Activation of STAT5 by IL-4 relies on Janus kinase function but not on receptor tyrosine phosphorylation, and can contribute to both cell proliferation and gene regulation. 
We have investigated mechanisms and consequences of STAT5 activation through the human IL-4 receptor (IL-4R). By functionally expressing receptor mutants in the murine pro-B cell line Ba/F3, we could show that phosphorylated tyrosine residues within the IL-4R alpha chain are dispensable for IL-4-induced STAT5 activity. However, disruption of a membrane-proximal proline-rich sequence motif ('box1') in either subunit of the bipartite IL-4R abolished not only ligand-induced tyrosine phosphorylation of Janus kinases JAK1 and JAK3, but also IL-4-triggered activation of STAT5 and concomitant cell proliferation. A dominant-negative version of STAT5b, but not of STAT5a, interfered with IL-4-induced DNA synthesis in Ba/F3 cells, suggesting an involvement of STAT5b in the control of cell proliferation through IL-4R. Reporter gene experiments finally showed that transcription from promoters of STAT5 target genes can be specifically induced by challenging cells with IL-4, and that both STAT5a and STAT5b can contribute to IL-4-triggered transcriptional control.