DOCK1 regulates the malignant biological behavior of endometrial cancer through c-Raf/ERK pathway

BACKGROUND

The effect of DOCK1 gene on the biological behavior of endometrial carcinoma cells and its related pathway has not been reported.

METHODS

The immunohistochemical method and western blot were utilized to analyze DOCK1 protein expression in endometrial tissues and cells, respectively. CCK-8, BrdU, transwell and flow cytometry were performed to analyze the effect of DOCK1 expression changes on the viability, proliferation, invasion, migration and apoptosis of endometrial cancer cells, respectively. The effects of DOCK1 gene on Bcl-2, MMP9, Ezrin, E-cadherin and c-RAF/ERK1/2 signaling pathway were evaluated by western blot. The xenograft models were constructed to analyze the effect of DOCK1 in vivo.

RESULTS

DOCK1 expression was increased in endometrial cancer tissues and cells compared with those in normal adjacent tissues and cells. DOCK1 knockout could inhibit the malignant biological behavior of endometrial cancer cells, while DOCK1 overexpression played the opposite effect. The expression of E-cadherin was upregulated and those of MMP9, Ezrin, Bcl-2, p-c-RAF (S338) and p-ERK1/2 (T202/Y204) were downregulated after DOCK1 knockout, while DOCK1 overexpression played the opposite effect. Additionally, Raf inhibitor LY3009120 reversed the function of DOCK1 on malignant biological behavior. In vivo experiment results showed that the growth and weight of transplanted tumors in nude mice were inhibited after DOCK1 knockout. The changes of E-cadherin, MMP9, Ezrin and Bcl-2 expressions in the transplanted tumors were consistent with those in vitro.

CONCLUSION

DOCK1 could enhance the malignant biological behavior of endometrial cancer cells, which might be through c-RAF/ERK1/2 signaling pathways in vitro and in vivo.

Establishment of a prognostic model to predict chemotherapy response and identification of RAC3 as a chemotherapeutic target in bladder cancer

Cisplatin-based chemotherapy is considered the primary treatment option for patients with advanced bladder cancer (BCa). However, the objective response rate to chemotherapy is often unsatisfactory, leading to a poor 5-year survival rate. Furthermore, current strategies for evaluating chemotherapy response and prognosis are limited and inefficient. In this study, we aimed to address these challenges by establishing a chemotherapy response type gene (CRTG) signature consisting of 9 genes and verified the prognostic value of this signature using TCGA and GEO BCa cohorts. The risk scores based on the CRTG signature were found to be associated with advanced clinicopathological status and demonstrated favorable predictive power for chemotherapy response in the TCGA cohort. Meanwhile, tumors with high risk scores exhibited a tendency toward a "cold tumor" phenotype. These tumors showed a low abundance of T cells, CD8+ T cells and cytotoxic lymphocytes, along with a high abundance of cancer-associated fibroblasts. Moreover, they displayed higher mRNA levels of these immune checkpoints: CD200, CD276, CD44, NRP1, PDCD1LG2 (PD-L2), and TNFSF9. Furthermore, we developed a nomogram that integrated the CRTG signature with clinicopathologic risk factors. This nomogram proved to be a more effective tool for predicting the prognosis of BCa patients. Additionally, we identified Rac family small GTPase 3 (RAC3) as a biomarker in our model. RAC3 was found to be overexpressed in chemoresistant BCa tissues and enhance the chemotherapeutic resistance of BCa cells in vitro and in vivo by regulating the PAK1-ERK1/2 pathway. In conclusion, our study presents a novel CRTG model for predicting chemotherapy response and prognosis in BCa. We also highlight the potential of combining chemotherapy with immunotherapy as a promising strategy for chemoresistant BCa and that RAC3 might be a latent target for therapeutic intervention.

Early diagnosis and prognostic potential of RAC3 in bladder tumor

BACKGROUND AND PURPOSE

Bladder tumors are among the most prevalent malignancies in the urinary system, and RAC3 has been linked to various types of cancer. This article seeks to explore the potential of RAC3 as both an early diagnostic marker for bladder tumors and a novel therapeutic target.

METHODS/PATIENTS

The expression of RAC3 in bladder tissue was detected using immunohistochemical staining. Additionally, the protein expression of RAC3 was measured and quantified through enzyme-linked immunosorbent assay (ELISA). Subsequently, the correlation between the expression level of RAC3 and bladder tumors was investigated through multifactorial analysis and survival analysis.

RESULTS

Our findings revealed that RAC3 expression was upregulated in bladder tumor tissues. Moreover, we observed higher levels of RAC3 expression in the serum and urine of patients with bladder tumors compared to those with non-bladder tumors. Additionally, we identified a significant positive correlation between RAC3 expression levels and the stage, degree of differentiation, and infiltration of bladder tumors. Importantly, high RAC3 expression emerged as an influential factor in the poor prognosis of bladder tumors, as patients with high RAC3 expression exhibited a lower overall survival rate than those with low RAC3 expression.

CONCLUSION

Based on our results, RAC3 shows promise as both a marker for early diagnosis of bladder tumors and a potential therapeutic target.

Pulsatilla saponin D regulates ras-related C3 botulinum toxin substrate 3 (RAC3) to overcome resistance to paclitaxel in lung adenocarcinoma cells

BACKGROUND

Paclitaxel, a tubulin-binding agent, is a Food and Drug Administration-approved first-line drug for the treatment of non-small cell lung cancer (NSCLC), for both squamous and non-squamous cell lung carcinoma, with paclitaxel/carboplatin + bevacizumab a common chemotherapy regimen for stage IV non-squamous NSCLC; however, primary or acquired resistance to paclitaxel is gradually increasing, leading to treatment failure.

METHODS

Our results show that Ras-related C3 botulinum toxin substrate 3 (RAC3) is overexpressed in cultured paclitaxel-resistant cells and that RAC3 expression levels are negatively correlated with sensitivity of lung adenocarcinoma cells to paclitaxel. Pulsatilla saponin D could inhibit RAC3 expression, and we hypothesize that it may block paclitaxel resistance. Further, we found that treatment with paclitaxel combined with Pulsatilla saponin D, can overcome lung adenocarcinoma cell resistance to paclitaxel alone in cell culture and mouse xenograft models.

Combinatorial genetics reveals the Dock1-Rac2 axis as a potential target for the treatment of NPM1;Cohesin mutated AML

Acute myeloid leukemia (AML) is driven by mutations that occur in numerous combinations. A better understanding of how mutations interact with one another to cause disease is critical to developing targeted therapies. Approximately 50% of patients that harbor a common mutation in NPM1 (NPM1cA) also have a mutation in the cohesin complex. As cohesin and Npm1 are known to regulate gene expression, we sought to determine how cohesin mutation alters the transcriptome in the context of NPM1cA. We utilized inducible Npm1cAflox/+ and core cohesin subunit Smc3flox/+ mice to examine AML development. While Npm1cA/+;Smc3Δ/+ mice developed AML with a similar latency and penetrance as Npm1cA/+ mice, RNA-seq suggests that the Npm1cA/+; Smc3Δ/+ mutational combination uniquely alters the transcriptome. We found that the Rac1/2 nucleotide exchange factor Dock1 was specifically upregulated in Npm1cA/+;Smc3Δ/+ HSPCs. Knockdown of Dock1 resulted in decreased growth and adhesion and increased apoptosis only in Npm1cA/+;Smc3Δ/+ AML. Higher Rac activity was also observed in Npm1cA/+;Smc3Δ/+ vs. Npm1cA/+ AMLs. Importantly, the Dock1/Rac pathway is targetable in Npm1cA/+;Smc3Δ/+ AMLs. Our results suggest that Dock1/Rac represents a potential target for the treatment of patients harboring NPM1cA and cohesin mutations and supports the use of combinatorial genetics to identify novel precision oncology targets.

Inflammatory and immune-related factor Caspase 1 contributes to the development of oral lichen planus

OBJECTIVE

In this study, we aimed at underlying the potential regulatory mechanism and overall biological functions of caspase 1 (CASP1) in oral lichen planus (OLP).

DESIGN

Buccal mucosa tissue samples were gained from healthy subjects or patients diagnosed with OLP. Immunochemical staining was applied to detect CASP1 in OLP tissues. Lipopolysaccharide (LPS) was used to construct OLP in vitro models. Cell counting kit-8 (CCK-8) and flow cytometry assay were applied to detecte cell viability and apoptosis.

RESULTS

The upregulation of CASP1 in OLP has been identified through comprehensive bioinformatics analysis and verified in clinical samples. In OLP tissues, inflammation-related factors, including tumor necrosis factor alpha (TNF-α), interleukin (IL)-1β, IL-6, and IL-18, were elevated and positively correlated with CASP1. In HaCaT cells, LPS stimulation induced CASP1 upregulation, suppressed cell viability, facilitated cell apoptosis, and elevated the levels of TNF-α, IL-1β, IL-6, and IL-18; silencing of CASP1 attenuated LPS-induced damages to HaCaT cells. Pearson's correlation analysis identified that 45 immune-related genes were positively correlated with CASP1; these 45 genes were enriched in the immune system process, associated with combined immunodeficiency, and spleen-specific and CD56 + NK cell-specific. PPI network among CASP1 and correlated immune-related factors was constructed, and CASP1 was positively correlated with RAC2, CYBB, and ARHGDIB. In HaCaT cells, LPS stimulation induced RAC2, CYBB, and ARHGDIB expression, whereas knocking down CASP1 attenuated LPS-induced increases in RAC2, CYBB, and ARHGDIB.

CONCLUSION

CASP1 is upregulated in OLP tissues. Knockdown of CASP1 in HaCaT cells could protect HaCaT cells from LPS-induced inflammatory injury. Comprehensive bioinformatics indicates that the interaction of CASP1 with RAC2, CYBB, and ARHGDIB, might be the potential molecular mechanism.

A gain-of-function RAC2 mutation is associated with bone-marrow hypoplasia and an autosomal dominant form of severe combined immunodeficiency

Severe combined immunodeficiencies (SCIDs) constitute a heterogeneous group of life-threatening genetic disorders that typically present in the first year of life. They are defined by the absence of autologous T cells and the presence of an intrinsic or extrinsic defect in the B-cell compartment. In three newborns presenting with frequent infections and profound leukopenia, we identified a private, heterozygous mutation in the RAC2 gene (p.G12R). This mutation was de novo in the index case, who had been cured by hematopoietic stem cell transplantation but had transmitted the mutation to her sick daughter. Biochemical assays showed that the mutation was associated with a gain of function. The results of in vitro differentiation assays showed that RAC2 is essential for the survival and differentiation of hematopoietic stem/progenitor cells. Therefore, screening for RAC2 gain-of-function mutations should be considered in patients with a SCID phenotype and who lack a molecular diagnosis.

Cdk1 phosphorylates the Rac activator Tiam1 to activate centrosomal Pak and promote mitotic spindle formation

Centrosome separation is critical for bipolar spindle formation and the accurate segregation of chromosomes during mammalian cell mitosis. Kinesin-5 (Eg5) is a microtubule motor essential for centrosome separation, and Tiam1 and its substrate Rac antagonize Eg5-dependent centrosome separation in early mitosis promoting efficient chromosome congression. Here we identify S1466 of Tiam1 as a novel Cdk1 site whose phosphorylation is required for the mitotic function of Tiam1. We find that this phosphorylation of Tiam1 is required for the activation of group I p21-activated kinases (Paks) on centrosomes in prophase. Further, we show that both Pak1 and Pak2 counteract centrosome separation in a kinase-dependent manner and demonstrate that they act downstream of Tiam1. We also show that depletion of Pak1/2 allows cells to escape monopolar arrest by Eg5 inhibition, highlighting the potential importance of this signalling pathway for the development of Eg5 inhibitors as cancer therapeutics.

Prenatal cocaine exposure increases synaptic localization of a neuronal RasGEF, GRASP-1 via hyperphosphorylation of AMPAR anchoring protein, GRIP

Prenatal cocaine exposure causes sustained phosphorylation of the synaptic anchoring protein, glutamate receptor interacting protein (GRIP1/2), preventing synaptic targeting of the GluR2/3-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs; J. Neurosci. 29: 6308–6319, 2009). Because overexpression of GRIP-associated neuronal rasGEF protein (GRASP-1) specifically reduces the synaptic targeting of AMPARs, we hypothesized that prenatal cocaine exposure enhances GRASP-1 synaptic membrane localization leading to hyper-activation of ras family proteins and heightened actin polymerization. Our results show a markedly increased GRIP1-associated GRASP-1 content with approximately 40% reduction in its rasGEF activity in frontal cortices (FCX) of 21-day-old (P21) prenatal cocaine-exposed rats. This cocaine effect is the result of a persistent protein kinase C (PKC)- and downstream Src tyrosine kinase-mediated GRIP phosphorylation. The hyperactivated PKC also increased membrane-associated GRASP-1 and activated small G-proteins RhoA, cdc42/Rac1 and Rap1 as well as filamentous actin (F-actin) levels without an effect on the phosphorylation state of actin. Since increased F-actin facilitates protein transport, our results suggest that increased GRASP-1 synaptic localization in prenatal cocaine-exposed brains is an adaptive response to restoring the synaptic expression of AMPA-GluR2/3. Our earlier data demonstrated that persistent PKC-mediated GRIP phosphorylation reduces GluR2/3 synaptic targeting in prenatal cocaine-exposed brains, we now show that the increased GRIP-associated GRASP-1 may contribute to the reduction in GluR2/3 synaptic expression and AMPAR signaling defects.

A role for the Myoblast city homologues Dock1 and Dock5 and the adaptor proteins Crk and Crk-like in zebrafish myoblast fusion

Myoblast fusion follows a defined sequence of events that is strikingly similar in vertebrates and invertebrates. Genetic analysis in Drosophila has identified many of the molecules that mediate the different steps in the fusion process; by contrast, the molecular basis of myoblast fusion during vertebrate embryogenesis remains poorly characterised. A key component of the intracellular fusion pathway in Drosophila is the protein encoded by the myoblast city (mbc) gene, a close homologue of the vertebrate protein dedicator of cytokinesis 1 (DOCK1,formerly DOCK180). Using morpholino antisense-oligonucleotide-mediated knockdown of gene activity in the zebrafish embryo, we show that the fusion of embryonic fast-twitch myoblasts requires the activities of Dock1 and the closely related Dock5 protein. In addition, we show that the adaptor proteins Crk and Crk-like (Crkl), with which Dock proteins are known to interact physically, are also required for myoblast fusion.

The PP2A-associated protein alpha4 plays a critical role in the regulation of cell spreading and migration

Compared with kinases, the role of protein phosphatases in regulating biological functions is less well understood. Here we show that alpha4, a non-catalytic subunit of the protein phosphatase 2A, plays a major role in the control of cell spreading, migration, and cytoskeletal architecture. Fibroblasts lacking alpha4 were impaired in their ability to spread and migrate compared with wild-type cells, whereas enforced expression of alpha4 promoted cell spreading and migration. These effects were not restricted to fibroblasts. Using a T cell-specific alpha4 transgenic mouse model, increased alpha4 expression was found to increase lymphocyte motility and chemotaxis. Elevated alpha4 expression results in an increase in the GTP-bound state of Rac1, and GTP-bound Rac1 was dramatically reduced in alpha4-deficient cells. A constitutively active mutant of Rac1 rescued the defects of cell spreading and migration caused by alpha4 deletion, while inhibition of Rac1 blocked the ability of alpha4 to promote cell migration. Together, these data define a novel role for the protein phosphatase 2A regulatory subunit alpha4 in the regulation of cell spreading and migration.

Roles of the Rac1 and Rac3 GTPases in human tumor cell invasion

Members of the Rho family of small GTPases have been shown to be involved in tumorigenesis and metastasis. Currently, most of the available information on the function of Rho proteins in malignant transformation is based on the use of dominant-negative mutants of these GTPases. The specificity of these dominant-negative mutants is limited however. In this study, we used small interfering RNA directed against either Rac1 or Rac3 to reduce their expression specifically. In line with observations using dominant-negative Rac1 in other cell types, we show that RNA interference-mediated depletion of Rac1 strongly inhibits lamellipodia formation, cell migration and invasion in SNB19 glioblastoma cells. Surprisingly however, Rac1 depletion has a much smaller inhibitory effect on SNB19 cell proliferation and survival. Interestingly, whereas depletion of Rac3 strongly inhibits SNB19 cell invasion, it does not affect lamellipodia formation and has only minor effects on cell migration and proliferation. Similar results were obtained in BT549 breast carcinoma cells. Thus, functional analysis of Rac1 and Rac3 using RNA interference reveals a critical role for these GTPases in the invasive behavior of glioma and breast carcinoma cells.

Rho and Rac promote acinar morphological changes, actin reorganization, and amylase secretion

Supramaximal stimulation of isolated pancreatic acini with specific agonists such as CCK induces the formation of large basolateral blebs, redistributes filamentous actin, and inhibits secretion. Rho family small G proteins are well documented for their function in actin reorganization that determines cell shape and have been suggested to play a role in secretion. Here, we determined whether Rho and Rac are involved in the morphological changes, actin redistribution, and inhibition of amylase secretion induced by high concentrations of CCK. Introduction of constitutively active RhoV14 and RacV12 but not Cdc42V12 in mouse pancreatic acini by adenoviral vectors stimulated acinar morphological changes including basolateral protrusions, increased the total amount of F-actin, and reorganized the actin cytoskeleton. Dominant-negative RhoN19, Clostridium botulinum C3 exotoxin, which inhibits Rho, and dominant-negative RacN17 all partially blocked CCK-induced acinar morphological changes and actin redistribution. To study the correlation between actin polymerization and acinar shape changes, two marine toxins were employed. Jasplakinolide, a reagent that facilitates actin polymerization and stabilizes F-actin, stimulated acinar basolateral protrusions, whereas latrunculin, which sequesters actin monomers, blocked CCK-induced acinar blebbing. Unexpectedly, RhoV14, RacV12, and jasplakinolide all increased amylase secretion by CCK from 30 pM to 10 nM. The data suggest that Rho and Rac are involved in CCK-evoked changes in acinar morphology, actin redistribution, and secretion and that inhibition of secretion by high concentrations of CCK is not directly coupled to the changes in acinar morphology.

Wave2 activates serum response element via its VCA region and functions downstream of Rac

WAVE2 is a member of the WASP/WAVE family of protein effectors of actin reorganization and cell movement. In this report, we demonstrate that WAVE2 overexpression induces serum response element (SRE) activation through serum response factor. A WAVE2 mutant lacking the VCA region did not induce SRE activation and actin polymerization. WAVE2-induced SRE activation was blocked by exposure of cells to Latrunculin A, or overexpression of actin mutant R62D. The DeltaVCA mutant inhibited Rac V12-induced SRE activation, suggesting that WAVE2 lies downstream of Rac. Similar deletion of the VCA domain of WASP attenuated Cdc42 V12-mediated SRE activation, suggesting that WAVE2 acts in relation to Rac as WASP acts in relation to Cdc42. WAVE2 overexpression did not activate NF-kappaB.

Rac1 mediates collapse of microvilli on chemokine-activated T lymphocytes

Lymphocytes circulate in the blood and upon chemokine activation rapidly bind, where needed, to microvasculature to mediate immune surveillance. Resorption of microvilli is an early morphological alteration induced by chemokines that facilitates lymphocyte emigration. However, the antecedent molecular mechanisms remain largely undefined. We demonstrate that Rac1 plays a fundamental role in chemokine-induced microvillar breakdown in human T lymphocytes. The supporting evidence includes: first, chemokine induces Rac1 activation within 5 s via a signaling pathway that involves Galphai. Second, constitutively active Rac1 mediates microvilli disintegration. Third, blocking Rac1 function by cell permeant C-terminal "Trojan" peptides corresponding to Rac1 (but not Rac2, Rho, or Cdc42) blocks microvillar loss induced by the chemokine stromal cell-derived factor 1alpha (SDF-1alpha). Furthermore, we demonstrate that the molecular mechanism of Rac1 action involves dephosphorylation-induced inactivation of the ezrin/radixin/moesin (ERM) family of actin regulators; such inactivation is known to detach the membrane from the underlying actin cytoskeleton, thereby facilitating disassembly of actin-based peripheral processes. Specifically, ERM dephosphorylation is induced by constitutively active Rac1 and stromal cell-derived factor 1alpha-induced ERM dephosphorylation is blocked by either the dominant negative Rac1 construct or by Rac1 C-terminal peptides. Importantly, the basic residues at the C terminus of Rac1 are critical to Rac1's participation in ERM dephosphorylation and in microvillar retraction. Together, these data elucidate new roles for Rac1 in early signal transduction and cytoskeletal rearrangement of T lymphocytes responding to chemokine.

Interaction between Dab1 and CrkII is promoted by Reelin signaling

Reelin-induced Dab1 tyrosine phosphorylation has been implicated in the regulation of neuronal positioning during brain development. The downstream consequences of Dab1 tyrosine phosphorylation are not fully understood, however. Here we identify CrkII, CrkL and Dock1 in complexes bound to tyrosine-phosphorylated Dab1, through mass spectrometry. The CrkII-Dab1 interaction requires tyrosine phosphorylation of Dab1 at residues 220 or 232 and is promoted by Reelin treatment of embryonic forebrain neurons. Unlike other CrkII binding proteins, such as paxillin and p130Cas, expression of Dab1 interfered with CrkII-dependent cell migration of Nara Bladder Tumor II (NBT-II) cells, in a tyrosine phosphorylation-site dependent manner. Overexpression of CrkIIGFP rescued the migration of these cells, suggesting that Dab1 makes Crk a limiting factor for migration. The Dock1-Dab1 association is indirect and requires CrkII. In organisms such as Drosophila melanogaster and Caenorhabditis elegans, signaling complexes, which contain Crk and Dock1 family members are conserved and act through Rac. We show that a rough-eye phenotype in Drosophila caused by exogenous expression of tyrosine-phosphorylated mouse Dab1RFP is partially rescued by a loss-of-function mutation in myoblast city, a Dock1-like gene in Drosophila. We propose a model that tyrosine-phosphorylated Dab1 engages the conserved Crk-Dock1-Rac signaling cassette, but when bound to Dab1 this signaling complex does not support migration.

Lipid Raft-Associated GTPase Signaling Controls Morphology and CD8+T Cell Stimulatory Capacity of Human Dendritic Cells

Their eponymous morphology and unique ability to activate naive T cells are hallmark features of dendritic cells (DCs). Specific properties of the actin cytoskeleton may define both characteristics. In search for regulators that coordinate DC phenotype and function, we observed strongly increased expression of the actin-remodeling GTPases Cdc42 and Rac1 during DC development from human stem cells. Cdc42 and Rac1 are constitutively active in immature DCs, and their activity is further up-regulated by maturational stimuli such as LPS or CD40L. Activation of Rac1 is associated with its rapid recruitment into lipid rafts. Cdc42 is not recruited into rafts, but readily activated by raft-associated moieties. The functional interplay of rafts, GTPases, and cortical actin is further shown by GTPase activation and actin remodeling after pharmacological disruption of lipid rafts and by the loss of the actin-based DC morphology by transfection of dominant-negative Cdc42 and Rac1. Both Cdc42 and Rac1 also control the transport of essential immunostimulatory molecules to the DC surface. Transfection with dominant-negative GTPases led to reduced surface expression of MHC class I and CD86. Consecutively, DCs display a reduced stimulatory capacity for CD8(+) T cells, whereas MHC class II-dependent stimulation of CD4(+) T cells remains unperturbed. We conclude that Cdc42 and Rac1 signaling controls DC morphology and conditions DCs for efficient CD8(+) T cell stimulation.

Regulation of dendritic spine motility and stability by Rac1 and Rho kinase: evidence for two forms of spine motility

Dendritic spines are major sites of excitatory synapses in the brain and display rapid motility, which is believed to be important for synapse formation and plasticity. Spine morphology was previously shown to be regulated by the Rho GTPases Rac1 and RhoA. Here, we analyzed the roles of Rac1 and a downstream effector of RhoA, Rho kinase, in controlling spine morphogenesis and their effects on spine motility and stability. Blockade of Rac1 induced long, thin spines and inhibited spine head growth, morphing, and stability. Spine head growth was more severely affected in mature spines. On the other hand, inhibition of Rho kinase induced new, long spines and protrusive motility. These data demonstrate that Rac1 and RhoA/Rho kinase pathways regulate different aspects of spine morphology, motility, and stability and presumably also different aspects of synaptic functions. Moreover, our data show that there are two different types of spine motility: protrusive motility and head morphing, which are differentially regulated by Rac1 and Rho kinase. We propose that these two different types of spine motility serve different functions in synaptogenesis and synapse maturation.

Differential Roles of WAVE1 and WAVE2 in Dorsal and Peripheral Ruffle Formation for Fibroblast Cell Migration

Cell migration is driven by actin polymerization at the leading edge of lamellipodia, where WASP family verprolin-homologous proteins (WAVEs) activate Arp2/3 complex. When fibroblasts are stimulated with PDGF, formation of peripheral ruffles precedes that of dorsal ruffles in lamellipodia. Here, we show that WAVE2 deficiency impairs peripheral ruffle formation and WAVE1 deficiency impairs dorsal ruffle formation. During directed cell migration in the absence of extracellular matrix (ECM), cells migrate with peripheral ruffles at the leading edge and WAVE2, but not WAVE1, is essential. In contrast, both WAVE1 and WAVE2 are essential for invading migration into ECM, suggesting that the leading edge in ECM has characteristics of both ruffles. WAVE1 is colocalized with ECM-degrading enzyme MMP-2 in dorsal ruffles, and WAVE1-, but not WAVE2-, dependent migration requires MMP activity. Thus, WAVE2 is essential for leading edge extension for directed migration in general and WAVE1 is essential in MMP-dependent migration in ECM.

Effects of large clostridial cytotoxins on activation of RBL 2H3-hm1 mast cells indicate common and different roles of Rac in FcepsilonRI and M1-receptor signaling

Using Rho GTPases-inhibiting clostridial cytotoxins, we showed recently in RBL cells that the GTPase Rac is involved in FcepsilonRI (high-affinity receptor for IgE) signaling and receptor-mediated calcium mobilization, including influx via calcium release-activated calcium channels. Here, we studied the role of Rho GTPases in muscarinic M1 receptor signaling in RBL 2H3-hm1 cells. Clostridium difficile toxin B, which inactivates Rho, Rac, and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac but not Rho, blocked M1-mediated exocytosis, indicating that Rac but not Rho is involved in the regulation of receptor-mediated exocytosis. Although antigen-induced FcepsilonRI stimulation caused tyrosine phosphorylation of the Rac guanine nucleotide exchange factor Vav, M1 stimulation by carbachol activated Rac independently of Vav. The Rac-inactivating toxins blocked M1 receptor-induced membrane translocation of the pleckstrin homology domain of protein kinase B, which is a phosphoinositide 3-kinase effector. The M1-induced calcium release from internal stores was not affected by toxin B; however, the subsequent calcium influx from the extracellular space was inhibited. The data suggest that besides capacitative calcium entry, the M1 signaling pathway activates further calcium entry channels with mechanisms that are not affected by the inhibition of Rac.

Defects in cytokinesis, actin reorganization and the contractile vacuole in cells deficient in RhoGDI

Rho GDP-dissociation inhibitors (RhoGDIs) modulate the cycling of Rho GTPases between active GTP-bound and inactive GDP-bound states. We identified two RhoGDI homologues in DICTYOSTELIUM: GDI1 shares 51-58% similarity to RhoGDIs from diverse species. GDI2 is more divergent (40-44% similarity) and lacks the N-terminal regulatory arm characteristic for RhoGDI proteins. Both are cytosolic proteins and do not relocalize upon reorganization of the actin cytoskeleton. Using a two-hybrid approach, we identified Rac1a/1b/1c, RacB, RacC and RacE as interacting partners for GDI1. Cells lacking GDI1 are multinucleate, grow slowly and display a moderate pinocytosis defect, but rates of phagocytosis are unaffected. Mutant cells present prominent actin-rich protrusions, and large vacuoles that are continuous with the contractile vacuole system. The actin polymerization response upon stimulation with cAMP was reduced, but the motile behavior toward the chemoattractant was unaffected. Our results indicate that GDI1 plays a central role in the regulation of signal transduction cascades mediated by Rho GTPases.

ced-10 Rac and mig-2 function redundantly and act with unc-73 trio to control the orientation of vulval cell divisions and migrations in Caenorhabditis elegans

Vulval development in the nematode Caenorhabditis elegans can be divided into a fate specification phase controlled in part by let-60 Ras, and a fate execution phase involving stereotypical patterns of cell division and migration controlled in part by lin-17 Frizzled. Since the small GTPase Rac has been implicated as a downstream target of both Ras and Frizzled and influences cytoskeletal dynamics, we investigated the role of Rac signaling during each phase of vulval development. We show that the Rac gene ced-10 and the Rac-related gene mig-2 are redundantly required for the proper orientation of certain vulval cell divisions, suggesting a role in spindle positioning. ced-10 Rac and mig-2 are also redundantly required for vulval cell migrations and play a minor role in vulval fate specification. Constitutively active and dominant-negative mutant forms of mig-2 cause vulval defects that are very similar to those seen in ced-10;mig-2 double loss-of-function mutants, indicating that they interfere with the functions of both ced-10 Rac and mig-2. Mutations in unc-73 (a Trio-like guanine nucleotide exchange factor) cause similar vulval defects, suggesting that UNC-73 is an exchange factor for both CED-10 and MIG-2. We discuss the similarities and differences between the cellular defects seen in Rac mutants and let-60 Ras or lin-17 Frizzled mutants.

Cell engulfment, no sooner ced than done

Three genes, ced-2, ced-5, and ced-10, are required for both cell corpse engulfment and distal tip cell migration in C. elegans. Recently, a fourth gene, ced-12, has been identified that is required for these two processes. ced-12 encodes a novel, conserved adaptor molecule involved in the activation of Rho/Rac/CDC42-like GTPases.

Laminin-10/11 and Fibronectin Differentially Regulate Integrin- dependent Rho and Rac Activation via p130Cas-CrkII-DOCK180 Pathway

The alpha(5) chain-containing laminin isoforms, laminins-10 and -11 (laminin-10/11), are the major components of the basement membrane, having potent cell-adhesive activity. We examined the cell-adhesive and integrin-mediated signaling activities of laminin-10/11 in comparison to fibronectin, the best characterized extracellular adhesive ligand. We found that laminin-10/11 are more active than fibronectin in promoting cell migration and preferentially activate Rac, not Rho, via the p130(Cas)-CrkII-DOCK180 pathway. Cells adhering to fibronectin develop stress fibers and focal contacts, whereas cells adhering to laminin-10/11 do not, consistent with the high cell migration-promoting activity of laminin-10/11. Pull-down assays of GTP-loaded Rac and Rho demonstrated the preferential activation of Rac on laminin-10/11, in contrast to the activation of Rho on fibronectin. Activation of Rac by laminin-10/11 was associated with the phosphorylation of p130(Cas) and an increased formation of a p130(Cas)-CrkII-DOCK180 complex. Cell migration on laminin-10/11 was suppressed by the expression of either a dominant-negative Rac or CrkII mutants defective in p130(Cas) or DOCK180 binding. This is the first report demonstrating a distinct activation of Rho family GTPases resulting from adhesion to different extracellular ligands.

Identification and kinetic analysis of the interaction between Nck-2 and DOCK180

Nck-2 is a newly identified adapter protein comprising three N-terminal SH3 domains and one C-terminal SH2 domain. We have identified in a yeast two-hybrid screen DOCK180, a signaling protein implicated in the regulation of membrane ruffling and migration, as a binding protein for Nck-2. Surface plasmon resonance analyses reveal that the second and the third SH3 domains interact with the C-terminal region of DOCK180. The interactions mediated by the individual SH3 domains, however, are much weaker than that of the full length Nck-2. Furthermore, a point mutation that inactivates the second or the third SH3 domain dramatically reduced the interaction of Nck-2 with DOCK180, suggesting that both SH3 domains contribute to the DOCK180 binding. A major Nck-2 binding site, which is recognized primarily by the third SH3 domain, has been mapped to residues 1819-1836 of DOCK180. Two additional, albeit much weaker, Nck-2 SH3 binding sites are located to DOCK180 residues 1793-1810 and 1835-1852 respectively. Consistent with the mutational studies, kinetic analyses by surface plasmon resonance suggest that two binding events with equilibrium dissociation constants of 4.15+/-1.9x10(-7) M and 3.24+/-1.9x10(-9) M mediate the binding of GST-Nck-2 to GST fusion protein containing the C-terminal region of DOCK180. These studies identify a novel interaction between Nck-2 and DOCK180. Furthermore, they provide a detailed analysis of a protein complex formation mediated by multiple SH3 domains revealing that tandem SH3 domains significantly enhance the weak interactions mediated by each individual SH3 domain.

alphavbeta5 integrin recruits the CrkII-Dock180-rac1 complex for phagocytosis of apoptotic cells

Integrin receptors are important for the phagocytosis of apoptotic cells. However, little is known about their function in mediating internalization, as previous studies used blocking antibodies for the inhibition of binding. Here we show that the alphavbeta5 receptor mediates both binding and internalization of apoptotic cells. Internalization is dependent upon signalling through the beta5 cytoplasmic tail, and engagement of the alphavbeta5 heterodimer results in recruitment of the p130cas-CrkII-Dock180 molecular complex, which in turn triggers Rac1 activation and phagosome formation. In addition to defining integrin-receptor signalling as critical for the internalization of apoptotic material, our results also constitute the first evidence in human cells that the CED-2-CED-5-CED-10 complex defined in Caenorhabditis elegans is functionally analagous to the CrkII-Dock180-Rac1 molecular complex in mammalian cells. By linking the alphavbeta 5 receptor to this molecular switch, we reveal an evolutionarily conserved signalling pathway that is responsible for the recognition and internalization of apoptotic cells by both professional and non-professional phagocytes.

Trio combines with dock to regulate Pak activity during photoreceptor axon pathfinding in Drosophila

Correct pathfinding by Drosophila photoreceptor axons requires recruitment of p21-activated kinase (Pak) to the membrane by the SH2-SH3 adaptor Dock. Here, we identify the guanine nucleotide exchange factor (GEF) Trio as another essential component in photoreceptor axon guidance. Regulated exchange activity of one of the two Trio GEF domains is critical for accurate pathfinding. This GEF domain activates Rac, which in turn activates Pak. Mutations in trio result in projection defects similar to those observed in both Pak and dock mutants, and trio interacts genetically with Rac, Pak, and dock. These data define a signaling pathway from Trio to Rac to Pak that links guidance receptors to the growth cone cytoskeleton. We propose that distinct signals transduced via Trio and Dock act combinatorially to activate Pak in spatially restricted domains within the growth cone, thereby controlling the direction of axon extension.

The neuronal architecture of Xenopus retinal ganglion cells is sculpted by rho-family GTPases in vivo

Dendritogenesis, axonogenesis, pathfinding, and target recognition are all affected in distinct ways when Xenopus retinal ganglion cells (RGCs) are transfected with constitutively active (ca), wild-type (wt), and dominant negative (dn) Rho-family GTPases in vivo. Dendritogenesis required Rac1 and Cdc42 activity. Moreover, ca-Rac1 caused dendrite hyperproliferation. Axonogenesis, in contrast, was inhibited by ca-Rac1. This phenotype was partially rescued by the coexpression of dn cyclin-dependent kinase (Cdk5), a proposed effector of Rac1, suggesting that Rac1 activity must be regulated tightly for normal axonogenesis. Growth cone morphology was particularly sensitive to dn-RhoA and wt-Cdc42 constructs. These also caused targeting errors, such as tectal bypass, suggesting that cytoskeletal rearrangements are involved in target recognition and are transduced by these pathways.

EPS8 and E3B1 transduce signals from Ras to Rac

The small guanine nucleotide (GTP)-binding protein Rac regulates mitogen-induced cytoskeletal changes and c-Jun amino-terminal kinase (JNK), and its activity is required for Ras-mediated cell transformation. Epistatic analysis placed Rac as a key downstream target in Ras signalling; however, the biochemical mechanism regulating the cross-talk among these small GTP-binding proteins remains to be elucidated. Eps8 (relative molecular mass 97,000) is a substrate of receptors with tyrosine kinase activity which binds, through its SH3 domain, to a protein designated E3b1/Abi-1. Here we show that Eps8 and E3b1/Abi-1 participate in the transduction of signals from Ras to Rac, by regulating Rac-specific guanine nucleotide exchange factor (GEF) activities. We also show that Eps8, E3b1 and Sos-1 form a tri-complex in vivo that exhibits Rac-specific GEF activity in vitro. We propose a model in which Eps8 mediates the transfer of signals between Ras and Rac, by forming a complex with E3b1 and Sos-1.

Rac1 disrupts p67phox/p40phox binding: a novel role for Rac in NADPH oxidase activation

Phagocytic cells possess a tightly regulated multicomponent enzyme complex, the NADPH oxidase, which produces superoxide, a reactive oxygen molecule that is an essential component of host defense against infection. Upon stimulation, a functional NADPH oxidase is assembled when the cytosolic proteins, Rac, p67phox, p47phox, and possibly p40phox, associate with the gp91phox and p22phox transmembrane proteins. Rac is a GTPase that in the GTP-bound state binds p67phox to activate NADPH oxidase. The function of p40phox is not known; it is believed to have a regulatory function in sequestering p67phox and p47phox in a cytosolic complex. We investigated binding interactions between p40phox, p67phox, and Rac and found that Rac1-GTP displaced p67phox bound to p40phox. In contrast, Cdc42, a GTPase homologous to Rac, did not displace p67phox from p40phox. A synthetic peptide corresponding to p67phox amino acids 170-199, a region identified previously as a Rac binding domain, significantly reduced the ability of Rac1-GTP to disrupt p67phox/p40phox binding. We hypothesize that Rac-GTP binds the p67phox N-terminal domain encompassing amino acids 170-199 that transmits a conformational change which causes p40phox to dissociate from its binding site in the p67phox C-terminus.

Cdc42 and Rac1 regulate the interaction of IQGAP1 with beta-catenin

IQGAP1, a target of Cdc42 and Rac1 small GTPases, directly interacts with beta-catenin and negatively regulates E-cadherin-mediated cell-cell adhesion by dissociating alpha-catenin from the cadherin-catenin complex in vivo (Kuroda, S., Fukata, M., Nakagawa, M., Fujii, K., Nakamura, T., Ookubo, T., Izawa, I., Nagase, T., Nomura, N., Tani, H., Shoji, I., Matsuura, Y., Yonehara, S., and Kaibuchi, K. (1998) Science 281, 832-835). Here we investigated how Cdc42 and Rac1 regulate the IQGAP1 function. IQGAP1 interacted with the amino-terminal region (amino acids 1-183) of beta-catenin, which contains the alpha-catenin-binding domain. IQGAP1 dissociated alpha-catenin from the beta-catenin-alpha-catenin complex in a dose-dependent manner in vitro. Guanosine 5'-(3-O-thio)triphosphate (GTPgammaS).glutathione S-transferase (GST)-Cdc42 and GTPgammaS. GST-Rac1 inhibited the binding of IQGAP1 to beta-catenin in a dose-dependent manner in vitro, whereas neither GDP.GST-Cdc42, GDP. GST-Rac1, nor GTPgammaS.GST-RhoA did. The coexpression of dominant active Cdc42 with IQGAP1 suppressed the dissociation of alpha-catenin from the cadherin-catenin complex induced by the overexpression of IQGAP1 in L cells expressing E-cadherin (EL cells). Consistent with this, the overexpression of either dominant negative Cdc42 or Rac1 resulted in the reduction of E-cadherin-mediated cell adhesive activity in EL cells. These results indicate that Cdc42 and Rac1 negatively regulate the IQGAP1 function by inhibiting the interaction of IQGAP1 with beta-catenin, leading to stabilization of the cadherin-catenin complex.

Microtubule targeting of substrate contacts promotes their relaxation and dissociation

We recently showed that substrate contact sites in living fibroblasts are specifically targeted by microtubules (Kaverina, I., K. Rottner, and J.V. Small. 1998. J. Cell Biol. 142:181-190). Evidence is now provided that microtubule contact targeting plays a role in the modulation of substrate contact dynamics. The results are derived from spreading and polarized goldfish fibroblasts in which microtubules and contact sites were simultaneously visualized using proteins conjugated with Cy-3, rhodamine, or green fluorescent protein. For cells allowed to spread in the presence of nocodazole the turnover of contacts was retarded, as compared with controls and adhesions that were retained under the cell body were dissociated after microtubule reassembly. In polarized cells, small focal complexes were found at the protruding cell front and larger adhesions, corresponding to focal adhesions, at the retracting flanks and rear. At retracting edges, multiple microtubule contact targeting preceded contact release and cell edge retraction. The same effect could be observed in spread cells, in which microtubules were allowed to reassemble after local disassembly by the application of nocodazole to one cell edge. At the protruding front of polarized cells, focal complexes were also targeted and as a result remained either unchanged in size or, more rarely, were disassembled. Conversely, when contact targeting at the cell front was prevented by freezing microtubule growth with 20 nM taxol and protrusion stimulated by the injection of constitutively active Rac, peripheral focal complexes became abnormally enlarged. We further found that the local application of inhibitors of myosin contractility to cell edges bearing focal adhesions induced the same contact dissociation and edge retraction as observed after microtubule targeting. Our data are consistent with a mechanism whereby microtubules deliver localized doses of relaxing signals to contact sites to retard or reverse their development. We propose that it is via this route that microtubules exert their well-established control on cell polarity.

Regulation of cell contraction and membrane ruffling by distinct signals in migratory cells

Cell migration and wound contraction requires assembly of actin into a functional myosin motor unit capable of generating force. However, cell migration also involves formation of actin-containing membrane ruffles. Evidence is provided that actin-myosin assembly and membrane ruffling are regulated by distinct signaling pathways in the migratory cell. Interaction of cells with extracellular matrix proteins or cytokines promote cell migration through activation of the MAP kinases ERK1 and ERK2 as well as the molecular coupling of the adaptor proteins p130CAS and c-CrkII. ERK signaling is independent of CAS/Crk coupling and regulates myosin light chain phosphorylation leading to actin-myosin assembly during cell migration and cell-mediated contraction of a collagen matrix. In contrast, membrane ruffling, but not cell contraction, requires Rac GTPase activity and the formation of a CAS/Crk complex that functions in the context of the Rac activating protein DOCK180. Thus, during cell migration ERK and CAS/Crk coupling operate as components of distinct signaling pathways that control actin assembly into myosin motors and membrane ruffles, respectively.

Distinct roles of profilin in cell morphological changes: microspikes, membrane ruffles, stress fibers, and cytokinesis

Here we report the functional importance of profilin in various actin-mediated morphological changes using H119E mutant profilin I, which is deficient only in actin binding. In the case of actin-protrusive structures from the plasma membrane, H119E-profilin was shown to suppress the formation of Cdc42-induced actin microspikes and Rac-induced membrane ruffles. Conversely, Rho-induced stress fiber formation seemed to occur independently of H119E-profilin introduction. Furthermore, H119E-profilin blocked cleavage furrow ingression and subsequent adhesion to the substratum during cell division, a process in which actin plays indispensable roles.

Cyclic strain stress-induced mitogen-activated protein kinase (MAPK) phosphatase 1 expression in vascular smooth muscle cells is regulated by Ras/Rac-MAPK pathways

Recently, we demonstrated that mechanical stress results in rapid phosphorylation or activation of platelet-derived growth factor receptors in vascular smooth muscle cells (VSMCs) followed by activation of mitogen-activated protein kinases (MAPKs) and AP-1 transcription factors (Hu, Y., Bock, G., Wick, G., and Xu, Q. (1998) FASEB J. 12, 1135-1142). Herein, we provide evidence that VSMC responses to mechanical stress also include induction of MAPK phosphatase-1 (MKP-1), which may serve as a negative regulator of MAPK signaling pathways. When rat VSMCs cultivated on a flexible membrane were subjected to cyclic strain stress (60 cycles/min, 5-30% elongation), induction of MKP-1 proteins and mRNA was observed in time- and strength-dependent manners. Concomitantly, mechanical forces evoked rapid and transient activation of all three members of MAPKs, i.e. extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal protein kinases (JNKs), or stress-activated protein kinases (SAPKs), and p38 MAPKs. Suramin, a growth factor receptor antagonist, completely abolished ERK activation, significantly blocked MKP-1 expression, but not JNK/SAPK and p38 MAPK activation, in response to mechanical stress. Interestingly, VSMC lines stably expressing dominant negative Ras (Ras N17) or Rac (Rac N17) exhibited a marked decrease in MKP-1 expression; the inhibition of ERK kinases (MEK1/2) by PD 98059 or of p38 MAPKs by SB 202190 resulted in a down-regulation of MKP-1 induction. Furthermore, overexpressing MKP-1 in VSMCs led to the dephosphorylation and inactivation of ERKs, JNKs/SAPKs, and p38 MAPKs and inhibition of DNA synthesis. Taken together, our findings demonstrate that mechanical stress induces MKP-1 expression regulated by two signal pathways, including growth factor receptor-Ras-ERK and Rac-JNK/SAPK or p38 MAPK, and that MKP-1 inhibits VSMC proliferation via MAPK inactivation. These results suggest that MKP-1 plays a crucial role in mechanical stress-stimulated signaling leading to VSMC growth and differentiation.

Integration of Rac-dependent regulation of cyclin D1 transcription through a nuclear factor-kappaB-dependent pathway

The small GTP-binding protein Rac1, a member of the Ras superfamily, plays a fundamental role in cytoskeleton reorganization, cellular transformation, the induction of DNA synthesis, and superoxide production. Cyclin D1 abundance is rate-limiting in normal G(1) phase progression, and the abundance of cyclin D1 is induced by activating mutations of both Ras and Rac1. Nuclear factor-kappaB (NF-kappaB) proteins consist of cytoplasmic hetero- or homodimeric Rel-related proteins complexed to a member of the IkappaB family of inhibitor proteins. In the current studies, activating mutants of Rac1 (Rac(Leu-61), Rac(Val-12)) induced cyclin D1 expression and the cyclin D1 promoter in NIH 3T3 cells. Induction of cyclin D1 by Rac1 required both an NF-kappaB and an ATF-2 binding site. Inhibiting NF-kappaB by overexpression of an NF-kappaB trans-dominant inhibitor (nonphosphorylatable IkappaBalpha) reduced cyclin D1 promoter activation by the Rac1 mutants, placing NF-kappaB in a pathway of Rac1 activation of cyclin D1. Specific amino acid mutations in the amino-terminal effector domain of Rac(Leu-61) had comparable effects on NF-kappaB transcriptional activity and activation of the cyclin D1 promoter. The NF-kappaB factors Rel A (p65) and NF-kappaB(1) (p50) induced the cyclin D1 promoter, requiring both the NF-kappaB binding site and the ATF-2 site. Stable overexpression of Rac(Leu-61) increased binding of Rel A and NF-kappaB(1) to the cyclin D1 promoter NF-kappaB site. Activation of Rac1 in NIH 3T3 cells induces both NF-kappaB binding and activity and enhances expression of cyclin D1 through an NF-kappaB and ATF-2 site in the proximal promoter, suggesting a critical role for NF-kappaB in cell cycle regulation through cyclin D1 and Rac1.

Differential activation of focal adhesion kinase, Rho and Rac by the ninth and tenth FIII domains of fibronectin

Fibronectins are widely expressed extracellular matrix ligands that are essential for many biological processes. Fibronectin-induced signaling pathways are elicited in diverse cell types when specific integrin receptors bind to the ninth and tenth FIII domains, FIII9-10. Integrin-mediated signal transduction involves activation of signaling pathways of the growth factor-dependent Ras-related small GTP-binding proteins Rho and Rac, and phosphorylation of focal adhesion kinase. We have dissected the requirement of FIII9 and FIII10 for Rho and Rac activity and phosphorylation of focal adhesion kinase in BHK fibroblasts and Swiss 3T3 cells. We demonstrate that FIII10 supports cell attachment but does not induce phosphorylation of focal adhesion kinase. In Swiss 3T3 cells, growth factor-independent phosphorylation of focal adhesion kinase and downstream adhesion events are dependent upon the presence of FIII9 in the intact FIII9-10 pair, whereas FIII10-mediated focal adhesion kinase phosphorylation requires a synergistic signal from growth factors. Furthermore, FIII10 is able to elicit cellular responses mediated by Rho, but not Rac, whereas FIII9-10 can elicit both Rho- and Rac-mediated responses. We propose that activation of specific integrin subunits by the FIII10 and FIII9-10 ligands elicits distinct signaling events. This may represent a general molecular mechanism for activation of receptor-specific signaling pathways by a multi-domain ligand.

Localization of p21-activated kinase 1 (PAK1) to pseudopodia, membrane ruffles, and phagocytic cups in activated human neutrophils

Leukocyte chemoattractants are known to stimulate signaling pathways that involve Rho family GTPases. Direct evidence for the regulation of the leukocyte cytoskeleton by Rho GTPases and their effector targets is limited. The p21-activated kinases (PAKs) are specific targets of activated GTP-bound Rac and Cdc42, and have been proposed as regulators of chemoattractant-driven actin cytoskeletal changes in fibroblasts. PAK1 colocalizes with F-actin to cortical actin structures in stimulated fibroblasts, and activated PAK1 mutants induce membrane ruffling and polarized cytoskeletal rearrangements. We investigated whether PAK1 was associated with remodeling of the actin cytoskeleton in activated human neutrophils. We monitored the redistribution of PAK1 and F-actin into the actin cytoskeleton after stimulation of human neutrophils with the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) or the particulate stimulus, opsonized zymosan (OZ). PAK1 exhibited a similar distribution as F-actin in fMLP-stimulated leukocytes, localizing in membrane ruffles and to lamellipodia at the leading edge of polarized cells. Addition of OZ induced phagocytic uptake of this particulate stimulus, and PAK1 re-localized to the F-actin-rich pseudopodia and phagocytic cups associated with this process. Once the OZ was internalized, there was little PAK1 localized around the ingested particles, suggesting that PAK1 may be regulating the cytoskeletal extensions and events required for engulfment of bacteria, but not the subsequent steps of internalization. Localization of PAK1 and F-actin in cytoskeletal structures was abolished by the actin polymerization inhibitor cytochalasin D and the phosphatidylinositol 3-kinase inhibitor wortmannin. Our data suggest that PAK1 may regulate a subset of cytoskeletal dynamics initiated by chemoattractant and phagocytic stimuli in human neutrophils.

Pak to the future

Members of the Pak family of serine/threonine kinases serve as targets for the small GTP-binding proteins Cdc42 and Rac and have been implicated in a wide range of biological activities. Recently, some exciting developments help elaborate the regulation of Pak activity and identify downstream signalling targets. These include the discovery of the Cool/Pix and Cat proteins, which modulate Pak signalling, and downstream kinases that modulate the organization of the actin cytoskeleton or gene expression. We present these recent findings and consider how these new regulators and targets could explain some of the cellular effects that have been attributed to Pak family members.

Tetratricopeptide repeat (TPR) motifs of p67(phox) participate in interaction with the small GTPase Rac and activation of the phagocyte NADPH oxidase

The small GTPase Rac functions as a molecular switch in several important cellular events including cytoskeletal reorganization and activation of the phagocyte NADPH oxidase, the latter of which leads to production of superoxide, a precursor of microbicidal oxidants. During formation of the active oxidase complex at the membrane, the GTP-bound Rac appears to interact with the N-terminal region of p67(phox), another indispensable activator that translocates from the cytosol upon phagocyte stimulation. Here we show that the p67(phox) N terminus lacks the CRIB motif, a well known Rac target, but contains four tetratricopeptide repeat (TPR) motifs with highly alpha-helical structure. Disruption of any of the N-terminal three TPRs, but the last one, results in defective interaction with Rac, while all the four are required for the NADPH oxidase activation. We also find that Arg-102 in the third repeat is likely involved in binding to Rac via an ionic interaction, and that replacement of this residue with Glu completely abrogates the capability of activating the oxidase both in vivo and in vitro. Thus the TPR motifs of p67(phox) are packed to function as a Rac target, thereby playing a crucial role in the active oxidase complex formation.

Roles of phosphatidylinositol 3-kinase and Rac in the nuclear signaling by tumor necrosis factor-alpha in rat-2 fibroblasts

We investigated the extent to which phosphatidylinositol 3-kinase (PI 3-kinase) and Rac, a member of the Rho family of small GTPases, are involved in the signaling cascade triggered by tumor necrosis factor (TNF)-alpha leading to activation of c-fos serum response element (SRE) and c-Jun amino-terminal kinase (JNK) in Rat-2 fibroblasts. Inhibition of PI 3-kinase by LY294002 or wortmannin, two specific PI 3-kinase antagonists, or co-transfection with a dominant negative mutant of PI 3-kinase dose-dependently blocked stimulation of c-fos SRE by TNF-alpha. Similarly, LY294002 significantly diminished TNF-alpha-induced activation of JNK, suggesting that nuclear signaling triggered by TNF-alpha is dependent on PI 3-kinase-mediated activation of both c-fos SRE and JNK. We also found nuclear signaling by TNF-alpha to be Rac-dependent, as demonstrated by the inhibitory effect of transient co-transfection with a dominant negative Rac mutant, RacN17. Our findings suggest that Rac is situated downstream of PI 3-kinase in the TNF-alpha signaling pathway to the nucleus, and we conclude that PI 3-kinase and Rac each plays a pivotal role in the nuclear signaling cascade triggered by TNF-alpha.

Cdc42, Rac1, and their effector IQGAP1 as molecular switches for cadherin-mediated cell-cell adhesion

Cell-cell adhesion is a dynamic process in various cellular and developmental situations. Cadherins, well-known Ca(2+)-dependent adhesion molecules, are thought to play a major role in the regulation of cell-cell adhesion. However, the molecular mechanism underlying the rearrangement of cadherin-mediated cell-cell adhesion is largely unknown. Cdc42 and Rac1, belonging to the Rho small GTPase family, have recently been shown to be involved in the regulation of cell-cell adhesion. In addition, IQGAP1, an effector for Cdc42 and Rac1, has been shown to regulate the cadherin function through interaction with beta-catenin, a molecule associated with cadherin. In this review, we will summarize the mode of action of Cdc42 and Rac1 as well as IQGAP1 as molecular switches for the cadherin function, and then discuss physiological processes in which the Cdc42/Rac1/IQGAP1 system may be involved.

Effect of potent and selective inhibitors of the Grb2 SH2 domain on cell motility

Cell motility has been correlated both with oncogenic invasiveness and metastatic potential. The development of selective inhibitors of motility has thus great potential importance. Grb2 is a SH2/SH3 domain-containing adaptor protein that links growth factor receptor tyrosine kinases to the Ras signaling pathway. We have developed specific small molecule inhibitors of the Grb2 SH2 domain as potential leads for drug discovery. Synthesis of the inhibitors and their effects on growth factor-induced growth in cells have been reported previously. In the current study, we establish that these inhibitors inhibit hepatocyte growth factor/scatter factor-induced A431 and Madin-Darby canine kidney cell motility and various cell motility-related events, including epidermal growth factor-induced ruffling of A431 cells and epidermal growth factor-induced translocation of the small GTPase Rac in these cells. We demonstrate for the first time a direct role for Grb2 in cell motility and indicate a new avenue for cancer therapeutics.

Separate roles for H-Ras and Rac in signaling by transforming growth factor (TGF)-beta. H-Ras is essential for activation of MAP kinase, partially required for transcriptional activation by TGF-beta, but not required for signaling of growth suppression by TGF-beta

The signaling components located downstream of the transforming growth factor (TGF)-beta receptor are poorly understood. We constructed adenoviral vectors expressing a dominant-negative form of either H-Ras (AdCARasY57) or Rac (AdCARacN17), and used them to examine the roles of H-Ras and Rac in TGF-beta signaling using arterial endothelial cells in primary culture, and several established cells including a mink lung epithelial cell line (Mv1Lu). The rapid activation of p42/44 MAP kinase (MAPK) by TGF-beta1 was eliminated completely, and transcriptional activation by TGF-beta1 of the plasminogen activator inhibitor-1 gene was reduced by 50% in both endothelial cells and Mv1Lu when they were infected with AdCARasY57. However, the antiproliferative effect of TGF-beta, as assessed by the induction of the mRNA for Cdk4/6-specific inhibitor p15INK4B and by DNA synthesis, was not affected in AdCARasY57-infected cells. A MAPK kinase (MEK)1/2 inhibitor, U0126 also abolished MAPK activation and partially inhibited transcriptional activation by TGF-beta, suggesting that MAPK may be partially involved in this pathway. MAPK activation, transcriptional activation and growth suppression by TGF-beta were all unaffected in cells infected with AdCARacN17, although the DNA synthesis elicited by serum mitogens was suppressed completely in the infected cells. Our data indicate that H-Ras is essential for mitogen-activated protein kinase activation, partly required for transcriptional activation by TGF-beta, but not critically involved in the signaling that exerts the antiproliferative effect of TGF-beta. The results also suggest that Rac may not serve as an essential molecule in signaling by TGF-beta in the cells tested.

Regulation of p21rac activation in human neutrophils

The small guanosine triphosphate (GTPase) p21rac is highly expressed in human neutrophils where it is thought to play a role in cytoskeletal reorganization and superoxide production. Using the p21rac binding domain of PAK (PAK-RBD) as an activation-specific probe, we have investigated agonist-stimulated activation of p21rac. Stimulation of neutrophils with the chemoattractants fMet-Leu-Phe (fMLP) or platelet-activating factor (PAF) induced an extremely rapid and transient p21rac activation, being optimal within 5 seconds. This activation correlates with the rapid changes of intracellular free Ca(2+) ([Ca(2+)](i)) stimulated by fMLP; however, changes in [Ca(2+)](i) were neither sufficient nor required for p21rac activation. Furthermore, fMLP-induced p21rac activation was not inhibited by broad tyrosine kinase inhibitors or specific inhibitors of ERK, p38 mitogen activated protein kinase, Src, or phosphatidylinositol 3-kinases. Surprisingly, the cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha did not cause p21rac activation or modulate fMLP-induced p21rac activation. AlF(-), a potent activator of heterotrimeric G-protein alpha-subunits, however, was found to activate p21rac. Stimulation of neutrophils with phorbol myristate acetate (PMA) strongly activated the respiratory burst, but did not induce p21rac activation, suggesting that superoxide production per se can occur independently of p21rac activation. These data suggest that in human granulocytes, G-protein coupled receptors, but not cytokine receptors, activate p21rac via a rapid, novel exchange-mechanism independently of changes in [Ca(2+)](i), tyrosine phosphorylation, or PI3K.

Rho guanine dissociation inhibitors: pivotal molecules in cellular signalling

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

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

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

Polarized distribution of endogenous Rac1 and RhoA at the cell surface

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