Experimental regulation of STAT gene expression reveals an involvement of STAT5 in interleukin-4-driven cell proliferation.

The precise roles of signal transducers and activators of transcription (STATs) in cytokine-triggered control of cell physiology are not sufficiently well understood. We have established cell lines in which the individual functional contributions of STAT6 and STAT5a/b to interleukin-(IL-) 3 and -4-dependent processes can be readily studied. Mutants of STAT6, STAT5a and 5b lacking the transcriptional transactivation domain were fused to the green fluorescent protein (GFP) and expressed in the murine pro-B cell line Ba/F3 in a regulatable fashion. The expression of these truncated STAT variants could be tightly controlled over a wide range by doxycycline in the medium. They specifically bound to cognate DNA elements upon cytokine stimulation and acted dominant-negatively on the transcription of respective reporter genes in response to IL-3 and -4. The system was applied to the question of STAT contributions to cytokine-dependent cell proliferation. Expression of dominant-negative STAT6 had no significant effect on cell growth in response to both IL-3 and IL-4. In contrast, truncated STAT5 interfered with cell proliferation in response to IL-3, and, interestingly, also to IL-4. The results support our earlier findings on a role of STAT5 in IL-4-induced intracellular signaling and indicate that STAT5b in particular is involved in IL-4 receptor-triggered control of cell proliferation.