Postnatal growth failure, short life span, and early onset of cellular senescence and subsequent immortalization in mice lacking the xeroderma pigmentosum group G gene.

The xeroderma pigmentosum group G (XP-G) gene (XPG) encodes a structure-specific DNA endonuclease that functions in nucleotide excision repair (NER). XP-G patients show various symptoms, ranging from mild cutaneous abnormalities to severe dermatological impairments. In some cases, patients exhibit growth failure and life-shortening and neurological dysfunctions, which are characteristics of Cockayne syndrome (CS). The known XPG protein function as the 3' nuclease in NER, however, cannot explain the development of CS in certain XP-G patients. To gain an insight into the functions of the XPG protein, we have generated and examined mice lacking xpg (the mouse counterpart of the human XPG gene) alleles. The xpg-deficient mice exhibited postnatal growth failure and underwent premature death. Since XPA-deficient mice, which are totally defective in NER, do not show such symptoms, our data indicate that XPG performs an additional function(s) besides its role in NER. Our in vitro studies showed that primary embryonic fibroblasts isolated from the xpg-deficient mice underwent premature senescence and exhibited the early onset of immortalization and accumulation of p53.

DNA binding mode of the Fab fragment of a monoclonal antibody specific for cyclobutane pyrimidine dimer.

Monoclonal antibodies specific for the cyclobutane pyrimidine dimer (CPD) are widely used for detection and quantification of DNA photolesions. However, the mechanisms of antigen binding by anti-CPD antibodies are little understood. Here we report NMR analyses of antigen recognition by TDM-2, which is a mouse monoclonal antibody specific for the cis - syn -cyclobutane thymine dimer (T[ c, s ]T). (31)P NMR and surface plasmon resonance data indicated that the epitope recognized by TDM-2 comprises hexadeoxynucleotides centered on the CPD. Chemical shift perturbations observed for TDM-2 Fab upon binding to d(T[ c, s ]T) and d(TAT[ c, s ]TAT) were examined in order to identify the binding sites for these antigen analogs. It was revealed that d(T[ c, s ]T) binds to the central part of the antibody-combining site, while the CPD-flanking nucleotides bind to the positively charged area of the V(H)domain via electrostatic interactions. By applying a novel NMR method utilizing a pair of spin-labeled DNA analogs, the orientation of DNA with respect to the antigen-binding site was determined: CPD-containing oligonucleotides bind to TDM-2 in a crooked form, draping the 3'-side of the nucleotides onto the H1 and H3 segments, with the 5'-side on the H2 and L3 segments. These data provide valuable information for antibody engineering of TDM-2.

Multistep nature of X-ray-induced neoplastic transformation in golden hamster embryo cells: expression of transformed phenotypes and stepwise changes in karyotypes.

We have examined the expression of transformed phenotypes and genetic changes associated with the expression of each transformed phenotype after X-ray irradiation. Unirradiated cells grown at a constant growth rate until 8 passages (population doubling number, 15) exhibited little morphological change and ceased to divide thereafter. X-irradiated cells escaped from senescence and showed morphological alteration and anchorage independence after a population doubling number of 20. The acquisition of tumorigenicity in nude mice was observed much later (35 population doublings after irradiation). From cytogenetic analysis, all anchorage-independent clones were consistently found to have trisomy of chromosome 7. Furthermore, cells derived from tumors contained three copies of chromosome 9q in addition to the trisomy of chromosome 7. We have not detected any augmented expression of v-Ha-ras- and v-myc-related oncogenes with RNA dot-blot analysis and could not find activation of any type of oncogenes by NIH3T3 transfection experiments. Our studies demonstrated that X-ray-induced neoplastic transformation is a multistep phenomenon and that the numerical change of specific chromosomes may play an important role in the expression of each transformed phenotype. The results suggest that different endogenous oncogenes, other than the ras gene family and myc oncogene, could be responsible for the progressive nature of neoplastic transformation.

Identification of cellular defect in UVS1, a UV-sensitive Chinese hamster ovary mutant cell line.

UVS1 is an intermediately UV-sensitive Chinese hamster ovary mutant originally isolated by its hypersensitivity to an anticancer drug, 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride. By cell fusion analysis, UVS1 complemented the UV sensitivity of the mouse lymphoma cell line US31 from the eighth complementation group of UV-sensitive rodent cell lines. By enzyme-linked immunosorbent assay we found that within 3 h after UV irradiation both pyrimidine dimers and (6-4)photoproducts in UVS1 were not removed from chromosomal DNA in UVS1 at all. Twenty-four h after UV irradiation the removal rate of (6-4)photoproducts was intermediate between CHO9, the parental cell line, and 43-3B, a UV-hypersensitive Chinese hamster ovary mutant of the complementation group 1, whereas the pyrimidine dimers in UVS1 were removed less efficiently as 43-3B. Alkaline elution assay showed that the incising activity to damaged DNA after UV irradiation of UVS1 was as low as that of 43-3B. The number of 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride-induced DNA interstrand cross-links of UVS1 was almost equal to that of 43-3B and about 1.5 times more than that of CHO9, suggesting that the gene products defective in UVS1 and 43-3B are essential for the excision repair of DNA damages produced by 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride.

Induction of cyclobutane pyrimidine dimers, pyrimidine(6-4)pyrimidone photoproducts, and Dewar valence isomers by natural sunlight in normal human mononuclear cells.

Immunocytochemistry was used for the direct measurement of cyclobutane pyrimidine dimers, (6-4) photoproducts, and Dewar isomers in normal human mononuclear cells following irradiation by natural sunlight or by a FS20 broad spectrum UVB sunlamp. The induction of each type of photoproduct was detected following 30-60 min sunlight exposure or with FS20 fluences as low as 50-100 Jm-2. With increasing FS20 fluences, there was a dose-dependent increase in the binding of pyrimidine dimer, (6-4) photoproduct, and Dewar isomer-specific monoclonal antibodies. The relative ratio of Dewar isomer to (6-4) photoproduct antibody binding sites was much higher following exposure to natural sunlight than to broad spectrum UVB. With the (6-4) monoclonal antibody, a small increase in binding sites was evident after a 1-h exposure to natural sunlight. This remained relatively constant with further exposure. These results are consistent with the hypothesis that, following irradiation with natural sunlight, the majority of (6-4) photoproducts are converted into Dewar valence isomers.

Hypersensitivity of human lymphocytes to UV-B and solar irradiation.

T-lymphocytes from three normal human donors, irradiated with broad-spectrum UV-B (peak emission, 312 nm), are 20-fold more sensitive than fibroblasts from four normal donors in a clonogenic assay. We have compared the formation of thymine cyclobutane dimers and pyrimidine-(6-4)-pyrimidone photoproducts following irradiation by UV-C (254 nm) and UV-B and studied killing at doses giving equal dimer formation. UV-B killing of fibroblasts appears to be associated with dipyrimidine photoproduct formation, whereas UV-B killing of lymphocytes is mediated by nondimer damage. Strand breakage following UV-B irradiation measured using the "Comet" assay (single cell gel electrophoresis) reflects this nondimer damage and has kinetics consistent with excisable damage. Lymphocytes from three excision-deficient xeroderma pigmentosum donors show reduced strand breakage and increased killing following UV-B irradiation, compared with lymphocytes from normal donors. We therefore suggest that UV-B kills human lymphocytes by excisable nondimer damage and that xeroderma pigmentosum lymphocytes are defective in its repair. The putative nondimer damage does not appear to be associated with radical attack, and the strand breakage is not a manifestation of apoptosis. A 1-min exposure of human lymphocytes in vitro to natural sunlight is sufficient to produce damage measurable by the Comet assay.

Different removal of ultraviolet photoproducts in genetically related xeroderma pigmentosum and trichothiodystrophy diseases.

To understand the heterogeneity in genetic predisposition to skin cancer in different nucleotide excision repair-deficient human syndromes, we studied repair of cyclobutane pyrimidine dimers (CPDs) and of pyrimidine(6-4)pyrimidone (6-4PP) photoproducts in cells from trichothiodystrophy (TTD) patients. TTD is not associated with increased incidence of skin cancer, although 50% of the patients are photosensitive and carry a defect in the nucleotide excision repair pathway, similar to Xeroderma pigmentosum patients. However, in striking contrast to TTD, Xeroderma pigmentosum is highly prone to cancer. To address this apparent paradox, two types of studies were conducted: (a) reactivation of UV-irradiated plasmids harboring actively transcribed reporter genes, with or without photolyase treatment before transfection of SV40-transformed fibroblasts; and (b) the kinetics of removal of UV-induced CPDs and 6-4PPs in genomic DNA by immunoblot analysis using lesion-specific mAbs in SV40-transformed and untransformed fibroblasts representative of all genetic TTD complementation groups. Results showed that all cell lines from photosensitive TTD patients efficiently express Cat or luciferase genes in transfected plasmids carrying non-CPD lesions, including 6-4PP, and display wild-type or near-wild-type (50-70% in 3 cell lines) 6-4PP repair in the overall genome after immunoblot analysis. However, CPD lesions (the repair of which is defective in the overall genome) also block the expression of the reporter gene in transfected plasmids. Two cell lines from nonphotosensitive TTD patients showed wild-type levels of repair for both photoproducts in overall genome. A model on the lesion-specific repair in the context of the molecular defect in TTD is proposed. The implication of the defective CPD repair and efficient 6-4PP repair subpathways in cancer prevention in TTD patients is discussed.

Respective roles of cyclobutane pyrimidine dimers, (6-4)photoproducts, and minor photoproducts in ultraviolet mutagenesis of repair-deficient xeroderma pigmentosum A cells.

The role of UV light-induced photoproducts in initiating base substitution mutation in human cells was examined by determining the frequency and spectrum of mutation in a supF tRNA gene in a shuttle vector plasmid transfected into DNA repair deficient cells (xeroderma pigmentosum complementation group A). To compare the role of two major UV-induced photoproducts, cis-syn cyclobutane-type pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs), each photoproduct was removed from UV-irradiated plasmid by photoreactivation before transfection. Removal of either CPDs or 6-4PPs by in vitro photoreactivation reduced the mutation frequency while keeping the mutation distribution and the predominance of G:C-A:T transitions as UV-irradiated plasmid without photoreactivation, indicating that both cytosine-containing CPDs and 6-4PPs were premutagenic lesions for G:C-A:T transitions. On the other hand, A:T-G:C transitions were not recovered from plasmids after the removal of 6-4PPs, whereas this type of mutation occurred at a significant level (11%) after the removal of CPDs. Thus, the premutagenic lesions for the A:T-G:C transition are 6-4PPs. Removal of both CPDs and 6-4PPs resulted in the disappearance of mutational hot spots and random distribution of mutation as observed in unirradiated control plasmids. However, the mutational spectrum of photoreactivated plasmids differed significantly from that of unirradiated plasmids. A characteristic feature is a high portion of A:T-T:A transversions (11%) in the photoreactivated plasmid. This mutation is due to nondipyrimidinic "minor" photoproducts, and the mutation spectrum suggests that TA*, the major photoproduct of thymidylyl-(3'-5')-deoxyadenosine, is the premutagenic lesion for this mutation. This is the first report revealing the distinct mutagenic roles of the major UV photoproducts and "minor" photoproducts by the use of (6-4)photolyase.

Transcriptionally active and inactive genes are similarly modified by chemical carcinogens or X-ray in normal human fibroblasts.

Chemical carcinogens and ionizing radiation induce DNA modifications and strand breaks in cells. This damage is reported to be affected by chromatin proteins or chromatin of a higher structure order. To compare the sensitivity of transcriptionally active and inactive genes on chromatin toward DNA-damaging agents, we treated normal human fibroblasts (WI-38) cells in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), X-ray, 4-hydroxyaminoquinoline 1-oxide or N-acetoxy-2-acetylaminofluorene, and high molecular weight DNA was isolated. After digestion with EcoRI to completion, the DNA was electrophoresed on an alkaline agarose gel, blotted on a nitrocellulose filter and hybridized with a transcriptionally active gene probe (human type I(alpha 2) procollagen gene) or an inactive gene probe (human beta-globin gene). The results show that both genes are similarly modified by these agents. Repair of DNA damage caused by MNNG also occurred similarly in collagen and beta-globin genes after removal of MNNG.

Antibodies specific for (6-4) DNA photoproducts: cloning, antibody modeling and construction of a single-chain Fv derivative.

We have investigated a series of four monoclonal antibodies that specifically recognize pyrimidine (6-4) pyrimidone photoproducts. One of these antibodies (64M4), bound all four possible pyrimidine-pyrimidone photoadducts with equal affinities whereas the others (64M2, 64M3 and 64M5) were selective for TC and TT sequences. In addition, 64M5 had the highest binding affinity for photodamaged DNA of the four [T. Mori et al., Photochem. Photobiol. 54 (1991) 225-232]. To help understand the differences between these antibodies, we have cloned and sequenced the variable region genes from all four. Comparing these sequences revealed that all four were highly similar to one another, although there were some differences in potential antigen-contact regions. To assess the influences of these sequence differences at the structural level, computer models were constructed for all four antibodies. Most of the sequence differences occurred in potential antigen contact regions, suggesting specific positions that might account for the observed differences in binding affinities and selectivities. A single-chain Fv derivative of 64M5 was therefore constructed and characterized to provide an experimental system in which structure-function relationships can be tested. This derivative could be isolated from Escherichia coli using two chromatographic steps and possessed the same binding specificity as the parent monoclonal antibody.

Crystal structure of the 64M-2 antibody Fab fragment in complex with a DNA dT(6-4)T photoproduct formed by ultraviolet radiation.

DNA photoproducts with (6-4) pyrimidine-pyrimidone adducts formed by ultraviolet radiation are implicated in mutagenesis and cancer, particularly skin cancer. The crystal structure of the Fab fragment of the murine 64M-2 antibody specific to DNA T(6-4)T photoproducts is determined as a complex with dT(6-4)T, a (6-4) pyrimidine-pyrimidone photodimer of dTpT, at 2.4 A resolution to a crystallographic R-factor of 0.199 and an R(free) value of 0.279. The 64M-2 Fab molecule is in an extended arrangement with an elbow angle of 174 degrees, and its five complementarity-determining regions, except L2, are involved in the ligand binding. The bound dT(6-4)T ligand adopting a ring structure with (6-4) linked 5' thymine-3' pyrimidone bases is fully accommodated in an antigen-binding pocket of about 15 Ax10 A. The 5'-thymine and 3'-pyrimidone bases are in half-chair and planar conformations, respectively, and are nearly perpendicular to each other. The 5'-thymine base is hydrogen-bonded to Arg95H and Ser96H, and is in van der Waals contact with Tyr100iH. The 3'-pyrimidone base is hydrogen-bonded to His35H, and is in contact with Trp33H. Three water molecules are located at the interface between the bases and the Fab residues. Hydrogen bonds involving these water molecules also contribute to Fab recognition of the dT(6-4)T bases. The sugar-phosphate backbone connecting the bases is surrounded by residues His27dL, Tyr32L, Ser92L, Trp33H, and Ser58H, but is not hydrogen-bonded to these residues.

Conformational multiplicity of the antibody combining site of a monoclonal antibody specific for a (6-4) photoproduct.

The antigen binding site of monoclonal antibody 64M5, which possesses a high degree of affinity for DNA containing pyrimidine (6-4) pyrimidone photoproducts, were investigated by use of stable-isotope-assisted NMR spectroscopy. A variety of 64M5 Fab fragments specifically labeled with 13C and 15N at backbone amide groups were prepared. Extensive assignments of amide resonances originating from the variable region of 64M5 were made by using 2D-HN(CO) measurements along with recombination of the heavy and light chains of 64M5. On the basis of chemical shift changes of the amide resonances caused upon addition of d(T[6-4]T) and d(GTAT[6-4]TATG), the binding sites of 64M5 Fab for the (6-4) photodimer and for the oligodeoxynucleotides flanking it were identified. It was revealed that the L1 and L3 segments, which are responsible for the binding to (6-4) photodimer, exhibit conformational multiplicities in the absence of antigens, and take different conformations between the d(T[6-4]T) and d(GTAT[6-4]TATG)-bound forms. On the basis of spectral comparison with another Fab fragment with a similarity in the amino acid sequence of the VL domain of 64M5, we suggest that the conformational multiplicities observed in the present study is caused by a substitution of an amino acid residue at the position of a key residue in L3 canonical structure, which leads to a preferable effect on the antigen binding, and by a specific combination of L1 and L3 canonical structures.

The spectrum of mitochondrial DNA deletions is a ubiquitous marker of ultraviolet radiation exposure in human skin.

We and colleagues have suggested that deletions of mitochondrial DNA may be useful as a biomarker of ultraviolet radiation exposure in skin. In this study using a southwestern approach involving monoclonal antibodies against thymine dimers we provide direct evidence for the presence of ultraviolet-induced damage in mitochondrial DNA purified from any nuclear DNA contamination. Previous studies have been limited, as they have focused on the frequency of a single mitochondrial DNA deletion. Therefore we have addressed the question of the spectrum of mitochondrial DNA deletions in skin and whether this can be used as an index of overall DNA damage. We have used a long polymerase chain reaction technique to determine the mitochondrial DNA deletion spectrum of almost the entire mitochondrial genome in 71 split skin samples in relation to sun exposure. There was a significant increase in the number of deletions with increasing ultraviolet exposure in the epidermis (Kruskal-Wallis test, p = 0.0015) but not the dermis (p = 0.6376). The findings in the epidermis are not confounded by any age-dependent increases in mitochondrial DNA deletions also detected by the long polymerase chain reaction technique. The large spectrum of deletions identified in our study highlights the ubiquitous nature and the high mutational load of mitochondrial DNA associated with ultraviolet exposure and chronologic aging. Compared with the detection of single deletions using competitive polymerase chain reaction, we show that long polymerase chain reaction is a sensitive technique and may therefore provide a more comprehensive, although not quantitative, index of overall mitochondrial DNA damage in skin.

Studies on epidermis reconstructed with and without melanocytes: melanocytes prevent sunburn cell formation but not appearance of DNA damaged cells in fair-skinned caucasians.

To assess the photoprotective role of melanocytes in the epidermis, we studied the effects of ultraviolet B on an epidermis reconstructed with and without melanocytes. To address more specifically the role of melanin in fair-skinned individuals, experiments were done with cells obtained from human skin of low phototypes (II-III). To study the effect of constitutive melanin and possibly that of newly synthesized melanin precursors, a single dose of ultraviolet B (0.10 or 0.15 J per cm2, corresponding to a 4-5 minimal erythema dose in vivo) was administered to reconstructs and the effects were monitored over the first 24 h. When reconstructs with and without melanocytes were compared, no difference was found for DNA damage/repair assessed with antibodies to cyclobutane pyrimidine dimers and 6-4 photoproducts. More necrotic/apoptotic cells, however, were noted 24 h following ultraviolet B irradiation in reconstructs lacking melanocytes. Twenty-four hours following ultraviolet B irradiation the number of necrotic/apoptotic cells and the number of cyclobutane pyrimidine dimer positive cells was coarsely concentration-dependent. The number of cyclobutane pyrimidine dimer positive cells, however, was independent of the type of reconstruct used (with/without melanocytes). In conclusion, low phototype melanocytes seem to protect epidermal basal cells against ultraviolet B-induced apoptosis/necrosis and may preserve the overall integrity of the epidermis after ultraviolet B irradiation. On the contrary, such melanocytes do not seem to have a protective role against DNA damage and may not prevent cancer.

The similarity of action spectra for thymine dimers in human epidermis and erythema suggests that DNA is the chromophore for erythema.

The location of DNA photodamage within the epidermis is crucial as basal layer cells are the most likely to have carcinogenic potential. We have determined the action spectra for DNA photodamage in different human epidermal layers in situ. Previously unexposed buttock skin was irradiated with 0.5, 1, 2, and 3 minimal erythema doses of monochromatic UVR at 280, 290, 300, 310, 320, 340, and 360 nm. Punch biopsies were taken immediately after exposure and paraffin sections were prepared for immunoperoxidase staining with a monoclonal antibody against thymine dimers that were quantitated by image analysis. Dimers were measured at two basal layer regions, the mid and the upper living epidermis. The slopes of dose-response curves were used to generate four action spectra, all of which had maxima at 300 nm. Dimer action spectra between 300 and 360 nm were independent of epidermal layer, indicating comparable epidermal transmission at these wavelengths. Furthermore, we observed 300 nm-induced dimers in dermal nuclei; however, there was a marked effect of epidermal layer between 280 and 300 nm, showing relatively poor transmission of 280 and 290 nm to the basal layer. These data indicate that solar UVB (approximately 295-320 nm) is more damaging to basal cells than predicted from transmission data obtained from human epidermis ex vivo. The epidermal dimer action spectra were compared with erythema action spectra determined from the same volunteers and ultraviolet radiation sources. Overall, these spectral comparisons suggest that DNA is a major chromophore for erythema in the 280-340 nm region.

Repair of 254 nm ultraviolet-induced (6-4) photoproducts: monoclonal antibody recognition and differential defects in xeroderma pigmentosum complementation groups A, D, and variant.

Repair kinetics of ultraviolet (UV) light-induced (6-4) photoproducts in xeroderma pigmentosum complementation group A, D, and variant cells were studied by the enzyme-linked immunosorbent assay (ELISA) using a specific monoclonal antibody raised against (6-4) photoproducts, together with unscheduled DNA synthesis (UDS) and loss of T4 endonuclease V-susceptible sites (ESS). Group AXP35KO cells completely failed to repair both ESS (cyclobutane pyrimidine dimers) and antibody-recognizing (6-4) photoproducts until tested 24 h after irradiation, and had 2% early-time UDS. Group DXP43KO cells showed about 10% removal of both (6-4) photoproducts and ESS in 24 h, despite showing a residually higher level of 40% early-time and cumulative UDS. Thus, the results substantiated the extreme UV hypersensitivity of XP group A and D cells. However, XP52KO variant cells exhibited the normal level of UDS and ESS loss, but a slightly reduced repair of antibody-recognizing (6-4) photoproducts at 6 and 12 h after irradiation, which may account for a small UV hypersensitivity of the XP variant cells.

Nucleotide excision repair genes are upregulated by low-dose artificial ultraviolet B: evidence of a photoprotective SOS response?

Nucleotide excision repair is a major mechanism of defense against the carcinogenic effects of ultraviolet light. Ultraviolet B causes sunburn and DNA damage in human skin. Nucleotide excision repair has been studied extensively and described in detail at the molecular level, including identification of many nucleotide excision repair-specific proteins and the genes encoding nucleotide excision repair proteins. In this study, normal human keratinocytes were exposed to increasing doses of ultraviolet B from fluorescent sunlamps, and the effect of this exposure on expression of nucleotide excision repair genes was examined. An RNase protection assay was performed to quantify transcripts from nucleotide excision repair genes, and a slot blot DNA repair activity assay was used to assess induction of the nucleotide excision repair pathway. The activity assay demonstrated that cyclobutane pyrimidine dimers were removed efficiently after exposure to low doses of ultraviolet B, but this activity was delayed significantly at higher doses. All nucleotide excision repair genes examined demonstrated a similar trend: ultraviolet B induces expression of nucleotide excision repair genes at low doses, but downregulates expression at higher doses. In addition, we show that pre-exposure of cells to low-dose ultraviolet protected keratinocytes from apoptosis following high-dose exposure. These data support the notion that nucleotide excision repair is induced in cells exposed to low doses of ultraviolet B, which may protect damaged keratinocytes from cell death; however, exposure to high doses of ultraviolet B downregulates nucleotide excision repair genes and is associated with cell death.

8-hydroxy-2'-deoxyguanosine is increased in epidermal cells of hairless mice after chronic ultraviolet B exposure.

8-Hydroxy-2'-deoxyguanosine (8-OHdG) is a mutation-prone (G:C to T:A transversion) DNA base-modified product generated by reactive oxygen species or photodynamic action. G:C to T:A transversions are observed in the p53 and ras genes of UVB-induced skin cancers of mice and in squamous and basal cell carcinomas of human skin exposed to sunlight. In the current study, 8-OHdG formation was evaluated in the epidermis of hairless mice after repeated exposure to UVB, and possible mechanisms involved were studied. Exposure of hairless mice to either 3.4 [2 minimal erythema dose (MED)] or 16.8 (10 MED) kJ/m2 of UVB three times a week for 2 wk induced a 2.5- or 6.1-fold increase, respectively, in the levels of 8-OHdG in DNA, compared to the unexposed controls. An immunohistochemical method using a monoclonal antibody specific for 8-OHdG showed stronger and more extensive staining in the nuclei of UV-irradiated epidermal cells than in those of nonirradiated cells. Western blots probed with antibodies against 4-hydroxy-2-nonenal-modified proteins confirmed the involvement of reactive oxygen species in the epidermal damage induced by chronic UVB exposure. 3-Nitro-L-tyrosine was detected in western blots in a concentration-dependent manner, suggesting that peroxynitrite derived from the reaction of nitric oxide and superoxide, both of which were probably released from inflammatory cells, was involved in modifying the DNA bases. Therefore, the formation of 8-OHdG after UVB exposure appears to be regulated by at least three pathways: photodynamic action, lipid peroxidation, and inflammation and may play a role in sunlight-induced skin carcinogenesis.

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.

The in situ repair kinetics of epidermal thymine dimers and 6-4 photoproducts in human skin types I and II.

We assessed the in situ time-dependent loss of epidermal thymine dimers and 6-4 photoproducts in skin types I and II after exposure to two minimal erythema doses of solar-simulating radiation on previously unexposed buttock skin. Using quantitative image analysis, we evaluated biopsy sections stained with monoclonal antibodies. We then made comparisons, in the same volunteers, with unscheduled DNA synthesis, which is a direct marker of overall excision repair. Removal of thymine dimers was slow (half-life = 33.3 h), with high levels of lesions still present 24 h post-irradiation; some lesions were still present at 7 d. In contrast, removal of 6-4 photoproducts was rapid (half-life = 2.3 h), the decay kinetics of which correlated better with the decline in epidermal unscheduled DNA synthesis (half-life = 7.1 h). These data show that as in mouse, monkey, and in vitro models, the 6-4 photolesion is repaired preferentially in human epidermis in situ. They also raise the possibility that poor thymine dimer repair may be a feature of skin types I and II, who are more prone to skin cancer than are types III and IV. There was an inverse relationship between the onset of erythema and 6-4 photoproduct repair, suggesting that this repair process initiates erythema.