COP1 Functions as a FoxO1 Ubiquitin E3 Ligase to Regulate FoxO1-mediated Gene Expression

Satomi Kato,Jixin Ding,Evan Pisck,Ulupi S Jhala,Keyong Du

doi:10.1074/jbc.m801011200

Satomi Kato, Jixin Ding + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m801011200

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Abstract

COP1 is a Ring-Finger E3 ubiquitin ligase that is involved in plant development, mammalian cell survival, growth, and metabolism. Here we report that COP1, whose expression is enhanced by insulin, regulates FoxO1 protein stability. We found that in Fao hepatoma cells, ectopic expression of COP1 decreased, whereas knockdown of COP1 expression increased the level of endogenous FoxO1 protein without impacting other factors such as C/EBPalpha and CREB (cAMP-response element-binding protein). We further showed that COP1 binds FoxO1, enhances its ubiquitination, and promotes its degradation via the ubiquitin-proteasome pathway. To determine the biological significance of COP1-mediated FoxO1 protein degradation, we have examined the impact of COP1 on FoxO1-mediated gene expression and found that COP1 suppressed FoxO1 reporter gene as well as FoxO1 target genes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, two key targets for FoxO1 in the regulation of gluconeogenesis, with corresponding changes of hepatic glucose production in Fao cells. We suggest that by functioning as a FoxO1 E3 ligase, COP1 may play a role in the regulation of hepatic glucose metabolism.

Highlights

In plants COP1 targets transcription factors including LAF1, HY5, HYH, and HFR1 [4] and photoreceptors including phytochrome A and cryptochrome [1, 5] for ubiquitin-dependent proteasomal degradation
Because COP1 interacts with TRB3 and because TRB3 has been shown to regulate glucose metabolism [10, 11], we set out to further investigate COP1 function in hepatoma cells
COP1 Expression Is Regulated by Insulin and cAMP in Fao Hepatoma Cells—COP1 interacts with TRB3 [9] and TRB3 has been shown to regulate glucose metabolism in the liver and muscle [10, 11]

Summary

Introduction

In plants COP1 targets transcription factors including LAF1, HY5, HYH, and HFR1 [4] and photoreceptors including phytochrome A and cryptochrome [1, 5] for ubiquitin-dependent proteasomal degradation. Phosphorylation by Akt provides multiple mechanisms to regulate FoxO1-mediated gene expression including nuclear exclusion, FoxO1 protein degradation (for a review, see Ref. 16), and disrupting interaction between FoxO1 and its coactivator PGC1 [18]. Because COP1 interacts with TRB3 and because TRB3 has been shown to regulate glucose metabolism [10, 11], we set out to further investigate COP1 function in hepatoma cells During these studies we uncovered that COP1 expression is induced by insulin and that COP1 modulates FoxO1 protein stability. Our data show that by down-regulation of FoxO1 COP1 regulates FoxO1 target genes such as G6Pase and PEPCK gene expression and, thereby, hepatic glucose production

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