TRANSCRIPTIONAL REGULATION OF MOUSE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR IL-3 LOCUS/

Granulocyte-macrophage (GM)-CSF and IL-3 are hemopoietic growth factor s whose genes are closely linked in both humans and mice, In humans, t he GM-CSF and IL-3 genes are regulated by a cyclosporin A-inhibitable enhancer located 3 kb upstream of the GM-CSF gene that is inducible by signals that mimic TCR activation, To search for a murine homologue o f this enhancer we probed mouse genomic DNA and located a 400-bp eleme nt 2 kb upstream of the mouse GM-CSF gene that was 76% homologous with the human GM-CSF enhancer, Like the human GM-CSF enhancer, this eleme nt formed a cyclosporin A-inhibitable DNase I-hypersensitive site in t he murine T cell line EL4 upon activation with phorbol ester and calci um ionophore. Transient transfection assays showed that this homologue of the human enhancer acted as an inducible enhancer of the thymidine kinase promoter, the mouse IL-3 promoter, and the human GM-CSF promot er. We observed, however, that the mouse GM-CSF promoter was significa ntly more active than the human GM-CSF promoter and found that it supp orted a level of activity equivalent to the combination of the human G M-CSF promoter and the human GM-CSF enhancer, Consequently, the activi ty of mouse GM-CSF promoter was not significantly elevated in the pres ence of the mouse GM-CSF enhancer. Because the mouse GM-CSF enhancer i s considerably less active than its human homologue we suggest that th e mouse GM-CSF gene has evolved with less dependence upon the upstream enhancer for its activation.