Fujitani Y, Hibi M, Fukada T, Takahashi-Tezuka M, Yoshida H, Yamaguchi T, Sugiyama K, Yamanaka Y, Nakajima K, Hirano T
Department of Molecular Oncology, Biomedical Research Center, Osaka University Medical School, Suita, Japan.
Oncogene. 1997 Feb 20;14(7):751-61. doi: 10.1038/sj.onc.1200907.
JAK is believed to be an essential tyrosine kinase that mediates signals from the cytokine receptor to its downstream events. JAK associates with the cytoplasmic domain of the type I cytokine receptor superfamily and upon the ligand stimulation it can be activated, resulting in the receptor phosphorylation. In signaling from gp130, a common signal transducer for the IL-6 family cytokines, STAT3, a transcription factor that contains an SH2 domain, is recruited by phosphotyrosines on gp130 and is subsequently phosphorylated by gp130-associated JAKs. In this study, we attempted to find a new target for JAK that is directly activated by JAK, independent of gp130 tyrosine phosphorylation, by using a yeast two-hybrid system. In the process we found that the JH2 domain of JAK1, JAK2 or JAK3 could specifically associate with the carboxy-terminal portion of STAT5, but not with STAT3 or STAT1. The interaction was confirmed using both a transient expression system in a cell line and a GST-fusion protein binding assay. Furthermore, we showed that the activation of STAT5 via gp130 did not need any phosphotyrosines on gp130 while that of STAT3 strictly depended on phosphotyrosines on gp130. Mutations of STAT5 that eliminated the interaction with JAK1 reduced the activation of STAT5 upon the gp130 stimulation, although such mutants could be still activated through erythropoietin receptor. These results indicate that STATs are activated through cytokine receptors by two distinct mechanisms, one dependent on receptor tyrosine phosphorylation and the other mediated by the JAK-STAT direct interaction.
JAK被认为是一种重要的酪氨酸激酶，可介导细胞因子受体信号传导至其下游事件。JAK与I型细胞因子受体超家族的胞质结构域相关联，在配体刺激后可被激活，导致受体磷酸化。在gp130（IL-6家族细胞因子的共同信号转导子）的信号传导中，STAT3（一种含有SH2结构域的转录因子）被gp130上的磷酸酪氨酸招募，随后被与gp130相关的JAK磷酸化。在本研究中，我们试图通过酵母双杂交系统找到一个由JAK直接激活的JAK新靶点，该靶点独立于gp130酪氨酸磷酸化。在此过程中，我们发现JAK1、JAK2或JAK3的JH2结构域可特异性地与STAT5的羧基末端部分结合，但不与STAT3或STAT1结合。使用细胞系中的瞬时表达系统和GST融合蛋白结合试验均证实了这种相互作用。此外，我们表明通过gp130激活STAT5不需要gp130上的任何磷酸酪氨酸，而STAT3的激活则严格依赖于gp130上的磷酸酪氨酸。消除与JAK1相互作用的STAT5突变体在gp130刺激后降低了STAT5的激活，尽管这些突变体仍可通过促红细胞生成素受体被激活。这些结果表明，STATs通过细胞因子受体由两种不同机制激活，一种依赖于受体酪氨酸磷酸化，另一种由JAK-STAT直接相互作用介导。
