mTORC2-mediated Akt Phosphorylation Research Articles

The aminopeptidase LAP3 suppression accelerates myogenic differentiation via the AKT-TFE3 pathway in C2C12 myoblasts.

Skeletal muscle maintenance depends largely on muscle stem cells (satellite cells) that supply myoblasts required for muscle regeneration and growth. The ubiquitin-proteasome system is the major intracellular protein degradation pathway. We previously reported that proteasome dysfunction in skeletal muscle significantly impairs muscle growth and development. Furthermore, the inhibition of aminopeptidase, a proteolytic enzyme that removes amino acids from the termini of peptides derived from proteasomal proteolysis, impairs the proliferation and differentiation ability of C2C12 myoblasts. However, no evidence has been reported on the role of aminopeptidases with different substrate specificities on myogenesis. In this study, therefore, we investigated whether the knockdown of aminopeptidases in differentiating C2C12 myoblasts affects myogenesis. The knockdown of the X-prolyl aminopeptidase 1, aspartyl aminopeptidase, leucyl-cystinyl aminopeptidase, methionyl aminopeptidase 1, methionyl aminopeptidase 2, puromycine-sensitive aminopeptidase, and arginyl aminopeptidase like 1 gene in C2C12 myoblasts resulted in defective myogenic differentiation. Surprisingly, the knockdown of leucine aminopeptidase 3 (LAP3) in C2C12 myoblasts promoted myogenic differentiation. We also found that suppression of LAP3 expression in C2C12 myoblasts resulted in the inhibition of proteasomal proteolysis, decreased intracellular branched-chain amino acid levels, and enhanced mTORC2-mediated AKT phosphorylation (S473). Furthermore, phosphorylated AKT induced the translocation of TFE3 from the nucleus to the cytoplasm, promoting myogenic differentiation through increased expression of myogenin. Overall, our study highlights the association of aminopeptidases with myogenic differentiation.

Combination of mTOR inhibitor PP242 and AMPK activator metformin exerts enhanced inhibitory effects on colorectal carcinoma cells in vitro by blocking multiple kinase pathways

The second-generation mammalian target of rapamycin (mTOR) inhibitor PP242 has demonstrated limited success in some rapamycin-insensitive tumours. We examined the therapeutic potential of combining PP242 with adenosine 50- monophosphate-activated protein kinase (AMPK) activator metformin, using a panel of colorectal carcinoma (CRC) cell lines. We found that the PP242 and metformin combination enhanced the suppression of CRC cell proliferation, colony formation, and cancer cell apoptosis induction. The effect of this combination was observed on AMPK phosphorylation. Western blotting showed that PP242 inhibited mTORC1 activation, as indicated by the reduced expression of its major substrate p-S6K1 and the partially reduced phosphorylation of eIF4E-binding protein 1 (4E-BP1). The inhibition of mTORC2-mediated AKT phosphorylation at Ser 473 (AKT Ser473) was transient and occurred in the first few hours of PP242 treatment; metformin exposure decreased the PP242 activity, counteracting AKT activation. We further demonstrated that this was related to direct AMPK-mediated phosphorylation of IRS-1 at Ser789. Thus, the combination of PP242 and metformin completely blocked the activity of both mTORC1 and mTORC2 kinase. This study suggests that this combination could be a more effective strategy for the treatment of CRC.

Protein kinase N2 mediates flow-induced endothelial NOS activation and vascular tone regulation.

Formation of NO by endothelial NOS (eNOS) is a central process in the homeostatic regulation of vascular functions including blood pressure regulation, and fluid shear stress exerted by the flowing blood is a main stimulus of eNOS activity. Previous work has identified several mechanosensing and -transducing processes in endothelial cells, which mediate this process and induce the stimulation of eNOS activity through phosphorylation of the enzyme via various kinases including AKT. How the initial mechanosensing and signaling processes are linked to eNOS phosphorylation is unclear. In human endothelial cells, we demonstrated that protein kinase N2 (PKN2), which is activated by flow through the mechanosensitive cation channel Piezo1 and Gq/G11-mediated signaling, as well as by Ca2+ and phosphoinositide-dependent protein kinase 1 (PDK1), plays a pivotal role in this process. Active PKN2 promoted the phosphorylation of human eNOS at serine 1177 and at a newly identified site, serine 1179. These phosphorylation events additively led to increased eNOS activity. PKN2-mediated eNOS phosphorylation at serine 1177 involved the phosphorylation of AKT synergistically with mTORC2-mediated AKT phosphorylation, whereas active PKN2 directly phosphorylated human eNOS at serine 1179. Mice with induced endothelium-specific deficiency of PKN2 showed strongly reduced flow-induced vasodilation and developed arterial hypertension accompanied by reduced eNOS activation. These results uncover a central mechanism that couples upstream mechanosignaling processes in endothelial cells to the regulation of eNOS-mediated NO formation, vascular tone, and blood pressure.

Abstract 657: Inhibition of mTORC1 by dihydroartemisinin

Abstract mTOR functions as two complexes, mTORC1 and mTORC2, and is a central controller for cell growth and metabolism. Both mTORC1 and mTORC2 are activated in numerous cancers, and have become the focus of anticancer drug development. Rapalogs and mTOR kinase inhibitors (TORKIs) have been developed. However, the anticancer efficacy of rapalogs is clinically weak or mild to most cancers, and the adverse effects of rapalogs and TORKIs are greatly concerned. So, it is imperative to discover and develop new generation of mTOR inhibitors. Dihydroartemisinin (DHA) is an anti-malarial agent showing good safety in both children and adults. Recently, we have shown that DHA, at pharmacological concentrations, inhibits proliferation and induces apoptosis in cells derived from various cancer types but not in normal or immortalized cell lines, implying that DHA is potentially a selective anticancer agent and may be repurposed for cancer therapy. Of interest, DHA potently suppresses mTORC1-mediated phosphorylation of S6K1 and 4E-BP1, but does not obviously affect mTORC2-mediated phosphorylation of Akt, which is often activated by classic mTOR inhibitors (rapalogs) via a feedback mechanism. The results suggest that DHA is a novel mTORC1 inhibitor. This study was set to determine how DHA inhibits mTORC1. Here we found that DHA did not bind to any mTORC1 components (mTOR, mLST8 or raptor) directly, as determined by in vivo [3H]-DHA radiolabeling. Besides, our molecular docking revealed that DHA did not have the ability to bind FKBP12 or mTOR kinase domain, in contrast to the positive controls rapamycin (binding FKBP12) and INK128 (binding mTOR kinase domain). These data suggest that DHA inhibits mTORC1 via indirect mechanism(s). Further research revealed that DHA altered neither the phosphorylation of IGF-1R/PI3K/PTEN and ERK, nor the expression of REDD1, but induced phosphorylation of AMPKα (T172). Interestingly, DHA-induced mTORC1 inhibition was profoundly attenuated by inhibition with Compound C (AMPK inhibitor) or ectopic expression of dominant negative AMPKα2 (AMPKα2-DN), indicating that activation of AMPK links to inhibition of mTORC1. Collectively, the results support that DHA inhibits mTORC1 by activating AMPK. (Supported by the Feist-Weiller Cancer Center, LSU Health Sciences Center, Shreveport, LA, USA.) Citation Format: Wang Liu, Jun Luo, Yoshinobu Odaka, Shile Huang. Inhibition of mTORC1 by dihydroartemisinin [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 657.

Genetic immunodeficiency and autoimmune disease reveals a WRC-independent role for Hem1 in mTORC2 regulation

Abstract Inborn errors of immunity (IEI) are genetic disorders that cause life-threatening infections, lymphoproliferation, autoimmunity, or combinations of these features. We describe five patients from four families with a novel IEI characterized by recurrent infections, atopic disease, lymphoproliferation, cytokine overproduction, and autoimmunity. All patients harbored loss-of-function (LOF) variants in the gene NCKAP1L, which encodes the hematopoietic-specific Hem1 protein. Hem1 is a required component of the WAVE regulatory complex (WRC), a critical mediator of Arp2/3-dependent actin polymerization. Three LOF mutations impair interactions between Hem1 and other WRC components, resulting in degradation of the complex. A fourth mutation in Hem1 (M371V) abrogates activation of the WRC by Arf1 GTPase, resulting in WRC deficiency. Loss of Hem1 function disrupts Arp2/3-dependent actin polymerization, immune synapse formation, lamellipodia formation, and migration. Diminished cortical actin barriers in Hem1-deficient cells enable increased degranulation, cytokine secretion, and immune hyperresponsiveness leading to lymphoproliferation. Surprisingly, Hem1 deficiency causes reduced mTORC2-mediated AKT phosphorylation independently of its role in the actin polymerization. Dysregulation of the mTORC2/AKT axis leads to impaired T cell proliferation and effector cytokine production upon T cell activation. Our data show that Hem1 governs two key regulatory complexes, the WRC and mTORC2, and provide a cellular explanation for the combined immunodeficiency and immune hyperresponsiveness observed in Hem1-deficient patients. This research was supported by the Intramural Research Program of the NIH, NIAID.

Phosphorylated Rho-GDP directly activates mTORC2 kinase towards AKT through dimerization with Ras-GTP to regulate cell migration.

mTORC2 plays critical roles in metabolism, cell survival, and actin cytoskeletal dynamics via phosphorylation of AKT. Despite its importance to biology and medicine, it is unclear how mTORC2-mediated AKT phosphorylation is controlled. Here, we identify an unforeseen principle by which a GDP-bound form of the conserved small G protein Rho GTPase directly activates mTORC2 in AKT phosphorylation in social amoebae Dictyostelium cells. Using biochemical reconstitution with purified proteins, we demonstrate that Rho-GDP promotes AKT phosphorylation by assembling the supercomplex with Ras-GTP and mTORC2. This supercomplex formation is controlled by chemoattractant-induced phosphorylation of Rho-GDP at serine 192 by GSK-3. Furthermore, Rho-GDP rescued defects in both mTORC2-mediated AKT phosphorylation and directed cell migration in Rho-null cells in a manner dependent on phosphorylation of serine 192. Thus, in contrast to the prevailing view that GDP-bound forms of G proteins are inactive, our study reveals that mTORC2-AKT signaling is activated by Rho-GDP.

Abstract 2961: Ganoderma lucidum extracts inhibit mTORC1/2 by activating AMPK and inhibiting IGFR/PI3K/Rheb in tumor cells

Abstract Ganoderma lucidum (G. lucidum) extracts, as dietary supplements, have been found to possess potent anticancer activity, which is attributed to the presence of polysaccharides and triterpenes. However, the molecular mechanism underlying the anticancer action of G. lucidum extracts remains to be elucidated. Here we show that ReishiMax GLp, containing both polysaccharides and triterpenes of G. lucidum (GLPT), inhibited cell proliferation and induced cell death in human lung cancer cells (A549 and A427), and concurrently suppressed the signaling pathways mediated by the mammalian target of rapamycin (mTOR), a central regulator of cell proliferation and survival. Interestingly, GLPT not only inhibited mTORC1-mediated phosphorylation of p70 S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), but also repressed mTORC2-mediated phosphorylation of Akt. Mechanistically, GLPT downregulated the phosphorylation and protein levels of insulin-like growth factor 1 receptor (IGFR) and phosphoinositide 3-kinase (PI3K), as well as the protein level of RAS homolog enriched in brain (Rheb). Besides, GLPT also activated AMP-activated protein kinase (AMPK) network. This is supported by the findings that GLPT increased the phosphorylation of AMPKα (T172), as well as its substrates tuberous sclerosis complex 2 (TSC2, S1387) and regulatory-associated protein of mTOR (raptor, S792). Ectopic expression of dominant negative AMPKα partially attenuated the inhibitory effect of GLPT on mTORC1, indicating that GLPT inhibits mTORC1 partly via activating AMPK. The results suggest that G. lucidum extracts execute the anticancer action at least partly by inhibiting the mTORC1/2 signaling through activation of AMPK and inhibition of IGFR/PI3K/Rheb in tumor cells. Supported by the Feist-Weiller Cancer Center, LSU Health Sciences Center, Shreveport, LA, USA. Citation Format: Shile Huang, Didem Sohretoglu, Chao Zhang, Jun Luo. Ganoderma lucidum extracts inhibit mTORC1/2 by activating AMPK and inhibiting IGFR/PI3K/Rheb in tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2961.

The Novel mTOR Complex 1/2 Inhibitor P529 Inhibits Human Lung Myofibroblast Differentiation.

Idiopathic pulmonary fibrosis is a progressive and deadly disorder with very few therapeutic options. Palomid 529 (8-(1-hydroxyethyl)-2-methoxy-3-(4-methoxybenzyloxy)-benzo[c]chromen-6-one; P529) is a novel dual inhibitor of mechanistic target of rapamycin complex 1/2 (mTORC1/2). In these studies, we investigated the effect of P529 on TGF-β-dependent signaling and myofibroblast differentiation. TGF-β-induced phosphorylation of the mTORC1 targets, p70 S6 kinase 1 (S6K1), and eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), were both dose dependently inhibited by P529 in human lung fibroblasts with maximal inhibition occurring between 10 and 20 μM. mTORC2-mediated phosphorylation of Akt at the S473 site was partially inhibited with a similar dose dependency, as was TGF-β-induced myofibroblast differentiation. Protein levels of TGF-β-induced fibronectin and collagen were similarly decreased by P529. At this dose, there was also inhibition of mRNA transcript levels for Col1 and α-SMA, suggesting inhibition of transcriptional activation. However, there was no effect of P529 on canonical TGF-β-induced Smad signaling, as assessed by receptor-associated Smad2/3 phosphorylation, Smad2/3/4 translocation, or Smad-driven gene expression, as assessed by Smad-binding element driven luciferase. Conversely, activation of mTORC1/2 signaling was dependent on TGF-β type I receptor (ALK5) signaling and on Smad2/3 expression. P529 treatment disrupted TGF-β-induced actin stress fiber formation during myofibroblast differentiation, the deposition of new extracellular fibronectin matrix, and linear wound closure by fibroblasts. Likewise, mTOR knockdown inhibited TGF-β-induced myofibroblast differentiation. In conclusion, P529 inhibits TGF-β-induced myofibroblast differentiation, actin stress fiber formation, and matrix protein expression and deposition. Inhibition of mTORC1/2 by P529 may be a promising approach to inhibit in vivo fibrosis. J. Cell. Biochem. 118: 2241-2249, 2017. © 2017 Wiley Periodicals, Inc.

Abstract 4655: Iron chelation inhibits mTORC1 signaling in tumor cells

Abstract Iron chelators are commonly used for treatment of iron overload diseases. Recent studies have shown that iron chelators possess potent anticancer activities. Iron chelators (ciclopirox and 3-AP) are in clinical trials for cancer treatment. However, the molecular mechanism underlying the anticancer effect of iron chelation is not well understood. Mammalian/mechanistic target of rapamycin (mTOR) is a master kinase, regulating cell growth/proliferation and survival through sensing environmental cues. mTOR functions as at least two complexes (mTORC1/2). Here we show that two iron chelators, ciclopirox olamine (CPX) and Dp44mT, inhibit tumor cell proliferation by suppressing mTORC1 signaling in human rhabdomyosarcoma (Rh30) and lung cancer (A549 and A427) cells. This is evidenced by the findings that treatment with CPX or Dp44mT inhibited the phosphorylation of 4E-BP1 and S6K1, two best known effector molecules of mTORC1, which could be blocked by addition of ferrous sulfate. Addition of ferrous sulfate prevented the growth inhibitory effect of CPX. Ectopic expression of constitutively active mTOR also conferred resistance to CPX-induced growth inhibition. Mechanistically, iron chelation inhibition of mTORC1 was partially linked to the activation of AMPK, a negative regulator of mTORC1. Overexpression of a dominant-negative AMPK (DN-AMPK) in Rh30 cells attenuated the inhibitory effect of CPX or Dp44mT on the phosphorylation of S6K1 and 4E-BP1. In addition, iron chelation also transiently stabilized HIF-1α and further upregulated the expression of its downstream effector Bnip3, another negative regulator of mTORC1. Knockdown of Bnip3 partially attenuated the inhibitory effect of iron chelation on mTORC1. Furthermore, we found that iron chelation increased the translocation of mTOR from the cytoplasm to the nucleus. The co-localization of mTOR with the third negative regulator of mTOR, PML (promyelocytic leukemia), increased in the nucleus upon iron chelation. Moreover, iron chelation did not obviously alter the phosphorylation of IGF-1 receptor, PI3K or PTEN. The effect of iron chelation on mTORC2-mediated phosphorylation of Akt was cell line dependent. Taken together, the results suggest that the anticancer effect of iron chelation is in part related to inhibition of mTORC1 signaling, and iron chelation inhibits mTORC1 signaling via multiple mechanisms. (Supported by the FWCC, LSU Health Sciences Center) Citation Format: Chaowei Shang, Hongyu Zhou, Shile Huang. Iron chelation inhibits mTORC1 signaling in tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4655.

Yin Yang 1 promotes mTORC2-mediated AKT phosphorylation.

Yin Yang 1 (YY1) regulates both gene expression and protein modifications, and has shown a proliferative role in cancers. In this study, we demonstrate that YY1 promotes AKT phosphorylation at S473, a marker of AKT activation. YY1 expression positively correlated with AKT(S473) phosphorylation in a tissue microarray and cultured cells of breast cancer, but negatively associated with the distant metastasis-free survival of 166 breast cancer patients. YY1 promotes AKT phosphorylation at S473 through direct interaction with AKT, and the AKT-binding site is mapped to the residues G201-S226 on YY1. These residues are also involved in YY1 interaction with Mdm2, Ezh2, and E1A, and thus are designated as the oncogene protein binding (OPB) domain. YY1-promoted AKT phosphorylation relies on the OPB domain but is independent of either transcriptional activity of YY1 or the activity of phosphoinositide-3-kinases. We also determine that YY1-promoted mTORC2 access to AKT leads to its phosphorylation at S473. Importantly, a peptide based on the OPB domain blocks YY1 interaction with AKT and reduces AKT phosphorylation and cell proliferation. Thus, we demonstrate for the first time that YY1 promotes mTORC2-mediated AKT activation and disrupting YY1-AKT interaction by OPB domain-based peptide may represent a potential strategy for cancer therapy.

Abstract 670: Targeting PI3K/mTOR leads to MEK/ERK over-activation in pancreatic cancer through suppression of mTORC2

Abstract Significant attention has been given to molecular target therapies in pancreatic cancer due to the limited survival benefits provided by standard chemotherapy. The phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1/S6K pathway which is aberrantly stimulated in pancreatic ductal adenocarcinoma (PDAC) has emerged as a target for therapy. Inhibitors of PI3K and/or mTOR (PI3K/TOR-KIs) are been developed to target this pathway. However, mTORC1/S6K also mediates negative feedback loops that attenuate upstream signaling. Suppression of these feedback loops opposes the growth-suppressive effects of mTOR inhibitors and leads to drug resistance. Here, we demonstrate that treatment of a panel of pancreatic ductal adenocarcinoma (PDAC) cells including PANC-1, MiaPaCa-2, AsPC-1 and BxPC-3 with the dual PI3K/mTOR kinase inhibitor (PI3K/TOR-KI) NPV-BEZ23 induces a novel negative feedback loop leading to increased activity of MEK/ERK pathway in PDAC cells. Exposure of the above mentioned PDAC cells to NPV-BEZ235 potently blocked mTORC1 activation mTORC1/S6K activation (scored by S6 phosphorylation at Ser240/244), mTORC1/4E-BP1 (assayed by 4E-BP1 phosphorylation at Thr37/46) and mTORC2-mediated Akt phosphorylation at Ser473, in a concentration-dependent manner. Strikingly, NPV-BEZ235 markedly enhanced the MEK/ERK pathway in a dose-dependent manner. Maximal ERK over-activation coincided with complete inhibition of phosphorylation of Akt and 4E-BP1. Other PI3K/TOR-KIs, including PKI-587 and GDC-0980 also induced ERK over-activation. The MEK inhibitors PD0325901 or Trametinib prevented ERK over-activation induced by PI3K/TOR-KIs at least acutely. The combination of NPV-BEZ235 and PD0325901 caused a more pronounced inhibition of cell growth than that produced by each inhibitor individually. Mechanistic studies assessing PI3K activity in single PDAC cells using a PIP3 fluorescent reporter indicated that NPV-BEZ235, PKI-587 and GDC-0980 act through a PI3K-independent pathway. Doses of PI3K/TOR-KIs that enhanced MEK/ERK activation coincided with those that inhibited mTORC2-mediated Akt phosphorylation on Ser473, suggesting a role of mTORC2. Knockdown of Rictor markedly increased baseline levels of ERK phosphorylation and treatment with NVP-BEZ235 did not produce further enhancement of ERK activation. These results imply that Rictor or mTORC2 exerts feedback inhibition of the MEK/ERK pathway in pancreatic cancer cells. We propose that dual PI3K/mTOR inhibitors suppress a novel negative feedback loop mediated by mTORC2 thereby leading to enhanced MEK/ERK pathway activity in pancreatic cancer cells. The therapeutic effectiveness of PI3K/TOR-KIs inhibitors in PDAC and possibly other malignancies can be diminished by activation of MEK/ERK that opposes their anti-proliferative effects and leads to drug resistance. Note: This abstract was not presented at the meeting. Citation Format: Heloisa P. Soares, Ming Ming, Michelle Mellon, Steven H. Young, Liang Han, James Sinnet-Smith, Enrique Rozengurt. Targeting PI3K/mTOR leads to MEK/ERK over-activation in pancreatic cancer through suppression of mTORC2. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 670. doi:10.1158/1538-7445.AM2015-670

Abstract A109: Dual mTOR/PI3K inhibitors induce compensatory MEK/ERK activation in human pancreatic cancer cells: A mechanism contributing to drug resistance

Abstract Background: The phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1/S6K pathway is aberrantly activated in premalignant pancreatic lesions and in pancreatic ductal adenocarcinoma (PDAC) tissues. The pathway functions downstream of RAS (mutated in 90% of PDAC) and plays a key role in tyrosine kinase receptor signaling. Consequently, mTORC1 and the upstream components of the cascade are attractive therapeutic targets in PDAC. However, mTORC1 also mediates potent negative feedback loops that restrain upstream signaling via insulin/IGF receptor and other tyrosine kinase receptors, as shown with first and second generation of mTOR inhibitors. Dual PI3K and mTOR inhibitors (PI3K/TOR-KIs) have been developed to block completely mTORC1 and mTORC2 and prevent upstream pathway activation. Surprisingly, we report here that dual PI3K/TOR-KIs induce robust MEK/ERK over-activation in PDAC cells via a PI3K-independent mechanism. Results: Treatment of PANC-1 or MiaPaCa-2 pancreatic cancer cells with NPV-BEZ235 (0.005-1µM), a clinically relevant dual PI3K/TOR-KI, potently blocked mTORC1/S6K activation (scored by S6 phosphorylation at Ser240/244), mTORC1/4E-BP1 (assayed by 4E-BP1 phosphorylation at Thr37/46) and mTORC2-mediated Akt phosphorylation at Ser473, in a concentration-dependent manner. Strikingly, exposure to NPV-BEZ235 markedly enhanced the increase in the phosphorylation of ERK at Thr202 and Tyr204. Maximal ERK over-activation (~3 fold) coincided with complete inhibition of phosphorylation of Akt and 4E-BP1 (0.1-0.5 µM NPV-BEZ235). ERK over-activation was demonstrated when PDAC cells were stimulated with serum growth factors or insulin and neurotensin as well as when different PI3K/TOR-KIs (PKI-587, GDC-0980) were used instead of NPV-BEZ235. In order to prove that dual PI3K/TOR-KIs over-activate ERK through a novel PI3K-independent pathway in PDAC cells, we determined the effect of NEV-BEZ235, PKI-587 and GDC-0980 on PI3K-generated accumulation of PIP3 in the plasma membrane of single MiaPaCa-2 cells, using distribution of Akt-PH-GFP to monitor PIP3. Stimulation with insulin induced striking translocation of the PIP3 sensor to the plasma membrane, indicative of robust PI3K activation. Exposure to NEV-BEZ235, PKI-587 or GDC-0980 completely prevented the translocation of Akt-PH-GFP to the plasma membrane, providing conclusive evidence that dual PI3K/mTOR inhibitors induce rapid MEK/ERK activation in PDAC cells with suppressed PI3K activity. Additionally, we showed that treatment with the MEK inhibitors U126 or PD0325901 (1-5 µM) prevented ERK over-activation induced by PI3K/TOR-KIs. PD0325901 released feedback inhibition of RAS/RAF, as revealed by over-phosphorylation of MEK in MiaPaCa-2 or PANC-1 cells. Interestingly, NVP-BEZ235 induced further enhancement of MEK phosphorylation in PDAC cells treated with PD-0325901, implying that dual PI3K/mTOR inhibitor enhanced Raf/MEK in cells without ERK-mediated negative feedbacks loops. Finally, in order to examine whether the over-activation of the ERK pathway counterbalances the growth-suppressive effect of PI3K/TOR-KIs, PDAC cells were treated with PI3K/TOR-KIs (NPV-BEZ235 or PKI-587 or GDC-0980) and PD0325901 or a combination of either PI3K/TOR-KIs and PD0325901. The combination of these drugs caused a more pronounced inhibition of cell growth than that produced by each inhibitor added individually. Conclusion: Collectively, our results highlight the importance of discovering novel signaling crosstalk to anticipate mechanisms of tumor resistance to new drugs. The capability of predicting drug resistance can assist in developing rational and effective strategies for developing combination therapies in PDAC. Citation Format: Heloisa P. Soares, Ming Ming, James Sinnett-Smith, Michelle Mellon, Enrique Rozengurt. Dual mTOR/PI3K inhibitors induce compensatory MEK/ERK activation in human pancreatic cancer cells: A mechanism contributing to drug resistance. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A109.

MLST8 Promotes mTOR-Mediated Tumor Progression.

The activity of the mechanistic target of rapamycin (mTOR) is elevated in various types of human cancers, implicating a role in tumor progression. However, the molecular mechanisms underlying mTOR upregulation remain unclear. In this study, we found that the expression of mLST8, a required subunit of both mTOR complex 1 (mTORC1) and complex 2 (mTORC2), was upregulated in several human colon and prostate cancer cell lines and tissues. Knockdown of mLST8 significantly suppressed mTORC1 and mTORC2 complex formation, and it also inhibited tumor growth and invasiveness in human colon carcinoma (HCT116) and prostate cancer (LNCaP) cells. Overexpression of mLST8 induced anchorage-independent cell growth in normal epithelial cells (HaCaT), although mLST8 knockdown had no effect on normal cell growth. mLST8 knockdown reduced mTORC2-mediated phosphorylation of AKT in both cancer and normal cells, whereas it potently inhibited mTORC1-mediated phosphorylation of 4E-BP1 specifically in cancer cells. These results suggest that mLST8 plays distinct roles in normal and cancer cells, depending upon its expression level, and that mLST8 upregulation may contribute to tumor progression by constitutively activating both the mTORC1 and mTORC2 pathways.

Dual PI3K/mTOR Inhibitors Induce Rapid Overactivation of the MEK/ERK Pathway in Human Pancreatic Cancer Cells through Suppression of mTORC2.

The PI3K/AKT/mTOR pathway, which is aberrantly stimulated in many cancer cells, has emerged as a target for therapy. However, mTORC1/S6K also mediates negative feedback loops that attenuate upstream signaling. Suppression of these feedback loops opposes the growth-suppressive effects of mTOR inhibitors and leads to drug resistance. Here, we demonstrate that treatment of PANC-1 or MiaPaCa-2 pancreatic ductal adenocarcinoma (PDAC) cells with the dual PI3K/mTOR kinase inhibitor (PI3K/TOR-KI) BEZ235 blocked mTORC1/S6K activation (scored by S6 phosphorylation at Ser(240/244)), mTORC1/4E-BP1 (assayed by 4E-BP1 phosphorylation at Thr(37/46)), and mTORC2-mediated AKT phosphorylation at Ser(473), in a concentration-dependent manner. Strikingly, BEZ235 markedly enhanced the MEK/ERK pathway in a dose-dependent manner. Maximal ERK overactivation coincided with complete inhibition of phosphorylation of AKT and 4E-BP1. ERK overactivation was induced by other PI3K/TOR-KIs, including PKI-587 and GDC-0980. The MEK inhibitors U126 or PD0325901 prevented ERK overactivation induced by PI3K/TOR-KIs. The combination of BEZ235 and PD0325901 caused a more pronounced inhibition of cell growth than that produced by each inhibitor individually. Mechanistic studies assessing PI3K activity in single PDAC cells indicate that PI3K/TOR-KIs act through a PI3K-independent pathway. Doses of PI3K/TOR-KIs that enhanced MEK/ERK activation coincided with those that inhibited mTORC2-mediated AKT phosphorylation on Ser(473), suggesting a role of mTORC2. Knockdown of RICTOR via transfection of siRNA markedly attenuated the enhancing effect of BEZ235 on ERK phosphorylation. We propose that dual PI3K/mTOR inhibitors suppress a novel negative feedback loop mediated by mTORC2, thereby leading to enhanced MEK/ERK pathway activity in pancreatic cancer cells.

Tu1973 The Novel Observation of Irreversible Electroporation Phenomenon on Gastrointestinal Cancer Cells in Physiological Conditions, With Sub-Micrometer Resolution by Scanning Ion Conductance Microscopy

G A A b st ra ct s enhanced MEK/ERK activation coincided with those that inhibited mTORC2-mediated Akt phosphorylation on Ser473, suggesting a role of mTORC2. Knockdown of Rictor markedly increased baseline levels of ERK phosphorylation and treatment with NVP-BEZ235 did not produce further enhancement of ERK activation. These results imply that Rictor or mTORC2 exerts feedback inhibition of the MEK/ERK pathway in pancreatic cancer cells. Conclusion: We propose that dual PI3K/mTOR inhibitors suppress a novel negative feedback loop mediated by mTORC2 thereby leading to enhanced MEK/ERK pathway activity in pancreatic cancer cells.

Tu1975 H2S Inhibition of Cystathionine-β-Synthase (CBS) Using Novel Prodrug Decreases Colorectal Cancer Xenograft Growth With Less Toxicity Than Aminooxyacetic Acid (AOAA)

G A A b st ra ct s enhanced MEK/ERK activation coincided with those that inhibited mTORC2-mediated Akt phosphorylation on Ser473, suggesting a role of mTORC2. Knockdown of Rictor markedly increased baseline levels of ERK phosphorylation and treatment with NVP-BEZ235 did not produce further enhancement of ERK activation. These results imply that Rictor or mTORC2 exerts feedback inhibition of the MEK/ERK pathway in pancreatic cancer cells. Conclusion: We propose that dual PI3K/mTOR inhibitors suppress a novel negative feedback loop mediated by mTORC2 thereby leading to enhanced MEK/ERK pathway activity in pancreatic cancer cells.

Abstract 5453: Dual mTOR/PI3K inhibitors induce ERK pathway overactivation in pancreatic cell lines: Rationale for combinatory therapy

Abstract Pancreatic ductal adenocarcinoma (PDAC), which comprises 90% of all human pancreatic cancers, is a devastating disease, with overall 5-year survival rate of only 3-5%. As the current therapies offer very limited survival benefits, novel molecular therapeutic targets and strategies are urgently needed to treat this aggressive disease. The complementary activation of phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1 and MEK/ERK signaling pathways in many cancer cells, including PDAC, potently stimulates their proliferation and survival. However, mTORC1 also mediates negative feedback loops that restrain upstream signaling via insulin/IGF receptor and other tyrosine kinase receptors. Suppression of these feedback loops by selective mTORC1 inhibitors, e.g. by rapamycin and its analogs, unleashes over-activation of the PI3K/Akt pathway that potentially opposes the anti-proliferative effects of mTOR inhibitors. This prompted the development of active-site mTORC1/2 kinase inhibitors (TOR-KIs) and dual PI3K and mTOR inhibitors (PI3K/TOR-KIs). It is not known whether PI3K/TOR-KIs induce any effect on the activity of the ERK pathway in PDAC cells. Consequently, we treated serum-starved cultures of PDAC cells (MiaPaca-2 and PANC-1) with increasing concentrations of the dual PI3K/TOR-KI NPV-BEZ235 followed by stimulation with insulin and neurotensin, a potent mitogenic combination for these cells. As expected, prior exposure to NPV-BEZ235 potently inhibited mTORC1 activation (scored by S6 phosphorylation at Ser-240/244) and mTORC2-mediated Akt phosphorylation at Ser-473, in a concentration-dependent manner. Strikingly, we also demonstrate, for the first time, that exposure to NPV-BEZ235 markedly enhanced the phosphorylation of ERK at Thr-202 and Tyr-204. Maximal ERK over-activation coincided with complete inhibition of Akt phosphorylation at Ser-473 (produced at 100-500 ηM NPV-BEZ235). ERK over-activation was also seen when PDAC cells were stimulated with 2% fetal bovine serum and when a different PI3K/TOR-KI (PKI-587) was used instead of NPV-BEZ235. Treatment with the MEK inhibitors U126 or PD0325901 (1-5 μM) prevented ERK over-activation induced by PI3K/TOR-KIs. Crucially, a combination of NPV-BEZ235 and PD0325901 caused a more pronounced inhibition of cell growth than that produced by each inhibitor added individually. Thus, the elucidation of novel feedbacks loops and crosstalk mechanisms can assist in developing rational and effective strategies for developing combination therapies in PDAC. Citation Format: Heloisa P. Soares, Ming Ming, Michelle Mellon, James Sinnet-Smith, Enrique Rozengurt. Dual mTOR/PI3K inhibitors induce ERK pathway overactivation in pancreatic cell lines: Rationale for combinatory therapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5453. doi:10.1158/1538-7445.AM2014-5453

Tu1919 Diagnostic Accuracy of Carcinoembryonic Antigen (CEA) in Cyst Fluid Analysis in Histologically Confirmed Pancreatic Cysts: Results From a Large, Multicenter Cohort Study

Background: The phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1/S6K pathway plays a pivotal role in the proliferation and survival of pancreatic ductal adenocarcinoma (PDAC) cells and is aberrantly activated in pancreatic cancer tissues. In addition to growth-promoting signaling, mTORC1/S6K also mediates negative feedback loops that restrain upstream signaling via insulin/IGF receptor and other tyrosine kinase receptors. Suppression of these feedback loops by selective mTORC1 inhibitors, e.g. by rapamycin and its analogs, unleashes overactivation of the PI3K/Akt pathway that potentially oppose the anti-proliferative effects of mTOR inhibitors. This prompted the development of active-site mTORC1/2 kinase inhibitors (TOR-KIs) and dual PI3K and mTOR inhibitors (PI3K/TOR-KIs). Recently, we reported that TOR-KIs induce an unexpected increase in the activity of the ERK pathway in PDAC cells through a PI3K-independent pathway. Here, we examined whether PI3K/TOR-KIs also induce ERK pathway over-activation in PDAC cells. Results: To determine the effect of dual PI3K/TOR inhibition on ERK activation, we treated serum-starved cultures of PDAC cells (MiaPaca-2 and PANC-1) with increasing concentrations of the dual PI3K/TOR-KI NPVBEZ235 for 2 h followed by stimulation with insulin and neurotensin, a potent mitogenic combination for these cells. As expected, prior exposure to NPV-BEZ235 potently blocked mTORC1 activation (scored by S6 phosphorylation at Ser-240/244) and mTORC2-mediated Akt phosphorylation at Ser-473, in a concentration-dependent manner. Strikingly, we also demonstrate, for the first time that exposure to NPV-BEZ235 markedly enhanced the increase in the phosphorylation of ERK at Thr-202 and Tyr-204. Maximal ERK over-activation coincided with complete inhibition of Akt phosphorylation at Ser-473 (produced at 100500 ηM NPV-BEZ235). ERK over-activation was also seen when PDAC cells were stimulated with 2% fetal bovine serum instead of insulin and neurotensin and when a different PI3K/ TOR-KI (PKI-587) was used instead of NPV-BEZ235. Treatment with the MEK inhibitors U126 or PD0325901 (1-5 μM) prevented ERK over-activation induced by PI3K/TOR-KIs. In order to examine whether the over-activation of the ERK pathway counterbalances the growth-suppressive effect of PI3K/TOR-KIs, PDAC cells were treated with NPV-BEZ235, PD0325901 or a combination of NPV-BEZ235 and PD0325901. The combination of these drugs caused a more pronounced inhibition of cell growth than that produced by each inhibitor added individually. Conclusion: Collectively, our results highlight the importance of discovering novel signaling crosstalk to anticipate mechanisms of tumor resistance to new drugs. The capability of predicting drug resistance can assist in developing rational and effective strategies for developing combination therapies in PDAC.

Tu1915 Suppression of the mTOR/PI3K Pathway Promotes ERK Pathway Activation in Human Pancreatic Cancer Cells

Background: The phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1/S6K pathway plays a pivotal role in the proliferation and survival of pancreatic ductal adenocarcinoma (PDAC) cells and is aberrantly activated in pancreatic cancer tissues. In addition to growth-promoting signaling, mTORC1/S6K also mediates negative feedback loops that restrain upstream signaling via insulin/IGF receptor and other tyrosine kinase receptors. Suppression of these feedback loops by selective mTORC1 inhibitors, e.g. by rapamycin and its analogs, unleashes overactivation of the PI3K/Akt pathway that potentially oppose the anti-proliferative effects of mTOR inhibitors. This prompted the development of active-site mTORC1/2 kinase inhibitors (TOR-KIs) and dual PI3K and mTOR inhibitors (PI3K/TOR-KIs). Recently, we reported that TOR-KIs induce an unexpected increase in the activity of the ERK pathway in PDAC cells through a PI3K-independent pathway. Here, we examined whether PI3K/TOR-KIs also induce ERK pathway over-activation in PDAC cells. Results: To determine the effect of dual PI3K/TOR inhibition on ERK activation, we treated serum-starved cultures of PDAC cells (MiaPaca-2 and PANC-1) with increasing concentrations of the dual PI3K/TOR-KI NPVBEZ235 for 2 h followed by stimulation with insulin and neurotensin, a potent mitogenic combination for these cells. As expected, prior exposure to NPV-BEZ235 potently blocked mTORC1 activation (scored by S6 phosphorylation at Ser-240/244) and mTORC2-mediated Akt phosphorylation at Ser-473, in a concentration-dependent manner. Strikingly, we also demonstrate, for the first time that exposure to NPV-BEZ235 markedly enhanced the increase in the phosphorylation of ERK at Thr-202 and Tyr-204. Maximal ERK over-activation coincided with complete inhibition of Akt phosphorylation at Ser-473 (produced at 100500 ηM NPV-BEZ235). ERK over-activation was also seen when PDAC cells were stimulated with 2% fetal bovine serum instead of insulin and neurotensin and when a different PI3K/ TOR-KI (PKI-587) was used instead of NPV-BEZ235. Treatment with the MEK inhibitors U126 or PD0325901 (1-5 μM) prevented ERK over-activation induced by PI3K/TOR-KIs. In order to examine whether the over-activation of the ERK pathway counterbalances the growth-suppressive effect of PI3K/TOR-KIs, PDAC cells were treated with NPV-BEZ235, PD0325901 or a combination of NPV-BEZ235 and PD0325901. The combination of these drugs caused a more pronounced inhibition of cell growth than that produced by each inhibitor added individually. Conclusion: Collectively, our results highlight the importance of discovering novel signaling crosstalk to anticipate mechanisms of tumor resistance to new drugs. The capability of predicting drug resistance can assist in developing rational and effective strategies for developing combination therapies in PDAC.

VEGF Signals through ATF6 and PERK to Promote Endothelial Cell Survival and Angiogenesis in the Absence of ER Stress

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) initiates IRE1α, ATF6, and PERK cascades, leading to a transcriptional/translational response known as unfolded protein response (UPR). Here we show that VEGF activates UPR mediators through a PLCγ-mediated crosstalk with the mTORC1 complex without accumulation of unfolded proteins in the ER. Activation of ATF6 and PERK contributes to the survival effect of VEGF on endothelial cells (ECs) by positively regulating mTORC2-mediated phosphorylation of AKT on Ser473, which is required for full activity of AKT. Low levels of CHOP allow ECs to evade the proapoptotic effect of this UPR product. Depletion of PLCγ, ATF6, or eIF2α dramatically inhibited VEGF-induced vascularization in mouse Matrigel plugs, suggesting that the ER and the UPR machinery constitute components of the VEGF signaling circuit that regulates EC survival and angiogenesis, extending their role beyond adaptation to ER stress.