title: Angiogenesis is regulated by a novel mechanism: pro- and antiangiogenic proteins are organized into separate platelet alpha granules and differentially released.
abstract: Platelets, in addition to their function in hemostasis, play an important role in wound healing and tumor growth. Because platelets contain angiogenesis stimulators and inhibitors, the mechanisms by which platelets regulate angiogenesis remain unclear. As platelets adhere to activated endothelium, their action can enhance or inhibit local angiogenesis. We therefore suspected a higher organization of angiogenesis regulators in platelets. Using double immunofluorescence and immunoelectron microscopy, we show that pro- and antiangiogenic proteins are separated in distinct subpopulations of alpha-granules in platelets and megakaryocytes. Double immunofluorescence labeling of vascular endothelial growth factor (VEGF) (an angiogenesis stimulator) and endostatin (an angiogenesis inhibitor), or for thrombospondin-1 and basic fibroblast growth factor, confirms the segregation of stimulators and inhibitors into separate and distinct alpha-granules. These observations motivated the hypothesis that distinct populations of alpha-granules could undergo selective release. The treatment of human platelets with a selective PAR4 agonist (AYPGKF-NH(2)) resulted in release of endostatin-containing granules, but not VEGF-containing granules, whereas the selective PAR1 agonist (TFLLR-NH(2)) liberated VEGF, but not endostatin-containing granules. In conclusion, the separate packaging of angiogenesis regulators into pharmacologically and morphologically distinct populations of alpha-granules in megakaryocytes and platelets may provide a mechanism by which platelets can locally stimulate or inhibit angiogenesis.
authors.affiliation: Translational Medicine Division, Brigham and Women's Hospital, Boston, MA 02115, USA. jitaliano@rics.bwh.harvard.edu
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Italiano
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Richardson
authors.forename: Sunita
authors.initials: S
authors.lastname: Patel-Hett
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Battinelli
authors.forename: Alexander
authors.initials: A
authors.lastname: Zaslavsky
authors.forename: Sarah
authors.initials: S
authors.lastname: Short
authors.forename: Sandra
authors.initials: S
authors.lastname: Ryeom
authors.forename: Judah
authors.initials: J
authors.lastname: Folkman
authors.forename: Giannoula L
authors.initials: GL
authors.lastname: Klement
mesh_terms: D001792:Blood Platelets / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism* / Q000648:ultrastructure; D016253:Microscopy, Immunoelectron; D018919:Neovascularization, Physiologic; D010447:Peptide Hydrolases / Q000378:metabolism
chemical_list: D010447:Peptide Hydrolases