Role of the Urokinase Receptor (uPAR) in the Cross-Talk of Hematopoietic Stem Cells with the Bone Marrow Microenvironment

The urokinase-type plasminogen activator receptor (uPAR) is a cell-surface receptor involved in cell adhesion and migration. uPAR binds urokinase (uPA) and vitronectin (VN) and interacts with integrins and chemotaxis receptors. Soluble forms of uPAR (suPAR) have been detected in human plasma and urine. A cleaved form of suPAR (c-suPAR), lacking the N-terminal domain and exposing the sequence SRSRY (aa 88–92), stimulates cell migration by activating fMLP receptors. We recently demonstrated uPAR involvement in G-CSF-induced CD34⁺ hematopoietic stem cell (HSC) mobilization. We also demonstrated that c-suPAR could induce mobilization of hematopoietic stem/progenitor cells in mice. Since HSC mobilization and homing to bone marrow (BM) are mirror image processes which utilize the same mediators and similar signaling pathways, we investigated whether uPAR and its ligands could play a role in regulating CD34⁺ HSC interactions with the BM stroma, thus also contributing to HSC homing and engraftment to the BM. We found expression of uPA and VN in cultures of human BM stroma cells. Interestingly, stroma cells also produced suPAR and high amounts of c-suPAR, exposing the chemotactic SRSRY sequence. The role of the different soluble forms of uPAR produced by stroma cells in regulating HSC interactions with the BM microenvironment was analyzed by long term cultures (LTC) of BM and G-CSF mobilized CD34⁺ HSCs, in the presence of suPAR or the uPAR-derived uPAR_84–95 peptide, corresponding to the active site of c-suPAR. Both suPAR and the uPAR_84–95 peptide increased the number of adherent and released clonogenic progenitors from LTC of BM and G-CSF mobilized HSCs. To elucidate the mechanism of suPAR and c-suPAR effects on CD34⁺ HSC interactions with the stromal microenvironment, in vitro adhesion and proliferation assays were performed on CD34⁺ KG1 cells. suPAR treatment determined a significant increase in CD34⁺ KG1 cell adhesion whereas c-suPAR increased cell proliferation. Taken together, our results indicate that BM stroma produces soluble forms of uPAR that regulate CD34⁺ HSC interactions with BM microenvironment, their local proliferation and trafficking from and to BM.