Rac GTPase instructs nuclear factor-kappaB activation by conveying the SCF complex and IkBalpha to the ruffling membranes

Effects of the Escherichia coli Bacterial Toxin Cytotoxic Necrotizing Factor 1 on Different Human and Animal Cells: A Systematic Review

Cytotoxic necrotizing factor 1 (CNF1) is a bacterial virulence factor, the target of which is represented by Rho GTPases, small proteins involved in a huge number of crucial cellular processes. CNF1, due to its ability to modulate the activity of Rho GTPases, represents a widely used tool to unravel the role played by these regulatory proteins in different biological processes. In this review, we summarized the data available in the scientific literature concerning the observed in vitro effects induced by CNF1. An article search was performed on electronic bibliographic resources. Screenings were performed of titles, abstracts, and full-texts according to PRISMA guidelines, whereas eligibility criteria were defined for in vitro studies. We identified a total of 299 records by electronic article search and included 76 original peer-reviewed scientific articles reporting morphological or biochemical modifications induced in vitro by soluble CNF1, either recombinant or from pathogenic Escherichia coli extracts highly purified with chromatographic methods. Most of the described CNF1-induced effects on cultured cells are ascribable to the modulating activity of the toxin on Rho GTPases and the consequent effects on actin cytoskeleton organization. All in all, the present review could be a prospectus about the CNF1-induced effects on cultured cells reported so far.

Activation of STAT transcription factors by the Rho-family GTPases

The Rho-family of small GTPases are biological molecular switches that are best known for their regulation of the actin cytoskeleton. Through their activation and stimulation of downstream effectors, the Rho-family control pathways involved in cellular morphology, which are commonly activated in cancer cell invasion and metastasis. While this makes them excellent potential therapeutic targets, a deeper understanding of the downstream signalling pathways they influence will be required for successful drug targeting. Signal transducers and activators of transcription (STATs) are a family of transcription factors that are hyper-activated in most cancer types and while STATs are widely understood to be activated by the JAK family of kinases, many additional activators have been discovered. A growing number of examples of Rho-family driven STAT activation, largely of the oncogenic family members, STAT3 and STAT5, are being identified. Cdc42, Rac1, RhoA, RhoC and RhoH have all been implicated in STAT activation, contributing to Rho GTPase-driven changes in cellular morphology that lead to cell proliferation, invasion and metastasis. This highlights the importance and therapeutic potential of the Rho-family as regulators of non-canonical activation of STAT signalling.

Bacterial cyclomodulins: types and roles in carcinogenesis

Various studies confirmed that bacterial infections contribute to carcinogenesis through the excessive accumulation of reactive oxygen species (ROS) and the expression of toxins that disrupt the cell cycle phases, cellular regulatory mechanisms and stimulate the production of tumorigenic inflammatory mediators. These toxins mimic carcinogens which act upon key cellular targets and result in mutations and genotoxicities. The cyclomodulins are bacterial toxins that incur cell cycle modulating effects rendering the expressing bacterial species of high carcinogenic potentiality. They are either cellular proliferating or cell cycle arrest cyclomodulins. Notably, cyclomodulins expressing bacterial species have been linked to different human carcinomas. For instance, Escherichia coli species producing the colibactin were highly prevalent among colorectal carcinoma patients, CagA⁺ Helicobacter pylori species were associated with MALT lymphomas and gastric carcinomas and Salmonella species producing CdtB were linked to hepatobiliary carcinomas. These species stimulated the overgrowth of pre-existing carcinomas and induced hyperplasia in in vivo animal models suggesting a role for the cyclomodulins in carcinogenesis. Wherefore, the prevalence and mode of action of these toxins were the focus of many researchers and studies. This review discusses different types of bacterial cyclomodulins highlighting their mode of action and possible role in carcinogenesis.

Cytoskeleton Dynamics in Peripheral T Cell Lymphomas: An Intricate Network Sustaining Lymphomagenesis

Defects in cytoskeleton functions support tumorigenesis fostering an aberrant proliferation and promoting inappropriate migratory and invasive features. The link between cytoskeleton and tumor features has been extensively investigated in solid tumors. However, the emerging genetic and molecular landscape of peripheral T cell lymphomas (PTCL) has unveiled several alterations targeting structure and function of the cytoskeleton, highlighting its role in cell shape changes and the aberrant cell division of malignant T cells. In this review, we summarize the most recent evidence about the role of cytoskeleton in PTCLs development and progression. We also discuss how aberrant signaling pathways, like JAK/STAT3, NPM-ALK, RhoGTPase, and Aurora Kinase, can contribute to lymphomagenesis by modifying the structure and the signaling properties of cytoskeleton.

Gut Microbiota and Colon Cancer: A Role for Bacterial Protein Toxins?

Accumulating evidence indicates that the human intestinal microbiota can contribute to the etiology of colorectal cancer. Triggering factors, including inflammation and bacterial infections, may favor the shift of the gut microbiota from a mutualistic to a pro-carcinogenic configuration. In this context, certain bacterial pathogens can exert a pro-tumoral activity by producing enzymatically-active protein toxins that either directly induce host cell DNA damage or interfere with essential host cell signaling pathways involved in cell proliferation, apoptosis, and inflammation. This review is focused on those toxins that, by mimicking carcinogens and cancer promoters, could represent a paradigm for bacterially induced carcinogenesis.

Oral and intestinal bacterial exotoxins: Potential linked to carcinogenesis

Growing evidence suggests that imbalances in resident microbes (dysbiosis) can promote chronic inflammation, immune-subversion, and production of carcinogenic metabolites, thus leading to neoplasia. Yet, evidence to support a direct link of individual bacteria species to human sporadic cancer is still limited. This chapter focuses on several emerging bacterial toxins that have recently been characterized for their potential oncogenic properties toward human orodigestive cancer and the presence of which in human tissue samples has been documented. These include cytolethal distending toxins produced by various members of gamma and epsilon Proteobacteria, Dentilisin from mammalian oral Treponema, Pasteurella multocida toxin, two Fusobacterial toxins, FadA and Fap2, Bacteroides fragilis toxin, colibactin, cytotoxic necrotizing factors and α-hemolysin from Escherichia coli, and Salmonella enterica AvrA. It was clear that these bacterial toxins have biological activities to induce several hallmarks of cancer. Some toxins directly interact with DNA or chromosomes leading to their breakdowns, causing mutations and genome instability, and others modulate cell proliferation, replication and death and facilitate immune evasion and tumor invasion, prying specific oncogene and tumor suppressor pathways, such as p53 and β-catenin/Wnt. In addition, most bacterial toxins control tumor-promoting inflammation in complex and diverse mechanisms. Despite growing laboratory evidence to support oncogenic potential of selected bacterial toxins, we need more direct evidence from human studies and mechanistic data from physiologically relevant experimental animal models, which can reflect chronic infection in vivo, as well as take bacterial-bacterial interactions among microbiome into consideration.

The Escherichia coli protein toxin cytotoxic necrotizing factor 1 induces epithelial mesenchymal transition

Some toxigenic bacteria produce protein toxins with carcinogenic signatures, which either directly damage DNA or stimulate signalling pathways related to cancer. So far, however, only a few of them have been proved to favour the induction or progression of cancer. In this work, we report that the Rho-activating Escherichia coli protein toxin, cytotoxic necrotising factor 1 (CNF1), induces epithelial to mesenchymal transition (EMT) in intestinal epithelial cells. EMT is a crucial step in malignant tumour conversion and invasiveness. In the case of CNF1, it occurs by up-regulation of the transcription factors ZEB1 and Snail1, delocalisation of E-cadherin and β-catenin, activation of the serine/threonine kinase mTOR, accelerated wound healing, and invasion. However, our results highlight that nontransformed epithelial cells entail the presence of inflammatory factors, in addition to CNF1, to acquire a mesenchymal-like behaviour. All this suggests that the surrounding microenvironment, as well as the cell type, dramatically influences the CNF1 ability to promote carcinogenic traits.

Bacterial Toxins and Targeted Brain Therapy: New Insights from Cytotoxic Necrotizing Factor 1 (CNF1)

Pathogenic bacteria produce toxins to promote host invasion and, therefore, their survival. The extreme potency and specificity of these toxins confer to this category of proteins an exceptionally strong potential for therapeutic exploitation. In this review, we deal with cytotoxic necrotizing factor (CNF1), a cytotoxin produced by Escherichia coli affecting fundamental cellular processes, including cytoskeletal dynamics, cell cycle progression, transcriptional regulation, cell survival and migration. First, we provide an overview of the mechanisms of action of CNF1 in target cells. Next, we focus on the potential use of CNF1 as a pharmacological treatment in central nervous system’s diseases. CNF1 appears to impact neuronal morphology, physiology, and plasticity and displays an antineoplastic activity on brain tumors. The ability to preserve neural functionality and, at the same time, to trigger senescence and death of proliferating glioma cells, makes CNF1 an encouraging new strategy for the treatment of brain tumors.

Breast cancer-associated gene 3 interacts with Rac1 and augments NF-κB signaling in vitro, but has no effect on RANKL-induced bone resorption in vivo

Breast cancer-associated gene 3 (BCA3) is a recently identified adaptor protein whose functions are still being defined. BCA3 has been reported to be an important regulator of nuclear factor-κB (NF-κB) signaling. It has also been reported to interact with the small GTPase, Rac1. Consistent with that observation, in the present study, BCA3 was found to interact with nuclear Rac1 in 293 cells and influence NF-κB signaling. Additional experiments revealed that depending on cell type, BCA3 augmented, attenuated or had no effect on NF-κB signaling in vitro. Since canonical NF-κB signaling is a critical downstream target from activated receptor activator of nuclear factor κB (RANK) that is required for mature osteoclast formation and function, BCA3 was selectively overexpressed in osteoclasts in vivo using the cathepsin K promoter and the response to exogenous receptor activator of nuclear factor κB ligand (RANKL) administration was examined. Despite its ability to augment NF-κB signaling in other cells, transgenic animals injected with high-dose RANKL had the same hypercalcemic response as their wild-type littermates. Furthermore, the degree of bone loss induced by a 2-week infusion of low-dose RANKL was the same in both groups. Combined with earlier studies, the data from our study data indicate that BCA3 can affect NF-κB signaling and that BCA3 plays a cell-type dependent role in this process. The significance of the BCA3/NF-κB interaction in vivo in bone remains to be determined.

Activated Rac1 regulates the degradation of IκBα and the nuclear translocation of STAT3-NFκB complexes in starved cancer cells

In several human tumors, signal transducer and activator of transcription 3 (STAT3) and nuclear factor κB (NFκB) are activated and interact; how these STAT3–NFκB complexes are transported to the nucleus is not fully understood. In this study, we found that Rac1 was activated in starved cancer cells and that activated Rac1 coexisted with STAT3 and NFκB. Rac1 knockdown and overexpression of the dominant-negative mutant Rac1N19 inhibited the degradation of IκBα, an inhibitor of NFκB. MG132, an inhibitor of the ubiquitin proteasome pathway, increased the amount of non-phosphorylated IκBα, but not serine-phosphorylated IκBα, indicating that IκBα degradation by Rac1 in starved cancer cells is independent of IκBα serine phosphorylation by IKK. Rac1 knockdown also inhibited the nuclear translocation of STAT3–NFκB complexes, indicating that this translocation requires activated Rac1. We also demonstrated that the mutant STAT3 Y705F could form complexes with NFκB, and these unphosphorylated STAT3–NFκB complexes translocated into the nucleus and upregulated the activity of NFκB in starved cancer cells, suggesting that phosphorylation of STAT3 is not essential for its translocation. To our knowledge, this is the first study demonstrating the crucial role of Rac1 in the function of STAT3–NFκB complexes in starved cancer cells and implies that targeting Rac1 may have future therapeutic significance in cancer therapy.

Cell-to-cell propagation of the bacterial toxin CNF1 via extracellular vesicles: potential impact on the therapeutic use of the toxin

Eukaryotic cells secrete extracellular vesicles (EVs), either constitutively or in a regulated manner, which represent an important mode of intercellular communication. EVs serve as vehicles for transfer between cells of membrane and cytosolic proteins, lipids and RNA. Furthermore, certain bacterial protein toxins, or possibly their derived messages, can be transferred cell to cell via EVs. We have herein demonstrated that eukaryotic EVs represent an additional route of cell-to-cell propagation for the Escherichia coli protein toxin cytotoxic necrotizing factor 1 (CNF1). Our results prove that EVs from CNF1 pre-infected epithelial cells can induce cytoskeleton changes, Rac1 and NF-κB activation comparable to that triggered by CNF1. The observation that the toxin is detectable inside EVs derived from CNF1-intoxicated cells strongly supports the hypothesis that extracellular vesicles can offer to the toxin a novel route to travel from cell to cell. Since anthrax and tetanus toxins have also been reported to engage in the same process, we can hypothesize that EVs represent a common mechanism exploited by bacterial toxins to enhance their pathogenicity.

Moringa oleifera Flower Extract Suppresses the Activation of Inflammatory Mediators in Lipopolysaccharide-Stimulated RAW 264.7 Macrophages via NF-κB Pathway

Aim of Study. Moringa oleifera Lam. (M. oleifera) possess highest concentration of antioxidant bioactive compounds and is anticipated to be used as an alternative medicine for inflammation. In the present study, we investigated the anti-inflammatory activity of 80% hydroethanolic extract of M. oleifera flower on proinflammatory mediators and cytokines produced in lipopolysaccharide- (LPS-) induced RAW 264.7 macrophages. Materials and Methods. Cell cytotoxicity was conducted by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Nitric oxide (NO) production was quantified through Griess reaction while proinflammatory cytokines and other key inflammatory markers were assessed through enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Results. Hydroethanolic extract of M. oleifera flower significantly suppressed the secretion and expression of NO, prostaglandin E2 (PGE2), interleukin- (IL-) 6, IL-1β, tumor necrosis factor-alpha (TNF-α), nuclear factor-kappa B (NF-κB), inducible NO synthase (iNOS), and cyclooxygenase-2 (COX-2). However, it significantly increased the production of IL-10 and IκB-α (inhibitor of κB) in a concentration dependent manner (100 μg/mL and 200 μg/mL). Conclusion. These results suggest that 80% hydroethanolic extract of M. oleifera flower has anti-inflammatory action related to its inhibition of NO, PGE2, proinflammatory cytokines, and inflammatory mediator's production in LPS-stimulated macrophages through preventing degradation of IκB-α in NF-κB signaling pathway.

Switching Rho GTPase activation into effective antibacterial defenses requires the caspase-1/IL-1beta signaling axis

The monitoring of the activation state of Rho GTPases has emerged as a potent innate immune mechanism for detecting pathogens. In the March issue of PLOS Pathogens, we show that the activation of Rho GTPases by the CNF1 toxin during E. coli-triggered bacteremia leads to a GR1(+)cell-mediated efficient bacterial clearing and improves host survival. Host alarm requires the Caspase-1/IL-1beta signaling axis. Furthermore, we discover that pathogenic bacteria have the capacity to block immune responses via the expression of the α-hemolysin pore-forming toxin. In this commentary, we will comment on these findings and highlight the questions raised by this example of attack-defense mechanisms used alternatively by the pathogen and the host during blood infection.

Functional compartmentalisation of NF-kB-associated proteins in A431 cells

NF-kB proteins belong to a family of ubiquitous transcription factors involved in a number of cellular responses. While the pathways of NF-kB activation and input into the regulation of gene activity have been comprehensively investigated, its cytoplasmic functions are poorly understood. In this study we addressed effects of the compartmentalisation of NF-kB proteins RelA/p65 and p50 in relation to the inhibitor IkB-a, using fibronectin (FN) and epidermal growth factor (EGF) for environmental stimulation of epidermoid carcinoma A431 cells. We thus assessed the presence of NF-kB family proteins in the cytosol, membrane, nuclear and cytoskeletal fractions with a special attention to the cytoskeletal fraction to define whether NFkB was active or not. Sub-cellular fractionation demonstrated that the proportion of RelA/p65 differed in diverse sub-cellular fractions, and that the cytoskeleton harboured about 7% thereof. Neither the nuclear nor the cytoskeleton fraction did contain IkB-a. The cytoskeleton binding of RelA/p65 and p50 was further confirmed by co-localisation and electron microscopy data. During 30-min EGF stimulation similar dynamics were found for RelA/p65 and IkB-a in the cytosol, RelA/p65 and p50 in the nucleus and p50 and IkB-a in the membrane. Furthermore, EGF stimulation for 30 min resulted in a threefold accumulation of RelA/p65 in cytoskeletal fraction. Our results suggest that nuclear-, membrane- and cytoskeleton-associated NF-kB are dynamic and comprise active pools, whereas the cytoplasmic is more constant and likely non-active due to the presence of IkB-a. Moreover, we discovered the existence of a dynamic, IkB-a-free pool of RelA/p65 associated with cytoskeletal fraction, what argues for a special regulatory role of the cytoskeleton in NF-kB stimulation.

N-methyl pyrrolidone (NMP) inhibits lipopolysaccharide-induced inflammation by suppressing NF-κB signaling

OBJECTIVE

N-methyl pyrrolidone (NMP), a small bioactive molecule, stimulates bone formation and inhibits osteoclast differentiation and bone resorption. The present study was aimed to evaluate the anti-inflammatory potentials of NMP on the inflammatory process and the underlying molecular mechanisms in RAW264.7 macrophages.

MATERIALS AND METHODS

RAW264.7 macrophages and mouse primary bone marrow macrophages (mBMMs) were used as an in vitro model to investigate inflammatory processes. Cells were pre-treated with or without NMP and then stimulated with lipopolysaccharides (LPS). The productions of cytokines and NO were determined by proteome profiler method and nitrite analysis, respectively. The expressions of nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were measured by Western blotting and/or qPCR. Western blot, ELISA-base reporter assay, and immunofluorescence were used to evaluate the activation of MAP kinases and NF-κB.

RESULTS

LPS-induced mRNA expressions of TNF-α, IL-1β, IL-6, iNOS, and COX-2 were inhibited by NMP in a dose-dependent manner. NMP also suppressed the LPS-increased productions of iNOS and NO. The proteome profiler array showed that several cytokines and chemokines involved in inflammation and up-regulated by LPS stimulation were significantly down-regulated by NMP. Additionally, this study shows that the effect of NMP is mediated through down-regulation of NFκB pathway.

CONCLUSIONS

Our results show that NMP inhibits the inflammatory mediators in macrophages by an NFκB-dependent mechanism, based on the epigenetical activity of NMP as bromodomain inhibitor. In the light of its action on osteoblast and osteoclast differentiation process and its anti-inflammatory potential, NMP might be used in inflammation-related bone loss.

Key network approach reveals new insight into Alzheimer's disease

Alzheimer's disease (AD) is a severe neurodegenerative disorder without curative treatment. Extensive data on pathological molecular processes have been accumulated over the last years. These data combined allows a systems biology approach to identify key regulatory elements of AD and to establish a model descriptive of the disease process which can be used for the development of therapeutic agents. In this study, the authors propose a closed network that uses a set of nodes (amyloid beta, tau, beta-secretase, glutamate, cyclin-dependent kinase 5, phosphoinositide 3-kinase and hypoxia-induced factor 1 alpha) as key elements of importance to the pathogenesis of AD. The proposed network, in total 39 nodes, is able to become a novel tool capable of providing new insights into AD, such as feedback loops. Further, it highlights interconnections between pathways and identifies their combination for therapy of AD.

Rosiglitazone inhibits hepatic insulin resistance induced by chronic pancreatitis and IKK-β/NF-κB expression in liver

OBJECTIVES

This study aimed to investigate the influence of rosiglitazone on hepatic insulin resistance and the expressions of IκB kinase-β (IKK-β)/nuclear factor-κB (NF-κB) in chronic pancreatitis (CP).

METHODS

After CP was induced in rats, rosiglitazone and GW9662 were administered at the doses of 4 and 2 mg/kg per day for 4 weeks, respectively. Then, glucose and insulin tolerance tests were performed. Hepatocytes were isolated for the glucose release experiments. Determination of the IKK-β, NF-κB, and Ser307p-insulin receptor substrates-1 (Ser307p-IRS-1) expression in the liver was performed.

RESULTS

The increased plasma glucose, reduced insulin sensitivity, and the capacity of insulin to suppress glucose release in hepatocytes were observed in CP rats. The IKK-β, NF-κB, and Ser307p-IRS-1 expressions were significantly higher in the liver of CP rats than in sham-operated rats (P < 0.05). Rosiglitazone treatment significantly improved hepatic insulin sensitivity and inhibited the IKK-β, NF-κB, and Ser307p-IRS-1 expressions in the liver (P < 0.05). Counteraction with peroxisome proliferator-activated receptor-γ by GW9662 attenuated the aforementioned effects of rosiglitazone.

CONCLUSIONS

Rosiglitazone attenuates hepatic insulin resistance induced by CP. The inhibition of hepatic IKK-β and NF-κB expressions via peroxisome proliferator-activated receptor-γ may be involved in the therapeutic effect of rosiglitazone.

Newcastle disease virus interaction in targeted therapy against proliferation and invasion pathways of glioblastoma multiforme

Glioblastoma multiforme (GBM), or grade IV glioma, is one of the most lethal forms of human brain cancer. Current bioscience has begun to depict more clearly the signalling pathways that are responsible for high-grade glioma initiation, migration, and invasion, opening the door for molecular-based targeted therapy. As such, the application of viruses such as Newcastle disease virus (NDV) as a novel biological bullet to specifically target aberrant signalling in GBM has brought new hope. The abnormal proliferation and aggressive invasion behaviour of GBM is reported to be associated with aberrant Rac1 protein signalling. NDV interacts with Rac1 upon viral entry, syncytium induction, and actin reorganization of the infected cell as part of the replication process. Ultimately, intracellular stress leads the infected glioma cell to undergo cell death. In this review, we describe the characteristics of malignant glioma and the aberrant genetics that drive its aggressive phenotype, and we focus on the use of oncolytic NDV in GBM-targeted therapy and the interaction of NDV in GBM signalling that leads to inhibition of GBM proliferation and invasion, and subsequently, cell death.

Enhancement of mitochondrial ATP production by the Escherichia coli cytotoxic necrotizing factor 1

Mitochondria are dynamic organelles that constantly change shape and structure in response to different stimuli and metabolic demands of the cell. The Escherichia coli protein toxin cytotoxic necrotizing factor 1 (CNF1) has recently been reported to influence mitochondrial activity in a mouse model of Rett syndrome and to increase ATP content in the brain tissue of an Alzheimer's disease mouse model. In the present work, the ability of CNF1 to influence mitochondrial activity was investigated in IEC-6 normal intestinal crypt cells. In these cells, the toxin was able to induce an increase in cellular ATP content, probably due to an increment of the mitochondrial electron transport chain. In addition, the CNF1-induced Rho GTPase activity also caused changes in the mitochondrial architecture that mainly consisted in the formation of a complex network of elongated mitochondria. The involvement of the cAMP-dependent protein kinase A signaling pathway was postulated. Our results demonstrate that CNF1 positively affects mitochondria by bursting their energetic function and modifying their morphology.

Rho protein GTPases and their interactions with NFκB: crossroads of inflammation and matrix biology

The RhoGTPases, with RhoA, Cdc42 and Rac being major members, are a group of key ubiquitous proteins present in all eukaryotic organisms that subserve such important functions as cell migration, adhesion and differentiation. The NFκB (nuclear factor κB) is a family of constitutive and inducible transcription factors that through their diverse target genes, play a major role in processes such as cytokine expression, stress regulation, cell division and transformation. Research over the past decade has uncovered new molecular links between the RhoGTPases and the NFκB pathway, with the RhoGTPases playing a positive or negative regulatory role on NFκB activation depending on the context. The RhoA–NFκB interaction has been shown to be important in cytokine-activated NFκB processes, such as those induced by TNFα (tumour necrosis factor α). On the other hand, Rac is important for activating the NFκB response downstream of integrin activation, such as after phagocytosis. Specific residues of Rac1 are important for triggering NFκB activation, and mutations do obliterate this response. Other upstream triggers of the RhoGTPase–NFκB interactions include the suppressive p120 catenin, with implications for skin inflammation. The networks described here are not only important areas for further research, but are also significant for discovery of targets for translational medicine.

DLC1 suppresses NF-κB activity in prostate cancer cells due to its stabilizing effect on adherens junctions

DLC1 (Deleted in Liver Cancer 1) gene encodes a RhoGTPase-activating protein (RhoGAP), which exerts most of its tumor suppressor functions through suppression of small Rho GTPases proteins RhoA, RhoB, RhoC and to some degree Cdc42, but not Rac. RhoGTPases are implicated in NF-κB activation in highly invasive prostate carcinoma (PCA), with consequences on cell proliferation, survival and metastatic capacity. Here we demonstrate that DLC1 transduction in two androgen-independent (AI) and highly metastatic PCA cell lines negatively regulates NF-κB activity in a GAP- and α-catenin-dependent manner. Expressed DLC1 protein suppresses the phosphorylation of NF-κB inhibitor, IκBα, causes its relocation from membrane ruffles into cytoplasm and attenuates its ubiquitination and subsequent degradation. DLC1-mediated NF-kB suppression and its effects are comparable to NF-κB inhibition using either shRNA knockdown or peptide inhibitor. Expression of transduced DLC1 suppressed the expression of NF-κB mediated genes. Such effects were found to be reliant on presence of calcium, indicating that the observed modifications are dependent on, and enabled by DLC-mediated stabilization of adherens junctions. These results expand the multitude of DLC1 interactions with other genes that modulate its oncosuppressive function, and may have potential therapeutic implications.

The E. coli CNF1 as a pioneering therapy for the central nervous system diseases

The Cytotoxic Necrotizing Factor 1 (CNF1), a protein toxin from pathogenic E. coli, modulates the Rho GTPases, thus, directing the organization of the actin cytoskeleton. In the nervous system, the Rho GTPases play a key role in several processes, controlling the morphogenesis of dendritic spines and synaptic plasticity in brain tissues. This review is focused on the peculiar property of CNF1 to enhance brain plasticity in in vivo animal models of central nervous system (CNS) diseases, and on its possible application in therapy.

The cytotoxic necrotizing factor of Yersinia pseudotuberculosis (CNFY) enhances inflammation and Yop delivery during infection by activation of Rho GTPases

Some isolates of Yersinia pseudotuberculosis produce the cytotoxic necrotizing factor (CNF_Y), but the functional consequences of this toxin for host-pathogen interactions during the infection are unknown. In the present study we show that CNF_Y has a strong influence on virulence. We demonstrate that the CNF_Y toxin is thermo-regulated and highly expressed in all colonized lymphatic tissues and organs of orally infected mice. Most strikingly, we found that a cnfY knock-out variant of a naturally toxin-expressing Y. pseudotuberculosis isolate is strongly impaired in its ability to disseminate into the mesenteric lymph nodes, liver and spleen, and has fully lost its lethality. The CNF_Y toxin contributes significantly to the induction of acute inflammatory responses and to the formation of necrotic areas in infected tissues. The analysis of the host immune response demonstrated that presence of CNF_Y leads to a strong reduction of professional phagocytes and natural killer cells in particular in the spleen, whereas loss of the toxin allows efficient tissue infiltration of these immune cells and rapid killing of the pathogen. Addition of purified CNF_Y triggers formation of actin-rich membrane ruffles and filopodia, which correlates with the activation of the Rho GTPases, RhoA, Rac1 and Cdc42. The analysis of type III effector delivery into epithelial and immune cells in vitro and during the course of the infection further demonstrated that CNF_Y enhances the Yop translocation process and supports a role for the toxin in the suppression of the antibacterial host response. In summary, we highlight the importance of CNF_Y for pathogenicity by showing that this toxin modulates inflammatory responses, protects the bacteria from attacks of innate immune effectors and enhances the severity of a Yersinia infection.

Various toxins and effector proteins of bacterial pathogens have been found to manipulate eukaryotic cell machineries to promote persistence and proliferation within their hosts. Many of these virulence factors target small Rho GTPases, but their role in pathogenesis is often unknown. Here, we addressed the expression and functional consequences of the CNF_Y toxin found in some isolates of Y. pseudotuberculosis. We found that CNF_Y besides modulating the cell cytoskeleton by activation of the GTPases RhoA, Rac1 and Cdc42, contributes to increased inflammation and tissue damage. Moreover, CNF_Y increases the ability of Yersinia to prevent the attack of the immune system, by enhancing the delivery of antiphagocytic and cytotoxic effectors into professional phagocytes. Our findings provide the first insights into the multi-functional action and severe consequences of the CNF_Y toxin on the inflammatory response and disease-associated tissue damage during the natural course of the infection.

The cytotoxic necrotizing factor 1 from E. coli: a janus toxin playing with cancer regulators

Certain strains of Escherichia coli have been indicated as a risk factor for colon cancer. E. coli is a normal inhabitant of the human intestine that becomes pathogenic, especially in extraintestinal sites, following the acquisition of virulence factors, including the protein toxin CNF1. This Rho GTPases-activating toxin induces dysfunctions in transformed epithelial cells, such as apoptosis counteraction, pro-inflammatory cytokines’ release, COX2 expression, NF-kB activation and boosted cellular motility. As cancer may arise when the same regulatory pathways are affected, it is conceivable to hypothesize that CNF1-producing E. coli infections can contribute to cancer development. This review focuses on those aspects of CNF1 related to transformation, with the aim of contributing to the identification of a new possible carcinogenic agent from the microbial world.

Pleiotropic roles of bile acids in metabolism

Enzymatic oxidation of cholesterol generates numerous distinct bile acids that function both as detergents that facilitate digestion and absorption of dietary lipids, and as hormones that activate four distinct receptors. Activation of these receptors alters gene expression in multiple tissues, leading to changes not only in bile acid metabolism but also in glucose homeostasis, lipid and lipoprotein metabolism, energy expenditure, intestinal motility and bacterial growth, inflammation, liver regeneration, and hepatocarcinogenesis. This review covers the roles of specific bile acids, synthetic agonists, and their cognate receptors in controlling these diverse functions, as well as their current use in treating human diseases.

The Rho-GEF Trio regulates a novel pro-inflammatory pathway through the transcription factor Ets2

Inflammation is characterized by endothelium that highly expresses numerous adhesion molecules to trigger leukocyte extravasation. Central to this event is increased gene transcription. Small Rho-GTPases not only control the actin cytoskeleton, but are also implicated in gene regulation. However, in inflammation, it is not clear how this is regulated. Here, we show that the guanine-nucleotide exchange factor Trio expression is increased upon inflammatory stimuli in endothelium. Additionally, increased Trio expression was found in the vessel wall of rheumatoid arthritis patients. Trio silencing impaired VCAM-1 expression. Finally, we excluded that Trio-controlled VCAM-1 expression used the classical NFκB or MAP-kinase pathways, but rather acts on the transcriptional level by increasing phosphorylation and nuclear translocalization of Ets2. These data implicate Trio in regulating inflammation and provide novel targets for therapeutic purposes to treat inflammatory diseases such as rheumatoid arthritis.

Ubiquitylation of active Rac1 by the E3 ubiquitin-ligase HACE1

Rho GTPases undergo ubiquitylation and degradation via the ubiquitin-proteasome pathway. We now report in the November issue of Developmental Cell that the E3 ubiquitin-ligase HACE1 catalyzes the ubiquitylation of GTP-bound Rac1. Depletion of HACE1 leads to an increase of Rac1 activity. We have proposed that HACE1 limits Rac1 activity in cells, a regulation that is usurped by some pathogenic bacteria for efficient invasion of host cell monolayers. We here review these findings in parallel with the regulation of RhoA by the ubiquitin and proteasome system (UPS) and discuss the impact of these regulations on the capacity of Rho GTPases to signal.

Rac1 signalling modulates a STAT5/BCL-6 transcriptional switch on cell-cycle-associated target gene promoters

Gene expression depends on binding of transcriptional regulators to gene promoters, a process controlled by signalling pathways. The transcriptional repressor B-cell lymphoma (BCL)-6 downregulates genes involved in cell-cycle progression and becomes inactivated following phosphorylation by the Rac1 GTPase-activated protein kinase PAK1. Interestingly, the DNA motifs recognized by BCL-6 and signal transducers and activators of transcription 5 (STAT5) are similar. Because STAT5 stimulation in epithelial cells can also be triggered by Rac1 signalling, we asked whether both factors have opposing roles in transcriptional regulation and whether Rac1 signalling may coordinate a transcription factor switch. We used chromatin immunoprecipitation to show that active Rac1 promotes release of the repressor BCL-6 while increasing binding of STAT5A to a BCL-6-regulated reporter gene. We further show in colorectal cell lines that the endogenous activation status of the Rac1/PAK1 pathway correlated with the phosphorylation status of BCL-6 and STAT5A. Three cellular genes (cyclin D2, p15^INK4B, small ubiquitin-like modifier 1) were identified to be inversely regulated by BCL-6 and STAT5A and responded to Rac1 signalling with increased expression and corresponding changes in promoter occupancy. Together, our data show that Rac1 signalling controls a group of target genes that are repressed by BCL-6 and activated by STAT5A, providing novel insights into the modulation of gene transcription by GTPase signalling.

Bacterial effectors: learning on the fly

A common defining characteristic of pathogenic bacteria is the expression of a repertoire of effector molecules that have been named virulence factors. These bacterial factors include a -variety of proteins, such as toxins that are internalized by receptors and translocate across endosomal membranes to reach the cytosol, as well as others that are introduced directly into the cell by means of bacterial secretory apparatuses. Given the importance of these effectors for understanding bacterial pathogenicity, significant effort has been made to dissect their molecular mechanisms of action and their respective roles during infection. Herein we will discuss how Drosophila have been used as a model system to study these important microbial effectors, and to understand their contribution to pathogenicity.

Ubiquitination: Added complexity in Ras and Rho family GTPase function

The regulation of the small GTPases leading to their membrane localization has long been attributed to processing of their C-terminal CAAX box. As deregulation of many of these GTPases have been implicated in cancer and other disorders, prenylation and methylation of this CAAX box has been studied in depth as a possibility for drug targeting, but unfortunately, to date no drug has proved clinically beneficial. However, these GTPases also undergo other modifications that may be important for their regulation. Ubiquitination has long been demonstrated to regulate the fate of numerous cellular proteins and recently it has become apparent that many GTPases, along with their GAPs, GeFs and GDis, undergo ubiquitination leading to a variety of fates such as re-localization or degradation. in this review we focus on the recent literature demonstrating that the regulation of small GTPases by ubiquitination, either directly or indirectly, plays a considerable role in controlling their function and that targeting these modifications could be important for disease treatment.

The interferon-gamma-induced GTPase, mGBP-2, inhibits tumor necrosis factor alpha (TNF-alpha) induction of matrix metalloproteinase-9 (MMP-9) by inhibiting NF-kappaB and Rac protein

Matrix metalloproteinase-9 (MMP-9) is important in numerous normal and pathological processes, including the angiogenic switch during tumor development and tumor metastasis. Whereas TNF-α and other cytokines up-regulate MMP-9 expression, interferons (IFNs) inhibit MMP-9 expression. We found that IFN-γ treatment or forced expression of the IFN-induced GTPase, mGBP-2, inhibit TNF-α-induced MMP-9 expression in NIH 3T3 fibroblasts, by inhibiting MMP-9 transcription. The NF-κB transcription factor is required for full induction of MMP-9 by TNF-α. Both IFN-γ and mGBP-2 inhibit the transcription of a NF-κB-dependent reporter construct, suggesting that mGBP-2 inhibits MMP-9 induction via inhibition of NF-κB-mediated transcription. Interestingly, mGBP-2 does not inhibit TNF-α-induced degradation of IκBα or p65/RelA translocation into the nucleus. However, mGBP-2 inhibits p65 binding to a κB oligonucleotide probe in gel shift assays and to the MMP-9 promoter in chromatin immunoprecipitation assays. In addition, TNF-α activation of NF-κB in NIH 3T3 cells is dependent on Rac activation, as evidenced by the inhibition of TNF-α induction of NF-κB-mediated transcription by a dominant inhibitory form of Rac1. A role for Rac in the inhibitory action of mGBP-2 on NF-κB is further shown by the findings that mGBP-2 inhibits TNF-α activation of endogenous Rac and constitutively activate Rac can restore NF-κB transcription in the presence of mGBP-2. This is a novel mechanism by which IFNs can inhibit the cytokine induction of MMP-9 expression.

The role of ubiquitylation and degradation in RhoGTPase signalling

Rho-like guanosine triphosphatases (RhoGTPases) control many aspects of cellular physiology through their effects on the actin cytoskeleton and on gene transcription. Signalling by RhoGTPases is tightly coordinated and requires a series of regulatory proteins, including guanine-nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and guanine-nucleotide dissociation inhibitors (GDIs). GEFs and GAPs regulate GTPase cycling between the active (GTP-bound) and inactive (GDP-bound) states, whereas GDI is a cytosolic chaperone that binds inactive RhoGTPases. Like many other proteins, RhoGTPases are subject to degradation following the covalent conjugation of ubiquitin. There have been increasing indications that ubiquitylation of small GTPases occurs in a regulated fashion, primarily upon activation, and is an important means to control signalling output. Recent work has identified cellular proteins that control RasGTPase and RhoGTPase ubiquitylation and degradation, allowing us to amend the canonical model for GTPase (in)activation. Moreover, accumulating evidence for indirect regulation of GTPase function through the ubiquitylation of GTPase regulators makes this post-translational modification a key feature of GTPase-dependent signalling pathways. Here, we will discuss these recent insights into the regulation of RhoGTPase ubiquitylation and their relevance for cell signalling.

Strain-specific activation of the NF-kappaB pathway by GRA15, a novel Toxoplasma gondii dense granule protein

NF-κB is an integral component of the immune response to Toxoplasma gondii. Although evidence exists that T. gondii can directly modulate the NF-κB pathway, the parasite-derived effectors involved are unknown. We determined that type II strains of T. gondii activate more NF-κB than type I or type III strains, and using forward genetics we found that this difference is a result of the polymorphic protein GRA15, a novel dense granule protein which T. gondii secretes into the host cell upon invasion. A GRA15-deficient type II strain has a severe defect in both NF-κB nuclear translocation and NF-κB-mediated transcription. Furthermore, human cells expressing type II GRA15 also activate NF-κB, demonstrating that GRA15 alone is sufficient for NF-κB activation. Along with the rhoptry protein ROP16, GRA15 is responsible for a large part of the strain differences in the induction of IL-12 secretion by infected mouse macrophages. In vivo bioluminescent imaging showed that a GRA15-deficient type II strain grows faster compared with wild-type, most likely through its reduced induction of IFN-γ. These results show for the first time that a dense granule protein can modulate host signaling pathways, and dense granule proteins can therefore join rhoptry proteins in T. gondii's host cell-modifying arsenal.

Interaction and co-localization of JC virus large T antigen and the F-box protein β-transducin-repeat containing protein

Lytic infection and transformation of cultured cells by JC virus (JCV) require five tumor proteins, which interact with factors regulating critical cellular processes. We demonstrate that JCV large T antigen (TAg) binds the F-box proteins β-transducin-repeat containing protein-1 and 2 (βTrCP1/2). These interactions involve a phosphodegron (DpSGX(2-4)pS) found in βTrCP substrates. TAg stability is unaltered, suggesting TAg is a pseudo-substrate. βTrCP and TAg co-localize in the cytoplasm, and a functional SCF complex is required. We examined whether TAg influences the levels of β-catenin, a βTrCP substrate. We were unable to demonstrate that TAg elevates β-catenin as previously reported, and a mutant TAg unable to bind βTrCP also had no detectable effect on β-catenin stability. Results presented in this study link JCV TAg to the cellular degradation complex, SCF(βTrCP1/2). Proteasomal degradation is essential for proper regulation of cellular functions, and interference with proteasomal pathways highlights possible JCV pathogenic and oncogenic mechanisms.

Human intestinal microbiota: cross-talk with the host and its potential role in colorectal cancer

In this review, we discuss the multifactorial role of intestinal microbiota in colorectal cancer. The peculiar metabolism of dietary compounds of the individual microbiota complement, its overall immunostimulation and immunomodulatory activity, and eventually the production of toxins that perturb the regulation of cell growth, define the balance of positive and negative risk factors for colorectal cancer development. Moreover, shaping the composition of the human intestinal microbiota, diet has an indirect impact in determining the balance between health and disease. The integration of diet, microbial, and host factors in a system approach is mandatory to determine the overall balance of risk and protective factors for colorectal cancer onset.

Escherichia coli cytotoxic necrotizing factor 1 (CNF1): toxin biology, in vivo applications and therapeutic potential

CNF1 is a protein toxin produced by certain pathogenic strains of Escherichia coli. It permanently activates the regulatory Rho, Rac, and Cdc42 GTPases in eukaryotic cells, by deamidation of a glutamine residue. This modification promotes new activities in cells, such as gene transcription, cell proliferation and survival. Since the Rho GTPases play a pivotal role also in several processes in vivo, the potentiality of CNF1 to act as a new pharmacological tool has been explored in experimental animals and in diverse pathological contexts. In this review, we give an update overview on the potential in vivo applications of CNF1.

The Rac GTPase-activating bacterial protein toxin CNF1 induces analgesia up-regulating mu-opioid receptors

Cytotoxic Necrotizing Factor 1 (CNF1) is a protein toxin from Escherichia coli that constitutively activates the Rho, Rac and Cdc42 GTPases. These regulatory proteins oscillate between a cytosolic GDP-bound inactive form and a membrane-linked GTP-bound active form, orchestrating the actin cytoskeleton assembly and dynamics. We herein describe, for the first time, the ability of CNF1 to potently counteract the formalin-induced inflammatory pain in mice. The analgesic response due to CNF1 requires both the sustained activation of the Rac GTPase, with consequent cerebral actin cytoskeleton remodeling, and the up-regulation of the mu-opioid receptors (MORs), the most important receptors controlling pain perception. The crucial role of Rac is proved by the lack of analgesic activity in mice challenged with a recombinant CNF1, in which the enzymatic activity was abolished by substituting serine with cysteine at position 866. The importance of MORs is proved by the inability of CNF1 to induce any analgesic effect in MORs knockout mice and by the ability of naloxone to antagonize the analgesic effects. Furthermore, it is worth noting that the analgesic effect in mice occurs after both peripheral and central administration of CNF1. Hence, taken altogether, our findings provide new insights into the comprehension of intracellular mechanisms involved in pain modulation, and indicate this bacterial protein toxin as a novel tool in the field of pain control. Conceivably, this might pave the way for new therapeutic strategies.

Rac1 signaling modulates BCL-6-mediated repression of gene transcription

Rac1 is a member of the Rho family of small GTPases that not only regulates signaling pathways involved in cell adhesion and migration but also regulates gene transcription. Here we show that the transcriptional repressor BCL-6 is regulated by Rac1 signaling. Transfection of active Rac1 mutants into colorectal DLD-1 cells led to increased expression of a BCL-6-controlled luciferase reporter construct. Conversely, inhibition of endogenous Rac1 activation by the Rac1 inhibitor NSC23766 decreased reporter activity. Moreover, BCL-6 lost its typical localization to nuclear dots upon activation of Rac1 and became predominantly soluble in a non-chromatin-bound cell fraction. Rac1 signaling also regulated the expression of endogenous BCL-6-regulated genes, including the p50 precursor NF-kappaB1/p105 and the cell adhesion molecule CD44. Interestingly, these effects were not stimulated by the alternative splice variant Rac1b. The mechanism of BCL-6 inhibition does not involve formation of a stable Rac1/BCL-6 complex and is independent of Rac-induced reactive oxygen species production or Jun NH(2)-terminal kinase activation. We show that PAK1 mediates inhibition downstream of Rac and can directly phosphorylate BCL-6. Together, these data provide substantial evidence that Rac1 signaling inhibits the transcriptional repressor BCL-6 in colorectal cells and reveal a novel pathway that links Rac1 signaling to the regulation of gene transcription.

Endogenous galectin-3 is localized in membrane lipid rafts and regulates migration of dendritic cells

This study reveals a function of endogenous galectin-3, an animal lectin recognizing beta-galactosides, in regulating dendritic cell motility both in vitro and in vivo, which to our knowledge is unreported. First, galectin-3-deficient (gal3(-/-)) bone marrow-derived dendritic cells exhibited defective chemotaxis compared to gal3(+/+) cells. Second, cutaneous dendritic cells in gal3(-/-) mice displayed reduced migration to draining lymph nodes upon hapten stimulation compared to gal3(+/+) mice. Moreover, gal3(-/-) mice were impaired in the development of contact hypersensitivity relative to gal3(+/+) mice in response to a hapten, a process in which dendritic cell trafficking to lymph nodes is critical. In addition, defective signaling was detected in gal3(-/-) cells upon chemokine receptor activation. By immunofluorescence microscopy, we observed that galectin-3 is localized in membrane ruffles and lamellipodia in stimulated dendritic cells and macrophages. Furthermore, galectin-3 was enriched in lipid raft domains under these conditions. Finally, we determined that ruffles on gal3(-/-) cells contained structures with lower complexity compared to gal3(+/+) cells. In view of the participation of membrane ruffles in signal transduction and cell motility, we conclude that galectin-3 regulates cell migration by functioning at these structures.

Guanylate binding protein-1 inhibits spreading and migration of endothelial cells through induction of integrin alpha4 expression

Human guanylate binding protein-1 (GBP-1) is a large GTPase that is induced by inflammatory cytokines and acts antiangiogenically through the inhibition of endothelial cell proliferation and migration. In this study, we detected that GBP-1-expressing cells show a significantly reduced spreading and migration on fibronectin matrices. Investigating possible mechanisms of these effects, we found that integrin alpha(4) (ITGA4) was consistently up-regulated at both the RNA and protein level in GBP-1-expressing cell cultures. Inhibition of cell spreading and migration by GBP-1 was dependent on the binding of ITGA4 to fibronectin. The inflammatory cytokines IL-1beta and TNF-alpha induced ITGA4 expression in HUVECs and inhibited spreading and migration. Knockdown of GBP-1 by shRNA abrogated inflammatory cytokine induced ITGA4 expression and restored spreading and migration capabilities of the cells. These results show that inhibition of endothelial cell spreading and migration by inflammatory cytokines is mediated by GBP-1 through induction of ITGA4 expression. Endothelial cell migration is a key process during angiogenesis. Therefore, ITGA4 may be a novel molecular target to modulate angiogenesis in human disease.

The Rho-activating CNF1 toxin from pathogenic E. coli: a risk factor for human cancer development?

Nowadays, there is increasing evidence that some pathogenic bacteria can contribute to specific stages of cancer development. The concept that bacterial infection could be involved in carcinogenesis acquired a widespread interest with the discovery that H. pylori is able to establish chronic infections in the stomach and that this infection is associated with an increased risk of gastric adenocarcinoma and mucosa associated lymphoid tissue lymphoma. Chronic infections triggered by bacteria can facilitate tumor initiation or progression since, during the course of infection, normal cell functions can come under the control of pathogen factors that directly manipulate the host regulatory pathways and the inflammatory reactions.Renowned publications have recently corroborated the molecular mechanisms that link bacterial infections, inflammation and cancer, indicating certain strains of Escherichia coli as a risk factor for patients with colon cancer. E. coli is a normal inhabitant of the human intestine that becomes highly pathogenic following the acquisition of virulence factors, including a protein toxin named cytotoxic necrotizing factor 1 (CNF1). This toxin permanently activates the small GTP-binding proteins belonging to the Rho family, thus promoting a prominent polymerization of the actin cytoskeleton as well as a number of cellular responses, including changes in protein expression and functional modification of the cell physiology. CNF1 is receiving an increasing attention as a putative factor involved in transformation because of its ability to: (i) induce COX2 expression, an immediate-early gene over-expressed in some type of cancers; (ii) induce a long-lasting activation of the transcription factor NF-kB, a largely accepted marker of tumor cells; (iii) protect epithelial cells from apoptosis; (iv) ensue the release of pro-inflammatory cytokines in epithelial and endothelial cells; and (v) promote cellular motility. As cancer may arise through dysfunction of the same regulatory systems, it seems likely that CNF1-producing E. coli infections can contribute to tumor development.This review focuses on the aspects of CNF1 activity linked to cell transformation with the aim of contributing to the identification of a possible carcinogenic agent from the microbial world.

Rho GTPase-activating bacterial toxins: from bacterial virulence regulation to eukaryotic cell biology

Studies on the interactions of bacterial pathogens with their host have provided an invaluable source of information on the major functions of eukaryotic and prokaryotic cell biology. In addition, this expanding field of research, known as cellular microbiology, has revealed fascinating examples of trans-kingdom functional interplay. Bacterial factors actually exploit eukaryotic cell machineries using refined molecular strategies to promote invasion and proliferation within their host. Here, we review a family of bacterial toxins that modulate their activity in eukaryotic cells by activating Rho GTPases and exploiting the ubiquitin/proteasome machineries. This family, found in human and animal pathogenic Gram-negative bacteria, encompasses the cytotoxic necrotizing factors (CNFs) from Escherichia coli and Yersinia species as well as dermonecrotic toxins from Bordetella species. We survey the genetics, biochemistry, molecular and cellular biology of these bacterial factors from the standpoint of the CNF1 toxin, the paradigm of Rho GTPase-activating toxins produced by urinary tract infections causing pathogenic Escherichia coli. Because it reveals important connections between bacterial invasion and the host inflammatory response, the mode of action of CNF1 and its related Rho GTPase-targetting toxins addresses major issues of basic and medical research and constitutes a privileged experimental model for host-pathogen interaction.

Cytotoxic necrotizing factor 1 prevents apoptosis via the Akt/IkappaB kinase pathway: role of nuclear factor-kappaB and Bcl-2

Cytotoxic necrotizing factor 1 (CNF1) is a protein toxin produced by some pathogenic strains of Escherichia coli that specifically activates Rho, Rac, and Cdc42 GTPases. We previously reported that this toxin prevents the ultraviolet-B-induced apoptosis in epithelial cells, with a mechanism that remained to be defined. In this work, we show that the proteasomal degradation of the Rho GTPase is necessary to achieve cell death protection, because inhibition of Rho degradation abolishes the prosurvival activity of CNF1. We hypothesize that Rho inactivation allows the activity of Rac to become dominant. This in turn leads to stimulation of the phosphoinositide 3-kinase/Akt/IkappaB kinase/nuclear factor-kappaB prosurvival pathway and to a remarkable modification in the architecture of the mitochondrial network, mainly consisting in the appearance of elongated and interconnected mitochondria. Importantly, we found that Bcl-2 silencing reduces the ability of CNF1 to protect cells against apoptosis and that it also prevents the CNF1-induced mitochondrial changes. It is worth noting that the ability of a bacterial toxin to induce such a remodeling of the mitochondrial network is herein reported for the first time. The possible pathophysiological relevance of this finding is discussed.

Crucial role of Rho-nuclear factor-kappaB axis in angiotensin II-induced renal injury

This study was performed to determine the effectiveness of the Rho kinase inhibitor and NF-kappaB inhibitor in renal injury of ANG II-infused hypertensive rats. Male Sprague-Dawley rats, maintained on a normal diet, received either a sham operation (n = 7) or continuous ANG II infusion (120 ng/min) subcutaneously via minipumps. The ANG II-infused rats were further subdivided into three subgroups (n = 7 each) to receive one of the following treatments during the entire period: vehicle, Rho kinase inhibitor (fasudil; 3 mg.kg(-1).day(-1) ip), or NF-kappaB inhibitor (parthenolide; 1 mg.kg(-1).day(-1) ip). After 12 days of ANG II infusion, systolic blood pressure (BP; 208 +/- 7 vs. 136 +/- 3 mmHg), Rho kinase activity, NF-kappaB activity, renal ANG II contents (160 +/- 25 vs. 84 +/- 14 pg/g), monocytic chemotactic protein (MCP) 1 mRNA, interstitial macrophage infiltration, transforming growth factor-beta1 (TGF-beta1) mRNA, interstitial collagen-positive area, urinary protein excretion (43 +/- 6 vs. 11 +/- 2 mg/day), and urinary albumin excretion were significantly enhanced compared with the Sham group. While fasudil or parthenolide did not alter systolic BP (222 +/- and 190 +/- 21, respectively), both treatments completely blocked ANG II-induced enhancement of NF-kappaB activity, renal ANG II contents (103 +/- 11 and 116 +/- 21 pg/g, respectively), MCP1 mRNA, interstitial macrophage infiltration, TGF-beta1 mRNA, interstitial collagen-positive area, urinary protein excretion (28 +/- 6 and 23 +/- 3 mg/day, respectively), and urinary albumin excretion. Importantly, parthenolide did not alter ANG II-induced Rho kinase activation although fasudil abolished ANG II-induced Rho kinase activation. These data indicate that the Rho-NF-kappaB axis plays crucial roles in the development of ANG II-induced renal injury independently from BP regulation.

Enhancement of learning and memory after activation of cerebral Rho GTPases

The mechanism whereby the morphology and connectivity of the dendritic tree is regulated depends on an actin dynamics that, in turn, is controlled by Rho GTPases, a family of small GTP-binding proteins encompassing Rho, Rac, and Cdc42 subfamilies. Cytotoxic necrotizing factor 1 (CNF1), a protein toxin from Escherichia coli, constitutively activates Rho GTPases, thus leading to remodeling of the actin cytoskeleton in intact cells. Here, we show that the modulation of cerebral RhoA and Rac1 activity induced by CNF1 in mice leads to (i) rearrangement of cerebral actin cytoskeleton, (ii) enhanced neurotransmission and synaptic plasticity, and (iii) improved learning and memory in various behavioral tasks. The effects persist for weeks and are not observed in mice treated with a recombinant CNF1, in which the enzymatic activity was abolished by substituting serine to cysteine at position 866. The results suggest that learning ability can be improved through pharmacological manipulation of neural connectivity.

A multinucleating Escherichia coli cytotoxin perturbs cell cycle in cultured epithelial cells

Pathogenic Escherichia coli strains produce a number of virulence-associated factors, among which cytotoxic necrotizing factor 1 (CNF1). CNF1 is a chromosomally encoded toxin that permanently activates the small GTP-binding proteins of the Rho family (Rho, Rac and Cdc42) by catalizing their deamidation at a specific glutamine residue. This activation modulates a high number of cellular functions, including the reorganization of the actin cytoskeleton, the promotion of cell spreading and the multinucleation. Indeed, accumulating evidence indicates that, in addition to the well-characterized Ras GTPases, also Rho family proteins are crucial in different points of cell cycle regulation. Here, we report that CNF1 induces a block of the cell cycle at the G(2)/M transition in epithelial cell line HEp-2, and up-regulates cyclin B1 and p53 proteins confining them in the cytoplasm region. The ability of CNF1 to perturb cell cycle progression could play a role in E. coli pathogenicity.

Escherichia coli cytotoxic necrotizing factor 1 blocks cell cycle G2/M transition in uroepithelial cells

Evidence is accumulating that a growing number of bacterial toxins act by modulating the eukaryotic cell cycle machinery. In this context, we provide evidence that a protein toxin named cytotoxic necrotizing factor 1 (CNF1) from uropathogenic Escherichia coli is able to block cell cycle G(2)/M transition in the uroepithelial cell line T24. CNF1 permanently activates the small GTP-binding proteins of the Rho family that, beside controlling the actin cytoskeleton organization, also play a pivotal role in a large number of other cellular processes, including cell cycle regulation. The results reported here show that CNF1 is able to induce the accumulation of cells in the G(2)/M phase by sequestering cyclin B1 in the cytoplasm and down-regulating its expression. The possible role played by the Rho GTPases in the toxin-induced cell cycle deregulation has been investigated and discussed. The activity of CNF1 on cell cycle progression can offer a novel view of E. coli pathogenicity.

Is the Rac GTPase-activating toxin CNF1 a smart hijacker of host cell fate?

The term mitotic catastrophe (MC) was coined to describe the mammalian cell death caused by aberrant mitosis. MC occurs with features that are fundamentally different from those typifying other forms of cell death, including apoptosis. We report here for the first time that the Rac-activating toxin CNF1 interferes with the occurrence of MC and leads to aneuploidy and multinucleation. This seems to be in line with the anti-apoptotic activity of the toxin and consistent with the hypothesis that points at CNF1 as a toxin bearing a carcinogenic potential.