Integrin signaling and cell growth control

Involvement of alphavbeta 3 integrin and disruption of endothelial fibronectin network during the adhesion of the human ovarian adenocarcinoma cell line IGROV1 on the human umbilical vein cell extracellular matrix

Like the majority of tumor cells, ovarian cancer cell growth is critically dependent on their neovascularization. Adhesion molecules and cellular events that lead to ovarian tumor cell interactions with endothelial extracellular matrix surrounding the vasculature are poorly identified. To understand the role of alphavbeta3 integrin and its ligand fibronectin in this process, we used in vitro coculture models with IGROV1 human ovarian adenocarcinoma cell line and human umbilical vein endothelial cells (HUVEC). Adhesion assays revealed a strong ability of IGROV1 cells to adhere to HUVEC-ECM. alphavbeta3 is mainly implicated and seems to cooperate with alpha5beta1 integrin in this event. Immunofluorescence staining revealed the presence of alphavbeta3 and alpha5beta1 in IGROV1 cells adhering on HUVEC-ECM at regions of cell sub-stratum contacts. Furthermore, our data showed the absence of fibronectin staining in IGROV1 cells and the disruption of the HUVEC-ECM fibrillar fibronectin network under IGROV1 cell influence. In situ experiments in ovarian neoplastic tissue corroborated the absence of fibronectin in the tumor and its strong detection in vasculature. These findings suggest the active participation of alphavbeta3 and alpha5beta1 integrins and the reorganization of endothelial fibronectin during the adhesion of IGROV1 cells to HUVEC-ECM whereas IGROV1 cells seem to be unable to synthesize fibronectin. Thus, fibronectin integrin receptors expressed by ovarian tumor cells and endothelial fibronectin may be of importance in ovarian carcinoma neovascularization and during tumor-vasculature interactions.

Protein and surface effects on monocyte and macrophage adhesion, maturation, and survival

Cell adhesion and maturation can be affected by the protein adsorption profile on the surface of an implanted biomaterial. In this study we have investigated how surface chemistry and adsorbed proteins can modulate monocyte and macrophage adhesion, IL-13-induced foreign-body giant cell formation, and apoptosis in vitro. Compared to a dimethylsilane-modified surface (DM), a surface modified with RGD peptides had no effect on adhesion density, foreign-body giant cell (FBGC) formation, or apoptosis in nondepleted serum conditions. The depletion of specific adhesive proteins affected adhesion, FBGC formation, and apo- ptosis. While the depletion of fibronectin and vitronectin had no overall effect compared to nondepleted serum conditions, the depletion of IgG from serum caused a significant decrease in initial adherent cell density [1000 +/- 200 compared to 2460 +/- 590 (p = 0.02)], a significant decrease in FBGC formation [2% compared to 17% (p = 0.02)], and a significant increase in the level of apoptosis [57% compared to 32% (p = 0.01)] on DM. The lowered initial adherent cell density on DM was not observed on the RGD surface, indicating that the RGD surface promotes increased initial adhesion. However, the RGD surface does not affect FBGC formation (i.e., macrophage fusion) or levels of apoptosis, which remained comparable to those on the DM surfaces at days 7 and 10.

Microfilament-dependent movement of the beta3 integrin subunit within focal contacts of endothelial cells

To gain insight into the dynamic properties of focal contacts, we induced expression of green fluorescent protein-tagged beta3 integrin (GFP-beta3) and actinin-1 (GFP-actinin-1) in endothelial cells. Both tagged proteins localize with alpha(v)beta3 integrin in focal contacts distributed towards the periphery of transfected cells. Labeled focal contacts migrate at about 0.1 mm/min in stationary live endothelial cells. We compared beta3 integrin and actinin-1 dynamics in focal contacts by using fluorescence recovery after photobleaching. Recovery of signal in bleached focal contacts that have incorporated actinin-1 is rapid and occurs within less than 4 min. This recovery is energy-dependent. In contrast, recovery of bleached focal contacts that contain GFP-beta3 integrin takes longer than 30 min. Yet, when a narrow stripe of fluorescence is bleached across a beta3 integrin-labeled focal contact, recovery is complete within 16 min. The latter recovery is energy-dependent and is blocked not only by actin-filament disrupting drugs but also by a myosin light chain kinase inhibitor. Thus, integrins are not immobile when incorporated into focal contacts, as some have suggested. We propose that integrins are mobile within the confines of focal contacts and that this mobility is supported by an actin-associated molecular motor.

Proteolytic events of wound-healing--coordinated interactions among matrix metalloproteinases (MMPs), integrins, and extracellular matrix molecules

During wound-healing, cells are required to migrate rapidly into the wound site via a proteolytically generated pathway in the provisional matrix, to produce new extracellular matrix, and, subsequently, to remodel the newly formed tissue matrix during the maturation phase. Two classes of molecules cooperate closely to achieve this goal, namely, the matrix adhesion and signaling receptors, the integrins, and matrix-degrading and -processing enzymes, the matrix metalloproteinases (MMPs). There is now substantial experimental evidence that blocking key molecules of either group will prevent or seriously delay wound-healing. It has been known for some time now that cell adhesion by means of the integrins regulates the expression of MMPs. In addition, certain MMPs can bind to integrins or other receptors on the cell surface involved in enzyme activation, thereby providing a mechanism for localized matrix degradation. By proteolytically modifying the existing matrix molecules, the MMPs can then induce changes in cell behavior and function from a state of rest to migration. During wound repair, the expression of integrins and MMPs is simultaneously up-regulated. This review will focus on those aspects of the extensive knowledge of fibroblast and keratinocyte MMPs and integrins in biological processes that relate to wound-healing.

Rac regulates vascular endothelial growth factor stimulated motility

During angiogenesis endothelial cells migrate towards a chemotactic stimulus. Understanding the mechanism of endothelial cell migration is critical to the therapeutic manipulation of angiogenesis and ultimately cancer prevention. Vascular endothelial growth factor (VEGF) is a potent chemotactic stimulus of endothelial cells during angiogenesis. The endothelial cell signal transduction pathway of VEGF represents a potential target for cancer therapy, but the mechanisms of post-receptor signal transduction including the roles of rho family GTPases in regulating the cytoskeletal effects of VEGF in endothelial cells are not understood. Here we analyze the mechanisms of cell migration in the mouse brain endothelial cell line (bEND3). Stable transfectants containing a tetracycline repressible expression vector were used to induce expression of Rac mutants. Endothelial cell haptotaxis was stimulated by constitutively active V12Rac on collagen and vitronectin coated supports, and chemotaxis was further stimulated by VEGF. Osteopontin coated supports were the most stimulatory to bEND3 haptotaxis, but VEGF was not effective in further increasing migration on osteopontin coated supports. Haptotaxis on support coated with collagen, vitronectin, and to a lesser degree osteopontin was inhibited by N17 Rac. N17 Rac expression blocked stimulation of endothelial cell chemotaxis by VEGF. As part of the chemotactic stimulation, VEGF caused a loss of actin organization at areas of cell-cell contact and increased stress fiber expression in endothelial cells which were directed towards pores in the transwell membrane. N17 Rac prevented the stimulation of cell-cell contact disruption and the stress fiber stimulation by VEGF. These data demonstrate two pathways of regulating endothelial cell motility, one in which Rac is activated by matrix/integrin stimulation and is a crucial modulator of endothelial cell haptotaxis. The other pathway, in the presence of osteopontin, is Rac independent. VEGF stimulated chemotaxis, is critically dependent on Rac activation. Osteopontin was a potent matrix activator of motility, and perhaps one explanation for the absence of a VEGF plus osteopontin effect is that osteopontin stimulated motility was inhibitory to the Rac pathway.

Phagocyte spreading and phagocytosis in the compound ascidian Botryllus schlosseri: evidence for an integrin-like, RGD-dependent recognition mechanism

The involvement of integrins in phagocyte spreading and phagocytosis was investigated in the compound ascidian Botryllus schlosseri. The number of spreading cells was significantly reduced when adhesion occurred in the presence of the tetrapeptide Arg--Gly--Asp--Ser (RGDS), but not of Arg--Gly--Glu--Ser (RGES) indicating the involvement of RGD-mediated adhesion mechanisms in phagocyte spreading. The significant decrease of the fraction of spreading cells in the presence of Botryllus blood plasma suggests the presence of RGD-containing molecules in the blood of our species. The increase in the same index when blood plasma-coated slides as well as fibrinogen- and fibronectin-coated coverslips were used, fits with the above hypothesis. Adhesion in the presence of RGDS leads to a consistent alteration of the actin cytoskeleton, in agreement with the known role of integrin adhesion in microfilament organization. Phagocytosis was greatly reduced by RGDS in the incubation medium, but not by RGES, and was significantly increased by coating yeast cells with fibronectin or blood plasma. Both spreading and phagocytic capability were severely inhibited by wortmannin, suggesting the importance of phosphatidylinositol-3-kinase in integrin-mediated signal transduction in ascidians.

Analysis of luteal tissue inhibitor of metalloproteinase-1, -2, and -3 during prostaglandin F(2alpha)-induced luteolysis

Increased matrix metalloproteinase (MMP) expression and activities help to mediate tissue involution through increasing extracellular matrix remodeling and promoting dedifferentiation and, ultimately, apoptosis. Therefore, we hypothesized that prostaglandin (PG) F(2alpha) administration would decrease expression of the tissue inhibitor of metalloproteinase (TIMP)-1, -2, and -3 and effectively increase the MMP:TIMP ratio, leading to glandular involution. In experiment 1, we tested the effects of PGF(2alpha) administration (Day 10 postestrus; Day 0 = estrus) on luteal TIMP-1, -2, and -3 mRNA and protein expression. Corpora lutea were collected at 0, 15, or 30 min or at 1, 2, 4, 6, 12, 24, and 48 h following PGF(2alpha) administration (n = 3-9 animals/time point). Following PGF(2alpha) administration, TIMP-1 mRNA levels decreased (P < 0.05) at 1 and 2 h relative to 0 h (controls), then increased to levels greater than controls at 4 and 6 h. In contrast, TIMP-2 and -3 mRNA levels did not decrease following PGF(2alpha) administration. The TIMP-1, -2, and -3 proteins were localized to large luteal cells (LLCs) within control (untreated) tissues. However, histodepletion of TIMP-1 within LLCs was evident within 30 min (earliest time point collected) following PGF(2alpha) injection and continued through 48 h. Luteal concentration of TIMP-1, as determined by RIA, was decreased (P < 0.05) by 15 min (earliest time point collected) following PGF(2alpha) administration and remained low through 48 h. In contrast, TIMP-2 and -3 immunolocalization was not altered by PGF(2alpha) administration. Experiment 2 was conducted to determine if PGF(2alpha) could initiate the preceding changes in TIMP-1 in early (Day 3) corpora lutea that can bind PGF(2alpha) but are refractory to its luteolytic effects. Serum concentrations of progesterone and luteal concentrations of TIMP-1 mRNA and protein were similar at 0 and 6 h after PGF(2alpha) injection on Day 3 postestrus. These data suggest that an early and sustained effect of PGF(2alpha) is the specific depletion of TIMP-1 within LLCs that are capable of responding to the luteolytic action of PGF(2alpha). This action may increase the MMP:TIMP-1 ratio, creating an environment that favors extracellular matrix degradation and, thereby, facilitates both functional and structural regression.

The v-erbA oncogene blocks expression of alpha2/beta1 integrin a normal inhibitor of erythroid progenitor proliferation

T2EC are chicken erythrocytic progenitors that balance between self-renewal and differentiation as a function of response to specific growth factors. Their transformation by the v-erbA oncogene locks them into the self-renewal program. We show here that the expression of the VLA-2 integrin alpha2 subunit mRNA is downregulated by v-erbA and that VLA-2 engagement and clustering, brought about by treatment with an alpha2-specific antibody or by culture on the VLA-2 ligand collagen I, inhibits T2EC proliferation. From competition studies using antibodies, VLA-2 was shown to be involved in the collagen-induced response. While engagement of VLA-2 inhibited proliferation, it was not sufficient to induce differentiation. The transformation of T2EC by v-erbA decreased their interaction with collagen I and the VLA-2 brake on cell proliferation, which may account for the increased proliferation potential of transformed erythrocytic progenitors and for their shedding into the blood of infected chickens. Our data suggest that the interaction between erythroid progenitors and collagen, mediated by VLA-2, play a major role in the control of erythropoiesis in vitro and that this pathway is a target of the v-erbA oncogene.

beta1 and beta2 integrins activate different signalling pathways in monocytes

Integrin-mediated signals play an important but poorly understood role in regulating many leucocyte functions. In monocytes and macrophages, integrins of the beta2 subfamily are involved in cell-cell interactions that are important for migration of the cells through the endothelium and also for phagocytosis. On the other hand, in the same cells, beta1 integrin-mediated adhesion to extracellular matrix proteins results in a strong induction of immediate early genes that are important in inflammation. To investigate the signalling pathways from these two types of integrin in monocytic cells, THP-1 cells were selectively stimulated via beta1 or beta2 integrins by cross-linking each type of receptor with specific monoclonal antibodies or their natural ligands. The involvement of extracellular signal-regulated kinase (ERK), Syk and phosphoinositide 3-kinase (PI-3K) was then analysed. Nuclear factor kappaB (NF-kappaB) activation was also detected in THP-1 cells transiently transfected with an NF-kappaB-driven luciferase reporter gene. We found that binding of both types of integrin to their natural ligands activated ERK in a Syk- and PI-3K-dependent manner. Yet, cross-linking of integrins by anti-beta1 antibodies caused activation of ERK while that by anti-beta2 antibodies did not. Also both types of integrin activated NF-kappaB. However, PI-3K was required for beta1 integrin-, but not beta2 integrin-, mediated NF-kappaB activation. In addition, inhibition of PI-3K with wortmannin and LY294002 blocked beta1 integrin-mediated NF-kappaB activation, but did not affect that mediated by beta2 integrin. These data suggest that distinct integrins activate different signalling pathways in monocytic cells.

Induction of cell death in T lymphocytes by invasin via beta1-integrin

Ligand binding to beta1-integrins exerts multiple effects on cells of the immune system including adhesion, spreading, haptotaxis and costimulation of T cells activated by anti-CD3. Here we show that a high-affinity ligand for beta1-integrins, the invasin (Inv) protein of Yersinia pseudotuberculosis, can induce cell death in T lymphocytes via a rapid process. Partially purified native Inv protein and an Inv fusion protein caused apoptotic/necrotic caspase-independent cell death in T lymphocytes as determined by phosphatidylserine exposure on the cell surface, uptake of propidium iodide, labeling of DNA strand breaks and presence of DNA ladder. Inv-induced cell death was mediated via beta1-integrins as indicated by the fact that Inv bound to the beta1-integrin subunit (CD29), that anti-beta(1)-integrin antibodies blocked Inv-induced cell death and that Inv-induced cell death was absent in two beta1-integrin- cell lines produced by different procedures. Killing via beta1-integrins represents a novel pathway for cell death in T lymphocytes.

Matrix metalloproteinase (2, 9, and 14) expression, localization, and activity in ovine corpora lutea throughout the estrous cycle

Members of the matrix metalloproteinase (MMP) family collectively degrade extracellular matrix (ECM) and help regulate luteal function. The objectives of these experiments were to characterize the mRNA expression, localization, and activity of MMPs 2, 9, and 14 in ovine corpora lutea (CL). Ovine CL were collected on Days 2, 4, 10, and 15 of the estrous cycle (Day 0 = estrus). Messenger RNA transcripts for MMPs 2 and 14 were detected using Northern analysis; however, expression of MMP-9 was undetectable. Expression of MMP-14 mRNA (membrane type-1 MMP) was increased (P < 0.05) on Day 4; whereas, expression of MMP-2 mRNA was highest (P < 0.05) on Day 10, which corresponded to the observed increases in gelatinolytic activity in luteal homogenates as measured by a fluroscein-labeled gelatin substrate assay. MMP 2 and 9 proteins were localized predominantly to large luteal cells (LLCs), whereas MMP-14 was localized primarily to cells other than LLCs as demonstrated by immunohistochemistry. Immunolocalization of MMP-2 to putative endothelial cells was also observed on Day 15. Localization of MMP activity was determined using in situ zymography. Luteal tissues contained gelatinolytic activity primarily localized pericellularly to various cell types, including LLCs. These results support the hypothesis that ECM remodeling occurs throughout the luteal phase and may help potentiate cellular migration, differentiation, angiogenesis, and growth factor bioavailability.

Enteropathogenic Escherichia coli induces modification of the focal adhesions of infected host cells

Enteropathogenic Escherichia coli (EPEC) is a human-specific pathogen that causes severe diarrhoea in young children. The disease involves intimate interaction between the pathogen and the brush border of enterocytes. During infection, EPEC uses a type III secretion system (TTSS) to inject several proteins into the infected cells, and these effector proteins modify specific processes in the host cell. We show that, upon infection, EPEC induces detachment of the infected host cells from the substratum, modification of focal adhesions (FA) in the infected cells and specific dephosphorylation of focal adhesion kinase (FAK). We also show that EPEC-induced cell detachment is dependent on FAK expression by the infected cells. Finally, we demonstrate that cell detachment, FA modification and FAK dephosphorylation are dependent on functional TTSS in the infecting EPEC. These results suggest that EPEC is using its TTSS to inject protein(s) into the infected cells, which can induce FAK dephosphorylation, as well as FAK-dependent FA modification and cell detachment. These processes are specific and probably play an important role in EPEC virulence.

Disruption of C-terminal cytoplasmic domain of betaPS integrin subunit has dominant negative properties in developing Drosophila

We have analyzed a set of new and existing strong mutations in the myospheroid gene, which encodes the betaPS integrin subunit of Drosophila. In addition to missense and other null mutations, three mutants behave as antimorphic alleles, indicative of dominant negative properties. Unlike null alleles, the three antimorphic mutants are synthetically lethal in double heterozygotes with an inflated (alphaPS2) null allele, and they fail to complement very weak, otherwise viable alleles of myospheroid. Two of the antimorphs result from identical splice site lesions, which create a frameshift in the C-terminal half of the cytoplasmic domain of betaPS. The third antimorphic mutation is caused by a stop codon just before the cytoplasmic splice site. These mutant betaPS proteins can support cell spreading in culture, especially under conditions that appear to promote integrin activation. Analyses of developing animals indicate that the dominant negative properties are not a result of inefficient surface expression, or simple competition between functional and nonfunctional proteins. These data indicate that mutations disrupting the C-terminal cytoplasmic domain of integrin beta subunits can have dominant negative effects in situ, at normal levels of expression, and that this property does not necessarily depend on a specific new protein sequence or structure. The results are discussed with respect to similar vertebrate beta subunit cytoplasmic mutations.

Integrin-induced epidermal growth factor (EGF) receptor activation requires c-Src and p130Cas and leads to phosphorylation of specific EGF receptor tyrosines

Integrin-mediated cell adhesion cooperates with growth factor receptors in the control of cell proliferation, cell survival, and cell migration. One mechanism to explain these synergistic effects is the ability of integrins to induce phosphorylation of growth factor receptors, for instance the epidermal growth factor (EGF) receptor. Here we define some aspects of the molecular mechanisms regulating integrin-dependent EGF receptor phosphorylation. We show that in the early phases of cell adhesion integrins associate with EGF receptors on the cell membrane in a macromolecular complex including the adaptor protein p130Cas and the c-Src kinase, the latter being required for adhesion-dependent assembly of the macromolecular complex. We also show that the integrin cytoplasmic tail, c-Src kinase, and the p130Cas adaptor protein are required for phosphorylation of EGF receptor in response to integrin-mediated adhesion. We show that integrins induce phosphorylation of EGF receptor on tyrosine residues 845, 1068, 1086, and 1173, but not on residue 1148, a major site of phosphorylation in response to EGF. In addition we find that integrin-mediated adhesion increases the amount of EGF receptor expressed on the cell surface. Therefore these data indicate that integrin-mediated adhesion induces assembly of a macromolecular complex containing c-Src and p130Cas and leads to phosphorylation of specific EGF receptor tyrosine residues.

Differential integrin expression by cardiac fibroblasts from hypertensive and exercise-trained rat hearts

The cardiac fibroblast is the principal cell type responsible for extracellular matrix (ECM) synthesis in the heart during growth and pathophysiological conditions. A dynamic interaction exists between the cardiac ECM and fibroblasts that is sensitive to the local mechanical and chemical tissue environment. We propose here that cardiac fibroblasts structurally and functionally adapt to changing local environments by altering their expression of receptor integrins. Changes in the extracellular environment are communicated in part by integrins, which link the ECM to the cell and regulate phenotype and function. In this report, we analyze integrin protein expression, migration and gel contraction by cardiac fibroblasts from rats subjected to 10 weeks of treadmill exercise (XTR), experimental hypertension (HYP) or controls (CONT). Immunoprecipitation shows that beta1 protein increases in XTR and HYP. Also, alpha1 and alpha2 integrins are lower in XTR and HYP, and alpha5 integrin is higher in XTR and lower in HYP. Functional assays show that XTR and HYP migrate slower on collagen, while XTR migrate faster and HYP slower on fibronectin. Cell isolation procedure, population expansion number or a general adaptation to culture conditions does not explain the differences observed. No significant differences in collagen gel contraction are detected. These results indicate that cardiac fibroblasts retain their in vivo patterns in vitro for a limited number of population expansions. This tissue-specific phenotype is exhibited in early passage (< or =6). However, by late passage (>8), cells begin to show adaptation to the in vitro conditions. These results show that cardiac fibroblasts respond to changing environments in pathophysiological conditions by modulating integrin expression, which is associated with changes in cell migration. They also suggest a pragmatic use for primary cardiac fibroblasts as a model to study the cardiac matrix remodeled by physiological (exercise) and pathological (hypertension) stressors.

Effect of protein kinase inhibitors on the stretch-elicited c-Fos and c-Jun up-regulation in human PDL osteoblast-like cells

Osteoblastic cells transduce signals of mechanical loading that plays a key role in maintaining bone formation. In an attempt to elucidate the biochemical events associated with the conversion of mechanical stress to biological outcome, we examined cultured human periodontal ligament (hPDL) osteoblastic cells exposed to continuous stretch, in terms of cellular parameters correlating known signaling cascades to the initial phase of osteoblast-specific transcriptional control. Time-course experiments revealed that mechanical stretch-loaded hPDL cells exhibit a very rapid and relatively sustained increase in the abundance of the immediate-early gene products, c-Fos and c-Jun, components of the activator protein-1 (AP-1) transcription factor. Moreover, this increase in protein levels was accompanied by hyperphosphorylation and thereby potentiation of c-Jun, the principal modulator of AP-1 activity. Importantly, these inductive effects were partly or completely abolished by pre-incubating the cells with SB 203580, PD 098059, and the novel compound Y-27632, inhibitors of p38 mitogen-activated protein kinase (MAPK), MAPK kinase (MEK), and Rho-associated protein kinase (RhoK), respectively. These results consolidate AP-1 as the pivotal downstream effector in the early response of hPDL cells to continuous mechanical stretching, via the coordinate stimulation of de novo synthesis and post-translational regulation of AP-1 proteins. This "integrating" function of AP-1 is mediated through a mechanotransduction circuit that incorporates elements of well-defined upstream signaling protein kinase systems.

Cloning and characterization of human syndecan-3

Syndecans are cell-surface heparan sulfate proteoglycans, which perform a variety of functions in the cell. Most important, they are co-receptors for growth factors and mediate cell-cell and cell-matrix interactions. Four syndecans (syndecan 1-4) have been described in different species. The aim of this work was the cloning and characterization of human syndecan-3. The human syndecan-3 sequence has high homology to the rat and mouse sequences, with the exception of the 5'-region. Syndecan-3 mRNA is mostly expressed in the nervous system, the adrenal gland, and the spleen. When different cell lines were transiently transfected with full-length syndecan-3 cDNA, it was localized to the membrane and induced the formation of long filopodia-like structures, microspikes, and varicosities. Consequently, the actin cytoskeleton was re-organized, since actin staining was mostly found in the cellular extensions and at the cell periphery, co-localizing with the syndecan-3 staining. The development of the phenotype depended on the presence of sugar chains, as transfected glycosaminoglycan-deficient Chinese hamster ovary (CHO) 745 cells did not show these structural changes, nor did transfected CHO K1 cells in the presence of heparin. The similarity of the cloned DNA sequence with that of other mammalian species and the high expression in the nervous system led us to the assumption that human syndecan-3 could perform comparable functions to those described for syndecan-3 in rat and mouse. Additionally, transient transfection experiments suggest a role of human syndecan-3 in the organization of cell shape by affecting the actin cytoskeleton, possibly by transferring signals from the cell surface in a sugar-dependent mechanism.

Phospholipase Cgamma binds alpha1beta1 integrin and modulates alpha1beta1 integrin-specific adhesion

Integrin adhesion receptors have been implicated in bidirectional signal transduction. The dynamic regulation of integrin affinity and avidity as well as post-ligand effects involved in outside-in signaling depends on the interaction of integrins with cytoskeletal and signaling proteins. In this study, we attempted to identify cytoplasmic binding partners of alpha(1)beta(1) integrin. We were able to show that cell adhesion to alpha(1)beta(1)-specific substrates results in the association of phospholipase Cgamma (PLCgamma) with the alpha(1)beta(1) integrin independent of PLCgamma tyrosine phosphorylation. Using peptide-binding assays, the membrane proximal sequences within the alpha(1)beta(1) integrin subunits were identified as binding sites for PLCgamma. In particular, the conserved sequence of beta(1) subunit binds the enzyme very efficiently. Because purified PLCgamma also binds the integrin peptides, binding seems to be direct. Inhibition of PLC by leads to reduced cell adhesion on alpha(1)beta(1)-specific substrates. Cells lacking the conserved domain of the alpha(1) subunit fail to respond to the PLC inhibition, indicating that this domain is necessary for PLC-dependent adhesion modulation of alpha(1)beta(1) integrin.

A comparative study of primary and immortalized cell adhesion characteristics to modified polymer surfaces: toward the goal of effective re-epithelialization

Facilitating tissue regeneration or replacement requires development of synthetic surfaces that promote cell adhesion, migration, and proliferation. Two successful approaches have been to incorporate minimal cell adhesion recognition sequences at the biomaterial surface and to integrate the entire adhesion molecule into a compatible synthetic matrix. While adhesion assays using immortalized cell lines are important in evaluating synthetic materials, cell type and source play a significant role in the ability of such models to mimic real tissues. Models that utilize multiple cell types or primary cells are more representative of native tissues than models that use single cell types or primary cells. In this study we investigated primary respiratory epithelial cell (REC) adhesion to modified fluoropolymers incorporating simple functional groups and minimal peptide recognition sequences, and we evaluated the potential of hybrid biopolymer materials to support adhesion and proliferation. X-ray photoelectron spectroscopy (XPS) was used to verify substrate surface composition. Significant differences were found in the adhesion characteristics of primary REC and in the A549 lung carcinoma cell line. Model systems composed of multiple cell types and/or primary cells necessarily represent increased levels of complexity for an investigation of cellular responses to synthetic surfaces. When evaluating biomaterials, adhesion studies using immortalized cell lines cannot necessarily be extrapolated to normal cell behavior.

Integrin-linked kinase, a promising cancer therapeutic target: biochemical and biological properties

Integrin-linked kinase (ILK) is an ankyrin repeat-containing Ser/Thr kinase that interacts with the cytoplasmic domains of beta(1) and beta(3) integrins. ILK is widely expressed in tissues throughout the body, and, as might be expected, appears to mediate a diversity of functions relating to its role in coupling integrins and growth factor receptors to downstream signaling pathways. Through its downstream targets protein kinase B/Akt and glycogen synthase kinase-3beta, ILK appears to be involved in several oncogenesis-related events, including suppression of apoptosis and promotion of cell survival, as well as cell migration and invasion. Over-expression of ILK in epithelial cells results in anchorage-independent cell growth with increased cell cycle progression. Inoculation of nude mice with ILK over-expressing cells leads to tumor formation. Furthermore, increased ILK expression and activity have been correlated with malignancy in several human tumor types, including breast, prostate, brain, and colon carcinomas. Based on these findings, ILK represents an excellent therapeutic target for the prevention of tumor progression. Here, we provide an overview of the physical and biochemical properties of ILK, and present data describing the impact of small-molecule ILK inhibitors on several ILK-mediated cellular functions.

Rap1 functions as a key regulator of T-cell and antigen-presenting cell interactions and modulates T-cell responses

Activation of T cells by antigen requires adhesive interactions with antigen-presenting cells (APC) in which leukocyte function-associated antigen 1 (LFA-1) and intercellular adhesion molecules (ICAMs) are important. However, it is not well understood what signaling molecules regulate this process and how the modulation of adhesive events influences T-cell activation. Here we show that Rap1 is activated in T cells in an antigen-dependent manner and accumulated at the contact site of T-cell and antigen-loaded APC. Inhibition of Rap1 activation by a dominant-negative Rap1 or SPA-1, a Rap1 GTPase-activating protein, abrogates LFA-1-ICAM-1-mediated adhesive interactions with antigen-pulsed APC and the subsequent T-cell-receptor triggering and interleukin-2 production. Conversely, augmented antigen-dependent Rap1 activation by the expression of wild-type Rap1 enhances these responses but culminates in apoptosis by Fas and FasL. Thus, Rap1 functions as a key regulator of T-cell and APC interactions and modulates T-cell responses from productive activation to activation-induced cell death by regulating the strength of adhesive interactions. Moreover, constitutive Rap1 activation rendered T cells unresponsive with accumulation of p27(Kip1). Our study indicates that the activation state of Rap1 has a decisive effect on the T-cell response to antigen.

Gonadotropin stimulation of MLS human epithelial ovarian carcinoma cells augments cell adhesion mediated by CD44 and by alpha(v)-integrin

OBJECTIVE

The goal of this work was to evaluate the involvement of gonadotropins in the regulation of adhesion of human epithelial ovarian carcinoma. We studied two pathways that were previously implicated in the metastatic implantation of ovarian carcinoma to the peritoneum, namely hyaluronan-CD44 and RGD-integrin mediated adhesion.

METHODS

Two cell lines derived from human epithelial ovarian carcinoma (MLS and OC238) were stimulated with luteinizing hormone (LH) and/or follicle stimulating hormone (FSH). Expression of CD44 was evaluated by Western blotting. Expression of alpha(v)-integrins was studied by RT-PCR and Northern blot. Integrin and CD44 mediated adhesion of the cells was analyzed using culture plates coated either with a thrombin derived RGD containing peptide or fibronectin for integrin mediated adhesion or with hyaluronan for CD44 mediated adhesion.

RESULTS

MLS cells stimulated with either LH or FSH showed increased adhesion to culture plates coated with hyaluronan, as well as to culture plates coated with fibronectin or with a thrombin derived RGD containing peptide. In these cells, gonadotropin stimulation led to induced expression of the integrin subunit alpha(v) and CD44, the cell surface hyaluronan receptor. On the other hand, OC238 cells showed no expression of the integrin subunit alpha(v) and no hormonal effect on the expression of CD44. Accordingly, adhesion of OC238 cells on either RGD or CD44 was not affected by hormonal stimulation.

CONCLUSION

Elevated levels of gonadotropins may in some cases facilitate peritoneal metastatic dissemination of ovarian cancer by increasing cell adhesion, the first essential step in the invasion process.

Microfilament disruption in a noncycling organized tissue, the corneal endothelium, initiates mitosis

The adult corneal endothelium represents a noncycling cell population that resides as a monolayer on its basement membrane, Descemet's membrane. Evidence is presented for the first time, showing that mitotic regulation in this organized tissue, residing on its natural basement membrane, is coupled to microfilament integrity. When mitotically quiescent rat corneal endothelia are organ cultured in medium containing serum and cytochalasin B, low levels of mitosis are initiated. Supplementing the culture medium with either insulin or IGF-2 augments this response and results in increased cell density within the tissue monolayer. Fluorescence microscopy of actin using TRITC-conjugated phalloidin revealed that cellular circumferential microfilament bundles appear unaffected by cytochalasin B treatment, whereas the cytoplasmic microfilaments appear to be completely disrupted. These results suggest the possibility that the actin cytoskeleton is involved with the regulation of cell growth in the corneal endothelium.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Cell Junction Dynamics in the Testis: Sertoli-Germ Cell Interactions and Male Contraceptive Development

Spermatogenesis is an intriguing but complicated biological process. However, many studies since the 1960s have focused either on the hormonal events of the hypothalamus-pituitary-testicular axis or morphological events that take place in the seminiferous epithelium. Recent advances in biochemistry, cell biology, and molecular biology have shifted attention to understanding some of the key events that regulate spermatogenesis, such as germ cell apoptosis, cell cycle regulation, Sertoli-germ cell communication, and junction dynamics. In this review, we discuss the physiology and biology of junction dynamics in the testis, in particular how these events affect interactions of Sertoli and germ cells in the seminiferous epithelium behind the blood-testis barrier. We also discuss how these events regulate the opening and closing of the blood-testis barrier to permit the timely passage of preleptotene and leptotene spermatocytes across the blood-testis barrier. This is physiologically important since developing germ cells must translocate across the blood-testis barrier as well as traverse the seminiferous epithelium during their development. We also discuss several available in vitro and in vivo models that can be used to study Sertoli-germ cell anchoring junctions and Sertoli-Sertoli tight junctions. An in-depth survey in this subject has also identified several potential targets to be tackled to perturb spermatogenesis, which will likely lead to the development of novel male contraceptives.

The role of proteases in fibronectin matrix remodeling in thyroid epithelial cell monolayer cultures

Fischer rat thyroid (FRT) cells organize a matrix of extracellular fibronectin (FN) fibrils, which undergoes extensive remodeling according to cell culture confluence. In non-confluent cells FN forms a fibrillar array associated with the ventral cell surface. However, basal FN is progressively removed in confluent cultures and substituted by non-fibrillar FN deposits at lateral cell domains in regions of cell-cell contacts. FRT cells secrete and expose on the plasma membrane the tissue-type plasminogen activator and, in serum-free cultures, plasminogen induces a rapid loss of FN fibrils. Incubation with plasmin inhibitors greatly reduces this effect. FRT cells also express annexin II, a plasminogen receptor, suggesting that plasmin activity is associated with the pericellular enviroment. This is in agreement with the observation that a great reduction in FN degradation is observed if the cells are pre-incubated with carboxypeptidase B, which prevents plasminogen binding to the cells. A gelatinolytic activity with a molecular weigth equivalent to MMP-2 has been demonstrated by zymography of culture media, and the presence of MMP-2 and MT1-MMP on the cell plasma membrane has been detected by immunofluorescence. These results indicate that in the FN remodeling process, occurring during FRT epithelium maturation, both plasmin-dependent (tPA activated) and plasmin-independent proteolytic activities are involved.

Molecular biology of nervous system tumors

Many genetic alterations that contribute to CNS tumorigenesis and progression have been identified. One goal of such studies is to identify loci that would serve as diagnostic prognostic markers or both. A significant advance is the observation that chromosome 1p loss identified anaplastic oligodendroglioma and a subset of high-grade glioma patients who responded to chemotherapy and had longer survival times. Combined 1p and 19q loss was a predictor of prolonged survival of patients having pure oligodendrogliomas. Such markers eventually may be used to identify patients upfront who would benefit from treatment, while sparing patients who would not benefit. Although many molecular participants involved in the biologic pathways that promote proliferation, angiogenesis, and invasion have been elucidated, there are still many gaps in clinicians' knowledge. It is expected that the use of the human genome project information and databases such as SAGEmap, in combination with techniques such as cDNA arrays and proteomics, will facilitate greatly the identification of novel genes that contribute to CNS tumors. cDNA arrays and tissue arrays will permit the construction of CNS-specific screening tools that will permit the identification of tumor-specific mutations and alterations so that patient-specific therapies can be designed.

Angiotensin receptors: signaling, vascular pathophysiology, and interactions with ceramide

Angiotensin II (ANG II) is a pleiotropic vasoactive peptide that binds to two distinct receptors: the ANG II type 1 (AT(1)) and type 2 (AT(2)) receptors. Activation of the renin-angiotensin system (RAS) results in vascular hypertrophy, vasoconstriction, salt and water retention, and hypertension. These effects are mediated predominantly by AT(1) receptors. Paradoxically, other ANG II-mediated effects, including cell death, vasodilation, and natriuresis, are mediated by AT(2) receptor activation. Our understanding of ANG II signaling mechanisms remains incomplete. AT(1) receptor activation triggers a variety of intracellular systems, including tyrosine kinase-induced protein phosphorylation, production of arachidonic acid metabolites, alteration of reactive oxidant species activities, and fluxes in intracellular Ca(2+) concentrations. AT(2) receptor activation leads to stimulation of bradykinin, nitric oxide production, and prostaglandin metabolism, which are, in large part, opposite to the effects of the AT(1) receptor. The signaling pathways of ANG II receptor activation are a focus of intense investigative effort. We critically appraise the literature on the signaling mechanisms whereby AT(1) and AT(2) receptors elicit their respective actions. We also consider the recently reported interaction between ANG II and ceramide, a lipid second messenger that mediates cytokine receptor activation. Finally, we discuss the potential physiological cross talk that may be operative between the angiotensin receptor subtypes in relation to health and cardiovascular disease. This may be clinically relevant, inasmuch as inhibitors of the RAS are increasingly used in treatment of hypertension and coronary heart disease, where activation of the RAS is recognized.

Characterization of the mouse integrin subunit alpha10 gene and comparison with its human homologue. Genomic structure, chromosomal localization and identification of splice variants

Alpha10beta1 is a collagen-binding integrin expressed by chondrocytes [Camper et al. (1998) J. Biol. Chem. 273: 20383-20389]. In the present study, the mouse alpha10 gene was isolated from a sCos1 SVJ library and the genomic structure and chromosomal localization was determined. The alpha10 gene consists of 30 translated exons spanning a region of approximately 18 kb genomic DNA. The sequences of all exon/intron borders follow the consensus "gt-ag" rule. A transcription start site, determined by primer extension analysis, was located 38 nucleotides upstream of the initiation ATG site. The 5' flanking region of the transcription start site lacked a TATA-box. The first exon contained, in addition to 38 untranslated nucleotides, the ATG translation start site and the major part of the signal peptide. The alpha10 gene was mapped to mouse chromosome 3 by fluorescence in situ hybridization and is the only integrin subunit localized to this chromosome. When we investigated the expression of alpha10 by PCR we found that both mouse and human articular chondrocytes express extracellular splice variants of the alpha10 subunit. In mouse, exon 26 was extended into the intron by 62 nt, generating a truncated alpha10-chain. In human, exon 25 consisted of 114 nt which were alternately spliced in or out.

Transforming growth factor beta1 (TGF-beta1) promotes endothelial cell survival during in vitro angiogenesis via an autocrine mechanism implicating TGF-alpha signaling

Mouse capillary endothelial cells (1G11 cell line) embedded in type I collagen gels undergo in vitro angiogenesis. Cells rapidly reorganize and form capillary-like structures when stimulated with serum. Transforming growth factor beta1 (TGF-beta1) alone can substitute for serum and induce cell survival and tubular network formation. This TGF-beta1-mediated angiogenic activity depends on phosphatidylinositol 3-kinase (PI3K) and p42/p44 mitogen-activated protein kinase (MAPK) signaling. We showed that specific inhibitors of either pathway (wortmannin, LY-294002, and PD-98059) all suppressed TGF-beta1-induced angiogenesis mainly by compromising cell survival. We established that TGF-beta1 stimulated the expression of TGF-alpha mRNA and protein, the tyrosine phosphorylation of a 170-kDa membrane protein representing the epidermal growth factor (EGF) receptor, and the delayed activation of PI3K/Akt and p42/p44 MAPK. Moreover, we showed that all these TGF-beta1-mediated signaling events, including tubular network formation, were suppressed by incubating TGF-beta1-stimulated endothelial cells with a soluble form of an EGF receptor (ErbB-1) or tyrphostin AG1478, a specific blocker of EGF receptor tyrosine kinase. Finally, addition of TGF-alpha alone poorly stimulated angiogenesis; however, by reducing cell death, it strongly potentiated the action of TGF-beta1. We therefore propose that TGF-beta1 promotes angiogenesis at least in part via the autocrine secretion of TGF-alpha, a cell survival growth factor, activating PI3K/Akt and p42/p44 MAPK.

Collagen VI deficiency affects the organization of fibronectin in the extracellular matrix of cultured fibroblasts

Fibronectin is one of the main components of the extracellular matrix and associates with a variety of other matrix molecules including collagens. We demonstrate that the absence of secreted type VI collagen in cultured primary fibroblasts affects the arrangement of fibronectin in the extracellular matrix. We observed a fine network of collagen VI filaments and fibronectin fibrils in the extracellular matrix of normal murine and human fibroblasts. The two microfibrillar systems did not colocalize, but were interconnected at some discrete sites which could be revealed by immunoelectron microscopy. Direct interaction between collagen VI and fibronectin was also demonstrated by far western assay. When primary fibroblasts from Col6a1 null mutant mice were cultured, collagen VI was not detected in the extracellular matrix and a different pattern of fibronectin organization was observed, with fibrils running parallel to the long axis of the cells. Similarly, an abnormal fibronectin deposition was observed in fibroblasts from a patient affected by Bethlem myopathy, where collagen VI secretion was drastically reduced. The same pattern was also observed in normal fibroblasts after in vivo perturbation of collagen VI-fibronectin interaction with the 3C4 anti-collagen VI monoclonal antibody. Competition experiments with soluble peptides indicated that the organization of fibronectin in the extracellular matrix was impaired by added soluble collagen VI, but not by its triple helical (pepsin-resistant) fragments. These results indicate that collagen VI mediates the three-dimensional organization of fibronectin in the extracellular matrix of cultured fibroblasts.

Activation of the extracellular signal-regulated protein kinase (ERK) cascade by membrane-type-1 matrix metalloproteinase (MT1-MMP)

The mechanisms underlying membrane-type-1 matrix metalloproteinase (MT1-MMP)-dependent induction of cell migration were investigated. Overexpression of MT1-MMP induced a marked increase in cell migration, this increase being dependent on the presence of the cytoplasmic domain of the protein. MT1-MMP-dependent migration was inhibited by a mitogen-activated protein kinase kinase 1 inhibitor, suggesting the involvement of the extracellular signal-regulated protein kinase (ERK) cascade in the induction of migration. Accordingly, MT1-MMP overexpression induced the activation of ERK, this process being also dependent on the presence of its cytoplasmic domain. MT1-MMP-induced activation of both migration and ERK required the catalytic activity of the enzyme as well as attachment of the cells to matrix proteins. The MT1-MMP-dependent activation of ERK was correlated with the activation of transcription through the serum response element, whereas other promoters were unaffected. Taken together, these results indicate that MT1-MMP trigger important changes in cellular signal transduction events, leading to cell migration and to gene transcription, and that these signals possibly originate from the cytoplasmic domain of the protein.

Regulation of nucleocytoplasmic trafficking by cell adhesion receptors and the cytoskeleton

It has become widely accepted that adhesion receptors can either directly activate, or significantly modulate, many of the signaling cascades initiated by circulating growth factors. An interesting recent development is the realization that adhesion receptors and their cytoskeletal partners can regulate the trafficking of signaling proteins between the cytoplasm and nucleus. Cell adhesion molecule control of nucleocytoplasmic trafficking allows adhesion to influence many cell decisions, and highlights the diversity of nuclear import and export mechanisms.

Regulation of human lung fibroblast phenotype and function by vitronectin and vitronectin integrins

Myofibroblasts, characterised by high expression of α-smooth muscle actin (α-SMA), are important and transient cells in normal wound healing but are found in increased number in various pathological conditions of the lung including asthma and pulmonary fibrosis. The mechanisms that regulate the myofibroblast phenotype are unknown but are likely to involve signals from the extracellular matrix transmitted via specific integrins. Vitronectin is a glycoprotein released during inflammation and has been shown to regulate the phenotype of vascular smooth muscle cells via αv and β1 integrins. In the current study we have examined whether vitronectin influences the phenotype and function of normal human lung fibroblasts (HFL-1). Incubation of HFL-1 cells with vitronectin induced a concentration-dependent reduction in α-SMA expression. By contrast, function-blocking monoclonal antibodies to the vitronectin integrins αv, β1, αvβ3 and αvβ5 induced the expression of α-SMA and its organization into stress fibers. Expression of α-SMA induced by all function-blocking monoclonal antibodies was abrogated by inhibition of protein kinase C and phosphatidylinositol-3 kinase, but the effects of inhibition of other signalling pathways was integrin dependent. Exposure to other extracellular matrix proteins such as fibronectin, collagen or their integrins did not influence expression of α-SMA. The expression and organization of α-SMA induced by exposure to function-blocking antibodies was translated into an augmented capacity of HFL-1 cells to contract fibroblast populated collagen gels. By contrast, contraction of collagen gels following incubation with vitronectin was not significantly different to control. This study has shown that vitronectin influences the phenotype and behaviour of HFL-1 cells by downregulating the expression of α-SMA and reducing their contractile ability. By contrast, occupancy of specific integrins by function-blocking antibodies upregulated the expression of α-SMA and induced the formation of functional stress fibers capable of contracting collagen gels. These results suggest that vitronectin modulates the fibroblast-myofibroblast phenotype, implying an important role in the remodelling process during lung development or response to injury.

Syndecan-4 and focal adhesion function

Two groups have now reported the viability of mice that lack syndecan-4. These mice have wound healing/angiogenesis problems, and fibroblasts from these animals differ in adhesion and migration from normal. This is consistent with recent in vitro data indicating a need for signaling via syndecan-4 for focal adhesion formation, and reports that overexpression of proteins that bind syndecan-4 can modify cell adhesion and migration.

Raf-MEK-Erk cascade in anoikis is controlled by Rac1 and Cdc42 via Akt

Signals from the extracellular matrix are essential for the survival of many cell types. Dominant-negative mutants of two members of Rho family GTPases, Rac1 and Cdc42, mimic the loss of anchorage in primary mouse fibroblasts and are potent inducers of apoptosis. This pathway of cell death requires the activation of both the p53 tumor suppressor and the extracellular signal-regulated mitogen-activated protein kinases (Erks). Here we characterize the proapoptotic Erk signal and show that it differs from the classically observed survival-promoting one by the intensity of the kinase activation. The disappearance of the GTP-bound forms of Rac1 and Cdc42 gives rise to proapoptotic, moderate activation of the Raf-MEK-Erk cascade via a signaling pathway involving the kinases phosphatidlyinositol 3-kinase and Akt. Moreover, concomitant activation of p53 and inhibition of Akt are both necessary and sufficient to signal anoikis in primary fibroblasts. Our data demonstrate that the GTPases of the Rho family control three major components of cellular signal transduction, namely, p53, Akt, and Erks, which collaborate in the induction of apoptosis due to the loss of anchorage.

Functions of the adapter protein Cas: signal convergence and the determination of cellular responses

Since Cas was first identified as a highly phosphorylated 130 kilodalton protein that associated with the v-Src and v-Crk-oncoproteins, considerable effort has been made to determine its function. Its predicted role as a scaffolding molecule based on its domain structure has been largely confirmed. Through its ability to undergo rapid changes in phosphorylation, subcellular localization and association with heterologous proteins, Cas may spatially and temporally regulate the function of its binding partners. Numerous proteins have been identified that bind to Cas in vitro and/or in vivo, but in only a few cases is there an understanding of how Cas may function in these protein complexes. To date, Cas-Crk and Cas-Src complexes have been most frequently implicated in Cas function, particularly in regards to processes involving regulation of the actin cytoskeleton and proliferation. These and other Cas protein complexes contribute to the critical role of Cas in cell adhesion, migration, proliferation and survival of normal cycling cells. However, under conditions in which these processes are deregulated, Cas appears to play a role in oncogenic transformation and perhaps metastasis. Therefore, in its capacity as an adapter protein, Cas serves as a point of convergence for many distinct signaling inputs, ultimately contributing to the generation of specific cellular responses.

Biodegradable polymer scaffolds for cartilage tissue engineering

Cartilage defects are common, painful conditions and none of the currently available treatment options are satisfactory. Tissue engineering techniques involving scaffolds made from biodegradable synthetic polymers hold great promise for the future. These materials can be manufactured in an injectable form for minimally invasive procedures or in a preformed state to treat large irreparable lesions including arthritis. The mechanical and biologic properties of synthetic polymers can be tailored to different clinical applications and engineering strategies. The scaffold serves as a mechanical substrate for cells and bioactive factors and can help direct and organize the process of regeneration. The ultimate goal of tissue engineering is to recapitulate normal organogenesis to create histologically and functionally normal tissue. A review of the characteristics and potential of synthetic polymers shows that these substances will play a major role in treating cartilage disorders.

Decorin inhibits macrophage colony-stimulating factor proliferation of macrophages and enhances cell survival through induction of p27(Kip1) and p21(Waf1)

Decorin is a small proteoglycan that is ubiquitous in the extracellular matrix of mammalian tissues. It has been extensively demonstrated that decorin inhibits tumor cell growth; however, no data have been reported on the effects of decorin in normal cells. Using nontransformed macrophages from bone marrow, results of this study showed that decorin inhibits macrophage colony-stimulating factor (M-CSF)-dependent proliferation by inducing blockage at the G(1) phase of the cell cycle without affecting cell viability. In addition, decorin rescues macrophages from the induction of apoptosis after growth factor withdrawal. Decorin induces the expression of the cdk inhibitors p21(Waf1) and p27(Kip1). Using macrophages from mice where these genes have been disrupted, inhibition of proliferation mediated by decorin is related to p27(Kip1) expression, whereas p21(Waf1) expression is necessary to protect macrophages from apoptosis. Decorin also inhibits M-CSF-dependent expression of MKP-1 and extends the kinetics of ERK activity, which is characteristic when macrophages become activated instead of proliferating. The effect of decorin on macrophages is not due to its interaction with epidermal growth factor or interferon-gamma receptors. Furthermore, decorin increases macrophage adhesion to the extracellular matrix, and this may be partially responsible for the expression of p27(Kip1) and the modification of ERK activity, but not for the increased cell survival.

Epidermal growth factor receptor signaling and the invasive phenotype of ovarian carcinoma cells

BACKGROUND

Most (70%-100%) ovarian carcinomas express high levels of the epidermal growth factor receptor (EGFR). To examine the relationship between EGFR and the invasive phenotype, we assessed integrin expression, adhesion, matrix metalloproteinase (MMP) activity, and migration in ovarian cancer cells in which EGFR expression was modified.

METHODS

NIH:OVCAR-8 human ovarian carcinoma cells were transfected with an expression vector containing the human EGFR complementary DNA in an antisense orientation (EGFR-antisense cells) or the vector alone (vector control cells). We compared vector control and EGFR-antisense cells for cell morphology and adhesion by light microscopy, expression of alpha(6)- and alpha(3)-integrin subunits by flow cytometry, MMP and tissue inhibitor of MMP (TIMP) activity by zymography, and migration by a wound migration assay. In some experiments, EGFR kinase activity in parental cells was inhibited by treatment with PD153035. All statistical tests were two-sided.

RESULTS

EGFR-antisense cells were morphologically distinct from vector control cells and had a selective decrease in adhesion to laminin-1 that was not observed with vector control cells (P = .008) or on other extracellular matrix substrates. Compared with vector control cells, cell surface alpha(6)-integrin expression decreased by approximately 80% (difference = 78.7%; 95% confidence interval [CI] = 77.8% to 79.6), MMP-9 activity decreased by approximately 50%, and TIMP activity increased by approximately 50% in EGFR-antisense cells. Vector control cells were highly motile (5.51 arbitrary distance unit; 95% CI = 4.98 to 6.04), whereas the EGFR-antisense cells were not (0.99 arbitrary distance units; 95% CI = 0.38 to 1.60). The morphology and integrin profile of NIH:OVCAR-8 parental cells treated with PD153035 were similar to those of the EGFR-antisense cells.

CONCLUSIONS

Reduced EGFR expression in ovarian carcinoma cells decreased their adhesion to laminin-1, expression of the alpha(6)-integrin subunit (a well-characterized laminin-1 receptor), and MMP-9 activity. These data support the hypothesis that EGFR overexpression in ovarian cancer cells results in multiple phenotypic changes that enhance the invasive phenotype.

Positive regulation of cell-cell and cell-substrate adhesion by protein kinase A

Integrin receptor activation is an important regulatory mechanism for cell-substrate and cell-cell adhesion. In this study, we explore a signaling pathway activated by mAb 12G10, an antibody that can activate β1 integrins and induce integrin-mediated cell-cell and cell-substrate adhesion. We have found that the cAMP-dependent protein kinase (PKA) is required for both mAb 12G10-induced cell-cell and cell-substrate adhesion of HT-1080 cells. Binding of mAb 12G10 to β1 integrins stimulates an increase in intracellular cAMP levels and PKA activity, and a concomitant shift in the localization of the PKA type II regulatory subunits from the cytoplasm to areas where integrins expressing the 12G10 epitope are located. MAb 12G10-induced cell-cell adhesion was mimicked by a combination of clustering β1 integrins and elevating PKA activity with Sp-adenosine-3′,5′-cyclic monophosphorothioate or forskolin. We also show that two processes required for HT-1080 cell-cell adhesion, integrin clustering and F-actin polymerization are both dependent on PKA. Taken together, our data suggest that PKA plays a key role in the signaling pathway, resulting from activation of β1 integrins, and that this enzyme may be required for upregulation of cell-substrate and cell-cell adhesion.

Rho family GTPases regulate VEGF-stimulated endothelial cell motility

Migration of endothelial cells induced by vascular endothelial growth factor (VEGF) is a critical step in angiogenesis. Stimulation of motility by growth factors such as VEGF requires interaction with the signal transduction pathways activated by the extracellular matrix (ECM). Here we demonstrate that the Rac GTPase is the critical intersection activated by type 1 collagen ECM and VEGF during stimulation of endothelial cell motility. To analyze the role of the Rho family GTPases in VEGF-stimulated endothelial cell chemotaxis and ECM-stimulated haptotaxis, we transduced the respective fusion proteins in human foreskin dermal endothelial cells using a Tat peptide from the human immunodeficiency virus Tat protein. VEGF signaling required Rac activation during chemotaxis, and Rac and Cdc42 were activated during haptotaxis on type I collagen. Similar to VEGF, Rac activation induced an increase in endothelial cell stress fiber and focal adhesion. Surprisingly, Rho activation was not present in collagen-induced haptotaxis or stimulation of chemotaxis by VEGF, although Rho induced stress fibers and focal adhesions similar to Rac activation. The result of constitutive Rho activation was an inhibition of haptotaxis. Thus, Rac is required and sufficient for the activation of endothelial cell haptotaxis and VEGF-stimulated chemotaxis.

Mammary epithelial cell-cycle progression via the alpha(2)beta(1) integrin: unique and synergistic roles of the alpha(2) cytoplasmic domain

The alpha(2)beta(1) integrin supports cell-cycle progression of mammary epithelial cells adherent to type I collagen matrices. Integrin collagen receptors containing the alpha(2) cytoplasmic domain stimulated expression of cyclin E and cyclin-dependent kinase (cdk)2, resulting in cyclin E/cdk2 activation in the absence of growth factors other than insulin. Integrin collagen receptors in which the alpha(2) cytoplasmic domain was replaced by the alpha(1) cytoplasmic domain or an alpha(2) subunit cytoplasmic domain truncated after the GFFKR sequence failed to stimulate cyclin E/cdk2 activation or entry into S phase in the absence of growth factors. Although overexpression of cyclins D or E or cdk2 in cells expressing the integrin collagen receptor with the alpha(1)-integrin cytoplasmic domain did not restore G(1) progression when mammary epithelial cells adhered to type I collagen, co-expression of cyclin E and cdk2 did rescue the ability of the transfectants to enter S phase. Activation of cyclin E/cdk2 complex by mammary epithelial cells required synergy between adhesion mediated by an integrin collagen receptor containing the alpha(2)-integrin subunit cytoplasmic domain and the insulin receptor.

Specific residues within the alpha 2 integrin subunit cytoplasmic domain regulate migration and cell cycle progression via distinct MAPK pathways

The alpha(2) integrin subunit cytoplasmic domain is necessary for epidermal growth factor (EGF)-stimulated chemotactic migration and insulin-dependent entry into S-phase of mammary epithelial cells adherent to type I collagen. Truncation mutants revealed that the seven amino acids, KYEKMTK, in addition to the GFFKR motif were sufficient for these functions. Mutation of tyrosine 1134 to alanine inhibited the ability of the cells to phosphorylate p38 MAPK and to migrate in response to EGF but had only a modest effect on the ability of the cells to induce sustained phosphorylation of the ERK MAPK, to up-regulate cyclin E and cdk2 expression, and to enter S-phase when adherent to type I collagen. Conversely, mutation of the lysine 1136 inhibited the ability of the cells to increase cyclin E and cdk2 expression, to maintain long term phosphorylation of the ERK MAPK, and to enter S-phase but had no effect on the ability of the cells to phosphorylate the p38 MAPK or to migrate on type I collagen in response to EGF. Methionine 1137 was essential for both migration and entry into S-phase. Thus, distinctly different structural elements of the alpha(2) integrin cytoplasmic domain are required to engage the signaling pathways leading to cell migration or cell cycle progression.

Integrin alphavbeta3 promotes anchorage-dependent apoptosis in human intestinal carcinoma cells

A population of cells surviving during prolonged incubation in suspension (anoikis-negative cells) were selected from the original anoikis-positive human intestinal carcinoma cell line Caco-2. Anoikis-negative cells are characterized by a strong transcriptional downregulation of the alphav-integrin chain as detected by FACS analysis, RT-PCR and Northern blotting. This finding suggested that alphav-integrin generates a signal stimulating apoptosis of Caco-2 cells upon their detachment from the extracellular matrix. Two lines of evidence supporting this suggestion were provided. First, activation of the alphavbeta3 integrin on Caco-2 cells by their treatment with an alphavbeta3-specific monoclonal antibody resulted in marked stimulation of anoikis. Second, treatment of Caco-2 cells with alphav-specific antisense oligonucleotide resulted in downregulation of the expression of alphav chain and in elevated resistance of these cells to anoikis. Thus, for the first time, our data prove that alphavbeta3 integrin can be an active transducer of apoptosis-stimulating signals generated in response to disruption of the cell-matrix contacts.

Effect of the tetrapeptide RGDS on somatic embryogenesis in Daucus carota

The present work examines the requirement for RGD-binding sites, such as those in the plasma membrane protein integrin during somatic embryogenesis in carrot (Daucus carota L. cv. Scarlet nantes). It is possible to assay for this requirement by competitively inhibiting binding of the site to the natural RGD-containing ligand by adding an excess of synthetic RGDS. We found that treatment of carrot callus cultures with RGDS (2.7 mM) inhibited the development of a normal shoot pole in carrot somatic embryos. The structures that formed contained separate zones of highly cytoplasmic and vacuolate cells and no evidence of embryonic organization, although occasionally a root-like structure was observed. If the aspartic acid residue in the peptide was replaced by a chemically similar amino acid (glutamic acid), the resultant somatic embryos were indistinguishable from those developing in untreated cultures. These effects are similar to those reported in animal systems where the protein receptor involved has been identified as integrin. Our results are thus consistent with the conclusion that a binding site for a motif similar to the integrin-binding site in a variety of animal extracellular matrix proteins exists in plants and appears to be important in somatic embryo development in carrot.

AlphaIIb's cytoplasmic domain is not required for ligand-induced clustering of integrin alphaIIbbeta3

The platelet integrin alphaIIbbeta3 exhibits bidirectional signaling, in that intracellular messengers enable adhesive macromolecules to bind to its ectodomain, while ligation promotes the association of cytoskeletal proteins with its cytoplasmic domains. In order to understand the linkage between these distant regions, we investigated the effects of receptor occupancy on the solution structure of both full-length recombinant alphaIIbbeta3 and alphaIIbDelta991beta3, an integrin truncation mutant which lacks one cytoplasmic domain. Lysates of (35)S-labeled human A549 cells expressing either full-length alphaIIbbeta3 or alphaIIbDelta991beta3 were examined by sucrose density gradient sedimentation followed by immunoprecipitation to determine the distributions of integrin protomers and oligomers. Recombinant alphaIIbbeta3 exhibited a weight-average sedimentation coefficient, S(w)=11.3+/-1.4 S with 73% sedimenting as protomers/dimers (9.1+/-1.0 S) and 27% as oligomers (15.4+/-0.4 S). Truncation mutant alphaIIbDelta991beta3 exhibited a similar pattern with 65% sedimenting as protomers/dimers. Upon ligation with eptifibatide, both full-length alphaIIbbeta3 and alphaIIbDelta991beta3 sedimented mainly at >14 S, indicating 2-3-fold increased oligomerization. Thus we have demonstrated that alphaIIb's cytoplasmic region is not required for integrin clustering, a key event in outside-in signaling.