Abstract
Impaired cell cholesterol trafficking in Niemann-Pick type C (NPC) disease results in the first known instance of impaired regulation of the ATP-binding cassette transporter A1 (ABCA1), a lipid transporter mediating the rate-limiting step in high density lipoprotein (HDL) formation, as a cause of low plasma HDL-cholesterol in humans. We show here that treatment of human NPC1(-/-) fibroblasts with the liver X receptor (LXR) agonist TO-901317 increases ABCA1 expression and activity in human NPC1(-/-) fibroblasts, as indicated by near normalization of efflux of radiolabeled phosphatidylcholine and a marked increase in efflux of cholesterol mass to apoA-I. LXR agonist treatment prior to and during apoA-I incubation resulted in reduction in filipin staining of unesterified cholesterol in late endosomes/lysosomes, as well as cholesterol mass, in NPC1(-/-) cells. HDL species in human NPC disease plasma showed the same pattern of diminished large, cholesterol-rich alpha-1 HDL particles as seen in isolated heterozygous ABCA1 deficiency. Incubating NPC1(-/-) fibroblasts with the LXR agonist normalized the pattern of HDL particle formation by these cells. ABCG1, another LXR target gene involved in cholesterol efflux to HDL, also showed diminished expression in NPC1(-/-) fibroblasts and increased expression upon LXR agonist treatment. These results suggest that NPC1 mutations can be largely bypassed and that NPC1 protein function is non-essential for the trafficking and removal of cellular cholesterol if the down-stream defects in ABCA1 and ABCG1 regulation in NPC disease cells are corrected using an LXR agonist.
Highlights
Consistent with impaired regulation of cholesterol synthesis and esterification and low density lipoprotein (LDL) receptor activity in this disorder (13–15), we previously demonstrated that basal and cholesterol-stimulated expression of ATP-binding cassette transporter A1 (ABCA1) is diminished in fibroblasts from a patient with Niemann-Pick type C (NPC) disease, leading to impaired lipidation of apolipoprotein A-I (16)
Diminished ABCA1 Expression in NPC1Ϫ/Ϫ Human Fibroblasts Is Corrected by liver X receptor (LXR) Agonists—We previously demonstrated low basal and cholesterol-stimulated levels of ABCA1 mRNA and protein in human NPC1Ϫ/Ϫ fibroblasts (16), consistent with impaired oxysterol- and LXR-target gene regulation in this disease
As shown previously (16), ABCA1 protein levels were low in NPC1Ϫ/Ϫ cells grown to confluence in 10% fetal bovine serum or loaded with non-lipoprotein cholesterol, when compared with NPC1ϩ/ϩ fibroblasts (Fig. 1)
Summary
HDL-C is an integral feature of NPC disease. To our knowledge NPC disease represents the first known condition of decreased HDL formation and low HDL-C as a consequence of impaired ABCA1 regulation, rather than mutation. Earlier work by Liscum and Faust (15) had shown that addition of the oxysterol 25-hydroxycholesterol could normalize the rate of cholesterol esterification in human NPC disease fibroblasts, despite concomitant suppression of endogenous cholesterol synthesis in these cells These results suggested the oxysterol might facilitate the mobilization of stored cholesterol from the late endosome/lysosome compartment to the endoplasmic reticulum. Frolov et al (18) reported impaired oxysterol generation in human NPC1 and NPC2 disease fibroblasts, and decreased accumulation of total cholesterol mass and filipin staining of late endosomes/lysosomes in human NPC disease cells incubated with LDL-containing serum in the presence of 25- or 27-hydroxycholesterol Together, these results suggest that correction of oxysterol-dependent gene regulation normalizes cholesterol trafficking even in the presence of mutations in the genes encoding NPC1 or NPC2. These results suggest LXR agonists might greatly improve or possibly normalize the trafficking and overaccumulation of cholesterol and other lipids in NPC disease
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
