The anticonvulsive Phenhydan® suppresses extrinsic cell death.

Different forms of regulated cell death-like apoptosis and necroptosis contribute to the pathophysiology of clinical conditions including ischemia-reperfusion injury, myocardial infarction, sepsis, and multiple sclerosis. In particular, the kinase activity of the receptor-interacting serine/threonine protein kinase 1 (RIPK1) is crucial for cell fate in inflammation and cell death. However, despite its involvement in pathological conditions, no pharmacologic inhibitor of RIPK1-mediated cell death is currently in clinical use. Herein, we screened a collection of clinical compounds to assess their ability to modulate RIPK1-mediated cell death. Our small-scale screen identified the anti-epilepsy drug Phenhydan® as a potent inhibitor of death receptor-induced necroptosis and apoptosis. Accordingly, Phenhydan® blocked activation of necrosome formation/activation as well as death receptor-induced NF-κB signaling by influencing the membrane function of cells, such as lipid raft formation, thus exerting an inhibitory effect on pathophysiologic cell death processes. By targeting death receptor signaling, the already FDA-approved Phenhydan® may provide new therapeutic strategies for inflammation-driven diseases caused by aberrant cell death.

Necroptosis and ferroptosis are alternative cell death pathways that operate in acute kidney failure.

Ferroptosis is a recently recognized caspase-independent form of regulated cell death that is characterized by the accumulation of lethal lipid ROS produced through iron-dependent lipid peroxidation. Considering that regulation of fatty acid metabolism is responsible for the membrane-resident pool of oxidizable fatty acids that undergo lipid peroxidation in ferroptotic processes, we examined the contribution of the key fatty acid metabolism enzyme, acyl-CoA synthetase long-chain family member 4 (ACSL4), in regulating ferroptosis. By using CRISPR/Cas9 technology, we found that knockout of Acsl4 in ferroptosis-sensitive murine and human cells conferred protection from erastin- and RSL3-induced cell death. In the same cell types, deletion of mixed lineage kinase domain-like (Mlkl) blocked susceptibility to necroptosis, as expected. Surprisingly, these studies also revealed ferroptosis and necroptosis are alternative, in that resistance to one pathway sensitized cells to death via the other pathway. These data suggest a mechanism by which one regulated necrosis pathway compensates for another when either ferroptosis or necroptosis is compromised. We verified the synergistic contributions of ferroptosis and necroptosis to tissue damage during acute organ failure in vivo. Interestingly, in the course of pathophysiological acute ischemic kidney injury, ACSL4 was initially upregulated and its expression level correlated with the severity of tissue damage. Together, our findings reveal ACSL4 to be a reliable biomarker of the emerging cell death modality of ferroptosis, which may also serve as a novel therapeutic target in preventing pathological cell death processes.

The pseudokinase MLKL mediates programmed hepatocellular necrosis independently of RIPK3 during hepatitis.

Although necrosis and necroinflammation are central features of many liver diseases, the role of programmed necrosis in the context of inflammation-dependent hepatocellular death remains to be fully determined. Here, we have demonstrated that the pseudokinase mixed lineage kinase domain-like protein (MLKL), which plays a key role in the execution of receptor-interacting protein (RIP) kinase-dependent necroptosis, is upregulated and activated in human autoimmune hepatitis and in a murine model of inflammation-dependent hepatitis. Using genetic and pharmacologic approaches, we determined that hepatocellular necrosis in experimental hepatitis is driven by an MLKL-dependent pathway that occurs independently of RIPK3. Moreover, we have provided evidence that the cytotoxic activity of the proinflammatory cytokine IFN-γ in hepatic inflammation is strongly connected to induction of MLKL expression via activation of the transcription factor STAT1. In summary, our results reveal a pathway for MLKL-dependent programmed necrosis that is executed in the absence of RIPK3 and potentially drives the pathogenesis of severe liver diseases.

Inhibition of regulated cell death by cell-penetrating peptides.

Development of the means to efficiently and continuously renew missing and non-functional proteins in diseased cells remains a major goal in modern molecular medicine. While gene therapy has the potential to achieve this, substantial obstacles must be overcome before clinical application can be considered. A promising alternative approach is the direct delivery of non-permeant active biomolecules, such as oligonucleotides, peptides and proteins, to the affected cells with the purpose of ameliorating an advanced disease process. In addition to receptor-mediated endocytosis, cell-penetrating peptides are widely used as vectors for rapid translocation of conjugated molecules across cell membranes into intracellular compartments and the delivery of these therapeutic molecules is generally referred to as novel prospective protein therapy. As a broad coverage of the enormous amount of published data in this field is unrewarding, this review will provide a brief, focused overview of the technology and a summary of recent studies of the most commonly used protein transduction domains and their potential as therapeutic agents for the treatment of cellular damage and the prevention of regulated cell death.

Synchronized renal tubular cell death involves ferroptosis.

Receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis is thought to be the pathophysiologically predominant pathway that leads to regulated necrosis of parenchymal cells in ischemia-reperfusion injury (IRI), and loss of either Fas-associated protein with death domain (FADD) or caspase-8 is known to sensitize tissues to undergo spontaneous necroptosis. Here, we demonstrate that renal tubules do not undergo sensitization to necroptosis upon genetic ablation of either FADD or caspase-8 and that the RIPK1 inhibitor necrostatin-1 (Nec-1) does not protect freshly isolated tubules from hypoxic injury. In contrast, iron-dependent ferroptosis directly causes synchronized necrosis of renal tubules, as demonstrated by intravital microscopy in models of IRI and oxalate crystal-induced acute kidney injury. To suppress ferroptosis in vivo, we generated a novel third-generation ferrostatin (termed 16-86), which we demonstrate to be more stable, to metabolism and plasma, and more potent, compared with the first-in-class compound ferrostatin-1 (Fer-1). Even in conditions with extraordinarily severe IRI, 16-86 exerts strong protection to an extent which has not previously allowed survival in any murine setting. In addition, 16-86 further potentiates the strong protective effect on IRI mediated by combination therapy with necrostatins and compounds that inhibit mitochondrial permeability transition. Renal tubules thus represent a tissue that is not sensitized to necroptosis by loss of FADD or caspase-8. Finally, ferroptosis mediates postischemic and toxic renal necrosis, which may be therapeutically targeted by ferrostatins and by combination therapy.

T-cell immunoglobulin and mucin domain 2 (TIM-2) is a target of ADAM10-mediated ectodomain shedding.

T-cell immunoglobulin and mucin domain (TIM)-2 is expressed on activated B cells. Here, we provide evidence that murine TIM-2 is a target of ADAM10-mediated ectodomain shedding, resulting in the generation of a soluble form of TIM-2. We identified ADAM10 but not ADAM17 as the major sheddase of TIM-2, as shown by pharmacological ADAM10 inhibition and with ADAM10-deficient and ADAM17-deficient murine embryonic fibroblasts. Ionomycin-induced or 2'(3')-O-(4-benzoylbenzoyl) ATP triethylammonium salt-induced shedding of TIM-2 was abrogated by deletion of 10 juxtamembrane amino acids from the stalk region but not by deletion of two further N-terminally located blocks of 10 amino acids, indicating a membrane-proximal cleavage site. TIM-2 lacking the intracellular domain was cleaved after ionomycin or 2' (3')-O-(4-benzoylbenzoyl) ATP triethylammonium salt treatment, indicating that this domain was not involved in the regulation of ectodomain shedding. Moreover, TIM-2 shedding was negatively controlled by calmodulin. Shed and soluble TIM-2 interacted with H-ferritin. In summary, we describe TIM-2 as a novel target for ADAM10-mediated ectodomain shedding, and reveal the involvement of ADAM proteases in cellular iron homeostasis.

Soluble T cell immunoglobulin and mucin domain (TIM)-1 and -4 generated by A Disintegrin And Metalloprotease (ADAM)-10 and -17 bind to phosphatidylserine.

T cell immunoglobulin and mucin domain 1 and 4 (TIM-1 and -4) proteins serve as phosphatidylserine receptors to engulf apoptotic cells. Here we show that human TIM-1 and TIM-4 proteins are targets of A Disintegrin And Metalloprotease (ADAM)-mediated ectodomain shedding resulting in soluble forms of TIM-1 and TIM-4. We identified ADAM10 and ADAM17 as major sheddases of TIM-1 and TIM-4 as shown by protease-specific inhibitors, the ADAM10 prodomain, siRNA and ADAM10/ADAM17 deficient murine embryonic fibroblasts (MEFs). TIM-1 and TIM-4 lacking the intracellular domain were efficiently cleaved after ionomycin- and PMA-treatment, indicating that the intracellular domain was not necessary for ectodomain shedding. Soluble TIM-1 and -4 were able to bind to phosphatidylserine, suggesting that soluble TIM-1 and -4 might act as negative regulators of cellular TIM-1 and -4. In summary, we describe TIM-1 and TIM-4 as novel targets for ADAM10- and ADAM17-mediated ectodomain shedding.

A disintegrin and metalloprotease (ADAM) 10 and ADAM17 are major sheddases of T cell immunoglobulin and mucin domain 3 (Tim-3).

T cell immunoglobulin and mucin domain 3 (Tim-3) dampens the response of CD4(+) and CD8(+) effector T cells via induction of cell death and/or T cell exhaustion and enhances the ability of macrophages to clear pathogens via binding to galectin 9. Here we provide evidence that human Tim-3 is a target of A disintegrin and metalloprotease (ADAM)-mediated ectodomain shedding resulting in a soluble form of Tim-3. We identified ADAM10 and ADAM17 as major sheddases of Tim-3 as shown by ADAM-specific inhibitors and the ADAM10 pro-domain in HEK293 cells and ADAM10/ADAM17-deficient murine embryonic fibroblasts. PMA-induced shedding of Tim-3 was abrogated by deletion of amino acids Glu(181)-Asp(190) of the stalk region and Tim-3 lacking the intracellular domain was not efficiently cleaved after PMA stimulation. Surprisingly, a single lysine residue within the intracellular domain rescues shedding of Tim-3. Shedding of endogenous Tim-3 was found in primary human CD14(+) monocytes after PMA and ionomycin stimulation. Importantly, the recently described down-regulation of Tim-3 from Toll-like receptor-activated CD14(+) monocytes was caused by ADAM10- and ADAM17-mediated shedding. Inhibition of Tim-3 shedding from lipopolysaccharide-induced monocytes did not influence lipopolysaccharide-induced TNFα and IL-6 but increases IL-12 expression. In summary, we describe Tim-3 as novel target for ADAM-mediated ectodomain shedding and suggest a role of Tim-3 shedding in TLR-mediated immune responses of CD14(+) monocytes.

Effects of blockade of peripheral interleukin-6 trans-signaling on hippocampus-dependent and independent memory in mice.

Besides functions of the interleukin-6 (IL-6)/gp130 cytokine family in immunology, IL-6 signaling has influence on memory processes. IL-6 acts on target cells via a membrane-bound IL-6 receptor (IL-6R) and subsequent association with the signal-transducing protein gp130. While gp130 is expressed on all cells in the body, IL-6R is expressed in only on few cells such as hepatocytes and some leukocytes. Cells lacking IL-6R were shown not to be responsive to the cytokine. Interestingly, a soluble form of the IL-6R in complex with IL-6 can stimulate cells that do not express the membrane-bound IL-6R. This signaling pathway has been called IL-6 trans-signaling. IL-6 trans-signaling can specifically be blocked by a soluble gp130 protein (sgp130Fc) without affecting IL-6 classic signaling via the membrane-bound IL-6R. Transgenic mice expressing sgp130Fc in the blood, but not in the central nervous system, were analyzed for hippocampus-dependent and independent memory, together with exploratory- and anxiety-related behavior. Transgenic animals did not show impaired hippocampus-dependent or independent learning and memory. However, compared to wild-type animals, they showed reduced exploratory behavior and an increased thermal pain threshold, indicating that these effects depend on IL-6 trans-signaling. These results bear important consequences for the therapeutic blockade of IL-6 activity in autoimmune diseases.