HUMAN PLATELETS AS A MODEL FOR THE BINDING AND DEGRADATION OF THROMBOPOIETIN

Recent studies have shown that plasma thrombopoietin (TPO) levels appe ar to be directly regulated by platelet mass and that removal of plasm a TPO by platelets via binding to the c-Mpl receptor is involved in th e clearance of TPO in rodents. To help elucidate the role of platelets in the clearance of TPO in humans, we studied the in vitro specific b inding of recombinant human TPO (rhTPO) to human platelet-rich plasma (PRP), washed platelets (WP), and cloned c-Mpl. Using a four-parameter fit and/or Scatchard analysis, the approximate affinity of rhTPO for its receptor, which was calculated from multiple experiments using dif ferent PRP preparations, was between 128 and 846 pmol/L, with similar to 25 to 224 receptors per platelet. WP preparations gave an affinity of 260 to 540 pmol/L, with similar to 25 to 35 receptors per platelet, and erythropoietin failed to compete with I-125-rhTPO for binding to WP. Binding and dissociation studies conducted with a BiaCore apparatu s yielded an affinity of 350 pmol/L for rhTPO binding to cloned c-Mpl receptors. The ability of PRP to bind and degrade I-125-rhTPO was both time- and temperature-dependent and was blocked by the addition of ex cess cold rhTPO. Analysis of platelet pellets by sodium dodecyl sulfat e-polyacrylamide gel electrophoresis showed that I-125-rhTPO was degra ded into a major fragment of similar to 45 to 50 kD. When I-125-rhTPO was incubated with a platelet homogenate at pH = 7.4, a degradation pa ttern similar to intact. platelets was observed. Together, these data show that human platelets specifically bind rhTPO with high affinity, internalize, and then degrade the rhTPO. (C) 1997 by The American Soci ety of Hematology.