A comparative study on the removal of cellular lipids from Landschütz ascites cells by human plasma apolipoproteins

The effects of human plasma lipoprotein-proteins on the removal of cellular lipids from Landschütz ascites cells were studied. Cellular lipids were labeled by injecting mice previously injected with ascites with either [3H]cholesterol or [3H]choline. Apoproteins from very low density (apoC-I, C-II, and C-111) and high density (apoA-I and A-II) lipoproteins were used. Each of the apoproteins alone was ineffective in removing cellular [3H]cholesterol. However, when synthetic phosphatidylcholines of known composition were added to each apoprotein and the experiments were repeated using either apoprotein-lipid mixtures or ultracentrifugally isolated complexes, the removal of sterol was considerably enhanced. Complexes of saturated phosphatidylcholines with apoA-II, apoC-I, or apoC-III were the most effective in releasing cellular sterol. Apoprotein-phospholipid complexes were much less effective in removing cellular [3H]phosphatidylcholine than the free apoproteins; apoA-I and apoC-I were the best of the five apoproteins studied. When a comparison was made of the adsorption of iodinated apoproteins to ascites cells, 3 to 4 times more apoA-II and apoC-III were bound than apoA-I. The binding of apoproteins was time and temperature dependent. Approximately 50% of the radioactivity that remained in the washed cells was removed with trypsin. To determine if the counts remaining in the trypsin-treated cells were internalized, identical experiments were performed using human erythrocytes, cells that do not exhibit pinocytosis. Again, approximately 50% of the radioactivity of the iodinated apoproteins was not released by trypsin. Succinylation of apoA-II not only destroys its phospholipid-binding properties but also its adsorption to red cells. These results suggest that the plasma apoproteins differ in their ability to remove cellular lipids and bind to both ascites and red cell membranes, and possibly to specific phospholipids, in such a way that only a part of the apoprotein is degraded with proteases.