Biomedical subjects

N Kieffer

Publications and source records attributed to N Kieffer.

At least 19 recordsLinked to original sources

PubMed2001

Probing conformational changes in the I-like domain and the cysteine-rich repeat of human beta 3 integrins following disulfide bond disruption by cysteine mutations: identification of cysteine 598 involved in alphaIIbbeta3 activation.

We have investigated receptor function and epitope expression of recombinant alpha(IIb)beta(3) mutated at Cys(177) or Cys(273) in the I-like domain as well as Cys(598), located in the fourth repeat of the membrane-proximal cysteine-rich region and mutated in a Glanzmann's thrombasthenia type II patient. The beta(3) mutants beta(3)C177A, beta(3)C273A, and beta(3)C598Y exhibited a decreased electrophoretic mobility in SDS-polyacrylamide gel electrophoresis under nonreducing conditions, confirming the disruption of the respective disulfide loops. Despite reduced surface expression, the alpha(IIb)beta(3)C177A, alpha(IIb)beta(3)C273A, and alpha(IIb)beta(3)C598Y receptors mediated cell adhesion to immobilized fibrinogen and translocated into focal adhesion plaques. The beta(3)C598Y mutation, but not the beta(3)C177A or beta(3)C273A mutations, induced spontaneous binding of the ligand mimetic monoclonal antibody PAC-1, while the beta(3)C177A and beta(3)C273A mutants exhibited reduced complex stability in the absence of Ca(2+). Epitope mapping of function-blocking monoclonal antibodies (mAbs) allowed the identification of two distinct subgroups; mAbs A2A9, pl2-46, 10E5, and P256 did not interact with alpha(IIb)beta(3)C273A and bound only weakly to alpha(IIb)beta(3)C177A, while mAbs AP2, LM609 and 7E3 bound normally to mutant alpha(IIb)beta(3)C273A, but interacted only weakly with mutant alpha(IIb)beta(3)C177A. Furthermore, a cryptic epitope recognized by mAb 4D10G3 and not exposed on wild type alpha(IIb)beta(3) became accessible only on mutant alpha(IIb)beta(3)C177A and was mapped to the 60-kDa chymotrypsin fragment of beta(3). Finally, the ligand-induced binding site (LIBS) epitopes AP5, D3, LIBS1, and LIBS2 were spontaneously expressed on all three mutants independent of RGDS or dithiothreitol treatment. Our results provide evidence that disruption of a single cysteine disulfide bond in the cysteine-rich repeat domain, but not in the I-like domain, activates integrin alpha(IIb)beta(3). In contrast, disruption of each of the disulfide bonds in the two long insertions of the I-like domain predicted to be in close contact with the alpha subunit beta-propeller domain affect the stability of the alpha(IIb)beta(3) heterodimer and inhibit complex-specific mAb binding without affecting the RGD binding capacity of the metal ion-dependent adhesion site-like domain.

Animals↗

PubMed2001

Green fluorescent protein (GFP) tagged to the cytoplasmic tail of alphaIIb or beta3 allows the expression of a fully functional integrin alphaIIb(beta3): effect of beta3GFP on alphaIIb(beta3) ligand binding.

Using green fluorescent protein (GFP) as an autofluorescent tag, we report the first successful visualization of a beta3 integrin in a living cell. GFP fused in frame to the cytoplasmic tail of either alphaIIb or beta3 allowed normal expression, heterodimerization, processing and surface exposure of alphaIIbGFPbeta3 and alphaIIb(beta3)GFP receptors in Chinese hamster ovary (CHO) cells. Direct microscopic observation of the autofluorescent cells in suspension following antibody-induced alphaIIb(beta3) capping revealed an intense autofluorescent cap corresponding to unlabelled immunoclustered GFP-tagged alphaIIb(beta3). GFP-tagged alphaIIbbeta3 receptors mediated fibrinogen-dependent cell adhesion, were readily detectable in focal adhesions of unstained living cells and triggered p125(FAK) tyrosine phosphorylation similar to wild-type alphaIIb(beta3) (where FAK corresponds to focal adhesion kinase). However, GFP tagged to beta3, but not to alphaIIb, induced spontaneous CHO cell aggregation in the presence of soluble fibrinogen, as well as binding of the fibrinogen mimetic monoclonal antibody PAC1 in the absence of alphaIIb(beta3) receptor activation. Time-lapse imaging of living transfectants revealed a characteristic redistribution of GFP-tagged alphaIIb(beta3) during the early stages of cell attachment and spreading, starting with alphaIIb(beta3) clustering at the rim of the cell contact area, that gradually overlapped with the boundary of the attached cell, and, with the onset of cell spreading, to a reorganization of alphaIIb(beta3) in focal adhesions. Taken together, our results demonstrate that (1) fusion of GFP to the cytoplasmic tail of either alphaIIb or beta3 integrin subunits allows normal cell surface expression of a functional receptor, and (2) structural modification of the beta3 integrin cytoplasmic tail, rather than the alphaIIb subunit, plays a major role in alphaIIb(beta3) affinity modulation. With the successful direct visualization of functional alphaIIb(beta3) receptors in living cells, the generation of autofluorescent integrins in transgenic animals will become possible, allowing new approaches to study the dynamics of integrin functions.

Animals↗

PubMed2001

A calreticulin-like molecule from the human hookworm Necator americanus interacts with C1q and the cytoplasmic signalling domains of some integrins.

Calreticulin was recently identified as a hookworm (Necator americanus) allergen, implying secretion, and contact with cells of the immune system, or significant worm attrition in the tissues of the host. As human calreticulin has been shown to bind to and neutralize the haemolytic activity of the complement component C1q, and to be putatively involved in integrin-mediated intracellular signalling events in platelets, it was of interest to determine whether a calreticulin from a successful nematode parasite of humans, with known immune modulatory and antihaemostatic properties, exhibited a capacity to interfere with complement activation and to interact with integrin domains associated with cell signalling in platelets and other leucocytes. We can now report that recombinant calreticulin failed to demonstrate significant calcium binding capacity, which is a hallmark of calreticulins in general and may indicate inappropriate folding following expression in a prokaryote. Nevertheless, recombinant calreticulin retained sufficient molecular architecture to bind to, and inhibit the haemolytic capacity of, human C1q. Furthermore, recombinant calreticulin reacted in surface plasmon resonance analysis (SPR) with peptides corresponding to cytoplasmic signalling domains of the integrins alphaIIb and alpha5, in a calcium independent manner. SPR was also used to ratify the specificity of a polyclonal antibody to hookworm calreticulin, which was then used to assess the stage specificity of expression of the native molecule (in comparison with reverse transcriptase-polymerase chain reaction), to indicate its apparent secretion, and to purify native calreticulin from worm extracts by affinity chromatography. This development will allow the functional tests described above to be repeated for native calreticulin, to ascertain its role in the host-parasite relationship.

Animals↗

PubMed2001

Evidence from site-directed mutagenesis that the cytoplasmic domain of the beta3 subunit influences the conformational state of the alphaVbeta3 integrin ectodomain.

In order to explore the mechanisms leading to conformational changes of the vitronectin receptor alphavbeta3 following ligand or divalent cation binding, we have investigated the expression of epitopes known as ligand-induced binding sites (LIBS) on beta3 cytoplasmic tail mutants expressed in CHO cells. Truncation of the entire beta3 cytoplasmic domain induced constitutive LIBS exposure on alphavbeta3 and alphaIIbeta3. Deletion of the C-terminal NITY759 sequence or disruption of the NPLY747 motif by a Y747A substitution impaired extracellular conformational changes on alphavbeta3 following RGDS, echistatin or Mn2+ binding, whereas the substitutions Y747F, Y759A or Y759F allowed normal LIBS exposure. Furthermore, metabolic energy depletion totally prevented Mn2+-dependent LIBS exposure, but had only a minor effect on RGDS-induced conformational changes. Our results demonstrate that the structural integrity of the NPLY747 motif in the beta3 cytoplasmic domain, rather than potential phosphorylation of Tyr747 or Tyr759, is a prerequisite for conformational changes within the alphavbeta3 ectodomain, and suggest that two different mechanisms are responsible for RGDS- and Mn2+-dependent conformational changes.

Amino Acid Motifs↗

PubMed2001

A naturally occurring point mutation in the beta3 integrin MIDAS-like domain affects differently alphavbeta3 and alphaIIIbbeta3 receptor function.

We have investigated the effect of a new Leu196Pro mutation, identified in the MIDAS-like domain of the beta3 integrin subunit in a patient with type II Glanzmann thrombasthenia, on beta3 integrin receptor function. Expression of the mutant beta3Pro196 subunit in CHO cells, either associated with recombinant human alphaIIb or alphav, resulted in normal biosynthesis of beta3 and heterodimerization with alphav or alphaIIb, but selectively interfered with alphaIIbbeta3 maturation and transport to the cell surface. Functional analysis of the beta3 mutant receptors revealed strong inhibition of alphavbeta3-mediated cell spreading on immobilized fibrinogen, focal contact formation, p125FAK phosphorylation and fibrin clot retraction, as opposed to normal alphaIIbbeta3-mediated cell interaction with immobilized fibrinogen, focal contact translocation and signaling. In contrast, antibody- or DTT-activated mutant aIIbbeta3 was unable to bind soluble fibrinogen or the ligand mimetic PAC-1 monoclonal antibody, but underwent a conformational change following RGD peptide binding as demonstrated by AP5-LIBS epitope expression. These results suggest that (1) the highly conserved TL196T motif in the beta3 integrin subunit is located in a domain structurally important for the exposure of a functional binding site for soluble fibrinogen; and (2) that the MIDAS-like contact site in beta3 is not involved in alphaIIbbeta3-mediated cell adhesion to immobilized fibrinogen, while it is essential for alphavbeta3-mediated interaction with this ligand.

Amino Acid Sequence↗

PubMed2000

The alpha 3(IV)185-206 peptide from noncollagenous domain 1 of type IV collagen interacts with a novel binding site on the beta 3 subunit of integrin alpha Vbeta 3 and stimulates focal adhesion kinase and phosphatidylinositol 3-kinase phosphorylation.

We have recently identified integrin alpha(v)beta(3) and the associated CD47/integrin-associated protein (IAP) together with three other proteins as the potential tumor cell receptors for the alpha(3) chain of basement membrane type IV collagen (Shahan, T.A., Ziaie, Z., Pasco, S., Fawzi, A., Bellon, G., Monboisse, J. C., and Kefalides, N. A. (1999) Cancer Res. 59, 4584-4590). Using different cell lines expressing alpha(v)beta(3), alpha(IIb)beta(3), and/or CD47 and a liquid phase receptor capture assay, we now provide direct evidence that the synthetic and biologically active alpha3(IV)185-206 peptide, derived from the alpha3(IV) chain, interacts with the beta(3) subunit of integrin alpha(v)beta(3), independently of CD47. Increased alpha3(IV) peptide binding was observed on transforming growth factor-beta(1)-stimulated HT-144 cells shown to up-regulate alpha(v)beta(3) independently of CD47. Also, incubation of HT-144 melanoma cells in suspension induced de novo exposure of ligand-induced binding site epitopes on the beta(3) subunit similar to those observed following Arg-Gly-Asp-Ser (RGDS) stimulation. However, RGDS did not prevent HT-144 cell attachment and spreading on the alpha3(IV) peptide, suggesting that the alpha3(IV) binding domain on the beta(3) subunit is distinct from the RGD recognition site. alpha3(IV) peptide binding to HT-144 cells in suspension stimulated time-dependent tyrosine phosphorylation, while the RGDS peptide did not. Two major phosphotyrosine proteins of 120-130 and 85 kDa were immunologically identified as focal adhesion kinase and phosphatidylinositol 3-kinase (PI3-kinase). A direct involvement of PI3-kinase in alpha3(IV)-dependent beta(3) integrin signaling could be documented, since pretreatment of HT-144 cells with wortmannin, a PI3-kinase inhibitor, reverted the known inhibitory effect of alpha3(IV) on HT-144 cell proliferation as well as membrane type 1-matrix metalloproteinase gene expression. These results provide evidence that the alpha3(IV)185-206 peptide, by directly interacting with the beta(3) subunit of alpha(v)beta(3), activates a signaling cascade involving focal adhesion kinase and PI3-kinase.

Amino Acid Sequence↗

PubMed2000

Cloning of an isoform of integrin-linked kinase (ILK) that is upregulated in HT-144 melanoma cells following TGF-beta1 stimulation.

We have shown previously that integrin-linked kinase (ILK) is upregulated in human HT-144 melanoma cells following TGF-beta1 stimulation. Using mRNA from TGF-beta1 stimulated HT-144 cells and reverse transcriptase polymerase chain reaction, we have isolated a cDNA encoding a protein highly homologous to ILK. Sequencing of the full-length 1359 base pair cDNA and polypeptide translation revealed that this protein, designated ILK-2, differs from the known ILK (hereafter called ILK-1) by only four amino acids, while the cDNA sequence diverges by 102 nucleotides, thus excluding that ILK-2 is an allelic variant of ILK-1. Expression of ILK-2 mRNA was observed in metastatic human HT-144 melanoma and HT-1080 fibrosarcoma cell lines, but not in normal human tissues. Moreover, stimulation of HT-144 cells with TGF-beta1, but not with EGF, PDGF-AB or insulin, induced a selective overexpression of ILK-2 mRNA as compared to ILK-1 mRNA. Bacterially-expressed GST/ILK-2 autophosphorylated and labeled myelin basic protein as well as a recombinant GST/beta3 integrin cytoplasmic tail peptide. Transfection of either ILK-2 or ILK-1 cDNA into the non-metastatic melanoma cell line SK-Mel-2, expressing exclusively ILK-1, induced anchorage independent cell growth and cell proliferation, as demonstrated by growth in soft agar. Our data provide evidence that ILK-2 is a new isoform of ILK-1 that is expressed in some highly invasive tumor cell lines but not in normal adult human tissues and whose expression is regulated by TGF-beta1.

Amino Acid Sequence↗

PubMed2000

Interaction between the cytodomains of the alpha 3 and beta 1 integrin subunits regulates remodelling of adhesion complexes on laminin.

The first step of laminin 1-induced signal transduction is initiated by the formation of alpha 6 beta 1 integrin-specific adhesion complexes. In contrast, on other laminin isoforms the adhesion complexes are alpha 3 beta 1 integrin-specific due to a transdominant regulation of the alpha 6 beta 1 integrin by the alpha 3 beta 1 integrin. To determine the mechanism of this regulation, peptides representing the cytoplasmic domain of the alpha 3 or alpha 6 integrin subunits were microinjected together with recombinant enhanced green fluorescence protein into live fibroblasts. Microinjection of the alpha 3 integrin peptide to laminin 1-adherent cells displaying alpha 6 beta 1 integrin-specific adhesion complexes resulted in the disengagement of the alpha 6 beta 1 integrin, while microinjection of green fluorescence protein alone or in combination with the alpha 6 integrin cytodomain had no effect. Further surface plasmon resonance studies revealed that the cytodomain of the beta 1 integrin subunit interacts with low affinity with the cytoplasmic tail of the alpha 3 integrin subunit, but not with that of several other alpha subunits including alpha 6. These results imply that the cytoplasmic tails of the integrin alpha subunits play a critical role in the regulation of integrin-induced signal transduction. In particular, the intracellular tail of the alpha 3 integrin subunit controls the formation of adhesion complexes in cells adhering to laminins.

Amino Acid Sequence↗

PubMed1999

Autocrine TGF-beta-regulated expression of adhesion receptors and integrin-linked kinase in HT-144 melanoma cells correlates with their metastatic phenotype.

We have previously shown that 2 human melanoma cell lines, the metastatic HT-144 and the non-metastatic SK-Mel-2 cells, exhibit marked in vitro heterogeneity with respect to integrin expression, migration and invasion potential. Here, we provide evidence that HT-144 melanoma cells, but not SK-Mel-2 cells, undergo a reversible transition to a fibroblastoid morphology following treatment with either their own serum-free acidified conditioned medium or biologically active exogenous TGF-beta1, thus identifying TGF-beta as an autocrine regulator of the spindle shape morphology of HT-144 melanoma cells. The fibroblastoid phenotype correlated with up-regulated beta1 and beta3 integrin and down-regulated E-cadherin expression, as shown by flow cytometry, Western blot and RT-PCR, as well as up-regulated expression of the matrix metalloproteinase MMP-9, as demonstrated by zymography. Our data further illustrate the TGF-beta1-dependent up-regulation of integrin-linked kinase and the nuclear translocation of beta-catenin, 2 intracellular proteins involved in integrin and cadherin signaling.

Antigens, CD↗

PubMed1999

Divalent cations differentially regulate integrin alphaIIb cytoplasmic tail binding to beta3 and to calcium- and integrin-binding protein.

We have used recombinant or synthetic alphaIIb and beta3 integrin cytoplasmic peptides to study their in vitro complexation and ligand binding capacity by surface plasmon resonance. alpha.beta heterodimerization occurred in a 1:1 stoichiometry with a weak KD in the micromolar range. Divalent cations were not required for this association but stabilized the alpha.beta complex by decreasing the dissociation rate. alpha.beta complexation was impaired by the R995A substitution or the KVGFFKR deletion in alphaIIb but not by the beta3 S752P mutation. Recombinant calcium- and integrin-binding protein (CIB), an alphaIIb-specific ligand, bound to the alphaIIb cytoplasmic peptide in a Ca2+- or Mn2+-independent, one-to-one reaction with a KD value of 12 microM. In contrast, in vitro liquid phase binding of CIB to intact alphaIIbbeta3 occurred preferentially with Mn2+-activated alphaIIbbeta3 conformers, as demonstrated by enhanced coimmunoprecipitation of CIB with PAC-1-captured Mn2+-activated alphaIIbbeta3, suggesting that Mn2+ activation of intact alphaIIbbeta3 induces the exposure of a CIB-binding site, spontaneously exposed by the free alphaIIb peptide. Since CIB did not stimulate PAC-1 binding to inactive alphaIIbbeta3 nor prevented activated alphaIIbbeta3 occupancy by PAC-1, we conclude that CIB does not regulate alphaIIbbeta3 inside-out signaling, but rather is involved in an alphaIIbbeta3 post-receptor occupancy event.

Antigens, CD↗

PubMed1998

R to Q amino acid substitution in the GFFKR sequence of the cytoplasmic domain of the integrin IIb subunit in a patient with a Glanzmann's thrombasthenia-like syndrome.

The integrin IIbbeta3 mediates platelet aggregation through its fibrinogen and adhesive protein-binding properties. Particular interest concerns the role of the cytoplasmic domains of IIb and beta3. We now report the molecular analysis of IIbbeta3 from a patient with a Glanzmann's thrombasthenia-like syndrome for whom the principal characteristics are an approximate 50% total platelet content of IIbbeta3 but with a much lower proportion in the surface pool (Hardisty et al, Blood 80:696, 1992). Polymerase chain reaction (PCR) single-strand conformational polymorphism and DNA sequencing showed a heterozygous mutation giving rise to amino acid substitution R995 to Q in the GFFKR sequence of the cytoplasmic domain of IIb. Reverse transcriptase-PCR and polymorphism analysis only detected mRNA for the mutated allele of the IIb gene and a single allele of the beta3 gene in his platelets, suggesting other unidentified defects. Site-directed mutagenesis followed by transient expression of the mutated IIb together with wild-type beta3 in Cos-7 cells resulted in a markedly decreased expression of the complex at the cell surface when compared with cells transfected with wild-type IIb and beta3. Flow cytometry with PAC-1 and a stable Chinese hamster ovary-transfected cell line showed that the mutated receptor was not locked into a high activation state, although it became so in the presence of the activating antibody, anti-LIBS6. This is the first reported natural mutation in the highly conserved GFFKR sequence of the IIb cytoplasmic domain.

Amino Acid Substitution↗

PubMed1998

Distinct involvement of beta3 integrin cytoplasmic domain tyrosine residues 747 and 759 in integrin-mediated cytoskeletal assembly and phosphotyrosine signaling.

We have investigated the structural requirements of the beta3 integrin subunit cytoplasmic domain necessary for tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin during alphav beta3-mediated cell spreading. Using CHO cells transfected with various beta3 mutants, we demonstrate a close correlation between alphav beta3-mediated cell spreading and tyrosine phosphorylation of FAK and paxillin, and highlight a distinct involvement of the NPLY747 and NITY759 motifs in these signaling processes. Deletion of the NITY759 motif alone was sufficient to completely prevent alphav beta3-dependent focal contact formation, cell spreading, and FAK/paxillin phosphorylation. The single Y759A substitution induced a strong inhibitory phenotype, while the more conservative, but still phosphorylation-defective, Y759F mutation restored wild type receptor function. Alanine substitution of the highly conserved Tyr747 completely abolished alphav beta3-dependent formation of focal adhesion plaques, cell spreading, and FAK/paxillin phosphorylation, whereas a Y747F substitution only partially restored these events. As none of these mutations affected receptor-ligand interaction, our results suggest that the structural integrity of the NITY759 motif, rather than the phosphorylation status of Tyr759 is important for beta3-mediated cytoskeleton reorganization and tyrosine phosphorylation of FAK and paxillin, while the presence of Tyr at residue 747 within the NPLY747 motif is required for optimal beta3 post-ligand binding events.

Amino Acid Sequence↗

PubMed1998

Ser752 mutation to Pro or Ala in the beta3 integrin subunit differentially affects the kinetics of cell spreading to von Willebrand factor and fibrinogen.

The beta3 cytoplasmic domain of the alpha v beta3 integrin is essential for intracellular signals required for cytoskeletal rearrangements. Expression of beta3Ser752Pro mutation in heterologous cells profoundly affects cell spreading and beta3 localization into focal contacts. However, the beta3Ser752Ala substitution mostly restores normal integrin functions, suggesting that the presence of Pro is responsible for the receptor's loss of function. To further assess the role of the Ser752 of the beta3 cytoplasmic domain in the cytoskeletal organization of adherent cells, we developed a computer-assisted method of image analysis allowing the automatic classification of spread cells according to the quantitative analysis of their cell morphology. We compared adhesion and spreading to von Willebrand factor (vWF) or fibrinogen (Fg) of cells expressing beta3 wild type, beta3Ser752Pro or beta3Ser752Ala mutated integrin subunit as a chimeric alpha v beta3 receptor. The beta3Ser752Ala substitution did not impair the general ability of cells to spread, but resulted in a delayed and reduced spreading on both vWF and Fg. Moreover, the beta3Ser752Ala mutation produced modifications of the morphology of spread cells, suggesting a disorganization of their cytoskeleton. Attachment studies showed that the beta3Ser752Ala mutation did not modify the capacity of cells to attach to the substrate, indicating no change in the ligand binding affinity of the alpha v beta3 integrin. Furthermore, we identified a slight defect of beta3Ser752Pro cell attachment to vWF and Fg, beside their impairment of spreading. Taken together, these results suggest a role of Ser752 of the beta3 cytoplasmic domain in the optimal cytoskeletal organization of adherent cells.

Alanine↗

PubMed1997

A three amino acid deletion in glycoprotein IIIa is responsible for type I Glanzmann's thrombasthenia: importance of residues Ile325Pro326Gly327 for beta3 integrin subunit association.

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin alphaIIb beta3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325pro326Gly327 --> Met) and creating a new BspHI restriction site. Expression of the mutated integrin beta3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length beta3 protein with an apparent molecular weight identical to wild-type beta3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant beta3 protein with endogenous alpha v was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the alpha vbeta3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type beta3, did not restore the ability of the recombinant mutant beta3 protein to associate with alpha v , suggesting that the Ile-Pro-Gly motif is located in a beta3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.

Algeria↗

PubMed1997

Human neutrophil elastase proteolytically activates the platelet integrin alphaIIbbeta3 through cleavage of the carboxyl terminus of the alphaIIb subunit heavy chain. Involvement in the potentiation of platelet aggregation.

Neutrophil elastase (NE) and cathepsin G are two serine proteinases released concomitantly by stimulated polymorphonuclear neutrophils. We previously demonstrated that while NE by itself does not activate human platelets, it strongly enhances the weak aggregation induced by a threshold concentration of cathepsin G (threshold of cathepsin G) (Renesto, P., and Chignard, M. (1993) Blood 82, 139-144). The aim of this study was to delineate the molecular mechanisms involved in this potentiation process. Two main pieces of data prompted us to focus on the activation of the platelet fibrinogen receptor, the alphaIIbbeta3 integrin. First, previous studies have shown this integrin to be particularly prone to proteolytic regulation of its function. Second, we found that the potentiating activity of NE on the threshold of cathepsin G-induced platelet aggregation was strictly dependent on the presence of exogenous fibrinogen. Using flow cytometry analysis, NE was shown to trigger a time-dependent binding of PAC-1 and AP-5, two monoclonal antibodies specific for the activated and ligand-occupied conformers of alphaIIbbeta3. Furthermore, the potentiated aggregation was shown to result from an increased capacity of platelets to bind fibrinogen. Indeed, the combination of NE and threshold of cathepsin G increased the binding of PAC-1 approximately 5.5-fold over basal values measured on nontreated platelets, whereas this binding raised only by approximately 3-fold in threshold of cathepsin G-stimulated platelets (p < 0.05). By contrast, phosphatidic acid accumulation, pleckstrin phosphorylation, and calcium mobilization produced by the combination of NE and threshold of cathepsin G were not significantly different from those measured with threshold of cathepsin G alone (p > 0.05), indicating that the phospholipase C/protein kinase C pathway is not involved in the potentiation of aggregation. The foregoing data, as well as the requirement of catalytically active NE to trigger alphaIIbbeta3 activation and potentiate threshold of cathepsin G-initiated platelet aggregation, led us to examine whether the structure of this integrin was affected by NE. Immunoblot and flow cytometry analysis revealed a limited proteolysis of the carboxyl terminus of the alphaIIb subunit heavy chain (alphaIIbH), as judged by the disappearance of the epitope for the monoclonal antibody PMI-1. Mass spectrometry studies performed on a synthetic peptide mapping over the cleavage domain of alphaIIbH predicted the site of proteolysis as located between Val837 and Asp838. Treatment by NE of ATP-depleted platelets or Chinese hamster ovary cells expressing human recombinant alphaIIbbeta3 clearly established that activation of the integrin was independent of signal transduction events and was concomitant with the proteolysis of alphaIIbH. In support of this latter observation, a close correlation was observed between the kinetics of proteolysis of alphaIIbH on platelets and that of expression of the ligand binding activity of alphaIIbbeta3 (r2 = 0.902, p </= 0. 005). However, only a subpopulation ( approximately 25%) of the proteolyzed alphaIIbbeta3 appeared to fully express the ligand binding capacity. Altogether, these results demonstrate that NE up-regulates the fibrinogen binding activity of alphaIIbbeta3 through a restricted proteolysis of the alphaIIb subunit, and that this process is relevant for the potentiation of platelet aggregation.

Amino Acid Sequence↗

PubMed1996

Up-regulated expression of the beta3 integrin and the 92-kDa gelatinase in human HT-144 melanoma cell tumors grown in nude mice.

In situ changes in the repertoire of integrins and proteolytic enzymes have been demonstrated during melanoma metastasis. To investigate whether established human melanoma cell lines, injected into nude mice, could undergo phenotypic changes similar to those observed in in situ lesions, we studied 3 melanoma cell lines of distinct metastatic origin, adherent HT-144 and SK-MEL-2 cells, and non-adherent SK-MEL-1 cells for integrin expression, proteolytic enzyme repertoire and invasive potential after in vitro culture. Heterogeneity in integrin expression, such as elevated levels in alpha(v)beta3 in SK-MEL-1 and SK-MEL-2 cells and low expression in HT-144 cells, correlated with their in vitro invasiveness, since only the adherent HT-144 and SK-MEL-2 cells were able to invade Matrigel, and in addition, secreted a 72-kDa gelatinase. In contrast, no similar correlation could be established in nude mice, as all 3 cell lines, including the non-adherent SK-MEL-1 cells, were tumorigenic when injected s.c., while only HT-144 consistently produced experimental lung metastasis. Immunochemical analysis of the integrin profile in s.c. xenografts revealed over-expression of alpha(v), beta1 and beta3 integrins exclusively in HT-144 cells, as well as increased expression of beta3 in HT-144 cell lung metastases, as confirmed by PCR analysis using species-specific primers, while zymography and Western-blot analysis demonstrated de novo expression of the 92-kDa gelatinase MMP-9 in HT-144 xenografts. Our results highlight a positive correlation between up-regulated beta3 integrin and MMP-9 expression in human HT-144 melanoma cell tumors grown in nude mice.

Animals↗

PubMed1996

alphaIIb beta 3-integrin mediated adhesion of human platelets to a fibrinogen matrix triggers phospholipase C activation and phosphatidylinositol 3',4'-biphosphate accumulation.

This study focused on the variations in phosphoinositide metabolism depending upon alphaIIbbeta3-integrin/fibrinogen interaction without previous activation of platelet agonist receptors. We found that adhesion of resting human platelets to immobilized fibrinogen stimulates phosphatidic acid production and a concomitant decrease in phosphatidylinositol 4',5'-bisphosphate. These results, and the absence of a transphosphatidylation reaction, argue in favor of the activation of a phospholipase C. Moreover, we observed the accumulation of phosphatidylinositol 3',4'-bisphosphate in adherent platelets as a consequence of the activation of a phosphatidylinositol 3-kinase. This effect was inhibited by ADP scavengers. Our results demonstrate that in adherent platelets, whereas phosphatidylinositol 3-kinase activation is controlled by both alphaIIbbeta-integrin engagement and released ADP, phospholipase C stimulation is triggered only by alphaIIbbeta-integrin/fibrinogen interaction.

Adenosine Diphosphate↗

PubMed1996

Serine 752 in the cytoplasmic domain of the beta 3 integrin subunit is not required for alpha v beta 3 postreceptor signaling events.

A naturally occurring point mutation (Ser752Pro substitution) in the beta subunit cytoplasmic domain of the platelet fibrinogen receptor GPIIb-IIIa (integrin alpha IIb beta 3), causing Glanzmann's thrombasthenia, has been shown to abrogate bidirectional transmembrane signaling of GPIIb-IIIa when expressed in heterologous cells (Chen YP, 1994, Blood 84, 1857-1865). As the vitronectin receptor alpha v beta 3 constitutively mediates cell attachment to RGD containing extracellular matrix proteins, the purpose of this study was to explore the regulatory role of Ser752 in alpha v beta 3 vitronectin receptor function, by cotransfecting recombinant human alpha v cDNA together with human beta 3 mutant cDNA into Chinese hamster ovary (CHO) cells. CHO cells expressing wild type human alpha v beta 3 acquired the ability to attach and spread on fibrinogen and von Willebrand factor, in contrast to non transfected CHO cells that only bound to vitronectin and fibronectin. Overexpression of a truncated recombinant beta 3 subunit (beta 3 delta 744) generated alpha v (hamster) beta 3 (human) chimers that mediated attachment but lost the ability to promote cell spreading on vitronectin, von Willebrand factor and fibrinogen, and to concentrate in focal contact sites, demonstrating a negative effect of beta 3 delta 744 on alpha v beta 3 dependent postreceptor occupancy events. Transfection of beta 3Ser752Pro reproduced the same negative effect as beta 3 delta 744, whereas beta 3Ser752Ala restored normal receptor function by allowing pronounced attachment and spreading on fibrinogen and von Willebrand factor. Our results provide evidence that (1) the C-terminal cytoplasmic domain of beta 3 (amino acids 744-762) is essential for alpha v beta 3 integrin postreceptor occupancy events; (2) within this domain, the Ser752Pro mutation affects alpha v beta 3 postreceptor occupancy events by preventing cell spreading and focal contact localization; (3) the defective receptor function of the vitronectin receptor alpha v beta 3 is due to the presence of Pro752, rather than the absence of Ser752, as a Ser to Ala substitution at position 752 restores normal beta 3 integrin cell spreading and adhesive plaque formation.

Alanine↗