The long-term objective of this research is to prevent atherosclerosis by understanding more completely the mechanisms by which plasma low density lipoprotein (LDL) and lipoproteins rich in apolipoprotein E (apo E) are cleared from plasma and how they interact with cells. The principal routs for LDL catabolism is through tissue uptake by the LDL receptor. However, evidence from several sources suggests that other lipoprotein receptors may also be present, especially in liver. This project focuses on defining further the role of a 109 kD Hep G2 liver cell protein which binds LDL and apo E-rich postprandial lipoproteins. It has been purified by HPLC, three tryptic peptides have been microsequenced, and it has bee identified as nucleolin by both sequence data and immunological comparison with authenthic nucleolin. Nucleolin is known to shuttle between the nucleus and cytoplasm and may carry important extranuclear information for the regulation of transcription. The present proposal will test 2 hypotheses: 1. That lipoprotein binding to nucleolin regulates cholesterol metabolism. 2. That the growth-promoting properties of lipoproteins are mediated in part through nucleolin. The ability of anti-nucleolin IgG to prevent lipoprotein binding to cultured cells in addition to Hep G2 cells will be determined. Regulation of hydroxymethylglutaryl CoA reductase and acyl cholesterol acyl transferase will studied. The effect of antibody to alter lipoprotein turnover in intact animals will be established. Finally, cells lacking the LDL receptor and expressing little nucleolin will be transfected with nucleolin cDNA in order to determine what effect a primary increase in nucleolin has on lipoprotein binding and cholesterol metabolism. Conversely, the effect of lipoproteins on nucleolin expression and shuttling will be determined.
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