Iron Transport into Mycobacterium Avium-containing Phagosomes from an Nramp1(Gly169)-transfected RAW264.7 Macrophage Cell Line

Nramp1 is an important determinant of innate resistance of macrophages to the growth of intracellular microorganisms. We previously showed that Nramp1 functions to transport iron from the cytoplasm into phagosomes of Mycobacterium avium-infected macrophages. The purpose of this investigation was to further characterize the factors that regulate Nramp1-mediated iron transport into phagosomes. Treatment of Nramp1(Gly169) macrophages with the lysomotrophic agents chloroquine or ammonium chloride reduced the import of iron significantly. We found that macrophage-activating cytokines, including TNF-alpha, IFN-gamma, IL-1alpha, and GM-CSF, when added prior to M. avium, increased the transport of iron into the phagosome. This increase in iron transport was not a result of an increased amount of Nramp1 protein in the phagosome nor to new protein synthesis. Treatment of Nramp1(Gly169)-transfected macrophages with inhibitors of protein kinase C (PKC) diminished the import of iron into the phagosomes. Iron import was inhibited by an anti-Nramp1 antibody against the putative fourth outer-loop region of Nramp1 but not by an anti-Nramp1 antibody against the carboxy terminus. The significance of these results on the orientation of Nramp1 in the phagosome membrane and on the transport of iron is discussed.