Integrin cytoplasmic interactions and bidirectional transmembrane signalling

Regulation of fibronectin matrix deposition and cell proliferation by the PINCH-ILK-CH-ILKBP complex

Alteration in renal glomerular mesangial cell growth and fibronectin matrix deposition is a hallmark of glomerulosclerosis, which ultimately leads to end-stage renal failure. We have previously shown that the expression of integrin-linked kinase (ILK), a cytoplasmic component of the cell-extracellular matrix contacts, is increased in mesangial cells in human patients with diabetic nephropathy. We show here that ILK forms a complex with PINCH and CH-ILKBP in primary mesangial cells, which are co-clustered at fibrillar adhesions, sites that are involved in fibronectin matrix deposition. To investigate functional significance of the PINCH-ILK-CH-ILKBP complex formation, we expressed the PINCH-binding N-terminal fragment and the CH-ILKBP-binding C-terminal fragment of ILK, respectively, in mesangial cells by using an adenoviral expression system. Overexpression of either the N-terminal fragment or the C-terminal fragment of ILK effectively inhibited the PINCH-ILK-CH-ILKBP complex formation. Inhibition of the PINCH-ILK-CH-ILKBP complex formation significantly reduced fibronectin matrix deposition and inhibited cell proliferation. These results indicate that the PINCH-ILK-CH-ILKBP complex is critically involved in the regulation of mesangial fibronectin matrix deposition and cell proliferation, and suggest that it may potentially serve as a useful target in the therapeutic control of progressive renal failure and other pathological processes involving abnormal cell proliferation and fibronectin matrix deposition.

Integrin alpha(v)beta3 expression confers on tumor cells a greater propensity to metastasize to bone

The reasons why tumor cells metastasize to bone remain obscure. There is some evidence to support the theory that integrins (acting as cell surface adhesion receptors) play a role in mediating metastasis in certain organs. Here, we report that overexpression of a functionally active integrin alpha(v)b3 in Chinese hamster ovary (CHO) tumor cells drastically increased the incidence, number, and area of bone metastases in nude mice compared with those observed in mock-transfected CHO cells (CHO dhfr+) or in CHO cells expressing a functionally inactive integrin alpha(v)b3 (CHO beta3Delta744). Moreover, a breast cancer cell line (B02) established from bone metastases caused by MDA-MB-231 cells constitutively overexpressed integrin alpha(v)b3, whereas the cell surface expression level of other integrins remained unchanged. In vivo, the extent of bone metastases in B02-bearing mice was significantly increased compared with that of MDA-MB-231-bearing mice. In vitro, B02 cells and CHO cells expressing a functionally active integrin alpha(v)b3 exhibited substantially increased invasion of and adhesion to mineralized bone, bone sialoprotein, and collagen compared with those found with MDA-MB-231, CHO dhfr+, and CHO beta3Delta744 cells, respectively. Overall, our findings suggest that integrin alpha(v)b3 expression in tumor cells accelerates the development of osteolytic lesions, presumably through increased invasion of and adhesion to bone.

The characteristics of integrins expression in decidualized human endometrial stromal cell induced by 8-Br-cAMP in in vitro

Integrins are heterodimeric glycoproteins that have been found to undergo dynamic temporal and spatial changes in the endometrium during the menstrual cycle and in early pregnancy. Specificity of integrins is known to be different in human endometrial stromal cells and decidual cells. These shifts of integrins suggested to play an important role in embryo implantation and can be modulated by progesterone, cAMP derivatives, and cytokines. The mechanisms of decidualization and its precise physiological role are still not clearly understood and in vitro systems could provide an alternative that overcomes limitations of studying such complex biological phenomena in vivo at the time of implantation. This study was undertaken to establish an in vitro model system for human decidualization using 8-bromo-cAMP and to investigate the characteristics of stromal integrin expression in vitro by 8-Br-cAMP. Endometrial stromal cells were isolated and cultured, and then were induced to decidualize by 0.5 mM 8-Br-cAMP for 15 days. Immunofluorescence staining and flow cytometric analyses of the integrin subunits (alpha1, alpha4, alpha5, alpha6, beta1 and alphavbeta3) were performed at day 9. In the presence of 8-Br-cAMP, the staining intensity of alphavbeta3 was significantly higher than control and measurements for alpha1, alpha4, alpha5, alpha6, and beta1 were similar. Immunofluorescent localization of the integrins reflected the differences obtained from the flow cytometric analyses described above. In summary, the expression of alphavbeta3 integrin increased in stromal cells in vitro decidualized by 8-Br-cAMP and this up-regulation of alphavbeta3 integrin expression during decidualization might influence on human implantation.

Transmembrane signal transduction of the alpha(IIb)beta(3) integrin

Integrins are composed of noncovalently bound dimers of an alpha- and a beta-subunit. They play an important role in cell-matrix adhesion and signal transduction through the cell membrane. Signal transduction can be initiated by the binding of intracellular proteins to the integrin. Binding leads to a major conformational change. The change is passed on to the extracellular domain through the membrane. The affinity of the extracellular domain to certain ligands increases; thus at least two states exist, a low-affinity and a high-affinity state. The conformations and conformational changes of the transmembrane (TM) domain are the focus of our interest. We show by a global search of helix-helix interactions that the TM section of the family of integrins are capable of adopting a structure similar to the structure of the homodimeric TM protein Glycophorin A. For the alpha(IIb)beta(3) integrin, this structural motif represents the high-affinity state. A second conformation of the TM domain of alpha(IIb)beta(3) is identified as the low-affinity state by known mutational and nuclear magnetic resonance (NMR) studies. A transition between these two states was determined by molecular dynamics (MD) calculations. On the basis of these calculations, we propose a three-state mechanism.

Role of integrin-linked kinase in leukocyte recruitment

Chemokines modulate leukocyte integrin avidity to coordinate adhesion and subsequent transendothelial migration, although the sequential signaling pathways involved remain poorly characterized. Here we show that integrin-linked kinase (ILK), a 59-kDa serine-threonine protein kinase that interacts principally with beta(1) integrins, is highly expressed in human mononuclear cells and is activated by exposure of leukocytes to the chemokine monocyte chemoattractant protein-1. Biochemical inhibitor studies show that chemokine-triggered activation of ILK is downstream of phosphoinositide 3-kinase. In functional assays under physiologically relevant flow conditions, overexpression of wild-type ILK in human monocytic cells diminishes beta(1) integrin/vascular cell adhesion molecule-1-dependent firm adhesion to human endothelial cells. These data implicate ILK in the dynamic signaling events involved in the regulation of leukocyte integrin avidity for endothelial substrates.

Promoter characterization and genomic organization of the gene encoding integrin-linked kinase 1

Integrin-linked kinase (ILK)-1 is a 59-kDa serine-threonine protein kinase, which associates with the cytoplasmic domain of beta 1, beta 2 and beta 3 integrins and acts as a receptor proximal kinase regulating integrin-mediated signal transduction. We have recently identified an isoform of ILK (ILK-2), which is expressed, in a TGF-beta 1-dependent manner, in a highly invasive tumor cell line but not in normal adult tissues. In contrast, ILK-1 is ubiquitously expressed in normal tissues and is up-regulated in various tumors independent of TGF-beta 1. Here, we report the structural organization and the promoter activity of the human ILK-1 gene, contained within a 8.8-kb genomic fragment cloned from a human BAC library. The mature protein is encoded by 13 exons. The last coding exon contains the entire 3' UTR of the ILK-1 gene, which overlaps with the complementary 3' UTR sequence of the TAF2H gene, a TATA box binding protein-associated factor. A major transcriptional initiation start site was found 138 bp upstream of exon 1 in close proximity to a consensus initiator element (Inr). The ILK gene is transcribed by a TATA-less and CAAT-less promoter with typical features of housekeeping genes. The promoter activity was characterized by a luciferase reporter assay and the minimal sequence conferring promoter activity was 349 bp in size and located immediately upstream of exon 1.

Defective Rac-mediated proliferation and survival after targeted mutation of the beta1 integrin cytodomain

Cell matrix adhesion is required for cell proliferation and survival. Here we report that mutation by gene targeting of the cytoplasmic tail of beta1 integrin leads to defective proliferation and survival both in vivo and in vitro. Primary murine embryonic fibroblasts (MEFs) derived from mutant homozygotes display defective cell cycle coupled to impaired activation of the FAK-PI3K-Akt and Rac-JNK signaling pathways. Expression in homozygous MEFs of a constitutively active form of Rac is able to rescue proliferation, survival, and JNK activation. Moreover, although showing normal Erk phosphorylation, mutant cells fail to display Erk nuclear translocation upon fibronectin adhesion. However, expression of the constitutively activated form of Rac restores Erk nuclear localization, suggesting that adhesion-dependent Rac activation is necessary to integrate signals directed to promote MAPK activity. Altogether, our data provide the evidence for an epistatic interaction between the beta1 integrin cytoplasmic domain and Rac, and indicate that this anchorage-dependent signaling pathway is crucial for cell growth control.

Adhesion molecules

The exact mechanisms by which serous ovarian cancer cells invade through their underlying basement membrane or are released from the surface of the ovary have yet to be elucidated. This process undoubtedly has a complex molecular basis that most likely involves multiple cell surface receptors, basement membrane components, intercellular adhesion molecules, and signaling from the cell [137]. One possible mechanism by which ovarian carcinoma tumor cells may alter their basement membrane is by the synthesis and secretion of proteolytic enzymes that degrade their basement membranes [88-94, 138]. Alternatively, metastatic ovarian carcinoma cells may decrease their synthesis and/or secretion of ECM molecules. Additional studies are required to determine whether the more aggressive behavior of malignant ovarian carcinoma cells, compared to normal ovarian epithelial cells, is related to an altered cellular response towards ECM molecules, perhaps due to alterations in adhesion molecules/receptors. A further elucidation of the mechanisms by which serous ovarian carcinoma cells regulate their expression of ECM molecules and adhesion molecules/receptors will help in our understanding of the invasion and metastasis of tumor cells. Members of several families of adhesion molecules have been described that seem to be important in the progression of ovarian carcinoma, including CD44, integrins, and E-cadherin. Due to the complexity of this disease, it is likely that other adhesion molecules will also be implicated in the adhesion, migration, invasion, growth, proliferation, and apoptosis of ovarian carcinoma cells. Our group and others have shown that CD44 and the beta 1 integrin subunit play fundamental roles in the adhesion and migration of ovarian carcinoma cells to mesothelial cells and their associated pericellular matrix. Subsequent to the initial adhesion, the ovarian carcinoma cells may migrate through the layer of mesothelial cells, penetrate through the underlying basement membrane, invade into the tissue, and establish a secondary site of growth. Further studies will be required in order to fully understand the relationship of each adhesion molecule and their ligand(s) in the progression of this disease. Once the adhesion molecules and their ligand(s) for each step of the progression of this disease have been identified, it should be possible to develop reagents that can inhibit these interactions. Then, when ovarian carcinoma cells can no longer interact with mesothelial cells and their associated ECM, the dissemination of ovarian carcinoma cells in vivo may be prevented.

Regulation of cell motility on polymer substrates via "dynamic," cell internalizable, ligand microinterfaces

In vivo, motile epidermal tissues frequently encounter ligand microinterfaces that are dynamic, owing to rapid cell-mediated substrate phagocytosis. In this study, we have examined cell motility phenomena in response to the adhesion ligand, collagen, which was presented on cell-internalizable 400-nm colloidal gold microcarriers. Normal human keratinocytes were seeded onto collagen-adsorbed poly(lactide-co-glycolide) (PLGA) films that were predeposited with varying densities of the ligand-associated microcarriers (LAMs), such that the overall ligand density and the ligand loading per microcarrier were invariant. Cells seeded on LAMs exhibited rapid and distinct cytoskeletal redistribution resulting in numerous filopodial extensions, indicative of activation of cell motility processes. We report that the population-averaged cell migration rate of cells, mu, was increased from 2 microm2/min on collagen substrates lacking the microcarriers, to approximately 50 microm2/min on collagen presenting LAMs. An increase in LAM density to 1.2 LAMs per microm2 was found to maximize mu, as well as the directional persistence and speed of cell migration, whereas very high LAM densities saturated cell internalization and diminished migration rates. The cell-LAM interactions essential to enhanced migration were ligand-activated, as mu; values were reduced 400% on internalizable microcarriers lacking ligands; moreover, cooperative ligand elicitation was possible, for example, when 10 microg/mL soluble fibronectin was introduced as a costimulant of keratinocytes, yielding the highest reported mu values (80 microm2/min) on collagen LAM-PLGA substrates. Notably, cell migration rates were severely repressed when cell internalization processes were challenged through covalent conjugation of ligand carriers to the substrate, indicating that signals from cell-LAM binding alone were inadequate for elevated levels of cell migration. Further analysis indicated that the presence of LAMs did not alter the protease-resistant adhesivity between the cell and the underlying substrate, suggesting that the activation governing ligand interactions likely arose at the cell-LAM interface rather than the cell-substrate interface. This study highlights the novel use of secondary ligand-presenting microscale/nanoscale depots at polymer substrates to elicit, via dynamic cell internalization processes, significantly enhanced levels of cell migration over traditional interfaces with ligand-bearing substrates.

Age-related loss of rooting capability in Arabidopsis thaliana and its reversal by peptides containing the Arg-Gly-Asp (RGD) motif

We describe here an experimental system to study the age-related decline of adventitious root formation in Arabidopsis thaliana L. (Heynh), ecotype Landsberg erecta (Ler). The system is based on the different rooting capacity of hypocotyls from de-rooted juvenile (12-day-old) and adult (26-day-old) plants. Hypocotyls from de-rooted juvenile plants rooted readily within a week of culture, and the rooting process was not dependent on exogenous auxin. In contrast, hypocotyls from de-rooted adult plants rooted poorly and only after longer periods of time. Exogenously applied auxin had no effect on rooting of hypocotyls from de-rooted adult plants. Rooting capacity, although correlated with the transition to flowering, did not depend on this transition. Root induction declined in a similar manner when the transition to flowering was delayed, either genetically with the fve mutant or physiologically with short days. The results showed that rooting of hypocotyls from de-rooted adult plants depended on the effect of peptides containing the RGD motif. Both the percentage of rooting and the number of roots were largely increased when the hypocotyls were treated transiently with the RGD peptide. The effect of the RGD peptide was a necessary, but not sufficient, condition for rooting of hypocotyls from de-rooted adult plants.

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.

Enhanced branching morphogenesis in mammary glands of mice lacking cell surface beta1,4-galactosyltransferase

Development of the mammary gland is influenced both by the systemic hormonal environment and locally through cell-cell and cell-extracellular matrix (ECM) interactions. We have previously demonstrated aberrant mammary gland morphogenesis in transgenic mice with elevated levels of the long isoform of beta1,4-galactosyltransferase 1 (GalT), a proportion of which is targeted to the plasma membrane, where it plays a role in cell-ECM interactions. Here, we show that mammary glands of mice lacking the long GalT isoform exhibit a complementary phenotype. Cell-surface GalT activity was reduced by over 60%, but because the short GalT isoform is intact, total GalT activity was reduced only slightly relative to wild type. Mammary glands from long GalT-null mice were characterized by excess branching, and this phenotype was accompanied by altered expression of laminin chains. Laminin alpha1 and alpha3 were reduced 2.4- and 3.0-fold, respectively, while expression of laminin gamma2 was elevated 2.3-fold. The expression and cleavage of laminin gamma2 have been correlated with branching and cell migration, and Western blotting revealed an altered pattern in gamma2 cleavage products in long GalT-null mammary glands. We then examined the expression of metalloproteases that cleave laminins or that have been shown to play a role in mammary gland morphogenesis. Expression of MT1-MMP, a membrane-bound protease that can cleave laminin gamma2, was elevated 5.5-fold in the long GalT-nulls. MMP 7 was also elevated 5.1-fold. Our results suggest that expression of surface GalT is important for the proper regulation of matrix expression and deposition, which in turn regulates the proper branching morphogenesis of the mammary epithelial ductal system.

Phosphorylation of beta3 integrin controls ligand binding strength

The cytoplasmic domain of beta(3) integrin contains tyrosines at positions 747 and 759 in domains that have been implicated in regulation of alpha(v)beta(3) function and that serve as potential substrates for Src family kinases. The phosphorylation level of beta(3) integrin was modulated using a temperature-sensitive v-Src kinase. Increased beta(3) phosphorylation abolished alpha(v)beta(3)- but not alpha(5)beta(1)-mediated adhesion to fibronectin. alpha(v)beta(3)-Mediated cell adhesion was restored by the expression of beta(3) containing Y747F or Y759F mutations but not by wild type beta(3) integrin. Thus, phosphorylation of the cytoplasmic domain of beta(3) is a negative regulator of alpha(v)beta(3)-fibronectin binding strength.

Integrin alphavbeta1 is a receptor for foot-and-mouth disease virus

Infection by field strains of Foot-and-mouth disease virus (FMDV) is initiated by binding to certain species of arginine-glycine-aspartic acid (RGD)-dependent integrin including alphavbeta3 and the epithelial integrin alphavbeta6. In this report we show that the integrin alphavbeta1, when expressed as a human/hamster heterodimer on transfected CHOB2 cells, is a receptor for FMDV. Virus binding and infection mediated by alphavbeta1 was inefficient in the presence of physiological concentrations of calcium and magnesium but were significantly enhanced by reagents that activate the integrin and promote ligand binding. The ability of chimeric alpha5/alphav integrin subunits, in association with the beta1 chain, to bind FMDV and mediate infection matched the ligand binding specificity of alphavbeta1, not alpha5beta1, thus providing further evidence for the receptor role of alphavbeta1. In addition, data are presented suggesting that amino acid residues near the RGD motif may be important for differentiating between the binding specificities of alphavbeta1 and alphavbeta6.

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.

Dissection of HEF1-dependent functions in motility and transcriptional regulation

Cas-family proteins have been implicated as signaling intermediaries in diverse processes including cellular attachment, motility, growth factor response, apoptosis and oncogenic transformation. The three defined Cas-family members (p130Cas, HEF1/Cas-L and Efs/Sin) are subject to multiple forms of regulation (including cell-cycle- and cell-attachment-mediated post-translational modification and cleavage) that complicate elucidation of the function of specific Cas proteins in defined biological processes. To explore the biological role of HEF1 further, we have developed a series of cell lines in which HEF1 production is regulated by an inducible promoter. In this system, HEF1 production rapidly induces changes in cellular morphology and motility, enhancing cell speed and haptotaxis towards fibronectin in a process partially dependent on intact ERK and p38 MAPK signaling pathways. Finally, cDNA expression array analysis and subsequent studies indicate that HEF1 production increases levels of mRNA transcripts encoding proteins that are associated with motility, cell transformation and invasiveness, including several metalloproteinases, MLCK, p160ROCK and ErbB2. Upregulation of such proteins suggests mechanisms through which misregulation of HEF1 may be involved in cancer progression.

Molecular mechanisms of IL-2 gene regulation following costimulation through LFA-1

The integrin LFA-1 serves as an accessory molecule in T cell activation. In addition to its well-known role as an adhesion molecule, LFA-1 can contribute to T cell activation and up-regulation of IL-2 gene expression. However, the specific mechanisms by which LFA-1 influences T cell activation have not been elucidated. Therefore, we examined the impact of LFA-1:ICAM-1 interactions on transcriptional and posttranscriptional IL-2 gene regulation, using a costimulation-negative cell line transfected with MHC class II alone, or in combination with ICAM-1 or B7-1. IL-2 transcription was assessed utilizing transgenic mice expressing an IL-2 promoter luciferase reporter construct crossed to DO11.10 TCR-transgenic mice, and IL-2 mRNA stability was evaluated by real-time RT-PCR. Comparison of naive and previously activated T cells demonstrates a dramatic increase in IL-2-luciferase transcription in activated T cells that can, in part, be attributed to downstream signaling events. Costimulation through LFA-1 enhances transcription of the transgenic reporter construct across a wide Ag dose range, but does not affect IL-2 mRNA stability. In contrast, CD28 costimulation is clearly mediated through up-regulation of IL-2 transcription and through enhancement of mRNA stability. These results indicate that the primary pathway whereby engagement of LFA-1 through its ligand ICAM-1 up-regulates IL-2 gene expression is through enhanced IL-2 transcription, in the absence of any effect on IL-2 mRNA stabilization.

Molecular properties of fibrin-based matrices for promotion of angiogenesis in vitro

The molecular properties of fibrin-based matrices, such as fibrillar structure and covalent modifications with adhesion domains, influence the angiogenic behavior of human umbilical vein endothelial cells (HUVECs) in vitro. The fibrillar structure of fibrin-based matrices was influenced by pH but not by covalent incorporation of exogenous adhesion domains. Native fibrin-based matrices polymerized at pH 10 formed organized and longitudinally oriented fibrin fibrils, which provided a good angiogenic substrate for endothelial cells. Furthermore, upon covalent incorporation of the model ligand L1Ig6, which binds to the integrin most prominently expressed on the surface of angiogenic endothelial cells, alpha(v)beta3, these matrices became angiogenesis-promoting when polymerized at physiological pH. The amount of incorporation of L1Ig6 into the matrices depended on the fibrinogen concentration on all three fibrin chains. Soluble forms of L1Ig6 diffused rapidly out of the matrix. Most important, L1Ig6-modified matrices were very specific in inducing the angiogenic phenotype of HUVECs, whereas control cells did not differentiate on these matrices. Our results indicate that artificial extracellular matrices can influence cell behavior in two ways. One way is based on the three-dimensional fibril structure of the matrix molecules themselves, and the other is due to providing specific binding sites for direct cell-matrix interactions that lead to the activation of second-messenger cascades and thus promoting angiogenic differentiation.

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.

Binding of Paxillin to the alpha 9 Integrin Cytoplasmic Domain Inhibits Cell Spreading

alpha(9)beta(1) integrin is a member of the beta(1) integrin family, plays an important role in extravasation of neutrophils at sites of acute inflammation, and is required for the normal development of the lymphatic system. The alpha(9) and alpha(4) integrin subunits are most closely related and form a subfamily of integrin alpha subunits. Previously, we have reported that the alpha(4) cytoplasmic domain directly and tightly binds paxillin, an intracellular signaling adaptor molecule. This interaction accounts for some of the unusual functional responses to alpha(4) integrin-mediated cell adhesion, including stimulation of cell migration and inhibition of cell spreading and focal adhesion formation. In the current studies, we have examined the interaction between the alpha(9) cytoplasmic domain and paxillin. Here we report that the alpha(9) cytoplasmic domain binds paxillin directly and tightly and that the alpha(9)-paxillin association inhibits cell spreading. We have identified amino acid residues in the alpha(9) cytoplasmic domain, Trp(999) and Trp(1001), that are critical for paxillin binding, and alanine substitution of either Trp(999) or Trp(1001) blocks paxillin binding. Furthermore, these mutations also reverse the effect of the alpha(9) cytoplasmic domain on cell spreading. Thus, the alpha(9) and alpha(4) integrin subunits form a paxillin-binding subfamily of integrin alpha subunits, and direct binding of paxillin to the alpha(9) cytoplasmic domain mediates some of the biological activities of the alpha(9)beta(1) integrin.

Mapping critical sites in collagen II for rational design of gene-engineered proteins for cell-supporting materials

Collagen II is the most abundant protein of cartilage and forms a network of fibrils extended by proteoglycans that enables cartilage to resist pressure. The surface of the collagen fibril serves as a platform for the attachment of collagen IX, growth factors, and cells. In this study we examined the mechanism of the interaction of chondrocytes with recombinant versions of procollagen II, in which one of the four blocks of 234 amino acids that define repeating D periods of the collagen triple helix has been deleted. Analysis of the attachment of chondrocytes to collagen II variants with deleted D periods indicated that the collagen II monomer contains randomly distributed sites critical for cell binding. However, as was shown by spreading and migration assays, the D4 period, which is between residues 703 to 936, contains amino acids critical for cell motility. We also showed that binding, spreading, and migration of chondrocytes through three-dimensional nanofibrillar collagenous matrices are controlled by an interaction of the collagen triple helix with beta1 integrins. The results of this study provide a basis for the rational design of a scaffold containing genetically engineered collagen with a high density of specific sites of interaction.

Dual role of H-Ras in regulation of lymphocyte function antigen-1 activity by stromal cell-derived factor-1alpha: implications for leukocyte transmigration

We investigated the role of H-Ras in chemokine-induced integrin regulation in leukocytes. Stimulation of Jurkat T cells with the CXC chemokine stromal cell-derived factor-1alpha (SDF-1alpha) resulted in a rapid increase in the phosphorylation, i.e., activation of extracellular signal receptor-activated kinase (ERK) but not c-Jun NH(2)-terminal kinase or p38 kinase, and phosphorylation of Akt, reflecting phosphatidylinositol 3-kinase (PI3-K) activation. Phosphorylation of ERK in Jurkat cells was enhanced and attenuated by expression of dominant active (D12) or inactive (N17) forms of H-Ras, respectively, while N17 H-Ras abrogated SDF-1alpha-induced Akt phosphorylation. SDF-1alpha triggered a transient regulation of adhesion to intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 mediated by lymphocyte function antigen-1 (LFA-1) and very late antigen-4 (VLA-4), respectively, and a rapid increase in LFA-1 binding to soluble ICAM-1.Ig, which was inhibited by D12 but not N17 H-Ras. Both D12 and N17 H-Ras abrogated the regulation of LFA-1 but not VLA-4 avidity, and impaired LFA-1-mediated transendothelial chemotaxis but not VLA-4-dependent transmigration induced by SDF-1alpha. Analysis of the mutant Jurkat J19 clone revealed LFA-1 with constitutively high affinity and reduced ERK phosphorylation, which were partially restored by expression of active H-Ras. Inhibition of PI3-K blocked the up-regulation of Jurkat cell adhesion to ICAM-1 by SDF-1alpha, whereas inhibition of mitogen-activated protein kinase kinase impaired the subsequent down-regulation and blocking both pathways abrogated LFA-1 regulation. Our data suggest that inhibition of initial PI3-K activation by inactive H-Ras or sustained activation of an inhibitory ERK pathway by active H-Ras prevail to abolish LFA-1 regulation and transendothelial migration induced by SDF-1alpha in leukocytes, establishing a complex and bimodal involvement of H-Ras.

A functional comparison of mutations in integrin β cytoplasmic domains

Cell adhesion is a multistep process that requires the interaction of integrins with their ligands in cell attachment, the activation of integrin-triggered signals, and cell spreading. Integrin β subunit cytoplasmic domains (β tails) participate in regulating each of these steps; however, it is not known whether the same or different regions within β tails are required. We generated a panel of amino acid substitutions within the β1 and β3 cytoplasmic domains to determine whether distinct regions within β tails regulate different steps in adhesion. We expressed these β cytoplasmic domains in the context of interleukin 2 (IL-2) receptor (tac) chimeras and tested their ability to activate tyrosine phosphorylation, to regulate β1 integrin conformation and to inhibit β1 integrin function in cell attachment and spreading. We found that many of the mutant β1 and β3 chimeras either had no effect on these parameters or dramatically inhibited the function of the β tail in most assays. However, one set of analogous Ala substitutions in the β1 and β3 tails differentially affected the ability of the tac-β1 and tac-β3 chimeras to activate tyrosine phosphorylation. The tac-β1 mutant containing Ala substitutions for the VTT motif did not signal, whereas the analogous tac-β3 mutant was able to activate tyrosine phosphorylation, albeit not to wild-type levels. We also identified a few mutations that inhibited β tail function in only a subset of assays. Ala substitutions for the Val residue in the VTT motif of the β1 tail or for the conserved Asp and Glu residues in the membrane-proximal region of the β3 tail greatly diminished the ability of tac-β1 and tac-β3 to inhibit cell spreading, but had minimal effects in other assays. Ala substitutions for the Trp and Asp residues in the conserved WDT motif in the β1 tail had dramatic effects on the ability of tac-β1 to regulate integrin conformation and function in cell spreading, but had no or intermediate effects in other assays. The identification of mutations in the β1 and β3 tails that specifically abrogated the ability of these β tails to regulate β1 integrin conformation and function in cell spreading suggests that distinct protein interactions with β tails regulate β cytoplasmic domain function in these processes.

Binding of a fibrinogen mimetic stabilizes integrin alphaIIbbeta3's open conformation

The platelet integrin alphaIIbbeta3 is representative of a class of heterodimeric receptors that upon activation bind extracellular macromolecular ligands and form signaling clusters. This study examined how occupancy of alphaIIbbeta3's fibrinogen binding site affected the receptor's solution structure and stability. Eptifibatide, an integrin antagonist developed to treat cardiovascular disease, served as a high-affinity, monovalent model ligand with fibrinogen-like selectivity for alphaIIbbeta3. Eptifibatide binding promptly and reversibly perturbed the conformation of the alphaIIbbeta3 complex. Ligand-specific decreases in its diffusion and sedimentation coefficient were observed at near-stoichiometric eptifibatide concentrations, in contrast to the receptor-perturbing effects of RGD ligands that we previously observed only at a 70-fold molar excess. Eptifibatide promoted alphaIIbbeta3 dimerization 10-fold more effectively than less selective RGD ligands, as determined by sedimentation equilibrium. Eptifibatide-bound integrin receptors displayed an ectodomain separation and enhanced assembly of dimers and larger oligomers linked through their stalk regions, as seen by transmission electron microscopy. Ligation with eptifibatide protected alphaIIbbeta3 from SDS-induced subunit dissociation, an effect on electrophoretic mobility not seen with RGD ligands. Despite its distinct cleft, the open conformer resisted guanidine unfolding as effectively as the ligand-free integrin. Thus, we provide the first demonstration that binding a monovalent ligand to alphaIIbbeta3's extracellular fibrinogen-recognition site stabilizes the receptor's open conformation and enhances self-association through its distant transmembrane and/or cytoplasmic domains. By showing how eptifibatide and RGD peptides, ligands with distinct binding sites, each affects alphaIIbbeta3's conformation, our findings provide new mechanistic insights into ligand-linked integrin activation, clustering and signaling.

Serine 785 phosphorylation of the β1 cytoplasmic domain modulates β1A-integrin-dependent functions

The integrin β1 cytoplasmic domain plays a key role in a variety of integrin-mediated events including adhesion, migration and signaling. A number of studies suggest that phosphorylation may modify the functional state of the cytoplasmic domain, but these studies frequently only examine the effect of substituting amino acid mimics that cannot be phosphorylated. We now demonstrate, using site directed mutagenesis, that substituting either an unphosphorylated (S to M) or a phosphorylated (S to D) mimic in place of serine can modify integrin function. Specifically, we show that expressing a residue that mimics a dephosphorylated form of the protein promotes cell spreading and directed cell migration, whereas a residue mimicking a phosphorylated form of the protein promotes attachment but inhibits cell spreading or migration. The significance of these observations is strengthened by the fact that the β1 mutations display the same properties in both a fibroblast cell line (GD25) and a teratocarcinoma cell line (F9). The results indicate that changes in the phosphorylation state of S785 modulates β1 integrin function.

Polarized distribution of alpha5 integrin in dendrites of hippocampal and cortical neurons

The distribution of immunoreactivity for the alpha5 subunit of the fibronectin receptor was evaluated in adult rat brain with particular interest in the cellular localization of immunostaining in the hippocampal formation and neocortex. Beyond localization to neuronal perikarya and short dendritic fragments within most brain areas, alpha5 immunoreactivity (-ir) was particularly dense within primary apical dendrites of pyramidal cells in both hippocampus and neocortex and within the dendritic arbors of cerebellar Purkinje cells. In hippocampal and cortical pyramidal cells, immunostaining was clearly polarized: alpha5-ir was not detectable in basal dendrites in hippocampal neurons and was limited to proximal arbors or absent from basal dendrites in pyramidal cells in superficial and deep layers of neocortex. Beyond this, alpha5-ir was distributed within the dendritic ramifications of the dentate gyrus granule cells and within perikarya and dendrites of occasional nonpyramidal neurons. Developmental studies demonstrated that, in both hippocampus and neocortex, alpha5-ir appears first within perikarya and is distributed to dendrites during the second postnatal week. These results are in accord with the broad hypothesis that integrins contribute to apical-basal differences in dendrites and that the integrin fibronectin (alpha5beta1) receptor, in particular, contributes to some late developing features of dendritic structure or function.

Modulation of integrin signal transduction by ILKAP, a protein phosphatase 2C associating with the integrin-linked kinase, ILK1

ILKAP, a protein serine/threonine (S/T) phosphatase of the PP2C family, was isolated in a yeast two-hybrid screen baited with integrin-linked kinase, ILK1. Association of ILK1 and ILKAP was independent of the catalytic activity of either partner, as assayed in co-precipitation and two-hybrid experiments. Condi tional expression of ILKAP in HEK 293 cells resulted in selective inhibition of ECM- and growth factor-stimulated ILK1 activity, but did not inhibit Raf-1 kinase activity. A catalytic mutant of ILKAP, H154D, did not inhibit ILK1 kinase activity. Two cellular targets of ILK1, glycogen synthase kinase 3 beta (GSK3beta) and protein kinase B (PKB)/AKT, were differentially affected by ILKAP-mediated inhibition of ILK1. Catalytically active, but not mutant ILKAP, strongly inhibited insulin-like growth factor-1-stimulated GSK3beta phosphorylation on Ser9, but did not affect phosphorylation of PKB on Ser473, suggesting that ILKAP selectively affects ILK-mediated GSK3beta signalling. Consistent with this, active, but not H154D mutant or the related PP2Calpha, selectively inhibited transactivation of a Tcf/Lef reporter gene, TOPFlash, in 293 cells. We propose that ILKAP regulates ILK1 activity, targeting ILK1 signalling of Wnt pathway components via modulation of GSK3beta phosphorylation.

Injury and EGF mediate the expression of alpha6beta4 integrin subunits in corneal epithelium

Our goal was to evaluate the role of epidermal growth factor and injury on the expression of integrin subunits alpha6(alpha6) and beta4(beta4). An in vitro wound model was used to evaluate corneal wound repair and cellular migration. Primary rabbit corneal epithelial cell cultures were serum-starved and injured in the presence or absence of EGF or tyrphostin AG1478, an inhibitor of EGF receptor kinase activity. Repair was monitored morphologically and expression was analyzed using in situ hybridization and immunohistochemistry accompanied by confocal microscopy. The addition of EGF to cell cultures induced a dose-dependent increase in beta4 mRNA expression but the constitutive expression of alpha6 was several fold greater. In the wounded cultures there was a rapid change in expression at the edge of the wound that was enhanced with EGF. In our model there was an increase in beta4 and alpha6 protein in migrating cells. Changes in integrin expression were accompanied by a transient increase in activation of the EGF receptor. The addition of tyrphostin inhibited migration of cells and wound repair, the activation of the EGF receptor and phosphorylation of beta4 in the cytoplasm. These data indicate that the activation of the EGF receptor plays a critical role in the regulation of integrin receptors and the mediation of cellular migration.

Role of the cytoplasmic domain of the beta-subunit of integrin alpha(v)beta6 in infection by foot-and-mouth disease virus

Field isolates of foot-and-mouth disease virus (FMDV) are believed to use RGD-dependent integrins as cellular receptors in vivo. Using SW480 cell transfectants, we have recently established that one such integrin, alpha(v)beta6, functions as a receptor for FMDV. This integrin was shown to function as a receptor for virus attachment. However, it was not known if the alpha(v)beta6 receptor itself participated in the events that follow virus binding to the host cell. In the present study, we investigated the effects of various deletion mutations in the beta6 cytoplasmic domain on infection. Our results show that although loss of the beta6 cytoplasmic domain has little effect on virus binding, this domain is essential for infection, indicating a critical role in postattachment events. The importance of endosomal acidification in alpha(v)beta6-mediated infection was confirmed by experiments showing that infection could be blocked by concanamycin A, a specific inhibitor of the vacuolar ATPase.

A new focal adhesion protein that interacts with integrin-linked kinase and regulates cell adhesion and spreading

Integrin-linked kinase (ILK) is a multidomain focal adhesion (FA) protein that functions as an important regulator of integrin-mediated processes. We report here the identification and characterization of a new calponin homology (CH) domain-containing ILK-binding protein (CH-ILKBP). CH-ILKBP is widely expressed and highly conserved among different organisms from nematodes to human. CH-ILKBP interacts with ILK in vitro and in vivo, and the ILK COOH-terminal domain and the CH-ILKBP CH2 domain mediate the interaction. CH-ILKBP, ILK, and PINCH, a FA protein that binds the NH(2)-terminal domain of ILK, form a complex in cells. Using multiple approaches (epitope-tagged CH-ILKBP, monoclonal anti-CH-ILKBP antibodies, and green fluorescent protein-CH-ILKBP), we demonstrate that CH-ILKBP localizes to FAs and associates with the cytoskeleton. Deletion of the ILK-binding CH2 domain abolished the ability of CH-ILKBP to localize to FAs. Furthermore, the CH2 domain alone is sufficient for FA targeting, and a point mutation that inhibits the ILK-binding impaired the FA localization of CH-ILKBP. Thus, the CH2 domain, through its interaction with ILK, mediates the FA localization of CH-ILKBP. Finally, we show that overexpression of the ILK-binding CH2 fragment or the ILK-binding defective point mutant inhibited cell adhesion and spreading. These findings reveal a novel CH-ILKBP-ILK-PINCH complex and provide important evidence for a crucial role of this complex in the regulation of cell adhesion and cytoskeleton organization.

The NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform

Beta3 integrin adhesion molecules play important roles in wound repair and the regulation of vascular development and three beta3 integrin isoforms (beta3-A, -B, -C) have been described so far. Surface expression of beta3 integrins is dynamically regulated through internalization of beta3 integrins, however, the molecular mechanisms are understood incompletely. To evaluate the role of the cytoplasmic domain of beta3 integrins for internalization, we have generated single chain chimeras with variant and mutated forms of beta3 cytoplasmic domains. Upon transient transfection into chinese hamster ovary cells, it was found that the beta3-A chimera had strongly reduced cell surface expression compared with the corresponding beta3-B, or beta3-C fusion proteins, or the tail-less constructs, whereas steady state levels of all chimeras were near identical. Studies employing cytoplasmic domain mutants showed that the NITY motif at beta3-A 756–759 is critical for plasma membrane expression of beta3-A. Furthermore, delivery of beta3-A to the cell surface was specifically modulated by the cytoplasmic protein beta3-endonexin, a previously described intracellular protein. Coexpression of the native, long form of beta3-endonexin, which does not interact with the beta3 tail, acted as a dominant negative inhibitor of beta3-A-internalization and enhanced steady-state surface expression of the beta3-A-chimera. Furthermore, anti-beta3 antibody-induced internalization of the native beta3 integrin (alpha(IIb)beta3 was dramatically reduced for the Tyr(759)-Ala substitution mutant (alpha(IIb)beta3) (Y759A) and expression of the long isoform of beta3-endonexin substantially decreased the internalization of wild-type alpha(IIb)beta3. Thus, the NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform and beta3-endonexin appears to couple the beta3-A isoform to a specific receptor-recycling pathway.

Development of mammary gland requires normal beta 1-integrin function

To study the role of beta 1-integrins in mammary gland development we have generated transgenic mice expressing a dominant negative mutant of the beta 1-integrin chain in the mammary epithelium. The transgenic glands presented a delayed development in pregnancy and lactation due to decreased epithelial cell proliferation and increased apoptosis, whereas at the beginning of lactation, expression of milk proteins, WAP and beta-casein was diminished. In correlation with transgene expression, the basement membrane component, laminin, and the beta 4 integrin were accumulated at the lateral surface of luminal epithelial cells, revealing defects in polarization. Our data show that beta 1-integrins are involved in vivo in the control of proliferation, apoptosis, differentiation, and maintenance of baso-apical polarity of mammary epithelium.

The molecular adapter SLP-76 relays signals from platelet integrin alphaIIbbeta3 to the actin cytoskeleton

Platelet adhesion to fibrinogen through integrin alpha(IIb)beta(3) triggers actin rearrangements and cell spreading. Mice deficient in the SLP-76 adapter molecule bleed excessively, and their platelets spread poorly on fibrinogen. Here we used human platelets and a Chinese hamster ovary (CHO) cell expression system to better define the role of SLP-76 in alpha(IIb)beta(3) signaling. CHO cell adhesion to fibrinogen required alpha(IIb)beta(3) and stimulated tyrosine phosphorylation of SLP-76. SLP-76 phosphorylation required coexpression of Syk tyrosine kinase and stimulated association of SLP-76 with the adapter, Nck, and with the Rac exchange factor, Vav1. SLP-76 expression increased lamellipodia formation induced by Syk and Vav1 in adherent CHO cells (p < 0.001). Although lamellipodia formation requires Rac, SLP-76 functioned downstream of Rac by potentiating adhesion-dependent activation of PAK kinase (p < 0.001), a Rac effector that associates with Nck. In platelets, adhesion to fibrinogen stimulated the association of SLP-76 with the SLAP-130 adapter and with VASP, a SLAP-130 binding partner implicated in actin reorganization. Furthermore, SLAP-130 colocalized with VASP at the periphery of spread platelets. Thus, SLP-76 functions to relay signals from alpha(IIb)beta(3) to effectors of cytoskeletal reorganization. Therefore, deficient recruitment of specific adapters and effectors to sites of adhesion may explain the integrin phenotype of SLP-76(-/-) platelets.

A novel syndrome of variant leukocyte adhesion deficiency involving defects in adhesion mediated by beta1 and beta2 integrins

Leukocyte adhesion deficiency type I (LAD-1) is a disorder associated with severe and recurrent bacterial infections, impaired extravascular targeting and accumulation of myeloid leukocytes, altered wound healing, and significant morbidity that is caused by absent or greatly diminished surface expression of integrins of the beta2 class. We report clinical features and analysis of functions of cells from a patient with a myelodysplastic syndrome and infectious complications similar to those in the severe form of LAD-1, but whose circulating neutrophils displayed normal levels of beta2 integrins. Analysis of adhesion of these cells to immobilized ligands and to endothelial cells and assays of cell-cell aggregation and chemotaxis demonstrated a profound defect in adhesion mediated by beta2 integrins indicative of a variant form of LAD-1. A novel cell line established from Epstein-Barr virus-transformed lymphoblasts from the subject demonstrated deficient beta2 integrin-dependent adhesive function similar to that of the primary leukocytes. In addition, these cells had markedly impaired beta1 integrin-dependent adhesion. Sequence analysis and electrophoretic mobility of beta1 and beta2 proteins from the cell line demonstrated that the defects were not a result of structural abnormalities in the integrin subunit chains themselves and suggest that the adhesive phenotype of these cells is due to one or more abnormalities of inside-out signaling mechanisms that regulate the activity of integrins of these classes. These features define a unique LAD-1 variant syndrome that may reveal important insights that are generally relevant to inside-out signaling of integrins, a molecular process that is as yet incompletely understood.

Rat seminiferous epithelium contains a unique junction (Ectoplasmic specialization) with signaling properties both of cell/cell and cell/matrix junctions

The seminiferous epithelium contains unique actin related cell-cell junctions, termed ectoplasmic specializations (ESs). Turnover of these junctions is fundamental to sperm release and to movement of spermatocytes from basal to adluminal compartments of the epithelium during spermatogenesis. In this study we report several novel observations related to the spatial and temporal distribution of integrin-related signaling molecules at ESs. We confirm the presence of beta(1)-integrin at these sites and further demonstrate co-localization of integrin linked kinase (ILK). beta(1)-Integrin and ILK were shown by immunoprecipitation to associate in whole cell lysates of seminiferous epithelium. This observation provides the first evidence for a direct beta(1)-integrin/ILK interaction in noncultured epithelium. Pan-cadherin and beta-catenin antibodies did not react at ESs. Rather, antibodies reacted with desmosome-like junctions that are present both at basal junctional complexes between Sertoli cells and at sites of attachment to spermatogenic cells. Focal adhesion kinase (FAK), a known integrin-associated molecule, did not codistribute with beta(1)-integrins and did not associate with these adhesion molecules in immunoprecipitation studies. Although FAK was expressed in the epithelium, it appeared to be limited to the cytoplasm of early spermatogenic cells. Significantly, polyclonal antibodies against phosphotyrosine-containing residues reacted strongly at ESs, with highest levels detected during sperm release and turnover of basal junction complexes. Our observations indicate that ESs share cell signaling features both of cell-cell junctions and of cell-extracellular matrix junctions.

A redox site involved in integrin activation

Integrin adhesion receptors contain an on/off switch that regulates ligand binding affinity and cell adhesion. The switch from "off" to "on" is commonly referred to as integrin activation. The objective of this study was to gain insight into the nature of the on/off switch in platelet integrin alpha(IIb)beta(3). Here, we show that a select group of the cysteines, located within the extracellular cysteine-rich domain of the beta subunit, remain unpaired. These unpaired cysteine residues exhibit the properties of a redox site involved in integrin activation. Alterations to the redox site prevent the inter-conversion between resting and active integrin. Altogether, the study establishes integrin as a direct target for redox modulation, revealing an unappreciated link between cell adhesion and redox biology.

Enhancing effect of troglitazone on porcine adipocyte differentiation in primary culture: a comparison with dexamethasone

OBJECTIVE

This study compares the effects of the thiazolidinedione, troglitazone (TGZ), dexamethasone (DEX), and DEX plus TGZ on preadipocyte differentiation and the expression of CCAAT/enhancer-binding protein-alpha (C/ EBPalpha) and peroxisome proliferator-activated receptor-gamma (PPARgamma).

RESEARCH METHODS AND PROCEDURES

Adipose tissue was obtained from postnatal pigs to isolate stromal-vascular cells. First, we applied 1, 5, or 10 microM TGZ and 10% fetal bovine serum for 3 days and counted the number of recruited preadipocytes. Next, we used either 10 microM TGZ, 80 nM DEX, or DEX plus TGZ with 10% fetal bovine serum for 3 days and then switched to serum-free medium with insulin for 6 days. On day 3 of culture, we counted preadipocytes, and on days 3 and 6 of culture, we performed immunostaining and Western blot analysis to determine the expression of C/EBPalpha and PPARgamma proteins. On day 9 of culture, we stained for lipids with oil red-O and measured the activity of glycerol phosphate dehydrogenase.

RESULTS

DEX and TGZ equally enhanced recruitment of preadipocytes and late differentiation, but these effects were not additive with DEX plus TGZ treatment. However, TGZ and DEX had a differential effect on morphogenesis; DEX-treated adipocytes were larger and organized in loose clusters, whereas TGZ-treated cells were smaller and formed compact clusters. Both agents increased C/EBPalpha expression but in a temporally distinct manner. DEX was a better inducer than TGZ, and its effect was early and temporary. However, treatment with either TGZ or DEX did not change PPARgamma protein expression as evaluated by a Western blotting, but immunocytochemistry showed a tendency for increased numbers of PPARgamma positive cells.

DISCUSSION

TGZ and DEX equally enhance early and late adipocyte differentiation, possibly by using some common pathways for preadipocyte recruitment. The differential effect on morphogenesis implies a potential differential effect on the expression of extracellular matrix components. C/EBPalpha may be the critical transcription factor involved in TGZ- and DEX-induced adipogenesis.

The LIM-only protein DRAL/FHL2 binds to the cytoplasmic domain of several alpha and beta integrin chains and is recruited to adhesion complexes

LIM proteins contain one or more double zinc finger structures (LIM domains) mediating specific contacts between proteins that participate in the formation of multiprotein complexes. We report that the LIM-only protein DRAL/FHL2, with four and a half LIM domains, can associate with alpha(3A), alpha(3B), alpha(7A), and several beta integrin subunits as shown in yeast two-hybrid assays as well as after overexpression in human cells. The amino acid sequence immediately following the conserved membrane-proximal region in the integrin alpha subunits or the C-terminal region with the conserved NXXY motif of the integrin beta subunits are critical for binding DRAL/FHL2. Furthermore, the DRAL/FHL2 associates with itself and with other molecules that bind to the cytoplasmic domain of integrin alpha subunits. Deletion analysis of DRAL/FHL2 revealed that particular LIM domains or LIM domain combinations bind the different proteins. These results, together with the fact that full-length DRAL/FHL2 is found in cell adhesion complexes, suggest that it is an adaptor/docking protein involved in integrin signaling pathways.

Integrin cytoplasmic domain-binding proteins

Integrins are a large family of cell surface receptors that mediate cell adhesion and influence migration, signal transduction, and gene expression. The cytoplasmic domains of integrins play a pivotal role in these integrin-mediated cellular functions. Through interaction with the cytoskeleton, signaling molecules, and other cellular proteins, integrin cytoplasmic domains transduce signals from both the outside and inside of the cell and regulate integrin-mediated biological functions. Identification and functional analyses of integrin cytoplasmic domain-binding proteins have been pursued intensively. In recent years, more cellular proteins have been reported to directly interact with integrin cytoplasmic domains and some of these interactions may play important roles in integrin-mediated biological responses. Integrin (β) chains, for example, interact with actin-binding proteins (e.g. talin and filamin), which form mechanical links to the cytoskeleton. These and other proteins (e.g. FAK, ILK and novel proteins such as TAP20) might also link integrins to signaling mechanisms and, in some cases (e.g. JAB1) mediate integrin-dependent gene regulation.

Disruption of integrin-dependent adhesion and survival of endothelial cells by recombinant adenovirus expressing isolated beta integrin cytoplasmic domains

We explored the possibility of using a genetic approach to inhibit integrin-mediated endothelial cell adhesion and survival. We constructed recombinant adenoviruses (Ads) expressing chimeric proteins consisting of the cytoplasmic and transmembrane domains of integrin beta1 (CH1), beta3 (CH3) or the beta1 transmembrane domain alone (CH2) connected to the extracellular domain of L3T4 placed under the control of the CMV promoter (AdCMV) or the endothelial cell specific Tie-1 promoter (AdTie). All constructs were expressed in a dose- and time-dependent manner with over 90% of cells expressing the constructs within 24 h (AdCMVs) or 72 h (AdTies) after infection. Confluent monolayers of HUVEC infected with AdCMVCH1 or AdCMVCH3 detached from the substrate in a time- and dose-dependent manner with over 95% of the cells being detached 2 days (AdCMVs) or 3 to 4 days (AdTies) after infection. Cell detachment was preceded by the disruption of focal adhesions and reorganization of the actin cytoskeleton and was associated with a reduced ligand-binding activity of beta1, while cell surface density of beta1 integrins remained unchanged. Detached cells failed to re-adhere to different matrix proteins, without, however, any specificity toward beta1 or beta3 integrin-mediated adhesion. Upon detachment, HUVEC rapidly died by apoptosis. These results demonstrate that dominant negative inhibition of integrin function is an effective approach to disrupt endothelial cell adhesion and survival in vitro.

Up-regulation of alpha 2 beta 1 integrin cell-surface expression protects A431 cells from epidermal growth factor-induced apoptosis

High epidermal growth factor (EGF) concentration (10(-8) M) induces inhibition of A431 cell proliferation, resulting in part from an apoptotic process. For some cells escaping this process, proliferation was associated with a decrease in apoptosis. Moreover, these surviving cells displayed marked morphological changes consisting of filopodia formation and cell aggregation. Disrupting cell-cell contacts by lowering extracellular calcium concentration reversed the resistance process, suggesting that apoptosis protection by aggregation may involve intercellular adhesion and cell-cell survival signals probably mediated by calcium-requiring molecules such as integrins. From a panel of integrins tested, only alpha 2 beta 1 integrin cell-surface expression was up-regulated after high apoptotic EGF treatment, and this up-regulation was not observed under a growth-stimulatory EGF concentration (10(-11) M). Double-labeling analysis (alpha 2 beta 1/DNA) implicated alpha 2 beta 1 integrin in the resistance process since 99% of cells that up-regulated alpha 2 beta 1 integrin survived a high dose of EGF. Moreover, the involvement of alpha 2 beta 1 integrin up-regulation in the survival of A431 cells that escape EGF-induced apoptosis was verified using the blocking anti-alpha 2 beta 1 integrin antibody, which was shown to decrease the survival of EGF-stimulated cells. Furthermore, under our culture conditions, alpha 2 beta 1 integrin-dependent cell-cell adhesion can be inhibited without affecting other cell-adhesive interactions, suggesting that alpha 2 beta 1 integrin is involved more directly in cell-cell interaction than in cell-substrate adhesion. Our results provide evidence that EGF-induced up-regulation of alpha 2 beta 1 integrin contributes to the enhancement of cell-cell adhesion, leading to cell aggregate formation, which permits the escape of A431 cells to EGF-induced death by alpha 2 beta 1 integrin signaling.

The cytoplasmic tyrosines of integrin subunit beta1 are involved in focal adhesion kinase activation

We have previously shown that mutation of the two tyrosines in the cytoplasmic domain of integrin subunit beta1 (Y783 and Y795) to phenylalanines markedly reduces the capability of beta1A integrins to mediate directed cell migration. In this study, beta1-dependent cell spreading was found to be delayed in GD25 cells expressing beta1A(Y783/795F) compared to that in wild-type GD25-beta1A. Focal adhesion kinase (FAK) tyrosine phosphorylation and activation were severely impaired in response to beta1-dependent adhesion in GD25-beta1A(Y783/795F) cells compared to that in wild-type GD25-beta1A or mutants in which only a single tyrosine was altered (beta1A(Y783F) or beta1A(Y795F)). Phosphorylation site-specific antibodies selective for FAK phosphotyrosine 397 indicated that the defect in FAK phosphorylation via beta1A(Y783/795F) lies at the level of the initial autophosphorylation step. Indeed, beta1A-dependent tyrosine phosphorylation of tensin and paxillin was lost in the beta1A(Y783/795F) cells, consistent with the impairment in FAK activation. In contrast, p130(CAS) overall tyrosine phosphorylation was unaffected by the beta1 mutations. Despite the defect in beta1-mediated FAK activation, FAK was still localized to focal adhesions. Taken together, the phenotype of the GD25-beta1A(Y783/795F) cells resembles, but is distinct from, the phenotype observed in FAK-null cells. These observations argue that tyrosines 783 and 795 within the cytoplasmic tail of integrin subunit beta1A are critical mediators of FAK activation and cell spreading in GD25 cells.

Immobilised echistatin promotes platelet adhesion and protein tyrosine phosphorylation

Echistatin, a 5000-Da disintegrin, is a strong competitive inhibitor of platelet alpha(IIb)beta(3) binding to fibrinogen. In addition to its antiplatelet activity, echistatin also exhibits activating properties by inducing a switch of alpha(IIb)beta(3) conformation towards an active state. However, soluble echistatin, which is a monomeric ligand, provides only receptor affinity modulation, but it is unable to activate integrin-dependent intracellular signals. Since proteins may exhibit a multivalent functionality as a result of their absorption to a substrate, in this study we evaluated whether immobilised echistatin is able to stimulate platelet adhesion and signalling. The immobilisation process led to an increase of echistatin affinity for integrin(s) expressed on resting platelets. Unlike the soluble form, immobilised echistatin bound at comparable extent either unstimulated or ADP-activated platelets. Furthermore, echistatin presented in this manner was effective in stimulating integrin-dependent protein tyrosine phosphorylation. Platelets adhering to immobilised echistatin showed a pattern of total tyrosine phosphorylated proteins resembling that of fibrinogen-attached platelets. In particular, solid-phase echistatin induced a strong phosphorylation of tyrosine kinases pp72(syk) and pp125(FAK). Inhibitors of platelet signalling, such as apyrase, prostaglandin E(1), cytochalasin D and bisindolylmaleimide, while not affecting platelet adhesion to immobilised echistatin, abolished pp125(FAK) phosphorylation. This suggests that signals activating protein kinase C function, dense granule secretion and cytoskeleton assembly might be involved in echistatin-induced pp125(FAK) phosphorylation.

Altered electrophysiological responses to mechanical stimulation and abnormal signalling through alpha5beta1 integrin in chondrocytes from osteoarthritic cartilage

OBJECTIVE

To establish whether chondrocytes from normal and osteoarthritic human articular cartilage recognize and respond to pressure induced mechanical strain in a similar manner.

DESIGN

Chondrocytes, extracted from macroscopically normal and osteoarthritic human articular cartilage obtained from knee joints at autopsy, were grown in monolayer culture and subjected to cyclical pressure-induced strain (PIS) in the absence or presence of anti-integrin antibodies, agents known to block ion channels and inhibitors of key molecules involved in the integrin-associated signalling pathways. The response of the cells to mechanical stimulation was assessed by measuring changes in membrane potential.

RESULTS

Unlike chondrocytes from normal articular cartilage, which showed a membrane hyperpolarization response to PIS, chondrocytes from osteoarthritic cartilage responded by membrane depolarization. The mechanotransduction pathway involves alpha5beta1 integrins, stretch-activated ion channels, tyrosine kinases and phospholipase C but the actin cytoskeleton and protein kinase C, which are important in the membrane hyperpolarization response in normal chondrocytes, are not necessary for membrane depolarization in osteoarthritic chondrocytes in response to PIS.

CONCLUSION

Chondrocytes derived from osteoarthritic cartilage show a different signalling pathway via alpha5beta1 integrin in response to mechanical stimulation which may be of importance in the production of phenotypic changes recognized to be present in diseased cartilage.

Peptides corresponding to the epidermal growth factor-like domain of mouse fertilin: synthesis and biological activity

A key step leading to fertilization is the binding of sperm to the egg plasma membrane. When a mammalian sperm reaches the egg plasma membrane, fertilin, an extracellular sperm membrane protein, is believed to bind to an egg plasma membrane receptor mediating fusion. Fertilin is composed of two subunits, and each subunit contains several domains, i.e., metalloprotease, disintegrin, epidermal growth factor (EGF)-like and fusion domains. This investigation examined the role of the EGF-like domains of mouse fertilin alpha and fertilin beta. Peptides corresponding to the N-terminal subdomain, containing four cysteines, and the C-terminal subdomain, containing two cysteines, were synthesized by solid-phase synthesis methods. Disulfide bonds were formed regioselectively according to the canonical EGF-like disulfide pattern. The activity of these peptides and their linear counterparts were tested for activity in a mouse in vitro fertilization assay. One peptide, 4a, corresponding to the cystine-constrained N-terminal subdomain of fertilin beta, had an activating effect on fertilization. The fertilization rate (number of eggs fertilized), fertilization index (number of sperm fused per egg), and level of polyspermy (three or more sperm fused per egg) increased in the presence of 500 microM 4a (12, 56, and 190%, respectively). Its linear counterpart, 4b, had no effect on in vitro fertilization. These data suggest that the EGF-like domain of fertilin beta has a function in sperm-egg binding and fusion. Previously, it has been shown that the fertilin beta disintegrin domain has a role in sperm-egg binding. Considered together, these studies suggest that fertilin is a modular, multidomain protein with more than one mechanism of action. This modularity may be used to design inhibitors of fertilin-receptor interactions that have high specificities for the fertilization process.

Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail

alpha-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the exposure of ligand binding sites. In the present study, the regulation of alpha(IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzymes and with possible regulatory domains in the cytosolic tail of the beta(3) subunit. Compared with intact platelets, the permeabilized platelets preserved >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand binding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3), but a reversed peptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds irreversibly to the catalytic domain of protein kinase C. Peptides corresponding to parts of the protein C inhibitor and beta(2)-glycoprotein I were used as negative controls and failed to interfere with ligand binding. Possible target domains for protein kinases are present in the cytoplasmic tail of the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (residues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 755 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/-1%, and 97+/-2% (n=3) at 500 micromol/L peptide, respectively. These observations indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in beta(3) that contribute to the regulation of this integrin.

The epithelial integrin alphavbeta6 is a receptor for foot-and-mouth disease virus

Field isolates of foot-and-mouth disease virus (FMDV) have been shown to use the RGD-dependent integrin alphavbeta3 as a cellular receptor on cultured cells. However, several other RGD-dependent integrins may have the potential to act as receptors for FMDV in vivo. Of these, alphavbeta6 is a likely candidate for use as a receptor by FMDV as it is expressed on epithelial cells, which correlates with the tissue tropism of the virus. In this report, we show that human colon carcinoma cells (SW480) that are normally nonpermissive for FMDV become susceptible to infection as a result of transfection with the integrin beta6 subunit and expression of alphavbeta6 at the cell surface. Integrin alphavbeta6 is the major site for virus attachment on the beta6-transfected cells, and binding to alphavbeta6 serves to increase the rate of virus entry into these cells. In addition, we show that virus binding and infection of the beta6-transfected cells is mediated through an RGD-dependent interaction that is specifically inhibited by a monoclonal antibody (10D5) that recognizes alphavbeta6. These studies establish a role for alphavbeta6 as a cellular receptor for FMDV.

Talin controls the exit of the integrin alpha 5 beta 1 from an early compartment of the secretory pathway

Talin is a major cytosolic protein that links the intracellular domains of beta1 and beta3 integrins to the cytoskeleton. It is required for focal adhesion assembly. However, its downregulation not only slows down cell spreading and organization of focal adhesions but also impairs the maturation of some beta1 integrins, including the fibronectin receptor alpha5beta1. To investigate this, we characterized the beta1 integrin synthesized in cells expressing talin anti-sense RNA (AT22 cells). We identified a large intracellular pool of beta1 integrins that is abnormally accumulated in an earlier compartment of the secretory pathway. In this report, we show that in talin-deficient AT22 cells, the aberrant glycosylation of integrin receptors is accompanied by a delay in the export of the integrin alpha5beta1. In normal cells, talin was found associated with beta1 integrins in an enriched membrane fraction containing Golgi and endoplasmic reticulum. Finally, microinjection of anti-talin antibodies resulted in accumulation of the integrins within the cells. These data strongly suggest that talin plays a specific role in the export of newly synthesized integrins. We propose that talin binding to the integrin may disclose a diphenylalanine export signal, which is present in the membrane-proximal GFFKR motif conserved in all integrin alpha chains.