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PubMed2005

Selective down-regulation of the alpha6-integrin subunit in melanocytes by UVB light.

In vivo, melanocytes bind to laminin (LM) molecules of the basement membrane (BM) via the integrins alpha3beta1 and alpha6beta1, and they adhere to neighbouring keratinocytes via E-cadherin. Only few studies have addressed the impact of ultraviolet (UV) light on the interaction of melanocytes with their microenvironment. In this report, we examined the influence of UVB irradiation on the expression of the most important melanocyte-adhesion molecules (E-, N-cadherin, alpha2-, alpha3-, alpha5-, alpha6-, alphaV-, beta1-, beta3-integrins and ICAM-1) in vitro by flow cytometry. We were able to demonstrate that the alpha6-integrin subunit is selectively and reversibly down-regulated by UVB in a dwzm 150ose-dependent manner. In comparison, keratinocytes lacked UVB-inducible alterations in the expression of alpha6-integrin. In the presence of LM-1, the UVB-induced down-regulation of alpha6-integrin in melanocytes was significantly reduced. Moreover, LM-1 increased the resistance of melanocytes to UVB-induced cell death, as measured by annexinV-binding analysis. This effect was reversed by preincubation with an alpha6-integrin-blocking antibody. By immunofluorescence, we could demonstrate that UVB leads to a dose-dependent internalization of alpha6-integrin, providing an obvious explanation for the down-regulation on the outer cell surface observed by flow cytometry. We suggest that adhesion to LM-1 through alpha6-integrin represents a protective mechanism for melanocytes to withstand UVB damage. Through alpha6-integrin internalization, sunburns might alter the interaction between melanocytes and the BM, resulting in apoptosis induced by loss of anchorage (anoikis). Repeated sunburns may then lead to the selection of a population of melanocytes which are capable of anchorage-independent survival, culminating in solar nevogenesis and melanoma development.

Annexin A5↗

PubMed2004

Transforming growth factor-beta 1-induced apoptosis is blocked by beta 1-integrin-mediated mitogen-activated protein kinase activation in human hepatoma cells.

Growth factors and extracellular matrices cooperatively regulate cellular behavior. However, the interactions between transforming growth factor-beta 1 (TGF-beta 1) and integrins in hepatic cells are not fully understood. We investigated the effects of beta 1-integrin on TGF-beta 1-regulated growth of hepatoma cells. Human hepatoma cell lines HepG2, Huh7, and Hep3B were stably transfected with beta 1-integrin, and the parental, and mock- and beta 1-integrin-transfected hepatoma cells were treated with TGF-beta 1. Modulation of apoptosis and pathways involved in the process were investigated. TGF-beta 1 suppressed the growth of hepatoma cells, and apoptosis was observed in Hep3B and Huh7. Hepatoma cells transfected with beta 1-integrin were protected from TGF-beta 1-induced apoptosis. Mitogen-activated protein (MAP) kinase inhibitors, PD98059, SB203580, and SP600125, abolished this protective effect of beta 1-integrin, but herbimycin A and wortmannin were ineffective. Hepatoma cells overexpressing beta 1-integrin showed increased activities of MAP kinases, and TGF-beta 1 induced sustained activation of MAP kinases in these cells, but only transient activation in mock-transfected cells. These data suggest that MAP kinases activated by beta 1-integrin provide a strong anti-apoptotic signal during TGF-beta 1-induced apoptosis in human hepatoma cells. Therefore beta 1-integrin-mediated signals may contribute to the development and progression of hepatocellular carcinoma.

Apoptosis↗

PubMed1991

Cultured human fibroblasts contain a large pool of precursor beta 1-integrin but lack an intracellular pool of mature subunit.

Previous work has shown the presence of an important intracellular pool of beta 1-integrin subunit in human skin fibroblasts as detected with monoclonal antibody DH12 [De Strooper, B., Van der Schueren, B., Jaspers, M., Saison, M., Spaepen, M., Van Leuven, F., Van den Berghe, H. & Cassiman, J. J. (1989) J. Histochem. Cytochem. 37,299-307]. To analyze this more quantitatively, a radioimmunoassay with radioiodinated monoclonal antibody was developed. The total amount of specific binding sites for monoclonal antibody DH12 on skin fibroblasts was between 0.8-1.5 x 10(6)/cell. After permeabilizing the cells with digitonin, a threefold increase in specific binding was observed, which suggested that about 60% of the total amount of beta 1-subunit was localized intracellularly. From pulse/chase experiments, it was deduced that an important pool of precursor subunit, as defined by its sensitivity to endoglycosidase treatment, existed in fibroblasts. Since in steady-state-labeling conditions, at least three to four times more precursor than mature subunit was immunoprecipitated with monoclonal antibody DH12, we suggested that the intracellular pool of beta 1-integrin subunit is mainly precursor pool. This precursor pool contains a degradation compartment and a maturation compartment. Other investigators have found evidence for a recirculating pool of mature integrin in Chinese hamster ovary cells. Therefore, the presence of a recirculating pool of integrin in human fibroblasts was also considered. The data obtained with mAb DH12 showed that less than 10% of the surface pool of integrin was internalized by endocytosis. Since, however, cross linking of beta 1-integrins with polyclonal antibodies leads to rapid endocytosis of most of the integrin, it remains possible that the quantitatively small effect was actually an artefact induced by the divalent mAb. We conclude that the intracellular pool of beta 1-integrins observed in our previous studies consists of precursor and that in skin fibroblasts no mature beta 1-integrin is available intracellularly for rapid quantitative modulations at the cell surface.

Acetylglucosaminidase↗

PubMed1997

Molecular characterisation of integrin-procollagen C-propeptide interactions.

The carboxyl-terminal propeptide of type I procollagen (CPP-I) plays a key role in regulation of collagen fibrillogenesis, and may exert feedback control of collagen biosynthesis. We have previously shown that CPP-I is a ligand for the integrin alpha2beta1 [Weston, S. A., Hulmes, D. J. S., Mould, A. P., Watson, R. B. & Humphries, M. J. (1994) Identification of the integrin alpha2beta1 as a cell surface receptor for the C-propeptide of type I procollagen, J. Biol. Chem. 269, 20982-20986] suggesting that some of the phenotypic effects of C-propeptides may be mediated by adhesion receptors. Here we have extended this work to study the molecular basis of this interaction. We have broadened the ligand range by demonstrating that the C-terminal propeptide of type II procollagen supports alpha2beta1-mediated binding of NHS human fibroblasts in cell attachment assays. Also, we have used function-blocking antibodies in cell attachment and solid-phase binding assays with purified integrin to expand the CPP-I receptor family, showing that integrin alpha1beta1 is also a receptor for CPP-I. Integrin alpha-subunit A-domains are known to be major ligand-binding sites and recombinant alpha1 and alpha2 subunit A-domains were able to bind CPP-I. Finally we have shown that peptides corresponding to potential integrin-binding sequences in CPP-I do not mediate integrin-CPP-I adhesion. Taken together, these studies indicate that the interactions between C-propeptides and integrins are more numerous than previously reported, that C-propeptides are a new class of molecule which bind to A-domains, and that the integrin-C-propeptide interaction does not utilise established peptide motifs.

Amino Acid Sequence↗

PubMed1996

Luminal and glandular endometrial epithelium express integrins differentially throughout the menstrual cycle: implications for implantation, contraception, and infertility.

PROBLEM: Integrins belong to a family of cell adhesion molecules that are present on virtually all cells. The temporal and spatial expression of these important proteins on the human endometrium suggests that certain integrins may participate in the cascade of molecular events leading to successful implantation. METHODS: Using immunohistochemistry, we studied the expression of 12 different integrins in up to 600 samples of human endometrium throughout the menstrual cycle. Intensity and distribution of staining was determined using the semiquantitative HSCORE, with specific focus on the differences between glandular and luminal expression. RESULTS: We noted that the glandular and luminal epithelium undergo independent alterations in integrin expression throughout the menstrual cycle. Specifically, glandular epithelium express certain integrins only during the window of implantation, while luminal epithelium down-regulate certain integrins during this time. The expression of one integrin (the alpha v beta 3 vitronectin receptor) on both luminal and glandular epithelium coincides with the time of embryo attachment; aberrant expression of this integrin is associated with infertility. CONCLUSION: It appears that the endometrium is a unique tissue with regard to the number of integrins that undergo temporal and spatial changes during the menstrual cycle. These data may offer new directions for the development of a novel contraceptive approach targeted to the endometrium as well as a better understanding of occult causes of infertility in women.

Contraception, Immunologic↗

PubMed2006

Integrin beta(1A) upregulates p27 protein amount at the post-translational level in human hepatocellular carcinoma cell line SMMC-7721.

Integrins mediate many fundamental cellular processes by binding to components of the extracellular matrix. We showed previously that integrin beta(1A) could inhibit cell proliferation. Integrin beta(1A) stimulated the promoter activity of p21(cip1) and enhanced its transcription in SMMC-7721 cells. In this study, we demonstrated that integrin beta(1A) upregulated p27(kip1) at the post-translational level in SMMC-7721 cells. Our results showed that integrin beta(1A) increased the p27 protein amount, both in cytoplasm and nucleus, but did not affect the p27 mRNA amount. Cycloheximide treatment experiment revealed that the half-life of p27 protein was prolonged in integrin beta1A overexpressing cells, indicating that integrin beta(1A) inhibited the degradation of p27 protein. Our data also provided evidence that both the proteasome and calpain were involved in the degradation of p27 protein in SMMC-7721 cells. Integrin beta(1A) decreased the Skp2 expression and repressed the activity of calpain during G1 phase in SMMC-7721 cells. Taken together, these results indicated that integrin beta(1A) might upregulate the protein amount of p27 through repressing Skp2-dependent proteasome degradation and calpain-mediated proteolysis in SMMC-7721 cells.

Calpain↗

PubMed1998

Integrins and development: how might these receptors regulate differentiation of the lens.

Integrins transduce both internal signals and signals from the matrix. These interactions between integrins, their extracellular matrix ligands, and their cytoskeletal partners play an important role in the regulation of cellular differentiation. We have shown them to be important in lens cell differentiation. In the lens capsule there is a compartmentalization of matrix components with fibronectin, primarily localized to the anterior capsule, and tenascin in the posterior capsule. Integrins are developmentally regulated in the lens. alpha 5 beta 1 integrin, like fibronectin, is primarily associated with the lens epithelial cells, where together they are likely to be important in regulation of adhesion and proliferation. alpha 6A beta 1, the integrin laminin receptor, is expressed at its highest levels in the equatorial epithelium and the peripheral fiber cells, both migratory populations. Because laminin is uniformly distributed in the lens capsule, such changes in alpha 6A integrin expression are likely critical to the cell's ability to regulate its response to laminin in the matrix. The organization of cytoskeletal molecules associated with the integrin cytoplasmic face also changes with development. In the epithelial regions of the lens, where the initiation of lens cell differentiation occurs, expression of the cytoskeletal proteins involved in cell-substrate interactions, talin, alpha-actinin, and the signaling proteins, are high. In the fiber cell region of the lens, where the cells establish stable cell-cell contacts, vinculin predominates and becomes highly associated with the cytoskeletal fraction. The role of integrins in lens development is not only regulated by changes in the expression of different integrin receptors but is also closely correlated with the expression and organization of the molecules with which they associate.

Animals↗

PubMed2001

Integrin pattern and effect on contraction in cultured testicular peritubular myoid cells.

PROBLEM: Neither the integrin pattern nor the biological functions of integrins have been extensively documented in human cultured testicular peritubular myoid cells (TPMC). The integrin pattern and the presence of some proteins of the immunoglobulin superfamily on human TPMC as well as the role of integrins in TPMC contraction were examined. METHOD OF STUDY: Integrin expression was evaluated by immunofluorescence and FACS analysis. To assess the role of integrin in TPMC contraction, human and rat cells were added to a collagen gel system and exposed to contractile stimuli. RESULTS: The immunofluorescence and cytofluorimetric analysis showed that human cultured TPMC express alpha1, alpha2, alpha3, alpha4, alpha5, alpha6, alphav, beta1, beta3, and beta4 integrin subunits, and significant amounts of intercellular adhesion molecule-1 (ICAM-1), whereas they do not present alpha4, beta2, beta7 subunits, nor intercellular adhesion molecule-2 (ICAM-2) and neural cell adhesion molecule (NCAM). The preincubation of human cells with an anti-beta1 mAb and of rat cells with a polyclonal anti-beta1 antibody inhibited TPMC contraction induced by different contractile stimuli. CONCLUSION: Our investigation documented a broad integrin pattern on human cultured TPMC as well as a role for integrins in human and rat TPMC contraction.

Adult↗

PubMed1999

Targeted disruption of fibronectin-integrin interactions in human gingival fibroblasts by the RI protease of Porphyromonas gingivalis W50.

Cell surface integrins mediate interactions between cells and their extracellular matrix and are frequently exploited by a range of bacterial pathogens to facilitate adherence and/or invasion. In this study we examined the effects of Porphyromonas gingivalis proteases on human gingival fibroblast (HGF) integrins and their fibronectin matrix. Culture supernatant from the virulent strain W50 caused considerably greater loss of the beta1 integrin subunit from HGF in vitro than did that of the beige-pigmented strain W50/BE1. Prior treatment of the W50 culture supernatant with the protease inhibitor Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) blocked its effects on cultured cells, indicating that this process is proteolytically mediated. Purified arginine-specific proteases from P. gingivalis W50 were able to mimic the effects of the whole-culture supernatant on loss of beta1 integrin expression. However purified RI, an alpha/beta heterodimer in which the catalytic chain is associated with an adhesin chain, was 12 times more active than RIA, the catalytic monomer, in causing loss of the alpha5beta1 integrin (fibronectin receptor) from HGF. No effect was observed on the alphaVbeta3 integrin (vitronectin receptor). The sites of action of RI and RIA were investigated in cells exposed to proteases pretreated with TLCK to inactivate the catalytic component. Use of both monoclonal antibody 1A1, which recognizes only the adhesin chain of RI, and a rabbit antibody against P. gingivalis whole cells indicated localization of RI on the fibroblasts in a clear, linear pattern typical of that seen with fibronectin and alpha5beta1 integrin. Exact colocalization of RI with fibronectin and its alpha5beta1 receptor was confirmed by double labeling and multiple-exposure photomicroscopy. In contrast, RIA bound to fibroblasts in a weak, patchy manner, showing only fine linear or granular staining. It is concluded that the adhesin component of RI targets the P. gingivalis arginine-protease to sites of fibronectin deposition on HGF, contributing to the rapid loss of both fibronectin and its main alpha5beta1 integrin receptor. Given the importance of integrin-ligand interactions in fibroblast function, their targeted disruption by RI may represent a novel mechanism of damage in periodontal disease.

Animals↗

PubMed1999

Structure of adenovirus complexed with its internalization receptor, alphavbeta5 integrin.

The three-dimensional structure of soluble recombinant integrin alphavbeta5 bound to human adenovirus types 2 and 12 (Ad2 and -12) has been determined at approximately 21-A resolution by cryoelectron microscopy (cryo-EM). The alphavbeta5 integrin is known to promote Ad cell entry. Cryo-EM has shown that the integrin-binding RGD (Arg-Gly-Asp) protrusion of the Ad2 penton base protein is highly mobile (P. L. Stewart, C. Y. Chiu, S. Huang, T. Muir, Y. Zhao, B. Chait, P. Mathias, and G. R. Nemerow, EMBO J. 16:1189-1198, 1997). Sequence analysis indicated that the Ad12 RGD surface loop is shorter than that of Ad2 and probably less flexible, hence more suitable for structural characterization of the Ad-integrin complex. The cryo-EM structures of the two virus-receptor complexes revealed a ring of integrin density above the penton base of each virus serotype. As expected, the integrin density in the Ad2 complex was diffuse while that in the Ad12 complex was better defined. The integrin consists of two discrete subdomains, a globular domain with an RGD-binding cleft approximately 20 A in diameter and a distal domain with extended, flexible tails. Kinetic analysis of Ad2 interactions with alphavbeta5 indicated approximately 4.2 integrin molecules bound per penton base at close to saturation. These results suggest that the precise spatial arrangement of five RGD protrusions on the penton base promotes integrin clustering and the signaling events required for virus internalization.

Adenoviruses, Human↗

PubMed2005

A novel integrin alpha5beta1 binding domain in module 4 of connective tissue growth factor (CCN2/CTGF) promotes adhesion and migration of activated pancreatic stellate cells.

BACKGROUND: Connective tissue growth factor (CCN2) is upregulated in pancreatic fibrosis and desmoplastic pancreatic tumours. CCN2 interacts with integrin alpha5beta1 on pancreatic stellate cells (PSC) in which it stimulates fibrogenesis, adhesion, migration, and proliferation. AIM: To determine the structural domain(s) in CCN2 that interact with integrin alpha5beta1 to regulation PSC functions. METHODS: Primary activated rat PSC were tested for their adherence to isoforms of CCN2 comprising modules 1-4 (CCN2(1-4)), modules 3-4 (CCN2(3-4)), module 3 alone (CCN2(3)), or module 4 alone (CCN2(4)). Adhesion studies were performed in the presence of EDTA, divalent cations, anti-integrin alpha5beta1 antibodies, CCN2 synthetic peptides, or heparin, or after pretreatment of the cells with heparinase, chondroitinase, or sodium chlorate. CCN2 integrin alpha5beta1 binding was analysed in cell free systems. The ability of CCN2(1-4), CCN2(3-4), or CCN2(4) to stimulate PSC migration was evaluated in the presence of anti-integrin alpha5beta1 or heparin. RESULTS: PSC adhesion was stimulated by CCN2(1-4), CCN2(3-4), or CCN2(4) and supported by Mg2+ but not Ca2+. CCN2(4) supported PSC adhesion or migration were blocked by anti-integrin alpha5beta1 antibodies or by treatment of cells with heparinase or sodium chlorate. A direct interaction between CCN2(4) and integrin alpha5beta1 was demonstrated in cell free assays. The sequence GVCTDGR in module 4 mediated the binding between CCN2(4) and integrin alpha5beta1 as well as CCN2(4) mediated PSC adhesion and migration. CONCLUSIONS: A GVCTDGR sequence in module 4 of CCN2 is a novel integrin alpha5beta1 binding site that is essential for CCN2 stimulated functions in PSC and which represents a new therapeutic target in PSC mediated fibrogenesis.

Animals↗

PubMed2005

Chemokine receptor CXCR4-beta1 integrin axis mediates tumorigenesis of osteosarcoma HOS cells.

It is known that beta1 integrins mediate the migratory response of cells to chemokine stimulation. Also, both beta1 integrins and chemokines have roles in tumor development. In the present study, the beta1 integrin-chemokine axis is assessed using human osteosarcoma (HOS) transfectant cells expressing the CXCR4 receptor for chemokine SDF-1 (CXCL12). We first identified in vitro the specific beta1 integrins that mediated the migratory response to SDF-1 stimulation. Results showed that on collagen type I and laminin, the chemotactic response to SDF-1 was predominantly mediated by alpha2beta1 integrin. On fibronectin, SDF-1-stimulated chemotaxis involved both alpha4beta1 and alpha5beta1 integrins. A comparison of the transfectant clones expressing CXCR4 at low, intermediate, and high levels and the control transfectant revealed that the transfectant clones migratory response in vitro and their ability to form tumors in vivo was related to their levels of CXCR4 expression. In addition, treatment by injection with mAbs to CXCR4, integrin alpha2beta1, or integrin alpha5beta1 effectively inhibited the growth of HOS-CXCR4 transfectant cells in vivo. Therefore, our results show that the beta1 integrins that mediated the migratory response were also functionally linked to the enhanced tumor growth of CXCR4-expressing HOS transfectant cells.

Animals↗

PubMed2003

Effects of thrombin on interactions between beta3-integrins and extracellular matrix in platelets and vascular cells.

The beta3-integrin family consists of alphaIIbbeta3 (also known as glycoprotein IIb/IIIa) and alpha(v)beta3. alphaIIbbeta3 is found on platelets and megakaryocytes and has an essential role in hemostasis. alpha(v)beta3 has a broader distribution, and it functions in angiogenesis, neointimal formation after vascular injury, and leukocyte trafficking. There are important interactions between thrombin and beta3-integrins relative to both "inside-out" (integrin activation) and "outside-in" (modification of cellular events by ligand binding to integrins) signaling. Thrombin, by binding to G protein-coupled, protease-activated receptors, is a potent activator of alphaIIbbeta3. Conversely, outside-in signaling through alphaIIbbeta3 amplifies events initiated by thrombin and is necessary for full platelet spreading, platelet aggregation, granule secretion, and the formation of a stable platelet thrombus. In smooth muscle cells, alpha(v)beta3-integrins influence various responses to thrombin, including proliferation, c-Jun NH2-terminal kinase-1 activation, and focal adhesion formation. Other interactions between beta3-integrins and thrombin include beta3-integrin promotion of the generation of thrombin by localizing prothrombin to cellular surfaces and/or enhancing the formation of procoagulant microparticles and the requirement of beta3-integrin function for platelet-dependent clot retraction. In summary, there is increasing evidence that interactions between beta3-integrins and thrombin play important roles in the regulation of hemostatic and vascular functions.

Animals↗

PubMed1998

Beta1 integrins participate in the hypertrophic response of rat ventricular myocytes.

Multiple signaling pathways have been implicated in the hypertrophic response of ventricular myocytes, yet the importance of cell-matrix interactions has not been extensively examined. Integrins are cell-surface molecules that link the extracellular matrix to the cellular cytoskeleton. They can function as cell signaling molecules and transducers of mechanical information in noncardiac cells. Given these properties and their abundance in cardiac cells, we evaluated the hypothesis that beta1 integrin function is involved in the alpha1-adrenergic mediated hypertrophic response of neonatal rat ventricular myocytes. The hypertrophic response of this model required interaction with extracellular matrix proteins. Specificity of these results was confirmed by demonstrating that ventricular myocytes plated onto an anti-beta1 integrin antibody supported the hypertrophic gene response. Adenovirus-mediated overexpression of beta1 integrin augmented the myocyte hypertrophic response when assessed by protein synthesis and atrial natriuretic factor production, a marker gene of hypertrophic induction. DNA synthesis was not altered by integrin overexpression. Transfection of cultured cardiac myocytes with either the ubiquitously expressed beta1A integrin or the cardiac/skeletal muscle-specific beta1 isoform (beta1D) activated reporter expression from both the atrial natriuretic factor and myosin light chain-2 ventricular promoters, genetic markers of ventricular cell hypertrophy. Finally, suppression of integrin signaling by overexpression of free beta1 integrin cytoplasmic domains inhibited the adrenergically mediated atrial natriuretic factor response. These findings show that integrin ligation and signaling are involved in the cardiac hypertrophic response pathway.

Adenoviridae↗

PubMed1991

Novel integrin alpha and beta subunit cDNAs identified in airway epithelial cells and lung leukocytes using the polymerase chain reaction.

The integrins are a large group of cell surface glycoproteins that mediate cell-matrix and cell-cell adhesive interactions. Integrins play a role in normal lung development, in host defense against pulmonary infection, and in the pathogenesis of the adult respiratory distress syndrome. Integrins are heterodimers consisting of one alpha subunit and one beta subunit. We identified consensus sequences within integrin subunits and used oligonucleotide primers based on these sequences to amplify cDNA by the polymerase chain reaction (PCR). We previously reported the use of this homology PCR technique for the identification of one novel integrin beta subunit, beta 6, from guinea pig airway epithelial cells. Here we demonstrate that primers based on alpha subunit consensus sequences can also be used for homology PCR. We have used the alpha and beta subunit primers to amplify and clone a large variety of integrin partial cDNAs from several cell types and species. Comparison of the deduced amino acid sequences reveals a high degree of cross-species conservation (86 to 98% identity). One alpha subunit (identified in guinea pig airway epithelial cells) and one beta subunit (identified in rabbit leukocytes obtained by bronchoalveolar lavage and in human and mouse leukocyte cell lines) have novel sequences that are related to but clearly distinct from all previously reported integrin sequences (24 to 61% identity). These novel cDNAs are very likely to encode previously unsequenced integrin subunit proteins that are expressed in the lung. Homology PCR is a powerful technique for the identification of known and novel integrin alpha and beta subunit cDNAs in cells from the lung and other organs.

Amino Acid Sequence↗

PubMed1998

Extracellular signal-regulated kinase and c-Jun NH2-terminal kinase activation by mechanical stretch is integrin-dependent and matrix-specific in rat cardiac fibroblasts.

Integrins, which connect the cytoskeleton to the extracellular matrix and mediate a variety of signaling cascades, may transduce mechanical stimuli into biochemical signals. We studied integrin- and matrix-dependent activation of extracellular signal-regulated kinase (ERK2), c-Jun NH2-terminal kinase (JNK1), and p38 in response to 4% static biaxial stretch in rat cardiac fibroblasts. ERK2 and JNK1, but not p38, were rapidly activated by stretch when the fibroblasts were allowed to synthesize their own matrices. When the cells were limited to specific matrix substrates, ERK2 and JNK1 were differentially activated: ERK2 was only activated when the cells were plated on fibronectin, while JNK1 was activated when the cells were plated on fibronectin, vitronectin, or laminin. Plating cells on collagen before stretching did not activate either kinase. Adhesion to all matrices was integrin-dependent because it could be blocked by inhibitors of specific integrins. ERK2 activation could be blocked with a combination of anti-alpha4 and -alpha5 antibodies and an arginine-glycine-aspartic acid (RGD) peptide, while the antibodies or peptide used separately failed to block ERK2 activation. This result suggests that at least two integrins, alpha4beta1 and an RGD-directed, non-alpha5beta1 integrin, activate ERK2 in response to mechanical stimulation. Activation of JNK1 could not be blocked with the inhibitors, suggesting that an RGD-independent integrin or integrins other than alpha4beta1 can activate JNK1 in cells adherent to fibronectin. This study demonstrates that integrins act as mechanotransducers, providing insight into potential mechanisms for in vivo responses to mechanical stimuli.

Animals↗

PubMed1999

Characterization of integrin expression in islets isolated from hamster, canine, porcine, and human pancreas.

The reasons for the failure of clinical islet transplantation remain obscure. Islet isolation, however, exposes the islet to variety of cellular stresses, including disruption of the cell-matrix relationship, an event associated with apoptosis. The cell-matrix relationship is characterized by an interaction between cell surface integrin receptors and matrix molecules of the surrounding basement membrane (BM). The purpose of this study was to characterize integrin expression and the distribution of the peri-insular BM in human, porcine, canine, and hamster pancreas, and after routine islet isolation. Whereas islets in the porcine pancreas do not have a demonstrable BM, islets in the human, canine, and hamster pancreas have an almost continuous BM with very little direct exocrine to endocrine cell-cell contact. After islet isolation, the BM was destroyed, only to be reestablished during the period of culture. In the pancreas of all four species, integrin alpha3 was expressed only on islet cells, and integrin alpha5 was present on islet cells as well as on acinar, centroacinar, and duct cells. Integrin alphaV was detected only in human and canine pancreas. Integrin beta1 was demonstrated only in the human pancreas. In isolated islets, integrin alpha3, alpha5, and alphaV expression decreased during the culture period and the intensity of the staining was observed to be coincident with the distribution of the BM. In summary, this is the first report of integrin expression in hamster, canine, porcine, and human islets. After islet isolation, the altered islet cell-matrix relationship is reflected both in the decrease in integrin expression and in the destruction of the peri-insular BM. These profound changes will need to be considered as the process of islet isolation for transplantation is refined. (J Histochem Cytochem 47:499-506, 1999)

Adult↗

PubMed2004

Structures of integrin domains and concerted conformational changes in the bidirectional signaling mechanism of alphaIIbbeta3.

Integrins are heterodimeric type I transmembrane cell-adhesive receptors whose affinity for ligands is regulated by tertiary and quaternary conformational changes that are transmitted from the cytoplasmic tails to the extracellular ectodomains during the transition from the inactive to the active state. Receptor occupancy initiates further structural alterations that transduce signals across the plasma membrane and result in receptor clustering and recruitment of signaling molecules and cytoskeletal rearrangements at the integrin's cytoplasmic domains. The large distance between the intracellular cytoplasmic domains and the ligand-binding site, which in an extended conformation spans more that 200 A, imposes a complex mechanism of interdomain communication for the bidirectional information flow across the plasma membrane. Significant progress has recently been made in elucidating the crystal and electron microscopy structures of integrin ectodomains in its unliganded and liganded states, and the nuclear magnetic resonance solution structures of stalk domains and the cytoplasmic tails. These structures revealed the location of sites that are functionally important and provided the basis for defining new models of integrin activation and signaling through bidirectional conformational changes, and for understanding the structural basis of the cation-dependent ligand-binding specificity of integrins. Platelet integrin alphaIIbbeta3 has served as a paradigm for many aspects of the structure and function of integrins The aim of this minireview is to combine recent structural and biochemical studies on integrin receptors that converge into a model of the tertiary and quaternary conformational changes in alphaIIbbeta3 and other homologous integrins that propagate inside-out and outside-in signals.

Amino Acid Sequence↗