the secretion of these enzymes via mass spectrometry of enriched subcellular microsomal fractions. Under each condition, fractions enriched for secretory organelles were processed for tandem mass spectrometry, resulting in the identification of peptides that originate from 1,081 proteins, 254 of which-many of them hypothetical proteins-were predicted to play direct roles in the secretory pathway. d-Xylose induction led to an increase in specific small GTPases known to be associated with polarized growth, exocytosis, and endocytosis. Moreover, the endoplasmic-reticulum-associated degradation (ERAD) components Cdc48 and all 14 of the 20S proteasomal subunits were recruited to the secretory organelles. In conclusion, induction of extracellular enzymes results in specific changes in the secretory subproteome of A. niger, and the most prominent change found in this study was the recruitment of the 20S proteasomal subunits to the secretory organelles.

Integrin Cytoplasmic domain-Associated Protein-1 (ICAP-1) promotes migration of myoblasts and affects focal adhesions

Integrin Cytoplasmic domain-Associated Protein-1 (ICAP-1) binds specifically to the beta1 integrin subunit cytoplasmic domain. We observed that RNAi-induced knockdown of ICAP-1 reduced migration of C2C12 myoblasts on the beta1 integrin ligand laminin and that overexpression of ICAP-1 increased this migration. In contrast, migration on the beta3 integrin ligand vitronectin was not affected. ICAP-1 knockdown also greatly diminished migration of microvascular endothelial cells on collagen. The number of central focal adhesions in C2C12 cells on laminin was reduced by ICAP-1 knockdown and increased by ICAP-1 overexpression. Previously, we demonstrated that ICAP-1 binds to the ROCK-I kinase and translocates ROCK-I to the plasma membrane. We show here that the ROCK kinase inhibitor Y27362 reduces migration on laminin and causes a loss of central focal adhesions, similarly as ICAP-1 knockdown. ICAP-1 and ROCK were co-immune-precipitated from C2C12 cells, and in cells that overexpressed ICAP-1, YFP-ROCK was translocated to membrane ruffles. These results indicate that ICAP-1 regulates beta1 integrin-dependent cell migration by affecting the pattern of focal adhesion formation. This is likely due to ICAP-1-induced translocation of ROCK to beta1 integrin attachment sites.

Simvastatin inhibits TNFα-induced invasion of human cardiac myofibroblasts via both MMP-9-dependent and -independent mechanisms

Statins can reduce adverse myocardial remodeling independently of their cholesterol-lowering ability. We have previously reported that simvastatin inhibits tumor necrosis factor-alpha (TNFalpha)-induced cardiac myofibroblast invasion and MMP-9 secretion, key events in this remodeling process. The aim of the present study was to investigate the mechanisms underlying this effect. Selective MMP-9 gene silencing with siRNA oligonucleotides revealed that myofibroblast invasion through a Matrigel barrier (Boyden chamber assay) was MMP-9-dependent. In contrast, cell migration (in the absence of Matrigel) was MMP-9-independent. Simvastatin, a commonly prescribed statin, inhibited both invasion and migration of myofibroblasts and disrupted the actin cytoskeleton as determined by confocal microscopy of rhodamine-phalloidin staining. All these effects of simvastatin were mimicked by the Rho-kinase inhibitor Y27632. TNFalpha activated the ERK-1/2, p38 MAPK, PI-3-kinase and NF-kappaB pathways but not the JNK pathway, as determined by immunoblotting with phospho-specific antibodies. Quantitative RT-PCR revealed that TNFalpha-induced MMP-9 mRNA expression was substantially reduced by pharmacological inhibitors of the ERK-1/2, PI-3-kinase and NF-kappaB pathways. However, none of the signal transduction pathways studied was influenced by simvastatin treatment. Moreover, despite reducing MMP-9 secretion, simvastatin had no effect on MMP-9 promoter activity (luciferase reporter assay) and actually increased MMP-9 mRNA levels. In summary, simvastatin reduces TNFalpha-induced invasion of human cardiac myofibroblasts through two distinct mechanisms: (i) by attenuating cell migration via Rho-kinase inhibition and subsequent cytoskeletal disruption, and (ii) by decreasing MMP-9 secretion via a post-transcriptional mechanism.

Effects of constitutively active GTPases on fibroblast behavior

The GTP-binding proteins RhoA, Cdc42 and Rac1 regulate the organization and turnover of the cytoskeleton and cell-matrix adhesions, structures bridging cells to their support, and translating forces, external or generated within the cell. To investigate the specific requirements of Rho GTPases for biomechanical activities of clonal cell populations, we compared side-by-side stable lines of human fibroblasts expressing constitutively active (CA) RhoA, Cdc42 or Rac1. There was no marked effect of any CA GTPase on cell adhesion to different extracellular matrix proteins. Cell spreading was CA Rho GTPase specific and independent of the extracellular matrix proteins allowing adhesion. Mechanical properties were dramatically restricted by CA RhoA on bi- and in tri-dimensional surroundings, were boosted by CA Rac1 on bi-dimensional surroundings only, and were not or marginally affected by CA Cdc42. In conclusion, the action of Rho GTPases appears to depend on the task cells are performing.

Wisteria floribunda gall extract inhibits cell migration in mouse B16F1 melanoma cells by regulating CD44 expression and GTP-RhoA activity

Extracts from galls grown on Wisteria floribunda are used as an anti-tumoral preparation in oriental traditional medicine. Here, we investigated the molecular mechanism of this anti-tumoral effect by first examining whether the extract inhibited cell migration in a B16 cell-based wound healing assay. The gall extract delayed wound healing in a dose- and time-dependent manner, indicating that one or more components of the fraction inhibited cell migration. Examination of two molecules known to be involved in metastasis, CD44, and RhoA-GTP, revealed that the gall extract decreased CD44 expression in a concentration-dependent manner, and also increased RhoA-GTP activity in comparison to untreated controls. Taken together, these results suggest that the Wisteria gall extract may inhibit cancer cell migration via inhibition of CD44 mRNA expression and activation of the GTP-RhoA protein.

Integrin engagement differentially modulates epithelial cell motility by RhoA/ROCK and PAK1

Integrin-ligand binding regulates tumor cell motility and invasion. Cell migration also involves the Rho GTPases that control the interplay between adhesion receptors and the cytoskeleton. We evaluated how specific extracellular matrix ligands modulate Rho GTPases and control motility of human squamous cell carcinoma cells. On laminin-5 substrates, the epithelial cells rapidly spread and migrated, but on type I collagen the cells spread slowly and showed reduced motility. We found that RhoA activity was suppressed in cells attached to laminin-5 through the alpha3 integrin receptor. In contrast, RhoA was strongly activated in cells bound to type I collagen and this was mediated by the alpha2 integrin. Inhibiting the RhoA pathway by expression of a dominant-negative RhoA mutant or by directly inhibiting ROCK, reduced focal adhesion formation and enhanced cell migration on type I collagen. Cdc42 and Rac and their downstream target PAK1 were activated following adhesion to laminin-5. PAK1 activation induced by laminin-5 was suppressed by expression of a dominant-negative Cdc42. Moreover, constitutively active PAK1 stimulated migration on collagen I substrates. Our results indicate that in squamous epithelial cells, collagen-alpha2beta1 integrin binding activates RhoA, slowing cell locomotion, whereas laminin-5-alpha3beta1 integrin interaction inhibits RhoA and activates PAK1, stimulating cell migration. The data demonstrate that specific ligand-integrin pairs regulate cell motility differentially by selectively modulating activities of Rho GTPases and their effectors.

Tech: a RhoA GEF selectively expressed in hippocampal and cortical neurons

Recent studies implicating the Rho family of small G proteins in the regulation of neuronal morphology have focused attention on identifying key components of Rho signaling pathways in neurons. To this end, we have conducted studies aimed at defining the localization and function of Tech, a Rho guanine nucleotide exchange factor (GEF) family member that is highly enriched in brain. We have found that Tech is selectively expressed in cortical and hippocampal neurons with prominent Tech immunostaining apparent in the cell bodies and dendrites of these cells. In vitro studies with prototypical members of the major Rho subfamilies, RhoA, Rac1 and Cdc42, indicate that Tech binds selectively to and activates RhoA. To assess whether Tech may be involved in the regulation of neuronal morphology, we examined the effects of Tech constructs on the morphology of cortical neurons grown in primary culture. We found that a constitutively active Tech construct, Tech 245DeltaC, decreases the number of dendritic processes present on these neurons. This reduction appears to be mediated by activation of RhoA as it is blocked by insertion of a point mutation into the DH domain of Tech which blocks its ability to activate RhoA or coexpression of a dominant negative RhoA construct. As Tech protein levels increase during post-natal development and remain at peak levels into adulthood, these results indicate that Tech regulates RhoA signaling pathways in developing and mature forebrain neurons.

Cytoglobin/STAP, its unique localization in splanchnic fibroblast-like cells and function in organ fibrogenesis

Cytoglobin/stellate cell activation-associated protein (Cygb/STAP) consists of a new class of hexacoordinate globin superfamily, which was recently discovered by a proteome analysis on the rat hepatic stellate cells. Unlike haemoglobin, myoglobin, and neuroglobin, Cygb/STAP is ubiquitously expressed in several organs, although its detailed localization has not been clarified. Immunohistochemistry and immunoelectron microscopy revealed that Cygb/STAP is uniquely localized in fibroblast-like cells in splanchnic organs, namely the vitamin A-storing cell lineage, but neither in epithelial cells, endothelial cells, muscle cells, blood cells, macrophages, nor dermal fibroblasts. The expression of Cygb/STAP was upregulated in fibrotic lesions of the pancreas and kidney in which activated fibroblast-like cells or myofibroblasts are known to increase in number. In cultured hepatic stellate cells, Cygb/STAP expression was augmented by the stimulation with sera, platelet-derived growth factor-BB, and transforming growth factor-beta 1. Overexpression of Cygb/STAP in NIH 3T3 cells induced the cells to lessen migratory activities and increase the expression of collagen alpha1(I) mRNA. These results indicate that Cygb/STAP is a tissue globin uniquely localized in splanchnic fibroblastic cell lineage and may play a role in fibrotic organ disorder.

Early activation of p160ROCK by pressure overload in rat heart

We investigated the effects of acute pressure overload on activation of p160(ROCK) in rat myocardium. Constriction of transverse aorta, controlled to increase peak systolic pressure of ascending aorta by approximately 40 mmHg, induced a rapid association of RhoA with Dbl-3 and p160(ROCK). The binding of p160(ROCK) to RhoA was rapidly increased, peaking at 30 min (approximately 3.5-fold), but reduced to lower levels (approximately 1.9-fold) by 60 min of pressure overload. The activity of immunoprecipitated p160(ROCK) toward myosin light chain increased approximately 2.5-fold within 10 min but decreased to lower levels (approximately 1.6-fold) after 60 min of pressure overload. Confocal microscopic analysis indicated that pressure overload induced the formation of aggregates of p160(ROCK) and RhoA along the longitudinal axis of cardiac myocytes. Immunoelectron microscopic analysis showed that pressure overload induced the association of p160(ROCK) and RhoA to Z-line, T-tubule, and subsarcolemmal areas. The rapid activation of p160(ROCK) by pressure overload and its aggregation in subcellular structures involved in transmission of mechanical force suggest a role for this enzyme in the mechanobiochemical transduction in the myocardium.

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 integrin cytoplasmic domain-associated protein ICAP-1 binds and regulates Rho family GTPases during cell spreading

Using two-hybrid screening, we isolated the integrin cytoplasmic domain-associated protein (ICAP-1), an interactor for the COOH terminal region of the beta1A integrin cytoplasmic domain. To investigate the role of ICAP-1 in integrin-mediated adhesive function, we expressed the full-length molecule in NIH3T3 cells. ICAP-1 expression strongly prevents NIH3T3 cell spreading on extracellular matrix. This inhibition is transient and can be counteracted by coexpression of a constitutively activated mutant of Cdc42, suggesting that ICAP-1 acts upstream of this GTPase. In addition, we found that ICAP-1 binds both to Cdc42 and Rac1 in vitro, and its expression markedly inhibits activation of these GTPases during integrin-mediated cell adhesion to fibronectin as detected by PAK binding assay. In the attempt to define the molecular mechanism of this inhibition, we show that ICAP-1 reduces both the intrinsic and the exchange factor-induced dissociation of GDP from Cdc42; moreover, purified ICAP-1 displaces this GTPase from cellular membranes. Together, these data show for the first time that ICAP-1 regulates Rho family GTPases during integrin-mediated cell matrix adhesion, acting as guanine dissociation inhibitor.

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.

Identification of Rho GTPase-dependent sites in the Dbl homology domain of oncogenic Dbl that are required for transformation

The Dbl family guanine-nucleotide exchange factors (GEFs) for Rho GTPases share the structural array of a Dbl homology (DH) domain in tandem with a Pleckstrin homology (PH) domain. For oncogenic Dbl, the DH domain is responsible for the GEF activity, and the DH-PH module constitutes the minimum structural unit required for cellular transformation. To understand the structure-function relationship of the DH domain, we have investigated the role of specific residues of the DH domain of Dbl in interaction with Rho GTPases and in Dbl-induced transformation. Alanine substitution mutagenesis identified a panel of DH mutants made in the alpha1, alpha6, and alpha9 regions and the PH junction site that suffer complete or partial loss of GEF activity toward Cdc42 and RhoA. Kinetic and binding analysis of these mutants revealed that although most displayed decreased k(cat) values in the GEF reaction, the substrate binding activities of T506A and R634A were significantly reduced. E502A, Q633A, and N673A/D674A, on the other hand, retained the binding capability to the Rho GTPases but lost the GEF catalytic activity. In general, the in vitro GEF activity of the DH mutants correlated with the in vivo Cdc42- and RhoA-activating potential, and the GEF catalytic efficiency mirrored the transforming activity in NIH 3T3 cells. Moreover, the N673A/D674A mutant exhibited a potent dominant-negative effect on serum-induced cell growth and caused retraction of actin structures. These studies identify important sites of the DH domain involved in binding or catalysis of Rho proteins and demonstrate that maintaining a threshold of GEF catalytic activity, in addition to the Rho GTPase binding activity, is essential for efficient transformation by oncogenic Dbl.