Severely altered cholesterol homeostasis in macrophages lacking apoE and SR-BI

Abstract

Mice deficient in scavenger receptor class B type I (SR-BI) and apolipoprotein E (apoE) [double knockout (DKO) mice] develop dyslipidemia, accelerated atherosclerosis, and myocardial infarction, and die prematurely. We examined effects of apoE and SR-BI deficiency on macrophage cholesterol homeostasis. DKO macrophages had increased total cholesterol (TC) stores (220-380 mg/mg protein) compared with apoE 2/2 cells (40 mg/mg), showed significant lysosomal lipid engorgement, and increased their TC by 34% after exposure to HDL. DKO macrophages from apoE 2/2 mice reconstituted with DKO bone marrow showed less cholesterol accumulation (89 mg/mg), suggesting that the dyslipidemia of DKO mice explains part of the cellular cholesterol defect. However, analyses of DKO and apoE 2/2 macrophages from transplanted apoE 2/2 mice revealed a role for macrophage SR-BI, inasmuch as the TC in DKO macrophages increased by 10% in the presence of HDL, whereas apoE 2/2 macrophage TC decreased by 33%. After incubation with HDL, the free cholesterol (FC) increased by 29% in DKO macrophages, and decreased by 8% in apoE 2/2 cells, and only DKO cells had FC in large peri-nuclear pools. Similar trends were observed with apoA-I as an acceptor. Thus, the abnormal cholesterol homeostasis of DKO macrophages is due to the plasma lipid environment of DKO mice and to altered trafficking of macrophage cholesterol. Both factors are likely to contribute to the accelerated atherosclerosis in DKO mice.-

Enhanced Foam Cell Formation, Atherosclerotic Lesion Development, and Inflammation by Combined Deletion of ABCA1 and SR-BI in Bone Marrow–Derived Cells in LDL Receptor Knockout Mice on Western-Type Diet

“…However, studies from Yancey et al demonstrated that macrophages with a combined deficiency of SR-BI and apoE display a reduced efflux capacity and accumulate free cholesterol in lysosomes. 20 Moreover, recently Cuchel et al showed that free cholesterol mobilization in vivo in response to reconstituted HDL infusion is primarily mediated by SR-BI and not ABCA1 or ABCG1. 21 In addition, overexpression of apoAI in heterozygous LDLr KO mice can protect against atherosclerosis in the absence of macrophage ABCG1 and ABCA1.…”

High-density lipoprotein therapeutics and cardiovascular prevention

“…34 SR-BI (also known as the hepatic HDL receptor) contributes significantly to lipid transfer from cells overloaded with cholesterol, such as arterial macrophages, and therefore may play a central role in atherogenesis. 35 As free (unesterified) cholesterol translocates across the cell membrane, HDL esterifies it by adding a fatty acid chain via the action of lecithin cholesterol acyl-transferase (LCAT). 32 The nonpolar cholesteryl esters are then stored in the particle core.…”

Macrophage SR-BI regulates LPS-induced pro-inflammatory signaling in mice and isolated macrophages

“…When fed a Western diet, mice lacking both SR-BI and apoE showed enhanced macrophage cholesterol accumulation ( 15 ). The dysregulation of cholesterol homeostasis in these cells was attributed to a disruption of macrophage cholesterol effl ux and lysosomal cholesterol traffi cking ( 15 ). Recent studies from our laboratory revealed that SR-BI can promote macrophage cholesterol effl ux to HDL ( 16 ).…”

“…SR-BI has been recognized as a mediator of bidirectional cholesterol fl ux between macrophages and extracellular acceptors ( 12,14 ). When fed a Western diet, mice lacking both SR-BI and apoE showed enhanced macrophage cholesterol accumulation ( 15 ). The dysregulation of cholesterol homeostasis in these cells was attributed to a disruption of macrophage cholesterol effl ux and lysosomal cholesterol traffi cking ( 15 ).…”

“…Animals were maintained in a pathogen-free facility with 12 h light/dark cycle and free access to food and water. SR-BI-null mice were obtained from Jackson Laboratory (Monty Krieger, MIT) ( 22 ) and backcrossed for 10 generations into a C57BL/6 background (MacRae F. Linton, Vanderbilt University) ( 15 ). ELISA sets for cytokine determination were purchased from BD Biosciences (San Jose, CA).…”