Induction of DNA strand breaks in normal human fibroblasts exposed to monochromatic ultraviolet and visible wavelengths in the 240-546 nm range

Irradiation with 365 nm and 405 nm wavelength shows differences in DNA damage of swine pancreatic islets

INTRODUCTION

3D printing is being used more extensively in modern biomedicine. One of the problems is selecting a proper crosslinking method of bioprinted material. Amongst currently used techniques we can distinguish: physical crosslinking (e.g. Ca2+ and Sr2+) and chemical crosslinking-the UV light crosslinking causing the biggest discussion. UV radiation is selectively absorbed by DNA, mainly in the UV-B region but also (to some extent) in UV-A and UV-C regions. DNA excitement results in typical photoproducts. The amount of strand breaks may vary depending on the period of exposition, it can also differ when cells undergo incubation after radiation.

AIM

The aim of this study was to show whether and how the time of irradiation with 405 nm and 365 nm wavelengths affect DNA damage in cell lines and micro-organs (pancreatic islets).

MATERIALS AND METHODS

The degree of DNA damage caused by different wavelengths of radiation (405 nm and 365 nm) was evaluated by a comet assay. The test was performed on fibroblasts, alpha cells, beta cells and porcine pancreatic islets after 24 hours incubation period. Samples without radiation treatment were selected as a control group. Results analysis consisted of determining the percent of cells with damaged DNA and the tail intensity evaluation.

RESULTS

The degree of DNA damage in pancreatic islets after exposure to 405 nm wavelength oscillated between 2% and 6% depending on the tested time period (10 - 300 seconds). However, treating islets using 365 nm wavelength resulted in damage up to 50%. This clearly shows significantly less damage when using 405 nm wavelength. Similar results were obtained for the tested cell lines.

CONCLUSIONS

Crosslinking with 405 nm is better for pancreatic islets than crosslinking with 365 nm UV light.

Understanding the physiological effects of UV-C light and exploiting its agronomic potential before and after harvest

There is an abundant literature about the biological and physiological effects of UV-B light and the signaling and metabolic pathways it triggers and influences. Much less is known about UV-C light even though it seems to have a lot of potential for being effective in less time than UV-B light. UV-C light is known since long to exert direct and indirect inhibitory and damaging effects on living cells and is therefore commonly used for disinfection purposes. More recent observations suggest that UV-C light can also be exploited to stimulate the production of health-promoting phytochemicals, to extent shelf life of fruits and vegetables and to stimulate mechanisms of adaptation to biotic and abiotic stresses. Clearly some of these effects may be related to the stimulating effect of UV-C light on the production of reactive oxygen species (ROS) and to the stimulation of antioxidant molecules and mechanisms, although UV-C light could also trigger and regulate signaling pathways independently from its effect on the production of ROS. Our review clearly underlines the high potential of UV-C light in agriculture and therefore advocates for more work to be done to improve its efficiency and also to increase our understanding of the way UV-C light is perceived and influences the physiology of plants.

Structural insights into DNA repair by RNase T--an exonuclease processing 3' end of structured DNA in repair pathways

DNA repair mechanisms are essential for preservation of genome integrity. However, it is not clear how DNA are selected and processed at broken ends by exonucleases during repair pathways. Here we show that the DnaQ-like exonuclease RNase T is critical for Escherichia coli resistance to various DNA-damaging agents and UV radiation. RNase T specifically trims the 3' end of structured DNA, including bulge, bubble, and Y-structured DNA, and it can work with Endonuclease V to restore the deaminated base in an inosine-containing heteroduplex DNA. Crystal structure analyses further reveal how RNase T recognizes the bulge DNA by inserting a phenylalanine into the bulge, and as a result the 3' end of blunt-end bulge DNA can be digested by RNase T. In contrast, the homodimeric RNase T interacts with the Y-structured DNA by a different binding mode via a single protomer so that the 3' overhang of the Y-structured DNA can be trimmed closely to the duplex region. Our data suggest that RNase T likely processes bulge and bubble DNA in the Endonuclease V-dependent DNA repair, whereas it processes Y-structured DNA in UV-induced and various other DNA repair pathways. This study thus provides mechanistic insights for RNase T and thousands of DnaQ-like exonucleases in DNA 3'-end processing.

Skin photoaging and the role of antioxidants in its prevention

Photoaging of the skin depends primarily on the degree of ultraviolet radiation (UVR) and on an amount of melanin in the skin (skin phototype). In addition to direct or indirect DNA damage, UVR activates cell surface receptors of keratinocytes and fibroblasts in the skin, which leads to a breakdown of collagen in the extracellular matrix and a shutdown of new collagen synthesis. It is hypothesized that dermal collagen breakdown is followed by imperfect repair that yields a deficit in the structural integrity of the skin, formation of a solar scar, and ultimately clinically visible skin atrophy and wrinkles. Many studies confirmed that acute exposure of human skin to UVR leads to oxidation of cellular biomolecules that could be prevented by prior antioxidant treatment and to depletion of endogenous antioxidants. Skin has a network of all major endogenous enzymatic and nonenzymatic protective antioxidants, but their role in protecting cells against oxidative damage generated by UV radiation has not been elucidated. It seems that skin's antioxidative defence is also influenced by vitamins and nutritive factors and that combination of different antioxidants simultaneously provides synergistic effect.

Quantification of UV-induced cyclobutane pyrimidine dimers using an oligonucleotide chip assay

A lesion-specific enzyme-induced DNA strand break assay was developed for an oligonucleotide chip for the determination of UVB-induced cyclobutane pyrimidine dimers (CPDs). A 20-mer of fluorophore-labeled and biotinylated oligonucleotide was immobilized on the chip. CPDs in DNA on the chip were formed by UVB irradiation (312 nm). T4 endonuclease V (T4N5) was used to excise the CPD site as T4N5 sensitively and specifically detects CPDs. The fluorophore-labeled DNA fragments were detected by a laser-induced fluorescence (LIF) detection system. The number of CPDs induced by UVB was determined based on a mathematical equation obtained from a predetermined calibration curve. The yield of UVB-induced CPDs was 1.73 CPDs per megabase per (kJ/m(2)). The reliability of this value was proved by its similarity to reference values obtained from gel electrophoresis. The developed assay has strong potential to quantify most kinds of UV-induced DNA lesions.

A novel single-stranded DNA-specific 3'-5' exonuclease, Thermus thermophilus exonuclease I, is involved in several DNA repair pathways

Single-stranded DNA (ssDNA)-specific exonucleases (ssExos) are expected to be involved in a variety of DNA repair pathways corresponding to their cleavage polarities; however, the relationship between the cleavage polarity and the respective DNA repair pathways is only partially understood. To understand the cellular function of ssExos in DNA repair better, genes encoding ssExos were disrupted in Thermus thermophilus HB8 that seems to have only a single set of 5'-3' and 3'-5' ssExos unlike other model organisms. Disruption of the tthb178 gene, which was expected to encode a 3'-5' ssExo, resulted in significant increase in the sensitivity to H(2)O(2) and frequency of the spontaneous mutation rate, but scarcely affected the sensitivity to ultraviolet (UV) irradiation. In contrast, disruption of the recJ gene, which encodes a 5'-3' ssExo, showed little effect on the sensitivity to H(2)O(2), but caused increased sensitivity to UV irradiation. In vitro characterization revealed that TTHB178 possessed 3'-5' ssExo activity that degraded ssDNAs containing deaminated and methylated bases, but not those containing oxidized bases or abasic sites. Consequently, we concluded that TTHB178 is a novel 3'-5' ssExo that functions in various DNA repair systems in cooperation with or independently of RecJ. We named TTHB178 as T. thermophilus exonuclease I.

High-dose long wave visible light induces perinuclear vacuolization in vivo but does not result in early photoageing and apoptosis

With the advancing widespread use of photodynamic therapy, questions have arisen about the necessity to protect the adjacent healthy skin from high-dose long-wave light. The aim of the present study was to investigate the effects of high dose visible light on the skin of healthy volunteers with focus on apoptosis, DNA damage, inflammation, melanogenesis and induction of matrix metalloproteinases (MMP). Fourteen healthy volunteers were included and irradiated daily on their buttocks with 1300 kJ/m2 long wave visible light (560-780 nm) on five consecutive days with a cumulative dose of 6500 kJ/m2. In each volunteer six biopsies were taken before and 24 h after irradiation on days 1, 2, 3 and 5 and on day 8 and 12. Frozen and paraffin sections were investigated by measuring parameters for photodamage (apoptosis, p53, phosphorylated c-Jun), skin ageing (phosphorylated c-Jun, MMP-1, elastin content) melanogenesis (Melan A). Although no sunburn cells were seen, a significant increase in perinuclear vacuolization was noted (P < 0.0003) from day 5 till 7 days after the last irradiation. There was no expression of phosphorylated c-Jun, whereas the expression of p53, Melan A, MMP-1 and elastin content did not change. High-dose visible light induces a significant increase in perinuclear vacuolization, but does not result in apoptosis, photodamage or early induction of skin ageing.

Phototherapy with turquoise versus blue light

Preterm jaundiced infants were treated by phototherapy with a new turquoise fluorescent lamp. This was more effective in reducing plasma total bilirubin in relation to light irradiance than the ubiquitously used blue fluorescent lamp.

Inhibitory effects of long-wave ultraviolet radiation of isolated chicken liver nuclei on the Mg2+-dependent transition of chromatin structure

Since the turbidity of nuclear suspensions is known to be correlated with the nuclear morphology, the effects of long-wave ultraviolet (UVA) radiation on Mg2+-dependent structural transition of chromatin in isolated chicken liver nuclei were monitored by measuring the relative turbidity of nuclear suspensions. UVA radiation of the nuclei inhibited the Mg2+-dependent change in relative turbidity of nuclear suspensions in a UVA dose-dependent manner under aerobic conditions but not under N2 conditions. No inhibitory effect of UVA radiation on the change in relative turbidity was observed in the presence of 50 mM NaN3, which scavenges singlet oxygen (1O2) and hydroxyl radicals (*OH). In contrast, 100 mM dimethyl sulfoxide, which primarily scavenges *OH, did not show the inhibitory effect of UVA radiation. The amounts of DNA-protein crosslinks increased with UVA dose under aerobic conditions but not under N2 conditions. The present study showed that UVA radiation of isolated nuclei inhibited the Mg2+-dependent unfolding of condensed chromatin and that O2 is likely to be involved in this process. Furthermore, the formation of DNA-protein crosslinks may contribute to the inhibition.

Escherichia coli exonuclease III enhances long PCR amplification of damaged DNA templates

Recent development of the long PCR technology has provided an invaluable tool in many areas of molecular biology. However, long PCR amplification fails whenever the DNA template is imperfectly preserved. We report that Escherichia coli exonuclease III, a major repair enzyme in bacteria, strikingly improves the long PCR amplification of damaged DNA templates. Escherichia coli exonuclease III permitted or improved long PCR amplification with DNA samples submitted to different in vitro treatments known to induce DNA strand breaks and/or apurinic/apyrimidinic (AP) sites, including high temperature (99 degrees C), depurination at low pH and near-UV radiation. Exonuclease III also permitted or improved amplification with DNA samples that had been isolated several years ago by the phenol/chloroform method. Amelioration of long PCR amplification was achieved for PCR products ranging in size from 5 to 15.4 kb and with DNA target sequences located either within mitochondrial DNA or the nuclear genome. Exonuclease III increased the amplification of damaged templates using either rTth DNA polymerase alone or rTth plus Vent DNA polymerases or TAQ: plus PWO: DNA polymerases. However, exonuclease III could not improve PCR amplification from extensively damaged DNA samples. In conclusion, supplementation of long PCR mixes with E.COLI: exonuclease III may represent a major technical advance whenever DNA samples have been partly damaged during isolation or subsequent storage.

The role of the spectral sensitivity curve in the selection of relevant biological dosimeters for solar UV monitoring

To estimate the risk of enhanced UV-B radiation due to stratospheric ozone depletion, phage T7 and uracil thin-layer biological dosimeters have been developed, which weight the UV irradiance according to induced DNA damage. To study the molecular basis of the biological effects observed after UV irradiation, the spectral sensitivity curves of the two dosimeters and induction of the two major DNA photoproducts, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts ((6-4)PDs), in phage T7 have been determined for polychromatic UV sources. CPDs and (6-4)PDs are determined by lesion-specific monoclonal antibodies in an immunodotblot assay. Phage T7 and uracil biological dosimeters together with a Robertson-Berger (RB) meter have been used for monitoring environmental radiation from the polar region to the equator. The biologically effective dose (BED) established with the three different dosimeters increases according to the changes in the solar angle and ozone column, but the degree of the change differs significantly. The results can be explained based on the different spectral sensitivities of the dosimeters. A possible method for determining the trend of the increase in the biological risk due to ozone depletion is suggested.

Illumination of human keratinocytes in the presence of the sunscreen ingredient Padimate-O and through an SPF-15 sunscreen reduces direct photodamage to DNA but increases strand breaks

On illumination with simulated sunlight, the UVB-absorbing sunscreen chemical 2-ethylhexyl-4-dimethylaminobenzoate (Padimate-O) generates excited species which inflict non-ligatable strand breaks on DNA in vitro and it also becomes mutagenic to yeast in vivo. Padimate-O is known to penetrate human skin but its effects on human cells are not clear. Here, we first simulate the sunlight which penetrates human skin and use it to illuminate human keratinocytes. The DNA damage observed in terms of UV-endonuclease-sensitive sites (ESS) and direct strand breaks per kilobase (kb) of DNA per joule per square metre agrees well with that predicted from action spectra based on monochromatic light. Using plasmid DNA in vitro, we find a very similar pattern of results. Next, we simulate the spectrum that results when the incident light is first attenuated by a film of sunscreen (SPF-15; 2 mg/cm(2)) containing benzophenone-3 (a UVA absorber), octyl methoxycinnamate (a UVB absorber), and Padimate-O. If the sunscreen is not in contact with keratinocytes it reduces direct DNA damage from sunlight (ESS). However, any Padimate-O in contact with the cells substantially increases indirect damage (strand breaks) even though the film of sunscreen reduces direct photodamage. We estimate that applying an SPF-15 sunscreen which contains Padimate-O to human skin followed by exposure to only 5 minimum erythemal doses (MED) of sunlight could, while suppressing the formation of ESS, increase strand breaks in cells under the epidermis by at least 75-fold compared to exposure to 1 MED in the absence of sunscreen.

Activation of flavin-containing oxidases underlies light-induced production of H2O2 in mammalian cells

Violet-blue light is toxic to mammalian cells, and this toxicity has been linked with cellular production of H2O2. In this report, we show that violet-blue light, as well as UVA, stimulated H2O2 production in cultured mouse, monkey, and human cells. We found that H2O2 originated in peroxisomes and mitochondria, and it was enhanced in cells overexpressing flavin-containing oxidases. These results support the hypothesis that photoreduction of flavoproteins underlies light-induced production of H2O2 in cells. Because H2O2 and its metabolite, hydroxyl radicals, can cause cellular damage, these reactive oxygen species may contribute to pathologies associated with exposure to UVA, violet, and blue light. They may also contribute to phototoxicity often encountered during light microscopy. Because multiphoton excitation imaging with 1,047-nm wavelength prevented light-induced H2O2 production in cells, possibly by minimizing photoreduction of flavoproteins, this technique may be useful for decreasing phototoxicity during fluorescence microscopy.

Transmittance spectra and theoretical sun protection factors for a series of sunscreen-containing sun care products

Sun care products containing sunscreens are widely used, but consumers are generally unaware of the important differences in the ability of these lotions to block exposure to the ultraviolet A (UVA) portion of the solar spectrum. The purpose of this study was to determine the transmittance spectra, with particular emphasis on the UVA portion of the spectrum, for a variety of commercially available sun care products, to determine Commission Internationale d'Eclairage (CIE) erythema effectiveness spectra and to compare these with information in the product label. The transmittance spectra for a sample of sun care products were measured spectrophotometrically. These values were convoluted with the CIE erythema action spectrum and the sunlight spectra determined for solar noon on June 21 at 0 degrees and 50 degrees N latitude to produce CIE effectiveness spectra. The UVA transmitted through the sun care products that claimed UVA protection on the bottle label varied from as little as 6% to as much as 52%. In addition, it was determined from the CIE effectiveness spectra that any erythema induced following application of the tested lotions would be caused by the UVA portion of the solar spectrum for all, but one, of the products examined. The results of this study emphasize the necessity for better guidance to the consumer as to the ability of sun care products to provide protection against UVA exposure.

Physiological doses of ultraviolet irradiation induce DNA strand breaks in cultured human melanocytes, as detected by means of an immunochemical assay

An immunochemical assay, i.e. sandwich enzyme-linked immunosorbent assay, has been modified to detect UV-induced damage in cellular DNA of monolayer-grown human melanocytes. The method is based on the binding of a monoclonal antibody to single-stranded DNA. The melanocytes derived from human foreskin of skin type II individuals were suspended and exposed to UVA, UVB, solar-simulated light or gamma-rays. Following physiological doses of UVA, UVB or solar-simulated light, a dose-related DNA unwinding comprising a considerable number of single-strand breaks (ssb) was observed. No correlation was found between different seeded cell densities or different culturing periods and the UVA sensitivity of the cells. After UVA irradiation, 0.07 ssb/10(10) Da/kJ/m2 were detected and after UVB irradiation 1.9 ssb/10(10) Da/kJ/m2 were seen. One minimal erythema dose of solar-simulated light induced 2.25 ssb/10(10) Da. Our results from melanocytes expressed in ssb/Da DNA are comparable and have the same sensitivity toward UVA as well as toward UVB as nonpigmented skin cells. As low doses of UVA have already been shown to induce detectable numbers of ssb, this assay is of great interest for further investigations about the photoprotecting and/or photosensitizing effects of melanins in human melanocytes derived from different skin types.

Photogenotoxicity of skin phototumorigenic fluoroquinolone antibiotics detected using the comet assay

The fluoroquinolone (FQ) antibiotics photosensitize human skin to solar UV radiation and are reported to photosensitize tumor formation in mouse skin. As tumor initiation will not occur without genotoxic insult, we examined the potential of ciprofloxacin, lomefloxacin, fleroxacin, BAYy3118 (a recently developed monofluorinated quinolone) and a nalidixic acid to photosensitize DNA damage in V79 hamster fibroblasts in vitro. Cells were exposed to 37.5 kJ/m2 UVA (320-400 nm; glass filtered Sylvania psoralen + UVA (PUVA) tubes; calibrated Waldmann radiometer) at 4 degrees C in the presence of FQ and immediately afterwards embedded in agarose, lysed and placed in an electrophoretic field at pH 12. Under these denaturing conditions, the presence of DNA single-strand breaks (SSB), alkali-labile sites (ALS) and double-strand breaks (DSB) can be visualized as DNA migrating away from the nucleus (characteristic "comet" appearance) after staining with a specific fluorochrome. At FQ concentrations that induced minimal loss of cell viability (neutral red uptake assay) the compounds tested induced comets with a rank order of BAYy3118 > norfloxacin > ciprofloxacin > lomefloxacin > fleroxacin > nalidixic acid. If cells were incubated after treatment for 1 h at 37 degrees C, the comet score decreased, suggesting efficient removal of SSB/ALS/DSB. Addition of the DNA polymerase(alpha) inhibitor, aphidicolin, to cells treated with either ciprofloxacin alone or ciprofloxacin + UVA resulted in an accumulation of SSB due to the endo/exonuclease steps of excision repair. We have demonstrated that the FQ are photogenotoxic in mammalian cells but the FQ-photosensitized SSB are efficiently repaired. Preliminary evidence that ciprofloxacin photosensitizes the formation of DNA lesions warranting excision repair may indicate production of more mutagenic lesions.

Near-UV-induced absorbance change and photochemical decomposition of ergosterol in the plasma membrane of the yeast Saccharomyces cerevisiae

When cells of the yeast Saccharomyces cerevisiae were exposed to near-UV (300-400 nm), their absorption spectra changed slightly within the range 220-300 nm with increasing dosage. Difference spectra, calculated by substracting the curve recorded in cells exposed to near-UV from the curve of unexposed cells, decreased with increasing dosage over a broad band with peaks at 272, 282 and 295 nm and a shoulder at 265 nm. These peaks were in agreement with the absorption maxima of ergosterol, which is one of the major components of the plasma membrane of yeast. Near-UV radiation induced a simultaneous decrease in absorption spectra and reduction of ergosterol content in the plasma membrane. Photochemical decomposition of ergosterol by near-UV radiation was revealed in vivo, although ergosterol is generally known to be photoconverted to previtamin D2 industrially by UV radiation in vitro. In order to remove photosensitizers, liposomes were prepared from phospholipids and glycolipids, with or without ergosterol from purified yeast plasma membranes. Liposomal ergosterol in the orientated state was photochemically decomposed by near-UV radiation but ergosterol in the disorientated state in a homogeneous solution was not. Near-UV radiation also induced a decrease in activity of membrane-bound ATPase. Dose-response curves for the reduction of ATPase activity were similar to that for decomposition of ergosterol, suggesting that near-UV caused membrane function damage.

In vitro photooxidation of nucleic acids by ultraviolet A radiation

Although ultraviolet A radiation (UVA, 315-400 nm) has been shown to induce oxidative stress in mammalian cells and skin, the critical chromophore(s) and molecular target(s) involved have not been identified. We examined the role of oxidative damage to nucleic acids induced by UVA. To assess photooxidation of cellular DNA and RNA by UVA, we irradiated human skin fibroblasts with up to 765 kJ/m2 UVA. Cellular DNA and RNA were isolated immediately and enzymatically hydrolyzed to nucleosides. 8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), a primary oxidation product in DNA, and 8-oxo-7,8-dihydroguanosine (8-oxoG), resulting from hydroxylation of guanine in RNA, were measured by HPLC with electrochemical detection. We determined that irradiation of skin fibroblasts with levels of UVA that produced moderate toxicity also resulted in significant levels of guanine hydroxylation in RNA. Lower levels of photooxidation were observed in DNA. These results suggest that measurement of guanine hydroxylation in nucleic acids, particularly in cellular RNA, may be a powerful tool for investigating the photobiological activity of UVA.

Biochemical alterations in inflammatory periodontal diseases I. Poly (ADP-ribose) synthetase activity in gingiva and gingival fibroblasts from humans with periodontitis

Periodontal diseases are characterized in part by generation of oxygen free radicals, which can cause breaks in cellular DNA strands. Repair of damaged DNA is dependent upon the synthesis of poly (ADP-ribose)(PADPR) catalyzed by PADPR synthetase, an enzyme known to be activated by the broken ends of DNA strands. We measured the activities of PADPR synthetase and of PADPR glycohydrolase, which degrades PADPRS, in gingival biopsy specimens from 16 sites with adult periodontitis and 12 clinically healthy control sites. The results indicated that sites with periodontitis displayed markedly reduced PADPR synthetase activity compared with healthy control sites, whereas PADPR glycohydrolase activity was not changed. The mean specific activity of PADPR synthetase for the diseased specimens was one-sixth of that of the healthy specimens (p < 0.001). The PADPR synthetase activity was negatively correlated with the Gingival Index (rs = -0.60), pocket depth (rs = -0.70) and bleeding upon probing (rs = -0.72). Cultured fibroblasts derived from clinically characterized healthy and diseased gingival sites reflected similar patterns of enzyme activity. The mean specific activity of PADPR synthetase for the diseased-site cultures (n = 9) was 56 +/- 7% (p < 0.001) of the cultures from healthy control sites (n = 6). These results suggest that a reduced level of PADPR synthetase activity is associated with increased inflammation and periodontal destruction, and that the ability to synthesize PADPR is compromised in adult periodontitis.

Targeted gene transfer in eucaryotic cells by dye-assisted laser optoporation

The blue beam of an Argon laser (488 nm) has been focused on the cell membrane in the presence of phenol-red, an usual component of cell culture media, through a 100 x objective. At the site of the beam impact, due probably to local temperature changes, the cell membrane modifies its permeability. As a consequence of the hit, circular areas, whose radius may be apparently regulated by changing the irradiation time and/or the radiation intensity (energy), appear on the wall, last for a short time and fade spontaneously within 1-2 minutes. No evident sings of cell injury or hurt have been observed afterward. Plasmid DNA, purposely added to culture fluid, easily slips in the cytoplasm; utilizing such approach, thereafter indicated as "optoporation', we have successfully transfected two genes, namely beta-galactosidase and chloramphenicol-acetyl-transferase in murine NIH3T3 fibroblasts. Therefore optoporation represents an additional procedure for gene transfer with several advantages over already available methods: (1) it only takes advantage of the presence of phenol-red, a normal cell medium component, with no need of addition of extraneous substances; (2) it is a very mild treatment virtually suitable for any cell type and (3) it allows transfection of selected cells even in the presence of cells of different type (providing that they are morphologically distinguishable).

Induction and repair of DNA damage in UV-irradiated human lymphocytes. Spectral differences and repair kinetics

The alkaline elution assay has been employed to study the induction and repair kinetics of DNA damage in human lymphocytes after irradiation with biologically relevant doses of UVB (297 and 302 nm) or UVA (365 nm) radiation. At 365 nm, when the predominant lesions are single-strand breaks, the rate of lesion induction was 1.5 x 10(-3) per 10(8) Da per kJ m-2. The number of breaks decayed with a half-life of about 50 min after a dose of 20 kJ m-2. In the UVB region, cyclobutyl pyrimidine dimers and 6-4 photoproducts are formed, both of which are repairable via the nucleotide excision repair pathway. By using repair inhibitors, the rate of induction of such lesions at 297 and 302 nm was found to be 0.07 per 10(8) Da per J m-2. Lesions were removed with a half-life of about 100 min. Mathematical modelling of the excision repair process revealed a time-dependent polymerization-ligation rate: after an initial lag phase the polymerization-ligation rate increased, reaching 50% of its maximum rate at 80-100 min after the start of repair incubation. This course of development might be due to a damage-associated regulation of DNA precursors synthesis.

Variation in radiosensitivity due to cell age and split-dose recovery in polykaryons induced by cytochalasin

We have investigated the properties of an in vitro cell survival assay that uses as its endpoint the ability to form polyploid cells (polykaryons) in the presence of cytochalasin B (CB). The criterion for survival is that a polykaryon-forming unit (PFU) must reach the arbitrary DNA content of at least 16C. The age-dependence of PFU sensitivity to 137Cs irradiation was determined using V79-379A cells synchronized at mitosis. Cells assayed as PFUs demonstrated much less variation in radiosensitivity with age than did clonogens, but the changes in curve shape were qualitatively similar. In both assays mitotic cells yielded an exponential survival curve while that obtained at 5 h (mid-late S) had a marked quadratic component. Owing to the small overall variation in PFU survival with age, at doses greater than about 25 Gy the surviving fraction at 5 h was lower than in mitosis. In both V79-379A and HeLa S3 cells, PFUs demonstrated a capacity for split-dose recovery and yielded recovery ratios at 2.6 at 50 Gy in V79 and 1.5 at 20 Gy in HeLa. Since these ratios were much lower than in clonogens at the same dose, we suggest that this is consistent with an association that we have previously demonstrated between PFU response and the clonogenic initial slope. In an attempt to clarify the DNA lesions to which PFUs may be sensitive, we determined PFU response following exposure to 254-nm UV irradiation. In contrast with ionizing radiation, PFU response to UV was very similar to that of clonogens. This suggests that following UV exposure the absence of cytokinesis in polykaryons may confer less protection than in the case of ionizing radiation, possibly due to fundamental differences in the spectrum of DNA lesions produced.

DNA repair synthesis following irradiation with 254-nm and 312-nm ultraviolet light is not diminished in fibroblasts from patients with dysplastic nevus syndrome

The DNA excision repair capacity of 23 primary fibroblast lines from patients with dysplastic nevus syndrome was investigated and DNA repair synthesis ("unscheduled DNA synthesis") was determined after UV exposure. Seventeen fibroblast lines from normal donors served as controls. The dose/response experiments included up to ten dose levels and two wavelength ranges: UV-C (using a low-pressure mercury lamp emitting predominantly 254-nm light) and UV-B (artificial "sunlamp" radiation centering around 312-nm light). For each dose level, silver grains over fibroblast nuclei were counted by visual inspection. Twelve cell lines were also evaluated for both UV wavelength ranges using a new semi-automatic image analyzing system. This system included components for rapid sequential identification of both fibroblast nuclei and silver grains sited above them. Silver grains over 100 nuclei were determined for each UV dose level. Dose/response curves were established and analyzed by linear regression. As a quantitative term for assessing DNA excision repair capacity of a cell line we calculated the linear increase (G0) in the number of grains per nucleus, when the UV dose was multiplied by the factor e (i.e. 2.72). The sensitivity of grain detection and resolution of overlapping grains was approximately threefold better in visual than in automatic counting, especially when there were more than 70 grains over nuclei. The time required for visual counting, however, was tenfold that of automatic counting. The variance-weighted mean G0v.w of all fibroblast lines from patients with dysplastic nevus syndrome was found to be 79.1 (+/- 1.8- grains/nucleus, that of fibroblast lines from normal donors was 74.2 (+/- 1.7) grains/nucleus. This difference revealed a slightly better repair capability for cell lines from patients but was at the borderline of detection and, therefore, should not be overinterpreted. From the experimental accuracy achieved by determination of the variance-weighted means of the two groups, we would have been able to detect a difference of 7 and more grains [> 2 x (sigma normal+sigma patients)]. The variance-weighted mean G0v.w of all fibroblast lines from patients with dysplastic nevus syndrome was found to be 76.4 (+/- 1.4) grains/nucleus, whereas that of fibroblast lines from normal donors was only 66.6 (+/- 1.8) grains/nucleus. This difference was statistically significant and, contrary to expectation, revealed better, not worse post-UV DNA repair capability in cell lines from patients that in those from normal donors.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cloning of the human gene SUVCC2 associated with mutagenesis following the induction of non-dimer DNA damages by solar UV radiation

A mutant cell line, DRP 512, sensitive to the induction of non-dimer DNA damages produced by solar UV radiation was derived from ICR 2A frog cells. In addition, the DRP 512 cells exhibited an abnormally high level of ouabain-resistant mutants after exposure to solar UV. A level of 1.1. mutants per 10(6) survivors per kJ m-2 was measured for ICR 2A whereas the yield was 4.2 mutants per 10(6) survivors per kJ m-2 for the solar-UV-sensitive cell line. The DRP 512 cells were transfected with human DNA and a secondary transformant obtained in which normal solar UV sensitivity and mutation induction were restored. DNA from this secondary transformant was used to construct a genomic DNA library from which a recombinant phage was isolated containing the human gene capable of restoring normal solar UV sensitivity and mutation induction to DRP 512. This gene has been designated SUVCC2.

A multitherapy resistance factor from melanoma reveals that killing by near UV is different from genotoxic agents

A diffusible multitherapy resistance factor (MTRF) is produced by Cloudman S91 melanoma cells in vitro. The MTRF decreases sensitivity of the target cell line, S91/amel, to gamma-irradiation, UVC (200-280 nm) and mitomycin C (MMC). In the present study, we demonstrate that MTRF also increases the survival of S91/amel after exposure to actinomycin D (AMD) and vinblastine (VBL). The MTRF is thus effective when target cells have been exposed to five genotoxic agents that act by different mechanisms. It does not alter the response to the same five agents of the S91/I3 producer cells, which are presumably saturated with the factor. The factor has no effect on the survival of S91/amel cells that have been exposed to lethal doses of near monochromatic UVB (280-320 nm) or UVA (320-400 nm) or to polychromatic FS20 lamps. The lack of effectiveness of MTRF after cells have been exposed to near (300-400 nm) UV radiation indicates that in this wavelength range, S91 melanoma cells are killed by mechanisms that are different from the lethal effects of the five genotoxic agents (gamma-irradiation, UVC, MMC, AMD and VBL) to which the target cells demonstrate a response.

Induction of slowly developing alkali-labile sites in human P3 cell DNA by UVA and blue- and green-light photons: action spectrum

Action spectra (365-520 nm) for the formation of DNA single-strand breaks (SSB) and slowly developing alkali-labile sites (SDALS) in human teratocarcinoma P3 cells in culture were determined. Induction of SDALS results from the absorption of blue- and green-light photons. The spectrum has a broad peak that is maximal between 400 nm to 500 nm and declines sharply above and below these wavelength regions. Negligible yields of SDALS were produced by photons at wavelengths of 365 nm or shorter and at 520 nm or longer, whereas for SSB, the action ioffeases with shorter wavelength throughout the whole spectral range studied. The configuration of the SDALS action spectrum suggests that the primary chromophore, and therefore possibly the photosensitizer, is a mixture of porphyrin and flavin residues.

Correlation of UVC and UVB cytotoxicity with the induction of specific photoproducts in T-lymphocytes and fibroblasts from normal human donors

By using specific monoclonal antibodies in situ and a computer-assisted image analysis system we have determined the relative induction of cyclobutane dimers, (6-4) photoproducts and Dewar isomers in human mononuclear cells and fibroblasts following irradiation with UVC, broad-spectrum UVB and narrow-spectrum UVB. The lamps produced these lesions in different proportions, with broad-spectrum UVB inducing a greater combined yield of (6-4) photoproducts and Dewar isomers per cyclobutane dimer than UVC or narrow-spectrum UVB. The relative induction ratios of (6-4) photoproducts compared to cyclobutane dimers were 0.15, 0.21 and 0.10 following irradiation with UVC, broad- or narrow-spectrum UVB, respectively. Although Dewar isomers were induced by UVC, their relative rate of formation compared to cyclobutane dimers was significantly greater after irradiation with either broad-spectrum or narrow-spectrum UVB. These values were 0.001, 0.07 and 0.07, respectively. With each lamp source, we have determined the survival of normal human T-lymphocytes and fibroblasts at fluences, which induce equivalent yields of cyclobutane dimers, (6-4) photoproducts or (6-4) photoproducts plus Dewar isomers. Killing of fibroblasts appears to be associated with (6-4) photoproduct formation, whereas killing of T-lymphocytes seems to be mediated by combined (6-4) plus Dewar yields. These results emphasize the need to study the biological effects of UVB because cellular responses may be different from those following UVC irradiation.

Use of a monoclonal antibody to detect DNA damage caused by the anticancer drug cis-diamminedichloroplatinum (II) in vivo and in vitro

A monoclonal antibody, MAb62-5, was prepared and used to detect DNA damage due to the anticancer drug cis-diamminedichloroplatinum (II) (or cisplatin). ELISA competition indicated that the binding of MAb62-5 to cisplatin-DNA was competitively inhibited (50% control) by 210 nM of cisplatin bound to DNA, cisplatin/nucleotide (D/N) = 0.2. Using a DNA mobility shift assay, MAb62-5 binding activity was inhibited by 50% by approximately 50-fold molar excess of cisplatin-DNA adducts (D/N = 0.08), whereas there was less than 5% inhibition by UV-DNA adducts or mock-treated DNA. In addition, MAb62-5 showed a similar affinity to the cisplatin-DNA adducts as compared to an endogenous cisplatin-damaged DNA recognition protein. Using ELISA with this antibody, we have demonstrated a 2-fold enhancement in excision repair of cisplatin-DNA adducts in resistant HeLa cells. This is supported by the measurement of repair-associated DNA strand breaks using alkaline elution and host cell reactivation of transfected plasmid DNA carrying cisplatin damage. These findings also provide explanation for the complexicity of immunoassay in cells.

Enhanced excision repair of DNA damage due to cis-diamminedichloroplatinum(II) in resistant cervix carcinoma HeLa cells

We have previously reported a cisplatin-resistant HeLa cell line which exhibits overproduction of nuclear proteins preferential for cisplatin-modified DNA (Chao et al., Cancer Res. 51:601-605, 1991; Biochem. J. 277: 875-878, 1991). In this study, excision repair of cisplatin-DNA adducts in a resistant and a revertant cell lines was investigated using in situ detection of cisplatin-DNA adducts by an immunoassay and the measurement of repair-associated DNA strand breaks by a sensitive alkaline elution method. The resistant cells exhibited a 2-fold decrease in the accumulation of cisplatin-DNA adducts; whereas, the revertant cells showed a similar level of cisplatin-DNA adducts as the parental cells in the parallel experiment. Immediately following cisplatin treatment, the resistant and the revertant cells accumulated respectively approximately 50% and 90% cisplatin-DNA adducts of the parental cells. However, the kinetic patterns of repair rate following peak accumulation of cisplatin-DNA adducts (which took approximately 4 h) was the same in the three cell lines. This finding was supported by the measurement of repair-associated DNA strand breaks using alkaline elution which showed 1.6- and 1.5-fold increase in the resistant and the revertant cells respectively. In addition, following transfection with plasmid DNA carrying cisplatin damage, the resistant and the revertant cells displayed a 2.4- and 1.4-fold enhancement in host cell reactivation, respectively. Furthermore, the acquired resistance in HeLa cells was partially reversed by nontoxic aphidicolin, a DNA polymerase-alpha and DNA repair inhibitor. The results strongly suggest the improved excision repair of cisplatin-DNA adducts as a mechanism of phenotypic resistance of cells to cisplatin.

Are enzymatically produced single-strand breaks involved in UV-induced inactivation of plasmid DNA?

pBR322 plasmid DNA was exposed to 254 nm UV radiation and examined for enzymatically produced single-strand break (sbb) and double-strand break (dsb) formation by treatment with an extract containing the proteins of Escherichia coli (AB1157 (uvrA+ recA+) and AB2480 (uvrA- recA-)). Enzymatic conversion of the 254 nm-induced lesions into ssbs on treatment with an extract from AB1157 was observed, but not conversion into dsbs. The rate of enzymatic ssb formation in the AB1157 extract is initially higher than in the AB2480 extract, the sbb formation levels off leading to plateau values with increasing incubation time. The rate of ssb formation in the AB2480 extract is initially lower, but does not level off, and the ssb yield becomes larger at longer incubation times than that with the AB1157 extract. The biological inactivation of the plasmids was measured as a function of 254 nm fluence by transformation of E. coli AB1157 and AB2480. Inactivation with AB2480 is mainly due to a single photoproduct, a cyclobutane-type pyrimidine dimer, per DNA molecule. Inactivation with AB1157 occurs with a quantum yield which is virtually identical with that of the plateau values of enzymatic ssb formation, as measured by incubation in the AB1157 extract. A possible interpretation is that the formation of irreparable ssbs is the lethal step in the sequence of events leading to inactivation of plasmid DNA in the repair wild-type strain. The quantum yield of inactivation is 10-20 times smaller for transformation of AB1157 than for AB2480, indicating that enzymatic repair of photolesions of the plasmid occurs in AB1157.

Apparent alterations in the early stage of excision repair of UV-induced DNA damages in a HeLa mutant cell line that is resistant to genotoxic stresses

We have previously reported a cisplatin-resistant HeLa cell line featuring a cross-resistance to genotoxic stresses including ultraviolet (UV) radiation and an enhancement of plasmid reactivation. In this study, excision repair of UV-DNA adducts in this resistant cell line was investigated. Using alkaline elution analysis, this resistant cell line showed a 2-fold enhancement in damage incision-associated DNA strand breaks. Using a gel mobility shift assay, the resistant cells exhibited a 3-fold increase in nuclear proteins which specifically recognize UV-damaged DNA. However, the rate of repair synthesis in the resistant cells appeared to be the same as in their parental counterparts. Thus, recognition and incision activities, the early stage of excision repair, are altered in the resistant cells. The results suggest that phenotypic cross-resistance of this cell line to UV is probably due to an improved excision repair of UV-DNA adducts which is defective in xeroderma pigmentosum group A cells. The results are consistent with the conclusion that the early stage, including recognition and incision, of excision repair is critical in determining cellular sensitivity or resistance to DNA damage.

Photosensitized formation of 8-hydroxydeoxyguanosine in cellular DNA by riboflavin

Photosensitized formation of 8-hydroxydeoxyguanosine (oh8dG) in DNA by riboflavin has recently been shown in vitro. The present study describes the formation of oh8dG in cellular DNA by photo-irradiation of cultured mammalian cells in the presence of riboflavin. Formation of oh8dG was dependent on the concentration of riboflavin as well as the duration of photoirradiation. These results suggest that photosensitized formation of oh8dG in DNA by riboflavin may be involved in photocarcinogenesis.

Photochemistry of nucleic acids in cells

A survey of the recent aspects of the main photoreactions induced by far-UV radiation in cellular DNA is reported. This mostly includes the formation of cyclobutadipyrimidines, pyrimidine(6-4)pyrimidone photoadducts and related Dewar valence isomers in various eukaryotic and prokaryotic cells, as monitored by using either specific or more general assays. Information is also provided on mechanistic aspects regarding the formation of 5,6-dihydro-5-(alpha-thyminyl) thymine, the so-called "spore photoproduct" within far-UV-irradiated bacterial spores. The second major topic of the review deals with the effects of near-UV radiation and visible light on cellular DNA which are mostly mediated by photosensitizers. The main photoreactions of furocoumarins with DNA, one major class of photosensitizers used in the phototherapy of skin diseases, involve a [2 + 2] cycloaddition to the thymine bases according to an oxygen-independent mechanism. In contrast a second type of photosensitized reaction which appears to play a major role in the genotoxic effects of both near-UV and visible light requires the presence of oxygen. The photodynamic effects which are mediated by either still unidentified endogenous photosensitizers or defined exogenous photosensitizers lead to the formation of a wide spectrum of DNA modifications including base damage, oligonucleotide strand breaks and DNA-protein cross-links.

Effect of vitamin E on cytotoxicity, DNA single strand breaks, chromosomal aberrations, and mutation in Chinese hamster V-79 cells exposed to ultraviolet-B light

The effect of pretreatment with vitamin E on cytotoxicity, DNA single strand breaks, and chromosomal aberrations as well as on mutation induced by ultraviolet-B light (UV-B) was investigated in Chinese hamster V-79 cells. Cellular pretreatment with non-toxic levels of 25 microM alpha-tocopherol succinate (vitamin E) for 24 h prior to exposure resulted in a 10-fold increase in cellular levels of alpha-tocopherol. Using a colony-forming assay, this pretreatment decreased the cytotoxicity of UV-B light. However, alkaline elution assays demonstrated that pretreatment with vitamin E did not affect the number of DNA single strand breaks caused by UV-B light. In addition, UV-B exposure produced a dose-dependent induction of chromosomal aberrations and mutations at the HGPRT locus, and neither of these actions of UV-B was influenced by pretreatment with the vitamin. These results suggest that vitamin E protects cells from UV-B-induced cytotoxicity, possibly through its ability to scavenge free radicals. The results also suggest that the extent of genotoxicity induced by UV-B light may not correlate directly with the cytotoxic action of this wavelength region in sunlight.

Skin photosensitizing agents and the role of reactive oxygen species in photoaging

In this paper, the role of reactive oxygen species in photoaging is presented. Many photosensitizing agents are known to generate reactive oxygen species (singlet oxygen (1O2), superoxide anion (O2.-) and .OH radicals). Although photoaging (dermatoheliosis) of human skin is caused by UVB and UVA radiation, the hypothesis tested here in the pathogenesis of photoaging of human skin is the free radical theory involving the generation of reactive oxygen species by UVA (320-400 nm) radiation and their damaging oxidative effects on cutaneous collagen and other model proteins. The UVA-generated reactive oxygen species cause cross-linking of proteins (e.g. collagen), oxidation of sulfydryl groups causing disulfide cross-links, oxidative inactivation of certain enzymes causing functional impairment of cells (fibroblasts, keratinocytes, melanocytes, Langerhans cells) and liberation of proteases, collagenase and elastase. The skin-damaging effects of UVA appear to result from type II, oxygen-mediated photodynamic reactions in which UVA or near-UV radiation in the presence of certain photosensitizing chromophores (e.g., riboflavin, porphyrins, nicotinamide adenine dinucleotide phosphate (NADPH), etc.) leads to the formation of reactive oxygen species (1O2, O2.-, .OH). Four specific observations are presented to illustrate the concept: (1) the production of 1O2 and O2.- by UVB, UVA and UVA plus photosensitizing agents (such as riboflavin, porphyrin and 3-carbethoxypsoralens) as a function of UV exposure dose, the sensitizer concentration and the pH of the irradiated solution; (2) the formation of protein cross-links in collagen, catalase and superoxide dismutase by 1O2 and O2.- (.OH) and the resulting denaturation of proteins and enzyme activities as a function of UVA exposure dose; (3) the protective role of selective quenchers of 1O2 and O2.- (e.g. alpha-tocopherol acetate, beta-carotene, sodium azide, ascorbic acid, etc.) against the photoinactivation of enzymes and the prevention of the protein cross-linking reaction; (4) the possible usefulness of certain antioxidants or quenchers that interact with the UVA-induced generation of reactive oxygen species in the amelioration of the process of photoaging.

Effects of ultraviolet B irradiation on cutaneous leishmaniasis

Protection against many infectious diseases is mediated by cellular immunity in the competent host. Ultraviolet (UV) radiation, a component of sunlight, is a potent suppressor of cell-mediated immune responses. Suzanne Holmes Giannini discusses the possible relevance of ambient levels of UVB to pathogenesis and immunity in infectious diseases, with special reference to cutaneous leishmaniasis.

Non-dimer DNA damage in Chinese hamster V-79 cells exposed to ultraviolet-B light

To understand and characterize non-dimer DNA damage and cytotoxicity induced by ultraviolet-B light (UV-B, 290-320 nm), an alkaline elution technique for analysis of DNA damage was used on Chinese hamster V-79 cells. Ultraviolet-B exposure produced a dose-dependent induction of DNA single strand breaks and DNA-protein crosslinks; however, there was an absence of DNA-DNA interstrand crosslinks. Neither of these types of DNA damage were repaired within a a 24 h incubation of the cells following a single UV-B exposure; rather the damage increased. Using a colony forming assay, we found that UV-B exposure resulted in an increase of cytotoxicity in a dose-dependent fashion. In addition, UV-B exposure inhibited DNA and RNA synthesis. The role of non-dimer DNA damage in the cytotoxicity induced by UV-B is discussed.

Changes in cell cycle distribution of V79 Chinese hamster fibroblasts after irradiation at different wavelengths

Changes in cell cycle distribution of V79 Chinese hamster fibroblasts were investigated at different wavelengths between 254 and 313 nm. The fluences applied led to surviving fractions of 0.61. In all cases, the S fraction was temporarily increased within 8-12 h after irradiation, whereas the G1 fraction was decreased. The maximum deviations from the initial values did not significantly depend on the wavelength.

Ultraviolet light induces double-strand breaks in DNA of cultured human P3 cells as measured by neutral filter elution

Neutral filter elution at pH 7.2 and 9.6 was used to measure the induction of DNA lesions in human P3 teratocarcinoma cells by monochromatic 254-, 270-, 313-, 334-, 365-, and 405-nm radiation and by 60 gamma rays. In this assay DNA double-strand breaks (dsb) increase the rate of elution of DNA from cell lysates on a filter. Yields of dsb as measured by this procedure were determined by using a calibration of the assay that correlates elution parameters with number of dsb caused by disintegration of 125I incorporated into the DNA. Analysis of fluence responses obtained by using the calibrated assay indicated that the number of dsb induced per dalton of DNA as measured by this assay is proportional to the square of the fluence at all the energies of radiation studied, implying that the induction of these lesions may be a two-hit event. Analysis of the relative efficiencies for the induction of dsb by ultraviolet radiation, corrected for quantum efficiency, revealed a spectrum that coincided closely with that for the induction of single-strand breaks (ssb) in the same cells, having a close fit with the spectrum of nucleic acid in the UVC and UVB region below 313 nm, and a shoulder in the UVA region. It was calculated, however, that there may be too few ssb for dsb to result from randomly distributed closely opposed ssb.

Photochemistry of DNA using 193 nm excimer laser radiation

Photoproducts in double-stranded DNA induced by 193 nm radiation have been investigated. Double-stranded, supercoiled pBR322 DNA in buffered aqueous solution was exposed to varying fluences of 193 nm radiation from an ArF excimer laser. The quantum yields for formation of cyclobutylpyrimidine dimers, frank strand breaks and alkali labile sites were calculated from the conversion of supercoiled (Form I) DNA to relaxed (Form II) DNA after treatment with Micrococcus luteus dimer-specific endonuclease, no treatment, or treatment with alkali and heat, respectively. The quantum yields were 1.65 (+/- 0.03) X 10(-3) for pyrimidine dimers, 9.4 (+/- 3.2) X 10(-5) for frank strand breaks and 9.6 (+/- 3.6) X 10(-5) for alkali labile sites. The quantum yields for pyrimidine dimers and strand breaks and alkali labile sites were not affected by 10 nM mannitol. The relative quantum yields for these DNA photoproducts induced by 193 nm radiation differed markedly from those produced by 254 nm radiation.

[Cellular effects of the ultraviolet components of sunlight]

The discussion on the possible increase of solar UV on earth due to the destruction of the stratospheric ozone has led to a renewed interest in the action of ultraviolet radiation on biological systems. The paper deals with changes at the cellular level stressing particularly molecular alterations in deoxyribonucleic acid, the carrier of genetic information. The most important repair processes by which lesions are removed or bypassed are described. It is also discussed whether the effectivity of a complex spectrum can be predicted on the basis of measurements with monochromatic radiation. Furthermore, possible consequences on human health are outlined which may be derived from cellular studies.

Effects of UV radiation of cells

UV radiation interacts with mammalian cells in a very complex manner, although DNA appears to be the main chromophore. Recent literature within this field is reviewed. The review is concentrated on the following main topics: Chromophores for UV action, photoproducts in DNA, repair of UV-induced DNA damage, wavelength interactions, inactivation, mutagenesis, transformation and protection mechanisms against UV damage.

Convex curvatures of alkaline elution profiles of DNA from human cells irradiated with 405 nm UVA: evidence for induction of slowly developing alkali-labile sites

The alkaline (pH 12.1) elution profiles of DNA from human P3 cells exposed to monochromatic 405 nm UVA radiation deviate from exponential: on a logarithmic plot of eluted fraction of DNA vs time of elution, the rate of elution accelerates for the first 6 h. Following this period, the profiles become exponential. In contrast, the elution profiles of DNA after 520 nm green light or ionizing radiation exposures (x- and gamma rays, and fission spectrum neutrons) are always strictly exponential, evidence that the convex profiles were not due to an artifact caused by elution technique. Holding the DNA at pH 12.1 for 6 h after 405-nm exposures before initiating elution resulted in profiles that were close to exponential, with slopes similar to the final slopes observed following the 6-h elution period in the original experiments. This is evidence that some DNA breaks develop slowly during the first 6 h of elution, as a result of exposure to alkali. Therefore, the DNA lesions induced by 405-nm light as measured by the alkaline elution technique are apparently heterogeneous and include a major class of alkali-labile sites that develop slowly during incubation at pH 12.1. Convex profiles also occur following exposure of the cells to visible light at 434 and 512 nm.

Inhibition and recovery of semiconservative DNA synthesis in normal and solar UV sensitive ICR 2A frog cell lines following the induction of non-dimer DNA damage by sunlamp UV greater than 315 nm

Cultures of the solar UV-sensitive cell lines, DRP 36 and DRP 153, and of the parental ICR 2A cell line, were exposed to 150 kJ/m2 of sunlamp UV greater than 315 nm plus photoreactivating light. This treatment resulted in the induction primarily of non-dimer DNA damage. Following either a 0, 3, 6, 12 or 24 h incubation, the cultures were pulse-labelled with [3H]thymidine, and the synthesis of different size classes of replicon intermediates measured using the alkaline step elution assay. For all three cell lines tested, an immediate depression of low molecular weight DNA synthesis was observed. This was followed by an inhibition of all size classes of replicon intermediates. Within 12 h following irradiation, recovery of DNA synthesis was observed, which was generally most apparent for low molecular weight DNA. The ICR 2A cells exhibited a nearly full recovery in all size classes of DNA synthesized by 24 h. However, a much smaller recovery of DNA synthesis was detected for the DRP 36 and DRP 153 cultures. This continued inhibition was primarily in the synthesis of full replicon size DNA, and was most pronounced for the DRP 36 cells. Hence, it appears that replicon chain elongation continues to be inhibited in these solar UV-sensitive cell lines long after irradiation.

DNA-protein crosslinking in normal and solar UV-sensitive ICR 2A frog cell lines exposed to solar UV-radiation

DNA-protein crosslinks (DPC) were measured following exposure to the solar UV wavelengths produced by a fluorescent sunlamp in ICR 2A frog cells and two solar UV-sensitive mutants derived from this cell line. Approx. 5-7 DPC per 10(10) dalton were induced in these cells by either 150 kJ/m2 of sunlamp UV greater than 315 nm plus photoreactivating light (PRL) or 10 kJ/m2 of sunlamp UV greater than 295 nm. The irradiated cells were then incubated for 0-24 h and the level of DPC measured using alkaline elution. It was found for the ICR 2A cells exposed to sunlamp UV greater than 315 nm that the level of DPC increased about 3-fold during a 2-h postirradiation incubation and then decreased. The mutant cell lines also showed an enhancement in the level of DPC following irradiation, although it was much less pronounced and the levels decreased much more rapidly. In a similar fashion, the level of DPC increased in ICR 2A cells exposed to sunlamp UV greater than 295 nm with more than a 5-fold enhancement after a 4-h incubation. Once again, the mutant cell lines showed an increase in the level of DPC that was smaller and more transient than the effect in the ICR 2A cells. These results suggests that this enhancement in DPC may be indicative of a process that plays a role in cellular survival following solar UV-irradiation.

Effect of age on endogenous DNA single-strand breakage, strand break induction and repair in the adult housefly, Musca domestica

It has been suggested that genomic alterations involving DNA damage and the ability to repair such damage play an important role in cellular senescence. In this study, endogenous DNA single-strand breaks, the susceptibility of DNA to induced strand breakage and the capacity to repair these breaks were compared in postmitotic cells from young (3-day-old) and old (23-day-old) houseflies. DNA single-strand breaks did not accumulate during normal aging in the housefly. However, cells of the old flies exhibited a greater sensitivity to single-strand breakage induced by gamma-radiation and UV light. The capacity to repair these exogenously induced single-strand breaks declined with age. Results do not support the view that DNA single-strand breaks are a causal factor in aging in the housefly. An age-related increase in the susceptibility to undergo single-strand breakage suggests alterations in chromatin during the aging process.