Background

Low-density lipoproteins (LDL) are detergent-like globules which transport cholesterol and other lipids in blood.  Their outer hydrophilic shell is composed mainly of phospholipids, free cholesterol, and a protein (apoB-100), while their lipophilic core contains cholesterol esters, and triglycerides.  During the oxidation of LDL polyunsaturated fatty esters are transformed into reactive oxidized lipids, many of which attach themselves to the LDL protein.  The resulting oxidatively modified (ox) LDL is recognized by specialized receptors on the surface of macrophage cells.  Receptor recognition promotes unregulated uptake (endocytosis) of the oxLDL by the cells.   Partial digestion of the damaged LDL leads to the accumulation of large quantities of cholesterol esters that coalesce into droplets giving the cells the appearance of being foam-filled.  The resulting "foam cells" accumulate in the subendothelial space of artery walls leading to the formation of fatty streaks, the earliest form of atherosclerotic plaques.

Our discovery that the level of isoLG-derived modification of blood proteins is closely correlated with cardiovascular disease is consistent with a seminal role for lipid-based oxidative protein modifications in the development of atherosclerosis.  Our more recent observations indicate that certain oxidized lipids themselves are also recognized and endocytized through macrophage receptors.

The Molecular Structure of oxLDL

We have made considerable progress in identifying the molecular structures of oxidized lipids and their protein adducts that are formed upon oxidative damage of LDL.  Nevertheless, our understanding remains incomplete.  Furthermore, oxLDL is not a single defined chemical entity.  Depending on the extent of oxidation different products are generated.  Some of these form adducts with proteins, and the adducts can be transformed by further oxidative modifications.  Therefore, a major thrust of our studies continues to be identification of the molecular structures of LDL-derived oxidized lipids and their protein adducts.

• Group Contact: Suresh Palani
• Collaborators: H. Hoff, L. Sayre

We are developing immunoassays that allow the convenient monitoring of isoLG-protein adduct and specific oxidized phospholipid levels in human blood.  This involves the preparation of specific antigens by unambiguous syntheses, genertion of antibodies, and clinical evaluation of their utility for identifying individuals with cardiovascular disease and for monitoring the efficacy of therapeutic interventions.  For example, we previously raised antibodies against antigens that were obtained by reacting iso[4]LGE2 with a protein.  This antigen is undoubtedly a complex mixture of adducts.  Consequntly, the antibodies recognize a variety of adduct structures, and this increases the chances for crossreactions of the antibodies and consequently a high level of background immunoreactivity.  We now know that isoLG-derived protein-bound hydroxylactams are stable endproducts from the reaction of isoLGs with protein.  To obtain antibodies that specifically recognize a single type of adduct, we are undertaking the total synthesis of an isoLG-derived hydroxylactam and derived protein adduct.  It is our hope that antibodies raised against this adduct will provide a more sensitive and specific immunoassay for detecting cardiovascular disease-related oxidative injury with a simple blood test.  Such a minimally invasive clinical tool would be valuable for detecting disease at an early stage when therapeutic intervention is most likely to have a successful outcome.

• Group Contacts: Mingjiang Sun, James Laird
• Collaborator: D. Sprecher

We previously showed that the adduct formed by the reaction of a levuglandin with LDL is specifically recognized by the same macrophage receptor that causes endocytosis of oxLDL.  Determining the biological activities associated with specific lipid-derived protein modifications remains an are of great interest for the Lipid Research Group.  In addition, with collaborators at UCLA and The Cleveland Clinic Foundation, we recently identified a large family of oxidized phospholipids that show biological activities which could contribute to cardiovascular disease.  These include activation of endothelial cells to bind monocytes, activation of platelets, and binding with the macrophage CD36 receptor.  Studies on the biological mechanisms and consequences of these activities are the subject of an intense effort involving The Lipid Research Group and  collaborators at UCLA and The Cleveland Clinic Foundation.

• Group Contacts: Eugenia Baytreva, Mingjiang Sun
• Collaborators: H. Hoff, J. Berliner, G. Subbanagounder, S. Hazen, E. Podrez