Haemin uptake and use as an iron source by Candida albicans: role of CaHMX1-encoded haem oxygenase

Abstract

Candida albicans, unlike Saccharomyces cerevisiae, was able to use extracellular haemin as an iron source. Haemin uptake kinetics by C. albicans cells showed two phases: a rapid phase of haemin binding (with a K d of about 0?2 mM) followed by a slower uptake phase. Both phases were strongly induced in iron-deficient cells compared to iron-rich cells. Haemin uptake did not depend on the previously characterized reductive iron uptake system and siderophore uptake system. CaHMX1, encoding a putative haem oxygenase, was shown to be required for iron assimilation from haemin. A double DCahmx1 mutant was constructed. This mutant could not grow with haemin as the sole iron source, although haemin uptake was not affected. The three different iron uptake systems (reductive, siderophore and haemin) were regulated independently and in a complex manner. CaHMX1 expression was induced by iron deprivation, by haemin and by a shift of temperature from 30 to 37˚C. CaHMX1 expression was strongly deregulated in a Defg1 mutant but not in a Dtup1 mutant. C. albicans colonies forming on agar plates with haemin as the sole iron source showed a very unusual morphology. Colonies were made up of tubular structures that were organized into a complex network. The effect of haemin on filamentation was increased in the double DCahmx1 mutant. This study provides the first experimental evidence that haem oxygenase is required for iron assimilation from haem by a pathogenic fungus. INTRODUCTIONIron is required by most living systems. Iron availability plays a critical role in the host-pathogen relationship and in the virulence of bacteria and fungi [reviewed by Howard (1999) and Payne (1993)]. In the host, iron is predominantly intracellular, occurring as haem, iron-sulphur proteins or ferritin, with smaller quantities in other iron proteins. The limited amounts of extracellular iron are tightly held by proteins such as transferrin in the serum and lactoferrin on body surfaces. The withholding of iron by the host has been shown to block virulent infections by many pathogens, and high-affinity iron acquisition by pathogens often promotes virulent infections (Ratledge & Dover, 2000). Candida albicans is a saprophytic organism for humans; it can be found inhabiting external surfaces of the body, including the skin and the mucosae of the gut and mouth in more than half of the normal population. In some settings C. albicans invades tissue and disseminates systemically. Prolonged neutropenia, following cancer chemotherapy or bone marrow transplantation, is a major risk factor for invasive fungal infections, and Candida species are the most frequently implicated organisms (Rex et al., 1995). Recurrent oropharyngeal and oesophageal candidiasis is encountered in most AIDS patients (Laine & Bonacini, 1994). There has been a rapid rise in the frequency of drug resistance in clinical C. albicans infections (Rex et al., 1995); hence, there is a pressing need to identify new drug targets.The first studies about iron uptake by C. albicans emphasized the possi...

Cap2-HAP Complex Is a Critical Transcriptional Regulator That Has Dual but Contrasting Roles in Regulation of Iron Homeostasis in Candida albicans

“…We also included an additional 15 ironrelated genes that showed Cap2-dependent up-regulation (six genes) or down-regulation (nine genes), for which the L_H_Fe data were not available. Interestingly, eight of the 18 up-regulated genes in the CAP2/cap2⌬ data have been shown experimentally to be involved in iron uptake pathways, viz., SIT1 encoding siderophore transporter (72) , FET3, FTR1, FRP1, CFL5, and FRE9 encoding reductive iron uptake (73,74), and RBT5 and HMX1 being part of heme uptake (35,75,76) pathways (Fig. 4C).…”

Candida albicans, Cryptococcus neoformans or Aspergillus fumigatus Induces an Antifungal Activity in Mouse Serum, which is Different from Transferrin

“…It has been reported that hemin, one of the heme proteins, can be used as a source of iron [22,29]. Thus, antifungal activity induced in the mouse serum could be due to the restriction of use of iron originated from heme protein.…”

Effects of hemin, CO2, and pH on the branching of Candida albicans filamentous forms

“…Y → H/PH switches are induced by temperatures between 35°C and 42°C [8,9], blood serum [10], hemoglobin at 1 mg/ml concentration (approximately 15.5 μM with 62 μM heme content; [11]), hemin (protoporphyrin IX with ferric ion or oxidized heme, which is released from hemoglobin; [12]) at 5-100 μg/ml (equals to 7.7-153.4 μM; [13]) or at 50 μM (equals to 32.6 μg/ml) concentrations [14], CO 2 [15,16], hypoxia + high CO 2 level [17], alkaline pH [15,18], the available carbon sources [8], and nitrogen starvation [18].…”

“…Besides environmental factors, the acquisition of iron plays a central role in fungal metabolism and, therefore, it is considered as a key element in the invasion of host organisms by pathogenic fungi including C. albicans [14,27]. In the human body, iron is present exclusively in bound forms, e.g., in hemoglobin, transferrin, and ferritin [14,[27][28][29][30].…”

“…In the human body, iron is present exclusively in bound forms, e.g., in hemoglobin, transferrin, and ferritin [14,[27][28][29][30]. Nevertheless, C. albicans has a remarkable arsenal to extract iron from its hosts, including the internalization and degradation of hemoglobin and hemin [14,28,[31][32][33]. In the circulating blood, free hemoglobin and hemin cannot be found under normal physiological conditions due to the hemoglobin-binding and heme-binding capacities of haptoglobin and hemopexin, respectively [18].…”