Abstract 2083: Apolipoprotein A-II Increases Cholesterol Efflux Capacity Of HDL Via The Apolipoprotein A-I C-terminus

The ability of high-density lipoprotein (HDL) to promote cellular cholesterol efflux (CEC) is a more robust predictor of cardiovascular disease protection than its plasma quantity. Previously, we found that fully lipidated HDL containing apolipoprotein A-II (APOA2) promotes cholesterol efflux via the ATP binding cassette transporter (ABCA1). This was surprising given that ABCA1 is thought to primarily interact with lipid-poor apolipoproteins. Having previously focused on isolated lipoproteins, we moved into human plasma with the hypothesis APOA2 can enhance ABCA1-mediated CEC in this more complex environment. Human plasma was incubated with increasing amounts of lipid-free APOA2. The samples were assayed for CEC from macrophages +/- cAMP for ABCA1 stimulation. APOA2 dose dependently increased whole plasma CEC - due mostly to HDL as the effect persisted in apoB-depleted plasma. Next, plasma was incubated with lipid-free APOA1 or APOA2, immediately fractionated by size exclusion chromatography and each fraction assayed for CEC. In unmodified plasma, cholesterol effluxed to peaks corresponding to LDL, HDL and lipid-free apolipoproteins with the latter exclusively increased by ABCA1 expression, as expected. When lipid-free APOA1 was added, efflux to LDL and HDL remained largely unchanged. Strikingly, when APOA2 was added, APOA2 increased ABCA1-mediated cholesterol efflux to the fully lipidated HDL fractions. Proteomic analyses indicated that APOA2 increased in HDL though some APOA1 was retained. To explore the mechanism, we reconstituted HDL particles with APOA1 that either contained or lacked its C-terminal lipid binding helix. APOA2 lost the ability to stimulate ABCA1 efflux to HDL when the C-terminal domain of APOA1 was deleted. Our current hypothesis is that APOA2 displaces the C-terminal helix of APOA1 from the HDL surface which can then interact with ABCA1 - much like it does in lipid-poor APOA1. Reconstituted forms of APOA1 are under development as a therapy to rapidly clear cholesterol from coronary arteries in patients with acute coronary syndrome. Our work suggests APOA2 may be a novel target given its dual benefits for cholesterol efflux.