高脂饮食豚鼠高密度脂蛋白代谢的特点

目的研究豚鼠高脂饮食后高密度脂蛋白代谢的特点,并与大鼠进行比较。方法将豚鼠和大鼠分别随机分为正常组（NC）和高脂组（HF）,正常组均给予普通饲料,高脂组给予高脂饲料诱导10周后,测定血清LDL-C、HDL-C水平,HDL3/HDL2比值和LCAT、CETP的表达;采用real-time RT-PCR方法检测肝脏SR-BI表达的变化。结果与正常组相比,豚鼠高脂组血清HDL-C水平显著升高,高密度脂蛋白亚型HDL3/HDL2的比值升高,血清CETP表达均显著增加,血清LCAT表达下降,肝脏SR-BI mRNA表达水平是正常组的2.27倍。而相同高脂饲料条件下,大鼠的上述指标均无明显变化。结论豚鼠摄入高脂饮食后HDL代谢与大鼠有所不同,主要表现为血清HDL-C升高,肝脏SR-BI受体表达增加,高密度脂蛋白亚型组分发生变化,大颗粒HDL2含量相对减少,小颗粒HDL3堆积,其机制与血清CETP、LCAT的变化密切相关。     

Molecular cloning,expression, and hormonal regulation of the chicken microsomal triglyceride transfer protein

During an egg-laying cycle, oviparous animals transfer massive amounts of triglycerides, the major lipid component of very low density lipoprotein (VLDL), from the liver to the developing oocytes. A major stimulus for this process is the rise in estrogen associated with the onset of an egg-laying cycle. In mammals, the microsomal triglyceride transfer protein (MTP) is required for VLDL assembly and secretion. To enable studies to determine if MTP plays a role in basal and estrogen-stimulated VLDL assembly and secretion in an oviparous vertebrate, we have cloned and sequenced the chicken MTP cDNA. This cDNA encodes a protein of 893 amino acids with an N-terminal signal sequence. The primary sequence of chicken MTP is, on average, 65% identical to that of mammalian homologs, and 23% identical to the Drosophila melanogaster protein. We have obtained a clone of chicken embryo fibroblast cells that stably express the avian MTP cDNA and show that these cells display MTP activity as measured by the transfer of a fluorescently labeled neutral lipid. As in mammals, chicken MTP is localized to the endoplasmic reticulum as revealed by indirect immunofluorescence and by the fact that its N-linked oligosaccharide moiety remains sensitive to endoglycosidase H. Endogenous, enzymatically active MTP is also expressed in an estrogen receptor-expressing chicken hepatoma cell line that secretes apolipoprotein B-containing lipoproteins. In this cell line and in vivo, the expression and activity of MTP are not influenced by estrogen. Therefore, up-regulation of MTP in the liver is not required for the increased VLDL assembly during egg production in the chicken. This indicates that MTP is not rate-limiting, even for the massive estrogen-induced secretion of VLDL accompanying an egg-laying cycle.     

Impact of apolipoprotein A1- or lecithin:cholesterol acyltransferase-deficiency on white adipose tissue metabolic activity and glucose homeostasis in mice

High density lipoprotein (HDL) has attracted the attention of biomedical community due to its well-documented role in atheroprotection. HDL has also been recently implicated in the regulation of islets of Langerhans secretory function and in the etiology of peripheral insulin sensitivity. Indeed, data from numerous studies strongly indicate that the functions of pancreatic β-cells, skeletal muscles and adipose tissue could benefit from improved HDL functionality. To better understand how changes in HDL structure may affect diet-induced obesity and type 2 diabetes we aimed at investigating the impact of Apoa1 or Lcat deficiency, two key proteins of peripheral HDL metabolic pathway, on these pathological conditions in mouse models. We report that universal deletion of apoa1 or lcat expression in mice fed western-type diet results in increased sensitivity to body-weight gain compared to control C57BL/6 group. These changes in mouse genome correlate with discrete effects on white adipose tissue (WAT) metabolic activation and plasma glucose homeostasis. Apoa1-deficiency results in reduced WAT mitochondrial non-shivering thermogenesis. Lcat-deficiency causes a concerted reduction in both WAT oxidative phosphorylation and non-shivering thermogenesis, rendering lcat^?/? mice the most sensitive to weight gain out of the three strains tested, followed by apoa1^?/? mice. Nevertheless, only apoa1^?/? mice show disturbed plasma glucose homeostasis due to dysfunctional glucose-stimulated insulin secretion in pancreatic β-islets and insulin resistant skeletal muscles. Our analyses show that both apoa1^?/? and lcat^?/? mice fed high-fat diet have no measurable Apoa1 levels in their plasma, suggesting no direct involvement of Apoa1 in the observed phenotypic differences among groups.     

SC-435, an ileal apical sodium-codependent bile acid transporter inhibitor alters mRNA levels and enzyme activities of selected genes involved in hepatic cholesterol and lipoprotein metabolism in guinea pigs

We have demonstrated that SC-435, an apical sodium codependent bile acid transporter (ASBT) inhibitor, lowers plasma low-density lipoprotein cholesterol (LDL-C) concentrations in guinea pigs. The purpose of this study was to further examine the hypocholesterolemic effects of SC-435, by measuring the activity and RNA expression of regulatory enzymes of hepatic cholesterol and lipoprotein metabolism. In addition, the use of a combination (COMBO) therapy with simvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, was also tested. Male Hartley guinea pigs were randomly allocated to one of three diets (n=10 per group), for 12 weeks. The control diet contained no ASBT inhibitor or simvastatin. The monotherapy diet (ASBTi) contained 0.1% of SC-435. The COMBO therapy consisted of a lower dose of SC-435 (0.03%) and 0.05% simvastatin. Cholesterol ester transfer protein (CETP) and HMG-CoA reductase mRNA abundance were determined using RT-PCR techniques. Hepatic HMG-CoA reductase and cholesterol 7-hydroxylase (CYP7) activities were measured by radioisotopic methods. Compared to the control group, CETP activity was 34% and 56% lower with ASBTi and COMBO, respectively. Similarly, CETP mRNA expression was reduced by 36% and 73% in ASBTi and COMBO groups, respectively. Cholesterol 7-hydroxylase and HMG-CoA reductase activities were increased 2-fold with ASBTi and COMBO treatments, respectively. Likewise, HMG-CoA reductase mRNA expression was increased 33% with ASBTi treatment. These results suggest that both SC-435 monotherapy and combination therapy lower LDL cholesterol concentrations by altering both hepatic cholesterol homeostasis and the intravascular processing of lipoproteins in guinea pigs.     

High density lipoprotein inhibits the activation of sterol regulatory element-binding protein-1 in cultured cells

A link between cellular uptake of high density lipoprotein (HDL) and regulation of sterol regulatory element-binding protein-1 (SREBP-1) was investigated in vitro. HDL decreased nuclear SREBP-1 levels as well as SREBP-1 target gene expression in HepG2 and HEK293 cells. However, HDL did not repress an exogenously expressed, constitutively active form of SREBP-1. HDL increased cellular cholesterol levels, and cellular cholesterol depletion by methyl-β-cyclodextrin abolished the effects of HDL. These results suggest that HDL inhibits the activation of SREBP-1 through a cholesterol-dependent mechanism, which may play an important role in regulating lipid synthetic pathways mediated by SREBP-1.     

Cholesteryl ester transfer protein inhibition as a strategy to reduce cardiovascular risk

Human and rabbit plasma contain a cholesteryl ester transfer protein (CETP) that promotes net mass transfers of cholesteryl esters from high density lipoproteins (HDL) to other plasma lipoprotein fractions. As predicted, inhibition of CETP in both humans and rabbits increases the concentration of cholesterol in the potentially protective HDL fraction, while decreasing it in potentially proatherogenic non-HDL fractions. Inhibition of CETP in rabbits also inhibits the development of diet-induced atherosclerosis. However, use of the CETP inhibitor torcetrapib in humans did not reduce atheroma in three imaging trials and caused an excess of deaths and cardiovascular events in a large clinical outcome trial. The precise explanation for the harm caused by torcetrapib is unknown but may relate to documented, potentially harmful effects unrelated to inhibition of CETP. More recently, a trial using the weak CETP inhibitor dalcetrapib, which raises HDL levels less effectively than torcetrapib and does not lower non-HDL lipoprotein levels, was terminated early for reasons of futility. There was no evidence that dalcetrapib caused harm in that trial. Despite these setbacks, the hypothesis that CETP inhibitors will be antiatherogenic in humans is still being tested in studies with anacetrapib and evacetrapib, two CETP inhibitors that are much more potent than dalcetrapib and that do not share the off-target adverse effects of torcetrapib.     

Dancing with the sterols: Critical roles for ABCG1, ABCA1, miRNAs, and nuclear and cell surface receptors in controlling cellular sterol homeostasis

ATP binding cassette (ABC) transporters represent a large and diverse family of proteins that transport specific substrates across a membrane. The importance of these transporters is illustrated by the finding that inactivating mutations within 17 different family members are known to lead to specific human diseases. Clinical data from humans and/or studies with mice lacking functional transporters indicate that ABCA1, ABCG1, ABCG4, ABCG5 and ABCG8 are involved in cholesterol and/or phospholipid transport. This review discusses the multiple mechanisms that control cellular sterol homeostasis, including the roles of microRNAs, nuclear and cell surface receptors and ABC transporters, with particular emphasis on recent findings that have provided insights into the role(s) of ABCG1. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).     

The effect of apolipoprotein E polymorphism on plasma cholesteryl ester transfer protein activity in type 2 diabetic patients

We studied the relationship between apo E polymorphism and cholesteryl ester transfer protein (CETP) activity in 127 type 2 diabetic patients who did not take lipid lowering drugs. Furthermore, we studied the relationship between apo E and cholesteryl ester transfer protein (CETP) in modulating plasma triglyceride and HDLcholesterol. Apo E genotypes were determined by PCR-RFLP, and CETP activity was measured using an exogenous way. Our results showed that the CETP activity increased significantly in the E2 carrier group compared to E4 carriers and E3/E3 homozygous (84.7 ± 43.9 vs. 62.5 ± 35.9 vs. 52.6 ± 23.6 nmol CE/ml/2h, respectively; p = 0.015). However, there was no association between apo E polymorphism and lipid parameter variations. Even after adjustment for CETP activity, the results remained unchanged, showing that CETP did not step in the relationship between apo E and lipid parameter variations. In conclusion there is an association between apo E polymorphism and CETP activity, and this association did not affect the relationship between apo E polymorphism and triglyceride and HDLcholesterol concentrations. Published in Russian in Molekulyarnaya Biologiya, 2008, Vol. 42, No. 6, pp. 931–935. The text was submitted by the authors in English.