Protective effect of topically applied olive oil against photocarcinogenesis following UVB exposure of mice

Reactive oxygen species have been shown to play a role in ultraviolet light (UV)-induced skin carcinogenesis. Vitamin E and green tea polyphenols redu ce experimental skin cancers in mice mainly because of their antioxidant pr operties. Since olive oil has also been reported to be a potent antioxidant , we examined its effect on UVB-induced skin carcinogenesis in hairless mic e. Extra-virgin olive oil was applied topically before or after repeated ex posure of mice to UVB. The onset of UVB-induced skin tumors was delayed in mice painted with olive oil compared with UVB control mice. However, with i ncreasing numbers of UVB exposures, differences in the mean number of tumor s between UVB control mice and mice pretreated with olive oil before UVB ex posure (pre-UVB group) were lost. In contrast, mice that received olive oil after UVB exposure (post-UVB group) showed significantly lower numbers of tumors per mouse than those in the UVB control group throughout the experim ental period. The mean number of tumors per mouse In the UVB control, pre-U VB and post-UVB groups was 7.33, 6.69 and 2.64, respectively, in the first experiment, and 8.53, 9.53 and 3.36 in the second experiment. Camellia oil was also applied, using the same experimental protocol, but did not have a suppressive effect. Immunohistochemical analysis of DNA damage in the form of cyclobutane pyrimidine dimers (CPD), (6-4) photoproducts and 8-hydroxy-2 '-deoxyguanosine (8-OHdG) in samples taken 30 min after a single exposure o f UVB showed no significant difference between UVB-irradiated control mice and the pre-UVB group. In the post-UVB group, there were lower levels of 8- OHdG in epidermal nuclei, but the formation of CPD and (6-4) photoproducts did not differ. Exposure of olive oil to UVB before application abrogated t he protective effect on 8-OHdG formation. These results indicate that olive oil topically applied after UVB exposure can effectively reduce UVB-induce d murine skin tumors, possibly via its antioxidant effects in reducing DNA damage by reactive oxygen species, and that the effective component may be labile to UVB.