Unraveling the Role of Modified HDL in Atherogenesis

PROJECT SUMMARY Cardiovascular disease (CVD) is the leading cause of death globally, with atherosclerosis being a major cause of CVD. High density lipoproteins (HDL) have many cardioprotective functions that help to decrease risk of developing atherosclerosis. Facilitating reverse cholesterol transport (RCT), or the delivery of peripheral cholesterol from macrophages to the liver for net excretion, is one of HDL’s major cardioprotective functions. While HDL-cholesterol (HDL-C) levels are inversely correlated with CVD risk, therapeutics targeted to raise HDL- C levels have not shown clinical benefit. It has therefore been suggested that HDL function may be a better indicator of CVD outcomes rather than HDL-C levels themselves. Previous studies have shown that HDL loses some of its key cardioprotective functions when it becomes oxidatively modified by reactive aldehydes. However, the mechanisms by which modified HDL promotes the pathogenesis of atherosclerosis are not well understood. Studies from our laboratory have shown that HDL modified by acrolein, a reactive aldehyde found in atherosclerotic plaque, exhibited a decreased ability to facilitate early and late steps in RCT. Additionally, in contrast to native HDL, incubation of murine macrophages with HDL modified by other reactive aldehydes (e.g. malondialdehyde) resulted in decreased macrophage migration and increased reactive oxygen species generation -- processes that are crucial for initiating atherosclerosis. Based on these findings, we have designed two Aims to test the global hypothesis that modified forms of HDL contribute to the progression of atherosclerosis. In Aim 1, we will investigate how modified forms of HDL prevent the last steps of reverse cholesterol transport. First, we will use cultured hepatocytes to determine whether modification of HDL by acrolein and MDA prevent SRBI-mediated selective uptake of HDL-C (Aim 1.1). Further, we will examine how SR-BI-mediated signaling pathways that trigger selective uptake of HDL-C may become impaired when HDL is modified (Aim 1.2). Finally, we will perform macrophage-to-feces reverse cholesterol transport assays in ApoA-I knock-out mice (a model with very low circulating HDL levels) to determine the in vivo relevance of modified forms of HDL in circulation. In Aim 2, we will investigate the physiological role of modified HDL in atherosclerosis progression. Specifically, we will determine if modified forms of HDL are unable to prevent the progression of atherosclerosis in Western diet-fed ApoA-I knock-out mice where the LDL receptor has also been knocked out by CRISPR (to create a hypercholesterolemic background). The outcomes of these studies will provide a better understanding of the mechanisms by which modified HDL promotes atherogenic pathways which could help identify novel therapeutic strategies for preventing atherosclerosis.