Rapamycin Is a Potent Inhibitor of Skin Tumor Promotion by 12-O-Tetradecanoylphorbol-13-Acetate

Aberrant activation of PI3K/Akt signaling has been implicated in the development and progression of multiple human cancers. During the process of skin tumor promotion induced by treatment with the phorbol ester TPA, activation of epidermal Akt occurs as well as several downstream effectors of Akt, including the activation of mTORC1. Rapamycin, an established mTORC1 inhibitor, was used to further explore the role of mTORC1 signaling in epithelial carcinogenesis, specifically during the tumor promotion stage. Rapamycin blocked TPA-induced activation of mTORC1 as well as several downstream targets. In addition, TPA-induced epidermal hyperproliferation and hyperplasia were inhibited in a dose-dependent manner with topical rapamycin treatments. Immunohistochemical analyses of the skin from mice in this multiple treatment experiment revealed that rapamycin also significantly decreased the number of infiltrating macrophages, T-cells, neutrophils, and mast cells seen in the dermis following TPA treatment. Using a two-stage skin carcinogenesis protocol with DMBA as initiator and TPA as the promoter, rapamycin (5–200 nmol per mouse given topically 30 minutes prior to TPA) exerted a powerful anti-promoting effect, reducing both tumor incidence and tumor multiplicity. Moreover, topical application of rapamycin to existing papillomas induced regression and/or inhibited further growth. Overall, the data indicate that rapamycin is a potent inhibitor of skin tumor promotion and suggest that signaling through mTORC1 contributes significantly to the process of skin tumor promotion. The data also suggest that blocking this pathway either alone or in combination with other agents targeting additional pathways may be an effective strategy for prevention of epithelial carcinogenesis.