Effect of size and competition by lipoproteins and apolepoproteins on the uptake of chylomicrons and chylomicron remnants by hepatoma cells in culture

Abstract

Remnants were prepared from rabbit lymph chylomicrons, labeled with radioactive retinyl esters, by hydrolyzing the triglycerides with purified bovine milk lipoprotein lipase. Uptake of these remnants as well as that of large ( d > 100 nm) and small ( d = 50–100 nm) chylomicrons were studied with FU 5 AH hepatoma cells in monolayer cultures. By analyzing the uptake kinetics according to a Michaelis-Menten type of model we found that the K m values for large chylomicrons, small chylomicrons and remnants were similar, ranging from 0.2 to 0.6 nM. The maximal rates of uptake for these fractions were 3.2, 67 and 20 fmol lipoprotein/mg cell protein per h, respectively with the highest uptake rates for the small chylomicrons. A three-fold change in cholesterol content of the large chylomicrons, brought about by infusing cholesterol into the rabbit during lymph collection had no effect on uptake rates. A progressive decrease in size caused by lipolysis to various degrees resulted in increased cellular uptake rates. The remnant uptake was competitively inhibited by, chylomicrons, very low density, high density and low density lipoproteins in the order of increasing K I . Apoproteins of high denisty and very low density lipoproteins also were inhibitory. From studies with isolated apoproteins the maximal inhibitory activity was found in the apoprotein C fraction. This study suggests a possible apoprotein C-mediated recognition of chylomicrons and their remnants by the Fu 5 AH hepatoma cells.