83  2001 American Society for Photobiology 0031-8655/00 $5.00+0.00 Photochemistry and Photobiology, 2001, 73(1): 83–89 Attenuation of DNA Damage in the Dermis and Epidermis of the Albino Hairless Mouse by Chronic Exposure to Ultraviolet-A and -B Radiation ¶ David L. Mitchell*, Michelle Byrom, Stephanie Chiarello and Megan G. Lowery Department of Carcinogenesis, The University of Texas M.D. Anderson Cancer Center, Smithville, TX Received 4 May 2000; accepted 18 October 2000 ABSTRACT Mammalian skin is vulnerable to the photocarcinogenic and photoaging effects of solar UV radiation and defends itself using a variety of photoprotective responses includ- ing 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 frequen- cy 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 attenua- tion was dependent on dose, wavelength and the type of damage induced. Although epidermal thickening was im- portant, our data suggest that UV protective compounds other than melanin may be involved in mitigating the damaging effects of sunlight in the skin. INTRODUCTION Solar UV radiation (UVR) causes skin cancer in man. Skin cancer, like all other cancers, is a genetic disorder with ini- tiation, promotion and progression phases associated with loss of genomic stability. Chronic exposure to sunlight has been linked to basal and squamous cell carcinomas that ap- pear on exposed parts of the body; acute exposure in ado- lescence has been suggested as a risk factor for melanoma (1). We are routinely exposed to low levels of solar ultra- violet-B (UVB)² (290–320 nm) and ultraviolet-A (UVA) (320–400 nm) radiation, the dose depending on geographical location, time of day, occupation and recreational or cos- metic behavior. In order to develop strategies for the pre- ¶Posted on the website on 22 November 2000. *To whom correspondence should be addressed at: The University of Texas M.D. Anderson Cancer Center, Science Park/Research Divi- sion, P.O. Box 389, Smithville, TX 78957, USA. Fax: 512-237-2437; e-mail: dmitch@io.com ² Abbreviations: CPD, cyclobutane pyrimidine dimer; MAA, myco- sporine-like amino acid; MED; minimal erythema dose; (6–4)PD, pyrimidine(6–4)pyrimidone dimer; RIA, radioimmunoassay; TE buffer, Tris–ethylenediaminetetraacetic acid buffer; UVA, ultra- violet-A radiation; UVB, ultraviolet-B radiation; UVR, ultraviolet radiation. vention of skin cancer a detailed understanding of how the skin protects itself against DNA damage would be beneficial. Dimerizations between adjacent pyrimidine bases in DNA are the predominant premutagenic events leading to the ini- tiation of basal and squamous cell carcinomas (2). The pre- dominant photoproducts occurring at these sites are the cy- clobutane pyrimidine dimer (CPD), pyrimidine(6–4)pyrimi- done dimer ([6–4]PD) and the Dewar photoisomer of the (6– 4)PD. Transition mutations (C → T) and tandem double mutations (CC → TT) arising at sites of paired pyrimidines or in a pyrimidine tract have been found in the p53 tumor suppressor gene of 50% of human basal cell carcinomas (3) and are considered the ‘‘signature’’ mutations of UVB radiation. As in humans, C → T transition mutations flourish in the p53 gene of murine skin tumors (4,5) and it is prob- able that in mice, as in men, the CPD and (6–4)PD are im- portant photocarcinogenic determinants. The mouse skin model has been used for many years to investigate the mech- anisms of multistage carcinogenesis (6) and chronic expo- sure of the albino hairless (Skh-1) mouse to UVB radiation has become the paradigm for laboratory studies on photo- carcinogenesis (7) and photoaging (8). In many ways tu- morigenesis in mouse and human skin is very similar (9). Although preventable, exposure to sunlight is hardly avoidable and, in fact, is essential for our sustenance and well being (e.g. vitamin D metabolism). Mammalian skin has evolved photoprotective mechanisms to reduce the ef- fective dose of UVR to the genetic material that include primarily a UV filter at the surface (i.e. the stratum corne- um), a diffusible UV-absorbing pigment in the epidermis (i.e. melanin) and inducible free-radical scavengers. Chronic exposure to low levels of sunlight activates and enhances these photoprotective mechanisms and thus reduces the risk of skin cancer. The capacity of chronic and acute UV ex- posures to protect against erythema in human and mouse skin has been studied in some detail, yet little is known about how chronic exposure to sunlight might affect DNA damage formation. Vink and coworkers (10,11) analyzed the induction, repair and accumulation of CPD in the skin of UVB-irradiated hairless mice receiving a short course of chronic UVB using immunohistochemical techniques. Mice were exposed daily for 11 days to 1.5 kJ/m 2 UVB radiation (i.e. 0.5–1  minimal erythema dose [MED]) from a broad spectrum source, and CPD was quantified immediately and at 24 h after irradiation in suprabasal and basal cells in the epidermis. They found that CPD accumulated in epidermal