Regulation of integrin function and cellular adhesion

Enhanced cellular osteogenic differentiation on CoFe2O4/P(VDF-TrFE) nanocomposite coatings under static magnetic field

Cellular responses can be regulated and manipulated through combining stimuli-responsive biomaterial with external stimulus. In this present, the magneto-responsive CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings were designed to understand cell behaviors of preosteoblasts, as well as get insight into the underlying mechanism of osteogenic differentiation under static magnetic field (SMF). CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings with differential magnetic property (low, medium and high magnetization) were prepared by incorporation of different mass fraction of CoFe₂O₄ nanoparticles (6%, 13 %, 20 %) into P(VDF-TrFE) matrix. Cell experiments indicated that all nanocomposite coatings with the assistance of SMF could promote the cell attachment, proliferation and osteogenic differentiation of MC3T3-E1 cells. Among different nanocomposite coatings, low magnetization coating (6%) showed a higher ALP activity and gene expression of Runx2, Col-I, OCN. Molecular biology assays demonstrated that the combination of nanocomposite coatings and SMF could significantly up-regulate the expression level of α2β1 integrin and p-ERK. Whereas, the addition of inhibitor U0126 down-regulated sharply the expression level of p-ERK, which indicated that cellular osteogenic differentiation of MC3T3-E1 cells was governed through α2β1 integrin-mediated MEK/ERK signaling pathways during CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings were combined with SMF. This work provided a promising strategy to enhance cellular osteogenic differentiation through a remote-control manner, which exhibited great potential in the application of bone tissue repair and regeneration.

Human T-Cell Leukemia Virus Type 1 (HTLV-1) bZIP Factor Upregulates the Expression of ICAM-1 To Facilitate HTLV-1 Infection

Human T-cell leukemia virus type 1 (HTLV-1) causes multiple pathological effects, ranging from a form of leukemia to a spectrum of inflammation-mediated diseases. These diseases arise from one or several infected CD4⁺ T cells among thousands acquiring proliferation and survival advantages and ultimately becoming pathogenic. Given the low incidence of HTLV-1-associated diseases among carriers, such cellular evolutionary processes appear to occur rarely. Therefore, infectious spread of HTLV-1 within the T-cell population may be one underlying factor influencing disease development. Free HTLV-1 virions are poorly infectious, so infection of T cells relies on direct contact between infected and target cells. Following contact, virions pass to target cells through a virological synapse or cellular conduits or are transferred to target cells within an extracellular matrix. Lymphocyte functioning antigen 1 (LFA-1) on the surface of the target cell engaging with its ligand, ICAM-1, on the surface of the infected cell (effector cell) initiates and stabilizes cell-cell contact for infection. We found that stable expression of an HTLV-1 accessory protein, HTLV-1 bZIP factor (HBZ), in Jurkat T cells increases homotypic aggregation. This phenotype was attributed to elevated ICAM-1 expression in the presence of HBZ. Using a single-cycle replication-dependent luciferase assay, we found that HBZ expression in Jurkat cells (used as effector cells) increases HTLV-1 infection. Despite this effect, HBZ could not replace the critical infection-related functions of the HTLV-1 regulatory protein Tax. However, in HTLV-1-infected T cells, knockdown of HBZ expression did lead to a decrease in infection efficiency. These overall results suggest that HBZ contributes to HTLV-1 infectivity.IMPORTANCE Human T-cell leukemia virus type 1 (HTLV-1) causes a variety of diseases, ranging from a fatal form of leukemia to immune-mediated inflammatory diseases. These diseases occur rarely, arising from one or a small subset of virally infected cells infrequently evolving into a pathogenic state. Thus, the process of HTLV-1 cell-to-cell transmission within the host helps influence the probability of disease development. HTLV-1 primarily infects T cells and initially spreads within this cell population when virally infected T cells dock to uninfected target T cells and then transfer HTLV-1 virus particles to the target cells. Here we found that the viral protein HTLV-1 bZIP factor (HBZ) promotes infectivity. HBZ accomplishes this task by increasing the surface abundance of a cellular adhesion protein known as intercellular adhesion molecule 1 (ICAM-1), which helps initiate and stabilize contact (docking) between infected and target T cells. These results define a novel and unexpected function of HBZ, diverging from its defined functions in cellular survival and proliferation.

Effect of surface topography and bioactive properties on early adhesion and growth behavior of mouse preosteoblast MC3T3-E1 cells

The effects of bioactive properties and surface topography of biomaterials on the adhesion and spreading properties of mouse preosteoblast MC3T3-E1 cells was investigated by preparation of different surfaces. Poly lactic-co-glycolic acid (PLGA) electrospun fibers (ES) were produced as a porous rough surface. In our study, coverslips were used as a substrate for the immobilization of 3,4-dihydroxyphenylalanine (DOPA) and collagen type I (COL I) in the preparation of bioactive surfaces. In addition, COL I was immobilized onto porous electrospun fibers surfaces (E-COL) to investigate the combined effects of bioactive molecules and topography. Untreated coverslips were used as controls. Early adhesion and growth behavior of MC3T3-E1 cells cultured on the different surfaces were studied at 6, 12, and 24 h. Evaluation of cell adhesion and morphological changes showed that the all the surfaces were favorable for promoting the adhesion and spreading of cells. CCK-8 assays and flow cytometry revealed that both topography and bioactive properties were favorable for cell growth. Analysis of β1, α1, α2, α5, α10 and α11 integrin expression levels by immunofluorescence, real-time RT-PCR, and Western blot and indicated that surface topography plays an important role in the early stage of cell adhesion. However, the influence of topography and bioactive properties of surfaces on integrins is variable. Compared with any of the topographic or bioactive properties in isolation, the combined effect of both types of properties provided an advantage for the growth and spreading of MC3T3-E1 cells. This study provides a new insight into the functions and effects of topographic and bioactive modifications of surfaces at the interface between cells and biomaterials for tissue engineering.

Involvement of CD137 ligand signaling in neural stem cell death

CD137 is a member of the tumor necrosis factor/nerve growth factor receptor superfamily. Interaction of CD137 with its ligand (CD137L) affects the apoptosis, proliferation and differentiation of immune cells. Interestingly, the CD137 receptor/ligand system involves the bi-directional transduction of signals. The expression of CD137 and its ligand is not restricted to immune organs, but can also be detected in a wide variety of tissues such as the brain, kidney, lung and heart. However, its role in brain is largely unknown. This study was performed to determine the role of CD137L reverse signaling in the apoptosis of neural stem cells. We identified the expression of CD137 and its ligand in C17.2 neural stem cells derived from mouse embryonic cerebellum. We found that the activation of CD137L reverse signaling by CD137 resulted in a decrease in cell adhesion to the fibronectin-coated culture basement, thus causing detachment-induced cell death. Furthermore, we showed that the cell death induced by CD137 was completely ameliorated by integrin activators and caspase inhibitors. Therefore we suggest that CD137L reverse signaling exerts a pro-apoptotic effect by suppressing integrin-mediated survival signals in neural stem cells.

Conformational epitopes at cadherin calcium-binding sites and p120-catenin phosphorylation regulate cell adhesion

The activity state of E-cadherin is controlled by conformational epitopes at interfaces between different EC domains, which are coupled to p120-catenin phosphorylation. Dephosphorylation activates adhesion, whereas phosphorylation inhibits activation. p120-dependent changes in the physical state of E-cadherin regulate epithelial cell morphogenesis.

We investigated changes in cadherin structure at the cell surface that regulate its adhesive activity. Colo 205 cells are nonadhesive cells with a full but inactive complement of E-cadherin–catenin complexes at the cell surface, but they can be triggered to adhere and form monolayers. We were able to distinguish the inactive and active states of E-cadherin at the cell surface by using a special set of monoclonal antibodies (mAbs). Another set of mAbs binds E-cadherin and strongly activates adhesion. In other epithelial cell types these activating mAbs inhibit growth factor–induced down-regulation of adhesion and epithelial morphogenesis, indicating that these phenomena are also controlled by E-cadherin activity at the cell surface. Both types of mAbs recognize conformational epitopes at different interfaces between extracellular cadherin repeat domains (ECs), especially near calcium-binding sites. Activation also induces p120-catenin dephosphorylation, as well as changes in the cadherin cytoplasmic domain. Moreover, phospho-site mutations indicate that dephosphorylation of specific Ser/Thr residues in the N-terminal domain of p120-catenin mediate adhesion activation. Thus physiological regulation of the adhesive state of E-cadherin involves physical and/or conformational changes in the EC interface regions of the ectodomain at the cell surface that are mediated by catenin-associated changes across the membrane.

Tetraspanin CD151 maintains vascular stability by balancing the forces of cell adhesion and cytoskeletal tension

Tetraspanin CD151 is highly expressed in endothelial cells and regulates pathologic angiogenesis. However, the mechanism by which CD151 promotes vascular morphogenesis and whether CD151 engages other vascular functions are unclear. Here we report that CD151 is required for maintaining endothelial capillary-like structures formed in vitro and the integrity of endothelial cell-cell and cell-matrix contacts in vivo. In addition, vascular permeability is markedly enhanced in the absence of CD151. As a global regulator of endothelial cell-cell and cell-matrix adhesions, CD151 is needed for the optimal functions of various cell adhesion proteins. The loss of CD151 elevates actin cytoskeletal traction by up-regulating RhoA signaling and diminishes actin cortical meshwork by down-regulating Rac1 activity. The inhibition of RhoA or activation of cAMP signaling stabilizes CD151-silenced or -null endothelial structure in vascular morphogenesis. Together, our data demonstrate that CD151 maintains vascular stability by promoting endothelial cell adhesions, especially cell-cell adhesion, and confining cytoskeletal tension.

Integrins α2β1 and α11β1 regulate the survival of mesenchymal stem cells on collagen I

Although mesenchymal stem cells (MSCs) are the natural source for bone regeneration, the exact mechanisms governing MSC crosstalk with collagen I have not yet been uncovered. Cell adhesion to collagen I is mostly mediated by three integrin receptors - α1β1, α2β1 and α11β1. Using human MSC (hMSC), we show that α11 subunit exhibited the highest basal expression levels but on osteogenic stimulation, both α2 and α11 integrins were significantly upregulated. To elucidate the possible roles of collagen-binding integrins, we applied short hairpin RNA (shRNA)-mediated knockdown in hMSC and found that α2 or α11 deficiency, but not α1, results in a tremendous reduction of hMSC numbers owing to mitochondrial leakage accompanied by Bcl-2-associated X protein upregulation. In order to clarify the signaling conveyed by the collagen-binding integrins in hMSC, we analyzed the activation of focal adhesion kinase, extracellular signal-regulated protein kinase and serine/threonine protein kinase B (PKB/Akt) kinases and detected significantly reduced Akt phosphorylation only in α2- and α11-shRNA hMSC. Finally, experiments with hMSC from osteoporotic patients revealed a significant downregulation of α2 integrin concomitant with an augmented mitochondrial permeability. In conclusion, our study describes for the first time that disturbance of α2β1- or α11β1-mediated interactions to collagen I results in the cell death of MSCs and urges for further investigations examining the impact of MSCs in bone conditions with abnormal collagen I.

Surface oxide net charge of a titanium alloy: modulation of fibronectin-activated attachment and spreading of osteogenic cells

In the current study, we have altered the surface oxide properties of a Ti6Al4V alloy using heat treatment or radiofrequency glow discharge (RFGD) in order to evaluate the relationship between the physico-chemical and biological properties of the alloy's surface oxide. The effects of surface pretreatments on the attachment of cells from two osteogenic cell lines (MG63 and MC3T3) and a mesenchymal stem cell line (C3H10T1/2) to fibronectin adsorbed to the alloy were measured. Both heat and RFGD pretreatments produced a several-fold increase in the number of cells that attached to fibronectin adsorbed to the alloy at a range of coating concentrations (0.001-10nM FN) for each cell line tested. An antibody (HFN7.1) directed against the central integrin binding domain of fibronectin produced a 65-70% inhibition of cell attachment to fibronectin-coated disks, indicating that cell attachment to the metal discs was dependent on fibronectin binding to cell integrin receptors. Both treatments also accelerated the cell spreading response manifested by extensive flattening and an increase in mean cellular area. The treatment-induced increases in the cell attachment activity of adsorbed fibronectin were correlated with previously demonstrated increases in Ti6Al4V oxide negative net surface charge at physiological pH produced by both heat and RFGD pretreatments. Since neither treatment increased the adsorption mass of fibronectin, these findings suggest that negatively charged surface oxide functional groups in Ti6Al4V can modulate fibronectin's integrin receptor activity by altering the adsorbed protein's conformation. Our results further suggest that negatively charged functional groups in the surface oxide can play a prominent role in the osseointegration of metallic implant materials.

Integrin signaling in epithelial cells

Although most cells of adult mammals express multiple different integrins, particular types of cells have a characteristic repertoire of integrin expression. Benign and malignant epithelial cells use specific integrins to allow the epithelial microenvironment to modulate a wide variety of cell functions, including cell survival, proliferation, morphogenesis, differentiation, motility, invasion and metastasis. An important concept emerging from the data on integrin signal transduction is that integrin signaling impinges on pathways downstream of other receptors, creating elaborate intracellular signaling networks. This review highlights signal transduction functions of epithelial integrins, with particular emphasis on signaling pathways underlying some of the most important functions of epithelium.

Melanoma cell migration is upregulated by tumour necrosis factor-alpha and suppressed by alpha-melanocyte-stimulating hormone

We reported recently that the inflammatory cytokine tumour necrosis factor α (TNF-α) can upregulate integrin expression, cell attachment and invasion of cells through fibronectin in a human melanoma cell line (HBL). Furthermore, the actions of TNF-α were suppressed by the addition of an anti-inflammatory peptide α-melanocyte-stimulating hormone (α-MSH). In the current study, we extend this work investigating to what extent TNF-α might stimulate melanoma invasion by promoting cell migration and whether α-MSH is also inhibitory. Two human melanoma cell lines were examined in vitro (HBL and C8161) using a scratch migration assay. Analysis using either time-lapse video microscopy or imaging software analysis of migrating ‘fronts’ of cells revealed that C8161 cells migrated more rapidly than HBL cells. However, when cells were stimulated with TNF-α both cell types responded with a significant increase in migration distance over a 16–26 h incubation time. α-Melanocyte-stimulating hormone had an inhibitory effect on TNF-α-stimulated migration for HBL cells, completely blocking migration at 10⁻⁹ M. In contrast, C8161 cells did not respond to α-MSH (as these cells have a loss-of-function melanocortin-1 receptor). However, stable transfection of C8161 cells with the wild-type melanocortin-1 receptor produced cells whose migration was significantly inhibited by α-MSH. In addition, the use of a neutralising antibody to the β₁-integrin subunit significantly reduced migration in both cell types. This data therefore supports an inflammatory environment promoting melanoma cell migration, and in addition shows that α-MSH can inhibit inflammatory stimulated migration. The data also support a fundamental role of the β₁-integrin receptor in melanoma cell migration.

Melanoma cell attachment, invasion, and integrin expression is upregulated by tumor necrosis factor alpha and suppressed by alpha melanocyte stimulating hormone

We have previously shown alpha-melanocyte stimulating hormone to protect melanocytes and melanoma cells from the proinflammatory actions of tumor necrosis factor-alpha. The aim of the study was to extend this work to look into the influence of tumor necrosis factor-alpha on melanoma cell attachment, invasion, and integrin expression and ask to what extent alpha-melanocyte stimulating hormone might protect cells from tumor necrosis factor-alpha stimulation of increased integrin expression. HBL human melanoma cells were studied under resting and stressed conditions using tumor necrosis factor-alpha as a proinflammatory cytokine. Functional information on the actions of tumor necrosis factor-alpha on melanoma cells was obtained by examining the strength of attachment of melanoma cells to substrates and the ability of melanoma cells to invade through fibronectin. alpha3, alpha4, and beta1 integrin expression was detected by Western immunoblotting and the ability of alpha-melanocyte stimulating hormone to oppose the actions of tumor necrosis factor-alpha was studied on HBL cell attachment, invasion, and integrin subunit expression. Our results show that tumor necrosis factor-alpha increases the number of melanoma cells attaching to collagen (types I and IV) and tissue culture polystyrene, increases ability to invade through fibronectin, and upregulates the expression of alpha3 (28%), alpha4 (90%), and beta1 (65%) integrin subunit expression. In contrast, alpha-melanocyte stimulating hormone reduced cell attachment, invasion, and integrin expression and opposed the stimulatory effects of tumor necrosis factor-alpha. In conclusion this study provides further evidence of alpha-melanocyte stimulating hormone acting to "protect" melanoma cells from proinflammatory cytokine action. Our data support a hypothesis that an inflammatory environment would promote melanoma invasion and that the anti-invasive actions of alpha-melanocyte stimulating hormone are consistent with its working in an anti-inflammatory capacity.

Evidence for sequential utilization of fibronectin, vitronectin, and collagen during fibroblast-mediated collagen contraction

Contraction plays a major role in wound healing and is inevitably mediated through the mechanical interaction of fibroblast cytoskeleton and integrins with their extracellular matrix ligands. Cell-matrix attachment is critical for such events. In human dermal fibroblasts most such interactions are mediated by the beta1-type integrins. This study investigated the role played by key components in this system, notably fibronectin, vitronectin, and integrin subcomponents alpha2 and alpha5, which recognize collagen and fibronectin. Inhibition of adhesion through these ligands was studied either by antibody blocking or with fibronectin and/or vitronectin depletion. Functional effects of inhibition were monitored as force generation in collagen-glycosaminoglycan (IntegraTM) sponges, over 20 hours using a culture force monitor. Dose and time-course inhibition studies indicated that initial attachment and force generation (approx. 0-5 hours postseeding) was through fibronectin receptors and this was followed by vitronectin ligand and receptor utilization (4 hours onward). Utilization of the collagen integrin subcomponent alpha2 appeared to be increasingly important between 6 and 16 hours and dominant thereafter. Additionally, there was evidence for functional interdependence between the three ligand systems fibronectin, vitronectin, and collagen. We propose that there is a short cascade of sequential integrin-ligand interactions as cells attach to, extend through, and eventually contract their matrix. (WOUND REP REG 2002;10:-408)

Effect of divalent cations on the affinity and selectivity of alpha4 integrins towards the integrin ligands vascular cell adhesion molecule-1 and mucosal addressin cell adhesion molecule-1: Ca2+ activation of integrin alpha4beta1 confers a distinct ligand specificity

A microtiter plate assay measuring the binding of cells expressing integrins alpha4beta1 or alpha4beta7 to VCAM-1 and MAdCAM-1, expressed as Ig fusion proteins, was used to explore the interplay between the variables of integrin beta-chain, identity and density of ligand, and identity and concentration of activating cations. Both Mn2+ and Mg2+ supported binding of either integrin to either ligand. Ca2+ supported only the binding of alpha4beta1 to VCAM-Ig. Cation concentrations required for half-maximal binding (EC50) ranged from 0.8-280 microM for Mn2+ and 0.8-30 mM for Mg2+, being thus 2-3 logs lower for Mn2+ compared to Mg2+ independent of ligand. EC50 values for binding of alpha4beta1 to VCAM-Ig were 30-45-fold lower compared to MAdCAM-Ig, while alpha4beta7 showed an opposite 3-15-fold selectivity for MAdCAM-Ig over VCAM-Ig. The density of ligand required for adhesion via alpha4beta1 was markedly lower with Mn2+ versus Mg2+, and with VCAM-Ig versus MAdCAM-Ig. These results were interpreted in terms of a coupled equilibrium model, in which binding of activating metal ions and of integrin ligands each stabilizes activated integrin. We conclude that Mn2+ and Mg2+ bind to common regulatory sites with different affinities, producing similar activated states of the integrin. The resulting activated alpha4beta1 binds more strongly to VCAM-Ig versus MAdCAM-Ig by 30-45-fold, while similarly activated alpha4beta7 binds more strongly to MAdCAM-Ig versus VCAM-Ig by 3-15-fold. Inhibition studies showed that Ca2+ also binds to regulatory sites on both integrins. However, the Ca2+-activated state of alpha4beta1 is distinct from that achieved by Mn2+ and Mg2+, possessing increased selectivity for binding to VCAM-1 versus MAdCAM-1.

Ex vivo expansion of umbilical cord blood (UCB) CD34(+) cells alters the expression and function of alpha 4 beta 1 and alpha 5 beta 1 integrins

We have investigated the influence of ex vivo expansion of human CD34(+) cord blood cells on the expression and function of adhesion molecules involved in the homing and engraftment of haematopoietic progenitors. Ex vivo expansion of umbilical cord blood CD34(+) cells for 6 d in the presence of interleukin 3 (IL-3), IL-6 and stem cell factor (SCF) or IL-11, SCF and Flt-3L resulted in increased expression of alpha 4, alpha 5, beta 1, alpha M and beta 2 integrins. However, a significant decrease in the adhesion of progenitor cells to fibronectin was observed after the ex vivo culture (adhesion of granulocyte-macrophage colony-forming units (CFU-GM) was 22 +/- 4% in fresh cells versus 5 +/- 2% and 2 +/- 2% in each combination of cytokines). Incubation with the beta 1 integrin-activating antibody TS2/16 restored adhesion to fibronectin. Transplantation of ex vivo expanded umbilical cord blood CD34(+) cells was associated with an early delayed engraftment in non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice. Incubation of cells with the monoclonal antibody TS2/16 before transplantation almost completely abrogated NOD/SCID repopulating ability of both fresh and expanded CD34(+) cells. The seeding efficiency of fresh and expanded CD34(+) cells was similar, but markedly reduced after incubation with the TS2/16 monoclonal antibody. Our results show that functional activation of beta 1 integrins could overcome the decreased very late antigen (VLA)-4- and VLA-5-mediated adhesion observed after ex vivo expansion of haematopoietic progenitors. However, in vivo, these effects induced an almost complete abrogation of the homing and repopulating ability of CD34(+) UCB cells.

Cell attachment modulation by radiation from a pulsed light diode (lambda = 820 nm) and various chemicals

BACKGROUND AND OBJECTIVE

Adhesive interactions between cells and extracellular matrices play a regulative role in wound repair processes. The objective of this investigation is to study the mechanisms of light action on cellular adhesion in vitro. The adhesion of HeLa cells to a glass matrix is evaluated after irradiation with a pulsed near-infrared (IR) diode and treatment with various chemicals.

STUDY DESIGN/MATERIALS AND METHODS

A semiconductor diode (820 +/- 10 nm, 10Hz, 16--120 J/m(2)) is used for irradiation of the cell suspension. In parallel experiments, various chemicals (mannitol, melatonin, ethanol, ascorbic acid, superoxide dismutase, catalase, rotenone, azide, dinitrophenol (DNP), methylene blue, and hydrogen peroxide) are added to the cell suspension before or after the irradiation procedure. The cell-glass adhesion is studied by using the adhesion assay technique (Lasers Surg. Med. 1996;18:171).

RESULTS

It has been found that cell-glass adhesion increases in a dose-dependent manner after irradiation. The treatment of the cells with antioxidants (free radical scavengers), e.g., mannitol, melatonin, ethanol, and ascorbic acid, as well as with the ionophore DNP, eliminated the light effect. The respiratory chain inhibitors rotenone and azide strongly modified the light effect, depending on the dose. The oxidative agents hydrogen peroxide (in a low concentration) and methylene blue increased the cell adhesion. Superoxide dismutase did not modify the light effect. The effect of the catalase (stimulative or suppressive) was dependent on its concentration and treatment sequence. Preirradiation was found to decrease (or normalize to the control level) the suppressive effects of some chemicals.

CONCLUSION

The results obtained are evidence that first, pulsed IR radiation with certain parameters modulates the cell-matrix attachment. second, free radical and redox processes are involved in the cell-matrix interaction, probably at some stage(s) of the photosignal transduction. Third, both types of the primary reactions in the respiratory chain, namely, the increase of the electron flow and production of the reactive oxygen species, cause a transient oxidative stress in the cytoplasm.

Distinct hematopoietic support by two human stromal cell lines

OBJECTIVE

The hematopoietic microenvironment is complex, and the role of myofibroblast in its function is crucial. In order to obtain a stable model reflecting this particular cell type, we have previously established human bone marrow cell lines from primary myofibroblastic Stro1(+) population (pStro1(+)). We placed HPV16 E6 and E7 expression under the control of different promoters. Here, we have characterized and studied the hematopoietic support for two cell lines corresponding to the promoters alpha-SM (alphaSM-56 line) and SV40 (SV40-56 line).

MATERIALS AND METHODS

The expression profile was analyzed at the RNA level by gene array and at the protein level by Western blot, flow cytometry, and ELISA. Hematopoietic support determined using colony-forming unit (CFU) and stroma-adherent colony-forming cell (SA-CFC) assays.

RESULTS

The phenotype of cell lines was not significantly modified compared with primary myofibroblastic cells. They secreted a broad spectrum of hematopoietic cytokines and nonspecific mediators. The two lines allowed the growth of hematopoietic precursors and had different support capabilities.

CONCLUSIONS

We have extensively characterized two novel human bone marrow stromal cell lines. They retained a myofibroblastic phenotype and have substantial but different hematopoietic support capabilities. These lines provided a basis for determining stromal factors involved in stem-cell regulation.

Analysis of integrin expression in U2OS cells cultured on various calcium phosphate ceramic substrates

Earlier we observed that calcium phosphate (Ca-P)-coated implant substrates stimulated the differentiation of osteoblast-like cells compared to uncoated substrates. This suggests that this difference in osteogenic induction is due to the chemical composition of the substratum. We hypothesized that Ca-P coatings modulate integrin expression patterns, because those receptors are the sensors of the cell. Therefore, in the present study we quantitatively analyzed integrin expression of osteosarcoma cells and their proliferation behavior on various well-defined Ca-P substrates. For this study we used the osteosarcoma cell line U2OS. Five groups of substrates were used: thermanox (Th), uncoated titanium (Ti), dense sintered hydroxyapatite (HA), and two Ca-P-coated titanium discs (TiHA-O% and TiHA-5%). At day 5, cell numbers were significantly lower (p < 0.05) for both types of Ca-P-coated titanium substrates compared to the other substrates. There were no significant differences between HA and uncoated titanium. From day 5 to 8, accumulated cell number was ranking highest to lowest HA > Th = Ti > TiHA-0% > TiHA-5%. Integrin expression at day 5 and day 8 of incubation was analyzed by flow cytometry for integrin subunits beta 1, alpha 3, alpha 4, alpha 5, alpha 6, and alpha v. Fluorescence-activated cell sorting (FACS) analysis showed that the cells express high levels of beta 1, low levels of alpha 4, alpha 5, and alpha 6, and moderate levels of alpha 3 and alpha v integrin subunits on the various biomaterial substrates. Minor differences in integrin expression between the various substrates were seen. Therefore, the observed differences in proliferation between the coatings may reside in modulating the functional properties of integrins.

Sialidase treatment exposes the beta1-integrin active ligand binding site on HL60 cells and increases binding to fibronectin

The migration of neutrophils from the circulation to areas of inflammation is the result of the sequential activation of multiple cellular adhesion molecules. beta1-Integrins are cell surface glycoproteins and the class of adhesion molecules responsible for binding to the extracellular matrix. The goal of this study was to determine the contribution of glycosylation, specifically the presence of sialic acid, to beta1-integrin adhesion in a neutrophil model. beta1-Integrins on differentiated HL60 cells were remodeled by treatment with the exoglycosidases, sialidase and beta-galactosidase. beta1-Integrin activity was determined by measuring adherence to the extracellular matrix protein fibronectin. The expression of beta1-integrins, beta2-integrins and activated beta1-integrins was determined by flow cytometry. Remodeling of beta1-integrins by treatment with sialidase increased adhesion by greater than 1,000%. Flow cytometric analysis of remodeled beta1-integrins demonstrated an increased expression of the activated beta1-integrin, but only minor increases in the expression of total beta1- and beta2-integrins. We postulate that glycosidase treatment increases adhesion and expression of activated beta1-integrins by exposure of the normally hidden ligand-binding site. The glycosylation of beta1-integrins on neutrophils may act to hide the ligand-binding site in unstimulated cells thereby contributing to the affinity modulation observed in neutrophil beta1-integrin function.

Interferon-mediated block in cell cycle and altered integrin expression in a differentiated salivary gland cell line (HSG) cultured on Matrigel

Sjögren's syndrome (SS), an idiopathic, autoimmune exocrinopathy, is partly characterized by diminished salivary flow, acinar cell atrophy, and increased expression of several cytokines. Several in vivo characteristics of the sialoadenitis are also evident in a human salivary gland ductal epithelial cell line (HSG) treated with cytokines. HSG cells differentiate to the acinar phenotype when cultured on Matrigel (Becton Dickinson, Bedford, MA), a basement membrane extract. To elucidate mechanisms of salivary gland pathology, the effects of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on cell cycle progression and integrin expression were evaluated in HSG acinarlike cells. Flow cytometry experiments showed that cytokine treatment for 2 days arrested cells in G(1) phase of the cell cycle, and this preceded significant morphologic changes and decreased viability. Whereas only modest cytokine-mediated increases in protein expression for the alpha 3 and beta 1 integrin subunits were seen by immunoprecipitation, a form of alpha 3 integrin displaying enhanced electrophoretic mobility was evident after 6 days of cytokine treatment. To our knowledge, this is the first report demonstrating an IFN-mediated alteration in the electrophoretic mobility of integrin subunits. From this study, it was evident that the combination of IFN-gamma and TNF-alpha resulted in a block in G(1) phase for acinar cells before accumulation of the alpha 3 integrin variant or significant degenerative cellular changes. Information from the present and previous studies suggests that cytokines may alter the pattern of integrin expression and block cell cycle progression in salivary gland cells grown in three-dimensional acinarlike clusters. These experiments may provide a new cell culture model to study the effects of cytokines in normal and diseased salivary glands, including SS.

Beta1 integrin-mediated T cell adhesion and cell spreading are regulated by calpain

To investigate the function of calpain in T cells, we sought to determine the role of this protease in cellular events mediated by beta1 integrins. T cell receptor cross-linked or phorbol ester-stimulated T cells binding to immobilized fibronectin induce the translocation of calpain to the cytoskeletal/membrane fraction of these cells. Such translocation of calpain is associated with proteolytic modification of protein tyrosine phosphatase 1B, increased cellular adhesion, and dramatic alterations in cellular morphology. However, affinity-related increases in T cell adhesion induced by the anti-beta1 integrin antibody 8A2 occur in a calpain-independent manner and in the absence of morphological shape changes. Furthermore, calpain undergoes activation in response to either alpha4beta1 or alpha5beta1 integrin binding to fibronectin in appropriately stimulated T cells, and calpain II as well as protein tyrosine phosphatase 1B accumulates at sites of focal contact formation. Inhibition of calpain activity not only inhibits the proteolytic modification of protein tyrosine phosphatase 1B, but also decreases the ability of T cells to adhere to and spread on immobilized fibronectin. Thus, we describe a potential regulatory role for calpain in beta1 integrin-mediated signaling events associated with T cell adhesion and cell spreading on fibronectin.

Galectin-8 binding to integrins inhibits cell adhesion and induces apoptosis

The interaction of cells with the extracellular matrix regulates cell adhesion, motility, growth, survival and differentiation through integrin-mediated signal transduction. Here we demonstrate that galectin-8, a secreted mammalian (beta)-galactoside binding protein, inhibits adhesion of human carcinoma (1299) cells to plates coated with integrin ligands, and induces cell apoptosis. Pretreatment of the cells with Mn(2+), which increases the affinity of integrins for their ligands, abolished the inhibitory effects of galectin-8. The inhibitory effects of galectin-8 were specific and were not mimicked by plant lectins or other galectins (galectin-1 and galectin-3). In accordance with its anti-adhesive effects, transfection of galectin-8 cDNA into 1299 cells significantly reduced (by 75%) colony formation, when compared to the number of colonies formed by cells transfected with an empty vector. Affinity chromatography over immobilized galectin-8 indicated that few membrane proteins interacted with galectin-8 in a sugar-dependent manner. Microsequencing and western immunoblotting revealed that (alpha)(3)(beta)(1)integrin derived from 1299 as well as other cells (e.g. HeLa and human endothelial cells) is a major galectin-8 binding-protein. Furthermore, immunoprecipitation and immunohistochemical studies suggested that endogenous galectin-8, secreted from 1299 cells, forms complexes with (alpha)(3)(beta)(1) integrins expressed on the surface of 1299 cells. Galectin-8 also interacts with other members of the integrin family, like (alpha)(6)(beta)(1)integrins. In contrast, galectin-8 only minimally interacts with (alpha)(4)or (beta)(3)integrins. We propose that galectin-8 is an integrin binding-protein that interacts to a different extent with several, but not all members of the integrin family. Binding of galectin-8 modulates integrin interactions with the extracellular matrix and thus regulates cell adhesion and cell survival.

Adhesion of aspirated tumor cells to extracellular matrix proteins: a new methodology utilizing fine-needle aspiration

BACKGROUND

Cell adhesion molecules mediate the interactions of cells with other cells and with extracellular matrix components. Such interactions may be important in the development of tumor invasion and metastasis. This article describes a new approach to the evaluation of tumor cell-matrix interactions by utilizing fine-needle aspiration of resected tumors.

METHODS

Fine-needle aspiration was performed on 15 fresh surgical specimens of various types of carcinomas. After partial purification by isotonic Percoll centrifugation, tumor cell adhesion to collagen Type IV, laminin, and fibronectin was evaluated by counting cytologically malignant cells adhering to matrix-coated plastic substrates. Frozen tissue sections of the corresponding tumors were studied simultaneously for immunohistochemical expression of alpha-2, alpha-3, alpha-4, and alpha-5 integrin subunit expression. Results of the immunohistochemical staining then were compared with the adhesion data for particular tumors.

RESULTS

In general, the majority of the tumors exhibited little or no adhesion to collagen or laminin, but several tumors showed marked adhesion to fibronectin. Striking differences were noted between some tumors of the same histologic subtype. Competitive inhibition studies performed with two of the tumors (a large cell carcinoma and a renal cell carcinoma) showed decreased adhesion to fibronectin in the presence of anti-alpha-5, suggesting at least a partial role for the alpha-5-beta 1 fibronectin receptor in mediating the adhesion of these tumors to fibronectin. All the tumors examined exhibited strong immunohistochemical expression of the alpha-2 and alpha-3 integrin subunits, and all were negative for alpha-4. Three of the tumors showed weak expression of alpha-5, two of which (a squamous cell carcinoma and a renal cell carcinoma) were the tumors that showed the greatest adhesion to fibronectin.

CONCLUSIONS

Quantitative adhesion data can be obtained using cell suspensions prepared from fine-needle aspirates, and there are marked differences in adhesive properties between particular tumors. Although two of the tumors showed a correlation between adhesion to fibronectin and immunohistochemical expression of the alpha-5 integrin subunit, matrix adhesion does not necessarily correlate with immunohistochemical expression of adhesion molecule receptors. In the future, this methodology potentially could be of value in determining which patients may benefit from therapies aimed at modifying tumor cell-matrix interactions.

Design and evaluation of benzophenone-containing conformationally constrained ligands as tools for photoaffinity scanning of the integrin alphaVbeta3-ligand bimolecular interaction

Integrins are cell-surface adhesion molecules involved in mediating cell-extracellular matrix interactions. High-resolution structural data are not available for these heterodimeric receptors. In order to generate tools for photoaffinity scanning of the RGD-binding site of human integrin alphaVbeta3. new conformationally constrained ligands were designed. The ligands were based on five different cyclic peptidic or peptidomimetic scaffolds with high affinity for alphaVbeta3. A single photoreactive group (a benzophenone moiety) was introduced at different positions relative to the RGD triad. In addition, an 125I or a biotin group was introduced as a reporting tag. Twenty-four cyclic ligands were prepared and their binding affinity for alphaVbeta3 was determined. In most cases, the modifications resulted in a 5- to 500-fold decrease in affinity relative to the unmodified scaffold. Analogs representing three of the five families were screened for their cross-linking efficiency. Ligands with submicromolar affinities cross-linked efficiently and specifically to the integrin receptor, whereas ligands with weaker affinities gave specific cross-linking, but with lower efficiency. Almost all of the screened ligands cross-linked predominantly to the beta3 subunit.

Changes in the levels of integrin and focal adhesion kinase (FAK) in human melanoma cells following 532 nm laser treatment

Despite the increase in laser therapy, concern remains that sublethal treatment of pre-malignant lesions may adversely affect the biological behaviour of surviving cells. Integrin receptors mediate interaction of cells with the extracellular matrix and their occupation leads to focal adhesion kinase (FAK) activation. Using our previously established model we have now investigated subcellular changes and compared integrin and FAK concentrations, the degree of FAK phosphorylation and its association with the beta1 integrin in laser vs. non-laser treated cells. We treated cells with laser generated from a frequency doubled Q-switched (Nd:YAG) laser system (532 nm) at 0.4 J/cm2 twice per week for 4 weeks. Using cell lysates we performed Western immunoblotting 24 hr later to detect integrin subunits and FAK proteins and immunoprecipitation to investigate FAK phosphorylation and its association with beta1. Cell morphology was examined using electron microscopy. SK23 and G361 cells exhibited an 3.4- and 11.2-fold increase, respectively, in FAK protein following laser treatment. FAK phosphorylation in SK23 cells was increased by 82%, whereas FAK phosphorylation in G361 cells was reduced slightly (2%). Furthermore, both alpha3 and 4 integrins were up-regulated, by approximately 4-fold and 7- to 9-fold, respectively. In addition, the beta1 integrin was proteolysed in both cell lines and the levels of FAK associated with beta1 was increased (2.1- and 2.7-fold, respectively). Finally, laser treatment of SK23 cells caused an increased number of cell processes. Sublethal 532 nm laser light thus induces changes in integrin and FAK concentrations and subsequently influences cellular attachment and morphology.

Analysis of C-cadherin regulation during tissue morphogenesis with an activating antibody

The regulation of cadherin-mediated adhesion at the cell surface underlies several morphogenetic processes. To investigate the role of cadherin regulation in morphogenesis and to begin to analyze the molecular mechanisms of cadherin regulation, we have screened for monoclonal antibodies (mAbs) that allow us to manipulate the adhesive state of the cadherin molecule. Xenopus C-cadherin is regulated during convergent extension movements of gastrulation. Treatment of animal pole tissue explants (animal caps) with the mesoderm-inducing factor activin induces tissue elongation and decreases the strength of C-cadherin-mediated adhesion between blastomeres (Brieher, W.M., and B.M. Gumbiner. 1994. J. Cell Biol. 126:519-527). We have generated a mAb to C-cadherin, AA5, that restores strong adhesion to activin-treated blastomeres. This C-cadherin activating antibody strongly inhibits the elongation of animal caps in response to activin without affecting mesodermal gene expression. Thus, the activin-induced decrease in C-cadherin adhesive activity appears to be required for animal cap elongation. Regulation of C-cadherin and its activation by mAb AA5 involve changes in the state of C-cadherin that encompass more than changes in its homophilic binding site. Although mAb AA5 elicited a small enhancement in the functional activity of the soluble C-cadherin ectodomain (CEC1-5), it was not able to restore cell adhesion activity to mutant C-cadherin lacking its cytoplasmic tail. Furthermore, activin treatment regulates the adhesion of Xenopus blastomeres to surfaces coated with two other anti-C-cadherin mAbs, even though these antibodies probably do not mediate adhesion through a normal homophilic binding mechanism. Moreover, mAb AA5 restores strong adhesion to these antibodies. mAb AA5 only activates adhesion of blastomeres to immobilized CEC1-5 when it binds to C-cadherin on the cell surface. It does not work when added to CEC1-5 on the substrate. Together these findings suggest that the regulation of C-cadherin by activin and its activation by mAb AA5 involve changes in its cellular organization or interactions with other cell components that are not intrinsic to the isolated protein.

Covalent immobilization of recombinant human alphavbeta3 integrin on a solid support with retention of functionality

Alphavbeta3 is the major receptor mediating the attachment of osteoclasts to bone surface and plays a critical role in bone resorption and remodeling. Interfering with alphavbeta3 binding inhibits osteoclast-mediated bone resorption, and thus demonstrates the potential utility of alphavbeta3 antagonists for therapy of osteoporosis. This report describes the generation of an alphavbeta3 affinity column which was created to enable screening of collections of large numbers of ligands, e.g., combinatorial libraries (previously prepared by us), to sort and identify ligands with the highest affinity for alphavbeta3. We demonstrate that covalent immobilization of the heterodimeric alphavbeta3 receptor can be achieved with retention of characteristic ligand binding properties. Human alphavbeta3 was isolated from human embryonic kidney cells (HEK 293) that stably express high levels of the recombinant receptor and then affinity purified to homogeneity. Purified alphavbeta3 receptor was linked covalently to activated CH-Sepharose 4B beads. Quantification of immobilized functional receptor was determined by Scatchard analysis. The immobilized functional receptor maintains binding properties similar to the membrane-embedded and soluble receptor. The immobilized receptor also can be used to select the highest affinity ligand from among a mixture of peptides which differ in their binding affinity, structure, and hydrophobicity, both when the peptides are loaded in equimolar concentrations in a mixture and when the concentration of the highest affinity ligand is reduced 10-fold. Liquid chromatography-mass spectrometry was utilized to confirm selective ligand binding and to demonstrate that preferential binding was not due to nonspecific hydrophobic interactions with immobilized alphavbeta3 receptor or the affinity column. This approach may be of general use for affinity-based screening of ligands for other integrins and should enable practical screening of combinatorial libraries containing large numbers of potential ligands for the human alphavbeta3 integrin receptor, including linear peptides, cyclic peptides, and peptidomimetics.

Fibronectin and collagen I matrixes promote calcification of vascular cells in vitro, whereas collagen IV matrix is inhibitory

Vascular calcification is a frequent component of atherosclerosis, yet the pathological mechanisms that regulate its formation are poorly understood. Calcification of the vessel wall may represent a process by which cells that normally exhibit a smooth muscle phenotype differentiate into cells that exhibit an osteoblast-like phenotype. One of the determinants of cellular phenotype is extracellular matrix; thus, we undertook the current study to evaluate the influence of extracellular matrix on calcification of vascular cells in vitro. Cell lines derived from bovine aortic media were divided into 1 of 3 groups: those that did not mineralize, those that mineralized slowly, or those that mineralized rapidly. When slowly mineralizing cells were plated onto matrix produced by rapidly mineralizing cells, the time required for mineralization decreased from 33+/-3.0 days to 7.8+/-1.3 days. Matrix produced by rapidly mineralizing cells was found to contain 3 times the amount of collagen I and fibronectin but 70% less collagen IV than nonmineralizing clones. When slowly mineralizing cells were cultured on purified collagen I or fibronectin, mineralized nodule formation, calcium incorporation, von Kossa staining, and alkaline phosphatase activity increased. In contrast, culturing slowly mineralizing cells on purified collagen IV inhibited these mineralization parameters. Furthermore, blocking antibodies to alpha5 integrins significantly inhibited the fibronectin-mediated increases in alkaline phosphatase activity, indicating that integrin-based signaling may be involved. These data suggest that matrix composition can regulate development of arterial calcification and that a subpopulation of vascular cells preferentially produces positively regulating matrix components.

Rac regulates integrin-mediated spreading and increased adhesion of T lymphocytes

Leukocyte adhesion to the extracellular matrix (ECM) is tightly controlled and is vital for the immune response. Circulating lymphocytes leave the bloodstream and adhere to ECM components at sites of inflammation and lymphoid tissues. Mechanisms for regulating T-lymphocyte-ECM adhesion include (i) an alteration in the affinity of cell surface integrin receptors for their extracellular ligands and (ii) an alteration of events following postreceptor occupancy (e.g., cell spreading). Whereas H-Ras and R-Ras were previously shown to affect T-cell adhesion by altering the affinity state of the integrin receptors, no signaling molecule has been identified for the second mechanism. In this study, we demonstrated that expression of an activated mutant of Rac triggered dramatic spreading of T cells and their increased adhesion on immobilized fibronectin in an integrin-dependent manner. This effect was not mimicked by expression of activated mutant forms of Rho, Cdc42, H-Ras, or ARF6, indicating the unique role of Rac in this event. The Rac-induced spreading was accompanied by specific cytoskeletal rearrangements. Also, a clustering of integrins at sites of cell adhesion and at the peripheral edges of spread cells was observed. We demonstrate that expression of RacV12 did not alter the level of expression of cell surface integrins or the affinity state of the integrin receptors. Moreover, our results indicate that Rac plays a role in the regulation of T-cell adhesion by a mechanism involving cell spreading, rather than by altering the level of expression or the affinity of the integrin receptors. Furthermore, we show that the Rac-mediated signaling pathway leading to spreading of T lymphocytes did not require activation of c-Jun kinase, serum response factor, or pp70(S6 kinase) but appeared to involve a phospholipid kinase.

Serine and threonine phosphorylation of the paxillin LIM domains regulates paxillin focal adhesion localization and cell adhesion to fibronectin

We have previously shown that the LIM domains of paxillin operate as the focal adhesion (FA)-targeting motif of this protein. In the current study, we have identified the capacity of paxillin LIM2 and LIM3 to serve as binding sites for, and substrates of serine/threonine kinases. The activities of the LIM2- and LIM3-associated kinases were stimulated after adhesion of CHO.K1 cells to fibronectin; consequently, a role for LIM domain phosphorylation in regulating the subcellular localization of paxillin after adhesion to fibronectin was investigated. An avian paxillin-CHO.K1 model system was used to explore the role of paxillin phosphorylation in paxillin localization to FAs. We found that mutations of paxillin that mimicked LIM domain phosphorylation accelerated fibronectin-induced localization of paxillin to focal contacts. Further, blocking phosphorylation of the LIM domains reduced cell adhesion to fibronectin, whereas constitutive LIM domain phosphorylation significantly increased the capacity of cells to adhere to fibronectin. The potentiation of FA targeting and cell adhesion to fibronectin was specific to LIM domain phosphorylation as mutation of the amino-terminal tyrosine and serine residues of paxillin that are phosphorylated in response to fibronectin adhesion had no effect on the rate of FA localization or cell adhesion. This represents the first demonstration of the regulation of protein localization through LIM domain phosphorylation and suggests a novel mechanism of regulating LIM domain function. Additionally, these results provide the first evidence that paxillin contributes to "inside-out" integrin-mediated signal transduction.

VCAM-1 is internalized by a clathrin-related pathway in human endothelial cells but its alpha 4 beta 1 integrin counter-receptor remains associated with the plasma membrane in human T lymphocytes

Lymphocyte extravasation involves a step(s) of de-adhesion to allow trans- and subendothelial migration in response to inflammatory signals. We show here that ligated VCAM-1 was rapidly internalized (t1/2 14.5 min) in ECV 304 endothelial cells and in TNF-alpha-primed human umbilical vein-derived endothelial cells (t1/2 11.2 min). The process required energy (ATP), intracellular Ca2+, an intact cytoskeletal network and active protein kinases. The internalization of VCAM-1 involved a clathrin-dependent pathway based on the observations that 1) it was inhibited in cells treated with lysosomotropic agents or with a hypertonic concentration of sucrose, and 2) internalized VCAM-1 colocalized with clathrin. In contrast, the cross-linked alpha 4 beta 1 integrin counter-receptor of VCAM-1 remained associated with the plasma membrane of purified peripheral T and Jurkat cells. Our results suggest a model where VCAM-1 would initially participate in the retention of T cells to the endothelium by binding alpha 4 beta 1 integrin. Lymphocyte de-adhesion would be facilitated as a result of the internalization of VCAM-1. The persistent cell surface expression of alpha 4 beta 1 integrin would allow the migrating T cells to interact with and receive signal(s) from its fibronectin ligand of the extracellular matrix.

Nerve-induced disruption and reformation of beta1-integrin aggregates during development of the neuromuscular junction

The earliest biochemical change detected during synaptogenesis is a local elimination of muscle basal lamina proteins. To explore whether this provides signal(s) that regulate postsynaptic differentiation, we examined the effects of innervation on the distribution of beta1-integrins, which were initially present in scattered aggregates complexed with basal lamina ligands. These beta1-integrin aggregates disappear along paths of nerve contact as their basal lamina ligands are eliminated. New accumulations of these proteins then form during assembly of the postsynaptic apparatus. The new beta1-integrin aggregates at developing synapses form partly via a redistribution of mobile molecules on muscle surface. We thus consider whether (a) the removal of integrins' basal lamina ligands alters their cytoplasmic ligand-interactions, causing the dissociation of integrin clusters, and (b) this receptor modulation helps to transduce local changes in pericellular protease activity into cytoplasmic signals that control postsynaptic differentiation.

NPXY motifs control the recruitment of the alpha5beta1 integrin in focal adhesions independently of the association of talin with the beta1 chain

With the exception of the divergent beta4 and beta8 chains, the integrin beta subunit cytoplasmic domains are short and highly conserved sequences. Consensus motifs are found among the different cytoplasmic beta chains. Experiments using chimeric receptors demonstrated that the 47 amino acids of the beta1 subunit cytoplasmic domain contain sufficient information to target integrins to adhesion plaques. Three clusters of amino acids, named cyto-1, cyto-2 and cyto-3, seem to contribute to this localization. Cyto-2 and cyto-3 exhibit NPXY motifs. At present, the exact function of these motifs remains unknown but it is likely that these sequences are involved in protein-protein interactions. Although NPXY motifs often act as internalization signals at the cytoplasmic tail of membrane receptors, our previous results showed that the two NPXY motifs are not responsible for the alpha5beta1 integrin endocytosis. Herein, we address the question of the role of the two highly conserved NPXY motifs found in the beta1 cytoplasmic domain, and which correspond to the conserved domains cyto-2 and cyto-3. We demonstrate that, within the integrin beta1 cytoplasmic tail, the two NPXY motifs are required for the recruitment of the integrin in focal adhesions. In addition, our results indicate that these two motifs control but do not belong to the talin-binding sites. Finally, the analysis of the phenotypes of NPXY mutants reveals that the interaction of talin with the beta1 cytosolic domain is not sufficient to target the integrins to focal adhesions.

Human neutrophil elastase proteolytically activates the platelet integrin alphaIIbbeta3 through cleavage of the carboxyl terminus of the alphaIIb subunit heavy chain. Involvement in the potentiation of platelet aggregation

Neutrophil elastase (NE) and cathepsin G are two serine proteinases released concomitantly by stimulated polymorphonuclear neutrophils. We previously demonstrated that while NE by itself does not activate human platelets, it strongly enhances the weak aggregation induced by a threshold concentration of cathepsin G (threshold of cathepsin G) (Renesto, P., and Chignard, M. (1993) Blood 82, 139-144). The aim of this study was to delineate the molecular mechanisms involved in this potentiation process. Two main pieces of data prompted us to focus on the activation of the platelet fibrinogen receptor, the alphaIIbbeta3 integrin. First, previous studies have shown this integrin to be particularly prone to proteolytic regulation of its function. Second, we found that the potentiating activity of NE on the threshold of cathepsin G-induced platelet aggregation was strictly dependent on the presence of exogenous fibrinogen. Using flow cytometry analysis, NE was shown to trigger a time-dependent binding of PAC-1 and AP-5, two monoclonal antibodies specific for the activated and ligand-occupied conformers of alphaIIbbeta3. Furthermore, the potentiated aggregation was shown to result from an increased capacity of platelets to bind fibrinogen. Indeed, the combination of NE and threshold of cathepsin G increased the binding of PAC-1 approximately 5.5-fold over basal values measured on nontreated platelets, whereas this binding raised only by approximately 3-fold in threshold of cathepsin G-stimulated platelets (p < 0.05). By contrast, phosphatidic acid accumulation, pleckstrin phosphorylation, and calcium mobilization produced by the combination of NE and threshold of cathepsin G were not significantly different from those measured with threshold of cathepsin G alone (p > 0.05), indicating that the phospholipase C/protein kinase C pathway is not involved in the potentiation of aggregation. The foregoing data, as well as the requirement of catalytically active NE to trigger alphaIIbbeta3 activation and potentiate threshold of cathepsin G-initiated platelet aggregation, led us to examine whether the structure of this integrin was affected by NE. Immunoblot and flow cytometry analysis revealed a limited proteolysis of the carboxyl terminus of the alphaIIb subunit heavy chain (alphaIIbH), as judged by the disappearance of the epitope for the monoclonal antibody PMI-1. Mass spectrometry studies performed on a synthetic peptide mapping over the cleavage domain of alphaIIbH predicted the site of proteolysis as located between Val837 and Asp838. Treatment by NE of ATP-depleted platelets or Chinese hamster ovary cells expressing human recombinant alphaIIbbeta3 clearly established that activation of the integrin was independent of signal transduction events and was concomitant with the proteolysis of alphaIIbH. In support of this latter observation, a close correlation was observed between the kinetics of proteolysis of alphaIIbH on platelets and that of expression of the ligand binding activity of alphaIIbbeta3 (r2 = 0.902, p </= 0. 005). However, only a subpopulation ( approximately 25%) of the proteolyzed alphaIIbbeta3 appeared to fully express the ligand binding capacity. Altogether, these results demonstrate that NE up-regulates the fibrinogen binding activity of alphaIIbbeta3 through a restricted proteolysis of the alphaIIb subunit, and that this process is relevant for the potentiation of platelet aggregation.

CD44: structure, function, and association with the malignant process

CD44 is a ubiquitous multistructural and multifunctional cells surface adhesion molecule involved in cell-cell and cell-matrix interactions. Twenty exons are involved in the genomic organization of this molecule. The first five and the last 5 exons are constant, whereas the 10 exons located between these regions are subjected to alternative splicing, resulting in the generation of a variable region. Differential utilization of the 10 variable region exons, as well as variations in N-glycosylation, O-glycosylation, and glycosaminoglycanation (by heparan sulfate or chondroitin sulfate), generate multiple isoforms (at least 20 are known) of different molecular sizes (85-230 kDa). The smallest CD44 molecule (85-95 kDa), which lacks the entire variable region, is standard CD44 (CD44s). As it is expressed mainly on cells of lymphohematopoietic origin, CD44s is also known as hematopoietic CD44 (CD44H). CD44s is a single-chain molecule composed of a distal extracellular domain (containing, the ligand-binding sites), a membrane-proximal region, a transmembrane-spanning domain, and a cytoplasmic tail. The molecular sequence (with the exception of the membrane-proximal region) displays high interspecies homology. After immunological activation, T lymphocytes and other leukocytes transiently upregulate CD44 isoforms expressing variant exons (designated CD44v). A CD44 isform containing the last 3 exon products of the variable region (CD44V8-10, also known as epithelial CD44 or CD44E), is preferentially expressed on epithelial cells. The longest CD44 isoform expressing in tandem eight exons of the variable region (CD44V3-10) was detected in keratinocytes. Hyaluronic acid (HA), an important component of the extracellular matrix (ECM), is the principal, but by no means the only, ligand of CD44. Other CD44 ligands include the ECM components collagen, fibronectin, laminin, and chondroitin sulfate. Mucosal addressin, serglycin, osteopontin, and the class II invariant chain (Ii) are additional, ECM-unrelated, ligands of the molecule. In many, but not in all cases, CD44 does not bind HA unless it is stimulated by phorbol esters, activated by agonistic anti-CD44 antibody, or deglycosylated (e.g., by tunicamycin). CD44 is a multifunctional receptor involved in cell-cell and cell-ECM interactions, cell traffic, lymph node homing, presentation of chemokines and growth factors to traveling cells, and transmission of growth signals. CD44 also participates in the uptake and intracellular degradation of HA, as well as in transmission of signals mediating hematopoiesis and apoptosis. Many cancer cell types as well as their metastases express high levels of CD44. Whereas some tumors, such as gliomas, exclusively express standard CD44, other neoplasms, including gastrointestinal cancer, bladder cancer, uterine cervical cancer, breast cancer and non-Hodgkin's lymphomas, also express CD44 variants. Hence CD44, particularly its variants, may be used as diagnostic or prognostic markers of at least some human malignant diseases. Furthermore, it has been shown in animal models that injection of reagents interfering with CD44-ligand interaction (e.g., CD44s- or CD44v-specific antibodies) inhibit local tumor growth and metastatic spread. These findings suggest that CD44 may confer a growth advantage on some neoplastic cells and, therefore, could be used as a target for cancer therapy. It is hoped that identification of CD44 variants expressed on cancer but not on normal cells will lead to the development of anti-CD44 reagents restricted to the neoplastic growth.

Regulation of leukocyte integrin function: affinity vs. avidity

Leukocytes circulate freely in the bloodstream until receiving signals which activate adhesive mechanisms essential for immune responsiveness. Key mediators of these adhesion events are heterodimeric cell surface receptors called integrins. It is now apparent that several components may contribute to successful integrin-mediated adhesion: alterations in individual receptors lead to enhanced affinity for ligand; integrin clustering causes an increase in avidity; by spreading, the adhering cell is less susceptible to shear force. Model systems have allowed us to examine the contribution of each of these factors in generating adhesion. In more physiologically relevant situations, it can now be questioned whether integrin-mediated adhesion is regulated via alterations in receptor affinity or avidity, or whether both these mechanisms are involved.