Meis1 induces FLT3 transcription and permits responsiveness to FL. (A-B) Proliferation of 2 progenitor lines immortalized by Hoxa9 and 2 progenitor lines immortalized by coexpressed Hoxa9 and Meis1, cultured in the presence of various cytokines (GM-CSF, FL, IL-7, or SCF plus GCSF). Cells were enumerated 7 days after plating in the indicated cytokines (A), and stained for morphology by Wright-Giemsa (B). (C) Northern blotting demonstrates expression of FLT3 and IL-7-R specifically in progenitors immortalized by coexpressed Hoxa9 plus Meis1, and immunoblotting demonstrates specific production of FLT3 in Hoxa9-immortalized progenitors that coexpress Meis1. (D) Immunoblot demonstrating that FL-induced MAP kinase (MAPK) phosphorylation is restricted to progenitors coexpressing Hoxa9 plus Meis1, while SCF induces receptor phosphorylation and MAP kinase phosphorylation in progenitors immortalized by Hoxa9 alone, as well as those immortalized by Hoxa9 plus Meis1. (E) Confirmation of microarray analysis by semiquantitative PCR, using 1:10 serial dilutions of cDNA as template. (F) Expression of Meis1 in 3 different Hoxa9-immortalized progenitor lines induces expression of FLT3 (inset) and robust proliferation in response to recombinant FL as the sole cytokine.