ADP ribosylation factor 6 (Arf6) acts through FilGAP protein to down-regulate Rac protein and regulates plasma membrane blebbing

RSK/GSK3-mediated phosphorylation of FilGAP regulates chemotactic cancer invasion

Cell migration plays a crucial role in various biological processes, such as gastrulation, immune response, and cancer metastasis. In response to chemoattractant-like growth factors, cells form protrusions and migrate toward the source of the signal. Rho family small GTPase Rac is a key regulator of cell migration by stimulating actin polymerization to generate lamellipodia, flat membrane protrusions at the leading edge of migrating cells. FilGAP (ARHGAP24), a Rac-specific GTPase-activating protein (GAP), suppresses lamellipodia formation, and controls tumor cell migration. In this study, we found that FilGAP is phosphorylated downstream of epidermal growth factor (EGF) signaling. Upon EGF stimulation, FilGAP is phosphorylated at Ser625 by p90 ribosomal S6 kinase (RSK) and then at Ser621 by glycogen synthase kinase 3 (GSK3). Phosphorylation of FilGAP induces its dissociation from actin filaments. We identified a novel actin-localization domain of FilGAP that is essential for stabilizing cell adhesion. Additionally, we found that phosphorylation of FilGAP inhibits its lamellipodia suppression activity. Finally, we showed the expression of nonphosphorylatable FilGAP mutant, but not wild-type FilGAP, reduced cell migration speed and persistence toward the EGF gradient. Taken together, our results suggest that phosphorylation of FilGAP downstream of EGF-signaling plays a critical role in regulating chemotactic tumor cell migration by controlling cell-matrix adhesion and protrusion formation.

FilGAP regulates tumor growth in Glioma through the regulation of mTORC1 and mTORC2

The mechanistic target of rapamycin (mTOR) is a serine/threonine protein kinase that forms the two different protein complexes, known as mTORC1 and mTORC2. mTOR signaling is activated in a variety of tumors, including glioma that is one of the malignant brain tumors. FilGAP (ARHGAP24) is a negative regulator of Rac, a member of Rho family small GTPases. In this study, we found that FilGAP interacts with mTORC1/2 and is involved in tumor formation in glioma. FilGAP interacted with mTORC1 via Raptor and with mTORC2 via Rictor and Sin1. Depletion of FilGAP in KINGS-1 glioma cells decreased phosphorylation of S6K and AKT. Furthermore, overexpression of FilGAP increased phosphorylation of S6K and AKT, suggesting that FilGAP activates mTORC1/2. U-87MG, glioblastoma cells, showed higher mTOR activity than KINGS-1, and phosphorylation of S6K and AKT was not affected by suppression of FilGAP expression. However, in the presence of PI3K inhibitors, phosphorylation of S6K and AKT was also decreased in U-87MG by depletion of FilGAP, suggesting that FilGAP may also regulate mTORC2 in U-87MG. Finally, we showed that depletion of FilGAP in KINGS-1 and U-87MG cells significantly reduced spheroid growth. These results suggest that FilGAP may contribute to tumor growth in glioma by regulating mTORC1/2 activities.

Arf6 as a therapeutic target: Structure, mechanism, and inhibitors

ADP-ribosylation factor 6 (Arf6), a small G-protein of the Ras superfamily, plays pivotal roles in multiple cellular events, including exocytosis, endocytosis, actin remodeling, plasma membrane reorganization and vesicular transport. Arf6 regulates the progression of cancer through the activation of cell motility and invasion. Aberrant Arf6 activation is a potential therapeutic target. This review aims to understand the comprehensive function of Arf6 for future cancer therapy. The Arf6 GEFs, protein structure, and roles in cancer have been summarized. Comprehending the mechanism underlying Arf6-mediated cancer cell growth and survival is essential. The structural features of Arf6 and its efforts are discussed and may be contributed to the discovery of future novel protein-protein interaction inhibitors. In addition, Arf6 inhibitors and mechanism of action are listed in the table. This review further emphasizes the crucial roles in drug resistance and attempts to offer an outlook of Arf6 in cancer therapy.

FilGAP, a GAP for Rac1, down-regulates invadopodia formation in breast cancer cells

Invadopodia are protrusive structures that mediate the extracellular matrix (ECM) degradation required for tumor invasion and metastasis. Rho small GTPases regulate invadopodia formation, but the molecular mechanisms of how Rho small GTPase activities are regulated at the invadopodia remain unclear. Here we have identified FilGAP, a GTPase-activating protein (GAP) for Rac1, as a negative regulator of invadopodia formation in tumor cells. Depletion of FilGAP in breast cancer cells increased ECM degradation and conversely, overexpression of FilGAP decreased it. FilGAP depletion promoted the formation of invadopodia with ECM degradation. In addition, FilGAP depletion and Rac1 overexpression increased the emergence of invadopodia induced by epidermal growth factor, whereas FilGAP overexpression suppressed it. Overexpression of GAP-deficient FilGAP mutant enhanced invadopodia emergence as well as FilGAP depletion. The pleckstrin-homology (PH) domain of FilGAP binds phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2], which is distributed on membranes of the invadopodia. FilGAP localized to invadopodia in breast cancer cells on the ECM, but FilGAP mutant lacking PI(3,4)P2-binding showed low localization. Similarly, the decrease of PI(3,4)P2 production reduced the FilGAP localization. Our results suggest that FilGAP localizes to invadopodia through its PH domain binding to PI(3,4)P2 and down-regulates invadopodia formation by inactivating Rac1, inhibiting ECM degradation in invasive tumor cells.Key words: invadopodia, breast carcinoma, Rac1, FilGAP, PI(3,4)P2.

The roles of Cdc42 and Rac1 in the formation of plasma membrane protrusions in cancer epithelial HeLa cells

The inducible model of clones generated from the cervical cancer epithelial HeLa cell line has shown the role of DOCK10 as a guanine-nucleotide exchange factor for Rho GTPases Cdc42 and Rac1 and as an inducer of filopodia and plasma membrane (PM) ruffles. In this model, constitutively active (CA) mutants of Cdc42 and Rac1 promote filopodia and ruffles, respectively, as in other models. DOCK9 also induces filopodia and ruffles, although ruffling activity is less prominent. By exploiting this model further, the aim of this work is to provide a more complete understanding of the role of Cdc42 and Rac1, and their interactions with DOCK10 and DOCK9, in regulation of PM protrusions. New clones have been generated from HeLa, including single clones expressing one form of wild-type (WT) or dominant negative (DN) Cdc42 or Rac1, and double clones co-expressing one of them together with either DOCK10 or DOCK9. Expression of WT- and DN-Cdc42 induced filopodia. WT-Cdc42 and, especially, DN-Cdc42 also gave rise to veil protrusions, which were neutralized by DOCK10. Moreover, DN-Cdc42 stimulated the emergence of ruffles, further increased by DOCK10, and WT-Cdc42 also augmented ruffles in presence of DOCK9 and DOCK10. WT-Rac1 greatly increased PM blebbing, as did DN-Rac1 more moderately. In both cases, blebs were enhanced by DOCK10. DN-Rac1 and CA-Rac1 moderately raised filopodia, and DOCK10 and DOCK9 had opposed effects on filopodia (up and down, respectively) in presence of WT-Rac1. As conclusions, we highlight that Cdc42 promotes filopodia regardless of its conformational state, and Rac1 induces blebs in conformations other than CA, especially WT-Rac1, in the inducible HeLa clones. The model could be useful to learn more about the mechanisms underlying PM protrusions.

FilGAP, a GAP protein for Rac, regulates front-rear polarity and tumor cell migration through the ECM

Migrating tumor cells are characterized by a sustained front-rear asymmetry, with a front enriched in filamentous actin, which is induced by Rho small GTPase Rac. Regulation of Rac activity by its regulators should be required for effective motility. Here, we show that FilGAP, a GTPase-activating protein (GAP) for Rac, controls front-rear polarity and contributes to maintain effective tumor cell migration through the extracellular matrix (ECM). Overexpression of FilGAP in breast cancer cells induced polarized morphology and led to increased migration speed in collagen matrices, while depletion of FilGAP impaired the cell polarity and migration. FilGAP localizes to the cell front through its pleckstrin-homology (PH) domain in a phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent manner and appears to inactivate Rac at its site. We found that the affinity of PH domain to PIP3 is critically involved in the maintenance of cell polarity. Moreover, small GTPase ADP-ribosylation factor 6 (Arf6), which binds to the FilGAP PH domain, also regulates FilGAP-mediated cell polarity and migration of breast cancer cells. We propose that FilGAP regulates front-rear polarity through its PIP3 and Arf6 binding in tumor cell migration through the ECM.

Tumor-Derived Apoptotic Vesicles: With Death They Do Part

Tumor cells release lipid particles known as extracellular vesicles (EV) that contribute to cancer metastasis, to the immune response, and to thrombosis. When tumors are exposed to radiation or chemotherapy, apoptotic vesicles (ApoVs) are released in abundance as the plasma membrane delaminates from the cytoskeleton. Recent studies have suggested that ApoVs are distinct from the EVs released from living cells, such as exosomes or microvesicles. Depending on their treatment conditions, tumor-released ApoV have been suggested to either enhance or suppress anti-cancer immunity. In addition, tumor-derived ApoV possess procoagulant activity that could increase the thrombotic state in cancer patients undergoing chemotherapy or radiotherapy. Since ApoVs are one of the least appreciated type of EVs, we focus in this review on the distinctive characterization of tumor ApoVs and their proposed mechanistic effects on cancer immunity, coagulation, and metastasis.

ARF6 mediates nephrin tyrosine phosphorylation-induced podocyte cellular dynamics

ADP-ribosylation factor 6 (ARF6) is a small GTPase necessary for regulating cellular structure, motility, and vesicle trafficking. In several cellular systems, ARF6 was shown to regulate actin dynamics in coordination with Rac1, a Rho small GTPase. We examined the function of ARF6 in the kidney podocyte because Rac1 was implicated in kidney diseases involving this cell. We found that ARF6 expression was enriched in human podocytes and that it modulated podocyte cytoskeletal dynamics through a functional interaction with nephrin, an intercellular junction protein necessary for podocyte injury-induced signaling requiring activation by tyrosine phosphorylation of its cytoplasmic domain. ARF6 was necessary for nephrin activation-induced ruffling and focal adhesion turnover, possibly by altering Rac1 activity. In podocyte-specific Arf6 (ARF6_PodKO) knockout mice, ARF6 deficiency did not result in a spontaneous kidney developmental phenotype or proteinuria after aging. However, ARF6_PodKO mice exhibited distinct phenotypes in two in vivo glomerular injury models. In the protamine sulfate perfusion model, which induced acute podocyte effacement, ARF6_PodKO mice were protected from podocyte effacement. In the nephrotoxic serum nephritis model, which induced immune-complex mediated injury, ARF6_PodKO mice exhibited aggravated proteinuria. Together, these observations suggest that while ARF6 is necessary for nephrin tyrosine phosphorylation-induced cytoskeletal dynamics in cultured podocytes, ARF6 has pleotropic podocyte roles in vivo, where glomerular injury-specific mechanisms might activate distinct signaling pathways that dictate whether ARF6 activity is beneficial or deleterious for maintaining the integrity of the glomerular filtration barrier.

The small GTPase Arf6 regulates sea urchin morphogenesis

The small GTPase Arf6 is a conserved protein that is expressed in all metazoans. Arf6 remodels cytoskeletal actin and mediates membrane protein trafficking between the plasma membrane in its active form and endosomal compartments in its inactive form. While a rich knowledge exists for the cellular functions of Arf6, relatively little is known about its physiological role in development. This study examines the function of Arf6 in mediating cellular morphogenesis in early development. We dissect the function of Arf6 with a loss-of-function morpholino and constitutively active Arf6-Q67L construct. We focus on the two cell types that undergo active directed migration: the primary mesenchyme cells (PMCs) that give rise to the sea urchin skeleton and endodermal cells that form the gut. Our results indicate that Arf6 plays an important role in skeleton formation and PMC migration, in part due to its ability to remodel actin. We also found that embryos injected with Arf6 morpholino have gastrulation defects and embryos injected with constitutively active Arf6 have endodermal cells detached from the gut epithelium with decreased junctional cadherin staining, indicating that Arf6 may mediate the recycling of cadherin. Thus, Arf6 impacts cells that undergo coordinated movement to form embryonic structures in the developing embryo.

The RacGAP protein FilGAP is a negative regulator of chemokine-promoted lymphocyte migration

Rho family small GTPases regulate lymphocyte migration induced by chemokines. However, how lymphocyte migration is regulated by Rho GTPases remains to be elucidated. Here, we identified FilGAP, a Rac-specific GAP, as a negative regulator of lymphocyte polarization and migration. Depletion of FilGAP in mouse pro-B BAF cells increased cellular elongation and membrane protrusion after stimulation of the cells with SDF-1α, which caused increased migration speed. Although FilGAP is detectable both at the front and rear of polarized cells, FilGAP appears to be concentrated at the tip of retracting lamellae of moving lymphocytes. Moreover, depletion of FilGAP increased activation of Rac at the front of polarized cells. Thus, FilGAP may inhibit lamellae extension at the front of moving lymphocytes.

Src Family Tyrosine Kinase Signaling Regulates FilGAP through Association with RBM10

FilGAP is a Rac-specific GTPase-activating protein (GAP) that suppresses lamellae formation. In this study, we have identified RBM10 (RNA Binding Motif domain protein 10) as a FilGAP-interacting protein. Although RBM10 is mostly localized in the nuclei in human melanoma A7 cells, forced expression of Src family tyrosine kinase Fyn induced translocation of RBM10 from nucleus into cell peripheries where RBM10 and FilGAP are co-localized. The translocation of RBM10 from nucleus appears to require catalytic activity of Fyn since kinase-negative Fyn mutant failed to induce translocation of RBM10 in A7 cells. When human breast carcinoma MDA-MB-231 cells are spreading on collagen-coated coverslips, endogenous FilGAP and RBM10 were localized at the cell periphery with tyrosine-phosphorylated proteins. RBM10 appears to be responsible for targeting FilGAP at the cell periphery because depletion of RBM10 by siRNA abrogated peripheral localization of FilGAP during cell spreading. Association of RBM10 with FilGAP may stimulate RacGAP activity of FilGAP. First, forced expression of RBM10 suppressed FilGAP-mediated cell spreading on collagen. Conversely, depletion of endogenous RBM10 by siRNA abolished FilGAP-mediated suppression of cell spreading on collagen. Second, FilGAP suppressed formation of membrane ruffles induced by Fyn and instead produced spiky cell protrusions at the cell periphery. This protrusive structure was also induced by depletion of Rac, suggesting that the formation of protrusions may be due to suppression of Rac by FilGAP. We found that depletion of RBM10 markedly reduced the formation of protrusions in cells transfected with Fyn and FilGAP. Finally, depletion of RBM10 blocked FilGAP-mediated suppression of ruffle formation induced by EGF. Taken together, these results suggest that Src family tyrosine kinase signaling may regulate FilGAP through association with RBM10.

Phosphorylation of Serine 402 Regulates RacGAP Protein Activity of FilGAP Protein

FilGAP is a Rho GTPase-activating protein (GAP) that specifically regulates Rac. FilGAP is phosphorylated by ROCK, and this phosphorylation stimulates its RacGAP activity. However, it is unclear how phosphorylation regulates cellular functions and localization of FilGAP. We found that non-phosphorylatable FilGAP (ST/A) mutant is predominantly localized to the cytoskeleton along actin filaments and partially co-localized with vinculin around cell periphery, whereas phosphomimetic FilGAP (ST/D) mutant is diffusely cytoplasmic. Moreover, phosphorylated FilGAP detected by Phos-tag is also mainly localized in the cytoplasm. Of the six potential phosphorylation sites in FilGAP tested, only mutation of serine 402 to alanine (S402A) resulted in decreased cell spreading on fibronectin. FilGAP phosphorylated at Ser-402 is localized to the cytoplasm but not at the cytoskeleton. Although Ser-402 is highly phosphorylated in serum-starved quiescent cells, dephosphorylation of Ser-402 is accompanied with the cell spreading on fibronectin. Treatment of the cells expressing wild-type FilGAP with calyculin A, a Ser/Thr phosphatase inhibitor, suppressed cell spreading on fibronectin, whereas cells transfected with FilGAP S402A mutant were not affected by calyculin A. Expression of constitutively activate Arf6 Q67L mutant stimulated membrane blebbing activity of both non-phosphorylatable (ST/A) and phosphomimetic (ST/D) FilGAP mutants. Conversely, depletion of endogenous Arf6 suppressed membrane blebbing induced by FilGAP (ST/A) and (ST/D) mutants. Our study suggests that Arf6 and phosphorylation of FilGAP may regulate FilGAP, and phosphorylation of Ser-402 may play a role in the regulation of cell spreading on fibronectin.

FilGAP, a Rho–ROCK-regulated GAP for Rac, controls adherens junctions in MDCK cells

Rho family small GTPases are essential for the formation of adherens junctions in epithelial cells. Here, we found that FilGAP (also known as ARHGAP24), a Rac-specific Rho GTPase-activating protein, promoted the formation of adherens junctions in Madin–Darby canine kidney (MDCK) cells. Knockdown of FilGAP by siRNA stimulated the disassembly and migration of MDCK cells induced by hepatocyte growth factor (HGF). By contrast, forced expression of FilGAP induced accumulation of E-cadherin at adherens junctions. Endogenous FilGAP colocalized with E-cadherin at adherens junctions, and depletion of FilGAP reduced the amount of E-cadherin expressed at the surface. The Rac GAP domain of FilGAP was necessary for the suppression of cell scattering induced by HGF. In agreement with this, siRNA-mediated knockdown of both Rac1 and FilGAP suppressed cell scattering induced by HGF. Forced expression of Rho kinase (ROCK, of which there are two isoforms ROCK1 and ROCK2) induced the accumulation of E-cadherin at the adherens junction, and depletion of FilGAP prevented the accumulation of E-cadherin. Moreover, wild-type FilGAP but not a non-phosphorylatable FilGAP mutant rescued the accumulation of E-cadherin at adherens junctions. These results suggest that FilGAP might regulate cell–cell adhesion through inactivation of Rac downstream of Rho–ROCK-signaling in MDCK cells.

Supervillin binds the Rac/Rho-GEF Trio and increases Trio-mediated Rac1 activation

We investigated cross-talk between the membrane-associated, myosin II-regulatory protein supervillin and the actin-regulatory small GTPases Rac1, RhoA, and Cdc42. Supervillin knockdown reduced Rac1-GTP loading, but not the GTP loading of RhoA or Cdc42, in HeLa cells with normal levels of the Rac1-activating protein Trio. No reduction in Rac1-GTP loading was observed when supervillin levels were reduced in Trio-depleted cells. Conversely, overexpression of supervillin isoform 1 (SV1) or, especially, isoform 4 (SV4) increased Rac1 activation. Inhibition of the Trio-mediated Rac1 guanine nucleotide exchange activity with ITX3 partially blocked the SV4-mediated increase in Rac1-GTP. Both SV4 and SV1 co-localized with Trio at or near the plasma membrane in ruffles and cell surface projections. Two sequences within supervillin bound directly to Trio spectrin repeats 4-7: SV1-171, which contains N-terminal residues found in both SV1 and SV4 and the SV4-specific differentially spliced coding exons 3, 4, and 5 within SV4 (SV4-E345; SV4 amino acids 276-669). In addition, SV4-E345 interacted with the homologous sequence in rat kalirin (repeats 4-7, amino acids 531-1101). Overexpressed SV1-174 and SV4-E345 affected Rac1-GTP loading, but only in cells with endogenous levels of Trio. Trio residues 771-1057, which contain both supervillin-interaction sites, exerted a dominant-negative effect on cell spreading. Supervillin and Trio knockdowns, separately or together, inhibited cell spreading, suggesting that supervillin regulates the Rac1 guanine nucleotide exchange activity of Trio, and potentially also kalirin, during cell spreading and lamellipodia extension.

FilGAP, a Rac-specific Rho GTPase-activating protein, is a novel prognostic factor for follicular lymphoma

FilGAP, a Rho GTPase-activating protein (GAP), acts as a mediator of Rho/ROCK (Rho-associated protein kinase)-dependent amoeboid movement, and its knockdown results in Rac-driven mesenchymal morphology. Herein, we focus on the possible roles of FilGAP expression in normal and malignant lymphocytes. Eighty-three cases of follicular lymphoma (FL), 84 of diffuse large B-cell lymphoma (DLBCL), and 25 of peripheral T-cell lymphoma (PTCL), as well as 10 of normal lymph nodes, were immunohistochemically investigated. In normal lymph nodes, FilGAP immunoreactivity was significantly higher in lymphocytes in the mantle zone as compared to those in the germinal center and paracortical areas. In contrast, the expression levels of both cytoplasmic and perinuclear Rac1 were significantly lower in the germinal center as compared to paracortical regions, suggesting that changes in the FilGAP/Rac axis may occur in B-cell lineages. In malignant lymphomas, FilGAP expression was significantly higher in B-cell lymphomas than PTCL, and the immunohistochemical scores were positively correlated with cytoplasmic Rac1 scores in FL and DLBCL, but not in PTCL. Patients with FL and germinal center B-cell-like (GCB)-type DLBCL showing high FilGAP scores had poor overall survival rates as compared to the low-score patients. Moreover, multivariate Cox regression analysis showed that a high FilGAP score was a significant and independent unfavorable prognostic factor in FL, but not in DLBCL. In conclusion, FilGAP may contribute to change in cell motility of B-lymphocytes. In addition, its expression appears to be useful for predicting the behavior of B-cell lymphoma, in particular FL.

Elevated Slit2 Activity Impairs VEGF-Induced Angiogenesis and Tumor Neovascularization in EphA2-Deficient Endothelium

Angiogenic remodeling during embryonic development and in adult tissue homeostasis is orchestrated by cooperative signaling between several distinct molecular pathways, which are often exploited by tumors. Indeed, tumors upregulate proangiogenic molecules while simultaneously suppressing angiostatic pathways to recruit blood vessels for growth, survival, and metastatic spread. Understanding how cancers exploit proangiogenic and antiangiogenic signals is a key step in developing new, molecularly targeted antiangiogenic therapies. While EphA2, a receptor tyrosine kinase (RTK), is required for VEGF-induced angiogenesis, the mechanism through which these pathways intersect remains unclear. Slit2 expression is elevated in EphA2-deficient endothelium, and here it is reported that inhibiting Slit activity rescues VEGF-induced angiogenesis in cell culture and in vivo, as well as VEGF-dependent tumor angiogenesis, in EphA2-deficient endothelial cells and animals. Moreover, blocking Slit activity or Slit2 expression in EphA2-deficient endothelial cells restores VEGF-induced activation of Src and Rac, both of which are required for VEGF-mediated angiogenesis. These data suggest that EphA2 suppression of Slit2 expression and Slit angiostatic activity enables VEGF-induced angiogenesis in vitro and in vivo, providing a plausible mechanism for impaired endothelial responses to VEGF in the absence of EphA2 function.

IMPLICATIONS

Modulation of angiostatic factor Slit2 by EphA2 receptor regulates endothelial responses to VEGF-mediated angiogenesis and tumor neovascularization.