Control of cholesterol turnover in the mouse

The results in rodent models of atherosclerosis are not interchangeable: the influence of diet and strain

The determinant factors for the development of atherosclerosis in response to dietary cholesterol were examined in two animal models to assess the comparability of results. We studied 128 male Apo E(-/-) and 128LDLr(-/-) mice randomly assigned to baseline (n=8) and 5 groups (n=24 each) that differed only in their dietary fat and cholesterol supplements. At 10, 16, 24 and 32 weeks of age, 8 animals from each group were sequentially sacrificed and the variables analyzed. The lesion sizes changed at different rates but they were predictable and did not differ in complexity. We observed, however, significant differences between strains, particularly in the constitutive expression of liver genes, their metabolic response to dietary cholesterol, their feeding behaviour, their glucose tolerance and the gain in body weight. Both strains presented characteristics that resemble steatohepatitis but manifestations were more severe in LDLr(-/-) mice. The divergent responses indicate that the choice of the diet and the model should be carefully considered in atherosclerosis studies and extrapolations interpreted with caution.

Characterization of liver disease and lipid metabolism in the Niemann-Pick C1 mouse

Niemann-Pick type C1 (NPC1) disease is an autosomal-recessive cholesterol-storage disorder characterized by liver dysfunction, hepatosplenomegaly, and progressive neurodegeneration. The NPC1 gene is expressed in every tissue of the body, with liver expressing the highest amounts of NPC1 mRNA and protein. A number of studies have now indicated that the NPC1 protein regulates the transport of cholesterol from late endosomes/lysosomes to other cellular compartments involved in maintaining intracellular cholesterol homeostasis. The present study characterizes liver disease and lipid metabolism in NPC1 mice at 35 days of age before the development of weight loss and neurological symptoms. At this age, homozygous affected (NPC1(-/-)) mice were characterized with mild hepatomegaly, an elevation of liver enzymes, and an accumulation of liver cholesterol approximately four times that measured in normal (NPC1(+/+)) mice. In contrast, heterozygous (NPC1(+/-)) mice were without hepatomegaly and an elevation of liver enzymes, but the livers had a significant accumulation of triacylglycerol. With respect to apolipoprotein and lipoprotein metabolism, the results indicated only minor alterations in NPC1(-/-) mouse serum. Finally, compared to NPC1(+/+) mouse livers, the amount and processing of SREBP-1 and -2 proteins were significantly increased in NPC1(-/-) mouse livers, suggesting a relative deficiency of cholesterol at the metabolically active pool of cholesterol located at the endoplasmic reticulum. The results from this study further support the hypothesis that an accumulation of lipoprotein-derived cholesterol within late endosomes/lysosomes, in addition to altered intracellular cholesterol homeostasis, has a key role in the biochemical and cellular pathophysiology associated with NPC1 liver disease.

Cholesterol Transport by the Placenta: Placental Liver X Receptor Activity as a Modulator of Fetal Cholesterol Metabolism?

Cholesterol is an important sterol in mammals. Defects in cholesterol synthesis or intracellular routing have devastating consequences already in utero: the Smith-Lemli-Opitz syndrome, desmosterolosis and Niemann-Pick C1 disease provide examples of severe human inherited diseases caused by mutations in cholesterol metabolism genes. On the other hand, elevated plasma cholesterol concentrations are associated with the development of atherosclerosis which represents a major health risk in Western societies. Moreover, several studies indicate that development of atherosclerosis may already start during fetal life. Hence, a carefully balanced regulation of cholesterol metabolism appears of critical importance for both the development of the fetus and health of the adult. In the adult, the liver X receptor is a key regulator of cholesterol metabolism. Its target genes regulate cellular cholesterol efflux and thereby modulate whole-body cholesterol fluxes. LXR and several of its target genes have recently been demonstrated to be expressed in the placenta, which would provide a means to control delivery of maternal cholesterol to the fetus. Here we discuss the potential role of the placenta in the regulation of fetal cholesterol homeostasis and strategies to influence maternal-fetal cholesterol transfer.

Analysis of HSD3B7 knockout mice reveals that a 3alpha-hydroxyl stereochemistry is required for bile acid function

Primary bile acids are synthesized from cholesterol in the liver and thereafter are secreted into the bile and small intestine. Gut flora modify primary bile acids to produce secondary bile acids leading to a chemically diverse bile acid pool that is circulated between the small intestine and liver. A majority of primary and secondary bile acids in higher vertebrates have a 3alpha-hydroxyl group. Here, we characterize a line of knockout mice that cannot epimerize the 3beta-hydroxyl group of cholesterol and as a consequence synthesize a bile acid pool in which 3beta-hydroxylated bile acids predominate. This alteration causes death in 90% of newborn mice and decreases the absorption of dietary cholesterol in surviving adults. Negative feedback regulation of bile acid synthesis mediated by the farnesoid X receptor (FXR) is disrupted in the mutant mice. We conclude that the correct stereochemistry of a single hydroxyl group at carbon 3 in bile acids is required to maintain their physiologic and regulatory functions in mammals.

Generation of ENU-Induced Mouse Mutants with Hypocholesterolemia: Novel Tools for Dissecting Plasma Lipoprotein Homeostasis

Pathologic plasma lipoprotein cholesterol levels play a key role in the development and pathogenesis of human atherosclerotic cardiovascular diseases. Plasma cholesterol homeostasis is regulated by genetic predispositions and environmental factors. Animal models showing aberrant plasma cholesterol levels are used for the identification and analysis of novel causative genes. Here, we searched for inherited hypocholesterolemia phenotypes in randomly mutant mice which may contribute to the detection of disease protective alleles. In the Munich ENU mouse mutagenesis project, clinical chemistry blood analysis was carried out on more than 15,500 G1 offspring and 230 G3 pedigrees of chemically mutagenized inbred C3H mice to detect dominant and recessive mutations leading to a decreased plasma total cholesterol level. We identified 66 animals consistently showing hypocholesterolemia. Transmission of the altered phenotype to the subsequent generations led to the successful establishment of 14 independent hypocholesterolemic lines. Line-specific differences were detected by clinical chemistry analysis of plasma HDL cholesterol, LDL cholesterol and triglycerides. Thus, we successfully established a novel panel of ENU-derived mutant mouse lines for their use in the identification of alleles selectively influencing the plasma cholesterol homeostasis. Such findings may be subsequently used for humans and other species.

Molecular regulation of HDL metabolism and function: implications for novel therapies

HDL metabolism represents a major target for the development of therapies intended to reduce the risk of atherosclerotic cardiovascular disease. HDL metabolism is complex and involves dissociation of HDL apolipoprotein and HDL cholesterol metabolism. Advances in our understanding of the molecular regulation of HDL metabolism, macrophage cholesterol efflux, and HDL function will lead to a variety of novel therapeutics.

Mechanisms for cellular cholesterol transport: defects and human disease

This review summarizes the mechanisms of cellular cholesterol transport and monogenic human diseases caused by defects in intracellular cholesterol processing. In addition, selected mouse models of disturbed cholesterol trafficking are discussed. Current pharmacological strategies to prevent atherosclerosis are largely based on altering cellular cholesterol balance and are introduced in this context. Finally, because of the organizing potential of cholesterol in membranes, disturbances in cellular cholesterol transport have implications for a wide variety of human diseases, of which selected examples are given.

Emerging roles of the intestine in control of cholesterol metabolism

The liver is considered the major “control center” for maintenance of whole body cholesterol homeostasis. This organ is the main site for de novo cholesterol synthesis, clears cholesterol-containing chylomicron remnants and low density lipoprotein particles from plasma and is the major contributor to high density lipoprotein (HDL; good cholesterol) formation. The liver has a central position in the classical definition of the reverse cholesterol transport pathway by taking up periphery-derived cholesterol from lipoprotein particles followed by conversion into bile acids or its direct secretion into bile for eventual removal via the feces. During the past couple of years, however, an additional important role of the intestine in maintenance of cholesterol homeostasis and regulation of plasma cholesterol levels has become apparent. Firstly, molecular mechanisms of cholesterol absorption have been elucidated and novel pharmacological compounds have been identified that interfere with the process and positively impact plasma cholesterol levels. Secondly, it is now evident that the intestine itself contributes to fecal neutral sterol loss as a cholesterol-secreting organ. Finally, very recent work has unequivocally demonstrated that the intestine contributes significantly to plasma HDL cholesterol levels. Thus, the intestine is a potential target for novel anti-atherosclerotic treatment strategies that, in addition to interference with cholesterol absorption, modulate direct cholesterol excretion and plasma HDL cholesterol levels.

Hepatic lipid accumulation in apolipoprotein C-I-deficient mice is potentiated by cholesteryl ester transfer protein

The impact of apolipoprotein C-I (apoC-I) deficiency on hepatic lipid metabolism was addressed in mice in the presence or the absence of cholesteryl ester transfer protein (CETP). In addition to the expected moderate reduction in plasma cholesterol levels, apoCIKO mice showed significant increases in the hepatic content of cholesteryl esters (+58%) and triglycerides (+118%) and in biliary cholesterol concentration (+35%) as compared with wild-type mice. In the presence of CETP, hepatic alterations resulting from apoC-I deficiency were enforced, with up to 58% and 302% increases in hepatic levels of cholesteryl esters and triglycerides in CETPTg/apoCIKO mice versus CETPTg mice, respectively. Biliary levels of cholesterol, phospholipids, and bile acids were increased by 88, 77, and 20%, respectively, whereas total cholesterol, HDL cholesterol, and triglyceride concentrations in plasma were further reduced in CETPTg/apoCIKO mice versus CETPTg mice. Finally, apoC-I deficiency was not associated with altered VLDL production rate. In line with the previously recognized inhibition of lipoprotein clearance by apoC-I, apoC-I deficiency led to decreased plasma lipid concentration, hepatic lipid accumulation, and increased biliary excretion of cholesterol. The effect was even greater when the alternate reverse cholesterol transport pathway via VLDL/LDL was boosted in the presence of CETP.

CETP expression enhances liver HDL-cholesteryl ester uptake but does not alter VLDL and biliary lipid secretion

The aim of this work was to study how CETP expression affects whole body cholesterol homeostasis. Thus, tissue uptake and plasma removal rates of labeled HDL-cholesteryl ester (CE), VLDL secretion rates, and biliary lipid secretion and fecal bile acid content were compared between human CETP transgenic (Tg) and non-transgenic (nTg) mice fed with a standard diet. CETP Tg mice exhibited increased HDL-CE plasma fractional catabolic rate and uptake by the liver, adrenals, adipose tissue and spleen. HDL fractions from both CETP Tg and from nTg mice were removed faster from the plasma of CETP expressing than from nTg mice, suggesting a direct role of CETP in accelerating tissue CE uptake. However, neither hepatic output of VLDL cholesterol and triglycerides nor biliary lipid and fecal bile acid excretion were changed in CETP Tg compared to nTg mice. CETP Tg mice also showed enhanced hepatic cholesterol content. Steady state cholesterol homeostasis was probably preserved through the downregulation of hepatic HMG-CoA reductase and LDL receptor expression. In conclusion, although CETP expression facilitates cholesteryl ester tissue uptake, it does not alter biliary lipid and fecal bile acid excretion, the mandatory final step of the reverse cholesterol transport.

Cholesterol substrate pools and steroid hormone levels are normal in the face of mutational inactivation of NPC1 protein

Mutational inactivation of NPC1 largely blocks the movement of LDL-derived cholesterol from the lysosome to the metabolically active, cytosolic pool of sterol that is the substrate for steroid hormone production. Such a block might, in theory, lead to deficiencies in circulating levels of testosterone, progesterone, and corticosterone. However, there are at least two other sources for cellular cholesterol, de novo synthesis and scavenger receptor class B type I-mediated uptake of HDL cholesteryl ester (CE). In this study, we measured the rates of net cholesterol acquisition by these three pathways in the adrenal, ovary, and testis. In all three organs, the majority (81-98%) of cholesterol acquisition came from the selective uptake of CE from HDL and de novo synthesis. Furthermore, in the npc1(-/-)mouse, the cytosolic storage pool of CE in a tissue such as the adrenal remained constant (approximately 25 mg/g). As a result of these alternative pathways, the plasma concentrations of testosterone (3.5 vs. 2.5 ng/ml), progesterone (8.5 vs. 6.7 ng/ml), and corticosterone (391 vs. 134 ng/ml) were either the same or elevated in the npc1(-/-)mouse, compared with the control animal. Thus, impairment of cholesterol acquisition through the NPC1-dependent, clathrin-coated pit pathway did not limit the availability of cholesterol substrate for steroid hormone synthesis in the steroidogenic cells.

Lanosterol synthase mutations cause cholesterol deficiency-associated cataracts in the Shumiya cataract rat

The Shumiya cataract rat (SCR) is a hereditary cataractous strain. It is thought that the continuous occurrence of poorly differentiated epithelial cells at the bow area of the lens forms the pathophysiological basis for cataract formation in SCRs. In this study, we attempted to identify the genes associated with cataract formation in SCRs by positional cloning. Genetic linkage analysis revealed the presence of a major cataract locus on chromosome 20 as well as a locus on chromosome 15 that partially suppressed cataract onset. Hypomorphic mutations were identified in genes for lanosterol synthase (Lss) on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on chromosome 15, both of which function in the cholesterol biosynthesis pathway. A null mutation for Lss was also identified. Cataract onset was associated with the specific combination of Lss and Fdft1 mutant alleles that decreased cholesterol levels in cataractous lenses to about 57% of normal. Thus, cholesterol insufficiency may underlie the deficient proliferation of lens epithelial cells in SCRs, which results in the loss of homeostatic epithelial cell control of the underlying fiber cells and eventually leads to cataractogenesis. These findings may have some relevance to other types of cataracts, inborn defects of cholesterol synthesis, and the effects of cholesterol-lowering medication.

Reverse cholesterol transport and cholesterol efflux in atherosclerosis

Reverse cholesterol transport (RCT) is a pathway by which accumulated cholesterol is transported from the vessel wall to the liver for excretion, thus preventing atherosclerosis. Major constituents of RCT include acceptors such as high-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), and enzymes such as lecithin:cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), hepatic lipase (HL) and cholesterol ester transfer protein (CETP). A critical part of RCT is cholesterol efflux, in which accumulated cholesterol is removed from macrophages in the subintima of the vessel wall by ATP-binding membrane cassette transporter A1 (ABCA1) or by other mechanisms, including passive diffusion, scavenger receptor B1 (SR-B1), caveolins and sterol 27-hydroxylase, and collected by HDL and apoA-I. Esterified cholesterol in the HDL is then delivered to the liver for excretion. In patients with mutated ABCA1 genes, RCT and cholesterol efflux are impaired and atherosclerosis is increased. In studies with transgenic mice, disruption of ABCA1 genes can induce atherosclerosis. Levels of HDL are inversely correlated with incidences of cardiovascular disease. Supplementation with HDL or apoA-I can reverse atherosclerosis by accelerating RCT and cholesterol efflux. On the other hand, pro-inflammatory factors such as interferon-gamma (IFN-gamma), endotoxin, tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta), can be atherogenic by impairing RCT and cholesterol efflux, according to in vitro studies. RCT and cholesterol efflux play a major role in anti-atherogenesis, and modification of these processes may provide new therapeutic approaches to cardiovascular disease. Further research on new modifying factors for RCT and cholesterol efflux is warranted.

Brain cholesterol synthesis in mice is affected by high dose of simvastatin but not of pravastatin

On a global scale, there is an increasing tendency for a more aggressive treatment of hypercholesterolemia. Minor effects of statins on brain cholesterol metabolism have been reported in some in vivo animal studies, and it seems that this is due to a local effect of the drug. We treated male mice of the inbred strain C57/BL6 with a high daily dose of lipophilic simvastatin (100 mg/kg b.wt.) or hydrophilic pravastatin (200 mg/kg b.wt.) or vehicle (controls) by oral gavage for 3 days. To compare the impact of both statins on brain cholesterol synthesis and degradation, levels of cholesterol, its precursor lathosterol, and its brain metabolite 24(S)-hydroxycholesterol as well as statin concentrations were determined in whole-brain lipid extracts using mass spectrometry. The expression of 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase mRNA and of other target genes were evaluated using real-time reverse transcription-polymerase chain reaction. In addition, analysis of liver and serum samples was performed. Similar levels of simvastatin and pravastatin were detected in whole-brain homogenates. Cholesterol contents in the brain, liver, and serum were not affected by high-dose statin treatment. Whereas brain cholesterol precursor levels were reduced in simvastatin-treated animals only, no effect was observed on the formation of the brain cholesterol metabolite, 24(S)-hydroxycholesterol. Polymerase chain reaction analysis revealed that mRNA expression of HMG-CoA reductase and ATP-binding cassette transporter A1 in the brain was significantly up-regulated in simvastatin-treated animals compared with pravastatin-treated or control animals. We conclude that, under the present experimental conditions, brain cholesterol synthesis is significantly affected by short-term treatment with high doses of lipophilic simvastatin, whereas whole-brain cholesterol turnover is not disturbed.

Hepatic expression of ABC transporters G5 and G8 does not correlate with biliary cholesterol secretion in liver transplant patients

The adenosine triphosphate (ATP)-binding cassette (ABC)-transporters ABCG5 and ABCG8 have been shown to mediate hepatic and intestinal excretion of cholesterol. In various (genetically modified) murine models, a strong relationship was found between hepatic expression of ABCG5/ABCG8 and biliary cholesterol content. Our study aimed to relate levels of hepatic expression of ABCG5 and ABCG8 to biliary excretion of cholesterol in man. From 24 patients who had received a liver transplant, bile samples were collected daily after transplantation over a 2-week period to determine biliary composition. Expression of ABCG5, ABCG8, MDR3, and BSEP was assessed by real-time polymerase chain reaction (PCR) in liver biopsy specimens collected before and after transplantation. Levels of hepatic ABCG5, ABCG8, and MDR3 messenger RNA (mRNA) were strongly correlated. After transplantation, the biliary secretion rate of cholesterol continuously increased, coinciding with gradual increases in bile salt and phospholipid secretion. In contrast, hepatic levels of ABCG5 and ABCG8 mRNA remained unchanged. Surprisingly, no correlation was found between the hepatic expression of ABCG5 and ABCG8 and rates of biliary cholesterol secretion, normalized for biliary phospholipid secretion. As expected, the concentration of biliary phospholipids correlated well with MDR3 expression. In conclusion, the strong relationship between ABCG5 and ABCG8 gene expression is consistent with the coordinate regulation of both genes, and in line with heterodimerization of both proteins into a functional transporter. Hepatic ABCG5/ABCG8 expression, at least during the early phase after transplantation, is not directly related to biliary cholesterol secretion in humans. This finding suggests the existence of alternative pathways for the hepatobiliary transport of cholesterol that are not controlled by ABCG5/ABCG8.

X-linked adrenoleukodystrophy mice demonstrate abnormalities in cholesterol metabolism

The neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD) is caused by ABCD1 mutations and characterized by very long-chain fatty acid (VLCFA) accumulation. Cholesterol-lowering normalized VLCFA in fibroblasts and plasma of X-ALD patients. We show that in cultured cells, cholesterol-loading induces ABCD1. In X-ALD mice, plasma cholesterol is elevated and not further increasable by cholesterol-feeding, whereas hepatic HMG-CoA reductase and Abcd2 are downregulated. Upon cholesterol modulation, brain VLCFA increased in X-ALD mice, but decreased in controls. In murine X-ALD fibroblasts, cholesterol-lowering did not normalize VLCFA. Thus, ALDP-deficiency and VLCFA are linked to cholesterol but species differences complicate evaluating cholesterol-lowering drugs in X-ALD mice.

The gene encoding the human ileal bile acid-binding protein (I-BABP) is regulated by peroxisome proliferator-activated receptors

Peroxisome proliferator-activator receptors (PPAR) are involved in cholesterol homeostasis through the regulation of bile acids synthesis, composition, and reclamation. As ileal bile acid-binding protein (I-BABP) is thought to play a crucial role in the enterohepatic circulation of bile acids, we investigated whether I-BABP gene expression could also be affected by PPAR. Indeed, treatment with the PPARalpha-PPARbeta/delta agonist bezafibrate led to the up-regulation of I-BABP mRNA levels in the human intestine-derived Caco-2 cells. Cotransfections of the reporter-linked human I-BABP promoter (hI-BABP-2769/+44) together with PPAR and RXR expression vectors demonstrated that the fibrate-mediated induction of the I-BABP gene is dependent on PPARalpha or PPARbeta/delta. Using progressive 5' deletions of the hI-BABP promoter and sequence analysis, we identified a putative PPAR-binding site located at the position -198 and -186 upstream of the transcription initiation site. Electrophoretic mobility shift assays showed that the PPAR/RXR heterodimer can specifically bind to this PPRE-like motif. The deletion of the PPRE within the hI-BABP promoter abolished the PPAR-mediated transactivation in transient transfection assays. The regulation of the I-BABP promoter by PPAR appears species-specific, as the mouse I-BABP promoter, which lacks a conserved PPRE, was not responsive to exogenous PPAR expression in the presence of bezafibrate. Our findings show that the I-BABP gene may be a novel target for PPAR in humans and further emphasize the role for PPAR in the control of bile acid homeostasis.

Niemann-Pick C1 expression is not regulated by the amount of cholesterol flowing through cells in the mouse

The Niemann-Pick C1 (NPC1) protein functions to regulate the transport of cholesterol from late endosomes/lysosomes to other cellular compartments after lipoprotein uptake through the coated-pit pathway. The present study examines the relative expression of NPC1 mRNA and NPC1 protein in different tissues of the mouse in relation to the uptake of total cholesterol carried in chylomicron remnants (CMr-TC), low density lipoproteins (LDL-TC), cholesteryl ester carried in high density lipoproteins (HDL-CE), and cholesterol synthesis. Results from this study demonstrate that the highest relative expression of NPC1 is in the liver, which is also the tissue with the highest uptake of CMr-TC, LDL-TC, HDL-CE, and cholesterol synthesis. However, there was no similar relation in the remaining tissues. To examine the relative expression of NPC1 in relation to the amount of cholesterol that flowed through the coated-pit pathway, mice were fed a diet supplemented with increasing amounts of cholesterol or cholestyramine. The results from this study demonstrated that there was no relation between the relative expression of NPC1 and the amount of cholesterol that flowed through the coated-pit pathway. We conclude that the relative expression of NPC1 is not regulated by the flow of cholesterol through cells in the mouse and is therefore constitutive.

Caveolin-1 is not required for murine intestinal cholesterol transport

Caveolin-1 (CAV1) is the structural protein of the filamentous coat that decorates the cytoplasmic surface of each caveola. Cell culture studies have implicated CAV1 in playing an important role in intracellular cholesterol trafficking. In addition, it has been reported that CAV1 forms a detergent-resistant protein complex with Annexin-2 in enterocytes that can be disrupted by the cholesterol absorption inhibitor ezetimibe, suggesting a possible role for CAV1 in cholesterol absorption. In this report, we have evaluated cholesterol homeostasis in Cav1 knock-out mice. Deletion of CAV1 does not result in either a compensatory increase of CAV2 or CAV3 in intestine. In addition, Cav1 knock-out mice display normal mRNA and protein levels of Annexin-2 or the putative cholesterol transport protein Niemann-Pick C1-like 1 (NPC1L1) in proximal intestinal mucosa. Fractional cholesterol absorption and fecal neutral sterol excretion are statistically similar in Cav1 knock-out mice and their wild-type littermates. Moreover, oral administration of ezetimibe is equally effective in decreasing cholesterol absorption in Cav1 null mice and wild-type controls. The mRNA expression levels of genes sensitive to intracellular cholesterol concentration (ATP-binding cassette transporters ABCA1 and ABCG5, hydroxymethylglutaryl-CoA synthase and the LDL receptor) are similarly altered in the proximal intestinal mucosa of Cav1 null and wild-type mice following ezetimibe treatment. These results demonstrate that CAV1 is not required for cholesterol absorption or ezetimibe sensitivity in the mouse.

Membrane cholesterol content accounts for developmental differences in surface B cell receptor compartmentalization and signaling

Recent studies argue for an important role for cholesterol in maintaining plasma membrane heterogeneity and influencing a variety of cellular processes, including signaling, adhesion, and permeability. Here, we document that tolerance-sensitive transitional immature B cells maintain significantly lower membrane unesterified cholesterol levels than mature-stage splenic B cells. In addition, the relatively low level of cholesterol in transitional immature B cells impairs compartmentalization of their B cell receptor (BCR) into cholesterol-enriched domains following BCR aggregation and reduces their ability to sustain certain aspects of BCR signaling as compared with mature B cells. These studies establish an unexpected difference in the lipid composition of peripheral transitional immature and mature B cells and point to a determining role for development-associated differences in cholesterol content for the differential responses of these B cells to BCR engagement.

Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer's disease

Extracellular amyloid plaques, intracellular neurofibrillary tangles, and loss of basal forebrain cholinergic neurons in the brains of Alzheimer's disease (AD) patients may be the end result of abnormalities in lipid metabolism and peroxidation that may be caused, or exacerbated, by beta-amyloid peptide (Abeta). Apolipoprotein E (apoE) is a major apolipoprotein in the brain, mediating the transport and clearance of lipids and Abeta. ApoE-dependent dendritic and synaptic regeneration may be less efficient with apoE4, and this may result in, or unmask, age-related neurodegenerative changes. The increased risk of AD associated with apoE4 may be modulated by diet, vascular risk factors, and genetic polymorphisms that affect the function of other transporter proteins and enzymes involved in brain lipid homeostasis. Diet and apoE lipoproteins influence membrane lipid raft composition and the properties of enzymes, transporter proteins, and receptors mediating Abeta production and degradation, tau phosphorylation, glutamate and glucose uptake, and neuronal signal transduction. The level and isoform of apoE may influence whether Abeta is likely to be metabolized or deposited. This review examines the current evidence for diet, lipid homeostasis, and apoE in the pathogenesis of AD. Effects on the cholinergic system and response to cholinesterase inhibitors by APOE allele carrier status are discussed briefly.

Cholesterol homeostasis in neurons and glial cells

Cholesterol is highly enriched in the brain compared to other tissues. Essentially all cholesterol in the brain is synthesized endogenously since plasma lipoproteins are unable to cross the blood-brain barrier. Cholesterol is transported within the central nervous system in the form of apolipoprotein E-containing lipoprotein particles that are secreted mainly by glial cells. Cholesterol is excreted from the brain in the form of 24-hydroxycholesterol. Apolipoprotein E and cholesterol have been implicated in the formation of amyloid plaques in Alzheimer's disease. In addition, the progressive neurodegenerative disorder Niemann-Pick C disease is characterized by defects in intracellular trafficking of cholesterol.

Molecular, Anatomical, and Biochemical Events Associated with Neurodegeneration in Mice with Niemann-Pick Type C Disease

In Niemann-Pick type C (NPC) disease, cholesterol associated with either apoE or apoB100 is taken up by cells in all tissues, including the central nervous system, through clathrin-coated pits and becomes trapped in late endosomes and lysosomes. This study defines the functional, biochemical, and molecular events that ensue as nerve cell death occurs. In mice homozygous for a mutation in NPC1, neuromuscular dysfunction begins at 5 weeks and death occurs at 13 weeks of age. Cholesterol accumulates in every tissue in the body. Purkinje cell loss in the cerebellum begins at 3 to 4 weeks of age and is nearly complete by 11 weeks. This neurodegeneration in the cerebellum is associated with increases in the levels of mRNA for caspase 1, caspase 3, NPC2, LipA, apoE, apoD, glial fibrillary acidic protein, and tumor necrosis factor-alpha, but not for most target genes of the LXR nuclear receptors. The level for apoER2 is significantly reduced. These studies show there is a compensatory increase in NPC2 and LipA in an attempt to overcome the physiological defect caused by the mutation. Nevertheless, neurodegeneration proceeds utilizing apoptosis with activation of glial cells, increased apoE and apoD synthesis, and increased cholesterol turnover across the CNS.

Structural analyses reveal phosphatidyl inositols as ligands for the NR5 orphan receptors SF-1 and LRH-1

Vertebrate members of the nuclear receptor NR5A subfamily, which includes steroidogenic factor 1 (SF-1) and liver receptor homolog 1 (LRH-1), regulate crucial aspects of development, endocrine homeostasis, and metabolism. Mouse LRH-1 is believed to be a ligand-independent transcription factor with a large and empty hydrophobic pocket. Here we present structural and biochemical data for three other NR5A members-mouse and human SF-1 and human LRH-1-which reveal that these receptors bind phosphatidyl inositol second messengers and that ligand binding is required for maximal activity. Evolutionary analysis of structure-function relationships across the SF-1/LRH-1 subfamily indicates that ligand binding is the ancestral state of NR5A receptors and was uniquely diminished or altered in the rodent LRH-1 lineage. We propose that phospholipids regulate gene expression by directly binding to NR5A nuclear receptors.

Thematic review series: brain Lipids. Cholesterol metabolism in the central nervous system during early development and in the mature animal

Unesterified cholesterol is an essential structural component of the plasma membrane of every cell. During evolution, this membrane came to play an additional, highly specialized role in the central nervous system (CNS) as the major architectural component of compact myelin. As a consequence, in the human the mean concentration of unesterified cholesterol in the CNS is higher than in any other tissue (approximately 23 mg/g). Furthermore, even though the CNS accounts for only 2.1% of body weight, it contains 23% of the sterol present in the whole body pool. In all animals, most growth and differentiation of the CNS occurs in the first few weeks or years after birth, and the cholesterol required for this growth apparently comes exclusively from de novo synthesis. Currently, there is no evidence for the net transfer of sterol from the blood into the brain or spinal cord. In adults, the rate of synthesis exceeds the need for new structural sterol, so that net movement of cholesterol out of the CNS must take place. At least two pathways are used for this excretory process, one of which involves the formation of 24(S)-hydroxycholesterol. Whether or not changes in the plasma cholesterol concentration alter sterol metabolism in the CNS or whether such changes affect cognitive function in the brain or the incidence of dementia remain uncertain at this time.

Increased fecal neutral sterol loss upon liver X receptor activation is independent of biliary sterol secretion in mice

BACKGROUND & AIMS

Reverse cholesterol transport (RCT) is defined as high-density lipoprotein (HDL)-mediated flux of excess cholesterol from peripheral cells to liver, followed by secretion into bile and disposal via the feces. Various steps of this pathway are controlled by the liver X receptor (LXR). We addressed the role of the intestine in LXR-dependent stimulation of fecal cholesterol excretion.

METHODS

To segregate biliary from intestine-derived cholesterol, wild-type and Mdr2 P-glycoprotein-deficient mice ( Mdr2 -/- ), which are unable to secrete cholesterol into bile, were treated with the LXR agonist GW3965.

RESULTS

Treatment with GW3965 increased biliary cholesterol secretion by 74% in wild-type mice but had no effect in Mdr2 -/- mice. LXR activation increased fecal neutral sterol excretion 2.1-fold in wild-type mice. Surprisingly, an identical increase was observed in Mdr2 -/- mice. Fractional cholesterol absorption was reduced on LXR activation in both strains but was more pronounced in Mdr2 -/- mice, coinciding with reduced Npc111 expression. Intestinal gene expression of ATP-binding cassette transporters (Abc) Abca1 , Abcg1 , Abcg5 , and Abcg8 was strongly induced upon LXR activation in both strains, whereas expression of HMGCoA reductase , controlling cholesterol synthesis, remained unaffected. Additionally, LXR activation stimulated the excretion of plasma-derived [ 3 H]cholesterol into the fecal neutral sterol fraction in Mdr2 -/- mice.

CONCLUSIONS

Increased fecal cholesterol loss upon LXR activation is independent of biliary cholesterol secretion in mice. An important part of excess cholesterol is excreted directly via the intestine, supporting the existence of an alternative, quantitatively important route for cholesterol disposal.

Reduced cholesterol absorption upon PPARdelta activation coincides with decreased intestinal expression of NPC1L1

Peroxisome proliferator-activated receptors (PPARs) control the transcription of genes involved in lipid metabolism. Activation of PPARdelta may have antiatherogenic effects through the increase of plasma HDL, theoretically promoting reverse cholesterol transport from peripheral tissues toward the liver for removal via bile and feces. Effects of PPARdelta activation by GW610742 were evaluated in wild-type and Abca1-deficient (Abca1(-/-)) mice that lack HDL. Treatment with GW610742 resulted in an approximately 50% increase of plasma HDL-cholesterol in wild-type mice, whereas plasma cholesterol levels remained extremely low in Abca1(-/-) mice. Yet, biliary cholesterol secretion rates were similar in untreated wild-type and Abca1(-/-) mice and unaltered upon treatment. Unexpectedly, PPARdelta activation led to enhanced fecal neutral sterol loss in both groups without any changes in intestinal Abca1, Abcg5, Abcg8, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase expression. Moreover, GW610742 treatment resulted in a 43% reduction of fractional cholesterol absorption in wild-type mice, coinciding with a significantly reduced expression of the cholesterol absorption protein Niemann-Pick C1-like 1 (Npc1l1) in the intestine. PPARdelta activation is associated with increased plasma HDL and reduced intestinal cholesterol absorption efficiency that may be related to decreased intestinal Npc1l1 expression. Thus, PPARdelta is a promising target for drugs aimed to treat or prevent atherosclerosis.

Expression and regulation of the sterol half-transporter genes ABCG5 and ABCG8 in rats

The ABCG5 and ABCG8 genes encode half-transporter proteins that heterodimerize to form a transporter of plant sterols and cholesterol. The purpose of this study was to examine the expression and regulation of ABCG5 and ABCG8 at the mRNA level in Sprague-Dawley rats. Both ABCG5 and ABCG8 mRNA were expressed primarily in rat small intestine and liver, and gender-specific differences in expression were observed. The effects of treatment with a battery of microsomal enzyme inducers on ABCG5 and ABCG8 mRNA were examined; most treatments had no effect, but of three PXR ligands, PCN was an effective inducer, spironolactone was repressive, and dexamethasone was ineffective. The effects of a 1% cholesterol diet on the regulation of rat ABCG5 and ABCG8 were also examined, and compared with those in C57BL/6 mice. Cholesterol caused a suppression of ABCG5 and ABCG8 mRNA in rat liver, but the same treatment increased the expression of these genes in mouse liver. ABCG5 and ABCG8 mRNA was also induced by cholesterol in rat ileum, but not mouse ileum. These results suggest variation between rats and mice in regulatory mechanisms controlling ABCG5 and ABCG8 expression, and may explain some differences in lipid metabolism observed between these two species.

Hypercholesterolemia in ENU-induced mouse mutants

Hypercholesterolemia is caused by multiple environmental factors and genetic predispositions, and plays an important role in the development and pathogenesis of various human diseases. In this study, we aimed to establish randomly mutant mouse lines showing hypercholesterolemia for their further use in the detection of novel causative alleles. In the Munich ENU Mouse Mutagenesis Project, clinical chemistry blood analysis was performed on more than 15,000 G1 mice and 230 G3 pedigrees of chemically mutagenized mice to detect dominant and recessive mutations leading to an increased plasma total cholesterol level. Using inbred C3HeB/FeJ mice we identified more than 100 animals consistently showing hypercholesterolemia. Transmission of the altered phenotype to the subsequent generations led to the production of nine hypercholesterolemic lines. A single line showed further obvious deviations in the analysis of additional clinical chemistry blood parameters. Thus, the lines produced will contribute to the search for alleles that selectively cause primary hypercholesterolemia.

High-level expression of ABCG5 and ABCG8 attenuates diet-induced hypercholesterolemia and atherosclerosis in Ldlr−− mice

Transgenic mice expressing human ABCG5 (G5) and ABCG8 (G8) have decreased fractional absorption and increased biliary secretion of cholesterol, but their plasma cholesterol levels are unchanged (males) or modestly reduced (females). To determine whether increased expression of G5 and G8 can ameliorate hypercholesterolemia in mice lacking LDL receptors (LDLRs), we examined the effects of G5G8 transgene expression on cholesterol metabolism and atherosclerosis in Ldlr-/- mice. In chow-fed Ldlr-/- mice, the G5G8 transgene reduced fractional absorption of dietary cholesterol by 50% and increased biliary cholesterol levels by 60% but did not affect plasma cholesterol levels. On a Western diet (21% fat, 0.2% cholesterol), G5G8Tg; Ldlr-/- mice had a 30% reduction in the level of hepatic cholesterol and 45% lower plasma cholesterol levels than the Ldlr-/- mice. After 6 months on the Western diet, the atherosclerotic lesion area in the aortic root and arch was approximately 70% lower in the G5G8Tg;Ldlr-/- than in the Ldlr-/- mice and was correlated with the plasma cholesterol levels. These results demonstrate that increased expression of G5 and G8 attenuates diet-induced hypercholesterolemia in Ldlr-/- mice, resulting in a significant reduction in plasma levels of cholesterol and aortic atherosclerotic lesion area.

The role of scavenger receptor class B type I (SR-BI) in lipid trafficking. defining the rules for lipid traders

The scavenger receptor class B type I (SR-BI) is a 509-amino acid, 82 kDa glycoprotein, with two cytoplasmic C- and N-terminal domains separated by a large extracellular domain. The aim of this review is to define the role of SR-BI as a lipoprotein receptor responsible for selective uptake of cholesteryl esters (CE) from high density lipoprotein (HDL) and low density lipoprotein (LDL) and free cholesterol (FC) efflux to lipoprotein acceptors. These activities depend on lipoprotein binding to its extracellular domain and subsequent lipid exchange at the plasma membrane. CE selective uptake supplies cholesterol to liver and steroidogenic tissues, for biliary cholesterol secretion and steroid hormone synthesis. Genetically modified mice have confirmed SR-BI's major role in tissue cholesterol uptake and in reverse cholesterol transport, i.e. cholesterol turnover. Accordingly, cellular cholesterol level, estrogens and trophic hormones regulate SR-BI expression by both transcriptional and post-transcriptional mechanisms. Importantly, mouse SR-BI overexpression has both corrective and preventive effects on atherosclerosis. Human SR-BI has very similar tissue distribution, binding properties and lipid transfer activities compared to rodent SR-BI. However, human plasma has most of its cholesterol in LDL. Thus, there is considerable interest to develop anti-atherogenic strategies involving human SR-BI-mediated increases in reverse cholesterol transport through HDL and/or LDL.

Low-Density Lipoprotein (LDL) Receptor/Transferrin Fusion Protein:In VivoProduction and Functional Evaluation as a Potential Therapeutic Tool for Lowering Plasma LDL Cholesterol

A soluble form of human low-density lipoprotein receptor (LDL-R) fused in frame with rabbit transferrin (LDL-Rs(hu)/Tf(rab)) is assessed in vivo as a therapeutic tool for lowering plasma LDL cholesterol. The cDNA encoding LDL-Rs(hu)/Tf(rab) is expressed in mice, using a hydrodynamics-based gene transfer procedure. The transgene is transcribed in the liver of transduced animals and the corresponding protein is secreted into the bloodstream. Circulating LDL-Rs(hu)/Tf(rab) binds LDL specifically, thus indicating that it is correctly processed through the cellular compartments in vivo. More importantly, the expression of LDL-Rs(hu)/Tf(rab) allows the removal of injected human (125)I-labeled LDL ((123)I-LDL) from the bloodstream of transduced CD1 mice, which show faster LDL plasma clearance, anticipating by approximately 90 min the same clearance value observed in control animals. A similar effect is observed in transduced LDL-R(-/-) mice, in which the clearance of injected human LDL depends solely on the presence of circulating LDL-Rs(hu) /Tf(rab). In these animals the extent of plasma LDL clearance is directly related to the concentration of LDL-Rs(hu)/Tf(rab) in the blood. Finally, LDL-Rs(hu)/Tf(rab) does not alter the pattern of LDL organ distribution: in transduced animals, as well as in control animals, liver and bladder are the predominantly labeled organs after (123)I-LDL injection. However, LDL-Rs(hu)/Tf(rab) has a quantitative effect on LDL tissue deposition: in treated animals LDL-Rs(hu)/Tf(rab) determines an increase in radioactivity in the liver at early times after (123)I-LDL injection and a progressive labeling of the bladder, starting 20 min after injection.

Lipoprotein cholesteryl ester production, transfer, and output in vivo in humans

Our aim was to identify and quantify the major in vivo pathways of lipoprotein cholesteryl ester transport in humans. Normal (n = 7), bile fistula (n = 5), and familial hypercholesterolemia (FH; n = 1) subjects were studied. Each received isotopic free cholesterol in HDL, LDL, or particulate form, along with another isotope of free or esterified cholesterol or mevalonic acid. VLDL, intermediate density lipoprotein (IDL), LDL, HDL, blood cells, and bile were collected for up to 6 days for analysis of radioactivity and mass of free and esterified cholesterol. These raw data were subjected to compartmental analysis using the SAAM program. Results in all groups corroborated net transport of free cholesterol to the liver from HDL, shown previously in fistula subjects. New findings revealed that 70% of ester was produced from free cholesterol in HDL and 30% from free cholesterol in LDL, IDL, and VLDL. No evidence was found for tissue-produced ester in plasma. There was net transfer of cholesteryl ester to VLDL and IDL from HDL and considerable exchange between LDL and HDL. Irreversible ester output was from VLDL, IDL, and LDL, but very little was from HDL, suggesting that selective and holoparticle uptakes of HDL ester are minor pathways in humans. It follows that 1) they contribute little to reverse transport, 2) very high HDL would not result from defects thereof, and 3) the clinical benefit of high HDL is likely explained by other mechanisms. Reverse transport in the subjects with bile fistula and FH was facilitated by ester output to the liver from VLDL plus IDL.

Hepatic ABCG5 and ABCG8 Overexpression Increases Hepatobiliary Sterol Transport but Does Not Alter Aortic Atherosclerosis in Transgenic Mice

The individual roles of hepatic versus intestinal ABCG5 and ABCG8 in sterol transport have not yet been investigated. To determine the specific contribution of liver ABCG5/G8 to sterol transport and atherosclerosis, we generated transgenic mice that overexpress human ABCG5 and ABCG8 in the liver but not intestine (liver G5/G8-Tg) in three different genetic backgrounds: C57Bl/6, apoE-KO, and low density lipoprotein receptor (LDLr)-KO. Hepatic overexpression of ABCG5/G8 enhanced hepatobiliary secretion of cholesterol and plant sterols by 1.5-2-fold, increased the amount of intestinal cholesterol available for absorption and fecal excretion by up to 27%, and decreased the accumulation of plant sterols in plasma by approximately 25%. However, it did not alter fractional intestinal cholesterol absorption, fecal neutral sterol excretion, hepatic cholesterol concentrations, or hepatic cholesterol synthesis. Consequently, overexpression of ABCG5/G8 in only the liver had no effect on the plasma lipid profile, including cholesterol, HDL-C, and non-HDL-C, or on the development of proximal aortic atherosclerosis in C57Bl/6, apoE-KO, or LDLr-KO mice. Thus, liver ABCG5/G8 facilitate the secretion of liver sterols into bile and serve as an alternative mechanism, independent of intestinal ABCG5/G8, to protect against the accumulation of dietary plant sterols in plasma. However, in the absence of changes in fractional intestinal cholesterol absorption, increased secretion of sterols into bile induced by hepatic overexpression of ABCG5/G8 was not sufficient to alter hepatic cholesterol balance, enhance cholesterol removal from the body or to alter atherogenic risk in liver G5/G8-Tg mice. These findings demonstrate that overexpression of ABCG5/G8 in the liver profoundly alters hepatic but not intestinal sterol transport, identifying distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism.

Cholesteryl ester transfer protein modulates the effect of liver X receptor agonists on cholesterol transport and excretion in the mouse

Human plasma, unlike mouse plasma, contains the cholesteryl ester transfer protein (CETP) that may influence the reverse cholesterol transport. Liver X receptor (LXR), an oxysterol-activated nuclear receptor induces CETP transcription via a direct repeat 4 element in the CETP gene promoter. The aim of the study was to assess in vivo the impact of LXR activation on CETP expression and its consequences on plasma lipid metabolism and hepatic and bile lipid content. Wild-type and humanized mice expressing CETP were treated for five days with T0901317 LXR agonist. This treatment produced marked rises in both hepatic CETP mRNA and plasma CETP activity levels. Interestingly, the LXR agonist-mediated, 2-fold rise in both total and HDL cholesterol levels in treated wild-type mice was not observed in CETPTg mice, and the accumulation of cholesterol in the liver of CETPTg mice was reversed by LXR agonist treatment. Moreover, LXR activation induced a 2-fold increase in hepatic LDL-receptor expression in wild-type and CETPTg mice, and it produced a significantly greater rise in biliary cholesterol concentration in CETPTg mice as compared with wild-type mice. In conclusion, induction of CETP constitutes a major determinant of the effect of LXR agonists on cholesterol transport and excretion.

Sexually dimorphic metabolism of branched-chain lipids in C57BL/6J mice

Despite the importance of branched chain lipid oxidation in detoxification, almost nothing is known regarding factors regulating peroxisomal uptake, targeting, and metabolism. One peroxisomal protein, sterol carrier protein-x (SCP-x), is thought to catalyze a key thiolytic step in branched chain lipid oxidation. When mice with substantially lower hepatic levels of SCP-x were tested for susceptibility to dietary stress with phytol (a phytanic acid precursor and peroxisome proliferator), livers of phytol-fed female but not male mice i). accumulated phytol metabolites (phytanic acid, pristanic acid, and Delta-2,3-pristanic acid); ii). exhibited decreased fat tissue mass and increased liver mass/body mass; iii). displayed signs of histopathological lesions in the liver; and iv). demonstrated significant alterations in hepatic lipid distributions. Moreover, both male and female mice exhibited phytol-induced peroxisomal proliferation, as demonstrated by liver morphology and upregulation of the peroxisomal protein catalase. In addition, levels of liver fatty acid binding protein, along with SCP-2 and SCP-x, increased, suggesting upregulation mediated by phytanic acid, a known ligand agonist of the peroxisomal proliferator-activated receptor alpha. In summary, the present work establishes a role for SCP-x in branched chain lipid catabolism and demonstrates a sexual dimorphic response to phytol, a precursor of phytanic acid, in lipid parameters and hepatotoxicity.

Genetic Approaches to Studying Mouse Models of Human Seizure Disorders

In conclusion, we have discussed a reverse genetics approach to studying seizure disorders in mice (Fig. 1), employing a targeted mutagenesis method to exploit the genetic defects identified in human epilepsy families. After detailed characterization of the nature of the human mutation and the mouse counterpart gene, a targeting vector containing the human disease allele is created. The endogenous mouse gene is replaced by the human disease allele through homologous recombination in ES cells, leading to the generation of chimeric animals. Mice carrying one copy or both copies of the human mutation can be bred to study the phenotypic effect of heterozygous and homozygous mutations. At this stage, one may want to split the newly created mice into two groups. One group will go through seizure phenotyping tests, while the other group will be used to generate disease allele-carrying mice on a different genetic background. Phenotypic characterization of mice on different inbred strains includes behavioral monitoring and EEG analysis looking for the occurrence of spontaneous seizures, as well as routine cage examination looking for handling-provoked seizure and ECT- and PTZ- induced seizure paradigms looking for sensitivity to these stimuli. A complete evaluation of the seizure phenotype at the whole-animal level establishes the relevance of the mouse model to the human condition. Further investigation including imaging, electrophysiology and AED response in these mouse models will shed light on the mechanistic basis of the convulsive disorder. Current epilepsy research in mouse genetics offers promise for understanding the molecular mechanisms that underlie epileptogenesis in humans. A large-scale forward genetic effort to create novel mouse mutants with seizure phenotypes by in vivo chemical mutagenesis with ethyl-nitroso urea (ENU) is underway at the Jackson Laboratory (http://www.jax.org/nmf/). Genetic mapping and isolation of the affected genes in these seizure-prone models will provide additional molecular pathways involved in seizures. The mutant mice generated through both forward and reverse genetic approaches will be a valuable resource for the biomedical community to study epilepsy at the molecular level and to characterize the pathological consequences of seizures in the whole organism.

Ontogenesis and regulation of cholesterol metabolism in the central nervous system of the mouse

These studies characterized the ontogenesis and regulation of cholesterol turnover in the central nervous system (CNS) of mice. During the first 3 weeks after birth, the CNS grew rapidly and equaled 5% of body weight. The cholesterol pool in this tissue expanded at a rate of 0.26 mg/day and the CNS synthesized sterol at a rate of 0.28 mg/day. In mature mice between 13 and 26 weeks of age, there was a marked decrease in these parameters including a reduction in the relative size of the CNS to 1.7% of body weight, a decrease in the rate of sterol accretion to 0.012 mg/day, and a reduction in the rate of cholesterol synthesis to 0.035 mg/day. Deletion of the NPC1 and CYP46A1 proteins markedly altered cholesterol metabolism in the CNS. However, changes in the plasma cholesterol concentration or loss of function of ATP-binding cassette AI transporter (ABCA1), scavenger receptor class B, type I (SR-BI), low-density lipoprotein receptor (LDLR), APOE or APOAI had no effect on sterol turnover in the brain. Thus, during early development, cholesterol comes entirely from local synthesis. In the adult, however, synthesis exceeds the need for structural cholesterol so that there is a constant excretion of sterol from the CNS into the plasma at a rate of about 0.023 mg/day.

Quantitation of two pathways for cholesterol excretion from the brain in normal mice and mice with neurodegeneration

Although the pool of cholesterol in the adult central nervous system (CNS) is large and of constant size, little is known of the process(es) involved in regulation of sterol turnover in this pool. In 7-week-old mice, net excretion of cholesterol from the brain equaled 1.4 mg/day/kg body weight, and from the whole animal was 179 mg/day/kg. Deletion of cholesterol 24-hydroxylase, an enzyme highly expressed in the CNS, did not alter brain growth or myelination, but reduced sterol excretion from the CNS 64% to 0.5 mg/day/kg. In mice with a mutation in the Niemann-Pick C gene that had ongoing neurodegeneration, sterol excretion from the CNS was increased to 2.3 mg/day/kg. Deletion of cholesterol 24-hydroxylase activity in these animals reduced net excretion only 22% to 1.8 mg/day/kg. Thus, at least two different pathways promote net sterol excretion from the CNS. One uses cholesterol 24-hydroxylase and may reflect sterol turnover in large neurons in the brain. The other probably involves the movement of cholesterol or one of its metabolites across the blood-brain barrier and may more closely mirror sterol turnover in pools such as glial cell membranes and myelin.

Liver X receptor signaling pathways in cardiovascular disease

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are members of the nuclear receptor family of proteins that are critical for the control of lipid homeostasis in vertebrates. The endogenous activators of these receptors are oxysterols and intermediates in the cholesterol biosynthetic pathway. LXRs serve as cholesterol sensors that regulate the expression of multiple genes involved in the efflux, transport, and excretion of cholesterol. Recent studies have outlined the importance of LXR signaling pathways in the development of metabolic disorders such as hyperlipidemia and atherosclerosis. Synthetic LXR agonists inhibit the development of atherosclerosis in murine models, an effect that is likely to result from the modulation of both metabolic and inflammatory gene expression. These observations identify the LXR pathway as a potential target for therapeutic intervention in human cardiovascular disease.

Lanosterol metabolism and sterol regulatory element binding protein (SREBP) expression in male germ cell maturation

Expression of genes involved in cholesterol biosynthesis in male germ cells is insensitive to the negative cholesterol feedback regulation, in contrast to cholesterol level-sensitive/sterol regulatory element binding protein (SREBP)-dependent gene regulation in somatic cells. The role of sterol regulatory element binding proteins in spermatogenic cells was an enigma until recently, when a soluble, 55kDa cholesterol-insensitive form of SREBP2 (SREBP2gc) was discovered [Mol. Cell. Endocrinol. 22 (2002) 8478], being translated from a germ cell-specific SREBP2 mRNA. Our RT-PCR results also show that SREBP2 as well as SREBP1c mRNAs are detectable in prepubertal and postpubertal male germ cells while SREBP1a is not detected. Surprisingly, three SREBP2 immunoreactive proteins (72, 63 and 55kDa), that are not present in mouse liver nuclei, reside in testis nuclei of prepubertal and adult mice. The 55kDa protein is likely SREBP2gc, the other two isoforms are novel. HPLC measurements in liver and testes of fasted prepubertal and postpubertal mice showed no significant difference in cholesterol level. However, FF-MAS and lanosterol/testis-meiosis activating sterol (T-MAS) intermediates that are detectable mainly in testes, increase in fasted postpubertal mice which coincides well with the elevated level of 68kDa SREBP2. Similar to SREBP2gc, the two novel SREBP2 immunoreactive proteins seem to be insensitive to the level of cholesterol.

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.

Pharmacological interference with intestinal bile acid transport reduces plasma cholesterol in LDL receptor/apoE deficiency

Reduction of plasma cholesterol by statins is fundamental to prevent coronary heart disease. Such therapy is often sub-optimal, however, particularly in patients with reduced LDL receptors (familial hypercholesterolemia), and novel or adjuvant therapies are therefore warranted. Cholesterol elimination is profoundly influenced by the rate of its conversion to bile acids (BA), regulated by the enzyme Cyp7a1. Induced fecal loss of BA by resin treatment reduces plasma cholesterol, presumably through induction of hepatic LDL receptors (LDLR). We here describe the effect of PR835, a drug belonging to a new class of lipid-lowering agents that inhibit the Slc10a2 protein, the intestinal transporter responsible for active uptake of BA. Treatment reduced plasma cholesterol by 40% in mice devoid of both the LDLR and its ligand, apoE, while triglycerides and HDL cholesterol were unchanged. Cyp7a1 enzyme activity and mRNA were induced several-fold, and hepatic HMG CoA reductase mRNA increased, mirroring an induced synthesis of BA and cholesterol. The addition of a statin potentiated the effect, leading to reductions of plasma total and LDL cholesterol by 64% and 70%, respectively. These effects could not be attributed to induction of other known hepatic lipoprotein receptors and indicate the presence of new points of targeting in lipid-lowering therapy.

Outsourcing in the brain: do neurons depend on cholesterol delivery by astrocytes?

Brain function depends on the cooperation between highly specialized cells. Neurons generate electrical signals and glial cells provide structural and metabolic support. Here, I propose a new kind of job-sharing between neurons and astrocytes. Recent studies on primary cultures of highly purified neurons from the rodent central nervous system (CNS) suggest that, during development, neurons reduce or even abandon cholesterol synthesis to save energy and import cholesterol from astrocytes via lipoproteins. The cholesterol shuttle may be restricted to compartments distant from the soma including synapses and may be regulated by electrical activity. Testing these hypotheses will help to improve our still insufficient understanding of brain cholesterol metabolism and its role in neurodegeneration.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Inhibition of cholesterol absorption by SCH 58053 in the mouse is not mediated via changes in the expression of mRNA for ABCA1, ABCG5, or ABCG8 in the enterocyte

Intestinal cholesterol absorption is a major determinant of plasma low density lipoprotein-cholesterol (LDL-C) concentrations. Ezetimibe (SCH 58235) and its analogs SCH 48461 and SCH 58053 are novel potent inhibitors of cholesterol absorption whose mechanism of action is unknown. These studies investigated the effect of SCH 58053 on cholesterol metabolism in female 129/Sv mice. In mice fed a low cholesterol rodent diet containing SCH 58053, cholesterol absorption was reduced by 46% and fecal neutral sterol excretion was increased 67%, but biliary lipid composition and bile acid synthesis, pool size, and pool composition were unchanged. When the dietary cholesterol content was increased either 10- or 50-fold, those animals given SCH 58053 manifested lower hepatic and biliary cholesterol concentrations than did their untreated controls. Cholesterol feeding increased the relative mRNA level for adenosine triphosphate-binding cassette transporter A1 (ABCA1), ABC transporter G5 (ABCG5), and ABC transporter G8 (ABCG8) in the jejunum, and of ABCG5 and ABCG8 in the liver, but the magnitude of this increase was generally less if the mice were given SCH 58053. We conclude that the inhibition of cholesterol absorption effected by this new class of agents is not mediated via changes in either the size or composition of the intestinal bile acid pool, or the level of mRNA expression of proteins that facilitate cholesterol efflux from the enterocyte, but rather may involve disruption of the uptake of luminal sterol across the microvillus membrane.