Environmental risk factors for skin cancer

UVA radiation is highly mutagenic in cells that are unable to repair 7,8-dihydro-8-oxoguanine in Saccharomyces cerevisiae

UVA (320-400 nm) radiation constitutes >90% of the environmentally relevant solar UV radiation, and it has been proposed to have a role in skin cancer and aging. Because of the popularity of UVA tanning beds and prolonged periods of sunbathing, the potential deleterious effect of UVA has emerged as a source of concern for public health. Although generally accepted, the impact of DNA damage on the cytotoxic, mutagenic, and carcinogenic effect of UVA radiation remains unclear. In the present study, we investigated the sensitivity of a panel of yeast mutants affected in the processing of DNA damage to the lethal and mutagenic effect of UVA radiation. The data show that none of the major DNA repair pathways, such as base excision repair, nucleotide excision repair, homologous recombination, and postreplication repair, efficiently protect yeast from the lethal action of UVA radiation. In contrast, the results show that the Ogg1 DNA glycosylase efficiently prevents UVA-induced mutagenesis, suggesting the formation of oxidized guanine residues. Furthermore, sequence analysis of UVA-induced canavanine-resistant mutations reveals a bias in favor of GC-->TA events when compared with spontaneous or H(2)O(2)-, UVC-, and gamma-ray- induced canavanine-resistant mutations in the WT strain. Taken together, our data point out a major role of oxidative DNA damage, mostly 7,8-dihydro-8-oxoguanine, in the genotoxicity of UVA radiation in the yeast Saccharomyces cerevisiae. Therefore, the capacity of skin cells to repair 7,8-dihydro-8-oxoguanine may be a key parameter in the mutagenic and carcinogenic effect of UVA radiation in humans.

UVA-induced cyclobutane pyrimidine dimers form predominantly at thymine-thymine dipyrimidines and correlate with the mutation spectrum in rodent cells

Ligation-mediated PCR was employed to quantify cyclobutane pyrimidine dimer (CPD) formation at nucleotide resolution along exon 2 of the adenine phosphoribosyltransferase (aprt) locus in Chinese hamster ovary (CHO) cells following irradiation with either UVA (340-400 nm), UVB (295-320 nm), UVC (254 nm) or simulated sunlight (SSL; lambda > 295 nm). The resulting DNA damage spectrum for each wavelength region was then aligned with the corresponding mutational spectrum generated previously in the same genetic target. The DNA sequence specificities of CPD formation induced by UVC, UVB or SSL were very similar, i.e., in each case the overall relative proportion of this photoproduct forming at TT, TC, CT and CC sites was approximately 28, approximately 26, approximately 16 and approximately 30%, respectively. Furthermore, a clear correspondence was noted between the precise locations of CPD damage hotspots, and of 'UV signature' mutational hotspots consisting primarily of C-->T and CC-->TT transitions within pyrimidine runs. However, following UVA exposure, in strong contrast to the above situation for UVC, UVB or SSL, CPDs were generated much more frequently at TT sites than at TC, CT or CC sites (57% versus 18, 11 and 14%, respectively). This CPD deposition pattern correlates well with the strikingly high proportion of mutations recovered opposite TT dipyrimidines in UVA- irradiated CHO cells. Our results directly implicate the CPD as a major promutagenic DNA photoproduct induced specifically by UVA in rodent cells.

Immune regulation by polysaccharides: implications for skin cancer

UV radiation causes sunburn, premature aging of the skin and is the major environmental carcinogen for squamous cell and basal cell skin cancer in humans. Besides causing mutations in DNA, UV radiation contributes to carcinogenesis by suppressing immune responses to highly antigenic, newly arising neoplasms. Strategies aimed at preventing UV-induced immune suppression, the mechanism of action of the agents used, and the significance of immune protection for prevention of skin cancer are reviewed. This review focuses on the use of plant polysaccharides to prevent immune damage triggered by UV radiation, an approach that goes beyond absorption of UV radiation by sunscreens as a means of reducing tissue damage. The efficacy and mechanism of action of these agents in preserving T cell-mediated immunity to model antigens in human beings and in laboratory animals are discussed.

Body site specific incidence of basal and squamous cell carcinoma in an exposed population, Townsville, Australia

Most data on body site distribution of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) do not take into account the surface proportion occupied by body sites experiencing differing amounts of ultraviolet radiation. The recording of BCC and SCC is heterogenous and body sites are not standardized. This study was undertaken to assess the magnitude of the incidence rates of skin cancers at different body sites in a population which experiences high ultraviolet exposure and involved all primary care services in Townsville over 6 weeks in June 1993. The annual site-specific age-standardized (World Population) incidence rates of histologically diagnosed lesions, determined from data where lesions were recorded on a body map, were adjusted for surface area and expressed per 100,000 body units (BU). Relative site densities of lesions were also calculated. Annual incidence rates for BCC per 100,000 BU on the most exposed face (ears, nose and cheeks) were 25,893 (95% confidence interval (c.i.) 18,837-32,950), 13,222 (95% c.i., 8273-18,171) on the less exposed face (forehead,eyebrow, chin and jaw) and 27,837 (95% c.i., 12,560-43,115) on the least exposed face (area within the orbit and nasolabial fold). Incidence rates for SCC rates were highest on the less exposed face, 5843 (95% c.i., 2627-9058) most exposed face, 4200 (95% c.i., 1274-7126) and the exposed upper limb, 3786 (95% c.i., 2783-4789). The relative site density of histologically confirmed BCC was 14 on the most exposed face in males compared with 5 in females and 11 around the eyes (least exposed) in males and 9 in females compared with the body as a whole. Adjustment for body surface proportion demonstrates that highly exposed body sites are at very high risk. The magnitude of the incidence rates on these sites is attributed to the combination of a susceptible population and high ambient ultraviolet radiation (UVR).