ROLE OF KIT-LIGAND IN PROLIFERATION AND SUPPRESSION OF APOPTOSIS IN MAST-CELLS - BASIS FOR RADIOSENSITIVITY OF WHITE SPOTTING AND STEEL MUTANT MICE

The receptor tyrosine kinase Kit and its cognate ligand KL/steel facto r are encoded at the white spotting (W) and Steel (Sl) loci of the mou se, respectively. Mutations at both the W and the Sl loci affect hemat opoiesis including the stem cell hierarchy, erythropoiesis, and mast c ells, as well as gametogenesis and melanogenesis. In addition, mutant mice display an increased sensitivity to lethal doses of irradiation. The role of KL/c-Kit in cell proliferation and survival under conditio ns of growth factor-deprivation and gamma-irradiation was studied by u sing bone marrow-derived mast cells (BMMC) as a model. Whereas apoptos is induced by growth factor deprivation in BMMC is a stochastic proces s and follows zero order kinetics, gamma-irradiation-induced apoptosis is an inductive process and follows higher order kinetics. In agreeme nt with these results, gamma-irradiation-induced apoptosis in BMMC was shown to be dependent on p53 whereas apoptosis induced by deprivation is partly dependent on p53, implying that there are other mechanisms mediating apoptosis in KL-deprived BMMC. In the presence and in the ab sence of serum, KL stimulated proliferation by promoting cell cycle pr ogression. The presence of KL was required only during the early part of the G(1) phase for entry into the S phase. At concentrations lower than those required for proliferation, KL suppressed apoptosis induced by both growth factor-deprivation and gamma-irradiation, and internuc leosomal DNA fragmentation characteristic of apoptosis. The ability of KL to suppress apoptosis was independent of the phase of the cell cyc le in which the cells were irradiated and suppression of apoptosis was a prerequisite for subsequent cell cycle progression. Moreover, addit ion of KL to gamma-irradiated and growth factor-deprived cells could b e delayed for up to 1 h after irradiation or removal of growth factors when cells became irreversibly committed to apoptosis. KL and IL-3 in duce suppression of apoptosis in mast cells by different mechanisms ba sed on the observations of induction of bcl-2 gene expression by IL-3 but not by KL. It is proposed that the increased sensitivity of W and Sl mutant mice to lethal irradiation results from paucity of the apopt osis suppressing and proliferative effects of KL.