Differential response of non-transferrin bound iron uptake in rat liver cells on long-term and short-term treatment with iron

Background:Uptake of non-transferrin-bound iron by the liver is important a s a clearance mechanism in iron overload. In contrast to physiological upta ke via receptor-mediated endocytosis of transferrin, no regulatory mechanis ms for this process are known. This study compares the influence of long-te rm and shortterm depletion and loading of hepatocytes with iron on the upta ke of non-transferrin bound iron, its affinity, specificity and the interac tion with the transferrin-mediated pathways. Methods: Rats were fed iron-de ficient, normal and 3,5,5-trimethylhexanoyl-ferrocene-containing diets to o btain livers with the corresponding desired status and the hepatocytes from these livers were used for transport studies. Hepatocytes from normal rats were depleted or loaded with iron by short-term treatment with desferrioxa mine or ferric ammonium citrate, respectively. Uptake of non-transferrin bo und iron was assayed from ferric citrate and from ferric diethylene triammi ne pentaacetate. Results: Uptake of non-transferrin-bound iron in hepatocytes could be seen as consisting of a high-affinity (K-m = 600 nM) and a low-affinity componen t. Whereas in normal and in iron-starved rats the high-affinity component w as more prominent, it disappeared altogether in hepatocytes from rats with iron overload resulting from prolonged feeding with TMH-ferrocene-enriched diet. Overloading also led to loss of inhibition by diferric transferrin, w hich occured in starved as well as normal cells. In contrast, shortterm iro n-depletion of isolated hepatocytes with desferrioxamine had only a weak st imulatory effect, whereas treatment with ferric ammonium citrate strongly i ncreased the uptake rates. However, the inhibition by diferric transferrin also disappeared. In both cases, uptake of non-transferrin bound iron was i nhibited by apotransferrin. Conclusions: Non-transferrin bound iron uptake in liver cells is apparently regulated by the iron status of the liver. The mode of response to iron lo ading depends on the method of loading in terms of time course and the form of iron used. It cannot be explained by the behavior of the iron regulator y protein, and it is complex, seeming to involve more than one transport sy stem.