A conserved RGD (Arg-Gly-Asp) motif in the transferrin receptor is required for binding to transferrin.
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ABSTRACT:
SUBMITTER: Dubljevic V
PROVIDER: S-EPMC1220324 | biostudies-other | 1999 Jul
REPOSITORIES: biostudies-other
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Project description:PurposeAdvanced glycation endproduct (AGE) formation on the basement membrane of retinal capillaries has been previously described but the impact of these adducts on capillary endothelial cell function vascular repair remains uncertain. This investigation has evaluated retinal microvascular endothelial cells (RMECs) growing on AGE-modified fibronectin (FN) and determined how this has an impact on cell-substrate interactions and downstream oxidative responses and cell survival.MethodsRMECs were grown on methylglyoxal-modified FN (AGE-FN) or native FN as a control. RMEC attachment and spreading was quantified. In a separate treatment, the AGE-FN substrate had Arg-Gly-Asp-Ser (RGDS) or scrambled peptide added before seeding. Phosphorylation of focal adhesion kinase (FAK) and alpha5beta1 integrin localization was assessed and apoptosis evaluated. In a subset of RMECs that remained attached to the AGE-FN substrate, the production of superoxide (O(2) (-)) was assayed using dihydroethidium (DHE) fluorescence or lucigenin, in the presence or absence of NADPH. The specificity of the O(2) (-) assays was confirmed by inhibition in the presence of polyethylene-glycol-superoxide dismutase (PEG-SOD). AGE-mediated changes to mRNAs encoding key basement membrane proteins and regulatory enzymes were investigated using real-time RT-PCR.ResultsAGE-FN reduced RMEC attachment and spreading when compared to FN controls (p<0.001). RGDS peptide enhanced cell attachment on AGE-FN (p<0.001), while the scrambled peptide had no effect. FAK phosphorylation in AGE-exposed RMECs was reduced in a time-dependent fashion, while alpha5beta1 integrin-immunoreactivity became focal at the basal membrane. AGE-exposure induced apoptosis, a response significantly prevented by RGDS peptide. AGE-exposure caused a significant increase in basal O(2) (-) and NADPH-stimulated production by RMECs (p<0.01), while AGE-FN also increased basement membrane associated mRNA expression (p<0.05).ConclusionsAGE substrate modifications impair the function of retinal capillary endothelium and their reparative potential in response to diabetes-related insults. Arginine-specific modifications alter vital endothelial cell interactions with the substrate. This phenomenon could play an important role in dysfunction and nonperfusion of retinal capillaries during diabetes.
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Project description:The Rho ADP-ribosylating C3 exoenzyme (C3bot) is a bacterial protein toxin devoid of a cell-binding or -translocation domain. Nevertheless, C3 can efficiently enter intact cells, including neurons, but the mechanism of C3 binding and uptake is not yet understood. Previously, we identified the intermediate filament vimentin as an extracellular membranous interaction partner of C3. However, uptake of C3 into cells still occurs (although reduced) in the absence of vimentin, indicating involvement of an additional host cell receptor. C3 harbors an Arg-Gly-Asp (RGD) motif, which is the major integrin-binding site, present in a variety of integrin ligands. To check whether the RGD motif of C3 is involved in binding to cells, we performed a competition assay with C3 and RGD peptide or with a monoclonal antibody binding to β1-integrin subunit and binding assays in different cell lines, primary neurons, and synaptosomes with C3-RGD mutants. Here, we report that preincubation of cells with the GRGDNP peptide strongly reduced C3 binding to cells. Moreover, mutation of the RGD motif reduced C3 binding to intact cells and also to recombinant vimentin. Anti-integrin antibodies also lowered the C3 binding to cells. Our results indicate that the RGD motif of C3 is at least one essential C3 motif for binding to host cells and that integrin is an additional receptor for C3 besides vimentin.
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Project description:We have previously demonstrated [Lu, Williams, Deadman, Salmon, Kakkar, Wilkinson, Baruch, Authi and Rahman (1994) Biochem. J. 304, 929-936] the preferential antagonism of the interactions of the integrin alpha IIb beta 3 on activated platelets with three immobilized glycoprotein ligands (fibrinogen, fibronectin and von Willebrand factor) by a selected panel of snake-venom RGD (Arg-Gly-Asp)-containing proteins including the disintegrins kistrin and elegantin, and the neurotoxin variant dendroaspin. Kistrin and dendroaspin, although structurally unrelated, contain similar amino acids flanking the tripeptide RGD and behaved as identical antagonists preferentially inhibiting platelet adhesion to immobilized fibrinogen as opposed to fibronectin. In contrast, elegantin, which shares extensive sequence similarity with kistrin but has different amino acids around the tripeptide RGD, preferentially inhibited platelet adhesion to immobilized fibronectin as opposed to fibrinogen. To develop further insights into the mechanisms underlying the preferential antagonism shown by the venom proteins in the adhesion studies, we, in the present study, sought to determine the binding properties of kistrin, elegantin and dendroaspin to the alpha IIb beta 3 complex by radioligand kinetic and competition studies. In direct binding experiments, both kistrin and dendroaspin were observed to bind to a single class of binding site on ADP-activated platelets with apparent equilibrium dissociation constant (Kdapp) values of 42 +/- 2 nM and 21 +/- 6 nM respectively. In competition studies, dendroaspin blocked the binding of 125I-labelled kistrin to ADP-activated platelets in a simple competitive manner, with an apparent equilibrium inhibition constant (Kiapp) of 143 +/- 14 nM, from which an indirect Kdapp = 22 nM for dendroaspin was determined. This result suggests that kistrin and dendroaspin bind to the same site on the integrin alpha IIb beta 3 consistent with their similar inhibitory properties. In contrast, elegantin recognized two classes of binding sites on the alpha IIb beta 3 complex with Kdapp values of 10.5 +/- 0.8 nM and 175 +/- 10 nM, and, unlike dendroaspin, did not inhibit the binding of 125I-labelled kistrin to ADP-activated platelets. However, in reciprocal experiments both kistrin and dendroaspin inhibited the binding of 125I-elegantin to ADP-activated platelets in a non-competitive manner, with Kiapp values of 34 +/- 3 nM and 21 +/- 2 nM respectively. Thus elegantin appears to interact with distinct but interacting sites on the alpha IIb beta 3 complex from the binding site of kistrin and dendroaspin, consistent with its distinctive inhibitory preferences as shown in platelet adhesion studies.(ABSTRACT TRUNCATED AT 400 WORDS)
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