Regulation of ferritin and transferrin synthesis in hepatocytes depending on iron status of rats

Abstract

In order to examine the control mechanism of ferritin (Fr) and transferrin (Tf) synthesis depending on intracellular iron levels, the rate of ¹⁴C-leucine incorporation into those proteins was investigated by cell culture of isolated hepatocytes obtained from iron deficient, iron injected and control rats. The effects of iron (ferric ammonium citrate: FeAC) and (diferric Tf: 2FeTf) or desferrioxamine (Dfo) in culture media were also examined. Serum iron, TIBC and Hb levels of iron deficient rats, which were fed an iron deficient diet for 2 and 4 weeks were significantly lower than the control. However serum iron and TIBC levels of iron injected rats which had received 30 and 45 mg iron as iron dextran 18 h before sacrifice were approximately ten times higher than the control group.
The time course of ¹⁴C-leucine incorporation into Fr and Tf was investigated at the 60, 120 and 180 minutes stages of the culture. Fr synthesis was increased by the amounts of iron injected, whereas Tf synthesis showed a negativeresponse to iron. The ¹⁴C activities in Fr and Tf detected from culture media were proportional to those in hepatocytes. The percentage of nonglycosylated Fr was 82.0-91.4% for total Fr in the culture media in every experiment, which was measured by the affinity of glycosylated Fr to Con A-Sepharose (Con A). This result suggested the leakage of cytosol Fr through the cell membrane instead of specific secretion of the sialated protein. The efficiency of Fr and Tf synthesis was positively or negatively proportional to cellular iron contents respectively. And the curves of ¹⁴C-leucine incorporation into both proteins, calculated as the sum of those in hepatocytes and culture media intersected at the point between the 30 mg iron injected and control groups. The addition of FeAC or 2FeTf into the culture media had an indistinct effect on Fr and Tf synthesis, whereas there was a significant decrease for Fr and a slight increase for Tf formations in the Dfo supplement. These results showed the influence of cellular iron levels in Fr and Tf synthesis.