雷帕霉素靶蛋白与线粒体稳态

0 Comments

Rheb and mammalian target of rapamycin in mitochondrial homoeostasis

Abstract

摘要

Mitochondrial dysfunction has been associated with various diseases, such as cancer, myopathies, neurodegeneration and obesity. Mitochondrial homoeostasis is achieved by mechanisms that adapt the number of mitochondria to that required for energy production and for the supply of metabolic intermediates necessary to sustain cell growth. Simultaneously, mitochondrial quality control mechanisms are in place to remove malfunctioning mitochondria. In the cytoplasm, the protein complex mTORC1 couples growth-promoting signals with anabolic processes, in which mitochondria play an essential role. Here, we review the involvement of mTORC1 and Rheb in mitochondrial homoeostasis. The regulatory processes downstream of mTORC1 affect the glycolytic flux and the rate of mitophagy, and include regulation of the transcription factors HIF1α and YY1/PGC-1α. We also discuss how mitochondrial function feeds back on mTORC1 via reactive oxygen species signalling to adapt metabolic processes, and highlight how mTORC1 signalling is integrated with the unfolded protein response in mitochondria, which in Caenorhabditis elegans is mediated via transcription factors such as DVE-1/UBL-5 and ATFS-1.

线粒体功能障碍与多种疾病有关,如癌症、肌病、神经退行性疾病和肥胖。线粒体稳定性是通过机制来实现的,这种机制使线粒体的数量适应能量生产和维持细胞生长所需的代谢中间物的供应。同时,线粒体质量控制机制已到位,以删除故障线粒体。在细胞质中,mTORC1蛋白质复合体将促生长信号与合成代谢过程联系起来,线粒体在其中起重要作用。本文就 mTORC1和 Rheb 在线粒体内环境稳定性中的作用进行综述。mTORC1的下游调控过程影响糖酵解通量和吞噬率,包括调控转录因子 hif1和 y1/pgc-1。我们还讨论了线粒体功能如何通过活性氧类信号反馈 mTORC1来适应新陈代谢过程,并强调 mTORC1信号是如何与线粒体中的未折叠蛋白反应结合在一起的,这在秀丽隐桿线虫中是通过 DVE-1/UBL-5和 ATFS-1等转录因子介导的。

2. Introduction

2. 引言

Metabolic processes in a cell require the continuous input of energy in the form of ATP. In the presence of nutrients and oxygen, mitochondria are the major suppliers of ATP. The metabolic process inside mitochondria that uses energy released during the oxidation of nutrients to produce ATP is called oxidative phosphorylation (OXPHOS). Depending on the cell type, mitochondria use derivatives of carbohydrates, fats or amino acids to fuel the tricarboxylic acid (TCA) cycle. When glucose is used as the primary energy source, it is processed into two molecules of pyruvate via the glycolytic pathway. These will enter the mitochondrial matrix (MM) where they will be converted to acetyl-CoA. Oxidation of this metabolite generates electrons for transport along the respiratory chain in the inner mitochondrial membrane (IMM). This transport of electrons results in the pumping of H+ from the MM to the intermembrane space (IMS), generating a pH gradient and a membrane potential (Δψm) across the IMM. Energy from H+ ions that flow down the electrochemical gradient is used by the ATP synthase for the conversion of ADP and Pi to ATP. As a by-product of OXPHOS, reactive oxygen species (ROS) are produced in the mitochondria. ROS produced in low levels have a signalling function in the cell, but high levels are damaging to mitochondria. Damaged mitochondria produce excess ROS and this leads to a vicious cycle of more damaged mitochondria with concomitant ROS production. Expression of uncoupling proteins allows H+ ions to cross the IMM without production of ATP in order to generate heat or lower the Δψm to prevent ROS formation. Apart from the generation of ATP, the electrochemical gradient is also used for import of proteins and metabolites that are used to sustain the function of the mitochondria themselves. The function of mitochondria also includes the formation of building blocks for amino acid and fatty acid synthesis, storage of Ca2+ and regulation of apoptosis (reviewed in [1]). Obviously, because of the central role of ATP in metabolic processes and the damaging effects of excess ROS there must be a strict coupling between the import of energy sources into the cell, metabolic pathways and the functionality of mitochondria.

细胞的新陈代谢过程需要以 ATP 的形式不断地输入能量。在有营养和氧气的情况下,线粒体是 ATP 的主要供应者。线粒体内的代谢利用营养物质氧化过程中释放的能量来产生 ATP,这种氧化磷酸化叫做氧化磷(OXPHOS)。根据不同的细胞类型,线粒体利用碳水化合物、脂肪或氨基酸的衍生物来维持三元羧酸循环。当葡萄糖作为主要能源时,它通过糖酵解途径被加工成两个丙酮酸分子。这些将进入线粒体基质,在那里他们将转化为乙酰辅酶 a。这种代谢产物的氧化产生电子,通过呼吸链在线粒体内膜中传递。这种电子的输送导致 h + 从 MM 泵到膜间隙,产生 pH 梯度和跨 IMM 的膜电位。从 h + 离子流向电化梯度的能量被 ATP 合成酶用于 ADP 和 Pi 转化为 ATP。作为 OXPHOS 的副产品,活性氧类(ROS)在线粒体中产生。低水平的活性氧在细胞中有信号功能,但高水平的活性氧会破坏线粒体。受损的线粒体产生过多的活性氧,这导致更多受损的线粒体与随之而来的活性氧产生的恶性循环。解偶联蛋白的表达允许 h + 离子在不产生 ATP 的情况下通过 IMM 以产生热量或降低 m 来阻止 ROS 的形成。除了产生 ATP,电化梯度还用于进口蛋白质和代谢物,这些蛋白质和代谢物用于维持线粒体本身的功能。线粒体的功能还包括形成氨基酸和脂肪酸合成的组成部分,Ca2 + 的储存和调节细胞凋亡。显然,由于 ATP 在代谢过程中的中心作用和过量活性氧的破坏性影响,在能量源进入细胞、代谢途径和线粒体功能之间必须有一个严格的耦合。

Mitochondrial homoeostasis depends on control mechanisms to regulate the number of mitochondria as well as their quality. Cells respond to an increase in metabolic demand by increasing their number of mitochondria (reviewed in [2,3]). Under conditions of low metabolic activity an excess of mitochondria can selectively be removed via a specialized form of autophagy, named mitophagy (reviewed in [4]). Quality control of mitochondria is ensured via various mechanisms. First, mitochondria undergo continuous cycles of fusion and fission that may support their function by preventing stochastic loss of metabolic substrates or mitochondrial DNA (reviewed in [5]). Second, when stress situations, such as mutation of mitochondrial DNA or excessive ROS production induced by physiological stimuli, result in the presence of damaged or improperly folded proteins, a mitochondrial unfolded protein response (UPRmt) is triggered. This response leads to transcriptional activation of mitochondrial chaperone proteins that reduce the number of misfolded proteins (reviewed in [6]). Finally, quality of mitochondria is also maintained via the mitophagic pathway, which prevents the production of excess ROS. Ultimately, if mitochondrial damage becomes too pervasive, the cell undergoes apoptosis by activation of caspases, triggered by the release of cytochrome c from the IMS [7]. As the vast majority of mitochondrial proteins are encoded by the nuclear genome, signalling pathways are required to regulate nuclear transcription of mitochondrial genes such that optimal mitochondrial function ensues. An important signalling pathway in this regulation is the target of rapamycin (TOR) signalling pathway. The kinase TOR is a critical player in the tight coupling of cellular metabolism and mitochondria in various organisms (reviewed in [8,9]). The scope here is to review the role of mammalian TOR (mTOR) and discuss the various modes by which mTOR influences these organelles.

线粒体的稳定性依赖于控制机制来调节线粒体的数量和质量。细胞通过增加线粒体的数量来应对代谢需求的增加(见文献2,3)。在低代谢活性的条件下,过剩的线粒体可以通过一种特殊形式的自噬被选择性地去除,这种形式被称为吞噬(见[4])。通过各种机制保证线粒体的质量控制。首先,线粒体经历连续的融合和裂变周期,这可能通过防止代谢底物或线粒体脱氧核糖核酸的随机损失来支持它们的功能。其次,当压力情况,如突变的线粒体脱氧核糖核酸或过多的活性氧产生的生理刺激,导致存在损坏或不当折叠的蛋白质,线粒体未折叠蛋白反应(UPRmt)被触发。这种反应导致线粒体伴侣蛋白的转录激活,从而减少错误折叠蛋白的数量。最后,线粒体的质量也可以通过噬细胞途径来维持,这种途径可以阻止过量活性氧的产生。最终,如果线粒体损伤变得过于普遍,细胞通过激活半胱氨酸蛋白酶进行凋亡,这种激活是由 IMS 释放细胞色素 c 引起的[7]。由于绝大多数线粒体蛋白质是由核基因组编码的,因此需要通过信号通路来调节线粒体基因的核转录,从而产生最佳的线粒体功能。一个重要的信号途径在这个调节是雷帕霉素(TOR)的目标信号途径。在各种生物体中,激酶 TOR 是细胞代谢和线粒体紧密耦合的关键因子(见文献[8,9])。这里的范围是回顾哺乳动物 TOR (mTOR)的作用,并讨论 mTOR 影响这些细胞器的各种模式。

3. Mammalian target of rapamycin signalling pathway

3. 雷帕霉素靶蛋白信号通路

TOR is a highly conserved serine–threonine kinase that plays an important role in cell growth, autophagy and metabolism in response to growth factors, nutrients, hypoxia and energy stress (figure 1). In mammalian cells, the kinase exists in two complexes. Complex 1 (mTORC1) consists of mTOR, Raptor, mLST8/GβL and PRAS40. This complex integrates growth factor signalling with the availability of nutrients and can be selectively inhibited by the fungicidal macrolide rapamycin. The second complex (mTORC2) consists of mTOR, Rictor, Sin1 and mLST8/GβL and is largely insensitive to nutrients and rapamycin (reviewed in [10]).

TOR 是一种高度保守的丝氨酸苏氨酸激酶,在细胞生长、自噬和新陈代谢中发挥重要作用,以应对生长因子、营养物质、缺氧和能量胁迫(图1)。在哺乳动物细胞中,这种激酶存在于两种复合体中。配合物1(mTORC1)由 mTOR、 Raptor、 mlst8/g l 和 PRAS40组成。这种复合物将生长因子信号与营养素的有效性结合起来,并且可以被大环内酯雷帕霉素选择性地抑制。第二种复合物(mTORC2)由 mTOR、 Rictor、 Sin1和 mlst8/g l 组成,对营养物质和雷帕霉素不敏感(见文献[10])。

Figure 1.
Figure 1. mTOR signalling pathway. Overview of major upstream regulatory signals of mTORC1 and major downstream effectors of mTORC1. Insulin and growth factors stimulate mTORC1 activity by binding growth factor receptors and stimulating downstream signalling. Activation leads to PIP3 generation by PI3K. PIP3 recruits PDK1, which in turn activates PKB. PKB represses the GAP activity of the TSC1/2 complex towards Rheb, which leads to GTP-loaded active Rheb and subsequent activation of mTORC1. Growth factors can simultaneously activate MAPK signalling, which stimulates mTORC1 activity via the inactivation of the TSC1/2 complex and the direct phosphorylation of Raptor. Finally, growth factors stimulate mTORC2 activity in a PI3K-dependent manner, but by an unknown mechanism. Amino acids also stimulate mTORC1 activity by activating the Rag GTPases. This activation recruits mTORC1 to the lysosomes where it is in close proximity to Rheb. Negative regulation of mTORC1 activity occurs when cells are deprived of oxygen or nutrients. Reduced availability of oxygen stabilizes HIF1α and thereby activates the REDD1/2 proteins that stimulate the TSC1/2 complex. Nutrient depletion stimulates the TSC1/2 complex via the activation of AMPK via either LKB1 activation or an increase in AMP levels. In addition, AMPK directly inhibits mTORC1 activity via the phosphorylation of Raptor. Upon activation, mTORC1 regulates a subset of downstream effects such as protein synthesis via S6K and 4EBP, autophagy via Ulk1 phosphorylation and energy homoeostasis via PGC-1α and YY1.图1。mTOR 信号通路。mTORC1的主要上游调控信号和主要下游效应因子概述。胰岛素和生长因子通过结合生长因子受体和刺激下游信号通路刺激 mTORC1的活性。激活导致 PI3K 产生 PIP3。PIP3招募 PDK1,而 PDK1又激活 PKB。PKB 抑制 TSC1/2复合物对 Rheb 的 GAP 活性,从而导致 gtp 负载活性 Rheb 和 mTORC1的激活。生长因子可以同时激活 MAPK 信号通过 TSC1/2复合物的失活和 Raptor 的直接磷酸化刺激 mTORC1的活性。最后,生长因子以 pi3k 依赖的方式刺激 mTORC2的活性,但是是通过一种未知的机制。氨基酸还通过激活 Rag GTPases 来刺激 mTORC1的活性。这种激活使 mTORC1靠近 Rheb 的溶酶体。当细胞缺氧或缺乏营养物质时,mTORC1活性出现负调节。氧气供应的减少稳定了 hif1,从而激活了能刺激 TSC1/2复合体的 reddit 1/2蛋白质。营养损耗通过激活 AMPK 或 LKB1激活或增加 AMP 水平来刺激 TSC1/2复合物。此外,AMPK 通过 Raptor 的磷酸化直接抑制 mTORC1的活性。在激活过程中,mTORC1通过 S6K 和4EBP 调节蛋白质合成,通过 Ulk1磷酸化调节自噬,通过 pgc-1和 y1调节能量稳态。

3.1. Mammalian target of rapamycin complex 1 regulation by growth factors

3.1. 生长因子对雷帕霉素复合物靶蛋白1的调控

Growth factors signal to mTORC1 through the phosphoinositide 3-kinase/protein kinase B (PI3K/PKB) pathway and the Ras/mitogen-activated protein kinase (MAPK) pathway. For example, activation of insulin and insulin-like growth factor 1 (IGF-1) receptors induces binding and phosphorylation of adapter proteins such as insulin receptor substrate 1 (IRS1), which then transmits the signal to the PI3K and the MAPK pathways. For the PI3K pathway, phosphorylation of phosphatidylinositol-(4,5)-biphosphate (PtdIns(4,5)P2; PIP2) by activated PI3K leads to the generation of PtdIns(3,4,5)P3 (PIP3) at the plasma membrane and the subsequent activation of phosphoinositide-dependent kinase-1 (PDK1) [11,12]. PtdIns(3,4,5)P3 also recruits PKB to the plasma membrane (reviewed in [13]) and following activation by PDK1, PKB phosphorylates and inactivates the tuberous sclerosis complex 1/2 (TSC1/TSC2) complex [14]. TSC2 harbours a GTPase-activating protein (GAP) domain in its C-terminal region, which stimulates the GTPase activity of the small GTPase Rheb [1519]. When TSC2 is inhibited, the resulting increase in GTP-bound Rheb will activate mTORC1. In turn, mTORC1 phosphorylates and activates its downstream substrate ribosomal protein S6 kinase 1 (S6K1), leading to 5′ TOP mRNA translation and cell growth [20]. S6K1 also phosphorylates and inactivates IRS1, thereby providing a negative feedback loop [21]. Phosphorylation of the other well-studied substrate of mTORC1, 4EBP1, releases it from the eukaryotic initiation factor eIF4E, thereby allowing eIF4E to initiate cap-dependent translation and to stimulate proliferation [20,22].

生长因子通过磷酸肌醇3激酶/蛋白激酶 b (PI3K/PKB)途径和 ras/丝裂原活化蛋白激酶(MAPK)途径向 mTORC1传递信号。例如,激活胰岛素和胰岛素样生长因子1(IGF-1)受体,诱导接头蛋白如胰岛素受体底物1(IRS1)的结合和磷酸化,然后将信号传递给 PI3K 和 MAPK 通路。在 PI3K 通路中,通过激活 PI3K 使磷脂酰肌醇-(4,5)-二磷酸二酯(PtdIns (4,5) P2; PIP2磷酸化,在质膜上产生 PtdIns (3,4,5) P3(PIP3) ,随后激活磷酸肌醇依赖性激酶 -1(PDK1)[11,12]。PtdIns (3,4,5) P3也会将 PKB 激活到质膜上(参见文献[13]) ,然后通过 PDK1激活 PKB,PKB 磷酸化并使结节性硬化症1/2(TSC1/TSC2)复合体失活[14]。TSC2在其 c 末端含有一个 GTPase 激活蛋白(GAP)结构域,该结构域能够激活小 GTPase Rheb [15-19]的 GTPase 活性。当 TSC2受到抑制时,gtp 结合的 Rheb 的增加会激活 mTORC1。反过来,mTORC1磷酸化并激活其下游底物核糖体蛋白质 S6激酶1(S6K1) ,导致5′ TOP mRNA 的翻译和细胞生长[20]。S6K1也使 IRS1磷酸化和失活,从而提供一个负反馈回路[21]。另一个被充分研究的 mTORC1,4EBP1底物的磷酸化作用,使它从 eIF4E 真核起始因子释放出来,从而允许 eIF4E 启动帽依赖的翻译并刺激增殖[20,22]。

The Ras/MAPK pathway is triggered via translocation of the adapter protein Grb2 with SOS, a Ras-specific guanine nucleotide exchange factor (GEF) that associates with Grb2, to the membrane. In the case of insulin/IGF receptor signalling, this involves IRS1. Once activated, MAPK will enhance p90 ribosomal S6 kinase. Both kinases have a dual role in the activation of mTORC1. First, they have both been reported to phosphorylate and inactivate TSC2 [23,24]. Secondly, they phosphorylate the mTORC1 complex protein Raptor on distinct sites [25,26].

Ras/MAPK 通路是通过适配蛋白 Grb2与 SOS 的转运而触发的,SOS 是一种与 Grb2相关的 ras 特异性鸟苷酸交换因子(GEF)。在胰岛素/胰岛素样生长因子受体信号转导的情况下,这涉及到 IRS1。一旦激活,MAPK 将增强 p90核糖体 S6激酶。这两种激酶在 mTORC1的激活中具有双重作用。首先,他们都被报道磷酸化和灭活 TSC2[23,24]。其次,他们在不同的位点磷酸化 mTORC1复合蛋白 Raptor [25,26]。

3.2. Mammalian target of rapamycin complex 1 regulation by nutrients

3.2. 哺乳动物雷帕霉素复合物靶蛋白1对营养物质的调节

As alluded to above, the mTORC1 pathway integrates growth factor signalling with nutritional status. It is important to note here that the negative regulation of mTORC1 caused by the lack of nutrients is dominant over positive inputs coming from growth factors. Remarkably, nutrient deprivation does not affect signalling components upstream of mTORC1, such as the insulin receptor or PKB. Multiple pathways have been delineated that serve to signal a shortage of one or more nutrients (reviewed in [27]). A lack of amino acids leads to a block of mTORC1 via a mechanism that is distinct from that arising from an insufficiency in carbohydrates [28]. Furthermore, various pathways often cooperate to fine-tune the response to a given type of nutrient. A major mechanism via which amino acid availability results in mTORC1 activity involves the heterodimeric Rag GTPases (RagAB/CD) [29]. The Rag GTPases are localized on lysosomes/Rab7 positive vesicles via their interaction with the so-called Ragulator complex (p18–p14–MP1–HBXIP–C7Orf59 complex), which has GEF activity towards RagA/B [30,31]. If cells are starved of amino acids, RagA/B is in its GDP-bound state and RagC/D is in its GTP-bound state, which results in cytoplasmic localization of mTORC1. When cells are replenished with amino acids, RagC/D becomes GDP-bound and RagA/B becomes GTP-bound. This will target mTOR to lysosomes via the GTP-dependent interaction of RagA/B with Raptor. The co-localization with Rheb, which also resides at lysosomes, then results in mTORC1 activation. It should be kept in mind that other proteins, such as VPS34 and MAP4K3, are also stimulated by amino acids and required for mTORC1 activity [3235].

如上所述,mTORC1通路将生长因子信号与营养状况结合在一起。值得注意的是,营养素缺乏引起的 mTORC1的负调节在来自生长因子的正投入中占主导地位。值得注意的是,缺乏营养并不影响 mTORC1上游的信号组件,如胰岛素受体或 PKB。多种途径已经被描述出来,用来表明一种或多种营养素的短缺(参见文献27)。缺乏氨基酸通过一种与碳水化合物不足不同的机制导致 mTORC1的阻滞。此外,各种途径往往相互协作,微调对特定营养类型的反应。氨基酸可利用性导致 mTORC1活性的一个主要机制涉及异二聚体 Rag GTPases (RagAB/CD)[29]。Rag GTPases 通过与所谓的 Ragulator 复合物(p18-p14-MP1-HBXIP-C7Orf59复合物)相互作用定位于溶酶体/rab7阳性囊泡上,该复合物对 RagA/B 具有 GEF 活性[30,31]。当细胞缺乏氨基酸时,RagA/B 处于 gdp 结合状态,RagC/D 处于 gtp 结合状态,导致 mTORC1细胞定位。当细胞补充氨基酸后,RagC/D 与 gdp 结合,RagA/B 与 gtp 结合。这将通过 RagA/B 与 Raptor 之间依赖于 gtp 的相互作用将 mTOR 作用于溶酶体。它与同样位于溶酶体的 Rheb 共定位,然后导致 mTORC1的激活。应该记住的是,其他蛋白质,如 VPS34和 MAP4K3,也受到氨基酸的刺激,需要 mTORC1活性[32-35]。

The AMP-activated protein kinase (AMPK) signals carbohydrate insufficiency to mTORC1. Glucose deprivation, interference in glycolysis by addition of 2-deoxy-D-glucose or mitochondrial inhibition with e.g. rotenone, all decrease mTORC1 activity by lowering the ATP concentration in a cell [28,36]. A decline in cellular ATP levels and associated rise in cellular AMP levels lead to the activation of AMPK. Activated AMPK stimulates catabolic processes, such as the uptake and metabolism of glucose and fatty acids, and inhibits anabolic processes, such as the synthesis of fatty acids, glycogen, cholesterol and protein synthesis (reviewed in [37,38]). The activated kinase inhibits these processes by phosphorylating multiple target proteins. Among these is TSC2, whose phosphorylation results in lower Rheb-GTP levels, and consequently suppression of mTORC1 activity [3941]. The observation that cells lacking TSC2 remain partially sensitive to energy stress suggested that AMPK has additional substrates in the mTORC1 pathway. Indeed, the mTORC1 complex member Raptor has been identified as another AMPK substrate [42]. AMPK directly phosphorylates Raptor in an LKB1-dependent manner leading to the binding of 14-3-3 proteins to Raptor and subsequent inhibition of mTORC1. A number of studies report that interfering in mitochondrial activity leads to a tighter interaction of Raptor and mTOR and a decrease in mTORC1 activity [43,44]. Although the precise mechanism behind this interaction is not resolved, it may reflect an effect of AMPK-mediated Raptor phosphorylation and subsequent conformational change in the complex.

AMP活化蛋白激酶蛋白激酶(AMPK)向 mTORC1发出糖供应不足的信号。葡萄糖剥夺、2- 脱氧 d- 葡萄糖干扰糖酵解或线粒体抑制,如鱼藤酮,都通过降低细胞内 ATP 浓度降低 mTORC1活性[28,36]。细胞 ATP 水平的下降和与之相关的细胞 AMP 水平的上升导致 AMPK 的激活。激活 AMPK 刺激分解代谢过程,如葡萄糖和脂肪酸的摄取和代谢,并抑制合成代谢过程,如脂肪酸,糖原,胆固醇和蛋白质合成(审查[37,38])。激活的激酶通过磷酸化多个靶蛋白来抑制这些过程。其中 TSC2,其磷酸化导致较低的 Rheb-GTP 水平,从而抑制 mTORC1活性[39-41]。缺乏 TSC2的细胞对能量胁迫部分敏感,提示 AMPK 在 mTORC1途径中有额外的底物。事实上,mTORC1复合物成员猛禽已被确定为另一个 AMPK 底物[42]。AMPK 以 lkb1依赖的方式直接磷酸化猛禽,导致14-3-3蛋白结合猛禽并随后抑制 mTORC1。许多研究报道,线粒体活性的干扰导致了 Raptor 和 mTOR 之间更紧密的相互作用和 mTORC1活性的降低[43,44]。虽然这种相互作用背后的确切机制尚不清楚,但它可能反映了 ampk 介导的 Raptor 磷酸化作用以及复合体中随后的构象改变的影响。

Inhibition of mTORC1 lowers protein translation, which is a major energy-consuming process in the cell. Thus, AMPK lowers the ATP demand in cells via the inhibition of mTORC1. If the energy balance is not restored, energy stress will lead to the induction of autophagy. This process, in which cellular components are included in double-membrane vesicles (autophagosomes), which subsequently fuse with lysosomes, serves to generate sufficient metabolites. Here, activation of AMPK and inhibition of mTOR act in concert to stimulate the autophagy initiating kinase Ulk1 (reviewed in [45,46]). When nutrients are sufficient, mTORC1 phosphorylates Ulk1 and thereby prevents its interaction with AMPK. However, a decrease in mTORC1 activity upon starvation allows AMPK to interact with Ulk1 and stimulate it by phosphorylation. Under energy stress both Ulk1 and AMPK negative cells are impaired in their autophagy response. The importance of this response is seen in cells that are devoid of Ulk1 or contain Ulk1 mutant protein that cannot be phosphorylated by AMPK. These cells are more susceptible to apoptosis upon nutrient deprivation [47]. A similar effect is seen when TSC2−/− cells are deprived of glucose [36]. These cells are highly dependent on glucose for their survival. However, mTORC1 inhibition with rapamycin during glucose deprivation prolongs survival of these cells through an OXPHOS-dependent mechanism. This requires that glutamine is present as an energy source, which is converted to glutamate via glutamine dehydrogenase to fuel the TCA cycle. Thus, rapamycin limits ATP usage to a level that can be sustained. In summary, AMPK and mTORC1 are sensors of the energy status of a cell and together form a switch that has control over the use of anabolic versus catabolic processes in a cell.

抑制 mTORC1降低蛋白质翻译,这是细胞中一个主要的能量消耗过程。因此,AMPK 通过抑制 mTORC1降低细胞对 ATP 的需求。如果能量平衡没有恢复,能量压力将导致自噬的诱导。在这个过程中,细胞成分被包含在双膜囊泡(自噬体)中,这些囊泡随后与溶酶体融合,产生足够的代谢物。在这里,AMPK 的激活和 mTOR 的抑制协同作用刺激自噬启动激酶 Ulk1(见文献[45,46])。当营养素充足时,mTORC1磷酸化 Ulk1,从而阻止其与 AMPK 的相互作用。然而,在饥饿状态下 mTORC1活性的降低使 AMPK 与 Ulk1相互作用并通过磷酸化来刺激它。在能量胁迫下,Ulk1和 AMPK 阴性细胞的自噬反应都受损。这种反应的重要性在缺乏 Ulk1或含有不能被 AMPK 磷酸化的 Ulk1突变蛋白的细胞中可见。这些细胞在缺乏营养时更容易发生凋亡[47]。当 TSC2-/-细胞被剥夺葡萄糖时,也可以看到类似的效果。这些细胞的生存高度依赖于葡萄糖。然而,在葡萄糖剥夺期间,雷帕霉素抑制 mTORC1通过 oxphos 依赖机制延长这些细胞的存活时间。这就需要谷氨酰胺作为能量来源,通过谷氨酰胺脱氢酶转化为谷氨酸,为 TCA 循环提供能量。因此,雷帕霉素将 ATP 的使用限制在一个可以持续的水平。总之,AMPK 和 mTORC1是细胞能量状态的传感器,一起形成一个开关,控制细胞中合成代谢和分解代谢过程的使用。

3.3. Mammalian target of rapamycin complex 1 regulation by oxygen levels

3.3. 哺乳动物雷帕霉素复合物靶蛋白1对氧水平的调节

Hypoxia leads to a change in the rate of metabolism in order to decrease ATP consumption and subsequently reduce the cellular oxygen demand to maintain metabolic homoeostasis. Hypoxia regulates mTORC1 activity via three different mechanisms. First, hypoxia leads to a decline in ATP levels and accumulation of AMP. This inhibits mTORC1 signalling via AMPK-induced activation of TSC2 and phosphorylation of Raptor as discussed above. Second, the mitochondrial pro-apoptotic proteins BNIP3 and BNIP3L (also known as Nix) are involved in the rapid inhibitory effect of hypoxia on mTORC1. BNIP3 and BNIP3L interact with Rheb and decrease GTP levels on Rheb, thereby inactivating mTORC1 [48]. Finally, under hypoxic conditions prolyl hydroxylation of hypoxia-inducible factor (HIF)1α by the prolyl hydroxylase domain proteins (PHD1–3) is inhibited. This prevents the marking of HIF1α for degradation by the von Hippel–Lindau tumour suppressor protein and leads to stabilization of HIF1α. The induction of HIF1α levels in cells exposed to hypoxic conditions is mTORC1-dependent and can be reversed by treatment of the cells with rapamycin [49,50]. Stabilized HIF1α binds to the constitutively expressed HIF1β to form an active HIF transcription complex, which induces the expression of REDD1 and REDD2. It has been shown that REDD1 and 14-3-3 proteins compete to bind to TSC2. Relieving the inhibitory action of 14-3-3 proteins on TSC2 leads to the stabilization of the TSC complex and subsequent inhibition of mTORC1 activity. This decrease in mTORC1 activity generates a negative feedback response by inducing the degradation of HIF1α resulting in the normalization of HIF1α levels in order to stop the acute response to hypoxia. REDD1 can inhibit mTORC1 signalling even in the presence of constitutive active PKB, indicating that the response of cells to hypoxic conditions overrides the response to mitogenic stimuli [51,52].

低氧导致代谢速率的改变,以减少 ATP 的消耗,从而降低细胞氧需求,以维持代谢稳定。缺氧通过三种不同的机制调节 mTORC1的活性。首先,缺氧导致 ATP 水平下降和 AMP 的积累。这抑制 mTORC1信号通过 ampk 诱导的 TSC2活化和磷酸化的猛禽如上所述。其次,线粒体促凋亡蛋白 BNIP3和 BNIP3L (又称 Nix)参与了缺氧对 mTORC1的快速抑制作用。BNIP3和 BNIP3L 与 Rheb 相互作用,降低 Rheb 的 GTP 水平,从而失活 mTORC1[48]。最后,在低氧条件下,低氧诱导因子(HIF)1的 prolyl 羟化结构域蛋白(PHD1-3)被抑制。这就防止了 Hippel-林道肿瘤抑制蛋白降解的 hif1标记,并导致了 hif1的稳定。在缺氧条件下,诱导细胞中的 hif1水平是 mtorc1依赖性的,可以通过雷帕霉素治疗细胞逆转。稳定的 hif1与组成性表达的 hif1结合,形成一个活跃的 HIF 转录复合物,诱导 reddit 1和 reddit 2的表达。已有研究表明,reddit 1和14-3-3蛋白竞争结合 TSC2。解除14-3-3蛋白对 TSC2的抑制作用,可使 TSC 复合物稳定化,继而抑制 mTORC1的活性。这种 mTORC1活性的下降通过诱导低氧诱导因子1的降解产生负反馈反应,导致低氧诱导因子1水平的正常化,从而停止对缺氧的急性反应。即使在组成型活性 PKB 存在的情况下,reddit 1也能抑制 mTORC1信号,这表明细胞对低氧条件的反应超过了对有丝分裂刺激的反应[51,52]。

4. Mitochondria as a target for mammalian target of rapamycin complex 1 signalling

4. 线粒体作为哺乳动物雷帕霉素复合物1信号转导靶标

mTORC1 increases cellular mass via its effect on multiple processes such as protein translation, ribosome biogenesis and mitochondrial biogenesis. With respect to mitochondrial function, mTORC1 appears to play a role at multiple levels. Processes that involve mTORC1 are mitochondrial biogenesis, direct regulation of mitochondrial proteins, regulation of uptake and utilization of carbohydrates and regulation of mitophagy. We discuss these processes in detail below.

mTORC1通过影响蛋白质翻译、核糖体生成和线粒体生成等多种过程,增加细胞质量。就线粒体功能而言,mTORC1似乎在多个水平发挥作用。涉及 mTORC1的过程是线粒体生物发生,直接调节线粒体蛋白质,调节碳水化合物的摄取和利用,调节食欲。我们将在下面详细讨论这些过程。

4.1. Mammalian target of rapamycin complex 1 activates transcription of genes for mitochondrial biogenesis

4.1. 哺乳动物雷帕霉素复合物靶蛋白1激活线粒体生物发生基因的转录

mTORC1 activity is important for stimulation of transcription of genes involved in mitochondrial biogenesis. One of the master regulators controlling these genes is the peroxisome-proliferator-activated receptor coactivator-1α (PGC-1α) [53]. Long-term inhibition of mTORC1 with rapamycin was found to decrease PGC-1α-mediated gene transcription in muscle cells in vitro. As a result, mitochondrial DNA content and oxygen consumption were lowered. An opposite effect was seen in TSC2−/− cells, where mTORC1 activity is constitutively high. mTORC1 appears to mediate its effect via the yin-yang 1 (YY1) transcription factor. YY1 can associate with the scaffolding protein Raptor in mTORC1 and is a direct substrate for mTOR. Phosphorylation by mTORC1 enhances the interaction between YY1 and PGC-1α, which stimulates the association of PGC-1α with mitochondrial genes [54]. In line with this finding, muscle-specific deletion of YY1 in mice results in decreased mitochondrial protein content, decreased OXPHOS and eventual exercise intolerance [55]. Overexpression of PGC-1α in muscle appears to be sufficient to increase OXPHOS capacity and leads to improved insulin sensitivity during ageing, further demonstrating the importance of the PGC-1α/YY1 complex for mitochondrial function [56]. The stimulatory effect of mTORC1 on PGC-1α/YY1-induced transcription is also evident following muscle-specific deletion of mTOR [57] or Raptor [58]. In both cases, progressive muscular dystrophy is seen with altered mitochondrial morphology and a decrease in oxidative capacity.

mTORC1活性是促进线粒体生物发生相关基因转录的重要因素。控制这些基因的主要调节因子之一是过氧化物酶体增殖物激活受体辅激活因子 -1(pgc-1)[53]。雷帕霉素长期抑制 mTORC1可以降低 pgc-1介导的肌肉细胞基因转录。因此,线粒体脱氧核糖核酸的含量和耗氧量都降低了。在 TSC2-/-细胞中观察到相反的效果,mTORC1的活性持续地高。mTORC1似乎通过阴阳1(YY1)转录因子介导其作用。YY1可与 mTORC1中的鹰架蛋白 Raptor 结合,是 mTOR 的直接底物。mTORC1的磷酸化增强了 YY1和 pgc-1之间的相互作用,促进了 pgc-1和线粒体基因的关联[54]。与这一发现相一致的是,YY1在小鼠肌肉中的特异性缺失导致线粒体蛋白质含量降低,OXPHOS 减少,最终导致运动耐受不良[55]。肌肉中 pgc-1的过度表达似乎足以提高 OXPHOS 能力,并导致老化过程中胰岛素敏感性的改善,进一步证明了 pgc-1/y1复合物对线粒体功能的重要性[56]。mTORC1对 pgc-1/yy1诱导的转录的刺激作用在肌肉特异性 mTOR [57]或 Raptor [58]缺失后也有明显的表现。在这两种情况下,进行性的肌肉萎缩症可以看到线粒体形态的改变和氧化能力的下降。

Although the studies above hint at a relatively simple and linear pathway from mTOR via PGC-1α/YY1 towards mitochondrial biogenesis, other studies demonstrate tissue-specific effects and more complex interactions between components of the mTORC1 pathway and mitochondrial biogenesis (table 1). For example, deletion of Raptor in mature adipocytes results in lean mice with a reduction in size and number of adipocytes, but little or no effect on the mass of other tissues [61]. The leanness is most likely to result from increased expression of the mitochondrial uncoupling protein UCP1 in white adipose tissue (WAT), rather than from reduced food intake, lipolysis or physical activity. These mice are protected from diet-induced obesity and maintain better insulin sensitivity. In WAT, as well as in muscle, PKB signalling is enhanced, demonstrating non-cell autonomous effects. Increased PKB activity is most likely due to the disruption of the negative feedback loop from S6K1 to IRS1. Furthermore, while studies in cell lines indicate that the mTORC1 target S6K1 was not involved in PGC-1α/YY1-induced mitochondrial gene transcription [44,54], studies in mice show that whole-body deletion of S6K1 causes a profound increase in the mitochondrial content of skeletal muscle and adipocytes. This is accompanied by increased expression of PGC-1α and mitochondrial genes including those for uncoupling proteins (UCP1 and UCP3) [62]. Mice lacking S6K1 are protected from high-fat diet-induced obesity, which is at least partially explained by increased OXPHOS of triglycerides. Remarkably, loss of S6K1 in skeletal muscle or other tissues results in energy stress as evident from increased levels of AMP and increased AMPK activity [59]. Under energy stress, perhaps induced by high levels of UCP1, increased AMPK rather than mTORC1 activity appears to promote the increase in mitochondrial mass and elevated PGC-1α expression in S6K1-deficient skeletal muscle [60].

虽然上述研究提示 mTOR 通过 pgc-1/YY1通向线粒体生物发生有一个相对简单和线性的途径,但其他研究表明 mTORC1途径的组成部分与线粒体生物发生之间具有组织特异性效应和更复杂的相互作用(表1)。例如,成熟脂肪细胞中 Raptor 的缺失导致瘦老鼠的体型和脂肪细胞数量减少,但对其他组织的质量影响很小或没有影响[61]。瘦肉最有可能是由于线粒体解偶联蛋白 UCP1在白色脂肪组织(WAT)中的表达增加,而不是由于食物摄入量减少、脂肪分解或体力活动减少。这些小鼠受到保护,免受饮食诱导的肥胖,并保持较好的胰岛素敏感性。在 WAT,以及在肌肉,PKB 信号增强,证明非细胞自主效应。PKB 活性的增加很可能是由于从 S6K1到 IRS1的负反馈回路的中断。此外,对细胞系的研究表明,mTORC1靶点 S6K1不参与 pgc-1/yy1诱导的线粒体基因转录[44,54] ,而对小鼠的研究表明,S6K1的全身缺失导致骨骼肌和脂肪细胞的线粒体含量显著增加。这是伴随着 pgc-1和包括解偶联蛋白(UCP1和 UCP3)在内的线粒体基因表达增加[62]。缺乏 S6K1的小鼠可以避免高脂肪饮食诱导的肥胖,这至少可以部分解释为甘油三酯 OXPHOS 增加。值得注意的是,骨骼肌或其他组织中 S6K1的丢失导致能量应激,这与 AMP 水平升高和 AMPK 活性升高有关[59]。在能量胁迫下,可能是由于 UCP1水平升高,AMPK 活性增加而不是 mTORC1活性增加,促进了 s6k1缺陷骨骼肌线粒体质量的增加和 pgc-1表达的增加[60]。

Table 1.
表一

Overview of effects of mTORC1 and downstream effectors on mitochondrial biogenesis. For each study, the cell/tissue type, the target and the method used to interfere with the target are summarized as well as the effects observed. MEFs, mouse embryonic fibroblasts.

mTORC1及其下游效应子对线粒体生物发生的影响。对于每项研究,细胞/组织的类型,目标和干扰目标的方法,以及观察到的影响进行了总结。小鼠胚胎成纤维细胞。View inline 内联查看View popup 查看弹出窗口

4.2. Mammalian target of rapamycin complex 1 regulates mitochondrial activity via phosphorylation of mitochondrial proteins

4.2. 哺乳动物雷帕霉素复合物靶蛋白1通过线粒体蛋白磷酸化调节线粒体活性

The second level at which mTORC1 may affect mitochondrial function is via direct modification of mitochondrial proteins. This hypothesis is supported by findings that mTORC1 can associate with mitochondria and by studies with rapamycin [44,63,64]. Using a cell sorting approach, Schieke et al. [44] found a positive correlation between mTORC1 activity, Δψm, maximal oxidative capacity and cellular ATP levels in Jurkat cells. Blocking mTORC1 by a 12-h rapamycin treatment lowers Δψm and maximal oxidative capacity without a measurable effect on mitochondrial mass. Interestingly, phosphorylation of many mitochondrial proteins is diminished, but the functional significance of this has not been established. Ramanathan & Schreiber [64] used a shorter rapamycin treatment to exclude transcriptional effects. They found that rapamycin lowers mitochondrial respiration and induces a shift from OXPHOS towards glycolysis as measured by metabolic profiling. They propose that this occurs through a mechanism that involves the interaction of mTOR with the voltage-dependent anion-selective channel (VDAC1) and the anti-apoptotic mitochondrial transmembrane protein Bcl-XL. VDAC1 is located in the OMM where it regulates the Δψm as a component of the mitochondrial permeability transition pore. Via phosphorylation by mTOR, Bcl-XL may stimulate the permeability of VDAC1 for TCA substrates. Strikingly, the effect of rapamycin can be mimicked with a Bcl inhibitor [64]. However, regulation of VDAC1 is complex and also PKB has been shown to regulate this channel in a glucose- and hexokinase-dependent manner [65].

mTORC1可能影响线粒体功能的第二个层次是通过直接修饰线粒体蛋白。mTORC1与线粒体相关的发现和雷帕霉素的研究支持了这一假说。通过细胞分类的方法,Schieke 等人发现 Jurkat 细胞中 mTORC1活性、 m、最大氧化能力和细胞 ATP 水平之间存在正相关。用12小时雷帕霉素处理阻断 mTORC1可降低 m 和最大氧化能力,但对线粒体质量没有明显影响。有趣的是,许多线粒体蛋白质的磷酸化作用减弱了,但其功能意义尚未确定。Ramanathan & Schreiber [64]使用了较短的雷帕霉素处理来排除转录效应。他们发现,雷帕霉素降低线粒体呼吸,并诱导从 OXPHOS 向糖酵解的转变,这是通过代谢轮廓来衡量的。他们提出,这是通过一个涉及 mTOR 与电压依赖性阴离子选择性通道(VDAC1)和抗凋亡线粒体跨膜蛋白 Bcl-XL 相互作用的机制发生的。VDAC1位于线粒体膜上,作为线粒体通透性转换孔的一个组成部分,它调节着膜的表达。Bcl-XL 可能通过 mTOR 的磷酸化作用,刺激 TCA 基质 VDAC1的通透性。引人注目的是,雷帕霉素的作用可以与 Bcl 抑制剂[64]相似。然而,VDAC1的调节是复杂的,而且 PKB 已被证明以葡萄糖和己糖激酶依赖的方式调节这个通道[65]。

4.3. Mammalian target of rapamycin complex 1 is involved in balancing glycolytic flux with mitochondrial respiration

4.3. 哺乳动物雷帕霉素靶蛋白1参与平衡糖酵解通量和线粒体呼吸

To sustain cell growth, mTORC1 affects mitochondrial function via regulation of uptake and utilization of carbohydrates. As part of the PI3K/PKB signalling network, mTORC1 can stimulate glucose uptake via increasing the presence and/or activity of the glucose transporter Glut1 on the plasma membrane [6668]. Importantly, proliferating cells use glucose not only for ATP production but also to provide intermediates for the synthesis of lipids and nucleic acids. To this end, rapidly proliferating cells exhibit enhanced aerobic glycolysis and reduced OXPHOS, a phenomenon that in tumour cells is known as the Warburg effect (reviewed in [69]). This means that cells prevent the conversion of pyruvate derived from glycolysis into acetyl-CoA in mitochondria. Using microarray analysis and metabolomics approaches, mTORC1 has been shown to increase glycolytic flux via HIF1α-mediated transcription of glycolytic genes [67]. Among these upregulated genes is pyruvate dehydrogenase kinase 1 (PDHK1), which phosphorylates mitochondrial pyruvate dehydrogenase and thereby inactivates the pyruvate dehydrogenase complex. As a consequence, pyruvate is metabolized to lactate instead of acetyl-CoA. Other mechanisms such as phosphorylation of PDHK1 and pyruvate kinase-M2 (PKM2) and hydroxylation of PKM2 [70] also contribute to the shunting of pyruvate into lactate [71,72]. The increased glycolytic flux arising from increased glucose uptake and/or increased expression of glycolytic genes may have a feed-forward effect on mTORC1 activity. It has been reported that the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) can bind Rheb and thereby sequester Rheb away from mTORC1 [7375]. When the glycolytic flux is high, increased levels of glyceraldehyde-3-phosphate will release Rheb from GAPDH, thus activating mTORC1. During tumorigenesis, activation of PI3K/mTORC1 signalling by activation of oncogenes or loss of tumour suppressor genes can also lead to induction of aerobic glycolysis. As a consequence, tumorigenic cells can survive better during phases of insufficient oxygen supply that they are likely to experience in developing tumours and during which glycolysis is further stimulated via stabilization of HIF1α (reviewed in [76]).

为了维持细胞生长,mTORC1通过调节碳水化合物的摄取和利用影响线粒体功能。作为 PI3K/PKB 信号网络的一部分,mTORC1可以通过增加质膜葡萄糖转运蛋白 Glut1的存在和/或活性来刺激葡萄糖摄取。重要的是,增殖细胞不仅利用葡萄糖产生 ATP,而且还为脂类和核酸的合成提供中间体。为此,快速增殖的细胞表现出增强的有氧糖酵解和降低的 OXPHOS,这种现象在肿瘤细胞中被称为沃伯格效应(见[69])。这意味着细胞阻止了来自糖酵解的丙酮酸转化为线粒体中的乙酰辅酶 a。利用微阵列分析和代谢组学方法,mTORC1已被证明能通过 hif1介导的糖酵解基因转录增加糖酵解通量[67]。在这些上调的基因中有丙酮酸脱氢酶激酶1(PDHK1) ,它能磷酸化线粒体丙酮酸脱氢酶,从而抑制丙酮酸脱氢酶复合体。因此,丙酮酸代谢成乳酸而不是乙酰辅酶 a。其他机制如 PDHK1和丙酮酸激酶 m2(PKM2)的磷酸化和 PKM2的羟化也有助于丙酮酸向乳酸分流[71,72]。葡萄糖摄取增加和/或糖酵解基因表达增加引起的糖酵解通量增加可能对 mTORC1活性有前馈作用。有报道糖酵解酶甘油醛 -3- 磷酸脱氢酶(GAPDH)能结合 Rheb,从而将 Rheb 从 mTORC1中隔离出来[73-75]。当糖酵解通量较高时,增加的甘油醛 -3- 磷酸会从 GAPDH 释放出 Rheb,从而激活 mTORC1。在肿瘤发生过程中,通过激活原癌基因或丢失抑癌基因激活 PI3K/mTORC1信号通路也可导致有氧糖酵解的诱导。因此,致瘤细胞可以在氧气供应不足的阶段更好地存活,这个阶段可能会发展成肿瘤,并且在此期间通过 hif1的稳定进一步刺激糖酵解(参见[76])。

Owing to the export of citrate from mitochondria for fatty acid synthesis, cells under the Warburg effect need to replenish their TCA intermediates (anaplerosis). Glutamine is a major source for this in proliferating cells. This occurs via glutaminolysis to form α-ketoglutarate, which after conversion to oxaloacetate is used to restore citrate levels. Intriguingly, the process of glutaminolysis activates mTORC1 via the Rag GTPases and may explain the addiction of tumour cells to glutamine [77,78].

由于柠檬酸从线粒体输出到脂肪酸合成,在沃伯格效应下的细胞需要补充 TCA 中间体。谷氨酰胺是增殖细胞中的主要来源。这通过谷氨酰胺分解形成-酮戊二酸,转化为草酰乙酸后用于恢复柠檬酸水平。有趣的是,谷氨酰胺溶解过程通过 Rag GTPases 激活 mTORC1,并可能解释肿瘤细胞对谷氨酰胺的依赖。

4.4. Mammalian target of rapamycin complex 1 and mitophagy

4.4. 雷帕霉素复合物1哺乳动物靶蛋白与噬菌体

Mitophagy can be part of a developmental programme, such as terminal differentiation of erythrocytes, or be induced by specific conditions, such as hypoxia. In addition, damaged mitochondria are also selectively degraded under normoxic conditions, which prevents accumulation of damaged mitochondria that may cause cell death. Interference in this process may, for example, lead to neuronal degeneration [79]. Selective degradation of depolarized mitochondria involves their recognition by BNIP3 and BNIP3L, which both interact with the autophagosomal membrane protein LC3. BNIP3 and BNIP3L protein levels are upregulated in a HIF1α-dependent manner under hypoxic conditions [80] and during terminal erythroid differentiation [81], suggesting that upregulation of these proteins can trigger mitophagy. BNIP3L prepares mitochondria for autophagic degradation by controlling mitochondrial localization of the E3 ubiquitin ligase Parkin upon depolarization of mitochondria [82]. Translocation of Parkin also depends on the PTEN-induced putative kinase PINK1. PINK1 is imported into all mitochondria where it is maintained at low levels by degradation. If mitochondrial function is impaired, PINK1 is stabilized and accumulates on the OMM of depolarized mitochondria to recruit Parkin [83]. Parkin ubiquitinates mitochondrial proteins, including VDAC1, that serve as a recognition mark for the autophagic machinery [83,84]. Ubiquitinated mitochondrial proteins are bound by p62 (also known as sequestosome 1), which is a multi-functional protein that also interacts with LC3 and thereby promotes association with autophagosomal membranes (figure 2) [85]. Other ubiquitin-binding proteins, such as HDAC6 and NBR1, may also be involved in this process [86,87]. Much of our understanding of the molecular mechanisms of mitophagy has come from studies with CCCP, a mitochondrial uncoupling agent that acutely affects the entire population of mitochondria. Studying mitophagy under more physiological conditions is hard (reviewed in [4]). However, it is reassuring that novel mass spectrometry approaches do support a role for PINK1 and Parkin in mitophagy in vivo in Drosophila [88].

吞噬细胞可能是发育过程的一部分,例如红细胞的终末分化,或者是特定条件的诱导,例如缺氧。此外,在常氧条件下,受损的线粒体也会选择性地降解,从而防止受损线粒体的积累,这可能导致细胞死亡。例如,这个过程中的干扰可能会导致神经元的退化[79]。去极化线粒体的选择性降解涉及到 BNIP3和 BNIP3L 对它们的识别,这两者都与自噬体膜蛋白 LC3相互作用。BNIP3和 BNIP3L 蛋白水平在低氧条件下[80]和红细胞分化末期[81]以 hif1依赖的方式上调,提示这些蛋白的上调可以触发噬细胞。BNIP3L 通过控制线粒体去极化过程中 E3泛素连接酶的线粒体定位,为自噬降解准备线粒体[82]。Parkin 的转运也依赖于 pten 诱导的可能的激酶 PINK1。PINK1进入所有的线粒体,通过降解维持在低水平。如果线粒体功能受损,PINK1稳定并积累在去极化线粒体的 OMM 上,以恢复 Parkin [83]。Parkin 泛素化包括 VDAC1在内的线粒体蛋白,作为自噬机制的识别标记[83,84]。泛素化线粒体蛋白是由 p62(也称为序列体1)结合,这是一种多功能蛋白,也与 LC3相互作用,从而促进自噬体膜(图2)[85]。其他泛素结合蛋白,如 HDAC6和 NBR1,也可能参与这一过程[86,87]。我们对于食肉的分子机制的理解大多来自于对 cpcp 的研究,ccp 是一种线粒体解偶联剂,它对线粒体的整个种群产生了剧烈的影响。在更多的生理条件下研究吞噬细胞是困难的。然而,令人欣慰的是,新的质谱法方法确实支持 PINK1和 Parkin 在果蝇体内的噬菌体中的作用。

Figure 2.
Figure 2. Mitochondrial priming in mitophagy. PINK1 accumulation on the outer mitochondrial membrane recruits Parkin to the mitochondria. In addition, BNIP3 and BNIP3L on the OMM also recruit Parkin to the mitochondria. In turn, Parkin will ubiquitinate mitochondrial proteins, such as VDAC1. Ubiquitinated proteins are recognized by p62, which interacts with LC3 on autophagosomes. In addition, BNIP3 and BNIP3L also interact with the autophagosomal protein LC3, thereby inducing association with the autophagosomal membrane. The increase in ROS production induced by BNIP3 and BNIP3L via the depolarization of ΔΨm will inhibit mTORC1 activity. In addition, the inactivation of Rheb by BNIP3 and BNIP3L will inhibit mTORC1 activity. MM, mitochondrial matrix; IMM, inner mitochondrial membrane; IMS, intermembrane space; OMM, outer mitochondrial membrane; ROS, reactive oxygen species.图2。线粒体启动在食肉动物中的作用。在线粒体外膜上的 PINK1积累使 Parkin 成为线粒体的新成员。此外,OMM 上的 BNIP3和 BNIP3L 也能促进 Parkin 参与线粒体的分化。反过来,Parkin 将普遍存在线粒体蛋白,如 VDAC1。泛素化蛋白被 p62识别,p62通过自噬体与 LC3相互作用。此外,BNIP3和 BNIP3L 还与自噬体蛋白 LC3相互作用,从而诱导与自噬体膜的联系。BNIP3和 BNIP3L 通过 m 的去极化增加 ROS 的产生,抑制 mTORC1的活性。此外,BNIP3和 BNIP3L 对大黄酸的失活也会抑制 mTORC1的活性。线粒体基质; IMM,线粒体内膜; IMS,膜间隙; OMM,线粒体外膜; ROS,活性氧类。

As a specialized form of autophagy, mitophagy requires suppression of mTORC1 activity and activation of Ulk1 [47]. As indicated above, BNIP3 and BNIP3L prevent mTORC1 activation by directly binding to and inactivating Rheb [48]. Whether p62 also has a role in mTORC1 inactivation is less clear. Upon amino acid stimulation, p62 activates mTORC1 by interacting with Raptor and the Rag GTPases at lysosomes [89]. Translocation of p62 from lysosomes to mitochondria may thus lead to inhibition of mTOR. However, p62 has multiple interaction partners and the relation of p62 and mTORC1 in the process of autophagy is complex [46].

作为一种特殊形式的自噬,吞噬需要抑制 mTORC1的活性和 Ulk1的激活[47]。如上所述,BNIP3和 BNIP3L 通过直接结合和失活 Rheb 防止 mTORC1的激活[48]。P62是否也参与 mTORC1的失活尚不清楚。在氨基酸刺激下,p62通过与 Raptor 和溶酶体上 Rag GTPases 相互作用激活 mTORC1[89]。因此,p62从溶酶体向线粒体移位可能导致 mTOR 的抑制。然而,p62具有多个相互作用伙伴,p62和 mTORC1在自噬过程中的关系很复杂[46]。

A recent paper shows increased mitophagy when mammalian cells are shifted from glucose-containing medium to glucose-free medium supplemented with glutamine [90]. This change in medium results in a shift from glycolysis to OXPHOS without an alteration of mitochondrial mass, which is accompanied by a selective increase in mitochondrial protein turnover. When the mTOR activator Rheb is transiently overexpressed, a decrease in mitochondrial mass is seen in both types of medium. This is accompanied by an increase in autophagy markers, including BNIP3L, and induces maturation of LC3 that localizes to mitochondria. Also, Rheb is detected at the OMM and can be co-immunoprecipitated with LC3 and BNIP3L. Together, these data show that increased OXPHOS results in a Rheb/BNIP3L-mediated mitochondrial renewal that prevents accumulation of damaged mitochondria. As expected, Rheb stimulates mTORC1, indicating that inhibition of mTORC1 is not a prerequisite for mitophagy under all circumstances. This is consistent with findings in a number of tumour cell lines where high levels of autophagy are found in the presence of active mTOR signalling [91]. A possible explanation may be that under nutrient-limiting conditions ROS are generated that directly activate autophagy via Atg4-mediated processing of LC3 [92].

最近的一篇论文表明,当哺乳动物细胞从含葡萄糖的培养基转移到含谷氨酰胺的无葡萄糖培养基时,吞噬能力增强。培养基中的这种变化导致了从糖酵解向 OXPHOS 的转变,而没有线粒体质量的改变,伴随着线粒体蛋白质周转的选择性增加。当 mTOR 激活剂 Rheb 短暂过度表达时,两种培养基中线粒体质量均下降。这是伴随着自噬标记的增加,包括 BNIP3L,并诱导 LC3成熟,定位于线粒体。同时,在 OMM 处检测到流变血红蛋白,可与 LC3和 BNIP3L 共同免疫沉淀。总之,这些数据表明增加 OXPHOS 导致 rheb/bnip3l 介导的线粒体更新,防止受损线粒体积累。正如所料,Rheb 刺激 mTORC1,这表明在任何情况下抑制 mTORC1都不是吞噬细胞的先决条件。这与许多肿瘤细胞系的研究结果一致,这些肿瘤细胞系存在高水平的自噬现象,并且存在活跃的 mTOR 信号传导[91]。一个可能的解释是,在营养限制条件下,活性氧通过 atg4介导的 LC3[92]的处理直接激活自噬。

5. Feedback of mitochondrial function on mammalian target of rapamycin complex 1 activity

5. 线粒体功能对雷帕霉素复合物1活性的反馈作用

Homoeostasis depends on feedback systems that allow for balanced adaptive changes. This is also seen in the case of mTOR and mitochondrial function. Under favourable conditions, mTOR promotes mitochondrial biogenesis via transcriptional regulation. However, the increase in mitochondrial mass should be adjusted to nutrient availability in order to prevent malfunctioning of mitochondria. Likewise, starvation responses should lead to a diminished number of mitochondria, but complete removal of mitochondria would be detrimental to a cell. Signalling pathways that coordinate these processes are only partially elucidated and may affect mTOR itself or other elements of the mTOR pathway. Below we briefly discuss recent insights into two mechanisms that involve mitochondrial signalling to mTORC1, namely ROS and the UPRmt.

稳态依赖于允许平衡的适应性变化的反馈系统。这在 mTOR 和线粒体功能的情况下也可以看到。在有利的条件下,mTOR 通过转录调控促进线粒体的生物合成。然而,线粒体质量的增加应该调整到营养的可用性,以防止线粒体的故障。同样,饥饿反应应该导致线粒体数量减少,但完全去除线粒体将不利于细胞。协调这些过程的信号通路只得到部分阐明,并可能影响 mTOR 本身或 mTOR 通路的其他元素。下面我们将简要讨论最近对涉及线粒体向 mTORC1发送信号的两种机制的见解,即 ROS 和 UPRmt。

5.1. Mammalian target of rapamycin complex 1 activity is regulated by reactive oxygen species

5.1. 哺乳动物雷帕霉素复合物靶蛋白1的活性受活性氧类调节

Mitochondrial respiration is a major source for generation of ROS by complexes I, II and III in the mitochondrial electron transport chain (mETC) [93]. The effects of ROS on proteins include oxidation of cysteines and carbonylation. The cellular effects of ROS are complex with dose and cell-type-dependent aspects. Low levels of ROS in a cell can stimulate growth factor-mediated signalling, whereas higher levels induce cell cycle arrest or cell death. Enhanced ROS production is seen under various conditions and context-dependent mechanisms are used to downregulate ROS itself and/or ROS production [94]. Although many effects of ROS on mTORC1 activity are cell-type dependent, there are indications that the effect of ROS on mTORC1 is concentration dependent. Activity of mTORC1 is induced by low levels of ROS, while mild and high levels inhibit mTORC1 activity [95]. ROS can activate mTORC1 via oxidation of cysteine groups. This is seen following treatment of cells with compounds such as phenylarsene oxide (PAO), that specifically induce disulfide bonds. Reducing agents have the opposite effect [96]. ROS and agents like PAO may either oxidize cysteine groups in the mTORC1 complex itself [96] or act at upstream levels, for example the TSC1/2 complex [97]. Mechanisms by which mild and high levels of ROS inactivate mTORC1 are stimulus and cell-type dependent. ROS, generated by mitochondrial depolarization with CCCP, inhibit mTORC1 in an N-acetylcysteine-dependent manner, suggesting an involvement of cysteine oxidation [82]. Recently, astrin has been implicated as a mediator of oxidative stress-induced mTORC1 inhibition. Oxidative stress induced by arsenite but also other stresses, for example heat shock, enhance the association of astrin with raptor and consequently diminish the amount of mTOR-bound Raptor [98]. The astrin–raptor complex translocates to stress granules, which are non-membrane-bound cytoplasmic compartments [99]. This results in a decreased mTORC1 activity and can have an anti-apoptotic effect in cancer cells. Reactivation of mTORC1 after resolution of stress can occur by the release of Raptor from stress granules. The dual specificity kinase DYRK3 both helps in dissolving stress granules and, in addition, has a more direct role in mTORC1 activation via phosphorylation of PRAS40 [100]. Most probably, different stresses induce stress granule assembly via distinct molecular mechanisms. How ROS induce stress granules is not exactly known, but it may involve halted translation via phosphorylation of the initiation factor eIF2. Indeed, indications for such a scenario have been documented in Caenorhabditis elegans (see below).

线粒体呼吸是一个主要来源的复合体产生活性氧的 i,II 和 III 在线粒体电子传递链。ROS 对蛋白质的影响包括半胱氨酸的氧化和羰基化。活性氧的细胞效应是复杂的,具有剂量和细胞类型依赖性。细胞中低水平的活性氧可以刺激生长因子介导的信号传导,而高水平的活性氧则会导致细胞周期阻滞或细胞死亡。在各种条件下,活性氧的产生得到了增强,并且使用了依赖环境的机制来下调活性氧本身和/或活性氧的产生[94]。尽管 ROS 对 mTORC1活性的许多影响是细胞类型依赖性的,但有迹象表明 ROS 对 mTORC1的影响是浓度依赖性的。mTORC1的活性由低水平的活性氧诱导,而轻度和高水平抑制 mTORC1的活性[95]。ROS 可以通过半胱氨酸基团的氧化激活 mTORC1。这可以在用化合物如苯胂氧化物(PAO)治疗细胞后看到,该化合物特异性地诱导二硫键。还原剂具有相反的效果[96]。ROS 和 PAO 等试剂可以氧化 mTORC1络合物本身的半胱氨酸基团,也可以作用于上游,例如 TSC1/2络合物[97]。活性氧轻度和高水平抑制 mTORC1的机制是刺激和细胞型依赖。线粒体去极化产生的活性氧,以 n- 乙酰半胱氨酸依赖的方式抑制 mTORC1,提示半胱氨酸氧化[82]。近年来,天冬氨酸被认为是氧化应激诱导的 mTORC1抑制的介导者。由亚砷酸盐引起的氧化应激,还有其他的应力,例如热冲击,增强了天体素与猛禽的结合,从而减少了受 mtor- 束缚的猛禽的数量[98]。星形-猛禽复合体位于应力颗粒上,为非膜结合的胞质室[99]。这导致 mTORC1活性下降,并能在癌细胞中产生抗凋亡作用。解除应力后的 mTORC1可以通过应力颗粒释放 Raptor 而重新活化。双特异性激酶 DYRK3不仅有助于溶解应力颗粒,而且通过磷酸化的 PRAS40[100]在 mTORC1活化中有更直接的作用。最有可能的是,不同的应力通过不同的分子机制诱导应力颗粒组装。ROS 如何诱导应激颗粒尚不清楚,但它可能涉及通过磷酸化的起始因子 eIF2来阻止翻译。事实上,这种情况的迹象已经在秀丽隐桿线虫中被记录下来了。

It should be noted that various other mechanisms have been reported that cells employ to inhibit mTORC1 after ROS challenge. First, ROS activate stress-activated kinases, for example JNK, that inactivate PI3K signalling via inhibitory phosphorylation of IRS1 [101]. Second, ROS can activate AMPK. ROS can do so via mitochondrial depolarization, which increases levels of AMP. Alternatively, this can occur in an AMP-independent manner [95,102], which, for example, takes place under hypoxic conditions [103].

值得注意的是,已有多种其他机制的报道表明,细胞在 ROS 攻击后抑制 mTORC1。首先,ROS 激活应激激活的激酶,例如 JNK,通过抑制 IRS1的磷酸化使 PI3K 信号通路失活[101]。其次,ROS 可以激活 AMPK。活性氧可以通过线粒体去极化来达到这一目的,去极化可以增加 AMP 的水平。或者,这可以发生在一个 amp 独立的方式[95,102] ,例如,发生在低氧条件下[103]。

The inactivation of mTORC1 by ROS represents a feedback mechanism, because it can result in a reduction of the number of mitochondria via mitophagy, and thus prevent further increases in ROS formation [82]. In a similar regulatory mechanism in yeast, nitrogen starvation promotes ROS-induced mitophagy, keeping the number of mitochondria to a minimum to meet energy requirements and simultaneously prevent the production of excess ROS [104].

ROS 对 mTORC1的失活代表了一种反馈机制,因为它可以通过吞噬作用减少线粒体的数量,从而阻止 ROS 形成的进一步增加[82]。在酵母中类似的调节机制中,氮饥饿促进了 ROS 诱导的吞噬作用,使线粒体的数量保持在最低水平以满足能量需求,同时防止产生过量的 ROS [104]。

It should be kept in mind that the inactivation of mTORC1 by ROS is only part of an integral programme by which cells prevent excessive damage. Enhanced ROS production is also counteracted by the antioxidant pathway consisting of JNK and FOXO that leads to upregulation of manganese superoxide dismutase (MnSOD), which converts H2O2 to H2O and O2, [105]. Another important transcription factor is NRF2, which controls antioxidant genes such as γ-glutamyl-cysteine ligase, involved in glutathione production. In unstressed cells, NRF2 is degraded following ubiquitination by the E3 ligase KEAP1. Oxidative stress modifies KEAP1 leading to stabilization of NRF2 and its nuclear accumulation [106]. Interestingly, mTORC1 can stimulate NRF2 via direct phosphorylation as a feedback mechanism to protect cells against acute oxidative stress [107].

应当牢记,ROS 对 mTORC1的失活只是细胞防止过度损伤的整体方案的一部分。提高活性氧的产生也被 JNK 和 FOXO 的抗氧化途径所抵消,后者导致锰超氧化物歧化酶的上调,将 H2O2转化为 H2O 和 O2,[105]。另一个重要的转录因子是 NRF2,它控制抗氧化基因,如谷氨酰半胱氨酸连接酶,参与谷胱甘肽的生产。在非应激细胞中,E3连接酶 KEAP1使 NRF2泛素化后降解。氧化应激修饰 KEAP1,从而稳定 NRF2及其核聚集[106]。有趣的是,mTORC1可以通过直接磷酸化作为反馈机制刺激 NRF2来保护细胞免受急性氧化应激的侵袭。

5.2. Mitochondrial unfolded protein response and mammalian target of rapamycin complex 1

5.2. 线粒体未折叠蛋白反应和哺乳动物雷帕霉素复合物靶蛋白1

Various mitochondrial stress situations can lead to accumulation of misfolded proteins in mitochondria, which triggers the UPRmt. The UPR is a well-characterized stress response in eukaryotic cells that relies on molecular chaperones. These chaperones prevent aggregation and promote efficient (re)folding and assembly of newly synthesized and stress denatured proteins. Furthermore, they can also assist in the degradation of irreversibly misfolded or misassembled proteins (reviewed in [6]). A number of genes involved have been identified in an RNAi screen for suppressors of the UPRmt in C. elegans [108111]. The results suggest a model in which proteolysis of mitochondrial proteins and export of the resulting degradation products induce nuclear accumulation of a transcriptional complex consisting of UBL-5 and DVE-1. In addition, diminished import of a basic leucine zipper transcription factor, named ATFS-1, into mitochondria allows ATFS-1 to transfer to the nucleus [112]. Together, these proteins enhance transcription of mitochondrial chaperones and restore protein folding. Interestingly, the RNAi screen also identified the orthologue of Rheb. The exact function of Rheb in this pathway is not clear and initially Rheb and TOR were suggested to act as negative regulators of the nuclear distribution of DVE-1/UBL-5 upon mitochondrial stress [110]. More recent data indicate that repression of cytosolic translation by inhibition of Rheb or mTOR may prevent the induction of mitochondrial stress [108]. This is based on the finding that ROS generated as a consequence of mitochondrial stress slows translation via phosphorylation of the eukaryotic translation factor eIF2α. This decreased translation results in a concomitant diminished requirement for protein folding via a reduction of the import of mETC components [108]. Thus, ROS-mediated inhibition of translation via eIF2α acts in parallel to the UPRmt to protect cells against mitochondrial dysfunction. It is probable that inhibition of mTOR acts in a similar fashion and by inhibition of translation lowers the UPRmt.

不同的线粒体应激状态可以导致线粒体中错误折叠的蛋白质积累,从而触发 UPRmt。UPR 是真核细胞应激反应的一个重要特征,它依赖于分子伴侣。这些分子伴侣阻止聚集,促进新合成和应力变性蛋白质的高效折叠和组装。此外,它们还可以帮助降解不可逆地错误折叠或错误组装的蛋白质(见[6])。在针对秀丽隐杆线虫 UPRmt 抑制子的 rna 干扰筛选中,已经确定了一些相关基因。这些结果表明,线粒体蛋白质的蛋白质水解和产生的降解产物的出口诱导核聚集的转录复合物组成的 UBL-5和 DVE-1模式。此外,减少进口的碱性亮氨酸拉链转录因子,命名 ATFS-1,到线粒体允许 ATFS-1转移到细胞核[112]。同时,这些蛋白质增强线粒体伴侣的转录和恢复蛋白质折叠。有趣的是,rna 干扰筛选也确定了 Rheb 的直系同源序列。Rheb 在这一通路中的确切功能尚不清楚,初步认为 Rheb 和 TOR 可能是线粒体应激时 DVE-1/UBL-5核分布的负性调节因子[110]。最近的数据表明,抑制胞质翻译的 Rheb 或 mTOR 可能会阻止线粒体应激的诱导[108]。这是基于这样的发现: 由于线粒体压力而产生的活性氧通过磷酸化真核翻译因子 eif2来减缓转化。这种减少的翻译导致同时减少需要蛋白质折叠的进口 mETC 组分[108]。因此,ros 介导的通过 eif2的翻译抑制作用与 UPRmt 平行,以保护细胞免受线粒体功能障碍。可能 mTOR 的抑制作用与此类似,通过抑制翻译可以降低 UPRmt。

The mammalian UPRmt appears to operate in a fashion similar to the one described for C. elegans, although fewer components of this signalling pathway have been described. The transcription factor that has been identified as a crucial element is a bZIP protein, CHOP, that heterodimerizes with C/EBPβ [113]. Together, these proteins drive the upregulation of mitochondrial stress response proteins, such as the chaperonin Hsp60 and the MM protease ClpP [113,114]. In addition, a separate stress response in the IMS acts to increase CHOP expression [115]. IMS stress also results in ROS production but does not cause a reduction in Δψm. ROS activate PKB, which in turn activates the oestrogen receptor ERα leading to upregulation of the IMS protease Htra2 and the transcription factor NRF1.

哺乳动物的 UPRmt 似乎以一种与线虫相似的方式运作,尽管这种信号传导途径的成分较少被描述。被确定为关键元件的转录因子是 bZIP 蛋白 CHOP,它与 c/ebp [113]异二聚。这些蛋白共同驱动线粒体应激反应蛋白的上调,如伴侣蛋白 Hsp60和 MM 蛋白酶 ClpP [113,114]。此外,IMS 中的单独应激反应会增加 CHOP 表达[115]。IMS 的压力也会导致活性氧的产生,但不会导致 m 的减少。活性氧激活蛋白激酶 PKB,这反过来激活雌激素受体内质网,导致 IMS 蛋白酶 Htra2和转录因子 NRF1的上调。

Even though the signalling components of the mammalian UPRmt are incompletely characterized, the importance of UPRmt for health is apparent. The UPRmt resulting from a nuclear–mitochondrial protein imbalance (i.e. the stoichiometric balance between nuclear and mitochondrial-encoded proteins) promotes longevity in mice and C. elegans[116]. The nuclear–mitochondrial protein imbalance can result from mutation of mitochondrial ribosomal genes or structural components of the ETC, but can also be induced by various drugs. For example, induction of UPRmt occurs following an increase in cytoplasmic nicotinamide adenine dinucleotide levels in C. elegans, where it promotes longevity [117]. The fact that UPRmt-dependent longevity can be seen in long-lived mutants from the insulin pathway is indicative of separate pathways. However, rapamycin treatment of worms was found to induce UPRmt and the resulting longevity depends on genes functioning in the UPRmt. These findings are reminiscent of studies in Drosophila, where a link between longevity and translational control of certain nuclear-encoded mitochondrial proteins is documented. Here, dietary restriction results in diminished TOR activity and upregulation of 4E-BP. This specifically enhances translation of complexes I and IV proteins that are important in mediating the lifespan extension [118]. It will be interesting to see whether inducing UPRmt or modulation of mitochondrial quality by other means can be employed to promote healthy ageing.

尽管哺乳动物的信号组成部分 UPRmt 是不完全的特点,UPRmt 的重要性是显而易见的健康。由于核线粒体蛋白质失衡(即核蛋白质和线粒体编码蛋白质之间的化学计量平衡)而导致的 UPRmt 促进了小鼠和秀丽隐杆线虫的寿命[116]。线粒体核-线粒体蛋白失衡可能是由于线粒体核糖体基因突变或 ETC 结构成分突变引起的,也可能是由多种药物引起的。例如,当秀丽隐杆线虫细胞质内烟酰胺腺嘌呤二核苷酸水平升高时,就会诱导紫外线修饰酶,从而延长寿命[117]。事实上,uprmt 依赖的长寿可见于来自胰岛素途径的长寿突变体,这表明存在不同的通路。然而,雷帕霉素治疗蠕虫被发现诱导 UPRmt 和由此产生的寿命取决于基因功能的 UPRmt。这些发现让人想起果蝇的研究,在果蝇的研究中,长寿和某些核编码的线粒体蛋白质的翻译控制之间的联系被记录下来。在这里,饮食限制导致 TOR 活性下降和4E-BP 的上调。这特别地增强了复合物 i 和 IV 蛋白质的翻译,这些蛋白质在延长寿命中起着重要作用[118]。看看诱导 UPRmt 或通过其他方式调节线粒体质量是否可以用来促进健康老龄化将是有趣的。

6. Conclusion and perspective

6. 总结和展望

Defective mitochondrial homoeostasis can result in tissue-specific effects, eventually causing cancer, neurodegenerative diseases, for example Parkinson’s disease, or muscle syndromes, for example dystonia [79]. In this review, we describe how mTOR contributes to mitochondrial homoeostasis (figure 3). mTOR can stimulate mitochondrial biogenesis and function. Simultaneously, mTOR is subjected to the various outputs of mitochondria, including ATP, ROS and metabolic intermediates. In all cases, mTOR functions in complex, cell-type-specific networks with numerous feedback systems. Recent developments reveal that Rheb, the well-known activator of mTOR, functions independently in the control of mitochondrial turnover [90]. Astrin has been identified as a mediator of ROS-induced translocation of Raptor to stress granules [98], while DYRK3 was shown to function in the reactivation following stress [100]. Classical forward genetic screens in C. elegans are continuing to identify novel proteins that function in mitochondrial homoeostasis by either acting in the UPRmt or modifying this response [108]. With rapid technical developments in metabolic profiling, cell culture systems using patient-derived cells and techniques to visualize ROS inside cells we will hopefully reach a more complete understanding of the reciprocal signalling of mTORC1 and mitochondria and be able to offer new prospects for treatment of mitochondria-related diseases.

线粒体内环境稳定性缺陷可导致组织特异性效应,最终导致癌症、神经退行性疾病(如帕金森氏症)或肌肉综合征(如肌张力障碍)。在这篇综述中,我们描述了 mTOR 如何促进线粒体内环境稳定性(图3)。mTOR 能刺激线粒体的生物发生和功能。同时,mTOR 受制于线粒体的各种输出,包括 ATP、 ROS 和代谢中间产物。在所有情况下,mTOR 功能复杂,细胞类型具体的网络与众多的反馈系统。最近的发展表明,Rheb,著名的 mTOR 激活剂,在控制线粒体周转中独立发挥作用[90]。[98] ,DYRK3在应激后的再激活中发挥作用[100]。经典的秀丽隐杆线虫正在继续鉴定新的蛋白质,这些蛋白质在线粒体内环境稳定性中起作用,它们要么在 UPRmt 中起作用,要么通过修饰这种反应[108]。随着新陈代谢分析技术的迅速发展,利用病人来源的细胞和可视化细胞内活性氧的技术的细胞培养系统,我们有希望对 mTORC1和线粒体的相互信号传导有更全面的了解,并能为治疗线粒体相关疾病提供新的前景。

Figure 3.
Figure 3. Integrated regulation of mTORC1 activity and mitochondrial function. Growth factors stimulate mTORC1 activity via the inactivation of TSC1/TSC2 complex. Amino acids stimulate mTORC1 activity via the Rag GTPases. Hypoxia inhibits mTORC1 activity via HIF1α stabilization and subsequent REDD1/2 transcription. Energy stress inhibits mTORC1 activity via AMPK. mTORC1 activity regulates mitochondrial homoeostasis via four targets; Glut1, YY1/PGC-1α, Ulk1 and VDAC1. Mitochondrial function regulates mTORC1 by feedback mechanisms. Mitochondrial function regulates HIF1α, AMPK, BNIP3(L) and ROS, which all impact on mTORC1 activity. Low levels of ROS stimulate mTORC1 activity and mild to high levels of ROS inhibit mTORC1 activity. This dual regulation is indicated with a dot. Arrows indicate stimulatory effects. Cross bars indicate inhibitory effects.图3。mTORC1活性与线粒体功能的整合调控。生长因子通过 TSC1/TSC2复合物的失活刺激 mTORC1的活性。氨基酸通过 Rag GTPases 刺激 mTORC1的活性。缺氧通过 hif1稳定和随后的 reddit 1/2转录抑制 mTORC1活性。能量胁迫通过 AMPK 抑制 mTORC1活性。mTORC1通过4个靶点调节线粒体内稳态: Glut1、 y1/pgc-1、 Ulk1和 VDAC1。线粒体功能通过反馈机制调节 mTORC1。线粒体功能调节 hif1、 AMPK、 BNIP3(l)和 ROS,这些都对 mTORC1活性有影响。低水平的活性氧刺激 mTORC1的活性,轻微到高水平的活性氧抑制 mTORC1的活性。这种双重调节是用一个点来表示的。箭头表示刺激效果。交叉酒吧表明抑制作用。

发表评论

您的电子邮箱地址不会被公开。 必填项已用*标注