Characterization of human high-density lipoprotein subclasses LP A-I and LP A-I/A-II and binding to HepG2 cells

Abstract Plasma HDL can be classified according to their apolipoprotein content into at least two types of lipoprotein particles: lipoproteins containing both apo A-I and apo A-II (LP A-I/A-II) and lipoproteins with apo A-I but without apo A-II (LP A-I). LP A-I and LP A-I/A-II were isolated by immuno-affinity chromatography. LP A-I has a higher cholesterol content and less protein compared to LP A-I/A-II. The average particle mass of LP A-I is higher (379 kDa) than the average particle weight of LP A-I/A-II (269 kDa). The binding of 125I-LP A-I to HepG2 cells at 4°C, as well as the uptake of [3H]cholesteryl ether-labelled LP A-I by HepG2 cells at 37° C, was significantly higher than the binding and uptake of LP A-I/A-II. It is likely that both binding and uptake are mediated by apo A-I. Our results do not provide evidence in favor of a specific role for apo A-II in the binding and uptake of HDL by HepG2 cells

Dietary trans fatty acids increase serum cholesterylester transfer protein activity in man

The average diet may provide some 8–10 g/day of unsaturated fatty acids with a trans double bond. Previous studies showed that dietary trans fatty acids may simultaneously raise low-density lipoprotein (LDL) cholesterol and reduce high-density lipoprotein (HDL) cholesterol. Human plasma contains a protein (CETP) which transfers cholesterylesters from HDL to lipoproteins of lower density. We hypothesized that CETP could play a role in the effect of trans fatty acids on lipoproteins and measured the activity levels of CETP in serum samples from a 9-week study in which 55 volunteers were fed three controlled diets with different fatty acid profiles. Mean activity was 114 (% of reference serum) after consumption of a high trans fatty acid diet, as opposed to 96 after linoleic acid and 97 after stearic acid (P < 0.02). We conclude that the increased activity of CETP may contribute to the rise in LDL cholesterol and the fall in HDL cholesterol seen on diets with high contents of trans fatty acids

Induction of adrenal scavenger receptor BI and increased high density lipoprotein-cholesteryl ether uptake by in vivo inhibition of hepatic lipase

Hepatic lipase (HL) and scavenger receptor type B class I (SR-BI) have both been implicated in high density lipoprotein (HDL)-cholesteryl ester uptake in cholesterol-utilizing tissues. Inactivation of HL by gene-directed targeting in mice results in up-regulation of SR-BI expression in adrenal gland (Wang, N., Weng, W., Breslow, J. L., and Tall, A. R. (1996) J. Biol. Chem. 271, 21001-21004). The net effect on HDL-cholesteryl ester uptake is not known. We determined the impact of acute in vivo inhibition of rat adrenal HL activity by antibodies on SR-BI expression and on human and rat HDL-[3H]cholesteryl ether (CEth) uptake in the adrenal gland. Rat HDL was isolated from rats in which HL activity had been inhibited for 1 h. The rats were studied under basal conditions (not ACTH-treated) and after previous treatment with ACTH for 6 days (ACTH-treated). Intravenous injection of anti-HL resulted in 70% lowering of adrenal HL activity in both conditions which were maintained for at least 8 h. In not ACTH-treated rats, inhibition of adrenal HL increased adrenal SR-BI mRNA (5.2-fold) and mass (1. 6-fold) within 4 h. HL inhibition resulted in 41% and 14% more adrenal accumulation of human HDL-[3H]CEth during 4 and 24 h, respectively. The adrenal uptake of rat HDL-[3H]CEth increased by 68%, 4 h after the antibody injection. ACTH treatment increased total adrenal HL activity from 3.7 +/- 0.5 milliunits to 34.0 +/- 17. 2 milliunits, as well as adrenal SR-BI mRNA from 2.9 +/- 0.7 arbitrary units (A.U.) to 86.8 +/- 41.1 A.U. and SR-BI mass from 7.7 +/- 1.8 A.U. to 63.16 +/- 46.7 A.U. The human HDL-[3H]CEth uptake by adrenals was also significantly increased from 0.58 +/- 0.11% of injected dose to 7.24 +/- 1.58% of injected dose. Inhibition of adrenal HL activity did not result in further induction of SR-BI expression and did not affect human HDL-[3H]CEth uptake. These findings indicate that SR-BI expression may be influenced by changes in HL activity. HL activity is not needed for the SR-BI-mediated HDL-cholester

Elevation of plasma phospholipid transfer protein in transgenic mice increases VLDL secretion

Two lipid transfer proteins are active in human plasma, cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP). Mice by nature do not express CETP. Additional inactivation of the PLTP gene resulted in reduced secretion of VLDL and subsequently in decreased susceptibility to diet-induced atherosclerosis. The aim of this study is to assess possible effects of differences in PLTP expression on VLDL secretion in mice that are proficient in CETP and PLTP. We compared human CETP transgenic (huCETPtg) mice with mice expressing both human lipid transfer proteins (huCETPtg/huPLTPtg). Plasma cholesterol in huCETPtg mice was 1.5-fold higher compared with huCETPtg/huPLTPtg mice (P < 0.001). This difference was mostly due to a lower HDL level in the huCETPtg/huPLTPtg mice, which subsequently could lead to the somewhat decreased CETP activity and concentration that was found in huCETPtg/huPLTPtg mice (P < 0.05). PLTP activity was 2.8-fold increased in these animals (P < 0.001). The human PLTP concentration was 5 microg/ml. Moderate overexpression of PLTP resulted in a 1.5-fold higher VLDL secretion compared with huCETPtg mice (P < 0.05). The composition of nascent VLDL was similar in both strains. These results indicate that elevated PLTP activity in huCETPtg mice results in an increase in VLDL secretion. In addition, PLTP overexpression decreases plasma HDL cholesterol as well as CETP

Evaluation of phospholipid transfer protein and cholesteryl ester transfer protein as contributors to the generation of pre beta-high-density lipoproteins

High-density lipoproteins (HDLs) are considered anti-atherogenic because they mediate peripheral cell cholesterol transport to the liver for excretion and degradation. An important step in this reverse cholesterol-transport pathway is the uptake of cellular cholesterol by a specific subclass of small, lipid-poor apolipoprotein A-I particles designated pre beta-HDL. The two lipid-transfer proteins present in human plasma, cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), have both been implicated in the formation of pre beta-HDL. In order to investigate the relative contribution of each of these proteins, we used transgenic mouse models. Comparisons were made between human CETP transgenic mice (huCETPtg), human PLTP transgenic mice (huPLTPtg) and mice transgenic for both lipid-transfer proteins (huCETPtg/huPLTPtg). These animals showed elevated plasma levels of CETP activity, PLTP activity or both activities, respectively. We evaluated the generation of pre beta-HDL in mouse plasma by immunoblotting and crossed immuno-electrophoresis. Generation of pre beta-HDL was equal in huCETPtg and wild-type mice. In contrast, in huPLTPtg and huCETPtg/huPLTPtg mice, pre beta-HDL generation was 3-fold higher than in plasma from either wild-type or huCETPtg mice. Our findings demonstrate that, of the two plasma lipid-transfer proteins, PLTP rather than CETP is responsible for the generation of pre beta-HDL. These data support the hypothesis of a role for PLTP in the initial stage of reverse cholesterol transport

Human plasma phospholipid transfer protein increases the antiatherogenic potential of high density lipoproteins in transgenic mice

Plasma phospholipid transfer protein (PLTP) transfers phospholipids between lipoprotein particles and alters high density lipoprotein (HDL) subfraction patterns in vitro, but its physiological function is poorly understood. Transgenic mice that overexpress human PLTP were generated. Compared with wild-type mice, these mice show a 2.5- to 4.5-fold increase in PLTP activity in plasma. This results in a 30% to 40% decrease of plasma levels of HDL cholesterol. Incubation of plasma from transgenic animals at 37 degrees C reveals a 2- to 3-fold increase in the formation of pre-beta-HDL compared with plasma from wild-type mice. Although pre-beta-HDL is normally a minor subfraction of HDL, it is known to be a very efficient acceptor of peripheral cell cholesterol and a key mediator in reverse cholesterol transport. Further experiments show that plasma from transgenic animals is much more efficient in preventing the accumulation of intracellular cholesterol in macrophages than plasma from wild-type mice, despite lower total HDL concentrations. It is concluded that PLTP can act as an antiatherogenic factor preventing cellular cholesterol overload by generation of pre-beta-HDL

Acute elevation of plasma PLTP activity strongly increases pre-existing atherosclerosis

Objective - A transgenic mouse model was generated that allows conditional expression of human PLTP, based on the tetracycline-responsive gene system, to study the effects of an acute increase in plasma PLTP activity as may occur in inflammation. Methods and Results - The effects of an acute elevation of plasma PLTP activity on the metabolism of apolipoprotein B-containing lipoproteins and on diet-induced pre-existing atherosclerosis were determined in mice displaying a humanized lipoprotein profile (low-density lipoprotein receptor knockout background). Induced expression of PLTP strongly increases plasma VLDL levels in LDL receptor knockout mice, whereas VLDL secretion is not affected. The elevation in plasma triglyceride levels is explained by a PLTP-dependent inhibition of VLDL catabolism, which is caused, at least partly, by a decreased lipoprotein lipase activity. Together with the decreased plasma HDL levels, the acutely increased PLTP expression results in a highly atherogenic lipoprotein profile. Induction of PLTP expression leads to a further increase in size of pre-existing atherosclerotic lesions, even on a chow diet. In addition, the lesions contain more macrophages and less collagen relative to controls, suggesting a less stable lesion phenotype. Conclusion - In conclusion, acute elevation of PLTP activity destabilizes atherosclerotic lesions and aggravates pre-existing atherosclerosis