Reduced absorption of saturated fatty acids and resistance to diet-induced obesity and diabetes by ezetimibe-treated and Npc1l1-/- mice

Ezetimibe prevents hepatic steatosis induced by a high-fat but not a high-fructose diet

Nonalcoholic fatty liver disease is the most frequent liver disease. Ezetimibe, an inhibitor of intestinal cholesterol absorption, has been reported to ameliorate hepatic steatosis in human and animal models. To explore how ezetimibe reduces hepatic steatosis, we investigated the effects of ezetimibe on the expression of lipogenic enzymes and intestinal lipid metabolism in mice fed a high-fat or a high-fructose diet. CBA/JN mice were fed a high-fat diet or a high-fructose diet for 8 wk with or without ezetimibe. High-fat diet induced hepatic steatosis accompanied by hyperinsulinemia. Treatment with ezetimibe reduced hepatic steatosis, insulin levels, and glucose production from pyruvate in mice fed the high-fat diet, suggesting a reduction of insulin resistance in the liver. In the intestinal analysis, ezetimibe reduced the expression of fatty acid transfer protein-4 and apoB-48 in mice fed the high-fat diet. However, treatment with ezetimibe did not prevent hepatic steatosis, hyperinsulinemia, and intestinal apoB-48 expression in mice fed the high-fructose diet. Ezetimibe decreased liver X receptor-α binding to the sterol regulatory element-binding protein-1c promoter but not expression of carbohydrate response element-binding protein and fatty acid synthase in mice fed the high-fructose diet, suggesting that ezetimibe did not reduce hepatic lipogenesis induced by the high-fructose diet. Elevation of hepatic and intestinal lipogenesis in mice fed a high-fructose diet may partly explain the differences in the effect of ezetimibe.

Ezetimibe impairs uptake of dietary cholesterol oxidation products and reduces alterations in hepatic cholesterol metabolism and antioxidant function in rats

Dietary cholesterol oxidation products (COP) induce various adverse effects, including development of atherosclerosis, modulation of lipid metabolism, and unfavorable changes in the antioxidant system. Therefore, we examined the effects of ezetimibe, a cholesterol absorption inhibitor on hepatic cholesterol metabolism and down-regulation of the antioxidant system in rats fed COP. Rats were fed a purified diet containing 0.3 % COP with or without ezetimibe (0.07 mg/100 g body weight) for 27 days. Levels of COP in both the plasma and liver were lowered by ezetimibe through promotion of COP excretion into the feces. Reflecting this effect, an increase in the arteriosclerotic index and a reduction in the mRNA expression of hepatic cholesterol biosynthesis transcripts by dietary COP were observed. Moreover, the ferric reducing ability of the plasma also was significantly higher in rats fed COP plus ezetimibe than in those fed COP alone. Finally, we also observed that ezetimibe enhanced the down-regulation of hepatic fatty acid synthesis in rats fed COP. Thus, ezetimibe, which inhibits the absorption of dietary COP from the small intestine, may exert preventive effects on dietary COP-induced disruption of cholesterol and fatty acid metabolism in the liver and down-regulation of the antioxidant system.

Monascus-fermented yellow pigments monascin and ankaflavin showed antiobesity effect via the suppression of differentiation and lipogenesis in obese rats fed a high-fat diet

Monascus-fermented monascin and ankaflavin are found to strongly inhibit differentiation and lipogenesis and stimulate lipolysis effects in a 3T3-L1 preadipocyte model, but the in vivo regulation mechanism is unclear. This study uses obese rats caused by a high-fat diet to examine the effects of daily monascin and ankaflavin feeding (8 weeks) on antiobesity effects and modulation of differentiation, lipogenesis, and lipid absorption. The results show that monascin and ankaflavin had a significant antiobesity effect, which should result from the modulation of monascin and ankaflavin on the inhibition of differentiation by inhibiting CCAT/enhancer-binding protein β (C/EBPβ) expression (36.4% and 48.3%) and its downstream peroxisome proliferator-activated receptor γ (PPARγ) (55.6% and 64.5%) and CCAT/enhancer-binding protein α (C/EBPα) expressions (25.2% and 33.2%) and the inhibition of lipogenesis by increasing lipase activity (14.0% and 10.7%) and decreasing heparin releasable lipoprotein lipase (HR-LPL) activity (34.8% and 30.5%). Furthermore, monascin and ankaflavin are the first agents found to suppress Niemann-Pick C1 Like 1 (NPC1L1) protein expression (73.6% and 26.1%) associated with small intestine tissue lipid absorption. Importantly, monascin and ankaflavin are not like monacolin K, which increases creatine phosphokinase (CPK) activity, known as a rhabdomyolysis indicator.

Synergistic interaction of dietary cholesterol and dietary fat in inducing experimental steatohepatitis

The majority of patients with nonalcoholic fatty liver disease (NAFLD) have "simple steatosis," which is defined by hepatic steatosis in the absence of substantial inflammation or fibrosis and is considered to be benign. However, 10%-30% of patients with NAFLD progress to fibrosing nonalcoholic steatohepatitis (NASH), which is characterized by varying degrees of hepatic inflammation and fibrosis, in addition to hepatic steatosis, and can lead to cirrhosis. The cause(s) of progression to fibrosing steatohepatitis are unclear. We aimed to test the relative contributions of dietary fat and dietary cholesterol and their interaction on the development of NASH. We assigned C57BL/6J mice to four diets for 30 weeks: control (4% fat and 0% cholesterol); high cholesterol (HC; 4% fat and 1% cholesterol); high fat (HF; 15% fat and 0% cholesterol); and high fat, high cholesterol (HFHC; 15% fat and 1% cholesterol). The HF and HC diets led to increased hepatic fat deposition with little inflammation and no fibrosis (i.e., simple hepatic steatosis). However, the HFHC diet led to significantly more profound hepatic steatosis, substantial inflammation, and perisinusoidal fibrosis (i.e., steatohepatitis), associated with adipose tissue inflammation and a reduction in plasma adiponectin levels. In addition, the HFHC diet led to other features of human NASH, including hypercholesterolemia and obesity. Hepatic and metabolic effects induced by dietary fat and cholesterol together were more than twice as great as the sum of the separate effects of each dietary component alone, demonstrating significant positive interaction.

CONCLUSION

Dietary fat and dietary cholesterol interact synergistically to induce the metabolic and hepatic features of NASH, whereas neither factor alone is sufficient to cause NASH in mice.

Cholesterol metabolism and the pathogenesis of non-alcoholic steatohepatitis

Emerging experimental and human evidence has linked altered hepatic cholesterol homeostasis and free cholesterol (FC) accumulation to the pathogenesis of non-alcoholic steatohepatits (NASH). This review focuses on cellular mechanisms of cholesterol toxicity involved in liver injury and on alterations in cholesterol homeostasis promoting hepatic cholesterol overload in NASH. FC accumulation injures hepatocytes directly, by disrupting mitochondrial and endoplasmic reticulum (ER) membrane integrity, triggering mitochondrial oxidative injury and ER stress, and by promoting generation of toxic oxysterols, and indirectly, by inducing adipose tissue dysfunction. Accumulation of oxidized LDL particles may also activate Kupffer and hepatic stellate cells, promoting liver inflammation and fibrogenesis. Hepatic cholesterol accumulation is driven by a deeply deranged cellular cholesterol homeostasis, characterized by elevated cholesterol synthesis and uptake from circulating lipoproteins and by a reduced cholesterol excretion. Extensive dysregulation of cellular cholesterol homeostasis by nuclear transcription factors sterol regulatory binding protein (SREBP)-2, liver X-receptor (LXR)-α and farnesoid X receptor (FXR) plays a key role in hepatic cholesterol accumulation in NASH. The therapeutic implications and opportunities for normalizing cellular cholesterol homeostasis in these patients are also discussed.

Effects of ezetimibe on visceral fat in the metabolic syndrome: a randomised controlled study

BACKGROUND

Although visceral obesity, a key abnormality in the metabolic syndrome, is an important risk for cardiovascular diseases, reduction in visceral fat is hard to achieve despite intensive efforts directed at lifestyle modification. The present study was designed to investigate whether ezetimibe, an inhibitor of intestinal cholesterol absorption through its binding to Niemann-Pick C1-like 1, reduces visceral fat in patients with metabolic syndrome.

MATERIALS AND METHODS

Seventy-eight outpatients (63·7 ± 10·4 years old) with metabolic syndrome were enroled and randomly assigned to receive either ezetimibe (10 mg/day) or nothing for 6 months. Changes in visceral fat were assessed by computed tomography.

RESULTS

Treatment with ezetimibe significantly improved lipid profiles. Visceral fat was decreased 7·2%, from 161·3 ± 58·6 cm(2) to 148·4 ± 52·7 cm(2) (P < 0·05), and adiponectin was increased 7·7%, from 3·61 ± 3·10 μg/mL to 3·86 ± 3·62 μg/mL (P < 0·05), after ezetimibe therapy; these beneficial effects were not observed in the control group. The increase in the adiponectin level was correlated with the reduction in visceral fat after ezetimibe treatment. Furthermore, ezetimibe reduced fasting insulin levels (P < 0·05) and improved the homoeostasis model assessment of insulin resistance (HOMA-IR) (P < 0·05).

CONCLUSIONS

Ezetimibe reduces visceral fat with beneficial effects on adiponectin and insulin resistance in patients with metabolic syndrome, suggesting a new therapeutic approach in such patients.

Modulation of lipid metabolism with the overexpression of NPC1L1 in mouse liver

Niemann-Pick C1-like 1 protein (NPC1L1), a transporter crucial in intestinal cholesterol absorption, is expressed in human liver but not in murine liver. To elucidate the role of hepatic NPC1L1 on lipid metabolism, we overexpressed NPC1L1 in murine liver utilizing adenovirus-mediated gene transfer. C57BL/6 mice, fed on normal chow with or without ezetimibe, were injected with NPC1L1 adenovirus (L1-mice) or control virus (Null-mice), and lipid analyses were performed five days after the injection. The plasma cholesterol levels increased in L1-mice, and FPLC analyses revealed increased cholesterol contents in large HDL lipoprotein fractions. These fractions, which showed α-mobility on agarose electrophoresis, were rich in apoE and free cholesterol. These lipoprotein changes were partially inhibited by ezetimibe treatment and were not observed in apoE-deficient mice. In addition, plasma and VLDL triglyceride (TG) levels decreased in L1-mice. The expression of microsomal triglyceride transfer protein (MTP) was markedly decreased in L1-mice, accompanied by the reduced protein levels of forkhead box protein O1 (FoxO1). These changes were not observed in mice with increased hepatic de novo cholesterol synthesis. These data demonstrate that cholesterol absorbed through NPC1L1 plays a distinct role in cellular and plasma lipid metabolism, such as the appearance of apoE-rich lipoproteins and the diminished VLDL-TG secretion.

Inhibition of intestinal cholesterol absorption decreases atherosclerosis but not adipose tissue inflammation

Adipose tissue inflammation is associated with insulin resistance and increased cardiovascular disease risk in obesity. We previously showed that addition of cholesterol to a diet rich in saturated fat and refined carbohydrate significantly worsens dyslipidemia, insulin resistance, adipose tissue macrophage accumulation, systemic inflammation, and atherosclerosis in LDL receptor-deficient (Ldlr(-/-)) mice. To test whether inhibition of intestinal cholesterol absorption would improve metabolic abnormalities and adipose tissue inflammation in obesity, we administered ezetimibe, a dietary and endogenous cholesterol absorption inhibitor, to Ldlr(-/-) mice fed chow or high-fat, high-sucrose (HFHS) diets without or with 0.15% cholesterol (HFHS+C). Ezetimibe blunted weight gain and markedly reduced plasma lipids in the HFHS+C group. Ezetimibe had no effect on glucose homeostasis or visceral adipose tissue macrophage gene expression in the HFHS+C fed mice, although circulating inflammatory markers serum amyloid A (SSA) and serum amyloid P (SSP) levels decreased. Nevertheless, ezetimibe treatment led to a striking (>85%) reduction in atherosclerotic lesion area with reduced lesion lipid and macrophage content in the HFHS+C group. Thus, in the presence of dietary cholesterol, ezetimibe did not improve adipose tissue inflammation in obese Ldlr(-/-) mice, but it led to a major reduction in atherosclerotic lesions associated with improved plasma lipids and lipoproteins.

Double-blind randomized clinical trial of the effects of ezetimibe on postprandial hyperlipidaemia and hyperglycaemia

AIM

Ezetimibe selectively blocks intestinal cholesterol absorption by inhibiting Niemann-Pick C1-like 1 (NPC1L1) and reducing LDL cholesterol (LDL-C). In animals, ezetimibe reversed diet-induced obesity, liver steatosis, and insulin resistance. In humans, its potential effects on liver steatosis and insulin resistance have been suggested. We investigated the effects of ezetimibe on postprandial hyperlipidaemia and hyperglycaemia in obese subjects with dyslipidaemia in a double-blind randomized crossover trial.

METHODS

Twenty obese men with hypertriglyceridaemia were assigned randomly to an ezetimibe- or a placebo-precedence-treated group. Subjects in the ezetimibe group were treated with ezetimibe (10 mg/day) for the first 4 weeks, followed by a 4-week interval and then treated with placebo for another 4 weeks. The placebo group received these treatments in reverse order. Subjects were requested to fast for at least 12 hours and then received a standard meal. Blood samples were collected at 0, 30, 60, 120, 240, 360 and 480 minutes after the meal on Days 0, 28, 56 and 84 and were used to measure the lipid and glucose metabolism markers.

RESULTS

Ezetimibe significantly decreased the postprandial serum triglyceride excursion (p=0.01) and fasting serum LDL-C, remnant-like particles(RLP) and ApoB48 levels (p<0.05). Postprandial glucose excursion, serum insulin levels, serum glucose-dependent insulinotropic polypeptide (GIP) and active glucagon-like peptide-1 (GLP-1) were not significantly affected by ezetimibe treatment.

CONCLUSION

Ezetimibe restored the postprandial dysregulation of lipid but did not affect glucose metabolism in a double-blind randomized crossover trial.

Visualization of lipid metabolism in the zebrafish intestine reveals a relationship between NPC1L1-mediated cholesterol uptake and dietary fatty acid

The small intestine is the primary site of dietary lipid absorption in mammals. The balance of nutrients, microorganisms, bile, and mucus that determine intestinal luminal environment cannot be recapitulated ex vivo, thus complicating studies of lipid absorption. We show that fluorescently labeled lipids can be used to visualize and study lipid absorption in live zebrafish larvae. We demonstrate that the addition of a BODIPY-fatty acid to a diet high in atherogenic lipids enables imaging of enterocyte lipid droplet dynamics in real time. We find that a lipid-rich meal promotes BODIPY-cholesterol absorption into an endosomal compartment distinguishable from lipid droplets. We also show that dietary fatty acids promote intestinal cholesterol absorption by rapid re-localization of NPC1L1 to the intestinal brush border. These data illustrate the power of the zebrafish system to address longstanding questions in vertebrate digestive physiology.

Recent advances in small bowel diseases: Part II

As is the case in all areas of gastroenterology and hepatology, in 2009 and 2010 there were many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed, and the important advances in basic science as well as clinical applications were considered. In Part II we review six topics: absorption, short bowel syndrome, smooth muscle function and intestinal motility, tumors, diagnostic imaging, and cystic fibrosis.

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.

Ezetimibe ameliorates intestinal chylomicron overproduction and improves glucose tolerance in a diet-induced hamster model of insulin resistance

Ezetimibe is a cholesterol uptake inhibitor that targets the Niemann-Pick C1-like 1 cholesterol transporter. Ezetimibe treatment has been shown to cause significant decreases in plasma cholesterol levels in patients with hypercholesterolemia and familial hypercholesterolemia. A recent study in humans has shown that ezetimibe can decrease the release of atherogenic postprandial intestinal lipoproteins. In the present study, we evaluated the mechanisms by which ezetimibe treatment can lower postprandial apoB48-containing chylomicron particles, using a hyperlipidemic and insulin-resistant hamster model fed a diet rich in fructose and fat (the FF diet) and fructose, fat, and cholesterol (the FFC diet). Male Syrian Golden hamsters were fed either chow or the FF or FFC diet ± ezetimibe for 2 wk. After 2 wk, chylomicron production was assessed following intravenous triton infusion. Tissues were then collected and analyzed for protein and mRNA content. FFC-fed hamsters treated with ezetimibe showed improved glucose tolerance, decreased fasting insulin levels, and markedly reduced circulating levels of TG and cholesterol in both the LDL and VLDL fractions. Examination of triglyceride (TG)-rich lipoprotein (TRL) fractions showed that ezetimibe treatment reduced postprandial cholesterol content in TRL lipoproteins as well as reducing apoB48 content. Although ezetimibe did not decrease TRL-TG levels in FFC hamsters, ezetimibe treatment in FF hamsters resulted in decreases in TRL-TG. Jejunal apoB48 protein expression was lower in ezetimibe-treated hamsters. Reductions in jejunal protein levels of scavenger receptor type B-1 (SRB-1) and fatty acid transport protein 4 were also observed. In addition, ezetimibe-treated hamsters showed significantly lower jejunal mRNA expression of a number of genes involved in lipid synthesis and transport, including srebp-1c, sr-b1, ppar-γ, and abcg1. These data suggest that treatment with ezetimibe not only inhibits cholesterol uptake, but may also alter intestinal function to promote improved handling of dietary lipids and reduced chylomicron production. These, in turn, promote decreases in fasting and postprandial lipid levels and improvements in glucose homeostasis.

Dietary fat is a lipid source in 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (TCDD)-elicited hepatic steatosis in C57BL/6 mice

2,3,7,8-Tetrachlorodibenzo-ρ-dioxin (TCDD) increases fatty acid (FA) transport and FA levels resulting in hepatic steatosis in mice. Diet as a source of lipids was investigated using customized diets, stearoyl-CoA desaturase 1 (Scd1) null mice, and (14)C-oleate (18:1n9) uptake studies. C57BL/6 mice fed with 5, 10, or 15% fat or 50, 60 or 70% carbohydrate diets exhibited increased relative liver weight following gavage with 30 µg/kg TCDD for 168 h. Hepatic lipid extract analysis from mice fed with 5, 10, and 15% fat diets identified a dose-dependent increase in total FAs induced by TCDD. Mice fed with fat diet also exhibited a dose-dependent increase in the dietary essential linoleic (18:2n6) and α-linolenic (18:3n3) acids. No dose-dependent FA increase was detected on carbohydrate diets, suggesting dietary fat as a source of lipids in TCDD-induced steatosis as opposed to de novo lipogenesis. TCDD also induced oleate levels threefold in Scd1 null mice that are incapable of desaturating stearate (18:0). This is consistent with oleate representing > 90% of all monounsaturated FAs in rodent chow. Moreover, TCDD increased hepatic (14)C-oleate levels twofold in wild type and 2.4-fold in Scd1 null mice concurrent with the induction of intestinal and hepatic lipid transport genes (Slc27a, Fabp, Ldlr, Cd36, and Apob). In addition, computational scanning identified putative dioxin response elements and in vivo ChIP-chip analysis revealed regions of aryl hydrocarbon receptor (AhR) enrichment in lipid transport genes differentially regulated by TCDD. Collectively, these results suggest the AhR mediates increased uptake of dietary fats that contribute to TCDD-elicited hepatic steatosis.

Therapeutic effects of ezetimibe for non-alcoholic steatohepatitis in fatty liver shionogi-ob/ob mice

AIM

  An effective therapy for non-alcoholic steatohepatitis has yet to be defined. This study examined the therapeutic effects of ezetimibe, a lipid-lowering medication, on steatosis and hepatic fibrosis in fatty liver Shionogi ob/ob (FLS-ob) mice.

METHODS

  Low-dose (0.2 mg/kg body weight) and high-dose (1.0 mg/kg body weight) of ezetimibe were administered to FLS-ob mice orally for 12 weeks.

RESULTS

  Administration of ezetimibe significantly and dose-dependently decreased liver cholesterol content. The area of hepatic fibrosis and hepatic hydroxyproline content in the low- and high-dose groups were significantly decreased compared with controls. Areas of α-smooth muscle actin positivity and F4/80 positivity were significantly decreased in a dose-dependent manner. Percentages of 8-hydroxy-2-deoxyguanosine-positive cells in low- and high-dose groups were significantly decreased compared with those in controls, and 8-hydroxy-2-deoxyguanosine DNA content and thiobarbituric acid reactive substances in the high-dose group was also significantly decreased compared to controls. Gene expression levels of procollagen I and transforming growth factor β1 mRNA levels were lower in the low- and high-dose groups than in controls. Tumor necrosis factor-α and sterol regulatory element-binding protein 1c mRNA levels were also lower in the low- and high-dose groups than in controls.

CONCLUSIONS

  Ezetimibe attenuated steatosis and liver fibrosis by reducing oxidative stress and lipid peroxidation and suppressing activated hepatic stellate cells and Kupffer cells.

Ezetimibe: its novel effects on the prevention and the treatment of cholesterol gallstones and nonalcoholic Fatty liver disease

The cholesterol absorption inhibitor ezetimibe can significantly reduce plasma cholesterol concentrations by inhibiting the Niemann-Pick C1-like 1 protein (NPC1L1), an intestinal sterol influx transporter that can actively facilitate the uptake of cholesterol for intestinal absorption. Unexpectedly, ezetimibe treatment also induces a complete resistance to cholesterol gallstone formation and nonalcoholic fatty liver disease (NAFLD) in addition to preventing hypercholesterolemia in mice on a Western diet. Because chylomicrons are the vehicles with which the enterocytes transport cholesterol and fatty acids into the body, ezetimibe could prevent these two most prevalent hepatobiliary diseases possibly through the regulation of chylomicron-derived cholesterol and fatty acid metabolism in the liver. It is highly likely that there is an intestinal and hepatic cross-talk through the chylomicron pathway. Therefore, understanding the molecular mechanisms whereby cholesterol and fatty acids are absorbed from the intestine could offer an efficacious novel approach to the prevention and the treatment of cholesterol gallstones and NAFLD.

Impact of murine intestinal apolipoprotein A-IV expression on regional lipid absorption, gene expression, and growth

Apolipoprotein A-IV (apoA-IV) is synthesized by intestinal enterocytes during lipid absorption and secreted into lymph on the surface of nascent chylomicrons. A compelling body of evidence supports a central role of apoA-IV in facilitating intestinal lipid absorption and in regulating satiety, yet a longstanding conundrum is that no abnormalities in fat absorption, feeding behavior, or weight gain were observed in chow-fed apoA-IV knockout (A4KO) mice. Herein we reevaluated the impact of apoA-IV expression in C57BL6 and A4KO mice fed a high-fat diet. Fat balance and lymph cannulation studies found no effect of intestinal apoA-IV gene expression on the efficiency of fatty acid absorption, but gut sac transport studies revealed that apoA-IV differentially modulates lipid transport and the number and size of secreted triglyceride-rich lipoproteins in different anatomic regions of the small bowel. ApoA-IV gene deletion increased expression of other genes involved in chylomicron assembly, impaired the ability of A4KO mice to gain weight and increase adipose tissue mass, and increased the distal gut hormone response to a high-fat diet. Together these findings suggest that apoA-IV may play a unique role in integrating feeding behavior, intestinal lipid absorption, and energy storage.

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.

Hepatic Niemann-Pick C1-like 1

PURPOSE OF REVIEW

The discovery of Niemann-Pick C1-like 1 (NPC1L1) and ezetimibe, a drug that lowers intestinal cholesterol absorption, has contributed to the recognition of the intestine as an important organ in whole-body cholesterol homeostasis. Unfortunately, the majority of the studies on NPC1L1 have been conducted in rodent models, which, in contrast to humans, do not express this protein in the liver. Thus the function of NPC1L1 in the liver is still not defined in detail. In this review, we discuss some of the recent progress in the understanding of the role of hepatic NPC1L1 in cholesterol metabolism.

RECENT FINDINGS

Mice expressing human NPC1L1 in the liver have decreased biliary cholesterol concentration, suggesting the involvement of this protein in the hepatic reabsorption of biliary cholesterol. Studies in gallstone patients have shown that only women have decreased hepatic NPC1L1 expression, suggesting a possible role for the sex-related differences in cholesterol gallstone disease. Also, several transcription factors (e.g., sterol regulatory element-binding protein 2, hepatocyte nuclear factor 1α) appear to modulate the expression of NPC1L1.

SUMMARY

Evidence suggests the involvement of NPC1L1 in biliary cholesterol uptake, HDL metabolism and cholesterol gallstone disease. Although difficult, studies in humans are required to further elucidate the function of this protein in the liver.

Ezetimibe decreases SREBP-1c expression in liver and reverses hepatic insulin resistance in mice fed a high-fat diet

Ezetimibe inhibits intestinal cholesterol absorption, thereby reducing serum cholesterol. Recent studies suggest that ezetimibe affects liver steatosis and insulin resistance. We investigated the impact of ezetimibe on insulin sensitivity and glucose metabolism in C57BL/6 mice. We analyzed 4 mouse groups fed the following diets: normal chow (4% fat) for 12 weeks, normal chow for 10 weeks followed by normal chow plus ezetimibe for 2 weeks, high-fat chow (32% fat) for 12 weeks, and high-fat chow for 10 weeks followed by high-fat chow plus ezetimibe for 2 weeks. In the normal chow + ezetimibe group, ezetimibe had no impact on body weight, fat mass, lipid metabolism, liver steatosis, glucose tolerance, or insulin sensitivity. In the high-fat chow + ezetimibe group, ezetimibe had no impact on body weight or fat mass but significantly decreased serum low-density lipoprotein cholesterol, triglyceride, and glutamate pyruvate transaminase levels; liver weight; hepatic triglyceride content; and hepatic cholesterol content and increased the hepatic total bile acid content. In association with increases in IRS-2 and Akt phosphorylation, ezetimibe ameliorated hepatic insulin resistance in the high-fat chow + ezetimibe group, but had no effect on insulin sensitivity in primary cultured hepatocytes. A DNA microarray and Taqman polymerase chain reaction revealed that ezetimibe up-regulated hepatic SREBP2 and SHP expression and down-regulated hepatic SREBP-1c expression. SHP silencing mainly in the liver worsened insulin resistance, and ezetimibe protected against insulin resistance induced by down-regulation of SHP. Ezetimibe down-regulated SREBP-1c in the liver and reversed hepatic insulin resistance in mice fed a high-fat diet.

Effects of adiposity on plasma lipid response to reductions in dietary saturated fatty acids and cholesterol

Dietary SFA and cholesterol are major targets for reducing plasma total and LDL cholesterol as a strategy to decrease cardiovascular disease risk. However, many studies show that excess adiposity attenuates the expected lipid and lipoprotein response to a plasma cholesterol-lowering diet. Diets low in SFA and cholesterol are less effective in improving the lipid profile in obese individuals and in patients with metabolic syndrome. In contrast, lean persons are more responsive to reductions in dietary SFA and cholesterol. Multiple mechanisms likely contribute to the altered plasma lipid responses to dietary changes in individuals with excess adiposity. The greater rate of hepatic cholesterol synthesis in obese individuals suppresses the expression of hepatic LDL receptors (LDLR), thereby reducing hepatic LDL uptake. Insulin resistance develops as a result of adipose-tissue induced inflammation, causing significant changes in enzymes necessary for normal lipid metabolism. In addition, the LDLR-mediated uptake in obesity is attenuated by alterations in neuroendocrine regulation of hormonal secretions (e.g. growth hormone, thyroid hormone, and cortisol) as well as the unique gut microbiota, the latter of which appears to affect lipid absorption. Reducing adipose tissue mass, especially from the abdominal region, is an effective strategy to improve the lipid response to dietary interventions by reducing inflammation, enhancing insulin sensitivity, and improving LDLR binding. Thus, normalizing adipose tissue mass is an important goal for maximizing the diet response to a plasma cholesterol-lowering diet.

Efficacy of long-term ezetimibe therapy in patients with nonalcoholic fatty liver disease

BACKGROUND

Hyperlipidemia, insulin resistance, and oxidative stress can heavily contribute to the initiation and progression of nonalcoholic fatty liver disease (NAFLD). Currently, there is no established treatment for this disease. Recently, several studies have shown that ezetimibe (EZ), a lipid-lowering drug, attenuates liver steatosis in an experimental NAFLD model. This study was designed to assess the efficacy of long-term EZ monotherapy in patients with NAFLD.

METHODS

A total of 45 patients with newly diagnosed liver biopsy-proven NAFLD were treated with EZ (10 mg/day) for 24 months. NAFLD-related biochemical parameters, imaging by computerized tomography, and liver biopsy were studied before and after treatment.

RESULTS

Ezetimibe therapy significantly improved NAFLD-related metabolic parameters including visceral fat area, fasting insulin, homeostasis model assessment of insulin resistance (HOMA-R), triglycerides, total cholesterol, low-density lipoprotein cholesterol (LDL-Ch), oxidative-LDL, the net electronegative charge modified-LDL, profiles of lipoprotein particle size and fatty acids component, and estimated desaturase activity. EZ therapy also significantly lowered serum alanine aminotransferase and high-sensitivity C-reactive protein levels, whereas no significant changes were found in serum type IV collagen 7S, adiponectin, leptin, and resistin levels. Histological features of steatosis grade (P = 0.0003), necroinflammatory grade (P = 0.0456), ballooning score (P = 0.0253), and NAFLD activity score (NAS) (P = 0.0007) were significantly improved from baseline. However, the fibrosis stage was not significantly (P = 0.6547) changed.

CONCLUSION

The results in this study suggest that the long-term EZ therapy can lead to improvement in metabolic, biochemical, and histological abnormalities of NAFLD. Therefore, EZ may be a promising agent for treatment of NAFLD.

Effect of ezetimibe on incretin secretion in response to the intestinal absorption of a mixed meal

Ezetimibe is a potent inhibitor of cholesterol absorption by enterocytes. Although ezetimibe minimally affects the absorption of triglyceride, it is unknown whether ezetimibe affects the secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). It has been shown that ezetimibe-treated mice are protected from diet-induced insulin resistance. Since GIP and GLP-1 promote the actions of insulin, we hypothesized that ezetimibe may affect the secretion of GIP and GLP-1 by enteroendocrine cells into lymph in response to the intestinal absorption of a mixed meal (Ensure). To test this hypothesis, we used the lymph fistula rat model to determine GIP and GLP-1 concentrations in lymph during the 2 h after the infusion of Ensure. Ezetimibe significantly reduced lymphatic cholesterol output during fasting, without coincident decreases in glucose, protein, and triglyceride outputs. However, ezetimibe did not influence cholesterol output after infusion of Ensure. Interestingly, ezetimibe significantly reduced the secretion of both GIP and GLP-1 into lymph after the infusion of Ensure. Therefore, the inhibitory effect of ezetimibe on GIP and GLP-1 secretion by enteroendocrine cells occurs outside of the effects of glucose, protein, or triglyceride secretion by the intestine.

Identification of novel inhibitors of dietary lipid absorption using zebrafish

Pharmacological inhibition of dietary lipid absorption induces favorable changes in serum lipoprotein levels in patients that are at risk for cardiovascular disease and is considered an adjuvant or alternative treatment with HMG-CoA reductase inhibitors (statins). Here we demonstrate the feasibility of identifying novel inhibitors of intestinal lipid absorption using the zebrafish system. A pilot screen of an unbiased chemical library identified novel compounds that inhibited processing of fluorescent lipid analogues in live zebrafish larvae. Secondary assays identified those compounds suitable for testing in mammals and provided insight into mechanism of action, which for several compounds could be distinguished from ezetimibe, a drug used to inhibit cholesterol absorption in humans that broadly inhibited lipid absorption in zebrafish larvae. These findings support the utility of zebrafish screening assays to identify novel compounds that target complex physiological processes.

Niemann-Pick C1-Like 1 deletion in mice prevents high-fat diet-induced fatty liver by reducing lipogenesis

Niemann-Pick C1-Like 1 (NPC1L1) mediates intestinal absorption of dietary and biliary cholesterol. Ezetimibe, by inhibiting NPC1L1 function, is widely used to treat hypercholesterolemia in humans. Interestingly, ezetimibe treatment appears to attenuate hepatic steatosis in rodents and humans without a defined mechanism. Over-consumption of a high-fat diet (HFD) represents a major cause of metabolic disorders including fatty liver. To determine whether and how NPC1L1 deficiency prevents HFD-induced hepatic steatosis, in this study, we fed NPC1L1 knockout (L1-KO) mice and their wild-type (WT) controls an HFD, and found that 24 weeks of HFD feeding causes no fatty liver in L1-KO mice. Hepatic fatty acid synthesis and levels of mRNAs for lipogenic genes are substantially reduced but hepatic lipoprotein-triglyceride production, fatty acid oxidation, and triglyceride hydrolysis remain unaltered in L1-KO versus WT mice. Strikingly, L1-KO mice are completely protected against HFD-induced hyperinsulinemia under both fed and fasted states and during glucose challenge. Despite similar glucose tolerance, L1-KO relative WT mice are more insulin sensitive and in the overnight-fasted state display significantly lower plasma glucose concentrations. In conclusion, NPC1L1 deficiency in mice prevents HFD-induced fatty liver by reducing hepatic lipogenesis, at least in part, through attenuating HFD-induced insulin resistance, a state known to drive hepatic lipogenesis through elevated circulating insulin levels.

Dietary cholesterol reverses resistance to diet-induced weight gain in mice lacking Niemann-Pick C1-Like 1

Niemann-Pick C1-Like 1 (NPC1L1) mediates intestinal cholesterol absorption. NPC1L1 knockout (L1-KO) mice were recently shown to be resistant to high-fat diet (HFD)-induced obesity in one study, which was contrary to several other studies. Careful comparison of dietary compositions in these studies implies a potential role of dietary cholesterol in regulating weight gain. To examine this potential, wild-type (WT) and L1-KO mice were fed one of three sets of diets for various durations: (1) a HFD without added cholesterol for 5 weeks; (2) a high-carbohydrate diet with or without added cholesterol for 5 weeks; or (3) a synthetic HFD with or without added cholesterol for 18 weeks. We found that L1-KO mice were protected against diet-induced weight gain only on a diet without added cholesterol but not on a diet containing 0.16% or 0.2% (w/w) cholesterol, an amount similar to a typical Western diet, regardless of the major energy source of the diet. Food intake and intestinal fat absorption were similar between the two genotypes. Intestinal cholesterol absorption was blocked, and fecal cholesterol excretion increased in L1-KO mice. Under all diets, L1-KO mice were protected from hepatosteatosis. In conclusion, increasing dietary cholesterol restores diet-induced weight gain in mice deficient in NPC1L1-dependent cholesterol absorption.

NPC1L1 and cholesterol transport

The polytopic transmembrane protein, Niemann-Pick C1-Like 1 (NPC1L1), is enriched in the apical membrane of small intestine absorptive enterocytes where it mediates extracellular sterol transport across the brush border membrane. It is essential for intestinal sterol absorption and is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that lowers blood cholesterol in humans. NPC1L1 is also highly expressed in human liver. The hepatic function of NPC1L1 may be to limit excessive biliary cholesterol loss. NPC1L1-dependent sterol uptake seems to be a clathrin-mediated endocytic process and is regulated by cellular cholesterol content. Recently, NPC1L1 inhibition has been shown to have beneficial effects on components of the metabolic syndrome, such as obesity, insulin resistance, and fatty liver, in addition to atherosclerosis.

Molecular mechanisms of ezetimibe-induced attenuation of postprandial hypertriglyceridemia

AIM

Postprandial hypertriglyceridemia (PHTG) has been shown repeatedly to be associated with metabolic syndrome and atherosclerotic cardiovascular diseases. We have recently reported that ezetimibe inhibits PHTG in patients with type IIb hyperlipidemia. Ezetimibe was also reported to atten-uate PHTG in combination with low-dose statins in patients with obesity or metabolic syndrome. We reported CD36-deficient (CD36KO) mice as a new model for PHTG, in which the synthesis of chylomicron (CM) in the small intestines is enhanced. In the current study, we investigated the effect of ezetimibe on PHTG in this mouse model of metabolic syndrome.

METHODS

Wild-type (WT) mice fed a western diet, and CD36KO mice fed a normal chow diet, respectively, were treated for 3 weeks with and without ezetimibe, followed by an evaluation of triglyceride (TG) concentrations by enzymatic method and by high performance liquid chromatogra-phy (HPLC) as well as those of and apolipoprotein (Apo) B-48 in plasma and intestinal lymph after oral fat loading with olive oil. Intestinal mucosa was also harvested to evaluate the transcriptional regulation of the genes involved in the intestinal production of ApoB-containing lipoproteins.

RESULTS

Ezetimibe dramatically reduced PHTG in both WT and CD36KO mice. HPLC analysis of plasma showed that the decrease in TG content in CM and CM remnants-sized particles contributed to this suppression, suggesting that CM production in the small intestines might be reduced after ezetimibe treatment. Intestinal lymph was collected after oral fat loading in ezetimibe-treated and non-treated mice. Both TG content and ApoB-48 mass were decreased in ezetimibe-treated mice. The quantitative RT-PCR of intestinal mucosa showed down-regulation of the mRNA expression of FATP4 and ApoB in both groups along with FABP2, DGAT1, DGAT2 and SCD1 in WT mice at postprandial state after ezetimibe treatment.

CONCLUSION

Ezetimibe alone reduces PHTG by blocking both the absorption of cholesterol and the intracellular trafficking and metabolism of long-chain fatty acids in enterocytes, resulting in the reduction of the formation of ApoB-48 which is necessary for the ApoB48-containing lipoprotein production in the small intestines.

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.

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.

Transporters as drug targets: discovery and development of NPC1L1 inhibitors

The potent cholesterol absorption inhibitor ezetimibe was developed as a first-in-class drug for treating hypercholesterolemia even before its molecular target, Niemann-Pick C1-like 1 (NPC1L1), had been identified. The NPC1L1 protein mediates sterol transport across the enterocyte brush border membrane and is essential for intestinal cholesterol absorption, a major pathway controlling whole-body cholesterol homeostasis. An elucidation of the mechanism underlying NPC1L1-dependent cholesterol absorption would greatly facilitate the discovery and development of new cholesterol-lowering agents for treating hypercholesterolemia and other cholesterol-related metabolic disorders.

Ezetimibe improves postprandial hyperlipidaemia in patients with type IIb hyperlipidaemia

BACKGROUND

Postprandial hyperlipidaemia is known to be a high-risk factor for atherosclerotic disease because of rapid and lasting accumulations of triglyceride-rich lipoproteins and remnants. The Niemann-Pick C1-Like 1 (NPC1L1) protein acts as an intestinal cholesterol transporter and ezetimibe, which inhibits NPC1L1, has been used in patients with hypercholesterolaemia. We investigated effects of ezetimibe on fasting lipid and lipoprotein profiles and postprandial hyperlipidaemia in patients with type IIb hyperlipidaemia.

MATERIALS AND METHODS

Ezetimibe 10 mg per day was administered in ten patients with type IIb hyperlipidaemia for 2 months, and lipid and lipoprotein profiles were examined during fasting and after an oral fat loading (OFL) test.

RESULTS

In the fasting state, ezetimibe significantly decreased not only total cholesterol, low density lipoprotein (LDL)-cholesterol and apolipoproteinB-100 (apoB-100) levels but triglycerides (TG), apoB-48 and remnant lipoprotein cholesterol (RemL-C) levels. High performance liquid chromatography analysis showed that ezetimibe decreased cholesterol and TG levels in the very low density lipoprotein (VLDL) and LDL size ranges as well as apoB-100 levels, suggesting a decrease in numbers of VLDL and LDL particles. After OFL, ezetimibe decreased the area under the curve for TG, apoB-48 and RemL-C. Ezetimibe decreased postprandial elevations of cholesterol and TG levels in the chylomicrons (CM) size range, suggesting that the postprandial production of CM particles was suppressed by ezetimibe.

CONCLUSIONS

These findings suggest that ezetimibe improves fasting lipoprotein profiles and postprandial hyperlipidaemia by suppressing intestinal CM production in patients with type IIb hyperlipidaemia and such treatment may prove to be effective in reducing atherosclerosis.

Peroxisome proliferator-activated receptor delta activation leads to increased transintestinal cholesterol efflux

Peroxisome proliferator-activated receptor delta (PPARdelta) is involved in regulation of energy homeostasis. Activation of PPARdelta markedly increases fecal neutral sterol secretion, the last step in reverse cholesterol transport. This phenomenon can neither be explained by increased hepatobiliary cholesterol secretion, nor by reduced cholesterol absorption. To test the hypothesis that PPARdelta activation leads to stimulation of transintestinal cholesterol efflux (TICE), we quantified it by intestine perfusions in FVB mice treated with PPARdelta agonist GW610742. To exclude the effects on cholesterol absorption, mice were also treated with cholesterol absorption inhibitor ezetimibe or ezetimibe/GW610742. GW601742 treatment had little effect on plasma lipid levels but stimulated both fecal neutral sterol excretion ( approximately 200%) and TICE ( approximately 100%). GW610742 decreased intestinal Npc1l1 expression but had no effect on Abcg5/Abcg8. Interestingly, expression of Rab9 and LIMPII, encoding proteins involved in intracellular cholesterol trafficking, was increased upon PPARdelta activation. Although treatment with ezetimibe alone had no effect on TICE, it reduced the effect of GW610742 on TICE. These data show that activation of PPARdelta stimulates fecal cholesterol excretion in mice, primarily by the two-fold increase in TICE, indicating that this pathway provides an interesting target for the development of drugs aiming at the prevention of atherosclerosis.

Opposing Gatekeepers of Apical Sterol Transport: Niemann-Pick C1-Like 1 (NPC1L1) and ATP-Binding Cassette Transporters G5 and G8 (ABCG5/ABCG8)

Cholesterol is essential for the growth and function of all mammalian cells, but abnormally elevated levels of circulating low-density lipoprotein cholesterol (LDL-C) are a major risk factor for the development of atherosclerotic cardiovascular disease (ASCVD). For many years, statin drugs have been used to effectively lower LDL-C, but ASCVD still persists in most of the world. Hence, additional LDL-C lowering is now recommended, and the search for therapeutic strategies that work in synergy with statins has now begun. Intestinal absorption and biliary excretion of cholesterol represent two major pathways and continue to show promise as druggable processes. Importantly, both of these complex physiological pathways are tightly regulated by key proteins located at the apical surface of the small intestine and the liver. One of these proteins, the target of ezetimibe Niemann-Pick C1-Like 1 (NPC1L1), was recently identified to be essential for intestinal cholesterol absorption and protect against excessive biliary sterol loss. In direct opposition of NPC1L1, the heterodimer of ATP-binding cassette transporters G5 and G8 (ABCG5/ABCG8) has been shown to be critical for promoting biliary cholesterol secretion in the liver, and has also been proposed to play a direct role in intestinal disposal of sterols. The purpose of this review is to summarize the current state of knowledge regarding the function of these opposing apical cholesterol transporters, and provide a framework for future studies examining these proteins.