Activation of human monocytes induces differential resistance to apoptosis with rapid down regulation of caspase-8/FLICE

Increased Circulating CD14+ Monocytes in Patients with Psoriatic Arthritis Presenting Impaired Apoptosis Activity

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis primarily affecting peripheral and axial joints. The osteolytic effect in the damaged joint is mediated by osteoclast activation. We aimed to investigate differential gene expression in peripheral CD14+ monocytes between patients with psoriatic arthritis (n = 15) and healthy controls (HCs; n = 15). Circulating CD14+ monocytes were isolated from peripheral blood mononuclear cells using CD14+ magnetic beads. Cell apoptosis was measured via Annexin V using flow cytometry. The gene expression profiling was analyzed via microarray (available in the NCBI GEO database; accession number GSE261765), and the candidate genes were validated using PCR. The results showed a higher number of peripheral CD14+ monocytes in patients with PsA than in the HCs. By analyzing the microarray data, identifying the differentially expressed genes, and conducting pathway enrichment analysis, we found that the apoptosis signaling pathway in CD14+ cells was significantly impaired in patients with PsA compared to the HCs. Among the candidate genes in the apoptotic signaling pathway, the relative expression level of cathepsin L was confirmed to be significantly lower in the PsAs than in the HCs. We concluded that the numbers of peripheral CD14+ monocytes increased, and their apoptosis activity was impaired in patients with PsA, which could lead to enhanced macrophage maturation and osteoclast activation. The resistance of apoptotic death in peripheral CD14+ monocytes may contribute to active joint inflammation in PsA.

Uncoupled pyroptosis and IL-1β secretion downstream of inflammasome signaling

Inflammasomes are supramolecular platforms that organize in response to various damage-associated molecular patterns and pathogen-associated molecular patterns. Upon activation, inflammasome sensors (with or without the help of ASC) activate caspase-1 and other inflammatory caspases that cleave gasdermin D and pro-IL-1β/pro-IL-18, leading to pyroptosis and mature cytokine secretion. Pyroptosis enables intracellular pathogen niche disruption and intracellular content release at the cost of cell death, inducing pro-inflammatory responses in the neighboring cells. IL-1β is a potent pro-inflammatory regulator for neutrophil recruitment, macrophage activation, and T-cell expansion. Thus, pyroptosis and cytokine secretion are the two main mechanisms that occur downstream of inflammasome signaling; they maintain homeostasis, drive the innate immune response, and shape adaptive immunity. This review aims to discuss the possible mechanisms, timing, consequences, and significance of the two uncoupling preferences downstream of inflammasome signaling. While pyroptosis and cytokine secretion may be usually coupled, pyroptosis-predominant and cytokine-predominant uncoupling are also observed in a stimulus-, cell type-, or context-dependent manner, contributing to the pathogenesis and development of numerous pathological conditions such as cryopyrin-associated periodic syndromes, LPS-induced sepsis, and Salmonella enterica serovar Typhimurium infection. Hyperactive cells consistently release IL-1β without LDH leakage and pyroptotic death, thereby leading to prolonged inflammation, expanding the lifespans of pyroptosis-resistant neutrophils, and hyperactivating stimuli-challenged macrophages, dendritic cells, monocytes, and specific nonimmune cells. Death inflammasome activation also induces GSDMD-mediated pyroptosis with no IL-1β secretion, which may increase lethality in vivo. The sublytic GSDMD pore formation associated with lower expressions of pyroptotic components, GSDMD-mediated extracellular vesicles, or other GSDMD-independent pathways that involve unconventional secretion could contribute to the cytokine-predominant uncoupling; the regulation of caspase-1 dynamics, which may generate various active species with different activities in terms of GSDMD or pro-IL-1β, could lead to pyroptosis-predominant uncoupling. These uncoupling preferences enable precise reactions to different stimuli of different intensities under specific conditions at the single-cell level, promoting cooperative cell and host fate decisions and participating in the pathogen "game". Appropriate decisions in terms of coupling and uncoupling are required to heal tissues and eliminate threats, and further studies exploring the inflammasome tilt toward pyroptosis or cytokine secretion may be helpful.

Temporal Analysis Reveals the Transient Differential Expression of Transcription Factors That Underlie the Trans-Differentiation of Human Monocytes to Macrophages

The activation of monocytes and their trans-differentiation into macrophages are critical processes of the immune response. Prior work has characterized the differences in the expression between monocytes and macrophages, but the transitional process between these cells is poorly detailed. Here, we analyzed the temporal changes of the transcriptome during trans-differentiation of primary human monocytes into M0 macrophages. We find changes with many transcription factors throughout the process, the vast majority of which exhibit a maximally different expression at the intermediate stages. A few factors, including AP-1, were previously known to play a role in immunological transitions, but most were not. Thus, these findings indicate that this trans-differentiation requires the dynamic expression of many transcription factors not previously discussed in immunology, and provide a foundation for the delineation of the molecular mechanisms associated with healthy or pathological responses that involve this transition.

Nociceptin/orphanin FQ opioid receptor (NOP) selective ligand MCOPPB links anxiolytic and senolytic effects

Accumulation of senescent cells may drive age-associated alterations and pathologies. Senolytics are promising therapeutics that can preferentially eliminate senescent cells. Here, we performed a high-throughput automatized screening (HTS) of the commercial LOPAC®Pfizer library on aphidicolin-induced senescent human fibroblasts, to identify novel senolytics. We discovered the nociceptin receptor FQ opioid receptor (NOP) selective ligand 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole (MCOPPB, a compound previously studied as potential anxiolytic) as the best scoring hit. The ability of MCOPPB to eliminate senescent cells in in vitro models was further tested in mice and in C. elegans. MCOPPB reduced the senescence cell burden in peripheral tissues but not in the central nervous system. Mice and worms exposed to MCOPPB also exhibited locomotion and lipid storage changes. Mechanistically, MCOPPB treatment activated transcriptional networks involved in the immune responses to external stressors, implicating Toll-like receptors (TLRs). Our study uncovers MCOPPB as a NOP ligand that, apart from anxiolytic effects, also shows tissue-specific senolytic effects.

Role of Kynurenine Pathway in Oxidative Stress during Neurodegenerative Disorders

Neurodegenerative disorders are chronic and life-threatening conditions negatively affecting the quality of patients’ lives. They often have a genetic background, but oxidative stress and mitochondrial damage seem to be at least partly responsible for their development. Recent reports indicate that the activation of the kynurenine pathway (KP), caused by an activation of proinflammatory factors accompanying neurodegenerative processes, leads to the accumulation of its neuroactive and pro-oxidative metabolites. This leads to an increase in the oxidative stress level, which increases mitochondrial damage, and disrupts the cellular energy metabolism. This significantly reduces viability and impairs the proper functioning of central nervous system cells and may aggravate symptoms of many psychiatric and neurodegenerative disorders. This suggests that the modulation of KP activity could be effective in alleviating these symptoms. Numerous reports indicate that tryptophan supplementation, inhibition of KP enzymes, and administration or analogs of KP metabolites show promising results in the management of neurodegenerative disorders in animal models. This review gathers and systematizes the knowledge concerning the role of metabolites and enzymes of the KP in the development of oxidative damage within brain cells during neurodegenerative disorders and potential strategies that could reduce the severity of this process.

The gasdermins, a protein family executing cell death and inflammation

The gasdermins are a family of recently identified pore-forming effector proteins that cause membrane permeabilization and pyroptosis, a lytic pro-inflammatory type of cell death. Gasdermins contain a cytotoxic N-terminal domain and a C-terminal repressor domain connected by a flexible linker. Proteolytic cleavage between these two domains releases the intramolecular inhibition on the cytotoxic domain, allowing it to insert into cell membranes and form large oligomeric pores, which disrupts ion homeostasis and induces cell death. Gasdermin-induced pyroptosis plays a prominent role in many hereditary diseases and (auto)inflammatory disorders as well as in cancer. In this Review, we discuss recent developments in gasdermin research with a focus on mechanisms that control gasdermin activation, pore formation and functional consequences of gasdermin-induced membrane permeabilization.

Whole blood assay as a model for in vitro evaluation of inflammasome activation and subsequent caspase-mediated interleukin-1 beta release

Processing of pro-interleukin (IL)-1β and IL-18 is regulated by multiprotein complexes, known as inflammasomes. Inflammasome activation results in generation of bioactive IL-1β and IL-18, which can exert potent pro-inflammatory effects. Our aim was to develop a whole blood-based assay to study the inflammasome in vitro and that also can be used as an assay in clinical studies. We show whole blood is a suitable milieu to study inflammasome activation in primary human monocytes. We demonstrated that unprocessed human blood cells can be stimulated to activate the inflammasome by the addition of adenosine 5'-triphosphate (ATP) within a narrow timeframe following lipopolysaccharide (LPS) priming. Stimulation with LPS resulted in IL-1β release; however, addition of ATP is necessary for "full-blown" inflammasome stimulation resulting in high IL-1β and IL-18 release. Intracellular cytokine staining demonstrated monocytes are the major producers of IL-1β in human whole blood cultures, and this was associated with activation of caspase-1/4/5, as detected by a fluorescently labelled caspase-1/4/5 probe. By applying caspase inhibitors, we show that both the canonical inflammasome pathway (via caspase-1) as well as the non-canonical inflammasome pathway (via caspases-4 and 5) can be studied using this whole blood-based model.

Contribution of innate immune cells to pathogenesis of severe influenza virus infection

Influenza A viruses (IAVs) cause respiratory illness of varying severity based on the virus strains, host predisposition and pre-existing immunity. Ultimately, outcome and recovery from infection rely on an effective immune response comprising both innate and adaptive components. The innate immune response provides the first line of defence and is crucial to the outcome of infection. Airway epithelial cells are the first cell type to encounter the virus in the lungs, providing antiviral and chemotactic molecules that shape the ensuing immune response by rapidly recruiting innate effector cells such as NK cells, monocytes and neutrophils. Each cell type has unique mechanisms to combat virus-infected cells and limit viral replication, however their actions may also lead to pathology. This review focuses how innate cells contribute to protection and pathology, and provides evidence for their involvement in immune pathology in IAV infections.

Human caspase-4 and caspase-5 regulate the one-step non-canonical inflammasome activation in monocytes

Monocytes promote the early host response to infection releasing key pro-inflammatory cytokines, such as IL-1β. The biologically inactive IL-1β precursor is processed to active form by inflammasomes, multi-protein complexes activating caspase-1. Human monocytes exhibit an unconventional one-step pathway of inflammasome activation in response to lipopolysaccharide (LPS) alone. Although this lineage-restricted mechanism is likely to contribute to the pathology of endotoxin shock, signalling pathways regulating this mechanism are currently unknown. Here we report that caspase-4 and caspase-5 mediate IL-1α and IL-1β release from human monocytes after LPS stimulation. Although caspase-4 remains uncleaved, caspase-5 undergoes rapid processing upon LPS treatment. We also identify an additional caspase-5 cleavage product in LPS-stimulated monocytes, which correlates with IL-1 secretion. This one-step pathway requires Syk activity and Ca²⁺ flux instigated by CD14/TLR4-mediated LPS internalization. Identification of caspase-4/5 as the key determinants of one-step inflammasome activation in human monocytes provides potential targets for therapeutic intervention in endotoxin shock.

Human monocytes exhibit an unconventional one-step pathway of inflammasome activation and IL-1 release in response to LPS. Here the authors show that it is mediated by caspases 4 and 5, and characterize caspase 5 cleavage, Syk and calcium signalling as key mediators of this pathway.

Fibronectin peptides as potential regulators of hepatic fibrosis through apoptosis of hepatic stellate cells

The turnover of extracellular matrix (ECM) components can generate signals that regulate several cellular functions such as proliferation, differentiation, and apoptosis. During liver injury, matrix metalloproteases (MMPs) production is enhanced and increased levels of peptides derived from extracellular matrix proteins can be generated. Synthetic peptides with sequences present in extracellular matrix proteins were previously found to induce both stimulating and apoptotic effects on several cell types including the inflammatory cells monocytes/macrophages. Therefore, in inflammatory liver diseases, locally accumulated peptides could be also important in regulating hepatic fibrosis by inducing apoptosis of hepatic stellate cells (HSC), the primary cellular source of extracellular matrix components. Here, we describe the apoptotic effect of fibronectin peptides on the cell line of human hepatic stellate cells LX-2 based on oligonucleosomal DNA fragmentation, caspase-3 and -9 activation, Bcl-2 depletion, and accumulation of Bax protein. We also found that these peptides trigger the activation of Src kinase, which in turn mediated the increase of JNK and p38 activities. By the use of specific inhibitors we demonstrated the involvement of Src, JNK, and p38 in apoptosis induced by fibronectin peptides on HSC. Moreover, fibronectin peptides increased iNOS expression in human HSC, and specific inhibition of iNOS significantly reduced the sustained activity of JNK and the programmed cell death caused by these peptides. Finally, the possible regulatory effect of fibronectin peptides in liver fibrosis was further supported by the ability of these peptides to induce metalloprotease-9 (MMP-9) expression in human monocytes.

Granzyme B expression is enhanced in human monocytes by TLR8 agonists and contributes to antibody-dependent cellular cytotoxicity

FcγRs are critical mediators of mAb cancer therapies, because they drive cytotoxic processes upon binding of effector cells to opsonized targets. Along with NK cells, monocytes are also known to destroy Ab-coated targets via Ab-dependent cellular cytotoxicity (ADCC). However, the precise mechanisms by which monocytes carry out this function have remained elusive. In this article, we show that human monocytes produce the protease granzyme B upon both FcγR and TLR8 activation. Treatment with TLR8 agonists elicited granzyme B and also enhanced FcγR-mediated granzyme B production in an additive fashion. Furthermore, monocyte-mediated ADCC against cetuximab-coated tumor targets was enhanced by TLR8 agonist treatment, and this enhancement of ADCC required granzyme B. Hence we have identified granzyme B as an important mediator of FcγR function in human monocytes and have uncovered another mechanism by which TLR8 agonists may enhance FcγR-based therapies.

Deficient spontaneous in vitro apoptosis and increased tmTNF reverse signaling-induced apoptosis of monocytes predict suboptimal therapeutic response of rheumatoid arthritis to TNF inhibition

Introduction

In vitro apoptosis of peripheral monocytes in rheumatoid arthritis (RA) is disturbed and influenced by cytokine production and transmembrane TNF (tmTNF) reverse signaling. The goal of the study was the analysis of the predictive value of the rate of in vitro apoptosis for the therapeutic response to anti-TNF treatment.

Methods

Spontaneous and tmTNF reverse signaling-induced apoptosis were determined in vitro in monocytes from 20 RA patients prior to initiation of therapeutic TNF inhibition with etanercept, and the subsequent clinical response was monitored.

Results

Spontaneous in vitro apoptosis was significantly reduced in RA patients compared to controls. Deficiency in spontaneous apoptosis was associated with an insufficient therapeutic response according to the European League Against Rheumatism (EULAR) response criteria and less reduction of the disease activity determined by disease activity score (DAS) 28. High susceptibility to reverse signaling-induced apoptosis was also associated with less efficient reduction in the DAS28. Of note, a strong negative correlation between the two apoptotic parameters was discernible, possibly indicative of two pathogenetically relevant processes counter-regulating each other.

tmTNF reverse signaling induced in vitro production of soluble IL1-RI and IL-1RII only in monocytes not deficient in spontaneous apoptosis, and the levels of soluble IL1-RII were found to be predictive of a good clinical response to Etanercept.

Conclusion

Although tmTNF reverse signaling is able to induce apoptosis of RA monocytes in vitro, this process appears to occur in vitro preferentially in patients with suboptimal therapeutic response. Resistance to spontaneous in vitro apoptosis, in contrast, is a predictor of insufficient response to treatment.

Arcobacter butzleri induces a pro-inflammatory response in THP-1 derived macrophages and has limited ability for intracellular survival

Recent case reports have identified Arcobacter (A.) butzleri to be another emerging pathogen of the family Campylobacteraceae causing foodborne diseases. However, little is known about its interaction with the human immune system. As macrophages act as first defense against bacterial infections, we studied for the first time the impact of A. butzleri on human macrophages using THP-1 derived macrophages as an in vitro infection model. Our investigations considered the inflammatory response, intracellular survival and activation of caspases as potential virulence mechanisms employed by A. butzleri. Induction of IL-1α, IL-1ß, IL-6, IL-8, IL-12ß and TNFα demonstrated a pro-inflammatory response of infected macrophages towards A. butzleri. gentamycin protection assays revealed the ability of A. butzleri strains to survive and resist the hostile environment of phagocytic immune cells for up to 22 h. Moreover, initial activation of intitiator- (CASP8) as well as effector caspases (CASP3/7) was observed without the onset of DNA damage, suggesting a potential counter regulation. Intriguingly, we recognized distinct strain specific differences in invasion and survival capabilities. This suggests the existence of isolate dependent phenotype variations and different virulence potentials as known for other intestinal pathogens such as Salmonella enterica ssp.

Anti-apoptotic genes in the survival of monocytic cells during infection

Macrophages are cells of the immune system that protect organisms against invading pathogens by fulfilling critical roles in innate and adaptive immunity and inflammation. They originate from circulating monocytes and show a high degree of heterogeneity, which reflects the specialization of function given by different anatomical locations. Differentiation of monocytes towards a macrophage phenotype is also accompanied by an increase of resistance against various apoptotic stimuli, a required characteristic that allows macrophages to accomplish their function in a stressful environment.

Apoptosis, a form of programmed cell death, is a tightly regulated process, needed to maintain homeostasis by balancing proliferation with cellular demise. Caspases, a family of cysteine proteases that are highly conserved in multicellular organisms, function as central regulators of apoptosis. FLIP (FLICE-inhibitory protein), anti-apoptotic members of the Bcl2 family and inhibitors of apoptosis (IAP) are the main three groups of anti-apoptotic genes that counteract caspase activation through both the extrinsic and intrinsic apoptotic pathways.

Modulation of the apoptotic machinery during viral and bacterial infections, as well as in various malignancies, is a wellestablished mechanism that promotes the survival of affected cells. The involvement of anti-apoptotic genes in the survival of monocytes/macrophages, either physiological or pathological, will be described in this review. How viral and bacterial infections that target cells of the monocytic lineage affect the expression of anti-apoptotic genes is important in understanding the pathological mechanisms that lead to manifested disease. The latest therapeutic approaches that target anti-apoptotic genes will also be discussed.

Monocytes are resistant to apoptosis in systemic juvenile idiopathic arthritis

We investigated whether circulating monocytes from patients with systemic juvenile idiopathic arthritis (SJIA) are resistant to apoptosis and which apoptotic pathway(s) may mediate this resistance. A microarray analysis of peripheral blood mononuclear cells (PBMC) of SJIA samples and RT-PCR analysis of isolated monocytes showed that monocytes from active SJIA patients express transcripts that imply resistance to apoptosis. SJIA monocytes incubated in low serum show reduced annexin binding and diminished FasL up-regulation compared to controls. SJIA monocytes are less susceptible to anti-Fas-induced apoptosis and, upon activation of the mitochondrial pathway with staurosporine, show diminished Bid cleavage and Bcl-w down-regulation compared to controls. Exposure to SJIA plasma reduces responses to apoptotic triggers in normal monocytes. Thus, SJIA monocytes are resistant to apoptosis due to alterations in both the extrinsic and intrinsic apoptosis pathways, and circulating factors associated with active SJIA may confer this phenotype.

Microarray analysis of amphotericin B-treated THP-1 monocytic cells identifies unique gene expression profiles among lipid and non-lipid drug formulations

The effects of deoxycholate amphotericin B (DAmB), amphotericin B lipid complex (ABLC), and amphotericin B colloidal dispersion (ABCD) on mRNA profiles from 218 genes in treated THP-1 monocytes were compared. Sixty-one genes were up-regulated and 8 were down-regulated by one or more of the AmB formulations. Fifty-three genes were up-regulated by DAmB while 24 and 18 genes were up-regulated by ABCD and ABLC, respectively. DAmB and ABCD up-regulated many pro-inflammatory genes, whereas ABLC did not. All three formulations up-regulated multiple categories of genes including the anti-apoptosis gene BIRC3 and the chemokine CXCL9 (MIG). Based on representative genes, DAmB activated more signaling pathways than ABCD or ABLC. Quantitative real time PCR confirmed array up-regulation of representative pro-inflammatory genes IL-8, CCL20 (MIP-3alpha), and CXCL2 (MIP-2alpha). Our study represents the first larger scale comparative gene expression profiling which may provide additional rationales for clinical side effects of each amphotericin B formulation.

Up-regulation of Bcl-2 through ERK phosphorylation is associated with human macrophage survival in an estrogen microenvironment

Estrogen is a known immunomodulator with pleiotropic effects on macrophage function that partly accounts for the gender bias observed in numerous autoimmune, cardiovascular, and neurodegenerative disorders. The effect of estrogen on the survival of human macrophages is largely unknown, and in this study we demonstrate that 17beta-estradiol (E2) provokes a death response in human THP-1 macrophages by initiating Bax translocation from cytosol to the mitochondria; however, a concomitant up-regulation of Bcl-2 creates a Bax to Bcl-2 ratio favorable for Bcl-2, thus ensuring cell survival. Both Bcl-2 up-regulation and Bax translocation are estrogen receptor-dependent events; however, Bcl-2 augmentation but not Bax translocation is dependent on Ca(2+) increase, activation of protein kinase C, and ERK phosphorylation. This estrogen-induced Bcl-2 increase is crucial for the survival of THP-1 macrophages as well as that of human peripheral blood monocyte-derived macrophages, which is evident from E2-induced cell death under small interfering RNA-mediated Bcl-2 knockdown conditions. Hence, this study demonstrates that E2-induced Bcl-2 up-regulation is a homeostatic survival mechanism necessary for the manifestation of immunomodulatory effect of estrogen on human macrophages.

Exposure of myeloid dendritic cells to exogenous or endogenous IL-10 during maturation determines their longevity

Dendritic cells (DC) are essential for the initiation of primary adaptive immune responses, and their functionality is strongly down-modulated by IL-10. Both innate and adaptive immune signals trigger the up-regulation of antiapoptotic Bcl-2 family members to facilitate the survival of DCs after maturation. However, whether IL-10 alters the expression of apoptotic-related genes in maturing DCs has not been determined. In this study, we demonstrate that spontaneous apoptosis rapidly occurred in myeloid DCs exposed to exogenous IL-10 upon maturation. Microarray analysis indicates that IL-10 suppressed the induction of three antiapoptotic genes, bcl-2, bcl-x, and bfl-1, which was coincident with the increased sensitivity of mature DCs to spontaneous apoptosis. IL-10 markedly inhibited the accumulation of steady state Bcl-2 message and protein in myeloid DCs activated through TLRs or TNFR family members, whereas exogenous IL-10 affected Bcl-x(L) expression in a moderate manner. In contrast, bcl-2 expression of plasmacytoid DCs was less sensitive to the effects of IL-10. We further show that autocrine IL-10 significantly limited the longevity of myeloid DCs and altered the expression kinetics of Bcl-2 but not Bcl-x(L) in maturing DCs. We conclude that the degree of IL-10 exposure and/or the level of endogenous IL-10 production upon myeloid DC maturation play a critical role in determining DC longevity. This regulatory mechanism of IL-10 is associated with the dynamic control of antiapoptotic Bcl-2 proteins.

A constitutive endogenous osteopontin production is important for macrophage function and differentiation

Macrophages are involved in the pathological process underlying atherosclerosis and constitutively express the multifunctional protein osteopontin which has important exogenous effects on these cells. However, the effect of the endogenous osteopontin expression on macrophage function has been sparsely studied. To shed light on the importance of the endogenous osteopontin expression, RAW 264.7 macrophage-like cells were silenced in osteopontin expression using RNAi. The cells were analysed for basic functions including attachment, migration, apoptosis and for the expression of macrophage differentiation markers and cytokines. The macrophages with silenced osteopontin expression showed impaired migration and an increased rate of serum starvation-induced apoptosis as compared to osteopontin-producing control cells. Furthermore, the cells with silence osteopontin expression had an altered phenotype with monocyte-like characteristics, including decreased expression of macrophage scavenger receptor A type 1. The altered phenotype of these cells could not be reversed by presence of extracellular osteopontin. In addition the cells with silenced osteopontin expression had a lower expression of IL-12 mRNA and the anti-apoptotic Flip mRNA. We conclude that a constitutive endogenous osteopontin production is important for proper basic functions of macrophages and our study indicates that the constitutive osteopontin production is involved in maintaining macrophages in a differentiated phenotype.

CCL18/PARC stimulates hematopoiesis in long-term bone marrow cultures indirectly through its effect on monocytes

Chemokines play a role in regulating hematopoietic stem cell function, including migration, proliferation, and retention. We investigated the involvement of CCL18 in the regulation of bone marrow hematopoiesis. Treatment of human long-term bone marrow cultures (LTBMCs) with CCL18 resulted in significant stimulation of hematopoiesis, as measured by the total number of hematopoietic cells and their committed progenitors produced in culture. Monocytes/macrophages, whose survival was almost doubled in the presence of CCL18 compared with controls, were the primary cells mediating this effect. Conditioned media from CCL18-treated mature monocytes fostered colony-promoting activity that increased the number of colonies formed by hematopoietic progenitor cells. Gene expression profiling of CCL18-stimulated monocytes demonstrated more than 200 differentially expressed genes, including those regulating apoptosis (caspase-8) and proliferation (IL-6, IL-15, stem cell factor [SCF]). Up-regulation of these cytokines was confirmed on the protein expression level. The contribution of SCF and IL-6 in CCL18-mediated stimulatory activity for hematopoiesis was confirmed by SCF- and IL-6-blocking antibodies that significantly inhibited the colony-promoting activity of CCL18-stimulated conditioned medium. In addition to the effect on monocytes, CCL18 facilitated the formation of the adherent layer in LTBMCs and increased the proliferation of stromal fibroblast-like cells.

Neuroprotective effect of an antioxidant in ischemic brain injury: involvement of neuronal apoptosis

The production of reactive oxygen species (ROS) has been implicated in reperfusion injury after cerebral ischemia, and antioxidant enzymes are believed to be among the major mechanisms by which the cells counteract the deleterious effect of ROS after cerebral ischemia. ROS also mediate the mitochondrial signaling pathway that may lead to apoptosis following cerebral ischemia. The recent development and availability of transgenic and knockout mutant rodents that either overexpress or are deficient in antioxidant genes have provided powerful tools for dissecting the molecular and cellular mechanisms of signaling pathways, direct oxidative damage, or both that are involved in ischemic brain injury. This article focuses on the contribution of ROS or an antioxidant system to the molecular pathway of postischemic apoptosis following transient focal cerebral ischemia by using transgenic mice that overexpress the cytosolic antioxidant copper/zinc superoxide dismutase.

The kinder side of killer proteases: caspase activation contributes to neuroprotection and CNS remodeling

Caspases are a family of cysteine proteases that are expressed as inactive zymogens and undergo proteolytic maturation in a sequential manner in which initiator caspases cleave and activate the effector caspases 3, 6 and 7. Effector caspases cleave structural proteins, signaling molecules, DNA repair enzymes and proteins which inhibit apoptosis. Activation of effector, or executioner, caspases has historically been viewed as a terminal event in the process of programmed cell death. Emerging evidence now suggests a broader role for activated caspases in cellular maturation, differentiation and other non-lethal events. The importance of activated caspases in normal cell development and signaling has recently been extended to the CNS where these proteases have been shown to contribute to axon guidance, synaptic plasticity and neuroprotection. This review will focus on the adaptive roles activated caspases in maintaining viability, the mechanisms by which caspases are held in check so as not produce apoptotic cell death and the ramifications of these observations in the treatment of neurological disorders.

Evaluation of the apoptogenic potential of hard metal dust (WC–Co), tungsten carbide and metallic cobalt

The present study aimed at comparing in vitro the apoptogenic properties of metallic cobalt (Co), tungsten carbide (WC) and tungsten carbide-cobalt (WC-Co) in conditions known to cause genotoxicity. Human peripheral blood mononucleated cells were incubated with 2.0-6.0 microg/ml of Co alone or mixed with WC particles and 33.3-100.0 microg/ml WC alone for up to 24 h. Under these culture conditions the majority (60%) of the cobalt metal particles were almost immediately solubilised in the culture medium, while WC remained under the form of particles that were progressively phagocytosed by monocytes. Apoptosis was assessed by Annexin-V staining, flow cytometry and analysis of DNA fragmentation by ELISA. Metallic Co-particles induced apoptosis in vitro. Furthermore, although so far considered as biologically inert, WC particles also induced apoptosis. When compared with its individual components WC-Co displayed an additive apoptotic effect in the DNA fragmentation assay. Apoptosis induced by WC particles was found largely dependent on caspase-9 activation and occurred presumably in monocytes, while that induced by Co involved both caspase-9 and -8 activation. The data suggest that apoptosis induced by the tested WC-Co mixture results from the additive effects of WC apoptosis induced in monocytes and Co-specific apoptosis in both monocytes and lymphocytes. The apoptogenic properties of these metals may be important in the mechanism of lung pathologies induced by the cobalt-containing particles.

Effects of a redox-active agent on lymphocyte activation and early gene expression patterns

Antioxidants can inhibit the proliferation of T lymphocytes induced by mitogens. This has been postulated to be due to their scavenging of reactive oxygen species which may act as second messengers in the antigen-induced signaling cascade leading to cell proliferation. When added concurrently with various mitogens, the thiol pyrrolidine dithiocarbamate (PDTC) inhibited the subsequent proliferation of lymphocytes. The extracellular copper chelator bathocuproine disulfonic acid (BCPS) increased the amount of PDTC needed for inhibition. We sought to determine the mechanism by which the two different treatments, PDTC (0.4 microM, copper-dependent) and PDTC (20 microM with BCPS, redox-sensitive) affected proliferation. We found that both inhibited the increase in expression of many of the genes, including IL-2 and MKP-2, that were induced early after stimulation of lymphocytes with phorbol myristate acetate and ionomycin. The inhibition of MKP-2 may have contributed to the enhancement observed by the thiol of mitogen-induced ERK phosphorylation. Of the two redox-sensitive, IL-2 regulating transcription factors, NF-kappaB and AP-1, the mitogen-induced activity of the former was inhibited by PDTC. Treatment of unstimulated cells with PDTC induced the expression of many genes, most notably several metallothioneins and heat shock proteins, and this may provide an alternative explanation for the inhibition of cellular proliferation.

Monocytes Stimulated with Group B Streptococci or Interferons Release Tumour Necrosis Factor-Related Apoptosis-Inducing Ligand

Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a cytotoxic member of the TNF family. Some reports have shown that TRAIL is released from cells in a soluble form. In this work, we have investigated the mechanism of release of TRAIL from monocytes. First, we show that whole gram-positive, gram-negative and mycoplasmal bacteria as well as lipopolysaccharide (LPS), interferon-alpha (IFN-alpha), -beta and -gamma all induced upregulation of TRAIL on the surface of human monocytes. Next, we show that IFN-alpha, -beta and -gamma all induced a dose-dependent release of TRAIL, giving significant amounts of soluble TRAIL after 2 days. Of the bacteria, only the Group B streptococcus COH-1 (GBS) induced release of TRAIL and concomittantly induced IFN-alpha. Monocytes stimulated with GBS or IFN-alpha also showed extensive cell death. When monocyte apoptosis was prevented by interleukin-1, GM-CSF, LPS or the caspase inhibitor zVADfmk, the IFN-alpha-induced release of TRAIL was reduced, whereas agents inducing necrosis caused increased release of TRAIL. LPS also prevented release of TRAIL from GBS-stimulated monocytes. The release of TRAIL from IFN-alpha-stimulated monocytes was reduced by inhibitors of both cysteine and metalloproteases. We conclude that bacteria and IFN induce upregulation of membrane TRAIL and that release of TRAIL is associated with cell death.

Estrogen stimulates microglia and brain recovery from hypoxia-ischemia in normoglycemic but not diabetic female mice

Diabetic hyperglycemia increases ischemic brain damage in experimental animals and humans. The mechanisms are unclear but may involve enhanced apoptosis in penumbral regions. Estrogen is an established neuroprotectant in experimental stroke. Our previous study demonstrated that female diabetic db/db mice suffered less damage following cerebral hypoxia-ischemia (H/I) than male db/db mice. Here we investigated the effects of diabetes and estrogen apoptotic gene expression following H/I. Female db/db and nondiabetic (+/?) mice were ovariectomized (OVX) and treated with estrogen or vehicle prior to H/I; brains were analyzed for damage and bcl-2 family gene expression. OVX increased ischemic damage in +/? mice; estrogen reduced tissue injury and enhanced antiapoptotic gene expression (bcl-2 and bfl-1). db/db mice demonstrated more damage, without increased bcl-2 mRNA; bfl-1 expression appeared at 48 hours of recovery associated with infarction. To our knowledge, this is the first description of bfl-1 in the brain with localization to microglia and macrophages. Early induction of bfl-1 expression in +/? mouse brain was associated with microglia; delayed bfl-1 expression in diabetic brain was in macrophages bordering the infarct. Furthermore, estrogen replacement stimulated early postischemic expression of bcl-2 and bfl-1 and reduced damage in normoglycemic animals but failed to protect the diabetic brain.

Estrogen stimulates microglia and brain recovery from hypoxia-ischemia in normoglycemic but not diabetic female mice

Diabetic hyperglycemia increases ischemic brain damage in experimental animals and humans. The mechanisms are unclear but may involve enhanced apoptosis in penumbral regions. Estrogen is an established neuroprotectant in experimental stroke. Our previous study demonstrated that female diabetic db/db mice suffered less damage following cerebral hypoxia-ischemia (H/I) than male db/db mice. Here we investigated the effects of diabetes and estrogen apoptotic gene expression following H/I. Female db/db and nondiabetic (+/?) mice were ovariectomized (OVX) and treated with estrogen or vehicle prior to H/I; brains were analyzed for damage and bcl-2 family gene expression. OVX increased ischemic damage in +/? mice; estrogen reduced tissue injury and enhanced antiapoptotic gene expression (bcl-2 and bfl-1). db/db mice demonstrated more damage, without increased bcl-2 mRNA; bfl-1 expression appeared at 48 hours of recovery associated with infarction. To our knowledge, this is the first description of bfl-1 in the brain with localization to microglia and macrophages. Early induction of bfl-1 expression in +/? mouse brain was associated with microglia; delayed bfl-1 expression in diabetic brain was in macrophages bordering the infarct. Furthermore, estrogen replacement stimulated early postischemic expression of bcl-2 and bfl-1 and reduced damage in normoglycemic animals but failed to protect the diabetic brain.

Simultaneous activation of apoptosis and inflammation in pathogenesis of septic shock: a hypothesis

Sepsis, a widely prevalent disease with increasing morbidity and mortality, is thought to result from uncontrolled inflammatory responses to microbial infection and/or components. However, failure of several experimental anti-inflammatory therapies has necessitated re-evaluation of the paradigm underlying the pathogenesis of this complex disorder. Apoptotic cell death forms a second dominant feature of septic shock in patients and animal models. Anti-apoptotic strategies may protect animals from septic death. However, simultaneous occurrence of apoptosis and inflammation is necessary for septic death. At the cellular level, apoptosis plays a central role in the development of the lymphoid system and regulation of immune responses. Immune activation renders cells refractory to apoptosis while apoptosis of activated lymphocytes is an important immunoregulatory mechanism. Factors such as complement factor 5a, caspase-1 and mitogen-activated protein kinase, which participate in apoptosis as well as pro-inflammatory pathways, may be responsible for simultaneous activation of apoptosis and inflammation in sepsis. Further identification of other similar biochemical events capable of co-activating inflammation and apoptosis may provide new targets for therapy of this hitherto untreatable disease.

Shikonin, a component of chinese herbal medicine, inhibits chemokine receptor function and suppresses human immunodeficiency virus type 1

Shikonin is a major component of zicao (purple gromwell, the dried root of Lithospermum erythrorhizon), a Chinese herbal medicine with various biological activities, including inhibition of human immunodeficiency virus (HIV) type 1 (HIV-1). G protein-coupled chemokine receptors are used by HIV-1 as coreceptors to enter the host cells. In this study, we assessed the effects of shikonin on chemokine receptor function and HIV-1 replication. The results showed that, at nanomolar concentrations, shikonin inhibited monocyte chemotaxis and calcium flux in response to a variety of CC chemokines (CCL2 [monocyte chemoattractant protein 1], CCL3 [macrophage inflammatory protein 1alpha], and CCL5 [regulated upon activation, normal T-cell expressed and secreted protein]), the CXC chemokine (CXCL12 [stromal cell-derived factor 1alpha]), and classic chemoattractants (formylmethionyl-leucine-phenylalanine and complement fraction C5a). Shikonin down-regulated surface expression of CCR5, a primary HIV-1 coreceptor, on macrophages to a greater degree than the other receptors (CCR1, CCR2, CXCR4, and the formyl peptide receptor) did. CCR5 mRNA expression was also down-regulated by the compound. Additionally, shikonin inhibited the replication of a multidrug-resistant strain and pediatric clinical isolates of HIV in human peripheral blood mononuclear cells, with 50% inhibitory concentrations (IC(50)s) ranging from 96 to 366 nM. Shikonin also effectively inhibited the replication of the HIV Ba-L isolate in monocytes/macrophages, with an IC(50) of 470 nM. Our results suggest that the anti-HIV and anti-inflammatory activities of shikonin may be related to its interference with chemokine receptor expression and function. Therefore, shikonin, as a naturally occurring, low-molecular-weight pan-chemokine receptor inhibitor, constitutes a basis for the development of novel anti-HIV therapeutic agents.

Vitamin C inhibits FAS-induced apoptosis in monocytes and U937 cells

The FAS receptor-FAS ligand system is a key apoptotic pathway for cells of the immune system. Ligation of the FAS-receptor (CD95) induces apoptosis by activation of pro-caspase-8 followed by downstream events, including an increase in reactive oxygen species (ROS) and the release of proapoptotic factors from the mitochondria, leading to caspase-3 activation. We investigated the role of vitamin C in FAS-mediated apoptosis and found that intracellular accumulation of pharmacologic concentrations of vitamin C inhibited FAS-induced apoptosis in the monocytic U937 cell line and in fresh human monocytes. Cells were loaded with vitamin C by exposure to dehydroascorbic acid (DHA), thereby circumventing in vitro artifacts associated with the poor transport and pro-oxidant effects of ascorbic acid (AA). Vitamin C inhibition of FAS-mediated apoptosis was associated with reduced activity of caspase-3, -8, and -10, as well as diminished levels of ROS and preservation of mitochondrial membrane integrity. Mechanistic studies indicated that the major effect of vitamin C was inhibition of the activation of caspase-8 with no effect on it enzymatic activity. An independent action of high intracellular concentrations of vitamin C on mitochondrial membrane stabilization was also detected. These studies illuminate the nature of redox-dependent signaling in FAS-induced apoptosis of human monocytes and suggest that vitamin C can modulate the immune system by inhibiting FAS-induced monocyte death.

Fas (CD95) induces proinflammatory cytokine responses by human monocytes and monocyte-derived macrophages

Fas (CD95, APO-1) is regarded as the prototypical cell death receptor of the TNFR superfamily. Fas-induced apoptosis is generally considered to be a noninflammatory process, contributing to the silent resolution of immune and inflammatory responses. However, accumulating evidence indicates that Fas may also induce cellular activation signals. We hypothesized that Fas could activate proinflammatory cytokine responses by normal human monocytes and macrophages. Monocytes were isolated by negative immunoselection from the PBMC fraction of venous blood from healthy volunteers, and monocyte-derived macrophages were cultivated in vitro. Both monocytes and monocyte-derived macrophages released TNF-alpha and IL-8 following Fas ligation, and conditioned medium from Fas-activated monocytes and macrophages induced the directed migration of neutrophils in a chemotaxis assay. Fas-induced monocyte cytokine responses were associated with monocyte apoptosis, nuclear translocation of NF-kappaB, and cytokine gene expression and were blocked by caspase inhibition but not by inhibition of IL-1beta signaling. In contrast, Fas-induced macrophage cytokine responses occurred in the absence of apoptosis and were caspase independent, indicating maturation-dependent differences in the Fas signaling pathways that lead to proinflammatory cytokine induction. Rather than contributing to the resolution of inflammation, Fas ligation on circulating monocytes and tissue macrophages may induce proinflammatory cytokine responses that can initiate acute inflammatory responses and tissue injury.

Induction of apoptosis in monocytes by Mycobacterium leprae in vitro: a possible role for tumour necrosis factor-alpha

A diverse range of infectious organisms, including mycobacteria, have been reported to induce cell death in vivo and in vitro. Although morphological features of apoptosis have been identified in leprosy lesions, it has not yet been determined whether Mycobacterium leprae modulates programmed cell death. For that purpose, peripheral blood mononuclear cells obtained from leprosy patients were stimulated with different concentrations of this pathogen. Following analysis by flow cytometry on 7AAD/CD14+ cells, it was observed that M. leprae induced apoptosis of monocyte-derived macrophages in a dose-dependent manner in both leprosy patients and healthy individuals, but still with lower efficiency as compared to M. tuberculosis. Expression of tumour necrosis factor-alpha (TNF-alpha), Bax-alpha, Bak mRNA and TNF-alpha protein was also detected in these cultures; in addition, an enhancement in the rate of apoptotic cells (and of TNF-alpha release) was noted when interferon-gamma was added to the wells. On the other hand, incubation of the cells with pentoxifylline impaired mycobacterium-induced cell death, the secretion of TNF-alpha, and gene expression in vitro. In addition, diminished bacterial entry decreased both TNF-alpha levels and the death of CD14+ cells, albeit to a different extent. When investigating leprosy reactions, an enhanced rate of spontaneous apoptosis was detected as compared to the unreactive lepromatous patients. The results demonstrated that M. leprae can lead to apoptosis of macrophages through a mechanism that could be at least partially related to the expression of pro-apoptotic members of the Bcl-2 protein family and of TNF-alpha. Moreover, while phagocytosis may be necessary, it seems not to be crucial to the induction of cell death by the mycobacteria.

NF-kappa B-dependent assembly of an enhanceosome-like complex on the promoter region of apoptosis inhibitor Bfl-1/A1

Expression of the prosurvival Bcl-2 homologue Bfl-1/A1 is induced by NF-kappa B-activating stimuli, while B and T cells from c-rel knockout mice show an absolute defect in bfl-1/a1 gene activation. Here, we demonstrate NF-kappa B-dependent assembly of an enhanceosome-like complex on the promoter region of bfl-1. Binding of NF-kappa B subunit c-Rel to DNA nucleated the concerted binding of transcription factors AP-1 and C/EBP beta to the 5'-regulatory region of bfl-1. Optimal stability of the complex was dependent on proper orientation and phasing of the NF-kappa B site. Chromatin immunoprecipitation analyses demonstrated that T-cell activation triggers in vivo binding of endogenous c-Rel, c-Jun, C/EBP beta, and HMG-IC to the bfl-1 regulatory region, coincident with selective recruitment of coactivators TAFII250 and p300, SWI/SNF chromatin remodeling factor component BRG-1, and basal transcription factors TATA-binding protein (TBP) and TFIIB, as well as hyperacetylation of histones H3 and H4. These results highlight a critical role for NF-kappa B in bfl-1 transcription and point to the need for a complex and precise regulatory network to control bfl-1 expression. To our knowledge, this is the first demonstration of enhanceosome-mediated regulation of a cell death inhibitor.

The prodomain of interleukin 1alpha interacts with elements of the RNA processing apparatus and induces apoptosis in malignant cells

Interleukin 1alpha (IL-1alpha), a 33 kDa precursor, is cleaved releasing the 17 kDa carboxyl-terminal cytokine IL-1alpha to which all of the biological properties of IL-1alpha have been attributed. We investigated the potential independent properties of the remaining 16 kDa IL-1alpha amino-terminal propiece by expression in human tumor and primary human cell lines. The IL-1alpha propiece produced apoptosis in malignant but not normal cell lines. A minimal fragment comprised of amino acids 55-108 was required for apoptosis. Deletion and mutation studies identified an extended nuclear localization sequence required for nuclear localization, induction of apoptosis and concentration of the IL-1alpha propiece in interchromatin granule clusters, concentrations of proteins in the RNA splicing and processing pathways. The IL-1alpha propiece interacted with five known components of the RNA splicing/processing pathway, suggesting that the mechanism of action may involve changes in RNA splicing or processing. Expression of the IL-1alpha propiece caused a shift in the ratio of Bcl-Xl/Bcl-Xs toward the apoptotic direction. Our findings indicate that the IL-1alpha propiece induces apoptosis in a range of tumor cells and likely operates through a mechanism involving the RNA processing apparatus and the alternate splicing of apoptosis regulatory proteins.

In vitro detection of apoptosis in monocytes/macrophages infected with human coronavirus

Human coronavirus (HCoV) strain 229E infection, but not HCoV strain OC43 infection, of monocytes/macrophages from healthy donors and patients with multiple sclerosis in remission resulted in increased apoptosis, as measured by DNA changes and annexin V staining. Apoptosis correlated with the differential release of infectious virus. HCoV strain 229E titers were 10(3.5) to 10(6) 50% tissue culture-infective doses (TCID(50))/ml, and HCoV strain OC43 titers were only 10(1.2) to 10(2.7) TCID(50)/ml.

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.

The effector phase of physiological cell death relies exclusively on the posttranslational activation of resident components

Inhibitors of transcription and translation can protect cells from physiological cell deaths induced by a variety of stimuli. These observations have been taken to suggest that de novo macromolecular synthesis may be an essential component of the cell death process. Paradoxically, the same inhibitors, at higher concentrations, themselves trigger the death of cells. Previously, we have mapped a conserved and ordered sequence of events that exerts physiological cell death. Diverse signals converge to activate this lethal pathway, composed of a proteolytic cascade of caspases and subsequent cyclin-dependent kinases. Here we report that inhibitors of nuclear gene expression, when they block cell death, act upstream of this lethal process to prevent its activation. In contrast, when cell death is triggered by high doses of the inhibitors, these same essential molecules are activated, despite the essentially complete blockade of macromolecular synthesis. This inhibitor-induced death response is associated with the release of cytochrome c from mitochondria and the activation of apical caspase 9 and is blocked by overexpression of Bcl-2. These data demonstrate that all essential molecules that exert lethality already are resident within cells and are activated posttranslationally upon stimulation. De novo macromolecular synthesis pertains idiosyncratically only to upstream, modulatory elements of particular death responses.

Shear stress and material surface effects on adherent human monocyte apoptosis

Monocytes play a critical role as both phagocytes and mediators of inflammatory responses in the prevention of cardiovascular device-related infections. However, persistent infection of these devices still occurs and may be attributed to deleterious cellular alterations resulting from monocyte interactions with a foreign material in an environment of dynamic flow. Thus, the effects of both shear stress and adhesion to material surfaces on human monocyte apoptosis were investigated. A rotating disk system generated physiologically relevant shear stress levels (0-14 dyn/cm(2)), and shear-related apoptosis occurring in adherent monocytes was characterized. Using annexin V analysis, apoptosis of polyurethane-adherent monocytes under shear for 4 h increased to levels >70% with increasing shear in a near-linear fashion (r2 = 0.713). It was qualitatively confirmed using confocal microscopy that filamentous (F)-actin distribution was altered, that DNA fragmentation occurred, and that activated caspases were involved in shear-induced apoptosis. Static studies determined that spontaneous apoptosis was material-dependent over 72 h by demonstrating marked differences between apoptosis of monocytes adherent to a polyurethane compared to an alkyl-modified glass. Treatment with TNF-alpha augmented this material dependency in a dose-dependent fashion over time. F-actin content of TNF-alpha-treated cells decreased to <62% of untreated cells. We conclude that concomitant effects from both material surfaces and dynamic flow mediate human monocyte apoptosis and may have serious implications in the context of implanted cardiovascular device infection.

Auto-protective redox buffering systems in stimulated macrophages

BACKGROUND

Macrophages, upon encounter with micro-organisms or stimulated by cytokines, produce various effector molecules aimed at destroying the foreign agents and protecting the organism. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are front line molecules exerting strong cytotoxic activities against micro-organisms and many cells, including macrophages themselves. Using cells of the murine macrophage cell line (RAW 264.7) stimulated in vitro with lipopolysaccharide (LPS) and/or interferon (IFN-gamma), which induce strong endogenous NO production, we examined by which mechanisms a fraction of activated macrophages protect themselves from nitrosative stress and manage to escape destruction?

RESULTS

We observed that survivors (10-50% depending on the experiments) had acquired a resistant phenotype being capable to survive when further exposed in vitro to an apoptosis inducing dose of the NO donor compound DETA-NO. These cells expressed an increased steady-state levels of Mn SOD, CuZn SOD and catalase mRNA (130-200%), together with an increased activity of the corresponding enzymes. Intracellular concentration of glutathione was also increased (x 3.5 fold at 6 hours, still maintained x 5.2 fold at 48 hours). Neither mRNA for glutathione peroxydase, gamma-glutamylcysteine synthase and glutathione reductase, nor thioredoxine and thioredoxine reductase, were significantly modified. Additional experiments in which RAW 264.7 cells were stimulated with LPS and/or IFN-gamma in the presence of relatively specific inhibitors of both Mn and Cu/Zn SOD, aminotriazol (ATZ) catalase inhibitor and buthionine sulfoximine (BSO) glutathione inhibitor, showed that inhibiting LPS-induced up-regulation of intracellular redox buffering systems also prevented acquisition of the resistant phenotype.

CONCLUSIONS

Our data suggest a direct causal relationship between survival of a fraction of macrophages and a up-regulation of key sets of auto-protective intracellular redox buffering systems, occurring simultaneously with modulation of expression of apoptotic molecules of the Bcl2-Bcl-XL/Bax-Bad family.

Interleukin-4 inhibits tumor necrosis factor-alpha-induced and spontaneous apoptosis of biomaterial-adherent macrophages

Biocompatibility of implanted materials is determined by the host foreign-body response, which is comprised of cellular (adherent monocytes and macrophages) and soluble (secreted cytokines) components. Modulating the presence, activity or both of adherent macrophages may increase or decrease the biocompatibility of implants because these cells remain adherent to the implant surface and fuse to form foreign-body giant cells (FBGCs), leading to failure of the implant. An attractive mechanism of eliminating these cells is through the induction of apoptosis; therefore ways of inducing or inhibiting apoptosis of biomaterial-adherent inflammatory cells are being investigated. We hypothesized that interleukin-4 (IL-4) promotes macrophage survival by inhibiting tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis. We found that TNF-alpha induces apoptosis in a time- and dose-dependent manner, whereas IL-4 inhibits TNF-alpha-induced and spontaneous apoptosis of biomaterial-adherent macrophages. Blocking experiments and evaluation of shedding of soluble TNF receptor type I (TNF-RI) demonstrated that endogenous TNF-alpha production is responsible for spontaneous apoptosis of biomaterial-adherent cells and that IL-4 inhibits this apoptosis by increasing levels of shedding of soluble TNF-RI. These findings suggest that TNF-alpha and IL-4 play key roles in determining the fate of biomaterial-adherent cells and that fusion of macrophages into FBGCs is a mechanism for promoting inflammatory-cell survival on implanted materials.

Activation of the MEK/MAPK pathway is involved in bryostatin1-induced monocytic differenciation and up-regulation of X-linked inhibitor of apoptosis protein

Induction of monocytic differentiation by bryostatin1 (bryo1) conferred on THP-1 leukemia cells the ability to resist Z-LLL-CHO-induced apoptosis. The mechanism of resistance developed during this process was investigated. Apoptosis resistance was associated with an enhanced expression of X-linked inhibitor of apoptosis protein (XIAP), an endogenous caspase inhibitor, in differentiated THP-1 cells. Bryo1 also increased the level of c-IAP-1, yet decreased the level of c-IAP-2 in THP-1 cells, indicating that distinct regulatory mechanisms are operative. In addition, treatment of THP-1 cells with bryo1 induced a rapid and sustained activation of MEK, prior to the upregulation of XIAP and monocytic differentiation. Pretreatment of THP-1 cells with MEK inhibitors (U0126 and PD98059) prior to bryo1 induction blocked the expression of both XIAP and the c-fms product (M-CSF receptor), a hallmark of monocytic differentiation, but not Bcl-2. In addition, the expression of XIAP in bryo1-treated cells was inhibited by CAPE, a NF-kappaB-specific inhibitor, indicating that its expression is under the transcriptional regulation of NF-kappaB downstream of the MEK/MAPK pathway. The importance of XIAP in mediating apoptosis resistance was illustrated in cells transiently transfected with XIAP, which conferred on THP-1 cells the ability to resist Z-LLL-CHO-induced apoptosis. These findings suggest that the expression of XIAP is linked to monocytic differentiation in bryo1-treated THP-1 cells and represents one of the potential antiapoptotic mechanisms acquired during this process.

GM-CSF: a strong arteriogenic factor acting by amplification of monocyte function

We investigated the role of the colony stimulating factor for monocytes (GM-CSF) to test the hypothesis whether prolongation of the monocyte's life cycle will support arteriogenesis (rapid growth of preexisting collateral arteries). This appeared logical in view of our discovery that circulating monocytes play an important part in the positive remodeling of small preexisting arterioles into arteries to compensate for arterial occlusions (arteriogenesis) and especially following our findings that MCP-1 markedly increases the speed of arteriogenesis. The continuous infusion of GM-CSF for 7 days into the proximal stump of the acutely occluded femoral artery of rabbits by osmotic minipump produced indeed a marked arteriogenic response as demonstrated by an increase (2-fold) in number and size of collateral arteries on postmortem angiograms and by the increase of maximal blood flow during vasodilation measured in vivo by blood pump perfusion of the hindquarter (5-fold). When GM-CSF and MCP-1 were simultaneously infused the effects on arteriogenesis were additive on angiograms as well as on conductance. GM-CSF was also able to widen the time window of MCP-1 activity: MCP-1 treatment alone was ineffective when given after the third week following occlusion. When administered together with GM-CSF about 80% of normal maximal conductance of the artery that was replaced by collaterals were achieved, a result that was not reached before by any other experimental treatment. Experiments with cells isolated from treated animals showed that monocyte apoptosis was markedly reduced. In addition we hypothesize that GM-CSF may aid in releasing pluripotent monocyte (stem-) cells from the bone marrow into the circulation. In contrast to MCP-1, GM-CSF showed no activity on monocyte transmigration through- and also no influence on monocyte adhesion to cultured endothelial cells. In conclusion we have discovered a new function of the hemopoietic stem cell factor GM-CSF, which is also a powerful arteriogenic peptide that acts via prolongation of the life cycle of monocytes/macrophages.

Apoptosis by pan-caspase inhibitors in lipopolysaccharide-activated macrophages

Although apoptosis has been observed in macrophages during the course of infections, the mechanism of apoptosis in activated macrophages is not fully understood. This study shows that pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (ZVAD) or t-butyloxycarbonyl-Asp-fluoromethylketone (Boc-D) caused the death of lipopolysaccharide (LPS)-activated macrophages and RAW 264.7 cells with apoptotic features. The apoptosis was also observed in lipoprotein-treated bacteria but not in CpG oligonucleotide- or flagellin-treated macrophages, indicating a difference of cellular responses downstream of different Toll-like receptors. Consistent with the induction of cell death by pan-caspase inhibitors, no activation of known caspases was detected in LPS-ZVAD-treated cells, suggesting an involvement of unknown proapoptotic caspases in the cell death. ZVAD inhibited the activation of extracellular signal-regulated kinase (ERK) and p38 but not of nuclear factor (NF)-kappa B induced by LPS, suggesting that the ZVAD-sensitive molecule lies upstream of the ERK and p38 pathways but downstream of the divergent site of NF-kappa B and mitogen-activated protein kinases. Our results demonstrate that apoptosis of macrophages induced by LPS+ZVAD is independent from the known proapoptotic caspases and suggest that activity of an unidentified ZVAD-sensitive molecule(s) is involved in the survival of LPS-activated macrophages.

Oxidative stress signalling in the apoptosis of Jurkat T-lymphocytes

The pathways of transduction of oxidative stress signals have been studied using the Jurkat T cell model. The oxidative stress was induced by exposure of the cells to 100 microM H(2)O(2). DNA damage was detected within 15 min after commencement of treatment. DNA damage repair occurred within about 1 h in cells exposed to oxidative stress for 15 min. In continuous exposure to stress, DNA repair was slower and control levels of DNA integrity were not reached. DNA repair did not involve gene transcription. H(2)O(2) at 100 microM caused cell death by necrosis as well as by apoptosis. Both these processes were induced by 15 min exposure to the stress stimulus. However, some important differences were found between necrosis and apoptosis. Necrosis was more rapid, began within an hour of treatment and continued to increase during the full duration of the experiment. But apoptosis was seen after 4 h from treatment and was conspicuous between 6 and 20 h after the start of treatment. The necrotic phase preceded apoptosis, although these did show an overlap. In the necrotic phase, Bcl-2, Caspase 8 genes were down regulated. The 6-20 h phase characterised by a marked increase in apoptosis is accompanied by the up regulation of both Bcl-2 and Caspase genes. Expression of the Fas and p53 genes was not altered in either phase. We also analysed the levels of expression of the scavenging genes whose gene products are involved in detoxification. No modulation of the antioxidant enzymes, catalase, Cu/Zn superoxide dismutase and glutathione peroxidase was detectable.

Tumor necrosis factor modulates fibroblast apoptosis, PMN recruitment, and osteoclast formation in response to P. gingivalis infection

P. gingivalis is an important oral pathogen, which has been closely linked to periodontal disease as well as lesions of endodontic origin. Both infections are associated with a decrease in fibroblast numbers, formation of an inflammatory infiltrate, and bone resorption. The goal of this study was to investigate the role that the host response plays in the capacity of P. gingivalis to stimulate fibroblast apoptosis, PMN recruitment, and osteoclastogenesis. This was accomplished by the use of an in vivo calvarial model in mice with targeted deletion of TNF receptors p55 and p75 and matched wild-type mice. The results indicate that P. gingivalis induces fibroblast apoptosis in vivo and establish for the first time that this involves the stimulation of a host response. Moreover, bacteria-stimulated PMN recruitment and osteoclastogenesis were also dependent upon the host response. The results suggest that much of the damage caused by P. gingivalis infection, including fibroblast apoptosis, at least under some circumstances, results from stimulation of the host response rather than the direct effect of bacterial products. Furthermore, this may represent a more general mechanism by which bacterial challenge induces apoptosis of matrix-producing cells through the induction of TNF.

Superoxide during reperfusion contributes to caspase-8 expression and apoptosis after transient focal stroke

BACKGROUND AND PURPOSE

Reactive oxygen species produced during reperfusion may play a detrimental role in focal cerebral ischemia (FCI). We examined the protein expression of caspase-8, which plays a major role in both Fas-dependent and cytochrome c-dependent apoptotic pathways after FCI with or without reperfusion. Caspase-8 expression after transient FCI was compared between wild-type and transgenic mice that overexpress the cytosolic antioxidant copper/zinc superoxide dismutase (SOD1).

METHODS

Adult male CD-1 mice were subjected to 1 hour of FCI and reperfusion or to permanent FCI by intraluminal blockade of the middle cerebral artery. DNA fragmentation was evaluated by genomic DNA gel electrophoresis. Caspase-8 expression was analyzed by Western blot.

RESULTS

Caspase-8 was significantly induced 4 hours after transient FCI and remained at an increased level until 24 hours, whereas it was not modified after permanent FCI. Genomic DNA gel electrophoresis showed DNA laddering in a pattern similar to that seen in apoptosis, with a small amount of background smear 24 hours after transient FCI, whereas 25 hours of permanent FCI resulted in less DNA laddering with a strong background smear. Caspase-8 induction was significantly reduced in SOD1 transgenic mice compared with wild-type mice 4 hours after transient FCI.

CONCLUSIONS

The results suggest that increased reactive oxygen species production during reperfusion may contribute to the induction of caspase-8, thereby exacerbating apoptosis after FCI.

The Fas-FasL death receptor and PI3K pathways independently regulate monocyte homeostasis

Peripheral blood-derived monocytes spontaneously undergo apoptosis mediated by Fas-Fas ligand (FasL) interactions. Activation of monocytes by LPS or TNF-alpha prevents spontaneous monocyte apoptosis through an unknown mechanism. Here, we demonstrate that LPS and TNF-alpha up-regulate Flip and suppress spontaneous Fas-FasL mediated monocyte apoptosis and caspase 8 and 3 activation. Flip was responsible for this protection, since inhibition of Flip by antisense oligonucleotides in the presence of LPS or TNF-alpha restored monocyte sensitivity to spontaneous apoptosis. We also investigated whether the PI3K pathway contributes to the suppression of spontaneous monocyte apoptosis mediated by LPS and TNF-alpha. Monocytes treated with a reversible PI3K inhibitor (LY294002) displayed enhanced apoptosis, while LPS and TNF-alpha partially protected against apoptosis mediated by LY294002. However, direct suppression of Fas-FasL interactions by addition of neutralizing anti-FasL antibody did not further suppress LY294002-induced apoptosis in the presence of LPS or TNF-alpha. Collectively, these data demonstrate that LPS or TNF-alpha protect monocytes from death receptor-mediated apoptosis through the up-regulation of Flip, but not apoptosis initiated by inhibition of the PI3K pathway.

Macrophages are eliminated from the injured peripheral nerve via local apoptosis and circulation to regional lymph nodes and the spleen

The present study investigated the fate of macrophages in peripheral nerves undergoing Wallerian degeneration, especially their disappearance from the injured nerves after phagocytosis of axonal and myelin debris. Wallerian degeneration was induced in adult male C57Bl/6 mice by transecting the right sciatic nerve. Five days after transection, the male sciatic nerves were transplanted into female recipient mice by placing them exactly parallel to the host sciatic nerves. Nerves of the female recipient mice were also transected to induce breakdown of the blood-nerve barrier in the host animal. Apoptosis was assessed by morphological, immunohistochemical (activated caspase-3), and molecular (DNA fragmentation) methods in transplanted, recipient, and in control nerves. A subpopulation of macrophages within the degenerating nerves died locally by apoptosis in each experiment. The fate of the male macrophages within the transplanted nerves and the host organism was investigated by in situ hybridization with a Y-chromosome-specific DNA probe (145SC5). In situ hybridization specifically stained cells within the transplanted male nerve. Y-chromosome-positive cells were detected not only inside the transplanted nerve, but also inside the female host nerve, the perineurial tissue, the local perineurial blood vessels, draining lymph nodes and the spleen of the female host, suggesting hematogenous as well as lymphatic elimination of macrophages from the injured nerve. These data indicate that local apoptosis and systemic elimination via circulation to the local lymph nodes and the spleen are involved in the disappearance of macrophages from the injured peripheral nervous system.