Effect of age on the formation and repair of UV photoproducts in human skin in situ

Männer- und Frauenhaut – Genderaspekte

Männer- und Frauenhaut sind unterschiedlich. Männerhaut ist dicker, hat mehr Talgdrüsen und Oberflächenlipide, eine höhere, nicht so divergente Keimzahl wie Frauen, niedrigere transepidermale Wasserverlust (TEWL)-Werte und einen niedrigeren pH-Wert. Die Wundheilung und das Niveau von Antioxidantien ist bei Frauenhaut besser. Männerhaut hat mehr Kollagen, altert an anderer Lokalisation, bei Männern finden sich mehr Falten an der Stirn, bei Frauen mehr Falten perioral. Studien sollten die Auswertung der Daten nach Geschlechtern mit beachten.

Age and insulin-like growth factor-1 impact PCNA monoubiquitination in UVB-irradiated human skin

Nonmelanoma skin cancers occur primarily in individuals over the age of 60 and are characterized by an abundance of ultraviolet (UV) signature mutations in keratinocyte DNA. Though geriatric skin removes UV photoproducts from DNA less efficiently than young adult skin, it is not known whether the utilization of other prosurvival but potentially mutagenic DNA damage tolerance systems such as translesion synthesis (TLS) is altered in older individuals. Using monoubiquitination of the replicative DNA polymerase clamp protein PCNA (proliferating cell nuclear antigen) as a biochemical marker of TLS pathway activation, we find that UVB exposure of the skin of individuals over the age of 65 results in a higher level of PCNA monoubiquitination than in the skin of young adults. Furthermore, based on previous reports showing a role for deficient insulin-like growth factor-1 (IGF-1) signaling in altered UVB DNA damage responses in geriatric human skin, we find that both pharmacological inhibition of the IGF-1 receptor (IGF-1R) and deprivation of IGF-1 potentiate UVB-induced PCNA monoubiquitination in both human skin ex vivo and keratinocytes in vitro. Interestingly, though the TLS DNA polymerase Pol eta can accurately replicate the major photoproducts induced in DNA by UV radiation, we find that it fails to accumulate on chromatin in the absence of IGF-1R signaling and that this phenotype is correlated with increased mutagenesis in keratinocytes in vitro. Thus, altered IGF-1/IGF-1R signaling in geriatric skin may predispose epidermal keratinocytes to carry out a more mutagenic form of DNA synthesis following UVB exposure.

Biomarkers of Cellular Senescence and Skin Aging

Cellular senescence is an irreversible growth arrest that occurs as a result of different damaging stimuli, including DNA damage, telomere shortening and dysfunction or oncogenic stress. Senescent cells exert a pleotropic effect on development, tissue aging and regeneration, inflammation, wound healing and tumor suppression. Strategies to remove senescent cells from aging tissues or preneoplastic lesions can delay tissue dysfunction and lead to increased healthspan. However, a significant hurdle in the aging field has been the identification of a universal biomarker that facilitates the unequivocal detection and quantification of senescent cell types in vitro and in vivo. Mammalian skin is the largest organ of the human body and consists of different cell types and compartments. Skin provides a physical barrier against harmful microbes, toxins, and protects us from ultraviolet radiation. Increasing evidence suggests that senescent cells accumulate in chronologically aged and photoaged skin; and may contribute to age-related skin changes and pathologies. Here, we highlight current biomarkers to detect senescent cells and review their utility in the context of skin aging. In particular, we discuss the efficacy of biomarkers to detect senescence within different skin compartments and cell types, and how they may contribute to myriad manifestations of skin aging and age-related skin pathologies.

Children sustain high levels of skin DNA photodamage, with a modest increase of serum 25-hydroxyvitamin D3 , after a summer holiday in Northern Europe

BACKGROUND

Childhood solar ultraviolet radiation (UVR) exposure increases the risk of skin cancer in adulthood, which is associated with mutations caused by UVR-induced cyclobutane pyrimidine dimers (CPD). Solar UVR is also the main source of vitamin D, essential for healthy bone development in children.

OBJECTIVES

To assess the impact of a 12-day Baltic Sea (54° N) beach holiday on serum 25-hydroxyvitamin D₃ [25(OH)D₃ ] and CPD in 32 healthy Polish children (skin types I-IV).

METHODS

Blood and urine were collected before and after the holiday and assessed for 25(OH)D₃ and excreted CPD, respectively, and personal UVR exposure was measured. Diaries were used to record sunbathing, sunburn and sunscreen use. Before- and after-holiday skin redness and pigmentation were measured by reflectance spectroscopy.

RESULTS

The average ± SD daily exposure UVR dose was 2·4 ± 1·5 standard erythema doses (SEDs), which is borderline erythemal. The mean concentration of 25(OH)D₃ increased (× 1·24 ± 0·19) from 64·7 ± 13·3 to 79·3 ± 18·7 nmol L^-1 (P < 0·001). Mean CPD increased 12·6 ± 10·0-fold from 26·9 ± 17·9 to 248·9 ± 113·4 fmol μmol^-1 creatinine (P < 0·001). Increased 25(OH)D₃ was accompanied by a very much greater increase in DNA damage associated with carcinogenic potential. Overall, skin type had no significant effects on behavioural, clinical or analytical outcomes, but skin types I/II had more CPD (unadjusted P = 0·0496) than skin types III/IV at the end of the holiday.

CONCLUSIONS

Careful consideration must be given to the health outcomes of childhood solar exposure, and a much better understanding of the risk-benefit relationships of such exposure is required. Rigorous photoprotection is necessary for children, even in Northern Europe.

Impact of Age and Insulin-Like Growth Factor-1 on DNA Damage Responses in UV-Irradiated Human Skin

The growing incidence of non-melanoma skin cancer (NMSC) necessitates a thorough understanding of its primary risk factors, which include exposure to ultraviolet (UV) wavelengths of sunlight and age. Whereas UV radiation (UVR) has long been known to generate photoproducts in genomic DNA that promote genetic mutations that drive skin carcinogenesis, the mechanism by which age contributes to disease pathogenesis is less understood and has not been sufficiently studied. In this review, we highlight studies that have considered age as a variable in examining DNA damage responses in UV-irradiated skin and then discuss emerging evidence that the reduced production of insulin-like growth factor-1 (IGF-1) by senescent fibroblasts in the dermis of geriatric skin creates an environment that negatively impacts how epidermal keratinocytes respond to UVR-induced DNA damage. In particular, recent data suggest that two principle components of the cellular response to DNA damage, including nucleotide excision repair and DNA damage checkpoint signaling, are both partially defective in keratinocytes with inactive IGF-1 receptors. Overcoming these tumor-promoting conditions in aged skin may therefore provide a way to lower aging-associated skin cancer risk, and thus we will consider how dermal wounding and related clinical interventions may work to rejuvenate the skin, re-activate IGF-1 signaling, and prevent the initiation of NMSC.

Insight in DNA Repair of UV-induced Pyrimidine Dimers by Chromatographic Methods

UV-induced formation of pyrimidine dimers in DNA is a major deleterious event in both eukaryotic and prokaryotic cells. Accumulation of cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts can lead to cell death or be at the origin of mutations. In skin, UV induction of DNA damage is a major initiating event in tumorigenesis. To counteract these deleterious effects, all cell types possess DNA repair machinery, such as nucleotide excision repair and, in some cell types, direct reversion. Different analytical approaches were used to assess the efficiency of repair and decipher the enzymatic mechanisms. We presently review the information provided by chromatographic methods, which are complementary to biochemical assays, such as immunological detection and electrophoresis-based techniques. Chromatographic assays are interesting in their ability to provide quantitative data on a wide range of damage and are also valuable tools for the identification of repair intermediates.

A new hair follicle-derived human epidermal model for the evaluation of sunscreen genoprotection

Induction of skin cancer is the most deleterious effect of excessive exposure to sunlight. Accurate evaluation of sunscreens to protect the genome is thus of major importance. In particular, the ability of suncare products to prevent the formation of DNA damage should be evaluated more directly since the Sun Protection Factor is only related to erythema induction. For this purpose, we developed an in vitro approach using a recently characterized reconstituted human epidermis (RHE) model engineered from hair follicle. The relevance of this skin substitute in terms of UV-induced genotoxicity was compared to ex vivo explants exposed to solar-simulated radiation (SSR). The yield of bipyrimidine photoproducts, their rate of repair, and the induction of apoptosis were very similar in both types of skin samples. In order to evaluate the protection afforded by sunscreen against DNA damage, bipyrimidine photoproducts were quantified in tissue models following SSR exposure in the presence or absence of a SPF50+ formula. A rather high DNA protection factor of approximately 20 was found in RHE, very similar to that determined for explants. Thus, RHE is a good surrogate to human skin, and also a convenient and useful tool for investigation of the genoprotection of sunscreens.

The variety of UV-induced pyrimidine dimeric photoproducts in DNA as shown by chromatographic quantification methods

Induction of DNA damage is one of the major consequences of exposure to solar UV radiation in living organisms. UV-induced DNA photoproducts are mostly pyrimidine dimers, including cyclobutane pyrimidine dimers, pyrimidine (6-4) pyrimidone photoproducts and Dewar valence isomers. In the last few decades, a large number of methods have been developed for the quantification of these pyrimidine dimers. The present review emphasizes the contribution of chromatographic techniques to our better understanding of the basic DNA photochemistry and the better description of damage in cells.

Urinary levels of thymine dimer as a biomarker of exposure to ultraviolet radiation in humans during outdoor activities in the summer

The incidence of skin cancer is rising rapidly in many countries, presumably due to increased leisure time exposure to solar ultraviolet radiation (UVR). UVR causes DNA lesions, such as the thymine dimer (T=T), which have been causatively linked to the development of skin cancer. T=T is clearly detectable in urine and may, thereby, be a potentially valuable biomarker of UVR exposure. The objective of this study was to evaluate the relationship between UVR exposure and urinary levels of T=T in a field study involving outdoor workers. Daily ambient and personal exposure of 52 beach lifeguards and agricultural workers to UVR were determined (employing 656 personal polysulphone dosimeters). In 22 of these subjects, daily urinary T=T levels (120 samples) were measured, the area of skin exposed calculated and associations assessed utilizing mixed statistical models. The average daily UVR dose was approximately 600 J/m(2) (7.7 standard erythemal doses), i.e. about 20% of ambient UVR. T=T levels were correlated to UVR dose, increasing by about 6 fmol/µmol creatinine for each 100 J/m(2) increase in dose (average of the three preceding days). This is the first demonstration of a relationship between occupational UVR exposure and urinary levels of a biomarker of DNA damage. On a population level, urinary levels of T=T can be used as a biomarker for UVR exposure in the field.

Quantification of ultraviolet radiation-induced DNA damage in the urine of Swedish adults and children following exposure to sunlight

CONTEXT

DNA damage following exposure to ultraviolet radiation (UVR) is important in skin cancer development. The predominant photoproduct, cyclobutane thymine dimer (T=T), is repaired and excreted in the urine, where it provides a biomarker of exposure.

OBJECTIVE

To quantify urinary T=T levels after recreational sunlight exposure in adults and children.

METHODS

Average UVR doses were measured with personal dosimeters. Urinary T=T was analysed with (32)P-postlabelling.

RESULTS

Background levels of T=T increased significantly following exposure to sunlight. Amounts of T=T in urine of children and adults were not significantly different after adjusting for area of skin exposed and physiological differences. UVR dose and amounts of T=T correlated for both adults and children.

CONCLUSION

Recreational exposure to sunlight in Sweden induces levels of DNA damage, clearly detectable in urine.

Inhibition of UVB-induced oxidative damage and apoptotic biochemical changes in human lymphocytes by 2,5-dihydroxy-3-undecyl-1,4-benzoquinone (embelin)

PURPOSE

The present study investigates the inhibition of Ultraviolet B (UVB, 290-320 nm) radiation-induced oxidative damage in peripheral blood human lymphocytes by embelin extracted from Embelia ribes.

MATERIALS AND METHODS

Embelin was extracted, purified and characterized. Prior to inhibitory assessment, a maximum concentration of embelin that was non-toxic was determined. Six experimental groups, including respective controls were made to assess the inhibitory effect of embelin for the selected concentrations of 10 and 20 μg/ml. For the experimental groups; lymphocytes (1 × 10(6) cells) were pre-treated with the chosen concentration of embelin for a period of 60 min and then exposed to UVB for 30 min. UVB radiation inhibitory effect of embelin assessed by measuring antioxidant and lipid peroxidation levels, deoxyribonucleic acid (DNA) damage, reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) at scheduled time points after irradiation.

RESULTS

Pre-treatment of lymphocytes with embelin prevents UVB-induced oxidative damage. An increase in antioxidant levels in irradiated cells in the presence of embelin and UV absorbance of embelin could be the reason for the decrease in lipid peroxidation level and prevention of DNA damage by UVB radiation.

CONCLUSION

Embelin prevents oxidative stress induced by UVB irradiation via its antioxidant property.

Epigallocatechin-3-gallate decreases UVA-induced HPRT mutations in human skin fibroblasts accompanied by increased rates of senescence and apoptosis

Our study was designed to determine the protective effect of epigallocatechin-3-gallate (EGCG) on cultured human skin fibroblasts (HSFs) from multiple ultraviolet A (UVA) irradiation-induced hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutant colony formation and its underlying mechanisms. In our study, the mutation frequency of the HPRT gene was examined by mutagenesis assay. Cell senescence was determined by histochemical staining of senescence-associated β-galactosidase. The apoptosis rate was detected by flow cytometry. EGCG decreased the UVA-induced HPRT gene mutation frequency by 47.85%. However, EGCG further increased the number of senescent cells by 38.92% and the apoptosis rate by 56.92% in HSFs. The photo-protective effect of EGCG on multiple UVA-exposed HSFs is related to a significant reduction in UVA-induced HPRT mutant cells. This may be caused by the induction of damaged cells to proceed to senescence and apoptosis.

Assessment of cyclobutane pyrimidine dimers by digital photography in human skin

UV-mediated DNA damage and repair are important mechanisms in research on UV-induced carcinogenesis. UV-induced DNA-damage and repair can be determined by immunohistochemical staining of photoproduct positive nuclei of keratinocytes in the epidermis. We developed a new method of analysing and quantifying thymine dimer (TT-CPD) positive cells in the epidermis. Normal skin of healthy controls was exposed to UVB ex vivo and in vivo. Skin samples were immunohistochemically stained for TT-CPDs. Digital images of the epidermis were quantified for TT-CPDs both visually and digitally. There was a UVB-dose dependent induction of TT-CPDs present in the ex vivo UVB-irradiated skin samples. The linear measurement range of the digital quantification was increased compared to the manual counting. The average 24-hour repair rate of the initiated TT-CPDs elicited by the UVB irradiation at T=0 of the 8 HCs showed a 34% decrease of TT-CPD photoproducts by the manual counting method and a 51% decrease determined by digital counting. The digital quantification method improves immunohistochemical quantification of DNA photo damage. It is more sensitive in measuring the extent of DNA-damage per nucleus.

Cellular stress response pathways and ageing: intricate molecular relationships

Ageing is driven by the inexorable and stochastic accumulation of damage in biomolecules vital for proper cellular function. Although this process is fundamentally haphazard and uncontrollable, senescent decline and ageing is broadly influenced by genetic and extrinsic factors. Numerous gene mutations and treatments have been shown to extend the lifespan of diverse organisms ranging from the unicellular Saccharomyces cerevisiae to primates. It is becoming increasingly apparent that most such interventions ultimately interface with cellular stress response mechanisms, suggesting that longevity is intimately related to the ability of the organism to effectively cope with both intrinsic and extrinsic stress. Here, we survey the molecular mechanisms that link ageing to main stress response pathways, and mediate age-related changes in the effectiveness of the response to stress. We also discuss how each pathway contributes to modulate the ageing process. A better understanding of the dynamics and reciprocal interplay between stress responses and ageing is critical for the development of novel therapeutic strategies that exploit endogenous stress combat pathways against age-associated pathologies.

Investigation of the effect of UV irradiation on DNA damage: comparison between skin cancer patients and normal volunteers

BACKGROUND

Susceptibility to environmental carcinogenesis is the consequence of a complex interplay between intrinsic hereditary factors and actual exposure to potential carcinogenic agents. Exposure to sunlight is the primary etiological agent for basal cell carcinoma (BCC).

AIM

The aim of this study was to determine the effects of different ultraviolet (UV) doses on DNA damage in epidermal keratinocytes in vivo and to elucidate if patients with BCC are more susceptible to UV-induced DNA damage in comparison with normal healthy volunteers in response to solar simulator radiation (SSR).

MATERIALS AND METHODS

Skin biopsies obtained post-UV irradiation from both normal healthy volunteers and BCC patients were analyzed for DNA damage, using immunohistochemical approach with TDM-2 antibody, which binds specifically to cyclobutane pyrimidine dimmers (CPDs).

RESULTS

In both normal volunteers and BCC patients, the peak of CPD-positive cells occurred at 4.5 h post-SSR. There was a statistically significant difference in CPD positivity between BCC patients and normal volunteers, at time points (from 4.5 h to 48 h post-SSR). For a given dose of SSR based on each individual minimal erythema dose (MED), a greater number of CPD-positive cells could be shown.

CONCLUSIONS

This study has shown for the first time and in vivo in human volunteers that BCC patients are more susceptible to UV-induced DNA damage in comparison with normal healthy volunteers.

Repair of UV dimers in skin DNA of patients with basal cell carcinoma

Epidemiologic studies suggest that exposure to sunlight is the primary etiologic agent for basal cell carcinoma. Formation of UV-induced DNA damage is believed to be a crucial event in the process leading to skin cancer. In this study, repair of photoproducts in DNA was followed in the skin of patients with basal cell carcinoma and control subjects. The subjects were exposed to 800 J/m(2) Commission Internationale de 1'Eclairag of solar-simulating radiation on buttock skin. Biopsies were taken at 0 hour, 24 hours, and 3 weeks after the exposure. Two cyclobutane pyrimidine dimers, TT=C and TT=T, were measured using a sensitive (32)P-postlabeling assay. Initial levels of both TT=C and TT=T differed between individuals in both groups. The levels of TT=T in patients with basal cell carcinoma and controls were similar (9.9 +/- 4.0 and 9.2 +/- 2.9 products per 10(6) normal nucleotides), whereas the level of TT=C was significantly lower in controls than in patients with basal cell carcinoma (6.2 +/- 3.1 versus 10.9 +/- 4.5 products per 10(6) normal nucleotides). The fractions of TT=T remaining after 24 hours and 3 weeks were significantly higher in patients with basal cell carcinoma (72% and 11%) compared with controls (48% and 5%). A slower removal in patients with basal cell carcinoma than in controls was indicated also for TT=C (52% versus 42% remaining at 24 hours); however, the difference between groups was not significant. When including data from our previously reported small-scale study, the fraction of dimers remaining at 24 hours was significantly higher in patients with basal cell carcinoma for both TT=C and TT=T. The data suggest that patients with basal cell carcinoma have a reduced capacity to repair UV-induced DNA lesions.

The protective role of melanin against UV damage in human skin

Human skin is repeatedly exposed to UVR that influences the function and survival of many cell types and is regarded as the main causative factor in the induction of skin cancer. It has been traditionally believed that skin pigmentation is the most important photoprotective factor, as melanin, besides functioning as a broadband UV absorbent, has antioxidant and radical scavenging properties. Besides, many epidemiological studies have shown a lower incidence for skin cancer in individuals with darker skin compared to those with fair skin. Skin pigmentation is of great cultural and cosmetic importance, yet the role of melanin in photoprotection is still controversial. This article outlines the major acute and chronic effects of UVR on human skin, the properties of melanin, the regulation of pigmentation and its effect on skin cancer prevention.

Differential repair of UVB-induced cyclobutane pyrimidine dimers in cultured human skin cells and whole human skin

Cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are the two main classes of mutagenic DNA damages induced by UVB radiation. Numerous studies have been devoted so far to their formation and repair in human cells and skin. However, the biochemical methods used often lack the specificity that would allow the individual study of each of the four CPDs and 6-4PPs produced at TT, TC, CT and CC dinucleotides. In the present work, we applied an HPLC-mass spectrometry assay to study the formation and repair of CPDs and 6-4PPs photoproducts in primary cultures of human keratinocytes and fibroblasts as well as in whole human skin. We first observed that the yield of dimeric lesions was slightly higher in fibroblasts than in keratinocytes. In contrast, the rate of global repair was higher in the last cell type. Moreover, removal of DNA photoproducts in skin biopsies was found to be slower than in both cultured skin cells. In agreement with previous works, the repair of 6-4PPs was found to be more efficient than that of CPDs in the three types of samples, with no observed difference between the removal of the TT and TC derivatives. In contrast, a significant influence of the nature of the two modified pyrimidines was observed on the repair rate of CPDs. The decreasing order of removal efficiency was the following: C<>T>C<>C>T<>C>T<>T. These data, together with the known intrinsic mutational properties of the lesions, would support the reported UV mutation spectra. A noticeable exception concerns CC dinucleotides that are mutational hotspots with an UV-specific CC to TT tandem mutation, although related bipyrimidine photoproducts are produced in low yields and efficiently repaired.

Changes in DNA repair during aging

DNA is a precious molecule. It encodes vital information about cellular content and function. There are only two copies of each chromosome in the cell, and once the sequence is lost no replacement is possible. The irreplaceable nature of the DNA sets it apart from other cellular molecules, and makes it a critical target for age-related deterioration. To prevent DNA damage cells have evolved elaborate DNA repair machinery. Paradoxically, DNA repair can itself be subject to age-related changes and deterioration. In this review we will discuss the changes in efficiency of mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER) and double-strand break (DSB) repair systems during aging, and potential changes in DSB repair pathway usage that occur with age. Mutations in DNA repair genes and premature aging phenotypes they cause have been reviewed extensively elsewhere, therefore the focus of this review is on the comparison of DNA repair mechanisms in young versus old.

Cyclobutane pyrimidine dimers are predominant DNA lesions in whole human skin exposed to UVA radiation

Solar UV radiation is the most important environmental factor involved in the pathogenesis of skin cancers. The well known genotoxic properties of UVB radiation (290-320 nm) mostly involve bipyrimidine DNA photoproducts. In contrast, the contribution of more-abundant UVA radiation (320-400 nm) that are not directly absorbed by DNA remains poorly understood in skin. Using a highly accurate and quantitative assay based on HPLC coupled with tandem mass spectrometry, we determined the type and the yield of formation of DNA damage in whole human skin exposed to UVB or UVA. Cyclobutane pyrimidine dimers, a typical UVB-induced DNA damage, were found to be produced in significant yield also in whole human skin exposed to UVA through a mechanism different from that triggered by UVB. Moreover, the latter class of photoproducts is produced in a larger amount than 8-oxo-7,8-dihydro-2'-deoxyguanosine, the most common oxidatively generated lesion, in human skin. Strikingly, the rate of removal of UVA-generated cyclobutane pyrimidine dimers was lower than those produced by UVB irradiation of skin. Finally, we compared the formation yields of DNA damage in whole skin with those determined in primary cultures of keratinocytes isolated from the same donors. We thus showed that human skin efficiently protects against UVB-induced DNA lesions, whereas very weak protection is afforded against UVA. These observations emphasize the likely role played by the UVA-induced DNA damage in skin carcinogenesis and should have consequences for photoprotection strategies.

Age related shift in the mutation spectra of germline and somatic NF2 mutations: hypothetical role of DNA repair mechanisms

It has been suggested that somatic mutations that accumulate due to an age related decline in the efficiency of DNA repair mechanisms might contribute to the increased incidence of cancer in older people. However, there is little direct evidence for this phenomenon. The spectra of germline and somatic mutations can be compared in cancer genes that cause inherited tumour syndromes and sporadic tumours, respectively. In addition, mosaic patients reflect the nature of mutations that occur in early development. Hence, we hypothesised that the "temporal mutation record" of a human cancer gene might provide insight into mechanisms of mutagenesis in the germline, in early development, and in adulthood. We compared the ratio of frameshift to nonsense mutations in three diseases that are related to the NF2 tumour suppressor gene: classic neurofibromatosis 2 (NF2), caused by germline NF2 mutations; mosaic NF2; and unilateral sporadic vestibular schwannoma (USVS), caused by somatic NF2 inactivation. Nonsense mutations predominated in both classic and mosaic NF2, but the ratio of nonsense to frameshift mutations was reversed in USVS. Moreover, in USVS patients, the ratio of somatic frameshift to nonsense mutations increased significantly with increasing age at diagnosis. This pattern is consistent with an age related decline in the efficiency of DNA repair mechanisms. Similar studies for other familial cancer genes may provide further evidence for this hypothesis.

Aged human skin removes UVB-induced pyrimidine dimers from the epidermis more slowly than younger adult skin in vivo

Although many studies have been reported on the repair of ultraviolet light (UV)-induced cyclobutane-type pyrimidine dimers (CPDs) in DNA, the effects of aging on the removal of UV-induced CPDs from the human skin epidermis in vivo remains uncertain. Therefore, we employed immunoblotting and immunohistochemical methods using monoclonal antibodies (TDM-2) to CPDs to study age-related differences in the time required for the in vivo removal of UVB-induced CPDs. The flexure surfaces of the upper arms of five young men were exposed to UVB light at a fluence of 35 and 700 mJ/cm2, and four older men were also irradiated with the same doses of UVB mentioned above. Each area of skin was biopsied before and immediately after irradiation, and at 4, 24 h, 2 and 4 days after irradiation in the younger group; and before and immediately after irradiation, and at 24 h, 4, 7, and 14 days after irradiation in the older group. A total of 108 DNA samples were taken from the epidermis of 108 biopsied specimens. These samples were immunoblotted using TDM-2 and the intensities of the immunoprecipitates were measured by photodensitometer. Our results show that the CPDs had been removed from the epidermis at 4 days after irradiation at either dose in the younger group, and between 7-14 days after irradiation in the aged group. The results of our immunohistochemical studies were consistent with those of our immunoblotting studies, and indicated that basal cells repair CPDs more quickly than prickle cells in the epidermis except the amounts at 24 h after UVB irradiation, and that the CPDs were removed by epidermal turnover after the nucleotide excision repair (NER). Our results showed age-associated decline in the NER in vivo, indicating high risk of UV-associated skin cancer.

Gene expression and DNA repair in progeroid syndromes and human aging

Human progeroid syndromes are caused by mutations in single genes accelerating some but not all features of normal aging. Most progeroid disorders are linked to defects in genome maintenance, and while it remains unknown if similar processes underlie normal and premature aging, they provide useful models for the study of aging. Altered transcription is speculated to play a causative role in aging, and is involved in the pathology of most if not all progeroid syndromes. Previous studies demonstrate that there is a similar pattern of gene expression changes in primary cells from old and Werner syndrome compared to young suggesting a presence of common cellular aging mechanisms in old and progeria. Here we review the role of transcription in progeroid syndromes and discuss the implications of similar transcription aberrations in normal and premature aging.

Fibromodulin Gene Transcription Is Induced by Ultraviolet Irradiation, and Its Regulation Is Impaired in Senescent Human Fibroblasts

Cells undergoing replicative senescence display an altered pattern of gene expression. Senescent fibroblasts show significant changes in the expression of mRNAs encoding extracellular matrix-remodeling proteins; among these mRNAs, the mRNA encoding fibromodulin is highly decreased in these cells. To understand the molecular basis of this phenomenon, we explored the regulatory mechanisms of the human fibromodulin gene. We found that fibromodulin gene promoter contains a cis-element, crucial for its basal expression, that forms a DNA-protein complex when exposed to nuclear extracts from exponentially growing human fibroblasts and not to extracts from cells undergoing senescence by repeated in vitro passages or by mild oxidative stress. The purification of this complex showed that it contains the damage-specific DNA-binding protein DDB-1. The latter is known to be induced by UV irradiation; therefore we checked whether fibromodulin gene promoter is regulated upon the exposure of the cells to UV rays. The results showed that, in exponentially growing fibroblasts, the promoter efficiency is increased by UV irradiation and the DDB-1-containing complex is robustly enriched in cells exposed to UV light. Accordingly, in these experimental conditions the endogenous fibromodulin mRNA accumulates to very high levels. On the contrary, senescent cells did not show any activation of the fibromodulin gene promoter, any induction of the DDB-1-containing complex, or any accumulation of fibromodulin mRNA. These phenomena are accompanied in senescent cells by a decrease of the UV-damaged DNA binding activity.

Gene expression responses to DNA damage are altered in human aging and in Werner Syndrome

The accumulation of DNA damage and mutations is considered a major cause of cancer and aging. While it is known that DNA damage can affect changes in gene expression, transcriptional regulation after DNA damage is poorly understood. We characterized the expression of 6912 genes in human primary fibroblasts after exposure to three different kinds of cellular stress that introduces DNA damage: 4-nitroquinoline-1-oxide (4NQO), gamma-irradiation, or UV-irradiation. Each type of stress elicited damage specific gene expression changes of up to 10-fold. A total of 85 genes had similar changes in expression of 3-40-fold after all three kinds of stress. We examined transcription in cells from young and old individuals and from patients with Werner syndrome (WS), a segmental progeroid condition with a high incidence of cancer, and found various age-associated transcriptional changes depending upon the type of cellular stress. Compared to young individuals, both WS and old individuals had similarly aberrant transcriptional responses to gamma- and UV-irradiation, suggesting a role for Werner protein in stress-induced gene expression. Our results suggest that aberrant DNA damage-induced gene regulation may contribute to the aging process and the premature aging in WS.

DNA end joining becomes less efficient and more error-prone during cellular senescence

Accumulation of somatic mutations is thought to contribute to the aging process. Genomic instability has been shown to increase during aging, suggesting an aberrant function of DNA double-strand break (DSB) repair. Surprisingly, DSB repair has not been examined with respect to cellular senescence. Therefore, we have studied the ability of young, presenescent, and senescent normal human fibroblasts to repair DSBs in transfected DNA by using a fluorescent reporter substrate. We have found that the efficiency of end joining is reduced up to 4.5 fold in presenescent and senescent cells, relative to young cells. Sequence analysis of end junctions showed that the frequency of precise ligation was higher in young cells, whereas end joining in old cells was associated with extended deletions. These results indicate that end joining becomes inefficient and more error-prone during cellular senescence. Furthermore, the ability to use microhomologies for end joining was compromised in senescent cells, suggesting that young and senescent cells may use different end joining pathways. We hypothesize that inefficient and aberrant end joining is a likely mechanism underlying the age-related genomic instability and higher incidence of cancer in the elderly.

Effect of the spectral range of a UV lamp on the production of cyclobutane pyrimidine dimers in human skin in situ

BACKGROUND

Ultraviolet (UV) irradiation has a broad spectrum of biological effects and a capacity to initiate skin carcinogenesis through DNA damage. The effect of different wave bands of UV light on the production of DNA damage in human skin in situ was studied with a broadband UV-B lamp TL-12 and a narrowband UV-B lamp TL-01.

METHODS

Eight psoriasis patients participated in the study. Their minimal erythema dose was assessed separately for the two UV-B wave band ranges. Test areas of buttock skin were irradiated with the two spectrally differing lamps using erythemally equivalent UV doses of 40 and 80 mJ/cm2 CIE (Commission International de I'Eclairage). Punch biopsies were taken from the irradiated areas, and UV-induced DNA lesions (cyclobutane pyrimidine dimers, CPDs) in the skin were analyzed with a 32P high-performance liquid chromatography postlabelling method.

RESULTS

No UV source-dependent differences in the induced levels of CPDs were detected in this study.

CONCLUSION

CPD production with broadband TL-12 and narrowband TL-01 UV-B lamps in situ did not differ when erythemally equivalent UV doses were used. The preliminary result needs to be confirmed in a larger study.

Single nucleotide polymorphisms in the XPG gene: determination of role in DNA repair and breast cancer risk

In this study we determined the effect of single nucleotide polymorphisms in the XPG gene on DNA repair and breast cancer susceptibility. Ninety individuals, with previously studied DNA repair rate at 24 hr of 2 types of UV-specific cyclobutane pyrimidines dimers (CPDs) in skin were genotyped for XPG polymorphism at codon 1104 (exon 15 G>C; Asp > His). The repair rate of TT=C dimer was similar in both wild-type GG homozygotes and GC heterozygotes, whereas, for TT=T, dimer repair was non-significantly (Student's t-test, p = 0.34) lower in GC heterozygotes than wild-type GG homozygotes. Genotyping of 220 breast cancer cases and 308 controls for the same single nucleotide polymorphism in exon 15 of the XPG gene exhibited marginally significant increased frequency of the variant allele (chi(2) 3.84, p = 0.05; OR 1.33, 95% CI 1.0-1.8) in cases (C-allele 0.29) compared to controls (C-allele 0.24). Combined heterozygote and variant homozygote genotype frequency was also higher in cases than controls (chi(2) 4.79, p = 0.03; OR 1.50, 95%CI 1.04-2.16).

Similar UV responses are seen in a skin organ culture as in human skin in vivo

Ultraviolet radiation (UVR) plays an important role in the development of non-melanoma skin cancer. Most tumors develop in chronically sun-exposed skin, most often in cosmetically sensitive locations, where in vivo experiments may be difficult to perform. In this study, we describe a skin organ culture model with preserved normal morphology and intact response to UVR. Skin explants from chronically sun-exposed and non-sun-exposed skin were irradiated with artificial UVA+UVB with and without topical sunscreen. UV-induced DNA damage, epidermal p53 response and repair kinetics were analyzed using immunohistochemistry. Four hours after UV-irradiation epidermal keratinocytes showed a strong immunoreactivity for thymine-dimers. Gradual repair during an incubation time resulted in few residual thymine-dimers after 48 h. Repair appeared to be more efficient in chronically sun-exposed skin compared with non-sun-exposed skin. There was also an accumulation of p53 protein in epidermal keratinocytes, peaking at 4-24 h after irradiation. Large interindividual differences with respect to formation and repair of thymine-dimers as well as induction and duration of the p53 response were observed. Skin explants treated with topical sunscreen prior to UV-irradiation showed a clear reduction of thymine-dimers and p53 expression. The epidermal UV-responses and repair kinetics in organ-cultured skin were similar to what was found in vivo. Our data suggest that organ-cultured skin provides a valuable tool for studies of UV-induced epidermal responses in chronically sun-exposed skin.

Repeated ultraviolet exposure affords the same protection against DNA photodamage and erythema in human skin types II and IV but is associated with faster DNA repair in skin type IV

We have investigated the photoprotective properties of induced pigmentation using erythema and epidermal DNA photodamage as endpoints. Previously unexposed buttock skin of 12 young, healthy adults (six skin type II and six skin type IV) was exposed daily (Monday to Friday) for 2 wk (days 1-12) with 0.65 minimal erythema dose of solar simulated radiation. Mean skin type IV minimal erythema dose was 1.8-fold greater than for skin type II. Compared to skin type II, solar simulated radiation treatments produced less erythema and more tanning in skin type IV. To assess DNA photodamage, biopsies were taken and prepared for paraffin sections that were stained with a monoclonal antibody for thymine dimers. Thymine dimers were quantified by image analysis. The single exposure data (0.65 and 2 minimal erythema dose) showed that DNA damage was related to physical dose (J per cm2) independent of skin type. Our data also showed that DNA photodamage accumulates in both skin types with repeated, suberythemal doses of solar simulated radiation. On day 12, there were more thymine dimers in skin type IV than skin type II, again indicating that physical rather than biologic dose determines the level of DNA damage. Comparisons on days 12 and 19, however, showed a much greater loss of thymine dimers in skin type IV, suggesting better thymine dimer repair. Protection factors for erythema and thymine dimers were calculated and shown to be about 2 in both skin types. This provides further indirect evidence that DNA is a chromophore for erythema, but also suggests that a tan may not be the major factor in natural photoprotection.

In situ repair of cyclobutane pyrimidine dimers in skin and melanocytic nevi of cutaneous melanoma patients

The development of cutaneous malignant melanoma (CMM) and its precursor lesions, melanocytic nevi, has been linked to sun exposure. Cyclobutane pyrimidine dimers (CPDs) are the majority of DNA lesions induced by sun exposure. In our study, we investigated if CMM patients have impaired ability to repair CPDs in skin as well as in melanocytic nevi. The repair kinetics were followed up to 3 weeks after exposure to 40 mJ/cm(2) of solar simulating radiation. Altogether 12 CMM patients and 10 healthy controls were included in our study. Buttock skin biopsies were taken at 0 hr, 48 hr and 3 weeks after UV exposure, whereas melanocytic nevi and surrounding skin biopsies were taken only at 0 hr and 3 weeks. The CPD levels were measured by a (32)P-postlabeling method. The results showed that the repair rate of CPDs in neither the skin nor the nevi was significantly different between the CMM patients and the control group. For both groups, the repair rate of TT = C was faster than that for TT = T. The important finding is that about 10% of the initial TT = T damage remained unrepaired after 3 weeks, and was detectable in normal epidermis as well as in nevi of all subjects. We also found that the amount of TT = C and TT = T at 0 hr in nevi was significantly lower than that in surrounding skin (Wilcoxon rank sum test, p < 0.05).

Ultraviolet photoproduct levels in melanocytic nevi and surrounding epidermis in human skin in situ

Melanocytic nevi are localized benign proliferations of melanocytes. The number of nevi has been shown to be the major risk marker for the development of cutaneous melanoma. This study compares the induction of photoproducts in nevi and in surrounding skin after exposure to solar-simulating radiation. Cyclobutane pyrimidine dimers (TT=T and TT=C) and 6-4 photoproducts (TT-T and TT-C) were measured in 20 nevi and 20 surrounding skin samples obtained from 14 subjects, using a 32P-postlabeling method. The amount of all four types of photoproducts in nevi was found to be 3-5-fold lower than that in surrounding skin, and the difference was statistically significant (paired t test, p < 0.01). In nevi, the photoproduct level was significantly associated with the color of nevi (the lowest level in the darkest color of nevi; r = -0.86, p < 0.01 for TT=T; r = -0.68, p < 0.01 for TT=C). Our findings suggest that the magnitude of the DNA damage is not a sole risk marker for the development of cutaneous melanoma.

Cyclobutane thymidine dimers are present in human urine following sun exposure: quantitation using 32P-postlabeling and high-performance liquid chromatography

Cyclobutane thymidine dimer (T=T) is the major DNA photoproduct formed in human skin after solar radiation. We have developed a 32P-postlabeling method suitable for quantitating T=T in human urine with a detection limit of about 0.5 fmol per 10 microl urine. The method was used in the present study to measure the daily T=T urinary level of two volunteers over a 15 d period, including frequent sun exposures ranging from 0 to 5 h daily. T=T was not detected before or immediately (4 h) after the initial sun exposure but was first observed in urine samples collected 18 h after the initial exposure. Thereafter, urinary T=T levels gradually increased up to a peak reached about 3 d after the maximum sun exposure. The levels decreased during the following days but were still detectable 8 d after the last sun exposure. About 70-75% decrease in excreted T=T was observed after 8 d. The T=T levels measured in urine were lower but in the same order of magnitude as the levels expected after a theoretical calculation based on previous published results and reasonable assumptions. This study shows the occurrence of cyclobutane thymidine dimers in human urine after skin exposure to solar radiation.

Levels and repair of cyclobutane pyrimidine dimers and 6-4 photoproducts in skin of sporadic basal cell carcinoma patients

The 32P-postlabeling method was applied to measure directly the levels and repair rates of specific cyclobutane pyrimidine dimers and 6-4 photoproducts in 10 basal cell carcinoma patients and 10 controls matched on age, skin type, and gender after exposure to 400 J per m2 of solar simulating radiation on previously unexposed buttock skin. The results showed an identical level of photoproducts at 0 h after solar simulating radiation in the basal cell carcinoma group and the control group. Erythemal response correlated with the repair of cyclobutane pyrimidine dimers within 24 h in both groups, i.e., repair was faster in those with a strong erythemal reaction. The basal cell carcinoma patients showed a somewhat slower repair of photoproducts in skin compared with the controls, but the result was not significant. Photoproducts formed at the TTC sites were repaired faster than those at the TTT sites for both cyclobutane pyrimidine dimers and 6-4 photoproducts in the basal cell carcinoma group and in the controls.

Cutaneous melanoma patients have normal repair kinetics of ultraviolet-induced DNA repair in skin in situ

The DNA lesions induced by ultraviolet radiation include cyclobutane pyrimidine dimers and 6-4 photoproducts. We investigated whether cutaneous melanoma patients have an impaired ability to repair their ultraviolet-induced photolesions. Seventeen patients with melanoma and 13 healthy controls took part in this study. Both groups received a dose of 40 mJ per cm2 Commission Internationale de l'Eclairage of solar simulating radiation on previously unexposed buttock skin. Skin biopsies were taken at 0 h, 24 h, and 48 h after ultraviolet exposure. A 32P-postlabeling method was used to measure both cyclobutane pyrimidine dimers and 6-4 photoproducts in skin. Cyclobutane pyrimidine dimers and 6-4 photoproduct levels did not differ in the melanoma patients from those in the control group at any time point post-ultraviolet radiation. The repair rate of cyclobutane dimer TT=C was faster than that for TT=T both at 24 h and 48 h postirradiation in both groups, providing evidence of site-specific repair (p < 0.05). We conclude that patients with melanoma have a normal ultraviolet-induced DNA repair capacity in skin in situ.