哺乳動(dòng)物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)是一種絲氨酸/蘇氨酸蛋白激酶,，在細(xì)胞生長(zhǎng),、分化,、增殖,、遷移和存活上扮演重要角色,。雷帕霉素常用于組織移植排斥,，先前有研究證實(shí)TOR是雷帕霉素的靶標(biāo)，后來(lái)又有發(fā)現(xiàn)TOR有抗癌效果,。2002年在酵母中發(fā)現(xiàn)的雷帕霉素不敏感的TOR信號(hào)途徑（rapamycin-insensitive TOR pathway）,，并沒(méi)有將此蛋白革名，反而使人們更加確信mTOR存在于兩種完全不同的多蛋白復(fù)合體—— mTORC1和mTORC2中,。
最近兩篇熱點(diǎn)封面介紹說(shuō)兩個(gè)實(shí)驗(yàn)室發(fā)現(xiàn)一種哺乳動(dòng)物mTORC2,，即雷帕霉素不敏感復(fù)合體,。馬薩諸塞州理工研究所David Sabatini實(shí)驗(yàn)室發(fā)現(xiàn)新的結(jié)合蛋白R(shí)ictor。在一篇補(bǔ)充文章中,，瑞士Basel大學(xué)Michael Hall研究小組發(fā)現(xiàn)mTORC2在酵母和人類(lèi)中是保守的,，也包括Rictor。一篇后繼文章中,，Sabatini實(shí)驗(yàn)室發(fā)現(xiàn)與mTORC2有關(guān)的第二個(gè)激酶——PDK2（PDK2引起Akt/PKB第473位絲氨酸磷酸化）丟失,。
Akt/PKB是癌癥研究的關(guān)鍵靶標(biāo)，因此越發(fā)將TORC2推向焦點(diǎn),。Sabatini說(shuō)：“因?yàn)锳kt是如此一個(gè)有名的激酶,，在癌癥和糖尿病中作用很多，許多人都開(kāi)始研究（mTORC2）,。”這有利于弄清mTORC2的調(diào)節(jié)機(jī)制以及設(shè)計(jì)特異的抑制劑,。
耶魯大學(xué)蘇兵說(shuō)：“TORC1和TORC2復(fù)合體是新出現(xiàn)的概念，非常有趣,。”mTOR結(jié)合蛋白似乎定義了兩條不同的途徑,。
在發(fā)現(xiàn)Rictor以前，已經(jīng)證實(shí)mTOR與蛋白R(shí)aptor結(jié)合,，形成后來(lái)被稱(chēng)為mTORC1的復(fù)合物,，調(diào)節(jié)細(xì)胞生長(zhǎng)。Rictor的出現(xiàn)提示了mTOR具有其它功能,。“我們先前沒(méi)有推測(cè)過(guò)Rictor的作用,，”Sabatini說(shuō)，但是“偶然觀察”到RNAi將果蠅中TOR沉默,，Rictor可以影響Akt/PKB的磷酸化,。
蘇兵說(shuō)這是一個(gè)特大發(fā)現(xiàn)，“S473被普遍用作Akt活性的抑制劑,，但是長(zhǎng)期以來(lái)都不清楚引起此位點(diǎn)磷酸化的激酶是哪種,。”確實(shí)，TOR同一性進(jìn)入了白熱化討論,。
近期涌現(xiàn)的研究結(jié)果支持mTORC2是忽略的激酶,。比如，蘇兵實(shí)驗(yàn)室最近敲除小鼠mTORC2的另一個(gè)成分——mSIN1,，發(fā)現(xiàn)Akt/PKB的S473的磷酸化受到抑制,。范德比爾特大學(xué)Mark Magnuson小組最近敲除小鼠的Rictor，得到了相似的結(jié)果,。
Magnuson說(shuō)：“仍有一些大問(wèn)題,，比如mTORC2不同組分的作用是什么？”Hall認(rèn)為未知因素還包括mTORC2的上下游物質(zhì),。敲除mTORC2,，對(duì)于許多已知的Akt/PKB靶標(biāo),，如GSK3、TSC2和S6K沒(méi)有作用,，只是會(huì)影響Forkhead轉(zhuǎn)錄因子Foxo1和Foxo3,。這些提示mTORC2只是影響Akt/PKB的特定部分，不是全部活性,。
因?yàn)閙SIN1和Rictor被敲除的小鼠是胚胎致死的,，加大了研究的困難程度。Rictor敲除的小鼠胚胎發(fā)育開(kāi)始時(shí)正常,，在E11.5天死亡,，沒(méi)有任何形態(tài)學(xué)差異，只有胎盤(pán)缺陷和細(xì)胞惡化,。Magnuson由此產(chǎn)生疑問(wèn)：“單單的Akt磷酸化能夠解釋這些表型嗎,？”
雷帕霉素何去何從？
雷帕霉素受到越來(lái)越多的重視,，被認(rèn)為是潛在的治療癌癥藥物,，但是mTORC2的出現(xiàn)提示雷帕霉素似乎只是抑制mTOR的部分功能。尋找mTOR特異抑制劑開(kāi)始引發(fā)新的研究熱潮,。蘇兵說(shuō)人們已經(jīng)拭圖用各種方法阻斷mTOR2途徑,，但是不能期望通過(guò)mTOR1阻斷細(xì)胞基礎(chǔ)的能量代謝功能。
Sabatini實(shí)驗(yàn)室最近在研究使雷帕霉素作用于mTORC2更長(zhǎng)久的物質(zhì)時(shí)發(fā)現(xiàn),，在幾種細(xì)胞中雷帕霉素可以降低mTORC2的表達(dá)量,。“這是矛盾的但也是光明的，”Sabatini說(shuō),，因?yàn)檫@個(gè)現(xiàn)象提示雷帕霉素有望用于一些細(xì)胞中阻斷Akt,。
從上世紀(jì)80年代觀察到雷帕霉素的抗腫瘤作用，到現(xiàn)在尋找mTOR的側(cè)面,，Hall說(shuō),，“一種很重要的物質(zhì)這么久都沒(méi)有被弄清，是一種很有魅力的挑戰(zhàn),。”
英文原文：
For mTOR, Clarification and Confusion
The mammalian target of rapamycin (mTOR) was already an established player in cell growth
Size and Shape: An evolving model of the mTOR signaling network posits two major branches. Rapamycin sensitive mTORC1 regulates cell size, and mTORC2, through actin organization, regulates cell shape.
when a discovery late in 2004 stepped up interest in the molecule. TOR was originally found as a target to rapamycin, a drug used to suppress tissue-graft rejection, that was also found to have some tantalizing antitumor effects. The discovery of a rapamycin-insensitive TOR pathway in 2002 in yeast didn't quash the name of the protein, but rather led to the realization that mTOR exists in two distinct multiprotein complexes, mTORC1 and mTORC2.
In the Hot Papers featured here, two labs discovered a mammalian component of mTORC2, the rapamycin-insensitive complex. David Sabatini's lab at the Massachusetts Institute of Technology found the new binding protein Rictor. In a complementary paper, Michael Hall's group at the University of Basel showed mTORC2 to be conserved in yeast and mammals, and to also include Rictor. Then, in a follow-up paper, Sabatini's lab discovered that the mTORC2 appears to be the missing second kinase, PDK2, which phosphorylates serine 473 (S473) on Akt/PKB.
Akt/PKB is a crucial target in cancer research, pushing TORC2 further into the spotlight. "Because Akt is such a famous kinase and it has so many different roles in cancer and diabetes, a lot of people are now running into [mTORC2]" says Sabatini. Consequently, the push is on to figure out how mTORC2 is regulated, what other functions it might have, and to design specific drugs to inhibit it.
Debated Kinase Found?
"TORC1 and TORC2 complexes are new emerging concepts that are very exciting," says Bing Su of Yale University. It is particularly interesting, he adds, that mTOR's binding partner seems to define two distinct pathways.
Before the discovery of Rictor, it was known that mTOR binds to the protein Raptor, forming what was later to be called mTORC1 and regulating cell growth. But with Rictor on the scene, it soon became apparent that mTOR had other functions. "We originally had no guesses what Rictor did," says Sabatini, but "a chance observation" showed that RNAi on Drosophila TOR and Rictor could affect Akt/PKB phosphorylation.
This was a huge find, says Su. "S473 has been widely used as an indicator of Akt activity, but which kinase phosphorylated this site has been unknown for a very long time." Indeed, the identity had been hotly debated.
Recent results back up mTORC2 as this missing kinase. For example, Su's lab recently knocked out a new mTORC2 component, mSIN1, in mice, finding that this inhibits S473 phosphorylation on Akt/PKB. Mark Magnuson's group at Vanderbilt University recently knocked out Rictor in mice with similar results on Akt/PKB. "There are still huge unanswered questions," says Magnuson, "like what are the functions of the different components of mTORC2?"
Hall agrees, adding that it's also unclear what's exactly upstream and downstream of mTORC2. Knocking out mTORC2 had no effect on several well-known Akt/PKB targets, such as GSK3, TSC2, and S6K, and affected only the Forkhead transcription factors Foxo1 and Foxo3. 4,5 This suggests that mTORC2 might affect only Akt/PKB specificity and not overall activity. "We so far know enough to [say] that mTORC2 is of fundamental importance," Hall says, "but I think what we know is only the tip of the iceberg."
Indeed, mSIN1 and Rictor knockout mice are embryonic lethal, making study difficult. Rictor knockouts begin to develop normally and then die at day E11.5 without gross structural differences, but with placental defects and indications of cellular degeneration. This led Magnuson to ask: "Does the phosphorylation of simply Akt explain this phenotype? I'm not sure we know that."
Whither Rapamycin?
Rapamycin received a lot of attention as a possible cancer treatment drug, but with the discovery of mTORC2, rapamycin appears to only inhibit some mTOR functions. The search for mTORC2-specific inhibitors is therefore garnering a lot of attention, says Su. "It makes a lot of sense to try to block the mTORC2 pathway," he adds. "You may not want to even block fundamental energy or metabolism functions of cells through mTORC1."
Sabatini says he thinks we haven't heard the last from rapamycin. His lab recently looked into what prolonged exposure to rapamycin does to mTORC2, finding that it can reduce mTORC2 levels over time in some cell types. "This is still controversial but looks promising," says Sabatini, since it suggests that rapamycin might be useful for blocking Akt in some cells.
Building from observations that began in the 1980's with the tumor-fighting abilities of rapamycin, to now seeing a different side of mTOR, Hall says, "It's fascinating that something that seems so important now, went undiscovered for so long."
