Cell death in sepsis: a matter of how, when, and where

Dysregulated cell death in several tissues is intimately involved in the pathogenesis of sepsis and contributes to multiple organ failure. Whether cell death during sepsis occurs by necrosis or apoptosis may depend on the cell type as well as the disease stage and is therefore a matter of intense debate. While lymphocyte apoptosis contributes to immunosuppression in sepsis, recent evidence suggests that necrosis of hepatocytes predominates in septic patients with liver dysfunction and correlates with poor survival. These distinct modes of cell death might have different consequences for the inflammatory response but are also critical for therapeutic interventions and the disease outcome. Understanding the complexity of death processes employing recently available serum biomarkers of cell death could lead to novel therapeutic approaches and assist in the steering of sepsis treatment

Mechanisms of Cell Death in Acute Liver Failure

Acute liver failure (ALF) can be the consequence of various etiologies, that might vary between different geographic regions. Most frequent are intoxications with acetaminophen, viral hepatitis, or liver damage of unknown origin. ALF occurs when the extent of hepatocyte death exceeds the regenerative capacity of the liver. The mode of liver cell death that is predominantly induced in ALF, i.e., apoptosis or necrosis, is still controversial and presumably determined by the etiology, duration, and magnitude of liver injury. Severe liver damage involves oxidative stress and depletion of ATP resulting in necrosis. In contrast, maintenance of ATP stores is required for the execution of apoptosis. Recent data suggest that necrosis resulting from severe liver damage is associated with poor outcome of ALF patients. Discrimination between apoptosis and necrosis might be therefore useful for the identification of ALF patients requiring liver transplantation. Identification of the molecular cell death mechanisms remains an important issue not only for early prediction of ALF outcome, but also for therapeutic interventions. In view of the pleiotropic functions of critical mediators of cell death and tissue regeneration, a particular challenge will be to reduce hepatocellular death without inhibiting the regenerative capacity of the liver. Here, we review the molecular mechanisms of hepatocyte injury and the pathways leading to apoptosis and necrosis, which might represent potential diagnostic and therapeutic targets in ALF

Differential Regulation and ATP Requirement for Caspase-8 and Caspase-3 Activation during CD95- and Anticancer Drug–induced Apoptosis

Apoptosis is induced by different stimuli, among them triggering of the death receptor CD95, staurosporine, and chemotherapeutic drugs. In all cases, apoptosis is mediated by caspases, although it is unclear how these diverse apoptotic stimuli cause protease activation. Two regulatory pathways have been recently identified, but it remains unknown whether they are functionally independent or linked to each other. One is mediated by recruitment of the proximal regulator caspase-8 to the death receptor complex. The other pathway is controlled by the release of cytochrome c from mitochondria and the subsequent ATP-dependent activation of the death regulator apoptotic protease-activating factor 1 (Apaf-1). Here, we report that both pathways can be dissected by depletion of intracellular ATP. Prevention of ATP production completely inhibited caspase activation and apoptosis in response to chemotherapeutic drugs and staurosporine. Interestingly, caspase-8, whose function appeared to be restricted to death receptors, was also activated by these drugs under normal conditions, but not after ATP depletion. In contrast, inhibition of ATP production did not affect caspase activation after triggering of CD95. These results suggest that chemotherapeutic drug–induced caspase activation is entirely controlled by a receptor-independent mitochondrial pathway, whereas CD95-induced apoptosis can be regulated by a separate pathway not requiring Apaf-1 function

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

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

Bax/Bak-independent mitochondrial depolarization and reactive oxygen species induction by sorafenib overcome resistance to apoptosis in renal cell carcinoma

Renal cell carcinoma (RCC) is polyresistant to chemo- and radiotherapy or biologicals including TNF-related apoptosis inducing ligand (TRAIL). Sorafenib, a multikinase inhibitor approved for the treatment of RCC, has been shown to sensitize cancer cells toward TRAIL-induced apoptosis, in particular by downregulation of the Bak-inhibitory Bcl 2 family protein Mcl 1. Here, we demonstrate that sorafenib overcomes TRAIL resistance in RCC by a mechanism that does not rely on Mcl 1 downregulation. Instead, sorafenib induces a rapid dissipation of the mitochondrial membrane potential (ΔΨ(m)) that is accompanied by the accumulation of reactive oxygen species (ROS). Loss of ΔΨ(m) and ROS production induced by sorafenib are independent of caspase activities and do not depend on the presence of the pro-apoptotic Bcl 2 family proteins Bax or Bak indicating that both events are functionally up-stream of the mitochondrial apoptosis signaling cascade. More intriguingly, we find that it is sorafenib-induced ROS accumulation that enables TRAIL to activate caspase 8 in RCC. This leads to apoptosis that involves activation of an amplification loop via the mitochondrial apoptosis pathway. Thus, our mechanistic data indicate that sorafenib bypasses central resistance mechanisms through a direct induction of ΔΨ(m) breakdown and ROS production. Activation of this pathway might represent a useful strategy to overcome the cell-inherent resistance to cancer therapeutics including TRAIL in multiresistant cancers such as RCC

Raptinal bypasses BAX, BAK, and BOK for mitochondrial outer membrane permeabilization and intrinsic apoptosis

Most antineoplastic chemotherapies eliminate cancer cells through activation of the mitochondria-controlled intrinsic apoptotic pathway. Therein, BAX, BAK, and/or BOK function as the essential pore-forming executioners of mitochondrial outer membrane permeabilization (MOMP). The activation threshold of BAX and BAK also correlates inversely with the required strength of an apoptotic stimulus to induce MOMP and thereby effectively determines a cell's readiness to undergo apoptosis. Consequently, the 'gatekeepers' BAX and BAK emerged as therapeutic targets, but functional or genetic loss renders BAX/BAK-targeting strategies prone to fail. Here, we show that the small molecule Raptinal overcomes this limitation by triggering cytochrome c release in a BAX/BAK/BOK-independent manner. Raptinal exerts a dual cytotoxic effect on cancer cells by rapid activation of the intrinsic apoptotic pathway and simultaneous shutdown of mitochondrial function. Together with its efficacy to eliminate cancer cells in vivo, Raptinal could be useful in difficult-to-treat cancer entities harboring defects in the intrinsic apoptosis pathway

CD152 (CTLA-4) Determines the Unequal Resistance of Th1 and Th2 Cells against Activation-induced Cell Death by a Mechanism Requiring PI3 Kinase Function

Survival of antigen-experienced T cells is essential for the generation of adaptive immune responses. Here, we show that the genetic and antibody-mediated inactivation of CD152 (cytotoxic T lymphocyte antigen 4) in T helper (Th) effector cells reduced the frequency of nonapoptotic cells in a completely Fas/Fas ligand (FasL)–dependent manner. CD152 cross-linking together with stimulation of CD3 and CD28 on activated Th2 cells prevented activation-induced cell death (AICD) as a result of reduced Fas and FasL expression. Apoptosis protection conferred by CD152 correlated with the up-regulation of Bcl-2 and was mediated by phosphatidylinositol 3 kinase, which prevented FasL expression through the inhibitory phosphorylation of Forkhead transcription factor FKHRL1. We show that signals induced by CD152 act directly on activated T lymphocytes and, due to its differential surface expression on activated Th1 and Th2 cells, induce resistance to AICD mainly in Th2 cells