Activated JAK2 binds to unphosphorylated STAT5; cytokine treatment of cells leads to JAK2 activation and promotes binding of JAK2 to unphosphorylated STAT5.
STAT5 proteins are considered the main targets of IL-3, IL-5 and GM-CSF signaling (Mui et al. 1995a, Mui et al. 1995b, Ihle, 2001), but other members of this family including STAT3 and STAT1 (Chin et al. 1996) can be involved, the STAT family member activated appears to depend on the cell line used in the study, rather than the cytokine (Reddy et al. 2000). IL-5 and GM-CSF increase STAT3 and 5 signaling (Caldenhoven et al. 1995, Stout et al. 2004).
Unphosphorylated STATs are cytoplasmic; tyrosine phosphorylation facilitates dimerization and translocation to the nucleus where they act as transcription factors. STATs were originally described as ligand-induced transcription factors in interferon-treated cells, subsequently they were shown to be critical in many signal transduction pathways associated with cytokines and neurokines including several interleukins, the interferons, erythropoietin, prolactin, growth hormone, oncostatin M (OSM), and ciliary neurotrophic factor (Darnell 1997, Reddy et al. 2000). JAK-STAT signaling is widely accepted as a primary signaling route for receptors that share the common beta subunit (Bc).
The role of the receptor itself in STAT5 binding is somewhat controversial because while STAT proteins can be recruited to tyrosine phosphorylated receptors via their SH2 domains (Greenlund et al. 1995, Li et al. 1997) binding of STAT5 to Bc has not been formally demonstrated (Guthridge et al. 1998), though tyrosine-phosphorylated peptides of Bc have been demonstrated to associate with STAT5, and anti-Bc or phosphotyrosine antibodies inhibited GM-CSF induced STAT5 DNA binding activity (Sakurai et al. 2000). Binding of JAK2 to STAT5 can occur in vitro when no receptor is present (Flores-Morales et al. 1998). STAT5 activation was seen when all six conserved cytoplasmic tyrosines in Bc were mutated to P (Okuda et al. 1997), but a C-terminal deletion mutant of Bc while able to activate JAK2 was unable to activate STAT5 (Smith et al. 1997). These observations suggest that JAK2 activation is a critical step in STAT signaling from Bc-containing receptors, but other factors may be required. It is not clear whether Bc is directly involved or not in STAT5 activation, but the specificity for particular STAT members is believed to be determined by STAT docking sites present on the receptor molecules, not JAK kinase preference (Reddy et al. 2000).