Regulation of inflammation by homeostatic cholesterol transport pathways.

Our body's response to infection and injury is centrally regulated by inflammation, a protective physiological response which is characterized by redness, swelling, heat, and pain, yet helps eliminate infection, clear out dead cells and tissues, and initiate tissue repair. However, perturbations in pathways that control inflammation can give rise to several chronic diseases including atherosclerosis, rheumatoid arthritis, and diabetes amongst others. Therefore, inflammation needs to be tightly regulated. As we age, our ability to regulate different physiological processes, including how we process dietary nutrients, declines, therefore exhibiting metabolic configurations previously unseen by the immune system. Although our understanding of the major drivers of inflammation has increased, however, detailed metabolic pathways that regulate inflammation remain unknown. The main aim of this proposal is to understand how cholesterol metabolism regulates inflammation. Increased cholesterol deposition is often a consequence of sedentary lifestyle and our studies suggest that cholesterol also contributes to the progression of the inflammatory process through mechanisms hitherto unidentified. We will study in-depth how cholesterol drives inflammation and how we may be able to develop treatments to prevent or slow this event. Cholesterol has a significant impact on health and treatment costs because of the associated risk of cardiovascular disease. Thus, understanding the roles of cholesterol in inflammation will help design better strategies to treat disease effectively.

Over the past several years, emerging evidence has suggested the tight regulation of immune cell function and inflammatory pathways by cellular lipid metabolism. Inflammasome, a multi-protein complex, is recognised as having pivotal roles in inflammatory and autoimmune diseases. Assembly of the NLRP3 inflammasome leads to caspase-1 processing and IL-1b secretion, which initiate an inflammatory reaction with implications in several conditions including cardiovascular disease as demonstrated by the recent CANTOS trial. The cellular metabolic pathways that regulate inflammasome are not well defined. Our exciting new data suggest that cellular cholesterol trafficking licenses NLRP3 activation. Cholesterol requirement of cells is achieved through biosynthesis and uptake programs. In an intricate pathway involving lysosomal cholesterol transporter NPC1, the sterol gets unequally distributed across intracellular compartments. Specifically, we demonstrated that blockade of NPC1-dependent cholesterol efflux through the late-endosome/lysosome pathway dampens NLRP3 activation (Journal of Cell Biology, in revision). We now propose to decipher detailed cholesterol transport pathways that are central to inflammasome activation and gain new understanding into the modulation of NLRP3 activation by cholesterol metabolism. By employing biochemical approaches, we will characterize the sub-cellular organelle to which cholesterol must localize for optimal inflammasome activation. Moreover, we will use the experimental model of monosodium urate (MSU) crystal-induced peritonitis to shed light on the physiological role of the identified cholesterol trafficking pathways in NLRP3 activation. Together, our studies will reveal novel metabolic regulators of the NLRP3 inflammasome and identify upstream targets to manage overt inflammation in clinically important diseases.

The research proposed in this application focuses particularly on understanding the regulation of inflammasome by cholesterol metabolism and transport. Thus, first and foremost, this research will be of interest to a broad spectrum of scientists working in the area of inflammation. Inflammasomes have been recognized as potential targets for inflammatory diseases. For instance, drugs targeting inflammasome effector cytokine IL-1b have proved effective for patients with cryopyrin-associated periodic syndromes (CAPS), rheumatoid arthritis, and cardiovascular disease. Therefore, our studies described herein have the potential to attract pharmaceutical research by those companies who are interested in developing compounds to dampen inflammation. Although the first focus may be on diseases with defective lipid metabolism, such therapies can potentially be used for a range of inflammatory and autoimmune diseases. The creative experiments we have designed, and the discoveries we make, will inspire other researchers to investigate similar mechanisms. In the long term, advances in understanding inflammatory cascades could result in potential economic benefits as a result of reduced healthcare support required. Moreover, as cholesterol is mainly obtained from dietary sources, our research will further inform how food and nutrition can affect health and impact disease risk, and how dietary metabolites affect processes that influence health. This might eventually translate into economic benefits for food manufacturers.

Since potential therapies from these studies may be useful for a range of inflammatory and autoimmune diseases, these would significantly enhance the quality of life for these patients. The policymakers including the World Health Organization are also likely to benefit from this research. Additionally, medical charities may be influenced by our research and this may inform funding priorities.

An additional benefit is the training of a junior researcher who will work on the project and develop invaluable skills in cell biology, molecular biology, biochemistry, and immunology. The aim would be to help develop an interested and enthusiastic scientist to an academic career and produce high-quality research. The training they will receive in the project will enable them to develop transferable skills as well as the discipline-specific expertise to contribute to future educational, scientific, and commercial competitiveness of the UK.