Abstract
BackgroundHuman exposure to Libby amphibole (LA) asbestos increases risk of lung cancer, mesothelioma, and non-malignant respiratory disease. This study evaluated potency and time-course effects of LA and positive control amosite (AM) asbestos fibers in male F344 rats following nose-only inhalation exposure.MethodsRats were exposed to air, LA (0.5, 3.5, or 25.0 mg/m3 targets), or AM (3.5 mg/m3 target) for 10 days and assessed for markers of lung inflammation, injury, and cell proliferation. Short-term results guided concentration levels for a stop-exposure study in which rats were exposed to air, LA (1.0, 3.3, or 10.0 mg/m3), or AM (3.3 mg/m3) 6 h/day, 5 days/week for 13 weeks, and assessed 1 day, 1, 3, and 18 months post-exposure. Fibers were relatively short; for 10 mg/m3 LA, mean length of all structures was 3.7 μm and 1 % were longer than 20 μm.ResultsTen days exposure to 25.0 mg/m3 LA resulted in significantly increased lung inflammation, fibrosis, bronchiolar epithelial cell proliferation and hyperplasia, and inflammatory cytokine gene expression compared to air. Exposure to 3.5 mg/m3 LA resulted in modestly higher markers of acute lung injury and inflammation compared to AM. Following 13 weeks exposure, lung fiber burdens correlated with exposure mass concentrations, declining gradually over 18 months. LA (3.3 and 10.0 mg/m3) and AM produced significantly higher bronchoalveolar lavage markers of inflammation and lung tissue cytokines, Akt, and MAPK/ERK pathway components compared to air control from 1 day to 3 months post-exposure. Histopathology showed alveolar inflammation and interstitial fibrosis in all fiber-exposed groups up to 18 months post-exposure. Positive dose trends for incidence of alveolar epithelial hyperplasia and bronchiolar/alveolar adenoma or carcinoma were observed among LA groups.ConclusionsInhalation of relatively short LA fibers produced inflammatory, fibrogenic, and tumorigenic effects in rats which replicate essential attributes of asbestos-related disease in exposed humans. Fiber burden, inflammation, and activation of growth factor pathways may persist and contribute to lung tumorigenesis long after initial LA exposure. Fiber burden data are being used to develop a dosimetry model for LA fibers, which may provide insights on mode of action for hazard assessment.Electronic supplementary materialThe online version of this article (doi:10.1186/s12989-016-0130-z) contains supplementary material, which is available to authorized users.
Highlights
Human exposure to Libby amphibole (LA) asbestos increases risk of lung cancer, mesothelioma, and non-malignant respiratory disease
Numbers of bronchoalveolar lavage fluid (BALF) alveolar macrophages were not significantly affected by exposure, whereas neutrophils were observed in all AM and LA exposure groups, and were significantly higher in the 25.0 mg/m3 LA-exposed group compared to all other groups (Fig. 1)
Protein and lactate dehydrogenase (LDH) levels were significantly higher in the 3.5 mg/m3 LA-exposed group compared with air, 0.5 mg/m3 LA, and 3.5 mg/m3 AMexposed groups, indicating greater lung toxicity of LA compared with AM at the same mass concentration
Summary
Human exposure to Libby amphibole (LA) asbestos increases risk of lung cancer, mesothelioma, and non-malignant respiratory disease. Results: Ten days exposure to 25.0 mg/m3 LA resulted in significantly increased lung inflammation, fibrosis, bronchiolar epithelial cell proliferation and hyperplasia, and inflammatory cytokine gene expression compared to air. LA (3.3 and 10.0 mg/m3) and AM produced significantly higher bronchoalveolar lavage markers of inflammation and lung tissue cytokines, Akt, and MAPK/ERK pathway components compared to air control from 1 day to 3 months post-exposure. Occupational exposure to LA is associated with significantly increased risk of respiratory disease, including non-malignant respiratory disease (such as localized and diffuse pleural thickening, asbestosis, and autoimmune disease), lung cancer, and mesothelioma, compared to background rates within the U.S population [3,4,5,6,7,8,9,10,11]. Outside of Libby, exposure to LA is a continuing risk since the contaminated vermiculite ore was shipped to locations around the nation for processing and used in a variety of applications, especially attic insulation which may be present in homes throughout the United States [1]
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