Homologous Low Density Lipoprotein Research Articles

Deposition and mobilization of cholesterol ester in cultured human skin fibroblasts

Human skin fibroblasts in culture served as a model system to study intracellular cholesterol ester deposition in mesenchymal cells. Confluent cultures were exposed to homologous low density lipoprotein alone and together with chloroquine. In the presence of low density lipoprotein alone, even at half circulating serum concentrations, cellular free cholesterol increased no more than 12%, while the increase in cholesterol ester ranged from 13--100% during 48 h of incubation. Addition of chloroquine to the culture medium containing low density lipoprotein resulted in a very marked increase in cholesterol ester and the ratio of cellular esterified cholesterol to free cholesterol rose up to 2.2. In the presence of chloroquine the sum of uptake and degradation of 125I-labeled low density lipoprotein was enhanced and at higher chloroquine concentrations the more pronounced inhibition of degradation resulted in the intracellular retention of undegraded protein. Upon removal of the chloroquine-containing medium, there was a slight fall in the cellular cholesterol after 24 h incubation in a medium containing 10% fetal calf serum. Replacement of the fetal calf serum by lipoprotein-deficient serum and a mixture of high density apolipoprotein and sphingomyelin increased very significantly the loss of total cholesterol from the cells. At the same time the rate of loss of the retained labeled low density lipoprotein was also increased. The present results provide an adequate and reproducible model system for the study of cholesterol accumulation in human mesenchymal cells, which is one of the basic changes in atheromatosis. The availability of cholesterol ester laden cells also provides a good system for the study of agents active in cholesterol removal.

The binding of low density lipoprotein by liver membranes in the pig

Porcine liver membranes are capable of high affinity binding of homologous low density lipoproteins (LDL). Binding is time and temperature dependant and substrate saturable. High affinity binding sites are half saturated at 11 μg/ml lipoprotein-protein. The binding of 125I-LDL is inhibited by unlabelled homologous LDL, very low density lipoproteins (VLDL) and high density lipoproteins (HDL) and also be human LDL and HDL, but not by unrelated proteins tested. The binding and displacement patterns with membranes from several other porcine tissues are similar to those of liver membranes. These results suggest the presence of “lipoprotein binding sites” in liver membranes which recognize structural features common to the lipoproteins and further indicate that liver membranes are not unique in their ability to bind LDL.