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Infection and Immunity, July 2007, p. 3498-3505, Vol. 75, No. 7
0019-9567/07/$08.00+0     doi:10.1128/IAI.00232-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Characterization of Prostaglandin E2 Production by Candida albicans{triangledown}

John R. Erb-Downward1 and Mairi C. Noverr2*

Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, Michigan 48109-0642,1 Department of Immunology and Microbiology, Wayne State University School of Medicine, Detroit, Michigan 482012

Received 12 February 2007/ Returned for modification 3 March 2007/ Accepted 23 April 2007


   ABSTRACT
Top
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Candida albicans produces lipid metabolites that are functionally similar to host prostaglandins. These studies, using mass spectrometry, demonstrate that C. albicans produces authentic prostaglandin E2 (PGE2) from arachidonic acid. Maximal PGE2 production was achieved at 37°C in stationary-phase culture supernatants and in cell-free lysates generated from stationary-phase cells. Interestingly, PGE2 production is inhibited by both nonspecific cyclooxygenase and lipoxygenase inhibitors but not by inhibitors specific for the cyclooxygenase 2 isoenzyme. The C. albicans genome does not possess a cyclooxygenase homolog; however, several genes that may play a role in prostaglandin production from C. albicans were investigated. It was found that a C. albicans fatty acid desaturase homolog (Ole2) and a multicopper oxidase homolog (Fet3) play roles in prostaglandin production, with ole2/ole2 and fet3/fet3 mutant strains exhibiting reduced PGE2 levels compared with parent strains. This work demonstrates that the synthesis of PGE2 in C. albicans proceeds via novel pathways.


   INTRODUCTION
Top
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The eicosanoids are a family of 20-carbon fatty acid metabolites that include prostaglandins and leukotrienes. Prostaglandin E2 (PGE2) is an oxygenated metabolite of arachidonic acid (AA) and is produced via a two-step process beginning with the initial action of a cyclooxygenase (COX-1 or COX-2) to form PGH2 followed by specific PGE synthases in the mammalian host (4, 16, 42, 43; reviewed in reference 26). PGE2 is a potent regulator of host immune responses, with the ability to elicit both pro- and anti-inflammatory responses, depending on the target cell. PGE2 acts via one of four different G-protein-coupled receptors (EP1 to EP4). PGE2 can inhibit Th1-type immune responses, phagocytosis, and lymphocyte proliferation (3, 24, 34, 41). PGE2 can also promote Th2-type responses, immunoglobulin E production, and tissue eosinophilia (6, 10, 15, 17, 35, 36). During a Candida infection, the development of a Th1 response results in protection and clearance while a Th2 response is nonprotective, leading to chronic or disseminating disease (23). Therefore, PGE2 may be an important factor in shifting host immune responses towards those that promote fungal colonization and chronic infection.

During the interaction of C. albicans with the host, lipid mediators coming from both the host and Candida have the potential to influence immune responses. Candida produces both endogenous oxylipins (the generic term for an oxygenated polyunsaturated fatty acid) and novel eicosanoid products from exogenous AA (8, 30). One of these fungal oxylipins isolated from Candida supernatants exhibits cross-reactivity with host PGE2 (30). This compound, termed PGEx for "PGE cross-reactive compound," is bioactive on mammalian cells in vitro similar to PGE2, indicating that the fungal oxylipins can modulate host immune responses. Our laboratory and others have previously reported that host PGE2 and fungal PGEx enhance the morphogenesis of C. albicans (18, 30). Candidal oxylipin production is also upregulated during biofilm formation (1). Similarly, mammalian eicosanoid inhibitors also inhibit candidal oxylipin production, morphogenesis, and biofilm formation (2, 30). These last two observations suggest the presence of an eicosanoid/oxylipin pathway in C. albicans that plays a role in the control of morphogenesis and biofilm formation. These physiological processes are important in the colonization of host tissues and indwelling medical devices (reviewed in references 31 and 45).

C. albicans does not contain AA as part of its fatty acid repertoire; however, C. albicans is known to cause the release of AA from host tissues (5). Additionally, supplementation of C. albicans cultures with exogenous AA significantly increased PGEx production (28). Taken together, the evidence suggests an important role for eicosanoid-derived oxylipins in C. albicans; thus, the objective of this study was to determine whether Candida produces authentic PGE2 from AA and to identify which growth variables influence PGE2 production.


   MATERIALS AND METHODS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Strains. The C. albicans strains used in this study include the following: CHN1 (a clinical isolate), SC5314 (a prototrophic parental strain) (13), CAF2-1 (an auxotrophic parental strain, URA3/ura3{Delta}::imm434), CAI4 (an auxotrophic parental strain, ura3{Delta}::imm434/ura3{Delta}::imm434) (13), JA2 (same as CAI4, but ole2{Delta}::hisG/ole2{Delta}::hisG URA3 hisG (20), JA2r (same as JA2, but with a plasmid containing an OLE2 overexpression vector, pAP5) (20), and a fet3{Delta} mutant (same as CAI4, but fet3::hisG/fet3::hisG-URA3::hisG) (9).

Prostaglandin purification and reverse-phase HPLC. C. albicans strain CHN1 was grown in Sabouraud dextrose broth (SDB) for 72 h. Cells were washed twice with 50 ml of phosphate-buffered saline (PBS), pH 6.5, and transferred to a 125-ml culture flask at a concentration of 2 x 107 cells/ml. Cultures were treated with or without 500 µM AA and shaken at 37°C overnight. Supernatants were then passed over PGE2 affinity columns, and the purified material was collected and dried under N2 gas to prepare for separation using reverse-phase high-performance liquid chromatography (HPLC). Reverse-phase HPLC analysis was carried out using a Waters 600 HPLC system with a 5-µl sample loop and a Waters Symmetry 2.1- by 150-mm analytical column (Waters Corp., Milford, MA). Prior to sample analysis, prostaglandin standards (Cayman Chemicals, Ann Arbor, MI) were separated using an elution gradient starting at 75:25:0.1 (water/acetonitrile/acetic acid) for 10 min, followed by a linear shift to 0:100:0.1 (water/acetonitrile/acetic acid) over 80 min. The elution time of PGE2 was determined, and the system was set to collect a 1-min fraction containing the PGE2 peak. Samples were resuspended in 25 µl of 50% methanol in water and separated in the same manner as that described above. The organic compounds were extracted from this fraction by the addition of 500 µl HPLC-grade ethyl acetate (Sigma-Aldrich, St. Louis, MO), followed by vortexing to mix the two layers. The phases were allowed to separate, and the organic phase was removed and transferred to a V vial coated with silane. This step was repeated, and the contents of the vial were dried in a 40°C water bath under a stream of grade 5 nitrogen.

Characterization of Candida albicans PGE2 by MS. To analyze the molecular structure of the C. albicans putative PGE2 compound, C. albicans cultures were treated with or without AA, the cells were spun out, and the prostaglandins were purified as described above. The dry sample was then stored at –80°C. All samples for mass spectrometry (MS) were tested the following day.

Liquid chromatography-tandem MS/MS (LC-MS/MS) was carried out using ThermoFinnigan Surveyor HPLC (San Jose, CA) interfaced directly with the electrospray ionization source of a ThermoFinnigan LTQ linear ion trap MS (Thermo-Electron Corp., San Jose, CA). The samples were resuspended in a methanol-water solution (1:1, vol/vol), and 20-µl aliquots were injected onto a Phenomenex Luna phenyl-hexyl column (Phenomenex Corp., Torrence, CA) (2.00 by 150 mm; 3-µm inside diameter). The mobile-phase solvents were 10 mM ammonium acetate, pH 8.5, and methanol. The compounds were separated and eluted from the analytical column with a linear gradient first of 50% to 60% methanol over 12 min and then of 60% to 90% methanol over 2 min at a flow rate of 0.3 ml/min, as described previously (48). The column was heated to 50°C, and the sample tray was cooled to 4°C throughout the analysis.

Determination of prostaglandin concentration by ELISA. To determine the optimal PGE2 production conditions, we monitored the production in different strains of C. albicans over time at different temperatures in both whole cells and lysates. C. albicans strains CHN1 (a clinical isolate) and SC5314 were grown overnight at 30°C in SDB (1% enzymatic digest of casein, 2% dextrose; Difco, Detroit, MI). For whole-cell assays, the overnight cultures were used to inoculate fresh SDB to an optical density (OD) at 600 nm of 0.2. Cultures were grown at 25°C or 37°C to an OD600 of 0.6 to 0.8. Peroxide-free AA (Cayman Chemicals, Ann Arbor, MI) was added to a final concentration of 500 µM, and growth was continued at room temperature or 37°C. Samples were removed at various time points, centrifuged, and stored at –20°C until an analysis of the PGE2 concentration was conducted. Culture supernatants were analyzed for prostaglandin production using a monoclonal PGE2 enzyme-linked immunosorbent assay (ELISA; Cayman Chemicals, Ann Arbor, MI). This ELISA kit is highly sensitive and detects as little as 15 pg/ml of PGE2. Controls included heat-killed (HK) cultures (boiled for 6 h), cultures grown without AA, and AA alone. Background levels of PGE2 detected in buffer plus AA alone at time zero were subtracted from experimental samples (~50 pg/ml). In addition, cell viability was measured by dilution plating samples onto Sabouraud dextrose agar plates.

For PGE2 determination in rat serum, C. albicans strain SC5314 was grown overnight in SDB at 30°C. Candida cells were counted and added to the serum at a concentration of 108 cells/ml and incubated at 37°C for 4 h. No exogenous AA was added. Normal whole rat serum was purchased from Zymed (San Francisco, CA), while analbuminemic rat serum was isolated from Nagase analbuminemic rats (27). A PGE metabolite kit was used to measure PGE2 levels in serum supernatants (Cayman Chemicals, Ann Arbor, MI). PGE2 is rapidly degraded in vivo and in ex vivo animal fluids, such as serum, into an unstable intermediate (15-keto-13,14-dihydro-PGE2). The PGE metabolite kit converts this unstable intermediate into a stable measurable derivative, which serves as a marker for PGE2 production. This enzyme immunoassay (EIA) kit is highly sensitive and detects as little as 2 pg/ml of PGE metabolites.

Preparation of C. albicans cell-free lysates. For lysates, C. albicans was grown for 24 h at 37°C in SDB. Cells were counted using a hemacytometer and concentrated to 2 x 109 cells/ml. Cells were washed twice in 25 ml 1x PBS, and 1 ml of the concentrated culture was resuspended in 1.5 ml lysis buffer (1 mM EDTA in distilled water). Approximately 500 µl acid-washed glass beads (Sigma-Aldrich, St. Louis, MO) was added, and cells were vortexed at high speed for 1 min and then incubated on ice for 1 min for five cycles. Lysates were collected, and the beads were washed with an additional 500 µl of lysis buffer. The two samples were pooled and then centrifuged at 14,000 rpm for 30 min at 4°C to remove cell debris. For prostaglandin production, 100 µl of lysate was incubated with 500 µM AA for 90 min at 37°C. To determine the efficiency of the lysis procedure, lysate cell debris pellets were resuspended, and the remaining viable cells were counted by dilution plating onto Sabouraud dextrose agar plates. The efficiency of lysis ranged from 97 to 98% for all samples. The protein concentrations of the lysates were normalized to a standard using a bicinchoninic acid protein assay kit (Pierce, Rockford, IL) prior to the addition of AA for prostaglandin production. Lysates were analyzed for prostaglandin production using a monoclonal PGE2 ELISA (Cayman Chemicals, Ann Arbor, MI). Controls included boiled lysates (boiled for 6 h), lysates without AA, and AA alone. Background levels of PGE2 detected in buffer plus AA alone at time zero were subtracted from experimental samples (~50 pg/ml).

COX and LO inhibitors. Cyclooxygenase (COX) inhibitors (aspirin, indomethacin, resveratrol, and CAY10404) and a lipoxygenase (LO) inhibitor (nordihydroguaiaretic acid [NDGA]) were purchased from Cayman Chemicals (Ann Arbor, MI). All inhibitors were dissolved in dimethyl sulfoxide for stock solutions and diluted in 1x PBS or 1 mM EDTA prior to addition to C. albicans whole cells or lysates, respectively.

Statistical analysis. The Student's t test (two-tailed with unequal variance) was used to analyze the significance of differences between the two experimental groups. Data with a P value of 0.05 or less were considered significant.


   RESULTS AND DISCUSSION
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The analysis of Candida PGEx using LC-MS/MS determined that PGEx (Fig. 1A) possessed a mass and an elution time that were identical to those of the commercial PGE2 standard (Fig. 1B). In the absence of AA, there was no detectable production of PGEx from C. albicans supernatants (data not shown); however, in the presence of AA, the PGEx product was readily detectable. Further analysis using MS/MS (Fig. 1B and inset) revealed that the fragmentation patterns for the PGEx and the PGE2 standard were identical. Although a small amount of auto-oxidation takes place, the Candida PGE2 is distinguishable from a product of the auto-oxidation of AA, since 8-iso-PGE2 elutes at a different time than PGE2 (Fig. 1C). Thus, these data demonstrate conclusively that C. albicans is capable of synthesizing authentic PGE2 from exogenous AA.


Figure 1
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  		FIG. 1. MS of PGE2 from C. albicans. C. albicans strain CHN1 was incubated with AA, and putative PGE2 was purified from the supernatants and subjected to LC-MS/MS. (A) Elution profile of PGEx focusing on the m/z range of 350.5 to 351.5. (B) Elution profile of a purified PGE2 standard. Insets in panels A and B demonstrate the MS/MS fragmentation pattern seen at the major peak at 6.4 min. (C) Elution profile of an 8-iso-PGE2 standard (a stereoisomer of PGE2 formed by auto-oxidation of AA).

 
C. albicans PGE2 production and accumulation in culture supernatants from whole cells increased over time, reaching maximum levels at 48 h and 37°C for both strains CHN1 and SC5314 (Fig. 2a). Growth curves for these samples with or without the addition of AA to SDB were similar; therefore, the increased PGE2 production observed in the presence of AA is not due to increased culture growth (data not shown). To test whether the AA levels found in serum would be sufficient for Candida prostaglandin production in vivo, we measured PGE2 production by Candida in the presence of serum ex vivo without the addition of exogenous AA (Fig. 2b). It is known that PGE2 is rapidly metabolized by serum proteins (12, 47), so to measure the levels of PGE2 produced by Candida incubated in serum and to quantify these unstable by-products, a PGE metabolite assay was employed. Serum from analbuminemic rats was also tested, since serum albumin is involved not only in the breakdown of PGE2 but also in the limitation of the bioavailability of fatty compounds by binding free fatty acids (including AA and PGE2) (14). Indeed, we observed PGE2 production by live Candida in the presence of analbuminemic rat serum without AA but not normal rat serum (Fig. 2b). This indicates that sufficient quantities of AA can be found in serum for Candida to potentially produce PGE2 in vivo using host AA. In addition, PGE2 could be detected when lysates were incubated with exogenous AA, indicating that enzymatic activity may occur intracellularly (Fig. 2c). Furthermore, significantly less PGE2 was detected in the supernatants of HK C. albicans and boiled lysates, indicating the presence of a denaturable enzymatic pathway for prostaglandin production (Fig. 2a and c). There was an unexpectedly high signal from controls containing the fatty acid alone (perhaps the products of the auto-oxidation of AA or perhaps simply nonspecific binding); however, significantly more PGE2 was detected in the presence of C. albicans. The difference in the background signals observed with AA alone and in the presence of boiled lysates may be a result of the binding of denatured lysate proteins to AA or its by-products.


Figure 2
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  		FIG. 2. PGE2 production in C. albicans whole cells and cell-free lysates. (a) C. albicans strains SC5314 and CHN1 were grown in SDB at room temperature or 37°C with or without 500 µM AA and sampled at various times. Controls included no AA, AA alone, and HK Candida (boiled, 6 h). (b) For whole-cell prostaglandin production in serum, C. albicans strain SC5314 was incubated in normal or analbuminemic rat serum for 4 h at 37°C. (c) For Candida cell-free lysates, 24-h cultures of C. albicans strains SC5314 and CHN1 were lysed by vortexing with acid-washed glass beads in 1 mM EDTA buffer. Lysates were centrifuged, protein concentrations were normalized to a standard, and the lysates were incubated at 37°C with or without 500 µM AA for 90 min. Controls included buffer alone, buffer with AA alone, and lysates without AA. Both whole-cell supernatants and cell lysates were assayed for PGE2 production using a monoclonal PGE2 EIA kit (Cayman Chemicals, Ann Arbor, MI), while PGE2 production in serum was measured using a PGE metabolite EIA kit (Cayman Chemicals, Ann Arbor, MI). Experiments were performed and measured in duplicate, and values are the averages of results of two to three independent experiments. *, P < 0.05, live versus HK C. albicans.

 
We next examined the effects of various COX and LO inhibitors on the PGE2 production by C. albicans. Inhibitors were added to the cultures at the same time as the AA. Interestingly, both the nonselective COX inhibitors (aspirin, indomethacin, and resveratrol) and the lipoxygenase inhibitor (NDGA) inhibited PGE2 production in whole cells at 48 h in a dose-dependent manner without significantly affecting fungal viability (Fig. 3a and data not shown). NDGA does not inhibit prostaglandin production in mammalian systems; however, in Candida, this molecule proved to be an effective inhibitor. Both NDGA and resveratrol share similar structural features in that both are plant polyphenols that are antioxidant defense compounds, which may explain their inhibitory effects on fungal PGE2 production. A highly selective COX-2 inhibitor (CAY10404; Cayman Chemicals, Ann Arbor, MI) had no effect on PGE2 production by whole cells, supporting the idea that drug-mediated inhibition of PGE2 production is at the enzymatic level and is not due to nonspecific toxicity effects (Fig. 3a). Both COX and LO inhibitors also diminished PGE2 production in lysates (Fig. 3b). In addition, CAY10404 exhibited a slight but insignificant inhibitory effect on lysates but not on whole cells, suggesting either that this drug is unable to gain access to the C. albicans prostaglandin biosynthetic machinery or that it loses effectiveness once inside the cell (Fig. 3b). Overall, these data suggest that an enzyme exists in C. albicans that is structurally distinct from both mammalian COX and LO enzymes but that may contain some features of both types of enzymes. BLAST searches of the Candida genome have not yielded any genes or proteins with any significant homology to mammalian eicosanoid biosynthetic enzymes (44). Therefore, the search for the enzymes involved in prostaglandin biosynthesis has been difficult. Recently, two homologs of the Saccharomyces cerevisiae {Delta}9 fatty acid stearyl-coenzyme A desaturase gene (OLE1) were characterized for C. albicans and termed OLE1 and OLE2 (20). As expected, OLE1p exhibits fatty acid desaturase activity (required for oleic acid synthesis), and the gene encoding it is essential in Candida. However, a mutant lacking both alleles of OLE2 had no apparent phenotype and was viable. It was suggested that this gene might play a role in fungal prostaglandin production (20). We therefore examined PGE2 production in JA2 (ole2/ole2) and a reconstituted mutant strain, JA2r (ole2/ole2 pAM2-OLE2). Compared with CAF2-1 (OLE2/OLE2), mutant strain JA2 (ole2/ole2) produced less PGE2 in whole cells at 48 h (Fig. 4a). There were no differences in the levels of growth between the strains (data not shown). This defect was rescued in the reconstituted strain JA2r (ole2/ole2 pAM2-OLE2). In addition, PGE2 production in the mutant strain was decreased by approximately 25% in C. albicans cell-free lysates compared with the parent strain or reconstituted mutant (Fig. 4b). This suggests that OLE2 plays a role in candidal prostaglandin production. Ole2 is a putative {Delta}9 desaturase; this type of enzyme is involved in unsaturated fatty acid production in yeast. However, Ole2 must exhibit some other enzymatic activity during prostaglandin production because the unsaturated fatty acid precursor was provided in these experiments. BLAST searches using the Ole2 protein sequence revealed the presence of two conserved domains, a {Delta}9 desaturase domain and a cytochrome B domain. Cytochrome B enzymes exhibit a wide range of properties and function in a large number of different redox processes (22). Therefore, Ole2 may be involved in PGE2 production at the level of oxidation of AA. However, the lack of complete inhibition of prostaglandin production in the OLE2 mutant indicates the involvement of other enzymes in fungal PGE2 synthesis.


Figure 3
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  		FIG. 3. Effect of COX and LO inhibitors on C. albicans PGE2 production in whole cells and lysates. C. albicans whole cells (a) and cell-free lysates (b) were treated as described in the legend to Fig. 2. Inhibitors dissolved in dimethyl sulfoxide were added at the time of AA addition. Both whole-cell supernatants and cell-free lysates (with protein concentrations normalized) were assayed for PGE2 production using a monoclonal PGE2 EIA kit (Cayman Chemicals, Ann Arbor, MI). Experiments were performed in duplicate, and results are the averages for two independent experiments which were repeated three times with similar results. Initials indicate which drugs exhibit statistically significant inhibition at each concentration. A, aspirin; I or Indo, indomethacin; N, NDGA; R or Res, resveratrol; C or Cay, CAY10404. *, P < 0.05, no inhibitor versus inhibitor.

 

Figure 4
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  		FIG. 4. Role of C. albicans OLE2 in PGE2 production. (a) C. albicans strain CAF2-1 (parent), JA2 (ole2/ole2), and JA2r (ole2/ole2 pAP5-OLE2 [overexpression plasmid]) whole cells (a) or cell-free lysates (b) were treated as described in the legend to Fig. 2 (20). Both whole-cell supernatants and lysates (with protein concentrations normalized) were assayed for PGE2 production using a monoclonal PGE2 EIA kit (Cayman Chemicals, Ann Arbor, MI). The amount of detectable oxidized AA observed in control samples containing AA alone for each time point was subtracted from the experimental samples. The percentage of control PGE2 production was determined by the formula [(PGE2 produced by mutant strain)/(PGE2 produced by parental strain)] x 100. Experiments were performed in duplicate, and results are the averages for three independent experiments. *, P < 0.05, CAF2-1 versus JA2.

 
Previous studies of prostaglandin production by Cryptococcus neoformans have implied that the enzyme laccase plays a primary role in the early stages of prostaglandin synthesis (J. Erb-Downward, unpublished observation). Cryptococcal laccase is a member of the multicopper oxidase family of polyphenol oxidases and is known to be critical for melanin formation and virulence (33, 46). One remarkable feature of the inhibitors resveratrol and NDGA is that they are both polyphenols with structures very similar to that of caffeic acid, a substrate for the cryptococcal laccase enzyme (40). Candida can also melanize in vivo, and laccase activity can be detected in cytoplasmic yeast extracts (25). Candida does possess a family of laccase homologs (the Fet family of multicopper oxidases), with several exhibiting >45% similarity to Lac1 at the protein level (data not shown). This family of oxidases also includes the Fet proteins (Fet3 and Fet5) in S. cerevisiae, which are ferroxidases involved in iron transport (7, 39). S. cerevisiae Fet3 exhibits polyphenol oxidase activity as well, indicating that it possesses enzymatic activity similar to that of C. neoformans laccase (37). To test the role of C. albicans laccase homologs in prostaglandin production, PGE2 levels were measured in a FET3 null mutant (fet3/fet3) (9). The fet3/fet3 mutant strain produced less PGE2 in both whole cells and cell-free lysates than did wild-type C. albicans, with no differences in cell growth (Fig. 5a and b and data not shown). This indicates that Fet3 also may play a role in PGE2 production, possibly in a role similar to that which has been observed for C. neoformans (Erb-Downward, unpublished observation). PGE2 production was not completely abrogated in the fet3/fet3 mutant strain, which indicates that other enzymes are involved. Studies are currently under way to construct null mutants with other C. albicans FET family genes and to test for defects in PGE2 production.


Figure 5
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  		FIG. 5. Role of C. albicans FET3 in PGE2 production. (b) C. albicans strain SC5314 and fet3/fet3 mutant whole cells (a) or cell-free lysates (b) were treated as described in the legend to Fig. 2 (9). Both whole-cell supernatants and lysates (with protein concentrations normalized) were assayed for PGE2 production using a monoclonal PGE2 EIA kit (Cayman Chemicals, Ann Arbor, MI). The amount of detectable oxidized AA observed in control samples containing AA alone for each time point was subtracted from the experimental samples. The percentage of control PGE2 production was determined by the formula [(PGE2 produced by mutant strain)/(PGE2 produced by parental strain)] x 100). Experiments were performed in duplicate, and results represent the average of three independent experiments. *, P < 0.05, SC5314 versus fet3/fet3 mutant.

 
C. albicans poses a significant clinical threat to both immunocompetent and immunocompromised individuals. Chronic colonization of mucosal surfaces with Candida is common, with carriage rates among healthy adults ranging from 30 to 70% (19, 38). Mechanisms by which C. albicans persists at mucosal surfaces in the face of an adaptive immune response are relatively unknown. We have demonstrated that C. albicans can produce an immune system signaling compound (PGE2) using fatty acids available within the host. PGE2 is known to downregulate Th1 inflammatory responses and promote Th2-type responses or allergic responses, which are nonprotective during candidiasis (3, 11, 21, 32, 36). This local dampening of Th1 responses may also facilitate chronic mucosal persistence via the maintenance of immunologic "ignorance" in an otherwise immunocompetent host. Once conditions favor the outgrowth of C. albicans (as would occur with immunocompromise or antibiotic treatment), PGE2 production may play a role as a virulence factor by downregulating the innate effector phase or the protective Th1 and response to the infection.

Mammalian inhibitors of eicosanoid production are effective at inhibiting C. albicans PGE2 production. In addition, this class of inhibitors is also effective in inhibiting morphogenesis and biofilm formation as well as exhibiting antifungal activity on biofilms (1, 2, 29). The development of drugs that specifically target the fungal prostaglandin pathways may be one strategy to combat fungal colonization and infection. The identification of enzymes (encoded by OLE2 and FET3) that participate at some level of PGE2 biosynthesis will provide insights into the biochemistry of fungal prostaglandin production. However, a "prostaglandin null" mutant, which would definitively address the role of fungal eicosanoids in the pathogenesis of fungal diseases, remains to be discovered.


   ACKNOWLEDGMENTS
 
We thank Gary Huffnagle (University of Michigan) for insightful discussions and scholarly feedback on the manuscript. We also thank Raimund Eck (Hans Knoll Institute) for generously providing us with the C. albicans fet3{Delta} (fet3/fet3) strain and Joachim Ernst (Institut fur Medizinische Mikrobiologie) for his generous donation of C. albicans strains JA2 (OLE2 mutant; ole2/ole2) and JA2r (reconstituted OLE2 mutant; ole2/ole2 pAM2). The analbuminemic rat serum was a generous donation from the laboratory of George Kaysen (UC Davis) and was collected and sent to us by Tjien Dwyer.

This work was supported by a startup fund provided by Wayne State University and a Francis Foundation grant (M.C.N.) and NIH-NHLBI T32 HL007749-11 (J.E.D.).


   FOOTNOTES
 
* Corresponding author. Mailing address: Department of Immunology and Microbiology, Wayne State University School of Medicine, Detroit, MI 48201. Phone: (313) 577-9746. Fax: (313) 577-1155. E-mail: mnoverr{at}med.wayne.edu Back

{triangledown} Published ahead of print on 30 April 2007. Back

Editor: A. Casadevall


   REFERENCES
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Infection and Immunity, July 2007, p. 3498-3505, Vol. 75, No. 7
0019-9567/07/$08.00+0     doi:10.1128/IAI.00232-07
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