Different (direct and indirect) mechanisms for the induction of DNA-protein crosslinks in human cells by far- and near-ultraviolet radiations (290 and 405 nm)

An overview of the bacterial microbiome of public transportation systems-risks, detection, and countermeasures

When we humans travel, our microorganisms come along. These can be harmless but also pathogenic, and are spread by touching surfaces or breathing aerosols in the passenger cabins. As the pandemic with SARS-CoV-2 has shown, those environments display a risk for infection transmission. For a risk reduction, countermeasures such as wearing face masks and distancing were applied in many places, yet had a significant social impact. Nevertheless, the next pandemic will come and additional countermeasures that contribute to the risk reduction are needed to keep commuters safe and reduce the spread of microorganisms and pathogens, but also have as little impact as possible on the daily lives of commuters. This review describes the bacterial microbiome of subways around the world, which is mainly characterized by human-associated genera. We emphasize on healthcare-associated ESKAPE pathogens within public transport, introduce state-of-the art methods to detect common microbes and potential pathogens such as LAMP and next-generation sequencing. Further, we describe and discuss possible countermeasures that could be deployed in public transportation systems, as antimicrobial surfaces or air sterilization using plasma. Commuting in public transport can harbor risks of infection. Improving the safety of travelers can be achieved by effective detection methods, microbial reduction systems, but importantly by hand hygiene and common-sense hygiene guidelines.

UV-induced DNA Damage in Skin is Reduced by CaSR Inhibition

The epidermis maintains a cellular calcium gradient that supports keratinocyte differentiation from its basal layers (low) to outer layers (high) leading to the development of the stratum corneum, which resists penetration of UV radiation. The calcium‐sensing receptor (CaSR) expressed in keratinocytes responds to the calcium gradient with signals that promote differentiation. In this study, we investigated whether the CaSR is involved more directly in protection from UV damage in studies of human keratinocytes in primary culture and in mouse skin studied in vivo. siRNA‐directed reductions in CaSR protein levels in human keratinocytes significantly reduced UV‐induced direct cyclobutane pyrimidine dimers (CPD) by ~80% and oxidative DNA damage (8‐OHdG) by ~65% compared with control transfected cells. Similarly, in untransfected cells, the CaSR negative modulator, NPS‐2143 (500 nm), reduced UV‐induced CPD and 8‐OHdG by ~70%. NPS‐2143 also enhanced DNA repair and reduced reactive oxygen species (ROS) by ~35% in UV‐exposed keratinocytes, consistent with reduced DNA damage after UV exposure. Topical application of NPS‐2143 also protected hairless Skh:hr1 mice from UV‐induced CPD, oxidative DNA damage and inflammation, similar to the reductions observed in response to the well‐known photoprotection agent 1,25(OH)₂D₃ (calcitriol). Thus, negative modulators of the CaSR offer a new approach to reducing UV‐induced skin damage.

ROS-Scavenging Therapeutic Hydrogels for Modulation of the Inflammatory Response

Although reactive oxygen species (ROS) are essential for cellular processes, excessive ROS could be a major cause of various inflammatory diseases because of the oxidation of proteins, DNA, and membrane lipids. It has recently been suggested that the amount of ROS could thus be regulated to treat such physiological disorders. A ROS-scavenging hydrogel is a promising candidate for therapeutic applications because of its high biocompatibility, 3D matrix, and ability to be modified. Approaches to conferring antioxidant properties to normal hydrogels include embedding ROS-scavenging catalytic nanoparticles, modifying hydrogel polymer chains with ROS-adsorbing organic moieties, and incorporating ROS-labile linkers in polymer backbones. Such therapeutic hydrogels can be used for wound healing, cardiovascular diseases, bone repair, ocular diseases, and neurodegenerative disorders. ROS-scavenging hydrogels could eliminate oxidative stress, accelerate the regeneration process, and show synergetic effects with other drugs or therapeutic molecules. In this review, the mechanisms by which ROS are generated and scavenged in the body are outlined, and the effects of high levels of ROS and the resulting oxidative stress on inflammatory diseases are described. Next, the mechanism of ROS scavenging by hydrogels is explained depending on the ROS-scavenging agents embedded within the hydrogel. Lastly, the recent achievements in the development of ROS-scavenging hydrogels to treat various inflammation-associated diseases are presented.

Establishment and Characterization of a Unilateral UV-Induced Photoreceptor Degeneration Model in the C57Bl/6J Mouse

Purpose

To investigate whether UV irradiation of the mouse eye can induce photoreceptor degeneration, producing a phenotype reminiscent of the rd10 mouse, left eyes of female C57Bl/6J mice were irradiated with a UV LED array (370 nm). A lens was placed between the cornea and LED, allowing illumination of about one-third of the retina. The short-term and long-term effects on the retina were evaluated.

Methods

First, a dose escalation study, in which corneal dosages between 2.8 and 9.3 J/cm² were tested, was performed. A dosage of 7.5 J/cm² was chosen for the following characterization study. Before and after irradiation slit-lamp examinations, full-field electroretinography, spectral domain optical coherence tomography and macroscopy were performed. After different time spans (5 days to 12 weeks) the animals were sacrificed and the retinae used for immunohistochemistry or multielectrode array testing. Right eyes served as untreated controls.

Results

In treated eyes, spectral domain optical coherence tomography revealed a decrease in retinal thickness to 53%. Full-field electroretinography responses decreased significantly from day 5 on in treated eyes. Multielectrode array recordings revealed oscillatory potentials with a mean frequency of 5.2 ± 0.6 Hz in the illuminated area. Structural changes in the retina were observed in immunohistochemical staining.

Conclusions

UV irradiation proved to be efficient in inducing photoreceptor degeneration in the mouse retina, while leaving the other retinal layers largely intact. The irradiated area of treated eyes can be identified easily in spectral domain optical coherence tomography and in explanted retinae.

Translational Relevance

This study provides information on anatomic and functional changes in UV-treated retina, enabling the use of this model for retinitis pigmentosa-like diseases in animals suited for experimental retinal surgery.

Cutaneous malignant melanoma incidences analyzed worldwide by sex, age, and skin type over personal Ultraviolet-B dose shows no role for sunburn but implies one for Vitamin D3

Because the incidence of cutaneous malignant melanoma (CMM) was reported to increase with increasing terrestrial UVR (290-400 nm) doses in the US back in 1975 and a recent publication showed no association exists with UVR exposure at all, we set out to fully elucidate the role of UVR in CMM. To achieve this goal, we analyzed the CMM incidences over latitude and estimated the average personal UVR dose in the US and numerous countries (> 50) on 5 continents around the world. Using data from the International Agency for Research on Cancer in 2005, we performed worldwide analysis of CMM over UVR dose by sex, age group (0-14, 15-29, 30-49, 50-69, 70-85+) and Fitzpatrick skin types I-VI. Surprisingly, increasing UVR doses, which represent erythemally-weighted doses comprised primarily of UVB (290-315 nm) radiation, did not significantly correlate with increasing CMM incidence for people with any skin type anywhere in the world. Paradoxically, we found significant correlations between increasing CMM and decreasing UVB dose in Europeans with skin types I-IV. Both Europeans and Americans in some age groups have significant increasing CMM incidences with decreasing UVB dose, which shows UVB is not the main driver in CMM and suggests a possible role for lower cutaneous vitamin D₃ levels and UVA (315-400 nm) radiation. CMM may be initiated or promoted by UVA radiation because people are exposed to it indoors through windows and outdoors through some sunscreen formulations. Thus, our findings may explain why some broad-spectrum sunscreen formulations do not protect against getting CMM.

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.

Plant self-defense mechanisms against oxidative injury and protection of the forest by planting trees of triploids and tetraploids

The depletion of the ozone layer, and the resulting substantial increase in incident ultraviolet (UV) irradiation and subsequent oxygen radical formation on the Earth, have caused an extensive variety of damage to the world's forests. Superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px), which scavenge harmful oxygen radicals and inhibit lipid peroxides, were examined in two types of Japanese cedars, black pines, and cypresses, namely those with leaves showing premature withering, shedding, or dying and those with leaves not showing these effects prematurely. The effect of homogenates from these trees on lipid peroxide formation in a reaction system which UV light induces was also studied. The results indicate that strong black pines have significantly higher SOD activities than ordinary black pines, the leaves of which prematurely wither or die. Remarkably, trees that had triploid or tetraploid chromosomes showed higher SOD levels than diploid trees and markedly inhibited lipid peroxide formation since the SOD gene resides on a chromosome. This was especially true of plus trees of Japanese cypress, some of which had five times higher SOD activities than common Japanese cypresses although GSH-Px appears to play less of a role in this regard. Rice leaves and osmunda which are resistant to UV damage showed markedly higher SOD and GSH-Px activity. Our experiments suggest that the trees that have high SOD can protect themselves by scavenging oxygen radicals induced by UV irradiation and inhibit harmful lipid peroxide formation. In order to protect forests from oxidative damage by UV light, we should plant trees of natural mutants and artificially crossed triploids and tetraploids.

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.

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.

Influence of UVA pre-exposure on UVB-induced erythema. A chromometric study

We examined the influence of ultraviolet A (UVA) pre-exposure on UVB minimal erythemal dose in 9 Caucasian subjects. Three zones were tested. One zone received only UVB, the second zone received a low UVA dose+UVB, and the third zone received a high UVA dose+UVB. Each zone was divided into 9 circles receiving increasing doses of UVB in order to obtain 3 different UVA-exposed series of 9 circles. Visual and chromometric readings were performed 24 h later. Pre-exposure to UVA caused variations in the slope of the dose-response curve (colorimetric index as a function of the UVB dose). In relation to UVB erythema, these variations indicated a protective effect for 6/9 subjects and an aggressive effect for 3/9 subjects. No predictive criteria were found for inclusion within a group.

Photochemical reactions and phototoxicity of sterols: novel self-perpetuating mechanisms for lipid photooxidation

Sterols are important lipid components that may contribute to phototoxicity. We have found that phototoxic response in earthworms is related to sterols extractable with lipophilic solvents. The photochemically active compounds in worm lipids are 5,7,9(11),22-ergostatetraen-3 beta-ol (9-DHE) and 5,7,9(11)-cholestartien-3 beta-ol (9-DDHC), respectively. Human skin lipids are known to contain 9-DHE. We have also found 9-DDHC in human skin, which is reported here for the first time. In the presence of an excess of the corresponding 5,7-dienes (ergosterol of 7-dehydrocholesterol), these photoactive sterols constitute a self-regenerating source of singlet molecular oxygen (1O2) during irradiation in vivo or in vitro with UVA (315-400 nm). The quantum yield for photosensitization of 1O2 by 9-DHE was estimated to be 0.09. The 1O2 is scavenged by the dienes and the rate constant for 1O2 quenching by ergosterol was found to be 1.2 x 10(7) M-1 s-1 in methyl t-butyl ether (MTBE). This scavenging ultimately leads to the production of 5,8-endoperoxide and hydrogen peroxide. Photochemically induced superoxide radical was also produced on irradiation of sterol 5,7,9-trienes and trapped with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The production of singlet oxygen, peroxides and radicals by the sterols may be significant in the cell damaging and tumor promoting action of UVA light on skin.

Induction of 8-oxo-7,8-dihydro-2'-deoxyguanosine by ultraviolet radiation in calf thymus DNA and HeLa cells

The levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) in purified calf thymus DNA and HeLa cells were measured following exposure to either UVC, UVB or UVA wavelengths. This DNA damage was quantitated using HPLC coupled with an electrochemical detector. The 8-oxodGuo was induced in purified DNA in a linear dose-dependent fashion by each portion of the UV spectrum at yields of 100, 0.46 and 0.16 8-oxodGuo per 10(5) 2'-deoxyguanosine (dGuo) per kJ/m2 for UVC, UVB and UVA, respectively. However, the amount of 8-oxodGuo in HeLa cells irradiated with these UV sources decreased to approximately 2.0, 0.013 and 0.0034 8-oxodGuo per 10(5) dGuo per kJ/m2, respectively. In contrast, the levels of cyclobutyl pyrimidine dimers were similar in both irradiated DNA and cells. Therefore, 8-oxodGuo is induced in cells exposed to wavelengths throughout the UV spectrum although it appears that protective precesses exist within cells that reduce the UV-induced formation of this oxidative DNA damage. Cell survival was also measured and the number of dimers or 8-oxodGuo per genome per lethal event determined. These calculations are consistent with the conclusion that dimers play a major role in cell lethality for UVC- or UVB-irradiated cells but only a minor role in cells exposed to UVA wavelengths. In addition, it was found that the relative yield of 8-oxodGuo to dimers increased nearly 1000-fold in both UVA-irradiated cells and DNA compared with cells subjected to either UVC or UVB. These results are supportive of the hypothesis that 8-oxodGuo, and possible other forms of oxidative damage, play an important role in the induction of biological effects caused by wavelengths in the UVA portion of the solar spectrum.

Identification of possible reactive oxygen species involved in ultraviolet radiation-induced oxidative DNA damage

We have previously demonstrated that each region of the ultraviolet (UV) spectrum (UVA, UVB, and UVC) induces the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) in purified calf thymus DNA and HeLa cells in a fluence-dependent manner. In the present study, we further characterize the possible reactive oxygen species (ROS) that are involved in the induction of 8-oxodGuo by UV radiation. Sodium azide, a singlet oxygen (1O2) scavenger though its quenching effect on HO. was also reported, inhibited 8-oxodGuo production in calf thymus DNA exposed to UVA, UVB, or UVC in a concentration-dependent fashion with maximal quenching effect of over 90% at a concentration of 10 mM. Catalase, at a concentration of 50 U/ml, reduced the yields of UVA- and UVB-induced 8-oxodGuo formation by approximately 50%, but had little effect on UVC-induced 8-oxodGuo production. In contrast, 50 U/ml of superoxide dismutase (SOD) did not affect induction of 8-oxodGuo by any portion of the UV spectrum. Hydroxyl radical (HO.) scavengers mannitol and dimethylsulfoxide (DMSO) moderately reduced the levels of 8-oxodGuo induced by UVA and UVB, but not those by UVC. Instead, mannitol and DMSO enhanced the formation of 8-oxodGuo induced by UVC. These results suggest that certain types of ROS are involved in UV-induced 8-oxodGuo formation with 1O2 playing the predominant role throughout the UV spectrum. Except for UVC, other ROS such as hydrogen peroxide (H2O2) and HO. may also be involved in UVA- and UVB-induced oxidative DNA damage. Superoxide anion appears not to participate in UV-induced oxidation of guanosine in calf thymus DNA, as SOD did not display any quenching effects.

UV-induced cutaneous photobiology

Ultraviolet radiation (UVR) present in sunlight is a major environmental factor capable of affecting human health and well being. The organ primarily affected by UVR is the skin, which is composed of a variety of different cell types. Here, UVR is needed for production of active vitamin D as well as producing undesirable effects such as sunburn, premature cutaneous photoaging, and promoting skin cancer development. Depending on the radiation dose, UVR influences virtually every cutaneous cell type investigated differently. Since the end of the nineteenth century, sun exposure has been known to induce skin cancer, which is now the human malignancy with the most rapidly increasing incidence. In several experimental models, mid-range UVR has been demonstrated to be the major cause of UV-induced cutaneous tumors. The stratospheric ozone layer protecting the terrestrial surface from higher quantum energy solar radiation is being damaged by industrial activities resulting in the possibility of increased UVR exposure in the future. Investigations in the field of experimental dermatology have shown that within the skin an immunosurveillance system exists that may be able to detect incipient neoplasms and to elicit a host responses against it. This article reviews the literature on studies designed to investigate the effects of UVR on cutaneous cellular components, with special focus on the immune system within the skin and the development of UV-induced cancer.

Wavelength dependence of laser-induced DNA damage in lymphocytes observed by single-cell gel electrophoresis

Human lymphocytes from a cell culture were irradiated with laser pulses of an Xe-Cl excimer laser (308 nm) or a dye laser at wavelengths from 312 to 640 nm. After conversion of photoinduced DNA damage into DNA strand breaks the "comet assay" (single-cell gel electrophoresis) was used to determine the number of photons required to induce detectable DNA damage. Between 308 and 450 nm the number of photons necessary for DNA damage increases about 10,000 times from 2.14 x 10(8) to 2.85 x 10(12) photons. Between 308 and 318 nm the number of photons necessary to induce detectable DNA damage is proportional to e0.45 lambda. Between 340 and 450 nm it is proportional to e0.05 lambda. No detectable damage was induced at 540 and 640 nm after irradiation with 9900 kJ m-2. Two-photon effects are unlikely to play a significant role.

Indoor tanning: risks, benefits, and future trends

The indoor tanning industry is a $1 billion-a-year business in the United States, and use of these high-intensity UVA tanning devices continues to grow. Many members of the medical community are concerned about the safety of these largely unregulated devices, but tanning salon operators state such concerns are unfounded and unproved. A comprehensive review of current knowledge of the operation and effects of indoor tanning is presented. The effects of UV radiation (especially UVA) on experimental animals and human beings are examined. Both acute and chronic effects are delineated from experimental sources as well as from epidemiologic studies and clinical observations. Existing safety regulations are described. Information on operator knowledge and compliance with existing safety regulations is reviewed.

Ultraviolet A-induced lipid peroxidation and antioxidant defense systems in cultured human skin fibroblasts

Cultured human skin fibroblasts from healthy donors were irradiated with 180 kJ.m-2 ultraviolet (UV) A (320-400 nm) and assayed for thiobarbituric acid-reactive substances (TBARS), taken as an indicator of lipid peroxidation. Antioxidant defenses, including total glutathione (GSH) levels, superoxide dismutase (SOD), glutathione peroxidase (GSHPx), and catalase (Cat) activities were simultaneously assayed before and after irradiation. For the various donors, with different activities of these antioxidant systems before irradiation, TBARS correlated positively with SOD activity and negatively with Cat activity, whereas no correlation with GSH level or GSHPx activity was found. These data support the view that O2- is generated by UVA irradiation. They also suggest that H2O2, arising from O2- dismutation by SOD is not completely removed by Cat. Thus, the sensitivity of human fibroblasts to UVA-induced lipid peroxidation depends on a balance between SOD and Cat activities. After UVA irradiation, Cat activity was strongly inhibited, whereas GSH level was slightly decreased. By contrast, GSHPx and SOD activity remained unchanged after UVA irradiation.

Genomic damage and its repair in young and aging brain

A brief review of the available information concerning age-related genomic (DNA) damage and its repair, with special reference to brain tissue, is presented. The usefulness of examining the validity of DNA-damage and repair hypothesis of aging in a postmitotic cell like neuron is emphasized. The limited number of reports that exist on brain seem to overwhelmingly support the accumulation of DNA damage with age. However, results regarding the age-dependent decline in DNA-repair capacity are conflicting and divided. The possible reasons for these discrepancies are discussed in light of the gathering evidence, including some human genetic disorders, to indicate how complex is the DNA-repair system in higher animals. It is suggested that assessment of repair potential of neurons with respect to a specific damage in a specific gene might yield more definitive answers about the DNA-repair process and its role in aging.

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.

The function of melanin or six blind people examine an elephant

The pigment melanin is found in all living kingdoms and in many different structures and forms. When its various functions are examined separately, its behaviors seem disparate and conflicting. It has a clear role in camouflage and sexual display. Other major roles are examined critically. It can act as a sun screen but is not a very effective one. It can also scavenge active chemical species, but this, too, is not done very effectively. It produces active radicals that can damage DNA. It binds to drugs in ways that are either beneficial or deleterious. Aside from camouflage, its other roles can be brought together by a unifying hypothesis as first proposed by Proctor and McGinness nearly 20 years ago. Melanin is envisaged as an energy transducer with the properties of an amorphous semiconductor. It can absorb many different types of energy and dissipate them in the form of heat. However, if the energy input is too great, the output can be expressed in the form of activated chemical species that can damage cellular macromolecules resulting in cell death, mutations and cancer. The protective aspect of melanin in dark skin is seen as resulting from its high concentration and its confinement to ellipsoidal and densely packed organelles that can effectively shield the nucleus. In light skin, its radical nature is seen as potentially participating in the carcinogenic process, particularly when overwhelmed by intense episodes of sunburn.

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.

Enhanced expression of protein kinase C gene caused by solar radiation

Natural solar radiation (5 min of midday exposure in mid July, latitude 42 degrees N) induces protein kinase C mRNA almost two-fold in human epithelioid P3 cells in culture. This response is the same as that following tumor promotion by chemicals. The result indicates a possible role of promotion by solar UV radiation.

Genotoxicity of singlet oxygen

Singlet oxygen, 1O2 (1 delta g), fulfills essential prerequisites for a genotoxic substance, like hydroxyl radicals and other oxygen radicals: it can react efficiently with DNA and it can be generated inside cells, e.g. by photosensitization and enzymatic oxidation. As might be anticipated from the non-radical character of singlet oxygen, the pattern of DNA modifications it produces is very different from that caused by hydroxyl radicals. While hydroxyl radicals produce DNA strand breaks and sites of base loss (AP sites) in high yield and react with all four bases of DNA, singlet oxygen generates predominantly modified guanine residues and few strand breaks and AP sites. There is now convincing evidence that a major product of base modification caused by singlet oxygen is 8-hydroxyguanine (7,8-dihydro-8-oxoguanine). Indeed, the recently reported miscoding properties of 8-hydroxyguanine can explain the predominant type of mutations observed when DNA modified by singlet oxygen is replicated in cells. There are also strong indications that singlet oxygen generated by photosensitization can act as an ultimate DNA modifying species inside cells. However, indirect genotoxic mechanisms involving other reactive oxygen species produced from singlet oxygen are also possible and appear to predominate in some cases. The cellular defense system against oxidants consists of effective singlet oxygen scavengers such as carotenoids. The observation that carotenoids can inhibit neoplastic cell transformation when administered not only together with but also after the application of chemical or physical carcinogens might indicate a role of singlet oxygen in tumor promotion that could be independent of the direct or indirect DNA damaging properties.

Comparison of initial yields of DNA-to-protein crosslinks and single-strand breaks induced in cultured human cells by far- and near-ultraviolet light, blue light and X-rays

The initial yields of DNA-to-protein crosslinks (dpc) caused by ionizing and nonionizing radiations were compared, with emphasis upon values within the biological dose ranges (D0). Induction of dpc in cold (0-0.5 degrees C) human P3 teratocarcinoma cells was measured by using alkaline elution techniques after exposure to monochromatic UVC (254 nm), UVB (313 nm), UVA (365 and 405 nm), and blue light (434 nm). UVC and UVB light induced detectable numbers (about 100 dpc per cell per D0). Monochromatic UVA radiations produced yields about 8 times higher than UVC or UVB (for 365 nm, about 1500 dpc per cell per D0) Similar results at low doses were obtained for measurements of single-strand breaks induced by the different radiations. The action spectra for dpc were closely similar. The biological significance of these relatively high numbers of DNA lesions caused by environmental nonionizing radiation that readily penetrates into human skin is not understood.

Does exposure of human skin in situ to 385 or 405 nm UV induce pyrimidine dimers in DNA?

A previous report [Freeman et al. (1986) Photochem. Photobiol. 43S, 93S] indicated that irradiation of human skin in situ with 385 or 405 nm radiation produced detectable levels of pyrimidine dimers in DNA. Since these wavelengths are absorbed poorly by DNA, these results suggested that DNA damage was sensitized by other absorbing molecules present in skin. Examination of two experimental aspects of the previous work indicates that (1) the static gel electrophoresis method for DNA dispersion used in lesion determination gave accurate values of the levels of induced dimers, and (2) the DNA damage apparently induced by 385 nm was actually induced by shorter wavelength UV present in the 20 nm bandpass beam of the monochromator. The current results indicate that monochromatic 385 and 405 nm radiation are ineffective in dimer production in human skin in situ.

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.

New trends in photobiology. The interaction of UVA radiation with cultured cells

Recent work concerning the interaction of UVA radiation (320-380 nm) with cultured cells is reviewed with particular emphasis on the involvement of cellular oxidative stress in the biological effects of this radiation on eucaryotic cells. Possible chromophores are considered and their role in generation of various oxidant species including hydrogen peroxide, superoxide anion, singlet oxygen and hydroxyl radical. DNA and membranes are discussed as possible targets for the lethal action of long wavelength radiation. Four mechanisms of cellular defence are proposed: (1) DNA repair; (2) antioxidant enzymes; (3) endogenous free radical quenchers; (4) inducible protection.

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.

Induction of DNA-protein crosslinks in melanotic cloudman S91 mouse melanoma cells and EMT6 mouse mammary carcinoma cells by monochromatic 254 and 405 nm light

The rates of induction of DNA-protein crosslinks (DPC) by monochromatic radiation at 254 and 405 nm were compared in melanotic S91 mouse melanoma cells and EMT6 mouse mammary carcinoma cells. At 254 nm, the rates of induction of DPC are the same in the two cell lines, whereas, at 405 nm, the rate of induction of DPC in the melanotic cells is considerably less than that in the nonmelanotic cells. Since the major difference in the two cell lines with respect to absorption is melanin, the latter finding implies that intracellular melanin can protect against this DNA damage caused by a component of environmental carcinogenic solar radiation.

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.

Induction of sister-chromatid exchanges by direct and indirect agents in a human teratoma cell line

We have extended the characterization of the P3 cell line, derived from a human epithelial teratocarcinoma, by studying the induction of sister-chromatid exchanges (SCEs) by direct and indirect carcinogens. Several direct acting carcinogens produce a dose-dependent increase in SCEs. Most notably, N-methyl-N'-nitro-N-nitrosoguanidine and 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene produce increases in SCEs at doses comparable to those used to induce mutations at the hypoxanthine-guanine phosphoribosyl transferase locus. The indirect carcinogens elicit SCEs only when the P3 cells are cocultured with cells capable of metabolizing the indirect carcinogens to the active form. Human breast carcinoma (BJ-015) and rat hepatoma (RL-12) cells are equally efficient in activating polycyclic aromatic hydrocarbons to the active form. This cell-mediated induction of SCEs is obtained when the P3 cells are incubated with live, X-irradiated, or UV-irradiated BJ or RL cells. This P3 cell line is thus equally suitable to study the induction of mutations or the induction of SCEs with direct and indirect carcinogens.

Photosensitized DNA breaks and DNA-to-protein crosslinks induced in human cells by antitumor agent gilvocarcin V

The antitumor agent gilvocarcin V (GV) is photoactivated to a genotoxic form by low fluences of near-ultraviolet radiation. Activation of GV by monochromatic 450-nm radiation causes two specific DNA changes in human P3 cells in culture as shown by alkaline elution techniques: single-strand breaks (i.e., alkali-labile sites plus frank strand scissions) and DNA-to-protein covalent bond crosslinks. When GV is present with the cells during irradiation, the yields of these damages are increased. Fluence and concentration studies show that the induction of both DNA lesions occurs at unusually low concentrations of drug and fluences of radiation. Both breaks and crosslinks are readily detectable after exposure to less than 100 kJ m-2 of 405 nm-radiation at a GV concentration of 7.5 X 10(-9) M. These results indicate a possible potential for use of GV in human tumor photochemotherapy.