Role of apoA-I, ABCA1, LCAT, and SR-BI in the biogenesis of HDL.

The concentration, composition, shape, and size of plasma high-density lipoprotein (HDL) are determined by numerous proteins that influence its biogenesis, remodeling, and catabolism. The discoveries of the HDL receptor (scavenger receptor class B type I, SR-BI) and the ABCA1 (ATP-binding cassette transporter A1) lipid transporter provided two missing links that were necessary to understand the biogenesis and some of the functions of HDL. Existing data indicate that functional interactions between apoA-I and ABCA1 are necessary for the initial lipidation of apoA-I. Through a series of intermediate steps, lipidated apoA-I proceeds to form discoidal HDL particles that can be converted to spherical particles by the action of lecithin:cholesterol acyltransferase (LCAT). Discoidal and spherical HDL can interact functionally with SR-BI and these interactions lead to selective lipid uptake and net efflux of cholesterol and thus remodel HDL. Defective apoA-I/ABCA1 interactions prevent lipidation of apoA-I that is necessary for the formation of HDL particles. In the same way, specific mutations in apoA-I or LCAT prevent the conversion of discoidal to spherical HDL particles. The interactions of lipid-bound apoA-I with SR-BI are affected in vitro by specific mutations in apoA-I or SR-BI. Furthermore, deficiency of SR-BI affects the lipid and apolipoprotein composition of HDL and is associated with increased susceptibility to atherosclerosis. Here we review the current status of the pathway of HDL biogenesis and mutations in apoA-I, ABCA1, and SR-BI that disrupt different steps of the pathway and may lead to dyslipidemia and atherosclerosis in mouse models. The phenotypes generated in experimental mouse models for apoA-I, ABCA1, LCAT, SR-BI, and other proteins of the HDL pathway may facilitate early diagnosis of similar phenotypes in the human population and provide guidance for proper treatment.