LDL-induced cytotoxicity and its inhibition by HDL in human vascular smooth muscle and endothelial cells in culture

Abstract Human aortic medial smooth muscle cells (SMC) and umbilical vein endothelial cells (EC) in culture were exposed to various concentrations of plasma low density (LDL) and high density (HDL) lipoproteins prepared from normolipemic donors in order to assess their effects on cell growth. So that the effects of each lipoprotein could be evaluated separately and in combination, lipoproteins were added to culture medium containing lipoprotein deficient serum (LPDS, d > 1.25 g/ml at a protein concentration of 4.5 mg/ml of medium). The addition of LDL at cholesterol concentrations of 160 μg/ml of culture medium, resulted in significant reductions in both the number of SMC and EC cells per dish within 3 days of exposure ( P P This cytotoxic phenomenon was dose-related, and only at LDL cholesterol concentrations equal to or below 50 μ/ml were no marked changes observed. In contrast, HDL at all concentrations tested produced no such deleterious effects. Autoradiographic assessment of DNA synthesis confirmed these findings. After 48 h of continuous exposure to tritiated thymidine, labeling indexes reached much lower plateaus in the LDL-treated groups. The addition of HDL inhibited this LDL-induced cytotoxicity. However, only at a low LDL concentration (100 pg LDL cholesterol/ml of medium) could HDL completely prevent toxicity. At the higher LDL concentration tested (250 μ/ml of medium) the addition of HDL up to an HDL/LDL ratio of unity could not prevent the LDL-induced effects. When LDL was added to lipid depleted serum (LDS at 4.5 mg protein/ml culture medium, prepared by ether extraction of all lipids, with the exception of phospholipids) instead of LPDS, only slight growth inhibition was noted. This suggests that protection against LDL toxicity by HDL (and LDS) may involve protein and phospholipid components.