The identification of intestinal scavenger receptor class B, type I (SR-BI) by expression cloning and its role in cholesterol absorption

Genome-wide association study and meta-analysis of phytosterols identifies a novel locus for serum levels of campesterol

Sitosterolemia is a rare inherited disorder caused by mutations in the ABCG5/ABCG8 genes. These genes encode proteins involved in the transport of plant sterols. Mutations in these genes lead to decreased excretion of phytosterols, which can accumulate in the body and lead to a variety of health problems, including premature coronary artery disease. We conducted the first genome-wide association study (GWAS) in the Middle East/North Africa population to identify genetic determinants of plant sterol levels in Qatari people. GWAS was performed on serum levels of β-sitosterol and campesterol using the Metabolon platform from Qatar Biobank (QBB) and genome sequence data provided by Qatar Genome Program. A trans-ancestry meta-analysis of data from our Qatari cohort with summary statistics from a previously published large cohort (9758 subjects) of European ancestry was conducted. Using conditional analysis, we identified two independent single nucleotide polymorphisms associated with β-sitosterol (rs145164937 and rs4299376), and two others with campesterol (rs7598542 and rs75901165) in the Qatari population in addition to previously reported variants. All of them map to the ABCG5/8 locus except rs75901165 which is located within the Intraflagellar Transport 43 (IFT43) gene. The meta-analysis replicated most of the reported variants, and our study provided significant support for the association of variants in SCARB1 and ABO with sitosterolemia. Evaluation of a polygenic risk score devised from European GWAS data showed moderate performance when applied to QBB (adjusted-R2 = 0.082). These findings provide new insights into the genetic architecture of phytosterol metabolism while showing the importance including under-represented populations in future GWAS studies.

SR-BI regulates the synergistic mast cell response by modulating the plasma membrane-associated cholesterol pool

The high-affinity IgE receptor FcεRI is the mast cell (MC) receptor responsible for the involvement of MCs in IgE-associated allergic disorders. Activation of the FcεRI is achieved via crosslinking by multivalent antigen (Ag) recognized by IgE resulting in degranulation and proinflammatory cytokine production. In comparison to the T- and B-cell receptor complexes, for which several co-receptors orchestrating the initial signaling events have been described, information is scarce about FcεRI-associated proteins. Additionally, it is unclear how FcεRI signaling synergizes with input from other receptors and how regulators affect this synergistic response. We found that the HDL receptor SR-BI (gene name: Scarb1/SCARB1) is expressed in MCs, functionally associates with FcεRI, and regulates the plasma membrane cholesterol content in cholesterol-rich plasma membrane nanodomains. This impacted the activation of MCs upon co-stimulation of the FcεRI with receptors known to synergize with FcεRI signaling. Amongst them, we investigated the co-activation of the FcεRI with the receptor tyrosine kinase KIT, the IL-33 receptor, and GPCRs activated by adenosine or PGE2. Scarb1-deficient bone marrow-derived MCs showed reduced cytokine secretion upon co-stimulation conditions suggesting a role for plasma membrane-associated cholesterol regulating respective MC activation. Mimicking Scarb1 deficiency by cholesterol depletion employing MβCD, we identified PKB and PLCγ1 as cholesterol-sensitive proteins downstream of FcεRI activation in bone marrow-derived MCs. When MCs were co-stimulated with stem cell factor (SCF) and Ag, PLCγ1 activation was boosted, which could be mitigated by cholesterol depletion and SR-BI inhibition. Similarly, SR-BI inhibition attenuated the synergistic response to PGE2 and anti-IgE in the human ROSAKIT WT MC line, suggesting that SR-BI is a crucial regulator of synergistic MC activation.

Mechanisms of ingested polystyrene micro-nanoplastics (MNPs) uptake and translocation in an in vitro tri-culture small intestinal epithelium

Micro and nanoplastics (MNPs) are now ubiquitous contaminants of food and water. Many cellular and animal studies have shown that ingested MNPs can breach the intestinal barrier to reach the circulation. To date however, the cellular mechanisms involved in intestinal absorption of MNPs have not been investigated with physiologically relevant models, and thus remain unknown. We employed in vitro simulated digestion, a tri-culture small intestinal epithelium model, and a panel of inhibitors to assess the contributions of the possible mechanisms to absorption of 26 nm carboxylated polystyrene (PS26C) MNPs. Inhibition of ATP synthesis reduced translocation by only 35 %, suggesting uptake by both active endocytic pathways and passive diffusion. Translocation was also decreased by inhibition of dynamin and clathrin, suggesting involvement of clathrin mediated endocytosis (CME) and fast endophilin-mediated endocytosis (FEME). Inhibition of actin polymerization also significantly reduced translocation, suggesting involvement of macropinocytosis or phagocytosis. However, inhibition of the Na+-H+ exchanger had no effect on translocation, thus ruling out macropinocytosis. Together these results suggest uptake by passive diffusion as well as by active phagocytosis, CME, and FEME pathways. Further studies are needed to assess uptake mechanisms for other environmentally relevant MNPs as a function of polymer, surface chemistry, and size.

Intestinal Pgc1α ablation protects from liver steatosis and fibrosis

Background & Aims

The gut-liver axis modulates the progression of metabolic dysfunction-associated steatotic liver disease (MASLD), a spectrum of conditions characterised by hepatic steatosis and a progressive increase of inflammation and fibrosis, culminating in metabolic dysfunction-associated steatohepatitis. Peroxisome proliferator-activated receptor-gamma coactivator 1α (Pgc1α) is a transcriptional co-regulator of mitochondrial activity and lipid metabolism. Here, the intestinal-specific role of Pgc1α was analysed in liver steatosis and fibrosis.

Methods

We used a mouse model in which Pgc1α was selectively deleted from the intestinal epithelium. We fed these mice and their wild-type littermates a Western diet to recapitulate the major features of liver steatosis (after 2 months of diet) and metabolic dysfunction-associated steatohepatitis (after 4 months of diet). The chow diet was administered as a control diet.

Results

In humans and mice, low expression of intestinal Pgc1α is inversely associated with liver steatosis, inflammation, and fibrosis. Intestinal disruption of Pgc1α impairs the transcription of a wide number of genes, including the cholesterol transporter Niemann-Pick C1-like 1 (Npc1l1), thus limiting the uptake of cholesterol from the gut. This results in a lower cholesterol accretion in the liver and a decreased production of new fatty acids, which protect the liver from lipotoxic lipid species accumulation, inflammation, and related fibrotic processes.

Conclusions

In humans and mice, intestinal Pgc1α induction during Western diet may be another culprit driving hepatic steatosis and fibrosis. Here, we show that enterocyte-specific Pgc1α ablation protects the liver from steatosis and fibrosis by reducing intestinal cholesterol absorption, with subsequent decrease of cholesterol and de novo fatty acid accumulation in the liver.

Impact and implications

Liver diseases result from several insults, including signals from the gut. Although the incidence of liver diseases is continuously increasing worldwide, effective drug therapy is still lacking. Here, we showed that the modulation of an intestinal coactivator regulates the liver response to a Western diet, by limiting the uptake of dietary cholesterol. This results in a lower accumulation of hepatic lipids together with decreased inflammation and fibrosis, thus limiting the progression of liver steatosis and fibrosis towards severe end-stage diseases.

The TICE Pathway: Mechanisms and Potential Clinical Applications

PURPOSE OF REVIEW

Transintestinal cholesterol excretion (TICE) is a non-biliary pathway that excretes excess cholesterol from the body through feces. This article focuses on the research progress of the TICE pathway in the last few years, including the discovery process of the TICE pathway, its molecular mechanism, and potential clinical applications.

RECENT FINDINGS

Cholesterol homeostasis is vital for cardiovascular diseases, stroke, and neurodegenerative diseases. Beyond the cholesterol excretion via hepatobiliary pathway, TICE contributes significantly to reverse cholesterol transport ex vivo and in vivo. Nuclear receptors are ligand-activated transcription factors that regulate cholesterol metabolism. The farnesoid X receptor (FXR) and liver X receptor (LXR) activated, respectively, by oxysterols and bile acids promote intestinal cholesterol secretion through ABCG5/G8. Nutrient regulators and intestinal flora also modulate cholesterol secretion through the TICE pathway. TICE allows direct elimination of plasma cholesterol, which may provide an attractive therapeutic targets. TICE pathway may provide a potential target to stimulate cholesterol elimination and reduce the risk of cardiovascular diseases.

Cross-Linking Mass Spectrometry Uncovers Interactions Between High-Density Lipoproteins and the SARS-CoV-2 Spike Glycoprotein

High-density lipoprotein (HDL) levels are reduced in patients with coronavirus disease 2019 (COVID-19), and the extent of this reduction is associated with poor clinical outcomes. While lipoproteins are known to play a key role during the life cycle of the hepatitis C virus, their influence on coronavirus (CoV) infections is poorly understood. In this study, we utilize cross-linking mass spectrometry (XL-MS) to determine circulating protein interactors of the severe acute respiratory syndrome (SARS)-CoV-2 spike glycoprotein. XL-MS of plasma isolated from patients with COVID-19 uncovered HDL protein interaction networks, dominated by acute-phase serum amyloid proteins, whereby serum amyloid A2 was shown to bind to apolipoprotein (Apo) D. XL-MS on isolated HDL confirmed ApoD to interact with SARS-CoV-2 spike but not SARS-CoV-1 spike. Other direct interactions of SARS-CoV-2 spike upon HDL included ApoA1 and ApoC3. The interaction between ApoD and spike was further validated in cells using immunoprecipitation-MS, which uncovered a novel interaction between both ApoD and spike with membrane-associated progesterone receptor component 1. Mechanistically, XL-MS coupled with data-driven structural modeling determined that ApoD may interact within the receptor-binding domain of the spike. However, ApoD overexpression in multiple cell-based assays had no effect upon viral replication or infectivity. Thus, SARS-CoV-2 spike can bind to apolipoproteins on HDL, but these interactions do not appear to alter infectivity.

Genome-wide meta-analysis of phytosterols reveals five novel loci and a detrimental effect on coronary atherosclerosis

Phytosterol serum concentrations are under tight genetic control. The relationship between phytosterols and coronary artery disease (CAD) is controversially discussed. We perform a genome-wide meta-analysis of 32 phytosterol traits reflecting resorption, cholesterol synthesis and esterification in six studies with up to 9758 subjects and detect ten independent genome-wide significant SNPs at seven genomic loci. We confirm previously established associations at ABCG5/8 and ABO and demonstrate an extended locus heterogeneity at ABCG5/8 with different functional mechanisms. New loci comprise HMGCR, NPC1L1, PNLIPRP2, SCARB1 and APOE. Based on these results, we perform Mendelian Randomization analyses (MR) revealing a risk-increasing causal relationship of sitosterol serum concentrations and CAD, which is partly mediated by cholesterol. Here we report that phytosterols are polygenic traits. MR add evidence of both, direct and indirect causal effects of sitosterol on CAD.

Current therapeutics against HCV

Hepatitis C is a positive stranded enveloped RNA virus belonging to the Flaviviridae family. HCV infection leads to severe liver diseases, cirrhosis and hepatocellular carcinoma worldwide. Although treatments have been available for a while, due to its complexity and genetic diversity, only few are reported to be effective against all HCV genotypes. Here, we review the HCV life cycle and its immunogenic potential and various mechanisms via which the virus interferes in the signalling process. A comprehensive overview of current anti-HCV therapeutics, such as, Direct Acting Antiviral (DAA) as well as Host Targeting Agents (HTA), along with their scope, known mechanism of action and limitations are presented.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-021-00697-0.

Muricholic Acids Promote Resistance to Hypercholesterolemia in Cholesterol-Fed Mice

BACKGROUND AND AIMS

Hypercholesterolemia is a major risk factor for atherosclerosis and cardiovascular diseases. Although resistant to hypercholesterolemia, the mouse is a prominent model in cardiovascular research. To assess the contribution of bile acids to this protective phenotype, we explored the impact of a 2-week-long dietary cholesterol overload on cholesterol and bile acid metabolism in mice.

METHODS

Bile acid, oxysterol, and cholesterol metabolism and transport were assessed by quantitative real-time PCR, western blotting, GC-MS/MS, or enzymatic assays in the liver, the gut, the kidney, as well as in the feces, the blood, and the urine.

RESULTS

Plasma triglycerides and cholesterol levels were unchanged in mice fed a cholesterol-rich diet that contained 100-fold more cholesterol than the standard diet. In the liver, oxysterol-mediated LXR activation stimulated the synthesis of bile acids and in particular increased the levels of hydrophilic muricholic acids, which in turn reduced FXR signaling, as assessed in vivo with Fxr reporter mice. Consequently, biliary and basolateral excretions of bile acids and cholesterol were increased, whereas portal uptake was reduced. Furthermore, we observed a reduction in intestinal and renal bile acid absorption.

CONCLUSIONS

These coordinated events are mediated by increased muricholic acid levels which inhibit FXR signaling in favor of LXR and SREBP2 signaling to promote efficient fecal and urinary elimination of cholesterol and neo-synthesized bile acids. Therefore, our data suggest that enhancement of the hydrophilic bile acid pool following a cholesterol overload may contribute to the resistance to hypercholesterolemia in mice. This work paves the way for new therapeutic opportunities using hydrophilic bile acid supplementation to mitigate hypercholesterolemia.

Transkingdom network analysis provides insight into host-microbiome interactions in Atlantic salmon

BACKGROUND

The Atlantic salmon gut constitutes an intriguing system for studying host-microbiota interactions due to the dramatic environmental change salmon experiences during its life cycle. Yet, little is known about the role of interactions in this system and there is a general deficit in computational methods for integrative analysis of omics data from host-microbiota systems.

METHODS

We developed a pipeline to integrate host RNAseq data and microbial 16S rRNA amplicon sequencing data using weighted correlation network analysis. Networks are first inferred from each dataset separately, followed by module detections and finally robust identification of interactions via comparisons of representative module profiles. Through the use of module profiles, this network-based dimensionality reduction approach provides a holistic view into the discovery of potential host-microbiota symbionts.

RESULTS

We analyzed host gene expression from the gut epithelial tissue and microbial abundances from the salmon gut in a long-term feeding trial spanning the fresh-/salt-water transition and including two feeds resembling the fatty acid compositions available in salt- and fresh-water environments, respectively. We identified several host modules with significant correlations to both microbiota modules and variables such as feed, growth and sex. Although the strongest associations largely coincided with the fresh-/salt-water transition, there was a second layer of correlations associating smaller host modules to both variables and microbiota modules. Hence, we identify extensive reprogramming of the gut epithelial transcriptome and large scale coordinated changes in gut microbiota composition associated with water type as well as evidence of host-microbiota interactions linked to feed.

Advances in fatty acids nutrition in dairy cows: from gut to cells and effects on performance

High producing dairy cows generally receive in the diet up to 5-6% of fat. This is a relatively low amount of fat in the diet compared to diets in monogastrics; however, dietary fat is important for dairy cows as demonstrated by the benefits of supplementing cows with various fatty acids (FA). Several FA are highly bioactive, especially by affecting the transcriptome; thus, they have nutrigenomic effects. In the present review, we provide an up-to-date understanding of the utilization of FA by dairy cows including the main processes affecting FA in the rumen, molecular aspects of the absorption of FA by the gut, synthesis, secretion, and utilization of chylomicrons; uptake and metabolism of FA by peripheral tissues, with a main emphasis on the liver, and main transcription factors regulated by FA. Most of the advances in FA utilization by rumen microorganisms and intestinal absorption of FA in dairy cows were made before the end of the last century with little information generated afterwards. However, large advances on the molecular aspects of intestinal absorption and cellular uptake of FA were made on monogastric species in the last 20 years. We provide a model of FA utilization in dairy cows by using information generated in monogastrics and enriching it with data produced in dairy cows. We also reviewed the latest studies on the effects of dietary FA on milk yield, milk fatty acid composition, reproduction, and health in dairy cows. The reviewed data revealed a complex picture with the FA being active in each step of the way, starting from influencing rumen microbiota, regulating intestinal absorption, and affecting cellular uptake and utilization by peripheral tissues, making prediction on in vivo nutrigenomic effects of FA challenging.

A new system to evaluate characteristics of Niemann-Pick C1 Like 1-mediated cholesterol transport using Xenopus laevis oocytes

Niemann-Pick C1 Like 1 (NPC1L1) is known to be involved in the intestinal absorption of cholesterol. For evaluating the function of NPC1L1, cell lines such as Caco-2, Madin-Darby canine kidney (MDCK) II, and McA-RH7777 have been used in previous studies, but the detailed molecular mechanism of transport has not been elucidated. In this study, the characteristics of cholesterol transport via NPC1L1 were investigated using a Xenopus laevis oocyte expression system in addition to a conventional cell line with stable expression. The transport activity of cholesterol uptake was increased in NPC1L1-overexpressed MDCK cells compared with that in mock cells, but MDCK cells expressed endogenous NPC1L1 and had high cholesterol transport activity. On the other hand, cRNA-injected oocytes expressed NPC1L1 after culturing for 5-6 days. The transport activity of cholesterol uptake was increased in NPC1L1 cRNA-injected oocytes compared with that in water-injected oocytes. In addition, the uptake of cholesterol was decreased in the presence of ezetimibe, an NPC1L1 inhibitor, in cRNA-injected oocytes but not in control oocytes, indicating that endogenous NPC1L1 is not expressed in oocytes. Furthermore, cholesterol uptake was substantially decreased in NPC1L1 L216A cRNA-injected oocytes compared with that in NPC1L1 cRNA-injected oocytes, indicating that leucine at position 216 of NPC1L1 is important for cholesterol transport and that an oocyte expression system is useful for mutant analysis. These results indicate that the oocyte expression system is useful for evaluating the characteristics of NPC1L1-mediated cholesterol transport and may contribute to the elucidation of the detailed molecular mechanism of cholesterol transport via NPC1L1.

Egg phospholipids exert an inhibitory effect on intestinal cholesterol absorption in mice

BACKGROUND/OBJECTIVES

Consumption of cholesterol-rich foods, such as eggs, has a minimal effect on circulating cholesterol levels in healthy humans. To gain insight, we investigated whether phospholipids rich in eggs (EPL) interfere with intestinal cholesterol absorption in vivo.

MATERIALS/METHODS

To investigate the acute effect of EPL on intestinal cholesterol absorption, male C57BL/6J mice were orally administered with 6, 11, or 19 mg of EPL for three days. We also tested the effect of chronic EPL consumption on cholesterol metabolism in the small intestine and the liver in mice with diet-induced hypercholesterolemia. Male C57BL/6J mice were fed a high fat/high cholesterol (HF/HC; 35% fat, 0.25% cholesterol, w/w) diet for 4 weeks to induce hypercholesterolemia, and subsequently the mice were either fed 0, 0.4 or 0.8% (w/w) of EPL for 6 weeks.

RESULTS

Intestinal cholesterol absorption was significantly decreased by the highest dose of acute EPL administration compared to control. Chronic EPL supplementation did not significantly alter intestinal cholesterol absorption nor plasma levels of total cholesterol and low-density lipoprotein cholesterol. In the small intestine and the liver, EPL supplementation minimally altered the expression of genes which regulate cellular cholesterol levels.

CONCLUSION

Although chronic EPL consumption was not able to counteract hypercholesterolemia in HF/HC-fed mice, acute EPL administration decreased intestinal cholesterol absorption. This study provides in vivo evidence that acute administration of PLs in eggs prevent cholesterol absorption in the intestine, suggesting a mechanism for a minimal effect of egg consumption on circulating cholesterol levels.

Inhibitory effects of Tiliacora triandra (Colebr.) Diels on cholesterol absorption

Background Natural supplements and herbal medicines have been attracted to use for managing elevated cholesterol levels. Tiliacora triandra (Colebr.) Diels (TT) or Yanang (in Thai) is commonly used as an ingredient in various types of Thai foods. In this study, we investigated the effect of methanolic TT leaf extract on cholesterol absorption by measuring the uptake and the efflux of cholesterol and cholesterol micellar solubility. In addition, we tested the effect of TT leaf extract on pancreatic lipase activity. Methods The uptake and efflux of cholesterol was determined by quantification of radioactivity in differentiated Caco-2 cells after treatment with radioactive cholesterol. Cholesterol mixed micelles were prepared for cholesterol uptake, efflux and solubility studies. The pancreatic lipase activity was determined using 4-methylumbelliferyl oleate as a substrate. Results Our finding showed that TT extract decreased the uptake of cholesterol by approximately 48% but did not affect the efflux of cholesterol. TT inhibited pancreatic lipase activity with the IC50 at 273.5 μg/mL and also decreased cholesterol micellar solubility. Conclusions These findings suggest that TT leaf extract seems to be a potential candidate as cholesterol-lowering agents.

The Impact of Egg Nutrient Composition and Its Consumption on Cholesterol Homeostasis

Nutrient deficiencies and excess are involved in many aspects of human health. As a source of essential nutrients, eggs have been used worldwide to support the nutritional needs of human societies. On the other hand, eggs also contain a significant amount of cholesterol, a lipid molecule that has been associated with the development of cardiovascular diseases. Whether the increase of egg consumption will lead to elevated cholesterol absorption and disruption of cholesterol homeostasis has been a concern of debate for a while. Cholesterol homeostasis is regulated through its dietary intake, endogenous biosynthesis, utilization, and excretion. Recently, some research interests have been paid to the effects of egg consumption on cholesterol homeostasis through the intestinal cholesterol absorption. Nutrient components in eggs such as phospholipids may contribute to this process. The goals of this review are to summarize the recent progress in this area and to discuss some potential benefits of egg consumption.

Intestinal CD36 and Other Key Proteins of Lipid Utilization: Role in Absorption and Gut Homeostasis

Several proteins have been implicated in fatty acid (FA) transport by enterocytes including the scavenger receptor CD36 (SR-B2), the scavenger receptor B1 (SR-B1) a member of the CD36 family and the FA transport protein 4 (FATP4). Here, we review the regulation of enterocyte FA uptake and its function in lipid absorption including prechylomicron formation, assembly and transport. Emphasis is given to CD36, which is abundantly expressed along the digestive tract of rodents and humans and has been the most studied. We also address the pleiotropic functions of CD36 that go beyond lipid absorption and metabolism to include recent evidence of its impact on intestinal homeostasis and barrier maintenance. Areas of progress involving contribution of membrane phospholipid remodeling and of cytosolic FA-binding proteins, FABP1 and FABP2 to fat absorption will be covered. © 2018 American Physiological Society. Compr Physiol 8:493-507, 2018.

Cholesterol trafficking and raft-like membrane domain composition mediate scavenger receptor class B type 1-dependent lipid sensing in intestinal epithelial cells

Scavenger receptor Class B type 1 (SR-B1) is a lipid transporter and sensor. In intestinal epithelial cells, SR-B1-dependent lipid sensing is associated with SR-B1 recruitment in raft-like/ detergent-resistant membrane domains and interaction of its C-terminal transmembrane domain with plasma membrane cholesterol. To clarify the initiating events occurring during lipid sensing by SR-B1, we analyzed cholesterol trafficking and raft-like domain composition in intestinal epithelial cells expressing wild-type SR-B1 or the mutated form SR-B1-Q445A, defective in membrane cholesterol binding and signal initiation. These features of SR-B1 were found to influence both apical cholesterol efflux and intracellular cholesterol trafficking from plasma membrane to lipid droplets, and the lipid composition of raft-like domains. Lipidomic analysis revealed likely participation of d18:0/16:0 sphingomyelin and 16:0/0:0 lysophosphatidylethanolamine in lipid sensing by SR-B1. Proteomic analysis identified proteins, whose abundance changed in raft-like domains during lipid sensing, and these included molecules linked to lipid raft dynamics and signal transduction. These findings provide new insights into the role of SR-B1 in cellular cholesterol homeostasis and suggest molecular links between SR-B1-dependent lipid sensing and cell cholesterol and lipid droplet dynamics.

Detection of high biliary and fecal viral loads in patients with chronic hepatitis C virus infection

BACKGROUND

The life cycle of the hepatitis C virus (HCV) is closely associated with lipid metabolism. Recently, NPC1L1 (a cholesterol transporter) has been reported to function as an HCV receptor. This receptor is expressed in the hepatocyte canalicular membrane and in the intestine; serving as a key transporter for the cholesterol enterohepatic cycle.

OBJECTIVES

We hypothesized that HCV might have a similar cycle, so we aimed to study the presence of HCV in bile and stools of infected patients.

MATERIALS AND METHODS

Blood, feces, and duodenal bile samples were collected from patients infected with HCV. The biliary viral load was normalized to the bile salt concentration of each sample and the presence of HCV core protein was also evaluated. A total of 12 patients were recruited. HCV RNA was detected in the bile from ten patients.

RESULTS

The mean viral load was 2.5log10IU/60mg bile salt. In the stool samples, HCV RNA was detected in ten patients (mean concentration 2.7log10IU/g of feces).

CONCLUSIONS

HCV RNA is readily detectable and is present at relatively high concentrations in the bile and stool samples of infected patients. This may be relevant as a source of infection in men who have sex with men. Biliary HCV secretion may perhaps play a role in the persistence of viral infection via an enterohepatic cycle of the virus or intrahepatic spread.

Intestinal scavenger receptor class B type I as a novel regulator of chylomicron production in healthy and diet-induced obese states

The small intestine contributes to diabetic dyslipidemia through the overproduction of apolipoprotein B48 (apoB48)-containing chylomicron particles. An important regulator of chylomicron generation is dietary lipid absorption, underlining the potential involvement of intestinal lipid transporters for developing dyslipidemia. Intestinal expression of scavenger receptor class B type I (SR-BI) has been found to be upregulated in animal models of insulin resistance. Here we characterized the potential importance of SR-BI in contributing to chylomicron production and postprandial hypertriglyceridemia in vivo. Postprandial triglyceride (TG)-rich lipoprotein (TRL) production was characterized in hamsters treated with the SR-BI inhibitor to block lipid transport-1 (BLT-1) under healthy conditions or conditions of diet-induced obesity and dyslipidemia. BLT-1 (1 mg/kg) or vehicle was administered acutely in chow-fed hamsters or gavaged twice daily over 10 days during high-fructose, high-fat, high-cholesterol (FFC) feeding. Effects of acute SR-BI inhibition by BLT-1 were confirmed in healthy fat-loaded rats. Finally, plasma lipid levels were compared between SR-BI(-/-) mice and their wild-type counterparts fed either chow or a 12-wk high-fat diet. Acute BLT-1 treatment reduced postprandial plasma and TRL TG levels in healthy hamsters and rats. Chronic BLT-1 treatment of FFC-fed hamsters blunted diet-induced weight gain and fasting hypertriglyceridemia, and lowered postprandial TRL-TG, -cholesterol, and -apoB48 levels. Finally, SR-BI(-/-) mice displayed lower plasma and TRL TG levels relative to wild type, and diet-induced weight gain and postprandial hypertriglyceridemia were hindered in SR-BI(-/-) mice. We conclude that intestinal SR-BI is a critical regulator of postprandial lipoprotein production, emphasizing its potential as a target for preventing diabetic dyslipidemia.

Specific cellular incorporation of a pyrene-labelled cholesterol: lipoprotein-mediated delivery toward ordered intracellular membranes

In the aim of testing tools for tracing cell trafficking of exogenous cholesterol, two fluorescent derivatives of cholesterol, 22-nitrobenzoxadiazole-cholesterol (NBD-Chol) and 21-methylpyrenyl-cholesterol (Pyr-met-Chol), with distinctive chemico-physical characteristics, have been compared for their cell incorporation properties, using two cell models differently handling cholesterol, with two incorporation routes. In the Caco-2 cell model, the cholesterol probes were delivered in bile salt micelles, as a model of intestinal absorption. The two probes displayed contrasting behaviors for cell uptake characteristics, cell staining, and efflux kinetics. In particular, Pyr-met-Chol cell incorporation involved SR-BI, while that of NBD-Chol appeared purely passive. In the PC-3 cell model, which overexpresses lipoprotein receptors, the cholesterol probes were delivered via the serum components, as a model of systemic delivery. We showed that Pyr-met-Chol-labelled purified LDL or HDL were able to specifically deliver Pyr-met-Chol to the PC-3 cells, while NBD-Chol incorporation was independent of lipoproteins. Observations by fluorescence microscopy evidenced that, while NBD-Chol readily stained the cytosolic lipid droplets, Pyr-met-Chol labelling led to the intense staining of intracellular structures of membranous nature, in agreement with the absence of detectable esterification of Pyr-met-Chol. A 48 h incubation of PC-3 cells with either Pyr-met-Chol-labelled LDL or HDL gave same staining patterns, mainly colocalizing with Lamp1, caveolin-1 and CD63. These data indicated convergent trafficking downwards their respective receptors, LDL-R and SR-BI, toward the cholesterol-rich internal membrane compartments, late endosomes and multivesicular bodies. Interestingly, Pyr-met-Chol staining of these structures exhibited a high excimer fluorescence emission, revealing their ordered membrane environment, and indicating that Pyr-met-Chol behaves as a fair cholesterol tracer regarding its preferential incorporation into cholesterol-rich domains. We conclude that, while NBD-Chol is a valuable marker of cholesterol esterification, Pyr-met-Chol is a reliable new lipoprotein fluorescent marker which allows to probe specific intracellular trafficking of cholesterol-rich membranes.

SR-BI mediates high density lipoprotein (HDL)-induced anti-inflammatory effect in macrophages

High density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake from lipoproteins into the liver as well as cholesterol efflux from macrophages to HDL. Recently, strong evidence has demonstrated the anti-inflammatory effect of HDL, although the mechanism of action is not fully understood. In this study, we showed that the anti-inflammatory effects of HDL are dependent on SR-BI expression in THP-1 macrophages. Consistent with earlier findings, pretreatment of macrophages with HDL abolished LPS-induced TNFα production. HDL also inhibited LPS-induced NF-κB activation. In addition, knockdown of SR-BI or inhibition of SR-BI ligand binding abolished the anti-inflammatory effect of HDL. SR-BI is a multi-ligand receptor that binds to modified lipoproteins as well as native HDL. Since modified lipoproteins have pro-inflammatory properties, it is unclear whether SR-BI activated by modified HDL has an anti- or pro-inflammatory effect. Glycated HDL induced NF-κB activation and cytokine production in macrophages in vitro, suggesting a pro-inflammatory effect for modified HDL. Moreover, inhibition of SR-BI function or expression potentiated glycated HDL-induced TNF-α production, suggesting an anti-inflammatory effect for SR-BI. In conclusion, SR-BI plays an important function in regulating HDL-mediated anti-inflammatory response in macrophages.

Scavenger receptor class B, type I (Scarb1) deficiency promotes osteoblastogenesis but stunts terminal osteocyte differentiation

Scavenger receptor class B type I (SR-BI), the Scarb1 gene product, is a high-density lipoprotein (HDL) receptor which was shown to influence bone metabolism. Its absence in mice is associated with alterations of the glucocorticoid/adrenocorticotropic hormone axis, and translated in high bone mass and enhanced bone formation. Since the cellular alterations underlying the enhanced bone formation remain unknown, we investigated Scarb1-deficient marrow stromal cells (MSC) behavior in vitro. No difference in HDL3, cholesteryl ester (CE) or estradiol (E) association/binding was measured between Scarb1-null and wild-type (WT) cells. Scarb1 genic expression was down-regulated twofold following osteogenic treatment. Neither WT nor null cell proliferation was influenced by HDL3 exposure whereas this condition decreased genic expression of osteoblastic marker osterix (Sp7), and osteocyte markers sclerostin (Sost) and dentin matrix protein 1 (Dmp1) independently of genotype. Sost and Dmp1 basal expression in null cells was 40% and 50% that of WT cells; accordingly, osteocyte density was 20% lower in vertebrae from Scarb1-null mice. Genic expression of co-receptors for Wnt signaling, namely LDL-related protein (Lrp) 5 and Lrp8, was increased, respectively, by two- and threefold, and of transcription target-genes axis inhibition protein 2 (Axin2) and lymphoid enhancer-binding factor 1 (Lef1) over threefold. Gene expression of Wnt signaling agonist Wnt5a and of the antagonist dickkopfs-related protein 1 (Dkk1) were found to be increased 10- to 20-fold in null MSC. These data suggest alterations of Wnt pathways in Scarb1-deficient MSC potentially explaining their enhanced function, hence contributing to the high bone mass observed in these mice.

High Density Lipoprotein: Assembly, Structure, Cargo, and Functions

Cardiovascular disease (CVD) is the leading cause of death globally. For close to four decades, we have known that high density lipoprotein (HDL) levels are inversely correlated with the risk of CVD. HDL is a complex particle that consists of proteins, phospholipids, and cholesterol and has the ability to carry micro-RNAs. HDL is constantly undergoing remodelling throughout its life-span and carries out many functions. This review summarizes many of the different aspects of HDL from its assembly, the receptors it interacts with, along with the functions it performs and how it can be altered in disease. While HDL is a key cholesterol efflux particle, this review highlights the many other important functions of HDL in the innate immune system and details the potential therapeutic uses of HDL outside of CVD.

Hepatitis C virus entry

Hepatitis C virus (HCV) is a hepatotropic virus and a major cause of chronic hepatitis and liver disease worldwide. Initial interactions between HCV virions and hepatocytes are required for productive viral infection and initiation of the viral life cycle. Furthermore, HCV entry contributes to the tissue tropism and species specificity of this virus. The elucidation of these interactions is critical, not only to understand the pathogenesis of HCV infection, but also to design efficient antiviral strategies and vaccines. This review summarizes our current knowledge of the host factors required for the HCV-host interactions during HCV binding and entry, our understanding of the molecular mechanisms underlying HCV entry into target cells, and the relevance of HCV entry for the pathogenesis of liver disease, antiviral therapy, and vaccine development.

Regulation of cholesterol homeostasis

Hypercholesterolemia is an important risk factor for cardiovascular disease. It is caused by a disturbed balance between cholesterol secretion into the blood versus uptake. The pathways involved are regulated via a complex interplay of enzymes, transport proteins, transcription factors and non-coding RNA's. The last two decades insight into underlying mechanisms has increased vastly but there are still a lot of unknowns, particularly regarding intracellular cholesterol transport. After decades of concentration on the liver, in recent years the intestine has come into focus as an important control point in cholesterol homeostasis. This review will discuss current knowledge of cholesterol physiology, with emphasis on cholesterol absorption, cholesterol synthesis and fecal excretion, and new (possible) therapeutic options for hypercholesterolemia.

Intestinal SR-BI does not impact cholesterol absorption or transintestinal cholesterol efflux in mice

Reverse cholesterol transport (RCT) can proceed through the classic hepatobiliary route or through the nonbiliary transintestinal cholesterol efflux (TICE) pathway. Scavenger receptor class B type I (SR-BI) plays a critical role in the classic hepatobiliary route of RCT. However, the role of SR-BI in TICE has not been studied. To examine the role of intestinal SR-BI in TICE, sterol balance was measured in control mice and mice transgenically overexpressing SR-BI in the proximal small intestine (SR-BI(hApoCIII-ApoAIV-Tg)). SR-BI(hApoCIII-ApoAIV-Tg) mice had significantly lower plasma cholesterol levels compared with wild-type controls, yet SR-BI(hApoCIII-ApoAIV-Tg) mice had normal fractional cholesterol absorption and fecal neutral sterol excretion. Both in the absence or presence of ezetimibe, intestinal SR-BI overexpression had no impact on the amount of cholesterol excreted in the feces. To specifically study effects of intestinal SR-BI on TICE we crossed SR-BI(hApoCIII-ApoAIV-Tg) mice into a mouse model that preferentially utilized the TICE pathway for RCT (Niemann-Pick C1-like 1 liver transgenic), and likewise found no alterations in cholesterol absorption or fecal sterol excretion. Finally, mice lacking SR-BI in all tissues also exhibited normal cholesterol absorption and fecal cholesterol disposal. Collectively, these results suggest that SR-BI is not rate limiting for intestinal cholesterol absorption or for fecal neutral sterol loss through the TICE pathway.

Role of the gut in lipid homeostasis

Intestinal lipid transport plays a central role in fat homeostasis. Here we review the pathways regulating intestinal absorption and delivery of dietary and biliary lipid substrates, principally long-chain fatty acid, cholesterol, and other sterols. We discuss the regulation and functions of CD36 in fatty acid absorption, NPC1L1 in cholesterol absorption, as well as other lipid transporters including FATP4 and SRB1. We discuss the pathways of intestinal sterol efflux via ABCG5/G8 and ABCA1 as well as the role of the small intestine in high-density lipoprotein (HDL) biogenesis and reverse cholesterol transport. We review the pathways and genetic regulation of chylomicron assembly, the role of dominant restriction points such as microsomal triglyceride transfer protein and apolipoprotein B, and the role of CD36, l-FABP, and other proteins in formation of the prechylomicron complex. We will summarize current concepts of regulated lipoprotein secretion (including HDL and chylomicron pathways) and include lessons learned from families with genetic mutations in dominant pathways (i.e., abetalipoproteinemia, chylomicron retention disease, and familial hypobetalipoproteinemia). Finally, we will provide an integrative view of intestinal lipid homeostasis through recent findings on the role of lipid flux and fatty acid signaling via diverse receptor pathways in regulating absorption and production of satiety factors.

Respective contributions of intestinal Niemann-Pick C1-like 1 and scavenger receptor class B type I to cholesterol and tocopherol uptake: in vivo v. in vitro studies

The intestinal absorption of cholesterol and lipid micronutrients such as vitamin E has been shown to share some common pathways. The present study aims to further compare the uptake of cholesterol ([3H]cholesterol v. 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3-ol (NBD-cholesterol)) and tocopherol in Caco-2 TC-7 cells and in mouse intestine, with special focus on the respective roles of scavenger receptor class B type I (SR-BI) and Niemann-Pick C1-like 1 (NPC1L1). Conversely to NBD-cholesterol, the uptakes of [3H]cholesterol and tocopherol by Caco-2 cells were impaired by both block lipid transport-1 and ezetimibe, which inhibit SR-BI and NPC1L1, respectively. These inhibitions occurred only when cholesterol or tocopherol was delivered to cells included in micelles that contained biliary acid and at least oleic acid as a lipid. In vivo, after 2 h of digestion in mice, the uptake of the two cholesterol analogues and of tocopherol all showed distinct patterns along the duodenum-jejunum axis. [3H]Cholesterol uptake, which correlated closely to NPC1L1 mRNA expression in wild-type (wt) mice, was strongly inhibited by ezetimibe. Intestinal SR-BI overexpression did not change NPC1L1 expression and led to a significant increase in [3H]cholesterol uptake in the distal jejunum. Conversely, neither ezetimibe treatment nor SR-BI overexpression had an effect on NBD-cholesterol uptake. However, in contrast with SR-BI mRNA expression, tocopherol absorption increased strongly up to the distal jejunum in wt mice where it was specifically inhibited by ezetimibe, and was increased in the proximal intestine of intestinal SR-BI-overexpressing mice. Thus, cholesterol and tocopherol uptakes share common pathways in cell culture models, but display different in vivo absorption patterns associated with distinct contributions of SR-BI and NPC1L1.

Targeting the ABCB4 gene to control cholesterol homeostasis

INTRODUCTION

Multidrug resistance 3 (MDR3) P-glycoprotein is a lipid floppase that is encoded by the ATP-binding cassette sub-family B member 4 (ABCB4) gene and plays a crucial role in proper bile formation by transporting phosphatidylcholine across the canalicular plasma membrane of the hepatocyte into bile. The relevance of this function is underscored by the severe pathology that develops in patients with ABCB4 deficiency. This deficiency leads to the destruction of hepatocytes and cholangiocytes by bile salts, because their cytolytic action is not reduced by formation of mixed micelles with phospholipid.

AREAS COVERED

Evidence that phospholipid secretion into bile is also essential for biliary cholesterol secretion as cholesterol dissolves much better in mixed micelles of bile salts and phospholipid than in pure bile salt micelles. As a consequence, net biliary cholesterol secretion depends on the amount of phospholipid secreted and hence, the expression of ABCB4 indirectly determines biliary cholesterol output.

EXPERT OPINION

It can be argued that upregulation of the ABCB4 gene expression may not only be beneficial for liver pathology in patients with partial ABCB4 deficiency, but also for the prevention of gallstone formation and optimal cholesterol disposition in a much larger population.

Exoplasmic cysteine Cys384 of the HDL receptor SR-BI is critical for its sensitivity to a small-molecule inhibitor and normal lipid transport activity

The HDL receptor, scavenger receptor, class B, type I (SR-BI), is a homooligomeric cell surface glycoprotein that controls HDL structure and metabolism by mediating the cellular selective uptake of lipids, mainly cholesteryl esters, from HDL. The mechanism underlying SR-BI-mediated lipid transfer, which differs from classic receptor-mediated endocytosis, involves a two-step process (binding followed by lipid transport) that is poorly understood. Our previous structure/activity analysis of the small-molecule inhibitor blocker of lipid transport 1 (BLT-1), which potently (IC(50) ∼ 50 nM) blocks SR-BI-mediated lipid transport, established that the sulfur in BLT-1's thiosemicarbazone moiety was essential for activity. Here we show that BLT-1 is an irreversible inhibitor of SR-BI, raising the possibility that cysteine(s) in SR-BI interact with BLT-1. Mass spectrometric analysis of purified SR-BI showed two of its six exoplasmic cysteines have free thiol groups (Cys251 and Cys384). Converting Cys384 (but not Cys251) to serine resulted in complete BLT-1 insensitivity, establishing that the unique molecular target of BLT-1 inhibition of cellular SR-BI dependent lipid transport is SR-BI itself. The C384S substitution reduced the receptor's intrinsic lipid uptake activity by approximately 60% without dramatically altering its surface expression, homooligomerization, or HDL binding. Thus, a small-molecule screening approach identified a key residue in SR-BI involved in lipid transport, providing a powerful springboard into the analyses of the structure and mechanism of SR-BI, and highlighting the power of this approach for such analyses.

Niemann-pick C1-like 1 (NPC1L1) protein in intestinal and hepatic cholesterol transport

Increased blood cholesterol is an independent risk factor for atherosclerotic cardiovascular disease. Cholesterol homeostasis in the body is controlled mainly by endogenous synthesis, intestinal absorption, and hepatic excretion. Niemann-Pick C1-Like 1 (NPC1L1) is a polytopic transmembrane protein localized at the apical membrane of enterocytes and the canalicular membrane of hepatocytes. It functions as a sterol transporter to mediate intestinal cholesterol absorption and counter-balances hepatobiliary cholesterol excretion. NPC1L1 is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that is widely used in treating hypercholesterolemia. Recent findings suggest that NPC1L1 deficiency or ezetimibe treatment also prevents diet-induced hepatic steatosis and obesity in addition to reducing blood cholesterol. Future studies should focus on molecular mechanisms underlying NPC1L1-dependent cholesterol transport and elucidation of how a cholesterol transporter modulates the pathogenesis of metabolic diseases.

Aldosterone production in human adrenocortical cells is stimulated by high-density lipoprotein 2 (HDL2) through increased expression of aldosterone synthase (CYP11B2)

Adrenal aldosterone production is regulated by physiological agonists at the level of early and late rate-limiting steps. Numerous studies have focused on the role of lipoproteins including high-density lipoprotein (HDL) as cholesterol providers in this process; however, recent research suggests that HDL can also act as a signaling molecule. Herein, we used the human H295R adrenocortical cell model to study the effects of HDL on adrenal aldosterone production and CYP11B2 expression. HDL, especially HDL2, stimulated aldosterone synthesis by increasing expression of CYP11B2. HDL treatment increased CYP11B2 mRNA in both a concentration- and time-dependent manner, with a maximal 19-fold increase (24 h, 250 μg/ml of HDL). Effects of HDL on CYP11B2 were not additive with natural agonists including angiotensin II or K(+). HDL effects were likely mediated by a calcium signaling cascade, because a calcium channel blocker and a calmodulin kinase inhibitor abolished the CYP11B2-stimulating effects. Of the two subfractions of HDL, HDL2 was more potent than HDL3 in stimulating aldosterone and CYP11B2. Further studies are needed to identify the active components of HDL, which regulate aldosterone production.

Phytosterols and phytosterolemia: gene-diet interactions

Phytosterol intake is recommended as an adjunctive therapy for hypercholesterolemia, and plant sterols/stanols can reduce cholesterol absorption at the intestinal lumen through the Niemann-Pick C1 Like 1 (NPC1L1) transporter pathway by competitive solubilization in mixed micelles. Phytosterol absorption is of less magnitude than cholesterol and is preferably secreted in the intestinal lumen by ABCG5/G8 transporters. Therefore, plasma levels of plant sterols/stanols are negligible compared with cholesterol, under an ordinary diet. The mechanisms of cholesterol and plant sterols absorption and the whole-body pool of sterols are discussed in this chapter. There is controversy about treatment with statins inducing further increase in plasma non-cholesterol sterols raising concerns about the safety of supplementation of plant sterols to such drugs. In addition, increase in plant sterols has also been reported upon consumption of plant sterol-enriched foods, regardless of other treatments. Rare mutations on ABCG5/G8 transporters affecting cholesterol/non-cholesterol extrusion, causing sitosterolemia with xanthomas and premature atheroslerotic disease are now known, and cholesterol/plant sterols absorption inhibitor, ezetimibe, emerges as the drug that reduces phytosterolemia and promotes xanthoma regression. On the other hand, common polymorphisms affecting the NPC1L1 transporter can interfere with the action of ezetimibe. Gene-diet interactions participate in this intricate network modulating the expression of genetic variants on specific phenotypes and can also affect the individual response to the hypolipidemic treatment. These very interesting aspects promoted a great deal of research in the field.

Adipocyte modulation of high-density lipoprotein cholesterol

BACKGROUND

Adipose harbors a large depot of free cholesterol. However, a role for adipose in cholesterol lipidation of high-density lipoprotein (HDL) in vivo is not established. We present the first evidence that adipocytes support transfer of cholesterol to HDL in vivo as well as in vitro and implicate ATP-binding cassette subfamily A member 1 (ABCA1) and scavenger receptor class B type I (SR-BI), but not ATP-binding cassette subfamily G member 1 (ABCG1), cholesterol transporters in this process.

METHODS AND RESULTS

Cholesterol efflux from wild-type, ABCA1(-/-), SR-BI(-/-), and ABCG1(-/-) adipocytes to apolipoprotein A-I (apoA-I) and HDL3 were measured in vitro. 3T3L1 adipocytes, labeled with (3)H-cholesterol, were injected intraperitoneally into wild-type, apoA-I transgenic, and apoA-I(-/-) mice, and tracer movement onto plasma HDL was monitored. Identical studies were performed with labeled wild-type, ABCA1(-/-), or SR-BI(-/-) mouse embryonic fibroblast adipocytes. The effect of tumor necrosis factor-alpha on transporter expression and cholesterol efflux was monitored during adipocyte differentiation. Cholesterol efflux to apoA-I and HDL3 was impaired in ABCA1(-/-) and SR-BI(-/-) adipocytes, respectively, with no effect observed in ABCG1(-/-) adipocytes. Intraperitoneal injection of labeled 3T3L1 adipocytes resulted in increased HDL-associated (3)H-cholesterol in apoA-I transgenic mice but reduced levels in apoA-I(-/-) animals. Intraperitoneal injection of labeled ABCA1(-/-) or SR-BI(-/-) adipocytes reduced plasma counts relative to their respective controls. Tumor necrosis factor-alpha reduced both ABCA1 and SR-BI expression and impaired cholesterol efflux from partially differentiated adipocytes.

CONCLUSIONS

These data suggest a novel metabolic function of adipocytes in promoting cholesterol transfer to HDL in vivo and implicate adipocyte SR-BI and ABCA1, but not ABCG1, in this process. Furthermore, adipocyte modulation of HDL may be impaired in adipose inflammatory disease states such as type 2 diabetes mellitus.

Protein mediators of sterol transport across intestinal brush border membrane

Dysregulation of cholesterol balance contributes significantly to atherosclerotic cardiovascular disease (ASCVD), the leading cause of death in the United States. The intestine has the unique capability to act as a gatekeeper for entry of cholesterol into the body, and inhibition of intestinal cholesterol absorption is now widely regarded as an attractive non-statin therapeutic strategy for ASCVD prevention. In this chapter we discuss the current state of knowledge regarding sterol transport across the intestinal brush border membrane. The purpose of this work is to summarize substantial progress made in the last decade in regards to protein-mediated sterol trafficking, and to discuss this in the context of human disease.

Intestinal lipid absorption

Our knowledge of the uptake and transport of dietary fat and fat-soluble vitamins has advanced considerably. Researchers have identified several new mechanisms by which lipids are taken up by enterocytes and packaged as chylomicrons for export into the lymphatic system or clarified the actions of mechanisms previously known to participate in these processes. Fatty acids are taken up by enterocytes involving protein-mediated as well as protein-independent processes. Net cholesterol uptake depends on the competing activities of NPC1L1, ABCG5, and ABCG8 present in the apical membrane. We have considerably more detailed information about the uptake of products of lipid hydrolysis, the active transport systems by which they reach the endoplasmic reticulum, the mechanisms by which they are resynthesized into neutral lipids and utilized within the endoplasmic reticulum to form lipoproteins, and the mechanisms by which lipoproteins are secreted from the basolateral side of the enterocyte. apoB and MTP are known to be central to the efficient assembly and secretion of lipoproteins. In recent studies, investigators found that cholesterol, phospholipids, and vitamin E can also be secreted from enterocytes as components of high-density apoB-free/apoAI-containing lipoproteins. Several of these advances will probably be investigated further for their potential as targets for the development of drugs that can suppress cholesterol absorption, thereby reducing the risk of hypercholesterolemia and cardiovascular disease.

Influence of class B scavenger receptors on cholesterol flux across the brush border membrane and intestinal absorption

To learn more about how the step of cholesterol uptake into the brush border membrane (BBM) of enterocytes influences overall cholesterol absorption, we measured cholesterol absorption 4 and 24 h after administration of an intragastric bolus of radioactive cholesterol in mice with scavenger receptor class B, type 1 (SR-BI) and/or cluster determinant 36 (CD36) deleted. The cholesterol absorption efficiency is unaltered by deletion of either one or both of the receptors. In vitro determinations of the cholesterol uptake specific activity of the BBM from the mice reveal that the scavenger receptors facilitate cholesterol uptake into the proximal BBM. It follows that cholesterol uptake into the BBM is not normally rate-limiting for the cholesterol absorption process in vivo; a subsequent step, such as NPC1L1-mediated transfer from the BBM into the interior of the enterocyte, is rate-limiting. The absorption of dietary cholesterol after 4 h in mice lacking SR-BI and/or CD36 and fed a high-fat/high-cholesterol diet is delayed to more distal regions of the small intestine. This effect probably arises because ATP binding cassette half transporters G5 and G8-mediated back flux of cholesterol from the BBM to the lumen of the small intestine limits absorption and causes the local cholesterol uptake facilitated by SR-BI and CD36 to become rate-limiting under this dietary condition.

Mechanisms and consequences of impaired lipid trafficking in Niemann-Pick type C1-deficient mammalian cells

Niemann-Pick C disease is a fatal progressive neurodegenerative disorder caused in 95% of cases by mutations in the NPC1 gene; the remaining 5% of cases result from mutations in the NPC2 gene. The major biochemical manifestation of NPC1 deficiency is an abnormal sequestration of lipids, including cholesterol and glycosphingolipids, in late endosomes/lysosomes (LE/L) of all cells. In this review, we summarize the current knowledge of the NPC1 protein in mammalian cells with particular focus on how defects in NPC1 alter lipid trafficking and neuronal functions. NPC1 is a protein of LE/L and is predicted to contain thirteen transmembrane domains, five of which constitute a sterol-sensing domain. The precise function of NPC1, and the mechanism by which NPC1 and NPC2 (both cholesterol binding proteins) act together to promote the movement of cholesterol and other lipids out of the LE/L, have not yet been established. Recent evidence suggests that the sequestration of cholesterol in LE/L of cells of the brain (neurons and glial cells) contributes to the widespread death and dysfunction of neurons in the brain. Potential therapies include treatments that promote the removal of cholesterol and glycosphingolipids from LE/L. Currently, the most promising approach for extending life-span and improving the quality of life for NPC patients is a combination of several treatments each of which individually modestly slows disease progression.

Cholesterol and non-cholesterol sterol transporters: ABCG5, ABCG8 and NPC1L1: a review

1. Whole-body sterol (cholesterol and xenosterol) balance is delicately regulated by the gastrointestinal tract and liver, which control sterol absorption and excretion, respectively, in addition to the contribution to the cholesterol pool by whole-body cholesterol synthesis. In the past ten years enormous strides have been made not only in establishing that specific transporters mediate the entry and exit of sterols and how these may regulate selective sterol access to the body pools, but also in how these pathways operate to integrate these physiological pathways. 2. The entry of sterols from the gastrointestinal and biliary canalicular lumen into the body is mediated by NPC1L1, which was discovered by a novel method, via a genomics-bioinformatics approach. 3. Identification of the genetic basis responsible for causing sitosterolaemia, characterized by plant sterol accumulation, led to the identification of two half-transporters (ABCG5 and ABCG8) that normally efflux plant sterols (and cholesterol) into the intestinal and biliary lumen for faecal excretion. 4. The objective of this review is to provide up-to-date knowledge on genomics, proteomics and function of these two transporter systems.

Modulation of intestinal cholesterol absorption by high glucose levels: impact on cholesterol transporters, regulatory enzymes, and transcription factors

Growing evidence suggests that the small intestine may contribute to excessive postprandial lipemia, which is highly prevalent in insulin-resistant/Type 2 diabetic individuals and substantially increases the risk of cardiovascular disease. The aim of the present study was to determine the role of high glucose levels on intestinal cholesterol absorption, cholesterol transporter expression, enzymes controlling cholesterol homeostasis, and the status of transcription factors. To this end, we employed highly differentiated and polarized cells (20 days of culture), plated on permeable polycarbonate filters. In the presence of [(14)C]cholesterol, glucose at 25 mM stimulated cholesterol uptake compared with Caco-2/15 cells supplemented with 5 mM glucose (P < 0.04). Because combination of 5 mM glucose with 20 mM of the structurally related mannitol or sorbitol did not change cholesterol uptake, we conclude that extracellular glucose concentration is uniquely involved in the regulation of intestinal cholesterol transport. The high concentration of glucose enhanced the protein expression of the critical cholesterol transporter NPC1L1 and that of CD36 (P < 0.02) and concomitantly decreased SR-BI protein mass (P < 0.02). No significant changes were observed in the protein expression of ABCA1 and ABCG8, which act as efflux pumps favoring cholesterol export out of absorptive cells. At the same time, 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity was decreased (P < 0.007), whereas ACAT activity remained unchanged. Finally, increases were noted in the transcription factors LXR-alpha, LXR-beta, PPAR-beta, and PPAR-gamma along with a drop in the protein expression of SREBP-2. Collectively, our data indicate that glucose at high concentrations may regulate intestinal cholesterol transport and metabolism in Caco-2/15 cells, thus suggesting a potential influence on the cholesterol absorption process in Type 2 diabetes.

Metabolomic strategies to identify tissue-specific effects of cardiovascular drugs

BACKGROUND

The number of patients eligible for cardiovascular therapies in general is forecast to increase substantially in the coming decades. However, the current list of potential future cardiovascular blockbuster drugs is alarmingly short. There is thus a clear need for innovative strategies to increase the efficiency of drug development pipelines by establishing new sensitive biomarkers to monitor drug efficacy and safety in the context of complexity of lipoprotein metabolism targeted by the cardiovascular drugs.

METHODS

Metabolomics is a discipline dedicated to the systematic study of small molecules in cells, tissues and biofluids. Since lipids (including cholesterol), as well as other metabolites, are key constituents of lipoprotein particles and are thus part of the complex lipoprotein metabolism that includes exchange of lipids and metabolites with peripheral tissues, cardiovascular drug safety and efficacy needs to be addressed in the context of systemic lipid metabolism.

RESULTS/CONCLUSION

Metabolomics, lipidomics in particular, is expected to make an important impact on the discovery and development of cardiovascular therapies.