SUMO modifies GßL and mediates mTOR signaling.

The mechanistic target of rapamycin (mTOR) signaling is influenced by multiple regulatory proteins and post-translational modifications; however, underlying mechanisms remain unclear. Here, we report a novel role of small ubiquitin-like modifier (SUMO) in mTOR complex assembly and activity. By investigating the SUMOylation status of core mTOR components, we observed that the regulatory subunit, GßL (G protein ß-subunit-like protein, also known as mLST8), is modified by SUMO1, 2, and 3 isoforms. Using mutagenesis and mass spectrometry, we identified that GßL is SUMOylated at lysine sites K86, K215, K245, K261, and K305. We found that SUMO depletion reduces mTOR-Raptor (regulatory protein associated with mTOR) and mTOR-Rictor (rapamycin-insensitive companion of mTOR) complex formation and diminishes nutrient-induced mTOR signaling. Reconstitution with WT GßL but not SUMOylation-defective KR mutant GßL promotes mTOR signaling in GßL-depleted cells. Taken together, we report for the very first time that SUMO modifies GßL, influences the assembly of mTOR protein complexes, and regulates mTOR activity.

Bioactive peptide inhibits acute myeloid leukemia cell proliferation by downregulating ALKBH5-mediated m6A demethylation of EIF4EBP1 and MLST8 mRNA.

PURPOSE: N6-methyladenosine (m6A), the most prevalent mRNA modification, plays an essential role in tumorigenesis. Notably, increasing interest has been directed to bioactive peptides (BPs) with antitumor activities. Here, we set out to investigate the potential of the BP-regulated ALKBH5/MLST8/EIF4EBP1 axis on prevention and treatment of acute myeloid leukemia (AML). METHODS: The biological effects of BP on AML cells were detected by MTT and ApoLive-Glo™ multiplex assays. The role of BP in tumor growth was determined by a subcutaneous xenograft model. The ALKBH5/MLST8/EIF4EBP1 axis was identified as a potential BP target in AML via methylated RNA immunoprecipitation sequencing (MeRIP-seq) combined with RNA sequencing (RNA-seq). Western blot, RT-qPCR, MeRIP-qPCR, dual-luciferase reporter and RNA stability assays were performed to validate the function and mode of action of the BP-regulated ALKBH5/MLST8/EIF4EBP1 axis. The clinical relevance of the BP-regulated ALKBH5/MLST8/EIF4EBP1 axis in AML was confirmed by TCGA data analysis. RESULTS: We found that BP can inhibit AML cell proliferation and promote apoptosis in vitro, and repress AML tumor growth in vivo. Mechanistically, we found that BP downregulated ALKBH5 expression, which in turn repressed m6A demethylation of MLST8 and EIF4EBP1 mRNAs. Reduction of the m6A levels of MLST8 and EIF4EBP1 facilitated MLST8 and EIF4EBP1 mRNA decay, resulting in inhibition of AML cell proliferation. Furthermore, we found that the BP-regulated ALKBH5/MLST8/EIF4EBP1 axis closely correlates with AML patient prognosis. CONCLUSIONS: Our data indicate that BP can inhibit acute myeloid leukemia cell proliferation by downregulating ALKBH5-mediated m6A demethylation of EIF4EBP1 and MLST8 mRNAs, which may have potential to prevent and treat this disease.

Upregulated YB-1 protein promotes glioblastoma growth through a YB-1/CCT4/mLST8/mTOR pathway.

Y-box-binding protein 1 (YB-1) is a multifunctional RNA binding protein involved in virtually every step of RNA metabolism. However, the functions and mechanisms of YB-1 in one of the most aggressive cancers, glioblastoma, are not well understood. In this study, we found that YB-1 protein was markedly overexpressed in glioblastoma and acted as a critical activator of both mTORC1 and mTORC2 signaling. Mechanistically, YB-1 bound the 5'UTR of CCT4 mRNA to promote the translation of CCT4, a component of the CCT chaperone complex, that in turn activated the mTOR signaling pathway by promoting mLST8 folding. In addition, YB-1 autoregulated its own translation by binding to its 5'UTR, leading to sustained activation of mTOR signaling. In patients with glioblastoma, high protein expression of YB-1 correlated with increased expression of CCT4 and mLST8 and activated mTOR signaling. Importantly, the administration of RNA decoys specifically targeting YB-1 in a mouse xenograft model resulted in slower tumor growth and better survival. Taken together, these findings uncover a disrupted proteostasis pathway involving a YB-1/CCT4/mLST8/mTOR axis in promoting glioblastoma growth, suggesting that YB-1 is a potential therapeutic target for the treatment of glioblastoma.

CDK1/FBXW7 facilitates degradation and ubiquitination of MLST8 to inhibit progression of renal cell carcinoma.

Recent studies have reported that MLST8 is upregulated in many malignant tumors. Nevertheless, the underlying molecular mechanism is still unclear. The aim of this work was to investigate how MLST8 contributes to the development and progression of clear cell renal cell carcinoma (ccRCC). MLST8 is an oncogenic protein in the TCGA database and ccRCC clinical specimens. We also ascertain that MLST8 interacts with FBXW7, which was universally regarded as an E3 ubiquitin ligase. MLST8 can be degraded and ubiquitinated by tumor suppressor FBXW7. FBXW7 recognizes a consensus motif (T/S) PXX (S/T/D/E) of MLST8 and triggers MLST8 degradation via the ubiquitin-proteasome pathway. Strikingly, the activated cyclin dependent kinase 1 (CDK1) kinase engages in the MLST8 phosphorylation required for FBXW7-mediated degradation. In vitro, we further prove that MLST8 is an essential mediator of FBXW7 inactivation-induced tumor growth, migration, and invasion. Furthermore, the MLST8 and FBXW7 proteins are negatively correlated in human renal cancer specimens. Our findings suggest that MLST8 is a putative oncogene that functions via interaction with FBXW7, and inhibition MLST8 could be a potential future target in ccRCC treatment.

Genomic Database Analysis for Head and Neck Cancer Prevention Targets: MTOR Signal Transduction Pathway.

BACKGROUND: Type II diabetes agents have anticancer effects on head and neck squamous cell carcinoma (HNSCC). The mechanistic target of rapamycin (MTOR) pathway represents a putative target. MATERIALS AND METHODS: We interrogated an Affymetrix HNSCC dataset for MTOR-related gene expression. RESULTS: MTOR expression itself was unchanged, but various related genes demonstrated differential expression. Pathway promoters ras homolog (RHEB), MTOR-associated protein (MLST8), and ribosomal protein S6 kinase B1 (RPS6KB1) were up-regulated. Expression of growth suppressors tuberous sclerosis complex 2 (TSC2), programmed cell death 4 (PDCD4), and BCL2 apoptosis regulator-associated agonist of cell death (BAD) were reduced in HNSCC. Upstream, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), AKT serine/threonine kinase 1 (AKT1), and phosphatase and tensin homolog (PTEN) were up-regulated in cancer. CONCLUSION: Several MTOR pathway promoters and tumor suppressors were found to be differentially expressed, favoring MTOR pathway up-regulation in HNSCC. Genomic databases can be interrogated to identify intervention targets and endpoints in HNSCC trials.

STAT3-mediated MLST8 gene expression regulates cap-dependent translation in cancer cells.

Signal transducer and activator of transcription 3 (STAT3) regulates cell growth, cell survival, angiogenesis, metastasis of cancer cells, and cancer immune evasion by regulating gene expression as a transcription factor. However, the effect of STAT3 on translation is almost unknown. We demonstrated that STAT3 acts as a trans-acting factor for MLST8 gene expression and the protein level of mLST8, a core component of mechanistic target of rapamycin complex 1 and 2 (mTORC1/2), positively regulates the mTORC1/2 downstream pathways. Suppression of STAT3 by siRNA attenuated 4E-BP1 phosphorylation, cap-dependent translation, and cell proliferation in a variety of cancer cells. In HCT116 cells, STAT3 knockdown-induced decreases in 4E-BP1 and AKT phosphorylation levels were further attenuated by MLST8 knockdown or recovered by mLST8 overexpression. STAT3 knockdown-induced G2/M phase arrest was partially restored by co-knockdown of 4EBP1, and the attenuation of cell proliferation was enhanced by the expression of an mTORC1-mediated phosphorylation-defective mutant of 4E-BP1. ChIP and promoter mapping using a luciferase reporter assay showed that the -951 to -894 bp of MLST8 promoter seems to include STAT3-binding site. Overall, these results suggest that STAT3-driven MLST8 gene expression regulates cap-dependent translation through 4E-BP1 phosphorylation in cancer cells.

Enhanced mLST8 Expression Correlates with Tumor Progression in Hepatocellular Carcinoma.

BACKGROUND: The mechanistic target of rapamycin (mTOR) pathway, containing mTOR complex 1 (mTORC1) and mTORC2, is dysregulated in multiple cancers, including hepatocellular carcinoma (HCC). Mammalian lethal with sec-13 protein 8 (mLST8) is a shared constituent of both mTORC1 and mTORC2, yet little is known regarding its role in HCC development. METHODS: mLST8 expression was detected in a total of 186 pairs of HCC and adjacent non-tumor specimens. The correlation between mLST8 level and clinicopathological features or prognostic significance were analyzed. The role of mLST8 on biological functions was also preliminarily studied. RESULTS: The study revealed that the mLST8 level was dramatically higher in HCC specimens than in adjacent non-tumor specimens. mLST8 overexpression positively correlated with tumor size, differentiation, and vessel invasion. Cases with elevated mLST8 level had more unfavorable overall survival (OS) and disease-free survival (DFS) than those with downregulated mLST8 level. Multivariate analysis demonstrated that mLST8 upregulation was an independent predictive marker for OS and DFS. Calibration curves from nomogram models indicated an excellent coherence between nomogram prediction and actual situation. Decision curve analysis proved that mLST8-based nomograms presented much higher predictive accuracy when compared with conventional clinical staging systems. Mechanistically, mLST8 enhanced cell proliferation and invasion through the AKT (protein kinase B) pathway. CONCLUSIONS: Our study demonstrates that mLST8 exerts an oncogenic role in HCC and may become a promising prognostic biomarker and therapeutic target for HCC patients.

Cross-talk between ribosome biogenesis, translation, and mTOR in CD133+ 4/CD44+ prostate cancer stem cells.

OBJECTIVE: To investigate the gene expression profile of CSCs and to explore the key pathways and specific molecular signatures involved in the characteristic of CSCs. MATERIALS AND METHODS: CD133+ /CD44+ CSCs and bulk population (non-CSCs) were isolated from DU-145 cells using fluorescence-activated cell sorting (FACS). We used Illumina HumanHT-12 v4 Expression to investigate gene expression profiling of CSCs and non-CSCs. Protein-protein interaction (PPI) network analysis was performed using the STRING database. Biomarkers selected based on gene expression profiling were visually analyzed using immunofluorescence staining method. An image analysis program, ImageJ®, was used for the analysis of fluorescence intensity. RESULTS: In microarray analysis, we found that many ribosomal proteins and translation initiation factors that constitute the mTOR complex were highly expressed. PPI analysis using the 33 genes demonstrated that there was a close interaction between ribosome biogenesis, translation, and mTOR signaling. The fluorescence amount of mTOR and MLST8 were higher in CSCs compared to non-CSCs. CONCLUSIONS: The increase in a number of genes associated with ribosome biogenesis, translation, and mTOR signaling may be important to evaluate prognosis and determine treatment approach for prostate cancer (PCa). A better understanding of the molecular pathways associated with CSCs may be promising to develop targeted therapies to prolong survival in PCa.

Structural and functional analysis of the role of the chaperonin CCT in mTOR complex assembly.

The mechanistic target of rapamycin (mTOR) kinase forms two multi-protein signaling complexes, mTORC1 and mTORC2, which are master regulators of cell growth, metabolism, survival and autophagy. Two of the subunits of these complexes are mLST8 and Raptor, ß-propeller proteins that stabilize the mTOR kinase and recruit substrates, respectively. Here we report that the eukaryotic chaperonin CCT plays a key role in mTORC assembly and signaling by folding both mLST8 and Raptor. A high resolution (4.0 Å) cryo-EM structure of the human mLST8-CCT intermediate isolated directly from cells shows mLST8 in a near-native state bound to CCT deep within the folding chamber between the two CCT rings, and interacting mainly with the disordered N- and C-termini of specific CCT subunits of both rings. These findings describe a unique function of CCT in mTORC assembly and a distinct binding site in CCT for mLST8, far from those found for similar ß-propeller proteins.

Disruption of the Scaffolding Function of mLST8 Selectively Inhibits mTORC2 Assembly and Function and Suppresses mTORC2-Dependent Tumor Growth In Vivo.

mTOR is a serine/threonine kinase that acts in two distinct complexes, mTORC1 and mTORC2, and is dysregulated in many diseases including cancer. mLST8 is a shared component of both mTORC1 and mTORC2, yet little is known regarding how mLST8 contributes to assembly and activity of the mTOR complexes. Here we assessed mLST8 loss in a panel of normal and cancer cells and observed little to no impact on assembly or activity of mTORC1. However, mLST8 loss blocked mTOR association with mTORC2 cofactors RICTOR and SIN1, thus abrogating mTORC2 activity. Similarly, a single pair of mutations on mLST8 with a corresponding mutation on mTOR interfered with mTORC2 assembly and activity without affecting mTORC1. We also discovered a direct interaction between mLST8 and the NH2-terminal domain of the mTORC2 cofactor SIN1. In PTEN-null prostate cancer xenografts, mLST8 mutations disrupting the mTOR interaction motif inhibited AKT S473 phosphorylation and decreased tumor cell proliferation and tumor growth in vivo. Together, these data suggest that the scaffolding function of mLST8 is critical for assembly and activity of mTORC2, but not mTORC1, an observation that could enable therapeutic mTORC2-selective inhibition as a therapeutic strategy. SIGNIFICANCE: These findings show that mLST8 functions as a scaffold to maintain mTORC2 integrity and kinase activity, unveiling a new avenue for development of mTORC2-specific inhibitors.

Magnesium transporter protein solute carrier family 41 member 1 suppresses human pancreatic ductal adenocarcinoma through magnesium-dependent Akt/mTOR inhibition and bax-associated mitochondrial apoptosis.

The aim of this study was to identify the function of the Mg2+ transporter protein solute carrier family 41 member 1 SLC41A1 in pancreatic ductal adenocarcinoma and the underlying mechanisms. A total of 27 solute carrier proteins were differentially expressed in pancreatic ductal adenocarcinoma. Three of these proteins were correlated with clinical outcomes in patients, among which SLC41A1 was downregulated in tumour. Overexpression of SLC41A1 suppressed orthotopic tumour growth in a mouse model and reduced the cell proliferation, colony formation, and invasiveness of KP3 and Panc-1 cells, which may have been associated with the increased population of apoptotic-prone cells. Overexpression of SLC41A1 reduced the mitochondrial membrane potential, induced Bax while suppressed Bcl-2 expression. Suppression of Bax abrogated the tumour-suppressive effects of SLC41A1. Furthermore, overexpression of SLC41A1 promoted Mg2+ efflux and suppressed Akt/mTOR activity, which is the upstream regulator of Bax and Bcl-2. An increase in Akt activity and supplementation with Mg2+ abolished SLC41A1-induced tumour suppression. The results of this study suggest that SLC41A1 may be a potential target for the treatment of pancreatic ductal adenocarcinoma.

Taurine Promotes Milk Synthesis via the GPR87-PI3K-SETD1A Signaling in BMECs.

Taurine, a ß-aminosulfonic acid, exerts many cellular physiological functions. It is still unknown whether taurine can regulate milk synthesis in the mammary gland. Therefore, in this study we investigated the effects and mechanism of taurine on milk synthesis in mammary epithelial cells (MECs). Bovine MECs (BMECs) cultured in FBS-free OPTI-MEMImedium were treated with taurine (0, 0.08, 0.16, 0.24, 0.32, and 0.4 mM). Taurine treatment led to increased milk protein and fat synthesis, mTOR phosphorylation, and SREBP-1c protein expression, in a dose-dependent manner, with an apparent maximum at 0.24 mM. Gene function study approaches revealed that the GPR87-PI3K-SETD1A signaling was required for taurine to increase the mTOR and SREBP-1c mRNA levels. Taurine stimulated GPR87 expression and cell membrane localization in a dose dependent manner, suggesting a sensing mechanism of GPR87 to extracellular taurine. Collectively, these data demonstrate that taurine promotes milk synthesis via the GPR87-PI3K-SETD1A signaling.

Sodium fluoride induces splenocyte autophagy via the mammalian targets of rapamycin (mTOR) signaling pathway in growing mice.

Fluoride is known to impair organism's development and function via adverse effects, and autophagy plays a regulation role in human or animal health and disease. At present, there are no reports focused on fluoride-induced autophagy in the animal and human spleen. The objective of this study was to investigate sodium fluoride (NaF)-induced splenocyte autophagy and the potential mechanism via regulation of p-mTOR in growing mice by using the methods of transmission electron microscopy (TEM), immunohistochemistry (IHC), quantitative real-time polymerase chain reaction (qRT-PCR) and western blot. A total of 240 ICR mice were equally allocated into four groups with intragastric administration of distilled water in the control group and 12, 24, 48 mg/kg NaF solution in the experimental groups for 42 days. Results revealed that NaF increased autophagosomes or autolysosomes in spleen. Simultaneously, the autophagy marker LC3 brown punctate staining was increased with NaF dosage increase. On the other hand, NaF caused inhibition of mTOR activity, which was characterized by down-regulation of PI3K, Akt and mTOR mRNA and protein expression levels. And the suppression of mTOR activity in turn resulted in the significantly increased of ULK1 and Atg13 expression levels. Concurrently, NaF increased the levels of mRNA and protein expression of autophagy markers LC3, Beclin1, Atg16L1, Atg12, Atg5 and decreased the mRNA and protein expression levels of p62. The above-mentioned findings verify that NaF induces autophagy via mTOR signaling pathway. The inhibition of mTOR activity and alteration of autophagy-related genes and proteins are the potential molecular mechanism of NaF-induced splenocyte autophagy.

Construction of competitive endogenous RNA network reveals regulatory role of long non-coding RNAs in type 2 diabetes mellitus.

Increasing epidemic of type 2 diabetes mellitus (T2DM) and its comorbidities makes it urgent to understand the pathogenesis and regulatory mechanism. However, little is known about the regulatory role of lncRNAs in diabetes. Here, we constructed a T2DM-related competitive endogenous RNA (ceRNA) network (DMCN) to explore biological function of lncRNAs during the development of diabetes mellitus. This network contained 351 nodes including 98 mRNAs, 86 microRNAs and 167 lncRNAs. Functional analysis showed that the mRNAs in DMCN were annotated into some diabetes-related pathways. Furthermore, mTOR-centred subnetwork was extracted and ncRNA-involved mTOR pathway was established. Finally, we validated that NEAT1 was potentially communicated with mTOR signalling target protein mLST8 via the association with miR-181b. These findings provide significant insight into lncRNA regulatory network in T2DM.

Ectopic expression of Arabidopsis Target of Rapamycin (AtTOR) improves water-use efficiency and yield potential in rice.

The target of Rapamycin (TOR) present in all eukaryotes is a multifunctional protein, regulating growth, development, protein translation, ribosome biogenesis, nutrient, and energy signaling. In the present study, ectopic expression of TOR gene of Arabidopsis thaliana in a widely cultivated indica rice resulted in enhanced plant growth under water-limiting conditions conferring agronomically important water-use efficiency (WUE) trait. The AtTOR high expression lines of rice exhibited profuse tillering, increased panicle length, increased plant height, high photosynthetic efficiency, chlorophyll content and low ∆13C. Δ13C, which is inversely related to high WUE, was as low as 17‰ in two AtTOR high expression lines. These lines were also insensitive to the ABA-mediated inhibition of seed germination. The significant upregulation of 15 stress-specific genes in high expression lines indicates their contribution to abiotic stress tolerance. The constitutive expression of AtTOR is also associated with significant transcriptional upregulation of putative TOR complex-1 components, OsRaptor and OsLST8. Glucose-mediated transcriptional activation of AtTOR gene enhanced lateral root formation. Taken together, our findings indicate that TOR, in addition to its multiple cellular functions, also plays an important role in response to abiotic stress and potentially enhances WUE and yield related attributes.