Multiwalled Carbon Nanotubes of Varying Size Lead to DNA Methylation Changes That Correspond to Lung Inflammation and Injury in a Mouse Model

Diversity in physicochemical properties of engineered multiwalled carbon nanotubes (MWCNTs) increases the complexity involved in interpreting toxicity studies of these materials. Studies indicate that epigenetic changes could be at least partially involved in MWCNTs-induced pro-inflammatory and fibrotic lung pathology. Therefore, we examined distinct methylation changes in response to MWCNTs of varied sizes to identify potential epigenetic biomarkers of MWCNTs exposure and disease progression. CS7BL/6 mice were exposed via oropharyngeal instillation to a single dose (50 mu g) to one of three differently sized MWCNTs: "narrow short" (NS), "wide short" (WS), and "narrow long" (NL). Vehicle-treated control mice received dispersion media (DM) only. Whole lung lavage fluid (LLF) and lung tissue were collected 24 h and 7 days postexposure to evaluate pro-inflammatory cytokines, epigenetic, or histological responses at acute and subchronic intervals, respectively. Luminometric methylation assay and pyrosequencing were used to measure global DNA methylation as well as promoter methylation of inflammation and fibrosis-related genes, respectively. Pro-inflammatory cytokines, including IL-1 beta, IL 6, and TNF-alpha, were measured using enzyme-linked immunosorbant assay, while airway thickening and interstitial collagen accumulation were measured in 7-day lung tissue using laser scanning cytometry. Distinct patterns of methylation (i.e., IL-1 beta, IL-6, and TNF-alpha) among the different sized MWCNTs at 24 h postexposure corresponded to some pro-inflammatory cytokine measurements from whole LLF. Fibrosis-related gene, Thy-1, was significantly hypermethylated after exposures to WS and NL MWCNTs, while only NL MWCNTs induced significantly lower global DNA methylation. After 7 days, a hierarchy in airway thickness and interstitial collagen deposition was observed: NS < WS < NL. However, only airway thickness was significantly greater in the WS and NL MWCNTs-exposed groups than the DM-exposed group. These data suggest that methylation changes could be involved in the initial immune response of inflammation and tissue remodeling that precedes lung disease in response to different MWCNTs sizes.