Intestinal absorption of dietary cholesteryl ester is decreased but retinyl ester absorption is normal in carboxyl ester lipase knockout mice

Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade

It is now well established that vitamins D, E, and K and carotenoids are not absorbed solely through passive diffusion. Broad-specificity membrane transporters such as SR-BI (scavenger receptor class B type I), CD36 (CD36 molecule), NPC1L1 (Niemann Pick C1-like 1) or ABCA1 (ATP-binding cassette A1) are involved in the uptake of these micronutrients from the lumen to the enterocyte cytosol and in their secretion into the bloodstream. Recently, the existence of efflux pathways from the enterocyte back to the lumen or from the bloodstream to the lumen, involving ABCB1 (P-glycoprotein/MDR1) or the ABCG5/ABCG8 complex, has also been evidenced for vitamins D and K. Surprisingly, no membrane proteins have been involved in dietary vitamin A uptake so far. After an overview of the metabolism of fat-soluble vitamins and carotenoids along the gastrointestinal tract (from the mouth to the colon where interactions with microbiota may occur), a focus is placed on the identified and candidate proteins participating in the apical uptake, intracellular transport, basolateral secretion and efflux back to the lumen of fat-soluble vitamins and carotenoids in enterocytes. This review also highlights the mechanisms that remain to be identified to fully unravel the pathways involved in fat-soluble vitamin and carotenoid intestinal absorption.

Vitamin A, systemic T-cells, and the eye: Focus on degenerative retinal disease

The first discovered vitamin, vitamin A, exists in a range of forms, primarily retinoids and provitamin carotenoids. The bioactive forms of vitamin A, retinol and retinoic acid, have many critical functions in body systems including the eye and immune system. Vitamin A deficiency is associated with dysfunctional immunity, and presents clinically as a characteristic ocular syndrome, xerophthalmia. The immune functions of vitamin A extend to the gut, where microbiome interactions and nutritional retinoids and carotenoids contribute to the balance of T cell differentiation, thereby determining immune status and contributing to inflammatory disease around the whole body. In the eye, degenerative conditions affecting the retina and uvea are influenced by vitamin A. Stargardt’s disease (STGD1; MIM 248200) is characterised by bisretinoid deposits such as lipofuscin, produced by retinal photoreceptors as they use and recycle a vitamin A-derived chromophore. Age-related macular degeneration features comparable retinal deposits, such as drusen featuring lipofuscin accumulation; and is characterised by parainflammatory processes. We hypothesise that local parainflammatory processes secondary to lipofuscin deposition in the retina are mediated by T cells interacting with dietary vitamin A derivatives and the gut microbiome, and outline the current evidence for this. No cures exist for Stargardt’s or age-related macular degeneration, but many vitamin A-based therapeutic approaches have been or are being trialled. The relationship between vitamin A’s functions in systemic immunology and the eye could be further exploited, and further research may seek to leverage the interactions of the gut-eye immunological axis.

Vitamin A fortification: Recent advances in encapsulation technologies

Vitamin A is an essential micronutrient whose deficiency is still a major health concern in many regions of the world. It plays an essential role in human growth and development, immunity, and vision, but may also help prevent several other chronic diseases. The total amount of vitamin A in the human diet often falls below the recommended dietary allowance of approximately 900-1000 μ $ \umu $ g/day for a healthy adult. Moreover, a significant proportion of vitamin A may be degraded during food processing, storage, and distribution, thereby reducing its bioactivity. Finally, the vitamin A in some foods has a relatively low bioavailability, which further reduces its efficacy. The World Health Organization has recommended fortification of foods and beverages as a safe and cost-effective means of addressing vitamin A deficiency. However, there are several factors that must be overcome before effective fortified foods can be developed, including the low solubility, chemical stability, and bioavailability of this oil-soluble vitamin. Consequently, strategies are required to evenly disperse the vitamin throughout food matrices, to inhibit its chemical degradation, to avoid any adverse interactions with any other food components, to ensure the food is palatable, and to increase its bioavailability. In this review article, we discuss the chemical, physical, and nutritional attributes of vitamin A, its main dietary sources, the factors contributing to its current deficiency, and various strategies to address these deficiencies, including diet diversification, biofortification, and food fortification.

Hepatic stellate cells in physiology and pathology

Hepatic stellate cells (HSCs) comprise a minor cell population in the liver but serve numerous critical functions in the normal liver and in response to injury. HSCs are primarily known for their activation upon liver injury and for producing the collagen-rich extracellular matrix in liver fibrosis. In the absence of liver injury, HSCs reside in a quiescent state, in which their main function appears to be the storage of retinoids or vitamin A-containing metabolites. Less appreciated functions of HSCs include amplifying the hepatic inflammatory response and expressing growth factors that are critical for liver development and both the initiation and termination of liver regeneration. Recent single-cell RNA sequencing studies have corroborated earlier studies indictaing that HSC activation involves a diverse array of phenotypic alterations and identified unique HSC populations. This review serves to highlight these many functions of HSCs, and to briefly describe the recent genetic tools that will help to thoroughly investigate the role of HSCs in hepatic physiology and pathology.

The Power of Plasticity-Metabolic Regulation of Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are resident non-parenchymal liver pericytes whose plasticity enables them to regulate a remarkable range of physiologic and pathologic responses. To support their functions in health and disease, HSCs engage pathways regulating carbohydrate, mitochondrial, lipid, and retinoid homeostasis. In chronic liver injury, HSCs drive hepatic fibrosis and are implicated in inflammation and cancer. To do so, the cells activate, or transdifferentiate, from a quiescent state into proliferative, motile myofibroblasts that secrete extracellular matrix, which demands rapid adaptation to meet a heightened energy need. Adaptations include reprogramming of central carbon metabolism, enhanced mitochondrial number and activity, endoplasmic reticulum stress, and liberation of free fatty acids through autophagy-dependent hydrolysis of retinyl esters that are stored in cytoplasmic droplets. As an archetype for pericytes in other tissues, recognition of the HSC's metabolic drivers and vulnerabilities offer the potential to target these pathways therapeutically to enhance parenchymal growth and modulate repair.

How unsaturated fatty acids and plant stanols affect sterols plasma level and cellular membranes? Review on model studies involving the Langmuir monolayer technique

The Langmuir monolayer technique has long been known for its usefulness to study the interaction between molecules and mimic cellular membranes to understand the mechanism of action of biologically relevant molecules. In this review we summarize the results that provided insight into the potential mechanism for lowering the plasma level of cholesterol by hypocholesterolemic substances (unsaturated fatty acids (UFAs) and phytocompounds) - in the aspect of prevention of atherosclerosis - and their effects on model biomembranes. The results on UFAs/cholesterol (oxysterols) interactions indicate that these systems are miscible and strongly interacting, contrary to immiscible systems containing saturated fatty acids. Lowering of cholesterol plasma level by UFAs was attributed to the strong affinity between UFAs and sterols, resulting in the formation of high stability complexes, in which sterols were bound and eliminated from the body. Studies on the effect of UFAs and plant sterols/stanols on simplified biomembranes (modeled as cholesterol/DPPC system) indicated that the studied hypocholesterolemic substances modify the biophysical properties of model membrane, affecting its fluidity and interactions between membrane components. Both UFAs and plant sterols/stanols were found to loosen interactions between DPPC and cholesterol and decrease membrane rigidity caused by the excess cholesterol in biomembrane, thus compensating strong condensing effect of cholesterol and restoring proper membrane fluidity, which is of utmost importance for normal cells functioning. The agreement between model - in vitro - studies and biological results prove the usefulness of the Langmuir monolayer technique, which helps in understanding the mode of action of biologically relevant substances.

Pancreatic lipase-related protein 2 is responsible for the increased hepatic retinyl ester hydrolase activity in vitamin A-deficient mice

The formation and hydrolysis of hepatic retinyl esters (RE) is a key process in maintaining serum retinol homeostasis. During vitamin A deficiency, the activity of RE hydrolases (REH) in liver increases to cope with the reduced dietary vitamin A intake. However, it remains unclear which REH is the main enzyme responsible for RE hydrolysis in the liver under physiological conditions. Our previous studies have shown that pancreatic lipase-related protein 2 (PLRP2) is conditionally expressed in the liver and may be involved in the hydrolysis of hepatic RE. In the current study, we generated Plrp2^-/- mice using transcription activator-like effector nuclease technology to investigate the role of PLRP2 in the metabolism of hepatic RE. Compared with the mice fed normal diet, the hepatic REH activity of wild-type (WT) mice fed vitamin A-deficient diet (VAD) increased significantly, while this activity did not increase in Plrp2^-/- mice fed VAD. Plrp2^-/- mice showed higher residual RE content in liver and lower serum retinol level, compared with WT mice fed VAD. Hepatic metabolic profiling from ¹ H NMR-based metabolomics suggested that Plrp2^-/- mice were more sensitive to VAD. Docking analysis and enzyme activity assay revealed that retinyl palmitate was the substrate with higher affinity for PLRP2. Our results indicate that Plrp2 can be activated in the liver and is responsible for the increased REH activity in the liver of mice fed VAD.

Mechanisms of Transport and Delivery of Vitamin A and Carotenoids to the Retinal Pigment Epithelium

Vision depends on the delivery of vitamin A (retinol) to the retina. Retinol in blood is bound to retinol-binding protein (RBP). Retinal pigment epithelia (RPE) cells express the RBP receptor, STRA6, that facilitates uptake of retinol. The retinol is then converted to retinyl esters by the enzyme lecithin:retinol acyltransferase. The esters are the substrate for RPE65, an enzyme that produces 11-cis retinol, which is converted to 11-cis retinaldehyde for transport to the photoreceptors to form rhodopsin. The dietary xanthophylls, lutein (LUT) and zeaxanthin (ZEA), accumulate in the macula of the eye, providing protection against age-related macular degeneration. To reach the macula, carotenoids cross the RPE. In blood, xanthophylls and β-carotene mostly associate with high-density lipoprotein (HDL) and low-density lipoprotein (LDL), respectively. Studies using a human RPE cell model evaluate the kinetics of cell uptake when carotenoids are delivered in LDL or HDL. For LUT and β-carotene, LDL delivery result in the highest rate of uptake. HDL is more effective in delivering ZEA (and meso-ZEA). This selective HDL-mediated uptake of ZEA, via a scavenger receptor and LDL-mediated uptake of LUT and β-carotene provides a mechanism for the selective accumulation of ZEA > LUT and xanthophylls over β-carotene in the macula.

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.

Long-Chain Metabolites of Vitamin E: Metabolic Activation as a General Concept for Lipid-Soluble Vitamins?

Vitamins E, A, D and K comprise the class of lipid-soluble vitamins. For vitamins A and D, a metabolic conversion of precursors to active metabolites has already been described. During the metabolism of vitamin E, the long-chain metabolites (LCMs) 13'-hydroxychromanol (13'-OH) and 13'-carboxychromanol (13'-COOH) are formed by oxidative modification of the side-chain. The occurrence of these metabolites in human serum indicates a physiological relevance. Indeed, effects of the LCMs on lipid metabolism, apoptosis, proliferation and inflammatory actions as well as tocopherol and xenobiotic metabolism have been shown. Interestingly, there are several parallels between the actions of the LCMs of vitamin E and the active metabolites of vitamin A and D. The recent findings that the LCMs exert effects different from that of their precursors support their putative role as regulatory metabolites. Hence, it could be proposed that the mode of action of the LCMs might be mediated by a mechanism similar to vitamin A and D metabolites. If the physiological relevance and this concept of action of the LCMs can be confirmed, a general concept of activation of lipid-soluble vitamins via their metabolites might be deduced.

The role of the carboxyl ester lipase (CEL) gene in pancreatic disease

The enzyme carboxyl ester lipase (CEL), also known as bile salt-dependent or -stimulated lipase (BSDL, BSSL), hydrolyzes dietary fat, cholesteryl esters and fat-soluble vitamins in the duodenum. CEL is mainly expressed in pancreatic acinar cells and lactating mammary glands. The human CEL gene resides on chromosome 9q34.3 and contains a variable number of tandem repeats (VNTR) region that encodes a mucin-like protein tail. Although the number of normal repeats does not appear to significantly influence the risk for pancreatic disease, single-base pair deletions in the first VNTR repeat cause a syndrome of endocrine and exocrine dysfunction denoted MODY8. Hallmarks are low fecal elastase levels and pancreatic lipomatosis manifesting before the age of twenty, followed by development of diabetes and pancreatic cysts later in life. The mutant protein forms intracellular and extracellular aggregates, suggesting that MODY8 is a protein misfolding disease. Recently, a recombined allele between CEL and its pseudogene CELP was discovered. This allele (CEL-HYB) encodes a chimeric protein with impaired secretion increasing five-fold the risk for chronic pancreatitis. The CEL gene has proven to be exceptionally polymorphic due to copy number variants of the CEL-CELP locus and alterations involving the VNTR. Genome-wide association studies or deep sequencing cannot easily pick up this wealth of genetic variation. CEL is therefore an attractive candidate gene for further exploration of links to pancreatic disease.

Molecular Basis for Vitamin A Uptake and Storage in Vertebrates

The ability to store and distribute vitamin A inside the body is the main evolutionary adaptation that allows vertebrates to maintain retinoid functions during nutritional deficiencies and to acquire new metabolic pathways enabling light-independent production of 11-cis retinoids. These processes greatly depend on enzymes that esterify vitamin A as well as associated retinoid binding proteins. Although the significance of retinyl esters for vitamin A homeostasis is well established, until recently, the molecular basis for the retinol esterification enzymatic activity was unknown. In this review, we will look at retinoid absorption through the prism of current biochemical and structural studies on vitamin A esterifying enzymes. We describe molecular adaptations that enable retinoid storage and delineate mechanisms in which mutations found in selective proteins might influence vitamin A homeostasis in affected patients.

Branched Fatty Acid Esters of Hydroxy Fatty Acids Are Preferred Substrates of the MODY8 Protein Carboxyl Ester Lipase

A recently discovered class of endogenous mammalian lipids, branched fatty acid esters of hydroxy fatty acids (FAHFAs), possesses anti-diabetic and anti-inflammatory activities. Here, we identified and validated carboxyl ester lipase (CEL), a pancreatic enzyme hydrolyzing cholesteryl esters and other dietary lipids, as a FAHFA hydrolase. Variants of CEL have been linked to maturity-onset diabetes of the young, type 8 (MODY8), and to chronic pancreatitis. We tested the FAHFA hydrolysis activity of the CEL MODY8 variant and found a modest increase in activity as compared with that of the normal enzyme. Together, the data suggest that CEL might break down dietary FAHFAs.

Lysosomal Acid Lipase Hydrolyzes Retinyl Ester and Affects Retinoid Turnover

Lysosomal acid lipase (LAL) is essential for the clearance of endocytosed cholesteryl ester and triglyceride-rich chylomicron remnants. Humans and mice with defective or absent LAL activity accumulate large amounts of cholesteryl esters and triglycerides in multiple tissues. Although chylomicrons also contain retinyl esters (REs), a role of LAL in the clearance of endocytosed REs has not been reported. In this study, we found that murine LAL exhibits RE hydrolase activity. Pharmacological inhibition of LAL in the human hepatocyte cell line HepG2, incubated with chylomicrons, led to increased accumulation of REs in endosomal/lysosomal fractions. Furthermore, pharmacological inhibition or genetic ablation of LAL in murine liver largely reduced in vitro acid RE hydrolase activity. Interestingly, LAL-deficient mice exhibited increased RE content in the duodenum and jejunum but decreased RE content in the liver. Furthermore, LAL-deficient mice challenged with RE gavage exhibited largely reduced post-prandial circulating RE content, indicating that LAL is required for efficient nutritional vitamin A availability. In summary, our results indicate that LAL is the major acid RE hydrolase and required for functional retinoid homeostasis.

Complexity of vitamin E metabolism

Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.

Cholesterol Absorption and Metabolism

Inhibitors of cholesterol absorption have been sought for decades as a means to treat and prevent cardiovascular diseases (CVDs) associated with hypercholesterolemia. Ezetimibe is the one clear success story in this regard, and other compounds with similar efficacy continue to be sought. In the last decade, the laboratory mouse, with all its genetic power, has become the premier experimental model for discovering the mechanisms underlying cholesterol absorption and has become a critical tool for preclinical testing of potential pharmaceutical entities. This chapter briefly reviews the history of cholesterol absorption research and the various gene candidates that have come under consideration as drug targets. The most common and versatile method of measuring cholesterol absorption is described in detail along with important considerations when interpreting results, and an alternative method is also presented. In recent years, reverse cholesterol transport (RCT) has become an area of intense new interest for drug discovery since this process is now considered another key to reducing CVD risk. The ultimate measure of RCT is sterol excretion and a detailed description is given for measuring neutral and acidic fecal sterols and interpreting the results.

Vitamin A Absorption, Storage and Mobilization

It is well established that chylomicron remnant (dietary) vitamin A is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. Since retinol-binding protein 4 (RBP4), a protein that is required for mobilizing stored vitamin A, is synthesized solely by hepatocytes and not HSCs, it similarly is not known how vitamin A is transferred from HSCs to hepatocytes. Although it has long been thought that RBP4 is absolutely essential for delivering vitamin A to tissues, recent research has proven that this notion is incorrect since total RBP4-deficiency is not lethal. In addition to RBP4, vitamin A is also found in the circulation bound to lipoproteins and as retinoic acid bound to albumin. It is not known how these different circulating pools of vitamin A contribute to the vitamin A needs of different tissues. In our view, better insight into these three issues is required to better understand vitamin A absorption, storage and mobilization. Here, we provide an up to date synthesis of current knowledge regarding the intestinal uptake of dietary vitamin A, the storage of vitamin A within the liver, and the mobilization of hepatic vitamin A stores, and summarize areas where our understanding of these processes is incomplete.

Revisiting Human Cholesterol Synthesis and Absorption: The Reciprocity Paradigm and its Key Regulators

Hypercholesterolemia is a major risk factor for cardiovascular disease. Cholesterol homeostasis in the body is governed by the interplay between absorption, synthesis, and excretion or conversion of cholesterol into bile acids. A reciprocal relationship between cholesterol synthesis and absorption is known to regulate circulating cholesterol in response to dietary or therapeutic interventions. However, the degree to which these factors affect synthesis and absorption and the extent to which one vector shifts in response to the other are not thoroughly understood. Also, huge inter-individual variability exists in the manner in which the two systems act in response to any cholesterol-lowering treatment. Various factors are known to account for this variability and in light of recent experimental advances new players such as gene-gene interactions, gene-environmental effects, and gut microbiome hold immense potential in offering an explanation to the complex traits of inter-individual variability in human cholesterol metabolism. In this context, the objective of the present review is to provide an overview on cholesterol metabolism and discuss the role of potential factors such as genetics, epigenetics, epistasis, and gut microbiome, as well as other regulators in modulating cholesterol metabolism, especially emphasizing the reciprocal relationship between cholesterol synthesis and absorption. Furthermore, an evaluation of the implications of this push-pull mechanism on cholesterol-lowering strategies is presented.

Revisiting Human Cholesterol Synthesis and Absorption: The Reciprocity Paradigm and its Key Regulators

Hypercholesterolemia is a major risk factor for cardiovascular disease. Cholesterol homeostasis in the body is governed by the interplay between absorption, synthesis, and excretion or conversion of cholesterol into bile acids. A reciprocal relationship between cholesterol synthesis and absorption is known to regulate circulating cholesterol in response to dietary or therapeutic interventions. However, the degree to which these factors affect synthesis and absorption and the extent to which one vector shifts in response to the other are not thoroughly understood. Also, huge inter-individual variability exists in the manner in which the two systems act in response to any cholesterol-lowering treatment. Various factors are known to account for this variability and in light of recent experimental advances new players such as gene-gene interactions, gene-environmental effects, and gut microbiome hold immense potential in offering an explanation to the complex traits of inter-individual variability in human cholesterol metabolism. In this context, the objective of the present review is to provide an overview on cholesterol metabolism and discuss the role of potential factors such as genetics, epigenetics, epistasis, and gut microbiome, as well as other regulators in modulating cholesterol metabolism, especially emphasizing the reciprocal relationship between cholesterol synthesis and absorption. Furthermore, an evaluation of the implications of this push-pull mechanism on cholesterol-lowering strategies is presented.

New insights into the molecular mechanism of intestinal fatty acid absorption

BACKGROUND

Dietary fat is one of the most important energy sources of all the nutrients. Fatty acids, stored as triacylglycerols (also called triglycerides) in the body, are an important reservoir of stored energy and derived primarily from animal fats and vegetable oils.

DESIGN

Although the molecular mechanisms for the transport of water-insoluble amphipathic fatty acids across cell membranes have been debated for many years, it is now believed that the dominant means for intestinal fatty acid uptake is via membrane-associated fatty acid-binding proteins, that is, fatty acid transporters on the apical membrane of enterocytes.

RESULTS

These findings indicate that intestinal fatty acid absorption is a multistep process that is regulated by multiple genes at the enterocyte level, and intestinal fatty acid absorption efficiency could be determined by factors influencing intraluminal fatty acid molecules across the brush border membrane of enterocytes. To facilitate research on intestinal, hepatic and plasma triacylglycerol metabolism, it is imperative to establish standard protocols for precisely and accurately measuring the efficiency of intestinal fatty acid absorption in humans and animal models. In this review, we will discuss the chemical structure and nomenclature of fatty acids and summarize recent progress in investigating the molecular mechanisms underlying the intestinal absorption of fatty acids, with a particular emphasis on the physical chemistry of intestinal lipids and the molecular physiology of intestinal fatty acid transporters.

CONCLUSIONS

A better understanding of the molecular mechanism of intestinal fatty acid absorption should lead to novel approaches to the treatment and the prevention of fatty acid-related metabolic diseases that are prevalent worldwide.

Absorption of vitamin A and carotenoids by the enterocyte: focus on transport proteins

Vitamin A deficiency is a public health problem in most developing countries, especially in children and pregnant women. It is thus a priority in health policy to improve preformed vitamin A and/or provitamin A carotenoid status in these individuals. A more accurate understanding of the molecular mechanisms of intestinal vitamin A absorption is a key step in this direction. It was long thought that β-carotene (the main provitamin A carotenoid in human diet), and thus all carotenoids, were absorbed by a passive diffusion process, and that preformed vitamin A (retinol) absorption occurred via an unidentified energy-dependent transporter. The discovery of proteins able to facilitate carotenoid uptake and secretion by the enterocyte during the past decade has challenged established assumptions, and the elucidation of the mechanisms of retinol intestinal absorption is in progress. After an overview of vitamin A and carotenoid fate during gastro-duodenal digestion, our focus will be directed to the putative or identified proteins participating in the intestinal membrane and cellular transport of vitamin A and carotenoids across the enterocyte (i.e., Scavenger Receptors or Cellular Retinol Binding Proteins, among others). Further progress in the identification of the proteins involved in intestinal transport of vitamin A and carotenoids across the enterocyte is of major importance for optimizing their bioavailability.

Absence of diabetes and pancreatic exocrine dysfunction in a transgenic model of carboxyl-ester lipase-MODY (maturity-onset diabetes of the young)

Background

CEL-MODY is a monogenic form of diabetes with exocrine pancreatic insufficiency caused by mutations in CARBOXYL-ESTER LIPASE (CEL). The pathogenic processes underlying CEL-MODY are poorly understood, and the global knockout mouse model of the CEL gene (CELKO) did not recapitulate the disease. We therefore aimed to create and phenotype a mouse model specifically over-expressing mutated CEL in the pancreas.

Methods

We established a monotransgenic floxed (flanking LOX sequences) mouse line carrying the human CEL mutation c.1686delT and crossed it with an elastase-Cre mouse to derive a bitransgenic mouse line with pancreas-specific over-expression of CEL carrying this disease-associated mutation (TgCEL). Following confirmation of murine pancreatic expression of the human transgene by real-time quantitative PCR, we phenotyped the mouse model fed a normal chow and compared it with mice fed a 60% high fat diet (HFD) as well as the effects of short-term and long-term cerulein exposure.

Results

Pancreatic exocrine function was normal in TgCEL mice on normal chow as assessed by serum lipid and lipid-soluble vitamin levels, fecal elastase and fecal fat absorption, and the normoglycemic mice exhibited normal pancreatic morphology. On 60% HFD, the mice gained weight to the same extent as controls, had normal pancreatic exocrine function and comparable glucose tolerance even after resuming normal diet and follow up up to 22 months of age. The cerulein-exposed TgCEL mice gained weight and remained glucose tolerant, and there were no detectable mutation-specific differences in serum amylase, islet hormones or the extent of pancreatic tissue inflammation.

Conclusions

In this murine model of human CEL-MODY diabetes, we did not detect mutation-specific endocrine or exocrine pancreatic phenotypes, in response to altered diets or exposure to cerulein.

Retinoids: Potent regulators of metabolism

Retinoids (vitamin A and its analogs) are highly potent regulators of cell differentiation, cell proliferation, and apoptosis. Because of these activities, retinoids have been most extensively studied in the contexts of embryonic development and of proliferative diseases, especially cancer and skin disease. Recently, there has been considerable new research interest focused on gaining understanding of the roles that retinoids and/or retinoid-related proteins may have in the development of metabolic diseases, primarily obesity, diabetes, and dyslipidemia. This review will summarize recent advances that have been made in these areas, focusing on the role of retinoids in modulating adipogenesis, the roles of retinoids and retinoid-related proteins as signaling molecules linking obesity with the development of type II diabetes, the roles of retinoids in pancreatic β-cell biology/insulin secretion, and the actions of retinoids in hepatic steatosis.

Low-density lipoprotein, its susceptibility to oxidation and the role of lipoprotein-associated phospholipase A2 and carboxyl ester lipase lipases in atherosclerotic plaque formation

An increased level of low-density lipoprotein (LDL) is a very well established risk factor of coronary artery disease (CAD). Unoxidized LDL is an inert transport vehicle of cholesterol and other lipids in the body and is thought to be atherogenic. Recently it has been appreciated that oxidized products of LDL are responsible for plaque formation properties previously attributed to the intact particle. The goal of this article is to review the recent understanding of the LDL oxidation pathway. The role of oxidized products and key enzymes (lipoprotein-associated phospholipase A2 and carboxyl ester lipase) are also extensively discussed in the context of clinical conditions.

Bile salt-stimulated lipase plays an unexpected role in arthritis development in rodents

OBJECTIVE

The present study aimed to explore the hypothesis that bile salt-stimulated lipase (BSSL), in addition to being a key enzyme in dietary fat digestion during early infancy, plays an important role in inflammation, notably arthritis.

METHODS

Collagen-induced arthritis (CIA) and pristane-induced arthritis (PIA) in rodents are commonly used experimental models that reproduce many of the pathogenic mechanisms of human rheumatoid arthritis, i.e. increased cellular infiltration, synovial hyperplasia, pannus formation, and erosion of cartilage and bone in the distal joints. We used the CIA model to compare the response in BSSL wild type (BSSL-WT) mice with BSSL-deficient 'knock-out' (BSSL-KO) and BSSL-heterozygous (BSSL-HET) littermates. We also investigated if intraperitoneal injection of BSSL-neutralizing antibodies affected the development or severity of CIA and PIA in mice and rats, respectively.

RESULTS

In two consecutive studies, we found that BSSL-KO male mice, in contrast to BSSL-WT littermates, were significantly protected from developing arthritis. We also found that BSSL-HET mice were less prone to develop disease compared to BSSL-WT mice, but not as resistant as BSSL-KO mice, suggesting a gene-dose effect. Moreover, we found that BSSL-neutralizing antibody injection reduced both the incidence and severity of CIA and PIA in rodents.

CONCLUSION

Our data strongly support BSSL as a key player in the inflammatory process, at least in rodents. It also suggests the possibility that BSSL-neutralizing agents could serve as a therapeutic model to reduce the inflammatory response in humans.

Vitamin A metabolism: an update

Retinoids are required for maintaining many essential physiological processes in the body, including normal growth and development, normal vision, a healthy immune system, normal reproduction, and healthy skin and barrier functions. In excess of 500 genes are thought to be regulated by retinoic acid. 11-cis-retinal serves as the visual chromophore in vision. The body must acquire retinoid from the diet in order to maintain these essential physiological processes. Retinoid metabolism is complex and involves many different retinoid forms, including retinyl esters, retinol, retinal, retinoic acid and oxidized and conjugated metabolites of both retinol and retinoic acid. In addition, retinoid metabolism involves many carrier proteins and enzymes that are specific to retinoid metabolism, as well as other proteins which may be involved in mediating also triglyceride and/or cholesterol metabolism. This review will focus on recent advances for understanding retinoid metabolism that have taken place in the last ten to fifteen years.

A single dose of c9,t11 or t10,c12 conjugated linoleic acid isomers perturbs vitamin A metabolism in mice

Conjugated linoleic acid (CLA) is a polyunsaturated fatty acid that has numerous biologic activities. Previous studies in rodents demonstrated that chronic intake of CLA t10,c12 or CLA c9,t11 isomers perturbs the metabolism of retinoids (vitamin A and its derivatives). Specifically, although both isomers increased liver retinoid levels, only CLA t10,c12 also stimulated hepatic retinol secretion into the bloodstream. Given that retinoid homeostasis in mammalian serum and tissues is crucial to maintain health, it is important to gain more insights into the mode of action of this nutrient-nutrient interaction. Here we hypothesized that an acute administration of either CLA isomer may also influence vitamin A metabolism. By gavaging wild-type and retinol-binding protein knockout mice with an oral bolus of radiolabeled retinol containing 1 of these 2 isomers, we showed that both CLA t10,c12 and CLA c9,t11 rapidly enhance hepatic uptake of dietary vitamin A and its resecretion from the liver in the form of retinol bound to retinol-binding protein. Indeed, in mice lacking this protein, the sole specific carrier for retinol in the circulation, this latter effect was blunted. In addition, by using a pharmacologic inhibitor of the clearance of chylomicrons, which distribute recently ingested vitamin A and lipids throughout the body, we provided evidence that CLA intake might rapidly enhance intestinal absorption of dietary vitamin A. These data demonstrate the existence of multiple levels of interaction between dietary CLA and retinoid metabolism and warrant further studies to understand the molecular mechanisms underlying these effects and their implications for human health.

Mechanisms involved in the intestinal absorption of dietary vitamin A and provitamin A carotenoids

Vitamin A is an essential nutrient for humans and is converted to the visual chromophore, 11-cis-retinal, and to the hormone, retinoic acid. Vitamin A in animal-derived foods is found as long chain acyl esters of retinol and these are digested to free fatty acids and retinol before uptake by the intestinal mucosal cell. The retinol is then reesterified to retinyl esters for incorporation into chlylomicrons and absorbed via the lymphatics or effluxed into the portal circulation facilitated by the lipid transporter, ABCA1. Provitamin A carotenoids such as β-carotene are found in plant-derived foods. These and other carotenoids are transported into the mucosal cell by scavenger receptor class B type I (SR-BI). Provitamin A carotenoids are partly converted to retinol by oxygenase and reductase enzymes and the retinol so produced is available for absorption via the two pathways described above. The efficiency of vitamin A and carotenoid intestinal absorption is determined by the regulation of a number of proteins involved in the process. Polymorphisms in genes for these proteins lead to individual variability in the metabolism and transport of vitamin A and carotenoids. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Comparative Structures and Evolution of Vertebrate Carboxyl Ester Lipase (CEL) Genes and Proteins with a Major Role in Reverse Cholesterol Transport

Bile-salt activated carboxylic ester lipase (CEL) is a major triglyceride, cholesterol ester and vitamin ester hydrolytic enzyme contained within pancreatic and lactating mammary gland secretions. Bioinformatic methods were used to predict the amino acid sequences, secondary and tertiary structures and gene locations for CEL genes, and encoded proteins using data from several vertebrate genome projects. A proline-rich and O-glycosylated 11-amino acid C-terminal repeat sequence (VNTR) previously reported for human and other higher primate CEL proteins was also observed for other eutherian mammalian CEL sequences examined. In contrast, opossum CEL contained a single C-terminal copy of this sequence whereas CEL proteins from platypus, chicken, lizard, frog and several fish species lacked the VNTR sequence. Vertebrate CEL genes contained 11 coding exons. Evidence is presented for tandem duplicated CEL genes for the zebrafish genome. Vertebrate CEL protein subunits shared 53-97% sequence identities; demonstrated sequence alignments and identities for key CEL amino acid residues; and conservation of predicted secondary and tertiary structures with those previously reported for human CEL. Phylogenetic analyses demonstrated the relationships and potential evolutionary origins of the vertebrate CEL family of genes which were related to a nematode carboxylesterase (CES) gene and five mammalian CES gene families.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

The mPlrp2 and mClps genes are involved in the hydrolysis of retinyl esters in the mouse liver

Retinyl esters are the major chemical forms of vitamin A stored in the liver, and can be delivered to peripheral tissues for conversion into biologically active forms. The function and regulation of the hepatic genes that are potentially involved in catalyzing the hydrolysis of retinyl esters remain unclear. Here we show that two lipid hydrolytic genes, pancreatic-related protein 2 (mPlrp2) and procolipase (mClps), expressed specifically in the mouse pancreas, are associated with the ratio of S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy). Light illumination deficiency or administration of 5'-AMP elevated the ratio of AdoMet to AdoHcy and induced the expression in the liver of mPlrp2 and mClps, which was blocked by all-trans retinoic acid. Mice fed a vitamin A-free diet exhibited increased activation of hepatic mPlrp2 and mClps expression, which was associated with increased methylation of histone H3K4 residues located near the mPlrp2 and mClps promoters. Inhibition of hepatic mPlrp2 and mClps expression by a methylase inhibitor, methylthioadenosine, markedly decreased plasma retinol levels in these mice. The activated hepatic stellate cell (HSC)-T6 cell line specifically expressed mClps and mPlrp2. Inhibition of mClps gene expressions by short hairpin RNA (shRNA) decreased hydrolysis of retinyl esters in the HSC-T6 cell line. These data suggest that the conditional expression of mPlrp2 and mClps is involved in the hydrolysis of retinyl esters in the mouse liver.

Reverse cholesterol transport is elevated in carboxyl ester lipase-knockout mice

Mechanisms to increase reverse cholesterol transport (RCT) and biliary sterol disposal are currently sought to prevent atherosclerosis. Previous work with HepG2 cells and primary hepatocytes showed that carboxyl ester lipase (CEL), a broad-spectrum lipase secreted by pancreas and liver, plays an important role in hydrolysis of high-density lipoprotein (HDL) cholesteryl esters (CEs) after selective uptake by hepatocytes. The effect of CEL on RCT of HDL cholesterol was assessed by measuring biliary and fecal disposal of radiolabeled HDL-CE in control and Cel(-/-) mice. Radiolabeled CE was increased 3-fold in hepatic bile of Cel(-/-) mice, and the mass of CE in gall bladder bile was elevated. Total radiolabeled transport from plasma to hepatic bile was more rapid in Cel(-/-) mice. Fecal disposal of radiolabel from HDL-CE, as well as total sterol mass, was markedly elevated for Cel(-/-) mice, primarily due to more CE. RCT of macrophage CE was also increased in Cel(-/-) mice, as measured by excretion of radiolabel from injected J774 cells. Increased sterol loss was compensated by increased cholesterol synthesis in Cel(-/-) mice. Together, the data demonstrate significantly increased RCT in the absence of CEL and suggest a novel mechanism by which to manipulate plasma cholesterol flux.

Pancreatic function in carboxyl-ester lipase knockout mice

BACKGROUND/AIMS

CEL-MODY is a monogenic form of diabetes and exocrine pancreatic insufficiency due to mutations in the carboxyl-ester lipase (CEL) gene. We aimed to investigate endocrine and exocrine pancreatic function in CEL knockout mice (CELKO).

METHODS

A knockout mouse model with global targeted deletion of CEL was investigated physiologically and histopathologically, and compared to littermate control CEL+/+ mice at 7 and 12 months on normal chow and high-fat diets (HFD), i.e. 42 and 60% fat by calories.

RESULTS

CELKO+/+ and -/- mice showed normal growth and development and normal glucose metabolism on a chow diet. Female CEL-/- mice on 60% HFD, on the other hand, had increased random blood glucose compared to littermate controls (p = 0.02), and this was accompanied by a reduction in glucose tolerance that did not reach statistical significance. In these mice there was also islet hyperplasia, however, α- and β-islet cells appeared morphologically normal and pancreatic exocrine function was also normal.

CONCLUSION

Although we observed mild glucose intolerance in female mice with whole-body knockout of CEL, the full phenotype of human CEL-MODY was not reproduced, suggesting that the pathogenic mechanisms involved are more complex than a simple loss of CEL function. and IAP.

Cholesterol absorption and metabolism

Inhibitors of cholesterol absorption have been sought for decades as a means to treat and prevent cardiovascular diseases associated with hypercholesterolemia. Ezetimibe is the one clear success story in this regard, and other compounds with similar efficacy continue to be sought. In the last decade, the laboratory mouse, with all its genetic power, has become the premier experimental model for discovering the mechanisms underlying cholesterol absorption and has become a critical tool for preclinical testing of potential pharmaceutical entities. This chapter briefly reviews the history of cholesterol absorption research and the various gene candidates that have come under consideration as drug targets. The most common and versatile method of measuring cholesterol absorption is described in detail along with important considerations when interpreting results, and an alternative method is also presented. In recent years, reverse cholesterol transport has become an area of intense new interest for drug discovery since this process is now considered another key to reducing cardiovascular disease risk. The ultimate measure of reverse cholesterol transport is sterol excretion and a detailed description is given for measuring neutral and acidic fecal sterols and interpreting the results.

Lipases in the pancreatic beta-cell: implications for insulin secretion

Lipids have been implicated in beta-cell stimulus-secretion coupling. In such a role, lipases in beta-cells would be required to generate lipid coupling factors. We have shown previously that glucose stimulates lipolysis in rodent islets. In addition, lipolysis and diacylglycerol lipase activity in islets are abolished by orlistat, an irreversible lipase inhibitor with a broad specificity for substrates. Moreover, orlistat dose-dependently inhibits glucose- and forskolin-stimulated insulin secretion, while leaving glucose oxidation and the rise in the ATP/ADP ratio intact. In an effort to identify beta-cell lipase(s), we found that HSL (hormone-sensitive lipase), the rate-limiting enzyme for acylglycerol hydrolysis in adipocytes, is expressed in rodent beta-cells. To resolve the role of this lipase, we have created global and beta-cell-specific knockout mice. Although our line of global HSL-knockout mice is moderately glucose-intolerant owing to reduced peripheral insulin sensitivity and exhibits normal islet metabolism and insulin secretion, other HSL-knockout lines have displayed impaired insulin secretion under certain conditions. In contrast, beta-cell-specific HSL-knockout mice, which are less prone to genetic redundancy, are hyperglycaemic, presumably caused by a perturbation of first-phase insulin secretion. Thus studies by us and others demonstrate that lipases, such as HSL, play a regulatory role in beta-cell stimulus-secretion coupling.

Plasma carboxyl ester lipase activity modulates apolipoprotein B-containing lipoprotein metabolism in a transgenic mouse model

Pancreatic carboxyl ester lipase (CEL) is in the plasma of many mammals, including humans and rats, but not mice. In vitro, CEL hydrolyzes cholesterol esters of apolipoprotein B-containing lipoproteins (apo B-Lp). To study the effect of CEL on metabolism of apo B-Lp and atherosclerosis in vivo, apo E-knockout (EKO) mice, which have high plasma levels of apo B-Lp and are prone to atherosclerosis, were made to secrete CEL into plasma by introducing a transgene containing a liver-specific promoter and rat CEL complementary DNA. Plasma CEL activity in EKO-CEL mice was comparable with that found in rats. Evidence of modification of apo B-Lp by plasma CEL in vivo was an increase in the free cholesterol to cholesterol ester ratio of apo B-Lp from mice on chow or a Western-type diet. In addition, plasma total cholesterol levels were elevated in EKO-CEL mice, with the elevation found exclusively in the apo B-Lp fraction. Associated with the increase in steady-state apo B-Lp levels was an increase in the plasma half-life of very low-density lipoproteins (VLDL) in EKO-CEL mice, measured by the clearance rate of injected VLDL. Interestingly, despite the increase of apo B-Lp, the atherosclerotic lesion did not differ between EKO and EKO-CEL mice on a Western-type diet. In summary, our results demonstrate that plasma CEL modulates apo B-Lp metabolism in vivo, resulting in reduced VLDL clearance and elevated plasma cholesterol levels.

Carboxyl ester lipase from either mother's milk or the pancreas is required for efficient dietary triglyceride digestion in suckling mice

Because dietary fats provide an important source of energy in the newborn, the efficient digestion of dietary fats is critical to their well-being. Despite the importance of dietary fat digestion, newborns have a deficiency of pancreatic triglyceride lipase, the predominant digestive lipase in adults. The efficient dietary fat digestion in newborns suggests that other lipases must compensate for the lack of pancreatic triglyceride lipase. In this study, we test the hypothesis that breast milk, pancreatic carboxyl ester lipase (CEL), or both contribute to dietary fat digestion in the newborn. To test this hypothesis, we determined the amount and composition of fecal fat in wild-type and CEL-deficient newborns nursed by either wild-type or CEL-deficient dams. We tested all genetic permutations of the nursing pairs. An interaction between the genotype of the dam and of the pup determined the amount of fecal fat (P < 0.001). Fecal fat was highest in CEL-deficient pups nursed by CEL-deficient dams. Furthermore, only the feces from the CEL-deficient pups nursed by CEL-deficient dams contained undigested lipids. Even with increased fecal fats, the CEL-deficient pups had normal weight gain. Our results demonstrate that CEL contributes significantly to dietary triglyceride digestion whether it originates from mother's milk or pancreatic secretions. However, only the absence of both mother's milk and pancreatic CEL produces fat maldigestion. The absence of a single CEL source makes no difference in the efficiency of dietary fat absorption.

Bile salt-dependent lipase interacts with platelet CXCR4 and modulates thrombus formation in mice and humans

Bile salt-dependent lipase (BSDL) is an enzyme involved in the duodenal hydrolysis and absorption of cholesteryl esters. Although some BSDL is transported to blood, the role of circulating BSDL is unknown. Here, we demonstrate that BSDL is stored in platelets and released upon platelet activation. Because BSDL contains a region that is structurally homologous to the V3 loop of HIV-1, which binds to CXC chemokine receptor 4 (CXCR4), we hypothesized that BSDL might bind to CXCR4 present on platelets. In human platelets in vitro, both BSDL and a peptide corresponding to its V3-like loop induced calcium mobilization and enhanced thrombin-mediated platelet aggregation, spreading, and activated alpha(IIb)beta(3) levels. These effects were abolished by CXCR4 inhibition. BSDL also increased the production of prostacyclin by human endothelial cells. In a mouse thrombosis model, BSDL accumulated at sites of vessel wall injury. When CXCR4 was antagonized, the accumulation of BSDL was inhibited and thrombus size was reduced. In BSDL(-/-) mice, calcium mobilization in platelets and thrombus formation were attenuated and tail bleeding times were increased in comparison with those of wild-type mice. We conclude that BSDL plays a role in optimal platelet activation and thrombus formation by interacting with CXCR4 on platelets.

Carboxyl ester lipase deficiency exacerbates dietary lipid absorption abnormalities and resistance to diet-induced obesity in pancreatic triglyceride lipase knockout mice

This study evaluated the contributions of carboxyl ester lipase (CEL) and pancreatic triglyceride lipase (PTL) in lipid nutrient absorption. Results showed PTL deficiency has minimal effect on triacylglycerol (TAG) absorption under low fat dietary conditions. Interestingly, PTL(-)(/)(-) mice displayed significantly reduced TAG absorption compared with wild type mice under high fat/high cholesterol dietary conditions (80.1 +/- 3.7 versus 91.5 +/- 0.7%, p < 0.05). Net TAG absorption was reduced further to 61.1 +/- 3.8% in mice lacking both PTL and CEL. Cholesterol absorption was 41% lower in PTL(-/-) mice compared with control mice (p < 0.05), but this difference was not exaggerated in PTL(-/-), CEL(-/-) mice. Retinyl palmitate absorption was reduced by 45 and 60% in PTL(-/-) mice (p < 0.05) and PTL(-/-), CEL(-/-) mice (p < 0.01), respectively. After 15 weeks of feeding, the high fat/high cholesterol diet, wild type, and CEL(-/-) mice gained approximately 24 g of body weight. However, body weight gain was 6.2 and 8.6 g less (p < 0.01) in PTL(-/-) and PTL(-/-), CEL(-/-) mice, respectively, despite their consumption of comparable amounts of the high fat/high cholesterol diet. The decrease body weight gain in PTL(-/-) and PTL(-/-), CEL(-/-) mice was attributed to their absorption of fewer calories from the high fat/high cholesterol diet, thereby resulting in less fat mass accumulation than that observed in wild type and CEL(-/-) mice. Thus, this study documents that PTL and CEL serve complementary functions, working together to mediate the absorption of a major portion of dietary fat and fat-soluble vitamin esters. The reduced lipid absorption efficiency due to PTL and CEL inactivation also resulted in protection against diet-induced obesity.

Regulation of Intestinal Cholesterol Absorption

The identification of defective structures in the ATP-binding cassette (ABC) transporters ABCG5 and ABCG8 in patients with sitosterolemia suggests that these two proteins are an apical sterol export pump promoting active efflux of cholesterol and plant sterols from enterocytes back into the intestinal lumen for excretion. The newly identified Niemann-Pick C1-like 1 (NPC1L1) protein is also expressed at the apical membrane of enterocytes and plays a crucial role in the ezetimibe-sensitive cholesterol absorption pathway. These findings indicate that cholesterol absorption is a multistep process that is regulated by multiple genes at the enterocyte level and that the efficiency of cholesterol absorption may be determined by the net effect between influx and efflux of intraluminal cholesterol molecules crossing the brush border membrane of the enterocyte. Combination therapy using cholesterol absorption (NPC1L1) inhibitor (ezetimibe) and 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitors (statins) provides a powerful novel strategy for the prevention and treatment of hypercholesterolemia.