Transcriptional activation of inflammasome components by Libby amphibole and the role of iron

Abstract

The induction of the NALP3 inflammasome complex is shown to be necessary for the development of fibrosis after asbestos exposure. Libby amphibole (LA) induces lung inflammation and fibrosis, while complexation of iron (Fe) on fibers inhibits inflammation. In this study we examined the ability of LA to induce the inflammasome cascade and the role of Fe in modulating inflammasome activity. Spontaneously hypertensive rats were exposed intratracheally to either saline (300 μl), deferoxamine (Def) (1 mg), FeCl₃ (21 μg), LA (0.5 mg), Fe-loaded LA (Fe + LA), or LA + Def. Activities of oxidative stress-sensitive enzymes, expression of inflammasome-specific genes, and cytokine proteins in bronchoalveolar lavage fluid were analyzed. Lung enzymes at 4 h and 24 h post-exposure were unchanged. LA increased lung expression of genes including interleukin-1β (IL-1β), cathepsin-B, ASC, NALP3, interleukin (IL)-6 and NFκB. LA+Fe significantly reduced IL-1β and NFκB with a trend of reduction in ASC, NALP3, cathepsin-B and IL-6 expression. Def treatment did not reverse the inhibitory effect of Fe on IL-1β and ASC but reversed IL-6 expression. CCL-7, CCL-12, CXCL-3 and COX-2 were induced by LA while LA+Fe tended to reduce these responses. Phosphorylation of ERK but not MEK was increased at 4 h after LA but not LA+Fe exposure. In conclusion, components of the NALP3 inflammasome are transcriptionally activated acutely during LA-induced inflammation. The key inflammatory regulators IL-1β and NFκB were inhibited in the presence of surface-complexed Fe possibly through decreased ERK signaling upstream of the NALP3 inflammasome. The inflammasome activation by LA may contribute to fibrosis, and Fe may reduce this response and alter compensatory mechanisms in individuals exposed to LA.

Keywords::

• Libby amphibole
• lung inflammation
• inflammasome
• iron

Acknowledgements

We thank Mr. Geremy Knapp of the US EPA for analysis of inflammasome-specific transcriptional changes using PCR arrays. We also thank Drs. Michael Madden, Gary Hatch, and Maureen Gwinn of the US EPA for their review of the manuscript.

Declaration of interest

This work was supported by EPA/UNC Toxicology Research Program, Training Agreement CR833237, with the Curriculum in Toxicology, University of North Carolina at Chapel Hill.