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Arsenic exposure increases the risk of atherosclerosis, the gradual occlusion of the large arteries with fibro-fatty plaque. While epidemiologic data provide convincing evidence this is true at higher exposures, it is unclear whether this may occur at low arsenic exposures, near the maximum contaminant level of 10 ppb. We have previously shown that 200 ppb arsenite in the drinking water increased the atherosclerosis in apolipoprotein E knock-out (apoE-/-) mice after 13 weeks, but the effects of lower concentrations were unknown. Therefore, here, we analyzed the effects of oral exposure to arsenite from 10 to 200 ppb after 13 weeks. Importantly, we found that even at the lowest concentration of arsenite, there was a significant increase in atherosclerotic plaque size. In our previous studies, we found that arsenite exposure resulted in decreased smooth muscle cells (SMCs) and collagen within the plaque. This change is indicative of a less stable phenotype that could increase the risk of rupture and subsequently, myocardial infarct or stroke in humans. In addition, we observed that lipid increased within the plaque without concomitant increase in macrophage content, suggesting that the macrophages were retaining more lipid intracellularly. We also assessed these plaque components in apoE-/- mice exposed to 10-200 ppb arsenite. Interestingly, we observed that macrophage lipid accumulation occurred at lower concentrations than the decreased SMC/collagen content. Together these data suggest that in the apoE-/- model, low arsenite concentrations are pro-atherogenic and that macrophage lipid homeostasis is more sensitive to arsenite-induced perturbation than the SMCs.

Arsenic exposure has been linked to an increased incidence of atherosclerosis. Previously, we have shown in vitro and in vivo that arsenic inhibits transcriptional activation of the liver X receptors (LXRs), key regulators of lipid homeostasis. Therefore, we evaluated the role of LXRα in arsenic-induced atherosclerosis using the apoE(-/-) mouse model. Indeed, deletion of LXRα protected apoE(-/-) mice against the proatherogenic effects of arsenic. We have previously shown that arsenic changes the plaque composition in apoE(-/-) mice. Arsenic decreased collagen content in the apoE(-/-) model, and we have observed the same diminution in LXRα(-/-)apoE(-/-) mice. However, the collagen-producing smooth muscle cells (SMCs) were decreased in apoE(-/-), but increased in LXRα(-/-)apoE(-/-). Although transcriptional activation of collagen remained the same in SMC from both genotypes, arsenic-exposed LXRα(-/-)apoE(-/-) plaques had increased matrix metalloproteinase activity compared with both control LXRα(-/-)apoE(-/-) and apoE(-/-), which could be responsible for both the decrease in plaque collagen and the SMC invasion. In addition, arsenic increased plaque lipid accumulation in both genotypes. However, macrophages, the cells known to retain lipid within the plaque, were unchanged in arsenic-exposed apoE(-/-) mice, but decreased in LXRα(-/-)apoE(-/-). We confirmed in vitro that these cells retained more lipid following arsenic exposure and are more sensitive to apoptosis than apoE(-/-). Mice lacking LXRα are resistant to arsenic-enhanced atherosclerosis, but arsenic-exposed LXRα(-/-)apoE(-/-) mice still present a different plaque composition pattern than the arsenic-exposed apoE(-/-) mice., (© The Author 2014. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

Arsenic is a widespread environmental contaminant to which millions of people are exposed worldwide. Exposure to arsenic is epidemiologically linked to increased cardiovascular disease, such as atherosclerosis. However, the effects of moderate concentrations of arsenic on atherosclerosis formation are unknown. Therefore, we utilized an in vivo ApoE(-/-) mouse model to assess the effects of chronic moderate exposure to arsenic on plaque formation and composition in order to facilitate mechanistic investigations. Mice exposed to 200 ppb arsenic developed atherosclerotic lesions, a lower exposure than previously reported. In addition, arsenic modified the plaque content, rendering them potentially less stable and consequently, potentially more dangerous. Moreover, we observed that the lower exposure concentration was more atherogenic than the higher concentration. Arsenic-enhanced lesions correlated with several proatherogenic molecular changes, including decreased liver X receptor (LXR) target gene expression and increased proinflammatory cytokines. Significantly, our observations suggest that chronic moderate arsenic exposure may be a greater cardiovascular health risk than previously anticipated.