The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression
Jun Nakae, … , David L. Silver, Domenico Accili
Jun Nakae, … , David L. Silver, Domenico Accili
Published November 1, 2001
Citation Information: J Clin Invest. 2001;108(9):1359-1367. https://doi.org/10.1172/JCI12876.
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Article

The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression

Abstract

Type 2 diabetes is characterized by the inability of insulin to suppress glucose production in the liver and kidney. Insulin inhibits glucose production by indirect and direct mechanisms. The latter result in transcriptional suppression of key gluconeogenetic and glycogenolytic enzymes, phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6p). The transcription factors required for this effect are incompletely characterized. We report that in glucogenetic kidney epithelial cells, Pepck and G6p expression are induced by dexamethasone (dex) and cAMP, but fail to be inhibited by insulin. The inability to respond to insulin is associated with reduced expression of the forkhead transcription factor Foxo1, a substrate of the Akt kinase that is inhibited by insulin through phosphorylation. Transduction of kidney cells with recombinant adenovirus encoding Foxo1 results in insulin inhibition of dex/cAMP–induced G6p expression. Moreover, expression of dominant negative Foxo1 mutant results in partial inhibition of dex/cAMP–induced G6p and Pepck expression in primary cultures of mouse hepatocyes and kidney LLC-PK1-FBPase+ cells. These findings are consistent with the possibility that Foxo1 is involved in insulin regulation of glucose production by mediating the ability of insulin to decrease the glucocorticoid/cAMP response of G6p.

Authors

Jun Nakae, Tadahiro Kitamura, David L. Silver, Domenico Accili

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(a) Subcellular localization of Foxo1 and Foxo3 in LLC cells. Cells at a...
(a) Subcellular localization of Foxo1 and Foxo3 in LLC cells. Cells at approximately 50% confluence were transiently transfected with c-Myc–tagged Foxo1 or c-Myc–tagged Foxo3. After transfection, cells were seeded into four-well slide culture chambers, cultured overnight in serum-free medium, and incubated in the absence (upper panels) or presence (lower panels) of dex/cAMP for 16 hours. Thereafter, they were treated with insulin (100 nM) for the indicated periods of time. Epitope-tagged Foxo1 and Foxo3 were visualized with anti–c-Myc mAb and FITC-conjugated anti–mouse IgG. At least 200 transfected cells were visually scored for localization of transfected proteins in each experiment. Data represent mean ± SEM from three independent transfection experiments. *P < 0.01 between the number of cells with nuclear staining in Foxo1- and Foxo3-expressing cells by one-factor ANOVA. (b) Insulin-induced Foxo1 phosphorylation. LLC cells were transduced with adenovirus encoding WT Foxo1. After 24 hours, cells were stimulated with insulin (100 nM) for the indicated lengths of time. At the end of the incubation, cells were harvested and detergent extracts were subjected to immunoprecipitation with anti-HA Ab, followed by sequential immunoblotting with anti–phospho-specific Ab’s or anti-Foxo1 Ab, as indicated next to each panel. A representative experiment is shown.