Potentiation of the mutagenicity and recombinagenicity of bleomycin in yeast by unconventional intercalating agents

Apricot kernels' extract and amygdalin alter bleomycin-induced Ty1 retrotransposition, mitotic gene conversion in the trp-5 locus and reverse point mutations in ilv1-92 allele in Saccharomyces cerevisiae

The present study aims to analyze the effect of apricot kernels' extract (AKE) and amygdalin (AMY) on bleomycin-induced genetic alternations. Five endpoints were analyzed: cell survival, Ty1 retrotransposition, mitotic gene conversion in the trp-5 locus, reverse point mutations in ilv1-92 allele, and mitotic crossing-over in the ade2 locus. The present work provides the first experimental evidence that bleomycin induces Ty1 retrotransposition in Saccharomyces cerevisiae. New data is obtained that the degree of DNA protection of AMY and AKE depends on the studied genetic event. AKE has been found to provide significant protection against bleomycin-induced Ty1 retrotransposition due to better-expressed antioxidant potential. On the other side, AMY better-expressed protection against bleomycin-induced mitotic gene conversion and reverse mutations may be attributed to the activation of the repair enzymes.

Origin, Regulation, and Fitness Effect of Chromosomal Rearrangements in the Yeast Saccharomyces cerevisiae

Chromosomal rearrangements comprise unbalanced structural variations resulting in gain or loss of DNA copy numbers, as well as balanced events including translocation and inversion that are copy number neutral, both of which contribute to phenotypic evolution in organisms. The exquisite genetic assay and gene editing tools available for the model organism Saccharomyces cerevisiae facilitate deep exploration of the mechanisms underlying chromosomal rearrangements. We discuss here the pathways and influential factors of chromosomal rearrangements in S. cerevisiae. Several methods have been developed to generate on-demand chromosomal rearrangements and map the breakpoints of rearrangement events. Finally, we highlight the contributions of chromosomal rearrangements to drive phenotypic evolution in various S. cerevisiae strains. Given the evolutionary conservation of DNA replication and recombination in organisms, the knowledge gathered in the small genome of yeast can be extended to the genomes of higher eukaryotes.

The Antibiotic Trimethoprim Displays Strong Mutagenic Synergy with 2-Aminopurine

We show that trimethoprim (TMP), an antibiotic in current use, displays a strong synergistic effect on mutagenesis in Escherichia coli when paired with the base analog 2-aminopurine (2AP), resulting in a 35-fold increase in mutation frequencies in the rpoB-Rif^r system. Combination therapies are often employed both as antibiotic treatments and in cancer chemotherapy. However, mutagenic effects of these combinations are rarely examined. An analysis of the mutational spectra of TMP, 2AP, and their combination indicates that together they trigger a response via an alteration in deoxynucleoside triphosphate (dNTP) ratios that neither compound alone can trigger. A similar, although less strong, response is seen with the frameshift mutagen ICR191 and 2AP. These results underscore the need for testing the effects on mutagenesis of combinations of antibiotics and chemotherapeutics.

Mutagenesis: Interactions with a parallel universe

Unexpected observations in mutagenesis research have led to a new perspective in this personal reflection based on years of studying mutagenesis. Many mutagens have been thought to operate via a single principal mechanism, with secondary effects usually resulting in only minor changes in the observed mutation frequencies and spectra. For example, we conceive of base analogs as resulting in direct mispairing as their main mechanism of mutagenesis. Recent studies now show that in fact even these simple mutagens can cause very large and unanticipated effects both in mutation frequencies and in the mutational spectra when used in certain pair-wise combinations. Here we characterize this leap in mutation frequencies as a transport to an alternate universe of mutagenesis.

9-AAA inhibits growth and induces apoptosis in human melanoma A375 and rat prostate adenocarcinoma AT-2 and Mat-LyLu cell lines but does not affect the growth and viability of normal fibroblasts

The present study found that, similarly to 5-fluorouracil, low concentrations (1-10 µM) of 9-aminoacridine (9-AAA) inhibited the growth of the two rat prostate cancer AT-2 and Mat-LyLu cell lines and the human melanoma A375 cell line. However, at the same concentrations, 9-AAA had no effect on the growth and apoptosis of normal human skin fibroblasts (HSFs). The differences between the cellular responses of the AT-2 and Mat-LyLu cell lines, which differ in malignancy, were found to be relatively small compared with the differences between normal HSFs and the cancer cell lines. Visible effects on the cell growth and survival of tumor cell lines were observed after 24-48 h of treatment with 9-AAA, and increased over time. The inhibition of cancer cell growth was found to be due to the gradually increasing number of cells dying by apoptosis, which was observed using two methods, direct counting and FlowSight analysis. Simultaneously, cell motile activity decreased to the same degree in cancer and normal cells within the first 8 h of incubation in the presence of 9-AAA. The results presented in the current study suggest that short-lasting tests for potential anticancer substances can be insufficient; which may result in cell type-dependent differences in the responses of cells to tested compounds that act with a delay being overlooked. The observed differences in responses between normal human fibroblasts and cancer cells to 9-AAA show the requirement for additional studies to be performed simultaneously on differently reacting cancer and normal cells, to determine the molecular mechanisms responsible for these differences.

A source of artifact in the lacZ reversion assay in Escherichia coli

The lacZ reversion assay in Escherichia coli measures point mutations that occur by specific base substitutions and frameshift mutations. The tester strains cannot use lactose as a carbon source (Lac(-)), and revertants are easily detected by growth on lactose medium (Lac(+)). Six strains identify the six possible base substitutions, and five strains measure +G, -G, -CG, +A and -A frameshifts. Strong mutagens give dose-dependent increases in numbers of revertants per plate and revertant frequencies. Testing compounds that are arguably nonmutagens or weakly mutagenic, we often noted statistically significant dose-dependent increases in revertant frequency that were not accompanied by an absolute increase in numbers of revertants. The increase in frequency was wholly ascribable to a declining number of viable cells owing to toxicity. Analysis of the conditions revealed that the frequency of spontaneous revertants is higher when there are fewer viable cells per plate. The phenomenon resembles "adaptive" or "stress" mutagenesis, whereby lactose revertants accumulate in Lac(-) bacteria under starvation conditions in the absence of catabolite repression. Adaptive mutation is observed after long incubation and might be expected to be irrelevant in a standard assay using 48-h incubation. However, we found that elevated revertant frequencies occur under typical assay conditions when the bacterial lawn is thin, and this can cause increases in revertant frequency that mimic chemical mutagenesis when treatments are toxic but not mutagenic. Responses that resemble chemical mutagenesis were observed in the absence of mutagenic treatment in strains that revert by different frameshift mutations. The magnitude of the artifact is affected by cell density, dilution, culture age, incubation time, catabolite repression and the age and composition of media. Although the specific reversion assay is effective for quickly distinguishing classes of mutations induced by potent mutagens, its utility for discerning effects of weak mutagens may be compromised by the artifact.

Antioxidant, antimutagenic, and anticarcinogenic effects of Papaver rhoeas L. extract on Saccharomyces cerevisiae

The aim of this work was to analyze the antioxidant and antimutagenic/anticarcinogenic capacity of Papaver rhoeas L. water extract against standard mutagen/carcinogen methyl methanesulfonate (MMS) and radiomimetic zeocin (Zeo) on a test system Saccharomyces cerevisiae. The following assays were used: 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, quantitative determination of superoxide anion (antireactive oxygen species [antiROS test]), DNA topology assay, D7ts1 test--for antimutagenic--and Ty1 transposition test--for anticarcinogenic effects. Strong pro-oxidative capacity of Zeo was shown to correlate with its well-expressed mutagenic and carcinogenic properties. The mutagenic and carcinogenic effects of MMS were also confirmed. Our data concerning the antioxidant activity of P. rhoeas L. extract revealed that concentration corresponding to IC(50) in the DPPH assay possessed the highest antioxidant activity in the antiROS biological assay. It was also observed that a concentration with 50% scavenging activity expressed the most pronounced antimutagenic properties decreasing Zeo-induced gene conversion twofold, reverse mutation fivefold, and total aberrations fourfold. The same concentration possessed well-expressed anticarcinogenic properties measured as reduction of MMS-induced Ty1 transposition rate fivefold and fourfold when Zeo was used as an inductor. Based on the well-expressed antioxidant, antimutagenic, and anticarcinogenic properties obtained in this work, the P. rhoeas L. extract could be recommended for further investigations and possible use as a food additive.

Prediction of noncovalent Drug/DNA interaction using computational docking models: studies with over 1350 launched drugs

Noncovalent chemical/DNA interactions, for example, intercalation and groove-binding, may be more important to genomic integrity than previously appreciated, and there may very well be genotoxic consequences of that binding. It is of importance, then, to develop methods allowing a determination or prediction of such interactions. This would have particular utility in the pharmaceutical industry where genotoxicity is, for the most part, disallowed in new drug entities. We have previously used DNA docking simulations to assess if molecules had structure and charge characteristics which could accommodate noncovalent binding via, for example, electrostatic/hydrogen bonding. We here extend those earlier studies by examining a series of over 1,350 "launched" drugs for ability to noncovalently bind 10 different DNA sequences using two computational programs: Autodock and Surflex. These drugs were also evaluated for binding to the crystallographic ATP-binding site of human topoisomerase II. The results obtained clearly demonstrate multiple series of noncovalent DNA binding structure activity relationships which would not have been predicted based on cursory structural examination. Many drugs within these series are genotoxic although not via any commonly recognized structural covalent alerts. The present studies confirm previously implicated features such as N-dialkyl groups and specific N-aryl ketones as potential genotoxic chemical moieties acting through noncovalent mechanisms. These initial studies provide considerable evidence that DNA intercalation may be an important, largely overlooked, source of drug-induced genotoxicity and further suggest involvement of topoisomerase in that genotoxicity.

Adaptive response to hydrogen peroxide in yeast: induction, time course, and relationship to dose-response models

The assay for trp5 gene conversion and ilv1-92 reversion in Saccharomyces cerevisiae strain D7 was used to characterize the induction of an adaptive response by hydrogen peroxide (H(2)O(2)). Effects of a small priming dose on the genotoxic effects of a larger challenge dose were measured in exponential cultures and in early stationary phase. An adaptive response, indicated by smaller convertant and revertant frequencies after the priming dose, occurred at lower priming and challenge doses in young, well-aerated cultures. Closely spaced priming doses from 0.000975 to 2 mM, followed by a 1 mM challenge, showed that the induction of the adaptive response is biphasic. In exponential cultures it was maximal with a priming dose of 0.125-0.25 mM. Very small priming doses were insufficient to induce the adaptive response, whereas higher doses contributed to damage. A significant adaptive response was detected when the challenge dose was administered 10-20 min after the priming exposure. It was fully expressed within 45 min, and the yeast began to return to the nonadapted state after 4-6 hr. Because of the similarity of the biphasic induction to hormetic curves and the proposal that adaptive responses are a manifestation of hormesis, we evaluated whether the low doses of H(2)O(2) that induce the adaptive response show a clear hormetic response without a subsequent challenge dose. Hormesis was not evident, but there was an apparent threshold for genotoxicity at or slightly below 0.125 mM. The results are discussed with respect to linear, threshold, and hormesis dose-response models.

Phenolphthalein induces centrosome amplification and tubulin depolymerization in vitro

Aneuploidy is a major cause of human reproductive failure and plays a large role in cancer. Phenolphthalein (PHT) induces tumors in rodents but its primary mechanism does not seem to be DNA damage. In heterozygous TSG-p53(®) mice, PHT induces lymphomas and also micronuclei (MN), many containing kinetochores (K), implying chromosome loss (aneuploidy). The induction of aneuploidy would be compatible with the loss of the normal p53 gene seen in the lymphomas. In this study, we confirm PHT's aneugenicity and determine the aneugenic mechanism of PHT by combining traditional genetic toxicology assays with image and flow cytometry methods. The data revealed that PHT induces tubulin polymerization abnormalities and deregulates the centrosome duplication cycle causing centrosome amplification. We also show that one of the consequences of these events is apoptosis.

Saccharomyces cerevisiae as a model system to study the response to anticancer agents

The development of new strategies for cancer therapeutics is indispensable for the improvement of standard protocols and the creation of other possibilities in cancer treatment. Yeast models have been employed to study numerous molecular aspects directly related to cancer development, as well as to determine the genetic contexts associated with anticancer drug sensitivity or resistance. The budding yeast Saccharomyces cerevisiae presents conserved cellular processes with high homology to humans, and it is a rapid, inexpensive and efficient compound screening tool. However, yeast models are still underused in cancer research and for screening of antineoplastic agents. Here, the employment of S. cerevisiae as a model system to anticancer research is discussed and exemplified. Focusing on the important determinants in genomic maintenance and cancer development, including DNA repair, cell cycle control and epigenetics, this review proposes the use of mutant yeast panels to mimic cancer phenotypes, screen and study tumor features and synthetic lethal interactions. Finally, the benefits and limitations of the yeast model are highlighted, as well as the strategies to overcome S. cerevisiae model limitations.