Chartreusin, an antitumor glycoside antibiotic, induces DNA strand scission

Indispensable role of microbes in anticancer drugs and discovery trends

Recent years have seen an increased focus on the advancement of naturally derived products for the treatment of cancer. Since the beginning of recorded history, nature has provided a variety of medicinal agents, and an overwhelming number of drugs that we have today are derived from natural sources. Such natural agents are prominently used to treat several diseases such as diabetes, malaria, Alzheimer's, pulmonary disorders, etc. with cancer being the highlight of this review. Due to the rapid development of resistance to chemotherapeutic drugs, the hunt for effective novel drugs is still a paramount concern in cancer treatment. Moreover, many chemotherapy drugs typically have high toxicity and adverse side effects, which necessitates the need to develop anti-tumor drugs that can be employed to treat deadly tumors with fewer negative effects on health and better efficacy. Isolation of several chemotherapeutic drugs has been conducted from a wide range of natural sources which include plants, microbes, fungi, and marine microorganisms. Considering the trends of previous decades, microbial diversity has grown to play a significant role in the formulation of pharmaceuticals and drugs, especially antibiotics and anti-cancer medications. Microbe-derived antitumor antibiotics such as anthracycline, epothilones, bleomycin, actinomycin, and staurosporine are amongst the widely used cancer chemotherapeutic agents. This review deals majorly with microbe-derived anticancer drugs taking into account their derivatives, mechanism of action, isolation procedures, limitations, and tumors targeted by them. This article also reports the phase of clinical study these drugs are undergoing. Moreover, it intends to portray the indispensable part that these microbes have been playing since time immemorial in the odyssey of chemotherapeutic agents. KEY POINTS: • Microbial diversity contributes heavily towards the formulation of anticancer drugs. • Polypeptides, carbohydrates, and alkaloids are prevalent microbe-based drug classes. • Microbe-derived anticancer agents target various sarcomas, carcinomas, and lymphomas.

Electrochemical monitoring of ROS generation by anticancer agents: the case of chartreusin

Generation of ROS by anticancer agents is monitored using solid state electrochemical techniques.

Identification of a novel topoisomerase inhibitor effective in cells overexpressing drug efflux transporters

Background

Natural product structures have high chemical diversity and are attractive as lead structures for discovery of new drugs. One of the disease areas where natural products are most frequently used as therapeutics is oncology.

Method and Findings

A library of natural products (NCI Natural Product set) was screened for compounds that induce apoptosis of HCT116 colon carcinoma cells using an assay that measures an endogenous caspase-cleavage product. One of the apoptosis-inducing compounds identified in the screen was thaspine (taspine), an alkaloid from the South American tree Croton lechleri. The cortex of this tree is used for medicinal purposes by tribes in the Amazonas basin. Thaspine was found to induce conformational activation of the pro-apoptotic proteins Bak and Bax, mitochondrial cytochrome c release and mitochondrial membrane permeabilization in HCT116 cells. Analysis of the gene expression signature of thaspine-treated cells suggested that thaspine is a topoisomerase inhibitor. Inhibition of both topoisomerase I and II was observed using in vitro assays, and thaspine was found to have a reduced cytotoxic effect on a cell line with a mutated topoisomerase II enzyme. Interestingly, in contrast to the topoisomerase II inhibitors doxorubicin, etoposide and mitoxantrone, thaspine was cytotoxic to cell lines overexpressing the PgP or MRP drug efflux transporters. We finally show that thaspine induces wide-spread apoptosis in colon carcinoma multicellular spheroids and that apoptosis is induced in two xenograft mouse models in vivo.

Conclusions

The alkaloid thaspine from the cortex of Croton lechleri is a dual topoisomerase inhibitor effective in cells overexpressing drug efflux transporters and induces wide-spread apoptosis in multicellular spheroids.

Crystal structure of chartreusin derivative A132

The crystal structure of chartreusin derivative A132 (benzilidene chartreusin) has been determined by single-crystal X-ray diffraction. The space group is C2 with unit cell dimensions, a=18.482(4), b=8.749(3), c=43.906(2) A, beta=94.87(2) degrees, and the structure was refined to R-factors of 0.2365 (6585 all unique reflections) and 0.087 (2914 reflections with F(o)>4 sigma(F(o))) by a full-matrix least-squares method. There are two molecules in an asymmetric unit. Both molecules have similar structures, which are favorable to bind with DNA in the minor groove. A modeling study of the A132-DNA complex based on the X-ray structures suggests that the sugar moiety of A132 may play an important role in recognizing the sequence of DNA base pairs.

Biochemical characterisation of elsamicin and other coumarin-related antitumour agents as potent inhibitors of human topoisomerase II

Elsamicin (EM) is a recently discovered antitumour agent that is structurally related to several other compounds displaying anticancer activities, including chartreusin (CT), chrysomycin V (CV) and M (CM), gilvocarcin V (GV) and ravidomycin (RM). The biochemical events resulting in cytotoxicity for most of these compounds have not been clearly elucidated. There is some evidence that GV and CT bind to DNA and that GV is photosensitive, causing DNA damage. Therefore, we investigated the effects of these chemicals on DNA in cells and on pBR322 plasmid DNA. Using alkaline elution techniques, we found that all these compounds induced, to a different extent, DNA breakage in the human lung adenocarcinoma A549 cell line. In addition, all either bound to or intercalated into DNA, as indicated by their ability to alter the electrophoretic migration of DNA in agarose gels. Using the P4 unknotting assay, EM, CT, CV, CM, GV and RM were found to be potent inhibitors of the catalytic activity of topoisomerase II (topo II). Their potencies were compared with the known topo II inhibitors teniposide (VM-26) and doxorubicin (DX). EM was the most potent, with an IC50 of 0.4 mumol/l followed in order by CV, GV, and CT. VM-26 was the least potent with an IC50 of 15 mumol/l. It was concluded from these results that EM, GV, CV, CM and CT are capable of inhibiting topo II and that EM is the most potent inhibitor of topo II yet discovered.

Experimental and modelling studies on the DNA cleavage by elsamicin A

The ability of elsamicin A, an antitumour antibiotic, to cleave DNA in the presence of ferrous iron and reducing agents, has been analysed using experimental and theoretical approaches. Experimentally, the antibiotic causes DNA breakage in the presence of ferrous ions and a reducing agent. The DNA-cleaving activity appears to be partially blocked by the action of superoxide dismutase and catalase. These results indicate that the elsamicin aglycone moiety (chartarin) can be involved in the production of free radicals. We have performed a broad theoretical study based in the quantum-mechanical framework, which allow us to determine the redox properties of elsamicin that lead to the generation of radical species. Our results clearly show that elsamicin acts as a true catalyst in the production of superoxide radicals. Moreover, it is suggested that the oxidation/reduction mechanism of the aglycone moiety of elsamicin (a lactone), leading to DNA breakage, is different from the mechanism followed by other well-known anti-cancer drugs, whose chromophore is a quinone.

Product analyses in DNA strand scission by antitumor antibiotic elsamicin A

Elsamicin A is an antitumor antibiotic with fascinating chemical structure and a good candidate for pharmaceutical development. Molecular mechanism of DNA backbone cleavage mediated by Fe(II)-elsamicin A has been examined. Product analysis using DNA sequencing gels and HPLC reveals the production of damaged DNA fragments bearing 3'-/5'-phosphate and 3'-phosphoglycolate termini associated with formation of free base. In addition, hydrazine-trapping experiments indicate that C-4' hydroxylated abasic sites are formed concomitant with DNA degradation by Fe(II)-elsamicin A. The results lead to the conclusion that the hydroxyl radical formed in Fe(II)-elsamicin A plus dithiothreitol system oxidizes the deoxyribose moiety via hydrogen abstraction predominantly at the C-4' carbon of the deoxyribose backbone and ultimately produces strand breakage of DNA.

Map of chartreusin and elsamicin binding sites on DNA

Three DNA restriction fragments designated tyrT, 102-mer and 70-mer, have been used as substrates for footprinting studies using DNase I in the presence of the structurally similar antibiotics chartreusin and elsamicin A. The sequence-selective binding sites of the antibiotics can be mapped in regions which are rich in guanine + cytosine. Chartreusin and elsamicin appear to recognize and bind preferentially to sequences containing a CpG step. Regions containing a TpG step also seem to be a good binding site. The binding of elsamicin to these sites appears to be more concentration-dependent. A comparative analysis is performed of the sizes and locations of the different binding sites, aimed to infer whether the different biological effects of chartreusin and elsamicin A can be correlated to differences in their sequence-selective binding to DNA.

Specific binding of chartreusin, an antitumor antibiotic, to DNA

Chartreusin, an antitumor and antibacterial antibiotic, was found to inhibit negatively superhelical DNA-relaxation catalyzed by prokaryotic topoisomerase I and conversion of the superhelical DNA into unit length linear form catalyzed by single-strand-specific S1 nuclease. The inhibitory effect of the agent was due to the binding to DNA causing the alteration of tertiary structure. To characterize the binding specificity, we investigated the protection of DNA against cleavages by various restriction endonucleases. It was evidenced that the binding of the agent is not at random and correlates to the sequence 5'CGC 3' 3'GCG 5' on DNA stretch.