作为寿命、衰老和细胞衰老调控因子的研究进展

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mTOR as Regulator of Lifespan, Aging, and Cellular Senescence: A Mini-ReviewmTOR

Abstract

摘要

The mechanistic target of rapamycin (mTOR) network is an evolutionary conserved signaling hub that senses and integrates environmental and intracellular nutrient and growth factor signals to coordinate basic cellular and organismal responses such as cell growth, proliferation, apoptosis, and inflammation depending on the individual cell and tissue. A growing list of evidence suggests that mTOR signaling influences longevity and aging. Inhibition of the mTOR complex 1 (mTORC1) with rapamycin is currently the only known pharmacological treatment that increases lifespan in all model organisms studied. This review discusses the potential mechanisms how mTOR signaling controls lifespan and influences aging-related processes such as cellular senescence, metabolism, and stem cell function. Understanding these processes might provide novel therapeutic approaches to influence longevity and aging-related diseases.

雷帕霉素(mTOR)网络是一个进化保守的信号枢纽,它感知和整合环境和细胞内的营养和生长因子信号,以协调基本的细胞和生物体的反应,如细胞生长、增殖、凋亡和炎症,取决于个别细胞和组织。越来越多的证据表明 mTOR 信号影响长寿和衰老。雷帕霉素对 mTOR 复合物1(mTORC1)的抑制作用是目前唯一已知的药物治疗方法,可以延长所有研究的模式生物的寿命。本文综述了 mTOR 信号通路如何控制寿命并影响细胞衰老、代谢和干细胞功能等衰老相关过程的潜在机制。了解这些过程可能会提供新的治疗方法,以影响长寿和老化相关疾病。


Introduction

引言

The mechanistic (formerly mammalian) target of rapamycin (mTOR) is an evolutionary conserved serine-threonine kinase that senses and integrates a diverse set of environmental and intracellular signals, such as growth factors and nutrients to direct cellular and organismal responses [1]. The name TOR (target of rapamycin) is derived from its inhibitor rapamycin, which was initially isolated in the 1970s from a soil bacterium on Rapa Nui (Easter Island). Rapamycin, also known as sirolimus, forms a complex with FK506-binding protein 12 (FKBP12) and in this form inhibits the activity of mTOR. Rapamycin was first described as an antifungal drug and used to inhibit the growth of yeast, but was later found to potently decrease proliferation of T lymphocytes [1]. We now know that the role of mTOR goes far beyond proliferation and coordinates a cell-tailored metabolic program to control cell growth and many biological processes including aging, cellular senescence, and lifespan. Rapamycin is currently the only known pharmacological substance to prolong lifespan in all studied model organisms and the only one in mammals. This review focuses on the role of the mTOR network in aging-related processes and discusses underlying molecular mechanisms that are controlled by mTOR. We discuss recent studies that may allow us to better understand the clinical consequences of mTOR inhibition in rapamycin-treated individuals.

雷帕霉素(mTOR)的机械性(以前是哺乳动物的)靶蛋白是一种进化上保守的丝氨酸苏氨酸激酶,它能感知并整合一系列不同的环境和细胞内信号,如生长因子和营养物质,引导细胞和有机体的反应[1]。TOR (雷帕霉素的靶)这个名字来源于它的抑制剂雷帕霉素,雷帕霉素最初是在20世纪70年代从 Rapa Nui (复活节岛)的一个土壤细菌中分离出来的。雷帕霉素,也被称为西罗莫司,与 fk506结合蛋白12(FKBP12)形成复合物,并以此形式抑制 mTOR 的活性。雷帕霉素最初被描述为一种抗真菌药物,用于抑制酵母菌的生长,但后来发现雷帕霉素可以有效地减少 t 淋巴细胞的增殖[1]。我们现在知道,mTOR 的作用远远超出了细胞增殖和协调细胞定制的代谢程序,以控制细胞生长和许多生物过程,包括老化,细胞衰老和寿命。雷帕霉素是目前唯一已知的在所有研究的模式生物中延长寿命的药理物质,也是哺乳动物中唯一的一种。本文综述了 mTOR 网络在衰老相关过程中的作用,并讨论了由 mTOR 控制的潜在分子机制。我们讨论最近的研究,可能使我们更好地了解 mTOR 抑制雷帕霉素治疗个人的临床后果。

mTOR Signaling

mTOR 信令

mTOR belongs to the phosphatidylinositol-3 kinases (PI3K)-related kinase (PIKK) family that is present as catalytic subunit in at least two protein complexes: mTOR complex 1 (mTORC1) and mTORC2 [1]. mTORC1 is composed of three core components: mTOR, Raptor (regulatory protein associated with mTOR), and mLST8 (mammalian lethal with Sec13 protein 8). In addition, mTORC1 comprises of DEPTOR (DEP domain containing mTOR interacting protein) and PRAS40 (proline-rich Akt substrate of 40 kDa), which are inhibitory subunits [1]. Numerous growth factor receptors such as the insulin receptor or the epidermal growth factor receptor activate tyrosine kinase adaptor molecules at the cell membrane leading to the recruitment of the class I family of PI3K to the receptor complex (Fig. 1). Following receptor engagement, PI3K phosphorylates phosphatidylinositol 4,5-bisphosphate (PI[3,4,5]P2) to generate phosphatidylinositol-3,4,5-trisphosphate (PI[3,4,5]P3) that recruits and activates the serine-threonine kinase Akt (also known as protein kinase B) via phosphorylation on threonine 308 by phosphoinositide-dependent protein kinase 1 (PDPK1) [2,3]. mTORC2 is also activated by PI3K through PI(3,4,5)P3[4] and phosphorylates Akt on serine 473, which is important for full activation and substrate specificity of Akt [3]. A main target of Akt is tuberous sclerosis 2 (TSC2). TSC2 forms a heterodimeric complex with TSC1 and inhibits mTORC1. Phosphorylation of TSC2 at threonine 1462 (Thr1462) by Akt blocks its GTPase-activating protein (GAP) activity for the small GTPase RAS homologue enriched in brain (Rheb), which therefore remains in a GTP-bound state and activates mTORC1 [1,3]. Thus, class I PI3K activation finally leads to mTORC1 activation through the inhibition of TSC2. Interestingly, the class II PI3K β (PI3KC2β) synthesizes phosphatidylinositol 3,4-bisphosphate (PI[3,4]P2) under growth factor-deprived conditions to inhibit mTORC1 on the lysosome [5].

mTOR 属于磷脂酰肌醇 -3激酶(PI3K)相关激酶(PIKK)家族,至少在 mTOR 复合物1(mTORC1)和 mTORC2[1]中以催化亚基的形式存在。mTORC1由三个核心成分组成: mTOR、 Raptor (与 mTOR 相关的调节蛋白)和 mLST8(哺乳动物致死的 Sec13蛋白8)。另外,mTORC1包括 detor (含 mTOR 相互作用蛋白的 DEP 结构域)和 PRAS40(富含脯氨酸的 Akt 底物,40 kDa) ,它们是抑制亚基[1]。许多生长因子受体,如胰岛素受体或表皮生长因子受体激活细胞膜上的酪氨酸激酶适配体分子,导致 i 类 PI3K 家族到受体复合体的补充(图1)。在受体结合后,PI3K 磷酸化磷脂酰肌醇4,5-二磷酸盐(PI [3,4,5] P2)产生磷脂酰肌醇 -3,4,5-三磷酸(PI [3,4,5] P3) ,通过依赖于肌醇的蛋白激酶1(dpk1)激活苏氨酸308上的磷酸化作用,招募和激活丝氨酸苏氨酸激酶 Akt (又称蛋白激酶 b)。mTORC2也被 PI3K 通过 PI (3,4,5) P3[4]激活,并在丝氨酸473上磷酸化 Akt,这对 Akt 的完全激活和底物特异性很重要。Akt 的主要目标是结节性硬化症2(TSC2)。TSC2与 TSC1形成异二聚体复合物并抑制 mTORC1。Akt 在苏氨酸1462(Thr1462)处磷酸化 TSC2,阻断了脑内富集的小型 GTPase RAS 同源物(Rheb)的 GTPase 激活蛋白(GAP)活性,因此仍处于 gtp 结合状态并激活 mTORC1[1,3]。因此,i 类 PI3K 激活通过抑制 TSC2最终导致 mTORC1的激活。有趣的是,II 类 PI3K β (pi3kc2β)在生长因子剥夺条件下合成磷脂酰肌醇3,4-二磷酸盐(PI [3,4] P2) ,抑制溶酶体上的 mTORC1[5]。

Fig. 1
图一

The mTOR signaling network. For details, see text.

mTOR 信令网络。有关详细信息,请参阅文本。/WebMaterial/ShowPic/928382

mTORC1 activation occurs on cellular organelles such as peroxisomes or lysosomes [6]. The activation of mTOR on the lysosome is best understood: mTORC1 binds to a complex consisting of the v-ATPase, Ragulator, and SLC38A9 [1] (Fig. 1). Our current understanding of full mTORC1 activation requires the presence of main nutrient and energy sources: amino acids, glucose, lipids, oxygen, and a high ATP/AMP ratio [7]. mTORC1 senses amino acid sufficiency, especially leucine and arginine on the lysosome via Ras-related GTPases (Rag) and arginine by SLC38A9. In addition, arginine, independently of Rag family members, inhibits lysosomal localization of TSC2 to stimulate mTORC1 activity [8]. Low levels of glucose-6-phosphate decrease mTORC1 activity by stimulating its binding to hexokinase 2 (HK2). A direct interaction of the lipid molecule phosphatidic acid with mTORC1 is also assumed as precondition for mTORC1 activation. Oxygen deprivation stimulates expression of DDIT4 (DNA damage-inducible transcript 4; also known as REDD1 or RTP801), which inhibits mTORC1 via TSC2 [2]. Thus, the presence of these nutrients and energy is supposed to be required for full mTORC1 activation. However, this concept has been established currently only in heavily transformed cancer cell lines such as HeLa cells and remains to be shown in primary cell types. Thus, it is possible that distinct cells require only specific signals to allow activation of mTORC1. Moreover, during homeostasis under nonproliferating conditions in vivo, most cells do not show active mTORC1 or mTORC2 signaling. This is in stark contrast to in vitro cell culture, where these pathways are always active under normal growth-promoting conditions. While rapamycin through FKBP12 directly inhibits mTORC1, mTORC2 is insensitive to acute rapamycin treatment [9]. Like mTORC1, mTORC2 also contains mTOR and mLST8, but contains Rictor (rapamycin-insensitive companion of mTOR), DEPTOR, as well as the regulatory subunits mSin1 and Protor1/2 [1]. Prolonged rapamycin treatment can abrogate mTORC2 signaling in some cells and in mice in vivo [9,10]; nevertheless the growth-promoting functions of mTOR inhibitors are thought to be mediated by mTORC1.

mTORC1的激活发生在细胞器上,如过氧化物酶体或溶酶体[6]。对溶酶体上 mTOR 的激活最好的理解是: mTORC1与一个由 v-ATPase、 Ragulator 和 SLC38A9组成的复合体结合(图1)。我们目前对 mTORC1完全活化的理解需要主要的营养和能量来源: 氨基酸、葡萄糖、脂类、氧气和高 ATP/AMP 比值[7]。mTORC1通过与 ras 相关的 GTPases (Rag)和 SLC38A9的精氨酸感受氨基酸的充足性,特别是在溶酶体上的亮氨酸和精氨酸。此外,精氨酸独立于 Rag 家族成员,抑制 TSC2的溶酶体定位以刺激 mTORC1的活性[8]。低水平的葡萄糖-6-磷酸通过刺激其与己糖激酶2(HK2)的结合而降低 mTORC1的活性。脂质分子磷脂酸与 mTORC1的直接相互作用也被认为是 mTORC1活化的前提。缺氧刺激 DDIT4(DNA 损伤诱导转录4,也称为 reddit 或 RTP801)的表达,通过 TSC2抑制 mTORC1[2]。因此,这些营养物质和能量的存在被认为是完全活化 mTORC1所必需的。然而,这一概念目前只建立在大量转化的癌细胞系,如 HeLa 细胞,并仍有待于显示在初级细胞类型。因此,有可能不同的细胞只需要特定的信号就可以激活 mTORC1。此外,在非增殖条件下的体内稳态过程中,大多数细胞不表现出活跃的 mTORC1或 mTORC2信号。这与体外细胞培养形成鲜明对比,在正常的促生长条件下,这些通路总是活跃的。而雷帕霉素通过 FKBP12直接抑制 mTORC1,mTORC2对雷帕霉素急性治疗不敏感[9]。和 mTORC1一样,mTORC2也含有 mTOR 和 mLST8,但含有 Rictor (mTOR 对雷帕霉素不敏感的伴侣)、 detor,以及调节亚基 mSin1和 Protor1/2[1]。长期雷帕霉素治疗可以阻断某些细胞和小鼠体内的 mTORC2信号传导[9,10] ; 然而,mTOR 抑制剂的促生长作用被认为是通过 mTORC1介导的。

Identification of the mTOR Network as Lifespan Regulator

mTOR 网络作为寿命调节器的辨识

The first indications that mTOR is a regulator of lifespan stem from experiments with the nematode Caenorhabditis elegans[11] and the fruit fly Drosophila melanogaster[12]. Afterwards, mTOR could also be linked to have a role in controlling lifespan in the yeast strain Saccharomyces cerevisiae [13]. In this simple organism, inhibition of mTOR with rapamycin doubled the lifespan (defined as survival time of nondividing cells). The interest in the mTOR network as regulator of aging and lifespan was strengthened by the finding that rapamycin extents the lifespan of genetically heterogeneous mice at three independent test locations by about 10-18% depending on sex [14]. Interestingly, treatment was only started late when the mice were 600 days of age equivalent to roughly 60 years of age in a human person. This proposes that inhibition of mTOR in the elderly might be enough to prolong life. The findings were confirmed and extended in mice, in which rapamycin treatment started earlier [15]. However, they failed to substantially observe larger effects on longevity. The maximal lifespan extension seems to be dose-dependent [16]. A lot of other reports confirmed mTOR as lifespan regulator in mice [14,17,18,19,20,21]. Deleting the mTORC1 substrate S6K1 similarly increases lifespan, but only in female and not male mice [22]. These findings in total suggest an evolutionary conserved role of mTOR as longevity regulator. The lifespan-enhancing effects of mTOR inhibitors have been linked to mTORC1 inhibition, whereas inhibition of mTORC2 might even be detrimental, because mTORC2 controls insulin-mediated suppression of hepatic gluconeogenesis [10,23].

最初的迹象表明 mTOR 是一种寿命调节剂,这是通过对秀丽隐桿线虫线虫和果蝇黑腹果蝇的实验得出的。之后,mTOR 也可能在控制酵母菌株酿酒酵母的寿命方面发挥作用。在这个简单的生物体中,雷帕霉素对 mTOR 的抑制使其寿命延长了一倍(定义为不分裂细胞的存活时间)。由于发现雷帕霉素在三个独立实验地点延长了遗传异质小鼠的寿命约10-18% ,这加强了人们对 mTOR 网络作为衰老和寿命调节因子的兴趣[14]。有趣的是,只有当老鼠600天的年龄相当于人类大约60岁的时候,治疗才开始得比较晚。这提示老年人抑制 mTOR 可能足以延长寿命。这些发现在小鼠身上得到了证实和延伸,雷帕霉素治疗开始得更早[15]。然而,他们没有实质性地观察到对长寿的更大影响。最大寿命延长似乎是剂量依赖性的[16]。许多其他的报告证实 mTOR 是老鼠寿命调节器[14,17,18,19,20,21]。删除 mTORC1底物 S6K1同样可以延长寿命,但仅限于雌性小鼠而非雄性小鼠[22]。这些发现总体上表明 mTOR 在进化过程中具有长寿调节因子的保守作用。mTORC2抑制剂的延长寿命作用与 mTORC1抑制有关,而 mTORC2抑制甚至可能是有害的,因为 mTORC2控制胰岛素介导的肝糖异生的抑制[10,23]。

mTOR, Lifespan, Aging, and the Mechanisms

mTOR,寿命,衰老和机制

It is now accepted that mTOR inhibition increases lifespan; yet, the mechanism through which this occurs is still uncertain. mTORC1 inhibition may not delay aging itself, but may delay age-related diseases [24,25,26,27]. However, many researchers directly link the longevity effects of mTOR inhibitors to a decrease in aging. Aging is generally characterized by a progressive loss of physiological integrity, which leads to impaired functions, and therefore increases vulnerability to death thus limiting lifespan [28]. Conserved hallmarks of aging have recently been proposed and include telomere attrition, epigenetic alterations, genomic instability, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication [28,29]. The mTOR network is known to regulate some of these aging hallmarks as described below. Ultimately, the prominence of mTORC1 signaling in aging likely reflects its exceptional capacity to regulate such a wide variety of key cellular functions (Fig. 2).

现在人们已经认识到 mTOR 抑制可以延长寿命; 然而,其发生的机制仍然不确定。mTORC1抑制剂可能不会延缓衰老本身,但可能会延缓与年龄有关的疾病[24,25,26,27]。然而,许多研究人员直接将 mTOR 抑制剂的长寿效应与衰老的减少联系起来。衰老通常是生理完整性逐渐丧失的拥有属性,这会导致功能受损,因此增加了死亡的可能性,从而限制了寿命[28]。保守的衰老特征最近被提出,包括端粒磨损,表观遗传改变,基因组不稳定,蛋白质停滞丢失,缺乏营养感知,线粒体功能障碍,细胞衰老,干细胞衰竭,和改变细胞间通讯[28,29]。众所周知,mTOR 网络控制着以下所述的一些老化标志。最终,mTORC1信号在衰老过程中的突出可能反映了其异常的能力来调节如此广泛的关键细胞功能(图2)。

Fig. 2
图二

The role of mTORC1 in longevity and aging. The mechanisms of how mTORC1 regulates longevity and aging.

mTORC1在长寿和衰老中的作用。 mTORC1调节长寿和衰老的机制。/WebMaterial/ShowPic/928381

mRNA Translation

翻译

mTORC1 regulates cap-dependent and cap-independent translation of mRNAs by phosphorylating the translation inhibitors eIF4E-binding protein 1 (4E-BP1) and 4E-BP2, which then release eukaryotic translation initiation factor 4E (eIF4E). mTORC1 is particularly potent in promoting translation initiation of mRNAs containing a 5′ terminal oligopyrimidine tract (5′ TOP) or a pyrimidine-rich translational element (PRTE); many of those encode translation- and ribosomal-related proteins but also metabolism-related genes [30]. In addition, ribosomal protein S6 kinase 1 (S6K1) and S6K2 are two other well-characterized targets of mTORC1-mediated phosphorylation, which subsequently phosphorylate and activate the ribosomal protein S6 to stimulate protein translation [1]. Several studies suggest that a general decrease in mRNA translation caused by mTORC1 inhibition slows aging. Mechanistically, this is explained by reducing the accumulation of proteotoxic and oxidative stress [1,22]. Deletion of the translational regulator S6K1 prolongs lifespan in mammals, although a direct link to altered global translation has not been shown so far [22]. Hence, although counterintuitive at first thought, reducing mRNA translation might permit easier endogenous protein degradation or repair to preserve homeostasis when facing oxidative damage as well as protein aggregation during ageing [31].

mTORC1通过磷酸化翻译抑制剂 eIF4E 结合蛋白1(4E-BP1)和4E-BP2来调节 mrna 的 cap 依赖性和 cap 非依赖性翻译,这些翻译抑制剂随后会释放 eIF4E 真核起始因子4E。mTORC1在促进含有5′末端寡核苷酸(5′ TOP)或富嘧啶翻译元件(PRTE)的 mrna 的翻译起始方面特别有效,其中许多编码翻译和核糖体相关蛋白,但也编码代谢相关基因[30]。此外,核糖体蛋白质 S6激酶1(S6K1)和 S6K2是 mtorc1介导的磷酸化的另外两个特征明显的靶点,它随后磷酸化并激活核糖体蛋白质 S6以刺激蛋白质翻译[1]。一些研究表明,由 mTORC1抑制引起的 mRNA 翻译的普遍减少延缓衰老。从机理上讲,这可以通过减少蛋白毒素和氧化应激的积累来解释。转录调节因子 S6K1的缺失延长了哺乳动物的寿命,尽管到目前为止还没有证据表明这与全局转录改变有直接联系。因此,尽管最初的想法违背直觉,减少信使核糖核酸的翻译可能使内源性蛋白质降解或修复更容易保持体内平衡,当面临氧化损伤以及老化过程中的蛋白质聚集[31]。

Autophagy and Mitochondria

自噬与线粒体

The starvation-induced degradation of cytosolic components known as autophagy is crucial in providing substrates for energy production under catabolic conditions of limited nutrient supply and removal of damaged organelles [32]. mTORC1 controls the Ser/Thr kinase ULK1, which regulates autophagosomal formation and autophagic flux [32]. mTORC1 actively suppresses autophagy by phosphorylating ULK1 and, accordingly, inhibition of mTORC1 induces autophagy. It has been suggested that autophagy might decline with age resulting in the accumulation of damaged proteins and organelles such as mitochondria; however, the underlying reason remains unclear. mTORC1 inhibition could slow aging by stimulating autophagy, which clears old and dysfunctional mitochondria (mitophagy), the accumulation of which is linked to aging and aging-related diseases [1]. Additionally, mTORC1 regulates mitochondrial functions [33]. On the one hand, mTORC1 controls the cellular energy metabolism by increasing glycolytic flux and simultaneously limiting oxidative phosphorylation (a process called aerobic glycolysis or Warburg effect in cancer cells). On the other hand, mTORC1 increases mitochondrial functions and stimulates mitochondrial biogenesis through PGC-1α and the transcription factor YY-1 in some cells [1]. It is important to preserve the function and number of mitochondria during aging, but the cumulative role of mTORC1 in these processes is complex and might depend on the individual tissue.

饥饿诱导的细胞溶质组分自噬的降解是在有限的营养供应和去除受损细胞器的分解代谢条件下提供能量产生的基质的关键。mTORC1控制 Ser/Thr 激酶 ULK1,它调节自噬体的形成和自噬通量[32]。mTORC1通过磷酸化 ULK1积极抑制自噬,从而抑制 mTORC1诱导自噬。有人认为,随着年龄的增长,自噬可能会减少,导致受损的蛋白质和细胞器(如线粒体)的积累; 然而,其根本原因仍不清楚。mTORC1抑制可以通过刺激自噬来延缓衰老,自噬清除老化和功能失调的线粒体(吞噬) ,线粒体的积累与衰老和衰老相关的疾病有关[1]。此外,mTORC1调节线粒体功能[33]。一方面,mTORC1通过增加糖酵解通量和同时限制氧化磷酸化来控制细胞能量代谢(这个过程被称为有氧糖酵解或癌细胞中的 Warburg 效应)。另一方面,mTORC1在某些细胞中通过 pgc-1α 和转录因子 YY-1增加线粒体功能并刺激线粒体生物合成。在衰老过程中保持线粒体的功能和数量是重要的,但 mTORC1在这些过程中的累积作用是复杂的,可能取决于个体组织。

Stem Cell and Immune Function

干细胞与免疫功能

A decline in stem cell number and function might be a critical cause in age-related dysfunction of tissue homeostasis [34]. mTORC1 inhibition may preserve adult stem cell function in various tissues [31]. For example, treatment of old mice with rapamycin enhances intestinal stem cell function indirectly by reducing mTORC1 signaling in Paneth cells, which creates a better intestinal niche for stem cells [35]. In contrast, during calorie restriction, which enhances lifespan (see below), mTORC1 is induced in intestinal stem cells to allow their expansion [36]. Similarly, mTORC1 controls the adaptive transition of quiescent muscle stem cells from G0 arrest to an alert state that is important to respond to injury-induced systemic signals [37].

干细胞数量和功能的下降可能是年龄相关的组织内稳态功能障碍的关键原因[34]。抑制 mTORC1可以保存各种组织中的成体干细胞功能[31]。例如,用雷帕霉素治疗老年小鼠,通过减少 Paneth 细胞中 mTORC1信号通路间接增强肠道干细胞功能,从而为干细胞创造一个更好的肠道生态位。相比之下,在延长寿命的卡路里限制期间(见下文) ,mTORC1在肠道干细胞中被诱导,使其得以扩张[36]。类似地,mTORC1控制静止的肌肉干细胞从 G0停止到警戒状态的适应性转变,这对于应对损伤诱导的全身信号非常重要[37]。

Optimal immune functions are evidently critical during aging for maintaining organismal fitness against pathogens, cancers, or other diseases. mTORC1 has many central and often divergent functions in the innate and adaptive immune system to enhance or limit inflammation or immune responses [2,38,39,40]. Importantly, mTORC1 inhibition is used as immunosuppressive therapy to limit T cell activation and prevent transplant rejection after organ transplantation. In contrast, inhibition of mTORC1 augments CD8+ T cell memory responses that are critical for viral defense [41]. The role and activity of mTORC1 in immune cells during aging is, however, not well studied. One study reported that an age-related decline in hematopoietic stem cell (HSC) function, which may contribute to anemia, poor vaccination, or enhanced tumorigenesis, is associated with an increased mTORC1 activity in HSCs from old mice [42]. Hence, rapamycin restores self-renewal and hematopoiesis of HSCs, which enables effective vaccination of old mice against a lethal challenge with influenza virus [42]. In summary, although mTORC1 inhibition clearly enhances overall lifespan, it may exert positive and negative functions on stem and immune cells that may differentially impact aging.

在老化过程中,最佳的免疫功能显然对维持机体对抗病原体、癌症或其他疾病的适应能力至关重要。mTORC1在先天性和后天免疫系统中有许多中枢和不同的功能,以增强或限制炎症或免疫反应[2,38,39,40]。重要的是,mTORC1抑制剂被用于免疫抑制治疗,以限制 t 细胞活化和预防移植排斥反应后的器官移植。相比之下,mTORC1的抑制增强了 CD8 + t 细胞的记忆反应,而这些反应对病毒防御至关重要[41]。然而,mTORC1在免疫细胞衰老过程中的作用和活性尚未得到很好的研究。一项研究报道,与年龄相关的造血干细胞功能下降,可能导致贫血、疫苗接种不良或增强肿瘤发生,与老年小鼠造血干细胞中 mTORC1活性增加有关[42]。因此,雷帕霉素能够恢复造血干细胞的自我更新和造血功能,这使得老年小鼠能够有效地接种疫苗,对抗流感病毒的致命挑战[42]。总之,虽然 mTORC1抑制明显地提高了整体寿命,但它可能对干细胞和免疫细胞发挥积极和消极的作用,这可能对衰老产生不同的影响。

Cellular Senescence

细胞衰老

Cellular senescence is historically defined as an irreversible cell cycle exit, whilst preserving cell viability [34]. Cellular senescence has been suggested to function as a tumor suppressor mechanism and promotor of tissue remodeling after wounding [34,43]. However, senescent cells may also directly contribute to aging [28,34]. Senescent cells show marked changes in morphology including an enlarged size, irregular cell shape, prominent and sometimes multiple nuclei, accumulation of mitochondrial and lysosomal mass, increased granularity and highly prominent stress fibers that are accompanied by shifts in metabolism and a failure of autophagy. Interestingly, many of these phenotypes are regulated by mTORC1 in various cell types [44,45]. Senescent cells secrete proinflammatory and pro-oxidant signals, which can cause inflammation, and due to a suppression of apoptosis, they occupy key cellular niches [28,46]. Due to these mechanisms, senescent cells steadily accumulate with age and contribute to aging-related diseases and morbidity [34]. Hence, clearance of senescent cells improves aging-related disorders [47].

细胞衰老历来被定义为一个不可逆的细胞周期退出,同时保持细胞活力[34]。细胞衰老被认为是肿瘤抑制机制和促进伤后组织重塑的机制[34,43]。然而,衰老细胞也可能直接促进衰老[28,34]。衰老细胞具有明显的形态学变化,包括体积增大、细胞形态不规则、细胞核突出、有时有多个细胞核、线粒体和溶酶体聚集、细胞粒度增大、应力纤维高度突出,伴有代谢变化和自噬失败。有趣的是,许多这些表型在各种细胞类型中都受到 mTORC1的调控[44,45]。衰老细胞分泌促炎症和促氧化信号,可以引起炎症,由于细胞凋亡的抑制,他们占据关键的细胞龛[28,46]。由于这些机制,衰老细胞随着年龄稳定地积累,并有助于与衰老有关的疾病和发病率[34]。因此,清除衰老细胞可以改善与衰老有关的疾病[47]。

Senescence-Associated Secretory Phenotype Regulated by mTORC1

mTORC1调控衰老相关分泌表型的研究

The secretion of proinflammatory mediators by senescent cells contributes to aging and has been termed senescence-associated secretory phenotype (SASP). Recent data identified a main role of mTORC1 to promote the SASP [48,49]. Rapamycin blunts the proinflammatory phenotype of senescent cells by specifically suppressing translation of the membrane-bound cytokine IL1A [48]. This reduction of IL1A diminishes transcription of inflammatory genes regulated by the proinflammatory transcription factors NF-κB. In parallel, mTOR controls translation of MK2, which in turn phosphorylates the RNA-binding protein ZFP36L1 during senescence. This phosphorylation of ZFP36L1 inhibits its ability to degrade transcripts of numerous SASP components. Thus, rapamycin activates ZFP36L1 to induce SASP component degradation [49]. The reason for mTORC1 activation in senescent cells might result from defects in amino acid and growth factor sensing [50]. Senescent human fibroblasts induced by stress, replicative exhaustion, or oncogene activation, show constitutive mTORC1 activation, which is resistant to serum and amino acid starvation. This is mediated in part by a depolarization of the plasma membrane resulting in a failure to abrogate growth factor signaling. Moreover, increased autophagy provides high amino acid levels to support mTORC1 activation [50]. Additionally, two proteins important for the DNA damage repair, O-6-methylguanine-DNA methyltransferase (MGMT) and N-myc downstream-regulated gene 1 (NDRG1) are negatively regulated by mTORC1 in senescent mice and cells [51]. These results in toto indicate that mTORC1 inhibitors inhibit the SASP through various mutually nonexclusive mechanisms.

衰老细胞分泌促炎性介质促进衰老,被称为衰老相关分泌表型(SASP)。最近的数据表明 mTORC1的主要作用是促进 SASP [48,49]。雷帕霉素通过特异性抑制膜结合细胞因子 IL1A [48]的翻译降低衰老细胞的促炎症表型。IL1A 的减少减少了炎症因子 NF-κB 调节的炎症基因的转录。同时,mTOR 控制 MK2的翻译,这反过来在衰老过程中磷酸化 rna 结合蛋白 ZFP36L1。这种磷酸化的 ZFP36L1抑制其能力降解转录品的许多 SASP 组件。因此,雷帕霉素激活 ZFP36L1诱导 SASP 组分降解[49]。衰老细胞中 mTORC1激活的原因可能是由于缺乏氨基酸和生长因子感应[50]。衰老的人成纤维细胞在应激、复制性衰竭或原癌基因激活的作用下,表现出组成型的 mTORC1激活,对血清和氨基酸饥饿具有抗性。这在一定程度上是由去极化的质膜导致失败,以废除生长因子信号。此外,增加的自噬提供了高水平的氨基酸来支持 mTORC1的活化[50]。另外,在衰老的小鼠和细胞中,对 DNA 损伤修复起重要作用的蛋白质 o-6- 甲基鸟嘌呤-DNA 甲基转移酶(MGMT)和 N-myc 下游调节基因1(NDRG1)受到 mTORC1的负调节[51]。这些结果表明 mTORC1抑制剂通过多种互斥机制抑制 SASP。

Metabolic Reprogramming in Senescence

衰老过程中的代谢重编程

Despite maintaining a nondividing state, senescent cells display a high metabolic rate [52]. Metabolic changes characteristic of replicative senescence often show a shift to glycolytic metabolism away from oxidative phosphorylation (which is also observed in proliferative cells), despite a marked increase in mitochondrial mass and markers of mitochondrial activity. This might stem from a rise in lysosomal pH as a consequence of proton pump failure, which leads to an inability to get rid of damaged organelles such as mitochondria caused by a failure of autophagy. Dysfunctional mitochondria not cleared by autophagy in senescent cells produce reactive oxygen species, which cause cellular damage including DNA damage. mTORC1 has been postulated as main driver of these metabolic changes [1], which are overlapping with the functions described above in the section Autophagy and Mitochondria. Hence, rapamycin treatment prevents metabolic stress and delays cellular senescence.

尽管保持不分裂状态,衰老细胞显示出高代谢率。尽管线粒体质量和线粒体活性标志物明显增加,但复制性衰老特征的代谢变化常常表现为糖酵解代谢的转变,远离氧化磷酸化(在增殖细胞中也可观察到)。这可能源于质子泵失效导致溶酶体 pH 值升高,从而导致无法清除受损细胞器,如自噬失败引起的线粒体。衰老细胞中的自噬不能清除功能障碍的线粒体产生活性氧类,这会导致细胞损伤,包括 DNA 损伤。mTORC1被认为是这些代谢变化的主要驱动因素[1] ,这些代谢变化与上面在自噬和线粒体一节中描述的功能重叠。因此,雷帕霉素治疗可以预防代谢应激和延缓细胞衰老。

Calorie Restriction

卡路里限制

There is one other intervention besides mTOR inhibitors that extends lifespan in a lot of organisms: caloric restriction (CR). CR is defined as a reduction in nutrient intake without malnutrition. Because mTORC1 has an important role in sensing energy, nutrients, and insulin intracellularly and on the organismal level [1,53], this fueled the speculation that the lifespan-enhancing effects of CR are at least partly mediated by decreased mTORC1 signaling. Indeed, CR-like regimens do not additionally lengthen the lifespan of yeast or worms when mTOR is inhibited, indicating overlapping mechanisms [12,13,54]. In contrast, mTORC1 inhibition and CR show additive effects on lifespan in flies [55]. In rhesus monkeys, CR has universal health benefits and but no clear-cut effects on survival [56]. Whether CR prolongs lifespan in human is currently unknown. In some animals including distinct strains of mice, CR does not always correlate with lifespan extension, although it consistently improves health [57]. Most CR studies (as well as mouse experiments in general) are designed in an ad libitum fashion; the animals can eat as much as they want. Hence, these studies may actually analyze more the effects of overfeeding, which is known to promote obesity-associated pathologies in our society [58].

除了 mTOR 抑制剂之外,还有一种干预措施可以延长许多生物体的寿命: 热量限制(CR)。CR 的定义是在没有营养不良的情况下减少营养素的摄入。由于 mTORC1在感知能量、营养物质和胰岛素方面具有重要作用,这使人们进一步推测,认为肌肉萎缩症延长寿命的作用至少部分是通过降低 mTORC1信号传导的。事实上,当 mTOR 被抑制时,类 cr 的方案并没有额外延长酵母或蠕虫的寿命,这表明了重叠机制[12,13,54]。相比之下,mTORC1抑制剂和 CR 对果蝇的寿命表现出加性效应[55]。在恒河猴身上,CR 具有普遍的健康益处,但对生存没有明显的影响。CR 是否能延长人的寿命目前尚不清楚。在一些动物中,包括不同品系的老鼠,CR 并不总是与寿命延长相关,虽然它始终改善健康[57]。大多数 CR 研究(包括一般的老鼠实验)都是随意设计的,动物可以想吃多少就吃多少。因此,这些研究实际上可能更多地分析过度饮食的影响,而过度饮食已知会促进我们社会中与肥胖相关的疾病。

Clinical Use of mTOR Inhibitors

mTOR 抑制剂的临床应用

The finding that inhibition of mTOR prolongs lifespan and postpones onset of age-associated diseases in mammals spurred the interest to develop mTOR inhibitors as drugs to augment human longevity. However, the side effect profiles of mTOR inhibitors are a major cause of concern. Because pharmacological inhibition of mTOR is an FDA-approved clinical principle, there is a wealth of information about the known side effects of mTOR inhibitors (rapalogs) in humans [59,60]. Importantly, the introduction of mTOR inhibitors as a therapeutic regimen generated a new spectrum of side effects that resulted in 20-40% dropout rates in phase III clinical trials [61]. The most common side effects are immunosuppression, hyperglycemia, and dyslipidemia as well as interstitial pneumonitis [59]. Most of these side effects are dosage-dependent and may regress with lower dosage. Because mTOR inhibitors are mostly given in combination with other drugs such as steroids or proliferation inhibitors in already severely diseased patients, preventive mTOR inhibitor therapy in the general healthy population might be more tolerable. Moreover, the immunosuppressive properties of mTOR inhibitors for T cell responses might be particularly strong for graft-reactive alloantigens, but not for pathogen-specific responses such as infections [62]. Indeed, mTOR inhibitors might be future longevity drugs, because a clinical trial in elderly healthy humans receiving the mTORC1 inhibitor everolimus at a very low dose showed safety and even improved immune function [24]. Moreover, alternate dosing regimens may still promote longevity but reduce side effects [63,64].

mTOR 抑制剂能延长哺乳动物的寿命,推迟年龄相关疾病的发生,这一发现促使人们开发 mTOR 抑制剂作为延长人类寿命的药物。然而,mTOR 抑制剂的副作用是一个主要的原因。由于 mTOR 的药理抑制作用是 fda 批准的临床原则,因此关于 mTOR 抑制剂在人体中已知的副作用有大量的信息[59,60]。重要的是,mTOR 抑制剂作为治疗方案的引入产生了一系列新的副作用,导致第三阶段临床试验中20-40% 的退出率[61]。最常见的副作用是免疫抑制、高血糖、血脂异常以及间质性肺病。这些副作用大多数是剂量依赖性的,并可能随着剂量的降低而消退。由于 mTOR 抑制剂大多与其他药物联合使用,如在已经严重患病的患者中使用类固醇或增殖抑制剂,因此在一般健康人群中使用预防性 mTOR 抑制剂治疗可能更容易忍受。此外,mTOR 抑制剂对 t 细胞反应的免疫抑制特性可能对移植物反应性同种抗原特别强,但对于病原体特异性反应,如感染[62]则不然。事实上,mTOR 抑制剂可能是未来的长寿药物,因为在接受非常低剂量的 mTORC1抑制剂依维莫司的老年健康人中进行的临床试验显示了安全性,甚至改善了免疫功能[24]。此外,交替给药方案仍然可以延长寿命,但减少副作用[63,64]。

Outlook

展望

mTOR inhibitors are currently the only known pharmacological intervention that increases lifespan in all experimental animal models tested. This raises the prospect that someday it might be possible to therapeutically increase lifespan, slow aging, or decrease aging-related diseases in humans. Nevertheless, we are currently only at the beginning of that journey. Further knowledge of the actual lifespan-enhancing effects of mTOR inhibition might generate more precise ways to promote longevity. For example, differential isoforms and splice variants exist for the individual mTOR pathway members; their role in normal cellular functions and for aging is currently poorly defined. Because RNA splicing is required for longevity in C. elegans downstream of the TOR pathway, this gap of knowledge should be closed [65]. Moreover, distinct mTORC1 and mTORC2 complexes may exist inside the cell at different locations that perform different functions that may differentially affect longevity [66].

mTOR 抑制剂是目前唯一已知的在所有实验动物模型中延长寿命的药理干预。这就提出了这样的前景: 有朝一日,在人类中,有可能在治疗上延长寿命,延缓衰老,或者减少与衰老有关的疾病。然而,我们目前只是处于这一旅程的开始。进一步了解 mTOR 抑制剂的实际延长寿命的作用可能会产生更精确的延长寿命的方法。例如,不同的亚型和剪接变异体存在于单个 mTOR 通路成员中; 它们在正常细胞功能和衰老中的作用目前还没有明确的定义。由于在 TOR 通路的下游,秀丽隐杆线虫的长寿需要 RNA 剪接,这种知识的鸿沟应该被弥合[65]。此外,不同的 mTORC1和 mTORC2复合物可能存在于细胞内不同的位置,执行不同的功能,可能不同地影响寿命[66]。

In conclusion, defining the exact mechanisms and tissues in which mTOR inhibition promotes longevity and reduces age-related diseases is of scientific and general but also socioeconomic importance.

总之,确定 mTOR 抑制促进长寿和减少与年龄有关的疾病的确切机制和组织具有科学和普遍的重要性,但也具有社会经济意义。

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