Abstract 1302: Targeting mTORC1 by epidermal overexpression of PRAS40(T246A) suppresses TPA-induced epidermal hyperplasia during tumor promotion.

Abstract Akt activation is an important event during the early stages of skin tumor promotion by diverse tumor promoting stimuli. Transgenic mice overexpressing either wild-type Akt1 (Aktwt) or a constitutively active form of Akt1 (Aktmyr) exhibit dramatically enhanced susceptibility to two-stage skin carcinogenesis and skin tumor promotion. Several downstream Akt targets are modulated (i.e., increased phosphorylation) in keratinocytes during tumor promotion, including the mammalian target of rapamycin (mTORC1). Recent experiments show that inhibiting mTORC1 by using topical rapamycin in both wild-type and BK5.Aktwt transgenic mice can effectively block TPA-induced activation of mTORC1 signaling, epidermal hyperproliferation and skin tumor promotion in a dose-dependent manner. Treatment with rapamycin also inhibited TPA-induced proliferation and migration of labeling retaining cells (LRCs) of the bulge region from the hair follicles to upper epidermal compartments. To further examine the importance of mTORC1 signaling in epithelial carcinogenesis, we generated BK5.PRAS40mt (PRAS40T246A) mice using the BK5 vector that targets expression to the basal layer of epidermis. PRAS40 was originally identified as a novel Akt substrate with a phosphorylation site at Thr246 and acts as a negative regulator when bound to mTORC1. PRAS40T246A expressed in BK5.PRAS40mt mice remained bound to raptor even after TPA treatment as determined by immunoprecipitation analysis. BK5.PRAS40mt mice displayed reduced phosphorylation of both S6K (T389) and 4EBP1 (S65) in epidermis following treatment with TPA. Furthermore, these transgenic mice were significantly less sensitive to TPA-induced epidermal proliferation (measured by both epidermal thickness and labeling index) compared to nontransgenic siblings. TPA-mediated migration and proliferation of LRCs was also inhibited in BK5.PRAS40mt mice. BK5.PRAS40mt mice also expressed lower levels of cell cycle proteins in epidermis after TPA treatment. Collectively, the current data further support the hypothesis that downstream signaling pathways through mTORC1 contribute significantly to the process of skin tumor promotion through effects on proliferation. Supported by CA37111 and 129409. Citation Format: Okkyung Rho, John DiGiovanni. Targeting mTORC1 by epidermal overexpression of PRAS40(T246A) suppresses TPA-induced epidermal hyperplasia during tumor promotion. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1302. doi:10.1158/1538-7445.AM2013-1302