title: Protein C supports platelet binding and activation under flow: role of glycoprotein Ib and apolipoprotein E receptor 2.
abstract: BACKGROUND
  Activated protein C (APC) regulates thrombin generation and inhibits apoptosis. Endothelial protein C receptor (EPCR)-bound protein C is activated by thrombomodulin-bound thrombin. APC inactivates coagulation factors (F)Va/VIIIa and generates cytoprotective signaling downstream of protease-activated receptor-1 (PAR-1). Binding of APC to EPCR both modifies and induces PAR-1 signaling, but it is unknown if protein C interacts with cells in an alternative manner.
  
  
  OBJECTIVE
  To determine whether platelets possess receptors for protein C that can generate intracellular signals.
  
  
  RESULTS
  Immobilized protein C or APC supported platelet adhesion, lamellipodia formation and elevation of intracellular Ca(2+). Adhesion of platelets to protein C or APC was inhibited by soluble recombinant apolipoprotein E receptor 2' (ApoER2') and by receptor-associated protein (RAP), an inhibitor of the low-density lipoprotein receptor family. Under shear, surface-bound protein C supported platelet adhesion and aggregation in a glycoprotein (GP)Ibalpha-dependent manner, and adhesion of platelets to immobilized protein C was abrogated by the addition of soluble forms of ApoER2' or RAP. APC bound to purified recombinant ApoER2' or GPIbalpha.
  
  
  CONCLUSIONS
  Our data demonstrate that activation of platelets with rapid intracellular signaling caused by binding to immobilized protein C or APC occurs via mechanisms that require ApoER2 and GPIbalpha and that APC directly binds to purified ectodomains of the receptors ApoER2 and GPIbalpha. These findings imply that protein C and APC may directly promote cell signaling in other cells by binding to ApoER2 and/or GPIbalpha.
authors.affiliation: Department of Biomedical Engineering, Oregon Health & Science University, Portland, OR 97239, USA.
authors.forename: T C
authors.initials: TC
authors.lastname: White
authors.forename: M A
authors.initials: MA
authors.lastname: Berny
authors.forename: E I
authors.initials: EI
authors.lastname: Tucker
authors.forename: R T
authors.initials: RT
authors.lastname: Urbanus
authors.forename: P G
authors.initials: PG
authors.lastname: de Groot
authors.forename: J A
authors.initials: JA
authors.lastname: Fernández
authors.forename: J H
authors.initials: JH
authors.lastname: Griffin
authors.forename: A
authors.initials: A
authors.lastname: Gruber
authors.forename: O J T
authors.initials: OJ
authors.lastname: McCarty
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002448:Cell Adhesion; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation; D006801:Humans; D007700:Kinetics; D026502:LDL-Receptor Related Proteins; D008954:Models, Biological; D010973:Platelet Adhesiveness; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism*; D011486:Protein C / Q000378:metabolism*; D011973:Receptors, LDL / Q000378:metabolism; D018110:Receptors, Lipoprotein / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D026502:LDL-Receptor Related Proteins; D019038:Platelet Glycoprotein GPIb-IX Complex; D011486:Protein C; D011973:Receptors, LDL; D018110:Receptors, Lipoprotein; C098773:low density lipoprotein receptor-related protein 8; D002118:Calcium