Date Published: January 30, 2014
Publisher: Public Library of Science
Author(s): Rym Ben-Othman, Andrew R. Flannery, Danilo C. Miguel, Diane M. Ward, Jerry Kaplan, Norma W. Andrews, Chinmay K. Mukhopadhyay.
Leishmania parasites infect macrophages, cells that play an important role in organismal iron homeostasis. By expressing ferroportin, a membrane protein specialized in iron export, macrophages release iron stored intracellularly into the circulation. Iron is essential for the intracellular replication of Leishmania, but how the parasites compete with the iron export function of their host cell is unknown. Here, we show that infection with Leishmania amazonensis inhibits ferroportin expression in macrophages. In a TLR4-dependent manner, infected macrophages upregulated transcription of hepcidin, a peptide hormone that triggers ferroportin degradation. Parasite replication was inhibited in hepcidin-deficient macrophages and in wild type macrophages overexpressing mutant ferroportin that is resistant to hepcidin-induced degradation. Conversely, intracellular growth was enhanced by exogenously added hepcidin, or by expression of dominant-negative ferroportin. Importantly, dominant-negative ferroportin and macrophages from flatiron mice, a mouse model for human type IV hereditary hemochromatosis, restored the infectivity of mutant parasite strains defective in iron acquisition. Thus, inhibition of ferroportin expression is a specific strategy used by L. amazonensis to inhibit iron export and promote their own intracellular growth.
Intracellular parasites must obtain essential nutrients from their host cells. Iron is a vital nutritional resource for mammalian hosts and also for many pathogens, acting as an essential cofactor of proteins and enzymes involved in metabolic pathways. Under physiological conditions, iron is normally found in the insoluble, oxidized Fe3+ form associated with carrier proteins such as transferrin and ferritin. After endocytosis mediated by transferrin receptors, Fe3+ is reduced to the soluble Fe2+ form and translocated to the cytosol. In the cytosol, iron may be utilized by the host, stored as complex with ferritin, or exported out of the cells. Thus, the pool of available iron within mammalian cells is determined by a carefully orchestrated balance between uptake through plasma membrane and endosomal receptors/transporters and cellular export.
Genes involved in iron acquisition play a critical role in pathogen virulence, as extensively demonstrated in bacteria  and more recently in the protozoan parasite Leishmania amazonensis, . In mammals Leishmania is an obligate intracellular parasite of macrophages, replicating within PVs with properties of phagolysosomes. Utilization of the macrophage as host cell has important implications for how Leishmania gains access to iron. Macrophages from the reticuloendothelial system play a fundamental role in iron recycling in vivo, through the process of erythrophagocytosis. Heme released from phagocytosed senescent red blood cells is translocated to the macrophage cytosol, where iron is extracted through the activity of heme oxygenase. Cytosolic iron is then utilized by the cell, stored as a complex with ferritin, or exported by Fpn1 . In this study we showed that L. amazonensis directly interferes with the iron export function of macrophages, by inhibiting cell surface expression of Fpn1. This Leishmania-driven process is associated with increased total macrophage iron content and stimulation of parasite intracellular replication (Figure 9).