Niemann-Pick type II fibroblasts exhibit impaired cholesterol esterification in response to sphingomyelin hydrolysis

Aggregated low density lipoprotein induces tissue factor by inhibiting sphingomyelinase activity in human vascular smooth muscle cells

BACKGROUND

Our previous results demonstrated that aggregated low density lipoprotein (agLDL) induces tissue factor (TF) expression and activation through Rho A translocation in human vascular smooth muscle cells (VSMC). We also previously demonstrated that membrane sphingomyelin (SM) content is higher in agLDL-exposed VSMC than in control cells. The main enzymes regulating cellular SM content are the family of sphingomyelinases (Smases) that hydrolize SM to phosphorylcholine and ceramide (CER).

OBJECTIVES

We wished to investigate whether agLDL has the ability to modulate acidic- (A-) and neutral (N-) Smase activity and whether or not this effect is related to the upregulatory effect of agLDL on Rho A translocation and TF activation in human VSMC.

METHODS AND RESULTS

By measuring generated [(14)C]-phosphorylcholine, we found that agLDL significantly decreased A-Smase and specially N-Smase activity. Pharmacological Smase inhibitors increased Rho A and TF. Specific loss-of-function of A-Smase or N-Smase 1 (N1-Smase) by siRNA treatment (500 nmol L(-1), 12 hours) dramatically increased membrane Rho A protein levels (5- and 3-fold, respectively). Concomitantly, TF protein expression and TF procoagulant activity were also increased. Inhibition of A-Smase or N-Smase activity by agLDL, siRNA-anti A- or N1-Smase or pharmacological treatment significantly increased the SM content of vascular cells. The inhibition of SM synthesis by fumonisin B(1) (FB(1)) prevented the upregulatory effect of agLDL on TF.

CONCLUSIONS

These results demonstrate that inhibition of both A- and N1-Smase might explain the upregulatory effect of agLDL on TF activation, and suggest that this effect is related, at least in part, to membrane SM enrichment.

Ezetimibe inhibits expression of acid sphingomyelinase in liver and intestine

Ezetimibe inhibits cholesterol absorption in the intestine. Sphingomyelin has strong interactions with cholesterol. We investigated the effects of ezetimibe on Sphingomyelinase (SMase) expression in intestine and liver. After feeding rats with ezetimibe (5 mg/kg per day) for 14 days, acid SMase activities in the liver and in the proximal part of small intestine were reduced by 34 and 25%, respectively. Alkaline SMase (alk-SMase) was increased in the proximal part of the small intestine. Administration of lower doses of ezetimibe reduced acid SMase only in the liver by 14% (P < 0.05). In cell culture studies, ezetimibe decreased acid SMase activity in Hep G2 and Caco-2 cells dose-dependently. The reductions were more rapid for Hep G2 cells than for Caco-2 cells. Western blot showed that acid SMase protein was decreased in both Hep G2 and Caco-2 cells by 100 microM ezetimibe. The SM content was increased in Hep G2 cells but not Caco-2 cells, and total cholesterol content was increased in both cell lines 24 h after stimulation with 100 microM ezetimibe. Mevastatin, the inhibitor of cholesterol synthesis, induced a mild increase in acid SMase activity in Hep G2 cells but not Caco-2 cells. Following the reduction of acid SMase, ezetimibe at high dose slightly increased alk-SMase activity. In conclusion, the study demonstrates an inhibitory effect of ezetimibe on acid SMase activity and expression in both liver and intestine.

Intracellular cholesterol transport

Intracellular cholesterol transport is essential for the maintenance of cholesterol homeostasis. Many aspects of cholesterol metabolism are well-known, including its synthesis in the endoplasmic reticulum, its extracellular transport in plasma lipoproteins, its uptake by the low-density lipoprotein receptor, and its regulation of SREBP and LXR transcription factors. These fundamental pathways in cholesterol metabolism all rely on its proper intracellular distribution among subcellular organelles and the plasma membrane. Transport involving the ER and endosomes is essential for cholesterol synthesis, uptake, and esterification, whereas cholesterol catabolism by enzymes in mitochondria and ER generates steroids, bile acids, and oxysterols. Cholesterol is a highly hydrophobic lipid that requires specialized transport in the aqueous cytosol, involving either vesicles or nonvesicular mechanisms. The latter includes hydrophobic cavity transporters such as StAR-related lipid transfer (START) proteins. Molecular understanding of intracellular cholesterol trafficking has lagged somewhat behind other aspects of cholesterol metabolism, but recent advances have defined some transport pathways and candidate proteins. In this review, we discuss cholesterol transport among specific intracellular compartments, emphasizing the relevance of these pathways to cholesterol homeostasis.

Biotinylated theta-toxin derivative as a probe to examine intracellular cholesterol-rich domains in normal and Niemann-Pick type C1 cells

BCtheta is a proteolytically nicked and biotinylated derivative of a cholesterol binding protein perfringolysin O (theta-toxin), and has been used to detect cholesterol-rich domains at the plasma membrane (PM). Here we show that by modifying the cell fixation condition, BCtheta can also be used to detect cholesterol-rich domains intracellularly. When cells were processed for PM cholesterol staining, the difference in BCtheta signals between the CT43 (CT) cell, a mutant Chinese hamster ovary cell line lacking the Niemann-Pick type C1 (NPC1) protein, and its parental cell 25RA (RA) was minimal. However, when cells were fixed with 4% paraformaldehyde, they became permeable to BCtheta. Under this condition, BCtheta mainly stained cholesterol-rich domains inside the cells, with the signal being much stronger in CT cells than in RA cells. The sensitivity of BCtheta staining was superior to that of filipin staining. The staining of cholesterol-rich domain(s) inside RA cells was sensitive to beta-cyclodextrin treatment, while most of the staining inside CT cells was relatively resistant to cyclodextrin treatment. Clear differences in intracellular BCtheta staining were also seen between the normal and mutant NPC1 fibroblasts of human or mouse origin. Thus, BCtheta is a powerful tool for visually monitoring cholesterol-rich domains inside normal and NPC cells.

Fate of endogenously synthesized cholesterol in Niemann-Pick type C1 cells

Mammalian cells obtain cholesterol via two pathways: endogenous synthesis in the endoplasmic reticulum and exogenous sources mainly through the low density lipoprotein (LDL) receptor pathway. We performed pulse-chase experiments to monitor the fate of endogenously synthesized cholesterol and showed that, after reaching the plasma membrane from the endoplasmic reticulum, the newly synthesized cholesterol eventually accumulates in an internal compartment in Niemann-Pick type C1 (NPC1) cells. Thus, the ultimate fate of endogenously synthesized cholesterol in NPC1 cells is the same as LDL-derived cholesterol. However, the time required for endogenous cholesterol to accumulate internally is much slower than LDL-derived cholesterol. Different pathways thus govern the post-plasma membrane trafficking of endogenous cholesterol and LDL-derived cholesterol to the internal compartment. Results using the inhibitor N-butyldeoxynojirimycin, which depletes cellular complex glycosphingolipids, demonstrates that the cholesterol trafficking defect in NPC1 cells is not caused by ganglioside accumulation. The ultimate cause of death in NPC disease is progressive neurological deterioration in the central nervous system, where the major source of cholesterol is derived from endogenous synthesis. Our current study provides a plausible link between defects in intracellular trafficking of endogenous cholesterol and the etiology of Niemann-Pick type C disease.

U18666A inhibits intracellular cholesterol transport and neurotransmitter release in human neuroblastoma cells

To determine if neurochemical function might be impaired in cell models with altered cholesterol balance, we studied the effects of U18666A (3-beta-[(2-diethyl-amino)ethoxy]androst-5-en-17-one) on intracellular cholesterol metabolism in three human neuroblastoma cell lines (SK-N-SH, SK-N-MC, and SH-SY5Y). U18666A (< or =0.2 microg/ml) completely inhibited low density lipoprotein (LDL)-stimulated cholesterol esterification in SK-N-SH cells, while cholesterol esterification stimulated by 25-hydroxycholesterol or bacterial sphingomyelinase was unaffected or partially inhibited, respectively. U18666A also blocked LDL-stimulated downregulation of LDL receptor and caused lysosomal accumulation of cholesterol as measured by filipin staining. U18666A treatment for 18 h resulted in 70% inhibition of K+-evoked norepinephrine release in phorbol ester-differentiated SH-SY5Y cells, while release stimulated by the calcium ionophore A23187 was only slightly affected. These results suggest that U 18666A may preferentially block a voltage-regulated Ca2+ channel involved in norepinephrine release and that alterations in neurotransmitter secretion might be a feature of disorders such as Niemann-Pick Type C, in which intracellular cholesterol transport and distribution are impaired.

Niemann-Pick C disease: cholesterol handling gone awry

Niemann-Pick C disease (NPC) is a debilitating, recessive disorder in humans that causes unrelenting neurological deterioration and is complicated by the presence of lipid-laden foamy cells in the major organs of the body. NPC fibroblasts cultured with an excess of low density lipoprotein (LDL) abnormally sequester cholesterol in their lysosomes. Biochemical analyses of NPC cells suggest an impairment in the intracellular transport of cholesterol to post-lysosomal destinations occurs in NPC. The recent identification of the NPC gene, NPC1, provides a definitive diagnosis of the disease and a means of studying this key component of intracellular cholesterol transport and homeostasis.

Decreased membrane fluidity and unsaturated fatty acids in Niemann-Pick disease type C fibroblasts

Niemann-Pick disease type C (NP-C) is an autosomal recessive disorder characterized by the sequestration and trapping of endocytosed cholesterol in lysosomes. The NPC1 gene on chromosome 18 was recently identified but its physiological function remains unknown. We have studied the lipid compositions of cultured human NP-C fibroblasts and mouse SPM-3T3 cell line derived from the C57BL/KsJ NP-C model mouse, which belongs to the same complementation group. Fibroblasts derived from apparently normal age-matched individuals and a subline of SPM-3T3 cells which restores cholesterol metabolism by transfer of human chromosome 18 were used as controls. Levels of free cholesterol in whole cell homogenates increased about 1.5-fold in human NP-C fibroblasts and mouse SPM-3T3 cells, while in the plasma membrane, cholesterol content did not significantly change in NP-C fibroblasts but rather decreased in SPM-3T3 cells. The total phospholipid content did not significantly change; however, among phospholipid head groups, increases in sphingomyelin and decreases in other classes were observed in human NP-C fibroblasts and mouse SPM-3T3 cells. The ratios of saturated fatty acids to unsaturated fatty acids increased in both human and mouse cells. The increase was also confirmed in the plasma membrane fraction of SPM-3T3 cells. Membrane fluidity was examined using a 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probe. The DPH anisotropy values were markedly increased in NP-C fibroblasts and in SPM-3T3 cells. The results suggest that a NP-C mutation causes complex alterations in cellular lipid contents and biophysical properties of the membrane.

Nova Scotia Niemann-Pick disease (type D): clinical study of 20 cases

Patients with Niemann-Pick type D have been traced to a single Acadian ancestor in Nova Scotia. The objective of this study was to describe the clinical course. A cohort of children with Niemann-Pick type D was identified by chart review. Some children were seen and a telephone interview with the remaining parents was conducted. Twenty children with Niemann-Pick type D were identified. The female to male ratio was 2:1. Five children had severe neonatal jaundice. Early milestones were normal in the majority. Neurologic symptoms generally developed between 5 and 10 years of age with a mean age of 7.2 years at diagnosis. Seizures developed in all between 4.5 and 16 years of age (mean, 10.5 yr), and were followed by significant physical and mental deterioration. The age at death ranged between 11 and 22.5 years (mean, 14.8 yr). In 61%, bronchopneumonia was the cause of death. There is significant variability in the presentation and clinical course of Niemann-Pick type D.

Altered expression of caveolin-1 and increased cholesterol in detergent insoluble membrane fractions from liver in mice with Niemann-Pick disease type C

Niemann-Pick type C (NPC) is an autosomal recessive disease characterized by impaired cholesterol homeostasis due to a defect in the intracellular transport of unesterified cholesterol. While accumulation of lysosomal cholesterol is the most apparent microscopic finding, cholesterol has also been shown to accumulate in the trans-cisternae of the Golgi apparatus. Caveolin-1, a cholesterol-binding protein that cycles between the Golgi apparatus and the plasma membrane, has been hypothesized to participate in cholesterol transport. Using the BALB/c murine model for NPC disease, we found that the expression of caveolin-1 in total liver homogenates from heterozygous and homozygous affected animals was altered. Immunoblot analysis of liver homogenates from heterozygous mice revealed that caveolin-1 is significantly (p < 0.005) elevated, 4.9 fold, compared to normal mice. Total liver homogenates from homozygous affected mice also had a significant (p < 0.05) increase in caveolin-1 expression, 1.6 fold, compared to normal mice. Immunohistochemical staining of liver cross-sections revealed that the increased caveolin-1 was localized to sinusoidal endothelial cells in heterozygous mice. The Triton insoluble floating fraction (TIFF) was isolated using liver from each genotype and analyzed for caveolin-1 expression. Caveolin-1 in the TIFF from heterozygous mice was significantly (p < 0.01) elevated, 1.8 fold, compared to normal and homozygous affected animals; normal and homozygous affected animals, however, were not significantly different from each other. The TIFF isolated from homozygous affected mice revealed a 15 fold increase in unesterified cholesterol compared to the TIFF isolated from heterozygous and normal mice. In summary, these findings demonstrate an altered expression of caveolin-1 in liver from heterozygous and homozygous NPC mice and increased concentration of cholesterol from TIFF in homozygous affected NPC mice. The identification of these alterations in the TIFF suggests involvement of detergent insoluble membrane structures, possibly caveolae and/or detergent insoluble glycosphingolipid-enriched complexes (DIGs), in the cholesterol trafficking defect in this disorder.

Increased expression of caveolin-1 in heterozygous Niemann-Pick type II human fibroblasts

Human Niemann-Pick type II fibroblasts, which encompass the panethnic type C (NPC) and Nova Scotia Acadian type D (NPD) variants, exhibit altered expression of caveolin-1 protein when examined by immunoblotting using an anti-caveolin-1 monoclonal antibody. Unexpectedly, caveolin-1 in heterozygous fibroblasts was significantly elevated as much as 10-fold compared to caveolin-1 in normal and homozygous affected fibroblasts. Homozygous NPC fibroblasts expressed caveolin-1 levels similar to those in normal fibroblasts, while the expression of caveolin-1 in homozygous NPD fibroblasts was slightly elevated. Northern analysis indicates that normal fibroblasts and NPC heterozygous fibroblasts have similar amounts of caveolin-1 mRNA, while NPC homozygous fibroblasts have significantly less caveolin-1 mRNA. In contrast, heterozygous and homozygous NPD fibroblasts exhibit increased levels of caveolin-1 mRNA. These novel findings suggest that caveolin-1 containing subcellular structures are involved in the pathophysiology of Niemann-Pick type II disease. Furthermore, altered caveolin-1 protein expression may serve as a useful marker for the diagnosis of carriers of NPC or NPD.

Acyl-coenzyme A:cholesterol acyltransferase

Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

Selective killing of human monocytes and cytokine release provoked by sphingomyelinase (beta-toxin) of Staphylococcus aureus

The best-known activity of Staphylococcus aureus sphingomyelinase C, alias beta-toxin, is as a hemolysin that provokes hot-cold lysis of erythrocytes which contain substantial amounts of sphingomyelin in the plasma membrane. Sheep erythrocytes are most susceptible, and we found that one hemolytic unit, representing the toxin concentration that elicits 50% hemolysis of 2.5 X 10(8) erythrocytes per ml, corresponds to 0.05 enzyme units or to approximately 0.25 microg of sphingomyelinase per ml. The cytotoxic action of beta-toxin on nucleated cells has not been described in any detail before, and the present investigation was undertaken to fill this information gap. We now identify beta-toxin as a remarkably potent monocytocidal agent. At a concentration of 0.001 U/ml, corresponding to approximately 5 ng/ml, beta-toxin killed over 50% of human monocytes (10(6) cells per ml) within 60 min. By contrast, 1 to 5 microg of beta-toxin per ml had no cytocidal effects on human granulocytes, fibroblasts, lymphocytes, or erythrocytes. A selective monocytocidal action was also observed with sphingomyelinase C from Bacillus cereus and a Streptomyces sp., whereas phospholipase A2 and phospholipase D at 100 U/ml were without effect. Monocytes succumbing to the action of beta-toxin processed and released interleukin-1beta, soluble interleukin-6 receptor, and soluble CD14 into the supernatant. Thus, monocyte killing by beta-toxin is associated with cytokine-related events that are important for the initiation and progression of infectious disease. These findings uncover a potentially important role for sphingomyelinase as a determinant of microbial pathogenicity.

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 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).

Regulation of intracellular cholesterol metabolism is defective in lymphoblasts from Niemann-Pick type C and type D patients

Regulation of intracellular cholesterol metabolism has been studied in Epstein-Barr virus-transformed lymphoblasts from patients with Niemann-Pick type C (NPC) and the Nova Scotia type D (NPD) disease. Addition of LDL to normal lymphoblasts cultured in lipoprotein-deficient medium increased cholesterol esterification 10-fold (to a maximum of 1.0 nmol/h/mg protein at 15 h), while little stimulation was seen in NPC cells. The response by NPD lymphoblasts was intermediate, reaching approximately half of normal values by 14-24 h. Lymphoblasts from both NPC and NPD obligate heterozygotes exhibited 50% of normal LDL-stimulated cholesterol esterification at 6 h, when activity was < 10% of normal values in patient cells. Fluorescence staining with filipin indicated excessive intracellular accumulation of LDL-derived cholesterol in both NPC and NPD lymphoblasts. Downregulation of LDL receptor mRNA levels by LDL, measured by S1 nuclease protection assay, was also impaired in NP lymphoblasts and fibroblasts (NPC > NPD), although a similar rate of receptor protein down-regulation by LDL (t1/2 = 10-15 h) was observed in normal and NP lymphoblasts. In contrast, LDL down-regulation of 3-hydroxy-3-methylglutaryl-CoA reductase mRNA did not appear to be affected in NP cells: LDL produced a 3-fold (lymphoblasts) or > 10-fold (fibroblasts) decrease by 12 h in both normal and affected cells. Thus, NPC and NPD lymphoblasts exhibit distinct defects in cholesterol esterification and storage, similar to those observed in mutant fibroblasts. Other regulatory responses are also impaired in NPC lymphoblasts but appear to be less affected in NPD cells. Lymphoblasts should provide a valuable immortalized cell line model for study of defective regulation of cholesterol esterification and transport in Niemann-Pick type II disease, and may also be suitable for diagnosis and carrier detection.