The physiological response of thrombopoietin (c‐Mpl ligand) to thrombocytopenia in the rat

Abstract

Summary. It has been suggested that circulating levels of thrombopoietin (TPO) are determined primarily by platelet and megakaryocyte clearance of TPO and not by changes in hepatic TPO production. The experimental evidence accumulated so far to support this hypothesis is incomplete. We have therefore developed a new model of non-immune thrombocytopenia in the rat and used it to assess the relationship of TPO (c-mpl ligand) to the platelet mass. 14 d following the administration of busulphan, the platelet count reached a nadir of <2% of its initial value and remained at this level for up to 6 d. Circulating TPO was measured by two different bioassays which were sensitive enough to measure normal levels of TPO and levels rose from 106 Ϯ 29 pg/ml in animals with a normal platelet count to 2015 Ϯ 544 pg/ml in those with thrombocytopenia. These elevated levels of TPO were solely a response to the low platelet count since transfusion of a normal mass of platelets into the thrombocytopenic animals returned the TPO levels exactly to normal. The increase in TPO levels in thrombocytopenic animals was not due to increased TPO production since the thrombocytopenic animals did not show any increase in TPO mRNA in total or polysome-associated hepatic RNA. Rather, rat platelets were able to bind and stoichiometrically remove TPO from thrombocytopenic plasma via high-affinity receptors (K d ¼ 38 Ϯ 10 pM; 233 Ϯ 32 receptors/platelet). These results serve as a proof that the circulating level of TPO is determined not by alterations in TPO transcription or translation but by the ability of the platelet mass to bind and remove TPO from the circulation.

The biology of thrombopoietin and thrombopoietin receptor agonists

“…There does not seem to be any significant increase in TPO gene transcription in the liver in thrombocytopenic animals [40]. This regulation of platelet production by TPO is similar to that by which neutrophil and monocyte production is regulated by G-CSF and M-CSF in normal physiology [48].…”

“…In normal physiology TPO is made at a constant rate in the liver, released into the circulation without any storage form, and then rapidly cleared from the circulation by platelets and possibly also megakaryocytes, leaving a basal level of thrombopoietin in the circulation [6,40]. There is no cytokine or clinical scenario that increases TPO mRNA or TPO production [41].…”

Risk factors and kinetics of thrombocytopenia associated with bortezomib for relapsed, refractory multiple myeloma

“…2 Circulating TPO is cleared via binding to circulating platelets and bone marrow megakaryocytes. 3,4 Marrow megakaryopoiesis is then regulated by the circulating platelet count as well as by marrow megakaryocyte content. When circulating platelet counts or megakaryocyte content of the marrow decreases as a result of cytotoxic injury, peripheral TPO levels increase due to decreased TPO clearance.…”

Thrombocytopenia caused by the development of antibodies to thrombopoietin

“…TPO is produced in a constitutive fashion in the liver, and its production is not increased in thrombocytopenic disorders such as immune thrombocytopenic purpura (ITP). 2 In animals in which the TPO gene or the TPO receptor, c-Mpl, has been eliminated in a homozygous fashion, bone marrow megakaryocyte progenitors are reduced to 5% of normal and the platelet count to 10% to 15% of normal. 3,4 Levels of white blood cells (WBCs) and red blood cells (RBCs) are not altered, but their bone marrow precursors are reduced to 30% to 40% of normal.…”