近日，來(lái)自美國(guó)北卡羅來(lái)納大學(xué)教堂山分校的研究人員發(fā)現(xiàn),，Mdm2內(nèi)RING結(jié)構(gòu)域的突變使其喪失了E3泛素連接酶活性，卻增強(qiáng)了p53的轉(zhuǎn)錄活性及p53-p300之間的相互作用。相關(guān)研究成果于5月29日發(fā)表在PLoS ONE上。
p53基因是一種抑癌基因,，是細(xì)胞生長(zhǎng)周期中的負(fù)調(diào)節(jié)因子，與細(xì)胞周期的調(diào)控,、DNA修復(fù)、細(xì)胞分化,、細(xì)胞凋亡等重要的生物學(xué)功能息息相關(guān),。p53主要被E3泛素連接酶Mdm2所調(diào)節(jié)，Mdm2泛素化p53，然后由蛋白酶體所降解,。Mdm2除了具有泛素連接酶的作用外,，現(xiàn)在已經(jīng)廣泛的認(rèn)為它還能夠抑制p53的轉(zhuǎn)錄活性，這主要是通過(guò)它結(jié)合并掩蓋p53的反式激活結(jié)構(gòu)域所致,。
以往的研究認(rèn)為,，Mdm2抑制p53活性的能力與Mdm2的泛素連接酶活性無(wú)關(guān)，僅僅是通過(guò)Mdm2與之結(jié)合所致,。但是2007年的一項(xiàng)研究否認(rèn)了這個(gè)觀點(diǎn),，研究人員在小鼠細(xì)胞內(nèi)Mdm2的RING結(jié)構(gòu)域敲入了一個(gè)半胱氨酸到丙氨酸的點(diǎn)突變(C462A)。因此,，老鼠的胚胎成纖維細(xì)胞具有這種突變,。后續(xù)研究發(fā)現(xiàn)，這種突變的Mdm2失去了E3泛素連接酶活性,，而且它仍能夠結(jié)合到p53,，但是它卻喪失了抑制p53的作用。
在這項(xiàng)研究里,，研究人員Hilary V. Clegg等人利用Mdm2C462A 小鼠模型,，進(jìn)一步研究了Mdm2的RING結(jié)構(gòu)域?qū)53的影響。研究發(fā)現(xiàn),，在體內(nèi)Mdm2C462A蛋白不僅不會(huì)抑制p53,，對(duì)比于完全缺乏Mdm2，Mdm2C462A卻增強(qiáng)了p53對(duì)其靶基因p21/CDKN1A,、MDM2,、BAX、NOXA及14-3-3σ的轉(zhuǎn)錄活性,。
除此以外,，他們還發(fā)現(xiàn)， Mdm2C462A 促進(jìn)了p53與乙?；D(zhuǎn)移酶CBP/p300之間的相互作用,，而且 Mdm2C462A 不能與它的同源物及Mdmx異聚化。這表明Mdm2抑制p300-p53之間的相互作用,，Mdm2與Mdmx之間的相互作用都需要完整地RING結(jié)構(gòu)域,。
為此，Hilary V. Clegg表示,，該研究能夠幫助我們更好的理解Mdm2-p53通路復(fù)雜的調(diào)節(jié)機(jī)制,，并對(duì)靶向Mdm2的化療藥物的開發(fā)具有重要的影響。（生物谷Deepblue編譯）
doi: 10.1371/journal.pone.0038212
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Mdm2 RING Mutation Enhances p53 Transcriptional Activity and p53-p300 Interaction
Hilary V. Clegg, Yoko Itahana, Koji Itahana, Sundhar Ramalingam, Yanping Zhang.
The p53 transcription factor and tumor suppressor is regulated primarily by the E3 ubiquitin ligase Mdm2, which ubiquitinates p53 to target it for proteasomal degradation. Aside from its ubiquitin ligase function, Mdm2 has been believed to be capable of suppressing p53's transcriptional activity by binding with and masking the transactivation domain of p53.The ability of Mdm2 to restrain p53 activity by binding alone, without ubiquitination, was challenged by a 2007 study using a knockin mouse harboring a single cysteine-to-alanine point mutation (C462A) in Mdm2's RING domain. Mouse embryonic fibroblasts with this mutation, which abrogates Mdm2's E3 ubiquitin ligase activity without affecting its ability to bind with p53, were unable to suppress p53 activity.In this study, we utilized the Mdm2C462A mouse model to characterize in further detail the role of Mdm2's RING domain in the control of p53. Here, we show in vivo that the Mdm2C462A protein not only fails to suppress p53, but compared to the complete absence of Mdm2, Mdm2C462A actually enhances p53 transcriptional activity toward p53 target genes p21/CDKN1A, MDM2, BAX, NOXA, and 14-3-3σ.In addition, we found that Mdm2C462A facilitates the interaction between p53 and the acetyltransferase CBP/p300, and it fails to heterodimerize with its homolog and sister regulator of p53, Mdmx, suggesting that a fully intact RING domain is required for Mdm2's inhibition of the p300-p53 interaction and for its interaction with Mdmx.These findings help us to better understand the complex regulation of the Mdm2-p53 pathway and have important implications for chemotherapeutic agents targeting Mdm2, as they suggest that inhibition of Mdm2's E3 ubiquitin ligase activity may be sufficient for increasing p53 activity in vivo, without the need to block Mdm2-p53 binding.
