Inhibition of ribonucleic acid synthesis by myxin

New prodrugs and analogs of the phenazine 5,10-dioxide natural products iodinin and myxin promote selective cytotoxicity towards human acute myeloid leukemia cells

Novel chemotherapeutic strategies for acute myeloid leukemia (AML) treatment are called for. We have recently demonstrated that the phenazine 5,10-dioxide natural products iodinin (3) and myxin (4) exhibit potent and hypoxia-selective cell death on MOLM-13 human AML cells, and that the N-oxide functionalities are pivotal for the cytotoxic activity. Very few structure-activity relationship studies dedicated to phenazine 5,10-dioxides exist on mammalian cell lines and the present work describes our efforts regarding in vitro lead optimizations of the natural compounds iodinin (3) and myxin (4). Prodrug strategies reveal carbamate side chains to be the optimal phenol-attached group. Derivatives with no oxygen-based substituent (-OH or -OCH3) in the 6th position of the phenazine skeleton upheld potency if alkyl or carbamate side chains were attached to the phenol in position 1. 7,8-Dihalogenated- and 7,8-dimethylated analogs of 1-hydroxyphenazine 5,10-dioxide (21) displayed increased cytotoxic potency in MOLM-13 cells compared to all the other compounds studied. On the other hand, dihalogenated compounds displayed high toxicity towards the cardiomyoblast H9c2 cell line, while MOLM-13 selectivity of the 7,8-dimethylated analogs were less affected. Further, a parallel artificial membrane permeability assay (PAMPA) demonstrated the majority of the synthesized compounds to penetrate cell membranes efficiently, which corresponded to their cytotoxic potency. This work enhances the understanding of the structural characteristics essential for the activity of phenazine 5,10-dioxides, rendering them promising chemotherapeutic agents.

Resistance-Nodulation-Division Efflux Pump, LexABC, Contributes to Self-Resistance of the Phenazine Di-N-Oxide Natural Product Myxin in Lysobacter antibioticus

Antibiotic-producing microorganisms have developed several self-resistance mechanisms to protect them from autotoxicity. Transporters belonging to the resistance- nodulation-division (RND) superfamily commonly confer multidrug resistance in Gram-negative bacteria. Phenazines are heterocyclic, nitrogen-containing and redox-active compounds that exhibit diverse activities. We previously identified six phenazines from Lysobacter antibioticus OH13, a soil bacterium emerging as a potential biocontrol agent. Among these phenazines, myxin, a di-N-oxide phenazine, exhibited potent activity against a variety of microorganisms. In this study, we identified a novel RND efflux pump gene cluster, designated lexABC, which is located far away in the genome from the myxin biosynthesis gene cluster. We found a putative LysR-type transcriptional regulator encoding gene lexR, which was adjacent to lexABC. Deletion of lexABC or lexR gene resulted in significant increasing susceptibility of strains to myxin and loss of myxin production. The results demonstrated that LexABC pump conferred resistance against myxin. The myxin produced at lower concentrations in these mutants was derivatized by deoxidation and O-methylation. Furthermore, we found that the abolishment of myxin with deletion of LaPhzB, which is an essential gene in myxin biosynthesis, resulted in significant downregulation of the lexABC. However, exogenous supplementation with myxin to LaPhzB mutant could efficiently induce the expression of lexABC genes. Moreover, lexR mutation also led to decreased expression of lexABC, which indicates that LexR potentially positively modulated the expression of lexABC. Our findings reveal a resistance mechanism against myxin of L. antibioticus, which coordinates regulatory pathways to protect itself from autotoxicity.

Total synthesis and antileukemic evaluations of the phenazine 5,10-dioxide natural products iodinin, myxin and their derivatives

A new efficient total synthesis of the phenazine 5,10-dioxide natural products iodinin and myxin and new compounds derived from them was achieved in few steps, a key-step being 1,6-dihydroxyphenazine di-N-oxidation. Analogues prepared from iodinin, including myxin and 2-ethoxy-2-oxoethoxy derivatives, had fully retained cytotoxic effect against human cancer cells (MOLM-13 leukemia) at atmospheric and low oxygen level. Moreover, iodinin was for the first time shown to be hypoxia selective. The structure-activity relationship for leukemia cell death induction revealed that the level of N-oxide functionality was essential for cytotoxicity. It also revealed that only one of the two phenolic functions is required for activity, allowing the other one to be modified without loss of potency.

Bioactive natural products from Lysobacter

The gliding Gram-negative Lysobacter bacteria are emerging as a promising source of new bioactive natural products. These ubiquitous freshwater and soil microorganisms are fast growing, simple to use and maintain, and genetically amenable for biosynthetic engineering. This Highlight reviews a group of biologically active and structurally distinct natural products from the genus Lysobacter, with a focus on their biosyntheses. Although Lysobacter sp. are known as prolific producers of bioactive natural products, detailed molecular mechanistic studies of their enzymatic assembly have been surprisingly scarce. We hope to provide a snapshot of the important work done on the lysobacterial natural products and to provide useful information for future biosynthetic engineering of novel antibiotics in Lysobacter.

DNA strand cleavage by the phenazine di-N-oxide natural product myxin under both aerobic and anaerobic conditions

Heterocyclic N-oxides are an interesting class of antitumor agents that selectively kill the hypoxic cells found in solid tumors. The hypoxia-selective activity of the lead compound in this class, tirapazamine, stems from its ability to undergo intracellular one-electron reduction to an oxygen-sensitive drug radical intermediate. In the presence of molecular oxygen, the radical intermediate is back-oxidized to the parent molecule. Under hypoxic conditions, the extended lifetime of the drug radical intermediate enables its conversion to a highly cytotoxic DNA-damaging intermediate via a "deoxygenative" mechanism involving the loss of oxygen from one of its N-oxide groups. The natural product myxin is a phenazine di-N-oxide that displays potent antibiotic activity against a variety of organisms under aerobic conditions. In light of the current view of heterocyclic N-oxides as agents that selectively operate under hypoxic conditions, it is striking that myxin was identified from Sorangium extracts based upon its antibiotic properties under aerobic conditions. Therefore, we set out to examine the molecular mechanisms underlying the biological activity of myxin. We find that myxin causes bioreductively activated, radical-mediated DNA strand cleavage under both aerobic and anaerobic conditions. Our evidence indicates that strand cleavage occurs via a deoxygenative metabolism. We show that myxin displays potent cytotoxicity against the human colorectal cancer cell line HCT-116 under both aerobic and anaerobic conditions that is comparable to the cell-killing properties of tirapazamine under anaerobic conditions. This work sheds light on the processes by which the naturally occurring aromatic N-oxide myxin gains its potent antibiotic properties under aerobic conditions. Furthermore, these studies highlight the general potential for aromatic N-oxides to undergo highly cytotoxic deoxygenative metabolism following enzymatic one-electron reduction under aerobic conditions.

Mode of action of quindoxin and substituted quinoxaline-di-N-oxides on Escherichia coli

The effect of quindoxin on the synthesis of deoxyribonucleic acid (DNA), ribonucleic acid, and protein in Escherichia coli KL 399 was examined under aerobic and anaerobic conditions. In the absence of oxygen the synthesis of DNA was completely inhibited by 10 ppm of quindoxin, whereas the syntheses of ribonucleic acid and protein were not affected. Quinoxalin-di-N-oxides (QdNO) induce degradation of DNA in both proliferating and non-proliferating cells. polA, recA, recB, recC, exrA, and uvrA mutants were more susceptible than the corresponding repair-proficient strains. All strains were more resistant in the presence of oxygen. Quindoxin was reduced to quinoxalin-N-oxide by intact E. coli cells or by a cell-free E. coli extract. Electron spin resonance measurements demonstrated the generation of free radicals during the reduction of quindoxin. Oxygen or deficiency of energy sources impaired the antibiotic activity and the reduction of QdNO. The QdNO reductase activity was demonstrated to be lower in QdNO-resistant mutants than in the susceptible parent strain. Based on these results it is concluded that an intermediate of reduction, probably a free radical, is responsible for the lethal effect of quindoxin. With three independent techniques no evidence has been found for binding of quindoxin to DNA.

Interactions of heteroaromatic compounds with nucleic acids. 1. The influence of heteroatoms and polarizability on the base specificity of intercalating ligands

We have examined the origins of base specificity in intercalating ligands by studying the interaction with DNA of a series of proflavine and acridine orange analogs differing in the heteroatoms present in the chromophore. Base specificity was determined by differential dialysis of the dye against DNA samples of differing G-C content. We find that G-C specificity increases as the visible absorbance band of the chromophore moves to longer wavelength, implying a relation between specificity and polarizability of the chromophore. This can be rationalized by recognizing that the G-C pair is more polar than A-T, and should therefore interact more favorably with an easily polarized ring system. We find in addition that dimethylation of the chromophore amino groups increases specificity which we discuss in terms of steric and coupled steric-electronic contributions. Our results also bear on the origin of G-C specificity in binding actinomycin to DNA. Some of the compounds studied are as G-C specific as actinomycin, yet they lack hydrogen-bonding functions as plausible determinants of specificity. This observation gives new life to the hypothesis that the specificity of actinomycin is determined primarily by preferential interaction of the chromophore with a G-C pair.