Interleukin 1β Regulation of the System xc- Substrate-specific Subunit, xCT, in Primary Mouse Astrocytes Involves the RNA-binding Protein HuR

Fluoride Exposure Increases the Activity of the Cystine/Glutamate Exchanger in Glia Cells

Fluoride exposure in drinking water has been widely related to impairment of cognitive function. Even though this ion has been described as neurotoxic for more than two decades, the molecular mechanisms of fluoride neurotoxicity are not fully understood, however, increasing evidence suggests that glial cells are the site of early injury in fluoride neurotoxicity. Nevertheless, a convergence point of many studies is the effect on glutamatergic neurotransmission and the generation of reactive oxygen species. In this context, we evaluated here the expression and regulation of the cystine/glutamate exchanger upon fluoride exposure since this transporter is in the interface between excitotoxicity and the antioxidant response. We demonstrate here the functional expression of the cystine /glutamate exchanger in both the U373 human glioblastoma cells and chick cerebellar Bergmann glia cells. Using a [3H]-L-Glutamate uptake assay, we demonstrate that fluoride increases the activity of the exchanger in a time and dose-dependent manner. This augmentation is mitigated by the antioxidant Trolox. To gain insight into fluoride neurotoxicity mechanisms, we evaluated its effect on human antigen R, a RNA binding protein, that binds to the 3'-UTR region of exchanger mRNA increasing its half time life. An increase in human antigen R protein was recorded after a 6 h fluoride exposure, suggesting that this ion regulates the exchanger through this RNA-binding protein. Furthermore, we show that fluoride exposure increases both the exchanger and human antigen R mRNAs half-life. These results provide insights into fluoride neurotoxicity mechanisms and support the notion of a central role of glial cells in neuronal glutamatergic transmission disruption that leads to neuronal cell death.

Nrf2 Regulates Basal Glutathione Production in Astrocytes

Astrocytes produce and export glutathione (GSH), an important thiol antioxidant essential for protecting neural cells from oxidative stress and maintaining optimal brain health. While it has been established that oxidative stress increases GSH production in astrocytes, with Nrf2 acting as a critical transcription factor regulating key components of the GSH synthetic pathway, the role of Nrf2 in controlling constitutive GSH synthetic and release mechanisms remains incompletely investigated. Our data show that naïve primary mouse astrocytes cultured from the cerebral cortices of Nrf2 knockout (Nrf2-/-) pups have significantly less intracellular and extracellular GSH levels when compared to astrocytes cultured from Nrf2 wild-type (Nrf2+/+) pups. Key components of the GSH synthetic pathway, including xCT (the substrate-specific light chain of the substrate-importing transporter, system xc-), glutamate-cysteine ligase [catalytic (GCLc) and modifying (GCLm) subunits], were affected. To wit: qRT-PCR analysis demonstrates that naïve Nrf2-/- astrocytes have significantly lower basal mRNA levels of xCT and both GCL subunits compared to naïve Nrf2+/+ astrocytes. No change in mRNA levels of glutathione synthetase (GS) or the GSH exporting transporter, Mrp1, was found. Western blot analysis reveals reduced protein levels of both subunits of GCL, while (seleno)cystine uptake into Nrf2-/- astrocytes was reduced compared to Nrf2+/+ astrocytes, confirming decreased system xc- activity. These findings suggest that Nrf2 regulates the basal production of GSH in astrocytes through constitutive transcriptional regulation of GCL and xCT.

5-methylcytosine methylation of MALAT1 promotes resistance to sorafenib in hepatocellular carcinoma through ELAVL1/SLC7A11-mediated ferroptosis

Emerging evidence demonstrates that long non-coding RNAs (lncRNAs) play a crucial role in sorafenib resistance in hepatocellular carcinoma (HCC), and lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) is a dysregulated lncRNA in sorafenib-resistant HCC cells. However, the underlying regulatory mechanisms of MALAT1 in sorafenib-resistant HCC cells remain unclear. In the present study, we demonstrated that 5-methylcytosine (m5C) methylation catalyzed by NSUN2 and ALYREF contributed to the RNA stability and upregulation of MALAT1. The NSUN2/ALYREF/MALAT1 signaling axis was activated in sorafenib-resistant cells, and the upregulation of MALAT1 inhibited sorafenib-induced ferroptosis to drive sorafenib resistance. Mechanistically, MALAT1 maintained the mRNA stability of SLC7A11 by directly binding to ELAVL1 and stimulating its cytoplasmic translocation. Furthermore, we explored a new synergetic strategy for the treatment of HCC by combining MALAT1 inhibitor MALAT1-IN1 with sorafenib. The results demonstrated that MALAT1-IN1 significantly enhanced sorafenib efficacy for the treatment of HCC both in vitro and in vivo. Collectively, our work brings new insights into the epigenetic mechanisms of sorafenib resistance and offers an alternative therapeutic strategy targeting ferroptosis for sorafenib-resistant HCC patients.

RNA binding protein PUM2 promotes IL-1β-induced apoptosis of chondrocytes via regulating FOXO3 expression

Objective

RNA-binding proteins (RBPs) have been recently proven to be involved in the pathogenesis of several diseases. However, few studies elaborated RBPs in regulating osteoarthritis. This study aims to define the function and mechanism of RBPs-PUM2 in chondrocyte apoptosis during osteoarthritis.

Methods

Cartilage tissue samples and human juvenile chondrocyte cell line C28/I2 were collected for further study. PUM2 expression in the human tissues and cells was determined using qRT-PCR. Chondrocyte viability and apoptosis were determined by MTT and flow cytometry. ROS generation was determined by flow cytometry. The regulation of PUM2 on FOXO3 translation was evaluated by RNA immunoprecipitation, RNA pull-down, and Luciferase gene reporter analysis.

Results

PUM2 is upregulated in both cartilage tissue of osteoarthritis patients and IL-1β-stimulated chondrocytes. PUM2 overexpression reduces cell viability and promotes cell apoptosis and ROS generation of chondrocytes. PUM2 silencing increases cell viability and ameliorates cell apoptosis as well as ROS generation in chondrocytes induced by IL-1β. PUM2 inhibits FOXO3 expression via binding its mRNA 3'-UTR. PUM2 forms a signaling axis with FOXO3 in IL-1β induced chondrocyte damage.

Conclusion

PUM2 is upregulated in cartilage tissue of osteoarthritis and positively regulates chondrocytes apoptosis through controlling FOXO3 protein expression.

Decreased xCT activity in patients associated with Helicobacter pylori infection

Objective: In animals, Helicobacter pylori (Hp)-induced gastric injury is accompanied by a decrease in the activity of the cysteine/glutamate transporter (xCT), which regulates extracellular glutamate levels. However, the impact of xCT activity in patients with Hp infection remains unclear. This study aims to investigate variations of xCT activity in the gastric mucosa of patients with Hp infection and to provide a clinical basis for identifying targets related to Hp infection. Methods: Our study included a total of 67 patients with gastritis, which consisted of 44 Hp-negative and 23 Hp-positive peptic ulcer cases. The inclusion criteria used to select patients were as follows: gastric histology was determined with a gastroscope, antral biopsies were taken for urease tests, and pathology and culture were performed for analysis of Hp-colonization. The clinical characteristics of the patients were obtained, the expressions of microRNAs and xCT protein were detected using immune histochemical analysis, and the concentration of glutamate in their gastric secretion was determined. Results: The findings revealed that xCT expression was significantly lower in Hp-positive patients as compared to Hp-negative individuals, which was accompanied by a decrease in glutamate concentration in gastric juice. We also discovered a high expression of microRNAs that have been shown to negatively regulate xCT expression, in Hp-positive patients. Conclusion: Reduced xCT activity in patients may play an important role in gastric ulcers caused by Hp infection. Our findings suggest that the microRNA/xCT pathway could be a potential treatment target for Hp-infection-related ulcers.

Validation of a System xc– Functional Assay in Cultured Astrocytes and Nervous Tissue Samples

Disruption of the glutamatergic homeostasis is commonly observed in neurological diseases and has been frequently correlated with the altered expression and/or function of astrocytic high-affinity glutamate transporters. There is, however, a growing interest for the role of the cystine-glutamate exchanger system xc– in controlling glutamate transmission. This exchanger is predominantly expressed in glial cells, especially in microglia and astrocytes, and its dysregulation has been documented in diverse neurological conditions. While most studies have focused on measuring the expression of its specific subunit xCT by RT-qPCR or by Western blotting, the activity of this exchanger in tissue samples remains poorly examined. Indeed, the reported use of sulfur- and carbon-radiolabeled cystine in uptake assays shows several drawbacks related to its short radioactive half-life and its relatively high cost. We here report on the elaborate validation of a method using tritiated glutamate as a substrate for the reversed transport mediated by system xc–. The uptake assay was validated in primary cultured astrocytes, in transfected cells as well as in crude synaptosomes obtained from fresh nervous tissue samples. Working in buffers containing defined concentrations of Na+, allowed us to differentiate the glutamate uptake supported by system xc– or by high-affinity glutamate transporters, as confirmed by using selective pharmacological inhibitors. The specificity was further demonstrated in primary astrocyte cultures from transgenic mice lacking xCT or in cell lines where xCT expression was genetically induced or reduced. As such, this assay appears to be a robust and cost-efficient solution to investigate the activity of this exchanger in physiological and pathological conditions. It also provides a reliable tool for the screening and characterization of new system xc– inhibitors which have been frequently cited as valuable drugs for nervous disorders and cancer.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Local translation in perisynaptic and perivascular astrocytic processes - a means to ensure astrocyte molecular and functional polarity?

Together with the compartmentalization of mRNAs in distal regions of the cytoplasm, local translation constitutes a prominent and evolutionarily conserved mechanism mediating cellular polarization and the regulation of protein delivery in space and time. The translational regulation of gene expression enables a rapid response to stimuli or to a change in the environment, since the use of pre-existing mRNAs can bypass time-consuming nuclear control mechanisms. In the brain, the translation of distally localized mRNAs has been mainly studied in neurons, whose cytoplasmic protrusions may be more than 1000 times longer than the diameter of the cell body. Importantly, alterations in local translation in neurons have been implicated in several neurological diseases. Astrocytes, the most abundant glial cells in the brain, are voluminous, highly ramified cells that project long processes to neurons and brain vessels, and dynamically regulate distal synaptic and vascular functions. Recent research has demonstrated the presence of local translation at these astrocytic interfaces that might regulate the functional compartmentalization of astrocytes. In this Review, we summarize our current knowledge about the localization and local translation of mRNAs in the distal perisynaptic and perivascular processes of astrocytes, and discuss their possible contribution to the molecular and functional polarity of astrocytes.

Nutritional ketosis as an intervention to relieve astrogliosis: Possible therapeutic applications in the treatment of neurodegenerative and neuroprogressive disorders

Nutritional ketosis, induced via either the classical ketogenic diet or the use of emulsified medium-chain triglycerides, is an established treatment for pharmaceutical resistant epilepsy in children and more recently in adults. In addition, the use of oral ketogenic compounds, fractionated coconut oil, very low carbohydrate intake, or ketone monoester supplementation has been reported to be potentially helpful in mild cognitive impairment, Parkinson’s disease, schizophrenia, bipolar disorder, and autistic spectrum disorder. In these and other neurodegenerative and neuroprogressive disorders, there are detrimental effects of oxidative stress, mitochondrial dysfunction, and neuroinflammation on neuronal function. However, they also adversely impact on neurone–glia interactions, disrupting the role of microglia and astrocytes in central nervous system (CNS) homeostasis. Astrocytes are the main site of CNS fatty acid oxidation; the resulting ketone bodies constitute an important source of oxidative fuel for neurones in an environment of glucose restriction. Importantly, the lactate shuttle between astrocytes and neurones is dependent on glycogenolysis and glycolysis, resulting from the fact that the astrocytic filopodia responsible for lactate release are too narrow to accommodate mitochondria. The entry into the CNS of ketone bodies and fatty acids, as a result of nutritional ketosis, has effects on the astrocytic glutamate–glutamine cycle, glutamate synthase activity, and on the function of vesicular glutamate transporters, EAAT, Na⁺, K⁺-ATPase, K_ir4.1, aquaporin-4, Cx34 and K_ATP channels, as well as on astrogliosis. These mechanisms are detailed and it is suggested that they would tend to mitigate the changes seen in many neurodegenerative and neuroprogressive disorders. Hence, it is hypothesized that nutritional ketosis may have therapeutic applications in such disorders.

IL-1β induces rod degeneration through the disruption of retinal glutamate homeostasis

BACKGROUND

Age-related macular degeneration is characterized by the accumulation of subretinal macrophages and the degeneration of cones, but mainly of rods. We have previously shown that Mononuclear Phagocytes-derived IL-1β induces rod photoreceptor cell death during experimental subretinal inflammation and in retinal explants exposed to IL-1β but the mechanism is unknown.

METHODS

Retinal explants were culture in the presence of human monocytes or IL-1β and photoreceptor cell survival was analyzed by TUNEL labeling. Glutamate concentration and transcription levels of gene involved in the homeostasis of glutamate were analyzed in cell fractions of explant cultured or not in the presence of IL-1β. Glutamate receptor antagonists were evaluated for their ability to reduce photoreceptor cell death in the presence of IL1-β or monocytes.

RESULTS

We here show that IL-1β does not induce death in isolated photoreceptors, suggesting an indirect effect. We demonstrate that IL-1β leads to glutamate-induced rod photoreceptor cell death as it increases the extracellular glutamate concentrations in the retina through the inhibition of its conversion to glutamine in Müller cells, increased release from Müller cells, and diminished reuptake. The inhibition of non-NMDA receptors completely and efficiently prevented rod apoptosis in retinal explants cultured in the presence of IL-1β or, more importantly, in vivo, in a model of subretinal inflammation.

CONCLUSIONS

Our study emphasizes the importance of inflammation in the deregulation of glutamate homeostasis and provides a comprehensive mechanism of action for IL-1β-induced rod degeneration.

xCT knockdown in human breast cancer cells delays onset of cancer-induced bone pain

Cancers in the bone produce a number of severe symptoms including pain that compromises patient functional status, quality of life, and survival. The source of this pain is multifaceted and includes factors secreted from tumor cells. Malignant cells release the neurotransmitter and cell-signaling molecule glutamate via the oxidative stress-related cystine/glutamate antiporter, system x_C^-, which reciprocally imports cystine for synthesis of glutathione and the cystine/cysteine redox cycle. Pharmacological inhibition of system x_C^- has shown success in reducing and delaying the onset of cancer pain-related behavior in mouse models. This investigation describes the development of a stable siRNA-induced knockdown of the functional trans-membrane system x_C^- subunit xCT ( SLC7A11) in the human breast cancer cell line MDA-MB-231. Clones were verified for xCT knockdown at the transcript, protein, and functional levels. RNAseq was performed on a representative clone to comprehensively examine the transcriptional cellular signature in response to xCT knockdown, identifying multiple differentially regulated factors relevant to cancer pain including nerve growth factor, interleukin-1, and colony-stimulating factor-1. Mice were inoculated intrafemorally and recordings of pain-related behaviors including weight bearing, mechanical withdrawal, and limb use were performed. Animals implanted with xCT knockdown cancer cells displayed a delay until the onset of nociceptive behaviors relative to control cells. These results add to the body of evidence suggesting that a reduction in glutamate release from cancers in bone by inhibition of the system x_C^- transporter may decrease the severe and intractable pain associated with bone metastases.

Inhibiting STAT3 in a murine model of human breast cancer-induced bone pain delays the onset of nociception

Aggressive breast cancer subtypes utilize system x_c^-, a membrane antiporter, to import cystine for glutathione synthesis and maintenance of redox homeostasis, in turn releasing glutamate as a metabolic pro-nociceptive by-product. Metastatic breast cancers establish themselves at distal sites including bone, where changes in extracellular glutamate levels contribute to cancer-induced bone pain. We previously established that stearically blocking system x_c^- activity with sulfasalazine delays the onset of nociceptive behaviours and that xCT, the functional antiporter subunit, is positively regulated by signal transducer and activator of transcription 3 (STAT3). In the current investigation, a murine xenograft cancer-induced bone pain model was applied to examine whether pharmacological inhibition of phosphorylated STAT3 (pSTAT3) induces changes in nociception. A high glutamate-releasing, xCT/pSTAT3 over-expressing human breast cancer cell line was selected for injection into the distal epiphysis of the right femur of female nude mice. A 14-day regimen of intraperitoneal injections with either vehicle or the novel STAT3 inhibitor DR-1-55 commenced three weeks after initial intrafemoral bone injection. Nociceptive behaviours were temporally monitored by automated von Frey, dynamic weight bearing and open-field testing for the duration of the study, beginning at the baseline. Prior to sacrifice and at ethical end point, tumour-induced osteolytic lesions were radiographically assessed. Treatment with DR-1-55 significantly delayed the onset and severity of spontaneous and induced nociceptive behaviours, also decreasing human SLC7A11 ( xCT) mRNA levels in tumour-bearing limbs without altering osteolysis. In addition, two pro-inflammatory cytokines released by this cell line, interleukin 6 and interleukin 1β, were also down-regulated at the mRNA level in response to DR-1-55 treatment in vivo, with lower human interleukin 6 levels detected in the host circulation. This study demonstrates that targeting pSTAT3 may be a viable therapeutic means to manage cancer-induced bone pain, alone or in combination with stearic system x_c^- blockers.

Interleukin-1β Protects Neurons against Oxidant-Induced Injury via the Promotion of Astrocyte Glutathione Production

Interleukin-1β (IL-1β), a key cytokine that drives neuroinflammation in the Central Nervous System (CNS), is enhanced in many neurological diseases/disorders. Although IL-1β contributes to and/or sustains pathophysiological processes in the CNS, we recently demonstrated that IL-1β can protect cortical astrocytes from oxidant injury in a glutathione (GSH)-dependent manner. To test whether IL-1β could similarly protect neurons against oxidant stress, near pure neuronal cultures or mixed cortical cell cultures containing neurons and astrocytes were exposed to the organic peroxide, tert-butyl hydroperoxide (t-BOOH), following treatment with IL-1β or its vehicle. Neurons and astrocytes in mixed cultures, but not pure neurons, were significantly protected from the toxicity of t-BOOH following treatment with IL-1β in association with enhanced GSH production/release. IL-1β failed to increase the GSH levels or to provide protection against t-BOOH toxicity in chimeric mixed cultures consisting of IL-1R1^+/+ neurons plated on top of IL-1R1^−/− astrocytes. The attenuation of GSH release via block of multidrug resistance-associated protein 1 (MRP1) transport also abrogated the protective effect of IL-1β. These protective effects were not strictly an in vitro phenomenon as we found an increased striatal vulnerability to 3-nitropropionic acid-mediated oxidative stress in IL-1R1 null mice. Overall, our data indicate that IL-1β protects neurons against oxidant injury and that this likely occurs in a non-cell-autonomous manner that relies on an increase in astrocyte GSH production and release.

Tumour-Derived Glutamate: Linking Aberrant Cancer Cell Metabolism to Peripheral Sensory Pain Pathways

Background

Chronic pain is a major symptom that develops in cancer patients, most commonly emerging during advanced stages of the disease. The nature of cancer-induced pain is complex, and the efficacy of current therapeutic interventions is restricted by the dose-limiting side-effects that accompany common centrally targeted analgesics.

Methods

This review focuses on how up-regulated glutamate production and export by the tumour converge at peripheral afferent nerve terminals to transmit nociceptive signals through the transient receptor cation channel, TRPV1, thereby initiating central sensitization in response to peripheral disease-mediated stimuli.

Results

Cancer cells undergo numerous metabolic changes that include increased glutamine catabolism and over-expression of enzymes involved in glutaminolysis, including glutaminase. This mitochondrial enzyme mediates glutaminolysis, producing large pools of intracellular glutamate. Up-regulation of the plasma membrane cystine/glutamate antiporter, system xc-, promotes aberrant glutamate release from cancer cells. Increased levels of extracellular glutamate have been associated with the progression of cancer-induced pain and we discuss how this can be mediated by activation of TRPV1.

Conclusion

With a growing population of patients receiving inadequate treatment for intractable pain, new targets need to be considered to better address this largely unmet clinical need for improving their quality of life. A better understanding of the mechanisms that underlie the unique qualities of cancer pain will help to identify novel targets that are able to limit the initiation of pain from a peripheral source–the tumour.

Cysteine, Glutathione, and Thiol Redox Balance in Astrocytes

This review discusses the current understanding of cysteine and glutathione redox balance in astrocytes. Particular emphasis is placed on the impact of oxidative stress and astrocyte activation on pathways that provide cysteine as a precursor for glutathione. The effect of the disruption of thiol-containing amino acid metabolism on the antioxidant capacity of astrocytes is also discussed.

Redox-Related Epigenetic Mechanisms in Glioblastoma: Nuclear Factor (Erythroid-Derived 2)-Like 2, Cobalamin, and Dopamine Receptor Subtype 4

Glioblastoma is an exceptionally difficult cancer to treat. Cancer is universally marked by epigenetic changes, which play key roles in sustaining a malignant phenotype, in addition to disease progression and patient survival. Studies have shown strong links between the cellular redox state and epigenetics. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a redox-sensitive transcription factor that upregulates endogenous antioxidant production, and is aberrantly expressed in many cancers, including glioblastoma. Methylation of DNA and histones provides a mode of epigenetic regulation, and cobalamin-dependent reactions link the redox state to methylation. Antagonists of dopamine receptor subtype 4 (D₄ receptor) were recently shown to restrict glioblastoma stem cell growth by downregulating trophic signaling, resulting in inhibition of functional autophagy. In addition to stimulating glioblastoma stem cell growth, D₄ receptors have the unique ability to catalyze cobalamin-dependent phospholipid methylation. Therefore, D₄ receptors represent an important node in a molecular reflex pathway involving Nrf2 and cobalamin, operating in conjunction with redox status and methyl group donor availability. In this article, we describe the redox-related effects of Nrf2, cobalamin metabolism, and the D₄ receptor on the regulation of the epigenetic state in glioblastoma.