Cholesterol from serum and cholesterol-phospholipid dispersions by isolated rat hepatocytes in vitro

The rate of uptake of cholesterol by isolated rat hepatocytes from incubation medium containing rat serum was low and did not increase by the prolongation of incubation time up to 2 hr at 4 degree. However, the hepatocytes took up cholesterol very actively at 37.5 degree from serum as well as from a cholesterol-phospholipid dispersion. The cholesterol uptake by the cells from liposomes increased linearly with the increase of the cholesterol concentration of liposomes, but that from serum reached the maximum at about 1.5 mu mol/ml cholesterol and remained at a plateau level at higher concentrations. Cholesterol was transfered from every class of serum lipoprotein, although the uptake ratio was the highest from the LDL fraction. As the lipoprotein concentration was increased, the uptake ratio decreased in the same way as that observed in the incubation with whole serum. The concentration of labeled ester cholesterol in the medium decreased more rapidly than free cholesterol during incubation. Cholesterol in the hepatocytes, however, was found mainly as the free form, suggesting that ester cholesterol taken up by the cells was rapidly hydrolyzed. Cholesterol accumulation was not observed in hepatocytes incubated in medium containing serum, but their cholesterol content was increased after the incubation in medium with liposomes of high cholesterol concentration. Therefore the uptake of cholesterol by hepatocytes from serum lipoproteins is probably carried out by a process different from that of uptake from liposomes which is considered to be a physicochemical phenomenon.