Modulation of transferrin synthesis, transferrin receptor expression, iNOS expression and NO production in mouse macrophages by cytokines, either alone or in combination

Abstract

Iron, an essential element for all living organisms, is central importance in a number of crucial metabolic pathways, including the regulation of immune function. Iron delivery to cells is accomplished by the complexing of iron to transferrin (Tf), a monomeric iron-binding protein in the plasma, followed by specific binding of Tf to cell-surface receptors, endocytosis of the receptor-ligand complexes and ultimately, release of iron from endosomal vesicles to the cytoplasm. The purpose of this study was to evaluate the effect of cytokines, alone and in combination, on the factors that can affect the iron delivery in thioglycollate-elicited macrophages. In this study, IFNγ induced a marked increase in Tf synthesis by macrophages, while IL-1, IL-6 and TNFα produced a more modest increase. Combinations of these cytokines were shown to be less effective in promoting macrophage Tf synthesis than the cytokines by themselves. IFNγ alone and in combination with other cytokines was effective in inducing nitrite (NO) production and inducible nitric oxide synthetase (iNOS) expression in macrophages, while IL-1, TNFα and IL- 6 individually, as well as in various combinations, were not. While all tested cytokines individually and in combination inhibited the expression of the transferrin receptor (TfR) on macrophages, IFNγ alone and in combination with other cytokines most strongly repressed the TfR expression. TfR localization in macrophages after IFNγ stimulation showed that TfR fluorescence was most intense in the perinuclear region after 6 hours and scattered diffusely throughout the cytoplasm after 24 hours. This data suggests that IFNγ may enhance iron uptake during the early phase of macrophage activation, and in later phases, down-regulate TfR expression by inducing NO, thus contributing to intracellular oxidative stress reduction.