title: Platelets exposed to elevated levels of endogenous thrombopoietin in vivo have a reduced response to megakaryocyte growth and development factor in vitro.
abstract: Thrombopoietin (TPO), or megakaryocyte growth and development factor (MGDF), has been shown to potentiate the sensitivity of normal human platelets to various agonists in vitro. The present study investigated the functional and biochemical properties of platelets from mice rendered thrombocytopenic by sublethal irradiation with regard to the reactivity to recombinant murine MGDF (rmMGDF) in vitro. During the course of reversible thrombocytopenia following irradiation, platelets from irradiated mice which had lower platelet counts and reciprocally higher plasma TPO levels showed lower reactivity to rmMGDF in agonist-induced platelet aggregation. Intravenous injections of recombinant soluble murine c-Mpl (sMpl), which has the ability to capture TPO, after irradiation restored the reactivity of platelets at the platelet nadir to rmMGDF. On the other hand, platelets prepared from normal mice 3 h after a single intravenous injection of pegylated rmMGDF did not respond to rmMGDF. There was a marked decrease in c-Mpl and Janus kinase 2 (JAK2) in platelets from irradiated mice at the platelet nadir. Similar results were observed with platelets from mice administered pegylated rmMGDF. JAK2 was only moderately decreased, however, in platelets from mice given sMpl after irradiation. These results indicate that exposure of platelets to increased endogenous TPO levels in vivo in thrombocytopenic mice leads to a reduction in the platelet reactivity to rmMGDF in vitro. Further, these results suggest that the c-Mpl-mediated signaling pathway, which is essential for the priming effect of rmMGDF, is defective in thrombocytopenic murine platelets.
authors.affiliation: Pharmaceutical Development Laboratory, Kirin Brewery Co, Ltd, Takasaki, Gunma, Japan.
authors.forename: U
authors.initials: U
authors.lastname: Nishiyama
authors.forename: H
authors.initials: H
authors.lastname: Morita
authors.forename: Y
authors.initials: Y
authors.lastname: Torii
authors.forename: T
authors.initials: T
authors.lastname: Kuwaki
authors.forename: E
authors.initials: E
authors.lastname: Shimizu
authors.forename: H
authors.initials: H
authors.lastname: Akahori
authors.forename: T
authors.initials: T
authors.lastname: Kato
authors.forename: H
authors.initials: H
authors.lastname: Miyazaki
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000187:drug effects* / Q000502:physiology / Q000528:radiation effects; D002455:Cell Division / Q000187:drug effects / Q000528:radiation effects; D004195:Disease Models, Animal; D053614:Janus Kinase 2; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D009363:Neoplasm Proteins; D010974:Platelet Aggregation / Q000187:drug effects / Q000528:radiation effects; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; D011994:Recombinant Proteins / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology; D015398:Signal Transduction / Q000502:physiology; D013921:Thrombocytopenia / Q000378:metabolism / Q000503:physiopathology; D013926:Thrombopoietin / Q000378:metabolism / Q000494:pharmacology* / Q000528:radiation effects
chemical_list: D009363:Neoplasm Proteins; D011518:Proto-Oncogene Proteins; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; D011994:Recombinant Proteins; C082218:MPL protein, human; D013926:Thrombopoietin; D011505:Protein-Tyrosine Kinases; C507924:JAK2 protein, human; C507926:Jak2 protein, mouse; D053614:Janus Kinase 2