From the Cover: Arsenic Induces Accumulation of α-Synuclein: Implications for Synucleinopathies and Neurodegeneration.

Synucleinopathies, including Parkinson's disease (PD), are neurodegenerative diseases characterized by accumulation of α-synuclein (SYN), a small neuronal protein with prion like properties that plays a central role in PD pathogenesis. SYN can misfold and generate toxic oligomers/aggregates, which can be cytotoxic. Environmental arsenic (As)-containing pesticide use correlates with increased incidence of PD. Moreover, because As exposure can lead to inhibition of autophagic flux we hypothesize that As can facilitate the accumulation of toxic SYN oligomers/aggregates and subsequent increases in markers of autophagy. We therefore examined the role of As in the oligomerization of SYN, and the consequences thereof. Chronic exposure of SH-SY5Y cells overexpressing SYN to As caused a dose-dependent oligomerization of SYN, with concomitant increases in protein ubiquitination and expression of other stress markers (protein glutathione binding, γ-GCS, light chain 3 (LC3)-I/II, P62, and NAD(P)H dehydrogenase quinone 1), indicative of an increased proteotoxic stress. Immunocytochemical analyses revealed an accumulation of SYN, and it's colocalization with LC3, a major autophagic protein. Mice exposed to As (100 ppb) for 1 month, exhibited elevated SYN accumulation in the cortex and striatum, and elevations in protein ubiquitination and LC3-I and II levels. However, tyrosine hydroxylase (TH), an indicator of dopaminergic cell density, was upregulated in the As exposed animals. Because SYN can inhibit TH function, and As can decrease monoamine levels, As exposure possibly leads to compensatory mechanisms leading to an increase in TH expression. Our findings suggest that susceptible individuals may be at higher risk of developing synucleinopathies and/or neurodegeneration due to environmental As exposure.

Altered Hepatic Transport by Fetal Arsenite Exposure in Diet-Induced Fatty Liver Disease.

Non-alcoholic fatty liver disease can result in changes to drug metabolism and disposition potentiating adverse drug reactions. Furthermore, arsenite exposure during development compounds the severity of diet-induced fatty liver disease. This study examines the effects of arsenite potentiated diet-induced fatty liver disease on hepatic transport in male mice. Changes were detected for Mrp2/3/4 hepatic transporter gene expression as well as for Oatp1a4/2b1/1b2. Plasma concentrations of Mrp and Oatp substrates were increased in arsenic exposure groups compared with diet-only controls. In addition, murine embryonic hepatocytes and adult primary hepatocytes show significantly altered transporter expression after exposure to arsenite alone: a previously unreported phenomenon. These data indicate that developmental exposure to arsenite leads to changes in hepatic transport which could increase the risk for ADRs during fatty liver disease.

Methylation of inorganic arsenic by murine fetal tissue explants.

Although it is generally believed that the developing fetus is principally exposed to inorganic arsenic and the methylated metabolites from the maternal metabolism of arsenic, little is known about whether the developing embryo can autonomously metabolize arsenic. This study investigates inorganic arsenic methylation by murine embryonic organ cultures of the heart, lung, and liver. mRNA for AS3mt, the gene responsible for methylation of arsenic, was detected in all embryonic tissue types studied. In addition, methylated arsenic metabolites were generated by all three tissue types. The fetal liver explants yielded the most methylated arsenic metabolites (∼7% of total arsenic/48 h incubation) while the heart, and lung preparations produced slightly greater than 2% methylated metabolites. With all tissues the methylation proceeded mostly to the dimethylated arsenic species. This has profound implications for understanding arsenic-induced fetal toxicity, particularly if the methylated metabolites are produced autonomously by embryonic tissues.

Effects of Arsenite Exposure during Fetal Development on Energy Metabolism and Susceptibility to Diet-Induced Fatty Liver Disease in Male Mice.

BACKGROUND: Chronic exposure to arsenicals at various life stages and across a range of exposures has been implicated in cardiometabolic and liver disease, but disease predisposition from developmental exposures remains unclear. OBJECTIVES: In utero and post-weaning exposure to trivalent arsenic (AsIII) was examined on the background of a Western-style diet to determine whether AsIII exposure affects metabolic disease. METHODS: Male Swiss Webster mice were exposed to 100 ppb AsIII in utero, after weaning, or both. Ad libitum access to a Western-style diet was provided after weaning, and the plasma metabolome, liver histopathology, liver enzyme activity, and gene expression were analyzed. RESULTS: Hepatic lipid composition and histopathology revealed that developmental AsIII exposure exacerbated Western-style diet-induced fatty liver disease. Continuous AsIII exposure increased cardiometabolic risk factors including increased body weight, insulin resistance, hyperglycemia, and plasma triglycerides. AsIII exposure produced a decrease in the intermediates of glycolysis and the TCA cycle while increasing ketones. Hepatic isocitrate dehydrogenase activity was also decreased, which confirmed disruption of the TCA cycle. Developmental AsIII exposure increased the expression of genes involved in fatty acid synthesis, lipogenesis, inflammation, and packaging of triglycerides, suggesting an increased acetyl coenzyme A (acetyl-CoA) load. CONCLUSIONS: In utero and continuous early-life exposure to AsIII disrupted normal metabolism and elevated the risk for fatty liver disease in mice maintained on a high-fat diet. Our findings suggest that individuals exposed to AsIII during key developmental periods and who remain exposed to AsIII on the background of a Western-style diet may be at increased risk for metabolic disease later in life.

Arsenite Disrupts Zinc-Dependent TGFß2-SMAD Activity During Murine Cardiac Progenitor Cell Differentiation.

TGFß2 (transforming growth factor-ß2) is a key growth factor regulating epithelial to mesenchymal transition (EMT). TGFß2 triggers cardiac progenitor cells to differentiate into mesenchymal cells and give rise to the cellular components of coronary vessels as well as cells of aortic and pulmonary valves. TGFß signaling is dependent on a dynamic on and off switch in Smad activity. Arsenite exposure of 1.34 µM for 24-48 h has been reported to disrupt Smad phosphorylation leading to deficits in TGFß2-mediated cardiac precursor differentiation and transformation. In this study, the molecular mechanism of acute arsenite toxicity on TGFß2-induced Smad2/3 nuclear shuttling and TGFß2-mediated cardiac EMT was investigated. A 4-h exposure to 5 µM arsenite blocks nuclear accumulation of Smad2/3 in response to TGFß2 without disrupting Smad phosphorylation or nuclear importation. The depletion of nuclear Smad is restored by knocking-down Smad-specific exportins, suggesting that arsenite augments Smad2/3 nuclear exportation. The blockage in TGFß2-Smad signaling is likely due to the loss of Zn(2+) cofactor in Smad proteins, as Zn(2+) supplementation reverses the disruption in Smad2/3 nuclear translocation and transcriptional activity by arsenite. This coincides with Zn(2+) supplementation rescuing arsenite-mediated deficits in cardiac EMT. Thus, zinc partially protects cardiac EMT from developmental toxicity by arsenite.