Granulocyte-macrophage colony-stimulating factor provokes RAS activation and transcription of c-fos through different modes of signaling

Granulocyte-macrophage colony-stimulating factor (GM-CSF) provokes a proliferative response and induction of early-response genes such as c-fos in target cells, It also induces rapid tyrosine phosphorylation of cellular proteins, including the beta subunit (beta c) of its functional receptor. However, locations and functions of phosphorylated tyrosine residues within the beta c are unclear, To elucidate the mechanism of the human GMCSF receptor signal transduction, mutational analyses were made of the cytoplasmic domain of the beta c, using murine BA/F3 cells. Deletion of the conserved box 1 motif resulted in loss of tyrosine phosphorylation of the beta c, thereby indicating an essential role for this motif in activating the tyrosine kinase which phosphorylates beta c. A C-terminal truncated mutant at position 589 activated the c-fos promoter, and this activation was diminished by a substitution at tyrosine 577 (Tyr(577)). However, the same substitution in the full-length beta c did not completely abrogate the c-fos promoter activation, hence, redundant signaling pathways probably exist, When we analyzed signaling molecules functioning downstream of the beta c we found that Tyr(577) is essential for Shc phosphorylation, while tyrosine phosphorylation of PTP1D was mediated through Tyr(577) as well as through other site(s). We suggest that GM-CSF stimulates at least two modes of signals leading to has activation, an event which ultimately gives rise to promoter activation of c-fos.