Inhibition of FOXO3 Tumor Suppressor Function by βTrCP1 through Ubiquitin-Mediated Degradation in a Tumor Mouse Model

Wen-Bin Tsai,Yiyu Zou,Keiko Nakayama,Mickey C-T Hu,Young Min Chung,See-Hyoung Park,Zhaohui Xu,Sue-Hwa Lin,Alexander Swarbrick

doi:10.1371/journal.pone.0011171

Abstract

BackgroundThe ubiquitin-proteasome system is the primary proteolysis machine for controlling protein stability of the majority of regulatory proteins including those that are critical for cancer development. The forkhead box transcription factor FOXO3 plays a key role in regulating tumor suppression; however, the control of FOXO3 protein stability remains to be established. It is crucial to elucidate the molecular mechanisms underlying the ubiquitin-mediated degradation of FOXO3 tumor suppressor.Methodology and Principal FindingsHere we show that βTrCP1 oncogenic ubiquitin E3-ligase interacts with FOXO3 and induces its ubiquitin-dependent degradation in an IκB kinase-β phosphorylation dependent manner. Silencing βTrCP1 augments FOXO3 protein level, resulting in promoting cellular apoptosis in cancer cells. In animal models, increasing FOXO3 protein level by silencing βTrCP1 suppresses tumorigenesis, whereas decreasing FOXO3 by over-expressing βTrCP1 promotes tumorigenesis and tumor growth in vivo.Conclusions/SignificanceThis is a unique demonstration that the βTrCP1-mediated FOXO3 degradation plays a crucial role in tumorigenesis. These findings significantly contribute to understanding of the control of FOXO3 stability in cancer cells and may provide opportunities for developing innovative anticancer therapeutic modalities.

Highlights

FOXO3 is a member of the forkhead box class O (FOXO) transcription factors which have been shown to play critical roles in modulating a number of cellular processes, such as metabolism, differentiation, and transformation in animal cells [1,2,3,4,5,6,7]
While we examined the expression of FOXO3 protein in various cancer cell lines, we unexpectedly found that the level of FOXO3 protein appeared to be inversely correlated with the level of bTrCP1 protein in various cancer cell lines, suggesting that bTrCP1 E3 Ub-ligase may contribute to a decrease of FOXO3
Over-expression of mutant bTrCP1DF-myc, which contains a deletion at the N-terminal F-box domain (a.a. 17-52) that abolishes its E3 ligase activity [26], increased the protein level of FOXO3 as compared to that in cells cotransfected with the FOXO3-Flag and bTrCP1-myc in the absence of proteasome inhibitor MG-132 (Figure S1B), suggesting that the expression of bTrCP1DF may inhibit the degradation of FOXO3 protein mediated by endogenous bTrCP1

Summary

Introduction

FOXO3 (or FOXO3a) is a member of the forkhead box class O (FOXO) transcription factors which have been shown to play critical roles in modulating a number of cellular processes, such as metabolism, differentiation, and transformation in animal cells [1,2,3,4,5,6,7]. We further show that downregulation of FOXO3 protein level by ectopic expression of bTrCP1 in breast cancer cells promotes tumor proliferation or tumorigenesis.

Results

Conclusion