Control of Foxo1 Gene Expression by Co-activator P300

Anne R Wondisford,Lishou Xiong,Evan Chang,Shumei Meng,David J Meyers,Mingsong Li,Philip A Cole,Ling He

doi:10.1074/jbc.m113.540500

Anne R Wondisford, Lishou Xiong + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m113.540500

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Abstract

FOXO1 is an important downstream mediator of the insulin signaling pathway. In the fed state, elevated insulin phosphorylates FOXO1 via AKT, leading to its nuclear exclusion and degradation. A reduction in nuclear FOXO1 levels then leads to suppression of hepatic glucose production. However, the mechanism leading to expression of Foxo1 gene in the fasted state is less clear. We found that Foxo1 mRNA and FOXO1 protein levels of Foxo1 were increased significantly in the liver of mice after 16 h of fasting. Furthermore, dibutyrl cAMP stimulated the expression of Foxo1 at both mRNA and protein level in hepatocytes. Because cAMP-PKA regulates hepatic glucose production through cAMP-response element-binding protein co-activators, we depleted these co-activators using adenoviral shRNAs. Interestingly, only depletion of co-activator P300 resulted in the decrease of Foxo1 mRNA and FOXO1 protein levels. In addition, inhibition of histone acetyltransferase activity of P300 significantly decreased hepatic Foxo1 mRNA and FOXO1 protein levels in fasted mice, as well as fasting blood glucose levels. By characterization of Foxo1 gene promoter, P300 regulates the Foxo1 gene expression through the binding to tandem cAMP-response element sites in the proximal promoter region of Foxo1 gene.

Highlights

The mechanism driving hepatic gene expression of Foxo1 in the fasted state remains unclear
Fasting Resulted in a Marked Increase of Hepatic Foxo1 mRNA and FOXO1 Protein Levels—Because phosphorylation of FOXO1 by activated AKT in the insulin signaling pathway results in FOXO1 nuclear exclusion and degradation in the fed state [12, 13], it is conceivable that fasting might increase FOXO1 levels due to its important role in stimulating gluconeogenesis and maintaining euglycemia [7,8,9]
The inhibition of Foxo1 transcription by actinomycin D blocked induction of FOXO1 protein levels by cAMP (Fig. 2c). These data suggest that the cAMP-PKA pathway activates Foxo1 gene expression and that the activation of the gluconeogenic enzyme profile by glucagon stimulation during fasting is mediating at least in part by an increase in Foxo1 gene expression

Summary

Background

The mechanism driving hepatic gene expression of Foxo in the fasted state remains unclear. FOXO1 is important in inhibition of hepatic gluconeogenesis by insulin; insulin inhibits FOXO1 activity through the PI3K/ AKT signaling pathway [12, 13] Another layer of FOXO1 regulation is via acetylation of the cAMP-response element-binding protein (CREB) co-activators P300 and CBP (14 –17). Phosphorylation of FOXO1 by insulin leads to its nuclear exclusion and degradation in the fed state [12, 13], yet the mechanism driving Foxo expression in the fasted state remains unclear. Phosphorylation of CRTC2 at Ser-171 by insulin leads to its nuclear exclusion and degradation [19] Considering these studies, we wanted to test the hypothesis that elevated fasting glucagon levels increase. We have examined the potential role of CREB co-activators in increasing Foxo gene expression in in vitro and in vivo experiments that model the fasting state

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION