Characterization of ApoB, E Receptor Function in the Luteinized Ovary

Recent findings from this laboratory have led to the suggestion that the hormone-producing cells of the rat luteinized ovary in situ may obtain a large share of low density lipoprotein (LDL) cholesterol without actually internalizing the intact lipoprotein particles. We have shown that the lipoproteins are trapped at the surface of the luteal cells in a rich network of "microvillar channels" and have theorized that these channel membranes, with their large surface area for interacting with lipoprotein particles, may function in the cholesterol transfer process. In the current study, we try to establish what proportion of the human (h)LDL-cholesterol transfer in the in situ perfused tissue occurs by a classical apoB, E receptor-mediated process versus a surface extraction process. We examine the tissue for the presence of apoB, E receptors, and characterize the structural/functional interaction of hLDL with the apoB, E receptor utilizing a variety of modified hLDL particles as probes. Then, using nonmetabolizable radiolabels for both the protein and cholesteryl ester moieties of these LDL probes, we attempt to quantify the extent to which apoB, E receptors in the ovary contribute to the uptake of hLDL-cholesterol during steroidogenesis. Our experiments show that although the luteinized ovary contains apoB, E receptor protein, hLDL interacts with the tissue atypically. That is, despite modifications of LDL amino acid residues to prevent interaction with the apoB, E receptor, the modified ligands continue to contribute cholesterol for luteal cell internalization and/or steroidogenesis. We conclude, therefore, that in this tissue much of the LDL-cholesterol is not delivered by the apoB, E receptor pathway.