IQGAP1 Expression Research Articles

IQGAP1 Mediates VE-Cadherin–Based Cell–Cell Contacts and VEGF Signaling at Adherence Junctions Linked to Angiogenesis

Vascular endothelial growth factor (VEGF) induces angiogenesis by stimulating reactive oxygen species (ROS) production primarily through the VEGF receptor-2 (VEGFR2). One of the initial responses in established vessels to stimulate angiogenesis is loss of vascular endothelial (VE)-cadherin-based cell-cell adhesions; however, little is known about the underlying mechanisms. IQGAP1 is a novel VEGFR2 binding protein, and it interacts directly with actin, cadherin, and beta-catenin, thereby regulating cell motility and morphogenesis. Confocal microscopy analysis shows that IQGAP1 colocalizes with VE-cadherin at cell-cell contacts in unstimulated human endothelial cells (ECs). VEGF stimulation reduces staining of IQGAP1 and VE-cadherin at the adherens junction without affecting interaction of these proteins. Knockdown of IQGAP1 using siRNA inhibits localization of VE-cadherin at cell-cell contacts, VEGF-stimulated recruitment of VEGFR2 to the VE-cadherin/beta-catenin complex, ROS-dependent tyrosine phosphorylation of VE-cadherin, which is required for loss of cell-cell contacts and capillary tube formation. IQGAP1 expression is increased in a mouse hindlimb ischemia model of angiogenesis. IQGAP1 is required for establishment of cell-cell contacts in quiescent ECs. To induce angiogenesis, it may function to link VEGFR2 to the VE-cadherin containing adherens junctions, thereby promoting VEGF-stimulated, ROS-dependent tyrosine phosphorylation of VE-cadherin and loss of cell-cell contacts.

Overexpression and diffuse expression pattern of IQGAP1 at invasion fronts are independent prognostic parameters in ovarian carcinomas

IQGAP1 is a multifunctional protein involved in actin cytoskeleton assembly and E-cadherin-mediated cell adhesion. To determine the role of IQGAP1 in ovarian tumors, we evaluated IQGAP1 expression by immunohistochemistry in 17 adenomas, 30 borderline tumors and 80 adenocarcinomas and its relation with patient survival. IQGAP1 was overexpressed in adenocarcinomas compared with adenomas and borderline tumors. Enhanced immunostaining in invasive tumor fronts was categorized as focal or diffuse. The diffuse expression pattern correlated with high histological grade and clinicopathological stages. IQGAP1 overexpression and diffuse invasion pattern were significantly associated with poor prognosis by multivariate analysis. Our findings suggest the involvement of IQGAP1 in the progression and spread of ovarian adenocarcinomas. Overexpression and diffuse expression pattern of IQGAP1 are potentially useful independent molecular predictors of highly aggressive ovarian carcinomas.

Possible mechanism of metastasis in lung adenocarcinomas with a micropapillary pattern

Micropapillary differentiation in adenocarcinomas has recently been associated with poor prognosis because these tumors are more likely to metastasize. However, no clear explanation exists as to why the presence of a micropapillary pattern is associated with metastasis. A case of primary lung adenocarcinoma with a prominent micropapillary pattern is presented here, with special reference to the immunohistochemical expression of the E-cadherin-mediated system and IQGAP1. Histologically, the tumor was diagnosed as a moderately differentiated papillary adenocarcinoma, showing an extensive micropapillary pattern, with intrapulmonary metastases, pulmonary disseminations, lymphovascular invasions, and lymph node metastases. Immunohistochemically, positive staining for the adhesion molecules E-cadherin, alpha-catenin, and beta-catenin was detected in both the micropapillary and non-micropapillary areas, whereas IQGAP1 was detected in the micropapillary, but not in the non-micropapillary, area. The adhesive function of E-cadherin depends on the integrity of the entire cadherin-catenin-actin network, and thus the expression of IQGAP1 may lead to adherens junction disassembly, and consequently, the release of carcinoma cells organizing in a micropapillary pattern. This is the first report to suggest correlation between adenocarcinoma with a micropapillary pattern and the presence of adhesion molecules, and offers an intriguing first glimpse on the role of the micropapillary pattern in the process of metastasis.

IQGAPs are differentially expressed and regulated in polarized gastric epithelial cells.

IQGAPs, GTPase-activating proteins with an IQ motif, are thought to regulate many actin cytoskeleton-based activities through interactions with Cdc42 and Rac. Recently, Cdc42 was implicated in regulation of gastric parietal cell HCl secretion, and IQGAP2 was immunolocalized with Cdc42 to F-actin-rich intracellular canalicular membranes of isolated gastric parietal cells in primary culture. Here we sought to define distribution and localization of IQGAP1 and IQGAP2 in major oxyntic (acid-secreting) gastric mucosal cell types and to determine whether secretory agonists modulate these proteins. Differential staining protocols were used to identify different cell populations (parietal, chief, surface/pit, and mucous neck cells) in semi-intact glands isolated from rabbit gastric mucosae and to characterize these same cells after dispersion and fractionation on isopycnic density gradients with simultaneous staining for F-actin, H+-K+-ATPase, and GSII lectin-binding sites. There was a pronounced increase in intracellular F-actin staining in dispersed chief cells, apparently from internalization of F-actin-rich apical membranes that normally abut the gland lumen. Therefore, other membrane-associated proteins might also be redistributed by disruption of cell-cell contacts. Western blot analyses were used to quantitate relative concentrations of IQGAPs in defined mucosal cell fractions, and gastric glands were used for in situ localizations. We detected uniform levels of IQGAP2 expression in oxyntic mucosal cells with predominant targeting to regions of cell-cell contact and nuclei of all cell types. IQGAP2 was not detected in parietal cell intracellular canaliculi. IQGAP1 expression was variable and targeted predominantly to the cortex of chief and mucous neck cells. Parietal cells expressed little or no IQGAP1 vs. other mucosal cell types. Phosphoprotein affinity chromatography, isoelectric focusing, and phosphorylation site analyses indicated that both IQGAP1 and IQGAP2 are phosphoproteins potentially regulated by [Ca2+]i/PKC and cAMP signaling pathways, respectively. Stimulation of glands with carbachol, which elevates [Ca2+]i and activates PKC, induced apparent translocation of IQGAP1, but not IQGAP2, to apical poles of chief (zymogen) and mucous neck cells. This response was mimicked by PMA but not by ionomycin or by elevation of [cAMP]i with forskolin. Our observations support a novel, PKC-dependent role for IQGAP1 in regulated exocytosis and suggest that IQGAP2 may play a more general role in regulating cell-cell interactions and possibly migration within the gastric mucosa.

IQGAP1, a novel vascular endothelial growth factor receptor binding protein, is involved in reactive oxygen species--dependent endothelial migration and proliferation.

Endothelial cell (EC) proliferation and migration are important for reendothelialization and angiogenesis. We have demonstrated that reactive oxygen species (ROS) derived from the small GTPase Rac1-dependent NAD(P)H oxidase are involved in vascular endothelial growth factor (VEGF)-mediated endothelial responses mainly through the VEGF type2 receptor (VEGFR2). Little is known about the underlying molecular mechanisms. IQGAP1 is a scaffolding protein that controls cellular motility and morphogenesis by interacting directly with cytoskeletal, cell adhesion, and small G proteins, including Rac1. In this study, we show that IQGAP1 is robustly expressed in ECs and binds to the VEGFR2. A pulldown assay using purified proteins demonstrates that IQGAP1 directly interacts with active VEGFR2. In cultured ECs, VEGF stimulation rapidly promotes recruitment of Rac1 to IQGAP1, which inducibly binds to VEGFR2 and which, in turn, is associated with tyrosine phosphorylation of IQGAP1. Endogenous IQGAP1 knockdown by siRNA shows that IQGAP1 is involved in VEGF-stimulated ROS production, Akt phosphorylation, endothelial migration, and proliferation. Wound assays reveal that IQGAP1 and phosphorylated VEGFR2 accumulate and colocalize at the leading edge in actively migrating ECs. Moreover, we found that IQGAP1 expression is dramatically increased in the VEGFR2-positive regenerating EC layer in balloon-injured rat carotid artery. These results suggest that IQGAP1 functions as a VEGFR2-associated scaffold protein to organize ROS-dependent VEGF signaling, thereby promoting EC migration and proliferation, which may contribute to repair and maintenance of the functional integrity of established blood vessels.

IQGAP2 functions as a GTP-dependent effector protein in thrombin-induced platelet cytoskeletal reorganization

AbstractHuman blood platelets are anucleate cells whose response to extracellular stimuli results in actin cytoskeleton rearrangements, thereby providing the critical initial step in the regulation of hemostasis. The serine protease α-thrombin, known to activate platelets by cleavage of a family of protease-activated receptors (PARs), is the most potent physiologic activator of human platelets, though downstream effector proteins uniquely linked to platelet cytoskeletal actin polymerization remain largely uncharacterized. The gene encoding the putative rac1/cdc42 effector protein IQGAP2 was identified within the PAR gene cluster at 5q13, flanked telomeric byPAR1and encompassingPAR3. Immunofluorescence microscopy demonstrated IQGAP2 expression in filopodial extensions of activated platelets and colocalized with F-actin in lamellipodia and filopodia of IQGAP2-transfected COS1 cells. Platelet activation by α-thrombin, but not saturating concentrations of fibrillar collagen or adenosine 5′-diphosphate, uniquely assemble an IQGAP2/arp2/3–actin cytoplasmic complex, an association regulated by guanosine triphosphate rac1 ([GTP]rac1) but not by [GTP]cdc42. Likewise, only thrombin-activated platelets resulted in rapid translocation of IQGAP2 to the platelet cytoskeleton. These observations identify a physiologic scaffolding function for IQGAP2 and establish the presence of a functional genomic unit in humans uniquely evolved to regulate thrombin-induced platelet cytoskeletal actin reorganization.

Immunohistochemical analysis of IQGAP1 expression in human colorectal carcinomas: its overexpression in carcinomas and association with invasion fronts

In histopathological sections, it is frequently observed that carcinoma cells move in and invade the stroma as coherent cell nests, rather than single cells. We have called this type of movement ‘cohort migration (CM)’ and developed an in vitro model, in which human colon adenocarcinoma cells move as coherent cell sheets when stimulated with naturally occurring motogenic factor, hepatocyte growth factor/scatter factor (HGF/SF). In this CM model, localized release from cell–cell adhesion is essential for cell movement. Recently, we have shown that IQGAP1, a target molecule of Cdc42 and Rac1 small GTPases, is involved in this localized release from the E-cadherin-based cell–cell adhesion during CM. In this study, we examined expression of IQGAP1 immunohistochemically in human colorectal tissues and found that IQGAP1 was overexpressed in carcinoma tissues compared with normal counterparts. Within the carcinoma tissue, IQGAP1 tended to be expressed more at the invasion front than at the upper portions, and higher levels of expression were observed in deeper two-thirds of carcinoma tissues than in the superficial one-third. This expression pattern showing stronger signals in deeper portions was most apparent in advanced carcinomas that invaded into the subserosa. These findings supported a role of IQGAP1 in colon carcinoma invasion.

Localization of IQGAP1 is inversely correlated with intercellular adhesion mediated by e-cadherin in gastric cancers.

Down-regulation of E-cadherin function is characteristic of cancer cells and might involve the small G-protein Rho family, including Rac1 and Cdc42. IQGAP1 has been reported to be one of the target proteins of Rac1 and Cdc42. To elucidate the role of IQGAP1 in cancer-cell adhesion, its expression was investigated in 47 cases of human gastric cancer by immunohistochemistry and Western blot upon protein fractionation, especially in comparison with E-cadherin and catenin expression. In the non-cancerous columnar epithelium of the stomach, IQGAP1, as well as E-cadherin/catenin, was expressed at the cell-cell boundary. IQGAP1 was frequently observed diffusely in the cytoplasm in intestinal-type tumors (20/22 cases) but was expressed at the cell membrane in diffuse-type tumors (19/25 cases), thus showing significant association with tumor differentiation (p < 0.01). Interestingly, membranous expression of IQGAP1 was inversely correlated with that of E-cadherin (p < 0.05) or alpha-catenin (p < 0.001). These observations were consistent with the Western blot results following protein fractionation. IQGAP1 was dominantly expressed in the soluble fraction in differentiated tumors; however, in undifferentiated tumors, it was mostly in the insoluble fraction. In contrast, both E-cadherin and alpha-catenin were detected only in the insoluble fraction. Thus, subcellular localization of IQGAP1 from the cytoplasm to the cell membrane was correlated with E-cadherin dysfunction and tumor dedifferentiation in gastric carcinogenesis.

Identification of cadherin-11 down-regulation as a common response of astrocytoma cells to Transforming Growth Factor-α

Transforming growth factor-α (TGF-α) and its receptor are frequently co-expressed in high-grade astrocytomas, suggesting a role for TGF-α autocrine/paracrine loops in the malignant progression of astrocytomas. To identify genes that may be critical in mediating TGF-α impact on the malignant progression of astrocytomas, we have used cDNA arrays to investigate TGF-α effects on the gene expression profile of U-373 MG glioblastoma cells. We found that in these cells ∼ 50% of the TGF-α regulated genes code for cell motility/invasion-related proteins. TGF-α action on the expression of four of these proteins, α-catenin, IQGAP1, RhoA, and cadherin-11, was further investigated by immunoblotting in four astrocytoma cell lines and in normal astrocytes. The results demonstrate that the effects of TGF-α on IQGAP1, α-catenin, and RhoA expression are cell-line dependent. On the other hand, under TGF-α treatment, cadherin-11 expression is consistently decreased in all astrocytoma cell lines tested but is increased in normal astrocytes. In addition, we found that cadherin-11 is consistently down-regulated in astrocytomas versus normal brain tissues. Altogether, these results suggest that the down-regulation of cadherin-11 is a frequent molecular event in the neoplastic transformation of astrocytes and that this down-regulation may be initiated and/or amplified by TGF-α autocrine/paracrine loops during tumor progression.

An IQGAP-related gene is activated during tentacle formation in the simple metazoan Hydra.

Differentiation of body column epithelial cells into tentacle epithelial cells in Hydra is accompanied by changes in both cell shape and cell-cell contact. The molecular mechanism by which epithelial cells acquire tentacle cell characteristics is unknown. Here we report that expression of a Hydra homologue of the mammalian IQGAP1 protein is strongly upregulated during tentacle formation. Like mammalian IQGAP, Hydra IQGAP1 contains an N-terminal calponin-homology domain, IQ repeats and a conserved C terminus. In adult polyps a high level of Hydra IQGAP1 mRNA is detected at the basis of tentacles. Consistent with a role in tentacle formation, IQGAP1 expression is activated during head regeneration and budding at a time when tentacles are emerging. The observations support the previous hypothesis that IQGAP proteins are involved in cytoskeletal as well as cell-cell contact rearrangements.

Helicobacter pylori infection upregulates IQGAP-1 expression in primary cultures of human antral epithelial cells

Helicobacter pylori infection upregulates IQGAP-1 expression in primary cultures of human antral epithelial cells