Silencing the constitutive active transcription factor CREB by the LKB1-SIK signaling cascade
Silencing the constitutive active transcription factor CREB by the LKB1-SIK signaling cascade
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DOI：
10.1111/j.1742-4658.2006.05291.x
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发表时间：
2006-06-01
期刊：
影响因子：
5.4
通讯作者：
Okamoto, Mitsuhiro
中科院分区：
文献类型：
--
作者：
Katoh, Yoshiko;Takemori, Hiroshi;Okamoto, Mitsuhiro
环腺苷酸反应元件（CRE）结合蛋白（CREB）是已知的激活转录时，其丝氨酸133磷酸化。两项独立的研究表明存在Ser 133非依赖性激活。一项研究确定了一种抑制CREB的激酶，即盐诱导激酶（SIK）;另一项研究分离出一种新型CREB特异性共激活因子，即调节CREB活性的转导子（TORC），可上调CREB活性。这两个相反的信号是由这样的事实，即，SIK磷酸化TORC，并诱导其核输出。由于LKB 1已被报道是一个上游激酶的IK，我们使用LKB 1缺陷的HeLa细胞，以进一步阐明TOC依赖的CREB激活。在LKB 1的情况下，SIK不能磷酸化TORC，这导致CRE活性的组成型激活。在HeLa细胞中过表达LKB 1以可调节的方式提高了CRE依赖的转录。在LKB 1阳性HEK 293细胞中，10 nM星形孢菌素对激酶级联的灭活也诱导了不受调节的组成性激活的CRE活性。用星形孢菌素处理完全抑制了SIK激酶活性，而对SIK中LKB 1-磷酸化位点的磷酸化水平或LKB 1的另一个靶点AMPK的活性没有任何显著影响。CREB在LKB 1缺陷细胞或星形孢菌素处理的细胞中的组成性激活不伴随CREB在Ser 133处的磷酸化。结果表明，LKB 1及其下游的SIK通过TORC的磷酸化在抑制CREB活性中发挥重要作用，这种沉默可能是CREB活性调节所不可或缺的。
Cyclic AMP responsive element (CRE)-binding protein (CREB) is known to activate transcription when its Ser133 is phosphorylated. Two independent investigations have suggested the presence of Ser133-independent activation. One study identified a kinase, salt-inducible kinase (SIK), which repressed CREB; the other isolated a novel CREB-specific coactivator, transducer of regulated CREB activity (TORC), which upregulated CREB activity. These two opposing signals are connected by the fact that SIK phosphorylates TORC and induces its nuclear export. Because LKB1 has been reported to be an upstream kinase of SIK, we used LKB1-defective HeLa cells to further elucidate TORC-dependent CREB activation. In the absence of LKB1, SIK was unable to phosphorylate TORC, which led to constitutive activation of CRE activity. Overexpression of LKB1 in HeLa cells improved the CRE-dependent transcription in a regulated manner. The inactivation of kinase cascades by 10 nM staurosporine in LKB1-positive HEK293 cells also induced unregulated, constitutively activated, CRE activity. Treatment with staurosporine completely inhibited SIK kinase activity without any significant effect on the phosphorylation level at the LKB1-phosphorylatable site in SIK or the activity of AMPK, another target of LKB1. Constitutive activation of CREB in LKB1-defective cells or in staurosporine-treated cells was not accompanied by CREB phosphorylation at Ser133. The results suggest that LKB1 and its downstream SIK play an important role in silencing CREB activity via the phosphorylation of TORC, and such silencing may be indispensable for the regulated activation of CREB.