ReviewIn vivo macrophage-specific RCT and antioxidant and antiinflammatory HDL activity measurements: New tools for predicting HDL atheroprotection
Introduction
Clinical and epidemiological studies have demonstrated an inverse correlation between the concentration of plasma high-density lipoprotein cholesterol (HDLc) and the incidence of atherosclerotic cardiovascular disease [1]. However, the relationship between HDLc and atherosclerosis is complex. ApoA-I Milano is a naturally occurring mutant of apoA-I which is associated with low HDLc and no evidence of coronary heart disease [2]. Further, a significant number of cardiovascular events occur in subjects with normal HDLc and LDL cholesterol (LDLc) [3].
The most widely accepted mechanistic explanation for the HDL-mediated cardioprotective effects is that HDL promotes cholesterol efflux from lipid-laden macrophages located in the arterial wall and delivers that cholesterol to the liver, from where it will be partly eliminated through bile and feces, a process termed reverse cholesterol transport (RCT) [1]. Several attempts have been made to determine RCT [4]. Several of these studies assessed total RCT by measuring centripetal cholesterol efflux, and demonstrated that apoA-I-deficiency and overexpression of CETP or SR-BI, despite causing a major reduction in HDLc, did not affect cholesterol efflux from any extrahepatic tissue in mice [4], [5], [6]. Similarly, hepatobiliary cholesterol transport was not impaired in ABCA1-deficient mice [4], [5], [6]. A major limitation of these studies is the lack of information on the contribution of macrophages to this fecal cholesterol output, since macrophage-specific RCT is a minor contributor to total RCT but macrophages are the most important cholesterol-accumulating cell in atherosclerosis. To address this question directly in experimental models, a recently developed approach in mice measures macrophage-specific RCT by tracing the reverse [3H]cholesterol transport from lipid-laden macrophages to feces in vivo[4].
Also, HDL is believed to protect against atherosclerosis by inhibiting oxidative modification of LDL and its proinflammatory properties [3]. An antioxidant mechanism of HDL may result from its ability to accept phospholipid-containing hydroperoxides and other lipid peroxidation products from oxidized LDL [3] and there is evidence that HDL have antiinflammatory effects in vivo[4], [7]. It is currently believed that a significant part of these HDL properties are related to their associated components (including apoA-I, PON1 and PAF-AH). Various mechanisms including oxidation, glycation, homocysteinylation or enzymatic degradation induce structural modifications of HDL and alter its inflammatory response [3], [8], [9], [10].
However, despite the increasing use of macrophage-dependent RCT and antioxidant and antiinflammatory HDL tests, conclusive evidence for these HDL-mediated atheroprotection processes in vivo has not been established. The main objective of this study was to review the studies that measured in vivo macrophage-specific RCT and HDL antioxidant and antiinflammatory functions in genetically engineered mice to provide evidence that these major HDL antiatherogenic properties correlate closely with changes in atherosclerosis suceptibility.
Section snippets
Adenosine triphosphate-binding cassette transporter A1 (ABCA1)
The first comparison of macrophage-specific RCT between ABCA1-deficient and wild-type mice provided direct evidence that ABCA1 deficiency in mice reduces RCT from macrophages to feces in vivo[11]. The findings of three recent reports confirmed and extended the knowledge on the contribution of macrophage ABCA1 to the entire macrophage RCT pathway [12], [13], [14]. The significant defect in macrophage-specific RCT after injection of ABCA1-deficient macrophages [11], [12], [13] and the increased
ApoA-I
Overexpression of apoA-I in mice has a protective role against atherosclerosis [5], [6]. This is associated with a promotion of macrophage-specific RCT in vivo[5], [6] and an increased ability of apoA-I to remove oxidant molecules from LDL [10]. Importantly, absence of apoA-I in LDLR- and apobec-deficient mice impaired RCT from macrophages to feces as well as HDL antioxidant and antiinflammatory function in vivo and increased atherosclerosis [25] (Table 2).
ApoA-II
HDL isolated from murine apoA-II
Paraoxonase 1 (PON1)
Growing evidence suggests that PON1 reduces oxidative stress due to its ability to protect against lipid oxidation and protects against cardiovascular diseases [33]. However, the relationship between PON1 polymorphisms and cardiovascular diseases is controversial [33]. A direct role for PON1 in determining HDL antioxidant and antiinflammatory properties and atherosclerosis was established in PON1-deficient mice [10]. Further, HDL from human PON1 transgenic mice protected against LDL oxidation
Cholesteryl ester transfer protein (CETP)
Human CETP expression in transgenic mice did not change biliary cholesterol excretion [45]. Also, CETP expression in mice did not impair RCT from macrophages to feces in vivo despite a reduction in plasma HDLc [46], [47]. Further, macrophage cholesterol efflux did not correlate with atherosclerosis susceptibility in ovariectomized human CETP transgenic mice [48]. However, human CETP expression promoted the macrophage-dependent RCT rate in the setting of effective apoB-100 clearance [46]. HDL
SR-BI
Liver SR-BI mediates the selective uptake of cholesteryl ester from HDL and promotes RCT from macrophages to feces in vivo despite reduced HDLc [54], [55]. Consistently, SR-BI-deficient mice showed impaired excretion of macrophage-derived cholesterol in feces [55]. SR-BI-deficient mice showed impaired removal of oxidized cholesterol from HDL, reduced PON1 activity and increased oxidative stress [23]. These results are fully consistent with the experiments in SR-BI transgenic and knockout mice
Therapeutic approaches to improving HDL function in vivo
Several HDL-based strategies are being used to promote the effects of HDL function for the development of new therapeutic agents. These include CETP inhibitors, peroxisome proliferator-activated receptor agonists, LXR agonists, nicotinic acid-based formulations and apoA-I-directed therapies [2], [61], [62], [63], [64], [65]. The failure of the torcetrapib clinical trial both in terms of clinical outcomes and ability to improve atheroma burden cast doubts on the estrategy of increasing HDLc to
Conclusions and perspectives
It is increasingly evident that the macrophage-dependent RCT and antioxidant and antiinflammatory functions of HDL constitute two major antiatherogenic HDL properties. Data from genetically engineered mice indicate that these HDL functions are more related to atherosclerosis susceptibility than HDLc. These major HDL functions predict atherosclerosis susceptibility in 22 out of 30 experimental models, whereas HDLc only predicts atherogenesis in 8 out of 30 models. At present, it is difficult to
Acknowledgment
This work was funded by the grant FIS PI06/0551. CIBER de Diabetes y Enfermedades Metabólicas Asociadas is an Instituto de Salud Carlos III project.
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