Induction of local denaturation in DNA in vitro by phleomycin and caffeine

Interaction of bleomycin with a bent DNA fragment

The interaction of bleomycin with a kinetoplast DNA fragment has been examined using various footprinting techniques. This DNA adopts a bent structure and displays an unusually low gel mobility on account of its phased runs of adenines. The bleomycin-cobalt complex increases the mobility of this DNA fragment, in contrast with other DNAs which show a decreased rate of gel migration, suggesting that the antibiotic removes DNA bending, possibly via an unwinding mechanism. Removal of the bending is confirmed by hydroxy-radical footprinting which produces a more even ladder of bands in the presence of the ligand. Cleavage by bleomycin is at the sequence G-pyrimidine, though not all such sites are affected to the same extent and some cutting is found at GA and GG. DNase I footprinting confirms the antibiotic-binding sites but reveals that some strong cleavage sites do not yield footprints. Bleomycin renders adenines on the 3' side of its cleavage sites (GT, GC and GA) hyper-reactive to diethyl pyrocarbonate.

Recombinagenicity of caffeine for Candida albicans

Caffeine at concentrations of 0.5 x 10(-2) M or higher inhibited cell replication and induced gene segregations in Candida albicans cultured on defined complete medium. Both responses increased incrementally with increasing caffeine concentrations, and were more severe during incubation at 37 degrees C than 25 degrees C; at 37 degrees C, caffeine levels above 1.5 x 10(-2) M caused cellular inactivation. Caffeine effects occurred only under conditions permitting cell growth, and their magnitudes were greater for unbudded than budding cells, were influenced by cellular genetic backgrounds, and were unaffected by the presence of adenine in the medium. Evaluations of segregations for recessive auxotrophic markers of a four member linkage group carried heterozygously in a cis arrangement in treated cells established that induced segregants arise through either reciprocal or nonreciprocal recombinations. The frequency distributions of classes of reciprocal and nonreciprocal recombinants for these markers conformed with those previously obtained following induction by ultraviolet radiation, indicating that the probabilities of recombinational events within the chromosomal regions defined by the markers are not biased by the differences in kinds of initial DNA lesions caused by the two recombinagens. A panel of four protoplast fusion hybrids considered deficient for DNA repair because of enhanced susceptibilities to UV induced cellular inactivation and mitotic recombination exhibited corresponding increased sensitivities to caffeine, signifying that DNA damage induced by caffeine is subject to repair. Caffeine did not affect behavior of a variant strain exhibiting high frequency phenotypic switching between minute smooth and large rough colonial forms, and no evidence for mutagenicity of the drug was obtained with systems for detection of forward or reverse mutations. The mechanism of caffeine's recombinagenicity, and the implications of that property for genetic studies of C. albicans are discussed.

Studies on DNA binding of caffeine and derivatives: evidence of intercalation by DNA-unwinding experiments

Caffeine and derivatives are compounds with pleiotropic effects on the genetic material which are supposed to originate from drugs binding to DNA. Here we show, by using two different topological methods, that methylated oxypurines, at biologically relevant concentrations, unwind DNA in a fashion similar to known intercalators. Methylated oxypurines could be ranked by decreasing unwinding potency: 8-methoxycaffeine greater than 8-ethoxycaffeine greater than 8-chlorocaffeine greater than caffeine greater than theophylline. These findings confirm, with a different assay, interaction of caffeine with DNA and add additional support to an intercalative mode of binding of these drugs to DNA.

Diethylpyrocarbonate and permanganate provide evidence for an unusual DNA conformation induced by binding of the antitumour antibiotics bleomycin and phleomycin

DNA structural changes induced by bleomycin have been investigated using diethylpyrocarbonate and permanganate as probes under conditions in which the antibiotic binds to, but does not cut the DNA. Diethyl-pyrocarbonate shows an enhanced reaction with adenines in the presence of the antibiotic in the sequences GTA greater than GCA greater than GAA, on the 3' side of the drug cutting site (GPy). Permanganate ions display an enhanced reactivity at the second pyrimidine of the sequence GPyPy. The results are consistent with a model in which bleomycin distorts the structure of the base pair on the 3' side of its binding site.

Sequence-selective binding of phleomycin to DNA

The binding of phleomycin and bleomycin to DNA has been investigated by studying their effects on cleavage by DNAase I and micrococcal nuclease. In the presence of cobalt, cleavage of DNA by the antibiotics is suppressed, yet they still provide protection from nuclease attack in regions surrounding the drug cleavage sites. We conclude that cleavage by phleomycin occurs at bonds around which the antibiotic is already selectively bound.

Chromosomal aberrations induced by caffeine in somatic ganglia of Drosophila melanogaster

Nerve ganglia of third-instar larvae were treated with various doses of caffeine (5 X 10(-4), 10(-3), 5 X 10(-3), 10(-2) and 2 X 10(-2) M) for 2 h at 25 +/- 1 degrees C. The ganglia were fixed at set time intervals after treatment so that the effect of caffeine in different stages of the cell cycle could be observed. Chromatid aberrations were induced only when the caffeine was administered in G2 or approaching mitosis. No aberrations were observed after treatment in S or early G2. In relation to the different doses administered, a threshold effect was evidenced, the number of aberrations increasing in a marked way at doses exceeding 5 X 10(-3) M. These data indicate that the effect observed in Drosophila melanogaster is similar to that described by Kihlman in animals and plants treated with caffeine at temperatures below 30 degrees C. Results obtained in non-cytological tests (non-disjunction, chromosome loss, lethal recessives, dominant lethals) have so far given incomplete indications as to the mutagenicity of caffeine in Drosophila. The results we have obtained with the cytological test seem to contribute to a better definition of the mutagenicity.

Interaction of caffeine with the DNA of Chinese hamster cells

Incubation of Chinese hamster cells with labelled caffeine leads to transfer of radioactivity to DNA. This association occurs during the S phase of the cell cycle and involves parental as well as newly synthesised strands. The replacement of thymidine by BrdUrd prevents the incorporation radioactivity from caffeine into the DNA strands containing BrdUrd. Thymine is the only base which becomes labelled and data suggesting the participation of methyl groups of caffeine in the biosynthesis of thymine are presented. Ultraviolet irradiation increases the incorporation of radioactivity from caffeine to DNA.

Caffeine

Most of the population of the world is exposed to caffeine to a greater or lesser extent since it occurs in a number of plants used in the preparation of widely consumed drinks, and has in addition a limited therapeutic use. Chromosomal abnormalities are induced by caffeine in both plant cells and in mammalian cells in culture and it also has some anti-mitotic activity. DNA-repair processes sensitive to caffeine have been demonstrated in a number of cell systems and it has been shown to affect a wide range of other cellular processes. Caffeine has potent mutagenic effects in Escherichia coli and other micro-organisms both when acting alone and in combination with other mutagens. However its mutagenic activity in Drosophila has been disputed and the available evidence suggests that it is neither mutagenic in mammals nor synergistic with other mutagens although at very high doses it appears to have some teratogenic activity in mammals.

The mechanism of DNA breakage by phleomycin in vitro

Phleomycin induces DNA breakage in vitro in the presence of the sulphydryl compound dithiothreitol. The reaction appears to be free radical-mediated, and requires oxygen and metal ions. Reaction rate is limited by the concentration of oxygen, which is converted to hydrogen peroxide during DNA breakage. However there is no net change in the sulphydryl compound. The proposed reaction mechanism involves metal ion/oxygen-catalysed oxidation of dithiothreitol to its free radical form, which reacts with phleomycin, leading to formation of activated phleomycin and regeneration of free sulphydryl. Free phleomycin is converted to an inactive form, but activation of phleomycin bound to DNA leads to DNA breakage.

Caffeine enhancement of digestion of DNA by nuclease S1

The activity of Aspergillus orzae nuclease S1 on DNA has been investigated under varying pH and metal ion conditions. Nuclease S1 was found to preferentially digest denatured DNA. With native DNA as substrate the enzyme could only digest the DNA when caffeine was added to the reaction mixture. The enzyme was more active in sodium acetate buffer (pH 4.5), than in either standard saline citrate (PH 7.0) or sodium phosphate buffer (pH 6.8). Caffeine was also found to affect the thermal stability of DNA, resulting in a melting profile characterized by two transitions. The first transition (poorly defined) was below the normal melting temperature of the DNA, while the next transition was at the normal melting temperature of the DNA, while the next transition was at the normal melting temperature of the DNA. The susceptibility of caffeine-treated DNA to nuclease digestion seems to be a result of the local unwinding that caffeine causes in the regions of DNA that melt in the first transition. This selective destabilization presumably sensitizes the unwound regions to nuclease hydrolysis. The hydrolysates of the DNA digested by nuclease S1 were subjected first to ion exchange chromatography followed by paper chromatography. The results from this partial characterization of the digestion products showed that they contain mononucleotides as well as oligonucleotides of varying lengths. The base composition of the mononucleotide digests suggests that caffeine has greater preference for interacting with A-T base-pairs in DNA.