Toxicity assessment of zinc oxide nanoparticles using sub-acute and sub-chronic murine inhalation models

Abstract

BackgroundAlthough ZnO nanoparticles (NPs) are used in many commercial products and the potential for human exposure is increasing, few in vivo studies have addressed their possible toxic effects after inhalation. We sought to determine whether ZnO NPs induce pulmonary toxicity in mice following sub-acute or sub-chronic inhalation exposure to realistic exposure doses.MethodsMice (C57Bl/6) were exposed to well-characterized ZnO NPs (3.5 mg/m3, 4 hr/day) for 2 (sub-acute) or 13 (sub-chronic) weeks and necropsied immediately (0 wk) or 3 weeks (3 wks) post exposure. Toxicity was assessed by enumeration of total and differential cells, determination of total protein, lactate dehydrogenase activity and inflammatory cytokines in bronchoalveolar lavage (BAL) fluid as well as measurements of pulmonary mechanics. Generation of reactive oxygen species was assessed in the lungs. Lungs were evaluated for histopathologic changes and Zn content. Zn concentration in blood, liver, kidney, spleen, heart, brain and BAL fluid was measured.ResultsAn elevated concentration of Zn2+ was detected in BAL fluid immediately after exposures, but returned to baseline levels 3 wks post exposure. Dissolution studies showed that ZnO NPs readily dissolved in artificial lysosomal fluid (pH 4.5), but formed aggregates and precipitates in artificial interstitial fluid (pH 7.4). Sub-acute exposure to ZnO NPs caused an increase of macrophages in BAL fluid and a moderate increase in IL-12(p40) and MIP-1α, but no other inflammatory or toxic responses were observed. Following both sub-acute and sub-chronic exposures, pulmonary mechanics were no different than sham-exposed animals.ConclusionsOur ZnO NP inhalation studies showed minimal pulmonary inflammation, cytotoxicity or lung histopathologic changes. An elevated concentration of Zn in the lung and BAL fluid indicates dissolution of ZnO NPs in the respiratory system after inhalation. Exposure concentration, exposure mode and time post exposure played an important role in the toxicity of ZnO NPs. Exposure for 13 wks with a cumulative dose of 10.9 mg/kg yielded increased lung cellularity, but other markers of toxicity did not differ from sham-exposed animals, leading to the conclusion that ZnO NPs have low sub-chronic toxicity by the inhalation route.

Significance of Intratracheal Instillation Tests for the Screening of Pulmonary Toxicity of Nanomaterials

“…Seiffert et al [27] conducted an intratracheal instillation test of silver nanoparticles, and the exposure induced a transient KC concentration in BALF, while the chemokine expression pattern was accompanied by neutrophil influx in the lung. An inhalation exposure of ZnO nanoparticles with low toxicity did not induce neutrophil influx or an increase the concentration of KC in BALF [28].…”

Zinc oxide nanoparticles exhibit cytotoxicity and genotoxicity through oxidative stress responses in human lung fibroblasts and <em>Drosophila melanogaster</em>

“…27 Studies reported that ZnO NPs cause no toxicity to human cells and only low subchronic toxicity to rodents. [28][29][30] Furthermore, a previous study using Drosophila had reported no toxicity observed upon the ingestion of ZnO NPs. 21 On the other hand, another earlier study assessing the genotoxicity and oxidative stress induced by ZnO NPs in Drosophila showed a weak genotoxicity of ZnO NPs.…”

Central nervous system toxicity of metallic nanoparticles

“…Therefore, it is difficult to predict the toxicological behavior of NPs in living organisms by in vitro models alone. 145,146 There are still many unanswered questions concerning nanoneurotoxicity. For instance, after bypassing the BBB, where do NPs go?…”