Toxicogenomics analysis of mouse lung responses following exposure to titanium dioxide nanomaterials reveal their disease potential at high doses.

Titanium dioxide nanoparticles (TiO2NPs) induce lung inflammation in experimental animals. In this study, we conducted a comprehensive toxicogenomic analysis of lung responses in mice exposed to six individual TiO2NPs exhibiting different sizes (8, 20 and 300nm), crystalline structure (anatase, rutile or anatase/rutile) and surface modifications (hydrophobic or hydrophilic) to investigate whether the mechanisms leading to TiO2NP-induced lung inflammation are property specific. A detailed histopathological analysis was conducted to investigate the long-term disease implications of acute exposure to TiO2NPs. C57BL/6 mice were exposed to 18, 54, 162 or 486 µg of TiO2NPs/mouse via single intratracheal instillation. Controls were exposed to dispersion medium only. Bronchoalveolar lavage fluid (BALF) and lung tissue were sampled on 1, 28 and 90 days post-exposure. Although all TiO2NPs induced lung inflammation as measured by the neutrophil influx in BALF, rutile-type TiO2NPs induced higher inflammation with the hydrophilic rutile TiO2NP showing the maximum increase. Accordingly, the rutile TiO2NPs induced higher number of differentially expressed genes. Histopathological analysis of lung sections on Day 90 post-exposure showed increased collagen staining and fibrosis-like changes following exposure to the rutile TiO2NPs at the highest dose tested. Among the anatase, the smallest TiO2NP of 8nm showed the maximum response. The anatase TiO2NP of 300nm was the least responsive of all. The results suggest that the severity of lung inflammation is property specific; however, the underlying mechanisms (genes and pathways perturbed) leading to inflammation were the same for all particle types. While the particle size clearly influenced the overall acute lung responses, a combination of small size, crystalline structure and hydrophilic surface contributed to the long-term pathological effects observed at the highest dose (486 µg/mouse). Although the dose at which the pathological changes were observed is considered physiologically high, the study highlights the disease potential of certain TiO2NPs of specific properties.

Introduction

Globally, titanium dioxide nanoparticles (TiO2NPs) are among the most widely produced and used nanomaterials (NMs). The estimated global annual production of TiO2NPs is currently near 12,500 tons (1,2). This value is expected to reach 2.5 million metric tons by 2025 (3). Owing to their unique physico-chemical properties, TiO2NPs are used in diverse applications including consumer, industrial and biomedical fields. The extensive synthesis and pervasive use of TiO2NPs has resulted in unprecedented avenues for human exposure to these materials in the environment and through the use of consumer products. Exposure to TiO2NPs via inhalation leads to pulmonary inflammation, emphysema and lung injury in experimental rodents (4,5). Due to their nanosize, TiO2NPs penetrate deep into the highly vascularised areas of lungs and persist in lungs for weeks after the last exposure (6). In addition, TiO2NPs deposited in rodent lungs translocate to blood, liver, heart, lymph nodes, spleen and other organs (7–10). These results imply that exposure to TiO2NPs can negatively impact health of the organisms exposed.

Acute pulmonary responses in rodents exposed to TiO2NPs are greatly influenced by their primary particle size (11–14), surface area, surface charge, surface coatings (15–18) and their crystalline structure (19,20). These results suggest that a combination of physico-chemical properties influence the pulmonary outcome of exposure to TiO2NPs. It is yet to be determined whether one of these properties is more important than the others and whether the underlying mechanisms of the observed pulmonary responses differ by the properties of TiO2NPs.

We have previously employed toxicogenomics tools to characterise the pulmonary transcriptomic responses in mice exposed via inhalation (21) or instillation to TiO2NPs of different sizes, surface coatings and TiO2NPs embedded in paint (6). We have shown that all types of TiO2NPs induce pulmonary inflammation via the same mechanisms; however, the severity of response varies with their specific properties (22). The transcriptomic studies discussed above were limited to few TiO2NP types and for now, it is not clear whether the results from those studies can be generalised to all TiO2NP variants. In the present study, we conducted a comprehensive investigation of lung toxicogenomic responses in mice exposed individually to six different types of TiO2NPs varying in size, crystalline structure and surface coatings to further our understanding of the underlying mechanisms of TiO2NP-induced lung responses. Acute, subchronic and chronic post-exposure time points were included along with a range of doses. The TiO2NP types included anatase TiO2NPs of 8, 20, and 300nm, mixed anatase/rutile TiO2NP of 20nm and rutile TiO2NPs of 20nm with hydrophilic or hydrophobic surfaces. Mice were exposed via single intratracheal instillation to 18, 54, 162 or 486 µg/animal doses of individual TiO2NPs. Although the 486 µg/animal dose is physiologically high, it was included in the study to determine whether exposure to TiO2NPs results in lung pathology at higher doses. Samples were collected at 1, 28 and 90 days post-exposure. Bronchoalveolar lavage fluid (BALF) cellularity, histopathology, particle localization in lungs (by transmission electron microscopy (TEM) and Cytoviva nanoscale hyperspectral microscopy) were assessed at all three post-exposure time points. Global gene expression profiles were generated for all doses, at Day 1 and Day 28 post-exposure time points. Pathway tools were employed to characterise the molecular pathways perturbed following exposure to TiO2NPs. Disease similarity tools were employed to determine whether the altered gene expression profiles are associated with any known lung disease.

Methods

TiO2NPs investigated

A set of six TiO2NPs of varying physico-chemical properties were investigated in the current study (Table 1): anatase TiO2NPs of three different sizes, 8, 20 and 300nm (aTiO28, aTiO220 and aTiO2300), mixed anatase and rutile TiO2NPs of 20nm (raTiO220) and rutile TiO2NPs of 20nm with hydrophilic [rTiO220 (HY)] or hydrophobic [rTiO220 (HP)] surfaces. All six particles were obtained from the Organisation for Economic Co-operation and Development through the sponsorship program. The aTiO28 is commercially known as Hombikat UV100 and is available through Sachtleben Chemie GmbH, Germany. It is used as a semiconductor catalyst in photocatalytic applications. The aTiO220 and aTiO2300 are commercially available as PC105 and tiona, respectively, from Cristal Global (http://www.cristal.com), whereas, raTiO220 is commercially known as P25 and is available from Evonik Degussa GmbH, Germany. The rTiO220 (HY) and rTiO220 (HP) are known as UV Titan M262 and UV Titan M212, respectively, and are available from Sachtleben Chemie GmbH, Germany. The selected anatase TiO2NPs are used in building materials and in air purification systems for their photocatalytic activity. They are also used in self-cleaning fabrics, glass and other products. The raTiO220 NPs are commonly used as catalyst carriers, active components for photocatalytic reactions and as heat stabilizers in silicone rubber. The two functionalised TiO2NPs—rTiO220 (HY) and rTiO220 (HP)—are used in skin and sun care products. The hydrophobic rTiO220 (HY) is also used in manufacturing waterproof and stain resistant textiles (23,24).

Table 1.

Physico-chemical properties of TiO2NPs.

Physico-chemical properties of TiO2NPs

The particles aTiO28, raTiO220, rTiO220 (HY) and rTiO220 (HP) have been extensively characterised by the European Commission’s Joint Research Centre (JRC) and reported in Rasmussen et al. (25). In brief, aTiO28 and raTiO220 are relatively pure. Negligible amount of silica (Si), aluminium (Al), and sulphur (S) was identified in aTiO28. The rTiO220 (HY) and rTiO220 (HP) contain 91.3% and 92.7% Ti, respectively, and are suggested to contain relatively higher levels of Al, Si, and S. In addition, gas chromatography–mass spectrometry analysis shows the presence of dimethicone (~2 weight %) coating on rTiO220(HY) and glycerol (~1–2 weight %) on rTiO220(HP), which contribute to the hydrophobic and hydrophilic nature of the rTiO220 (HY) and rTiO220 (HP), respectively (25). Extensive characterisations of aTiO220 and aTiO2300 have not been reported. The details of the particle characterisations are summarized in Table 1.

Preparation of TiO2NP stocks for the exposure

A pilot study was conducted to assess the dispersion of TiO2NPs in MilliQ water, 2% serum or mouse BALF. The results revealed (data not shown) that with the exception of raTiO220, all other TiO2NPs are best dispersed in MilliQ water. Thus, MilliQ water was used as particle dispersion vehicle for the experiments. For the purposes of consistency, the raTiO220 was also prepared in MilliQ water. A stock suspension of 6mL for each type of TiO2NPs at a concentration of 12.15mg/mL was made in MilliQ water. The particle suspension was dispersed by sonicating the samples using an S-450D sonifier (Branson Ultrasonics Corp., Danbury, CT, USA) at 10% amplitude for a total of 16min on ice with alternating 10s pulses and pauses in an enclosed chamber.

TEM analysis

The size and agglomeration state of TiO2NPs in dry state were analysed using TEM analysis (Tecnai G2 F20 FETEM, FEI, Hillsboro, Oregon, USA) at the Nano Imaging Facility of Carleton University. Samples were stirred in filtered ethanol and sonicated for 5min (ultrasonic water bath). A drop of this suspension was deposited on copper TEM grids (200-mesh copper grids covered with Formvar/Carbon film (Agar Scientific, Essex, UK), air-dried and analysed using TEM.

Dynamic light scattering analysis

The stock suspensions of the TiO2NPs at a concentration of 50 µg/ mL were prepared in MilliQ water and sonicated as described above. Prior to the dynamic light scattering (DLS) analysis with the Nicomp 380 ZLS instrument (Nicomp Particle Sizing Systems, Santa Barbara, CA, USA), samples were manually mixed and diluted 1 in 100 with 0.22 µm-filtered MilliQ water and were sonicated again for 5min in a water bath sonicator. The DLS instrument was calibrated using 90nm and 240nm standard calibration beads, and the analysis was conducted according to the DLS manufacturers’ instructions. The value of the number-weighted particle size distribution was calculated assuming that the particles are spheres of uniform density that scatter light according to classical Mie theory (26).

Animal handling

Five-to-seven-week-old female C57BL/6 mice (Jackson Laboratory, Bar Harbor, ME, USA) were housed in autoclaved cages with a 12-h light–dark cycle. All mice were given food and water ad libitum throughout the experiment. Animals were weighed before beginning the exposure and once every week during the course of the experiment. Mice were housed in pairs. All animal procedures were conducted in accordance with the guidelines of the Canadian Council for Animal Care and approved by Health Canada’s Animal Care Committee.

The particle suspensions were dispersed as described above under preparation of TiO2NP stocks section. The 486 µg/40µL stock suspensions of the TiO2NPs were diluted to obtain 18, 54, and 162 µg per 40 µL and dispersed by sonication for an additional minute. The doses 18, 54, 162 and 486 μg/mouse are equivalent to 1.5, 5, 15, and 45 working days at the Danish occupational exposure level for TiO2 (6.0mg Ti/m3~9.75mg TiO2/m3), respectively. Although physiologically high, the dose 486 µg/mouse was included in the study to determine whether TiO2NPs induce pathological changes at the higher doses. The suspensions were used immediately after the preparation. MilliQ water devoid of particles was used to expose control animals.

Exposure of mice and harvesting of biological samples

Each treatment group consisted of 10 animals. Mice in the experimental groups received a single intratracheal instillation of 18 µg (very low), 54 µg (low dose), 162 µg (medium dose) or 486 µg (high dose) of TiO2NPs in a 40-µL suspension prepared as described above, followed by 150 μl air with a 250 µL SGE glass syringe (250F-LT-GT, MicroLab, Aarhus, Denmark). Intratracheal instillation was performed under anaesthesia with 5% isoflurane (Isoflo, Esteve Farma, Carnaxide, Portugal) in 100% oxygen. Control mice received 40 µL of vehicle (MilliQ water) only. Following instillation, mice were kept under observation until they recovered from anaesthesia. Mice were killed on Days 1, 28 and 90 post-exposure. Blood, BALF and lung tissue were collected. One set of five experimental and five control mice were lavaged and the left lobe of lung from these mice were cut into two pieces, snap frozen in liquid nitrogen and stored in cryogenic vials at −80°C until analysis. The other set of five experimental and five control mice were not lavaged. Whole lungs from these mice were fixed in formalin (three mice/group) or glutaraldehyde (two mice/group) for histopathology and TEM analysis, respectively. Various end points investigated at the different post-exposure time points are listed in Figure 1.

Figure 1.

Schematic representation of the toxicity end points assessed at various post-exposure time points.

BALF analysis

One set of five mice from control and treated groups were lavaged as described in Poulsen et al. (27). In brief, a total of 2mL of 0.9% sterile saline was injected in lungs and was recovered. The BALF was immediately put on ice until further processing. BALF was centrifuged at 4°C and 400 g for 10min to collect cells, which were resuspended in 0.5mL PBS.

Differential cell counts

The BALF cytospins were prepared as described by Poulsen et al. (27) previously. The cytospins were fixed with two sprays of Sheldon Cell Fix and air-dried and stained with haematoxylin and eosin (H & E). A total of 500 cells were counted per cytospin to identify mononuclear cells, neutrophils and lymphocytes using an optical microscope (Olympus BH2, Olympus Optical Company Ltd, Tokyo, Japan). A non-parametric one-way analysis of variance (ANOVA) with a post-hoc Tukey-type test (28) was conducted to examine the effect of doses compared with the matched control using Origin version 8 (Northampton, MA, USA). Two-way ANOVA was conducted that examined the effect of particle and dose for the three post-exposure time points. Data were analysed using the R statistical computing environment (29). The model assumptions for normality and the constant variance assumption were tested using the Anderson–Darling statistic for normality (30) and the Leven’s test for homogeneity of variance (31). However, these assumptions were not satisfied. As a result, the analysis was conducted on the ranks. As the interaction term was significant (P < 0.0001), the post hoc analysis was conducted on the pairwise comparisons. The Holm–Sidak procedure (32) was used to adjust the P values for multiple testing. The estimate and standard error was reported using the untransformed data.

Cellular toxicity assays

Cell-free BALF supernatants were assessed for evidence of lung injury at 1- and 90-day post-exposure time points.

β-N-Acetylglucosaminidase activity assay

Commercially available β-N-acetylglucosaminidase (NAG) activity colorimetric assay kit (Biovision, CA, USA) was used according to the manufacturer’s instructions. In this assay, 125 µL standard synthetic p-nitrophenol derivative (pNP) or BALF were incubated for 30min at 37°C in the presence of NAG substrate, when the reaction was terminated by adding a stop solution. The release of pNP was determined by measuring the absorbance at 400nm.

Alkaline phosphatase activity assay

Commercially available alkaline phosphatase (ALP) activity fluorometric assay kit (Biovision, CA, USA) was used according to the manufacturer’s instructions. Briefly, 120 µL BALF or standard ALP enzymes were incubated with 4-methylumbelliferyl phosphate disodium salt (MUP) for 30min at room temperature, when the reaction was terminated by adding a stop solution. The fluorescence was measured at Ex/Em = 360nm/440nm using a fluorescence microtitre plate reader.

TEM of lung tissues

TEM analysis of lung tissues was conducted through the services available at the Electron Microscopy Core Laboratory, Cell Imaging and Histology Core Facility, University of Ottawa Heart Institute, Ottawa, Canada. In brief, lung tissues from control and TiO2NP exposed (two mice/group) mice collected on Days 1, 28 or 90 post-exposure time points were perfused and fixed with 2.5 % glutaraldehyde (electron microscopy sciences, Hatfield, PA, USA) and 2% paraformaldehyde solution (electron microscopy sciences, Hatfield, PA, USA). The fixed lung tissue samples were cut into small pieces, rinsed with 0.15M phosphate buffer and 0.15M sodium cacodylate buffer, post-fixed in 2 % osmium tetroxide (Polysciences, Inc., Warrington, PA, USA) and 0.05M potassium ferricyanide (Sigma–Aldrich Canada Ltd., Oakville, ON, Canada) in 0.12M sodium cacodylate buffer (pH 7.2) for 2h. The osmofication was followed by three rinses with Milli-Q and en bloc staining with 1% uranyl acetate (Leica Microsystems, Ultrastain-1, Buffalo Grove, IL, USA) in Milli-Q water overnight at 4°C. The samples were gradually dehydrated in ethanol on the following day and embedded in Epon according to the standard procedures (TAAB Laboratories Equipment, TAAB 812 resin kit, West Berkshire, UK). Embedded samples were ultrathin-sectioned on a Leica EM UC6 ultramicrotome (Leica Microsystems GmbH, Wetzlar, Germany). The resulting sections were stained with uranyl acetate and lead citrate before they were visualised using TEM.

Total RNA extraction and purification

Total RNA was isolated from random sections of the left lung lobe (n = 5 per experimental group) using TRIzol reagent (Invitrogen, Carlsbad, CA, USA) and purified using RNeasy Plus Mini kits (Qiagen, Mississauga, ON, Canada) according to the manufacturer’s instructions. Total RNA concentration was measured using a NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific Inc., Wilmington, DE, USA), and RNA quality and integrity was assessed using an Agilent 2100 Bioanalyzer (Agilent Technologies, Inc., Mississauga, ON, Canada) according to the manufacturer’s instructions. All samples showed RNA integrity numbers above seven.

Microarray hybridization

Total RNA (250ng) samples from individual mice (n = 5 per experimental and control group) and from Universal Mouse Reference total RNA (UMRR; Agilent Technologies, Inc., Mississauga, ON, Canada) were used to synthesise double-stranded complementary DNA (cDNA), which was then used to synthesise Cyanine-labelled cRNAs using Quick Amp Labeling Kit (Agilent Technologies Inc., Mississauga, ON, Canada) according to the manufacturer’s instructions. cRNAs from experimental (control and TiO2NPs-exposed) groups were labelled with cyanine 5-CTP and reference cRNAs were labelled with cyanine 3-CTP using a T7 RNA polymerase in vitro transcription kit (Agilent Technologies Inc., Mississauga, ON, Canada) and purified using RNeasy Mini kits (Qiagen, Mississauga, ON, Canada). An equimolar amount of reference cRNA was mixed with each experimental cRNA sample and was hybridised to Agilent mouse 4×44k oligonucleotide microarrays (Agilent Technologies Inc., Mississauga, ON, Canada) for 17h in a hybridisation chamber at 65°C with a rotation speed of 10rpm. At the end of hybridization, arrays were scanned on an Agilent G2505B scanner according to manufacturer’s protocols (Agilent Technologies Inc., Mississauga, ON, Canada). Gene expression data from the scanned images were extracted using Agilent Feature Extraction software version 9.5.3.1.

Statistical analysis of microarray data

A reference randomised block design (33) was used to analyse microarray data, normalised using LOcally WEighted Scatterplot Smoothing (LOWESS) regression modelling method and statistical significance of the differentially expressed genes was determined using MicroArray ANalysis Of VAriance (MAANOVA) in R statistical software (http://www.r-project.org). The Fs statistic (34) was used to test the treatment effects compared with the matched controls exposed to vehicle only and P values were estimated by the permutation method using residual shuffling. In order to minimise any false positives, the false discovery rate (FDR) multiple testing correction (35) was applied. The fold changes of gene expression were calculated considering the least square means. Genes showing expression changes of at least 1.5-fold in either direction compared with their matched controls and showing FDR P ≤ 0.05 were considered as significantly differentially expressed genes (DEGs) and were used in all downstream analyses. All microarray data have been deposited in the NCBI gene expression omnibus database and can be accessed via the accession number GSE81570.

Functional and pathway analysis of DEGs

Functional gene ontology (GO) analysis of the DEGs was performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) v6.7. Benjamini–Hochberg-corrected GO processes, with a Fisher’s exact P ≤ 0.05 considered to be significantly enriched (over-represented). Specific biological pathways associated with the DEGs were identified using Ingenuity Pathway Analysis (IPA, Ingenuity Systems, Redwood City, CA, USA). Pathways with a Fisher’s exact P value of ≤0.05 were considered for the interpretation of results.

All DEGs from the highest dose of 486 µg/mouse groups were mined against the genomic data repositories in NextBio (http://ehsrb.nextbio.com) to identify curated studies of diseases with similar gene profiles, gene ranking and consistency in fold change directionality. Pairwise gene signature correlations and rank-based enrichment statistics were employed to calculate the NextBio scores for each disease. The disease (associated with TiO2NPs exposure) that ranked the highest was given a score of 100 and the rest were normalised accordingly. The meta-analysis function in NextBio was used to compare the DEGs associated with most enriched GOs from TiO2NP-exposed lung to those reflective of lung fibrosis from the curated datasets available in NextBio.

Histopathology

For histopathology examinations, whole lungs from control and TiO2NP-exposed mice (n = 3) were perfused and fixed in 10% formalin, dehydrated with graded alcohol and paraffin embedded prior to staining with H & E. Three mice per dose group per particle type for a total of 90 slides at each post-exposure time point were assessed. Lung sections from the 162 and 486 µg/mouse dose groups were also stained with Masson trichrome stain to assess collagen deposition. The service was carried out in the PALM Histology Core Facility, University of Ottawa, Ottawa, Ontario, Canada.

Results

Since the 18 µg/animal dose showed non-significant changes in the BALF inflammatory cell profile as measured by the BALF differential cell counts and since it did not induce any observable cellular toxicity as measured by the NAG or ALP assays, this dose group was not included in the microarray experiments and is not discussed in the Results section below.

Characterization of TiO2NPs

The primary particle size and morphology of aTiO28, aTiO220, aTiO2300, raTiO220, rTiO220 (HY) and rTiO220 (HP) were determined by TEM analysis. The average diameter of the majority of each particle type obtained from TEM analysis showed values close to those reported by the manufacturer, except for aTiO2300 (Table 1). Some differences in sizes and particle shapes were observed. Whereas aTiO28, raTiO220 and aTiO2300 were spherical in shape, the other TiO2NPs appeared elongated. The diameter of the cross sections ranged from 18nm to 52nm for the raTiO220 and rTiO220 (HY). The diameters of the elongated or rod shaped aTiO220 and rTiO220 (HP) were ~30nm and ~35–70nm, respectively, in the longest cross sections. Diameters of aTiO2300 were much smaller (100–120nm) than reported by the manufacturer. The size of the aTiO2300 was confirmed using nanoparticle tracking analysis (data not shown) by NanoSight LM20 (NanoSight, Amesbury, UK). All TiO2NPs were found agglomerated.

DLS analysis

DLS analysis of the particle suspension revealed that all TiO2NPs agglomerate in water. The number-weighted particle aggregate sizes are presented in Table 1. In suspended form, the rTiO220 (HP) had the smallest particle aggregate size of 39.3±7.3nm compared with 242.9±53.1nm or 242.9±56.4nm observed for aTiO220 and raTiO220, respectively. The number-weighted particle sizes for aTiO28, aTiO2300 and rTiO220 (HY) were 148.4±38.3, 130.1±49.8nm and 110.1±27.9, respectively.

Post-exposure detection of TiO2NPs in lung tissues

TEM analysis was used to detect TiO2NPs in the ultrastructures of the lung tissues collected on Day 1 and Day 28 post-exposure. TEM images of vehicle only-exposed lung tissues did not reveal any particles on Day 1 (Supplementary Figure 1A-a and A-b) or on Day 28 (Supplementary Figure 1B-a and B-b) post-exposure. In general, all TiO2NPs were found mostly in an agglomerated state in lungs both at Day 1 and Day 28 post-exposure at all the doses tested (Supplementary Figure 1 presents few representative TEM images of lungs exposed to individual TiO2NP type). The alveolar space (Supplementary Figure 1A-c, A-g and A-k), endothelium and epithelial membranes (Supplementary Figure 1A-d, A-h and A-l), macrophages (Supplementary Figure 1A-e and A-m), lamellar bodies (Supplementary Figure 1A-j), cytoplasm (Supplementary Figure 1A-i and A-n) and caveolin and near collagen fibres (Supplementary Figure 1A-f) were some of the structures of lungs where TiO2NPs were found on Day 1 post-exposure.

TiO2NPs were mainly observed in alveolar space near the microvilli (Supplementary Figure 1B-c, B-f, B-k), within the microvilli (Supplementary Figure 1B-d, B-f and B-n) and inside the vesicles (Supplementary Figure 1B-e, B-l and B-m), in samples collected on Day 28 post-exposure. At this time point, particles were mostly found in a non-agglomerated state suggesting that over time, the larger agglomerates may be breaking down.

BALF cellularity

Differential cell counts.

BALF from five mice from each control and exposed groups was assessed for differential cell counts. Details of the total and differential cell counts are summarised in Figure 2 and in Supplementary Table 1. As such, no general trend in increase in total cell number was observed; however, a non-significant dose-dependent increase in the total number of cells in BALF was observed on Day 1 post-exposure for raTiO220 NPs (Figure 2A). On Day 28 post-exposure, only the rutile/anatase and rutile groups showed increased number of BALF cells at the high dose (Figure 2B).

Figure 2.

Differential cell counts in BALF fluid on (A) Day 1 post-exposure and (B) Day 28 post-exposure. TCN, MCN, NEU and LYM represent total cell count, monocytes, neutrophils and lymphocytes, respectively. * P ≤ 0.05: Significant compared with the matched controls by a non-parametric one-way ANOVA with a post- hoc Tukey-type experimental comparison test. A pairwise comparison was performed using post-hoc test following a significant interaction that examined the effects of particle and dose for the two post-exposure time points. γP ≤ 0.05: Significant compared with aTiO28; ζP ≤ 0.05: Significant compared with rTiO220 (HY).

The mononuclear cells, neutrophils and lymphocytes accounted for 98–99.7%, 0.3–1.8% and ~1.0% of the total cell population of control BALF, respectively. According to the cellular profiles shown in Figure 2A, a dose-dependent decrease in the number of mononuclear cells (Figure 2A-b) and concomitant increase in the number of neutrophils (Figure 2A-c) was observed at Day 1 post-exposure, regardless of the type of TiO2NPs. The highest increase in neutrophils was observed in mouse lungs exposed to rTiO220 (HP) (Supplementary Table 1 and Figure 2A-c). The changes in the number of lymphocytes at any dose were not significant (Figure 2A-d).

The changes in the number of mononuclear cells following exposure to rutile/anatase or rutile NPs were observed but not for the anatase NPs on Day 28 post-exposure (Figure 2B-b); a dramatic decrease in the number of BALF neutrophils was observed for all groups (Figure 2B-c). Moreover, a dose-dependent increase in the total number of lymphocytes was observed for all TiO2NP groups on Day 28 post-exposure; although only the changes at medium and high doses were significant (Figure 2B-d).

Microarray analysis

Gene expression analysis

Global lung gene expression profiles derived from a total of 240 microarrays were used to assemble the data. Two main comparisons were made: (i) aTiO28, aTiO220 and aTiO2300 were compared to understand size-related effects and (ii) raTiO220, rTiO220 (HY) and rTiO220 (HP) were compared to understand the influence of surface modification. Each individual data set was analysed separately to identify the specific changes in gene expression compared with the matched vehicle controls. A list of all DEGs (upregulated and downregulated genes) on Day 1 and Day 28 following exposure to six individual types of TiO2NPs are presented in Supplementary Table 2. Figure 3A summarises the number of upregulated and downregulated genes for all doses and post-exposure time points for each particle type.

Figure 3.

(A) Differentially expressed genes (FDR P value ≤ 0.05) following exposure to TiO2NPs on Day 1 and on Day 28. (B) Hierarchical clustering.

Gene expression changes induced by the anatase type of TiO2NPs

A total of 225, 105 and 85 unique DEGs (Figure 3A, Supplementary Table 3a–c. The DEGs that were common to more than one dose were only counted once.) were found in the lungs of mice exposed to aTiO28, aTiO220 and aTiO2300, respectively, on Day 1 post-exposure time point. Specific to individual dose groups, aTiO28 did not have any DEGs at the low (54 µg) dose; whereas 8 (all upregulated) and 224 (185 genes upregulated and 39 downregulated) DEGs were found in the 162 and 486 µg dose groups, respectively. The most DEGs were mainly associated with the inflammatory and acute phase signalling pathways. The large fold changes were observed for members of the Serum amyloid A (Saa); Saa3 (38.0-fold), Saa2 (3.4-fold) and Saa1 (8.9-fold). The others included tissue inhibitor of metalloproteinase 1 (Timp1, 10.1-fold), chemokine (C–C motif) ligand (Ccl) 2 (Ccl2, 6.3-fold), Ccl7 (6.8 fold), lipocalin 2 (Lcn2, 5.1-fold), chemokine (C–X–C motif) ligand (Cxcl) 1 (Cxcl1, 4.0-fold), Cxcl10 (3.8-fold) and Cxcl5 (3.6-fold). Fewer DEGs were observed for the aTiO220 type at all doses and post-exposure time points; a total of 4, 4 and 72 upregulated and 0, 0 and 30 downregulated DEGs in the 54, 162 and 486 µg dose groups were observed, respectively, at Day 1 post-exposure. Saa3 (79.6-fold), Saa1 (10.1-fold), Timp1 (11.7-fold), Ccl2 (4.7-fold) and Lcn2 (9.0-fold) were significantly differentially expressed for aTiO220. The lungs of mice exposed to aTiO2300 showed a total of 85 unique genes differentially expressed. Among these 4, 6 and 69 DEGs were upregulated and 2, 0 and 12 were downregulated in 54, 162 and 486 µg dose groups, respectively. A large number of inflammation-associated genes including, Saa3 (71.2-fold), Saa2 (4.3-fold), Saa1 (8.4-fold), Timp1 (6.6-fold), Ccl2 (2.8-fold), Ccl7 (2.8-fold), Ccl17 (2.6-fold), Lcn2 (8.7-fold), Cxcl1 (2.1-fold), Cxcl10 (1.9-fold) and Cxcl5 (3.4-fold) were significantly expressed in mouse lungs exposed to aTiO2300 at the highest dose.

At Day 28, the response was largely reversed for all three particle types. A total of 8, 5 and 25 DEGs were (Figure 3A, Supplementary Table 3d–f) found in the lungs of mice exposed to aTiO28, aTiO220 and aTiO2300, respectively, at Day 28 post-exposure. The aTiO28 group showed 2 (upregulated) and 8 (3 upregulated and 5 downregulated) DEGs in the 162 µg and 486 µg dose groups, respectively. The aTiO220 NPs induced differential expression of 5 (3 upregulated and 2 downregulated) and 3 (2 upregulated and 1 downregulated) genes in each of the 54 and 486 µg dose groups, respectively, and none in the 162 µg dose group. Nr1d1(2-fold), Spon2 (−2.1-fold) and Nr1d1(1.9-fold) genes associated with inflammation were altered following exposure to aTiO28 and aTiO220, respectively. In the mice treated with aTiO2300, 1, 4 and 12 DEGs were upregulated and 4, 7 and 11 genes were downregulated in the 54 µg, 162 µg and 486 µg dose groups, respectively. Myl9 (1.6-fold) associated with muscle contractile activity and Hspa1a (-1.8-fold) and KFL4/KFL9 (−1.8-fold) genes associated with carcinoma were significantly perturbed following aTiO2300 exposure; none of the DEGs in the aTiO2300-exposed lungs was associated with inflammation.

Gene expression changes induced by the rutile type of TiO2NPs

In contrast to the anatase type TiO2NPs described above, the transcriptomic responses to rutile type raTiO220, rTiO220 (HY) and rTiO220 (HP) were robust. A total of 719, 104 and 354 genes were (Supplementary Table 3g–i) differentially expressed in the lungs of mice exposed to raTiO220, rTiO220 (HY) and rTiO220 (HP), respectively, on Day 1 post-exposure. The mixed unmodified raTiO220-induced 33 DEGS (31 upregulated and 2 downregulated) in the 54 µg, 108 DEGs (84 upregulated and 24 downregulated) in the 162 µg and 714 DEGs (401 upregulated and 313 downregulated) in the 486 µg dose groups (Supplementary Table 3g–i). The rTiO220 (HY) modified with aluminum oxide and dimethicone induced fewer DEGs compared with raTiO220 and rTiO220 (HP) at all the doses tested; 4, 25 and 69 genes were upregulated and 4, 15 and 35 genes were downregulated for the 54 µg, 162 µg and 486 µg dose groups (Supplementary Table 3g–i). In comparison, the rTiO220 (HP) modified with aluminum oxide and glycerol induced significantly higher number of DEGs at all three doses and post-exposure time points; a total of 70, 134 and 177 genes were upregulated and 11, 17 and 102 DEGs were downregulated on Day 1 post-exposure in the 54 µg, 162 µg and 486 µg dose groups, respectively.

At the highest dose, the genes associated with inflammation including, Saa3 (64.5, 41.6, 87.6-fold), Saa2 (5.6, 5.1, 5.4-fold), Saa1 (13.0, 5.2, 11-fold), Ccl7 (4, 4, 6.2 fold), Ccl17 (3.5, 3.5, 3-fold), Lcn2 (9.3, 5.9, 8.5-fold) and Cxcl1 (2.2, 3.8, 3.2-fold) showed the highest fold changes following exposure to raTiO220, rTiO220 (HY) or rTiO220 (HP), respectively. The Ccl2 (6.3-fold) showed significant fold change following exposure to rTiO220 (HP) only. In addition, the expression of interleukin 1 receptor, type II (Il1r2, 3.3, 2.7-fold), (Il1B, 1.9, 2.2-fold), cytochrome P450 family 7 subfamily B member 1 (Cyp7b1, 3.3, 3.4-fold) and placenta growth factor (Pgf, 2.2, 2.0-fold) responsible for fibrosis were also upregulated following exposure to raTiO220 and rTiO220 (HP), respectively, and Timp1 (19.8, 5.5, 12.7-fold) and Cd14 antigen, (Cd14—1.9, 1.8, 2.3-fold) were differentially expressed in all three rutile groups, raTiO220 and rTiO220 (HP) and rTiO220 (HY). Serine (or cysteine) peptidase inhibitor, clade E, member 1, (Serpine1, 2.6- fold) was upregulated only in the raTiO220 group.

Fewer genes were altered at Day 28 post-exposure to raTiO220 (Supplementary Table 3j); none in the 54 µg dose group, and 1 upregulated and 1 DEGs downregulated in the 162 µg group. In the 486 µg group, 9 were upregulated and 7 DEGs were downregulated. Ly6f (2.8-fold), Trem2 (2.2-fold), Nr1d2 (1.9-fold) and Spon2 (−2.3-fold) were among the genes differentially expressed at the highest dose that were associated with inflammation. Several genes were persistently altered at Day 28 following exposure to rTiO220 (HY) (Supplementary Table 3k); 3 (1 upregulated, 2 downregulated), 13 (9 upregulated, 4 downregulated) and 34 (25 upregulated, 9 downregulated) genes showed differential expression in the 54, 162 and 486 µg groups, respectively. In response to rTiO220 (HP), 9 (2 upregulated and 7 downregulated), 21 (13 upregulated and 8 downregulated) and 148 (131 upregulated and 17 downregulated) DEGS were found in the 54, 162 and 486 µg dose groups, respectively (Supplementary Table 3l). Several genes associated with inflammation and acute phase signalling were persistently altered at Day 28 following exposure to rTiO220 (HY) and rTiO220 (HP). These included Saa3 (27.7 and 7.0-fold), Lcn2 (7.9 and 6.3-fold), Cxcl5 (5.4 and 5.7-fold), Ccl7 (4.0 and 5.0-fold), Cxcl1 (3.6 and 2.6-fold), Clec5a (2.5 and 2.5-fold) and Cd14 (2.1 and 1.8-fold), respectively, at the high dose. In addition, pro-fibrotic genes including Cd14 (2.1-fold), Timp1 (2.5-fold) and Cyp7b1 (2.2-fold) following rTiO220 (HY) and Ccl2 (4.1-fold), Csf2 (2.1-fold), Cd14 (1.8-fold), Adora1 (1.7-fold) and Il1rn (1.5-fold) following rTiO220 (HP) showed largest changes at the high dose.

In addition to a separate assessment of the individual data sets, a hierarchical clustering of genes differentially expressed in at least one experimental condition was conducted to determine particle, dose, or time-dependent expression patterns (Figure 3B). The analysis revealed that the expression patterns were similar for individual mice within the same treatment groups. Two main clusters were observed: Cluster-1 consisted of all doses of the five TiO2NPs for Day 28 post-exposure time point excluding raTiO220, along with Day 1 post-exposure samples of rTiO220 (HY) from the low-dose group. The Cluster-2 included Day 1 samples of all six TiO2NP types along with Day 28 post-exposure samples of raTiO220 from all dose groups and Day 28 post-exposure samples of rTiO220 (HY) from the high-dose group. The results suggested that the observed responses at the gene expression level were mainly post-exposure time-dependent and particle-specific differences were not observed (Figure 3B).

Influence of size on gene expression

Size-related effects were assessed by comparing gene expression responses to aTiO28, aTiO220 and aTiO2300; Supplementary Table 3a–f lists DEGs from this group of samples. In general, the number of DEGs at Day 1 post-exposure was remarkably lower for the aTiO2300 exhibiting surface area of 10 m2/g (85 genes) and was highest for the aTiO28 exhibiting surface area of 229–235 m2/g (225 genes) among all dose groups. In comparison, the aTiO220 with the surface area of 90 m2/g showed 105 DEGs. A VENN analysis of all DEGs from all three anatase types showed that no genes in common in the low-dose group (54 µg), and three genes in the 162 µg and 7 genes in the 486 µg groups (Supplementary Figure 2). The common genes included Saa3, Saa1, Timp1, Lcn2, Ccl2, and Bcan, all of which are associated with the process of lung inflammation. In addition, Il1r2 a pro-fibrotic gene was common to all (Supplementary Table 3). On Day 28 post-exposure, only one DEG at the highest dose (Supplementary Figure 2B) was common to all anatase groups.

Functional pathway analysis

The DEGs were analysed to identify specific biological functions, processes or pathways perturbed following exposure. In agreement with the total number of DEGs, a large number of perturbed biological processes were observed in the 162 and 486 µg dose groups. The list of all biological processes affected at Day 1 post-exposure for all doses are summarised in Supplementary Table 4. Immune/inflammation responses were the top processes affected following exposure to all anatase TiO2NPs, with enrichment of GO annotations such as inflammatory response (GO:0006954), response to wounding (GO:0009611), defense response (GO:0006952), chemotaxis (GO:0006935), taxis (GO:0042330) and behaviour (GO:0007610). In addition, genes in the high-dose group of the aTiO2300 also perturbed steroid metabolic process (GO: 0008202). The largest significant changes were observed in mice exposed to aTiO28. No biological processes were significantly altered in aTiO220-exposed lungs. Two biological processes—transcription (GO:0006350) and rhythmic process (GO:0048511)—were altered in aTiO2300 group at Day 28 post-exposure. There were no significantly altered biological processes for the other two anatase types.

The microarray data were further analysed through Kyoto Encyclopedia of Genes and Genomes (KEGG) and Ingenuity Pathway Analysis (IPA) to understand the implications of the altered genes in lung diseases and disorders. The list of all KEGG pathways affected at Day 1 post-exposure is provided in Supplementary Table 4; the cytokine–cytokine receptor interaction (mmu04060), chemokine signalling pathway (mmu04062) and NOD-like receptor signalling (mmu04621) pathways implicated in the inflammation process were among the top pathways affected. In addition, the terpenoid backbone biosynthesis (mmu00900) and p53 signalling pathway (mmu04115) were altered in response to aTiO28 and aTiO220, and extracellular matrix (ECM)–receptor interaction (mmu04512) and focal adhesion (mmu04510) were perturbed in response to aTiO28. This response was not seen in Day 28 post-exposure samples. A heat map of enriched (−log P value ≥ 1.3) canonical pathways (IPA) regulated by the DEGs at Day 1 and Day 28 are presented in Figure 4. IPA revealed unique association of DEGs in the high-dose group of aTiO220 with the p53 signalling pathway and the ataxia telangiectasia mutated signalling pathway, implying DNA damage response. The p53 signalling pathway was among the significantly perturbed KEGG pathways that was associated with DEGs of aTiO220 and aTiO28. The DEGs in the high-dose group of aTiO28 and aTiO2300 showed significant association with the pathways of hepatic fibrosis and interleukin 17 signalling in fibroblasts, suggesting a potential for these particles to induce lung fibrosis.

Figure 4.

Schematic heat map depicting dose-dependent enrichment in canonical pathways in lungs exposed to aTiO28, aTiO220 or aTiO2300 at Day 1 and Day 28 post-exposure.

Only a few altered canonical pathways that included acute phase signalling, glucocorticoid receptor signalling and FXR/VDR/LXR/RXR activation were significantly perturbed on Day 28 for all three anatase TiO2NPs.

The results showed that the altered DEGs in this group are involved in inflammation, oxidative stress and fibrosis. aTiO28, aTiO220 and aTiO2300 induced 24, 12 and 24 DEGs associated with inflammation; 13, 4 and 8 DEGs associated with fibrosis and 2, 7 and 5 DEGs associated with oxidative stress in the 486 µg dose group (Supplementary Figure 3).These genes were no longer significantly expressed on Day 28 post-exposure.

Influence of surface modification on gene expression

For the rutile raTiO220, rTiO220 (HY) and rTiO220 (HP) NPs, transcriptomic response seemed to be influenced by the type of surface modifications. While the rutile/anatase raTiO220 induced the largest number of DEGs at the high dose on Day 1, rTiO220 (HP) with hydrophilic surface modification induced large number of DEGs at all doses and post-exposure time points. There were 1, 15 and 36 genes common to raTiO220, rTiO220 (HY) and rTiO220 (HP) groups at the low, medium and high doses, respectively, which were mainly associated with inflammation (Supplementary Figure 4A). There were 5 and 15 DEGs common to the rTiO220 (HY) and rTiO220 (HP) exposure groups, respectively, at the medium and high dose at Day 28 post-exposure. Only 1 and 2 DEGs were common to the three rutile NPs at medium and high doses and none at the low dose (Supplementary Figure 4B).

The biological processes perturbed on Day 1 and Day 28 post-exposure for all doses are summarised in Supplementary Table 5. Immune/inflammation responses were the top biological processes affected on Day 1 post-exposure, with enrichment of GO annotations, such as inflammatory response (GO:0006954), response to wounding (GO:0009611), defense response (GO:0006952), chemotaxis (GO:0006935), taxis (GO:0042330) and behaviour (GO:0007610) (Supplementary Table 5-a). DEGs in the high-dose group of the rTiO220 and rTiO220 (HY) were also associated with steroid metabolic process (GO:0008202). Processes corresponding to cell adhesion were perturbed in the high-dose raTiO220 group.

At Day 28, significant alterations in the immune/inflammation responses (Supplementary Table 5-b), which were associated with inflammatory response (GO:0006954), response to wounding (GO:0009611), defense response (GO:0006952), chemotaxis (GO:0006935), taxis (GO:0042330) and behaviour (GO:0007610) were seen only in rTiO220 (HP) group at the highest dose. raTiO220 perturbed the alteration of regulation of transcription (GO:0045449), RNA metabolism (GO:0051252), circadian rhythm (GO:0007623) and rhythmic processes (GO:0048511). No significant changes in the biological processes were observed in rTiO220 (HY) group at any dose at this late time point.

The list of all KEGG pathways affected at Day 1 and Day 28 post-exposure is provided in Supplementary Table 5; the cytokine–cytokine receptor interaction (mmu04060), chemokine signalling pathway (mmu04062) and NOD-like receptor signalling (mmu04621) pathways implicated in the inflammation process were among the top pathways affected. In addition, the terpenoid backbone biosynthesis (mmu00900), ECM-receptor interaction (mmu04512), focal adhesion (mmu04510) and p53 signalling pathway (mmu04115) were altered in lungs in response to raTiO220.

At Day 28 post-exposure, only rTiO220 (HP) and rTiO220 (HY) showed perturbations in the inflammatory (cytokine–cytokine receptor interaction (mmu04060) and chemokine signalling (mmu04062)) pathways. In addition, rTiO220 (HP) showed perturbations in toll-like receptor signalling pathway (mmu04620) and NOD-like receptor signalling (mmu04621) and lysosome (mmu04142) pathways.

Heat map of enriched (−log P ≥ 1.3) canonical pathways regulated by the DEGs at Day 1 and Day 28 post-exposure are presented in Figure 5. The top five canonical pathways associated with DEGs on Day 1 include acute phase response signalling, agranulocyte adhesion and diapedesis, granulocyte adhesion and diapedesis, role of hypercytokinemia/hyperchemokinemia in pathogenesis and communication between innate and adaptive immune cells. In addition to inflammatory pathways, hepatic fibrosis/hepatic stellate cell activation and hepatic cholestasis were perturbed even in the low-dose groups of the rutile raTiO220 and rTiO220 (HP) and in the medium- and high-dose groups of the rTiO220 (HY) at Day 1 post-exposure (Figure 4). Altered inflammatory pathways such as acute phase response signalling, agranulocyte adhesion and diapedesis, granulocyte adhesion and diapedesis were also observed at Day 28 post-exposure for all rutile NPs. At this this time point, the fibrosis pathway was altered only in the high-dose group of the rTiO220 (HY) and rTiO220 (HP).

Figure 5.

Schematic heat map depicting dose-dependent enrichment in canonical pathways in lungs exposed to raTiO220, rTiO220 (HY) and rTiO220 (HP) at Day 1 and Day 28 post-exposure.

The results also showed that raTiO220 induced highest number of DEGs associated with fibrotic (25 DEGs at the high dose) and oxidative stress (14 DEGs at the high dose) response categories on Day 1 (Supplementary Figure 5); however, these genes were no longer significantly expressed on Day 28 post-exposure. The rTiO220 (HY) and rTiO220 (HP) induced 6 and 11 DEGs associated with fibrosis on Day 1, respectively. Six of those induced by rTiO220 (HY) were also observed on Day 28. Only three DEGs were induced by rTiO220 (HP) on Day 28, which were mainly associated with inflammation.

Identification of human diseases associated with perturbed gene expression profiles

Diseases and functions predicted from IPA showed that the diseases and functions related to inflammation were activated in lungs treated with the anatase TiO2NPs on Day 1 post-exposure (Supplementary Figure 6). In addition to diseases and functions related to inflammation, diseases and functions related to DNA damage were also activated in lungs treated with aTiO220. These diseases and functions were no longer activated on Day 28 post-exposure. In the rutile/anatase and rutile-exposed groups, the diseases and functions related to inflammation, fibrosis and oxidative stress were activated in the 162 µg and 486 µg dose groups (Supplementary Figure 6) on Day 1 post-exposure. These diseases and functions related to inflammation, fibrosis and oxidative stress were only activated in the 486 µg dose groups on Day 28 post-exposure.

DEGs from the 162 and 486 µg dose groups of all six particle types were analysed using the NextBio Human Atlas (Supplementary Table 6) to identify their potential association with known human diseases. The analysis showed that respiratory disorders, viral infectious diseases, bacterial infectious disease, parasitic disease, and nutritional and metabolic disease are among the top disease categories associated with the DEGs. The sub-categories in the respiratory disorders category included severe acute respiratory syndrome, infectious disease of lung, lower respiratory tract infection, pneumonia, interstitial lung disease, injury of lung, asthma, pulmonary hypertension, chronic obstructive pulmonary disease and fibrosis of lung.

A recently published INHAND system was used to classify lung morphology and lung lesions.(36) The H & E-stained slides were blinded for the type of TiO2NPs. The detailed lung lesion inventory is provided in Supplementary Table 7. For each lesion category, a numeric score from 0 to 5 was assigned based on a subjective assessment of the lesion severity, which corresponded to lesion not present, minimal, mild, moderate, marked and severe, respectively.

The results showed that lung sections from vehicle-treated control mice on Day 1 post-exposure are well ventilated with a few macrophages distributed diffusely in the alveolar spaces (Figure 6A–a). Infiltration of alveolar macrophages and neutrophils was observed in the treated groups in a dose-dependent manner (Figure 6Ab–m). The alveolar macrophages were heavily laden with NPs. Alveolar inflammation or alveolitis was assessed by the presence of granulomatous and pyogranulomatous like structures dominated by large numbers of macrophages and neutrophils, respectively. Mild macrophage–neutrophilic alveolitis was also observed in the 162 µg dose group (Figure 6A-b–g). The degree of alveolitis in centriacinar area regions was more prominent in the 486 µg dose group for all particle types (Figure 6A-h–m).

Figure 6.

Histopathology of lungs exposed to TiO2NPs on Day 1 and Day 28 post-exposure. Large arrowhead: infiltration of inflammatory cells; small arrowhead: particle-laden macrophages; arrows: presence of NPs; and blue arrowhead: deposition of collagen associated with fibrosis.

The lung sections remained well ventilated on Day 28 post-exposure (Figure 6B-a). A dose-dependent increase in the alveolar macrophages was observed for some particle types mainly at 162 (Figure 6B-b–g) or 486 µg doses (Figure 6B-n–s); however, this change was less significant compared with the Day 1 post-exposure groups (Supplementary Table 7-b). Peribronchiolar and perivascular infiltrations of lymphocytes were observed in the 162 µg and 486 µg dose groups, except in mice exposed to aTiO2300 (Supplementary Table 7-b). Focal to multifocal granulomatous and pyogranulomatous inflammation were more pronounced in rTiO220 (HY)- and rTiO220 (HP)-exposed lungs in the 162 µg (Figure 6B-f–g) and 486 µg dose groups (Figure 6B-r–s). Lung sections were stained with Masson’s trichrome staining to assess collagen deposition on Day 28 post-exposure in the medium- and high-dose groups. Masson’s trichrome staining showed that collagen depositions were minimal in lung sections treated with TiO2NPs in the 162 µg dose groups (Figure 6B-h–m). Collagen depositions were higher in rTiO220 (HY)- and rTiO220 (HP)-exposed lungs in the 486 µg dose groups (Figure 6B-t–y) compared with other lung sections treated with other TiO2NP types.

Long-term effects of exposure to TiO2NPs

aTiO28, raTiO220, rTiO220 (HY) and rTiO220 (HP) showed larger transcriptomic responses compared with other particle types, suggesting some TiO2NPs may be more harmful than others. Thus, a series of experiments were conducted at 90-day post-exposure to determine whether differential transcriptomics responses among the TiO2NPs eventually translates into disease manifestation for some TiO2NPs. TEM analysis of lung sections (Figure 7) showed that an appreciable fraction of TiO2NPs deposited via single intratracheal instillation were retained in lung tissues up to 90 days post-exposure regardless of the specific size or surface modifications, in the 162 and 486 µg dose groups (Figure 7-a–l). The TiO2NPs were mostly found dispersed in the cytoplasm and some were localized to phagocytic vacuoles (Figure 7-e–g). In lung sections from the aTiO28 and rTiO220 (HP)-exposed groups, particles were found inside the nuclei (Figure 7-d, 7–l).

Figure 7.

TEM analysis of lung tissue sections on Day 90 post-exposure and detection of TiO2NPs in different areas of lungs. Red arrows: presence of NPs; N: nucleus; and V: vacuole.

BALF cellularity analysis revealed small increases in total cell number (Supplementary Figure 7A-a and B-a), macrophages (Supplementary Figure 7A-b and B-b) and lymphocytes (Supplementary Figure 7A-c and B-c) at 90-day post-exposure, indicating persistent but subtle inflammation. No general trends specific to particle types were observed.

To further understand the nature of the response observed at Day 90 post-exposure, cytotoxicity was assessed using β-NAG activity and ALP activity (37) in cell-free BALF supernatants collected on Day 1 and Day 90 post-exposure. Increased β-NAG activity is indicative of macrophage lysosomal activation (38). Increased ALP activity is used as a marker of (39) pathological pulmonary conditions, in which epithelial damage is a central feature of the pathogenesis, such as acute lung injury, pulmonary fibrosis, viral pneumonia and particulate matter-induced lung toxicity (40). The results showed that the β-NAG (Supplementary Figure 8A-a and A-b) activity increased significantly on Day 1 following exposure to aTiO28 and rTiO220 (HY) at the medium and high doses and in lungs exposed to rTiO220 (HP) at the high-dose group in comparison with their matched controls. The ALP activity (Supplementary Figure 8B-a and B-b) was significantly higher on Day 1 in lungs of mice exposed to aTiO28 and rTiO220 (HP) at the medium and high doses and in lungs exposed to aTiO28and rTiO220 at the high-dose group compared with their matched controls. The increased β-NAG or ALP activities on Day 1 (Supplementary Figure 8B-a and B-b) were reduced significantly on Day 90.

In alignment with the observations above, the lung sections from the treated animals showed an increased infiltration of alveolar macrophages throughout but particularly in centriacinar area alveolar spaces at 90 Day post-exposure (Figure 8). Perivascular and peribronchiolar infiltrations of inflammatory cells were often observed irrespective of the type of TiO2NPs (Figure 8). Alveolar or septal fibroblast proliferation and granulomatous alveolar inflammation dominated by macrophages and lymphocytes were observed occasionally in treated lungs. Control mice showed only a subtle and diffuse infiltration of alveolar macrophages (Figure 8A-a–c and B-a–c). Masson’s trichrome staining for collagen revealed the presence of mild fibrosis in lungs exposed to aTiO28 (Figure 8A-j and A-p), rTiO220 (HY) (Figure 8B-k and B-q) and rTiO220 (HP) (Figure 8B-l and B-r). A mild and focal type 2 pneumocyte hypertrophy and hyperplasia, indicative of repair of alveolar epithelium was found only in lungs exposed to medium dose of rTiO220 (HP) (data not shown).

Figure 8.

Histopathology of lung tissue exposed to TiO2NPs on Day 90. Large arrowhead: infiltration of inflammatory cells; small arrowhead: particle-laden macrophages; arrow: presence of NPs; and blue arrowhead: fibrosis-like changes.

Discussion

Property-specific responses

As stated earlier in the introduction above, it is a well-known fact that the primary particle size and the resulting surface area influence the toxicity induced by several classes of NMs including TiO2NPs. For TiO2NP variants, lung exposures leading to acute pulmonary inflammation is suggested to be predominantly dependent on size but also on several other properties such as the surface modifications. However, it is not known whether the underlying mechanisms of pulmonary inflammation are the same for all and it is also not clear whether subtle differences in the severity of acute pulmonary inflammation observed for some TiO2NP variants (22) translate into a disease phenotype at a later post-exposure time point. Thus, in the present study, we conducted a detailed assessment of lung responses to six different types of TiO2NPs with varying sizes, modifications and crystalline structures. Although the TiO2NP types investigated in the study were not synthesised in a controlled manner to vary in only one property at a time, from the results of the comprehensive assessment conducted, it can be concluded that the underlying mechanisms of acute pulmonary inflammation induced by TiO2NPs are the same regardless of their properties, which is in alignment with the previous study results published by this research group (22). Also in alignment were the results showing that while size may be the primary determinant of the toxic potential of TiO2NPs, surface modifications influence the severity of the response with hydrophilic surface being more inflammogenic compared with the other types. Halappanavar et al. (22) compared the lung transcriptomic responses in mice exposed to rutile TiO2NPs of 10nm, 38nm and 10nm TiO2NPs with hydrophilic surface and showed that at the transcriptional level, 10nm particles were more responsive with highest number of DEGs. However, surface amination of 10nm TiO2NPs resulting in hydrophilic surface increased the severity of the response (larger fold changes in inflammation-associated genes). The extensive transcriptomic profiling and histopathological analysis conducted in the present study confirm these findings. In the present study, the number of DEGs at Day 1 post-exposure to anatase-type TiO2NPs was remarkably lower for the aTiO2300 with the surface area of 10 m2/g (85 genes) and was highest for the aTiO28 exhibiting 229–235 m2/g (225 genes). While the primary particle size may have been instrumental in the lung responses induced by the anatase type TiO2NPs, for the rutile types [raTiO220, rTiO220 (HY) and rTiO220 (HP)], mixed properties including surface modifications and crystallinity appeared to have influenced the responses. New to the present study was the finding that anatase TiO2NPs were less responsive compared with the rutile/anatase and rutile TiO2NPs at the transcriptional level. Although a direct comparison of responses induced by the anatase and rutile types was not possible as they vary in their surface properties, the overall biological response of mice exposed to anatase type TiO2NPs was less compared with the rutile ones. Previous studies have shown that crystalline structure of TiO2NPs plays an important role in toxicity induced by these particles (16,18).

Among the rutile types, all three of them were of 20nm in size and exhibited a surface area between 51 and 57 m2/g, which is relatively less compared with the surface area of anatase aTiO220 90 m2/g. It was observed that the number of DEGs induced by rTiO220 (HY) were similar to the number of DEGs induced by aTiO220; however, rTiO220 (HP) induced considerably higher number of genes at all doses and post-exposure time points compared with the other rutile and anatase TiO2NP types. These results suggest that the transcriptional responses to rutile type TiO2NPs are influenced by their surface modifications and that the hydrophilic surface positively impacts the lung responses. Although all rutile types are similar in size and surface area, mixed rutile/anatase type exhibited significantly larger transcriptomic response at the highest dose tested. The raTiO220 has been previously shown to exert higher toxicity compared with the other rutile or anatase only TiO2NPs (41–43). The higher activity of raTiO220 compared with that of either pure rutile type can be suggested to be due to the synergistic effects between rutile and anatase crystalline phases (44); however, this could not be confirmed since the present study did not include an unmodified rutile TiO2 type.

The results of the gene expression analysis were in agreement in general with the BALF cellularity and inflammatory cell infiltration. However, the BALF results were not sensitive enough to identify the subtle differences between the responses induced by the different particle types, implying that gene expression profiling may be a sensitive tool to identify property-specific toxicity attributes of particles. Although the results confirm that particle size for the unmodified and hydrophilic surface for the modified are the important properties playing a role in the toxicity induced by TiO2NPs, more studies involving TiO2NPs of a range of sizes and surface properties are required to affirm this knowledge.

The larger response of rTiO220 (HP) at the gene expression level was also reflected in the results of the histopathology showing persistent inflammation, alveolitis and granulomatous/pyogranulomatous inflammatory lesions reflective of mild fibrosis. Higher toxicity following exposure to hydrophilic rTiO220 has been shown in rats exposed via inhalation (5). Teubl et al. (45) showed that compared with rTiO220 (HY), the rTiO220 (HP) induces pronounced generation of reactive oxygen species by decreasing the mitochondrial membrane potential in human buccal epithelial TR146 cells (46). Surface-coated NMs are shown to exert higher toxicity compared with the pristine types; silica-coated rutile TiO2NPs elicit higher pulmonary neutrophilia, in in vivo and in vitro models (5). Rutile TiO2NPs coated with alumina or alumina/silica induce higher inflammation compared with the uncoated TiO2NPs in Sprague Dawley rats (47). In contrast to the above, a study by Warheit et al. showed that the uncoated raTiO220 are more harmful than the alumina or alumina/silica coated TiO2NPs in rats exposed by intratracheal instillation, suggesting that the crystalline structure may also be important (16,18). The latter results reflect what we have observed in this study with raTiO220 showing highest number of differentially expressed genes acutely after the exposure at the highest dose tested.

Although it was not possible to elucidate the exact reasons why hydrophilic surface renders particles more toxic, we speculate that the nature of the surface coating of the core NPs and resulting surface charge may be playing a role in determining NP–cell interactions, such as altering the cellular membrane permeability and cellular localisation upon uptake. NMs that are strongly hydrophobic in nature may non-specifically interact with the cell membrane leading to their internalisation. Preferential cellular uptake of positively charged NMs could be occurring due to the strong electrostatic interaction with the negatively charged cellular membrane (48). Studies also have shown a differential cellular localisation of the hydrophilic TiO2NPs compared with the hydrophobic TiO2NPs (49). The poor interactions with the lipid bilayer may also prolong the life of hydrophilic NPs in blood, which will allow them to reach specific sites and cause toxicity.

Pulmonary inflammation and fibrosis-like changes

The different types of TiO2NPs tested in this study confirmed our previous findings (22) that initial lung responses to TiO2NPs leading to lung inflammation involves similar mechanisms, which includes activation of acute phase signalling and granulocyte and agranulocyte diapedesis pathways resulting in cytokine/chemokine storm and infiltration of neutrophils into lung fluid. However, the magnitude of the observed response was higher and persistent for rutile types and even more pronounced in rTiO220 (HP) group, suggesting that the extent of the response is property specific. Since rTiO220 (HP)-induced expression changes in few of the genes that have known association with lung fibrosis, we compared the gene expression profiles generated from the present study with the gene profiles of lungs of mice exposed to carbon nanotubes that are known to induce lung fibrosis in the same mouse model at similar doses (27,50). A direct comparison of the 69 fibrosis-associated genes described in Poulsen et al. and Snyder-Talkington et al. study (27,50), with DEGs from the present study revealed a low level of concordance with very few overlapping genes (Table 2), which included Timp1, Ccl17, Serpine1, Sod2, Plaur1, Ccl2, Il1rn, Il1b, Ctsk and C3. Considering the fact that TiO2NPs are generally viewed as inert, the subtle pro-fibrotic gene responses suggest that histopathological observations of fibrotic-like changes at the high doses (162 µg and 486 µg/animal) of TiO2NPs may still be reflecting the early phase responses to disease development and that a full manifestation of fibrosis may require recurring exposure to TiO2NPs at similar dose levels. Alveolar metaplasia has been reported in rat but not in mice following exposure to TiO2NPs in previous studies (51,52). Retention of TiO2NPs in human lung has been reported following cumulative exposures in TiO2NP production plants (53,54). In some of these human cases, exposure to rutile TiO2NPs resulted in mild subpleural fibrosis (6,55–58). However, the specific properties of the TiO2NPs that the humans were exposed to are not identified. The results of the present study suggest that the hydrophilic surface may render TiO2NPs with pro-fibrotic potential in the long term when exposed to sufficiently high doses.

Table 2.

Differentially expressed genes (FDR-adjusted P value ≤ 0.05 and fold change ≥ 1.5 or ≤ −1.5.) associated with lung fibrosis.

aTiO28	aTiO220	aTiO2300	raTiO220	rTiO220(HY)	rTiO220 (HP)	aTiO28	aTiO220	aTiO2300	raTiO220	rTiO220 (HY)	rTiO220 (HP)
24 h	24 h	24 h	24 h	24 h	24 h	28 days	28 days	28 days	28 days	28 days	28 days
Adora2b	Ccl2	C3	Arg1	Ccl17	Arg1					Ccl17	C3
Arg1	Nfkbia	Ccl2	Ccl17	Plaur	C3					Timp1	Ccl2
Ccl2	Timp1	Ccl17	Cd74	Timp1	Ccl2						Ctsb
Ccl17		Il1b	Ctsk		Ccl17						Ctsk
Ctsk		Il1rn	Cx3cl1		Ctsk						Il1rn
Hbegf		S100a4	Hbegf		Fn1						Mmp12
Il1rn		Timp1	Il1b		Il1b						Rgs16
Serpine1			Il1rn		Il1rn						S100a4
Socs1			Lgals3		Lgals3						Tlr2
Socs3			Mmp8		Rgs16						Tnfaip3
Timp1			Plat		S100a4
			Plaur		Serpine1
			Rassf1		Socs3
			Serpine1		Sod2
			Socs3		Timp1
			Sod2
			Timp1

Apart from the predominant inflammogenic gene expression changes, several genes associated with the p53 signalling pathway was found to be specifically altered following exposure to aTiO28 and aTiO220 NPs. TEM analysis also found particles in the nucleus following exposure to aTiO28 and rTiO220 (HP). These results suggest that some of them may have a potential to induce DNA damage. There are not many in vivo studies that have investigated the DNA damage potential of TiO2NPs. It was shown that a 21-nm anatase/rutile (59) and a 33-nm aTiO2NPs (60) induce DNA damage and genetic instability in blood cells of mice exposed orally over a short period of time. Intragastric administration of 75nm aTiO2NPs to male Sprague Dawley rats for 30 days (4) was shown to induce genotoxic effects in the bone marrow. However, positive genotoxic effects have not been observed in rodent lungs exposed to TiO2NPs via inhalation (61,62). Although many studies have investigated the genotoxic effects of TiO2NPs using in vitro models, the results are not consistent. For example, AMES assays showed no increases in the mutagenic potential of aTiO2NPs in vitro models. However, DNA damage analysis using comet assays showed that uncoated 20–50nm aTiO2NPs tested at a concentration of 100 µg/mL induced DNA breaks in BEAS2B cells, bottlenose dolphin leucocytes, human gastric adenocarcinoma cells, human epidermal cells and human embryonic kidney (HEK293) cells (6,55–57). One other study showed positive DNA damage potential of 33nm aTiO2NPs in bone marrow and liver of CBAB6F1 mice exposed via gavage (60). However, at present, the properties of TiO2NPs or factors responsible for their DNA damaging potential are not understood.

Conclusions

In conclusion, a comprehensive assessment of lung responses was conducted following exposure to six types of TiO2NPs exhibiting differences in size, crystalline structure and surface modifications across a range of doses and post-exposure time points. The results confirmed our previous findings that the underlying mechanisms leading to TiO2NP-induced lung inflammation acutely after exposure are the same regardless of their properties; however, the results suggested that the long-term pathological effects may be influenced by their specific properties such as crystallinity and surface modifications. Specifically, from the six types tested, a combination of properties such as rutile structure and hydrophilic surface can be deemed as pathologically more active compared to other types. Although TiO2NPs are generally regarded as inert, the results from the study suggest that unfavourable and chronic exposures to very high doses of certain TiO2NP types may result in tissue injury and initiate pro-fibrotic changes in lungs over a long period of time.

Supplementary data

Supplementary Tables 1 to 7 and Supplementary Figures 1 to 8 are available at Mutagenesis Online.

Funding

This work was supported by Health Canada’s Genomics Research and Development Initiative and Chemicals Management Plan.