The alphavbeta5 integrin functions as an endocytic receptor for vitronectin

Role of irisin in bone diseases

Bone diseases are common among middle-aged and elderly people, and harm to activities of daily living (ADL) and quality of life (QOL) for patients. It is crucial to search for key regulatory factors associated with the development of bone diseases and explore potential therapeutic targets for bone diseases. Irisin is a novel myokine that has been discovered in recent years. Accumulating evidence indicates that irisin has beneficial effects in the treatment of various diseases such as metabolic, cardiovascular and neurological disorders, especially bone-related diseases. Recent studies had shown that irisin plays the role in various bone diseases such as osteoarthritis, osteoporosis and other bone diseases, suggesting that irisin may be a potential molecule for the prevention and treatment of bone diseases. Therefore, in this review, by consulting the related domestic and international literature of irisin and bone diseases, we summarized the specific regulatory mechanisms of irisin in various bone diseases, and provided a systematic theoretical basis for its application in the diagnosis and treatment of the bone diseases.

Mitotic Activation Around Wound Edges and Epithelialization Repair in UVB-Induced Capsular Cataracts

Purpose

Ultraviolet B (UVB) has been well documented to induce capsular cataracts; however, the mechanism of the lens epithelial cell-mediated repair process after UVB irradiation is not fully understood. The purpose of this study was to better understand lens epithelial cell repair after UVB-induced epithelium damage.

Method

C57BL/6J mice were irradiated by various doses of UVB. Lens morphology and lens capsule opacity were monitored by slit lamp, darkfield microscopy, and phase-contrast microscopy. Lens epithelial cell mitotic activation and cell apoptosis were measured by immunohistochemistry. Lens epithelial ultrastructure was analyzed by transmission electron microscopy.

Results

UVB irradiation above a dose of 2.87 kJ/m2 triggered lens epithelial cell apoptosis and subcapsular cataract formation, with a ring-shaped structure composed of multilayered epithelial cell clusters manifesting a dense ring-shaped capsular cataract. The epithelial cells immediately outside the edge of the ring-shaped aggregates transitioned to mitotically active cells and performed wound healing through the epithelialization process. However, repairs ceased when lens epithelial cells made direct contact, and scar-like tissue in the center of the anterior capsule remained even by 6 months after UVB irradiation.

Conclusions

Our present study demonstrates that normally quiescent lens epithelial cells can be reactivated for epithelialization repair in response to UV-induced damage.

Nutrient scavenging in cancer

While cancer cell proliferation depends on access to extracellular nutrients, inadequate tumour perfusion means that glucose, amino acids and lipids are often in short supply. To overcome this obstacle to growth, cancer cells utilize multiple scavenging strategies, obtaining macromolecules from the microenvironment and breaking them down in the lysosome to produce substrates for ATP generation and anabolism. Recent studies have revealed four scavenging pathways that support cancer cell proliferation in low-nutrient environments: scavenging of extracellular matrix proteins via integrins, receptor-mediated albumin uptake and catabolism, macropinocytic consumption of multiple components of the tumour microenvironment and the engulfment and degradation of entire live cells via entosis. New evidence suggests that blocking these pathways alone or in combination could provide substantial benefits to patients with incurable solid tumours. Both US Food and Drug Administration (FDA)-approved drugs and several agents in preclinical or clinical development shut down individual or multiple scavenging pathways. These therapies may increase the extent and durability of tumour growth inhibition and/or prevent the development of resistance when used in combination with existing treatments. This Review summarizes the evidence suggesting that scavenging pathways drive tumour growth, highlights recent advances that define the oncogenic signal transduction pathways that regulate scavenging and considers the benefits and detriments of therapeutic strategies targeting scavenging that are currently under development.

Synthesis and biological evaluation of RGD and isoDGR peptidomimetic-α-amanitin conjugates for tumor-targeting

RGD-α-amanitin and isoDGR-α-amanitin conjugates were synthesized by joining integrin ligands to α-amanitin via various linkers and spacers. The conjugates were evaluated for their ability to inhibit biotinylated vitronectin binding to the purified α_Vβ₃ receptor, retaining good binding affinity, in the same nanomolar range as the free ligands. The antiproliferative activity of the conjugates was evaluated in three cell lines possessing different levels of α_Vβ₃ integrin expression: human glioblastoma U87 (α_Vβ₃+), human lung carcinoma A549 (α_Vβ₃−) and breast adenocarcinoma MDA-MB-468 (α_Vβ₃−). In the U87, in the MDA-MB-468, and partly in the A549 cancer cell lines, the cyclo[DKP-isoDGR]-α-amanitin conjugates bearing the lysosomally cleavable Val-Ala linker were found to be slightly more potent than α-amanitin. Apparently, for all these α-amanitin conjugates there is no correlation between the cytotoxicity and the expression of α_Vβ₃ integrin. To determine whether the increased cytotoxicity of the cyclo[DKP-isoDGR]-α-amanitin conjugates is governed by an integrin-mediated binding and internalization process, competition experiments were carried out in which the conjugates were tested with U87 (α_Vβ₃+, α_Vβ₅+, α_Vβ₆−, α₅β₁+) and MDA-MB-468 (α_Vβ₃−, α_Vβ₅+, α_Vβ₆+, α₅β₁−) cells in the presence of excess cilengitide, with the aim of blocking integrins on the cell surface. Using the MDA-MB-468 cell line, a fivefold increase of the IC₅₀ was observed for the conjugates in the presence of excess cilengitide, which is known to strongly bind not only α_Vβ₃, but also α_Vβ₅, α_Vβ₆, and α₅β₁. These data indicate that in this case the cyclo[DKP-isoDGR]-α-amanitin conjugates are possibly internalized by a process mediated by integrins different from α_Vβ₃ (e.g., α_Vβ₅).

Blood vitronectin is a major activator of LIF and IL-6 in the brain through integrin-FAK and uPAR signaling

We defined how blood-derived vitronectin (VTN) rapidly and potently activates leukemia inhibitory factor (LIF) and pro-inflammatory interleukin 6 (IL-6) in vitro and after vascular injury in the brain. Treatment with VTN (but not fibrinogen, fibronectin, laminin-111 or collagen-I) substantially increased LIF and IL-6 within 4 h in C6-astroglioma cells, while VTN^-/- mouse plasma was less effective than that from wild-type mice. LIF and IL-6 were induced by intracerebral injection of recombinant human (rh)VTN in mice, but induction seen upon intracerebral hemorrhage was less in VTN^-/- mice than in wild-type littermates. In vitro, VTN effects were inhibited by RGD, αvβ3 and αvβ5 integrin-blocking peptides and antibodies. VTN activated focal adhesion kinase (FAK; also known as PTK2), whereas pharmacological- or siRNA-mediated inhibition of FAK, but not PYK2, reduced the expression of LIF and IL-6 in C6 and endothelial cells and after traumatic cell injury. Dominant-negative FAK (Y397F) reduced the amount of injury-induced LIF and IL-6. Pharmacological inhibition or knockdown of uPAR (also known as PLAUR), which binds VTN, also reduced cytokine expression, possibly through a common target of uPAR and integrins. We propose that VTN leakage into tissues promotes inflammation. Integrin-FAK signaling is therefore a novel IL-6 and LIF regulation mechanism relevant to the inflammation and stem cell fields.

Heavy subunit of cell surface Gal/GalNAc lectin (Hgl) undergoes degradation via endo-lysosomal compartments in Entamoeba histolytica

The human gut parasite Entamoeba histolytica uses a multifunctional virulence factor, Hgl, a cell surface transmembrane receptor subunit of Gal/GalNAc lectin that contributes to adhesion, invasion, cytotoxicity and immune response in the host. At present, the physiologic importance of Hgl receptor is mostly known for pathogenicity of E. histolytica. However, the molecular mechanisms of Hgl trafficking events and their association with the intracellular membrane transport machinery are largely unknown. We used biochemical and microscopy-based assays to understand the Hgl trafficking in the amoebic trophozoites. Our results suggest that the Hgl is constitutively degraded through delivery into amoebic lysosome-like compartments. Further, we also observed that the Hgl was significantly colocalized with amoebic Rab GTPases such as EhRab5, EhRab7A, and EhRab11B. While, we detected association of Hgl with all these Rab GTPases in early vacuolar compartments, only EhRab7A remains associated with Hgl till its transport to amoebic lysosome-like compartments.

The fibronectin III-1 domain activates a PI3-Kinase/Akt signaling pathway leading to αvβ5 integrin activation and TRAIL resistance in human lung cancer cells

Background

Fibronectin is a mechanically sensitive protein which is organized in the extracellular matrix as a network of interacting fibrils. The lung tumor stroma is enriched for fibronectin which is thought to contribute to metastasis and drug resistance. Fibronectin is an elastic, multi-modular protein made up of individually folded domains, some of which can stretch in response to increased mechanical tension. Very little is known about the relationship of fibronectin’s unfolded domains to lung cancer resistance to chemotherapy. In the present study, we evaluated the impact of unfolding the first Type III domain of fibronectin (FnIII-1c) on TNF-related apoptosis inducing ligand (TRAIL) resistance.

Methods

NCI-H460 non-small cell lung cancer cells were treated with FnIII-1c then assessed for TRAIL-induced apoptosis. Subsequent analysis of FnIII-1c-mediated signaling pathways was also completed. Human non-small cell lung cancer tissue sections were assessed for the expression of vitronectin by immunohistochemistry.

Results

FnIII-1c inhibited TRAIL-induced activation of caspase 8 and subsequent apoptosis in NCI-H460 lung cancer cells. FnIII-1c treatment was associated with the activation of the phosphatidylinositol-3-kinase/alpha serine/threonine kinase (PI3K/Akt) pathway and the αvβ5 integrin receptor for vitronectin, both of which were required for TRAIL resistance. Immunohistochemical staining of sections from non-small cell lung cancers showed that vitronectin was localized around blood vessels and in the tumor-stroma interface.

Conclusions

Unfolding of Type III domains within the fibronectin matrix may promote TRAIL resistance through the activation of a PI3K/Akt/αvβ5 signaling axis and point to a novel mechanism by which changes in secondary structure of fibronectin contribute to cancer cell resistance to apoptosis.

Research advances on structure and biological functions of integrins

Integrins are an important family of adhesion molecules that were first discovered two decades ago. Integrins are transmembrane heterodimeric glycoprotein receptors consisting of α and β subunits, and are comprised of an extracellular domain, a transmembrane domain, and a cytoplasmic tail. Therein, integrin cytoplasmic domains may associate directly with numerous cytoskeletal proteins and intracellular signaling molecules, which are crucial for modulating fundamental cell processes and functions including cell adhesion, proliferation, migration, and survival. The purpose of this review is to describe the unique structure of each integrin subunit, primary cytoplasmic association proteins, and transduction signaling pathway of integrins, with an emphasis on their biological functions.

Lysosomal trafficking of TGFBIp via caveolae-mediated endocytosis

Transforming growth factor-beta-induced protein (TGFBIp) is ubiquitously expressed in the extracellular matrix (ECM) of various tissues and cell lines. Progressive accumulation of mutant TGFBIp is directly involved in the pathogenesis of TGFBI-linked corneal dystrophy. Recent studies reported that mutant TGFBIp accumulates in cells; however, the trafficking of TGFBIp is poorly understood. Therefore, we investigated TGFBIp trafficking to determine the route of its internalization and secretion and to elucidate its roles in the pathogenesis of granular corneal dystrophy type 2 (GCD2). Our data indicate that newly synthesized TGFBIp was secreted via the endoplasmic reticulum/Golgi-dependent secretory pathway, and this secretion was delayed in the corneal fibroblasts of patients with GCD2. We also found that TGFBIp was internalized by caveolae-mediated endocytosis, and the internalized TGFBIp accumulated after treatment with bafilomycin A1, an inhibitor of lysosomal degradation. In addition, the proteasome inhibitor MG132 inhibits the endocytosis of TGFBIp. Co-immunoprecipitation revealed that TGFBIp interacted with integrin α_Vβ₃. Moreover, treatment with arginine-glycine-aspartic acid (RGD) tripeptide suppressed the internalization of TGFBIp. These insights on TGFBIp trafficking could lead to the identification of novel targets and the development of new therapies for TGFBI-linked corneal dystrophy.

In vitro assessment of the effects of vedolizumab binding on peripheral blood lymphocytes

Vedolizumab (VDZ) is a humanized monoclonal antibody in development for the treatment of inflammatory bowel disease. VDZ binds to the α4β7 integrin complex and inhibits its binding to mucosal addressin cell adhesion molecule-1 (MAdCAM-1), thus preventing lymphocyte extravasation to gut mucosal tissues. To understand whether VDZ has additional effects that may affect its overall safety as a therapeutic molecule, we examined other potential actions of VDZ. In vitro assays with human peripheral blood lymphocytes demonstrated that VDZ fails to elicit cytotoxicity, lymphocyte activation, and cytokine production from memory T lymphocytes and does not interfere with the suppressive ability of regulatory T cells. Furthermore, we demonstrated that VDZ induces internalization of α4β7 and that the integrin is rapidly re-expressed and fully functional after VDZ withdrawal. These studies provide insight into the mechanisms underlying the observed safety profile of VDZ in clinical trials.

Endocytic trafficking of laminin is controlled by dystroglycan and is disrupted in cancers

The dynamic interactions between cells and basement membranes serve as essential regulators of tissue architecture and function in metazoans, and perturbation of these interactions contributes to the progression of a wide range of human diseases, including cancers. Here, we reveal the pathway and mechanism for the endocytic trafficking of a prominent basement membrane protein, laminin-111 (referred to here as laminin), and their disruption in disease. Live-cell imaging of epithelial cells revealed pronounced internalization of laminin into endocytic vesicles. Laminin internalization was receptor mediated and dynamin dependent, and laminin proceeded to the lysosome through the late endosome. Manipulation of laminin receptor expression revealed that the dominant regulator of laminin internalization is dystroglycan, a laminin receptor that is functionally perturbed in muscular dystrophies and in many cancers. Correspondingly, laminin internalization was found to be deficient in aggressive cancer cells displaying non-functional dystroglycan, and restoration of dystroglycan function strongly enhanced the endocytosis of laminin in both breast cancer and glioblastoma cells. These results establish previously unrecognized mechanisms for the modulation of cell-basement-membrane communication in normal cells and identify a profound disruption of endocytic laminin trafficking in aggressive cancer subtypes.

Cadherin 99C regulates apical expansion and cell rearrangement during epithelial tube elongation

Apical and basolateral determinants specify and maintain membrane domains in epithelia. Here, we identify new roles for two apical surface proteins - Cadherin 99C (Cad99C) and Stranded at Second (SAS) - in conferring apical character in Drosophila tubular epithelia. Cad99C, the Drosophila ortholog of human Usher protocadherin PCDH15, is expressed in several embryonic tubular epithelial structures. Through loss-of-function and overexpression studies, we show that Cad99C is required to regulate cell rearrangement during salivary tube elongation. We further show that overexpression of either Cad99C or SAS causes a dramatic increase in apical membrane at the expense of other membrane domains, and that both proteins can do this independently of each other and independently of mislocalization of the apical determinant Crumbs (Crb). Overexpression of Cad99C or SAS results in similar, but distinct effects, suggesting both shared and unique roles for these proteins in conferring apical identity.

Integrin αvβ5 in endothelial cells of rat splenic sinus: an immunohistochemical and ultrastructural study

Localization of integrins β1-8, α1, α2, α3, α5, α6 and αv in sinus endothelial cells of the rat spleen was examined by immunofluorescence microscopy. Labeling for anti-integrin β5 and integrin αv was detected and colocalized in the entire circumference of endothelial cells. Labeling for integrin β5, vinculin and actin filaments demonstrated that they lay close to each other in the basal part of the endothelial cells. Although the other integrin βs, including integrin β1 and integrins α1, α2, α3, α5 and α6 in combination with integrin β1, were localized in leukocytes, slightly large cells, megakaryocytes and/or platelets in the sinus lumen and splenic cords, they were not detected in endothelial cells. Labeling for vitronectin, a component of the extracellular-matrix-binding integrin αvβ5, was strongly stained in the periphery of the wall of sinuses, as was collagen IV and, in addition, was localized in the cytoplasm of endothelial cells. Ultrastructural localization of integrin β5, vitronectin and clathrin was examined by immunogold electron microscopy to elucidate the involvement of integrin αvβ5 in the endocytosis of vitronectin in sinus endothelial cells. Electron microscopy with detergent extraction revealed abundant coated pits and coated vesicles in endothelial cells. Immunogold labeling for vitronectin was present in pits, vesicles and the stacked endoplasmic reticulum. Double-labeling for integrin β5 or integrin αv and clathrin revealed that they were colocalized in some vesicles in close proximity to the apical and lateral plasma membrane of the endothelial cells. The possible functional roles of integrin αvβ5 in endothelial cells of the splenic sinus are discussed.

MS-Monitored Conjugation of Poly(ethylene glycol) Monomethacrylate to RGD Peptides

Development of biologically active polymers is an active area of research due to their applications in varied and diverse fields of biomedical research: cell adhesion, tissue proliferation, and drug delivery. Recent advances in chemical modification allow fine-tuning of the properties of biomedical polymers to improve their applications: blood circulation half-life, stimuli-responsive degradation, site-specific targeting, drug loading, etc. In this article, convergent synthesis of polymerizable macromonomers bearing a site-specific ligand (RGD peptide) using a low molecular weight MA-poly(ethylene glycols) (PEGs) is presented. The method affords macromonomers useful as the starting materials to produce biomedical polymers. We found matrix assisted laser desorption/ionization mass spectromerty convenient in monitoring the conjugation process via step-by-step following of PEG modification.

Construction, expression, and purification of recombinant αVβ5 integrin

A recombinant integrin expression system has been created for the large-scale production of αVβ5 integrin extracellular domains that take advantage of Fos and Jun dimerization for expression in bacterial, insect, and mammalian cells. This utilizes an all-in-one vector, pQE-TriSystem, with molecular machinery for parallel expression without the need of additional subcloning. Optimal expression in HEK293 cells was determined by a time course analysis. The heterodimer was purified in a one-step nickel column purification scheme, and the sequence and functional state were confirmed by mass spectrometry and inhibition assays, respectively. The yields of αVβ5 integrin obtained are in quantities suitable for multiple applications including structural biology and functional assays.

iTRAQ-Based and Label-Free Proteomics Approaches for Studies of Human Adenovirus Infections

Both isobaric tags for relative and absolute quantitation (iTRAQ) and label-free methods are widely used for quantitative proteomics. Here, we provide a detailed evaluation of these proteomics approaches based on large datasets from biological samples. iTRAQ-label-based and label-free quantitations were compared using protein lysate samples from noninfected human lung epithelial A549 cells and from cells infected for 24 h with human adenovirus type 3 or type 5. Either iTRAQ-label-based or label-free methods were used, and the resulting samples were analyzed by liquid chromatography (LC) and tandem mass spectrometry (MS/MS). To reduce a possible bias from quantitation software, we applied several software packages for each procedure. ProteinPilot and Scaffold Q+ software were used for iTRAQ-labeled samples, while Progenesis LC-MS and ProgenesisF-T2PQ/T3PQ were employed for label-free analyses. R² correlation coefficients correlated well between two software packages applied to the same datasets with values between 0.48 and 0.78 for iTRAQ-label-based quantitations and 0.5 and 0.86 for label-free quantitations. Analyses of label-free samples showed higher levels of protein up- or downregulation in comparison to iTRAQ-labeled samples. The concentration differences were further evaluated by Western blotting for four downregulated proteins. These data suggested that the label-free method was more accurate than the iTRAQ method.

Differential interactions of Streptococcus gordonii and Staphylococcus aureus with cultured osteoblasts

The impedance of normal osteoblast function by microorganisms is at least in part responsible for the failure of dental or orthopedic implants. Staphylococcus aureus is a major pathogen of bone, and exhibits high levels of adhesion and invasion of osteoblasts. In this article we show that the commensal oral bacterium Streptococcus gordonii also adheres to and is internalized by osteoblasts. Entry of S. gordonii cells had typical features of phagocytosis, similar to S. aureus, with membrane protrusions characterizing initial uptake, and closure of the osteoblast membrane leading to engulfment. The sensitivities of S. gordonii internalization to inhibitors cytochalasin D, colchicine and monensin indicated uptake through endocytosis, with requirement for actin accumulation. Internalization levels of S. gordonii were enhanced by expression of S. aureus fibronectin-binding protein A (FnBPA) on the S. gordonii cell surface. Lysosomal-associated membrane protein-1 phagosomal membrane marker accumulated with intracellular S. aureus and S. gordonii FnBPA, indicating trafficking of bacteria into the late endosomal/lysosomal compartment. Streptococcus gordonii cells did not survive intracellularly for more than 12 h, unless expressing FnBPA, whereas S. aureus showed extended survival times (>48 h). Both S. aureus and S. gordonii DL-1 elicited a rapid interleukin-8 response by osteoblasts, whereas S. gordonii FnBPA was slower. Only S. aureus elicited an interleukin-6 response. Hence, S. gordonii invades osteoblasts by a mechanism similar to that exhibited by S. aureus, and elicits a proinflammatory response that may promote bone resorption.

Cell adhesion and its endocytic regulation in cell migration during neural development and cancer metastasis

Cell migration is a crucial event for tissue organization during development, and its dysregulation leads to several diseases, including cancer. Cells exhibit various types of migration, such as single mesenchymal or amoeboid migration, collective migration and scaffold cell-dependent migration. The migration properties are partly dictated by cell adhesion and its endocytic regulation. While an epithelial-mesenchymal transition (EMT)-mediated mesenchymal cell migration requires the endocytic recycling of integrin-mediated adhesions after the disruption of cell-cell adhesions, an amoeboid migration is not dependent on any adhesions to extracellular matrix (ECM) or neighboring cells. In contrast, a collective migration is mediated by both cell-cell and cell-ECM adhesions, and a scaffold cell-dependent migration is regulated by the endocytosis and recycling of cell-cell adhesion molecules. Although some invasive carcinoma cells exhibit an EMT-mediated mesenchymal or amoeboid migration, other cancer cells are known to maintain cadherin-based cell-cell adhesions and epithelial morphology during metastasis. On the other hand, a scaffold cell-dependent migration is mainly utilized by migrating neurons in normal developing brains. This review will summarize the structures of cell adhesions, including adherens junctions and focal adhesions, and discuss the regulatory mechanisms for the dynamic behavior of cell adhesions by endocytic pathways in cell migration in physiological and pathological conditions, focusing particularly on neural development and cancer metastasis.

A genome-wide association study identifies two novel promising candidate genes affecting Escherichia coli F4ab/F4ac susceptibility in swine

Enterotoxigenic Escherichia coli (ETEC) expressing F4 fimbria is the major pathogenic bacteria causing diarrhoea in neonatal and post-weaning piglets. Previous studies have revealed that the susceptibility to ETEC F4ab/F4ac is an autosomal Mendelian dominant trait and the loci controlling the F4ab/F4ac receptor are located on SSC13q41, between markers SW207 and S0283. To pinpoint these loci and further validate previous findings, we performed a genome-wide association study (GWAS) using a two generation family-based population, consisting of 301 piglets with phenotypes of susceptibility to ETEC F4ab/F4ac by the vitro adhesion test. The DNA of all piglets and their parents was genotyped using the Illumina PorcineSNP60 BeadChip, and 50,972 and 50,483 SNPs were available for F4ab and F4ac susceptibility, respectively, in the association analysis after quality control. In summary, 28 and 18 significant SNPs (p<0.05) were detected associated with F4ab and F4ac susceptibility respectively at genome-wide significance level. From these significant findings, two novel candidate genes, HEG1 and ITGB5, were firstly identified as the most promising genes underlying F4ab/F4ac susceptibility in swine according to their functions and positions. Our findings herein provide a novel evidence for unravelling genetic mechanism of diarrhoea risk in piglets.

Degradation of internalized αvβ5 integrin is controlled by uPAR bound uPA: effect on β1 integrin activity and α-SMA stress fiber assembly

Myofibroblasts (Mfs) that persist in a healing wound promote extracellular matrix (ECM) accumulation and excessive tissue contraction. Increased levels of integrin αvβ5 promote the Mf phenotype and other fibrotic markers. Previously we reported that maintaining uPA (urokinase plasminogen activator) bound to its cell-surface receptor, uPAR prevented TGFβ-induced Mf differentiation. We now demonstrate that uPA/uPAR controls integrin β5 protein levels and in turn, the Mf phenotype. When cell-surface uPA was increased, integrin β5 levels were reduced (61%). In contrast, when uPA/uPAR was silenced, integrin β5 total and cell-surface levels were increased (2–4 fold). Integrin β5 accumulation resulted from a significant decrease in β5 ubiquitination leading to a decrease in the degradation rate of internalized β5. uPA-silencing also induced α-SMA stress fiber organization in cells that were seeded on collagen, increased cell area (1.7 fold), and increased integrin β1 binding to the collagen matrix, with reduced activation of β1. Elevated cell-surface integrin β5 was necessary for these changes after uPA-silencing since blocking αvβ5 function reversed these effects. Our data support a novel mechanism by which downregulation of uPA/uPAR results in increased integrin αvβ5 cell-surface protein levels that regulate the activity of β1 integrins, promoting characteristics of the persistent Mf.

Force- and Ca2+-dependent internalization of integrins in cultured endothelial cells

The effects of mechanical force applied to the integrin clusters at focal contacts were examined in cultured human umbilical vein endothelial cells. When a fibronectin-coated glass bead was attached to the apical cell surface, focal contacts formed beneath the bead that became linked to focal contacts at the basal cell membrane by actin stress fibers in 5 minutes. Integrin dynamics at the basal focal contacts were monitored in live cells in response to a localized mechanical stimulus generated by displacing the glass bead. Traction force transmitted to the basal focal contacts through the stress fibers was monitored by measuring the deformation of the polyacrylamide gel substratum. The force declined in a few seconds, probably owing to decreases in the elastic modulus of the stress fibers. This transient mechanical stimulus caused the dephosphorylation of paxillin and disassembly of integrin clusters at the basal cell membrane in 20 minutes. The disassembly was mediated mainly by clathrin-dependent endocytosis of integrins. The integrin internalization was inhibited in Ca2+- and K+-free solution, and by phenylarsine oxide, a phosphatase inhibitor. These results suggest that a transient mechanical stimulus applied to focal contacts induces Ca2+-dependent dephosphorylation of some proteins, including paxillin, and facilitates clathrin-dependent endocytosis of integrins.

Pharmacoproteomic analysis of a novel cell-permeable peptide inhibitor of tumor-induced angiogenesis

P11, a novel peptide ligand containing a PDZ-binding motif (Ser-Asp-Val) with high affinity to integrin α(v)β(3) was identified from a hexapeptide library (PS-SPCL) using a protein microarray chip-based screening system. Here, we investigated the inhibitory mechanism of P11 (HSDVHK) on tumor-induced angiogenesis via a pharmacoproteomic approach. P11 was rapidly internalized by, human umbilical vein endothelial cells (HUVECs) via an integrin α(v)β(3)-mediated event. Caveolin and clathrin appeared to be involved in the P11 uptake process. The cell-penetrating P11 resulted in suppression of bFGF-induced HUVEC proliferation in a dose-dependent manner. Phosphorylation of extracellular-signal regulated kinase (ERK1/2) and mitogen-activated protein kinase kinase (MEK) in bFGF-stimulated HUVECs was inhibited by cell-permeable P11. Proteomic analysis via antibody microarray showed up-regulation of p53 in P11-treated HUVECs, resulting in induction of apoptosis via activation of caspases-3, -8, and -9. Several lines of experimental evidence strongly suggest that the molecular mechanism of P11, a novel anti-angiogenic agent, inhibits bFGF-induced HUVEC proliferation via mitogen-activated protein kinase kinase and extracellular-signal regulated kinase inhibition as well as p53-mediated apoptosis related with activation of caspases.

Morphological and chemical mechanisms of elongated mineral particle toxicities

Much of our understanding regarding the mechanisms for induction of disease following inhalation of respirable elongated mineral particles (REMP) is based on studies involving the biological effects of asbestos fibers. The factors governing the disease potential of an exposure include duration and frequency of exposures; tissue-specific dose over time; impacts on dose persistence from in vivo REMP dissolution, comminution, and clearance; individual susceptibility; and the mineral type and surface characteristics. The mechanisms associated with asbestos particle toxicity involve two facets for each particle's contribution: (1) the physical features of the inhaled REMP, which include width, length, aspect ratio, and effective surface area available for cell contact; and (2) the surface chemical composition and reactivity of the individual fiber/elongated particle. Studies in cell-free systems and with cultured cells suggest an important way in which REMP from asbestos damage cellular molecules or influence cellular processes. This may involve an unfortunate combination of the ability of REMP to chemically generate potentially damaging reactive oxygen species, through surface iron, and the interaction of the unique surfaces with cell membranes to trigger membrane receptor activation. Together these events appear to lead to a cascade of cellular events, including the production of damaging reactive nitrogen species, which may contribute to the disease process. Thus, there is a need to be more cognizant of the potential impact that the total surface area of REMP contributes to the generation of events resulting in pathological changes in biological systems. The information presented has applicability to inhaled dusts, in general, and specifically to respirable elongated mineral particles.

Caveolin limits membrane microdomain mobility and integrin-mediated uptake of fibronectin-binding pathogens

Staphylococcus aureus, which is a leading cause of hospital-acquired infections, binds via fibronectin to integrin α5β1, a process that can promote host colonization in vivo. Integrin engagement induces actin cytoskeleton rearrangements that result in the uptake of S. aureus by non-professional phagocytic cells. Interestingly, we found that fibronectin-binding S. aureus trigger the redistribution of membrane microdomain components. In particular, ganglioside GM1 and GPI-linked proteins were recruited upon integrin β1 engagement, and disruption of membrane microdomains blocked bacterial internalization. Several membrane-microdomain-associated proteins, such as flotillin-1 and flotillin-2, as well as caveolin, were recruited to sites of bacterial attachment. Whereas dominant-negative versions of flotillin-2 did not affect bacterial attachment or internalization, cells deficient for caveolin-1 (Cav1(-/-)) showed increased uptake of S. aureus and other Fn-binding pathogens. Recruitment of membrane microdomains to cell-associated bacteria was unaltered in Cav1(-/-) cells. However, fluorescence recovery after photobleaching (FRAP) revealed an enhanced mobility of membrane-microdomain-associated proteins in the absence of caveolin-1. Enhanced membrane microdomain mobility and increased uptake of S. aureus was repressed by expression of wild-type caveolin-1, but not caveolin-1 G83S, which harbors a point mutation in the caveolin scaffolding domain. Similarly, chemical or physical stimulation of membrane fluidity led to increased uptake of S. aureus. These results highlight a crucial role for caveolin-1 in negative regulation of membrane microdomain mobility, thereby affecting endocytosis of bacteria-engaged integrins. This process might not only limit host cell invasion by integrin-binding bacterial pathogens, but might also be physiologically relevant for integrin-mediated cell adhesion.

Dual sources of vitronectin in the human lower urinary tract: synthesis by urothelium vs. extravasation from the bloodstream

Vitronectin (VN), secreted into the bloodstream by liver hepatocytes, is known to anchor epithelial cells to basement membranes through interactions with cell surface integrin receptors. We report here that VN is also synthesized by urothelial cells of urothelium in vivo and in vitro. In situ hybridization, dideoxy sequencing, immunohistochemistry, and ELISA of urothelial cell mRNA, cDNA, tissue, and protein extracts demonstrated that the VN gene is active in vivo and in vitro. The expression of VN by urothelium is hypothesized to constitute one of several pathways that anchor basal cells to an underlying substratum and explains why urothelial cells adhere to glass and propagate under serum-free conditions. Therefore, two sources of VN in the human urinary bladder are recognized: 1) localized synthesis by urothelial cells and 2) extravasation of liver VN through fenestrated capillaries. When human plasma was fractionated by denaturing heparin affinity chromatography, VN was isolated in a biologically active form that supported rapid spreading of urothelial cells in vitro under serum-free conditions. This activity was inhibited by the matricellular protein SPARC via direct binding of VN to SPARC through a Ca(+2)-dependent mechanism. A novel form of VN, isolated from the same heparin affinity chromatography column and designated as the VN(c) chromatomer, also supported cell spreading but failed to interact with SPARC. Therefore, the steady-state balance among urothelial cells, their extracellular milieu, and matricellular proteins constitutes a principal mechanism by which urothelia are anchored to an underlying substrata in the face of constant bladder cycling.

Enhancement of peptide coupling to hydroxyapatite and implant osseointegration through collagen mimetic peptide modified with a polyglutamate domain

Hydroxyapatite (HA) is a widely-used biomaterial for bone repair due to its high degree of osteoconductivity. However, strategies for improving HA performance by functionalizing surfaces with bioactive factors are limited. In this study, we explored the use of a HA-binding domain (heptaglutamate, "E7") to facilitate coupling of the collagen mimetic peptide, DGEA, to two types of HA-containing materials, solid HA disks and electrospun polycaprolactone matrices incorporating nanoparticulate HA. We found that the E7 domain directed significantly more peptide to the surface of HA and enhanced peptide retention on both materials in vitro. Moreover, E7-modified peptides were retained in vivo for at least two months, highlighting the potential of this mechanism as a sustained delivery system for bioactive peptides. Most importantly, E7-DGEA-coupled HA, as compared with DGEA-HA, enhanced the adhesion and osteoblastic differentiation of mesenchymal stem cells, and also increased new bone formation and direct bone-implant contact on HA disks implanted into rat tibiae. Collectively, these results support the use of E7-DGEA peptides to promote osteogenesis on HA substrates, and further suggest that the E7 domain can serve as a universal tool for anchoring a wide variety of bone regenerative molecules to any type of HA-containing material.

Ubiquitination of alpha 5 beta 1 integrin controls fibroblast migration through lysosomal degradation of fibronectin-integrin complexes

Cell migration requires endocytosis and recycling of integrins, but it is not known whether degradation of these membrane proteins is involved. Here we demonstrate that in migrating cells, a fraction of the endocytosed fibronectin receptor, alpha 5 beta 1 integrin, is sorted into multivesicular endosomes together with fibronectin and degraded in lysosomes. This sorting requires fibronectin-induced ubiquitination of the alpha 5 subunit, and the activity of the endosomal sorting complex required for transport (ESCRT) machinery, which interacts with alpha 5 beta 1 integrin. Importantly, we demonstrate that both alpha 5 ubiquitination and ESCRT functions are required for proper migration of fibroblasts. We propose that ligand-mediated degradation of alpha 5 beta 1 integrin via the ESCRT pathway is required in order to prevent endosomal accumulation of ligand-bound integrins that might otherwise form nonproductive adhesion sites. Fibronectin and alpha 5 beta 1 integrin therefore are trafficked to lysosomes in a similar way to growth factors and their receptors.

Integrin alpha5beta1 function is regulated by XGIPC/kermit2 mediated endocytosis during Xenopus laevis gastrulation

During Xenopus gastrulation α5β1 integrin function is modulated in a temporally and spatially restricted manner, however, the regulatory mechanisms behind this regulation remain uncharacterized. Here we report that XGIPC/kermit2 binds to the cytoplasmic domain of the α5 subunit and regulates the activity of α5β1 integrin. The interaction of kermit2 with α5β1 is essential for fibronectin (FN) matrix assembly during the early stages of gastrulation. We further demonstrate that kermit2 regulates α5β1 integrin endocytosis downstream of activin signaling. Inhibition of kermit2 function impairs cell migration but not adhesion to FN substrates indicating that integrin recycling is essential for mesoderm cell migration. Furthermore, we find that the α5β1 integrin is colocalized with kermit2 and Rab 21 in embryonic and XTC cells. These data support a model where region specific mesoderm induction acts through kermit2 to regulate the temporally and spatially restricted changes in adhesive properties of the α5β1 integrin through receptor endocytosis.

Collagen I matrix turnover is regulated by fibronectin polymerization

Extracellular matrix (ECM) remodeling occurs during normal homeostasis and also plays an important role during development, tissue repair, and in various disease processes. ECM remodeling involves changes in the synthesis, deposition, and degradation of ECM molecules. ECM molecules can be degraded extracellularly, as well as intracellularly following endocytosis. Our data show that the ECM protein fibronectin is an important regulator of ECM remodeling. We previously showed that agents that inhibit the polymerization of fibronectin into ECM fibrils promote the loss of preexisting fibronectin matrix and accelerate fibronectin endocytosis and degradation. In this paper we show that inhibition of fibronectin polymerization leads to the loss of collagen I matrix fibrils and a corresponding increase in the levels of endocytosed collagen I. In contrast, manipulations that stabilize fibronectin matrix fibrils, such as caveolin-1 depletion, stabilize collagen I matrix fibrils and cause a decrease in ECM collagen I endocytosis. Our data also show that endocytosis of ECM collagen I is regulated by both beta1 integrins and Endo180/urokinase plasminogen activator associated protein (uPARAP). Unexpectedly, Endo180/uPARAP was also shown to promote the endocytosis of fibronectin from the ECM. These data demonstrate that fibronectin polymerization regulates the remodeling of ECM collagen I, in part, by regulating collagen I endocytosis. Furthermore, these data show that processes that regulate ECM deposition coordinately regulate the removal of proteins from the ECM. These data highlight the complexity of ECM remodeling. This multifaceted regulatory process may be important to ensure tight regulation of ECM fibronectin and collagen I levels.

Definitive endoderm derived from human embryonic stem cells highly express the integrin receptors alphaV and beta5

Human embryonic stem cells (hESCs) can be directed to differentiate into a number of endoderm cell types, however mature functional cells have yet to be produced in vitro. This suggests that there may be important factors that have yet to be described, which may be essential for the proper derivation of these cells. One such factor is the integrin mediated interactions between a cell and the extracellular matrix (ECM). On this basis, the present study investigated the role of the ECM in the directed differentiation of hESCs to definitive endoderm via analysis of integrin gene expression. The results showed that definitive endoderm can be efficiently and effectively derived from hESCs in a feeder free, single defined ECM of laminin. Analysis of integrin expression also showed that definitive endoderm highly express the integrins alphaV and beta5, which have the ability to bind to vitronectin, whilst expression of the pluripotency related laminin binding integrins alpha3, alpha6 and beta4 were downregulated. This suggested a potential role of vitronectin binding integrins in the development of definitive endoderm.

Immobilization of RGD peptide on HA coating through a chemical bonding approach

In this work, Arg-Gly-Asp (RGD) sequence containing peptide was immobilized on hydroxyapatite (HA) coatings through a chemical bonding approach in two steps, surface modification with 3-aminopropyltriethoxysilane (APTES) and RGD immobilization. The results indicate that RGD has been successfully immobilized on HA coatings. Comparing with physical adsorption coatings, the chemically bonded RGD on the coatings shows much better anti-wash-out ability. Since RGD is able to recognize cell-membrane integrins on biointerfaces, the present method will be an effective way to favor interaction of cells with HA coatings.

Processing of MIA protein during melanoma cell migration

MIA (melanoma inhibitory activity) protein, identified as a small 11 kDa protein highly expressed and secreted by malignant melanoma cells, plays an important functional role in melanoma development, progression and tumor cell invasion. Recent data describe a direct interaction of MIA protein with cell adhesion receptors integrin alpha(4)beta(1) and integrin alpha(5)beta(1) and extracellular matrix molecules. By modulating integrin activity MIA protein mediates detachment of melanoma cells from surrounding structures resulting in enhanced invasive and migratory potential. However, until today a detailed understanding of the processes of MIA function is missing. In this study, we show that after binding of MIA protein to integrin alpha(5)beta(1), MIA protein is internalized together with this cell adhesion receptor at the cell rear. This mechanism enables tumor cells to migrate in a defined direction as appropriate for invasion processes. Treatment of melanoma cells with PKC-inhibitors strongly reduced internalization of MIA protein. Endocytosis is followed by dissociation of MIA-integrin complexes. In acidic vesicles MIA protein is degraded while integrins are recycled. Treatment of melanoma cells with MIA inhibitory peptides almost completely blocked the MIA protein uptake into cells. As MIA protein has a major contribution to the aggressive characteristics of malignant melanoma in particular to formation of metastasis, it is important to elucidate the MIA functional mechanism in tumor cells to find novel therapeutic strategies in the fight against skin cancer.

Quantitative proteomics identifies a Dab2/integrin module regulating cell migration

Clathrin-associated endocytic adapters recruit cargoes to coated pits as a first step in endocytosis. We developed an unbiased quantitative proteomics approach to identify and quantify glycoprotein cargoes for an endocytic adapter, Dab2. Surface levels of integrins beta1, alpha1, alpha2, and alpha3 but not alpha5 or alphav chains were specifically increased on Dab2-deficient HeLa cells. Dab2 colocalizes with integrin beta1 in coated pits that are dispersed over the cell surface, suggesting that it regulates bulk endocytosis of inactive integrins. Depletion of Dab2 inhibits cell migration and polarized movement of integrin beta1 and vinculin to the leading edge. By manipulating intracellular and surface integrin beta1 levels, we show that migration speed correlates with the intracellular integrin pool but not the surface level. Together, these results suggest that Dab2 internalizes integrins freely diffusing on the cell surface and that Dab2 regulates migration, perhaps by maintaining an internal pool of integrins that can be recycled to create new adhesions at the leading edge.

Clustering and internalization of integrin alphavbeta3 with a tetrameric RGD-synthetic peptide

Integrin alpha(v)beta(3) is overexpressed on neoendothelial cells and frequently on tumor cells. We have developed a peptide-like scaffold (regioselectively addressable functionalized template, RAFT), which holds four cyclo(-RGDfK-) (cRGD) motifs and proved that this molecule (called regioselectively addressable functionalized template-arginine-glycine-aspartic acid, RAFT-RGD) targets integrin alpha(v)beta(3) in vitro and in vivo. Using fluorescence correlation spectroscopy (FCS), we measured the constant of affinity (K(D)) of the RAFT-RGD for purified integrins. K(D) values rose from 3.87 nmol/l for RAFT-RGD to 41.70 nmol/l for cyclo(-RGDfK-). In addition, RAFT-RGD inhibited alpha(v)beta(3) lateral mobility in the cell membrane, probably due to the formation of integrin clusters as demonstrated by fluorescence recovery after photobleaching (FRAP). This was confirmed by electronic microscopy data, which established the formation of molecular complexes containing two integrins in the presence of RAFT-RGD but not cRGD or regioselectively addressable functionalized template-arginine-alanine- aspartic acid (RAFT-RAD). Using an enzyme-linked immunosorbent assay (ELISA), we proved that 1 micromol/l RAFT-RGD increased by 79% alpha(v)beta(3) internalization via clathrin-coated vesicles. Conversely, cRGD was internalized without modifying alpha(v)beta(3) internalization. Although RGD has been known for >20 years, this is the first study to formerly establish the relationships among multimeric presentation, increased affinity, and subsequent integrin-mediated cointernalization. These results strongly support the rationale for using multimeric RGD-peptides as targeting vectors for imaging, diagnosis, or therapy of cancers.

Cell adhesion and response to synthetic nanopatterned environments by steering receptor clustering and spatial location

During adhesion and spreading, cells form micrometer-sized structures comprising transmembrane and intracellular protein clusters, giving rise to the formation of what is known as focal adhesions. Over the past two decades these structures have been extensively studied to elucidate their organization, assembly, and molecular composition, as well as to determine their functional role. Synthetic materials decorated with biological molecules, such as adhesive peptides, are widely used to induce specific cellular responses dependent on cell adhesion. Here, we focus on how surface patterning of such bioactive materials and organization at the nanoscale level has proven to be a useful strategy for mimicking both physical and chemical cues present in the extracellular space controlling cell adhesion and fate. This strategy for designing synthetic cellular environments makes use of the observation that most cell signaling events are initiated through recruitment and clustering of transmembrane receptors by extracellular-presented signaling molecules. These systems allow for studying protein clustering in cells and characterizing the signaling response induced by, e.g., integrin activation. We review the findings about the regulation of cell adhesion and focal adhesion assembly by micro- and nanopatterns and discuss the possible use of substrate stiffness and patterning in mimicking both physical and chemical cues of the extracellular space.

Caveolin-1-dependent beta1 integrin endocytosis is a critical regulator of fibronectin turnover

beta1 integrins are major cell surface receptors for fibronectin. Some integrins, including beta1 integrins, are known to undergo constitutive endocytosis and recycling. Integrin endocytosis/recycling has been implicated in the regulation of cell migration. However, the mechanisms by which integrin endocytosis/recycling regulates cell migration, and other biological consequences of integrin trafficking are not completely understood. We previously showed that turnover of extracellular matrix (ECM) fibronectin occurs via receptor-mediated endocytosis. Here, we investigate the biological relevance of beta1 integrin endocytosis to fibronectin matrix turnover. First, we demonstrate that beta1 integrins, including alpha5beta1 play an important role in endocytosis and turnover of matrix fibronectin. Second, we show that caveolin-1 constitutively regulates endocytosis of alpha5beta1 integrins, and that alpha5beta1 integrin endocytosis can occur in the absence of fibronectin and fibronectin matrix. We also show that downregulation of caveolin-1 expression by siRNA results in marked reduction of beta1 integrin and fibronectin endocytosis. Hence, caveolin-1-dependent beta1 integrin and fibronectin endocytosis plays a critical role in fibronectin matrix turnover, and may contribute to abnormal ECM remodeling that occurs in fibrotic disorders.

Integrin trafficking and its role in cancer metastasis

Enhanced levels of expression of certain integrins, and a consequent increase in specific integrin signals, have been linked to cancer cell progression. Dysfunctional integrin signaling is thought to be involved, at least in part, in mediating the detachment of tumor cells from neighboring cells while providing enhanced survival and proliferative capabilities which allow such disseminating tumor cells to grow in new, foreign, microenvironments. Cell biologists have known for some time that integrin heterodimers are endocytosed from the plasma membrane in to the cytoplasm with some of this receptor later being exocytosed back to the cell surface; a cellular mechanism referred to as 'trafficking'. Although extensive research within the integrin field has elucidated key signal transduction pathways as being involved in integrin-mediated cellular behavior, both in normal and transformed cells, it is only relatively recently that the importance of integrin trafficking in modulating cellular function has been demonstrated. This review aims to identify the major trafficking molecules found to play a functional role in cancer cell behavior with special emphasis on the importance of integrin trafficking during neoplastic cell migration and invasion; vital components of the metastatic process.

Inhibition of caveolar uptake, SV40 infection, and beta1-integrin signaling by a nonnatural glycosphingolipid stereoisomer

Caveolar endocytosis is an important mechanism for the uptake of certain pathogens and toxins and also plays a role in the internalization of some plasma membrane (PM) lipids and proteins. However, the regulation of caveolar endocytosis is not well understood. We previously demonstrated that caveolar endocytosis and β1-integrin signaling are stimulated by exogenous glycosphingolipids (GSLs). In this study, we show that a synthetic GSL with nonnatural stereochemistry, β-d-lactosyl-N-octanoyl-l-threo-sphingosine, (1) selectively inhibits caveolar endocytosis and SV40 virus infection, (2) blocks the clustering of lipids and proteins into GSLs and cholesterol-enriched microdomains (rafts) at the PM, and (3) inhibits β1-integrin activation and downstream signaling. Finally, we show that small interfering RNA knockdown of β1 integrin in human skin fibroblasts blocks caveolar endocytosis and the stimulation of signaling by a GSL with natural stereochemistry. These experiments identify a new compound that can interfere with biological processes by inhibiting microdomain formation and also identify β1 integrin as a potential mediator of signaling by GSLs.

Integrin traffic

Cell adhesion, migration and the maintenance of cell polarity are all processes that depend on the correct targeting of integrins and the dynamic remodelling of integrin-containing adhesion sites. The importance of the endo/exocytic cycle of integrins as a key regulator of these functions is increasingly recognized. Several recent publications have provided mechanistic insight into how integrin traffic is regulated in cells. Increasing evidence suggests that small GTPases such as Arf6 and members of the Rab family control integrin internalization and recycling back to the plasma membrane along microtubules. The fine tuning of these trafficking events seems to be mediated by specific guanine-nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs). In addition, several kinases regulate integrin traffic. The identification of their substrates has demonstrated how these kinases regulate integrin traffic by controlling small GTPases or stabilizing cytoskeletal tracks that are crucial for efficient traffic of integrins to the plasma membrane.

The Arf6 GEF GEP100/BRAG2 regulates cell adhesion by controlling endocytosis of beta1 integrins

The small GTPase Arf6 has been shown to regulate the post-endocytic trafficking of a subset of membrane proteins, including beta1 integrins, and inhibition of Arf6 function impairs both cell adhesion and motility. The activity of Arf GTPases is regulated by a large family of guanine nucleotide exchange factors (GEFs). Arf-GEP100/BRAG2 is a GEF with reported specificity for Arf6 in vitro, but it is otherwise poorly characterized. Here we report that BRAG2 exists in two ubiquitously expressed isoforms, which we call BRAG2a and BRAG2b, both of which can activate Arf6 in vivo. Depletion of endogenous BRAG2 by siRNA leads to dramatic effects in the cell periphery; one such effect is an accumulation of beta1 integrin on the cell surface and a corresponding enhancement of cell attachment and spreading on fibronectin-coated substrates. In contrast, depletion of Arf6 leads to intracellular accumulation of beta1 integrin and reduced adhesion and spreading. These findings suggest that Arf6 regulates both endocytosis and recycling of beta1 integrins and that BRAG2 functions selectively to activate Arf6 during integrin internalization.

Syndecan-1 regulates alphavbeta5 integrin activity in B82L fibroblasts

B82L mouse fibroblasts respond to fibronectin or vitronectin via a syndecan-1-mediated activation of the alphavbeta5 integrin. Cells attached to syndecan-1-specific antibody display only filopodial extension. However, the syndecan-anchored cells extend lamellipodia when the antibody-substratum is supplemented with serum, or low concentrations of adsorbed vitronectin or fibronectin, that are not sufficient to activate the integrin when plated alone. Integrin activation is blocked by treatment with (Arg-Gly-Asp)-containing peptides and function-blocking antibodies that target alphav integrins, as well as by siRNA-mediated silencing of beta5 integrin expression. In addition, alphavbeta5-mediated cell attachment and spreading on high concentrations of vitronectin is blocked by competition with recombinant syndecan-1 ectodomain core protein and by downregulation of mouse syndecan-1 expression by mouse-specific siRNA. Taking advantage of the species-specificity of the siRNA, rescue experiments in which human syndecan-1 constructs are expressed trace the activation site to the syndecan-1 ectodomain. Moreover, both full-length mouse and human syndecan-1 co-immunoprecipitate with the beta5 integrin subunit, but fail to do so if the syndecan is displaced by competition with soluble, recombinant syndecan-1 ectodomain. These results suggest that the ectodomain of the syndecan-1 core protein contains an active site that assembles into a complex with the alphavbeta5 integrin and regulates alphavbeta5 integrin activity.

Role of αvβ5 integrin receptor in endocytosis of crocidolite and its effect on intracellular glutathione levels in human lung epithelial (A549) cells

Crocidolite, containing 27% iron by weight, is the most carcinogenic form of asbestos. Crocidolite fibers are endocytized by alphavbeta5 integrin receptors in rabbit pleural mesothelial cells. We show here that crocidolite fibers are endocytized in human lung epithelial (A549) cells and in primary small airway epithelial (SAEC) cells. Presence of the integrin alphavbeta5 blocking antibody, P1F6, significantly reduced the uptake of crocidolite fibers in A549 cells. Thus, the integrin alphavbeta5 receptor is involved in endocytosis of crocidolite fibers in A549 cells as well. Previously, it has been observed that asbestos fibers lead to changes in the intracellular redox environment, i.e. a marked decrease in intracellular glutathione concentrations and an increase in the extracellular glutathione in A549 cells. In addition, the decrease in intracellular glutathione was found to be largely independent of iron present on the surface of the fiber. A549 cells were treated with crocidolite in the presence of endocytosis inhibitor cytochalasin D. Our data indicate that, upon preventing endocytosis, we were able to reverse the decrease in total intracellular glutathione. The decrease in total intracellular glutathione could also be prevented in the presence of the monoclonal antibody P1F6. Thus, we observed that endocytosis of crocidolite fibers via integrin alphavbeta5 receptor is linked to the marked decrease in total intracellular glutathione in A549 cells.

The effect of the addition of a polyglutamate motif to RGD on peptide tethering to hydroxyapatite and the promotion of mesenchymal stem cell adhesion

Mimicking endogenous bone-binding proteins, RGD peptides have been synthesized with polyacidic amino acid domains in order to ionically tether the peptides to bone-like synthetic biomaterials, including hydroxyapatite (HA). However, a direct comparison of unmodified RGD with polyacidic-conjugated RGD has not been performed, and thus a benefit for the acidic domain has not been established. We evaluated the peptide/HA bond of RGD peptides with and without an attached polyglutamate sequence (E(7)), as well as examined mesenchymal stem cell (MSC) adhesion and morphology as they were affected by the conjugated peptide. We found that significantly more E(7)RGD was bound to HA than RGD at all coating concentrations tested, and moreover, more E(7)RGD was retained on the HA surface even after extended washing in serum-free media. Consistent with in vitro results, higher levels of E(7)RGD than RGD remained on HA that had been implanted in vivo for 24 h, indicating that the polyacidic domain improved peptide-binding efficiency. At several peptide concentrations, E(7)RGD increased cell adhesion compared to RGD surfaces, establishing a biological benefit for the E(7) modification. In addition, HA pre-coated sequentially with low-density E(7)RGD (1-10 microg/ml) and serum (FBS) stimulated cell adhesion and spreading, compared to either coating alone, suggesting that an ionic linkage allows for the potential adsorption of serum proteins to unoccupied sites, which may be important for bone formation in vivo. Collectively, these results suggest that tethering peptides to HA via a polyglutamate domain is an effective method for improving the peptide/HA bond, as well as for enhancing MSC adhesion.