Danielle J. Padilla-Carlin

Effect of Size-Fractionation on the Toxicity of Amosite and Libby Amphibole Asbestos

Abnormally high incidences of asbestos-related pulmonary disease have been reported in residents of Libby, Montana, because of occupational and environmental exposure to asbestos-contaminated vermiculite. The mechanism by which Libby amphibole (LA) causes pulmonary injury is not known. The purpose of this study is to compare the cellular stress responses induced in primary human airway epithelial cells (HAECs) exposed to a respirable size fraction (≤ 2.5 μm) of Libby amphibole (LA(2.5)) to a similar size fraction of a reference amphibole sample amosite (AM(2.5)). HAEC were exposed to 0, 2.64, 13.2, or 26.4 μg/cm(2) AM(2.5) or LA(2.5) or to equivalent doses of unfractionated amosite (AM) or LA for 2 or 24 h. Comparable messenger RNA transcript levels were observed for interleukin-8, cyclooxygenase-2, and heme oxygenase-1 in HAEC following a 24-h exposure to AM or LA. Conversely, exposure to AM(2.5) resulted in a 4- to 10-fold greater induction in these proinflammatory mediators compared with LA(2.5) after 24 h. Evaluation of the expression of 84 additional genes involved in cellular stress and toxicity responses confirmed a more robust response for AM(2.5) compared with LA(2.5) on an equal mass basis. Differences in total surface area (TSA) by gas adsorption, total particle number, or oxidant generation by the size-fractionated particles did not account for the observed difference in response. In summary, AM(2.5) and LA(2.5) are at least as potent in stimulating production of proinflammatory cytokines as unfractionated AM and LA. Interestingly, AM(2.5) was more potent at inducing a proinflammatory response than LA(2.5). This difference could not be explained by differences in mineral contamination between the two samples, TSA, or oxidant generation by the samples.

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.

The role of cardiovascular disease-associated iron overload in Libby amphibole-induced acute pulmonary injury and inflammation

Pulmonary toxicity induced by asbestos is thought to be mediated through redox-cycling of fiber-bound and bioavailable iron (Fe). We hypothesized that Libby amphibole (LA)-induced cute lung injury will be exacerbated in rat models of cardiovascular disease (CVD)-associated Fe-overload and oxidative stress. Healthy male Wistar Kyoto (WKY), spontaneously hypertensive (SH) and SH heart failure (SHHF) rats were intratracheally instilled with 0.0, 0.25 or 1.0 mg/rat LA and examined at 1 day, 1 week or 1 month. Although histologically it was not possible to distinguish severity differences between strains in LA-induced initial inflammation and later fibrosis, quantitative assessment of biomarkers showed strain-related differences. LA-induced neutrophilic inflammation was reversible in WKY but persisted more in SH and SHHF. Lung MIP-2 mRNA increased only in WKY at 1 day in response to LA but not in SH and SHHF. Bronchoalveolar lavage fluid (BALF) protein increased in SH but not WKY at 1 week and 1 month, while γ-glutamyltransferase and N-acetyl-β-D-glucosaminidase activities increased in all strains (WKY>SH=SHHF). BALF ferritin levels were high at baseline and increased following LA exposure only in SH and SHHF. Ferritin heavy chain mRNA increased only in SHHF at 1 day. At 1 month ferritin light chain mRNA declined from already high baseline levels in SHHF but increased in WKY and SH suggesting its differential involvement in LA-induced injury in Fe-overload. Unlike WKY, both SHHF and SH failed to increase the lung lining antioxidant, ascorbate, in response to LA. We conclude that underlying CVD-associated Fe-overload is likely linked to persistent lung injury, inflammation and antioxidant decompensation following LA exposure in rats.

Comparison of functional, biochemical, and morphometric alterations in the lungs of 4 rat strains and hamsters following repeated intratracheal instillation of crocidolite asbestos.

ABSTRACT Four rat strains and hamsters were exposed to 0.7 mg crocidolite asbestos/g lung once/week for 3 weeks by intratracheal instillation (IT). Pulmonary function, biochemistry, and morphometry were evaluated at 3 and 6 months after IT. Each rat strain, but not the hamster, exhibited elevated lung volumes. Quasistatic compliance in rats and hamsters was reduced 15%–40% and 25%–50%, respectively. Diffusing capacity for carbon monoxide was elevated in the rats, but in hamsters, it was reduced at both time points. Hydroxyproline was increased in the rat strains but not in hamsters. Lung protein/dry weight was not altered in most of the rat strains and in hamsters at both time points. The linear mean intercept value was increased in Fischer 344 (F344) rats (3 and 6 months) and Long Evans rats (6 months), whereas in hamsters only at 6 months. Surface area was unchanged in both species. Specific density for parenchymal tissue was reduced for F344 rats at both time points, but alveolar density values did not change overall relative to species and time. The correlated functional and morphological changes in the hamster appeared more consistent with human asbestosis. Divergent lung responses in different species and strains should be considered when selecting laboratory animal models for studies related to asbestos exposure.