title: Platelets express and release osteocalcin and co-localize in human calcified atherosclerotic plaques.
abstract: BACKGROUND
  Although vascular-calcification mechanisms are only partially understood, the role of circulating calcifying cells and non-collagenous bone matrix proteins in the bone-vascular axis is emerging. In spite of the fact that platelets represent a cellular interface between hemostasis, inflammation and atherosclerosis, and have a myeloid precursor, a possible involvement in the modulation of vascular calcification has rarely been investigated. We investigated if osteocalcin (OC) is released by platelets and described OC expression in patients with carotid artery occlusive disease.
  
  
  METHODS
  Expression and release of OC were determined by Western blot, immunofluorescence, fluorescence-activated cell sorting (FACS) and ELISA in human resting and activated platelets and megakaryocytes. Co-localization of platelet aggregates, macrophages, OC and calcifications was studied in carotid endarterectomy specimens and normal tissues.
  
  
  RESULTS
  Human platelets expressed OC and co-localized with CD63 in δ-granules. Upon activation with an endogenous mechanism, platelets released OC in the extracellular medium. Expression of OC in megakaryocytes suggested lineage specificity. The OC count in circulating platelets and the released amount were significantly higher in patients with carotid artery occlusive disease than in healthy controls (P < 0.0001) in spite of similar serum levels. In atherosclerotic plaques, OC strongly overlapped with CD41+ platelets in the early stage of calcification, but this was not seen in normal tissues. CD68+OC+ cells were present at the periphery of the calcified zone.
  
  
  CONCLUSIONS
  Given the active role played by platelets in the atherosclerotic process, the involvement of OC release from platelets in atherosclerotic lesions and the impact of genetic and cardiovascular risk factors in mediating bone-marrow preconditioning should be investigated further.
authors.affiliation: Section of Clinical Pathology and Human Reproduction Pathology Service, Department of Histology, University of Padova, Padova, Italy. carlo.foresta@unipd.it
authors.forename: C
authors.initials: C
authors.lastname: Foresta
authors.forename: G
authors.initials: G
authors.lastname: Strapazzon
authors.forename: L
authors.initials: L
authors.lastname: De Toni
authors.forename: F
authors.initials: F
authors.lastname: Fabris
authors.forename: F
authors.initials: F
authors.lastname: Grego
authors.forename: G
authors.initials: G
authors.lastname: Gerosa
authors.forename: S
authors.initials: S
authors.lastname: Vettore
authors.forename: A
authors.initials: A
authors.lastname: Garolla
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D015153:Blotting, Western; D002340:Carotid Artery Diseases / Q000097:blood* / Q000473:pathology / Q000601:surgery; D016022:Case-Control Studies; D002469:Cell Separation / Q000379:methods; D016894:Endarterectomy, Carotid; D004797:Enzyme-Linked Immunosorbent Assay; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006801:Humans; D008297:Male; D008533:Megakaryocytes / Q000378:metabolism; D015675:Osteocalcin / Q000097:blood*; D058226:Plaque, Atherosclerotic; D015539:Platelet Activation; D040201:Platelet Membrane Glycoprotein IIb / Q000097:blood; D022142:Secretory Vesicles / Q000378:metabolism; D060149:Tetraspanin 30 / Q000097:blood; D061205:Vascular Calcification / Q000097:blood* / Q000473:pathology / Q000601:surgery
chemical_list: C554586:CD63 protein, human; D040201:Platelet Membrane Glycoprotein IIb; D060149:Tetraspanin 30; D015675:Osteocalcin