title: Protein kinase C and cyclic AMP regulate reversible exposure of binding sites for fibrinogen on the glycoprotein IIB-IIIA complex of human platelets.
abstract: Platelet aggregation is mediated via binding of fibrinogen to sites on the membrane glycoprotein IIB-IIIA complex which become exposed when the cells are stimulated. We report here evidence of a dynamic and reversible exposure of binding sites for fibrinogen. In the absence of fibrinogen, exposed sites (B*) gradually lose their capacity to bind fibrinogen and close (Bo). On stimulation with platelet-activating factor (PAF, 500 nM) at 22 degrees C, closing of B* is enhanced by agents that raise cyclic AMP levels (10 ng of prostaglandin I2/ml; 5 mM-theophylline), inhibit protein kinase C (PKC; 25 microM-sphingosine; 1 microM-staurosporine), or disrupt the energy supply (30 mM-2-deoxy-D-glucose + 1 mM-CN-), or by raising the temperature to 37 degrees C. Conversely, activation of PKC 1 microM-1,2-dioctanoyl-sn-glycerol; 55 nM-phorbol 12-myristate 13-acetate) and an increase in intracellular [Ca2+] (100 nM-ionomycin + extracellular Ca2+) oppose the disappearance of B*. Phosphorylation of the 47 kDa protein illustrates the tight coupling between PKC and B* under all conditions tested, except when the cyclic AMP level is raised, and B* is converted to Bo without affecting PKC activity. Although the increase in PKC activity is much smaller with ADP or even absent upon stimulation with adrenaline, the control of B* is equally sensitive to modulation of cyclic AMP and PKC activity. We conclude that PAF, ADP and adrenaline regulate exposure of fibrinogen binding sites through a common mechanism consisting of two independent pathways, one dominated by PKC and the other by an as yet unidentified cyclic AMP-sensitive step.
authors.affiliation: Department of Haematology, University Hospital Utrecht, The Netherlands.
authors.forename: G
authors.initials: G
authors.lastname: van Willigen
authors.forename: J W
authors.initials: JW
authors.lastname: Akkerman
mesh_terms: D000244:Adenosine Diphosphate / Q000502:physiology; D001665:Binding Sites; D001792:Blood Platelets / Q000378:metabolism*; D000242:Cyclic AMP / Q000502:physiology*; D004837:Epinephrine / Q000502:physiology; D005340:Fibrinogen / Q000378:metabolism*; D006801:Humans; D010972:Platelet Activating Factor / Q000502:physiology; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction; D013696:Temperature
chemical_list: D010972:Platelet Activating Factor; D010980:Platelet Membrane Glycoproteins; D000244:Adenosine Diphosphate; D005340:Fibrinogen; D000242:Cyclic AMP; D011493:Protein Kinase C; D004837:Epinephrine