Considerable evidence indicates a crucial role of Akt and mTORC1 signaling in cancer growth and progression. Previously, we discovered that Akt and mTORC1 signaling pathways play an important role in epithelial multistage carcinogenesis using the mouse skin model, especially during the tumor promotion stage. Targeting mTORC1 with rapamycin effectively blocked mTORC1 downstream signaling, epidermal hyperproliferation induced by TPA in a dose-dependent manner in both wild-type and BK5.AktWT transgenic mice. Rapamycin effectively inhibited TPA skin tumor promotion in TPA-hypersensitive BK5.AktWT transgenic mice although higher doses were required to achieve similar levels of inhibition compared to wild-type mice. Elevated Akt expression led to a significant expansion of the label retaining cells (LRCs) as well as a significant increase in K15 expressing cells in the hair follicle of transgenic mice that coincided with expression of phospho-Akt, phospho-S6K and phospho-PRAS40 compared to wild-type mice. Furthermore, TPA-mediated proliferation and migration of LRCs to the interfollicular epidermis from the hair follicle were significantly inhibited by rapamycin. PRAS40 was originally identified as a novel Akt substrate through Akt-mediated phosphorylation at Thr246. Recently, PRAS40 has been identified as a negative regulator when bound to mTORC1. We have used the bovine keratin 5 (BK5) promoter to generate transgenic mice that overexpress a mutant form of PRAS40 (PRAS40T246A) to further examine the importance of mTORC1 signaling in epithelial carcinogenesis. BK5.PRAS40T246A transgenic mice were significantly less sensitive to TPA-induced epidermal hyperproliferation and hyperplasia measured by both epidermal thickness and labeling index. Overexpression of mutant PRAS40T246A in the epidermis led to a significant suppression of skin tumor development by 63% as well as reductions in tumor weight and tumor size compared to wild-type mice. Immunoprecipitation analysis showed that PRAS40T246A in BK5.PRAS40T246A mice remained bound to raptor even after TPA treatment. In addition, BK5.PRAS40T246A mice displayed reduced TPA-induced mTORC1 signaling and cell cycle progression, but increased autophagy signaling and elevated apoptotic cells in the epidermis following TPA treatment. Furthermore, TPA-induced proliferation and migration of LRCs was also effectively inhibited in BK5.PRAS40T246A mice. Collectively, the current data demonstrate that mTORC1 downstream signaling pathways contribute significantly to the process of skin tumor promotion leading to a potential target for chemoprevention of cancer. Supported by CA037111 and CA0129409.
Citation Format: Okkyung Rho, Jiyoon Cho, John DiGiovanni. Targeting mTORC1 suppresses proliferation of keratinocyte stem cells and inhibits skin tumor promotion in mice. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5357. doi:10.1158/1538-7445.AM2014-5357