Reduced ABCA1-Mediated Cholesterol Efflux and Accelerated Atherosclerosis in Apolipoprotein E–Deficient Mice Lacking Macrophage-Derived ACAT1

Abstract

Background— Macrophage acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) and apolipoprotein E (apoE) have been implicated in regulating cellular cholesterol homeostasis and therefore play critical roles in foam cell formation. Deletion of either ACAT1 or apoE results in increased atherosclerosis in hyperlipidemic mice, possibly as a consequence of altered cholesterol processing. We have studied the effect of macrophage ACAT1 deletion on atherogenesis in apoE-deficient (apoE −/− ) mice with or without the restoration of macrophage apoE. Methods and Results— We used bone marrow transplantation to generate apoE −/− mice with macrophages of 4 genotypes: apoE +/+ /ACAT1 +/+ (wild type), apoE +/+ /ACAT1 −/− (ACAT −/− ), apoE −/− /ACAT1 +/+ (apoE −/− ), and apoE −/− /ACAT1 −/− (2KO). When macrophage apoE was present, plasma cholesterol levels normalized, and ACAT1 deficiency did not have significant effects on atherogenesis. However, when macrophage apoE was absent, ACAT1 deficiency increased atherosclerosis and apoptosis in the proximal aorta. Cholesterol efflux to apoA-I was significantly reduced (30% to 40%; P <0.001) in ACAT1 −/− peritoneal macrophages compared with ACAT1 +/+ controls regardless of apoE expression. 2KO macrophages had a 3- to 4-fold increase in ABCA1 message levels but decreased ABCA1 protein levels relative to ACAT1 +/+ macrophages. Microarray analyses of ACAT1 −/− macrophages showed increases in proinflammatory and procollagen genes and decreases in genes regulating membrane integrity, protein biosynthesis, and apoptosis. Conclusions— Deficiency of macrophage ACAT1 accelerates atherosclerosis in hypercholesterolemic apoE −/− mice but has no effect when the hypercholesterolemia is corrected by macrophage apoE expression. However, ACAT1 deletion impairs ABCA1-mediated cholesterol efflux in macrophages regardless of apoE expression. Changes in membrane stability, susceptibility to apoptosis, and inflammatory response may also be important in this process.

Apoptotic Cell Death and Efferocytosis in Atherosclerosis

“…Interestingly, the modulation of cholesterol transport in macrophages affects the expression of molecules implicated in efferocytosis such as MFGE8. 49 Moreover, using immunohistochemistry, we have demonstrated a decrease of MFGE8 protein expression in macrophages around the necrotic core. 50 These data suggest that changes in macrophage phenotype during atherogenesis and foam cell formation might substantially alter efferocytosis.…”

“…47 Moreover, Ogden et al showed that IgM is required for optimal complement dependent efferocytosis of ACs. 48 Besides competitive inhibition with oxLDL, oxidative stress 44 and transformation into foam cells 49 also impair the capacity to engulf apoptotic debris. Interestingly, the modulation of cholesterol transport in macrophages affects the expression of molecules implicated in efferocytosis such as MFGE8.…”

TNF-alpha stimulates the ACAT1 expression in differentiating monocytes to promote the CE-laden cell formation

“…Experimentally, in a mouse model for atherosclerosis, partial inhibition of ACAT1 activity by an ACAT1-specific inhibitor K604 reduced the CE content at the atherosclerogenic lesions and caused regression of the atherosclerotic plaques without altering the overall serum cholesterol levels in the treated animals (46). However, complete inhibition of ACAT1 by gene inactivation in macrophages produced severe cytotoxicity and enlarged the atherosclerotic lesions (47,48). In addition, when rodent macrophage cell lines were grown under conditions with minimal cellular cholesterol efflux (a situation that may occur in advanced atherosclerotic lesions), addition of isotype nonspecific ACAT inhibitors to the cells caused undesirable build up of cellular free (unesterified) cholesterol and led to the cells undergoing apoptosis (49).…”

Lactadherin Deficiency Leads to Apoptotic Cell Accumulation and Accelerated Atherosclerosis in Mice

“…1,16 This is in agreement with a recent finding showing a downregulation of Mfge8 mRNA expression in a subset of free cholesterol-loaded macrophages. 17 In addition, apoptotic cell death as detected by TUNEL staining occurred more frequently in areas with low Mfge8 expression in advanced human lesions (Data Supplement Figure III).…”