ABCA1/G1 and LXRs in Atherogenesis

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Despite the success of LDL lowering therapies there is a need for new treatments to reduce the large burden of residual atherosclerotic cardiovascular disease. Increasing beneficial HDL functions is one potential approach. HDL infusion therapies and small molecule LXR activators, induce cholesterol efflux from macrophage foam cells and reduce atherosclerosis in animal models. However, the underlying protective mechanisms are incompletely understood and this has delayed clinical development. Cholesterol efflux pathways appear to exert anti-atherogenic effects by suppressing inflammatory responses in myeloid cells. The efflux of cholesterol to ApoA-1 and HDL is facilitated by the ATP binding cassette transporters ABCA1 and ABCG1, which are induced by LXRs. Our recent studies in mice with myeloid cell deficiency of these transporters have revealed a major role of cholesterol efflux pathways in suppressing the inflammasome. These mice showed inflammasome activation in macrophages and neutrophils. Unexpectedly, they also displayed prominent neutrophil extracellular traps (NETs) in lesions. Deficiency of inflammasome components reduced lesion area and abolished NETs, showing for the first time that inflammasome activation promotes lesional NETosis. The recent CANTOS trial has highlighted the importance of inflammasome activation and IL-1b production in human coronary heart disease. Other studies have shown a role of NETosis in atherogenesis and plaque instability. Thus, our studies showing that HDL and cholesterol efflux pathways can suppress these processes have major translational potential, especially in conditions where ABCA1/G1 are suppressed and HDL levels are low, such as Type 2 diabetes. The goal of this proposal is to evaluate mechanisms linking cholesterol efflux pathways to atherogenic inflammation. Aim 1 will explore mechanisms linking ABCA1/G1-mediated cholesterol efflux to inflammasome activation, atherogenesis and NETosis. Aim 2 will explore the mechanisms and significance of rHDL- mediated cholesterol efflux in macrophage inflammation. Aim 3 will assess new mechanisms connecting LXR activation to suppression of atherogenic inflammation. These studies may provide novel mechanistic insights stimulating the development of new treatments for atherosclerosis.