Dynamin participates in focal extracellular matrix degradation by invasive cells

Modulation of cytoskeletal dynamics by mammalian nucleoside diphosphate kinase (NDPK) proteins

Nucleoside diphosphate kinase (NDPK) proteins comprise a family of ten human isoforms that participate in the regulation of multiple cellular processes via enzymatic and nonenzymatic functions. The major enzymatic function of NDPKs is the generation of nucleoside triphosphates, such as guanosine triphosphate (GTP). Mechanisms behind the nonenzymatic NDPK functions are not clear but likely involve context-dependent signaling roles of NDPK within multi-protein complexes. This is most evident for NDPK-A, which is encoded by the human NME1 gene, the first tumor metastasis suppressor gene to be identified. Understanding which protein interactions are most relevant for the biological and metastasis-related functions of NDPK will be important in the potential utilization of NDPK as a disease target. Accumulating evidence suggests that NDPK interacts with and affects various components and regulators of the cytoskeleton, including actin-binding proteins, intermediate filaments, and cytoskeletal attachment structures (adherens junctions, desmosomes, and focal adhesions). We review the existing literature on this topic and highlight outstanding questions and potential future directions that should clarify the impact of NDPK on the different cytoskeletal systems.

Dynamin1 is a novel target for IRSp53 protein and works with mammalian enabled (Mena) protein and Eps8 to regulate filopodial dynamics

Filopodia are dynamic actin-based structures that play roles in processes such as cell migration, wound healing, and axonal guidance. Cdc42 induces filopodial formation through IRSp53, an Inverse-Bin-Amphiphysins-Rvs (I-BAR) domain protein. Previous work from a number of laboratories has shown that IRSp53 generates filopodia by coupling membrane protrusion with actin dynamics through its Src homology 3 domain binding partners. Here, we show that dynamin1 (Dyn1), the large guanosine triphosphatase, is an interacting partner of IRSp53 through pulldown and Förster resonance energy transfer analysis, and we explore its role in filopodial formation. In neuroblastoma cells, Dyn1 localizes to filopodia, associated tip complexes, and the leading edge just behind the anti-capping protein mammalian enabled (Mena). Dyn1 knockdown reduces filopodial formation, which can be rescued by overexpressing wild-type Dyn1 but not the GTPase mutant Dyn1-K44A and the loss-of-function actin binding domain mutant Dyn1-K/E. Interestingly, dynasore, an inhibitor of Dyn GTPase, also reduced filopodial number and increased their lifetime. Using rapid time-lapse total internal reflection fluorescence microscopy, we show that Dyn1 and Mena localize to filopodia only during initiation and assembly. Dyn1 actin binding domain mutant inhibits filopodial formation, suggesting a role in actin elongation. In contrast, Eps8, an actin capping protein, is seen most strongly at filopodial tips during disassembly. Taken together, the results suggest IRSp53 partners with Dyn1, Mena, and Eps8 to regulate filopodial dynamics.

Expression of the adaptor protein Tks5 in human cancer: prognostic potential

Tks5 (tyrosine kinase substrate with 5 SH3 domains) is an adaptor protein which cooperates with Src tyrosine kinase to promote the formation of protease-enriched, actin-based projections known as invadopodia, which are utilized by invasive cancer cells to degrade the extracellular matrix (ECM). We previously identified a Src-Tks5-Nck pathway which promotes invadopodium formation and ECM proteolysis in melanoma and breast cancer cells. We therefore sought to investigate the significance of Tks5 expression in human cancers. This was undertaken retrospectively through an immunohistochemical evaluation in tissue microarray cores and through data mining of the public database, Oncomine. Here we showed that Tks5 was expressed at higher levels in the microarray cores of breast, colon, lung and prostate cancer tissues compared to the levels in normal tissues. Importantly, mining of Oncomine datasets revealed a strong correlation between Tks5 mRNA overexpression in a number of cancers with increased metastatic events and a poorer prognosis. Collectively, these findings suggest a clinical association of Tks5 expression in human cancers. It identifies the importance for further investigations in examining the full potential of Tks5 as a relevant prognostic marker in a select number of cancers which may have implications for future targeted therapies.

Regulation of dynamin oligomerization in cells: the role of dynamin-actin interactions and its GTPase activity

Dynamin is a 96-kDa protein that has multiple oligomerization states that influence its GTPase activity. A number of different dynamin effectors, including lipids, actin filaments, and SH3-domain-containing proteins, have been implicated in the regulation of dynamin oligomerization, though their roles in influencing dynamin oligomerization have been studied predominantly in vitro using recombinant proteins. Here, we identify higher order dynamin oligomers such as rings and helices in vitro and in live cells using fluorescence lifetime imaging microscopy (FLIM). FLIM detected GTP- and actin-dependent dynamin oligomerization at distinct cellular sites, including the cell membrane and transition zones where cortical actin transitions into stress fibers. Our study identifies a major role for direct dynamin-actin interactions and dynamin's GTPase activity in the regulation of dynamin oligomerization in cells.

Control of MT1-MMP transport by atypical PKC during breast-cancer progression

Dissemination of carcinoma cells requires the pericellular degradation of the extracellular matrix, which is mediated by membrane type 1-matrix metalloproteinase (MT1-MMP). In this article, we report a co-up-regulation and colocalization of MT1-MMP and atypical protein kinase C iota (aPKCι) in hormone receptor-negative breast tumors in association with a higher risk of metastasis. Silencing of aPKC in invasive breast-tumor cell lines impaired the delivery of MT1-MMP from late endocytic storage compartments to the surface and inhibited matrix degradation and invasion. We provide evidence that aPKCι, in association with MT1-MMP-containing endosomes, phosphorylates cortactin, which is present in F-actin-rich puncta on MT1-MMP-positive endosomes and regulates cortactin association with the membrane scission protein dynamin-2. Thus, cell line-based observations and clinical data reveal the concerted activity of aPKC, cortactin, and dynamin-2, which control the trafficking of MT1-MMP from late endosome to the plasma membrane and play an important role in the invasive potential of breast-cancer cells.

ADF/cofilin promotes invadopodial membrane recycling during cell invasion in vivo

Localized F-actin disassembly by ADF/cofilin drives invadopodial membrane recycling through endolysosomes, which promotes efficient cell transmigration through the basement membrane.

Invadopodia are protrusive, F-actin–driven membrane structures that are thought to mediate basement membrane transmigration during development and tumor dissemination. An understanding of the mechanisms regulating invadopodia has been hindered by the difficulty of examining these dynamic structures in native environments. Using an RNAi screen and live-cell imaging of anchor cell (AC) invasion in Caenorhabditis elegans, we have identified UNC-60A (ADF/cofilin) as an essential regulator of invadopodia. UNC-60A localizes to AC invadopodia, and its loss resulted in a dramatic slowing of F-actin dynamics and an inability to breach basement membrane. Optical highlighting indicated that UNC-60A disassembles actin filaments at invadopodia. Surprisingly, loss of unc-60a led to the accumulation of invadopodial membrane and associated components within the endolysosomal compartment. Photobleaching experiments revealed that during normal invasion the invadopodial membrane undergoes rapid recycling through the endolysosome. Together, these results identify the invadopodial membrane as a specialized compartment whose recycling to form dynamic, functional invadopodia is dependent on localized F-actin disassembly by ADF/cofilin.

The significance of dynamin 2 expression for prostate cancer progression, prognostication, and therapeutic targeting

Dynamin 2 (Dyn2) is essential for intracellular vesicle formation and trafficking, cytokinesis, and receptor endocytosis. In this study, we investigated the implication of Dyn2 as a prognostic marker and therapeutic target for progressive prostate cancer (PCA). We evaluated Dyn2 protein expression by immunohistochemistry in two cohorts: men with localized PCA treated by retropubic radical prostatectomy (n = 226), and men with advanced/castrate-resistant PCA (CRPC) treated by transurethral resection of prostate (TURP) (n = 253). The role of Dyn2 in cell invasiveness was assessed by in vitro and in vivo experiments using androgen-responsive and refractory PCA preclinical models. Dyn2 expression was significantly increased across advanced stages of PCA compared to benign prostate tissue (P < 0.0001). In the CRPC cohort, high Dyn2 was associated with higher Gleason score (P = 0.004) and marginally with cancer-specific mortality (P = 0.052). In preclinical models, Dyn2 gene silencing significantly reduced cell migration and invasion in vitro, as well as tumor size and lymph node metastases in vivo. In isolated PCA cells, Dyn2 was found to regulate focal adhesion turnover, which is critical for cell migration; this mechanism requires full Dyn2 compared to mutants deficient in GTPase activity. In conclusion, Dyn2 overexpression is associated with neoplastic prostate epithelium and is associated with poor prognosis. Inhibition of Dyn2 prevents cell invasiveness in androgen-responsive and -refractory PCA models, supporting the potential benefit of Dyn2 to serve as a therapeutic target for advanced PCA.

Essential function of dynamin in the invasive properties and actin architecture of v-Src induced podosomes/invadosomes

The large GTPase dynamin plays a key role in endocytosis but is also localized at numerous actin rich sites. We investigated dynamin functions at podosomes/invadosomes, actin-based cellular adhesion structures implicated in tissue invasion. Podosomes/invadosomes are constituted of long F-actin bundles perpendicular to the substratum (actin cores), connected to randomly arranged F-actin fibers parallel to the substratum (actin cloud). We show here that dynamin depletion in v-Src-transformed fibroblasts triggers a massive disorganization of podosomes/invadosomes (isolated or in rosettes), with a corresponding inhibition of their invasive properties. The action of dynamin at podosomes/invadosomes requires a functional full-length protein, suggesting that the effects of dynamin at these sites and in membrane remodelling during endocytosis are mediated by similar mechanisms. In order to determine direct effect of dynamin depletion on invadosome, an optogenetic approach based on the photosensitizer KillerRed was developed. Acute dynamin photo-inactivation leads to a very rapid disorganization of invadosome without affecting focal adhesions. Dynamin therefore is a key regulator of the architecture of actin in podosomes/invadosomes.

N'-[4-(dipropylamino)benzylidene]-2-hydroxybenzohydrazide is a dynamin GTPase inhibitor that suppresses cancer cell migration and invasion by inhibiting actin polymerization

Dynasore, a specific dynamin GTPase inhibitor, suppresses lamellipodia formation and cancer cell invasion by destabilizing actin filaments. In search for novel dynamin inhibitors that suppress actin dynamics more efficiently, dynasore analogues were screened. N'-[4-(dipropylamino)benzylidene]-2-hydroxybenzohydrazide (DBHA) markedly reduced in vitro actin polymerization, and dose-dependently inhibited phosphatidylserine-stimulated dynamin GTPase activity. DBHA significantly suppressed both the recruitment of dynamin 2 to the leading edge in U2OS cells and ruffle formation in H1299 cells. Furthermore, DBHA suppressed both the migration and invasion of H1299 cells by approximately 70%. Furthermore, intratumoral DBHA delivery significantly repressed tumor growth. DBHA was much less cytotoxic than dynasore. These results strongly suggest that DBHA inhibits dynamin-dependent actin polymerization by altering the interactions between dynamin and lipid membranes. DBHA and its derivative may be potential candidates for potent anti-cancer drugs.

Dynamin rings: not just for fission

The GTPase dynamin has captivated researchers for over two decades, even managing to establish its own research field. Dynamin's allure is partly due to its unusual biochemical properties as well as its essential role in multiple cellular processes, which include the regulation of clathrin-mediated endocytosis and of actin cytoskeleton. On the basis of the classic model, dynamin oligomerization into higher order oligomers such as rings and helices directly executes the final fission reaction in endocytosis, which results in the generation of clathrin-coated vesicles. Dynamin's role in the regulation of actin cytoskeleton is mostly explained by its interactions with a number of actin-binding and -regulating proteins; however, the molecular mechanism of dynamin's action continues to elude us. Recent insights into the mechanism and role of dynamin oligomerization in the regulation of actin polymerization point to a novel role for dynamin oligomerization in the cell.

Integrin-specific control of focal adhesion kinase and RhoA regulates membrane protrusion and invasion

Cell invasion through extracellular matrix (ECM) is a hallmark of the metastatic cascade. Cancer cells require adhesion to surrounding tissues for efficient migration to occur, which is mediated through the integrin family of receptors. Alterations in expression levels of β1 and β3 integrins have previously been reported in a number of human cancers. However, whether there are specific roles for these ubiquitous receptors in mediating cell invasion remains unclear. Here we demonstrate that loss of β1 but not β3 integrins leads to increased spread cell area and focal adhesion number in cells on 2D immobilized fibronectin. Increased adhesion numbers in β1 knockdown cells correlated with decreased cell migration on 2D surfaces. Conversely, cells depleted of β1 integrins showed increased migration speed on 3D cell-derived matrix as well as in 3D organotypic cultures and inverted invasion assays. This increased invasive potential was also seen in cells lacking β3 integrin but only in 3D cultures containing fibroblasts. Mechanistically, in situ analysis using FRET biosensors revealed that enhanced invasion in cells lacking β1 integrins was directly coupled with reduced activation of focal adhesion kinase (FAK) and the small GTPase RhoA resulting in formation of enhanced dynamic protrusions and increased invasion. These reductions in FAK-RhoA signal activationwere not detected in β3 knockdown cells under the same conditions. This data demonstrates a specific role for β1 integrins in the modulation of a FAK-RhoA-actomyosin signaling axis to regulate cell invasion through complex ECM environments.

Atypical protein kinase C in cell motility

Cell motility is defined as cell movement in the three-dimensional space leading to repositioning of the cell. Atypical protein kinase C (aPKC, including ζ and λ/ι) are a subfamily of PKC. Different from classic PKC and novel PKC, the activation of atypical PKC is not dependent on diacylglycerol or calcium. PKCζ can be activated by lipid components, such as phosphatidylinositols, phosphatidic acid, arachidonic acid, and ceramide. Both phosphatidylinositol (3,4,5)-trisphosphate and PDK1 are necessary for the complete and stable activation of PKCζ. Atypical PKC is involved in the regulation of cell polarization, directional sensing, formation of filopodia, and cell motility. It is essential for migration and invasion of multiple cancer cell types. Particularly, atypical PKC has been found in the regulation of the motility of hematopoietic cells. It also participates in the regulation of proteolytic activity of podosomes and invadopodia. It has been found that atypical PKC can work coordinately with other PKC subfamily members and other signaling pathways. Research on the roles of atypical PKC in cell motility may lead to new therapeutic strategies for cancer and other diseases.

Impact of flavonoids on matrix metalloproteinase secretion and invadopodia formation in highly invasive A431-III cancer cells

Metastasis is a major cause of mortality in cancer patients. Invadopodia are considered to be crucial structures that allow cancer cells to penetrate across the extracellular matrix (ECM) by using matrix metalloproteinases (MMPs). Previously, we isolated a highly invasive A431-III subline from parental A431 cells by Boyden chamber assay. The A431-III cells possess higher invasive and migratory abilities, elevated levels of MMP-9 and an enhanced epithelial-mesenchymal transition (EMT) phenotype. In this study, we discovered that A431-III cells had an increased potential to form invadopodia and an improved capacity to degrade ECM compared with the original A431 cells. We also observed enhanced phosphorylation levels of cortactin and Src in A431-III cells; these phosphorylated proteins have been reported to be the main regulators of invadopodia formation. Flavonoids, almost ubiquitously distributed in food plants and plant food products, have been documented to exhibit anti-tumor properties. Therefore, it was of much interest to explore the effects of flavonoid antioxidants on the metastatic activity of A431-III cells. Exposure of A431-III cells to two potent dietary flavonoids, namely luteolin (Lu) and quercetin (Qu), caused inhibition of invadopodia formation and decrement in ECM degradation. We conclude that Lu and Qu attenuate the phosphorylation of cortactin and Src in A431-III cells. As a consequence, there ensues a disruption of invadopodia generation and the suppression of MMP secretion. These changes, in concert, bring about a reduction in metastasis.

Combined p21-activated kinase and farnesyltransferase inhibitor treatment exhibits enhanced anti-proliferative activity on melanoma, colon and lung cancer cell lines

Background

Farnesyltransferase inhibitors (FTIs) are anticancer agents with a spectrum of activity in Ras-dependent and independent tumor cellular and xenograph models. How inhibition of protein farnesylation by FTIs results in reduced cancer cell proliferation is poorly understood due to the multiplicity of potential FTase targets. The low toxicity and oral availability of FTIs led to their introduction into clinical trials for the treatment of breast cancer, hematopoietic malignancy, advanced solid tumor and pancreatic cancer treatment, and Hutchinson-Gilford Progeria Syndrome. Although their efficacy in combinatorial therapies with conventional anticancer treatment for myeloid malignancy and solid tumors is promising, the overall results of clinical tests are far below expectations. Further exploitation of FTIs in the clinic will strongly rely on understanding how these drugs affect global cellular activity.

Methods

Using FTase inhibitor I and genome-wide chemical profiling of the yeast barcoded deletion strain collection, we identified genes whose inactivation increases the antiproliferative action of this FTI peptidomimetic. The main findings were validated in a panel of cancer cell lines using FTI-277 in proliferation and biochemical assays paralleled by multiparametric image-based analyses.

Results

ABC transporter Pdr10 or p-21 activated kinase (PAK) gene deletion increases the antiproliferative action of FTase inhibitor I in yeast cells. Consistent with this, enhanced inhibition of cell proliferation by combining group I PAK inhibition, using IPA3, with FTI-277 was observed in melanoma (A375MM), lung (A549) and colon (HT29), but not in epithelial (HeLa) or breast (MCF7), cancer cell lines. Both HeLa and A375MM cells show changes in the nuclear localization of group 1 PAKs in response to FTI-277, but up-regulation of PAK protein levels is observed only in HeLa cells.

Conclusions

Our data support the view that group I PAKs are part of a pro-survival pathway activated by FTI treatment, and group I PAK inactivation potentiates the anti-proliferative action of FTIs in yeast as well as in cancer cells. These findings open new perspectives for the use of FTIs in combinatorial strategies with PAK inhibitors in melanoma, lung and colon malignancy.

Critical role of transient activity of MT1-MMP for ECM degradation in invadopodia

Focal degradation of extracellular matrix (ECM) is the first step in the invasion of cancer cells. MT1-MMP is a potent membrane proteinase employed by aggressive cancer cells. In our previous study, we reported that MT1-MMP was preferentially located at membrane protrusions called invadopodia, where MT1-MMP underwent quick turnover. Our computer simulation and experiments showed that this quick turnover was essential for the degradation of ECM at invadopodia (Hoshino, D., et al., (2012) PLoS Comp. Biol., 8: e1002479). Here we report on characterization and analysis of the ECM-degrading activity of MT1-MMP, aiming at elucidating a possible reason for its repetitive insertion in the ECM degradation. First, in our computational model, we found a very narrow transient peak in the activity of MT1-MMP followed by steady state activity. This transient activity was due to the inhibition by TIMP-2, and the steady state activity of MT1-MMP decreased dramatically at higher TIMP-2 concentrations. Second, we evaluated the role of the narrow transient activity in the ECM degradation. When the transient activity was forcibly suppressed in computer simulations, the ECM degradation was heavily suppressed, indicating the essential role of this transient peak in the ECM degradation. Third, we compared continuous and pulsatile turnover of MT1-MMP in the ECM degradation at invadopodia. The pulsatile insertion showed basically consistent results with the continuous insertion in the ECM degradation, and the ECM degrading efficacy depended heavily on the transient activity of MT1-MMP in both models. Unexpectedly, however, low-frequency/high-concentration insertion of MT1-MMP was more effective in ECM degradation than high-frequency/low-concentration pulsatile insertion even if the time-averaged amount of inserted MT1-MMP was the same. The present analysis and characterization of ECM degradation by MT1-MMP together with our previous report indicate a dynamic nature of MT1-MMP at invadopodia and the importance of its transient peak in the degradation of the ECM.

Dynamin 2 potentiates invasive migration of pancreatic tumor cells through stabilization of the Rac1 GEF Vav1

The large GTPase Dynamin 2 (Dyn2) is markedly upregulated in pancreatic cancer, is a potent activator of metastatic migration, and is required for Rac1-mediated formation of lamellipodia. Here we demonstrate an unexpected mechanism of Dyn2 action in these contexts via direct binding to the Rac1 guanine nucleotide exchange factor (GEF) Vav1. Surprisingly, disruption of the Dyn2-Vav1 interaction targets Vav1 to the lysosome for degradation via an interaction with the cytoplasmic chaperone Hsc70, resulting in a dramatic reduction of Vav1 protein stability. Importantly, a specific mutation in Vav1 near its Dyn2-binding C-terminal Src homology 3 (SH3) domain prevents Hsc70 binding, resulting in a stabilization of Vav1 levels. Dyn2 binding regulates the interaction of Vav1 with Hsc70 to control the stability and subsequent activity of this oncogenic GEF. These findings elucidate how Dyn2 activates Rac1, lamellipod protrusion, and invasive cellular migration and provide insight into how this specific Vav is ectopically expressed in pancreatic tumors.

HDAC6 deacetylase activity is required for hypoxia-induced invadopodia formation and cell invasion

Despite significant progress in the cancer field, tumor cell invasion and metastasis remain a major clinical challenge. Cell invasion across tissue boundaries depends largely on extracellular matrix degradation, which can be initiated by formation of actin-rich cell structures specialized in matrix degradation called invadopodia. Although the hypoxic microenvironment within solid tumors has been increasingly recognized as an important driver of local invasion and metastasis, little is known about how hypoxia influences invadopodia biogenesis. Here, we show that histone deacetylase 6 (HDAC6), a cytoplasmic member of the histone deacetylase family, is a novel modulator of hypoxia-induced invadopodia formation. Hypoxia was found to enhance HDAC6 tubulin deacetylase activity through activation of the EGFR pathway. Activated HDAC6, in turn, triggered Smad3 phosphorylation resulting in nuclear accumulation. Inhibition of HDAC6 activity or knockdown of the protein inhibited both hypoxia-induced Smad3 activation and invadopodia formation. Our data provide evidence that hypoxia influences invadopodia formation in a biphasic manner, which involves the activation of HDAC6 deacetylase activity by EGFR, resulting in enhanced Smad phosphorylation and nuclear accumulation. The identification of HDAC6 as a key participant of hypoxia-induced cell invasion may have important therapeutic implications for the treatment of metastasis in cancer patients.

Cancer cell metabolism regulates extracellular matrix degradation by invadopodia

Transformed cancer cells have an altered metabolism, characterized by a shift towards aerobic glycolysis, referred to as 'the Warburg phenotype'. A change in flux through mitochondrial OXPHOS and cytosolic pathways for ATP production and a gain of capacity for biomass production in order to sustain the needs for altered growth and morphodynamics are typically involved in this global rewiring of cancer cell metabolism. Characteristically, these changes in metabolism are accompanied by enhanced uptake of nutrients like glucose and glutamine. Here we focus on the relationship between cell metabolism and cell dynamics, in particular the formation and function of invadopodia, specialized structures for focal degradation of the extracellular matrix. Since we recently found presence of enzymes that are active in glycolysis and associated pathways in invadopodia, we hypothesize that metabolic adaptation and invadopodia formation are linked processes. We give an overview on the background for this idea and show for the first time that extracellular matrix degradation by invadopodia can be differentially manipulated, without effects on cell proliferation, by use of metabolic inhibitors or changes in nutrient composition of cell culture media. We conclude that cell metabolism and carbohydrate availability, especially pyruvate, are involved in fuelling of invadopodia formation and activity.

The signalling factor PI 3-kinase is a specific regulator of the clathrin-independent dynamin-dependent endocytosis of IL-2 receptors

Receptor-mediated endocytosis is an essential process used by eukaryotic cells to internalise many molecules. Several clathrin-independent endocytic routes exist but the molecular mechanism of each pathway remains to be uncovered. This study focuses on a clathrin-independent, dynamin-dependent pathway used by interleukin 2 receptors (IL-2R), essential players of the immune response. Rac1 and its targets the p21-activated kinases (Pak) are specific regulators of this pathway, acting on cortactin and actin polymerization. Here, our study reveals a dual and specific role of phosphatidylinositol 3-kinase (PI 3-kinase) in IL-2R endocytosis. Firstly, the inhibition of the catalytic activity of PI 3-kinase strongly affects IL-2R endocytosis, in contrast to transferrin (Tf) uptake, a marker of the clathrin-mediated pathway. Moreover, Vav2, a GTPase exchange factor (GEF) induced upon PI 3-kinase activation, is specifically involved in IL-2R entry. The second action of PI 3-kinase is via its regulatory subunit, p85α, which binds to and recruits Rac1 during IL-2R internalisation. Indeed, the overexpression of a p85α mutant missing the Rac1 binding motif, leads to the specific inhibition of IL-2R endocytosis. The inhibitory effect of this p85α mutant could be rescued by the overexpression of either Rac1 or the active form of Pak, indicating that p85α acts upstream of the Rac1-Pak cascade. Finally, biochemical and fluorescent microscopy techniques reveal an interaction between p85α, Rac1 and IL-2R that is enhanced by IL-2. In summary our results point out a key role of class I PI 3-kinase in IL-2R endocytosis that creates a link with IL-2 signalling.

Dynamin: expanding its scope to the cytoskeleton

The large GTPase dynamin is well known for its actions on budded cellular membranes to generate vesicles, most often, clathrin-coated endocytic vesicles. The scope of cellular processes in which dynamin-mediated vesicle formation occurs, has expanded to include secretory vesicle formation at the Golgi, from other endosomes and nonclathrin structures, such as caveolae, as well as membrane remodeling during exocytosis and vesicle fusion. An intriguing new facet of dynamin's sphere of influence is the cytoskeleton. Cytoskeletal filament networks maintain cell shape, provide cell movement, execute cell division and orchestrate vesicle trafficking. Recent evidence supports the hypothesis that dynamin influences actin filaments and microtubules via mechanisms that are independent of its membrane-remodeling activities. This chapter discusses this emerging evidence and considers possible mechanisms of action.

Synergistic modulation of cellular contractility by mixed extracellular matrices

The extracellular matrix (ECM) is known to provide various physicochemical cues in directing cell behavior including composition, topography, and dimensionality. Physical remodeling of the ECM has been documented in a variety of cancers. In breast cancer, the increased deposition of matrix proteins, their crosslinking, and alignment create a stiffer microenvironment that activates cell contractility and promotes cancer invasion. In this paper, we sought to study the collective influence of ECM composition and density on the contractile mechanics of human MDA-MB-231 cells making use of the recently established trypsin deadhesion assay. Using collagen and fibronectin-coated surfaces of varying density, we show that cell contractility is tuned in a density-dependent manner, with faster deadhesion on fibronectin-coated surfaces compared to collagen-coated surfaces under identical coating densities. The deadhesion responses are significantly delayed when cells are treated with the myosin inhibitor blebbistatin. By combining collagen and fibronectin at two different densities, we show that mixed ligand surfaces synergistically modulate cell contractility. Finally, we show that on fibroblast-derived 3D matrices that closely mimic in vivo matrices, cells are strongly polarized and exhibit faster deadhesion compared to the mixed ligand surfaces. Together, our results demonstrate that ECM composition, density, and 3D organization collectively regulate cell contractility.

Quantitative measurement of invadopodia-mediated extracellular matrix proteolysis in single and multicellular contexts

Cellular invasion into local tissues is a process important in development and homeostasis. Malregulated invasion and subsequent cell movement is characteristic of multiple pathological processes, including inflammation, cardiovascular disease and tumor cell metastasis. Focalized proteolytic degradation of extracellular matrix (ECM) components in the epithelial or endothelial basement membrane is a critical step in initiating cellular invasion. In tumor cells, extensive in vitro analysis has determined that ECM degradation is accomplished by ventral actin-rich membrane protrusive structures termed invadopodia. Invadopodia form in close apposition to the ECM, where they moderate ECM breakdown through the action of matrix metalloproteinases (MMPs). The ability of tumor cells to form invadopodia directly correlates with the ability to invade into local stroma and associated vascular components. Visualization of invadopodia-mediated ECM degradation of cells by fluorescent microscopy using dye-labeled matrix proteins coated onto glass coverslips has emerged as the most prevalent technique for evaluating the degree of matrix proteolysis and cellular invasive potential. Here we describe a version of the standard method for generating fluorescently-labeled glass coverslips utilizing a commercially available Oregon Green-488 gelatin conjugate. This method is easily scaled to rapidly produce large numbers of coated coverslips. We show some of the common microscopic artifacts that are often encountered during this procedure and how these can be avoided. Finally, we describe standardized methods using readily available computer software to allow quantification of labeled gelatin matrix degradation mediated by individual cells and by entire cellular populations. The described procedures provide the ability to accurately and reproducibly monitor invadopodia activity, and can also serve as a platform for evaluating the efficacy of modulating protein expression or testing of anti-invasive compounds on extracellular matrix degradation in single and multicellular settings.

ATAT1/MEC-17 acetyltransferase and HDAC6 deacetylase control a balance of acetylation of alpha-tubulin and cortactin and regulate MT1-MMP trafficking and breast tumor cell invasion

Invasive tumor cells use proteases to degrade and migrate through the stromal environment consisting of a 3D network of extracellular matrix macromolecules. In particular, MT1-MMP, a membrane-anchored metalloproteinase, is critical during cancer cell invasion. MT1-MMP is stored in endosomal compartments and then delivered to invadopodia, the specialized plasma membrane domains of invasive cancer cells endowed with extracellular matrix-degradation capacity. In macrophages, traffic of MT1-MMP vesicles to invadopodia-related podosomes requires microtubules. We previously found that in breast tumor MDA-MB-231 cells an increase of microtubule and cortactin acetylation upon inhibition of HDAC6 correlates with a decrease of matrix degradation and invasion in three-dimensional collagen I gel. Here, we investigated the role of the recently identified α-tubulin N-acetyltransferase 1 ATAT1 in invasive MDA-MB-231 cells. We found that the dynamics and distribution of MT1-MMP-positive endosomes require regulation of acetylation levels. We observed that ATAT1 tubulin acetyltransferase binds and regulates cortactin acetylation levels. In addition, ATAT1 colocalizes with cortactin at the adherent surface of the cells and it is required for 2D migration and invasive migration of MDA-MB-231 cells in collagen matrix. All together, our data indicate that a balance of acetylation and deaceylation by ATAT1/HDAC6 enzymes with opposite activities regulates the migratory and invasive capacities of breast tumor cells.

Spatiotemporal regulation of Src and its substrates at invadosomes

In the past decade, substantial progress has been made in understanding how Src family kinases regulate the formation and function of invadosomes. Invadosomes are organized actin-rich structures that contain an F-actin core surrounded by an adhesive ring and mediate invasive migration. Src kinases orchestrate, either directly or indirectly, each phase of the invadosome life cycle including invadosome assembly, maturation and matrix degradation and disassembly. Complex arrays of Src effector proteins are involved at different stages of invadosome maturation and their spatiotemporal activity must be tightly regulated to achieve effective invasive migration. In this review, we highlight some recent progress and the challenges of understanding how Src is regulated temporally and spatially to orchestrate the dynamics of invadosomes and mediate cell invasion.

Focal adhesion kinases in adhesion structures and disease

Cell adhesion to the extracellular matrix (ECM) is essential for cell migration, proliferation, and embryonic development. Cells can contact the ECM through a wide range of matrix contact structures such as focal adhesions, podosomes, and invadopodia. Although they are different in structural design and basic function, they share common remodeling proteins such as integrins, talin, paxillin, and the tyrosine kinases FAK, Pyk2, and Src. In this paper, we compare and contrast the basic organization and role of focal adhesions, podosomes, and invadopodia in different cells. In addition, we discuss the role of the tyrosine kinases, FAK, Pyk2, and Src, which are critical for the function of the different adhesion structures. Finally, we discuss the essential role of these tyrosine kinases from the perspective of human diseases.

NG2-mediated Rho activation promotes amoeboid invasiveness of cancer cells

The aim of this study was to analyze the potential role of NG2 chondroitin sulfate proteoglycan in amoeboid morphology and invasiveness of cancer cells. In the highly metastatic amoeboid cell lines A3 and A375M2, siRNA-mediated down-regulation of NG2 induced an amoeboid-mesenchymal transition associated with decreased invasiveness in 3D collagen and inactivation of the GTPase Rho. Conversely, the expression of NG2 in mesenchymal sarcoma K2 cells as well as in A375M2 cells resulted in an enhanced amoeboid phenotype associated with increased invasiveness and elevated Rho-GTP levels. Remarkably, the amoeboid-mesenchymal transition in A375M2 cells triggered by NG2 down-regulation was associated with increased extracellular matrix-degrading ability, although this was not sufficient to compensate for the decreased invasive capability caused by down-regulated Rho/ROCK signaling. Conversely, in K2 cells with overexpression of NG2, the ability to degrade the extracellular matrix was greatly reduced. Taken together, we suggest that NG2-mediated activation of Rho leading to effective amoeboid invasiveness is a possible mechanism through which NG2 could contribute to tumor cell invasion and metastasis.

pH regulators in invadosomal functioning: proton delivery for matrix tasting

Invadosomes are actin-rich finger-like cellular structures sensing and interacting with the surrounding extracellular matrix (ECM) and involved in its proteolytic remodeling. Invadosomes are structures distinct from other adhesion complexes, and have been identified in normal cells that have to cross tissue barriers to fulfill their function such as leukocytes, osteoclasts and endothelial cells. They also represent features of highly aggressive cancer cells, allowing them to escape from the primary tumor, to invade surrounding tissues and to reach systemic circulation. They are localized to the ventral membrane of cells grown under 2-dimensional conditions and are supposed to be present all around cells grown in 3-dimensional matrices. Indeed invadosomes are key structures in physiological processes such as inflammation and the immune response, bone remodeling, tissue repair, but also in pathological conditions such as osteopetrosis and the development of metastases. Invadosomes are subdivided into podosomes, found in normal cells, and into invadopodia specific for cancer cells. While these two structures exhibit differences in organization, size, number and half-life, they share similarities in molecular composition, participation in cell-matrix adhesion and promoting matrix degradation. A key determinant in invadosomal function is the recruitment and release of proteases, such as matrix metalloproteinases (MMPs), serine proteases and cysteine cathepsins, together with their activation in a tightly controlled and highly acidic microenvironment. Therefore numerous pH regulators such as V-ATPases and Na(+)/H(+) exchangers, are found in invadosomes and are directly involved in their constitution as well as their functioning. This review focuses on the participation of pH regulators in invadosome function in physiological and pathological conditions, with a particular emphasis on ECM remodeling by osteoclasts during bone resorption and by cancer cells.

Pathological roles of invadopodia in cancer invasion and metastasis

Invadopodia are actin-rich membrane protrusions formed by invasive cancer cells. Invadopodia mediate the focal degradation of pericellular extracellular matrix (ECM) by the localized proteolytic activity of matrix metalloproteinases (MMPs). Over the last 2 decades, much progress has been made in identifying the molecular components of invadopodia and understanding the molecular mechanisms underlying their formation. Although the physiological and pathological roles of invadopodia have long been elusive, emerging evidence has begun to reveal their importance in local invasion during cancer metastasis. This review highlights recent findings on the roles of invadopodia in cancer invasion and metastasis and discusses the possibility of and strategies for targeting invadopodia formation for the development of novel anticancer therapeutics.

Dynamin 2 mediates PDGFRα-SHP-2-promoted glioblastoma growth and invasion

Dynamin 2 (Dyn2), a large GTPase, is involved in receptor tyrosine kinase (RTK)-promoted cell migration. However, the molecular mechanisms by which Dyn2 regulates RTK-induced cell migration have not been established. Recently, we reported that tyrosine-protein phosphatase non-receptor type 11 (SHP-2) and phosphatidylinositol 3-kinase (PI3K) mediate platelet-derived growth factor receptor-α (PDGFRα)-promoted glioma tumor growth and invasion. Here, we show that Dyn2 is an effector downstream of the PDGFRα-PI3K/SHP-2 signaling in glioma cells. Depletion of endogenous Dyn2 by short hairpin RNAs (shRNAs) inhibited PDGFRα-stimulated phosphorylation of Akt, Erk1/2, Rac1 and Cdc42 activities, glioma cell migration and survival in vitro and tumor growth and invasion in the brains of mice. Dyn2 binds to SHP-2 and PI3K and colocalizes with PDGFRα at the invasive fronts in PDGF-A-stimulated glioma cells. Inhibition of SHP-2 by siRNA knockdown abrogated Dyn2 association with activated PDGFRα and PDGFRα activation of Rac1 and Cdc42, and glioma cell migration, thereby establishing a link between SHP-2 interaction with Dyn2 and the PDGFRα signaling. Furthermore, a dominant-negative SHP-2 C459S mutant inhibited PDGF-A-stimulated glioma cell migration, phosphorylation of Dyn2 and concomitantly blocked PDGFRα-induced Src activation. Inhibition of Src by Src inhibitors attenuated PDGF-A-stimulated phosphorylation of Akt and Dyn2 and glioma cell migration. Additionally, mutations of binding sites to PI3K, SHP-2 or Src of PDGFRα impaired PDGFRα-stimulated phosphorylation of Akt and Dyn2, and Dyn2 association with activated PDGFRα. Taken together, this study identifies Dyn2 as an effector that mediates PDGFRα-SHP-2-induced glioma tumor growth and invasion, suggesting that targeting the PDGFRα-SHP-2-Dyn2 pathway may be beneficial to patients with malignant glioblastomas.

Filamin A controls matrix metalloproteinase activity and regulates cell invasion in human fibrosarcoma cells

Filamins are an important family of actin-binding proteins that, in addition to bundling actin filaments, link cell surface adhesion proteins, signaling receptors and channels to the actin cytoskeleton, and serve as scaffolds for an array of intracellular signaling proteins. Filamins are known to regulate the actin cytoskeleton, act as mechanosensors that modulate tissue responses to matrix density, control cell motility and inhibit activation of integrin adhesion receptors. In this study, we extend the repertoire of filamin activities to include control of extracellular matrix (ECM) degradation. We show that knockdown of filamin increases matrix metalloproteinase (MMP) activity and induces MMP2 activation, enhancing the ability of cells to remodel the ECM and increasing their invasive potential, without significantly altering two-dimensional random cell migration. We further show that within filamin A, the actin-binding domain is necessary, but not sufficient, to suppress the ECM degradation seen in filamin-A-knockdown cells and that dimerization and integrin binding are not required. Filamin mutations are associated with neuronal migration disorders and a range of congenital malformations characterized by skeletal dysplasia and various combinations of cardiac, craniofacial and intestinal anomalies. Furthermore, in breast cancers loss of filamin A has been correlated with increased metastatic potential. Our data suggest that effects on ECM remodeling and cell invasion should be considered when attempting to provide cellular explanations for the physiological and pathological effects of altered filamin expression or filamin mutations.

Polarised apical-like intracellular sorting and trafficking regulates invadopodia formation and degradation of the extracellular matrix in cancer cells

Invadopodia are proteolytically active protrusions formed by invasive tumoral cells when grown on an extracellular matrix (ECM) substratum. A current challenge is to understand how proteolytic activity is so precisely localised at discrete sites of the plasma membrane to produce focalised ECM degradation at invadopodia. Indeed, a number of components including metalloproteases need to be directed to invadopodia to ensure proper segregation of proteolytic activities. We recently found invadopodia to feature the properties of cholesterol-rich membrane domains (a.k.a. lipid drafts) and that ECM degradation depends on the tight control of cholesterol homeostasis. Since apically directed polarised sorting and transport in epithelial cells relies on segregation of proteins into lipid rafts at the Golgi complex, we hypothesised that invadopodia-dependent ECM degradation might also rely on lipid raft-dependent polarised transport routes. To investigate this issue we undertook a three-pronged approach. First, we found that microtubule depolymerisation, which is known to disrupt polarised transport in polarised cells, strongly inhibited invadopodia formation, while not affecting overall protein transport. In the second approach we found that glycosylphosphatidylinositol-anchored green fluorescent protein (an apical model protein), but not vesicular stomatitis virus G-protein or influenza virus hemagglutinin (both model basolateral model cargoes), was transported to sites of ECM degradation. Finally, RNAi-mediated knock-down of proteins known to specifically regulate polarised apical or basolateral transport in epithelial cells, such as caveolin 1 and annexin XIIIB or clathrin, respectively, demonstrated that the selective inhibition of the apical, but not the basolateral, transport route impairs invadopodia formation and ECM degradation. Taken together, our findings suggest that invadopodia are apical-like membrane domains, where signal transduction and local membrane remodelling events might be temporally and spatially confined via selective raft-dependent apical transport routes.

Establishment and validation of computational model for MT1-MMP dependent ECM degradation and intervention strategies

MT1-MMP is a potent invasion-promoting membrane protease employed by aggressive cancer cells. MT1-MMP localizes preferentially at membrane protrusions called invadopodia where it plays a central role in degradation of the surrounding extracellular matrix (ECM). Previous reports suggested a role for a continuous supply of MT1-MMP in ECM degradation. However, the turnover rate of MT1-MMP and the extent to which the turnover contributes to the ECM degradation at invadopodia have not been clarified. To approach this problem, we first performed FRAP (Fluorescence Recovery after Photobleaching) experiments with fluorescence-tagged MT1-MMP focusing on a single invadopodium and found very rapid recovery in FRAP signals, approximated by double-exponential plots with time constants of 26 s and 259 s. The recovery depended primarily on vesicle transport, but negligibly on lateral diffusion. Next we constructed a computational model employing the observed kinetics of the FRAP experiments. The simulations successfully reproduced our FRAP experiments. Next we inhibited the vesicle transport both experimentally, and in simulation. Addition of drugs inhibiting vesicle transport blocked ECM degradation experimentally, and the simulation showed no appreciable ECM degradation under conditions inhibiting vesicle transport. In addition, the degree of the reduction in ECM degradation depended on the degree of the reduction in the MT1-MMP turnover. Thus, our experiments and simulations have established the role of the rapid turnover of MT1-MMP in ECM degradation at invadopodia. Furthermore, our simulations suggested synergetic contributions of proteolytic activity and the MT1-MMP turnover to ECM degradation because there was a nonlinear and marked reduction in ECM degradation if both factors were reduced simultaneously. Thus our computational model provides a new in silico tool to design and evaluate intervention strategies in cancer cell invasion.

Prevention of invasion is important in cancer therapy. MT1-MMP is a membrane protein involved in degradation of ECM (extracellular matrix) that is highly expressed at invadopodia, which are small protrusions of cancer cells. ECM degradation by MT1-MMP at invadopodia is hypothesized as the initial step of cancer cell invasion. However, MT1-MMP is inhibited by the endogenous inhibitor TIMP-2, so continuous turnover of MT1-MMP at the surface of invadopodia would be required. In agreement, it has been reported that the blockade of vesicle transport, which is one mechanism involved in the turnover, blocked the ECM degradation. However, the turnover rate of MT1-MMP at invadopodia and the extent to which the turnover is critical for the degradation of ECM have not been clarified. In this report we measured the turnover rate of MT1-MMP at a single invadopodium and found rapid turnover rates with time constants of 26 s and 259 s, which primarily depended on the vesicle transport. A computational model was constructed based on the observed kinetics. If we blocked the rapid turnover, the ECM degradation was blocked both experimentally and in simulations. These results established the role of the rapid turnover of MT1-MMP in the ECM degradation at invadopodia.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Formation of atypical podosomes in extravillous trophoblasts regulates extracellular matrix degradation

Throughout pregnancy the cytotrophoblast, the stem cell of the placenta, gives rise to the differentiated forms of trophoblasts. The two main cell lineages are the syncytiotrophoblast and the invading extravillous trophoblast. A successful pregnancy requires extravillous trophoblasts to migrate and invade through the decidua and then remodel the maternal spiral arteries. Many invasive cells use specialised cellular structures called invadopodia or podosomes in order to degrade extracellular matrix. Despite being highly invasive cells, the presence of invadapodia or podosomes has not previously been investigated in trophoblasts. In this study these structures have been identified and characterised in extravillous trophoblasts. The role of specialised invasive structures in trophoblasts in the degradation of the extracellular matrix was compared with well characterised podosomes and invadopodia in other invasive cells and the trophoblast specific structures were characterised by using a sensitive matrix degradation assay which enabled visualisation of the structures and their dynamics. We show trophoblasts form actin rich protrusive structures which have the ability to degrade the extracellular matrix during invasion. The degradation ability and dynamics of the structures closely resemble podosomes, but have unique characteristics that have not previously been described in other cell types. The composition of these structures does not conform to the classic podosome structure, with no distinct ring of plaque proteins such as paxillin or vinculin. In addition, trophoblast podosomes protrude more deeply into the extracellular matrix than established podosomes, resembling invadopodia in this regard. We also show several significant pathways such as Src kinase, MAPK kinase and PKC along with MMP-2 and 9 as key regulators of extracellular matrix degradation activity in trophoblasts, while podosome activity was regulated by the rigidity of the extracellular matrix.

Dynamin, a membrane-remodelling GTPase

Dynamin, the founding member of a family of dynamin-like proteins (DLPs) implicated in membrane remodelling, has a critical role in endocytic membrane fission events. The use of complementary approaches, including live-cell imaging, cell-free studies, X-ray crystallography and genetic studies in mice, has greatly advanced our understanding of the mechanisms by which dynamin acts, its essential roles in cell physiology and the specific function of different dynamin isoforms. In addition, several connections between dynamin and human disease have also emerged, highlighting specific contributions of this GTPase to the physiology of different tissues.

V2 vasopressin receptor (V2R) mutations in partial nephrogenic diabetes insipidus highlight protean agonism of V2R antagonists

Inactivating mutations of the V2 vasopressin receptor (V2R) cause cross-linked congenital nephrogenic diabetes insipidus (NDI), resulting in renal resistance to the antidiuretic hormone AVP. In two families showing partial NDI, characterized by an apparently normal response to diagnostic tests and an increase in the basal ADH levels suggesting AVP resistance, we have identified two V2R mutations, Ser-333del and Y128S. Both mutant V2Rs, when expressed in COS-7 cells, show partial defects in vasopressin-stimulated cAMP accumulation and intracellular localization. The inhibition of internalization does not rescue their localization. In contrast, the non-peptide V2R antagonists OPC41061 and OPC31260 partially rescue the membrane localization and basal function of these V2R mutants, whereas they inhibit the basal activity of the wild-type V2R. These results indicate that a partial loss of function of Ser-333del and Y128S mutant V2Rs results from defective membrane trafficking. These findings further indicate that V2R antagonists can act as protean agonists, serving as pharmacological chaperones for inactivating V2R mutants and also as inverse agonists of wild-type receptors. We speculate that this protean agonism could underlie the possible dual beneficial effects of the V2R antagonist: improvement of hyponatremia with heart failure or polycystic kidney disease and potential rescue of NDI.

The endocytic protein GRAF1 is directed to cell-matrix adhesion sites and regulates cell spreading

GTPase regulator associated with focal adhesion kinase-1 (GRAF1) interacted with endocytic and adhesion proteins, and GRAF1 endocytic activity was up-regulated in spreading cells and concentrated at the leading edge of migrating cells. Depletion of GRAF1 resulted in profound defects in cell spreading. GRAF1 remodeled membrane microdomains at adhesions, aiding membrane turnover during cell morphological changes.

The rho GTPase-activating protein GTPase regulator associated with focal adhesion kinase-1 (GRAF1) remodels membranes into tubulovesicular clathrin-independent carriers (CLICs) mediating lipid-anchored receptor endocytosis. However, the cell biological functions of this highly prevalent endocytic pathway are unclear. In this article, we present biochemical and cell biological evidence that GRAF1 interacted with a network of endocytic and adhesion proteins and was found enriched at podosome-like adhesions and src-induced podosomes. We further demonstrate that these sites comprise microdomains of highly ordered lipid enriched in GRAF1 endocytic cargo. GRAF1 activity was upregulated in spreading cells and uptake via CLICs was concentrated at the leading edge of migrating cells. Depletion of GRAF1, which inhibits CLIC generation, resulted in profound defects in cell spreading and migration. We propose that GRAF1 remodels membrane microdomains at adhesion sites into endocytic carriers, facilitating membrane turnover during cell morphological changes.

Specific tyrosine phosphorylation sites on cortactin regulate Nck1-dependent actin polymerization in invadopodia

Invadopodia are matrix-degrading membrane protrusions in invasive carcinoma cells enriched in proteins that regulate actin polymerization. The on-off regulatory switch that initiates actin polymerization in invadopodia requires phosphorylation of tyrosine residues 421, 466, and 482 on cortactin. However, it is unknown which of these cortactin tyrosine phosphorylation sites control actin polymerization. We investigated the contribution of individual tyrosine phosphorylation sites (421, 466, and 482) on cortactin to the regulation of actin polymerization in invadopodia. We provide evidence that the phosphorylation of tyrosines 421 and 466, but not 482, is required for the generation of free actin barbed ends in invadopodia. In addition, these same phosphotyrosines are important for Nck1 recruitment to invadopodia via its SH2 domain, for the direct binding of Nck1 to cortactin in vitro, and for the FRET interaction between Nck1 and cortactin in invadopodia. Furthermore, matrix proteolysis-dependent tumor cell invasion is dramatically inhibited in cells expressing a mutation in phosphotyrosine 421 or 466. Together, these results identify phosphorylation of tyrosines 421 and 466 on cortactin as the crucial residues that regulate Nck1-dependent actin polymerization in invadopodia and tumor cell invasion, and suggest that specifically blocking either tyrosine 421 or 466 phosphorylation might be effective at inhibiting tumor cell invasion in vivo.

Cortactin: a multifunctional regulator of cellular invasiveness

Branched actin assembly is critical for a variety of cellular processes that underlie cell motility and invasion, including cellular protrusion formation and membrane trafficking. Activation of branched actin assembly occurs at various subcellular locations via site-specific activation of distinct WASp family proteins and the Arp2/3 complex. A key branched actin regulator that promotes cell motility and links signaling, cytoskeletal and membrane trafficking proteins is the Src kinase substrate and Arp2/3 binding protein cortactin. Due to its frequent overexpression in advanced, invasive cancers and its general role in regulating branched actin assembly at multiple cellular locations, cortactin has been the subject of intense study. Recent studies suggest that cortactin has a complex role in cellular migration and invasion, promoting both on-site actin polymerization and modulation of autocrine secretion. Diverse cellular activities may derive from the interaction of cortactin with site-specific binding partners.

Growth cone morphology and spreading are regulated by a dynamin-cortactin complex at point contacts in hippocampal neurons

Neuronal growth cone (GC) migration and targeting are essential processes for the formation of a neural network during embryonic development. Currently, the mechanisms that support directed motility of GCs are not fully defined. The large GTPase dynamin and an interacting actin-binding protein, cortactin, have been localized to GCs, although the function performed by this complex is unclear. We have found that cortactin and the ubiquitous form of dynamin (Dyn) 2 exhibit a striking co-localization at the base of the transition zone of advancing GCs of embryonic hippocampal neurons. Confocal and total internal reflection fluorescence microscopies demonstrate that this basal localization represents point contacts. Exogenous expression of wild-type Dyn2 and cortactin leads to large, exceptionally flat, and static GCs, whereas disrupting this complex has no such effect. We find that excessive GC spreading is induced by Dyn2 and cortactin over-expression and substantial recruitment of the point contact-associated, actin-binding protein α-actinin1 to the ventral GC membrane. The distributions of other point contact proteins such as vinculin or paxillin appear unchanged. Immunoprecipitation experiments show that both Dyn2 and cortactin reside in a complex with α-actinin1. These findings provide new insights into the role of Dyn2 and the actin cytoskeleton in GC adhesion and motility.

Oncogenic Src requires a wild-type counterpart to regulate invadopodia maturation

The proto-oncogene Src tyrosine kinase (Src) is overexpressed in human cancers and is currently a target of anti-invasive therapies. Activation of Src is an essential catalyst of invadopodia production. Invadopodia are cellular structures that mediate extracellular matrix (ECM) proteolysis, allowing invasive cell types to breach confining tissue barriers. Invadopodia assembly and maturation is a multistep process, first requiring the targeting of actin-associated proteins to form pre-invadopodia, which subsequently mature by recruitment and activation of matrix metalloproteases (MMPs) that facilitate ECM degradation. We demonstrate that active, oncogenic Src alleles require the presence of a wild-type counterpart to induce ECM degradation at invadopodia sites. In addition, we identify the phosphorylation of the invadopodia regulatory protein cortactin as an important mediator of invadopodia maturation downstream of wild-type Src. Distinct phosphotyrosine-based protein-binding profiles in cells forming pre-invadopodia and mature invadopodia were identified by SH2-domain array analysis. These results indicate that although elevated Src kinase activity is required to target actin-associated proteins to pre-invadopodia, regulated Src activity is required for invadopodia maturation and matrix degradation activity. Our findings describe a previously unappreciated role for proto-oncogenic Src in enabling the invasive activity of constitutively active Src alleles.

A novel and specific NADPH oxidase-1 (Nox1) small-molecule inhibitor blocks the formation of functional invadopodia in human colon cancer cells

The NADPH oxidase (Nox) proteins catalyze the regulated formation of reactive oxygen species (ROS), which play key roles as signaling molecules in several physiological and pathophysiological processes. ROS generation by the Nox1 member of the Nox family is necessary for the formation of extracellular matrix (ECM)-degrading, actin-rich cellular structures known as invadopodia. Selective inhibition of Nox isoforms can provide reversible, mechanistic insights into these cellular processes in contrast to scavenging or inhibition of ROS production. Currently no specific Nox inhibitors have been described. Here, by high-throughput screening, we identify a subset of phenothiazines, 2-acetylphenothiazine (here referred to as ML171) (and its related 2-(trifluoromethyl)-phenothiazine) as nanomolar, cell-active, and specific Nox1 inhibitors that potently block Nox1-dependent ROS generation, with only marginal activity on other cellular ROS-producing enzymes and receptors including the other Nox isoforms. ML171 also blocks the ROS-dependent formation of ECM-degrading invadopodia in colon cancer cells. Such effects can be reversed by overexpression of Nox1 protein, which is suggestive of a selective mechanism of inhibition of Nox1 by this compound. These results elucidate the relevance of Nox1-dependent ROS generation in mechanisms of cancer invasion and define ML171 as a useful Nox1 chemical probe and potential therapeutic agent for inhibition of cancer cell invasion.

c-Src-mediated phosphorylation of NoxA1 and Tks4 induces the reactive oxygen species (ROS)-dependent formation of functional invadopodia in human colon cancer cells

Reactive oxygen species (ROS) generated by the NADPH oxidase system have been shown to be necessary for the invadopodia formation and function. We show here that the abolishment of Src-mediated phosphorylation of NoxA1 and Tks4 blocks their binding, decreases Nox1-dependent ROS generation, and inhibits the invadopodia formation and ECM degradation.

The NADPH oxidase family, consisting of Nox1-5 and Duox1-2, catalyzes the regulated formation of reactive oxygen species (ROS). Highly expressed in the colon, Nox1 needs the organizer subunit NoxO1 and the activator subunit NoxA1 for its activity. The tyrosine kinase c-Src is necessary for the formation of invadopodia, phosphotyrosine-rich structures which degrade the extracellular matrix (ECM). Many Src substrates are invadopodia components, including the novel Nox1 organizer Tks4 and Tks5 proteins. Nox1-dependent ROS generation is necessary for the maintenance of functional invadopodia in human colon cancer cells. However, the signals and the molecular machinery involved in the redox-dependent regulation of invadopodia formation remain unclear. Here, we show that the interaction of NoxA1 and Tks proteins is dependent on Src activity. Interestingly, the abolishment of Src-mediated phosphorylation of Tyr110 on NoxA1 and of Tyr508 on Tks4 blocks their binding and decreases Nox1-dependent ROS generation. The contemporary presence of Tks4 and NoxA1 unphosphorylable mutants blocks SrcYF-induced invadopodia formation and ECM degradation, while the overexpression of Tks4 and NoxA1 phosphomimetic mutants rescues this phenotype. Taken together, these results elucidate the role of c-Src activity on the formation of invadopodia and may provide insight into the mechanisms of tumor formation in colon cancers.

Nck1 and Grb2 localization patterns can distinguish invadopodia from podosomes

Invadopodia are matrix-degrading ventral cell surface structures formed in invasive carcinoma cells. Podosomes are matrix-degrading structures formed in normal cell types including macrophages, endothelial cells, and smooth muscle cells that are believed to be related to invadopodia in function. Both invadopodia and podosomes are enriched in proteins that regulate actin polymerization including proteins involved in N-WASp/WASp-dependent Arp2/3-complex activation. However, it is unclear whether invadopodia and podosomes use distinct mediators for N-WASp/WASp-dependent Arp2/3-complex activation. We investigated the localization patterns of the upstream N-WASp/WASp activators Nck1 and Grb2 in invadopodia of metastatic mammary carcinoma cells, podosomes formed in macrophages, and degradative structures formed in Src-transformed fibroblasts and PMA-stimulated endothelial cells. We provide evidence that Nck1 specifically localizes to invadopodia, but not to podosomes formed in macrophages or degradative structures formed in Src-transformed fibroblasts and PMA-stimulated endothelial cells. In contrast, Grb2 specifically localizes to degradative structures formed in Src-transformed fibroblasts and PMA-stimulated endothelial cells, but not invadopodia or podosomes formed in macrophages. These findings suggest that distinct upstream activators are responsible for N-WASp/WASp activation in invadopodia and podosomes, and that all these ventral cell surface degradative structures have distinguishing molecular as well as structural characteristics. These patterns of Nck1 and Grb2 localization, identified in our study, can be used to sub-classify ventral cell surface degradative structures.