The role of inflammasomes in asbestos-induced mesothelial to fibroblastic transition

Malignant Mesothelioma (MM) is a fatal disease with a low median survival between 8 to 12 months after diagnosis. MM has a long latency period (10-60 years), is causally related to asbestos exposure, and is refractory to all available modes of therapy. Despite the causal association between asbestos exposure and MM however, the mechanisms by which asbestos induces this deadly disease remain unclear. Chronic inflammation due to the presence of asbestos fibers is believed to play an important role in all aspects of MM pathogenesis, from development to progression and resistance. Chronic inflammation has been shown to promote dysregulated wound repair, fibrosis and epithelial to mesenchymal transition (EMT). One of the inflammatory pathways that asbestos activates is the inflammasome (a multiprotein scaffold that assembles in response to various stimuli to facilitate the activation of caspase-1), which has been implicated in several chronic inflammatory diseases and disorders. The nucleotide binding oligomerization domain (NOD) like receptor containing a pyrin domain 3 (NLRP3) inflammasome, both as a whole or via its components [NLRP3, apoptosis related speck-like protein containing a CARD (caspase activating and recruitment domain) (ASC) and caspase-1] as well as its products, IL-1β and IL-18, has been implicated in the development of EMT during chronic inflammation. Asbestos fibers, especially the amphiboles, are non-biodegradable and thus persist in tissues of the body for years after exposure. In mesothelial cells, the squamous epitheliallike cells that line the serosal cavities of the body, from which MM originates, asbestos chronically activates the NLRP3 inflammasome. Asbestos also activates the NLRP3 inflammasome in human macrophages that can lead to the establishment of a chronic inflammation environment. We therefore hypothesized that asbestos dependent regulation of the inflammasome played a role in mesothelial to fibroblastic transition to facilitate eventual neoplastic transformation of the mesothelial cells. Using in vitro models, siRNA knockdown approaches as well as in vivo models of asbestos exposure utilizing inflammasome component knockout mice, we demonstrate that asbestos-induced reactive oxygen species generation modulates the redox state of the endogenous antioxidant, thioredoxin, causing its dissociation from thioredoxin interacting protein to promote activation of the inflammasome. We also show that the inflammasome plays a role in asbestos-induced mesothelial to fibroblastic transition (MFT) (a form of EMT occurring in the mesothelial cells) both in vitro and in vivo with a requirement for caspase-1 in vivo to promote thickening of the submesothelium. Through our studies, we have identified tissue factor pathway inhibitor 2 (TFPI2) and fibroblast growth factor 2 (FGF2) as molecules that are upregulated in response to asbestos exposure with potential roles in the progression of asbestos-induced MFT. There is a dearth of diagnostic biomarkers that enable early detection of MM, thus with further studies these two molecules could be explored as biomarkers of asbestos exposure/disease progression. TFPI2 levels were downregulated in response to blockage of IL-1β signaling and thus could be harnessed as a potential marker for therapy efficiency with further studies.