beta1 integrin and organized actin filaments facilitate cardiomyocyte-specific RhoA-dependent activation of the skeletal alpha-actin promoter

Activation of RhoA GTPase causes actin filament bundling into stress fibers, integrin clustering, and focal adhesion formation through its action on actin cytoskeleton organization. RhoA also regulates transcriptional activity of serum response factor (SRF). Recent studies in NIH 3T3 fibroblasts have shown that SRF activation by RhoA does not require an organized cytoskeleton and may be regulated by G-actin level. In cardiac myocytes, the organization of actin fibers into myofibrils is one of the primary characteristics of cardiac differentiation and hypertrophy. The primary purpose of this study was to examine if RhoA regulates SRF-dependent gene expression in neonatal cardiomyocytes in a manner different from that observed in fibroblasts. Our results show that RhoA-dependent skeletal alpha-actin promoter activation requires beta1 integrin and a functional cytoskeleton in cardiomyocytes but not in NIH 3T3 fibroblasts. Activation of the alpha-actin promoter by RhoA is greatly potentiated (up to 15-fold) by co-expression of the integrin beta1A or beta1D isoform but is significantly reduced by 70% with a co-expressed dominant negative mutant of beta1 integrin. Furthermore, clustering of beta1 integrin with anti-beta1 integrin antibodies potentiates synergistic RhoA and beta1 integrin activation of the alpha-actin promoter. Cytochalasin D and latrunculin B, inhibitors of actin polymerization, significantly reduced RhoA-induced activation of the alpha-actin promoter. Jasplakinolide, an actin polymerizing agent, mimics the synergistic effect of RhoA and beta1 integrin on the actin promoter. These observations support the concept that RhoA regulates SRF-dependent cardiac gene expression through cross-talk with beta1 integrin signal pathway via an organized actin cytoskeleton.

Characterization of p190RhoGEF, a RhoA-specific guanine nucleotide exchange factor that interacts with microtubules

Rho family GTPases control numerous cellular processes including cytoskeletal reorganization and transcriptional activation. Rho GTPases are activated by guanine nucleotide exchange factors (GEFs) which stimulate the exchange of bound GDP for GTP. We recently isolated a putative GEF, termed p190RhoGEF that binds to RhoA and, when overexpressed in neuronal cells, induces cell rounding and inhibits neurite outgrowth. Here we show that the isolated tandem Dbl homology/pleckstrin homology domain of p190RhoGEF activates RhoA in vitro, but not Rac1 or Cdc42, as determined by GDP release and protein binding assays. In contrast, full-length p190RhoGEF fails to activate RhoA in vitro. When overexpressed in intact cells, however, p190RhoGEF does activate RhoA with subsequent F-actin reorganization and serum response factor-mediated transcription. Immunofluorescence studies show that endogenous p190RhoGEF localizes to distinct RhoA-containing regions at the plasma membrane, to the cytosol and along microtubules. In vitro and in vivo binding experiments show that p190RhoGEF directly interacts with microtubules via its C-terminal region adjacent to the catalytic Dbl homology/pleckstrin homology domain. Our results indicate that p190RhoGEF is a specific activator of RhoA that requires as yet unknown binding partners to unmask its GDP/GTP exchange activity in vivo, and they suggest that p190RhoGEF may provide a link between microtubule dynamics and RhoA signaling.

RhoA/rho-associated kinase mediates fibroblast contractile force generation

The intracellular signals governing contractile force generation by non-muscle cells remain uncertain. Our aim was to test the hypothesis that the rhoA/rho-associated kinase signaling pathway is a principal mediator of contractile force generation in non-muscle cells. We measured myosin II regulatory light chain (MLC) phosphorylation and directly quantitated force generation by chicken embryo fibroblasts in the absence and presence of selective inhibitors of rhoA, and its downstream effector, rho-associated kinase. Inactivation of rhoA, with C3 transferase, inhibited serum-stimulated MLC phosphorylation and contractile force generation. Y-27632, an inhibitor of rho-associated kinase, reduced basal contractile tension, and inhibited both serum and endothelin-1 stimulated MLC phosphorylation and contractile force generation. The results of this study provide novel evidence indicating that the rhoA/rho-associated kinase signaling pathway is a principal mediator of MLC phosphorylation and consequent contractile force generation by non-muscle cells.

Cdc42Hs facilitates cytoskeletal reorganization and neurite outgrowth by localizing the 58-kD insulin receptor substrate to filamentous actin

Cdc42Hs is involved in cytoskeletal reorganization and is required for neurite outgrowth in N1E-115 cells. To investigate the molecular mechanism by which Cdc42Hs regulates these processes, a search for novel Cdc42Hs protein partners was undertaken by yeast two-hybrid assay. Here, we identify the 58-kD substrate of the insulin receptor tyrosine kinase (IRS-58) as a Cdc42Hs target. IRS-58 is a brain-enriched protein comprising at least four protein-protein interaction sites: a Cdc42Hs binding site, an Src homology (SH)3-binding site, an SH3 domain, and a tryptophan, tyrptophan (WW)-binding domain. Expression of IRS-58 in Swiss 3T3 cells leads to reorganization of the filamentous (F)-actin cytoskeleton, involving loss of stress fibers and formation of filopodia and clusters. In N1E-115 cells IRS-58 induces neurite outgrowth with high complexity. Expression of a deletion mutant of IRS-58, which lacks the SH3- and WW-binding domains, induced neurite extension without complexity in N1E-115 cells. In Swiss 3T3 cells and N1E-115 cells, IRS-58 colocalizes with F-actin in clusters and filopodia. An IRS-58(1267N) mutant unable to bind Cdc42Hs failed to localize with F-actin to induce neurite outgrowth or significant cytoskeletal reorganization. These results suggest that Cdc42Hs facilitates cytoskeletal reorganization and neurite outgrowth by localizing protein complexes via adaptor proteins such as IRS-58 to F-actin.

Overexpression of RhoA mRNA is associated with advanced stage in testicular germ cell tumour

OBJECTIVE

To clarify the role of Rho small GTP-binding protein (Rho) in the progression of testicular germ cell tumour (GCT), by examining the expression levels of mRNAs of Rho genes in testicular GCT.

PATIENTS AND METHODS

The mRNA levels of the RhoA, RhoB and RhoC genes were analysed in the surgical specimens of testicular GCT tissues from 45 consecutive Japanese patients, and in the corresponding unaffected tissue originating from the same patient, using reverse transcription-polymerase chain reaction. The expression levels in tumour tissues were compared with those in unaffected tissues and the relationship between their expression levels in tumours and tumour stage evaluated. The expression levels of mRNAs of the Rho genes were also evaluated between tumours with seminoma only, and mixed tumours with seminoma and nonseminoma.

RESULTS

The mRNA levels of RhoA were greater in tumour tissues than in unaffected tissues of the resected testis (P < 0.01); the mRNAs of RhoB and RhoC were not detected in either tissue. The increase in RhoA mRNA levels was related to tumour stage (P < 0.05). The mRNA levels of RhoA in seminomatous and nonseminomatous areas where both were present were higher than those in tumours with seminoma only (P < 0.05).

CONCLUSIONS

These results suggest that RhoA is involved in testicular germinal epithelial carcinogenesis and progression in testicular GCT, indicating that RhoA may be a useful prognostic marker for progression in testicular GCT.

Enhancement of Rho/Rho-kinase system in regulation of vascular smooth muscle contraction in tachycardia-induced heart failure

OBJECTIVE

The Rho/Rho-kinase system regulates Ca(2+) sensitivity in vascular smooth muscle. A new drug, Y-27632, specifically inhibits Rho-kinase and hence decreases the phosphorylation of myosin light chain, thus reducing contraction. Here, we compare the effects of Y-27632 and nifedipine on the vasoconstrictor response of the femoral artery in heart failure.

METHODS

Heart failure (HF) was produced by chronic rapid RV pacing (250 bpm, 28 days, six dogs). Indo1-AM was loaded into endothelium-denuded femoral artery segments for measuring intracellular [Ca(2+)]. Tension and changes in intracellular [Ca(2+)] [the change in the ratio (418 nm/468 nm) of Indo1 fluorescence (F(ratio))] were simultaneously measured in Krebs-Ringer solution.

RESULTS

In HF: (i) norepinephrine (10 microM) produced greater tension (784+/-52 g/cm(2)) than in control (502+/-64 g/cm(2)) despite a similar increase in F(ratio), indicating increased Ca(2+) sensitivity in vascular smooth muscle; (ii) nifedipine attenuated this enhanced response by only a maximum of 27% at 1 micromol/l with a 56% reduction in F(ratio); (iii) Y-27632 attenuated it by a maximum of 80% at 100 micromol/l without a significant change in F(ratio); (iv) RhoA protein and mRNA expression levels in the femoral artery were up-regulated by +110% and +56%, respectively, while those of Rho-kinase were unchanged.

CONCLUSIONS

The Ca(2+)-sensitizing mechanism involving the Rho/Rho-kinase system may be deeply involved in the enhanced arterial vasoconstriction seen in HF. Since Y-27632 attenuated this response in small arteries, it shows potential as a novel, potent vasodilator for the treatment of HF.

Rho GTPase activity modulates Pseudomonas aeruginosa internalization by epithelial cells

The Gram-negative pathogen Pseudomonas aeruginosa invades epithelial cells in vivo and in vitro. We have examined the pathway(s) by which epithelial cells internalize P. aeruginosa strain PA103 using Madin-Darby canine kidney (MDCK) cells. We have recently demonstrated that P. aeruginosa internalization occurs by an actin-dependent Toxin B-inhibited pathway which becomes downregulated as epithelial cells become polarized, suggesting that one or more of the Rho family GTPases is involved in bacterial internalization. Here, we demonstrate that activation of the Rho family GTPases by cytotoxic necrotizing factor 1 (CNF-1) stimulates P. aeruginosa internalization. Examination of the roles of the individual Rho family GTPases in internalization shows that expression of a constitutively active allele of RhoA (RhoAV14), but not of constitutively active Rac1 (Rac1V12) or Cdc42 (Cdc42V12), is sufficient to increase uptake of PA103pscJ. This relative increase persists when bacterial infection is established at the basolateral surface of polarized cells, suggesting that the effect of RhoAV14 is not simply due to its known ability to disrupt tight junction integrity in polarized cells. RhoAV14-mediated stimulation of bacterial uptake is actin dependent as it is abrogated by exposure to latrunculin A. We also find that endogenous Rho GTP levels in epithelial cells are increased by infection with an internalized strain of P. aeruginosa; conversely, a poorly internalized isogenic strain expressing the bacterial anti-internalization protein ExoT causes decreased Rho GTP levels. Experimental inhibition of Rho, either by expressing dominant negative RhoAN19 or by inhibiting native Rho using a membrane permeable fusion construct of a Rho-specific inhibitor, C3 ADP-ribosyltransferase, does not inhibit PA103pscJ internalization in MDCK or HeLa cells. Models consistent with these data are presented.

Regulation of mitogen-activated protein kinases in cardiac myocytes through the small G protein Rac1

Small guanine nucleotide-binding proteins of the Ras and Rho (Rac, Cdc42, and Rho) families have been implicated in cardiac myocyte hypertrophy, and this may involve the extracellular signal-related kinase (ERK), c-Jun N-terminal kinase (JNK), and/or p38 mitogen-activated protein kinase (MAPK) cascades. In other systems, Rac and Cdc42 have been particularly implicated in the activation of JNKs and p38-MAPKs. We examined the activation of Rho family small G proteins and the regulation of MAPKs through Rac1 in cardiac myocytes. Endothelin 1 and phenylephrine (both hypertrophic agonists) induced rapid activation of endogenous Rac1, and endothelin 1 also promoted significant activation of RhoA. Toxin B (which inactivates Rho family proteins) attenuated the activation of JNKs by hyperosmotic shock or endothelin 1 but had no effect on p38-MAPK activation. Toxin B also inhibited the activation of the ERK cascade by these stimuli. In transfection experiments, dominant-negative N17Rac1 inhibited activation of ERK by endothelin 1, whereas activated V12Rac1 cooperated with c-Raf to activate ERK. Rac1 may stimulate the ERK cascade either by promoting the phosphorylation of c-Raf or by increasing MEK1 and/or -2 association with c-Raf to facilitate MEK1 and/or -2 activation. In cardiac myocytes, toxin B attenuated c-Raf(Ser-338) phosphorylation (50 to 70% inhibition), but this had no effect on c-Raf activity. However, toxin B decreased both the association of MEK1 and/or -2 with c-Raf and c-Raf-associated ERK-activating activity. V12Rac1 cooperated with c-Raf to increase expression of atrial natriuretic factor (ANF), whereas N17Rac1 inhibited endothelin 1-stimulated ANF expression, indicating that the synergy between Rac1 and c-Raf is potentially physiologically important. We conclude that activation of Rac1 by hypertrophic stimuli contributes to the hypertrophic response by modulating the ERK and/or possibly the JNK (but not the p38-MAPK) cascades.

Integrin-mediated adhesion regulates cell polarity and membrane protrusion through the Rho family of GTPases

Integrin-mediated adhesion is a critical regulator of cell migration. Here we demonstrate that integrin-mediated adhesion to high fibronectin concentrations induces a stop signal for cell migration by inhibiting cell polarization and protrusion. On fibronectin, the stop signal is generated through alpha 5 beta 1 integrin-mediated signaling to the Rho family of GTPases. Specifically, Cdc42 and Rac1 activation exhibits a biphasic dependence on fibronectin concentration that parallels optimum cell polarization and protrusion. In contrast, RhoA activity increases with increasing substratum concentration. We find that cross talk between Cdc42 and Rac1 is required for substratum-stimulated protrusion, whereas RhoA activity is inhibitory. We also show that Cdc42 activity is inhibited by Rac1 activation, suggesting that Rac1 activity may down-regulate Cdc42 activity and promote the formation of stabilized rather than transient protrusion. Furthermore, expression of RhoA down-regulates Cdc42 and Rac1 activity, providing a mechanism whereby RhoA may inhibit cell polarization and protrusion. These findings implicate adhesion-dependent signaling as a mechanism to stop cell migration by regulating cell polarity and protrusion via the Rho family of GTPases.

cGMP-dependent protein kinase phosphorylates and inactivates RhoA

Small GTPase Rho and cGMP/cGMP-dependent protein kinase (cGK) pathways exert opposing effects in specific systems such as vascular contraction and growth. However, the direct interaction between these pathways has remained elusive. We demonstrate that cGK phosphorylates RhoA in vitro at Ser188, the same residue phosphorylated by cAMP-dependent protein kinase. In HeLa cells transfected with constitutively active cGK (C-cGK), stress fiber formation induced by lysophosphatidic acid or V14RhoA was blocked. By contrast, C-cGK failed to inhibit stress fiber formation in cells transfected with mutant RhoA with substitution of Ser188 to Ala. C-cGK did not affect actin reorganization induced by Rac1 or Rho-associated kinase, one of the effectors for RhoA. Furthermore, C-cGK expression inhibited the membrane translocation of RhoA. Collectively, our findings suggest that cGK phosphorylates RhoA at Ser188 and inactivates RhoA signaling. The physiological relevance of the direct interaction between RhoA and cGK awaits further investigation.

Galpha(13) mediates activation of a depolarizing chloride current that accompanies RhoA activation in both neuronal and nonneuronal cells

Loss of membrane potential (membrane depolarization) is one of the earliest and most striking responses of quiescent cells to stimulation with serum or G protein-coupled receptor (GPCR) agonists such as lysophosphatidic acid and thrombin. Membrane depolarization is due to the activation of a chloride conductance. While this response has received relatively little attention in the past, it is clear that the acute loss of membrane potential may have important physiological consequences. However, the dissection of the underlying G protein pathway and the establishment of cause-effect relationships have remained elusive to date. Here we report that, in neuronal cells, the depolarizing chloride current invariably accompanies GPCR-induced activation of RhoA and subsequent neurite retraction, and neither of these events requires phosphoinositide hydrolysis or Ca2+ mobilization. Through antibody microinjections and a genetic approach, we demonstrate that activation of the chloride conductance is mediated by Galpha(13) in a RhoA-independent manner in both neuronal cells and fibroblasts. We further show that, in neuronal cells, this newly described Galpha(13) pathway may profoundly modulate membrane excitability during RhoA-regulated neurite remodeling.

Accumulation of RhoA, RhoB, RhoG, and Rac1 in fibroblasts from Tangier disease subjects suggests a regulatory role of Rho family proteins in cholesterol efflux

Tangier disease (TD) is an inherited disorder of lipid metabolism characterized by very low high density lipoprotein (HDL) plasma levels, cellular cholesteryl ester accumulation and reduced cholesterol excretion in response to HDL apolipoproteins. Molecular defects in the ATP binding cassette transporter 1 (ABCA1) have recently been identified as the cause of TD. ABCA1 plays a key role in the translocation of cholesterol across the plasma membrane, and defective ABCA1 causes cholesterol storage in TD cells. However, the exact relationship of many of the biochemical and morphological abnormalities in TD to ABCA1 is unknown. Since small GTP-binding proteins are important regulators of many cellular functions, we characterized these proteins in normal and TD fibroblasts using the [alpha-32P]GTP overlay technique and Western blotting of SDS and isoelectric focusing gels. Our results indicate that GTP-binding proteins of the Rho family (RhoA, RhoB, RhoG, Rac-1) are enriched in fibroblasts from TD patients. The accumulation of small G proteins may have potential implications for the TD phenotype and the regulation of cholesterol excretion in TD cells.

Smooth muscle differentiation marker gene expression is regulated by RhoA-mediated actin polymerization

Smooth muscle cell (SMC) differentiation is regulated by a complex array of local environmental cues, but the intracellular signaling pathways and the transcription mechanisms that regulate this process are largely unknown. We and others have shown that serum response factor (SRF) contributes to SMC-specific gene transcription, and because the small GTPase RhoA has been shown to regulate SRF, the goal of the present study was to test the hypothesis that RhoA signaling is a critical mechanism for regulating SMC differentiation. Coexpression of constitutively active RhoA in rat aortic SMC cultures significantly increased the activity of the SMC-specific promoters, SM22 and SM alpha-actin, whereas coexpression of C3 transferase abolished the activity of these promoters. Inhibition of either stress fiber formation with the Rho kinase inhibitor Y-27632 (10 microm) or actin polymerization with latrunculin B (0.5 microm) significantly decreased the activity of SM22 and SM alpha-actin promoters. In contrast, increasing actin polymerization with jasplakinolide (0.5 microm) increased SM22 and SM alpha-actin promoter activity by 22-fold and 13-fold, respectively. The above interventions had little or no effect on the transcription of an SRF-dependent c-fos promoter or on a minimal thymidine kinase promoter that is not SRF-dependent. Taken together, the results of these studies indicate that in SMC, RhoA-dependent regulation of the actin cytoskeleton selectively regulates SMC differentiation marker gene expression by modulating SRF-dependent transcription. The results also suggest that RhoA signaling may serve as a convergence point for the multiple signaling pathways that regulate SMC differentiation.

Structure-activity relationships in flexible protein domains: regulation of rho GTPases by RhoGDI and D4 GDI

The guanine dissociation inhibitors RhoGDI and D4GDI inhibit guanosine 5'-diphosphate dissociation from Rho GTPases, keeping these small GTPases in an inactive state. The GDIs are made up of two domains: a flexible N-terminal domain of about 70 amino acid residues and a folded 134-residue C-terminal domain. Here, we characterize the conformation of the N-terminal regions of both RhoGDI and D4GDI using a series of NMR experiments which include (15)N relaxation and amide solvent accessibility measurements. In each protein, two regions with tendencies to form helices are identified: residues 36 to 58 and 9 to 20 in RhoGDI, and residues 36 to 57 and 20 to 25 in D4GDI. To examine the functional roles of the N-terminal domain of RhoGDI, in vitro and in vivo functional assays have been carried out with N-terminally truncated proteins. These studies show that the first 30 amino acid residues are not required for inhibition of GDP dissociation but appear to be important for GTP hydrolysis, whilst removal of the first 41 residues completely abolish the ability of RhoGDI to inhibit GDP dissociation. The combination of structural and functional studies allows us to explain why RhoGDI and D4GDI are able to interact in similar ways with the guanosine 5'-diphosphate-bound GTPase, but differ in their ability to regulate GTP-bound forms; these functional differences are attributed to the conformational differences of the N-terminal domains of the guanosine 5'-diphosphate dissociation inhibitors. Therefore, the two transient helices, appear to be associated with different biological effects of RhoGDI, providing a clear example of structure-activity relationships in a flexible protein domain.

Dual stimulation of Ras/mitogen-activated protein kinase and RhoA by cell adhesion to fibronectin supports growth factor-stimulated cell cycle progression

In cellular transformation, activated forms of the small GTPases Ras and RhoA can cooperate to drive cells through the G1 phase of the cell cycle. Here, we show that a similar but substrate-regulated mechanism is involved in the anchorage-dependent proliferation of untransformed NIH-3T3 cells. Among several extracellular matrix components tested, only fibronectin supported growth factor-induced, E2F-dependent S phase entry. Although all substrates supported the mitogen-activated protein kinase (MAPK) response to growth factors, RhoA activity was specifically enhanced on fibronectin. Moreover, induction of cyclin D1 and suppression of p21(Cip/Waf) occurred specifically, in a Rho-dependent fashion, in cells attached to fibronectin. This ability of fibronectin to stimulate both Ras/MAPK- and RhoA-dependent signaling can explain its potent cooperation with growth factors in the stimulation of cell cycle progression.

Structure of the BH domain from graf and its implications for Rho GTPase recognition

Cellular signaling by small G-proteins is down-regulated by GTPase-activating proteins (GAPs), which increase the rate of GTP hydrolysis. The GTPase regulator associated with focal adhesion kinase (Graf) exhibits GAP activity toward the RhoA and Cdc42 GTPases, but is only weakly active toward the closely related Rac1. We determined the crystal structure of a 231-residue fragment of Graf (GrafGAP), a domain containing the GAP activity, at 2.4-A resolution. The structure clarifies the boundaries of the functional domain and yields insight to the mechanism of substrate recognition. Modeling its interaction with substrate suggested that a favorable interaction with Glu-95 of Cdc42 (Glu-97 of RhoA) would be absent with the corresponding Ala-95 of Rac1. Indeed, GrafGAP activity is diminished approximately 40-fold toward a Cdc42 E95A mutant, whereas a approximately 10-fold increase is observed for a Rac1 A95E mutant. The GrafGAP epitope that apparently interacts with Glu-95(Glu-97) contains Asn-225, which was recently found mutated in some myeloid leukemia patients. We conclude that position 95 of the GTPase is an important determinant for GrafGAP specificity in cellular function and tumor suppression.

Rho-kinase/ROCK is involved in cytokinesis through the phosphorylation of myosin light chain and not ezrin/radixin/moesin proteins at the cleavage furrow

The small GTPase Rho and one of its targets, Rho-kinase (also termed ROK or ROCK), are implicated in various cellular functions including stress fiber formation, smooth muscle contraction, tumor cell invasion and cell motility. We have previously reported that Rho-kinase accumulates at the cleavage furrow during cytokinesis in several cultured cells. Here, using Rho-kinase inhibitors, Y-27632 and HA1077, we found that Rho-kinase is responsible for the phosphorylation of myosin regulatory light chain at Ser19 in the cleavage furrow during cytokinesis. On the other hand, phosphorylation of ezrin/radixin/moesin (ERM) proteins at the cleavage furrow was enhanced by the addition of the above Rho-kinase inhibitors. Treatment with Y-27632 strongly enhanced the accumulation of Rho-kinase but not RhoA and citron kinase at the cleavage furrow. Furthermore, the furrow ingression in cytokinesis was significantly prolonged in the presence of Y-27632. These results suggest that Rho-kinase is involved in the progression of cytokinesis through the phosphorylation of several proteins including myosin light chain at the cleavage furrow.

Role of RhoA and Rho kinase in lysophosphatidic acid-induced endothelial barrier dysfunction

In the present study, the roles of the small GTPase RhoA and its target Rho kinase in endothelial permeability were investigated in vitro. We have shown previously that, in addition to a rise in the intracellular Ca(2+) concentration ([Ca(2+)](i)), RhoA is involved in the prolonged thrombin-induced barrier dysfunction. To study the role of RhoA and Rho kinase more specifically, endothelial cells were stimulated with lysophosphatidic acid (LPA), a commonly used RhoA activator. LPA induced a 2- to 3-fold increase in the passage of horseradish peroxidase (HRP) across endothelial monolayers that lasted for several hours, whereas thrombin induced a 5- to 10-fold increase. Comparable to the thrombin-induced barrier dysfunction, the LPA-induced barrier dysfunction was accompanied by a reorganization of the F-actin cytoskeleton and the formation of focal attachment sites. LPA induced only a transient increase in myosin light-chain (MLC) phosphorylation, which returned to basal level within 10 minutes. In endothelial cells, [Ca(2+)](i) was not elevated by LPA. Chelation of Ca(2+)(i) ions by 1, 2-bis(2-aminophenoxy)ethane-N:,N:,N:',N:'-tetraacetic acid did not prevent the LPA-induced passage of HRP. Apparently, a low degree of MLC kinase activation occurred, because the MLC kinase inhibitor KT5926 reduced the levels of both basal and LPA-stimulated HRP passage. Inhibition of RhoA by the C3 transferase from Clostridium botulinum inhibited the LPA-induced cytoskeletal changes and prevented the LPA-induced HRP passage. Inhibition of Rho kinase by Y-27632 completely prevented the LPA-induced increase in HRP passage without affecting basal permeability. These data indicate that LPA-induced endothelial hyperpermeability occurs without a change in [Ca(2+)](i) and requires activation of RhoA and Rho kinase.

Dissection of Ras-dependent signaling pathways controlling aggressive tumor growth of human fibrosarcoma cells: evidence for a potential novel pathway

Activation of multiple signaling pathways is required to trigger the full spectrum of in vitro and in vivo phenotypic traits associated with neoplastic transformation by oncogenic Ras. To determine which of these pathways are important for N-ras tumorigenesis in human cancer cells and also to investigate the possibility of cross talk among the pathways, we have utilized a human fibrosarcoma cell line (HT1080), which contains an endogenous mutated allele of the N-ras gene, and its derivative (MCH603c8), which lacks the mutant N-ras allele. We have stably transfected MCH603c8 and HT1080 cells with activating or dominant-negative mutant cDNAs, respectively, of various components of the Raf, Rac, and RhoA pathways. In previous studies with these cell lines we showed that loss of mutant Ras function results in dramatic changes in the in vitro phenotypic traits and conversion to a weakly tumorigenic phenotype in vivo. We report here that only overexpression of activated MEK contributed significantly to the conversion of MCH603c8 cells to an aggressive tumorigenic phenotype. Furthermore, we have demonstrated that blocking the constitutive activation of the Raf-MEK, Rac, or RhoA pathway alone is not sufficient to block the aggressive tumorigenic phenotype of HT1080, despite affecting a number of in vitro-transformed phenotypic traits. We have also demonstrated the possibility of bidirectional cross talk between the Raf-MEK-ERK pathway and the Rac-JNK or RhoA pathway. Finally, overexpression of activated MEK in MCH603c8 cells appears to result in the activation of an as-yet-unidentified target(s) that is critical for the aggressive tumorigenic phenotype.

A morphological study of the expression of the small G protein RhoA in resting and activated MDCK cells

The small G protein Rho subfamily controls several cellular events such as growth, movement, proliferation and differentiation by rearranging actin and cytoskeleton proteins. Most of these effects are mediated by the activation of growth factor and extracellular matrix molecule receptors, suggesting a role for Rho molecules in the transduction pathway of these receptors. Despite the importance of Rho peptides in fundamental cellular events, data on their subcellular immunolocalisation are sparse: here we investigated the expression and subcellular localisation of RhoA in resting (cultured on plastic) and activated (Matri-cell or hepatocyte growth factor) MDCK cells by immunoperoxidase and immunogold techniques. Resting MDCK cells contain detectable amounts of RhoA mainly localised in the cytoplasm; RhoA expression is significantly enhanced by Matri-cell substrates that promote translocation of RhoA at the membrane level. This enhancing effect is reduced after exposure to hepatocyte growth factor.

The protein tyrosine phosphatase Shp-2 regulates RhoA activity

Remodeling of filamentous actin into distinct arrangements is precisely controlled by members of the Rho family of small GTPases [1]. A well characterized member of this family is RhoA, whose activation results in reorganization of the cytoskeleton into thick actin stress fibers terminating in integrin-rich focal adhesions [2]. Regulation of RhoA is required to maintain adhesion in stationary cells, but is also critical for cell spreading and migration [3]. Despite its biological importance, the signaling events leading to RhoA activation are not fully understood. Several independent studies have implicated tyrosine phosphorylation as a critical event upstream of RhoA [4]. Consistent with this, our recent studies have demonstrated the existence of a protein tyrosine phosphatase (PTPase), sensitive to the dipeptide aldehyde calpeptin, acting upstream of RhoA [5]. Here we identify the SH2 (Src homology region 2)-containing PTPase Shp-2 as a calpeptin-sensitive PTPase, and show that calpeptin interferes with the catalytic activity of Shp-2 in vitro and with Shp-2 signaling in vivo. Finally, we show that perturbation of Shp-2 activity by a variety of genetic manipulations results in raised levels of active RhoA. Together, these studies identify Shp-2 as a PTPase acting upstream of RhoA to regulate its activity and contribute to the coordinated control of cell movement.

The trio guanine nucleotide exchange factor is a RhoA target. Binding of RhoA to the trio immunoglobulin-like domain

Trio is a complex protein containing two guanine nucleotide exchange factor domains each with associated pleckstrin homology domains, a serine/threonine kinase domain, two SH3 domains, an immunoglobulin-like domain, and spectrin-like repeats. Trio was originally identified as a LAR tyrosine phosphatase-binding protein and is involved in actin remodeling, cell migration, and cell growth. Herein we provide evidence that Trio not only activates RhoA but is also a RhoA target. The RhoA-binding site was mapped to the Trio immunoglobulin-like domain. RhoA isoprenylation is necessary for the RhoA-Trio interaction, because mutation of the RhoA carboxyl-terminal cysteine residue blocked binding. The existence of an intramolecular functional link between RhoA activation and RhoA binding is suggested by the finding that Trio exchange activity enhanced RhoA binding to Trio. Furthermore, immunofluorescence studies of HeLa cells showed that although ectopically expressed Trio was evenly distributed within the cell, co-expression of Trio with RhoA resulted in relocalization of Trio into punctate structures. Relocalization was not observed with Trio constructs lacking the immunoglobulin-like domain, indicating that RhoA acts to regulate Trio localization via binding to the immunoglobulin-like domain. We propose that Trio-mediated RhoA activation and subsequent RhoA-mediated relocalization of Trio functions to modulate and coordinate Trio signaling.

The Yersinia protein kinase A is a host factor inducible RhoA/Rac-binding virulence factor

The pathogenic yersiniae inject proteins directly into eukaryotic cells that interfere with a number of cellular processes including phagocytosis and inflammatory-associated host responses. One of these injected proteins, the Yersinia protein kinase A (YpkA), has previously been shown to affect the morphology of cultured eukaryotic cells as well as to localize to the plasma membrane following its injection into HeLa cells. Here it is shown that these activities are mediated by separable domains of YpkA. The amino terminus, which contains the kinase domain, is sufficient to localize YpkA to the plasma membrane while the carboxyl terminus of YpkA is required for YpkAs morphological effects. YpkAs carboxyl-terminal region was found to affect the levels of actin-containing stress fibers as well as block the activation of the GTPase RhoA in Yersinia-infected cells. We show that the carboxyl-terminal region of YpkA, which contains sequences that bear similarity to the RhoA-binding domains of several eukaryotic RhoA-binding kinases, directly interacts with RhoA as well as Rac (but not Cdc42) and displays a slight but measurable binding preference for the GDP-bound form of RhoA. Surprisingly, YpkA binding to RhoA(GDP) affected neither the intrinsic nor guanine nucleotide exchange factor-mediated GDP/GTP exchange reaction suggesting that YpkA controls activated RhoA levels by a mechanism other than by simply blocking guanine nucleotide exchange factor activity. We go on to show that YpkAs kinase activity is neither dependent on nor promoted by its interaction with RhoA and Rac but is, however, entirely dependent on heat-sensitive eukaryotic factors present in HeLa cell extracts and fetal calf serum. Collectively, our data show that YpkA possesses both similarities and differences with the eukaryotic RhoA/Rac-binding kinases and suggest that the yersiniae utilize the Rho GTPases for unique activities during their interaction with eukaryotic cells.

Hepatoma cell migration through a mesothelial cell monolayer is inhibited by cyclic AMP-elevating agents via a Rho-dependent pathway

1-Oleoyl lysophosphatidic acid (LPA) induces transmonolayer migration (in vitro invasion) of rat ascites hepatoma MM1 cells and their morphological changes leading to the migration. We have previously shown that an LPA analog, palmitoyl cyclic phosphatidic acid (Pal-cPA), suppresses transmonolayer migration of MM1 cells by rapidly increasing the intracellular cyclic AMP (cAMP) concentration. We report here that various cAMP-elevating agents, including dibutyryl cAMP, forskolin, cholera toxin and 3-isobutyl-1-methylxanthine, consistently inhibited LPA-induced transmonolayer migration of MM1 cells. Moreover, pull-down assays for GTP-bound, active RhoA demonstrated that the blockage by cAMP-elevating agents of morphological changes leading to the migration was probably mediated through inhibiting RhoA activation.

Synaptojanin 2, a novel Rac1 effector that regulates clathrin-mediated endocytosis

The small GTPase Rac has been implicated in a wide range of cellular processes, including the organization of the actin cytoskeleton, transcriptional control and endocytic vesicle trafficking [1-3]. The signaling components that mediate these functions downstream of Rac largely remain to be identified. In this study, we have identified synaptojanin 2, a polyphosphoinositide phosphatase as a novel Rac1 effector. Synaptojanin 2 directly and specifically interacts with Rac1 in a GTP-dependent manner. Expression of constitutively active Rac1 caused the translocation of synaptojanin 2 from the cytoplasm to the plasma membrane. Both activated Rac1 and a membrane-targeted version of synaptojanin 2 inhibited endocytosis of the epidermal growth factor (EGF) and transferrin receptors, a process that is known to be dependent on polyphosphoinositide lipids. Endocytosis of growth factor receptors is thought to play an important role in the regulation of cell proliferation. Thus, these results suggest that synaptojanin 2 may mediate the inhibitory effect of Rac1 on endocytosis and could contribute to Rac1-mediated control of cell growth.

Activation of RhoA by association of Galpha(13) with Dbl

RhoA is a small G protein that is implicated in the regulation of the actin cytoskeleton, gene expression, and cell cycle progression. It is activated by many agonists whose receptors are linked to heterotrimeric G proteins, but the mechanisms are incompletely understood. In this study, we show that the constitutively active alpha-subunit of the heterotrimeric G protein G(13) associated with the Rho family guanine nucleotide exchange factor Dbl in NIH 3T3 cells and that this resulted in activation of RhoA. This activation was not seen with wild-type Galpha(13) or if Dbl and active Galpha(13) were expressed separately and mixed. In contrast, coexpression of constitutively active Galpha(q) with Dbl did not lead to their association and caused a weak activation of RhoA that was no greater than that observed with wild-type Galpha(q). These findings illustrate that activated Galpha(13) and Dbl can associate in vivo and that this leads to Rho activation.

Leptin promotes invasiveness of kidney and colonic epithelial cells via phosphoinositide 3-kinase-, rho-, and rac-dependent signaling pathways

Leptin plays a key role regulating food intake, body weight and fat mass. These critical parameters are associated with an increased risk for digestive and mammary gland cancer in the Western population. Here we determined whether leptin contributes to the invasive phenotype of colonic and kidney epithelial cells at various stages of the neoplastic progression. First, leptin potently (EC50 = 10-30 ng/ml) induces invasion of collagen gels by premalignant familial adenomatous colonic cells PC/AA/C1 and nontumorigenic MDCK kidney epithelial cells, their src-transformed counterparts, and the human adenocarcinoma colonic cells LoVo and HCT-8/S11. Leptin and its Ob-Rb receptors were consistently identified by RT-PCR and immunoblotting in these cell lines, as well as in human colonic epithelial crypts, polyps, colonic tumor resections, and adjacent mucosa. Leptin-induced invasion was effectively blocked by pharmacological inhibitors of several downstream signaling pathways involved in cell transformation, namely, JAK2 tyrosine kinase (AG490), phosphoinositide PI3'-kinase (wortmannin and LY294002), mTOR kinase (rapamycin), and protein kinases C (GF109203X, Gö6976). Accordingly, leptin induces transient elevation of the PI3'-kinase lipid products in JAK2 immunoprecipitates prepared from parental MDCK cells. The leptin effect on invasion was potentiated by the activated form of the small GTPase RhoA and was abrogated by dominant negative mutants of RhoA, Rac1, and the p110alpha of PI3'-K. Our data indicate that leptin may exert a local and beneficial effect on migration of normal colonic epithelial cells and reparation of the inflamed or wounded digestive mucosa. We also emphasize a new role for leptin, linking the nutritional and body fat status to digestive cancer susceptibility by stimulating the invasive capacity of colonic epithelial cells at early stages of neoplasia. This finding has potential clinical implications for colon cancer progression and management of obesity.

Defective platelet aggregation in polycythaemia vera is not caused by impaired calcium signaling, phospholipase D activation or decreased amounts of focal adhesion proteins

We have previously demonstrated that platelets in polycythaemia vera (PV) exhibit decreased aggregation after stimulation with platelet activating factor (PAF) and reduced expression of GPIIIa on both resting and stimulated platelets. In the present study, we investigated if these results were related to changes in the mobilization of intracellular calcium, activation of phospholipase D (PLD) or amounts of GPIIIa and the intracellular tyrosine kinases Fak, Syk, Grb2, Shc and rhoA. Intracellular calcium levels were not different in resting platelets from 14 PV patients and 15 healthy controls (median 43 nmol/L, range 10-114, vs. 36 nmol/L, range 10-119). After stimulation with PAF (1 micromol/L) an equal increase was seen (125 nmol/L for PV platelets, range 67-257, vs. 113 nmol/L for controls, range 60-250). Also formation of phosphatidyl ethanol (PEt) was similar after exposure to 0.5 U/ml thrombin (0.28% PEt of total phospholipid, range 0.16-1.10, vs. 0.24 for controls, range 0.11-2.3) and 1 micromol/L PMA (0.25, range 0.16-0.32, vs. 0.14, range 0.09-0.6). In contrast to the reduced amount of GPIIIa on the surface of PV platelets, immunoblotting on whole cell lysates showed no reduction in PV patients compared to controls, indicating the possibility of an impaired incorporation of GPIIIa to the cell membrane. Levels of Fak, Syk, Shc, Grb2 and rhoA appeared equal in patients and controls. Similar intracellular proteins were tyrosine phosphorylated after stimulation with thrombin, PAF and PMA. In summary, defective platelet aggregation after stimulation with PAF is caused by neither defective mobilization of intracellular calcium nor, in contrast to the situation in PV granulocytes, an impaired activation of PLD. Moreover, no apparent differences in the intracellular amounts of Fak, Syk Shc, Grb2 and rhoA could be detected between PV and control platelets.

Evidence that beta3 integrin-induced Rac activation involves the calpain-dependent formation of integrin clusters that are distinct from the focal complexes and focal adhesions that form as Rac and RhoA become active

Interaction of integrins with the extracellular matrix leads to transmission of signals, cytoskeletal reorganizations, and changes in cell behavior. While many signaling molecules are known to be activated within Rac-induced focal complexes or Rho-induced focal adhesions, the way in which integrin-mediated adhesion leads to activation of Rac and Rho is not known. In the present study, we identified clusters of integrin that formed upstream of Rac activation. These clusters contained a Rac-binding protein(s) and appeared to be involved in Rac activation. The integrin clusters contained calpain and calpain-cleaved beta3 integrin, while the focal complexes and focal adhesions that formed once Rac and Rho were activated did not. Moreover, the integrin clusters were dependent on calpain for their formation. In contrast, while Rac- and Rho-GTPases were dependent on calpain for their activation, formation of focal complexes and focal adhesions by constitutively active Rac or Rho, respectively, occurred even when calpain inhibitors were present. Taken together, these data are consistent with a model in which integrin-induced Rac activation requires the formation of integrin clusters. The clusters form in a calpain-dependent manner, contain calpain, calpain-cleaved integrin, and a Rac binding protein(s). Once Rac is activated, other integrin signaling complexes are formed by a calpain-independent mechanism(s).

Direct genetic demonstration of G alpha 13 coupling to the orphan G protein-coupled receptor G2A leading to RhoA-dependent actin rearrangement

G2A is an orphan G protein-coupled receptor (GPCR), expressed predominantly in T and B cells and homologous to a small group of GPCRs of unknown function expressed in lymphoid tissues. G2A is transcriptionally induced in response to diverse stimuli, and its ectopic expression suppresses transformation of B lymphoid precursors by BCR-ABL. G2A induces morphological transformation of NIH 3T3 fibroblasts. Microinjection of constructs encoding G2A into Swiss 3T3 fibroblasts induces actin reorganization into stress fibers that depends on RhoA, but not CDC42 or RAC. G2A elicits RhoA-dependent transcriptional activation of serum response factor. Direct evaluation of RhoA activity demonstrates elevated levels of RhoA-GTP in G2A-expressing cells. Microinjection of embryonic fibroblasts derived from various G alpha knockout mice establishes a requirement for G alpha 13 but not G alpha 12 or G alpha q/11 in G2A-induced actin rearrangement. In conclusion, G2A represents a family of GPCRs expressed in lymphocytes that may link diverse stimuli to cytoskeletal reorganization and transcriptional activation through a pathway involving G alpha 13 and RhoA.

Overexpression of wild-type RhoA produces growth arrest by disrupting actin cytoskeleton and microtubules

We have investigated the role of Rho GTPase in cell growth by generating stable cells that express the wild-type RhoA (RhoA(wt)) under the control of an inducible promoter. Induction of RhoA(wt) had a biphasic effect on the actin cytoskeleton. At low levels of expression, RhoA(wt) stimulated the assembly of actin stress fibers without affecting cell growth. At high levels, there was a paradoxical disruption of the actin cytoskeleton accompanied by a growth arrest. Cell cycle analysis revealed a dual block at the G(1)/S and G(2)/M checkpoints. The G(1)/S arrest correlated with the accumulation of p21(Cip1), resulting in the inhibition of cdk2 activity, whereas the G(2)/M block correlated with the loss of microtubules. The cyclin B level and the cdc2 kinase activity, however, were increased, suggesting that the progression through mitosis rather than entry into the G(2)/M is defective when RhoA(wt) is overexpressed. Similar cell cycle defects and the loss of microtubules were observed after a cytochalasin D treatment, indicating that the ability of RhoA to regulate the integrity of actin cytoskeleton may be critical for the cell cycle transition through both the G(1)/S and M phase checkpoints.

Potentiation of Rho-A-mediated lysophosphatidic acid activity by hyperinsulinemia

We have shown previously that insulin promotes phosphorylation and activation of farnesyltransferase and geranylgeranyltransferase (GGTase) II. We have now examined the effect of insulin on geranylgeranyltransferase I in MCF-7 breast cancer cells. Insulin increased GGTase I activity 3-fold and augmented the amounts of geranylgeranylated Rho-A by 18%. Both effects of the insulin were blocked by an inhibitor of GGTase I, GGTI-286. The insulin-induced increases in the amounts of geranylgeranylated Rho-A resulted in potentiation of the Rho-A-mediated effects of lysophosphatidic acid (LPA) on a serum response element-luciferase construct. Preincubation of cells with insulin augmented the LPA-stimulated serum response element-luciferase activation to 12-fold, compared with just 6-fold for LPA alone (p < 0.05). The potentiating effect of insulin was dose-dependent, inhibited by GGTI-286 and not mimicked by insulin-like growth factor-1. We conclude that insulin activates GGTase I, increases the amounts of geranylgeranylated Rho-A protein, and potentiates the Rho-A-dependent nuclear effects of LPA in MCF-7 breast cancer cells.

RhoA prenylation is required for promotion of cell growth and transformation and cytoskeleton organization but not for induction of serum response element transcription

The importance of post-translational geranylgeranylation of the GTPase RhoA for its ability to induce cellular proliferation and malignant transformation is not well understood. In this manuscript we demonstrate that geranylgeranylation is required for the proper cellular localization of V14RhoA and for its ability to induce actin stress fiber and focal adhesion formation. Furthermore, V14RhoA geranylgeranylation was also required for suppressing p21(WAF) transcription, promoting cell cycle progression and cellular proliferation. The ability of V14RhoA to induce focus formation and enhance plating efficiency and oncogenic Ras anchorage-dependent growth was also dependent on its geranylgeranylation. The only biological activity of V14RhoA that was not dependent on its prenylation was its ability to induce serum response element transcriptional activity. Furthermore, we demonstrate that a farnesylated form of V14RhoA was also able to bind RhoGDI-1, was able to induce cytoskeleton organization, proliferation, and transformation, and was just as potent as geranylgeranylated V14RhoA at suppressing p21(WAF) transcriptional activity. These results demonstrate that RhoA geranylgeranylation is required for its biological activity and that the nature of the lipid modification is not critical.

Vav2 activates Rac1, Cdc42, and RhoA downstream from growth factor receptors but not beta1 integrins

The Rho family of GTPases plays a major role in the organization of the actin cytoskeleton. These G proteins are activated by guanine nucleotide exchange factors that stimulate the exchange of bound GDP for GTP. In their GTP-bound state, these G proteins interact with downstream effectors. Vav2 is an exchange factor for Rho family GTPases. It is a ubiquitously expressed homologue of Vav1, and like Vav1, it has previously been shown to be activated by tyrosine phosphorylation. Because Vav1 becomes tyrosine phosphorylated and activated following integrin engagement in hematopoietic cells, we investigated the tyrosine phosphorylation of Vav2 in response to integrin-mediated adhesion in fibroblasts and epithelial cells. However, no tyrosine phosphorylation of Vav2 was detected in response to integrin engagement. In contrast, treating cells with either epidermal growth factor or platelet-derived growth factor stimulated tyrosine phosphorylation of Vav2. We have examined the effects of overexpressing either wild-type or amino-terminally truncated (constitutively active) forms of Vav2 as fusion proteins with green fluorescent protein. Overexpression of either wild-type or constitutively active Vav2 resulted in prominent membrane ruffles and enhanced stress fibers. These cells revealed elevated rates of cell migration that were inhibited by expression of dominant negative forms of Rac1 and Cdc42. Using a binding assay to measure the activity of Rac1, Cdc42, and RhoA, we found that overexpression of Vav2 resulted in increased activity of each of these G proteins. Expression of a carboxy-terminal fragment of Vav2 decreased the elevation of Rac1 activity induced by epidermal growth factor, consistent with Vav2 mediating activation of Rac1 downstream from growth factor receptors.

Cytotoxic necrotizing factor type 1 of uropathogenic Escherichia coli kills cultured human uroepithelial 5637 cells by an apoptotic mechanism

Pathogenic Escherichia coli associated with urinary tract infections (UTIs) in otherwise healthy individuals frequently produce cytotoxic necrotizing factor type 1 (CNF1), a member of the family of bacterial toxins that target the Rho family of small GTP-binding proteins. To gain insight into the function of CNF1 in the development of E. coli-mediated UTIs, we examined the effects of CNF1 intoxication on a panel of human cell lines derived from physiologically relevant sites (bladder, ureters, and kidneys). We identified one uroepithelial cell line that exhibited a distinctly different CNF1 intoxication phenotype from the prototypic one of multinucleation without cell death that is seen when HEp-2 or other epithelial cells are treated with CNF1. The 5637 bladder cell line detached from the growth surface within 72 h of CNF1 intoxication, a finding that suggested frank cytotoxicity. To determine the basis for the unexpected toxic effect of CNF1 on 5637 cells, we compared the degree of toxin binding, actin fiber formation, and Rho modification with those CNF1-induced events in HEp-2 cells. We found no apparent difference in the amount of CNF1 bound to 5637 cells and HEp-2 cells. Moreover, CNF1 modified Rho, in vivo and in vitro, in both cell types. In contrast, one of the classic responses to CNF1 in HEp-2 and other epithelial cell lines, the formation of actin stress fibers, was markedly absent in 5637 cells. Indeed, actin stress fiber induction by CNF1 did not occur in any of the other human bladder cell lines that we tested (J82, SV-HUC-1, or T24). Furthermore, the appearance of lamellipodia and filopodia in 5637 cells suggested that CNF1 activated the Cdc42 and Rac proteins. Finally, apoptosis was observed in CNF1-intoxicated 5637 cells. If our results with 5637 cells reflect the interaction of CNF1 with the transitional uroepithelium in the human bladder, then CNF1 may be involved in the exfoliative process that occurs in that organ after infection with uropathogenic E. coli.

Permissive role of protein kinase C alpha but not protein kinase C delta in sphingosine 1-phosphate-induced Rho A activation in C2C12 myoblasts

Rho GTPases participate in various important signaling pathways and have been implicated in myogenic differentiation. Here the first evidence is provided that in C2C12 myoblasts sphingosine 1-phosphate (SPP) rapidly and transiently induced membrane association of Rho A in a pertussis toxin-insensitive manner. The bioactive lipid preferentially relocalized the GTPase to Golgi-enriched membrane. Translocation of Rho A was abolished by inhibition or down-regulation of protein kinase C (PKC). Notably, treatment with Gö6976, an inhibitor of conventional PKCs, which selectively blocked PKC alpha in these cells, prevented SPP-induced Rho A translocation. Conversely rottlerin, a selective inhibitor of PKC delta, was without effect, demonstrating that SPP signaling to Rho A involves PKC alpha but not PKC delta activation. This novel functional relationship between the two proteins may have a role in SPP-mediated regulation of downstream effectors.

The Yersinia Ser/Thr protein kinase YpkA/YopO directly interacts with the small GTPases RhoA and Rac-1

Pathogenic bacteria of the genus Yersinia counteract host defense by interfering with eukaryotic signal transduction pathways. YpkA of Yersinia pseudotuberculosis shares significant homology with eukaryotic Ser/Thr protein kinases, is translocated into the host cell and has been shown to be an essential virulence factor in a mouse infection model. In this study, we identify the small GTPases RhoA and Rac-1 as eukaryotic binding partners of YpkA and its homolog YopO of Yersinia enterocolitica. We demonstrate that the interaction is independent of phosphorylation of YpkA and nucleotide loading state of the GTPases. The interaction with RhoA and Rac-1 might provide an important clue to how YpkA interferes with eukaryotic signaling on a molecular level.

The characterization of phospholipase D in FRTL-5 thyroid cells

We previously demonstrated that TSH activates phospholipase D (PLD) in Fischer rat thyroid line (FRTL)-5 cells. To date, two types of mammalian phosphatidylcholine-specific PLD cDNAs, designated as PLD-1 and PLD-2, have been cloned. The present study determined the PLD isoform composition in FRTL-5 thyroid cells and which isoform is regulated by TSH. PLD-1 is activated by small molecular weight G-proteins, such as ADP-ribosylation factor (ARF) and RhoA family members, while PLD-2 is relatively independent of such stimuli. We established the presence of PLD-1 and PLD-2 by Western blot analysis and compared PLD activity in cytosol, membranes and combined fractions in the presence and absence of GTPgammaS. The membrane fraction showed very little activity in the absence of GTPgammaS, but this activity increased approximately 5-fold (P<0.05, ANOVA) in the presence of GTPgammaS. Maximal PLD activity was seen with the combination of membrane plus cytosolic fractions (which contained ARF and RhoA) where the addition of GTPgammaS increased PLD activity approximately 8-fold (P<0.05, ANOVA). To determine the relative activities of PLD-1 and PLD-2 in FRTL-5 thyroid cells, cell-free PLD assays were performed in the presence of GTPgammaS or GDPbetaS with varying concentrations of phosphatidylinositol 4,5-bisphosphate (PIP(2)). PLD-2 contributed only approximately 19% of the total amount of PLD activity in the membranes and PLD-1 was the predominant PLD isoform. TSH stimulated PLD-1 activity by up to 2. 3-fold over control values (P<0.01, ANOVA). To establish the dependence of PLD-1 on small molecular weight G-proteins, the translocations of ARF and RhoA to the membrane fractions was determined after stimulation by TSH. Both ARF and RhoA were maximally translocated to the membrane fraction after 10 min incubation with 100 microU/ml TSH by approximately 1.7- and 2.3-fold over control values, respectively (P<0.02 and P<0.03, ANOVA). It is concluded that TSH stimulates PLD-1 activity in FRTL-5 thyroid cells and this is accompanied by the translocation of ARF and RhoA to the membrane fraction.

Mechanism of nucleotide release from Rho by the GDP dissociation stimulator protein

Guanine nucleotide dissociation stimulator (GDS) promotes the release of tightly bound GDP from various Ras superfamily proteins, including RhoA, Rac1, K-Ras, Rap1A, and Rap1B. It displays no significant sequence homology to other known exchange factors for small G-proteins. Studies are reported here of the mechanism of GDS-mediated nucleotide release from RhoA using a combination of equilibrium and stopped-flow kinetic measurements, employing fluorescent N-methylanthraniloyl (mant) derivatives of GDP and 2'-deoxyGDP. It is proposed that GDS operates by an associative displacement mechanism where stimulated nucleotide release from the Rho.mantGDP complex occurs via a transiently populated ternary complex (Rho.GDS.mantGDP). In kinetic experiments where excess GDS was mixed with the Rho.mantGDP complex, stimulated mantGDP dissociation rates of 1 s(-)(1) were measured during a single turnover, representing a 5000-fold enhancement over the intrinsic rate of mantGDP dissociation from Rho. The stable, nucleotide-free binary complex Rho.GDS was isolated. When the Rho.GDS complex was mixed with excess mantGDP, a biphasic increase in fluorescence occurred, the observed rate constants of which both reached saturating values at high mantGDP concentrations. This is compelling evidence that an isomerization of the Rho.GDS.mantGDP ternary complex is an important feature of the mechanism of nucleotide release.

Rho GTPases mediate the regulation of cochlear outer hair cell motility by acetylcholine

Outer hair cells are the mechanical effectors of the cochlear amplifier, an active process that improves the sensitivity and frequency discrimination of the mammalian ear. In vivo, the gain of the cochlear amplifier is regulated by the efferent neurotransmitter acetylcholine through the modulation of outer hair cell motility. Little is known, however, regarding the molecular mechanisms activated by acetylcholine. In this study, intracellular signaling pathways involving the small GTPases RhoA, Rac1, and Cdc42 have been identified as regulators of outer hair cell motility. Changes in cell length (slow motility) and in the amplitude of electrically induced movement (fast motility) were measured in isolated outer hair cells patch clamped in whole-cell mode, internally perfused through the patch pipette with different inhibitors and activators of these small GTPases while being externally stimulated with acetylcholine. We found that acetylcholine induces outer hair cell shortening and a simultaneous increase in the amplitude of fast motility through Rac1 and Cdc42 activation. In contrast, a RhoA- and Rac1-mediated signaling pathway induces outer hair cell elongation and decreases fast motility amplitude. These two opposing processes provide the basis for a regulatory mechanism of outer hair cell motility.

Activation of Rho signaling contributes to lysophosphatidic acid-induced contraction of intact ileal smooth muscle of guinea-pig

To elucidate the possible role of Rho A/Rho-kinase on lysophosphatidic acid (LPA)-induced contraction in intact guinea-pig ileal smooth muscle, we examined effects of pretreatment with a specific inhibitor of Rho-kinase (Y-27632) on the LPA-induced contraction and MLC20 phosphorylation. In addition, we investigated whether LPA actually elicits an activation of Rho A by studying subcellular distribution of Rho A in unstimulated and stimulated smooth muscles by LPA. LPA induced a less intense, but sustained, contraction compared with ACh, and was accompanied by significant increases in MLC20 phosphorylation. The effects of LPA on tension and MLC20 phosphorylation were inhibited by Y-27632. The ACh-induced contraction, but not increases in MLC20 phosphorylation, was partially inhibited by Y-27632. High K+-induced contraction was unaffected by the inhibitor. LPA stimulated translocation of Rho A from the cytosol to the membrane fraction of the muscle. Translocation of Rho A was also induced by ACh and high K+. These results suggest that LPA-induced contraction of intact ileal smooth muscle is dominated through activation of Rho A and Rho-kinase and subsequent increases in MLC20 phosphorylation.

The yeast exchange assay, a new complementary method to screen for Dbl-like protein specificity: identification of a novel RhoA exchange factor

The target Rho GTPases of many guanine nucleotide exchange factors (GEFs) of the Dbl family remain to be identified. Here we report a new method: the yeast exchange assay (YEA), a rapid qualitative test to perform a wide range screen for GEF specificity. In this assay based on the two-hybrid system, a wild type GTPase binds to its effector only after activation by a specific GEF. We validated the YEA by activating GTPases by previously reported GEFs. We further established that a novel GEF, GEF337, activates RhoA in the YEA. GEF337 promoted nucleotide exchange on RhoA in vitro and promoted F-actin stress fiber assembly in fibroblasts, characteristic of RhoA activation.

Inhibition of RhoA by p120 catenin

RhoA organizes actin stress fibres and is necessary for cell transformation by oncogenes such as src and ras. Moreover, RhoA is implicated in cadherin clustering during the formation of adherens junctions. The catenin p120 has also been implicated in cadherin clustering through an unknown mechanism. Here we show that p120 selectively inhibits RhoA activity in vitro and in vivo. RhoA inhibition and the interaction of p120 with cadherins are mutually exclusive, suggesting a mechanism for regulating the recruitment and exchange of RhoA at nascent cell-cell contacts. By affecting RhoA activation, p120 could modulate cadherin functions, including suppression of invasion, neurite extension and junction formation.

Small guanosine triphosphatase Rho/Rho-associated kinase as a novel regulator of intracellular redistribution of lysosomes in invasive tumor cells

To investigate the role of RhoA on the intracellular membrane dynamics of lysosomes in rat hepatoma cells (MM1), we analyzed the localization of lysosomal aspartic proteinase cathepsin D by confocal immunofluorescence microscopy in the dominant active RhoA-transfected cells. Here we show that the transfection of the dominant active form of human small guanosine triphosphatase (GTPase) RhoA in MMI cells, a highly invasive cell line, causes the redistribution and spreading of small punctate structures stained for cathepsin D throughout the cytoplasm. We found that the microtubule organization was markedly different in the two cell lines: uniformly developed and polymerized microtubule filaments were seen in the mock transfectants; however, the dynamic organization of microtubules was less pronounced in the active RhoA transfectants. Furthermore, we found for the first time that a selective inhibitor of Rho-associated kinase (p160ROCK), Y-27632, impeded the subcellular spreading of cathepsin D staining and promoted reclustering of cathepsin D toward the perinuclear region in the active RhoA-transfected cells. To our knowledge, this is the first indication that the RhoA/ROCK-mediated signaling pathway is involved in the intracellular membrane dynamics of lysosomes by regulating the cytoskeletal microtubule organization as well as the actin cytoskeletons.

Nuclear signaling by Rac and Rho GTPases is required in the establishment of epithelial planar polarity in the Drosophila eye

BACKGROUND

The small GTPases Rac and Rho act as cellular switches in many important biological processes. In the fruit fly Drosophila, RhoA participates in the establishment of planar polarity, a process mediated by the receptor Frizzled (Fz). Thus far, analysis of Rac in this process has not been possible because of the absence of mutant Rac alleles. Here, we have investigated the role of Rac and Rho in establishing the polarity of ommatidia in the Drosophila eye.

RESULTS

By expressing a dominant negative or a constitutively activated form of Rac1, we interfered specifically with Rac signaling and disrupted ommatidial polarity. The resulting defects were similar to the loss/gain-of-function phenotypes typical of tissue-polarity genes. Through genetic interaction and rescue experiments involving a polarity-specific, loss-of-function dishevelled (dsh) allele, we found that Rac1 acts downstream of Dsh in the Fz signaling pathway, but upstream of, or in parallel to, RhoA. Rac signaled to the nucleus through the Jun N-terminal kinase (JNK) cascade in this process. By generating point mutations in the effector loop of RhoA, we found that RhoA also signals to the nucleus during the establishment of ommatidial polarity. Nevertheless, Rac and RhoA activated transcription of distinct target genes.

CONCLUSIONS

Rac is specifically required downstream of Dsh in the Fz pathway. It functions upstream or in parallel to RhoA and both signal to the nucleus, through distinct effectors, to establish planar polarity in the Drosophila eye.

Distinct roles of ROCK (Rho-kinase) and MLCK in spatial regulation of MLC phosphorylation for assembly of stress fibers and focal adhesions in 3T3 fibroblasts

ROCK (Rho-kinase), an effector molecule of RhoA, phosphorylates the myosin binding subunit (MBS) of myosin phosphatase and inhibits the phosphatase activity. This inhibition increases phosphorylation of myosin light chain (MLC) of myosin II, which is suggested to induce RhoA-mediated assembly of stress fibers and focal adhesions. ROCK is also known to directly phosphorylate MLC in vitro; however, the physiological significance of this MLC kinase activity is unknown. It is also not clear whether MLC phosphorylation alone is sufficient for the assembly of stress fibers and focal adhesions. We have developed two reagents with opposing effects on myosin phosphatase. One is an antibody against MBS that is able to inhibit myosin phosphatase activity. The other is a truncation mutant of MBS that constitutively activates myosin phosphatase. Through microinjection of these two reagents followed by immunofluorescence with a specific antibody against phosphorylated MLC, we have found that MLC phosphorylation is both necessary and sufficient for the assembly of stress fibers and focal adhesions in 3T3 fibroblasts. The assembly of stress fibers in the center of cells requires ROCK activity in addition to the inhibition of myosin phosphatase, suggesting that ROCK not only inhibits myosin phosphatase but also phosphorylates MLC directly in the center of cells. At the cell periphery, on the other hand, MLCK but not ROCK appears to be the kinase responsible for phosphorylating MLC. These results suggest that ROCK and MLCK play distinct roles in spatial regulation of MLC phosphorylation.

The role of Mg2+ cofactor in the guanine nucleotide exchange and GTP hydrolysis reactions of Rho family GTP-binding proteins

The biological activities of Rho family GTPases are controlled by their guanine nucleotide binding states in cells. Here we have investigated the role of Mg(2+) cofactor in the guanine nucleotide binding and hydrolysis processes of the Rho family members, Cdc42, Rac1, and RhoA. Differing from Ras and Rab proteins, which require Mg(2+) for GDP and GTP binding, the Rho GTPases bind the nucleotides in the presence or absence of Mg(2+) similarly, with dissociation constants in the submicromolar concentration. The presence of Mg(2+), however, resulted in a marked decrease in the intrinsic dissociation rates of the nucleotides. The catalytic activity of the guanine nucleotide exchange factors (GEFs) appeared to be negatively regulated by free Mg(2+), and GEF binding to Rho GTPase resulted in a 10-fold decrease in affinity for Mg(2+), suggesting that one role of GEF is to displace bound Mg(2+) from the Rho proteins. The GDP dissociation rates of the GTPases could be further stimulated by GEF upon removal of bound Mg(2+), indicating that the GEF-catalyzed nucleotide exchange involves a Mg(2+)-independent as well as a Mg(2+)-dependent mechanism. Although Mg(2+) is not absolutely required for GTP hydrolysis by the Rho GTPases, the divalent ion apparently participates in the GTPase reaction, since the intrinsic GTP hydrolysis rates were enhanced 4-10-fold upon binding to Mg(2+), and k(cat) values of the Rho GTPase-activating protein (RhoGAP)-catalyzed reactions were significantly increased when Mg(2+) was present. Furthermore, the p50RhoGAP specificity for Cdc42 was lost in the absence of Mg(2+) cofactor. These studies directly demonstrate a role of Mg(2+) in regulating the kinetics of nucleotide binding and hydrolysis and in the GEF- and GAP-catalyzed reactions of Rho family GTPases. The results suggest that GEF facilitates nucleotide exchange by destabilizing both bound nucleotide and Mg(2+), whereas RhoGAP utilizes the Mg(2+) cofactor to achieve high catalytic efficiency and specificity.

Activation of RhoA by thrombin in endothelial hyperpermeability: role of Rho kinase and protein tyrosine kinases

Endothelial cells (ECs) actively regulate the extravasation of blood constituents. On stimulation by vasoactive agents and thrombin, ECs change their cytoskeletal architecture and small gaps are formed between neighboring cells. These changes partly depend on a rise in [Ca(2+)](i) and activation of the Ca(2+)/calmodulin-dependent myosin light chain kinase. In this study, mechanisms that contribute to the thrombin-enhanced endothelial permeability were further investigated. We provide direct evidence that thrombin induces a rapid and transient activation of RhoA in human umbilical vein ECs. Under the same conditions, the activity of the related protein Rac was not affected. This was accompanied by an increase in myosin light chain phosphorylation, the generation of F-actin stress fibers, and a prolonged increase in endothelial permeability. Inhibition of the RhoA target Rho kinase with the specific inhibitor Y-27632 reduced all of these effects markedly. In the presence of Y-27632, the thrombin-enhanced permeability was additionally reduced by chelation of [Ca(2+)](i) by BAPTA. These data indicate that RhoA/Rho kinase and Ca(2+) represent 2 pathways that act on endothelial permeability. In addition, the protein tyrosine kinase inhibitor genistein reduced thrombin-induced endothelial permeability without affecting activation of RhoA by thrombin. Our data support a model of thrombin-induced endothelial permeability that is regulated by 3 cellular signal transduction pathways.

G13alpha-mediated PYK2 activation. PYK2 is a mediator of G13alpha -induced serum response element-dependent transcription

G(12)alpha/G(13)alpha transduces signals from G-protein-coupled receptors to stimulate growth-promoting pathways and the early response gene c-fos. Within the c-fos promoter lies a key regulatory site, the serum response element (SRE). Here we show a critical role for the tyrosine kinase PYK2 in muscarinic receptor type 1 and G(12)alpha/G(13)alpha signaling to an SRE reporter gene. A kinase-inactivate form of PYK2 (PYK2 KD) inhibits muscarinic receptor type 1 signaling to the SRE and PYK2 itself triggers SRE reporter gene activation through a RhoA-dependent pathway. Placing PYK2 downstream of G-protein activation but upstream of RhoA, the expression of PYK2 KD blocks the activation of an SRE reporter gene by GTPase-deficient forms of G(12)alpha or G(13)alpha but not by RhoA. The GTPase-deficient form of G(13)alpha triggers PYK2 kinase activity and PYK2 tyrosine phosphorylation, and co-expression of the RGS domain of p115 RhoGEF inhibits both responses. Finally, we show that in vivo G(13)alpha, although not G(12)alpha, readily associates with PYK2. Thus, G-protein-coupled receptors via G(13)alpha activation can use PYK2 to link to SRE-dependent gene expression.

G2A is an oncogenic G protein-coupled receptor

G2A is a heptahelical cell surface protein that has recently been described as a potential tumor suppressor, based on its ability to counteract transformation of pre-B cells and fibroblasts by Bcr-Abl, an oncogenic tyrosine kinase. We have isolated cDNAs encoding G2A in the course of screening libraries for clones that cause oncogenic transformation of NIH3T3 fibroblasts. When expressed at high levels in NIH3T3 cells by retroviral transduction, G2A induced a full range of phenotypes characteristic of oncogenic transformation, including loss of contact inhibition, anchorage-independent survival and proliferation, reduced dependence on serum, and tumorigenicity in mice. When expressed by transfection, G2A greatly enhanced the ability of a weakly oncogenic form of Raf-1 to transform NIH3T3 cells. These results demonstrate that G2A is potently oncogenic both on its own and in cooperation with another oncogene. Expression of G2A in fibroblasts and endothelial cells resulted in changes in cell morphology and cytoskeleton structure that were equivalent to those induced by the G protein subunit Galpha13. Transformation of NIH3T3 cells via G2A expression was completely suppressed by co-expression of LscRGS, a GTPase activating protein that suppresses signaling by Galpha12 and Galpha13. Hyperactivity of Galpha12 or Galpha13 has previously been shown to result in activation of Rho GTPases. G2A expression resulted in activation of Rho, and transformation via G2A was suppressed by a dominant negative form of RhoA. These results indicate that G2A may be directly coupled to Galpha13, and that it is the activation of this Rho-activating Galpha protein which is responsible for the ability of G2A to transform fibroblasts.