Heavy metal and pesticide exposure: A mixture of potential toxicity and carcinogenicity

Environmental pollutants are recognized as one of the major concerns for public health and responsible for various forms of neurological disorders. Some of the common sources of environmental pollutants related to neurotoxic manifestations are industrial waste, pesticides, automobile exhaust, laboratory waste, and burning of terrestrial waste. Among various environmental pollutants, particulate matter, ultrafine particulate matter, nanoparticles, and lipophilic vaporized toxicant (acrolein) easily cross the blood–brain barrier, activate innate immune responses in the astrocytes, microglia, and neurons, and exert neurotoxicity. Growing shreds of evidence from human epidemiological studies have correlated the environmental pollutants with neuroinflammation, oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction, myelin sheath disruption, and alterations in the blood–brain barrier anatomy leading to cognitive dysfunction and poor quality of life. These environmental pollutants also considerably cause developmental neurotoxicity, exhibit teratogenic effect and mental growth retardance, and reduce IQ level. Until now, the exact mechanism of pollutant-induced neurotoxicity is not known, but studies have shown interference of pollutants with the endogenous antioxidant defense system, inflammatory pathway (Nrf2/NF-kB, MAPKs/PI3K, and Akt/GSK3β), modulation of neurotransmitters, and reduction in long-term potentiation. In the current review, various sources of pollutants and exposure to the human population, developmental neurotoxicity, and molecular mechanism of different pollutants involved in the pathogenesis of different neurological disorders have been discussed.

Environmental neurotoxic pollutants: review

Recent advancements in rapid analysis of pesticides using nano biosensors: A present and future perspective

While a number of genetic mutations are associated with Parkinson's disease (PD), it is also widely acknowledged that the environment plays a significant role in the etiology of neurodegenerative diseases. Epidemiological evidence suggests that occupational exposure to pesticides (e.g., dieldrin, paraquat, rotenone, maneb, and ziram) is associated with a higher risk of developing PD in susceptible populations. Within dopaminergic neurons, environmental chemicals can have an array of adverse effects resulting in cell death, such as aberrant redox cycling and oxidative damage, mitochondrial dysfunction, unfolded protein response, ubiquitin-proteome system dysfunction, neuroinflammation, and metabolic disruption. More recently, our understanding of how pesticides affect cells of the central nervous system has been strengthened by computational biology. New insight has been gained about transcriptional and proteomic networks, and the metabolic pathways perturbed by pesticides. These networks and cell signaling pathways constitute potential therapeutic targets for intervention to slow or mitigate neurodegenerative diseases. Here we review the epidemiological evidence that supports a role for specific pesticides in the etiology of PD and identify molecular profiles amongst these pesticides that may contribute to the disease. Using the Comparative Toxicogenomics Database, these transcripts were compared to those regulated by the PD-associated neurotoxicant MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). While many transcripts are already established as those related to PD (alpha-synuclein, caspases, leucine rich repeat kinase 2, and parkin2), lesser studied targets have emerged as "pesticide/PD-associated transcripts" [e.g., phosphatidylinositol glycan anchor biosynthesis class C (Pigc), allograft inflammatory factor 1 (Aif1), TIMP metallopeptidase inhibitor 3, and DNA damage inducible transcript 4]. We also compared pesticide-regulated genes to a recent meta-analysis of genome-wide association studies in PD which revealed new genetic mutant alleles; the pesticides under review regulated the expression of many of these genes (e.g., ELOVL fatty acid elongase 7, ATPase H+ transporting V0 subunit a1, and bridging integrator 3). The significance is that these proteins may contribute to pesticide-related increases in PD risk. This review collates information on transcriptome responses to PD-associated pesticides to develop a mechanistic framework for quantifying PD risk with exposures.

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Elucidating Conserved Transcriptional Networks Underlying Pesticide Exposure and Parkinson's Disease: A Focus on Chemicals of Epidemiological Relevance.

Endocrine disruption (ED) and endocrine disruptors (EDs) emerged as scientific concepts in 1995, after numerous chemical pollutants were found to be responsible for reproductive dysfunction. The World Health Organization established in the United Nations Environment Programme a list of materials, plasticizers, pesticides, and various pollutants synthesized from petrochemistry that impact not only reproduction, but also hormonal functions, directly or indirectly. Cells communicate via either chemical or electrical signals transmitted within the endocrine or nervous systems. To investigate whether hormone disruptors may also interfere directly or indirectly with the development or functioning of the nervous system through either a neuroendocrine or a more general mechanism, we examined the scientific literature to ascertain the effects of EDs on the nervous system, specifically in the categories of neurotoxicity, cognition, and behaviour. To date, we demonstrated that all of the 177 EDs identified internationally by WHO are known to have an impact on the nervous system. Furthermore, the precise mechanisms underlying this neurodisruption have also been established. It was previously believed that EDs primarily function via the thyroid. However, this study presents substantial evidence that approximately 80 % of EDs operate via other mechanisms. It thus outlines a novel concept: EDs are also neurodisruptors (NDs) and can be collectively termed endocrine and nervous disruptors (ENDs). Most of ENDs are derived from petroleum residues, and their various mechanisms of action are similar to those of "spam" in electronic communications technologies. Therefore, ENDs can be considered as an instance of spam in a biological context.

Endocrine disruptors also function as nervous disruptors and can be renamed endocrine and nervous disruptors (ENDs).

Mitochondria are sensitive targets of environmental chemicals. Dieldrin (DLD) is an organochlorine pesticide that remains a human health concern due to high lipid bioaccumulation, and it has been epidemiologically associated to an increased risk for Parkinson's disease (PD). As mitochondrial dysfunction is involved in the etiology of PD, this study aimed to determine whether DLD impaired mitochondrial bioenergetics in dopaminergic cells. Rat immortalized dopaminergic N27 cells were treated for 24 or 48h with one dose of either a solvent control, 2.5, 25, or 250μM DLD. Dopaminergic cells treated with 250μM DLD showed increased Casp3/7 activity at 24 and 48h. DLD also caused a dose dependent reduction in cell viability of ∼25-30% over 24h. No significant effects on cell viability, apoptosis, nor cytotoxicity were detected at 24 or 48h with 2.5μM DLD. Following a 24h exposure to 2.5 and 25μM DLD, viable cells were subjected to a mitochondrial stress test using the Seahorse XFe24 Extracellular Flux Analyzer. Following three independent experiments conducted for rigor, dopaminergic cells that were treated with 2.5 and 25μM DLD consistently showed a reduction in maximum respiration and spare capacity compared to the control group. Molecular responses were measured to determine mechanisms of DLD-induced mitochondrial dysfunction. There were no changes in transcripts associated with mitochondrial membrane potential and permeability (e.g. Ant, Hk1, Tspo, Vdac), nor PI3 K/Akt/mTor signaling or mitochondrial-associated apoptotic factors (Bax, Bcl2, Casp3). However, transcript levels for Chop/Gadd153 (DNA Damage Inducible Transcript 3), an apoptotic gene activated following endoplasmic reticulum (ER) stress, were 3-fold higher in N27 cells treated with DLD, suggesting that DLD-induced mitochondrial dysfunction is related to ER stress. Dopamine cells were also assessed for changes in tyrosine hydroxylase (TH) protein, which did not differ among treatments. This study demonstrates that DLD impairs oxidative respiration in dopamine cells, and ER stress is hypothesized to be associated with the DLD-induced mitochondrial dysfunction. This is important as ER stress is also linked to PD. This study presents mechanistic insight into pesticide-induced mitochondrial dysfunction using a chemical that is reported to be associated to a higher risk for neurodegenerative disease.

Dieldrin-induced neurotoxicity involves impaired mitochondrial bioenergetics and an endoplasmic reticulum stress response in rat dopaminergic cells

The phenylpyrazole fipronil is an insecticide that inhibits gamma-amino-butyric acid (GABA) ionotropic receptors in the central nervous system. Experimental evidence suggests that fipronil acts as a neurotoxin and it is implicated in neurodegenerative diseases; however, the mechanisms of neurotoxicity are not fully elucidated. The objective of this study was to quantify mechanisms of fipronil-induced neurotoxicity in dopamine cells. Rat primary immortalized mesencephalic dopaminergic cells (N27) were treated with fipronil (0.25 up to 500 mu M depending on the assay). We measured endpoints related to mitochondrial bioenergetics, mitophagy, mitochondrial membrane potential, and ATP production in addition to discerning transcriptome responses to the pesticide. Fipronil reduced cell viability at 500 mu M after 24 h exposure and caspase 3/7 activity was significant increased after 6 and 12 h by 250 and 500 mu M fipronil. Subsequent endpoints were thus assessed at concentrations that were below cytotoxicity. We measured oxidative respiration of N27 cells following a 24 h exposure to one dose of either 0.25, 2.5, 25, or 50 mu M fipronil. Oxygen consumption rates (OCR) were not different between vehicle-control and 0.25 or 2.5 mu M fipronil treatments, but there was a similar to 40-60 % reduction in basal respiration, as well as reduced oligomycin-induced ATP production at 50 mu M. The reduction in OCR is hypothesized to be related to lower mitochondrial mass due to mitophagy. Mitochondrial membrane potential was also sensitive to fipronil, and it was compromised at concentrations of 2.5 mu M and above. To further elucidate the mechanisms linked to neurotoxicity, we conducted transcriptomics in dopamine cells following treatment with 25 mu M fipronil. Fipronil suppressed transcriptional networks associated with mitochondria (damage, depolarization, permeability, and fission), consistent with its effects on mitochondrial membrane potential. Altered gene networks also included those related to Alzheimer disease, inflammatory disease, nerve fiber degeneration, and neurofibrillary tangles. This study clarifies molecular targets of fipronil-induced neurotoxicity and supports, through multiple lines of evidence, that fipronil acts as a mitochondrial toxicant in dopamine cells. This is relevant to neurodegenerative diseases like Parkinson's disease as exposure to fipronil is associated with the progressive loss of nigrostriatal dopaminergic neurons in rodents.

Mitochondrial and transcriptome responses in rat dopaminergic neuronal cells following exposure to the insecticide fipronil.

The role of ketones in metabolic health has progressed over the past two decades, moving from what was perceived as a simple byproduct of fatty acid oxidation to a central player in a multiplicity of disease states. Previous work with hyperpolarized (HP) 13C has shown that ketone production can be detected when using precursors that labeled acetyl-CoA at the C1 position, often in tissues that are not normally recognized as ketogenic. Here, we assay metabolism of HP [2-13C]pyruvate in the perfused mouse liver, a classic metabolic testbed where nutritional conditions can be precisely controlled. Livers perfused with long-chain fatty acids or the medium-chain fatty acid octanoate showed no evidence of ketogenesis in the 13C spectrum. In contrast, addition of dichloroacetate, a potent inhibitor of pyruvate dehydrogenase kinase, resulted in significant production of both acetoacetate and 3-hydroxybutyrate from the pyruvate precursor. This result indicates that ketones are readily produced from carbohydrates, but only in the case where pyruvate dehydrogenase activity is upregulated.

/pdf/detecting-de-novo-hepatic-ketogenesis-using-hyperpolarized-2-310kdhox.pdf

Detecting de novo Hepatic Ketogenesis Using Hyperpolarized [2-13C] Pyruvate

The pancreas functions as both an endocrine and exocrine gland. The endocrine component consists of the islets of Langerhans, whereas the exocrine tissue produces digestive enzymes through the ductal system. Although the majority of Type I Diabetes research has focused on the islets, the exocrine tissue is likely implicated in T1D as well. This research utilizes a novel method to investigate exocrine pancreas metabolism with a pancreas perfusion system. I hypothesized that dissolution dynamic nuclear polarization (dDNP) combined with the pancreas perfusion system would provide real-time measurements of central carbon metabolic turnover in the exocrine pancreas. DNP is a phenomenon in which a transfer of high spin polarization greatly enhances the signal in the resulting nuclear magnetic resonance (NMR) spectra. This research compares dDNP measurements in the exocrine pancreas in C57BL/6 male mice under varying glucose conditions. It also examines central carbon metabolism using this system in the T1D mouse model NOD.Rag1-/-.AI4α/β and the NOD.Rag1-/- control. After cannulation of the common bile duct, the exocrine pancreas was perfused for 30 minutes with perfusate containing either 3mM or 16.7mM glucose. Tissue viability was confirmed throughout the perfusion with oxygen consumption measurements. dDNP was performed using the trityl radical and 4mM [1-13C]pyruvate. NMR spectra were collected on a 600MHz instrument with a 10mm [13C]-optimized cryoprobe and analyzed with Bruker TopSpin. Several metabolites were identified and quantified in all measured spectra, demonstrating the detection of real-time metabolic turnover. Ultimately, this work demonstrates that dissolution dynamic nuclear polarization can be used to evaluate metabolic flux in the pancreas in multiple mouse models. Future will investigate the real-time metabolic response of the exocrine pancreas to insulin concentrations in the T1D disease model.
 
 
 A.Rushin: None. M.Mcleod: None. M.Ragavan: None. M.Merritt: None.
 
 
 
 National Institutes of Health (T32 DK10876) National Science Foundation (DMR-1644779)


1366-P: Examining Central Carbon Metabolic Flux in the Perfused Exocrine Pancreas Using Dissolution Dynamic Nuclear Polarization

Insulin resistance is a major contributor to nonalcoholic fatty liver disease (NAFLD) . NAFLD is a progressive disease that leads to impaired hepatic function and liver cirrhosis. Increased ketogenesis has been correlated with a protective phenotype in mice and humans. During our study we identified differences in acetyl-CoA partitioning between the oxidative tricarboxylic acid cycle (TCA) , and ketogenesis. 8 week old C57Bl6 mice were fed a high fat or low fat (60% or 10% fat by calories respectively) diet for 16 weeks before performing liver perfusion in 12-14 hours fasted mice with 1 mM [D15]-octanoate fatty acid tracer. The effluent perfusate was collected and GC-MS analysis was performed to identify the distribution of deuterium label. Enrichments were greater in ketones (˜70%) than the TCA cycle (˜20%) with the unexpected determination that terminal acetyl-CoA units derived from fatty acids were preferentially used for ketogenesis as compared to internal acetyl-CoA units (Figure 1) . These changes were confirmed with fractional enrichment analysis and metabolic modeling of the ketogenic pathway and TCA cycle. This finding elucidates differential handling of acetyl-CoA that could be exploited for therapeutic benefit to NAFLD.
 
 
 
 M.Mcleod: None. M.Ragavan: None. A.Rushin: None. M.Merritt: None. A.G.Giacalone: None.


Anna Rushin

Papers

Dieldrin-induced neurotoxicity involves impaired mitochondrial bioenergetics and an endoplasmic reticulum stress response in rat dopaminergic cells

Mitochondrial and transcriptome responses in rat dopaminergic neuronal cells following exposure to the insecticide fipronil.

Detecting de novo Hepatic Ketogenesis Using Hyperpolarized [2-13C] Pyruvate

1366-P: Examining Central Carbon Metabolic Flux in the Perfused Exocrine Pancreas Using Dissolution Dynamic Nuclear Polarization

1350-P: Elucidation of Terminal Fatty Acid–Derived Acetyl-CoA Preference for Ketogenic Flux