Endothelial Cell Injury Caused by Candida albicans Is Dependent on Iron

Although it is known that Candida albicans causes endothelial cell injury, in vitro and in vivo, the mechanism by which this process occurs remains unknown. Iron is critical for the induction of injury in many types of host cells. Therefore, we investigated the role of iron in Candida-induced endothelial cell injury. We found that pretreatment of endothelial cells with the iron chelators phenanthroline and deferoxamine protected them from candidal injury, even though the organisms germinated and grew normally. Loading endothelial cells with iron reversed the cytoprotective effects of iron chelation. Moreover, chelation of endothelial cell iron significantly reduced phagocytosis of C. albicans by these cells, while candidal adherence to chelator-treated endothelial cells was slightly enhanced. Since endothelial cell phagocytosis of C. albicans is required for endothelial cell injury to occur, inhibition of phagocytosis is likely the principal mechanism of the cytoprotective effects of iron chelation. The production of toxic reactive oxygen intermediates by host cells is known to be inhibited by iron chelation. Therefore, we investigated whether treating endothelial cells with antioxidants could mimic the cytoprotective effects of iron chelation. Neither extracellular nor membrane-permeative antioxidants reduced candidal injury of endothelial cells. Furthermore, depleting endothelial cells of the endogenous antioxidant glutathione did not render them more susceptible to damage by C. albicans. These results suggest that candidal injury of endothelial cells is independent of the production of reactive oxygen intermediates and that the cytoprotective effects of iron chelation are not due to inhibition of the synthesis of these toxic intermediates.During the process of hematogenous dissemination, Candida albicans must first cross the endothelial cell lining of the vasculature to invade the tissue parenchyma. One mechanism by which the organism may escape from the vascular compartment is by causing endothelial cell injury. This injury likely results in exposure of the subendothelial cell basement membrane, which may enhance candidal adherence and facilitate tissue invasion (23). We have been investigating the mechanisms by which C. albicans injures endothelial cells in vitro. Previously, we found that phagocytosis of the organism by endothelial cells is required for endothelial cell damage to occur (8, 9). This phagocytosis requires both intact endothelial cell microfilaments and microtubules. In addition, although endothelial cells are able to phagocytize killed organisms (35), only the phagocytosis of live, germinating organisms causes endothelial cell injury (8).After endothelial cells phagocytize C. albicans, the organism may injure the endothelial cells by several potential mechanisms. It is possible that phospholipases and/or proteinases secreted by C. albicans injure host cells (6, 18, 38). Another possibility is that C. albicans causes endothelial cell injury by an iron-dependent process. For example, endothelial cells are known to synthesize and release superoxide anions during phagocytic activity (11, 14). Iron is required for the assembly of enzymes, such as xanthine oxidase (33) and cytochromes (20), that catalyze the synthesis of these reactive oxygen intermediates. In addition, iron serves as a cofactor, which converts these anions to highly reactive hydroxyl radicals (21) which cause cellular damage (10, 24). Finally, iron is required for the function of nitric oxide synthase, an enzyme that can catalyze the synthesis of cytotoxic concentrations of nitric oxide in some cell types (31).In this study, we used the iron chelators phenanthroline and deferoxamine to examine the role of iron in endothelial cell injury caused by C. albicans. We found that chelation of endothelial cell iron protected these cells from injury by C. albicans. The cytoprotective effects of iron chelation were likely due to reduced phagocytosis of this fungus by endothelial cells. Furthermore, we found that C. albicans damages endothelial cells by a process that is likely independent of the production of reactive oxygen intermediates.(This work was presented in part at the 35th Interscience Conference on Antimicrobial Agents and Chemotherapy, September 1995, San Francisco, Calif.)