Stimulatory effect of topical application of caffeine on UVB-induced apoptosis in the epidermis of p53 and Bax knockout mice.

3165

In earlier studies, we found that treatment of SKH-1 mice with caffeine topically immediately after exposure to UVB enhanced UVB-induced apoptosis, but topical application of caffeine in the absence of UVB exposure had no effect (Lu et al, Oncol. Res. 13: 61-70, 2002). In addition, topical applications of caffeine to SKH-1 mice previously treated with UVB (“high risk” initiated mice) inhibited the subsequent formation of skin tumors and selectively increased apoptosis in the tumors but not in non-tumor areas of the epidermis (Lu et al, Proc. Nat. Acad. Sci. (USA) 99:12455-12460, 2002). Since UVB-induced tumors possess a high frequency of p53 mutations, the data suggested the possible presence of a caffeine-inducible p53-independent pathway for the regulation of apoptosis in the tumors. In the present study, shaved male or female p53(-/-) C57BL/6J mice and their wild-type littermates were irradiated once with UVB (60 mJ/cm2). The UVB-induced increase in apoptotic sunburn cells in p53(-/-) mice at 6-10 hr after exposure to UVB was only 10-30% of that observed after treatment of p53(+/+) mice with UVB. Topical applications of caffeine immediately after UVB to female p53(+/+) or female p53(-/-) mice enhanced the UVB-induced increase in apoptosis 6 hr later by 127% and 563%, respectively. Similar results were observed in male p53(-/-) mice and by using caspase 3 (active form) immunoreactive positive cells as the biomarker for apoptosis. In another study, shaved female Bax(-/-) C57BL/6J mice and their wild-type littermates were irradiated once with UVB (60 mJ/cm2). The UVB-induced increase in apoptotic sunburn cells in Bax(-/-) mice at 6 hr after exposure to UVB was only 14% of that observed after treatment of Bax(+/+) mice with UVB. Topical application of caffeine immediately after irradiation of Bax(+/+) or Bax(-/-) mice with UVB enhanced the UVB-induced increases in apoptotic sunburn cells at 6 hr by 214% and 467%, respectively, and topical application of caffeine immediately after irradiation of Bax(+/+) or Bax(-/-) mice with UVB enhanced the UVB-induced increase in caspase 3 (active form) positive cells at 6 hr by 253% and 750%, respectively. The results indicate that UVB-induced increases in apoptosis in the epidermis of wild-type mice are predominantly (but not entirely) by p53- and Bax-dependent pathways and that topical application of caffeine can enhance UVB-induced increases in apoptosis by p53- and Bax- independent pathways. (Supported in part by NIH Grants No. CA80759 and CA88961)