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磷脂酰肌醇3-激酶(PI3Ks)信號(hào)通路相關(guān) 磷脂酰肌醇3-激酶(PI3Ks)蛋白家族參與細(xì)胞增殖、分化、凋亡和葡萄糖轉(zhuǎn)運(yùn)等多種細(xì)胞功能的調(diào)節(jié)。PI3K活性的增加常與多種癌癥相關(guān)。PI3K磷 酸化磷脂酰肌醇PI(一種膜磷脂)肌醇環(huán)的第3位碳原子。PI在細(xì)胞膜組分中所占比例較小，比磷脂酰膽堿、磷脂酰乙醇胺和磷脂酰絲氨酸含量少。但在腦細(xì)胞 膜中，含量較為豐富，達(dá)磷脂總量的10%。 PI的肌醇環(huán)上有5個(gè)可被磷酸化的位點(diǎn)，多種激酶可磷酸化PI肌醇環(huán)上的4th和5th位點(diǎn)，因而通常在這兩位點(diǎn)之一或兩位點(diǎn)發(fā)生磷酸化修飾，尤其發(fā)生 在質(zhì)膜內(nèi)側(cè)。通常，PI-4,5-二磷酸(PIP2)在磷脂酶C的作用下，產(chǎn)生二酰甘油(DAG)和肌醇-1,4,5-三磷酸。PI3K轉(zhuǎn)移一個(gè)磷酸基團(tuán) 至位點(diǎn)3，形成的產(chǎn)物對(duì)細(xì)胞的功能具有重要的影響。譬如，單磷酸化的PI-3-磷酸，能刺激細(xì)胞遷移(cell trafficking)，而未磷酸化的則不能。PI-3,4-二磷酸則可促進(jìn)細(xì)胞的增殖(生長)和增強(qiáng)對(duì)凋亡的抗性，而其前體分子PI-4-磷酸則不 然。PIP2轉(zhuǎn)換為PI-3,4,5-三磷酸，可調(diào)節(jié)細(xì)胞的黏附、生長和存活。 PI3K可分為3類，其結(jié)構(gòu)與功能各異。其中研究最廣泛的為I類PI3K, 此類PI3K為異源二聚體，由一個(gè)調(diào)節(jié)亞基和一個(gè)催化亞基組成。調(diào)節(jié)亞基含有SH2和SH3結(jié)構(gòu)域，與含有相應(yīng)結(jié)合位點(diǎn)的靶蛋白相作用。該亞基通常稱為 p85, 參考于第一個(gè)被發(fā)現(xiàn)的亞型(isotype),然而目前已知的6種調(diào)節(jié)亞基，大小從50至110kDa不等。催化亞基有4種，即p110α, β,δ,γ，而δ僅限于白細(xì)胞，其余則廣泛分布于各種細(xì)胞中。 PI3K的活化很大程度上參與到靠近其質(zhì)膜內(nèi)側(cè)的底物。多種生長因子和信號(hào)傳導(dǎo)復(fù)合 物，包括成纖維細(xì)胞生長因子(FGF)、血管內(nèi)皮生長因子(VEGF)、人生長因子(HGF)、血管位蛋白I(Ang1)和胰島素都能啟始PI3K的激活 過程。這些因子激活受體酪氨酸激酶(RTK)，從而引起自磷酸化。受體上磷酸化的殘基為異源二聚化的PI3Kp85亞基提供了一個(gè)停泊位點(diǎn) (docking site)。然而在某些情況下，受體磷酸化則會(huì)介導(dǎo)募集一個(gè)接頭蛋白(adaptor protein)。比如，當(dāng)胰島素激活其受體后，則必須募集一個(gè)胰島素受體底物蛋白(IRS)，來促進(jìn)PI3K的結(jié)合。相似的，當(dāng)整連蛋白 integrin(非RTK)被激活后，粘著斑激酶(FAK) 則作為接頭蛋白，將PI3K通過其p85停泊。但在以上各情形下，p85亞基的SH2和SH3結(jié)構(gòu)域均在一個(gè)磷酸化位點(diǎn)與接頭蛋白結(jié)合。PI3K募集到活 化的受體后，起始多種PI中間體的磷酸化。與癌腫尤其相關(guān)的PI3K轉(zhuǎn)化PIP2為PIP3。 PIP3作為錨定物(anchor) 許多蛋白含有一個(gè)Pleckstrin Homology(PH)結(jié)構(gòu)域，因而可使其與PI-3,4-P2或PI-3,4,5-P3相結(jié)合。這種相互作用可以控制蛋白與膜結(jié)合的時(shí)間與定位，通過 這種方式來調(diào)節(jié)蛋白的活性。蛋白與脂質(zhì)間的這種相互作用亦可能引起蛋白構(gòu)像的變化而改變蛋白的功能。PI3K激活的結(jié)果是在質(zhì)膜上產(chǎn)生第二信使PIP3, PIP3與細(xì)胞內(nèi)含有PH結(jié)構(gòu)域的信號(hào)蛋白AKT和PDK1(phosphoinositide dependent kinase-1)結(jié)合, 促使PDK1磷酸化AKT蛋白的Ser308導(dǎo)致AKT活化。其他PDK1的底物還包括PKC(蛋白激酶C)、S6K(p70S6)和 SGK(serum/glucocorticoid regulated kinases) 。AKT, 亦稱為蛋白激酶B(PKB)，是PI3K下游主要的效應(yīng)物。AKT可分為3種亞型(AKT1、AKT2、AKT3或PKBα, PKBβ,PKBγ)，3種亞型的功能各異，但也有重疊。 AKT的作用 活化的AKT通過磷酸化多種酶、激酶和轉(zhuǎn)錄因子等下游因子，進(jìn)而調(diào)節(jié)細(xì)胞的功能。譬如，AKT刺激葡萄糖的代謝：AKT激活A(yù)S160(AKT底 物，160kDa)，進(jìn)而促進(jìn)GLUT4轉(zhuǎn)座和肌細(xì)胞對(duì)葡萄糖的吸收。AKT也磷酸化GSK3β而抑制其活性，從而促進(jìn)葡萄糖的代謝和調(diào)節(jié)細(xì)胞的周期。 AKT磷酸化TSC1/2(tuberous sclerosis complex)，可阻止其對(duì)小G蛋白R(shí)heb(Ras homology enriched in brain)的負(fù)調(diào)控，進(jìn)而使得Rheb富集以及對(duì)納巴霉素(rapamycin)敏感的mTOR復(fù)合體(mTORC1)的活化。這些作用可激活蛋白的翻 譯，增強(qiáng)細(xì)胞的生長。 AKT通過下游多種途徑對(duì)靶蛋白進(jìn)行磷酸化而發(fā)揮抗凋亡作用。ATK激活I(lǐng)kB激酶(IKKα),導(dǎo)致NF-κB的抑制劑 IκB的降解, 從而使NF-κB從細(xì)胞質(zhì)中釋放出來進(jìn)行核轉(zhuǎn)位, 激活其靶基因而促進(jìn)細(xì)胞的存活。AKT磷酸化Bcl-2家族成員BAD，使其與14-3-3結(jié)合而阻止其與Bcl-XL結(jié)合起始凋亡。此外，AKT能抑制 蛋白水解酶caspase-9的活性而阻止凋亡級(jí)聯(lián)反應(yīng)的激活。腫瘤抑制因子p53為一轉(zhuǎn)錄因子，調(diào)控凋亡、DNA修復(fù)和細(xì)胞周期的停滯。Akt能通過磷 酸化P53結(jié)合蛋白MDM2影響P53的活性, 磷酸化的MDM2轉(zhuǎn)位到細(xì)胞核與P53結(jié)合, 通過增加P53蛋白的降解而影響細(xì)胞存活。Forkhead轉(zhuǎn)錄因子FOXO1( 或FKHR)調(diào)節(jié)涉及多種細(xì)胞功能基因的表達(dá)，包括凋亡、DNA修復(fù)和 細(xì)胞周期的停滯和葡萄糖代謝等，AKT磷酸化FOXO1，抑制其核轉(zhuǎn)位而阻止其轉(zhuǎn)錄激活作用。 PTEN: 一個(gè)關(guān)鍵磷酸酶 PTEN (phosphatase and tensin homology deleted on chromosome 10),在廣泛的人類腫瘤中發(fā)生基因突變或缺失。PTEN是一個(gè)PIP3-磷酸酶，與PI3K的功能相反，它可以通過去磷酸化將PIP3轉(zhuǎn)變?yōu)镻I- 4,5-P2。PTEN可減少AKT的活化而阻止所有由AKT調(diào)控的下游信號(hào)傳導(dǎo)事件。通過產(chǎn)生PIP2, PTEN的功能遠(yuǎn)不止僅僅阻止AKT的作用。PIP2作為磷脂酶Cβ(PLCβ)的底物，產(chǎn)生DAG和IP3作為第二信使，升高胞內(nèi)鈣離子的水平并激活蛋 白激酶C(PKC)。與膜結(jié)合的PIP2也能介導(dǎo)(增加)多種離子通道的活性，包括鈣離子、鉀離子和鈉離子通道。PIP也參與到膜泡的形成以及細(xì)胞骨架與 膜的相互作用過程中。PIP2還影響多種參與脂代謝的酶活性，包括磷脂酶D和神經(jīng)酰胺-1-磷酸。通過從PIP產(chǎn)生PIP2, PTEN廣泛的影響著細(xì)胞的多種功能途徑。PTEN在減弱PIP3信號(hào)傳導(dǎo)而增加PIP2水平的過程中，發(fā)揮著重要的作用。 PI3K-AKT-mTOR is one of the three major signalling pathways that have been identified as important in cancer. mTOR is a key kinase downstream of PI3K/AKT, which regulates tumor cell proliferation, growth, survival and angiogenesis. Cancer cells escape normal biochemical systems regulating the balance between apoptosis (suicide) and survival. PI3K-AKT-mTOR generally acts to promote survival through inhibition of proapoptotic factors and activation of anti-apoptotic factors. Through phosphorylation, PI3K-AKT-mTOR inhibits the activity of proapoptotic members while activating anti-apoptotic members. To negatively regulate PI3K, cells contain PTEN phosphatase. A reduction in PTEN expression indirectly stimulates PI3K-AKT-mTOR activity thereby contributing to oncogenesis in human. Recent data suggests that the PI3K-AKT-mTOR signaling pathway plays an important role in cancer stem cell self-renewal and resistance to chemotherapy or radiotherapy, which is believed to be the root of treatment failure and cancer recurrence, as well as metastasis. 癌癥中的PI3K-mTOR信號(hào)途徑 Figure 1 | Minding your Ps: the PtdIns(4,5)P2–PtdIns(3,4,5)P3 cycle. Phosphatidylinositol phosphates are composed of a membrane-associated phosphatidic acid group and a glycerol moiety that is linked to a cytosolic phosphorylated inositol head group. Phosphatidylinositol 3-kinase (PI3K) can phosphorylate PtdIns(4,5)P2 (PIP2) at the D3 position to form the second messenger PtdIns(3,4,5)P3 (PIP3). Phosphorylation at the D3 position is necessary for binding to the pleckstrin-homology domain of AKT (not shown). Dephosphorylation of PIP3 to regenerate PIP2 is accomplished by the 3-phosphatase PTEN. Additionally, PIP3 can be dephosphorylated at the D5 position by SHIP1 or SHIP2 to generate PtdIns(3,4)P2, another potential second messenger. AKT信號(hào)的調(diào)控 Figure 3 |. Activation of AKT is initiated by membrane translocation,which occurs after cell stimulation and PtdIns(3,4,5)P3 (PIP3) production. Localization of AKT to the plasma membrane is accomplished by an interaction between its pleckstrin-homology (PH) domain and PIP3. At the membrane, association with carboxy-terminal modulator protein (CTMP) prevents AKT from becoming phosphorylated and fully active. Phosphorylation of CTMP by an asyet unidentified kinase releases CTMP from AKT and allows AKT to be phosphorylated by PDK1 and PDK2 at Thr308 and Ser473, respectively. Phosphorylation at these two sites causes ful activation of AKT. C2, C2 domain; CD, catalytic domain; p85 BD, p85-binding domain. PI3K信號(hào)通路 Figure 3 | Activation of class IA phosphatidylinositol 3-kinases (PI3Ks) occurs through stimulation of receptor tyrosine kinases (RTKs) and the concomitant assembly of receptor–PI3K complexes. These complexes localize at the membrane where the p110 subunit of PI3K catalyses the conversion of PtdIns(4,5)P2 (PIP2) to PtdIns(3,4,5)P3 (PIP3). PIP3 serves as a second messenger that helps to activate AKT. Through phosphorylation, activated AKT mediates the activation and inhibition of several targets, resulting in cellular growth, survival and proliferation through various mechanisms. Additionally, PI3K has been shown to regulate the activity of other cellular targets, such as the serum and glucocorticoid-inducible kinase (SGK), the small GTP-binding proteins RAC1 and CDC42, and protein kinase C (PKC), in an AKT-independent manner through poorly characterized mechanisms. The activity of these targets leads to survival, cytoskeletal rearrangement and transformation. GSK3 |
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