Cell motility mediated by rho and Rho-associated protein kinase plays a critical role in intrahepatic metastasis of human hepatocellular carcinoma

Human hepatocellular carcinoma (HCC) can invade the portal vein and metastasizes to other parts of the liver even at a relatively early stage of the disease, with less tumor spread occurring outside the liver. This intrahepatic metastasis is the main cause of liver failure and death in HCC patients. To analyze the mechanisms of intrahepatic metastasis we have constructed metastatic models using orthotopic implantation of human HCC cell lines. Five HCC cell lines formed liver tumors after injection into the livers of SCID mice, and of those 5 cell lines, Li7 and KYN-2 cells also resulted in vascular tumor thrombi and intrahepatic metastasis. These 2 cell lines had markedly higher cell motilities than the other 3 cell lines in vitro. Their motilities appeared to be Rho-mediated; serum and lysophosphatidic acid (LPA) evoked actin reorganization and motility of Li7 cells, and C3 exoenzyme exposure reduced the motility of both serum-stimulated Li7 cells and KYN-2 cells. Dominant negative and active forms of p160 Rho-associated coiled-coil forming protein kinase (p160ROCK), one of the downstream effectors of Rho, were separately and stably introduced into Li7 cells. Dominant active p160ROCK transfectants showed increased motility that was independent of serum and LPA, and dominant negative p160ROCK transfectants showed reduced motility under stimulation. Furthermore, implantation of dominant negative p160ROCK transfectants resulted in a reduced metastatic rate in vivo compared with the parent cells or a control transfectant. These findings indicate that cell motility mediated by the Rho/p160ROCK signaling pathway plays a critical role in intrahepatic metastasis of human HCC.

Dichotomous regulation of myosin phosphorylation and shape change by Rho-kinase and calcium in intact human platelets

Both Rho-kinase and the Ca(2+)/calmodulin-dependent myosin light chain (MLC) kinase increase the phosphorylation of MLC. We show that upon thrombin receptor stimulation by low-dose thrombin or the peptide ligand YFLLRNP, or upon thromboxane receptor activation by U46619, shape change and MLC phosphorylation in human platelets proceed through a pathway that does not involve an increase in cytosolic Ca(2+). Under these conditions, Y-27632, a specific Rho-kinase inhibitor, prevented shape change and reduced the stimulation of MLC-phosphorylation. In contrast, Y-27632 barely affected shape change and MLC-phosphorylation by adenosine diphosphate (ADP), collagen-related peptide, and ionomycin that were associated with an increase in cytosolic Ca(2+) and inhibited by BAPTA-AM/EGTA treatment. Furthermore, C3 exoenzyme, which inactivates Rho, inhibited preferentially the shape change induced by YFLLRNP compared with ADP and ionomycin. The results indicate that the Rho/Rho-kinase pathway is pivotal in mediating the MLC phosphorylation and platelet shape change by low concentrations of certain G protein-coupled platelet receptors, independent of an increase in cytosolic Ca(2+). Our study defines 2 alternate pathways, Rho/Rho-kinase and Ca(2+)/calmodulin-regulated MLC-kinase, that lead independently of each other through stimulation of MLC-phosphorylation to the same physiological response in human platelets (ie, shape change).

Signaling from Rho to the actin cytoskeleton through protein kinases ROCK and LIM-kinase

The actin cytoskeleton undergoes extensive remodeling during cell morphogenesis and motility. The small guanosine triphosphatase Rho regulates such remodeling, but the underlying mechanisms of this regulation remain unclear. Cofilin exhibits actin-depolymerizing activity that is inhibited as a result of its phosphorylation by LIM-kinase. Cofilin was phosphorylated in N1E-115 neuroblastoma cells during lysophosphatidic acid-induced, Rho-mediated neurite retraction. This phosphorylation was sensitive to Y-27632, a specific inhibitor of the Rho-associated kinase ROCK. ROCK, which is a downstream effector of Rho, did not phosphorylate cofilin directly but phosphorylated LIM-kinase, which in turn was activated to phosphorylate cofilin. Overexpression of LIM-kinase in HeLa cells induced the formation of actin stress fibers in a Y-27632-sensitive manner. These results indicate that phosphorylation of LIM-kinase by ROCK and consequently increased phosphorylation of cofilin by LIM-kinase contribute to Rho-induced reorganization of the actin cytoskeleton.

Regulation of Rho protein binding to membranes by rhoGDI: inhibition of releasing activity by physiological ionic conditions

The Rho GDP dissociation inhibitor (GDI) is an ubiquitously expressed regulatory protein involved in the cycling of Rho proteins between membrane-bound and soluble forms. Here, we characterized the Rho solubilization activity of a glutathione S-transferase (GST) - GDI fusion protein in a cell-free system derived from rat kidney. Addition of GST-GDI to kidney brush border membranes resulted in the specific release of Cdc42 and RhoA from the membranes, while RhoB and Ras were not extracted. The release of Cdc42 and RhoA by GST-GDI was dose dependent and saturable with about 50% of both RhoA and Cdc42 extracted. The unextracted Rho proteins were tightly bound to membranes and could not be solubilized by repeated GST-GDI treatment. These results demonstrated that kidney brush border membranes contained two populations of RhoA and Cdc42. Furthermore, the GST-GDI solubilizing activity on membrane-bound Cdc42 and RhoA was abolished at physiological conditions of salt and temperature in all tissues examined. When using bead-immobilized GST-GDI, KCl did not reduced the binding of Rho proteins. However, washing brush border membranes with KCl prior treatment by GST-GDI inhibited the extraction of Rho proteins. Taken together, these results suggest that the binding of GDI to membrane-bound Cdc42 and RhoA occurs easily under physiological ionic strength conditions, but a complementary factor is required to extract these proteins from membranes. These observations suggest that the shuttling activity of GDI upon Rho proteins could be normally downregulated under physiological conditions.

Calpain mediates integrin-induced signaling at a point upstream of Rho family members

Integrin-induced adhesion leads to cytoskeletal reorganizations, cell migration, spreading, proliferation, and differentiation. The details of the signaling events that induce these changes in cell behavior are not well understood but they appear to involve activation of Rho family members which activate signaling molecules such as tyrosine kinases, serine/threonine kinases, and lipid kinases. The result is the formation of focal complexes, focal adhesions, and bundles and networks of actin filaments that allow the cell to spread. The present study shows that mu-calpain is active in adherent cells, that it cleaves proteins known to be present in focal complexes and focal adhesions, and that overexpression of mu-calpain increased the cleavage of these proteins, induced an overspread morphology and induced an increased number of stress fibers and focal adhesions. Inhibition of calpain with membrane permeable inhibitors or by expression of a dominant negative form of mu-calpain resulted in an inability of cells to spread or to form focal adhesions, actin filament networks, or stress fibers. Cells expressing constitutively active Rac1 could still form focal complexes and actin filament networks (but not focal adhesions or stress fibers) in the presence of calpain inhibitors; cells expressing constitutively active RhoA could form focal adhesions and stress fibers. Taken together, these data indicate that calpain plays an important role in regulating the formation of focal adhesions and Rac- and Rho-induced cytoskeletal reorganizations and that it does so by acting at sites upstream of both Rac1 and RhoA.

Opposite regulation of transepithelial electrical resistance and paracellular permeability by Rho in Madin-Darby canine kidney cells

Small GTPase Rho has been thought to be important for the formation and the maintenance of tight junction in epithelial cells, but the role of Rho in the regulation of barrier function of tight junction is not well understood. We here examined whether Rho was involved in the barrier function of tight junction in Madin-Darby canine kidney (MDCK) cells. The activation of prostaglandin EP3beta receptor, coupled to a Rho activation pathway, induced the increase in transepithelial electrical resistance (TER) but the increase in paracellular flux of mannitol in the preformed monolayer of the MDCK cells expressing the EP3beta receptor. This effect of the EP3 receptor was mimicked by the expression of constitutively active RhoA but not by active Rac1 in MDCK cells, using an isopropyl-beta-D-thiogalactoside-inducible expression system. On the other hand, the activation of EP3beta receptor suppressed the elevation of TER and the decrease in paracellular mannitol flux during Ca(2+) switch-induced tight junction formation, whereas the expression of active RhoA or Rac1 did not apparently affect the TER development in the Ca(2+) switch. These results demonstrate that the EP3 receptor and active RhoA regulate permeabilities of ionic and nonionic molecules in opposite directions in the preformed monolayer, and the EP3 receptor suppresses the elevation of TER during the tight junction formation.

Differential expression of the small GTP-binding proteins RhoA, RhoB, Cdc42u and Cdc42b in developing rat neocortex

Studies with cultured cells indicate that small GTPases of the Rho family may be involved in cell proliferation, differentiation, as well as migration. Therefore, we have studied the expression of four members of this protein family, i.e., RhoA, RhoB, the ubiquitous Cdc42u, and brain specific Cdc42b, during the embryonic and early postnatal development of rat neocortex. A clear isoform specificity of expression was found during the prenatal development. Thus, RhoA and Cdc42u were present in the proliferation zone while RhoB and Cdc42b were expressed only in the cortical plate where neural cells settle and differentiate. After birth, this isoform specificity quickly disappeared so that already at postnatal day 8 the adult pattern of expression was present. Our findings of a differential expression of the small GTP-binding proteins RhoA, RhoB, Cdc42u and Cdc42b in developing brain neocortex suggest isoform specific functions during neurogenesis and differentiation.

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.

Cell and tissue-type specific expression of Ras-related GTPase RhoB

Ras-homologous (Rho) GTPases are involved in the regulation of a variety of cellular processes such as the organization of the actin cytoskeleton, malignant transformation and genotoxic stress-induced signaling. Here we show that, among the family of Rho GTPases, specifically rhoB mRNA expression is rapidly induced upon UV-irradiation, whereas the level of rac 1 and cdc42 mRNA is not affected. Increase in rhoB mRNA was accompanied by a approximately 4-fold increase in the amount of membrane-bound RhoB protein. Basal expression of rhoB mRNA appears to be cell-type specific with low amounts in rodent NIH 3T3, V79, H4IIE, CHO and human HaCat cells and comparably high amounts in monkey COS, human HeLa and HepG2 cells. In rabbit tissues, exceptionally high levels of rhoB mRNA and RhoB protein were found in lung whereas its expression was quite low in heart, liver, spleen and kidney. Variations in rhoB mRNA expression level are not due to cell-type specific differences in rhoB mRNA stability as shown by inhibitor experiments. However, transiently transfected rhoB promoter CAT construct was expressed at significantly higher level in HeLa and HepG2 as compared to NIH 3T3 and CHO cells. Thus, cell-type specific differences in the level of rhoB mRNA are likely to be due to variations in the transcriptional activity of the rhoB gene. The data indicate that, among the family of Rho GTPases, only the expression of rhoB is rapidly stimulated by genotoxic stress. Furthermore basal rhoB expression appears to be regulated in a cell and tissue-type specific manner. This may be related to yet unknown tissue-specific physiological function of RhoB.

Cleavage and nuclear translocation of the caspase 3 substrate Rho GDP-dissociation inhibitor, D4-GDI, during apoptosis

While investigating endonucleases potentially involved in apoptosis, an antisera was raised to bovine deoxyribonuclease II, but it recognized a smaller protein of 26 kDa protein in a variety of cell lines. The 26 kDa protein underwent proteolytic cleavage to 22 kDa concomitantly with DNA digestion in cells induced to undergo apoptosis. Sequencing of the 26 kDa protein identified it as the Rho GDP-dissociation inhibitor D4-GDI. Zinc, okadaic acid, calyculin A, cantharidin, and the caspase inhibitor z-VAD-fmk, all prevented the cleavage of D4-GDI, DNA digestion, and apoptosis. The 26 kDa protein resided in the cytoplasm of undamaged cells, whereas following cleavage, the 22 kDa form translocated to the nucleus. Human D4-GDI, and D4-GDI mutated at the caspase 1 or caspase 3 sites, were expressed in Chinese hamster ovary cells which show no detectable endogenous D4-GDI. Mutation at the caspase 3 site prevented D4-GDI cleavage but did not inhibit apoptosis induced by staurosporine. The cleavage of D4-GDI could lead to activation of Jun N-terminal kinase which has been implicated as an upstream regulator of apoptosis in some systems. However, the results show that the cleavage of D4-GDI and translocation to the nucleus do not impact on the demise of the cell.

Involvement of RhoA and its interaction with protein kinase C and Src in CCK-stimulated pancreatic acini

We evaluated intracellular pathways responsible for the activation of the small GTP-binding protein Rho p21 in rat pancreatic acini. Intact acini were incubated with or without CCK and carbachol, and Triton X-100-soluble and crude microsomes were used for Western immunoblotting. When a RhoA-specific antibody was used, a single band at the location of 21 kDa was detected. CCK (10 pM-10 nM) and carbachol (0.1-100 microM) dose dependently increased the amount of immunodetectable RhoA with a peak increase occurring at 3 min. High-affinity CCK-A-receptor agonists JMV-180 and CCK-OPE (1-1,000 nM) did not increase the intensities of the RhoA band, suggesting that stimulation of RhoA is mediated by the low-affinity CCK-A receptor. Although an increase in RhoA did not require the presence of extracellular Ca2+, the intracellular Ca2+ chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM abolished the appearance of the RhoA band in response to CCK and carbachol. The Gq protein inhibitor G protein antagonist-2A (10 microM) and the phospholipase C (PLC) inhibitor U-73122 (10 microM) markedly reduced RhoA bands in response to CCK. The protein kinase C (PKC) activator phorbol ester (10-1,000 nM) dose dependently increased the intensities of the RhoA band, which were inhibited by the PKC inhibitor K-252a (1 microM). The pp60(c-src) inhibitor herbimycin A (6 microM) inhibited the RhoA band in response to CCK, whereas the calmodulin inhibitor W-7 (100 microM) and the phosphoinositide 3-kinase inhibitor wortmannin (6 microM) had no effect. RhoA was immunoprecipitated with Src, suggesting association of RhoA with Src. Increases in mass of this complex were observed with CCK stimulation. In permeabilized acini, the Rho inhibitor Clostridium botulinum C3 exoenzyme dose dependently inhibited amylase secretion evoked by a Ca2+ concentration with an IC50 of C3 exoenzyme at 1 ng/ml. We concluded that the small GTP-binding protein RhoA p21 exists in pancreatic acini and appears to be involved in the mediation of pancreatic enzyme secretion evoked by CCK and carbachol. RhoA pathways are involved in the activation of PKC and Src cascades via Gq protein and PLC.

Geranylgeranylpyrophosphate plays a key role for the G1 to S transition in vascular smooth muscle cells

Pravastatin, a HMG-CoA reductase inhibitor was found to inhibit DNA synthesis of vascular smooth muscle cells (VSMC) in a dose-dependent manner. Flow cytometric analysis demonstrated that pravastatin induced G1 arrest. Mevalonate restored the inhibitory effect of pravastatin on DNA synthesis and on cell cycle progression, suggesting the importance of mevalonate itself and/or its metabolites in VSMC proliferation. The major intermediate metabolites of mevalonate, geranylgeranyl-pyrophosphate (GGPP), farnesyl pyrophosphate (FPP) and IPP (isopentenyl pyrophosphate) were prepared in the form of liposomes, and the effects of GGPP, FPP and IPP on pravastatin induced inhibition of VSMC proliferation and G1 arrest were examined. Only GGPP restored the pravastatin-induced inhibition of DNA synthesis and G1 arrest. Pravastatin inhibited translocation of Rho small GTPase from cytosol to membrane. By the addition of GGPP, Rho small GTPase are geranylgeranylated and translocated to membranes during G1/S transition. These data suggest that GGPP, rather than FPP or IPP, is an essential metabolite among mevalonic acid metabolites for VSMC proliferation and the G1/S transition.

Syndecan-4 signals cooperatively with integrins in a Rho-dependent manner in the assembly of focal adhesions and actin stress fibers

The assembly of focal adhesions and actin stress fibers by cells plated on fibronectin depends on adhesion-mediated signals involving both integrins and cell-surface heparan sulfate proteoglycans. These two cell-surface receptors interact with different domains of fibronectin. To attempt to identify the heparan sulfate proteoglycans involved, we used fibronectin-null (FN-/-) mouse fibroblasts to eliminate the contribution of endogenous fibronectin during the analysis. FN-/- fibroblasts plated on the cell-binding domain of fibronectin or on antibodies directed against mouse beta1 integrin chains attach but fail to spread and do not form focal adhesions or actin stress fibers. When such cells are treated with antibodies directed against the ectodomain of mouse syndecan-4, they spread fully and assemble focal adhesions and actin stress fibers indistinguishable from those seen in cells plated on intact fibronectin. These results identify syndecan-4 as a heparan sulfate proteoglycan involved in the assembly process. The antibody-stimulated assembly of focal adhesions and actin stress fibers in cells plated on the cell-binding domain of fibronectin can be blocked with C3 exotransferase, an inhibitor of the small GTP-binding protein Rho. Treatment of cells with lysophosphatidic acid, which activates Rho, results in full spreading and assembly of focal adhesions and actin stress fibers in fibroblasts plated on the cell-binding domain of fibronectin. We conclude that syndecan-4 and integrins can act cooperatively in generating signals for cell spreading and for the assembly of focal adhesions and actin stress fibers. We conclude further that these joint signals are regulated in a Rho-dependent manner.

Lymphocyte migration through brain endothelial cell monolayers involves signaling through endothelial ICAM-1 via a rho-dependent pathway

Lymphocyte extravasation into the brain is mediated largely by the Ig superfamily molecule ICAM-1. Several lines of evidence indicate that at the tight vascular barriers of the central nervous system (CNS), endothelial cell (EC) ICAM-1 not only acts as a docking molecule for circulating lymphocytes, but is also involved in transducing signals to the EC. In this paper, we examine the signaling pathways in brain EC following Ab ligation of endothelial ICAM-1, which mimics adhesion of lymphocytes to CNS endothelia. ICAM-1 cross-linking results in a reorganization of the endothelial actin cytoskeleton to form stress fibers and activation of the small guanosine triphosphate (GTP)-binding protein Rho. ICAM-1-stimulated tyrosine phosphorylation of the actin-associated molecule cortactin and ICAM-1-mediated, Ag/IL-2-stimulated T lymphocyte migration through EC monolayers were inhibited following pretreatment of EC with cytochalasin D. Pretreatment of EC with C3 transferase, a specific inhibitor of Rho proteins, significantly inhibited the transmonolayer migration of T lymphocytes, endothelial Rho-GTP loading, and endothelial actin reorganization, without affecting either lymphocyte adhesion to EC or cortactin phosphorylation. These data show that brain vascular EC are actively involved in facilitating T lymphocyte migration through the tight blood-brain barrier of the CNS and that this process involves ICAM-1-stimulated rearrangement of the endothelial actin cytoskeleton and functional EC Rho proteins.

Cell growth inhibition by farnesyltransferase inhibitors is mediated by gain of geranylgeranylated RhoB

Recent results have shown that the ability of farnesyltransferase inhibitors (FTIs) to inhibit malignant cell transformation and Ras prenylation can be separated. We proposed previously that farnesylated Rho proteins are important targets for alternation by FTIs, based on studies of RhoB (the FTI-Rho hypothesis). Cells treated with FTIs exhibit a loss of farnesylated RhoB but a gain of geranylgeranylated RhoB (RhoB-GG), which is associated with loss of growth-promoting activity. In this study, we tested whether the gain of RhoB-GG elicited by FTI treatment was sufficient to mediate FTI-induced cell growth inhibition. In support of this hypothesis, when expressed in Ras-transformed cells RhoB-GG induced phenotypic reversion, cell growth inhibition, and activation of the cell cycle kinase inhibitor p21WAF1. RhoB-GG did not affect the phenotype or growth of normal cells. These effects were similar to FTI treatment insofar as they were all induced in transformed cells but not in normal cells. RhoB-GG did not promote anoikis of Ras-transformed cells, implying that this response to FTIs involves loss-of-function effects. Our findings corroborate the FTI-Rho hypothesis and demonstrate that gain-of-function effects on Rho are part of the drug mechanism. Gain of RhoB-GG may explain how FTIs inhibit the growth of human tumor cells that lack Ras mutations.

Functional design in the actin cytoskeleton

Changes in cell shape, anchorage and motility are all associated with the dynamic reorganisation of the architectural arrays of actin filaments that make up the actin cytoskeleton. The relative expression of these functionally different actin filament arrays is intimately linked to the pattern of contacts that a cell develops with its extracellular substrate. Cell polarity is acquired by the development of an asymmetric pattern of substrate contacts, effected in a specific, site-directed manner by the delivery of adhesion-site modulators along microtubules.

Regulation of the small GTP-binding protein Rho by cell adhesion and the cytoskeleton

Soluble factors from serum such as lysophosphatidic acid (LPA) are thought to activate the small GTP-binding protein Rho based on their ability to induce actin stress fibers and focal adhesions in a Rho-dependent manner. Cell adhesion to extracellular matrices (ECM) has also been proposed to activate Rho, but this point has been controversial due to the difficulty of distinguishing changes in Rho activity from the structural contributions of ECM to the formation of focal adhesions. To address these questions, we established an assay for GTP-bound cellular Rho. Plating Swiss 3T3 cells on fibronectin-coated dishes elicited a transient inhibition of Rho, followed by a phase of Rho activation. The activation phase was greatly enhanced by serum. In serum-starved adherent cells, LPA induced transient Rho activation, whereas in suspended cells Rho activation was sustained. Furthermore, suspended cells showed higher Rho activity than adherent cells in the presence of serum. These data indicate the existence of an adhesion-dependent negative-feedback loop. We also observed that both cytochalasin D and colchicine trigger Rho activation despite their opposite effects on stress fibers and focal adhesions. Our results show that ECM, cytoskeletal structures and soluble factors all contribute to regulation of Rho activity.

Participation of small GTPases in dorsal closure of the Drosophila embryo: distinct roles for Rho subfamily proteins in epithelial morphogenesis

The Rho subfamily of Ras-related small GTPases participates in a variety of cellular events including organization of the actin cytoskeleton and signalling by c-Jun N-terminal kinase and p38 kinase cascades. These functions of the Rho subfamily are likely to be required in many developmental events. We have been studying the participation of the RHO subfamily in dorsal closure of the Drosophila embryo, a process involving morphogenesis of the epidermis. We have previously shown that Drac1, a Rho subfamily protein, is required for the presence of an actomyosin contractile apparatus believed to be driving the cell shape changes essential to dorsal closure. Expression of a dominant negative Drac1 transgene causes a loss of this contractile apparatus from the leading edge of the advancing epidermis and dorsal closure fails. We now show that two other Rho subfamily proteins, Dcdc42 and RhoA, as well as Ras1 are also required for dorsal closure. Dcdc42 appears to have conflicting roles during dorsal closure: establishment and/or maintenance of the leading edge cytoskeleton versus its down regulation. Down regulation of the leading edge cytoskeleton may be controlled by the serine/threonine kinase DPAK, a potential Drac1/Dcdc42 effector. RhoA is required for the integrity of the leading edge cytoskeleton specifically in cells flanking the segment borders. We have begun to characterize the interactions of the various small GTPases in regulating dorsal closure and find no evidence for the hierarchy of Rho subfamily activity described in some mammalian cell types. Rather, our results suggest that while all Ρ subfamily p21s tested are required for dorsal closure, they act largely in parallel.

A built-in arginine finger triggers the self-stimulatory GTPase-activating activity of rho family GTPases

Signal transduction through the Rho family GTPases requires regulated cycling of the GTPases between the active GTP-bound state and the inactive GDP-bound state. Rho family members containing an arginine residue at position 186 in the C-terminal polybasic region were found to possess a self-stimulatory GTPase-activating protein (GAP) activity through homophilic interaction, resulting in significantly enhanced intrinsic GTPase activities. This arginine residue functions effectively as an "arginine finger" in the GTPase activating reaction to confer the catalytic GAP activity but is not essential for the homophilic binding interactions of Rho family proteins. The arginine 186-mediated negative regulation seems to be absent from Cdc42, a Rho family member important for cell-division cycle regulation, of lower eukaryotes, yet appears to be a part of the turn-off machinery of Cdc42 from higher eukaryotes. Introduction of the arginine 186 mutation into S. cerevisiae CDC42 led to phenotypes consistent with down-regulated CDC42 function. Thus, specific Rho family GTPases may utilize a built-in arginine finger, in addition to RhoGAPs, for negative regulation.

Cytoskeletal changes induced by GRAF, the GTPase regulator associated with focal adhesion kinase, are mediated by Rho

Graf, the GTPase regulator associated with focal adhesion kinase was previously shown to have GAP activity for Ρ A and Cdc42 in vitro (Hildebrand et al 1996 Mol. Cell Biol. 16: 3169–3178). In this study we sought to determine whether Graf acted at the level of Cdc42, Rho, or both in vivo and whether Graf was a signal terminator or transducer for these proteins. Microinjection of Graf cDNA into subconfluent Swiss 3T3 cells (in the presence of serum) has marked effects on cell shape and actin localization. Graf expression causes clearing of stress fibers followed by formation of long actin based filopodial-like extensions. Similar phenotypes were observed following injection of the Rho-inhibitor, C3 into these cells. The Graf response was dependent on GAP activity, since injection of Graf cDNA containing point mutations in the GAP domain (R236Q or N351V) which block enzymatic activity, does not confer this phenotype. Injection of Graf into Swiss 3T3 cells in which Rho has been down-regulated by serum starvation has no effect on cell morphology. Using this system, we demonstrate that Graf blocks sphingosine-1-phosphate (SPP) stimulated (Rho-mediated) stress fiber formation. Conversely, Graf expression does not inhibit bradykinin stimulated (Cdc42-mediated) filopodial extensions. These data indicate that Graf is a GAP for Rho in vivo. To further substantiate these results we examined the effect of Graf over-expression on Rho-mediated neurite retraction in nerve growth factor (NGF)-differentiated PC12 cells. In PC12 cells, which express relatively high levels of endogenous Graf, overexpression of Graf (but not Graf containing the R236Q mutation) enhances SPP-induced neurite retraction. These data indicate the possibility that Graf may be an effector for Rho in certain cell types.

Functional interaction between RhoB and the transcription factor DB1

RhoB has been implicated in cell growth control, actin regulation, adhesion-dependent viability, and gene expression, but its effector functions are poorly defined. Prenylation is important for the physiological functions of Rho proteins, so to identify RhoB effector functions we identified proteins whose interaction was sensitive to prenylation. Here we report the investigation of one such protein, an ubiquitously expressed transcription factor termed DB1 that was originally cloned as a Tax-activated regulator of the IL3 promoter. The RhoB-binding domain in DB1 was located in a functionally undefined region upstream and separable from its zinc finger DNA binding domain. DB1 interacted strongly with prenylated RhoB but weakly with RhoA and not at all with H-Ras. Functional interaction was supported by the identification of prenylated species of RhoB in the nuclear membrane and in an intranuclear laminar region, where they were available for DB1 association in principle, and by the ability of RhoB to inhibit transcriptional activation by DB1, whereas RhoA or Ras had little or no effect, respectively. The results of this study suggest a novel mechanism by which certain Rho proteins may regulate transcription, through sequestration of a transcription factor.

A role for rhoB in the delamination of neural crest cells from the dorsal neural tube

The differentiation of neural crest cells from progenitors located in the dorsal neural tube appears to involve three sequential steps: the specification of premigratory neural crest cell fate, the delamination of these cells from the neural epithelium and the migration of neural crest cells in the periphery. BMP signaling has been implicated in the specification of neural crest cell fate but the mechanisms that control the emergence of neural crest cells from the neural tube remain poorly understood. To identify molecules that might function at early steps of neural crest differentiation, we performed a PCR-based screen for genes induced by BMPs in chick neural plate cells. We describe the cloning and characterization of one gene obtained from this screen, rhoB, a member of the rho family GTP-binding proteins. rhoB is expressed in the dorsal neural tube and its expression persists transiently in migrating neural crest cells. BMPs induce the neural expression of rhoB but not the more widely expressed rho family member, rhoA. Inhibition of rho activity by C3 exotoxin prevents the delamination of neural crest cells from neural tube explants but has little effect on the initial specification of premigratory neural crest cell fate or on the later migration of neural crest cells. These results suggest that rhoB has a role in the delamination of neural crest cells from the dorsal neural tube.

Fluid shear-induced mechanical signaling in MC3T3-E1 osteoblasts requires cytoskeleton-integrin interactions

Mechanical stimulation of bone induces new bone formation in vivo and increases the metabolic activity and gene expression of osteoblasts in culture. We investigated the role of the actin cytoskeleton and actin-membrane interactions in the transmission of mechanical signals leading to altered gene expression in cultured MC3T3-E1 osteoblasts. Application of fluid shear to osteoblasts caused reorganization of actin filaments into contractile stress fibers and involved recruitment of beta1-integrins and alpha-actinin to focal adhesions. Fluid shear also increased expression of two proteins linked to mechanotransduction in vivo, cyclooxygenase-2 (COX-2) and the early response gene product c-fos. Inhibition of actin stress fiber development by treatment of cells with cytochalasin D, by expression of a dominant negative form of the small GTPase Rho, or by microinjection into cells of a proteolytic fragment of alpha-actinin that inhibits alpha-actinin-mediated anchoring of actin filaments to integrins at the plasma membrane each blocked fluid-shear-induced gene expression in osteoblasts. We conclude that fluid shear-induced mechanical signaling in osteoblasts leads to increased expression of COX-2 and c-Fos through a mechanism that involves reorganization of the actin cytoskeleton. Thus Rho-mediated stress fiber formation and the alpha-actinin-dependent anchorage of stress fibers to integrins in focal adhesions may promote fluid shear-induced metabolic changes in bone cells.

Cellularization in Drosophila melanogaster is disrupted by the inhibition of rho activity and the activation of Cdc42 function

Regulation of cytoskeletal dynamics is essential for cell shape change and morphogenesis. Drosophila melanogaster embryos offer a well-defined system for observing alterations in the cytoskeleton during the process of cellularization, a specialized form of cytokinesis. During cellularization, the actomyosin cytoskeleton forms a hexagonal array and drives invagination of the plasma membrane between the nuclei located at the cortex of the syncytial blastoderm. Rho, Rac, and Cdc42 proteins are members of the Rho subfamily of Ras-related G proteins that are involved in the formation and maintenance of the actin cytoskeleton throughout phylogeny and in D. melanogaster. To investigate how Rho subfamily activity affects the cytoskeleton during cellularization stages, embryos were microinjected with C3 exoenzyme from Clostridium botulinum or with wild-type, constitutively active, or dominant negative versions of Rho, Rac, and Cdc42 proteins. C3 exoenzyme ADP-ribosylates and inactivates Rho with high specificity, whereas constitutively active dominant mutations remain in the activated GTP-bound state to activate downstream effectors. Dominant negative mutations likely inhibit endogenous small G protein activity by sequestering exchange factors. Of the 10 agents microinjected, C3 exoenzyme, constitutively active Cdc42, and dominant negative Rho have a specific and indistinguishable effect: the actomyosin cytoskeleton is disrupted, cellularization halts, and embryogenesis arrests. Time-lapse video records of DIC imaged embryos show that nuclei in injected regions move away from the cortex of the embryo, thereby phenocopying injections of cytochalasin or antimyosin. Rhodamine phalloidin staining reveals that the actin-based hexagonal array normally seen during cellularization is disrupted in a dose-dependent fashion. Additionally, DNA stain reveals that nuclei in the microinjected embryos aggregate in regions that correspond to actin disruption. These embryos halt in cellularization and do not proceed to gastrulation. We conclude that Rho activity and Cdc42 regulation are required for cytoskeletal function in actomyosin-driven furrow canal formation and nuclear positioning.

Adenovirus endocytosis requires actin cytoskeleton reorganization mediated by Rho family GTPases

Adenovirus (Ad) endocytosis via alphav integrins requires activation of the lipid kinase phosphatidylinositol-3-OH kinase (PI3K). Previous studies have linked PI3K activity to both the Ras and Rho signaling cascades, each of which has the capacity to alter the host cell actin cytoskeleton. Ad interaction with cells also stimulates reorganization of cortical actin filaments and the formation of membrane ruffles (lamellipodia). We demonstrate here that members of the Rho family of small GTP binding proteins, Rac and CDC42, act downstream of PI3K to promote Ad endocytosis. Ad internalization was significantly reduced in cells treated with Clostridium difficile toxin B and in cells expressing a dominant-negative Rac or CDC42 but not a H-Ras protein. Viral endocytosis was also inhibited by cytochalasin D as well as by expression of effector domain mutants of Rac or CDC42 that impair cytoskeletal function but not JNK/MAP kinase pathway activation. Thus, Ad endocytosis requires assembly of the actin cytoskeleton, an event initiated by activation of PI3K and, subsequently, Rac and CDC42.

Coxiella burnetii induces reorganization of the actin cytoskeleton in human monocytes

Coxiella burnetii, an obligate intracellular bacterium which survives in myeloid cells, causes Q fever in humans. We previously demonstrated that virulent C. burnetii organisms are poorly internalized by monocytes compared to avirulent variants. We hypothesized that a differential mobilization of the actin cytoskeleton may account for this distinct phagocytic behavior. Scanning electron microscopy demonstrated that virulent C. burnetii stimulated profound and polymorphic changes in the morphology of THP-1 monocytes, consisting of membrane protrusions and polarized projections. These changes were transient, requiring 5 min to reach their maximum extent and vanishing after 60 min of incubation. In contrast, avirulent variants of C. burnetii did not induce any significant changes in cell morphology. The distribution of filamentous actin (F-actin) was then studied with a specific probe, bodipy phallacidin. Virulent C. burnetii induced a profound and transient reorganization of F-actin, accompanied by an increase in the F-actin content of THP-1 cells. F-actin was colocalized with myosin in cell protrusions, suggesting that actin polymerization and the tension of actin-myosin filaments play a role in C. burnetii-induced morphological changes. In addition, contact between the cell and the bacterium seems to be necessary to induce cytoskeleton reorganization. Bacterial supernatants did not stimulate actin remodeling, and virulent C. burnetii organisms were found in close apposition with F-actin protrusions. The manipulation of the actin cytoskeleton by C. burnetii may therefore play a critical role in the internalization strategy of this bacterium.

Non-Ras targets of farnesyltransferase inhibitors: focus on Rho

Farnesyltransferase inhibitors (FTIs) are a novel class of cancer therapeutics whose development was based on the discovery that the function of oncogenic Ras depends upon its posttranslational farnesylation. Significantly, experiments in animal models have shown that FTIs have promise as nontoxic cancer therapeutics. However, cell biological studies have suggested that FTIs may act at a level beyond that of suppressing Ras function, so the exact mechanism of action has emerged as a question of major interest. Here, we review evidence that proteins other than Ras are important targets for inhibition, summarize findings suggesting a role for farnesylated Rho proteins prompted by studies on RhoB, and suggest a new model for how FTIs exert their biological effects. The 'FTI-Rho hypothesis' proposes that FTIs act in part by altering Rho-dependent cell adhesion signals which are linked to pathways controlling cell cycle and cell survival and which are subverted or defective in neoplastic cells. This model offers a novel framework for addressing the questions about FTI biology, including the basis for lack of toxicity to normal cells, cytotoxic versus cytostatic effects on tumor cells, and the persistence and drug resistance of malignant cells in FTI-treated animals.

Tyrosine-phosphorylation-dependent and rho-protein-mediated control of cellular phosphatidylinositol 4,5-bisphosphate levels

The polyphosphoinositide PtdIns(4,5)P2, best known as a substrate for phospholipase C isozymes, has recently been recognized to be involved in a variety of other cellular processes. The aim of this study was to examine whether the cellular levels of this versatile phospholipid are controlled by tyrosine phosphorylation. The studies were performed in human embryonic kidney (HEK)-293 cells stably expressing the M3 muscarinic acetylcholine receptor. Inhibition of tyrosine phosphatases by pervanadate induced an up-to-approx.-2. 5-fold increase in the total cellular level of PtdIns(4,5)P2, which was both time- and concentration-dependent. In contrast, the tyrosine kinase inhibitors, genistein and tyrphostin 23, caused a rapid and specific fall in the cellular PtdIns(4,5)P2 level and prevented the stimulatory effect of pervanadate on PtdIns(4,5)P2 formation. Inactivation of Rho proteins by Clostridium difficile toxin B caused a similar fall in the HEK-293 cell PtdIns(4,5)P2 level, which was not altered by additional genistein treatment. Furthermore, toxin B treatment abolished the pervanadate-induced increase in PtdIns(4,5)P2 levels. As PtdIns(4,5)P2 is an essential stimulatory cofactor for phospholipase D (PLD) enzymes, we finally examined the effects of the agents regulating PtdIns(4,5)P2 levels on PLD activity in HEK-293 cells. Inhibition of tyrosine phosphatases by pervanadate caused an increase in PLD activity, which was susceptible to genistein and tyrphostin 23, and which was abolished by prior treatment with toxin B. In conclusion, the data presented indicate that the cellular level of the multifunctional phospholipid, PtdIns(4,5)P2, in HEK-293 cells is controlled by a tyrosine-kinase-dependent mechanism and that this process apparently involves Rho proteins, as found similarly for tyrosine-phosphorylation-induced PLD activation.

Post-transcriptional regulation of endothelial nitric oxide synthase mRNA stability by Rho GTPase

The mechanism by which 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors increase endothelial nitric oxide synthase (eNOS) expression is unknown. To determine whether changes in isoprenoid synthesis affects eNOS expression, human endothelial cells were treated with the HMG-CoA reductase inhibitor, mevastatin (1-10 microM), in the presence of L-mevalonate (200 microM), geranylgeranylpyrophosphate (GGPP, 1-10 microM), farnesylpyrophosphate (FPP, 5-10 microM), or low density lipoprotein (LDL, 1 mg/ml). Mevastatin increased eNOS mRNA and protein levels by 305 +/- 15% and 180 +/- 11%, respectively. Co-treatment with L-mevalonate or GGPP, but not FPP or LDL, reversed mevastatin's effects. Because Rho GTPases undergo geranylgeranyl modification, we investigated whether Rho regulates eNOS expression. Immunoblot analyses and [35S]GTPgammaS-binding assays revealed that mevastatin inhibited Rho membrane translocation and GTP binding activity by 60 +/- 5% and 78 +/- 6%, both of which were reversed by co-treatment with GGPP but not FPP. Furthermore, inhibition of Rho by Clostridium botulinum C3 transferase (50 microg/ml) or by overexpression of a dominant-negative N19RhoA mutant increased eNOS expression. In contrast, activation of Rho by Escherichia coli cytotoxic necrotizing factor-1 (200 ng/ml) decreased eNOS expression. These findings indicate that Rho negatively regulates eNOS expression and that HMG-CoA reductase inhibitors up-regulate eNOS expression by blocking Rho geranylgeranylation, which is necessary for its membrane-associated activity.

Rho directs activation-associated changes in rat hepatic stellate cell morphology via regulation of the actin cytoskeleton

Hepatic stellate cell activation, thought to play a key role in fibrosis of the liver, is characterized by changes in cellular morphology. The intracellular signals regulating morphological alterations associated with stellate cell activation are uncertain. The ras-like guanosine triphosphate-binding protein, rho, has recently emerged as an important regulator of the actin cytoskeleton, and consequently cell morphology. The aim of this study was to test the hypothesis that rho signaling pathways direct activation-associated morphological changes in stellate cells by regulating the actin cytoskeleton. The morphology and actin cytoskeleton of primary rat hepatic stellate cells were studied with phase contrast, differential interference contrast, and epifluorescence microscopy. Immunohistochemistry and immunoblot analysis were used to examine rho expression and activity, respectively. Quiescent and activated stellate cells were investigated in the absence and presence of C3 transferase, a bacterial toxin that specifically inhibits rho. Stellate cell activation was characterized by the development of prominent intracellular fibers, and the loss of dendrite-like processes and perinuclear retinoid droplets. Moreover, activation was accompanied by the formation of prominent actin stress fibers and focal adhesions. Both rho expression and activity were demonstrated in stellate cells. C3 transferase blocked and reversed, both activation-associated morphological alterations and activation-associated changes in the actin cytoskeleton, in quiescent and activated stellate cells, respectively. These results indicate that rho directs activation-associated changes in rat hepatic stellate cell morphology via regulation of the actin cytoskeleton.

Lovastatin modulates in vivo and in vitro the plasminogen activator/plasmin system of rat proximal tubular cells: role of geranylgeranylation and Rho proteins

Interstitial fibrosis is one of the most deleterious events during the progression of renal deterioration after renal mass reduction. In vivo, hydroxymethylglutaryl CoA reductase inhibitors (HRI) were shown to reduce progression of glomerulosclerosis, but the mechanisms are still unclear. The present study investigates, in vivo, whether lovastatin, a potent HRI, was able to modulate the plasminogen-plasmin pathway, one of the most efficient systems involved in extracellular matrix remodeling, and characterizes in vitro the cellular mechanisms of these effects. Proximal tubules freshly isolated from rats treated for 2 d with lovastatin (4 mg/kg per d) showed increased tissue-type plasminogen activator (tPA) and urokinase (uPA) activities and antigens. Incubation with lovastatin (5 microM) of proximal tubules isolated from untreated rats induced an increase in tPA and uPA and a decrease in plasminogen activator inhibitor-1 (PAI-1) activities. In vitro, supernatants, cytosols, and membranes of renal proximal tubular cells in primary cultures had no detectable uPA activity, and lovastatin (0.1 to 10 microM) induced an increase in tPA and a decrease in PAI-1 activities and antigens. These effects were reversed by mevalonate and geranylgeranyl-pyrophosphate (GGPP) but not by farnesyl-pyrophosphate or LDL cholesterol. C3 exoenzyme, an inhibitor of the geranylgeranylated-activated Rho protein, reproduced the effect of lovastatin on tPA and PAI- activity and blocked its reversion by GGPP. The effect of lovastatin was associated with a disruption of cellular actin stress fibers, which was reversed by GGPP and reproduced by C3 exoenzyme. In conclusion, HRI can modify the fibrinolytic potential of proximal tubules, most likely via inhibition of geranylgeranylated Rho protein and disruption of the cytoskeleton. The resulting increase of proteolytic activity of tubular cells may serve to prevent extracellular matrix deposition and renal interstitial fibrosis.

Chemokine-induced monocyte transmigration requires cdc42-mediated cytoskeletal changes

The ras-related small GTPases of the rho family coordinate the assembly of complex cytoskeletal structures crucial for cell motility and polarization. Cdc42 controls filopodia formation in some cell types; however, other physiological functions, agonists and subsequent signaling cascades remain to be elucidated. Expression of cdc42 mutants in monocytic cells showed that CC chemokines regulate the rearrangement of the actin cytoskeleton via cdc42, i.e. formation of filopodia-like projections was induced by CC chemokines or dominant active cdc42, while dominant inactive cdc42 prevented its stimulation with CC chemokines. Both cdc42 mutants inhibited CC chemokine-induced monocyte migration across bare filters or across filters coated with endothelium, implicating cdc42 activity and its effector functions in chemotaxis. In contrast, cdc42 mutants did not affect the rho-dependent activation integrin avidity by CC chemokines. The chemokine-induced signaling pathways involved phosphoinositide 3-kinase upstream of cdc42, as shown by inhibition of cytoskeletal reorganization with wortmannin. These data identify CC chemokines as physiological agonists of cdc42 and reveal its functional importance in chemotaxis and extravasation of monocytes.

GTPgammaS-induced actin polymerisation in vitro: ATP- and phosphoinositide-independent signalling via Rho-family proteins and a plasma membrane-associated guanine nucleotide exchange factor

In a cell-free system from neutrophil cytosol GTP(γ)S can induce an increase in the number of free filament barbed ends and massive actin polymerisation and cross-linking. GTP(γ)S stimulation was susceptible to an excess of GDP, but not Bordetella pertussis toxin and could not be mimicked by aluminium fluoride, myristoylated GTPgammaS. Gialpha2 or Gbeta1gamma2 subunits of trimeric G proteins. In contrast, RhoGDI and Clostridium difficile toxin B (inactivating Rho family proteins) completely abrogated the effect of GTPgammaS. When recombinant, constitutively activated and GTPgammaS-loaded Rac1, RhoA, or Cdc42 proteins alone or in combination were probed at concentrations >100 times the endogenous, however, they were ineffective. Purified Cdc42/Rac-interactive binding (CRIB) domain of WASP or C3 transferase did not prevent actin polymerisation by GTPgammaS. The action of GTPgammaS was blocked by mM [Mg2+], unless a heat- and trypsin-sensitive component present in neutrophil plasma membrane was added. Liberation of barbed ends seems therefore to be mediated by a toxin B-sensitive cytosolic Rho-family protein, requiring a membrane-associated guanine nucleotide exchange factor (GEF) for its activation by GTPgammaS under physiologic conditions. The inefficiency of various protein kinase and phosphatase inhibitors (staurosporine, genistein, wortmannin, okadaic acid and vanadate) and removal of ATP by apyrase, suggests that phosphate transfer reactions are not required for the downstream propagation of the GTPgammaS signal. Moreover, exogenously added phosphoinositides failed to induce actin polymerisation and a PtdIns(4,5)P2-binding peptide did not interfere with the response to GTPgammaS. The speed and simplicity of the presented assay applicable to protein purification techniques will facilitate the further elucidation of the molecular partners involved in actin polymerisation.

RhoB is stabilized by transforming growth factor beta and antagonizes transcriptional activation

Transforming growth factor beta (TGF-beta) is the prototype for an evolutionarily conserved superfamily of secreted factors implicated in diverse biological phenomena. The pleiotropic responses to TGF-beta are initiated by a heteromeric receptor complex that binds and phosphorylates downstream effectors. Among these, the Smads have been extensively studied. However, less attention has been directed toward alternative downstream effectors and their participation in TGF-beta signal transduction. We show that TGF-beta promotes accumulation of the labile monomeric GTPase RhoB by antagonizing its normal proteolytic destruction, presumably via the 26 S proteasome. RhoB accumulates in its isoprenylated form. Transient overexpression of wild type RhoB but not its dominant negative mutant RhoB-N19 antagonizes TGF-beta-mediated transcriptional activation. These results suggest a novel mechanism of regulation by TGF-beta and implicate RhoB as a negative regulator of TGF-beta signal transduction.

Interaction of Rho1p target Bni1p with F-actin-binding elongation factor 1alpha: implication in Rho1p-regulated reorganization of the actin cytoskeleton in Saccharomyces cerevisiae

The RHO1 gene encodes a homolog of mammalian RhoA small G protein in the yeast Saccharomyces cerevisiae. We have shown that Bni1p is one of the downstream targets of Rho1p and regulates reorganization of the actin cytoskeleton through the interaction with profilin, an actin monomer-binding protein. A Bni1p-binding protein was affinity purified from the yeast cytosol fraction and was identified to be Tef1p/Tef2p, translation elongation factor 1alpha (EF1alpha). EF1alpha is an essential component of the protein synthetic machinery and also possesses the actin filament (F-actin)-binding and -bundling activities. EF1alpha bound to the 186 amino acids region of Bni1p, located between the FH1 domain, the proline-rich profilin-binding domain, and the FH2 domain, of which function is not known. The binding of Bni1p to EF1alpha inhibited its F-actin-binding and -bundling activities. The BNI1 gene deleted in the EF1alpha-binding region did not suppress the bni1 bnr1 mutation in which the actin organization was impaired. These results suggest that the Rho1p-Bni1p system regulates reorganization of the actin cytoskeleton through the interaction with both EF1alpha and profilin.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

Characterization of graf, the GTPase-activating protein for rho associated with focal adhesion kinase. Phosphorylation and possible regulation by mitogen-activated protein kinase

Graf is a GTPase-activating protein for Rho that interacts with focal adhesion kinase and co-localizes with the actin cytoskeleton (Hildebrand, J. D., Taylor, J. M. and Parsons, J. T. (1996) Mol. Cell. Biol. 16, 3169-3178). We examined the expression and regulation of Graf as a prelude to understanding the role of Graf in mediating signal transduction in vivo. We demonstrated that Graf is a ubiquitously expressed 95-kDa protein with high levels observed in heart and brain and cells derived from these tissues. Stimulation of PC12 cells with epidermal growth factor or nerve growth factor induced a phosphatase-reversible mobility shift upon gel electrophoresis, indicative of phosphorylation. In vitro, purified mitogen-activated protein (MAP) kinase catalyzed the phosphorylation of Graf on serine 510, suggesting that Graf phosphorylation may be mediated through MAP kinase signaling. In addition, the mutation of serine 510 to alanine inhibited the epidermal growth factor-induced mobility shift of mutant Graf protein in vivo, consistent with serine 510 being the site of in vivo phosphorylation. Based on these data we suggest that phosphorylation of Graf by MAP kinase or related kinases may be a mechanism by which growth factor signaling modulates Rho-mediated cytoskeletal changes in PC12 and perhaps other cells.

Agents that inhibit Rho, Rac, and Cdc42 do not block formation of actin pedestals in HeLa cells infected with enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) induces formation of actin pedestals in infected host cells. Agents that inhibit the activity of Rho, Rac, and Cdc42, including Clostridium difficile toxin B (ToxB), compactin, and dominant negative Rho, Rac, and Cdc42, did not inhibit formation of actin pedestals. In contrast, treatment of HeLa cells with ToxB inhibited EPEC invasion. Thus, Rho, Rac, and Cdc42 are not required for assembly of actin pedestals; however, they may be involved in EPEC uptake by HeLa cells.

The N-terminal part of the enzyme component (C2I) of the binary Clostridium botulinum C2 toxin interacts with the binding component C2II and functions as a carrier system for a Rho ADP-ribosylating C3-like fusion toxin

The binary actin-ADP-ribosylating Clostridium botulinum C2 toxin consists of the enzyme component C2I and the binding component C2II, which are separate proteins. The active component C2I enters cells through C2II by receptor-mediated endocytosis and membrane translocation. The N-terminal part of C2I (C2IN), which consists of 225 amino acid residues but lacks ADP-ribosyltransferase activity, was identified as the C2II contact site. A fusion protein (C2IN-C3) of C2IN and the full-length C3-like ADP-ribosyltransferase from Clostridium limosum was constructed. The fusion protein C2IN-C3 ADP-ribosylated Rho but not actin in CHO cell lysates. Together with C2II, C2IN-C3 induced complete rounding up of CHO and HeLa cells after incubation for 3 h. No cell rounding was observed without C2II or with the original C3-like transferase from C. limosum. The data indicate that the N-terminal 225 amino acid residues of C2I are sufficient to cause the cellular uptake of C. limosum transferase via the binding component of C2II, thereby increasing the cytotoxicity of the C3-like exoenzyme several hundred-fold.

Exogenous phospholipase D generates lysophosphatidic acid and activates Ras, Rho and Ca2+ signaling pathways

BACKGROUND

Phospholipase D (PLD) hydrolyzes phospholipids to generate phosphatidic acid (PA) and a free headgroup. PLDs occur as both intracellular and secreted forms; the latter can act as potent virulence factors. Exogenous PLD has growth-factor-like properties, in that it induces proto-oncogene transcription, mitogenesis and cytoskeletal changes in target cells. The underlying mechanism is unknown, although it is generally assumed that PLD action is mediated by PA serving as a putative second messenger.

RESULTS

In quiescent fibroblasts, exogenous PLD (from Streptomyces chromofuscus) stimulated accumulation of the GTP-bound form of Ras, activation of mitogen-activated protein (MAP) kinase and DNA synthesis, through the pertussis-toxin-sensitive inhibitory G protein Gi. Furthermore, PLD mimicked bioactive lysophospholipids (but not PA) in inducing Ca2+ mobilization, membrane depolarization and Rho-mediated neurite retraction. PLD action was mediated by Iysophosphatidic acid (LPA) derived from Iysophosphatidylcholine acting on cognate G-protein-coupled LPA receptor(s). There was no evidence for the involvement of PA in mediating the effects of exogenous PLD.

CONCLUSIONS

Our results provide a molecular explanation for the multiple cellular responses to exogenous PLDs. These PLDs generate bioactive LPA from pre-existing Iysophosphatidylcholine in the outer membrane leaflet, resulting in activation of G-protein-coupled LPA receptors and consequent activation of Ras, Rho and Ca2+ signaling pathways. Unscheduled activation of LPA receptors may underlie, at least in part, the known pathogenic effects of exogenous PLDs.

Sequence analysis of a 34.7-kb DNA segment from the genome of Buchnera aphidicola (endosymbiont of aphids) containing groEL, dnaA, the atp operon, gidA, and rho

Buchnera aphidicola is a prokaryotic endosymbiont of the aphid Schizaphis graminum. From past and present nucleotide sequence analyses of the B. aphidicola genome, we have assembled a 34. 7-kilobase (kb) DNA segment. This segment contains genes coding for 32 open reading frames (ORFs), which corresponded to 89.9% of the DNA. All of these ORFs could be identified with homologous regions of the Escherichia coli genome. The order of the genes with established functions was groELS-trmE-rnpA-rpmH-dnaA-dnaN-gyrB-atpCDGAH FEB-gidA-fdx-hscA- hscB-nifS-ilvDC-rep-trxA-rho. The order of genes in small DNA fragments was conserved in both B. aphidicola and E. coli. Most of these fragments were in approximately the same region of the E. coli genome. The latter organism, however, contained many additional inserted genes within and between the fragments. The results of the B. aphidicola genome analyses indicate that the endosymbiont has many properties of free-living bacteria.

PRK1 is targeted to endosomes by the small GTPase, RhoB

RhoB has been shown to be an endosomal GTPase both by immunocytochemistry and electron microscopy, however, its role in endocytosis is unknown. Elucidation of the cellular roles of other members of this superfamily of signaling proteins has come with the identification of their downstream partners. We show here that the recently isolated serine/threonine kinase PRK1 is targeted to the endosomal compartment by RhoB. This is established both through immunofluorescence and cell fractionation. PRK1 is shown to interact with activated RhoB in cells and is localized to endosomes through its Rho-binding HR1 domain. Translocation of PRK1 to the endosomal compartment by RhoB is accompanied by a shift in the electrophoretic mobility of the kinase indicative of an accompanying activation.

Rho GTPases and the actin cytoskeleton

The actin cytoskeleton mediates a variety of essential biological functions in all eukaryotic cells. In addition to providing a structural framework around which cell shape and polarity are defined, its dynamic properties provide the driving force for cells to move and to divide. Understanding the biochemical mechanisms that control the organization of actin is thus a major goal of contemporary cell biology, with implications for health and disease. Members of the Rho family of small guanosine triphosphatases have emerged as key regulators of the actin cytoskeleton, and furthermore, through their interaction with multiple target proteins, they ensure coordinated control of other cellular activities such as gene transcription and adhesion.