The surface of articular cartilage contains a progenitor cell population

It is becoming increasingly apparent that articular cartilage growth is achieved by apposition from the articular surface. For such a mechanism to occur, a population of stem/progenitor cells must reside within the articular cartilage to provide transit amplifying progeny for growth. Here, we report on the isolation of an articular cartilage progenitor cell from the surface zone of articular cartilage using differential adhesion to fibronectin. This population of cells exhibits high affinity for fibronectin, possesses a high colony-forming efficiency and expresses the cell fate selector gene Notch 1. Inhibition of Notch signalling abolishes colony forming ability whilst activated Notch rescues this inhibition. The progenitor population also exhibits phenotypic plasticity in its differentiation pathway in an embryonic chick tracking system, such that chondroprogenitors can engraft into a variety of connective tissue types including bone, tendon and perimysium. The identification of a chondrocyte subpopulation with progenitor-like characteristics will allow for advances in our understanding of both cartilage growth and maintenance as well as provide novel solutions to articular cartilage repair.

Fibronectin fibrillogenesis, a cell-mediated matrix assembly process

The extracellular matrix provides a framework for cell adhesion, supports cell movement, and serves to compartmentalize tissues into functional units. Fibronectin is a core component of many extracellular matrices where it regulates a variety of cell activities through direct interactions with cell surface integrin receptors. Fibronectin is synthesized by many adherent cells which then assemble it into a fibrillar network. The assembly process is integrin-dependent and fibronectin-integrin interactions initiate a step-wise process involving conformational activation of fibronectin outside and organization of the actin cytoskeleton inside. During assembly, fibronectin undergoes conformational changes that expose fibronectin-binding sites and promote intermolecular interactions needed for fibril formation. In this review, the main steps of fibronectin assembly are described and recent studies on fibronectin conformational changes are discussed.

Interaction between leukemic-cell VLA-4 and stromal fibronectin is a decisive factor for minimal residual disease of acute myelogenous leukemia

Bone-marrow minimal residual disease (MRD) causes relapse after chemotherapy in patients with acute myelogenous leukemia (AML). We postulate that the drug resistance is induced by the attachment of very late antigen (VLA)-4 on leukemic cells to fibronectin on bone-marrow stromal cells. We found that VLA-4-positive cells acquired resistance to anoikis (loss of anchorage) or drug-induced apoptosis through the phosphatidylinositol-3-kinase (PI-3K)/AKT/Bcl-2 signaling pathway, which is activated by the interaction of VLA-4 and fibronectin. This resistance was negated by VLA-4-specific antibodies. In a mouse model of MRD, we achieved a 100% survival rate by combining VLA-4-specific antibodies and cytosine arabinoside (AraC), whereas AraC alone prolonged survival only slightly. In addition, overall survival at 5 years was 100% for 10 VLA-4-negative patients and 44.4% for 15 VLA-4-positive patients. Thus, the interaction between VLA-4 on leukemic cells and fibronectin on stromal cells may be crucial in bone marrow MRD and AML prognosis.

Cancer-associated fibroblasts promote directional cancer cell migration by aligning fibronectin

Cancer-associated fibroblasts (CAFs) are major components of the carcinoma microenvironment that promote tumor progression. However, the mechanisms by which CAFs regulate cancer cell migration are poorly understood. In this study, we show that fibronectin (Fn) assembled by CAFs mediates CAF-cancer cell association and directional migration. Compared with normal fibroblasts, CAFs produce an Fn-rich extracellular matrix with anisotropic fiber orientation, which guides the cancer cells to migrate directionally. CAFs align the Fn matrix by increasing nonmuscle myosin II- and platelet-derived growth factor receptor α-mediated contractility and traction forces, which are transduced to Fn through α5β1 integrin. We further show that prostate cancer cells use αv integrin to migrate efficiently and directionally on CAF-derived matrices. We demonstrate that aligned Fn is a prominent feature of invasion sites in human prostatic and pancreatic carcinoma samples. Collectively, we present a new mechanism by which CAFs organize the Fn matrix and promote directional cancer cell migration.

Rab25 associates with alpha5beta1 integrin to promote invasive migration in 3D microenvironments

Here, we report a direct interaction between the beta1 integrin cytoplasmic tail and Rab25, a GTPase that has been linked to tumor aggressiveness and metastasis. Rab25 promotes a mode of migration on 3D matrices that is characterized by the extension of long pseudopodia, and the association of the GTPase with alpha5beta1 promotes localization of vesicles that deliver integrin to the plasma membrane at pseudopodial tips as well as the retention of a pool of cycling alpha5beta1 at the cell front. Furthermore, Rab25-driven tumor-cell invasion into a 3D extracellular matrix environment is strongly dependent on ligation of fibronectin by alpha5beta1 integrin and the capacity of Rab25 to interact with beta1 integrin. These data indicate that Rab25 contributes to tumor progression by directing the localization of integrin-recycling vesicles and thereby enhancing the ability of tumor cells to invade the extracellular matrix.

Loss of E-cadherin promotes ovarian cancer metastasis via alpha 5-integrin, which is a therapeutic target

E-cadherin loss is frequently associated with ovarian cancer metastasis. Given that adhesion to the abdominal peritoneum is the first step in ovarian cancer dissemination, we reasoned that down-regulation of E-cadherin would affect expression of cell matrix adhesion receptors. We show here that inhibition of E-cadherin in ovarian cancer cells causes up-regulation of alpha(5)-integrin protein expression and transcription. When E-cadherin was blocked, RMUG-S ovarian cancer cells were able to attach and invade more efficiently. This greater efficiency could, in turn, be inhibited both in vitro and in vivo with an alpha(5)beta(1)-integrin-blocking antibody. When E-cadherin is silenced, alpha(5)-integrin is up-regulated through activation of an epidermal growth factor receptor/FAK/Erk1-mitogen-activated protein kinase-dependent signaling pathway and not through the canonical E-cadherin/beta-catenin signaling pathway. In SKOV-3ip1 ovarian cancer xenografts, which express high levels of alpha(5)-integrin, i.p. treatment with an alpha(5)beta(1)-integrin antibody significantly reduced tumor burden, ascites, and number of metastasis and increased survival by an average of 12 days when compared with IgG treatment (P < 0.0005). alpha(5)-Integrin expression was detected by immunohistochemistry in 107 advanced stage ovarian cancers using a tissue microarray annotated with disease-specific patient follow-up. Ten of 107 tissues (9%) had alpha(5)-integrin overexpression, and 39% had some level of alpha(5)-integrin expression. The median survival for patients with high alpha(5)-integrin levels was 26 months versus 35 months for those with low integrin expression (P < 0.05). Taken together, we have identified alpha(5)-integrin up-regulation as a molecular mechanism by which E-cadherin loss promotes tumor progression, providing an explanation for how E-cadherin loss increases metastasis. Targeting this integrin could be a promising therapy for a subset of ovarian cancer patients.

Structure of integrin alpha5beta1 in complex with fibronectin

The membrane-distal headpiece of integrins has evolved to specifically bind large extracellular protein ligands, but the molecular architecture of the resulting complexes has not been determined. We used molecular electron microscopy to determine the three-dimensional structure of the ligand-binding headpiece of integrin alpha5beta1 complexed with fragments of its physiological ligand fibronectin. The density map for the unliganded alpha5beta1 headpiece shows a 'closed' conformation similar to that seen in the alphaVbeta3 crystal structure. By contrast, binding to fibronectin induces an 'open' conformation with a dramatic, approximately 80 degrees change in the angle of the hybrid domain of the beta subunit relative to its I-like domain. The fibronectin fragment binds to the interface between the beta-propeller and I-like domains in the integrin headpiece through the RGD-containing module 10, but direct contact of the synergy-region-containing module 9 to integrin is not evident. This finding is corroborated by kinetic analysis of real-time binding data, which shows that the synergy site greatly enhances k(on) but has little effect on the stability or k(off) of the complex.

The fibronectin-binding integrins alpha5beta1 and alphavbeta3 differentially modulate RhoA-GTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis

We have studied the formation of different types of cell matrix adhesions in cells that bind to fibronectin via either alpha5beta1 or alphavbeta3. In both cases, cell adhesion to fibronectin leads to a rapid decrease in RhoA activity. However, alpha5beta1 but not alphavbeta3 supports high levels of RhoA activity at later stages of cell spreading, which are associated with a translocation of focal contacts to peripheral cell protrusions, recruitment of tensin into fibrillar adhesions, and fibronectin fibrillogenesis. Expression of an activated mutant of RhoA stimulates alphavbeta3-mediated fibrillogenesis. Despite the fact that alpha5beta1-mediated adhesion to the central cell-binding domain of fibronectin supports activation of RhoA, other regions of fibronectin are required for the development of alpha5beta1-mediated but not alphavbeta3-mediated focal contacts. Using chimeras of beta1 and beta3 subunits, we find that the extracellular domain of beta1 controls RhoA activity. By expressing both beta1 and beta3 at high levels, we show that beta1-mediated control of the levels of beta3 is important for the distribution of focal contacts. Our findings demonstrate that the pattern of fibronectin receptors expressed on a cell dictates the ability of fibronectin to stimulate RhoA-mediated organization of cell matrix adhesions.

Rab25 and CLIC3 collaborate to promote integrin recycling from late endosomes/lysosomes and drive cancer progression

Here we show that Rab25 permits the sorting of ligand-occupied, active-conformation α5β1 integrin to late endosomes/lysosomes. Photoactivation and biochemical approaches show that lysosomally targeted integrins are not degraded but are retrogradely transported and recycled to the plasma membrane at the back of invading cells. This requires CLIC3, a protein upregulated in Rab25-expressing cells and tumors, which colocalizes with active α5β1 in late endosomes/lysosomes. CLIC3 is necessary for release of the cell rear during migration on 3D matrices and is required for invasion and maintenance of active Src signaling in organotypic microenvironments. CLIC3 expression predicts lymph node metastasis and poor prognosis in operable cases of pancreatic ductal adenocarcinoma (PDAC). The identification of CLIC3 as a regulator of a recycling pathway and as an independent prognostic indicator in PDAC highlights the importance of active integrin trafficking as a potential drive to cancer progression in vivo.

Controlling integrin specificity and stem cell differentiation in 2D and 3D environments through regulation of fibronectin domain stability

The extracellular matrix (ECM) exerts powerful control over many cellular phenomena, including stem cell differentiation. As such, design and modulation of ECM analogs to ligate specific integrin is a promising approach to control cellular processes in vitro and in vivo for regenerative medicine strategies. Although fibronectin (FN), a crucial ECM protein in tissue development and repair, and its RGD peptide are widely used for cell adhesion, the promiscuity with which they engage integrins leads to difficulty in control of receptor-specific interactions. Recent simulations of force-mediated unfolding of FN domains and sequences analysis of human versus mouse FN suggest that the structural stability of the FN's central cell-binding domains (FN III9-10) affects its integrin specificity. Through production of FN III9-10 variants with variable stabilities, we obtained ligands that present different specificities for the integrin alpha(5)beta(1) and that can be covalently linked into fibrin matrices. Here, we demonstrate the capacity of alpha(5)beta(1) integrin-specific engagement to influence human mesenchymal stem cell (MSC) behavior in 2D and 3D environments. Our data indicate that alpha(5)beta(1) has an important role in the control of MSC osteogenic differentiation. FN fragments with increased specificity for alpha(5)beta(1) versus alpha(v)beta(3) results in significantly enhanced osteogenic differentiation of MSCs in 2D and in a clinically relevant 3D fibrin matrix system, although attachment/spreading and proliferation were comparable with that on full-length FN. This work shows how integrin-dependant cellular interactions with the ECM can be engineered to control stem cell fate, within a system appropriate for both 3D cell culture and tissue engineering.

Ovarian cancer spheroids use myosin-generated force to clear the mesothelium

Dissemination of ovarian tumors involves the implantation of cancer spheroids into the mesothelial monolayer on the walls of peritoneal and pleural cavity organs. Biopsies of tumors attached to peritoneal organs show that mesothelial cells are not present under tumor masses. We have developed a live, image-based in vitro model in which interactions between tumor spheroids and mesothelial cells can be monitored in real time to provide spatial and temporal understanding of mesothelial clearance. Here we provide evidence that ovarian cancer spheroids utilize integrin- and talin- dependent activation of myosin and traction force to promote mesothelial cells displacement from underneath a tumor cell spheroid. These results suggest that ovarian tumor cell clusters gain access to the sub-mesothelial environment by exerting force on the mesothelial cells lining target organs, driving migration and clearance of the mesothelial cells.

PEGylated PLGA-based nanoparticles targeting M cells for oral vaccination

To improve the efficiency of orally delivered vaccines, PEGylated PLGA-based nanoparticles displaying RGD molecules at their surface were designed to target human M cells. RGD grafting was performed by an original method called "photografting" which covalently linked RGD peptides mainly on the PEG moiety of the PCL-PEG, included in the formulation. First, three non-targeted formulations with size and zeta potential adapted to M cell uptake and stable in gastro-intestinal fluids, were developed. Their transport by an in vitro model of the human Follicle associated epithelium (co-cultures) was largely increased as compared to mono-cultures (Caco-2 cells). RGD-labelling of nanoparticles significantly increased their transport by co-cultures, due to interactions between the RGD ligand and the beta(1) intregrins detected at the apical surface of co-cultures. In vivo studies demonstrated that RGD-labelled nanoparticles particularly concentrated in M cells. Finally, ovalbumin-loaded nanoparticles were orally administrated to mice and induced an IgG response, attesting antigen ability to elicit an immune response after oral delivery.

Neuropilin-1/GIPC1 signaling regulates alpha5beta1 integrin traffic and function in endothelial cells

Neuropilin 1 (Nrp1) is a coreceptor for vascular endothelial growth factor A165 (VEGF-A165, VEGF-A164 in mice) and semaphorin 3A (SEMA3A). Nevertheless, Nrp1 null embryos display vascular defects that differ from those of mice lacking either VEGF-A164 or Sema3A proteins. Furthermore, it has been recently reported that Nrp1 is required for endothelial cell (EC) response to both VEGF-A165 and VEGF-A121 isoforms, the latter being incapable of binding Nrp1 on the EC surface. Taken together, these data suggest that the vascular phenotype caused by the loss of Nrp1 could be due to a VEGF-A164/SEMA3A-independent function of Nrp1 in ECs, such as adhesion to the extracellular matrix. By using RNA interference and rescue with wild-type and mutant constructs, we show here that Nrp1 through its cytoplasmic SEA motif and independently of VEGF-A165 and SEMA3A specifically promotes alpha5beta1-integrin-mediated EC adhesion to fibronectin that is crucial for vascular development. We provide evidence that Nrp1, while not directly mediating cell spreading on fibronectin, interacts with alpha5beta1 at adhesion sites. Binding of the homomultimeric endocytic adaptor GAIP interacting protein C terminus, member 1 (GIPC1), to the SEA motif of Nrp1 selectively stimulates the internalization of active alpha5beta1 in Rab5-positive early endosomes. Accordingly, GIPC1, which also interacts with alpha5beta1, and the associated motor myosin VI (Myo6) support active alpha5beta1 endocytosis and EC adhesion to fibronectin. In conclusion, we propose that Nrp1, in addition to and independently of its role as coreceptor for VEGF-A165 and SEMA3A, stimulates through its cytoplasmic domain the spreading of ECs on fibronectin by increasing the Rab5/GIPC1/Myo6-dependent internalization of active alpha5beta1. Nrp1 modulation of alpha5beta1 integrin function can play a causal role in the generation of angiogenesis defects observed in Nrp1 null mice.

c-Met overexpression is a prognostic factor in ovarian cancer and an effective target for inhibition of peritoneal dissemination and invasion

The hepatocyte growth factor receptor c-Met is a receptor tyrosine kinase that plays an important role in tumor growth by activating mitogenic signaling pathways. The goal of this study was to evaluate the role of c-Met in the biology of ovarian cancer and to determine its potential as a therapeutic target. c-Met protein expression was detected by immunohistochemistry in 138 advanced-stage ovarian cancers using a tissue microarray annotated with disease-specific patient follow-up. Fifteen of 138 (11%) tissues had c-Met overexpression. Median survival for patients with high c-Met levels was 17 months versus 32 months (P = 0.001) for patients with low c-Met expression. Infection of SKOV-3ip1 cells with an adenovirus expressing a small interfering RNA (siRNA) against c-Met efficiently inhibited c-Met protein and mRNA expression as well as extracellular signal-regulated kinase and phosphatidylinositol 3-kinase signaling. It also inhibited adhesion to different extracellular matrix components, human primary mesothelial cells, and full-thickness human peritoneum and, in vivo, to mouse peritoneum. This was paralleled by a significant reduction in alpha(5) and beta(1) integrin protein and mRNA expression as well as a reduction of urokinase and matrix metalloproteinase (MMP)-2/MMP-9 activity. In SKOV-3ip1 ovarian cancer xenografts, i.p. treatment with the c-Met siRNA significantly reduced tumor burden, ascites formation, protease activity, and the number of peritoneal implants but not tumor size or angiogenesis. These results suggest that c-Met overexpression is a prognostic factor in ovarian cancer and that targeting c-Met in vivo inhibits peritoneal dissemination and invasion through an alpha(5)beta(1) integrin-dependent mechanism. Therefore, c-Met should be explored further as a therapeutic target in ovarian cancer.

The enhancement of chondrogenic differentiation of human mesenchymal stem cells by enzymatically regulated RGD functionalities

A thiol-acrylate photopolymerization was used to incorporate enzymatically cleavable peptide sequences into PEG hydrogels to induce chondrogenic differentiation of encapsulated human mesenchymal stem cells (hMSCs). An adhesive sequence, RGD, was designed with an MMP-13 specific cleavable linker. RGD promotes survival of hMSCs encapsulated in PEG gels and has shown to induce early stages of chondrogenesis, while its persistence can limit complete differentiation. Therefore, an MMP-13 cleavage site was incorporated into the peptide sequence to release RGD mimicking the native differentiation timeline. Active MMP-13 production of encapsulated hMSCs was seen to increase from day 9 to 14 and only in chondrogenic differentiating cultures. Seeded hMSCs attached to the material prior to enzymatic cleavage, but a significant population of the cells detach after cleavage and release of RGD. Finally, hMSCs encapsulated in RGD-releasing gels produce 10 times as much glycosaminoglycan as cells with uncleavable RGD functionalities, by day 21 of culture. Furthermore, 75% of the cells stain positive for collagen type II deposition where RGD is cleavable, as compared to 19% for cultures where RGD persists. Collectively, these data provide evidence that temporal regulation of integrin-binding peptides is important in the design of niches in differentiating hMSCs to chondrocytes.

alpha v beta3 and alpha5beta1 integrin recycling pathways dictate downstream Rho kinase signaling to regulate persistent cell migration

Accumulating evidence suggests that integrin recycling regulates cell migration. However, the lack of reagents to selectively target the trafficking of individual heterodimers, as opposed to endocytic transport as a whole, has made it difficult to define the contribution made by particular recycling pathways to directional cell movement. We show that autophosphorylation of protein kinase D1 (PKD1) at Ser⁹¹⁶ is necessary for its association with αvβ3 integrin. Expression of PKD1^916A or the use of mutants of β3 that do not bind to PKD1 selectively inhibits short-loop, Rab4-dependent recycling of αvβ3, and this suppresses the persistence of fibroblast migration. However, we report that short-loop recycling does not directly contribute to fibroblast migration by moving αvβ3 to the cell front, but by antagonizing α5β1 recycling, which, in turn, influences the cell's decision to migrate with persistence or to move randomly.

Proteomic analysis of integrin-associated complexes identifies RCC2 as a dual regulator of Rac1 and Arf6

The binding of integrin adhesion receptors to their extracellular matrix ligands controls cell morphology, movement, survival, and differentiation in various developmental, homeostatic, and disease processes. Here, we report a methodology to isolate complexes associated with integrin adhesion receptors, which, like other receptor-associated signaling complexes, have been refractory to proteomic analysis. Quantitative, comparative analyses of the proteomes of two receptor-ligand pairs, alpha(4)beta(1)-vascular cell adhesion molecule-1 and alpha(5)beta(1)-fibronectin, defined both core and receptor-specific components. Regulator of chromosome condensation-2 (RCC2) was detected in the alpha(5)beta(1)-fibronectin signaling network at an intersection between the Rac1 and adenosine 5'-diphosphate ribosylation factor 6 (Arf6) subnetworks. RCC2 knockdown enhanced fibronectin-induced activation of both Rac1 and Arf6 and accelerated cell spreading, suggesting that RCC2 limits the signaling required for membrane protrusion and delivery. Dysregulation of Rac1 and Arf6 function by RCC2 knockdown also abolished persistent migration along fibronectin fibers, indicating a functional role for RCC2 in directional cell movement. This proteomics workflow now opens the way to further dissection and systems-level analyses of adhesion signaling.

Mesenchymal stem cell migration is regulated by fibronectin through α5β1-integrin-mediated activation of PDGFR-β and potentiation of growth factor signals

Cell migration during vascular remodelling is regulated by crosstalk between growth factor receptors and integrin receptors, which together coordinate cytoskeletal and motogenic changes. Here, we report extracellular matrix (ECM)-directed crosstalk between platelet-derived growth factor receptor (PDGFR)-β and α5β1-integrin, which controls the migration of mesenchymal stem (stromal) cells (MSCs). Cell adhesion to fibronectin induced α5β1-integrin-dependent phosphorylation of PDGFR-β in the absence of growth factor stimulation. Phosphorylated PDGFR-β co-immunoprecipitated with α5-integrin and colocalised with α5β1-integrin in the transient tidemarks of focal adhesions. Adhesion to fibronectin also strongly potentiated PDGF-BB-induced PDGFR-β phosphorylation and focal adhesion kinase (FAK) activity, in an α5β1-integrin-dependent manner. PDGFR-β-induced phosphoinositide 3-kinase (PI3K) and Akt activity, actin reorganisation and cell migration were all regulated by fibronectin and α5β1-integrin. This synergistic relationship between α5β1-integrin and PDGFR-β is a fundamental determinant of cell migration. Thus, fibronectin-rich matrices can prime PDGFR-β to recruit mesenchymal cells at sites of vascular remodelling.

Integrin-specific signaling pathways controlling focal adhesion formation and cell migration

The fibronectin (FN)-binding integrins alpha4beta1 and alpha5beta1 confer different cell adhesive properties, particularly with respect to focal adhesion formation and migration. After analyses of alpha4+/alpha5+ A375-SM melanoma cell adhesion to fragments of FN that interact selectively with alpha4beta1 and alpha5beta1, we now report two differences in the signals transduced by each receptor that underpin their specific adhesive properties. First, alpha5beta1 and alpha4beta1 have a differential requirement for cell surface proteoglycan engagement for focal adhesion formation and migration; alpha5beta1 requires a proteoglycan coreceptor (syndecan-4), and alpha4beta1 does not. Second, adhesion via alpha5beta1 caused an eightfold increase in protein kinase Calpha (PKCalpha) activation, but only basal PKCalpha activity was observed after adhesion via alpha4beta1. Pharmacological inhibition of PKCalpha and transient expression of dominant-negative PKCalpha, but not dominant-negative PKCdelta or PKCzeta constructs, suppressed focal adhesion formation and cell migration mediated by alpha5beta1, but had no effect on alpha4beta1. These findings demonstrate that different integrins can signal to induce focal adhesion formation and migration by different mechanisms, and they identify PKCalpha signaling as central to the functional differences between alpha4beta1 and alpha5beta1.

Breast cancer cells in three-dimensional culture display an enhanced radioresponse after coordinate targeting of integrin alpha5beta1 and fibronectin

Tactics to selectively enhance cancer radioresponse are of great interest. Cancer cells actively elaborate and remodel their extracellular matrix (ECM) to aid in survival and progression. Previous work has shown that beta1-integrin inhibitory antibodies can enhance the growth-inhibitory and apoptotic responses of human breast cancer cell lines to ionizing radiation, either when cells are cultured in three-dimensional laminin-rich ECM (3D lrECM) or grown as xenografts in mice. Here, we show that a specific alpha heterodimer of beta1-integrin preferentially mediates a prosurvival signal in human breast cancer cells that can be specifically targeted for therapy. 3D lrECM culture conditions were used to compare alpha-integrin heterodimer expression in malignant and nonmalignant cell lines. Under these conditions, we found that expression of alpha5beta1-integrin was upregulated in malignant cells compared with nonmalignant breast cells. Similarly, we found that normal and oncofetal splice variants of fibronectin, the primary ECM ligand of alpha5beta1-integrin, were also strikingly upregulated in malignant cell lines compared with nonmalignant acini. Cell treatment with a peptide that disrupts the interactions of alpha5beta1-integrin with fibronectin promoted apoptosis in malignant cells and further heightened the apoptotic effects of radiation. In support of these results, an analysis of gene expression array data from breast cancer patients revealed an association of high levels of alpha5-integrin expression with decreased survival. Our findings offer preclinical validation of fibronectin and alpha5beta1-integrin as targets for breast cancer therapy.

Galectin binding to Mgat5-modified N-glycans regulates fibronectin matrix remodeling in tumor cells

Oncogenic signaling stimulates the dynamic remodeling of actin microfilaments and substrate adhesions, essential for cell spreading and motility. Transformation is associated with increased expression of beta1,6GlcNAc-branched N-glycans, products of Golgi beta1,6-acetylglucosaminyltransferase V (Mgat5) and the favored ligand for galectins. Herein we report that fibronectin fibrillogenesis and fibronectin-dependent cell spreading are deficient in Mgat5(-/-) mammary epithelial tumor cells and inhibited in Mgat5(+/+) cells by blocking Golgi N-glycan processing with swainsonine or by competitive inhibition of galectin binding. At an optimum dosage, exogenous galectin-3 added to Mgat5(+/+) cells activates focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K), recruits conformationally active alpha5beta1-integrin to fibrillar adhesions, and increases F-actin turnover. RGD peptide inhibits PI3K-dependent fibronectin matrix remodeling and fibronectin-dependent cell motility, while galectin-3 stimulates and overrides the inhibitory effects of RGD. Antibodies to the galectin-3 N-terminal oligomerization domain stimulate alpha5beta1 activation and recruitment to fibrillar adhesions in Mgat5(+/+) cells, an effect that is blocked by disrupting galectin-glycan binding. Our results demonstrate that fibronectin polymerization and tumor cell motility are regulated by galectin-3 binding to branched N-glycan ligands that stimulate focal adhesion remodeling, FAK and PI3K activation, local F-actin instability, and alpha5beta1 translocation to fibrillar adhesions.

Cell adhesion to fibrillin-1 molecules and microfibrils is mediated by alpha 5 beta 1 and alpha v beta 3 integrins

Fibrillins are the major glycoprotein components of microfibrils that form a template for tropoelastin during elastic fibrillogenesis. We have examined cell adhesion to assembled purified microfibrils, and its molecular basis. Human dermal fibroblasts exhibited Arg-Gly-Asp and cation-dependent adhesion to microfibrils and recombinant fibrillin-1 protein fragments. Strong integrin alpha 5 beta 1 interactions with fibrillin ligands were identified, but integrin alpha v beta 3 also contributed to cell adhesion. Fluorescence-activated cell sorting analysis confirmed the presence of abundant alpha 5 beta 1 and some alpha v beta 3 receptors on these cells. Adhesion to microfibrils and to Arg-Gly-Asp containing fibrillin-1 protein fragments induced signaling events that led to cell spreading, altered cytoskeletal organization, and enhanced extracellular fibrillin-1 deposition. Differences in cell shape when plated on fibrillin or fibronectin implied substrate-specific alpha 5 beta 1-mediated cellular responses. An Arg-Gly-Asp-independent cell adhesion sequence was also identified within fibrillin-1. Adhesion and spreading of smooth muscle cells on fibrillin ligands was enhanced by antibody-induced beta1 integrin activation. A375-SM melanoma cells bound Arg-Gly-Asp-containing fibrillin-1 protein fragments mainly through alpha v beta 3, whereas HT1080 cells used mainly alpha 5 beta 1. This study has shown that fibrillin microfibrils mediate cell adhesion, that alpha 5 beta 1 and alpha v beta 3 are both important but cell-specific fibrillin-1 receptors, and that cellular interactions with fibrillin-1 influence cell behavior.

Angiopoietin-2 stimulates breast cancer metastasis through the alpha(5)beta(1) integrin-mediated pathway

Acquisition of a metastatic phenotype by breast cancer cells includes alternations of multigenic programs that permit tumor cells to metastasize to distant organs. Here, we report that angiopoietin-2 (Ang2), a known growth factor, is capable of promoting breast cancer cell invasion leading to metastasis. Analysis of 185 primary human breast cancer specimens that include 97 tumors showing lymph node and/or distant metastasis reveals a significant correlation between the expression of Ang2 and E-cadherin, Snail, metastatic potential, tumor grade, and lymph-vascular invasion during breast cancer progression. Using a xenograft model, we show that overexpression of Ang2 in poorly metastatic MCF-7 breast cancer cells suppresses expression of E-cadherin and induces Snail expression and phosphorylation of Akt and glycogen synthase kinase-3beta (GSK-3beta) promoting metastasis to the lymph nodes and lung. In cell culture, Ang2 promotes cell migration and invasion in Tie2-deficient breast cancer cells through the alpha(5)beta(1) integrin/integrin-linked kinase (ILK)/Akt, GSK-3beta/Snail/E-cadherin signaling pathway. Inhibition of ILK and the alpha(5)beta(1) integrin abrogates Ang2 modulation of Akt, GSK-3beta, Snail, and E-cadherin and Ang2-stimulated breast cancer cell migration and invasion. Together, these results underscore the significant contribution of Ang2 in cancer progression, not only by stimulating angiogenesis but also by promoting metastasis, and provide a mechanism by which breast cancer cells acquire an enhanced invasive phenotype contributing to metastasis.

CCN3 (NOV) is a novel angiogenic regulator of the CCN protein family

CCN3 (NOV) is a matricellular protein of the CCN family, which also includes CCN1 (CYR61), CCN2 (CTGF), CCN4 (WISP-1), CCN5 (WISP-2), and CCN6 (WISP-3). During development, CCN3 is expressed widely in derivatives of all three germ layers, and high levels of expression are observed in smooth muscle cells of the arterial vessel wall. Altered expression of CCN3 has been observed in a variety of tumors, including hepatocellular carcinomas, Wilm's tumors, Ewing's sarcomas, gliomas, rhabdomyosarcomas, and adrenocortical carcinomas. To understand its biological functions, we have investigated the activities of purified recombinant CCN3. We show that in endothelial cells, CCN3 supports cell adhesion, induces directed cell migration (chemotaxis), and promotes cell survival. Mechanistically, CCN3 supports human umbilical vein endothelial cell adhesion through multiple cell surface receptors, including integrins alphavbeta3, alpha5beta1, alpha6beta1, and heparan sulfate proteoglycans. In contrast, CCN3-induced cell migration is dependent on integrins alphavbeta3 and alpha5beta1, whereas alpha6beta1 does not play a role in this process. Although CCN3 does not contain a RGD sequence, it binds directly to immobilized integrins alphavbeta3 and alpha5beta1, with half-maximal binding occurring at 10 nm and 50 nm CCN3, respectively. Furthermore, CCN3 induces neovascularization when implanted in rat cornea, demonstrating that it is a novel angiogenic inducer. Together, these findings show that CCN3 is a ligand of integrins alphavbeta3 and alpha5beta1, acts directly upon endothelial cells to stimulate pro-angiogenic activities, and induces angiogenesis in vivo.

CD97, an adhesion receptor on inflammatory cells, stimulates angiogenesis through binding integrin counterreceptors on endothelial cells

CD97, a membrane protein expressed at high levels on inflammatory cells and some carcinomas, is a member of the adhesion G protein-coupled receptor family, whose members have bipartite structures consisting of an extracellular peptide containing adhesion motifs noncovalently coupled to a class B 7-transmembrane domain. CD97alpha, the extracellular domain of CD97, contains 3 to 5 fibrillin class 1 epidermal growth factor (EGF)-like repeats, an Arg-Gly-Asp (RGD) tripeptide, and a mucin stalk. We show here that CD97alpha promotes angiogenesis in vivo as demonstrated with purified protein in a directed in vivo angiogenesis assay (DIVAA) and by enhanced vascularization of developing tumors expressing CD97. These data suggest that CD97 can contribute to angiogenesis associated with inflammation and tumor progression. Strong integrin alpha5beta1 interactions with CD97 have been identified, but alpha v beta3 also contributes to cell attachment. Furthermore, soluble CD97 acts as a potent chemoattractant for migration and invasion of human umbilical vein endothelial cells (HUVECs), and this function is integrin dependent. CD97 EGF-like repeat 4 is known to bind chondroitin sulfate. It was found that coengagement of alpha5beta1 and chondroitotin sulfate proteoglycan by CD97 synergistically initiates endothelial cell invasion. Integrin alpha5beta1 is the first high-affinity cellular counterreceptor that has been identified for a member within this family of adhesion receptors.