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

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 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.

Differential Regulation of ATP Binding Cassette Protein A1 Expression and ApoA-I Lipidation by Niemann-Pick Type C1 in Murine Hepatocytes and Macrophages

“…Here, we showed that apoA-I lipidation and HDL secretion are also increased in Npc-1 hepatocytes, in keeping with the increased HDL cholesterol levels and formation of larger HDL particles determined by fast protein liquid chromatography in Npc1-null mice (8,13). In contrast, human NPC homozygote subjects apparently exhibit a moderate decrease in plasma HDL levels (18). It is unclear whether this discrepancy reflects the effects of chronic illness, a species difference, or methodologies for lipoprotein quantification.…”

“…Deficiency Increases the Lipidation of Newly Synthesized ApoA-I in Hepatocytes-Whereas Npc1 inactivation is well known to impair intracellular cholesterol traffic and decrease cholesterol efflux to apoA-I in macrophages or fibroblasts (6,18,31,32), its effect on the lipidation of apoA-I in liver, the major site of HDL synthesis, is unknown. Here, we evaluated the effect of Npc1 inactivation on the lipidation of endogenous apoA-I in hepatocytes that were labeled with either endogenously synthesized lipids or exogenous lipids delivered by lipoproteins.…”

Apolipoprotein E-Containing Lipoproteins Protect Neurons from Apoptosis via a Signaling Pathway Involving Low-Density Lipoprotein Receptor-Related Protein-1

“…RAP was purified using a GSTrap FF column, thrombin, and a HiTrap Benzamidine FF column (GE Healthcare). Human apoA1 was prepared from human plasma (Choi et al, 2003) and was provided by Dr. G. A. Francis (University of Alberta, Edmonton, Alberta, Canada). A colony of ApoE Ϫ/Ϫ mice was established at the University of Alberta from mice obtained from The Jackson Laboratory (Bar Harbor, ME).…”

Pregnane X receptor (PXR) activation: A mechanism for neuroprotection in a mouse model of Niemann–Pick C disease

“…Mutations in the NPC1 gene are responsible for Ϸ95% of human NPC disease. NPC1 loss-of-function mutants exhibit marked impairment of low-density lipoprotein (LDL) cholesterol esterification and mobilization of newly hydrolyzed LDL cholesterol to the plasma membrane (2)(3)(4), resulting in lysosomal sequestration of LDL cholesterol, delayed down-regulation of the LDL receptor and de novo cholesterol biosynthesis, and impaired ABCA1-mediated cholesterol efflux (5)(6)(7). Despite recent progress in characterizing the biochemical and genetic defects in NPC disease, the mechanisms underlying the neurodegenerative phenotype are not well understood.…”