Impaired ABCA1-dependent Lipid Efflux and Hypoalphalipoproteinemia in Human Niemann-Pick type C Disease

Teddy Chan,Hong Y Choi,Barbara Karten,Jean E Vance,Wenda L Greer,Randall A Heidenreich,Gordon A Francis,William S Garver

doi:10.1074/jbc.m304553200

Abstract

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 overaccumulation of cellular lipids in this disorder.

Highlights

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
Niemann-Pick type C disease is characterized by the accumulation of low density lipoprotein (LDL)-derived cholesterol in late endosomes/lysosomes and an inability to regulate normally three central mechanisms of cholesterol homeostasis: delivery of unesterified cholesterol to the endoplasmic reticulum for esterification by acyl-CoA:cholesterol acyltransferase, regulation of cholesterol synthesis by 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and regulation of LDL receptor activity (1, 9 –11)
In the current studies we demonstrate that regulation of another pivotal mediator of cholesterol homeostasis, ATP-binding cassette transporter A1 (ABCA1), is impaired in human NPC1-deficient fibroblasts

Summary

Introduction

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. In the current studies we characterized apoA-I-mediated efflux of phospholipids and cholesterol from distinct cellular pools, binding of apoA-I, and regulation of ABCA1 expression in normal (NPC), NPC1ϩ/Ϫ, and NPC1Ϫ/Ϫ human fibroblasts, and we correlated our findings with the plasma lipid profiles of NPC patients.

Results

Conclusion