Mechanisms Underlying the Delayed Activation of the Cap1 Transcription Factor in Candida albicans following Combinatorial Oxidative and Cationic Stress Important for Phagocytic Potency

Abstract

Following phagocytosis, microbes are exposed to an array of antimicrobial weapons that include reactive oxygen species (ROS) and cationic fluxes. This is significant as combinations of oxidative and cationic stresses are much more potent than the corresponding single stresses, triggering the synergistic killing of the fungal pathogen Candida albicans by “stress pathway interference.” Previously we demonstrated that combinatorial oxidative plus cationic stress triggers a dramatic increase in intracellular ROS levels compared to oxidative stress alone. Here we show that activation of Cap1, the major regulator of antioxidant gene expression in C. albicans, is significantly delayed in response to combinatorial stress treatments and to high levels of H2O2. Cap1 is normally oxidized in response to H2O2; this masks the nuclear export sequence, resulting in the rapid nuclear accumulation of Cap1 and the induction of Cap1-dependent genes. Here we demonstrate that following exposure of cells to combinatorial stress or to high levels of H2O2, Cap1 becomes trapped in a partially oxidized form, Cap1OX-1. Notably, Cap1-dependent gene expression is not induced when Cap1 is in this partially oxidized form. However, while Cap1OX-1 readily accumulates in the nucleus and binds to target genes following high-H2O2 stress, the nuclear accumulation of Cap1OX-1 following combinatorial H2O2 and NaCl stress is delayed due to a cationic stress-enhanced interaction with the Crm1 nuclear export factor. These findings define novel mechanisms that delay activation of the Cap1 transcription factor, thus preventing the rapid activation of the stress responses vital for the survival of C. albicans within the host.

Cell biology of Candida albicans–host interactions

“…Multiple individual imposed toxic stresses have been shown to exert synergistic effects on the ability to kill Candida . These studies illustrate the importance of the Hog1 MAPK and the Cap1 transcription factor in the regulation of combinatorial stress responses [ 60 , 61 ].…”

Specificity of the osmotic stress response in Candida albicans highlighted by quantitative proteomics

“…In particular, host defence mechanisms are perceived as stresses by the pathogen. Myeloid cells attempt to kill fungal cells using a range of toxic reactive oxygen and nitrogen species 9 , and it was recently shown that a combination of cationic (NaCl) and oxidative stresses are particularly lethal for pathogenic Candida species 10 , 11 . Therefore, an understanding of stress response mechanisms could potentially lead to therapeutic approaches that augment host defence.…”

Stress Adaptation

“…Strikingly, studies investigating the mechanistic basis underlying the exquisite sensitivity of C. albicans to such combinations of stress have revealed that exposure to cationic stress prevents this pathogen from mounting an oxidative stress response. The oxidative stress regulon in C. albicans is largely regulated by the Cap1 AP-1-like transcription factor (214), and Cap1 fails to be activated following exposure to combinatorial oxidative and cationic stress (211, 215). This phenomenon, which has been termed “ stress pathway interference ” (211) (Figure 3), contrasts starkly with that of stress cross-protection in which exposure to one stress protects against the subsequent exposure to a different stress (104).…”

“…First, cations inhibit catalase activity which triggers significant increases in intracellular ROS levels following combinations of cationic and oxidative stresses (211). Such high levels of ROS trap Cap1 in a partially oxidised form that fails to induce target antioxidant-encoding genes (215). Second, cations stimulate the interaction of Cap1 with the nuclear export factor Crm1, which results in significant delays in the H 2 O 2 -induced nuclear accumulation of this transcription factor.…”