Despite very low plasma levels of HDL, carriers of the
apolipoprotein AI Arg173 --> Cys mutation apoAI(Milano) (AIM) have no apparent increase in risk for atherosclerotic
vascular disease. HDL
apolipoprotein species in AIM carriers include apoAI-AII heterodimers, previously found to confer the enhanced ability of tyrosyl radical-oxidized HDL to mobilize
cholesterol for removal from cultured cells. To determine whether enhanced mobilization of
cholesterol by
apoprotein species in AIM explains a cardioprotective action of this mutation, we examined the ability of
lipid-free and
lipid-bound AIM and AIM-AII heterodimers to deplete
cholesterol from cultured cells. Free AIM and AIM-AII heterodimers showed a decreased capacity to act as acceptors of
cholesterol from
cholesterol-loaded human fibroblasts compared with native apoAI but similar capacities to deplete fibroblasts of the pool of
cholesterol 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 compared at equivalent rHDL
protein levels, but similar abilities of these rHDL to deplete cell
cholesterol were seen when compared at equivalent
phospholipid levels. Spherical rHDL generated using the whole
lipid fraction of HDL and apoAI or AIM showed similar capacities to deplete total and ACAT-accessible cell
cholesterol when compared at similar
protein levels, but an increased capacity of AIM-containing particles was seen when compared at equivalent
phospholipid levels. Unlike the apoAI-AII heterodimer in tyrosylated HDL, AIM-AII heterodimer-containing spherical rHDL showed no increased capacity 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 form in discoidal rHDL to enhance the mobilization of cellular
cholesterol when compared to AIM in its free or
lipid-bound forms. Any increase in depletion of cellular
cholesterol by
lipid-bound AIM in spherical rHDL appears related to altered
phospholipid-binding rather than intrinsic
cholesterol-mobilizing characteristics of this
protein compared to native apoAI. The lack of major differences in these studies in
cholesterol mobilization by native apoAI and AIM, or by apoAIM-AII heterodimers, suggests that any protection against
atherosclerosis conferred by this mutation is likely related to other beneficial vascular effects of AIM.