p190B RhoGAP regulates endothelial-cell-associated proteolysis through MT1-MMP and MMP2

The Muscarinic Acetylcholine M2 Receptor-Induced Nitration of p190A by eNOS Increases RhoA Activity in Cardiac Myocytes

p190RhoGAP, which exists in two paralogs, p190RhoGAP-A (p190A) and p190RhoGAP-B (p190B), is a GTPase activating protein (GAP) contributing to the regulation of the cellular activity of RhoGTPases. Recent data showed that M2 muscarinic acetylcholine receptor (M2R) stimulation in neonatal rat cardiac myocytes (NRCM) induces the binding of p190RhoGAP to the long isoform of the regulator of G protein signaling 3 (RGS3L). This complex formation alters the substrate preference of p190RhoGAP from RhoA to Rac1. By analyzing carbachol-stimulated GAP activity, we show herein that p190A, but not p190B, alters its substrate preference in NRCM. Based on data that the RhoGAP activity of p190A in endothelial cells is diminished upon nitration by endothelial nitric oxide synthase (eNOS)-derived peroxynitrite, we studied whether carbachol-induced NO/peroxynitrite formation contributes to the carbachol-induced RhoA activation in NRCM. Interestingly, the carbachol-induced RhoA activation in NRCM was suppressed by the eNOS-preferring inhibitor L-NIO as well as the non-selective NOS inhibitor L-NAME. Using L-NIO, we firstly verified the carbachol-induced NO production concurrent with eNOS activation and, secondly, the carbachol-induced nitration of p190A in NRCM. By co-immunoprecipitation, the carbachol-induced complex formation of eNOS, p190A, RGS3L and caveolin-3 was detected. We thus conclude that the NO production by M2R-induced eNOS activation in caveolae in NRCM is required for the nitration of p190A, leading to the binding to RGS3L and the change in substrate preference from RhoA to Rac1. In line with this interpretation, the disruption of caveolae in NRCM by methyl-β-cyclodextrin suppressed carbachol-induced RhoA activation in NRCM to a similar extent as the inhibition of NO production.

Actin cytoskeleton in angiogenesis

Actin, one of the most abundant intracellular proteins in mammalian cells, is a critical regulator of cell shape and polarity, migration, cell division, and transcriptional response. Angiogenesis, or the formation of new blood vessels in the body is a well-coordinated multi-step process. Endothelial cells lining the blood vessels acquire several new properties such as front-rear polarity, invasiveness, rapid proliferation and motility during angiogenesis. This is achieved by changes in the regulation of the actin cytoskeleton. Actin remodelling underlies the switch between the quiescent and angiogenic state of the endothelium. Actin forms endothelium-specific structures that support uniquely endothelial functions. Actin regulators at endothelial cell-cell junctions maintain the integrity of the blood-tissue barrier while permitting trans-endothelial leukocyte migration. This review focuses on endothelial actin structures and less-recognised actin-mediated endothelial functions. Readers are referred to other recent reviews for the well-recognised roles of actin in endothelial motility, barrier functions and leukocyte transmigration. Actin generates forces that are transmitted to the extracellular matrix resulting in vascular matrix remodelling. In this review, we attempt to synthesize our current understanding of the roles of actin in vascular morphogenesis. We speculate on the vascular bed specific differences in endothelial actin regulation and its role in the vast heterogeneity in endothelial morphology and function across the various tissues of our body.

Identification of an inhibitory domain in GTPase-activating protein p190RhoGAP responsible for masking its functional GAP domain

The GTPase-activating protein (GAP) p190RhoGAP (p190A) is encoded by ARHGAP35 which is found mutated in cancers. p190A is a negative regulator of the GTPase RhoA in cells and must be targeted to RhoA-dependent actin-based structures to fulfill its roles. We previously identified a functional region of p190A called the PLS (protrusion localization sequence) required for localization of p190A to lamellipodia but also for regulating the GAP activity of p190A. Additional effects of the PLS region on p190A localization and activity need further characterization. Here, we demonstrated that the PLS is required to target p190A to invadosomes. Cellular expression of a p190A construct devoid of the PLS (p190AΔPLS) favored RhoA inactivation in a stronger manner than WT p190A, suggesting that the PLS is an autoinhibitory domain of p190A GAP activity. To decipher this mechanism, we searched for PLS-interacting proteins using a two-hybrid screen. We found that the PLS can interact with p190A itself. Coimmunoprecipitation experiments demonstrated that the PLS interacts with a region in close proximity to the GAP domain. Furthermore, we demonstrated that this interaction is abolished if the PLS harbors cancer-associated mutations: the S866F point mutation and the Δ865-870 deletion. Our results are in favor of defining PLS as an inhibitory domain responsible for masking the p190A functional GAP domain. Thus, p190A could exist in cells under two forms: an inactive closed conformation with a masked GAP domain and an open conformation allowing p190A GAP function. Altogether, our data unveil a new mechanism of p190A regulation.

Proteomic Profiling of Retinoblastoma-Derived Exosomes Reveals Potential Biomarkers of Vitreous Seeding

Retinoblastoma (RB) is the most common tumor of the eye in early childhood. Although recent advances in conservative treatment have greatly improved the visual outcome, local tumor control remains difficult in the presence of massive vitreous seeding. Traditional biopsy has long been considered unsafe in RB, due to the risk of extraocular spread. Thus, the identification of new biomarkers is crucial to design safer diagnostic and more effective therapeutic approaches. Exosomes, membrane-derived nanovesicles that are secreted abundantly by aggressive tumor cells and that can be isolated from several biological fluids, represent an interesting alternative for the detection of tumor-associated biomarkers. In this study, we defined the protein signature of exosomes released by RB tumors (RBT) and vitreous seeding (RBVS) primary cell lines by high resolution mass spectrometry. A total of 5666 proteins were identified. Among these, 5223 and 3637 were expressed in exosomes RBT and one RBVS group, respectively. Gene enrichment analysis of exclusively and differentially expressed proteins and network analysis identified in RBVS exosomes upregulated proteins specifically related to invasion and metastasis, such as proteins involved in extracellular matrix (ECM) remodeling and interaction, resistance to anoikis and the metabolism/catabolism of glucose and amino acids.

Impaired autophagic degradation of lncRNA ARHGAP5-AS1 promotes chemoresistance in gastric cancer

Chemoresistance remains the uppermost disincentive for cancer treatment on account of many genetic and epigenetic alterations. Long non-coding RNAs (lncRNAs) are emerging players in promoting cancer initiation and progression. However, the regulation and function in chemoresistance are largely unknown. Herein, we identified ARHGAP5-AS1 as a lncRNA upregulated in chemoresistant gastric cancer cells and its knockdown reversed chemoresistance. Meanwhile, high ARHGAP5-AS1 expression was associated with poor prognosis of gastric cancer patients. Intriguingly, its abundance is affected by autophagy and SQSTM1 is responsible for transporting ARHGAP5-AS1 to autophagosomes. Inhibition of autophagy in chemoresistant cells, thus, resulted in the upregulation of ARHGAP5-AS1. In turn, it activated the transcription of ARHGAP5 in the nucleus by directly interacting with ARHGAP5 promoter. Interestingly, ARHGAP5-AS1 also stabilized ARHGAP5 mRNA in the cytoplasm by recruiting METTL3 to stimulate m⁶A modification of ARHGAP5 mRNA. As a result, ARHGAP5 was upregulated to promote chemoresistance and its upregulation was also associated with poor prognosis in gastric cancer. In summary, impaired autophagic degradation of lncRNA ARHGAP5-AS1 in chemoresistant cancer cells promoted chemoresistance. It can activate the transcription of ARHGAP5 in the nucleus and stimulate m⁶A modification of ARHGAP5 mRNA to stabilize ARHGAP5 mRNA in the cytoplasm by recruiting METTL3. Therefore, targeting ARHGAP5-AS1/ARHGAP5 axis might be a promising strategy to overcome chemoresistance in gastric cancer.

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

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

SIRT1 suppresses the migration and invasion of gastric cancer by regulating ARHGAP5 expression

Gastric cancer (GC) ranks among the top five malignant tumors worldwide by the incidence and mortality rate. However, the mechanisms underlying its progression are poorly understood. In this study, we investigated the role of SIRT1, a class III deacetylase, in the invasion and metastasis of GC. Here, we found that knockdown of SIRT1 promoted GC cell migration and invasion in vitro and metastasis in vivo. Forced expression of SIRT1 in GC cells had the opposite effects. Then, we used mRNA microarray to identify the target genes that are regulated by SIRT1 and found that ARHGAP5 was downregulated by SIRT1. The results of the mRNA microarray were confirmed in several GC cell lines. Furthermore, SIRT1 inhibited the expression of ARHGAP5 by physically associating with transcription factor c-JUN and deacetylating and inhibiting the transcriptional activity of c-JUN. Then the expression dynamics and clinical significance of ARHGAP5 were analyzed using clinical samples and database. The expression of ARHGAP5 was increased in GC, and positively correlated with tumor size, tumor infiltration, lymph node metastasis, and clinical stage. And multivariate analyses indicated that ARHGAP5 served as an independent prognostic marker of GC. In addition, the biological effects of ARHGAP5 in SIRT1-mediated inhibition of GC migration and invasion were investigated using both in vitro and in vivo models. Silencing of ARHGAP5 considerably inhibited the migration and invasion of GC, and ARHGAP5 was found to be involved in the SIRT1-mediated inhibition of GC migration and invasion. Our results indicate that SIRT1 suppresses migration and invasion of GC by downregulating ARHGAP5 through an interaction with c-JUN, and these phenomena represent a novel mechanism of the antitumor action of SIRT1.

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

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

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

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

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

VEGF-A stimulates podosome-mediated collagen-IV proteolysis in microvascular endothelial cells

Podosomes are dynamic cell-matrix contact structures that combine several key abilities, including adhesion, matrix degradation and mechanosensing. These actin-based cytoskeletal structures have been mostly studied in monocytic cells, but much less is known about those formed in other lineages. In this study, we characterise podosomes in capillary-derived microvascular endothelial cells. We identify two types of podosomes: constitutive podosomes that form in the absence of specific stimulation and induced podosomes that arise in response to the angiogenic factor VEGF-A. Constitutive and VEGF-A-induced podosomes share similar components but exhibit marked differences in terms of gelatinolytic activity. We also show that the extracellular matrix proteins laminin and collagen-IV are key determinants of the VEGF-A response, but neither collagen-I nor fibronectin are conducive for podosome induction. Moreover, only collagen-IV elicits the formation of proteolytically active podosomes through a mechanism involving increased Src phosphorylation, p190RhoGAP-B (also known as ARHGAP5) relocalisation and MT1-MMP (also known as MMP14) cell surface exposure at podosome sites. We hypothesise that by promoting podosome formation, VEGF-A enables endothelial cells to overcome the basement membrane barrier to allow sprouting outwards from the existing vasculature.

MiR-744 functions as a proto-oncogene in nasopharyngeal carcinoma progression and metastasis via transcriptional control of ARHGAP5

Nasopharyngeal carcinoma (NPC) is a highly invasive and metastasis-prone epithelial cancer. The paucity of effective treatment strategies for recurrent and metastatic NPC is the major cause for stagnating survival rate of NPC. Therefore, it's urgent to understand the molecular mechanisms underlying NPC progression and identify novel avenues for targeted therapy. It has emerged recently that microRNAs are potential pro-tumorigenic or tumor-suppressive factors that participate in oncogenesis. In this study, we found that miR-744 expression was upregulated in NPC specimens compared to nasopharyngeal epithelium (NPE) tissue, and miR- 744 upregulation was significantly associated with TNM stage, tumorigenesis and metastasis. Functional studies revealed that miR-744 acts as a novel tumor promotor in NPC. Moreover, we determined that miR-744 targets ARHGAP5 (Rho GTPase activating protein 5), a protumorigenic gene, by directly interacting with its promoter and thereby regulating its expression at transcriptional level. Reintroduction of ARHGAP5 resembled the effects of miR-744 and silencing of ARHGAP5 clearly abrogated miR-744-induced enhancement of cell migration and invasion. High level of ARHGAP5 was positively correlated with that of miR-744 and with advanced stages of NPC, as well as with lymph node metastasis. Taken together, these data reveal for the first time that miR-744 exerts its proto-oncogenic function by directly targeting ARHGAP5 promoter. This newly identified miR-744/ARHGAP5 pathway provides further insight into the progression and metastasis of NPC and indicates potential novel therapeutic targets for NPC.

The microenvironment controls invadosome plasticity

Invadosomes are actin-based structures involved in extracellular matrix degradation. Invadosomes is a term that includes podosomes and invadopodia, which decorate normal and tumour cells, respectively. They are mainly organised into dots or rosettes, and podosomes and invadopodia are often compared and contrasted. Various internal or external stimuli have been shown to induce their formation and/or activity. In this Commentary, we address the impact of the microenvironment and the role of matrix receptors on the formation, and dynamic and degradative activities of invadosomes. In particular, we highlight recent findings regarding the role of type I collagen fibrils in inducing the formation of a new linear organisation of invadosomes. We will also discuss invadosome plasticity more generally and emphasise its physio-pathological relevance.

Podosomes and invadopodia: tools to breach vascular basement membrane

The vascular basement membrane (BM) is a thin and dense cross-linked extracellular matrix layer that covers and protects blood vessels. Understanding how cells cross the physical barrier of the vascular BM will provide greater insight into the potentially critical role of vascular BM breaching in cancer extravasation, leukocyte trafficking and angiogenic sprouting. In the last year, new evidence has mechanistically linked the breaching of vascular BM with the formation of specific cellular micro-domains known as podosomes and invadopodia. These structures are specialized cell-matrix contacts with an inherent ability to degrade the extracellular matrix. Specifically, the formation of podosomes or invadopodia was shown as an important step in vascular sprouting and tumor cell extravasation, respectively. Here, we review and comment on these recent findings and explore the functions of podosomes and invadopodia within the context of pathological processes such as tumor dissemination and tumor angiogenesis.

p53 in cell invasion, podosomes, and invadopodia

Cell invasion of the extracellular matrix is prerequisite to cross tissue migration of tumor cells in cancer metastasis, and vascular smooth muscle cells in atherosclerosis. The tumor suppressor p53, better known for its roles in the regulation of cell cycle and apoptosis, has ignited much interest in its function as a suppressor of cell migration and invasion. How p53 and its gain-of-function mutants regulate cell invasion remains a puzzle and a challenge for future studies. In recent years, podosomes and invadopodia have also gained center stage status as veritable apparatus specialized in cell invasion. It is not clear, however, whether p53 regulates cell invasion through podosomes and invadopodia. In this review, evidence supporting a negative role of p53 in podosomes formation in vascular smooth muscle cells will be surveyed, and signaling nodes that may mediate this regulation in other cell types will be explored.

Rho GAPs and GEFs: controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.

Dense fibrillar collagen is a potent inducer of invadopodia via a specific signaling network

High-density fibrillar collagen matrix induces invadopodia formation in both fibroblasts and carcinoma cell lines through a kindlin2-dependent mechanism that drives local ECM remodeling.

Cell interactions with the extracellular matrix (ECM) can regulate multiple cellular activities and the matrix itself in dynamic, bidirectional processes. One such process is local proteolytic modification of the ECM. Invadopodia of tumor cells are actin-rich proteolytic protrusions that locally degrade matrix molecules and mediate invasion. We report that a novel high-density fibrillar collagen (HDFC) matrix is a potent inducer of invadopodia, both in carcinoma cell lines and in primary human fibroblasts. In carcinoma cells, HDFC matrix induced formation of invadopodia via a specific integrin signaling pathway that did not require growth factors or even altered gene and protein expression. In contrast, phosphoproteomics identified major changes in a complex phosphosignaling network with kindlin2 serine phosphorylation as a key regulatory element. This kindlin2-dependent signal transduction network was required for efficient induction of invadopodia on dense fibrillar collagen and for local degradation of collagen. This novel phosphosignaling mechanism regulates cell surface invadopodia via kindlin2 for local proteolytic remodeling of the ECM.

The roles of akt isoforms in the regulation of podosome formation in fibroblasts and extracellular matrix invasion

Mesenchymal cells employ actin-based membrane protrusions called podosomes and invadopodia for cross-tissue migration during normal human development such as embryogenesis and angiogenesis, and in diseases such as atherosclerosis plaque formation and cancer cell metastasis. The Akt isoforms, downstream effectors of phosphatidylinositol 3 kinase (PI3K), play crucial roles in cell migration and invasion, but their involvement in podosome formation and cell invasion is not known. In this study, we have used Akt1 and/or Akt2 knockout mouse embryonic fibroblasts and Akt3-targeted shRNA to determine the roles of the three Akt isoforms in Src and phorbol ester-induced podosome formation, and extracellular matrix (ECM) digestion. We found that deletion or knockdown of Akt1 significantly reduces Src-induced formation of podosomes and rosettes, and ECM digestion, while suppression of Akt2 has little effect. In contrast, Akt3 knockdown by shRNA increases Src-induced podosome/rosette formation and ECM invasion. These data suggest that Akt1 promotes, while Akt3 suppresses, podosome formation induced by Src, and Akt2 appears to play an insignificant role. Interestingly, both Akt1 and Akt3 suppress, while Akt2 enhances, phorbol ester-induced podosome formation. These data show that Akt1, Akt2 and Akt3 play different roles in podosome formation and ECM invasion induced by Src or phorbol ester, thus underscoring the importance of cell context in the roles of Akt isoforms in cell invasion.

Podosomes as novel players in endothelial biology

Podosomes and invadopodia, collectively known as invadosomes, are specialized cell-matrix contacts with an inherent ability to degrade extracellular matrix. Their occurrence in either normal (podosomes) or cancer cells (invadopodia) is thus traditionally associated with cell invasiveness and tissue remodelling. These specialized micro-domains of the plasma membrane are characterized by enrichment of F-actin, cortactin and metalloproteases. Recent developments in the field show that, under some circumstances, vascular endothelial cells (ECs) can be induced to form this kind of peculiar structures. Cultured ECs contain either 0.5-1-μm-wide individual podosomes or 5 to 10 μm wide ring-like clusters of podosomes (podosome rosettes). The formation of individual podosomes or podosome rosettes in ECs can be induced by soluble factors, such as TGFβ, VEGF, TNFα or pharmacological agents, such as phorbol esters. Recently, the evidence of the existence of such structures in vascular endothelium has been provided by ex vivo observation. Endothelial podosome rosettes have recently been functionally linked to arterial remodelling and sprouting angiogenesis. Concerted efforts aim now at confirming the relevance of endothelial podosomes in these patho-physiological processes in vivo. In the current review, we will introduce some general considerations regarding ECs in the vascular system. From there on, we will review the various EC types where podosomes have been described and the state-of-art knowledge hitherto generated regarding endothelial podosome features.

Inhibition of osteoclast bone resorption activity through osteoprotegerin-induced damage of the sealing zone

Bone remodeling is dependent on the dynamic equilibrium between osteoclast-mediated bone resorption and osteoblast-mediated osteogenesis. The sealing zone is an osteoclast-specific cytoskeletal structure, the integrity of which is critical for osteoclast-mediated bone resorption. To date, studies have focused mainly on the osteoprotegerin (OPG)‑induced inhibition of osteoclast differentiation through the OPG/receptor activator of the nuclear factor kappa-B ligand (RANKL)/RANK system, which affects the bone resorption of osteoclasts. However, the effects of OPG on the sealing zone have not been reported to date. In this study, the formation of the sealing zone was observed by Hoffman modulation contrast (HMC) microscopy and confocal laser scanning microscopy. The effects of OPG on the existing sealing zone and osteoclast-mediated bone resorption activity, as well as the regulatory role of genes involved in the formation of the sealing zone were examined by immunofluorescence staining, HMC microscopy, quantitative reverse transcription polymerase chain reaction (RT-qPCR), western blot analysis and scanning electron microscopy. The sealing zone was formed on day 5, with belt-like protuberances at the cell edge and scattered distribution of cell nuclei, but no filopodia. The sealing zone was intact in the untreated control group. However, defects in the sealing zone were observed in the OPG-treated group (20 ng/ml) and the structure was absent in the groups treated with 40 and 80 ng/ml OPG. The podosomes showed a scattered or clustered distribution between the basal surface of the osteoclasts and the well surface. Furthermore, resorption lacunae were not detected in the 20 ng/ml OPG-treated group, indicating the loss of osteoclast-mediated bone resorption activity. Treatment with OPG resulted in a significant decrease in the expression of Arhgef8/Net1 and DOCK5 Rho guanine nucleotide exchange factors (RhoGEFs), 10 of 18 RhoGTPases (RhoA, RhoB, cdc42v1, cdc42v2, RhoU/Wrch1, RhoF/Rif, Rac2, RhoG, Rnd1 and RhoBTB1), ROCK1 and ROCK2. In conclusion, podosome distribution was affected by the OPG-induced inhibition of the expression of genes in the RhoGTPase signaling pathway. This resulted in damage to or destruction of the sealing zone, thus inhibiting osteoclast-mediated bone resorption activity.

Ionizing radiation-inducible miR-494 promotes glioma cell invasion through EGFR stabilization by targeting p190B rhoGAP

MicroRNAs (miRNAs) play an important role in various stages of tumor progression. miR-494, which we had previously identified as a miRNA induced by ionizing radiation (IR) in the glioma cell line U-251, was observed to enhance invasion of U-251 cells by activating MMP-2. The miR-494-induced invasive potential was accompanied by, and dependent on, epidermal growth factor receptor (EGFR) upregulation and the activation of its downstream signaling constituents, Akt and ERK. The upregulation of EGFR by miR-494 involved the suppression of lysosomal protein turnover. Among the putative target proteins tested, p190B RhoGAP (p190B) was downregulated by miR-494, and its reduced expression was responsible for the increase in EGFR expression. A reporter assay using a luciferase construct containing p190B 3'-untranslated region (3'UTR) confirmed that p190B is a direct target of miR-494. Downregulation of p190B by small interfering RNA (siRNA) transfection closely mimicked the outcomes of miR-494 transfection, and showed increased EGFR expression, MMP-2 secretion, and invasion. Ectopic expression of p190B suppressed the miR-494-induced EGFR upregulation and invasion promotion, thereby suggesting that p190B depletion is critical for the invasion-promoting action of miR-494. Collectively, our results suggest a novel function for miR-494 and its potential application as a target to control invasiveness in cancer therapy.

Importance of RhoGTPases in formation, characteristics, and functions of invadosomes

Podosomes and invadopodia (collectively known as invadosomes) are specialized plasma-membrane actin-based microdomains that combine adhesive properties with matrix degrading and/or mechanosensor activities. These organelles have been extensively studied in vitro and current concerted efforts aim at establishing their physiological relevance and subsequent association with human diseases. Proper functioning of the bone, immune, and vascular systems is likely to depend on these structures while their occurrence in cancer cells appears to be linked to tumor metastasis. The elucidation of the mechanisms driving invadosome assembly is a prerequisite to understanding their role in vivo and ultimately to controlling their functions. Adhesive and soluble ligands act via transmembrane receptors that propagate signals to the cytoskeleton via small G proteins of the Rho family, assisted by tyrosine kinases and scaffold proteins to induce invadosome formation and rearrangements. Oncogene expression and cell-cell interactions may also trigger their assembly. Manipulation of the signals that regulate invadosome formation and dynamics could therefore be a strategy to interfere with their functions in a multitude of pathological settings, such as excessive bone breakdown, infections, vascular remodeling, transendothelial diapedesis, and metastasis.

Extracellular matrix rigidity controls podosome induction in microvascular endothelial cells

BACKGROUND INFORMATION

Podosomes are actin-based structures involved in cell adhesion, migration, invasion and extracellular matrix degradation. They have been described in large vessel endothelial cells, but nothing is known concerning microvascular endothelial cells. Here, we focussed on liver sinusoidal endothelial cells (LSECs), fenestrated microvascular cells that play major roles in liver physiology. Liver fibrosis induces a dedifferentiation of LSECs leading notably to a loss of fenestrae. Because liver fibrosis is associated with increased matrix stiffness, and because substrate stiffness is known to regulate the actin cytoskeleton, we investigated the impact of matrix rigidity on podosome structures in LSECs.

RESULTS

Using primary LSECs, we demonstrated that microvascular endothelial cells are able to form constitutive podosomes. Podosome presence in LSECs was independent of cytokines such as transforming growth factor-β or vascular endothelial growth factor, but could be modulated by matrix stiffness. As expected, LSECs lost their differentiated phenotype during cell culture, which was paralleled by a loss of podosomes. LSECs however retained the capacity to form active podosomes following detachment/reseeding or actin-destabilising drug treatments. Finally, constitutive podosomes were also found in primary microvascular endothelial cells from other organs.

CONCLUSIONS

Our results show that microvascular endothelial cells are able to form podosomes without specific stimulation. Our data suggest that the major determinant of podosome induction in these cells is substrate rigidity.

Physiological type I collagen organization induces the formation of a novel class of linear invadosomes

This study shows that fibrillar collagen I is the physiological inducer of a novel class of invadosomes, which we named “linear invadosomes.” They are dependent on the scaffold protein Tks5 and are able to degrade extracellular matrix elements. Moreover, we demonstrate that they are β1- and β3-integrin independent, unlike classical invadosomes.

Invadosomes are F-actin structures capable of degrading the matrix through the activation of matrix metalloproteases. As fibrillar type I collagen promotes pro-matrix metalloproteinase 2 activation by membrane type 1 matrix metalloproteinase, we aimed at investigating the functional relationships between collagen I organization and invadosome induction. We found that fibrillar collagen I induced linear F-actin structures, distributed along the fibrils, on endothelial cells, macrophages, fibroblasts, and tumor cells. These structures share features with conventional invadosomes, as they express cortactin and N-WASP and accumulate the scaffold protein Tks5, which proved essential for their formation. On the basis of their ability to degrade extracellular matrix elements and their original architecture, we named these structures “linear invadosomes.” Interestingly, podosomes or invadopodia were replaced by linear invadosomes upon contact of the cells with fibrillar collagen I. However, linear invadosomes clearly differ from classical invadosomes, as they do not contain paxillin, vinculin, and β1/β3 integrins. Using knockout mouse embryonic fibroblasts and RGD peptide, we demonstrate that linear invadosome formation and activity are independent of β1 and β3 integrins. Finally, linear invadosomes also formed in a three-dimensional collagen matrix. This study demonstrates that fibrillar collagen I is the physiological inducer of a novel class of invadosomes.

Ethanol increases p190RhoGAP activity, leading to actin cytoskeleton rearrangements

We previously reported that cells chronically exposed to ethanol show alterations in actin cytoskeleton organization and dynamics in primary cultures of newborn rat astrocytes, a well-established in vitro model for foetal alcohol spectrum disorders. These alterations were attributed to a decrease in the cellular levels of active RhoA (RhoA-GTP), which in turn was produced by an increase in the total RhoGAP activity. We here provide evidence that p190RhoGAPs are the main factors responsible for such increase. Thus, in astrocytes chronically exposed to ethanol we observe: (i) an increase in p190A- and p190B-associated RhoGAP activity; (ii) a higher binding of p190A and p190B to RhoA-GTP; (iii) a higher p120RasGAP-p190A RhoGAP complex formation; and (iv) the recruitment of both p190RhoGAPs to the plasma membrane. The simultaneous silencing of both p190 isoforms prevents the actin rearrangements and the total RhoGAP activity increase triggered both by ethanol. Therefore, our data directly points p190RhoGAPs as ethanol-exposure molecular targets on glial cells of the CNS.

The kinesin KIF9 and reggie/flotillin proteins regulate matrix degradation by macrophage podosomes

Podosomes are actin-based matrix contacts in a variety of cell types. This study identifies the kinesin KIF9 and reggie/flotillin proteins as novel regulators of macrophage podosomes and shows that their interaction through the unique C-terminal domain of KIF9 is critical for the matrix-degrading ability of these structures.

Podosomes are actin-based matrix contacts in a variety of cell types, most notably monocytic cells, and are characterized by their ability to lyse extracellular matrix material. Besides their dependence on actin regulation, podosomes are also influenced by microtubules and microtubule-dependent transport processes. Here we describe a novel role for KIF9, a previously little-characterized member of the kinesin motor family, in the regulation of podosomes in primary human macrophages. We find that small interfering RNA (siRNA)/short-hairpin RNA–induced knockdown of KIF9 significantly affects both numbers and matrix degradation of podosomes. Overexpression and microinjection experiments reveal that the unique C-terminal region of KIF9 is crucial for these effects, presumably through binding of specific interactors. Indeed, we further identify reggie-1/flotillin-2, a signaling mediator between intracellular vesicles and the cell periphery, as an interactor of the KIF9 C-terminus. Reggie-1 dynamically colocalizes with KIF9 in living cells, and, consistent with KIF9-mediated effects, siRNA-induced knockdown of reggies/flotillins significantly impairs matrix degradation by podosomes. In sum, we identify the kinesin KIF9 and reggie/flotillin proteins as novel regulators of macrophage podosomes and show that their interaction is critical for the matrix-degrading ability of these structures.

Sodium fluoride induces podosome formation in endothelial cells

BACKGROUND INFORMATION

Fluoride is a well-known G-protein activator. Exposure of cultured cells to its derivatives results in actin cytoskeleton remodelling. Podosomes are actin-based structures endowed with adhesion and matrix-degradation functions. This study investigates actin cytoskeleton reorganization induced by fluoride in endothelial cells.

RESULTS

Treatment of cultured endothelial cells with sodium fluoride (NaF) results in a rapid and potent stimulation of podosome formation. Furthermore, we show that Cdc42 (cell-division cycle 42), Rac1 and RhoA activities are stimulated in NaF-treated cells. However, podosome assembly is dependent on Cdc42 and Rac1, but not RhoA. Although the sole activation of Cdc42 is sufficient to induce individual podosomes, a balance between RhoGTPase activities regulates podosome formation in response to NaF, which in this case are often found in groups or rosettes. As in other models, podosome formation in endothelial cells exposed to NaF also involves Src. Finally, we demonstrate that NaF-induced podosomes are fully competent for matrix protein degradation.

CONCLUSIONS

Taken together, our findings establish NaF as a novel inducer of podosomes in endothelial cells in vitro.

Podosome regulation by Rho GTPases in myeloid cells

Myeloid cells form a first line of defense against infections. They migrate from the circulation to the infected tissues by adhering to and subsequently crossing the vascular wall. This process requires precise control and proper regulation of these interactions with the environment is therefore crucial. Podosomes are the most prominent adhesion structures in myeloid cells. Podosomes control both the adhesive and migratory properties of myeloid cells and the regulation of podosomes is key to the proper functioning of these cells. Here we discuss the regulation of podosomes by Rho GTPases, well known regulators of adhesion and migration, focusing on myeloid cells. In addition, the regulation of podosomes by GTPase regulators such as GEFs and GAPs, as well as the effects of some Rho GTPase effector pathways, will be discussed.

KIF5B and KIF3A/KIF3B kinesins drive MT1-MMP surface exposure, CD44 shedding, and extracellular matrix degradation in primary macrophages

The matrix metalloproteinase (MMP) MT1-MMP plays pivotal roles in leukocyte physiology such as monocyte diapedesis, dendritic cell migration, and T-cell homing. MT1-MMP is a surface-anchored "master switch" proteinase that cleaves a variety of substrates including extracellular matrix components, matrix receptors, and also other MMPs. However, little is known about the mechanisms enabling intracellular trafficking and exposure of MT1-MMP on the cell surface. We now show that, in primary human macrophages, MT1-MMP-positive vesicles travel bidirectionally along microtubules, in a process regulated by KIF5B and KIF3A/KIF3B kinesins. SiRNA-induced knockdown revealed that transport by KIF5B and KIF3A/KIF3B is crucial for delivery of MT1-MMP to the cell surface and also for surface-associated functions of MT1-MMP, such as shedding of the matrix receptors CD44 and syndecan-1 or degradation of extracellular matrix at podosomes. These data show that kinesin-mediated intracellular transport of MT1-MMP is a pivotal process that allows macrophages to dynamically modify their pericellular environment. These data also identify specific kinesins as potential targets for the early manipulation of MT1-MMP activity in tissues.

Simultaneous visualization of protumorigenic Src and MT1-MMP activities with fluorescence resonance energy transfer

Both Src kinase and membrane type 1 matrix metalloproteinase (MT1-MMP) play critical roles in cancer invasion and metastasis. It is not clear, however, how the spatiotemporal activation of these two critical enzymes is coordinated in response to an oncogenic epithelial growth factor (EGF) stimulation. Here, we have visualized the activities of Src and MT1-MMP concurrently in a single live cell by combining two fluorescence resonance energy transfer (FRET) pairs with distinct spectra: (a) cyan fluorescent protein (CFP) and yellow FP (YFP), and (b) orange FP (mOrange2) and red FP (mCherry). The new FRET pair, mOrange2 and mCherry, was first characterized in vitro and in cultured mammalian cells. When integrated with the CFP/YFP pair, this new pair allowed the revelation of an immediate, rapid, and relatively dispersed Src activity. In contrast, the MT1-MMP activity displayed a slow increase at the cell periphery, although Src was shown to play a role upstream to MT1-MMP globally. This difference in the activation patterns of MT1-MMP and Src in response to EGF is further confirmed using an optimized MT1-MMP biosensor capable of being rapidly cleaved by MT1-MMP. The results indicate that although Src and MT1-MMP act globally in the same signaling pathway, their activations differ in space and time upon EGF stimulation, possibly mediated by different sets of intermediates at different subcellular locations. Our results also showed the potential of mOrange2/mCherry as a new FRET pair, together with the popular variants of CFP and YFP, for the simultaneous visualization of multiple molecular activities in a single live cell.

Navigating ECM barriers at the invasive front: the cancer cell-stroma interface

A seminal event in cancer progression is the ability of the neoplastic cell to mobilize the necessary machinery to breach surrounding extracellular matrix barriers while orchestrating a host stromal response that ultimately supports tissue-invasive and metastatic processes. With over 500 proteolytic enzymes identified in the human genome, interconnecting webs of protease-dependent and protease-independent processes have been postulated to drive the cancer cell invasion program via schemes of daunting complexity. Increasingly, however, a body of evidence has begun to emerge that supports a unifying model wherein a small group of membrane-tethered enzymes, termed the membrane-type matrix metalloproteinases (MT-MMPs), plays a dominant role in regulating cancer cell, as well as stromal cell, traffic through the extracellular matrix barriers assembled by host tissues in vivo. Understanding the mechanisms that underlie the regulation and function of these metalloenzymes as host cell populations traverse the dynamic extracellular matrix assembled during neoplastic states should provide new and testable theories regarding cancer invasion and metastasis.

TGFbeta-induced endothelial podosomes mediate basement membrane collagen degradation in arterial vessels

Podosomes are specialized plasma-membrane actin-based microdomains that combine adhesive and proteolytic activities to spatially restrict sites of matrix degradation in in vitro assays, but the physiological relevance of these observations remain unknown. Inducible rings of podosomes (podosome rosettes) form in cultured aortic cells exposed to the inflammatory cytokine TGFbeta. In an attempt to prove the existence of podosomes in living tissues, we developed an ex vivo endothelium observation model. This system enabled us to visualize podosome rosettes in the endothelium of native arterial vessel exposed to biologically active TGFbeta. Podosomes induced in the vessel appear similar to those formed in cultured cells in terms of molecular composition, but in contrast to the latter, arrange in a protruding structure that is similar to invadopodia. Local degradation of the basement membrane scaffold protein collagen-IV, is observed underneath the structures. Our results reveal for the first time the presence of podosome rosettes in the native endothelium and provide evidence for their capacity to degrade the basement membrane, opening up new avenues to study their role in vascular pathophysiology. We propose that podosome rosettes are involved in arterial vessel remodeling.

Haploinsufficiency for p190B RhoGAP inhibits MMTV-Neu tumor progression

INTRODUCTION

Rho signaling regulates key cellular processes including proliferation, survival, and migration, and it has been implicated in the development of many types of cancer including breast cancer. P190B Rho GTPase activating protein (RhoGAP) functions as a major inhibitor of the Rho GTPases. P190B is required for mammary gland morphogenesis, and overexpression of p190B in the mammary gland induces hyperplastic lesions. Hence, we hypothesized that p190B may play a pivotal role in mammary tumorigenesis.

METHODS

To investigate the effects of loss of p190B function on mammary tumor progression, p190B heterozygous mice were crossed with an MMTV-Neu breast cancer model. Effects of p190B deficiency on tumor latency, multiplicity, growth, preneoplastic progression and metastasis were evaluated. To investigate potential differences in tumor angiogenesis between the two groups, immunohistochemistry to detect von Willebrand factor was performed and quantified. To examine gene expression of potential mediators of the angiogenic switch, an angiogenesis PCR array was utilized and results were confirmed using immunohistochemistry. Finally, reciprocal transplantation of tumor fragments was performed to determine the impact of stromal deficiency of p190B on tumor angiogenesis.

RESULTS

P190B deficiency reduced tumor penetrance (53% of p190B+/-Neu mice vs. 100% of p190B+/+Neu mice formed tumors) and markedly delayed tumor onset by an average of 46 weeks. Tumor multiplicity was also decreased, but an increase in the number of preneoplastic lesions was detected indicating that p190B deficiency inhibited preneoplastic progression. Angiogenesis was decreased in the p190B heterozygous tumors, and expression of a potent angiogenic inhibitor, thrombospondin-1, was elevated in p190B+/-Neu mammary glands. Transplantation of p190B+/-Neu tumor fragments into wild-type recipients restored tumor angiogenesis. Strikingly, p190B+/+Neu tumor fragments were unable to grow when transplanted into p190B+/-Neu recipients.

CONCLUSIONS

These data suggest that p190B haploinsufficiency in the epithelium inhibits MMTV-Neu tumor initiation. Furthermore, p190B deficiency in the vasculature is responsible, in part, for the inhibition of MMTV-Neu tumor progression.

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

BACKGROUND

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

METHODS

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

RESULTS

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

GENERAL SIGNIFICANCE

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

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

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

[The endothelial podosome]

Podosomes are highly dynamic adhesion microdomains formed at the ventral membrane of some monocyte-derived cells. Structurally, their most distinguishing feature is their two-part architecture, consisting in a core of F-actin and actin-associated proteins, surrounded by a ring structure consisting of plaque proteins as well as signalling proteins. In addition to the presence of specific markers, they are distinguished from other adhesion structures by the presence of metalloproteases, endowing them with the ability to degrade the extracellular matrix. Invadopodia are related structures, of similar molecular composition but of distinct architecture, made by fibroblasts or epithelial cells transformed by the v-src oncogene or aggressive carcinoma cells. Such membrane-associated cellular devices, now named invadosomes, are thought to have a central role in mediating polarized migration in cells that cross anatomical boundaries. Podosomes have now been shown to form in endothelial cells, non monocytic and non tumoral cells, endowed with tissue invasive activities during vascular remodelling. Here, we summarize the recent advances and developments in this field, discuss how endothelial podosomes combine specificities of monocytic podosomes and invadopodia and provide our provisional outlook into the future understanding of endothelial podosomes.

The myoendothelial junction: breaking through the matrix?

Within the vasculature, specialized cellular extensions from endothelium (and sometimes smooth muscle) protrude through the extracellular matrix where they interact with the opposing cell type. These structures, termed myoendothelial junctions, have been cited as a possible key element in the control of several vascular physiologies and pathologies. This review will discuss observations that have led to a focus on the myoendothelial junction as a cellular integration point in the vasculature for both homeostatic and pathological conditions and as a possible independent signaling entity. We will also highlight the need for novel approaches to studying the myoendothelial junction in order to comprehend the cellular biology associated with this structure.