Delineation of the Role of Pre-β ₁ -HDL in Cholesterol Efflux Using Isolated Pre-β ₁ -HDL

OBJECTIVE: The role of pre-beta1-high density lipoprotein (pre-beta1-HDL) in cholesterol efflux was investigated by separating human plasma into purified pre-beta1-HDL and pre-beta1-HDL-deficient plasma by using a monoclonal antibody specifically reacting with pre-beta1-HDL. METHODS AND RESULTS: When compared with whole plasma, pre-beta1-HDL-deficient plasma was equally efficient in promoting cholesterol efflux from human skin fibroblasts and THP-1 human macrophage cells. When added at the same apolipoprotein A-I concentration, pre-beta1-HDL was less effective than whole plasma in promoting cholesterol efflux from fibroblasts but equally effective in promoting cholesterol efflux from THP-1 cells. However, pre-beta1-HDL-deficient plasma reconstituted with 16% pre-beta1-HDL was more active than whole plasma, demonstrating that pre-beta1-HDL does promote cholesterol efflux actively. The amount of cellular cholesterol present in reisolated pre-beta1-HDL was 1.5- to 2-fold greater after incubation of the cells with whole plasma than after incubation of the cells with pre-beta1-HDL-deficient plasma or plasma treated with the anti-pre-beta1-HDL antibody. However, the anti-pre-beta1-HDL antibody did not inhibit cholesterol efflux. CONCLUSIONS: We conclude that whereas pre-beta1-HDL is capable of taking up cellular cholesterol, its presence in plasma is not essential for cholesterol efflux, at least in vitro. Instead, pre-beta1-HDL may be the first product of apolipoprotein A-I lipidation during the formation of HDL but may not play a major role in transferring cellular cholesterol to HDL.