The Removal of Cellular Lipids from Landschütz Ascites Cells and Smooth Muscle Cells in Culture

Abstract

Although the function of the plasma high density lipoproteins (HDL) is unknown, Glomset (1,2) has postulated that plasma HDL and plasma lecithin:cholesterol acyltransferase (LCAT) play an important role in the transport of unesterified cholesterol from other lipoproteins and peripheral tissues to the liver. It is postulated that the cholesterol exchanges in an non-enzymic manner between plasma lipoproteins and membranes and HDL. The unesterified cholesterol in the HDL is then esterified by the plasma enzyme, LCAT. The cholesteryl esters formed are then transported via HDL to the liver where they are either converted to bile acids and secreted in the bile or incorporated into lipoproteins and released into the circulation. Evidence that cells can, indeed, lose their membrane cholesterol to the plasma lipoproteins and, in particular HDL, has been demonstrated by a number of investigators (3–7). Stein and Stein (5) found that [³H]cholesterol was removed from Landschütz ascites cells incubated in the presence of either LDL or HDL. However, HDL was much more effective in removing cellular sterol. Although lipid-free HDL (apoHDL) was also effective, the release of sterol was greatly increased when apo HDL were mixed with either rat liver phosphatidylcholine or sphingomyelin. In these earlier studies (5), no attempt was made to use the isolated apoproteins or phospholipids of known composition. In the present communication, we have used purified apoproteins from both the high density and very low density lipoproteins (VLDL) and have measured the removal of cellular lipids from both Landschütz ascites cells and smooth muscle cells. The findings provide evidence that the various apoproteins differ in their ability to remove cellular lipids.