哺乳動物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)是一種絲氨酸/蘇氨酸蛋白激酶，在細胞生長,、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TOR是雷帕霉素的靶標,，后來又有發(fā)現TOR有抗癌效果,。2002年在酵母中發(fā)現的雷帕霉素不敏感的TOR信號途徑（rapamycin-insensitive TOR pathway），并沒有將此蛋白革名,，反而使人們更加確信mTOR存在于兩種完全不同的多蛋白復合體—— mTORC1和mTORC2中,。
最近兩篇熱點封面介紹說兩個實驗室發(fā)現一種哺乳動物mTORC2，即雷帕霉素不敏感復合體,。馬薩諸塞州理工研究所David Sabatini實驗室發(fā)現新的結合蛋白Rictor,。在一篇補充文章中，瑞士Basel大學Michael Hall研究小組發(fā)現mTORC2在酵母和人類中是保守的,，也包括Rictor,。一篇后繼文章中，Sabatini實驗室發(fā)現與mTORC2有關的第二個激酶——PDK2（PDK2引起Akt/PKB第473位絲氨酸磷酸化）丟失,。
Akt/PKB是癌癥研究的關鍵靶標,，因此越發(fā)將TORC2推向焦點。Sabatini說：“因為Akt是如此一個有名的激酶,，在癌癥和糖尿病中作用很多,，許多人都開始研究（mTORC2）。”這有利于弄清mTORC2的調節(jié)機制以及設計特異的抑制劑,。
耶魯大學蘇兵說：“TORC1和TORC2復合體是新出現的概念,，非常有趣。”mTOR結合蛋白似乎定義了兩條不同的途徑,。
在發(fā)現Rictor以前,，已經證實mTOR與蛋白Raptor結合，形成后來被稱為mTORC1的復合物,，調節(jié)細胞生長,。Rictor的出現提示了mTOR具有其它功能。“我們先前沒有推測過Rictor的作用,，”Sabatini說,，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,。
蘇兵說這是一個特大發(fā)現,，“S473被普遍用作Akt活性的抑制劑,，但是長期以來都不清楚引起此位點磷酸化的激酶是哪種。”確實,，TOR同一性進入了白熱化討論,。
近期涌現的研究結果支持mTORC2是忽略的激酶。比如,，蘇兵實驗室最近敲除小鼠mTORC2的另一個成分——mSIN1,，發(fā)現Akt/PKB的S473的磷酸化受到抑制。范德比爾特大學Mark Magnuson小組最近敲除小鼠的Rictor,，得到了相似的結果,。
Magnuson說：“仍有一些大問題，比如mTORC2不同組分的作用是什么,？”Hall認為未知因素還包括mTORC2的上下游物質,。敲除mTORC2，對于許多已知的Akt/PKB靶標,，如GSK3,、TSC2和S6K沒有作用，只是會影響Forkhead轉錄因子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。
因為mSIN1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)育開始時正常，在E11.5天死亡,，沒有任何形態(tài)學差異,，只有胎盤缺陷和細胞惡化。Magnuson由此產生疑問：“單單的Akt磷酸化能夠解釋這些表型嗎,？”
雷帕霉素何去何從,？
雷帕霉素受到越來越多的重視，被認為是潛在的治療癌癥藥物,，但是mTORC2的出現提示雷帕霉素似乎只是抑制mTOR的部分功能,。尋找mTOR特異抑制劑開始引發(fā)新的研究熱潮。蘇兵說人們已經拭圖用各種方法阻斷mTOR2途徑,，但是不能期望通過mTOR1阻斷細胞基礎的能量代謝功能,。
Sabatini實驗室最近在研究使雷帕霉素作用于mTORC2更長久的物質時發(fā)現，在幾種細胞中雷帕霉素可以降低mTORC2的表達量,。“這是矛盾的但也是光明的,，”Sabatini說，因為這個現象提示雷帕霉素有望用于一些細胞中阻斷Akt,。
從上世紀80年代觀察到雷帕霉素的抗腫瘤作用,，到現在尋找mTOR的側面,，Hall說，“一種很重要的物質這么久都沒有被弄清,，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."
