Endotoxins Inhibit Endocytotic Catabolism of Low–Density Lipoproteins in Hep G2 Cells

The effects of endotoxins on the uptake and degradation of low-density lipoproteins in Hep G2, a well-differentiated human hepatoma cell line, were studied. The results showed that incubation of Hep G2 cells with 125I-labeled low-density lipoprotein in the presence of endotoxins caused decreased uptake and degradation of 125I-labeled low-density lipoprotein. The inhibitory effects of endotoxins on the uptake and degradation of 125I-labeled low-density lipoprotein were dose and time dependent. With a monoclonal low-density lipoprotein receptor antibody, it was found that endotoxins interfered with both low-density lipoprotein receptor-mediated and non-low-density lipoprotein receptor-mediated uptake. If, however, the cells were pretreated with endotoxins for 1 or 24 hr and then incubated with new medium without endotoxins, no inhibitory effect on the subsequent uptake and degradation of 125I-labeled low-density lipoprotein occurred. Endotoxins had no toxic effects on Hep G2 cells as judged by [3H]thymidine incorporation and by determination of cell growth. Also, endotoxins did not under our experimental conditions induce oxidative modification of low-density lipoprotein. Furthermore, reisolated low-density lipoprotein that had previously been incubated with endotoxin was catabolized to a lower extent by Hep G2 cells than was control low-density lipoprotein. We speculate that the inhibitory effect of endotoxins on cellular low-density lipoprotein catabolism is due to the formation of endotoxin-low-density lipoprotein complexes, which interfere with the binding of low-density lipoprotein to the cell surface.