Tyrosine phosphorylation of the vascular endothelial-growth-factor receptor-2 (VEGFR-2) is modulated by Rho proteins

The effects of Rho-specific modifying toxins on the tyrosine phosphorylation of endothelial cell proteins were investigated. Incubation of the cells with the Rho-activating toxin cytotoxic necrotizing factor 1 (CNF1) induced a marked increase in the tyrosine phosphorylation of a number of signalling intermediates of the vascular endothelial growth factor (VEGF)-mediated cascade, including focal adhesion kinase, paxillin, phospholipase Cgamma1 and a Shc-associated protein of 195 kDa. Both CNF1- and VEGF-dependent tyrosine phosphorylation of these proteins were significantly reduced by prior incubation with C3 transferase, a known inhibitor of RhoA function, suggesting a Rho-dependent mechanism. The stimulation of endothelial cells with CNF1 resulted in a marked increase in the tyrosine phosphorylation of the VEGF receptor (VEGFR)-2, which was correlated with a stimulation of its kinase activity and with its association with downstream tyrosine phosphorylated proteins. The stimulatory effect of CNF1 was specific for VEGFR-2 since the phosphotyrosine content of VEGFR-1 was not affected by the toxin. Transient overexpression of a dominant-active RhoA mutant also induced an increase in the tyrosine phosphorylation of the VEGFR-2, whereas overexpression of a dominant-inactive form of the protein was without effect. Taken together, these results indicate that Rho proteins may play an important role in angiogenesis by modulating the tyrosine phosphorylation levels of VEGFR-2.

Reconstitution of membranes simulating "glycosignaling domain" and their susceptibility to lyso-GM3

GM3 ganglioside at the surface of mouse melanoma B16 cells is clustered and organized with signal transducer molecules c-Src, Rho A, and focal adhesion kinase (FAK) to form a membrane unit separable from caveolae, which are enriched in cholesterol and caveolin but do not contain GM3 or the above three signal transducers. The GM3-enriched membrane units are involved in GM3-dependent cell adhesion coupled with activation of c-Src, Rho A, and FAK and are termed the "glycosphingolipid signaling domain" or the "glycosignaling domain" (GSD). In order to assess the essential components that display GSD function, membranes with properties similar to those of GSD were reconstituted using GM3, sphingomyelin, and c-Src, with or without other lipid components. The reconstituted membrane thus prepared displayed GM3-dependent adhesion to plates coated with Gg3 or anti-GM3 antibody, resulting in enhanced c-Src phosphorylation (c-Src phosphorylation response). This response in reconstituted membrane depends on GM3 concentration and was not observed when GM3 was absent or replaced with other gangliosides GM1 or GD1a, or with LacCer. The GM3-dependent c-Src phosphorylation response was enhanced when cholesterol and phosphatidylcholine were added. Although GM3, sphingomyelin, and c-Src are essential for GSD function, a small quantity of cholesterol and phosphatidylcholine may act as an auxiliary factor to stabilize membrane. GSD function in terms of GM3-dependent adhesion and signaling was blocked in the presence of lyso-GM3 or its analogue but not psychosine, lactosyl-sphingosine, or lyso-phosphatidylcholine. Such susceptibility of reconstituted GSD to lyso-GM3 and other lyso compounds is the same as GSD of original B16 cells. Thus, functional organization of the reconstituted membrane closely simulates that of GSD in B16 cells, which is based on clustered GM3 organized with c-Src as the essential components.

Modulation of Rho and cytoskeletal protein attachment to membranes by a prenylcysteine analog

The GTPases Rho regulate the assembly of polymerized actin structures. Their C-terminal sequences end with the CAAX motif that undergo a lipidation of the cysteine residue. Analogs to the C-terminal ends of Rho proteins, N-acetyl-S-all-trans, trans-farnesyl-L-cysteine and N-acetyl-S-all-trans-geranylgeranyl-L-cysteine, wereused to analyze the role of prenylation in their membrane association. Silver-stained gels indicated that N-acetyl-S-all-trans-geranylgeranyl-L-cysteine treatment released only a few proteins of 20, 46, and 60 kDa. Western blot analysis showed that N-acetyl-S-all-trans-geranylgeranyl-L-cysteine released RhoB (10%), RhoA (28%), and Cdc42 (95%) from membranes, whereas N-acetyl-S-all-trans and trans-farnesyl-L-cysteine did not. Rab1, which possesses two geranylgeranyl groups, was also strongly extracted by N-acetyl-S-all-trans-geranylgeranyl-L-cysteine, whereas Ras, which is farnesylated, was not. Furthermore, N-acetyl-S-all-trans-geranylgeranyl-L-cysteine was very efficient (95%) in dissociating actin and tubulin from membranes but not integral membrane protein P-glycoprotein and sodium/phosphate cotransporter NaP(i)-2. The extraction of Rho and cytoskeletal proteins occurred below the critical micellar concentration of N-acetyl-S-all-trans-geranylgeranyl-L-cysteine. Membrane treatments with 0.7 m KI totally extracted actin, whereas 70% of Cdc42 was released. Actin was, however, insoluble in Triton X-100-treated membranes, whereas this detergent extracted (80%) Cdc42. These data show that Rho proteins and actin are not physically bound together and suggest that their extraction from membranes by N-acetyl-S-all-trans-geranylgeranyl-L-cysteine likely occurs via different mechanisms.

Somatostatin interferes with thyrotropin-induced G1-S transition mediated by cAMP-dependent protein kinase and phosphatidylinositol 3-kinase. Involvement of RhoA and cyclin E x cyclin-dependent kinase 2 complexes

cAMP-mediated cell proliferation is a complex process that involves multiple pathways. Using a cAMP-dependent cell system, FRTL-5 thyroid cells, we have previously demonstrated the existence of a precise autocrine loop in the control of cell proliferation that involves the positive effector thyrotropin (TSH) and the general inhibitor somatostatin. In search of the regulatory mechanisms responsible for the TSH and somatostatin control of cell proliferation, we analyzed the cell cycle regulatory proteins and the cellular pathways involved in the action of both signals. The results show that specific inhibition of cAMP-dependent protein kinase (PKA) and phosphatidylinositol (PI) 3-kinase blocks independently TSH-induced FRTL-5 cell proliferation and that somatostatin interferes with both signals. Each pathway activates different proteins required for G(1)/S progression. Thus, PKA is responsible for the TSH-induction of 3-hydroxy-3-methylglutaryl-CoA reductase mRNA levels, RhoA activation, and down-regulation of p27(kip1). These correlated events are necessary for FRTL-5 cell proliferation after TSH stimulation. Moreover, TSH through PKA pathway increases cyclin-dependent kinase 2 levels, whereas PI 3-kinase signaling increases cyclin E levels. Together, both pathways finally converge, increasing the formation and activation of cyclin E x cyclin-dependent kinase 2 complexes and the phosphorylation of the retinoblastoma protein, two important steps in the transition from G(1) to S phase in growth-stimulated cells. Somatostatin exerts its antiproliferative effect inhibiting more upstream the TSH stimulation of PKA and PI 3-kinase, interfering with the TSH-mediated increases of intracellular cAMP levels by inactivation of adenylyl cyclase activity. Together, these results suggest the existence of a PKA-dependent pathway and a new PKA-independent PI 3-kinase pathway in the TSH/cAMP-mediated proliferation of FRTL-5 thyroid cells.

Receptor-activated phospholipase D is present in caveolin-3-enriched light membranes of C2C12 myotubes

Caveolin-3 (cav-3) is a key structural component of caveolar membrane in skeletal muscle. Cav-3-enriched light membrane (CELM) fractions obtained from C2C12 myotubes contain phospholipase D1 (PLD1) and its major regulators, RhoA and protein kinase Calpha (PKCalpha). All these proteins were found bound to cav-3. An in vivo assay of PLD activity, which allows to localize the reaction product in CELMs, indicated that the enzyme associated to this membrane microdomain was active. Moreover, bradykinin (BK), thrombin and phorbol 12-myristate 13-acetate induced rapid stimulation of PLD activity in CELMs. The cav-3-PLD1 complex was not affected by BK treatment, whereas the agonist induced a marked increase of RhoA association with cav-3. Furthermore, BK-induced PLD activation in CELMs was dependent, at least in part, on PKCalpha.

Activation of cdc42, rac, PAK, and rho-kinase in response to hepatocyte growth factor differentially regulates epithelial cell colony spreading and dissociation

Hepatocyte growth factor (HGF), the ligand for the Met receptor tyrosine kinase, is a potent modulator of epithelial-mesenchymal transition and dispersal of epithelial cells, processes that play crucial roles in tumor development, invasion, and metastasis. Little is known about the Met-dependent proximal signals that regulate these events. We show that HGF stimulation of epithelial cells leads to activation of the Rho GTPases, Cdc42 and Rac, concomitant with the formation of filopodia and lamellipodia. Notably, HGF-dependent activation of Rac but not Cdc42 is dependent on phosphatidylinositol 3-kinase. Moreover, HGF-induced lamellipodia formation and cell spreading require phosphatidylinositol 3-kinase and are inhibited by dominant negative Cdc42 or Rac. HGF induces activation of the Cdc42/Rac-regulated p21-activated kinase (PAK) and c-Jun N-terminal kinase, and translocation of Rac, PAK, and Rho-dependent Rho-kinase to membrane ruffles. Use of dominant negative and activated mutants reveals an essential role for PAK but not Rho-kinase in HGF-induced epithelial cell spreading, whereas Rho-kinase activity is required for the formation of focal adhesions and stress fibers in response to HGF. We conclude that PAK and Rho-kinase play opposing roles in epithelial-mesenchymal transition induced by HGF, and provide new insight regarding the role of Cdc42 in these events.

ADP-ribosylation factor 6 regulates actin cytoskeleton remodeling in coordination with Rac1 and RhoA

In this study, we have documented an essential role for ADP-ribosylation factor 6 (ARF6) in cell surface remodeling in response to physiological stimulus and in the down regulation of stress fiber formation. We demonstrate that the G-protein-coupled receptor agonist bombesin triggers the redistribution of ARF6- and Rac1-containing endosomal vesicles to the cell surface. This membrane redistribution was accompanied by cortical actin rearrangements and was inhibited by dominant negative ARF6, implying that bombesin is a physiological trigger of ARF6 activation. Furthermore, these studies provide a new model for bombesin-induced Rac1 activation that involves ARF6-regulated endosomal recycling. The bombesin-elicited translocation of vesicular ARF6 was mimicked by activated Galphaq and was partially inhibited by expression of RGS2, which down regulates Gq function. This suggests that Gq functions as an upstream regulator of ARF6 activation. The ARF6-induced peripheral cytoskeletal rearrangements were accompanied by a depletion of stress fibers. Moreover, cells expressing activated ARF6 resisted the formation of stress fibers induced by lysophosphatidic acid. We show that the ARF6-dependent inhibition of stress fiber formation was due to an inhibition of RhoA activation and was overcome by expression of a constitutively active RhoA mutant. The latter observations demonstrate that activation of ARF6 down regulates Rho signaling. Our findings underscore the potential roles of ARF6, Rac1, and RhoA in the coordinated regulation of cytoskeletal remodeling.

Regulation of urokinase receptor transcription by Ras- and Rho-family GTPases

How cell adhesion is coordinated with extracellular proteolysis is a key question in understanding cell migration. Potentially, the small GTP-binding proteins that affect actin organisation and signal transduction may also regulate the expression of genes associated with extracellular proteolysis. We investigated the ability of Ras, Rac-1, Cdc42Hs, and RhoA to regulate transcription from the1.55-kb promoter region of the human urokinase plasminogen activator receptor (uPAR) gene. Constitutively active V12 H-Ras and Rho-A stimulated uPAR transcription while Cdc42Hs and Rac-1 did not. The use of Ras effector-loop mutants indicated that signalling via multiple Ras-effectors is necessary for the maximum activation of transcription.

Rasputin, the Drosophila homologue of the RasGAP SH3 binding protein, functions in ras- and Rho-mediated signaling

The small GTPase Ras plays an important role in many cellular signaling processes. Ras activity is negatively regulated by GTPase activating proteins (GAPs). It has been proposed that RasGAP may also function as an effector of Ras activity. We have identified and characterized the Drosophila homologue of the RasGAP-binding protein G3BP encoded by rasputin (rin). rin mutants are viable and display defects in photoreceptor recruitment and ommatidial polarity in the eye. Mutations in rin/G3BP genetically interact with components of the Ras signaling pathway that function at the level of Ras and above, but not with Raf/MAPK pathway components. These interactions suggest that Rin is required as an effector in Ras signaling during eye development, supporting an effector role for RasGAP. The ommatidial polarity phenotypes of rin are similar to those of RhoA and the polarity genes, e.g. fz and dsh. Although rin/G3BP interacts genetically with RhoA, affecting both photoreceptor differentiation and polarity, it does not interact with the gain-of-function genotypes of fz and dsh. These data suggest that Rin is not a general component of polarity generation, but serves a function specific to Ras and RhoA signaling pathways.

Stimulation of phosphatidylinositol-4-phosphate 5-kinase by Rho-kinase

The serine/threonine kinase Rho-kinase was recently identified as a downstream effector of the small GTPase Rho, mediating effects of Rho on the actin cytoskeleton. Also phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) has been implicated in the regulation of actin polymerization. As the synthesis of PI(4,5)P(2) has been suggested to be affected by Rho proteins, we investigated whether Rho-kinase is involved in the control of PI(4,5)P(2) levels. Overexpression of RhoA in HEK-293 cells increased phosphatidylinositol 4-phosphate (PI4P) 5-kinase activity and concomitantly enhanced cellular PI(4,5)P(2) levels, whereas overexpression of the Rho-inactivating C3 transferase decreased both PI4P 5-kinase activity and PI(4,5)P(2) levels. These effects of RhoA could be mimicked by overexpression of wild-type Rho-kinase and of the constitutively active catalytic domain of Rho-kinase, Rho-kinase-CAT. In contrast, a kinase-deficient mutant of Rho-kinase had no effect on PI4P 5-kinase activity. Importantly, the increase in PI4P 5-kinase activity and PI(4,5)P(2) levels by wild-type Rho-kinase, but not by Rho-kinase-CAT, was completely prevented by coexpression of C3 transferase, indicating that the effect of Rho-kinase was under the control of endogenous Rho. In cell lysates, addition of recombinant RhoA and Rho-kinase-CAT stimulated PI4P 5-kinase activity. Finally, the increase in PI(4,5)P(2) levels induced by both Rho-kinase-CAT and RhoA was reversed by the Rho-kinase inhibitor HA-1077. Our data suggest that Rho-kinase is involved in the Rho-controlled synthesis of PI(4,5)P(2) by PI4P 5-kinase.

Vav2 is an activator of Cdc42, Rac1, and RhoA

Vav and Vav2 are members of the Dbl family of proteins that act as guanine nucleotide exchange factors (GEFs) for Rho family proteins. Whereas Vav expression is restricted to cells of hematopoietic origin, Vav2 is widely expressed. Although Vav and Vav2 share highly related structural similarities and high sequence identity in their Dbl homology domains, it has been reported that they are active GEFs with distinct substrate specificities toward Rho family members. Whereas Vav displayed GEF activity for Rac1, Cdc42, RhoA, and RhoG, Vav2 was reported to exhibit GEF activity for RhoA, RhoB, and RhoG but not for Rac1 or Cdc42. Consistent with their distinct substrate targets, it was found that constitutively activated versions of Vav and Vav2 caused distinct transformed phenotypes when expressed in NIH 3T3 cells. In contrast to the previous findings, we found that Vav2 can act as a potent GEF for Cdc42, Rac1, and RhoA in vitro. Furthermore, we found that NH(2)-terminally truncated and activated Vav and Vav2 caused indistinguishable transforming actions in NIH 3T3 cells that required Cdc42, Rac1, and RhoA function. In addition, like Vav and Rac1, we found that Vav2 activated the Jun NH(2)-terminal kinase cascade and also caused the formation of lamellipodia and membrane ruffles in NIH 3T3 cells. Finally, Vav2-transformed NIH 3T3 cells showed up-regulated levels of Rac-GTP. We conclude that Vav2 and Vav share overlapping downstream targets and are activators of multiple Rho family proteins. Therefore, Vav2 may mediate the same cellular consequences in nonhematopoietic cells as Vav does in hematopoietic cells.

Rho and Rac exert antagonistic functions on spreading of macrophage-derived multinucleated cells and are not required for actin fiber formation

Multinucleated giant cells (MNGC) derived from avian blood monocytes present, like osteoclasts, an unusual cytoskeletal organization characterized by (1) cortical rings of actin filaments, (2) unique adhesion structures called podosomes and (3) vinculin containing focal complexes which are not visibly connected to F-actin structures. The Rho family of small GTPases plays an essential role in the regulation and organization of cellular cytoskeletal structures including F-actin and vinculin associated structures. Using bacterial toxins such as modified exoenzyme C3 (C3B) and toxin B or overexpression of constitutively active Rac and Rho proteins fused to the green fluorescent protein (GFP), we show that Rac and Rho play antagonistic roles in regulating the morphology of osteoclast-like cells. Inhibition of Rho by C3B triggered MNGC spreading whereas activated Rho promoted cell retraction. However, inhibition or activation of Rho led to complete disorganization of fibrillar actin structures, including podosomes. Toxin B inhibition of Rho, Rac and Cdc42 induced a time dependent F-actin and vinculin reorganization. Initially, actin fibers with associated adhesion plaques formed and disappeared subsequently. Finally, only small focal complexes remained at the MNGC periphery before retracting. At the time when actin fibers formed, we observed that Rac was already inhibited by toxin B. By combining C3B treatment and overexpression of a dominant negative form of Rac (N17Rac), we show that the formation of these focal adhesion and actin fiber structures required neither Rho nor Rac activity. Moreover, our results show that podosomes are extremely unstable structures since any modifications of Rho or Rac activity resulted in their dissociation.

Effects of insulin on prenylation as a mechanism of potentially detrimental influence of hyperinsulinemia

To investigate the cause and effect relationship between hyperinsulinemia and the increased amounts of farnesylated p21Ras, we performed hyperinsulinemic euglycemic clamps in normal weight volunteers as well as in normal mice and dogs. Insulin infusions significantly raised the amounts of farnesylated p21Ras in the white blood cells of humans, in liver samples of mice and dogs, and in aorta samples of mice. Obese hyperinsulinemic individuals and dogs (made hyperinsulinemic by surgical diversion of the pancreatic outflow from the portal vein into the vena cava) displayed increased amounts of farnesylated p21Ras before the hyperinsulinemic clamps. Infusions of insulin did not alter the already increased levels of farnesylated p21Ras in these experimental models. To further investigate the role of acquired insulin resistance in modulating insulin's effect on p21Ras prenylation, we induced insulin resistance in rats by glucosamine infusion. Insulin-resistant glucosamine-treated animals displayed significantly increased farnesylated p21Ras in response to insulin infusion compared to that in control saline-treated animals. Transgenic models of insulin resistance (heterozygous insulin receptor substrate-1 knockout mice, A-ZIP/F-1 fatless mice, and animals overexpressing glutamine:fructose-6-phosphate amidotransferase) contained increased amounts of farnesylated p21Ras. We conclude that hyperinsulinemia, either endogenous (a prominent feature of insulin resistance) or produced by infusions of insulin, increases the amounts of farnesylated p21Ras in humans, mice, and dogs. This aspect of insulin action may represent one facet of the molecular mechanism of the potentially detrimental influence of hyperinsulinemia.

BETA 2 integrin signaling in leukocytes

Members of the beta 2 integrin family are the dominating integrins expressed on leukocytes, and they play a major role in leukocyte cell-cell and cell-matrix adhesions during inflammation and other immune responses. Beta 2 integrins are signaling receptors, but they are also targets of and are functionally affected by intracellular signals. Accordingly, researchers usually discuss two types of signaling by beta 2 integrins (and integrins in general): transmission of signals into the cell following binding of ligands or counter-receptors to the integrins (outside-in signaling), and regulation of the avidity and conformation of integrins by signals generated by other receptors within the cell (inside-out signaling). In this review, our aim is to summarize what is known about the capacity of beta 2 integrins to generate outside-in signaling in leukocytes, in particular polymorphonuclear neutrophils. Results in the literature clearly demonstrate that one of the earliest events in beta 2 integrin signaling is activation of non-receptor tyrosine kinases, which in turn triggers downstream activation of various signaling pathways that affect different functional responses of the cell. We also discuss molecules of potential importance in beta 2 integrin signaling.

Ezrin links syndecan-2 to the cytoskeleton

The syndecan family of heparan sulfate proteoglycans is known to associate with the actin cytoskeleton, possibly transducing signals from the extracellular matrix. In the search for proteins that could mediate the association of syndecan-2 with the actin cytoskeleton we found that ezrin, a protein which links membrane receptors to the cytoskeleton, coimmunoprecipitated with syndecan-2 in COS-1 cells. In vitro assays indicated a direct association between the amino-terminal domain of ezrin and the cytoplasmic domain of syndecan-2. Confocal microscopy showed colocalization of ezrin and syndecan-2 in actin-rich microspikes in COS-1 cells. The syndecan-2/ezrin protein complex was resistant to 0.2% Triton X-100 extraction but the syndecan-2/amino-terminal domain of ezrin complex was not, which indicated that carboxi-terminal domain of ezrin is involved in the cytoskeleton anchorage of this protein complex. Additionally we observed that the activation of rhoA GTPase increased syndecan-2 insolubility in 0.2% Triton X-100 and syndecan-2/ezrin association. Taken together, these results indicate that ezrin connects syndecan-2 to the actin cytoskeleton.

Up-regulation of p21- and RhoA-activated protein kinases in human pregnant myometrium

The role of small ras homologous GTP-binding proteins in the regulation of smooth muscle contractility has become increasingly apparent but there is still little information about the presence of these proteins in human uterine smooth muscle. Messenger RNAs for p21-activated protein kinase isoforms (PAK1, PAK2, and PAK3) were detectable in both nonpregnant and pregnant human myometrial tissue. However, PAK3 protein was not detectable and the proteins for PAK1 and PAK2 were only detectable in pregnant tissue. Moreover there was a large increase in the constitutively active p34 protein fragment of PAK2 in pregnant tissue. Protein expression of RhoA-activated protein kinases isoforms (ROK1 and ROK2) also increased during pregnancy. Stimulation of RhoA signaling in pregnant myometrial tissue with lysophosphatic acid (LPA) increased the level of myosin light chain (MLC20) phosphorylation. Preincubation of the tissue with C3 toxin inhibited LPA-stimulated MLC20 phosphorylation and lowered the basal phosphorylation level of MLC20. Thus ROKS and PAKS have the potential to regulate uterine contractility and/or load-bearing during human pregnancy.

Unique in vivo associations with SmgGDS and RhoGDI and different guanine nucleotide exchange activities exhibited by RhoA, dominant negative RhoA(Asn-19), and activated RhoA(Val-14)

We compared the in vivo characteristics of hemagglutinin (HA)-tagged RhoA, dominant negative RhoA(Asn-19), and activated RhoA(Val-14) stably expressed in Chinese hamster ovary (CHO) cells. Proteins co-precipitating with these HA-tagged GTPases were identified by peptide sequencing or by Western blotting. Dominant negative RhoA(Asn-19) co-precipitates with the guanine nucleotide exchange factor (GEF) SmgGDS but does not detectably interact with other expressed GEFs, such as Ost or Dbl. SmgGDS co-precipitates minimally with wild-type RhoA and does not detectably associate with RhoA(Val-14). The guanine nucleotide dissociation inhibitor RhoGDI co-precipitates with RhoA, and to a lesser extent with RhoA(Val-14), but does not detectably co-precipitate with RhoA(Asn-19). Wild-type RhoA is predominantly in the [(32)P]GDP-bound form, RhoA(Val-14) is predominantly in the [(32)P]GTP-bound form, and negligible levels of [(32)P]GDP or [(32)P]GTP are bound to RhoA(Asn-19) in (32)P-labeled cells. Immunofluorescence analyses indicate that HA-RhoA(Asn-19) is excluded from the nucleus and cell junctions. Microinjection of SmgGDS cDNA into CHO cells stably expressing HA-RhoA causes HA-RhoA to be excluded from the nucleus and cell junctions, similar to the distribution of RhoA(Asn-19). Our findings indicate that the expression of RhoA(Asn-19) may specifically inhibit signaling pathways that rely upon the SmgGDS-dependent activation of RhoA.

Cerivastatin suppresses lipopolysaccharide-induced ICAM-1 expression through inhibition of Rho GTPase in BAEC

We investigated the effect of cerivastatin on lipopolysaccharide (LPS)-induced intercellular adhesion molecule-1 (ICAM-1) expression in bovine aortic endothelial cells. Cerivastatin suppressed LPS-induced ICAM-1 mRNA expression. Cotreatment with geranylgeranylpyrophosphate reversed the effect of cerivastatin. Because Rho undergoes geranylgeranyl modification, we elucidated whether Rho is involved in LPS-induced ICAM-1 expression. Inhibition of Rho activity by Clostridium botulinum C3 transferase or by overexpression of RhoA T19N, a dominant-negative mutant of RhoA, decreased LPS-induced ICAM-1 expression. Although cerivastatin up-regulated endothelial nitric oxide synthase (eNOS), inhibition of nitric oxide (NO) synthesis by cotreatment with N(omega)-nitro-l-arginine methyl ester (L-NAME) exhibited no influence on the effect of cerivastatin. The present results indicate that cerivastatin prevents LPS-induced ICAM-1 expression in endothelial cells via inhibition of Rho activity. This inhibitory effect is likely unrelated to up-regulation of eNOS.

Regulation of endocytic traffic by rho family GTPases

Endocytosis is a complicated yet highly efficient process that involves the uptake and processing of cargoes, ranging from small molecules, to activated signalling receptors, to whole microorganisms. Regulation of endocytic pathways is poorly understood. Recent evidence suggests that the Rho GTPase family of signalling proteins is intimately involved in endocytic traffic, providing novel insights into the control mechanisms that govern this process.

Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo

The development and structural plasticity of dendritic arbors are governed by several factors, including synaptic activity, neurotrophins and other growth-regulating molecules. The signal transduction pathways leading to dendritic structural changes are unknown, but likely include cytoskeleton regulatory components. To test whether GTPases regulate dendritic arbor development, we collected time-lapse images of single optic tectal neurons in albino Xenopus tadpoles expressing dominant negative or constitutively active forms of Rac, Cdc42 or RhoA. Analysis of images collected at two-hour intervals over eight hours indicated that enhanced Rac activity selectively increased branch additions and retractions, as did Cdc42 to a lesser extent. Activation of endogenous RhoA decreased branch extension without affecting branch additions and retractions, whereas dominant-negative RhoA increased branch extension. Finally, we provide data suggesting that RhoA mediates the promotion of normal dendritic arbor development by NMDA receptor activation.

Differential effect of Rac and Cdc42 on p38 kinase activity and cell cycle progression of nonadherent primary mouse fibroblasts

The Rho GTPases play an important role in transducing signals linking plasma membrane receptors to the organization of the cytoskeleton and also regulate gene transcription. Here, we show that expression of constitutively active Ras or Cdc42, but not RhoA, RhoG, and Rac1, is sufficient to cause anchorage-independent cell cycle progression of mouse embryonic fibroblasts. However, in anchorage free conditions, whereas activation of either Cdc42 or Ras results in cyclin A transcription and cell cycle progression, Cdc42 is not required for Ras-mediated cyclin A induction, and the two proteins act in a synergistic manner in this process. Surprisingly, the ability of Cdc42 to induce p38 MAPK activity in suspended mouse embryonic fibroblast was impaired. Moreover, inhibition of p38 activity allowed Rac1 to induce anchorage-independent cyclin A transcription, indicating that p38 MAPK has an inhibitory function on cell cycle progression of primary fibroblasts. Finally, a Rac mutant, which is unable to induce lamellipodia and focal complex formation, promoted cyclin A transcription in the presence of SB203580, suggesting that the organization of the cytoskeleton is not required for anchorage-independent proliferation. This demonstrates a novel function for Cdc42, distinct from that of Rac1, in the control of cell proliferation.

Activation of rho through a cross-link with polyamines catalyzed by Bordetella dermonecrotizing toxin

The small GTPase Rho, which regulates a variety of cell functions, also serves as a specific substrate for bacterial toxins. Here we demonstrate that Bordetella dermonecrotizing toxin (DNT) catalyzes cross-linking of Rho with ubiquitous polyamines such as putrescine, spermidine and spermine. Mass spectrometric analyses revealed that the cross-link occurred at Gln63, which had been reported to be deamidated by DNT in the absence of polyamines. Rac1 and Cdc42, other members of the Rho family GTPases, were also polyaminated by DNT. The polyamination, like the deamidation, markedly reduced the GTPase activity of Rho without affecting its GTP-binding activity, indicating that polyaminated Rho behaves as a constitutively active analog. Moreover, polyamine-linked Rho, even in the GDP-bound form, associated more effectively with its effector ROCK than deamidated Rho in the GTP-bound form and, when microinjected into cells, induced the anomalous formation of stress fibers indistinguishable from those seen in DNT-treated cells. The results imply that the polyamine-linked Rho, transducing signals to downstream ROCK in a novel GTP-independent manner, plays an important role in DNT cell toxicity.

Activation of the guanine nucleotide exchange factor Dbl following ACK1-dependent tyrosine phosphorylation

Signals triggered by diverse receptors modulate the activity of Rho family proteins, although the regulatory mechanism remains largely unknown. On the basis of their biochemical activity as guanine nucleotide exchange factors (GEFs), Dbl family proteins are believed to be implicated in the regulation of Rho family GTP-binding proteins in response to a variety of extracellular stimuli. Here we show that GEF activity of full-length proto-Dbl is enhanced upon tyrosine phosphorylation. When transiently coexpressed with the activated form of the non-receptor tyrosine kinase ACK1, a downstream target of Cdc42, Dbl became tyrosine-phosphorylated. In vitro GEF activity of Dbl toward Rho and Cdc42 was augmented following tyrosine phosphorylation. Moreover, accumulation of the GTP-bound form of Rho and Rac within the cell paralleled ACK-1-dependent tyrosine phosphorylation of Dbl. Consistently, activation of c-Jun N-terminal kinase downstream of Rho family GTP-binding proteins was also enhanced when Dbl was tyrosine-phosphorylated. Collectively, these findings suggest that the tyrosine kinase ACK1 may act as a regulator of Dbl, which in turn activates Rho family proteins.

Essential roles of Drosophila RhoA in the regulation of neuroblast proliferation and dendritic but not axonal morphogenesis

The pleiotropic functions of small GTPase Rho present a challenge to its genetic analysis in multicellular organisms. We report here the use of the MARCM (mosaic analysis with a repressible cell marker) system to analyze the function of RhoA in the developing Drosophila brain. Clones of cells homozygous for null RhoA mutations were specifically labeled in the mushroom body (MB) neurons of mosaic brains. We found that RhoA is required for neuroblast (Nb) proliferation but not for neuronal survival. Surprisingly, RhoA is not required for MB neurons to establish normal axon projections. However, neurons lacking RhoA overextend their dendrites, and expression of activated RhoA causes a reduction of dendritic complexity. Thus, RhoA is an important regulator of dendritic morphogenesis, while distinct mechanisms are used for axonal morphogenesis.

Human RhoA/RhoGDI complex expressed in yeast: GTP exchange is sufficient for translocation of RhoA to liposomes

The human small GTPase, RhoA, expressed in Saccharomyces cerevisiae is post-translationally processed and, when co-expressed with its cytosolic inhibitory protein, RhoGDI, spontaneously forms a heterodimer in vivo. The RhoA/RhoGDI complex, purified to greater than 98% at high yield from the yeast cytosolic fraction, could be stoichiometrically ADP-ribosylated by Clostridium botulinum C3 exoenzyme, contained stoichiometric GDP, and could be nucleotide exchanged fully with [3H]GDP or partially with GTP in the presence of submicromolar Mg2+. The GTP-RhoA/RhoGDI complex hydrolyzed GTP with a rate constant of 4.5 X 10(-5) s(-1), considerably slower than free RhoA. Hydrolysis followed pseudo-first-order kinetics indicating that the RhoA hydrolyzing GTP was RhoGDI associated. The constitutively active G14V-RhoA mutant expressed as a complex with RhoGDI and purified without added nucleotide also bound stoichiometric guanine nucleotide: 95% contained GDP and 5% GTP. Microinjection of the GTP-bound G14V-RhoA/RhoGDI complex (but not the GDP form) into serum-starved Swiss 3T3 cells elicited formation of stress fibers and focal adhesions. In vitro, GTP-bound-RhoA spontaneously translocated from its complex with RhoGDI to liposomes, whereas GDP-RhoA did not. These results show that GTP-triggered translocation of RhoA from RhoGDI to a membrane, where it carries out its signaling function, is an intrinsic property of the RhoA/RhoGDI complex that does not require other protein factors or membrane receptors.

The essential role of H2O2 in the regulation of intracellular Ca2+ by epidermal growth factor in rat-2 fibroblasts

We have investigated a new mechanism by which epidermal growth factor (EGF) increases intracellular Ca(2+) ([Ca(2+)](i)) in Rat-2 fibroblasts. EGF induced a transient increase of [Ca(2+)](i), and sustained Ca(2+) increase disappeared in the absence of extracellular Ca(2+). However, EGF had no effect on the formation of inositol phosphates. Expression of N17Rac or scrape-loading of C3 transferase blocked the elevation of [Ca(2+)](i) by EGF, but not by lysophosphatidic acid (LPA). EGF increased intracellular H(2)O(2), with a maximal increase at 5 min, which was blocked by catalase, scrape-loading of C3 transferase, or expression of N17Rac. H(2)O(2) scavengers, catalase and N-acetyl-L-cysteine, also blocked the Ca(2+) response to EGF, but not to LPA. In the presence of EGTA, preincubation with EGF completely inhibited subsequent Ca(2+) response to extracellular H(2)O(2) and vice versa. Incubation with EGF or phosphatidic acid abolished subsequent elevation of [Ca(2+)](i) by phosphatidic acid or EGF, respectively. Furthermore, preincubation with LPA inhibited the subsequent Ca(2+) response to EGF, but not vice versa. These results suggested that intracellular H(2)O(2) regulated by Rac and RhoA, but not inositol phosphates, was responsible for the EGF-stimulated elevation of [Ca(2+)](i). It was also suggested that EGF cross talked with LPA in the regulation of [Ca(2+)](i) by producing intracellular H(2)O(2).

Myotonic dystrophy protein kinase (DMPK) induces actin cytoskeletal reorganization and apoptotic-like blebbing in lens cells

DMPK, the product of the DM locus, is a member of the same family of serine-threonine protein kinases as the Rho-associated enzymes. In DM, membrane inclusions accumulate in lens fiber cells producing cataracts. Overexpression of DMPK in cultured lens epithelial cells led to apoptotic-like blebbing of the plasma membrane and reorganization of the actin cytoskeleton. Enzymatically active DMPK was necessary for both effects; inactive mutant DMPK protein did not produce either effect. Active RhoA but not constitutive GDP-state mutant protein produced similar effects as DMPK. The similar actions of DMPK and RhoA suggest that they may function in the same regulatory network. The observed effects of DMPK may be relevant to the removal of membrane organelles during normal lens differentiation and the retention of intracellular membranes in DM lenses.

RhoA function in lamellae formation and migration is regulated by the alpha6beta4 integrin and cAMP metabolism

Clone A colon carcinoma cells develop fan-shaped lamellae and exhibit random migration when plated on laminin, processes that depend on the ligation of the alpha6beta4 integrin. Here, we report that expression of a dominant negative RhoA (N19RhoA) in clone A cells inhibited alpha6beta4-dependent membrane ruffling, lamellae formation, and migration. In contrast, expression of a dominant negative Rac (N17Rac1) had no effect on these processes. Using the Rhotekin binding assay to assess RhoA activation, we observed that engagement of alpha6beta4 by either antibody-mediated clustering or laminin attachment resulted in a two- to threefold increase in RhoA activation, compared with cells maintained in suspension or plated on collagen. Antibody-mediated clustering of beta1 integrins, however, actually suppressed Rho A activation. The alpha6beta4-mediated interaction of clone A cells with laminin promoted the translocation of RhoA from the cytosol to membrane ruffles at the edges of lamellae and promoted its colocalization with beta1 integrins, as assessed by immunofluorescence microscopy. In addition, RhoA translocation was blocked by inhibiting phosphodiesterase activity and enhanced by inhibiting the activity of cAMP-dependent protein kinase. Together, these results establish a specific integrin-mediated pathway of RhoA activation that is regulated by cAMP and that functions in lamellae formation and migration.

Deamidation of RhoA glutamine 63 by the Escherichia coli CNF1 toxin requires a short sequence of the GTPase switch 2 domain

CNF1, a toxin produced by pathogenic Escherichia coli strains, deamidates the RhoA GTP-binding protein glutamine 63 and impairs RhoGAP-mediated GTP hydrolysis resulting in RhoA permanent activation. Using peptides derived from the RhoA sequence, we found that DTAGQEDYDRL (corresponding to RhoA 59-69 residues) was the minimum RhoA-derived peptide which could be deamidated in vitro by the CNF1 catalytic domain (CNF1-Cter). Site-directed mutagenesis outside the RhoA 59-69 sequence had no influence on glutamine 63 deamidation by CNF1-Cter. RhoA proteins with substitutions L57G, D65G, Y66G, or R70G were not affected in their ability to be deamidated by CNF1-Cter, whereas this was abolished by the R68G substitution. Arginine 68 is part of the DYDRL motif that is strictly conserved in Rho, Rac, and Cdc42 but not in other small GTP-binding proteins consistent with the observation that only Rho, Rac, and Cdc42 can be modified by CNF1.

Integrin signaling's potential for mediating gene expression in hypertrophying skeletal muscle

Overloaded skeletal muscle undergoes dramatic shifts in gene expression, which alter both the phenotype and mass. Molecular biology techniques employing both in vivo and in vitro hypertrophy models have demonstrated that mechanical forces can alter skeletal muscle gene regulation. This review's purpose is to support integrin-mediated signaling as a candidate for mechanical load-induced hypertrophy. Research quantifying components of the integrin-signaling pathway in overloaded skeletal muscle have been integrated with knowledge regarding integrins role during development and cardiac hypertrophy, with the hope of demonstrating the pathway's importance. The role of integrin signaling as an integrator of mechanical forces and growth factor signaling during hypertrophy is discussed. Specific components of integrin signaling, including focal adhesion kinase and low-molecular-weight GTPase Rho are mentioned as downstream targets of this signaling pathway. There is a need for additional mechanistic studies capable of providing a stronger linkage between integrin-mediated signaling and skeletal muscle hypertrophy; however, there appears to be abundant justification for this type of research.

Inhibition of Rho-associated kinase blocks agonist-induced Ca2+ sensitization of myosin phosphorylation and force in guinea-pig ileum

Ca2+ sensitization of smooth muscle contraction involves the small GTPase RhoA, inhibition of myosin light chain phosphatase (MLCP) and enhanced myosin regulatory light chain (LC20) phosphorylation. A potential effector of RhoA is Rho-associated kinase (ROK). The role of ROK in Ca2+ sensitization was investigated in guinea-pig ileum. Contraction of permeabilized muscle strips induced by GTPgammaS at pCa 6.5 was inhibited by the kinase inhibitors Y-27632, HA1077 and H-7 with IC50 values that correlated with the known Ki values for inhibition of ROK. GTPgammaS also increased LC20 phosphorylation and this was prevented by HA1077. Contraction and LC20 phosphorylation elicited at pCa 5.75 were, however, unaffected by HA1077. Pre-treatment of intact tissue strips with HA1077 abolished the tonic component of carbachol-induced contraction and the sustained elevation of LC20 phosphorylation, but had no effect on the transient or sustained increase in [Ca2+]i induced by carbachol. LC20 phosphorylation and contraction dynamics suggest that the ROK-mediated increase in LC20 phosphorylation is due to MLCP inhibition, not myosin light chain kinase activation. In the absence of Ca2+, GTPgammaS stimulated 35S incorporation from [35S]ATPgammaS into the myosin targeting subunit of MLCP (MYPT). The enhanced thiophosphorylation was inhibited by HA1077. No thiophosphorylation of LC20 was detected. These results indicate that ROK mediates agonist-induced increases in myosin phosphorylation and force by inhibiting MLCP activity through phosphorylation of MYPT. Under Ca2+-free conditions, ROK does not appear to phosphorylate LC20 in situ, in contrast to its ability to phosphorylate myosin in vitro. In particular, ROK activation is essential for the tonic phase of agonist-induced contraction.

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

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

Molecular characterization of the Caenorhabditis elegans Rho GDP-dissociation inhibitor

GDP-dissociation inhibitors (GDIs) form one of the classes of regulatory proteins that modulate the cycling of the Ras superfamily of GTPases between active GTP-bound and inactive GDP-bound states. We report here the characterization of the Caenorhabditis elegans RhoGDI (CeRhoGDI) as part of our investigations into Rho-GTPase signalling pathways that are involved in nematode development. CeRhoGDI is a 23-kDa protein that is localized predominantly in the cytosol. CeRhoGDI interacts only with the lipid-modified forms of C. elegans Rho-GTPases, CeRhoA, CeRac1 and Cdc42Ce, in vitro and is able to solubilize the membrane-bound forms of these GTPases. CeRhoGDI recognizes the GTPases in both GTP- and GDP-bound forms; hence it inhibits both the guanine-nucleotide dissociation and GTP-hydrolysis activities. The inhibitory activity towards the GTP-bound GTPases is weak compared with that towards GDP-bound GTPases. CeRhoGDI is expressed throughout development and is highly expressed in marginal and vulval epithelial cells, in sperm cells and spicules. Taken together, our results suggest that CeRhoGDI may be involved in specific morphogenetic events mediated by the C. elegans Rho-GTPases.

Modulation of endocytic traffic in polarized Madin-Darby canine kidney cells by the small GTPase RhoA

Efficient postendocytic membrane traffic in polarized epithelial cells is thought to be regulated in part by the actin cytoskeleton. RhoA modulates assemblies of actin in the cell, and it has been shown to regulate pinocytosis and phagocytosis; however, its effects on postendocytic traffic are largely unexplored. To this end, we expressed wild-type RhoA (RhoAWT), dominant active RhoA (RhoAV14), and dominant inactive RhoA (RhoAN19) in Madin-Darby canine kidney (MDCK) cells expressing the polymeric immunoglobulin receptor. RhoAV14 expression stimulated the rate of apical and basolateral endocytosis, whereas RhoAN19 expression decreased the rate from both membrane domains. Polarized basolateral recycling of transferrin was disrupted in RhoAV14-expressing cells as a result of increased ligand release at the apical pole of the cell. Degradation of basolaterally internalized epidermal growth factor was slowed in RhoAV14-expressing cells. Although apical recycling of immunoglobulin A (IgA) was largely unaffected in cells expressing RhoAV14, transcytosis of basolaterally internalized IgA was severely impaired. Morphological and biochemical analyses demonstrated that a large proportion of IgA internalized from the basolateral pole of RhoAV14-expressing cells remained within basolateral early endosomes and was slow to exit these compartments. RhoAN19 and RhoAWT expression had little effect on these postendocytic pathways. These results indicate that in polarized MDCK cells activated RhoA may modulate endocytosis from both membrane domains and postendocytic traffic at the basolateral pole of the cell.

Spire contains actin binding domains and is related to ascidian posterior end mark-5

Spire is a maternal effect locus that affects both the dorsal-ventral and anterior-posterior axes of the Drosophila egg and embryo. It is required for localization of determinants within the developing oocyte to the posterior pole and to the dorsal anterior corner. During mid-oogenesis, spire mutants display premature microtubule-dependent cytoplasmic streaming, a phenotype that can be mimicked by pharmacological disruption of the actin cytoskeleton with cytochalasin D. Spire has been cloned by transposon tagging and is related to posterior end mark-5, a gene from sea squirts that encodes a posteriorly localized mRNA. Spire mRNA is not, however, localized to the posterior pole. SPIRE also contains two domains with similarity to the actin monomer-binding WH2 domain, and we demonstrate that SPIRE binds to actin in the interaction trap system and in vitro. In addition, SPIRE interacts with the rho family GTPases RHOA, RAC1 and CDC42 in the interaction trap system. Thus, our evidence supports the model that SPIRE links rho family signaling to the actin cytoskeleton.

Cytotoxic necrotizing factor type 2 produced by pathogenic Escherichia coli deamidates a gln residue in the conserved G-3 domain of the rho family and preferentially inhibits the GTPase activity of RhoA and rac1

Cytotoxic necrotizing factor types 1 and 2 (CNF1 and -2) produced by pathogenic Escherichia coli strains have 90% conserved residues over 1,014-amino-acid sequences. Both CNFs are able to provoke a remarkable increase in F-actin structures in cultured cells and covalently modify the RhoA small GTPases. In this study, we demonstrated that CNF2 reduced RhoA GTPase activity in the presence and absence of P122(RhoGAP). Subsequently, peptide mapping and amino acid sequencing of CNF2-modified FLAG-RhoA produced in E. coli revealed that CNF2 deamidates Q63 of RhoA-like CNF1. In vitro incubation of the C-terminal domain of CNF2 with FLAG-RhoA resulted also in deamidation of the FLAG-RhoA, suggesting that this region contains the enzymatic domain of CNF2. An oligopeptide antibody (anti-E63) which specifically recognized the altered G-3 domain of the Rho family reacted with glutathione S-transferase (GST)-RhoA and GST-Rac1 but not with GST-Cdc42 when coexpressed with CNF2. In addition, CNF2 selectively induced accumulation of GTP form of FLAG-RhoA and FLAG-Rac1 but not of FLAG-Cdc42 in Cos-7 cells. Taken together, these results indicate that CNF2 preferentially deamidates RhoA Q63 and Rac1 Q61 and constitutively activates these small GTPases in cultured cells. In contrast, anti-E63 reacted with GST-RhoA and GST-Cdc42 but not with GST-Rac1 when coexpressed with CNF1. These results indicate that CNF2 and CNF1 share the same catalytic activity but have distinct substrate specificities, which may reflect their differences in toxic activity in vivo.

Impact of amino acids 22-27 of Rho-subfamily GTPases on glucosylation by the large clostridial cytotoxins TcsL-1522, TcdB-1470 and TcdB-8864

Here we report data describing some principles of the interaction between small GTP-binding proteins and large Clostridial cytotoxins (LCTs). Our investigation was based on the differential glucosylation of Rac1 versus RhoA by LCTs TcsL-1522, TcdB-1470 and TcdB-8864. Chimeric RhoA/Rac1 proteins and GTPases mutated at defined regions or single amino acids were used as substrates. Starting with chimeric Rac/Rho proteins we demonstrated that proteins containing the N-terminal 73 amino acids of Rac1 (but not those of RhoA) were efficiently glucosylated. Within this stretch, three regions differ significantly in Rac1 and RhoA. Regions containing amino acids 41-45 and 50-54 had no effect on toxin induced glucosylation, whereas amino acids 22-27 had a drastic impact on the potential of all three toxins to covalently modify the GTPases. Point mutations K25T of RhoA (numbering according to Rac1) and K27A of Cdc42 significantly increased glucosylation by the cytotoxins; introduction of lysines at the equivalent positions of Rac1 hindered modification. Our experiments demonstrate the influence of this charged residue on GTPase-LCT interactions. Amino acids 22-27 are part of the transition between the alpha1-helix to the switch I region of small GTP-binding proteins; both are known structures for specificity determination of the interactions with physiologic partners. Comparing these structures with data from our investigation we suggest that TcsL-1522, TcdB-1470 and TcdB-8864 mimic aspects of the physiologic interactions of small GTP-binding proteins.

RhoA-induced changes in fibroblasts cultured on organic monolayers

Substantial previous work indicates that adherent cell morphology in culture is modulated by surface chemistry. Activation of the intracellular small molecular weight GTPase, RhoA, has recently been shown to play an essential role in controlling initiation of key integrin-mediated events in surface adhesion and proliferation. RhoA is interconvertible between an active, membrane-bound form and an inactive, cytosolic RhoGDI-bound form in response to integrin stimulation. This study reports the use of self-assembled functionalized organic alkylthiol monolayers (SAMs) as well-defined cell culture substrates to investigate the relationships between surface chemistry, RhoA activation and subsequent cell morphological and molecular level signal transduction responses in cells attaching to derivatized SAMs. Well-controlled alkylthiol surface chemistries were used to monitor and modulate the activation state of RhoA in attaching cells. Activation states were determined indirectly by fractionating cell lysates into membrane and cytosolic fractions by ultracentrifugation. Western blots were then performed, showing RhoA localization to be surface chemistry-dependent. RhoGDI levels and its intracellular localization were also shown to be surface-chemistry dependent. Cells cultured on -CH3 terminated SAMs, which normally exhibit a low-growth phenotype, were transfected with a constitutively active mutant form of RhoA. Subsequent cell morphological changes were observed on SAM surfaces by fluorescence microscopy. Results support surface chemistry influences on the activation state of RhoA mediated by adsorbed proteins and distinct changes in adherent cell morphology resulting from modulation of this activation state.

Involvement of the small GTPases XRhoA and XRnd1 in cell adhesion and head formation in early Xenopus development

The Rho family of small GTPases regulates a variety of cellular functions, including the dynamics of the actin cytoskeleton, cell adhesion, transcription, cell growth and membrane trafficking. We have isolated the first Xenopus homologs of the Rho-like GTPases RhoA and Rnd1 and examined their potential roles in early Xenopus development. We found that Xenopus Rnd1 (XRnd1) is expressed in tissues undergoing extensive morphogenetic changes, such as marginal zone cells involuting through the blastopore, somitogenic mesoderm during somite formation and neural crest cells. XRnd1 also causes a severe loss of cell adhesion in overexpression experiments. These data and the expression pattern suggest that XRnd1 regulates morphogenetic movements by modulating cell adhesion in early embryos. Xenopus RhoA (XRhoA) is a potential XRnd1 antagonist, since overexpression of XRhoA increases cell adhesion in the embryo and reverses the disruption of cell adhesion caused by XRnd1. In addition to the potential roles of XRnd1 and XRhoA in the regulation of cell adhesion, we find a role for XRhoA in axis formation. When coinjected with dominant-negative BMP receptor (tBR) in the ventral side of the embryo, XRhoA causes the formation of head structures resembling the phenotype seen after coinjection of wnt inhibitors with dominant-negative BMP receptor. Since dominant-negative XRhoA is able to reduce the formation of head structures, we propose that XRhoA activity is essential for head formation. Thus, XRhoA may have a dual role in the embryo by regulating cell adhesion properties and pattern formation.

Tyrosine kinase-regulated small GTPase translocation and the activation of phospholipase D in HL60 granulocytes

We focus on the mechanisms of regulation of phospholipase D (PLD) activity. Three agonists known to stimulate PLD activity, fMet-Leu-Phe (fMLP), phorbol 12-myristate 13-acetate (PMA) and V4+-OOH, induced a differential translocation of ADP-ribosylation factor (ARF), RhoA, and protein kinase Calpha (PKCalpha), all cofactors for PLD activation. Whereas fMLP recruited all three proteins to membranes, V4+-OOH only elicited RhoA translocation and PMA induced ARF and PKCalpha translocation. Three tyrosine kinases inhibitors, ST-638, methyl 2,5-dihydroxycinnamate, and genistein reduced fMLP-stimulated PLD activity by up to 80%. Furthermore, tyrosine kinase inhibitors reduced the fMLP-induced increase of GTPgammaS-stimulated PLD activity in membranes and recruitment of ARF, RhoA, and PKCalpha to the membrane fraction. The data suggest that a tyrosine phosphorylation event is located upstream of the translocation of ARF, RhoA, and PKCalpha in the signaling pathway leading to PLD activation by fMLP. RO 31-8220, a specific inhibitor of PKC, reduced PMA-induced PLD activity by 80% in intact HL60 granulocytes but enhanced fMLP-stimulated PLD activity by 60%. Although PMA alone had no effect on RhoA recruitment to the membrane fraction, in the presence of RO 31-8220 the levels of membrane-bound RhoA were increased. The levels of membrane-bound ARF and PKCalpha were unaffected by RO 31-8220 during PMA stimulation. In contrast, fMLP-induced recruitment of ARF and RhoA was insensitive to RO 31-8220 but PKCalpha translocation was increased. We propose that RhoA translocation may be regulated by PKC in an ATP-independent manner. Furthermore, increased fMLP-induced PKCalpha translocation in the presence of RO 31-8220 may partially account for the synergistic activation of PLD observed when both fMLP and RO 31-8220 are used together in intact HL60 cells.

Activation of ERM proteins in vivo by Rho involves phosphatidyl-inositol 4-phosphate 5-kinase and not ROCK kinases

When activated, ERM (ezrin, radixin, moesin) proteins are recruited to the plasma membrane, with concomitant carboxy-terminal threonine phosphorylation, where they crosslink actin filaments to the plasma membrane to form microvilli (reviewed in [1] [2] [3] [4] [5]). Here, we report that, when NIH3T3 or HeLa cells were transfected with a constitutively active mutant of the small GTPase RhoA (V14RhoA), microvilli were induced and the level of carboxy-terminal threonine-phosphorylated ERM proteins (CPERM) [6] [7] increased approximately 30-fold. This increase was not observed following transfection of constitutively active forms of two other Rho-family GTPases, Rac1 and Cdc42, or of a direct effector of Rho, Rho-kinase (also known as ROKalpha or ROCK-II) [8] [9] [10]. The V14RhoA-induced phosphorylation of ERM proteins was not suppressed by Y-27632, a specific inhibitor of ROCK kinases including Rho-kinase [11]. Overexpression of another direct effector of Rho, phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) type Ialpha [12] [13] [14], but not a kinase-inactive mutant [15], increased approximately sixfold the level of CPERM, and induced microvilli. Together with the previous finding that the PI4P5K product phosphatidylinositol 4,5-bisphosphate (PIP(2)) activates ERM proteins in vitro [16], our data suggest that PIP(2), and not ROCK kinases, is involved in the RhoA-dependent activation of ERM proteins in vivo. The active state of ERM proteins is maintained through threonine phosphorylation by as yet undetermined kinases, leading to microvillus formation.

Neurotrophin binding to the p75 receptor modulates Rho activity and axonal outgrowth

While the neurotrophin receptor p75NTR is expressed by many developing neurons, its function in cells escaping elimination by programmed cell death remains unclear. The lack of intrinsic enzymatic activity of p75NTR prompted a search for protein interactors expressed in the developing retina, which resulted in the identification of the GTPase RhoA. In transfected cells, p75NTR activated RhoA, and neurotrophin binding abolished RhoA activation. In cultured neurons, inactivation of Rho proteins mimicked the effect of neurotrophins by increasing the rate of neurite elongation. In vivo, axonal outgrowth was retarded in mice carrying a mutation in the p75NTR gene. These results indicate that p75NTR modulates in a ligand-dependent fashion the activity of intracellular proteins known to regulate actin assembly.

RhoA inactivation enhances endothelial barrier function

The modulation of endothelial barrier function is thought to be a function of contractile tension mediated by the cell cytoskeleton, which consists of actomyosin stress fibers (SF) linked to focal adhesions (FA). We tested this hypothesis by dissociating SF/FA with Clostridium botulinum exoenzyme C3 transferase (C3), an inhibitor of the small GTP-binding protein RhoA. Bovine pulmonary artery endothelial cell (EC) monolayers given C3, C3 + thrombin, thrombin, or no treatment were examined using a size-selective permeability assay and quantitative digital imaging measurements of SF/FA. C3 treatment disassembled SF/FA, stimulated diffuse myosin II immunostaining, and reduced the phosphotyrosine (PY) content of paxillin and 130- to 140-kDa proteins that included p125(FAK). C3-treated monolayers displayed a 60-85% decline in F-actin content and a 170-300% increase in EC surface area with enhanced endothelial barrier function. This activity correlated with reorganization of F-actin and PY protein(s) to beta-catenin-containing cell-cell junctions. Because C3 prevented the thrombin-induced formation of myosin ribbons, SF/FA, and the increased PY content of proteins, these characteristics were Rho dependent. Our data show that C3 inhibition of Rho proteins leads to cAMP-like characteristics of reduced SF/FA and enhanced endothelial barrier function.

Rho GTPases control migration and polarization of adhesion molecules and cytoskeletal ERM components in T lymphocytes

Motile lymphocytes adopt a polarized morphology with different adhesion molecules (ICAM, CD43 and CD44) and ERM actin-binding proteins concentrated on the uropod, a slender posterior appendage with important functions in cell-cell interactions and lymphocyte recruitment. We have studied the role of Rho family of GTPases (Rho, Rac and Cdc42) in the control of lymphocyte polarity and migration by analyzing the effects of exogenously introduced Rho GTPase mutants. Transfection of T cell lines that constitutively display a polarized motile morphology with activated mutants of RhoA, Rac1 and Cdc42 impaired cell polarization. A guanosine nucleotide exchange factor for Rac, Tiam-1, induced the same effect as activated Rac1. Conversely, dominant negative forms of the three GTP-binding proteins induced a polarized phenotype in constitutively round-shaped T cells with redistribution of ICAM-3 and moesin to the uropod in an integrin-dependent manner. On the other hand, overexpression of dominant negative Cdc42 and activated mutants of all three Rho GTPases significantly inhibited SDF-1alpha-induced T cell chemotaxis. Together, these data demonstrate that Rho GTPases regulate lymphocyte polarization and chemokine-induced migration, and underscore the key role of Cdc42 in lymphocyte directional migration.

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

Dbs was identified initially as a transforming protein and is a member of the Dbl family of proteins (>20 mammalian members). Here we show that Dbs, like its rat homolog Ost and the closely related Dbl, exhibited guanine nucleotide exchange activity for the Rho family members RhoA and Cdc42, but not Rac1, in vitro. Dbs transforming activity was blocked by specific inhibitors of RhoA and Cdc42 function, demonstrating the importance of these small GTPases in Dbs-mediated growth deregulation. Although Dbs transformation was dependent upon the structural integrity of its pleckstrin homology (PH) domain, replacement of the PH domain with a membrane localization signal restored transforming activity. Thus, the PH domain of Dbs (but not Dbl) may be important in modulating association with the plasma membrane, where its GTPase substrates reside. Both Dbs and Dbl activate multiple signaling pathways that include activation of the Elk-1, Jun, and NF-kappaB transcription factors and stimulation of transcription from the cyclin D1 promoter. We found that Elk-1 and NF-kappaB, but not Jun, activation was necessary for Dbl and Dbs transformation. Finally, we have observed that Dbl and Dbs regulated transcription from the cyclin D1 promoter in a NF-kappaB-dependent manner. Previous studies have dissociated actin cytoskeletal activity from the transforming potential of RhoA and Cdc42. These observations, when taken together with those of the present study, suggest that altered gene expression, and not actin reorganization, is the critical mediator of Dbl and Rho family protein transformation.

The structural basis of Rho effector recognition revealed by the crystal structure of human RhoA complexed with the effector domain of PKN/PRK1

The small G protein Rho has emerged as a key regulator of cellular events involving cytoskeletal reorganization. Here we report the 2.2 A crystal structure of RhoA bound to an effector domain of protein kinase PKN/PRK1. The structure reveals the antiparallel coiled-coil finger (ACC finger) fold of the effector domain that binds to the Rho specificity-determining regions containing switch I, beta strands B2 and B3, and the C-terminal alpha helix A5, predominantly by specific hydrogen bonds. The ACC finger fold is distinct from those for other small G proteins and provides evidence for the diverse ways of effector recognition. Sequence analysis based on the structure suggests that the ACC finger fold is widespread in Rho effector proteins.

Rho-specific binding and guanine nucleotide exchange catalysis by KIAA0380, a dbl family member

Several guanine nucleotide exchange factors (GEFs) for Rho-GTPases have been identified, all of them containing a Dbl homology (DH) and pleckstrin homology (PH) domain, but exhibiting different specificities to the Rho family members, Rho, Rac and Cdc42. We report here that KIAA0380, a protein with a tandem DH/PH domain, an amino-terminal PDZ domain and a regulator of G protein signalling (RGS) homology domain, is a specific GEF for RhoA, but not for Rac1 and Cdc42, as determined by GDP release, guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) binding and protein binding assays. When expressed in J82 cells, DH/PH domain-containing forms of KIAA0380 induced actin stress fibers, whereas expression of the RGS homology domain prevented lysophosphatidic acid (LPA)-induced stress fiber formation.

Monoglucosylation of RhoA at threonine 37 blocks cytosol-membrane cycling

The small GTPases Rho, Rac, and Cdc42 are monoglucosylated at effector domain amino acid threonine 37/35 by Clostridium difficile toxins A and B. Glucosylation renders the Rho proteins inactive by inhibiting effector coupling. To understand the functional consequences, effects of glucosylation on subcellular distribution and cycling of Rho GTPases between cytosol and membranes were analyzed. In intact cells and in cell lysates, glucosylation leads to a translocation of the majority of RhoA GTPase to the membranes whereas a minor fraction is monomeric in the cytosol without being complexed with the guanine nucleotide dissociation inhibitor (GDI-1). Rho complexed with GDI-1 is not substrate for glucosylation, and modified Rho does not bind to GDI-1. However, a membranous factor inducing release of Rho from the GDI complex makes cytosolic Rho available as a substrate for glucosylation. The binding of glucosylated RhoA to the plasma membranes is saturable, competable with unmodified Rho-GTPgammaS guanosine 5'-O-(3-thiotriphosphate), and takes place at a membrane protein with a molecular mass of about 70 kDa. Membrane-bound glucosylated Rho is not extractable by GDI-1 as unmodified Rho is, leading to accumulation of modified Rho at membranous binding sites. Thus, in addition to effector coupling inhibition, glucosylation also inhibits Rho cycling between cytosol and membranes, a prerequisite for Rho activation.

Regulation of the RhoA pathway in human endothelial cell spreading on type IV collagen: role of calcium influx

We have shown that nonvoltage-operated Ca(2+) entry regulates human umbilical vein endothelial cell adhesion, migration, and proliferation on type IV collagen. We now demonstrate a requirement for Ca(2+) influx for activation of the RhoA pathway during endothelial cell spreading on type IV collagen. Reorganization of actin into stress fibers was complete when the cells where fully spread at 90 minutes. No actin organization into stress fibers was seen in endothelial cells plated on type I collagen, indicating a permissive effect of type IV collagen. CAI, a blocker of nonvoltage-operated Ca(2+) channels, prevented development of stress fiber formation in endothelial cells on type IV collagen. This permissive effect was augmented by Ca(2+) influx, as stimulated by 0. 5 microM thapsigargin or 0.1 microM ionomycin, yielding faster development of actin stress fibers. Ca(2+) influx and actin rearrangement in response to thapsigargin and ionomycin were abrogated by CAI. Activated, membrane-bound RhoA is a substrate for C3 exoenzyme which ADP-ribosylates and inactivates RhoA, preventing actin stress fiber formation. Pretreatment of endothelial cells with C3 exoenzyme prevented basal and thapsigargin-augmented stress fiber formation. While regulation of Ca(2+) influx did not alter RhoA translocation, it reduced in vitro ADP-ribosylation of RhoA (P(2)<0. 05), suggesting Ca(2+) influx is needed for RhoA activation during spreading on type IV collagen; no Ca(2+) regulated change in RhoA was seen in HUVECs spreading on type I collagen matrix. Blockade of Ca(2+) influx of HUVEC spread on type IV collagen also reduced tyrosine phosphorylation of p190Rho-GAP and blocked thapsigargin-enhanced binding of p190Rho-GAP to focal adhesion kinase. Thus, Ca(2+) influx is necessary for RhoA activation and for linkage of the RhoA/stress fiber cascade to the focal adhesion/focal adhesion kinase pathway during human umbilical vein endothelial cell spreading on type IV collagen.

[Probable involvement of the augmented agonist-induced Ca2+ sensitization of airway smooth muscle contraction in the pathogenesis of airway hyperresponsiveness]

Nonspecific airway hyperresponsiveness (AHR) is a common feature of allergic bronchial asthmatics, but the underlying mechanism (s) of AHR have yet to be elucidated. The importance of AHR in the pathogenesis of asthma has been suggested by its relevance to the severity of this disease. There is thus a need to understand the underlying mechanisms of AHR for the sake of asthma therapy. In the present minireview, we discussed the involvement of the augmented agonist-induced Ca2+ sensitization of airway smooth muscle contraction in the pathogenesis of AHR. Treatment with acetylcholine (ACh) of a beta-escin-permeabilized intrapulmonary bronchial smooth muscle of the rat induced a stronger contractile force even when the Ca2+ concentration was clamped at 1 microM. The ACh-induced Ca2+ sensitization of myofilaments was found to be significantly greater in antigen-induced airway hyperresponsive rats than in control rats. The ACh-induced Ca2+ sensitization was completely blocked by treatment with Clostridium botulinum C3 exoenzyme, an inactivator of the Rho family proteins. Moreover, the protein level of RhoA in the intrapulmonary bronchi was demonstrated to be significantly increased in the airway hyperresponsive rats. Thus, the increased airway smooth muscle contractility observed in asthmatics may be related to the augmented agonist-induced, Rho-mediated Ca2+ sensitization of myofilaments.