Lipid Homeostasis and Lipoprotein Secretion in Niemann-Pick C1-deficient Hepatocytes

Agnes Kulinski,Jean E Vance

doi:10.1074/jbc.m610001200

Abstract

Niemann-Pick C (NPC) disease is a fatal inherited disorder characterized by an accumulation of cholesterol and other lipids in late endosomes/lysosomes. Although this disease is considered to be primarily a neurodegenerative disorder, many NPC patients suffer from liver disease. We have investigated alterations that occur in hepatic lipid homeostasis using primary hepatocytes isolated from NPC1-deficient mice. The cholesterol content of Npc1(-/-) hepatocytes was 5-fold higher than that of Npc1(+/+) hepatocytes; phospholipids and cholesteryl esters also accumulated. In contrast, the triacylglycerol content of Npc1(-/-) hepatocytes was 50% lower than of Npc1(+/+) hepatocytes. We hypothesized that the cholesterol sequestration induced by NPC1 deficiency might inhibit very low density lipoprotein secretion. However, this process was enhanced by NPC1 deficiency and the secreted particles were enriched in cholesteryl esters. We investigated the mechanisms responsible for these changes. The synthesis of phosphatidylcholine, cholesteryl esters, and cholesterol in hepatocytes was increased by NPC1 deficiency and the amount of the mature form of sterol response element-binding protein-1 was also increased. These observations indicate that the enhanced secretion of lipoproteins from NPC1-deficient hepatocytes is due, at least in part, to increased lipid synthesis.

Highlights

Niemann-Pick type C (NPC)2 disease is an inherited autosomal, recessive disorder that is characterized by an accumulation of cholesterol and other lipids in tissues
Accumulation of Lipids in NPC1-deficient Hepatocytes—The distribution of unesterified cholesterol in primary hepatocytes isolated from 5-week-old Npc1ϩ/ϩ and Npc1Ϫ/Ϫ mice was assessed by staining the cells with filipin [26, 31], an agent that detects un-esterified cholesterol but not cholesteryl esters (CE)
The amount of the processed form of SREBP-2 was not increased by NPC1 deficiency (Fig. 8). These data indicate that the mechanism by which very low density lipoproteins (VLDLs) secretion is increased in NPC1-deficient hepatocytes is by increased synthesis of CE, cholesterol and/or PC, and stimulation of lipogenesis induced by increased amounts of mature SREBP-1. In these studies we have examined alterations in the synthesis of several lipids and VLDLs in response to NPC1 deficiency in hepatocytes isolated from the mouse model of NPC disease

Summary

Introduction

Niemann-Pick type C (NPC)2 disease is an inherited autosomal, recessive disorder that is characterized by an accumulation of cholesterol and other lipids in tissues. Accumulation of Lipids in NPC1-deficient Hepatocytes—The distribution of unesterified cholesterol in primary hepatocytes isolated from 5-week-old Npc1ϩ/ϩ and Npc1Ϫ/Ϫ mice was assessed by staining the cells with filipin [26, 31], an agent that detects un-esterified cholesterol but not CE.

Results

Conclusion