Transforming Growth Factor-β2 Is a Transcriptional Target for Akt/Protein Kinase B via Forkhead Transcription Factor

Ahmed A Samatar,Luquan Wang,Asra Mirza,Sandra Koseoglu,Suxing Liu,Chandra C Kumar

doi:10.1074/jbc.m203686200

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 beta2 (TGF-beta2) promoter. TGF-beta inhibits cell proliferation and induces apoptosis in many cell types, and acquisition of TGF-beta resistance is linked to tumorigenesis. In this study, we show that activated Akt down-regulates TGF-beta2 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-beta2 promoter in normal and Akt-transformed cell lines. In human breast and pancreatic tumors, activated Akt expression correlated with down-regulation of TGF-beta 2 mRNA levels. A number of tumor cells expressing activated Akt were responsive to TGF-beta addition, indicating the presence of an intact TGF-beta-signaling pathway. These results suggest that repression of TGF-beta 2 promoter activity in cells expressing activated Akt may play a role in promoting tumorigenesis and escape from the growth-inhibitory and/or apoptotic effects of TGF-beta.

Highlights

Akt/protein kinase B is a cellular homologue of the viral oncogene v-Akt of the transforming retrovirus Akt-8 [1]
transforming growth factor ␤2 (TGF-␤2) mRNA Is Down-regulated in Tumor Cells Expressing Activated Akt—we examined the relationship between Akt activation and TGF-␤2 mRNA levels in several human tumor cell lines
In this study we have shown that TGF-␤2 promoter contains three consensus binding sites for forkhead transcription factor and is down-regulated by activated Akt expression

Summary

Introduction

Akt/protein kinase B is a cellular homologue of the viral oncogene v-Akt of the transforming retrovirus Akt-8 [1]. Under conditions of growth factor deprivation, the phosphatidylinositol 3-kinase/ Akt pathway is inactivated, and forkhead factor is unphosphorylated, leading to activation of genes associated with programmed cell death. Genes that might be targets for transcriptional regulation by the forkhead transcription factors have been identified in several systems. These include the insulin-like growth factor-binding protein 1 (IGFBP-1)1 [9], Fas ligand (FasL) [10], Bim [11], and p27 [12]. We show that TGF-␤2 promoter contains three consensus sequences for forkhead transcription factor binding, and TGF-␤2 is a transcriptional target for Akt/phosphatidylinositol 3-kinase signaling pathway. TGF-␤2 mRNA levels are down-regulated in several human breast and pancreatic cell lines expressing activated Akt

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