Human Apolipoprotein C-I Accounts for the Ability of Plasma High Density Lipoproteins to Inhibit the Cholesteryl Ester Transfer Protein Activity

Abstract

The aim of the present study was to identify the protein that accounts for the cholesteryl ester transfer protein (CETP)-inhibitory activity that is specifically associated with human plasma high density lipoproteins (HDL). To this end, human HDL apolipoproteins were fractionated by preparative polyacrylamide gradient gel electrophoresis, and 30 distinct protein fractions with molecular masses ranging from 80 down to 2 kDa were tested for their ability to inhibit CETP activity. One single apolipoprotein fraction was able to completely inhibit CETP activity. The N-terminal sequence of the 6-kDa protein inhibitor matched the N-terminal sequence of human apoC-I, the inhibition was completely blocked by specific anti-apolipoprotein C-I antibodies, and mass spectrometry analysis confirmed the identity of the isolated inhibitor with full-length human apoC-I. Pure apoC-I was able to abolish CETP activity in a concentration-dependent manner and with a high efficiency (IC 50 ‫؍‬ 100 nmol/liter). The inhibitory potency of total delipidated HDL apolipoproteins completely disappeared after a treatment with anti-apolipoprotein C-I antibodies, and the apoC-I deprivation of native plasma HDL by immunoaffinity chromatography produced a mean 43% rise in cholesteryl ester transfer rates. The main localization of apoC-I in HDL and not in low density lipoprotein in normolipidemic plasma provides further support for the specific property of HDL in inhibiting CETP activity.Cholesteryl ester transfer protein (CETP), 1 a member of the lipid transfer/lipopolysaccharide-binding protein family promotes the exchange of neutral lipid species, i.e. cholesteryl esters and triglycerides, between plasma lipoproteins. In addition to the concentration of CETP in the plasma compartment, a number of other parameters, including the concentration and composition of lipoprotein substrates, can affect the velocity and the extent of the CETP-mediated neutral lipid transfer reaction. In earlier kinetic studies, Barter and Jones (1) and Ihm and colleagues (2) reported that the interactions of CETP with low density lipoproteins (LDL) versus high density lipoproteins (HDL) markedly differ. Hence, at a constant amount of HDL, the rate of the CETP-mediated cholesteryl ester exchange rises with increasing amounts of LDL until a maximal constant value is reached with high LDL to HDL cholesteryl ester ratios. Although at a fixed amount of LDL the rate of cholesteryl ester exchange also tended to increase with moderate amounts of HDL, a potent and significant inhibition of the CETP-mediated lipid transfer process was observed with higher HDL/LDL cholesterol ratios (1, 2). Initially, the inhibitory effect of HDL has been explained in terms of a greater interaction of CETP with HDL than with LDL, favoring the HDL-HDL exchanges over the LDL-HDL exchanges (1, 2). More recently, purified HDL apolipoproteins were shown to modulate the cholesteryl ester transfer reaction, and in particular apoA-I and apoA-II; i.e. the two major HDL apolipoproteins were alternatively des...

Plasma kinetics of VLDL and HDL apoC-I in normolipidemic and hypertriglyceridemic subjects

“…It is interesting that recent work by Gautier et al (16) has provided convincing evidence that apoC-I is the protein in HDL which accounts for the CETP-inhibitory activity that is specifically associated with human HDL. This suggests that apoC-I may be very important in determining plasma HDL-C levels, particularly when it is associated with HDL.…”

“…Although the biological function of apoC-I in humans has not been completely elucidated, in vitro experiments have demonstrated that apoC-I has the capacity to: a ) activate LCAT (7)(8)(9); b ) inhibit lipoprotein lipase (10,11), hepatic lipase (12,13), and phospholipase A 2 (14); and c ) inhibit cholesterol ester transfer activity (15,16). ApoC-I has also been shown to play an important role in plasma TRL metabolism by inhibiting the binding and/or uptake of triglyceride emulsions or VLDL by the LDL receptor (17,18), the LDL receptor-related protein (LRP) (19,20), and the VLDL receptor (21).…”

Metabolism and Modification of Apolipoprotein B-Containing Lipoproteins Involved in Dyslipidemia and Atherosclerosis

“…146) On the other hand, apoC-I is an inhibitor of CETP. 147,148) Transgenic mice overexpressing human apoC-I exhibit elevated levels of cholesterol and TG owing to the accumulation of cholesterol-enriched VLDL, VLDL remnants, and LDL. 149,150) In human apoC-I-transgenic mice, the elevated lipid levels are primarily associated with decreased uptake of apoB-containing lipoproteins by the liver rather than with disturbed lipolysis of lipoproteins.…”

Control of cholesteryl ester transfer protein activity by sequestration of lipid transfer inhibitor protein in an inactive complex

“…Physiologically, CETP may be regulated by at least two proteins. Apolipoprotein C-I, which resides primarily on HDL, has been reported to inhibit CETP in vitro, and studies with transgenic animals have demonstrated its ability to suppress CETP activity in vivo (6,7). Its mode of action is via modification of the surface charge of HDL, resulting in weakened CETP-HDL interactions and thus suppression of lipid transfer events with HDL (8).…”