Apolipoprotein M affecting lipid metabolism or just catching a ride with lipoproteins in the circulation?

Apolipoprotein M (apoM) is a novel apolipoprotein found mainly in high-density lipoproteins (HDL). Its function is yet to be defined. ApoM (25 kDa) has a typical lipocalin ?-barrel fold and a hydrophobic pocket. Retinoids bind apoM but with low affinity and may not be the natural ligands. ApoM retains its signal peptide, which serves as a hydrophobic anchor to the lipoproteins. This prevents apoM from being lost in the urine. Approximately 5% of HDL carries an apoM molecule. ApoM in plasma (1 μM) correlates strongly with both low-density lipoprotein (LDL) and HDL cholesterol, suggesting a link to cholesterol metabolism. However, in casecontrol studies, apoM levels in patients with coronary heart disease (CHD) and controls were similar, suggesting apoM levels not to affect the risk for CHD in humans. Experiments in transgenic mice suggested apoM to have antiatherogenic properties; possible mechanisms include increased formation of pre-? HDL, enhanced cholesterol mobilization from foam cells, and increased antioxidant properties. Received 28 November 2008; received after revision 15 December 2008; accepted 16 December 2008     

Net transport of cholesterol from cells of the human EA.hy 926 endothelial cell line to high density lipoproteins

EA.hy 926 cells, a human endothelial cell line, show characteristics of differentiated endothelial cells. The cells express saturable binding of apo E-free¹²⁵I-high density lipoprotein₃ (HDL₃). B_max increased from 71 to 226 ng HDL₃ bound/mg cell protein after cholesterol loading of the confluent endothelial cells with cationized low density lipoprotein (LDL). The affinity did not change after cholesterol enrichment (K_d was 37 g HDL₃ protein/ml for control cells and 31 g/ml, for loaded cells). Incubation of cholesterol-loaded EA.hy 926 cells with native HDL and LDL had different effects on cellular cholesterol levels. Incubation with HDL decreased both esterified and unesterified cellular cholesteryl, but LDL did not change total cellular cholesterol However, LDL tended to increase cellular cholesteryl esters, with a concomitant decrease of unesterified, cellular cholesterol. Incubation of endothelial cells with both HDL and LDL also resulted in decreased total cellular cholesterol levels. These data show that cationized LDL-loaded human endothelial EA.hy 926 cells can be used to study the net transport of cellular cholesterol to HDL, the first step in reverse cholesterol transport.     

ABCA2: a candidate regulator of neural transmembrane lipid transport

Studies in the past years have implicated multispan transmembrane transport molecules of the ATP binding cassette (ABC) transporter family in cellular lipid export processes. The prototypic ABC transporter ABCA1 has recently been demonstrated to act as a major facilitator of cellular cholesterol and phospholipid export. Moreover, the transporter ABCA4 (ABCR) plays a pivotal role in retinaldehyde processing, and ABCA3 has recently implicated in lung surfactant processing. These pioneering observations have directed considerable attention to the A subfamily of ABC proteins. ABCA2 is the codefining member of the ABC A-transporter subclass. Although known for some time, it was not until recently that its complete molecular structure was established. Unlike other ABC A-subfamily members, ABCA2 is predominantly expressed in the brain and neural tissues. The unique expression profile together with available structural data suggest roles for this largest known ABC protein in neural transmembrane lipid export. Received 31 January 2002; received after revision 11 March 2002; accepted 11 March 2002     

Cholesterol feeding to rats does not modulate the expression of binding sites for HDL on liver membranes

The binding of HDL, Apo-E-free, was studied in rats fed a cholesterol rich diet for 2, 4 and 7 days. Plasma cholesterol increased up to 16-fold (from 55 to 900 mg/dl); liver cholesterol was also raised, from 0.5 to 16 mg/g of tissue. The HDL binding to membrane preparations was not affected while the binding of beta VLDL was reduced to about 50% of the controls. These data show, therefore, that liver binding sites for HDL are refractory to regulation by dietary cholesterol.     

Structural studies of discoidal lipoprotein A-I

Apolipoprotein A-I (apoA-I) is a major exchangeable apolipoprotein of high-density lipoproteins (HDLs), and plays an important role in reverse cholesterol transport. This process involves transport of cholesterol from peripheral tissues to the liver for processing, thereby eliminating excess cholesterol from the body. The function of apoA-I and its interaction with other components of HDL, including lecithin-cholesterol acyltransferase, seems to be closely linked to its structural plasticity. ApoA-I is likely to undergo changes in its structure and orientation between the various HDL subclasses and, therefore, knowledge of the precise structure of apoA-I is essential for understanding its role in the antiatherogenic properties of HDL. This review focuses on the role of apoA-I in reverse cholesterol transport and the work done by various groups to determine the structure of apoA-I in discoidal HDL particles.     

Cholesterol feeding to rats does not modulate the expression of binding sites for HDL on liver membranes

Summary The binding of HDL, Apo-E-free, was studied in rats fed a cholesterol rich diet for 2, 4 and 7 days. Plasma cholesterol increased up to 16-fold (from 55 to 900 mg/dl); liver cholesterol was also raised, from 0.5 to 16 mg/g of tissue. The HDL binding to membrane preparations was not affected while the binding of VLDL was reduced to about 50% of the controls. These data show, therefore, that liver binding sites for HDL are refractory to regulation by dietary cholesterol.     

The nucleotide receptor P2Y13 is a key regulator of hepatic High-Density Lipoprotein (HDL) endocytosis

Cell surface receptors for high-density lipoprotein (HDL) on hepatocytes are major partners in the regulation of cholesterol homeostasis. We recently identified a cell surface ATP synthase as a high-affinity receptor for HDL apolipoprotein A-I (apoA-I) on human hepatocytes. Stimulation of this ectopic ATP synthase by apoA-I triggered a low-affinity-receptor-dependent HDL endocytosis by a mechanism strictly related to the generation of ADP. This suggests that nucleotide G-protein- coupled receptors of the P2Y family are molecular components in this pathway. Only P2Y₁ and P2Y₁₃ are present on the membrane of hepatocytes. Using both a pharmacological approach and small interference RNA, we identified P2Y₁₃ as the main partner in hepatic HDL endocytosis, in cultured cells as well as in situ in perfused mouse livers. We also found a new important action of the antithrombotic agent AR-C69931MX as a strong activator of P2Y₁₃-mediated HDL endocytosis. Received 9 May 2005; received after revision 24 June 2005; accepted 1 September 2005     

Marked elevation of HDL-cholesterol in cold-adapted golden hamsters

Golden hamsters with spontaneous hypercholesterolemia at 22 degrees C developed a further increase in plasma cholesterol when they were maintained at 6 degrees C. This hypercholesterolemia was associated with a redistribution of plasma cholesterol between VLDL and HDL. Plasma cholesterol transported in the VLDL decreased while cholesterol in the HDL increased by 45%. The LDL profile was not significantly modified.     

Marked elevation of HDL-cholesterol in cold-adapted golden hamsters

Summary Golden hamsters with spontaneous hypercholesterolemia at 22°C developed a further increase in plasma cholesterol when they were maintained at 6°C. This hypercholesterolemia was associated with a redistribution of plasma cholesterol between VLDL and HDL. Plasma cholesterol transported in the VLDL decreased while cholesterol in the HDL increased by 45%. The LDL profile was not significantly modified.     

Cholesteryl ester transfer protein and its inhibition

Cholesteryl ester transfer protein (CETP) is a plasma glycoprotein that facilitates the transfer of cholesteryl esters from the atheroprotective high density lipoprotein (HDL) to the proatherogenic low density lipoprotein cholesterol (LDL) and very low density lipoprotein cholesterol (VLDL) leading to lower levels of HDL but raising the levels of proatherogenic LDL and VLDL. Inhibition of CETP is considered a potential approach to treat dyslipidemia. However, discussions regarding the role of CETP-mediated lipid transfer in the development of atherosclerosis and CETP inhibition as a potential strategy for prevention of atherosclerosis have been controversial. Although many animal studies support the hypothesis that inhibition of CETP activity may be beneficial, negative phase III studies on clinical endpoints with the CETP inhibitor torcetrapib challenged the future perspectives of CETP inhibitors as potential therapeutic agents. The review provides an update on current understanding of the molecular mechanisms involved in CETP activity and its inhibition.     

辛伐他汀联合用药研究进展

动脉粥样硬化是心脑血管疾病的主要病理学基础,他汀类药物是抗动脉粥样硬化的基石.辛伐他汀降低总胆固醇（TC）和低密度脂蛋白胆固醇（LDL-C）作用明显,同时也降低甘油三酯（TG）和升高高密度脂蛋白（HDL）水平,是目前临床上应用最广泛的降脂药之一.诸多专家学者对辛伐他汀的联合用药也进行了广泛深入的研究,认为其不仅在抗动脉粥样硬化领域具有确切的疗效,而且在不稳定性心绞痛、冠心病的二级预防、高血压、早期糖尿病肾病以及骨质疏松等疾病中的治疗也具有重要的意义.     

Effect of Royal Jelly on serum lipids in experimental animals and humans with atherosclerosis

The primary objective of this review was to assess the size and consistency of Royal Jelly (RJ) effect on serum lipids in experimental animals and humans. The data from animal studies were pooled, where possible, and statistically evaluated by Student's t-test. Meta-analysis was used for the evaluation of human trials. It was found that RJ significantly decreased serum and liver total lipids and cholesterol levels in rats and rabbits and also retarded the formation of atheromas in the aorta of rabbits fed a hyperlipemic diet. Meta-analysis of the controlled human trials of RJ to reduce hyperlipidemia showed a significant reduction in total serum lipids and cholesterol levels and normalization of HDL and LDL as determined from decrease in β/α lipoproteins. The best available evidence suggests that RJ at approximately 50 to 100 mg per day, decreased total serum cholesterol levels by about 14%, and total serum lipids by about 10% in the group of patients studied.     

Role of the ubiquitin–proteasome system in the regulation of P2Y13 receptor expression: impact on hepatic HDL uptake

The protective effect of high density lipoproteins (HDL) against atherosclerosis is mainly attributed to their capacity to transport excess cholesterol from peripheral tissues back to the liver for further elimination into the bile, a process called reverse cholesterol transport (RCT). Recently, the importance of the P2Y₁₃ receptor (P2Y₁₃-R) was highlighted in HDL metabolism since HDL uptake by the liver was decreased in P2Y₁₃-R deficient mice, which translated into impaired RCT. Here, we investigated for the first time the molecular mechanisms regulating cell surface expression of P2Y₁₃-R. When transiently expressed, P2Y₁₃-R was mainly detected in the endoplasmic reticulum (ER) and strongly subjected to proteasome degradation while its homologous P2Y₁₂ receptor (P2Y₁₂-R) was efficiently targeted to the plasma membrane. We observed an inverse correlation between cell surface expression and ubiquitination level of P2Y₁₃-R in the ER, suggesting a close link between ubiquitination of P2Y₁₃-R and its efficient targeting to the plasma membrane. The C-terminus tail exchange between P2Y₁₃-R and P2Y₁₂-R strongly restored plasma membrane expression of P2Y₁₃-R, suggesting the involvement of the intra-cytoplasmic tail of P2Y₁₃-R in expression defect. Accordingly, proteasomal inhibition increased plasma membrane expression of functionally active P2Y₁₃-R in hepatocytes, and consequently stimulated P2Y₁₃-R-mediated HDL endocytosis. Importantly, proteasomal inhibition strongly potentiated HDL hepatic uptake (>200 %) in wild-type but not in P2Y₁₃-R-deficient mice, thus reinforcing the role of P2Y₁₃-R expression in regulating HDL metabolism. Therefore, specific inhibition of the ubiquitin–proteasome system might be a novel powerful HDL therapy to enhance P2Y₁₃-R expression and consequently promote the overall RCT.     

MDR1, cholesterol certification and cell growth: a comparative study in normal and multidrug-resistant KB cell lines

The product of the MDR1 gene (P-gp) has been implicated in the transport of cholesterol from plasma membrane to endoplasmic reticulum for esterification. In previous studies on leukemia cell lines, we suggested that cholesterol esterification may regulate the rate of cell growth and that the MDR1 gene might be involved in this process by modulating intracellular cholesterol esters levels. To further investigate this matter, the rate of cell growth, cholesterol metabolism, expression of the MDR1 gene, and P-gp activity were compared in KB cell lines displaying differences in expression and function of P-gp (drug-sensitive phenotype versus MDR phenotype). The rate of cell growth correlated with cholesterol esterification in all KB cell lines, whereas the over-expression of MDR1 observed in the MDR cell lines was not always associated with an increased capacity of cells to esterify cholesterol. Two known inhibitors of P-gp activity, progesterone and verapamil, strongly inhibited both cholesterol esterification and cell proliferation in all KB cell lines, but they affected intracellular accumulation of labeled vinblastine only in MDR cell lines. These results further support a role for cholesterol esters in the regulation of cell growth and suggest that the P-gp expressed in MDR KB cells is not involved in the general process leading to cholesterol esterification. Received 14 February 2000; received after revision 10 April 2000; accepted 8 May 2000     

Cell surface adenylate kinase activity regulates the F1-ATPase/P2Y13-mediated HDL endocytosis pathway on human hepatocytes

We have previously demonstrated on human hepatocytes that apolipoprotein A-I binding to an ecto-F₁-ATPase stimulates the production of extracellular ADP that activates a P2Y₁₃-mediated high-density lipoprotein (HDL) endocytosis pathway. Therefore, we investigated the mechanisms controlling the extracellular ATP/ADP level in hepatic cell lines and primary cultures to determine their impact on HDL endocytosis. Here we show that addition of ADP to the cell culture medium induced extracellular ATP production that was due to adenylate kinase and nucleoside diphosphokinase activities, but not to ATP synthase activity. We further observed that in vitro modulation of both ecto-NDPK and AK activities could regulate the ADP-dependent HDL endocytosis. But interestingly, only AK appeared to naturally participate in the pathway by consuming the ADP generated by the ecto-F₁-ATPase. Thus controlling the extracellular ADP level is a potential target for reverse cholesterol transport regulation. Received 13 July 2006; received after revision 29 August 2006; accepted 19 September 2006     

Close correlation between levels of cholesterol and free fatty acids in lymphoid cells

Summary A close correlation was found between the levels of free cholesterol and free fatty acids in lymphoid cells from thymus, spleen or lymph node of mice and guinea-pigs. This relationship suggests a possible role of cholesterol regulating the fatty acid levels in lymphoid cells.     

Mylip makes an Idol turn into regulation of LDL receptor

High blood low-density-lipoprotein (LDL) cholesterol is a serious health problem among an increased number of patients in the Western world. Statins and other cholesterol lowering drugs have proven to be beneficial as therapy but are not optimal and show adverse effects in some patients. The LDL receptor is a crucial determinant of cholesterol metabolism in the body and amenable for drug interventions. Novel insights into the physiology of this receptor come from studies on the ubiquitination and degradation of LDL receptor by the ubiquitin ligase Mylip/Idol that is induced in cells by the nuclear receptor, LXR. This may open up new possibilities in the future to influence LDL receptor levels and cholesterol metabolism pharmacologically in various diseases.     

In vitro lipid synthesis by lathyrogen-treated L-929 fibroblasts.

Aside from cholesterol, cholesterol esters and lyso-lecithin, the de novo lipid synthesic mechanisms which operate in cells grown in the presence of beta-amino-propionitrile are largely depressed and suggest that there may be in operation specific metabolic control mechanisms for regulation of cellular lipid composition.