Increased hepatobiliary and fecal cholesterol excretion upon activation of the liver X receptor is independent of ABCA1

The ATP-binding cassette transporter ABCA1 is essential for high density lipoprotein (HDL) formation and considered rate-controlling for reverse cholesterol transport. Expression of the Abca1 gene is under control of the liver X receptor (LXR). We have evaluated effects of LXR activation by the synthetic agonist T0901317 on hepatic and intestinal cholesterol metabolism in C57BL/6J and DBA/1 wild-type mice and in ABCA1-deficient DBA/1 mice. In wild-type mice, T0901317 increased expression of Abca1 in liver and intestine, which was associated with an approximately 60% rise in HDL. Biliary cholesterol excretion rose 2.7-fold upon treatment, and fecal neutral sterol output was increased by 150-300%. Plasma cholesterol levels also increased in treated Abca1(-/-) mice (+120%), but exclusively in very low density lipoprotein-sized fractions. Despite the absence of HDL, hepatobiliary cholesterol output was stimulated upon LXR activation in Abca1(-/-) mice, leading to a 250% increase in the biliary cholesterol/phospholipid ratio. Most importantly, fecal neutral sterol loss was induced to a similar extent (+300%) by the LXR agonist in DBA/1 wild-type and Abca1(-/-) mice. Expression of Abcg5 and Abcg8, recently implicated in biliary excretion of cholesterol and its intestinal absorption, was induced in T0901317-treated mice. Thus, activation of LXR in mice leads to enhanced hepatobiliary cholesterol secretion and fecal neutral sterol loss independent of (ABCA1-mediated) elevation of HDL and the presence of ABCA1 in liver and intestine.     

A potent synthetic LXR agonist is more effective than cholesterol loading at inducing ABCA1 mRNA and stimulating cholesterol efflux.

The LXR nuclear receptors are intracellular sensors of cholesterol excess and are activated by various oxysterols. LXRs have been shown to regulate multiple genes of lipid metabolism, including ABCA1 (formerly known as ABC1). ABCA1 is a lipid pump that effluxes cholesterol and phospholipid out of cells. ABCA1 deficiency causes extremely low high density lipoprotein (HDL) levels, demonstrating the importance of ABCA1 in the formation of HDL. The present work shows that the acetyl-podocarpic dimer (APD) is a potent, selective agonist for both LXRalpha (NR1H3) and LXRbeta (NR1H2). In transient transactivation assays, APD was approximately 1000-fold more potent, and yielded approximately 6-fold greater maximal stimulation, than the widely used LXR agonist 22-(R)-hydroxycholesterol. APD induced ABCA1 mRNA levels, and increased efflux of both cholesterol and phospholipid, from multiple cell types. Gas chromatography-mass spectrometry measurements demonstrated that APD stimulated efflux of endogenous cholesterol, eliminating any possible artifacts of cholesterol labeling. For both mRNA induction and stimulation of cholesterol efflux, APD was found to be more effective than was cholesterol loading. Taken together, these data show that APD is a more effective LXR agonist than endogenous oxysterols. LXR agonists may therefore be useful for the prevention and treatment of atherosclerosis, especially in the context of low HDL levels.     

Correction of apolipoprotein A-I-mediated lipid efflux and high density lipoprotein particle formation in human Niemann-Pick type C disease fibroblasts

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.     

Activation of PPARdelta alters lipid metabolism in db/db mice

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARalpha and PPARgamma, relatively little is known about the most widely expressed PPAR subtype, PPARdelta. Here we show that treatment of insulin resistant db/db mice with the PPARdelta agonist L-165041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L-165041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARdelta ligand, but was increased by a PPARgamma agonist. These data suggest both that PPARdelta is involved in the regulation of cholesterol metabolism in db/db mice and that PPARdelta ligands could potentially have therapeutic value.     

Enlargement of high density lipoprotein in mice via liver X receptor activation requires apolipoprotein E and is abolished by cholesteryl ester transfer protein expression

The factors involved in the generation of larger high density lipoprotein (HDL) particles, HDL1 and HDLc, are still not well understood. Administration of a specific synthetic liver X receptor (LXR) agonist, T0901317, in mice resulted in an increase of not only HDL cholesterol but also HDL particle size (Cao, G., Beyer, T. P., Yang, X. P., Schmidt, R. J., Zhang, Y., Bensch, W. R., Kauffman, R. F., Gao, H., Ryan, T. P., Liang, Y., Eacho, P. I., and Jiang, X. C. (2002) J. Biol. Chem. 277, 39561-39565). We have investigated the roles that apoE and CETP may play in this process. We treated apoE-deficient, cholesterol ester transport protein (CETP) transgenic, and wild type mice with various doses of the LXR agonist and monitored their HDL levels. Fast protein liquid chromatography and apolipoprotein analysis revealed that in apoE knockout mouse plasma, there was neither induction of larger HDL formation nor increase of HDL cholesterol, suggesting that apoE is essential for the LXR agonist effects on HDL metabolism. In CETP transgenic mice, CETP expression completely abolished LXR agonist-mediated HDL enlargement and greatly attenuated HDL cholesterol levels. Analysis of HDL particles by electron microscope and nondenaturing gel electrophoresis revealed similar findings. In apoE-deficient mice, LXR agonist also produced a significant increase in very low density lipoprotein/low density lipoprotein cholesterol and apolipoprotein B content. Our studies provide direct evidence that apoE and CETP are intimately involved in the accumulation of the enlarged HDL (HDL1 or HDLc) particles in mice.     

Cellular cholesterol efflux is modulated by phospholipid-derived signaling molecules in familial HDL deficiency/Tangier disease fibroblasts

Familial HDL deficiency (FHD) is the heterozygous form of Tangier disease (TD). Mutations of the ABCA1 gene cause FHD and TD. FHD/TD cells are unable to normally efflux cholesterol onto nascent HDL particles, which are rapidly catabolized. TD fibroblasts have an abnormal pattern of PLC and PLD activation following cell stimulation with HDL(3) or apolipoprotein A-I (apoA-I). We examined cellular cholesterol efflux in FHD and TD fibroblasts by phospholipid-derived-molecules, compared with that of normal cells. We used the PKC agonist 1,2-dioctanoylglycerol (DOG) and phorbol myristate acetate (PMA) to activate PKC, calphostin C, and GO 6976, as inhibitors of PKC; phosphatidic acid (PA), which is the product of PLD, and lysophosphatidic acid (LPA), phosphatidylcholine, sphingomyelin, and beta-cyclodextrin to investigate their potential effect in modulating cellular cholesterol efflux in [(3)H]cholesterol-labeled and cholesterol-loaded fibroblasts. Phosphatidylcholine, sphingomyelin, and beta-cyclodextrin promoted cholesterol efflux in an identical fashion in control, FHD, or TD fibroblasts. In a dose-dependent fashion, DOG (0-200 microM) increased apoA-I-mediated cellular cholesterol efflux by 40% in controls, 71% in FHD, and 242% in TD cells. PMA similarly increased cholesterol efflux to a maximum of 256% in controls, 182% in FHD, and 191% in TD cells. This effect was inhibited by calphostin C. PA (100 microM) also increased cholesterol efflux by 25% in control, 44% in FHD, and 100% in TD cells. Conversely, LPA reduced cholesterol efflux in a dose-dependent fashion in control and FHD cells (-50%, 200 microM) but not in TD cells, where efflux was increased by 140%. Propranolol (100 microM) significantly increased cholesterol efflux at 24 h in all three cell lines. n-Butanol partially decreased the DOG-mediated increase in cholesterol efflux. The inhibitory effect of calphostin C on DOG-stimulated cholesterol efflux could be partially overcome by propranolol, suggesting that PA is a downstream mediator of PKC-stimulated cholesterol efflux.We conclude that PLC and PLD activities are required for apoA-I-mediated cellular cholesterol efflux, and modulating cellular PA concentration can correct, at least partially, the cholesterol efflux defect in FHD and TD.     

Synthetic LXR agonist inhibits the development of atherosclerosis in New Zealand White rabbits

The nuclear receptors Liver X receptors, LXRα and LXRβ, regulate cholesterol and triglyceride metabolism. We and others have previously reported that synthetic LXR agonists reduced atherosclerosis in models of mouse with no detectable plasma cholesteryl ester transfer protein (CETP) activity, which plays an important role in reverse cholesterol transport. In the present study, we investigated the effect of LXR activation in rabbits to elucidate the influence of CETP activity. First, we cloned rabbit LXRs cDNA. The data indicated that rabbit LXRα was mostly highly expressed in the liver, whereas LXRβ expression was ubiquitous. Next, we investigated the effect of LXR agonist on lipid levels. Treatment with LXR agonist T0901317 increased plasma CETP activity and consequently elevated LDL, but no change in HDL. High cholesterol (HC) diet-feeding, which is thought to provide oxysterols as the natural agonists, could also increase expression of CETP and other LXR target genes. Finally, we tested T0901317 in the atherosclerosis intervention study. Chronic administration of T0901317 significantly reduced atherosclerosis in HC diet-fed rabbits despite less favorable lipid profiles, i.e. increases of plasma triglycerides and no change of HDL. T0901317 induced ATP-binding cassette transporters ABCA1 and ABCG1 and suppressed inflammatory genes expression in the aorta, suggesting that direct actions of LXR agonist on vascular gene expression are likely to contribute to the antiatherogenic effect. The present work strongly supports the idea that LXR agonists could be beneficial as therapeutic agents for treatment of atherosclerosis.     

In vivo modulation of HDL phospholipid has opposing effects on SR-BI- and ABCA1-mediated cholesterol efflux

The effects of in vivo modulation of HDL phospholipid (PL) on scavenger receptor class BI (SR-BI)- and ATP binding cassette transporter 1 (ABCA1)-mediated efflux were examined by overexpressing either endothelial lipase (EL) or phosphatidylserine phospholipase (PS-PLA1) in human apolipoprotein A-I (apoA-I) transgenic mice. Overexpression of EL led to large reductions in the serum PL/apoA-I ratio (-60%), total cholesterol (TC; -89%), and HDL cholesterol (-91%). Relative to the serum before overexpression of EL, the efflux potential of the serum via SR-BI decreased by 90% and ABCA1-mediated efflux increased by 63%. In contrast to overexpression of EL, overexpression of PS-PLA1 led to increases in the PL/apoA-I ratio (88%), TC (78%), HDL cholesterol (57%), and HDL size. The efflux potential of the serum increased by 60% via SR-BI and decreased by 57% via ABCA1. There were significant positive correlations between SR-BI-mediated efflux and a number of serum parameters, including PL/apoA-I ratio, PL, TC, free cholesterol (FC), and HDL cholesterol. In striking contrast, the same correlations were seen with ABCA1-mediated efflux, but the relationships were inverse. In summary, in vivo modulation of HDL PL content affects ABCA1- and SR-BI-mediated efflux in a reciprocal manner. These findings indicate that the type of lipase acting on HDL in vivo will determine which FC efflux pathway the HDL serves. Additionally, the extent of lipolysis will determine the efficiency of FC removal via this pathway.     

Activation of the constitutive androstane receptor decreases HDL in wild-type and human apoA-I transgenic mice

The nuclear hormone receptor constitutive androstane receptor (CAR, NR1I3) regulates detoxification of xenobiotics and endogenous molecules, and has been shown to be involved in the metabolism of hepatic bile acids and cholesterol. The goal of this study was to address potential effects of CAR on the metabolism of HDL particles, key components in the reverse transport of cholesterol to the liver. Wild-type (WT) mice, transgenic mice expressing human apolipoprotein A-I (HuAITg), and CAR-deficient (CAR(-/-)) mice were treated with the specific CAR agonist 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). CAR activation decreased HDL cholesterol and plasma apolipoprotein A-I (apoA-I) levels in both WT and HuAITg mice, but not CAR(-/-) mice. Both mouse apoA-I and human apoA-I were decreased by more than 40% after TCPOBOP treatment, and kinetic studies revealed that the production rate of HDL is reduced in TCPOBOP-treated WT mice. In transient transfections, TCPOBOP-activated CAR decreased the activity of the human apoA-I promoter. Although loss of CAR function did not alter HDL levels in normal chow-fed mice, HDL cholesterol, apoA-I concentration, and apoA-I mRNA levels were increased in CAR(-/-) mice relative to WT mice when both were fed a high-fat diet. We conclude that CAR activation in mice induces a pronounced decrease in circulating levels of plasma HDL, at least in part through downregulation of apoA-I gene expression.     

Influence of calcic and magnesic sulphurous thermal water on the metabolism of lipoproteins in the rat

We have studied in rats fed hypercholesterolemic diet the action of calcic and magnesic sulphurous water from Capvern on the modification of the lipoproteins metabolism caused by hypercholesterolemia. The rats subjected to a hypercholesterolemic diet with thermal water of Capvern was found to have a plasma level of cholesterol significantly less increased (P less than 0.01) compared to those subjected to the same diet with ordinary drinking water (25%). We demonstrated after 105 days of experimentation on tested rats that thermal water may affect the cholesterol catabolism by increased level of cholesterol HDL (52%) and stabilizing level of cholesterol LDL comparatively to the controls. These data suggest that the thermal water from Capvern enhanced the transformation of cholesterol to biliary acids and their biliary secretion. A possible relationship between the influence of the thermal water and the metabolism of lipoproteins would be explained by a possible increase of hepatic receptors which identify apolipoproteins B (LDL) and E (HDLc) on cholesterol fed rats, suggesting a great synthesis of nascent apolipoproteins HDL which are antiatherogenic.     

Behaviour of phospholipase modified-HDL towards cultured hepatocytes. II. Increased cell cholesterol storage and bile acid synthesis

Human total HDL (hydrated density 1.070-1.210), HDL2 (1.070-1.125), HDL3 (1.125-1.210) or HDL separated by heparin affinity chromatography were treated with or without purified phospholipase A2 from Crotalus adamanteus. Control and treated HDL were reisolated and were then incubated with cultured hepatocytes. 1. Mass measurements evidenced a time-dependent cholesterol enrichment in hepatocytes cultured in the absence of lipoproteins. Addition of HDL2 still enhanced by 25% the cell cholesterol content and down-regulated endogenous sterol synthesis in similar proportions. Conversely, HDL3 slightly decreased the amount of free cholesterol in hepatocytes (-12%). 2. Incubations with phospholipase A2-treated HDL resulted in a 35%-50% increase of both the cellular cholesterol esterification and the cholesterylester accumulation, when compared to cells cultured in the presence of control-HDL. This effect was observed with HDL2, HDL3 and combining the data with all subfractions. 3. Cultured hepatocytes secreted cholic and beta-muricholic acids as major bile acids and HDL2 showed a tendency to stimulate their secretion. Phospholipase treatment of HDL again induced an increased production by hepatocytes of those two bile acids. Thus, whereas HDL2 and HDL3 display different behaviours with respect to cell cholesterol content, neosynthesis and bile acid secretion, their modifications by phospholipases always orientate the cell sterol metabolism in the same direction: increased cholesterylester accumulation and bile acid production.     

Lysosomal acid lipase deficiency impairs regulation of ABCA1 gene and formation of high density lipoproteins in cholesteryl ester storage disease

ATP-binding cassette transporter A1 (ABCA1) mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, and its expression is regulated primarily by oxysterol-dependent activation of liver X receptors. We previously reported that ABCA1 expression and HDL formation are impaired in the lysosomal cholesterol storage disorder Niemann-Pick disease type C1 and that plasma HDL-C is low in the majority of Niemann-Pick disease type C patients. Here, we show that ABCA1 regulation and activity are also impaired in cholesteryl ester storage disease (CESD), caused by mutations in the LIPA gene that result in less than 5% of normal lysosomal acid lipase (LAL) activity. Fibroblasts from patients with CESD showed impaired up-regulation of ABCA1 in response to low density lipoprotein (LDL) loading, reduced phospholipid and cholesterol efflux to apolipoprotein A-I, and reduced α-HDL particle formation. Treatment of normal fibroblasts with chloroquine to inhibit LAL activity reduced ABCA1 expression and activity, similar to that of CESD cells. Liver X receptor agonist treatment of CESD cells corrected ABCA1 expression but failed to correct LDL cholesteryl ester hydrolysis and cholesterol efflux to apoA-I. LDL-induced production of 27-hydroxycholesterol was reduced in CESD compared with normal fibroblasts. Treatment with conditioned medium containing LAL from normal fibroblasts or with recombinant human LAL rescued ABCA1 expression, apoA-I-mediated cholesterol efflux, HDL particle formation, and production of 27-hydroxycholesterol by CESD cells. These results provide further evidence that the rate of release of cholesterol from late endosomes/lysosomes is a critical regulator of ABCA1 expression and activity, and an explanation for the hypoalphalipoproteinemia seen in CESD patients.     

Abnormal high density lipoproteins from patients with liver disease regulate cholesterol metabolism in cultured human skin fibroblasts

Apolipoprotein B (apoB) of plasma low density lipoproteins (LDL) binds to high affinity receptors on many cell types. A minor subclass of high density lipoproteins (HDL), termed HDL1, which contains apoE but lacks apoB, binds to the same receptor. Bound lipoproteins are engulfed, degraded, and regulate intracellular cholesterol metabolism and receptor activity. The HDL of many patients with liver disease is rich in apoE. We tested the hypothesis that such patient HDL would reduce LDL binding and would themselves regulate cellular cholesterol metabolism. Normal HDL had little effect on binding, uptake, and degradation of 125I-labeled LDL by cultured human skin fibroblasts. Patient HDL (d 1.063-1.21 g/ml) inhibited these processes, and in 15 of the 25 samples studied there was more than 50% inhibition at 125I-labeled LDL and HDL protein concentrations of 10 micrograms/ml and 25 micrograms/ml, respectively. There was a significant negative correlation between the percentage of 125I-labeled LDL bound and the apoE content of the competing HDL (r = -0.54, P less than 0.01). Patient 125I-labeled HDL was also taken up and degraded by the fibroblasts, apparently through the LDL-receptor pathway, stimulated cellular cholesterol esterification, increased cell cholesteryl ester content, and suppressed cholesterol synthesis and receptor activity. We conclude that LDL catabolism by the receptor-mediated pathway may be impaired in liver disease and that patient HDL may deliver cholesterol to cells.     

The effects of dietary fish oil on hepatic high density and low density lipoprotein receptor activities in the rat

Rats were fed either a standard ration diet or that diet supplemented with 8% by wt of a marine fish oil or safflower oil. After 10 days, plasma triacylglycerols, total cholesterol, high density lipoprotein (HDL) cholesterol, hepatic cholesterol and fatty acid synthesis and hepatic low density lipoprotein (LDL) receptor activity were significantly depressed while HDL receptor activity was significantly increased in rats fed fish oil. Fish oil-induced effects on cholesterol metabolism in the rat therefore include reciprocal changes in the activities of hepatic LDL and HDL receptors.     

Studies on LXR- and FXR-mediated effects on cholesterol homeostasis in normal and cholic acid-depleted mice

As previously reported by us, mice with targeted disruption of the CYP8B1 gene (CYP8B1-/-) fail to produce cholic acid (CA), upregulate their bile acid synthesis, reduce the absorption of dietary cholesterol and, after cholesterol feeding, accumulate less liver cholesterol than wild-type (CYP8B1+/+) mice. In the present study, cholesterol-enriched diet (0.5%) or administration of a synthetic liver X receptor (LXR) agonist strongly upregulated CYP7A1 expression in CYP8B1-/- mice, compared to CYP8B1+/+ mice. Cholesterol-fed CYP8B1-/- mice also showed a significant rise in HDL cholesterol and increased levels of liver ABCA1 mRNA. A combined CA (0.25%)/cholesterol (0.5%) diet enhanced absorption of intestinal cholesterol in both groups of mice, increased their liver cholesterol content, and reduced their expression of CYP7A1 mRNA. The ABCG5/G8 liver mRNA was increased in both groups of mice, but cholesterol crystals were only observed in bile from the CYP8B1+/+ mice. The results demonstrate the cholesterol-sparing effects of CA: enhanced absorption and reduced conversion into bile acids. Farnesoid X receptor (FXR)-mediated suppression of CYP7A1 in mice seems to be a predominant mechanism for regulation of bile acid synthesis under normal conditions and, as confirmed, able to override LXR-mediated mechanisms. Interaction between FXR- and LXR-mediated stimuli might also regulate expression of liver ABCG5/G8.     

The RICO rat (genetically hypercholesteremic): a good model for testing a food substance or a drug specific for a key enzyme of cholesterol metabolism]

The genetically hypercholesterolemic RICO rat: a good model for testing a food substance or a drug specific for a key enzyme involved in cholesterol metabolism? The genetically hypercholesterolemic RICO rat, whose cholesterolemia is situated between 1.3 and 1.5 mg x mL(-1), possibly reaching 2 mg x mL(-1), after the addition of cholesterol to its food, possesses a different lipoprotein spectrum than man, because approximatively 70% of the plasma cholesterol is carried by HDL (28% of which are carried by the light HDL1 subfraction, rich in apolipoproteinE (apoE). The effects of certain substances in food (carbohydrates, cholesterol, allyldisulfide, etc.) or drugs (ethinylestradiol, streptozotocin, statins, inhibitors of ACAT, etc.) on the cholesterolemia of the rat were studied, in relation to certain important parameters of cholesterol metabolism (LDLr, VLDL liver secretion, activities of lipolytic enzymes: LPL, HL, etc.). The increase in a number of LDL receptors (LDLr) in the RICO rat, induced by ethinylestradiol, streptozotocin, etc., provokes an important decrease in the apoE-rich HDL concentration, filtered out by its receptors. This decrease is observed in man for LDL. Simvastatin, which stimulates LDLr in man and not in rat, lowers the level of LDL in man and has no effect on the cholesterolemia of the RICO rat. In rat and man, the concentration of plasma cholesterol is inversely proportional to the rate of cholesterol synthesis in the organism and to its plasma turnover rate. The concentration of cholesterol in the plasma carried by the HDL1 of the rat, is however, proportional to hepatic cholesterogenesis. This fraction is positively correlated to the activity of hepatic lipase (HL) and negatively to the activity of lipoprotein lipase (LPL), released by heparin. These data demonstrate the importance of the liver and lipolytic enzymes in the intraplasmatic hydrolysis of HDL3 (precursors of HDL1), murine particles that can be considered similar to human LDL.     

ABCA1-dependent mobilization of lysosomal cholesterol requires functional Niemann-Pick C2 but not Niemann-Pick C1 protein

Niemann-Pick disease type C (NPC) is caused by mutations leading to loss of function of NPC1 or NPC2 proteins, resulting in accumulation of unesterified cholesterol in late endosomes and lysosomes. We previously reported that expression of the ATP-binding cassette transporter A1 (ABCA1) is impaired in human NPC1(-/-) fibroblasts, resulting in reduced HDL particle formation and providing a mechanism for the reduced plasma HDL cholesterol seen in the majority of NPC1 patients. We also found that treatment of NPC1(-/-) fibroblasts with an agonist of liver X-receptor corrects ABCA1 expression and HDL formation and reduces lysosomal cholesterol accumulation. We have confirmed that ABCA1 expression is also reduced in NPC2(-/-) cells, and found that α-HDL particle formation is impaired in these cells. To determine whether selective up-regulation of ABCA1 can correct lysosomal cholesterol accumulation in NPC disease cells and HDL particle formation, we produced and infected NPC1(-/-) and NPC2(-/-) fibroblasts with an adenovirus expressing full-length ABCA1 and enhanced green fluorescent protein (AdABCA1-EGFP). ABCA1-EGFP expression in NPC1(-/-) fibroblasts resulted in normalization of cholesterol efflux to apolipoprotein A-I (apoA-I) and α-HDL particle formation, plus a marked reduction in filipin staining of unesterified cholesterol in late endosomes/lysosomes. In contrast, AdABCA1-EGFP treatment of NPC2(-/-) fibroblasts to normalize ABCA1 expression had no effect on cholesterol efflux to apoA-I or accumulation of excess cholesterol in lysosomes, and only partially corrected α-HDL formation by these cells. These results suggest that correction of ABCA1 expression can bypass the mutation of NPC1 but not NPC2 to mobilize excess cholesterol from late endosomes and lysosomes in NPC disease cells. Expression of ABCA1-EGFP in NPC1(-/-) cells increased cholesterol available for esterification and reduced levels of HMG-CoA reductase protein, effects that were abrogated by co-incubation with apoA-I. A model can be generated in which ABCA1 is able to mobilize cholesterol, to join the intracellular regulatory pool or to be effluxed for HDL particle formation, either directly or indirectly from the lysosomal membrane, but not from the lysosomal lumen. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).     

Use of low-density and high-density lipoproteins in undifferentiated porcine granulosa cells

Granulosa cells aspirated from medium-sized porcine ovarian follicles (3-5 mm) in short-term incubation responded to the addition of both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) with increased accumulation of progesterone. HDL was more potent than LDL in enhancing progesterone secretion. When granulosa cells were cultured under serum-free conditions for 72 h, HDL but not LDL exhibited a dose-dependent enhancement of progesterone secretion. Addition of insulin to the cells greatly potentiated the stimulatory effect of LDL on progesterone accumulation, while the response to HDL was only slightly increased. Granulosa cells in culture degraded LDL but not HDL. Addition of insulin enhanced LDL degradation. Exposure of cells in culture to chloroquine, an inhibitor of lysosomal function, completely blocked LDL degradation and LDL-induced stimulation of steroidogenesis. The stimulatory effect of HDL was not affected by chloroquine. We interpret these findings to indicate that granulosa cells derive cholesterol from LDL by means of lysosomal degradation, which is not required for use of cholesterol from HDL. Monensin, a carboxylic ionophore that interrupts recycling of LDL receptors, prevented LDL-enhanced progesterone accumulation but not HDL-induced stimulation. This provides evidence that HDL-induced stimulation of steroidogenesis does not involve LDL receptors. We conclude that HDL present in follicular fluid is capable of providing cholesterol to granulosa cells for steroidogenesis. The stimulatory effect of HDL is not due to the presence of apoprotein E, an HDL component that binds to the LDL receptor. A unique HDL pathway that does not involve LDL receptors and lysosomal degradation may operate in porcine granulosa cells.