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

Abstract

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.

Keywords:

• Carbon nanotubes
• mitochondria
• genotoxicity
• epithelial
• in vitro

Acknowledgements

The authors would like to acknowledge Greg Solomon, Jason Malphurs, and Nicole Reeves for their assistance with ultradeep sequencing, Jianying Li for statistical analysis, and Dr Michael Dykstra for ultrastructural imaging. The authors would also like to acknowledge Annette Rice and the NIEHS Clinical Research Unit for the recruitment and bronchoscopy of healthy donors to obtain bronchial cell samples, as well as Wesley Gladwell and Jacqui Marzec for their technical assistance with this study. The authors also acknowledge the assistance of Dr. James Samet at the US Environmental Protection Agency (EPA) in acquiring the darkfield microscopy images used in the Supplemental Information. Finally, The authors wish to acknowledge the assistance and expertise of Dr. Scott Randell and his laboratory in the Cystic Fibrosis Center the University of North Carolina Chapel Hill.

Disclosure statement

The authors report no conflict of interest.

Additional information

Funding

This research was supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences (NIEHS).