Journal of Lipid Research, Vol 30, 1421-1428, Copyright © 1989 by Lipid Research, Inc.

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A novel sterol-regulated surface protein on chicken fibroblasts

K Hayashi, S Ando, S Stifani and WJ Schneider
Department of Biochemistry, University of Alberta, Edmonton, Canada.

In the laying hen, two different receptors for apolipoprotein B (apoB)- containing lipoproteins are expressed on somatic cells and oocytes, respectively. The somatic protein has an apparent Mr of 130,000, while the oocyte receptor is a 95-kDa protein (1989. K. Hayashi, J. Nimpf, and W. J. Schneider, J. Biol. Chem. 264:3131-3139). In order to investigate the yet unresolved relationship between these two proteins, we applied immunoblotting with anti-receptor antibodies to extracts of oocytes and chicken embryo fibroblasts. IgG fractions that recognize the 95-kDa oocyte receptor did not cross-react with the somatic receptor; however, chicken fibroblasts as well as ovarian granulosa cells that had been exposed to sterols (cholesterol and 25-OH- cholesterol) or low density lipoprotein (LDL) were shown to express a novel immunoreactive protein with an apparent Mr of 110,000. This protein is localized on the cell surface, and is unable to bind apoB- containing lipoproteins. The formation of the 110-kDa protein in fibroblasts is induced in time- and concentration-dependent fashion by sterols, concomitant with a progressive decrease in the amount of functional 130-kDa receptor protein. Following its induction, exposure of cells to LDL, but not to high density lipoprotein, caused the disappearance of the immunoreactive protein. Furthermore, the production of the 110-kDa protein did not require protein synthesis. These data are compatible with the notion that this novel receptor- related, nonfunctional protein is a truncated intermediate in the degradation pathway for the 130-kDa apoB receptor, and that the truncation generates antigenic epitope(s) shared by the 95-kDa oocyte receptor and the 110-kDa protein, but not expressed on the somatic receptor.
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