| Co-Investigator(Kenkyū-buntansha) |
ASANO Shigetaka Institute of Medical Science, The University of Tokyo, Professor, 医科学研究所, 教授 (50134614)
IWAKURA Yoichiro Institute of Medical Science, The University of Tokyo, Professor, 医科学研究所, 教授 (10089120)
NOSAKA Tetsuya Institute of Medical Science, The University of Tokyo, Visiting Associate Professor, 医科学研究所, 客員助教授 (30218309)
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| Research Abstract |
Cytokines have two major characteristics; functional redundancy of multiple cytokines and pleiotropic functions of a single cytokine. We previously presented the first evidence that a shared subunit of cytokine receptors is responsible for the functional redundancy of cytokines. On the other hand, the molecular basis of pleiotropic functions of cytokines has not been elucidated. Among signaling pathways stimulated by cytokine receptors, recently identified JAK-STAT pathway is now extensively studied. There are four JAK kinases and seven STAT transcription factors. Each STAT protein activates many target genes, which may explain. pleiotropic functions of cytokines. However, it has been difficult to identify the direct biological outcome of STAT activation because cytokine stimulation leads to simultaneous activation of many signaling pathways including the JAK-STAT pathway.
We were interested in STAT5 molecules because STAT5 is activated by a variety of cytokines with pleiotropic functio
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ns. To study direct biological outcomes of STAT5 activation, we attempted to identify a constitutively active STAT5 using PCR-driven random mutagenesis followed by retrovirus-mediated expression screening, and identified constitutively active forms of STAT5. 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 hours, or differentiation followed by cell death. In these cells, mRNA expression of negative regulators downstream of STAT5 including CIS. JAB/SOCS-1/SSI-1, and p21ィイD1WAF1/Cip1ィエD1 were highly induced, and the expression of these negative regulators persisted for a and long period. Of the STAT5-regulated genes, constitutive expression of JAB/SOCS-1/SSI-1 was sufficient to induce apoptosis of Ba/F3 cells, while p21ィイD1WAF1/Cip1ィエD1 induced macrophage differentiation of these cells. Constitutive expression of pim-1 was sufficient to induce IL-3-independent growth of Ba/F3 cells. In addition, the constitutively active STAT5 induced macrophage differentiation of M1 leukemic cells, where STAT5-driven IL-6 production induced M1 differentiation through a autocrine fashion. 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. Less
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