Anthrax lethal toxin represses glucocorticoid receptor (GR) transactivation by inhibiting GR-DNA binding in vivo

Systems biology approach to understand the interplay between Bacillus anthracis and human host genes that leads to CVDs

Humans infected with invasive Bacillus anthracis (B. anthracis) have a very poor prognosis and are at high risk for developing cardiovascular diseases (CVDs) and shock. Several bacterial elements probably have significant pathogenic roles in this pathogenic process of anthrax. In our current work, we have analysed the molecular level interactions between B. anthracis and human genes to understand the interplay during anthrax that leads to the CVDs. Our results have shown dense interactions between the functional partners in both host and the B. anthracis Gene interaction network (GIN). The functional enrichment analysis indicated that the clusters in the host GIN had genes related to hypoxia and autophagy in response to the lethal toxin; and genes related to adherens junction and actin cytoskeleton in response to edema toxin play a significant role in multiple stages of the disease. The B. anthracis genes BA_0530, guaA, polA, rpoB, ribD, secDF, metS, dinG and human genes ACTB, EGFR, EP300, CTNNB1, ESR1 have shown more than 50 direct interactions with the functional partners and hence they can be considered as hub genes in the network and they are observed to have important roles in CVDs. The outcome of our study will help to understand the molecular pathogenesis of CVDs in anthrax. The hub genes reported in the study can be considered potential drug targets and they can be exploited for new drug discovery.

Anthrax Pathogenesis

Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis. The bacterium's major virulence factors are (a) the anthrax toxins and (b) an antiphagocytic polyglutamic capsule. These are encoded by two large plasmids, the former by pXO1 and the latter by pXO2. The expression of both is controlled by the bicarbonate-responsive transcriptional regulator, AtxA. The anthrax toxins are three polypeptides-protective antigen (PA), lethal factor (LF), and edema factor (EF)-that come together in binary combinations to form lethal toxin and edema toxin. PA binds to cellular receptors to translocate LF (a protease) and EF (an adenylate cyclase) into cells. The toxins alter cell signaling pathways in the host to interfere with innate immune responses in early stages of infection and to induce vascular collapse at late stages. This review focuses on the role of anthrax toxins in pathogenesis. Other virulence determinants, as well as vaccines and therapeutics, are briefly discussed.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Glucocorticoid regulation of inflammation and its functional correlates: from HPA axis to glucocorticoid receptor dysfunction

Enhanced susceptibility to inflammatory and autoimmune disease can be related to impairments in HPA axis activity and associated hypocortisolism, or to glucocorticoid resistance resulting from impairments in local factors affecting glucocorticoid availability and function, including the glucocorticoid receptor (GR). The enhanced inflammation and hypercortisolism that typically characterize stress-related illnesses, such as depression, metabolic syndrome, cardiovascular disease, or osteoporosis, may also be related to increased glucocorticoid resistance. This review focuses on impaired GR function as a molecular mechanism of glucocorticoid resistance. Both genetic and environmental factors can contribute to impaired GR function. The evidence that glucocorticoid resistance can be environmentally induced has important implications for management of stress-related inflammatory illnesses and underscores the importance of prevention and management of chronic stress. The simultaneous assessment of neural, endocrine, and immune biomarkers through various noninvasive methods will also be discussed.

Cellular and physiological effects of anthrax exotoxin and its relevance to disease

Bacillus anthracis, the causative agent of anthrax, secretes a tri-partite exotoxin that exerts pleiotropic effects on the host. The purification of the exotoxin components, protective antigen, lethal factor, and edema factor allowed the rapid characterization of their physiologic effects on the host. As molecular biology matured, interest focused on the molecular mechanisms and cellular alterations induced by intoxication. Only recently have researchers begun to connect molecular and cellular knowledge back to the broader physiological effects of the exotoxin. This review focuses on the progress that has been made bridging molecular knowledge back to the exotoxin’s physiological effects on the host.

Respiratory syncytial virus represses glucocorticoid receptor-mediated gene activation

Respiratory syncytial virus (RSV) is a common cause of bronchiolitis in infants. Although antiinflammatory in nature, glucocorticoids have been shown to be ineffective in the treatment of RSV-induced bronchiolitis and wheezing. In addition, the effectiveness of glucocorticoids at inhibiting RSV-induced proinflammatory cytokine production in cell culture has been questioned. In this study, we have investigated the effect of RSV infection on glucocorticoid-induced gene activation in lung epithelium-derived cells. We show that RSV infection inhibits dexamethasone induction of three glucocorticoid receptor (GR)-regulated genes (glucocorticoid-inducible leucine zipper, FK506 binding protein, and MAPK phosphatase 1) in A549, BEAS-2B cells, and primary small airway epithelial cells. UV irradiation of the virus prevents this repression, suggesting that viral replication is required. RSV is known to activate the nuclear factor κB (NFκB) pathway, which is mutually antagonistic towards the GR pathway. However, specific inhibition of NFκB had no effect on the repression of GR-induced genes by RSV infection, indicating that RSV repression of GR is independent of NFκB. RSV infection of A549 cells does not alter GR protein levels or GR nuclear translocation but does reduce GR binding to the promoters of the glucocorticoid responsive genes analyzed in this study. Repression of GR by RSV infection may account for the apparent clinical ineffectiveness of glucocorticoids in RSV bronchiolitis therapy. In addition, this data adds to our previously published data suggesting that GR may be a general target for infectious agents. Identifying the mechanisms through which this suppression occurs may lead to the development of novel therapeutics.

The glucocorticoid receptor: a revisited target for toxins

The hypothalamic-pituitary-adrenal (HPA) axis activation and glucocorticoid responses are critical for survival from a number of bacterial, viral and toxic insults, demonstrated by the fact that removal of the HPA axis or GR blockade enhances mortality rates. Replacement with synthetic glucocorticoids reverses these effects by providing protection against lethal effects. Glucocorticoid resistance/insensitivity is a common problem in the treatment of many diseases. Much research has focused on the molecular mechanism behind this resistance, but an area that has been neglected is the role of infectious agents and toxins. We have recently shown that the anthrax lethal toxin is able to repress glucocorticoid receptor function. Data suggesting that the glucocorticoid receptor may be a target for a variety of toxins is reviewed here. These studies have important implications for glucocorticoid therapy.

Cellular and systemic effects of anthrax lethal toxin and edema toxin

Anthrax lethal toxin (LT) and edema toxin (ET) are the major virulence factors of anthrax and can replicate the lethality and symptoms associated with the disease. This review provides an overview of our current understanding of anthrax toxin effects in animal models and the cytotoxicity (necrosis and apoptosis) induced by LT in different cells. A brief reexamination of early historic findings on toxin in vivo effects in the context of our current knowledge is also presented.

Bacillus anthracis lethal toxin represses MMTV promoter activity through transcription factors

We have recently shown that the anthrax lethal toxin (LeTx) selectively represses nuclear hormone receptors. In this study, we found that LeTx repressed the activation of the mouse mammary tumor virus promoter related to overexpression of the transcription factors hepatocyte nuclear factor 3, octamer-binding protein 1, and c-Jun. LeTx transcriptional repression was associated with a decrease in the protein levels of these transcription factors in a lethal factor protease activity-dependent manner. Early administration of LeTx antagonists partially or completely abolished the repressive effects of LeTx. In contrast to the rapid cleavage of mitogen-activated protein kinase kinases by LeTx, the degradation of these transcription factors occurred at a relatively late stage after LeTx treatment. In addition, LeTx repressed phorbol-12-myristate-13-acetate-induced mouse mammary tumor virus promoter activity and phorbol 12-myristate 13-acetate induction of endogenous c-Jun protein. Collectively, these findings suggest that transcription factors are intracellular targets of LeTx and expand our understanding of the molecular action of LeTx at a later stage of low-dose exposure.

The role of glucocorticoids and progestins in inflammatory, autoimmune, and infectious disease

A bidirectional communication exists between the CNS and the immune system. The autonomic nervous system, through neurotransmitters and neuropeptides, works in parallel with the hypothalamic-pituitary-adrenal axis through the actions of glucocorticoids to modulate inflammatory events. The immune system, through the action of cytokines and other factors, in turn, activates the CNS to orchestrate negative-feedback mechanisms that keep the immune response in check. Disruption of these interactions has been associated with a number of syndromes including inflammatory, autoimmune, and cardiovascular diseases, metabolic and psychiatric disorders, and the development of shock. The hypothalamic-pituitary-gonadal axis also plays an important part in regulating immunity through the secretion of sex hormones. Although numerous studies have established a role for immunomodulation by estrogen and testosterone, the role of progesterone is less well understood. Progesterone is crucial for reproductive organ development and maintenance of pregnancy, and more recent studies have clearly shown its role as an important immune regulator. The main focus of this review will be about the role of steroid hormones, specifically glucocorticoids and progesterone, in inflammatory responses and infectious diseases and how dysregulation of their actions may contribute to development of autoimmune and inflammatory disease.

The hypothalamic-pituitary-adrenal axis in critical illness

Hypothalamic-pituitary-adrenal (HPA) axis function is crucial to maintain and restore homeostasis. The HPA axis does not function in isolation. Rather, the HPA axis modulates and reacts to signals from endocrine, neural, and immune systems. Cortisol is the major glucocorticoid secreted by the human adrenal cortex. Its actions are largely mediated by the glucocorticoid receptor. The potent anti-inflammatory actions of glucocorticoids led to their use in critically ill patients. Metaanalyses of these early studies (before 1985) concluded that large glucocorticoid doses had no effect and were potentially detrimental. More recently, the pendulum has swung in the opposite direction based on the concept that critically ill patients may have relative adrenal insufficiency and/or acquired glucocorticoid resistance. However, inconsistent diagnostic criteria, heterogeneity of subjects, variable nutritional status, and pre-existing conditions preclude formulating definitive conclusions regarding glucocorticoid use among critically patients. Diagnosing adrenal insufficiency in the critically ill patient remains challenging. To resolve the issue, our challenge is to develop physiologically relevant tools to assess glucocorticoid action and GR function at the cellular level.

The large clostridial toxins from Clostridium sordellii and C. difficile repress glucocorticoid receptor activity

We have previously shown that Bacillus anthracis lethal toxin represses glucocorticoid receptor (GR) transactivation. We now report that repression of GR activity also occurs with the large clostridial toxins produced by Clostridium sordellii and C. difficile. This was demonstrated using a transient transfection assay system for GR transactivation. We also report that C. sordellii lethal toxin inhibited GR function in an ex vivo assay, where toxin reduced the dexamethasone suppression of the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha). Furthermore, the glucocorticoid antagonist RU-486 in combination with C. sordellii lethal toxin additively prevented glucocorticoid suppression of TNF-alpha. These findings corroborate the fact that GR is a target for the toxin and suggest a physiological role for toxin-associated GR repression in inflammation. Finally, we show that this repression is associated with toxins that inactivate p38 mitogen-activated protein kinase (MAPK).

Manipulation of host signalling pathways by anthrax toxins

Infectious microbes face an unwelcoming environment in their mammalian hosts, which have evolved elaborate multicelluar systems for recognition and elimination of invading pathogens. A common strategy used by pathogenic bacteria to establish infection is to secrete protein factors that block intracellular signalling pathways essential for host defence. Some of these proteins also act as toxins, directly causing pathology associated with disease. Bacillus anthracis, the bacterium that causes anthrax, secretes two plasmid-encoded enzymes, LF (lethal factor) and EF (oedema factor), that are delivered into host cells by a third bacterial protein, PA (protective antigen). The two toxins act on a variety of cell types, disabling the immune system and inevitably killing the host. LF is an extraordinarily selective metalloproteinase that site-specifically cleaves MKKs (mitogen-activated protein kinase kinases). Cleavage of MKKs by LF prevents them from activating their downstream MAPK (mitogen-activated protein kinase) substrates by disrupting a critical docking interaction. Blockade of MAPK signalling functionally impairs cells of both the innate and adaptive immune systems and induces cell death in macrophages. EF is an adenylate cyclase that is activated by calmodulin through a non-canonical mechanism. EF causes sustained and potent activation of host cAMP-dependent signalling pathways, which disables phagocytes. Here I review recent progress in elucidating the mechanisms by which LF and EF influence host signalling and thereby contribute to disease.