Role of c-KIT expression level and phosphatidylinositol 3-kinase activation in survival and proliferative responses of early myeloid cells.

The receptor tyrosine kinase c-KIT and its ligand Stem Cell Factor (SCF) are critical in haemopoiesis but pathways linking receptor activation to specific responses in progenitor cells are still unclear. We have investigated the role of c-KIT expression level and the phosphatidylinositol 3-kinase (PI3-K) pathway in survival and cell division of early myeloid cells in response to SCF. Two factor-dependent murine early myeloid cell lines, FDC-P1 and Myb-immortalised haemopoietic cells (MIHC), were transduced to express wild-type c-KIT or a mutant form of the receptor (Y721F) that lacks the major recruitment site for the p85 regulatory subunit of PI3-K. Several clones expressing different receptor levels were analysed in each case. Growth of cells expressing either the wild-type or Y721F mutant KIT was strongly dependent on receptor level within the physiological range. Using an assay that allows quantitative measurement of the contributions of cell survival and cell division, diminished cell growth in response to SCF under limiting conditions of receptor copy number or PI3-K recruitment was shown to be almost entirely due to decreased cell survival. Further studies with the PI3-K inhibitor LY294002 indicated that PI3-K activation was also required for cell division. Alternate binding and/or indirect activation of PI3-K could support cell division mediated by Y721F mutant KIT, but was insufficient for the survival response.