Utilization of ferroproteins byCandida albicans during candidastasis by apotransferrin

Iron at the Centre of Candida albicans Interactions

Iron is an absolute requirement for both the host and most pathogens alike and is needed for normal cellular growth. The acquisition of iron by biological systems is regulated to circumvent toxicity of iron overload, as well as the growth deficits imposed by iron deficiency. In addition, hosts, such as humans, need to limit the availability of iron to pathogens. However, opportunistic pathogens such as Candida albicans are able to adapt to extremes of iron availability, such as the iron replete environment of the gastrointestinal tract and iron deficiency during systemic infection. C. albicans has developed a complex and effective regulatory circuit for iron acquisition and storage to circumvent iron limitation within the human host. As C. albicans can form complex interactions with both commensal and pathogenic co-inhabitants, it can be speculated that iron may play an important role in these interactions. In this review, we highlight host iron regulation as well as regulation of iron homeostasis in C. albicans. In addition, the review argues for the need for further research into the role of iron in polymicrobial interactions. Lastly, the role of iron in treatment of C. albicans infection is discussed.

Iron Metabolism and the Inflammatory Response

Iron (Fe) is essential to almost all organisms, as required by cells to satisfy metabolic needs and accomplish specialized functions. Its ability to exchange electrons between different substrates, however, renders it potentially toxic. Fine tune-mechanisms are necessary to maintain Fe homeostasis and, as such, to prevent its participation into the Fenton reaction and generation of oxidative stress. These are particularly important in the context of inflammation/infection, where restricting Fe availability to invading pathogens is one, if not, the main host defense strategy against microbial growth. The ability of Fe to modulate several aspects of the immune response is associated with a number of "costs" and "benefits", some of which have been described in this review. © 2017 IUBMB Life, 69(6):442-450, 2017.

The identification of surface interaction of apotransferrin with Candida albicans

Our recent data indicate that apotransferrin, an iron-chelating protein, has anti-candidal activity by binding to the Candida albicans surface rather than just simple iron-chelation. Following that study, in this present study, we investigated the nature of the candidal surface substance that is responsible for the anticandidal activity by using (59)Fe(3+)-apotransferrin and biological assay methods. Data resulting from the binding studies showed that the yeast cells had one class of binding sites as analyzed by the Scatchard equation, and the binding was specific as determined by competitive binding assay with unlabeled and labeled transferrin. All these observations indicate that there is a substance(s) that mediates the binding. Thus, a mannoprotein-like substance was extracted from C. albicans surface using hot water-treatment. Radioisotope binding study revealed that the substance blocked the transferrin binding. At 25 μg of IHS (inhibitory substance) addition, there was 65 % inhibition of the transferrin binding to C. albicans (5 × 10(7) cells/ml) (P < 0.05). The blockage of the transferrin binding disrupted the anticandidal activity of transferrin, resulting in a full recovery from growth inhibition. These results explain our previous observation that there is partial growth inhibition when C. albicans interacts directly with iron-saturated transferrin (100 %). Thus, it was concluded that a candidate for transferrin receptor is involved in the anticandidal activity of transferrin when in direct contact with C. albicans.

Apotransferrin has a second mechanism for anticandidal activity through binding of Candida albicans

It has been reported that transferrin has antibacterial and antifungal activities via iron chelation in the environment surrounding the microbes. In the present study, we investigated whether the binding of transferrin to Candida albicans mediates growth inhibition. By using cultures that contained iron-free (apo)transferrin glycoprotein either in contact with candidal cells or separated from candidal cells by a dialysis membrane, we distinguished the growth inhibition by transferrin-cell interaction from that of simple iron chelation. Maximal growth inhibition always occurred when the apotransferrin interacted directly with the cells. Additionally, there was partial inhibition even when candidal cells were in contact with iron-saturated transferrin. Binding studies with (59)Fe(3+) radiolabeled-transferrin indicated that the apo-protein can bind to the candidal cell surface. The binding sites were saturable and it was dose dependent. Chemicals (hydrogen peroxide, dithiothreitol, sodium dodecyl sulfate) blocked transferrin binding to C. albicans, and among the three, hydrogen peroxide (HP) was the most effective for the blocking. When HP-treated yeast cells were added to the culture that was pretreated with apotransferrin, candidal cell growth increased by 5-fold as compared to the growth of HP-untreated candidal cells under apotransferrin-regulation (P < 0.05). Combined all data together, it was concluded that transferrin has a second mechanism of anticandidal activity that is mediated by binding to the surface of C. albicans yeast cells.

Candida albicans, Cryptococcus neoformans or Aspergillus fumigatus induces an antifungal activity in mouse serum, which is different from transferrin

It has been reported that administration of Candida albicans into mouse induces an antifungal activity in serum, which has been identified as transferrin. In the present study, we show that not only C. albicans, but also other fungus such as Cryptococcus neoformans or Aspergillus fumigatus similarly can induce an antifungal activity in mouse serum. This antifungal activity was inhibited by the addition of ferrous ion, indicating that the growth inhibition of C. albicans was due to deficiency of ferrous ion, which may be caused by transferrin. Indeed, addition of transferrin in an in vitro assay system using RPMI1640 culture medium inhibited the growth of C. albicans, C. neoformans or A. fumigatus. However, when C. albicans was grown in RPMI1640 medium with 10% fetal bovine serum (FBS), transferrin was unable to inhibit the growth of C. albicans, in sharp contrast, when C. albicans treated mouse serum was added instead of FBS, the growth of the organism was inhibited. Similar results were obtained when C. neoformans or A. fumigatus was used. Taken together, the results suggest that antifungal activity induced by C. albicans, C. neoformans or A. fumigatus was not due to transferrin but likely due to other unknown serum proteins, which may cut off the source of iron for the growth of these fungi.

Candida albicans can utilize siderophore during candidastasis caused by apotransferrin

Ability of iron acquisition of pathogenic microorganisms functions as a virulence factor. Candida albicans, a fungal pathogen that requires iron for growth, is susceptible to growth retardation by high-affinity iron binding proteins such as transferrin. Recently, we reported that C. albicans could utilize the heme as a part of heme-containing proteins dissociated by heme oxygenase, CaHMX1. In search of another pathway that C. albicans can use to bypass the growth regulation produced by iron limitation, this present study examined utilization of non-candidal siderophores such as Desferal and rhodotorulic acid (RA) for acquisition of inorganic iron by the fungus. C. albicans secreting no siderophores was cultured in iron-free (pretreated with apotransferrin for 24 h) (culture medium). Once growth of the yeast reached stasis from iron starvation, a siderophore was added to the culture media. Results showed that cultures containing apotransferrin within a dialysis membrane recovered growth to the level of untreated controls, whereas C. albicans yeast cells in direct contact with soluble iron-free (apo) transferrin recovered growth only partially. When static growth from iron limitation was reached, the addition of siderophore-apotransferrin complex to culture medium also permitted the yeast to recover growth from apotransferrin growth regulation. All the data show that C. albicans can utilize the non-candidal siderophores for iron acquisition under transferrin regulation as can pathogenic bacteria.

Ginsenoside Rg1 helps mice resist to disseminated candidiasis by Th1 type differentiation of CD4+ T cell

Ginsenosides, the most important component isolated from Panax ginseng, exhibits a variety of biological activities. Particularly, ginsenoside Rg1 is known to have various immune-modulating activities such as increase of immune activity of T helper (Th) cells. In this present work, we investigated the effect of the Rg1 on Candida albicans growth. Results showed that direct interaction of the Rg1 to C. albicans yeast cells resulted in no growth inhibition as tested by agar diffusion susceptibility method. Reversely, mice given the Rg1 intraperitoneally (i.p.) before intravenous (i.v.) challenge with live C. albicans yeast cells protected the mice to experimental disseminated candidiasis. By kidney candidal CFU (colony forming unit) determination, the disease severity of the Rg1-treated mice was confirmed far less than Rg1-untreated control mice. The protection was transferable by CD4+ T cells (RGCD4T) isolated from Rg1-treated mice. ELISA analysis revealed that there were cytokine inductions of IFNgamma, IL-2, IL-4 and IL-10 from the RGCD4T, demonstrating the Th1-lineage development of predominant IFNgamma and IL-2 production. Anti-mouse IFNgamma antibody treatment of Rg1-treated mice abolished the protection to disseminated disease. Our studies show that ginsenoside Rg1 helps the host resists disseminated candidiasis by the CD4(+) T cell-mediated immune response led from a Th1-dominant cytokine response.