Abstract
We have previously characterized a 95-kDa plasma membrane receptor for low and very low density lipoproteins in chicken oocytes (George, R., Barber, D. L., and Schneider, W. J. (1987) J. Biol. Chem. 262, 16838-16847). We now report that somatic cells of chickens, such as fibroblasts, express a different receptor for these lipoproteins. This receptor has a Mr of 130,000 and is part of a regulatory system for cholesterol homeostasis analogous to the low density lipoprotein receptor pathway in mammalian cells. Oocytes produce only the 95-kDa receptor, while fibroblasts synthesize exclusively the 130-kDa receptor. In addition to their different Mr values, another distinctive feature of the two proteins was revealed by ligand blotting experiments: the oocyte receptor bound rabbit beta-VLDL (a class of apolipoprotein-B and -E containing lipoprotein particles), whereas the fibroblast receptor did not. Furthermore, polyclonal rabbit antibodies that recognize the oocyte 95-kDa receptor failed to cross-react with the 130-kDa protein on fibroblasts [corrected]. We suggest that different receptors have evolved in the chicken in order to facilitate the deposition of lipids into oocytes (i.e. yolk formation) with concomitant maintenance of cholesterol homeostasis in extraoocytic tissues.
Highlights
We have previously characterized a 95-kDa plasma stimulates cholesterol esterification, and leads to a decrease membrane receptor for low and very low density lipo- in the number of low density lipoprotein (LDL) receptors
We suggest that different metabolism in cultured aortic smooth muscle cells [9,10,11] and receptors have evolvedin the chicken in order tofacil- fibroblasts [11].It was concluded that none of the extraheitate the deposition of lipids into oocytes (Le. yolk patic pigeon cell lines expressed a functional LDL receptor formation) with concomitant maintenance of choles- pathway [11], and thatcholesterol homeostasis is maintained terol homeostasis in extraoocytic tissues
The major means to maintain cholesterol homeostasis in most, if not all, extrahepatic mammalian cells is the expression of the low density lipoprotein (LDL)’ receptor pathway [1].Receptor-mediated endocytosis of LDL leads to lysosomal degradation and hydrolysis of cholesteryl esters, andthe liberated cholesterol inhibits cellular cholesterol synthesis
Summary
The major means to maintain cholesterol homeostasis in most, if not all, extrahepatic mammalian cells is the expression of the low density lipoprotein (LDL)’ receptor pathway [1].Receptor-mediated endocytosis of LDL leads to lysosomal degradation and hydrolysis of cholesteryl esters, andthe liberated cholesterol inhibits cellular cholesterol synthesis,. Ligand and Western blotting were carried out in buffer containing 75 mM Tris-HC1, pH 7.4, 2 mM CaClZand5% (w/v) bovine serum albumin as blocking agent as described previously [12]; 1251-goaatnti-rabbit IgG was used as second antibody in Western blots [12].The concentration and specificradioactivity of the ligands used in the incubation mixtures are indicated in the figure legends. Incorporation of YCJAcetic Acid into f4CJCholesterolby Cultured Cells-After the cells had been incubated with ["CC]acetic acid (1pCi/ ml) as indicated, the medium was immediately transferred to a glass tube and each monolayer was washed three times with 2 ml each of phosphate-buffered saline containing 2mg/ml bovine serum albumin and thendissolvedin 1ml of 0.1N NaOH. The protein contents of samples containing Triton X-100 and of lipoproteins were measured by a modification of the Lowry procedure as described previously [27] with bovine serum albumin as standard
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