Gonadotropin Regulation of Apolipoprotein E Production by Steroidogenic Cells
Apolipoprotein E (apo E) is a 34-kD protein that is found in association with various lipoproteins, including chylomycron remnants, very low density lipoproteins (VLDL), and certain subclasses of high-density lipoprotein (HDLc) (1, 2). Apo E plays an important role in cholesterol metabolism and thus has been the subject of much investigation. First, it functions as a ligand for the receptor-mediated uptake of plasma lipoproteins by the B/E (LDL) receptor of peripheral cells and the E-receptor on hepatocytes (3). Second, it serves as the ligand for the recently described LDL receptor-related protein (LRP) of hepatic and peripheral cells (4). Third, apo E can serve as a distributor of lipids between cells in tissue and as a mediator of reverse transport of cholesterol from peripheral cells back to the liver (2), especially in species lacking the cholesterol ester transfer protein. Apo E is synthesized predominantly in the liver, but unlike the other apoproteins, it is also produced in most extrahepatic tissues, including brain, spleen, and the steroidogenic organs (adrenal, testis, and ovary) (5, 6). In individual extrahepatic tissues, apo E could serve some or all the functions mentioned previously. The steroidogenic tissues have a great demand for cholesterol as the substrate for steroid hormone production. A role for apo E as a tissue distributor of lipids would likely be identified in them.
KeywordsGranulosa Cell Cholera Toxin Peripheral Cell mRNA Content Cholesterol Ester Transfer Protein
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