Urokinase-Type Plasminogen Activator Production by Cultured Megakaryocytes and Blood Outgrowth Endothelial Cells in the Quebec Platelet Disorder.

Abstract

The Quebec Platelet Disorder (QPD) is an unusual α-granule protein disorder, associated with increased expression and storage of urokinase-type plasminogen activator (u-PA) in platelets and platelet-dependent, accelerated clot lysis. The increased u-PA in QPD platelets, and normal to increased levels in QPD plasma, led us to investigate the production of u-PA by cultured QPD megakaryocytes (MKs) and by QPD blood outgrowth endothelial cells (BOECs). Our objectives were to 1) characterize production of u-PA and other platelet proteins by normal and QPD MKs at different stages of differentiation, and 2) determine if the abnormal production of u-PA in the QPD extended to endothelial cells. Thrombopoietin-stimulated MKs were obtained by culturing CD34+ progenitor cells isolated from peripheral blood. BOECs were obtained from fibronectin-adherent peripheral blood mononuclear cells. Cell lysates and culture media were harvested from MKs on days 7 and 13, and from BOECs on day 11. Proteins were quantified by ELISA. MK differentiation was evaluated by flow cytometry for αIIbβ3 expression. For BOECs, endothelial lineage differentiation was assessed by immunostaining for acetylated low-density lipoprotein uptake and Ulex europaeus agglutinin binding. QPD and normal control BOECs were indistinguishable in morphology and differentiation during culture, and their content of u-PA was low (ng u-PA/mg cellular protein, mean ± SD for n = 9: QPD 3.5 ± 1.1, control 3.1 ± 1.5, p = 0.54), without detectable u-PA-PAI-1 (plasminogen activator inhibitor 1) complexes. QPD BOECs also produced normal amounts of the control secretory proteins PAI-1, multimerin 1 and von Willebrand factor (VWF). The morphology, viability, proliferation and differentiation of QPD and control MKs grown in culture were indistinguishable. By 7 days of MK culture, QPD and control cells had secreted similar, low quantities of u-PA into the culture media (ng u-PA/106cells, mean ± SD for n = 4: QPD 0.3 ± 0.1, control 0.4 ± 0.1; p = 0.18). However, by day 13 of MK culture (when most cells expressed αIIbβ3), QPD cultures contained significantly more u-PA in their culture media (ng u-PA/106 cells: QPD 20.5 ± 9.8, control 0.2 ± 0.1, p < 0.05 compared to controls and to day 7 QPD samples). The increased production of u-PA in late stage QPD MK cultures was associated with increased intracellular u-PA (ng u-PA/106 cells: QPD 9.6 ± 6.3, control 0.03 ± 0.02, p < 0.05) and increases in intracellular and secreted u-PA-PAI-1 complexes (p < 0.05). In contrast, the levels of PAI-1, VWF, thrombospondin-1 and platelet factor 4 were normal in day 13 QPD MK cultures. Taken together, these data indicate that low levels of u-PA are normally produced by MKs and BOECs isolated from peripheral blood progenitors. The increased u-PA in differentiating QPD MKs and peripheral blood platelets, and normal levels of u-PA in QPD BOECs and early MK progenitors, indicate that the molecular defect in this disorder induces a cell-type restricted defect in u-PA regulation that is manifested during the later stages of MK differentiation. The molecular nature of this congenital defect in u-PA regulation is under investigation.