Point mutations in TOR confer Rheb-independent growth in fission yeast and nutrient-independent mammalian TOR signaling in mammalian cells

Advanced gynecologic malignancies treated with a combination of the VEGF inhibitor bevacizumab and the mTOR inhibitor temsirolimus

Background: Bevacizumab and temsirolimus are active agents in gynecologic tumors. Temsirolimus attenuates upregulation of HIF-1α levels, a resistance mechanism for antiangiogenics, and targets the PI3-kinase/AKT/mTOR axis, commonly aberrant in these tumors

Patients and Methods: We analyzed safety and responses in 41 patients with gynecologic cancers treated as part of a Phase I study of bevacizumab and temsirolimus.

Results: Median age of the 41 women was 60 years (range, 33-80 years); median number of prior systemic therapies was 4 (1-11). Grade 3 or 4 treatment-related toxicities included: thrombocytopenia (10%), mucositis (2%), hypertension (2%), hypercholesterolemia (2%), fatigue (7%), elevated aspartate aminotransferase (2%), and neutropenia (2%). Twenty-nine patients (71%) experienced no treatment-related toxicity greater than grade 2. Full FDA-approved doses of both drugs (bevacizumab 15mg/kg IV Q3weeks and temsirolimus 25mg IV weekly) were administered without dose-limiting toxicity. Eight patients (20%) achieved stable disease (SD) ≥ 6 months and 7 patients (17%), a partial response (PR) [total = 15/41 patients (37%)]. Eight of 13 patients (62%) with high-grade serous histology (ovarian or primary peritoneal) achieved SD ≥ 6 months/PR.

Conclusion: Bevacizumab and temsirolimus was well tolerated. Thirty-seven percent of heavily-pretreated patients achieved SD ≥ 6 months/PR, suggesting that this combination warrants further study.

Mechanisms of ATM Activation

The ataxia-telangiectasia mutated (ATM) protein kinase is a master regulator of the DNA damage response, and it coordinates checkpoint activation, DNA repair, and metabolic changes in eukaryotic cells in response to DNA double-strand breaks and oxidative stress. Loss of ATM activity in humans results in the pleiotropic neurodegeneration disorder ataxia-telangiectasia. ATM exists in an inactive state in resting cells but can be activated by the Mre11-Rad50-Nbs1 (MRN) complex and other factors at sites of DNA breaks. In addition, oxidation of ATM activates the kinase independently of the MRN complex. This review discusses these mechanisms of activation, as well as the posttranslational modifications that affect this process and the cellular factors that affect the efficiency and specificity of ATM activation and substrate phosphorylation. I highlight functional similarities between the activation mechanisms of ATM, phosphatidylinositol 3-kinases (PI3Ks), and the other PI3K-like kinases, as well as recent structural insights into their regulation.

mTOR pathway in colorectal cancer: an update

The mammalian target of rapamycin (mTOR) has emerged as a potential target for drug development, particularly due to the fact that it plays such a crucial role in cancer biology. In addition, next-generation mTOR inhibitors have become available, marking an exciting new phase in mTOR-based therapy. However, the verdict on their therapeutic efectiveness remains unclear. Here we review phosphatidylinositol-3-kinase (PI3K)/Akt/mTOR signaling as one of the primary mechanisms for sustaining tumor outgrowth and metastasis, recent advances in the development of mTOR inhibitors, and current studies addressing mTOR activation/inhibition in colorectal cancer (CRC). We will also discuss our recent comparative study of diferent mTOR inhibitors in a population of colon cancer stem cells (CSCs), and current major challenges for achieving individualized drug therapy using kinase inhibitors.

Rheb protein binds CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase) protein in a GTP- and effector domain-dependent manner and influences its cellular localization and carbamoyl-phosphate synthetase (CPSase) activity

Rheb small GTPases, which consist of Rheb1 and Rheb2 (also known as RhebL1) in mammalian cells, are unique members of the Ras superfamily and play central roles in regulating protein synthesis and cell growth by activating mTOR. To gain further insight into the function of Rheb, we carried out a search for Rheb-binding proteins and found that Rheb binds to CAD protein (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase), a multifunctional enzyme required for the de novo synthesis of pyrimidine nucleotides. CAD binding is more pronounced with Rheb2 than with Rheb1. Rheb binds CAD in a GTP- and effector domain-dependent manner. The region of CAD where Rheb binds is located at the C-terminal region of the carbamoyl-phosphate synthetase domain and not in the dihydroorotase and aspartate transcarbamoylase domains. Rheb stimulated carbamoyl-phosphate synthetase activity of CAD in vitro. In addition, an elevated level of intracellular UTP pyrimidine nucleotide was observed in Tsc2-deficient cells, which was attenuated by knocking down of Rheb. Immunostaining analysis showed that expression of Rheb leads to increased accumulation of CAD on lysosomes. Both a farnesyltransferase inhibitor that blocks membrane association of Rheb and knockdown of Rheb mislocalized CAD. These results establish CAD as a downstream effector of Rheb and suggest a possible role of Rheb in regulating de novo pyrimidine nucleotide synthesis.

The structural basis for mTOR function

The phosphoinositide 3-kinase (PI3K) related protein kinases (PIKKs) are a family of protein kinases with a diverse range of vital cellular functions. Recent high-resolution crystal structures of the protein kinase mTOR suggest general architectural principles that are likely to be common to all of the PIKKs. Furthermore, the structures make clear the close relationship of the PIKKs to the PI3Ks. However, the structures also make clear the unique features of mTOR that enable its substrate specificity. The active site is deeply recessed and flanked by structural elements unique to the PIKKs, namely, the FRB domain, the LST8 binding element, and a C-terminal stretch of helices known as the FATC domain. The FRB has a conserved element in it that is part of a bipartite substrate recognition mechanism that is probably characteristic of all of the PIKKs. The FRB also binds the mTOR inhibitor rapamycin that has been referred to as an allosteric inhibitor, implying that this inhibitor is actually a competitive inhibitor of the protein substrate. This bipartite substrate-binding site also helps clarify how rapamycin can result in substrate-specific inhibition.

[Expression and clinical significance of serine-threonine kinase/mammalian target of rapamycin/p70 S6K signal path- way in oral squamous cell carcinoma]

OBJECTIVE

To investigate the expressions of serine-threonine kinase (Akt)/mammalian target of rapamycin (mTOR)/p70 S6K in oral squamous cell carcinoma (OSCC) and provide references for early diagnosis and prognosis evalua- tion of OSCC.

METHODS

A total of 51 cases of OSCC, 10 cases of paracancerous mucosa, and 10 cases of normal oral mucosa were collected. The expressions of Akt/mTOR/p70 S6K in these cases were detected using the SP method of immunohisto- chemistry. The correlation between their expressions in OSCC was also analyzed.

RESULTS

The positive expressions ofp-Akt, p-mTOR, and p70 S6K in OSCC were significantly higher than those in normal oral mucosa and paracancerous mucosa. The expressions of p-Akt, p-mTOR, and p70 S6K in OSCC were not correlated with age, gender, and clinical stage; by comparison, these expressions were correlated with lymph node metastasis and pathological grade. Strong positive correlations were also observed between the expressions ofp-Akt, p-mTOR, and p70 S6K in OSCC.

CONCLUSION

Akt/mTOR/p70 S6K signaling molecules exhibit active expressions in OSCC and may be implicated in the occurrence and development of OSCC.

[Expression and clinical significance of serine-threonine kinase/mammalian target of rapamycin/p70 S6K signal path- way in oral squamous cell carcinoma]

OBJECTIVE

To investigate the expressions of serine-threonine kinase (Akt)/mammalian target of rapamycin (mTOR)/p70 S6K in oral squamous cell carcinoma (OSCC) and provide references for early diagnosis and prognosis evalua- tion of OSCC.

METHODS

A total of 51 cases of OSCC, 10 cases of paracancerous mucosa, and 10 cases of normal oral mucosa were collected. The expressions of Akt/mTOR/p70 S6K in these cases were detected using the SP method of immunohisto- chemistry. The correlation between their expressions in OSCC was also analyzed.

RESULTS

The positive expressions ofp-Akt, p-mTOR, and p70 S6K in OSCC were significantly higher than those in normal oral mucosa and paracancerous mucosa. The expressions of p-Akt, p-mTOR, and p70 S6K in OSCC were not correlated with age, gender, and clinical stage; by comparison, these expressions were correlated with lymph node metastasis and pathological grade. Strong positive correlations were also observed between the expressions ofp-Akt, p-mTOR, and p70 S6K in OSCC.

CONCLUSION

Akt/mTOR/p70 S6K signaling molecules exhibit active expressions in OSCC and may be implicated in the occurrence and development of OSCC.

Co-option of the piRNA pathway for germline-specific alternative splicing of C. elegans TOR

Many eukaryotic genes contain embedded antisense transcripts and repetitive sequences of unknown function. We report that male germline-specific expression of an antisense transcript contained in an intron of C. elegans Target of Rapamycin (TOR, let-363) is associated with (1) accumulation of endo-small interfering RNAs (siRNAs) against an embedded Helitron transposon and (2) activation of an alternative 3' splice site of TOR. The germline-specific Argonaute proteins PRG-1 and CSR-1, which participate in self/nonself RNA recognition, antagonistically regulate the generation of these endo-siRNAs, TOR mRNA levels, and 3' splice-site selection. Supply of exogenous double-stranded RNA against the region of sense/antisense overlap reverses changes in TOR expression and splicing and suppresses the progressive multigenerational sterility phenotype of prg-1 mutants. We propose that recognition of a "nonself" intronic transposon by endo-siRNAs/the piRNA system provides physiological regulation of expression and alternative splicing of a host gene that, in turn, contributes to the maintenance of germline function across generations.

Development of synchronous VHL syndrome tumors reveals contingencies and constraints to tumor evolution

BACKGROUND

Genomic analysis of multi-focal renal cell carcinomas from an individual with a germline VHL mutation offers a unique opportunity to study tumor evolution.

RESULTS

We perform whole exome sequencing on four clear cell renal cell carcinomas removed from both kidneys of a patient with a germline VHL mutation. We report that tumors arising in this context are clonally independent and harbour distinct secondary events exemplified by loss of chromosome 3p, despite an identical genetic background and tissue microenvironment. We propose that divergent mutational and copy number anomalies are contingent upon the nature of 3p loss of heterozygosity occurring early in tumorigenesis. However, despite distinct 3p events, genomic, proteomic and immunohistochemical analyses reveal evidence for convergence upon the PI3K-AKT-mTOR signaling pathway. Four germline tumors in this young patient, and in a second, older patient with VHL syndrome demonstrate minimal intra-tumor heterogeneity and mutational burden, and evaluable tumors appear to follow a linear evolutionary route, compared to tumors from patients with sporadic clear cell renal cell carcinoma.

CONCLUSIONS

In tumors developing from a germline VHL mutation, the evolutionary principles of contingency and convergence in tumor development are complementary. In this small set of patients with early stage VHL-associated tumors, there is reduced mutation burden and limited evidence of intra-tumor heterogeneity.

mTORC1 drives HIF-1α and VEGF-A signalling via multiple mechanisms involving 4E-BP1, S6K1 and STAT3

Recent clinical trials using rapalogues in tuberous sclerosis complex (TSC) show regression in volume of typically vascularised tumours including angiomyolipomas (AMLs) and sub-ependymal giant cell astrocytomas (SEGAs). By blocking mechanistic/mammalian target of rapamycin complex 1 (mTORC1) signalling, rapalogue efficacy is likely to occur in part through suppression of hypoxia inducible factors (HIFs) and vascular endothelial growth factors (VEGFs). We show that rapamycin reduces HIF-1α protein levels, and to a lesser extent VEGF-A levels, in renal cystadenoma cells in a Tsc2+/− mouse model. We establish that mTORC1 drives HIF-1α protein accumulation through enhanced transcription of HIF-1α mRNA, a process that is blocked by either inhibition or knockdown of signal transducer and activation of transcription 3 (STAT3). Furthermore, we demonstrate that STAT3 is directly phosphorylated by mTORC1 on Ser727 during hypoxia, promoting HIF-1α mRNA transcription. mTORC1 also regulates HIF-1α synthesis on a translational level via co-operative regulation of both initiation factor 4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase-1 (S6K1), whilst HIF-1α degradation remains unaffected. We therefore propose that mTORC1 drives HIF-1α synthesis in a multi-faceted manner through 4E-BP1/eIF4E, S6K1 and STAT3. Interestingly, we observe a disconnect between HIF-1α protein levels and VEGF-A expression. While both S6K1 and 4E-BP1 regulate HIF-1α translation, VEGF-A is primarily under the control of 4E-BP1/eIF4E. S6K1 inhibition reduces HIF-1α but not VEGF-A expression, suggesting that mTORC1 mediates VEGF-A expression via both HIF-1α-dependent and -independent mechanisms. Our work has important implications for the treatment of vascularised tumours, where mTORC1 acts as a central mediator of STAT3, HIF-1α, VEGF-A and angiogenesis via multiple signalling mechanisms.

Glucose activates TORC2-Gad8 protein via positive regulation of the cAMP/cAMP-dependent protein kinase A (PKA) pathway and negative regulation of the Pmk1 protein-mitogen-activated protein kinase pathway

The target of rapamycin (TOR) kinase belongs to the highly conserved eukaryotic family of phosphatidylinositol 3-kinase-related kinases. TOR proteins are found at the core of two evolutionary conserved complexes, known as TORC1 and TORC2. In fission yeast, TORC2 is dispensable for proliferation under optimal growth conditions but is required for starvation and stress responses. TORC2 has been implicated in a wide variety of functions; however, the signals that regulate TORC2 activity have so far remained obscure. TORC2 has one known direct substrate, the AGC kinase Gad8, which is related to AKT in human cells. Gad8 is phosphorylated by TORC2 at Ser-546 (equivalent to AKT Ser-473), leading to its activation. Here, we show that glucose is necessary and sufficient to induce Gad8 Ser-546 phosphorylation in vivo and Gad8 kinase activity in vitro. The glucose signal that activates TORC2-Gad8 is mediated via the cAMP/PKA pathway, a major glucose-sensing pathway. By contrast, Pmk1, similar to human extracellular signal-regulated kinases and a major stress-induced mitogen activated protein kinase (MAPK) in fission yeast, inhibits TORC2-dependent Gad8 phosphorylation and activation. Inhibition of TORC2-Gad8 also occurs in response to ionic or osmotic stress, in a manner dependent on the cAMP/PKA and Pmk1-MAPK signaling pathways. Our findings highlight the significance of glucose availability in regulation of TORC2-Gad8 and indicate a novel link between the cAMP/PKA, Pmk1/MAPK, and TORC2-Gad8 signaling.

Fission yeast arrestin-related trafficking adaptor, Arn1/Any1, is ubiquitinated by Pub1 E3 ligase and regulates endocytosis of Cat1 amino acid transporter

The Tsc1–Tsc2 complex homologous to human tuberous sclerosis complex proteins governs amino acid uptake by regulating the expression and intracellular distribution of amino acid transporters in Schizosaccharomyces pombe. Here, we performed a genetic screening for molecules that are involved in amino acid uptake and found Arn1 (also known as Any1). Arn1 is homologous to ART1, an arrestin-related trafficking adaptor (ART) in Saccharomyces cerevisiae, and contains a conserved arrestin motif, a ubiquitination site, and two PY motifs. Overexpression of arn1⁺ confers canavanine resistance on cells, whereas its disruption causes hypersensitivity to canavanine. We also show that Arn1 regulates endocytosis of the Cat1 amino acid transporter. Furthermore, deletion of arn1⁺ suppresses a defect of amino acid uptake and the aberrant Cat1 localization in tsc2Δ. Arn1 interacts with and is ubiquitinated by the Pub1 ubiquitin ligase, which is necessary to regulate Cat1 endocytosis. Cat1 undergoes ubiquitinations on lysine residues within the N-terminus, which are mediated, in part, by Arn1 to determine Cat1 localization. Correctively, Arn1 is an ART in S. pombe and contributes to amino acid uptake through regulating Cat1 endocytosis in which Tsc2 is involved.

FMRP S499 is phosphorylated independent of mTORC1-S6K1 activity

Hyperactive mammalian target of rapamycin (mTOR) is associated with cognitive deficits in several neurological disorders including tuberous sclerosis complex (TSC). The phosphorylation of the mRNA-binding protein FMRP reportedly depends on mTOR complex 1 (mTORC1) activity via p70 S6 kinase 1 (S6K1). Because this phosphorylation is thought to regulate the translation of messages important for synaptic plasticity, we explored whether FMRP phosphorylation of the S6K1-dependent residue (S499) is altered in TSC and states of dysregulated TSC-mTORC1 signaling. Surprisingly, we found that FMRP S499 phosphorylation was unchanged in heterozygous and conditional Tsc1 knockout mice despite significantly elevated mTORC1-S6K1 activity. Neither up- nor down-regulation of the mTORC1-S6K1 axis in vivo or in vitro had any effect on phospho-FMRP S499 levels. In addition, FMRP S499 phosphorylation was unaltered in S6K1-knockout mice. Collectively, these data strongly suggest that FMRP S499 phosphorylation is independent of mTORC1-S6K1 activity and is not altered in TSC.

A diverse array of cancer-associated MTOR mutations are hyperactivating and can predict rapamycin sensitivity

Genes encoding components of the PI3K-AKT-mTOR signaling axis are frequently mutated in cancer, but few mutations have been characterized in MTOR, the gene encoding the mTOR kinase. Using publicly available tumor genome sequencing data, we generated a comprehensive catalog of mTOR pathway mutations in cancer, identifying 33 MTOR mutations that confer pathway hyperactivation. The mutations cluster in six distinct regions in the C-terminal half of mTOR and occur in multiple cancer types, with one cluster particularly prominent in kidney cancer. The activating mutations do not affect mTOR complex assembly, but a subset reduces binding to the mTOR inhibitor DEPTOR. mTOR complex 1 (mTORC1) signaling in cells expressing various activating mutations remains sensitive to pharmacologic mTOR inhibition, but is partially resistant to nutrient deprivation. Finally, cancer cell lines with hyperactivating MTOR mutations display heightened sensitivity to rapamycin both in culture and in vivo xenografts, suggesting that such mutations confer mTOR pathway dependency.

mTOR signaling and its roles in normal and abnormal brain development

Target of rapamycin (TOR) was first identified in yeast as a target molecule of rapamycin, an anti-fugal and immunosuppressant macrolide compound. In mammals, its orthologue is called mammalian TOR (mTOR). mTOR is a serine/threonine kinase that converges different extracellular stimuli, such as nutrients and growth factors, and diverges into several biochemical reactions, including translation, autophagy, transcription, and lipid synthesis among others. These biochemical reactions govern cell growth and cause cells to attain an anabolic state. Thus, the disruption of mTOR signaling is implicated in a wide array of diseases such as cancer, diabetes, and obesity. In the central nervous system, the mTOR signaling cascade is activated by nutrients, neurotrophic factors, and neurotransmitters that enhances protein (and possibly lipid) synthesis and suppresses autophagy. These processes contribute to normal neuronal growth by promoting their differentiation, neurite elongation and branching, and synaptic formation during development. Therefore, disruption of mTOR signaling may cause neuronal degeneration and abnormal neural development. While reduced mTOR signaling is associated with neurodegeneration, excess activation of mTOR signaling causes abnormal development of neurons and glia, leading to brain malformation. In this review, we first introduce the current state of molecular knowledge of mTOR complexes and signaling in general. We then describe mTOR activation in neurons, which leads to translational enhancement, and finally discuss the link between mTOR and normal/abnormal neuronal growth during development.

S. pombe TORC1 activates the ubiquitin-proteasomal degradation of the meiotic regulator Mei2 in cooperation with Pat1 kinase

Target of rapamycin (TOR) kinase regulates cell metabolism and growth, acting as a subunit of two multi-protein complexes, TORC1 and TORC2. Known TORC substrates are either kinases or general factors involved in growth control. Here, we show that fission yeast TORC1, which promotes vegetative growth and suppresses sexual development, can phosphorylate Mei2 (a specific factor involved in switching the cell fate) in vitro. Alanine substitutions at the nine Mei2 phosphorylation sites stabilize the protein and promote mating and meiosis in vivo. We found that Mei2 is polyubiquitylated in vivo in a TORC1-dependent manner. Based on these data, we propose that TORC1 contributes to the suppression of sexual development by phosphorylating Mei2, in addition to controlling the cellular metabolic status.

Activating mTOR mutations in a patient with an extraordinary response on a phase I trial of everolimus and pazopanib

Understanding the genetic mechanisms of sensitivity to targeted anticancer therapies may improve patient selection, response to therapy, and rational treatment designs. One approach to increase this understanding involves detailed studies of exceptional responders: rare patients with unexpected exquisite sensitivity or durable responses to therapy. We identified an exceptional responder in a phase I study of pazopanib and everolimus in advanced solid tumors. Whole-exome sequencing of a patient with a 14-month complete response on this trial revealed two concurrent mutations in mTOR, the target of everolimus. In vitro experiments demonstrate that both mutations are activating, suggesting a biologic mechanism for exquisite sensitivity to everolimus in this patient. The use of precision (or "personalized") medicine approaches to screen patients with cancer for alterations in the mTOR pathway may help to identify subsets of patients who may benefit from targeted therapies directed against mTOR.

Isp7 is a novel regulator of amino acid uptake in the TOR signaling pathway

TOR proteins reside in two distinct complexes, TOR complexes 1 and 2 (TORC1 and TORC2), that are central for the regulation of cellular growth, proliferation, and survival. TOR is also the target for the immunosuppressive and anticancer drug rapamycin. In Schizosaccharomyces pombe, disruption of the TSC complex, mutations in which can lead to the tuberous sclerosis syndrome in humans, results in a rapamycin-sensitive phenotype under poor nitrogen conditions. We show here that the sensitivity to rapamycin is mediated via inhibition of TORC1 and suppressed by overexpression of isp7(+), a member of the family of 2-oxoglutarate-Fe(II)-dependent oxygenase genes. The transcript level of isp7(+) is negatively regulated by TORC1 but positively regulated by TORC2. Yet we find extensive similarity between the transcriptome of cells disrupted for isp7(+) and cells mutated in the catalytic subunit of TORC1. Moreover, Isp7 regulates amino acid permease expression in a fashion similar to that of TORC1 and opposite that of TORC2. Overexpression of isp7(+) induces TORC1-dependent phosphorylation of ribosomal protein Rps6 while inhibiting TORC2-dependent phosphorylation and activation of the AGC-like kinase Gad8. Taken together, our findings suggest a central role for Isp7 in amino acid homeostasis and the presence of isp7(+)-dependent regulatory loops that affect both TORC1 and TORC2.

Phosphorylation of the TOR ATP binding domain by AGC kinase constitutes a novel mode of TOR inhibition

AGC kinase–mediated phosphorylation of the TOR kinase reduces its activity and results in physiologically significant changes in TOR signalling in both yeast and human cells.

TOR (target of rapamycin) signaling coordinates cell growth, metabolism, and cell division through tight control of signaling via two complexes, TORC1 and TORC2. Here, we show that fission yeast TOR kinases and mTOR are phosphorylated on an evolutionarily conserved residue of their ATP-binding domain. The Gad8 kinase (AKT homologue) phosphorylates fission yeast Tor1 at this threonine (T1972) to reduce activity. A T1972A mutation that blocked phosphorylation increased Tor1 activity and stress resistance. Nitrogen starvation of fission yeast inhibited TOR signaling to arrest cell cycle progression in G1 phase and promoted sexual differentiation. Starvation and a Gad8/T1972-dependent decrease in Tor1 (TORC2) activity was essential for efficient cell cycle arrest and differentiation. Experiments in human cell lines recapitulated these yeast observations, as mTOR was phosphorylated on T2173 in an AKT-dependent manner. In addition, a T2173A mutation increased mTOR activity. Thus, TOR kinase activity can be reduced through AGC kinase–controlled phosphorylation to generate physiologically significant changes in TOR signaling.

Hypoxia-inducible factor 1 regulation through cross talk between mTOR and MT1-MMP

Hypoxia-inducible factor 1 (HIF-1) plays a key role in the cellular adaptation to hypoxia. Although HIF-1 is usually strongly suppressed by posttranslational mechanisms during normoxia, HIF-1 is active and enhances tumorigenicity in malignant tumor cells that express the membrane protease MT1-MMP. The cytoplasmic tail of MT1-MMP, which can bind a HIF-1 suppressor protein called factor inhibiting HIF-1 (FIH-1), promotes inhibition of FIH-1 by Mint3 during normoxia. To explore possible links between HIF-1 activation by MT1-MMP/Mint3 and tumor growth signals, we surveyed a panel of 252 signaling inhibitors. The mTOR inhibitor rapamycin was identified as a possible modulator, and it inhibited the mTOR-dependent phosphorylation of Mint3 that is required for FIH-1 inhibition. A mutant Mint3 protein that cannot be phosphorylated exhibited a reduced ability to inhibit FIH-1 and promoted tumor formation in mice. These data suggest a novel molecular link between the important hub proteins MT1-MMP and mTOR that contributes to tumor malignancy.

TORC1 signaling is governed by two negative regulators in fission yeast

The target of rapamycin (TOR) is a highly conserved protein kinase that regulates cell growth and metabolism. Here we performed a genome-wide screen to identify negative regulators of TOR complex 1 (TORC1) in Schizosaccharomyces pombe by isolating mutants that phenocopy Δtsc2, in which TORC1 signaling is known to be up-regulated. We discovered that Δnpr2 displayed similar phenotypes to Δtsc2 in terms of amino acid uptake defects and mislocalization of the Cat1 permease. However, Δnpr2 and Δtsc2 clearly showed different phenotypes in terms of rapamycin supersensitivity and Isp5 transcription upon various treatments. Furthermore, we showed that Tor2 controls amino acid homeostasis at the transcriptional and post-transcriptional levels. Our data reveal that both Npr2 and Tsc2 negatively regulate TORC1 signaling, and Npr2, but not Tsc2, may be involved in the feedback loop of a nutrient-sensing pathway.

The reverse, but coordinated, roles of Tor2 (TORC1) and Tor1 (TORC2) kinases for growth, cell cycle and separase-mediated mitosis in Schizosaccharomyces pombe

Target of rapamycin complexes (TORCs), which are vital for nutrient utilization, contain a catalytic subunit with the phosphatidyl inositol kinase-related kinase (PIKK) motif. TORC1 is required for cell growth, while the functions of TORC2 are less well understood. We show here that the fission yeast Schizosaccharomyces pombe TORC2 has a cell cycle role through determining the proper timing of Cdc2 Tyr15 dephosphorylation and the cell size under limited glucose, whereas TORC1 restrains mitosis and opposes securin–separase, which are essential for chromosome segregation. These results were obtained using the previously isolated TORC1 mutant tor2-L2048S in the phosphatidyl inositol kinase (PIK) domain and a new TORC2 mutant tor1-L2045D, which harbours a mutation in the same site. While mutated TORC1 and TORC2 displayed diminished kinase activity and FKBP12/Fkh1-dependent rapamycin sensitivity, their phenotypes were nearly opposite in mitosis. Premature mitosis and the G2–M delay occurred in TORC1 and TORC2 mutants, respectively. Surprisingly, separase/cut1—securin/cut2 mutants were rescued by TORC1/tor2-L2048S mutation or rapamycin addition or even Fkh1 deletion, whereas these mutants showed synthetic defect with TORC2/tor1-L2045D. TORC1 and TORC2 coordinate growth, mitosis and cell size control, such as Wee1 and Cdc25 do for the entry into mitosis.

TGF-β/Smad3 activates mammalian target of rapamycin complex-1 to promote collagen production by increasing HIF-1α expression

Transforming growth factor (TGF)-β is a major mediator of kidney fibrosis. In the past decade it was recognized that, besides canonical Smad signaling, many other signaling pathways participate in the process of TGF-β-induced fibrogenesis. One such pathway involves mammalian target of rapamycin complex (mTORC)1. We recently reported that the hypoxia-inducible factor (HIF)-1 is essential for TGF-β-induced collagen expression regardless of ambient oxygen tension. A modulator of HIF expression other than oxygen tension is mTORC1. We therefore sought to evaluate a possible role for mTORC1 activity in TGF-β-induced fibrogenesis. mTORC1 activity was increased in human mesangial cells treated with TGF-β in a TGF-β receptor-dependent manner. Short hairpin (sh)RNA to Smad3 decreased, while overexpression of Smad3 increased, the mTORC1 activity, suggesting that TGF-β stimulation of mTORC1 also requires Smad3. Pretreatment with rapamycin or shRNA for a regulatory molecule of mTORC1, Raptor, reduced TGF-β-induced COL1A2-luc activity and collagen I protein expression. mTORC1 inhibition also prevented the TGF-β-stimulated increase in both hypoxia-responsive element (HRE) activity and HIF-1α protein expression, while activation of mTORC1 by active Rheb increased basal but not TGF-β-induced HRE activity. shRNA to Smad3 reduced HRE activity, while overexpression of Smad3 increased HIF-1α protein expression and activity in an mTORC1-dependent manner. Lastly, overexpression of HIF-1α bypassed the inhibitory effect of mTORC1 blockade on collagen expression. These results suggest that Smad3/mTORC1 interaction to promote HIF-1 expression is a key step in normoxic kidney fibrogenesis.

TORC2 is required to maintain genome stability during S phase in fission yeast

DNA damage can occur due to environmental insults or intrinsic metabolic processes and is a major threat to genome stability. The DNA damage response is composed of a series of well coordinated cellular processes that include activation of the DNA damage checkpoint, transient cell cycle arrest, DNA damage repair, and reentry into the cell cycle. Here we demonstrate that mutant cells defective for TOR complex 2 (TORC2) or the downstream AGC-like kinase, Gad8, are highly sensitive to chronic replication stress but are insensitive to ionizing radiation. We show that in response to replication stress, TORC2 is dispensable for Chk1-mediated cell cycle arrest but is required for the return to cell cycle progression. Rad52 is a DNA repair and recombination protein that forms foci at DNA damage sites and stalled replication forks. TORC2 mutant cells show increased spontaneous nuclear Rad52 foci, particularly during S phase, suggesting that TORC2 protects cells from DNA damage that occurs during normal DNA replication. Consistently, the viability of TORC2-Gad8 mutant cells is dependent on the presence of the homologous recombination pathway and other proteins that are required for replication restart following fork replication stalling. Our findings indicate that TORC2 is required for genome integrity. This may be relevant for the growing amount of evidence implicating TORC2 in cancer development.

IκB kinase complex (IKK) triggers detachment-induced autophagy in mammary epithelial cells independently of the PI3K-AKT-MTORC1 pathway

Adherent cells require proper integrin-mediated extracellular matrix (ECM) engagement for growth and survival; normal cells deprived of proper ECM contact undergo anoikis. At the same time, autophagy is induced as a survival pathway in both fibroblasts and epithelial cells upon ECM detachment. Here, we further define the intracellular signals that mediate detachment-induced autophagy and uncover an important role for the IκB kinase (IKK) complex in the induction of autophagy in mammary epithelial cells (MECs) deprived of ECM contact. Whereas the PI3K-AKT-MTORC1 pathway activation potently inhibits autophagy in ECM-detached fibroblasts, enforced activation of this pathway is not sufficient to suppress detachment-induced autophagy in MECs. Instead, inhibition of IKK, as well as its upstream regulator, MAP3K7/TAK1, significantly attenuates detachment-induced autophagy in MECs. Furthermore, function-blocking experiments corroborate that both IKK activation and autophagy induction result from decreased ITGA3-ITGB1 (α3β1 integrin) function. Finally, we demonstrate that pharmacological IKK inhibition enhances anoikis and accelerates luminal apoptosis during acinar morphogenesis in three-dimensional culture. Based on these results, we propose that the IKK complex functions as a key mediator of detachment-induced autophagy and anoikis resistance in epithelial cells.

mTOR kinase structure, mechanism and regulation

The mammalian target of rapamycin (mTOR), a phosphoinositide 3-kinase-related protein kinase, controls cell growth in response to nutrients and growth factors and is frequently deregulated in cancer. Here we report co-crystal structures of a complex of truncated mTOR and mammalian lethal with SEC13 protein 8 (mLST8) with an ATP transition state mimic and with ATP-site inhibitors. The structures reveal an intrinsically active kinase conformation, with catalytic residues and a catalytic mechanism remarkably similar to canonical protein kinases. The active site is highly recessed owing to the FKBP12-rapamycin-binding (FRB) domain and an inhibitory helix protruding from the catalytic cleft. mTOR-activating mutations map to the structural framework that holds these elements in place, indicating that the kinase is controlled by restricted access. In vitro biochemistry shows that the FRB domain acts as a gatekeeper, with its rapamycin-binding site interacting with substrates to grant them access to the restricted active site. Rapamycin-FKBP12 inhibits the kinase by directly blocking substrate recruitment and by further restricting active-site access. The structures also reveal active-site residues and conformational changes that underlie inhibitor potency and specificity.

Rheb activation in subventricular zone progenitors leads to heterotopia, ectopic neuronal differentiation, and rapamycin-sensitive olfactory micronodules and dendrite hypertrophy of newborn neurons

Mammalian target of rapamycin (mTOR) hyperactivity in perinatal neural progenitor cells (NPCs) of tuberous sclerosis complex 1 (Tsc1) heterozygote mice leads to heterotopia and abnormal neuronal morphogenesis as seen in patients with tuberous sclerosis. Considering that pathological hyperactive mTOR also occurs in individuals carrying no genetic mutations, we examined whether increasing mTOR activity in neonatal NPCs of wild-type mice would recapitulate the above phenotypes. Electroporation of a plasmid encoding constitutively active Ras-homolog enriched in brain (Rheb(CA)) into subventricular zone NPCs increased mTOR activity in newborn cells. At 19 d post-electroporation (dpe), heterotopia and ectopic cells with a neuronal morphology were observed along the migratory path [rostral migratory stream (RMS)] and in the olfactory bulb (OB). These ectopic cells displayed action potentials and received synaptic inputs identifying them as synaptically integrated neurons. RMS heterotopias contained astrocytes, neurons, and entrapped neuroblasts. Immunostaining at 3 dpe revealed the presence of Mash1(+) Olig2(-) cells in the migratory route accompanied by ectopic neuronal differentiation and altered direction and speed of neuroblast migration at 7 dpe, suggesting a non-cell-autonomous disruption of migration. At >19 dpe, newborn Rheb(CA)-expressing neurons displayed altered distribution and formed micronodules in the OB. In addition, they displayed increased dendritic complexity along with altered membrane biophysics and increased frequency of GABAergic synaptic inputs. OB heterotopia, micronodules, and dendrite hypertrophy were notably prevented by rapamycin treatment, suggesting their mTOR dependence. Collectively, these data show that increasing mTOR activity in neonatal NPCs of wild-type mice recapitulate the pathologies observed in Tsc1 mutant mice. In addition, increased mTOR activity in individuals without known mutations could significantly impact neurogenesis and circuit formation.

Mutations in critical domains confer the human mTOR gene strong tumorigenicity

Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that regulates cell growth, proliferation, and survival. mTOR is frequently activated in human cancers and is a commonly sought anticancer therapeutic target. However, whether the human mTOR gene itself is a proto-oncogene possessing tumorigenicity has not been firmly established. To answer this question, we mutated evolutionarily conserved amino acids, generated eight mutants in the HEAT repeats (M938T) and the FAT (W1456R and G1479N) and kinase (P2273S, V2284M, V2291I, T2294I, and E2288K) domains of mTOR, and studied their oncogenicity. On transient expression in HEK293T cells, these mTOR mutants displayed elevated protein kinase activities accompanied by activated mTOR/p70S6K signaling at varying levels, demonstrating the gain of function of the mTOR gene with these mutations. We selected P2273S and E2288K, the two most catalytically active mutants, to further examine their oncogenicity and tumorigenicity. Stable expression of the two mTOR mutants in NIH3T3 cells strongly activated mTOR/p70S6K signaling, induced cell transformation and invasion, and remarkably, caused rapid tumor formation and growth in athymic nude mice after subcutaneous inoculation of the transfected cells. This study confirms the oncogenic potential of mTOR suggested previously and demonstrates for the first time its tumorigenicity. Thus, beyond the pivotal position of mTOR to relay the oncogenic signals from the upstream phosphatidylinositol 3-kinase/Akt pathway in human cancer, mTOR is capable potentially of playing a direct role in human tumorigenesis if mutated. These results also further support the conclusion that mTOR is a major therapeutic target in human cancers.

HIF-independent role of prolyl hydroxylases in the cellular response to amino acids

Hypoxia-inducible factor (HIF) prolyl hydroxylases (PHDs) are α-ketoglutarate (αKG)-dependent dioxygenases that function as cellular oxygen sensors. However, PHD activity also depends on factors other than oxygen, especially αKG, a key metabolic compound closely linked to amino-acid metabolism. We examined the connection between amino-acid availability and PHD activity. We found that amino-acid starvation leads to αKG depletion and to PHD inactivation but not to HIF stabilization. Furthermore, pharmacologic or genetic inhibition of PHDs induced autophagy and prevented mammalian target of rapamycin complex 1 (mTORC1) activation by amino acids in a HIF-independent manner. Therefore, PHDs sense not only oxygen but also respond to amino acids, constituting a broad intracellular nutrient-sensing network.

Psk1, an AGC kinase family member in fission yeast, is directly phosphorylated and controlled by TORC1 and functions as S6 kinase

Target of rapamycin (TOR), an evolutionarily conserved serine/threonine protein kinase, plays pivotal roles in several important cellular processes in eukaryotes. In the fission yeast Schizosaccharomyces pombe, TOR complex 1 (TORC1), which includes Tor2 as a catalytic subunit, manages the switch between cell proliferation and differentiation by sensing nutrient availability. However, little is known about the direct target of TORC1 that plays key roles in nutrient-dependent TORC1 signaling in fission yeast. Here we report that in fission yeast, three AGC kinase family members, named Psk1, Sck1 and Sck2, which exhibit high homology with human S6K1, are phosphorylated under nutrient-rich conditions and are dephosphorylated by starvation conditions. Among these, Psk1 is necessary for phosphorylation of ribosomal protein S6. Furthermore, Psk1 phosphorylation is regulated by TORC1 in nutrient-dependent and rapamycin-sensitive manners in vivo. Three conserved regulatory motifs (the activation loop, the hydrophobic and the turn motifs) in Psk1 are phosphorylated and these modifications are required for Psk1 activity. In particular, phosphorylation of the hydrophobic motif is catalyzed by TORC1 in vivo and in vitro. Ksg1, a homolog of PDK1, is also important for Psk1 phosphorylation in the activation loop and for its activity. The TORC1 components Pop3, Toc1 and Tco89, are dispensable for Psk1 regulation, but disruption of pop3(+) causes an increase in the sensitivity of TORC1 to rapamycin. Taken together, these results provide convincing evidence that TORC1/Psk1/Rps6 constitutes a nutrient-dependent signaling pathway in fission yeast.

Genetic variations in a PTEN/AKT/mTOR axis and prostate cancer risk in a Chinese population

Backgroud

Genetic variations in a PTEN/AKT/mTOR signaling axis may influence cellular functions including cell growth, proliferation and apoptosis, and then increase the individual’s risk of cancer. Accordingly, we explore the association between single nucleotide polymorphisms (SNPs) of these genes and prostate cancer (PCa) in our Chinese population.

Methods

Subjects were recruited from 666 PCa patients and 708 cancer-free controls, and eight SNPs in the PTEN/AKT/mTOR axis were determined by the TaqMan assay. Odds ratios (OR) and 95% confidence intervals (95% CI) were evaluated by logistic regression.

Results

We observed significant associations between PCa risk and mTOR rs2295080 [P = 0.027, OR = 0.85, 95%CI = 0.74–0.98], and AKT2 rs7254617 (P = 0.003, OR = 1.35, 95%CI = 1.11–1.64). When estimated these two SNPs together, the combined genotypes with 2–4 risk alleles (rs2295080 T and rs7254617 A alleles) were associated with an increased risk of PCa compared with 0–1 risk alleles, which was more pronounced among subgroups of age >71 years, smokers, drinkers and no family history of cancer. Results of stratified analyses by cliniopathological parameters revealed that the frequencies of the combined genotypes with 2–4 risk alleles in advanced stage were significantly higher than in localized stage(P = 0.022), but there was no significant association in Gleason score and PSA level.

Conclusion

Our results indicate, for the first time that the two variants in AKT2 and mTOR, particularly the joint genotypes with 2–4 risk alleles may influence PCa susceptibility and progression in Chinese, and the association appeared to be more strong in the subgroup of smokers and drinkers.

TORC2 and the AGC kinase Gad8 regulate phosphorylation of the ribosomal protein S6 in fission yeast

TOR (Target Of Rapamycin) signalling coordinates cell growth and division in response to changes in the nutritional environment of the cell. TOR kinases form two distinct complexes: TORC1 and TORC2. In mammals, the TORC1 controlled S6K1 kinase phosphorylates the ribosomal protein S6 thereby co-ordinating cell size and nutritional status. We show that the Schizosaccharomyces pombe AGC kinase Gad8 co-immunoprecipitates with the ribosomal protein S6 (Rps6) and regulates its phosphorylation status. It has previously been shown that Gad8 is phosphorylated by TORC2. Consistent with this, we find that TORC2 as well as TORC1 modulates Rps6 phosphorylation. Therefore, S6 phosphorylation in fission yeast actually represents a read-out of the combined activities of TORC1 and TORC2. In contrast, we find that the in vivo phosphorylation status of Maf1 (a repressor of RNA polymerase III) specifically correlates with TORC1 activity.

TORC1 of fission yeast is rapamycin-sensitive

The target of rapamycin (TOR) protein kinase plays central roles in the regulation of cell growth in response to nutritional availability. TOR forms two distinct multiprotein complexes termed TOR complex 1 (TORC1) and TORC2. Typically, only the activity of TORC1 is inhibited by the immunosuppressant rapamycin. Although rapamycin strongly inhibits cell growth of the budding yeast Saccharomyces cerevisiae through inhibition of TORC1, growth of the fission yeast Schizosaccharomyces pombe appears to be resistant to rapamycin. Here, we demonstrate that rapamycin inhibits the kinase activity of S. pombe TORC1 in vitro in a similar manner to TORC1 of other organisms. We furthermore show that incomplete inhibition of TORC1 by rapamycin underlies the apparent rapamycin resistance of S. pombe. In the presence of caffeine, which potentially lowers TORC1 activity, the growth of wild-type S. pombe cells is sensitive to rapamycin in a TORC1-dependent manner. Moreover, treatment of S. pombe cells with rapamycin plus caffeine induces starvation-specific gene expression and autophagy, similarly to cells with reduced TORC1 activity. These results indicate that rapamycin does inhibit TORC1 in S. pombe, but the inhibition is not sufficient to cause a growth defect. These findings establish a universal action of rapamycin on TORC1 inhibition.

Glutaminolysis activates Rag-mTORC1 signaling

Amino acids control cell growth via activation of the highly conserved kinase TORC1. Glutamine is a particularly important amino acid in cell growth control and metabolism. However, the role of glutamine in TORC1 activation remains poorly defined. Glutamine is metabolized through glutaminolysis to produce α-ketoglutarate. We demonstrate that glutamine in combination with leucine activates mammalian TORC1 (mTORC1) by enhancing glutaminolysis and α-ketoglutarate production. Inhibition of glutaminolysis prevented GTP loading of RagB and lysosomal translocation and subsequent activation of mTORC1. Constitutively active Rag heterodimer activated mTORC1 in the absence of glutaminolysis. Conversely, enhanced glutaminolysis or a cell-permeable α-ketoglutarate analog stimulated lysosomal translocation and activation of mTORC1. Finally, cell growth and autophagy, two processes controlled by mTORC1, were regulated by glutaminolysis. Thus, mTORC1 senses and is activated by glutamine and leucine via glutaminolysis and α-ketoglutarate production upstream of Rag. This may provide an explanation for glutamine addiction in cancer cells.

Nutrient limitations alter cell division control and chromosome segregation through growth-related kinases and phosphatases

In dividing fission yeast Schizosaccharomyces pombe cells, the balance between Wee1 kinase and Cdc25 phosphatase which control the cyclin-dependent kinase (CDK) at the G2-M transition determines the rod-shaped cell length. Under nitrogen source starvation or glucose limitation, however, cell size determination is considerably modulated, and cell size shortening occurs for wild-type cells. For several mutants of kinases or phosphatases, including CDK, target of rapamycin complex (TORC) 1 and 2, stress-responsive mitogen-activated protein kinase (MAPK) Sty1/Spc1, MAPK kinase Wis1, calcium- and calmodulin-dependent protein kinase kinase-like Ssp1, and type 2A and 2A-related phosphatases inhibitor Sds23, this cell shortening does not normally occur. In tor1 and ssp1 mutants, cell elongation is observed. Sds23 that binds to and inhibits 2A and 2A-related phosphatases is synergistic with Ssp1 in the cell size determination and survival under low glucose and nitrogen source. Tor2 (TORC1) is required for growth, whereas Tor1 (TORC2) is needed for determining division size according to different nutrient conditions. Surprisingly, in growth-diminished tor2 mutant or rapamycin-treated cells, the requirement of separase/Cut1-securin/Cut2 essential for chromosome segregation is greatly alleviated. By contrast, defects of tor1 with secruin/cut2 or overproduction of Cut1 are additive. While Tor1 and Tor2 are opposite in their apparent functions, both may actually coordinate cell division with growth in response to the changes in nutrients.

The Vam6 and Gtr1-Gtr2 pathway activates TORC1 in response to amino acids in fission yeast

The Rag family of GTPases has been implicated in the TORC1 activation in Drosophila and in mammalian cells in response to amino acids. We have investigated the role of the Rag GTPases Gtr1 and Gtr2 in TORC1 regulation in Schizosaccharomyces pombe. Fission yeast Gtr1 and Gtr2 are non-essential proteins that enhance cell growth in the presence of amino acids in the medium. The function of Gtr1 and Gtr2 in nutrient signaling is further supported by the observation that even in rich medium the deletion of either gene results in the promotion of mating, meiosis and sporulation, consistent with the downregulation of TORC1. We show that Gtr1 and Gtr2 colocalize with TORC1 in vacuoles, where TORC1 is presumably activated. Epistasis analyses indicated that Gtr1 and Gtr2 function downstream of Vam6 and upstream of TORC1 in response to amino acid signals. Our data demonstrate the existence of an evolutionarily conserved pathway with the Vam6 and Gtr1-Gtr2 pathway activating TORC1, which in turns stimulates cell growth and inhibits sexual differentiation.

TOR complex 2 (TORC2) in Dictyostelium suppresses phagocytic nutrient capture independently of TORC1-mediated nutrient sensing

The TOR protein kinase functions in two distinct complexes, TOR complex 1 (TORC1) and 2 (TORC2). TORC1 is required for growth in response to growth factors, nutrients and the cellular energy state; TORC2 regulates AKT signaling, which can modulate cytoskeletal polarization. In its ecological niche, Dictyostelium engulf bacteria and yeast for nutrient capture. Despite the essential role of TORC1 in control of cellular growth, we show that nutrient particle capture (phagocytosis) in Dictyostelium is independent of TORC1-mediated nutrient sensing and growth regulation. However, loss of Dictyostelium TORC2 components Rictor/Pia, SIN1/RIP3 and Lst8 promotes nutrient particle uptake; inactivation of TORC2 leads to increased efficiency and speed of phagocytosis. In contrast to phagocytosis, we show that macropinocytosis, an AKT-dependent process for cellular uptake of fluid phase nutrients, is not regulated by either of the TOR complexes. The integrated and balanced regulation of TORC1 and TORC2 might be crucial in Dictyostelium to coordinate growth and energy needs with other essential TOR-regulated processes.

Rhb1 regulates the expression of secreted aspartic protease 2 through the TOR signaling pathway in Candida albicans

Candida albicans is a major fungal pathogen in humans. In C. albicans, secreted aspartyl protease 2 (Sap2) is the most highly expressed secreted aspartic protease in vitro and is a virulence factor. Recent research links the small GTPase Rhb1 to C. albicans target of rapamycin (TOR) signaling in response to nitrogen availability. The results of this study show that Rhb1 is related to cell growth through the control of SAP2 expression when protein is the major nitrogen source. This process involves various components of the TOR signaling pathway, including Tor1 kinase and its downstream effectors. TOR signaling not only controls SAP2 transcription but also affects Sap2 protein levels, possibly through general amino acid control. DNA microarray analysis identifies other target genes downstream of Rhb1 in addition to SAP2. These findings provide new insight into nutrients, Rhb1-TOR signaling, and expression of C. albicans virulence factor.

Target of rapamycin (TOR) in nutrient signaling and growth control

TOR (Target Of Rapamycin) is a highly conserved protein kinase that is important in both fundamental and clinical biology. In fundamental biology, TOR is a nutrient-sensitive, central controller of cell growth and aging. In clinical biology, TOR is implicated in many diseases and is the target of the drug rapamycin used in three different therapeutic areas. The yeast Saccharomyces cerevisiae has played a prominent role in both the discovery of TOR and the elucidation of its function. Here we review the TOR signaling network in S. cerevisiae.

PPIs reverse multidrug resistance to chemotherapy in gastric cancer by inhibiting vacuolar H+-ATPase signal pathway

Multidrug resistance (MDR) in gastric cancer influences the effects of chemotherapy. The change in tumor microenvironment is intimately associated with multidrug resistance. Acidification and hypoxia are the two major characteristics of tumor microenvironment. The glucose metabolism in hypoxic conditions by the neoplasms leads to an intercellular pH drift towards acidity. But tumor cells can maintain intracellular to neutro-alkalinity. Vacuolar H⁺-ATPases may be involved in this process by regulating H⁺ excretion. Acidification of extracellular pH would trigger the intracellular PI3K/Akt signal pathway, activate mTOR, up-regulate the expression of HIF-1α, and promote the expression of P-gp and MRP1, resulting in multidrug resistance in gastric cancer. PPIs may change the tumor microenvironment by inhibiting the expression of vacuolar H⁺-ATPases, trigger the PI3K/Akt/mTOR/HIF-1α signal pathway and reverse multidrug resistance to chemotherapy in gastric cancer.

AAV transduction of dopamine neurons with constitutively active Rheb protects from neurodegeneration and mediates axon regrowth

There are currently no therapies that provide either protection or restoration of neuronal function for adult-onset neurodegenerative diseases such as Parkinson's disease (PD). Many clinical efforts to provide such benefits by infusion of neurotrophic factors have failed, in spite of robust effects in preclinical assessments. One important reason for these failures is the difficulty, due to diffusion limits, of providing these protein molecules in sufficient amounts to the intended cellular targets in the central nervous system. This challenge suggests an alternative approach, that of viral vector transduction to directly activate the intracellular signaling pathways that mediate neurotrophic effects. To this end we have investigated the ability of a constitutively active form of the GTPase Rheb, an important activator of mammalian target of rapamycin (mTor) signaling, to mediate neurotrophic effects in dopamine neurons of the substantia nigra (SN), a population of neurons affected in PD. We find that constitutively active hRheb(S16H) induces many neurotrophic effects in mice, including abilities to both preserve and restore the nigrostriatal dopaminergic axonal projections in a highly destructive neurotoxin model. We conclude that direct viral vector transduction of vulnerable neuronal populations to activate intracellular neurotrophic signaling pathways offers promise for the treatment of neurodegenerative disease.

Future directions in renal cell carcinoma: 2011 and beyond

Emerging from a largely cytokine-based era, the last several years have witnessed a dramatic change in the therapeutic landscape of renal cancer. Molecularly targeted and antiangiogenic agents now form the backbone of most therapeutic strategies for patients with advanced renal cell carcinoma (RCC). Although the next few years may not see such broad paradigm shifts, there remains significant room for improvement in the care of patients with RCC. This review discusses challenges that face physicians and researchers as well as innovations that may contribute to improving the therapeutic outcomes for patients with RCC.

Control of mTORC1 signaling by the Opitz syndrome protein MID1

Mutations in the MID1 gene are causally linked to X-linked Opitz BBB/G syndrome (OS), a congenital disorder that primarily affects the formation of diverse ventral midline structures. The MID1 protein has been shown to function as an E3 ligase targeting the catalytic subunit of protein phosphatase 2A (PP2A-C) for ubiquitin-mediated degradation. However, the molecular pathways downstream of the MID1/PP2A axis that are dysregulated in OS and that translate dysfunctional MID1 and elevated levels of PP2A-C into the OS phenotype are poorly understood. Here, we show that perturbations in MID1/PP2A affect mTORC1 signaling. Increased PP2A levels, resulting from proteasome inhibition or depletion of MID1, lead to disruption of the mTOR/Raptor complex and down-regulated mTORC1 signaling. Congruously, cells derived from OS patients that carry MID1 mutations exhibit decreased mTORC1 formation, S6K1 phosphorylation, cell size, and cap-dependent translation, all of which is rescued by expression of wild-type MID1 or an activated mTOR allele. Our findings define mTORC1 signaling as a downstream pathway regulated by the MID1/PP2A axis, suggesting that mTORC1 plays a key role in OS pathogenesis.

Non-canonical functions of the tuberous sclerosis complex-Rheb signalling axis

The protein products of the tuberous sclerosis complex (TSC) genes, TSC1 and TSC2, form a complex, which inhibits the small G-protein, Ras homolog enriched in brain (Rheb). The vast majority of research regarding these proteins has focused on mammalian Target of Rapamycin (mTOR), a target of Rheb. Here, we propose that there are clinically relevant functions and targets of TSC1, TSC2 and Rheb, which are independent of mTOR. We present evidence that such non-canonical functions of the TSC-Rheb signalling network exist, propose a standard of evidence for these non-canonical functions, and discuss their potential clinical and therapeutic implications for patients with TSC and lymphangioleiomyomatosis (LAM).

Activating mutations of TOR (target of rapamycin)

Mammalian target of rapamycin (mTOR) is a key regulator of eukaryotic cell growth. In particular, mTORC1, one of the two complexes that contain mTOR, is involved in the regulation of protein synthesis, proliferation, cell cycle and autophagy. Hyperactivation of the mTOR signaling pathway is observed in human cancer. A variety of approaches including deletion analysis, yeast genetic screens and mining of human cancer genome databases were taken that resulted in the identification of activating mutations of TOR. These studies suggest that the FAT, FRB and kinase domains are the three regions of TOR where activating mutations can be identified. Within the kinase domain, the mutations are clustered in three hot spots that are all located in the kinase active site that was deduced by the alignment with PI3K. One of the hot spots corresponds to the region where PI3K oncogenic mutations have been identified. These results are beginning to provide important insights into the mechanism of activation of mTOR.

TOR and PKA pathways synergize at the level of the Ste11 transcription factor to prevent mating and meiosis in fission yeast

Background

In the fission yeast Schizosaccharomyces pombe, the TOR (target of rapamycin) and PKA (protein kinase A) signaling transduction pathways regulate the expression of genes required for cell growth and sexual differentiation in response to the nutritional environment. Inhibition of Tor2 signaling results in the induction of genes involved in sexual differentiation, and the cells undergo mating and meiosis, even under good nutritional conditions. The same phenotype is observed in mutants in which the PKA pathway is inactive. By contrast, Tor2 overexpression or mutations that hyperactivate PKA signaling impair sexual differentiation, even under poor nutritional conditions. Accordingly, a very important question is to understand the molecular mechanism by which these two pathways coordinately regulate gene expression in response to nutrients.

Methodology/Principal Findings

Here we demonstrate that TOR and PKA pathways operate coordinately to negatively regulate sexual differentiation by inhibiting the nuclear accumulation of the Ste11 transcription factor. However, the Tor2 pathway is unable to block the nuclear localization of Ste11 under good nutritional conditions when the PKA pathway is inactive. Using microarray analyses, we found that both pathways inhibit sexual differentiation by blocking ste11-dependent gene expression.

Conclusions/Significance

We conclude that both the PKA and the TOR pathways inhibit Ste11 nuclear accumulation to repress Ste11-dependent gene expression. However, the PKA pathway plays a quantitatively more important role than the TOR pathway in this process.

Structure of the human mTOR complex I and its implications for rapamycin inhibition

The mammalian target of rapamycin complex 1 (mTORC1) regulates cell growth in response to the nutrient and energy status of the cell, and its deregulation is common in human cancers. Little is known about the overall architecture and subunit organization of this essential signaling complex. We have determined the three-dimensional (3D) structure of the fully assembled human mTORC1 by cryo-electron microscopy (cryo-EM). Our analyses reveal that mTORC1 is an obligate dimer with an overall rhomboid shape and a central cavity. The dimeric interfaces are formed by interlocking interactions between the mTOR and raptor subunits. Extended incubation with FKBP12-rapamycin compromises the structural integrity of mTORC1 in a stepwise manner, leading us to propose a model in which rapamycin inhibits mTORC1-mediated phosphorylation of 4E-BP1 and S6K1 through different mechanisms.

Single amino-acid changes that confer constitutive activation of mTOR are discovered in human cancer

Mammalian target of rapamycin (mTOR) is a serine/threonine kinase that regulates a variety of cellular functions such as growth, proliferation and autophagy. In a variety of cancer cells, overactivation of mTOR has been reported. In addition, mTOR inhibitors, such as rapamycin and its derivatives, are being evaluated in clinical trials as anticancer drugs. However, no active mutants of mTOR have been identified in human cancer. Here, we report that two different point mutations, S2215Y and R2505P, identified in human cancer genome database confer constitutive activation of mTOR signaling even under nutrient starvation conditions. S2215Y was identified in large intestine adenocarcinoma whereas R2505P was identified in renal cell carcinoma. mTOR complex 1 prepared from cells expressing the mutant mTOR after nutrient starvation still retains the activity to phosphorylate 4E-BP1 in vitro. The cells expressing the mTOR mutant show increased percentage of S-phase cells and exhibit resistance to cell size decrease by amino-acid starvation. The activated mutants are still sensitive to rapamycin. However, they show increased resistance to 1-butanol. Our study points to the idea that mTOR activating mutations can be identified in a wide range of human cancer.

N-Formyl-3,4-methylenedioxy-benzylidene-gamma-butyrolaetam, KNK437 induces caspase-3 activation through inhibition of mTORC1 activity in Cos-1 cells

The mammalian target of rapamycin complex 1 (mTORC1: mTOR-raptor interaction) and heat shock protein 70 (Hsp70) regulate various cellular processes and are crucial for the progression of many cancers and metabolic diseases. In the recent study, we reported that interaction of Hsp70 with tuberous sclerosis complex 1 (TSC1) regulated apoptosis. This study was designed to elucidate the underlying mechanism in Cos-1 cells. Here, we show that N-formyl-3,4-methylenedioxy-benzylidene-gamma-butyrolaetam (KNK437), which inhibits the expression level of Hsp70, abrogated phosphorylation of mTOR and S6K in response to insulin, and inhibited mTORC1 activity via disruption of an interaction between mTOR and raptor. In addition, KNK437 did not alter TSC1/2 complex formation. Furthermore, KNK437 inhibited the mTOR-raptor interaction on the outer membrane of the mitochondria and triggered caspase-3 activation. A reduction in the level of Hsp70 could result in the inhibition of the mTORC1 signaling pathway, thereby inducing apoptosis.