- Open Access
Progressive severe lung injury by zinc oxide nanoparticles; the role of Zn2+ dissolution inside lysosomes
© Cho et al; licensee BioMed Central Ltd. 2011
- Received: 13 April 2011
- Accepted: 6 September 2011
- Published: 6 September 2011
Large production volumes of zinc oxide nanoparticles (ZnONP) might be anticipated to pose risks, of accidental inhalation in occupational and even in consumer settings. Herein, we further investigated the pathological changes induced by ZnONP and their possible mechanism of action.
Two doses of ZnONP (50 and 150 cm2/rat) were intratracheally instilled into the lungs of rats with assessments made at 24 h, 1 wk, and 4 wks after instillation to evaluate dose- and time-course responses. Assessments included bronchoalveolar lavage (BAL) fluid analysis, histological analysis, transmission electron microscopy, and IgE and IgA measurement in the serum and BAL fluid. To evaluate the mechanism, alternative ZnONP, ZnONP-free bronchoalveolar lavage exudate, and dissolved Zn2+ (92.5 μg/rat) were also instilled to rats. Acridine orange staining was utilized in macrophages in culture to evaluate the lysosomal membrane destabilization by NP.
ZnONP induced eosinophilia, proliferation of airway epithelial cells, goblet cell hyperplasia, and pulmonary fibrosis. Bronchocentric interstitial pulmonary fibrosis at the chronic phase was associated with increased myofibroblast accumulation and transforming growth factor-β positivity. Serum IgE levels were up-regulated by ZnONP along with the eosinophilia whilst serum IgA levels were down-regulated by ZnONP. ZnONP are rapidly dissolved under acidic conditions (pH 4.5) whilst they remained intact around neutrality (pH 7.4). The instillation of dissolved Zn2+ into rat lungs showed similar pathologies (eg., eosinophilia, bronchocentric interstitial fibrosis) as were elicited by ZnONP. Lysosomal stability was decreased and cell death resulted following treatment of macrophages with ZnONP in vitro.
We hypothesise that rapid, pH-dependent dissolution of ZnONP inside of phagosomes is the main cause of ZnONP-induced diverse progressive severe lung injuries.
- Goblet Cell
- Airway Epithelial Cell
- Intratracheal Instillation
- Eosinophilic Inflammation
Zinc oxide nanoparticles (ZnONP) are utilised in many commercial products including cosmetics, paints, textiles, food additives, and personal hygiene products. Because ZnONP are translucent and highly effective in protection against ultraviolet A and B radiation, they are important ingredients of sunscreens and moisturizers . ZnONP is widely used as an ingredient of paints and coating and finishing materials in products and buildings because they provide long-term protection from ultraviolet light . ZnONP have also been used as a dietary supplement in human and livestock because Zinc can stimulate immune systems and act in an anti-inflammatory way [3, 4]. ZnONP has external uses as antibacterial agents in ointments, lotions, mouthwashes, and surface coatings to prevent microorganism growth .
There are few toxicity reports on ZnONP despite their widespread use and potential for use in various applications. Toxicity studies of ZnONP have mainly focused on dermal toxicity, of relevance due to the inclusion of ZnONP within materials that are directly applied to skin. Penetration of ZnONP through normal skin was limited to the stratum corneum in porcine  and human models . Exposure of human skin epithelial cells to ZnONP produced severe cytotoxicity accompanied by oxidative stress and genotoxicity . Few studies have been reported concerning the in vivo toxicity of ZnONP although intratracheal instillation of ZnONP (50 - 70 nm) in Sprague-Dawley rats induced cytotoxicity and neutrophilic inflammation at 24 h after instillation .
Previously, we assessed the pulmonary inflammogenicity of a large panel of NP, including ZnONP and found that metal oxide NP elicited diverse patterns of inflammation with different cellular bases, at both the acute and chronic phase . In the present study, we extended these experiments to specifically evaluate the mechanisms of eosinophilic inflammation and pulmonary fibrosis induced by ZnONP instillation.
We briefly described materials and methods in the main text but the detailed methods were described in the Additional file 1.
Characterization and dispersion of NP
Characterization of nanoparticles.
Nanostructure and Amorphous Materials Inc.
Nanostructure and Amorphous Materials Inc.
Nanostructure and Amorphous Materials Inc.
Diameter (nm) examined by TEM
10.7 ± 0.7
137 ± 9.2
5.3 ± 0.4
30.5 ± 1.8
Surface area (m2/g)a
Mass (μg) per 150 cm2 (high dose- rat)
Mass (μg) per 50 cm2 (low dose- rat)
Mass (μg) per 15 cm2 (high dose- mouse)
Zn2+ (μg) equivalent to 50 cm2 (rat study)
Hydrodynamic size (nm) in
2855 ± 773
4833 ± 625
1210 ± 471
787 ± 324
3925 ± 715
5683 ± 510
2236 ± 407
1166 ± 344
PBS (5% rat serum)
423 ± 24
282 ± 124
93 ± 4
119.1 ± 39.6
PBS (5% mouse serum)
229 ± 187
83 ± 44
Zeta potential (mV) in
PBS (5% rat serum)
-27.1 ± 1.4
-25.9 ± 0.6
-26.0 ± 5.0
-28.5 ± 5.2
PBS (5% mouse serum)
-18.9 ± 1.5
-21.4 ± 0.8
Durability of ZnONP
To evaluate the biopersistence of ZnONP in vivo, ZnONP were incubated with artificial lysosomal fluid (ALF)  and artificial pulmonary interstitial fluid (Gamble's solution) . As a control particle, the rutile form of TiO2NP (30.5 ± 1.8 nm) was purchased from Nanostructure and Amorphous Materials Inc. (Houston, TX, USA) (Table 1). ALF (pH 4.5) and Gamble's solution (pH 7.4) were prepared as previously described [11, 13]. ZnONP and TiO2NP were incubated with ALF or Gamble's solution at 5 mg/ml for 24 h at 37°C with gentle shaking. After 24 h, 50 mg of suspensions were centrifuged at 13 000 × g for 30 min and the supernatant discarded. After the final wash with distilled water (DW), pellets were air dried and resuspended in 5 ml of DW. NP suspensions were then weighed and calculated by subtraction of the weight of the container and the same volume of distilled water. There is potential for some error in this method due a small fraction of small NP that could remain in suspension after centrifugation and are lost on washing.
Intratracheal instillation of ZnONP
Female Wistar rats (200 - 250 g) were humanely maintained and handled in accordance with the UK Home Office Animals Scientific Procedures Act. Intratracheal instillation was performed as previously described method . ZnONP were instilled at a surface area dose of 50 or 150 cm2/rat, and 5% rat serum in saline was used as the vehicle control (n = 5 - 7 per group). Large agglomerates of ZnONP (diameter- 4,380 nm) in saline without rat serum also instilled at 150 cm2 per rat to evaluate the effects of agglomeration on eosinophilia. We used surface area as a dose metric rather than mass because surface area has been known as a better descriptor of potential of NP to cause toxicity in vitro and in vivo . To evaluate the time-course of the consequent inflammation, rats were sacrificed at 24 h, 1 wk, and 4 wks after instillation. Preparation of bronchoalveolar lavage (BAL) fluid and analysis for LDH and total protein was performed as previously described method .
ELISA for pro-inflammatory mediators
Measurements of cytokines (TNF-α, IL-1β, IL-13, and TGF-β) and chemokines (MIP-2 and eotaxin) were performed in non-diluted BAL fluid following the manufacturer's instructions [IL-13 ELISA was obtained from Invitrogen (Camarillo, CA, USA) and other assays were from R&D Systems (Minneapolis, MN, USA)]. The detection limits of ELISA kit was as follow: TNF-α- 5 pg/ml; IL-1β- 5 pg/ml; TGF-β- 1.7 - 15.4 pg/ml; MIP-2- 0.5 - 2.7 pg/ml; eotaxin- 3 pg/ml; IL-13- 1.5 pg/ml.
IgE and IgA ELISA in the serum and BAL fluid
To evaluate the serum immunoglobulin E (IgE) levels, ZnONP were instilled into rats (n = 4) at 150 cm2 per rat and blood was taken via the tail vein at day 1 and week 1, 2, 3, and 4 after instillation. Serum was then collected and diluted 1 in 10 with PBS. Total serum IgE and IgA levels were determined using a rat IgE ELISA set and rat IgA ELISA set, respectively (all from BD Biosciences, Oxford, UK).
Histological analysis (H&E, PSR, and PAS staining)
At each time point, histological analysis of lung tissues and picrosirius red (PSR) staining was performed as previously describe method . For detection of goblet cells which contain mucin, periodic acid-Schiff (PAS) (Sigma-Aldrich) staining was performed according to standard methods. The quantitative image analysis of PAS-positive cells was performed using Image-Pro Plus (Media Cybernetics, MD, USA). PAS-positive signals in the airways were separately evaluated according to airway diameter; airways smaller than 1 mm were considered to be small airways or bronchioles and larger than 1 mm were considered to be a large airway or bronchi . The total area of PAS-positive cells was divided by the total area of epithelial cells including basement membrane. The data were expressed as percentage of PAS-positive area versus total epithelial area.
Immunohistochemistry for eotaxin, TGF-β, and α-SMA
Immunohistochemical staining for eotaxin, transforming growth factor-beta (TGF-β), and alpha-smooth muscle actin (α-SMA) was performed on lung sections. The detailed method for immunohistochemistry was described in Additional file 1.
Transmission electron microscopy (TEM)
TEM was used to evaluate ultra-structural changes in the lungs induced by instillation of ZnONP. Lungs of vehicle control and ZnONP treated rats 4 wks after instillation were fixed with 1.5% glutaraldehyde in 0.1 M cacodylate buffer, stained en bloc with uranyl acetate, and embedded in epoxy resin. Ultra-thin (60 nm) sections were cut, stained with uranyl acetate and lead citrate, and examined with a TEM (JEM-1200EX II, JEOL, Tokyo, Japan).
Instillation of alternative ZnONP
To evaluate whether the eosinophilic inflammation was induced by specific types of ZnONP, we instilled another type of ZnONP (designated ZnONPalt) into rats. ZnONPalt were purchased from Nanostructural and Amorphous Materials, Inc. (Houston, TX, USA) (Table 1). ZnONPalt were instilled into female Wistar rats at 310 μg per rat, which is the same mass of 150 cm2 per rat as the ZnONP from NanoScale Corporation. Four rats were used for each treatment group. After 24 h, rats were euthanized and BAL collection and analysis was performed as described above.
Instillation of ZnONP-free BAL extract to rats
To evaluate whether inflammatory mediators produced by ZnONP instillation can produce similar pathologies to that seen with ZnONP treatment, we extracted ZnONP-free BAL fluid and instilled this in rats. The detailed method was described in Additional file 1.
Instillation of dissolved Zn2+ to rats
To evaluate the effects of dissolved Zn2+ in the acidic solution, 1 mg/ml of ZnONP in HCl-acidified saline were dissolved at a pH of 4.5. After 1 wk, ZnONP-free supernatant was collected by three rounds of centrifugation at 13000 × g. The supernatant was filtered three times through a 0.22 μm filter (Millipore, Cork, Ireland) to exclude possible bacterial contamination. The concentration of Zn2+ of supernatant was measured by inductively coupled plasma-atomic emission spectrometry (ICP-OES) (Perkin Elmer Optima 5300 DV ICP-OES). The pH of the dissolved Zn2+ was 6.5, which was less acidic than 0.9% saline (pH 5.5). Thereafter 92.5 and 277.5 μg of Zn2+ were instilled intratracheally into rats and cytological and histological evaluation was performed at 24 h and 4 wks after instillation. The small discrepancy between the figure of 92.5 μg that was used in the experiment and the 82.8 μg that should have been used, was the result of an error where we originally calculated the equivalent Zn2+ dose for ZnONP based on the zinc metal weight rather than zinc oxide weight.
Aspiration of ZnONP into mice
To evaluate whether the eosinophilic inflammation was a species- and strain-specific phenomenon, ZnONP were aspirated into C57BL/6 and BALB/c mice. ZnONP were dispersed in 5% heat-inactivated mouse serum (collected from healthy C57BL/6 mice) to a dose of 150 cm2/ml and 100 μl (15 cm2 ZnONP) were aspirated into the lungs of mice which were sacrificed 24 h later. As a control, NiONP (Table 1) known to cause acute neutrophilic inflammation  were aspirated at the same surface area dose. Four mice per each group were used for cytological evaluation. Eotaxin (R&D systems) and IL-13 (Invitrogen) levels were measured in the BAL samples as described above. TEM was also applied to evaluate the ultra-structural changes in the lung with the same method described above.
Acridine orange staining
To evaluate the lysosomal membrane destabilization by NP, acridine orange staining was applied to THP-1 cells. Human monocytic cell line THP-1 was obtained from American Type Culture Collection (ATCC) and cultured at 37°C with 5% CO2 in RPMI containing 10% FBS, 2 mM L-glutamine (Life Technologies, Paisley, UK), 100 IU/ml penicillin, and 100 U/ml streptomycin (Life Technologies). THP-1 cells (1 × 106 cells/ml) were seeded to a μ-Dish35 mm, high (Thistle Scientific Ltd., Glasgow, UK) and differentiated using 10 ng/ml of phorbol myristate acetate (PMA; Sigma-Aldrich) for 48 h. After activation, cells were washed three times with PBS and stained with 5 μg/ml acridine orange (Sigma-Aldrich) for 15 min. Cells were then washed three times with PBS and were treated with ZnONP (10 cm2/ml; 20 μg/ml) for 24 h. As a control, TiO2NP (10 cm2/ml; 36 μg/ml) were used. Cells were examined and photographed in a Leica SP5 confocal microscope (Leica Microsystems, Buckinghamshire, UK). Cytotoxicity was also measured using a lactated dehydrogenase assay kit according to the manufacturer's manual (Roche Diagnostics Ltd.).
Data are expressed as mean ± S.D. and were analyzed with GraphPad InStat software (Version 3, GraphPad Software, Inc., La Jolla, CA). To compare each treatment group, one-way analysis of variance with Tukey's post hoc pairwise comparisons was applied. Student t-test was applied for comparison between vehicle control and ZnONP treatment group in C57BL/6 and BALB/c mice or ZnONPalt treatment group. We considered p < 0.05 to be statistically significant.
Characterization of ZnONP
ZnONP showed a "hard agglomerates" which is not readily dispersed without any stresses (mechanical or sonication) in both PBS and DW (Table 1). However, when ZnONP were dispersed with serum protein, they showed a "soft agglomerates" which is readily dispersed without any stresses (mechanical or sonication) because of the protein corona on the surface of NP. The zeta potential of ZnONP in PBS was determined as -27.13 ± 1.36 mV. Endotoxin levels of ZnONP suspensions and rat serum were below the lower detection limit (0.1 EU/ml) whilst 5% mouse serum was calculated to contain 0.09 ± 0.02 EU/ml.
Durability of ZnONP
Differential cell counts in the BAL fluid
Total protein and LDH in the BAL fluid
The levels of total protein and LDH in the BAL were significantly increased at 24 h after instillation and were comparable to controls thereafter except for total protein at 1 wk with low-dose ZnONP (Figures 2H and 2I).
Pro-inflammatory cytokine levels in BAL fluid
The level of IL-1β was significantly increased 24 h and 1 wk after instillation of 150 cm2 ZnONP (Figure 2J). Eotaxin expression was significantly increased only at 24 h with both doses whilst IL-13 was increased at 24 h and 1 wk with the 150 cm2 dose only (Figures 2K and 2L). The concentrations of IL-1β, eotaxin, and IL-13 in the BAL were dose-related. The levels of TGF-β in the BAL peaked at 24 h and were still significantly increased at 1 and 4 wks after instillation. However, levels of TNF-α and MIP-2 showed no significant changes compared to controls (data not shown).
IgE and IgA levels in the serum and BAL fluid
ZnONP induced diverse pathological lung lesions both at the acute and chronic phase. The representative pathological lesions could be classified as eosinophilic inflammation, airway epithelial cell injury, regenerative proliferation, goblet cell hyperplasia, and pulmonary fibrosis with atelectasis (collapse of lung tissue affecting part or all of a lung).
Airway epithelial cell injury and goblet cell hyperplasia
Instillation of agglomerated ZnONP
Large poorly-dispersed agglomerates of ZnONP (diameter- 4,380 nm) produced around 91,000 eosinophils (1.3%) in the BAL whilst well-dispersed ZnONP (diameter- 242.9 nm) produced 595,000 eosinophils (36.7%) in the BAL (Additional file 4).
Study with alternative ZnONP
Effects of NP-free BAL fluid on inflammation in rat lungs
NP-free BAL fluid collected 24 h after instillation of ZnONP into rats was instilled into naïve rat lungs. There was no inflammation at 1 and 4 wks after instillation of NP-free BAL fluid from ZnONP-exposed lungs (data not shown).
Inflammatory pattern of dissolved Zn2+ after instillation into rat lungs
Inflammatory profile in the BAL after ZnONP aspiration into mice
Destabilization of lysosomes and cytotoxicity in macrophages exposed to ZnONP in vitro
In previous studies we found that zinc oxide nanoparticles (ZnONP) were highly fibrogenic and caused an eosinophil exudate into the BAL, a finding that is highly unusual and possibly unique following particle exposure . The current paper set out to determine the likely mechanism of the effects of ZnONP. This paper is notable by its lack of systematic inclusion of benchmark or control particles. The reason is that we have already published extensive findings on the ZnONP response compared to other metal oxides and control NP . The aim of the present paper was solely to investigate the detail of the ZnONP-induced response with a view to better understanding the mechanism whereby they cause such severe pathological effects in rat lungs following a single intratracheal instillation.
In this study, ZnONP were well dispersed with serum protein. Stability of NP depends on a balance between attractive and repulsive forces between particles . Incubation of NP with serum protein forms a protein corona which acts as steric stabilizer preventing agglomeration . When NP deposit in the lung, surfactant proteins and lipids are adsorbed onto the NP forming a lipoprotein corona. Therefore, intratracheal instillation of NP dispersed with serum protein partially mimics the interaction of NP with the lung surface environment. To minimize xenogeneic effects, the serum protein from the same strain animals as those used in the experiments was used as a dispersion medium.
All NP in this study showed negative charge by zeta potential measurement because of the negatively charged protein corona which is the actual charge that is encountered by cells . The recognition by phagocytes facilitates phagocytosis which ingests NP into phagosomes which are then acidified by phagosome/lysosomes fusion. Because ZnONP showed fast dissolution in acidic solution, they are not likely to persistent in the acid milieu of the phagolysosomes. Therefore the effect of surface charge is likely to be less important for high-solubility NP although insoluble positively charged NP are more toxic than neutral or negatively charged NP .
ZnONP induced pulmonary eosinophilia from 24 h to 4 wks after a single intratracheal instillation without any prior sensitisation process. There are published studies showing eosinophilic inflammation in animals following various sensitisation procedures; particles such as TiO2NP  and ambient particulate matter  have been implicated in enhancing development of the murine model of allergic asthma. In addition, instillation or intravenous injection of Sephadex beads (complex of cross-linked dextran polymers; diameter: 20 - 50 μm) also reported to cause eosinophilic inflammation in the lung . However, the eosinophilia caused by Sephadex beads may be produced by their extremely large size because ultrasonication of this particle produced only a transient neutrophilic inflammation . In addition, the Sephadex beads are polymer particles which are fundamentally different from ZnONP used in this study. NP-free BAL exudates collected 24 h after rats were instilled intratracheally with ZnONP at 150 cm2 did not produce any inflammatory reaction either at 1 wk or 4 wks. Considering that eosinophilia peaked 1 wk after instillation and pulmonary fibrosis was mature 4 wks after instillation, the lack of inflammation caused by NP-free BAL exudate instillation suggest that BAL fluid might be too diluted to produce the pathologies caused by ZnONP instillation.
In mice, ZnONP aspiration caused eosinophilia but eosinophils were not found in the alveoli (BAL fluid) but were present in the alveolar interstitium. The interstitial type of eosinophilia was consistent with the elevated levels of eotaxin and IL-13 in the BAL. Eosinophils in the interstitium are as likely to cause tissue injury as eosinophils in the bronchoalveolar space, if not more so .
We undertook a number of assays to investigate the mechanism of the complex and severe pathological syndrome seen following exposure to ZnONP. In the conventional rodent asthma model, recruited eosinophils are associated with airway remodelling including peribronchial fibrosis, smooth muscle hyperplasia, and mucus secretion with the involvement of eotaxin and IL-13 [24, 25]. In the ZnONP model we found that eotaxin and IL-13 were produced early in rats and mice exposed to ZnONP and these are key mediators of eosinophil recruitment [25, 26]. IL-13 is especially involved in the regulation of eosinophil infiltration, IgE synthesis, goblet cell hyperplasia, mucus hypersecretion, and sub-epithelial fibrosis in asthma [25, 27]. Therefore, IL-13 provoked by ZnONP might exert an important role on the wide spectrum of pathological effects seen here with ZnONP exposure.
Although the main inflammatory cells induced by ZnONP were eosinophils, PMN were also recruited during the acute phase. PMN have been regarded as the representative acute inflammatory cells playing a role in particle effects and their recruitment is highly correlated with the surface area dose of low-toxicity, low-solubility particles  and toxic particle such as crystalline silica . The pro-inflammatory cytokine recruiting PMN into ZnONP-exposed lungs was most likely IL-1β which is known to induce neutrophilic inflammation in the lung . However, PMN recruitment by ZnONP was confined to the 24 h time-point whilst significant IL-1β in BAL continued to the 1 week time-point at the highest dose.
Intratracheal instillation of ZnONP induced massive proliferation of airway epithelial cells and goblet cell hyperplasia. ZnONP were reported to be very cytotoxic to BEAS-2B cells in vitro by generating reactive oxygen species . In the present study, ZnONP dramatically increased the levels of LDH and total protein in the BAL at the acute phase indicating cell death and increased vascular permeability, respectively. Therefore, the proliferation of airway epithelial cells likely represents a regenerative response to the cytotoxicity induced by ZnONP. Proliferation of airway epithelial cells led to large-scale goblet cell hyperplasia. Goblet cell hyperplasia was most pronounced at 1 wk and had waned by 4 wks after instillation of ZnONP. Goblet cell hyperplasia plays an important role in protecting the airway from damage due to inhaled particles . The resulting increase in mucus flow traps inhaled particles and removes them from the airways by muco-ciliary clearance. In addition, goblet cells can be progenitors of ciliated cells to maintain mucus flow  and hyperplasia of goblet cells is reversible on cessation of administration . Hypersecretion of mucus by goblet cell hyperplasia is also a feature of airway injury including exposure to cigarette smoke, and sulphur dioxide, and in asthma where it can contribute to obstruction of airways . TGF-β is known to enhance goblet cell hyperplasia and mucus hyper-secretion in mice through an NF-κB-dependent mechanism . In this study, we found strong immunostaining for TGF-β at all time-points, but particularly at 1 and 4 wks after instillation when goblet cell hyperplasia was most pronounced. Consistent with the immunostaining results, total TGF-β concentration in the BAL was also increased at all time-points.
Massive pulmonary fibrosis with contraction and atelectasis was also induced by ZnONP instillation. Following ZnONP instillation collagen fibres, determined by PSR staining, were increased from 1 wk and were more marked at 4 wks. At 4 wks, TEM images showed that large swathes of collagen fibres were primarily located in the perivascular and alveolar interstitium co-localized with eosinophils. Myofibroblasts, the major source of extracellular matrix proteins and contractile forces during fibrogenesis , were also seen in the contracted and fibrotic lung lesions at 1 and 4 wks after treatment. TGF-β also is known to cause airway remodelling including peribronchial fibrosis and smooth muscle hyperplasia .
Instillation of ZnONP increased IgE levels in the serum but not in the BAL. Multi-walled carbon nanotubes , diesel exhaust particles , and ultrafine carbon black  have all been reported to increase serum IgE levels by an adjuvant-like mechanism in murine asthma models. Surprisingly, compared to previous studies, the increase in IgE levels by ZnONP were induced by just a single instillation without any sensitization process. The levels of IgA in the serum were decreased compared to vehicle control. Although there was no statistical significance, IgA levels in the BAL showed an increasing trend at 1 wk after instillation of ZnONP. Increases in mucosal secretory IgA were present in the BAL, and this might explain the decrease of IgA in serum if BAL IgA was derived from the vascular space. The increased IgA levels in the BAL might act be anti-inflammatory in inflamed lung . TGF-β is also known to induce IgA isotype expression by activating Smad3/4 complex translocation into the nucleus .
Our data on ZnONP durability in acid and neutral conditions suggests a mechanism for pathogenicity of ZnONP. ZnONP were stable at neutral pH or saline but very rapidly dissolved in the acidic artificial lysosomal fluid (pH 4.5). Thus ZnONP in the approximately neutral surfactant fluid or in the cytosol might be persistent. However, when ZnONP are internalized into the acid environment of the lysosome, they will be rapidly dissolved producing a high local concentration of Zn2+ ions. Zn2+ is one of the essential elements in cell homeostasis and remains in a bound form inside cells because free Zn2+ is very reactive and cytotoxic . The acute increase in free Zn2+ levels may damage lysosomes, allowing the contents to escape into the cytoplasm where they may damage other organelles leading to cell death [41, 42]. We suggest therefore that Zn2+ released from the phagolysosomes of dead or damaged cells is the source of the Zn2+ after ZnONP uptake in the lungs. The interaction of Zn2+ with cells generates oxidative stress and finally triggers cell death signalling cascades [41, 43]. When ZnONP were added to activated THP-1 cells (a differentiated macrophage cell line), lysosomes were destabilised by a mechanism which seems likely to involve dissolution of ZnONP under the acid condition of the lysosomes. Unlike ZnONP, TiO2NP showed no dissolution in acid or neutral conditions and when incubated with macrophages the fluorescence intensity of lysosomes was not reduced. The loss of lysosomal integrity induced by ZnONP was accompanied by cell death. A role for dissolved Zn2+ in the toxic mechanism of ZnONP is further supported by Muller et al.  who reported that ZnO nanowires were rapidly dissolved in the acidic pH of lysosomes causing structural changes in mitochondria and cell death including necrosis and apoptosis .
Instillation of dissolved Zn2+ from ZnONP treated under acid conditions in saline, produced eosinophilia 24 h after instillation. Moreover, the toxicity of soluble Zn2+ was much greater than similar mass dose of ZnONP based on mortality, number of eosinophils, and levels of LDH and total protein. The higher number of eosinophils by ZnONP than that of ZnONPalt might be due to the smaller primary particle size and their wider distribution inside of the lung. The chronic lung lesions caused by Zn2+ instillation were similar to those induced by ZnONP in terms of bronchocentric interstitial pulmonary fibrosis, goblet cell hyperplasia, and atelectasis.
There are many inhalation/instillation studies using zinc compounds although few papers on nanoparticles. No papers reported eosinophilia on exposure to zinc compounds except for our previous papers using ZnONP [10, 45] and one human case report with zinc oxide . The possible reason for this discrepancy is (1) species and strain differences, (2) location of eosinophils infiltration to the interstitium or airspaces, (3) dose, (4) polydispersity of particles, and (5) durability of particles. In particular the dispersed NP size might be important as we have shown by comparing well-dispersed and agglomerated NP which might specifically address the role of ZnONP agglomerate size. We noted that large poorly-dispersed agglomerates of ZnONP recruited much less number and percent eosinophils (1.3%) compared to well-dispersed ZnONP (37.7%). Based on this data we also conclude that micron-sized particles would not cause substantial eosinophilia in BAL following instillation. This finding is consistent with our previous study which showed no eosinophilic inflammation with ZnONPalt (137 nm) using same strain of rats and instillation technique used in here . In addition, other previous publications also showed no eosinophilic inflammation by nano-size ZnO (50 -70 nm) or micrometer-size ZnO (< 1000 nm) [9, 48]. These previous studies instilled ZnO without any dispersion which implies agglomeration into much larger particles whose compartmentation in the lung might differ from singlet NP or small agglomerates. Therefore, the eosinophilic inflammation by ZnONP may be elicited only by exposure to ZnONP well dispersed and at high doses.
ZnONP-induced eosinophilia, fibrosis, and goblet cell hyperplasia mediated by the soluble Zn2+ is to our knowledge a novel finding in rat lungs. Interestingly, one epidemiological study showed that the level of Zn2+ in ambient particulate matter was associated with asthma morbidity in USA . Therefore, ZnONP pose a unique and substantial hazard to the lungs and hygiene precautions and control of airborne exposure should be instituted in any situation with the potential for exposure in order to reduce the risks of the kinds of lung pathology described here.
Financial support was provided by the Medical Research Council of United Kingdom (MRC G0701323). CJS is supported by MRC Career Development Award (G0800340). KD acknowledges the support of the Colt Foundation.
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