1. ¹Cutaneous Biology Research Center, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts, USA
2. ²Pfizer La Jolla Global Research and Development, San Diego, California, USA
3. ³Department of Chemical Biology, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, New Jersey, USA
4. ⁴Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, Washington, USA

Correspondence: Dr Paul Nghiem, Division of Dermatology, Department of Medicine, University of Washington, 815 Mercer Street, Seattle, Washington 98109, USA. E-mail: pnghiem@u.washington.edu

⁵Current address: Dana-Farber Cancer Institute, Boston, Massachusetts 02115, USA.

⁶These authors contributed equally to this study.

⁷Current address: Division of Dermatology, Department of Medicine, University of Washington, Seattle, Washington 98109, USA

⁸Current address: Cylene Pharmaceuticals, San Diego, California 92121, USA

Received 19 June 2008; Revised 10 November 2008; Accepted 25 November 2008; Published online 26 February 2009.

Abstract

New approaches to prevent and reverse UV damage are needed to combat rising sunlight-induced skin cancer rates. Mouse studies have shown that oral or topical caffeine promotes elimination of UV-damaged keratinocytes through apoptosis and markedly inhibits subsequent skin cancer development. This potentially important therapeutic effect has not been studied in human skin cells. Here, we use primary human keratinocytes to examine which of several caffeine effects mediates this process. In these cells, caffeine more than doubled apoptosis after 75 mJ cm^-2 of ultraviolet light B (UVB). Selectively targeting two of caffeine's known effects did not alter UVB-induced apoptosis: inhibition of ataxia–telangiectasia mutated and augmentation of cyclic AMP levels. In contrast, siRNA against ataxia-telangiectasia and Rad3-related (ATR) doubled apoptosis after UV through a p53-independent mechanism. Caffeine did not further augment apoptosis after UVB in cells in which ATR had been specifically depleted, suggesting that a key target of caffeine in this effect is ATR. Inhibition of a central ATR target, checkpoint kinase 1 (Chk1), through siRNA or a new and highly specific inhibitor (PF610666) also augmented UVB-induced apoptosis. These data suggest that a relevant target of caffeine is the ATR–Chk1 pathway and that inhibiting ATR or Chk1 might have promise in preventing or reversing UV damage.