Caspase-3-dependent apoptosis of citreamicin ε-induced HeLa cells is associated with reactive oxygen species generation

Chemistry and biosynthesis of bacterial polycyclic xanthone natural products

Covering: up to the end of 2021Bacterial polycyclic xanthone natural products (BPXNPs) are a growing family of natural xanthones featuring a pentangular architecture with various modifications to the tricyclic xanthone chromophore. Their structural diversities and various activities have fueled biosynthetic and chemical synthetic studies. Moreover, their more potent activities than the clinically used drugs make them potential candidates for the treatment of diseases. Future unraveling of structure activity relationships (SARs) will provide new options for the (bio)-synthesis of drug analogues with higher activities. This review summarizes the isolation, structural elucidation and biological activities and more importantly, the recent strategies for the microbial biosynthesis and chemical synthesis of BPXNPs. Regarding their biosynthesis, we discuss the recent progress in enzymes that synthesize tricyclic xanthone, the protein candidates for structural moieties (methylene dioxygen bridge and nitrogen heterocycle), tailoring enzymes for methylation and halogenation. The chemical synthesis part summarizes the recent methodology for the division synthesis and coupling construction of achiral molecular skeletons. Ultimately, perspectives on the biosynthetic study of BPXNPs are discussed.

Bioactive Marine Xanthones: A Review

The marine environment is an important source of specialized metabolites with valuable biological activities. Xanthones are a relevant chemical class of specialized metabolites found in this environment due to their structural variety and their biological activities. In this work, a comprehensive literature review of marine xanthones reported up to now was performed. A large number of bioactive xanthone derivatives (169) were identified, and their structures, biological activities, and natural sources were described. To characterize the chemical space occupied by marine-derived xanthones, molecular descriptors were calculated. For the analysis of the molecular descriptors, the xanthone derivatives were grouped into five structural categories (simple, prenylated, O-heterocyclic, complex, and hydroxanthones) and six biological activities (antitumor, antibacterial, antidiabetic, antifungal, antiviral, and miscellaneous). Moreover, the natural product-likeness and the drug-likeness of marine xanthones were also assessed. Marine xanthone derivatives are rewarding bioactive compounds and constitute a promising starting point for the design of other novel bioactive molecules.

Albofungin and chloroalbofungin: antibiotic crystals with 2D but not 3D isostructurality

The potent antibiotics albofungin [systematic name: (1S,4R,8aR)-13-amino-1,15,16-trihydroxy-4-methoxy-12-methyl-3,4,8a,13-tetrahydro-1H-xantheno[4',3',2':4,5][1,3]benzodioxino[7,6-g]isoquinoline-14,17(2H,9H)-dione, C₂₇H₂₄N₂O₉, 1] and its chlorinated analogue chloroalbofungin (the 11-chloro analogue, C₂₇H₂₃ClN₂O₉, 2) have been crystallized following their isolation from the bacterial strain Streptomyces chrestomyceticus and their structures determined by single-crystal X-ray diffraction. The novel N-aminoquinolone molecular arrangement shows N-N bond lengths of 1.4202 (16) and 1.424 (2) Å in 1 and 2, respectively. The regiochemistry of chloro substitution in the A-ring is para to the quinolone O atom, with a C-Cl bond length of 1.741 (2) Å. The absolute stereochemistry at three chiral centres of the xanthone rings (i.e. 10S, 13R and 19R) is confirmed. Both compounds crystallize in chiral Sohncke space groups consistent with enantiopurity, but are not fully isostructural. A preserved supramolecular construct (SC) confers two-dimensional (2D) isostructurality, but the SC self-associates via either a twofold screw operation in 1, giving a monoclinic P2₁ structure, or a twofold rotation in 2, affording a monoclinic C2 structure with a doubled unit-cell axis.

Synergistic Effect of 1,3,6-Trihydroxy-4,5,7-Trichloroxanthone in Combination with Doxorubicin on B-Cell Lymphoma Cells and Its Mechanism of Action Through Molecular Docking

Background

The increasing rate of cancer chemoresistance and adverse side effects of therapy have led to the wide use of various chemotherapeutic combinations in cancer management, including lymphoid malignancy.

Objective

We investigated the effects of a combination of 1,3,6-trihydroxy-4,5,7-trichloroxanthone (TTX) and doxorubicin on the Raji lymphoma cell line.

Methods

Raji cells were treated with different concentrations of TTX, doxorubicin, or combinations thereof. Cancer cell growth inhibition was evaluated using 3-(4,5-dimethyltiazol-2-yl)-2,5- diphenyltetrazolium bromide/MTT assay to determine the half-maximal inhibitory concentration. Combination index values were calculated using CompuSyn (ComboSyn, Inc, Paramus, NJ). Molecular docking was conducted using a Protein-Ligand ANT System.

Results

The mean (SD) half-maximal inhibitory concentration values of TTX and doxorubicin were 15.948 (3.101) µM and 25.432 (1.417) µM, respectively. The combination index values of the different combinations ranged from 0.057 to 0.285, indicating strong to very strong synergistic effects. The docking study results reveal that TTX docks at the active site of Raf-1 and c-Jun N-kinase receptors with predicted free energies of binding of -79.37 and -75.42 kcal/mol, respectively.

Conclusions

The xanthone-doxorubicin combination showed promising in vitro activity against lymphoma cells. The results also indicate that the TTX and doxorubicin combination's effect was due to the interaction between TTX with Raf-1 and c-Jun N-kinase receptors, 2 determinants of doxorubicin resistance progression.

Antibacterial Aromatic Polyketides Incorporating the Unusual Amino Acid Enduracididine

The increasing incidence of infections caused by drug-resistant pathogens requires new efforts for the discovery of novel antibiotics. By screening microbial extracts in an assay aimed at identifying compounds interfering with cell wall biosynthesis, based on differential activity against a Staphylococcus aureus strain and its isogenic l-form, the potent enduracyclinones (1, 2), containing the uncommon amino acid enduracididine linked to a six-ring aromatic skeleton, were discovered from different Nonomuraea strains. The structures of 1 and 2 were established through a combination of derivatizations, oxidative cleavages, and NMR analyses of natural and ¹³C-¹⁵N-labeled compounds. Analysis of the biosynthetic cluster provides the combination of genes for the synthesis of enduracididine and type II polyketide synthases. Enduracyclinones are active against Gram-positive pathogens (especially Staphylococcus spp.), including multi-drug-resistant strains, with minimal inhibitory concentrations in the range of 0.0005 to 4 μg mL^-1 and with limited toxicity toward eukaryotic cells. The combined results from assays and macromolecular syntheses suggest a possible dual mechanism of action in which both peptidoglycan and DNA syntheses are inhibited by these molecules.

Influence of formulation of ZnO nanoblokes containing metallic ions dopants on their cytotoxicity and protective factors: An in vitro study on human skin cells exposed to UVA radiation

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The [Zn(O)/M] (M = Mg, Al, Ca, Ti) synthesize by thermal method and characterize.

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The metal dopants percolate into crystal lattice of ZnO and stable it.

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The [Zn(O)/M] shows very low amount of Zn⁺² release into culture medium.

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Intracellular reactive oxygen species generation decrease under UVA radiation.

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The [Zn(O)/M] protects of human skin cells against UVA radiation.

Application of ZnO nanoparticles in sunscreens exposes human skin with their adverse effects, which correlates to dissolution/translocation of free Zn⁺² ions. The possibility of decreasing solubility and therefore, reducing toxicity, by structural modifications have been discussed as a solution. The present investigation has developed new metallic lattices of ZnO to reduce cytotoxicity of ZnO nanoparticles. Novel metal-promoted Zn-based nanocomposites ([Zn(O)/M], M = Mg, Al, Ca, Ti) were synthesized and their physicochemical properties and their cytotoxicity were evaluated. Solubility and release studies showed that modification of ZnO structure decreases release of Zn⁺² into culture medium. XRD and UV absorbance analyses showed that metallic-dopants percolate into crystalline lattice of ZnO. This phenomenon is basic reason for stability of Zn-based network. Cell culture studies and MTT assay on human skin cells (HFF-1) exposed to UVA radiation showed that the level of protection of [Zn(O)/M] compounds were more than of [ZnO]. Dichlorofluoroscein diacetate-ROS assay and Zn⁺² release experiments indicated that [Zn(O)/M] nanocomposites decreased the level of ROS generation and Zn⁺² release in compared to ZnO, indicating higher safety of nanocomposites. This study shows that the synthesized Zn-based nanocomposites have potential to be used as safer and more effective sunscreens than ZnO.

Synthesis of the Pentacyclic Core of Citreamicin η

The citreamicins comprise a novel class of polycyclic xanthone natural products that have not yet yielded to total synthesis. A concise 11-step synthesis of the pentacyclic core of citreamicin η is now reported that features the use of a general approach for the synthesis of 1,4-dioxygenated xanthones. The synthesis also showcases improved techniques for effecting regioselective bromination of certain substituted phenols and coupling of acetylides with hindered ketones.

Pentacyclic antibiotics from a tidal mud flat-derived actinomycete

The combination of investigating a unique source of chemically prolific bacterium with an LC/MS-based bacterial strain selection approach resulted in the discovery of two new secondary metabolites, buanmycin (1) and buanquinone (2), from the culture of a marine Streptomyces strain, which was isolated from a tidal mudflat in Buan, Republic of Korea. The carbon backbone of buanmycin (1), comprising 20 quaternary carbons out of 30 total carbons, was determined via (13)C-(13)C COSY NMR analysis after labeling 1 with (13)C by culturing the bacterium with (13)C-glucose. The complete structure of 1 was confidently elucidated, primarily based on 1D and 2D NMR spectroscopic and X-ray crystallographic analysis, as that of a new pentacyclic xanthone. The absolute configuration of the α-methyl serine unit in 1 was established by applying the advanced Marfey's method. The structure of buanquinone (2) was determined to be a new pentacyclic quinone based on NMR and MS spectroscopic data. Buanmycin exhibited potent cytotoxicity against colorectal carcinoma cells (HCT-116) and gastric carcinoma cells (SNU-638) with submicromolar IC50 values and strongly inhibited the pathogenic Gram-negative bacterium Salmonella enterica (MIC = 0.7 μM). In particular, buanmycin demonstrated inhibition of sortase A, which is a promising target for antibiotic discovery.

Arixanthomycins A-C: Phylogeny-guided discovery of biologically active eDNA-derived pentangular polyphenols

Soil microbiomes are a rich source of uncharacterized natural product biosynthetic gene clusters. Here we use short conserved biosynthetic gene sequences (natural product sequence tags) amplified from soil microbiomes as phylogenetic markers to correlate genotype to chemotype and target the discovery of novel bioactive pentangular polyphenols from the environment. The heterologous expression of an environmental DNA-derived gene cluster (the ARX cluster), whose ketosynthase beta (KS_β) sequence tag was phylogenetically distinct from any known KS_β sequence, led to the discovery of the arixanthomycins. Arixanthomycin A (1) exhibits potent antiproliferative activity against human cancer cell lines.