Superoxide Dismutases inCandida albicans: Transcriptional Regulation and Functional Characterization of the Hyphal-inducedSOD5Gene

Abstract

Superoxide dismutases (SOD) convert superoxide radicals into less damaging hydrogen peroxide. The opportunistic human pathogen Candida albicans is known to express CuZnSOD (SOD1) and MnSOD (SOD3) in the cytosol and MnSOD (SOD2) in the mitochondria. We identified three additional CuZn-containing superoxide dismutases, SOD4, SOD5, and SOD6, within the sequence of the C. albicans genome. The transcription of SOD5 was up-regulated during the yeast to hyphal transition of C. albicans, and SOD5 was induced when C. albicans cells were challenged with osmotic or with oxidative stresses. SOD5 transcription was also increased when cells were grown on nonfermentable substrates as the only carbon source. The Rim101p transcription factor was required for all inductions observed, whereas the Efg1p transcription factor was specifically needed for serum-modulated expression. Deletion of SOD5 produced a viable mutant strain that showed sensitivity to hydrogen peroxide when cells were grown in nutrient-limited conditions. Sod5p was found to be necessary for the virulence of C. albicans in a mouse model of infection. However, the sod5 mutant strain showed the same resistance to macrophage attack as its parental strain, suggesting that the loss of virulence in not due to an increased sensitivity to macrophage attack. INTRODUCTIONAerobic eukaryotic pathogens can encounter superoxide radicals (O 2 Ϫ ) generated from several sources. These sources can be internal or external. An important internal source is the mitochondrial respiratory chain (Boveris, 1978;Casteilla et al., 2001;Lenaz, 2001), and thus the rate of respiration can have a significant impact on reactive oxygen species (ROS) production. A key external source of ROS encountered by pathogens is from phagocytes. The superoxide radical is the first intermediate in the oxidative burst generated in the phagosome, and this burst is thought to be involved in pathogen killing (Reeves et al., 2002). The superoxide radicals are known to inactivate [4Fe-4S] cluster-containing enzymes by oxidizing one iron and releasing it from the cluster (Liochev and Fridovich, 1994;Fridovich, 1995). Free iron can react with hydrogen peroxide to generate toxic hydroxyl radicals (OH Ϫ ) by Fenton chemistry (Fridovich, 1978;Meneghini, 1997). The hydroxyl and superoxide radicals react with cellular components, resulting in oxidation of proteins and nucleic acids as well as lipid peroxidation. These effects can lead to inactivation of enzymes, to disruption of membranes, to mutations, and ultimately to cell death Gutteridge, 1990, 1999).To reduce the harmful effects of superoxide radicals, cells express detoxifying enzymes. Superoxide dismutase (SOD) is an antioxidant enzyme involved in elimination of superoxide anions; it catalyzes the reaction: O 2 Ϫ ϩ O 2 Ϫ ϩ 2H ϩ 3 H 2 O 2 ϩ O 2 . Normally, H 2 O 2 is still toxic to the cell; therefore, another enzyme, catalase, converts it to water. Superoxide dismutases can be classified according to metal cofactor(s) bound to them: there are iron (FeSOD), mangan...

Superoxide dismutases and glutaredoxins have a distinct role in the response of Candida albicans to oxidative stress generated by the chemical compounds menadione and diamide

“…The ability to combat ROS is vital for microbial pathogenesis to either cope with ROS generated as a consequence of their own metabolism or escape from human phagocytic cells (Cabiscol et al 2000, Lamarre et al 2001, Hwang et al 2002, Martchenko et al 2004, Gonzalez-Parraga et al 2005. Some chemical compounds have been used to induce oxidative stress in microorganisms, such as menadione and diamide.…”

“…However, while sod1Δ is hypersensitive to killing by a macrophage cell line in vitro (Hwang et al 2002), the same was not observed for sod5Δ (Martchenko et al 2004), indicating that the virulence role of Sod5 may be due to a resistance to different phagocytic cells. Frohner et al (2009) recently demonstrated that the Sod4 and Sod5 produced by innate immune response cells are extremely important as ROS scavengers.…”

“…These enzymes possess antioxidant properties that catalyse the dismutation of O 2 -into molecular oxygen and H 2 O 2 , which can then be detoxified by Cat (Chaves et al 2007), thereby scavenging the toxic effects of O 2 (Frohner et al 2009). These Sods include cytoplasmic Sod1 and Sod3, mitochondrial Sod2 and the cell surface GP-anchored Sod4, Sod5 and Sod6 (Martchenko et al 2004, Frohner et al 2009, Bink et al 2011.…”

“…Some genes related to oxidative stress are also regulated during hyphae formation and appear to be somehow related to C. albicans morphogenesis (Hwang et al 2002, Nantel et al 2002, Martchenko et al 2004, Chaves et al 2007). In addition, Schroter et al (2000) analysed the amount of ROS produced when cells were incubated in 10% serum.…”

Candida albicansdimorphism as a therapeutic target

“…While neutrophils strongly inhibit growth (including the yeast-tohyphal transition) of C. albicans and can efficiently kill the fungus [19,45,46], yeast cells phagocytosed by macrophages may survive and produce hyphae, which pierce through the host membrane, rapidly killing the phagocyte and allowing C. albicans to escape [19,47]. The higher expression level of SOD5 (encoding an extracellular superoxide dismutase) in hyphae may counteract the oxidative burst of phagocytic cells and thus may help to survive attack of monocytes, macrophages and neutrophils [19,48,49]. Therefore, the hyphal transition is a critical event for escaping phagocytes.…”