| Abstract: | Granulocyte macrophage colony-stimulating factor (GM-CSF) binds to the high-affinity GM-CSF receptor (GMR) consisting of alpha and beta subunits and induces rapid tyrosine phosphorylation, activation of early response genes, and proliferation of hematopoietic cells. The alpha subunit is the primary cytokine binding component and the beta subunit is required for high-affinity binding as well as for signal transduction. Using tyrosine kinase inhibitors and cytoplasmic deletion mutants of the beta subunit, we obtained evidence that there are at least two distinct pathways downstream of the GMR in BA/F3 cell, one which is essential for proliferation, leads to the c-myc gene activation, and is sensitive to herbimycin and genistein. Activation of this pathway depends on the cytoplasmic region between amino acid positions 455 and 517 of the beta subunit. The second pathway, which leads to activation of c-fos and c-jun genes, is only partially sensitive to herbimycin, is resistant to genistein and depends on the region between amino acid positions 626 and 763 of the beta subunit. Unexpectedly, the c-fos mRNA induction was augmented by genistein. The enhanced expression of c-fos mRNA by genistein also occurred with stimulation with cAMP, PMA, or EGF in NIH3T3 cells. It thus seems likely that genistein affects a common pathway downstream of these signals. |
