Lipoprotein Particles in Cerebrospinal Fluid.

The brain is the most lipid-rich organ in the body, and the intricate interplay between lipid metabolism and pathologies associated with neurodegenerative disorders is being increasingly recognized. The brain is bathed in cerebrospinal fluid (CSF), which, like plasma, contains lipid-protein complexes called lipoproteins that are responsible for extracellular lipid transport. Multiple CSF lipoprotein populations exist, some of which are produced de novo in the central nervous system and others that appear to be generated from protein constituents that are produced in the periphery. These CSF lipoproteins are thought to play key roles in maintaining lipid homeostasis in the central nervous system, while little else is known due to their limited accessibility and their low abundance in CSF. Recent work has provided new insights into the compositional complexity of CSF lipoprotein families and their metabolism in cerebral circulation. The purpose of this review is to summarize our current state of knowledge on the composition, origin, and metabolism of CSF lipoproteins.

Clinical diversity and molecular mechanism of VPS35L-associated Ritscher-Schinzel syndrome.

PURPOSE: The Retriever subunit VPS35L is the third responsible gene for Ritscher-Schinzel syndrome (RSS) after WASHC5 and CCDC22. To date, only one pair of siblings have been reported and their condition was significantly more severe than typical RSS. This study aimed to understand the clinical spectrum and underlying molecular mechanism in VPS35L-associated RSS. METHODS: We report three new patients with biallelic VPS35L variants. Biochemical and cellular analyses were performed to elucidate disease aetiology. RESULTS: In addition to typical features of RSS, we confirmed hypercholesterolaemia, hypogammaglobulinaemia and intestinal lymphangiectasia as novel complications of VPS35L-associated RSS. The latter two complications as well as proteinuria have not been reported in patients with CCDC22 and WASHC5 variants. One patient showed a severe phenotype and the other two were milder. Cells established from patients with the milder phenotypes showed relatively higher VPS35L protein expression. Cellular analysis found VPS35L ablation decreased the cell surface level of lipoprotein receptor-related protein 1 and low-density lipoprotein receptor, resulting in reduced low-density lipoprotein cellular uptake. CONCLUSION: VPS35L-associated RSS is a distinct clinical entity with diverse phenotype and severity, with a possible molecular mechanism of hypercholesterolaemia. These findings provide new insight into the essential and distinctive role of Retriever in human development.

High-density lipoproteins mediate small RNA intercellular communication between dendritic cells and macrophages.

HDL are dynamic transporters of diverse molecular cargo and play critical roles in lipid metabolism and inflammation. We have previously reported that HDL transport both host and nonhost small RNAs (sRNA) based on quantitative PCR and sRNA sequencing approaches; however, these methods require RNA isolation steps which have potential biases and may not isolate certain forms of RNA molecules from samples. HDL have also been reported to accept functional sRNAs from donor macrophages and deliver them to recipient endothelial cells; however, using PCR to trace HDL-sRNA intercellular communication has major limitations. The present study aims to overcome these technical barriers and further understand the pathways involved in HDL-mediated bidirectional flux of sRNAs between immune cells. To overcome these technical limitations, SYTO RNASelect, a lipid-penetrating RNA dye, was used to quantify a) overall HDL-sRNA content, b) bidirectional flux of sRNAs between HDL and immune cells, c) HDL-mediated intercellular communication between immune cells, and d) HDL-mediated RNA export changes in disease. Live cell imaging and loss-of-function assays indicate that the endo-lysosomal system plays a critical role in macrophage storage and export of HDL-sRNAs. These results identify HDL as a substantive mediator of intercellular communication between immune cells and demonstrate the importance of endocytosis for recipient cells of HDL-sRNAs. Utilizing a lipid-penetrating RNA-specific fluorescence dye, we were able to both quantify the absolute concentration of sRNAs transported by HDL and characterize HDL-mediated intercellular RNA transport between immune cells.

LDL uptake-dependent phosphatidylethanolamine translocation to the cell surface promotes fusion of osteoclast-like cells.

Osteoporosis is associated with vessel diseases attributed to hyperlipidemia, and bone resorption by multinucleated osteoclasts is related to lipid metabolism. In this study, we generated low-density lipoprotein receptor (LDLR)/lectin-like oxidized LDL receptor-1 (LOX-1, also known as Olr1) double knockout (dKO) mice. We found that, like LDLR single KO (sKO), LDLR/LOX-1 dKO impaired cell-cell fusion of osteoclast-like cells (OCLs). LDLR/LOX-1 dKO and LDLR sKO preosteoclasts exhibited decreased uptake of LDL. The cell surface cholesterol levels of both LDLR/LOX-1 dKO and LDLR sKO osteoclasts were lower than the levels of wild-type OCLs. Additionally, the amount of phosphatidylethanolamine (PE) on the cell surface was attenuated in LDLR/LOX-1 dKO and LDLR sKO preosteoclasts, whereas the PE distribution in wild-type OCLs was concentrated on the filopodia in contact with neighboring cells. Abrogation of the ATP binding cassette G1 (ABCG1) transporter, which transfers PE to the cell surface, caused decreased PE translocation to the cell surface and subsequent cell-cell fusion. The findings of this study indicate the involvement of a novel cascade (LDLR∼ABCG1∼PE translocation to cell surface∼cell-cell fusion) in multinucleation of OCLs.

Charged Residues in the C-Terminal Domain of Apolipoprotein A-I Modulate Oligomerization.

Charged residues of the C-terminal domain of human apolipoprotein A-I (apoA-I) were targeted by site-directed mutagenesis. A series of mutant proteins was engineered in which lysine residues (Lys 195, 206, 208, 226, 238, and 239) or glutamate residues (Glu 234 and 235) were replaced by glutamine. The amino acid substitutions did not result in changes in secondary structure content or protein stability. Cross-linking and size-exclusion chromatography showed that the mutations resulted in reduced self-association, generating a predominantly monomeric apoA-I when five or six lysine residues were substituted. The rate of phosphatidylcholine vesicle solubilization was enhanced for all variants, with approximately a threefold rate enhancement for apoA-I lacking Lys 206, 208, 238, and 239, or Glu 234 and 235. Single or double mutations did not change the ability to protect lipolyzed low density lipoprotein from aggregation, but variants lacking >4 lysine residues were less effective in preventing lipoprotein aggregation. ApoA-I mediated cellular lipid efflux from wild-type mice macrophage foam cells was decreased for the variant with five lysine mutations. However, this protein was more effective in releasing cellular phosphatidylcholine and sphingomyelin from Abca1-null mice macrophage foam cells. This suggests that the mutations caused changes in the interaction with ABCA1 transporters and that membrane microsolubilization was primarily responsible for lipid efflux in cells lacking ABCA1. Taken together, this study indicates that ionic interactions in the C-terminal domain of apoA-I favor self-association and that monomeric apoA-I is more active in solubilizing phospholipid bilayers.

Reconstituted Discoidal High-Density Lipoproteins: Bioinspired Nanodiscs with Many Unexpected Applications.

PURPOSE OF REVIEW: Summarize the initial discovery of discoidal high-density lipoprotein (HDL) in human plasma and review more recent innovations that span the use of reconstituted nanodisc HDL for membrane protein characterization to its use as a drug carrier and a novel therapeutic agent for cardiovascular disease. RECENT FINDINGS: Using a wide variety of biophysical techniques, the structure and composition of endogenous discoidal HDL have now largely been solved. This has led to the development of new methods for the in vitro reconstitution of nanodisc HDL, which have proven to have a wide variety of biomedical applications. Nanodisc HDL has been used as a platform for mimicking the plasma membrane for the reconstitution and investigation of the structures of several plasma membrane proteins, such as cytochrome P450s and ABC transporters. Nanodisc HDL has also been designed as drug carriers to transport amphipathic, as well as hydrophobic small molecules, and has potential therapeutic applications for several diseases. Finally, nanodisc HDL itself like native discoidal HDL can mediate cholesterol efflux from cells and are currently being tested in late-stage clinical trials for cardiovascular disease. The discovery of the characterization of native discoidal HDL has inspired a new field of synthetic nanodisc HDL, which has offered a growing number of unanticipated biomedical applications.

Calpain-mediated ABCA1 degradation: post-translational regulation of ABCA1 for HDL biogenesis.

Helical apolipoproteins remove cellular phospholipid and cholesterol to generate nascent HDL and this reaction is the major source of plasma HDL. ABCA1 is mandatory and rate-limiting for this reaction. Besides regulation of the gene expression by transcriptional factors including LXR, AP2 and SREBP, the ABCA1 activity is regulated post-translationally by calpain-mediated proteolytic degradation of ABCA1 protein that occurs in the early endosome after its endocytosis. When the HDL biogenesis reaction is ongoing as helical apolipoproteins interact with ABCA1, ABCA1 becomes resistant to calpain and is recycled to cell surface after endocytosis. Biogenesis of HDL is most likely to take place on cell surface. Clearance rate of ABCA1 by this mechanism is also retarded by various factors that interact with ABCA1, such as α1-syntrophin, LXRß and calmodulin. Physiological relevance of the retardation by these factors is not entirely clear. Pharmacological inhibition of the calpain-mediated ABCA1 degradation results in the increase of the ABCA1 activity and HDL biogenesis in vitro and in vivo, and potentially suppresses atherogenesis. 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).

Apolipoprotein A-I in mouse cerebrospinal fluid derives from the liver and intestine via plasma high-density lipoproteins assembled by ABCA1 and LCAT.

Apolipoprotein (apo) A-I, the major structural protein of high-density lipoprotein (HDL), is present in human and mouse cerebrospinal fluid (CSF) despite its lack of expression in brain cells. To identify the origin of apoA-I in CSF, we generated intestine-specific and liver-specific Apoa1 knockout mice (Apoa1ΔInt and Apoa1Δliv mice, respectively). Lipoprotein profiles of Apoa1ΔInt and Apoa1ΔLiv mice resembled those of control littermates, whereas knockout of Apoa1 in both intestine and liver (Apoa1ΔIntΔLiv ) resulted in a 60-percent decrease in HDL-cholesterol levels, thus strongly mimicking the Apoa1-/- mice. Immunoassays revealed that mouse apoA-I was not present in the CSF of the Apoa1ΔIntΔLiv mice. Furthermore, apoA-I levels in CSF were highly correlated with plasma spherical HDL levels, which were regulated by ABCA1 and LCAT. Collectively, these results suggest that apoA-I protein in CSF originates in liver and small intestine and is taken up from the plasma.

Cholesterol homeostasis in ABCA1/LCAT double-deficient mouse.

We examined cholesterol homeostasis in mice with the two major cholesterol transport pathways for catabolism interrupted by disrupting abca1, lcat, or both. Plasma HDL markedly decreased in these genotype but LDL/VLDL decreased only in the double deficiency. Fractional catabolic rate of HDL increased in the order of wild type

Selective Correction of Genotype Yield by Probucol in HDL-Deficient Mice Propagation.

AIM: Probucol is a controversial drug to inhibit ATP-binding cassette transporter A1 (ABCA1) and to exhibit some positive clinical effects such as regression of xanthomas. It reportedly rescues female infertility in scavenger receptor BI-deficient mice. Here, we investigated the effect of probucol on propagation in HDL-deficient mice as alternative models for impaired HDL-mediated cholesterol delivery. METHODS: Propagation of ABCA1-deficient (Abca1-/-) mice and lecithin: cholesterol acyltransferase (LCAT)-deficient (Lcat-/-) mice were quantitatively observed under the probucol treatment. RESULTS: Abca1-/- and Lcat-/- mice appear with negligible plasma HDL concentration. Upon backcrossing Abc1+/- with the Abc1-/- mice and cross-breeding between Abc1+/- mice, the numbers of Abc1-/- weaned pups were reduced to 54.7% and to 57.1% from those expected by Mendelian genetics, respectively. Similarly, Lcat-/- weaned pups decreased to 67.7% and to 35.9% but only in the male. Probucol severely reduced plasma HDL-cholesterol to 5% in the wild-type mice, but showed no effects on their propagation. Probucol corrected the deflections of the genotype distribution in the weaned pups recovery in the LCAT-deficient mice propagation but not in the ABCA1-deficient mice while plasma HDL was kept negligible. Probucol had no effect on cholesterol content in the steroidogenic organs of the HDL-deficient mice, while it somewhat increased plasma corticosterone and expression of adrenal cortex HMG-CoA reductase, StAR, cytochrome P450scc, and VKORC1 indicating increase in the synthesis of cholesterol and steroid hormones and in vitamin K turn-over. However, no evident mechanistic background was indicated. CONCLUSIONS: Probucol corrected deflection of genotype distribution in propagation of the LCAT-deficient mice but not the ABCA1-deficient mice at the weaning stage, apparently not through normalization of hypoalphalipoproteinemia.

Pharmacological inhibition of ABCA1 degradation increases HDL biogenesis and exhibits antiatherogenesis.

Expression of ABCA1 is regulated by transcription of the gene and calpain-mediated proteolytic degradation, and inhibition ABCA1 degradation results in increased ABCA1 and HDL biogenesis in vitro. We examined whether this approach could be a potential antiatherogenic treatment. Although probucol inhibits both the activity and degradation of ABCA1, its oxidized products, spiroquinone and diphenoquinone, reduce degradation of ABCA1 without inhibiting its activity or altering transcription of the ABCA1 gene. Accordingly, both compounds enhanced apolipoprotein A-I/ABCA1-dependent generation of HDL in vitro, and increased hepatic ABCA1 and plasma HDL without increasing antioxidant activity in plasma when given to rabbits. Both compounds also decreased vascular lipid deposition in cholesterol-fed rabbits. We therefore conclude that stabilization of ABCA1 against calpain-mediated degradation is a novel and potentially important strategy to increase HDL formation and prevent atherosclerosis. Spiroquinone and diphenoquinone are potential seeds for development of such drugs.

Exposure to High Glucose Concentration Decreases Cell Surface ABCA1 and HDL Biogenesis in Hepatocytes.

AIM: To study atherosclerosis risk in diabetes, we investigated ATP-binding cassette transporter A1 (ABCA1) expression and high-density lipoprotein (HDL) biogenesis in the liver and hepatocytes under hyperglycemic conditions. METHODS AND RESULTS: In streptozotocin-induced diabetic mice, plasma HDL decreased while ABCA1 protein increased without changing its mRNA in the liver, only in the animals that responded to the treatment to show hypoinsulinemia and fasting hyperglycemia but not in the poor responders not showing those. To study the mechanism for this finding, hepatocytes were isolated from the control and diabetic mice, and they showed no difference in expression of ABCA1 protein, its mRNA, and HDL biogenesis in 1 g/l d-glucose but showed decreased HDL biogenesis in 4.5 g/l d-glucose although ABCA1 protein increased without change in its mRNA. Similar findings were confirmed in HepG2 cells with d-glucose but not with l-glucose. Thus, these cell models reproduced the in vivo findings in hyperglycemia. Labeling of cell surface protein revealed that surface ABCA1 decreased in high concentration of d-glucose in HepG2 cells despite the increase of cellular ABCA1 while not with l-glucose. Immunostaining of ABCA1 in HepG2 cells demonstrated the decrease of surface ABCA1 but increase of intracellular ABCA1 with high d-glucose. Clearance of ABCA1 was retarded both in primary hepatocytes and HepG2 cells exposed to high d-glucose but not to l-glucose, being consistent with the decrease of surface ABCA1. CONCLUSIONS: It is suggested that localization of ABCA1 to the cell surface is decreased in hepatocytes in hyperglycemic condition to cause decrease of HDL biogenesis.

ABCG1 and ABCG4 Suppress γ-Secretase Activity and Amyloid ß Production.

ATP-binding cassette G1 (ABCG1) and ABCG4, expressed in neurons and glia in the central nervous system, mediate cholesterol efflux to lipid acceptors. The relationship between cholesterol level in the central nervous system and Alzheimer's disease has been reported. In this study, we examined the effects of ABCG1 and ABCG4 on amyloid precursor protein (APP) processing, the product of which, amyloid ß (Aß), is involved in the pathogenesis of Alzheimer's disease. Expression of ABCG1 or ABCG4 in human embryonic kidney 293 cells that stably expressed Swedish-type mutant APP increased cellular and cell surface APP levels. Products of cleavage from APP by α-secretase and by ß-secretase also increased. The levels of secreted Aß, however, decreased in the presence of ABCG1 and ABCG4, but not ABCG4-KM, a nonfunctional Walker-A lysine mutant. In contrast, secreted Aß levels increased in differentiated SH-SY5Y neuron-like cells in which ABCG1 and ABCG4 were suppressed. Furthermore, Aß42 peptide in the cerebrospinal fluid from Abcg1 null mice significantly increased compared to the wild type mice. To examine the underlying mechanism, we analyzed the activity and distribution of γ-secretase. ABCG1 and ABCG4 suppressed γ-secretase activity and disturbed γ-secretase localization in the raft domains where γ-secretase functions. These results suggest that ABCG1 and ABCG4 alter the distribution of γ-secretase on the plasma membrane, leading to the decreased γ-secretase activity and suppressed Aß secretion. ABCG1 and ABCG4 may inhibit the development of Alzheimer's disease and can be targets for the treatment of Alzheimer's disease.

An inhibitor of acylCoA: cholesterol acyltransferase increases expression of ATP-binding cassette transporter A1 and thereby enhances the ApoA-I-mediated release of cholesterol from macrophages.

The effect of inhibition of acylCoA: cholesterol acyltransferase (ACAT) was studied on high density lipoprotein (HDL) metabolism. An inhibitor of ACAT, MCC-147, was given mouse peritoneal macrophages and expression of ATP-binding cassette transporter A1 (ABCA1) was examined. ABCA1 was increased both at the mRNA and protein levels, only when the cells are cholesterol-loaded and thereby the inhibitor decreased esterified cholesterol and increased unesterified cholesterol. In this condition, the ACAT inhibitor increased reversible binding of apoA-I to the cells and enhanced apoA-I-mediated release of cellular cholesterol and phospholipid, but did not influence nonspecific cellular cholesterol efflux to lipid microemulsion. It was therefore concluded that the ACAT inhibitor increased the release of cholesterol from the cholesterol-loaded macrophages by increasing the expression of ABCA1, putatively through shifting cholesterol distribution from the esterified to the free compartments.

Cholesteryl ester transfer protein expressed in lecithin cholesterol acyltransferase-deficient mice.

OBJECTIVE: Regulation of plasma cholesteryl ester transfer protein (CETP) concentration was studied in lecithin-cholesterol acyltransferase (LCAT)-knockout mice. METHODS AND RESULTS: LCAT-knockout mice were cross-bred with CETP transgenic mice. The offspring (n=63) were classified for LCAT genotype and plasma CETP levels (no CETP, low CETP, and high CETP). High density lipoprotein (HDL) decreased as LCAT decreased in each CETP-level group. In the lcat(+/+) and lcat(+/-) mice, plasma CETP varied from 0 to 30 micro g/mL, whereas it was <10 micro g/mL in the lcat(-/-) mice. HDL cholesterol and phospholipid decreased and HDL triglyceride and apolipoprotein B increased in CETP in the lcat(+/+) and lcat(+/-) mice, whereas there was no difference in HDL between low and high CETP. An effect of CETP on HDL was not detected in the lcat(-/-) mice because of the absence of mature HDL. Genomic DNA and mRNA of CETP were correlated and were similar in the lcat(-/-) and lcat(+/+) mice. Plasma CETP was correlated with its genomic DNA and mRNA, but the slope of the increase was much lower in the lcat(-/-) mice. Whereas plasma CETP mostly associates with HDL in the lcat(+/+) mouse, it is found free in the lcat(-/-) mouse. CONCLUSIONS: Plasma CETP is posttranscriptionally downregulated in the lcat(-/-) mice, presumably by its extremely low HDL.

Lipid accumulation in smooth muscle cells under LDL loading is independent of LDL receptor pathway and enhanced by hypoxic conditions.

OBJECTIVE: The effect of a variety of hypoxic conditions on lipid accumulation in smooth muscle cells (SMCs) was studied in an arterial wall coculture and monocultivation model. METHODS AND RESULTS: Low density lipoprotein (LDL) was loaded under various levels of oxygen tension. Oil red O staining of rabbit and human SMCs revealed that lipid accumulation was greater under lower oxygen tension. Cholesterol esters were shown to accumulate in an oxygen tension-dependent manner by high-performance liquid chromatographic analysis. Autoradiograms using radiolabeled LDL indicated that LDL uptake was more pronounced under hypoxia. This result holds in the case of LDL receptor-deficient rabbit SMCs. However, cholesterol biosynthesis and cellular cholesterol release were unaffected by oxygen tension. CONCLUSIONS: Hypoxia significantly increases LDL uptake and enhances lipid accumulation in arterial SMCs, exclusive of LDL receptor activity. Although the molecular mechanism is not clear, the model is useful for studying lipid accumulation in arterial wall cells and the difficult-to-elucidate events in the initial stage of atherogenesis.

Isoosmotic isolation of rat brain synaptic vesicles, some of which contain tyrosine hydroxylase.

Rat brain synaptic vesicles were isoosmotically isolated and examined for Mg(2+)-ATPase [EC 3.6.1.3.] and tyrosine hydroxylase [EC 1.14.16.2.] associated with the synaptic vesicles. Synaptosomes in 0.32 M sucrose were disrupted by freezing and thawing treatment, and the cytosol fraction was fractionated on a Sephacryl S-500 column with a mean exclusion size of 200 nm. Peak I at the void volume was a mixture of large vesicular membranes, small amounts of synaptic vesicles and coated vesicles, etc. Peak II consisted of non- and granulated synaptic vesicles of 35-40 nm diameter, and peak III of soluble proteins. The synaptic vesicles in peak II reacted with antibodies against the H(+)-ATPase A-subunit, vesicular acetylcholine transporter, and vesicular monoamine transporter. However, they showed little Mg(2+)-ATPase activity. Tyrosine hydroxylase was observed in either peak II or III on blotting with an anti-tyrosine hydroxylase antibody. These results imply that tyrosine hydroxylase exists in soluble and bound forms to synaptic vesicles in nerve terminals.

High density lipoprotein turnover is dependent on peroxisome proliferator-activated receptor α in mice.

AIM: Peroxisome proliferator-activated receptor (PPAR) α is a nuclear receptor that through sensing fatty acid metabolites as ligands regulates genes involved in lipid transport and metabolism (Atherosclerosis 205: 413, 2009). Disruption of the PPARα gene, however, may not lead to apparent changes in phenotypes, perhaps due to compensation by other members of the nuclear receptor superfamily. We characterized cholesterol homeostasis in PPARα-null mice on a C57BL/6 background with a focus on HDL metabolism. METHODS AND RESULTS: PPARα-null mice gained weight significantly more than wild-type mice, with more prominence in females, without changing the HDL-apoprotein clearance rate. Clearance of plasma HDL-cholesteryl ester was retarded, more prominently in females. Uptake of HDL-cholesteryl ester by the adrenal glands and ovaries was found to be decreased in female mutant mice. The ATP binding cassette transporter A1 (ABCA1), liver X receptor α and PPARδ mRNAs were decreased in the liver, and that encoding scavenger receptor B1 (SR-B1) decreased in the adrenal glands of these mice. Although the plasma lipoprotein profile did not show major differences, some subtle changes were found in the mutants, including HDL properties being consistent with its slow turnover. Cholesterol content in the organs was not significantly influenced except for a paradoxical increase in gonad glands. CONCLUSIONS: We conclude that the plasma HDL turnover rate is retarded in PPARα-null mice, perhaps more prominently in females.

On the mechanism for PPAR agonists to enhance ABCA1 gene expression.

Expression of ATP binding cassette transporter A1 (ABCA1), a major regulator of high density lipoprotein (HDL) biogenesis, is known to be up-regulated by the transcription factor liver X receptor (LXR) alpha, and expression is further enhanced by activation of the peroxisome proliferator activated receptors (PPARs). We investigated this complex regulatory network using specific PPAR agonists: four fibrates (fenofibrate, bezafibrate, gemfibrozil and LY518674), a PPAR delta agonist (GW501516) and a PPAR gamma agonist (pioglitazone). All of these compounds increased the expression of LXRs, PPARs and ABCA1 mRNAs, and associated apoA-I-mediated lipid release in THP-1 macrophage, WI38 fibroblast and mouse fibroblast. When mouse fibroblasts lacking expression of PPAR alpha were examined, the effects of fenofibrate and LY518674 were markedly diminished while induction by other ligands were retained. The PPAR alpha promoter was activated by all of these compounds in an LXR alpha-dependent manner, and partially in a PPAR alpha-dependent manner, in mouse fibroblast. The LXR responsive element (LXRE)-luciferase activity was enhanced by all the compounds in an LXR alpha-dependent manner in mouse fibroblast. This activation was exclusively PPAR alpha-dependent by fenofibrate and LY518674, but nonexclusively by the others. We conclude that PPARs and LXRs are involved in the regulation of ABCA1 expression and HDL biogenesis in a cooperative signal transduction pathway.

Effects of fibrate drugs on expression of ABCA1 and HDL biogenesis in hepatocytes.

Fibric acid-shaped drugs raise high-density lipoprotein (HDL) cholesterol by upregulating the HDL-related genes through activating peroxisome proliferater activated receptor (PPAR)-alpha. We investigated the effects of fibrates to induce expression of adenosine triphosphate-binding cassette transporter A1 (ABCA1) and increase HDL biogenesis in hepatocytes. Fenofibrate, bezafibrate, gemfibrozil, and LY518674 were tested for HepG2 cells and primary-cultured mouse hepatocytes. All the compounds examined increased ABCA1 expression and HDL biogenesis dependent on PPARalpha in association with the liver X receptor alpha upregulation. While fenofibrate and LY518674 showed exclusive dependency on PPARalpha for these activities, bezafibrate and gemfibrozil exhibited dependency on PPARbeta/delta and PPARgamma as well. On the other hand, cholesterol-enrichment of HDL may involve PPARgamma for fenofibrate and bezafibrate, and PPARbeta/delta for the fibrates examined except for bezafibrate. We concluded that fibrates enhance expression of ABCA1 in hepatocytes to contribute to increase of the HDL biogenesis in a PPAR-dependent manner, whether exclusively or nonexclusively on PPARalpha.