Abstract 1731: High Density Lipoprotein and ATP Binding Cassette Transporter G1 Protect Macrophages from Apoptosis during Phagocytosis of Apoptotic Cells
Recent studies have shown that ABCG1−/− and ABCA1−/−/ABCG1−/− macrophages show increased expression of inflammatory genes, reflecting cholesterol accumulation in the plasma membrane and increased signaling via TLR4. Furthermore, in vivo models of transporter deficiency have shown increased macrophage apoptosis and inflammatory cell infiltrates in atheroma and lung. To investigate the role of ABCA1 and ABCG1 in preserving the viability of macrophages during phagocytosis of apoptotic cells (efferocytosis), we fed apoptotic Jurkat cells to peritoneal macrophages from mice deficient in ABCA1 and/or ABCG1. While the phagocytic index was not appreciably altered, after 24h there was a dramatic increase in apoptosis in ABCA1−/−/ABCG1−/− > ABCG1−/− > ABCA1−/− > WT (51%, 41%, 18%, 9% apoptotic phagocytes, respectively). Increased apoptosis could not be explained by increased levels of free cholesterol, 7-ketocholesterol or ceramide signaling in efferocytes. Pre-incubation of macrophages with LPS to activate TLR4 increased apoptosis of efferocytes especially in the presence of transporter deficiency. It is known that apoptotic cells have high levels of oxidized phospholipids “oxPL” detectable with antibody E06. Staining of lung sections showed a marked increase in E06 reactive material in ABCG1−/− and ABCA1−/−/ABCG1−/− mice compared to wild type and ABCA1−/− mice. After loading macrophages with 10 or 50 μg/ml “oxPL” for 12h a similar pattern of increased apoptosis was observed (50 μg/ml “oxPL” - WT 19%, ABCA1−/− 23%, ABCG1−/− 30%, ABCA1−/−/ABCG1−/− 52%). Under these conditions the addition of HDL (50 μg/ml) rescued WT and ABCA1−/− but not ABCG1−/− and ABCA1−/−/ABCG1−/− macrophages. Efflux measurements of “oxPL” from WT and ABCG1−/− macrophages to HDL using the E06 antibody did not show a role of ABCG1 in “oxPL” efflux, suggesting that increased apoptosis could arise from cross-talk between increased TLR4 signaling due to ABCG1 deficiency and CD36 signaling elicited by oxPL. Thus, HDL and ABCG1 have a major role in preserving viability of efferocytes, suggesting an important mechanism of atherosclerotic plaque stabilization.