Rat High Density Lipoprotein Subfraction (HDL,) Uptake and Catabolism by Isolated Rat Liver Parenchymal Cells*

Rat liver parenchymal cell binding, uptake, and proteolytic degradation of rat ‘Z51-labeled high density lipoprotein (HDL) subfraction, HDL, (1.110 < d < 1.210 g/ml), in which apo-A-I is the major polypeptide, were investigated. Structural and metabolic integrity of the isolated cells was verified by trypan blue exclusion, low lactic dehydrogenase leakage, expected morphology, and gluconeogenesis from lactate and pyruvate. L261-labeled HDL, was incubated with 10 x lo* cells at 37” and 4” in albumin and Krebs-Henseleit bicarbonate buffer, pH 7.4. Binding and uptake were determined by radioactivity in washed cells. Proteolytic degradation was determined by trichloroacetic acid-soluble radioactivity in the incubation medium. At 37”, maximum HDL, binding (B,., ) and uptake occurred at 30 min with a B,,, of 31 ng/mg dry weight of cells. The apparent dissociation constant of the HDL, receptor system (K,) was 60 x 1Om8 M, based on M, = 28,000 of apo-A-I, the predominant rat HDL, protein. Proteolytic degradation showed a 15min lag and then constant proteolysis. After 2 hours 5.8% of incubated lZJI-labeled HDL, protein was degraded. Sixty per cent of cell radioactivity at 37” was trypsinreleasable. At 37”, ‘*4-labeled HDL, was incubated with cells in the presence of varying concentrations of native (cold) HDL,, very low density lipoproteins, and low density lipoproteins. Incubation with native HDL, resulted in greatest inhibition of ‘Y-labeled HDL, binding, uptake, and proteolytic degradation. When ‘Y-labeled HDL, was preincubated with increasing amounts of HDL, antiserum, binding and uptake by cells were decreased to complete inhibition. Cell binding, uptake, and proteolytic degradation of ‘*%labeled HDL, were markedly diminished at 4”. Less than 1 mM chloroquine enhanced ‘2SI-labeled HDL, proteolysis but at 5 mM or greater, chloroquine inhibited proteolysis with ‘2”I-labeled HDL, accumulattion in cells. L[U-“C]Lysine-labeled HDL, was bound, taken up, and degraded by cells as effectively as XZJI-labeled HDL,. These data suggest that liver cell binding, uptake, and proteolytic degradation of rat HDL, are actively performed and linked in the sequence:binding, then uptake, and finally proteolytic degradation. Furthermore, there may be a specific HDL, (lipoprotein A) receptor or recognition site(s) on the plasma membrane. Finally, our data further support our previous reports of the important role of liver lysosomes in proteolytic degradation of HDL,.