Inflammasomes: Fanning The Flames Of Malignant Mesothelioma Initiation

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PAMalignant mesothelioma (MM) is an aggressive and devastating cancer of the pleural/peritoneal mesothelium related to asbestos exposure. MM has a low survival rate (average: less than 12 months). Despite the causal relationship between asbestos and MM development, the exact mechanism by which asbestos causes MM is still poorly understood. There is an urgent need for the identification of mechanism(s) that may help in early detection and finding new treatment targets for prevention and treatment of MM.We have recently shown that asbestos exposure of human mesothelial cells (HMCs) leads to the activation of the NLRP3 inflammasome and a concomitant increase in the secretion of the inflammasome products, IL-1β and IL-18. Other studies have shown that IL-1β promotes the stemness of colon cancer cells and an epithelial to mesenchymal transition (EMT) of corneal endothelial cells. The activation of the inflammasome by asbestos is protracted in mesothelial cells and led us to hypothesize that the inflammasome and its products play a crucial role in the tumorigenesis of MM by promoting a mesothelial to fibroblastic transition (MFT) of HMCs. Our studies using EMT PCR array revealed that asbestos exposure results in the down regulation of E-cadherin and KRT19 among others in HMCs. Western blot analysis revealed decreases in expression of the epithelial markers, E-cadherin, Claudin-1 and β-catenin in response to asbestos exposure, while levels of the transcription factors ZEB-1, Snail and Slug (drivers of EMT) were increased in response to asbestos exposure in primary human pleural mesothelial cells. The secretion of MFT cytokines/mediators, IL-1β, IL-18, IL-6, IL-8, FGF2 and TFPI2 were also upregulated after asbestos exposure. Inhibition of NLRP3 by siRNA attenuated various parameters, suggesting a role for inflammasomes in the process. In vivo studies using an intra-peritoneal model of asbestos exposure in wild type and knockout mice (NLRP3-/-, ASC-/- and caspase-1-/-) also showed that asbestos exposure causes a thickening of the parietal peritoneal mesothelium over an 8 week period and an early increase in IL-1β and IL-18 levels in the peritoneal lavage fluid. An increase in collagen deposition and expression of vimentin and alpha smooth muscle actin was also observed, indicating a shift towards a mesenchymal phenotype in the mesothelium. Our results demonstrated that caspase-1 plays a significant role in asbestos-induced peritoneal wall thickening. Taken together, our results indicate that asbestos exposure activates the NLRP3 inflammasome thereby increasing secretion of the pro-inflammatory cytokines, IL-6 and IL-8 in an IL-1β dependent manner while compromising the fibrinolytic capacity of the mesothelium through the increased expression of TFPI2 and other protease inhibitors involved in fibrin clearance (e.g. SERPINE1). This altogether leads to MFT and may eventually result in MM development. This work is supported by funding from NIEHS grant RO1 ES021110Citation Format: Joyce K. Thompson, Maximilian B. MacPherson, Stacie L. Beuschel, Arti Shukla. Inflammasomes: fanning the flames of malignant mesothelioma initiation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3164. doi:10.1158/1538-7445.AM2015-3164