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“HACSαβγδφε" |导读： The mammalian (or mechanistic) target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that is known to sense the environmental and cellular nutrition and energy status. Diverse mitogens, growth factors, and nutrients stimulate the activation of the two mTOR complexes mTORC1 and mTORC2 to regulate diverse functions, such as cell growth, proliferation, development, memory, longevity, angiogenesis, autophagy, and innate as well as adaptive immune responses. Dysregulation of the mTOR pathway is frequently observed in various cancers and in genetic disorders, such as tuberous sclerosis complex(TSC) or cystic kidney disease. In this article, I will give an overview of the current understanding of mTOR signaling and its role in diverse tissues and cells. Genetic deletion of specific mTOR pathway proteins in distinct tissues and cells broadened our understanding of the cell-specific roles of mTORC1 and mTORC2. Inhibition of mTOR is an established therapeutic principle in transplantation medicine and in cancers, such as renal cell carcinoma. Pharmacological targeting of both mTOR complexes by novel drugs potentially expand the clinical applicability and efficacy of mTOR inhibition in various disease settings.
mTORC1 controls growth and proliferation by modulating mRNA translation through phosphorylation of the 4E-BP1, 2, and 3 and the S6K1 and 2. More specifically, the three 4E-BPs do not regulate cell size, but they block cell proliferation by inhibiting the translation of messenger RNAs that encode proteins involved in proliferation and cell cycle progression. In T lymphocytes, mTOR controls cell cycle progression from the G1 into S phase in IL-2-stimulated cells. The cyclin-dependent kinase (Cdk) enzymes, when associated with the G1 cyclins D and E, are rate-limiting for the entry into the S phase of the cell cycle. IL-2 activates Cdk by causing the elimination of the Cdk inhibitor protein p27Kip1, a process that is prevented by rapamycin in T cells. Moreover, mTORC2 activates the serum- and glucocorticoid-induced protein kinase-1 (SGK1), which in turn phosphorylates p27Kip1. Once phosphorylated, p27Kip1 is retained in the cytoplasm rendering it incapable of blocking Cdk1 or Cdk2 activation and therefore allowing entry into the cell cycle
Autophagy is a starvation-induced degradation of cytosolic components ranging from individual proteins (microautophagy) to entire organelles (macroautophagy). Autophagy is critical in providing substrates for energy production under conditions of limited nutrient supply. mTORC1 actively suppresses autophagy and, conversely, inhibition of mTORC1 strongly induces autophagy. In S. cerevisiae, TOR-dependent phosphorylation of autophagy-related 13 (Atg13) disrupts the Atg1–Atg13–Atg17 complex that triggers the formation of the autophagosome. The mammalian homologs of yeast Atg13 and Atg1, ATG13 and ULK1, bind to the 200 kDa FAK family kinase-interacting protein (FIP200; a putative ortholog of Atg17) and the mammalianspecifi c component ATG101. mTOR phosphorylates ATG13 and ULK1 to block autophagosome initiation.
As the mTOR pathway is critical for many basic aspects of cell biology, it is not surprising that mTOR also plays a prominent role in development.
For example, embryonic homozygous deletion of mTOR leads to a developmental arrest at E5.5. Moreover, mTOR−/− embryos show a defect in inner cell mass proliferation consistent with an inability to establish embryonic stem cells from mTOR-deficient embryos. The catalytic function of mTOR is critical for the embryonic development as knock-in mice carrying a mutation in the catalytic domain of mTOR die before embryonic day 6.5. Rheb, the essential upstream regulator of mTORC1, is likewise important for embryonic development. Interestingly, in contrast to mTOR or Raptor mutants, the inner cell mass of Rheb−/− embryos differentiate normally. Moreover, embryonic deletion of Rheb in neural progenitor cells （神经前体细胞）abolishes mTORC1 signaling in the developing brain and increases mTORC2 signaling. While embryonic and early postnatal brain development appears grossly normal in these mice, there are defects in myelination. These results suggest that mTORC1 signaling plays a role in selective cellular adaptations but is not decisive for general cellular viability.
TSC is an autosomal dominant disease caused by mutations in either TSC1 or TSC2. TSC is characterized by the presence of benign tumors called hamartomas, which within the brain are known as cortical tubers.
Neurological manifestations in TSC patients include epilepsy, mental retardation, and autistic features. In mice, Tsc2 haploin sufficiency causes aberrant retinogeniculate projections and the TSC2–Rheb–mTOR pathway controls axon guidance in the visual system.
In addition, dorsal root ganglial neurons (DRGs) in the peripheral nervous system activate mTOR following damage to enhance axonal growth capacity ( 42) .
Hence, the mTOR pathway has a central role in axon guidance, regeneration, and growth.
Human embryonic stem cells (hESCs), derived from blastocyst-stage embryos, can undergo long-term self-renewal and have the remarkable ability to differentiate into multiple cell types in the human body. A role for mTOR in these processes has recently been appreciated . mTOR integrates signals from extrinsic pluripotency-supporting factors and represses the transcriptional activities of a subset of developmental and growth inhibitory genes in hESCs. Repression of the developmental genes by mTOR is necessary for the maintenance of hESC pluripotency. A similar mechanism is operative in human amniotic fluid stem cells. On the other hand, it has been proposed that mTOR-mediated activation of S6K1 induces differentiation of pluripotent hESCs ( 48). In that line, mTORC1 activation is detrimental to stem cell maintenance in spermatogonial progenitor cells (SPCs).
mTOR regulates the metabolism, growth and survival of β-cells, the cardinal cells in the pancreas that produce insulin, a hormone that controls the level of glucose in the blood to regulate food intake. Studies in S6K1 knockout mice demonstrate a central positive role of mTOR/S6K1 signaling in β-cell growth and function. Indeed, these mice develop glucose intolerance despite increased insulin sensitivity. This is associated with depletion of the pancreatic insulin content, hypoinsulinemia and reduced β-cell mass suggesting that lack of S6K1 activity impairs β-cell growth and function. In addition, it has been shown that obesity develops in older hypothalamic Tsc1 knockout animals; however, young animals display a prominent gain-of-function β-cell phenotype prior to the onset of obesity. Young hypothalamic Tsc1 knockout animals display improved glycemic control due to mTOR-mediated enhancement of β-cell size and insulin production. Thus, mTOR disseminates a dominant signal to promote β cell/ islet size and insulin production, and this pathway is crucial for β-cell function and glycemic control.
The immune system is a complex network of cells that protect against disease by identifying and killing pathogens and tumor cells, but it is also implicated in homeostatic mechanisms like tissue remodeling and wound healing. A growing body of evidence indicates that in myeloid phagocytes (monocytes, macrophages, and myeloid DC; mDC) mTOR is crucially implicated in TLR signaling and might serve as a decision maker to control the cellular response to pathogens by modulating cytokines, chemokines, and type I interferon responses. Inhibition of mTOR by rapamycin in these cells promotes IL-12 and IL-23 production via the transcription factor NF- κB but blocks the release of IL-10 via Stat3. These results have been confirmed in kidney transplant patients in vivo. The most prominent transcriptional alterations in peripheral blood from rapamycin-treated kidney transplant recipients affect the innate immune cell compartment and hyperactivation of NF- κB-mediated proinfl ammatory pathways. Moreover, kidney transplant patients on rapamycin display an increased inflammatory and immunostimulatory potential of myeloid monocytes and dendritic cells in vivo compared with patients on calcineurin inhibitors. Moreover, rapamycin can augment inflammation and pulmonary injury by enhancing NF- κB activity in the lung of tobacco-exposed mice. In dendritic cells, autophagy facilitates the presentation of endogenous proteins on MHC class I and class II molecules. This leads to the activation of CD4+ T cells and connects autophagy in innate immune cells with enhanced adaptive immune responses. For example, in Mycobacterium tuberculosisinfected DCs, rapamycin-induced autophagy enhances the presentation of mycobacterial antigens.
Plasmacytoid DCs (pDCs) constitute a specialized cell population that produce large amounts of type I interferons (IFN) in response to viral infection via the activation of cytoplasmic receptors or TLRs. The mTOR pathway is important for the regulation of type I IFN production in murine pDCs. Inhibition of mTOR or its downstream mediators S6K1 and S6K2 during pDC activation block the phosphorylation and nuclear translocation of the transcription factor IRF-7, which results in impaired IFN- α and β production. In addition, translation of IRF-7 in pDCs is negatively regulated by the 4E-BP pathway downstream of mTOR. Hence, mTOR via its two downstream effectors, 4E-BP and S6K, controls translation and activation of IRF-7.
Peripheral CD4+ T helper (Th) lymphocytes are critical in regulating immune responses as well as autoimmune and infl ammatory diseases. Upon activation, naïve CD4+ Th cells differentiate into distinct effector subsets depending on the cytokine milieu . Recent data show that mTOR-deficient naïve CD4+ T cells are unable to differentiate into Th1, Th2, and Th17 cells, but preferentially develop into induced regulatory T (Treg) cells, which can potently suppress adaptive immune responses. In line, rapamycin is able to enrich Treg cells in vitro. mTORC2 is important in these processes as Rictor-deficient CD4+ T cells are unable to differentiate into Th2 cells demonstrating that mTORC2 is critical for Th2 differentiation. On the other hand, Rheb-defi cient T cells fail to generate Th17 responses in vitro and in vivo . The role of mTORC1 and mTORC2 for Th1 differentiation is currently under debate.
Another insight how mTOR regulates adaptive immunity in vivo can be deduced from recent experiments showing that mTOR influences the migratory properties of murine CD8+ T lymphocytes and the differentiation of CD8+ memory T cells. The migratory properties of naïve CD8+ T lymphocytes into the lymph nodes crucially depends on the constitutive expression of the chemokine receptor 7 (CCR7) and L -selectin (CD62L), which is controlled by the transcription factor Krüppel-like factor 2 (KLF2). mTOR negatively regulates KLF2, and therefore controls migration of activated CD8+ T lymphocytes in vivo. Memory CD8+ T cells are a critical component of protective immunity, and inducing effective memory T-cell responses is a major goal of vaccines against chronic infections and tumors. Recently, it was demonstrated that rapamycin promotes the generation of memory CD8+ T cells after viral or bacterial infection in vivo. Importantly, mTOR acts cell intrinsically to regulate memory T-cell differentiation.
The molecular and cellular mechanisms that regulate aging are currently under scrutiny because aging is linked to many human diseases.
The TOR pathway is emerging as a key regulator of aging and inhibition of mTOR by rapamycin or genetic deletion has been shown to expand life-span of invertebrates, including yeast, nematodes, and fruit flies. Even more strikingly, rapamycin extends median and maximal life span of male and female mice when feeding began at 600 days of age . Rapamycin is the only pharmacological substance so far, which has been shown to expand life span in a mammal species. Calorie restriction extends life in rhesus monkeys and inhibits mTOR signaling, however the underlying mechanism, how rapamycin exerts its life-extending effects is currently unknown but potentially involves S6K. It should be noted that the concentrations used in these mice (~80 ng/ml) were about ten times higher than the concentrations, which are currently used in human transplantation medicine.
In 1999, rapamycin was approved by the US Food and Drug Administration for the prevention of kidney allograft transplant rejection. Currently, rapamycin (sirolimus) and its derivative RAD0001 (everolimus) are mostly evaluated and used as alternative treatments in all organ and bone-marrow transplantations in lieu of calcineurin inhibitors (e.g., cyclosporine) which cause chronic renal allograft damage. Rapamycin-eluting stents are used for the prevention of in-stent restenosis (a thrombosis in a blood vessel) after percutaneous coronary revascularization in patients with coronary artery lesions. Three rapamycin analogs, CCI-779, RAD001, and AP23573 are currently in advanced clinical trials for the treatment of cancers. These include renal cell carcinomas, bone sarcomas, glioblastomas, mantle cell lymphomas, and endometrial carcinomas. Patients with TSC or sporadic lymphangioleiomyomatosis often develop benign renal neoplasms called angiomyolipomas, which impair renal function. In initial clinical trials, inhibition of mTOR showed some promise in leading to a partial angiomyolipoma regression.
Rapamycin was initially seen as a holy grail for cancer therapy; however, the potency of rapamycin as an anticancer drug in clinical trials was limited to the tumor types described above. The constricted success of rapamycin and the appreciation that mTORC1 has both rapamycin-sensitive and rapamycin-insensitive substrates, as well as the fi nding that mTORC2 activation is largely unaffected or even increased by acute cellular exposure to rapamycin promoted the development of novel active-site mTOR inhibitors that fully block both mTOR complexes. These novel ATP-competitive inhibitors have improved anticancer activity compared to rapamycin in a variety of solid tumor models in vitro and in vivo and at least three candidate compounds have entered clinical trials. They show a consistently potent effect against tumors that are driven by PI3K–Akt; however, the clinical effectiveness of these novel mTOR inhibitors remain to be determined. 简版： In the last years, there has been a tremendous amount of novel data establishing how the mTOR pathway is regulated by various environmental and intracellular molecules and how mTOR controls many different processes implicated in health and disease. Inhibition of mTOR is currently established in allogeneic transplantation and in certain forms of cancer. Novel applications for mTOR inhibitors, such as the generation of high number of Treg cells ex vivo for immunotherapy or the improvement of vaccines by the promotion of memory CD8+ T-cell responses, are currently evaluated. The identification of novel signaling pathways important for the control of mTOR in different tissues of the body may open further clinical applications of inhibiting mTOR in human disease.
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