Rho guanine dissociation inhibitors: pivotal molecules in cellular signalling

The small G proteins of the Ras family act as bimodal relays in the transfer of intracellular signals. This is a dynamic phenomenon involving a cascade of protein-protein interactions modulated by chemical modifications, structural rearrangements and intracellular relocalisations. Most of the small G proteins could be operationally defined as proteins having two conformational states, each of which interacts with different cellular partners. These two states are determined by the nature of the bound nucleotide, GDP or GTP. This capacity to cycle between a GDP-bound conformation and a GTP-bound conformation enables them to filter, to amplify or to temporise the upstream signals that they receive. Thus the control of this cycle is crucial. Membrane anchoring of the proteins in the Ras family is a prerequisite for their activity. Most of the proteins in the Rho/Rac and Rab subfamilies of Ras proteins cycle between cytosol and membranes. Then the control of membrane association/dissociation is an other important regulation level. This review will describe one family of crucial regulators acting on proteins in the Rho/Rac family-the Rho guanine nucleotide dissociation inhibitors, or RhoGDIs. As yet, only three RhoGDIs have been described: RhoGDI-1, RhoGDI-2 (or D4/Ly-GDI) and RhoGDI-3. RhoGDI 1 and 2 are cytosolic and participate in the regulation of both the GDP/GTP cycle and the membrane association/dissociation cycle of Rho/Rac proteins. The non-cytosolic RhoGDI-3 seems to act in a slightly different way.

Characterization of a Rac1 signaling pathway to cyclin D(1) expression in airway smooth muscle cells

We examined the importance of the Rho family GTPase Rac1 for cyclin D(1) promoter transcriptional activation in bovine tracheal myocytes. Overexpression of active Rac1 induced transcription from the cyclin D(1) promoter, whereas platelet-derived growth factor (PDGF)-induced transcription was inhibited by a dominant-negative allele of Rac1, suggesting that Rac1 functions as an upstream activator of cyclin D(1) in this system. Rac1 forms part of the NADPH oxidase complex that generates reactive oxygen species such as H(2)O(2). PDGF stimulated a substantial increase in intracellular reactive oxygen species, as measured by the fluorescence of dichlorofluorescein-loaded cells, and this was blocked by the glutathione peroxidase mimetic ebselen. Pretreatment with ebselen, catalase, and the flavoprotein inhibitor diphenylene iodonium each attenuated PDGF- and Rac1-mediated cyclin D(1) promoter activation, while having no effect on the induction of cyclin D(1) by mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase-1 (MEK1), the upstream activator of ERKs. Antioxidant treatment also inhibited PDGF-induced cyclin D(1) protein expression and DNA synthesis. Overexpression of an N-terminal fragment of p67(phox), a component of NADPH oxidase which interacts with Rac1, attenuated PDGF-induced cyclin D(1) promoter activity, whereas overexpression of the wild-type p67 did not. Finally, Rac1 was neither required nor sufficient for ERK activation. Taken together, these data suggest a model by which two distinct signaling pathways, the ERK and Rac1 pathways, positively regulate cyclin D(1) and smooth muscle growth.

Polarized distribution of endogenous Rac1 and RhoA at the cell surface

Rac1 and RhoA regulate membrane ruffling and stress fiber formation. Both molecules appear to exert their control from the plasma membrane. In fibroblasts stimulated with platelet-derived growth factor or lysophosphatidic acid, the reorganization of the cytoskeleton begins at specific sites on the cell surface. We now report that endogenous Rac1 and RhoA also have a polarized distribution at the cell surface. Cell fractionation and immunogold labeling show that in quiescent fibroblasts both of these molecules are concentrated in caveolae, which are plasma membrane domains that are associated with actin-rich regions of the cell. Treatment of these cells with platelet-derived growth factor stimulated the recruitment of additional Rac1 and RhoA to caveolae fractions, while lysophosphatidic acid only caused the recruitment of RhoA. We could reconstitute the recruitment of RhoA using either whole cell lysates or purified caveolae. Surprisingly, pretreatment of the lysates with exoenzyme C3 shifted both resident and recruited RhoA from caveolae to noncaveolae membranes. The shift in location was not caused by inactivation of the RhoA effector domain. Moreover, chimeric proteins containing the C-terminal consensus site for Rac1 and RhoA prenylation were constitutively targeted to caveolae fractions. These results suggest that the polarized distribution of Rho family proteins at the cell surface involves an initial targeting of the protein to caveolae and a mechanism for retaining it at this site.

Gas6-mediated survival in NIH3T3 cells activates stress signalling cascade and is independent of Ras

Gas6 is a growth factor membrane of the vitamin K-dependent family of proteins which is preferentially expressed in quiescent cells. Gas6 was identified as the ligand for Axl tyrosine kinase receptor family. Consistent with this, Gas6 was previously reported to induce cell cycle re-entry of serum-starved NIH3T3 cells and to prevent cell death after complete growth factor withdrawal, the survival effect being uncoupled from Gas6-induced mitogenesis. We have previously demonstrated that both Gas6 mitogenic and survival effects are mediated by Src and the phosphatidylinositol3-OH kinase (PI3K). Here we report that Ras is required for Gas6 mitogenesis but is dispensable for its survival effect. Gas6-induced survival requires the activity of the small GTPases of the Rho family, Rac and Rho, together with the downstream kinase Pak. Overexpression of the respective dominant negative constructs abrogates Gas6-mediated survival functions. Addition of Gas6 to serum starved cells results in the activation of AKT/PKB and in the phosphorylation of the Bcl-2 family member, Bad. By ectopic expression of a catalytically inactive form of AKT/PKB, we demonstrate that AKT/PKB is necessary for Gas6-mediated survival functions. We further show evidence that Gas6 stimulation of serum starved NIH3T3 cells results in a transient ERK, JNK/SAPK and p38 MAPK activation. Blocking ERK activation did not influence Gas6-induced survival, suggesting that such pathway is not involved in Gas6 protection from cell death. On the contrary we found that the late constitutive increase of p38 MAPK activity associated with cell death was downregulated in Gas6-treated NIH3T3 cells thus suggesting that Gas6 might promote survival by interfering with this pathway. Taken together the evidence here provided identity elements involved in Gas6 signalling more specifically elucidating the pathway responsible for Gas6-induced cell survival under conditions that do not allow cell proliferation.

Tyrosine phosphorylation of Vav stimulates IL-6 production in mast cells by a Rac/c-Jun N-terminal kinase-dependent pathway

This study investigates whether the guanine nucleotide exchange activity of Vav is linked to cytokine production in mast cells. Overexpression of Vav in the RBL-2H3 mast cell line resulted in the constitutive tyrosine phosphorylation and activation of Vav. We analyzed the functional effect of Vav overexpression on cytokine production. IL-2 and IL-6 mRNA levels were dramatically increased in Vav-overexpressing cells and correlated with increased NF-AT activity. Little or no effect was observed on the mRNA levels of IL-3, IL-4, GM-CSF, TNF-alpha, and TGF-beta. FcepsilonRI engagement did not further enhance IL-2 and IL-6 mRNA levels and only slightly enhanced NF-AT activity, but dramatically increased the mRNA levels of other tested cytokines. To understand the signal transduction required, we focused primarily on IL-6 induction by measuring mitogen-activated protein kinase activity and analyzing the effects of mutant or dominant negative forms of Vav, Rac1, and c-Jun N-terminal kinase-1 (JNK1). Vav overexpression resulted in the constitutive activation of JNK1 with little or no effect on p38 mitogen-activated protein kinase and ERK2. This was dependent on Vav-mediated activation of Rac1 as a Dbl domain-mutated Vav, inactive Rac N17, and inactive JNK1 down-regulated the Vav-induced JNK1 or IL-6 responses. Vav expression, but not expression of domain-mutated Vav, increased IL-6 secretion from nonimmortalized bone marrow-derived mast cells upon FcepsilonRI engagement. We conclude that Vav phosphorylation contributes to IL-6 induction in mast cells.

Role of RhoA activation in the growth and morphology of a murine prostate tumor cell line

Prostate cancer cells derived from transgenic mice with adenocarcinoma of the prostate (TRAMP cells) were treated with the HMG-CoA reductase inhibitor, lovastatin. This caused inactivation of the small GTPase RhoA, actin stress fiber disassembly, cell rounding, growth arrest in the G1 phase of the cell cycle, cell detachment and apoptosis. Addition of geranylgeraniol (GGOL) in the presence of lovastatin, to stimulate protein geranylgeranylation, prevented lovastatin's effects. That is, RhoA was activated, actin stress fibers were assembled, the cells assumed a flat morphology and cell growth resumed. The following observations support an essential role for RhoA in TRAMP cell growth: (1) TRAMP cells expressing dominant-negative RhoA (T19N) mutant protein displayed few actin stress fibers and grew at a slower rate than controls (35 h doubling time for cells expressing RhoA (T19N) vs 20 h for untransfected cells); (2) TRAMP cells expressing constitutively active RhoA (Q63L) mutant protein displayed a contractile phenotype and grew faster than controls (13 h doubling time). Interestingly, addition of farnesol (FOL) with lovastatin, to stimulate protein farnesylation, prevented lovastatin-induced cell rounding, cell detachment and apoptosis, and stimulated cell spreading to a spindle shaped morphology. However, RhoA remained inactive and growth arrest persisted. The morphological effects of FOL addition were prevented in TRAMP cells expressing dominant-negative H-Ras (T17N) mutant protein. Thus, it appears that H-Ras is capable of inducing cell spreading, but incapable of supporting cell proliferation, in the absence of geranylgeranylated proteins like RhoA.

Receptor for advanced glycation end products (RAGE)-mediated neurite outgrowth and activation of NF-kappaB require the cytoplasmic domain of the receptor but different downstream signaling pathways

Receptor for advanced glycation end products (RAGE) mediates neurite outgrowth in vitro on amphoterin-coated substrates. Ligation of RAGE by two other ligands, advanced glycation end products or amyloid beta-peptide, is suggested to play a role in cell injury mechanisms involving cellular oxidant stress and activation of the transcription factor NF-kappaB. However, the RAGE signaling pathways in neurite outgrowth and cell injury are largely unknown. Here we show that transfection of RAGE to neuroblastoma cells induces extension of filopodia and neurites on amphoterin-coated substrates. Furthermore, ligation of RAGE in transfected cells enhances NF-kappaB-dependent transcription. Both the RAGE-mediated neurite outgrowth and activation of NF-kappaB are blocked by deletion of the cytoplasmic domain of RAGE. Moreover, dominant negative Rac and Cdc42 but not dominant negative Ras inhibit the extension of neurites induced by RAGE-amphoterin interaction. In contrast, the activation of NF-kappaB is inhibited by dominant negative Ras but not Rac or Cdc42. These data suggest that distinct signaling pathways are used by RAGE to induce neurite outgrowth and regulate gene expression through NF-kappaB.

Stimulation of a vascular smooth muscle cell NAD(P)H oxidase by thrombin. Evidence that p47(phox) may participate in forming this oxidase in vitro and in vivo

Thrombin is a potent vascular smooth muscle cell (VSMC) mitogen. Because recent evidence implicates reactive oxygen intermediates (ROI) in VSMC proliferation in general and atherogenesis in particular, we investigated whether ROI generation is necessary for thrombin-induced mitogenesis. Treatment of human aortic smooth muscle cells with thrombin increased DNA synthesis, an effect that was antagonized by diphenyleneiodonium but not by other inhibitors of cellular oxidase systems. This effect of thrombin was accompanied by increased O-2 and H2O2 generation and NADH/NADPH consumption. ROI generation in response to thrombin pretreatment could also be blocked by diphenyleneiodonium, suggesting that the NAD(P)H oxidase was necessary for ROI generation and thrombin-induced mitogenesis. Because of observed differences between the VSMC and neutrophil oxidase, we examined whether the cytosolic components of the phagocytic NAD(P)H oxidase were present in VSMC. p47(phox) and Rac2 were present in VSMC. Furthermore, thrombin increased expression of p47(phox) and Rac2 and stimulated their translocation to the cell membrane. We examined whether p47(phox) might be similarly regulated in vivo in a rat aorta balloon injury model and found that p47(phox) protein was increased after injury. Immunocytochemistry localized expression of p47(phox) to the neointima and media of injured arteries. Our data demonstrate that generation of O-2 and H2O2 is required for thrombin-mediated mitogenesis in VSMC and that p47(phox) is regulated by thrombin in vitro and is associated with vascular lesion formation in vivo.

The myotonic dystrophy kinase-related Cdc42-binding kinase is involved in the regulation of neurite outgrowth in PC12 cells

The myotonic dystrophy kinase-related Cdc42-binding kinase (MRCKalpha) has been implicated in the morphological activities of Cdc42 in nonneural cells. Both MRCKalpha and the kinase-related Rho-binding kinase (ROKalpha) are involved in nonmuscle myosin light-chain phosphorylation and associated actin cytoskeleton reorganization. We now show that in PC12 cells, overexpression of the kinase domain of MRCKalpha and ROKalpha resulted in retraction of neurites formed on nerve growth factor (NGF) treatment, as observed with RhoA. However, introduction of kinase-dead MRCKalpha did not result in NGF-independent neurite outgrowth as observed with dominant negative kinase-dead ROKalpha or the Rho inhibitor C3. Neurite outgrowth induced by NGF or kinase-dead ROKalpha was inhibited by dominant negative Cdc42(N17), Rac1(N17), and the Src homology 3 domain of c-Crk, indicating the participation of common downstream components. Neurite outgrowth induced by either agent was blocked by kinase-dead MRCKalpha lacking the p21-binding domain or by a minimal C-terminal regulatory region consisting of the cysteine-rich domain/pleckstrin homology domain plus a region with homology to citron. The latter region alone was an effective blocker of NGF-induced outgrowth. These results suggest that although ROKalpha is involved in neurite retraction promoted by RhoA, the related MRCKalpha is conversely involved in neurite outgrowth promoted by Cdc42 and Rac.

Involvement of Cdc42 small G protein in cell-cell adhesion, migration and morphology of MDCK cells

The Rho small G protein family consists of the Rho, Rac, and Cdc42 subfamilies and regulates various cell functions through reorganization of the actin cytoskeleton. We previously showed that the Rho subfamily regulates the formation of stress fibers and focal adhesions whereas the Rac subfamily regulates the E-cadherin-based cell-cell adhesion in MDCK cells. We studied here the function of the Cdc42 subfamily, consisting of two members, Cdc42Hs and G25k, in cell adhesion, migration, and morphology of MDCK cells. For this purpose, we made and used MDCK cell lines stably expressing each of dominant active mutants of Cdc42Hs (sMDCK-Cdc42HsDA) and G25K (sMDCK-G25KDA). Actin filaments at the cell-cell adhesion sites increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Electron microscopic analysis revealed that sMDCK-Cdc42HsDA cells tightly contacted with each other throughout the lateral membranes. Moreover, both the HGF- and TPA-induced disruption of the cadherin-based cell-cell adhesion and the subsequent cell migration were inhibited in both sMDCK-Cdc42HsDA and -G25KDA cells. Co-expression of the dominant negative mutant of Rac1, a member of the Rac subfamily, with the dominant active mutant of Cdc42Hs did not inhibit the increased accumulation of actin filaments at the cell-cell adhesion sites. These results suggest that the Cdc42 subfamily is involved in the cadherin-based cell-cell adhesion in a manner independent of the Rac subfamily. Furthermore, the cells were frequently enveloped by the large multinuclear cells in both sMDCK-Cdc42HsDA and -G25KDA cells. Video microscopic analysis revealed that the cells were engulfed by the large cells during cytokinesis.

Cellular functions of TC10, a Rho family GTPase: regulation of morphology, signal transduction and cell growth

The small Ras-related GTPase, TC10, has been classified on the basis of sequence homology to be a member of the Rho family. This family, which includes the Rho, Rac and CDC42 subfamilies, has been shown to regulate a variety of apparently diverse cellular processes such as actin cytoskeletal organization, mitogen-activated protein kinase (MAPK) cascades, cell cycle progression and transformation. In order to begin a study of TC10 biological function, we expressed wild type and various mutant forms of this protein in mammalian cells and investigated both the intracellular localization of the expressed proteins and their abilities to stimulate known Rho family-associated processes. Wild type TC10 was located predominantly in the cell membrane (apparently in the same regions as actin filaments), GTPase defective (75L) and GTP-binding defective (31N) mutants were located predominantly in cytoplasmic perinuclear regions, and a deletion mutant lacking the carboxyl terminal residues required for post-translational prenylation was located predominantly in the nucleus. The GTPase defective (constitutively active) TC10 mutant: (1) stimulated the formation of long filopodia; (2) activated c-Jun amino terminal kinase (JNK); (3) activated serum response factor (SRF)-dependent transcription; (4) activated NF-kappaB-dependent transcription; and (5) synergized with an activated Raf-kinase (Raf-CAAX) to transform NIH3T3 cells. In addition, wild type TC10 function is required for full H-Ras transforming potential. We demonstrate that an intact effector domain and carboxyl terminal prenylation signal are required for proper TC10 function and that TC10 signals to at least two separable downstream target pathways. In addition, TC10 interacted with the actin-binding and filament-forming protein, profilin, in both a two-hybrid cDNA library screen, and an in vitro binding assay. Taken together, these data support a classification of TC10 as a member of the Rho family, and in particular, suggest that TC10 functions to regulate cellular signaling to the actin cytoskeleton and processes associated with cell growth.

Plant cell growth and differentiation may involve GAP regulation of Rac activity

Two Rac GTPase cDNAs, LjRac1 and LjRac2, were identified in the legume Lotus japonicus. Two-hybrid screening with dominant-constitutive mutations in the two Rac GTPases target three plant cDNAs, LjRacGAP1, LjRacGAP2 and LjRacGAP3, that encode putative GTPase activating proteins of Rho-GTPase subfamily members. Employing Rac antiserum, purified recombinant LjRac GTPases and recombinant LjRacGAP1, for ligand overlay assays, in vitro GAP affinity assays and GTPase activation, we confirmed that eukaryote Rac/RacGAP interplay is conserved in plants. In this investigation we have developed some tools that can be used to characterize the role of enhanced LjRac2 expression in developing root nodules.

Pak functions downstream of Dock to regulate photoreceptor axon guidance in Drosophila

The SH2/SH3 adaptor protein Dock has been proposed to transduce signals from guidance receptors to the actin cytoskeleton in Drosophila photoreceptor (R cell) growth cones. Here, we demonstrate that Drosophila p21-activated kinase (Pak) is required in a Dock pathway regulating R cell axon guidance and targeting. Dock and Pak colocalize to R cell axons and growth cones, physically interact, and their loss-of-function phenotypes are indistinguishable. Normal patterns of R cell connectivity require Pak's kinase activity and binding sites for both Dock and Cdc42/Rac. A membrane-tethered form of Pak (Pak(myr) acts as a dominant gain-of-function protein. Retinal expression of Pak(myr) rescues the R cell connectivity phenotype in dock mutants. These data establish Pak as a critical regulator of axon guidance and a downstream effector of Dock in vivo.

Identification of the stef gene that encodes a novel guanine nucleotide exchange factor specific for Rac1

The Rho family GTPases are involved in a variety of cellular events by changing the organization of actin cytoskeletal networks in response to extracellular signals. However, it is not clearly known how their activities are spatially and temporally regulated. Here we report the identification of a novel guanine nucleotide exchange factor for Rac1, STEF, which is related in overall amino acid sequence and modular structure to mouse Tiam1 and Drosophila SIF proteins. STEF protein contains two pleckstrin homology domains, a PDZ domain and a Dbl homology domain. The in vitro assay showed that STEF protein specifically enhanced the dissociation of GDP from Rac1 but not that from either RhoA or Cdc42. Expression of a truncated STEF protein in culture cells induced membrane ruffling with altered actin localization, which implies that this protein also activates Rac1 in vivo. The stef transcript was observed in restricted parts of mice, including cartilaginous tissues and the cortical plate of the central nervous system during embryogenesis. These findings suggested that STEF protein participates in the control of cellular events in several developing tissues, possibly changing the actin cytoskeletal network by activating Rac1.

Phosphoinositide 3-kinase-dependent and -independent activation of the small GTPase Rac2 in human neutrophils

The small GTPase Rac participates in various cellular events such as cytoskeletal reorganization. It has remained, however, largely unknown about intracellular signaling pathways for Rac activation because of the lack of a simple and reliable assay to estimate the activation. Here we describe a novel method to detect the GTP-bound, active Rac in cells by pulling it down with the Rac-binding domain of the protein kinase PAK. Experiments using this method reveal that stimulation of human neutrophils with the Gi-coupled receptor agonists N-formyl-methionyl-leucyl-phenylalanine (fMLP) and leukotriene B4 (LTB4) leads to a rapid and transient increase in the GTP-bound state of Rac2, whereas phorbol myristate acetate (PMA) causes a slow but more sustained activation of Rac2. Pretreatment of cells with pertussis toxin results in defective activation of Rac2 in response to fMLP and LTB4, indicating that coupling of the receptors to Gi plays a crucial role in the activation. Furthermore, the phosphoinositide 3-kinase (PI3K) inhibitors wortmannin and LY294002 block Rac2 activation elicited by the receptor agonists, but not that by PMA. Thus the Gi-coupled receptors likely mediate Rac2 activation via PI3K, whereas PMA activates Rac2 in a PI3K-independent manner.

Interplay between Rac and Rho in the control of substrate contact dynamics

BACKGROUND

Substrate anchorage and cell locomotion entail the initiation and development of different classes of contact sites, which are associated with the different compartments of the actin cytoskeleton. The Rho-family GTPases are implicated in the signalling pathways that dictate contact initiation, maturation and turnover, but their individual roles in these processes remain to be defined.

RESULTS

We monitored the dynamics of peripheral, Rac-induced focal complexes in living cells in response to perturbations of Rac and Rho activity and myosin contractility. We show that focal complexes formed in response to Rac differentiated into focal contacts upon upregulation of Rho. Focal complexes were dissociated by inhibitors of myosin-II-dependent contractility but not by an inhibitor of Rho-kinase. The downregulation of Rac promoted the enlargement of focal contacts, whereas a block in the Rho pathway not only caused a dissolution of focal contacts but also stimulated membrane ruffling and formation of new focal complexes, which were associated with the advance of the cell front.

CONCLUSIONS

Rac functions to signal the creation of new substrate contacts at the cell front, which are associated with the induction of ruffling lamellipodia, whereas Rho serves in the maturation of existing contacts, with both contact types requiring contractility for their formation. The transition from a focal complex to a focal contact is associated with a switch to Rho-kinase dependence. Rac and Rho also influence the development of focal contacts and focal complexes, respectively, through mutually antagonistic pathways.

IpaC induces actin polymerization and filopodia formation during Shigella entry into epithelial cells

Shigella proteins that are targeted to host cells by a type III secretion apparatus are essential for reorganization of the cytoskeleton during cell invasion. We have developed a semi-permeabilized cell assay that tests the effects of bacterial proteins on the actin cytoskeleton. The Shigella IpaC protein was found to induce the formation of filopodial and lamellipodial extensions in these semi-permeabilized cells. Microinjection of IpaC into cells, or cellular expression of IpaC also led to the formation of filopodial structures. Monoclonal antibodies (mAbs) directed against the C-terminus of IpaC inhibited the IpaC-induced extensions, whereas an anti-N-terminal IpaC mAb stimulated extensive lamellae formation. Shigella induced foci of actin polymerization in the permeabilized cells and these were inhibited by anti-C-terminal IpaC mAbs. Consistent with a role for IpaC in Shigella-induced cytoskeletal rearrangements during entry, stable transfectants expressing IpaC challenged with Shigella showed increased bacterial internalization. IpaC-induced extensions were inhibited by a dominant-interfering form of Cdc42 or the Cdc42-binding domain of WASP, whereas a dominant-interfering form of Rac resulted in inhibition of lamellae formation. We conclude that IpaC leads to activation of Cdc42 which in turn, causes activation of Rac, both GTPases being required for Shigella entry.

Subcellular localization of RhoA and ezrin at membrane ruffles of human endothelial cells: differential role of collagen and fibronectin

Endothelial cells and the regulation of their migration are of prime importance in many physiological and pathological processes such as angiogenesis. RhoA, an important Rho family member known to trigger actin reorganization, has been shown to mediate the formation of focal adhesions and stress fibers in quiescent fibroblasts. However, recent studies have emphasized its functional diversity and its implication in migration or metastatic processes in different cell types other than fibroblasts. Its role in endothelial cells is little known. In this study, we were interested by analyzing in human endothelial cells the subcellular redistribution of endogenous RhoA and the reorganization of cytoskeletal actin induced by two important extracellular matrix proteins, collagen and fibronectin. This paper shows a translocation of RhoA and its association with cortical actin in focal contact domains at membrane ruffles and at lamellipodia of spread or migrating endothelial cells, in the absence of any soluble mitogen stimulation. Furthermore, RhoA was found colocalized with ezrin, a member of the ERM family proteins newly described as important membrane-actin cytoskeleton linkers, at early membrane ruffles of endothelial cells spread on collagen but not on fibronectin. The present study points out that extracellular matrix, depending on the nature of its components, may promote distinct assemblies of focal contact constitutive proteins and strongly suggests that endothelial RhoA, like Rac1, may be an important mediator of matrix signaling pathway regulating endothelial cell adhesiveness and motility, independently of growth factor stimulation.

Monocyte adhesion and spreading on human endothelial cells is dependent on Rho-regulated receptor clustering

The GTPase Rho is known to mediate the assembly of integrin-containing focal adhesions and actin stress fibers. Here, we investigate the role of Rho in regulating the distribution of the monocyte-binding receptors E-selectin, ICAM-1, and VCAM-1 in human endothelial cells. Inhibition of Rho activity with C3 transferase or N19RhoA, a dominant negative RhoA mutant, reduced the adhesion of monocytes to activated endothelial cells and inhibited their spreading. Similar effects were observed after pretreatment of endothelial cells with cytochalasin D. In contrast, dominant negative Rac and Cdc42 proteins did not affect monocyte adhesion or spreading. C3 transferase and cytochalasin D did not alter the expression levels of monocyte-binding receptors on endothelial cells, but did inhibit clustering of E-selectin, ICAM-1, and VCAM-1 on the cell surface induced by monocyte adhesion or cross-linking antibodies. Similarly, N19RhoA inhibited receptor clustering. Monocyte adhesion and receptor cross-linking induced stress fiber assembly, and inhibitors of myosin light chain kinase prevented this response but did not affect receptor clustering. Finally, receptor clusters colocalized with ezrin/moesin/ radixin proteins. These results suggest that Rho is required in endothelial cells for the assembly of stable adhesions with monocytes via the clustering of monocyte-binding receptors and their association with the actin cytoskeleton, independent of stress fiber formation.

Arachidonic acid induces the activation of the stress-activated protein kinase, membrane ruffling and H2O2 production via a small GTPase Rac1

Arachidonic acid (AA) is generated via Rac-mediated phospholipase A2 (PLA2) activation in response to growth factors and cytokines and is implicated in cell growth and gene expression. In this study, we show that AA activates the stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in a time- and dose-dependent manner. Indomethacin and nordihydroguaiaretic acid, potent inhibitors of cyclooxygenase and lipoxygenase, respectively, did not exert inhibitory effects on AA-induced SAPK/JNK activation, thereby indicating that AA itself could activate SAPK/JNK. As Rac mediates SAPK/JNK activation in response to a variety of stressful stimuli, we examined whether the activation of SAPK/JNK by AA is mediated by Rac1. We observed that AA-induced SAPK/JNK activation was significantly inhibited in Rat2-Rac1N17 dominant-negative mutant cells. Furthermore, treatment of AA induced membrane ruffling and production of hydrogen peroxide, which could be prevented by Rac1N17. These results suggest that AA acts as an upstream signal molecule of Rac, whose activation leads to SAPK/JNK activation, membrane ruffling and hydrogen peroxide production.

Platelet-derived growth factor stimulation of mitogen-activated protein kinases and cyclin D1 promoter activity in cultured airway smooth-muscle cells. Role of Ras

We hypothesized that in bovine tracheal myocytes, growth factor treatment induces transcription from the cyclin D1 promoter that is dependent on the activation of both Ras and extracellular signal-related kinase (ERK). We found that platelet-derived growth factor (PDGF) treatment induced substantial activation of ERK2 that was blocked by expression of a dominant-negative Ha-Ras. Further, expression of a constitutively active Ha-Ras induced substantial ERK2 activity, consistent with the notion that Ras is required and sufficient for ERK activation. PDGF treatment induced only modest activation of the Jun amino terminal kinase-1 (JNK1) and p38 mitogen-activated protein kinases (MAPKs). Active Ras induced similar responses, implying that complete activation of the JNK and p38 pathways requires additional or alternative upstream signaling intermediates besides Ras. In contrast, expression of a constitutively active Rac1, an alternative guanosine triphosphatase involved in intracellular signaling, produced a high level of JNK1 activation, suggesting that Rac1 is an important upstream activator of JNK in this system. Active Ras and MAPK/ ERK kinase-1 (MEK1) (the upstream activator of ERK) each induced cyclin D1 promoter activity, whereas active stress-activated protein kinase/ERK kinase-1 (SEK1), an upstream activator of JNK, did not. Finally, the synthetic MEK inhibitor PD98059 blocked Ras-induced cyclin D1 promoter activity. Together, these data suggest that in bovine tracheal myocytes: (1) activation of MAPK by PDGF is dependent on Ras; (2) active Ras is sufficient for ERK activation but is insufficient for maximal activation of JNK or p38; (3) activation of Rac1 is sufficient for maximal JNK activation; and (4) Ras, MEK, and ERK constitute a distinct pathway to cyclin D1 transcriptional activation.

The IL-1 receptor and Rho directly associate to drive cell activation in inflammation

IL-1-stimulated mesenchymal cells model molecular mechanisms of inflammation. Binding of IL-1 to the type I IL-1 receptor (IL-1R) clusters a multi-subunit signaling complex at focal adhesion complexes. Since Rho family GTPases coordinately organize actin cytoskeleton and signaling to regulate cell phenotype, we hypothesized that the IL-1R signaling complex contained these G proteins. IL-1 stimulated actin stress fiber formation in serum-starved HeLa cells in a Rho-dependent manner and rapidly activated nucleotide exchange on RhoA. Glutathione S-transferase (GST) fusion proteins, containing either the full-length IL-1R cytosolic domain (GST-IL-1Rcd) or the terminal 68 amino acids of IL-1R required for IL-1-dependent signal transduction, specifically coprecipitated both RhoA and Rac-1, but not p21(ras), from Triton-soluble HeLa cell extracts. In whole cells, a small-molecular-weight G protein coimmunoprecipitated by anti-IL-1R antibody was a substrate for C3 transferase, which specifically ADP-ribosylates Rho GTPases. Constitutively activated RhoA, loaded with [gamma-32P]GTP, directly interacted with GST-IL-1Rcd in a filter-binding assay. The IL-1Rcd-RhoA interaction was functionally important, since a dominant inhibitory mutant of RhoA prevented IL-1Rcd-directed transcriptional activation of the IL-6 gene. Consistent with our previous data demonstrating that IL-1R-associated myelin basic protein (MBP) kinases are necessary for IL-1-directed gene expression, cellular incorporation of C3 transferase inhibited IL-1R-associated MBP kinase activity both in solution and in gel kinase assays. In summary, IL-1 activated RhoA, which was physically associated with IL-1Rcd and necessary for activation of cytosolic nuclear signaling pathways. These findings suggest that IL-1-stimulated, Rho-dependent cytoskeletal reorganization may cluster signaling molecules in specific architectures that are necessary for persistent cell activation in chronic inflammatory disease.

Rho GTPases are over-expressed in human tumors

Small GTPases of the Rho family are involved in the regulation of a variety of cellular processes, such as the organization of the microfilamental network, cell-cell contact and malignant transformation. To address the question of whether Rho proteins are involved in carcinogenesis in man, we compared their expression in tumors from colon, breast and lung with that of the corresponding normal tissue originating from the same patient. As shown by Rho-specific 32P-ADP-ribosylation, as well as Western-blot analysis, the amount of RhoA protein was largely increased in all 3 types of tumors tested. The most dramatic differences in the expression of Rho GTPases were observed in breast tissue. All breast tumors analyzed showed high levels of RhoA, Rac and Cdc42 proteins, whereas in the corresponding normal tissue these Rho proteins were hardly or not detectable. Progression of breast tumors from WHO grade I to grade III was accompanied by a significant average increase in RhoA protein. Overall, increase in the amount of Rho GTPases, in particular RhoA, appears to be a frequent event in different types of human tumors. This supports the view that Rho GTPases are involved in human carcinogenesis.

Activation of the leukocyte NADPH oxidase by protein kinase C in a partially recombinant cell-free system

The leukocyte NADPH oxidase is an enzyme present in phagocytes and B lymphocytes that when activated catalyzes the production of O-2 from oxygen at the expense of NADPH. A correlation between the activation of the oxidase and the phosphorylation of p47(PHOX), a cytosolic oxidase component, is well recognized in whole cells, and direct evidence for a relationship between the phosphorylation of this oxidase component and the activation of the oxidase has been obtained in a number of cell-free systems containing neutrophil membrane and cytosol. Using superoxide dismutase-inhibitable cytochrome c reduction to quantify O-2 production, we now show that p47(PHOX) phosphorylated by protein kinase C activates the NADPH oxidase not only in a cell-free system containing neutrophil membrane and cytosol, but also in a system in which the cytosol is replaced by the recombinant proteins p67(PHOX), Rac2, and phosphorylated p47(PHOX), suggesting that neutrophil plasma membrane plus those three cytosolic proteins are both necessary and sufficient for oxidase activation. In both the cytosol-containing and recombinant cell-free systems, however, activation by SDS yielded greater rates of O-2 production than activation by protein kinase C-phosphorylated p47(PHOX), indicating that a system that employs protein kinase C-phosphorylated p47(PHOX) as the sole activating agent, although more physiological than the SDS-activated system, is nevertheless incomplete.

The interaction between rac1 and its guanine nucleotide dissociation inhibitor (GDI), monitored by a single fluorescent coumarin attached to GDI

The interaction of rac with guanine nucleotide dissociation inhibitor protein (rhoGDI) is described, using GDI fluorescently labeled on its single cysteine with N-[2-(1-maleimidyl)ethyl]-7-diethylaminocoumarin-3-carboxamide (MDCC). The labeled GDI shows a 70% decrease in fluorescence emission on binding geranylgeranylated rac1.GDP and has an affinity for rac1 within a factor of 2 of the unlabeled GDI. The labeled GDI was used to determine the kinetic mechanism of the interaction by measuring the association and dissociation in real time. The kinetics are interpreted in terms of a two-step mechanism: binding of rac to GDI and then a conformational change of the complex with an overall dissociation constant of 0.4 nM. The conformational change has a rate constant of 7.3 s-1 (pH 7.5, 30 degrees C), and the reverse has a rate constant of 1.4 x 10(-)3 s-1. To overcome difficulties inherent in using and manipulating lipid-modified rac, we also used a combination of unmodified rac1, expressed in Escherichia coli and produced with C-terminal truncation (thus lacking the cysteine that is the site of lipid attachment), and farnesylated C-terminal peptide. This combination can mimic geranylgeranylated rac1, producing a complex with the coumarin-labeled GDI, and was used to examine the relative importance of different regions of rac1 in interaction with GDI.