Lipopolysaccharide mediates endothelial apoptosis by a FADD-dependent pathway

Macrophage migration inhibitory factor governs endothelial cell sensitivity to LPS-induced apoptosis

Human endothelial cells (EC) are typically resistant to the apoptotic effects of stimuli associated with lung disease. The determinants of this resistance remain incompletely understood. Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine produced by human pulmonary artery EC (HPAEC). Its expression increases in response to various death-inducing stimuli, including lipopolysaccharide (LPS). We show here that silencing MIF expression by RNA interference (MIF siRNA) dramatically reduces MIF mRNA expression and the LPS-induced increase in MIF protein levels, thereby sensitizing HPAECs to LPS-induced cell death. Addition of recombinant human MIF (rhMIF) protein prevents the death-sensitizing effect of MIF siRNA. A common mediator of apoptosis resistance in ECs is the death effector domain (DED)-containing protein, FLIP (FLICE-like inhibitory protein). We show that LPS induces a transcription-independent increase in the short isoform of FLIP (FLIP(s)). This increase is blocked by MIF siRNA but restored with the addition of recombinant MIF protein (rHMIF). While FLIP(s) siRNA also sensitizes HPAECs to LPS-induced death, the addition of rhMIF does not affect this sensitization, placing MIF upstream of FLIP(s) in preventing HPAEC death. These studies demonstrate that MIF is an endogenous pro-survival factor in HPAECs and identify a novel mechanism for its role in apoptosis resistance through the regulation of FLIP(s). These results show that MIF can protect vascular endothelial cells from inflammation-associated cell damage.

The toll-like receptor-4 (TLR-4) pathway and its possible role in the pathogenesis of Escherichia coli mastitis in dairy cattle

Mastitis is one of the most costly production diseases in the dairy industry that is caused by a wide array of microorganisms. In this review, we focus on the Gram-negative Escherichia coli infections that often occur at periods when the innate immune defence mechanisms are impaired (i.e., parturition through the first 60 days of lactation). There is substantial evidence demonstrating that at these periods, the expected influx of polymorphonuclear neutrophil leukocytes (PMN) into the mammary gland is delayed during inflammation after intramammary infection with E. coli. Here, we provide some hypotheses on the potential mechanisms of action on how the disease may develop under circumstances of immunosuppression, and describe the potential involvement of the toll-like receptor-4 signal transduction pathway in the pathogenesis of E. coli mastitis. In addition, some ideas are proposed to help prevent E. coli mastitis and potentially other diseases caused by Gram-negative infections in general.

Anti-vascular permeability of the cleaved reactive center loop within the carboxyl-terminal domain of C1 inhibitor

C1 inhibitor (C1INH), a member of the serine proteinase inhibitor (serpin) family, functions as an inhibitor of the complement and contact systems. Cleavage of the reactive center loop (RCL) within the carboxyl-terminal domain of C1INH (iC1INH), lacking of serpin function, induces a conformational change in the molecule. Our previous data demonstrated that active, intact C1INH prevents vascular permeability induced by gram-negative bacterial lipopolysaccharide (LPS). In this study, we investigate the role of RCL-cleaved, inactive C1INH (iC1INH) in vascular endothelial activation. In the cultured primary human umbilical vein endothelial cell (HUVEC) monolayer, iC1INH blocked LPS-induced cell injury by evaluated as transendothelial flux, cell detachment, and cytoskeletal disorganization. LPS-induced upregulation of vascular cell adhesion molecule-1 (VCAM-1) could be suppressed by treatment with iC1INH. Studies exploring the underlying mechanism of iC1INH-mediated suppression in VCAM-1 expression were related to reduction of NF-kappaB activation and nuclear translocation in an I kappa B alpha-dependent manner. The inhibitory effect was associated with stabilization of the NF-kappaB inhibitor I kappa B and reduction of inhibitor I kappa B kinase activity. In the model of endotoxin-induced mice, increased plasma leakage in local abdominal skin in response to LPS was reversed by treatment with iC1INH. Furthermore, systemic administration of LPS to mice resulted in increased microvascular permeability in multiple organs, which was reduced by iC1INH. These data provide evidence that iC1INH has an anti-vascular permeability independent on the serpin function.

Protection against lipopolysaccharide-induced myocardial dysfunction in mice by cardiac-specific expression of soluble Fas

The mechanisms responsible for myocardial dysfunction in the setting of sepsis remain undefined. Fas ligation with its cognate ligand (FasL) induces apoptosis and activates cellular inflammatory responses associated with tissue injury. We determined whether interruption of Fas/FasL interaction by cardiac-specific expression of soluble Fas (sFas), a competitive inhibitor of FasL, would improve myocardial dysfunction and inflammation in a lipopolysaccharide (LPS)-induced mouse model of sepsis. Wild-type (WT) and sFas transgenic mice were injected intraperitoneally with 10 mg/kg LPS or with an equivalent volume of saline. At 18 h after LPS administration, echocardiographic evaluation revealed a significant decrease in left ventricular fractional shortening in the WT mice, whereas the fractional shortening was preserved in the sFas mice. Activation of nuclear factor-kappa B (NF-kappaB) and the increase in the transcript levels of proinflammatory cytokines, tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6 resulting from LPS treatment were attenuated in the myocardium of sFas mice. sFas expression also inhibited LPS-induced upregulation of Toll-like receptor 4 (TLR-4) and inducible nitric oxide synthase (iNOS), and formation of peroxynitrite in the myocardium. LPS-induced increase in caspase-3/7 activity and apoptotic cell death were suppressed in sFas mice compared with WT mice. LPS-induced lung injury and increase in lung water content were also significantly reduced in sFas mice. These data indicate that neutralization of FasL by expression of sFas significantly preserves cardiac function and reduces inflammatory responses in the heart, suggesting that Fas/FasL signaling pathway is important in mediating the deleterious effects of LPS on myocardial function.

Lipopolysaccharide and interferon-gamma enhance Fas-mediated cell death in mouse vascular endothelial cells via augmentation of Fas expression

The effect of interferon (IFN)-gamma and/or lipopolysaccharide (LPS) on Fas-mediated cell death with anti-Fas agonistic antibody in vascular endothelial cells was examined using a mouse END-D cell line. Anti-Fas agonistic antibody exhibited cytotoxic actions on END-D cells. Fas-mediated cell death was enhanced by LPS or IFN-gamma. The combination of IFN-gamma and LPS significantly enhanced cell death compared to IFN-gamma or LPS alone. IFN-gamma and LPS augmented cell surface expression of Fas, but not tumour necrosis factor (TNF) receptor 1. Inhibitors of p38 mitogen-activated protein kinase (MAPK) prevented augmentation of Fas expression in IFN-gamma and LPS-treated END-D cells. IFN-gamma and LPS-treated END-D cells did not become susceptible to TNF-alpha or nitric oxide-mediated cytotoxicity. IFN-gamma and LPS thus appear to augment selectively Fas expression via activation of p38 MAPK and enhance Fas-mediated cell death in END-D cells. Furthermore, administration of IFN-gamma and LPS into mice induced in vivo expression of Fas on vascular endothelial cells and Fas ligand (FasL) on peripheral blood leucocytes. The relationship between enhancement of Fas-mediated cell death by IFN-gamma and LPS and the development of vascular endothelial injury is discussed.

The intrinsic apoptotic pathway is required for lipopolysaccharide-induced lung endothelial cell death

LPS has been implicated in the pathogenesis of endothelial cell death associated with Gram-negative bacterial sepsis. The binding of LPS to the TLR-4 on the surface of endothelial cells initiates the formation of a death-inducing signaling complex at the cell surface. The subsequent signaling pathways that result in apoptotic cell death remain unclear and may differ among endothelial cells in different organs. We sought to determine whether LPS and cycloheximide-induced cell death in human lung microvascular endothelial cells (HmVECs) was dependent upon activation of the intrinsic apoptotic pathway and the generation of reactive oxygen species. We found that cells overexpressing the anti-apoptotic protein Bcl-X(L) were resistant to LPS and cycloheximide-induced death and that the proapoptotic Bcl-2 protein Bid was cleaved following treatment with LPS. The importance of Bid was confirmed by protection of Bid-deficient (bid(-/-)) mice from LPS-induced lung injury. Neither HmVECs treated with the combined superoxide dismutase/catalase mimetic EUK-134 nor HmVECs depleted of mitochondrial DNA (rho(0) cells) were protected against LPS and cycloheximide-induced death. We conclude that LPS and cycloheximide-induced death in HmVECs requires the intrinsic cell death pathway, but not the generation of reactive oxygen species.

FADD negatively regulates lipopolysaccharide signaling by impairing interleukin-1 receptor-associated kinase 1-MyD88 interaction

Lipopolysaccharide (LPS) engages Toll-like receptor 4 (TLR4) on various cells to initiate inflammatory and angiogenic pathways. FADD is an adaptor protein involved in death receptor-mediated apoptosis. Here we report a role for FADD in regulation of TLR4 signals in endothelial cells. FADD specifically attenuates LPS-induced activation of c-Jun NH(2)-terminal kinase and phosphatidylinositol 3'-kinase in a death domain-dependent manner. In contrast, FADD-null cells show hyperactivation of these kinases. Examining physical associations of endogenous proteins, we show that FADD interacts with interleukin-1 receptor-associated kinase 1 (IRAK1) and MyD88. LPS stimulation increases IRAK1-FADD interaction and recruitment of the IRAK1-FADD complex to activated MyD88. IRAK1 is required for FADD-MyD88 interaction, as FADD does not associate with MyD88 in IRAK1-null cells. By shuttling FADD to MyD88, IRAK1 provides a mechanism for controlled and limited activation of the TLR4 signaling pathway. Functionally, enforced FADD expression inhibited LPS- but not vascular endothelial growth factor-induced endothelial cell sprouting, while FADD deficiency led to enhanced production of proinflammatory cytokines induced by stimulation of TLR4 and TLR2, but not TLR3. Reconstitution of FADD reversed the enhanced production of proinflammatory cytokines. Thus, FADD is a physiological negative regulator of IRAK1/MyD88-dependent responses in innate immune signaling.

TLR4-mediated survival of macrophages is MyD88 dependent and requires TNF-alpha autocrine signalling

Modulation of macrophage survival is a critical factor in the resolution of inflammatory responses. Exposure to LPS protects innate immune cells against apoptosis, although the precise pathways responsible for prolongation of macrophage survival remain to be fully established. The goal of this study was to characterize the mechanism of TLR4-mediated survival of murine bone marrow-derived macrophages upon M-CSF withdrawal in more detail. Using a combination of knockout mice and pharmacological inhibitors allowed us to show that TLR4 and TLR2 stimulation promotes long-term survival of macrophages in a MyD88-, PI3K-, ERK-, and NF-kappaB-dependent manner. LPS-induced long-term, but not short-term, survival requires autocrine signaling via TNF-alpha and is facilitated by a general cytoprotective program, similar to that mediated by M-CSF. TLR4-mediated macrophage survival is accompanied by a remarkable up-regulation of specific cell surface markers, suggesting that LPS stimulation leads to the differentiation of macrophages toward a mixed macrophage/dendritic cell-like phenotype.

Role of macrophages in LPS-induced osteoblast and PDL cell apoptosis

In periradicular lesions and periodontal disease, bacterial invasion leads to chronic inflammation resulting in disruption of the structural integrity of the periodontal ligament and progressive alveolar bone destruction. The pathogenesis of these conditions has been attributed not only to bacterial-induced tissue destruction but also to a defect in periodontal tissue repair. Accumulated data have also shown that lipopolysaccharide (LPS) can directly induce cell death or apoptosis in many cell types, including macrophages, osteoblasts, vascular endothelial cells, hepatocytes and myocytes. The present study hypothesized that bacterial LPS-induced apoptosis in osteoblasts and periodontal ligament fibroblasts (PDL cells) is an important contributing factor to the defect in periodontal tissue repair in periodontal and periapical disease. Macrophages have been shown to respond to bacterial LPS by increasing the production of proinflammatory cytokines. In addition, large numbers of macrophages are present in inflamed periodontal tissue. We speculated that macrophages were a potential candidate cell for mediating apoptosis in osteoblasts and PDL cells in response to bacteria-derived LPS. The macrophage-like cell line, RAW 264.7, was stimulated with LPS, and the conditioned medium was used to treat osteoblasts and PDL cells. Bacterial LPS had no direct apoptotic effect on mouse osteoblasts or PDL cells, whereas the conditioned medium from LPS-activated macrophages was able to induce apoptosis in these cells. To evaluate the contribution of tumor necrosis factor-alpha (TNF-alpha) released from macrophages on osteoblast and PDL cell apoptosis, cells were incubated with conditioned medium from LPS-treated macrophages in the presence and absence of anti-TNF-alpha neutralizing antibodies. TNF-alpha neutralizing antibody pretreatment inhibited the effect of conditioned medium from LPS-treated macrophages on osteoblast and PDL cell apoptosis in a dose-dependent manner. These results suggest that LPS could indirectly induce apoptosis in osteoblasts and PDL cells through the induction of TNF-alpha release from macrophages. These studies provide insight into a potential mechanism by which bacterial-derived LPS could contribute to defective periodontal and bone tissue repair in periodontal and periapical disease.

Role of Circulating Vascular Progenitors in Angiogenesis, Vascular Healing, and Pulmonary Hypertension

Accumulating evidence suggests that circulating progenitors contribute to vascular healing and remodeling under physiological and pathological conditions. Although there is growing enthusiasm for therapeutic and diagnostic application of bone marrow-derived progenitors, there are concerns that transplanted precursors or bone marrow cells may participate in the pathogenesis of unfavorable diseases such as cancer, retinopathy, and atherosclerosis. This review summarizes recent findings obtained from animal models to examine the roles of circulating vascular progenitor cells in angiogenesis, pulmonary hypertension, and vascular healing.

A comprehensive map of the toll-like receptor signaling network

Recognition of pathogen-associated molecular signatures is critically important in proper activation of the immune system. The toll-like receptor (TLR) signaling network is responsible for innate immune response. In mammalians, there are 11 TLRs that recognize a variety of ligands from pathogens to trigger immunological responses. In this paper, we present a comprehensive map of TLRs and interleukin 1 receptor signaling networks based on papers published so far. The map illustrates the possible existence of a main network subsystem that has a bow-tie structure in which myeloid differentiation primary response gene 88 (MyD88) is a nonredundant core element, two collateral subsystems with small GTPase and phosphatidylinositol signaling, and MyD88-independent pathway. There is extensive crosstalk between the main bow-tie network and subsystems, as well as feedback and feedforward controls. One obvious feature of this network is the fragility against removal of the nonredundant core element, which is MyD88, and involvement of collateral subsystems for generating different reactions and gene expressions for different stimuli.

ExoS of Pseudomonas aeruginosa induces apoptosis through a Fas receptor/caspase 8-independent pathway in HeLa cells

Pseudomonas aeruginosa infection is a serious complication in immunocompromised individuals and in patients with cystic fibrosis. We have previously shown that the type III secreted effector ExoS triggers apoptosis in various cultured cell lines via its ADP-ribosyltransferase (ADPRT) activity. The apoptosis process was further shown to involve intrinsic signalling pathway requiring c-Jun N-terminal kinase (JNK)-initiated mitochondrial pathway. In the present study, we investigated the role of Fas pathway activation in P. aeruginosa-induced apoptosis. P. aeruginosa infection resulted in caspase 8 cleavage in HeLa cells, which was inhibited by overexpression of a dominant negative version of Fas-associated death domain (FADD), suggesting that Fas pathway was activated. In fact, confocal laser scanning microscopy showed that P. aeruginosa induced clustering of FasR. In addition, the ADPRT activity of the ExoS was required for the induction of FasR clustering and caspase 8 cleavage. However, blocking the FasR-FasL interaction by antagonistic antibodies to FasR or to FasL had no effect on P. aeruginosa-induced caspase 8 and caspase 3 activation, neither did the silencing of FasR by small interfering RNA (siRNA), suggesting that caspase 8 activation through the FADD bypasses FasR/FasL-mediated signalling. Thus, FADD-mediated caspase 8 activation involves intracellular ExoS in an ADPRT-dependent manner. Furthermore, silencing of caspase 8 by siRNA did not interfere with P. aeruginosa-induced apoptosis, whereas it rendered HeLa cells markedly increased resistance towards FasL-induced apoptosis. In conclusion, our findings indicate that ExoS of P. aeruginosa induces apoptosis through a mechanism that is independent of Fas receptor/caspase 8 pathway.

Lipopolysaccharide signaling in endothelial cells

Sepsis is the systemic immune response to severe bacterial infection. The innate immune recognition of bacterial and viral products is mediated by a family of transmembrane receptors known as Toll-like receptors (TLRs). In endothelial cells, exposure to lipopolysaccharide (LPS), a major cell wall constituent of Gram-negative bacteria, results in endothelial activation through a receptor complex consisting of TLR4, CD14 and MD2. Recruitment of the adaptor protein myeloid differentiation factor (MyD88) initiates an MyD88-dependent pathway that culminates in the early activation of nuclear factor-kappaB (NF-kappaB) and the mitogen-activated protein kinases. In parallel, a MyD88-independent pathway results in a late-phase activation of NF-kappaB. The outcome is the production of various proinflammatory mediators and ultimately cellular injury, leading to the various vascular sequelae of sepsis. This review will focus on the signaling pathways initiated by LPS binding to the TLR4 receptor in endothelial cells and the coordinated regulation of this pathway.

The role of toll-like receptors in the pathogenesis and treatment of dermatological disease

Toll-like receptors (TLR) are crucial players in the innate immune response to microbial invaders. These receptors are expressed on immune cells, such as monocytes, macrophages, dendritic cells, and granulocytes. Importantly, TLR are not only expressed by peripheral blood cells, but their expression has been demonstrated in airway epithelium and skin, important sites of host-pathogen interaction. Host cells expressing TLR are capable of recognizing conserved pathogen-associated molecular patterns, such as lipopolysaccharide and CpG DNA, and their activation triggers signaling pathways that result in the expression of immune response genes and cytokine production. As TLR are instrumental in both launching innate immune responses and influencing adaptive immunity, regulation of TLR expression at sites of disease such as in leprosy, acne, and psoriasis may be important in the pathophysiology of these diseases. Furthermore, since TLR are vital players in infectious and inflammatory diseases, they have been identified as potential therapeutic targets. Indeed, synthetic TLR agonists such as imiquimod have already established utility in treating viral pathogens and skin cancers. In the future, it seems possible there may also be drugs capable of blocking TLR activation and thus TLR-dependent inflammatory responses, providing new treatment options for inflammatory diseases.

The role of endothelial cell apoptosis in inflammatory and immune diseases

The integrity of the endothelial lining of the vasculature is essential for vascular homeostasis and normal organ function. Endothelial injury or dysfunction has been implicated in the pathogenesis of diverse vascular diseases. Studies in vitro have demonstrated that a wide variety of stimuli can induce programmed cell death (apoptosis) of endothelial cells, and have suggested that apoptosis could be an important mechanism of vascular injury, resulting in vascular leak, inflammation, and coagulation. In this review, we focus on the potential role of endothelial apoptosis in the initiation and progression of inflammatory and immune disorders, reviewing human diseases and in vivo models in which endothelial cell apoptosis has been demonstrated. Although endothelial cell apoptosis is observed in many inflammatory and immune disorders, we find that there is, as yet, only limited experimental evidence demonstrating that it is critical to the pathogenesis of disease.

Negative regulation of toll-like receptor-mediated immune responses

Toll-like receptors (TLRs) are involved in host defence against invading pathogens, functioning as primary sensors of microbial products and activating signalling pathways that induce the expression of immune and pro-inflammatory genes. However, TLRs have also been implicated in several immune-mediated and inflammatory diseases. As the immune system needs to constantly strike a balance between activation and inhibition to avoid detrimental and inappropriate inflammatory responses, TLR signalling must be tightly regulated. Here, we discuss the various negative regulatory mechanisms that have evolved to attenuate TLR signalling to maintain this immunological balance.

Characterization of bovine FAS-associated death domain gene

The FAS-associated death domain (FADD) protein is an adapter/signaling molecule that has been shown to function in human cells to promote apoptosis and to inhibit NF-kappaB activation. Because of the critical role that apoptosis and NF-kappaB play in a variety of disease states, we mapped the bovine FADD gene, sequenced bovine FADD cDNA, and characterized its expression in endothelial cells (EC). Sequencing of bovine FADD revealed approximately 65 and 58% amino acid sequence identity to its human and murine homologues, respectively. Bovine FADD was mapped to chromosome 29 by radiation hybrid mapping. In addition, the functionality of bovine FADD was studied. Expression of a bovine FADD dominant-negative construct blocked bacterial lipopolysaccharide (LPS)- and TNF-alpha-induced apoptosis in bovine EC consistent with previous studies of human FADD. In contrast to human FADD, elevated expression of bovine FADD had no effect on LPS- or TNF-alpha-induced upregulation of NF-kappaB-dependent gene products as assayed by E-selectin expression. Thus, while the role of FADD in mediating apoptosis is conserved across species, its role in regulating NF-kappaB-dependent gene expression is not.

The expression and roles of Toll-like receptors in the biology of the human neutrophil

Neutrophils are amongst the first immune cells to arrive at sites of infection, where they initiate antimicrobial and proinflammatory functions, which serve to contain infection. Sensing and defeating microbial infections are daunting tasks as a result of their molecular heterogeneity; however, Toll-like receptors (TLRs) have emerged as key components of the innate-immune system, activating multiple steps in the inflammatory reaction, eliminating invading pathogens, and coordinating systemic defenses. Activated neutrophils limit infection via the phagocytosis of pathogens and by releasing antimicrobial peptides and proinflammatory cytokines and generating reactive oxygen intermediates. Through the production of chemokines, they additionally recruit and activate other immune cells to aid the clearance of the microbes and infected cells and ultimately, mount an adaptive immune response. In acute inflammation, influx of neutrophils from the circulation leads to extremely high cell numbers within tissues, which is exacerbated by their delayed, constitutive apoptosis caused by local inflammatory mediators, potentially including TLR agonists. Neutrophil apoptosis and safe removal by phagocytic cells limit tissue damage caused by release of neutrophil cytotoxic granule contents. This review addresses what is currently known about the function of TLRs in the biology of the human neutrophil, including the regulation of TLR expression, their roles in cellular recruitment and activation, and their ability to delay apoptotic cell death.

Targeted expression of maspin in tumor vasculatures induces endothelial cell apoptosis

Angiogenesis, the formation of new blood vessels, is required for normal tissue development and pathological conditions such as tumorigenesis. Most solid tumors can not grow beyond a few millimeters without the recruitment of neovessels since cancer cells require access to blood vessels for nutrients and to escape the local environment and metastasize to other tissue and organ sites. Targeting tumor vessel endothelium therefore should serve as an effective therapy for cancers. Maspin is a serpin that exhibits antiangiogenic properties. In this report, we show that when maspin overexpression is targeted in vivo to endothelial cells, it actively induces endothelial cell apoptosis. Intravascular administration of adenovirus-maspin to mice bearing mammary tumors disrupts tumor-induced angiogenesis. Interestingly, tumor neovessels become leaky after maspin treatment, whereas normal mature vessels are not affected by maspin treatment. We further demonstrate that maspin directly induces endothelial cell apoptosis in vitro, and this effect is maspin specific. The induction of apoptosis is accompanied by changes in the expression of Bcl-2 family genes and is blocked by caspase inhibitors. In addition, the apoptotic effect is mediated by intracellular maspin and is dependent on the RSL region of maspin. Furthermore, we have shown that transient overexpression of Bcl-2 protected the HUVECs from maspin-mediated apoptosis, and the presence of both maspin and Bax accelerated the apoptosis process. These findings demonstrate that neovascular endothelial cells are highly sensitive to maspin level inside the cells. This property can be used for targeted therapy against tumor angiogenesis and metastasis.

The protein structures that shape caspase activity, specificity, activation and inhibition

The death morphology commonly known as apoptosis results from a post-translational pathway driven largely by specific limited proteolysis. In the last decade the structural basis for apoptosis regulation has moved from nothing to 'quite good', and we now know the fundamental structures of examples from the initiator phase, the pre-mitochondrial regulator phase, the executioner phase, inhibitors and their antagonists, and even the structures of some substrates. The field is as well advanced as the best known of proteolytic pathways, the coagulation cascade. Fundamentally new mechanisms in protease regulation have been disclosed. Structural evidence suggests that caspases have an unusual catalytic mechanism, and that they are activated by apparently unrelated events, depending on which position in the apoptotic pathway they occupy. Some naturally occurring caspase inhibitors have adopted classic inhibition strategies, but other have revealed completely novel mechanisms. All of the structural and mechanistic information can, and is, being applied to drive therapeutic strategies to combat overactivation of apoptosis in degenerative disease, and underactivation in neoplasia. We present a comprehensive review of the caspases, their regulators and inhibitors from a structural and mechanistic point of view, and with an aim to consolidate the many threads that define the rapid growth of this field.

FLICE-like inhibitory protein (FLIP) protects against apoptosis and suppresses NF-kappaB activation induced by bacterial lipopolysaccharide

Bacterial lipopolysaccharide (LPS) via its activation of Toll-like receptor-4 contributes to much of the vascular injury/dysfunction associated with gram-negative sepsis. Inhibition of de novo gene expression has been shown to sensitize endothelial cells (EC) to LPS-induced apoptosis, the onset of which correlates with decreased expression of FLICE-like inhibitory protein (FLIP). We now have data that conclusively establish a role for FLIP in protecting EC against LPS-induced apoptosis. Overexpression of FLIP protected against LPS-induced apoptosis, whereas down-regulation of FLIP using antisense oligonucleotides sensitized EC to direct LPS killing. Interestingly, FLIP overexpression suppressed NF-kappaB activation induced by LPS, but not by phorbol ester, suggesting a specific role for FLIP in mediating LPS activation. Conversely, mouse embryo fibroblasts (MEF) obtained from FLIP -/- mice showed enhanced LPS-induced NF-kappaB activation relative to those obtained from wild-type mice. Reconstitution of FLIP-/- MEF with full-length FLIP reversed the enhanced NF-kappaB activity elicited by LPS in the FLIP -/- cells. Changes in the expression of FLIP had no demonstrable effect on other known LPS/Tlr-4-activated signaling pathways including the p38, Akt, and Jnk pathways. Together, these data support a dual role for FLIP in mediating LPS-induced apoptosis and NF-kappaB activation.

C1 inhibitor prevents Gram-negative bacterial lipopolysaccharide-induced vascular permeability

Gram-negative bacterial endotoxemia may lead to the pathological increase of vascular permeability with systemic vascular collapse, a vascular leak syndrome, multiple organ failure (MOF), and/or shock. Previous studies demonstrated that C1 inhibitor (C1INH) protects mice from lipopolysaccharide (LPS)-induced lethal septic shock via a direct interaction with LPS. Here, we report that C1INH blocked the LPS-induced increase in transendothelial flux through an endothelial monolayer. In addition, LPS-mediated detachment of cultured endothelial cells was prevented with C1INH. C1INH also inhibited LPS-induced endothelial cell apoptosis as demonstrated by suppression of DNA fragmentation and annexin V expression. As illustrated by laser scanning confocal microscopy, C1INH completely blocked the binding of fluorescein isothiocyanate (FITC)-LPS to human umbilical vein endothelial cells (HUVECs). C1INH protected from localized LPS-induced increased plasma leakage in C57BL/6J mice and in C1INH-deficient mice. Local vascular permeability in response to LPS was increased to a greater extent in C1INH-deficient mice compared with wild-type littermate controls and was reversed by treatment with C1INH. Systemic administration of LPS to mice resulted in increased vascular permeability, which was reduced by C1INH. Therefore, these studies demonstrate that C1INH, in addition to its role in suppression of LPS-mediated macrophage activation, may play an important role in the prevention of LPS-mediated increased vascular permeability, endothelial cell injury, and multiple organ failure.

Sustained lipopolysaccharide-induced lung inflammation in mice is attenuated by functional deficiency of the Fas/Fas ligand system

To determine whether the Fas/Fas ligand (FasL) (CD95/CD178) system contributes to the development of an inflammatory response in vivo, 2.5 microg of bacterial lipopolysaccharide (LPS; endotoxin) per g was administered intranasally to healthy mice (C57BL/6) and mutant mice deficient in either Fas (lpr mice) or FasL (gld mice). Sustained LPS-induced neutrophilic inflammation in the lungs was attenuated in both lpr and gld mice. These observations provide further evidence of a proinflammatory role for the Fas/FasL system in the lungs.

Adrenomedullin and adrenomedullin binding protein-1 attenuate vascular endothelial cell apoptosis in sepsis

OBJECTIVE

To determine whether vascular endothelial cell apoptosis occurs in the late stage of sepsis and, if so, whether administration of a potent vasodilatory peptide adrenomedullin and its newly reported specific binding protein (AM/AMBP-1) prevents sepsis-induced endothelial cell apoptosis.

SUMMARY BACKGROUND DATA

Polymicrobial sepsis is characterized by an early, hyperdynamic phase followed by a late, hypodynamic phase. Our recent studies have shown that administration of AM/AMBP-1 delays or even prevents the transition from the hyperdynamic phase to the hypodynamic phase of sepsis, attenuates tissue injury, and decreases sepsis-induced mortality. However, the mechanisms responsible for the beneficial effects of AM/AMBP-1 in sepsis remain unknown.

METHODS

Polymicrobial sepsis was induced by cecal ligation and puncture in adult male rats. Human AMBP-1 (40 microg/kg body weight) was infused intravenously at the beginning of sepsis for 20 minutes and synthetic AM (12 microg/kg body weight) was continuously administered for the entire study period using an Alzert micro-osmotic pump, beginning 3 hours prior to the induction of sepsis. The thoracic aorta and pulmonary tissues were harvested at 20 hours after cecal ligation and puncture (ie, the late stage of sepsis). Apoptosis was determined using TUNEL assay, M30 Cytodeath immunostaining, and electromicroscopy. In addition, anti-apoptotic Bcl-2 and pro-apoptotic Bax gene expression and protein levels were assessed by RT-PCR and Western blot analysis, respectively.

RESULTS

Vascular endothelial cells underwent apoptosis formation at 20 hours after cecal ligation and puncture as determined by three different methods. Moreover, partial detached endothelial cell in the aorta was observed. Bcl-2 mRNA and protein levels decreased significantly at 20 hours after the onset of sepsis while Bax was not altered. Administration of AM/AMBP-1 early after sepsis, however, significantly reduced the number of apoptotic endothelial cells. This was associated with significantly increased Bcl-2 protein levels and decreased Bax gene expression in the aortic and pulmonary tissues.

CONCLUSION

The above results suggest that vascular endothelial cell apoptosis occurs in late sepsis and the anti-apoptotic effects of AM/AMBP-1 appear to be in part responsible for their beneficial effects observed under such conditions.

Short-term delay of Fas-stimulated apoptosis by GM-CSF as a result of temporary suppression of FADD recruitment in neutrophils: evidence implicating phosphatidylinositol 3-kinase and MEK1-ERK1/2 pathways downstream of classical protein kinase C

Granulocyte/macrophage colony-stimulating factor (GM-CSF) inhibits Fas-induced apoptosis of neutrophils. However, the exact step in the apoptotic pathway blocked by GM-CSF remained unclear. Here, we found that pretreatment of neutrophils with GM-CSF inhibits the recruitment of Fas-associated protein with death domain (FADD) to Fas, abolishing the formation of the death-inducing signaling complex required for Fas-induced apoptosis. Two-dimensional electrophoresis revealed that GM-CSF modifies the ratio of FADD subspecies. These GM-CSF-triggered changes were abrogated, and Fas-induced apoptosis was restored by an inhibitor of classical protein kinase C (PKC), Go6976, and by the combination of a phosphatidylinositol 3-kinase (PI-3K) inhibitor, LY294002, and an inhibitor of mitogen-activated protein kinase kinase (MEK)1, PD98059. Go6976 blocked GM-CSF-elicited phosphorylation of Akt/PKB and extracellular signal-regulated kinase (ERK)1/2. These results indicated that GM-CSF suppresses Fas-induced neutrophil apoptosis by inhibiting FADD binding to Fas, through redundant actions of PI-3K and MEK1-ERK1/2 pathways downstream of classical PKC.

Molecular strategies to treat vascular diseases: circulating vascular progenitor cell as a potential target for prophylactic treatment of atherosclerosis

Atherosclerosis is responsible for more than half of all deaths in Western countries. Numerous studies have reported that accumulation of smooth muscle cells (SMCs) plays a principal role in atherogenesis, post-angioplasty restenosis and transplantation-associated vasculopathy. Although much effort has been devoted to targeting the migration and proliferation of medial SMCs, effective therapy to prevent occlusive vascular remodeling has not been established. Recently, it was suggested that bone marrow-derived precursors can give rise to vascular cells that contribute to the repair, remodeling, and lesion formation of the arterial wall under certain circumstances. This review highlights the recent findings on circulating vascular precursors and describes the potential therapeutic strategies for vascular diseases, targeting mobilization, homing, differentiation and proliferation of circulating progenitor cells.

Lipopolysaccharide initiates a TRAF6-mediated endothelial survival signal

Similar to tumor necrosis factor (TNF), bacterial lipopolysaccharide (LPS) elicits parallel apoptotic and antiapoptotic pathways in endothelial cells. The overall result is that there is minimal endothelial cell death in response to LPS without inhibition of the cytoprotective pathway. While the TNF-induced death and survival pathways have been relatively well elucidated, much remains to be learned about LPS signaling events in this regard. It is known that the transcription factor nuclear factor-κB (NF-κB) provides a critical cell survival signal in response to TNF, but is not an essential component of the LPS-induced survival pathway. The TNF receptor-associated factor 6 (TRAF6) is a major effector of multiple LPS-induced signals, including a c-Jun N-terminal kinase (JNK)-mediated apoptotic response. In this report we demonstrate that following LPS stimulation, TRAF6 also transmits an important endothelial cell survival signal in a situation of complete NF-κB blockade. In response to LPS, TRAF6 activates the phosphatidylinositol 3′-kinase (PI3K)/Akt pathway, but not ERK1/2 mitogen-activated protein kinases (MAPKs) in endothelial cells. Activation of PI3K signals a critical antiapoptotic pathway in response to LPS in endothelial cells, whereas ERK1/2 does not. Thus TRAF6 acts as a bifurcation point of the LPS-initiated death and survival signals in endothelial cells. (Blood. 2004;103:4520-4526)

Mechanisms involved in apoptosis of human macrophages induced by lipopolysaccharide from Actinobacillus actinomycetemcomitans in the presence of cycloheximide

Actinobacillus actinomycetemcomitans is a major periodontopathic bacterium with multiple virulence factors, including lipopolysaccharide (LPS). Previous reports have demonstrated that LPS induced apoptosis in a murine macrophage-like cell line, J744.1, as well as in peritoneal macrophages from C3H/HeN mice in the presence of cycloheximide (CHX). However, the detailed molecular mechanisms involved in the apoptosis of macrophages induced by LPS and CHX are not well known. To clarify the possible role of LPS in the induction of macrophage apoptosis, we investigated cell death induced by LPS from A. actinomycetemcomitans and CHX in human macrophage-like U937 cells, which were differentiated by 12-O-tetradecanoylphorbol 13-acetate (TPA), and also assessed the molecular mechanisms involved in the process. We found that TPA-differentiated U937 cells usually showed resistance to LPS-induced apoptosis. However, in the presence of CHX, LPS induced release of cytochrome c without modifying steady-state levels of Bcl-2, Bcl-xL, Bax, and Bak. Treatment with LPS in the presence of CHX also led to activation of caspase-3 and apoptosis via, in part, the CD14/toll-like receptor 4 (TLR4). The induction of cytochrome c release may have been due to dephosphorylation of Akt and Bad, which were cooperatively induced by CHX and LPS. However, endogenous tumor necrosis factor alpha- and Fas-induced signals, extracellular signal-regulated kinase kinase/mitogen-activated protein kinases and I-kappa B alpha/nuclear factor-kappa B (NF-kappa B) were not required for caspase-3-dependent apoptosis. These results emphasize the possible important role of the mitochondrial apoptotic pathway leading to caspase-3 activation in LPS-induced apoptosis of human macrophages in the presence of CHX.

C5a-induced gene expression in human umbilical vein endothelial cells

The endothelium plays a critical role in the inflammatory process. The complement activation product, C5a, is known to have proinflammatory effects on the endothelium, but the molecular mechanisms remain unclear. We have used cDNA microarray analysis to assess gene expression in human umbilical vein endothelial cells (HUVECs) that were stimulated with human C5a in vitro. Chip analyses were confirmed by reverse transcriptase-polymerase chain reaction and by Western blot analysis. Gene activation responses were remarkably similar to gene expression patterns of HUVECs stimulated with human tumor necrosis factor-alpha or bacterial lipopolysaccharide. HUVECs stimulated with C5a showed progressive increases in gene expression for cell adhesion molecules (eg, E-selectin, ICAM-1, VCAM-1), cytokines/chemokines, and related receptors (eg, VEGFC, IL-6, IL-18R). Surprisingly, HUVECs showed little evidence for up-regulation of complement-related genes. There were transient increases in gene expression associated with broad functional activities. The three agonists used also caused down-regulation of genes that regulate angiogenesis and drug metabolism. With a single exception, C5a caused little evidence of activation of complement-related genes. These studies indicate that endothelial cells respond robustly to C5a by activation of genes related to progressive expression of cell adherence molecules, and cytokines and chemokines in a manner similar to responses induced by tumor necrosis factor-alpha and lipopolysaccharide.

Keratinocyte growth factor 1 inhibits wound edge epithelial cell apoptosis in vitro

The ability of keratinocyte growth factor 1 to modulate apoptosis in the absence of proliferation was studied in vitro. A HaCaT scrape wound model was developed in which dense monolayers prior to wounding were cultured to quiescence in defined media with hydroxyurea at concentrations that blocked proliferation without loss of cell viability. Scrape wounding was then found to induce apoptosis, originating at the wound edge, but subsequently radiating away over a 24 h period to encompass areas not originally damaged. Keratinocyte growth factor 1 inhibited this radial progression of apoptosis in a concentration-dependent manner up to 20 ng per mL with induced migration present at the wound edge. The extent of this rescue was modulated by the concentration of Ca2+ prior to wounding. In control wound cultures apoptotic bodies were found in cells adjacent to the wound interface but were greatly reduced in keratinocyte-growth-factor-1-treated groups. Keratinocyte growth factor 1 receptor expression was significantly induced within two to three cell widths of the scraped wound edge, at levels far exceeding those found at the leading edge of a nonwounded epithelial sheet. Tumor necrosis factor alpha (1-5 ng per mL) or Escherichia coli lipopolysaccharide (10-50 ng per mL) exacerbated scrape-induced early apoptosis (1-4 h), but was largely ameliorated by coculture with keratinocyte growth factor 1. Keratinocyte growth factor 1 protection was associated with a reduction in both caspase-3 activation and cytokeratin-19 loss. Protected wound edges were also associated with the maintenance of e-cadherin expression and induction of beta1 integrin and actin stress fiber organization. These results suggest that keratinocyte growth factor 1 may play a role in limiting mechanically induced apoptotic processes at the epithelial wound edge in a manner that is distinct from its proliferative function.

A dominant role of Toll-like receptor 4 in the signaling of apoptosis in bacteria-faced macrophages

Conserved bacterial components potently activate host immune cells through transmembrane Toll-like receptors (TLRs), which trigger a protective immune response but also may signal apoptosis. In this study, we investigated the roles of TLR2 and TLR4 as inducers of apoptosis in Yersinia enterocolitica-infected macrophages. Yersiniae suppress activation of the antiapoptotic NF-kappaB signaling pathway in host cells by inhibiting inhibitory kappaB kinase-beta. This leads to macrophage apoptosis under infection conditions. Experiments with mouse macrophages deficient for TLR2, TLR4, or both receptors showed that, although yersiniae could activate signaling through both TLR2 and TLR4, loss of TLR4 solely diminished Yersinia-induced apoptosis. This suggests implication of TLR4, but not of TLR2, as a proapoptotic signal transducer in Yersinia-conferred cell death. In the same manner, agonist-specific activation of TLR4 efficiently mediated macrophage apoptosis in the presence of the proteasome inhibitor MG-132, an effect that was less pronounced for activation through TLR2. Furthermore, the extended stimulation of overexpressed TLR4 elicited cellular death in epithelial cells. A dominant-negative mutant of Fas-associated death domain protein could suppress TLR4-mediated cell death, which indicates that TLR4 may signal apoptosis through a Fas-associated death domain protein-dependent pathway. Together, these data show that TLR4 could act as a potent inducer of apoptosis in macrophages that encounter a bacterial pathogen.

Potentiation of trichothecene-induced leukocyte cytotoxicity and apoptosis by TNF-alpha and Fas activation

Trichothecene mycotoxins cause immunosuppression by inducing apoptosis in lymphoid tissue. Trichothecene-induced leukocyte apoptosis can be augmented by bacterial lipopolysaccharide (LPS) but the mechanisms involved in this potentiating effect are not completely understood. The objective of this study was to test the hypothesis that the trichothecene deoxynivalenol (DON, vomitoxin) can interact with LPS directly and other mediators or agonists associated with immune/inflammatory responses to induce apoptosis in primary murine leukocyte cultures. Primary leukocyte suspensions were prepared from murine thymus (TH), spleen (SP), bone marrow (BM) and Peyer's patches (PP) and then cultured with DON in the absence or presence of LPS, prostaglandin E2 (PGE2), anti-immunoglobulin (as antigen mimic), dexamethasone, Fas ligand, or TNF-alpha. Cytotoxicity and apoptosis were evaluated by MTT assay and morphologic assays, respectively. DON was found to inhibit LPS-induced proliferation and dexamethasone-induced apoptosis in SP cultures. In contrast, potentiation of DON-induced apoptosis and cytotoxicity was observed in BM cultures treated with anti-Fas and in TH cultures treated with TNF-alpha. When potentiation of DON-induced apoptosis by TNF-alpha was assessed using pharmacological inhibitors, generation of ROS, intracellular Ca2+, p38/SAPK, and caspase-3 activation were found to play roles. Taken together, these data demonstrate that LPS and its downstream mediators can interact with trichothecenes to modulate proliferative, cytotoxic and apoptotic outcomes in leukocytes in a tissue-specific manner.

A caspaselike activity is triggered by LPS and is required for survival of human dendritic cells

Bacterial endotoxin (lipopolysaccharide [LPS]) is a potent inducer of human dendritic cell (DC) maturation and survival. Here we show that immature DCs exposed to LPS trigger an early and sustained caspase-like activity, which can be blocked by zVAD (z-Val-Ala-Asp), in the absence of detectable caspase 8 and caspase 10 activation, or poly(ADP-ribose) polymerase (PARP)-cleaving activity. Preventing LPS-induced caspase-like activation in DC results in massive cell death. Importantly, triggering of the caspase-like activity is required for LPS-induced activation of extracellular signal-regulated kinases (ERKs) and for LPS-induced up-regulation of cFLIP (Fas-associating protein with death domain-like interleukin-1 beta-converting enzyme [FLICE]-like inhibitory protein). Therefore, a caspase-dependent pathway initiated by LPS controls survival of human DCs.

A role for caspase-1 in serum withdrawal-induced apoptosis of endothelial cells

Mouse lung endothelial cells (MLEC) and HUVEC were used under serum withdrawal (SW) conditions as a model of endothelial cell (EC) apoptosis. Apoptosis was quantified by time-lapse video microscopy. Mouse lung ECs from caspase-1(-/-) mice had significantly reduced rates of SW-induced apoptosis compared with wild-type mice, specifically implicating caspase-1 in proapoptotic signaling in ECs. SW conditions induced HUVEC apoptosis with concomitant activation of caspase-1. Further studies demonstrated that the caspase-1 inhibitors z-VAD and z-YVAD significantly reduced the rate of SW-induced HUVEC apoptosis. HUVEC, when transfected with caspase-1, showed a highly significant increase in apoptosis. SW was associated with increases in reactive oxygen species production that were significantly inhibited by the antioxidant N-acetyl-L-cysteine, although rates of apoptosis and caspase-1 activation were unaffected. These results demonstrate the involvement of caspase-1 in SW-induced EC apoptosis, independently of reactive oxygen species production.

Toll-like receptors

The innate immune system in drosophila and mammals senses the invasion of microorganisms using the family of Toll receptors, stimulation of which initiates a range of host defense mechanisms. In drosophila antimicrobial responses rely on two signaling pathways: the Toll pathway and the IMD pathway. In mammals there are at least 10 members of the Toll-like receptor (TLR) family that recognize specific components conserved among microorganisms. Activation of the TLRs leads not only to the induction of inflammatory responses but also to the development of antigen-specific adaptive immunity. The TLR-induced inflammatory response is dependent on a common signaling pathway that is mediated by the adaptor molecule MyD88. However, there is evidence for additional pathways that mediate TLR ligand-specific biological responses.

Beta blockade induces apoptosis in cultured capillary endothelial cells

Continuous beta blockade stimulates deposition of collagen in the pulmonary alveolar interstitium of adult rats. It also causes changes to the capillary endothelial cell compartment reminiscent of programmed cell death. To test whether beta blockade results in endothelial cell apoptosis, cultures of capillary endothelial cells were treated with both a wide-spectrum beta blocker and a beta-2-specific antagonist. Apoptosis was measured in these cultures using both terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling and annexin-V assays. Both forms of beta blockade stimulated programmed cell death in these cultures. To test whether the apoptotic effect of beta blockade was related to interstitial collagen deposition, capillary endothelial cells were cocultured with beta-blocked pulmonary fibroblast monolayers. Cocultured endothelial cells were substantially protected from apoptosis after beta blockade; coculture over plain tissue culture plastic or over exogenous collagen films had no effect on programmed cell death in endothelial cells. These results suggest that both pulmonary endothelial and interstitial cells are vulnerable to injury from beta blockade but that paracrine interactions between these cells may protect the peripheral lung from substantive damage.

Mechanisms of bacterial lipopolysaccharide-induced endothelial apoptosis

Gram-negative bacterial sepsis remains a common, life-threatening event. The prognosis for patients who develop sepsis-related complications, including the development of acute respiratory distress syndrome (ARDS), remains poor. A common finding among patients and experimental animals with sepsis and ARDS is endothelial injury and/or dysfunction. A component of the outer membrane of gram-negative bacteria, lipopolysaccharide (LPS) or endotoxin, has been implicated in the pathogenesis of much of the endothelial cell injury and/or dysfunction associated with these disease states. LPS is a highly proinflammatory molecule that elicits a wide array of endothelial responses, including the upregulation of cytokines, adhesion molecules, and tissue factor. In addition to activation, LPS induces endothelial cell death that is apoptotic in nature. This review summarizes the evidence for LPS-induced vascular endothelial injury and examines the molecular signaling pathways that activate and inhibit LPS-induced endothelial apoptosis. Furthermore, the role of apoptotic signaling molecules in mediating LPS-induced activation of endothelial cells will be considered.

The role of the endothelium in severe sepsis and multiple organ dysfunction syndrome

Severe sepsis, defined as sepsis with acute organ dysfunction, is associated with high morbidity and mortality rates. The development of novel therapies for sepsis is critically dependent on an understanding of the basic mechanisms of the disease. The pathophysiology of severe sepsis involves a highly complex, integrated response that includes the activation of a number of cell types, inflammatory mediators, and the hemostatic system. Central to this process is an alteration of endothelial cell function. The goals of this article are to (1) provide an overview of sepsis and its complications, (2) discuss the role of the endothelium in orchestrating the host response in sepsis, and (3) emphasize the potential value of the endothelium as a target for sepsis therapy.

Expression of dominant-negative Fas-associated death domain blocks human keratinocyte apoptosis and vesication induced by sulfur mustard

DNA damaging agents up-regulate levels of the Fas receptor or its ligand, resulting in recruitment of Fas-associated death domain (FADD) and autocatalytic activation of caspase-8, consequently activating the executioner caspases-3, -6, and -7. We found that human epidermal keratinocytes exposed to a vesicating dose (300 microm) of sulfur mustard (SM) exhibit a dose-dependent increase in the levels of Fas receptor and Fas ligand. Immunoblot analysis revealed that the upstream caspases-8 and -9 are both activated in a time-dependent fashion, and caspase-8 is cleaved prior to caspase-9. These results are consistent with the activation of both death receptor (caspase-8) and mitochondrial (caspase-9) pathways by SM. Pretreatment of keratinocytes with a peptide inhibitor of caspase-3 (Ac-DEVD-CHO) suppressed SM-induced downstream markers of apoptosis. To further analyze the importance of the death receptor pathway in SM toxicity, we utilized Fas- or tumor necrosis factor receptor-neutralizing antibodies or constructs expressing a dominant-negative FADD (FADD-DN) to inhibit the recruitment of FADD to the death receptor complex and block the Fas/tumor necrosis factor receptor pathway following SM exposure. Keratinocytes pretreated with Fas-blocking antibody or stably expressing FADD-DN and exhibiting reduced levels of FADD signaling demonstrated markedly decreased caspase-3 activity when treated with SM. In addition, the processing of procaspases-3, -7, and -8 into their active forms was observed in SM-treated control keratinocytes, but not in FADD-DN cells. Blocking the death receptor complex by expression of FADD-DN additionally inhibited SM-induced internucleosomal DNA cleavage and caspase-6-mediated nuclear lamin cleavage. Significantly, we further found that altering the death receptor pathway by expressing FADD-DN in human skin grafted onto nude mice reduces vesication and tissue injury in response to SM. These results indicate that the death receptor pathway plays a pivotal role in SM-induced apoptosis and is therefore a target for therapeutic intervention to reduce SM injury.

Differential usage of IkappaBalpha and IkappaBbeta in regulation of apoptosis versus gene expression

In this study we use the N-substituted benzamides declopramide (3-CPA) and N-acetyl declopramide (Na-3-CPA) to investigate the involvement of the transcription factor NF-kappaB in the induction of apoptosis and surface immunoglobulin kappa (Igkappa) expression in the mouse pre-B cell line 70Z/3. We first showed that 3-CPA-induced apoptosis at doses around 500 microM and that the 3-CPA-induced apoptosis could be suppressed by over-expression of the Bcl-2 protein. Na-3-CPA was shown to be non-apoptotic at doses up to 1-2 mM. On the other hand, Na-3-CPA inhibited LPS-induced Igkappa expression while 3-CPA had no effect. Further analysis showed that while 3-CPA inhibited breakdown of IkappaBalpha, Na-3-CPA inhibited breakdown of IkappaBbeta. In addition, we used a 70Z/3 cell line expressing a dominant negative IkappaBalpha (70Z/3(deltaNIkappaBalpha)). The 70Z/3(deltaNIkappaBalpha) cell line was shown to be more sensitive to apoptosis and cytotoxicity induced by 3-CPA as well as by LPS, probably due to a defect in NF-kappaB rescue mechanism. Taken together, our data implicate distinct roles for IkappaBalpha and IkappaBbeta in regulating various NF-kappaB activities.

Shiga-like toxin inhibition of FLICE-like inhibitory protein expression sensitizes endothelial cells to bacterial lipopolysaccharide-induced apoptosis

Shiga-like toxin (SLT) has been implicated in the pathogenesis of hemolytic uremic syndrome and its attendant endothelial cell (EC) injury. Key serotypes of Escherichia coli produce SLT-1 in addition to another highly pro-inflammatory molecule, lipopolysaccharide (LPS). It has previously been established that SLT-1 induces EC apoptosis and that LPS enhances this effect. LPS alone has no affect on human EC viability, and the mechanism for this enhancement remains unknown. In the present report, we demonstrate that SLT-1 sensitizes EC to LPS-induced apoptosis. Pretreatment with SLT-1 sensitized EC to LPS-induced apoptosis, whereas pretreatment with LPS did not influence SLT-1-induced apoptosis. SLT-1 exposure resulted in decreased expression of FLICE-like inhibitory protein (FLIP), an anti-apoptotic protein that has previously been shown to block LPS-induced apoptosis. This SLT-1-mediated decrease in FLIP expression preceded the onset of apoptosis elicited by SLT-1 alone or in combination with LPS. SLT-1-mediated decrements in FLIP expression correlated in a dose- and time-dependent manner with sensitization to LPS-induced apoptosis. Finally, transient or stable overexpression of FLIP protected against LPS enhancement of SLT-1-induced apoptosis, and this protection corresponded with sustained expression of FLIP. Together, these data suggest that SLT-1 sensitizes EC to LPS-induced apoptosis by inhibiting FLIP expression.

Lipopolysaccharide signals an endothelial apoptosis pathway through TNF receptor-associated factor 6-mediated activation of c-Jun NH2-terminal kinase

Inflammatory mediators such as TNF and bacterial LPS do not cause significant apoptosis of endothelial cells unless the expression of cytoprotective genes is blocked. In the case of TNF, the transcription factor NF-kappaB conveys an important survival signal. In contrast, even though LPS can also activate NF-kappaB, this signal is dispensable for LPS-inducible cytoprotective activity. LPS intracellular signals are transmitted through a member of the Toll-like receptor family, TLR4. This family of receptors transduces signals through a downstream molecule, TNFR-associated factor 6 (TRAF6). In this study, we demonstrate that the C-terminal fragment of TRAF6 (TRAF6-C) inhibits LPS-induced NF-kappaB nuclear translocation and c-Jun NH(2)-terminal kinase (JNK) activation in endothelial cells. In contrast, LPS activation of p38 kinase is not inhibited by TRAF6-C. TRAF6-C also inhibits LPS-initiated endothelial apoptosis, but potentiates TNF-induced apoptosis. LPS-induced loss of mitochondrial transmembrane potential, cytochrome c release, and caspase activation are all blocked by TRAF6-C. We demonstrate that TRAF6 signals apoptosis via JNK activation, since inhibition of JNK activation using a dominant-negative mutant also inhibits apoptosis. JNK inhibition blocks caspase activation, but the reverse is not true. Hence, JNK activation lies upstream of caspase activation in response to LPS stimulation.

MyD88 is involved in the signalling pathway for Taxol-induced apoptosis and TNF-alpha expression in human myelomonocytic cells

Taxol is an effective anti-tumour drug against a variety of tumour cells. Taxol directly induces apoptosis in addition to a G2/M cell cycle arrest. However, it remains poorly understood how Taxol induces apoptosis in tumour cells. Taxol induces the secretion of inflammatory cytokines in murine macrophages in a toll-like receptor-4 (TLR-4)-dependent manner in addition to its anti-tumour effects, but the effect of Taxol on human macrophages is controversial. In this study, we demonstrated that low doses (less than 1000 nmol/l) of Taxol induced the expression of tumour necrosis factor (TNF)-alpha in human myelomonocytic cells and that the induction of TNF-alpha mRNA was inhibited by dominant-negative myeloid differentiation protein (dnMyD88). Furthermore, we demonstrated that the same doses of Taxol induced apoptosis of the same myelomonocytic cells and that the Taxol-induced apoptosis was also inhibited by dnMyD88. In accordance with the previous reports, Taxol induced the expression of TNF-alpha and apoptosis in a TLR4-independent manner. These results suggest that TNF-alpha expression and apoptosis, both induced by Taxol in human myelomonocytic cells, share the signal transduction molecule MyD88.

Silica-induced apoptosis in murine macrophage: involvement of tumor necrosis factor-alpha and nuclear factor-kappaB activation

Alveolar macrophages play a critical role in silica-induced lung fibrosis. Silica exposure induces tumor necrosis factor (TNF)-alpha release and nuclear factor (NF)-kappaB activation, and apoptotic mechanisms have been implicated in silica-induced pathogenesis. To characterize potential relationships between these signaling events, we studied their induction in two murine macrophage cell lines. The RAW 264.7 macrophage cell line was more sensitive, and the IC-21 macrophage cell line more tolerant to silica exposure (0.2 or 1 mg/ml for 6 h) as evidenced by significantly higher apoptotic responses in RAW 264.7 (P < 0.05). RAW 264.7 macrophages exhibited enhanced TNF-alpha production and NF-kappaB activation in response to silica, whereas IC-21 macrophages did not produce TNF-alpha in response to silica and did not induce NF-kappaB nuclear binding. Inhibition of NF-kappaB in RAW 264.7 cells with BAY11-7082 significantly increased apoptosis while inhibiting TNF-alpha release. In addition, TNF-alpha and NF-kappaB activation, but not apoptosis, were induced by lipopolysaccharide (LPS) in both cell lines, and NF-kappaB inhibition reduced LPS-induced TNF-alpha release. These data suggest that TNF-alpha induction is dependent on NF-kappaB activation in both cell lines. However, silica can induce apoptosis in murine macrophages, independently of TNF-alpha stimulation, as in IC-21 macrophages. Furthermore, NF-kappaB activation in macrophages may play dual roles, both pro- and antiapoptotic during silica injury.

Lipopolysaccharide-induced apoptosis of endothelial cells and its inhibition by vascular endothelial growth factor

Endothelial injury is a major manifestation of septic shock induced by LPS. Recently, LPS was shown to induce apoptosis in different types of endothelial cells. In this study, we observed that pretreatment with vascular endothelial growth factor (VEGF), a known cell survival factor, blocked LPS-induced apoptosis in endothelial cells. We then further defined this LPS-induced apoptotic pathway and its inhibition by VEGF. We found that LPS treatment increased caspase-3 and caspase-1 activities and induced the cleavage of focal adhesion kinase. LPS also augmented expression of the pro-apoptotic protein Bax and the tumor suppressor gene p53. The pro-apoptotic Bax was found to translocate to the mitochondria from the cytosol following stimulation with LPS. Pretreatment of endothelial cells with VEGF inhibited the induction of both Bax and p53 as well as the activation of caspase-3. These data suggest that VEGF inhibits LPS-induced endothelial apoptosis by blocking pathways that lead to caspase activation.

Beta blockade induces apoptosis in cultured capillary endothelial cells

Continuous beta blockade stimulates deposition of collagen in the pulmonary alveolar interstitium of adult rats. It also causes changes to the capillary endothelial cell compartment reminiscent of programmed cell death. To test whether beta blockade results in endothelial cell apoptosis, cultures of capillary endothelial cells were treated with both a wide-spectrum beta blocker and a beta-2-specific antagonist. Apoptosis was measured in these cultures using both terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling and annexin-V assays. Both forms of beta blockade stimulated programmed cell death in these cultures. To test whether the apoptotic effect of beta blockade was related to interstitial collagen deposition, capillary endothelial cells were cocultured with beta-blocked pulmonary fibroblast monolayers. Cocultured endothelial cells were substantially protected from apoptosis after beta blockade; coculture over plain tissue culture plastic or over exogenous collagen films had no effect on programmed cell death in endothelial cells. These results suggest that both pulmonary endothelial and interstitial cells are vulnerable to injury from beta blockade but that paracrine interactions between these cells may protect the peripheral lung from substantive damage.

Divergence of apoptosis-inducing and preventing signals in bacteria-faced macrophages through myeloid differentiation factor 88 and IL-1 receptor-associated kinase members

The induction of apoptosis in host cells is a common strategy by which pathogenic bacteria interfere with the host immune response. The Yersinia enterocolitica outer protein P (YopP) inhibits activation of transcription factor NF-kappa B in macrophages, which suppresses NF-kappa B-dependent antiapoptotic activities. The simultaneous initiation of proapoptotic signaling by yersiniae infection or LPS treatment results in macrophage apoptosis. In this study, we used YopP as a tool to dissect survival- and death-inducing pathways in bacteria-faced macrophages. We cotransfected J774A.1 macrophages with expression plasmids for YopP and dominant-negative mutants of signal transmitters of the NF-kappa B cascade downstream from the LPS receptor complex. Dominant-negative myeloid differentiation factor 88 (MyD88) or IL-1R-associated kinase (IRAK) 2 diminished LPS-induced apoptosis in YopP-transfected macrophages, suggesting implication of MyD88 and IRAK2 in signaling cell death. In contrast, dominant-negative IRAK1 and TNFR-associated factor 6 (TRAF6) did not provide protection, but augmented LPS-mediated apoptosis in the absence of YopP, which indicates roles of IRAK1 and TRAF6 in the antiapoptotic signal relay of the NF-kappa B cascade. The distinct functions of IRAK members in macrophage survival were reflected by opposing effects of dominant-negative IRAK1 and IRAK2 on Y. enterocolitica-mediated apoptosis. Yersiniae- and LPS-dependent cell death were substantially attenuated by a specific caspase-8 inhibitory peptide or by dominant negative Fas-associated death domain protein (FADD). This suggests, that Yersinia-induced apoptosis involves a proapoptotic signal relay through MyD88 and IRAK2, which potentially targets the Fas-associated death domain protein/caspase-8 apoptotic pathway, whereas IRAK1 and TRAF6 counteract the bacteria-induced cytotoxic response by signaling macrophage survival.

Expression of transcriptional repressor ATF3/LRF1 in human atherosclerosis: colocalization and possible involvement in cell death of vascular endothelial cells

Vascular endothelial cell death contributes to the progression of atherosclerotic lesion, and several transcriptional regulators are involved in the process. Activating transcription factor 3/liver regenerating factor-1 (ATF3/LRF-1), a stress-inducible transcriptional repressor, was shown to be highly expressed in vascular endothelial cells and macrophages of human atherosclerotic lesions by immunohistological assay. The expression was colocalized in these cells which were positive for TdT-mediated dUTP nick-end labeling (TUNEL) and annexin V. Treatment of human umbilical vein endothelial cells (HUVECs) by tumor necrosis factor (TNF)-alpha, oxidized low density lipoprotein (oxLDL), and lysophosphatidylcholine (LPC) rapidly induced ATF3/LRF-1, which showed an increased DNA binding to the consensus ATF/CRE sequence by supershift of gel shift assay. Flow cytometry analysis and immunostaining analysis with TUNEL assay showed that ATF3/LRF-1 was highly expressed in cell death induced by these agents. Moreover, antisense ATF3/LRF-1 cDNA partly suppressed the cell death induced by TNF-alpha, oxLDL, and LPC. From these results, it is indicated that ATF3/LRF-1 is one of the immediate early response genes in vascular endothelial cells in response to atherogenic stimuli, and may play a role in the endothelial cell death associated with atherogenesis.

The Fas-associated death domain protein suppresses activation of NF-kappa B by LPS and IL-1 beta

Activation of NF-kappa B by bacterial LPS promotes the upregulation of proinflammatory cytokines that contribute to the pathogenesis of Gram-negative septic shock. LPS activation of NF-kappa B is dependent upon the interaction of two death domain-containing (DD-containing) proteins, MyD88 and IL-1 receptor-associated kinase IRAK. Another DD-containing protein, Fas-associated death domain (FADD), also binds MyD88 through respective DD-DD interactions. Although FADD has been classically described as a proapoptotic signaling molecule, several reports have implicated a role for FADD in mediating NF-kappa B activation. In the present report, we investigated whether FADD could mediate LPS activation of NF-kappa B. Overexpression of FADD blocked LPS-induced NF-kappa B activation, whereas absence of FADD enhanced activation of NF-kappa B by LPS. Further, LPS-induced expression of two NF-kappa B-dependent gene products, IL-6 and KC, was enhanced in FADD(-/-) mouse embryo fibroblasts (MEFs) compared with wild-type. This increase in NF-kappa B activity correlated with enhanced I kappa B degradation. FADD(-/-) MEFs were also resistant to NF-kappa B activation induced by IL-1 beta. Finally, reconstitution of full-length FADD in the FADD(-/-) MEFs completely reversed the enhanced activation of NF-kappa B elicited by either LPS or IL-1 beta. Together, these data indicate that FADD negatively regulates LPS- and IL-1 beta-induced NF-kappa B activation and that this regulation occurs upstream of I kappa B degradation.