Abstract 2481: Rapamycin is a potent inhibitor of skin tumor promotion by TPA

Abstract

Abstract The activation of Akt signaling occurs early during skin tumorigenesis. Data from our lab has demonstrated that the phorbol ester TPA leads to activation of Akt as well as several downstream effectors of Akt. Of particular interest is the Akt downstream signaling molecule, mTOR, which is elevated in the epidermis of wild-type mice following treatment with TPA. These studies suggest that activation of mTOR may contribute to skin tumor promotion. To further test this hypothesis, rapamycin, which is an established mTORC1 inhibitor, was used to further explore the role of mTOR signaling in epithelial carcinogenesis and specifically during tumor promotion. In a multiple treatment regimen, rapamycin was applied topically 30 min prior to TPA at doses of 1 umol, 100 nmol and 20 nmol. Rapamycin, in a dose dependent manner inhibited TPA induced mTOR activity as well as several downstream signaling proteins including p70S6KT389 and pS6S240/244 as well as p4EBP1T37/46, the FRAP/mTOR phosphorylation site of translational repressor protein 4EBP1. In addition, the highest dose of rapamycin caused a reduction in Akt-mediated phosphorylation of Akt downstream targets GSK3B as well as PRAS40 as assessed by western blotting using phosphospecific antibodies. These data suggest that multiple treatments with rapamycin at higher doses blocked both mTORC1 and mTORC2, while lower doses primarily affected mTORC1. Multiple treatment experiments evaluating the effects of rapamycin (5-200 nmol/mouse) on epidermal hyperplasia and epidermal labeling index, two short-term markers of tumor promotion by TPA, demonstrated a significant inhibition of both parameters in a dose dependent manner. Using a two-stage chemical skin carcinogenesis model with the standard DMBA/TPA protocol, the ability of rapamycin to block skin tumor promotion by TPA was evaluated. Rapamycin exerted a powerful anti-promoting effect reducing the average number of papillomas per mouse as well as the overall incidence of papillomas. In this regard, even a 5 nmol topical dose of rapamycin given 30 min prior to each TPA treatment produced approximately 50% inhibition of papilloma formation. Moreover, topical application of rapamycin to existing papillomas induced regression and/or inhibited growth. Skin tumors from this study were collected from TPA or TPA + rapamycin treated groups and are being evaluated for expression of proteins involved in cell cycle regulation as well as proteins involved in autophagy and apoptosis. The results of these experiments will be presented. Collectively, the current results indicate that signaling through mTORC1 contributes significantly to the process of skin tumor promotion, and that blocking this pathway may be an effective strategy for prevention of epithelial carcinogenesis. Supported by NIH grant CA37111. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2481.