Cholesterol exchange between fat cells, chylomicrons and plasma lipoproteins.

Isolated rat adipocytes were incubated with serum lipoproteins or lymphchylomicrons which contained 14c-labeled cholesterol. The specific activity of lipoprotein free cholesterol decreased and that of cellular free cholesterol increased linearly up to 7 h. At this time the cell cholesterol specific activity was only 11% of that of medium cholesterol indicating that the rate of exchange was slow. The specific activity of lipoprotein esterified cholesterol remained unchanged while that of cells showed a slight increase suggesting esterification of incorporated free cholesterol. No detectable change of the lipoprotein or cellular cholesterol concentration occurred indicating that the uptake of radioactive free cholesterol was due to exchange without net movement of sterol. The radioactive cholesterol was incorporated into both membrane fraction and the fat droplet of the adipocytes. The rate of cholesterol exchange was temperature-dependent but it was not influenced by the metabolic state of the cells and not by addition of metabolic inhibitors. Trypsin or pronase treatment of the cells were without influence on the rate of the exchange and denaturation of the plasma lipoproteins with formalin increased the rate of exchange. These results indicate that the exchange of cholesterol is a physical chemical process, which is not linked to energy metabolism of the cells, and which is not mediated by either specific lipoprotein receptors on fat cell membranes or pinocytic uptake of lipoproteins. The rate of free cholesterol exchange showed a linear correlation with the concentration of lipoprotein particles in the medium. The relative transfer rate was highest for chylomicrons and decreased in order chylomicron remnants greater than very low density lipoprotein greater than low density lipoprotein greater than high density lipoprotein. A saturation of the system could be obtained only with high density lipoprotein.