Cholesterol mobilization by free and lipid-bound ApoAI(Milano) and ApoAI(Milano)-ApoAII heterodimers

Despite very low plasma levels of HDL, carriers of the apolipoprotein AI Ar g173 --> Cys mutation apoAI(Milano) (AIM) have no apparent increase in risk for atherosclerotic vascular disease. HDL apolipoprotein species in AIM ca rriers include apoAI-AII heterodimers, previously found to confer the enhan ced ability of tyrosyl radical-oxidized HDL to mobilize cholesterol for rem oval from cultured cells, To determine whether enhanced mobilization of cho lesterol by apoprotein species in AIM explains a cardioprotective action of this mutation, we examined the ability of lipid-free and lipid-bound AIM a nd AIM-AII heterodimers to deplete cholesterol from cultured cells. Free AI M and AIM-AII heterodimers showed a decreased capacity to act as accepters of cholesterol from cholesterol-loaded human fibroblasts compared with nati ve apoAI but similar capacities to deplete fibroblasts of the pool of chole sterol available for esterification by acyl-CoA:cholesterol acyltransferase (ACAT). Discoidal reconstituted HDL (rHDL) containing apoAI depleted both of these cholesterol pools more readily than AIM-containing rHDL when compa red at equivalent rHDL protein levels, but similar abilities of these rHDL to deplete cell cholesterol were seen when compared at equivalent phospholi pid levels. Spherical rHDL generated using the whole lipid fraction of HDL and apoAI or AIM showed similar capacities to deplete total and ACAT-access ible cell cholesterol when compared at similar protein levels, but an incre ased capacity of AIM-containing particles was seen when compared at equival ent phospholipid levels. Unlike the apoAI-AII heterodimer in tyrosylated HD L, AIM-ALT. heterodimer-containing spherical rHDL showed no increased capac ity to deplete either of these pools of cholesterol. These results suggest a similar or better capacity of native apoAI in lipid-free or lipid-bound f orm in discoidal rHDL to enhance the mobilization of cellular cholesterol w hen compared to AIM in its free or lipid-bound forms. Any increase in deple tion of cellular cholesterol by lipid-bound AIM in spherical rHDL appears r elated to altered phospholipid-binding rather than intrinsic cholesterol-mo bilizing characteristics of this protein compared to native apoAI. The lack of major differences in these studies in cholesterol mobilization by nativ e apoAI and AIM, or by apoAIM-AII heterodimers, suggests that any protectio n against atherosclerosis conferred by this mutation is likely related to o ther beneficial vascular effects of AIM.