TGF-beta is a critical mediator of acute lung injury.

We have shown that the integrin alphavbeta6 activates latent TGF-beta in the lungs and skin. We show here that mice lacking this integrin are completely protected from pulmonary edema in a model of bleomycin-induced acute lung injury (ALI). Pharmacologic inhibition of TGF-beta also protected wild-type mice from pulmonary edema induced by bleomycin or Escherichia coli endotoxin. TGF-beta directly increased alveolar epithelial permeability in vitro by a mechanism that involved depletion of intracellular glutathione. These data suggest that integrin-mediated local activation of TGF-beta is critical to the development of pulmonary edema in ALI and that blocking TGF-beta or its activation could be effective treatments for this currently untreatable disorder.

Effect of antibody against integrin alpha4 on bleomycin-induced pulmonary fibrosis in mice.

Integrins are a family of transmembrane glycoproteins that can interact with components of the extracellular matrix. The alpha4beta1 and alpha4beta7 integrins are heterodimeric leukocyte cell surface molecules critical to their cell and matrix adhesive interactions. Evidence for a central role for the alpha4 integrins in leukocyte pathophysiology in the lung is well documented. In this study, we tested the hypothesis that neutralizing antibody for integrin alpha4 (PS2) may reduce bleomycin (BL)-induced lung fibrosis in vivo. Male C57BL/6 mice were injected intratracheally with saline (SA) or BL (0.08 U/mouse) followed by intraperitoneal injection of SA, isotype control antibody (1E6), or PS2 (100 microg) three times a week. Twenty-one days after the intratracheal instillation, mice were killed for bronchoalveolar lavage (BAL), biochemical, histopathological, and immunohistological analyses. Treatment with PS2 significantly reduced BL-induced increases in lung lipid peroxidation and hydroxyproline content. Lung histopathology also showed reduced fibrotic lesions in the BL-treated lungs by treatment with PS2. BL-treated mouse lungs also showed induction of cells with the myofibroblast phenotype, as indicated by the increased expression of alpha-smooth muscle actin (alphaSMA), whereas treatment with PS2 minimized the BL-induced alphaSMA expression. Furthermore, treatment with PS2 reduced the BL-induced increase in the BAL total cell number, and attenuated the BL-induced increase in the BAL protein level. It is concluded that integrin alpha4 may play an important role in BL-induced pulmonary fibrosis, and the use of anti-alpha4 antibody offers therapeutic antifibrotic potential in vivo.

Recombinant soluble transforming growth factor beta type II receptor ameliorates radiation enteropathy in mice.

BACKGROUND & AIMS: Transforming growth factor (TGF)-beta has been implicated in many fibrotic conditions. However, its mechanistic role in radiation toxicity is equivocal despite compelling correlative evidence. This study assessed whether in vivo administration of a soluble TGF-beta type II receptor (TbetaR-II) protein ameliorates intestinal radiation injury (radiation enteropathy). METHODS: A recombinant fusion protein, consisting of the extracellular portion of mouse TbetaR-II and the Fc portion of mouse immunoglobulin (Ig) G, was produced. A 5-cm segment of mouse ileum was exposed to 19 Gy x-radiation. TbetaR-II:Fc fusion protein (1 mg/kg every other day) or mouse IgG was administered from 2 days before to 6 weeks after irradiation. Radiation injury was assessed at 6 weeks using quantitative histology, morphometry, and immunohistochemistry. Collagen was measured colorimetrically, and TGF-beta1 messenger RNA was assessed with fluorogenic probe reverse-transcription polymerase chain reaction. RESULTS: Compared with IgG controls, TbetaR-II:Fc-treated mice exhibited less structural injury, preservation of mucosal surface area, and less intestinal wall fibrosis. Intestinal TGF-beta1 messenger RNA increased in TbetaR-II:Fc-treated mice, whereas TGF-beta immunoreactivity decreased. TbetaR-II:Fc treatment increased crypt cell proliferation but otherwise did not affect unirradiated intestine. CONCLUSIONS: Long-term modulation of TGF-beta with a TbetaR-II:Fc fusion protein is feasible and ameliorates radiation enteropathy. These data confirm the putative role of TGF-beta in intestinal radiation fibrosis.

Regulation of inflammation by collagen-binding integrins alpha1beta1 and alpha2beta1 in models of hypersensitivity and arthritis.

Adhesive interactions play an important role in inflammation by promoting leukocyte attachment and extravasation from the vasculature into the peripheral tissues. However, the importance of adhesion molecules within the extracellular matrix-rich environment of peripheral tissues, in which cells must migrate and be activated, has not been well explored. We investigated the role of the major collagen-binding integrins, alpha1beta1 and alpha2beta1, in several in vivo models of inflammation. mAb's against murine alpha1 and alpha2 were found to significantly inhibit effector phase inflammatory responses in animal models of delayed-type hypersensitivity (DTH), contact hypersensitivity (CHS), and arthritis. Mice that were alpha1-deficient also showed decreased inflammatory responses in the CHS and arthritis models when compared with wild-type mice. Decreased leukocyte infiltration and edema formation accompanied inhibition of antigen-specific models of inflammation, as nonspecific inflammation induced by croton oil was not inhibited. This study demonstrates the importance in vivo of alpha1beta1 and alpha2beta1, the collagen-binding integrins, in inflammatory diseases. The study also extends the role of integrins in inflammation beyond leukocyte attachment and extravasation at the vascular endothelial interface, revealing the extracellular matrix environment of peripheral tissues as a new point of intervention for adhesion-based therapies.

Reduction of bleomycin induced lung fibrosis by transforming growth factor beta soluble receptor in hamsters.

BACKGROUND: Transforming growth factor beta (TGF-beta) is a key mediator of collagen synthesis in the development of lung fibrosis. It has previously been shown that the administration of TGF-beta antibody and TGF-beta binding proteoglycan, decorin, reduced bleomycin (BL) induced lung fibrosis in animals. The present study was carried out to investigate whether intratracheal instillation of TGF-beta soluble receptor (TR) would minimise the BL induced lung fibrosis in hamsters. METHODS: The effect of a recombinant TR (TGFbetaRII) on the lung collagen accumulation was evaluated in a BL hamster model of pulmonary fibrosis. Animals were divided into four groups and intratracheally injected with saline or BL at 6.5 U/4 ml/kg followed by intratracheal instillation of phosphate buffered saline (PBS) or 4 nmol TR in 0.3 ml twice a week. Twenty days after the first intratracheal instillation the hamsters were killed for bronchoalveolar lavage (BAL) fluid, biochemical, and histopathological analyses. RESULTS: Treatment of hamsters with TR after intratracheal instillation of BL significantly reduced BL induced lung fibrosis as shown by decreases in the lung hydroxyproline level and prolyl hydroxylase activity, although they were still significantly higher than those of the saline control. Histopathological examination showed a considerable decrease in BL induced fibrotic lesions by TR treatment. However, TR did not prevent the BL induced increases in total cells and protein in the BAL fluid. CONCLUSIONS: These results suggest that TR has antifibrotic potential in vivo and may be useful in the treatment of fibrotic diseases where increased TGF-beta is associated with excess collagen accumulation.

Crystal structure of the alpha1beta1 integrin I-domain: insights into integrin I-domain function.

The alpha1beta1 integrin is a major cell surface receptor for collagen. Ligand binding is mediated, in part, through a 200 amino acid inserted 'I'-domain contained in the extracellular part of the integrin alpha chain. Integrin I-domains contain a divalent cation binding (MIDAS) site and require cations to interact with integrin ligands. We have determined the crystal structure of recombinant I-domain from the rat alpha1beta1 integrin at 2.2 A resolution in the absence of divalent cations. The alpha1 I-domain adopts the dinucleotide binding fold that is characteristic of all I-domain structures that have been solved to date and has a structure very similar to that of the closely related alpha2beta1 I-domain which also mediates collagen binding. A unique feature of the alpha1 I-domain crystal structure is that the MIDAS site is occupied by an arginine side chain from another I-domain molecule in the crystal, in place of a metal ion. This interaction supports a proposed model for ligand-induced displacement of metal ions. Circular dichroism spectra determined in the presence of Ca2+, Mg2+ and Mn2+ indicate that no changes in the structure of the I-domain occur upon metal ion binding in solution. Metal ion binding induces small changes in UV absorption spectra, indicating a change in the polarity of the MIDAS site environment.

Divalent cations stabilize the alpha 1 beta 1 integrin I domain.

Recent structural and functional analyses of alpha integrin subunit I domains implicate a region in cation and ligand binding referred to as the metal ion-dependent adhesion site (MIDAS). Although the molecular interactions between Mn2+ and Mg2+ and the MIDAS region have been defined by crystallographic analyses, the role of cation in I domain function is not well understood. Recombinant alpha 1 beta 1 integrin I domain (alpha1-I domain) binds collagen in a cation-dependent manner. We have generated and characterized a panel of antibodies directed against the alpha1-I domain, and selected one (AJH10) that blocks alpha 1 beta 1 integrin function for further study. The epitope of AJH10 was localized within the loop between the alpha 3 and alpha 4 helices which contributes one of the metal coordination sites of the MIDAS structure. Kinetic analyses of antibody binding to the I domain demonstrate that divalent cation is required to stabilize the epitope. Denaturation experiments demonstrate that cation has a dramatic effect on the stabilization of the I domain structure. Mn2+ shifts the point at which the I domain denatures from 3.4 to 6.3 M urea in the presence of the denaturant, and from 49.5 to 58.6 degrees C following thermal denaturation. The structural stability provided to the alpha1-I domain by divalent cations may contribute to augmented ligand binding that occurs in the presence of these cations.

Soluble transforming growth factor-beta type II receptor inhibits negative remodeling, fibroblast transdifferentiation, and intimal lesion formation but not endothelial growth.

Using the rat balloon catheter denudation model, we examined the role of transforming growth factor-beta (TGF-beta) isoforms in vascular repair processes. By en face in situ hybridization, proliferating and quiescent smooth muscle cells in denuded vessels expressed high levels of mRNA for TGF-beta1, TGF-beta2, TGF-beta3, and lower levels of TGF-beta receptor II (TGF-betaRII) mRNA. Compared with normal endothelium, TGF-beta1 and TGF-beta2, as well as TGF-betaRII, mRNA were upregulated in endothelium at the wound edge. Injected recombinant soluble TGF-betaRII (TGF-betaR:Fc) localized preferentially to the adventitia and developing neointima in the injured carotid artery, causing a reduction in intimal lesion formation (up to 65%) and an increase in lumen area (up to 88%). The gain in lumen area was largely due to inhibition of negative remodeling, which coincided with reduced adventitial fibrosis and collagen deposition. Four days after injury, TGF-betaR:Fc treatment almost completely inhibited the induction of smooth muscle alpha-actin expression in adventitial cells. In the vessel wall, TGF-betaR:Fc caused a marked reduction in mRNA levels for collagens type I and III. TGF-betaR:Fc had no effect on endothelial proliferation as determined by reendothelialization of the denuded rat aorta. Together, these findings identify the TGF-beta isoforms as major factors mediating adventitial fibrosis and negative remodeling after vascular injury, a major cause of restenosis after angioplasty.

Epitope mapping of a function-blocking beta 1 integrin antibody by phage display.

Integrins are a major class of cell surface receptors involved in cell-cell and cell-matrix adhesion and communication. Ha2/11 is a function-blocking anti-rat beta 1 integrin hamster IgM that should be a useful reagent for understanding beta 1 integrin function. We demonstrate that Ha2/11 cross reacts with human, Xenopus, and Drosophila beta 1 integrins, and use phage display to map the epitope for Ha2/11 to residues within the sequence LRSGEPQTF which lies 18 amino acids proximal to the putative I domain in beta 1 integrins. Monoclonal antibody mapping experiments, mutational analyses, and direct binding assays have implicated integrin I domains in both cation and ligand binding. Our data therefore suggest that Ha2/11 blocks beta 1 integrin function by interfering with I domain-mediated ligand binding.

Fibronectin type III repeats mediate RGD-independent adhesion and signaling through activated beta1 integrins.

Many cell-surface and extracellular matrix proteins contain multiple modular domains known as fibronectin type III (FNIII) repeats. Cells adhere to the extracellular matrix proteins fibronectin and tenascin in part by the interaction of certain integrins with the Arg-Gly-Asp (RGD) sequence, displayed on specific FNIII repeats. We have found that, after experimental activation of beta1 integrins, a number of cell types adhere and spread on FNIII repeats lacking RGD, derived from extracellular matrix proteins and cytokine receptors. Interaction between individual FNIII domains and beta1 integrins mediates focal adhesion kinase phosphorylation and subsequent stress fiber and focal contact formation. These data suggest that many FNIII-containing proteins may bind and signal through activated beta1 integrins, dramatically expanding the potential for integrin-dependent intercellular and cell-matrix communication.

A functional monoclonal antibody recognizing the human alpha 1-integrin I-domain.

Alpha 1 beta 1 heterodimer is a member of the integrin receptor superfamily that has been described to be involved in cell-matrix binding through its interaction with collagens, fibronectin and laminin. The alpha 1 integrin belongs to a subset of I-domain containing integrins that includes alpha M, alpha L, alpha X and alpha 2. In this study we describe an anti-alpha 1 mAb (FB12) that recognizes an epitope located in the human alpha 1 I-domain, since the mAb can bind to human, but not to rat, recombinant I-domain GST fusion protein. FB12 mAb efficiently and specifically inhibits the binding of activated human lymphocytes to laminin, collagen and fibronectin. These data support the notion that the alpha 1 I-domain itself has an important role in receptor-ligand binding. In particular, we show that alpha 1 integrin-dependent lymphocyte adhesion to fibronectin is I-domain mediated, at variance with the RGD-dependent adhesion which seems to be mediated by the beta 1 rather than the alpha 1 integrin chain. Lastly, the overexpression of the alpha 1-integrin by stromal cells and blood vessels of solid tumors may suggest a role for this integrin in tumor biology.

The alpha1beta1 integrin is expressed during neointima formation in rat arteries and mediates collagen matrix reorganization.

Remodeling of the extracellular matrix by activated mesenchymal cells (myofibroblasts) is a critical aspect of wound repair in all adult organs. Collagen-dependent gel contraction, a process requiring integrin function, is an established in vitro assay thought to mimic in vivo matrix remodeling. Numerous data have implicated the alpha2beta1 integrin in various cell types as the primary collagen receptor responsible for collagen gel contraction. However, evidence from the literature suggests that the major collagen binding integrin expressed on mesenchymally derived cells in situ is the alpha1beta1 integrin, not the alpha2beta1 integrin. In this report, we use a rat vascular injury model to illustrate that the alpha1beta1 integrin is the major collagen receptor expressed on vascular smooth muscle cells after injury. Using two smooth muscle cell lines, expressing either the alpha1beta1 integrin alone or both the alpha1beta1 and alpha2beta1 integrins, along with Chinese hamster ovary cells transfected with the alpha1 integrin, we demonstrate that alpha1beta1 supports not only collagen-dependent adhesion and migration, but also gel contraction. These data suggest that in vivo the alpha1beta1 integrin is a critical collagen receptor on mesenchymally derived cells potentially involved in matrix remodeling after injury.

Drosophila PS1 integrin is a laminin receptor and differs in ligand specificity from PS2.

We have expressed Drosophila position-specific (PS) integrins on the surfaces of Schneider S2 cells and tested for adhesion and spreading on various matrix molecules. We report that PS1 integrin is a laminin receptor and that PS1 and PS2 integrins promote cell spreading on two different Drosophila extracellular matrix molecules, laminin and tiggrin, respectively. The differing ligand specificities of these two integrins, combined with data on the in vivo expression patterns of the integrins and their ligands, lead to a model for the structure of integrin-dependent attachments in the pupal wings and embryonic muscles of Drosophila.

Drosophila integrins and their ligands.

The major advance during the past year was the identification of ligands for two of the previously known position-specific integrins in Drosophila. At the same time, two new Drosophila integrin subunits (one alpha and one beta) were discovered, and significant progress was made on developmental genetic analyses of integrin functions, shedding light on the roles of integrins in Drosophila development.

Three neighboring genes interact with the Broad-Complex and the Stubble-stubbloid locus to affect imaginal disc morphogenesis in Drosophila.

The Broad-Complex (BR-C) is a complex regulatory locus at 2B-5 on the X chromosome of Drosophila melanogaster. The wild-type BR-C products are apparent transcription factors necessary for imaginal disc morphogenesis. Alleles of the Stubble-stubbloid (Sb-sbd) locus at 89B9-10 act as dominant enhancers of broad alleles of the BR-C. Sb-sbd wild-type products are necessary for appendage elongation. We report, here, on three new loci implicated in imaginal disc morphogenesis based on their genetic interactions with both BR-C and/or Sb-sbd mutants. Enhancer of broad (E(br)) was identified as a dominant enhancer of the br1 allele of the BR-C and is a recessive lethal. Mapping of E(br) has led to the identification of two loci, blistered and l(2)B485, mutants of which interact with E(br) and the Sb-sbd locus. Blistered, but not l(2)B485, interacts strongly with the BR-C. Alleles of the blistered locus are viable and disrupt proper wing disc morphogenesis independent of genetic interactions. All three loci map within the 0.6-map unit interval between the genetic markers speck and Irregular facets and to the cytological region 60C5-6; 60E9-10 at the tip of chromosome 2R. Genetic evidence is consistent with the view that the BR-C regulates blistered.