A Study of High Density Lipoprotein (HDL) Receptors Expressed on a Human Hepatoma Cell Line Mahlavu

Abstract

High density lipoproteins (HDL) may play an important role in removing cholesterol from peripheral tissue and deliver it to the liver, acting as a shuttle in reverse cholesterol transport. The mode of interaction between the liver and HDL, however, has not yet been clarified.
We investigated catabolism of human HDL in a human hepatoma cell line Mahlavu. The binding of ¹²⁵I-Iabeled HDL at 4°C was time-dependent and reached completion within 2hr. The observed binding rates of ¹²⁵I-labeled HDL at 4°C and the uptake and degradation at 37°C indicated the presence of both specific and non-specific HDL binding sites. Non-specific binding occured in proportion to the concentration of ¹²⁵I-Iabeled HDL. Specific binding of ¹²⁵I-labeled HDL accounted for 60% (at 4°C) and 75% (at 37°C) of total binding capacity. The association and degradation of ¹²⁵I-labeled HDL was inhibited by the addition of unlabeled HDL, but not LDL, suggesting HDL specificity. The lysosomal degradation of ¹²⁵I-labeled HDL was time-dependent and inhibited only 10% and 20% by chloroquine at 50μM and 100μM concentrations, respectively. This finding suggested that HDL was metabolized through a non-lysosomal pathway.
There was a marked difference in the degradation modes of ¹²⁵I-labeled HDL between fibroblasts and Mahlavu cells. The degradation of ¹²⁵I-labeled HDL by Mahlavu cells increased over time, while only a small portion of ¹²⁵I-labeled HDL was degraded by fibroblasts. This data shows that hepatocytes such as Mahlavu cells have HDL receptors which are considerably different from those of fibroblasts. This cell line provides a good model for analyzing interactions between HDL and hepatocytes in humans.