Soluble forms of UPAR regulate adhesion, proliferation and survival of KG1 leukemic cells

The receptor (uPAR) of the urokinase-type plasminogen activator (uPA) serine-protease is crucial in cell migration processes, since it concentrates uPA proteolytic activity at the cell surface, thus allowing localized ECM degradation. uPAR also binds vitronectin (VN) and interacts with integrins, regulating their activity. uPAR is formed by three domains (DI, DII, and DIII) and is anchored to the cell surface through a glycosyl phosphatidylinositol (GPI) tail. The N-terminal DI can be removed by proteolytic cleavage, generating a shorter uPAR form (DIIDIII-uPAR). Both full-length and cleaved uPAR can be released by the cell surface as soluble forms (suPAR and DIIDIII-suPAR, respectively), which have been detected in human plasma and urine. Full-length suPAR is still able to interact with integrins, unlike DIIDIII-suPAR, which, however, stimulates cell migration by activating the receptors (fMLF-R) for fMLF. The levels of soluble uPAR forms increase in acute myeloid leukemia (AML) and decrease rapidly during chemotherapy. Thus, we investigated whether full-length and cleaved suPAR are able to affect adhesion, proliferation or survival of the AML KG1 cell line. in vitro adhesion assays showed that KG1 cells adhered efficiently to plastic-bound fibronectin (FN), which is largely present in bone marrow stroma, whereas they did not bind to plastic-bound vitronectin (VN). Cell treatment with suPAR induced a significant increase in KG1 cell adhesion to FN, whereas cell treatment with DIIDIII-suPAR did not exert any effect. Viceversa, proliferation in vitro assays showed that DIIDIII-suPAR significantly increased proliferation of KG1 cells, with an effect peaking at 48h, whereas suPAR did not show any influence. Interestingly, both full length and cleaved forms of soluble uPAR protected KG1 cells from apoptosis induced by serum starvation. Altogether, these results suggest that the soluble uPAR forms identified in sera from AML patients could play a role in the adhesion, proliferation and survival of leukemic cells, probably through their capability to interact with integrins and fMLF-Rs.