Abstract
We compared the metabolism of low density lipoprotein (LDL) in SV40-transformed smooth muscle cells (TSMCs) to that in nontransformed smooth muscle cells (SMCs). When SMCs were incubated in medium with 100 μg/ml LDL for 24 hours, they did not accumulate sudanophilic lipid droplets. On the other hand, when TSMCs were incubated in medium containing more than 100 μg/ml LDL, they accumulated a large amount of lipid droplets in their cytoplasm. When cells were incubated with 200 μg/ml LDL for 24 hours, cholesteryl ester levels significantly increased in TSMCs (18.3 ± 3.53 μg/mg protein), as compared with SMCs (2.40 ± 0.85 μg/mg protein). However, there was no difference in the cellular level of free cholesterol between the TSMCs and SMCs. Although the TSMCs and SMCs had a similar number of binding sites for LDL, the TSMCs demonstrated a markedly higher uptake of LDL labeled with 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate (Dil-LDL), compared with the SMCs. SMCs that had been pretreated with 100 μg/ml of unlabeled LDL for 24 hours showed a decreased uptake of Dil-LDL. In contrast, TSMCs incorporated Dil-LDL independently of the preincubation with 100 μg/ml LDL. The presence of brefeldin A, which may block the transport of glycoproteins from the ER to Golgi apparatus, had less of an effect on the uptake of LDL in the TSMCs than in the SMCs. These results suggest that SV40-transformed smooth muscle cells show an increased uptake of LDL independent of the cellular cholesterol level, which may induce the accumulation of lipid droplets in their cytoplasm. A LDL receptor-independent pathway may be related to the increased uptake of LDL in SV40-transformed smooth muscle cells.
