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J Bacteriol. Dec 2006; 188(24): 8658–8661.
Published online Oct 13, 2006. doi:  10.1128/JB.01253-06
PMCID: PMC1698250

Regulation of Aggregatibacter (Actinobacillus) actinomycetemcomitans Leukotoxin Secretion by Iron[down-pointing small open triangle]

Abstract

The gram-negative oral and systemic pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans produces a leukotoxin (LtxA) that is a member of the RTX (repeats in toxin) family of secreted bacterial toxins. We have recently shown that LtxA has the ability to lyse erythrocytes, which results in a beta-hemolytic phenotype on Columbia blood agar. To determine if LtxA is regulated by iron, we examined beta-hemolysis under iron-rich and iron-limiting conditions. Beta-hemolysis was suppressed in the presence of FeCl3. In contrast, strong beta-hemolysis occurred in the presence of the iron chelator deferoxamine. We found that secretion of LtxA was completely inhibited by free iron, but expression of ltxA was not regulated by iron. Free chromium, cobalt, and magnesium did not affect LtxA secretion. Other LtxA-associated genes were not regulated by iron. Thus, iron appears to play an important role in the regulation of LtxA secretion in A. actinomycetemcomitans in a manner independent of gene regulation.

Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans (30) is a highly adherent (10, 20) gram-negative pathogen that colonizes the oral cavity of humans and Old World primates. In humans, A. actinomycetemcomitans is the etiologic agent of localized aggressive periodontitis, a rapidly progressing oral disease that occurs primarily in adolescents (11, 35). A. actinomycetemcomitans is also a member of the HACEK group of bacteria, which is implicated in causing subacute infective endocarditis (5).

A. actinomycetemcomitans produces the RTX toxin leukotoxin (LtxA) as part of its array of virulence factors (23, 26, 27). LtxA is a secreted ~114-kDa protein that is homologous to Escherichia coli α-hemolysin (42, 43), Bordetella pertussis adenylate cyclase (12), and Mannheimia haemolytica leukotoxin (6, 13). RTX toxins are soluble proteins that destroy target cells by disrupting cell membranes (4, 32).

LtxA appears to act as an important virulence factor for A. actinomycetemcomitans by helping the bacterium evade the host immune response. The toxin has been reported to be highly specific for human and primate leukocytes (38, 40, 41). In addition, we recently showed that LtxA is able to lyse both human and nonprimate erythrocytes (3). As a result, LtxA-mediated erythrocyte lysis confers beta-hemolytic activity to A. actinomycetemcomitans (3). Erythrocyte lysis is one mechanism by which a bacterial pathogen could acquire iron.

Previous studies showed that LtxA production is regulated by oxygen (16, 24, 31, 37), but little is known about other factors that might regulate production or secretion of LtxA in A. actinomycetemcomitans. Because of our recent discovery that LtxA is able to lyse erythrocytes (3), we wondered whether leukotoxin also plays a role in iron acquisition. Proteins that are involved in iron uptake and metabolism are often regulated by iron. We show here that LtxA secretion is strongly affected by the presence of free iron. This represents the first report of iron-mediated regulation of LtxA in A. actinomycetemcomitans.

Beta-hemolysis under different iron conditions.

To determine if iron plays a role in LtxA regulation, we assayed beta-hemolysis under excess-iron and iron-limiting conditions on Columbia agar with 5% sheep blood (3). We found that, compared to results obtained with media with no added iron, beta-hemolysis was suppressed when A. actinomycetemcomitans was grown on Columbia agar containing 300 μM FeCl3 (Fig. (Fig.1).1). This difference was less pronounced with the highly leukotoxic strain JP2 (8, 36, 41) than with the minimally leukotoxic strain DF2200 (21), but it was highly reproducible and could be due to the greater total amount of LtxA produced by strain JP2. In contrast to the results obtained under high-iron conditions, beta-hemolysis was increased when an iron chelator, deferoxamine (DFO; 100 μM), was included in the medium (Fig. (Fig.1).1). Both strains JP2 and DF2200 showed strongly hemolytic phenotypes on the blood agar with DFO. We observed similar effects with another iron chelator, 2,2′-dipyridyl, but found that DFO was more effective (data not shown). Because LtxA is required for beta-hemolysis (3), these results suggest that LtxA production is regulated by iron.

FIG. 1.
Beta-hemolysis by A. actinomycetemcomitans on Columbia agar with 5% sheep blood. Strains JP2 and DF2200 were streaked onto medium containing 300 μM FeCl3, no additional components (0), or 100 μM DFO. Plates were incubated for 3 days at ...

Secretion of LtxA is regulated by iron.

To determine which step of LtxA production is regulated by iron, the highly leukotoxic A. actinomycetemcomitans strain JP2 was grown in AAGM broth (10) and Columbia broth (Accumedia, Baltimore, MD) in the presence or absence of excess FeCl3 (300 μM). After approximately 16 h of growth, bacterial cells were separated from the supernatant by centrifugation. Equal amounts of cell-associated and secreted protein (supernatant) were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis using antileukotoxin antibody (9). Figure Figure22 shows that without added FeCl3, LtxA is predominantly secreted, while a small amount remains associated with cells. In contrast, under conditions of excess iron, LtxA was not secreted but was instead associated with cells (Fig. (Fig.2).2). The results were identical for both AAGM and Columbia broth. Further, chromium (300 μM CrCl3 · 6H2O), cobalt (300 μM CoCl2 · 6H2O), and magnesium (300 μM MgCl2 · 6H2O) had no effect on LtxA secretion (Fig. (Fig.22).

FIG. 2.
Western blot analysis of secreted (super) and cell-associated (pellet) LtxA. Strain JP2 was grown in AAGM with or without the metal indicated at the left. Supernatant and cell pellet protein were resolved through an SDS-PAGE gel, transferred to a nitrocellulose ...

The above results suggest that LtxA secretion, rather than production per se, is regulated by iron. To confirm that ltxA synthesis is not affected by iron, we determined the levels of ltxA mRNA produced under normal and excess-iron growth conditions by quantitative real-time PCR (qRT-PCR) using a quantitative RT-PCR ReadyMix kit according to the manufacturer's protocol (Sigma, St. Louis, MO) (Table (Table1).1). Highly leukotoxic strains JP2 (36, 41) and NJ4500, a fresh clinical isolate (9, 21), were grown in AAGM broth with and without 300 μM FeCl3. Relative to the expression of housekeeping genes glyA (29) and gapdh (34) (Table (Table1),1), we found that the ratio of ltxA mRNA levels under excess-iron conditions to those under normal growth conditions was approximately 1 (Table (Table2).2). This result indicates that expression of ltxA is not affected by iron.

TABLE 1.
Primers and molecular beacons used for qRT-PCR analysisa
TABLE 2.
Expression level ratios of genes in A. actinomycetemcomitans grown under excess-iron and normal-iron conditionsa

We offer two models to explain how iron might regulate LtxA secretion in A. actinomycetemcomitans. First, modulation of secretion might occur through regulated expression of ltx secretion genes, such as ltxB, ltxD, or tdeA. LtxB and LtxD are part of the type I secretion system for LtxA (1, 14, 25). TdeA is a TolC-like protein we recently identified in A. actinomycetemcomitans as the putative outer membrane channel-forming component required for secretion of LtxA and drug efflux (8a). To test if the levels of ltxD and tdeA mRNA are affected by iron, we performed RT-PCR (Table (Table1)1) as described for ltxA. The levels of these genes were very similar under excess-iron and normal growth conditions (Table (Table2),2), indicating that differences in expression do not account for the complete lack of LtxA secretion under iron-rich conditions.

Second, iron may regulate the activity of a protein required for LtxA secretion. For example, binding of iron to TdeA may result in a structural change in the protein, preventing transport of LtxA through the pore. In support of this hypothesis, Andersen et al. found that the function of TolC was severely inhibited by divalent and trivalent cations (2). They noted that trivalent cations, such as Cr3+, were more potent because of irreversible binding of the metals to an aspartate ring that lines the periplasmic entrance of the assembled protein (2). Interestingly, a comparison of A. actinomycetemcomitans TdeA with E. coli TolC reveals that TdeA contains two glutamate residues with a relative location (in the last α-helix) and configuration (separated by two amino acid residues) similar to those of the two aspartate residues in TolC. Thus, in our model, iron might bind the negatively charged glutamate residues of TdeA, causing constriction of the pore and preventing secretion of LtxA.

Other RTX toxins have been shown to be regulated by iron. Interestingly, iron regulates expression of these other RTX toxins at the transcriptional level, which represents a mechanism different from what we have observed for A. actinomycetemcomitans. Marciel and Highlander (28) showed that transcription of Mannheimia haemolytica leukotoxin (Lkt) is increased in the presence of an iron chelator. Similarly, the RTX toxin of Neisseria meningitidis (FrpC) is up-regulated under iron-limiting conditions (39). Thus, whatever the mechanism, iron appears to play a negative regulatory role in RTX toxin production.

A. actinomycetemcomitans LtxA is both secreted (18, 19) and associated with the outer membrane of the cell (7, 9, 17, 22). We show here that secretion of LtxA is required for hemolysis, since iron affects secretion but not ltxA synthesis. Thus, it is tempting to speculate that the cell-associated and secreted forms of the toxin may play distinct functional roles (9).

Conclusions.

Suggesting a role for LtxA in iron acquisition may also shed light on the evolution of high leukotoxicity in A. actinomycetemcomitans. High leukotoxicity results from increased ltx promoter activity due to a 530-bp deletion in the promoter region (8). Interestingly, Hayashida et al. (15) found that none of the highly leukotoxic strains (JP2 type) of A. actinomycetemcomitans they examined was able to utilize hemoglobin as an iron source due to a nonfunctional hemoglobin binding protein A (HgpA). Because A. actinomycetemcomitans is unable to obtain iron from human transferrin or lactoferrin (15, 44) and does not produce siderophores (33, 44), hemoglobin may be an important physiological source of iron for A. actinomycetemcomitans. It is tempting to speculate that the 530-bp deletion was selected for in strains that contained a nonfunctional HgpA because the increased production of LtxA may have allowed these bacteria to lyse more erythrocytes and release a greater amount of other usable forms of iron.

Acknowledgments

We thank Luis Actis and Eric Rhodes for helpful comments and suggestions throughout this study. We thank Jeffrey Kaplan for his careful review of the manuscript.

This work was generously supported by grants from the National Institute of Dental and Craniofacial Research (R01 DE16133 to S.C.K. and F32 DE017828 to N.V.B.).

Footnotes

[down-pointing small open triangle]Published ahead of print on 13 October 2006.

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