Lipid‐Free Apolipoprotein A‐I Reduces Progression of Atherosclerosis by Mobilizing Microdomain Cholesterol and Attenuating the Number of CD131 Expressing Cells: Monitoring Cholesterol Homeostasis Using the Cellular Ester to Total Cholesterol Ratio

Abstract

BackgroundAtherosclerosis is a chronic inflammatory disorder whose development is inversely correlated with high‐density lipoprotein concentration. Current therapies involve pharmaceuticals that significantly elevate plasma high‐density lipoprotein cholesterol concentrations. Our studies were conducted to investigate the effects of low‐dose lipid‐free apolipoprotein A‐I (apoA‐I) on chronic inflammation. The aims of these studies were to determine how subcutaneously injected lipid‐free apoA‐I reduces accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without sustained elevations in plasma high‐density lipoprotein cholesterol concentrations.Methods and Results Ldlr −/− and Ldlr −/− apoA‐I −/− mice were fed a Western diet for a total of 12 weeks. After 6 weeks, a subset of mice from each group received subcutaneous injections of 200 μg of lipid‐free human apoA‐I 3 times a week, while the other subset received 200 μg of albumin, as a control. Mice treated with lipid‐free apoA‐I showed a decrease in cholesterol deposition and immune cell retention in the aortic root compared with albumin‐treated mice, regardless of genotype. This reduction in atherosclerosis appeared to be directly related to a decrease in the number of CD131 expressing cells and the esterified cholesterol to total cholesterol content in several immune cell compartments. In addition, apoA‐I treatment altered microdomain cholesterol composition that shifted CD131, the common β subunit of the interleukin 3 receptor, from lipid raft to nonraft fractions of the plasma membrane.ConclusionsApoA‐I treatment reduced lipid and immune cell accumulation within the aortic root by systemically reducing microdomain cholesterol content in immune cells. These data suggest that lipid‐free apoA‐I mediates beneficial effects through attenuation of immune cell lipid raft cholesterol content, which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation.

Apolipoprotein A‐I Modulates Atherosclerosis Through Lymphatic Vessel‐Dependent Mechanisms in Mice

“…Therefore, to directly assess the effect of the drug on the lymphatic vasculature per se, we performed intradermal injections, rather than subcutaneous injections as performed in previous work 37. Following the injection, apoA‐I is first retrieved in lymph before appearing in the blood circulation, thus acting upon the lymphatic vasculature as soon as it gets within the mouse.…”

“…59 Therefore, to directly assess the effect of the drug on the lymphatic vasculature per se, we performed intradermal injections, rather than subcutaneous injections as performed in previous work. 37 Following the injection, apoA-I is first retrieved in lymph before appearing in the blood circulation, thus acting upon the lymphatic vasculature as soon as it gets within the mouse. We observe longitudinal plaque regression in the thoracic aorta of Ldlr À/À mice following lipid-free apoA-I treatment, independently of cholesterol accumulation in lymph or plasma.…”

“…HDL and apoA‐I have been shown to modify atherogenic and antigen presentation properties by interfering with the cell membrane lipid raft 34, 35, 36. In the same vein, it has been demonstrated that apoA‐I acts in hypercholesterolemic mice by systemically reducing excess cellular cholesterol accumulation in membrane lipid rafts 37. The latter are tightly regulated microdomains contained in cell membranes and they form a platform responsible for organizing the signaling of receptors and proteins of various cell types.…”

“…[34][35][36] In the same vein, it has been demonstrated that apoA-I acts in hypercholesterolemic mice by systemically reducing excess cellular cholesterol accumulation in membrane lipid rafts. 37 The latter are tightly regulated microdomains contained in cell membranes and they form a platform responsible for organizing the signaling of receptors and proteins of various cell types. In a recent publication, in vitro treatment with apoA-I, on a tumor necrosis factor-a background, has been found to cause lymphangiogenesis.…”

MPP1-based mechanism of resting state raft organization in the plasma membrane. Is it a general or specialized mechanism in erythroid cells?

“…In atherosclerosis, transition of macrophages into foam cells following uptake of ox-LDL is raft dependent as ox-LDL receptors localize to raft domains [37,38]. Other data [39] which affects numerous types of signal transduction pathways that rely on microdomain integrity for assembly and activation. On the other hand, it was found [40] that 7-dehydrocholesterol, but not cholesterol or other sterols, promotes lipid raft/caveolae formation, leading to suppression of canonical TGF-b signaling and atherogenesis.…”