Regulation of thrombopoietin levels by c-mpl-mediated binding to platelets

Abstract

The involvement of platelets and the c-mpl receptor in the regulation of thrombopoietin (TPO) plasma concentrations and tissue mRNA levels was investigated in both normal mice and mice defective in c-mpl (c-mpl- /-). Although c-mpl-/- mice have fewer platelets and higher plasma TPO activity than normal mice, there was no increase in TPO mRNA levels as measured by an S1 nuclease protection assay. After the intravenous injection of 125I-TPO, specific uptake of radioactivity by the spleen and blood cells was present in the normal mice, but absent in the c-mpl- /- mice. Platelet-rich plasma (PRP) from normal mice was able to bind and internalize 125I-TPO, whereas PRP from c-mpl-/- mice lacked this ability. Analysis of 125I-TPO binding to normal PRP indicated that binding was specific and saturable, with an approximate affinity of 560 pmol/L and 220 receptors per platelet. PRP from normal mice was also able to degrade 125I-TPO into lower molecular weight fragments. After the intravenous injections, c-mpl-/- mice cleared a dose of 125I-TPO at a much slower rate than did normal mice. Injection of washed platelets from normal mice into c-mpl-/- mice resulted in a dramatic, but transient, decrease in plasma TPO levels. These data provide evidence that platelets regulate plasma TPO levels via binding to the c-mpl receptor on circulating platelets.

Thrombopoietin in Acute Liver Failure

“…Under normal circumstances, TPO is produced constitutively in the liver and serum levels of circulating TPO are regulated by the mass of platelets and megakaryocytes that internalize (via TPO receptors) and degrade TPO. 11,12 In this study, we did not observe the usual inverse relationship between serum TPO and platelet count, 1,2 suggesting that decreased platelet mass (and thereby decreased TPO clearance) might not be responsible for the increase in serum TPO levels observed. Our study was not designed to measure TPO production, but an increase in TPO production could be derived either from hepatic or extrahepatic sources.…”

Platelet c‐mpl expression is dysregulated in patients with essential thrombocythaemia but this is not of diagnostic value

“…Since TPO appears to be an important regulator of platelet number, it is possible that abnormalities of TPO regulation may contribute to the pathogenesis of ET. Studies have shown that there is an inversely proportional relationship between circulating TPO levels and platelet/megakaryocyte mass through cytokine binding to receptor expressed on the surface of these cells (Fielder et al, 1996;Kuter & Rosenberg, 1995). However, although this relationship is generally maintained in thrombocytopenia and normal steady-state haemopoiesis, it does not appear to hold true in thrombocythaemic situations.…”

Circulating thrombopoietin concentrations in thrombocytopenic patients, including cancer patients following chemotherapy, with or without peripheral blood progenitor cell transplantation

“…However, the correlation between TPO concentrations and platelet counts in the thrombocytopenic patients was weak, suggesting that factors other than platelet mass also contribute to the regulation of circulating TPO levels (Fig 1). The circulating TPO level is probably regulated by platelet mass through TPO binding to c-mpl on platelets (Fielder et al, 1996). Since TPO promotes proliferation and maturation of megakaryocytes, megakaryocyte mass may also regulate the circulating TPO level, possibly through TPO binding to c-mpl on megakaryocytes.…”