Apoptotic cells trigger a membrane-initiated pathway to increase ABCA1
Aaron M. Fond, Chang Sup Lee, Ira G. Schulman, Robert S. Kiss, Kodi S. Ravichandran
Aaron M. Fond, Chang Sup Lee, Ira G. Schulman, Robert S. Kiss, Kodi S. Ravichandran
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Research Article Cardiology

Apoptotic cells trigger a membrane-initiated pathway to increase ABCA1

Abstract

Macrophages clear millions of apoptotic cells daily and, during this process, take up large quantities of cholesterol. The membrane transporter ABCA1 is a key player in cholesterol efflux from macrophages and has been shown via human genetic studies to provide protection against cardiovascular disease. How the apoptotic cell clearance process is linked to macrophage ABCA1 expression is not known. Here, we identified a plasma membrane–initiated signaling pathway that drives a rapid upregulation of ABCA1 mRNA and protein. This pathway involves the phagocytic receptor brain-specific angiogenesis inhibitor 1 (BAI1), which recognizes phosphatidylserine on apoptotic cells, and the intracellular signaling intermediates engulfment cell motility 1 (ELMO1) and Rac1, as ABCA1 induction was attenuated in primary macrophages from mice lacking these molecules. Moreover, this apoptotic cell–initiated pathway functioned independently of the liver X receptor (LXR) sterol–sensing machinery that is known to regulate ABCA1 expression and cholesterol efflux. When placed on a high-fat diet, mice lacking BAI1 had increased numbers of apoptotic cells in their aortic roots, which correlated with altered lipid profiles. In contrast, macrophages from engineered mice with transgenic BAI1 overexpression showed greater ABCA1 induction in response to apoptotic cells compared with those from control animals. Collectively, these data identify a membrane-initiated pathway that is triggered by apoptotic cells to enhance ABCA1 within engulfing phagocytes and with functional consequences in vivo.

Authors

Aaron M. Fond, Chang Sup Lee, Ira G. Schulman, Robert S. Kiss, Kodi S. Ravichandran

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Figure 6

BAI1 signaling affects serum lipid levels in dyslipidemic Ldlr-deficient mice.

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BAI1 signaling affects serum lipid levels in dyslipidemic Ldlr-deficient...
(A) Control LR73 (gray), BAI1-overexpressing LR73 (dark blue), or HA-tagged BAI1-overexpressing (HA-BAI1) LR73 (light blue) cells were treated with apoptotic Jurkat cells and their efflux 3H-cholesterol to ApoA1 was assessed (mean ± SD). Representative of 3 independent experiments. (B) Schematic for the generation of Tg mice overexpressing BAI1. The Tg constructs engineered to express WT BAI1 and BAI1-AAA mutant (that fail to engage ELMO1-mediated downstream signaling) contain a loxP-flanked STOP cassette preceding the coding sequence for BAI1. These constructs were knocked into the Rosa26 locus of C57BL/6 ES cells, from which Tg mice were generated. Upon crossing with E2A-Cre–expressing mice and Cre-mediated removal of the STOP cassette, BAI1 and BAI1-AAA expression were attained. (C) Peritoneal macrophages from Tg BAI1 or Tg BAI1-AAA mice were incubated with apoptotic Jurkat cells. Abca1 upregulation (compared with untreated controls) was measured, and data are shown as normalized to littermate controls (WT) (mean ± SEM). *P < 0.05 (t test). (D) Representative assay of at least 2 independent experiments for the engulfment of apoptotic thymocytes by peritoneal macrophages isolated from Tg BAI1 or Tg BAI1-AAA mice (mean ± SD). Combined Tg BAI1 (n = 9–10 mice) and Tg BAI1-AAA (n = 6–7 mice) statistics are shown. *P < 0.05, P = 0.887 for BAI1-AAA vs. WT (t test). (E) Tg BAI1 Ldlr–/– mice were fed a Western diet for 15 weeks, and the ratios of HDL/total cholesterol and HDL/LDL in the serum were analyzed. *P < 0.05 (t test).