Increased cellular free cholesterol in macrophage-specific Abca1 knock-out mice enhances pro-inflammatory response of macrophages
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Macrophage-specific Abca1 knock-out (Abca1(-M/-M)) mice were generated to determine the role of macrophage ABCA1 expression in plasma lipoprotein concentrations and the innate immune response of macrophages. Plasma lipid and lipoprotein concentrations in chow-fed Abca1(-M/-M) and wild-type (WT) mice were indistinguishable. Compared with WT macrophages, Abca1(-M/-M) macrophages had a > 95% reduction in ABCA1 protein, failed to efflux lipid to apoA-I, and had a significant increase in free cholesterol (FC) and membrane lipid rafts without induction of endoplasmic reticulum stress. Lipopolysaccharide (LPS)-treated Abca1(-M/-M) macrophages exhibited enhanced expression of pro-inflammatory cytokines and increased activation of the NF-kappa B and MAPK pathways, which could be diminished by silencing MyD88 or by chemical inhibition of NF-kappa B or MAPK. In vivo LPS injection also resulted in a higher pro-inflammatory response in Abca1(-M/-M) mice compared with WT mice. Furthermore, cholesterol depletion of macrophages with methyl-beta-cyclodextrin normalized FC content between the two genotypes and their response to LPS; cholesterol repletion of macrophages resulted in increased cellular FC accumulation and enhanced cellular response to LPS. Our results suggest that macrophage ABCA1 expression may protect against atherosclerosis by facilitating the net removal of excess lipid from macrophages and dampening pro-inflammatory MyD88-dependent signaling pathways by reduction of cell membrane FC and lipid raft content.