Epigenetic Reprogramming in Atherosclerosis

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Project Summary/Abstract Epigenetic Reprogramming in Atherosclerosis: Occlusion of coronary arteries by atherosclerotic plaque is the leading cause of mortality and morbidity in the developed world, and is rapidly becoming so in lower income countries, with an estimated 7.4 million deaths globally due to coronary heart disease. Inappropriate lipid deposition in the sub-endothelial arterial space drives plaque progression by the recruitment and differentiation of immune cells, principally monocyte-derived macrophages who attempt to phagocytose these pathologic lipid deposits. Lipid deposition and disease risk is positively correlated with circulating levels of cholesterol carried by low density lipoprotein (LDL). Conversely, higher serum levels of high density lipoprotein (HDL) cholesterol and function associate with decreased risk. We have shown that as LDL derived lipids accumulate and crystallize in the sub-endothelial space and this triggers activation of a large cytoplasmic protein complex termed the NLRP3 inflammasome. This activation triggers subsequent processing and secretion of pro-inflammatory cytokines including IL-1β, that in a feed forward mechanism, recruits more immune cells to the developing plaque. Conversely, and more recently, we have discovered HDL triggers the activity of an anti-inflammatory transcriptional repressor termed ATF3. Most interestingly, we have now also found that Western diet feeding imparts a long-lived immune hyper- responsiveness of bone marrow myeloid progenitors and tissue resident macrophages. Transcriptional profiling using RNA sequencing identified that Western diet feeding induces a primed and hyper-inflammatory state of granulocyte / monocyte progenitors (GMPs) leading to their exacerbated responsiveness towards innate immune activators and their increased proliferation and activation in the bone marrow. In aggregate, our published and preliminary data suggests a hypothesis positing that the pathologic deposition of LDL derived lipid triggers signaling and transcriptional responses that are long lived and are, in part, encoded by an epigenetic reprogramming mechanism in macrophage progenitors and tissue resident macrophages. To explore this hypothesis we propose three specific aims: Aim-1: Define the nature of the long- lived inflammatory signal imparted by the pressure of dietary LDL hypercholesterolemia on macrophage progenitor cells as well as plaque macrophages in mouse models of atherosclerosis. Aim-2: Define if the long- lived hyper-immune state of macrophages is encoded by an epigenetic mechanism and influenced by NLRP3 inflammasome activation. Aim-3: To identify how Western diet induces transcriptional and epigenetic reprogramming of cells.