Signal transducers and activators of transcription (Stats) are activated by tyrosine phosphorylation in response to cytokines, and are thought to mediate many of their functional responses. Stat6 is activated in response to interleukin (IL)-4 and may contribute to various functions including mitogenesis, T-helper cell differentiation and immunoglobulin isotype switching. To evaluate the role of Stat6, we generated Stat6-null mice (Stat6 -/-) by gene disruption in embryonic stem cells. The mice were viable, indicating the lack of a non-redundant function in normal development. Although naive lymphoid cell development was normal, Stat6 -/- mice were deficient in IL-4-mediated functions including Th2 helper T-cell differentiation, expression of cell surface markers, and immunoglobulin class switching to IgE. In contrast, IL-4-mediated proliferation was only partly affected.

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Signal transducers and activators of transcription (STATs) play key roles in growth factor-mediated intracellular signal transduction. In the present study using a constitutively active STAT5 mutant, we show that STAT5 has pleiotropic functions regulating cell proliferation, differentiation and apoptosis in an IL-3-dependent Ba/F3 cell line. The mutant STAT5 possessed constitutive tyrosine phosphorylation and DNA binding activity, induced expression of bcl-xL and pim-1 in the absence of IL-3 in Ba/F3 cells, and rendered Ba/F3 cells factor-independent. Unexpectedly, IL-3 treatment of the factor-independent Ba/F3 cells expressing the constitutively active STAT5 resulted in apoptosis within 24 h, or differentiation followed by cell death. In these cells, mRNA expression of growth inhibitory genes downstream of STAT5 such as CIS, JAB/SOCS-1/ SSI-1, and p21 WAF1/Cip1 was highly induced, correlating with prolonged hyper-phosphorylation of the mutant STAT5 after IL-3 stimulation. Of the STAT5-regulated genes, we found that constitutive expression of JAB/ SOCS-1/SSI-1 was sufficient to induce apoptosis of Ba/ F3 cells, while p21 WAF1/Cip1 could induce differentiation of these cells. In contrast, constitutive expression of pim-1 was sufficient to induce IL-3-independent growth of Ba/F3 cells. These findings suggest that a single transcription factor regulates cell fate by varying the intensity and duration of the expression of a set of target genes.

STAT (signal transducers and activators of transcription) proteins are transcription factors which are activated by phosphorylation on tyrosine residues upon stimulation by cytokines. Seven members of the STAT family are known, including the closely related STAT5A and STAT5B, which are activated by various cytokines. Except for prolactin-dependent ␤-casein production in mammary gland cells, the biological consequences of STAT5 activation in various systems are not clear. We applied PCR-driven random mutagenesis and a retrovirus-mediated expression screening system to identify constitutively active forms of STAT5. By this strategy, we have identified a constitutively active STAT5 mutant which has two amino acid substitutions; one is located upstream of the putative DNA binding domain (H299R), and the other is located in the transactivation domain (S711F). The mutant STAT5 was constitutively phosphorylated on tyrosine residues, localized in the nucleus, and was transcriptionally active. Expression of the mutant STAT5 partially dispenses with interleukin 3 (IL-3) as a growth stimulant of IL-3-dependent cell lines. Further analyses of the mutant STAT5 have demonstrated that both of the mutations are required for nuclear localization, efficient transcriptional activation, and induction of IL-3-independent growth of an IL-3-dependent cell line, Ba/F3, and have indicated that a molecular basis for the constitutive activation is the stability of the phosphorylated form of the mutant STAT5.Stimulation of cytokine receptors leads to activation of multiple signal transduction pathways, including the Ras-Raf-MEK-mitogen-activated protein kinase (MAPK) and the JAK-STAT pathways (14,28,34,42,44). The latter signaling pathway was originally found downstream of the interferon receptors and is now recognized as a common pathway downstream of most cytokine receptors. Upon stimulation with cytokines, receptor-associated JAKs are activated and phosphorylate STAT factors on tyrosine residues. The phosphorylated STAT molecules then form homo-or heterodimers through SH2-mediated interactions and translocate into nuclei to activate transcription of various target genes. Seven members of the STAT family (STAT1 through 4, -5A, -5B, and -6) are known; STAT5A and STAT5B are closely related. With the exception of STAT4 and STAT6, which were shown to be specifically activated by only one or two cytokines, interleukin 12 (IL-12) or both IL-4 and IL-13, respectively (13, 15), most of the other STATs are activated by multiple cytokines. In particular, both STAT5A and STAT5B are activated by numerous cytokines, including prolactin, IL-2, IL-3, IL-5, IL-7, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, M-CSF, erythropoietin (Epo), thrombopoietin, and growth hormone (GH).Using the receptor for the human GM-CSF as a model system, members of our group previously showed that activation of the Ras-Raf-MEK-MAPK pathway inhibits apoptosis while the region of the GM-CSF receptor, which is responsible for activation of JAK2 and STAT5 an...