The role of angiopoietin-like 4 in inflammation

Abstract

Inflammation is a homeostatically fundamental feature of the host defence surveillance system made up of an army of sentinel cells that guard against foreign intrusion. At the onslaught of an attack, primary immune responders (macrophages and neutrophils) secrete a plethora of cytokines and chemokines that attract secondary responders (T and B cells) to the site of inflammation, to eliminate the intrusion and encourage tissue healing. The acute inflammatory stimulation is rapidly attenuated following tissue repair to prevent hypercytokinemia. However, the dysregulation of inflammatory stimulation can also lead to a chronic, erroneous attack on the host immune system, leading to autoimmune diseases and in some cases, cancer initiation. The chronic inflammation theory established by Rudolf Virchow in 1845 is perhaps the first that accurately described the inflammation-induced cancer phenomenon. Since the advancement of technologies, independent researchers have brought forth increasing novel evidence to support the role of inflammation in tumorigenesis. In the compilation of this thesis, we will explore the multi-faceted role of Angiopoietin-like 4 (ANGPTL4) in the regulation of the chemokine landscape during both acute and chronic inflammation.

In Part I, we shall investigate the role of ANGPTL4 in attenuating acute inflammation through the intermediate tristetraprolin (TTP). We show that ANGPTL4 deficiency resulted in an exacerbated inflammatory response in mice subjected to pro-inflammatory stimulation. Confirming the current paradigm, we also reveal that significant microbial divergence was only visible during inflammation and not at steady states. Using immortalized colonic epithelial cells, we connected ANGPTL4 and TTP in the inflammatory signaling axis, and show that ANGPTL4 is able to regulate the acute inflammatory landscape through both TTP-dependent and independent signaling pathways.

In Parts II and III, we demonstrate that ANGPTL4 regulates oxidative stress levels in the tumor and tumor microenvironment during chronic inflammation, and induces the epithelial-mesenchymal transition (EMT) process. Capitalizing on numerous cultures like A5RT3, MKN78 and HSC5, we first establish that TAK1 protected cells against TGFβ-induced EMT. TAK1 deficiency also promoted Rac1-Nox1 activation, increasing overall ROS production and augmented mesenchymal markers like Snai1 and E-cadherin. H2O2 augmented key signaling intermediates which are involved in cellular homeostasis and increased the invasiveness of epithelial cells. In addition, H2O2 also desensitized cells towards TGFβ stimulation by decreasing Smad3 and TAK1 expression. To underscore the medical relevance of ANGPTL4 in tumorigenesis, we reveal that ANGPTL4 drives EMT by elevating cellular energy flux through 14-3-3γ. In addition, 14-3-3γ promotes the transcription of E-cadherin by stabilizing the binding of Snai1 to its promoter during EMT.

These findings illustrate the importance of ANGPTL4 in regulating the inflammatory landscape during both acute and chronic inflammation, and in tumorigenesis, expanding its role outside of metabolism, insulin resistance, angiogenesis and wound healing.