title: Analyzing the mechanism of Rap1 activation in platelets: Rap1 activation is related to the release reaction mediated through the collagen receptor GPVI.
abstract: The abundant Rap1 in platelets becomes activated when these cells are stimulated by various agonists, but its function has remained unknown. In view of this, we developed an assay to quantitatively measure activated Rap1 and used it to determine relationships between Rap1 activation and several platelet functions: integrin alpha2beta1 activation, tyrosine phosphorylation, and the release reaction. We looked at how these processes are affected by the protein kinase C inhibitor BIMI, tyrosine kinase inhibitor PP2, PI 3-kinase inhibitor wortmannin, and ADP scavenger apyrase. In CRP (collagen related peptide)-activated platelets, all the inhibitors severely inhibited Rap1 activation, but had little effect on integrin alpha2beta1 activation, indicating that the integrin activation mechanism is different from the Rap1 activation mechanism, at least in GPVI-dependent activation. With p85alpha-null mouse platelets, we demonstrated that Rap1 activation involves PI 3-kinase p85alpha-dependent tyrosine phosphorylation. All the inhibitors similarly decreased Rap1 activation and the serotonin release reaction, and the inhibition of Rap1 activation was not due to the lack of released ADP. Our results indicate that platelet Rap1 activation is closely related to the release reaction and not to integrin alpha2beta1 activation in GPVI-mediated platelet activation.
authors.affiliation: Department of Protein Biochemistry, Institute of Life Science, Kurume University, Kurume, Fukuoka, Japan.
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Jung
authors.forename: Masaaki
authors.initials: M
authors.lastname: Ohnuma
authors.forename: Naohide
authors.initials: N
authors.lastname: Watanabe
authors.forename: Mamiko
authors.initials: M
authors.lastname: Sonoda
authors.forename: Makoto
authors.initials: M
authors.lastname: Handa
authors.forename: Masaaki
authors.initials: M
authors.lastname: Moroi
mesh_terms: D000730:Androstadienes / Q000494:pharmacology; D001081:Apyrase / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism*; D003094:Collagen / Q000378:metabolism; D006801:Humans; D038982:Integrin alpha2beta1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D010766:Phosphorylation; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011743:Pyrimidines / Q000494:pharmacology; D014443:Tyrosine / Q000378:metabolism; D000077191:Wortmannin; D020827:rap1 GTP-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: C412373:AG 1879; D000730:Androstadienes; D038982:Integrin alpha2beta1; D010980:Platelet Membrane Glycoproteins; D047428:Protein Kinase Inhibitors; D011743:Pyrimidines; C098341:platelet membrane glycoprotein VI; D014443:Tyrosine; D003094:Collagen; D001081:Apyrase; D020827:rap1 GTP-Binding Proteins; D000077191:Wortmannin