Rictor, a Novel Binding Partner of mTOR, Defines a Rapamycin-Insensitive and Raptor-Independent Pathway that Regulates the Cytoskeleton

AbstractThe mammalian TOR (mTOR) pathway integrates nutrient- and growth factor-derived signals to regulate growth, the process whereby cells accumulate mass and increase in size. mTOR is a large protein kinase and the target of rapamycin, an immunosuppressant that also blocks vessel restenosis and has potential anticancer applications. mTOR interacts with the raptor and GβL proteins [1–3] to form a complex that is the target of rapamycin. Here, we demonstrate that mTOR is also part of a distinct complex defined by the novel protein rictor (rapamycin-insensitive companion of mTOR). Rictor shares homology with the previously described pianissimo from D. discoidieum[4], STE20p from S. pombe[5], and AVO3p from S. cerevisiae[6, 7]. Interestingly, AVO3p is part of a rapamycin-insensitive TOR complex that does not contain the yeast homolog of raptor and signals to the actin cytoskeleton through PKC1 [6]. Consistent with this finding, the rictor-containing mTOR complex contains GβL but not raptor and it neither regulates the mTOR effector S6K1 nor is it bound by FKBP12-rapamycin. We find that the rictor-mTOR complex modulates the phosphorylation of Protein Kinase C α (PKCα) and the actin cytoskeleton, suggesting that this aspect of TOR signaling is conserved between yeast and mammals

ENHANCING AN OXIDATIVE “TROJAN HORSE” ACTION OF VITAMIN C WITH ARSENIC TRIOXIDE FOR EFFECTIVE SUPPRESSION OF KRAS-MUTANT CANCERS: A PROMISING PATH AT THE BEDSIDE

The turn-on mutations of the KRAS gene, coding a small GTPase coupling growth factor signaling, are contributing to nearly 25% of all human cancers, leading to highly malignant tumors with poor outcomes. Targeting of oncogenic KRAS remains a most challenging task in oncology. Recently, the specific G12C mutant KRAS inhibitors have been developed but with a limited clinical outcome because they acquire drug resistance. Alternatively, exploiting a metabolic breach of KRAS-mutant cancer cells related to a glucose-dependent sensitivity to oxidative stress is becoming a promising indirect cancer targeting approach. Here, we discuss the use of a vitamin C (VC) acting in high dose as an oxidative “Trojan horse” agent for KRAS-mutant cancer cells that can be potentiated with another oxidizing drug arsenic trioxide (ATO) to obtain a potent and selective cytotoxic impact. Moreover, we outline the advantages of VC’s non-natural enantiomer, D-VC, because of its distinctive pharmacokinetics and lower toxicity. Thus, the D-VC and ATO combination shows a promising path to treat KRAS-mutant cancers in clinical settings

Rictor Phosphorylation on the THR-1135 Site Does Not Require Mammalian Target of Rapamycin Complex 2

available in PMC 2012 January 1.In animal cells, growth factors coordinate cell proliferation and survival by regulating the phosphoinositide 3-kinase/Akt signaling pathway. Deregulation of this signaling pathway is common in a variety of human cancers. The PI3K-dependent signaling kinase complex defined as mammalian target of rapamycin complex 2 (mTORC2) functions as a regulatory Ser-473 kinase of Akt. We find that activation of mTORC2 by growth factor signaling is linked to the specific phosphorylation of its component rictor on Thr-1135. The phosphorylation of this site is induced by the growth factor stimulation and expression of the oncogenic forms of ras or PI3K. Rictor phosphorylation is sensitive to the inhibition of PI3K, mTOR, or expression of integrin-linked kinase. The substitution of wild-type rictor with its specific phospho-mutants in rictor null mouse embryonic fibroblasts did not alter the growth factor–dependent phosphorylation of Akt, indicating that the rictor Thr-1135 phosphorylation is not critical in the regulation of the mTORC2 kinase activity. We found that this rictor phosphorylation takes place in the mTORC2-deficient cells, suggesting that this modification might play a role in the regulation of not only mTORC2 but also the mTORC2-independent function of rictor. Mol Cancer Res; 8(6); 896–906.University of Texas M.D. Anderson Cancer Center (Fellow Trust fund)American Cancer Society (M.D. Anderson Cancer Center Breast Specialized Programs of Research Excellence (RSG-09-026-01CCG01))National Institutes of Health (U.S.) (NIH grant CA133522)National Institutes of Health (U.S.) (NIH grant AI104389

Tuberous Sclerosis Complex-1 Deficiency Attenuates Diet-Induced Hepatic Lipid Accumulation

Non-alcoholic fatty liver disease (NAFLD) is causally linked to type 2 diabetes, insulin resistance and dyslipidemia. In a normal liver, insulin suppresses gluconeogenesis and promotes lipogenesis. In type 2 diabetes, the liver exhibits selective insulin resistance by failing to inhibit hepatic glucose production while maintaining triglyceride synthesis. Evidence suggests that the insulin pathway bifurcates downstream of Akt to regulate these two processes. Specifically, mTORC1 has been implicated in lipogenesis, but its role on hepatic steatosis has not been examined. Here, we generated mice with hepatocyte-specific deletion of Tsc1 to study the effects of constitutive mTORC1 activation in the liver. These mice developed normally but displayed mild hepatomegaly and insulin resistance without obesity. Unexpectedly, the Tsc1-null livers showed minimal signs of steatosis even under high-fat diet condition. This ‘resistant’ phenotype was reversed by rapamycin and could be overcome by the expression of Myr-Akt. Moreover, rapamycin failed to reduce hepatic triglyceride levels in models of steatosis secondary to Pten ablation in hepatocytes or high-fat diet in wild-type mice. These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis. Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis. Specifically, mTORC1 activity induces a metabolic shift towards fat utilization and glucose production in the liver. These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism

The Combination of RAD001 and NVP-BEZ235 Exerts Synergistic Anticancer Activity against Non-Small Cell Lung Cancer In Vitro and In Vivo

The phosphoinositide 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling axis has emerged as a novel target for cancer therapy. Agents that inhibit PI3K, mTOR or both are currently under development. The mTOR allosteric inhibitor, RAD001, and the PI3K/mTOR dual kinase inhibitor, BEZ235, are examples of these agents. We were interested in developing strategies to enhance mTOR-targeted caner therapy. In this study, we found that BEZ235 alone effectively inhibited the growth of rapamycin-resistant cancer cells. Interestingly, the combination of sub-optimal concentrations of RAD001 and BEZ235 exerted synergistic inhibition of the growth of human lung cancer cells along with induction of apoptosis and G1 arrest. Furthermore, the combination was also more effective than either agent alone in inhibiting the growth of lung cancer xenografts in mice. The combination showed enhanced effects on inhibiting mTOR signaling and reducing the expression of c-Myc and cyclin D1. Taken together, our results suggest that the combination of RAD001 and BEZ235 is a novel strategy for cancer therapy

Growing roles for the mTOR pathway.

The mammalian TOR (mTOR) pathway is a key regulator of cell growth and proliferation and increasing evidence suggests that its deregulation is associated with human diseases, including cancer and diabetes. The mTOR pathway integrates signals from nutrients, energy status and growth factors to regulate many processes, including autophagy, ribosome biogenesis and metabolism. Recent work identifying two structurally and functionally distinct mTOR-containing multiprotein complexes and TSC1/2, rheb, and AMPK as upstream regulators of mTOR is beginning to reveal how mTOR can sense diverse signals and produce a myriad of responses. Introduction Rapamycin has had a story book trajectory: emerging in the 1970s from the soil of Easter Island [1], playing the starring role in the discovery of a fundamental biological pathway and rising to its current status as an important drug. The study of its mechanism of action has been full of unexpected and exciting findings, beginning with the odd way in which it acts. Rapamycin binds to the FKBP12 protein to form a drug-receptor complex that then interacts with and perturbs a large protein kinase called TOR (target of rapamycin) A tale of two mTOR complexes Until the introduction of RNA interference technology, the majority of work on the mammalian TOR pathway relied on rapamycin to probe mTOR biology. We now realize that rapamycin does not perturb all mTOR functions because mTOR exists in two distinct multi-protein complexes and only one binds to FKBP12-rapamycin How FKBP12-rapamycin inhibits the kinase activity of the raptor-mTOR complex is not understood. The drug does not displace GbL or raptor from mTOR but does strongly destabilize the raptor-mTOR interaction Growth control by raptor-mTOR Extensive work with rapamycin indicates that the raptormTOR complex positively regulates cell growth and that its inhibition causes a large decrease in cell size. The raptor branch of the mTOR pathway modulates a stunning number of major processes, including mRNA translation (reviewed in S6K1 is a famous protein in the TOR field. It was the first component of the pathway to be identified -even before the cloning of the mammalian and yeast TOR genes - Growing roles for the mTOR pathway Sarbassov, Ali and Sabatini 597 The function in translational control of 4E-BP1 is better understood and has been recently reviewed The paucity of direct substrates for raptor-mTOR remains a major obstacle to understanding how it connects at the molecular level to downstream growth processes, like ribosome biogenesis. The phosphorylation sites on S6K1 and 4E-BP1 are not conserved [21], suggesting that the mTOR kinase domain may not have inherent substrate specificity and that mTOR-associated proteins may determine substrate preference. Consistent with this notion, both S6K1 and 4E-BP1 contain a conserved short sequence called the TOS motif that raptor recognizes and that is required for efficient in vitro and in vivo phosphorylation by the raptor-mTOR comple

Gibberellic-acid-dependent expression of α-amylase in wheat aleurone cells is mediated by target of rapamycin (TOR) signaling

Target of rapamycin (TOR) signaling is an essential nutrient-dependent pathway controlling cell growth in all eukaryotes. TOR signaling is well characterized in yeast and animals but remains poorly investigated in plants. The hormonal action of gibberellic acid (GA) is a crucial factor for wheat germination by inducing the synthesis of α-amylase in wheat aleurone cells. Here we showed that GA promotes the activation of Triticum aestivum TOR (TaTOR) signaling as evidenced by increased phosphorylation of T. aestivum S6K1 (TaS6K1) on its conserved hydrophobic motif together with proteasomal degradation of growth-inhibitory factor Rht-1. GA-dependent activation of TaTOR signaling led to α-amylase synthesis and Rht-1 proteasomal degradation because both GA-dependent events were sensitive to TaTOR inhibition. Using antibodies specific to TaTOR, we successfully identified the presence of endogenous TaTOR protein in terminally differentiated wheat aleurone layers. Additionally, by examining the rapamycin-sensitive phosphorylation of S6K1 as a reliable indicator of endogenous TOR kinase activity, we demonstrated that the activity of TaTOR in aleurone layers is enhanced by GA. Importantly, this stimulation is not associated with the regulation of either TaTOR transcription or the accumulation of TaTOR protein. In yeast and pull-down assays, a robust interaction between TaS6K1 and the N terminus of Rht-1 (amino acids 1–234) was observed, a finding further supported by co-immunoprecipitation of endogenous Rht-1 and TaS6K1. Furthermore, the administration of mTOR inhibitors significantly attenuated GA-induced degradation of endogenous Rht-1 and prolonged the persistence of the complex formed by these two proteins. We propose that TaTOR-TaS6K1 signaling contributes to GA-dependent wheat germination by mediating α-amylase synthesis and controlling proteasomal degradation of Rht-1 in wheat aleurone cells

La germination du blé dépend de la cible végétale de la signalisation de la rapamycine

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