α-Toxin is a mediator of Staphylococcus aureus–induced cell death and activates caspases via the intrinsic death pathway independently of death receptor signaling

Abstract

Infections with Staphylococcus aureus, a common inducer of septic and toxic shock, often result in tissue damage and death of various cell types. Although S. aureus was suggested to induce apoptosis, the underlying signal transduction pathways remained elusive. We show that caspase activation and DNA fragmentation were induced not only when Jurkat T cells were infected with intact bacteria, but also after treatment with supernatants of various S. aureus strains. We also demonstrate that S. aureus–induced cell death and caspase activation were mediated by α-toxin, a major cytotoxin of S. aureus, since both events were abrogated by two different anti–α-toxin antibodies and could not be induced with supernatants of an α-toxin–deficient S. aureus strain. Furthermore, α-toxin–induced caspase activation in CD95-resistant Jurkat sublines lacking CD95, Fas-activated death domain, or caspase-8 but not in cells stably expressing the antiapoptotic protein Bcl-2. Together with our finding that α-toxin induces cytochrome c release in intact cells and, interestingly, also from isolated mitochondria in a Bcl-2-controlled manner, our results demonstrate that S. aureus α-toxin triggers caspase activation via the intrinsic death pathway independently of death receptors. Hence, our findings clearly define a signaling pathway used in S. aureus–induced cytotoxicity and may provide a molecular rationale for future therapeutic interventions in bacterial infections.

Staphylococcus aureus α-toxin-induced cell death: predominant necrosis despite apoptotic caspase activation

“…To investigate this hypothesis in more detail, we first analyzed hypodiploid nuclei formation induced by either a-toxin or by a supernatant of the cytotoxic S. aureus strain Wood 46 in the absence or presence of the broad spectrum caspase inhibitor zVAD-fmk. Similar to the results obtained with Jurkat Bcl-2 cells, 32 32 ), zVAD-fmk reduced hypodiploid nuclei formation by only 22 and 40%, respectively. In contrast, CD95-induced DNA fragmentation in Jurkat cells was almost completely blocked in the presence of zVAD-fmk ( Figure 1a).…”

“…32 During this work, however, we also noticed that although S. aureusinduced caspase activation was almost completely blocked in Bcl-2-overexpressing Jurkat cells, the formation of hypodiploid nuclei as measured by FACS analysis could only be partially prevented in these cells. These results indicated that caspases might not be required for S. aureus-induced DNA fragmentation and cell death.…”

“…In addition, we have recently demonstrated that only low a-toxin doses activate caspases, and that this activation is mediated via the intrinsic pathway independently of death receptors. 32 Thus, it appears that the mode of cell death critically depends on the concentration of a-toxin.…”

Exploiting death: apoptotic immunity in microbial pathogenesis

“…18,21,41,[55][56][57][58][59][60][61][62][63][64] Viruses exploit apoptotic processes as well, especially reducing the death of virally infected cells to facilitate intracellular viral replication. The viral reprogramming of cell death, in contrast, reflects a distinct approach.…”

Burkholderia cenocepacia J2315-mediated destruction of Staphylococcus aureus NRS77 biofilms.

“…The leukotoxin γ-hemolysin targets red blood cells and can cause massive hemolysis [50], while the monomeric α-toxin (or α-hemolysin) is highly cytotoxic and polymerizes to form a water channel in host cell membranes [51]. This leukotoxin leads to apoptosis through activation of a caspase cascade [52]. These leukotoxins extend bacterial infection by eliminating predatory immune cells and enhancing invasion of host tissue.…”