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

Abstract

Niemann-Pick type C1 (Npc1) protein inactivation results in lipid accumulation in late endosomes and lysosomes, leading to a defect of ATP binding cassette protein A1 (Abca1)-mediated lipid efflux to apolipoprotein A-I (apoA-I) in macrophages and fibroblasts. However, the role of Npc1 in Abca1-mediated lipid efflux to apoA-I in hepatocytes, the major cells contributing to HDL formation, is still unknown. Here we show that, whereas lipid efflux to apoA-I in Npc1-null macrophages is impaired, the lipidation of endogenously synthesized apoA-I by low density lipoprotein-derived cholesterol or de novo synthesized cholesterol or phospholipids in Npc1-null hepatocytes is significantly increased by about 1-, 3-, and 8-fold, respectively. The increased cholesterol efflux reflects a major increase of Abca1 protein in Npc1-null hepatocytes, which contrasts with the decrease observed in Npc1-null macrophages. The increased Abca1 expression is largely post-transcriptional, because Abca1 mRNA is only slightly increased and Lxr␣ mRNA is not changed, and Lxr␣ target genes are reduced. This differs from the regulation of Abcg1 expression, which is up-regulated at both mRNA and protein levels in Npc1-null cells. Abca1 protein translation rate is higher in Npc1-null hepatocytes, compared with wild type hepatocytes as measured by [35 S]methionine incorporation, whereas there is no difference for the degradation of newly synthesized Abca1 in these two types of hepatocytes. Cathepsin D, which we recently identified as a positive modulator of Abca1, is markedly increased at both mRNA and protein levels by Npc1 inactivation in hepatocytes but not in macrophages. Consistent with this, inhibition of cathepsin D with pepstatin A reduced the Abca1 protein level in both Npc1-inactivated and WT hepatocytes. Therefore, Abca1 expression is specifically regulated in hepatocytes, where Npc1 activity modulates cathepsin D expression and Abca1 protein translation rate. Niemann-Pick type C (NPC)2 disease is an autosomal lipid storage disease resulting from mutations in either the Npc1 (95% of families) or Npc2 genes (1-3). It is characterized by progressive hepatosplenomegaly and neurodegeneration, leading to premature death (4, 5). The exact function of both proteins remains unknown, but there is much evidence that they facilitate the transport of lipids, primarily cholesterol, from late endosome to the Golgi apparatus, endoplasmic reticulum (ER), mitochondria, and plasma membrane (6, 7). Thus the storage involves the accumulation of unesterified cholesterol, sphingolipids, and other lipids within the endosomal/lysosomal compartments of cells in almost all body tissues (8). Impaired cholesterol traffic in NPC disease cells prevents the normal down-regulation of endogenous cholesterol synthesis and the low density lipoprotein (LDL) receptor (9, 10), through the interruption of normal cholesterol regulation by sterol regulatory element-binding protein at the ER and the interruption of generation of LDL cholesterol-derived oxysterols in the mitochond...

In vivo antisense oligonucleotide reduction of NPC1 expression as a novel mouse model for Niemann Pick type C– associated liver disease

“…The NPC nih mouse has been shown to exhibit no change, 29 decreases, 18 or increases in serum cholesterol, HDL-cholesterol, 30 and LDL-cholesterol levels. [30][31][32] Knockdown of NPC1 Increases Serum ALT and AST, Indicating Liver Damage. We next determined if NPC1 ASO treatment induces liver damage by measuring serum levels of the liver enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST).…”

MCP-1 impacts RCT by repressing ABCA1, ABCG1, and SR-BI through PI3K/Akt posttranslational regulation in HepG2 cells

“…In contrast, tumor necrosis factor ␣ markedly decreases ABCA1 gene expression by attenuating the ABCA1 promoter activity transcriptionally via the nuclear factor kappa B pathway, but not the liver X receptor (LXR) pathway, and posttranslationally enhances the rate of ABCA1 degradation without attenuating the expression of LXR target genes, such as ABCG1 ( 24,34 ). In Npc1-null hepatocytes, the upregulation of ABCG1 expression is mainly transcriptional without changes on LXR ␣ mRNA, whereas ABCA1 expression is largely dependent on posttranscriptional mechanisms, including an increased translation rate and decreased degradation of ABCA1 by cathepsin D ( 35 ). Thus, the transcriptional factors regulating the expression of ABC transporters are not identical, except for the known LXR pathway.…”

Lysosomal Acid Lipase Deficiency Impairs Regulation of ABCA1 Gene and Formation of High Density Lipoproteins in Cholesteryl Ester Storage Disease

“…Previous studies indicated hepatic ABCA1 is a critical regulator of plasma HDL-C in mice (43) and that hepatocyte ABCA1 is not reduced in NPC1 Ϫ/Ϫ mice (44). The absence of low plasma HDL-C in LAL-deficient mice is consistent with these observations, because mouse hepatic ABCA1 is reported to be unresponsive to LXR stimulation (45,46) and would not be expected to show reduced expression in the face of reduced flux of cholesterol out of lysosomes and oxysterol generation with LAL deficiency.…”