Product of the steel locus suppresses apoptosis in hemopoietic cells. Comparison with pathways activated by granulocyte macrophage colony-stimulating factor.

Steel factor (SF), also referred to as Kit ligand, stem cell factor, or mast cell growth factor, is essential for the development of hematopoietic stem cells in vivo. It is shown here that SF is mainly a survival factor for hemopoietic cells with little if any proliferative effect. In contrast, granulocyte macrophage colony-stimulating factor (GM-CSF) acts both as a survival factor and as a potent growth factor. We have probed the pathways activated by SF and GM-CSF in suppression of active cell death (apoptosis) using two classes of inhibitors: Tyrphostins that are specific inhibitors of protein tyrosine kinase, and amiloride derivatives (5-(N,N-ethyl-n-isopropyl)amiloride and 5-(N,N-hexamethylene)amiloride) that have been designed as specific inhibitors of the Na+/H+ antiporter. Both SF-dependent and GM-CSF-dependent pathways are sensitive to inhibition by Tyrphostins with, nonetheless, a quantitative difference. All Tyrphostins tested are more potent inhibitors of c-Kit than of GM-CSF receptor triggered pathways, the most striking being Tyrphostin B42 that is 10 times more potent. In contrast to the discrepancy in Tyrphostin dose-response curves, titration curves for 5-(N-ethyl-n-isopropyl)amiloride and 5-(N,N-hexamethylene)amiloride are comparable in SF- or GM-CSF-stimulated cells. Furthermore, SF induces a rapid and sustained alkalinization of the intracellular pH, as assessed with the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5-carboxyfluorescein. Taken together, our data indicate that input from two distinct pathways with discrepancy in immediate early events, that of c-Kit and GM-CSF receptor, results in a common output, activation of the Na+/H+ antiporter and suppression of apoptosis by the two ligands.