Elevated soluble urokinase plasminogen activator receptor in plasma from patients with idiopathic myelofibrosis or polycythaemia vera

Extracellular proteolytic enzymes of the urokinase-type plasminogen activator (uPA) system and the family of metalloproteases play a crucial role in the matrix degradation and tissue remodelling processes characteristic of malignant disorders. The receptor for urokinase plasminogen activator (uPAR) serves to localise and intensify the action of uPA and is expressed on the surface of malignant as well as tumour stromal cells including fibroblasts. A soluble form of uPAR (suPAR) cleaved from its glycosyl-phosphatidylinositol anchor is detected in plasma from healthy individuals and increased levels of suPAR have been found in advanced malignancy, suggesting that suPAR may be a marker of extensive tissue remodelling. In an attempt to clarify whether suPAR might be a marker for bone marrow tissue remodelling we measured plasma suPAR levels in a patient cohort comprising 17 with myelofibrosis (MF), 17 with polycythaemia vera (PV), 15 with essential thrombocythaemia (ET), one with a transitional myeloproliferative disorder evolving from PV and 30 controls. Compared with controls suPAR levels were significantly higher in the patients (P < 0.0001) (median 3.35 vs. 2.32 μg L^-1). Moreover, in subgroup analyses including patients with MF, PV, and ET, respectively, suPAR levels differed significantly with the highest levels found in patients with MF and PV (MF vs. PV vs. ET; P = 0.0003). When comparing suPAR levels of the individual patient subgroups with controls, only suPAR levels of PV and MF patients were significantly increased (P < 0.0001). Sixty-five percent of patients with PV and MF (22/34) had suPAR plasma values that were above the mean +2 standard deviations (SD) of controls. The concentration of suPAR was significantly correlated to plasma lactate dehydrogenase, thrombomodulin, and complex of tPA:PAI-1 in the patients. There was no difference between patients and controls when comparing plasma uPA levels. Increased plasma suPAR levels in patients with chronic myeloproliferative disorders may reflect increased uPAR production in the bone marrow, leading to enhanced bone marrow remodelling.