Roles of JAK kinase in human GM-CSF receptor signals.

The IL-3 and GM-CSF (hGMR) receptors consist of two subunits, alpha and beta, both of which are members of the cytokine receptor superfamily. Phosphorylation of tyrosine residues of hGMR beta subunit and several cellular proteins are observed with hGM-CSF stimulation. We analyzed role of tyrosine residue of hGMR beta subunit and nature of tyrosine kinase, JAK2 in hGMR signals using several hGMR beta subunit mutants. In addition to box1 region, a membrane distal region (a.a. 544-589) of hGMR beta is required for c-fos activation. Only one tyrosine residue (Tyr577) exists within the region 544-589, and substitution of Tyr577 to phenylalanine in GMR beta 589 resulted in the loss of c-fos activation. In contrast, the same substitution in a wild type receptor did not affect GM-CSF-induced activities such as c-fos mRNA induction and proliferation but abolished Shc phosphorylation. These results suggest that the activation of Shc is not essential for c-fos activation and several tyrosine residues co-ordinate to activate c-fos activation. It is well documented that IL-3 or GM-CSF activates JAK2 in BA/F3 cells. However the role of JAK2 in IL-3/GM-CSF functions is largely unknown. We examined the role of JAK2 in GM-CSF-induced signaling pathways. Dominant negative JAK2 (delta JAK2) lacking the C-terminus kinase domain, suppressed IL-3/GM-CSF induced c-fos activation, c-myc activation and proliferation suggesting that JAK2 is involved in both signaling pathways. PTP1D and Shc are phosphorylated by IL-3/GM-CSF in BA/F3 cells, however these phosphorylation events were inhibited by expression of delta JAK2. Taken together, these results indicate that JAK2 is a primary kinase regulating all the known activities of GM-CSF. JAK2 mediates GM-CSF induced c-fos activation through receptor phosphorylation and Shc/PTP1D activation.