Uptake and degradation of low density lipoprotein by swine arterial smooth muscle cells with inhibition of cholesterol biosynthesis

Abstract

We have previously proposed on the basis of studies in hepatectomized animals that low density lipoproteins are degraded at a significant rate by peripheral tissues. To test the capacity of one peripheral cell type to catabolize low density lipoprotein, cultures of swine aortic smooth muscle cells were incubated with homologous 125I-labeled low density lipoprotein and uptake and degradation measured. Degradation of 125I-labeled low density lipoprotein to products soluble in trichloroacetic acid showed an initial lag period of 1–2 h after which the rate increased and remained linear for the following 15 h. Rates of degradation increased sharply with low density lipoprotein concentration over the lower range (from 0–25μg protein/ml) and then more slowly up to the highest concentration tested, 300 μg protein/ml. Even at very low concentrations, 1 μg low density lipoprotein protein/ml (less than 10% of the plasma low density lipoprotein concentration), the in vitro degradation rate (per kg of smooth muscle cells) exceeded the in vivo degradation rate (per kg of total body weight). To the extent that smooth muscle cells are representative of other peripheral cells, the results support the proposal that peripheral degradation of low density lipoprotein apoprotein may be quantitatively important. The rate of incorporation of labeled acetate into sterols was suppressed in cells incubated with whole serum, low density and very low density lipoproteins, or suspensions of free cholesterol. In this respect, the results were similar to those observed in human skin fibroblasts studied concurrently. However, high density lipoprotein inhibited sterol synthesis by about 25% in swine smooth muscle cells while it had no effect in human skin fibroblasts. Cells incubated overnight in lipoprotein-deficient medium showed a net decrease in cholesterol content which could be restored by incubating with low density lipoprotein but not with high density lipoprotein. At high concentrations of low density lipoprotein, when uptake and degradation proceed at very high rates, the amount of cholesterol delivered to the cell is in excess of observed net changes in cell cholesterol content. These findings suggest that there may be a very active “reverse cholesterol transport” in vivo carrying cholesterol from peripheral cells (in which low density lipoprotein is being degraded) back to the liver for ultimate metabolism and excretion.