Structure and Function of Apolipoprotein E in Cholesterol Homeostasis and Atherosclerosis

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Area of research
My laboratory focuses on the structure and function of apolipoprotein (apo) E, specifically its critical role in cholesterol homeostasis and atherosclerosis.

Significance
ApoE regulates the clearance of plasma lipoproteins by mediating their binding to lipoprotein receptors. High levels of plasma lipoproteins are associated with atherosclerosis, the leading cause of death in the United States.

Although the three common isoforms of human apoE (apoE2, apoE3, and apoE4) differ from one another by single amino acid substitutions, these differences have profound functional consequences. ApoE3 seems to be the normal isoform in all known functions, while apoE4 and apoE2 can each be dysfunctional. ApoE4 is associated with increased risk of atherosclerosis (and Alzheimerís disease) and apoE2 with type III hyperlipoproteinemia and, in some instances, with increased risk of atherosclerosis.

Approaches
We use biophysical techniques (X-ray diffraction and crystallography) in combination with cell and molecular biology and transgenic and gene knockout mice to study the molecular mechanisms of apoE in normal cholesterol metabolism and in atherosclerosis.

Previous Accomplishments
My laboratory described apoEís ligand function, determined its protein and gene sequences, mapped the amino acid residues involved in receptor binding, defined the three-dimensional structure of the ligand-binding domain, and identified mutations that established its role in the pathogenesis of type III hyperlipoproteinemia.

Questions/problems under study
We are pursuing several intriguing potential mechanisms to explain the differential effects of apoE3 and apoE4. Three specific areas of investigation are (1) the role of apoE in regulating plasma lipid and lipoprotein metabolism and in type III hyperlipoproteinemia, (2) the pathways responsible for remnant lipoprotein metabolism by the liver, and (3) the effect of low levels of high density lipoprotein cholesterol on coronary heart disease.

We also hypothesize that many detrimental effects of apoE4 are mediated by a reactive intermediate or molten globule. Thermal and chemical denaturation studies, performed in collaboration with Dr. Karl Weisgraber, have shown that apoE4 is less stable than apoE3 and is more prone to form a molten globule, and we are pursuing experiments to test this hypothesis.

I also direct an epidemiological study that has defined the major risk factor for heart disease in Turkey: genetically determined low levels of plasma high density lipoproteins. The genetic nature of this disorder is under investigation.