IQGAP1: a key regulator of adhesion and migration

IQGAP1 in microbial pathogenesis: Targeting the actin cytoskeleton

Microbial pathogens cause widespread morbidity and mortality. Central to the pathogens' virulence is manipulation of the host cell's cytoskeleton, which facilitates microbial invasion, multiplication, and avoidance of the innate immune response. IQGAP1 is a ubiquitously expressed scaffold protein that integrates diverse signaling cascades. Research has shown that IQGAP1 binds to and modulates the activity of multiple proteins that participate in bacterial invasion. Here, we review data that support a role for IQGAP1 in infectious disease via its ability to regulate the actin cytoskeleton. In addition, we explore other mechanisms by which IQGAP1 may be exploited by microbial pathogens.

A screen for novel phosphoinositide 3-kinase effector proteins

Class I phosphoinositide 3-kinases exert important cellular effects through their two primary lipid products, phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2)). As few molecular targets for PtdIns(3,4)P(2) have yet been identified, a screen for PI 3-kinase-responsive proteins that is selective for these is described. This features a tertiary approach incorporating a unique, primary recruitment of target proteins in intact cells to membranes selectively enriched in PtdIns(3,4)P(2). A secondary purification of these proteins, optimized using tandem pleckstrin homology domain containing protein-1 (TAPP-1), an established PtdIns(3,4)P(2) selective ligand, yields a fraction enriched in proteins of potentially similar lipid binding character that are identified by liquid chromatography-tandem MS. Thirdly, this approach is coupled to stable isotope labeling with amino acids in cell culture using differential isotope labeling of cells stimulated in the absence and presence of the PI 3-kinase inhibitor wortmannin. This provides a ratio-metric readout that distinguishes authentically responsive components from copurifying background proteins. Enriched fractions thus obtained from astrocytoma cells revealed a subset of proteins that exhibited ratios indicative of their initial, cellular responsiveness to PI 3-kinase activation. The inclusion among these of tandem pleckstrin homology domain containing protein-1, three isoforms of Akt, switch associated protein-70, early endosome antigen-1 and of additional proteins expressing recognized lipid binding domains demonstrates the utility of this strategy and lends credibility to the novel candidate proteins identified. The latter encompass a broad set of proteins that include the gene product of TBC1D2A, a putative Rab guanine nucleotide triphosphatase activating protein (GAP) and IQ motif containing GAP1, a potential tumor promoter. A sequence comparison of the former protein indicates the presence of a pleckstrin homology domain whose lipid binding character remains to be established. IQ motif containing GAP1 lacks known lipid interacting components and a preliminary analysis here indicates that this may exemplify a novel class of atypical phosphoinositide (aPI) binding domain.

Interaction between Core protein of classical swine fever virus with cellular IQGAP1 protein appears essential for virulence in swine

Here we show that IQGAP1, a cellular protein that plays a pivotal role as a regulator of the cytoskeleton interacts with Classical Swine Fever Virus (CSFV) Core protein. Sequence analyses identified residues within CSFV Core protein (designated as areas I, II, III and IV) that maintain homology to regions within the matrix protein of Moloney Murine Leukemia Virus (MMLV) that mediate binding to IQGAP1 [EMBO J, 2006 25:2155]. Alanine-substitution within Core regions I, II, III and IV identified residues that specifically mediate the Core-IQGAP1 interaction. Recombinant CSFV viruses harboring alanine substitutions at residues (207)ATI(209) (I), (210)VVE(212) (II), (213)GVK(215) (III), or (232)GLYHN(236) (IV) have defective growth in primary swine macrophage cultures. In vivo, substitutions of residues in areas I and III yielded viruses that were completely attenuated in swine. These data shows that the interaction of Core with an integral component of cytoskeletal regulation plays a role in the CSFV cycle.

S100P is a novel interaction partner and regulator of IQGAP1

Ca(2+)-binding proteins of the S100 family participate in intracellular Ca(2+) signaling by binding to and regulating specific cellular targets in their Ca(2+)-loaded conformation. Because the information on specific cellular targets of different S100 proteins is still limited, we developed an affinity approach that selects for protein targets only binding to the physiologically active dimer of an S100 protein. Using this approach, we here identify IQGAP1 as a novel and dimer-specific target of S100P, a member of the S100 family enriched in the cortical cytoskeleton. The interaction between S100P and IQGAP1 is strictly Ca(2+)-dependent and characterized by a dissociation constant of 0.2 μM. Binding occurs primarily through the IQ domain of IQGAP1 and the first EF hand loop of S100P, thus representing a novel structural principle of S100-target protein interactions. Upon cell stimulation, S100P and IQGAP1 co-localize at or in close proximity to the plasma membrane, and complex formation can be linked to altered signal transduction properties of IQGAP1. Specifically, the EGF-induced tyrosine phosphorylation of IQGAP1 that is thought to function in assembling signaling intermediates at IQGAP1 scaffolds in the subplasmalemmal region is markedly reduced in cells overexpressing S100P but not in cells expressing an S100P mutant deficient in IQGAP1 binding. Furthermore, B-Raf binding to IQGAP1 and MEK1/2 activation occurring downstream of IQGAP1 in EGF-triggered signaling cascades are compromised at elevated S100P levels. Thus, S100P is a novel Ca(2+)-dependent regulator of IQGAP1 that can down-regulate the function of IQGAP1 as a signaling intermediate by direct interaction.

Rho GTPases and exocytosis: what are the molecular links?

Delivery of proteins or lipids to the plasma membrane or into the extracellular space occurs through exocytosis, a process that requires tethering, docking, priming and fusion of vesicles, as well as F-actin rearrangements in response to specific extracellular cues. GTPases of the Rho family have been implicated as important regulators of exocytosis, but how Rho proteins control this process is an open question. In this review, we focus on molecular connections that drive Rho-dependent exocytosis in polarized and regulated exocytosis. Specifically, we present data showing that Rho proteins interaction with the exocyst complex and IQGAP mediates polarized exocytosis, whereas interaction with actin-binding proteins like N-WASP mediates regulated exocytosis.

miR-124a is important for migratory cell fate transition during gastrulation of human embryonic stem cells

Precise control of gene expression is of paramount importance for proper embryonic development. Although a number of microRNAs (miRNAs) has been implicated in fine-tuning mRNA translation during development, their exact roles for gastrulation, particularly in connection with functional targets, have yet to be clarified, with regard to stage-specific cell migration to form three embryonic germ layers. We found that miR-124a is expressed in human embryonic stem cells (hESC), but is gradually downregulated during embryoid body (EB) formation in vitro. We also provide evidence that SLUG and IQGAP1, which modulates rearrangement of the migratory cytoskeleton, are specific targets for miR-124a during EB formation. Furthermore, we show that the beginning of cell migration, a hallmark event in gastrulation, is tightly coupled with downregulation of miR-124a during EB formation and induction of SLUG and IQGAP1. Overexpressed miR-124a in hESC reduced expression of SLUG and IQGAP1 and blocked migratory cell behavior in EB. An expression level of MIXL1, associated with gastulation process, was also inversely correlated with expression of miR-124a. Taken together, our results strongly suggest that miR-124a may play an active role in inhibiting hESCs from differentiation into EB by downregulating expression of SLUG and IQGAP1, thereby maintaining stemness.

IQGAP1 and IQGAP2 are reciprocally altered in hepatocellular carcinoma

Background

IQGAP1 and IQGAP2 are homologous members of the IQGAP family of scaffold proteins. Accumulating evidence implicates IQGAPs in tumorigenesis. We recently reported that IQGAP2 deficiency leads to the development of hepatocellular carcinoma (HCC) in mice. In the current study we extend these findings, and investigate IQGAP1 and IQGAP2 expression in human HCC.

Methods

IQGAP1 and IQGAP2 protein expression was assessed by Western blotting and immunohistochemistry. IQGAP mRNA was measured by quantitative RT-PCR. The methylation status of the Iqgap2 promoter was determined by pyrosequencing of bisulfite-treated genomic DNA.

Results

IQGAP1 and IQGAP2 expression was reciprocally altered in 6/6 liver cancer cell lines. Similarly, immunohistochemical staining of 82 HCC samples showed that IQGAP2 protein expression was reduced in 64/82 (78.0%), while IQGAP1 was present in 69/82 (84.1%). No IQGAP1 staining was detected in 23/28 (82.1%) normal livers, 4/4 (100.0%) hepatic adenomas and 23/23 (100.0%) cirrhosis cases, while IQGAP2 was increased in 22/28 (78.6%), 4/4 (100.0%) and 23/23 (100.0%), respectively. Although the Iqgap2 promoter was not hypermethylated in HCC at any of the 25 CpG sites studied (N = 17), IQGAP2 mRNA levels were significantly lower in HCC specimens (N = 23) than normal livers (N = 6).

Conclusions

We conclude that increased IQGAP1 and/or decreased IQGAP2 contribute to the pathogenesis of human HCC. Furthermore, downregulation of IQGAP2 in HCC occurs independently of hypermethylation of the Iqgap2 promoter. Immunostaining of IQGAP1 and IQGAP2 may aid in the diagnosis of HCC, and their pharmacologic modulation may represent a novel therapeutic strategy for the treatment of liver cancer.

IQGAP1 is involved in post-ischemic neovascularization by regulating angiogenesis and macrophage infiltration

Background

Neovascularization is an important repair mechanism in response to ischemic injury and is dependent on inflammation, angiogenesis and reactive oxygen species (ROS). IQGAP1, an actin-binding scaffold protein, is a key regulator for actin cytoskeleton and motility. We previously demonstrated that IQGAP1 mediates vascular endothelial growth factor (VEGF)-induced ROS production and migration of cultured endothelial cells (ECs); however, its role in post-ischemic neovascularization is unknown.

Methodology/Principal Findings

Ischemia was induced by left femoral artery ligation, which resulted in increased IQGAP1 expression in Mac3⁺ macrophages and CD31⁺ capillary-like ECs in ischemic legs. Mice lacking IQGAP1 exhibited a significant reduction in the post-ischemic neovascularization as evaluated by laser Doppler blood flow, capillary density and α-actin positive arterioles. Furthermore, IQGAP1^−/− mice showed a decrease in macrophage infiltration and ROS production in ischemic muscles, leading to impaired muscle regeneration and increased necrosis and fibrosis. The numbers of bone marrow (BM)-derived cells in the peripheral blood were not affected in these knockout mice. BM transplantation revealed that IQGAP1 expressed in both BM-derived cells and tissue resident cells, such as ECs, is required for post-ischemic neovascularization. Moreover, thioglycollate-induced peritoneal macrophage recruitment and ROS production were inhibited in IQGAP1^−/− mice. In vitro, IQGAP1^−/− BM-derived macrophages showed inhibition of migration and adhesion capacity, which may explain the defective macrophage recruitment into the ischemic tissue in IQGAP1^−/− mice.

Conclusions/Significance

IQGAP1 plays a key role in post-ischemic neovascularization by regulating, not only, ECs-mediated angiogenesis but also macrophage infiltration as well as ROS production. Thus, IQGAP1 is a potential therapeutic target for inflammation- and angiogenesis-dependent ischemic cardiovascular diseases.

Cadherin-mediated intercellular adhesion and signaling cascades involving small GTPases

Epithelia form physical barriers that separate the internal milieu of the body from its external environment. The biogenesis of functional epithelia requires the precise coordination of many cellular processes. One of the key events in epithelial biogenesis is the establishment of cadherin-dependent cell-cell contacts, which initiate morphological changes and the formation of other adhesive structures. Cadherin-mediated adhesions generate intracellular signals that control cytoskeletal reorganization, polarity, and vesicle trafficking. Among such signaling pathways, those involving small GTPases play critical roles in epithelial biogenesis. Assembly of E-cadherin activates several small GTPases and, in turn, the activated small GTPases control the effects of E-cadherin-mediated adhesions on epithelial biogenesis. Here, we focus on small GTPase signaling at E-cadherin-mediated epithelial junctions.

Chronic ethanol exposure alters the levels, assembly, and cellular organization of the actin cytoskeleton and microtubules in hippocampal neurons in primary culture

The organization and dynamics of microtubules (MTs) and the actin cytoskeleton are critical for the correct development and functions of neurons, including intracellular traffic and signaling. In vitro ethanol exposure impairs endocytosis, exocytosis, and nucleocytoplasmic traffic in astrocytes and alters endocytosis in cultured neurons. In astrocytes, these effects relate to changes in the organization and/or function of MTs and the actin cytoskeleton. To evaluate this possibility in hippocampal cultured neurons, we analyzed if chronic ethanol exposure affects the levels, assembly, and cellular organization of both cytoskeleton elements and the possible underlying mechanisms of these effects by morphological and biochemical methods. In the experiments described below, we provide the first evidence that chronic alcohol exposure decreases the amount of both filamentous actin and polymerized tubulin in neurons and that the number of MTs in dendrites lowers in treated cells. Alcohol also diminishes the MT-associated protein-2 levels, which mainly localizes in the somatodendritic compartment in neurons. Ethanol decreases the levels of total Rac, Cdc42, and RhoA, three small guanosine triphosphatases (GTPases) involved in the organization and dynamics of the actin cytoskeleton and MTs. Yet when alcohol decreases the levels of the active forms (GTP bound) of Rac1 and Cdc42, it does not affect the active form of RhoA. We also investigated the levels of several effector and regulator molecules of these GTPases to find that alcohol induces heterogeneous results. In conclusion, our results show that MT, actin cytoskeleton organization, and Rho GTPase signaling pathways are targets for the toxic effects of ethanol in neurons.

Quantitative proteomic analysis of HepG2 cells treated with quercetin suggests IQGAP1 involved in quercetin-induced regulation of cell proliferation and migration

Quercetin, a wild distributed bioflavonoid, exhibits antitumor effects on murine models by inducing apoptosis and inhibiting growth of many cancer cell lines, while proteins involved in antitumor effects at proteomic level are still unclear. In our study, we used a quantitative proteomic strategy termed stable isotope labeling by amino acids in cell culture (SILAC)-mass spectrometry (MS) to study the differential proteomic profiling of HepG2 cells treated by quercetin. In all, there were 70 changed proteins among those quantified proteins in HepG2 cells treated by 50 microM quercetin for 48 h, and 14 proteins showed significant upregulation, whereas 56 proteins were downregulated. The functional classification of changed proteins includes signaling protein, protein synthesis, cytoskeleton, metabolism, etc. Of these, Ras GTPase-activating-like protein (IQGAP1) and beta-tubulin were found to be reduced at a large degree. The migration inhibition of HepG2 cells can be induced by quercetin, and the RNA and protein expression level of IQGAP1 and beta-tubulin were respectively decreased obviously in HepG2 cells exposed to quercetin for 48 h in the scratch migration assay. The downregulated expression of IQGAP1 and beta-tubulin by quercetin treatment correlated with cell migration ability, and quercetin probably inhibits HepG2 proliferation and migration through IQGAP1 and beta-tubulin expression changes and their interactions with other proteins.

Honokiol inhibits HepG2 migration via down-regulation of IQGAP1 expression discovered by a quantitative pharmaceutical proteomic analysis

Honokiol (HNK), a natural small molecular product, inhibited proliferation of HepG2 cells and exhibited anti-tumor activity in nude mice. In this article, we applied a novel sensitive stable isotope labeling with amino acids in cell culture-based quantitative proteomic method and a model of nude mice to investigate the correlation between HNK and the hotspot migration molecule Ras GTPase-activating-like protein (IQGAP1). The quantitative proteomic analysis showed that IQGAP1 was 0.53-fold down-regulated under 10 microg/mL HNK exposure for 24 h on HepG2 cells. Migration ability of HepG2 cells under HNK treatment was correlated with its expression level of IQGAP1. In addition, the biochemical validation on HepG2 cells and the tumor xenograft model further demonstrated that HNK decreased the expression level of IQGAP1 and its upstream proteins Cdc42/Rac1. These data supported that HNK can modulate cell adhesion and cell migration by acting on Cdc42/Rac1 signaling via IQGAP1 interactions with its upstream Cdc42/Rac1 proteins, which is a new molecular mechanism of HNK to exert its anti-tumor activity.

Apocynin derivatives interrupt intracellular signaling resulting in decreased migration in breast cancer cells

Cancer cells are defined by their ability to divide uncontrollably and metastasize to secondary sites in the body. Consequently, tumor cell migration represents a promising target for anticancer drug development. Using our high-throughput cell migration assay, we have screened several classes of compounds for noncytotoxic tumor cell migration inhibiting activity. One such compound, apocynin (4-acetovanillone), is oxidized by peroxidases to yield a variety of oligophenolic and quinone-type compounds that are recognized inhibitors of NADPH oxidase and may be inhibitors of the small G protein Rac1 that controls cell migration. We report here that while apocynin itself is not effective, apocynin derivatives inhibit migration of the breast cancer cell line MDA-MB-435 at subtoxic concentrations; the migration of nonmalignant MCF10A breast cells is unaffected. These compounds also cause a significant rearrangement of the actin cytoskeleton, cell rounding, and decreased levels of active Rac1 and its related G protein Cdc42. These results may suggest a promising new route to the development of novel anticancer therapeutics.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Overexpression of IQGAP1 in advanced colorectal cancer correlates with poor prognosis-critical role in tumor invasion

IQGAP1 is a multifunctional protein involved in actin cytoskeleton assembly and E-cadherin-mediated cell adhesion. We reported previously IQGAP1 overexpression in human colorectal carcinomas especially at the invasion front (IF) and that such overexpression tended to correlate with lymph node metastasis in advanced cases. Thus, in this study, we investigated the clinicopathological significance of IQGAP1 expression in 85 cases of pT2-3 colorectal carcinomas with special reference to its expression pattern and prognosis, followed by analysis of the role of IQGAP1 in cancer invasion in vitro. Quantitative reverse transcription-PCR showed significant upregulation of IQGAP1 in colorectal carcinomas compared with normal mucosa. Immunohistochemically, IQGAP1 expression pattern was classified into diffuse (20%), IF-associated (35.3%) and focal (44.7%). The diffuse pattern was associated with higher rates of distant metastasis. Patients with IQGAP1 overexpression and diffuse pattern had significantly shorter survival (p < 0.0001) than others, and the diffuse pattern was an independent predictor of poor survival by multivariate analysis. In vitro invasion assays using three human colon carcinoma cell lines showed that IQGAP1 siRNA significantly suppressed hepatocyte growth factor (HGF)-stimulated cell invasion. HGF reduced membranous localization of alpha-catenin, but did not alter localization of E-cadherin, beta-catenin and IQGAP1 in membranes. Suppression of IQGAP1 expression by siRNA did not alter membranous localization of alpha-catenin even in the presence of HGF. Our results indicate that IQGAP1 plays a critical role in colon cancer cell invasion, and therefore diffuse and high expression of IQGAP1 predicts poor prognosis in patients with colorectal carcinoma.

PKA-mediated phosphorylation of EPEC-Tir at serine residues 434 and 463: A novel pathway in regulating Rac1 GTPase function

Type-III or type-IV secretion systems of many Gram-negative bacterial pathogens inject effector proteins into host cells that modulate cellular functions in their favour. A preferred target of these effectors is the actin-cytoskeleton as shown by studies using the gastric pathogens Helicobacter pylori (H. pylori) and enteropathogenic Escherichia coli (EPEC). We recently developed a co-infection approach to study effector protein function and molecular mechanisms by which they highjack cellular signalling cascades. This is exemplified by our observation that EPEC profoundly blocks H. pylori-induced epithelial cell scattering and elongation, a disease-related event requiring the activity of small Rho GTPase Rac1. While this suppressive effect is dependent on the effector protein Tir and the outer-membrane protein Intimin, it unexpectedly revealed evidence for Tir-signalling independent of phosphorylation of Tir at tyrosine residues 454 and 474. Instead, our studies revealed a previously unidentified function for protein kinase A (PKA)-mediated phosphorylation of Tir at serine residues 434 and 463. We demonstrated that EPEC infection activates PKA for Tir phosphorylation. Activated PKA then phosphorylates Rac1 at its serine residue 71 associated with reduced GTP-load and inhibited cell elongation. Phosphorylation of Rho GTPases such as Rac1 might be an interesting novel strategy in microbial pathogenesis.

Matrix invasion by tumour cells: a focus on MT1-MMP trafficking to invadopodia

When migrating away from a primary tumour, cancer cells interact with and remodel the extracellular matrix (ECM). Matrix metalloproteinases (MMPs), and in particular the transmembrane MT1-MMP (also known as MMP-14), are key enzymes in tumour-cell invasion. Results from recent in vitro studies highlight that MT1-MMP is implicated both in the breaching of basement membranes by tumour cells and in cell invasion through interstitial type-I collagen tissues. Remarkably, MT1-MMP accumulates at invadopodia, which are specialized ECM-degrading membrane protrusions of invasive cells. Here we review current knowledge about MT1-MMP trafficking and its importance for the regulation of protease activity at invadopodia. In invasive cells, endocytosis of MT1-MMP by clathrin- and caveolae-dependent pathways can be counteracted by several mechanisms, which leads to protease stabilization at the cell surface and increased pericellular degradation of the matrix. Furthermore, the recent identification of cellular components that control delivery of MT1-MMP to invadopodia brings new insight into mechanisms of cancer-cell invasion and reveals potential pharmacological targets.

IQGAP1 and vimentin are key regulator genes in naturally occurring hepatotumorigenesis induced by oxidative stress

To identify key genes involved in the complex multistep process of hepatotumorigenesis, we reduced multivariate clinicopathological variables by using the Long-Evans Cinnamon rat, a model with naturally occurring and oxidative stress-induced hepatotumorigenesis. Gene expression patterns were analyzed serially by profiling liver tissues from rats of a naive status (4 weeks old), through to those with chronic hepatitis (26 and 39 weeks old) to tumor development (67 weeks old). Of 31 099 probe sets used for microarray analysis, 87 were identified as being upregulated in a stepwise manner during disease progression and tumor development. Quantitative real-time reverse transcription-polymerase chain reaction and statistical analyses verified that IQGAP1 and vimentin mRNA expression levels increased significantly throughout hepatotumorigenesis. A hierarchical clustering algorithm showed both genes clustered together and in the same cluster group. Immunohistochemical and western blot analyses showed similar increases in protein levels of IAGAP1 and vimentin. Finally, pathway analyses using text-mining technology with more comprehensive and recent gene-gene interaction data identified IQGAP1 and vimentin as important nodes in underlying gene regulatory networks. These findings enhance our understanding of the multistep hepatotumorigenesis and identification of target molecules for novel treatments.

RSK is a principal effector of the RAS-ERK pathway for eliciting a coordinate promotile/invasive gene program and phenotype in epithelial cells

The RAS-stimulated RAF-MEK-ERK pathway confers epithelial cells with critical motile and invasive capacities during development, tissue regeneration, and carcinoma progression, often via promoting the epithelial-mesenchymal transition (EMT). Many mechanisms by which ERK exerts this control remain elusive. We demonstrate that the ERK-activated kinase RSK is necessary to induce mesenchymal motility and invasive capacities in nontransformed epithelial and carcinoma cells. RSK is sufficient to induce certain motile responses. Expression profiling analysis revealed that a primary role of RSK is to induce transcription of a potent promotile/invasive gene program by FRA1-dependent and -independent mechanisms. The program enables RSK to coordinately modulate the extracellular environment, the intracellular motility apparatus, and receptors mediating communication between these compartments to stimulate motility and invasion. These findings uncover a mechanism whereby the RAS-ERK pathway controls epithelial cell motility by identifying RSK as a key effector, from which emanate multiple highly coordinate transcription-dependent mechanisms for stimulation of motility and invasive properties.

Golgi-derived CLASP-dependent microtubules control Golgi organization and polarized trafficking in motile cells

Microtubules are indispensable for Golgi complex assembly and maintenance, which are integral parts of cytoplasm organization during interphase in mammalian cells. Here, we show that two discrete microtubule subsets drive two distinct, yet simultaneous, stages of Golgi assembly. In addition to the radial centrosomal microtubule array, which positions the Golgi in the centre of the cell, we have identified a role for microtubules that form at the Golgi membranes in a manner dependent on the microtubule regulators CLASPs. These Golgi-derived microtubules draw Golgi ministacks together in tangential fashion and are crucial for establishing continuity and proper morphology of the Golgi complex. We propose that specialized functions of these two microtubule arrays arise from their specific geometries. Further, we demonstrate that directional post-Golgi trafficking and cell migration depend on Golgi-associated CLASPs, suggesting that correct organization of the Golgi complex by microtubules is essential for cell polarization and motility.

Reduction of Akt2 inhibits migration and invasion of glioma cells

Malignant gliomas have a tendency to invade diffusely into surrounding healthy brain tissues, thereby precluding their successful surgical removal. The serine/threonine kinase Akt2 is well known as an important regulator of cell survival and growth. In this study, we show that siRNA-mediated depletion of Akt2 inhibited migration and invasion of glioma cells. In addition, we demonstrate the mechanisms by which Akt2 functions to promote cell migration and invasion. Phosphorylation of cofilin, a critical step of actin polymerization, and phosphorylation of Girdin, essential for the integrity of the actin cytoskeleton and cell migration, were impaired. Furthermore, epidermal growth factor-induced ACAP1 phosphorylation and integrin beta1 phosphorylation were also blocked, consistent with defects in adhesion. Thus, Akt2 regulates both cell adhesion and cytoskeleton rearrangement during migration. Decreased MMP-9 expression in Akt2 knocked-down glioma cells was subsequently confirmed by Western blotting, consistent with the decreased invasion in vitro and in vivo. These results suggest that Akt2 contributes to glioma cells migration and invasion by regulating the formation of cytoskeleton, influencing adhesion and increasing expression of MMP-9. Our immunohistochemistry results by using human gliomas tissue sections also indicated that Akt2 expression was closely related with the malignancy of gliomas. This is coincident with our in vivo and in vitro results from cell lines. All of these results indicate that Akt2 is a critical factor in gliomas invasion. This study identifies that Akt2 is a potentially antiinvasion target for therapeutic intervention in gliomas.

Involvement of IQGAP3, a regulator of Ras/ERK-related cascade, in hepatocyte proliferation in mouse liver regeneration and development

The spatio-temporal regulation of hepatocyte proliferation is a critical issue in liver regeneration. Here, in normal and regenerating liver as well as in developing liver, we examined its expression/localization of IQGAP3, which was most recently reported as a Ras/Rac/Cdc42-binding proliferation factor associated with cell-cell contacts in epithelial-type cells. In parallel, the expression/localization of Rac/Cdc42-binding IQGAP1/2 was examined. IQGAP3 showed a specific expression in proliferating hepatocytes positive for the proliferating marker Ki-67, the levels of expressions of mRNAs and proteins were significantly increased in hepatocytes in liver regeneration and development. In immunofluorescence, IQGAP3 was highly enriched at cell-cell contacts of hepatocytes. IQGAP1 and IQGAP2 were exclusively expressed in Kupffer and sinusoidal endothelial cells, respectively, in normal, regenerating, and developing liver. The expression of IQGAP1, but not of IQGAP2, was increased in CCl4-induced (but not in partial hepatectomy-induced) liver regeneration. Exclusive expression/localization of IQGAP3 to hepatocytes in the liver likely reflects the specific involvement of the IQGAP3/Ras/ERK signaling cascade in hepatocyte proliferation in addition to the previously identified signaling pathways, possibly by integrating cell-cell contact-related proliferating signaling events. On the other hand, the Rac/Cdc42-binding properties of IQGAP1/2/3 may be related to the distinct modes of remodeling due to the different strategies which induced proliferation of liver cells; partial hepatectomy, CCl4 injury, or embryonic development. Thus, the functional orchestration of Ras and the Ras homologous (Rho) family proteins Rac/Cdc42 likely plays a critical role in liver regeneration and development.

G(alpha)12-mediated pathway promotes invasiveness of nasopharyngeal carcinoma by modulating actin cytoskeleton reorganization

The molecular mechanisms behind the aggressiveness of nasopharyngeal carcinoma (NPC), a highly invasive and metastatic head and neck malignancy, have not been made clear. In this study investigating these mechanisms, guanine nucleotide-binding protein alpha(12) subunit (G(alpha)(12)) signaling was found by microarray analysis to be increased in primary NPC cells and NPC-derived cell lines. Using small interfering RNA to knock down G(alpha)(12) in NPC cells resulted in a reduction in cell migration and invasion as well as a reversal in fibroblastoid morphology. Using microarray analysis, we also found a reduction in expression of key actin dynamics regulators and several epithelial-to-mesenchymal transition-related genes in G(alpha)(12)-depleted NPC cells. Knocking down one of those genes, IQ motif containing GTPase activating protein 1, reduced the migration and formation of adherens junctions and reversed the fibroblastoid morphology of NPC cells, as knocking down G(alpha)(12) was found to do. Immunohistochemical analysis found NPC tumors to have significantly greater levels of G(alpha)(12) protein than the normal basal epithelial cells. Quantitative real-time PCR analysis revealed a significant correlation between G(alpha)(12) mRNA levels and NPC lymph node metastasis. Together, our findings support a model in which activation of G(alpha)(12) signaling promotes tumorigenesis and progression of NPC by modulating actin cytoskeleton reorganization and expression of epithelial-to-mesenchymal transition-related genes. =

Rac1 and Rho contribute to the migratory and invasive phenotype associated with somatic E-cadherin mutation

Recent evidence suggests a close association between extracellular E-cadherin mutation in diffuse-type gastric carcinoma and the acquisition of a migratory phenotype of tumour cells. To characterize the cellular machinery that mediates the gain of motility of tumour cells with mutant E-cadherin, we turned to the small Rho GTPases Rac1 and Rho because they have been implicated in pathological processes including tumour cell migration and invasion. In the present study, we analyse the activity of Rac1 and Rho in relation to E-cadherin harbouring an in-frame deletion of exon 8 and prove for the first time that the mutation reduces the ability of E-cadherin to activate Rac1 and to inhibit Rho. We provide evidence that the lack of Rac1 activation observed in response to mutant E-cadherin influences the downstream signalling of Rac1, as is shown by the decrease in the binding of the Rac1 effector protein IQGAP1 to Rac1-GTP. Moreover, reduced membranous localization of p120-catenin in mutant E-cadherin expressing cells provides an explanation for the lack of negative regulation of Rho by mutant E-cadherin. Further, we show by time-lapse laser scanning microscopy and invasion assay that the enhanced motility and invasion associated with mutant E-cadherin is sensitive to the inhibition of Rac1 and Rho. Together, these findings present evidence that the mutation of E-cadherin influences Rac1 and Rho activation in opposite directions and that Rac1 and Rho are involved in the establishment of the migratory and invasive phenotype of tumour cells that have an E-cadherin mutation.

Selective and tunable gradient device for cell culture and chemotaxis study

This article describes a microfluidic device for cell culture and chemotaxis studies under various temporal and spatial concentration gradients of the medium or chemoattractant. Vertical membranes formed using in situ fabrication are employed to avoid fluid flow inside the cell observation chamber. Thus, the medium and chemoattractants are primarily provided by diffusion, maintaining cell-cell communication via secreted factors. Neutrophils were used to demonstrate the capability of the device for chemotaxis research. Experiments exhibited successful migration up a concentration gradient of interleukin 8.

Compartmentalization of redox signaling through NADPH oxidase-derived ROS

Reactive oxygen species (ROS) are generated in response to growth factors, cytokines, G protein-coupled receptor agonists, or shear stress, and function as signaling molecules in nonphagocytes. However, it is poorly understood how freely diffusible ROS can activate specific signaling, so-called "redox signaling." NADPH oxidases are a major source of ROS and now recognized to have specific subcellular localizations, and this targeting to specific compartments is required for localized ROS production. One important mechanism may involve the interaction of oxidase subunits with various targeting proteins localized in lamellipodial leading edge and focal adhesions/complexes. ROS are believed to inactivate protein tyrosine phosphatases, thereby establishing a positive-feedback system that promotes activation of specific redox signaling pathways involved in various functions. Additionally, ROS production may be localized through interactions of NADPH oxidase with signaling platforms associated with caveolae/lipid rafts, endosomes, and nucleus. These indicate that the specificity of ROS-mediated signal transduction may be modulated by the localization of Nox isoforms and their regulatory subunits within specific subcellular compartments. This review summarizes the recent progress on compartmentalization of redox signaling via activation of NADPH oxidase, which is implicated in cell biology and pathophysiologies.

Collagen I membranes for tendon repair: effect of collagen fiber orientation on cell behavior

Tendons have poor spontaneous regenerative capabilities, and complete regeneration is never achieved despite intensive remodeling. In this in vitro study, we characterized two multilamellar collagen I membranes differing in the arrangement of collagen fiber deposition (oriented vs. nonoriented) and compared their mechanical properties. Human dermal fibroblasts and tenocytes were seeded on the two membranes to evaluate the effect of fiber orientation on cell viability and cytoskeletal organization. Results demonstrate that the multilamellar collagen I membrane with oriented fibers has the better mechanical properties and affords optimum cell proliferation and adhesion. Its fiber arrangement provides an instructive pattern for cell growth and may serve to guide the alignment of cells migrating from the ends of a crushed or frayed tendon to obtain a strong, correctly structured tendon, thus providing a viable clinical option for tendon repair.

IQGAPs in cancer: a family of scaffold proteins underlying tumorigenesis

The IQGAP family comprises three proteins in humans. The best characterized is IQGAP1, which participates in protein-protein interactions and integrates diverse signaling pathways. IQGAP2 and IQGAP3 harbor all the domains identified in IQGAP1, but their biological roles are poorly defined. Proteins that bind IQGAP1 include Cdc42 and Rac1, E-cadherin, beta-catenin, calmodulin and components of the mitogen-activated protein kinase pathway, all of which are involved in cancer. Here, we summarize the biological functions of IQGAPs that may contribute to neoplasia. Additionally, we review published data which implicate IQGAPs in cancer and tumorigenesis. The cumulative evidence suggests IQGAP1 is an oncogene while IQGAP2 may be a tumor suppressor.

Tubulin tyrosination is required for the proper organization and pathfinding of the growth cone

Background

During development, neuronal growth cones integrate diffusible and contact guidance cues that are conveyed to both actin and microtubule (MT) cytoskeletons and ensure axon outgrowth and pathfinding. Although several post-translational modifications of tubulin have been identified and despite their strong conservation among species, their physiological roles during development, especially in the nervous sytem, are still poorly understood.

Methodology/Findings

Here, we have dissected the role of a post-translational modification of the last amino acid of the α-tubulin on axonal growth by analyzing the phenotype of precerebellar neurons in Tubulin tyrosin ligase knock-out mice (TTL^−/−) through in vivo, ex vivo and in vitro analyses. TTL^−/− neurons are devoid of tyrosinated tubulin. Their pathway shows defects in vivo, ex vivo, in hindbrains open-book preparations or in vitro, in a collagen matrix. Their axons still orient toward tropic cues, but they emit supernumerary branches and their growth cones are enlarged and exhibit an emission of mis-oriented filopodia. Further analysis of the TTL^−/− growth cone intracellular organization also reveals that the respective localization of actin and MT filaments is disturbed, with a decrease in the distal accumulation of Myosin IIB, as well as a concomitant Rac1 over-activation in the hindbrain. Pharmacological inhibition of Rac1 over-activation in TTL^−/− neurons can rescue Myosin IIB localization.

Conclusions/Significance

In the growth cone, we propose that tubulin tyrosination takes part in the relative arrangement of actin and MT cytoskeletons, in the regulation of small GTPases activity, and consequently, in the proper morphogenesis, organization and pathfinding of the growth cone during development.

IQGAP1 regulation and roles in cancer

IQGAP1 is a key mediator of several distinct cellular processes, in particular cytoskeletal rearrangements. Recent studies have implicated a potential role for IQGAP1 in cancer, supported by the over-expression and distinct membrane localisation of IQGAP1 observed in a range of tumours. IQGAP1 is thought to contribute to the transformed cancer cell phenotype by regulating signalling pathways involved in cell proliferation and transformation, weakening of cell:cell adhesion contacts and stimulation of cell motility and invasion. In this review we discuss these different functional and regulatory roles of IQGAP1 and its homologues in relation to their potential impact on tumourigenesis.

Proteomics analysis of liver pathological calcification suggests a role for the IQ motif containing GTPase activating protein 1 in myofibroblast function

To date the cellular and molecular mechanisms by which liver pathological calcifications occur and are regulated are poorly investigated. To study the mechanisms linked to their appearance, we performed a proteomics analysis of calcified liver samples. To this end, human liver biopsies collected in noncalcified (N), precalcified (P), and calcified (C) areas of the liver were subjected to weak ion exchange chromatography, SDS-PAGE, and LC-ESI MS/MS analyses. As we previously demonstrated that alpha-smooth muscle actin (α-SMA) expressing myofibroblasts were involved in liver pathological calcification, we performed a targeted analysis of actin cytoskeleton remodeling-related proteins. This revealed dramatic changes in protein expression patterns in the periphery of the calcified areas. More particularly, we found that IQGAP1 and IQGAP2 proteins were subjected to major expression changes. We show that IQGAP1 expression within P and C areas of the liver correlates with the high abundance of myofibroblasts and that IQGAP1 is specifically expressed in these cells. In addition, we find that IQGAP1 is part of a protein complex including β-catenin and Rac1 mainly in P and C regions of the liver. These results suggest that IQGAP1 may play a critical role in the regulation of cytoskeleton remodeling in liver myofibroblasts in response to liver injury and consequently impact on their function.

ADP-ribosylation factor 6 regulates glioma cell invasion through the IQ-domain GTPase-activating protein 1-Rac1-mediated pathway

A common pathobiological feature of malignant gliomas is the insidious infiltration of single tumor cells into the brain parenchyma, rendering these deadly tumors virtually incurable with available therapies. In this study, we report that ADP-ribosylation factor 6 (ARF6), a Ras superfamily small GTPase, is abundantly expressed in invasive human glioma cells. Cellular depletion of ARF6 by small interfering RNA decreased Rac1 activation, impaired HGF-stimulated and serum-stimulated glioma cell migration in vitro, and markedly decreased the invasive capacity of invasive glioma in the brain. Furthermore, ectopic expression of ARF6 in glioma cells promoted cell migration via the activation of Rac1. Upon stimulation of glioma cells with HGF, we show that IQ-domain GTPase-activating protein 1 (IQGAP1) is recruited and overlaps with ARF6 at the leading edge of migrating cells. However, cellular depletion of ARF6 abrogated this recruitment of IQGAP1 and attenuated the formation of surface protrusions. ARF6 forms complexes with Rac1 and IQGAP1 in glioma cells upon HGF stimulation, and knockdown of IQGAP1 significantly inhibits ARF6-induced Rac1 activation and cell migration. Taken together, these data suggest that ARF6-mediated Rac1 activation is essential for glioma cell invasion via a signaling pathway that requires IQGAP1.

Touch, grasp, deliver and control: functional cross-talk between microtubules and cell adhesions

Cross-talk between microtubule networks and sites of cell-matrix and cell-cell adhesion has profound impact on these structures and is essential for proper cell organization, polarization and motility. Components of adhesion sites can interact directly with microtubules or with proteins that specifically associate with microtubule plus ends and minus ends and in this way capture, stabilize or destabilize microtubules. In their turn, microtubules can serve as routes for delivery of structural and regulatory factors that control adhesion site turnover. In addition, the microtubule lattice or growing microtubule plus ends can serve as diffusional sinks that accumulate and scaffold regulatory molecules, thereby affecting their activity in the vicinity of adhesions. Combination of these mechanisms underlies the functional co-operation between microtubules and adhesion sites and defines their dynamic behavior.

Rho GTPases in hepatocellular carcinoma

Rho GTPases are major regulators of signal transduction pathways and play key roles in processes including actin dynamics, cell cycle progression, cell survival and gene expression, whose deregulation may lead to tumorigenesis. A growing number of in vitro and in vivo studies using tumor-derived cell lines, primary tumors and animal cancer models strongly suggest that altered Rho GTPase signaling plays an important role in the initiation as well as in the progression of hepatocellular carcinoma (HCC), one of the deadliest human cancers in the world. These alterations can occur at the level of the GTPases themselves or of one of their regulators or effectors. The participation into the tumorigenic process can occur either through the over-expression of one of these components which presents an oncogenic activity as illustrated with RhoA and C or through the attenuation of the expression of a component presenting tumor suppressor activity as for Cdc42 or the RhoGAP, DLC-1. Consequently, these observations reflect the heterogeneity and the complexity of liver carcinogenesis. Recently, pharmacological approaches targeting Rho GTPase signaling have been used in HCC-derived models with relative success but remain to be validated in more physiologically relevant systems. Therefore, therapeutic approaches targeting Rho GTPase signaling may provide a novel alternative for anti-HCC therapy.

Identification and characterization of Ibe, a novel type III effector protein of A/E pathogens targeting human IQGAP1

Enteropathogenic Escherichia coli (EPEC), atypical enteropathogenic Escherichia coli (ATEC) and enterohemorrhagic Escherichia coli (EHEC) belong to the family of attaching and effacing (A/E) pathogens. Pathogenicity is mediated by subversion of host cell functions involving type III secretion system (TTSS)-dependent effector proteins. In this study, we have identified and characterized a novel TTSS-dependent effector protein encoded at the 5'-end of the locus of enterocyte effacement (LEE) pathogenicity island (PAI) of ATEC strain 3431-4/86 (O8:H(-)). Using affinity purification we identified IQGAP1, a scaffolding protein involved in the regulation of the actin cytoskeleton, as a putative host cell target. Accordingly, we termed the novel effector protein 'Ibe' for IQGAP1-binding effector. The interaction of Ibe and IQGAP1 was confirmed by co-immunoprecipitation from ATEC-infected cells and immunofluorescence analysis, which revealed colocalization of Ibe and IQGAP1 in ATEC-induced pedestals and actin-rich membrane ruffles. This suggests that the putative effector function of Ibe is mediated via IQGAP1. The Ibe-independent recruitment of IQGAP1 to ATEC-induced pedestals implies a general role for IQGAP1 in the subversion of host cell functions during infection. Homologues of the novel effector Ibe are widely distributed among EPEC, ATEC and EHEC strains but are not necessarily genetically linked to the LEE as they have occasionally also been found to be encoded within lambdoid prophages.

Menin interacts with IQGAP1 to enhance intercellular adhesion of beta-cells

Multiple endocrine neoplasia type 1 (MEN1) is a dominantly inherited tumor syndrome that results from the mutation of the MEN1 gene that encodes protein menin. Stable overexpression of MEN1 has been shown to partially suppress the RAS-mediated morphological changes of NH3 fibroblast cells. Little is known about the molecular mechanisms by which menin decreases the oncogenic effects on cell morphology and other phenotypes. Here we showed that ectopic expression of menin in pretumor beta cells increases islet cell adhesion and reduces cell migration. Our further studies revealed that menin interacts with the scaffold protein, IQGAP1, reduces GTP-Rac1 interaction with IQGAP1 but increases E-cadherin/ß-catenin interaction with IQGAP1. Consistent with an essential role for menin in regulating ß cell adhesion in vivo, accumulations of β-catenin and E-cadherin are reduced at cell junctions in the islets from Men1-excised mice. Together, these results define a novel menin-IQGAP1 pathway that controls cell migration and cell-cell adhesion in endocrine cells.

The membrane-tubulating potential of amphiphysin 2/BIN1 is dependent on the microtubule-binding cytoplasmic linker protein 170 (CLIP-170)

Amphiphysins are BIN-amphiphysin-RVS (BAR) domain-containing proteins that influence membrane curvature in sites such as T-tubules in muscular cells, endocytic pits in neuronal as well as non-neuronal cells, and possibly cytoplasmic endosomes. This effect on lipid membranes is fulfilled by diverse amphiphysin 2/BIN1 isoforms, generated by alternative splicing and showing distinct structural and functional properties. In this study, our goal was to characterize the functional role of a ubiquitously expressed amphiphysin 2/BIN1 by the characterization of new molecular partners. We performed a two-hybrid screen with an isoform of amphiphysin 2/BIN1 expressed in HeLa cells. We identified CLIP-170 as an amphiphysin 2/BIN1-interacting molecule. CLIP-170 is a plus-end tracking protein involved in microtubule (MT) stability and recruitment of dynactin. The binding between amphiphysin 2/BIN1 and CLIP-170 is dependent on the N-terminal part of amphiphysin 2 (mostly the BAR domain) and an internal coiled-coil region of CLIP-170. This partnership was confirmed by GST pull-down assay and by co-immunoprecipitation in HeLa cells that express endogenous amphiphysin 2 (mostly isoforms 6, 9 and 10). When overexpressed in HeLa cells, amphiphysin 2/BIN1 leads to the formation of intracellular tubules which can closely align with MTs. After MT depolymerization by nocodazole, amphiphysin 2-stained tubules disappear, and reappear after nocodazole washout. Furthermore, depletion of CLIP-170 by RNAi induced a decrease in the proportion of cells with amphiphysin 2-stained tubules and an increase in the proportion of cells with no tubules. This result suggests the existence of a mechanistic link between the two types of tubules, which is likely to involve the +TIP protein, CLIP-170. Amphiphysin 2/BIN1 may be an anchoring point on membranes for CLIP-170, and consequently for MT. Then, the pushing force of polymerizing MT could help amphiphysin 2/BIN1 in its tubulation potential. We propose that amphiphysin 2/BIN1 participates in the tubulation of traffic intermediates and intracellular organelles first via its intrinsic tubulating potential and second via its ability to bind CLIP-170 and MT.

The IQGAP Iqg1 is a regulatory target of CDK for cytokinesis in Candida albicans

Cyclin-dependent kinases (CDKs) drive and coordinate multiple cell-cycle events, including construction and contraction of the actomyosin ring during cytokinesis. However, it remains unclear whether CDKs regulate cytokinesis by directly targeting components of the ring. In a search for proteins containing consensus CDK phosphorylation sites in Candida albicans, we found that the IQGAP Iqg1 contains two dense clusters of 19 such sites flanking the actin-interacting CH domain. Here, we show that Iqg1 is indeed a phosphoprotein that undergoes cell-cycle-dependent phosphorylation and can be phosphorylated by purified Clb-Cdc28 kinases in vitro. Mass spectrometry identified several phosphoserine and phosphothreonine residues among these CDK sites. Mutating 15 of the CDK phosphorylation sites with alanine markedly reduced Iqg1 phosphorylation in vivo. The 15A mutation greatly stabilized Iqg1, caused both premature assembly and delayed disassembly of the actomyosin ring, blocked Iqg1 interaction with the actin-nucleating proteins Bni1 and Bnr1, and resulted in defects in cytokinesis. Our data therefore strongly support the idea that the Cdc28 CDK regulates cytokinesis partly by directly phosphorylating the actomyosin ring component Iqg1.

Changes in regulation of cell-cell adhesion during tumor transformation

Cadherin-mediated cell-cell adhesion defines the integrity of most tissues. Cell-cell adherens junctions are dynamic structures whose functional state is regulated by interactions of cadherin with beta-catenin, p120, and actin cytoskeleton structures. Small GTPases of the Rho family and GTPase Rap1 play the central role in the formation and maintenance of cell-cell adhesion. Aberrant activation of signaling pathways, transcriptional repression of the E-cadherin gene, ectopic expression of N-cadherin, and disturbances in regulation of adhesive and transcriptional functions of beta-catenin stimulate tumor progression.

Compartmentalised MAPK pathways

The mitogen-activated protein kinase (MAPK) pathway provides cells with the means to interpret external signal cues or conditions, and respond accordingly. This cascade regulates many cell functions such as differentiation, proliferation and migration. Through modulation of both the amplitude and duration of MAPK signalling, cells can control their responses to the multiple activators of the pathway. In addition, recent work has highlighted the importance of the cellular compartment from which the signalling occurs. Cells have developed intricate systems that enable them to localise MAPK components to specific subcellular domains in response to a particular stimulus. Consequently, different factors can activate the same kinase in separate locations. Crucial to this ability are molecular scaffolds, which act as signalling modules for MAPKs, confining them to the desired compartment. The participation of the MAPK network in fundamental physiological processes, such as cell proliferation and inflammation, and the derangement of the homeostasis that occurs in disease processes, renders MAPK a highly desirable target for therapeutic intervention. As we enhance our comprehension of scaffolds and other regulatory molecules, novel targets for drug design may be discovered that will afford selective and specific MAPK modulation.

Cdc42 protein acts upstream of IQGAP1 and regulates cytokinesis in mouse oocytes and embryos

Cdc42 and Rac1 Rho family GTPases, and their interacting protein IQGAP1 are the key regulators of cell polarity. We examined the role of Cdc42 and IQGAP1 in establishing the polarity of mouse oocyte and regulation of meiotic and mitotic divisions. We showed that Cdc42 was localized on the microtubules of meiotic and mitotic spindle and in the cortex of mouse oocytes and cleaving embryos. IQGAP1 was present in the cytoplasm and cortex of growing and fully-grown oocytes. During maturation it disappeared from the cortex and during meiotic and mitotic cytokinesis it concentrated in the contractile ring. Toxin B inhibition of the binding activity of Cdc42 changed the localization of IQGAP1, inhibited emission of the first polar body, and caused disappearance of the cortical actin without affecting the migration of meiotic spindle. This indicates, that in maturing oocytes accumulation of cortical actin is not indispensable for spindle migration. In zygotes treated with toxin B actin cytoskeleton was rearranged and the first and/or subsequent cytokinesis were inhibited. Our results indicate that Cdc42 acts upstream of IQGAP1 and is involved in regulation of cytokinesis in mouse oocytes and cleaving embryos, rather than in establishing the polarity of the oocyte.

IQGAP3 regulates cell proliferation through the Ras/ERK signalling cascade

Proliferation of epithelial cells must be spatiotemporally regulated to maintain the organization of epithelial sheets. Here we show that the IQGAP family, comprising IQGAP1, 2 and 3, underlies lateral cell-cell contacts of epithelial cells. Of the three proteins, IQGAP3 is unique in that its expression is specifically confined to proliferating cells. Knockdown of IQGAP3 in cultured epithelial cells caused inhibition of proliferation and ERK activity. When exogenously expressed in quiescent cells, IQGAP3 was capable of inducing cell-cycle re-entry, which was completely inhibited by the MEK inhibitor U0126. Thus, IQGAP3 is necessary and sufficient for driving cell proliferation and ERK acts downstream of IQGAP3. Furthermore, IQGAP3 specifically interacted with the active, GTP-bound form of Ras, and in IQGAP3 knockdown cells, the activity of Ras, but not of other small GTPases, was inhibited. Thus, IQGAP3 regulates the promotion of cell proliferation through Ras-dependent ERK activation.

IQGAP1 activates Tcf signal independent of Rac1 and Cdc42 in injury and repair of bronchial epithelial cells

The process of injury and repair involves spreading, migration and cell proliferation. The functions of Rho GTPases and their effector IQGAP1 are poor known in this process of airway epithelium. In the present study, we employed a widely used in vitro model by scratching a monolayer of BECs. We found that scratching induced decreasing of the GTP-bound Rac1 and Cdc42, but increasing the amounts of IQGAP1 at different time points. Next, we confirmed that IQGAP1 interacted with the constitutively active Rac1 (Rac1(V12)) and Cdc42 (Cdc42(V12)) rather than the dominant negative Rac1 (Rac1(N17)) and Cdc42 (Cdc42(N17)). Over-expressions of wild type (WT) IQGAP1 and its mutant (T1050AX2), which was defective to interact with Rho GTPases, induced translocation of beta-catenin from the cytoplasm into the nucleus. These results activated Tcf/Lef and increased the expression levels of its target genes of c-myc and cyclin D1. Likewise, the amounts of c-myc and cyclin D1 increased after scratching. Our results suggested that IQGAP1 mediated cell proliferation through activating Tcf in a manner independent of Rac1 and Cdc42 in wound repair of BECs.

The interaction of IQGAP1 with the exocyst complex is required for tumor cell invasion downstream of Cdc42 and RhoA

Invadopodia are actin-based membrane protrusions formed at contact sites between invasive tumor cells and the extracellular matrix with matrix proteolytic activity. Actin regulatory proteins participate in invadopodia formation, whereas matrix degradation requires metalloproteinases (MMPs) targeted to invadopodia. In this study, we show that the vesicle-tethering exocyst complex is required for matrix proteolysis and invasion of breast carcinoma cells. We demonstrate that the exocyst subunits Sec3 and Sec8 interact with the polarity protein IQGAP1 and that this interaction is triggered by active Cdc42 and RhoA, which are essential for matrix degradation. Interaction between IQGAP1 and the exocyst is necessary for invadopodia activity because enhancement of matrix degradation induced by the expression of IQGAP1 is lost upon deletion of the exocyst-binding site. We further show that the exocyst and IQGAP1 are required for the accumulation of cell surface membrane type 1 MMP at invadopodia. Based on these results, we propose that invadopodia function in tumor cells relies on the coordination of cytoskeletal assembly and exocytosis downstream of Rho guanosine triphosphatases.