Inhibition of death receptor signaling by bacterial gut pathogens

Gastrointestinal bacterial pathogens such as enteropathogenic Escherichia coli, Salmonella and Shigella control inflammatory and apoptotic signaling in human intestinal cells to establish infection, replicate and disseminate to other hosts. These pathogens manipulate host cell signaling through the translocation of virulence effector proteins directly into the host cell cytoplasm, which then target various signaling pathways. Death receptors such as TNFR1, FAS and TRAIL-R induce signaling cascades that are crucial to the clearance of pathogens, and as such are major targets for inhibition by pathogens. This review focuses on what is known about how bacterial gut pathogens inhibit death receptor signaling to suppress inflammation and prevent apoptosis.     

A near death experience: Shigella manipulates host death machinery to silence innate immunity

Release of mitochondrial contents often triggers inflammation and cell death, and modulating this process can be advantageous to invading pathogens. In this issue of The EMBO Journal, Andree and colleagues reveal new findings that an intracellular bacterial pathogen exploits apoptotic machinery to suppress host immune signaling, yet avoids cell death. This study emphasizes the need to expand our understanding of the roles played by pro‐apoptotic proteins in non‐death scenarios.     

Salmonella AvrA Coordinates Suppression of Host Immune and Apoptotic Defenses via JNK Pathway Blockade

Salmonellae are bacterial pathogens that have evolved sophisticated strategies to evade host immune defenses. These strategies include the secretion of effector proteins into mammalian cells so as to subvert innate immune and apoptotic signaling pathways, thereby allowing Salmonella to avoid elimination. Here, we show that the secreted Salmonella typhimurium effector protein AvrA possesses acetyltransferase activity toward specific mitogen-activated protein kinase kinases (MAPKKs) and potently inhibits c-Jun N-terminal kinase (JNK) and NF-kappaB signaling pathways in both transgenic Drosophila and murine models. Furthermore, we show that AvrA dampens the proapoptotic innate immune response to Salmonella at the mouse intestinal mucosa. This activity is consistent with the natural history of Salmonella in mammalian hosts, where the bacteria elicit transient inflammation but do not destroy epithelial cells. Our findings suggest that targeting JNK signaling to dampen apoptosis may be a conserved strategy for intracellular pathogens.     

Co-evolution and exploitation of host cell signaling pathways by bacterial pathogens

Bacterial pathogens have evolved by combinations of gene acquisition, deletion, and modification, which increases their fitness. Additionally, bacteria are able to evolve in "quantum leaps" via the ability to promiscuously acquire new genes. Many bacterial pathogens - especially Gram-negative enteric pathogens - have evolved mechanisms by which to subvert signal transduction pathways of eukaryotic cells by expressing genes that mimic or regulate host protein factors involved in a variety of signaling cascades. This results in the ability to cause diseases ranging from tumor formation in plants to gastroenteritis and bubonic plague. Here, we present recent advances on mechanisms of bacterial pathogen evolution, including specific signaling cascades targeted by their virulence genes with an emphasis on the ubiquitin modification system, Rho GTPase regulators, cytoskeletal modulators, and host innate immunity. We also comment briefly on evolution of host defense mechanisms in place that limit disease caused by bacterial pathogens.     

Mechanisms and consequences of bacterial targeting by the autophagy pathway

Autophagy is a key component of our immune response to invading pathogens. Autophagic targeting of intracellular bacteria within vacuolar compartments or the cytosol helps to control bacterial replication in the host cell. The mechanism by which these invading pathogens are selectively targeted for degradation is of particular interest. Recently, several signaling factors have been shown to play roles in the specific targeting of bacteria by the autophagy pathway including: pattern recognition receptors, reactive oxygen species, ubiquitin and diacylglycerol. Here, we discuss these signaling factors and the consequences of bacterial targeting by autophagy during infection of host cells.     

Lack of effective anti-apoptotic activities restricts growth of Parachlamydiaceae in insect cells

The fundamental role of programmed cell death in host defense is highlighted by the multitude of anti-apoptotic strategies evolved by various microbes, including the well-known obligate intracellular bacterial pathogens Chlamydia trachomatis and Chlamydia (Chlamydophila) pneumoniae. As inhibition of apoptosis is assumed to be essential for a successful infection of humans by these chlamydiae, we analyzed the anti-apoptotic capacity of close relatives that occur as symbionts of amoebae and might represent emerging pathogens. While Simkania negevensis was able to efficiently replicate within insect cells, which served as model for metazoan-derived host cells, the Parachlamydiaceae (Parachlamydia acanthamoebae and Protochlamydia amoebophila) displayed limited intracellular growth, yet these bacteria induced typical features of apoptotic cell death, including formation of apoptotic bodies, nuclear condensation, internucleosomal DNA fragmentation, and effector caspase activity. Induction of apoptosis was dependent on bacterial activity, but not bacterial de novo protein synthesis, and was detectable already at very early stages of infection. Experimental inhibition of host cell death greatly enhanced parachlamydial replication, suggesting that lack of potent anti-apoptotic activities in Parachlamydiaceae may represent an important factor compromising their ability to successfully infect non-protozoan hosts. These findings highlight the importance of the evolution of anti-apoptotic traits for the success of chlamydiae as pathogens of humans and animals.     

Incoming pathogens team up with harmless 'resident' bacteria

Microbial diseases occur as a result of multifarious host-pathogen interactions. However, invading pathogens encounter a large number of different harmless and beneficial bacterial species, which colonize and reside in the host. Surprisingly, there has been little study of the possible interactions between incoming pathogens and the resident bacterial community. Recent studies have revealed that resident bacteria assist different types of incoming pathogens via a wide variety of mechanisms including cell-cell signaling, metabolic interactions, evasion of the immune response and a resident-to-pathogen switch. This calls for serious consideration of pathogen-microbe interactions in the host with respect to disease severity and progression.     

Akt inhibition upregulates FasL, downregulates c-FLIPs and induces caspase-8-dependent cell death in Jurkat T lymphocytes

In T lymphocytes, the role of Akt in regulating Fas/Fas ligand (FasL)-mediated apoptotic signaling and death is not clearly understood. In this study, we observed that inhibition of Akt causes enhanced expression of FasL mRNA and protein and increased death-inducing signaling complex (DISC) formation with Fas-associated death domain (FADD) and procaspase-8 recruitment. Also, caspase-8 was activated at the DISC with accompanying decrease in c-FLIPs expression. FasL neutralizing antibody significantly decreased apoptotic death in the Akt-inhibited T cells. Additionally, Akt inhibition-induced Fas signaling was observed to link to the mitochondrial pathway via Bid cleavage. Further, inhibition of caspase-8 activity effectively blocked the loss of mitochondrial membrane potential and DNA fragmentation, suggesting that DISC formation and subsequent caspase-8 activation are critical initiating events in Akt inhibition-induced apoptotic death in T lymphocytes. These data demonstrate yet another important survival function governed by Akt kinase in T lymphocytes, which involves the regulation of FasL expression and consequent apoptotic signaling.     

Suppression of host defense in compatible plant-Pseudomonas syringae interactions

Despite impressive advances in the study of plant resistance to pathogens, little is known about the molecular basis of plant susceptibility to virulent pathogens. Recent progress in susceptible plant-Pseudomonas syringae interactions has provided a glimpse into the battles fought between plants and bacterial pathogens. A key step for pathogenesis appears to be the suppression of host defenses. Suppression of host defenses, including basal defense, gene-for-gene resistance and nonhost resistance, is a key step for pathogenesis. Defense suppression is mediated by bacterial effector proteins, which are secreted through the type III secretion system, and by coronatine, a bacterial toxin that structurally and functionally mimics methyl jasmonate, a plant defense signaling molecule.     

Host-pathogen interactions: cheating the host by making new connections

Dynamic signaling networks are required to perform complex cellular processes. Structural and functional data now indicate the intriguing possibility that extracellular bacterial pathogens use catalytic scaffolds to assemble unique supramolecular signaling networks that effectively subvert key cellular processes in the host.     

The Versatility of Helicobacter pylori CagA Effector Protein Functions: The Master Key Hypothesis

Several bacterial pathogens inject virulence proteins into host target cells that are substrates of eukaryotic tyrosine kinases. One of the key examples is the Helicobacter pylori CagA effector protein which is translocated by a type‐IV secretion system. Injected CagA becomes tyrosine‐phosphorylated on EPIYA sequence motifs by Src and Abl family kinases. CagA then binds to and activates/inactivates multiple signaling proteins in a phosphorylation‐dependent and phosphorylation‐independent manner. A recent proteomic screen systematically identified eukaryotic binding partners of the EPIYA phosphorylation sites of CagA and similar sites in other bacterial effectors by high‐resolution mass spectrometry. Individual phosphorylation sites recruited a surprisingly high number of interaction partners suggesting that each phosphorylation site can interfere with many downstream pathways. We now count 20 reported cellular binding partners of CagA, which represents the highest quantitiy among all yet known virulence‐associated effector proteins in the microbial world. This complexity generates a highly remarkable and puzzling scenario. In addition, the first crystal structure of CagA provided us with new information on the function of this important virulence determinant. Here we review the recent advances in characterizing the multiple binding signaling activities of CagA. Injected CagA can act as a ‘master key’ that evolved the ability to highjack multiple host cell signalling cascades, which include the induction of membrane dynamics, actin‐cytoskeletal rearrangements and the disruption of cell‐to‐cell junctions as well as proliferative, pro‐inflammatory and anti‐apoptotic nuclear responses. The discovery that different pathogens use this common strategy to subvert host cell functions suggests that more examples will emerge soon.     

CD46 signaling in T cells: Linking pathogens with polarity

CD46 is a cell surface protein that regulates complement activity and is utilized as a receptor by numerous viral and bacterial pathogens that infect humans. CD46 is not just an entry site for pathogens, but can affect various cellular activities in response to pathogen binding that can have profound consequences for the host response to infection. The study of CD46 signaling in T cells has emerged as an exciting area of research that is shedding new light on how pathogens might manipulate the host immune response. This review will focus on our current understanding of CD46 signaling in T cell polarity and how this might influence disease outcome.     

Mithramycin A activates Fas death pathway in leukemic cell lines

Mithramycin A (MMA, trade name Plicamycin) can facilitate TNFα- (Tumor Necrosis Factor) and Fas ligand-induced apoptosis. Besides, several drugs play their anticancer effect through Fas apoptotic pathway. So we investigated the effect of MMA on Fas signaling. In this study we show that MMA induces apoptosis in Fas sensitive Jurkat cells and Fas resistant KG1a cells. This effect involves Fas apoptotic pathway: cell exposure to MMA leads to Fas clustering at the cell surface, DISC (Death Inducing Signaling Complex) formation and caspase cleavage. This phenomenon is independent of Fas ligand/Fas interaction and blockade of Fas death pathway partially inhibits MMA-induced apoptosis. Moreover the activation of Fas apoptotic pathway by MMA is correlated to the modulation of c-Flip_L expression. Finally, pre-treatment with sub-lethal doses of MMA sensitizes KG1a cells to chemotherapeutic agents. Thus all these results may have important implications to improve clinical treatments.     

Subversion of membrane transport pathways by vacuolar pathogens

Mammalian phagocytes control bacterial infections effectively through phagocytosis, the process by which particles engulfed at the cell surface are transported to lysosomes for destruction. However, intracellular pathogens have evolved mechanisms to avoid this fate. Many bacterial pathogens use specialized secretion systems to deliver proteins into host cells that subvert signaling pathways controlling membrane transport. These bacterial effectors modulate the function of proteins that regulate membrane transport and alter the phospholipid content of membranes. Elucidating the biochemical function of these effectors has provided a greater understanding of how bacteria control membrane transport to create a replicative niche within the host and provided insight into the regulation of membrane transport in eukaryotic cells.     

Evaluation of the Fas/FasL signaling pathway in diabetic rat testis

We investigated the role of the Fas/Fas ligand (FasL) signaling pathway in diabetic male infertility. Male rats were divided into two groups: a control group and a streptozotocin induced diabetic group. Thirty days after induction of diabetes, samples of testes were harvested and fixed in 10% formalin for light microscopy. Germ cell apoptosis was determined using the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate in situ nick end-labeling (TUNEL) and immunostaining of caspase 8 and active caspase 3. We also investigated the expressions of Fas and FasL using immunohistochemistry. Streptozotocin-induced diabetes caused severe histopathological damage and increased apoptotic tubule and apoptotic cell indices, caspase 8 and caspase 3 expressions, and Fas and FasL-immunopositive cells in the rat testes. We suggest that the Fas/FasL signaling pathway may play a role in male infertility caused by diabetes.     

Pathogen-mediated posttranslational modifications: A re-emerging field

Posttranslational modifications are increasingly recognized as key strategies used by bacterial and viral pathogens to modulate host factors critical for infection. A number of recent studies illustrate how pathogens use these posttranslational modifications to target central signaling pathways in the host cell, such as the NF-kB and MAP kinase pathways, which are essential for pathogens' replication, propagation, and evasion from host immune responses. These discoveries open new avenues for investigating the fundamental mechanisms of pathogen infection and the development of new therapeutics.     

TNF receptor p55 is required for elimination of inflammatory cells following control of intracellular pathogens.

The elimination of lymphocytes within inflammatory lesions is a critical component in the resolution of disease once pathogens have been cleared. We report here that signaling through the TNF receptor p55 (TNFRp55) is required to eliminate lymphocytes from lesions associated with intracellular pathogens. Thus, TNFRp55-/- mice, but not Fas-deficient mice, maintained inflammatory lesions associated with either Leishmania major or Rhodococcus equi infection, although they developed a Th1 response and controlled the pathogens. Inflammatory cells from either L. major- or R. equi-infected C57BL/6 mice were sensitive to TNF-induced apoptosis, and conversely the number of apoptotic cells in the lesions from TNFRp55-/- mice was dramatically reduced compared with wild-type mice. Furthermore, in vivo depletion of TNF in wild-type mice blocked lesion regression following R. equi infection. Taken together, our results suggest that signaling through the TNFRp55, but not Fas, is required to induce apoptosis of T cells within inflammatory lesions once pathogens are eliminated, and that in its absence lesions fail to regress.     

Mycobacterium tuberculosis subverts innate immunity to evade specific effectors

The macrophage is the niche of the intracellular pathogen Mycobacterium tuberculosis. Induction of macrophage apoptosis by CD4(+) or CD8(+) T cells is accompanied by reduced bacterial counts, potentially defining a host defense mechanism. We have already established that M. tuberculosis-infected primary human macrophages have a reduced susceptibility to Fas ligand (FasL)-induced apoptosis. To study the mechanisms by which M. tuberculosis prevents apoptotic signaling, we have generated a cell culture system based on PMA- and IFN-gamma-differentiated THP-1 cells recapitulating the properties of primary macrophages. In these cells, nucleotide-binding oligomerization domain 2 or TLR2 agonists and mycobacterial infection protected macrophages from apoptosis and resulted in NF-kappaB nuclear translocation associated with up-regulation of the antiapoptotic cellular FLIP. Transduction of a receptor-interacting protein-2 dominant-negative construct showed that nucleotide-binding oligomerization domain 2 is not involved in protection in the mycobacterial infection system. In contrast, both a dominant-negative construct of the MyD88 adaptor and an NF-kappaB inhibitor abrogated the protection against FasL-mediated apoptosis, showing the implication of TLR2-mediated activation of NF-kappaB in apoptosis protection in infected macrophages. The apoptosis resistance of infected macrophages might be considered as an immune escape mechanism, whereby M. tuberculosis subverts innate immunity signaling to protect its host cell against FasL(+)-specific cytotoxic lymphocytes.