THE UROKINASE-RECEPTOR (CD87) IS EXPRESSED IN CELLS OF THE MEGAKARYOBLASTIC LINEAGE

Megakaryocytopoiesis is governed in the bone marrow microenvironment b y cellular interactions that include various adhesion receptor systems and pericellular proteolysis for proper regulation of cell motility a nd differentiation. In order to define the role of cell surface molecu les required for these processes, we searched for protease receptors o n these cells. In an in vitro system utilizing different cell lines of the megakaryoblastic lineage (MEG-01, Dami). low level surface expres sion of the urokinase (uPA) receptor was noted. Following stimulation with phorbolester (PMA), a 3-6 fold higher expression of uPA receptor over a period of up to 5 days could be observed by fluorescent activat ed cell-sorting as well as by direct ligand-binding of amino-terminal fragment of uPA or vitronectin. Together with elevated expression of a lpha IIb beta 3-integrin (glycoprotein IIb/IIIa complex), double immun e-fluorescence staining of stimulated cells confirmed the increased ce ll surface localization of uPA receptor. Semi-quantitative RT-PCR, lig and blot analysis and measurement of cell-bound proteolytic activity r evealed a differentiation-dependent upregulation of the uPA receptor e xpression in megakaryoblastic cell lines as in monocytic cells. Due to its glycolipid anchorage, incubation with phosphatidylinositol-specif ic phospholipase C reduced uPA receptor-mediated ligand binding by abo ut 60%. uPA receptor mRNA was expressed in cultured megakaryocytes der ived from bone marrow, whereas no uPA receptor mRNA was detectable in platelets. These results indicate a differentiation-dependent increase in the expression of uPA receptor in megakaryoblastic cells. The char acteristics of surface expression and functionality of the receptor on megakaryocytic cells may influence their maturation by regulating cel lular communication in the hone marrow micro-environment.