The mTOR (mammalian target of rapamycin) kinase maintains integrity of mTOR complex 2

Discrete Mechanistic Target of Rapamycin Signaling Pathways, Stem Cells, and Therapeutic Targets

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that functions via its discrete binding partners to form two multiprotein complexes, mTOR complex 1 and 2 (mTORC1 and mTORC2). Rapamycin-sensitive mTORC1, which regulates protein synthesis and cell growth, is tightly controlled by PI3K/Akt and is nutrient-/growth factor-sensitive. In the brain, mTORC1 is also sensitive to neurotransmitter signaling. mTORC2, which is modulated by growth factor signaling, is associated with ribosomes and is insensitive to rapamycin. mTOR regulates stem cell and cancer stem cell characteristics. Aberrant Akt/mTOR activation is involved in multistep tumorigenesis in a variety of cancers, thereby suggesting that the inhibition of mTOR may have therapeutic potential. Rapamycin and its analogues, known as rapalogues, suppress mTOR activity through an allosteric mechanism that only suppresses mTORC1, albeit incompletely. ATP-catalytic binding site inhibitors are designed to inhibit both complexes. This review describes the regulation of mTOR and the targeting of its complexes in the treatment of cancers, such as glioblastoma, and their stem cells.

mTOR pathway - a potential therapeutic target in stroke

Stroke is ranked as the second leading cause of death worldwide and a major cause of long-term disability. A potential therapeutic target that could offer favorable outcomes in stroke is the mammalian target of rapamycin (mTOR) pathway. mTOR is a serine/threonine kinase that composes two protein complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), and is regulated by other proteins such as the tuberous sclerosis complex. Through a significant number of signaling pathways, the mTOR pathway can modulate the processes of post-ischemic inflammation and autophagy, both of which play an integral part in the pathophysiological cascade of stroke. Promoting or inhibiting such processes under ischemic conditions can lead to apoptosis or instead sustained viability of neurons. The purpose of this review is to examine the pathophysiological role of mTOR in acute ischemic stroke, while highlighting promising neuroprotective agents such as hamartin for therapeutic modulation of this pathway. The therapeutic potential of mTOR is also discussed, with emphasis on implicated molecules and pathway steps that warrant further elucidation in order for their neuroprotective properties to be efficiently tested in future clinical trials.

Role of mammalian target of rapamycin (mTOR) signalling in oncogenesis

The signalling system known as mammalian target of rapamycin (mTOR) is believed to be required for several biological activities involving cell proliferation. The serine-threonine kinase identified as mTOR recognises PI3K-AKT stress signals. It is well established in the scientific literature that the deregulation of the mTOR pathway plays a crucial role in cancer growth and advancement. This review focuses on the normal functions of mTOR as well as its abnormal roles in cancer development.

Natural compounds from botanical drugs targeting mTOR signaling pathway as promising therapeutics for atherosclerosis: A review

Atherosclerosis (AS) is a chronic inflammatory disease that is a major cause of cardiovascular diseases (CVDs), including coronary artery disease, hypertension, myocardial infarction, and heart failure. Hence, the mechanisms of AS are still being explored. A growing compendium of evidence supports that the activity of the mechanistic/mammalian target of rapamycin (mTOR) is highly correlated with the risk of AS. The mTOR signaling pathway contributes to AS progression by regulating autophagy, cell senescence, immune response, and lipid metabolism. Various botanical drugs and their functional compounds have been found to exert anti- AS effects by modulating the activity of the mTOR signaling pathway. In this review, we summarize the pathogenesis of AS based on the mTOR signaling pathway from the aspects of immune response, autophagy, cell senescence, and lipid metabolism, and comb the recent advances in natural compounds from botanical drugs to inhibit the mTOR signaling pathway and delay AS development. This review will provide a new perspective on the mechanisms and precision treatments of AS.

The Role of mTOR Signaling as a Therapeutic Target in Cancer

The aim of this review was to summarize current available information about the role of phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling in cancer as a potential target for new therapy options. The mTOR and PI3K/AKT/mTORC1 (mTOR complex 1) signaling are critical for the regulation of many fundamental cell processes including protein synthesis, cell growth, metabolism, survival, catabolism, and autophagy, and deregulated mTOR signaling is implicated in cancer, metabolic dysregulation, and the aging process. In this review, we summarize the information about the structure and function of the mTOR pathway and discuss the mechanisms of its deregulation in human cancers including genetic alterations of PI3K/AKT/mTOR pathway components. We also present recent data regarding the PI3K/AKT/mTOR inhibitors in clinical studies and the treatment of cancer, as well the attendant problems of resistance and adverse effects.

Direct physical interaction of active Ras with mSIN1 regulates mTORC2 signaling

Background

The mechanistic (or mammalian) target of rapamycin (mTOR), a Ser/Thr kinase, associates with different subunits forming two functionally distinct complexes, mTORC1 and mTORC2, regulating a diverse set of cellular functions in response to growth factors, cellular energy levels, and nutrients. The mechanisms regulating mTORC1 activity are well characterized; regulation of mTORC2 activity, however, remains obscure. While studies conducted in Dictyostelium suggest a possible role of Ras protein as a potential upstream regulator of mTORC2, definitive studies delineating the underlying molecular mechanisms, particularly in mammalian cells, are still lacking.

Methods

Protein levels were measured by Western blotting and kinase activity of mTORC2 was analyzed by in vitro kinase assay. In situ Proximity ligation assay (PLA) and co-immunoprecipitation assay was performed to detect protein-protein interaction. Protein localization was investigated by immunofluorescence and subcellular fractionation while cellular function of mTORC2 was assessed by assaying extent of cell migration and invasion.

Results

Here, we present experimental evidence in support of the role of Ras activation as an upstream regulatory switch governing mTORC2 signaling in mammalian cancer cells. We report that active Ras through its interaction with mSIN1 accounts for mTORC2 activation, while disruption of this interaction by genetic means or via peptide-based competitive hindrance, impedes mTORC2 signaling.

Conclusions

Our study defines the regulatory role played by Ras during mTORC2 signaling in mammalian cells and highlights the importance of Ras-mSIN1 interaction in the assembly of functionally intact mTORC2.

Diverse signaling mechanisms of mTOR complexes: mTORC1 and mTORC2 in forming a formidable relationship

Activation of Mechanistic target of rapamycin (mTOR) signaling plays a crucial role in tumorigenesis of numerous malignancies including glioblastoma (GB). The Canonical PI3K/Akt/mTOR signaling cascade is commonly upregulated due to loss of the tumor suppressorm PTEN, a phosphatase that acts antagonistically to the kinase (PI3K) in conversion of PIP2 to PIP3. mTOR forms two multiprotein complexes, mTORC1 and mTORC2 which are composed of discrete protein binding partners to regulate cell growth, motility, and metabolism. These complexes are sensitive to distinct stimuli, as mTORC1 is sensitive to nutrients while mTORC2 is regulated via PI3K and growth factor signaling. The main function of mTORC1 is to regulate protein synthesis and cell growth through downstream molecules: 4E-BP1 (also called EIF4E-BP1) and S6K. On the other hand, mTORC2 is responsive to growth factor signaling by phosphorylating the C-terminal hydrophobic motif of some AGC kinases like Akt and SGK and it also plays a crucial role in maintenance of normal and cancer cells through its association with ribosomes, and is involved in cellular metabolic regulation. mTORC1 and mTORC2 regulate each other, as shown by the fact that Akt regulates PRAS40 phosphorylation, which disinhibits mTORC1 activity, while S6K regulates Sin1 to modulate mTORC2 activity. Allosteric inhibitors of mTOR, rapamycin and rapalogs, remained ineffective in clinical trials of Glioblastoma (GB) patients, in part due to their incomplete inhibition of mTORC1 as well as unexpected activation of mTOR via the loss of negative feedback loops. In recent years, novel ATP binding inhibitors of mTORC1 and mTORC2 suppress mTORC1 activity completely by total dephosphorylation of its downstream substrate pS6K^Ser235/236, while effectively suppressing mTORC2 activity, as demonstrated by complete dephosphorylation of pAKT^Ser473. Furthermore by these novel combined mTORC1/mTORC2 inhibitors reduced the proliferation and self-renewal of GB cancer stem cells. However, a search of more effective way to target mTOR has generated a third generation inhibitor of mTOR, "Rapalink", that bivalently combines rapamycin with an ATP-binding inhibitor, which effectively abolishes the mTORC1 activity. All in all, the effectiveness of inhibitors of mTOR complexes can be judged by their ability to suppress both mTORC1/mTORC2 and their ability to impede both cell proliferation and migration along with aberrant metabolic pathways.

c-Jun N-terminal kinase (JNK)-mediated induction of mSin1 expression and mTORC2 activation in mesenchymal cells during fibrosis

Mammalian target of rapamycin complex 2 (mTORC2) has been shown to regulate mTORC1/4E-BP1/eIF4E signaling and collagen I expression in mesenchymal cells (MCs) during fibrotic activation. Here we investigated the regulation of the mTORC2 binding partner mammalian stress-activated protein kinase-interacting protein 1 (mSin1) in MCs derived from human lung allografts and identified a novel role for mSin1 during fibrosis. mSin1 was identified as a common downstream target of key fibrotic pathways, and its expression was increased in MCs in response to pro-fibrotic mediators: lysophosphatidic acid (LPA), transforming growth factor β, and interleukin 13. Fibrotic MCs had higher mSin1 protein levels than nonfibrotic MCs, and siRNA-mediated silencing of mSIN1 inhibited collagen I expression and mTORC1/2 activity in these cells. Autocrine LPA signaling contributed to constitutive up-regulation of mSin1 in fibrotic MCs, and mSin1 was decreased because of LPA receptor 1 siRNA treatment. We identified c-Jun N-terminal kinase (JNK) as a key intermediary in mSin1 up-regulation by the pro-fibrotic mediators, as pharmacological and siRNA-mediated inhibition of JNK prevented the LPA-induced mSin1 increase. Proteasomal inhibition rescued mSin1 levels after JNK inhibition in LPA-treated MCs, and the decrease in mSin1 ubiquitination in response to LPA was counteracted by JNK inhibitors. Constitutive JNK1 overexpression induced mSin1 expression and could drive mTORC2 and mTORC1 activation and collagen I expression in nonfibrotic MCs, effects that were reversed by siRNA-mediated mSIN1 silencing. These results indicate that LPA stabilizes mSin1 protein expression via JNK signaling by blocking its proteasomal degradation and identify the LPA/JNK/mSin1/mTORC/collagen I pathway as critical for fibrotic activation of MCs.

Who does TORC2 talk to?

The target of rapamycin (TOR) is a protein kinase that, by forming complexes with partner proteins, governs diverse cellular signalling networks to regulate a wide range of processes. TOR thus plays central roles in maintaining normal cellular functions and, when dysregulated, in diverse diseases. TOR forms two distinct types of multiprotein complexes (TOR complexes 1 and 2, TORC1 and TORC2). TORC1 and TORC2 differ in their composition, their control and their substrates, so that they play quite distinct roles in cellular physiology. Much effort has been focused on deciphering the detailed regulatory links within the TOR pathways and the structure and control of TOR complexes. In this review, we summarize recent advances in understanding mammalian (m) TORC2, its structure, its regulation, and its substrates, which link TORC2 signalling to the control of cell functions. It is now clear that TORC2 regulates several aspects of cell metabolism, including lipogenesis and glucose transport. It also regulates gene transcription, the cytoskeleton, and the activity of a subset of other protein kinases.

Drug discovery targeting the mTOR pathway

Mechanistic target of rapamycin (mTOR) is the kinase subunit of two structurally and functionally distinct large multiprotein complexes, referred to as mTOR complex 1 (mTORC1) and mTORC2. mTORC1 and mTORC2 play key physiological roles as they control anabolic and catabolic processes in response to external cues in a variety of tissues and organs. However, mTORC1 and mTORC2 activities are deregulated in widespread human diseases, including cancer. Cancer cells take advantage of mTOR oncogenic signaling to drive their proliferation, survival, metabolic transformation, and metastatic potential. Therefore, mTOR lends itself very well as a therapeutic target for innovative cancer treatment. mTOR was initially identified as the target of the antibiotic rapamycin that displayed remarkable antitumor activity in vitro Promising preclinical studies using rapamycin and its derivatives (rapalogs) demonstrated efficacy in many human cancer types, hence supporting the launch of numerous clinical trials aimed to evaluate the real effectiveness of mTOR-targeted therapies. However, rapamycin and rapalogs have shown very limited activity in most clinical contexts, also when combined with other drugs. Thus, novel classes of mTOR inhibitors with a stronger antineoplastic potency have been developed. Nevertheless, emerging clinical data suggest that also these novel mTOR-targeting drugs may have a weak antitumor activity. Here, we summarize the current status of available mTOR inhibitors and highlight the most relevant results from both preclinical and clinical studies that have provided valuable insights into both their efficacy and failure.

Role of mTORC1 Controlling Proteostasis after Brain Ischemia

Intense efforts are being undertaken to understand the pathophysiological mechanisms triggered after brain ischemia and to develop effective pharmacological treatments. However, the underlying molecular mechanisms are complex and not completely understood. One of the main problems is the fact that the ischemic damage is time-dependent and ranges from negligible to massive, involving different cell types such as neurons, astrocytes, microglia, endothelial cells, and some blood-derived cells (neutrophils, lymphocytes, etc.). Thus, approaching such a complicated cellular response generates a more complex combination of molecular mechanisms, in which cell death, cellular damage, stress and repair are intermixed. For this reason, animal and cellular model systems are needed in order to dissect and clarify which molecular mechanisms have to be promoted and/or blocked. Brain ischemia may be analyzed from two different perspectives: that of oxygen deprivation (hypoxic damage per se) and that of deprivation of glucose/serum factors. For investigations of ischemic stroke, middle cerebral artery occlusion (MCAO) is the preferred in vivo model, and uses two different approaches: transient (tMCAO), where reperfusion is permitted; or permanent (pMCAO). As a complement to this model, many laboratories expose different primary cortical neuron or neuronal cell lines to oxygen-glucose deprivation (OGD). This ex vivo model permits the analysis of the impact of hypoxic damage and the specific response of different cell types implicated in vivo, such as neurons, glia or endothelial cells. Using in vivo and neuronal OGD models, it was recently established that mTORC1 (mammalian Target of Rapamycin Complex-1), a protein complex downstream of PI3K-Akt pathway, is one of the players deregulated after ischemia and OGD. In addition, neuroprotective intervention either by estradiol or by specific AT2R agonists shows an important regulatory role for the mTORC1 activity, for instance regulating vascular endothelial growth factor (VEGF) levels. This evidence highlights the importance of understanding the role of mTORC1 in neuronal death/survival processes, as it could be a potential therapeutic target. This review summarizes the state-of-the-art of the complex kinase mTORC1 focusing in upstream and downstream pathways, their role in central nervous system and their relationship with autophagy, apoptosis and neuroprotection/neurodegeneration after ischemia/hypoxia.

Tumor suppressor Pdcd4 attenuates Sin1 translation to inhibit invasion in colon carcinoma

Programmed cell death 4 (Pdcd4), a tumor invasion suppressor, is frequently down-regulated in colorectal cancer and other cancers. In this study, we find that loss of Pdcd4 increases the activity of mammalian target of rapamycin complex 2 (mTORC2) and thereby upregulates Snail expression. Examining the components of mTORC2 showed that Pdcd4 knockdown increased the protein but not mRNA level of stress-activated-protein kinase interacting protein 1 (Sin1), which resulted from enhanced Sin1 translation. To understand how Pdcd4 regulates Sin1 translation, the SIN1 5’ untranslated region (5’UTR) was fused with luciferase reporter and named as 5’Sin1-Luc. Pdcd4 knockdown/knockout significantly increased the translation of 5’Sin1-Luc but not the control luciferase without the SIN1 5’UTR, suggesting that Sin1 5’UTR is necessary for Pdcd4 to inhibit Sin1 translation. Ectopic expression of wild type Pdcd4 and Pdcd4(157–469), a deletion mutant that binds to translation initiation factor 4A (eIF4A), sufficiently inhibited Sin1 translation, and thus suppressed mTORC2 kinase activity and invasion in colon tumor cells. By contrast, Pdcd4(157–469)(D253A,D418A), a mutant that does not bind to eIF4A, failed to inhibit Sin1 translation, and consequently failed to repress mTORC2 activity and invasion. In addition, directly inhibiting eIF4A with silvestrol significantly suppressed Sin1 translation and attenuated invasion. These results indicate that Pdcd4-inhibited Sin1 translation is through suppressing eIF4A, and functionally important for suppression of mTORC2 activity and invasion. Moreover, in colorectal cancer tissues, the Sin1 protein but not mRNA was significantly up-regulated while Pdcd4 protein was down-regulated, suggesting that loss of Pdcd4 might correlate with Sin1 protein level but not mRNA level in colorectal cancer patients. Taken together, our work reveals a novel mechanism by which Pdcd4 inhibits Sin1 translation to attenuatemTORC2 activity and thereby suppresses invasion.

Phosphoglycerate Kinase 1 Phosphorylates Beclin1 to Induce Autophagy

Autophagy is crucial for maintaining cell homeostasis. However, the precise mechanism underlying autophagy initiation remains to be defined. Here, we demonstrate that glutamine deprivation and hypoxia result in inhibition of mTOR-mediated acetyl-transferase ARD1 S228 phosphorylation, leading to ARD1-dependent phosphoglycerate kinase 1 (PGK1) K388 acetylation and subsequent PGK1-mediated Beclin1 S30 phosphorylation. This phosphorylation enhances ATG14L-associated class III phosphatidylinositol 3-kinase VPS34 activity by increasing the binding of phosphatidylinositol to VPS34. ARD1-dependent PGK1 acetylation and PGK1-mediated Beclin1 S30 phosphorylation are required for glutamine deprivation- and hypoxia-induced autophagy and brain tumorigenesis. Furthermore, PGK1 K388 acetylation levels correlate with Beclin1 S30 phosphorylation levels and poor prognosis in glioblastoma patients. Our study unearths an important mechanism underlying cellular-stress-induced autophagy initiation in which the protein kinase activity of the metabolic enzyme PGK1 plays an instrumental role and reveals the significance of the mutual regulation of autophagy and cell metabolism in maintaining cell homeostasis.

Discrete signaling mechanisms of mTORC1 and mTORC2: Connected yet apart in cellular and molecular aspects

Activation of PI3K/Akt/mTOR (mechanistic target of rapamycin) signaling cascade has been shown in tumorigenesis of numerous malignancies including glioblastoma (GB). This signaling cascade is frequently upregulated due to loss of the tumor suppressor PTEN, a phosphatase that functions antagonistically to PI3K. mTOR regulates cell growth, motility, and metabolism by forming two multiprotein complexes, mTORC1 and mTORC2, which are composed of special binding partners. These complexes are sensitive to distinct stimuli. mTORC1 is sensitive to nutrients and mTORC2 is regulated via PI3K and growth factor signaling. mTORC1 regulates protein synthesis and cell growth through downstream molecules: 4E-BP1 (also called EIF4E-BP1) and S6K. Also, mTORC2 is responsive to growth factor signaling by phosphorylating the C-terminal hydrophobic motif of some AGC kinases like Akt and SGK. mTORC2 plays a crucial role in maintenance of normal and cancer cells through its association with ribosomes, and is involved in cellular metabolic regulation. Both complexes control each other as Akt regulates PRAS40 phosphorylation, which disinhibits mTORC1 activity, while S6K regulates Sin1 to modulate mTORC2 activity. Another significant component of mTORC2 is Sin1, which is crucial for mTORC2 complex formation and function. Allosteric inhibitors of mTOR, rapamycin and rapalogs, have essentially been ineffective in clinical trials of patients with GB due to their incomplete inhibition of mTORC1 or unexpected activation of mTOR via the loss of negative feedback loops. Novel ATP binding inhibitors of mTORC1 and mTORC2 suppress mTORC1 activity completely by total dephosphorylation of its downstream substrate pS6K^Ser235/236, while effectively suppressing mTORC2 activity, as demonstrated by complete dephosphorylation of pAKT^Ser473. Furthermore, proliferation and self-renewal of GB cancer stem cells are effectively targetable by these novel mTORC1 and mTORC2 inhibitors. Therefore, the effectiveness of inhibitors of mTOR complexes can be estimated by their ability to suppress both mTORC1 and 2 and their ability to impede both cell proliferation and migration.

mTOR inhibitors in urinary bladder cancer

Despite the great scientific advances that have been made in cancer treatment, there is still much to do, particularly with regard to urinary bladder cancer. Some of the drugs used in urinary bladder cancer treatment have been in use for more than 30 years and show reduced effectiveness and high recurrence rates. There have been several attempts to find new and more effective drugs, to be used alone or in combination with the drugs already in use, in order to overcome this situation.The biologically important mammalian target of rapamycin (mTOR) pathway is altered in cancer and mTOR inhibitors have raised many expectations as potentially important anticancer drugs. In this article, the authors will review the mTOR pathway and present their experiences of the use of some mTOR inhibitors, sirolimus, everolimus and temsirolimus, in isolation and in conjunction with non-mTOR inhibitors cisplatin and gemcitabine, on urinary bladder tumour cell lines. The non-muscle-invasive cell line, 5637, is the only one that exhibits a small alteration in the mTOR and AKT phosphorylation after rapalogs exposure. Also, there was a small inhibition of cell proliferation. With gemcitabine plus everolimus or temsirolimus, the results were encouraging as a more effective response was noticed with both combinations, especially in the 5637 and T24 cell lines. Cisplatin associated with everolimus or temsirolimus also gave promising results, as an antiproliferative effect was observed when the drugs were associated, in particular on the 5637 and HT1376 cell lines. Everolimus or temsirolimus in conjunction with gemcitabine or cisplatin could have an important role to play in urinary bladder cancer treatment, depending on the tumour grading.

Point mutations of the mTOR-RHEB pathway in renal cell carcinoma

Aberrations in the mTOR (mechanistic target of rapamycin) axis are frequently reported in cancer. Using publicly available tumor genome sequencing data, we identified several point mutations in MTOR and its upstream regulator RHEB (Ras homolog enriched in brain) in patients with clear cell renal cell carcinoma (ccRCC), the most common histology of kidney cancer. Interestingly, we found a prominent cluster of hyperactivating mutations in the FAT (FRAP-ATM-TTRAP) domain of mTOR in renal cell carcinoma that led to an increase in both mTORC1 and mTORC2 activities and led to an increased proliferation of cells. Several of the FAT domain mutants demonstrated a decreased binding of DEPTOR (DEP domain containing mTOR-interacting protein), while a subset of these mutations showed altered binding of the negative regulator PRAS40 (proline rich AKT substrate 40). We also identified a recurrent mutation in RHEB in ccRCC patients that leads to an increase in mTORC1 activity. In vitro characterization of this RHEB mutation revealed that this mutant showed considerable resistance to TSC2 (Tuberous Sclerosis 2) GAP (GTPase activating protein) activity, though its interaction with TSC2 remained unaltered. Mutations in the FAT domain of MTOR and in RHEB remained sensitive to rapamycin, though several of these mutations demonstrated residual mTOR kinase activity after treatment with rapamycin at clinically relevant doses. Overall, our data suggests that point mutations in the mTOR pathway may lead to downstream mTOR hyperactivation through multiple different mechanisms to confer a proliferative advantage to a tumor cell.

Targeting molecules to medicine with mTOR, autophagy and neurodegenerative disorders

Neurodegenerative disorders are significantly increasing in incidence as the age of the global population continues to climb with improved life expectancy. At present, more than 30 million individuals throughout the world are impacted by acute and chronic neurodegenerative disorders with limited treatment strategies. The mechanistic target of rapamycin (mTOR), also known as the mammalian target of rapamycin, is a 289 kDa serine/threonine protein kinase that offers exciting possibilities for novel treatment strategies for a host of neurodegenerative diseases that include Alzheimer's disease, Parkinson's disease, Huntington's disease, epilepsy, stroke and trauma. mTOR governs the programmed cell death pathways of apoptosis and autophagy that can determine neuronal stem cell development, precursor cell differentiation, cell senescence, cell survival and ultimate cell fate. Coupled to the cellular biology of mTOR are a number of considerations for the development of novel treatments involving the fine control of mTOR signalling, tumourigenesis, complexity of the apoptosis and autophagy relationship, functional outcome in the nervous system, and the intimately linked pathways of growth factors, phosphoinositide 3-kinase (PI 3-K), protein kinase B (Akt), AMP activated protein kinase (AMPK), silent mating type information regulation two homologue one (Saccharomyces cerevisiae) (SIRT1) and others. Effective clinical translation of the cellular signalling mechanisms of mTOR offers provocative avenues for new drug development in the nervous system tempered only by the need to elucidate further the intricacies of the mTOR pathway.

Rapamycin inhibits mSin1 phosphorylation independently of mTORC1 and mTORC2

Current knowledge indicates that the mammalian target of rapamycin (mTOR) functions as two complexes, mTORC1 and mTORC2, regulating cell growth, proliferation, survival, differentiation, and motility. Recently mSin1 has been identified as a critical component of mTORC2, which is essential for phosphorylation of Akt and other signaling molecules. Studies have shown that rapamycin inhibits phosphorylation of mSin1. However, the underlying mechanism is unknown. Here we found that rapamycin inhibited phosphorylation of mSin1 potently and rapidly. Expression of rapamycin-resistant mutant of mTOR (mTOR-T), but not rapamycin-resistant and kinase dead mutant of mTOR (mTOR-TE), prevented rapamycin from inhibiting mSin1 phosphorylation, suggesting that rapamycin-induced dephosphorylation of mSin1 is mTOR-dependent. Surprisingly, ectopic expression of rapamycin-resistant and constitutively active p70 S6 kinase 1 (S6K1) did not confer resistance to rapamycin-induced dephosphorylation of mSin1. Furthermore, disruption of mTORC1 and mTORC2 by silencing raptor and rictor, respectively, or downregulation of S6K1 or Akt did not induce the dephosphorylation of mSin1 as rapamycin did. However, silencing mTOR or mLST8 mimicked the effect of rapamycin, inhibiting mSin1 phosphorylation. Our findings suggest that rapamycin inhibits mSin1 phosphorylation, which is independent of mTORC1 and mTORC2, but is possibly dependent on a new mTOR complex, which at least contains mTOR and mLST8.

Sestrin 3 protein enhances hepatic insulin sensitivity by direct activation of the mTORC2-Akt signaling

Sestrin proteins have been implicated in multiple biological processes including resistance to oxidative and genotoxic stresses, protection against aging-related pathologies, and promotion of metabolic homeostasis; however, the underlying mechanisms are incompletely understood. Some evidence suggests that sestrins may inhibit mTORC1 (mechanistic target of rapamycin complex 1) through inhibition of RagA/B GTPases or activation of AMPK; however, whether sestrins are also involved in mTORC2 regulation and function is unclear. To investigate the functions and mechanisms of Sestrin 3 (Sesn3), we generated Sesn3 liver-specific transgenic and knockout mice. Our data show that Sesn3 liver-specific knockout mice exhibit insulin resistance and glucose intolerance, and Sesn3 transgenic mice were protected against insulin resistance induced by a high-fat diet. Using AMPK liver-specific knockout mice, we demonstrate that the Sesn3 insulin-sensitizing effect is largely independent of AMPK. Biochemical analysis reveals that Sesn3 interacts with and activates mTORC2 and subsequently stimulates Akt phosphorylation at Ser473. These findings suggest that Sesn3 can activate Akt via mTORC2 to regulate hepatic insulin sensitivity and glucose metabolism.

Regulation of mTOR activity in Snell dwarf and GH receptor gene-disrupted mice

The involvement of mammalian target of rapamycin (mTOR) in lifespan control in invertebrates, calorie-restricted rodents, and extension of mouse lifespan by rapamycin have prompted speculation that diminished mTOR function may contribute to mammalian longevity in several settings. We show here that mTOR complex-1 (mTORC1) activity is indeed lower in liver, muscle, heart, and kidney tissue of Snell dwarf and global GH receptor (GHR) gene-disrupted mice (GHR-/-), consistent with previous studies. Surprisingly, activity of mTORC2 is higher in fasted Snell and GHR-/- than in littermate controls in all 4 tissues tested. Resupply of food enhanced mTORC1 activity in both controls and long-lived mutant mice but diminished mTORC2 activity only in the long-lived mice. Mice in which GHR has been disrupted only in the liver do not show extended lifespan and also fail to show the decline in mTORC1 and increase in mTORC2 seen in mice with global loss of GHR. The data suggest that the antiaging effects in the Snell dwarf and GHR-/- mice are accompanied by both a decline in mTORC1 in multiple organs and an increase in fasting levels of mTORC2. Neither the lifespan nor mTOR effects appear to be mediated by direct GH effects on liver or by the decline in plasma IGF-I, a shared trait in both global and liver-specific GHR-/- mice. Our data suggest that a more complex pattern of hormonal effects and intertissue interactions may be responsible for regulating both lifespan and mTORC2 function in these mouse models of delayed aging.

Cutting through the complexities of mTOR for the treatment of stroke

On a global basis, at least 15 million individuals suffer some form of a stroke every year. Of these individuals, approximately 800,000 of these cerebrovascular events occur in the United States (US) alone. The incidence of stroke in the US has declined from the third leading cause of death to the fourth, a result that can be attributed to multiple factors that include improved vascular disease management, reduced tobacco use, and more rapid time to treatment in patients that are clinically appropriate to receive recombinant tissue plasminogen activator. However, treatment strategies for the majority of stroke patients are extremely limited and represent a critical void for care. A number of new therapeutic considerations for stroke are under consideration, but it is the mammalian target of rapamycin (mTOR) that is receiving intense focus as a potential new target for cerebrovascular disease. As part of the phosphoinositide 3-kinase (PI 3-K) and protein kinase B (Akt) cascade, mTOR is an essential component of mTOR Complex 1 (mTORC1) and mTOR Complex 2 (mTORC2) to govern cell death involving apoptosis, autophagy, and necroptosis, cellular metabolism, and gene transcription. Vital for the consideration of new therapeutic strategies for stroke is the ability to understand how the intricate and complex pathways of mTOR signaling sometimes lead to disparate clinical outcomes.

TGFβ-induced deptor suppression recruits mTORC1 and not mTORC2 to enhance collagen I (α2) gene expression

Enhanced TGFβ activity contributes to the accumulation of matrix proteins including collagen I (α2) by proximal tubular epithelial cells in progressive kidney disease. Although TGFβ rapidly activates its canonical Smad signaling pathway, it also recruits noncanonical pathway involving mTOR kinase to regulate renal matrix expansion. The mechanism by which chronic TGFβ treatment maintains increased mTOR activity to induce the matrix protein collagen I (α2) expression is not known. Deptor is an mTOR interacting protein that suppresses mTOR activity in both mTORC1 and mTORC2. In proximal tubular epithelial cells, TGFβ reduced deptor levels in a time-dependent manner with concomitant increase in both mTORC1 and mTORC2 activities. Expression of deptor abrogated activity of mTORC1 and mTORC2, resulting in inhibition of collagen I (α2) mRNA and protein expression via transcriptional mechanism. In contrast, neutralization of endogenous deptor by shRNAs increased activity of both mTOR complexes and expression of collagen I (α2) similar to TGFβ treatment. Importantly, downregulation of deptor by TGFβ increased the expression of Hif1α by increasing translation of its mRNA. TGFβ-induced deptor downregulation promotes Hif1α binding to its cognate hypoxia responsive element in the collagen I (α2) gene to control its protein expression via direct transcriptional mechanism. Interestingly, knockdown of raptor to specifically block mTORC1 activity significantly inhibited expression of collagen I (α2) and Hif1α while inhibition of rictor to prevent selectively mTORC2 activation did not have any effect. Critically, our data provide evidence for the requirement of TGFβ-activated mTORC1 only by deptor downregulation, which dominates upon the bystander mTORC2 activity for enhanced expression of collagen I (α2). Our results also suggest the presence of a safeguard mechanism involving deptor-mediated suppression of mTORC1 activity against developing TGFβ-induced renal fibrosis.

Distinct signaling mechanisms of mTORC1 and mTORC2 in glioblastoma multiforme: a tale of two complexes

Mechanistic target of rapamycin (mTOR) is a serine-threonine kinase that functions via two multiprotein complexes, namely mTORC1 and mTORC2, each characterized by different binding partners that confer separate functions. mTORC1 function is tightly regulated by PI3-K/Akt and is sensitive to rapamycin. mTORC2 is sensitive to growth factors, not nutrients, and is associated with rapamycin-insensitivity. mTORC1 regulates protein synthesis and cell growth through downstream molecules: 4E-BP1 (also called EIF4E-BP1) and S6K. Also, mTORC2 is thought to modulate growth factor signaling by phosphorylating the C-terminal hydrophobic motif of some AGC kinases such as Akt and SGK. Recent evidence has suggested that mTORC2 may play an important role in maintenance of normal as well as cancer cells by virtue of its association with ribosomes, which may be involved in metabolic regulation of the cell. Rapamycin (sirolimus) and its analogs known as rapalogues, such as RAD001 (everolimus) and CCI-779 (temsirolimus), suppress mTOR activity through an allosteric mechanism that acts at a distance from the ATP-catalytic binding site, and are considered incomplete inhibitors. Moreover, these compounds suppress mTORC1-mediated S6K activation, thereby blocking a negative feedback loop, leading to activation of mitogenic pathways promoting cell survival and growth. Consequently, mTOR is a suitable target of therapy in cancer treatments. However, neither of these complexes is fully inhibited by the allosteric inhibitor rapamycin or its analogs. In recent years, new pharmacologic agents have been developed which can inhibit these complexes via ATP-binding mechanism, or dual inhibition of the canonical PI3-K/Akt/mTOR signaling pathway. These compounds include WYE-354, KU-003679, PI-103, Torin1, and Torin2, which can target both complexes or serve as a dual inhibitor for PI3-K/mTOR. This investigation describes the mechanism of action of pharmacological agents that effectively target mTORC1 and mTORC2 resulting in suppression of growth, proliferation, and migration of tumor and cancer stem cells.

IBP-mediated suppression of autophagy promotes growth and metastasis of breast cancer cells via activating mTORC2/Akt/FOXO3a signaling pathway

Interferon regulatory factor-4 binding protein (IBP) is a novel upstream activator of Rho GTPases. Our previous studies have shown that ectopic expression of IBP was correlated with malignant behaviors of human breast cancer cells, and invasive human breast cancer had high expression of IBP that promoted the proliferation of these cells. However, it remains unknown whether autophagy inhibition contributes to IBP-mediated tumorigenesis. In this study, we for the first time, reported that upregulation of IBP expression significantly suppressed the autophagy of breast cancer cells, and downregulation of IBP expression markedly induced autophagy of these cells. Further investigation revealed that IBP effectively counteracted autophagy by directly activating mammalian target of rapamycin complex 2 (mTORC2) and upregulating phosphorylation of Akt on ser473 and FOXO3a on Thr32. Moreover, IBP-mediated suppression of autophagy was dependent on mTORC2/Akt/FOXO3a signaling pathway. Finally, our results demonstrated that IBP-mediated breast cancer cell growth in vitro and in vivo was strongly correlated with suppression of mTORC2-dependent autophagy. These findings suggest that the anti-autophagic property of IBP has an important role in IBP-mediated tumorigenesis, and IBP may serve as an attractive target for treatment of breast cancer.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

The role of phospholipase D in modulating the MTOR signaling pathway in polycystic kidney disease

The mammalian target of rapamycin (mTOR) signaling pathway is aberrantly activated in polycystic kidney disease (PKD). Emerging evidence suggests that phospholipase D (PLD) and its product phosphatidic acid (PA) regulate mTOR activity. In this study, we assessed in vitro the regulatory function of PLD and PA on the mTOR signaling pathway in PKD. We found that the basal level of PLD activity was elevated in PKD cells. Targeting PLD by small molecule inhibitors reduced cell proliferation and blocked mTOR signaling, whereas exogenous PA stimulated mTOR signaling and abolished the inhibitory effect of PLD on PKD cell proliferation. We also show that blocking PLD activity enhanced the sensitivity of PKD cells to rapamycin and that combining PLD inhibitors and rapamycin synergistically inhibited PKD cell proliferation. Furthermore, we demonstrate that targeting mTOR did not induce autophagy, whereas targeting PLD induced autophagosome formation. Taken together, our findings suggest that deregulated mTOR pathway activation is mediated partly by increased PLD signaling in PKD cells. Targeting PLD isoforms with pharmacological inhibitors may represent a new therapeutic strategy in PKD.

Autoregulation of the mechanistic target of rapamycin (mTOR) complex 2 integrity is controlled by an ATP-dependent mechanism

Nutrients are essential for living organisms because they fuel biological processes in cells. Cells monitor nutrient abundance and coordinate a ratio of anabolic and catabolic reactions. Mechanistic target of rapamycin (mTOR) signaling is the essential nutrient-sensing pathway that controls anabolic processes in cells. The central component of this pathway is mTOR, a highly conserved and essential protein kinase that exists in two distinct functional complexes. The nutrient-sensitive mTOR complex 1 (mTORC1) controls cell growth and cell size by phosphorylation of the regulators of protein synthesis S6K1 and 4EBP1, whereas its second complex, mTORC2, regulates cell proliferation by functioning as the regulatory kinase of Akt and other members of the AGC kinase family. The regulation of mTORC2 remains poorly characterized. Our study shows that the cellular ATP balance controls a basal kinase activity of mTORC2 that maintains the integrity of mTORC2 and phosphorylation of Akt on the turn motif Thr-450 site. We found that mTOR stabilizes SIN1 by phosphorylation of its hydrophobic and conserved Ser-260 site to maintain the integrity of mTORC2. The optimal kinase activity of mTORC2 requires a concentration of ATP above 1.2 mM and makes this kinase complex highly sensitive to ATP depletion. We found that not amino acid but glucose deprivation of cells or acute ATP depletion prevented the mTOR-dependent phosphorylation of SIN1 on Ser-260 and Akt on Thr-450. In a low glucose medium, the cells carrying a substitution of SIN1 with its phosphomimetic mutant show an increased rate of cell proliferation related to a higher abundance of mTORC2 and phosphorylation of Akt. Thus, the homeostatic ATP sensor mTOR controls the integrity of mTORC2 and phosphorylation of Akt on the turn motif site.

Deconstructing mTOR complexes in regulation of Glioblastoma Multiforme and its stem cells

Atypical serine-threonine kinase, mTOR (mechanistic target of Rapamycin; originally coined "mammalian TOR"), exists in two distinct multi-protein complexes termed mTOR complex 1 (mTORC1) and 2 (mTORC2), that senses and integrates a variety of environmental signals to control organism growth and homeostasis via non-overlapping signaling pathways. mTOR belongs to the phosphoinositide 3-kinase (PI3-K)-related kinase family, and an aberrant activation of mTORC1 is a potential contributing factor in uncontrolled cell growth, proliferation, and survival of tumor cells via specific effects on cap-dependent translation initiation, as well as in a more sustained manner via advancing ribosome biogenesis. It is thereby shown to be deregulated in numerous pathological conditions including cancer, obesity, type 2 diabetes, and neurodegeneration. Notably, mTOR itself, or through its substrates, regulates stem cell differentiation and maintenance of plueropotency. mTORC2 has been linked to cytoskeletal reorganization and cell survival through Akt, and is crucial to many divergent physiological functions, which may include stem cell regulation.

The rationale of targeting mammalian target of rapamycin for ischemic stroke

Given the current limitation of therapeutic approach for ischemic stroke, a leading cause of disability and mortality in the developed countries, to develop new therapeutic strategies for this devastating disease is urgently necessary. As a serine/threonine kinase, mammalian target of rapamycin (mTOR) activation can mediate broad biological activities that include protein synthesis, cytoskeleton organization, and cell survival. mTOR functions through mTORC1 and mTORC2 complexes and their multiple downstream substrates, such as eukaryotic initiation factor 4E-binding protein 1, p70 ribosomal S6 kinase, sterol regulatory element-binding protein 1, hypoxia inducible factor-1, and signal transducer and activator transcription 3, Yin Ying 1, Akt, protein kinase c-alpha, Rho GTPase, serum-and gucocorticoid-induced protein kinase 1, etc. Specially, the role of mTOR in the central nervous system has been attracting considerable attention. Based on the ability of mTOR to prevent neuronal apoptosis, inhibit autophagic cell death, promote neurogenesis, and improve angiogenesis, mTOR may acquire the capability of limiting the ischemic neuronal death and promoting the neurological recovery. Consequently, to regulate the activity of mTOR holds a potential as a novel therapeutic strategy for ischemic stroke.

mTOR: a link from the extracellular milieu to transcriptional regulation of oligodendrocyte development

Oligodendrocyte development is controlled by numerous extracellular signals that regulate a series of transcription factors that promote the differentiation of oligodendrocyte progenitor cells to myelinating cells in the central nervous system. A major element of this regulatory system that has only recently been studied is the intracellular signalling from surface receptors to transcription factors to down-regulate inhibitors and up-regulate inducers of oligodendrocyte differentiation and myelination. The current review focuses on one such pathway: the mTOR (mammalian target of rapamycin) pathway, which integrates signals in many cell systems and induces cell responses including cell proliferation and cell differentiation. This review describes the known functions of mTOR as they relate to oligodendrocyte development, and its recently discovered impact on oligodendrocyte differentiation and myelination. A potential model for its role in oligodendrocyte development is proposed.

Beyond control of protein translation: what we have learned about the non-canonical regulation and function of mammalian target of rapamycin (mTOR)

Mammalian target of rapamycin (mTOR) is a serine-threonine kinase involved in almost every aspect of mammalian cell function. This kinase was initially believed to control protein translation in response to amino acids and trophic factors, and this function has become a canonical role for mTOR. However, mTOR can form two separate protein complexes (mTORCs). Recent advances clearly demonstrate that both mTORCs can respond to various stimuli and change myriad cellular processes. Therefore, our current view of the cellular roles of TORCs has rapidly expanded and cannot be fully explained without appreciating recent findings about the new modes of mTOR regulation and identification of non-canonical effectors of mTOR that contribute to transcription, cytoskeleton dynamics, and membrane trafficking. This review discusses the molecular details of these newly discovered non-canonical functions that allow mTORCs to control the cellular environment at multiple levels. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

mTOR: on target for novel therapeutic strategies in the nervous system

The mammalian target of rapamycin (mTOR), the key component of the protein complexes mTORC1 and mTORC2, plays a critical role in cellular development, tissue regeneration, and repair. mTOR signaling can govern not only stem cell development and quiescence but also cell death during apoptosis or autophagy. Recent studies highlight the importance of both traditional and newly recognized interactors of mTOR, such as p70S6K, 4EBP1, GSK-3β, REDD1/RTP801, TSC1/TSC2, growth factors, wingless, and forkhead transcription factors, that influence Alzheimer's disease, Parkinson's disease, Huntington's disease, tuberous sclerosis, and epilepsy. Targeting mTOR in the nervous system can offer exciting new avenues of drug discovery, but crucial to this premise is elucidating the complexity of mTOR signaling for robust and safe clinical outcomes.

The evolution of the TOR pathway and its role in cancer

The target of rapamycin (TOR) pathway is highly conserved among eukaryotes and has evolved to couple nutrient sensing to cellular growth. TOR is found in two distinct signaling complexes in cells, TOR complex 1 (TORC1) and TOR complex 2 (TORC2). These complexes are differentially regulated and act as effectors for the generation of signals that drive diverse cellular processes such as growth, proliferation, protein synthesis, rearrangement of the cytoskeleton, autophagy, metabolism and survival. Mammalian TOR (mTOR) is very important for development in embryos, while in adult organisms it is linked to aging and lifespan effects. In humans, the mTOR pathway is implicated in the tumorigenesis of multiple cancer types and its deregulation is associated with familial cancer syndromes. Because of its high biological relevance, different therapeutic strategies have been developed to target this signaling cascade, resulting in the emergence of unique pharmacological inhibitors that are either already approved for use in clinical oncology or currently under preclinical or clinical development. Multimodal treatment strategies that simultaneously target multiple nodes of the pathway and/or negative feedback regulatory loops may ultimately provide the best therapeutic advantage in targeting this pathway for the treatment of malignancies.

Shedding new light on neurodegenerative diseases through the mammalian target of rapamycin

Neurodegenerative disorders affect a significant portion of the world's population leading to either disability or death for almost 30 million individuals worldwide. One novel therapeutic target that may offer promise for multiple disease entities that involve Alzheimer's disease, Parkinson's disease, epilepsy, trauma, stroke, and tumors of the nervous system is the mammalian target of rapamycin (mTOR). mTOR signaling is dependent upon the mTORC1 and mTORC2 complexes that are composed of mTOR and several regulatory proteins including the tuberous sclerosis complex (TSC1, hamartin/TSC2, tuberin). Through a number of integrated cell signaling pathways that involve those of mTORC1 and mTORC2 as well as more novel signaling tied to cytokines, Wnt, and forkhead, mTOR can foster stem cellular proliferation, tissue repair and longevity, and synaptic growth by modulating mechanisms that foster both apoptosis and autophagy. Yet, mTOR through its proliferative capacity may sometimes be detrimental to central nervous system recovery and even promote tumorigenesis. Further knowledge of mTOR and the critical pathways governed by this serine/threonine protein kinase can bring new light for neurodegeneration and other related diseases that currently require new and robust treatments.

Mammalian target of rapamycin signaling in diabetic cardiovascular disease

Diabetes mellitus currently affects more than 170 million individuals worldwide and is expected to afflict another 200 million individuals in the next 30 years. Complications of diabetes as a result of oxidant stress affect multiple systems throughout the body, but involvement of the cardiovascular system may be one of the most severe in light of the impact upon cardiac and vascular function that can result in rapid morbidity and mortality for individuals. Given these concerns, the signaling pathways of the mammalian target of rapamycin (mTOR) offer exciting prospects for the development of novel therapies for the cardiovascular complications of diabetes. In the cardiovascular and metabolic systems, mTOR and its multi-protein complexes of TORC1 and TORC2 regulate insulin release and signaling, endothelial cell survival and growth, cardiomyocyte proliferation, resistance to β-cell injury, and cell longevity. Yet, mTOR can, at times, alter insulin signaling and lead to insulin resistance in the cardiovascular system during diabetes mellitus. It is therefore vital to understand the complex relationship mTOR and its downstream pathways hold during metabolic disease in order to develop novel strategies for the complications of diabetes mellitus in the cardiovascular system.