573 Immediate, but not delayed, topical caffeine suppresses UV carcinogenesis via blocking error-prone replication across DNA lesions

Ultraviolet (UV) radiation is the most prevalent carcinogen and is associated with nonmelanoma skin cancers whose annual incidence is 5.4 million in the U.S. Several large human epidemiological studies have shown that caffeine intake is associated with a dose-dependent decrease in risk of melanoma and nonmelanoma skin cancers. Based on currently available data, we estimate that approximately 400,000 cases of skin cancer are prevented annually in the U.S. by caffeinated beverage intake. We recently demonstrated that inhibition of the ATR kinase (which senses UV damage and regulates the replication checkpoint) is the relevant mechanism for the cancer-preventive effect of caffeine. However, it is unclear whether ATR should be inhibited “immediately” (minutes) or “long” (weeks) after UV, with translational implications in terms of optimizing the public health benefit. To determine the optimal timing for preventing carcinogenesis, we tested two different schedules of topical caffeine application to the skin of UV-sensitive (Xpc–/–) mice. In the “immediate” schedule, caffeine was topically applied within minutes after each UV irradiation (twice per week for 19 weeks). In the “delayed” schedule, topical application of caffeine (thrice weekly) was initiated after completing 14 weeks of UV treatment (results in mice with a high risk of developing skin cancer over the next several months in the absence of further UV irradiation). Strikingly, immediate caffeine, but not delayed caffeine, suppressed UV-induced skin cancer development. Epidermal cells isolated from UV-irradiated mouse skin showed that immediate caffeine prolongs S-phase arrest and augments apoptosis. Furthermore, in XPC-deficient fibroblasts, immediate caffeine increased the number of UV-induced lesions that abnormally remain on single-stranded DNA in S phase, indicating it inhibited DNA replication across UV lesions (lesion bypass). This finding suggests that caffeine blocks error-prone lesion bypass, thereby decreasing mutation incorporation and UV carcinogenesis.