NF-E2-related factor 2 activation in PC12 cells: its protective role in manganese-induced damage

Neurotoxic effects of heavy metal pollutants in the environment: Focusing on epigenetic mechanisms

The pollution of heavy metals (HMs) in the environment is a significant global environmental issue, characterized by its extensive distribution, severe contamination, and profound ecological impacts. Excessive exposure to heavy metal pollutants can damage the nervous system. However, the mechanisms underlying the neurotoxicity of most heavy metals are not completely understood. Epigenetics is defined as a heritable change in gene function that can influence gene and subsequent protein expression levels without altering the DNA sequence. Growing evidence indicates that heavy metals can induce neurotoxic effects by triggering epigenetic changes and disrupting the epigenome. Compared with genetic changes, epigenetic alterations are more easily reversible. Epigenetic reprogramming techniques, drugs, and certain nutrients targeting specific epigenetic mechanisms involved in gene expression regulation are emerging as potential preventive or therapeutic tools for diseases. Therefore, this review provides a comprehensive overview of epigenetic modifications encompassing DNA/RNA methylation, histone modifications, and non-coding RNAs in the nervous system, elucidating their association with various heavy metal exposures. These primarily include manganese (Mn), mercury (Hg), lead (Pb), cobalt (Co), cadmium (Cd), nickel (Ni), sliver (Ag), toxic metalloids arsenic (As), and etc. The potential epigenetic mechanisms in the etiology, precision prevention, and target therapy of various neurodevelopmental disorders or different neurodegenerative diseases are emphasized. In addition, the current gaps in research and future areas of study are discussed. From a perspective on epigenetics, this review offers novel insights for prevention and treatment of neurotoxicity induced by heavy metal pollutants.

Mechanisms of manganese-induced neurotoxicity and the pursuit of neurotherapeutic strategies

Chronic exposure to elevated levels of manganese via occupational or environmental settings causes a neurological disorder known as manganism, resembling the symptoms of Parkinson's disease, such as motor deficits and cognitive impairment. Numerous studies have been conducted to characterize manganese's neurotoxicity mechanisms in search of effective therapeutics, including natural and synthetic compounds to treat manganese toxicity. Several potential molecular targets of manganese toxicity at the epigenetic and transcriptional levels have been identified recently, which may contribute to develop more precise and effective gene therapies. This review updates findings on manganese-induced neurotoxicity mechanisms on intracellular insults such as oxidative stress, inflammation, excitotoxicity, and mitophagy, as well as transcriptional dysregulations involving Yin Yang 1, RE1-silencing transcription factor, transcription factor EB, and nuclear factor erythroid 2-related factor 2 that could be targets of manganese neurotoxicity therapies. This review also features intracellular proteins such as PTEN-inducible kinase 1, parkin, sirtuins, leucine-rich repeat kinase 2, and α-synuclein, which are associated with manganese-induced dysregulation of autophagy/mitophagy. In addition, newer therapeutic approaches to treat manganese's neurotoxicity including natural and synthetic compounds modulating excitotoxicity, autophagy, and mitophagy, were reviewed. Taken together, in-depth mechanistic knowledge accompanied by advances in gene and drug delivery strategies will make significant progress in the development of reliable therapeutic interventions against manganese-induced neurotoxicity.

Sex-dependent metal accumulation and immunoexpression of Hsp70 and Nrf2 in rats' brain following manganese exposure

Manganese (Mn), although important for multiple cellular processes, has posed environmental health concerns due to its neurotoxic effects. In recent years, there have been extensive studies on the mechanism of Mn-induced neuropathology, as well as the sex-dependent vulnerability to its neurotoxic effects. Nonetheless, cellular mechanisms influenced by sex differences in susceptibility to Mn have yet to be adequately characterized. Since oxidative stress is a key mechanism of Mn neurotoxicity, here, we have probed Hsp70 and Nrf2 proteins to investigate the sex-dependent changes following exposure to Mn. Male and female rats were administered intraperitoneal injections of MnCl2 (10 mg/kg and 25 mg/kg) 48 hourly for a total of eight injections (15 days). We evaluated changes in body weight, as well as Mn accumulation, Nrf2 and Hsp70 expression across four brain regions; striatum, cortex, hippocampus and cerebellum in both sexes. Our results showed sex-specific changes in body-weight, specifically in males but not in females. Additionally, we noted sex-dependent accumulation of Mn in the brain, as well as in expression levels of Nrf2 and Hsp70 proteins. These findings revealed sex-dependent susceptibility to Mn-induced neurotoxicity corresponding to differential Mn accumulation, and expression of Hsp70 and Nrf2 across several brain regions.

Upregulation of antioxidant and autophagy pathways via NRF2 activation protects spinal cord neurons from ozone damage

Ozone therapy can relieve multiple types of pain but exhibits potential neurotoxicity, the mechanism of which is unclear. The present study aimed to identify the role of nuclear factor (erythroid-derived-2)-related 2 (NRF2) in preventing spinal cord injury caused by ozone overdose. Primary neuronal cells were extracted from newborn Wistar rats and authenticated by immunofluorescence using anti-microtubule-associated protein 2 as a cell type-specific marker. Cell viability assay with different ozone concentrations (0, 10, 20, 30 and 40 µg/ml) was used to determine the concentration that caused primary neuron injury; 30 min of 40 µg/ml ozone therapy notably decreased cell viability to 71%. In order to test the effects of ozone, the cells were divided into five treatment groups [0-, 30- and 40 µg/ml ozone, tert-butylhydroquinone (tBHQ) + 40 µg/ml ozone (T40) and tBHQ (T0)]. Cells in the T40 and T0 groups received 40 µmol/l tBHQ on the fifth day of SCN cultivation. Reverse transcription-quantitative PCR and western blotting showed that protein expression levels of heme oxygenase-1 (HO-1) and mRNA expression levels of HO-1 and NRF2 were decreased. NRF2, ubiquitin-binding protein p62 and microtubule-associated proteins 1A/1B light chain 3B expression levels were decreased following treatment with 40 µg/ml ozone. Immunofluorescence showed that NRF2 nuclear expression levels also decreased following 40 µg/ml ozone treatment. However, cells in the T40 group did not display decreased NRF2 nuclear expression levels. Normal/Apoptotic/Necrotic Cell Detection kit revealed that necrosis rate increased following treatment with 40 µg/ml ozone; however, the T40 group did not exhibit this increased necrosis. At 40 µg/ml, ozone increased spinal cord neuron (SCN) death in vitro. Moreover, treatment with 40 µg/ml ozone damaged SCNs. The p62/NRF2/antioxidant response element pathway prevented such injury. tBHQ activated this pathway, upregulated autophagy and increased local nuclear NRF2 concentration, thus enhancing the antioxidant system to protect SCNs from injury caused by high concentrations of ozone.

[Protective Effect of Epalrestat against Oxidative Stress-induced Cytotoxicity]

Epalrestat (EPS), approved in Japan, is currently the only aldose reductase inhibitor that is available for the treatment of diabetic neuropathy. Recently, we found that EPS at near-plasma concentration increases the intracellular levels of glutathione (GSH) in rat Schwann cells. GSH, the most abundant non-protein thiol antioxidant in cells, is important for protection against oxidative stress. Oxidative stress is associated with the development and progression of many pathological conditions, such as atherosclerosis, diabetes, and neurodegeneration. In this study, we tested the hypothesis that EPS enhances resistance to oxidative stress, by using rat Schwann cells. To determine whether EPS protects Schwann cells from oxidative stress, we performed experiments by using radical generators, drugs, and heavy metals as the source of oxidative stress. EPS reduced the cytotoxicity induced by 2,2-azobis-[2-(2-imidazolin-2-yl) propane] dihydrochloride, 6-hydroxydopamine, cisplatin, palmitate, cadmium chloride, and manganese (II) sulfate, indicating that EPS plays a role in protecting cells from oxidative stress. We suggest that EPS has the potential to prevent the development and progression of disorders caused by oxidative stress.

Manganese increases Aβ and Tau protein levels through proteasome 20S and heat shock proteins 90 and 70 alteration, leading to SN56 cholinergic cell death following single and repeated treatment

Manganese (Mn) produces cholinergic neuronal loss in basal forebrain (BF) region that was related to cognitive dysfunction induced after single and repeated Mn treatment. All processes that generate cholinergic neuronal loss in BF remain to be understood. Mn exposure may produce the reduction of BF cholinergic neurons by increasing amyloid beta (Aβ) and phosphorylated Tau (pTau) protein levels, altering heat shock proteins' (HSPs) expression, disrupting proteasome P20S activity and generating oxidative stress. These mechanisms, described to be altered by Mn in regions different than BF, could lead to the memory and learning process alteration produced after Mn exposure. The research performed shows that single and repeated Mn treatment of SN56 cholinergic neurons from BF induces P20S inhibition, increases Aβ and pTau protein levels, produces HSP90 and HSP70 proteins expression alteration, and oxidative stress generation, being the last two effects mediated by NRF2 pathway alteration. The increment of Aβ and pTau protein levels was mediated by HSPs and proteasome dysfunction. All these mechanisms mediated the cell decline observed after Mn treatment. Our results are relevant because they may assist to reveal the processes leading to the neurotoxicity and cognitive alterations observed after Mn exposure.

Redox toxicology of environmental chemicals causing oxidative stress

Living organisms are surrounded with heavy metals such as methylmercury, manganese, cobalt, cadmium, arsenic, as well as pesticides such as deltamethrin and paraquat, or atmospheric pollutants such as quinone. Extensive studies have demonstrated a strong link between environmental pollutants and human health. Redox toxicity is proposed as one of the main mechanisms of chemical-induced pathology in humans. Acting as both a sensor of oxidative stress and a positive regulator of antioxidants, the nuclear factor erythroid 2-related factor 2 (NRF2) has attracted recent attention. However, the role NRF2 plays in environmental pollutant-induced toxicity has not been systematically addressed. Here, we characterize NRF2 function in response to various pollutants, such as metals, pesticides and atmospheric quinones. NRF2 related signaling pathways and epigenetic regulations are also reviewed.

Evaluation of Nrf2 with Exposure to Nanoparticles

Transcription factor Nrf2, nuclear factor (erythroid-derived 2)-like 2, is considered a master regulator of redox homeostasis and plays a central role in antioxidant and anti-inflammatory defence. It has been largely reported that oxidative stress is implicated in nanoparticle-induced toxicity with the involvement of Nrf2. Several basic methods for Nrf2 evaluation with exposure to nanoparticles are described in this chapter including real-time reverse transcription-polymerase chain reaction (RT-PCR), western blotting, immunofluorescence staining, electrophoretic mobility shift assay, DNase I footprinting, dimethylsulfate footprinting, protein pulse-chase analysis, and tert-butylhydroquinone treatment.

Paraquat and MPTP induce neurodegeneration and alteration in the expression profile of microRNAs: the role of transcription factor Nrf2

Both transcription factors (TFs) and microRNAs (miRNAs) can exert a widespread impact on gene expression. In the present study, we investigated the role of Nrf2 in paraquat-induced intracorporeal neurodegeneration and miRNA expression by exposing Nrf2 wild-type and knockout mice to paraquat (PQ) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Exposure to 10 mg/kg PQ or 30 mg/kg MPTP caused damage to nerve cells in the substantia nigra (SN) in both Nrf2 (+/+) and Nrf2 (−/−) ICR mice, which included cell morphological changes, detectable apoptosis and a significant reduction in the number of dopaminergic (DA) neurons. When mice were exposed to the same PQ dose of 10 mg/kg, significant fewer tyrosine hydroxylase (TH)-positive DA neurons were observed in the Nrf2 (−/−) mice than that in the Nrf2 (+/+) mice. Both Nrf2 deficiency and PQ or MPTP exposure could alter miRNA expression profile in the SN, suggesting the potential involvement of Nrf2 in the PQ-induced or MPTP-induced miRNA expression alteration. The expression of miR-380-3p was altered by the Nrf2-MPTP interaction effect. miR-380-3p/Sp3-mRNA pathway is likely part of the mechanism of MPTP-induced neurodegeneration. Collectively, our results corroborated the protective role of Nrf2 and also demonstrated the essential interaction of Nrf2 with miRNAs in intracorporal neurodegeneration induced by neurotoxicants.

The gene expression regulators Nrf2 and small regulatory RNA molecule miR-380-3p protect nerve cells from damage caused by neurotoxins. Huangyuan Li, at Fujian Medical University, China, and colleagues examined the effects of paraquat and MPTP (1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine), two chemicals known to induce parkinsonism, in mice lacking Nrf2, a key mediator of antioxidant and anti-inflammatory responses. They found that after MPTP or paraquat exposure, the extent of neuronal loss in the substantia nigra was greater in mice lacking Nrf2 than in wild-type controls. Moreover, Nrf2 deficiency prevented an upregulation of miR-380-3p following MPTP exposure. These findings implicate miR-380-3p in the mechanism through which Nrf2 protects the brain from Parkinson’s disease-related cell death and opens new avenues of investigation for developing more effective neuroprotective therapies.

Role of histone acetylation in activation of nuclear factor erythroid 2-related factor 2/heme oxygenase 1 pathway by manganese chloride

Manganese neurotoxicity is characterized by Parkinson-like symptoms with degeneration of dopaminergic neurons in the basal ganglia as the principal pathological feature. Manganese neurotoxicity studies may contribute to a good understanding of the mechanism of Parkinson's disease (PD). In this study, we first confirmed that MnCl₂ can promote the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) protein in the nucleus or cytoplasm while increasing the binding activity of Nrf2 and antioxidant response elements, further promoting the expression of downstream target gene heme oxygenase 1 (HO-1) and leading to increase levels of reactive oxygen species (ROS) and reduce the levels of reduced glutathione (GSH). Second, we investigated the role of histone acetylation in the activation of Nrf2/HO-1 pathway by manganese chloride in rat adrenal pheochromocytoma (PC12) cells. Histone acetyltransferase inhibitor (anacardic acid) and histone deacetylase inhibitor (trichostatin A, TSA) were used as pretreatment reagents to adjust the level of histone acetylation. Here, we show that downregulation of histone acetylation can inhibit Mn-induced Nrf2 nuclear translocation and further inhibits the Mn-activated Nrf2/HO-1 pathway. This downregulation also promotes manganese-induced increase of ROS and decrease of GSH in neurons. These results suggest that the downregulation of histone acetylation may play an important role in the neurotoxicity caused by manganese and that TSA may provide new ideas and targets in treating manganese-induced Parkinson's syndrome and PD.

Nrf2 protects against oxidative stress induced by SiO2 nanoparticles

Aim: The aim of our study was to explore the role of nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2) on the exposure of SiO2 nanoparticles (NPs) and its influence. Materials & methods: To understand the mechanism of NP-induced oxidative stress, the involvement of oxidative-stress-responding transcription factors and the Nrf2/antioxidant reactive element (ARE) signaling pathway in the toxicity of SiO2 NPs’ exposure was investigated via in vivo and in vitro models. Results: A549 cells showed a significant cytotoxic effect while A549-shNrf2 cells showed decreased cell viability after nm-SiO2 exposure. SiO2 NPs’ exposure activated the Nrf2/ARE signaling pathway. Nrf2−/− exposed mice showed increased reactive oxygen species, 8-hydroxyl deoxyguanosine level and decreased total antioxidant capacity. Nrf2/ARE signaling pathway activation disrupted, leading inhibition of heme oxygenase-1 and upregulation of PKR-like endoplasmic-reticulum-regulated kinase. Conclusion: Our findings suggested that Nrf2 could protect against oxidative stress induced by SiO2 NPs, and the Nrf2/ARE pathway might be involved in mild-to-moderate SiO2 NP-induced oxidative stress that was evident from dampened activity of Nrf2.

Dietary flavonoid genistein induces Nrf2 and phase II detoxification gene expression via ERKs and PKC pathways and protects against oxidative stress in Caco-2 cells

SCOPE

Flavonoids have well-known antioxidant, anti-inflammatory, and anti-cancer activities. Isoflavone genistein is considered a potent antioxidant agent against oxidative stress. Although several mechanisms have been proposed, a clear antioxidant mechanism of genistein is still remained to be answered.

METHODS AND RESULTS

In this study, we focused on the concerted effects on expression of Nrf2 and phase II enzyme pathway components. Transient transfection assays, RT-PCR and immunoblot analysis were performed to study its molecular mechanisms of action. In Caco-2 cells, treatment with genistein markedly attenuated H(2)O(2) -induced peroxide formation; this amelioration was reversed by buthionine sulfoximine(GCLC inhibitor) and zinc protoporphyrin(HO-1 inhibitor). Genistein increased HO-1 and GCLC mRNA and protein expression. Genistein treatment activated the ERK1/2 and PKC signaling pathway; therefore increased Nrf2 mRNA and protein expression. The roles of the ERK1/2 and PKC signaling pathway were determined using PD98059 (ERK1/2 inhibitor) and GF109203X (PKC inhibitor) and RNA interference directed against Nrf2. Both inhibitors and siNrf2 abolished genistein-induced HO-1 and GCLC protein expression. These results suggest the involvement of ERK1/2, PKC, and Nrf2 in inducing HO-1 and GCLC by genistein.

CONCLUSION

Our studies show that genistein up-regulated HO-1 and GCLC expression through the EKR1/2 and PKC /Nrf2 pathways during oxidative stress.

Manganese induces p21 expression in PC12 cells at the transcriptional level

Manganese is a common environmental and occupational pollutant. Excessive intake of manganese can cause toxicity known as manganism. Recently it has been demonstrated that unusual expression of cell cycle proteins and aberrant cell cycle progression in the central nervous system are involved in the pathogenesis of neurodegenerative diseases. The present studies were initiated to investigate whether p21 are induced after manganese exposure and its potential effects in vitro, with particular attention being given to understand the underlying regulatory mechanism of p21 induction by manganese in this process. We found that manganese induced DAergic cells injury and upregulation of p21 levels in nigrostriatal regions. Treatment of the PC12 cells with manganese resulted in a time- and concentration-dependent loss of cell viability. Analysis of cell cycle profile indicated that manganese blocked cell cycle progression by arresting the cell cycle at G2/M phase. Moreover, manganese treatment resulted in an increase in the mRNA and protein levels of p21, but did not have the same effect on other related factors. Silencing p21 by RNA interference showed a marked reversal of both G2/M arrest and the decrease in cell viability induced by manganese. Manganese did not stabilize the p21 protein and mRNA, and caused a marked increase in p21 mRNA levels together with an increase in its promoter activity, indicating a transcriptional mechanism. Overall, the in vivo and in vitro data suggest that exposure to manganese can increase p21 levels. An altered cell cycle status of PC12 cells can be induced by manganese through p21 up-regulation, and the induction of p21 occurs at the transcriptional level via promoter activation and mRNA induction.

3',4'-didemethylnobiletin induces phase II detoxification gene expression and modulates PI3K/Akt signaling in PC12 cells

Oxidative stress is considered a major cause of neurodegenerative disorders. In this work, we investigated the cytoprotective effects and mechanisms of the citrus flavonoid nobiletin (NOB) and its metabolite, 3',4'-didemethylnobiletin (3',4'-dihydroxy-5,6,7,8-tetramethoxyflavone; DTF), in PC12 cells. Both NOB and DTF exhibited strong potency in attenuating serum withdrawal- and H(2)O(2)-caused cell death and increased intracellular GSH level via upregulation of both catalytic and modifier subunits of glutamate-cysteine ligase (GCL). However, only DTF suppressed intracellular ROS accumulation in H(2)O(2)-treated cells, induced heme oxygenase-1 (HO-1) expression, and enhanced nuclear factor E2-related factor 2 (Nrf2) binding to the ARE. Nevertheless, DTF-mediated HO-1 upregulation was independent of Nrf2 activation because knockdown of Nrf2 expression by siRNA did not affect its expression. DTF suppressed NF-κB activation, and addition of NF-κB inhibitor, pyrrolidine dithiocarbamate or Bay 11-7082, synergistically enhanced DTF-mediated HO-1 expression, indicating that HO-1 induction is associated with NF-κB suppression. NOB and DTF also activated the ERK, JNK, and Akt pathways in PC12 cells that had undergone serum starvation. Addition of pharmacological kinase inhibitors, U0126, SP600125, and LY294002, caused cytotoxicity and the last significantly attenuated NOB- and DTF-mediated antiapoptotic actions, indicating the involvement of PI3K/Akt signaling in their cytoprotective effects. In conclusion, HO-1 and GCL upregulation and intrinsic ROS-scavenging activity may contribute to DTF-mediated cytoprotection. Furthermore, modulation of PI3K/Akt signaling is involved in channeling the DTF stimulus for cell survival against oxidative insults.

Manganese potentiates LPS-induced heme-oxygenase 1 in microglia but not dopaminergic cells: role in controlling microglial hydrogen peroxide and inflammatory cytokine output

Excessive manganese (Mn) exposure increases output of glial-derived inflammatory products, which may indirectly contribute to the neurotoxic effects of this essential metal. In microglia, Mn increases hydrogen peroxide (H(2)O(2)) release and potentiates lipopolysaccharide (LPS)-induced cytokines (TNF-α, IL-6) and nitric oxide (NO). Inducible heme-oxygenase (HO-1) plays a role in the regulation of inflammation and its expression is upregulated in response to oxidative stressors, including metals and LPS. Because Mn can oxidatively affect neurons both directly and indirectly, we investigated the effect of Mn exposure on the induction of HO-1 in resting and LPS-activated microglia (N9) and dopaminergic neurons (N27). In microglia, 24h exposure to Mn (up to 250 μM) had minimal effects on its own, but it markedly potentiated LPS (100 ng/ml)-induced HO-1 protein and mRNA. Inhibition of microglial HO-1 activity with two different inhibitors indicated that HO-1 is a positive regulator of the Mn-potentiated cytokine output and a negative regulator of the Mn-induced H(2)O(2) output. Mn enhancement of LPS-induced HO-1 does not appear to be dependent on H(2)O(2) or NO, as Mn+LPS-induced H(2)O(2) release was not greater than the increase induced by Mn alone and inhibition of iNOS did not change Mn potentiation of HO-1. However, because Mn exposure potentiated the LPS-induced nuclear expression of small Maf proteins, this may be one mechanism Mn uses to affect the expression of HO-1 in activated microglia. Finally, the potentiating effects of Mn on HO-1 appear to be glia-specific for Mn, LPS, or Mn+LPS did not induce HO-1 in N27 neuronal cells.