Characterising the role of ATF3 in inflammation and cancer – an insight into inflammatory bowel disease and colon cancer.

Abstract

ATF3 has far reaching influences in the spectrum between inflammation and cancer development and is therefore a critical component in host defence. The scope of this thesis extended initially from pioneering work relating to the negative regulation of TLR-induced inflammatory pathways by ATF3 in macrophages and dendritic cells. TLRs are also important regulators of macrophage survival, so in this work we saw an opportunity to bridge gaps in a complete understanding of macrophage survival pathways with potentially novel contributions by ATF3, a known regulator of cell cycle and apoptosis. Here, we demonstrate that loss of atf3 in murine macrophages resulted in a significant reduction in TLR4 mediated survival in vitro, corresponding with excessive apoptosis induced by increased expression of pro-apoptotic proteins Bak and Bax. This implicates ATF3 as an important regulator of macrophage viability via suppression of these pro-apoptotic genes, and a key factor in the maintenance of normal immune homeostasis. Following on from these initial findings, the focus of our study became broader and sought to model inflammatory disease and cancer in vivo in order to more clearly demonstrate the potential of ATF3 to down-regulate inflammation and its ability to dichotomously influence oncogenesis. In this regard, few other diseases are as intrinsically linked by inflammation as are inflammatory bowel disease (IBD) and colon cancer, which we believed that ATF3 could profoundly affect. Using administration of dextran sulfate sodium (DSS) in wild type and atf3 deficient mice, we were able to demonstrate significantly more severe inflammatory colitis, associated with increased expression of pro-inflammatory cytokines IL-6, TNFα and IL-17. In a similar fashion, increased tumour growth was observed in atf3 deficient mice treated with colitis-associated cancer inducing azoxymethane and DSS, or in APCmin/+ ATF3-/- compound mutants, which spontaneously develop intestinal tumours. Importantly, these tumours possessed marked reductions in ATF3 expression, a finding that closely correlated with measurement in our human colon cancer specimens. Together this implicated ATF3 as a novel protective factor and tumour suppressor gene in inflammatory bowel disease and colorectal cancer respectively. Through this work we hope to have contributed to an ever growing understanding of ATF3, helping to firmly establish it as a key regulator of all processes that bridge the gap between the onset of inflammatory disease and eventual onset of cancer, and beyond. We believe that our work highlights the possibility for future efforts to establish ATF3 as a key component of IBD and colon cancer pathogenesis via regulation of leukocyte gene expression, survival, trafficking and function; modulation of diverse cytokine networks; and participation in intrinsic tissue repair and tumour suppressor pathways within the epithelial compartment. Our efforts will hopefully set the stage to implicate ATF3 as a prognostic factor in the clinical outcome of IBD and colon cancer in humans, so that future therapeutic developments will hinge on their ability to moderate ATF3 expression or activity.