Potential Role for Extracellular Glutathione-Dependent Ferric Reductase in Utilization of Environmental and Host Ferric Compounds byHistoplasma capsulatum

Abstract

The mammalian host specifically limits iron during Histoplasma capsulatum infection, and fungal acquisition of iron is essential for productive infection. H. capsulatum expresses several iron acquisition mechanisms under iron-limited conditions in vitro. These components include hydroxamate siderophores, extracellular glutathione-dependent ferric reductase enzyme, extracellular nonproteinaceous ferric reductant(s), and cell surface ferric reducing agent(s). We examined the relationship between these mechanisms and a potential role for the extracellular ferric reductase in utilization of environmental and host ferric compounds through the production of free, soluble Fe(II). Siderophores and ferric reducing agents were coproduced under conditions of iron limitation. The H. capsulatum siderophore dimerum acid and the structurally similar basidiomycete siderophore rhodotorulic acid acted as substrates for the ferric reductase, and rhodotorulic acid removed Fe(III) bound by transferrin. The mammalian Fe(III)-binding compounds hemin and transferrin served both as substrates for the ferric reductase and as iron sources for yeast-phase growth at neutral pH. In the case of transferrin, there was a correlation between the level of iron saturation and efficacy for both of these functions. Our data are not consistent with an entirely pH-dependent mechanism of iron acquisition from transferrin, as has been suggested to occur in the macrophage phagolysosome. The foreign siderophore ferrioxamine B also acted as a substrate for the ferric reductase, while the foreign siderophore ferrichrome did not. Both ferrioxamine and ferrichrome served as iron sources for yeast-and mold-phase growth, the latter presumably by some other acquisition mechanism(s).

Survival of Aspergillus fumigatus in Serum Involves Removal of Iron from Transferrin: the Role of Siderophores

“…However, we cannot exclude the possibility that low-molecular-weight reductants also participate in iron acquisition, as has been shown for H. capsulatum (47) and C. neoformans (36). Direct evidence for a role of A. fumigatus siderophores in virulence awaits the study of siderophorenegative mutant strains.…”

“…Hydroxamate siderophores have been reported to remove iron from transferrin in other studies. These include rhodotorulic acid (47), a dihydroxamate siderophore with a pM of 21.9 (16), and aerobactin (25), a bacterial siderophore with a prototypical hydroxamate-citrate structure and a pM of 23.3 (16). Escherichia coli strains bearing the plasmid for aerobactin production can grow in the presence of transferrin, and virulence is associated with the synthesis of aerobactin (50).…”

The Aspergillus fumigatus Siderophore Biosynthetic Gene sidA , Encoding l -Ornithine N 5 -Oxygenase, Is Required for Virulence

“…For example, the ferrous iron transport system was found to be required for virulence of Candida albicans (40). In addition, low-molecularweight reductants have also been shown to participate in iron acquisition in Histoplasma capsulatum (49) and Cryptococcus neoformans (36). Both wild-type and ⌬sidA A. fumigatus strains produced measurable ferric reduction activity when grown in GA medium supplemented with 5 M FeCl 3 or in YM medium.…”

Histoplasma capsulatum Depends on De Novo Vitamin Biosynthesis for Intraphagosomal Proliferation

“…Without siderophore production, intracellular Histoplasma growth is hampered (43,44). In addition, Histoplasma produces extracellular iron reductases (41,42), including a ␥-glutamyltransferase (Ggt1) which causes a pH-independent release of iron from transferrin that is necessary for full Histoplasma virulence in phagocytes (45). Beyond iron acquisition and pyrimidine biosynthesis, little is known about the nutritional growth requirements of intracellular Histoplasma yeasts.…”

“…Full virulence of yeasts also depends on acquisition of iron. Histoplasma yeasts produce hydroxamate siderophores which can steal iron from transferrin (41,42), the most likely source of iron within the phagosome. Without siderophore production, intracellular Histoplasma growth is hampered (43,44).…”