Minimal lipidation of pre-beta HDL by ABCA1 results in reduced ability to interact with ABCA1

The aim of this study was to determine the role of ATP binding cassette transporter A1 (ABCA1) on generation of different-sized nascent HDLs.

HEK293 cells stably-transfected with ABCA1 (HEK293-ABCA1) or non-transfected (control) cells were incubated with lipid free 125I-apoA-I for 24 hours. Incubation of apoA-I with HEK293-ABCA1 cells, but not control cells, led to the formation of heterogeneous-sized, pre-beta migrating nascent HDL subpopulations (pre-beta1 to -4) that varied in size (7.1 to 15.7 nm), lipid, and apoA-I content. Kinetic studies suggested that all subpopulations were formed simultaneously, with no evidence for a precursor-product relationship between smaller and larger-sized particles. When isolated nascent pre-beta HDLs (pre-beta1 to -4) were added back to HEK293-ABCA1 cells, their ability to bind to ABCA1 and efflux lipid was severely compromised. Heat-denaturation of pre-beta1 HDL resulted in partial recovery of ABCA1 binding, suggesting that initial interaction of apoA-I with ABCA1 results in a constrained conformation of apoA-I that decreases subsequent binding.

Interaction of apoA-I with ABCA1 results in the simultaneous generation of pre-beta HDLs of discrete size and chemical composition. These nascent particles are poor substrates for subsequent lipidation by ABCA1 and presumably require additional non-ABCA1-mediated lipidation for further maturation.