The two aims of this proposal are focused on the lipoprotein cholesteryl ester (CE) selective uptake pathway. Selective uptake is a process in which CE is selectively transferred from lipoproteins into the cell without the uptake and degradation of the lipoprotein particle. This is a major pathway for the uptake of lipoprotein cholesteryl ester into steroidogenic tissues (about 90% in rodents) and may also play a significant role in the liver in reverse cholesterol transport. Although the selective uptake pathway has been known for some 15 years, there has been little success in determining the protein participants on the cell surface or the essential protein and lipid components on the lipoprotein particle. This proposal is based on two recent findings which now make it possible to evaluate important features of both the lipoprotein particle and a putative selective uptake receptor on the cell surface. The first finding is that steroidogenic tissues of apoA-I-deficient mice show an almost complete failure to accumulate cholesteryl ester, suggesting the hypothesis that apoA-I is essential for selective uptake of HDL-CE. This hypothesis will be tested by evaluating the ability of other apoproteins to substitute for apoA-I in vivo and by analyzing the selective uptake activities of apoA-I containing and apoA-I deficient HDL particles in cell culture. The second finding is that the recently cloned scavenger receptor B1 (SR-B1) can mediate selective uptake of HDL-CE when expressed in CHO cells. The hypothesis will be tested that this receptor is responsible for selective uptake of HDL-CE and LDL-CE into steroidogenic cells and the delivery of cholesterol for steroid production by expressing a transfected SR-B1 gene in adrenal cells. The SR-B1 expressing cells will be used to evaluate the apoprotein specificity for this process and to delineate domains of apoA-I that are important in native HDL particles. These studies will provide new and important information about a cellular process that is central to the normal functioning of steroidogenic cells and that contributes to the regulation of the plasma cholesterol concentration which is an important determinant of human health.
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