REGULATION OF TRANSFERRIN RECEPTOR EXPRESSION AND FERRITIN CONTENT IN HUMAN MONONUCLEAR PHAGOCYTES - COORDINATE UP-REGULATION BY IRON TRANSFERRIN AND DOWN-REGULATION BY INTERFERON-GAMMA

We have investigated the regulation of key human iron binding proteins in mononuclear phagocytes by IFNgamma and iron transferrin. In a previous study, we demonstrated that IFNgamma down-regulates the expression on human monocytes of transferrin receptors, the major source of iron for the cell. In the present study, we show that IFNgamma also downregulates the intracellular concentration of ferritin, the major iron storage protein in the cell. By radioimmunoassay, the mean ferritin content of nonactivated monocytes was 361+/-107 fg/monocyte (mean+/-SEM) whereas the mean ferritin content of IFNgamma-activated monocytes was 64+/-13 fg/monocyte, an 82% reduction with activation (P < 0.01, t test). Consistent with its downregulating effect on these iron proteins, IFNgamma treatment also results in decreased iron incorporation. IFNgamma-activated monocytes incorporated 33% less iron from Fe-59-transferrin than nonactivated monocytes (P < 0.05, t test). Gel filtration chromatography revealed that incorporated iron is located primarily in ferritin in both nonactivated and IFNgamma-activated monocytes. Ferritin in IFNgamma-activated monocytes is saturated with approximately three times as much Fe-59 as ferritin in nonactivated monocytes. We have also explored the effect of iron transferrin on transferrin receptor expression and intracellular ferritin content in human monocytes. We have found that iron transferrin markedly upregulates both transferrin receptor expression and intracellular ferritin content in both nonactivated (2.3- and 1.3-fold, respectively) and IFNgamma-activated (3.4- and 2.9-fold, respectively) monocytes. This study demonstrates that transferrin receptor expression and intracellular ferritin content in human monocytes is unidirectionally and coordinately upregulated by iron transferrin and unidirectionally and coordinately downregulated by IFNgamma.