Manganese treatment modulates the expression of peroxisome proliferator-activated receptors in astrocytoma and neuroblastoma cells

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.

Brain Expression, Physiological Regulation and Role in Motivation and Associative Learning of Peroxisome Proliferator-activated Receptor γ

Like other members of the superfamily of nuclear receptors, the peroxisome proliferator-activated receptor γ (PPARγ), is a ligand-activated transcription factor known for its insulin-sensitizing actions in the periphery. Despite only sparse evidence for PPARγ in the CNS, many reports suggest direct PPARγ-mediated actions in the brain. This study aimed to (i) map PPARγ expression in rodent brain areas, involved in the regulation of cognitive, motivational, and emotional functions, (ii) examine the regulation of central PPARγ by physiological variables (age, sex, obesity); (iii) chemotypically identify PPARγ-expressing cells in the frontal cortex (FC) and hippocampus (HP); (iv) study whether activation of PPARγ by pioglitazone (Pio) in FC and HP cells can induce target gene expression; and (v) demonstrate the impact of activated PPARγ on learning behavior and motivation. Immunoreactive PPARγ was detectable in specific sub-nuclei/subfields of the FC, HP, nucleus accumbens, amygdala, hypothalamus, thalamus, and granular layers of the cerebellum. PPARγ protein levels were upregulated during aging and in high fat diet-induced obesity. PPARγ mRNA expression was upregulated in the amygdala of females (but not males) that were made obese. Neural precursor cells, mature neurons, and astrocytes in primary FC and HP cultures were shown to express PPARγ. Pioglitazone dose-dependently upregulated PPARγ target genes in manner that was specific to the origin (FC or HP) of the cultures. Lastly, administration of Pio impaired motivation and associative learning. Collectively, we provide evidence for the presence of regulatable PPARγ in the brain and demonstrate their participation the regulation of key behaviors.

Trace Elements, PPARs, and Metabolic Syndrome

Metabolic syndrome (MetS) is a constellation of metabolic derangements, including central obesity, insulin resistance, hypertension, glucose intolerance, and dyslipidemia. The pathogenesis of MetS has been intensively studied, and now many factors are recognized to contribute to the development of MetS. Among these, trace elements influence the structure of proteins, enzymes, and complex carbohydrates, and thus an imbalance in trace elements is an independent risk factor for MetS. The molecular link between trace elements and metabolic homeostasis has been established, and peroxisome proliferator-activated receptors (PPARs) have appeared as key regulators bridging these two elements. This is because on one hand, PPARs are actively involved in various metabolic processes, such as abdominal adiposity and insulin sensitivity, and on the other hand, PPARs sensitively respond to changes in trace elements. For example, an iron overload attenuates hepatic mRNA expression of Ppar-α; zinc supplementation is considered to recover the DNA-binding activity of PPAR-α, which is impaired in steatotic mouse liver; selenium administration downregulates mRNA expression of Ppar-γ, thereby improving lipid metabolism and oxidative status in the liver of high-fat diet (HFD)-fed mice. More importantly, PPARs' expression and activity are under the control of the circadian clock and show a robust 24 h rhythmicity, which might be the reasons for the side effects and the clinical limitations of trace elements targeting PPARs. Taken together, understanding the casual relationships among trace elements, PPARs' actions, and the pathogenesis of MetS is of great importance. Further studies are required to explore the chronopharmacological effects of trace elements on the diurnal oscillation of PPARs and the consequent development of MetS.

Manganese supplementation increases adiponectin and lowers ICAM-1 and creatinine blood levels in Zucker type 2 diabetic rats, and downregulates ICAM-1 by upregulating adiponectin multimerization protein (DsbA-L) in endothelial cells

Blood and tissue levels of manganese (Mn) are lower in type 2 diabetic and atherosclerosis patients compared with healthy subjects. Adiponectin has anti-diabetic and anti-atherogenic properties. Impairment in Disulfide bond A-like protein (DsbA-L) is associated with low adiponectin levels and diabetes. This study investigates the hypothesis that the beneficial effects of Mn supplementation are mediated by adiponectin and DsbA-L. At 6 weeks of age, Male Zucker diabetic fatty rats (ZDF) were randomly divided into two groups: diabetic controls and Mn-supplemented diabetic rats. Each rat was supplemented with Mn (D+Mn, 16 mg/kg BW) or water (placebo, D+P) daily for 7 weeks by oral gavage. For cell culture studies, Human Umbilical Vein Endothelial Cells (HUVEC) or 3T3L1 adipocytes were pretreated with Mn (0-10 µM MnCl₂) for 24 h, followed by high glucose (HG, 25 mM) or normal glucose (5 mM) exposure for another 24 h. Mn supplementation resulted in higher adiponectin (p = 0.01), and lower ICAM-1 (p = 0.04) and lower creatinine (p = 0.04) blood levels compared to those in control ZDF rats. Mn-supplemented rats also caused reduced oxidative stress (ROS) and NADPH oxidase, and higher DsbA-L expression in the liver (p = 0.03) of ZDF rats compared to those in livers of control rats; however, Fe levels in liver were lower but not significant (p = 0.08). Similarly, treatment with high glucose (25 mM) caused a decrease in DsbA-L, which was prevented by Mn supplementation in HUVEC and adipocytes. Mechanistic studies with DsbA-L siRNA showed that the beneficial effects of Mn supplementation on ROS, NOX4, and ICAM-1 expression were abolished in DsbA-L knock-down HUVEC. These studies demonstrate that DsbA-L-linked adiponectin mediates the beneficial effects observed with Mn supplementation and provides evidence for a novel mechanism by which Mn supplementation can increase adiponectin and reduce the biomarkers of endothelial dysfunction in diabetes.

PPAR Gamma in Neuroblastoma: The Translational Perspectives of Hypoglycemic Drugs

Neuroblastoma (NB) is the most common and aggressive pediatric cancer, characterized by a remarkable phenotypic diversity and high malignancy. The heterogeneous clinical behavior, ranging from spontaneous remission to fatal metastatic disease, is attributable to NB biology and genetics. Despite major advances in therapies, NB is still associated with a high morbidity and mortality. Thus, novel diagnostic, prognostic, and therapeutic approaches are required, mainly to improve treatment outcomes of high-risk NB patients. Among neuroepithelial cancers, NB is the most studied tumor as far as PPAR ligands are concerned. PPAR ligands are endowed with antitumoral effects, mainly acting on cancer stem cells, and constitute a possible add-on therapy to antiblastic drugs, in particular for NB with unfavourable prognosis. While discussing clinical background, this review will provide a synopsis of the major studies about PPAR expression in NB, focusing on the potential beneficial effects of hypoglycemic drugs, thiazolidinediones and metformin, to reduce the occurrence of relapses as well as tumor regrowth in NB patients.

Partial protection of N-acetylcysteine against MPA-capped CdTe quantum dot-induced neurotoxicity in rat primary cultured hippocampal neurons

CdTe QD exposure caused death and apoptosis of rat primary cultured hippocampal neurons via generating reactive oxygen species and increasing intracellular calcium levels, which could be reversed by a common antioxidant NAC.

Regulation of URG4/URGCP and PPARα gene expressions after retinoic acid treatment in neuroblastoma cells

Neuroblastoma (NB), originating from neural crest cells, is the most common extracranial tumor of childhood. Retinoic acid (RA) which is the biological active form of vitamin A regulates differentiation of NB cells, and RA derivatives have been used for NB treatment. PPARα (peroxisome proliferator-activated receptor) plays an important role in the oxidation of fatty acids, carcinogenesis, and differentiation. URG4/URGCP gene is a proto-oncogene and that overexpression of URG4/URGCP is associated with metastasis and tumor recurrence in osteosarcoma. It has been known that URG4/URGCP gene is an overexpressed gene in hepatocellular carcinoma and gastric cancers. This study aims to detect gene expression patterns of PPARα and URG4/URGCP genes in SH-SY5Y NB cell line after RA treatment. Expressions levels of PPARα and URG4/URGCP genes were analyzed after RA treatment for reducing differentiation in SH-SY5Y NB cell line. To induce differentiation, the cells were treated with 10 μM RA in the dark for 3-10 days. Gene expression of URG4/URGCP and PPARα genes were presented as the yield of polymerase chain reaction (PCR) products from target genes compared with the yield of PCR products from the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene. SH-SY5Y cells possess small processes in an undifferentiated state, and after treatment with RA, the cells developed long neurites, resembling a neuronal phenotype. PPARα gene expression increased in RA-treated groups; URG4/URGCP gene expression decreased in SH-SY5Y cells after RA treatment compared with that in the control cells. NB cell differentiation might associate with PPARα and URG4/URGCP gene expression profile after RA treatment.

Peroxisome proliferator-activated receptor (PPAR)β/δ, a possible nexus of PPARα- and PPARγ-dependent molecular pathways in neurodegenerative diseases: Review and novel hypotheses

Peroxisome proliferator-activated receptors (PPARα, -β/δ and -γ) are lipid-activated transcription factors. Synthetic PPARα and PPARγ ligands have neuroprotective properties. Recently, PPARβ/δ activation emerged as the focus of a novel approach for the treatment of a wide range of neurodegenerative diseases. To fill the gap of knowledge about the role of PPARβ/δ in brain, new hypotheses about PPARβ/δ involvement in neuropathological processes are requested. In this paper, we describe a novel hypothesis, claiming the existence of tight interactions between the three PPAR isotypes, which we designate the "PPAR triad". We propose that PPARβ/δ has a central control of the PPAR triad. The majority of studies analyze the regulation only by one of the PPAR isotypes. A few reports describe the mutual regulation of expression levels of all three PPAR isotypes by PPAR agonists. Analysis of these studies where pairwise interactions of PPARs were described allows us to support the existence of the PPAR triad with central role for PPARβ/δ. In the present review, we propose the hypothesis that in a wide range of brain disorders, PPARβ/δ plays a central role between PPARα and PPARγ. Finally, we prove the advantages of the PPAR triad concept by describing hypotheses of PPARβ/δ involvement in the regulation of myelination, glutamate-induced neurotoxicity, and signaling pathways of reactive oxygen species/NO/Ca(2+).

Manganese supplementation reduces high glucose-induced monocyte adhesion to endothelial cells and endothelial dysfunction in Zucker diabetic fatty rats

Endothelial dysfunction is a hallmark of increased vascular inflammation, dyslipidemia, and the development of atherosclerosis in diabetes. Previous studies have reported lower levels of Mn(2+) in the plasma and lymphocytes of diabetic patients and in the heart and aortic tissue of patients with atherosclerosis. This study examines the hypothesis that Mn(2+) supplementation can reduce the markers/risk factors of endothelial dysfunction in type 2 diabetes. Human umbilical vein endothelial cells (HUVECs) were cultured with or without Mn(2+) supplementation and then exposed to high glucose (HG, 25 mm) to mimic diabetic conditions. Mn(2+) supplementation caused a reduction in monocyte adhesion to HUVECs treated with HG or MCP-1. Mn(2+) also inhibited ROS levels, MCP-1 secretion, and ICAM-1 up-regulation in HUVECs treated with HG. Silencing studies using siRNA against MnSOD showed that similar results were observed in MnSOD knockdown HUVECs following Mn(2+) supplementation, suggesting that the effect of manganese on monocyte adhesion to endothelial cells is mediated by ROS and ICAM-1, but not MnSOD. To validate the relevance of our findings in vivo, Zucker diabetic fatty rats were gavaged daily with water (placebo) or MnCl2 (16 mg/kg of body weight) for 7 weeks. When compared with placebo, Mn(2+)-supplemented rats showed lower blood levels of ICAM-1 (17%, p < 0.04), cholesterol (25%, p < 0.05), and MCP-1 (28%, p = 0.25). These in vitro and in vivo studies demonstrate that Mn(2+) supplementation can down-regulate ICAM-1 expression and ROS independently of MnSOD, leading to a decrease in monocyte adhesion to endothelial cells, and therefore can lower the risk of endothelial dysfunction in diabetes.

Manganese-induced oxidative DNA damage in neuronal SH-SY5Y cells: attenuation of thymine base lesions by glutathione and N-acetylcysteine

Manganese (Mn) is an essential trace element required for normal function and development. However, exposure to this metal at elevated levels may cause manganism, a progressive neurodegenerative disorder with neurological symptoms similar to idiopathic Parkinson's disease (IPD). Elevated body burdens of Mn from exposure to parental nutrition, vapors in mines and smelters and welding fumes have been associated with neurological health concerns. The underlying mechanism of Mn neurotoxicity remains unclear. Accordingly, the present study was designed to investigate the toxic effects of Mn(2+) in human neuroblastoma SH-SY5Y cells. Mn(2+) caused a concentration dependent decrease in SH-SY5Y cellular viability compared to controls. The LD50 value was 12.98 μM Mn(2+) (p<0.001 for control vs. 24h Mn treatment). Both TUNEL and annexin V/propidium iodide (PI) apoptosis assays confirmed the induction of apoptosis in the cells following exposure to Mn(2+) (2 μM, 62 μM or 125 μM). In addition, Mn(2+) induced both the formation and accumulation of DNA single strand breaks (via alkaline comet assay analysis) and oxidatively modified thymine bases (via gas chromatography/mass spectrometry analysis). Pre-incubation of the cells with characteristic antioxidants, either 1mM N-acetylcysteine (NAC) or 1mM glutathione (GSH) reduced the level of DNA strand breaks and the formation of thymine base lesions, suggesting protection against oxidative cellular damage. Our findings indicate that (1) exposure of SH-SY5Y cells to Mn promotes both the formation and accumulation of oxidative DNA damage, (2) SH-SY5Y cells with accumulated DNA damage are more likely to die via an apoptotic pathway and (3) the accumulated levels of DNA damage can be abrogated by the addition of exogenous chemical antioxidants. This is the first known report of Mn(2+)-induction and antioxidant protection of thymine lesions in this SH-SY5Y cell line and contributes new information to the potential use of antioxidants as a therapeutic strategy for protection against Mn(2+)-induced oxidative DNA damage.

Suppression of endogenous PPARγ increases vulnerability to methamphetamine-induced injury in mouse nigrostriatal dopaminergic pathway

RATIONALE

Methamphetamine is a commonly abused drug and dopaminergic neurotoxin. Repeated administration of high doses of methamphetamine induces programmed cell death, suppression of dopamine release, and reduction in locomotor activity. Previous studies have shown that pretreatment with peroxisome proliferator-activated receptor gamma (PPARγ) agonist reduced methamphetamine-induced neurodegeneration.

OBJECTIVES

The purpose of this study was to examine the role of endogenous PPARγ in protecting against methamphetamine toxicity.

METHODS

Adeno-associated virus (AAV) encoding the Cre recombinase gene was unilaterally injected into the left substantia nigra of loxP-PPARγ or control wild-type mice. Animals were treated with high doses of methamphetamine 1 month after viral injection. Behavioral tests were examined using rotarod and rotometer. In vivo voltammetry was used to examine dopamine release/clearance and at 2 months after methamphetamine injection.

RESULTS

Administration of AAV-Cre selectively removed PPARγ in left nigra in loxP-PPARγ mice but not in the wild-type mice. The loxP-PPARγ/AAV-Cre mice that received methamphetamine showed a significant reduction in time on the rotarod and exhibited increased ipsilateral rotation using a rotometer. The peak of dopamine release induced by local application of KCl and the rate of dopamine clearance were significantly attenuated in the left striatum of loxP-PPARγ/AAV-Cre animals. Tyrosine hydroxylase immunoreactivity was reduced in the left, compared to right, nigra, and dorsal striatum in loxP-PPARγ/AAV-Cre mice receiving high doses of methamphetamine.

CONCLUSION

A deficiency in PPARγ increases vulnerability to high doses of methamphetamine. Endogenous PPARγ may play an important role in reducing methamphetamine toxicity in vivo.

Peroxisome Proliferator-Activated Receptors (PPARs) as Potential Inducers of Antineoplastic Effects in CNS Tumors

The peroxisome proliferator-activated receptors (PPARs) are ligand-inducible transcription factors which belong to the superfamily of nuclear hormone receptors. In recent years it turned out that natural as well as synthetic PPAR agonists exhibit profound antineoplastic as well as redifferentiation effects in tumors of the central nervous system (CNS). The molecular understanding of the underlying mechanisms is still emerging, with partially controverse findings reported by a number of studies dealing with the influence of PPARs on treatment of tumor cells in vitro. Remarkably, studies examining the effects of these drugs in vivo are just beginning to emerge. However, the agonists of PPARs, in particular the thiazolidinediones, seem to be promising candidates for new approaches in human CNS tumor therapy.

PPARs in Human Neuroepithelial Tumors: PPAR Ligands as Anticancer Therapies for the Most Common Human Neuroepithelial Tumors

Neuroepithelial tumors represent a heterogeneous class of human tumors including benignant and malignant tumors. The incidence of central nervous system neoplasms ranges from 3.8 to 5.1 cases per 100,000 in the population. Among malignant neuroepithelial tumors, with regard to PPAR ligands, the most extensively studied were tumors of astrocytic origin and neuroblastoma. PPARs are expressed in developing and adult neuroepithelial cells, even if with different localization and relative abundance. The majority of malignant neuroepithelial tumors have poor prognosis and do not respond to conventional therapeutic protocols, therefore, new therapeutic approaches are needed. Natural and synthetic PPAR ligands may represent a starting point for the formulation of new therapeutic approaches to be used as coadjuvants to the standard therapeutic protocols. This review will focus on the major studies dealing with PPAR expression in gliomas and neuroblastoma and the therapeutic implications of using PPAR agonists for the treatment of these neoplasms.

Genotoxic and cytotoxic effects of manganese chloride in cultured human lymphocytes treated in different phases of cell cycle

Manganese (Mn) has a natural occurrence and is necessary during the initial periods of the development. However, in high concentrations, Mn can be related to neurodegenerative disorders. The aim of the present study was to evaluate the mutagenic potential of manganese chloride (MnCl2.4H2O). Comet assay and chromosome aberrations analysis were applied to determine the DNA-damaging and clastogenic effects of MnCl2.4H2O. Cultured human lymphocytes were treated with 15, 20 and 25 microM manganese chloride during the G1, G1/S, S (pulses of 1 and 6h), and G2 phases of the cell cycle. All tested concentrations were cytotoxic and reduced significantly the mitotic index in G1, G1/S and S (1 and 6h) treatments, while in G2 treatment only the higher concentrations (20 and 25 microM) showed cytotoxic effects. Clastogenicity and DNA damage were found only in treatments with the highest concentration (25 microM). Chromosome aberrations were found exclusively in the G2 phase of the cell cycle. The absence of polyploidy in mitosis, suggests that manganese does not affect the formation of the mitotic spindle with the concentrations tested. The genotoxicity found in G2 phase and in the comet assay can be related to the short time of treatment in both cases.

Metabolic and antioxidant system alterations in an astrocytoma cell line challenged with mitochondrial DNA deletion

Oxidative stress can induce mitochondrial dysfunction, mitochondrial DNA (mtDNA) depletion, and neurodegeneration, although the underlying mechanisms are poorly understood. The major mitochondrial antioxidant system that protects cells consists of manganese superoxide dismutase (MnSOD), glutathione peroxidase (GPx) and glutathione (GSH). To investigate the putative adaptive changes in antioxidant enzyme protein expression and targeting to mitochondria as mtDNA depletion occurs, we progressively depleted U87 astrocytoma cells of mtDNA by chronic treatment with ethidium bromide (EB, 50 ng/ml). Cellular MnSOD protein expression was markedly increased in a time-related manner while that of GPx showed time-related decreases. The mtDNA depletion also altered targeting or subcellular distribution of GPx, suggesting the importance of intact mtDNA in mitochondrial genome-nuclear genome signaling/communication. Cellular NADP(+)-ICDH activity also showed marked, time-related increases while their GSH content decreased. Thus, our findings suggest that interventions to elevate MnSOD, GPx, NADP(+)-ICDH, and GSH levels may protect brain cells from oxidative stress.