Transforming growth factor-β2 is a transcriptional target for Akt/protein kinase B via forkhead transcription factor

Abstract

Tumors evade cell death by constitutively activating cell survival pathways and suppressing intrinsic death machinery. Activation of cell survival pathways leads to transcriptional repression of genes associated with cell death and activation of ones promoting anti-apoptosis. Akt/protein kinase B phosphorylates forkhead transcription factors and prevents their nuclear localization, leading to repression of genes involved in apoptosis, such as Fas ligand (FasL). Using bioinformatic approaches, we have identified three consensus sequences for forkhead transcription factor binding in transforming growth factor β2 (TGF-β2) promoter. TGF-β inhibits cell proliferation and induces apoptosis in many cell types, and acquisition of TGF-β resistance is linked to tumorigenesis. In this study, we show that activated Akt down-regulates TGF-β promoter, and sequences within the promoter that are related to consensus forkhead binding sites are necessary for repression. Forkhead factor FKHRL1 binds in vitro to the three consensus sequences and can activate TGF-β2 promoter in normal and Akt-transformed cell lines. In human breast and pancreatic tumors, activated Akt expression correlated with down-regulation of TGF-β 2 mRNA levels. A number of tumor cells expressing activated Akt were responsive to TGF-β addition, indicating the presence of an intact TGF-β-signaling pathway. These results suggest that repression of TGF-β 2 promoter activity in cells expressing activated Akt may play a role in promoting tumorigenesis and escape from the growthinhibitory and/or apoptotic effects of TGF-β.