Ezetimibe restores biliary cholesterol excretion in mice expressing Niemann-Pick C1-Like 1 only in liver

Niemann-Pick C1-Like 1 (NPC1L1) is highly expressed in the small intestine across mammalian species and is the target of ezetimibe, a potent cholesterol absorption inhibitor. In humans, NPC1L1 is also expressed in the liver. We found that transgenic overexpression of NPC1L1 in the wild-type mouse liver inhibits biliary cholesterol secretion and raises blood cholesterol, which can be reversed by ezetimibe treatment. Unfortunately, the high expression of endogenous NPC1L1 in the intestine hampered a definitive establishment of the role of hepatic NPC1L1 in cholesterol metabolism and ezetimibe action in the liver because intestinal NPC1L1 dramatically influences cholesterol homeostasis and is a target of ezetimibe. To circumvent this obstacle, we crossed liver-specific NPC1L1 transgenic mice to NPC1L1 knockout (L1-KO) mice and created a mouse line expressing no endogenous NPC1L1, but human NPC1L1 in liver only (L1(LivOnly) mice). Compared to L1-KO mice, L1(LivOnly) mice on a 0.2% cholesterol diet showed significantly increased hepatic and plasma cholesterol, and despite a 90% reduction in biliary cholesterol excretion, their fecal cholesterol excretion remained completely unaltered. Remarkably, 4days of ezetimibe treatment significantly restored biliary cholesterol secretion in L1(LivOnly) mice. These findings demonstrated a direct role of hepatic NPC1L1 in regulating biliary cholesterol excretion and hepatic/blood cholesterol levels, and unequivocally established hepatic NPC1L1 as a target of ezetimibe.     

Ezetimibe interferes with cholesterol trafficking from the plasma membrane to the endoplasmic reticulum in CaCo-2 cells

Niemann-Pick C1-like 1 protein (NPC1L1) is the putative intestinal sterol transporter and the molecular target of ezetimibe, a potent inhibitor of cholesterol absorption. To address the role of NPC1L1 in cholesterol trafficking in intestine, the regulation of cholesterol trafficking by ezetimibe was studied in the human intestinal cell line, CaCo-2. Ezetimibe caused only a modest decrease in the uptake of micellar cholesterol, but markedly prevented its esterification. Cholesterol trafficking from the plasma membrane to the endoplasmic reticulum was profoundly disrupted by ezetimibe without altering the trafficking of cholesterol from the endoplasmic reticulum to the plasma membrane. Cholesterol oxidase-accessible cholesterol at the apical membrane was increased by ezetimibe. Cholesterol synthesis was modestly increased. Although the amount of cholesteryl esters secreted at the basolateral membrane was markedly decreased by ezetimibe, the transport of lipids and the number of lipoprotein particles secreted were not altered. NPC1L1 gene and protein expression were decreased by sterol influx, whereas cholesterol depletion enhanced NPC1L1 gene and protein expression. These results suggest that NPC1L1 plays a role in cholesterol uptake and cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. Interfering with its function will profoundly decrease the amount of cholesterol transported into lymph.     

Niemann-Pick C1 Like 1 (NPC1L1) is the intestinal phytosterol and cholesterol transporter and a key modulator of whole-body cholesterol homeostasis

Niemann-Pick C1 Like 1 (NPC1L1) is a protein localized in jejunal enterocytes that is critical for intestinal cholesterol absorption. The uptake of intestinal phytosterols and cholesterol into absorptive enterocytes in the intestine is not fully defined on a molecular level, and the role of NPC1L1 in maintaining whole body cholesterol homeostasis is not known. NPC1L1 null mice had substantially reduced intestinal uptake of cholesterol and sitosterol, with dramatically reduced plasma phytosterol levels. The NPC1L1 null mice were completely resistant to diet-induced hypercholesterolemia, with plasma lipoprotein and hepatic cholesterol profiles similar to those of wild type mice treated with the cholesterol absorption inhibitor ezetimibe. Cholesterol/cholate feeding resulted in down-regulation of intestinal NPC1L1 mRNA expression in wild type mice. NPC1L1 deficiency resulted in up-regulation of intestinal hydroxymethylglutaryl-CoA synthase mRNA and an increase in intestinal cholesterol synthesis, down-regulation of ABCA1 mRNA, and no change in ABCG5 and ABCG8 mRNA expression. NPC1L1 is required for intestinal uptake of both cholesterol and phytosterols and plays a major role in cholesterol homeostasis. Thus, NPC1L1 may be a useful drug target for the treatment of hypercholesterolemia and sitosterolemia.     

Ezetimibe improves liver steatosis and insulin resistance in obese rat model of metabolic syndrome

Non-alcoholic fatty liver disease (NAFLD) is associated with the metabolic syndrome characterized by dislipidemia and insulin resistance. We hypothesized that ezetimibe, an inhibitor of NPC1L1, improves these metabolic disorders in Zucker obese fatty rats (ZOF). Ezetimibe significantly lowered total cholesterol and triglycerides in ZOF with prominent reduction in the remnant lipoprotein fraction and small dense low density lipoprotein fraction. Moreover, lipid deposition and fibrosis of liver were decreased by ezetimibe. Interestingly, ezetimibe improved insulin and plasma glucose response after intraperitoneal glucose injection. Further, ezetimibe enhanced insulin signaling in cultured hepatocytes. Our results indicate the potential of ezetimibe in treating the metabolic syndrome and NAFLD.     

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

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

Niemann–Pick C1 Like 1 (NPC1L1) an intestinal sterol transporter

Niemann–Pick C1 Like 1 (NPC1L1) has been identified and characterized as an essential protein in the intestinal cholesterol absorption process. NPC1L1 localizes to the brush border membrane of absorptive enterocytes in the small intestine. Intestinal expression of NPC1L1 is down regulated by diets containing high levels of cholesterol. While otherwise phenotypically normal, Npc1l1 null mice exhibit a significant reduction in the intestinal uptake and absorption of cholesterol and phytosterols. Characterization of the NPC1L1 pathway revealed that cholesterol absorption inhibitor ezetimibe specifically binds to an extracellular loop of NPC1L1 and inhibits its sterol transport function. Npc1l1 null mice are resistant to diet-induced hypercholesterolemia, and when crossed with apo E null mice, are completely resistant to the development of atherosclerosis. Intestinal gene expression studies in Npc1l1 null mice indicated that no exogenous cholesterol was entering enterocytes lacking NPC1L1, which resulted in an upregulation of intestinal and hepatic LDL receptor and cholesterol biosynthetic gene expression. Polymorphisms in the human NPC1L1 gene have been found to influence cholesterol absorption and plasma low density lipoprotein levels. Therefore, NPC1L1 is a critical intestinal sterol uptake transporter which influences whole body cholesterol homeostasis.     

Inhibiting intestinal NPC1L1 activity prevents diet-induced increase in biliary cholesterol in Golden Syrian hamsters

Niemann-Pick C1-like 1 (NPC1L1) facilitates the uptake of sterols into the enterocyte and is the target of the novel cholesterol absorption inhibitor, ezetimibe. These studies used the Golden Syrian hamster as a model to delineate the changes in the relative mRNA expression of NPC1L1 and other proteins that regulate sterol homeostasis in the enterocyte during and following cessation of ezetimibe treatment and also to address the clinically important question of whether the marked inhibition of cholesterol absorption alters biliary lipid composition. In hamsters fed a low-cholesterol, low-fat basal diet, the abundance of mRNA for NPC1L1 in the small intestine far exceeded that in other regions of the gastrointestinal tract, liver, and gallbladder. In the first study, female hamsters were fed the basal diet containing ezetimibe at doses up to 2.0 mg.day(-1).kg body wt(-1). At this dose, cholesterol absorption fell by 82%, fecal neutral sterol excretion increased by 5.3-fold, and hepatic and intestinal cholesterol synthesis increased more than twofold, but there were no significant changes in either fecal bile acid excretion or biliary lipid composition. The ezetimibe-induced changes in intestinal cholesterol handling were reversed when treatment was withdrawn. In a second study, male hamsters were given a diet enriched in cholesterol and safflower oil without or with ezetimibe. The lipid-rich diet raised the absolute and relative cholesterol levels in bile more than fourfold. This increase was largely prevented by ezetimibe. These data are consistent with the recent finding that ezetimibe treatment significantly reduced biliary cholesterol saturation in patients with gallstones.     

Characterization of liver disease and lipid metabolism in the Niemann-Pick C1 mouse

Niemann-Pick type C1 (NPC1) disease is an autosomal-recessive cholesterol-storage disorder characterized by liver dysfunction, hepatosplenomegaly, and progressive neurodegeneration. The NPC1 gene is expressed in every tissue of the body, with liver expressing the highest amounts of NPC1 mRNA and protein. A number of studies have now indicated that the NPC1 protein regulates the transport of cholesterol from late endosomes/lysosomes to other cellular compartments involved in maintaining intracellular cholesterol homeostasis. The present study characterizes liver disease and lipid metabolism in NPC1 mice at 35 days of age before the development of weight loss and neurological symptoms. At this age, homozygous affected (NPC1(-/-)) mice were characterized with mild hepatomegaly, an elevation of liver enzymes, and an accumulation of liver cholesterol approximately four times that measured in normal (NPC1(+/+)) mice. In contrast, heterozygous (NPC1(+/-)) mice were without hepatomegaly and an elevation of liver enzymes, but the livers had a significant accumulation of triacylglycerol. With respect to apolipoprotein and lipoprotein metabolism, the results indicated only minor alterations in NPC1(-/-) mouse serum. Finally, compared to NPC1(+/+) mouse livers, the amount and processing of SREBP-1 and -2 proteins were significantly increased in NPC1(-/-) mouse livers, suggesting a relative deficiency of cholesterol at the metabolically active pool of cholesterol located at the endoplasmic reticulum. The results from this study further support the hypothesis that an accumulation of lipoprotein-derived cholesterol within late endosomes/lysosomes, in addition to altered intracellular cholesterol homeostasis, has a key role in the biochemical and cellular pathophysiology associated with NPC1 liver disease.     

禁食状态下MED1肝脏特异性敲除鼠呈现高脂血症

为研究MED1如何影响血浆脂质代谢,以MED1肝脏特异性敲除鼠 (MED1ΔLiv) 为动物模型,禁食0、24、48、72 h,通过H&E染色切片观察了MED1ΔLiv鼠肝脏的形态学变化,运用甘油三酯和胆固醇酶试剂盒及FPLC方法分析了MED1ΔLiv和对照鼠 (MED1fl/fl) 血浆甘油三酯和胆固醇的水平以及脂蛋白的分布情况。研究结果显示,禁食72 h,与MED1fl/fl和PPARα?/?对照鼠相比,MED1ΔLiv鼠肝脏无脂肪沉积。禁食24、48、72 h,与MED1fl/fl鼠相比,MED1Δ     

Ezetimibe blocks the internalization of NPC1L1 and cholesterol in mouse small intestine

The multiple transmembrane protein Niemann-Pick C1 like1 (NPC1L1) is essential for intestinal cholesterol absorption. Ezetimibe binds to NPC1L1 and is a clinically used cholesterol absorption inhibitor. Recent studies in cultured cells have shown that NPC1L1 mediates cholesterol uptake through vesicular endocytosis that can be blocked by ezetimibe. However, how NPC1L1 and ezetimibe work in the small intestine is unknown. In this study, we found that NPC1L1 distributed in enterocytes of villi and transit-amplifying cells of crypts. Acyl-CoA cholesterol acyltransferase 2 (ACAT2), another important protein for cholesterol absorption by providing cholesteryl esters to chylomicrons, was mainly presented in the apical cytoplasm of enterocytes. NPC1L1 and ACAT2 were highly expressed in jejunum and ileum. ACAT1 presented in the Paneth cells of crypts and mesenchymal cells of villi. In the absence of cholesterol, NPC1L1 was localized on the brush border of enterocytes. Dietary cholesterol induced the internalization of NPC1L1 to the subapical layer beneath the brush border and became partially colocalized with the endosome marker Rab11. Ezetimibe blocked the internalization of NPC1L1 and cholesterol and caused their retention in the plasma membrane. This study demonstrates that NPC1L1 mediates cholesterol entering enterocytes through vesicular endocytosis and that ezetimibe blocks this step in vivo.     

Multiple plasma membrane receptors but not NPC1L1 mediate high-affinity, ezetimibe-sensitive cholesterol uptake into the intestinal brush border membrane

We compared cholesterol uptake into brush border membrane vesicles (BBMV) made from the small intestines of either wild-type or Niemann-Pick C1-like 1 (NPC1L1) knockout mice to elucidate the contribution of NPC1L1 to facilitated uptake; this uptake involves cholesterol transport from lipid donor particles into the BBM of enterocytes. The lack of NPC1L1 in the BBM of the knockout mice had no effect on the rate of cholesterol uptake. It follows that NPC1L1 cannot be the putative high-affinity, ezetimibe-sensitive cholesterol transporter in the brush border membrane (BBM) as has been proposed by others. The following findings substantiate this conclusion: (I) NPC1L1 is not a brush border membrane protein but very likely localized to intracellular membranes; (II) the cholesterol absorption inhibitor ezetimibe and its analogues reduce cholesterol uptake to the same extent in wild-type and NPC1L1 knockout mouse BBMV. These findings indicate that the prevailing belief that NPC1L1 facilitates intestinal cholesterol uptake into the BBM and its interaction with ezetimibe is responsible for the inhibition of this process can no longer be sustained.     

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.     

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. Overconsumption 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.     

The G0/G1 Switch Gene 2 Is an Important Regulator of Hepatic Triglyceride Metabolism

Nonalcoholic fatty liver disease is associated with obesity and insulin resistance. Factors that regulate the disposal of hepatic triglycerides contribute to the development of hepatic steatosis. G₀/G₁ switch gene 2 (G0S2) is a target of peroxisome proliferator-activated receptors and plays an important role in regulating lipolysis in adipocytes. Therefore, we investigated whether G0S2 plays a role in hepatic lipid metabolism. Adenovirus-mediated expression of G0S2 (Ad-G0S2) potently induced fatty liver in mice. The liver mass of Ad-G0S2-infected mice was markedly increased with excess triglyceride content compared to the control mice. G0S2 did not change cellular cholesterol levels in hepatocytes. G0S2 was found to be co-localized with adipose triglyceride lipase at the surface of lipid droplets. Hepatic G0S2 overexpression resulted in an increase in plasma Low-density lipoprotein (LDL)/Very-Low-density (VLDL) lipoprotein cholesterol level. Plasma High-density lipoprotein (HDL) cholesterol and ketone body levels were slightly decreased in Ad-G0S2 injected mice. G0S2 also increased the accumulation of neutral lipids in cultured HepG2 and L02 cells. However, G0S2 overexpression in the liver significantly improved glucose tolerance in mice. Livers expressing G0S2 exhibited increased 6-(N-(7-nitrobenz-2-oxa-1-3-diazol-4-yl) amino)-6-deoxyglucose uptake compared with livers transfected with control adenovirus. Taken together, our results provide evidence supporting an important role for G0S2 as a regulator of triglyceride content in the liver and suggest that G0S2 may be a molecular target for the treatment of insulin resistance and other obesity-related metabolic disorders.     

Effects of phenotype, sex, and diet on plasma lipids in LA/N-cp rats

The LA/N-corpulent (cp) rat is a recently developed congenic strain which exhibits obesity. The effects of phenotype and sex on serum and lipoprotein lipid content were examined in LA/N-cp rats fed either a control or an atherogenic diet high in saturated fat and protein. Obese rats were pair-fed to equivalent lean animals. Results from this study indicate that sex, phenotype, and diet exert significant effects on plasma and lipoprotein cholesterol content. Plasma cholesterol levels were higher in obese compared with lean rats, females than in males, and rats consuming the atherogenic diet compared with the control diet. Plasma and lipoprotein triglyceride levels were significantly increased only in obese compared with lean animals. The increased plasma cholesterol and triglyceride was observed primarily in the chylomicron and very low density lipoprotein fractions. Increased levels of plasma cholesterol were not a result of increased dietary cholesterol absorption or increased liver cholesterol biosynthesis. These data suggest that LA/N-cp rats can serve as a unique rodent model for the study of the interrelationships between hyperlipidemia, obesity, and coronary heart disease.     

Liver X receptor agonist T0901317 reduces athero-sclerotic lesions in apoE-/- mice by up-regulating NPC1 expression

In this study, we studied the effect of liver X receptor (LXR) agonist T0901317 on Niemann-Pick C1 protein (NPC1) expression in apoE-/- mice. Male apoE-/- mice were randomized into 4 groups, baseline group (n=10), control group (n=14), treatment group (n=14) and prevention group (n=14). All of the mice were fed with a high-fat/high-cholesterol (HFHC) diet containing 15% fat and 0.25% cholesterol. The baseline group treated with vehicle was sacrificed after 8 weeks of the diet. The control group and the prevention group were treated with either vehicle or T0901317 daily by oral gavage for 14 weeks. The treatment group was treated with vehicle for 8 weeks, and then was treated with the agonist T0901317 for additional 6 weeks. Gene and protein expression was analyzed by real-time quantitative PCR, immunohistochemistry and Western blotting, respectively. Plasma lipid concentrations were measured by commercially enzymatic methods. We used RNA interference technology to silence NPC1 gene expression in THP-1 macrophage-derived foam cells and then detected the effect of LXR agonist T0901317 on cholesterol efflux. Plasma triglyceride (TG), total cholesterol (TC), high density lipoprotein cholesterol (HDL-C) and apoA-I concentrations were markedly increased in T0901317-treated groups. T0901317 treatment reduced the aortic atherosclerotic lesion area by 64.2% in the prevention group and 58.3% in the treatment group. LXR agonist treatment increased NPC1 mRNA expression and protein levels in the small intestine, liver and aorta of apoE-/- mice. Compared with the normal cells, cholesterol efflux of siRNA THP-1 macrophage-derived foam cells was significantly decreased, whereas cholesterol efflux of LXR agonist T0901317-treated THP-1 macrophage-derived foam cells was significantly increased. Our results suggest that LXR agonist T0901317 inhibits atherosclerosis development in apoE-/- mice, which is related to up-regulating NPC1 expression.     

Class B type I scavenger receptor is responsible for the high affinity cholesterol binding activity of intestinal brush border membrane vesicles

Recent studies have documented the importance of Niemann-Pick C1-like 1 protein (NPC1L1), a putative physiological target of the drug ezetimibe, in mediating intestinal cholesterol absorption. However, whether NPC1L1 is the high affinity cholesterol binding protein on intestinal brush border membranes is still controversial. In this study, brush border membrane vesicles (BBMV) from wild type and NPC1L1-/- mice were isolated and assayed for micellar cholesterol binding in the presence or absence of ezetimibe. Results confirmed the loss of the high affinity component of cholesterol binding when wild type BBMV preparations were incubated with antiserum against the class B type 1 scavenger receptor (SR-BI) in the reaction mixture similar to previous studies. Subsequently, second order binding of cholesterol was observed with BBMV from wild type and NPC1L1-/- mice. The inclusion of ezetimibe in these in vitro reaction assays resulted in the loss of the high affinity component of cholesterol interaction. Surprisingly, BBMVs from NPC1L1-/- mice maintained active binding of cholesterol. These results documented that SR-BI, not NPC1L1, is the major protein responsible for the initial high affinity cholesterol ligand binding process in the cholesterol absorption pathway. Additionally, ezetimibe may inhibit BBM cholesterol binding through targets such as SR-BI in addition to its inhibition of NPC1L1.     

Characterization of a hypertriglyceridemic transgenic miniature pig model expressing human apolipoprotein CIII

Hypertriglyceridemia has recently been considered to be an independent risk factor for coronary heart disease, in which apolipoprotein (Apo)CIII is one of the major contributory factors, as it is strongly correlated with plasma triglyceride levels. Although ApoCIII transgenic mice have been generated as an animal model for the study of hypertriglyceridemia, the features of lipoprotein metabolism in mice differ greatly from those in humans. Because of the great similarity between pigs and humans with respect to lipid metabolism and cardiovascular physiology, we generated transgenic miniature pigs expressing human ApoCIII by the transfection of somatic cells combined with nuclear transfer. The expression of human ApoCIII was detected in the liver and intestine of the transgenic pigs. As compared with nontransgenic controls, transgenic pigs showed significantly increased plasma triglyceride levels (83 ± 36 versus 38 ± 4 mg·dL(-1), P < 0.01) when fed a chow diet. Plasma lipoprotein profiling by FPLC in transgenic animals showed a higher peak in large-particle fractions corresponding to very low-density lipoprotein/chylomicrons when triglyceride content in the fractions was assayed. There was not much difference in cholesterol content in FPLC fractions, although a large low-density lipoprotein peak was identified in both nontransgenic and transgenic animals, resembling that found in humans. Further analysis revealed markedly delayed clearance of plasma triglyceride, accompanied by significantly reduced lipoprotein lipase activity in post-heparin plasma, in transgenic pigs as compared with nontransgenic controls. In summary, we have successfully generated a novel hypertriglyceridemic ApoCIII transgenic miniature pig model that could be of great value for studies on hyperlipidemia in relation to atherosclerotic disorders.     

Transgenic expression of CYP7A1 in LDL receptor-deficient mice blocks diet-induced hypercholesterolemia

Constitutive expression of a cholesterol-7alpha-hydroxylase (CYP7A1) transgene in LDL receptor-deficient mice blocked the ability of a cholesterol-enriched diet to increase plasma levels of apolipoprotein B-containing lipoproteins. LDL receptor-deficient mice expressing the CYP7A1 transgene exhibited complete resistance to diet-induced hypercholesterolemia and to the accumulation of cholesterol in the liver. Hepatic mRNA expression of liver X receptor-inducible ABCG5 and ABCG8 was decreased in CYP7A1 transgenic, LDL receptor-deficient mice fed a cholesterol-enriched diet. Thus, increased biliary cholesterol excretion could not account for the maintenance of cholesterol homeostasis. CYP7A1 transgenic, LDL receptor-deficient mice fed the cholesterol-enriched diet exhibited decreased jejunal Niemann-Pick C1-Like 1 protein (NPC1L1) mRNA expression, an important mediator of intestinal cholesterol absorption. A taurocholate-enriched diet also decreased NPC1L1 mRNA expression in a farnesoid X receptor-independent manner. Reduced expression of NPC1L1 mRNA was associated with decreased cholesterol absorption ( approximately 20%; P < 0.05) exhibited by CYP7A1 transgenic LDL receptor-deficient mice fed the cholesterol-enriched diet. The combined data show that enhanced expression of CYP7A1 is an effective means to prevent the accumulation of cholesterol in the liver and of atherogenic apolipoprotein B-containing lipoproteins in plasma.