STROMAL CELL-DERIVED FACTOR-1-ALPHA AND STEM-CELL FACTOR KIT-LIGAND SHARE SIGNALING PATHWAYS IN HEMATOPOIETIC PROGENITORS - A POTENTIAL MECHANISM FOR COOPERATIVE INDUCTION OF CHEMOTAXIS

Stromal cell-derived factor (SDF-1 alpha), the ligand for CXCR4, is a chemokine that acts as a potent chemoattractant for hemopoietic progen itor cells. Stem cell factor/kit ligand (SCF/KL), an early acting cyto kine, has recently been reported to enhance the chemotaxis induced by SDF-1 alpha. However, very little is known about downstream signaling events following these receptor-ligand interactions. To investigate th ese events, we utilized a model progenitor cell line, CTS, which expre sses both the CXCR4 and c-kit receptors. We observed strong Ca2+ mobil ization and enhancement of chemotaxis following treatment with SDF-1 a lpha or SCF/KL. A combination of these factors enhanced this chemotaxi s in CTS cells as well as in CD34(+) bone marrow cells, Prior treatmen t of CTS cells with pertussis toxin inhibited the SDF-1 alpha-induced chemotaxis, suggesting that SDF-1 alpha signaling involves a pertussis -sensitive G(1)-coupled protein, SDF-1 alpha treatment resulted in a r apid phosphorylation of the focal adhesion molecules RAFTK (related ad hesion focal tyrosine kinase), paxillin, and p130(cas), which then dec lined within minutes. SCF/KL alone or in combination with SDF-1 alpha induced a rapid and sustained effect on phosphorylation of these subst rates. SDF-1 alpha treatment resulted in a rapid and robust activation of p44/42 mitogen-activated protein kinase compared with the relative ly weak and delayed effect of SCF/KL treatment. Interestingly, a delay ed but sustained activation of mitogen-activated protein kinase activa tion was observed when the factors were used in combination. Such coop erativity in downstream signaling pathways may explain the enhanced ch emotaxis of progenitors observed with SDF-1 alpha in combination with SCF/KL.