Lipopolysaccharide induction of REDD1 is mediated by two distinct CREB-dependent mechanisms in macrophages

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.

RTP801/REDD1 Is Involved in Neuroinflammation and Modulates Cognitive Dysfunction in Huntington's Disease

RTP801/REDD1 is a stress-regulated protein whose levels are increased in several neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington's diseases (HD). RTP801 downregulation ameliorates behavioral abnormalities in several mouse models of these disorders. In HD, RTP801 mediates mutant huntingtin (mhtt) toxicity in in vitro models and its levels are increased in human iPSCs, human postmortem putamen samples, and in striatal synaptosomes from mouse models of the disease. Here, we investigated the role of RTP801 in the hippocampal pathophysiology of HD. We found that RTP801 levels are increased in the hippocampus of HD patients in correlation with gliosis markers. Although RTP801 expression is not altered in the hippocampus of the R6/1 mouse model of HD, neuronal RTP801 silencing in the dorsal hippocampus with shRNA containing AAV particles ameliorates cognitive alterations. This recovery is associated with a partial rescue of synaptic markers and with a reduction in inflammatory events, especially microgliosis. Altogether, our results indicate that RTP801 could be a marker of hippocampal neuroinflammation in HD patients and a promising therapeutic target of the disease.

RTP801/REDD1 contributes to neuroinflammation severity and memory impairments in Alzheimer's disease

RTP801/REDD1 is a stress-regulated protein whose upregulation is necessary and sufficient to trigger neuronal death. Its downregulation in Parkinson's and Huntington's disease models ameliorates the pathological phenotypes. In the context of Alzheimer's disease (AD), the coding gene for RTP801, DDIT4, is responsive to Aβ and modulates its cytotoxicity in vitro. Also, RTP801 mRNA levels are increased in AD patients' lymphocytes. However, the involvement of RTP801 in the pathophysiology of AD has not been yet tested. Here, we demonstrate that RTP801 levels are increased in postmortem hippocampal samples from AD patients. Interestingly, RTP801 protein levels correlated with both Braak and Thal stages of the disease and with GFAP expression. RTP801 levels are also upregulated in hippocampal synaptosomal fractions obtained from murine 5xFAD and rTg4510 mice models of the disease. A local RTP801 knockdown in the 5xFAD hippocampal neurons with shRNA-containing AAV particles ameliorates cognitive deficits in 7-month-old animals. Upon RTP801 silencing in the 5xFAD mice, no major changes were detected in hippocampal synaptic markers or spine density. Importantly, we found an unanticipated recovery of several gliosis hallmarks and inflammasome key proteins upon neuronal RTP801 downregulation in the 5xFAD mice. Altogether our results suggest that RTP801 could be a potential future target for theranostic studies since it could be a biomarker of neuroinflammation and neurotoxicity severity of the disease and, at the same time, a promising therapeutic target in the treatment of AD.

Redd1 knockdown prevents doxorubicin-induced cardiac senescence

Regulated in development and DNA damage response-1 (Redd1) is a stress-response gene that is transcriptionally induced by diverse stressful stimuli to influence cellular growth and survival. Although evidence suggests that aging may drive Redd1 expression in skeletal muscles, the expression patterns and functions of Redd1 in senescent cardiomyocytes remain unspecified. To address this issue, in vitro and in vivo models of cardiomyocyte senescence were established by administration of doxorubicin (Dox). Redd1 overexpression and knockdown was achieved in cultured H9c2 cardiomyocytes and mouse tissues using, respectively, lentivirals and adeno-associated virus 9 (AAV9) vectors. In the hearts of both aged (24 months old) and Dox-treated mice, as well as in Dox-exposed H9c2 cardiomyocytes, high Redd1 expression accompanied the increase in both cellular senescence markers (p16INK4a and p21) and pro-inflammatory cytokine expression indicative of a stress-associated secretory phenotype (SASP). Notably, Redd1 overexpression accentuated, whereas Redd1 silencing markedly attenuated, Dox-induced cardiomyocyte senescence features both in vitro and in vivo. Notably, AAV9-shRNA-mediated Redd1 silencing significantly alleviated Dox-induced cardiac dysfunction. Moreover, through pharmacological inhibition, immunofluorescence, and western blotting, signaling pathway analyses indicated that Redd1 promotes cardiomyocyte senescence as a downstream effector of p38 MAPK to promote NF-kB signaling via p65 phosphorylation and nuclear translocation.

Is REDD1 a metabolic double agent? Lessons from physiology and pathology

The Akt/mechanistic target of rapamycin (mTOR) signaling pathway governs macromolecule synthesis, cell growth, and metabolism in response to nutrients and growth factors. Regulated in development and DNA damage response (REDD)1 is a conserved and ubiquitous protein, which is transiently induced in response to multiple stimuli. Acting like an endogenous inhibitor of the Akt/mTOR signaling pathway, REDD1 protein has been shown to regulate cell growth, mitochondrial function, oxidative stress, and apoptosis. Recent studies also indicate that timely REDD1 expression limits Akt/mTOR-dependent synthesis processes to spare energy during metabolic stresses, avoiding energy collapse and detrimental consequences. In contrast to this beneficial role for metabolic adaptation, REDD1 chronic expression appears involved in the pathogenesis of several diseases. Indeed, REDD1 expression is found as an early biomarker in many pathologies including inflammatory diseases, cancer, neurodegenerative disorders, depression, diabetes, and obesity. Moreover, prolonged REDD1 expression is associated with cell apoptosis, excessive reactive oxygen species (ROS) production, and inflammation activation leading to tissue damage. In this review, we decipher several mechanisms that make REDD1 a likely metabolic double agent depending on its duration of expression in different physiological and pathological contexts. We also discuss the role played by REDD1 in the cross talk between the Akt/mTOR signaling pathway and the energetic metabolism.

Blocking the REDD1/TXNIP axis ameliorates LPS-induced vascular endothelial cell injury through repressing oxidative stress and apoptosis

The aim of the present study was to investigate the potential role of regulated in development and DNA damage response 1 (REDD1) in LPS-induced vascular endothelial injury by using human umbilical vein endothelial cells (HUVECs). We observed that REDD1 expression was apparently elevated in HUVECs after exposure to LPS. Additionally, elimination of REDD1 strikingly attenuated the secretion of the proinflammatory cytokines TNF-α, IL-6, IL-1β, and monocyte chemotactic protein-1 and the endothelial cell adhesion markers ICAM-1 and VCAM-1 that was induced by LPS stimulation. Subsequently, knockdown of REDD1 augmented cell viability but ameliorated lactate dehydrogenase release in HUVECs stimulated with LPS. Meanwhile, depletion of REDD1 effectively restricted LPS-induced HUVEC apoptosis, as exemplified by reduced DNA fragmentation, and it also elevated antiapoptotic Bcl-2 protein, concomitant with reduced levels of proapoptotic proteins Bax and cleaved caspase-3. Furthermore, repression of REDD1 remarkably alleviated LPS-triggered intracellular reactive oxygen species generation accompanied by decreased malondialdehyde content and increased the activity of the endogenous antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase. Most important, depletion of REDD1 protected HUVECs against inflammation-mediated apoptosis and oxidative damage partly through thioredoxin-interacting protein (TXNIP). Collectively, these findings indicate that blocking the REDD1/TXNIP axis repressed the inflammation-mediated vascular injury process, which may be closely related to oxidative stress and apoptosis in HUVECs, implying that the REDD1/TXNIP axis may be a new target for preventing the endothelial cell injury process.

Induction of REDD1 via AP-1 prevents oxidative stress-mediated injury in hepatocytes

Regulated in development and DNA damage responses 1 (REDD1) is an inducible gene in response to various stresses, which functions as a negative regulator of the mammalian target of rapamycin protein kinase in complex 1. In the present study, we identified the role of REDD1 under the oxidative stress-mediated hepatocyte injury and its regulatory mechanism. REDD1 protein was increased in H₂O₂ or tert-butylhydroperoxide (t-BHP)-treated hepatocytes_· H₂O₂ also elevated REDD1 mRNA levels. This event was inhibited by antioxidants such as diphenyleneiodonium chloride, N-acetyl-L-cysteine, or butylated hydroxy anisole. Interestingly, we found that H₂O₂-mediated REDD1 induction was transcriptionally regulated by activator protein-1 (AP-1), and that overexpression of c-Jun increased REDD1 protein levels and REDD1 promoter-driven luciferase activity. Deletion of the putative AP-1 binding site in proximal region of the human REDD1 promoter significantly abolished REDD1 transactivation by c-Jun. A NF-E2-related factor 2 activator, tert-butylhydroquinone treatment also elevated REDD1 levels, but it was independent on NF-E2-related factor 2 activation. Furthermore, we observed that REDD1 overexpression attenuated H₂O₂ or t-BHP-derived reactive oxygen species formation as well as cytotoxicity. Conversely, siRNA against REDD1 aggravated t-BHP-induced reactive oxygen species generation and cell death. In addition, we showed that REDD1 was induced by in vitro or in vivo ischemia/reperfusion model. Our results demonstrate that REDD1 induction by oxidative stress is mainly transcriptionally regulated by AP-1, and protects oxidative stress-mediated hepatocyte injury. These findings suggest REDD1 as a novel molecule that reduced susceptibility to oxidant-induced liver injury.

REDD-1 aggravates endotoxin-induced inflammation via atypical NF-κB activation

Regulated in development and DNA damage responses 1 (REDD-1), an inhibitor of mammalian target of rapamycin (mTOR), is induced by various cell stressors, including LPS, a major player in the pathogenesis of endotoxemic shock. However, the pathologic role of REDD-1 in endotoxemia is largely unknown. We found that LPS increased REDD-1 expression, nuclear transcription factor-κB (NF-κB) activation, and inflammation and that these responses were suppressed by REDD-1 knockdown and in REDD-1^+/- macrophages. REDD-1 overexpression stimulated NF-κB-dependent inflammation without additional LPS stimulation. REDD-1-induced NF-κB activation was independent of 2 classic IKK-dependent NF-κB pathways and the mTOR signaling pathway; however, REDD-1, particularly its C-terminal region (178-229), interacted with and sequestered IκBα, to elicit atypical NF-κB activation during the delayed and persistent phases of inflammation after stimulation. Moreover, REDD-1 knockdown mitigated vascular inflammation and permeability in endotoxemic mice, resulting in decreases in immune cell infiltration, systemic inflammation, caspase-3 activation, apoptosis, and consequent mortality. We further confirmed the inflammatory and cytotoxic effects of REDD-1 in endotoxemic REDD-1^+/- mice. Our data support the likelihood that REDD-1 exacerbates endotoxemic inflammation via atypical NF-κB activation by sequestering IκBα.-Lee, D.-K., Kim, J.-H., Kim, J., Choi, S., Park, M., Park, W., Kim, S., Lee, K.-S., Kim, T., Jung, J., Choi, Y. K., Ha, K.-S., Won, M.-H., Billiar, T. R., Kwon, Y.-G., Kim, Y.-M. REDD-1 aggravates endotoxin-induced inflammation via atypical NF-κB activation.

Inhibition of REDD1 Sensitizes Bladder Urothelial Carcinoma to Paclitaxel by Inhibiting Autophagy

Purpose: Regulated in development and DNA damage response-1 (REDD1) is a stress-related protein and is involved in the progression of cancer. The role and regulatory mechanism of REDD1 in bladder urothelial carcinoma (BUC), however, is yet unidentified.Experimental Design: The expression of REDD1 in BUC was detected by Western blot analysis and immunohistochemistry (IHC). The correlation between REDD1 expression and clinical features in patients with BUC were assessed. The effects of REDD1 on cellular proliferation, apoptosis, autophagy, and paclitaxel sensitivity were determined both in vitro and in vivo Then the targeted-regulating mechanism of REDD1 by miRNAs was explored.Results: Here the significant increase of REDD1 expression is detected in BUC tissue, and REDD1 is first reported as an independent prognostic factor in patients with BUC. Silencing REDD1 expression in T24 and EJ cells decreased cell proliferation, increased apoptosis, and decreased autophagy, whereas the ectopic expression of REDD1 in RT4 and BIU87 cells had the opposite effect. In addition, the REDD1-mediated proliferation, apoptosis, and autophagy are found to be negatively regulated by miR-22 in vitro, which intensify the paclitaxel sensitivity via inhibition of the well-acknowledged REDD1-EEF2K-autophagy axis. AKT/mTOR signaling initially activated or inhibited in response to silencing or enhancing REDD1 expression and then recovered rapidly. Finally, the inhibited REDD1 expression by either RNAi or miR-22 sensitizes BUC tumor cells to paclitaxel in a subcutaneous transplant carcinoma model in vivoConclusions: REDD1 is confirmed as an oncogene in BUC, and antagonizing REDD1 could be a potential therapeutic strategy to sensitize BUC cells to paclitaxel. Clin Cancer Res; 24(2); 445-59. ©2017 AACR.

Cacna1c in the Prefrontal Cortex Regulates Depression-Related Behaviors via REDD1

The CACNA1C gene that encodes the L-type Ca2+ channel (LTCC) Cav1.2 subunit has emerged as a candidate risk gene for multiple neuropsychiatric disorders including bipolar disorder, major depressive disorder, and schizophrenia, all marked with depression-related symptoms. Although cacna1c heterozygous (HET) mice have been previously reported to exhibit an antidepressant-like phenotype, the molecular and circuit-level dysfunction remains unknown. Here we report that viral vector-mediated deletion of cacna1c in the adult prefrontal cortex (PFC) of mice recapitulates the antidepressant-like effect observed in cacna1c HET mice using the sucrose preference test (SPT), forced swim test (FST), and tail suspension test (TST). Molecular studies identified lower levels of REDD1, a protein previously linked to depression, in the PFC of HET mice, and viral-mediated REDD1 overexpression in the PFC of these HET mice reversed the antidepressant-like effect in SPT and TST. Examination of downstream REDD1 targets found lower levels of active/phosphorylated Akt (S473) with no change in mTORC1 phosphorylation. Examination of the transcription factor FoxO3a, previously linked to depression-related behavior and shown to be regulated in other systems by Akt, revealed higher nuclear levels in the PFC of cacna1c HET mice that was further increased following REDD1-mediated reversal of the antidepressant-like phenotype. Collectively, these findings suggest that REDD1 in cacna1c HET mice may influence depression-related behavior via regulation of the FoxO3a pathway. Cacna1c HET mice thus serve as a useful mouse model to further study cacna1c-associated molecular signaling and depression-related behaviors relevant to human CACNA1C genetic variants.

Implication of REDD1 in the activation of inflammatory pathways

In response to endotoxemia, the organism triggers an inflammatory response, and the visceral adipose tissue represents a major source of proinflammatory cytokines. The regulation of inflammation response in the adipose tissue is thus of crucial importance. We demonstrated that Regulated in development and DNA damage response-1 (REDD1) is involved in inflammation. REDD1 expression was increased in response to lipopolysaccharide (LPS) in bone marrow derived macrophages (BMDM) and in epidydimal adipose tissue. Loss of REDD1 protected the development of inflammation, since the expression of proinflammatory cytokines (TNFα, IL-6, IL-1β) was decreased in adipose tissue of REDD1^−/− mice injected with LPS compared to wild-type mice. This decrease was associated with an inhibition of the activation of p38MAPK, JNK, NF-κB and NLRP3 inflammasome leading to a reduction of IL-1β secretion in response to LPS and ATP in REDD1^−/− BMDM. Although REDD1 is an inhibitor of mTORC1, loss of REDD1 decreased inflammation independently of mTORC1 activation but more likely through oxidative stress regulation. Absence of REDD1 decreases ROS associated with a dysregulation of Nox-1 and GPx3 expression. Absence of REDD1 in macrophages decreases the development of insulin resistance in adipocyte-macrophage coculture. Altogether, REDD1 appears to be a key player in the control of inflammation.

Is REDD1 a Metabolic Éminence Grise?

Regulated in development and DNA damage response 1 (REDD1) has been functionally linked to the control of diverse cellular processes due, at least in part, to its ability to repress mammalian or mechanistic Target of Rapamycin (mTOR) Complex-1 (mTORC1), a key protein complex controlled by hormonal and nutrient cues. Notably, emerging evidence suggests that REDD1 also regulates several pathways involved in modulating energy balance and metabolism. Herein, we discuss evidence implicating REDD1 as a key modulator of insulin action and metabolic function, including its potential contribution to mitochondrial biology and pancreatic islet function. Collectively, the available evidence suggests that REDD1 has a more prominent role in energy homeostasis than was previously thought, and implicates REDD1 as a potential therapeutic target for treatment of metabolic disorders.

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.