Activation of AMPK results in ubiquitin-dependent degradation of Foxo1a, a pro-diabetic transcription factor regulating hepatic glucose production

Foxo1a is an insulin-regulated transcription factor essentially involved in the regulation of hepatic glucose production and the promoter of the Glucose-6-phosphatase (G6Pase) gene has been found to be an important target of Foxo1a transcriptional activity. G6Pase is a rate-limiting enzyme of hepatic glucose production and G6Pase gene expression is comparably repressed by both insulin and activation of AMPK (AMP-activated kinase). AMPK is a central sensor of the cellular energy charge and metformin, a frequently prescribed antidiabetic drug is thought to affect hepatic glucose production at least in part through activation of AMPK. While insulin is known to inhibit G6Pase gene expression through protein kinase B dependent phosphorylation and nuclear exclusion of the transcription factor Foxo1a, we investigated the mechanism how activation of AMPK can repress the expression of G6Pase. We have previously demonstrated that activation of AMPK led to an almost complete and specific disappearance of Foxo1a protein that was correlated with inhibition of G6Pase gene expression. In this study we provide evidence that clasto-lactacystine, a specific inhibitor of proteasomal protein degradation partially reverses the inhibitory effect of AMPK on G6Pase gene expression. Furthermore, using specific antibodies, we were able to detect ubiquitin-reactive fragments in Foxo1a-immunoprecipitates following activation of AMPK, therefore indicating a proteasome-dependent degradation process.

In conclusion, our data suggest that activation of AMPK results in ubiquitin-dependent degradation of Foxo1a, thereby repressing G6Pase gene expression.