The Structure and Metabolism of Human Plasma Lipoproteins
Metabolic Compartments of Human Plasma Lipoproteins at Normo- and Hypertriclyceridemia
Tech Area / Field
3 Approved without Funding
National Research Center for Preventive Medicine, Russia, Moscow
- Institute of Biochemical Physics, Russia, Moscow
- University of Granada, Spain, Granada
Project summaryBesides of classical point of view of the primary role of cholesterol in the atherosclerotic injury of heart and brain vessels, the contribution of the metabolic disturbances of plasma triglyceride into disease development and manifestation becomes more and more prominent. These disturbances are characterized by the increased plasma triglyceride level, i.e. by hypertriglyceridemia. The major goal of the present study is to investigate the structure and function of human plasma lipoproteins as lipid transport system in blood vessel both at normo- and hypertriglyceridemia.
A lot of structural and functional considerations of these protein-lipid complexes has been done at the Department of Metabolic Disturbances, Research Centre for Preventive Medicine, such as lipoprotein composition and structure, the contribution of apolipoprotein constituents and dynamic state of the lipid phase into the metabolic transformation of lipoproteins via receptor- and enzyme-dependent pathways. Specifically, the mechanism of triglyceride hydrolysis by lipoprotein lipase (Dergunov et al. (1989) Biochim. Biophys. Acta 1005, 79-86) and the mode of interaction of the major "pro-atherogenic" lipoprotein particles, i.e. very low density (VLDL) and low density (LDL) lipoproteins with the LDL receptor in vitro were studied. The primary role of apolipoprotein E in these processes was described. These studies were performed with lipoproteins from patients with widely varied triglyceride content but nearly constant plasma cholesterol content. The complete study of various metabolic compartments which can influence each by other has to include, however, the investigation both "pro-atherogenic" VLDL and LDL particles and "anti-atherogenic" high density lipoproteins (HDL), in particular, their generation, catabolism and the exchange of the lipid constituents.
The scientists from Institute of Biochemical Physics which are also the Established Investigators at the Department of Chemical Enzymology of Moscow State University have a great experience in kinetic study of various biological processes, in the mathematical treatment of complex reactions. The major studies were performed with various enzymes, enzyme systems and receptors (Varfolomeev S.D., Gurevich K.G. (1999) Biokinetics, Moscow, Fair-press, 720 pages).
Three specific tasks will be operative under this study and more detailed description follows.
Task 1: role of structural organization of apolipoprotein E in the interaction of VLDL with the LDL receptor at normo- and hypertriglyceridemia.
All levels of the organization of apolipoprotein E in solution and in native and reconstituted very low (VLDL) and high (HDL) density lipoproteins will be studied to test the contribution of supramolecular apolipoprotein organization on the surface of triglyceride(TG)-rich lipoprotein particles into lipoprotein-receptor interaction and its impairment at atherogenic dyslipoproteinemias. The apoE structure will be studied in detail in model particles and in triglyceride-rich lipoproteins from normo- and hypertriglyceridemics both before and after their lypolytic degradation and differing in their ability to interact with the LDL receptor. For a first time, a possible competition between atherogenic VLDL and LDL particles for a binding to the LDL receptor, a possible positive influence of apoE-containing HDL particles on the binding process and a significance of this influence at hypertriglyceridemia will be estimated, based on the existence of two dependent pools of apoE in VLDL and HDL particles. These data are suggested to result in a principally new way of the regulation and correction of the interaction of VLDL and LDL particles with the LDL receptor, especially at hypertriglyceridemia.
Task 2: protein-lipid interactions in HDL and LCAT activity regulation.
For a first time, it is planned to study:
– the kinetic parameters of lecithin:cholesterol acyltransferase (LCAT)-catalyzed reaction with the substrate complexes consisting of inpidual apolipoprotein A-I, A-II, E with phospholipid, cholesterol or cholesteryl ester;
– the dynamic behaviour of lipid molecules in bulk bilayer and at protein/lipid interface in these substrate particles;
– the apolipoprotein conformation in these complexes.
The analysis and comparison of the data obtained probably will prove our hypothesis about direct control of the efficiency of formation and transport of cholesteryl esters by the physical state of lipid-protein interface (“boundary lipid”) in lipoprotein particles. The results obtained for two most critical reactions in reverse cholesterol transport will contribute to:
– the mechanism of the influence of various apolipoproteins on LCAT activity involved in the transformation of lipid constituents of lipoproteins specifically at hypertriglyceridemia when the content of apoA-I and apoE in plasma is changed;
– the prediction of modulation of the enzyme activity on the substrate level through the modulation of the structure of the lipid phase of the particle.
Task 3: role of HDL apolipoproteins in the exchange of hydrophobic lipids.
To study the initial steps of "reverse" cholesterol transport, a continuous assay of cholesteryl ester transfer protein (CETP) will be developed and, for a first time, the discoidal reconstituted HDL particles as a model of nascent HDL particles – the most efficient cellular cholesterol acceptor – will be used both as donor and acceptor particles in this reaction. The influence of various apolipoproteins and the physical state of the lipid phase on the efficiency of homo-exchange between discoidal HDL particles, as a single comparment for generation and transport of cholesteryl ester molecules will be investigated, instead of a traditional study of hetero-exchange between spherical HDL and VLDL or LDL particles. The contribution of homo- and hetero-exchange into overall exchange efficiency will be estimated, specifically at hypertriglyceridemia, when HDL deficiency often exists.
As a result, the contribution of the protein and lipid structural components into the steady-state level of particular lipoprotein species in VLDL, LDL and HDL metabolic compartments via enzyme- and receptor-dependent pathways will be evaluated. The whole set of the data will result in the concept of molecular therapy of hypertriglyceridemia – the less studied atherosclerosis risk factor that could improve the population health.
The active participation of the collaborator into the project assumes the access to intermediate and final project reports, the organization of joint seminars and workshops, the evaluation of the papers submitted to publication.
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