Cellular cholesteryl ester clearance. Relationship to the physical state of cholesteryl ester inclusions

The hypothesis that clearance of cellular cholesteryl ester deposits may be a function of the physical state of the stored lipid has been investigated. Cultured rat hepatoma cells were induced to store cholesteryl ester in either anisotropic inclusions by exposure to free cholesterol-rich phospholipid dispersions or isotropic inclusions by exposure to identical dispersions supplemented with oleic acid. Differential scanning calorimetry demonstrated an order/disorder transition at 43 degrees C for cholesteryl esters stored in anisotropic inclusions; the enthalpy of this transition was consistent with a smectic liquid crystalline to liquid transition. Lipids in cells with isotropic inclusions displayed no order/disorder transitions over the range 20-80 degrees C, indicating that the lipids are in a liquid state. The presence of oleic acid did not influence the mass of cholesteryl ester stored but increased the amount of stored triglyceride. Fatty acyl compositions of the cholesteryl esters were different under the two loading conditions; in particular, there was 38% cholesteryl oleate in anisotropic inclusions and 65% cholesteryl oleate in isotropic inclusions. Kinetics of cholesteryl ester clearance from cells with either anisotropic or isotropic inclusions were studied during a 12-h exposure to acceptors of free cholesterol. In both cases, cholesteryl ester clearance is essentially linear over 12 h and is directly proportional to the initial content of cholesteryl ester. However, the fraction of initial content of cholesteryl ester cleared in 12 h is 0.17 +/- 0.05 for cells with anisotropic inclusions and 0.34 +/- 0.09 for cells with isotropic inclusions. Our data demonstrate that the more rapid clearance of cholesteryl ester by cells with isotropic inclusions can be correlated with the physical state of the cholesteryl ester.