Low-density lipoproteins are degraded in HepG2 cells with low efficiency

In previous studies we have shown that in HepG2 cells, as compared with fibroblasts, the low-density lipoprotein (LDL) receptor is only weakly down-regulated upon incubation of the cells with LDL. To explain this difference in down-regulation of the LDL-receptor activity, we studied simultaneously the intracellular processing of 125I-labelled LDL in both cell lines. Upon incubation of HepG2 cells with 125I-LDL, the appearance of degradation products started at 90 min, whereas in fibroblasts this lag time was only 30 min. The degradation efficiency (representing the ratio degradation/cell association of LDL) in HepG2 was less than 50% of that in fibroblasts up to 5h of incubation at 37 degrees C. The longer lag time and low efficiency of the degradation of LDL in HepG2 cells were independent of the cell density. Pulse-chase experiments indicated that the internalization rate of surface-bound LDL in HepG2 cells is similar to that of fibroblasts. Endosomal loading of 125I-LDL by incubation at 18 degrees C for 4.5 h, followed by a shift to 37 degrees C, resulted in degradation of LDL within 30 min in fibroblasts, whereas in HepG2 cells the lag time of the degradation was 90 min. In parallel experiments using subcellular fractionation by Percoll-gradient centrifugation of homogenized cells and 125I-tyramine-cellobiose-labelled LDL, we observed that in both cell types LDL is equally rapidly shifted from a low- to a high-density compartment (within 15 min), representing the endosomal and the late-endosomal plus lysosomal compartment respectively. We conclude that in HepG2 cells the cell-bound LDL, upon internalization, goes through the intracellular itinerary at the same rate as in fibroblasts, but that either the fusion between late endosomes and lysosomes or the lysosomal degradation itself is proceeding at a lower efficiency. A low degradation rate of LDL may contribute to explain the relatively weak down-regulation of the LDL-receptor activity in HepG2 cells by LDL.