Study of the components of reverse cholesterol transport in lecithin:cholesterol acyltransferase deficiency

Probing the 121-136 domain of lecithin:cholesterol acyltransferase using antibodies

Lecithin:cholesterol acyltransferase (LCAT) catalyzes the esterification of plasma lipoprotein cholesterol in mammals as part of the reverse cholesterol transport pathway. Studies of the natural mutations of LCAT revealed a region that is highly sensitive to mutations (residues 121-136) and it is highly conserved in six animal species. The purpose of these studies was to investigate the reactivity of wild type and several mutated forms of LCAT, with a series polyclonal antibodies to further characterize this specific domain (residues 121-136). Two polyclonal antibodies directed against the whole enzyme, one against human plasma LCAT and the other against purified recombinant LCAT, and one site specific polyclonal antibody, directed against the 121-136 region of LCAT, were employed. All three antibodies reacted with a recombinant form of purified LCAT; however, only the polyclonal antibodies directed against the whole enzyme were able to recognize the LCAT when it was adsorbed to a hydrophobic surface in a solid phase immunoassay, or when bound to HDL in a sink immunoassay. These findings indicate that the epitope(s) of the 121-136 region are not accessible to antibodies under these conditions. Three mutant forms of LCAT, representing alterations in the 121-136 region, were also examined for their immunoreactivity with the same panel of antibodies and compared to the wild-type enzyme. These studies demonstrate that in its native configuration the 121-136 region of LCAT is likely to reside on a surface of LCAT. Furthermore, mutations within this region appear to markedly impact the exposure of epitopes at additional sites. These findings suggest that the 121-136 region could play an important role in enzyme interaction with its hydrophobic lipoprotein substrates as mutations within this region appear to alter enzyme conformation, catalytic activity, and the specificity of LCAT.

Effect of LpA-I composition and structure on cholesterol transfer between lipoproteins

The effect of high density lipoprotein composition on the rates of unesterified cholesterol exchange between low density lipoproteins (LDL) and well-defined homogeneous discoidal lipoproteins (LpA-I) reconstituted with phosphatidylcholine, cholesterol, and apolipoprotein A-I (apoA-I) has been investigated. LpA-I containing cholesterol and 2, 3, and 4 apoA-I molecules per particle differed in their ability to accept or donate cholesterol. A significant cholesterol exchange occurs between LDL and Lp2A-I (7.8 and 9.6 nm), while there is little or no cholesterol exchange detectable between LDL and Lp3A-I (10.8 and 13.4 nm) and Lp4A-I (17.0 nm) complexes. The cholesterol transfer from LDL to the cholesterol-free Lp2A-I (9.6 nm), Lp3A-I (13.4 nm), and Lp4A-I (17.0 nm) particles also shows significant cholesterol transfer to Lp2A-I, while there is no detectable transfer to Lp3- and 4A-I particles. The rates of cholesterol transfer to cholesterol-free and cholesterol-containing Lp2A-I appear to differ significantly. Cholesterol transfer from LDL to cholesterol-free Lp2A-I is zero order with respect to acceptor concentrations when the Lp2A-I/LDL ratio is above 10. Transfer rates from LDL to cholesterol-free Lp2A-I are faster for the smaller Lp2A-I (8.5 nm) than to the larger Lp2A-I (9.7 nm) and exhibit half-times (t1/2) at 25 degrees C of 4.0 and 5.3 h, respectively. In contrast, cholesterol transfer from LDL to cholesterol-containing Lp2A-I remains dependent upon acceptor concentrations to an acceptor/donor particle ratio of 80. In addition, transfer from LDL to cholesterol-containing Lp2A-I is faster to the 9.6 nm than to 7.8 nm particles, with t1/2 of 1.4 and 2.3 h, respectively. The rates of cholesterol transfer from Lp2A-I to LDL are higher than in the opposite direction, in particular for the small Lp2A-I (7.8 nm), which has a t1/2 of approximately 50 min. The results show that changes in the composition and structure of apoA-I-containing particles have a significant effect on inter-lipoprotein exchange of cholesterol. This suggests that the kinetics of cholesterol transfer to and from reconstituted discoidal LpA-I particles cannot be fully explained by passive aqueous diffusion.

Cell-derived unesterified cholesterol cycles between different HDLs and LDL for its effective esterification in plasma

Pulse-chase incubations of human plasma with [3H]cholesterol-laden skin fibroblasts or low density lipoproteins (LDL) and nondenaturing two-dimensional electrophoresis were used to study the transfer and esterification of cell-derived unesterified cholesterol (UC) in human plasma lipoproteins. Specific radioactivities ([3H]UC per microgram of UC) were calculated, and net cholesterol mass transfer was quantified using a fluoro-enzymatic assay to validate productive transfers of UC between high density lipoprotein (HDL) and LDL. Cellular UC was initially taken up by pre-beta 1-HDL and subsequently transferred in the sequence pre-beta 2-HDL-->pre-beta 3-HDL-->alpha-HDL-->LDL. During the first 5 minutes of this process, only 5% of cellular cholesterol was esterified in pre-beta 3-HDL and alpha-HDL; the remainder reached LDL as UC. Cellular UC accumulating in LDL was then redistributed to various HDL particles via two pathways: 1) the partially LDL receptor-mediated uptake and re-secretion of UC by cells and 2) the direct transfer of UC to HDL, mostly to alpha-HDL and a small amount to pre-beta-HDL. UC was not transferred from LDL to HDL after inhibition of lecithin:cholesterol acyltransferase (LCAT). The esterification of cellular [3H]cholesterol in plasma was competitively inhibited by the addition of excess unlabeled LDL but not of excess HDL. However, both excess LDL and excess HDL prevented the esterification of cell-derived cholesterol in apolipoprotein B-free plasma. This demonstrated that LDL is the major source of UC to the LCAT reaction and that the transfer of UC from LDL to HDL is LCAT dependent. In conclusion, the effective esterification of cell-derived cholesterol in plasma involves a rapid transfer of UC via HDL particles to LDL, from which it is distributed to pre-beta-HDL and alpha-HDL. Furthermore, we hypothesize that the transfer per se of cellular UC to LDL forms a cholesterol concentration gradient between cell membranes and HDL and thus a second, reverse cholesterol transport mechanism in addition to the esterification of cholesterol by LCAT.

Molecular species of phosphatidylcholine in familial lecithin-cholesterol acyltransferase deficiency: effect of enzyme supplementation

We studied the molecular species composition of plasma phosphatidylcholine (PC) in three patients with familial deficiency of lecithin-cholesterol acyltransferase in order to determine whether certain species are increased in the absence of this enzyme activity. Compared to normal plasma, the deficient plasma contained significantly higher percentages of 16:0-18:2 and 18:0-18:2 species and lower percentages of 16:0-20:4 and 18:0-20:4 species. The bulk of the total plasma PC as well as the abnormal composition of molecular species were found in the very-low-density and low-density lipoprotein (VLDL + LDL) fractions. When the deficient plasma was incubated with partially purified enzyme from normal human plasma, there was a significant reduction in the amounts of most major species, mainly in the VLDL + LDL fraction. When the selectivity factors were calculated by dividing the percentage contribution of each species for cholesterol esterification by its percentage concentration, the highest selectivity factors were found for 16:0-18:2, 18:1-18:1 and 18:1-18:2. The order of selectivity of the enzyme for various species was very similar to that obtained earlier using normal HDL as substrate. These results show that lecithin-cholesterol acyltransferase has significant effects on the molecular species composition of plasma PC and the deficiency of the enzyme results in accumulation of certain PC species normally used by the enzyme, as well as in abnormal distribution of these species among the lipoproteins.

Isolation and characterization of a novel lipoprotein particle from human placental extracts

While it is believed that placental tissue is very active in lipid metabolism, the nature of lipid containing particles secreted (if any) by this tissue is not known. Lipoprotein profile of human placental tissue was analysed by gel filtration, gel electrophoresis and electron microscopy. Our studies demonstrated the presence of lipoproteins with unusual compositions. A novel major lipoprotein (which eluted in the same position on plasma VLDL) was characterized. While this lipoprotein floated at density greater than 1.006 gr/ml and contained apo B (same as plasma VLDL) it differed from plasma VLDL in a) size, b) contining a significant amount of apo Al, and c) carried bulk of the cholesterol (80% in free form) and phospholipids. This study suggests that placental tissue might contain unique lipoproteins perhaps serving specific metabolic needs.

Lecithin:cholesterol acyltransferase in familial HDL deficiency (Tangier disease)

These studies were performed to investigate the relationship between the enzyme lecithin:cholesterol acyltransferase and plasma lipoproteins in Tangier disease, a condition characterized by a virtual absence of high-density lipoproteins (HDLs) and an accumulation of cholesteryl esters in peripheral tissues. Apolipoprotein A-I was nearly absent from the patient's plasma (1% of the normal levels were found). However, apolipoprotein A-I purified from the plasma of the Tangier disease patient, was found to activate both purified and the plasma enzyme. At lower apolipoprotein concentrations (up to 25 micrograms/ml), about twice the amount of Tangier apolipoprotein A-I was required to achieve a certain level of lecithin:cholesterol acyltransferase activity as compared with the activating potential of the normal apolipoprotein. Gel chromatography studies revealed that as in normal plasma, lecithin:cholesterol acyltransferase in Tangier plasma was associated with an HDL-size lipoprotein fraction. However, unlike in normal plasma, this lipoprotein complex (containing lecithin:cholesterol acyltransferase) was not removed from Tangier plasma by immunoaffinity chromatography utilizing immobilized anti-apolipoprotein A-I antibodies. Plasma incubation studies showed that free cholesterol was primarily supplied by LDL in normal plasma, whereas both LDL and VLDL donated the free cholesterol for lecithin:cholesterol acyltransferase reaction in Tangier plasma. The majority of the cholesteryl esters, generated during the incubation experiments, were transferred back to LDL in normal plasma, whereas in Tangier plasma both LDL and VLDL served as cholesteryl ester acceptors. The cholesteryl ester transfer from HDL to lower-density lipoproteins was lower in Tangier plasma as compared to this process in a normal control, suggesting that a minimal cholesteryl ester core may be required for the stability of HDL.