Airway barrier dysfunction induced by exposure to carbon nanotubes in vitro: which role for fiber length?

Single-chirality of single-walled carbon nanotubes (SWCNTs) through chromatography and its potential biological applications

Gel chromatography is used to separate single-chirality and selective-diameter SWCNTs. We also explore the use of photothermal therapy and biosensor applications based on single-chirality, selected-diameter, and unique geometric shape.

Occupational Exposure to Carbon Nanotubes and Carbon Nanofibres: More Than a Cobweb

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are erroneously considered as singular material entities. Instead, they should be regarded as a heterogeneous class of materials bearing different properties eliciting particular biological outcomes both in vitro and in vivo. Given the pace at which the industrial production of CNTs/CNFs is increasing, it is becoming of utmost importance to acquire comprehensive knowledge regarding their biological activity and their hazardous effects in humans. Animal studies carried out by inhalation showed that some CNTs/CNFs species can cause deleterious effects such as inflammation and lung tissue remodeling. Their physico-chemical properties, biological behavior and biopersistence make them similar to asbestos fibers. Human studies suggest some mild effects in workers handling CNTs/CNFs. However, owing to their cross-sectional design, researchers have been as yet unable to firmly demonstrate a causal relationship between such an exposure and the observed effects. Estimation of acceptable exposure levels should warrant a proper risk management. The aim of this review is to challenge the conception of CNTs/CNFs as a single, unified material entity and prompt the establishment of standardized hazard and exposure assessment methodologies able to properly feed risk assessment and management frameworks.

Toxicity of Single-Walled Carbon Nanotubes (SWCNTs): Effect of Lengths, Functional Groups and Electronic Structures Revealed by a Quantitative Toxicogenomics Assay

Single-walled carbon nanotubes (SWCNTs) are a group of widely used carbon-based nanomaterials (CNMs) with various applications, which raise increasing public concerns associated with their potential toxicological effect and risks on human and ecosystems. In this report, we comprehensively evaluated the nanotoxicity of SWCNTs with their relationship to varying lengths, functional groups and electronic structures, by employing both newly established quantitative toxicogenomics test, as well as conventional phenotypic bioassays. The objective is to reveal potential cellular toxicity and mechanisms of SWCNTs at the molecular level, and to probe their potential relationships with their morphological, surface, and electronic properties. The results indicated that DNA damage and oxidative stress were the dominant mechanisms of action for all SWCNTs and, the toxicity level and characteristics varied with length, surface functionalization and electronic structure. Distinguishable molecular toxicity fingerprints were revealed for the two SWCNTs with varying length, with short SWCNT exhibiting higher toxicity level than the long one. In terms of surface properties, SWCNT functionalization, namely carboxylation and hydroxylation, led to elevated overall toxicity, especially genotoxicity, as compared to unmodified SWCNT. Carboxylated SWCNT induced a greater toxicity than the hydroxylated SWCNT. The nucleus is likely the primary target site for long, short, and carboxylated SWCNTs and mechanical perturbation is likely responsible for the DNA damage, specifically related to degradation of the DNA double helix structure. Finally, dramatically different electronic structure-dependent toxicity was observed with metallic SWCNT exerting much higher toxicity than the semiconducting one that exhibited minimal toxicity among all SWCNTs.

Length-dependent toxicity of TiO2 nanofibers: mitigation via shortening

Length and aspect ratio represent important toxicity determinants of fibrous nanomaterials. We have previously shown that anatase TiO₂ nanofibers (TiO₂ NF) cause a dose-dependent decrease of cell viability as well as the loss of epithelial barrier integrity in polarized airway cell monolayers. Herein we have investigated the impact of fiber shortening, obtained by ball-milling, on the biological effects of TiO₂ NF of industrial origin. Long TiO₂ NF (L-TiO₂ NF) were more cytotoxic than their shortened counterparts (S-TiO₂ NF) toward alveolar A549 cells and bronchial 16HBE cells. Moreover, L-TiO₂ NF increased the permeability of 16HBE monolayers and perturbed the distribution of tight-junction proteins, an effect also mitigated by fiber shortening. Raw264.7 macrophages efficiently internalized shortened but not long NF, which caused cell stretching and deformation. Compared with L-TiO₂ NF, S-TiO₂ NF triggered a more evident macrophage activation, an effect suppressed by the phagocytosis inhibitor cytochalasin B. Conversely, a significant increase of inflammatory markers was detected in either the lungs or the peritoneal cavity of mice exposed to L-TiO₂ NF but not to S-TiO₂ NF, suggesting that short-term macrophage activation in vitro may not be always a reliable indicator of persistent inflammation in vivo. It is concluded that fiber shortening mitigates NF detrimental effects on cell viability and epithelial barrier competence in vitro as well as inflammation development in vivo. These data suggest that fiber shortening may represent an effective safe-by-design strategy for mitigating TiO₂ NF toxic effects.

Catechin and Procyanidin B2 Modulate the Expression of Tight Junction Proteins but Do Not Protect from Inflammation-Induced Changes in Permeability in Human Intestinal Cell Monolayers

The possibility of counteracting inflammation-related barrier defects with dietary compounds such as (poly)phenols has raised much interest, but information is still scarce. We have investigated here if (+)-catechin (CAT) and procyanidin B₂ (PB₂), two main dietary polyphenols, protect the barrier function of intestinal cells undergoing inflammatory stress. The cell model adopted consisted of co-cultured Caco-2 and HT29-MTX cells, while inflammatory conditions were mimicked through the incubation of epithelial cells with the conditioned medium of activated macrophages (MCM). The epithelial barrier function was monitored through trans-epithelial electrical resistance (TEER), and ROS production was assessed with dichlorofluorescein, while the expression of tight-junctional proteins and signal transduction pathways were evaluated with Western blot. The results indicated that MCM produced significant oxidative stress, the activation of NF-κB and MAPK pathways, a decrease in occludin and ZO-1 expression, and an increase in claudin-7 (CL-7) expression, while TEER was markedly lowered. Neither CAT nor PB₂ prevented oxidative stress, transduction pathways activation, ZO-1 suppression, or TEER decrease. However, PB₂ prevented the decrease in occludin expression and both polyphenols produced a huge increase in CL-7 abundance. It is concluded that, under the conditions adopted, CAT and PB₂ do not prevent inflammation-dependent impairment of the epithelial barrier function of intestinal cell monolayers. However, the two compounds modify the expression of tight-junctional proteins and, in particular, markedly increase the expression of CL-7. These insights add to a better understanding of the potential biological activity of these major dietary flavan-3-ols at intestinal level.

Multi-walled carbon nanotubes upregulate mitochondrial gene expression and trigger mitochondrial dysfunction in primary human bronchial epithelial cells

Nanomaterials are a relatively new class of materials that acquire novel properties based on their reduced size. While these materials have widespread use in consumer products and industrial applications, the potential health risks associated with exposure to them remain to be fully characterized. Carbon nanotubes are among the most widely used nanomaterials and have high potential for human exposure by inhalation. These nanomaterials are known to penetrate the cell membrane and interact with intracellular molecules, resulting in a multitude of documented effects, including oxidative stress, genotoxicity, impaired metabolism, and apoptosis. While the capacity for carbon nanotubes to damage nuclear DNA has been established, the effect of exposure on mitochondrial DNA (mtDNA) is relatively unexplored. In this study, we investigated the potential of multi-walled carbon nanotubes (MWCNTs) to impair mitochondrial gene expression and function in human bronchial epithelial cells (BECs). Primary BECs were exposed to sub-cytotoxic doses (up to 3 μg/ml) of MWCNTs for 5 d and assessed for changes in expression of all mitochondrial protein-coding genes, heteroplasmies, and insertion/deletion mutations (indels). Exposed cells were also measured for cytotoxicity, metabolic function, mitochondrial abundance, and mitophagy. We found that MWCNTs upregulated mitochondrial gene expression, while significantly decreasing oxygen consumption rate and mitochondrial abundance. Confocal microscopy revealed induction of mitophagy by 2 hours of exposure. Mitochondrial DNA heteroplasmy and insertion/deletion mutations were not significantly affected by any treatment. We conclude that carbon nanotubes cause mitochondrial dysfunction that leads to mitophagy in exposed BECs via a mechanism unrelated to its reported genotoxicity.

Comparative in Vitro Cytotoxicity of Realistic Doses of Benchmark Multi-Walled Carbon Nanotubes towards Macrophages and Airway Epithelial Cells

Multi-walled carbon nanotubes (MWCNT) have many outstanding physical and chemical properties that make them useful in many applications in nanotechnology. However, these properties are reported to be potentially harmful for the human body. The effects of low and realistic doses of three well-characterized preparations of MWCNT, obtained from the Joint Research Centre (JRC) (NM-400, NM-401, and NM-402), were assessed in two murine macrophage lines, Raw264.7, of peritoneal origin, and MH-S, derived from alveolar macrophages. Macrophage viability, evaluated with two distinct methods, was significantly lowered by NM-401 (needle-like, average length 4 μm, diameter 67 nm) with IC₅₀ values of 10 μg/cm², whereas NM-400 and NM-402 (tangled, average lengths 846–1372 nm, diameter 11 nm) had much smaller effects. In contrast, at 10 μg/cm², NM-400 and NM-402 induced the M1 marker Nos2 and, consistently, a sizable accumulation of nitrites in the medium, whereas NM-401 had no significant effect. None of the MWCNT preparations induced the M2 marker Arg1. Phagocytic activity, assessed in Raw264.7 macrophages, was significantly reduced in cells exposed to NM-401, but not to NM-400 or NM-402. When tested on Calu-3 bronchial epithelial cell monolayers, the three MWCNT preparations did not affect cell viability, but decreased the trans-epithelial electrical resistance at the maximal dose tested (80 μg/cm²), with the most evident effect detected for NM-401, even at 10 μg/cm². In conclusion, among the possible structural determinants of the toxic effects exerted by MWCNT towards macrophages and airway epithelial cells, shape and length appear the most relevant at low, realistic doses.

Shape-Related Toxicity of Titanium Dioxide Nanofibres

Titanium dioxide (TiO₂) nanofibres are a novel fibrous nanomaterial with increasing applications in a variety of fields. While the biological effects of TiO₂ nanoparticles have been extensively studied, the toxicological characterization of TiO₂ nanofibres is far from being complete. In this study, we evaluated the toxicity of commercially available anatase TiO₂ nanofibres using TiO₂ nanoparticles (NP) and crocidolite asbestos as non-fibrous or fibrous benchmark materials. The evaluated endpoints were cell viability, haemolysis, macrophage activation, trans-epithelial electrical resistance (an indicator of the epithelial barrier competence), ROS production and oxidative stress as well as the morphology of exposed cells. The results showed that TiO₂ nanofibres caused a cell-specific, dose-dependent decrease of cell viability, with larger effects on alveolar epithelial cells than on macrophages. The observed effects were comparable to those of crocidolite, while TiO₂ NP did not decrease cell viability. TiO₂ nanofibres were also found endowed with a marked haemolytic activity, at levels significantly higher than those observed with TiO₂ nanoparticles or crocidolite. Moreover, TiO₂ nanofibres and crocidolite, but not TiO₂ nanoparticles, caused a significant decrease of the trans-epithelial electrical resistance of airway cell monolayers. SEM images demonstrated that the interaction with nanofibres and crocidolite caused cell shape perturbation with the longest fibres incompletely or not phagocytosed. The expression of several pro-inflammatory markers, such as NO production and the induction of Nos2 and Ptgs2, was significantly increased by TiO₂ nanofibres, as well as by TiO₂ nanoparticles and crocidolite. This study indicates that TiO₂ nanofibres had significant toxic effects and, for most endpoints with the exception of pro-inflammatory changes, are more bio-active than TiO₂ nanoparticles, showing the relevance of shape in determining the toxicity of nanomaterials. Given that several toxic effects of TiO₂ nanofibres appear comparable to those observed with crocidolite, the possibility that they exert length dependent toxicity in vivo seems worthy of further investigation.

Mechanisms of lung fibrosis induced by carbon nanotubes: towards an Adverse Outcome Pathway (AOP)

Several experimental studies have shown that carbon nanotubes (CNT) can induce respiratory effects, including lung fibrosis. The cellular and molecular events through which these effects develop are, however, not clearly elucidated. The purpose of the present review was to analyze the key events involved in the lung fibrotic reaction induced by CNT and to assess their relationships. We thus address current knowledge and gaps with a view to draft an Adverse Outcome Pathway (AOP) concerning the fibrotic potential of CNT.

As for many inhaled particles, CNT can indirectly activate fibroblasts through the release of pro-inflammatory (IL-1β) and pro-fibrotic (PDGF and TGF-β) mediators by inflammatory cells (macrophages and epithelial cells) via the induction of oxidative stress, inflammasome or NF-kB. We also highlight here direct effects of CNT on fibroblasts, which appear as a new mode of toxicity relatively specific for CNT. Direct effects of CNT on fibroblasts include the induction of fibroblast proliferation, differentiation and collagen production via ERK 1/2 or Smad signaling. We also point out the physico-chemical properties of CNT important for their toxicity and the relationship between in vitro and in vivo effects. This knowledge provides evidence to draft an AOP for the fibrogenic activity of CNT, which allows developing simple in vitro models contributing to predict the CNT effects in lung fibrosis, and risk assessment tools for regulatory decision.

Toxicity determinants of multi-walled carbon nanotubes: The relationship between functionalization and agglomeration

The elucidation of toxicity determinants of multi-walled carbon nanotubes (MWCNT) is still incomplete. Functionalization with carboxyl groups is, however, commonly used to mitigate MWCNT toxicity, although the rationale for the mitigating effect has not been fully clarified yet. In this work, two optimized chemical vapor deposition methods were employed to obtain MWCNT of comparable length but different diameter, which were subsequently functionalized. For MWCNT of diameter larger than 40 nm, no detrimental effects on cell viability of macrophages were observed, while mild cytotoxicity was recorded for diameters between 15 and 40 nm, with a mitigating effect of functionalization. To investigate the factors responsible for the mitigation, we used the thinnest MWCNT preparation on different cell models, evaluating several endpoints, such as viability, production of nitric oxide (NO), expression of pro-inflammatory markers, the Trans-Epithelial Electrical Resistance (TEER), and clonogenic activity. Substantial mitigation of the changes caused by pristine MWCNT was observed not only with carboxyl- but also with amino-functionalized MWCNT, suggesting that negative or positive surface charge was not the main factor responsible for the effect. Instead, either functionalized preparation exhibited a stronger tendency to agglomerate that was strictly dependent on the presence of proteins. Moreover, we found that either carboxyl- or amino-functionalized MWCNT adsorbed a larger amount of serum proteins than pristine counterparts, with a distinctive pattern for each type of MWCNT. We propose, therefore, that the formation of larger agglomerates, dependent upon different protein coronae, contributes to mitigate the biological effects of functionalized MWCNT in protein-rich biological media.

Blood-brain barrier transport studies, aggregation, and molecular dynamics simulation of multiwalled carbon nanotube functionalized with fluorescein isothiocyanate

In this study, the ability of a multiwalled carbon nanotube functionalized with fluorescein isothiocyanate (MWCNT–FITC) was assessed as a prospective central nervous system-targeting drug delivery system to permeate the blood–brain barrier. The results indicated that the MWCNT–FITC conjugate is able to penetrate microvascular cerebral endothelial monolayers; its concentrations in the Transwell^® system were fully equilibrated after 48 hours. Cell viability test, together with phase-contrast and fluorescence microscopies, did not detect any signs of MWCNT–FITC toxicity on the cerebral endothelial cells. These microscopic techniques also revealed presumably the intracellular localization of fluorescent MWCNT–FITCs apart from their massive nonfluorescent accumulation on the cellular surface due to nanotube lipophilic properties. In addition, the 1,000 ps molecular dynamics simulation in vacuo discovered the phenomenon of carbon nanotube aggregation driven by van der Waals forces via MWCNT–FITC rapid dissociation as an intermediate phase.

Potential in vitro model for testing the effect of exposure to nanoparticles on the lung alveolar epithelial barrier

Pulmonary barrier function plays a pivotal role in protection from inhaled particles. However, some nano-scaled particles, such as carbon nanotubes (CNT), have demonstrated the ability to penetrate this barrier in animal models, resulting in an unusual, rapid interstitial fibrosis. To delineate the underlying mechanism and specific bio-effect of inhaled nanoparticles in respiratory toxicity, models of lung epithelial barriers are required that allow accurate representation of in vivo systems; however, there is currently a lack of consistent methods to do so. Thus, this work demonstrates a well-characterized in vitro model of pulmonary barrier function using Calu-3 cells, and provides the experimental conditions required for achieving tight junction complexes in cell culture, with trans-epithelial electrical resistance measurement used as a biosensor for proper barrier formation and integrity. The effects of cell number and serum constituents have been examined and we found that changes in each of these parameters can greatly affect barrier formation. Our data demonstrate that use of 5.0 × 10⁴ Calu-3 cells/well in the Transwell cell culture system, with 10% serum concentrations in culture media is optimal for assessing epithelial barrier function. In addition, we have utilized CNT exposure to analyze the dose-, time-, and nanoparticle property-dependent alterations of epithelial barrier permeability as a means to validate this model. Such high throughput in vitro cell models of the epithelium could be used to predict the interaction of other nanoparticles with lung epithelial barriers to mimic respiratory behavior in vivo, thus providing essential tools and bio-sensing techniques that can be uniformly employed.

Multiwalled carbon nanotubes induce altered morphology and loss of barrier function in human bronchial epithelium at noncytotoxic doses

Multiwalled carbon nanotubes (MWCNTs) have seen increasing application in consumer products over the past decade, resulting in an increasing risk of human exposure. While numerous toxicological studies have been performed using acute high doses of various carbonaceous nanomaterials, the effects of longer-term, low doses of MWCNTs remain relatively unexplored. This study examined bronchoscopy-derived healthy human bronchial epithelial cells exposed in submerged culture to noncytotoxic doses of MWCNTs over 7 days. Under these conditions, doses as low as 3 μg/mL caused altered cell morphology, superficially resembling fibroblasts. Electrical impedance of the epithelial monolayer was greatly reduced following MWCNT exposure. However, Western blot and polymerase chain reaction showed no elevated expression of the fibroblast markers, vimentin, α-smooth muscle actin, or fibronectin, indicating that a mechanism other than epithelial-mesenchymal transition may be responsible for the changes. Phalloidin and tubulin immunostaining showed disruption of the cytoskeleton, and confocal imaging showed a reduction of the tight junction proteins, zona occludens 1 and occludin. We propose that MWCNTs interfere with the cytoskeleton of the lung epithelium, which can result in a harmful reduction in barrier function over time, even at noncytotoxic doses.

Identifying contact-mediated, localized toxic effects of MWCNT aggregates on epithelial monolayers: a single-cell monitoring toxicity assay

Aggregates of multiwalled carbon nanotubes (MWCNT) impair the barrier properties of human airway cell monolayers. To resolve the mechanism of the barrier alteration, monolayers of Calu-3 human airway epithelial cells were exposed to aggregated MWCNT. At the cell-population level, trans-epithelial electrical resistance (TEER) was used as an indicator of barrier competence, caspase activity was assessed with standard biochemical assays, and cell viability was investigated by biochemical techniques and high-throughput screening (HTS) technique based on automated epifluorescence microscopy. At cell level, the response to MWCNT was investigated with confocal microscopy, by evaluating cell death (calcein/propidium iodide (PI)), proliferation (Ki-67), and apoptosis (caspase activity). At the cell-population level, exposure to aggregated MWCNT caused a decrease in TEER, which was not associated with a decrease in cell viability or onset of apoptosis even after an 8-d exposure. In contrast, confocal imaging demonstrated contact with MWCNT aggregates triggered cell death after 24 h of exposure. In the presence of a natural surfactant, both TEER decrease and contact-mediated toxicity were mitigated. With confocal imaging, increased proliferation and apoptosis were detected in Calu-3 cells next to the aggregates. Contact-mediated cytotoxicity was recorded in two additional cell lines (BEAS-2B and A549) derived from human airways. Similar results were confirmed by adopting two additional MWCNT preparations with different physico-chemical features. This indicates MWCNT caused localized damage to airway epithelial monolayers in vitro and altered the apoptotic and proliferative rate of epithelial cells in close proximity to the aggregates. These findings provide evidence on the pathway by which MWCNT aggregates impair airway barrier function, and support the use of imaging techniques as a possible regulatory-decision supporting tool to identify effects of aggregated nanomaterials not readily detected at cell population level.

Potential Occupational Risks Associated with Pulmonary Toxicity of Carbon Nanotubes

Given their remarkable properties, carbon nanotubes (CNTs) have made their way through various industrial and medicinal applications and the overall production of CNTs is expected to grow rapidly in the next few years, thus requiring an additional recruitment of workers. However, their unique applications and desirable properties are fraught with concerns regarding occupational exposure. The concern about worker exposure to CNTs arises from the results of recent animal studies. Short-term and sub-chronic exposure studies in rodents have shown consistent adverse health effects such as pulmonary inflammation, granulomas, fibrosis, genotoxicity and mesothelioma after inhalation or instillation of several types of CNTs. Furthermore, physicochemical properties of CNTs such as dispersion, functionalization and particle size can significantly affect their pulmonary toxicity. Risk estimates from animal studies necessitate implementation of protective measures to limit worker exposure to CNTs. Information on workplace exposure is very limited, however, studies have reported that CNTs can be aerosolized and attain respirable airborne levels during synthesis and processing activities in the workplace. Quantitative risk assessments from sub-chronic animal studies recommend the health-based need to reduce exposures below the recommended exposure limit of 1 µg/m³. Practice of prevention measures including the use of engineering controls, personal protective equipment, health surveillance program, safe handling and use, as well as worker training can significantly minimize worker exposure and improve worker health and safety.

Pulmonary toxicity and fibrogenic response of carbon nanotubes

Carbon nanotubes (CNTs) have been a subject of intensive research for a wide range of applications. However, because of their extremely small size and light weight, CNTs are readily inhaled into human lungs resulting in increased rates of pulmonary disorders, most notably fibrosis. Several studies have demonstrated the fibrogenic effects of CNTs given their ability to translocate into the surrounding areas in the lung causing granulomatous lesions and interstitial and sub-pleural fibrosis. However, the mechanisms underlying the disease process remain obscure due to the lack of understanding of the cellular interactions and molecular targets involved. Interestingly, certain physicochemical properties of CNTs have been shown to affect their respiratory toxicity, thereby becoming significant determinants of fibrogenesis. CNT-induced fibrosis involves a multitude of cell types and is characterized by the early onset of inflammation, oxidative stress and accumulation of extracellular matrix. Increased reactive oxygen species activate various cytokine/growth factor signaling cascades resulting in increased expression of inflammatory and fibrotic genes. Profibrotic growth factors and cytokines contribute directly to fibroblast proliferation and collagen production. Given the role of multiple players during the pathogenesis of CNT-induced fibrosis, the objective of this review is to summarize the key findings and discuss major cellular and molecular events governing pulmonary fibrosis. We also discuss the physicochemical properties of CNTs and their effects on pulmonary toxicities as well as various biological factors contributing to the development of fibrosis.

Morphological transformation induced by multiwall carbon nanotubes on Balb/3T3 cell model as an in vitro end point of carcinogenic potential

In this work we investigated the toxicological effects of nude and chemically functionalised (-NH(2), -OH and -COOH groups) multiwall carbon nanotubes (mwCNTs) using immortalised mouse fibroblasts cell line (Balb/3T3) as in vitro model, alternative to the use of animals, to assess basal cytotoxicity, carcinogenic potential, genotoxicity and cell interaction of nanomaterials (NM). Combining in vitro tests such as cell transformation assay and micronucleus with physicochemical and topological analysis, we obtained results showing no cytotoxicity and genotoxicity. Carcinogenic potential and mwCNTs interaction with cells were instead evident. We stressed the importance that different toxicological end points have to be considered when studying NM, therefore, assays able to detect long-term effects, such as carcinogenicity, must be taken into account together with a panel of tests able to detect more immediate effects like basal cytotoxicity or genotoxicity.

Formaldehyde interferes with airway epithelium integrity and functions in a dose- and time-dependent manner

Formaldehyde (HCHO) is a common indoor air pollutant. To assess its potential role and mechanism of action in asthma, we exposed the bronchial epithelial cell lines Calu-3 and 16HBE to HCHO (70-7000 μM) according to two exposure schedules (30 min and 24 h), before measuring cell viability, necrosis and apoptosis, reactive oxygen species production, cytokine release, as well as trans-epithelial electrical resistance (TEER) of cell monolayers. Whereas exposure to HCHO for 30 min had a limited effect on cell viability, exposure for 24h to 1400-7000 μM HCHO induced a pronounced dose-dependent cell death. The important decrease in cell viability observed after 24h exposure to the highest concentrations of HCHO (1400-7000 μM) was accompanied by important LDH release and ROS production, whereas a 4h exposure to lower HCHO concentrations (350 μM) induced cell apoptosis. Also, exposure to HCHO for 30 min dose-dependently inhibited basal and lipopolysaccharide-induced interleukin-6 (IL-6) and IL-8 production by bronchial epithelial cells. As well, HCHO triggered a dose- and time-dependent decrease in TEER of Calu-3 cell monolayers. The present work demonstrates that HCHO interferes with airway epithelium integrity and functions, and may thus modulate the onset and the severity of asthma. However, importantly, conditions of exposure to HCHO, e.g. level and duration, are determinant in the nature of the effects triggered by the pollutant.

Carbon nanotubes induce oxidative DNA damage in RAW 264.7 cells

The induction of DNA and chromosome damage following in vitro exposure to carbon nanotubes (CNT) was assessed on the murine macrophage cell line RAW 264.7 by means of the micronucleus (MN) and the comet assays. Exposures to two CNT preparations (single-walled CNT (SWCNT > 90%) and multiwalled CNT (MWCNT > 90%) were performed in increasing mass concentrations (0.01-100 microg/ml). The frequency of micronuclei was significantly increased in cells treated with SWCNT (at doses above 0.1 microg/ml), whereas MWCNT had the same effect at higher concentrations (1 microg/ml) (P < 0.05). The results of the comet assay revealed that the effects of treatment with SWCNT were detectable at all concentrations tested (1-100 microg/ml); oxidized purines increased significantly, whereas pyrimidines showed a significant increase (P < 0.001) only at the highest concentration (100 microg/ml). In cells treated with MWCNT, an increase in DNA migration due to the oxidative damage to purines was observed at a concentration of 1 and 10 microg/ml, whereas pyrimidines showed a significant increase only at the highest mass concentration tested. However, both SWCNT and MWCNT induced a statistically significant cytotoxic effect at the highest concentrations tested (P < 0.001). These findings suggest that both the MN and comet assays can reliably detect small amount of damaged DNA at both chromosome and nuclear levels in RAW 264.7 cells. Moreover, the modified version of the comet assay allows the specific detection of the induction of oxidative damage to DNA, which may be the underlying mechanism involved in the CNT-associated genotoxicity.