From the Cover: Arsenite Uncouples Mitochondrial Respiration and Induces a Warburg-like Effect inCaenorhabditis elegans

Abstract

Millions of people worldwide are chronically exposed to arsenic through contaminated drinking water. Despite decades of research studying the carcinogenic potential of arsenic, the mechanisms by which arsenic causes cancer and other diseases remain poorly understood. Mitochondria appear to be an important target of arsenic toxicity. The trivalent arsenical, arsenite, can induce mitochondrial reactive oxygen species production, inhibit enzymes involved in energy metabolism, and induce aerobic glycolysis in vitro, suggesting that metabolic dysfunction may be important in arsenic-induced disease. Here, using the model organism Caenorhabditis elegans and a novel metabolic inhibition assay, we report an in vivo induction of aerobic glycolysis following arsenite exposure. Furthermore, arsenite exposure induced severe mitochondrial dysfunction, including altered pyruvate metabolism; reduced steady-state ATP levels, ATP-linked respiration and spare respiratory capacity; and increased proton leak. We also found evidence that induction of autophagy is an important protective response to arsenite exposure. Because these results demonstrate that mitochondria are an important in vivo target of arsenite toxicity, we hypothesized that deficiencies in mitochondrial electron transport chain genes, which cause mitochondrial disease in humans, would sensitize nematodes to arsenite. In agreement with this, nematodes deficient in electron transport chain complexes I, II, and III, but not ATP synthase, were sensitive to arsenite exposure, thus identifying a novel class of gene-environment interactions that warrant further investigation in the human populace.

Swimming Exercise and Transient Food Deprivation in Caenorhabditis elegans Promote Mitochondrial Maintenance and Protect Against Chemical-Induced Mitotoxicity

“…5b , individual traces in Supplementary Fig. S3 ) 27 , but tFD and exercise groups were significantly protected (see also Table 1 ), again suggesting long-term benefits of limited intermittent food restriction aspect of our protocol. Overall our data reveal that exercise confers resistance to arsenite exposure (consistent with healthier mitochondria), but that specific benefits are more readily apparent 24 hours post training.…”

“…We focused our assessment of muscle mitochondrial morphology by imaging in the tail region, which features minimal interference from gut autofluorescence and in which mitochondrial morphology may decline faster than the anterior region as the animals age (unpublished observations). Decline in muscle mitochondria occurs during mammalian and C. elegans aging 24 , 25 , and in the presence of some mitochondrial toxicants 26 , 27 . This decline is characterized by mitochondrial networks becoming less organized, more sparse (fewer mitochondria) and by accumulation of large dysfunctional mitochondria 24 , 28 .…”

“…Both age and relative inactivity may compromise mitochondrial health, limiting the robustness of the organismal response to environmental toxins. For example, although the mechanism of action of arsenite is complex, it involves significant mitochondrial impairment 27 , 31 , 32 . Therefore, we tested whether the improved mitochondrial health of exercised animals protected them from lethality after a 24-hour exposure of arsenite.…”

“…Arsenic is a widespread drinking water contaminant to which millions of people worldwide are exposed every day at levels exceeding World Health Organization guidelines 46 . The mechanisms of arsenic toxicity are complex and not fully understood; however, mitochondria are an important target for arsenic toxicity, especially for the trivalent form (arsenite) used in this study 27 , 31 , 32 . The protection of exercised nematodes, with or without food, from arsenite toxicity demonstrated a dramatic and sustained protection compared to non-exercised controls.…”

The Role of Reactive Oxygen Species in Arsenic Toxicity

“…Chronic arsenite (75 ppb) exposure was shown to induce aerobic glycolysis while inhibiting mitochondrial oxidative phosphorylation in primary human cells and multiple cell lines (BEAS-2B, human prostate epithelial cell line (RWPE-1), human pulmonary epithelial carcinoma cell line (A549), primary human urothelial cells (HUC), and human dermal fibroblasts (HDF)) [182]. A similar phenomenon was also observed in Caenorhabditis elegans following 48 h arsenite exposure (50 to 500 µM) [183]. When human hepatocyte cells (HL-7702) were treated with different concentrations of arsenite (1 to 5 µM, 12 h), overproduction of ROS resulted from activated nicotine adenine disphosphonucleotide (NADPH) oxidase-mitochondria axis inactivated prolyl hydroxylases (PHDs), which led to protein accumulation of hIF-1α [184].…”