Pharmacologic doses of 17α-ethinyl estradiol have been reported to cause a marked lowering of plasma lipoprotein levels in the rat. The drop in plasma low density lipoprotein (LDL) is associated with enhanced uptake of LDL by the liver. In the current studies, we show that membranes prepared from livers of ethinyl estradiol-treated rats exhibit a 3- to 10-fold increase in saturable binding sites for human 125I-LDL. These binding sites resembled the LDL receptors previously described in extrahepatic human, mouse, and bovine cells in that they: 1) showed a marked preference for human LDL as opposed to human high density lipoprotein (HDL); 2) required calcium; 3) failed to bind LDL in which the lysine residues had been acetylated or methylated in vitro; and 4) were destroyed by pronase. Hepatic uptake of intravenously administered human 125I-HDL was 12-fold higher in estradiol-treated rats as compared with controls. Uptake of 125I-LDL was competitively inhibited by unlabeled human LDL, but not by human HDL. Moreover, methylated human 125I-HDL, which did not bind to the LDL site on membranes, was taken up by the liver in vivo at a 10-fold lower rate than 125I-LDL. These data suggest that the 125I-LDL membrane binding site that was detected in vitro mediated the uptake of 125I-LDL in vivo. Livers of untreated and ethinyl estradiol-treated rats also exhibited a saturable binding site for human 125I-HDL. This site bound human HDL much more avidly than human LDL. HDL binding was not increased by ethinyl estradiol treatment, did not require calcium, and was not destroyed by pronase. Whether the 125I-LDL and 125I-HDL binding sites function as hepatic lipoprotein receptors in untreated rats is not yet known.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Biological Chemistry|
|State||Published - Dec 1 1979|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology