The effect of ammonia, chloroquine, leupeptin, colchicine and cytochalasin B on degradation of high density lipoproteins in isolated rat hepatocytes

Degradation of 125I-labelled HDL ([125I]HDL) was measured in isolated rat hepatocytes that had been preincubated with [125I]HDL and then reincubated in fresh medium without [125I]HDL. About 5 % of the [125I]HDL associated with the cells in advance were degraded per hour at 37 °C. This in vitro degradation was inhibited about 50% by lysosomal inhibitors such as chloroquine, ammonia and leupeptin. Depolymerization of microtubuli by colchicine inhibited the degradation of [125I]HDL to about 65–75 % of the control cells. Cytochalasin B (CB), a destabilizer of microfilaments, had a less marked effect on the degradation in vitro. Degradation of [125I]HDL associated with cells in vivo after intravenous injection was also studied in isolated cells. About 8.5% of the [125I]HDL associated with the cells in vivo were degraded per hour in the isolated cells. The effects of ammonia, chloroquine, leupeptin and colchicine on HDL degradation were similar for [125I]HDL taken up in vivo and in vitro. Subcellular fractionation by centrifugation in sucrose gradients indicated that [125I]HDL associated with hepatocytes in vivo are primarily accumulated in lysosomes. [125I]HDL associated with the cells in vitro are located in organelles whose distribution coincides with that of 5′-nucleotidase. These organelles may be endocytic vesicles. It is concluded that the internalization of [125I]HDL in rat hepatocytes is relatively slow. The intracellular degradation of the apoproteins of HDL is at least partly lysosomal.