Biogenesis and speciation of nascent apoA-I-containing particles in various cell lines

It is generally thought that the large heterogeneity of human HDL confers antiatherogenic properties; however, the mechanisms governing HDL biogenesis and speciation are complex and poorly understood. Here, we show that incubation of exogenous apolipoprotein A-I (apoA-I) with fibroblasts, CaCo-2, or CHO-overexpressing ABCA1 cells generates only α-nascent apolipoprotein A-I-containing particles (α-LpA-I) with diameters of 8-20 nm, whereas human umbilical vein endothelial cells and ABCA1 mutant (Q597R) cells were unable to form such particles. Interestingly, incubation of exogenous apoA-I with either HepG2 or macrophages generates both α-LpA-I and preβ₁-LpA-I. Furthermore, glyburide inhibits almost completely the formation of α-LpA-I but not preβ₁-LpA-I. Similarly, endogenously secreted HepG2 apoA-I was found to be associated with both preβ₁-LpA-I and α-LpA-I; by contrast, CaCo-2 cells secreted only α-LpA-I. To determine whether α-LpA-I generated by fibroblasts is a good substrate for LCAT, isolated α-LpA-I as well as reconstituted HDL [r(HDL)] was reacted with LCAT. Although both particles had similar V_max (8.4 vs. 8.2 nmol cholesteryl ester/h/μg LCAT, respectively), the K_m value was increased 2-fold for α-LpA-I compared with r(HDL) (1.2 vs. 0.7 μM apoA-I). These results demonstrate that 1) ABCA1 is required for the formation of α-LpA-I but not preβ-LpA-I; and 2) α-LpA-I interacts efficiently with LCAT. Thus, our study provides direct evidence for a new link between specific cell lines and the speciation of nascent HDL that occurs by both ABCA1-dependent and -independent pathways.