Overexpression of PRAS40T246A in the Proliferative Compartment Suppresses mTORC1 Signaling, Keratinocyte Migration, and Skin Tumor Development

The proline-rich Akt (v-akt murine thymoma viral oncogene homolog 1) substrate of 40 kDa (PRAS40), an inhibitory component of the mTORC1 complex, was identified as an Akt substrate through phosphorylation at Thr246. Phosphorylation at this site releases PRAS40 from the mammalian/mechanistic target of rapamycin complex 1 (mTORC1) complex allowing increased activity. Targeted expression of a mutant form of PRAS40 (PRAS40T246A) in basal keratinocytes of mouse epidermis (BK5.PRAS40T246A mice) has allowed further examination of mTORC1-specific signaling in epithelial carcinogenesis. BK5.PRAS40T246A mice were resistant to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced epidermal hyperproliferation and skin tumor development. In transgenic mice, PRAS40T246A remained bound to raptor in keratinocytes even after treatment with TPA, consistent with reduced mTORC1 signaling and altered levels of cell cycle proteins. BK5.PRAS40T246A mice also displayed attenuated skin inflammation in response to TPA. Inhibition of mTORC1 in keratinocytes significantly inhibited their migration in vitro and, in addition, inhibited 12-O-tetradecanoylphorbol-13-acetate-induced proliferation and migration of bulge-region stem cells in vivo. Furthermore, targeted inhibition of mTORC1 in BK5.PRAS40T246A mice resulted in delayed wound healing. Decreased keratinocyte migration and impaired wound healing correlated with altered expression of epithelial-mesenchymal transition (EMT) markers and reduced smad signaling. Collectively, the current data using this unique mouse model provide further evidence that mTORC1 signaling in keratinocytes regulates key events in keratinocyte function and epithelial cancer development.