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

Efficacy and underlying mechanisms of berberine against lipid metabolic diseases: a review

Lipid-lowering therapy is an important tool for the treatment of lipid metabolic diseases, which are increasing in prevalence. However, the failure of conventional lipid-lowering drugs to achieve the desired efficacy in some patients, and the side-effects of these drug regimens, highlight the urgent need for novel lipid-lowering drugs. The liver and intestine are important in the production and removal of endogenous and exogenous lipids, respectively, and have an important impact on circulating lipid levels. Elevated circulating lipids predisposes an individual to lipid deposition in the vascular wall, affecting vascular function. Berberine (BBR) modulates liver lipid production and clearance by regulating cellular targets such as cluster of differentiation 36 (CD36), acetyl-CoA carboxylase (ACC), microsomal triglyceride transfer protein (MTTP), scavenger receptor class B type 1 (SR-BI), low-density lipoprotein receptor (LDLR), and ATP-binding cassette transporter A1 (ABCA1). It influences intestinal lipid synthesis and metabolism by modulating gut microbiota composition and metabolism. Finally, BBR maintains vascular function by targeting proteins such as endothelial nitric oxide synthase (eNOS) and lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1). This paper elucidates and summarizes the pharmacological mechanisms of berberine in lipid metabolic diseases from a multi-organ (liver, intestine, and vascular system) and multi-target perspective.

Resveratrol intervention attenuates chylomicron secretion via repressing intestinal FXR-induced expression of scavenger receptor SR-B1

Two common features of dietary polyphenols have hampered our mechanistic understanding of their beneficial effects for decades: targeting multiple organs and extremely low bioavailability. We show here that resveratrol intervention (REV-I) in high-fat diet (HFD)-challenged male mice inhibits chylomicron secretion, associated with reduced expression of jejunal but not hepatic scavenger receptor class B type 1 (SR-B1). Intestinal mucosa-specific SR-B1^-/- mice on HFD-challenge exhibit improved lipid homeostasis but show virtually no further response to REV-I. SR-B1 expression in Caco-2 cells cannot be repressed by pure resveratrol compound while fecal-microbiota transplantation from mice on REV-I suppresses jejunal SR-B1 in recipient mice. REV-I reduces fecal levels of bile acids and activity of fecal bile-salt hydrolase. In Caco-2 cells, chenodeoxycholic acid treatment stimulates both FXR and SR-B1. We conclude that gut microbiome is the primary target of REV-I, and REV-I improves lipid homeostasis at least partially via attenuating FXR-stimulated gut SR-B1 elevation.

From Congenital Disorders of Fat Malabsorption to Understanding Intra-Enterocyte Mechanisms Behind Chylomicron Assembly and Secretion

During the last two decades, a large body of information on the events responsible for intestinal fat digestion and absorption has been accumulated. In particular, many groups have extensively focused on the absorptive phase in order to highlight the critical "players" and the main mechanisms orchestrating the assembly and secretion of chylomicrons (CM) as essential vehicles of alimentary lipids. The major aim of this article is to review understanding derived from basic science and clinical conditions associated with impaired packaging and export of CM. We have particularly insisted on inborn metabolic pathways in humans as well as on genetically modified animal models (recapitulating pathological features). The ultimate goal of this approach is that "experiments of nature" and in vivo model strategy collectively allow gaining novel mechanistic insight and filling the gap between the underlying genetic defect and the apparent clinical phenotype. Thus, uncovering the cause of disease contributes not only to understanding normal physiologic pathway, but also to capturing disorder onset, progression, treatment and prognosis.

Ezetimibe impairs transcellular lipid trafficking and induces large lipid droplet formation in intestinal absorptive epithelial cells

Ezetimibe inhibits Niemann-Pick C1-like 1 (NPC1L1) protein, which mediates intracellular cholesterol trafficking from the brush border membrane to the endoplasmic reticulum, where chylomicron assembly takes place in enterocytes or in the intestinal absorptive epithelial cells. Cholesterol is a minor lipid constituent of chylomicrons; however, whether or not a shortage of cholesterol attenuates chylomicron assembly is unknown. The aim of this study was to examine the effect of ezetimibe, a potent NPC1L1 inhibitor, on trans-epithelial lipid transport, and chylomicron assembly and secretion in enterocytes. Caco-2 cells, an absorptive epithelial model, grown onto culture inserts were given lipid micelles from the apical side, and chylomicron-like triacylglycerol-rich lipoprotein secreted basolaterally were analyzed after a 24-h incubation period in the presence of ezetimibe up to 50 μM. The secretion of lipoprotein and apolipoprotein B48 were reduced by adding ezetimibe (30% and 34%, respectively). Although ezetimibe allowed the cells to take up cholesterol normally, the esterification was abolished. Meanwhile, oleic acid esterification was unaffected. Moreover, ezetimibe activated sterol regulatory element-binding protein 2 by approximately 1.5-fold. These results suggest that ezetimibe limited cellular cholesterol mobilization required for lipoprotein assembly. In such conditions, large lipid droplet formation in Caco-2 cells and the enterocytes of mice were induced, implying that unprocessed triacylglycerol was sheltered in these compartments. Although ezetimibe did not reduce the post-prandial lipid surge appreciably in triolein-infused mice, the results of the present study indicated that pharmacological actions of ezetimibe may participate in a novel regulatory mechanism for the efficient chylomicron assembly and secretion.

Volanesorsen in the Treatment of Familial Chylomicronemia Syndrome or Hypertriglyceridaemia: Design, Development and Place in Therapy

Severe hypertriglyceridaemia is associated with pancreatitis and chronic pancreatitis-induced diabetes. Familial chylomicronaemia syndrome (FCS) is a rare autosomal recessive disorder of lipid metabolism characterised by high levels of triglycerides (TGs) due to failure of chylomicron clearance. It causes repeated episodes of severe abdominal pain, fatigue and attacks of acute pancreatitis. There are few current options for its long-term management. The only universal long-term therapy is restriction of total dietary fat intake to <10-15% of daily calories (15 to 20g per day). Many patients have been treated with fibrates and statins with a variable response, but many remain susceptible to pancreatitis. Other genetic syndromes associated with hypertriglyceridaemia include familial partial lipodystrophy (FPLD). Targeting apolipoprotein C3 (apoC3) offers the ability to increase clearance of chylomicrons and other triglyceride-rich lipoproteins. Volanesorsen is an antisense oligonucleotide (ASO) inhibitor of apoC3, which reduces TG levels by 70–80% which has been shown also to reduce rates of pancreatitis and improve well-being in FCS and reduce TGs and improve insulin resistance in FPLD. It is now undergoing licensing and payer reviews. Further developments of antisense technology including small interfering RNA therapy to apoC3 as well as other approaches to modulating triglycerides are in development for this rare disorder.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

Mechanisms of Carotenoid Intestinal Absorption: Where Do We Stand?

A growing literature is dedicated to the understanding of carotenoid beneficial health effects. However, the absorption process of this broad family of molecules is still poorly understood. These highly lipophilic plant metabolites are usually weakly absorbed. It was long believed that β-carotene absorption (the principal provitamin A carotenoid in the human diet), and thus all other carotenoid absorption, was driven by passive diffusion through the brush border of the enterocytes. The identification of transporters able to facilitate carotenoid uptake by the enterocytes has challenged established statements. After a brief overview of carotenoid metabolism in the human upper gastrointestinal tract, a focus will be put on the identified proteins participating in the transport and the metabolism of carotenoids in intestinal cells and the regulation of these processes. Further progress in the understanding of the molecular mechanisms regulating carotenoid intestinal absorption is still required to optimize their bioavailability and, thus, their health effects.

Insulin increases cholesterol uptake, lipid droplet content, and apolipoprotein B secretion in CaCo-2 cells by upregulating SR-BI via a PI3K, AKT, and mTOR-dependent pathway

The actions of insulin on intestinal cholesterol absorption and lipoprotein secretion are not well understood. Herein, we determined the effects of insulin on the levels of cholesterol transporter scavenger receptor, class B, type I (SR-BI), cellular cholesterol uptake, intracellular lipid accumulation, and lipoprotein secretion in a cellular model of human intestinal epithelium.

METHODS

CaCo-2 cells were cultured to postconfluency in Transwell filters and stimulated with glucose (25 mM) in the presence or absence of insulin (100 nM) at their basolateral surface. SR-BI mRNA and protein levels were quantified by quantitative reverse transcription-PCR and immunoblot, respectively. Polarized localization of SR-BI was determined by cell surface proteins biotinylation and streptavidin precipitation. Activities of PI3K, AKT, mTOR, and SR-BI were pharmacologically antagonized. Cholesterol uptake was assessed by NBD-cholesterol incorporation. Apolipoprotein (apo) B concentration was quantified by ELISA. Subcellular localization of neutral lipids (BODIPY) and SR-BI (immunofluorescence) was determined by confocal microscopy.

RESULTS

In polarized CaCo-2 cells, insulin increased SR-BI at the mRNA and protein levels. SR-BI was exclusively present at apical cell surface, as indicated by biotinylation and confocal microscopy analysis. Glucose did not modify SR-BI abundance or subcellular localization. Effects of insulin on SR-BI levels were abrogated by PI3K, AKT, or mTOR pharmacological antagonism. Cholesterol uptake, neutral lipid abundance, and apo B secretion were increased by insulin in CaCo-2 cells, and these effects were prevented by SR-BI pharmacological antagonism with block lipid transport-1.

CONCLUSIONS

insulin promotes cholesterol uptake, intracellular lipid store, and apo B-containing lipoproteins secretion by SR-BI-dependent mechanisms in a model of human intestinal epithelium.

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.

Developments in intestinal cholesterol transport and triglyceride absorption

PURPOSE OF REVIEW

To discuss recent advances in research focused on intestinal lipid handling.

RECENT FINDINGS

An important strategy in reducing atherosclerosis and risk of cardiovascular events is to increase the rate of reverse cholesterol transport, including its final step; cholesterol excretion from the body. The rate of removal is determined by a complex interplay between the factors involved in regulation of intestinal cholesterol absorption. One of these factors is a process known as transintestinal cholesterol excretion. This pathway comprises transport of cholesterol directly from the blood, through the enterocyte, into the intestinal lumen. In humans, this pathway accounts for 35% of cholesterol excretion in the feces. Mechanistic studies in mice revealed that, activation of the bile acid receptor farnesoid X receptor increases cholesterol removal via the transintestinal cholesterol excretion pathway as well as decreases plasma cholesterol and triglyceride providing an interesting target for treatment of dyslipidemia in humans. The physical chemical properties of bile acids are under control of farnesoid X receptor and determine intestinal cholesterol and triglyceride solubilization as well as absorption, providing a direct link between these two important factors in the pathogenesis of cardiovascular disease. Besides bile acids, intestinal phospholipids are important for luminal lipid solubilization. Interestingly, phospholipid remodeling through LPCAT3 was shown to be pivotal for uptake of fatty acids by enterocytes, which may provide a mechanistic handle for therapeutic intervention.

SUMMARY

The importance of the intestine in control of cholesterol and triglyceride homeostasis is increasingly recognized. Recently, novel factors involved in regulation of cholesterol excretion and intestinal triglyceride and fatty acid uptake have been reported and are discussed in this short review.

Investigational therapies for hypercholesterolemia

INTRODUCTION

Cardiovascular morbidity and mortality are of increasing concern, not only to patients but also to the health care profession and service providers. The preventative benefit of treatment of dyslipidaemia is unquestioned but there is a large, so far unmet need to improve clinical outcome. There are exciting new discoveries of targets that may translate into improved clinical outcome. Areas covered: This review highlights some new pathways in cholesterol and triglyceride metabolism and examines new targets, new drugs and new molecules. The review includes the results of recent trials of relatively new drugs that have shown benefit in cardiovascular endpoint outcomes, drugs that have been licenced without endpoint trials yet available and new drugs that have not yet been licenced but have produced exciting results in animal studies and some in early phase 2 human studies. Expert opinion: The new areas that have been discovered as the cause of dyslipidaemia have opened up a host of new targets for new drugs including antisense RNA's, microRNA's and human monoclonal antibodies. The plethora of new targets and new drugs has made it an extraordinarily exciting time in the development of therapeutics to combat atherosclerosis.

Role of the Enterocyte in Fructose-Induced Hypertriglyceridaemia

Dietary fructose has been linked to an increased post-prandial triglyceride (TG) level; which is an established independent risk factor for cardiovascular disease. Although much research has focused on the effects of fructose consumption on liver-derived very-low density lipoprotein (VLDL); emerging evidence also suggests that fructose may raise post-prandial TG levels by affecting the metabolism of enterocytes of the small intestine. Enterocytes have become well recognised for their ability to transiently store lipids following a meal and to thus control post-prandial TG levels according to the rate of chylomicron (CM) lipoprotein synthesis and secretion. The influence of fructose consumption on several aspects of enterocyte lipid metabolism are discussed; including de novo lipogenesis; apolipoprotein B48 and CM-TG production; based on the findings of animal and human isotopic tracer studies. Methodological issues affecting the interpretation of fructose studies conducted to date are highlighted; including the accurate separation of CM and VLDL. Although the available evidence to date is limited; disruption of enterocyte lipid metabolism may make a meaningful contribution to the hypertriglyceridaemia often associated with fructose consumption.

High Dietary Fructose: Direct or Indirect Dangerous Factors Disturbing Tissue and Organ Functions

High dietary fructose is a major contributor to insulin resistance and metabolic syndrome, disturbing tissue and organ functions. Fructose is mainly absorbed into systemic circulation by glucose transporter 2 (GLUT2) and GLUT5, and metabolized in liver to produce glucose, lactate, triglyceride (TG), free fatty acid (FFA), uric acid (UA) and methylglyoxal (MG). Its extrahepatic absorption and metabolism also take place. High levels of these metabolites are the direct dangerous factors. During fructose metabolism, ATP depletion occurs and induces oxidative stress and inflammatory response, disturbing functions of local tissues and organs to overproduce inflammatory cytokine, adiponectin, leptin and endotoxin, which act as indirect dangerous factors. Fructose and its metabolites directly and/or indirectly cause oxidative stress, chronic inflammation, endothelial dysfunction, autophagy and increased intestinal permeability, and then further aggravate the metabolic syndrome with tissue and organ dysfunctions. Therefore, this review addresses fructose-induced metabolic syndrome, and the disturbance effects of direct and/or indirect dangerous factors on the functions of liver, adipose, pancreas islet, skeletal muscle, kidney, heart, brain and small intestine. It is important to find the potential correlations between direct and/or indirect risk factors and healthy problems under excess dietary fructose consumption.

The association between gallstone disease and plaque in the abdominopelvic arteries

Background:

The aim of this study was to assess the atheromatous plaque, in the abdominopelvic arteries as a marker of cardiac risk in patients with or without gallstone disease (GD).

Materials and Methods:

A total of 136 patients were enrolled in this cross-sectional study. Forty-eight patients had GD and the remaining 88 patients did not. The presence or absence of gallstones was noted during abdominal ultrasonography while vascular risk factors such as plaque formation, intima-media thickness, plaque calcification, mural thrombus, stenosis, aneurysm, and inflammation were recorded during an abdominopelvic computed tomography scan. In addition, percentage of the abdominopelvic aorta surface covered by atheromatous plaque was calculated.

Results:

The mean age of patients with GD and without GD was 50.81 ± 16.20 and 50.40 ± 12.43, respectively. Patients with GD were more likely to have diabetes mellitus, a higher body mass index (BMI) (P < 0.001), and higher cholesterol (P < 0.01), and low-density lipoprotein-cholesterol (P < 0.02) levels. No significant differences were found between the groups regarding other atherosclerotic risk factors. Patients with GD had significantly higher rates of the vascular risk factors as intima-media thickness, plaque formation, calcification, aneurysm, mural thrombosis, stenosis, and inflammation in all abdominal arterial segments other than aneurysm in the femoral arteries. In addition, patients with GD had severe atheromatous plaques in the abdominal aorta, common iliac, external iliac, and common femoral artery (CFA). In patients with GD, parameters of age, BMI, and systolic and diastolic blood pressure were all correlated with the severity of the atheromatous plaque in abdominal aorta, common iliac, external iliac, and CFA.

Conclusion:

We demonstrated a direct relationship between GD and abdominopelvic atheromatous plaque, which is a marker for increased cardiovascular risk, for the first time in the literature. Patients with GD exhibit greater abdominopelvic atherosclerosis and therefore, have a higher risk of cardiovascular disease.

Intestinal CREBH overexpression prevents high-cholesterol diet-induced hypercholesterolemia by reducing Npc1l1 expression

Objective

The transcription factor cyclic AMP-responsive element-binding protein H (CREBH, encoded by Creb3l3) is highly expressed in the liver and small intestine. Hepatic CREBH contributes to glucose and triglyceride metabolism by regulating fibroblast growth factor 21 (Fgf21) expression. However, the intestinal CREBH function remains unknown.

Methods

To investigate the influence of intestinal CREBH on cholesterol metabolism, we compared plasma, bile, fecal, and tissue cholesterol levels between wild-type (WT) mice and mice overexpressing active human CREBH mainly in the small intestine (CREBH Tg mice) under different dietary conditions.

Results

Plasma cholesterol, hepatic lipid, and cholesterol crystal formation in the gallbladder were lower in CREBH Tg mice fed a lithogenic diet (LD) than in LD-fed WTs, while fecal cholesterol output was higher in the former. These results suggest that intestinal CREBH overexpression suppresses cholesterol absorption, leading to reduced plasma cholesterol, limited hepatic supply, and greater excretion. The expression of Niemann–Pick C1-like 1 (Npc1l1), a rate-limiting transporter mediating intestinal cholesterol absorption, was reduced in the small intestine of CREBH Tg mice. Adenosine triphosphate-binding cassette transporter A1 (Abca1), Abcg5/8, and scavenger receptor class B, member 1 (Srb1) expression levels were also reduced in CREBH Tg mice. Promoter assays revealed that CREBH directly regulates Npc1l1 expression. Conversely, CREBH null mice exhibited higher intestinal Npc1l1 expression, elevated plasma and hepatic cholesterol, and lower fecal output.

Conclusion

Intestinal CREBH regulates dietary cholesterol flow from the small intestine by controlling the expression of multiple intestinal transporters. We propose that intestinal CREBH could be a therapeutic target for hypercholesterolemia.

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Plasma cholesterol, hepatic lipid, and gallstones were lower in CREBH Tg mice.

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Expression of intestinal Npc1l1 was reduced in CREBH Tg mice.

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CREBH directly down-regulates mouse Npc1l1 promoter activity.

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Intestinal CREBH regulates dietary cholesterol flow from the small intestine.

Identification of nonabsorbable inhibitors of the scavenger receptor-BI (SR-BI) for tissue-specific administration

The identification of a low-permeability scavenger receptor BI (SR-BI) inhibitor starting from the ITX-5061 template is described. Structure-activity and structure-permeability relationships were assessed for analogs leading to the identification of compound 8 as a potent and nonabsorbable SR-BI inhibitor.

Liver X Receptor Regulates Triglyceride Absorption Through Intestinal Down-regulation of Scavenger Receptor Class B, Type 1

BACKGROUND & AIMS

Reducing postprandial triglyceridemia may be a promising strategy to lower the risk of cardiovascular disorders associated with obesity and type 2 diabetes. In enterocytes, scavenger receptor class B, type 1 (SR-B1, encoded by SCARB1) mediates lipid-micelle sensing to promote assembly and secretion of chylomicrons. The nuclear receptor subfamily 1, group H, members 2 and 3 (also known as liver X receptors [LXRs]) regulate genes involved in cholesterol and fatty acid metabolism. We aimed to determine whether intestinal LXRs regulate triglyceride absorption.

METHODS

C57BL/6J mice were either fed a cholesterol-enriched diet or given synthetic LXR agonists (GW3965 or T0901317). We measured the production of chylomicrons and localized SR-B1 by immunohistochemistry. Mechanisms of postprandial triglyceridemia and SR-B1 regulation were studied in Caco-2/TC7 cells incubated with LXR agonists.

RESULTS

In mice and in the Caco-2/TC7 cell line, LXR agonists caused localization of intestinal SR-B1 from apical membranes to intracellular organelles and reduced chylomicron secretion. In Caco-2/TC7 cells, LXR agonists reduced SR-B1-dependent lipidic-micelle-induced Erk phosphorylation. LXR agonists also reduced intracellular trafficking of the apical apolipoprotein B pool toward secretory compartments. LXR reduced levels of SR-B1 in Caco-2/TC7 cells via a post-transcriptional mechanism that involves microRNAs.

CONCLUSION

In Caco-2/TC7 cells and mice, intestinal activation of LXR reduces the production of chylomicrons by a mechanism dependent on the apical localization of SR-B1.