calcimycin and arginyl-glycyl-aspartic-acid

We studied the effect of glycoprotein GPIIb/IIIa (integrin alpha IIb beta 3) receptor occupancy by adenosine 5',1-thiotriphosphate (ATP alpha S), a competitive inhibitor of the ADP receptor, by fibrinogen, and by peptides containing the RGD (Arg-Gly-Asp) sequence as RGDW (Arg-Gly-Asp-Trp), RGDS (Arg-Gly-Asp-Ser), or the negative control RGGW (Arg-Gly-Gly-Trp) on human platelet physiological functions: aggregation, ATP secretion, and [Ca2+]in. As the presence of a nucleotide binding site on GPIIb alpha has been demonstrated in platelets [N. J. Greco, N. Yamamoto, B. W. Jackson, N. N. Tandon, M. Moos, and G. A. Jamieson (1991) J. Biol. Chem. 266, 13627-13633], we studied the effect of ATP alpha S, which specifically binds to this site, on platelet activation. We observed that ATP alpha S inhibited aggregation by thrombin, ADP, PMA, and ionophore A23187. Moreover, ATP alpha S dose dependently inhibited ATP secretion by ionophore A23187 and Ca2+ transients by thrombin and vasopressin in both the presence and absence of external Ca2+. Fibrinogen, although induced by a potentiation of platelet aggregation, inhibited ATP secretion and [Ca2+]in elevation induced by low thrombin concentrations or by vasopressin, interfering with both Ca2+ entry and Ca2+ release by the intracellular stores. RGD peptides, which specifically bind to GPIIb/IIIa, inhibited aggregation, secretion, and Ca2+ transients by thrombin, whereas the negative control RGGW did not exert any effect. We conclude that the occupancy of the GPIIb/IIIa receptor binding sites modulates platelet function by giving an inhibitory outside-in signal in platelets, particularly effective in platelets stimulated with low agonist doses. We suggest that ATP alpha S, fibrinogen, or RGD compounds, by interacting with GPIIb/IIIa receptor, prime some intracellular negative feedback mechanisms, which prevent further activation of circulating platelets by low-intensity stimuli and intravascular aggregation.

Topics: Adenosine Triphosphate; Amino Acid Sequence; Blood Platelets; Calcimycin; Calcium; Fibrinogen; Humans; Kinetics; Oligopeptides; Platelet Activation; Platelet Aggregation; Platelet Aggregation Inhibitors; Platelet Glycoprotein GPIIb-IIIa Complex; Structure-Activity Relationship; Thionucleotides; Thrombin

Stabilization of a clot is dependent on fibrin cross-linking mediated by the transglutaminase, factor XIIIa (FXIIIa). In addition to fibrin stabilization, FXIIIa acts on a number of platelet-reactive proteins, including fibronectin and vitronectin, as well as the platelet proteins, glycoprotein (GP) IIb-IIIa, myosin, and actin. However, conditions inducing the platelet-activation dependent binding of FXIIIa have not been characterized nor have the sites mediating FXIIIa binding been identified. The generation of FXIIIa and consequent detection of FXIIIa on the platelet surface were compared with other thrombin-induced activation events; the rate at which FXIIIa bound to activated platelets was much slower than platelet degranulation or fibrin(ogen) binding. Whereas platelets could be rapidly induced to express a functional receptor for FXIIIa, the rate of FXIIIa binding to platelets is limited by the rate of conversion of FXIII to FXIIIa. Immunoprecipitation of radiolabeled platelets using polyclonal anti-FXIII A-chain antibody identified two proteins corresponding to GPIIb and GPIIIa. Preincubation of intact platelets with 7E3, a monoclonal antibody that blocks the fibrinogen binding site, or GRGDSP peptide inhibited FXIIIa binding by about 95% when measured by flow cytometry; FXIIIa binding to purified GPIIb-IIIa was also inhibited by 7E3. The binding of FXIIIa to purified GPIIb-IIIa was enhanced by the addition of fibrinogen, but not by that of fibronectin or thrombospondin, suggesting that FXIIIa also binds to fibrinogen associated with the complex. These observations suggest that activated platelets bearing FXIIIa may enhance stabilization of platelet-rich thrombi through surface-localized cross-linking events.

Topics: Adenosine Diphosphate; Amino Acid Sequence; Antibodies, Monoclonal; Antigens, CD; Arachidonic Acid; Blood Platelets; Calcimycin; Collagen; Epinephrine; Fibrinogen; Fibronectins; Flow Cytometry; Humans; In Vitro Techniques; Kinetics; Membrane Glycoproteins; Membrane Proteins; Oligopeptides; Platelet Activation; Platelet Membrane Glycoproteins; Reference Values; Tetradecanoylphorbol Acetate; Thrombasthenia; Thrombin; Thrombospondins; Time Factors; Transglutaminases