Exposure of Platelet Fibrinogen Receptors by Zinc Ions: Role of Protein Kinase C

Abstract

Previous studies demonstrated that Zn²⁺ at a concentration of 50 μM increases the number of fibrinogen receptors exposed on ADP-stimulated platelets and that higher concentrations of Zn²⁺ induce platelet aggregation that appears to be mediated by receptors associated with the glycoprotein llb/llla complex. The purpose of this study was to identify the mechanism by which Zn²⁺ modulates exposure of fibrinogen receptors on the surface of human washed platelets. We determined that Zn²⁺ (300-800 μM)-induced platelet aggregation that was not accompanied by the release of [¹⁴C]serotonin was not blocked by ADP scavenging enzymes and 5′-p-fluorosulfonylbenzoyl-adenosine, an affinity label for ADP binding sites, but it was inhibited by disintegrins, staurosporine, and EDTA. Zn²⁺ (50-200 μM) showed a synergistic effect on platelet aggregation and platelet release caused by ADP and N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine, a Zn²⁺ chelator, and inhibited ADP-induced platelet aggregation that was reversed by Zn²⁺ (50 μM). Zn²⁺ (200 μM) increased the number of fibrinogen binding sites and the affinity of albolabrin (a disintegrin isolated from Trimeresurus albolabris snake venom that has been shown to bind to the fibrinogen receptor) on ADP-activated platelets. On the other hand, Zn²⁺ (100-800 μM) did not increase fibrinogen binding to the purified receptor. Incubation of platelets with Zn²⁺ (200 μM) resulted in the phosphorylation of a 47-kDa protein that was blocked by staurosporine, an inhibitor of protein kinase C. In conclusion, Zn²⁺ ions activate protein kinase C and enhance fibrinogen receptor exposure on the surface of platelets stimulated by ADP.