Uptake of high-density lipoprotein-associated apoprotein A-I and cholesterol esters by 16 tissues of the rat in vivo and by adrenal cells and hepatocytes in vitro.

Abstract

The uptake of high-density lipoprotein (HDL)-associated apolipoprotein A-I and cholesterol esters was estimated in 16 tissues of the rat using rat HDL doubly labeled with nondegradable tracers; covalently attached 125I-tyramine-cellobiose traced apo-A-I, and [3H]cholesteryl linoleyl ether traced cholesterol esters. Both labels remained associated with the HDL fraction in the plasma, adequately traced their unlabeled counterparts, and were well trapped at their sites of uptake. Cholesteryl ether was taken up at a greater fractional rate than apo-A-I by adrenal, ovary, and liver: 7-fold, 4-fold, and 2-fold greater, respectively. The rates of uptake of cholesteryl ether and apo-A-I were about equal in the other tissues (except kidney). The disproportionate uptake of HDL cholesteryl ether relative to HDL apo-A-I was also observed in primary cultures of rat adrenal cells and hepatocytes. Uptake of both moieties in both cell types showed saturability. Both the absolute rate of uptake of [3H]cholesteryl ether and the ratio of ether uptake to apo-A-I uptake were greater in adrenal cells than in hepatocytes, consonant with the in vivo observations. Very similar results were obtained using HDL biologically labeled with [3H]cholesterol esters. The disproportionate uptake of [3H]cholesteryl ether was not significantly decreased by depletion of apo-E from the HDL nor by reductive methylation of the apo-E to block its recognition by receptors. However, apo-A-I uptake was decreased, suggesting that apo-E mediates the uptake of particles containing apo-A-I but does not contribute to the disproportionate uptake of [3H]cholesteryl ether.