Abstract 15: Proteomic Analysis of HDL from Inbred Strains of Mice Implicates APOE in Reduced Cholesterol Efflux Capacity via the ABCA1 Pathway

Abstract

Background: Human HDL’s efflux capacity-the ability to promote cholesterol efflux from macrophages-associates strongly and negatively with risk of future cardiac events. However, the molecular factors that regulate efflux capacity remain poorly understood. Methods and Results: We investigated the relationships between HDL’s size, protein cargo, and sterol efflux capacity, using HDLs from five inbred mouse strains with different HDL-cholesterol levels and susceptibilities to atherosclerosis. Like human HDL, mouse HDL carried >70 proteins linked to lipid metabolism, the acute-phase response, proteinase inhibition, and the immune system. Hierarchical cluster analysis recapitulated the genealogy of the mouse strains, indicating that the HDL proteome is under genetic control. HDL particle size from the different strains correlated strongly and positively with APOA2 content. Analysis of HDL isolated from Apoa2 -/- or Apoa2 transgenic mice confirmed that APOA2 is a major determinant of HDL particle size. Unexpectedly, APOE correlated strongly and negatively with in vitro efflux capacity, suggesting that it impairs the efflux capacity of HDL. Moreover, the efflux capacity of HDL isolated from Apoe -/- mice was significantly greater than that of HDL from wild-type mice. This difference resulted from a marked increase in sterol efflux by the ABCA1 pathway when we tested HDL from the mutant strain. Conclusions: Our observations demonstrate that APOA2 determines(or correlates with) HDL particle size and that APOE regulates HDL’s efflux capacity by a pathway selectively involving ABCA1. These findings may be clinically relevant, because HDL’s APOE content is a strong predictor of cardiovascular disease risk and because ABCA1 deficiency promotes unregulated sterol accumulation in human macrophages.