Effects of chronic low-dose ultraviolet B radiation on DNA damage and repair in mouse skin.

Chronic exposure to sunlight causes skin cancer in humans, yet little is known about how habitual exposure to low doses of ultraviolet B radiation (UVB) affects DNA damage in the skin. We treated Skh-1 hairless mice with daily doses of suberythemal UVB for 40 days and analyzed the amount and distribution of DNA photodamage using RIAs and immunofluorescence micrography. We found that DNA damage accumulated in mouse skin as a result of chronic irradiation and that this damage persisted in the dermis and epidermis for several weeks after the chronic treatment was terminated. Although the persistent damage was evenly distributed throughout the dermis, it remained in the epidermis as a small number of heavily damaged cells at the dermal-epidermal boundary. Rates of DNA damage induction and repair were determined at different times over the course of chronic treatment in response to a higher challenge dose of UVB light. The amount of damage induced by the challenge dose increased in response to chronic exposure, and excision repair of cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone dimers was significantly reduced. The sensitization of mouse epidermal DNA to photoproduct induction, the reduction in excision repair, and the accumulation of nonrepairable DNA damage in the dermis and epidermis suggest that chronic low-dose exposure to sunlight may significantly enhance the predisposition of mammalian skin to sunlight-induced carcinogenesis.

Effects of chronic exposure to ultraviolet B radiation on DNA repair in the dermis and epidermis of the hairless mouse.

It has previously been shown that chronic exposure to low fluences of ultraviolet B radiation reduced DNA repair capacity in mouse skin. In this study we now extend this to examine the concentration dependence and tissue dependence of this phenomenon. We found that (6-4) photoproducts were repaired considerably faster than cyclobutane dimers and that the kinetics for photoproduct removal were comparable in the dermis and epidermis. Chronic ultraviolet B irradiation significantly reduced the initial rate and extent of DNA repair. After low daily doses of ultraviolet B (6-4) photoproduct repair was most affected and after high daily doses the repair of both cyclobutane and (6-4) dimers was reduced. Whereas cyclobutane dimer repair was most affected in the dermis, reduced (6-4) photoproduct repair was observed in both tissues. The deleterious effects of chronic ultraviolet exposure were sustained for a considerable time after the chronic treatment ended.

Identification of a non-dividing subpopulation of mouse and human epidermal cells exhibiting high levels of persistent ultraviolet photodamage.

The distribution and persistence of cyclobutane pyrimidine dimers were investigated in mouse skin after chronic and acute exposures to ultraviolet-B radiation. We found that DNA damage accumulated in response to chronic irradiation and persisted in a unique set of epidermal cells located at the basal layer. Treatment with a tumor promoter caused the heavily damaged epidermal cells to divide and p53-immunopositive clusters to form within 24 h suggesting that these cells may be progenitors of the mutant p53 clusters associated with actinic keratoses and squamous cell carcinomas. In contrast to low fluence chronic irradiation, daily treatment with a higher fluence of ultraviolet-B produced extensive hyperplasia and considerably reduced penetration of photodamage. Exposure of chronically irradiated skin to an acute "sunburn dose" of ultraviolet-B also produced significant epidermal hyperplasia and resulted in complete loss of heavily damaged basal cells within 4 d postirradiation. The occurrence and distribution of cyclobutane dimers in human skin correlated well with putative sunlight exposure and resembled that observed in ultraviolet-B-irradiated mice. Heavily damaged basal cells were observed at various sites, including those receiving sporadic sunlight exposure, suggesting that these cells may play an important role in carcinoma formation in humans.

Rel/NF-kappaB represses bcl-2 transcription in pro-B lymphocytes.

The mechanisms controlling programmed cell death (PCD) during early B cell development are not well understood. Members of both the Bcl-2 family of apoptosis-related proteins and the nuclear factor-kappa B/Rel (NF-kappaB/Rel) family of transcription factors are expressed differentially during B cell development. To date, however, no direct interactions between these two families have been demonstrated. The FL5.12 cell line represents a model for progenitor B cell development. Such cells reproducibly undergo PCD upon IL-3 withdrawal. The signal to enter the apoptotic pathway is mediated by a shift in the ratio of Bcl-2:Bax. While bax levels remain constant, bcl-2 transcription rate, steady-state mRNA, and protein levels decrease. Analysis of the bcl-2 promoter reveals 3 kappaB sites functionally able to bind kappaB factors from FL5.12 nuclear extracts. Cotransfection studies demonstrate that NF-kappaB factors can repress bcl-2 transcription and that site-directed mutagenesis of the kappaB motifs abolishes this repression. These studies suggest that NF-kappaB mediates PCD in pro-B cells through transcriptional repression of the survival gene bcl-2, thus shifting the bcl-2:bax ratio in favor of death-promoting complexes.

Attenuation of DNA damage in the dermis and epidermis of the albino hairless mouse by chronic exposure to ultraviolet-A and -B radiation.

Mammalian skin is vulnerable to the photocarcinogenic and photoaging effects of solar UV radiation and defends itself using a variety of photoprotective responses including epidermal thickening, tanning and the induction of repair and antiradical systems. We treated Skh-1 albino hairless mice for 60 days with ultraviolet-A (UVA) or ultraviolet-B (UVB) radiation and measured the frequency of cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts induced by a single acute sunburn dose of UVB at different stages of the chronic treatment. We found that both UVA and UVB exposure produced a photoprotective response in the dermis and epidermis and that the degree of photoproduct attenuation was dependent on dose, wavelength and the type of damage induced. Although epidermal thickening was important, our data suggest that UV protective compounds other than melanin may be involved in mitigating the damaging effects of sunlight in the skin.

Resolution of UV-induced DNA damage in Xiphophorus fishes.

The genus Xiphophorus is an important model for investigating the etiology and genetics of sunlight-induced melanoma as well as other cancers. We investigated the role DNA damage plays in tumorigenesis in Xiphophorus using a variety of immunological techniques to examine the induction, distribution, and repair of the major photoproducts in DNA after exposure to solar (ultraviolet-B) radiation. We found that cyclobutane pyrimidine dimers (CPDs) were induced at 5- to 10-fold greater frequency than the (6-4) photoproduct ((6-4)PD) in Xiphophorus signum, and the efficiency of photoproduct formation was tissue-dependent, with the scales providing considerable photoprotection against both types of damage. Both of these lesions are efficiently repaired in the presence of visible light by photoenzymatic repair with CPDs repaired at about twice the rate of (6-4)PDs. Photoenzymatic repair of cyclobutane dimers is inducible by prior exposure to low levels of visible light and can be extremely rapid, with most of the lesions removed within 30 minutes. In the absence of light, dimers are removed by nucleotide excision repair with somewhat greater efficiency for the (6-4)PD compared with the CPD in most species. The relative efficiencies of nucleotide excision repair and photoenzymatic repair are tissue-specific and species-specific. The diverse photochemical and photobiological responses observed in Xiphophorus fishes suggest that heritable traits governing the induction and repair of DNA damage may be involved in the susceptibility of Xiphophorus hybrids to melanomagenesis.