The intracellular transport of low density lipoprotein-derived cholesterol is defective in Niemann-Pick type C fibroblasts

Niemann-Pick disease type C (NPC) is characterized by substantial intracellular accumulation of unesterified cholesterol. The accumulation of unesterified cholesterol in NPC fibroblasts cultured with low density lipoprotein (LDL) appears to result from the inability of LDL to stimulate cholesterol esterification in addition to impaired LDL-mediated downregulation of LDL receptor activity and cellular cholesterol synthesis. Although a defect in cholesterol transport in NPC cells has been inferred from previous studies, no experiments have been reported that measure the intracellular movement of LDL-cholesterol specifically. We have used four approaches to assess intracellular cholesterol transport in normal and NPC cells and have determined the following: (a) mevinolin-inhibited NPC cells are defective in using LDL-cholesterol for growth. However, exogenously added mevalonate restores cell growth equally in normal and NPC cells; (b) the transport of LDL-derived [3H]cholesterol to the plasma membrane is slower in NPC cells, while the rate of appearance of [3H]acetate-derived, endogenously synthesized [3H]cholesterol at the plasma membrane is the same for normal and NPC cells; (c) in NPC cells, LDL-derived [3H]cholesterol accumulates in lysosomes to higher levels than normal, resulting in defective movement to other cell membranes; and (d) incubation of cells with LDL causes an increase in cholesterol content of NPC lysosomes that is threefold greater than that observed in normal lysosomes. Our results indicate that a cholesterol transport defect exists in NPC that is specific for LDL-derived cholesterol.     

The intracellular transport of low density lipoprotein-derived cholesterol is inhibited in Chinese hamster ovary cells cultured with 3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one

In mammalian cells, low density lipoprotein (LDL) is bound, internalized, and delivered to lysosomes where LDL-cholesteryl esters are hydrolyzed to unesterified cholesterol. The mechanisms of intracellular transport of LDL-cholesterol from lysosomes to other cellular sites and LDL-mediated regulation of cellular cholesterol metabolism are unknown. We have identified a pharmacological agent, U18666A (3-beta-[2-diethyl-amino)ethoxy]androst-5-en-17-one), which impairs the intracellular transport of LDL-derived cholesterol in cultured Chinese hamster ovary (CHO) cells. U18666A blocks the ability of LDL-derived cholesterol to stimulate cholesterol esterification, and to suppress 3-hydroxy-3-methylglutaryl-coenzyme A reductase and LDL receptor activities. However, U18666A does not impair 25-hydroxycholesterol-mediated regulation of these processes. In addition, U18666A impedes the ability of LDL-derived cholesterol to support the growth of CHO cells. However, U18666A has only moderate effects on growth supported by non-lipoprotein cholesterol. LDL binding, internalization, and lysosomal hydrolysis of LDL-cholesteryl esters are not affected by the presence of U18666A. Analysis of intracellular cholesterol transport reveals that LDL-derived cholesterol accumulates in the lysosomes of U18666A-treated CHO cells which results in impaired movement of LDL-derived cholesterol to other cell membranes.     

Characterization of Chinese hamster ovary cells that are resistant to 3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one inhibition of low density lipoprotein-derived cholesterol metabolism

The pharmacological agent U18666A (3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one inhibits the intracellular transport of low density lipoprotein (LDL)-derived cholesterol in Chinese hamster ovary (CHO) cells. LDL-derived cholesterol accumulates in the lysosomes of U18666A-treated cells causing delayed LDL-mediated regulation of cellular cholesterol metabolism and impaired movement of LDL-derived cholesterol to other cell membranes. As a result of impaired LDL-derived cholesterol transport, LDL-dependent growth of CHO cells is also inhibited by U18666A. By selecting for cell growth in the presence of U18666A, we have identified a CHO cell line, designated U18R, that is resistant to U18666A-inhibition of LDL-derived cholesterol trafficking. When compared to parental CHO cells, U18R cells are relatively resistant to U18666A inhibition of LDL-derived cholesterol transport as well as LDL-mediated regulation of cellular cholesterol metabolism. In cell fusion experiments, the U18666A resistance observed in U18R cells displays a dominant phenotype. Identification of the U18666A-resistant factor may provide important insights toward the understanding of intracellular LDL-derived cholesterol regulation and trafficking.     

The transport of low density lipoprotein-derived cholesterol to the plasma membrane is defective in NPC1 cells

Niemann-Pick disease type C (NPC) is characterized by lysosomal storage of cholesterol and gangliosides, which results from defects in intracellular lipid trafficking. Most studies of NPC1 have focused on its role in intracellular cholesterol movement. Our hypothesis is that NPC1 facilitates the egress of cholesterol from late endosomes, which are where active NPC1 is located. When NPC1 is defective, cholesterol does not exit late endosomes; instead, it is carried along to lysosomal storage bodies, where it accumulates. In this study, we addressed whether cholesterol is transported from endosomes to the plasma membrane before reaching NPC1-containing late endosomes. Our study was conducted in Chinese hamster ovary cell lines that display the classical NPC biochemical phenotype and belong to the NPC1 complementation group. We used three approaches to test whether low density lipoprotein (LDL)-derived cholesterol en route to NPC1-containing organelles passes through the plasma membrane. First, we used cyclodextrins to measure the arrival of LDL cholesterol at the plasma membrane and found that the arrival of LDL cholesterol in a cyclodextrin-accessible pool was significantly delayed in NPC1 cells. Second, the movement of LDL cholesterol to NPC1-containing late endosomes was assessed and found to be normal in Chinese hamster ovary mutant 3-6, which exhibits defective movement of plasma membrane cholesterol to intracellular membranes. Third, we examined the movement of plasma membrane cholesterol to the endoplasmic reticulum and found that this pathway is intact in NPC1 cells, i.e. it does not pass through NPC1-containing late endosomes. Our data suggest that in NPC1 cells LDL cholesterol traffics directly through endosomes to lysosomes, bypassing the plasma membrane, and is trapped there because of dysfunctional NPC1.     

Role of Niemann-Pick type C1 protein in intracellular trafficking of low density lipoprotein-derived cholesterol

Niemann-Pick type C (NPC) is a disease that affects intracellular cholesterol-trafficking pathways. By cloning the hamster ortholog of NPC1, we identified the molecular lesions in two independently isolated Chinese hamster ovary cell mutants, CT60 and CT43. Both mutants lead to premature translational terminations of the NPC1 protein. Transfecting hamster NPC1 cDNA complemented the defects of the mutants. Investigation of the CT mutants, their parental cells, and an NPC1-stable transfectant allow us to present evidence that NPC1 is involved in a post-plasma membrane cholesterol-trafficking pathway. We found that the initial movement of low density lipoprotein (LDL)-derived cholesterol to the plasma membrane (PM) did not require NPC1. After reaching the PM and subsequent internalization, however, cholesterol trafficking back to the PM did involve NPC1. Both LDL-derived cholesterol and cholesterol originating from the PM accumulated in a dense, intracellular compartment in the CT mutants. Cholesterol movement from this compartment to the PM or endoplasmic reticulum was defective in the CT mutants. Our results functionally distinguish the dense, intracellular compartment from the early endocytic hydrolytic organelle and imply that NPC1 is involved in sorting cholesterol from the intracellular compartment back to the PM or to the endoplasmic reticulum.     

Toxoplasma gondii exploits host low-density lipoprotein receptor-mediated endocytosis for cholesterol acquisition

The obligate intracellular protozoan Toxoplasma gondii resides within a specialized parasitophorous vacuole (PV), isolated from host vesicular traffic. In this study, the origin of parasite cholesterol was investigated. T. gondii cannot synthesize sterols via the mevalonate pathway. Host cholesterol biosynthesis remains unchanged after infection and a blockade in host de novo sterol biosynthesis does not affect parasite growth. However, simultaneous limitation of exogenous and endogenous sources of cholesterol from the host cell strongly reduces parasite replication and parasite growth is stimulated by exogenously supplied cholesterol. Intracellular parasites acquire host cholesterol that is endocytosed by the low-density lipoprotein (LDL) pathway, a process that is specifically increased in infected cells. Interference with LDL endocytosis, with lysosomal degradation of LDL, or with cholesterol translocation from lysosomes blocks cholesterol delivery to the PV and significantly reduces parasite replication. Similarly, incubation of T. gondii in mutant cells defective in mobilization of cholesterol from lysosomes leads to a decrease of parasite cholesterol content and proliferation. This cholesterol trafficking to the PV is independent of the pathways involving the host Golgi or endoplasmic reticulum. Despite being segregated from the endocytic machinery of the host cell, the T. gondii vacuole actively accumulates LDL-derived cholesterol that has transited through host lysosomes.     

Isolation and characterization of Chinese hamster ovary cell mutants defective in intracellular low density lipoprotein-cholesterol trafficking

This paper reports the isolation and characterization of Chinese hamster ovary cell mutants defective in low density lipoprotein (LDL)-cholesterol trafficking. The parental cell line was 25-RA, which possesses LDL receptors and various cholesterogenic enzyme activities that are partially resistant to down regulation by exogenous sterols (Chang, T. Y., and J. S. Limanek. 1980. J. Biol. Chem. 255:7787-7795). Because these cells accumulate a large amount of intracellular cholesteryl ester when grown in medium containing 10% fetal calf serum, mutagenized populations of 25-RA cells were grown in the presence of a specific inhibitor of acyl-coenzyme A: cholesterol acyltransferase (ACAT), which depleted their cholesteryl ester stores. Without this cholesterol ester storage, 99% of 25-RA cells die after 5-d growth in cholesterol starvation medium, while the mutant cells, which accumulate free cholesterol intracellularly, survived. In two mutant clones chosen for characterization, activation of cholesteryl ester synthesis by LDL was markedly reduced in the mutant cells compared with 25-RA cells. This lack of activation of cholesterol ester synthesis in the mutant cells could not be explained by defective uptake and/or processing of LDL or by a decreased amount of ACAT, as determined by in vitro enzyme activity. Mutant cells grown in the presence of LDL contain numerous cytosolic particles that stain intensely with the fluorescent compound acridine orange, suggesting that they are acidic. The particles are also stained with filipin, a cholesterol-specific fluorescent dye. Indirect immunofluorescence with a monoclonal antibody specific for a lysosomal/endosomal fraction revealed a staining pattern that colocalized with the filipin signal. The mutant phenotype was recessive. The available evidence indicates that the mutant cells can take up and process LDL normally, but the hydrolyzed cholesterol accumulates in an acidic compartment, probably the lysosomes, where it can not be transported to its normal intracellular destinations.     

Pharmacological inhibition of the intracellular transport of low-density lipoprotein-derived cholesterol in Chinese hamster ovary cells

Mammalian cells, cultured in the presence of serum lipoproteins, acquire cholesterol necessary for growth from the uptake and lysosomal hydrolysis of low-density lipoproteins (LDL). The mechanism(s) of intracellular transport of LDL-derived cholesterol from lysosomes to other cellular sites is unknown. In this study, various pharmacological agents were assessed for their ability to inhibit the movement of LDL-cholesterol from lysosomes to the plasma membrane. The only pharmacological agent tested in these experiments that specifically inhibited LDL-cholesterol movement was U18666A. Ketoconazole impaired the intracellular transport of LDL-cholesterol; however, ketoconazole also had a general effect on cholesterol movement, since it impeded the desorption of endogenously synthesized cholesterol into the medium. Other drugs that affected cholesterol movement appeared to be nonspecific. Cholesterol transport from lysosomes to plasma membranes was not significantly altered by agents that affect lysosomal function or cytoskeletal organization, as well as energy poisons and cycloheximide.     

Intracellular transport of cholesterol in type C Niemann-Pick fibroblasts

The purpose of this study was to determine the capacity of Niemann-Pick type C (NPC) fibroblasts to transport cholesterol from the cell surface to intracellular membranes. This is relevant in light of the observations that NPC cells display a sluggish metabolism of LDL-derived cholesterol, a phenomenon which could be explained by a defective intracellular transport of cholesterol. Treatment of NPC cells for 4 h with 0.1 mg/ml of LDL failed to increase the incorporation of [14C]oleic acid into cholesterol [14C]oleate, an observation consistent with previous reports on this cell type (Pentchev et al. (1985) Proc. Natl. Acad. Sci. USA 82, 8247). Normal fibroblasts, however, displayed the classical upregulation (6-fold over control) of the endogenous esterification reaction in response to LDL exposure. Incubation of normal or NPC fibroblasts with sphingomyelinase (100 mU/ml; Staphylococcus aureus) led to a rapid and marked increase (9- and 10-fold for normal and NPC fibroblasts, respectively, after 4 h) in the esterification of plasma-membrane-derived [3H]cholesterol suggesting that sphingomyelin degradation forced a net transfer of cholesterol from the cell surface to the endoplasmic reticulum. The similar response in normal and mutant fibroblasts to the degradation of sphingomyelin suggests that plasma membrane cholesterol can be transported into the substrate pool of ACAT to about the same extent in these two cell types. Degradation of cell sphingomyelin in NPC fibroblasts also resulted in the movement of 20-25% of the cellular cholesterol from a cholesterol oxidase susceptible pool into oxidase-resistant pools, implying that a substantial amount of plasma membrane cholesterol was internalized after sphingomyelin degradation. This cholesterol internalization was not accompanied by an increased rate of membrane internalization, as measured by [3H]sucrose uptake. Although NPC cells showed a relative accumulation of unesterified cholesterol and a sluggish esterification of LDL-derived cholesterol when exposed to LDL, these cells responded like normal fibroblasts with regard to their capacity to transport cholesterol from the cell surface into intracellular sites in response to sphingomyelin degradation. It therefore appears that NPC cells, in contrast to the impaired intracellular movement of lipoprotein-derived cholesterol, do not display a general impairment of cholesterol transport between the cell surface and the intracellular regulatory pool of cholesterol.     

A functional role for vimentin intermediate filaments in the metabolism of lipoprotein-derived cholesterol in human SW-13 cells.

Numerous studies have indicated that cytoplasmic intermediate filaments (cIFs) can associate with cellular lipids. To determine if these interactions might have functional consequences, we have studied the lipid metabolism of human SW-13 adrenal tumor cell lines that either contain vimentin-type cIFs (vim+) or lack any detectable cIF network (vim-). Although there were no significant differences in phospholipid or glyceride synthesis, vim- cell lines had elevated levels of cholesterol synthesis and decreased cholesterol esterification, compared with vim+ cells. These differences in cholesterol synthesis and esterification were found to be due to an impaired ability of vim- cells to utilize low density lipoprotein (LDL)-derived cholesterol, although receptor-mediated endocytosis of LDL and the capacity of these cells to esterify endogenously produced cholesterol were not affected. Expression of a mouse vimentin cDNA in stably transfected cell lines, derived from vim- cells, restored the capacity of these cells to utilize LDL cholesterol. The uptake and metabolism of [3H]cholesterol linoleate-loaded LDL showed that the impaired ability of vim- cells to esterify LDL cholesterol was not associated with an accumulation of cellular free cholesterol but rather an increase in the appearance of [3H]cholesterol in the culture medium. These studies indicate that in SW-13 cells, the intracellular movement of LDL-derived cholesterol from the lysosome to the site of esterification is a vimentin-dependent process.     

Translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the endoplasmic reticulum for esterification may require common cellular factors involved in cholesterol egress from the acidic compartments (lysosomes/endosomes)

Using a stable cell line 25-RA derived from wild-type Chinese hamster ovary (CHO) cells as the parental cell, this laboratory previously reported the isolation and characterization of CHO cell mutants (cholesterol-trafficking or CT) defective in transporting LDL-derived cholesterol out of the acidic compartment(s) (lysosomes/endosomes) to the endoplasmic reticulum (ER) for esterification. In this report, we show that the CT mutation can be complemented by fusion with human cells; however, attempts to complement the CT defect through DNA transfection have resulted in a collection of stable cell lines designated as ST cells. Under cholesterol starvation condition, the ST cells exhibit an elevated rate of cholesterol ester biosynthesis (by 3- to 5-fold) compared to both the parental CHO cells and the CT cells. The phenotypes of the ST cells are stable. ST cells are thus new cell lines arisen from the CT cells. When the plasma membranes of the parental, CT, and ST cells are labelled with [³H]cholesterol, ST cells show rates of [³H]cholesterol esterification much higher than that observed in CT cells but lower than that observed in the parental CHO cells. This result shows that translocation of plasma membrane cholesterol to the ER for esterification is defective in the CT cells. This result also suggests that ST cells acquire increased cholesterol trafficking activity between the lysosome and the ER without mixing with the plasma membrane cholesterol pool. The characteristics of CT cells and ST cells reported here suggest that translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the ER for esterification may require common cellular factors involved in cholesterol egress from the acidic compartment(s) (lysosomes/endosomes).     

Low density lipoprotein (LDL)-mediated suppression of cholesterol synthesis and LDL uptake is defective in Niemann-Pick type C fibroblasts

One characteristic of type C Niemann-Pick (NPC) disease is the substantial intracellular accumulation of unesterified cholesterol. The increased cholesterol content in NPC fibroblasts which are grown in the presence of low density lipoproteins (LDL) has been postulated to be due to a deficiency in cellular cholesterol esterification. We have examined several aspects of LDL metabolism in NPC fibroblasts. We observe that LDL binding, internalization, and lysosomal hydrolysis of LDL cholesteryl esters are normal in NPC cells. As reported by Pentchev et al. (Pentchev, P. G., Comly, M. E., Kruth, H. S., Vanier, M. T., Wenger, D. A., Patel, S., and Brady, R. O. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 8247-8251), we find that LDL does not stimulate cholesterol esterification. However, we also show that LDL does not down-regulate cholesterol synthesis or LDL receptor activity as normal. In NPC cells, these processes are regulated normally by nonlipoprotein effectors, such as 25-hydroxycholesterol or mevalonate. Since NPC cells are not defective in lysosomal hydrolysis of LDL-derived cholesteryl esters, they must exhibit a different defect than Wolman's or cholesteryl ester storage diseases. We conclude that NPC cells are defective specifically in LDL-mediated regulation of cellular cholesterol metabolism. We suggest that the intracellular processing of LDL-derived cholesterol may be defective in NPC fibroblasts.     

Cholesterol efflux via HDL resecretion occurs when cholesterol transport out of the lysosome is impaired

Recently, we showed that holo HDL particle uptake and resecretion occur in physiologically relevant cell lines and that HDL uptake is mediated by scavenger receptor class B type I (SR-BI). Furthermore, we established that HDL resecretion is accompanied by [(3)H]cholesterol efflux. This study shows that HDL uptake and resecretion occur even when LDL uptake and cholesterol trafficking are disturbed. First, we used a set of inhibitors that block cholesterol transport out of the lysosome: chloroquine, imipramine, U18666A, and monensin. In all cases, HDL retroendocytosis occurred and HDL resecretion mediated [(3)H]cholesterol efflux, although to a lesser extent. Second, cell lines carrying somatic mutations in intracellular cholesterol transport were used: CHO 2-2 and CHO 3-6 cells accumulated LDL-derived lipid in the lysosome but showed all components of HDL retroendocytosis. SR-BI overexpression increased HDL uptake and resecretion and [(3)H]cholesterol efflux in these mutant cells. Finally, we used Niemann-Pick type C (NPC) patient fibroblast cells, which carry a defect in cholesterol transfer out of the lysosome. NPC fibroblast cells accumulate cholesterol in the lysosome as a result of a mutation in the NPC1 gene. Despite disturbed intracellular cholesterol transfer, NPC fibroblast cells exhibited HDL retroendocytosis and [(3)H]cholesterol efflux via HDL resecretion, although to a lesser extent. Thus, [(3)H]cholesterol efflux via HDL resecretion is independent of the cholesterol uptake pathway via the LDL receptor and may be an alternative way to remove excess cholesterol.     

D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol alters cellular cholesterol homeostasis by modulating the endosome lipid domains

D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) is a frequently used inhibitor of glycosphingolipid biosynthesis. However, some interesting characteristics of D-PDMP cannot be explained by the inhibition of glycolipid synthesis alone. In the present study, we showed that d-PDMP inhibits the activation of lysosomal acid lipase by late endosome/lysosome specific lipid, bis(monoacylglycero)phosphate (also called as lysobisphosphatidic acid), through alteration of membrane structure of the lipid. When added to cultured fibroblasts, D-PDMP inhibits the degradation of low-density lipoprotein (LDL) and thus accumulates both cholesterol ester and free cholesterol in late endosomes/lysosomes. This accumulation results in the inhibition of LDL-derived cholesterol esterification and the decrease of cell surface cholesterol. We showed that D-PDMP alters cellular cholesterol homeostasis in a glycosphingolipid-independent manner using L-PDMP, a stereoisomer of D-PDMP, which does not inhibit glycosphingolipid synthesis, and mutant melanoma cell which is defective in glycolipid synthesis. Altering cholesterol homeostasis by D-PDMP explains the unique characteristics of sensitizing multidrug resistant cells by this drug.     

Type C Niemann-Pick disease: dimethyl sulfoxide moderates abnormal LDL-cholesterol processing in mutant fibroblasts

Biochemical and cytochemical studies have revealed that abnormal processing of low-density-lipoprotein (LDL) cholesterol can be reversed in mutant Niemann-Pick C (NP-C) fibroblasts when 2% dimethyl sulfoxide (DMSO) is added to the culture medium. Both the excessive lysosomal accumulation of LDL cholesterol and the delayed induction of cellular homeostatic responses associated with the uptake of LDL by the mutant cells were substantially reversed by DMSO. DMSO appears to accelerate the intracellular mobilization of LDL-derived cholesterol through effects that may reflect enhanced membrane permeability or cholesterol solubilization.     

Type C Niemann-Pick disease. Lysosomal accumulation and defective intracellular mobilization of low density lipoprotein cholesterol

The intracellular accumulation of unesterified cholesterol was examined during 24 h of low density lipoprotein (LDL) uptake in normal and Niemann-Pick C fibroblasts by fluorescence microscopy with filipin staining and immunocytochemistry. Perinuclear fluorescence derived from filipin-sterol complexes was observed in both normal and mutant cells by 2 h. This perinuclear cholesterol staining reached its peak in normal cells at 6 h. Subsequent development of fluorescence during the remaining 18 h of LDL incubation was primarily limited to the plasma membrane region of normal cells. In contrast, mutant cells developed a much more intense perinuclear fluorescence throughout the entire 24 h of LDL uptake with little enhancement of cholesterol fluorescence staining in the plasma membranes. Direct mass measurements confirmed that internalized LDL cholesterol more readily replenishes the plasma membrane cholesterol of normal than of mutant fibroblasts. Perinuclear filipin-cholesterol fluorescence of both normal and mutant cells was colocalized with lysosomes by indirect immunocytochemical staining of lysosomal membrane protein. Abnormal sequestration of LDL cholesterol in mutant cells within a metabolically latent pool is supported by the finding that in vitro esterification of cellular cholesterol could be stimulated in mutant but not in normal cell homogenates by extensive disruption of the intracellular membranous structures of cells previously cultured with LDL. Deficient translocation of exogenously derived cholesterol from lysosomes to other intracellular membrane sites may be responsible for the delayed homeostatic responses associated with LDL uptake by mutant Niemann-Pick Type C fibroblasts.     

Synthesis and biochemical properties of a new photoactivatable cholesterol analog 7,7-azocholestanol and its linoleate ester in Chinese hamster ovary cell lines

We report the chemical synthesis of a new photoactivatable cholesterol analog 7,7-azocholestanol (AC) and its linoleate ester (ACL). We also examined the biochemical properties of the sterol and its ester by employing several different mutant Chinese hamster ovary (CHO) cell lines with defined abnormalities in cholesterol metabolism as tools. AC mimics cholesterol in supporting the growth of a mutant cell line (M19) that requires cholesterol for growth. In normal cells, tritiated ACL present in low-density lipoprotein (LDL) was hydrolyzed and reesterified in a manner similar to tritiated cholesteryl linoleate (CL) in LDL. Also, in the mutant cell line (AC29) lacking the enzyme acyl-coenzyme A:cholesterol acyltransferase or in the mutant cell line (CT60) defective in the Niemann-Pick type C1 protein, the hydrolysis of ACL in LDL was normal, but the reesterification of the liberated AC was defective. Therefore, the metabolism of ACL in LDL is very similar to that of CL in LDL. Tritium-labeled AC delivered to intact CHO cells as a cyclodextrin complex was shown to photoaffinity label several discrete polypeptides, including caveolin-1. These results demonstrate AC as an effective reagent for studying cholesterol-protein interactions involved in intracellular cholesterol trafficking.     

Identification of a pharmaceutical compound that partially corrects the Niemann-Pick C phenotype in cultured cells

Niemann-Pick C (NPC) is an autosomal recessive lysosomal lipid storage disease characterized by progressive central nervous system degeneration. In cultured human NPC fibroblasts, LDL-derived cholesterol accumulates in lysosomes and endosomes, LDL-cholesterol transport from endocytic compartments to other cellular compartments is delayed, and LDL does not elicit normal homeostatic responses. Currently, there is no therapy that delays the onset of neurological symptoms or prolongs the life span of NPC children. We have developed and implemented an amphotericin B-mediated cytotoxicity assay to screen for potential therapeutic drugs that induce cholesterol movement in cultured NPC cells. NPC cells are relatively resistant to amphotericin B killing due to intracellular sequestration of cellular cholesterol. The screen was carried out using simian virus 40-transformed ovarian granulosa cells from the npc (nih) mouse model of NPC disease. A library of 44240 compounds was screened and 55 compounds were identified that promote amphotericin B-mediated killing of NPC cells. One compound, NP-27, corrected the NPC phenotype by four different measures of cholesterol homeostasis. In addition to making NPC cells more sensitive to amphotericin B, NP-27 stimulated two separate cholesterol transport pathways and restored LDL stimulation of cholesterol esterification to near normal levels.     

The C-terminus of IcmT is essential for pore formation and for intracellular trafficking of Legionella pneumophila within Acanthamoeba polyphaga

We have shown previously that the five rib (release of intracellular bacteria) mutants of Legionella pneumophila are competent for intracellular replication but defective in pore formation-mediated cytolysis and egress from protozoan and mammalian cells. The rib phenotype results from a point mutation (deletion) DeltaG544 in icmT that is predicted to result in the expression of a protein truncated by 32 amino acids from the C-terminus. In contrast to the rib mutants that are capable of intracellular replication, an icmT null mutant was completely defective in intracellular replication within mammalian and protozoan cells, in addition to its defect in pore formation-mediated cytolysis. The icmT wild-type allele complemented the icmT null mutant for both defects of intracellular replication and pore formation-mediated cytolysis and egress from mammalian cells. In contrast, the icmTDeltaG544 allele complemented the icmT null mutant for intracellular growth, but not for the pore-forming activity. Consistent with their defect in pore formation-mediated cytotoxicity in vitro, both mutants failed to cause pulmonary inflammation in A/J mice. Interestingly, the rib mutant was severely defective in intracellular growth within Acanthamoeba polyphaga. Confocal laser scanning and electron microscopy confirmed that the rib mutant and the icmT null mutant were severely and completely defective, respectively, in intracellular growth in A. polyphaga, and the respective defects correlated with fusion of the bacterial phagosomes to lysosomes. Taken together, the data showed that the C-terminus domain of IcmT is essential for the pore-forming activity and is required for intracellular trafficking and replication within A. polyphaga, but not within mammalian cells.     

Trafficking defects in endogenously synthesized cholesterol in fibroblasts,macrophages, hepatocytes,and glial cells from Niemann-Pick type C1 mice

Niemann-Pick type C1 disease (NPC1) is an inherited neurovisceral lipid storage disorder, hallmarked by the intracellular accumulation of unesterified cholesterol and glycolipids in endocytic organelles. Cells acquire cholesterol through exogenous uptake and endogenous biosynthesis. NPC1 participation in the trafficking of LDL-derived cholesterol has been well studied; however, its role in the trafficking of endogenously synthesized cholesterol (endoCHOL) has received much less attention. Previously, using mutant Chinese hamster ovary cells, we showed that endoCHOL moves from the endoplasmic reticulum (ER) to the plasma membrane (PM) independent of NPC1. After arriving at the PM, it moves between the PM and internal compartments. The movement of endoCHOL from internal membranes back to the PM and the ER for esterification was shown to be defective in NPC1 cells. To test the generality of these findings, we have examined the trafficking of endoCHOL in four different physiologically relevant cell types isolated from wild-type, heterozygous, and homozygous BALB/c NPC1NIH mice. The results show that all NPC1 homozygous cell types (embryonic fibroblasts, peritoneal macrophages, hepatocytes, and cerebellar glial cells) exhibit partial trafficking defects, with macrophages and glial cells most prominently affected. Our findings suggest that endoCHOL may contribute significantly to the overall cholesterol accumulation observed in selective tissues affected by Niemann-Pick type C disease.