Abstract
Trafficking of hematopoietic stem and progenitor cells (HPC) is controlled by G protein coupled receptors (GPRs), particularly by the chemokine receptor CXCR4. However, homing of HPC does not exclusively depend on CXCR4 and its ligand SDF-1. In addition to chemokine receptors, also GPRs for non-peptide lipid mediators may be involved in HPC migration. In the present study, we demonstrate that sphingosine 1-phosphate (S1P) is chemotactic for human CD34+ HPC (optimum dose 5 μM), as measured in a modified Boyden chamber system. In CD34+ cell lines, the chemotactic response was even stronger and occurred at lower S1P concentrations (e.g., optimum dose 100 nM for Jurkat cells, 2-fold stronger migration compared to HPC). By RT-PCR, we measured mRNA expression of the 5 different S1P receptors (S1P1,2,3,4,5). All CD34+ hematopoietic cell lines analyzed (KG1, KG1a, Jurkat) expressed S1P1, a S1P receptor with known chemotactic activity, while CD34- cell lines (HL-60, THP-1) were negative for S1P1. In HL-60 cells, chemotaxis in response to S1P was even reduced compared to spontaneous migration, which might be due to the expression of S1P receptors other than S1P1 with known inhibitory effects, particularly S1P2. In mobilized peripheral blood CD34+ progenitor cells from different donors, S1P1 was consistently expressed in both CD34+CD38+ and more primitive CD34+CD38- HPC, while expression of S1P2,3,4,5 was variable. S1P induced also other typical responses of GPR-mediated signaling in CD34+ cell lines and HPC, such as polymerization of filamentous actin, as measured by flow cytometry after labeling of the cells with FITC-phalloidin. We conclude that S1P is a chemotactic factor for CD34+ HPC and CD34+ cell lines due to expression of the GPR S1P1. As megakaryocytes and platelets represent an abundant source of S1P in the bone marrow, our results suggest that S1P and its receptor S1P1 may contribute to trafficking and spatial distribution of HPC in the hematopoietic microenvironment.
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