FMS-LIKE TYROSINE KINASE-3 CATALYTIC DOMAIN CAN TRANSDUCE A PROLIFERATIVE SIGNAL IN FDC-P1 CELLS THAT IS QUALITATIVELY SIMILAR TO THE SIGNAL DELIVERED BY C-FMS

full length clone of murine fms-like tyrosine kinase 3 [flt3, also kno wn as fetal liver kinase 2 (flk2)] was constructed from sequences obta ined from a brain complementary DNA (cDNA) library and from cDNA prepa red from the cell line Tikaut. In the absence of a ligand to study the function of Flt3, a chimeric molecule was constructed comprising the extracellular domain of murine c-Fms and the transmembrane and cytopla smic domains of Flt3. A plasmid encoding the chimeric receptor was cot ransfected along with a plasmid conferring neomycin resistance into FD C-P1 cells that do not normally express c-fms or flt3 and require gran ulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin 3 for growth. Two types of clones were obtained following selection in CM-CSF and G418. Two of seven clones had the capacity for M-CSF-depend ent colony formation in semisolid medium, indicating that the cytoplas mic domain of Flt3 can transduce a proliferative signal. From the rema ining clones, M-CSF-dependent clonogenic cells could be selected by pr ior bulk liquid culture in M-CSF. It has been shown previously that th e GM-CSF-dependent proliferative capacity is strongly inhibited by M-C SF in FDC-P1 cells engineered to express full length c-fms. This pheno menon was also observed with FD/fms-flt3 cells that were clonogenic in M-CSF. Stimulation of FD/fms or FD/fms-flt3 cells in liquid culture b y M-CSF caused differentiation of a small proportion of cells along th e myelomonocytic pathway which was enhanced by the combination of M-CS F and GM-CSF. The similarity of the response of cells bearing either c -Fms or the Fms/Flt3 chimeric receptor to stimulation by M-CSF suggest s that Flt3 and c-Fms function through similar signaling pathways.