title: Constitutive expression and involvement of cyclooxygenase-2 in human megakaryocytopoiesis.
abstract: OBJECTIVE
  Cyclooxygenase-1 (COX-1), but not COX-2, is expressed in human platelets, and thromboxane A2 (TXA2) produced via COX-1 induces platelet aggregation. The objectives of this study were to investigate the expression of COX-1 and COX-2 during platelet differentiation and to determine whether these enzymes are involved in the differentiation.
  
  
  RESULTS
  CD34+ progenitor cells isolated from human cord blood were cultured with thrombopoietin and c-kit ligand. The cells differentiated into megakaryocytes (CD34-/CD41+) after 8 days of culture and into platelets (CD41+/prodium iodide-) after 14 days of culture. The CD34+cells expressed a trace of COX-1 gene and no COX-2 gene. On day 5, COX-2 gene expression was observed and continued throughout the remainder of the culture. COX-1 gene expression increased after 8 days of culture. The treatment of this liquid culture with indomethacin, a dual inhibitor of COX-1 and COX-2, and NS-398, a COX-2-specific inhibitor, suppressed megakaryocyte differentiation. In contrast, at a dose of 10(-7) M, mofezolac, which is a highly selective inhibitor of COX-1, did not affect differentiation. NS-398-induced suppression of megakaryocyte differentiation was partly abrogated by stable analogues of TXA2.
  
  
  CONCLUSIONS
  We report here that COX-2 and COX-1 are constitutively expressed in megakaryocytes, and TXA2 produced by COX-2 plays an important role in megakaryocytopoiesis.
authors.affiliation: Department of Cellular Physiological Chemistry, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan.
authors.forename: Nobuhito
authors.initials: N
authors.lastname: Tanaka
authors.forename: Takahiro
authors.initials: T
authors.lastname: Sato
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujita
authors.forename: Ikuo
authors.initials: I
authors.lastname: Morita
mesh_terms: D002454:Cell Differentiation; D002478:Cells, Cultured / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; D052246:Cyclooxygenase 2 Inhibitors; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D004790:Enzyme Induction; D005312:Fetal Blood / Q000166:cytology; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D006801:Humans; D007231:Infant, Newborn; D007527:Isoenzymes / Q000096:biosynthesis / Q000097:blood / Q000235:genetics / Q000502:physiology*; D007555:Isoxazoles / Q000494:pharmacology; D008533:Megakaryocytes / Q000201:enzymology*; D008565:Membrane Proteins; D009578:Nitrobenzenes / Q000494:pharmacology; D010976:Platelet Count; D039904:Pluripotent Stem Cells / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D011451:Prostaglandin-Endoperoxide Synthases / Q000096:biosynthesis / Q000097:blood / Q000235:genetics / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis; D019089:Stem Cell Factor / Q000494:pharmacology; D013449:Sulfonamides / Q000494:pharmacology; D034061:Thrombopoiesis / Q000502:physiology*; D013926:Thrombopoietin / Q000494:pharmacology; D013928:Thromboxane A2 / Q000096:biosynthesis / Q000502:physiology*
chemical_list: D052246:Cyclooxygenase 2 Inhibitors; D016861:Cyclooxygenase Inhibitors; D007527:Isoenzymes; D007555:Isoxazoles; D008565:Membrane Proteins; D009578:Nitrobenzenes; D012333:RNA, Messenger; D019089:Stem Cell Factor; D013449:Sulfonamides; C080955:N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide; D013928:Thromboxane A2; D013926:Thrombopoietin; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; C496511:PTGS1 protein, human; C496540:PTGS2 protein, human; D011451:Prostaglandin-Endoperoxide Synthases; C054999:mofezolac