Although high plasma concentrations of LDL-C (?bad cholesterol?) are associated with atherosclerotic cardiovascular disease (ACVD), statins reduce plasma LDL-C and with it ACVD. In contrast, high density lipoprotein-cholesterol (HDL-C; ?good cholesterol?) varies inversely with ACVD. However attempts to reduce ACVD via increased plasma HDL-C levels have failed. New evidence suggests that HDL quality is more important than quantity and that its ability to remove free cholesterol (FC) from macrophages (M?), an important cell type in ACVD, is its most important atheroprotective quality. This process, M?-FC efflux, initiates the FC transfer to the intestine for disposal?an atheroprotective process. Paradoxically, patients with very high plasma HDL-C levels are at high ACVD risk; the underlying mechanism is unknown, and currently there are no interventions that reverse high HDL-C levels in a cardioprotective way. We hypothesize that the underlying cause of ACVD in patients with very high plasma HDL-C levels is too much HDL that contains high amounts of FC, which transfers freely among cells and lipoproteins. This state makes FC highly bioavailable so that rather than removing FC from the arterial wall, FC-rich HDL transfers FC to arterial-wall M??an atherogenic process. Using a mouse model of ACVD with underlying high HDL-C levels (SR-B1-/- mouse) we plan to identify HDL-FC bioavailability as a driver of ACVD and show that treatment with an HDL-lowering bacterial protein (serum opacity factor), delivered with an adeno-associated virus prevents/reverses ACVD. Within this ACVD-HDL axis, we propose the following specific aims:
Aim 1 ?To compare the plasma clearance kinetics of wild-type and SR-B1-/- HDL-[3H]FC and cholesteryl ester (CE) in wild-type and SR-B1-/- mice, simultaneously identifying the tissue sites of [3H]FC and [3H]CE accretion, and the effects of AAVSOF vs. AAVGFP on these kinetics and tissue distributions.
Aim 2 a?To test the hypothesis that FC flux between HDL and J774 M? switches from efflux to influx with increasing HDL-FC bioavailability, which is a function of HDL particle concentration and HDL-FC content (mol% FC).
Aim 2 b?Concurrently with Aim 1, to test the hypothesis that HMGCoA reductase and ACAT activities decrease and increase, respectively, M?-FC content as effected by increasing HDL-FC bioavailability.
Aim 2 c? To test the hypothesis that increased HDL-FC bioavailability induces foam cell formation in J774 M?.
Aim 3 ?To test the hypothesis that reduction of HDL-FC by AAVSOF vs. AAVGFP delivery prevents and/or reverses atherosclerosis in SR-B1-/- mice. Completion of these aims will provide a compelling rationale ?for studies to determine whether high plasma HDL- FC is associated with ACVD in patients with high HDL-C and ?for the development of drugs that lower HDL-FC.

Public Health Relevance

We propose a new risk factor for atherosclerotic cardiovascular disease (ACVD)?too much free cholesterol (FC) carried in high density lipoproteins (HDL), the so-called ?good? cholesterol. We will uncover the mechanisms(s) by which FC in the context of high plasma levels of dysfunctional HDL, contributes to ACVD and show that a novel bacterial protein called serum opacify factor (SOF), which we discovered lowers plasma cholesterol levels, also reduces ACVD. Successful completion of our studies will provide a rationale for human studies that correlate high plasma FC levels with excess ACVD and for the development of small molecules that catalyze the SOF reaction in a way that reduces excess plasma FC levels in an cardioprotective way.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
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Atherosclerosis and Inflammation of the Cardiovascular System Study Section (AICS)
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Liu, Lijuan
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Methodist Hospital Research Institute
United States
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