Structure–Function Relation of Efomycines, a Family of Small-Molecule Inhibitors of Selectin Functions

RF-3192C and other polyketides from the marine endophytic Aspergillus niger ASSB4: structure assignment and bioactivity investigation

Chemical investigation of the methanolic extract of endophytic Aspergillus niger SB4, isolated from the marine alga Laurencia obtuse, afforded the pentacyclic polyketide, RF-3192C (1), the dimeric coumarin orlandin (2), fonsecin B (3), TMC-256A1 (4), cyclo-(Leu-Ala) (5), and cerebroside A (6).The chemical structure of RF-3192C (1) is assigned herein for the first time using 1D/2D NMR and HRESI-MS. Additionally, the revision of the NMR assignments of orlandin (2) was reported herein as well. Investigation of the antimicrobial activities of isolated compounds revealed the high activity of RF-3192C (1) against Pseudomonas aeruginosa and Bacillus subtilis, and moderate activity against yeast. Moreover, an in vitro cytotoxic activity against liver (HEPG2), cervical (HELA), lung (A549), prostate (PC3), and breast (MCF7) cancer cell lines of the isolated compounds was evaluated. The isolation and taxonomical characterization of the producing fungus was reported as well.

Selectins-The Two Dr. Jekyll and Mr. Hyde Faces of Adhesion Molecules-A Review

Selectins belong to a group of adhesion molecules that fulfill an essential role in immune and inflammatory responses and tissue healing. Selectins are glycoproteins that decode the information carried by glycan structures, and non-covalent interactions of selectins with these glycan structures mediate biological processes. The sialylated and fucosylated tetrasaccharide sLe^x is an essential glycan recognized by selectins. Several glycosyltransferases are responsible for the biosynthesis of the sLe^x tetrasaccharide. Selectins are involved in a sequence of interactions of circulated leukocytes with endothelial cells in the blood called the adhesion cascade. Recently, it has become evident that cancer cells utilize a similar adhesion cascade to promote metastases. However, like Dr. Jekyll and Mr. Hyde’s two faces, selectins also contribute to tissue destruction during some infections and inflammatory diseases. The most prominent function of selectins is associated with the initial stage of the leukocyte adhesion cascade, in which selectin binding enables tethering and rolling. The first adhesive event occurs through specific non-covalent interactions between selectins and their ligands, with glycans functioning as an interface between leukocytes or cancer cells and the endothelium. Targeting these interactions remains a principal strategy aimed at developing new therapies for the treatment of immune and inflammatory disorders and cancer. In this review, we will survey the significant contributions to and the current status of the understanding of the structure of selectins and the role of selectins in various biological processes. The potential of selectins and their ligands as therapeutic targets in chronic and acute inflammatory diseases and cancer will also be discussed. We will emphasize the structural characteristic of selectins and the catalytic mechanisms of glycosyltransferases involved in the biosynthesis of glycan recognition determinants. Furthermore, recent achievements in the synthesis of selectin inhibitors will be reviewed with a focus on the various strategies used for the development of glycosyltransferase inhibitors, including substrate analog inhibitors and transition state analog inhibitors, which are based on knowledge of the catalytic mechanism.

Efomycins K and L From a Termite-Associated Streptomyces sp. M56 and Their Putative Biosynthetic Origin

Two new elaiophylin derivatives, efomycins K (1) and L (2), and five known elaiophylin derivatives (3–7) were isolated from the termite-associated Streptomyces sp. M56. The structures were determined by 1D and 2D NMR and HR-ESIMS analyses and comparative CD spectroscopy. The putative gene cluster responsible for the production of the elaiophylin and efomycin derivatives was identified based on significant homology to related clusters. Phylogenetic analysis of gene cluster domains was used to provide a biosynthetic rational for these new derivatives and to demonstrate how a single biosynthetic pathway can produce diverse structures.

Dynamic Docking of Conformationally Constrained Macrocycles: Methods and Applications

Many natural products consist of large and flexible macrocycles that engage their targets via multiple contact points. This combination of contained flexibility and large contact area often allows natural products to bind at target surfaces rather than deep pockets, making them attractive scaffolds for inhibiting protein-protein interactions and other challenging therapeutic targets. The increasing ability to manipulate such compounds either biosynthetically or via semisynthetic modification means that these compounds can now be considered as starting points for medchem campaigns rather than solely as ends. Modern medchem benefits substantially from rational improvements made on the basis of molecular docking. As such, docking methods have been enhanced in recent years to deal with the complicated binding modalities and flexible scaffolds of macrocyclic natural products and natural product-like structures. Here, we comprehensively review methods for treating and docking these large macrocyclic scaffolds and discuss some of the resulting advances in medicinal chemistry.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

Molecular markers and targeted therapy of skin rejection in composite tissue allotransplantation

Skin rejection remains a major hurdle in reconstructive transplantation. We investigated molecular markers of skin rejection with particular attention to lymphocyte trafficking. Skin biopsies (n = 174) from five human hand transplant recipients were analyzed for rejection, characteristics of the infiltrate and lymphocytic adhesion markers. The cellular infiltrate predominantly comprised CD3+ T cells. CD68, Foxp3 and indoleamine 2, 3-dioxygenase expression and the CD4/CD8 increased with severity of rejection. Lymphocyte adhesion markers were upregulated upon rejection, intercellular adhesion molecule-1 and E-selectin correlated best with severity of rejection. Guided by the findings, a specific E- and P-selectin inhibitor was investigated for its effect on skin rejection in a rat hind limb allotransplant model. While efomycine M (weekly s.c. injection into the graft) alone had no effect, long-term allograft survival was achieved when combined with antithymocyte globulin and tacrolimus (control group without efomycine M rejected at postoperative day [POD] 61 +/- 1). Upregulation of lymphocyte trafficking markers correlates with severity of skin rejection and time after transplantation in human hand transplantation. Blocking E- and P-selectin in the skin holds potential to significantly prolong limb allograft survival.

Selectin inhibitors and their proposed role in ischemia and reperfusion

The selectin family of cellular adhesion molecules plays an important role in the cellular infiltration and molecular signaling associated with ischemia/reperfusion (I/R). Selectins are essential in the recruitment and infiltration of leukocytes to sites of inflammation, and consequently, selectin blockade represents an important area of current research in the potential alleviation of the cell-mediated injury associated with I/R. Previously, treatments targeted at only a single selectin have proven ineffective, due to compensation by uninhibited cell-adhesion molecules. However, pan-selectin antagonists - those inhibitors capable of blocking the actions of all three selectins - have demonstrated great potential in blocking the initial events in the leukocyte-endothelium adhesion cascade. A number of therapeutics have been developed, with the most promising results demonstrated by a class of non-oligosaccharide, small-molecule selectin antagonists. TB-1269 and OC-229 are two of the most promising of inhibitors in this class - they are capable of binding all three selectins, they have been demonstrated to reduce neutrophil infiltration following ischemia/reperfusion, and they have been associated with reduced tissue damage in experimental animal models of ischemia/reperfusion involving the liver, the heart, the kidneys, and the whole body. Furthermore, TBC-1269 has recently undergone successful phase I and phase IIa clinical trials for asthma and psoriasis. Though the timing of selectin inhibition is essential in attenuating leukocyte infiltration and cell-mediated injury, the transient blockade of selectin function, in a well-controlled setting, could be an extremely beneficial intervention in ischemia/reperfusion injury.

Therapeutic potential of selectin antagonists in psoriasis

Psoriasis is a systemic chronic inflammatory disorder. One of the major characteristics is an excess of infiltration of inflammatory cells, mainly lymphocytes, into the skin. Because the adhesion family of selectins is suggested to play a relevant role in this process, selectins have emerged as an interesting target for drug discovery and development in psoriasis. Different strategies targeting selectins have been described. This review discusses these approaches and summarises the current development of selectin antagonists for the treatment of psoriasis. An expert opinion will give the authors' personal opinion about selectin antagonism in psoriasis and which approach might be preferable.

Targeting leukocyte trafficking to inflamed skin - still an attractive therapeutic approach?

Research into leukocyte trafficking and its therapeutic exploitation appears to be a multistep process, just like the trafficking cascade itself. The initial euphoria evoked by an early understanding of the trafficking steps was followed by considerable disappointment following the clinical failure of the first selectin antagonist Cylexin (CY-1503), a sialyl Lewis(X) mimetic. The research area recovered and identified additional attractive pharmacological targets such as chemokine receptors and integrins. However, after lack of efficacy in anti-chemokine trials and the fatalities associated with anti VLA-4 therapy (Tysabri), the question arose again whether targeting leukocyte trafficking is really promising or whether such a complex, multistep process with many redundant and/or functionally overlapping molecules is simply too challenging to deal with. In this article, we delineate some pros and cons of this approach followed by a brief update on where we stand in the field and where we might move in the future.

Therapeutic targeting of molecules involved in leukocyte-endothelial cell interactions

Inflammation is traditionally viewed as a physiological reaction to tissue injury. Leukocytes contribute to the inflammatory response by the secretion of cytotoxic and pro-inflammatory compounds, by phagocytotic activity and by targeted attack of foreign antigens. Leukocyte accumulation in tissues is important for the initial response to injury. However, the overzealous accumulation of leukocytes in tissues also contributes to a wide variety of diseases, such as atherosclerosis, chronic inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, vasculitis, systemic inflammatory response syndrome, juvenile diabetes and psoriasis. Many therapeutic interventions target immune cells after they have already migrated to the site of inflammation. This review addresses different therapeutic strategies, used to reduce or prevent leukocyte-endothelial cell interactions and communication, in order to limit the progression of inflammatory diseases.

Diminished lymphocyte adhesion and alleviation of allergic responses by small-molecule- or antibody-mediated inhibition of L-selectin functions

Selectins are attractive targets for specific anti-inflammatory therapies. Using human lymphocytes as well as an L-selectin-transfected pre-B-cell line in dynamic flow chamber experiments, we could demonstrate that the small-molecule compound efomycine M blocks L-selectin-mediated lymphocyte rolling on sialylated Lewis(X), an action that was confirmed by plasmon resonance spectroscopy. Recruitment of naive lymphocytes to peripheral lymph nodes depends on L-selectin-mediated adhesion to high endothelial venules. We performed intravital microscopy studying lymphocyte rolling in peripheral lymph nodes and showed a 53% reduction (P=0.0006) of lymphocyte rolling in mice treated with efomycine M or a function-blocking antibody against L-selectin. In addition, the number of lymph node-homing T cells was reduced by >60% using either efomycine M or L-selectin-blocking antibodies. As recruitment of naive lymphocytes is a prerequisite for sensitization in T-cell-mediated immune reactions and allergic responses, mice were treated with efomycine M or an L-selectin-specific antibody during contact sensitization with DNFB. After adoptive transfer of corresponding T cells into non-sensitized recipient mice, the capacity of these cells to induce contact hypersensitivity was significantly reduced (P=0.0002 and P=0.0001, respectively). Our data demonstrate that it is possible, in principle, to diminish T-cell-mediated allergic reactions through interference with L-selectin functions during the early sensitization phase.