Endothelial mediation is necessary for subsequent hepatocyte uptake of transferrin

The authors previously have reported on the presence of transferrin (TF) receptors on liver endothelial cells and have shown that hepatic uptake of transferrin-iron (TF-Fe) complexes in the liver is mediated by the endothelium. We now provide evidence that this endothelial cell mediation may be necessary for hepatocyte uptake of TF-Fe complexes. Transport of TF-Fe from endothelial cell to hepatocyte was studied in mixed cell suspensions in which radiolabeled TF-Fe complexes were incubated at 37 degrees C with the two cell populations purified and then mixed in equal ratios. The mixtures were then refractionated at various times after incubation and cell-associated radioactivities measured. Radiolabeled TF was rapidly taken up by the endothelial cell fraction, but radioactivity began to decline in this fraction as it increased in the hepatocyte fraction. In double labeling experiments with 125I-TF-59Fe, both radiolabels moved across the endothelium in parallel fashion, indicating that Fe remains associated with TF during transcytosis. However, in hepatocytes the two radiolabels became dissociated, with Fe remaining cell-associated and TF being recycled. Hepatocyte uptake of processed TF was partially inhibitable by galactan and asialofetuin, indicating that hepatocyte uptake may occur via asialoglycoprotein receptors of hepatocyte.