Characterization of clathrin and Syk interaction upon Shiga toxin binding

Protein kinase signaling by Shiga Toxin subunits

Background:

Escherichia coli produces Shiga toxin (Stx), a pentamer composed of one A subunit and four B subunits. The B subunit of Stx (StxB) mediated the attachment of the holotoxin to the cell surface while the A subunit (StxA) has N-glycosidase activity, resulting in protein synthesis and cell death inhibition. Stx-induced cytotoxicity and apoptosis have been observed in various cell lines, although the signaling effectors are not precisely defined. Activated by protein kinases (PK), the signaling pathway in human tumors plays an oncogenic role. Tumor proliferation, survival, and metastasis are promoted by kinase receptors. In this regard, PK regulatory effects on the cellular constituents of the tumor microenvironment can affect immunosuppressive purposes.

Methods:

In this study, kinase inhibitors were used to evaluate the influence of Stx and its subunits on HeLa and Vero cells. Selective inhibitors of protein kinase C (PKC), CaM kinase (calmodulin kinase), protein kinase A (PKA), and protein kinase G (PKG) were used to compare the signaling activity of each subunit.

Results:

The ribotoxic activity in the target cells will lead to rapid protein synthesis inhibition and cell death in the mammalian host. The expression of Bcl2 family members was also assessed. Protein kinase signaling by Stx and its A and B subunits was induced by PKA, PKG, and PKC in HeLa cells. CaM kinase induction was significant in Vero cells. StxB significantly induced the pro-apoptotic Bax signaling factor in HeLa cells.

Conclusion:

The assessment of different signaling pathways utilized by Stx and its subunits could help in a better understanding of various cell death responses. The use of inhibitors can block cell damage and disease progression and create therapeutic compounds for targeted cancer therapy. Inhibition of these pathways is the primary clinical goal.

Cross-linking of glycosphingolipids at the plasma membrane: consequences for intracellular signaling and traffic

Glycosphingolipids (GSLs) are predominantly found in the outer leaflet of the plasma membrane, where they play a role in important processes such as cell adhesion, migration and signaling. However, by which mechanisms GSLs regulate these processes remains elusive. In this study, we therefore took advantage of the fact that some GSLs also serve as receptors for certain protein toxins, which rely on receptor binding for internalization and intoxication. Here, we demonstrate that Shiga and cholera toxins, which both possess multivalent GSL-binding capacity, induce dissociation of the cytosolic cPLA2α-AnxA1 complex in HeLa and HMEC-1 cells. The dissociation is mediated through an increase in cytosolic calcium levels and activation of the tyrosine kinase Syk. Ricin, a protein toxin that does not cross-link surface molecules, has no effect on the same complex. Importantly, we find that antibody-mediated cross-linking of Gb3 and GM1, the GSL receptors for Shiga and cholera toxin, respectively, also induces dissociation. These data demonstrate that cross-linking of GSLs at the plasma membrane mediates the intracellular signaling events resulting in dissociation of the complex. After dissociation, cPLA2α and AnxA1 are translocated to intracellular membranes where they are known to function in regulating membrane transport processes. In conclusion, we have characterized a novel mechanism for cell surface-induced initiation of intracellular signaling and transport events.

Escherichia coli O104:H4 Pathogenesis: an Enteroaggregative E. coli/Shiga Toxin-Producing E. coli Explosive Cocktail of High Virulence

A major outbreak caused by Escherichia coli of serotype O104:H4 spread throughout Europe in 2011. This large outbreak was caused by an unusual strain that is most similar to enteroaggregative E. coli (EAEC) of serotype O104:H4. A significant difference, however, is the presence of a prophage encoding the Shiga toxin, which is characteristic of enterohemorrhagic E. coli (EHEC) strains. This combination of genomic features, associating characteristics from both EAEC and EHEC, represents a new pathotype. The 2011 E. coli O104:H4 outbreak of hemorrhagic diarrhea in Germany is an example of the explosive cocktail of high virulence and resistance that can emerge in this species. A total of 46 deaths, 782 cases of hemolytic-uremic syndrome, and 3,128 cases of acute gastroenteritis were attributed to this new clone of EAEC/EHEC. In addition, recent identification in France of similar O104:H4 clones exhibiting the same virulence factors suggests that the EHEC O104:H4 pathogen has become endemically established in Europe after the end of the outbreak. EAEC strains of serotype O104:H4 contain a large set of virulence-associated genes regulated by the AggR transcription factor. They include, among other factors, the pAA plasmid genes encoding the aggregative adherence fimbriae, which anchor the bacterium to the intestinal mucosa (stacked-brick adherence pattern on epithelial cells). Furthermore, sequencing studies showed that horizontal genetic exchange allowed for the emergence of the highly virulent Shiga toxin-producing EAEC O104:H4 strain that caused the German outbreak. This article discusses the role these virulence factors could have in EAEC/EHEC O104:H4 pathogenesis.

Plasma membrane reorganization: A glycolipid gateway for microbes

Ligand-receptor interactions, which represent the core for cell signaling and internalization processes are largely affected by the spatial configuration of host cell receptors. There is a growing piece of evidence that receptors are not homogeneously distributed within the plasma membrane, but are rather pre-clustered in nanodomains, or clusters are formed upon ligand binding. Pathogens have evolved many strategies to evade the host immune system and to ensure their survival by hijacking plasma membrane receptors that are most often associated with lipid rafts. In this review, we discuss the early stage molecular and physiological events that occur following ligand binding to host cell glycolipids. The ability of various biological ligands (e.g. toxins, lectins, viruses or bacteria) that bind to glycolipids to induce their own uptake into mammalian cells by creating negative membrane curvature and membrane invaginations is explored. We highlight recent trends in understanding nanoscale plasma membrane (re-)organization and present the benefits of using synthetic membrane systems. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Lipid requirements for entry of protein toxins into cells

The plant toxin ricin and the bacterial toxin Shiga toxin both belong to a group of protein toxins having one moiety that binds to the cell surface, and another, enzymatically active moiety, that enters the cytosol and inhibits protein synthesis by inactivating ribosomes. Both toxins travel all the way from the cell surface to endosomes, the Golgi apparatus and the ER before the ribosome-inactivating moiety enters the cytosol. Shiga toxin binds to the neutral glycosphingolipid Gb3 at the cell surface and is therefore dependent on this lipid for transport into the cells, whereas ricin binds both glycoproteins and glycolipids with terminal galactose. The different steps of transport used by these toxins have specific requirements for lipid species, and with the recent developments in mass spectrometry analysis of lipids and microscopical and biochemical dissection of transport in cells, we are starting to see the complexity of endocytosis and intracellular transport. In this article we describe lipid requirements and the consequences of lipid changes for the entry and intoxication with ricin and Shiga toxin. These toxins can be a threat to human health, but can also be exploited for diagnosis and therapy, and have proven valuable as tools to study intracellular transport.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Dual phosphoproteomics and chemical proteomics analysis of erlotinib and gefitinib interference in acute myeloid leukemia cells

Small molecule inhibitors of protein kinases have emerged as a major class of therapeutic agents for the treatment of hematological malignancies. Both in vitro studies and patient case reports suggest therapeutic potential of the clinical kinase inhibitors erlotinib and gefitinib in acute myeloid leukemia (AML). The drugs' cellular modes of action in AML warrant further investigation as their primary therapeutic target, the epidermal growth factor receptor, is not expressed. We therefore performed SILAC-based quantitative mass spectrometry analyses to a depth of 10,975 distinct phosphorylation sites to characterize the phosphoproteome of KG1 AML cells and its regulation upon erlotinib and gefitinib treatment. Less than 50 site-specific phosphorylations changed significantly, indicating rather specific interference with AML cell signaling. Many drug-induced changes occurred within a network of tyrosine phosphorylated proteins that included Src family kinases (SFKs) and the tyrosine kinases Btk and Syk. We further performed quantitative chemical proteomics in KG1 cell extracts and identified SFKs and Btk as direct cellular targets of both erlotinib and gefitinib. Taken together, our data suggest that cellular perturbation of SFKs and/or Btk translates into rather specific signal transduction inhibition, which in turn contributes to the antileukemic activity of erlotinib and gefitinib in AML.

A novel mechanism involved in the pathogenesis of Graves ophthalmopathy (GO): clathrin is a possible targeting molecule for inhibiting local immune response in the orbit

INTRODUCTION

Excessive orbital fibroblast (OF) proliferation and extracellular matrix production, as well as inflammation resulting in the expansion and remodeling of orbital tissue, are characteristic of Graves ophthalmopathy (GO). Our aim was to analyze and inhibit signaling pathways in resident OF that are involved in GO. METHODS/MAIN OUTCOME MEASURES: Primary human OF were obtained from 12 patients with active, severe GO and from 12 healthy control subjects. The cells were characterized by immunofluorescence assay and flow cytometry. Tyrosine phosphorylation of cellular proteins was determined by Western blot techniques, immunoprecipitation, and protein identity with mass spectrometry. Cell proliferation was determined by 5-bromo-2-deoxyuridine incorporation, hyaluronan (HA) production was assessed by a HA-binding protein based assay, and intracellular reactive oxygen species (ROS) were determined by the dichlorofluorescein assay. Clathrin heavy-chain (CHC) expression was inhibited with small interfering RNA technology.

RESULTS

Tyrosine phosphorylation of CHC is constitutively increased in vitro in GO-derived OF, independent of serum or other stimulating factors. The proliferative and biosynthetic capabilities (production of HA, ROS) of GO-derived OF are significantly higher than those of OF from healthy control subjects. Down-regulation of CHC expression leads to a normalization of pathologically increased proliferation and production of HA and ROS in GO-derived OFs in vitro.

CONCLUSIONS

Our findings strongly suggest that clathrin and clathrin-mediated signaling pathways are involved in the inflammatory signal transduction of OF in GO. With the identification of clathrin, we report a new potential targeting molecule for specific pharmacological inhibition of the local inflammatory response characteristic of GO.

Escherichia coli Shiga Toxin Mechanisms of Action in Renal Disease

Shiga toxin-producing Escherichia coli is a contaminant of food and water that in humans causes a diarrheal prodrome followed by more severe disease of the kidneys and an array of symptoms of the central nervous system. The systemic disease is a complex referred to as diarrhea-associated hemolytic uremic syndrome (D(+)HUS). D(+)HUS is characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. This review focuses on the renal aspects of D(+)HUS. Current knowledge of this renal disease is derived from a combination of human samples, animal models of D(+)HUS, and interaction of Shiga toxin with isolated renal cell types. Shiga toxin is a multi-subunit protein complex that binds to a glycosphingolipid receptor, Gb3, on select eukaryotic cell types. Location of Gb3 in the kidney is predictive of the sites of action of Shiga toxin. However, the toxin is cytotoxic to some, but not all cell types that express Gb3. It also can cause apoptosis or generate an inflammatory response in some cells. Together, this myriad of results is responsible for D(+)HUS disease.

Shiga toxin increases formation of clathrin-coated pits through Syk kinase

Clathrin-dependent endocytosis is a main entry mechanism for the glycolipid-binding Shiga toxin (Stx), although clathrin-independent pathways are also involved. Binding of Stx to its receptor Gb3 not only is essential for Stx retrograde transport to the endoplasmic reticulum and toxicity but also activates signaling through the tyrosine kinase Syk. We previously described that Syk activity is important for Stx entry, but it remained unclear how this kinase modulates endocytosis of Stx. Here we characterized the effects of Stx and Syk on clathrin-coated pit formation. We found that acute treatment with Stx results in an increase in the number of clathrin-coated profiles as determined by electron microscopy and on the number of structures containing the endocytic AP-2 adaptor at the plasma membrane determined by live-cell spinning disk confocal imaging. These responses to Stx require functional Syk activity. We propose that a signaling pathway mediated by Syk and modulated by Stx leads to an increased number of endocytic clathrin-coated structures, thus providing a possible mechanism by which Stx enhances its own endocytosis.

Interplay between toxin transport and flotillin localization

The flotillin proteins are localized in lipid domains at the plasma membrane as well as in intracellular compartments. In the present study, we examined the importance of flotillin-1 and flotillin-2 for the uptake and transport of the bacterial Shiga toxin (Stx) and the plant toxin ricin and we investigated whether toxin binding and uptake were associated with flotillin relocalization. We observed a toxin-induced redistribution of the flotillins, which seemed to be regulated in a p38-dependent manner. Our experiments provide no evidence for a changed endocytic uptake of Stx or ricin in cells silenced for flotillin-1 or -2. However, the Golgi-dependent sulfation of both toxins was significantly reduced in flotillin knockdown cells. Interestingly, when the transport of ricin to the ER was investigated, we obtained an increased mannosylation of ricin in flotillin-1 and flotillin-2 knockdown cells. The toxicity of both toxins was twofold increased in flotillin-depleted cells. Since BFA (Brefeldin A) inhibits the toxicity even in flotillin knockdown cells, the retrograde toxin transport is apparently still Golgi-dependent. Thus, flotillin proteins regulate and facilitate the retrograde transport of Stx and ricin.