Signaling Functions of the Tyrosine Residues in the βc Chain of the Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Abstract

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) is a heterodimeric receptor expressed by myeloid lineage cells. Binding of GM-CSF activates at least one receptor-associated tyrosine kinase, JAK2, and rapidly induces tyrosine phosphorylation of the GMR βc-chain (GMRβ), but not the GMR α-chain (GMRα). To examine the role of GMRβ tyrosine phosphorylaiton, each of the 8 tyrosine residues in the cytoplasmic domain of the human GMRβ was mutated to phenylalanine (GMRβ-F8), and this mutant receptor was expressed with wild-type GMRα in the interleukin-3–dependent murine hematopoietic cell line, Ba/F3. GM-CSF induced tyrosine phosphorylation of multiple cellular proteins in cells expressing GMRβ-F8 , including JAK2 and STAT5. However, GM-CSF–induced tyrosine phosphorylation of both SHP2 and SHC was reduced or absent compared with wild-type. Next, a series of 8 receptors were generated, each containing only a single, restored, tyrosine residue. Tyrosine 577 was found to be sufficient to regenerate GM-CSF–dependent phosphorylation of SHC, and any of Y577, Y612, or Y695 was sufficient to regenerate GM-CSF–inducible phosphorylation of SHP2. Despite the signaling defect to SHC and SHP2, Ba/F3 cells expressing GMRβ-F8 were still able to proliferate in response to 10 ng/mL of human GM-CSF, although mitogenesis was impaired compared with wild-type GMRβ, and this effect was even more prominent at lower concentrations of GM-CSF (1 ng/mL). Overall, these results indicate that GMRβ tyrosine residues are not necessary for activation of the JAK/STAT pathway or for proliferation, viability, or adhesion signaling in Ba/F3 cells, although tyrosine residues significantly affect the magnitude of the response. However, specific tyrosine residues are needed for activation of SHC and SHP2.