MDMX inhibits Casein Kinase 1? activity and stimulates Wnt signaling

Casein kinase 1 alpha (CK1?) is a serine/threonine kinase with numerous functions, including regulating the Wnt/?-catenin and p53 pathways. CK1? has a well-established role in inhibiting the p53 tumor suppressor by binding to MDMX and stimulating MDMX-p53 interaction. MDMX purified from cells contains near-stoichiometric amounts of CK1?, suggesting that MDMX may in turn regulate CK1? function. We present evidence that MDMX is a potent competitive inhibitor of CK1? kinase activity (Ki=8 nM). Depletion of MDMX increases CK1? activity and ?-catenin S45 phosphorylation, whereas ectopic MDMX expression inhibits CK1? activity and ?-catenin phosphorylation. The MDMX acidic domain and zinc finger are necessary and sufficient for binding and inhibition of CK1?. P53 binding to MDMX disrupts an intramolecular auto-regulatory interaction and enhances its ability to inhibit CK1?. P53-null mice expressing the MDMXW200S/W201G mutant, defective in CK1? binding, exhibit reduced Wnt/?-catenin target gene expression and delayed tumor development. Therefore, MDMX is a physiological inhibitor of CK1? and has a role in modulating cellular response to Wnt signaling. The MDMX-CK1? interaction may account for certain p53-independent functions of MDMX.

酪蛋白激酶1α（Casein kinase 1 alpha, CK1α）是一类丝氨酸/苏氨酸激酶，具备多种生物学功能，包括调控Wnt/β-连环蛋白通路与p53信号通路。既往研究已明确，CK1α可通过结合MDMX并促进MDMX-p53相互作用，进而抑制肿瘤抑制因子p53的活性。从细胞中纯化获得的MDMX可结合近乎化学计量比的CK1α，这提示MDMX可能反向调控CK1α的功能。本研究证实，MDMX是CK1α激酶活性的强效竞争性抑制剂，其抑制常数（Ki）为8 nM。MDMX耗竭可增强CK1α活性以及β-连环蛋白S45位点的磷酸化水平，而异位表达MDMX则会抑制CK1α活性与β-连环蛋白的磷酸化。MDMX的酸性结构域与锌指结构域是其结合并抑制CK1α所必需且充分的结构基础。p53与MDMX的结合可破坏其分子内自身调控相互作用，进而增强MDMX对CK1α的抑制能力。表达CK1α结合缺陷型MDMXW200S/W201G突变体的p53基因敲除小鼠，其Wnt/β-连环蛋白通路靶基因的表达水平降低，且肿瘤发生进程延迟。综上，MDMX是CK1α的生理性抑制剂，在调控细胞对Wnt信号通路的应答中发挥重要作用。MDMX与CK1α的相互作用或可解释MDMX的部分不依赖p53的生物学功能。