Impaired ABCA1-dependent lipid efflux and hypoalphalipoproteinemia in human Niemann-Pick type C disease

The cholesterol trafficking defect in Niemann-Pick type C (NPC) disease leads to impaired regulation of cholesterol esterification, cholesterol synthesis, and low density lipoprotein receptor activity. The ATP-binding cassette transporter A1 (ABCA1), which mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, is also regulated by cell cholesterol content. To determine whether the Niemann-Pick C1 protein alters the expression and activity of ABCA1, we determined the ability of apolipoprotein A-I (apoA-I) to deplete pools of cellular cholesterol and phospholipids in human fibroblasts derived from NPC1^+/+, NPC1 ^+/-, and NPC1^-/- subjects. Efflux of low density lipoprotein-derived, non-lipoprotein, plasma membrane, and newly synthesized pools of cell cholesterol by apoA-I was diminished in NPC1^-/- cells, as was efflux of phosphatidylcholine and sphingomyelin. NPC1^+/- cells showed intermediate levels of lipid efflux compared with NPC1 ^+/+ and NPC1^-/- cells. Binding of apoA-I to cholesterol-loaded and non-cholesterol-loaded cells was highest for NPC1 ^+/- cells, with NPC1^+/+ and NPC1^-/- cells showing similar levels of binding. ABCA1 mRNA and protein levels increased in response to cholesterol loading in NPC1^+/+ and NPC1^+/- cells but showed low levels at base line and in response to cholesterol loading in NPC1^-/- cells. Consistent with impaired ABCA1-dependent lipid mobilization to apoA-I for HDL particle formation, we demonstrate for the first time decreased plasma HDL-cholesterol levels in 17 of 21 (81%) NPC1 ^-/- subjects studied. These results indicate that the cholesterol trafficking defect in NPC disease results in reduced activity of ABCA1, which we suggest is responsible for the low HDL-cholesterol in the majority of NPC subjects and partially responsible for the over-accumulation of cellular lipids in this disorder.