Conjugated eicosapentaenoic acid inhibits human topoisomerase IB with a mechanism different from camptothecin

Conjugated eicosapentaenoic acid (cEPA) has been found to have antitumor effects which has been ascribed to their ability to inhibit DNA topoisomerases and DNA polymerases. We here show that cEPA inhibits the catalytic activity of human topoisomerase I, but unlike camptothecin it does not stabilize the cleavable complex, indicating a different mechanism of action. cEPA inhibits topoisomerase by impeding the catalytic cleavage of the DNA substrate as demonstrated using specific oligonucleotide substrates, and prevents the stabilization of the cleavable complex by camptothecin. Preincubation of the inhibitor with the enzyme is required to obtain complete inhibition. Molecular docking simulations indicate that the preferred cEPA binding site is proximal to the active site with the carboxylic group strongly interacting with the positively charged K443 and K587. Taken together the results indicate that cEPA inhibitor does not prevent DNA binding but inhibits DNA cleavage, binding in a region close to the topoisomerase active site.     

Crystal structure of the resuscitation-promoting factor (DeltaDUF)RpfB from M. tuberculosis

Mycobacterium tuberculosis is able to establish a non-replicating state and survive in an intracellular habitat for years. Resuscitation of dormant M. tuberculosis bacteria is promoted by resuscitation-promoting factors (Rpfs), which are secreted from slowly replicating bacteria close to dormant bacteria. Here we report the crystal structure of a truncated form of RpfB (residues 194-362), the sole indispensable Rpf of the five Rpfs encoded in this bacterium genome. The structure, denoted as (DeltaDUF)RpfB, exhibits a comma-like shape formed by a lysozyme-like globular catalytic domain and an elongated G5 domain, which is widespread among cell surface binding proteins. The G5 domain, whose structure was previously uncharacterised, presents some peculiar features. The basic structural motif of this domain, which represents the tail of the comma-like structure, is a novel super-secondary-structure element, made of two beta-sheets interconnected by a pseudo-triple helix. This intricate organisation leads to the exposure of several backbone hydrogen-bond donors/acceptors. Mutagenesis analyses and solution studies indicate that this protein construct as well as the full-length form are elongated monomeric proteins. Although (DeltaDUF)RpfB does not self-associate, the exposure of structural elements (backbone H-bond donors/acceptors and hydrophobic side chains) that are usually buried in globular proteins is typically associated with adhesive properties. This suggests that the RpfB G5 domain has a cell-wall adhesive function, which allows the catalytic domain to be properly oriented for the cleavage reaction. Interestingly, sequence comparisons indicate that these structural features are also shared by G5 domains involved in biofilm formation.     

Helicobacter pylori Counteracts the Apoptotic Action of Its VacA Toxin by Injecting the CagA Protein into Gastric Epithelial Cells

Infection with Helicobacter pylori is responsible for gastritis and gastroduodenal ulcers but is also a high risk factor for the development of gastric adenocarcinoma and lymphoma. The most pathogenic H. pylori strains (i.e., the so-called type I strains) associate the CagA virulence protein with an active VacA cytotoxin but the rationale for this association is unknown. CagA, directly injected by the bacterium into colonized epithelium via a type IV secretion system, leads to cellular morphological, anti-apoptotic and proinflammatory effects responsible in the long-term (years or decades) for ulcer and cancer. VacA, via pinocytosis and intracellular trafficking, induces epithelial cell apoptosis and vacuolation. Using human gastric epithelial cells in culture transfected with cDNA encoding for either the wild-type 38 kDa C-terminal signaling domain of CagA or its non-tyrosine-phosphorylatable mutant form, we found that, depending on tyrosine-phosphorylation by host kinases, CagA inhibited VacA-induced apoptosis by two complementary mechanisms. Tyrosine-phosphorylated CagA prevented pinocytosed VacA to reach its target intracellular compartments. Unphosphorylated CagA triggered an anti-apoptotic activity blocking VacA-induced apoptosis at the mitochondrial level without affecting the intracellular trafficking of the toxin. Assaying the level of apoptosis of gastric epithelial cells infected with wild-type CagA⁺/VacA⁺ H. pylori or isogenic mutants lacking of either CagA or VacA, we confirmed the results obtained in cells transfected with the CagA C-ter constructions showing that CagA antagonizes VacA-induced apoptosis. VacA toxin plays a role during H. pylori stomach colonization. However, once bacteria have colonized the gastric niche, the apoptotic action of VacA might be detrimental for the survival of H. pylori adherent to the mucosa. CagA association with VacA is thus a novel, highly ingenious microbial strategy to locally protect its ecological niche against a bacterial virulence factor, with however detrimental consequences for the human host.     

Differences in the DNA replication of unicellular eukaryotes and metazoans: known unknowns

Although the basic mechanisms of DNA synthesis are conserved across species, there are differences between simple and complex organisms. In contrast to lower eukaryotes, replication origins in complex eukaryotes lack DNA sequence specificity, can be activated in response to stressful conditions and require poorly conserved factors for replication firing. The response to replication fork damage is monitored by conserved proteins, such as the TIPIN–TIM–CLASPIN complex. The absence of this complex induces severe effects on yeast replication, whereas in higher eukaryotes it is only crucial when the availability of replication origins is limiting. Finally, the dependence of DNA replication on homologous recombination proteins such as RAD51 and the MRE11–RAD50–NBS1 complex is also different; they are dispensable for yeast S‐phase but essential for accurate DNA replication in metazoans under unchallenged conditions. The reasons for these differences are not yet understood. Here, we focus on some of these known unknowns of DNA replication.     

New 7,8-ethylenedioxy-2,3-benzodiazepines as noncompetitive AMPA receptor antagonists

A series of 1-aryl-3,5-dihydro-7,8-ethylenedioxy-4H-2,3-benzodiazepin-4-ones 2a-f, were synthesized and screened as anticonvulsant agents in DBA/2 mice against sound-induced seizures. The new compounds display anticonvulsant properties although the ED(50) values are higher than those of prototypes 1-aryl-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-ones (1) and GYKI 52466, well-known noncompetitive AMPA receptor antagonists. Functional tests were performed to evaluate the antagonistic activity at the AMPA and kainate receptors.     

A series of novel, potent, and selective histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors offer a promising strategy for cancer therapy and the first generation HDAC inhibitors are currently in clinical trials. A structurally novel series of HDAC inhibitors based on the natural cyclic tetrapeptide Apicidin is described. Selected screening of the sample collection looking for L-2-amino-8-oxodecanoic acid (L-Aoda) derivatives identified a small acyclic lead molecule 1 with the unusual ketone zinc binding group. SAR studies around this lead resulted in optimization to potent, low molecular weight, selective, non-hydroxamic acid HDAC inhibitors, equipotent to current clinical candidates.     

Mouse profilin 2 regulates endocytosis and competes with SH3 ligand binding to dynamin 1

Mammalian profilins are abundantly expressed actin monomer-binding proteins, highly conserved with respect to their affinities for G-actin, poly-L-proline, and phosphoinositides. Profilins associate with a large number of proline-rich proteins; the physiological significance and regulation of which is poorly understood. Here we show that profilin 2 associates with dynamin 1 via the C-terminal proline-rich domain of dynamin and thereby competes with the binding of SH3 ligands such as endophilin, amphiphysin, and Grb2, thus interfering with the assembly of the endocytic machinery. We also present a novel role for the brain-specific mouse profilin 2 as a regulator of membrane trafficking. Overexpression of profilin 2 inhibits endocytosis, whereas lack of profilin 2 in neurons results in an increase in endocytosis and membrane recycling. Phosphatidylinositol 4,5-bisphosphate releases profilin 2 from the profilin 2-dynamin 1 complex as well as from the profilin 2-actin complex, suggesting that profilin 2 is diverging the phosphoinositide signaling pathway to actin polymerization as well as endocytosis.     

Interaction among proteins and peptide libraries in proteome analysis: pH involvement for a larger capture of species

When capturing proteins via combinatorial peptide ligand libraries, a method well known for drastically reducing the concentration of high-abundance proteins and substantially magnifying the signal of low-abundance species, thus leading to the discovery of a large number of proteins previously undetected in proteomes, we had constantly noticed that there would be a loss of species initially present in the untreated sample, to the tune of 5%, up to 15% in some cases. Such losses are a nuisance and hamper to some extent the unique performance of the method. In order to verify if such losses could be reduced and also to understand some mechanisms of the capture process, we introduce here an important variant to the capture operation, up to the present carried out in physiological saline at pH 7.2. In this novel protocol, the binding step is conducted at three different pH values, namely the standard one at pH 7.2, plus two additional processes, at acidic (pH 4.0) and alkaline (pH 9.3) pH values. Indeed the capture process is more extensive, with a number of additional species captured at the two pH extremes in sera and other proteomes. Interestingly, at pH 4.0 newly detected proteins were mostly acidic, while at the alkaline pH additional protein species were more evenly distributed throughout the pI range towards the alkaline area. The role of pH in the complex mechanism of binding among the hexapeptide library and the various proteomes being analyzed is discussed and evaluated. Due to significant changes in protein patterns with pH, recommendations are thus made to increase the possibility to find additional gene products illustrated by two examples (snake venom and leaf protein extract). Keeping under control the environmental pH when facing reproducibility studies or for comparative proteomics profiling is also a general rule suggested by this study.