A role for the inflammasome in STAT3-mediated gastric tumourigenesis

Abstract

The innate immune system is the body’s first line of defence against invading pathogens, and also detects signals of cellular damage that may indicate illness. Chronic dysfunction of this immune response can lead to permanent tissue damage and changes in cell function, which can eventually lead to cells evading the cell cycle and becoming cancerous. Gastric cancer, malignant disease of the stomach, is a product of long-term infection and inflammation in the stomach, often associated with infection by H. pylori, a common gastric pathogen. Gastric cancer remains one of the deadliest cancers worldwide, as detection methods and treatment options are limited. Investigation into how the immune system functions in the context of stomach cancer may help to establish targetted treatment options and earlier detection techniques. The pro-inflammatory cytokines IL-1β and IL-18 have previously been shown to play a role in the development of gastric cancer, as has the transcription factor STAT3. The studies described in this thesis address the potential for a link between these two key inflammation-associated molecules using a well-established mouse model for gastric cancer, in which gastric tumours develop spontaneously as a result of hyperactivation of STAT3. It was found that IL-18, but not IL-1β, was elevated in this STAT3-driven model for gastric tumourigenesis, and that, importantly, a key upstream mediator of IL-18 activation, caspase-1, displayed increased activity in the stomachs of these mice. Furthermore, when the gene for a key adaptor molecule, ASC, was deleted from these mice, there was a significant reduction in tumour burden. Collectively, this suggests that the inflammasome, a key upstream activator of IL-18 (and IL-1β), which comprises caspase-1, ASC and a pattern recognition receptor (PRR), plays a role in STAT3-mediated tumourigenesis. Investigations of the molecular mechanisms responsible for this reduction in tumour burden revealed that inflammation was unchanged in the presence or absence of ASC, but that there was a significant increase in cell death when the gene encoding ASC was deleted. This was accompanied by a significant increase in activation of caspase-8, a caspase involved in apoptotic cell death. Despite the critical role of ASC in the mouse model, its expression was not found to be upregulated in human disease. Gene expression of two PRRs that can form an inflammasome with ASC, AIM2 and NLRP3, was found to be significantly upregulated in human gastric cancer. Given this finding, genes for both these PRRs were deleted in mice that overexpressed STAT3 in order to determine if these molecules played a role in the development of gastric tumours. Deletion of NLRP3 resulted in a significant reduction in tumour burden, however deletion of AIM2 did not alter the phenotype. The reduction in tumour burden observed when NLRP3 was deleted was similar to the phenotype of mice lacking ASC – there was no change in the severity of inflammation but there was a significant increase in cell death. Overall, this work demonstrated a role for the inflammasome in STAT3-mediated gastric tumourigenesis, and demonstrated a new role for ASC in preventing cell death in a hyperactive STAT3 environment.