Biological effects of essential oils--a review

Since the middle ages, essential oils have been widely used for bactericidal, virucidal, fungicidal, antiparasitical, insecticidal, medicinal and cosmetic applications, especially nowadays in pharmaceutical, sanitary, cosmetic, agricultural and food industries. Because of the mode of extraction, mostly by distillation from aromatic plants, they contain a variety of volatile molecules such as terpenes and terpenoids, phenol-derived aromatic components and aliphatic components. In vitro physicochemical assays characterise most of them as antioxidants. However, recent work shows that in eukaryotic cells, essential oils can act as prooxidants affecting inner cell membranes and organelles such as mitochondria. Depending on type and concentration, they exhibit cytotoxic effects on living cells but are usually non-genotoxic. In some cases, changes in intracellular redox potential and mitochondrial dysfunction induced by essential oils can be associated with their capacity to exert antigenotoxic effects. These findings suggest that, at least in part, the encountered beneficial effects of essential oils are due to prooxidant effects on the cellular level.

Cadmium: cellular effects, modifications of biomolecules, modulation of DNA repair and genotoxic consequences (a review)

Cadmium is an important toxic environmental heavy metal. Occupational and environmental pollution with cadmium results mainly from mining, metallurgy industry and manufactures of nickel-cadmium batteries, pigments and plastic stabilizers. Important sources of human intoxication are cigarette smoke as well as food, water and air contaminations. In humans, cadmium exposures have been associated with cancers of the prostate, lungs and testes. Acute exposures are responsible for damage to these organs. Chronic intoxication is associated with obstructive airway disease, emphysema, irreversible renal failure, bone disorders and immuno-suppression. At the cellular level, cadmium affects proliferation, differentiation and causes apoptosis. It has been classified as a carcinogen by the International Agency for Research on Cancer (IARC). However, it is weakly genotoxic. Indirect effects of cadmium provoke generation of reactive oxygen species (ROS) and DNA damage. Cadmium modulates also gene expression and signal transduction, reduces activities of proteins involved in antioxidant defenses. Several studies have shown that it interferes with DNA repair. The present review focuses on the effects of cadmium in mammalian cells with special emphasis on the induction of damage to DNA, membranes and proteins, the inhibition of different types of DNA repair and the induction of apoptosis. Current data and hypotheses on the mechanisms involved in cadmium genotoxicity and carcinogenesis are outlined.

Reassembly of shattered chromosomes in Deinococcus radiodurans

Dehydration or desiccation is one of the most frequent and severe challenges to living cells. The bacterium Deinococcus radiodurans is the best known extremophile among the few organisms that can survive extremely high exposures to desiccation and ionizing radiation, which shatter its genome into hundreds of short DNA fragments. Remarkably, these fragments are readily reassembled into a functional 3.28-megabase genome. Here we describe the relevant two-stage DNA repair process, which involves a previously unknown molecular mechanism for fragment reassembly called 'extended synthesis-dependent strand annealing' (ESDSA), followed and completed by crossovers. At least two genome copies and random DNA breakage are requirements for effective ESDSA. In ESDSA, chromosomal fragments with overlapping homologies are used both as primers and as templates for massive synthesis of complementary single strands, as occurs in a single-round multiplex polymerase chain reaction. This synthesis depends on DNA polymerase I and incorporates more nucleotides than does normal replication in intact cells. Newly synthesized complementary single-stranded extensions become 'sticky ends' that anneal with high precision, joining together contiguous DNA fragments into long, linear, double-stranded intermediates. These intermediates require RecA-dependent crossovers to mature into circular chromosomes that comprise double-stranded patchworks of numerous DNA blocks synthesized before radiation, connected by DNA blocks synthesized after radiation.

Characterization of homologous recombination induced by replication inhibition in mammalian cells

To analyze relationships between replication and homologous recombination in mammalian cells, we used replication inhibitors to treat mouse and hamster cell lines containing tandem repeat recombination substrates. In the first step, few double-strand breaks (DSBs) are produced, recombination is slightly increased, but cell lines defective in non-homologous end-joining (NHEJ) affected in ku86 (xrs6) or xrcc4 (XR-1) genes show enhanced sensitivity to replication inhibitors. In the second step, replication inhibition leads to coordinated kinetics of DSB accumulation, Rad51 foci formation and RAD51-dependent gene conversion stimulation. In xrs6 as well as XR-1 cell lines, Rad51 foci accumulate more rapidly compared with their respective controls. We propose that replication inhibition produces DSBs, which are first processed by the NHEJ; then, following DSB accumulation, RAD51 recombination can act.

Photochemotherapy (PUVA) of psoriasis using 3-carbethoxypsoralen, a non-carcinogenic compound in mice

The carcinogenic risk of photochemotherapy (PUVA) with bi-functional furocoumarins such as 8-methoxypsoralen (8-MOP) which form cross-links in cellular DNA has initiated a search for active but less hazardous psoralens. A new compound, 3-carbethoxypsoralen (3-CPs), studied in the yeast Saccharomyces cerevisiae (eukaryote), has been shown to be very photoactive on DNA and to form only mono-additions to DNA. These lesions appear to be more easily repaired than the cross-links induced by 8-MOP. 3-CPs produces less nuclear genetic events such as nuclear mutations and mitotic crossovers, but more cytoplasmic 'petite' mutations (damage to mitochondrial DNA) than 8-MOP. In mice it was demonstrated that after local or intra-peritoneal administration, in contrast to 8-MOP, 3-CPs is non-toxic, non-erythematogenic, and non-carcinogenic. A study of ten psoriatic patients had shown that local applications of 3-CPs plus UV-A exhibit about the same therapeutic activity for the clearing of psoriatic lesions as local treatment with 8-MOP plus UV-A, but without any localized hyperpigmentation.

Role of Mitochondria in Radiation Responses: Epigenetic, Metabolic, and Signaling Impacts

Until recently, radiation effects have been considered to be mainly due to nuclear DNA damage and their management by repair mechanisms. However, molecular biology studies reveal that the outcomes of exposures to ionizing radiation (IR) highly depend on activation and regulation through other molecular components of organelles that determine cell survival and proliferation capacities. As typical epigenetic-regulated organelles and central power stations of cells, mitochondria play an important pivotal role in those responses. They direct cellular metabolism, energy supply and homeostasis as well as radiation-induced signaling, cell death, and immunological responses. This review is focused on how energy, dose and quality of IR affect mitochondria-dependent epigenetic and functional control at the cellular and tissue level. Low-dose radiation effects on mitochondria appear to be associated with epigenetic and non-targeted effects involved in genomic instability and adaptive responses, whereas high-dose radiation effects (>1 Gy) concern therapeutic effects of radiation and long-term outcomes involving mitochondria-mediated innate and adaptive immune responses. Both effects depend on radiation quality. For example, the increased efficacy of high linear energy transfer particle radiotherapy, e.g., C-ion radiotherapy, relies on the reduction of anastasis, enhanced mitochondria-mediated apoptosis and immunogenic (antitumor) responses.

Biological effects and repair of damage photoinduced by a derivative of psoralen substituted at the 3,4 reaction site: photoreactivity of this compound and lethal effect in yeast

A newly synthesized linear psoralen derivative, 3-carbethoxypsoralen is shown to bind to yeast nucleic acids after 365 nm light treatment. As compared to 8-methoxypsoralen, a well-known bifunctional furocoumarin, 3-carbethoxypsoralen exhibits a high photoaffinity for DNA in vivo. Both compounds bind and photoreact more efficiently in vivo than in vitro. In contrast to 8-methoxypsoralen, 3-carbethoxypsoralen does not form cross-links in yeast DNA as demonstrated by heat denaturation-reassociation studies at least in the range of doses used. Thus 3-carbethoxypsoralen reacts as a monofunctional compound. Wild-type cells of Saccharomyces cerevisiae are 6 times more resistant to 3-carbethoxypsoralen than to 8-methoxypsoralen plus 365 nm light treatment in terms of lethal effect. In comparison to angelicin, another monofunctional (but angular) furocoumarin, 3-carbethoxypsoralen is more photoreactive. When the photoaffinity for DNA of 8-methoxypsoralen and 3-carbethoxypsoralen are considered in relation to photoinduced cell killing, it is clear that monoadducts are very efficiently repaired in wild-type cells. In contrast to the additivity obtained with 8-methoxypsoralen, a synergistic interaction of the two different repair pathways blocked by the rad2 and the rad9 mutation is observed after 3-carbethoxypsoralen plus 365 nm light. Dark holding experiments show that the excision repair function which is present in wild-type and rad9-4 cells is important for dark recovery.

Recent reports on the effect of low doses of ionizing radiation and its dose-effect relationship

Recently, the risk associated with low doses of ionizing radiation has gained new interest. Here, we analyze and discuss the major differences between two reports recently published on this issue; the report of the French Academy of Sciences and of the French Academy of Medicine published in March 2005, and the BEIR VII-Phase 2 Report of the American National Academy of Sciences published as a preliminary version in July 2005. The conclusion of the French Report is that the linear no-threshold relationship (LNT) may greatly overestimate the carcinogenic effect of low doses (<100 mSv) and even more that of very low doses (<10 mSv), such as those delivered during X-ray examinations. Conversely, the conclusion of the BEIR VII report is that LNT should be used for assessing the detrimental effects of these low and very low doses. The causes of these diverging conclusions should be carefully examined. They seem to be mostly associated with the interpretation of recent biological data. The point of view of the French Report is that these recent data are incompatible with the postulate on which LNT is implicitly based, namely the constancy of the carcinogenic effect per unit dose, irrespective of dose and dose rate.

Relationship between lesions photoinduced by mono- and bi-functional furocoumarins in DNA and genotoxic effects in diploid yeast

The induction of genetic effects was studied in a diploid strain of Saccharomyces cerevisiae (D7) after treatments with the monofunctional furocoumarins 7-methylpyrido[3,4-c]psoralen (MePyPs), pyrido[3,4-c]psoralen (PyPs) and 3-carbethoxypsoralen (3-CPs) and the bifunctional furocoumarins 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP) in the presence of 365-nm radiation. The DNA photobinding of radioactively labelled MePyPs, 3-CPs, 5-MOP and 8-MOP was determined in parallel. The DNA-photobinding capacity was highest for MePyPs followed in decreasing order by 5-MOP, 3-CPs and 8-MOP. At a concentration of 5 microM and 4.2 kJ/m2 of 365-nm radiation approximately 160, 66, 60 and 16 adducts per 10(6) base pairs were formed by MePyPs, 5-MOP, 3-CPs and 8-MOP, respectively. The activity of MePyPs and PyPs for the induction of lethal effects lay in the same range as that of 5-MOP whereas 8-MOP was 3 times less active and 3-CPs showed very little activity. For the induction of mitotic gene conversion and genetically altered colonies including mitotic crossing-over the order of activity was about the same as that observed for the induction of lethal effects: MePyPs greater than 5-MOP greater than PyPs greater than 8-MOP much greater than 3-CPs. Nuclear reversions were induced most effectively by 5-MOP, 8-MOP being about 3 times less effective. Up to 4 and 6 kJ/m2 of 365-nm radiation, MePyPs and PyPs, respectively, were less mutagenic than 8-MOP but became more mutagenic at higher doses. At equal survival, the pyridopsoralens were, however, clearly less mutagenic than the bifunctional furocoumarins 8-MOP and 5-MOP. By plotting the genetic data versus the number of lesions induced in DNA, it was shown that the monoadducts induced by the monofunctional furocoumarins MePyPs and 3-CPs exert a relatively low potential for the induction of lethal and nuclear genetic events as compared to photoadditions induced by the bifunctional furocoumarins 8-MOP and 5-MOP. However, at a very high density, the monoadducts induced by MePyPs became as lethal and as mutagenic as the mixture of mono- and biadducts induced by 8-MOP and 5-MOP probably due to overloading of cellular repair capacities.(ABSTRACT TRUNCATED AT 400 WORDS)

Does scientific evidence support a change from the LNT model for low-dose radiation risk extrapolation?

The linear no-threshold (LNT) model has been widely used to establish international rules and standards in radiation protection. It is based on the notion that the physical energy deposition of ionizing radiation (IR) increases carcinogenic risk linearly with increasing dose (i.e., the carcinogenic effectiveness remains constant irrespective of dose) and, within a factor of two, also with dose-rate. However, recent findings have strongly put into question the LNT concept and its scientific validity, especially for very low doses and dose-rates. Low-dose effects are more difficult to ascertain than high-dose effects. Epidemiological studies usually lack sufficient statistical power to determine health risks from very low-dose exposures. In this situation, studies of the fundamental mechanisms involved help to understand and assess short- and long-term effects of low-dose IR and to evaluate low-dose radiation risks. Several lines of evidence demonstrate that low-dose and low dose-rate effects are generally lower than expected from high-dose exposures. DNA damage signaling, cell cycle checkpoint activation, DNA repair, gene and protein expression, apoptosis, and cell transformation differ qualitatively and quantitatively at high- and low-dose IR exposures, and most animal and epidemiological data support this conclusion. Thus, LNT appears to be scientifically invalid in the low-dose range.

The debate on the use of linear no threshold for assessing the effects of low doses

From December 2004 to July 2005, three reports on the effects of low doses of ionising radiation were released: ICRP (2004), the joint report of the French Academies of Science and Medicine (Tubiana et al 2005), and a report from the American Academy of Sciences (BEIR VII 2005). These reports quote the same recent articles on the biological effects of low doses, yet their conclusions diverge. The French report concludes that recent biological data show that the efficacy of defense mechanisms is modulated by dose and dose rate and that linear no threshold (LNT) is no longer plausible. The ICRP and the BEIR VII reports recognise that there are biologic arguments against LNT but feel that there are not sufficient biological proofs against it to change risk assessment methodology and subsequent regulatory policy based on LNT. They point out the remaining uncertainties and the lack of mechanistic explanations of phenomena such as low dose hyperlethality or the adaptive response. In this context, a critical analysis of the available data is necessary. The epidemiological data and the experimental data challenge the validity of the LNT hypothesis for assessing the carcinogenic effect of low doses, but do not allow its exclusion. Therefore, the main criteria for selecting the most reliable dose-effect relationship from a scientific point of view should be based on biological data. Their analysis should help one to understand the current controversy.

Interactions between magnetic nanowires and living cells: uptake, toxicity, and degradation

We report on the uptake, toxicity, and degradation of magnetic nanowires by NIH/3T3 mouse fibroblasts. Magnetic nanowires of diameters 200 nm and lengths between 1 and 40 μm are fabricated by controlled assembly of iron oxide (γ-Fe(2)O(3)) nanoparticles. Using optical and electron microscopy, we show that after 24 h incubation the wires are internalized by the cells and located either in membrane-bound compartments or dispersed in the cytosol. Using fluorescence microscopy, the membrane-bound compartments were identified as late endosomal/lysosomal endosomes labeled with lysosomal associated membrane protein (Lamp1). Toxicity assays evaluating the mitochondrial activity, cell proliferation, and production of reactive oxygen species show that the wires do not display acute short-term (<100 h) toxicity toward the cells. Interestingly, the cells are able to degrade the wires and to transform them into smaller aggregates, even in short time periods (days). This degradation is likely to occur as a consequence of the internal structure of the wires, which is that of a noncovalently bound aggregate. We anticipate that this degradation should prevent long-term asbestos-like toxicity effects related to high aspect ratio morphologies and that these wires represent a promising class of nanomaterials for cell manipulation and microrheology.

DNA cross-link-dependent RAD50/MRE11/NBS1 subnuclear assembly requires the Fanconi anemia C protein

Fanconi anemia (FA) is a cancer-predisposition syndrome characterized by hypersensitivity to interstrand-cross-link (ICL) inducers. FA hypersensitivity to ICL has been correlated with alterations in homologous recombination, non-homologous end-joining, telomere maintenance, DNA-damage assessment and checkpoint regulation, processes in which the components of the RAD50/MRE11/NBS1 (RMN) complex are involved. To better characterize the mechanisms by which ICL are processed in human cells and to gain insight into their toxicity in FA, we examined (i). the RMN complex assembling in response to the ICL inducers mitomycin C (MMC) and photoactivated 8-methoxypsoralen and (ii). the proficiency of FA cells to perform RMN activation in response to ICL inducers. We show here that ICL activates the assembly of the RMN proteins into subnuclear foci, and that their formation proceeds independently of ICL incision, a step mainly dependent on XP-F/ERCC1 heterodimer activity. Interestingly, FA cells were unable to form RMN foci in response to either ICL inducer. Analysis by pulsed-field gel electrophoresis and single-cell gel electrophoresis of MMC-treated cells showed that FA cells from complementation group C (FA-C cells, defective in the FANCC gene) form double-strand breaks and unhook MMC-induced ICL similarly to FANCC wild-type cells. These observations imply that the absence of RMN assembly in FA-C cells is not simply due to the absence of DNA ends produced as intermediates of ICL processing, and indicates a direct role for FANCC in RMN focus assembly in response to ICL inducers. Moreover, we show that the formation of foci, including BRCA1 and/or RAD51 proteins, is significantly delayed in FA cells. These alterations in the assembly of DNA-repair proteins in FA provide an interpretation for the DNA-damage processing anomalies observed in FA cells and for the genetic instability and the cancer predisposition of the syndrome.

Genetic effect of 3-carbethoxypsoralen, angelicin, psoralen and 8-methoxypsoralen plus 365-nm irradiation in Saccharomyces cerevisiae: induction of reversions, mitotic crossing-over, gene conversion and cytoplasmic "petite" mutations

The genetic effects of two mono-functional photosensitizing furocoumarins, 3-carbethoxypsoralen (3-CPs) and angelicin, were compared with those of two bi-functional furocoumarins, 8-methoxypsoralen and psoralen in Saccharomyces cerevisiae. A drug concentration of 5 X 10(-5) M plus various doses of 365-nm irradiation at a dose rate of 1.2 kJ m-2 min-1 were used. Per dose of 365-nm irradiation, the frequency of induced nuclear events such as gene mutation and mitotic recombination (conversion and crossing-over) is higher for the bi-functional than for the mono-functional compounds. The higher efficiency of the bi-functional furocoumarins is also evident when the frequency of mutants is expressed as a function of survival. However, the photo-addition of the 4 furocoumarins studied leads to the same response for the induction of recombinational events per viable cell. Amongst genetically altered colonies induced in the diploid strains D5 and D7, the colonies corresponding to the induction of crossing-over are effectively produced by bi-functional furocoumarins, but are rare (D7) or even absent (D5) after treatment with monofunctional furocoumarins. This suggests a certain specificity of genetic alterations produced by the bi-functional agents. 3-CPs is the most effective inducer on the cytoplasmic "petite" mutation in stationary phase cells per unit irradiation dose or per viable cell.

Photochemotherapy using pyridopsoralens

Aiming to decrease the acute side effects and genotoxic hazards of PUVA, pyrido (3,4-C) psoralen (PP) and 7-methyl pyrido (3,4-C) psoralen (MPP) were synthesized and studied. Their UVA maximum absorption lies at 325 and 330 nm, respectively. Their photostability is comparable to that of 8-MOP. They complex to DNA in the dark, and, in the presence of UVA, produce only monoadditions to DNA, as shown by fluorescence and DNA denaturation-renaturation studies. In diploid eukaryotic yeast they are more effective than 8-MOP for the induction of lethal effects and mitochondrial damage. Their mutagenic activity per unit dose of UVA is in the same range as that of 8-MOP. However, per viable cell they are clearly less mutagenic than 8-MOP. This difference is also observed for recombinogenic activity. No oxygen effect is observed. In mammalian cells the following ranges of effectiveness are found: inhibition of DNA synthesis in human fibroblasts: MPP greater than PP greater than 8-MOP; mutagenic activity in V79 Chinese hamster cells: MPP greater than PP greater than 8-MOP; cell transforming ability in C3H embryonic mouse cells: MPP greater than 8-MOP greater than PP as a function of UVA dose, and: 8-MOP greater than MPP greater than PP as a function of survival; induction of sister chromatic exchanges (SCE) per unit dose: MPP greater than PP greater than 8-MOP in the linear part of the induction curve, and : 8-MOP greater than PP greater than MPP at the maximum level of SCE obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Phototherapeutic, photobiologic, and photosensitizing properties of khellin

Khellin, whose chemical structure closely resembles that of psoralen, is reported to be an efficient drug for treating vitiligo when combined with ultraviolet A irradiation. Photobiological activity on yeast is found to be much lower than that of bifunctional psoralens such as 5-methoxypsoralen. In vitro experiments reveal that khellin is a poor photosensitizer. It behaves as a monofunctional agent with respect to DNA photoaddition. It does not photoinduce cross-links in DNA in vitro or in Chinese hamster cells in vivo. This behavior may explain the low photogenotoxicity in yeast and the lack of phototoxic erythemal response when treating vitiligo with khellin.

Lack of ceramide generation in TF-1 human myeloid leukemic cells resistant to ionizing radiation

The mechanism(s) by which ionizing radiation (IR) induces cell death is of fundamental importance in understanding cell sensitivity and resistance. Here we evaluated the response to IR of two subclones of the autonomous human erythromyeloblastic cell line TF-1: TF-1-34 (which expresses CD34) and TF-1-33 (which lacks CD34). In clonogenic assays, TF-1-34 cells were found to be relatively less sensitive to IR compared to TF-1-33 cells based on the D0 determination (3.01 vs 1.56 Gy). Furthermore, after IR at 12 Gy, TF-1-33 cell viability decreased by approximately 50% within 24 h, whereas TF-1-34 cell growth was unaffected during this time. Gradual loss of TF-1-34 cell viability was observed only after 48 h. Morphological and molecular analysis revealed that TF-1-33 cells died of apoptosis, and TF-1-34 cells of delayed reproductive cell death. While IR produced comparable amounts of DNA double strand breaks (DSB) in both cell lines, TF-1-34 retained DSB much longer than TF-1-33 suggesting that radioresistance and the defective apoptotic response of TF-1-34 cells was not related to a higher DNA repair capacity. However, the lack of an apoptotic response in TF-1-34 was correlated to the absence of a sphingomyelin (SM)-ceramide (CER) signaling pathway. Indeed, IR triggered in TF-1-33 cells but not in TF-1-34, SM hydrolysis (25% at 12 Gy) and CER generation (>50%) through the activation of neutral but not acid sphingomyelinase. Synthetic cell permeate CER induced apoptosis in both TF-1-33 and TF-1-34 cells. This study indicates that alterations of the SM-CER signaling pathway can significantly influence the cell death process as well as the survival of acute myeloid leukemia cells after IR exposure.

8-Methoxypsoralen plus 365 nm light effects and repair in yeast

Haploid wild-type and mutant cells of Saccharomyces carrying one of the single genes rad2-20 or rad9-4 and the double mutant rad2-20rad9-4 were tested for their response to a treatment with 8-methoxypsoralen plus 365 nm light using immediate and delayed plating techniques. The mutant defective in the excision of ultraviolet-induced pyrimidine dimers (rad2-20) as well as that presumably deficient in a recombinational repair system (rad9-4) are more sensitive than wild type cells. The double mutant (rad2-20rad9-4) demonstrates a higher sensitivity than each of the single mutants, indicating that at least two pathways are involved in the repair of the 8-methoxypsoralen plus 365 nm induced damages. In all cases survival curves have shoulders. The survival of wild type and rad9-4 cells is increased after dark holding whereas it remains constant for the rad2-20 mutant and for the double mutant. These results show that the induced damages are reparable. Respiratory deficient mutant (p-) were compared to the corresponding respiratory competent cells. It is shown that the respiratory function is required for the expression of the excision repair activity. The 8-methoxypsoralen plus 365 nm ligh treatment appears to be less effective than ultraviolet irradiation (254 nm) in the induction of the cytoplasmic 'petite' mutation at the same survival levels.

Cytotoxicity and gene induction by some essential oils in the yeast Saccharomyces cerevisiae

In order to get an insight into the possible genotoxicity of essential oils (EOs) used in traditional pharmacological applications we tested five different oils extracted from the medicinal plants Origanum compactum, Coriandrum sativum, Artemisia herba alba, Cinnamomum camphora (Ravintsara aromatica) and Helichrysum italicum (Calendula officinalis) for genotoxic effects using the yeast Saccharomyces cerevisiae. Clear cytotoxic effects were observed in the diploid yeast strain D7, with the cells being more sensitive to EOs in exponential than in stationary growth phase. The cytotoxicity decreased in the following order: Origanum compactum>Coriandrum sativum>Artemisia herba alba>Cinnamomum camphora>Helichrysum italicum. In the same order, all EOs, except that derived from Helichrysum italicum, clearly induced cytoplasmic petite mutations indicating damage to mitochondrial DNA. However, no nuclear genetic events such as point mutations or mitotic intragenic or intergenic recombination were induced. The capacity of EOs to induce nuclear DNA damage-responsive genes was tested using suitable Lac-Z fusion strains for RNR3 and RAD51, which are genes involved in DNA metabolism and DNA repair, respectively. At equitoxic doses, all EOs demonstrated significant gene induction, approximately the same as that caused by hydrogen peroxide, but much lower than that caused by methyl methanesulfonate (MMS). EOs affect mitochondrial structure and function and can stimulate the transcriptional expression of DNA damage-responsive genes. The induction of mitochondrial damage by EOs appears to be closely linked to overall cellular cytotoxicity and appears to mask the occurrence of nuclear genetic events. EO-induced cytotoxicity involves oxidative stress, as is evident from the protection observed in the presence of ROS inhibitors such as glutathione, catalase or the iron-chelating agent deferoxamine.