Upar soluble forms and hematopoietic stem cell transplantion

The urokinase (uPA)-type plasminogen activator receptor (uPAR) is expressed on the surface of various cell types, both in full-length and cleaved forms, lacking the N-terminal DI domain. uPAR binds uPA and vitronectin (VN) and regulates integrin activity. uPAR can be shed from the cell surface, generating full-length and cleaved soluble forms (suPAR and DIIDIII-suPAR, respectively). suPAR is still able to bind uPA and VN, unlike DIIDIII-suPAR, which, however, if exposing the sequence SRSRY (aa 88-92) at its N-terminus, is able to bind the chemotactic receptors for fMLF. Soluble forms of uPAR have been detected in human fluids. We previously demonstrated the involvement of uPAR soluble forms in G-CSF-induced human CD34+ hematopoietic stem cell (HSC) mobilization. Further, we demonstrated that DIIDIII-suPAR can induce mobilization of hematopoietic stem/progenitor cells in mice. Since HSC mobilization and homing to bone marrow (BM) are specular processes which utilize same mediators and similar signaling pathways, we investigated whether the soluble forms of uPAR could be also involved in HSC homing and engraftment to the BM. Firstly, we examined suPAR and DIIDIII-suPAR expression in cultures of human BM stroma cells. Interestingly, stroma cells produced suPAR and high amounts of the chemotactic DIIDIII-suPAR. We then evaluated the levels of both suPAR forms in sera from four healthy donors and from five patients before and after the pretransplant conditioning with chemotherapy. We found a significant increase only in DIIDIII-suPAR levels in sera from patients before conditioning, as compared to healthy donors; however, the chemotherapy conditioning significantly decreased circulating DIIDIIIsuPAR levels. We also examined the potential effects of the different soluble forms of uPAR in long term cultures (LTC) of G-CSF-mobilized CD34+ HSCs, in the presence of suPAR or of the uPAR84-95 peptide, corresponding to the active site of DIIDIII-suPAR. Both suPAR and the uPAR84-95 peptide increased the number of adherent clonogenic progenitors in LTC of G-CSF mobilized HSCs. Altogether, our results suggest that BM stroma produces soluble forms of uPAR which could contribute to the engraftment of CD34+ HSC to BM. According with our previous observation on the mobilizing effects of DIIDIII-suPAR, the circulating cleaved suPAR seems to be lowered by pretransplant conditioning, probably to allow CD34+ HSC homing.