G4 DNA present at human telomeric DNA contributes toward reduced sensitivity to γ-radiation induced oxidative damage, but not bulky adduct formation.

PURPOSE: DNA, the hereditary material of a human cell generally exists as Watson-Crick base paired double-stranded B-DNA. Studies suggest that DNA can also exist in non-B forms, such as four stranded G-quadruplexes (G4 DNA). Recently, our studies revealed that the regions of DNA that can fold into G-quadruplex structures are less sensitive to ionizing radiation (IR) compared to B-DNA. Importantly, we reported that the planar G-quartet of a G4 structure is shielded from radiation induced DNA breaks, while the single- and double-stranded DNA regions remained susceptible. Thus, in the present study, we investigate whether telomeric repeat DNA present at the end of telomere, known to fold into G4 DNA can protect from radiation induced damages including strand breaks, oxidation of purines and bulky adduct formation on DNA. MATERIALS AND METHODS: For plasmid irradiation assay, plasmids containing human telomeric repeat DNA sequence TTAGGG (0.8 kb or 1.8 kb) were irradiated with increasing doses of IR along with appropriate control plasmids and products were resolved on 1% agarose gel. Radioprotection was evaluated based on extent of conversion of supercoiled to nicked or linear forms of the DNA following irradiation. Formation of G-quadruplex structure on supercoiled DNA was evaluated based on circular dichroism (CD) spectroscopy studies. Cleavage of radiation induced oxidative damage and extent of formation of nicks was further evaluated using base and nucleotide excision repair proteins. RESULTS: Results from CD studies showed that the plasmid DNA harboring human telomeric repeats (TTAGGG) can fold into G-quadruplex DNA structures. Further, results showed that human telomeric repeat sequence when present on a plasmid can protect the plasmid DNA against IR induced DNA strand breaks, unlike control plasmids bearing random DNA sequence. CONCLUSIONS: Human telomeric repeat sequence when present on plasmids can fold into G-quadruplex DNA structures, and can protect the DNA against IR induced DNA strand breaks and oxidative damage. These results in conjunction with our previous studies suggest that telomeric repeat sequence imparts less sensitivity to IR and thus telomeres of chromosomes are protected from radiation.

Detection of the thietane precursor in the UVA formation of the DNA 6-4 photoadduct.

Notwithstanding the central biological role of the (6-4) photoadduct in the induction of skin cancer by sunlight, crucial mechanistic details about its formation have evaded characterization despite efforts spanning more than half a century. 4-Thiothymidine (4tT) has been widely used as an important model system to study its mechanism of formation, but the excited-state precursor, the intermediate species, and the time scale leading to the formation of the (6-4) photoadduct have remained elusive. Herein, steady-state and time-resolved spectroscopic techniques are combined with new and reported quantum-chemical calculations to demonstrate the excited state leading to the formation of the thietane intermediate, its rate, and the formation of the (6-4) photoadduct using the 5'-TT(4tT)T(4tT)TT-3' DNA oligonucleotide. Efficient, sub-1 ps intersystem crossing leads to the population of a triplet minimum of the thietane intermediate in as short as 3 ps, which intersystem crosses to its ground state and rearranges to form the (6-4) photoadduct.

Hydrated electrons induce the formation of interstrand cross-links in DNA modified by cisplatin adducts.

Double-stranded oligonucleotides containing cisplatin adducts, with and without a mismatched region, were exposed to hydrated electrons generated by gamma-rays. Gel electrophoresis analysis demonstrates the formation of cisplatin-interstrand crosslinks from the cisplatin-intrastrand species. The rate constant per base for the reaction between hydrated electrons and the double-stranded oligonucleotides with and without cisplatin containing a mismatched region was determined by pulse radiolysis to be 7 × 109 and 2 × 109 M-1 s-1, respectively. These results provide a better understanding of the radiosensitizing effect of cisplatin adducts in hypoxic tumors and of the formation of interstrand crosslinks, which are difficult for cells to repair.

Ultraviolet A light induces DNA damage and estrogen-DNA adducts in Fuchs endothelial corneal dystrophy causing females to be more affected.

Fuchs endothelial corneal dystrophy (FECD) is a leading cause of corneal endothelial (CE) degeneration resulting in impaired visual acuity. It is a genetically complex and age-related disorder, with higher incidence in females. In this study, we established a nongenetic FECD animal model based on the physiologic outcome of CE susceptibility to oxidative stress by demonstrating that corneal exposure to ultraviolet A (UVA) recapitulates the morphological and molecular changes of FECD. Targeted irradiation of mouse corneas with UVA induced reactive oxygen species (ROS) production in the aqueous humor, and caused greater CE cell loss, including loss of ZO-1 junctional contacts and corneal edema, in female than male mice, characteristic of late-onset FECD. UVA irradiation caused greater mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) damage in female mice, indicative of the sex-driven differential response of the CE to UVA, thus accounting for more severe phenotype in females. The sex-dependent effect of UVA was driven by the activation of estrogen-metabolizing enzyme CYP1B1 and formation of reactive estrogen metabolites and estrogen-DNA adducts in female but not male mice. Supplementation of N-acetylcysteine (NAC), a scavenger of reactive oxygen species (ROS), diminished the morphological and molecular changes induced by UVA in vivo. This study investigates the molecular mechanisms of environmental factors in FECD pathogenesis and demonstrates a strong link between UVA-induced estrogen metabolism and increased susceptibility of females for FECD development.

Genome-wide excision repair in Arabidopsis is coupled to transcription and reflects circadian gene expression patterns.

Plants are exposed to numerous DNA-damaging stresses including the exposure to ultraviolet (UV) component of solar radiation. They employ nucleotide excision repair to remove DNA-bulky adducts and to help eliminate UV-induced DNA lesions, so as to maintain their genome integrity and their fitness. Here, we generated genome-wide single-nucleotide resolution excision repair maps of UV-induced DNA damage in Arabidopsis at different circadian time points. Our data show that the repair of UV lesions for a large fraction of the genome is controlled by the joint actions of the circadian clock and transcription by RNA polymerase II. Our findings reveal very strong repair preference for the transcribed strands of active genes in Arabidopsis, and 10-30% of the transcription-coupled repair is circadian time-dependent. This dynamic range in nucleotide excision repair levels throughout the day enables Arabidopsis to cope with the bulky DNA lesion-inducing environmental factors including UV.

RecBCD (Exonuclease V) is inhibited by DNA adducts produced by cisplatin and ultraviolet light.

The presence of adducts on the DNA double-helix can have major consequences for the efficient functioning of DNA repair enzymes. E. coli RecBCD (exonuclease V) is involved in recombinational repair of double-strand breaks that are caused by defective DNA replication, DNA damaging agents and other factors. The holoenzyme possesses a bipolar helicase activity which helps unwind DNA from both 3'- and 5'-directions and is coupled with a potent exonuclease activity that is also capable of digesting DNA from both 3'- and 5'-ends. In this study, DNA sequences were damaged with cisplatin or UV followed by RecBCD treatment. DNA damaging agents such as cisplatin and UV induce the formation of intrastrand adducts in the DNA template. It was demonstrated that RecBCD degradation was inhibited by either cisplatin-damaged or UV-damaged DNA sequences. This is the first occasion that RecBCD has been demonstrated to be inhibited by DNA adducts induced by cisplatin or UV. In addition, we quantified the amounts of DNA remaining after RecBCD treatment and observed that the level of inhibition was concentration and dose dependent. A DNA-targeted 9-aminoacridinecarboxamide cisplatin analogue was also found to inhibit RecBCD activity.

Chemical repair activity of free radical scavenger edaravone: reduction reactions with dGMP hydroxyl radical adducts and suppression of base lesions and AP sites on irradiated plasmid DNA.

Reactions of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) with deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts were investigated by pulse radiolysis technique. Edaravone was found to reduce the dGMP hydroxyl radical adducts through electron transfer reactions. The rate constants of the reactions were greater than 4 × 10(8) dm(3) mol(-1) s(-1) and similar to those of the reactions of ascorbic acid, which is a representative antioxidant. Yields of single-strand breaks, base lesions, and abasic sites produced in pUC18 plasmid DNA by gamma ray irradiation in the presence of low concentrations (10-1000 µmol dm(-3)) of edaravone were also quantified, and the chemical repair activity of edaravone was estimated by a method recently developed by the authors. By comparing suppression efficiencies to the induction of each DNA lesion, it was found that base lesions and abasic sites were suppressed by the chemical repair activity of edaravone, although the suppression of single-strand breaks was not very effective. This phenomenon was attributed to the chemical repair activity of edaravone toward base lesions and abasic sites. However, the chemical repair activity of edaravone for base lesions was lower than that of ascorbic acid.

Physiological aspects of UV-excitation of DNA.

Solar ultraviolet (UV) radiation, mainly UV-B (280-315 nm), is one of the most potent genotoxic agents that adversely affects living organisms by altering their genomic stability. DNA through its nucleobases has absorption maxima in the UV region and is therefore the main target of the deleterious radiation. The main biological relevance of UV radiation lies in the formation of several cytotoxic and mutagenic DNA lesions such as cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers (DEWs), as well as DNA strand breaks. However, to counteract these DNA lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, excision repair, and mismatch repair (MMR). Photoreactivation involving the enzyme photolyase is the most frequently used repair mechanism in a number of organisms. Excision repair can be classified as base excision repair (BER) and nucleotide excision repair (NER) involving a number of glycosylases and polymerases, respectively. In addition to this, double-strand break repair, SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms to ensure genomic stability. This review concentrates on the UV-induced DNA damage and the associated repair mechanisms as well as various damage detection methods.

Synergistic cytotoxicity and DNA strand breaks in cells and plasmid DNA exposed to uranyl acetate and ultraviolet radiation.

Depleted uranium (DU) has a chemical toxicity that is independent of its radioactivity. The purpose of this study was to explore the photoactivation of uranyl ion by ultraviolet (UV) radiation as a chemical mechanism of uranium genotoxicity. The ability of UVB (302 nm) and UVA (368 nm) radiation to photoactivate uranyl ion to produce single strand breaks was measured in pBR322 plasmid DNA, and the presence of adducts and apurinic/apyrimidinic sites that could be converted to single strand breaks by heat and piperidine was analyzed. Results showed that DNA lesions in plasmid DNA exposed to UVB- or UVA-activated DU were only slightly heat reactive, but were piperidine sensitive. The cytotoxicity of UVB-activated uranyl ion was measured in repair-proficient and repair-deficient Chinese hamster ovary cells and human keratinocyte HaCaT cells. The cytotoxicity of co-exposures of uranyl ion and UVB radiation was dependent on the order of exposure and was greater than co-exposures of arsenite and UVB radiation. Uranyl ion and UVB radiation were synergistically cytotoxic in cells, and cells exposed to photoactivated DU required different DNA repair pathways than cells exposed to non-photoactivated DU. This study contributes to our understanding of the DNA lesions formed by DU, as well as their repair. Results suggest that excitation of uranyl ion by UV radiation can provide a pathway for uranyl ion to be chemically genotoxic in populations with dermal exposures to uranium and UV radiation, which would make skin an overlooked target organ for uranium exposures.

An immunohistochemical panel to assess ultraviolet radiation-associated oxidative skin injury.

Ultraviolet (UV) radiation results in a significant loss in years of healthy life, approximately 1.5 million disability-adjusted life years (DALYs), and is associated with greater than 60,000 deaths annually worldwide that are attributed to melanoma and other skin cancers. Currently, there are no standardized biomarkers or assay panels to assess oxidative stress skin injury patterns in human skin exposed to ionizing radiation. Using biopsy specimens from chronic solar UV-exposed and UV-protected skin, we demonstrate that UV radiation-induced oxidative skin injury can be evaluated by an immunohistochemical panel that stains 8-hydroxydeoxyguanosine (8-OH-dG) to assess DNA adducts, 4-hydroxy-2-nonenal (HNE) to assess lipid peroxidation, and advanced glycation end products (AGEs) to assess protein damage. We believe this panel contains the necessary cellular biomarkers to evaluate topical agents, such as sunscreens and anti-oxidants that are designed to prevent oxidative skin damage and may reduce UV-associated skin aging, carcinogenesis, and inflammatory skin diseases. We envision that this panel will become an important tool for researchers developing topical agents to protect against UV radiation and other oxidants and ultimately lead to reductions in lost years of healthy life, DALYs, and annual deaths associated with UV radiation.

Non-DSB clustered DNA lesions induced by ionizing radiation are largely responsible for the loss of plasmid DNA functionality in the presence of cisplatin.

The combination of cisplatin and ionizing radiation (IR) increases cell toxicity by both enhancing DNA damage and inhibiting repair mechanisms. Although the formation of cluster DNA lesions, particularly double-strand breaks (DSB) at the site of cisplatin-DNA-adducts has been reported to induce cell death, the contribution of DSB and non-DSB cluster lesions to the cellular toxicity is still unknown. Although both lesions are toxic, it is not always possible to measure their frequency and cell survival in the same model system. To overcome this problem, here, we investigate the effect of cisplatin-adducts on the induction of DSB and non-DSB cluster DNA lesions by IR and determine the impact of such lesions on plasmid functionality. Cluster lesions are two or more lesions on opposite DNA strands with a short distance such that error free repair is difficult or impossible. At a ratio of two cisplatin per plasmid, irradiation of platinated DNA in solution with (137)Cs γ-rays shows enhancements in the formation of DNA DSB and non-DSB cluster lesions by factors of 2.6 and 2.1, respectively, compared to unmodified DNA. However, in absolute terms, the yield for non-DSB cluster lesions is far larger than that for DSB, by a factor of 26. Unmodified and cisplatin-modified DNA were irradiated and subsequently transformed into Escherichia coli to give survival curves representing the functionality of the plasmid DNA as a function of radiation dose. Our results demonstrate that non-DSB cluster lesions are the only toxic lesions present at a sufficient frequency to account for the loss of DNA functionality. Our data also show that Frank-DSB lesions are simply too infrequent to account for the loss of DNA functionality. In conclusion, non-DSB cluster DNA damage is known to be difficult to repair and is probably the lesion responsible for the loss of functionality of DNA modified by cisplatin.

One-electron oxidation reactions of purine and pyrimidine bases in cellular DNA.

PURPOSE: The aim of this survey is to critically review the available information on one-electron oxidation reactions of nucleobases in cellular DNA with emphasis on damage induced through the transient generation of purine and pyrimidine radical cations. Since the indirect effect of ionizing radiation mediated by hydroxyl radical is predominant in cells, efforts have been made to selectively ionize bases using suitable one-electron oxidants that consist among others of high intensity UVC laser pulses. Thus, the main oxidation product in cellular DNA was found to be 8-oxo-7,8-dihydroguanine as a result of direct bi-photonic ionization of guanine bases and indirect formation of guanine radical cations through hole transfer reactions from other base radical cations. The formation of 8-oxo-7,8-dihydroguanine and other purine and pyrimidine degradation products was rationalized in terms of the initial generation of related radical cations followed by either hydration or deprotonation reactions in agreement with mechanistic pathways inferred from detailed mechanistic studies. The guanine radical cation has been shown to be implicated in three other nucleophilic additions that give rise to DNA-protein and DNA-DNA cross-links in model systems. Evidence was recently provided for the occurrence of these three reactions in cellular DNA. CONCLUSION: There is growing evidence that one-electron oxidation reactions of nucleobases whose mechanisms have been characterized in model studies involving aqueous solutions take place in a similar way in cells. It may also be pointed out that the above cross-linked lesions are only produced from the guanine radical cation and may be considered as diagnostic products of the direct effect of ionizing radiation.

RXRα ablation in epidermal keratinocytes enhances UVR-induced DNA damage, apoptosis, and proliferation of keratinocytes and melanocytes.

We show here that keratinocytic nuclear receptor retinoid X receptor-α (RXRα) regulates mouse keratinocyte and melanocyte homeostasis following acute UVR. Keratinocytic RXRα has a protective role in UVR-induced keratinocyte and melanocyte proliferation/differentiation, oxidative stress-mediated DNA damage, and cellular apoptosis. We discovered that keratinocytic RXRα, in a cell-autonomous manner, regulates mitogenic growth responses in skin epidermis through secretion of heparin-binding EGF-like growth factor, GM-CSF, IL-1α, and cyclooxygenase-2 and activation of mitogen-activated protein kinase pathways. We identified altered expression of several keratinocyte-derived mitogenic paracrine growth factors such as endothelin 1, hepatocyte growth factor, α-melanocyte stimulating hormone, stem cell factor, and fibroblast growth factor-2 in skin of mice lacking RXRα in epidermal keratinocytes (RXRα(ep-/-) mice), which in a non-cell-autonomous manner modulated melanocyte proliferation and activation after UVR. RXRα(ep-/-) mice represent a unique animal model in which UVR induces melanocyte proliferation/activation in both epidermis and dermis. Considered together, the results of our study suggest that RXR antagonists, together with inhibitors of cell proliferation, can be effective in preventing solar UVR-induced photocarcinogenesis.

UVR exposure sensitizes keratinocytes to DNA adduct formation.

UV radiation (UVR) and exposure to tobacco smoke, a source of polycyclic aromatic hydrocarbons (PAH), have been linked to skin carcinogenesis. UVR-mediated activation of the aryl hydrocarbon receptor (AhR) stimulates the transcription of CYP1A1 and CYP1B1, which encode proteins that convert PAH to genotoxic metabolites. We determined whether UVR exposure sensitized human keratinocytes to PAH-induced DNA adduct formation. UVR exposure induced CYP1A1 and CYP1B1 in HaCaT cells, an effect that was mimicked by photooxidized tryptophan (aTRP) and FICZ, a component of aTRP. UVR exposure or pretreatment with aTRP or FICZ also sensitized cells to benzo(a)pyrene (B[a]P)-induced DNA adduct formation. alphaNF, an AhR antagonist, suppressed UVR-, aTRP-, and FICZ-mediated induction of CYP1A1 and CYP1B1 and inhibited B[a]P-induced DNA adduct formation. Treatment with 17-AAG, an Hsp90 inhibitor, caused a marked decrease in levels of AhR; inhibited UVR-, aTRP-, and FICZ-mediated induction of CYP1A1 and CYP1B1; and blocked the sensitization of HaCaT cells to B[a]P-induced DNA adduct formation. FICZ has been suggested to be a physiologic ligand of the AhR that may have systemic effects. Hence, studies of FICZ were also carried out in MSK-Leuk1 cells, a model of oral leukoplakia. Pretreatment with alpha-naphthoflavone or 17-AAG blocked FICZ-mediated induction of CYP1A1 and CYP1B1, and suppressed the increased B[a]P-induced DNA adduct formation. Collectively, these results suggest that sunlight may activate AhR signaling and thereby sensitize cells to PAH-mediated DNA adduct formation. Antagonists of AhR signaling may have a role in the chemoprevention of photocarcinogenesis.

Damage control: DNA repair, transcription, and the ubiquitin-proteasome system.

The presence of DNA damage within an actively transcribed gene poses an immediate threat to cellular viability. Bulky DNA adducts, such as those induced by ultraviolet light, can profoundly influence patterns of gene expression by causing the irreversible arrest of RNA polymerase II at sites of DNA damage. It is critical that processes exist to either specifically repair transcribed genes or clear stalled RNA polymerase, so that general repair can occur and transcription resume. A growing body of evidence indicates that clearance of stalled polymerase is achieved, in part, by ubiquitin-mediated destruction of the largest subunit of RNA polymerase II. In this review, we shall discuss how an intimate connection between RNA polymerase II and the ubiquitylation machinery acts to restore normal transcription after DNA damage, and other forms of transcriptional arrest, has occurred.

Optimization of the comet assay for the sensitive detection of PUVA-induced DNA interstrand cross-links.

Psoralen plus ultraviolet A (PUVA), commonly used for the treatment of hyperproliferative skin disorders, has been found to be associated with an increased risk of squamous cell cancer. Interstrand cross-link (ICL) formation by PUVA treatment is considered the major factor contributing to the carcinogenesis. However, it remains unclear how PUVA causes, or promotes cancers, in humans. As an initial step in understanding the mechanisms of mutagenesis and carcinogenesis of PUVA photochemotherapy, we have optimized and subsequently utilized a modified alkaline comet assay involving a post-lysis gamma-irradiation at 9 Gy to sensitively measure the formation and repair of PUVA-induced ICLs in the immortalized human keratinocyte cell line HaCaT. A clear dose-dependent response of HaCaT cells to PUVA exposure was observed with a combination of a fixed UVA dose at 0.05 J/cm(2) and a dose of 8-methoxypsoralen ranging from 10 to 100 microM. Results also indicated that the ICL repair was concentration dependent. We have also demonstrated that PUVA-induced monoadduct formation, at an estimated ratio of 3:1 to ICLs in the present experimental conditions, does not interfere with the detection of the ICLs in the modified alkaline comet assay. Furthermore, comparison of the amount of ICL formation between the single-dose UVA treatment and a split-dose protocol was performed. The split-dose protocol was believed to generate more ICLs than the single-dose treatment, thus more effective in PUVA photochemotherapy. Our results demonstrate that comparable amounts of ICLs were formed in HaCaT cells for each dose of UVA, using either the split-dose or single-dose protocols.

Exposure of mice to cigarette smoke and/or light causes DNA alterations in heart and aorta.

Cigarette smoke (CS) is a major risk factor for cardiovascular diseases, cancer, and other chronic degenerative diseases. UV-containing light is the most ubiquitous DNA-damaging agent existing in nature, but its possible role in cardiovascular diseases had never been suspected before, although it is known that mortality for cardiovascular diseases is increased during periods with high temperature and solar irradiation. We evaluated whether exposure of Swiss CD-1 mice to environmental CS (ECS) and UV-C-covered halogen quartz lamps, either individually or in combination, can cause DNA damage in heart and aorta cells. Nucleotide alterations were evaluated by (32)P postlabeling methods and by HPLC-electrochemical detection. The whole-body exposure of mice to ECS considerably increased the levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) and of bulky DNA adducts in both heart and aorta. Surprisingly, even exposure to a light that simulated solar irradiation induced oxidatively generated damage in both tissues. The genotoxic effects of UV light in internal organs is tentatively amenable to formation of unidentified long-lived mutagenic products in the skin of irradiated mice. Nucleotide alterations were even more pronounced when the mice were exposed to smoke and/or light during the first 5 weeks of life rather than during adulthood for an equivalent period of time. Although the pathogenetic meaning is uncertain, DNA damage in heart and aorta may tentatively be related to cardiomyopathies and to the atherogenesis process, respectively.

An ultraviolet melting study of the stability of the DNA double helix in the NaDNA-bipyridyl-(ethylenediamine)platintum(II) complex.

Complexes of NaDNA with the bipyridyl-(ethylenediamine)platintum(II) (abbreviated [(bipy)Pt(en)]2+) molecular ion have been studied in solution via ultraviolet absorption experiments at 260 nm between 50 and 90 degrees C. These measurements, performed as a function of the molar ratio of the [(bipy)Pt(en)]2+ complex to DNA base pairs, show that the stability of the DNA double helix is increased by the formation of the DNA.[(bipy)Pt(en)]2+ complex: at a molar ratio of 0.33, the temperature at which the DNA double helix separates into two single strands is increased by about 15 degrees C.

Irradiation of DNA loaded with platinum containing molecules by fast atomic ions C(6+) and Fe(26+).

PURPOSE: In order to study the role of the Linear Energy Transfer (LET) of fast atomic ions in platinum-DNA complexes inducing breaks, DNA Plasmids were irradiated by C(6+) and Fe(26+) ions. MATERIAL AND METHODS: DNA Plasmids (pBR322) loaded with different amounts of platinum contained in a terpyridine-platinum molecule (PtTC) were irradiated by C(6+) ions and Fe(26+) ions. The LET values ranged between 13.4 keV/microm and 550 keV/microm. In some experiments, dimethyl sulfoxide (DMSO) was added. RESULTS: In all experiments, a significant increase in DNA strand breaks was observed when platinum was present. The yield of breaks induced per Gray decreased when the LET increased. The yield of single and double strand breaks per plasmid per track increased with the LET, indicating that the number of DNA breaks per Gray was related to the number of tracks through the medium. CONCLUSIONS: These findings show that more DNA breaks are induced by atomic ions when platinum is present. This effect increases for low LET heavy atoms. As DSB induction may induce cell death, these results could open new perspectives with the association of hadrontherapy and chemotherapy. Thus the therapeutic index might be improved by loading the tumour with platinum salts.

Photoirradiation of polycyclic aromatic hydrocarbons with UVA light - a pathway leading to the generation of reactive oxygen species, lipid peroxidation, and dna damage.

Polycyclic aromatic hydrocarbons (PAHs) are a class of genotoxic environmental contaminants. We have long been interested in determining the mechanisms by which PAHs induce genotoxicity. Although the metabolic activation of PAHs leading to biological activities has been well studied, the photo-induced activation pathway has seldom reported. In this paper, we review the study of photoirradiation of PAHs with UVA irradiation results in (i) cytotoxicity and DNA damage (ii) DNA single strand cleavage; (iii) formation of 8-hydroxy-2'-deoxyguanosine adduct (8-OHdG), and (iv) formation of lipid peroxidation. Evidence has been shown that these photobiological activities are mediated by reactive oxygen species (ROS).