Interaction of thrombopoietin with the platelet c‐mpl receptor in plasma: binding, internalization, stability and pharmacokinetics

Abstract

Summary. Thrombopoietin (TPO) is the primary regulator of platelet production and acts through binding its receptor, c-mpl, found on megakaryocyte progenitor cells, megakaryocytes and platelets. Circulating levels of TPO are regulated primarily by the clearance of TPO after it binds to c-mpl receptors on circulating platelets. In this study the interaction of TPO with the platelet c-mpl receptor has been analysed under physiological conditions using radiochemical and pharmacokinetic approaches.125 I-rHuTPO was prepared using a novel method of gentle iodination that preserved its biological activity and used to demonstrate that platelets, but not endothelial cells, have a single class of binding sites (56 6 17 binding sites/platelet) with high af®nity (K d 163 6 31 pM). Cross-linking experiments con®rmed that TPO, but not erythropoietin (EPO), speci®cally associated with the 95 kD platelet c-mpl receptor. Upon addition of TPO to platelets, 80% of the TPO binding sites were internalized within an hour and were not recycled. TPO that was not bound by platelets was stable for up to 6 d in both platelet-poor and platelet-rich plasma. Using unlabelled recombinant human TPO (rHuTPO), standard pharmacokinetic analysis demonstrated that platelets have an average TPO clearance of 1´24 6 0´38 ml/h/10 9 platelets and that TPO clearance was reduced by low temperature but not by a number of drugs or metabolic inhibitors. The maximal amount of TPO removed by platelets in vitro was identical to that predicted by the total number of TPO binding sites. These results provide a biochemical and pharmacokinetic basis for the clinical use of TPO and for understanding possible disorders of platelet production.

Incomplete restoration of Mpl expression in the mpl−/− mouse produces partial correction of the stem cell–repopulating defect and paradoxical thrombocytosis

“…Furthermore, because TPO is internalized and destroyed after binding to Mpl, plasma TPO levels are primarily regulated by megakaryocyte and platelet mass. [13][14][15][16][17][18][19][20] In murine hematopoietic stem cell gene transfer studies, ectopic expression of an Mpl transgene under the direction of a constitutive promoter produced dyshematopoieses with both inappropriate expansion of erythroid cells and the development of thrombocytopenia due to depressed plasma TPO levels. [21][22][23][24] Successful gene therapy of CAMT necessitates ectopic expression of Mpl in a manner that faithfully mimics its pattern in nature.…”

Blood Glucose Lowering Effect and Safety of the Aqueous Leaf Extracts of Zanha africana

“…The significant increase in white blood cells observed on intraperitoneal administration of plant extracts of Z. africana indicates a more accelerated production of these cells and a boosted immunity to mice treated by this extract [58][59][60]. This could be due to tissue damage caused by some constituents of this plant extract.…”

Effect Of Chromatographic Fraction of Portulaca Oleracea on Haematological and Plasma Biochemical Parameters In Male Albino Rats

“…Similar report was given by Adewusi and Afolayan (2009) in Pelargonium reniforme extract treated rats. Also, the insignificant change in the platelet count caused by fraction could be an indication that it does not has the potential to stimulate thrombopoietin production (Li et al, 1999) with the hemostatic capability of the blood maintaining the status quo since platelets mediate in the blood -clotting mechanism. Treatment of rats with fraction caused significant increases in total protein levels, which probably indicates that the buffering capacity of the blood and body fluid balance have been enhanced.…”