Rapid turnover of the mTOR complex 1 (mTORC1) repressor REDD1 and activation of mTORC1 signaling following inhibition of protein synthesis

RTP801 mediates transneuronal toxicity in culture via extracellular vesicles

Extracellular vesicles (EVs) play a crucial role in intercellular communication, participating in the paracrine trophic support or in the propagation of toxic molecules, including proteins. RTP801 is a stress-regulated protein, whose levels are elevated during neurodegeneration and induce neuron death. However, whether RTP801 toxicity is transferred trans-neuronally via EVs remains unknown. Hence, we overexpressed or silenced RTP801 protein in cultured cortical neurons, isolated their derived EVs (RTP801-EVs or shRTP801-EVs, respectively), and characterized EVs protein content by mass spectrometry (MS). RTP801-EVs toxicity was assessed by treating cultured neurons with these EVs and quantifying apoptotic neuron death and branching. We also tested shRTP801-EVs functionality in the pathologic in vitro model of 6-Hydroxydopamine (6-OHDA). Expression of RTP801 increased the number of EVs released by neurons. Moreover, RTP801 led to a distinct proteomic signature of neuron-derived EVs, containing more pro-apoptotic markers. Hence, we observed that RTP801-induced toxicity was transferred to neurons via EVs, activating apoptosis and impairing neuron morphology complexity. In contrast, shRTP801-EVs were able to increase the arborization in recipient neurons. The 6-OHDA neurotoxin elevated levels of RTP801 in EVs, and 6-OHDA-derived EVs lost the mTOR/Akt signalling activation via Akt and RPS6 downstream effectors. Interestingly, EVs derived from neurons where RTP801 was silenced prior to exposing them to 6-OHDA maintained Akt and RPS6 transactivation in recipient neurons. Taken together, these results suggest that RTP801-induced toxicity is transferred via EVs, and therefore, it could contribute to the progression of neurodegenerative diseases, in which RTP801 is involved.

Protective Role of Ethanol Extract of Cibotium barometz (Cibotium Rhizome) against Dexamethasone-Induced Muscle Atrophy in C2C12 Myotubes

Sarcopenia is a progressive muscle disease characterized by the loss of skeletal muscle mass, strength, function, and physical performance. Since the disease code was assigned, attention has been focused on natural products that can protect against muscle atrophy. Cibotium barometz (Cibotium Rhizome) has been used as an herbal medicine for the treatment of bone or joint diseases in Asian countries. However, no studies have identified the mechanism of action of Cibotium Rhizome on muscle atrophy related to sarcopenia at the site of myotubes. The aim of this study was to investigate the improvement effect of the ethanol extract of Cibotium Rhizome (ECR) on dexamethasone-induced muscle atrophy in an in vitro cell model, i.e., the C2C12 myotubes. High-performance liquid chromatography was performed to examine the phytochemicals in ECR. Seven peaks in the ECR were identified, corresponding to the following compounds: protocatechuic acid, (+)-catechin hydrate, p-coumaric acid, ellagic acid, chlorogenic acid, caffeic acid, and ferulic acid. In atrophy-like conditions induced by 100 μM dexamethasone for 24 h in C2C12, ECR increased the expression of the myosin heavy chain, p-Akt, the p-mammalian target of rapamycin (mTOR), p-p70S6K, and repressed the expression of regulated in development and DNA damage responses 1 (REDD1), kruppel-like factor 15 (KLF 15), muscle atrophy F-box, and muscle-specific RING finger protein-1 in C2C12. In addition, ECR alleviated dexamethasone-induced muscle atrophy by repressing REDD1 and KLF15 transcription in C2C12 myotubes, indicating the need for further studies to provide a scientific basis for the development of useful therapeutic agents using ECR to alleviate the effects of skeletal muscle atrophy or sarcopenia.

The stress-responsive protein REDD1 and its pathophysiological functions

Regulated in development and DNA damage-response 1 (REDD1) is a stress-induced protein that controls various cellular functions, including metabolism, oxidative stress, autophagy, and cell fate, and contributes to the pathogenesis of metabolic and inflammatory disorders, neurodegeneration, and cancer. REDD1 usually exerts deleterious effects, including tumorigenesis, metabolic inflammation, neurodegeneration, and muscle dystrophy; however, it also exhibits protective functions by regulating multiple intrinsic cell activities through either an mTORC1-dependent or -independent mechanism. REDD1 typically regulates mTORC1 signaling, NF-κB activation, and cellular pro-oxidant or antioxidant activity by interacting with 14-3-3 proteins, IκBα, and thioredoxin-interacting protein or 75 kDa glucose-regulated protein, respectively. The diverse functions of REDD1 depend on cell type, cellular context, interaction partners, and cellular localization (e.g., mitochondria, endomembrane, or cytosol). Therefore, comprehensively understanding the molecular mechanisms and biological roles of REDD1 under pathophysiological conditions is of utmost importance. In this review, based on the published literature, we highlight and discuss the molecular mechanisms underlying the REDD1 expression and its actions, biological functions, and pathophysiological roles.

The Case for the Target of Rapamycin Pathway as a Candidate Circadian Oscillator

The molecular mechanisms that drive circadian (24 h) rhythmicity have been investigated for many decades, but we still do not have a complete picture of eukaryotic circadian systems. Although the transcription/translation feedback loop (TTFL) model has been the primary focus of research, there are many examples of circadian rhythms that persist when TTFLs are not functioning, and we lack any good candidates for the non-TTFL oscillators driving these rhythms. In this hypothesis-driven review, the author brings together several lines of evidence pointing towards the Target of Rapamycin (TOR) signalling pathway as a good candidate for a non-TTFL oscillator. TOR is a ubiquitous regulator of metabolism in eukaryotes and recent focus in circadian research on connections between metabolism and rhythms makes TOR an attractive candidate oscillator. In this paper, the evidence for a role for TOR in regulating rhythmicity is reviewed, and the advantages of TOR as a potential oscillator are discussed. Evidence for extensive feedback regulation of TOR provides potential mechanisms for a TOR-driven oscillator. Comparison with ultradian yeast metabolic cycles provides an example of a potential TOR-driven self-sustained oscillation. Unanswered questions and problems to be addressed by future research are discussed.

Activation of Disulfide Redox Switch in REDD1 Promotes Oxidative Stress Under Hyperglycemic Conditions

The stress response protein regulated in development and DNA damage response 1 (REDD1) has been implicated in visual deficits in patients with diabetes. The aim here was to investigate the mechanism responsible for the increase in retinal REDD1 protein content that is observed with diabetes. We found that REDD1 protein expression was increased in the retina of streptozotocin-induced diabetic mice in the absence of a change in REDD1 mRNA abundance or ribosome association. Oral antioxidant supplementation reduced retinal oxidative stress and suppressed REDD1 protein expression in the retina of diabetic mice. In human retinal Müller cell cultures, hyperglycemic conditions increased oxidative stress, enhanced REDD1 expression, and inhibited REDD1 degradation independently of the proteasome. Hyperglycemic conditions promoted a redox-sensitive cross-strand disulfide bond in REDD1 at C150/C157 that was required for reduced REDD1 degradation. Discrete molecular dynamics simulations of REDD1 structure revealed allosteric regulation of a degron upon formation of the disulfide bond that disrupted lysosomal proteolysis of REDD1. REDD1 acetylation at K129 was required for REDD1 recognition by the cytosolic chaperone HSC70 and degradation by chaperone-mediated autophagy. Disruption of REDD1 allostery upon C150/C157 disulfide bond formation prevented the suppressive effect of hyperglycemic conditions on REDD1 degradation and reduced oxidative stress in cells exposed to hyperglycemic conditions. The results reveal redox regulation of REDD1 and demonstrate the role of a REDD1 disulfide switch in development of oxidative stress.

Cell plasticity in regeneration in the stomach and beyond

Recent studies using cell lineage-tracing techniques, organoids, and single-cell RNA sequencing analyses have revealed: 1) adult organs use cell plasticity programs to recruit progenitor cells to regenerate tissues after injury, and 2) plasticity is far more common than previously thought, even in homeostasis. Here, we focus on the complex interplay of normal stem cell differentiation and plasticity in homeostasis and after injury, using the gastric epithelium as a touchstone. We also examine common features of regenerative programs and discuss the evolutionarily conserved, stepwise process of paligenosis which reprograms mature cells into progenitors that can repair damaged tissue. Finally, we discuss how conserved plasticity programs may help us better understand pathological processes like metaplasia.

Loss of REDD1 prevents chemotherapy-induced muscle atrophy and weakness in mice

BACKGROUND

Chemotherapy is an essential treatment to combat solid tumours and mitigate metastasis. Chemotherapy causes side effects including muscle wasting and weakness. Regulated in Development and DNA Damage Response 1 (REDD1) is a stress-response protein that represses the mechanistic target of rapamycin (mTOR) in complex 1 (mTORC1), and its expression is increased in models of muscle wasting. The aim of this study was to determine if deletion of REDD1 is sufficient to attenuate chemotherapy-induced muscle wasting and weakness in mice.

METHODS

C2C12 myotubes were treated with carboplatin, and changes in myotube diameter were measured. Protein synthesis was measured by puromycin incorporation, and REDD1 mRNA and protein expression were analysed in myotubes treated with carboplatin. Markers of mTORC1 signalling were measured by western blot. REDD1 global knockout mice and wild-type mice were treated with a single dose of carboplatin and euthanized 7 days later. Body weight, hindlimb muscle weights, forelimb grip strength, and extensor digitorum longus whole muscle contractility were measured in all groups. Thirty minutes prior to euthanasia, mice were injected with puromycin to measure puromycin incorporation in skeletal muscle.

RESULTS

C2C12 myotube diameter was decreased at 24 (P = 0.0002) and 48 h (P < 0.0001) after carboplatin treatment. Puromycin incorporation was decreased in myotubes treated with carboplatin for 24 (P = 0.0068) and 48 h (P = 0.0008). REDD1 mRNA and protein expression were increased with carboplatin treatment (P = 0.0267 and P = 0.0015, respectively), and this was accompanied by decreased phosphorylation of Akt T³⁰⁸ (P < 0.0001) and S⁴⁷³ (P = 0.0006), p70S6K T³⁸⁹ (P = 0.0002), and 4E-binding protein 1 S⁶⁵ (P = 0.0341), all markers of mTORC1 activity. REDD1 mRNA expression was increased in muscles from mice treated with carboplatin (P = 0.0295). Loss of REDD1 reduced carboplatin-induced body weight loss (P = 0.0013) and prevented muscle atrophy in mice. REDD1 deletion prevented carboplatin-induced decrease of protein synthesis (P = 0.7626) and prevented muscle weakness.

CONCLUSIONS

Carboplatin caused loss of body weight, muscle atrophy, muscle weakness, and inhibition of protein synthesis. Loss of REDD1 attenuates muscle atrophy and weakness in mice treated with carboplatin. Our study illustrates the importance of REDD1 in the regulation of muscle mass with chemotherapy treatment and may be an attractive therapeutic target to combat cachexia.

REDD1 is a determinant of low-dose metronomic doxorubicin-elicited endothelial cell dysfunction through downregulation of VEGFR-2/3 expression

Low-dose metronomic chemotherapy (LDMC) inhibits tumor angiogenesis and growth by targeting tumor-associated endothelial cells, but the molecular mechanism has not been fully elucidated. Here, we examined the functional role of regulated in development and DNA damage responses 1 (REDD1), an inhibitor of mammalian target of rapamycin complex 1 (mTORC1), in LDMC-mediated endothelial cell dysfunction. Low-dose doxorubicin (DOX) treatment induced REDD1 expression in cultured vascular and lymphatic endothelial cells and subsequently repressed the mRNA expression of mTORC1-dependent translation of vascular endothelial growth factor receptor (Vegfr)-2/3, resulting in the inhibition of VEGF-mediated angiogenesis and lymphangiogenesis. These regulatory effects of DOX-induced REDD1 expression were additionally confirmed by loss- and gain-of-function studies. Furthermore, LDMC with DOX significantly suppressed tumor angiogenesis, lymphangiogenesis, vascular permeability, growth, and metastasis in B16 melanoma-bearing wild-type but not Redd1-deficient mice. Altogether, our findings indicate that REDD1 is a crucial determinant of LDMC-mediated functional dysregulation of tumor vascular and lymphatic endothelial cells by translational repression of Vegfr-2/3 transcripts, supporting the potential therapeutic properties of REDD1 in highly progressive or metastatic tumors.

Transient activation of mTORC1 signaling in skeletal muscle is independent of Akt1 regulation

The regulation of cellular protein synthesis is a critical determinant of skeletal muscle growth and hypertrophy in response to an increased workload such as resistance exercise. The mechanistic target of rapamycin complex 1 (mTORC1) and its upstream protein kinase Akt1 have been implicated as a central signaling pathway that regulates protein synthesis in the skeletal muscle; however, the precise molecular regulation of mTORC1 activity is largely unknown. This study employed germline Akt1 knockout (KO) mice to examine whether upstream Akt1 regulation is necessary for the acute activation of mTORC1 signaling in the plantaris muscle following mechanical overload. The phosphorylation states of S6 kinase 1, ribosomal protein S6, and eukaryotic translation initiation factor 4E‐binding protein 1 which show the functional activity of mTORC1 signaling, were significantly increased in the skeletal muscle of both wildtype and Akt1 KO mice following an acute bout (3 and 12 hr) of mechanical overload. Akt1 deficiency did not affect load‐induced alteration of insulin‐like growth factor‐1 (IGF‐1)/IGF receptor mRNA expression. Also, no effect of Akt1 deficiency was observed on the overload‐induced increase in the gene expressions of pax7 and myogenic regulatory factor of myogenin. These observations show that the upstream IGF‐1/Akt1 regulation is dispensable for the acute activation of mTORC1 signaling and regulation of satellite cells in response to mechanical overload.

IFNs Reset the Differential Capacity of Human Monocyte Subsets to Produce IL-12 in Response to Microbial Stimulation

Human primary monocytes are composed of a minor, more mature CD16+(CD14low/neg) population and a major CD16neg(CD14+) subset. The specific functions of CD16+ versus CD16neg monocytes in steady state or inflammation remain poorly understood. In previous work, we found that IL-12 is selectively produced by the CD16+ subset in response to the protozoan pathogen, Toxoplasma gondii In this study, we demonstrated that this differential responsiveness correlates with the presence of an IFN-induced transcriptional signature in CD16+ monocytes already at baseline. Consistent with this observation, we found that in vitro IFN-γ priming overcomes the defect in the IL-12 response of the CD16neg subset. In contrast, pretreatment with IFN-γ had only a minor effect on IL-12p40 secretion by the CD16+ population. Moreover, inhibition of the mTOR pathway also selectively increased the IL-12 response in CD16neg but not in CD16+ monocytes. We further demonstrate that in contrast to IFN-γ, IFN-α fails to promote IL-12 production by the CD16neg subset and blocks the effect of IFN-γ priming. Based on these observations, we propose that the acquisition of IL-12 responsiveness by peripheral blood monocyte subsets depends on extrinsic signals experienced during their developmental progression in vivo. This process can be overridden during inflammation by the opposing regulatory effects of type I and II IFN as well as the mTOR inhibition.

Insulin potentiates essential amino acids effects on mechanistic target of rapamycin complex 1 signaling in MAC-T cells

Different models of lactation offer conflicting evidence as to whether insulin signaling is required for AA to stimulate mechanistic target of rapamycin complex 1 (mTORC1) activity. We hypothesized that insulin potentiates essential AA stimulation of mTORC1 activity in the MAC-T mammary epithelial cell line. Here, our objective was to assess mTORC1 signaling activity in response to insulin and individual or grouped essential AA. Insulin and essential AA concentrations in the treatment medium ranged from normo- to supraphysiological, with insulin at 0, 1, 10, or 100 nmol/L and essential AA at approximately 0, 0.01, 0.05, 0.1, 1, or 3× reference plasma levels. Effects and interaction of insulin and total essential AA were tested in a 3 × 5 factorial design (n = 3 replicates/treatment); insulin and the individual AA Leu, Met, Ile, and Arg were likewise tested in 3 × 4 factorials (n = 4). As the remaining individual AA His, Lys, Phe, Thr, Trp, and Val were expected to not affect mTORC1, these were tested only at the highest insulin level, 100 nmol/L (n = 4). For all of these, linear and quadratic effects of total and individual AA were evaluated. Essential AA were subsequently grouped by their positive (Leu, Met, Ile, Arg, and Thr; TOR-AA) or absent-to-negative effects (His, Lys, Phe, Trp, and Val; NTOR-AA), and tested for interaction in a 2 × 2 factorial design (n = 4), with each AA at its respective 1× plasma level, and insulin held at 100 nmol/L. All experiments consisted of 1 h treatment incubation, followed by Western blotting of cell lysates to measure phosphorylation and abundance of the mTORC1 pathway proteins Akt (Ser473); ribosomal protein S6 kinase p70 (S6K1, Thr389); eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1, Ser65); and ribosomal protein S6 (S6, Ser240/244). The Akt phosphorylation was overall increased by insulin, with a possible negative interaction with both total essential AA and the individual AA Leu. Total essential AA also increased S6K1 and 4E-BP1 phosphorylation in an insulin-dependent manner. The individual AA Leu, Met, Ile, and Arg increased S6K1 phosphorylation in an insulin-dependent manner. Similarly, Met and Arg increased 4E-BP1 phosphorylation in an insulin-dependent manner. Histidine, Lys, Trp, and Val did not affect S6K1 phosphorylation. However, S6K1 phosphorylation was linearly increased by Thr and quadratically decreased by Phe. Relative to the phosphorylation of S6K1 when cells were incubated with no essential AA, the NTOR-AA group had no effect, whereas the TOR-AA increased phosphorylation to the same degree observed with all 10 essential AA. Overall, we have found that insulin is required for essential AA to stimulate mTORC1 activity in MAC-T cells. In addition, the AA responsible for the bulk of mTORC1 activation in MAC-T are limited to Leu, Met, Ile, Arg, and Thr.

A Dedicated Evolutionarily Conserved Molecular Network Licenses Differentiated Cells to Return to the Cell Cycle

Differentiated cells can re-enter the cell cycle to repair tissue damage via a series of discrete morphological and molecular stages coordinated by the cellular energetics regulator mTORC1. We previously proposed the term "paligenosis" to describe this conserved cellular regeneration program. Here, we detail a molecular network regulating mTORC1 during paligenosis in both mouse pancreatic acinar and gastric chief cells. DDIT4 initially suppresses mTORC1 to induce autodegradation of differentiated cell components and damaged organelles. Later in paligenosis, IFRD1 suppresses p53 accumulation. Ifrd1^-/- cells do not complete paligenosis because persistent p53 prevents mTORC1 reactivation and cell proliferation. Ddit4^-/- cells never suppress mTORC1 and bypass the IFRD1 checkpoint on proliferation. Previous reports and our current data implicate DDIT4/IFRD1 in governing paligenosis in multiple organs and species. Thus, we propose that an evolutionarily conserved, dedicated molecular network has evolved to allow differentiated cells to re-enter the cell cycle (i.e., undergo paligenosis) after tissue injury. VIDEO ABSTRACT.

The consequences of increased 4E-BP1 in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disease, characterized by cyst formation and growth. Hyperproliferation is a major contributor to cyst growth. At the nexus of regulating proliferation, is 4E-BP1. We demonstrate that ADPKD mouse and rat models, ADPKD patient renal biopsies and PKD1-/- cells exhibited hyperphosphorylated 4E-BP1, a biomarker of increased translation and proliferation. We hypothesized that expression of constitutively active 4E-BP1 constructs (4E-BP1F113A and 4E-BP1R13AF113A) would decrease proliferation and reduce cyst expansion. Utilizing the Pkd1RC/RC mouse, we determined the effect of 4E-BP1F113A on PKD. Unexpectedly, 4E-BP1F113A resulted in increased cyst burden and suppressed apoptosis markers, increased anti-apoptotic Bcl-2 protein and increased mitochondrial proteins. Exogenous 4E-BP1 enhanced proliferation, decreased apoptosis, increased anti-apoptotic Bcl-2 protein, impaired NADPH oxidoreductase activity, increased mitochondrial proteins and increased superoxide production in PKD patient-derived renal epithelial cells. Reduced 4E-BP1 expression suppressed proliferation, restored apoptosis and improved cellular metabolism. These findings provide insight into how cyst-lining cells respond to 4E-BP1.

Glucagon-Dependent Suppression of mTORC1 is Associated with Upregulation of Hepatic FGF21 mRNA Translation

Fibroblast growth factor 21 (FGF21) is a peptide hormone that acts to enhance insulin sensitivity and reverse many of the metabolic defects associated with consumption of a high-fat diet. Recent studies show that the liver is the primary source of FGF21 in the blood, and that hepatic FGF21 expression is upregulated by glucagon. Interestingly, glucagon acts to upregulate FGF21 production by primary cultures of rat hepatocytes and H4IIE and HepG2 hepatocarcinoma cells independent of changes in FGF21 mRNA abundance, suggesting that FGF21 protein expression is regulated post-transcriptionally. Based on these observations, the goal of the present study was to assess whether or not FGF21 mRNA is translationally regulated. The results show that FGF21 mRNA translation and secretion of the hormone are significantly upregulated in H4IIE cells exposed to 25 nM glucagon, independent of changes in FGF21 mRNA abundance. Furthermore, the glucagon-induced upregulation of FGF21 mRNA translation is associated with suppressed activity of the mechanistic target of rapamycin in complex 1 (mTORC1). Similarly, the results show that rapamycin-induced suppression of mTORC1 leads to upregulation of FGF21 mRNA translation with no change in FGF21 mRNA abundance. In contrast, activation of mTORC1 by refreshing the culture medium leads to downregulation of FGF21 mRNA translation. Notably, re-feeding fasted rats also leads to downregulation of FGF21 mRNA translation concomitantly with activation of mTORC1 in the liver. Overall, the findings support a model in which glucagon acts to upregulate FGF21 production by hepatocytes through suppression of mTORC1 and subsequent upregulation of FGF21 mRNA translation.

A pathway linking translation stress to checkpoint kinase 2 signaling in Neurospora crassa

Checkpoint kinase 2 (CHK-2) is a key component of the DNA damage response (DDR) pathway and its activation mechanism is evolutionarily conserved. We show that PERIOD-4 (PRD-4), the CHK-2 ortholog of Neurospora crassa, is part of an additional signaling pathway that is activated when protein translation is compromised. Translation stress induces phosphorylation of PRD-4 by an upstream kinase distinct from those of the DDR pathway. We present evidence that the activating kinase is mTOR. Translation stress is sensed via a decrease in levels of an unstable inhibitor that antagonizes phosphorylation of PRD-4.

Checkpoint kinase 2 (CHK-2) is a key component of the DNA damage response (DDR). CHK-2 is activated by the PIP3-kinase-like kinases (PI3KKs) ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related protein (ATR), and in metazoan also by DNA-dependent protein kinase catalytic subunit (DNA-PKcs). These DNA damage-dependent activation pathways are conserved and additional activation pathways of CHK-2 are not known. Here we show that PERIOD-4 (PRD-4), the CHK-2 ortholog of Neurospora crassa, is part of a signaling pathway that is activated when protein translation is compromised. Translation stress induces phosphorylation of PRD-4 by a PI3KK distinct from ATM and ATR. Our data indicate that the activating PI3KK is mechanistic target of rapamycin (mTOR). We provide evidence that translation stress is sensed by unbalancing the expression levels of an unstable protein phosphatase that antagonizes phosphorylation of PRD-4 by mTOR complex 1 (TORC1). Hence, Neurospora mTOR and PRD-4 appear to coordinate metabolic state and cell cycle progression.

M(en)TORship lessons on life and death by the integrated stress response

Cells employ pro-survival and pro-adaptive pathways to cope with different forms of environmental stress. When stress is excessive, and the damage caused by it is unsustainable, cells engage pro-death pathways, which are in place to protect the host from the deleterious effects of harmed cells. Two important pathways that determine the balance between survival and death of stressed cells are the integrated stress response (ISR) and the mammalian target of rapamycin (mTOR), both of which converge at the level of mRNA translation. The two pathways have established avenues of communication to control their activity and determine the fate of stressed cells in a context-dependent manner. The functional interplay between the ISR and mTOR may have significant ramifications in the development and treatment of human diseases such as diabetes, neurodegeneration and cancer.

Overexpression of the recently identified oncogene REDD1 correlates with tumor progression and is an independent unfavorable prognostic factor for ovarian carcinoma

Background

Regulated in development and DNA damage response (REDD1), a gene responding to hypoxia or multiple DNA damage events, was recently implicated in cancer development and progression. Previously, in vivo and in vitro experiments indicated that REDD1 functions as an oncogene in ovarian cancer cells. However, the role of REDD1 in cancer cell migration and invasion and in clinical significance of prognostic values is not examined in detail.

Methods

We detected the REDD1 protein expression by immunohistochemistry in 18 normal ovarian surface epithelium or fallopian tube epithelium specimens, 24 ovarian borderline tumors, and 229 ovarian cancers. Fisher’s exact test, logistic regression analysis, the Kaplan–Meier method, and the log-rank test were used to evaluate the association of REDD1 with clinical factors, overall survival and disease-free survival. The prognostic predictive value of REDD1 for ovarian cancer patients was evaluated using multivariate Cox proportional hazard regression models. REDD1 expression in HEY, HEY A8, SKOV3, SKOV3 ip1, OVCA429, OVCA433 and A2780 human ovarian epithelial cancer cell lines was detected by western blotting. The role of REDD1 in cell invasion and migration was assessed by transwell migration and invasion assays using SKOV3, A2780, HEY, HEYA8, and SKOV3-REDD1 with parental A2780-REDD1 HEY-REDD1i and HEY A8-REDD1i.

Results

High expression of REDD1 was observed in 35.4% of primary ovarian carcinoma samples. Overexpression of cytoplasmic REDD1 in ovarian cancer was significantly associated with serous carcinoma (P < 0.001), late-stage disease (P < 0.001), ascites (P < 0.001), and partial or non-response to chemotherapy (P < 0.001). High cytoplasmic expression of REDD1 was correlated with poorer overall survival (P < 0.001) and disease-free survival (P < 0.001). The multivariate Cox proportional hazards regression analysis indicated that patients with high cytoplasmic REDD1 expression had a high risk of death (P < 0.001) and high risk of an event (i.e., recurrence, progression, or death) (P < 0.001). REDD1 was first reported as an independent prognostic factor in ovarian cancer patients. In addition, REDD1 overexpression enhanced ovarian cancer cell migration and invasion.

Conclusion

REDD1 is an independent unfavorable prognostic factor in ovarian carcinoma and may promote ovarian cancer metastasis.

Electronic supplementary material

The online version of this article (10.1186/s13000-018-0754-4) contains supplementary material, which is available to authorized users.

Consumption of a high fat diet promotes protein O-GlcNAcylation in mouse retina via NR4A1-dependent GFAT2 expression

The incidence of type 2 diabetes, the most common cause of diabetic retinopathy (DR), is rapidly on the rise in developed countries due to overconsumption of calorie rich diets. Using an animal model of diet-induced obesity/pre-diabetes, we evaluated the impact of a diet high in saturated fat (HFD) on O-GlcNAcylation of retinal proteins, as dysregulated O-GlcNAcylation contributes to diabetic complications and evidence supports a role in DR. Protein O-GlcNAcylation was increased in the retina of mice fed a HFD as compared to littermates receiving control chow. Similarly, O-GlcNAcylation was elevated in retinal Müller cells in culture exposed to the saturated fatty acid palmitate or the ceramide analog Cer6. One potential mechanism responsible for elevated O-GlcNAcylation is increased flux through the hexosamine biosynthetic pathway (HBP). Indeed, inhibition of the pathway's rate-limiting enzyme glutamine-fructose-6-phosphate amidotransferase (GFAT) prevented Cer6-induced O-GlcNAcylation. Importantly, expression of the mRNA encoding GFAT2, but not GFAT1 was elevated in both the retina of mice fed a HFD and in retinal cells in culture exposed to palmitate or Cer6. Notably, expression of nuclear receptor subfamily 4 group A member 1 (NR4A1) was increased in the retina of mice fed a HFD and NR4A1 expression was sufficient to promote GFAT2 mRNA expression and O-GlcNAcylation in retinal cells in culture. Whereas palmitate or Cer6 addition to culture medium enhanced NR4A1 and GFAT2 expression, chemical inhibition of NR4A1 transactivation repressed Cer6-induced GFAT2 mRNA expression. Overall, the results support a model wherein HFD increases retinal protein O-GlcNAcylation by promoting NR4A1-dependent GFAT2 expression.

Deletion of the stress-response protein REDD1 promotes ceramide-induced retinal cell death and JNK activation

The role of dyslipidemia in the development of retinal dysfunction remains poorly understood. Using an animal model of diet-induced obesity/pre-type 2 diabetes, we investigated molecular defects in the retina arising from consumption of a diet high in saturated fats and sugars ( i.e., a Western diet). We found that feeding mice a Western diet increased the abundance of retinal sphingolipids, attenuated protein kinase B (Akt) phosphorylation, enhanced JNK activation, and increased retinal cell death. When we used palmitate or C6-ceramide (Cer) to assess sphingolipid-mediated signaling in cultured murine and human cells, we observed similar effects on Akt, JNK, and cell death. Furthermore, both Western diet and C6-Cer exposure enhanced expression of the stress-response protein regulated in development and DNA damage response 1 (REDD1) and loss of REDD1 increased C6-Cer-induced JNK activation and cell death. Exogenous REDD1 expression repressed JNK-mediated phosphorylation in cultured cells. We found that thioredoxin-interacting protein (TXNIP) expression was elevated in REDD1-deficient cell lines and C6-Cer promoted TXNIP expression in both wild-type and REDD1-deficient cells. Likewise, TXNIP knockdown attenuated JNK activation and caspase 3 cleavage after either C6-Cer exposure or REDD1 deletion. The results support a model wherein Cer-induced REDD1 expression attenuates TXNIP-dependent JNK activation and retinal cell death.-Dai, W., Miller, W. P., Toro, A. L., Black, A. J., Dierschke, S. K., Feehan, R. P., Kimball, S. R., Dennis, M. D. Deletion of the stress-response protein REDD1 promotes ceramide-induced retinal cell death and JNK activation.

DNA Damage Inducible Transcript 4 Gene: The Switch of the Metabolism as Potential Target in Cancer

DNA damage inducible transcript 4 (DDIT4) gene is expressed under stress situations turning off the metabolic activity triggered by the mammalian target of rapamycin (mTOR). Several in vitro and in vivo works have demonstrated the ability of DDIT4 to generate resistance to cancer therapy. The link between the metabolism suppression and aggressiveness features of cancer cells remains poorly understood since anti-mTOR agents who are part of the repertoire of drugs used for systemic treatment of cancer achieving variable results. Interestingly, the high DDIT4 expression is associated with worse outcomes compared to tumors with low DDIT4 expression, seen in a wide variety of solid and hematological tumors, which suggests the driver role of this gene and provide the basis to target it as part of a new therapeutic strategy. In this review, we highlight our current knowledge about the biology of DDIT4 and its role as a prognostic biomarker, encompassing the motives for the development of target drugs against DDIT4 as a better target than mTOR inhibitors.

Rapamycin Modulates Glucocorticoid Receptor Function, Blocks Atrophogene REDD1, and Protects Skin from Steroid Atrophy

Glucocorticoids have excellent therapeutic properties; however, they cause significant adverse atrophogenic effects. The mTORC1 inhibitor REDD1 has been recently identified as a key mediator of glucocorticoid-induced atrophy. We performed computational screening of a connectivity map database to identify putative REDD1 inhibitors. The top selected candidates included rapamycin, which was unexpected because it inhibits pro-proliferative mTOR signaling. Indeed, rapamycin inhibited REDD1 induction by glucocorticoids dexamethasone, clobetasol propionate, and fluocinolone acetonide in keratinocytes, lymphoid cells, and mouse skin. We also showed blunting of glucocorticoid-induced REDD1 induction by either catalytic inhibitor of mTORC1/2 (OSI-027) or genetic inhibition of mTORC1, highlighting role of mTOR in glucocorticoid receptor signaling. Moreover, rapamycin inhibited glucocorticoid receptor phosphorylation, nuclear translocation, and loading on glucocorticoid-responsive elements in REDD1 promoter. Using microarrays, we quantified a global effect of rapamycin on gene expression regulation by fluocinolone acetonide in human keratinocytes. Rapamycin inhibited activation of glucocorticoid receptor target genes yet enhanced the repression of pro-proliferative and proinflammatory genes. Remarkably, rapamycin protected skin against glucocorticoid-induced atrophy but had no effect on the glucocorticoid anti-inflammatory activity in different in vivo models, suggesting the clinical potential of combining rapamycin with glucocorticoids for the treatment of inflammatory diseases.

Loss of NEDD4 contributes to RTP801 elevation and neuron toxicity: implications for Parkinson's disease

Parkinson's disease (PD) is a disorder characterized by the degeneration of certain neuronal populations in the central and peripheral nervous system. One of the hallmarks of the disease is the toxic accumulation of proteins within susceptible neurons due to major impairment in the degradation/clearance protein systems.

RTP801 is a pro-apoptotic protein that is sufficient and necessary to induce neuronal death in cellular and animal models of PD. RTP801 is also upregulated in sporadic and parkin mutant PD brains. Here, we report the role of NEDD4, an E3 ligase involved in α-synuclein degradation and PD pathogenesis, in the regulation of RTP801 protein levels and toxicity. NEDD4 polyubiquitinates RTP801 in a cell-free system and in cellular cultures, and they interact physically. NEDD4 conjugates K63-ubiquitin chains to RTP801 and targets it for degradation. NEDD4 regulates RTP801 protein levels in both cultured cells and in the brain tissue. NEDD4 levels are diminished in nigral neurons from human PD brains. Interestingly, neurotoxin 6-OHDA decreases dramatically NEDD4 protein expression but elevates RTP801 protein levels. Moreover, NEDD4 protects neuronal PC12 cells from both 6-OHDA and RTP801-induced toxicity. In primary cortical neurons, NEDD4 knockdown toxicity is mediated by RTP801 since the double knockdown of RTP801 and NEDD4 abrogates the loss of phospho Ser473-Akt and the appearance of caspase-cleaved spectrin fragments.

Thus, NEDD4 ligase regulates RTP801 and is sensitive to PD-associated oxidative stress. This suggests that NEDD4 loss of function in PD could contribute importantly into neuronal death by elevating RTP801.

Influenza virus differentially activates mTORC1 and mTORC2 signaling to maximize late stage replication

Influenza A virus usurps host signaling factors to regulate its replication. One example is mTOR, a cellular regulator of protein synthesis, growth and motility. While the role of mTORC1 in viral infection has been studied, the mechanisms that induce mTORC1 activation and the substrates regulated by mTORC1 during influenza virus infection have not been established. In addition, the role of mTORC2 during influenza virus infection remains unknown. Here we show that mTORC2 and PDPK1 differentially phosphorylate AKT upon influenza virus infection. PDPK1-mediated phoshorylation of AKT at a distinct site is required for mTORC1 activation by influenza virus. On the other hand, the viral NS1 protein promotes phosphorylation of AKT at a different site via mTORC2, which is an activity dispensable for mTORC1 stimulation but known to regulate apoptosis. Influenza virus HA protein and down-regulation of the mTORC1 inhibitor REDD1 by the virus M2 protein promote mTORC1 activity. Systematic phosphoproteomics analysis performed in cells lacking the mTORC2 component Rictor in the absence or presence of Torin, an inhibitor of both mTORC1 and mTORC2, revealed mTORC1-dependent substrates regulated during infection. Members of pathways that regulate mTORC1 or are regulated by mTORC1 were identified, including constituents of the translation machinery that once activated can promote translation. mTORC1 activation supports viral protein expression and replication. As mTORC1 activation is optimal midway through the virus life cycle, the observed effects on viral protein expression likely support the late stages of influenza virus replication when infected cells undergo significant stress.

Drug-resistant influenza viruses commonly arise due to frequent genetic changes and current antiviral drugs are not highly efficient. These underscore the need for new antiviral therapies effective against influenza viruses. Understanding how influenza virus uses cellular proteins for infection can potentially identify novel targets for pharmacological intervention. Influenza virus modulates cellular pathways to promote its replication and avoid immune restriction. Here we reveal the interplay between the cellular protein mTOR, which functions in two distinct protein complexes, and influenza virus infection. mTOR complex 1 (mTORC1) is activated during influenza virus infection through a cascade of specific modifications, or phosphorylation events, and by reducing the levels of another cellular protein termed REDD1, which is an mTORC1 inhibitor. Activation of mTORC1 results in additional phosphorylation events that together promote viral protein expression and replication. On the other hand, mTOR complex 2 (mTORC2) phosphorylates AKT at a specific site during infection, which is a process mediated by the viral NS1 protein that is known to regulate viral-mediated cell death. Since these effects occur midway through the virus life cycle in the infected cell, mTORC1 and mTORC2 activation are likely important to regulate the cellular environment in order to facilitate the late stages of viral infection.

Molecular neurobiology of mTOR

Mammalian/mechanistic target of rapamycin (mTOR) is a serine-threonine kinase that controls several important aspects of mammalian cell function. mTOR activity is modulated by various intra- and extracellular factors; in turn, mTOR changes rates of translation, transcription, protein degradation, cell signaling, metabolism, and cytoskeleton dynamics. mTOR has been repeatedly shown to participate in neuronal development and the proper functioning of mature neurons. Changes in mTOR activity are often observed in nervous system diseases, including genetic diseases (e.g., tuberous sclerosis complex, Pten-related syndromes, neurofibromatosis, and Fragile X syndrome), epilepsy, brain tumors, and neurodegenerative disorders (Alzheimer's disease, Parkinson's disease, and Huntington's disease). Neuroscientists only recently began deciphering the molecular processes that are downstream of mTOR that participate in proper function of the nervous system. As a result, we are gaining knowledge about the ways in which aberrant changes in mTOR activity lead to various nervous system diseases. In this review, we provide a comprehensive view of mTOR in the nervous system, with a special focus on the neuronal functions of mTOR (e.g., control of translation, transcription, and autophagy) that likely underlie the contribution of mTOR to nervous system diseases.

Mitochondrial activation chemicals synergize with surface receptor PD-1 blockade for T cell-dependent antitumor activity

Although immunotherapy by PD-1 blockade has dramatically improved the survival rate of cancer patients, further improvement in efficacy is required to reduce the fraction of less sensitive patients. In mouse models of PD-1 blockade therapy, we found that tumor-reactive cytotoxic T lymphocytes (CTLs) in draining lymph nodes (DLNs) carry increased mitochondrial mass and more reactive oxygen species (ROS). We show that ROS generation by ROS precursors or indirectly by mitochondrial uncouplers synergized the tumoricidal activity of PD-1 blockade by expansion of effector/memory CTLs in DLNs and within the tumor. These CTLs carry not only the activation of mechanistic target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) but also an increment of their downstream transcription factors such as PPAR-gamma coactivator 1α (PGC-1α) and T-bet. Furthermore, direct activators of mTOR, AMPK, or PGC-1α also synergized the PD-1 blockade therapy whereas none of above-mentioned chemicals alone had any effects on tumor growth. These findings will pave a way to developing novel combinatorial therapies with PD-1 blockade.

The eIF2-alpha kinase HRI is a novel therapeutic target in multiple myeloma

Dexamethasone (dex) induces apoptosis in multiple myeloma (MM) cells and is a frontline treatment for this disease. However resistance to dex remains a major challenge and novel treatment approaches are needed. We hypothesized that dex utilizes translational pathways to promote apoptosis in MM and that specific targeting of these pathways could overcome dex-resistance. Global unbiased profiling of mRNA translational profiles in MM cells treated with or without dex revealed that dex significantly repressed eIF2 signaling, an important pathway for regulating ternary complex formation and protein synthesis. We demonstrate that dex induces the phosphorylation of eIF2α resulting in the translational upregulation of ATF4, a known eIF2 regulated mRNA. Pharmacologic induction of eIF2α phosphorylation via activation of the heme-regulated eIF2α kinase (HRI) induced apoptosis in MM cell lines and in primary MM cells from patients with dex-resistant disease. In addition, co-culture with marrow stroma failed to protect MM cells from apoptosis induced by targeting the eIF2 pathway. Combination therapy with rapamycin, an mTOR inhibitor, and BTdCPU, an activator of HRI, demonstrated additive effects on apoptosis in dex-resistant cells. Thus, specific activation of the eIF2α kinase HRI is a novel therapeutic target in MM that can augment current treatment strategies.

Regulation of protein and mRNA expression of the mTORC1 repressor REDD1 in response to leucine and serum

Expression of the mTORC1 repressor, Regulated in DNA Damage and Development 1 (REDD1), is elevated in skeletal muscle during various catabolic conditions including fasting, hindlimb immobilization, and sepsis. Conversely, REDD1 expression is suppressed by anabolic stimuli such as resistance exercise or nutrient consumption following a fast. Though it is known that nutrient consumption reduces REDD1 expression, it is largely unknown how nutrients and hormones individually contribute to the reduction in REDD1 expression. Therefore, the purpose of the present study was to determine how nutrients and hormones individually regulate REDD1 expression. HeLa cells were deprived of leucine or serum for 10 h, after which either leucine or serum was reintroduced to cell culture medium for 60 min. Re-supplementation of either leucine or serum resulted in a reduction in REDD1 protein levels by 34.8±5.8% and 54.1±3.4%, respectively, compared to the deprived conditions. Re-supplementation of leucine or serum to deprived cells also led to a reduction in REDD1 mRNA content by 49.1±2.7% and 65.0±1.4%, respectively, compared to the deprived conditions. Interestingly, rates of REDD1 protein degradation were unaffected by either leucine or serum re-supplementation, as assessed in cells treated with cycloheximide to block protein synthesis. Likewise, addition of leucine- or serum to cells treated with Actinomycin D to inhibit gene transcription failed to alter the rate of REDD1 mRNA degradation. The data indicate that the leucine or serum-induced suppression of REDD1 expression occurs independent of changes in the rate of degradation of either the REDD1 protein or mRNA. Thus, the leucine- or serum-induced suppression likely occurs through alternative mechanism(s) such as reduced REDD1 gene transcription and/or mRNA translation.

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Deprivation of leucine or serum induces REDD1 mRNA and protein expression.

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Re-supplementation of leucine or serum reduces REDD1 mRNA and protein expression.

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Nutrient deplete or replete conditions do not affect the degradation rate of REDD1.

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REDD1 expression is controlled through altered rates of transcription.

Emerging role for regulated in development and DNA damage 1 (REDD1) in the regulation of skeletal muscle metabolism

Since its discovery, the protein regulated in development and DNA damage 1 (REDD1) has been implicated in the cellular response to various stressors. Most notably, its role as a repressor of signaling through the central metabolic regulator, the mechanistic target of rapamycin in complex 1 (mTORC1) has gained considerable attention. Not surprisingly, changes in REDD1 mRNA and protein have been observed in skeletal muscle under various physiological conditions (e.g., nutrient consumption and resistance exercise) and pathological conditions (e.g., sepsis, alcoholism, diabetes, obesity) suggesting a role for REDD1 in regulating mTORC1-dependent skeletal muscle protein metabolism. Our understanding of the causative role of REDD1 in skeletal muscle metabolism is increasing mostly due to the availability of genetically modified mice in which the REDD1 gene is disrupted. Results from such studies provide support for an important role for REDD1 in the regulation of mTORC1 as well as reveal unexplored functions of this protein in relation to other aspects of skeletal muscle metabolism. The goal of this work is to provide a comprehensive review of the role of REDD1 (and its paralog REDD2) in skeletal muscle during both physiological and pathological conditions.

Caloric Restriction Normalizes Obesity-Induced Alterations on Regulators of Skeletal Muscle Growth Signaling

The objective of this study was to establish the impact of caloric restriction on high fat diet-induced alterations on regulators of skeletal muscle growth. We hypothesized that caloric restriction would reverse the negative effects of high fat diet-induced obesity on REDD1 and mTOR-related signaling. Following an initial 8 week period of HF diet-induced obesity, caloric restriction (CR ~30 %) was employed while mice continued to consume either a low (LF) or high fat (HF) diet for 8 weeks. Western analysis of skeletal muscle showed that CR reduced (p < 0.05) the obesity-related effects on the lipogenic protein, SREBP1. Likewise, CR reduced (p < 0.05) the obesity-related effects on the hyperactivation of mTORC1 and ERK1/2 signaling to levels comparable to the LF mice. CR also reduced (p < 0.05) obesity-induced expression of negative regulators of growth, REDD1 and cleaved caspase 3. These findings have implications for on the reversibility of dysregulated growth signaling in obese skeletal muscle, using short-term caloric restriction.

Short inter-set rest blunts resistance exercise-induced increases in myofibrillar protein synthesis and intracellular signalling in young males

What is the central question of this study? Does shorter rest between sets of resistance exercise promote a superior circulating hormonal and acute muscle anabolic response compared with longer rest periods? What is the main finding and its importance? We demonstrate that short rest (1 min) between sets of moderate-intensity, high-volume resistance exercise blunts the acute muscle anabolic response compared with a longer rest period (5 min), despite a superior circulating hormonal milieu. These data have important implications for the development of training regimens to maximize muscle hypertrophy. Manipulating the rest-recovery interval between sets of resistance exercise may influence training-induced muscle remodelling. The aim of this study was to determine the acute muscle anabolic response to resistance exercise performed with short or long inter-set rest intervals. In a study with a parallel-group design, 16 males completed four sets of bilateral leg-press and knee-extension exercise at 75% of one-repetition maximum to momentary muscular failure, followed by ingestion of 25 g of whey protein. Resistance exercise sets were interspersed by 1 min (n = 8) or 5 min of passive rest (n = 8). Muscle biopsies were obtained at rest, 0, 4, 24 and 28 h postexercise during a primed continuous infusion of l-[ring-(13) C6 ]phenylalanine to determine myofibrillar protein synthesis and intracellular signalling. We found that the rate of myofibrillar protein synthesis increased above resting values from 0 to 4 h postexercise with 1 (76%; P = 0.047) and 5 min inter-set rest (152%; P < 0.001) and was significantly greater in the 5 min inter-set rest group (P = 0.001). Myofibrillar protein synthesis rates at 24-28 h postexercise remained elevated above resting values (P < 0.05) and were indistinguishable between groups. Postexercise p70S6K(Thr389) and rpS6(Ser240/244) phosphorylation were reduced with 1 compared with 5 min inter-set rest, whereas phosphorylation of eEF2(Thr56) , TSC2(Thr1462) , AMPK(Thr172) and REDD1 protein were greater for 1 compared with 5 min inter-set rest. Serum testosterone was greater at 20-40 min postexercise and plasma lactate greater immediately postexercise for 1 versus 5 min inter-set rest. Resistance exercise with short (1 min) inter-set rest duration attenuated myofibrillar protein synthesis during the early postexercise recovery period compared with longer (5 min) rest duration, potentially through compromised activation of intracellular signalling.

Ezrin Inhibition Up-regulates Stress Response Gene Expression

Ezrin is a member of the ERM (ezrin/radixin/moesin) family of proteins that links cortical cytoskeleton to the plasma membrane. High expression of ezrin correlates with poor prognosis and metastasis in osteosarcoma. In this study, to uncover specific cellular responses evoked by ezrin inhibition that can be used as a specific pharmacodynamic marker(s), we profiled global gene expression in osteosarcoma cells after treatment with small molecule ezrin inhibitors, NSC305787 and NSC668394. We identified and validated several up-regulated integrated stress response genes including PTGS2, ATF3, DDIT3, DDIT4, TRIB3, and ATF4 as novel ezrin-regulated transcripts. Analysis of transcriptional response in skin and peripheral blood mononuclear cells from NSC305787-treated mice compared with a control group revealed that, among those genes, the stress gene DDIT4/REDD1 may be used as a surrogate pharmacodynamic marker of ezrin inhibitor compound activity. In addition, we validated the anti-metastatic effects of NSC305787 in reducing the incidence of lung metastasis in a genetically engineered mouse model of osteosarcoma and evaluated the pharmacokinetics of NSC305787 and NSC668394 in mice. In conclusion, our findings suggest that cytoplasmic ezrin, previously considered a dormant and inactive protein, has important functions in regulating gene expression that may result in down-regulation of stress response genes.

Interleukin-6 influences stress-signalling by reducing the expression of the mTOR-inhibitor REDD1 in a STAT3-dependent manner

Interleukin 6 (IL-6) is a pleiotropic cytokine and a strong activator of Mammalian Target of Rapamycin (mTOR). In contrast, mTOR activity is negatively regulated by Regulated in Development and DNA Damage Responses 1 (REDD1). Expression of REDD1 is induced by cellular stressors such as glucocorticoids and DNA damaging agents. We show that the expression of basal as well as stress-induced REDD1 is reduced by IL-6. The reduction of REDD1 expression by IL-6 is independent of proteasomal or caspase-mediated degradation of REDD1 protein. Instead, induction of REDD1 mRNA is reduced by IL-6. The regulation of REDD1 expression by IL-6 is independent of Phosphatidylinositide-3-Kinase (PI3K) and Mitogen-Activated Protein Kinase (MAPK) signalling but depends on the expression and activation of Signal Transducer and Activator of Transcription 3 (STAT3). Furthermore, the reduction of basal REDD1 expression by IL-6 correlates with IL-6-induced activation of mTOR signalling. Inhibition of STAT3 activation blocks IL-6-induced mTOR activation. In summary, we present a novel STAT3-dependent mechanism of both IL-6-induced activation of mTOR and IL-6-dependent reversion of stress-induced inhibition of mTOR activity.

Validation of a commercially available anti-REDD1 antibody using RNA interference and REDD1-/- mouse embryonic fibroblasts

REDD1 is a transcriptional target gene of p53 and HIF-1, and an inhibitor of mTOR (mechanistic target of rapamycin) complex 1 (mTORC1)-signaling through PP2A-dependent interaction, making it an important convergence point of both tumor suppression and cell growth pathways. In accordance with this positioning, REDD1 levels are transcriptionally upregulated in response to a variety of cellular stress factors such as nutrient deprivation, hypoxia and DNA damage. In the absence of such conditions, and in particular where growth factor signaling is activated, REDD1 expression is typically negligible; therefore, it is necessary to induce REDD1 prior to experimentation or detection in model systems. Here, we evaluated the performance of a commercially available polyclonal antibody recognizing REDD1 by Western blotting in the presence of thapsigargin, a pharmacological inducer of ER stress well known to upregulate REDD1 protein expression. Further, REDD1 antibody specificity was challenged in HEK-293 cells in the presence of RNA interference and with a REDD1 ^-/- mouse embryonic fibroblast knockout cell line. Results showed reproducibility and specificity of the antibody, which was upheld in the presence of thapsigargin treatment. We conclude that this antibody can be used to reliably detect REDD1 endogenous expression in samples of both human and mouse origin.

Iron depletion suppresses mTORC1-directed signalling in intestinal Caco-2 cells via induction of REDD1

Iron is an indispensable micronutrient that regulates many aspects of cell function, including growth and proliferation. These processes are critically dependent upon signalling via the mammalian or mechanistic target of rapamycin complex 1 (mTORC1). Herein, we test whether iron depletion induced by cell incubation with the iron chelator, deferoxamine (DFO), mediates its effects on cell growth through mTORC1-directed signalling and protein synthesis. We have used Caco-2 cells, a well-established in vitro model of human intestinal epithelia. Iron depletion increased expression of iron-regulated proteins (TfR, transferrin receptor and DMT1, divalent metal transporter, as predicted, but it also promoted a marked reduction in growth and proliferation of Caco-2 cells. This was strongly associated with suppressed mTORC1 signalling, as judged by reduced phosphorylation of mTOR substrates, S6K1 and 4E-BP1, and diminished protein synthesis. The reduction in mTORC1 signalling was tightly coupled with increased expression and accumulation of REDD1 (regulated in DNA damage and development 1) and reduced phosphorylation of Akt and TSC2. The increase in REDD1 abundance was rapidly reversed upon iron repletion of cells but was also attenuated by inhibitors of gene transcription, protein phosphatase 2A (PP2A) and by REDD1 siRNA — strategies that also antagonised the loss in mTORC1 signalling associated with iron depletion. Our findings implicate REDD1 and PP2A as crucial regulators of mTORC1 activity in iron-depleted cells and indicate that their modulation may help mitigate atrophy of the intestinal mucosa that may occur in response to iron deficiency.

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Cellular iron (Fe) depletion dramatically reduces growth of intestinal Caco-2 cells.

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mTORC1-directed signalling and protein synthesis are reduced in Fe-depleted cells.

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Fe deficiency induces expression and gain of REDD1 in a PP2A-dependent manner.

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PP2A inhibition blocks REDD1 gain and restores mTORC1 activity in Fe-depleted cells.

mTOR inhibition activates overall protein degradation by the ubiquitin proteasome system as well as by autophagy

Growth factors and nutrients enhance protein synthesis and suppress overall protein degradation by activating the protein kinase mammalian target of rapamycin (mTOR). Conversely, nutrient or serum deprivation inhibits mTOR and stimulates protein breakdown by inducing autophagy, which provides the starved cells with amino acids for protein synthesis and energy production. However, it is unclear whether proteolysis by the ubiquitin proteasome system (UPS), which catalyzes most protein degradation in mammalian cells, also increases when mTOR activity decreases. Here we show that inhibiting mTOR with rapamycin or Torin1 rapidly increases the degradation of long-lived cell proteins, but not short-lived ones, by stimulating proteolysis by proteasomes, in addition to autophagy. This enhanced proteasomal degradation required protein ubiquitination, and within 30 min after mTOR inhibition, the cellular content of K48-linked ubiquitinated proteins increased without any change in proteasome content or activity. This rapid increase in UPS-mediated proteolysis continued for many hours and resulted primarily from inhibition of mTORC1 (not mTORC2), but did not require new protein synthesis or key mTOR targets: S6Ks, 4E-BPs, or Ulks. These findings do not support the recent report that mTORC1 inhibition reduces proteolysis by suppressing proteasome expression [Zhang Y, et al. (2014) Nature 513(7518):440-443]. Several growth-related proteins were identified that were ubiquitinated and degraded more rapidly after mTOR inhibition, including HMG-CoA synthase, whose enhanced degradation probably limits cholesterol biosynthesis upon insulin deficiency. Thus, mTOR inhibition coordinately activates the UPS and autophagy, which provide essential amino acids and, together with the enhanced ubiquitination of anabolic proteins, help slow growth.

Aberrant REDD1-mTORC1 responses to insulin in skeletal muscle from Type 2 diabetics

The objective of this study was to establish whether alterations in the REDD1-mTOR axis underlie skeletal muscle insensitivity to insulin in Type 2 diabetic (T2D), obese individuals. Vastus lateralis muscle biopsies were obtained from lean, control and obese, T2D subjects under basal and after a 2-h hyperinsulinemic (40 mU·m(-2)·min(-1))-euglycemic (5 mM) clamp. Muscle lysates were examined for total REDD1, and phosphorylated Akt, S6 kinase 1 (S6K1), 4E-BP1, ERK1/2, and MEK1/2 via Western blot analysis. Under basal conditions [(-) insulin], T2D muscle exhibited higher S6K1 and ERK1/2 and lower 4E-BP1 phosphorylation (P < 0.05), as well as elevations in blood cortisol, glucose, insulin, glycosylated hemoglobin (P < 0.05) vs. lean controls. Following insulin infusion, whole body glucose disposal rates (GDR; mg/kg/min) were lower (P < 0.05) in the T2D vs. the control group. The basal-to-insulin percent change in REDD1 expression was higher (P < 0.05) in muscle from the T2D vs. the control group. Whereas, the basal-to-insulin percent change in muscle Akt, S6K1, ERK1/2, and MEK1/2 phosphorylation was significantly lower (P < 0.05) in the T2D vs. the control group. Findings from this study propose a REDD1-regulated mechanism in T2D skeletal muscle that may contribute to whole body insulin resistance and may be a target to improve insulin action in insulin-resistant individuals.

REDD1 Is Essential for Optimal T Cell Proliferation and Survival

REDD1 is a highly conserved stress response protein that is upregulated following many types of cellular stress, including hypoxia, DNA damage, energy stress, ER stress, and nutrient deprivation. Recently, REDD1 was shown to be involved in dexamethasone induced autophagy in murine thymocytes. However, we know little of REDD1’s function in mature T cells. Here we show for the first time that REDD1 is upregulated following T cell stimulation with PHA or CD3/CD28 beads. REDD1 knockout T cells exhibit a defect in proliferation and cell survival, although markers of activation appear normal. These findings demonstrate a previously unappreciated role for REDD1 in T cell function.

Proteomics-level analysis of myelin formation and regeneration in a mouse model for Vanishing White Matter disease

Vanishing white matter (VWM) is a recessive neurodegenerative disease caused by mutations in translation initiation factor eIF2B and leading to progressive brain myelin deterioration, secondary axonal damage, and death in early adolescence. Eif2b5(R132H/R132H) mice exhibit delayed developmental myelination, mild early neurodegeneration and a robust remyelination defect in response to cuprizone-induced demyelination. In the current study we used Eif2b5(R132H/R132H) mice for mass-spectrometry analyses, to follow the changes in brain protein abundance in normal- versus cuprizone-diet fed mice during the remyelination recovery phase. Analysis of proteome profiles suggested that dysregulation of mitochondrial functions, altered proteasomal activity and impaired balance between protein synthesis and degradation play a role in VWM pathology. Consistent with these findings, we detected elevated levels of reactive oxygen species in mutant-derived primary fibroblasts and reduced 20S proteasome activity in mutant brain homogenates. These observations highlight the importance of tight translational control to precise coordination of processes involved in myelin formation and regeneration and point at cellular functions that may contribute to VWM pathology. Eif2b5(R132H/R132H) mouse model for vanishing white matter (VWM) disease was used for mass spectrometry of brain proteins at two time points under normal conditions and along recovery from cuprizone-induced demyelination. Comparisons of proteome profiles revealed the importance of mitochondrial function and tight coordination between protein synthesis and degradation to myelination formation and regeneration, pointing at cellular functions that contribute to VWM pathology.

RTP801 Is Involved in Mutant Huntingtin-Induced Cell Death

RTP801 expression is induced by cellular stress and has a pro-apoptotic function in non-proliferating differentiated cells such as neurons. In several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease, elevated levels of RTP801 have been observed, which suggests a role for RTP801 in neuronal death. Neuronal death is also a pathological hallmark in Huntington's disease (HD), an inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. Currently, the exact mechanisms underlying mutant huntingtin (mhtt)-induced toxicity are still unclear. Here, we investigated whether RTP801 is involved in (mhtt)-induced cell death. Ectopic exon-1 mhtt elevated RTP801 mRNA and protein levels in nerve growth factor (NGF)-differentiated PC12 cells and in rat primary cortical neurons. In neuronal PC12 cells, mhtt also contributed to RTP801 protein elevation by reducing its proteasomal degradation rate, in addition to promoting RTP801 gene expression. Interestingly, silencing RTP801 expression with short hairpin RNAs (shRNAs) blocked mhtt-induced cell death in NGF-differentiated PC12 cells. However, RTP801 protein levels were not altered in the striatum of Hdh(Q7/Q111) and R6/1 mice, two HD models that display motor deficits but not neuronal death. Importantly, RTP801 protein levels were elevated in both neural telencephalic progenitors differentiated from HD patient-derived induced pluripotent stem cells and in the putamen and cerebellum of human HD postmortem brains. Taken together, our results suggest that RTP801 is a novel downstream effector of mhtt-induced toxicity and that it may be relevant to the human disease.

Mode of action and pharmacogenomic biomarkers for exceptional responders to didemnin B

Modern cancer treatment employs many effective chemotherapeutic agents originally discovered from natural sources. However, a significant challenge currently confronting clinical application is balancing systemic toxicity risk with therapeutic benefit. The cyclic depsipeptide didemnin B has demonstrated impressive anti-cancer activity in preclinical models. Clinical use has been approved but is limited by sparse patient responses combined with toxicity risk and an unclear mechanism of action. From a broad-scale effort to match antineoplastic natural products to their cellular activities, we found that didemnin B selectively induces rapid and wholesale apoptosis through dual inhibition of PPT1 and EEF1A1. Furthermore, empirical discovery of a small panel of exceptional responders to didemnin B allowed generation of a regularized regression model to extract a sparse-feature genetic biomarker capable of predicting sensitivity to didemnin B. This may facilitate patient selection that could enhance and expand therapeutic application of didemnin B against neoplastic disease.

Molecular mechanisms of mTOR regulation by stress

Tumors are prime examples of cell growth in unfavorable environments that elicit cellular stress. The high metabolic demand and insufficient vascularization of tumors cause a deficiency of oxygen and nutrients. Oncogenic mutations map to signaling events via mammalian target of rapamycin (mTOR), metabolic pathways, and mitochondrial function. These alterations have been linked with cellular stresses, in particular endoplasmic reticulum (ER) stress, hypoxia, and oxidative stress. Yet tumors survive these challenges and acquire highly energy-demanding traits, such as overgrowth and invasiveness. In this review we focus on stresses that occur in cancer cells and discuss them in the context of mTOR signaling. Of note, many tumor traits require mTOR complex 1 (mTORC1) activity, but mTORC1 hyperactivation eventually sensitizes cells to apoptosis. Thus, mTORC1 activity needs to be balanced in cancer cells. We provide an overview of the mechanisms contributing to mTOR regulation by stress and suggest a model wherein stress granules function as guardians of mTORC1 signaling, allowing cancer cells to escape stress-induced cell death.

A genome-wide siRNA screen in mammalian cells for regulators of S6 phosphorylation

mTOR complex1, the major regulator of mRNA translation in all eukaryotic cells, is strongly activated in most cancers. We performed a genome-wide RNAi screen in a human cancer cell line, seeking genes that regulate S6 phosphorylation, readout of mTORC1 activity. Applying a stringent selection, we retrieved nearly 600 genes wherein at least two RNAis gave significant reduction in S6-P. This cohort contains known regulators of mTOR complex 1 and is significantly enriched in genes whose depletion affects the proliferation/viability of the large set of cancer cell lines in the Achilles database in a manner paralleling that caused by mTOR depletion. We next examined the effect of RNAi pools directed at 534 of these gene products on S6-P in TSC1 null mouse embryo fibroblasts. 76 RNAis reduced S6 phosphorylation significantly in 2 or 3 replicates. Surprisingly, among this cohort of genes the only elements previously associated with the maintenance of mTORC1 activity are two subunits of the vacuolar ATPase and the CUL4 subunit DDB1. RNAi against a second set of 84 targets reduced S6-P in only one of three replicates. However, an indication that this group also bears attention is the presence of rpS6KB1 itself, Rac1 and MAP4K3, a protein kinase that supports amino acid signaling to rpS6KB1. The finding that S6 phosphorylation requires a previously unidentified, functionally diverse cohort of genes that participate in fundamental cellular processes such as mRNA translation, RNA processing, DNA repair and metabolism suggests the operation of feedback pathways in the regulation of mTORC1 operating through novel mechanisms.

Phosphorylation of eIF2α triggered by mTORC1 inhibition and PP6C activation is required for autophagy and is aberrant in PP6C-mutated melanoma

Amino acid deprivation promotes the inhibition of the kinase complex mTORC1 (mammalian target of rapamycin complex 1) and activation of the kinase GCN2 (general control nonrepressed 2). Signaling pathways downstream of both kinases have been thought to independently induce autophagy. We showed that these two amino acid-sensing systems are linked. We showed that pharmacological inhibition of mTORC1 led to activation of GCN2 and phosphorylation of the eukaryotic initiation factor 2α (eIF2α) in a mechanism dependent on the catalytic subunit of protein phosphatase 6 (PP6C). Autophagy induced by pharmacological inhibition of mTORC1 required PP6C, GCN2, and eIF2α phosphorylation. Although some of the PP6C mutants found in melanoma did not form a strong complex with PP6 regulatory subunits and were rapidly degraded, these mutants paradoxically stabilized PP6C encoded by the wild-type allele and increased eIF2α phosphorylation. Furthermore, these PP6C mutations were associated with increased autophagy in vitro and in human melanoma samples. Thus, these data showed that GCN2 activation and phosphorylation of eIF2α in response to mTORC1 inhibition are necessary for autophagy. Additionally, we described a role for PP6C in this process and provided a mechanism for PP6C mutations associated with melanoma.

Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables

Concurrent training is defined as simultaneously incorporating both resistance and endurance exercise within a periodized training regime. Despite the potential additive benefits of combining these divergent exercise modes with regards to disease prevention and athletic performance, current evidence suggests that this approach may attenuate gains in muscle mass, strength, and power compared with undertaking resistance training alone. This has been variously described as the interference effect or concurrent training effect. In recent years, understanding of the molecular mechanisms mediating training adaptation in skeletal muscle has emerged and provided potential mechanistic insight into the concurrent training effect. Although it appears that various molecular signaling responses induced in skeletal muscle by endurance exercise can inhibit pathways regulating protein synthesis and stimulate protein breakdown, human studies to date have not observed such molecular 'interference' following acute concurrent exercise that might explain compromised muscle hypertrophy following concurrent training. However, given the multitude of potential concurrent training variables and the limitations of existing evidence, the potential roles of individual training variables in acute and chronic interference are not fully elucidated. The present review explores current evidence for the molecular basis of the specificity of training adaptation and the concurrent interference phenomenon. Additionally, insights provided by molecular and performance-based concurrent training studies regarding the role of individual training variables (i.e., within-session exercise order, between-mode recovery, endurance training volume, intensity, and modality) in the concurrent interference effect are discussed, along with the limitations of our current understanding of this complex paradigm.

Endurance exercise induces REDD1 expression and transiently decreases mTORC1 signaling in rat skeletal muscle

Working muscle conserves adenosine triphosphate (ATP) for muscle contraction by attenuating protein synthesis through several different pathways. Regulated in development and DNA damage response 1 (REDD1) is one candidate protein that can itself attenuate muscle protein synthesis during muscle contraction. In this study, we investigated whether endurance exercise induces REDD1 expression in association with decreased mammalian target of rapamycin (mTOR) complex I (mTORC1) signaling and global protein synthesis in rat skeletal muscle. After overnight fasting, rats ran on a treadmill at a speed of 28 m/min for 60 min, and were killed before and immediately, 1, 3, 6, 12, and 24 h after exercise. REDD1 mRNA and corresponding protein levels increased rapidly immediately after exercise, and gradually decreased back to the basal level over a period of 6 h in the gastrocnemius muscle. Phosphorylation of mTOR Ser2448 and S6K1 Thr389 increased with the exercise, but diminished in 1–3 h into the recovery period after cessation of exercise. The rate of protein synthesis, as determined by the surface sensing of translation (SUnSET) method, was not altered by exercise in fasted muscle. These results suggest that REDD1 attenuates exercise‐induced mTORC1 signaling. This may be one mechanism responsible for blunting muscle protein synthesis during exercise and in the early postexercise recovery period.

We show that REDD1 expression is rapidly induced by an acute bout of endurance exercise in association with a decrease in mTORC1 signaling in rat muscle. The rate of mixed‐muscle protein synthesis was, however, not altered by exercise in fasted state. These results suggest that REDD1‐induced suppression of mTORC1 signaling may be one mechanism to blunt muscle protein synthesis during exercise and postexercise early recovery period.

Reduced REDD1 expression contributes to activation of mTORC1 following electrically induced muscle contraction

Regulated in DNA damage and development 1 (REDD1) is a repressor of mTOR complex 1 (mTORC1) signaling. In humans, REDD1 mRNA expression in skeletal muscle is repressed following resistance exercise in association with activation of mTORC1. However, whether REDD1 protein expression is also reduced after exercise and if so to what extent the loss contributes to exercise-induced activation of mTORC1 is unknown. Thus, the purpose of the present study was to examine the role of REDD1 in governing the response of mTORC1 and protein synthesis to a single bout of muscle contractions. Eccentric contractions of the tibialis anterior were elicited via electrical stimulation of the sciatic nerve in male mice in either the fasted or fed state or in fasted wild-type or REDD1-null mice. Four hours postcontractions, mTORC1 signaling and protein synthesis were elevated in fasted mice in association with repressed REDD1 expression relative to nonstimulated controls. Feeding coupled with contractions further elevated mTORC1 signaling, whereas REDD1 protein expression was repressed compared with either feeding or contractions alone. Basal mTORC1 signaling and protein synthesis were elevated in REDD1-null compared with wild-type mice. The magnitude of the increase in mTORC1 signaling was similar in both wild-type and REDD1-null mice, but, unlike wild-type mice, muscle contractions did not stimulate protein synthesis in mice deficient for REDD1, presumably because basal rates were already elevated. Overall, the data demonstrate that REDD1 expression contributes to the modulation of mTORC1 signaling following feeding- and contraction-induced activation of the pathway.

The order of exercise during concurrent training for rehabilitation does not alter acute genetic expression, mitochondrial enzyme activity or improvements in muscle function

Concurrent exercise combines different modes of exercise (e.g., aerobic and resistance) into one training protocol, providing stimuli meant to increase muscle strength, aerobic capacity and mass. As disuse is associated with decrements in strength, aerobic capacity and muscle size concurrent training is an attractive modality for rehabilitation. However, interference between the signaling pathways may result in preferential improvements for one of the exercise modes. We recruited 18 young adults (10 ♂, 8 ♀) to determine if order of exercise mode during concurrent training would differentially affect gene expression, protein content and measures of strength and aerobic capacity after 2 weeks of knee-brace induced disuse. Concurrent exercise sessions were performed 3x/week for 6 weeks at gradually increasing intensities either with endurance exercise preceding (END>RES) or following (RES>END) resistance exercise. Biopsies were collected from the vastus lateralis before, 3 h after the first exercise bout and 48 h after the end of training. Concurrent exercise altered the expression of genes involved in mitochondrial biogenesis (PGC-1α, PRC, PPARγ), hypertrophy (PGC-1α4, REDD2, Rheb) and atrophy (MuRF-1, Runx1), increased electron transport chain complex protein content, citrate synthase and mitochondrial cytochrome c oxidase enzyme activity, muscle mass, maximum isometric strength and VO_2peak. However, the order in which exercise was completed (END>RES or RES>END) only affected the protein content of mitochondrial complex II subunit. In conclusion, concurrent exercise training is an effective modality for the rehabilitation of the loss of skeletal muscle mass, maximum strength, and peak aerobic capacity resulting from disuse, regardless of the order in which the modes of exercise are performed.

RTP801/REDD1: a stress coping regulator that turns into a troublemaker in neurodegenerative disorders

Mechanistic target of Rapamycin (mTOR) pathway regulates essential processes directed to preserve cellular homeostasis, such as cell growth, proliferation, survival, protein synthesis and autophagy. Importantly, mTOR pathway deregulation has been related to many diseases. Indeed, it has become a hallmark in neurodegenerative disorders, since a fine-tuned regulation of mTOR activities is crucial for neuron function and survival. RTP801/REDD1/Dig2 has become one of the most puzzling regulators of mTOR. Although the mechanism is not completely understood, RTP801 inactivates mTOR and Akt via the tuberous sclerosis complex (TSC1/TSC2) in many cellular contexts. Intriguingly, RTP801 protects dividing cells from hypoxia or H₂O₂-induced apoptosis, while it sensitizes differentiated cells to stress. Based on experimental models of Parkinson’s disease (PD), it has been proposed that at early stages of the disease, stress-induced RTP801 upregulation contributes to mTOR repression, in an attempt to maintain cell function and viability. However, if RTP801 elevation is sustained, it leads to neuron cell death by a sequential inhibition of mTOR and Akt. Here, we will review RTP801 deregulation of mTOR in a context of PD and other neurodegenerative disorders.

Quantitative Lys-ϵ-Gly-Gly (diGly) proteomics coupled with inducible RNAi reveals ubiquitin-mediated proteolysis of DNA damage-inducible transcript 4 (DDIT4) by the E3 ligase HUWE1

Targeted degradation of proteins through the ubiquitin-proteasome system (UPS) via the activities of E3 ubiquitin ligases regulates diverse cellular processes, and misregulation of these enzymes contributes to the pathogenesis of human diseases. One of the challenges facing the UPS field is to delineate the complete cohort of substrates for a particular E3 ligase. Advances in mass spectrometry and the development of antibodies recognizing the Lys-ϵ-Gly-Gly (diGly) remnant from ubiquitinated proteins following trypsinolysis have provided a tool to address this question. We implemented an inducible loss of function approach in combination with quantitative diGly proteomics to find novel substrates of HUWE1 (HECT, UBA, and WWE domain containing 1, E3 ubiquitin protein ligase), an E3 ligase implicated in cancer and intellectual disabilities. diGly proteomics results led to the identification of DNA damage-inducible transcript 4 (DDIT4) as a putative HUWE1 substrate. Cell-based assays demonstrated that HUWE1 interacts with and regulates ubiquitination and stability of DDIT4. Together these data suggest a model in which HUWE1 mediates DDIT4 proteasomal degradation. Our results demonstrate proof of concept that inducible knockdown of an E3 ligase in combination with diGly proteomics provides a potentially advantageous method for identifying novel E3 substrates that may help to identify candidates for therapeutic modulation in the UPS.

Parkin loss of function contributes to RTP801 elevation and neurodegeneration in Parkinson's disease

Mutations in the PARK2 gene are associated with an autosomal recessive form of juvenile parkinsonism (AR-JP). These mutations affect parkin solubility and impair its E3 ligase activity, leading to a toxic accumulation of proteins within susceptible neurons that results in a slow but progressive neuronal degeneration and cell death. Here, we report that RTP801/REDD1, a pro-apoptotic negative regulator of survival kinases mTOR and Akt, is one of such parkin substrates. We observed that parkin knockdown elevated RTP801 in sympathetic neurons and neuronal PC12 cells, whereas ectopic parkin enhanced RTP801 poly-ubiquitination and proteasomal degradation. In parkin knockout mouse brains and in human fibroblasts from AR-JP patients with parkin mutations, RTP801 levels were elevated. Moreover, in human postmortem PD brains with mutated parkin, nigral neurons were highly positive for RTP801. Further consistent with the idea that RTP801 is a substrate for parkin, the two endogenous proteins interacted in reciprocal co-immunoprecipitates of cell lysates. A potential physiological role for parkin-mediated RTP801 degradation is indicated by observations that parkin protects neuronal cells from death caused by RTP801 overexpression by mediating its degradation, whereas parkin knockdown exacerbates such death. Similarly, parkin knockdown enhanced RTP801 induction in neuronal cells exposed to the Parkinson's disease mimetic 6-hydroxydopamine and increased sensitivity to this toxin. This response to parkin loss of function appeared to be mediated by RTP801 as it was abolished by RTP801 knockdown. Taken together these results indicate that RTP801 is a novel parkin substrate that may contribute to neurodegeneration caused by loss of parkin expression or activity.