Effects of chloroquine on the metabolism of phosphatidylcholine associated with low density lipoprotein in arterial smooth muscle cells

Abstract

Phospholipid associated with LDL (LDL-phospholipid) has been suggested to affect metabolism of LDL in arterial smooth muscle cells. However, the metabolism of LDL-phosphatidylcholine in these cells has not been well clarified. We compared the metabolic pathway of LDL-phosphatidylcholine with that of cholesteryl ester associated with LDL (LDL-cholesteryl ester) in rabbit arterial smooth muscle cells by incubating the cells in the absence or presence of chloroquine, an inhibitor of lysosomal function. When the cells were incubated with LDL-[ 3 H]cholesterol linoleate in the absence of chloroquine, 26.6 and 51.4 of incorporated radioactivity was found as cholesteryl ester in the lysosome-rich and microsome-rich fractions, respectively. When the cells were incubated in the presence of 50 μM chloroquine, the radioactivity found as cholesteryl ester in the lysosome-rich fraction increased to 45.5% while that in microsome-rich fraction decreased to 21.4%, indicating that LDL-cholesteryl ester accumulated in lysosomes as a consequence of inhibition of lysosomal function. When the cells were incubated with LDL-[ 14C]linoleoyl phosphatidylcholine in the absence of chloroquine, 25.1% of incorporated radioactivity was found as phosphatidylcholine in the lysosome-rich fraction and 24.8% in the cytosol-rich fraction. When the cells were incubated in the presence of chloroquine, phosphatidylcholine-associated radioactivity found in the lysosome-rich and cytosol-rich fractions changed only to 28.8% and 26.1%, respectively, showing that LDL-phosphatidylcholine did not accumulate in lysosomes when lysosomal function was inhibited. In conclusion, these data indicate that LDL-phosphatidylcholine, in contrast to LDL-cholesteryl ester, is not only hydrolized in lysosomes, but also at other subcellular sites.