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

Abstract

Aspergillus fumigatus is a filamentous fungus which can cause invasive disease in immunocompromised individuals. A. fumigatus can grow in medium containing up to 80% human serum, despite very low concentrations of free iron. The purpose of this study was to determine the mechanism by which A. fumigatus obtains iron from the serum iron-binding protein transferrin. In iron-depleted minimal essential medium (MEM), A. fumigatus growth was supported by the addition of holotransferrin (holoTf) or FeCl 3 but not by the addition of apotransferrin (apoTf). Proteolytic degradation of transferrin by A. fumigatus occurred in MEM-serum; however, transferrin degradation did not occur until late logarithmic phase. Moreover, transferrin was not degraded by A. fumigatus incubated in MEM-holoTf. Urea polyacrylamide gel electrophoresis showed that in MEM-holoTf, holoTf was completely converted to apoTf by A. fumigatus. In human serum, all of the monoferric transferrin was converted to apoTf within 8 h. Siderophores were secreted by A. fumigatus after 8 h of growth in MEM-serum and 12 h in MEM-holoTf. The involvement of small molecules in iron acquisition was confirmed by the fact that transferrin was deferrated by A. fumigatus even when physically separated by a 12-kDa-cutoff membrane. Five siderophores were purified from A. fumigatus culture medium, and the two major siderophores were identified as triacetylfusarinine C and ferricrocin. Both triacetylfusarinine C and ferricrocin removed iron from holoTf with an affinity comparable to that of ferrichrome. These data indicate that A. fumigatus survival in human serum in vitro involves siderophore-mediated removal of iron from transferrin. Proteolytic degradation of transferrin may play a secondary role in iron acquisition.

Frequent severe liver iron overload after stem cell transplantation and its possible association with invasive aspergillosis

“…Proteolytic degradation of transferrin by proteinases may play a secondary role in iron acquisition. 22 Indeed, the deletion of A. Nidulans gene sidA completely blocks triacetylfusarinine C and ferricrocin biosynthesis, and siderophore-deficient strains are unable to grow. 23 Two experimental findings in vivo can be related with the iron-dependency of Aspergillus: the preventive effect of endotoxin against Aspergillus infection in mice, 24 and the delayed lethal response to A. fumigatus infection in sarcoma 180 tumour-bearing mice.…”

Human Polymorphonuclear Leukocytes Inhibit Aspergillus fumigatus Conidial Growth by Lactoferrin-Mediated Iron Depletion

“…Microbial competition for scarce bioavailable iron has favored evolution of siderophores, small secreted molecules that bind extracellular iron with high affinity and return it to the microbe for uptake. A. fumigatus synthesizes at least five siderophores (48). Deletion of sidA, a gene involved in siderophore synthesis, results in attenuated virulence (49,50), and iron acquisition appears to be required for growth in human serum (48).…”

Iron acquisition in the cystic fibrosis lung and potential for novel therapeutic strategies

“…Despite reports of the inhibitory effects of lactoferrin and transferrin on pathogenic fungi (Almeida et al, 2009;Caza & Kronstad, 2013), Candida albicans and Aspergillus fumigatus can both utilize transferrin bound iron for growth using different mechanisms (Ramanan & Wang, 2000;Hissen et al, 2004). Siderophores are not involved in iron acquisition by Candida albicans; rather direct contact of Candida albicans with transferrin is required.…”

“…Iron is then released from transferrin by the activity of the reductase Fre10 and the permease Ftr1 (Ramanan & Wang, 2000). Aspergillus fumigatus, similar to the other CF pathogens, uses secreted siderophores to access iron from transferrin (Hissen et al, 2004;. Ferritin use as a sole iron source for fungal pathogens has been best characterized in Candida albicans and involves the adhesin Als3 in ferritin uptake with acidification as the probable mechanism for the release of iron (Almeida et al, 2008).…”