Effect of size fractionation on the toxicity of amosite and Libby amphibole asbestos

Toxicity of size separated chrysotile fibres: The relevance of the macrophage-endothelial axis crosstalk

Asbestos minerals have been widely exploited due to their physical-chemical properties, and chrysotile asbestos has accounted for about 95% of all asbestos commercially employed worldwide. The exposure to chrysotile, classified like other five amphibole asbestos species as carcinogenic to humans, represents a serious occupational and environmental hazard. Nevertheless, this mineral is still largely employed in about 65% of the countries worldwide, which still allow its "safe use". The complex mechanisms through which the mineral fibres induce toxicity are not yet completely understood. In this regard, the morphometric parameters of asbestos fibres (e.g., length, width, aspect ratio) are known for their fundamental role in determining the degree of pathogenicity. In this context, the potential toxicity of short chrysotile fibres remains widely debated due to the contradictory results from countless studies. Thus, the present study investigated the different toxicity mechanisms of two representative batches of short (length ≤5 µm) and long (length >5 µm) chrysotile fibres obtained by cryogenic milling. The fibre doses were based upon equal mass and size, since due to chrysotile ability to form bundles, it was not possible to calculate the number of fibers applied per cell. The cytotoxic, genotoxic, and pro-inflammatory potential of the two size-separated chrysotile fractions was investigated on human THP-1-derived macrophages and HECV endothelial cells, both separately and in a co-culture setup, mimicking the alveolar pro-inflammatory microenvironment, in time course experiments up to 1 week. Both chrysotile fractions displayed cytotoxic, genotoxic, and pro-inflammatory effects, with results comparable to the well-known damaging effects of crocidolite asbestos, or higher, as in the case of the longer chrysotile fraction. Furthermore, in presence of HECV, fibre-treated macrophages showed prolonged inflammation, indicating an interesting crosstalk between these cells able to sustain a low-grade chronic inflammation in the lung. In conclusion, these results help to shed light on some important open questions on the mechanisms of toxicity of chrysotile asbestos fibres.

Amphibole asbestos as an environmental trigger for systemic autoimmune diseases

A growing body of evidence supports an association between systemic autoimmune disease and exposure to amphibole asbestos, a form of asbestos typically with straight, stiff, needle-like fibers that are easily inhaled. While the bulk of this evidence comes from the population exposed occupationally and environmentally to Libby Amphibole (LA) due to the mining of contaminated vermiculite in Montana, studies from Italy and Australia are broadening the evidence to other sites of amphibole exposures. What these investigations have done, that most historical studies have not, is to evaluate amphibole asbestos separately from chrysotile, the most common commercial asbestos in the United States. Here we review the current and historical evidence summarizing amphibole asbestos exposure as a risk factor for autoimmune disease. In both mice and humans, amphibole asbestos, but not chrysotile, drives production of both antinuclear autoantibodies (ANA) associated with lupus-like pathologies and pathogenic autoantibodies against mesothelial cells that appear to contribute to a severe and progressive pleural fibrosis. A growing public health concern has emerged with revelations that a) unregulated asbestos minerals can be just as pathogenic as commercial (regulated) asbestos, and b) bedrock and soil occurrences of asbestos are far more widespread than previously thought. While occupational exposures may be decreasing, environmental exposures are on the rise for many reasons, including those due to the creation of windborne asbestos-containing dusts from urban development and climate change, making this topic an urgent challenge for public and heath provider education, health screening and environmental regulations.

Distinct Pro-Inflammatory Mechanisms Elicited by Short and Long Amosite Asbestos Fibers in Macrophages

While exposure to long amphibolic asbestos fibers (L > 10 µm) results in the development of severe diseases including inflammation, fibrosis, and mesothelioma, the pathogenic activity associated with short fibers (L < 5 µm) is less clear. By exposing murine macrophages to short (SFA) or long (LFA) fibers of amosite asbestos different in size and surface chemistry, we observed that SFA internalization resulted in pyroptotic-related immunogenic cell death (ICD) characterized by the release of the pro-inflammatory damage signal (DAMP) IL-1α after inflammasome activation and gasdermin D (GSDMD)-pore formation. In contrast, macrophage responses to non-internalizable LFA were associated with tumor necrosis factor alpha (TNF-α) release, caspase-3 and -7 activation, and apoptosis. SFA effects exclusively resulted from Toll-like receptor 4 (TLR4), a pattern-recognition receptor (PRR) recognized for its ability to sense particles, while the response to LFA was elicited by a multifactorial ignition system involving the macrophage receptor with collagenous structure (SR-A6 or MARCO), reactive oxygen species (ROS) cascade, and TLR4. Our findings indicate that asbestos fiber size and surface features play major roles in modulating ICD and inflammatory pathways. They also suggest that SFA are biologically reactive in vitro and, therefore, their inflammatory and toxic effects in vivo should not be underestimated.

Autoimmune antibodies and asbestos exposure: Evidence from Wittenoom, Western Australia

BACKGROUND

Studies comparing different forms of asbestos are rare, and limited by the failure to compare results with unexposed populations. We compare autoimmune responses among former workers and residents of the crocidolite mining and milling town of Wittenoom, Western Australia, with an unexposed population.

METHODS

ANA testing using indirect immunofluorescence was performed on randomly selected serum samples from Wittenoom workers or residents and compared with those from participants of another unexposed cohort study.

RESULTS

ANA scores were higher in the Wittenoom participants compared with Busselton and the odds of being ANA positive was fivefold greater among Wittenoom participants than Busselton (OR 5.5, 95%CI 2.3-13.0).

CONCLUSIONS

This study is the first to report increased ANA positivity among persons exposed exclusively to crocidolite. This finding of a high frequency of positive ANA tests among crocidolite-exposed subjects may be an indicator for an increased risk of systemic autoimmune diseases and needs further scrutiny.

Current Research and Opportunities to Address Environmental Asbestos Exposures

Asbestos-related diseases continue to result in approximately 120,000 deaths every year in the United States and worldwide. Although extensive research has been conducted on health effects of occupational exposures to asbestos, many issues related to environmental asbestos exposures remain unresolved. For example, environmental asbestos exposures associated with a former mine in Libby, Montana, have resulted in high rates of nonoccupational asbestos-related disease. Additionally, other areas with naturally occurring asbestos deposits near communities in the United States and overseas are undergoing investigations to assess exposures and potential health risks. Some of the latest public health, epidemiological, and basic research findings were presented at a workshop on asbestos at the 2014 annual meeting of the Society of Toxicology in Phoenix, Arizona. The following focus areas were discussed: a) mechanisms resulting in fibrosis and/or tumor development; b) relative toxicity of different forms of asbestos and other hazardous elongated mineral particles (EMPs); c) proper dose metrics (e.g., mass, fiber number, or surface area of fibers) when interpreting asbestos toxicity; d) asbestos exposure to susceptible populations; and e) using toxicological findings for risk assessment and remediation efforts. The workshop also featured asbestos research supported by the National Institute of Environmental Health Sciences, the Agency for Toxic Substances and Disease Registry, and the U.S. Environmental Protection Agency. Better protection of individuals from asbestos-related health effects will require stimulation of new multidisciplinary research to further our understanding of what constitutes hazardous exposures and risk factors associated with toxicity of asbestos and other hazardous EMPs (e.g., nanomaterials).

Comparative long-term toxicity of Libby amphibole and amosite asbestos in rats after single or multiple intratracheal exposures

In former mine workers of Libby, MT, exposure to amphibole-containing vermiculite was linked to increased rates of asbestosis, lung cancer, and mesothelioma. Although many studies showed adverse effects following exposure to Libby amphibole (LA; a mixture of winchite, richterite, and tremolite), little is known regarding the relative toxicity of LA compared to regulated asbestos, or regarding the risks associated with acute high-dose exposures relative to repeated low-dose exposures. In this study, pulmonary function, inflammation, and pathology were assessed after single or multiple intratracheal (IT) exposures of LA or a well-characterized amosite (AM) control fiber with equivalent fiber characteristics. Male F344 rats were exposed to an equivalent total mass dose (0.15, 0.5, 1.5, or 5 mg/rat) of LA or AM administered either as a single IT instillation, or as multiple IT instillations given every other week over a 13-wk period, and necropsied up to 20 mo after the initial IT. When comparing the two fiber types, in both studies LA resulted in greater acute neutrophilic inflammation and cellular toxicity than equal doses of AM, but long-term histopathological changes were approximately equivalent between fibers, suggesting that LA is at least as toxic as AM. In addition, although no dose-response relationship was discerned, mesothelioma or lung carcinomas were found after exposure to low and high dose levels of LA or AM in both studies. Conversely, when comparing studies, an equal mass dose given over multiple exposures instead of a single bolus resulted in greater chronic pathological changes in lung at lower doses, despite the initially weaker acute inflammatory response. Overall, these results suggest that there is a possibility of greater long-term pathological changes with repeated lower LA dose exposures, which more accurately simulates chronic environmental exposures.

In vitro determinants of asbestos fiber toxicity: effect on the relative toxicity of Libby amphibole in primary human airway epithelial cells

Background

An abnormally high incidence of lung disease has been observed in the residents of Libby, Montana, which has been attributed to occupational and environmental exposure to fibrous amphiboles originating from a nearby contaminated vermiculite mine. The composition of Libby amphibole (LA) is complex and minimal toxicity data are available. In this study, we conduct a comparative particle toxicity analysis of LA compared with standard reference asbestiform amphibole samples.

Methods

Primary human airway epithelial cells (HAEC) were exposed to two different LA samples as well as standard amphibole reference samples. Analysis of the samples included a complete particle size distribution analysis, calculation of surface area by electron microscopy and by gas adsorption and quantification of surface-conjugated iron and hydroxyl radical production by the fibers. Interleukin-8 mRNA levels were quantified by qRT-PCR to measure relative pro-inflammatory response induced in HAEC in response to amphibole fiber exposure. The relative contribution of key physicochemical determinants on the observed pro-inflammatory response were also evaluated.

Results

The RTI amosite reference sample contained the longest fibers and demonstrated the greatest potency at increasing IL-8 transcript levels when evaluated on an equal mass basis. The two LA samples and the UICC amosite reference sample consisted of similar particle numbers per milligram as well as similar particle size distributions and induced comparable levels of IL-8 mRNA. A strong correlation was observed between the elongated particle (aspect ratio ≥3:1) dose metrics of length and external surface area. Expression of the IL-8 data with respect to either of these metrics eliminated the differential response between the RTI amosite sample and the other samples that was observed when HAEC were exposed on an equal mass basis.

Conclusions

On an equal mass basis, LA is as potent as the UICC amosite reference sample at inducing a pro-inflammatory response in HAEC but is less potent than the RTI amosite sample. The results of this study show that the particle length and particle surface area are highly correlated metrics that contribute significantly to the toxicological potential of these amphibole samples with respect to the inflammogenic response induced in airway epithelial cells.

Length-dependent effect of single-walled carbon nanotube exposure in a dynamic cell growth environment of human alveolar epithelial cells

Despite the great use of nanomaterials for engineering and medical applications, nanomaterials may have adverse consequences by accidental exposure, because of their nanoscale size, composition and shape. Like many nanomaterials, carbon nanotubes (CNTs) have been used for many proven applications, but the size of the CNTs makes them more readily become airborne and can therefore create the risk of being inhaled by a worker. In this study, we evaluated single-walled CNT (SWCNT)-induced effects on cellular responses such as cell proliferation, inflammatory response and oxidative stress in dynamic cell growth condition. A dynamic cell growth environment was established to mimic the dynamic changes in the amount of circumferential and longitudinal expansion and contraction occurred during normal breathing movement in the lung. Two different length (short: outer diameter (OD) 1-2 nm, length 0.5-2 μm; long: OD 1-2 nm, length 5-30 μm) of SWCNTs were used at different exposure concentrations (5, 10 and 20 μg/ml) during the different exposure duration (24, 48 and 72 h). Dynamic environment facilitated altered interaction between SWCNTs and A549 monolayer. Cellular responses in dynamic condition were significantly different from those in static condition. Moreover, cellular responses were dependent on the length of SWCNTs both in static and dynamic cell growth conditions.

Silicon, a Possible Link between Environmental Exposure and Autoimmune Diseases: The Case of Rheumatoid Arthritis

Silicon is one of the most common chemicals on earth. Several compounds such as silica, asbestos, silicone or, nanoparticles are built from tetrahedral units with silicon as the central atom. Despite these, structural similarities, they have rarely been analyzed as a group. These compounds generate significant biological alterations that include immune hyperactivation, production of the reactive species of oxygen and tissue injury. These pathological processes may trigger autoimmune responses and lead to the development of rheumatoid arthritis. Populations at risk include those that constantly work in industrial process, mining, and agriculture as well as those that undergo silicone implants. Herein a review on the main features of these compounds and how they may induce autoimmune responses is presented.

Sumas Mountain chrysotile induces greater lung fibrosis in Fischer344 rats than Libby amphibole, El Dorado tremolite, and Ontario ferroactinolite

The physical properties of different types of asbestos may strongly affect health outcomes in exposed individuals. This study was designed to provide understanding of the comparative toxicity of naturally occurring asbestos (NOA) fibers including Libby amphibole (LA), Sumas Mountain chrysotile (SM), El Dorado tremolite (ED), and Ontario ferroactinolite (ON) cleavage fragments. Rat-respirable fractions (PM₂.₅) were prepared by water elutriation. Surface area was greater for SM (64.1 m²/g) than all other samples (range: 14.1-16.2 m²/g), whereas mean lengths and aspect ratios (ARs) for LA and SM were comparable and greater than ED and ON. Samples were delivered via a single intratracheal (IT) instillation at doses of 0.5 and 1.5mg/rat. One day post-IT instillation, low-dose NOA exposure resulted in a 3- to 4-fold increase in bronchoalveolar lavage fluid (BALF) cellularity compared with dispersion media (DM) controls, whereas high-dose exposure had a more severe effect on lung inflammation which varied by source. Although inducing less neutrophilic inflammation than ON and ED, exposure to either LA or SM resulted in a greater degree of acute lung injury. Three months post-IT instillation, most BALF parameters had returned to control levels, whereas the development of fibrosis persisted and was greatest in SM-exposed rats (SM > LA > ON > ED). These data demonstrate that fiber length and higher AR are directly correlated with the severity of fibrosis and that, in the rat, exposure to SM is more fibrogenic than LA which suggests that there may be cause for concern for people at risk of being exposed to NOA from the Sumas Mountain landslide.

Interactive toxicity of usnic acid and lipopolysaccharides in human liver HepG2 cells

Usnic acid (UA), a natural botanical product, is a constituent of some dietary supplements used for weight loss. It has been associated with clinical hepatotoxicity leading to liver failure in humans. The present study was undertaken to evaluate the interactive toxicity, if any, of UA with lipopolysaccarides (LPS), a potential contaminant of food, at low non-toxic concentrations. The human hepatoblastoma HepG2 cells were treated with the vehicle control and test agents, separately and in a binary mixture, for 24 h at 37°C in 5% CO2. After the treatment period, the cells were evaluated by the traditional biochemical endpoints of toxicity in combination with the toxicogenomic endpoints that included cytotoxicity, oxidative stress, mitochondrial injury and changes in pathway-focused gene expression profiles. Compared with the controls, low non-toxic concentrations of UA and LPS separately showed no effect on the cells as determined by the biochemical endpoints. However, the simultaneous mixed exposure of the cells to their binary mixture resulted in increased cytotoxicity, oxidative stress and mitochondrial injury. The pathway-focused gene expression analysis resulted in the altered expression of several genes out of 84 genes examined. Most altered gene expressions induced by the binary mixture of UA and LPS were different from those induced by the individual constituents. The genes affected by the mixture were not modulated by either UA or LPS. The results of the present study suggest that the interactions of low nontoxic concentrations of UA and LPS produce toxicity in HepG2 cells.

Graphene-based nanoplatelets: a new risk to the respiratory system as a consequence of their unusual aerodynamic properties

Graphene is a new nanomaterial with unusual and useful physical and chemical properties. However, in the form of nanoplatelets this new, emerging material could pose unusual risks to the respiratory system after inhalation exposure. The graphene-based nanoplatelets used in this study are commercially available and consist of several sheets of graphene (few-layer graphene). We first derived the respirability of graphene nanoplatelets (GP) from the basic principles of the aerodynamic behavior of plate-shaped particles which allowed us to calculate their aerodynamic diameter. This showed that the nanoplatelets, which were up to 25 μm in diameter, were respirable and so would deposit beyond the ciliated airways following inhalation. We therefore utilized models of pharyngeal aspiration and direct intrapleural installation of GP, as well as an in vitro model, to assess their inflammatory potential. These large but respirable GP were inflammogenic in both the lung and the pleural space. MIP-1α, MCP-1, MIP-2, IL-8, and IL-1β expression in the BAL, the pleural lavage, and cell culture supernatant from THP-1 macrophages were increased with GP exposure compared to controls but not with nanoparticulate carbon black (CB). In vitro, macrophages exposed to GP showed expression of IL-1β. This study highlights the importance of nanoplatelet form as a driver for in vivo and in vitro inflammogenicity by virtue of their respirable aerodynamic diameter, despite a considerable 2-dimensional size which leads to frustrated phagocytosis when they deposit in the distal lungs and macrophages attempt to phagocytose them. Our data suggest that nanoplatelets pose a novel nanohazard and structure-toxicity relationship in nanoparticle toxicology.

Long-term response of rats to single intratracheal exposure of Libby amphibole or amosite

In former mine workers and residents of Libby, Montana, exposure to amphibole-contaminated vermiculite has been associated with increased incidences of asbestosis and mesothelioma. In this study, long-term effects of Libby amphibole (LA) exposure were investigated relative to the well-characterized amosite asbestos in a rat model. Rat-respirable fractions of LA and amosite (aerodynamic diameter≤2.5 μm) were prepared by water elutriation. Male F344 rats were exposed to a single dose of either saline, amosite (0.65 mg/rat), or LA (0.65 or 6.5 mg/rat) by intratracheal (IT) instillation. One year after exposure, asbestos-exposed rats displayed chronic pulmonary inflammation and fibrosis. Two years postexposure, lung inflammation and fibrosis progressed in a time- and dose-dependent manner in LA-exposed rats, although the severity of inflammation and fibrosis was smaller in magnitude than in animals exposed to amosite. In contrast, gene expression of the fibrosis markers Col 1A2 and Col 3A1 was significantly greater in LA-exposed compared to amosite-exposed rats. There was no apparent evidence of preneoplastic changes in any of the asbestos-exposed groups. However, all asbestos-exposed rats demonstrated a significant increase in the expression of epidermal growth factor receptor (EGFR) 2 yr after instillation. In addition, only LA-exposed rats showed significant elevation in mesothelin (Msln) and Wilms' tumor gene (WT1) expression, suggesting possible induction of tumor pathways. These results demonstrate that a single IT exposure to LA is sufficient to induce significant fibrogenic, but not carcinogenic, effects up to 2 yr after exposure that differ both in quality and magnitude from those elicited by amosite administration at the same mass dose in F344 rats. Data showed that LA was on a mass basis less potent than amosite.

Subchronic pulmonary pathology, iron overload, and transcriptional activity after Libby amphibole exposure in rat models of cardiovascular disease

BACKGROUND

Surface-available iron (Fe) is proposed to contribute to asbestos-induced toxicity through the production of reactive oxygen species.

OBJECTIVE

Our goal was to evaluate the hypothesis that rat models of cardiovascular disease with coexistent Fe overload would be increasingly sensitive to Libby amphibole (LA)-induced subchronic lung injury.

METHODS

Male healthy Wistar Kyoto (WKY), spontaneously hypertensive (SH), and SH heart failure (SHHF) rats were intratracheally instilled with 0.0, 0.25, or 1.0 mg LA (with saline as the vehicle). We examined bronchoalveolar lavage fluid (BALF) and histological lung sections after 1 week, 1 month, or 3 months for pulmonary biomarkers and pathology. SHHF rats were also assessed at 6 months for pathological changes.

RESULTS

All animals developed concentration- and time-dependent interstitial fibrosis. Time-dependent Fe accumulation occurred in LA-laden macrophages in all strains but was exacerbated in SHHF rats. LA-exposed SHHF rats developed atypical hyperplastic lesions of bronchiolar epithelial cell origin at 3 and 6 months. Strain-related baseline differences existed in gene expression at 3 months, with persistent LA effects in WKY but not SH or SHHF rats. LA exposure altered genes for a number of pathways, including inflammation, immune regulation, and cell-cycle control. Cell-cycle control genes were inhibited after LA exposure in SH and SHHF but not WKY rats, whereas tumor suppressor genes were induced only in WKY rats. The inflammatory gene expression also was apparent only in WKY rats.

CONCLUSION

These data show that in Fe-overload conditions, progressive Fe accumulation occurs in fiber-laden macrophages within LA-induced lesions. Fe overload does not appear to contribute to chronic inflammation, and its role in hyperplastic lesion development requires further examination.

The role of iron in Libby amphibole-induced acute lung injury and inflammation

Complexation of host iron (Fe) on the surface of inhaled asbestos fibers has been postulated to cause oxidative stress contributing to in vivo pulmonary injury and inflammation. We examined the role of Fe in Libby amphibole (LA; mean length 4.99 µm ± 4.53 and width 0.28 µm ± 0.19) asbestos-induced inflammogenic effects in vitro and in vivo. LA contained acid-leachable Fe and silicon. In a cell-free media containing FeCl(3), LA bound #17 µg of Fe/mg of fiber and increased reactive oxygen species generation #3.5 fold, which was reduced by deferoxamine (DEF) treatment. In BEAS-2B cells exposure to LA, LA loaded with Fe (FeLA), or LA with DEF did not increase HO-1 or ferritin mRNA expression. LA increased IL-8 expression, which was reduced by Fe loading but increased by DEF. To determine the role of Fe in LA-induced lung injury in vivo, spontaneously hypertensive rats were exposed intratracheally to either saline (300 µL), DEF (1 mg), FeCl(3) (21 µg), LA (0.5 mg), FeLA (0.5 mg), or LA + DEF (0.5 mg). LA caused BALF neutrophils to increase 24 h post-exposure. Loading of Fe on LA but not chelation slightly decreased neutrophilic influx (LA + DEF > LA > FeLA). At 4 h post-exposure, LA-induced lung expression of MIP-2 was reduced in rats exposed to FeLA but increased by LA + DEF (LA + DEF > LA > FeLA). Ferritin mRNA was elevated in rats exposed to FeLA compared to LA. In conclusion, the acute inflammatory response to respirable fibers and particles may be inhibited in the presence of surface-complexed or cellular bioavailable Fe. Cell and tissue Fe-overload conditions may influence the pulmonary injury and inflammation caused by fibers.

Effect of gold nanorod surface chemistry on cellular response

Gold nanorods (GNRs) stabilized with cetyltrimethylammonium bromide (CTAB) and GNR functionalized via a ligand exchange method with either thiolated polyethylene glycol (PEG(5000)) or mercaptohexadecanoic acid (MHDA) were investigated for their stability in biological media and subsequent toxicological effects to HaCaT cells. GNR-PEG and GNR-MHDA exhibited minimal effects on cell proliferation, whereas GNR-CTAB reduced cell proliferation significantly due to the inherent toxicity of the cationic surfactant to cells. Cell uptake studies indicated relatively low uptake for GNR-PEG and high uptake for GNR-MHDA. Reverse transcriptase polymerase chain reaction (RT-PCR) revealed that GNR-PEG induced less significant and unique changes in the transcription levels of 84 genes related to stress and toxicity compared to GNR-MHDA. The results demonstrate that, although cell proliferation was not affected by both particles, there is a significant difference in gene expression in GNR-MHDA exposed cells, suggesting long-term implications for chronic exposure.

Pulmonary inflammatory and fibrotic responses in Fischer 344 rats after intratracheal instillation exposure to Libby amphibole

Increased incidences of asbestosis have been reported in workers from Libby, MT, associated with exposures to amphibole-contaminated vermiculite. In this study pulmonary and histopathological changes were investigated following Libby amphibole (LA) exposure in a rat model. Rat respirable fractions of LA and amosite (aerodynamic diameter <2.5 μm) were prepared by water elutriation. Male F344 rats were exposed to single doses of either saline (SAL), amosite (0.65 mg/rat), or LA (0.65 or 6.5 mg/rat) by intratracheal instillation. At times from 1 d to 3 mo after exposure, bronchoalveolar lavage (BAL) was performed and right and left lungs were removed for reverse-transcription polymerase chain reaction (RT-PCR) and histopathological analysis, respectively. Data indicated that 0.65 mg amosite resulted in a higher degree of pulmonary injury, inflammation, and fibrotic events than LA at the same mass dose. Exposure to either amosite or high dose LA resulted in higher levels of cellular permeability and injury, inflammatory enzymes, and iron binding proteins in both BAL fluid and lung tissue at most time points when compared to SAL controls. However, mRNA expression for some growth factors (e.g., platelet-derived growth factor [PDGF]-A and transforming growth factor [TGF]-1β), which contribute to fibrosis, were downregulated at several time points. Furthermore, histopathological examination showed notable thickening of interstitial areas surrounding the alveolar ducts and terminal bronchioles. On a mass dose basis, amosite produced a greater acute and persistent lung injury for at least 3 mo after exposure. However, further testing and analysis of LA are needed with regard to the dose metric to fully evaluate its potential fibrogenicity and carcinogenicity.