UPTAKE AND DEGRADATION OF PROTEINS IN ISOLATED RAT LIVER CELLS

Abstract

The uptake and the degradation of asialofetuin, formaldehyde-treated human serum albumin (HSA), and high-density lipoproteins (HDL) have been studied in isolated rat liver cells. Purified hepatocytes and nonparenchymal liver cells were prepared by differential centrifugation from an enzymatic ally prepared mixed liver cell suspension. 125I-labeled asialofetuin was taken up almost exclusively by hepatocytes in vitro, while 125I-HSA was taken up only by nonparenchymal liver cells. HDL, labeled with 3H-cholesterol in the cholesterol ester portion, was taken up both by hepatocytes and nonparenchymal liver cells. Asialofetuin and HDL seemed to be taken up by a saturable process in hepatocytes. The uptake of HSA also leveled off at increasing HSA concentrations, while the uptake of HDL by nonparenchymal cells did not exhibit saturation kinetics. Degradation of asialofetuin and HSA was followed by measuring acid-soluble radioactivity released to the medium from the cells. Degradation started 10–20 minutes after the addition of labeled proteins to the cells and was inhibited by chloroquine and ammonium ions. Isopycnic centrifugation studies revealed that part of the labeled asialofetuin in the cells was in lysosomes about 60 minutes after the start of the uptake of the protein. We conclude that enzymatically prepared rat liver cells carry out endocytosis and heterophagy of proteins. These processes can therefore be studied under controlled conditions in vitro.