Role of PI3K/Akt signaling in insulin‐like growth factor‐1 (IGF‐1) skin tumor promotion

Abstract Overexpression of human IGF‐1 with the bovine keratin 5 (BK5) promoter (BK5.IGF‐1 transgenic mice) induces persistent epidermal hyperplasia and leads to spontaneous skin tumor formation. In previous work, PI3K and Akt activities were found to be elevated in the epidermis of BK5.IGF‐1 transgenic mice compared to nontransgenic littermates. In the present study, we examined the importance of the PI3K/Akt signaling pathway in mediating the skin phenotype and the skin tumor promoting action of IGF‐1 in these mice. Western blot analyses with epidermal lysates showed that signaling components downstream of PI3K/Akt were altered in epidermis of BK5.IGF‐1 mice. Increased phosphorylation of GSK‐3 (Ser 9/21 ), TSC2(Thr 1462 ), and mTOR(Ser 2448 ) was observed. In addition, hypophosphorylation and increased protein levels of β‐catenin were observed in the epidermis of BK5.IGF‐1 mice. These data suggested that components downstream of Akt might be affected, including cell cycle machinery in the epidermis of BK5.IGF‐1 mice. Protein levels of cyclins (D1, E, A), E2F1, and E2F4 were all elevated in the epidermis of BK5.IGF‐1 mice. Also, immunoprecipitation experiments demonstrated an increase in cdk4/cyclin D1 and cdk2/cyclin E complex formation, suggesting increased cdk activity in the epidermis of transgenic mice. In further studies, the PI3K inhibitor, LY294002, significantly blocked IGF‐1‐mediated epidermal proliferation and skin tumor promotion in DMBA‐initiated BK5.IGF‐1 mice. In addition, inhibition of PI3K/Akt with LY294002 reversed many of the cell cycle related changes observed in untreated transgenic animals. Collectively, the current results supported the hypothesis that elevated PI3K/Akt activity and subsequent activation of one or more downstream effector pathways contributed significantly to the tumor promoting action of IGF‐1 in the epidermis of BK5.IGF‐1 mice. © 2005 Wiley‐Liss, Inc.