在外部細(xì)胞因子及內(nèi)部的酪氨酸激酶刺激的信號網(wǎng)絡(luò)下游,，信號傳導(dǎo)及轉(zhuǎn)錄激活因子5b（Stat5b）是一個(gè)關(guān)鍵性的位點(diǎn),。Stat5b的最大轉(zhuǎn)錄活化需要Ser和Tyr的磷酸化,。雖然Tyr磷酸化調(diào)節(jié)機(jī)制以及Stat5b的激活機(jī)制已經(jīng)被廣泛的研究,，但是Ser磷酸化的作用機(jī)制還需要被完全闡明,。近日,，美國德克薩斯大學(xué)阿爾帕索分校的Robert A. Kirken等人研究發(fā)現(xiàn)了一個(gè)新的可以調(diào)節(jié)Stat5與DNA結(jié)合以及轉(zhuǎn)錄活性的磷酸化位點(diǎn),。相關(guān)研究發(fā)表在3月22日的美國《生化周刊》（Journal of Biological Chemistry）上,。
STAT（Signal transducers and activators of transcription）即信號傳導(dǎo)及轉(zhuǎn)錄激活因子,含有SH2和SH3結(jié)構(gòu)域,可與特定的含磷酸化酪氨酸的肽段結(jié)合。當(dāng)STAT被磷酸化后,發(fā)生聚合成為同源或異源二聚體形式的活化的轉(zhuǎn)錄激活因子,進(jìn)入胞核內(nèi)與靶基因啟動(dòng)子序列的特定位點(diǎn)結(jié)合,促進(jìn)其轉(zhuǎn)錄,。
使用質(zhì)譜和磷酸化特異抗體研究,，研究人員發(fā)現(xiàn)Ser193是人類Stat5b中一個(gè)新發(fā)現(xiàn)的由細(xì)胞因子誘導(dǎo)的磷酸化位點(diǎn)。在應(yīng)答于幾種常見細(xì)胞因子包括白細(xì)胞介素2,，7,，9和15時(shí)，Stat5b pS193被發(fā)現(xiàn)于激活的PBMCs或者是淋巴細(xì)胞系,。運(yùn)動(dòng)及空間分析表明：Stat5b S193的磷酸化是迅速并且短暫的,，而且在Stat5b轉(zhuǎn)位到核內(nèi)之前就已經(jīng)在細(xì)胞的胞質(zhì)室內(nèi)發(fā)生了。此外,，可誘導(dǎo)的Stat5b S193磷酸化對抑制劑哺乳動(dòng)物雷帕霉素靶蛋白(mTOR)敏感,，而抑制蛋白磷酸酶2A(PP2A）后會(huì)引起S193的磷酸化。在HEK293細(xì)胞的重構(gòu)分析、定點(diǎn)突變和EMSA研究表明：Stat5b的最高轉(zhuǎn)錄活化需要S193的磷酸化,。事實(shí)上,，在幾種淋巴瘤細(xì)胞系、原發(fā)性白血病和淋巴瘤病人腫瘤細(xì)胞中都發(fā)現(xiàn)了Stat5b S193的組成性磷酸化,。
總的來說,，IL-2家族通過雷帕霉素敏感機(jī)理緊密控制Stat5b S193的磷酸化。而且,，S193組成性磷酸化與Stat5b原癌活性有關(guān),，因此Stat5b S193很可能會(huì)成為造血系統(tǒng)惡性腫瘤的新型治療靶點(diǎn)。（生物谷Deepblue編譯）
doi: 10.1074/jbc.M111.319756
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PMID:
Signal transducer and activator of transcription 5b (Stat5b) serine 193 is a novel cytokine induced phospho-regulatory site that is constitutively activated in primary hematopoietic malignancies
Abhisek Mitra1, Jeremy A. Ross, Georgialina Rodriguez, Zsuzsanna S. Nagy, Harry L. Wilson and Robert A. Kirken.
Signal transducer and activator of transcription 5b (Stat5b) is a critical node in the signaling network downstream of external (cytokines or growth factors) or internal (oncogenic tyrosine kinases) stimuli. Maximum transcriptional activation of Stat5b requires both tyrosine and serine phosphorylation. Although the mechanisms governing tyrosine phosphorylation and activation of Stat5b have been extensively studied, the role of serine phosphorylation remains to be fully elucidated.Using mass spectrometry and phospho-specific antibodies, we identified S193 as a novel site of cytokine induced phosphorylation within human Stat5b. Stat5b pS193 was detected in activated primary human PBMCs or lymphoid cell lines in response to several gamma common cytokines, including interleukin (IL)-2, -7, -9, and -15. Kinetic and spatial analysis indicated that Stat5b S193 phosphorylation was rapid, transient and occurred in the cytoplasmic compartment of the cell prior to Stat5b translocation to the nucleus. Moreover, inducible Stat5b S193 phosphorylation was sensitive to inhibitors of mammalian target of rapamycin (mTOR), whereas inhibition of protein phosphatase 2A (PP2A) induced phosphorylation of S193. Reconstitution assays in HEK293 cells in conjunction with site-directed mutagenesis, EMSA and reporter assays, indicated that pS193 is required for maximal Stat5b transcriptional activity. Indeed, Stat5b S193 was found constitutively phosphorylated in several lymphoid tumor cell lines as well as primary leukemia and lymphoma patient tumor cells.Taken together, IL-2 family cytokines tightly control Stat5b S193 phosphorylation through a rapamycin sensitive mechanism. Furthermore, constitutive S193 phosphorylation is associated with Stat5b proto-oncogenic activity and therefore may serve as a novel therapeutic target for treating hematopoietic malignancies.
