Signal-transducing Mechanisms Involved in Activation of the Platelet Collagen Receptor Integrin α2β1

• Published in: The Journal of biological chemistry 2000-03, Vol.275 (11), p.8016-8026
• Main Authors: Jung, Stephanie M., Moroi, Masaaki
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.275.11.8016

Evidence was obtained about the mechanism responsible for platelet integrin α2β activation by determining effects of various inhibitors on soluble collagen binding, a parameter to assess integrin α2β1 activation, in stimulated platelets. Agonists that can also activate platelet glycoprotein IIb/IIIa are able to activate integrin α2β1, but those operating via glycoprotein Ib cannot. Activation of α2β1 induced by low thrombin or collagen-related peptide concentrations was almost completely inhibited by apyrase, and the inhibitors wortmannin, 4-amino-5-(chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, bisindolylmaleimide I, and SQ29548 significantly inhibited it. Activation induced by high thrombin or collagen-related peptide concentrations was far less sensitive to these inhibitors. However, only wortmannin markedly inhibited ADP-induced integrin α2β1 activation, and this was not ADP concentration-dependent. These results suggest that at the low agonist concentrations, the released ADP would be a primary inducer of integrin α2β1 activation, while at the high agonist concentrations, there would be several pathways through which integrin α2β1 activation can be induced. Kinetic analyses revealed that ADP-induced platelets had about the same number of binding sites (Bmax) as thrombin-induced platelets, but their affinity (Kd) for soluble collagen was 3.7–12.7-fold lower, suggesting that activated integrin α2β1 induced by ADP is different from that induced by thrombin. The data are consistent with an activation mechanism involving released ADP and in which there exists two different states of activated integrin α2β1; these activated forms of integrin α2β1 would have different conformations that determine their ligand affinity.