Degradation of structurally modified DNAs by bleomycin group antibiotics

Genome Instability and Bleomycin Sensitivity Test

Procjena individualne osjetljivosti na mutagene često je dio istraživanja u epidemiološkim studijama koje prate pojavnost zloćudnih bolesti u populacijama. Posljedica djelovanja mutagena u genomu izloženih osoba jest nastanak osoba jest nastanak određene, manje ili veće, količine oštećenja, uvjetovane individualnim razlikama u osjetljivosti. Viša razina takve genomske nestabilnosti znači opasnost (rizik) od razvoja zloćudnih bolesti. Interindividualne razlike u odgovoru na mutagene obično se povezuju i s promijenjenom (većinom smanjenom) sposobnosti (kapacitetom) za popravak DNA. Citogenetičke studije su pokazale da je genom tumorskih stanica nestabilniji od normalnih, a time i skloniji akumuliranju oštećenja, bilo da je nestabilnost uzrokovana nasljeđem, izloženošću ili kombinacijom tih dvaju učinaka. U oboljelih ispitanika utvrđena je povećana učestalost kromatidnih i kromosomskih aberacija naspram normalne populacije te sklonost razvoju određenih vrsta neoplazija. U praćenju povezanosti promijenjenog odgovora i pojavnosti tumora služe nam različiti biomarkeri. Kao indirektni pokazatelji uspješnosti popravka DNA često se rabe testovi osjetljivosti na mutagene u kulturama limfocita periferne krvi. Jedan od takvih testova je i bleomicinski test. Radiomimetik i citostatik, a po strukturi glikopeptid, bleomicin se u stanici prevodi u aktivni oblik sposoban cijepati molekulu DNA što uzrokuje brojne jednolančane i dvolančane lomove. Kao jednostavna i jeftina metoda, zasniva se na utvrđivanju ukupnog broja jednolančanih lomova u kromosomima limfocita uzgajanih u staničnoj kulturi koji su u uvjetima in vitro tijekom kasne G2-faze staničnog ciklusa bili izloženi bleomicinu. Ovaj revijalni rad daje pregled utjecaja raznih faktora na rezultate samog testa i pokazuje njegovu široku primjenu u proučavanju genomske nestabilnosti koju najčešće uzrokuje kombinacija raznih faktora.

Noncovalent interactions with DNA: an overview

Over the last four decades, intense research has focused on the effects of small organic compounds that noncovalently bind to nucleic acids. These interactions have been shown to disrupt replication and/or transcription culminating in cellular death. Accordingly, DNA binding compounds have potential applications as anti-cancer and anti-viral agents. This report provides an overview of the different DNA-binding modes with an emphasis on DNA groove specificity for the groove-binding and intercalation modes. While most DNA-interacting agents selectively bind to DNA by either groove binding or intercalation, some compounds can exhibit both binding modes. The binding mode with the most favorable free energy for complex formation depends on the DNA sequence and structural features of the bound ligand.

Protective mechanisms against the antitumor agent bleomycin: lessons from Saccharomyces cerevisiae

Bleomycin is a small glycopeptide antibiotic used in combination therapy for the treatment of a few types of human cancer. The antitumor effect of bleomycin is most likely caused by its ability to bind to DNA and induce the formation of toxic DNA lesions via a free radical reactive (Fe.bleomycin) complex. However, the chemotherapeutic potential of bleomycin is limited, as it causes pulmonary fibrosis and tumor resistance at high doses. The chemical structure and modes of action of bleomycin have been extensively studied and these provide a foundation towards improving the therapeutic value of the drug. This review provides a first account of the current state of knowledge of the cellular processes that can allow the yeast Saccharomyces cerevisiae to evade the lethal effects of bleomycin. This model organism is likely to provide rapid clues in our understanding of bleomycin resistance in tumor cells.

Raman spectroscopy of an O(2)-Co(II)bleomycin-calf thymus DNA adduct: alternate polymer conformations

Bleomycin (Blm) is an antitumor agent which binds to specific sequences of DNA and as HO(2)-Fe(III)Blm causes single and double strand cleavage. In the present investigation, binding of O(2)-Co(II)Blm to a native DNA polymer, calf thymus DNA, was examined using conventional Raman spectroscopy. O(2)-Co(II)Blm is a model for O(2)-Fe(II)Blm, the direct precursor of HO(2)-Fe(III)Blm. Although the DNA polymer retained a predominant B-form structure, Raman spectral evidence was obtained for localized structural changes to A, C and Z-DNA forms. The presence of these alternate DNA forms within B-DNA implied the presence of B/A, B/C and B/Z junctions. The observed changes in DNA secondary structure were attributed to perturbation of structural water resulting from binding of O(2)-Co(II)Blm within the minor groove.

Iron(II) bleomycin-mediated degradation of a DNA-RNA heteroduplex

The effect of iron(II) bleomycin on a DNA-RNA heteroduplex was investigated using a substrate formed by reverse transcription of Escherichia coli 5S ribosomal RNA. Both strands of the heteroduplex were cleaved by FeII.BLM A2 at comparable concentrations; complete digestion of both strands was observed using 5 microM FeII.BLM A2. The DNA strand of the heteroduplex was cleaved predominantly at 5'-G-pyr-3' sites; the sites of cleavage of the DNA strand were a subset of those observed for the corresponding DNA strand of a DNA duplex of identical sequence. The sites of cleavage of the RNA strand of the heteroduplex involved both purines and pyrimidines and were found to be different than the sites of cleavage of the 5S rRNA alone, demonstrating that cleavage of the former must actually have involved heteroduplex recognition by FeII.BLM A2. Both the DNA and RNA strands of the heteroduplex were cleaved by FeII.BLM A2 in the presence of physiological concentrations of Mg2+, consistent with the possibility that DNA-RNA heteroduplexes may be therapeutically relevant targets for bleomycin.

Chromosomal sensitivity of human lymphocytes to bleomycin. Influence of antioxidant enzyme activities in whole blood and different blood fractions

The activity of the antioxidant enzymes catalase (CAT), peroxidases (POD), and superoxide dismutases (SOD) in whole blood and different blood fractions was analyzed in 20 normal human beings and correlated with the chromosomal sensitivity of lymphocytes to bleomycin (BLM) (measured as frequency of dicentric chromosomes per BLM dose). Our results demonstrate that both the physiologic activities of the enzymes and the chromosomal sensitivity to BLM exhibit an ample and significant interindividual variability. An inverse and linear correlation between chromosomal sensitivity to BLM and the concentration of 1) CAT and POD in plasma and 2) SOD in whole blood, erythrocytes, and plasma was found. On the other hand, the chromosomal sensitivity to BLM showed a direct correlation with the concentration of SOD and POD in mononuclear leukocytes. It is suggested that a determination of antioxidant enzyme (AOE) activities in a given cell population may serve to predict the chromosomal sensitivity to BLM.

Decreased protein kinase C activity is associated with programmed cell death (apoptosis) in freshly isolated rat hepatocytes

Apoptosis of freshly isolated rat hepatocytes was induced by either the omission of fetal bovine serum in the culture medium or addition of the protein kinase C inhibitors polymyxin B or staurosporine. The time-course of DNA breakdown into oligonucleosome-sized fragments and the activity of protein kinase C was determined. Hepatocytes were found to be sensitive to bleomycin which induced a high degree of DNA breakdown even within 30 min incubation. Both staurosporine and polymyxin B induced DNA degradation in hepatocytes after three hours incubation, an effect that was partially prevented by phorbol myristate acetate (PMA). After eight hours incubation, PMA failed to counteract this action and itself produced the apoptosis of rat hepatocytes. The results suggest the involvement of protein kinase C in hepatocyte survival.

Site-specific cleavage of RNA by Fe(II).bleomycin

Bleomycin is an antitumor agent whose activity has long been thought to derive from its ability to degrade DNA. Recent findings suggest that cellular RNA may be a therapeutically relevant locus. At micromolar concentrations, Fe(II)-bleomycin readily cleaved a Bacillus subtilis tRNAHis precursor in a highly selective fashion, but Escherichia coli tRNA(Tyr) precursor was largely unaffected even under more forcing conditions. Other substrates included an RNA transcript encoding a large segment of the reverse transcriptase from human immunodeficiency virus 1. RNA cleavage was oxidative, approximately 10-fold more selective than DNA cleavage, and largely unaffected by nonsubstrate RNAs. RNA sequence analysis suggested recognition of RNA tertiary structure, rather than recognition of specific sequences; subsets of nucleotides at the junction of single- and double-stranded regions were especially susceptible to cleavage. The ready accessibility of cellular RNAs to xenobiotic agents, the high selectivity of bleomycin action on RNAs, and the paucity of mechanisms for RNA repair suggest that RNA may be a therapeutically relevant target for bleomycin.

Molecular basis for potentiation of bleomycin-mediated degradation of DNA by polyamines. Experimental and molecular mechanical studies

The bleomycin-mediated degradation of DNA is stimulated (amplified) by certain DNA binding compounds, such as polyamines, that distort the double helix. Computer modelling studies suggest that putrescine (1), spermidine (2), and spermine (3) bind preferentially on the floor of the major groove of (dGdC)5.(dGdC)5. This interaction results in a bend of the oligomer helix toward the major groove and enlargement of the minor groove, both effects being in the order 1 less than 2 less than 3. These polyamine-induced distortions, as obtained from theoretical studies, parallel the experimental values of the amplification activities of 1-3 in the bleomycin-mediated degradation of poly(dGdC).poly(dGdC). The amplification mechanism of non-competitive binding of amplifier molecules in the major groove, and bleomycin in the minor groove, is proposed. It is suggested that the amplifier-induced conformational changes of the DNA helix increase affinity of the activated bleomycin complex toward the DNA minor groove and, consequently, result in an increased efficiency of the bleomycin-mediated degradation of the helix.

On the mechanism of topoisomerase I inhibition by camptothecin: evidence for binding to an enzyme-DNA complex

Camptothecin, a cytotoxic antitumor compound, has been shown to produce protein-linked DNA breaks mediated by mammalian topoisomerase I. We have investigated the mechanism by which camptothecin disrupts DNA processing by topoisomerase I and have examined the effect of certain structurally related compounds on the formation of a DNA-topoisomerase I covalent complex. Enzyme-mediated cleavage of supercoiled plasmid DNA in the presence of camptothecin was completely reversed upon the addition of exogenous linear DNA or upon dilution of the reaction mixture. Camptothecin and topoisomerase I produced the same amount of cleavage from supercoiled DNA or relaxed DNA. In addition, the alkaloid decreased the initial velocity of supercoiled DNA relaxation mediated by catalytic quantities of topoisomerase I. Inhibition occurred under conditions favoring processive catalysis as well as under conditions favoring distributive catalysis. By use of [3H]camptothecin and an equilibrium dialysis assay, the alkaloid was shown to bind reversibly to a DNA-topoisomerase I complex, but not to isolated enzyme or isolated DNA. These results are consistent with a model in which camptothecin reversibly traps an intermediate involved in DNA unwinding by topoisomerase I and thereby perturbs a set of equilibria, resulting in increased DNA cleavage. By examining certain compounds that are structurally related to camptothecin, it was found that the 20-hydroxy group, which has been shown to be essential for antitumor activity, was also necessary for stabilization of the covalent complex between DNA and topoisomerase I. In contrast, no such correlation existed for UV-light-induced cleavage of DNA by Cu(II)-camptothecin derivatives.