The function of the p190 Rho GTPase-activating protein is controlled by its N-terminal GTP binding domain

Cross Talk between ARF1 and RhoA Coordinates the Formation of Cytoskeletal Scaffolds during Chlamydia Infection

Chlamydia trachomatis is an obligate intracellular bacterium that has developed sophisticated mechanisms to survive inside its infectious compartment, the inclusion. Notably, Chlamydia weaves an extensive network of microtubules (MTs) and actin filaments to enable interactions with host organelles and enhance its stability. Despite the global health and economic burden caused by this sexually transmitted pathogen, little is known about how actin and MT scaffolds are integrated into an increasingly complex virulence system. Previously, we established that the chlamydial effector InaC interacts with ARF1 to stabilize MTs. We now demonstrate that InaC regulates RhoA to control actin scaffolds. InaC relies on cross talk between ARF1 and RhoA to coordinate MTs and actin, where the presence of RhoA downregulates stable MT scaffolds and ARF1 activation inhibits actin scaffolds. Understanding how Chlamydia hijacks complex networks will help elucidate how this clinically significant pathogen parasitizes its host and reveal novel cellular signaling pathways.

The Dimeric Form of HPV16 E6 Is Crucial to Drive YAP/TAZ Upregulation through the Targeting of hScrib

Simple Summary

Understanding the mechanisms of action of HPV oncoproteins is pivotal for the rationale development of anti-cancer drugs to treat HPV-related malignancies. The aim of the present study was to explore more in detail the mechanism of action of the HPV16 oncoprotein E6 that directly fosters the YAP/TAZ signaling pathway, a conserved cascade highly active in HPV-related cancers. We confirmed previous evidence about the importance of the PDZ-protein targeting in this process, highlighting here the importance of hScrib degradation, and discovered that the targeting of the Scribble module involves the dimeric form of HPV16 E6. The findings here presented extend our knowledge about the mechanism through which the oncoprotein E6 targets a PDZ-host factor to degradation in cancer cells.

Abstract

Human papillomavirus is the most common viral infectious agent responsible for cancer development in humans. High-risk strains are known to induce cancer through the expression of the viral oncogenes E6 and E7, yet we have only a partial understanding of the precise mechanisms of action of these viral proteins. Here we investigated the molecular mechanism through which the oncoprotein E6 alters the Hippo-YAP/TAZ pathway to trigger YAP/TAZ induction in cancer cells. By employing E6 overexpression systems combined with protein–protein interaction studies and loss-of-function approaches, we discovered that the E6-mediated targeting of hScrib, which supports YAP/TAZ upregulation, intimately requires E6 homodimerization. We show that the self-association of E6, previously reported only in vitro, takes place in the cytoplasm and, as a dimer, E6 targets the fraction of hScrib at the cell cortex for proteasomal degradation. Thus, E6 homodimerization emerges as an important event in the mechanism of E6-mediated hScrib targeting to sustain downstream YAP/TAZ upregulation, unraveling for the first time the key role of E6 homodimerization in the context of its transforming functions and thus paving the way for the possible development of E6 dimerization inhibitors.

p190A RhoGAP induces CDH1 expression and cooperates with E-cadherin to activate LATS kinases and suppress tumor cell growth

The ARHGAP35 gene encoding p190A RhoGAP (p190A) is significantly altered by both mutation and allelic deletion in human cancer, but the functional implications of such alterations are not known. Here, we demonstrate for the first time that p190A is a tumor suppressor using a xenograft mouse model with carcinoma cells harboring defined ARHGAP35 alterations. In vitro, restoration of p190A expression in carcinoma cells promotes contact inhibition of proliferation (CIP) through activation of LATS kinases and phosphorylation of the proto-oncogenic transcriptional co-activator YAP. In contrast, p190A forms harboring recurrent cancer mutations exhibit loss of function in modulating the Hippo pathway, inducing CIP, as well as attenuated suppression of tumor growth in mice. We determine that p190A promotes mesenchymal to epithelial transition (MET) and elicits expression of a cassette of epithelial adherens junction-associated genes in a cell density-dependent manner. This cassette includes CDH1 encoding E-cadherin, which amplifies p190A-mediated LATS activation and is necessary for CIP. Oppositely, we establish that p190A is obligatory for E-cadherin to activate LATS kinases and induce CIP. Collectively, this work defines a novel mechanism by which p190A and E-cadherin cooperate in modulating Hippo signaling to suppress tumor cell growth.

Investigation of the role and mechanism of ARHGAP5-mediated colorectal cancer metastasis

Background: Metastatic colorectal cancer (CRC) is a lethal disease; however, the underlying molecular mechanisms remain unclear and require further study. Methods: RNA-Seq, PCR, Western blotting, immunohistochemistry, ChIP and RNAi assays were performed to investigate Rho GTPase-activating protein 5 (ARHGAP5, aslo known as p190RhoGAP-B, p190-B) expression and the clinical relevance, functional roles and regulatory mechanisms of this protein using human CRC cells and tissues. In vivo, two cell-based xenograft models were used to evaluate the roles of ARHGAP5 in CRC metastasis. Results: Here, we report that ARHGAP5 expression is significantly increased in metastatic CRC tissues and is inversely associated with patient overall survival. The suppression of ARHGAP5 reduces CRC cell metastasis in vitro and in cell-based xenograft models. Furthermore, we show that ARHGAP5 promotes CRC cell epithelial-mesenchymal transition by negatively regulating RhoA activity. Mechanistically, cAMP response element-binding protein (CREB1) transcriptionally upregulates ARHGAP5 expression, and decreased miR-137 further contributes to ARHGAP5 mRNA stability in CRC. Conclusions: Overall, our study highlights the crucial function of ARHGAP5 in CRC metastasis, thus suggesting novel prognostic biomarkers and hypothetical therapeutic targets.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

PseudoGTPase domains in p190RhoGAP proteins: a mini-review

Pseudoenzymes generally lack detectable catalytic activity despite adopting the overall protein fold of their catalytically competent counterparts, indeed 'pseudo' family members seem to be incorporated in all enzyme classes. The small GTPase enzymes are important signaling proteins, and recent studies have identified many new family members with noncanonical residues within the catalytic cleft, termed pseudoGTPases. To illustrate recent discoveries in the field, we use the p190RhoGAP proteins as an example. p190RhoGAP proteins (ARHGAP5 and ARHGAP35) are the most abundant GTPase activating proteins for the Rho family of small GTPases. These are key regulators of Rho signaling in processes such as cell migration, adhesion and cytokinesis. Structural biology has complemented and guided biochemical analyses for these proteins and has allowed discovery of two cryptic pseudoGTPase domains, and the re-classification of a third, previously identified, GTPase-fold domain as a pseudoGTPase. The three domains within p190RhoGAP proteins illustrate the diversity of this rapidly expanding pseudoGTPase group.

The N-Terminal GTPase Domain of p190RhoGAP Proteins Is a PseudoGTPase

The pseudoGTPases are a rapidly growing and important group of pseudoenzymes. p190RhoGAP proteins are critical regulators of Rho signaling and contain two previously identified pseudoGTPase domains. Here we report that p190RhoGAP proteins contain a third pseudoGTPase domain, termed N-GTPase. We find that GTP constitutively purifies with the N-GTPase domain, and a 2.8-Å crystal structure of p190RhoGAP-A co-purified with GTP reveals an unusual GTP-Mg2+ binding pocket. Six inserts in N-GTPase indicate perturbed catalytic activity and inability to bind to canonical GTPase activating proteins, guanine nucleotide exchange factors, and effector proteins. Biochemical analysis shows that N-GTPase does not detectably hydrolyze GTP, and exchanges nucleotide only under harsh Mg2+ chelation. Furthermore, mutational analysis shows that GTP and Mg2+ binding stabilizes the domain. Therefore, our results support that N-GTPase is a nucleotide binding, non-hydrolyzing, pseudoGTPase domain that may act as a protein-protein interaction domain. Thus, unique among known proteins, p190RhoGAPs contain three pseudoGTPase domains.

ARHGAP42 is activated by Src-mediated tyrosine phosphorylation to promote cell motility

The tyrosine kinase Src acts as a key regulator of cell motility by phosphorylating multiple protein substrates that control cytoskeletal and adhesion dynamics. In an earlier phosphotyrosine proteomics study, we identified a novel Rho-GTPase activating protein, now known as ARHGAP42, as a likely biologically relevant Src substrate. ARHGAP42 is a member of a family of RhoGAPs distinguished by tandem BAR-PH domains lying N-terminal to the GAP domain. Like other family members, ARHGAP42 acts preferentially as a GAP for RhoA. We show that Src principally phosphorylates ARHGAP42 on tyrosine 376 (Tyr-376) in the short linker between the BAR-PH and GAP domains. The expression of ARHGAP42 variants in mammalian cells was used to elucidate its regulation. We found that the BAR domain is inhibitory toward the GAP activity of ARHGAP42, such that BAR domain deletion resulted in decreased active GTP-bound RhoA and increased cell motility. With the BAR domain intact, ARHGAP42 GAP activity could be activated by phosphorylation of Tyr-376 to promote motile cell behavior. Thus, phosphorylation of ARHGAP42 Tyr-376 is revealed as a novel regulatory event by which Src can affect actin dynamics through RhoA inhibition.

Regulation of Rho GTPase activity at the leading edge of migrating cells by p190RhoGAP

Cell migration, a key feature of embryonic development, immunity, angiogenesis, and tumor metastasis, is based on the coordinated regulation of actin dynamics and integrin-mediated adhesion. Rho GTPases play a major role in this phenomenon by regulating the onset and maintenance of actin-based protruding structures at cell leading edges (i.e. lamellipodia and filopodia) and contractile structures (i.e., stress fibers) at their trailing edge. While spatio-temporal analysis demonstrated the tight regulation of Rho GTPases at the migration front during cell locomotion, little is known about how the main regulators of Rho GTPase activity, such as GAPs, GEFs and GDIs, play a role in this process. In this review, we focus on a major negative regulator of RhoA, p190RhoGAP-A and its close isoform p190RhoGAP-B, which are necessary for efficient cell migration. Recent studies, including our, demonstrated that p190RhoGAP-A localization and activity undergo a complex regulatory mechanism, accounting for the tight regulation of RhoA, but also other members of the Rho GTPase family, at the cell periphery.

Interaction of p190A RhoGAP with eIF3A and Other Translation Preinitiation Factors Suggests a Role in Protein Biosynthesis

The negative regulator of Rho family GTPases, p190A RhoGAP, is one of six mammalian proteins harboring so-called FF motifs. To explore the function of these and other p190A segments, we identified interacting proteins by tandem mass spectrometry. Here we report that endogenous human p190A, but not its 50% identical p190B paralog, associates with all 13 eIF3 subunits and several other translational preinitiation factors. The interaction involves the first FF motif of p190A and the winged helix/PCI domain of eIF3A, is enhanced by serum stimulation and reduced by phosphatase treatment. The p190A/eIF3A interaction is unaffected by mutating phosphorylated p190A-Tyr³⁰⁸, but disrupted by a S296A mutation, targeting the only other known phosphorylated residue in the first FF domain. The p190A-eIF3 complex is distinct from eIF3 complexes containing S6K1 or mammalian target of rapamycin (mTOR), and appears to represent an incomplete preinitiation complex lacking several subunits. Based on these findings we propose that p190A may affect protein translation by controlling the assembly of functional preinitiation complexes. Whether such a role helps to explain why, unique among the large family of RhoGAPs, p190A exhibits a significantly increased mutation rate in cancer remains to be determined.

Cancer-associated mutations in the protrusion-targeting region of p190RhoGAP impact tumor cell migration

p190RhoGAP (p190A) is a negative regulator of RhoA and localizes to membrane protrusions, where its GAP activity is required for directional migration. Here, Binamé et al. identify the protrusion-localization sequence in p190A and show that cancer-associated mutations in this region affect p190A localization and function as well as tumor cell migration.

Spatiotemporal regulation of RhoGTPases such as RhoA is required at the cell leading edge to achieve cell migration. p190RhoGAP (p190A) is the main negative regulator of RhoA and localizes to membrane protrusions, where its GTPase-activating protein (GAP) activity is required for directional migration. In this study, we investigated the molecular processes responsible for p190A targeting to actin protrusions. By analyzing the subcellular localization of truncated versions of p190A in hepatocellular carcinoma cells, we identified a novel functional p190A domain: the protrusion localization sequence (PLS) necessary and sufficient for p190A targeting to leading edges. Interestingly, the PLS is also required for the negative regulation of p190A RhoGAP activity. Further, we show that the F-actin binding protein cortactin binds the PLS and is required for p190A targeting to protrusions. Lastly, we demonstrate that cancer-associated mutations in PLS affect p190A localization and function, as well as tumor cell migration. Altogether, our data unveil a new mechanism of regulation of p190A in migrating tumor cells.

A Point Mutation in p190A RhoGAP Affects Ciliogenesis and Leads to Glomerulocystic Kidney Defects

Rho family GTPases act as molecular switches regulating actin cytoskeleton dynamics. Attenuation of their signaling capacity is provided by GTPase-activating proteins (GAPs), including p190A, that promote the intrinsic GTPase activity of Rho proteins. In the current study we have performed a small-scale ENU mutagenesis screen and identified a novel loss of function allele of the p190A gene Arhgap35, which introduces a Leu1396 to Gln substitution in the GAP domain. This results in decreased GAP activity for the prototypical Rho-family members, RhoA and Rac1, likely due to disrupted ordering of the Rho binding surface. Consequently, Arhgap35-deficient animals exhibit hypoplastic and glomerulocystic kidneys. Investigation into the cystic phenotype shows that p190A is required for appropriate primary cilium formation in renal nephrons. P190A specifically localizes to the base of the cilia to permit axoneme elongation, which requires a functional GAP domain. Pharmacological manipulations further reveal that inhibition of either Rho kinase (ROCK) or F-actin polymerization is able to rescue the ciliogenesis defects observed upon loss of p190A activity. We propose a model in which p190A acts as a modulator of Rho GTPases in a localized area around the cilia to permit the dynamic actin rearrangement required for cilia elongation. Together, our results establish an unexpected link between Rho GTPase regulation, ciliogenesis and glomerulocystic kidney disease.

p190RhoGAP has cellular RacGAP activity regulated by a polybasic region

p190RhoGAP is a GTPase-activating protein (GAP) known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of the intrinsic GTPase activity of Rho. Although the GAP domain of p190RhoGAP stimulates the intrinsic' GTPase activity of several Rho family members (Rho, Rac, Cdc42) under in vitro conditions, p190RhoGAP is generally regarded as a GAP for RhoA in the cell. The cellular RacGAP activity of the protein has not been proven directly. We have previously shown that the in vitro RacGAP and RhoGAP activity of p190RhoGAP was inversely regulated through a polybasic region of the protein. Here we provide evidence that p190RhoGAP shows remarkable GAP activity toward Rac also in the cell. The cellular RacGAP activity of p190RhoGAP requires an intact polybasic region adjacent to the GAP domain whereas the RhoGAP activity is inhibited by the same domain. Our data indicate that through its alternating RacGAP and RhoGAP activity, p190RhoGAP plays a more complex role in the Rac-Rho antagonism than it was realized earlier.

ARHGAP21 is a RhoGAP for RhoA and RhoC with a role in proliferation and migration of prostate adenocarcinoma cells

BACKGROUND

Several Rho GTPase-activating proteins (RhoGAPs) are implicated in tumor progression through their effects on Rho GTPase activity. ARHGAP21 is a RhoGAP with increased expression in head and neck squamous cell carcinoma and with a possible role in glioblastoma tumor progression, yet little is known about the function of ARHGAP21 in cancer cells. Here we studied the role of ARHGAP21 in two prostate adenocarcinoma cell lines, LNCaP and PC3, which respectively represent initial and advanced stages of prostate carcinogenesis.

RESULTS

ARHGAP21 is located in the nucleus and cytoplasm of both cell lines and its depletion resulted in decreased proliferation and increased migration of PC3 cells but not LNCaP cells. In PC3 cells, ARHGAP21 presented GAP activity for RhoA and RhoC and induced changes in cell morphology. Moreover, its silencing altered the expression of genes involved in cell proliferation and cytoskeleton organization, as well as the endothelin-1 canonical pathway.

CONCLUSIONS

Our results reveal new functions and signaling pathways regulated by ARHGAP21, and indicate that it could contribute to prostate cancer progression.

Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin transmembrane receptors and their semaphorin ligands, as well as their co-receptors (Neuropilin, Integrin, VEGFR2, ErbB2, and Met kinase) are emerging as key regulatory proteins in a wide variety of developmental, regenerative, but also pathological processes. The diverse arenas of plexin function are surveyed, including roles in the nervous, cardiovascular, bone and skeletal, and immune systems. Such different settings require considerable specificity among the plexin and semaphorin family members which in turn are accompanied by a variety of cell signaling networks. Underlying the latter are the mechanistic details of the interactions and catalytic events at the molecular level. Very recently, dramatic progress has been made in solving the structures of plexins and of their complexes with associated proteins. This molecular level information is now suggesting detailed mechanisms for the function of both the extracellular as well as the intracellular plexin regions. Specifically, several groups have solved structures for extracellular domains for plexin-A2, -B1, and -C1, many in complex with semaphorin ligands. On the intracellular side, the role of small Rho GTPases has been of particular interest. These directly associate with plexin and stimulate a GTPase activating (GAP) function in the plexin catalytic domain to downregulate Ras GTPases. Structures for the Rho GTPase binding domains have been presented for several plexins, some with Rnd1 bound. The entire intracellular domain structure of plexin-A1, -A3, and -B1 have also been solved alone and in complex with Rac1. However, key aspects of the interplay between GTPases and plexins remain far from clear. The structural information is helping the plexin field to focus on key questions at the protein structural, cellular, as well as organism level that collaboratoria of investigations are likely to answer.

The GTPase TcRjl of the human pathogen Trypanosoma cruzi is involved in the cell growth and differentiation

The protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas Disease, undergoes through a complex life cycle where rounds of cell division and differentiation occur initially in the gut of triatominae vectors and, after transmission, inside of infected cells in vertebrate hosts. Members of the Ras superfamily of GTPases are molecular switches which play pivotal regulatory functions in cell growth and differentiation. We have previously described a novel GTPase in T. cruzi, TcRjl, which belongs to the RJL family of Ras-related GTP binding proteins. Here we show that most of TcRjl protein is found bound to GTP nucleotides and may be locked in this stage. In addition, we show that TcRjl is located close to the kinetoplast, in a region corresponding possibly to flagellar pocket of the parasite and the expression of a dominant-negative TcRjl construct (TcRjlS37N) displays a significative growth phenotype in reduced serum medium. Remarkably, overexpression of TcRjl inhibits differentiation of epimastigotes to trypomastigote forms and promotes the accumulation of intermediate differentiation stages. Our data suggest that TcRjl might play a role in the control of the parasite growth and differentiation.

ARHGAP25, a novel Rac GTPase-activating protein, regulates phagocytosis in human neutrophilic granulocytes

Members of the Rac/Rho family of small GTPases play an essential role in phagocytic cells in organization of the actin cytoskeleton and production of toxic oxygen compounds. GTPase-activating proteins (GAPs) decrease the amount of the GTP-bound active form of small GTPases, and contribute to the control of biologic signals. The number of potential Rac/RhoGAPs largely exceeds the number of Rac/Rho GTPases and the expression profile, and their specific role in different cell types is largely unknown. In this study, we report for the first time the properties of full-length ARHGAP25 protein, and show that it is specifically expressed in hematopoietic cells, and acts as a RacGAP both in vitro and in vivo. By silencing and overexpressing the protein in neutrophil model cell lines (PLB-985 and CosPhoxFcγR, respectively) and in primary macrophages, we demonstrate that ARHGAP25 is a negative regulator of phagocytosis acting probably via modulation of the actin cytoskeleton.

The Tumor Suppressor, p190RhoGAP, Differentially Initiates Apoptosis and Confers Docetaxel Sensitivity to Breast Cancer Cells

p190RhoGAP (p190) is a negative regulator of RhoGTPases and a putative tumor suppressor, whose mechanism of tumor suppression is poorly defined. Ectopic expression of p190 induces various morphological phenotypes, including multinucleation, dendrite-like formation, and chromatin condensation, suggesting an involvement in apoptosis. We examined the possibility that p190 can function as a tumor suppressor by regulating induction of apoptosis. We show that the predominant phenotype of p190 overexpression in a variety of cell lines is apoptosis, which is mediated through p190's regulation of Rho and caspases. The secondary phenotypes, multinucleation and dendrite-like formation, are determined by transformation status, not cell lineage, and appear to be intermediate phenotypes in the p190-induced apoptotic pathway. Finally, we show that p190 levels can regulate the apoptotic response of breast cancer cell lines to docetaxel through its regulation of Rho. Together, these findings suggest that one mechanism by which p190 can mediate its tumor-suppressive function is through regulation of Rho-activated cell death pathways and that this function can be exploited to optimize the action of cytoskeletal-based chemotherapeutics, such as the taxanes.

Caveolin-1-eNOS signaling promotes p190RhoGAP-A nitration and endothelial permeability

Caveolin-1–mediated inhibition of endothelial nitric oxide synthase signaling is essential for adherens junction integrity and endothelial permeability homeostasis.

Endothelial barrier function is regulated by adherens junctions (AJs) and caveolae-mediated transcellular pathways. The opening of AJs that is observed in caveolin-1^−/− (Cav-1^−/−) endothelium suggests that Cav-1 is necessary for AJ assembly or maintenance. Here, using endothelial cells isolated from Cav-1^−/− mice, we show that Cav-1 deficiency induced the activation of endothelial nitric oxide synthase (eNOS) and the generation of nitric oxide (NO) and peroxynitrite. We assessed S-nitrosylation and nitration of AJ-associated proteins to identify downstream NO redox signaling targets. We found that the GTPase-activating protein (GAP) p190RhoGAP-A was selectively nitrated at Tyr1105, resulting in impaired GAP activity and RhoA activation. Inhibition of eNOS or RhoA restored AJ integrity and diminished endothelial hyperpermeability in Cav-1^−/− mice. Thrombin, a mediator of increased endothelial permeability, also induced nitration of p120-catenin–associated p190RhoGAP-A. Thus, eNOS-dependent nitration of p190RhoGAP-A represents a crucial mechanism for AJ disassembly and resultant increased endothelial permeability.

Farnesyl transferase inhibitor treatment of breast cancer cells leads to altered RhoA and RhoC GTPase activity and induces a dormant phenotype

Farnesyl transferase inhibitors (FTIs) were shown to be effective in modulating tumor growth in Ras-transformed tumor cells. Recent studies have focused on Rho GTPases as putative targets of FTI action. Previously, we demonstrated that FTIs were effective in inhibiting the growth and invasiveness of RhoC GTPase-overexpressing inflammatory breast cancer (IBC) cells however, RhoC activity was increased. In this study, we examine the mechanisms of FTI action on breast cancer cells in culture through modulation of RhoC and RhoA GTPases. We found that FTI inhibition of breast cancer cell growth was reversible and resembled what has been described for an in vitro model of tumor cell dormancy. On FTI treatment, levels of active RhoA decreased significantly, whereas levels of active RhoC increased 3.8-fold. We studied the role of these two GTPases in a fibronectin and basic FGF-induced model of breast cancer cell dormancy. Hypoactivation of RhoA and hyperactivation of RhoC were seen to induce morphology and growth changes consistent with tumor cell dormancy in culture. In addition, the JNK/SAPK pathway was induced on FTI treatment. A pharmacologic inhibitor of the JNK/SAPK pathway significantly reduced the number of dormant cells. This study has implications for the use of FTIs as therapeutic agents as well as potential mechanisms for breast cancer cell dormancy.

Mitotic down-regulation of p190RhoGAP is required for the successful completion of cytokinesis

p190RhoGAP-A (p190) is a GTPase-activating protein known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of Rho intrinsic GTPase activity. We have previously shown that p190 protein levels are cell cycle-regulated, decreasing in mitosis, and that this decrease is mediated by the ubiquitin-proteasome pathway. In addition, overexpression of p190 results in decreased RhoGTP levels at the cleavage furrow during cytokinesis, p190 and the RhoGEF Ect2 play opposing roles in cytokinesis, and sustained levels of p190 in mitosis are associated with cytokinesis failure, all findings that suggest but do not directly demonstrate that completion of cytokinesis is dependent on reduced levels of p190. Here we report, using an RNAi reconstitution approach with a degradation-resistant mutant, that decreased p190 levels are required for successful cytokinesis. We also show that the multinucleation phenotype is dependent on p190 RhoGAP activity, determine that the N-terminal GBDS1 region is necessary and sufficient for p190 mitotic ubiquitination and degradation, and identify four N-terminal residues as necessary for the degradation of p190 in mitosis. Our data indicate that in addition to activation of RhoGEF(s), reduction of RhoGAP (p190) is a critical mechanism by which increased RhoGTP levels are achieved in late mitosis, thereby ensuring proper cell division.

Regulation of the substrate preference of p190RhoGAP by protein kinase C-mediated phosphorylation of a phospholipid binding site

The Rho family GTPases are stringently regulated through the action of a large family of GTPase activating proteins (GAPs) that stimulate their relatively weak intrinsic GTP hydrolyzing activity. The p190RhoGAPs, which include the p190A and p190B proteins, are potent and widely expressed GAPs acting on both Rho and Rac GTPases. We have observed that several acidic phospholipids inhibit the RhoGAP activity and promote the RacGAP activity of p190 proteins. In liposome binding assays we have demonstrated that binding of p190A to phospholipids is controlled by electrostatic interactions. Using mapping techniques, we determined that a small polybasic peptide stretch within p190A is a common site for both the phospholipid binding and PKC phosphorylation. Moreover, PKC-mediated phosphorylation of two amino acids (serine-1221 and threonine-1226) within this region of p190A prevents the binding and substrate specificity regulation by phospholipids. Transfection of COS-7 cells with mutant forms of p190A either unable to bind to phospholipids or unable to become phosphorylated induced distinct morphological changes. Together, these findings reveal a novel GAP regulatory mechanism in which phosphorylation indirectly alters GTPase substrate preference by affecting the interaction with acidic phospholipids. Our observations provide a potential mechanism of Rac/Rho antagonism described in several cellular functions.

Angiotensin II induces RhoA activation through SHP2-dependent dephosphorylation of the RhoGAP p190A in vascular smooth muscle cells

Angiotensin II (ANG II) is a major regulator of blood pressure that essentially acts through activation of ANG II type 1 receptor (AT1R) of vascular smooth muscle cells (VSMC). AT1R activates numerous intracellular signaling pathways, including the small G protein RhoA known to control several VSMC functions. Nevertheless, the mechanisms leading to RhoA activation by AT1R are unknown. RhoA activation can result from activation of RhoA exchange factor and/or inhibition of Rho GTPase-activating protein (GAP). Here we hypothesize that a RhoGAP could participate to RhoA activation induced by ANG II in rat aortic VSMC. The knockdown of the RhoGAP p190A by small interfering RNA (siRNA) abolishes the activation of RhoA-Rho kinase pathway induced after 5 min of ANG II (0.1 microM) stimulation in rat aortic VSMC. We then show that AT1R activation induces p190A dephosphorylation and inactivation. In addition, expression of catalytically inactive or phosphoresistant p190A mutants increases the basal activity of RhoA-Rho kinase pathway, whereas phosphomimetic mutant inhibits early RhoA activation by ANG II. Using siRNA and mutant overexpression, we then demonstrate that the tyrosine phosphatase SHP2 is necessary for 1) maintaining p190A basally phosphorylated and activated by the tyrosine kinase c-Abl, and 2) inducing p190A dephosphorylation and RhoA activation in response to AT1R activation. Our work then defines p190A as a new mediator of RhoA activation by ANG II in VSMC.

PKCdelta influences p190 phosphorylation and activity: events independent of PKCdelta-mediated regulation of endothelial cell stress fiber and focal adhesion formation and barrier function

BACKGROUND

We have shown that protein kinase Cdelta (PKCdelta) inhibition results in increased endothelial cell (EC) permeability and decreased RhoA activity; which correlated with diminished stress fibers (SF) and focal adhesions (FA). We have also shown co-precipitation of p190RhoGAP (p190) with PKCdelta. Here, we investigated if PKCdelta regulates p190 and whether PKCdelta-mediated changes in SF and FA or permeability were dependent upon p190.

METHODS

Protein-protein interaction and activity analyses were performed using co-precipitation assays. Analysis of p190 phosphorylation was performed using in vitro kinase assays. SF and FA were analyzed by immunofluorescence analyses. EC monolayer permeability was measured using electrical cell impedance sensor (ECIS) technique.

RESULTS

Inhibition of PKCdelta increased p190 activity, while PKCdelta overexpression diminished p190 activity. PKCdelta bound to and phosphorylated both p190FF and p190GTPase domains. p190 protein overexpression diminished SF and FA formation and RhoA activity. Disruption of SF and FA or increased permeability induced upon PKCdelta inhibition, were not attenuated in EC in which the p190 isoforms were suppressed individually or concurrently.

GENERAL SIGNIFICANCE

Our findings suggest that while PKCdelta can regulate p190 activity, possibly at the FF and/or GTPase domains, the effect of PKCdelta inhibition on SF and FA and barrier dysfunction occurs through a pathway independent of p190.

Tks5 recruits AFAP-110, p190RhoGAP, and cortactin for podosome formation

Podosome formation in vascular smooth muscle cells is characterized by the recruitment of AFAP-110, p190RhoGAP, and cortactin, which have specific roles in Src activation, local down-regulation of RhoA activity, and actin polymerization, respectively. However, the molecular mechanism that underlies their specific recruitment to podosomes remains unknown. The scaffold protein Tks5 is localized to podosomes in Src-transformed fibroblasts and in smooth muscle cells, and may serve as a specific recruiting adapter for various components during podosome formation. We show here that induced mislocalization of Tks5 to the surface of mitochondria leads to a major subcellular redistribution of AFAP-110, p190RhoGAP, and cortactin, and to inhibition of podosome formation. Analysis of a series of similarly mistargeted deletion mutants of Tks5 indicates that the fifth SH3 domain is essential for this recruitment. A Tks5 mutant lacking the PX domain also inhibits podosome formation and induces the redistribution of AFAP-110, p190RhoGAP, and cortactin to the perinuclear area. By expressing a catalytically inactive point mutant and by siRNA-mediated expression knock-down we also provide evidence that p190RhoGAP is required for podosome formation. Together our findings demonstrate that Tks5 plays a central role in the recruitment of AFAP-110, p190RhoGAP, and cortactin to drive podosome formation.

Rho GTPase activity modulates Wnt3a/beta-catenin signaling

Wnt proteins constitute a family of secreted signaling molecules that regulate highly conserved pathways essential for development and, when aberrantly activated, drive oncogenesis in a number of human cancers. A key feature of the most widely studied Wnt signaling cascade is the stabilization of cytosolic beta-catenin, resulting in beta-catenin nuclear translocation and transcriptional activation of multiple target genes. In addition to this canonical, beta-catenin-dependent pathway, Wnt3A has also been shown to stimulate RhoA GTPase. While the importance of activated Rho to non-canonical Wnt signaling is well appreciated, the potential contribution of Wnt3A-stimulated RhoA to canonical beta-catenin-dependent transcription has not been examined and is the focus of this study. We find that activated Rho is required for Wnt3A-stimulated osteoblastic differentiation in C3H10T1/2 mesenchymal stem cells, a biological phenomenon mediated by stabilized beta-catenin. Using expression microarrays and real-time RT-PCR analysis, we show that Wnt3A-stimulated transcription of a subset of target genes is Rho-dependent, indicating that full induction of these Wnt targets requires both beta-catenin and Rho activation. Significantly, neither beta-catenin stabilization nor nuclear translocation stimulated by Wnt3A is affected by inhibition or activation of RhoA. These findings identify Rho activation as a critical element of the canonical Wnt3A-stimulated, beta-catenin-dependent transcriptional program.

p190RhoGAP negatively regulates Rho activity at the cleavage furrow of mitotic cells

Previous studies demonstrated that p190RhoGAP (p190) negatively affects cytokinesis in a RhoGAP-dependent manner, suggesting that regulation of Rho may be a critical mechanism of p190 action during cytokinesis. P190 localizes to the cleavage furrow (CF) of dividing cells, and its levels decrease during late mitosis by an ubiquitin-mediated mechanism, consistent with the hypothesis that high RhoGTP levels are required for completion of cytokinesis. To determine whether RhoGTP levels in the CF are affected by p190 and to define the phase(s) of cytokinesis in which p190 is involved, we used FRET analysis alone or in combination with time-lapse microscopy. In normal cell division activated Rho accumulated at the cell equator in early anaphase and in the contractile ring, where it co-localized with p190. Real-time movies revealed that cells expressing elevated levels of p190 exhibited multiple cycles of abnormal CF site selection and ingression/regression, which resulted in failed or prolonged cytokinesis. This was accompanied by mislocalization of active Rho at the aberrant CF sites. Quantified data revealed that in contrast to ECT2 and dominate negative p190 (Y1283Ap190), which resulted in hyper-activated Rho, Rho activity in the CF was reduced by wild type p190 in a dose-dependent manner. These results suggest that p190 regulates cytokinesis through modulation of RhoGTP levels, thereby affecting CF specification site selection and subsequent ring contraction.

JNK-mediated phosphorylation of paxillin in adhesion assembly and tension-induced cell death by the adenovirus death factor E4orf4

The adenovirus type 2 Early Region 4 ORF4 (E4orf4) protein induces a caspase-independent death program in tumor cells involving changes in actin dynamics that are functionally linked to cell killing. Because an increase in myosin II-based contractility is needed for the death of E4orf4-expressing cells, we have proposed that alteration of cytoskeletal tension is part of the signals engaging the death pathway. Yet the mechanisms involved are poorly defined. Herein, we show that the Jun N-terminal kinase JNK is activated in part through a pathway involving Src, Rho, and ROCK (Rho kinase) and contributes to dysregulate adhesion dynamics and to kill cells in response to E4orf4. JNK supports the formation of atypically robust focal adhesions, which are bound to the assembly of the peculiar actomyosin network typifying E4orf4-induced cell death and which are required for driving nuclear condensation. Remarkably, the dramatic enlargement of focal adhesions, actin remodeling, and cell death all rely on paxillin phosphorylation at Ser-178, which is induced by E4orf4 in a JNK-dependent way. Furthermore, we found that Ser-178-paxillin phosphorylation is necessary to decrease adhesion turnover and to enhance the time residency of paxillin at focal adhesions, promoting its recruitment from an internal pool. Our results indicate that perturbation of tensional homeostasis by E4orf4 involves JNK-regulated changes in paxillin adhesion dynamics that are required to engage the death pathway. Moreover, our findings support a role for JNK-mediated paxillin phosphorylation in adhesion growth and stabilization during tension signaling.

Opposing roles of p190RhoGAP and Ect2 RhoGEF in regulating cytokinesis

Evidence suggests that p190RhoGAP (p190), a GTPase activating protein (GAP) specific for Rho, plays a role in cytokinesis. First, ectopic expression of p190 induces a multinucleated cellular phenotype. Second, endogenous p190 localizes to the cleavage furrow of dividing cells. Lastly, its levels are reduced in late mitosis by ubiquitin-mediated proteasomal degradation, consistent with the idea that low levels of p190 and high levels of active Rho are required for completion of cytokinesis. As with p190, RhoA and the RhoGEF, ECT2, have been localized to the cleavage furrow. These findings raise the question of whether p190 and ECT2 cooperate antagonistically to regulate the activity of Rho and contraction of the actomyosin ring during cytokinesis. Here we demonstrate ECT2 can, in a dose-dependent manner, reduce multinucleation induced by p190. Furthermore, endogenous p190 and ECT2 colocalize at the cleavage furrow of dividing cells and stably associate with one another in co-immunoprecipitation assays. Functional and physical interactions between p190 and ECT2 are reflected in the levels of Rho activity, as assessed by Rho pull-down assays. Together, these results suggest that co-regulation of Rho activity by p190RhoGAP and ECT2 in the cleavage furrow determines whether cells properly complete cytokinesis.

Identification of an autoinhibitory mechanism that restricts C1 domain-mediated activation of the Rac-GAP alpha2-chimaerin

Chimaerins are a family of GTPase activating proteins (GAPs) for the small G-protein Rac that have gained recent attention due to their important roles in development, cancer, neuritogenesis, and T-cell function. Like protein kinase C isozymes, chimaerins possess a C1 domain capable of binding phorbol esters and the lipid second messenger diacylglycerol (DAG) in vitro. Here we identified an autoinhibitory mechanism in alpha2-chimaerin that restricts access of phorbol esters and DAG, thereby limiting its activation. Although phorbol 12-myristate 13-acetate (PMA) caused limited translocation of wild-type alpha2-chimaerin to the plasma membrane, deletion of either N- or C-terminal regions greatly sensitize alpha2-chimaerin for intracellular redistribution and activation. Based on modeling analysis that revealed an occlusion of the ligand binding site in the alpha2-chimaerin C1 domain, we identified key amino acids that stabilize the inactive conformation. Mutation of these sites renders alpha2-chimaerin hypersensitive to C1 ligands, as reflected by its enhanced ability to translocate in response to PMA and to inhibit Rac activity and cell migration. Notably, in contrast to PMA, epidermal growth factor promotes full translocation of alpha2-chimaerin in a phospholipase C-dependent manner, but not of a C1 domain mutant with reduced affinity for DAG (P216A-alpha2-chimaerin). Therefore, DAG generation and binding to the C1 domain are required but not sufficient for epidermal growth factor-induced alpha2-chimaerin membrane association. Our studies suggest a role for DAG in anchoring rather than activation of alpha2-chimaerin. Like other DAG/phorbol ester receptors, including protein kinase C isozymes, alpha2-chimaerin is subject to autoinhibition by intramolecular contacts, suggesting a highly regulated mechanism for the activation of this Rac-GAP.

p190A RhoGAP is a glycogen synthase kinase-3-beta substrate required for polarized cell migration

The Rho GTPases are critical regulators of the actin cytoskeleton and are required for cell adhesion, migration, and polarity. Among the key Rho regulatory proteins in the context of cell migration are the p190 RhoGAPs (p190A and p190B), which function to modulate Rho signaling in response to integrin engagement. The p190 RhoGAPs undergo complex regulation, including phosphorylation by several identified kinases, interactions with phospholipids, and association with a variety of cellular proteins. Here, we have identified an additional regulatory mechanism unique to p190A RhoGAP that involves priming-dependent phosphorylation by glycogen synthase-3-beta (GSK-3beta), a kinase previously implicated in establishing cell polarity. We found that p190A-deficient fibroblasts exhibit a defect in directional cell migration reflecting a requirement for GSK-3beta-mediated phosphorylation of amino acids in the C-terminal "tail" of p190A. This phosphorylation leads to inhibition of p190A RhoGAP activity in vitro and in vivo. These studies identify p190A as a novel GSK-3beta substrate and reveal a mechanism by which GSK-3beta contributes to cellular polarization in directionally migrating cells via effects on Rho GTPase activity.

The PAR-6 polarity protein regulates dendritic spine morphogenesis through p190 RhoGAP and the Rho GTPase

The majority of excitatory synaptic transmission in the brain occurs at dendritic spines, which are actin-rich protrusions on the dendrites. The asymmetric nature of these structures suggests that proteins regulating cell polarity might be involved in their formation. Indeed, the polarity protein PAR-3 is required for normal spine morphogenesis. However, this function is independent of association with atypical protein kinase C (aPKC) and PAR-6. Here we show that PAR-6 together with aPKC plays a distinct but essential role in spine morphogenesis. Knockdown of PAR-6 inhibits spine morphogenesis, whereas overexpression of PAR-6 increases spine density, and these effects are mediated by aPKC. Using a FRET biosensor, we further show that p190 RhoGAP and RhoA act downstream of the PAR-6/aPKC complex. These results define a role for PAR-6 and aPKC in dendritic spine biogenesis and maintenance, and reveal an unexpected link between the PAR-6/aPKC complex and RhoA activity.

MKK6 phosphorylation regulates production of superoxide by enhancing Rac GTPase activity

Rac-dependent NADPH oxidases generate reactive oxygen species used in cell signaling and microbial killing or both. Whereas the mechanisms leading to NADPH oxidase activation are fairly well studied, the mechanisms that control downregulation of this enzyme complex remain unclear. We hypothesized that reactive oxygen species produced by NADPH oxidase may autoregulate the complex by inhibiting Rac activity. To this end, we searched for binding partners of Rac1 and identified a tyrosine-phosphorylated fragment of MKK6 that bound to Rac1 under redox-stress conditions. Constitutively active MKK6 interacted directly with Rac1 in vitro, and this interaction was enhanced when MKK6 was phosphorylated on tyrosine 219. Both Rac1 and Rac2 immunoprecipitated an MKK6 fragment under conditions that elevate cellular peroxide levels in 293 and RAW cells, respectively. Constitutively active and wild-type MKK6 enhanced Rac-GTPase activity in vitro, and their overexpression inhibited PMA-induced NADPH oxidase activation in RAW cells. In contrast, a Y219F mutant of MKK6 only partially enhanced Rac1 GTPase activity, and its overexpression did not alter PMA-induced NADPH oxidase activation in RAW cells. Last, MKK6 deficiency led to an increase in Rac1-GTP levels in brain tissue. Our findings suggest that MKK6 downregulates NADPH oxidase activity by enhancing Rac-GTPase activity.

Plasma membrane localization affects the RhoGAP specificity ofPseudomonasExoS

Pseudomonas aeruginosa ExoS (453 amino acids) is a bifunctional type III cytotoxin, comprising a Rho GTPase-activating protein domain (RhoGAP), and a 14-3-3 dependent ADP-ribosyltransferase domain. In addition, ExoS contains a membrane localization domain (termed MLD, residues 51-77) which localizes and traffics ExoS within intoxicated host cells. While membrane localization has been shown to be essential for ExoS to ADP-ribosylate Ras, the relationship between intracellular localization and expression of RhoGAP activity has not been addressed. In this study, loss of MLD function was observed to abolish expression of ExoS RhoGAP activity in HeLa cells. One mutation within the MLD (R56, R63, D70 mutated to N, RRD-->N) diminished plasma membrane localization and altered the cell rounding phenotype elicited by ExoS RhoGAP. In addition, cell rounding caused by ExoS-MLD(RRD-->N) was reversed by dominant active Rac1, but not dominant active Cdc42, indicating a switch in ExoS RhoGAP substrate specificity. Mutation of the C-terminal polybasic region abolished the ability of dominant active Rac1 to protect HeLa cells from expression of the RhoGAP activity of ExoS-MLD(RRD-->N). This study shows the importance of membrane localization in the targeting of Rho GTPases by ExoS RhoGAP.

Transforming growth factor beta regulates the expression of the M2 muscarinic receptor in atrial myocytes via an effect on RhoA and p190RhoGAP

Transforming growth factor beta (TGFbeta) signaling is involved in the development and regulation of multiple organ systems and cellular signaling pathways. We recently demonstrated that TGFbeta regulates the response of atrial myocytes to parasympathetic stimulation. Here, TGFbeta(1) is shown to inhibit expression of the M(2) muscarinic receptor (M(2)), which plays a critical role in the parasympathetic response of the heart. This effect is mimicked by overexpression of a dominant negative mutant of RhoA and by the RhoA kinase inhibitor Y27632, whereas adenoviral expression of a dominant activating-RhoA reverses TGFbeta inhibition of M(2) expression. TGFbeta(1) also mediates a decrease in GTP-bound RhoA and a reciprocal increase in the expression of the RhoA GTPase-activating protein, p190RhoGAP, whereas total RhoA is unchanged. Inhibition of M(2) promoter activity by TGFbeta(1) is mimicked by overexpression of p190RhoGAP, whereas a dominant negative mutant of p190RhoGAP reverses this effect of TGFbeta(1). In contrast to atrial myocytes, in mink lung epithelial cells, in which TGFbeta signaling through activation of RhoA has been previously identified, TGFbeta(1) stimulated an increase in GTP-bound RhoA in association with a reciprocal decrease in the expression of p190RhoGAP. Both effects demonstrated a similar dose dependence on TGFbeta(1). Thus TGFbeta regulation of M(2) muscarinic receptor expression is dependent on RhoA, and TGFbeta regulation of p190RhoGAP expression may be a cell type-specific mechanism for TGFbeta signaling through RhoA.

Role of Cdc42 in neurite outgrowth of PC12 cells and cerebellar granule neurons

Inactivation of Rho GTPases inhibited the neurite outgrowth of PC12 cells. The role of Cdc42 in neurite outgrowth was then studied by selective inhibition of Cdc42 signals. Overexpression of ACK42, Cdc42 binding domain of ACK-1, inhibited NGF-induced neurite outgrowth in PC12 cells. ACK42 also inhibited the neurite outgrowth of PC12 cells induced by constitutively activated mutant of Cdc42, but not Rac. These results suggest that Cdc42 plays an important role in mediating NGF-induced neurite outgrowth of PC12 cells. Inhibition of neurite outgrowth was also demonstrated using a cell permeable chimeric protein, penetratin-ACK42. A dominant negative mutant of Rac, RacN17 inhibited Cdc42-induced neurite outgrowth of PC12 cells suggesting that Rac acts downstream of Cdc42. Further studies, using primary-cultures of rat cerebellar granule neurons, showed that Cdc42 is also involved in the neurite outgrowth of cerebellar granule neurons. Both penetratin-ACK42 and Clostridium difficile toxin B, which inactivates all members of Rho GTPases strongly inhibited the neurite outgrowth of cerebellar granule neurons. These results show that Cdc42 plays a similar and essential role in the development of neurite outgrowth of PC12 cells and cerebellar granule neurons. These results provide evidence that Cdc42 produces signals that are essential for the neurite outgrowth of PC12 cells and cerebellar granule neurons.

p190 Rho-GTPase activating protein associates with plexins and it is required for semaphorin signalling

Plexins are transmembrane receptors for semaphorins, guiding cell migration and axon extension. Plexin activation leads to the disassembly of integrin-based focal adhesive structures and to actin cytoskeleton remodelling and inhibition of cell migration; however, the underlying molecular mechanisms are unclear. We consistently observe a transient decrease of cellular RhoA-GTP levels upon plexin activation in adherent cells. One of the main effectors of RhoA downregulation is p190, a ubiquitously expressed GTPase activating protein (GAP). We show that, in p190-deficient fibroblasts, the typical functional activities mediated by plexins (such as cell collapse and inhibition of integrin-based adhesion) are blocked or greatly impaired. Notably, the functional response can be rescued in these cells by re-expressing exogenous p190, but not a mutant form specifically lacking RhoGAP activity. We furthermore demonstrate that semaphorin function is blocked in epithelial cells, primary endothelial cells and neuroblasts upon treatment with small interfering RNAs that knockdown p190 expression. Finally, we show that p190 transiently associates with plexins, and its RhoGAP activity is increased in response to semaphorin stimulation. We conclude that p190-RhoGAP is crucially involved in semaphorin signalling to the actin cytoskeleton, via interaction with plexins.

GAPs in growth factor signalling

Approximately 2% of genes predicted by the sequenced human genome encode small GTPases and their regulators, highlighting the biological significance of regulated GTPase activity. Among the key GTPase regulators are the GTPase activating proteins (GAPs), which function to down-modulate active GTPases. Of the numerous identified GAPs, several have been implicated in signal transduction downstream of growth factors. In particular, GAPs for the Ras and Rho GTPases, which mediate a variety of receptor-transduced signals, appear to play an essential role in growth factor dependent GTPase regulation. Experimental studies of several of the GAPs have begun to elucidate mechanisms by which GAP activity is influenced by growth factor signaling, including direct phosphorylation, sub-cellular redistribution and protein degradation. Here, some of these mechanisms of GAP regulation in the context of signaling responses to growth factors are reviewed.

PARG1, a protein-tyrosine phosphatase-associated RhoGAP, as a putative Rap2 effector

Rap2 belongs to the Ras family of small GTP-binding proteins, but its specific signaling role is unclear. By yeast two-hybrid screening, we have found that the Caenorhabditis elegans ortholog of Rap2 interacts with a protein containing a Rho-GTPase-activating protein (RhoGAP) domain, ZK669.1a, whose human ortholog PARG1 exhibits RhoGAP activity in vitro. ZK669.1a and PARG1 share a homology region with previously unknown function, designated the ZK669.1a and PARG1 homology (ZPH) region. Here we show that the ZPH region of PARG1 mediates interaction with Rap2. PARG1 interacted with Rap2 in a GTP-dependent manner but not with Ras or Rap1. We also show that PARG1 and its mutant lacking the ZPH region induce typical cytoskeletal changes for Rho inactivation in fibroblasts. Rap2 suppressed this in vivo action of PARG1 but not that of the mutant PARG1. These results suggest that PARG1 is a putative specific effector of Rap2 to regulate Rho.

Characterization of a novel GTPase-activating protein associated with focal adhesions and the actin cytoskeleton

In the present study we characterize a novel RhoGAP protein (RC-GAP72) that interacts with actin stress fibers, focal adhesions, and cell-cell adherens junctions via its 185-amino acid C-terminal region. Overexpression of RC-GAP72 in fibroblasts induces cell rounding with partial or complete disruption of actin stress fibers and formation of membrane ruffles, lamellipodia, and filopodia. RC-GAP72 mutant truncated downstream of the GTPase-activating protein (GAP) domain retains the ability to stimulate membrane protrusions but fails to affect stress fiber integrity or induce cell retraction. A mutant protein consisting of the C terminus of RC-GAP72 and lacking the GAP domain does not exert any visible effect on cellular morphology. Inactivation of the GAP domain by a point mutation does not abolish the effect of RC-GAP72 on actin stress fibers but moderates its capability to induce membrane protrusions. Our data imply that the cytoskeletal localization of RC-GAP72 and its interaction with GTPases are essential for its effect on the integrity of actin stress fibers, whereas the induction of lamellipodia and filopodia depends on the activity of the GAP domain irrespective of binding to the actin cytoskeleton. We propose that RC-GAP72 affects cellular morphology by targeting activated Cdc42 and Rac1 GTPases to specific subcellular sites, triggering local morphological changes. The overall physiological functions of RC-GAP72 are presently unknown, yet our data suggest that RC-GAP72 plays a role in regulating cell morphology and cytoskeletal organization.

Rho-regulatory proteins in breast cancer cell motility and invasion

The importance of the Rho-GTPases in cancer progression, particularly in the area of metastasis, is becoming increasingly evident. This review will provide an overview of the role of the Rho-regulatory proteins in breast cancer metastatis.

RhoA/ROCK activation by growth hormone abrogates p300/histone deacetylase 6 repression of Stat5-mediated transcription

We demonstrate here that growth hormone (GH) stimulates the activation of RhoA and its substrate Rho kinase (ROCK) in NIH-3T3 cells. GH-stimulated formation of GTP-bound RhoA requires JAK2-dependent dissociation of RhoA from its negative regulator p190 RhoGAP. Inactivation of RhoA does not affect GH-stimulated JAK2 tyrosine phosphorylation nor p44/42 MAPK activity. However, RhoA and ROCK activities are required for GH-stimulated, Stat5-mediated transcription. RhoA-dependent enhancement of GH-stimulated, Stat5-mediated transcription is due to repression of histone deacetylase 6 activity recruited by transcription cofactor p300 that negatively regulates GH-stimulated, Stat5-mediated transcription. We also demonstrate that RhoA is the pivot for cAMP-dependent protein kinase inhibition of GH-stimulated, Stat5-mediated transcription as a consequence of cAMP-dependent protein kinase inactivation of RhoA through serine residue 188 of RhoA. We have therefore provided a novel mechanism by which a Ras-like small GTPase, RhoA, can regulate Stat5-mediated transcription.

A possible role for p190RhoGAP in PKCε-induced morphological effects

We have previously shown that protein kinase C (PKC) epsilon induces neurite outgrowth via its regulatory domain. This is accompanied by PKC-induced stress fibre loss. Here, we show that the regulatory domain (RD) of PKCepsilon induces processes also in NIH-3T3 fibroblasts, similar to what has been observed with p190RhoGAP. This study also shows that p190RhoGAP induces neurite outgrowth in SK-N-BE(2) neuroblastoma cells. We therefore investigated whether p190RhoGAP may be downstream of PKCepsilon. We could detect a co-localization of p190RhoGAP and PKCepsilon at membrane ruffles and an increased association between the proteins in fibroblasts treated with 12-O-tetradecanoylphorbol-13-acetate (TPA). The association is also seen in neuroblastoma cells, and nerve growth factor (NGF) treatment of SH-SYSY/TrkA cells decreases the interaction. However, overexpressed PKCepsilon did not coprecipitate overexpressed p190RhoGAP in CHO cells, indicating that the proteins do not interact directly. This raises the possibility that p190RhoGAP is involved in mediating the morphological effects of PKCepsilon.

p190RhoGAP is cell cycle regulated and affects cytokinesis

p190RhoGAP (p190), a Rho family GTPase-activating protein, regulates actin stress fiber dynamics via hydrolysis of Rho-GTP. Recent data suggest that p190 also regulates cell proliferation. To gain insights into the cellular process(es) affected by p190, we altered its levels by conditional or transient overexpression. Overexpression of p190 resulted in a multinucleated phenotype that was dependent on the GTPase-activating protein domain. Confocal immunofluorescence microscopy revealed that both endogenous and exogenous p190 localized to the newly forming and contracting cleavage furrow of dividing cells. However, overexpression of p190 resulted in abnormal positioning of the furrow specification site and unequal daughter cell partitioning, as well as faulty furrow contraction and multinucleation. Furthermore, levels of endogenous p190 protein were transiently decreased in late mitosis via an ubiquitin-mediated degradation process that required the NH₂-terminal GTP-binding region of p190. These results suggest that a cell cycle–regulated reduction in endogenous p190 levels is linked to completion of cytokinesis and generation of viable cell progeny.

PAK4 kinase is essential for embryonic viability and for proper neuronal development

The serine/threonine kinase PAK4 is a target for the Rho GTPase Cdc42 and has been shown to regulate cell morphology and cytoskeletal organization in mammalian cells. To examine the physiological and developmental functions of PAK4, we have disrupted the PAK4 gene in mice. The absence of PAK4 led to lethality by embryonic day 11.5, a result most likely due to a defect in the fetal heart. Striking abnormalities were also evident in the nervous systems of PAK4-deficient embryos. These embryos had dramatic defects in neuronal development and axonal outgrowth. In particular, spinal cord motor neurons and interneurons failed to differentiate and migrate to their proper positions. This is probably related to the role for PAK4 in the regulation of cytoskeletal organization and cell and/or extracellular matrix adhesion. PAK4-null embryos also had defects in proper folding of the caudal portion of the neural tube, suggesting an important role for PAK4 in neural tube development.

Engulfment of apoptotic cells is negatively regulated by Rho-mediated signaling

The rapid and efficient phagocytosis of apoptotic cells plays a critical role in preventing secondary necrosis, inflammation as well as in tissue remodeling and regulating immune responses. However, the molecular details of engulfment are just beginning to be elucidated. Among the Rho family GTPases, previous studies have implicated a role for Rac and Cdc42 in the uptake of apoptotic cells by phagocytes, yet the role of Rho has remained unclear. Here, we present evidence that Rho-GTP levels decrease during engulfment. RhoA seems to negatively affect basal engulfment, such that inhibition of Rho-mediated signaling in phagocytes enhanced the uptake of apoptotic targets. Activation of endogenous Rho or overexpression of constitutively active forms of Rho also inhibited engulfment. By testing mutants of RhoA that selectively activate downstream effectors, the Rho-kinase seemed to be primarily responsible for this inhibitory effect. Taken together, these data suggest that inhibition of Rho- and Rho-kinase-mediated signaling might be important during engulfment, which could have important implications for several clinical trials involving inhibition of the Rho kinase.

Rnd proteins function as RhoA antagonists by activating p190 RhoGAP

BACKGROUND

The Rnd proteins Rnd1, Rnd2, and Rnd3 (RhoE) comprise a unique branch of Rho-family G-proteins that lack intrinsic GTPase activity and consequently remain constitutively "active." Prior studies have suggested that Rnd proteins play pivotal roles in cell regulation by counteracting the biological functions of the RhoA GTPase, but the molecular basis for this antagonism is unknown. Possible mechanisms by which Rnd proteins could function as RhoA antagonists include sequestration of RhoA effector molecules, inhibition of guanine nucleotide exchange factors, and activation of GTPase-activating proteins (GAPs) for RhoA. However, effector molecules of Rnd proteins with such properties have not been identified.

RESULTS

Here we identify p190 RhoGAP (p190), the most abundant GAP for RhoA in cells, as an interactor with Rnd proteins and show that this interaction is mediated by a p190 region that is distinct from the GAP domain. Using Rnd3-RhoA chimeras and Rnd3 mutants defective in p190 binding, as well as p190-deficient cells, we demonstrate that the cellular effects of Rnd expression are mediated by p190. We moreover show that Rnd proteins increase the GAP activity of p190 toward GTP bound RhoA and, finally, demonstrate that expression of Rnd3 leads to reduced cellular levels of RhoA-GTP by a p190-dependent mechanism.

CONCLUSIONS

Our results identify p190 RhoGAPs as effectors of Rnd proteins and demonstrate a novel mechanism by which Rnd proteins function as antagonists of RhoA.

Central role for Rho in TGF-beta1-induced alpha-smooth muscle actin expression during epithelial-mesenchymal transition

New research suggests that, during tubulointerstitial fibrosis, alpha-smooth muscle actin (SMA)-expressing mesenchymal cells might derive from the tubular epithelium via epithelial-mesenchymal transition (EMT). Although transforming growth factor-beta(1) (TGF-beta(1)) plays a key role in EMT, the underlying cellular mechanisms are not well understood. Here we characterized TGF-beta(1)-induced EMT in LLC-PK(1) cells and examined the role of the small GTPase Rho and its effector, Rho kinase, (ROK) in the ensuing cytoskeletal remodeling and SMA expression. TGF-beta(1) treatment caused delocalization and downregulation of cell contact proteins (ZO-1, E-cadherin, beta-catenin), cytoskeleton reorganization (stress fiber assembly, myosin light chain phosphorylation), and robust SMA synthesis. TGF-beta(1) induced a biphasic Rho activation. Stress fiber assembly was prevented by the Rho-inhibiting C3 transferase and by dominant negative (DN) ROK. The SMA promoter was activated strongly by constitutively active Rho but not ROK. Accordingly, TGF-beta(1)-induced SMA promoter activation was potently abrogated by two Rho-inhibiting constructs, C3 transferase and p190RhoGAP, but not by DN-ROK. Truncation analysis showed that the first CC(A/T)richGG (CArG B) serum response factor-binding cis element is essential for the Rho responsiveness of the SMA promoter. Thus Rho plays a dual role in TGF-beta(1)-induced EMT of renal epithelial cells. It is indispensable both for cytoskeleton remodeling and for the activation of the SMA promoter. The cytoskeletal effects are mediated via the Rho/ROK pathway, whereas the transcriptional effects are partially ROK independent.

The formin-homology-domain-containing protein FHOD1 enhances cell migration

Formin-homology-domain-containing proteins interact with Rho-family GTPases and regulate actin cytoskeleton organization and gene transcription. FHOD1 is a member of this family, interacts with Rac1 and induces transcription from the serum response element. In this study, we examined the effects of FHOD1 expression on cytoskeletal organization and function in mammalian cells. FHOD1 proteins were stably expressed in WM35 melanoma cells and NIH-3T3 fibroblasts. Cells expressing full-length FHOD1 demonstrated an elongated phenotype compared with vector-transfected cells and cells expressing a truncated FHOD1 (1-421) that lacks the conserved FH1 and FH2 domains. Full-length FHOD1 co-localized with filamentous actin at cell peripheries. Cells transiently expressing a C-terminal FHOD1 truncation mutant (DeltaC, residues 1-1010), which lacks an autoinhibitory protein-protein interaction domain, displayed prominent stress fibers. FHOD1 (1-421) did not induce stress fibers but localized to membrane ruffles in a manner similar to the full-length protein, indicating that the FH1 and FH2 domains are required for stress fiber appearance. FHOD1 DeltaC (1-1010)-dependent stress fibers were sensitive to dominant-negative RacN17 and the RhoA and ROCK inhibitors, C3 transferase and Y-27632. Stable overexpression of full-length FHOD1 enhanced the migration of WM35 and NIH-3T3 cells to type-I collagen and fibronectin, respectively. Cells expressing FHOD1 (1-421) migrated similar to control cells. Integrin expression and activation were not affected by FHOD1 expression. Moreover, FHOD1 overexpression did not alter integrin usage during adhesion or migration. These data demonstrate that FHOD1 interacts with and regulates the structure of the cytoskeleton and stimulates cell migration in an integrin-independent manner.