Production of pyrimidine dimers in DNA of human skin exposed in situ to UVA radiation

Comparison of DNA damage responses following equimutagenic doses of UVA and UVB: a less effective cell cycle arrest with UVA may render UVA-induced pyrimidine dimers more mutagenic than UVB-induced ones

Mechanisms of UVA-mutagenesis remain a matter of debate. Earlier described higher rates of mutation formation per pyrimidine dimer with UVA than with UVB and other evidence suggested that a non-pyrimidine dimer-type of DNA damage contributes more to UVA- than to UVB-mutagenesis. However, more recently published data on the spectra of UVA-induced mutations in primary human skin cells and in mice suggest that pyrimidine dimers are the most common type of DNA damage-inducing mutations not only with UVB, but also with UVA. As this rebuts a prominent role of non-dimer type of DNA damage in UVA-mutagenesis, we hypothesized that the higher mutation rate at UVA-induced pyrimidine dimers, as compared to UVB-induced ones, is caused by differences in the way UVA- and UVB-exposed cells process DNA damage. Therefore, we here compared cell cycle regulation, DNA repair, and apoptosis in primary human fibroblasts following UVB- and UVA-irradiation, using the same physiologic and roughly equimutagenic doses (100-300 J m(-2) UVB, 100-300 kJ m(-2) UVA) we have used previously for mutagenesis experiments with the same type of cells. ELISAs for the detection of pyrimidine dimers confirmed that much fewer dimers were formed with these doses of UVA, as compared to UVB. We found that cell cycle arrests (intra-S, G1/S, G2/M), mediated at least in part by activation of p53 and p95, are much more prominent and long-lasting with UVB than with UVA. In contrast, no prominent differences were found between UVA and UVB for other anti-mutagenic cellular responses (DNA repair, apoptosis). Our data suggest that less effective anti-mutagenic cellular responses, in particular different and shorter-lived cell cycle arrests, render pyrimidine dimers induced by UVA more mutagenic than pyrimidine dimers induced by UVB.

Reduction of 8-oxodGTP in the nucleotide pool by hMTH1 leads to reduction in mutations in the human lymphoblastoid cell line TK6 exposed to UVA

UVA has been suggested to play an important role in UV-induced mutagenesis. The mechanisms by which UVA induces mutations are still a matter of debate. Our aim was to investigate the protective capacity of hMTH1, a nucleotide pool sanitization enzyme with 8-oxodGTPase activity. Human B lymphoblastoid cells were stably transfected with shRNA directed against hMTH1. Clonogenic survival, mutations, intracellular and extracellular levels of 8-oxodG (8-oxo-7, 8-dihydro-2'-deoxyguanosine) and dG in the nucleotide pool of UVA-irradiated transfected and non-transfected cells were investigated. Mutations were determined in the thymidine kinase locus. Intracellular 8-oxodG and dG were measured using a modified ELISA and HPLC, respectively, after extraction of the nucleotide pool and conversion of nucleotides to their corresponding nucleosides. 8-oxodG in the medium was measured using ELISA. UVA-induced mutations were significantly higher while the survival was slightly lower in transfected compared to non-transfected cells. The increased mutation rate in transfected cells at increased exposure correlated with enhanced levels of 8-oxodG in the nucleotide pool, and a somewhat reduced level of 8-oxodG in the medium. The results indicate that the nucleotide pool is a significant target for UVA-induced mutations and implicates that hMTH1 plays an important role in protecting cells from UVA-induced oxidative stress.

No formation of DNA double-strand breaks and no activation of recombination repair with UVA

Longwave UVA is an independent class I carcinogen. A complete understanding of UVA-induced DNA damage and how this damage is processed in skin cells is therefore of utmost importance. A particular question that has remained contentious is whether UVA induces DNA double-strand breaks (DSBs), either directly or through processing of other types of DNA damage, such as recombination repair of replication forks stalled at DNA photoproducts. We therefore studied activation of the recombination repair pathway by solar available doses of UVA and assessed formation of DNA DSBs in primary skin fibroblasts. We found that, unlike ionizing radiation or UVB, UVA does not activate the Fanconi anemia/BRCA DNA damage response pathway or the "recombinase" RAD51 in primary skin fibroblasts. The fact that this pathway mediates recombination repair of DNA DSBs suggests that DNA DSBs are not formed by UVA. This is further supported by findings that UVA did not induce DNA DSBs, as assayed by neutral single-cell electrophoresis or by formation of γ-H2AX nuclear foci, considered the most sensitive assay for DNA DSBs. The lack of sufficient evidence for formation of DNA DSBs underlines the pivotal role of UVA-induced DNA photoproducts in UVA mutagenesis and carcinogenesis.

Quantitation of ultraviolet-induced single-strand breaks using oligonucleotide chip

A simple, accurate and robust methodology was established for the direct quantification of ultraviolet (UV)-induced single-strand break (SSB) using oligonucleotide chip. Oligonucleotide chips were fabricated by covalently anchoring the fluorescent-labeled ssDNAs onto silicon dioxide chip surfaces. Assuming that the possibility of more than one UV-induced SSB to be generated in a small oligonucleotide is extremely low, SSB formation was investigated quantifying the endpoint probe density by fluorescence measurement upon UV irradiation. The SSB yields obtained based on the highly sensitive laser-induced fluorometric determination of fluorophore-labeled oligonucleotides were found to coincide well with that predicted from a theoretical extrapolation of the results obtained for plasmid DNAs using conventional agarose gel electrophoresis. The developed method has the potential to serve as a high throughput, sample-thrifty, and time saving tool to realize more realistic, and direct quantification of radiation and chemical-induced strand breaks. It will be especially useful for determining the frequency of SSBs or lesions convertible to SSBs by specific cleaving reagents or enzymes.

Mechanisms of mutation formation with long-wave ultraviolet light (UVA)

Long-wave ultraviolet (UV) A light is able to damage DNA, to cause mutations, and to induce skin cancer, but the exact mechanisms of UVA-induced mutation formation remain a matter of debate. While pyrimidine dimers are well established to mediate mutation formation with shortwave UVB, other types of DNA damage, such as oxidative base damage, have long been thought to be the premutagenic lesions for UVA mutagenesis. However, pyrimidine dimers can also be generated by UVA, and there are several lines of evidence that these are the most important premutagenic lesions not only for UVB- but also for UVA-induced mutation formation. C-->T transition mutations, which are generated by pyrimidine dimers, are called UV-signature mutations. They cannot be interpreted to be solely UVB-induced, as they are induced by UVA as well. Furthermore, there is no consistent evidence for a separate UVA-signature mutation that is only generated with UVA. We hypothesize that a weaker anti-mutagenic cellular response, but not a different type of DNA damage, may be responsible for a higher mutation rate per DNA photoproduct with UVA, as compared with UVB.

Low-intensity ultraviolet A irradiation of the lens capsule to remove lens epithelial cells during cataract surgery

PURPOSE

To assess the effectiveness and safety of low-intensity ultraviolet A (UVA) irradiation in removing lens epithelial cells (LECs) during cataract surgery and compare them with those of mechanical polishing and no treatment.

SETTING

Eyecove Ophthalmology Clinics, Pune, India.

METHODS

This prospective randomized double-masked study consisted of preoperative screening of 36 patients, of which 30 met the inclusion criteria and were recruited. The patients had routine cataract surgery. A bean-shaped capsulorhexis was performed. After the nucleus and cortex were removed, the capsular bag was irradiated from inside with low-intensity UVA in 1 group. A second group had mechanical polishing, and a third group was not treated. A small flap of the anterior capsule was removed in each patient. The flap was stained and mounted in a Fuchs-Rosenthal chamber. For estimation of effectiveness, the area of capsule covered with epithelial cells was estimated by examination under a light microscope. One day postoperatively, an examination was performed to assess the safety of each technique.

RESULTS

The area of the capsule from which the LECs were removed was significantly larger in the UVA-irradiation group than in the mechanical-polishing group (P = .001) and the no-treatment group (P = .001). There was no significant difference between the mechanical-polishing and no-treatment groups (P>.05). The area of the capsule flap that was covered with LECs was significantly less in the UVA-irradiation group than in the mechanical-polishing group (P = .017) and the no-treatment group (P = .001). The mechanical-polishing group and no-treatment group were not significantly different from each other (P>.05). Corneal edema was significantly less in the UVA-irradiation group than in the mechanical-polishing group (P<.001) and no-treatment group (P = .012). No patient in the UVA-irradiation group had postoperative lid edema; 8 patients in each of the other 2 groups had lid edema. The difference was statistically significant (P<.0001). Pupil size was significantly larger in the UVA-irradiation group than in the mechanical-polishing group and no-treatment group; the difference was significant (both P = .0001). There was no significant difference in pupil size in the mechanical-polishing group and no-treatment group. No significant difference was observed between the 3 groups in visual acuity, conjunctival edema, anterior chamber flare, and intraocular pressure.

CONCLUSION

Ultraviolet A irradiation of the capsular bag was effective and safe in removing LECs from the anterior capsule during cataract surgery.

Short- and long-wave UV light (UVB and UVA) induce similar mutations in human skin cells

While the mutagenic and carcinogenic properties of longwave UV light (UVA) are well established, mechanisms of UVA mutagenesis remain a matter of debate. To elucidate the mechanisms of mutation formation with UVA in human skin, we determined the spectra of UVA- and UVB-induced mutations in primary human fibroblasts. As with UVB, we found the majority of mutations to be C-to-T transitions also with UVA. For both UVA and UVB, these transitions were found within runs of pyrimidines, at identical hotspots, and with the same predilection for the nontranscribed strand. They also included CC-to-TT tandem mutations. Therefore, these mutations point to a major role of pyrimidine dimers not only in UVB but also in UVA mutagenesis. While some differences were noted, the similarity between the spectra of UVA- and UVB-induced mutations further supports similar mechanisms of mutation formation. A non-dimer type of DNA damage does not appear to play a major role in either UVA or UVB mutagenesis. Therefore, the previously reported increasing mutagenicity per dimer with increasing wavelengths cannot be due to non-dimer DNA damage. Differences in the cellular response to UVA and UVB, such as the less prominent activation of p53 by UVA, might determine a different mutagenic outcome of UVA- and UVB-induced dimers.

The photocarcinogenesis of antibiotic lomefloxacin and UVA radiation is enhanced in xeroderma pigmentosum group A gene-deficient mice

Lomefloxacin (LFLX) is phototoxic and phototumorigenic, but the mechanisms of phototumorigenesis of quinolone drugs have not been fully elucidated. Formation of cyclobutane pyrimidine dimers (CPD) by UVB radiation is primarily involved in the carcinogenesis of ultraviolet (UV) radiation. On the other hand, UVA region is responsible to photobiologic reactions of quinolone drugs. To know if CPD can be formed by UVA radiation in the presence of LFLX and is involved in the phototumorigenesis, we used xeroderma pigmentosum (XP) group A gene-deficient (XPA-/-) mouse, which is defective in nucleotide excision repair. XPA-/- and XPA+/+ mice were irradiated to 5 J per cm2-UVA with or without the administration of LFLX. In XPA-/- mice treated with LFLX, the first skin tumor appeared after exposures to 75 J per cm2 in 5 wk. In XPA+/+ mice treated with LFLX, the first tumor appeared after exposures to 345 J per cm2 in 23 wk. Immunohistochemically, CPD formation was observed after UVA-exposure in the skin of XPA+/+ as well as XPA-/- mice which had been given LFLX. The CPD disappeared, however, earlier from XPA+/+ mice than from XPA-/- mice. The acute inflammatory reaction after LFLX administration and exposure to UVA were greatly enhanced in XPA-/- mice. These results indicate that UVA exposure induces DNA damage in the form of CPD in the presence of LFLX, which exerts phototoxicity and phototumorigenesis.

Iatrogenic skin cancer: induction by psoralen/ultraviolet A and immunosuppression of organ transplant recipients

Photochemotherapy (psoralen/UVA (PUVA)) is an efficient therapeutic tool for a wide range of skin diseases. Concern, however, exists regarding the long-term carcinogenic effects of this treatment modality and, as a consequence, is being used less frequently. PUVA remains an important treatment in our therapeutic armamentarium but must be used with caution in those patients with risk factors and cumulative dose exposure must be limited. PUVA-induced cancers show features in common with skin cancers induced by immunosuppressed organ transplant recipients. Tumours in the latter group of individuals are, however, much more aggressive and difficult to manage.

Immunochemical detection of UV-induced DNA damage and repair

The application of an antiserum to ultraviolet radiation (UVR)-damaged DNA is presented. A novel experimental system was employed to ascertain the limits of detection for this antiserum. Using a DNA standard containing a known amount of dimer, the limits of detection were found to be 0.9 fmol of dimer. This was compared to a limit of 20-50 fmol dimer using gas chromatography-mass spectrometry (GC-MS). Induction of thymine dimers in DNA following UVR exposure, as assessed using this antiserum in an enzyme-linked immunosorbent assay (ELISA), was compared with GC-MS measurements. The ELISA method successfully demonstrated the induction of lesions in DNA irradiated either with UVC or UVB, although despite high sensitivity, no discernible binding was seen to UVA-irradiated DNA. The antiserum was also shown to be applicable to immunocytochemistry, localising damage in the nuclei of UVR exposed keratinocytes in culture. The ability of the antiserum to detect DNA damage in skin biopsies of individuals exposed to sub-erythemal doses of UVR was also demonstrated. Moreover, the subsequent removal of this damage, as evidenced by a reduction in antiserum staining, was noted in sections of biopsies taken in the hours following irradiation.

Photoprotection by sunscreens

Ultraviolet (UV) radiation is an etiologic factor for the development of nonmelanoma skin cancers and also possibly melanoma. The American Academy of Dermatology currently recommends the daily use of sunscreens with a sun protection factor (SPF) of 15 or above. The SPF numerical rating is an in vivo, standardized measure of sunscreen efficacy. SPF assessment predominantly reflects the ability of a product to screen UVB radiation. The physical and biologic properties of UVA radiation are relevant as UVA contributes to photoimmunosuppression and photocarcinogenesis. An ideal sunscreen assessment system would address both UVB and UVA protection. However, the SPF rating should continue to serve as the major determinant of sunscreen efficacy.

G2 phase cell cycle arrest in human skin following UV irradiation

The contribution of the short wavelength ultraviolet (UV) component of sunlight to the aetiology of skin cancer has been widely acknowledged, although its direct contribution to tumour initiation or progression is still poorly understood. The loss of normal cell cycle controls, particularly checkpoint controls, are a common feature of cancer. UV radiation causes both G1 and G2 phase checkpoint arrest in vitro cultured cells. In this study we have investigated the cell cycle responses to suberythemal doses of UV on skin. We have utilized short-term whole organ skin cultures, and multi parameter immunohistochemical and biochemical analysis to demonstrate that basal and suprabasal layer melanocytes and keratinocytes undergo a G2 phase cell cycle arrest for up to 48 h following irradiation. The arrest is associated with increased p16 expression but no apparent p53 involvement. This type of organ culture provides a very useful model system, combining the ease of in vitro manipulation with the ability to perform detailed molecular analysis in a normal tissue environment.

UVAI-induced edema and pyrimidine dimers in murine skin

The induction of edema and pyrimidine dimers in epidermal DNA was determined in the skin of SKH:HR1 mice exposed to graded doses of ultraviolet radiation AI (UVAI; 340-400 nm). Exposure to UVAI induced 1.6 +/- 0.08 x 10(-6) (mean +/- standard error of mean) pyrimidine dimers per 10(8) Da of DNA per J/m2. Edema in irradiated animals was determined as an increase in skinfold thickness. A dose of 1.8 x 10(6) J/m2 of UVAI that resulted in a 50% increase in skinfold thickness (SFT50%) would have induced 1.0 x 10(5) dimers per basal cell genome. A similar increase in SFT induced by full spectrum solar ultraviolet radiation (290-400 nm) would accompany the induction of 11.0 x 10(5) pyrimidine dimers per basal cell genome. These results support a hypothesis that UVAI-induced pathological changes of the skin are mediated through the formation of nondimer photoproducts.

The effect of suntan parlor exposure on delayed and contact hypersensitivity

Cutaneous and systemic immune function are believed to play an important role in cutaneous carcinogenesis. We therefore sought to determine whether the suntan parlor radiation sources commonly used in the United States cause measurable qualitative suppression of immune function and quantitative alterations in circulating T cell subpopulations. Subjects (n = 22) were recruited and randomly assigned to receive suntan parlor exposures (10 full-body UV exposures over a 2 week period, shielding only the right flexural arm) or no exposure. Baseline circulating T lymphocyte subpopulations (T helper lymphocyte, CD4; T suppressor/cytotoxic lymphocyte, CD8) were measured. Two weeks later (upon completion of UV exposures for those in this group), circulating T cell subpopulations were measured and dinitrochlorobenzene (DNCB) sensitization (in the UV group, on the UV-exposed buttock) was performed. Subsequent DNCB elicitation was performed in a bilateral fashion (in the UV group, on the right UV-shielded and the left UV-exposed upper arm). We found that subjects in the UV group demonstrated localized suppression of contact hypersensitivity sensitization and elicitation and also an increase in circulating CD8 cells when compared to the control group (P < or = 0.05). We conclude that suntan parlor exposures, as typically received in this country, suppress contact hypersensitivity and increase the circulating T suppressor/cytotoxic cell number quantity.

Influence of spectral and angular sensitivity on the readout of biological dosimeters

Biological systems used as biological dosimeters can possess different angular sensitivities from the detectors usually used in physical devices. A simple experimental setup has been developed and used to measure the angular sensitivity of uracil thin-layer biological dosimeters. Results of angular sensitivity measurements for uracil thin-layer dosimeters are presented using a Xe arc lamp as the UV source. According to the experiments described here, uracil thin-layer dosimeters show a cosine-type angular dependence. In several indoor experiments broadband UV meters are used to control the applied dose rate from a given artificial UV source. The experimental setup has been designed and used to verify experimentally the importance of spectral and angular sensitivity differences of biological and physical UV meters applied in biological experiments. Model calculations for two different irradiation systems, using different geometrical arrangements of artificial UV sources, are also presented. For these arrangements relative dose rates that could be measured with dosimeters of arbitrary spectral, but different angular sensitivity have been calculated.

The 0.8% ultraviolet B content of an ultraviolet A sunlamp induces 75% of cyclobutane pyrimidine dimers in human keratinocytes in vitro

Tanning lamps, emitting predominantly ultraviolet (UV) A, are used widely throughout the U.K. and other countries, but little is known about the long-term risks associated with their use, especially with respect to skin cancer. We have exposed normal human epidermal keratinocytes to a commercial tanning lamp and used the comet assay in association with DNA repair enzymes T4 endonuclease V and endonuclease III to investigate the relative yields of directly formed cyclobutane pyrimidine dimers (CPDs) and indirectly formed types of oxidative DNA damage. To put the risk of using tanning lamps into perspective, the sunbed used in this study (five Philips Performance 80W-R UVA tubes at a distance of 35 cm) was found to be approximately 0.7 times as potent at inducing CPDs as U.K. natural sunlight around noon on a fine summer day. This compares with a relative risk for CPD induction and erythema of 0.8 and 0.7 times, respectively, calculated from the relevant action spectra of tanning lamps and British noontime sunlight. To determine the relative contribution of UVB and UVA to the induction of CPDs and oxidative DNA damage, we modified the spectral output of the tanning lamps with a series of Schott WG UVB filters. The induction of CPDs was more dependent on the UVB component of the sunbed than oxidative types of damage. Schott WG UVB filters with 50% transmission at 305 nm reduced the yield of T4 endonuclease V sites by 42% while there was only a 17% decrease in the yield of endonuclease III sites. CPD induction was not completely abolished after irradiation through WG335 and WG345 nm filters despite there being no detectable UVB. From these data, it was estimated that, although the tanning lamps emitted only 0.8% of their total output in the UVB range, these wavelengths were responsible for the induction of over 75% of CPDs and 50% of the oxidative damage to DNA.

Human melanocytes and keratinocytes exposed to UVB or UVA in vivo show comparable levels of thymine dimers

Epidemiology shows a relationship between solar exposure and all types of skin cancer. Understanding the mechanisms of skin cancer requires knowledge of the photomolecular events that occur within the relevant epidermal cell types in vivo. Studies to date have focused on UVR-induced DNA lesions in keratinocytes, the majority epidermal cell population which gives rise to most skin cancers. Malignant melanoma, arising from melanocytes (5%-10% of epidermal cells), accounts for most skin cancer deaths. We report on new techniques to detect DNA photolesions in human epidermal melanocytes in situ. Previously nonexposed buttock skin of volunteers of skin types I/II was exposed to clinically relevant doses of narrow bandwidth UVB (300 nm) and UVA (320 nm, 340 nm, 360 nm) radiation. Biopsies were taken immediately afterwards and processed for routine histology. Microscope sections were prepared and double-stained with fluorescent-tagged monoclonal antibodies for thymine dimers and melanocytes. UVR dose-response curves for dimer levels within melanocyte nuclei were determined by image analysis and compared with dimer levels in adjacent basal cell keratinocytes. Our data show that UVB and UVA readily induce thymine dimers in melanocytes at levels that are comparable with those found in adjacent keratinocytes. This new technique will enable melanocyte specific studies, such as DNA repair kinetics, to be done in vivo.

Glutathione response after UVA irradiation in mitotic and postmitotic human skin fibroblasts and keratinocytes

Since Hayflick's pioneering work in the early sixties, human diploid fibroblasts have become a widely accepted in vitro model system. Recently, Bayreuther and co-workers extended this experimental approach showing that fibroblasts in culture resemble, in their design, the hemopoietic stem-cell differentiation system. They found that the chemical agent mitomycin C accelerates the differentiation pathway from mitotic to postmitotic fibroblasts. We measured the response of endogenous glutathione levels after UVA irradiation (320-400 nm) in mitotic and mitomycin C-induced postmitotic human skin fibroblasts and foreskin-derived keratinocytes. The initial levels in mitotic foreskin derived human fibroblasts were 14.4 nmol glutathione per mg protein, whereas a 30% higher value was obtained in matching foreskin-derived keratinocytes. Similar elevated levels of this important intracellular free radical scavenging system were found in fibroblasts of a donor suffering from xeroderma pigmentosum. Furthermore, three to four times higher levels of glutathione in mitomycin C-treated mitotic fibroblasts have been determined. In mitotic skin fibroblasts, UVA irradiation resulted in a depletion of glutathione up to 90% following a fluence of 1.0 MJ/m2 UVA radiation. Higher initial glutathione levels were found in keratinocytes and mitomycin C-treated skin fibroblasts. In these fibroblasts a 70% depletion was detected and a much lower depletion (10-20%) was seen in some keratinocyte cell lines following fluences up to 1.0 MJ/m2. The depletion in skin fibroblasts was retained after 24 h following a fluence of 0.75 MJ/m2 UVA light. In view of the fact that glutathione has been shown to be involved in a variety of metabolic processes and plays a role in cellular protection against UVA radiation, our results imply that the fibroblast differentiation system is a very useful tool to unravel the complex mechanism of UVA-induced oxidative stress.

Cancer incidence among Finnish patients with psoriasis treated with trioxsalen bath PUVA

BACKGROUND

Systemic methoxsalen PUVA increases nonmelanoma skin cancer risk in a dose-dependent manner, whereas trioxsalen bath PUVA treatment has been suggested to be less carcinogenic.

OBJECTIVE

Our purpose was to study the carcinogenicity of topical trioxsalen PUVA.

METHODS

We performed a record linkage study of 337 male and 190 female patients with psoriasis treated with trioxsalen bath PUVA during the period 1977 to 1988 and the Finnish Cancer Registry (cancer incidence in the period 1977 to 1993). The mean follow-up period per person was approximately 11 years. Data on the total cumulative UVA dose and other potentially carcinogenic treatments were collected from the patients' files. The standardized incidence ratio (SIR) was calculated, in which the expected number of cases was based on the national cancer incidence rates.

RESULTS

During the follow-up, 26 cancer cases were observed in the cohort versus 30 expected (SIR, 0.88; 95% confidence interval [CI], 0.57-1.28). The only primary sites showing high SIRs were cancer of the kidney (SIR, 3.56; 95% CI, 0.97-9.10) and non-Hodgkin's lymphoma (SIR, 2.94; 95% CI, 0.36-10.6). There was only one case of nonmelanoma skin cancer; the expected number was 0.8 (SIR, 1.26; 95% CI, 0.03-7.04). The average cumulative UVA dose was 65 J/cm2; 40 patients had received more than 200 J/cm2. The average number of treatments was 112; 65 patients had received more than 200 treatments.

CONCLUSION

No excess of squamous cell carcinoma of the skin was found in patients treated with trioxsalen bath PUVA. However, because of the small size of the cohort, only a sevenfold excess risk can be excluded. The possible associations between psoriasis or its treatment and kidney cancer and lymphoma need to be studied further in larger series.

Development of cutaneous tolerance to ultraviolet B during ultraviolet B phototherapy for psoriasis

During a schedule of multiple exposures to ultraviolet B radiation (UVB, 280-320 nm), skin develops a reduced sensitivity, variously called tolerance, photoadaptation, accommodation or acclimatization. In this study we have investigated the development of tolerance in the normal skin of a group of psoriatic patients during the course of UVB therapy. Tolerance was assessed by phototests carried out on non-lesional skin as frequently as possible throughout the treatment. Maximum tolerance was developed by the group of individuals most sensitive to UVB, which was twice that of the least sensitive group. The minimal perceptible erythema dose (MPE) increased rapidly in the first 2 weeks (220% per week) and reached a plateau by the eighth week of 800% above the baseline MPE dose. For the more sensitive patients there was a further increase in sensitivity (decrease in MPE dose) after the ninth week of continuous treatment. Tolerance to UVB also involves pigmentation in the first few weeks, but in these patients there was no evidence of hyperpigmentation by the end of treatment. While epidermal hyperplasia is most likely to play a leading role in the development of tolerance to UV, there is no reason to expect this protection to decrease under conditions of continuous exposure. Thus, accommodation to ultraviolet radiation (UVR) is not a monotonically increasing process but appears to alter the accepted reactions of human skin to UVR.

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.

A role for ultraviolet A in solar mutagenesis

It is well established that exposure to solar UVB (290-320 nm) gives rise to mutations in oncogenes and tumor suppressor genes that initiate the molecular cascade toward skin cancer. Although UVA (320-400 nm) has also been implicated in multistage photocarcinogenesis, its potential contribution to sunlight mutagenesis remains poorly characterized. We have determined the DNA sequence specificity of mutations induced by UVB (lambda > 290 nm), and by UVA (lambda > 350 nm), at the adenine phosphoribosyltransferase locus of Chinese hamster ovary cells. This has been compared to results previously obtained for stimulated sunlight (lambda > or = 310 nm) and 254-nm UVC in the same gene. We demonstrate that T-->G transversions, a generally rare class of mutation, are induced at high frequency (up to 50%) in UVA-exposed cells. Furthermore, this event comprises a substantial proportion of the simulated sunlight-induced mutant collection (25%) but is significantly less frequent (P < 0.05) in cells irradiated with either UVB (9%) or UVC (5%). We conclude that the mutagenic specificity of broad-spectrum solar light in rodent cells is not determined entirely by the UVB component and that UVA also plays an important role.

Ultraviolet irradiation induces c-fos but not c-Ha-ras proto-oncogene expression in human epidermis

The link between sun exposure and skin cancer is well established, but the mechanism of photocarcinogenesis is still incompletely understood. In vitro experimentation has shown that induction of the c-fos proto-oncogene occurs in cultured human keratinocytes after ultraviolet exposure, and c-Ha-ras mutations are commonly present in human skin neoplasms removed from chronically sun-exposed sites. In the present study, the effect of UV irradiation on the expression of these two proto-oncogenes was examined. The sun-protected volar forearm of six subjects was exposed to a standardized erythemogenic dose of solar-simulated light, and punch biopsies were obtained after 1 h and 24 h from the irradiated area and a nearby shielded area. Expression of c-fos, determined by in situ hybridization of histologic cross-sections, was detected in the basal and lower epidermal layers in all biopsies. However, at 1 h there was a marked increase that returned to baseline by 24 h. c-Ha-ras mRNA could not be detected by riboprobe hybridization in any of the biopsy specimens. Our data demonstrate transient induction of c-fos but not c-Ha-ras expression, at least at the timepoints studied, following a modest UV exposure in normal skin. This phenomenon may lead to the subsequent constitutive over-expression and super-inducibility of c-fos observed in cultured keratinocytes derived from photodamaged skin and may facilitate the development of skin cancer.

PUVA, UVB, psoriasis, and nonmelanoma skin cancer

This article reviews the English-language literature on the risk of nonmelanoma skin cancer from phototherapy (UVB) or photochemotherapy (PUVA) in the treatment of psoriasis. The evolution of the debate on the carcinogenic potential of these treatment is discussed, particularly with reference to PUVA. Available data indicate that therapeutic UVB has a low risk of producing cutaneous cancers, with the possible exception of those on male genital skin. However, there is a definite cutaneous carcinogenic risk from oral PUVA. The major associated risk factors are reviewed. We provide recommendations for the assessment and treatment of patients to minimize this risk.

Rising trends in melanoma. An hypothesis concerning sunscreen effectiveness

Incidence rates of melanoma have risen especially steeply since the mid-1970s. The two principal strategies for reduction of risk of melanoma and other skin cancers are sun avoidance and use of chemical sunscreens. Rising trends in the incidence of and mortality from melanoma have continued since the 1970s and 1980s, when sunscreens with high sun protection factors became widely used. Commonly used chemical sunscreens block ultraviolet B (UVB) but are virtually transparent to ultraviolet A (UVA), which makes up 90 to 95% of ultraviolet energy in the solar spectrum. Because sunscreens prevent erythema and sunburn, and inhibit accommodation of the skin to sunlight, their use may permit excessive exposure of the skin to portions of the solar spectrum other than UVB. If melanoma and basal cell carcinoma are initiated or promoted by solar radiation other than UVB, as laboratory data suggest, then UVB sunscreens might not be effective in preventing these cancers, and sunscreen use might increase the risk of their occurrence. Alternative explanations for the rapid rise in the incidence and mortality rates of melanoma, such as changes in patterns of recreational sun exposure, are discussed. Traditional means of limiting overexposure to the sun, such as wearing of hats and adequate clothing and avoidance of prolonged sunbathing, may be more prudent than reliance on chemical sunscreens.

An in vitro study on the effect of UVA radiation on human gingival fibroblasts

The recent availability of a 99.9% UVA source has made possible studies that show that low energy wavelengths, previously considered innocuous, significantly affected wound healing in hairless guinea pigs. Decreased wound tensile strength and a slower rate of wound contraction in irradiated animals were among the changes noted. Because of their advocated role in the wound healing process, fibroblasts were chosen to study the effects of pure UVA exposure at a cellular level. 3H-thymidine uptake levels were measured in 8 groups of fibroblast cultures (12 samples/group). The cultures were exposed to varying concentrations of pure UVA. Previously incorporated 14C-thymidine levels were used to compensate for differences in cell numbers between samples. At a fluence of 3.65 x 10(-3) watts/cm2, a significant decrease in 3H-thymidine incorporation (compared to controls) was seen for all exposure periods and there was a dose-dependent decrease only in 3H-thymidine uptake for cells exposed to 1-4 min of UVA. Using post-exposure incubations of 2-16 h, a time-dependent recovery of 3H-thymidine uptake was also demonstrated, from 40% of control at 4 h, to 75% at 8 h, and 99% at 16 h. The near-complete recovery at 16 h was seen in exposures up to 2.73 joules/cm2 (12 min), whereas higher concentrations showed only partial recovery. These studies demonstrated the deleterious, though reversible, effects of UVA on fibroblasts and suggest a possible pathophysiologic process for UVA's effect on wound healing in this animal model.

Do pyrimidine dimer yields correlate with erythema induction in human skin irradiated in situ with ultraviolet light (275-365 nm)?

Ultraviolet radiation produces erythema in human skin, and damages the DNA of living cells in skin. Previous work showed that broad-band UV-B (290-320 nm) radiation produced higher levels of cyclobutyl pyrimidine dimers in DNA of individuals with high UV-B sensitivity (low minimal erythema dose) than in subjects of low UV-B sensitivity [Freeman et al. (1986) J. Invest. Dermatol., 86, 34-36]. We examined the relationship between erythema induction and dimer yields in DNA of human skin irradiated in situ with narrow band radiation spanning the wavelength range 275-365 nm. We find that, in general, higher dimer yields are produced per incident photon in volunteers with higher susceptibility to erythema induced by radiation of the same wavelength.

Photocarcinogenicity of psoralens used in PUVA treatment: present status in mouse and man

There is good evidence that 8-methoxypsoralen is photocarcinogenic in psoriatic patients undergoing long-term photochemotherapy (PUVA) in the U.S.A. However, this conclusion has not been supported by two major European studies which have indicated that PUVA is a tumour promoter of damage initiated by other agents. Variation in PUVA treatment protocols in the U.S.A. and Europe may partly account for the different conclusions. There is much interest in the therapeutic potential of monofunctional psoralens. It is hoped that these may reduce long-term risk. Monofunctional and cross-linking psoralens have been shown to be photocarcinogenic in mouse skin. The relative risk of different compounds may be assessed in the mouse, but it is important to base comparisons on dose protocols that have been shown to be therapeutically effective.

Quantitation of ultraviolet radiation-induced cyclobutyl pyrimidine dimers in DNA by video and photographic densitometry

We have compared video and photographic methods for calculating the number of ultraviolet radiation (uv)-induced pyrimidine dimers in DNA from the bacteriophage T7 exposed to uv (0 to 800 J/m2) from an FS40 sunlamp. DNA was incubated with a pyrimidine dimer-specific Micrococcus luteus uv endonuclease, subjected to alkaline agarose gel electrophoresis, neutralized, and stained with ethidium bromide, and the DNA fluorescence was recorded either with a video camera or on photographic film. The slopes of the dose-response curves for the number of uv-endonuclease-sensitive sites per 10(3) bases (pyrimidine dimers) was 1.2 (+/- 0.1) X 10(-4) uv-endonuclease-sensitive sites per J/m2 for the video analysis and 1.3 (+/- 0.04) X 10(-4) uv-endonuclease-sensitive sites per J/m2 for the photographic analysis. Results for pyrimidine dimer determination by either method were statistically comparable.

Gel electrophoresis method for quantitation of gamma ray induced single- and double-strand breaks in DNA irradiated in vitro

We describe a method based on gel electrophoresis for the quantitation of strand breaks in DNA and demonstrate its application to the measurement of single- and double-strand breaks formed by gamma-rays for DNA irradiated in vitro. For single-strand breaks, our data span the dose range from 0.1 to 1 Gy, while for double-strand breaks doses were from 3 to 15 Gy. In agreement with results obtained using other techniques, we find that the dose response function for single-strand breaks is linear while the dose response function for double-strand breaks is curved, indicating that it is the sum of both linear and quadratic components. We discuss factors that determine the sensitivity of the method and indicate approaches to make possible the quantitation of strand breaks in the DNA of cells irradiated with sublethal doses.

Modification of sunburn cell production in 8-MOP sensitized mouse epidermis: a method of assessing UVA sunscreen efficacy

The UVA(320-400 nm) photoprotection of four commercially available sunscreens with different sun protection factors was evaluated in 8-methoxypsoralen (8-MOP) treated mouse epidermis with the number of sunburn cells (SBCs) as the end-point. SBC production in 8-MOP sensitized mouse epidermis with and without application of sunscreens is dose-related. The difference between the slopes of the dose-response curves of the control group and the four sunscreens was highly significant (p less than 0.001). It appears that the SBC production was modified both by the dose of the UVA irradiation administered and by the UVA photoprotection of the four sunscreens. It could be concluded that the number of SBCs in 8-MOP sensitized mouse epidermis is a useful parameter to quantify the degree of UVA photoprotection of sunscreens.

Variations in excision repair of UVB-induced pyrimidine dimers in DNA of human skin in situ

The excision repair kinetics of UVB (280-320 nm)*-induced pyrimidine dimers in DNA of human skin in situ was determined for seventeen volunteers using a dimer-specific endonuclease from Micrococcus luteus in conjunction with agarose gel electrophoresis. Removal of pyrimidine dimers from human skin could be detected within 6 h after irradiation and the average half-life for removal of pyrimidine dimers was 11.0 h (+/- 4.3 h). However, there was significant inter-individual variability of repair as indicated by a half-life coefficient of variation of 38%.