Cholesterol Transfer Protein Research Articles

A novel cholesterol transfer protein in cardiac sarcolemma. Purification and initial characterization.

In contrast to several sterol carrier proteins isolated from soluble cytosolic fractions, a cholesterol transfer protein (CHTP) with an apparent molecular weight of 73,000 was isolated from a cardiac sarcolemmal fraction by detergent solubilization, column chromatography, and preparative electrophoresis using nondissociating polyacrylamide gels. This protein must be reconstituted into an artificial membrane in order to mediate cholesterol transfer activity. For the expression of its full activity, CHTP must also be present in the membrane in a multimeric form, since the monomer was shown not to be active. We believe this novel protein might represent an important molecule in the regulation of the homeostasis of cholesterol in cardiac sarcolemma.

Luteinized rat ovaries contain a sterol carrier protein.

Highly luteinized rat ovaries contain a soluble 13,000 mol wt factor which binds [3H]cholesterol and enhances cholesterol transfer from phospholipid vesicles to mitochondria. The ovarian cholesterol transfer protein was purified 1200-fold. It has a basic isoelectric point, is heat stable, and has an amino acid composition similar to that reported for rat liver sterol carrier protein2. Antisera raised against preparations of rat liver sterol carrier protein2 precipitated labeled cholesterol associated with the ovarian 13,000 mol wt fraction. A line of partial identity was obtained when the ovarian fractions and purified rat liver sterol carrier protein2 were run against these antisera in double immunodiffusion. We conclude that luteinized rat ovaries contain a sterol carrier protein with properties similar to hepatic sterol carrier protein2. This protein may facilitate the intracellular distribution of cholesterol in ovarian cells.

Role of esterified cholesterol transfers in the regulation of plasma cholesterol esterification

The role of esterified cholesterol transfers in mediating the known effect of very low density lipoproteins (VLDL) on plasma cholesterol esterification has been examined. Advantage has been taken of the fact that pig plasma, in cintrast to human plasma, is deficient in activity of the protein that transfers esterified cholesterol between plasma lipoproteins. The experimental protocol consisted of a pre-incubation in which VLDL-deficient pig plasma or isolated pig lipoproteins were incubated with added VLDL, in the presence or absence of an exogenous source of the esterified cholesterol transfer protein. After pre-incubation, any VLDL that may have been present was removed and rates of cholesterol esterification measured in subsequent incubations of the recovered VLDL-depleted fractions. It was, thus, possible to assess the effect of VLDL and/or transfer protein activity on the capacity of other lipoproteins to act as substrates for cholesterol esterification. In the absence of transfer protein activity, the presence of VLDL in pre-incubations had no effect on the subsequently measured rate of cholesterol esterification. However, when pig plasma was supplemented with human lipoprotein-free plasma (a source of esterified cholesterol transfer protein activity) the presence of VLDL in pre-incubations resulted in both a reduction in the esterified cholesterol content of the re-isolated VLDL-deficient fraction and a markedly enhanced rate of cholesterol esterification in subsequent incubations of this fraction. It is concluded that the effect of VLDL on cholesterol esterification is dependent on the capacity of VLDL by acting as recipients of esterified cholesterol, to deplete the esterified cholesterol content of other lipoprotein fractions.

Changes in the density distribution of pig high density lipoproteins during incubation in vitro: Influence of esterified cholesterol transfer activity

Pig plasma, depleted of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) by ultracentrifugation, was incubated in vitro at 37°C in the presence of 0.002 M parachlormercuryphenyl sulfonate, a chemical inhibitor of lecithin: cholesterol acyltransferase. Incubation resulted in a consistent and significant shift in the density distribution of esterified cholesterol within the high density lipoprotein (HDL) fraction. After 24 h of incubation the esterified cholesterol in HDL of density > 1.125 g/ml was reduced by a mean of 28%, while that in HDL of density < 1.125 g/ml was increased by a mean of 77%. Both the rate and the magnitude of this redistribution were markedly enhanced when the incubation mixture contained rabbit lipoprotein-free plasma, a rich source of the esterified cholesterol transfer protein. When pig plasma, depleted of VLDL and LDL, was subjected to rate zonal ultracentrifugation, the HDL in non-incubated samples eluted as a single symmetrical peak in a position comparable to that of human HDL 3. After 24 h of incubation at 37°C the density of the HDL was reduced, with a peak midway between the positions of human HDL 3 and HDL 2. When VLDL- and LDL-depleted pig plasma were mixed with human lipoprotein-free plasma (also a source of esterified cholesterol transfer activity) and incubated for 24 h at 37°C, there was an even greater reduction in the density of pig HDL, which now eluted in a position comparable to that of human HDL 2. It was concluded that the capacity of the esterified transfer protein to redistribute esterified cholesterol between different lipoprotein particles may be of importance in the process of HDL interconversion.

A physiologic role for the esterified cholesterol transfer protein: In vivo studies in rabbits and pigs

The observation that pig plasma is deficient in esterified cholesterol transfer activity has been exploited in an attempt to establish an in vivo role for the esterified cholesterol transfer protein. The plasma high density lipoproteins (HDL) of pigs and also of rabbits (a species known to posses an active esterified cholesterol transfer protein) were labeled with 3H in the esterified cholesterol moiety and with 125I in the apoprotein moieties and reinjected into the respective species. In both rabbits and pigs, the removal of 125I from the recipient HDL fraction was parallel to that from the whole plasma, with negligible 125I appearing in other plasma lipoprotein fractions. In the pig, the removal of esterified 3H-cholesterol from the recipient HDL was very similar to that of 125I; there was only minimal appearance in other lipoproteins. In the rabbit, however, there was a major in vivo transfer of esterified 3H-cholesterol from HDL to other fractions. It has been concluded that an active esterified cholesterol transfer protein is probably necessary to achieve the in vivo transfer of esterified cholesterol from HDL to other plasma lipoproteins.

Regulation of human plasma lecithin:cholesterol acyltransferase activity by lipoprotein acceptor cholesteryl ester content.

Very low density lipoproteins and low density lipoproteins attain maximal cholesteryl ester contents during the incubation of human plasma and, under these conditions, both lecithin:cholesterol acyltransferase and transfer proteins are inhibited. These lipoproteins provide the major part of free cholesterol for the lecithin:cholesterol acyltransferase reaction, and are the major acceptors of cholesteryl ester generated by lecithin:cholesterol acyltransferase and transported to the lipoprotein acceptors by the transfer protein. The results obtained indicate that the concentration of acceptor limits esterification and transfer in plasma, and that in vivo these acceptors contain close to their maximal cholesteryl ester content. Human plasma end product acceptor lipoproteins have a composition similar to that of the ester-rich large low density lipoprotein characteristic of primate models of experimental atherosclerosis.

Kinetic studies of the transfer of esterified cholesterol between human plasma low and high density lipoproteins.

In vitro incubations (6 hr at 37 degrees C) of human low density lipoproteins (LDL), high density lipoproteins (HDL), and lipoprotein-free plasma revealed no significant net mass transfers of esterified cholesterol from either lipoprotein fraction to the other. Transfers of esterified [3H]cholesterol from LDL to HDL must therefore have represented a process of molecular exchange between the two fractions. In molar terms, the exchange increased with increasing incubation concentrations of LDL regardless of whether the HDL was increased in parallel, decreased, or kept constant. In direct contrast, with LDL kept constant, an increase in the concentration of HDL resulted in a decrease in the molar rate of exchange of esterified cholesterol between LDL and HDL. The data were then fitted to a mathematical model describing a physical model in which an esterified cholesterol transfer protein circulates in the plasma, interacting with lipoprotein particles into which it deposits and from which it picks up esterified cholesterol molecules. According to this model, to which the experimental data fit extremely well, the transfer protein had a much greater affinity for HDL than for LDL in a transfer process that was readily saturable by HDL but not by LDL.