Opinion: Bacterial toxins and cancer--a case to answer?

Since the discovery that Helicobacter pylori infection leads to gastric cancer, other chronic bacterial infections have been shown to cause cancer. The bacterial and host molecular mechanisms remain unclear. However, many bacteria that cause persistent infections produce toxins that specifically disrupt cellular signalling to perturb the regulation of cell growth or to induce inflammation. Other bacterial toxins directly damage DNA. Such toxins mimic carcinogens and tumour promoters and might represent a paradigm for bacterially induced carcinogenesis.     

The influence of bacterial toxins on the carcinogenesis

Bacterial infections may constitute an important risk factor of developing cancer disease. Molecular mechanisms by which bacteria contribute to cancer are extremely complex and still remain not fully understood. So far, it is generally accepted that Helicobacter pylori infections are associated with induction of gastric adenocarcinoma and MALT lymphoma. Two H. pylori toxins which modulate many cellular functions are VacA and CagA. So far, CagA is the only one known bacterial oncoprotein. However, many other bacteria produce toxins or effector proteins perturbing host cell homeostasis or/and evoking chronic inflammation. Both processes may be associated with tumour formation. Bacterial toxins which interfere, with various host signal transduction pathways, deregulate processes of cell division, proliferation and differentiation and modulate apoptosis. Some toxins cause even direct DNA damage. This review discuss the potential links between action of bacterial toxins and cancer.     

Bacterial intoxication evokes cellular senescence with persistent DNA damage and cytokine signalling

Cytolethal distending toxins (CDTs) are proteins produced and secreted by facultative pathogenic strains of Gram-negative bacteria with potentially genotoxic effects. Mammalian cells exposed to CDTs undergo cell type-dependent cell-cycle arrest or apoptosis; however, the cell fate responses to such intoxication are mechanistically incompletely understood. Here we show that both normal and cancer cells (BJ, IMR-90 and WI-38 fibroblasts, HeLa and U2-OS cell lines) that survive the acute phase of intoxication by Haemophilus ducreyi CDT possess the hallmarks of cellular senescence. This characteristic phenotype included persistently activated DNA damage signalling (detected as 53BP1/γH2AX⁺ foci), enhanced senescence-associated β-galactosidase activity, expansion of promyelocytic leukaemia nuclear compartments and induced expression of several cytokines (especially interleukins IL-6, IL-8 and IL-24), overall features shared by cells undergoing replicative or premature cellular senescence. We conclude that analogous to oncogenic, oxidative and replicative stresses, bacterial intoxication represents another pathophysiological stimulus that induces premature senescence, an intrinsic cellular response that may mechanistically underlie the 'distended' morphology evoked by CDTs. Finally, the activation of the two anticancer barriers, apoptosis and cellular senescence, together with evidence of chromosomal aberrations (micronucleation) reported here, support the emerging genotoxic and potentially oncogenic effects of this group of bacterial toxins, and warrant further investigation of their role(s) in human disease.     

Antibodies in oncology

Monoclonal antibodies (mAbs) have become one of the largest classes of new therapeutic agents approved for use in oncology, and have revolutionised the treatment of many human malignancies. Clinically useful mAbs can function through several different mechanisms, including inhibition of tumour-related signalling, induction of apoptosis, inhibition of angiogenesis, enhancing host immune response against cancer and targeted delivery of payloads (such as toxins, cytotoxic agents or radioisotopes) to the tumour site. The increasing knowledge of key molecules and cellular pathways involved in tumour induction and progression has led to a rise in the proportion of therapeutic mAbs entering clinical trials. These mAbs consist of various conventional or recombinant, murine, humanised, chimeric or fully human and fusion constructs. In this review, we provide an overview of mAbs approved for use in clinical oncology and those currently in clinical development. We also discuss the mechanisms of action of anti-cancer mAbs, as well as the antigen targets recognised by these antibodies.     

Splicing DNA-damage responses to tumour cell death

The ability of a tumour cell to evade programmed cell death (apoptosis) is crucial in the development of cancer. The process of apoptosis is complex and involves the careful interplay of a host of signalling molecules. Cellular stresses, such as DNA-damage, can initiate apoptosis through multiple pathways, all of which eventually lead to eradication of damaged cells that may otherwise go on to form a tumour. Moreover, the relevance of this to combating cancer is very strong since several therapeutic agents used to treat malignant disease utilize the cells' apoptotic machinery. The purpose of this review is to provide an insight into what we know about how apoptosis is initiated by DNA-damaging agents, how pro- and anti-apoptotic signals converge in the execution of cell death, and how such mechanisms can be perturbed in cancer.     

Calcium signalling during cell interactions with bacterial pathogens

The ability of a pathogenic microorganism to cause a disease is conditioned by its ability to colonise a given niche and implicates the expression of specific determinants, i.e. virulence factors, that allow the pathogen to adhere to or to invade epithelial cells. Diseases may be induced by bacteria that replicate extracellularly and alter the epithelial mucosa by producing toxins. Ca2+ signalling has been implicated in various steps of bacterial infection. Bacterial toxins can induce an increase in free cytosolic Ca2+ in host cells, itself required for the toxin-mediated effects. Such toxins, by diffusing in the extracellular media, can act at a distance from the site of infection and have a global effect on the integrity of the epithelium by promoting the expression of pro-inflammatory cytokines. Independent on toxins, bacteria can induce Ca2+ responses that play a role in cytoskeletal rearrangements required for cell binding or internalisation of the microorganism. In some instances, invasion of the epithelium may be followed by bacterial access to deeper tissue, dissemination to other organs, and sometimes persistence in host cells in a parasitic-like mode. Such strategies underline the pathogen abilities to control innate defence cells such as professional phagocytes, and may implicate the diversion of Ca(2+)-dependent cellular processes that normally result in killing of the ingested bacteria. Finally, bacterial pathogens can also induce the cell release of ATP, a Ca2+ agonist, that may expand bacterial cell signalling by a paracrine or autocrine route, leading to enhanced colonisation or enhanced host cell responses to the invading microorganism.     

Fusion of apoptosis-related protein Cytochrome c with anti-HER-2 single-chain antibody targets the suppression of HER-2+ breast cancer

Cancer treatment has gradually developed from toxic chemotherapy to targeted therapy with fewer side effects. Approximately 30% of breast cancer patients overexpress human epidermal growth factor receptor 2 (HER-2). Previous studies have successfully produced single-chain antibodies (scFv) targeting HER-2+ breast cancer; however, scFv have poor stability, easy aggregation and a shorter half-life, which have no significant effect on targeting therapy. Moreover, scFv has been considered as a drug delivery platform that can kill target cells by effector molecules. However, the functional killing domains of immunotoxins are mainly derived from plant or bacterial toxins, which have a large molecular weight, low tissue permeability and severe side effects. To address these concerns, we designed several apoptotic immune molecules to replace exogenous toxins using endogenous apoptosis-related protein DNA fragmentation factor 40 (DFF40) and tandem-repeat Cytochrome c base on caspase-3 responsive peptide (DEVD). Our results suggest that DFF40 or Cytc fusion scFv specifically targets HER-2 overexpressing breast cancer cells (SK-BR-3 and BT-474) rather than HER-2 negative cells (MDA-MB-231 and MCF-7). Following cellular internalization, apoptosis-related proteins inhibited tumour activity by initiating endogenous apoptosis pathways, which significantly reduced immunogenicity and toxic side effects. Therefore, we suggest that immunoapoptotic molecules may become potential drugs for targeted immunotherapy of breast cancer.     

Protein toxins from plants and bacteria: Probes for intracellular transport and tools in medicine

A number of protein toxins produced by bacteria and plants enter eukaryotic cells and inhibit protein synthesis enzymatically. These toxins include the plant toxin ricin and the bacterial toxin Shiga toxin, which we will focus on in this article. Although a threat to human health, toxins are valuable tools to discover and characterize cellular processes such as endocytosis and intracellular transport. Bacterial infections associated with toxin production are a problem worldwide. Increased knowledge about toxins is important to prevent and treat these diseases in an optimal way. Interestingly, toxins can be used for diagnosis and treatment of cancer.     

MicroRNAs as important players in regulating cancer through PTEN/PI3K/AKT signalling pathways

Cancer being the leading cause of mortality has become a great threat worldwide. Current cancer therapeutics lack specificity and have side effects due to a lack of understanding of the molecular mechanisms and signalling pathways involved in carcinogenesis. In recent years, researchers have been focusing on several signalling pathways to pave the way for novel therapeutics. The PTEN/PI3K/AKT pathway is one of the important pathways involved in cell proliferation and apoptosis, leading to tumour growth. In addition, the PTEN/PI3K/AKT axis has several downstream pathways that could lead to tumour malignancy, metastasis and chemoresistance. On the other hand, microRNAs (miRNAs) are important regulators of various genes leading to disease pathogenesis. Hence studies of the role of miRNAs in regulating the PTEN/PI3K/AKT axis could lead to the development of novel therapeutics for cancer. Thus, in this review, we have focused on various miRNAs involved in the carcinogenesis of various cancer via the PTEN/PI3K/AKT axis.     

Molecular weaponry: diverse effectors delivered by the Type VI secretion system

The Type VI secretion system is a widespread bacterial nanomachine, used to deliver toxins directly into eukaryotic or prokaryotic target cells. These secreted toxins, or effectors, act on diverse cellular targets, and their action provides the attacking bacterial cell with a significant fitness advantage, either against rival bacteria or eukaryotic host organisms. In this review, we discuss the delivery of diverse effectors by the Type VI secretion system, the modes of action of the so‐called ‘anti‐bacterial’ and ‘anti‐eukaryotic’ effectors, the mechanism of self‐resistance against anti‐bacterial effectors and the evolutionary implications of horizontal transfer of Type VI secretion system‐associated toxins. Whilst it is likely that many more effectors remain to be identified, it is already clear that toxins delivered by this secretion system represent efficient weapons against both bacteria and eukaryotes.     

Caveolin‐1 as a promoter of tumour spreading: when,how, where and why

Caveolae are non‐clathrin invaginations of the plasma membrane in most cell types; they are involved in signalling functions and molecule trafficking, thus modulating several biological functions, including cell growth, apoptosis and angiogenesis. The major structural protein in caveolae is caveolin‐1, which is known to act as a key regulator in cancer onset and progression through its role as a tumour suppressor. Caveolin‐1 can also promote cell proliferation, survival and metastasis as well as chemo‐ and radioresistance. Here, we discuss recent findings and novel concepts that support a role for caveolin‐1 in cancer development and its distant spreading. We also address the potential application of caveolin‐1 in tumour therapy and diagnosis.     

Reactive oxygen species in cancer

Abstract

Elevated rates of reactive oxygen species (ROS) have been detected in almost all cancers, where they promote many aspects of tumour development and progression. However, tumour cells also express increased levels of antioxidant proteins to detoxify from ROS, suggesting that a delicate balance of intracellular ROS levels is required for cancer cell function. Further, the radical generated, the location of its generation, as well as the local concentration is important for the cellular functions of ROS in cancer. A challenge for novel therapeutic strategies will be the fine tuning of intracellular ROS signalling to effectively deprive cells from ROS-induced tumour promoting events, towards tipping the balance to ROS-induced apoptotic signalling. Alternatively, therapeutic antioxidants may prevent early events in tumour development, where ROS are important. However, to effectively target cancer cells specific ROS-sensing signalling pathways that mediate the diverse stress-regulated cellular functions need to be identified. This review discusses the generation of ROS within tumour cells, their detoxification, their cellular effects, as well as the major signalling cascades they utilize, but also provides an outlook on their modulation in therapeutics.     

Exploiting cell death pathways by an E. coli cytotoxin: autophagy as a double-edged sword for the host

Cytotoxic necrotizing factor 1 is a bacterial protein toxin from Escherichia coli that is able to activate the Rho GTPases and to hinder apoptosis and mitotic catastrophe. Upon exposure to toxin, cells undergo a complex framework of changes, including cytoskeleton remodeling and multinucleation. These cells also show a high survival rate for long periods of time and improve both their macropinocytotic scavenging activities and microautophagy. Only at the very end, probably when "feeding" materials are exhausted, do these cells die by autophagy. Taking into account the complex role of bacterial protein toxins in the infectious processes, we indicate the CNF1 activity as a Janus-faced paradigm of those bacteria that hijack cell fate to their own benefit. This could somehow be linked to the hypothesized connection between certain bacterial toxins and cancer onset.     

Necrosis, a well-orchestrated form of cell demise: signalling cascades, important mediators and concomitant immune response

Necrosis has long been described as a consequence of physico-chemical stress and thus accidental and uncontrolled. Recently, it is becoming clear that necrotic cell death is as well controlled and programmed as caspase-dependent apoptosis, and that it may be an important cell death mode that is both pathologically and physiologically relevant. Necrotic cell death is not the result of one well-described signalling cascade but is the consequence of extensive crosstalk between several biochemical and molecular events at different cellular levels. Recent data indicate that serine/threonine kinase RIP1, which contains a death domain, may act as a central initiator. Calcium and reactive oxygen species (ROS) are main players during the propagation and execution phases of necrotic cell death, directly or indirectly provoking damage to proteins, lipids and DNA, which culminates in disruption of organelle and cell integrity. Necrotically dying cells initiate pro-inflammatory signalling cascades by actively releasing inflammatory cytokines and by spilling their contents when they lyse. Unravelling the signalling cascades contributing to necrotic cell death will permit us to develop tools to specifically interfere with necrosis at certain levels of signalling. Necrosis occurs in both physiological and pathophysiological processes, and is capable of killing tumour cells that have developed strategies to evade apoptosis. Thus detailed knowledge of necrosis may be exploited in therapeutic strategies.     

The multiple mechanisms of Ca2+ signalling by listeriolysin O, the cholesterol-dependent cytolysin of Listeria monocytogenes

Cholesterol-dependent cytolysins (CDCs) represent a large family of conserved pore-forming toxins produced by several Gram-positive bacteria such as Listeria monocytogenes, Streptococcus pyrogenes and Bacillus anthracis. These toxins trigger a broad range of cellular responses that greatly influence pathogenesis. Using mast cells, we demonstrate that listeriolysin O (LLO), a prototype of CDCs produced by L. monocytogenes, triggers cellular responses such as degranulation and cytokine synthesis in a Ca(2+)-dependent manner. Ca(2+) signalling by LLO is due to Ca(2+) influx from extracellular milieu and release of from intracellular stores. We show that LLO-induced release of Ca(2+) from intracellular stores occurs via at least two mechanisms: (i) activation of intracellular Ca(2+) channels and (ii) a Ca(2+) channels independent mechanism. The former involves PLC-IP(3)R operated Ca(2+) channels activated via G-proteins and protein tyrosine kinases. For the latter, we propose a novel mechanism of intracellular Ca(2+) release involving injury of intracellular Ca(2+) stores such as the endoplasmic reticulum. In addition to Ca(2+) signalling, the discovery that LLO causes damage to an intracellular organelle provides a new perspective in our understanding of how CDCs affect target cells during infection by the respective bacterial pathogens.     

New genotoxin shows diversity of bacterial attack mechanisms

Bacteria make a wide range of toxic products that interact with eukaryotic cellular machinery in a precise way. These toxins interfere with key eukaryotic processes, such as cellular signalling components, and some directly attack the genome. Nougayrède and colleagues have recently identified a novel hybrid peptide-polyketide compound from Escherichia coli that leads to DNA damage. This novel compound is produced by pathogenic and, most interestingly, commensal isolates. Although it is not yet clear how the peptide-polyketide compound functions at the molecular level, it is possible that it contributes to bacterial pathogenesis and bacterially induced carcinogenesis.     

Chemokines and cancer: migration, intracellular signalling and intercellular communication in the microenvironment

Inappropriate chemokine/receptor expression or regulation is linked to many diseases, especially those characterized by an excessive cellular infiltrate, such as rheumatoid arthritis and other inflammatory disorders. There is now overwhelming evidence that chemokines are also involved in the progression of cancer, where they function in several capacities. First, specific chemokine-receptor pairs are involved in tumour metastasis. This is not surprising, in view of their role as chemoattractants in cell migration. Secondly, chemokines help to shape the tumour microenvironment, often in favour of tumour growth and metastasis, by recruitment of leucocytes and activation of pro-inflammatory mediators. Emerging evidence suggests that chemokine receptor signalling also contributes to survival and proliferation, which may be particularly important for metastasized cells to adapt to foreign environments. However, there is considerable diversity and complexity in the chemokine network, both at the chemokine/receptor level and in the downstream signalling pathways they couple into, which may be key to a better understanding of how and why particular chemokines contribute to cancer growth and metastasis. Further investigation into these areas may identify targets that, if inhibited, could render cancer cells more susceptible to chemotherapy.