The study described in this paper shows that 125I-labelled low-density lipoproteins (LDL) interact with high- and low-affinity binding sites on human hepatoma (HepG2) cells. The former site is the LDL receptor and the latter is the lipoprotein-binding site (LBS). The association of 125I-labelled LDL and [3H]cholesteryl ethers–LDL with HepG2 cells revealed a 4-fold selective uptake of cholesteryl esters (CE) in a 4 h incubation period, which correlated with the depletion of CE mass in LDL. This selective uptake was not observed when the cells were incubated in the presence of a 100-fold excess of high-density lipoprotein 3, conditions where only the LDL receptor is being monitored. Also, no reduction in uptake was observed in the presence of IgG-C7, an anti-(LDL receptor) monoclonal antibody. Both findings indicate that the selective uptake occurs through the LBS and that the LBS contributes more to the entry of CE from LDL into the cell than does the LDL receptor. The fates of CE entering the cell via the LDL receptor and the LBS were also followed. To achieve this, LDL were labelled with [3H]cholesteryl oleate and the hydrolysis of [3H]cholesteryl oleate was monitored. The results indicated that 45% of the CE were hydrolysed after a 4 h incubation period, irrespective of the site of entry. Chloroquine (100 µM) was shown to inhibit hydrolysis, indicating that lysosomal enzymes were responsible for the hydrolysis of LDL–CE, whichever pathway was used. Thus our results reveal, for the first time, that the mass of CE entering the cell via the LBS is substantial and that hydrolysis of CE is by lysosomal enzyme activity. Overall, this suggests that the LBS has significant physiological importance.
- The Biochemical Society, London © 1996