Red Blood Cells Facilitate Reverse Cholesterol Transport.

Lecithin:cholesterol acyl transferase (LCAT)-dependent conversion of cholesterol (CH) to cholesteryl ester (CE), a key component of the reverse cholesterol transport (RCT) pathway, is essential for cholesterol processing. We hypothesized that red blood cells (RBCs) function in this pathway by facilitating phosphatidylcholine (PC) re-generation from LCAT-derived lysophosphatidylcholine (LPC). Addition of 14C-oleate to fresh RBCs resulted in an ATP-dependent incorporation of radiolabel into PC via the Lands pathway. Prior depletion of red cell LPC content reduced the incorporation of 14C-oleate into PC, which was restored by the addition of LPC. Reconstituted PC/cholesterol/apolipoprotein A-I (apoA-I) high-density lipoprotein (rHDL) was used as substrate for LCAT dependent conversion of CH to CE. Addition of LPC to this reaction mix inhibited CE production. The inhibition was overcome, however, by inclusion of RBCs, suggesting rHDL-generated LPC migration to RBCs. RBCs depleted of LPC increased their ability to generate 14C-PC from 14C-oleate in the presence of rHDL with LCAT, indicating that LCAT-derived LPC can be utilized as a substrate for PC production in RBCs. Radio-labeled CH associated with RBCs was recovered as CE only when rHDL substrate and LCAT were present, indicating RBC-associated CH migration to rHDL. When RBCs containing 14C-PC were incubated with rHDL and LCAT, PC transferred from RBCs to the rHDL particles. The interaction of apoA-I with the membrane lipid transporter ATP-Binding Cassette A1 protein (ABCA1) in cell membranes has been shown to play an essential role in the formation of HDL and facilitates RCT. Using monoclonal antibodies, we were able to show that RBCs contain ABCA7, closely related to ABCA1. Our data show that fluorescently labeled apoA-I binds to RBCs, suggesting that the interaction between ABCA7 and apoA-I may be important for HDL/RBC interaction. Together, our data supports an active role for RBCs, the major cell type in blood, in LCAT-mediated lipoprotein remodeling. Thus, RBCs represent a hitherto underappreciated component of the RCT pathway.